Nanostructured superhydrophobic coatings for solar panel applications

CHAPTER

Nanostructured superhydrophobic coatings for solar panel applications

12

Abhilasha Mishra1, Neha Bhatt1 and A.K. Bajpai2 1

Department of Chemistry, Graphic Era (Deemed to be University), Dehradun, India Bose Memorial Research Laboratory, Department of Chemistry, Government Autonomous Science College, Jabalpur, India

2

CHAPTER OUTLINE 12.1 Introduction ...................................................................................................397 12.2 Techniques used for superhydrophobic and antireflective coatings ...................404 12.2.1 Chemical vapor deposition ..........................................................404 12.2.2 Physical vapor deposition ............................................................405 12.2.3 Sol
gel method .........................................................................406 12.3 Characterization of surfaces ...........................................................................413 12.3.1 Wettability .................................................................................413 12.3.2 Surface texture, composition, and morphology ..............................415 12.3.3 Optical properties .......................................................................417 12.3.4 Hardness and scratch-resistant test..............................................417 12.3.5 Thermal analysis ........................................................................417 12.4 Conclusion ....................................................................................................418 Acknowledgment.....................................................................................................418 References .............................................................................................................418

12.1 INTRODUCTION Renewable energy, such as solar power, wind power, geothermal energy, hydroelectric energy, biomass, hydrogen, and fuel cells have provided interesting solutions for depleting conventional sources of energy. Photovoltaic (PV) systems are the most widespread alternative with low environmental impact [1]. Solar power was identified as the world’s leading source of additional power generating capacity in the year 2016. Solar PV capacity power produced 228 GW in the year of 2015 and increased to 303 GW in 2016 [2]. PV cells are made up of doping of boron and phosphorous on silicon, thus making it “p-type” and “n-type”

Nanomaterials-based Coatings. DOI: https://doi.org/10.1016/B978-0-12-815884-5.00012-0 © 2019 Elsevier Inc. All rights reserved.

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CHAPTER 12 Nanostructured superhydrophobic coatings

FIGURE 12.1 A schematic diagram showing construction and working of solar panel [4].

semiconductors, respectively. When the sunlight falls on the PN junction, the electrons move from n-type to p-type via the external path, and this flow of electrons provides the direct current, which is stored in the battery [3]. A schematic diagram showing construction and working of solar panel is presented in Fig. 12.1. The efficiency of solar PV cells depends on several factors, such as cell temperature, spectral distribution of sunlight, installing angles, weather conditions, density of radiation on that area, accumulation of dust or soil, shadowing effect, lifetime/aging effect, diodes and wiring, and uncertainty in manufacturers rating. Ability of the cover glass to allow the radiation to be absorbed or reflected is a major parameter which affects the efficiency of solar panel [5]. Accumulation of dust decreases the efficiency of the solar system because of the decreased absorption of the solar light. Dust means the pollution, fibers of cloth, soil particle carried by wind, smoke particles carried by vehicles, dust by pedestrian, water stains (salts), bird droppings, etc. These dust particles are present everywhere in the environment, and their accumulation cannot be easily prevented. The dust accumulation starts first by one layer accumulation and covers all the surface. After that the next layer accumulation starts on all over the surface, and this process is carried on until multilayer accumulations are formed [6,7]. In this way the incoming radiations are blocked and scattered by accumulated dust particles, and therefore a regular cleaning of PV solar panel is mandatory. Moreover, the accumulation of dust and rain water on glass surface causes corrosion of solar panel which also lowers the cells efficiency [8,9].

12.1 Introduction

Most of the solar power plants are situated in hot deserts and semiarid areas, because of the highest solar irradiation and least effects of shadow and clouds are present there. However, dust is a major problem in these areas also, and therefore the cleaning on regular basis is necessary. Most common methods used to clean these solar panels are washing with water and detergents. In some areas, robotic brush cleanings are also used. These methods, however, need large water tanks, pumps, labor, money, and much effort. These manual and robotic cleaning sometime become more critical, because of the shortage of water in the desert area and also include heavy cost of maintenance, cost of frequent cleaning, and scratches by robotic arm and by labor work [10,11]. The mimicking of self-cleaning tendency (hydrophobicity) of nature (lotus leaf, rose petals) has given the idea to reduce dust accumulation on PV surface [7], and this effect is called “lotus effect” or “superhydrophobicity.” If the tendency of water molecules to interact with one another is more than that with the surface, the condition is called hydrophobicity or water-repellent tendency. The extent of hydrophobicity is measured by drop shape analysis. The contact angle can be measured by image of sessile drop, the point of intersection between the drop counter and the projection of the surface. When the interface exists, the angle between the surface of liquid and the outline of the contact surface is described as the contact angle (θ). If the contact angle is .90 degrees, surface is hydrophobic and if .150 degrees, surface is superhydrophobic. Roll-off angle on a superhydrophobic surface should be less than 5 degrees [12]. Different types of wetting behavior of surfaces are shown in Fig. 12.2. Superhydrophobic coatings have many applications; they protect the coated surface or material from corrosion, contamination, prevention, and protection from other hazardous chemicals. [14]. Nanostructured transparent superhydrophobic coating on glass surfaces reduces the adhesion of dust particle and water toward surface and increases the cleaning efficiency. Some of the developed coatings work for few months in open environment, and are more applicable for the regions where the frequency of rain and wind speed is more [11].

FIGURE 12.2 Wetting behavior of a surface [13].

399

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CHAPTER 12 Nanostructured superhydrophobic coatings

In the case of optical surfaces the reflection loss depends on the refractive index. Refractive indices of glass and common plastic surfaces are in the range of 1.45
1.7. For antireflective (AR) surface the refractive index should decrease to B1.22. Natural substances with low refractive index are rare. So, the coating of AR material is used to reduce refractive index [15,16]. Antireflective coatings (ARCs) provide not only less reflectance but also more passivation to the surface. However, the surface passivation is better for refractive index above 2.3. The reflection from the surface also affects the efficiency of solar panel. So, the silicon surfaces are covered with single or double layers of ARCs to increase the efficiency. Solar cells work in a wide range of wavelength from 300 to 1200 nm. So, they need broadband ARC. Single-layer ARC allows only narrow size of wavelength radiation of spectrum. So, double-layer ARC (DARC) is normally used. In DARC a lower layer consists of high amount of silicon which gives more passivation to surface and upper layer consists of material, having low refractive index, to minimize the reflection of solar panels glass cover. ARCs can provide high photocurrent as compared to uncoated ARCs showing least reflectance [17]. An ideal homogeneous single-layer AR coating having an optical thickness 1=4 of one-quarter of a wavelength will have its refractive index nc ðna ns Þ1=2 (where na and ns are the refractive indices of the air and the substrate, respectively) [18]. Materials used in designing DARC are generally MgF2/CeO2, poly(allylamine hydrochloride)/poly(acrylic acid) by layer-by-layer (LBL) coating, SiO2/TiO2, SiN/SiO2, MgF2/ZnS, and MgF2/SiNx [17,19,20]. MgF2 is highly transparent, has lowest refractive index among inorganic materials (1.38), high chemical and mechanical durability, both weather resistant and scratch resistant, and also stables up to 1000 C. Generally, the porous MgF2 coatings or MgF2 nanoparticles can be prepared by MgCl2 and HF reaction. There are many other methods also for preparing MgF2 sol [21
23]. Porous silicon also provides greater amount of antireflectivity. It can be formed by etching methods. Porous silicon provides same reflectance as DARC. The porous silicon emitter fabrication on monocrystalline solar cell provides high efficiency (14%
15% increase in efficiency) as reported by Bilyalov et al. [24
27]. For AR purpose, coating should be scratch resistant and should have great adhesion property. [18]. The stability of coating material for a longer time is also important for the preparation of AR coatings [28]. SiO2 has good passivation and scratch-resistant property and is also stable at high temperature. TiO2 has chemical stability, mechanical hardness, less moisture absorption, and smooth coating process, which make it suitable for ARCs [29
31]. Dust has significant effect on the efficiency and power output of solar panel. To observe this experimentally, much research has been done, and the results represent the numerical data of the loss. Works done by some researchers on the effect of dust on solar PV are summarized in Table 12.1.

Table 12.1 Some studies on the effect of dust on solar panel’s efficiency. Location

Panel condition

Specifications

Result

Reference

University of Malaga, Spain

One cell was cleaned daily, another contained dust and only been cleaned by rain

December 15, 2008 to December 14, 2009, dry summer to rainfall30 degrees tilt angle

[32]

Jagan university, Saudi Arabia

Two panels

Bangalore, India

One was indoor setup with or without dust. Another is outdoor system with and without dust

30 and 55 degrees tilt angle. Duration—16 weeks, little rain occurred on 14th week. Temperature range—22 C
38 C during year, average humidity 70% and wind speed 15 km/h Tilt angle 13 degrees southward

Mean daily loss observed 4.4% Average monthly loss 2% The losses values observed minimum at solar noon, then increased to the maximum value (  75 degrees), and then again decreases The total reduction due to dust and accumulation of dust by little rain was calculated 10.4% by the 30 degrees tilted panel and 9.70% by 55 degrees tilted panel Current drop rate with respect to clean panel was more in indoor system. 5%
6% drop in power loss observed because of dust density of 1.4 g/m2 with compare of maximum possible power output comparing with cleaned photovoltaic system in outdoor unit Due to dust density of 7.155 g/m2 power loss measured 45%
55% of the maximum power output compared to cleaned panel in indoor system

[33]

[4]

(Continued)

Table 12.1 Some studies on the effect of dust on solar panel’s efficiency. Continued Location

Panel condition

Specifications

Result

Reference

Kuwait City, Kuwait

Two panels were placed in open environment. One panel was cleaned daily and second was monthly. Impurity accumulation increased in the month of April, may, October, and December by 90%, 72%, 27%, and 11.26%, respectively, and decreased in the months of November and June by 63% and 61.5%, respectively Three solar panels were used, with 5, 10, and 15 g dust load

Tilt angle—30 degrees from ground, facing south. Duration—1 year

The reduction on the panel efficiency was observed in the month of April, May, October, and December by 15.07%, 13.74%, 10.685%, and 8.742%, respectively, compared to clean panel

[34]

In this sun simulator used LED for simulating sunlight. The chamber is create by polycarbonate plate and used fan for air flow to create natural dust deposition

Maximum amount of degradation in voltage 8.03% by 15 g dust panel and minimum amount of deduction 3.85% by 5 g dust panel Vmax loss by 15 g dust panel— 13.33% Vmax loss by 5 g dust panel— 6.66% Short circuit current loss noticed 45.30% by 15 g dust panel and 26.17% by 5 g dust panel Imax loss by 15 g dust panel 44.41% Imax loss by 5 g dust panel 25.10% Power loss noticed 51.82% by 15 g dust panel and 30.10% by 5 g dust panel

[35]

Pakdasht County, Tehran

Bahawalpur, Pakistan

Two panels were cleaned regularly, and two remained accumulated by dust

Kathmandu Valley, Nepal

Among two panels one was cleaned daily and one was not

Bhopal, India

Two 36 W panel on stand for 1 year

Bangkok, Thailand

Two glass sheets and two 40 W PV cell were mounted on top of a building. One panel and glass pair cleaned daily and one pair left uncleaned for 30-day time duration

The tilt angle 28 degrees in southward direction for 3 months, that is, June, July, and August. The density deposited on the month of June was 0.786 mg/ cm2, July was 0.681 mg/cm2, and of August was 0.601 mg/cm2 Tilt angle 27 degrees. Time period—August 13, 2015
January 10, 2016. Dust deposition density ranges from 0.1047 to 9.6711 g/m2 The minimum and maximum solar intensity was during the time was found—210 and 985 W/m2 15 degrees facing to the south

Total output loss occurred was 22%, 15.5%, and 10.2%, respectively, for June, July, and august. 3% overall average efficiency lost due to dust deposition

[36]

Power reduction was calculated 29.76% with transmittance loss 2.52% on the first day and increased to 69.06% on the last day at 750 nm The reduction in power was observed 92% and reduction in efficiency observed 89% Transmittance of clean sheet was 90% and uncleaned sheet was reduced from 89.8% to 74.9%. The differences between the values of generated power by the clean and uncleaned modules are 0.03, 0.07, and 0.11 MJ for 7, 15, and 30 days of exposure, respectively. The conversion efficiencies of the clean PV module between 4.78% and 4.92% and the decrement of the uncleaned module efficiencies from 4.78% to 4.07%

[37]

[2]

[38]

404

CHAPTER 12 Nanostructured superhydrophobic coatings

12.2 TECHNIQUES USED FOR SUPERHYDROPHOBIC AND ANTIREFLECTIVE COATINGS To make the surface hydrophobic, it is required that the surface has to be rough at nanoscale. Techniques, such as lithography, etching, deposition, and selfassembly, are used to prepare nanoparticle for superhydrophobic surfaces. Deposition methods include sol
gel methodology, spin coating, spray coating, dip coating, chemical vapor deposition (CVD), electrochemical deposition, selfassembly, and anodization [39
41]. These methods can be divided in two approaches—top-down approach and bottom-up approach. In “top-down approach” the material is removed from surface by carving, machining, molding, etc. Methods used to prepare superhydrophobic surface in top-down approach are lithography, templating, micromachining, plasma treatment, etching, etc. There is no control on the structure of template. It could be uneven in structure with micro- or nanostructure development. In “bottom-up approach,” there is control on nanoscale structures, while in top-down approach, only desired micro/nanostructures can be developed. In this approach, material is added on the surface and micro- or nanostructures are created. This method contains CVD, LBL approach, sol
gel method, etc. [42].

12.2.1 CHEMICAL VAPOR DEPOSITION In the CVD technique, deposition occurs as a result of chemical reaction in a chamber, where heated materials, which are to be coated, are present and gases are flushed into the chamber. It is done in hot-wall reactor and cold-wall reactor, at high atmospheric pressure, with or without carrier gases at temperature ranging from 200 C to 1600 C. Nowadays, many enhanced techniques have been developed for CVD, which include ion, photon, plasma, and laser to increase or decrease the deposition rate. The advantage of CVD is that it can deposit a variety of materials and even that with high purity [43]. Schematic diagram of CVD is shown in Fig. 12.3. For fluorocarbons coating, their deposition is difficult by other methods because these types of polymers are insoluble in commonly used solvents. In CVD method, there is no need of solvents, and deposition can be done by without using solvent. Superhydrophobic film by CVD is currently a great area of interest. CVD normally results in flat and chemically homogeneous thin films. Chemical roughness can be produced by altering elements in CVD process. CVD deposition of carbon nanotubes (CNTs) produce a rough microstructure surface due to its specific shape (hexagonal network of carbon rolled up into cylinder). Though CNT is slightly hydrophilic, however, its fluorination by CF4 used in CVD increases its hydrophobicity, and contact angle  165 degrees. Polytetrafluoroethylene (PTFE) has very low surface energy with respect to water. Plasma-enhanced CVD (PECVD) is used to deposit PTFE thin films with contact

12.2 Techniques used for superhydrophobic and antireflective coatings

FIGURE 12.3 Schematic diagram of chemical vapor deposition [44].

angle .100 degrees with water. Fluorocarbon source (hexafluorobenzene) is introduced to increase the rough surface area and the contact angle (up to 160 degrees). Trimethylmethoxysilane is also used to produce hydrophobic surface by PECVD. Breakdown of trimethylmethoxysilane produces rough microstructure and the contact angle of 150 degrees. Hexamethylcyclotrisiloxane is also used to produce hydrophobic surface by PECVD. Furthermore, it also results in chains and rings of methylsiloxane on activation. This also leads to the superhydrophobic surface with contact angle up to 162 degrees [45
47].

12.2.2 PHYSICAL VAPOR DEPOSITION Inorganic materials such as metals, alloy, and mixture of organic compounds can be coated by the physical vapor deposition (PVD) method. It is basically used to coat high-temperature superconductor films. In PVD method the material is deposited in vacuum by condensation. A schematic presentation of PVD is shown in Fig. 12.4. Types of physical vapor deposition—PVD methods are differentiated from each other by the type of evaporation, nature of the deposited material, and plasma conditions at the time of deposition.

• Cathodic arc deposition—In this method a high-power electric arc is •

discharged from the source material, which blasts the material into a highly ionized vapor and deposited onto the substrate. Electron beam
physical vapor deposition (EB
PVD)—In EB
PVD method the anode is bombarded with electron beam (generated by electron guns) under high vacuum. The electron beam converts anode atoms into gaseous phase that is transported by diffusion and then deposited by condensation in solid form onto the preheated substrate. This method has high deposition rate, controlled composition and microstructure, and high thermal efficiency.

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CHAPTER 12 Nanostructured superhydrophobic coatings

FIGURE 12.4 Schematic diagram of physical vapor deposition [48].

• Evaporative deposition—In this method the material to be deposited is heated

•

•

to a high vapor pressure by electrically resistive heating in a “low” vacuum. Evaporation rate is higher than the deposition, which affects the quality of the coatings. Recently used method is plasma-activated evaporation method. In this method, evaporated particles which cross the plasma zone are ionized and can form the dense films. So, the plasma-activated evaporation method is more ideal. Pulsed laser deposition—In this method a high-power laser beam is kept in vacuum and strike to a material that is to be coated on the substrate. When the laser pulse is absorbed by the target, electromagnetic energy transformed into electronic excitation, then into thermal energy, chemical energy, and mechanical energy, and results evaporation, ablation, plasma formation due to excitation and ionization of the species ejected from the material by the laser photons. Material after these processes deposited on a substrate. Sputter deposition—In these method, argon atoms used to bombard the coating material, and the high-energy ions are deposited to the substrate. There are four kinds of sputtering methods—DC sputtering, magnetron sputtering, reactive sputtering, and plasma sputtering [49
51].

12.2.3 SOL
GEL METHOD Sol
gel technology has been developing in past 50 years. In this process the solution after polymerization and polycondensation transforms into solid network

12.2 Techniques used for superhydrophobic and antireflective coatings

structure. Ebelman was the first to report (1845) the formation of transparent materials from slow hydrolysis of an ester of silicic acid. Later in 1930 a sol
gel method for oxide layer on glass from metal precursor was also proposed elsewhere [52]. The sol
gel process is a versatile process for the preparation of superhydrophobic and ARCs. This method requires low cost, low temperature, and low pressure to produce nanoscale rough surface. Generally, silica- and titanium-based coatings are used in sol
gel method. Sol
gel process has many other applications. These are as follows: 1. Monoliths—These are the bulk gels without cracking and used for many optical lenses and devices. 2. Powders, grains, and spheres—They have controlled shape and size, which can be used as abrasive, catalysts, and fillers, for example, uranyl spheres are used as nuclear fuels. 3. Fibers—Sol
gel methods are used to prepare continuous, refractory, polycrystalline fibers that exhibit high strength and stiffness and high chemical durability. Ceramic fibers are used for strengthening of fiber polymer, metal and ceramic matrix composites. 4. Porous gels and membranes—Small size of pores provides permeability, ultrafiltration, and reverse osmosis. Porous gel granules, powders, and films are also used as desiccants and as catalyst supports. 5. Thin films and coatings—Protective coatings are prepared for corrosion resistance, self-cleaning, antibacterial, antiicing, and superhydrophobic surfaces. By alteration of coating material and method transmittance, reflectance, absorptivity, and color of the coatings can be varied. 6. Nanotechnology—Sol
gel method is one of methods to produce nanoparticles, nanopowder, nanocomposite, nanostructures for fabrication, thin film coating, and monoliths. Nanotechnology is very useful and new growing technology for the fabrication of protective nanocoatings and thin films [50]. The concept of nanotechnology was introduced by physics Nobel laureate Richard P Feynman in his famous lecture “There’s plenty of room at the bottom” in the meeting of the American Physical Society, December 1959. Nanoparticles are the agglomeration of atoms and molecule in a specific size (1
100 nm) and possess specific size-based properties. Nanoparticles in onedimensional form are referred as thin films; 2D nanoparticles are nanowires, nanotubes, and fibers; and 3D nanoparticles are nanocrystals, fullerene, precipitate, colloids, etc. Nanoparticles exist as dispersion, aerosol, colloid, and agglomeration. Nanoparticle sticks to each other in very short distance due to van der Waal forces, magnetic interaction, and electrostatic forces [23,53,54]. In sol
gel methodology the inorganic or metal
organic precursor (generally alkoxy group) is dissolved in water or organic solvent. Reaction of metal alkoxy group and metal hydroxyl group occurs, and condensation reaction produces inorganic polymer (contains M
O
M) in the nanoparticles forms. These

407

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CHAPTER 12 Nanostructured superhydrophobic coatings

nanoparticles link together and form three-dimensional networks, which on calcination produces porous nanomaterials with large surface area [23,39,40,55]. Reaction mechanism Step 1—Hydrolysis reaction MðORÞx 1 H2 O-MðOHÞx ðORÞn2x 1 xRðOHÞ

Step 2—Polymerization Dehydration condensation 2 M 2 OH 1 HO 2 M 2 - 2 M 2 O 2 M 2 1 H2 O

Step 3—Loss of alcohol, and condensation 2 M 2 OR 1 H 2 O 2 M 2 - 2 M 2 O 2 M 2 1 ROH

Silica alkoxides are immiscible with water, so alcohol or organic liquid is used to mix it. Reaction of silica precursor such as Si(OR)4 with alcohol is also very slow, so the acid or base is used as catalyst to start hydrolysis and condensation reaction. Weakly branched polymer formed by acid-catalyzed sol, and highly branched polymers are formed by base-catalyzed sol. It has been proved that base-catalyzed sol shows higher hardness and elastic behavior than acid-catalyzed sol. The branching or increase in chain length in alkoxy silane decreases the rate of reaction [56,57]. Silica nanoparticles have stimulated interest in hydrophobic coating due to its thermal and mechanical stability and have high surface roughness. On the rough nanoparticle-coated surface, another compound is layered to achieve low energy surface [58]. Some common precursors used in sol
gel method for silica coatings:

• • • • •

Tetra ethyl ortho silicate [59
62] Methyl trimthoxy silicate [59,60,63] Polydimethylsiloxane [61,64] PTFE [64] Octadecyltrimethoxysilane [64] Common precursors used for TiO2 sol
gel coatings:

• Titanium tetra chloride [65] • Titanium tetra isopropoxide [66] • Tetra butyl titanate [67] Precursor used for zinc oxide (ZnO) solcoatings:

• Zinc nitrate hexahydrate [64] • ZnO nanoparticles [68] • Zinc chloride [69] Polymeric nanoparticles are biodegradable and nontoxic.

12.2 Techniques used for superhydrophobic and antireflective coatings

FIGURE 12.5 Schematic diagram of dip-coating technique [73].

12.2.3.1 Dip-coating technique Dip coating is the easiest and fastest method for the preparation of thin film. The substrate is dipped on a solution, and solution naturally and homogeneously spreads on the surface of the substrate due to the effect of viscous drag, gravity force, and capillary rise. The substrate is withdrawn vertically at constant speed. Then evaporation takes place to solidify the final coating. The draining, evaporation, and hydrolysis steps represent the actual sol
gel transformation. In this method the inorganic species are concentrated by evaporation leading to aggregation, gelation. Disorder and variation in atmosphere will lead to inhomogeneity in coating. Many factors contribute to determine the final coated thin film. Dip coating can be fabricated by controlling the initial substrate surface, immersion speed, removal speed, dipping cycles numbers, solution composition, concentration and temperature and humidity of environment. The difference in the structures of condensed phase leads to difference in the structure of deposited films. For successful dip coating, it is necessary that the condensed phase should remain dispersed in sol and should avoid macroscopic gelation [70
72]. Schematic diagram of Dip-coating technique is shown in Fig. 12.5.

12.2.3.2 Spin-coating technique Spin-coating technique is used to prepare uniform thin films in the thickness range of micrometer to nanometer. The substrate is mounted on a chuck that rotates the sample, and the centrifugal force drives the liquid radically outward. Viscous force and surface tension are the main causes for the flat deposition on surface. Finally, the thin film is formed by the evaporation. Spin coating consists of several stages, such as fluid dispense, spin up, stable fluid outflow, spin off, and evaporation, respectively.

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CHAPTER 12 Nanostructured superhydrophobic coatings

FIGURE 12.6 Schematic diagram of spin-coating method [77].

The advantages of spin coating are to produce very fine, thin, and uniform coating, while the disadvantage is the difficulty with large area samples [74,75]. By spin-coating method the desired thickness of the film can be achieved. Thickness of the layer depends on many different parameters, and the following equation shows how these parameters affect thickness:     ρ 3η  m 1=3 h5 12 A  2ρA0 ω2 ρA0

where h is thickness, ρA is density of volatile liquid, η is viscosity of solution, m is rate of evaporation, and ω is angular speed. As evaporation rate is calculated experimentally, a simpler equation has been suggested as given below: h 5 Aω2B

B is a constant and experimentally calculated parameter. In most of the cases B is somewhere between 0.4 and 0.7. From this equation, it is clear that the higher the angular speed of the substrate, the thinner will be the film [76]. Schematic diagram of spin-coating method is shown in Fig. 12.6.

12.2.3.3 Spray-coating technique Spray coating is a widely known technique for body painting of the vehicles. The coating is applied by a device from which the liquid coating material is sprayed on the substrate surface. The fluid is atomized at the nozzle of the spray front head from where a continuous flow of spray droplets occurs. The system is completely pressurized by the air, which breaks the liquid into small spraying droplets at the nozzle. Coating quality depends on the wetting behavior, surface property, spray nozzle distance, coating speed, droplet size, and number of the sprayed layers. Schematic diagram of spray coating-method is shown at two different time of spray pass as shown in Fig. 12.7.

12.2 Techniques used for superhydrophobic and antireflective coatings

FIGURE 12.7 Schematic diagram of spray-coating method, shown at two different time of spray pass [78].

There are several types of spray-coating methods, which are as follows:

• Conventional flame spray process—Conventional flame spray contains the

•

•

•

wire flame spray method and the powder flame spray method. The characteristic of flame temperature depends on flame
gas ratio and pressure. In wire flame spray the spray wire material is melted in oxygen-fuel flame, fuel gas can be acetylene, propane, or hydrogen. The wire material is atomized and melted by air, and compressed air accelerates it to the substrate surface. In powder flame spray method the powder form of material is sprayed, and rest method is same as wire flame spray method. Electric arc wire spray process—In this method, no external or internal sources for flame or heat are needed. The arc is formed by two oppositely charged metallic wires, and this leads to the melting of the tip of metallic wire. Air atomizes the melted material and goes toward the surface. Electric arc coatings are widely used for fabricating zinc corrosion
resistant coatings. Plasma spray process—High-frequency arc is ignited, so the gas (He, H2, N2, or mixture) ionizes, and a plasma path is developed for few centimeters. The spraying material is injected to plasma, where it is melted and forcibly directed toward the substrate surface by gases. High-velocity oxy-fuel process—This process contains supersonic jet in which the fuels may be propane, propylene, and hydrogen mixed and burned in gas chamber. The speed of the particles is very high, which improves the coating characteristics. The velocity of melted powder increases up to about 550 m/s [78
80]. Some coating materials, their properties and coating methods have been summarized in Table 12.2.

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CHAPTER 12 Nanostructured superhydrophobic coatings

Table 12.2 Some coating materials, methods, and properties of coated surface. Material used Si:C:H Si:N:H TiOx SiNx:H MgF2 1 SiNx

Porous silicon SiO2 1 PEG 1 TritonX-100 SiO2 1 HMDS SiO2 1 MTMS Silica ormosil aerogel and siloxane hybrids Cu2O on SnO2 film

Property of coated surface Antireflective coating Antireflective coating Antireflective coating Antireflective coating Double-layer antireflective coating Antireflective Antireflective Superhydrophobic, antireflective Superhydrophobic, antireflective Superhydrophobic, antireflective Antireflective

Zeolite coating

Superhydrophobic, antireflective

Fluorinated silica SiO2 1 PMMA

Superhydrophobic Superhydrophobic, antireflective Superhydrophobic, antireflective Superhydrophobic Superhydrophobic

Fluorine doped SnO2 1 TMCS SiO2 1 PTMS C/SiO2 1 perfluorosilane

Coating method

References

PECVD

[81]

PECVD

[81]

Spray technique

[81]

PECVD

[82]

SiNx by PECVD and enhanced by MgF2 thermal evaporation Electrochemical etching Sol
gel method

[83]

Sol
gel method

[16]

Sol
gel method

[16]

Sol
gel method

[18]

SnO2—sol
gel method Cu2O-electrochemical deposition Layer-by-layer approach, sol
gel method Sol
gel method Sol
gel method

[84]

Sol
gel method

[88]

Sol
gel method C/SiO2—Sol
gel method Perfluorosilane—CVD

[89] [90]

[24] [18]

[85]

[86] [87]

HMDS, Hexamethyldisilazane; MTMS, methyltrimethoxysilane; PECVD, Plasma-enhanced chemical vapor deposition; PMMA, polymethyl methacrylate; PTMS, phenyl trimethoxysilane; TMCS, trimethylchlorosilane.

12.3 Characterization of surfaces

12.3 CHARACTERIZATION OF SURFACES 12.3.1 WETTABILITY The wettability of a surface can be determined by contact angle and contact angle hysteresis determination.

12.3.1.1 Contact angle When a liquid droplet is on the solid surface, the characteristic angle at equilibrium or the angle made by the liquid against solid is called static contact angle. The equipment used for contact angle measurement is contact angle goniometer. The contact angle of a liquid depends upon wettability of surface, which further depends on the surface tension of liquid and surface energy of the surface. Surface energy can be lowered by incorporating organic groups, such as
CF3,
CH3, and
CH2 [39,89].

12.3.1.2 Young’s equation for calculating contact angle Contact angle θ is given by the following Young’s equation, cosθ0 5

γ SA 2 γ SL γ LA

where γ SA, γ SL, γ LA are the surface energies solid against air, solid against liquid, and liquid against air, respectively. Young’s equation is based on the ideal, smooth, and flat surface. To add the effects of defects and roughness, some wettability modifications were done by Wenzel and Cassie
Baxter as mentioned next:

12.3.1.2.1 Wenzel equation For rough surface, Young’s model was modified, and Wenzel equation was derived for contact angle as follows: cosθ0 5 r

  γ SA 2 γ SL 5 rcosθ γ LA

where θ is the Young’s contact angle, and θ0 is the Wenzel’s contact angle, r is the roughness factor, which equals to the ratio of the actual and apparent surface areas; r 5 1, for solid, smooth, flat surface area, and r . 1, for rough surface area. Wenzel equation shows that hydrophobic surface becomes more hydrophobic ( . 90 degrees), and hydrophilic surface become more hydrophilic (,90 degrees) as the roughness, r, of the surface increases. Sometime the grooves of rough surface are filled by air bubbles, and this case was studied by Cassie
Baxter.

12.3.1.2.2 Cassie
Baxter equation As Wenzel equation is only valid for homogeneous surfaces, Cassie
Baxter modified it for heterogeneous surfaces. In Cassie surface model the water droplet is not able to wet the surface, and the air molecules are trapped in micro- or nanosized grooves. Here, the liquid surface interface consists of two phases, one is

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liquid
solid, and one is liquid
air. Heterogeneous surfaces are made up of two fractions with surface area f1 and f2. cosθ 5 f1 cosθ1 1 f2 cosθ2 f1 1 f2 5 1;

where θ is the contact angle, and θ1 and θ2 are the contact angles for phase 1 and phase 2, respectively, θ2 5 angle between air and liquid interface 5 180degrees f1 5 fSL ; So f2 5 1 2 fSL cosθ 5 fSL cosθ0 1 ð1 2 fSL Þcos180 cosθ 5 fSL cosθ0 1 ðfSL 2 1Þ

All the earlier described models can be schematically presented in Fig. 12.8. Contact angle hysteresis—Contact angle hysteresis is the measure of energy dissipation during the flow of liquid at tilted position. Contact angle hysteresis is the difference between advancing and receding contact angles as shown in Fig. 12.9. The difference is due to different metastable states and free energy. So, the true equilibrium contact angle is impossible to be measured [10,39,91,93,94].

FIGURE 12.8 The Wetting behavior of a liquid droplet on rough solid surface; (A) Young’s model; (B) Wenzel’s model; (C) Cassie’s model [91].

FIGURE 12.9 Advancing and receding contact angle [92].

12.3 Characterization of surfaces

Table 12.3 Critical surface tension of some halogenated functional groups. Surface constitution

Surface tension in dynes/cm [95]


CF3
CF2H
CF2CF2

CH3
CH2CH2

CClH
CH2

CCl2CH2


6 15 18 20
22 31 39 40

Table 12.4 Critical surface tension of some polymers. Polymer name

Surface tension value in dynes/cm [95]

Polystyrene Polytetrafluoroethylene Polyethylene Polytrifluoroethylene Poly(vinylidene fluoride) Polyvinyl fluoride Polyvinyl chloride Polyhexafluoropropylene

33.00 18.5 31.00 22.00 25.00 28.00 31.00 16.2

The surface atoms of a liquid have lesser bonds than the interior atoms. So, they possess higher energy than the interior atoms. This additional energy is known as free surface energy or surface tension (γ). Wettability can be determined by critical surface tension with the help of contact angle. “Critical surface tension is the surface tension at which the liquid completely wets the solid surface.” Zisman plotted the graph between cos θ (θ 5 contact angle) and surface tension of different organic liquids on same type of substrate. The intercept values give the critical surface tension. The critical surface tension of some halogenated functional group and some polymers is given in Tables 12.3 and 12.4, respectively [96].

12.3.2 SURFACE TEXTURE, COMPOSITION, AND MORPHOLOGY 12.3.2.1 Microscopy The surface morphology of coatings can be examined by various microscopic techniques. Scanning electron microscopy (SEM), transmission electron

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microscopy (TEM), and atomic force microscopy (AFM) are the most commonly used for surfaces characterization of superhydrophobic ARCs. SEM—A typical SEM micrograph shows the particle size, particle size distribution, optimized AR coating, smoothness, and cracked or crack-free surfaces with micro-, nanopores in different magnification ranges. In SEM, high-energy electrons are projected on the surface, and these electrons interact with the surface atoms and impart characteristic X-rays, which shows surface topology. SEM produces high-resolution images. Coated surface with micro/nanostructures makes large surface area so that more air particles exist on the surface, that’s why it reduces the interface area between the surface and liquid drop, and more hydrophobicity can be produced. TEM—The morphology and internal structure of various particles, such as SiO2, ZnO2, and TiO2 particles, are characterized by TEM. In TEM analysis the crystal structure of the sample interacts with electron beam by diffraction, and this intensity of diffraction depends on the orientation of the plane of atoms in a crystal. TEM images show the distribution and dispersion of nanoparticles on coatings. AFM—The nanoscale level surface roughness and uniformity of the coatings can be measured by AFM. The AFM works by scanning with a sharp probe over the surface, which feels the surface and provides the height and topology of surface. AFM has a big advantage that it can picture the image of almost any type of sample [97
99].

12.3.2.2 Infrared spectroscopy In infrared (IR) spectroscopy the IR radiations pass through the sample, and absorption of frequency or wavelength gives the spectrum. Some IR radiations are absorbed and some transmitted, and the resulting spectrum represents the molecular fingerprint of the sample. Coatings generally consist of resins, binders, and pigments after evaporation of the solvent. Furthermore, these components are subdivided into atoms with particular bonding and functional groups. The bonding structure, functional group, and organic molecules on the coating can be studied by FT-IR spectroscopy. Different functional groups show characteristic identical peaks [100].

12.3.2.3 X-ray photoelectron spectroscopy The chemical composition of the coatings is characterized by X-ray photoelectron spectroscopy (XPS). XPS spectra are obtained by irradiating a material with Xrays beam, and simultaneously measuring the kinetic energy and the number of electrons that escape from the material being analyzed. XPS requires ultrahigh vacuum conditions. It is a surface chemical analysis technique that is used to analyze the surface chemistry of thin films. XPS is used to measure

• Surface elemental composition • Empirical formula of pure materials

12.3 Characterization of surfaces

• Chemical and electronic state of every element which is present in the surface • Uniformity of elemental composition on top surface [101].

12.3.3 OPTICAL PROPERTIES 12.3.3.1 Ultraviolet
visible spectrophotometer Ultraviolet
visible spectrophotometer is used to measure the transmittance of the coatings. A sample is exposed to UV
visible light from UV light source. The detector records the ratio of intensities between the reference and the sample beams, and the amount of light absorbed by the sample in UV
visible region is determined [102].

12.3.3.2 Ellipsometer Ellipsometry is the use of polarized light to characterize the optical properties of materials in bulk and thin films. This is an optical measurement technique to measure the transmission and reflection properties when light are incident on the materials. The refractive index of AR coating on glass is measured with respect to wavelength using an ellipsometer. The advantages of ellipsometry are that it is a fast process with high precision of wavelength [103].

12.3.4 HARDNESS AND SCRATCH-RESISTANT TEST Mahadik et al. [18] tested the hardness and scratch resistance of coatings by using pencil hardness tester. This test is a constant-load scratch test that makes use of pencils of different hardness grades (9B
9H) as the scratch stylus. A fixed constant load within the pencil is applied on the sample surfaces, which leads to different hardness values. The pencil grade that doesn’t damage the coating is taken as its pencil hardness.

12.3.5 THERMAL ANALYSIS Thermogravimetric analyses of the coatings are carried out in the temperature range from room temperature to 1000 C in air atmosphere to evaluate the thermal stability at a heating rate 10 C/min. Thermal decomposition temperature of polymer is very necessary to know the thermal stability of coatings. The weight loss in the range of 25 C
200 C refers to the loss due to vaporization of the solvent, which is absorbed by the particles. Thermal oxidative decomposition of superhydrophobic alkyl group occurs in the range of 200 C
400 C. Since the solar panel is always exposed to atmosphere, the longevity and strength of the solar can be tested by exposing it to the open environment for long durations, where the natural condition, temperature, pressure, rain, wind, humidity, etc., can affect the coatings [18,87,104].

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12.4 CONCLUSION Increasing concern toward the future crisis of energy has increased the global attention toward exploring renewable energy sources. Solar PV technology is environment-friendly technique, but it has large issues with some inevitable environmental factors, such as dust and reflection due to cover glass, and many weather conditions, which reduces the output performance of solar panel. These issues can be addressed by preparing stable and durable transparent, superhydrophobic, AR, and adherent coatings on the cover glass with the inspiration of natural idea of micro/nanostructured materials on the surfaces, as can be observed in many plant surfaces. There are many growing technologies in this field in which either the coating substance is directly deposited on substrate as in CVD and PVD or a coating liquid is prepared and coated by employing methods, such as spray coating, spin coating, and dip coating. Sol
gel method for the preparation of coating liquid is used widely. The chemical composition and structure of the coating can be evaluated by IR and XPS techniques. The surface morphology of the coating can also be evaluated by using various microscopy techniques, such as SEM, TEM, and AFM. The contact angle measurement is used for the evaluation of wettability, and optical properties can also be measured by utilizing UV
visible spectrophotometer and ellipsometer. Enhancement and improvement is still needed to increase the durability and effectiveness of superhydrophobic coatings. By the development of these coatings the efficiency of solar PV cells will increase, which provide a comparable alternative to other nonrenewable or eco-unfriendly energy sources which have high efficiency. The smart surface coating technology can become a great step in the direction of producing and enhancing the use of environment-friendly solar energy resource.

ACKNOWLEDGMENT This research was supported by the Department of Science and Technology—Clean Energy Research Initiative, New Delhi, India.

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