13 A green alternative to counteract solar panel soiling was studied using nanocellulose composites. 14 Cellulose nanocrystals (CNC) synthesized from acid hydrolysis was blended with Polyvinyl 15 alcohol (PVA), and Polybutylene terephthalate (PBT) and powdered Silica (SiO2) were used to 16 modify its surface to produce a transparent, hydrophobic film. The objective was to test the 17 composite film as a self-cleaning cover for solar panels. FTIR peaks of OH at 3284cm-1, C-H at 18 2852 cm-1, C=C at 1654 cm-1, and C-O at 1087 cm-1, and a 74.38% crystallinity index using X-ray 19 Diffraction test confirmed the synthesis of CNC and presence of PVA. The OH peak's lower 20 intensity in the composite film and dim peaks at 2918 cm-1 2852 cm-1 confirmed PBT, and Silica's 21 layering effect. The composite film exhibited an average transmittance of 92.18% of the incident 22 light in the solar Photo-Voltaic working spectra (420nm-700nm). Results showed an average water 23 contact angle (WCA) of 87.25°. When subjected to photocatalytic and hydrolytic decay, the film 24 showed minimal changes in the FTIR plot, which confirmed its durability. The study proved the 25 scope of CNC/PVA with PBT/SiO2 films as transparent, hydrophobic, durable self-cleaning 26 covers against solar soiling but warranted further research to make suitable properties better. 27 28 Keywords: Cellulose and crystals (CNC), Polyvinyl alcohol (PVA), Thin-films, Self-cleaning, 29 Solar panels, Transparent, Hydrophobic, Poly butylene terephthalate (PBT) 30 31 32 1. Introduction 33 Electricity generation from solar energy using photovoltaic (PV) panels has been a growing field 34 of study in recent years as an eco-friendly, regenerative form of energy source. The current decade 35 is set to improve solar power generation in cost and energy efficiency, making solar power 36 affordable for application [1]. Solar power is harnessed using small modules of silicon p-n junction 37 semi-conductor or PV cells, which trap solar radiation, generally in the wavelength range of 420 38 to 700nm [2]. The major problem associated with this energy conversion panels’ blockage is 39 soiling with dust and other particulate matter, bird dropping, and dew. It becomes a nuisance, 40 especially in dust-prone cities, where PV cells are applied in street lightning, rendering timely 41 maintenance problematic and expensive. In the Kathmandu valley, soiling induced transmittance 42 losses up to 69.06% were observed with a power loss of 29.76% due to a dust deposition density 43 of 9.6711 g/m2 over a study period of several months in the dry season [3]. 44 Cleaning robots are required for timely maintenance, especially in large solar farms. These 45 mechanical robots are effective in areas with high soiling rates. In areas with limited access, 46 sprinkler systems are used, while commercially available anti-soiling coatings are employed 47 elsewhere [4,5]. Commercial coatings like fluorinated silanes are often used for anti-soiling 48 purposes but have a disadvantage in durability, as they dissipate quickly on exposure to the 49 atmosphere. So, hydrophobic coatings become expensive on the periodic application [6,7]. Some 50 inorganic non-biodegradable coating such as TiO 2-Fe2O3 is economical enough but at the same 51 time detrimental to the environment [8]. Therefore, a green alternative is needed for coating 52 technology or alternatives to address this problematic area. Since the durability of coatings comes 53 into question, eco-friendly superhydrophobic thin films were the key objectives of this research. 54 Thin-films (<1mm) could be used as a protective cover layer for solar panels. The film’s versatility 55 could technically be used for applications where self-cleaning or Lotus-effect and dust repelling 56 properties are required. 57 The contact angle (CA) value for hydrophobic pristine Polyethylene terephthalate (PET) is 75.3° 58 ± 1.2° [9]. However, to become super hydrophobic, CA must be >120 °, which can be achieved 59 by micro-and nano-corrugation either in the Cassie-Baxter (CB) state (which accounts for non-60 wettable water contact on the surface) and the Wenzel state (which accounts for wettable surface 61 contact). Ideally, for electronics, the CB state is preferred [10,11]. Low energy surface modifiers 62 like silane or silica have been conventionally used to create the CB state and achieve either 63 hydrophobicity or hydrophilicity [12, 13]. These modifiers also reduce the affinity of dust-water 64 on the substrate [12]. If the film is to be used for solar panels, then the transmittance should be 65 near 100%. So, a combination of self-cleaning, low dust water affinity, and near-perfect 66 transparency is required for the thin film. 67 Recently, CNC has shown to be versatile with facile synthesis and variegated applications. Some 68 of the standard synthesis techniques include thermo-mechanical and chemical treatments, which 69 produce different properties based on source, technique, and starting cellulose concentration 70 [14,15]. CNC is ideal for this application due to its convenient polymer making, accessibility, and 71 eco-friendly nature, which is why it was employed to prepare hydrophobic films [16]. Polyvinyl 72 alcohol (PVA) was used for its chemical resistance, optical properties, and bonding suitability with 73 CNC [17], making it easier to modify its surface for hydrophobicity. Further, PVA’s traditional 74 cloudiness in water can be treated with methods such as hot-pressing or chemical treatments to 75 improve its transparency to near-perfection [18]. Moreover, polybutylene terephthalate (PBT) and 76 silica (SiO 2) coating on the surface added more hydrophobicity. The films prepared were 77 characterized by Fourier transform Infrared spectroscopy (FTIR), X-ray diffractogram (XRD), 78 CA, and ellipsometry to optimize its better performance [16,17,18,19]. 79 2. Experimental 80 2.1 Materials 81 Microcrystalline cellulose (MCC) from Sigma Aldrich with 99 % purity (M w=18000g/mol) with 82 particle length ranging from 20µm to 100µm was used to nano fibrillate. Polymer matrix 83 crystalline PVA from Nippon Gosei with a 98 % purity and PBT from Zhishang was purchased 84 from Bandana Trade Concern (BTC) pvt ltd. Laboratory grade Toluene was used as a solvent for 85 surface modification, and Silica gel-g from ACME synthetic chemicals with 99.8 % purity was 86 also purchased from the BTC. 95 % by wt. of sulphuric acid (H 2SO4) from Fisher Scientific and 87 other necessary chemicals were used from the nanomaterial laboratory of Department of Applied 88 Sciences, Institute of Engineering, Pulchowk Campus. In this work, all chemicals and materials 89 were used without further modification and purification. 90 2.2 Preparation of CNC 91 CNC was prepared by acid-hydrolysis (64% by wt. H 2SO4) of MCC [20]. The time of hydrolysis 92 was vital to obtain the appropriate properties of the prepared CNC as longer hydrolysis time 93 renders the breaking of coarse aggregates in the CNC's cellulose matrix leading to larger axial 94 ratios, which is conducive to transparency [19,21]. 95 50g of MCC was mixed with 50ml of de-ionized distill water and subjected to dropwise addition 96 of 500ml of 64% by wt. H 2SO4 for a hydrolysis time of 1h. The hydrolysis was carried out at a 97 temperature maintained at 45℃ with a magnetic stirrer operating at 200-600 rpm, depending upon 98 the mixture's viscosity [16,19]. The hydrolysis was interrupted by diluting the solution 10-folds by 99 de-ionized distill water. 100 The solution was set to sit for 24h in a cool, dry environment. After the CNC suspension settled, 101 the supernatant liquid was decanted off, and the sediment was kept for further purification. CNC 102 at this time was highly acidic. The CNC suspension was centrifuged using a Virtuose PCF01 Lab 103 Grade Centrifuge at 4000 rpm at room temperature (25°C) for 20 minutes while replacing de-104 ionized distill water at every batch until the pH was 6.53 (~7) measured using a Measuretech PHS-105 3C digital pH meter [16,19]. A 0.22µm Whatman membrane filter was used in a vacuum filtration 106 process using a Joan VT335 vacuum filtration apparatus to remove any untreated MCC [22]. The 107 filtrate obtained, characterized by opalescence in appearance, was determined to be the prepared 108 CNC. The solution was kept in a 5℃ freezer for storage and for later use. 109 2.3 Preparation of PVA and CNC/PVA film 110 PVA was dissolved in de-ionized water (20g in 180ml) and heated at 60℃ for 5h for complete 111 dissolution, using a magnetic stirrer at 800 rpm [23]. The PVA/CNC blend was created using a 112 mixture of 80% PVA, 15% CNC suspension, and 5% Glycerol. CNC concentration was conducive 113 to modify for hydrophobicity in the later process. Glycerol was added to improve the dispersion 114 of CNC in the mixture. [17]. The mixture was kept at 60℃ and stirred for 30mins at 800 rpm. 115 The film of CNC/PVA blend was prepared by solution casting on a Boro-silicate circular glass 116 plate of approximately 80 mm diameter. Another batch was spin-coated at 500 rpm onto the same 117 type of glass plate. Both the solution cast and spin-coated glass were set to air-dry at 120℃ for 118 10mins in an oven, and for redundancy, one was left to dry overnight in a cool dry environment. 119 2.4 Surface Modification of CNC/PVA film with PBT/SiO 2 120 1g of silica-g with 2g of finely crushed Polybutylene terephthalate (PBT) was dissolved in 20ml 121 of anhydrous Toluene. The suspension was ultra-sonicated for 10mins at 25℃. The resultant 122 solution was kept in a glass petri dish, and the prepared films were dip-coated into it for 10mins 123 on each side and left to dry in the ambient air under the assumption that PBT coats a thin layer of 124 hydrophobic biodegradable plastic while silica enhances its hydrophobicity by surface 125 modification [9]. 126 2.5 Characterization 127 The film of CNC/PVA and the surface-modified film was analyzed for Fourier Transform Infrared 128 (FTIR) spectroscopy using Perkin Elmer (FTIR2000) in attenuated total reflectance (ATR) mode 129 within the wavenumber range of 4000 to 500 cm–1 [26] at the Central Department of Chemistry, 130 Tribhuvan University (TU, Kritipur, Nepal). FTIR was also conducted for durability tests of 131 CNC/PVA with PBT/SiO 2 after being exposed to photocatalytic and hydrolytic media. 132 The X-ray diffraction data were observed for CNC by using Bruker D2 Phaser Diffractometer at 133 Nepal Academy of Science and Technology (NAST, Lalitpur, Nepal). The radiation used in the 134 XRD was CuKα with a step of 0.02 and the powder sample was subjected to the diffraction from 135 rotation of 10° to 80°. Samples used for XRD analysis were prepared by freeze-drying the 136 suspension at -40°C of CNC via BIOBASE BK-FD10S Laboratory Freeze Drying Vacuum Freeze 137 Dryer/Lyophilizer at Nano-Laboratory IOE, Pulchowk [28,29]. The crystallinity index was 138 calculated using Segal’s method [30] from equation (1) by obtaining the peak height of the 139 crystalline region (I 002) and the amorphous region (I am) [29]. 140 𝐶𝐼=𝐼଴଴ଶ−𝐼௔௠ 𝐼଴଴ଶ×100% (1) 141 Spectroscopic ellipsometry was used to measure a change in light reﬂection when transmitted from 142 the ﬁlm using TF probe Armstrong tech Ellipsometer for a wavelength of 200 nm to 1100 nm. The 143 test was carried out at the Department of Natural Sciences, Kathmandu University (Kavre, Nepal). 144 Contact angle (CA) was conducted at the Department of Natural Sciences, Kathmandu University 145 using a contact-drop test apparatus Rame hart goniometer 200, tested with 4-6 µl of regular 146 distilled water. The angles were observed from both ends. 147 3. Results and Discussion 148 3.1 Macroscopic features of the CNC/PVA film 149 The obtained CNC/PVA film resembled a plastic-like consistency. The film obtained from air 150 dried (24h), spin coated on PET petri dish PVA/CNC film was qualitatively better. The film lacked 151 any inconsistency in concentration unlike solution-casted films and were easy to peel off from the 152 substrate unlike glass-substrate films. Fig. 1a and b are the macroscopic photographs of these films. 153 154 Figure 1. Prepared CNC/PVA film (a) before modification with SiO 2 and PBT (b) after modification with SiO 2 and 155 PBT. 156 These films were preliminarily tested for thickness using an Aarson Travelling Microscope (RSP-157 202) by observing Lycopodium powder on the top and bottom of the film. The thickness was 158 <1mm (~0.5mm). Also, the thickness obtained was verified using ellipsometer where the thickness 159 of 0.6mm was obtained [24]. 160 3.2 FTIR Spectroscopy of CNC/PVA and PVA/CNC film modified with PBT & SiO 2 161 FTIR plot of PVA/CNC shown in Fig.2 indicated the stretching vibration of OH at 3284 cm-1, 162 which could have offset from the peak of pure PVA. The characteristic cellulosic intra and 163 intermolecular H-bonding has been retained at broad bands between 3600-3000 cm-1. 164 165 Figure 2. FTIR spectra of a) CNC/PVA composite film (CNC/PVA) and b) CNC/PVA composite film modified with 166 SiO2 and PBT (CNC/PVA & SIO 2/PBT). 167 This peak is usually stretched from peaks at 3267 cm-1 [17, 33]. The addition of CNC has also 168 resulted in higher intensity stretch at the OH band at 3327 cm-1. Absorption bands at 2918 cm-1 169 and a sharp peak at 2852 cm-1 indicate C-H stretching, which indicates alkene's presence, which is 170 also commensurate to CNC/PVA [17, 33]. Bands at 1654 cm-1 are for C=C (alkene stretch), along 171 with 1375 cm-1, which might have been shifted from a higher wavenumber due to CNC's addition. 172 Small peaks at 1328 cm-1 and 1087 cm-1 designate C-O stretching signifying secondary alcohol 173 from PVA, and sharp peaks 1051 cm-1 and 1033 cm-1 also resemble the same. 174 The FTIR plot of CNC/PVA & SIO 2/PBT demonstrated specific changes in the peak intensities 175 compared to that of CNC/PVA. The broad peak previously observed at 3284 cm-1 is absent, i.e., 176 OH stretch is not retained in FTIR plot of CNC/PVA & SiO 2/PBT. The peak at 2918 cm-1 and 177 2852 cm-1 show lower intensity, which indicates the presence of PBT [25,26], while the peak 178 around 1654 cm-1 remains intact. 179 3.3 X-Ray Diffraction of the synthesized CNC 180 The X-ray diffraction from the samples was shown in Fig. 3. The peaks intensity obtained at 181 15.457° and 22.678° are designated typically for cellulose II, which has an antiparallel structure 182 4000 3500 3000 2500 2000 1500 1000 5001328 1087 C-O Stretch13751654 C=C2852 CH Stretching29183284 OH BandTransmittance (%) Wave number (1/cm) CNC/PVA CNC/PVA&SIO2/PBT [31]. For the incident radiation, I 002 plane, the 2θ = 20.1°, and I am = 16.3°. The crystallinity index 183 (CI) was calculated using equation (1) for those planes, and it was 74.38%, as predicted according 184 to acid concentration during the hydrolysis period. The CI value obtained is comparable to nano-185 cellulose reported in many works [27, 28, 29, 32]. The small deviations showed is due to the 186 aberrant hydrolysis process. 187 188 Figure 3: X-Ray diffractogram of CNC powder 189 3.4 Transmittance Test of the PVA/CNC film modified with PBT & SiO 2 190 The transparency value was obtained from ellipsometry. Data obtained from the test were plotted 191 against wavelength, and the trend of transmittance from 200 to 1100 nm wavelength is shown in 192 Fig. 4. It was observed that the film transmits consistently over the working spectra range of the 193 solar panel, i.e. (420-700 nm). The result was coherent with the transmittance of traditional 194 PVA/CNC [16]. The average transmittance of 92.18 % in the operating range is low compared to 195 the target (>95 %). This result was observed without chemical treatment or hot-pressing of PVA 196 to obtain high transparency [28]. 197 198 Figure 4 : Transmittance test plot of the CNC/PVA composite film modified with SiO 2 and PBT 199 3.5 Water Contact Angle Test for the PVA/CNC film modified with PBT & SiO 2 200 201 Figure 5: Photograph showing contact angle of test water droplets on PVA/CNC composite films modified with 202 SiO2 and PBT. 203 The water drop test indicated an apparent average CA of 87.8°. The test was carried out with 204 multiple readings in different directions for a single drop of tap water, as shown in Fig. 5. The 205 readings were averaged to indicate two opposite directions and a total of 4 experiments were made 206 for water CA. They were further averaged to obtain a mean CA of 87.8°. 207 The water CA was comparatively less than required (>120°) but higher than PET’s to enhance the 208 self-cleaning ability of the film [34]. These films also exhibited some wettability, which was 209 qualitatively assessed. 210 3.6 Durability Test for hydro catalytic and photocatalytic decay of CNC/PVA & PBT/SiO 2 211 films 212 The durability test was performed on the prepared PVA/CNC thin-films by exposing it to ambient 213 sunlight for three months in the dry winter season and the photo-catalytic degradation or chemical 214 changes were observed. The film was exposed to 5-7h in sunlight with a glass plate at an inclination 215 of 25° with the horizon facing the true South-East (SE). Similarly, hydrolytic degradation was 216 performed by placing the CNC/PVA thin film in a beaker full of collected rain-water and storing 217 it in a cool dry environment. The FTIR analysis of both samples were conducted and compared 218 with the original plot of fig. 2 and the results were plotted and shown in fig 6. [23]. 219 220 Figure 6: FTIR Spectra of CNC/PVA composite film, CNC/PVA film modified using SiO 2 and PBT, Modified film 221 subjected to hydrolytic decay and Modified film subjected to Photocatalytic Decay. 222 The stacked plot of individual FTIR plots indicated minimal changes observed in the PVA/CNC 223 modified by SiO 2 and PBT, along with its subsequent degradation in sunlight and water. These 224 plots showed that the prepared film was resilient against hydrolytic and photo-catalytic action 225 during the study period as there were no significant changes in the characteristic bands for films 226 exposed to sunlight. 227 Minimum changes were found in the hydrolytic decay. The small broad peak at 3284 cm-1 228 represented the O-H band starting to return to its original intensity. This change could have been 229 because of the degradation of the surface modifiers i.e. silica and PBT in water. The intensity of 230 peaks at 2918 and 2852 cm-1 also decreased, implying hydrolytic decay had further affected the 231 underlying substrate of the PVA/CNC film. The same effect was found to be prominent around 232 peaks of 1654 cm-1. 233 For the photo-catalytic decay, the effect of degradation was minimum. The film retained its PBT 234 coating under ambient air as the O-H band around the 3285 cm-1 was not affected. The peaks at 235 2918 and 2852 cm-1 also remained intact, suggesting minimal degradation of the surface modifier 236 i.e. PBT and SiO 2. However, at peaks below 1654 cm-1, the effects were similar to hydrolytic 237 decay, which signified the degradation of the underlying PVA/CNC substrate. 238 4. Conclusions and Recommendations 239 PVA and CNC composite films were successfully prepared and modified by silica and PBT to 240 achieve hydrophobicity and transparency by the solution casting method. FTIR tests of PVA/CNC 241 represented characteristic OH stretch at 3327 cm-1 and peaks at 2918 cm-1 and 2852 cm-1 while the 242 composite film had these peaks suppressed, confirming the PBT’s and silica’s layering effect. The 243 X-ray diffractogram indicated a CI value of 74.38 % which was substantiated for the synthesis of 244 nanocellulose. From this work, following conclusions were drawn: 245  The film exhibited an average transmittance of 92.18 % in the range of 420 nm to 700 nm, 246 which was within the optimal range for solar panels to function. 247  The water CA of PVA/CNC modified film was 87.8°, which was enough to exhibit 248 hydrophobicity. 249  The film showed minimal changes in photocatalytic and hydrolytic degradation during the study 250 period of 3 months, verifying its durability over commonly available coatings. 251 Finally, the prospect of thin films comprising PVA and CNC with surface modification by PBT 252 and silica as transparent, hydrophobic covers was concluded as promising. Silica-produced 253 surface roughness in this film could be enhanced further using silanes for better hydrophobicity. 254 Transparency improvement could be accomplished by proper techniques for CNC dispersion and 255 pre-treatment of PVA virtually yielding a transmittance of up to 97% [18]. The ideal film should 256 have a transmittance of ~99% (>95%). In terms of hydrophobicity, a water CA of >120° is 257 required by the surface to successfully exhibit the elusive Lotus- effect or self-cleaning property 258 required to counteract solar soiling [6]. Therefore, additional research in this hypothesis could 259 improve suitable properties for better application. 260 5.