Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy Harvesting Applications

*Sofia Henriques Ferreira, Ana Rovisco, Andreia dos Santos, Hugo Águas, Rui Igreja, Pedro Barquinha, Elvira Fortunato and Rodrigo Martins*

#### **Abstract**

The ever-growing global market for smart wearable technologies and Internet of Things (IoT) has increased the demand for sustainable and multifunctional nanomaterials synthesized by low-cost and energy-efficient processing technologies. Zinc oxide (ZnO) is a key material for this purpose due to the variety of facile methods that exist to produced ZnO nanostructures with tailored sizes, morphologies, and optical and electrical properties. In particular, ZnO nanostructures with a porous structure are advantageous over other morphologies for many applications because of their high specific surface area. In this chapter, a literature review on the latest progress regarding the synthesis and applications of ZnO with a porous morphology will be provided, with special focus on the synthesis by microwave hydrothermal method of these nanomaterials and their potential for application in energy harvesting devices. Nanogenerators of a composite made by polydimethylsiloxane (PDMS) and porous ZnO nanostructures were explored and optimized, with an output voltage of (4.5 ± 0.3) V being achieved for the best conditions. The daily life applicability of these devices was demonstrated by lighting up a commercial LED, by manually stimulating the nanogenerator directly connected to the LED.

**Keywords:** zinc oxide, microwave synthesis, porous nanostructures, energy harvesting devices

#### **1. Introduction**

Zinc oxide (ZnO) is an inorganic semiconductor material that has been applied in a wide range of applications over the last centuries [1]. The attraction to ZnO can be attributed to its remarkable optical and electronic characteristics. With a direct and wide bandgap of 3.37 eV and a large exciton binding energy of 60 meV at room temperature [2], ZnO has the potential to be applied in advanced electronic and optoelectronic devices with promising results, such as UV sensors [3, 4], transparent electrodes [5, 6], gas sensors [7], thin film transistors [8, 9], and solar cells [10–12].

Moreover, ZnO is a low-cost and biocompatible material with high photostability, high chemical and thermal stability, low toxicity, and a broad range of UV radiation absorption [13]. These properties allow ZnO to be applied in a wide range of applications besides electronic devices, such as skin ointments and sunscreens, rubber tires, paints, bioimaging, drug delivery, biosensors, antibacterial textiles, and photocatalysis for the degradation of pollutants in wastewaters [1, 14–19].

Due to its piezoelectric properties, ZnO nanostructures have also been widely explored for energy harvesting applications, being an important sustainable energy source [20]. The demand for wearable devices led to a high development of new energy sources. Nanogenerators have demonstrated the capability to power small electronic devices, appearing as a good alternative to batteries [21]. The most common nanogenerators are based on piezoelectric and/or triboelectric effects. In the piezoelectric nanogenerators, mechanical energy is converted into electrical energy through piezoelectric polarization resultant from strain [1]. The triboelectric effect results from the surface charges' generation subsequent from the friction between two different materials (with opposite triboelectric polarities) [22].

Materials with piezoelectric properties have the capability to convert mechanical energy into electrical energy [1]. Within the different piezoelectric materials, lead zirconate titanate (PZT) is the material that presented so far the highest piezoelectric coefficient (d33 = 593 pC N−1), still this material has a high toxicity [23, 24]. While presenting a much lower d33 value(≈ 10 pC N−1) [25–27], ZnO is a very good alternative, since it is not only sustainable and eco-friendly, as it can also be easily fabricated, while still presenting a good performance [28, 29].

Nanogenerators of different types of ZnO nanostructures (i.e., nanorods, nanoparticles, nanoflowers) have been reported [30–34]. For example, Saravanakumar et al. reported a nanogenerator fabricated using vertically grown ZnO nanowires with surrounding PDMS, with output values of 6 V/4 nA/0.39 nW cm−2 under finger bending [35]. Rahman et al. used ZnO nanoparticles dispersed into a PDMS film, achieving output values of 20 V/20 μA/20 μW, with finger tapping [36]. As another example, ZnO nanoflowers were mixed with multiwalled carbon nanotubes and PDMS, with an output of 75 V/3.2 μA/260 mW cm−2 being obtained. In this case, the devices were tested in the soles of human shoes with the force being applied by a person walking [37].

#### **1.1 Synthesis and applications of porous ZnO nanostructures**

Despite all the established applications of ZnO, the research involving this semiconductor has not yet diminished, mostly due to the continuing development of new synthesis technologies and applications. For instance, ZnO nanomaterials can be easily synthesized into tailored sizes and morphologies at low temperatures (< 200 °C) by a variety of methods, including chemical bath deposition [38], electrodeposition [39], chemical vapor deposition [40], electrospinning [41], laser assisted flow deposition [42], and solvothermal [16] or hydrothermal [43, 44] synthesis, either by conventional or microwave-assisted heating [4, 45].

Porous oxide semiconductor nanomaterials, particularly two-dimensional (2D) materials with nanoscale thickness, are promising candidates due to their usually large specific surface areas that can improve their performance in several applications [46–51]. These nanomaterials can inclusively assemble into three-dimensional (3D) hierarchically structures with controlled morphology and dimensions which can lead to novel properties and applications [52]. The self-assemble technique is a facile method to produce 3D hierarchical structures where low-dimension building units aggregate spontaneously into high-dimensional architectures. This technique

#### *Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

offers many advantages over other methods as it can be performed at low temperatures using low-cost materials while having high yield for scale production [52].

An indirect way to produce porous 3D ZnO structures has been recently developed by thermal decomposition of layered zinc hydroxide (LZH) precursors [52]. LZHs are usually composed of positively charged zinc hydroxyl layers intercalated by anions that balance the overall charge and water molecules [53]. The anions in LZH generally include CO3 2−, SO4 2−, NO3 − , Cl− , CH3COO− [50, 53–58]. These LZH precursors are fabricated with the desired morphology and then converted into porous ZnO nanomaterials by a calcination process at high temperatures [59]. During calcination, the precursors release gaseous molecules and, consequently, the original structure contracts and pores are formed throughout the structures [52].

LZHs are typically obtained via solution techniques, mainly hydrothermal methods where the materials' synthesis occurs in a basic medium that results from the addition of certain reagents, such as hexamethylenetetramine [60], ammonia [58, 61], and urea [52, 62–65]. Although the basic structure is similar in all LZHs, the sites occupied by the anions and water molecules are different and, as a result, the final morphology, crystal structure, interlayer distances, and thermal decomposition temperature differ depending on the anion type [66]. In particular, the LZH carbonate (LZHC) is composed of zinc hydroxide layers combined with carbonate ions and water molecules. During the synthesis of this material, a well-crystallized phase is typically obtained with an invariable distance between the LZH [53]. The resulting morphology of LZHC usually consists of 2D structures stacked in a hierarchical 3D arrangement. However, the synthesis of uniform LZHC 3D morphologies through a simple and fast hydrothermal method has not yet been fully explored.

For this purpose, hydrothermal synthesis assisted by microwave irradiation offers many advantages over conventional heating. In a synthesis assisted by conventional heating, the heat transfer occurs through a combination of conductive and convective mechanisms that result in a low heating rate and, consequently, long synthesis time [67]. Conventional heating method is also dependent on the thermal conduction of the material of which the reaction vessel walls are made. Moreover, the temperature maximum occurs on the vessel wall surface, as shown in **Figure 1**. All these factors can lead to a non-uniform heating of the reaction medium and, subsequently, originate a heterogeneity in the obtained products [1]. On the other hand, hydrothermal synthesis assisted by microwave irradiation allows for rapid and uniform heating since the heat transfer occurs directly from the microwaves to the molecules of the reaction's materials, as illustrated in **Figure 1**. This results in high reaction rates and a homogeneous and volumetric heating [68, 69].

The porous morphology of ZnO nanostructures obtained by calcination of LZHC significantly increases the materials' specific surface area [70] and, therefore,

these ZnO nanomaterials have been used in applications that benefit from this characteristic, such as photocatalysis [51, 52, 70], gas sensors [50, 54, 71–73], surface enhanced Raman scattering (SERS) substrates [74], dye-synthesized solar cells [44, 75, 76], and battery electrodes [65].

This work aims to demonstrate the potential of high surface area porous ZnO nanostructures for energy harvesting devices, showing original and novel results regarding the characterization of nanogenerators based on these structures. For that, 3D hierarchically structures composed of LZHC nanoplates were successfully synthesized through a facile, low-cost, and low temperature hydrothermal process assisted by microwave irradiation. Porous ZnO nanostructures were obtained by calcination of the LZHC at 700 °C for 2 h in air while maintaining the LZHC hierarchical 3D structure. Porous ZnO nanostructures were then embedded in PDMS and deposited by spin-coating technique on flexible substrates. Energy harvesting based on a micro-structured composite of porous ZnO nanostructures embedded in PDMS was investigated. The combination of using the porous ZnO nanostructures, which have piezoelectric properties, and triboelectricity resultant from the microstructuring leads to a performance improvement of the nanogenerators [37, 77]. To the best of our knowledge, porous ZnO nanostructures were for the first time used to fabricate a micro-structured PDMS/ZnO composite for energy harvesting devices.

#### **2. Materials and methods**

#### **2.1 Synthesis and characterization of porous ZnO nanostructures**

Porous ZnO nanostructures were synthesized by hydrothermal method assisted by microwave irradiation. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma-Aldrich 98%) and urea (CH4N2O, Sigma-Aldrich 99.0–100.5%) were used without further purification. In a typical synthesis, 0.05 M of zinc nitrate was first dissolved in de-ionized water, and after its total dissolution, urea was added to the aqueous solution. The molar ratio of zinc to urea was kept at 1:5. Then, 25 mL of the obtained solution was transferred to a 35 mL Pyrex vessel which was placed in a CEM Discovery SP microwave. The synthesis was carried out at 140 °C for 15 min under a power of 100 W.

After the synthesis, the resulting white precipitates were washed with deionized water followed by isopropanol and centrifuged at 4500 rpm for 5 min. This washing process was repeated three times. The powders were dried in air at room temperature for 48 h and then calcinated in air in a Nabertherm muffle furnace at 700 °C for 2 h with a heating rate of 250 °C h−1.

The crystallinity of the produced nanostructures was analyzed by X-ray diffraction (XRD) using a PANalytical's X'Pert PRO MRD X-ray diffractometer, with a monochromatic Cu Kα radiation source with wavelength 1.540598 Å. XRD measurements were carried out from 10 to 90° (2θ), with a scanning step size of 0.016°. The morphology of the LZHC precursor and porous ZnO nanostructures was evaluated by scanning electron microscopy (SEM) using a Carl Zeiss AURIGA CrossBeam FIB-SEM workstation equipped with an Oxford X-ray Energy Dispersive Spectrometer.

Differential scanning calorimetric (DSC) and thermogravimetry (TG) measurements of the synthesized product without any temperature treatment were carried out with a simultaneous thermal analyzer NETZSCH STA 449 F3 Jupiter. Approximately 20 mg of the synthesized powder was loaded into an open platinumrhodium crucible and heated in air from room temperature to 850 °C with a heating rate of 10 °C min−1.

*Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

Diffuse reflectance measurements of the porous ZnO nanostructures were performed at room temperature using a PerkinElmer lambda 950 UV/VIS/NIR spectrophotometer with a diffuse reflectance module with a 150 mm diameter integrating sphere, internally coated with Spectralon. The calibration of the system was achieved by using a standard Spectralon reflector sample as reference. The reflectance spectra were obtained from 350 to 800 nm.

#### **2.2 Fabrication and characterization of energy harvesting devices**

The devices were fabricated as described in references [27, 78] and the fabrication process is illustrated in **Figure 2**. Briefly, composites of porous ZnO nanostructures embedded in PDMS were produced with concentrations of 20, 25, and 30 wt%. Firstly, the nanostructures were mixed with the PDMS elastomer (from Dow Corning) and a volume of ethyl acetate (from Fluka-Honeywell) enough to ensure a homogeneous mixture of elastomer and nanostructures. The mixture was stirred until the evaporation of the solvent, and then a curing agent (Sylgard 184, from Dow Corning) was added in a weight ratio to the elastomer of 1:10 while stirring to obtain a homogeneous mixture. Two types of devices were produced, unstructured and micro-structured nanogenerators. The former was fabricated by spin-coating the mixture at 250 rpm for 90 s, with an acceleration of 100 rpm·s−1, on commercial substrates of polyethylene terephthalate (PET) with a layer of indium tin oxide (ITO) deposited on top (PET/ITO, from Kintec Company), whereas the latter was obtained by depositing the mixture in a similar way on acrylic molds (5 mm thick, from Dagol). The acrylic molds used were produced as described in reference [79].

The composites were then cured at 60 °C for 1 h. After the curing process, PET/ITO electrodes were placed on top of the composite films, as shown in **Figure 2**. The electrical characterization of the produced nanogenerators was performed by applying a mechanical stimulus in a contact area of 0.3 cm<sup>2</sup> with a pushing force of 2.3 N at different frequencies (0.5, 1, 1.5, and 2 pushes per second) with a home-made machine with a linear motor.

#### **Figure 2.**

*Fabrication schematic of a micro-structured nanogenerator based on a PDMS/ZnO composite film. Adapted from dos Santos et al. [78].*

#### **3. Porous ZnO nanostructures as a piezoelectric material for nanogenerators**

#### **3.1 Synthesis of porous ZnO nanostructures**

#### *3.1.1 Characterization of the LZHC precursor*

**Figure 3(a)** presents the X-ray diffractogram obtained from the final product of the hydrothermal synthesis prior to the calcination process. All the peaks from the diffractogram can be indexed to zinc hydroxide carbonate hydrate (Zn4(CO3) (OH)6·H2O) (ICDD 11–0287). The morphology of the precursor was observed in SEM and it is shown in **Figure 3(b)**. The SEM image reveals that the LZHC precursor obtained after only 15 min of microwave hydrothermal synthesis consists of many flower-like structures, with a few micrometers of diameter, composed of densely packed LZHC nanoplates with a few nanometers of thickness.

Differential scanning calorimetric (DSC) measurements were carried out in air from room temperature to 800 °C to analyze the conversion process of LZHC into ZnO. The DSC curve in **Figure 3(c)** shows two endothermic peaks at 64 °C and 266 °C. The peak at 64 °C corresponds to the removal of water that is weakly adsorbed to the LZHC nanostructures [80], resulting in a weight loss of 38.12%. The second peak at 266 °C results in a weight loss of 14.87% and it is associated with the release of water and carbon dioxide from the thermal decomposition of LZHC precursor [50, 59].

#### *3.1.2 Characterization of the porous ZnO nanostructures*

After the calcination process, the LZHC precursor was successfully converted into porous ZnO nanostructures, which can be inferred from the X-ray diffractograms of the samples obtained at 700 °C, depicted in **Figure 4(a)**. All the peaks in the diffractogram correspond to the hexagonal wurtzite ZnO structure (ICDD 36–1451). No characteristic peaks from any other impurities were detected, indicating that the LZHC precursor was completely converted into ZnO. SEM images of the calcinated product are presented in **Figure 4(b)** with different magnifications. The low magnification image shows that the morphology of the final ZnO product did not suffer significant changes when compared with the LZHC precursor, since ZnO nanoplates are still assembled into flower-like structures. However, when observing the high magnification SEM images, it is possible to see that the ZnO nanoplates present a porous structure with serrate edges and a wide pore size distribution.

The synthesis of LZHC precursor by urea-assisted hydrothermal method, followed by the calcination process to originate porous ZnO nanostructures, has been explained before in the literature [44, 65, 73, 80, 81]. **Figure 5** shows a simple schematic of the synthesis and transformation process of LZHC precursor into porous ZnO nanostructures. During the hydrothermal synthesis, urea is hydrolyzed leading to the formation of hydroxide (OH<sup>−</sup> ) and carbonate (CO3 2−) ions. Zn2+ ions from the added zinc salt react with both OH<sup>−</sup> and CO3 2− ions forming the LZHC precursor (Zn4(CO3)(OH)6·H2O). It has been reported that the surface of LZHC plates is hydrophobic whereas the lateral sides are hydrophilic, resulting in a vertical growth of this material and consequent plate-like morphology [76, 82, 83]. The agglomeration of these nanoplates into stable flower-like microstructures occurs to favor the minimization of surface energy by reducing exposed surface areas [83]. Under calcination at high temperature, LZHC is decomposed into ZnO by releasing

*Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

**Figure 3.** *(a) X-ray diffractogram, (b) SEM images and (c) TG/DSC curves of the LZHC precursor.*

#### **Figure 4.**

*(a) XRD diffractogram and (b) SEM images of porous ZnO nanostructures synthesized by hydrothermal method assisted by microwave irradiation followed by calcination at 700 °C for 2 h in air.*

H2O and CO2 in the form of gas, which leads to a contraction of the original structure which originates pores throughout the nanoplates and a consequent porous ZnO structure, as illustrated in **Figure 5** [84].

The UV–Vis diffuse reflectance of the produced ZnO samples is presented in **Figure 6**. The optical band gap *E*g was calculated by applying the Kubelka-Munk (K-M) method to the reflectance (*R*) data [85]. The K-M method is based on the following equation:

$$F(R) = \frac{\left(\mathbf{1} - R\right)^2}{2R} \tag{1}$$

#### **Figure 5.**

*Schematic of the hydrothermal synthesis assisted by microwave irradiation and calcination process of porous ZnO structures.*

The K-M function (*F(R)*) is proportional to the absorption coefficient (*α*). Therefore, by considering the Tauc relation, the following expressions can be obtained [86]:

$$F(R) \propto \alpha \propto \frac{\left(h\nu - E\_{\ll}\right)^{1/n}}{h\nu} \tag{2}$$

$$\left(F(R)h\nu\right)^{n} = A\left(h\nu - E\_{\p}\right) \tag{3}$$

where *A* is a constant and *n* is equal to 2 for semiconductors with direct allowed transitions [87]. As shown by the inset graph in **Figure 6**, the value of *E*g can be determined by extrapolating the linear part of the function curve with the energy axis. The estimated bandgap energy is 3.26 eV for ZnO nanostructures obtained at 700 °C, which is consistent with the values reported in the literature [52, 88].

#### **3.2 Fabrication of energy harvesting devices**

#### *3.2.1 Characterization of the PDMS/ZnO composite films*

Composites of porous ZnO nanostructures embedded in PDMS (PDMS/ZnO films) were fabricated. The composites were produced with a micro-structuring and in an unstructured form. SEM images of a micro-structured porous PDMS/ ZnO film are presented in **Figure 7(a)**. The array of aligned cones observed has an average height of 380 μm, an average diameter of 300 μm, and a gap around 100 μm. **Figure 7(b)** combines the XRD diffractogram of the porous ZnO nanostructures, the PDMS/ZnO composite film, and the pure PDMS film. As expected, even if presenting a much lower intensity, the hexagonal wurtzite ZnO structure (ICDD 36–1451) can be identified in the PDMS/ZnO composite, whereas the PDMS film presents an amorphous structure.

#### *3.2.2 Performance of the PDMS/ZnO nanogenerators*

To optimize the nanogenerator output, its performance was evaluated by varying the concentration of the porous ZnO nanostructures in the PDMS film. This study was performed with unstructured composites. Three concentrations were considered to produce the devices: 20, 25, and 30 wt%. **Figure 8(a)** presents the peak-to-peak output voltage of the nanogenerators. The electrical characterization of the nanogenerators was performed by applying a mechanical stimulus with a pushing force of 2.3 N at frequency of 2 pushes per second with a home-made

*Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

#### **Figure 6.**

*Reflectance spectra of the porous ZnO nanostructures with an inset graphic showing the obtained bandgap energy via the K-M function.*

#### **Figure 7.**

*(a) SEM images of a micro-structured PDMS/ZnO composite film, with the insets displaying closer views of the micro-cones. (b) XRD diffractogram of porous ZnO nanostructures, PDMS/ZnO composite film, and PDMS film.*

bending machine. The obtained results reveal an increase of the output voltage from 20 to 25 wt%, and then a decrease for 30 wt%. These results are in agreement to what was previously observed using the same approach for ZnO nanorods, where the optimal concentration for the nanogenerators output was also 25 wt% [78]. As such, to further characterize the nanogenerators, the concentration considered was 25 wt%.

In previous studies from our group [27, 78], an enhanced response was achieved by micro-structuring the composite, as shown in **Figure 7**, and, therefore, the same approach was adopted in this study. **Figure 8(b)** presents the output voltage

#### **Figure 8.**

*(a) Peak-to-peak output voltage for PDMS/ZnO composites with different concentrations of porous ZnO nanostructures. Note that each point was determined using the average output of 2–6 equal devices. (b) Output voltage for an unstructured and a micro-structured nanogenerator with a porous ZnO nanostructures concentration of 25 wt%. (c) Peak-to-peak voltage for different frequencies applying a pushing force of 2.3 N. (d) Output voltage from the optimized nanogenerator for 12,000 cycles.*

for this nanogenerator in comparison with the unstructured one. A peak-to-peak output voltage of (4.5 ± 0.3) V was obtained for the micro-structured nanogenerator against only (0.5 ± 0.2) V for the unstructured one. The micro-structuring can not only improve the force delivery into the nanostructures, leading to an increase of the piezoelectric effect, but it can also induce an extra triboelectric effect, as a consequence of the air gaps between the PDMS/ZnO composite micro-structures and the ITO electrode. These two effects originate an enhanced response of the micro-structured nanogenerator.

Considering the micro-structured nanogenerator with the best performance (25 wt%), the influence of varying the frequency of the stimulus was investigated while maintaining the applied force at 2.3 N. **Figure 8(c)** shows the peak-to-peak output voltage of the nanogenerator as function of the frequency, where the output voltage increases with increasing frequency. This trend has been observed by other groups, and it can be explained by the eventual accumulation of residual charges due to an inefficient neutralization of the induced charges provoked by a faster stimulation [89].

To study the potential of the nanogenerator in a daily life application, its stability along 12,000 cycles was also investigated. For this study, the stimulus was applied with a pushing force of 2.3 N while maintaining the frequency at 2 pushes per second. **Figure 8(d)** shows the output voltage along the pushing cycles, and no *Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

deterioration of its performance is observed. Instead, it is possible to detect a slight increase of the output voltage to (7.2 ± 0.1) V along the pushing cycles, which can also be related to charges accumulation.

#### *3.2.3 Proof-of-concept of the PDMS/ZnO nanogenerator*

To understand the applicability of the micro-structured PDMS/ZnO nanogenerator, it is important to study its performance when connected to external load resistances with different values (1 to 30 MΩ). This study was performed with a fixed pushing of 2.3 N at 2 pushes per second. **Figure 9(a)** presents the peak-to-peak output voltage and current, while **Figure 9(b)** shows the resultant instantaneous power density. An increase of the power density with increasing load resistance is observed until 10 MΩ, reaching a maximum value of 2.7 μW cm−2, after that a slight decrease is observed. Comparing to the recent results on PDMS/ZnO nanorods nanogenerators [78], the maximum power obtained here is just slightly lower, presenting the same order of magnitude. Its lower output is expected due the absence of a preferential direction for piezoelectric response (c-axes) in these nanostructures. Nevertheless, the synthesis of these porous ZnO nanostructures allows for a faster and low-cost fabrication of nanogenerators, since it is a rapid, simple, and high yield approach to obtain ZnO nanostructures.

Additionally, the nanogenerator output is very satisfactory, proven to be enough to light up a blue LED (2.8–4 V, 20 mA), by directly connecting the nanogenerator to the LED and manually stimulating the energy harvester, as shown in **Figures 9(c)** and **(d)** and Video 1 available from (can be viewed at) https:// youtu.be/JCT60ozKCX8. These results prove not only the applicability of these

#### **Figure 9.**

*Application of the micro-structured PDMS/ZnO nanogenerator, stimulated with a pushing force of 2.3 N at a frequency of 2 pushes per second varying the load resistance. Peak-to-peak (a) voltage and current outputs, and (b) correspondent power density for several load resistances. Note that each peak-to-peak value is an average of 5 measurements. (c and d) Nanogenerator directly lighting up a blue LED by applying manual force.*

nanogenerators in simple daily life applications but also demonstrate their potential to power wearable sensors or multifunctional platforms where these porous ZnO nanostructures are employed in more than one application.

#### **4. Conclusions**

In summary, porous ZnO nanostructures were successfully synthesized via a facile and fast hydrothermal method assisted by microwave irradiation and calcinated at 700 °C for 2 h in air. The effect of calcination temperature on the morphological, structural, and optical properties of the porous ZnO nanostructures was investigated. Nanogenerators based on a micro-structured composite of PDMS with embedded porous ZnO structures were successfully produced, reaching an output voltage of (4.5 ± 0.3) V. The devices proved to be very robust and stable by presenting no deterioration of their performance after 12,000 pushing cycles. An external load of 10 MΩ optimized the nanogenerators performance, reaching a power density of 2.7 μW cm−2. The capability of these nanogenerators to lighting up commercial LEDs, through direct connection and with a manual stimulus, was shown, demonstrating their potential for daily life applications.

#### **Acknowledgements**

This work is funded by National Funds through FCT - Portuguese Foundation for Science and Technology, Reference UIDB/50025/2020-2023 and FCT/ MCTES. This work also received funding from the European Community's H2020 programme under grant agreement No. 787410 (ERC-2018-AdG DIGISMART), No. 716510 (ERC-2016-StG TREND) and No. 952169 (SYNERGY, H2020-WIDESPREAD-2020-5, CSA). S. H. F. acknowledges the Portuguese Foundation for Science and Technology for the AdvaMTech PhD program scholarship PD/BD/114086/2015 and IDS-FunMat-INNO-2 project FPA2016/EIT/EIT RawMaterials Grant Agreement 17184.

#### **Conflict of interest**

The authors declare no conflict of interest.

#### **Notes/thanks/other declarations**

The authors would like to thank Ana Pimentel for the TG-DSC measurement and Daniela Nunes for the SEM images.

*Porous ZnO Nanostructures Synthesized by Microwave Hydrothermal Method for Energy… DOI: http://dx.doi.org/10.5772/intechopen.97060*

### **Author details**

Sofia Henriques Ferreira\*, Ana Rovisco\*, Andreia dos Santos, Hugo Águas, Rui Igreja, Pedro Barquinha, Elvira Fortunato and Rodrigo Martins\* CENIMAT/i3N, Department of Materials Science, NOVA School of Science and Technology (FCT-NOVA) and CEMOP/UNINOVA, NOVA University Lisbon, Caparica, Portugal

\*Address all correspondence to: sdl.ferreira@campus.fct.unl.pt; a.rovisco@campus.fct.unl.pt and rm@uninova.pt

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## **Chapter 7** Nanoporous Metallic Films

*Swastic and Jegatha Nambi Krishnan*

#### **Abstract**

Nanoporous metallic films are known to have high surface to volume ratio due to the presence of pores. The presence of pores and ligaments make them suitable for various critical applications like sensing, catalysis, electrodes for energy applications etc. Additionally, they also combine properties of metals like good electrical and thermal conductivity and ductility. They can be fabricated using top-down or bottom-up approaches also known as dealloying and templating which give the fabricator room to tailor properties according to need. In addition, they could find potential applications in many relevant fields in current scenario like drug delivery vehicles. However, there is a long way to go to extract its whole potential.

**Keywords:** metallic nanostructures, nanoporous metallic films, nano fabrication, optical sensing, catalysis

#### **1. Introduction**

Nanoscience is the study of phenomena and manipulation of materials at atomic, molecular and macromolecular scales (1 Bohr radius = 0.5292 Å ≈ 0.05 nm). Due to the influence of the negligible dimensions, materials exhibit remarkable functionality and phenomena. Here properties differ significantly from those at larger scale, because at this level, quantum mechanics and statistical mechanics come into play instead of classical mechanics, and the extremely high surface area to volume ratio of the particles modifies the electrical and chemical activity of the substance, thus the effective concentration of reactants confined in nanostructures may be very high. Typical nano-systems may contain from hundreds to tens of thousands of atoms.

Presently scientists and engineers are finding a wide variety of ways to deliberately make materials at the nano-scale to take advantage of their enhanced properties such as higher strength, lighter weight, increased control of light spectrum, and greater chemical reactivity than their larger-scale counterparts [1], and these products have various applications and a niche market in the fields of electronics, chemistry and biomedicine; semiconductor technology being of the most significance. Needless to say, this branch of science has a great of scope for research and innovation for years to come, on its way to increasing process efficiency, cost effectiveness, and broadening the range and accuracy of human perception.

Out of all the nanostructures, nanoporous film has attracted many research groups in the past decade due to the presence of nano holes in it which acts as a nanoparticle and increases the specific surface area. Also, the increase in chemical stability plays a role in the attraction of several research groups. In order to investigate it further various metals have been used to fabricate nanoporous metallic films. Nanoporous Ag cathode has been used photoelectrochemical carbon dioxide

#### **Figure 1.**

*SEM image: Electroless plated Au on e-beam evaporated Cu on silicon substrate [5].*

reduction [2] and nanoporous palladium have been used for reductive dichlorination [3]. But among all the metallic porous films, the nanoporous gold (NPG) film is used mostly due to its chemical stability and unique surface chemistry.

NPG film provides a suitable microenvironment for immobilization of biomolecules (like enzymes) by maintaining their biological activity and facilitates electron transfer between the immobilized proteins and electrode surfaces, leading to its intensive usage in electrochemical biosensors with enhanced analytical performance compared to other biosensor designs [4].

**Figure 1** shows surface morphology of NPG film which was manufactured by e-beam deposition of Cu on silicon substrate and then electroless plated with Au. The characteristics of Au nanoporous film such as high surface-to-volume ratio, high surface energy, ability to decrease proteins metal inter-particulate distance, and the functioning as electron conducting pathway between prosthetic groups and the electrode surface have been claimed as the reasons to facilitate electron transfer between redox proteins and electrode surfaces. And these are the properties that make the NPG film so coveted for the fabrication of electrochemical sensors and biosensors.

In this chapter, the processes to make nanoporous metallic films followed by effect on properties of the film are discussed. The applications of nanoporous metallic films and finally, the future scope are also elucidated.

#### **2. Fabrication methods**

The fabrication techniques for the preparation of nanoporous structures vary with the requirement of the application. The nanoporous films can be either etched out from an alloy (top-down approach), known as dealloying process or can be fabricated using a template and removing it after deposition of required metal on it (bottom-up approach), known as a templating process [6]. The stability of a nanoporous film is dictated by their pore and ligament sizes.

In dealloying, the undesirable material of the alloy is dissolved under appropriate corrosive condition leaving a stable porous desirable metal [7]. Dealloying can also

#### *Nanoporous Metallic Films DOI: http://dx.doi.org/10.5772/intechopen.95933*

be further divided into free corrosion dealloying and dealloying by using electrochemical methods. The key factor for choosing an alloy for dealloying by free corrosion is the parting limit which suggests that the composition of undesirable material should be higher than a threshold value. The parting limit for *Au Ag x x*−<sup>1</sup> found by experiments done by Newman [8] is o.4 for *x* . Selective dealloying has been used by various research groups to fabricate porous structures below 100 nm. The formation of pores during the dealloying process can be divided into three stages according to SAXS (small-angle X-ray scattering) analysis [9]. The time of each stage is inversely proportional to the dealloying temperature. In the first stage, there are some changes in the SAXS data but physically the commencement of pore formation cannot be detected. The second stage shows some physical changes with the increase in visibility of pits on the surface, subsequently, there is a drastic change in the intensity of the SAXS curve. The third stage corresponds to the growth of pores and a visible increase of ligaments in the structure until a stable structure is achieved. When strictly focusing on NPG films of some suitable alloys like AuAg [10], AuCu [11] and AuNi [12], the most commonly used is AuAg alloy.

Introduction of electric potential to facilitate the dealloying process have also been attempted and is known as dealloying by using an electrochemical method. The major component of this process is an anode (the alloy), cathode, reference electrode and electrolyte. Two common electrolytes for AuAg alloy are aqueous perchloric acid (HClO4) for relatively big pore size and neutral silver nitrate solution (AgNO3) for small pore size [13]. This method requires a more sophisticated set-up than free dealloying process but in return, more uniform nanoporous film and a higher degree of process control are achieved. Similar to parting limit, this method also requires a positive potential value known as critical potential (*Ec*). The selective dissolution takes place by the rapid increase of Ag dissolution rate. As would be explained later in this chapter, any factor which enhances the surface diffusivity would have an impact on pore as well as ligament size.

Dealloying by the electrochemical process has also been divided into two types, potentiostatic dealloying and galvanostatic dealloying. The experimental set-up for both potentiostatic and galvanostatic methods is the same. In potentiostatic dealloying, the potential value is kept just above the critical potential (*Ec*) value, which facilitates the gradual dissolution of Ag giving a robust and uniform structure at the end. Whereas in galvanostatic dealloying, the potential value starts above *Ec* and gradually increased till a maximum limit which is also known as cut-off potential. The two competing factors of Ag dissolution which increases the stress and Au diffusion which decreases the stress in NPG film contribute to the quality of pores in a NPG film [13]. Galvanostatic dealloying by controlling Ag dissolution rate and Au diffusion rate through a periodic increase in potential provides a more robust and crack-free NPG film when compared to that of potentiostatic dealloying.

Low pore size and high ligament size related to the high thermal and electrical conductivities is reported by various research groups [14, 15]. For a 1.3 μm thick NPG film, the ligament size can range from 22 to 155 nm [16]. Hakamada [17] while fabricating nanoporous Ni, Ni-Cu and Cu found an inverse correlation between the atomic ratio of Ni in alloy and ligament size. Another important factor that determines the ligament size is surface diffusion at the metal/electrolyte interface. Correlation between surface diffusion coefficient ( *Ds* ) and ligament size ( *d* ) is given by Equation [18];

$$D\_s = \frac{d^4kT}{32\,\gamma t a^4} \tag{1}$$

where *k* is Boltzmann constant, *T* is the absolute temperature, γ is the surface energy, *t* is the dealloying time and *a* is lattice parameter. According to the above

relationship, surface diffusion also depends on dealloying temperature and time. Qian and Chen [18] quantified the temperature dependence of NPG films by increasing temperature from −20ͦC to 25ͦC leading to an increase in diffusivity by two orders of magnitude. Apart from the above-mentioned factors, the dealloying process gets affected by properties of precursor alloy and dealloying solution [19, 20].

The disadvantages of dealloying is the effect of acids and bases used as a solution on the workforce as well as wastage of the dissolved metal. To cope with this problem, Zhang [21] have used ultrasonic irradiation (UI) to assist the dealloying process. This additional method uses lower acid concentration and simultaneously reduce environmental pollution. The UI reduces the surface energy which further enhances the diffusion leading to more coarsening of the ligaments [22]. This experiment proved that the coarsening rate of the ligament increases by introducing UI in dealloying process.

Similar to the use of ultrasonic irradiation, ultrasonic agitation has been used to achieve finer ligaments and pores of palladium-nickel nanoporous thin films [23]. The ultrasonic agitation reduced the time by a factor of 5 without disturbing the desired structure. There has been a similar effect of the magnetic field highlighted on the nanopores of Ag [24].

Another method for the fabrication of NPG films is the templating process. The templating process can be explained in two steps, the preparation of Au or Ag-Au coated core/shell particles followed by the removal of core material to get pure metal foam [25]. The preferred material for template assisted fabrication of NPG film is silica or polystyrene beads. This method gives a higher level of control over pore and ligament size as these would be dependent on the size of beads that can be readily controlled during template fabrication.

#### **3. Properties of nanoporous gold films**

#### **3.1 Mechanical properties**

Using the analogy of foam to describe nanoporous materials, mechanical properties of foam depends on the cell size similarly pore size dictates the mechanical behaviour of nanoporous materials. Though there is a resemblance between both structures, the effect of scale cannot be neglected and the equation of foams for mechanical behaviour cannot be applied. Also, the introduction of capillary actions and the plastic behaviour of ligaments cannot be unforeseen at lower dimensions. Hodge et al. [26] attempted to present an equation from experimental data for yield strength and it should be emphasized that as the ligament size approaches 1.0 μm the data begin to approach the Gibson and Ashby scaling prediction.

$$\boldsymbol{\sigma}^\* = \mathbf{C}\_s \left[ \boldsymbol{\sigma}\_o + \boldsymbol{k} \boldsymbol{L}^{-\frac{1}{2}} \right] \left( \frac{\boldsymbol{\rho}^\*}{\boldsymbol{\rho}} \right)^{3/2} \tag{2}$$

Where \* denotes foam properties and s denotes solid properties,*Cs* is a fitting coefficient,σ *<sup>o</sup>* is the bulk material yield strength (σ *<sup>s</sup>* ), *k* is the Hall–Petch-type coefficient for the theoretical yield strength of Au in the regime, ∗ / ρ ρ*<sup>s</sup>* is the ratio between densities of the porous structure and corresponding dense material and *L* is the ligament size. The real picture of what is happening at the nanoscale can only be found by experimenting, so experimental results of yield strength and tensile strength. On the experiment front, the results from pillar compression tests revealed that the yield strength comes closer to theoretical yield strength of Au when the size

#### *Nanoporous Metallic Films DOI: http://dx.doi.org/10.5772/intechopen.95933*

of pillars decreases. The tensile test on NPG revealed some macroscale brittleness in it which is opposite to the inherent ductile behaviour of Au [13]. This contradiction in behaviour has been checked through another test known as fracture toughness.

Another mechanical property which is of importance is fracture toughness. It was found that fracture toughness of NPG is low even though gold is inherently ductile [27]. But when the previous phenomena of a tensile test revealing the macroscale brittleness are combined with the above results, the contradictory behaviour becomes clear. In nanoporous films, the ligament acts as a pillar to support the structure. So, the combined behaviour of the structure is coming from the intrinsic behaviour of ligaments. Li and Sieradzki [28], also correlated the ligament size and fracture behaviour. Research groups also concluded the rupturing of ligaments below 100 nm in size [29, 30]. The change in facture behaviour has been observed when the amount of Ag was varied in the final product. For less than 1% of Ag in final nanoporous structure, the rupture is smoother than the other increased value of Ag suggesting the rupturing is intragranular for lesser Ag content [13]. This observation means for less than 1% Ag, the grain boundary strength is higher or the whole system is more brittle as it broke without showing a significant change in appearance. But when the amount of Ag is increased the crack propagates through grain boundaries. Though there is significant data for these behaviours, intensive research is required to fully understand the phenomena.

#### **3.2 Optical properties**

The optical properties of metals are dictated by the to and fro motion of the electrons in the outer shell of metal that are triggered by any electromagnetic radiation. The motion can simply be understood by imagining photoelectric effect. The surface electrons are known as surface plasmons (SPs). The variables in this phenomenon are metal since each metal releases a unique amount of energy which acts as the fingerprint of that metal and frequency of electromagnetic radiations. Therefore, by changing these variables a nanoporous structure can be used for a huge number of applications like sensors [31], medical imaging, diagnostics [32] etc. Based on the movement of surface plasmons (SPs), the optical characterization techniques have been classified into surface plasmon polaritons (SPPs) and localized SPRs (LSPRs). With the help of excitation from grating or prism couplers, SPPs are known to propagate for tens or hundreds of micrometers [33]. As the name suggests the second one, localized SPRs (LSPRs) are non-propagating type and since the resonance in a confined space has been associated with a strong electromagnetic field, LSPRs contribute to several significant phenomena like surface-enhanced Raman spectroscopy [34], phononic effects [31]. This strong electromagnetic field becomes more prominent when the nanostructures have sharp features.

So, an ideal nanostructure would be the one which supports both localized as well as propagating systems. The simultaneous presence of a planar structure and nanostructure makes nanoporous materials an ideal candidate with good optical properties. This bicontinuous structure facilitates high field enhancements and good directional control [13]. The relation between irradiation wavelength and propagation of NPR has also been established. The longer the laser wavelength, the farther the propagating SPRs [35].

#### **4. Applications of nanoporous gold films**

The nanoporous materials field has gained much attention from the industry due to its enormous specific surface area, well-defined pore sizes and functional sites [36]. Surely, these properties can be achieved for other nanostructures too, but the low capital, high throughput and ease of control of morphology involved in the manufacturing make nanoporous materials more attractive. Among all the metals used for nanoporous structures, Au stands as an outstanding material due to its high surface area (~10 m<sup>2</sup> /g), electrochemical activity, biocompatibility and ease of preparation [37, 38]. Due to the enormous surface to volume ratio of NPG, they have shown exceptional sensitivity and selectivity [39]. Particularly sensitivity becomes very crucial in medical or manufacturing safety field, concerning the placement of sensors on which sometimes many lives depend. This is the reason; NPG is finding its way into medical and manufacturing safety field more rapidly.

#### **4.1 Optical sensing**

As described in the previous section, the generation of surface plasmon resonance is due to the reduction of the dimension of metals to the scale of the mean free path of electrons [40]. When the electromagnetic radiations of the surroundings interact with electrons, there is inelastic scattering which depends on the pore size. In general, the smaller the pore size the higher the sensitivity [39].

Lang [41] studied the effect of varying nanoporosity on the enhancement of fluorescence. A new method of fabrication was introduced using a combination of dealloying and electroless plating to fabricate NPG structure with high ligament size. This enlarged ligament size facilitated the weakening of plasmon dampening leading to the enhancement in surface-enhanced fluorescence. It was further reported by Lang et al. [42] that fluorescent intensity of molecules absorbed on human-serum-albumin (HSA)-coated NPG films is inversely proportional to the nanopore size. The 45 times increase of fluorescence intensity was reported for a pore size of ~10 nm using this method. Whereas Zhang [43] fabricated a NPG film based optical sensor for sub-ppt detection of mercury ions. A Cy5-labelled aptamer NPG sensor was used with resonant excitation laser, to achieve 0.2 ppt Hg2+ sensitivity. This sensor could be extended further for detection of other heavy metal ions.

Similarly, there have been many studies on the surface-enhanced Raman spectroscopy. Zhang [44] have modified the nanoporous structures with wrinkles to include more "hot spots" for ultrahigh SERS enhancements. This was achieved with the help of thermal contractions of prestrained polystyrene microparticles (PS). The wrinkled NPG was found to have 100 times higher signal than the normal NPG films. Another interesting optical application was reported by Shih [5] where they have used NPG gold disks to sense chemical and find refractive index simultaneously. The NPG disks modified with octadecanethiol (ODT) and the surfaceenhanced near-infrared absorption (SENIRA) spectroscopy was used to detect hydrocarbon compounds from crude oil samples.

**Figure 2** shows the enhancement in SERS spectra when Au nanostructures are formed Pt substrates rather than on Cu substrate [45]. This research work also proves the importance of selection of substrate for use in optical sensing phenomena.

#### **4.2 Electrochemical sensing**

Electrochemical sensors are an electrode which goes through a redox reaction to detect the substance attached to the sensors. Now, the sensitivity and selectivity of the sensor become the prominent property to tune for respective applications. A schematic diagram of the sensor has been shown in **Figure 3**.

#### **Figure 2.**

*(A) SERS spectra constituting SERS signal from (1) bare e-beam Au sample and Cu substrate whose reaction times are (2) 0 min, (3) 2 min, (4) 18 h and (5) 24 h respectively. (B) SERS spectra obtained from Pt substrate whose reaction times are (1) 0 min, (2) 2 min, (3) 19 h and (4) 24 h respectively [45].*

#### **Figure 3.**

*Working of a NPG electrochemical sensor [46].*

Electrochemical sensors are being used in biomedical applications on a large scale due to its sensitivity and selectivity. Chen [47] fabricated an electrochemical NPG film sensor to detect glucose based on the current response. As it was observed in optical sensors, lower pore size gave better sensitivity for glucose in this electrochemical sensor. The sensor was fabricated with the help of dealloying method and then cyclic voltammogram (CV) curves for NPG was used to detect OH¯ adsorptions as it has direct correlations with electro-oxidation of glucose. In order to check the selectivity and sensitivity towards glucose of the sensor, glucose concentration was varied by keeping the pore size (18 nm) and current potential constant (0.1 and 0.3 V). On comparison, the current density at 0.1 V decreased while it increased linearly for 0.3 V proving the sensitivity of electrochemical sensor towards oxidation and subsequently towards the concentration of glucose. Additionally, the sensor was evident of excellent selectivity by avoiding interference caused by other substance present in the solution.

Electrochemical NPG sensor was used by a research group for the detection of DNA [48]. The biosensor showed an excellent sensitivity with a limit of detection up to 28 aM. The fact that the nanopores capture DNA and immobilizes it makes it more selective. Likewise, simple fabrication technique of dealloying makes it more feasible. Qui [49] went one step further to enzyme-modify NPG electrochemical biosensors to detect glucose and ethanol. The NPG was modified with the help of alcohol dehydrogenase (ADH) or glucose oxidase (GOD) that enhanced its sensitivity towards glucose and ethanol. The promising fact about these sensors is

even after leaving them for 1-month storage at 4ͦC, the ADH- and GOD- based biosensor lost only 5% and 4% efficiencies, respectively. In this connected world, where some product is manufactured at one place and then transported to another sustained efficiency is of prime importance.

#### **4.3 Catalysis**

Catalysis is another activity which is highly dependent on the surface area for its efficiency. The first catalytic activity of gold nanoparticles was reported to be back in 1987 when CO was oxidized far below room temperature [50]. Due to inherent inert behaviour of gold, this experimental result came as a surprise. Moreover, a nanostructure is constructed on a substrate. So, when a reaction was taking place of these nanostructures used to come off from the substrate as a result of poor adhesion. This is where NPG gained its importance in this field for its bicontinuous structure [51]. In case of oxidation, the high surface area acts as an important site for adsorption giving exposure to a higher number of reactant molecules to interact with the surface. Another reason for high oxidation behaviour is the presence of some amount of Ag in NPG films. It is known that Ag bind oxygen and activate them [52].

Shi et al. [53] used NPG functionalized with praseodymium-titania mixed oxide to catalyze water-gas shift reaction. Both electron energy loss spectroscopy (EELS) and flow reactor tests revealed that Pr-TiOx functionalized NPG is highly active as well as very stable to high temperature such as 180–400°C. This study exhibited the interaction between Au substrate and the oxide deposit which plays a vital role in the dissociation of water. The problem with the use of any nanostructure was decay in catalytic activity with time due to coarsening of the nanostructure. Use of Pr-TiOx formed a mixed Pr-TiOx solid solutions which prevented further coarsening of NPG making the catalyst stable to use for a long period. The catalytic activity of Au is also vital in recent reports of hydrogen fuel. Albeit the produced hydrogen contains a small amount of CO that can further deactivate the electrodes [54], a highly sensitive and selective catalyst is required for this purpose. NPG films form potential candidate for such catalytic applications.

Similar to CO oxidation, research has also been started in H2 oxidation. Qadir et al. reported very low H2 oxidation activity by bare np-Au [55]. The activity was manipulated by deposition of titania on the catalyst. This exercise also proves that tuning the amount of titania deposit can increase the oxidation activity of the structure.

#### **4.4 Biomolecular sensing**

Modified electrodes are being widely employed in modern electrochemistry for electrocatalytic reactions and as electrochemical sensors. Gold electrodes are useful to construct electrochemical sensors because of their chemical inertness. The well-established strategy of a self-assembled monolayer formation for immobilization of compounds onto gold surfaces are based on the attachment of thiol (-SH) or disulfide (-S-S-) functional groups to Au (111) [56].

In order to develop new reliable, efficient and functional micro/nanoscale devices, control over the surface properties is essential. The surface properties of microscale and nanoscale devices can easily be controlled and manipulated in a versatile manner through surface modification technology. The properties such as wetting, biocompatible, bioselective, optical and electronic characteristics of various inorganic and polymeric surfaces can be adjusted and controlled by modifying the surface.

#### **Figure 4.**

*STM image of gold on mica with surface modification by L-cysteine molecules [57].*

Furthermore, a wide variety of terminal functional groups such as amino group, carboxylic acid group can be implemented to detect the trace heavy metal ions, DNA, RNA or antibodies.

Surface modifications can be grouped into two broad categories: (a) Chemically or physically altering the atoms or molecules in the existing surface (treatment, etching, chemical modification) (b) Coating over the existing surface with a material having a new composition (solvent coating or thin film deposition by chemical vapour deposition, radiation grafting, chemical grafting or RF-plasmas).

The Self Assembled Monolayers (SAMs) are nanostructures that are formed by organic assemblies owing to the adsorption of molecular constituents from solution or gas phase onto the surface of solids or arrays on liquid phase. The molecules or ligands that form SAMs have a chemical functionality called "headgroup" which has a special affinity towards a substrate. Typically, the thickness of a SAM is typically 1–3 nm.

SAMs are well-suited for studies in nanoscience and nanotechnology because: They are easy to prepare. They do not require ultrahigh vacuum (UHV) or other specialized equipment. SAMs can be easily prepared by immersing the substrates into the known solution. They form on objects of all sizes and are critical components for stabilizing and adding function to preformed, nanometer-scale objects for example, thin films, nanowires, colloids, and other nanostructures. They can couple the external environment to the molecular, electronic and optical properties of metallic structures. The most extensively studied class of SAMs is derived from the adsorption of alkanethiols (-SH) on gold, silver, copper, palladium, and mercury. SAMs also provide a convenient approach for ultra-low-level analyte recognition and have been important in the development of electroanalytical devices and electrochemical sensors [56].

Above **Figure 4** shows the surface coverage of L-Cysteine molecules on NPGF and with the increased surface coverage L-Cysteine molecules would be able to trap more heavy metal ions leading to lesser limit of detection (LOD) [57].

#### **5. Future of NPG**

The future for NPG films is promising though there are many unanswered questions. Like the understanding of relationship between constituent of an alloy and its morphology after dealloying and pressure flow relationship NPG sieves as well as

**Figure 5.** *SEM image of nanoflower [45].*

its membrane architecture [58]. Also, the degree of sensitivity is going to play a vital role in coming days which will strengthen its foothold in the sophisticated sensing applications. The research work is already underway for different structures such as NPG leaves, nanowires, nanoflowers (**Figure 5**), etc. Looking at the promising optical and mechanical properties there is a long way to go before the full potential of NPG is reached. Additionally, with the onset of the decade where data is going to be of so much importance, correct data and large amount of data would be of high importance for precise decision-making purposes.

With the passage of time, the resources are becoming scarce triggering the requirement of tools which utilizes fewer materials to give more information. Also, selectivity would be of prime importance. For increased selectivity the sensors or catalysts should be manipulated from the bottom and this can be possible through NPG film like structures only. The use of minimal material would ensure less environmental impact. Hereby, the research should be more focused in areas like hydrogen fuel which is environmentally friendly and can pave way for potential applications in transportation industry.

#### **6. Conclusion**

Nanoporous metals is known to exhibit strong electrochemical, optical and mechanical properties due to their unique three-dimensional and quasi-periodic nanoporosity. Nevertheless, there are many challenges that remain. Performance of electrochemical sensors in energy applications depends strongly on their structure and composition [59]. So, new electrochemical fabrication method with the ability of tailoring of the size and shape of nanomaterials is required. Similarly, understanding of nanoporous metals with improved optical performances is necessary as it needs superior reproducibility, facile synthesis and excellent stability [13]. Additionally, in the field of medical research nanoporous metals are can be used for controlled drug-delivery [60]. Nanoporous metallic films would be attractive materials for future applications research that would result in huge advancement of the field of technology with strong conviction.

### **Acknowledgements**

The authors would like to thank BITS Pilani OPERA Award and Reliance Industries Limited Education assistantship.

### **Thanks/other declarations**

The authors do not have any difference in opinion. There is no conflict of interest.

### **Author details**

Swastic1 and Jegatha Nambi Krishnan2 \*

1 University of Leeds, Leeds, UK

2 BITS Pilani K K Birla Goa Campus, Goa, India,

\*Address all correspondence to: jegathak@goa.bits-pilani.ac.in

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 8**

## Nanoporous Carbon Materials toward Phenolic Compounds Adsorption

*Mahmoud Fathy Mubarak, Alshimaa Maher Ahmed and Sahar saad Gabr*

#### **Abstract**

Nanoporous carbon-based sorbents are used to generate a three-dimensional real-space model of the nanoporous structure using the concept of Gaussian random fields. This pore model is used to derive important pore size characteristics, which are cross-validated against the corresponding values from gas sorption analysis. After filling the model pore structure with an aqueous electrolyte and rearranging the ions *via* a Monte Carlo simulation for different applied adsorption potentials. In comparison to nanopores formed from solid-state membranes (e.g., silicon oxide, aluminum oxide, polymer membranes, glass, hafnium oxide, gold, etc.) and very recently 2D materials (e.g., boron nitride, molybdenum disulfide, etc.), those nanopores produced from carbon materials (e.g., graphene, carbon nanotubes (CNTs), diamond, etc.), especially those from graphene appear to be perfect for adsorption process. The thickness of carbon structures nanopores can be as thin as 0.35 nm, resembling the height of the base spacing. Moreover, the sizes of carbon structures nanopores can be precisely fabricated and tuned to around 1.0 nm, the similar size of many heavy metals and organic pollutants molecules. Furthermore, carbon materials are chemically stable and feature-rich surface chemistry. Therefore, various carbon nanopore sequencing techniques have been developed. Finally, in this chapter the adsorption of phenolic compounds on nanoporous carbon specifically the active carbon are overviewed and how to affect the heterogeneity of activated carbon surface, PH of the solution on the efficiency of adsorption.

**Keywords:** nanoporous carbon, phenolic compounds, adsorption, activation process, templating methods

#### **1. Introduction**

Contamination of water is one of the significant issues in the universe, that poses negative effects on individual and surroundings. The rising in industrial and human activities resulted in increasing the flowing of wastewater into water supplies [1, 2]. In the last years, the impacts of exposure of human and animals to chemicals in the ambiance especially the aquatics medium has taken the high interest of many scientists and decision-makers [3]. Among these chemicals, phenolic compounds are considered the most important due to their toxic effect on animals and humans

that result from their staying in the ambient for along time and then collect to cause that effect [3]. There are two types of phenolic compounds, natural compounds connected to the flowers and fruits colors and synthetic compounds used in daily humans life for various purposes [3]. Phenolic compounds are present in the effluents of various industries such as oil refining, petrochemicals, pharmaceuticals, coking operations, resin manufacturing, plastics, paint, pulp, paper, and wood products. Discharge of these compounds without treatment may lead to serious health risks to humans, animals, and aquatic systems [4]. The presence of these compounds is attributed to a breakdown of natural organic materials in the water, flows water away from farmland, and discharge of wastes resulting from industries and humans uses in water resources. The presence of these compounds in water results in the interaction of them with chemical, physical and biological variables inside the water that led to their conversion to other forms that have a dangerous effect than the original ones [3, 5, 6]. Phenol has been designated as a priority pollutant by the US Environmental Protection Agency (EPA) and the National Pollutant Release Inventory (NPRI) of Canada [3]. International regulatory bodies have set strict discharge limits for phenols for a sustainable environment. For example, the EPA has set a water purity standard of less than 1 ppb for phenol in surface water [7]. The toxicity levels usually are in the range of 9–25 mg/L for both humans and aquatic life [3, 8]. Phenolic compounds are categorized as very harmful contaminants due to their toxic effects and cancer diseases causing. Short -term exposure to these compounds results in irritation in human organs, headache, and inability to balance even at low content, while Long-term exposure to these compounds causing arise in the pressure of blood and very strong kidney and liver problems [9, 10].

Phenolic compounds removal from water is necessary to protect humans and aquatics from the pollution that those toxic compounds causing. Appearing a lot of methods used in the phenolic compounds removing will overcome the hazard problems connected to these chemicals and wastes discharge challenges, in addition to, the getting of additional value phenolic compounds as secondary products. A lot of technologies are used to remove phenolic compounds from wastewater successfully before it's disposal in water resources [3, 7].

Electrochemical oxidation [2, 3, 8], (electro)chemical coagulation [10], solvent extraction [3], bioremediation [10], and photocatalytic degradation [3, 8], Reverse osmosis and nanofiltration [2, 8], Chemical oxidation [2, 8], have been used for the treatment of wastewater from phenolic compounds for many years, but these techniques are very costly due to the requirements they needed in purification process as supplementary chemical materials and high input of energy, in addition to the undesired by-products produced through the treatment process. Therefore, the separation of phenolic compounds from wastewater requires the development and using energy-efficient and cheap methods [5, 8, 11–14]. In this Chapter, the adsorption method is very effective for that purpose. Adsorption is the most effective method for removing the organic and inorganic contaminants from wastewater because it is a very easy method to set up, low cost, no time consuming, the adsorbent used in the process not harmful to the environment and can be recovery and reused again without the decrease in the efficiency [10]. In the adsorption method, the removal of pollutants from water occurs by holding them on the adsorbent surface [2, 11–13]. Carbon-based nanomaterials such as fullerenes, carbon nanotubes (CNTs), graphene and its derivative compounds, nanodiamonds, and nanoporous carbons (NPC) such as activated carbon are the most popular nano adsorbent materials used for purification of water between scientists due to their harmless natural to the environment, abundance, simplicity of handling, and size and form that give them different properties [15–19]. In this chapter, nanoporous

*Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

carbons (NPC) is considered one of the most effective and economical adsorbents used in the separation of organic and inorganic contaminants from the aquatic environment due to their various properties such as high surface area and high porosity, in addition to they are inexpensive, abundance, generate from renewable sources, very thermally stable, and their perfect chemical resistance [20–22]. Furthermore, NPC can attract attention to used in many purposes due to its rule in decreasing the amount wastes in the environment through using them in Their preparation process. Their unique properties offer new opportunities in the area of inclusion chemistry, guest host interaction, and molecular manipulations, showcasing their potential impact in a wide range of research fields, such as adsorption, separation, catalysis, electronic devices, and drug delivery [18, 19, 21].

Activation process(such as physical or thermal activation and chemical activation methods) is one of the methods used to prepare nanoporous carbon but due to the disadvantages of this process, the vision has been directed to using the templating method in the preparation process [19, 23].

#### **1.1 The aim of this chapter**

To prepare nanoporous carbon materials (NPC) to use in phenolic compound removal, discussing preparation methods, properties of these materials especially activated carbon, and improving these properties to improve the performance of these materials in adsorption application by using templating methods.

#### **2. Phenolic compounds**

Nowadays there is a growing concern around the world constantly about the increasing volume of pollutants in the water and the removal of dangerous pollutants from wastewater is one of the most important environmental issues at present. Phenolic derivatives are among the common environmental pollutants. The extremely low concentration of these pollutants is an obstacle to water use. Phenols are toxic and carcinogenic that can cause a bad taste and smell in drinking water also harmful to human health [5, 24]. Phenolic compounds come to water from different various sources such as oil refineries, coal gasification sites, petrochemical units, and from the synthesis of plastics, paints, pesticides, insecticides, pharmaceutical, etc. according to European Union countries, the maximum concentration of phenols in the drinking water is limited to .5 ppb and in the USA to 1 ppb. Both the US Environmental Protection Agency (EPA) and the European Union (EU) involve indicated that nitrophenols and chlorophenols rank first on the pollutant list. The most widespread of phenolic compounds in water is chlorophenol that generates from the chlorination of aromatic compounds that present in water and soil. Phenols have weak acidic properties. (**Table 1**) discuss the basic information about most phenolic compounds [5, 6, 9].

Diverse technologies have been employed for the removal of phenolic compounds from a variety of water sources including steam distillation, aerobic and anaerobic biodegradation, oxidation by ozone, ion-exchange resins, adsorption, and membrane filtration [11]. But above-mentioned methods, adsorption is the most applied technique for water treatment due to its very simple technique as it works by adding the adsorbent to the polluted water and then target pollutants are adsorbed into the adsorbent, cost-effective, friendly environment and the availability of a wide range of adsorbents. The adsorption of phenol and its derivatives on nanoporous carbon especially activated carbon has become an important issue by many researchs [9, 25]. Adsorption is the most applied technique for the removal




**Table 1.** *The basic information about most phenolic compounds [24].*

of phenolic compounds as it is low cost with high efficiency and easy ergonomic design besides that activated carbon is the most applied adsorbent as it has an internal porous structure (consisting of pores of varying size) with large surface area and specific chemical structure of the surface. And the efficiency of adsorption capacity of phenolic compounds on the activated carbon accompanied by numbers of factors such as:


There are also important factors such as the type of precursors for Activated carbon preparation and the aqueous solubility of phenolic compounds [9, 11, 26].

Despite extensive studies on factors affecting phenol adsorption, the mechanism of its adsorption is unclear and should be further studied. In particular, the most controversial matter is the role of the presence of oxygen group on its surface in the uptake of phenols [24].

In this study, we have tried to explain the importance of the above factors and how to prepare suitable AC from cheap and available precursors to remove phenolic compounds.

#### **3. Historic perspective of nanoporous carbons**

Carbon is the most spreading element on the earth, it has distinct characteristics and can form many compounds with different properties. Carbon has been used for a long period in form of coal, charcoal, and carbon black. After that has been discovered a new process to improve the properties of carbon materials through

activation of charcoal. These new materials are called nanoporous carbon materials [13, 27, 28].

Carbon was used in past in form of charcoal or carbon black for many purposes:


However, a few years later, in 1794, an English sugar refinery successfully used wood charcoal for decolorization. This application remained a secret until 1812 when the first patent appeared in England, although from 1805 wood charcoal was used in a large-scale sugar refining facility in France for decolorizing syrups, and by 1808 all sugar refineries in Europe were using charcoal as a decolorizer [25, 30].

In 1811, it was proved that the efficiency of decolorization of sugar syrups by bone char was higher than wood char. In 1815, the majority of sugar refining facilities were using granular bone-derived char.

In 1817 Joseph de Cavaillon patented a method of regenerating used bone chars, but the method was not entirely successful [13, 23].

The first example of producing an activated carbon by a combination of thermal and chemical processes was constituted by Bussy In 1822 who demonstrated that the decolorizing abilities of carbons depended on:


At the beginning of the twentieth century, Raphael von Ostrejko who patented between 1900 and 1903 made a revolution by exploring two distinct methods for the production of nanoporous carbon materials (activated carbon materials) from the activation of charcoal. This scientific breakthrough caused an improving the performance of these carbon materials in many applications by formation a high porosity in carbon materials skeleton.

*Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

Because of these discoveries, the first factory for the production of activated carbon materials has been built in Ratibor and was became the oldest factory for activated carbon production in the world [28, 30].

The first application of activated carbon was in World War I, when it was used in manufacturing soldiers masks for protection against hazardous gases and vapors [25].

The production and search for new activated carbons have been boosted decade after a decade due to their fundamental role in various technological applications which are related to, namely, restricted environmental regulations, recovery of valuable chemical compounds, and catalyst support. Nowadays, the driving forces for the research in nanoporous carbons are related to the properties of the most recent carbon materials: fullerenes, carbon nanotubes, and graphene. However, the excellent properties of these novel carbon forms also fostered the interest in the traditional porous carbons and, in recent years, a considerable number of studies searching for new synthetic approaches have been published. The main objective is the preparation of highly porous materials with controlled porosity, and often also with tuned surface chemistry, to present enhanced behavior as, for example, electrode materials for supercapacitors [26, 28, 31, 32].

#### **4. Properties of nanoporous carbons**

Carbon is one of the most abundant elements on the Earth and plays a critical role in the bio- and ecosystems. Carbon has the unique capability of forming a variety of interesting materials exhibiting extraordinarily different physical and chemical properties [20, 22, 27]. Fullerene [33], carbonnanotubes [33], graphite [15, 34], and diamond [35] are examples. To improve performance, nanoporous structures have been introduced into carbon because nanopores can give a large surface area.

Porous materials have various properties than bulk materials have [27].


**Figure 1.** *Types of pores according to their accessibility to surroundings [29].*


IUPAC (International Union of Pure and Applied Chemistry) proposed the classification of pores according to their size:


Nanoporous materials are materials with pore size in the range of 1-100 nm [15, 21, 29].

Nanoporous materials have unique features such as high specific surface area, shape-selective effects, fluid permeability, large porosity, and ordered uniform pore configuration. Therefore these materials can be used for many purposes such as separation, sensing, and catalysis applications [19, 39].


#### **Table 2.**

*Properties of nanoporous materials [39].*

#### *Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

Various nanoporous materials with different properties such as surface area, porosity, pore size, thermal stability, etc. [9] are discussed in (**Table 2**).

The classification of pores discussed above is limited by the data of nitrogen adsorption–desorption at 77 k and that depends on: each pore size has a different mechanism of pore filling determined by isotherm profile [28].

	- ultramicropores (narrow micropores) filling occurs at low relative pressures (P/P0 < 0.01) and is controlled completely by the enhanced fluid–solid Adsorption interactions (enhancement of the adsorbentadsorbate interaction). This process is called (primary micropore filling).
	- supermicropores (wider micropores) filling occurs at a higher relative pressure (P/P0 in range of 0.01–0.15) and is controlled by cooperative fluid–solid interactions and fluid–fluid interactions.

#### **4.1 Surface chemistry of nanoporous carbons**

The main component of the nanoporous carbon skeleton is carbon atoms, but the basic structure of these materials also contains hydrogen and oxygen and may also include groups containing nitrogen, sulfur, or phosphorus, depending on the precursor, preparation route, and post-synthesis functionalization. Owing to the presence of unsaturated carbon atoms that are extremely reactive, these heteroatoms are primarily found at the edges of the basal planes. Due to particular interactions with the adsorptive and also the solvent in the case of solution adsorption, the elemental composition, and type of surface groups of a nanoporous carbon affect its efficiency in both gaseous and liquid phase processes. Properties such as hydrophobicity/hydrophilicity or acid/base action are extremely dependent on the surface chemistry of these materials [28, 37, 41, 42].

According to acid/base character, due to the presence of both acid and basic sites on their surface, nanoporous carbons are considered amphoteric materials. Thus, the materials may present net acid, basic or neutral surfaces depending on the amount and the power of all the surface groups [19, 38, 42].

Many methods can be used to evaluate nanoporous carbons surface chemistry and the best way to achieve a good characterization is using the supplementary techniques and incorporation between of the data analysis such as:


#### *4.1.1 Acidic surfaces*

The chemical nature of nanoporous carbons is determined by surface groups containing oxygen that are mostly located on the external surface or edges of the basal plane.

The amount of oxygen on the surface has a high effect on the nanoporous carbons, s adsorption abilities as these groups constitute the majority of adsorption surface.

These groups can be classified according to chemical nature into three categories: acidic, basic, neutral.

Carboxylic, lactone, phenol, carbonyl, pyrone, chromene, quinone, and ether groups are examples of oxygen-containing functional groups on the nanoporous carbons surface see (**Figure 2**).

The responsible for surface acidity is Functional groups such as:

Carboxylic acid or carboxylic anhydride, lactone, and phenolic hydroxyl.

These Oxygen-containing functionalities are created by oxidation of carbon surface. The most commonly used activation methods to introduce oxygencontaining acidic groups are oxidation by gases and aqueous oxidants.

	- oxidation at low temperature can be used to form strong acidic groups (carboxylic).
	- oxidation at high temperatures can be used to form a large number of weak acid groups (phenolic).

**Figure 2.** *Acidic and basic surface functional groups on a carbon basal plane [42].*

*Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

> ◦ Liquid phase Treatment: Nitric acid or nitric and sulfuric acid mixture are very effective oxidizing agents due to the introduction of a significant number of oxygenated acidic functionalities onto the carbon surface that mainly includes carboxylic, lactone, and phenolic hydroxyl groups.

A greater quantity of oxygen groups in form of carboxylic and phenolic hydroxyl groups are produced in liquid phase oxidation at much lower temperatures compared to the gas phase oxidation [37, 42].

#### *4.1.2 Basic surfaces*

Basicity of activated carbon can be associated with:


Chromene, ketone, and pyrone are oxygen-containing surface groups that respond to the nanoporous carbons' basicity (**Figure 2**).

The basic character of activated carbons, however, arises primarily from electrons of delocalized graphene-layer. It was proved that these electrons could act as Lewis bases.

The contribution of basal planes to carbon fundamentality has been studied by some researchers. Leon y Leon et al. studied the surface basicity of two carbon series and showed that solution protons can be adsorbed from oxygen-free carbon sites.

These sites are found on the basal plane of carbon crystallites in -electron-rich areas. Fundamental sites are therefore the Lewis type associated with the carbon structure itself [42].

Nitrogen-containing functionalities can be introduced through:


#### **Figure 3.**

*Types of nitrogen surface functional groups: (a) pyrrole, (b) primary amine, (c) secondary amine, (d) pyridine, (e) imine, (f) tertiary amine, (g) nitro, (h) nitroso, (i) amide, (j) pyridone, (k) pyridine-N-oxide, (l) quaternary nitrogen [42].*

#### *Nanopores*

Possible structures of the nitrogen functionalities include the following: amide group, imide group, lactame group, pyrrolic group, and pyridinic group; which are shown in (**Figure 3**). Nitrogen functionalities generally provide basic property, which can enhance the interaction between carbon surface and acid molecules such as, dipole–dipole, H-bonding, covalent bonding, and so on [37, 41, 42].

#### **4.2 Nanoporous carbons analysis**

Nanoporous carbons are used in various applications such as separation, catalysis and energy storage, and so on [19]. The properties of these materials depend on the application used. Therefore, characterization of these materials is very necessary to determine the properties of materials before use in experimental applications. Surface area, pore size, and porosity are important properties in the fields of catalysis, separation, batteries, gas and energy storage, and others.

As selectivity, diffusional rates and transport phenomena are important properties in catalyzed reactions, determining the pore structure in-depth is very necessary as it controls these properties.

Various techniques can be used for this purpose such as:


Each approach has a small applicability length scale for the study of pore size. The International Union of Pure and Applied Chemistry (IUPAC) gave a detailed overview of the various methods for characterizing pore size and their application range [43–45].

Gas adsorption is a common one among these, as it allows a wide variety of pore sizes to be examined, including the full range of micro-and mesopores. Moreover, as opposed to some of the methods described above, gas adsorption techniques are easy to use, are not harmful, and are not expensive [45].

#### *4.2.1 Adsorption process*

In general, adsorption is defined as the enrichment of molecules, atoms, or ions in the vicinity of an interface. In the case of gas/solid systems, adsorption takes place in the vicinity of the solid surface and outside the solid structure. The material in the adsorbed state is known as the (adsorbate), while the adsorptive is the same component in the fluid phase. The adsorption space is the space occupied by the adsorbate. Adsorption can be physical (physisorption) or chemical (chemisorption) [1, 5, 46].

#### *4.2.1.1 Chemical adsorption: (chemisorption)*

In chemisorption, the intermolecular forces involved lead to the formation of chemical bonds. When the molecules of the adsorptive penetrate the surface layer and enter the structure of the bulk solid, the term absorption is used. It is sometimes difficult or impossible to distinguish between adsorption and absorption: it is then convenient to use the wider term sorption which embraces both phenomena, and to use the derived terms sorbent, sorbate, and sorptive [1, 46].

#### *4.2.1.2 Physisorption: (physical adsorption)*

Is widely used for the surface and textural characterization of nanoporous materials (e.g. for textbooks and reviews see: Sanghi, Canevesi, Celzard, Thommes [40], and Fierro. 2020 [4]; Thommes 2015 [46]; Thommes 2014 [40]) [40]. Physisorption is a general phenomenon and occurs whenever an adsorbable gas (the adsorptive) is brought into contact with the surface of a solid (the adsorbent). The forces involved are the van der Waals forces. Physisorption in porous materials is governed by the interplay between the strength of fluid–wall and fluid–fluid interactions as well as the effects of confined pore space on the state and thermodynamic stability of fluids in narrow pores [6, 7, 40].

There has been considerable progress over the last two decades in understanding sorption phenomena in small pores, which in turn has contributed to substantial progress in physical adsorption.

The development and application of microscopic methods, such as functional density theory (DFT) of inhomogeneous fluids (e.g., nonlocal density functional theory, NLDFT) or computer simulation methods such as Monte Carlo (MC) and molecular dynamic (MD) simulations, is closely correlated with this advancement [29]. Among many porous materials, nanoporous carbons (NPC), with interpenetrated and regular nanopore systems, have recently triggered enormous research activities because of their fascinating chemical and physical properties, such as high specific surface area, tunable pore structure, catalytic activity, high thermal and chemical stability, intrinsic high electrical conductivity, low density, and wide availability. Therefore, they have been implemented in hydrogen storage, pollutant adsorption, energy storage,(i.e., batteries, supercapacitor), catalysts, energy conversion, and electrochemical devices [19, 27, 36, 37].

#### **5. Nanoporous carbon materials synthesis**

Various natural biomass such as cassava peel waste, chicken eggshell, seed shell, rubberwood sawdust, wood, peanut kernel, lignocellulose (biomass) materials, corn cob, Kraft lignin, scrap tires, textile waste, rice husk, palm shell, and sugar have been employed as precursors for the production of NPC. These sources are generally rich in carbon giving amorphous phases, and the plant wastes containing the cellulose are familiar to form graphitic nanostructures with high-temperature treatments [47].

Conventional porous carbon materials, such as activated carbon, have routinely been prepared by pyrolysis followed by the activation process of the organic precursors, such as coal, plant, wood, or polymers, at specific high temperatures [19].

#### **5.1 Activation**

Carbonaceous materials are activated to create porosity, controlled morphology, and functional groups on the surface. The pyrolysis process is generally carried out

before undergoing the activation process as the former process generates organic residues, which may block the porous channels of the final carbon materials. Physical and chemical activation are two preferred choices for the fabrication of nanoporous carbon materials from carbon-rich precursors, including waste materials [47, 48] (**Table 3**).

#### **5.2 Physical or thermal activation**

Physical activation is usually carried out In two consecutive heating stages: Carbonization of the raw material under the inert atmosphere (usually nitrogen) to devolatilize the raw material, accompanied by activation consisting of partial gasification of the char acquired with oxidizing agents (i.e. steam, carbon dioxide or a combination of both) leading to the creation of a porous network.

While carbonization normally occurs at temperatures between 400 and 600 °C, temperatures ranging from 800 to 1000 °C are needed for gasification.

It is also possible to skip the carbonization stage, depending on the raw material, and proceed directly to thermal activation [47, 48].

• CO2 is considered the preferred choice for physical activation due to the ease of handling, control of various parameters, and slow reaction rate. Instead of


#### **Table 3.**

*Appropriate precursors, kinetic of activation, and type of porosity are typically obtained for the most common activating agents [28].*

diffusional regulation which is quicker but contributes to external particle burning and, ultimately, to poor production of porosity. CO2 activation must occur in conditions that ensure chemical control (slow activation rate-days).

The reactions of steam and carbon dioxide with carbon are endothermic, thus:

To sustain the necessary high temperature, thermal activation requires an external energy supply [28, 31].

• Oxygen (or air) is not widely used as an oxidizing agent because its carbon reaction is highly exothermic and rapid, and instead of particle consumption, it is difficult to monitor and ensure porosity growth.

Oxygen activation is scarcely used because of this and the safety concerns associated with temperature regulation.

However, low amounts of oxygen (or air) may be added to steam or carbon dioxide during thermal activation to help sustain high temperatures by responding to the gases emitted during activation (i.e. CO and H2).

This strategy has the benefit of decreasing CO and H2 pressure, both inhibiting activation gases and increasing the triggering agent's partial pressure [28] (**Table 3**).

#### **5.3 Chemical activation**

Chemical activation normally needs just one heating step: the raw material is combined with an activating agent (e.g. ZnCl2, H3 PO4, KOH) and further treated at temperatures between 400 and 900 °C under a controlled atmosphere, depending on the activating agent selected. The activating agent helps to remove the residual water moieties from the raw materials by acting as a dehydrating agent and also assists as an oxidant. Both the processes affect the decomposition of precursors and rearrangement of the resulting carbon atoms into an aromatic framework (**Table 3**).

Chemical activation offers an additional advantage of introducing functional groups such as -COOH, -NH, or -OH on the surface of the porous carbon. However, the crystallinity of the sample after the chemical activation is reduced due to the continuous dehydration and the oxidation with the activating agent, which creates a lot of defect sites along the carbon walls of the final product [28, 47, 48].

The mechanism of pore formation depends, in this process, on the chemical agent:



#### **Table 4.**

*Advantages and dis advantages of activation processes [28, 48].*

atmosphere), but when chemical activation is applied, the activating reagent is incorporated into the particles which inhibit the anticipated contraction, i.e. the activating agent will act as a template for microporosity formation. [28, 31, 32, 48, 49].

Chemical activation has advantages over the physical process discussed in (**Table 4**).

#### **5.4 Other nanoporous carbons synthesis methods**

The chemical activation process using KOH, K2CO3, K2O, ZnCl2, KHCO3, H3PO4, etc., and their reaction are very useful to make nanopores, however, harsh reaction condition, cost of required chemicals and processes, residue, ununiformed pore distribution, and difficult control of pore size should be considered for upscaling the production and commercialization. Compared with chemical activation, physical activations utilizing mainly CO2 and steam activation usually has a high yield and bulk density but suffers from a relatively low surface area and pore volume due to the lower degree of carbon etching. Therefore, many researchers have studied the efficiency of other methods to fabricate nanoporous carbon materials.

Thus, hard- and soft-templating approaches have been successfully introduced for the preparation of NPC with well-defined pore structures and narrow pore size distributions. In this Chapter, the hard- and soft-templating synthesis are introduced as potential approaches for the preparation of NPC materials with a special emphasis on the progress and developments in the methodology.

Hard synthesis of templates requires the use of pre-synthesized organic or inorganic templates, while the soft synthesis of templates depends on the creation of nanostructures through self-assembling organic molecules [19, 23].

#### *5.4.1 Templating method*

Historically, Knox and co-workers, who demonstrated the synthesis of graphitic porous carbons for liquid chromatography separation by impregnation of spherical

#### *Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

porous silica gel particles with phenolic resin and subsequent carbonization and silica removal, first reported the templating process in 1986. This technique has gained considerable attention since then and different types of template carbons are synthesized. The resulting carbon synthesized by the templating method has a relatively narrow PSD and regulated architecture called a templated carbon.

The templated carbonization method permits one to control the carbon structure in terms of various aspects, such as pore structure, specific surface area, microscopic morphology, and graphitizability, which makes this method very attractive [48].

Porous materials are fabricated in several different ways. The Hard Template Method and Soft Template Method are the two most common methods to make porous materials [39, 50, 51].

#### *5.4.1.1 Hard template method*

The most common hard template synthetic route for mesoporous carbon materials was first reported by Knox et al. using a spherical solid gel as the template. Highly ordered NPC with oriented mesoporous structures can be obtained using the hard template method.

The hard template method includes the following steps: (a) synthesis of a suitable porous template; (b) introduction of a suitable carbon precursor into the template pores using the method of wet impregnation, chemical vapor deposition, or a combination of both methods; (c) polymerization and carbonization of the carbon precursor; and (d) removal of the inorganic template. Following these steps, porous carbon with a specific pore structure is formed [19, 48, 52].

Angelina Sterczyńska, Małgorzata Śliwińska-Bartkowiak, Małgorzata Zienkiewicz-Strzałka, Anna Deryło-Marczewska Synthesized Nanoporous Carbon (also called ordered mesoporous carbon material [OMC]) with a 4.6 nm pore size, and ordered silica porous matrix, SBA-15, with a 5.3 nm pore size [54].

Also, Dandan Guo, Jin Qian, Ranran Xin, Zhen Zhang, Wei Jiang, Gengshen Hu, Maohong Fan prepared Mesoporous carbons enriched with nitrogen by hard template method for supercapacitors. Where CCl4 and ethylenediamine (EDA)

**Figure 4.** *Preparation of mesoporous carbon using silica porous matrix [53].*

represent precursors whereas silica act as a hard template [53] (see **Figure 4**). While Wei Liu, Hong Yuan, and Yihu Ke Prepared ordered mesoporous carbon-based on soybean oil by using the hard template method where a hard template is represented by ordered mesoporous SiO2 molecular sieves (SBA-15) [55].

However, when extracting from the template, the sacrificing of the solid template and mesoporous NPC structures limits the usefulness of hard template synthesis. The use of soft template synthesis will overcome these constraints [51, 54, 56].

#### *5.4.1.2 The soft template method*

The soft template is a kind of surfactant, which has a strong interaction with the carbon source, and mesoporous carbons with different structures can be obtained through the soft template method. This method possesses good controllability and operability; as a result, it has very good application prospects. The mechanisms of the soft template method include a liquid-crystal template mechanism, a synergistic assembly mechanism, a "rod micellar" mechanism, and so on; these mechanisms have been widely recognized [48, 52].

Amphiphilic molecules, such as surfactants and block copolymers, have been extensively used as soft-templates in the synthesis of ordered mesoporous materials. The discovery of ordered mesoporous carbon materials appeared to have a great impact in this field because of the fascinating features of their unique physical and chemical properties which can surmount the shortcomings in various technological applications. Preparing these ordered mesoporous carbons can be difficult to achieve by a simple self-assembly method for many reasons, although recent reports have demonstrated that polymeric micelles can serve as templates for mesoporous carbons. The key requirements for a successful synthesis using the soft-templating method are (i) the ability of the precursor species, such as the copolymers and the carbon source, to self-assemble into nanostructured polymer composite, (ii) the presence of at least one performing species, and one carbon source, (iii) the stability of the pore-forming species which can endure the temperature required for thermally decomposing the

#### **Figure 5.**

*Soft-templating synthesis of carbon nitride and graphene materials. Route 1: (1-a) self-assembly of surfactant or block copolymer molecules (I) into micelles (I-1-a), (1-b) addition of a carbon nitride precursor and formation of micelle-precursor mesostructures (I-1-b), (1-c) initial condensation/ polymerization of the precursor, (1-d) further condensation and template elimination creating a nanoporous carbon nitride material (I-1-d), (1-c\*) initial condensation of the precursor conducted at a temperature higher than the decomposition point of the soft template, (1-d\*) further condensation but causing structural collapse. Route 2: (2-a) addition of a carbon precursor and production of individual micelle-carbon precursor units, (2-b) close-packing of these units on a substrate (side view, depicted in gray) forming a monolayer (1–2-b), (2-c) polymerization of the carbon precursor followed by carbonization, (2-d) graphitization giving nanoporous graphene sheets. Black lines represent the 2D building units, namely carbon nitride or graphene layers [19].*

*Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

carbon source during carbonization process, and finally (iv) the ability of the carbon source to form cross-linked polymers that can retain their nanostructures during the thermal decomposition. The synthesis principles of these self-assembled nanostructured mesoporous carbons open the way for the development of new strategies for materials in the future. Researchers have reported that only a few materials meet the requirements for the successful synthesis of ordered mesoporous carbons using a softtemplating approach [23, 50, 56]. Some of the research activities related to the softtemplating synthesis of polymeric structures are summarized [19] (see **Figure 5**).

#### **6. Activated carbon as the essential phenol removal adsorbent**

#### **6.1 The activated carbon precursors**

There is a wide range of raw materials that can be successfully used as a precursor for the preparation of activated carbon. Almost interesting precursors have been obtained from any carbonaceous materials such as agricultural waste, wood, petroleum coke, and industrial biomass. An important aspect in the preparation of activated carbon is the use of different parts of plants including the pulp, stems, shells, peels, flowers, fruits, seeds, stones, peels, and leaves. All these precursors can be carbonized and then activated under desired conditions to yield activated carbon [25]. The selection of the precursors is based mainly upon the following several factors:



**Table 5.**

*Properties of some raw materials and the properties of activated carbon generated [24].*

The characteristic of activated carbons such as physicochemical properties that responsible for carbon adsorption properties and other possible applications depend on selected carbon precursors in addition to the preparation method. Lately, it is longer than lignocellulosic resources and waste biomass is the most used precursors for the production of the activated carbon (**Table 5**). Summary of the properties of some raw materials and the properties of activated carbon generated [24, 57, 58].

The usage of lignocellulosic biomass in the generation of activated carbon has many features as it is renewable and most abundant in nature, inexpensive, and helps to dispose of its negative impact effect on the environment. Numerous reviews have been devoted to inexpensive precursors of activated carbon in recent years [58, 59].

#### **6.2 Generation of porosity and surface chemistry (activated carbon)**

All activated carbon is generally characterized by a porous structure with their high surface area, usually have few amounts of chemically bounded heteroatom oxygen, hydrogen, sulfur, and nitrogen. Beside may contain around 20% by weight of a mineral substance called ash content [24, 49]. It is known that the surface of activated carbon has a high heterogeneous phenomenon. AC surface heterogeneity comes from two various sources called geometrical and chemical sources. Geometric heterogeneity results from differences in the size and shape of pores, cracks, pits, and steps. Chemical heterogeneity is associated with different functional groups, especially the oxygen groups that are most often found at the edges of turbine crystals, as well as with various surface impurities. The heterogeneity of AC (Geometric, Chemical) surfaces affect unique adsorption properties. The chemical properties and structure of activated carbon and its structure can be changed depending on the type and nature of presence and number of oxygen functional groups on its surface [24, 25, 36, 59].

It's possible to produce activated carbon from all carbonaceous materials which its preparations involve two major steps: carbonization of the precursors followed by activation method as shown in (**Figure 6**). Carbonization means the conversions of raw materials at elevated temperatures into a highly stable carbon structure with an elementary and partially -developed pore structure. During this step, water and volatile substances are removed leaving the char behind. Followed by activation of char by physical or chemical activation to produce highly porous activated carbon. The generated activated carbon characterize by having a porous structure, high

**Figure 6.** *Synthesis of activated carbon [49].*

#### *Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

surface area, and highly reactive surface functionality. Physical activation involves the carbonization of the precursors in the presence of inert gas in the range between 500 to 900 °C followed by gasification of the resulting char with carbon dioxide. Steam, air, or mixtures of both can be also used as activating agents. Excessive temperatures lead to reduced carbon content, collapse its pore structures and increase ash generation. in physical activation, proper temperature and time are so important to achieve adequate pore development and the creation of functional groups. In chemical activation, the raw material is directly impregnated with an activating agent such as KOH, NaOH, H3PO4, H2SO4, HNO3, ZnCL2, and FeCL3, and the impregnated product is pyrolyzed at high temperature for a certain time and then product washed to remove the activating agent. The activating agent can contribute in the oxidation and gasification the carbon precursors to improve porosity and transform surface functional group. The pores generated from activation are usually identified as microspores and mesoporous [24, 28, 48]. Chemical activation has advantages among physical included in (a)-the low temperature of activation. (b) Well developed in the porous structures.

Different pore sizes (micro, meso, and macropores) are obtained depending on the nature and type of precursors, activating agent, and reaction conditions such as time and temperature. The properties of raw material such as its type and size, the type of activating agent, the ratio of mixing raw material with the activating agent, the conditions of heating in the furnace, will have a significant effect on the characteristics of the final product including surface area and pore size [24, 48].

However, first and foremost, the adsorption features of activated carbon are dictated by its chemical composition. The existence of hydrogen and oxygen groups on the surface of the activated carbon directly affects the adsorption performance. Original AC precursors, the activation process, or post-treatment after the preparation process can be the source of these surface groups. The oxygen groups are mainly formed on the surface of activated carbon following activation by air exposure or by relevant post-treatment [25, 57, 60, 61].

Carbon is more likely to chemisorb oxygen than any other species. Chemisorbed oxygen present on the surface of AC to form carbon and oxygen functional groups can be acidic, neutral, or basic. The formation of oxygen groups on the carbon surface is generated from the reaction with the activating agent used such as (H2So4, HNO3, H2O2) and other oxidizing gases like CO2 and O3. Among the factors affecting the nature of the surface group of oxygen, the temperature may be taken into account, as surface acidity is formed that includes carboxyl functional groups, carboxylic anhydrides, lactones, and phenol hydroxyls upon exposure to low temperature while basic surface that generated from delocalized π-electrons on a carbon basal plane like pyrone, quinone, and carbonyl generated at high tempertature [24].

To define the number of groups of oxygenated surfaces, The Boehm titration method is used. The basic functional groups are the most preferred than the acidic functional group for the adsorption of phenolic compounds. And some experimental methods like temperature-programmed desorption (TPD), infrared spectroscopy, acid–base titration, X-ray photoelectron spectroscopy (XPS), can be used to characterize surface-oxygen groups [24, 57].

#### **6.3 Role of surface heterogeneity on adsorption of phenol**

Although activated carbon has been investigated for a long time as an effective adsorbent of organic pollutants, the exact structure of the functional groups and the mechanism of phenolic compounds adsorption is not well understood yet.

Much information should be considered before applied adsorption of phenol such as:


The adsorption capacity of phenol on activated carbon depends on some factors such as:


And thus adsorption capacity increase with increasing specific surface area and porosity while it decreases by the solubility of phenolic compounds in water and increases the hydrophobicity of phenolic substituted. For example, phenolic compounds that having low solubility in water like p-cresol and p-nitrophenol are adsorbed on activated carbon than other phenols. On the other hand, chlorophenol, nitrophenol, cresol adsorbed greater on activated carbon than phenol and aminophenol due to their hydrophobic group. The adsorption of phenolic compounds onto the ACs mainly contribute to three types of interactions namely, (i) ππ dispersion interaction, (ii) the electron-donor–acceptor complex formation, and (iii) the hydrogen-bonding formation. The mechanism of adsorption of phenolic compounds toward activated carbon occurs through the formation of electron donor-acceptor complex between the aromatic ring of phenol and basic sites on the surface of activated carbon (basic surface oxygen complex and/or πelectron-rich sites on the basal planes). Therefore, the relative affinity between the carbon surface's basic characteristic and aromatic phenolic ring increases. Electron withdrawing of phenolic rings tends to form electron donor-acceptor complex between these ring and basic sites on the surface of activated carbon [24, 25, 42]. In the case of oxidation, the surface of activated carbon with a strong oxidizing agent leads to the *Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

formation of the acidic surface with a large quantity of carboxyl and phenolic groups with a small amount of carbonyl and chromene lead to inhibition of phenol adsorption. During the adsorption of phenol on activated carbon, these regions act as a donor and the aromatic rings of phenol as acceptors. Phenol adsorption onto the activated carbon is controlled by dispersive force between π electrons. The interaction of π-π dispersion occurs between basal planes of activated carbon and the phenol aromatic ring [24, 42]. The change in PH solution affects phenol adsorption. The adsorbed amount of PH decrease at low and high PH values. At low PH value, protons were added to compete with the adsorbate for the carbonyl sites leading to a reduction of adsorption of phenol at this value. Besides the surface chemistry of activated carbon, the pore structure also affects the adsorption process. The porosity of activated carbon has been considered an important factor in the adsorption processes of phenolic compounds from aqueous solutions. The adsorption capacity of small molecules such as phenol to the inner surface of carbon correlates with the content of micropores and BET surface area, while for mesoporous ACs, substituent group in the phenol and nature of the carbon controlled the phenol adsorption as well [24, 37, 38, 41].

#### **7. Conclusion**

Nanoporous carbon materials have an attractive rate performance in many applications of recent technology such as pollutant adsorption. In this chapter, the properties of nanoporous carbon and its various preparation methods are presented. Also, our choice of the preparation method, reaction conditions, and the precursor materials affect the properties of the resulting nanoporous structure. The adsorption of phenolic compounds from polluted water is one of the most common uses of nanoporous carbon, especially activated carbon in water treatment. Numerous factors are known to have an important influence on phenolic adsorption like the type of carbon structure, functional groups present on the surface, oxygen availability on its surface, pH value of the aqueous media, etc. Furthermore, there are several scientific papers reviewed - aspects most relevant to indicating today's trends and potential insights in elucidating the adsorption mechanisms of phenolic compounds on activated carbon.

#### **Appendices and nomenclature**

Nanoporous carbon materials: (NPC).

*Nanopores*

### **Author details**

Mahmoud Fathy Mubarak<sup>1</sup> \*, Alshimaa Maher Ahmed<sup>2</sup> and Sahar saad Gabr<sup>1</sup>

1 Petroleum Application Department, Egyptian Petroleum Research Institute, Cairo, Egypt

2 Department of Chemistry, Faculty of Science, Helwan University, Cairo, Egypt

\*Address all correspondence to: fathy8753@gmail.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Nanoporous Carbon Materials toward Phenolic Compounds Adsorption DOI: http://dx.doi.org/10.5772/intechopen.96380*

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## **Chapter 9** Graphene Nanopores

*Per A. Löthman*

### **Abstract**

Graphene is a two-dimensional, atomic thin, usually *impermeable* nanomaterial with astonishing electrical, magnetic and mechanical properties and can therefore at its own right be found in applications as sensors, energy storage or reinforcement in composite materials. By introducing *nanoscale pores* graphene alter and extend its properties beyond permeability. Graphene then resembles a nanoporous sensor, a nanoporous, atomic thin membrane which opens up for such varied applications such as water purification, industrial waste water treatment, mineral recovery, analytical chemistry separation, molecular size exclusion and supramolecular separations. Due to its nanoscopic size it can serve as nanofilters for ion separation even at ultralow nano- or picomolar concentrations. It is an obvious choice for DNA translocation, reading of the sequence of nucleotides in a DNA molecule, and other single molecular analyses as well for biomedical nanoscopic devices since dimensions of conventional membranes does not suffice in those applications. Even though graphene nanopores are known to be unstable against filling by carbon adatoms they can be stabilized by dangling bond bridging via impurity or foreign atoms resulting in a robust nanoporous material. Finally, graphene's already exceptional electronic properties, its charge carriers exhibit an unusual high mobility and ballistic transport even at 300 K, can be made even more favorable by the presence of nanopores; the semimetallic graphene turns into a semiconductor. In the pores, semiconductor bands with an energy gap of one electron volt coexist with localized states. This may enable applications such as nanoscopic transistors.

**Keywords:** graphene, carbon, DNA, bottom-up, translocation, sculpting

### **1. Introduction**

Pores are *ubiquitous* in nature, engineering and the natural sciences. We recall such diverse examples such as porous light weight metals, aluminum foams or metallic hollow spheres structures [1–4] that can save energy by reduced gas consumption, or cell membranes with ion-channels which constitute highly functional nanoporous structures of the cell. They are responsible for maintaining the required pressure gradient, ion-flux and ultimately nutrition and life itself in both monoand multicellular organisms. One of the simplest but highly ordered proteinaceous nanoporous membranes in nature are bacterial surface layers (s-layers), a spontaneously occurring protective layer on the surface of the bacterial cell. This regularly structured nanoporous membrane protects and regulates a minimum out- and influx of nutrients to the cell and can be used as templates to synthesize metallic nanoparticles in the lower nanorange (1,4 nm) or as a soft membrane in nanobiodevices [5]. Nanopores such as the protein hemolysin are found in cell membranes, acting as transport channels for ions or molecules in and out of cells [6–8]. The

selection mechanism of these membranes can be based on size exclusion as well as exclusion based on double layer overlap and dielectric exclusion [9]. S-layers are together with the cell membrane one of numerous *soft-matter* nanoporous materials. While cell membrane ion-channels or s-layers serve several life sustaining, protective biological functions they are not durable or mechanical stable and therefore not suitable for engineering applications. Quite the opposite are *solid state* nanopores which presents obvious advantages over their soft-matter counterparts. They are highly stable, exhibit controllable dimensional parameters such as channel length and diameter. Their surface characteristics can be altered and enable integration into devices and arrays.

A solid state material which has gained considerable recognition the last decade is *graphene* considered as one of the strongest and thinnest materials known. Monolayer Graphene possesses astonishing characteristics: Its electron mobility is 100 times higher than silicon; it conducts heat twice as good as diamond; its electrical conductivity is 13 times better than copper and it absorbs only 2.3% of reflecting light i.e. it is transparent; it is impenetrable even to the extent that the smallest atom (helium) cannot pass through a defect-free monolayer graphene sheet; and its high surface area of 2630 m<sup>2</sup> /g which means that with less than 3 grams you could fully cover an entire soccer field. It is a two-dimensional atomic thin allotrope of carbon consisting of a single layer of atoms arranged in a twodimensional hexagonal honeycomb structure [10, 11]. The name reflects the fact that the graphite allotrope of carbon consists of stacked graphene layers [12]. They are bound to each other by weak van der Waal forces which makes graphene an integral part of the 3D material graphite from which it was first isolated. Graphene was however not expected to exist in the free state i.e. as a single monoatomic layer. Scientists had argued convincingly that monoatomic thin 2D materials like graphene would be too thermodynamically unstable to exist. Thermal fluctuations would be as large as the force binding the atoms together, causing the structure to fall apart [10, 11]. However, carbon bonds are in fact strong enough and small enough that thermal fluctuations are not enough to destabilize graphene even at room temperature. Free-standing monolayer graphene was isolated in 2004 by Novoselov and Geim [13, 14] and follow-up investigations revealed several novel exciting properties [15, 16]. Graphene was considered as the new material of the future and Novoselov and Geim were awarded the Nobel Prize in Physics for their discovery and characterization of graphene.

Graphene has excellent *mechanical and electrical* properties: an atomic thin monolayer graphene has an inplane direction independent Young's modulus of 1 TPa and strength of 100 GPa [17–21].

Graphene's superior electrical properties are due to the fact that its charge carriers are massless Dirac fermions [10–12] with high mobility and ballistic transport even at highest electric-field and affected to only minor degree by chemical doping. Its extraordinary high electrical conductivity and its capacity to carry large currents at room temperature [22] makes it indeed an exciting material. Carbon atoms have four electrons available to make chemical bonds. Graphene is however only one atom thick and every atom in the crystal is bound to only three others. Each atom thus has one free electron available for electronic conduction which means that graphene by far exceed the electrical properties of metals. Since each graphene 2D lattice provides as many charge carriers as metals are only able to supply from bulk 3D atomic architectures, even when metals tend to have some electrons delocalized and shared in a "sea of electrons" among all atoms within a piece of metal, which makes graphene an extraordinary material in electronics [23]. In electronics graphene may act as scaffold on which parts that can act as distinct components may self-assemble into an electronic circuit. This is due to the fact that various molecules

#### *Graphene Nanopores DOI: http://dx.doi.org/10.5772/intechopen.98737*

can attach to the graphene surface due to its electronic structure and that chemical changes made to parts of the graphene sheet such that local electric properties can be fine-tuned and varied on the same surface along with additional properties such as permeability via nanopores (sculpting). Such nanometer-sized circuitry may one-day enable faster and smaller computational and electronic devices.

Moreover, the specific electrical properties of graphene in terms of conductivity are due to the fact that with one pz electron per atom in the model the valence band is fully occupied, while the conduction band is vacant. The two bands touch at the zone corners (the K point in the Brillouin zone), where there is a zero density of states but *no band gap*. The graphene sheet thus displays a *semimetallic* (or *zero-gap-semiconductor*) character, although the same cannot be said of a graphene sheet rolled into a carbon nanotube, due to its curvature. By introducing nanopores in graphene one can open up an energy band gap in a graphene sheet as described below.

Conceptually graphene represents a new class of materials; inorganic, twodimensional materials that are only one atom thin. Thereby graphene provides new incursions into low-dimensional physics which has always been a rich source for novel applications. Graphene does no longer requires any further proof of its importance in terms of fundamental physics, however, there is still room for extending, altering and improving graphene properties. As already mentioned *nanoscopic sculpting* such as nanolithography, manipulation by AFM [24] or an electron beam of a transmission electron microscope [25] is considered a promising venue to target properties of nanomaterials. In this way nanopores can be introduced into graphene. It has become an alternative route of materials development; instead of turning to a different class of graphene-based materials such as nanoribbons or nanocomposites, sculpting nanopores into graphene would further alter the already numerous and exceptional properties and extend the fields of applications. It would open up an energy band gap in a graphene sheet for the application as field effect transistors (FETs) [25–27]. Nanopores can turn *semimetallic* graphene into a *semiconductor* [28].

Nanoporous graphene exhibit a *periodic arrangement* with nanoscale diameters in the graphene membrane. Apart from Nano sculpting as mentioned above numerous methods, such as chemical etching [29], vapor deposition [30], and electron beam [31], have been developed to fabricate nanoporous materials and control pore dimensions. Moreover, it is expected that facile methods such as self-assembly of graphene are just as suitable for nanoporous graphene as it is for impermeable graphene or grapheneoxide [32–34].

The aim of this work is to illustrate how the properties and applications of the nanoscopic material graphene can be altered, improved and extended by introducing nanopores in the graphene layer.

#### **2. Ion transport through graphene nanopores**

Transport phenomena through ion exchange membranes have been investigated for several decades. When L. Michaelis first observed the effect of membrane charge on the ion transport through pores in the year 1926 [35, 36] there has been an continuous interest in this research field and the transport phenomena are now well understood for conventional dense ion exchange membranes. The current trend towards nanotechnology and miniaturization of devices, ion transport through solid state nanopores is gaining attention [37–41]. 2D materials such as graphene play an important role for applications in nanofluidic device, biosensing, and DNA translocation [42–45].

2D materials may have some limits in these applications because of the presence of intrinsic defects and low surface charge density. The ion selectivity may be influenced by the pore size distribution. These membranes exhibit low surface charge which limits rejection of ions. It is therefore important to optimize fabrication techniques combined with a thorough understanding of transport phenomena through a 2D interface. It is expected that transport under nano-confinement in 2D is expected to differ from highly charged ion exchange membranes.

A number of different physical transport processes occur in the pores of a membrane. The most relevant processes for ion transport and ion separation processes the most important are size exclusion, charge exclusion and dielectric exclusion [46]. Size exclusion occurs when the pore size of the membrane is comparable or smaller than the species to be retained.

Microfiltration (MF) membranes have relatively large pore sizes (0.1–10 μm), to separate smaller species from 1 to 100 nm (e.g. proteins, viruses), ultrafiltration (UF) membranes are used. Nanofiltration (NF) membranes (1–10 nm) are used for removal of salt, amino acid, and dye [47]. Dielectric exclusion is an ion rejection mechanism observed in NF membranes and typically dominates at <1 nm and effective up to about 2 nm pore size [46, 48]. This phenomena occurs at interfaces between media having different dielectric constants. The mutual interaction of ions at the surface and the induced bound electric charge at the interface leads to the dielectric exclusion. This also depends on pore geometry e.g. cylindrical pores have stronger exclusion compared to slit pores. Ion exchange membranes (IEM) are used for demineralization or deionization of water, energy conversion and energy storage in fuel cells, redox flow batteries [49, 50].

Other than these commercial membranes, nano-porous materials such as solid state nano-pores in synthetic membranes (SiNx, SiO2), *nano-porous graphene*, graphene oxide multi layers, metal organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and hybrid membranes can act as *ion selective membranes* depending on the ion concentration.

Konatham *et al.* studied water transport through monolayer graphene nanopores via molecular dynamics (MD) simulations. The pore diameter war as small as 7.5–14.5 Å in the monolayer graphene [51]. In this case ion exclusion was achieved up to a 7.5 Å pore diameter of non-functionalized (uncharged) pores. Larger pores cannot block the ions. Dielectric exclusion may also in this case be an important mechanism of exclusion for pore sizes close to 7.5 Å.

The ion rejection mechanism in pores is influenced by functionalized pores. Functionalization with carboxyl groups show improved ion rejection due to a higher free energy barrier towards water and ions. Because of this ion screening effect, the free energy barrier decreases with increasing salt concentrations in the bulk. Cohen-Tanugi *et al.* showed via MD simulation that multilayer *graphene membranes* can desalinate water more effectively than monolayer graphene. The salt rejection mechanism as a function of pore diameter, layer spacing and applied pressure was investigated. The smaller nanopores (3 Å) reject salt entirely compared to larger pores (4.5 Å) and highly aligned pores with multiple layers can even combine high salt rejection with high water flux [52].

Graphene pores supported on track etched PCTE membranes and pores were enlarged by oxidative etching in acidic potassium permanganate solution was investigated. The number of pores were 1012/cm<sup>2</sup> and the pore sizes were in the sub nanometer (0.40 ± 0.24 nm) range. For short oxidative etching times, the resulting membrane showed cation selective behavior because of steric exclusion and the negatively charged surface groups at the pores. A membrane potential around 4 mV was observed for 0.5 M KCl/0.1667 M KCl which is lower compared to the theoretical Nernst potential for this salt concentration ratio (28.1 mV). This indicates that the pore sizes may be larger than 0.4 nm as the selectivity is expected to be higher at 0.5 M KCl due to dielectric exclusion. The membrane potential decreased with increase in pore size obtained after longer oxidative etching times.

Oxygen (O2) plasma etching is an additional fabrication method to create graphene nanopores; CVD graphene transferred onto a SiN substrate with a 5 μm hole and subjected to O2 plasma etching. This generates nanometer sized holes in the graphene sheet which was confirmed by Raman spectroscopy. Nanopores fabricated via this method have shown very high *salt retention* at lower etching time due to the small pore size. The transport properties of ions through these nanoporous interfaces were investigated with or without externally applied electric fields [53–60].

Nanoporous graphene is *cation selective* [61, 62] and a non-linear current–voltage relationship has been detected [63]. A diode rectification effect has widely been observed especially in solid state conical nanopores [54, 56]. For graphene nanopores and other 2D materials (MoS2, h-BN) the rectification effects has been observed in both intrinsic and artificially made pores [61, 64–68]. Nanoporous graphene supported on PET shows an ion rectification effect due to the presence of *conical* nanopores as a result of *asymmetric etching* [64–68]. Applying an external potential and gating the graphene the ion selectivity can even be *tuned* [69].

Up to date most investigations were limited to sub-nanometer sized pores in graphene where the ion rejection was mainly dominated by steric exclusion.

The ion-selectivity of graphene nanopores favors K+ over Cl− even up to a pore diameter of 20 nm. The selectivity calculated by the Goldman-Hodgkin-Katz (GHK) model was around 100, a value much higher than the selectivity observed by Jain *et al.* for pore sizes of sub-nm level (0.4 nm) [63]. The membrane potential, specially for biological membranes is typically calculated using the GHK voltage Equation [70–76]. This equation is applicable for multiple permeating monovalent species and it takes into account the permeability of each specie. Furthermore, the selectivity was dependent on the pH. The cation to anion selectivity decreases with decreasing pH which was attributed to the protonation of surface charged groups e.g. carboxyl groups at the graphene edge. Interestingly, the membrane could differentiate between monovalent and divalent cations by conducting monovalent cations 5 times faster than divalent cations. Ion selective transport through graphene with pores larger than a nanometer was experimentally shown by Rollings and van Deursen et al. [74, 75].

Molecular dynamics simulations by Cohen-Tanugi *et al.* show that nanoporous free-standing graphene membranes are able to reject NaCl ions while letting water flow at permeabilities several orders of magnitude higher than conventional reverse osmosis membranes. The performance was studied as a function of pore size, chemical functionalization, and applied pressure. The results indicate that the membrane's ability to prevent the salt passage but allowing for water flow depends critically on pore diameter. Also chemical functional groups bonded to the edges of graphene pores suggests that commonly occurring hydroxyl groups can roughly double the water flux thanks to their hydrophilic character. Nanoporous graphene may play an important role for water purification. The maximum diameter for salt permeability is around 5.5 Å, that is, Na + and Cl- ions will pass through the membrane beyond this diameter [77].

Other molecular dynamics studies by Suk *et al.* found that pure water can continue to flow across graphene nanopores with diameters below 1 nm, and calculations suggest that the chemical functionalization of graphene nanopores could be tuned to selectively reject certain solvated ions [78–81].

#### **3. Single molecule analysis via graphene nanopores**

Nanopores resemble a class of a biosensor, allowing for highly sensitive detection of biomolecules including nucleic acids and proteins at single-molecule resolution [76, 82, 83]. Nanopore sensors have emerged as powerful devices for probing biomolecules and offer a novel platform for single molecule analysis and characterization. In particular, they have attracted significant attention as tools for high-throughput, robust, and low-error DNA sequencing. Especially graphene and other two dimensional (2D) materials are being investigated with respect to their integration into nanoscaled devices that may in the future sequence genomes. The successful implementation of solid-state nanopores in emerging third-generation DNA sequencing applications is contingent upon developing methods for scalable fabrication, high-accuracy output, and integration with low-noise electronic architectures.

In a nanopore ion currents and forces can be monitored as molecules pass through. This makes it possible to investigate a wide range of phenomena involving DNA, RNA and proteins. The solid-state nanopore increasingly proves to be a surprisingly versatile new single-molecule tool in biophysics and nanofluidics.

The high sensitivity of the nanopore comes from the characteristic structure of the nanopore: a nanometer scale pore as large as the size of the molecule of interest [84, 85]. It allows for detection of biomolecules even at a sub-nanomolar concentration level and discrimination of minute differences in molecular structure between different nucleotides [86–88]. The high sensitivity originates from the electric potential applied across the nanopore membrane which generates a highly concentrated electric field near the nanopore. Charged molecules pass through the narrow pore one molecule at a time [87].

The methods in use for biomolecule translocation detection include resistive pulse sensing [89–92], tunneling current detection [42, 93, 94] and optical sensing, [95–99]. In the resistive pulse technique (or Coulter-counter method), the nanopore acts as the only channel across the membrane for both ions and biomolecules, and partial blocking of the nanopore by a biomolecule is directly reflected in a perturbation in the measured ionic current. More precisely, a membrane containing a single nanopore is sandwiched between two reservoirs of electrolytic solution, such as aqueous potassium chloride. Ions are driven through the pore, as illustrated in **Figure 1**, by applying an electric potential difference across the membrane, resulting in an ionic current that can be measured. When the electrolyte contains larger charged molecules, such as DNA or proteins, these are also driven through the pore, causing a transient dip in the ionic current where each current dip represents a passage, or translocation, of a biomolecule, with the magnitude and duration of the pulse being indicative of the molecule's radius and length, respectively.

Moreover, slowing down the DNA translocation speed has been a major issue for nanopore sensing. The DNA translocation speed is a few orders of magnitude faster in the solid-state nanopore than in a biological nanopore for unknown reasons [95].

To slow down translocation speed in the solid-state nanopore, various engineering strategies have been enivsaged: dragging the molecule by strengthened interaction with the modified or chemically decorated pore surface. Here materials including aluminum oxide (Al2O3)–graphene etc. integrated with the nanopores slows down the DNA translocation by an enhanced Coulombic, specific, or hydrophobic nonspecific interaction between the nanopore surface and DNA [101, 102].

Various research groups investigated the influence of the number of nanoporous graphene layers on the DNA translocation; 1 or 2 layers thick [103], 1–8 layers thick [101] and 3–15 layers thick [102]. Initial DNA detection experiments were carried out, an important step towards DNA sequencing. In each case the nanopores could

**Figure 1.** *A DNA molecule translocating through a graphene nanopore [100].*

detect double stranded DNA molecules with lengths from 400 to 48,000 base pairs. Even membranes with significant variability in the baseline current levels were found to be viable for DNA detection. Interestingly the nanopores could differentiate between DNA that passed through the pore in an extended form and that which passed through in a folded form. Even though these developments are impressive, the central goal remains unsolved: is singlebase resolution with a graphene nanopore feasible? Also in this case all the different nanoporous graphene membranes did show that the translocation events are too fast to be resolved by the existing detection electronics.

#### **4. Mechanical and electrical properties of nanoporous graphene**

Nanoporous graphene is unique in that it exhibits both *electronic functionality* as a *tunable* semiconductor and *mechanical functionality* as a *tunable* molecular filter membrane. These properties combined in a single atomically-thin, mechanically robust platform makes nanoporous graphene a promising candidate for electronically active nanodevice applications [104–106].

As is noted, a better understanding of the structure–property relation would be of direct relevance to the structure design and function optimization in a variety of technological applications. The present study aims at the mechanical properties of nanoporous graphene membranes. Many attempts have been made to exploit the basic properties of nanoporous graphene membranes for functional applications. In this context Cohen-Tanugi *et al.* [105] used molecular dynamics simulations and continuum fracture mechanics in order to study the mechanical resilience of nanoporous graphene as a reverse osmosis membrane. The mechanical properties such as strength depend on the nanopore architecture in the nanoporous graphene materials and the nanopore diameter of the substrate .

An energy band can be opened by introducing nanopores in a graphene sheet for example for application as field effect transistors (FETs) [101, 102, 106]. Semimetallic graphene, the normal state of graphene, can turn into a semiconductor by introducing nanopores . The opening and tuning of a bandgap in nanoporous graphene membranes and the dependence of electronic properties on the structural parameters has been investigated in theory [102, 103].

The energy bandgap of semiconducting graphene nanopores, a chirality dependent scaling rules have been suggested. On the basis of extensive tight binding studies and simple geometric arguments, *Lee et al.* report that Pedersen scaling governs not only the energy bandgap but also the effective mass of the Bloch electron of the semiconducting graphene nanopores regardless of its chirality or the crystallography of pores when the nanopore areal fraction is low [102].

To open a tunable bandgap in graphene, which is required for semiconductor materials, has been desirable for novel applications of graphene. One strategy of constructing periodic nanopores in graphene to form graphene antidot lattices (GALs) has been extensively studied The electronic structure of graphene antidot lattices with zigzag hole edges was studied with first-principles calculations. It was revealed that half of the possible GAL patterns were unintentionally missed in the usual construction models used in earlier studies. With the complete models, the bandgap of the GALs was sensitive to the width W of the wall between the neighboring holes. A nonzero bandgap was opened in hexagonal GALs with even W, while the bandgap remained closed in those with odd W. Similar alternating gap opening/closing with W was also demonstrated in rhombohedral GALs. Moreover, analytical solutions of single-walled GALs were derived based on a tight-binding model to determine the location of the Dirac points and the energy dispersion, which confirmed the unique effect in GALs [103].

Hu *et al.* [101] investigated the mechanical behavior and fracture mechanism of nanoporous graphene NPG for porosities up to 80% and marked the transition of mechanical behavior at a critical porosity of ~15%. Carpenter *et al.* [102] analyzed the dependence of elastic properties on the architecture of graphene nanopore arrays (the pore arrangement, pore morphology, material density, and pore edge passivation), and further established the scaling law between modulus and relative density. Moreover, Liu *et al.* [106] carried out MD simulations to study the mechanical properties of nanoporous graphene with the pore size ranging from 0.4 nm to 1.3 nm, and for the first time revealed the relationships between mechanical properties (Young's modulus and fracture strength) and porosity. These investigations have a shed light upon deformation behavior and mechanical properties of nanoporous graphene.

#### **5. Fabrication of graphene nanopores**

Graphene seems to be an ideal material to create nanopores because it is mechanically and chemically robust even when being atomically thin. A defect free graphene layer is completely impermeable. For this reason, pore creation is necessary to investigate transport mechanisms through graphene nanopores as well as altered mechanical and electronic properties. Pore creation in this two dimensional material is challenging as it is difficult to handle this monolayer graphene without creating additional defects and cracks.

The first attempts to fabricate nanoporous graphene was based on the *top-down approach* in which the structures were created via electronbeam lithography (nano sculpting) or directly etched from graphene, patterned by using self-assembled etch masks as described in the upper part of **Figure 2**. The top-down method do however not lead to small nanopores that can open band gaps of roughly 1 eV (comparable to that in silicon, a conventional semiconductor material). The diameter should be less than 2 nm to meet this requirement [27, 108] for implementation in devices. Since this is beyond the structural resolution of top-down approaches successful attempts

#### **Figure 2.**

*Nanopores can be introduced in graphene by top-down or bottom-up strategies. Top-down approaches involve patterning and etching of graphene sheets. The resulting pore structure are often too large for device applications that require a band gap. Bottom-up assembly using molecular precursors can overcome this limitation as shown in the lower part [28, 107].*

to create graphene nanopores of the desired size was rather achieved via *bottom-up* approaches (lower part of **Figure 2**). Starting from molecular building blocks, such as DBBA (10,109-dibromo-9,99-bianthracene) and other halogenated molecular precursors [28, 109] very narrow graphene nanoribbons could be made with atomic precision when sublimed onto a single crystal Au(111) substrate. The molecules form linear polymer chains in ultra-high vacuum (UHV) at about 200°C and annealing at 400°C where the chains planarize and fuse. Other halogenated polycyclic aromatic hydrocarbon precursors lead to a large variety of structurally different graphene nanoribbons via similar on-surface reactions [102] and of the nanoribbons are not entirely straight i.e. a non- uniform width, prior to fusion, they produce graphene nanostructures with nanoscopic holes after an extra annealing at 450°C which is the case when diphenylsubstituted DBBA (DP-DBBA) is sublimed on Au(111) substrate in UHV. Several spectroscopic studies have shown that the resulting nanoporous graphene has a highly anisotropic electronic structure with a band gap of about 1 eV [110].

Additional bottom-up strategies use polyphenylene units through surfaceassisted coupling of halogenated molecular building blocks [108, 109] as well as chevron-shaped graphene nanoribbons fused to form nanoscale graphene nanopores [110]. Other bottom-up methods such as chemical etching [18], vapor deposition [25], and electron beam [26], have been developed to fabricate NPG materials and control the characteristic size. Moreover, it is even possible to realize a *single nanopore* in a graphene sheet as Rollings *et al.* have shown via fabricating a single nanopore supported on SiNx by an electrical pulse method [74].

Graphene nanopores are the material of choice for applications in nanodevices and in this context it should be mentioned that characterization where graphene are implemented is still a challenge. The short length scales (mostly around <50 nm) as well as the necessity of accurate alignment relative to the device structure, and high contact resistances, and in studies of electrical properties of nanoribbons the yield of working devices is often rather low. On the other hand nanoporous graphene could form larger electrically conducting domains, from which devices for electrical property measurements could be produced at yield as high as ~75% yield [110, 111].

#### **6. Conclusion**

It is obvious that for improvement and alteration of graphene properties introducing nanopores into the graphene sheets can open up novel fields of application for graphene. Nanoporous graphene design is of equal importance in terms of improving long-range order of nanopores, avoiding defects and establish accurate dimensional control and increasing yield. Here the bottom-up approach seem to have advantages over the top-down approach. Hopefully the potential of nanoporous graphene will stimulate chemists to develop new molecular precursors for nanoporous graphenes with various combinations of structural parameters (size, geometry and arrangement of pores). To induce periodic, atomically-precise nanopores and to tailor precise dimensions and electronic properties and to fabricate nanoporous graphene with complete atomic precision is a future goal. However the present highly anisotropic structure of nanoporous graphenes may also be of interest for spectroscopic studies. Graphene nanostructures with nanoscopic pores may be of interest for applications such as separation, sensing, and potentially even DNA sequencing. Since nanopores in graphene open up the band gap and makes the material semiconducting promising applications in mostly FETs but also water filtration, supercapacitors, biological analysis, DNA translocation and molecular sieving among others can be envisaged [112, 113]. It is because of the extended plethora of properties of nanoporous graphene that it is considered to be the next leap forward in carbon based nanomaterials research.

#### **Author details**

Per A. Löthman Foviatech GmbH, Hamburg, Germany

\*Address all correspondence to: per.loethman@foviatech.com

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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## *Edited by Sadia Ameen, M. Shaheer Akhtar and Hyung-Shik Shin*

The field of nanoporous materials has advanced significantly over the last two decades with new concepts and applications emerging all the time. This book is a comprehensive and easy-to-understand source of information on the latest developments in nanopore research. It is a collection of contributions from leading specialists in the subject that address topics such as synthetic methodologies, characterization techniques, and applications of nanopores. This book will appeal to a wide spectrum of readers, including students, professors, and professionals.

Published in London, UK © 2021 IntechOpen © Girolamo Sferrazza Papa / iStock

Nanopores

Nanopores

*Edited by Sadia Ameen,* 

*M. Shaheer Akhtar and Hyung-Shik Shin*