**3.1 Characterization of porous ZnO nanostructures**

**Figure 1(A)** shows the X-ray diffraction (XRD) pattern of ZnO nanostructures prepared via the calcination route explained in section 2.1. High intense sharp diffraction peaks are seen in **Figure 1(A)** matched with the wurtzite structure of ZnO ((JCPDS No. 89–7102) [34]. The Raman spectrum of the prepared ZnO nanostructures (given in **Figure 1(B)**) indicated a sharp band located at 437 cm−1 as a result of E2 (high) mode vibrations [35]. **Figure 1(C, D)** represents the deconvoluted X-ray photoelectron spectroscopy of Zn and O states present in the ZnO nanostructures. The Zn 2p states (given in **Figure 1(C)**) showed the presence of two peaks corresponding to the Zn 2p3/2 (at 1022 eV) and Zn 2p1/2 (at 1045 eV), respectively. A value of 23 eV is obtained for the difference between the peak positions of Zn 2p3/2, and Zn 2p1/2 states that matched with the reported ones and indicates that Zn possesses an oxidation state of +2 in the synthesized ZnO nanostructures [32]. The O 1 s spectrum evidences the broad peak centered at 532 eV (given in **Figure 1(D)**) arises from the oxygen content present in the wurtzite ZnO [35]. The field emission scanning electron micrographs (FE-SEM) of the as prepared ZnO nanostructures (given in **Figure 1(E)** and **(F)**) showed the presence of nanoparticles with a high amount of pores. The high magnification micrograph (**Figure 1(F)**) evidences the honeycomb-like porous ZnO nanostructures with pore sizes ranging from 200 to

#### **Figure 1.**

*(A) X-ray diffraction pattern, (B) laser Raman spectrum, (C) Zn 2p core-level X-ray photoelectron spectrum, and (D) O 1 score-level photoelectron spectrum of porous ZnO nanostructures and scanning electron micrographs of porous ZnO nanoparticles with (E) low and (F) high magnification.*

400 nm. The Brunauer-Emmet-Teller (BET) analysis using the Barrett–Joyner– Halenda (BJH) method of the ZnO nanostructures revealed the Type IV isotherm with a hysteresis (data not shown) highlighting the presence of mesoporous nature [36]. The prepared ZnO nanostructures possess a pore volume and surface area of about 0.028 cm3 /g, and 9.7 m2 /g. The obtained high surface area of the ZnO nanostructures is due to the role of PEG in the preparation. The guest-host chemistry of metal ions with polymer matrix (i.e., Zn2+ ions as a guest in the PEG host matrix) results in the formation of mesoporous ZnO nanostructures via decomposition of PEG during the calcination process.

#### **3.2 Characterization of copper stearate**

The formation of copper stearate via precipitation method was studied using Fourier transformed infra-red spectroscopy (FT-IR) and XRD analysis (given in **Figure 2(A,B)**). During the formation of copper stearate from stearic acid using copper salts, the carboxyl group's and hydrogen atom in the stearic acid is replaced with the Cu ions. **Figure 2(A)** compares the FT-IR spectra of copper stearate to that of stearic acid. The presence of vibration due to the COO- group in stearic acid is noticed via its characteristic peak at 1700 cm−1 and is disappeared/shifted towards a lower peak position at 1583 cm−1, respectively [37]. The sharp bands observed at 3352 (-OH) and 1046 cm−1 (C-O stretching) in the spectrum of stearic acid were disappeared in the FT-IR spectrum of copper stearate. Additionally, the FT-IR spectrum of copper stearate revealed the presence of characteristic bands at 2847, 2917, 1441, 720, and 880 cm−1 raised from the CH2, C-H, C-C, CH2 (rocking), and CH3 rocking vibrations, respectively [38, 39]. The presence of characteristic diffraction peaks from 5° to 20° in the XRD pattern (given in **Figure 2(B)**) confirmed the formation of copper stearate [32, 38].

#### **3.3 Characterization of ZnO/copper stearate composite films**

**Figure 2(C)** shows the FT-IR spectrum of bare ZnO nanostructures and ZnO/ copper stearate composite films. The presence of vibration bands centered at 518 and

**321**

**Figure 2.**

*stearate coatings.*

*Nanostructured Materials for the Development of Superhydrophobic Coatings*

437 cm−1 was raised from the Zn-O vibrations present in ZnO nanostructures [40]. The FT-IR spectrum of ZnO/copper stearate film given in **Figure 2(C)** shows almost all the characteristics vibration bands of ZnO nanostructures and copper stearate with slight variation in their peak positions, that is due to the interaction between the ZnO nanostructures and copper stearate via guest-host chemistry. **Figure 2(D)** shows the XRD pattern of the ZnO/copper stearate coatings. The peaks corresponding to the crystalline ZnO nanostructures were visible in the XRD pattern, whereas the peaks due to the copper stearate were diminished/not observed. This is due to the low crystallinity of copper stearate compared to ZnO nanostructures' highly crystalline nature. **Figure 3(A)** depicts the SEM image of spray-coated ZnO/copper stearate films. It is quite challenging to distinguish ZnO nanostructures in the spraycoated films due to the low weight percentage of ZnO to that of copper stearate and better dispersibility of ZnO nanostructures in the copper stearate due to the ultrasonication process. The size of the pores present in the ZnO/copper stearate coatings was found to be in the range from 100 to 300 nm determined using ImageJ software [41]. Here, it is noteworthy that micron-sized ZnO in similar coatings resulted in irregular surface formation due to the low dispersion index of micronsized ZnO. **Figure 3(B-D)** shows the elemental maps of Zn, O, and Cu components present in the ZnO/copper stearate coatings. The Zn map of the spray-coated films (shown in **Figure 3(B)**) indicated the presence of well-dispersed ZnO nanostructures in the copper stearate matrix. **Figure 3(C)** presents the oxygen map indicating the existence of array-like arrangement due to the chain-like structures of copper stearate in addition to the ZnO components of the coating. The mapping of copper elements in the spray coated films (given in **Figure 3(D)**) shows that the copper elements are randomly distributed in the films. Since the copper elements are attached

*(A) Fourier transform infrared spectrum and (B) X-ray diffraction pattern of copper stearate, (C) FT-IR spectrum of porous ZnO and ZnO/copper stearate coatings, (D) X-ray diffraction pattern of ZnO/copper* 

*DOI: http://dx.doi.org/10.5772/intechopen.96320*

*Nanostructured Materials for the Development of Superhydrophobic Coatings DOI: http://dx.doi.org/10.5772/intechopen.96320*

#### **Figure 2.**

*Novel Nanomaterials*

about 0.028 cm3

**Figure 1.**

/g, and 9.7 m2

PEG during the calcination process.

**3.2 Characterization of copper stearate**

the formation of copper stearate [32, 38].

**3.3 Characterization of ZnO/copper stearate composite films**

400 nm. The Brunauer-Emmet-Teller (BET) analysis using the Barrett–Joyner– Halenda (BJH) method of the ZnO nanostructures revealed the Type IV isotherm with a hysteresis (data not shown) highlighting the presence of mesoporous nature [36]. The prepared ZnO nanostructures possess a pore volume and surface area of

*(A) X-ray diffraction pattern, (B) laser Raman spectrum, (C) Zn 2p core-level X-ray photoelectron spectrum, and (D) O 1 score-level photoelectron spectrum of porous ZnO nanostructures and scanning electron* 

*micrographs of porous ZnO nanoparticles with (E) low and (F) high magnification.*

structures is due to the role of PEG in the preparation. The guest-host chemistry of metal ions with polymer matrix (i.e., Zn2+ ions as a guest in the PEG host matrix) results in the formation of mesoporous ZnO nanostructures via decomposition of

The formation of copper stearate via precipitation method was studied using Fourier transformed infra-red spectroscopy (FT-IR) and XRD analysis (given in **Figure 2(A,B)**). During the formation of copper stearate from stearic acid using copper salts, the carboxyl group's and hydrogen atom in the stearic acid is replaced with the Cu ions. **Figure 2(A)** compares the FT-IR spectra of copper stearate to that of stearic acid. The presence of vibration due to the COO- group in stearic acid is noticed via its characteristic peak at 1700 cm−1 and is disappeared/shifted towards a lower peak position at 1583 cm−1, respectively [37]. The sharp bands observed at 3352 (-OH) and 1046 cm−1 (C-O stretching) in the spectrum of stearic acid were disappeared in the FT-IR spectrum of copper stearate. Additionally, the FT-IR spectrum of copper stearate revealed the presence of characteristic bands at 2847, 2917, 1441, 720, and 880 cm−1 raised from the CH2, C-H, C-C, CH2 (rocking), and CH3 rocking vibrations, respectively [38, 39]. The presence of characteristic diffraction peaks from 5° to 20° in the XRD pattern (given in **Figure 2(B)**) confirmed

**Figure 2(C)** shows the FT-IR spectrum of bare ZnO nanostructures and ZnO/ copper stearate composite films. The presence of vibration bands centered at 518 and

/g. The obtained high surface area of the ZnO nano-

**320**

*(A) Fourier transform infrared spectrum and (B) X-ray diffraction pattern of copper stearate, (C) FT-IR spectrum of porous ZnO and ZnO/copper stearate coatings, (D) X-ray diffraction pattern of ZnO/copper stearate coatings.*

437 cm−1 was raised from the Zn-O vibrations present in ZnO nanostructures [40]. The FT-IR spectrum of ZnO/copper stearate film given in **Figure 2(C)** shows almost all the characteristics vibration bands of ZnO nanostructures and copper stearate with slight variation in their peak positions, that is due to the interaction between the ZnO nanostructures and copper stearate via guest-host chemistry. **Figure 2(D)** shows the XRD pattern of the ZnO/copper stearate coatings. The peaks corresponding to the crystalline ZnO nanostructures were visible in the XRD pattern, whereas the peaks due to the copper stearate were diminished/not observed. This is due to the low crystallinity of copper stearate compared to ZnO nanostructures' highly crystalline nature. **Figure 3(A)** depicts the SEM image of spray-coated ZnO/copper stearate films. It is quite challenging to distinguish ZnO nanostructures in the spraycoated films due to the low weight percentage of ZnO to that of copper stearate and better dispersibility of ZnO nanostructures in the copper stearate due to the ultrasonication process. The size of the pores present in the ZnO/copper stearate coatings was found to be in the range from 100 to 300 nm determined using ImageJ software [41]. Here, it is noteworthy that micron-sized ZnO in similar coatings resulted in irregular surface formation due to the low dispersion index of micronsized ZnO. **Figure 3(B-D)** shows the elemental maps of Zn, O, and Cu components present in the ZnO/copper stearate coatings. The Zn map of the spray-coated films (shown in **Figure 3(B)**) indicated the presence of well-dispersed ZnO nanostructures in the copper stearate matrix. **Figure 3(C)** presents the oxygen map indicating the existence of array-like arrangement due to the chain-like structures of copper stearate in addition to the ZnO components of the coating. The mapping of copper elements in the spray coated films (given in **Figure 3(D)**) shows that the copper elements are randomly distributed in the films. Since the copper elements are attached

#### **Figure 3.**

*Field emission scanning electron micrograph of (A) ZnO/copper stearate and (B-D) shows the elemental maps of Zn, O, and Cu present in these coatings and (E-F) 2D and 3D Atomic force micrograph of ZnO/copper stearate coatings.*

to the end of the chain-like structure of copper stearate. More probably, the copper ends were attached to the glass substrates due to their hydrophilic nature leaving the hydrophobic methyl group on the exterior surfaces. The 2D and 3D topographic analysis of the spray coated ZnO/copper stearate films analyzed by atomic force micrograph is shown in **Figure 3(E, F)**. These studies demonstrated the existence of porous and rough surfaces nature of the spray-coated ZnO/copper stearate films.

#### **3.4 Superhydrophobic properties of ZnO/copper stearate coatings**

The water contact angle measurement was carried out for determining the superhydrophobic properties of the ZnO/copper stearate coatings with various loading ratios of ZnO nanostructures in the copper stearate matrix. The plain glass substrates possess a WCA of 27.4°, indicating their hydrophilic surfaces [42]. The bare copper stearate coatings (with low surface energy) on glass substrates possess a WCA of 152.4°, demonstrating their hydrophobic properties. It is expected that the inclusion of porous ZnO nanostructures in the copper stearate matrix might alter their roughness and thus improves the hydrophobicity. The effect of porous ZnO nanostructures loading ratio on the water repellent properties of the ZnO/copper stearate coatings is summarized in **Figure 4(A)**. It displayed that the superhydrophobic effect was not obtained in the composite films with a loading ratio of ZnO nanostructures until 0.01 g. The ZnO/copper stearate coatings displayed the superhydrophobic properties with a WCA of 161° when the ZnO weight percentage is increased up to 0.14 g. This can be due to the improvements in the films' roughness by the inclusion of highly-porous ZnO nanostructures. The mechanism of superhydrophobic effect achieved in the spray-coated films can be described via the Cassie-Baxter model using the following relation [43]:

$$\cos\Theta^\* = \mathfrak{p}\_s \cos\Theta + \mathfrak{p}\_s - \mathbf{1} \dots \tag{1}$$

**323**

properties.

**Figure 4.**

*coatings.*

**stearate coatings**

*Nanostructured Materials for the Development of Superhydrophobic Coatings*

an increase in the loading ratio of porous ZnO. A low substantial fraction of about 0.094 was reached for the ZnO/copper stearate coatings with a ZnO loading of about 0.14 g. The observed decrease in solid fraction values indicated the enhancement in the surface roughness due to porous ZnO nanostructures' impregnation. A similar effect is not observed in films coated using non-porous ZnO or micron-sized ZnO powders. This further substantiates porous ZnO nanostructures' significance for modulating the roughness of copper stearate films to obtain superhydrophobic

*(A) Water contact angle, (B) floating characteristics, and (C) load-bearing properties of ZnO/copper stearate* 

**3.5 Floating and load-bearing characteristics of superhydrophobic ZnO/copper** 

Floating and load-bearing characteristics are major applications where superhydrophobic coatings for underwater robotics, monitoring water pollution, water quality analysis, and surveillance applications can be developed [44–46]. Based on the Archimedes principle, it is well known that any object (with density higher than water) will sink in water. However, water strider possesses the ability to float in water and stride freely on the water surface due to the hierarchical fibrous architecture of strider leg exhibiting superhydrophobic effect. The bare glass substrates immediately drown after placing in water solution, which can be explained using the Archimedes principle [47]. It is expected that superhydrophobic coatings can possess a floating nature. The floating characteristics of the spray-coated ZnO/copper stearate coatings (laid down) in the water solution is shown in **Figure 4(B)**. The fundamental mechanism of an object's floating nature will be achieved if the buoyancy force exerted by the object is higher than the drown force acts on it. The ZnO/copper stearate coatings on glass substrates float on water solution for more than a week without any noticeable sinking effects. Here, in addition to the buoyancy force, a curvature force acts on the surface of superhydrophobic coatings, acting against the drown force that enables them to float over a prolonged time. The superhydrophobic ZnO/

*DOI: http://dx.doi.org/10.5772/intechopen.96320*

Here θ\*, φs and θ represents the apparent contact angle, substantial fraction in contact with liquid, and Young's contact angle, respectively. The substantial fraction of the bare copper stearate coating is about 0.197, and these values decrease with

*Nanostructured Materials for the Development of Superhydrophobic Coatings DOI: http://dx.doi.org/10.5772/intechopen.96320*

#### **Figure 4.**

*Novel Nanomaterials*

**Figure 3.**

*stearate coatings.*

to the end of the chain-like structure of copper stearate. More probably, the copper ends were attached to the glass substrates due to their hydrophilic nature leaving the hydrophobic methyl group on the exterior surfaces. The 2D and 3D topographic analysis of the spray coated ZnO/copper stearate films analyzed by atomic force micrograph is shown in **Figure 3(E, F)**. These studies demonstrated the existence of porous and rough surfaces nature of the spray-coated ZnO/copper stearate films.

*Field emission scanning electron micrograph of (A) ZnO/copper stearate and (B-D) shows the elemental maps of Zn, O, and Cu present in these coatings and (E-F) 2D and 3D Atomic force micrograph of ZnO/copper* 

The water contact angle measurement was carried out for determining the superhydrophobic properties of the ZnO/copper stearate coatings with various loading ratios of ZnO nanostructures in the copper stearate matrix. The plain glass substrates possess a WCA of 27.4°, indicating their hydrophilic surfaces [42]. The bare copper stearate coatings (with low surface energy) on glass substrates possess a WCA of 152.4°, demonstrating their hydrophobic properties. It is expected that the inclusion of porous ZnO nanostructures in the copper stearate matrix might alter their roughness and thus improves the hydrophobicity. The effect of porous ZnO nanostructures loading ratio on the water repellent properties of the ZnO/copper stearate coatings is summarized in **Figure 4(A)**. It displayed that the superhydrophobic effect was not obtained in the composite films with a loading ratio of ZnO nanostructures until 0.01 g. The ZnO/copper stearate coatings displayed the superhydrophobic properties with a WCA of 161° when the ZnO weight percentage is increased up to 0.14 g. This can be due to the improvements in the films' roughness by the inclusion of highly-porous ZnO nanostructures. The mechanism of superhydrophobic effect achieved in the spray-coated films can be described via the

Here θ\*, φs and θ represents the apparent contact angle, substantial fraction in contact with liquid, and Young's contact angle, respectively. The substantial fraction of the bare copper stearate coating is about 0.197, and these values decrease with

θ∗ = ϕ θ + ϕ … s s cos cos – 1 (1)

**3.4 Superhydrophobic properties of ZnO/copper stearate coatings**

Cassie-Baxter model using the following relation [43]:

**322**

*(A) Water contact angle, (B) floating characteristics, and (C) load-bearing properties of ZnO/copper stearate coatings.*

an increase in the loading ratio of porous ZnO. A low substantial fraction of about 0.094 was reached for the ZnO/copper stearate coatings with a ZnO loading of about 0.14 g. The observed decrease in solid fraction values indicated the enhancement in the surface roughness due to porous ZnO nanostructures' impregnation. A similar effect is not observed in films coated using non-porous ZnO or micron-sized ZnO powders. This further substantiates porous ZnO nanostructures' significance for modulating the roughness of copper stearate films to obtain superhydrophobic properties.

### **3.5 Floating and load-bearing characteristics of superhydrophobic ZnO/copper stearate coatings**

Floating and load-bearing characteristics are major applications where superhydrophobic coatings for underwater robotics, monitoring water pollution, water quality analysis, and surveillance applications can be developed [44–46]. Based on the Archimedes principle, it is well known that any object (with density higher than water) will sink in water. However, water strider possesses the ability to float in water and stride freely on the water surface due to the hierarchical fibrous architecture of strider leg exhibiting superhydrophobic effect. The bare glass substrates immediately drown after placing in water solution, which can be explained using the Archimedes principle [47]. It is expected that superhydrophobic coatings can possess a floating nature. The floating characteristics of the spray-coated ZnO/copper stearate coatings (laid down) in the water solution is shown in **Figure 4(B)**. The fundamental mechanism of an object's floating nature will be achieved if the buoyancy force exerted by the object is higher than the drown force acts on it. The ZnO/copper stearate coatings on glass substrates float on water solution for more than a week without any noticeable sinking effects. Here, in addition to the buoyancy force, a curvature force acts on the surface of superhydrophobic coatings, acting against the drown force that enables them to float over a prolonged time. The superhydrophobic ZnO/

copper stearate coatings possess a trapped air film on their exterior surfaces leading to an excessive displaced volume of water. The entire phenomenon can be termed as super-buoyancy [48]. These studies demonstrated the plastron effect's vital role in the ZnO/copper stearate coatings' floating properties. The ZnO/copper stearate coatings' load-bearing characteristics are investigated by loading known mass (stapler pins) on their top surface during floating, as shown in **Figure 4(B)**. It showed that after loading a few stapler pins, there are no signs of sinking for the ZnO/copper stearate coatings. The effect of thickness of superhydrophobic ZnO/copper stearate coatings via different deposition times (180 to 1800 seconds) on their load-bearing properties is provided in **Figure 4(C)**. It showed that ZnO/copper stearate coatings with a high thickness (weight of about 0.062 g) remain floating and can bear about 0.3667 g (19 stapler pins) without sinking issues. **Figure 4(C)** illustrated that the ratio of net floating weight to the weight of ZnO/copper stearate coating decreases with increasing thickness. This can be explained based on the more inert/dead layers present in the coatings with high thickness. On the other hand, the capability of load-bearing is superior for thin coatings (deposition at 30 seconds) that can bear 52 pins (~333 times higher than their net weight). The findings on the floating and load-bearing characteristics of the superhydrophobic ZnO/copper stearate coatings can be applied for water floating micro-robots and surveillance applications.
