Fabrication of Nanowire Arrays CuO-Al2O3-TiO2 as p-Insulator-n Heterojunction for Photochemical Water Splitting

*Yu-Min Shen, Dipti Ranjan Sahu, Sheng-Chang Wang and Jow-Lay Huang*

#### **Abstract**

CuO@TiO2 nanowires were prepared with the use of a porous alumina membrane (PAM). The conventional thermal chemical vapor deposition (CVD) method was used for the deposition of TiO2 by decomposing titanium isopropoxide (Ti(O*i*Pr)4). The multilayer heterojunction is tested for possible photochemical water splitting application. The photo conversion properties of the CuO-Al2O3-TiO2 (p-insulator-n) heterojunction along with the microstructures and composition were characterized by Potentiostat, SEM and TEM, respectively. The shape of CuO nanowire arrays were hexagonal honeycombs and size was about 90 nm which depends on the uniform pore size of the PAM. The microstructural characterization showed that the nanosized CuO-Al2O3-TiO2 is a p-insulator-n heterojunction. The maximum photoconversion efficiency of 1.13 and 1.61% is obtained for CuO and CuO-Al2O3-TiO2 nanowire arrays junctions. An energy band diagram was introduced to explain the change in current during water splitting due to electron tunneling through the insulating layer.

**Keywords:** nanowires, porous alumina membrane, TiO2, CuO and photoelectrochemical, chemical vapour deposition (CVD)

#### **1. Introduction**

The use of TiO2 for photoelectrochemical (PEC) water splitting applications has been attracted a great deal attention from many research groups [1–8] since Fujishima et al. [9, 10] discovered that the hydrolysis of water in oxygen and hydrogen could be carried out by lighting anatase phase of titanium oxide (TiO2). This is because TiO2 has a wide band gap of ~3.2 eV, appropriate band-gap positions, strongly optical adsorption, and high chemical stability. However, the conversion efficiency of TiO2 is usually limited, due to large band-gap energy, which means that UV irradiation is need for photocatalytic activity to occur. Since UV light accounts for only 5% of the sun's energy compared to the 45% of visible, enhancing the photoconversion

efficiency of TiO2 has been a key objective of many research teams. H2 has been used to treat TiO2 in order to modify its optical absorption responsive band gap from UV to the visible wavelength [8, 11, 12], and narrow band gap semiconductor have also been used to avoid the problem of rapid electron-hole recombination [13–17]. For example, it is known that p-type copper oxide (CuO) can act as a hole conductor due to its narrow bandgap of only 1.4 eV. The use of copper oxides heterojunctions with TiO2 have been reported for enhanced hydrogen generation, due to the resulting shift in the adsorption light wavelength [18].

p-n heterojunctions with various structures have been developed for photoconversion applications, resulting in high photocurrent efficiencies and H2 generation [19–27]. The interface layers between p-type and n-type heterojunction semiconductors have significant effects on the resulting opto-electric properties, which are enhanced when the electrons at the conduction band (CB) were rapidly move from the n-type semiconductor to p-type semiconductor, or vice versa, which depends on the semiconductor band edge position. The interfaces of p-n heterojunction have recently been shown to have excellent photovoltaic properties, because the intermediate intrinsic layer prevents the injection of holes from the p-type semiconductor [28, 29]. Tian et al. [30] also indicated that p-type/intrinsic/n-type coaxial silicon nanowires have a high threshold voltage, as seen in the result of I-V characterization, due to the contributions from tunneling and avalanche mechanisms [31]. However, the influence of an insulator interface between p-n heterojunctions on the resulting water splitting behavior has not been examined.

An insulator layer between p-type and n-type semiconductors was prepared for this study by electrochemical deposition (ECD) and chemical vapor deposition (CVD), for the fabrication of the heterojunction structure. In the ECD process, the CuO/Al2O3 nanowire arrays were prepared with the use of template assistance process [32, 33]. The TiO2 particles were deposited by a thermal CVD process. An energy band structural diagram is used to explain the I-V characteristics of the resulting, CuO/Al2O3/TiO2 (p-insulator-n) heterojunction for water splitting applications.

#### **2. Experimental methods**

#### **2.1 Preparation of porous alumina membranes (PAMs)**

The porous alumina membranes (PAM) are prepared using high purity Al foils (99.9995%) with two step anodization process [34, 35]. The Al foil was degreased in ethanol, annealed in an argon, cleaned and electro-polished in a mixture of HClO4- C2H5OH (1:4 vol %) at 10°C, with a current density of 100 mA/cm2 for 1 min. The anodization was carried out in 0.3 sulfuric acid and 0.3 M oxalic acid at a constant voltage of 60 V using Pt foil as a counter electrode. After 3–6 h of anodization, the alumina film was selectively etched in a mixture of H3PO4-CrO3-H2O (2 g-3.5 mL-100 mL) at 70°C for 40 min. Afterwards second anodization was done under the same conditions for 18–24 h. The straight nano channel pore was obtained by dissolving the barrier layer using 5% H3PO4 solution at 60°C for 20 min.

#### **2.2 Preparation of CuO/PAMs nanowire arrays**

Before the deposition of the nanowire arrays, layers of Ti (10 nm) and Pt (100 nm) were coated onto one side of the membrane and stuck on the Cu substrate to serve as

*Fabrication of Nanowire Arrays CuO-Al2O3-TiO2 as p-Insulator-n Heterojunction… DOI: http://dx.doi.org/10.5772/intechopen.112454*

the working electrode (cathode). Pt foil (anode) and a saturated calomel electrode (SCE, 0.241V/NHE) served as the counter and reference electrode, respectively, in a three-electrode electrochemical deposition system. A potentiostat (263A, Princeton Applied Research) was used for electrochemical system to provide the potential. The PAMs/Ti/Pt/Cu were degassed using a mixture of 0.2 M CuSO4 and 0.15 M H3BO3 solution with pH = 2 (controlled by H2SO4) and deposited at −0.18 V/SCE for 2 h. After electrochemical deposition, the Cu/PAMs nanowires arrays were heated in air at 400°C for 12 h to obtain the CuO/PAMs nanowires arrays.

#### **2.3 Preparation of CuO@TiO2 heterojunction structure by CVD**

Thermal chemical vapor deposition (CVD) was used to grow the TiO2 coatings on the CuO nanowires. Before the coating process, the CuO/PAMs compound was stuck on the FTO glass using silver paste, and the PAM was removed by 1 M NaOH at 50°C for 5 min. The CuO/PAMs sample was placed on the graphite holder in the quartz tube under reduced pressure of 10−6 Torr. The growth parameter was set at 500°C for 15 min. The precursor, titanium isopropoxide (Ti(O*i*Pr)4),was introduced to decompose the TiO2 in the quartz tube at 35°C.

#### **2.4 Characterization of the CuO/PAMs nanowires arrays and CuO@TiO2**

The CuO/PAMs was immersed into the 1 M NaOH solution at 50°C for 5 min, and then cleaned with deionized water several times before the SEM observation. The TEM sample for CuO@TiO2 was dispersed in ethanol and subjected to ultrasonication for 10 min, then dropped on the Ni grid. To examine the photochemical response of CuO/PAM and CuO@TiO2 p-insulator-n junction, sliver wire was stuck on the substrates, and this was then covered with a non-transparent and non-conductive epoxy-resin. The photochemical current-voltage was characterized using a potentiostat, while the counter and reference electrode were platinum and saturated calomel (SCE, 0.242V) in 1 M NaOH solution. Sunlight was simulated using a 150 W xenon arc lamp (1.5 AM filter, Oriel).

#### **3. Results and discussion**

#### **3.1 Crystal structure and microstructure observation**

**Figure 1** presents the X-ray diffraction (XRD) spectra of the Cu/CuO nanowire arrays obtained after 400°C heat treatment for 12 h. It clearly shows the CuO peaks which correspond to the JCPDF card number 45-0937. The remaining Cu was due to the alumina template covering the surface of the Cu. The alumina template made it difficult for the oxygen to diffuse into it and to form CuO. **Figure 2(A)** presents the ordered pore array of AAO with 90–100 nm pore size obtained using two step anodination of Al foil under oxalic acid at 60 V [35]. The uniform shape of CuO nanowires were synthesized with the use of porous alumina membranes. **Figure 2(B)** shows the CuO nanowires after removing the PAM with NaOH solution. It is observed that the CuO nanowires were agglomerated and collapsed on both sides of AAO due to surface interaction. The SEM in **Figure 2(C)** represents the images of TiO2 particles attached to the CuO nanowire after the CVD process. **Figure 2(D)** indicates the clear view of formation of nanosize CuO nanowires on PAM.

**Figure 1.** *XRD pattern of Cu/CuO after 400°C heat treatment for 12 h.*

**Figure 2.**

*SEM images of (A) porous alumina membranes, (B) CuO nanowire arrays, and (C) CuO@TiO2 nanostructure, (D) clear view of nanosize CuO nanowire arrays.*

#### *Fabrication of Nanowire Arrays CuO-Al2O3-TiO2 as p-Insulator-n Heterojunction… DOI: http://dx.doi.org/10.5772/intechopen.112454*

**Figure 3** shows the TEM, high-resolution TEM, and fast Fourier transformation (FFT) images of a CuO@TiO2 nanowire. **Figure 3A** and **C** show that the particles cohered on the surface of nanowire. The d-spacings of the nanowire and particles were calculated as 0.242 nm and 0.446 nm, which confirm that the compounds were CuO (111) and TiO2 (110), respectively. The high resolution TEM image shows that the CuO nanowire was a single crystal, and that its growth direction was (111).

#### **Figure 3.**

*TEM images of a CuO@TiO2 nanowire (A) at low magnification, (B) EDS analysis of a CuO@TiO2 nanowire, (C) high resolution, and fast Fourier transformation images, (D) CuO/PAMs, (E) CuO@TiO2, and (F) TiO2 particles produced by thermal CVD.*

In addition, the CuO nanowire, CuO@TiO2 and TiO2 particles are also shown in the fast Fourier transformation images in **Figure 3C–E**. The impurity spots in **Figure 3D** show that the CuO nanowire was covered with TiO2 particles. However, some indistinct regions can also be seen on the CuO nanowire in the HR-TEM and hollow ring in FFT images (**Figure 3C** and **D**), and this was due to the amorphous porous alumina membranes. **Figure 3B** shows the EDS analysis of a CuO@TiO2 nanowire, which indicates the presence of the Al2O3 compound. This demonstrates that the insulator layer, i.e., PAM, was not completely removed during etching by 1M NaOH solution. The TEM and EDS results indicated that a p (CuO)-insulator (Al2O3)-n (TiO2) junction was successfully fabricated.

#### **3.2 Water splitting properties of CuO/Al2O3/TiO2 p-insulator-n heterojunction**

According to the water splitting theory, the optimum energy bandgap of the semiconductor should be in the range of 1.7–2.0 eV and, the band edges should be straddle water potential of 1.23 V/NHE. However, the photon adsorption of CuO is 1.4 eV, which do not provide enough energy to split water into hydrogen and oxygen. The wide band gap of TiO2 not only provides enough band edges to straddle the H2O reduction potentials but forms the donor level in the forbidden band to enable electron transition [13]. The photoelectrochemical and photocurrent characterization of the CuO/ PAMs nanowire arrays and CuO@TiO2 nanostructure by thermal CVD were carried out based on their current density and voltage properties under dark and UV illumination. **Figure 4(A)** presents the current density measurement of nanowire arrays with various applied potential under 150 W UV illumination. The CuO/PAMs nanowire arrays indicates p-type semiconductor behavior due to the signature of negative current in the figure. There is also a current density peak between 0.2 and 0.7 V/SCE due to the oxidation of the remaining Cu to Cu2+ as the potential increases to +1 V/SCE.

**Figure 4(B)** reveals the photochemical response of CuO@TiO2 nanowire arrays under 100 mW/cm<sup>2</sup> simulated solar illumination. The threshold voltages (Vth) of the CuO/PAMs and CuO@TiO2 samples were 0.67 and 0.89 V/SCE, while the maximum photocurrent densities were 1.12 and 1.83 mA/cm<sup>2</sup> respectively.

In order to examine the photoconversion efficiency (*η*) of each sample, the efficiency equation is expressed as follow [36]:

$$\eta\left(\text{@}\right) = \frac{J\_{ph}\left\lfloor \left(\mathbf{1}.2\mathbf{3} - \left|V\_{app}\right|\right) \right\rfloor}{I\_0} \times \mathbf{100} \tag{1}$$

Where *I*0 is the intensity of the incident light (100 mW/cm<sup>2</sup> , *V*app is the applied potential, *V*mea is the measured potential (obtained with a potentiostat, and *V*oc is the open circuit potential. **Figure 5** presents the photo conversion efficiency of CuO/ PAMs and CuO@TiO2 samples. The conversion efficiency of 1.61% at 0.8 V/SCE, was observed for CuOs@TiO2 heterojunction which is higher than that of 1.13 % at 0.67 V/SCE for CuO/PAMs.

#### **3.3 Interface behavior of the CuO/Al2O3/TiO2 (p-insulator-n) heterojunction**

Based on the photochemical properties outlined above, the results for the CuO@ TiO2 sample show that it has better performance than the CuO/PAMs sample [37]. Bandara et al. [13] reported that in a CuO-TiO2 water system the electrons and holes are generated on the conduction band (CB) and valence band (VB) when the TiO2 is *Fabrication of Nanowire Arrays CuO-Al2O3-TiO2 as p-Insulator-n Heterojunction… DOI: http://dx.doi.org/10.5772/intechopen.112454*

#### **Figure 4.**

*(A) Variation of current density versus applied potential measured using 150 W UV illuminations (AM 1.5 filter) and (B) photocurrent density vs applied potential.*

illuminated. The rapid electrons transfer from the CB of TiO2 to the CB of CuO occurs because the CB position of CuO is lower than that of TiO2. The threshold voltage (Vth) of the ideal CuO-TiO2 p-n junction should be set at 0 V/SCE. However, in our study, the photo response of CuO@TiO2 appeared at a relatively high Vth (0.9 V/SCE). This was due to the Al2O3 insulation layer between the CuO and TiO2 interlayers, which formed the heterojunctions, as shown in **Figure 6**. The band gaps of CuO, Al2O3, and TiO2 are 1.4, 8.8, and 3.2 eV, respectively. In this heterojunction, insulator (Al2O3) was used as an electron blocking layer. In order to balance Fermi level, (the interface between the p- and n-type) insulators started to bend when applying a bias due to the high band gap of this layer. This caused electrons to accumulate on the barrier

**Figure 5.** *Plot of photoconversion efficiency of versus applied potential of Cu/PAM and CuO@TiO2 nanowire arrays.*

**Figure 6.** *Energy band gap structural diagram of CuO-Al2O3-TiO2 heterojunctions.*

layer before the breakdown potential was reached. When the applied potential was increased to the breakdown level, at ~0.6 V/SCE, the rapidly increasing current was caused by electron tunneling.In this CuO-Al2O3-TiO2 heterojunction, the conduction band edge of CuO is more negative than TiO2. Upon illumination the photogenerated electrons (e− ) and holes (h+ ) are produced at the active sites of the CuO-TiO2 photocatalyst. The electrons in p-type CuO easily flow to the conduction band of TiO2,

whereas the photogenerated holes migrate in the opposite direction. CuO serves as electron reservoir by receiving electrons from TiO2, which suppresses the recombination of e− /h<sup>+</sup> and transfers the received electron.

#### **4. Application**

CuO based photoelectrodes have great potential for applications in photo electrochemical (PEC) water splitting which give useful information on factors that affect the photo electrochemical capability. Modifying the CuO structure revealed a significant influence on the enhancement of the photocurrent density and photostability. Further, copper containing TiO2 is cost-effective compared with noble metal loaded TiO2, for the photocatalytic hydrogen production. The CuO-TiO2 oxide system have promising photoactive optical and photocatalytic properties, as compared with TiO2 or CuO alone. CuO and TiO2 based junction have better charge mobility at the proximity to the junction of the electrode/electrolyte within the nanostructure. CuO-Al2O3-TiO2 nanocomposites represent the synergy effect for hydrogen production. This nanocomposites junction is very good for design of efficient catalyst for selective alcohol conversion to other valuable chemicals. The p-i-n heterojunctions with suitable band edge positions has the improved separation of the photogenerated charge which improved the photocatalytic activity. This junction has application for improved photocurrent behavior, the H2 generation rate by water splitting in a full PEC device (without application of a bias) and the solar-to-hydrogen efficiency.

#### **5. Conclusions**

CuO@TiO2 nanostructures were fabricated by a template assisted (PAMs) nanowires (CuO) growth and CVD process (TiO2). Observations of the microstructure show that the CuO@TiO2 nanostructure was composed of p-insulator-n heterojunctions. The I-V characteristics of the CuO/PAMs and CuO@TiO2 nanostructures show high threshold voltages of 0.67 and 0.89 V/SCE, with photoconversion efficiencies of 1.13% and 1.61%, respectively. The energy band structure diagram indicates that the rise in current during water splitting is due to electron tunneling through the insulation layer (Al2O3).

#### **Acknowledgements**

This work was supported by the National Science Council of Taiwan under grant NSC 101-2221-E-006-129.

#### **Author contributions statement**

DRS, SCW, JLH conceived the experiment(s), YMS conducted the experiment(s) and characterization. YMS and DRS wrote the manuscript. YMS, DRS and SCW analyzed the results.

All authors reviewed the manuscript.

### **Competing financial interests**

The authors declare no competing financial interests.

### **Author details**

Yu-Min Shen1 , Dipti Ranjan Sahu2 \*, Sheng-Chang Wang3 and Jow-Lay Huang1,4,5\*

1 Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan

2 Faculty of Health, Natural Resources and Applied Sciences, School of Natural and Applied Sciences, Department of Biology, Chemistry and Physics, Namibia University of Science and Technology, Windhoek, Namibia

3 Department of Mechanical Engineering, Southern Taiwan University of Science and Technology, Tainan, Taiwan

4 Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan, Taiwan

5 Hierarchical Green-Energy Materials (Hi-GEM) Research Center, National Cheng Kung University, Tainan, Taiwan

\*Address all correspondence to: dsahu@nust.na; jlh888@mail.ncku.edu.tw

© 2023 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.

*Fabrication of Nanowire Arrays CuO-Al2O3-TiO2 as p-Insulator-n Heterojunction… DOI: http://dx.doi.org/10.5772/intechopen.112454*

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## *Edited by Dipti Ranjan Sahu*

Nanofabrication is the process of assembling structures at the nanoscale with unique properties. This book describes proficient, low-cost, and robust nanofabrication techniques to produce nanostructures. It presents information on nanofabrication technology principles, methodologies, equipment, and processes, as well as discusses the fabrication of new structures for new applications. The nanofabrication techniques reviewed are applicable to different engineering processes, nano-electromechanical systems, biosensors, nanomaterials, photonic crystals, devices, and new structures. This book is a useful resource for students and professionals, including engineers, scientists, researchers, technicians, and technology managers.

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Nanofabrication Techniques - Principles, Processes and Applications

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Nanofabrication Techniques

Principles, Processes and Applications

*Edited by Dipti Ranjan Sahu*