Enhancing anti-reflective and hydrophobic properties of glass surfaces by nanostructuration and grafting of saturated carbon chains

Journal Pre-proofs Full Length Article Enhancing anti-reflective and Hydrophobic Properties of Glass Surfaces by Nanostructuration and Grafting of Saturated Carbon Chains Amina Kermad, Amel Hassani, Nesrine Hadjaj, Sabrina Sam, Sabrina Belaid, Samira Kaci, Yousseuf Touati, Samia Belhouse, Amar Manseri, Hamid Mennari, Belkhiri Sabrina, Amel Hamrani, Kahina Lasmi PII: DOI: Reference:

S0169-4332(19)33660-8 https://doi.org/10.1016/j.apsusc.2019.144843 APSUSC 144843

To appear in:

Applied Surface Science

Received Date: Revised Date: Accepted Date:

12 December 2018 2 November 2019 22 November 2019

Please cite this article as: A. Kermad, A. Hassani, N. Hadjaj, S. Sam, S. Belaid, S. Kaci, Y. Touati, S. Belhouse, A. Manseri, H. Mennari, B. Sabrina, A. Hamrani, K. Lasmi, Enhancing anti-reflective and Hydrophobic Properties of Glass Surfaces by Nanostructuration and Grafting of Saturated Carbon Chains, Applied Surface Science (2019), doi: https://doi.org/10.1016/j.apsusc.2019.144843

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Enhancing anti-reflective and Hydrophobic Properties of Glass Surfaces by Nanostructuration and Grafting of Saturated Carbon Chains Amina Kermada,b,*,[email protected], Amel Hassanic, Nesrine Hadjajc, Sabrina Sama, Sabrina Belaida, Samira Kacia, Yousseuf Touatid, Samia Belhousea, Amar Manseria, Hamid Mennaria, Belkhiri Sabrinac, Amel Hamrania, Kahina Lasmia aResearch

Center on Semiconductor Technology for energetic (CRTSE), thin films surface and

interface division CMSI, 02 Bd. Frantz-Fanon, B.P. 140, Alger-7 merveilles, Algiers, Algeria bResearch

Scientific and Technical Center on Physico-Chemical Analysis (CRAPC), BP 384, Siège ex-Pasna Zone Industrielle, Bou-Ismail CP 42004, Tipaza, Algeria

cChemistry Department, Sciences Faculty, M’hamed Bougara University, Boumerdes, Algeria Sciences 1L Université aboratoireA.deBelkaid Catalyse – Tlemcen, et Synthèse B.P. en 119 Chimie Tlemcen, Organique, Algérie Département de Chimie, Faculté des dLaboratory of Catalysis and Synthesis in Organic Chemistry, Chemistry Department, Faculty of sciences, Abou Bekr Belkaid- University of Tlemcen. Tlemcen, B.P. 119 Tlemcen, Algeria. *Corresponding

author:

Abstract: Herein we designed a simple method to fabricate an anti-reflective and hydrophobic glass surface, which used multistep strategy. Glass surface was nanotextured by etching process, using a facile hydrothermal processing, based on the interaction between ammonium hydroxide (NH4OH) and glass surface. Amine terminations were generated by silanization of the nanostructured glass surface using 3-aminopropyltriethoxysilane (APTES). Octa-decanoic acid was anchored on the amine nanotextured glass surface by using N-hydroxysuccinimide (NHS) in the presence of the coupling agent N-ethyl-N-(3-dimethylaminopropyl) carbodiimide (EDC) leading to saturated long carbon chains terminations on the glass surface. The surface characterizations of the glass samples and the effect of the molar concentration of the grafted acid were carried out, after each modification, by Fourier Transform Infrared Spectroscopy analyzes in ATR geometry (ATR-FTIR) and the transmittance of the glass was measured by UV-Visible spectroscopy. Scanning Electron Microscopy (SEM) was used to perform the morphology observations of the samples after etching. Surface hydrophobicity was assessed by contact angle measurements. A decrease in the wettability accompanied with an increase in transmission of nanotextured glass surfaces was noticed after grafting of octadecanoic acid on the surface of the textured glass, which attest the success of the acid grafting process, established in this study, to elaborate hydrophobic and anti-reflective layers. Keywords: Nanotextured glass, grafting, octa-decanoic acid.

1. Introduction The photovoltaic application around the world is faced with many risks of degradation during their external exposure by environmental factors (solar radiation, humidity, wind, rain, snow, dust and soiling ... etc.), which affect the different constituent parts of the module. And the contamination of the surface of the glasses in the panels is one of the main problems encountered in the field, because it is significantly reducing the transmission of light and thus affecting the performance of photovoltaic modules [1, 2]. The evaluation of the energy losses caused by the deposition of dust is widely spread in the literature, as a large number of studies have been conducted in this manner. We can cite a study which was carried out by A. Rao's et al. in order to evaluate the effect of dust accumulation on the conversion efficiency of a solar panel; for this, they traced the Current versus Voltage of a clean panel and another covered with dust, subjected to the same experimental conditions (illumination and temperature); the obtained results showed a shortening of short circuit (Isc) from 30 to 40% [3]. In the same path Sulaiman et al. have shown that the accumulation of dirt on the surface of a solar panel reduces its efficiency up to 85% [4]. Time exposure to dusty conditions plays a big role in the deterioration of the PV cells performance, however even a short time exposure was found to decrease its performance by about 12% compared to clean cells as it has been reported by Chaichan et al. [5]. The study carried out by Kazem et al. on the effect of fouling with several types of dust from different regions of northern Oman has shown that, for regions where dust has a high water content, the impact is more negative on the performance of solar panels [6]. Also the work done by M. Saidan et al. confirmed that the accumulation of dust particles reduces, both the short circuit current (Isc) and the output power, the average degradation rate of the efficiencies of the solar modules exposed to dust are; 6.24%, 11.8% and 18.74% calculated for exposure periods of one day, one week and one month respectively [7]. More recently, Tripathi et al. showed that the reduction in short-circuit current and open-circuit voltage was respectively 33.33% and 6.64% for 12 g of dust deposit on the surface of the module, the power output has also known a reduction up to 42% [8]. In order to preserve, improve and decrease the cost of maintenance of solar panels, several research studies have been carried out for the development of self-cleaning surfaces with a hydrophobic character [9-11]. In the photovoltaic field, in addition to the hydrophilic or hydrophobic character, the increase in the transmission of solar radiation in a panel is of considerable importance for the increase in energy efficiency. In this perspective, numerous

researches have focused on the development of anti-reflective layers, these last ones allow the improvement of the transmission [12]. Generally, the development of hydrophobic surfaces with an anti-reflective character is done in two stages. The first is to form a rough layer, which allows, on the one hand, to increase the mechanical stability of the elaborated layers and to give them the anti-reflective character, on the other hand. The second step is to chemically modify rough structures, in order to obtain a super hydrophobicity. However, in order to achieve mechanical durability; several new studies have adopted the nanotexturing of glass surfaces by hydrothermal route in an alkaline solution. This method appears as a simple and less expensive technique. In the study done by J. Xiong et al., a porous layer was created on a glass surface, by a simple alkali etching process, they showed that the morphology, optical and wetting properties were controlled by the original glass composition and etching time. Indeed, the etched glass showed high transparency, low reflectance and possess superhydrophilic and antifogging properties, by changing the hydrophilic porous surfaces into hydrophobic surfaces by a simple OTS SAM modification [13]. Another approach for the nanotexturing of glass surfaces was led by Verma et al.,in which a non-lithographic method was used to improve the performance of solar cells following the increase in transmission, they have reported that the maximum improvement of the transmission has been observed for the nanostructures of a 200 nm of thickness , also their obtained results showed that the surface of the glass becomes superhydrophilic with a contact angle of less than 5°, so the self-cleaning effect was confirmed by tests carried out for 22 days [14]. Ji et al. obtained a glass surface with the hierarchical textured morphology by one-step hydrothermal method in which the structure of the surface was controlled by the variation of the temperature and the concentration , they have showed that after modification with vinyltriethoxysilane, the glass surface exhibited a stable superhydrophobic with a high water contact angle of 155° and a low sliding angle of 5°[15]. Zhang et al. have reported a simple approach to the preparation of a nanotextured glass, their method involved using the hydrothermal process in an ammonium hydroxide solution and allowed obtaining nanotextured surfaces with an anti-reflective character, which exhibited a superhydrophilic character, the study shows that the main factors affecting glass transmission were the temperature and the treatment time, whereas the effect of the concentration of ammonium hydroxide is low, thus, the highest transmission area was obtained under optimized conditions and the transmittance can reach 98.4% near wavelength of 500 nm [16]. More recently, Matin et al. conducted their study on glass and plastic substrates, in their work, the hydrophilization of the glass surfaces was carried out by performing a

deposition of octa-decyl-trichlorosilane (ODTS), the results showed that the elaborated layer was superhydrophobic with a contact angle greater than 150°; nevertheless, the authors did not take into account to study the transmission [17]. According to this bibliographic overview, we note that most of the studies carried out are based on the use of organosilane compounds to achieve hydrophobization; however, in other areas of application namely textile, anticorrosion, etc…, numerous studies have proven the efficiency of the use of fatty acids for the development of super hydrophobic layers. Zhang et al. obtained a stable superhydrophobicity on aluminum alloy by a low-cost one-step method, by simply immersing the substrates in a solution containing hydrochloric acid and fatty acid molecules, the formation mechanism of such a surface was proposed on the base of the SEM morphology and EDS results, the resulting surface showed superhydrophobicity [18]. In the work done by Lakshmi et al., superhydrophobic surfaces on aluminum alloy substrates have been successfully fabricated by a simple low-cost two-step approach, in their study, the surface modification was achieved with alkanoic acids with carbon chain length C14, they noticed that the combination of a binary surface morphology and a low surface energy played a vital role in rendering super hydrophobicity to the surface [19]. Li et al. showed in their study that, the modification of Sapphire surface with stearic acid increased the value of the contact angle up to 107°, caused by the hydrophobic tail group of the stearic acid molecule [20]. More recently, Forooshani et al. have studied the modification of the surface of aluminum by different acids, namely: stearic acid, myristic acid and decanoic acid; the contact angle results have shown that the use of these acids allowed the successful formation of super hydrophobic layers and improved corrosion resistance and that the surfaces etched with stearic acid show better properties of superhydrophobicity with a contact angle of about 157 ° [21]. The objective of our study is the elaboration of transparent and antireflective layers on glass substrates, in order to do so, we rely on the work done by Ji-Fan Zhang [16], however, we are interested in transforming the hydrophilic character of the structured glass surface into a hydrophobic surface. Hydrophobic surface transformation consists in grafting molecules or groups containing terminations with hydrophobic properties, namely fatty acids, such as octadecanoic acid. The experimental protocol adopted in this study consists, for the first step, in a nanotexturing of glass surface using a facile hydrothermal processing, based on the interaction between ammonium hydroxide (NH4OH) and glass surface. The second step consists of functionalization of etched glass surfaces and grafting of saturated carbon chains. Hence, the

functionalization

was

initiated

by

silanization

of

fresh

etched

layer

using

3-

aminopropyltriethoxysilane (APTES), to form an amine terminated surface. Subsequently, the grafting of octa-decanoic acid has been done by using N-hydroxysuccinimide (NHS) and the coupling agent N-ethyl-N-(3-dimethylaminopropyl) carbodiimide (EDC), leading to saturated long carbon chains terminations on the glass surface. We note that the grafting was performed for several concentrations of octadecanoic acid in order to study its effect on the different properties in target, thus, and after each modification, the surface characterization of the glass samples was carried out by FTIR Fourier transform infrared spectroscopy analyses in ATR geometry (ATR-FTIR), scanning electron microscopy (SEM), UV-Visible spectrophotometer and contact angle measurements. It’s worth noting that these two last techniques allow revealing the properties of hydrophobicity and transmission of the layers developed.

2. Experimental 2.1 Reagents Glass

Slides

Ammoniumhydroxide

were

supplied

(NH4OH)

by

(≥99.

Super 99%),

Frost®Plus

by

Menzel-Glaser.

N-(3-Dimethylaminopropyl)-N-

ethylcarbodiimide hydrochloride (EDC) (98%) and N-Hydroxysuccinimide(NHS) (98%) were purchased from Sigma Aldrich. Octadecanoicacid (≥ 97%) was purchased from Merck.3aminopropyltriethoxysilane (APTES) (97%) from Aldrich and absolute ethanol was supplied by Carlo Erba. 2.2 Preparation of Nanotextured glass surface Glass slides were cleaned by using isopropanol and thoroughly dried under a nitrogen stream. Then, the cleaned glass was placed in a 30 ml Teflon container with 20 ml ammonium hydroxide (NH4OH) of a given concentration and sealed in a stainless-steel autoclave. After the treatment, it was left to cool down to room temperature. The treated glass sample was rinsed with deionized water and dried using a nitrogen stream, the glass slides were treated under different temperatures and treatment times. The temperature/times ranges for surface nanotexturing are summarized in the table below (Table I).

2.3. Functionalization of nanotextured glass layer The different functionalization steps are summarized in Scheme.1. The amine terminations were generated by silanization of the nanotextured glass surface using 3aminopropyltriethoxysilane (APTES). Subsequently, octadecanoic acid was anchored on the amine glass surface by using N-hydroxysuccinimide (NHS) in the presence of the coupling agent N-ethyl-N-(3-dimethylaminopropyl) carbodimide (EDC) leading to saturated long carbon chains terminations on the glass surface; the acid grafting was performed for different concentrations of octadecanoic acid. 2.4. Sample characterization Scanning electron microscopy (Philips) was used to perform the SEM observations of the samples. Fourier transform infrared (FT-IR) spectra were recorded at each stage of functionalization in ATR geometry, using IR Affinity-1S with a diamond crystal. The measurements were made between 400 and 4000 cm -1, with 45 scans and a resolution of 4 cm-1. Contact angle measurements were achieved with 3µl ultrapure water and a system of controlled geometry (Visiodrop), the error of measurement is estimated to 2◦. Optical transmission measurements were performed using a Specord 200 plus-Analytic Jena model UV-visible-nearinfrared spectrophotometer. 3. Results and discussion 3. 1 SEM observations after hydrothermal etching We were unable to determine the thickness of the nanotextured glass layer. This is mainly due to the performance of the equipment as it is a conventional scanning electron microscope. Indeed, during the measurement a load effect was generated which prevented the analysis of the glass sample in cross section. Fig.1 illustrates the SEM pictures, for the same magnification, of glass surfaces before (Fig. 1-a) and after (Fig. 1-b, 1-c, 1-d and 1-e) hydrothermal etching at different times and temperatures performed in fixed molar NH4OH solution concentration of 2.5 M. The SEM observations show that, before attack, the surface of the glass has a very smooth morphology. The SEM pictures, after attack, reveal a significant change in the surface of the glass. Indeed, at a temperature of 100°C for 4 hours, we observe the formation of pores; however, we note that the attack is not very homogeneous over the entire surface (Fig.1-b). Moreover, for the same temperature and for an attack time of 6 hours (Fig. 1-c), we notice the generation of an overlap between the pores, this is clearly due to the increase in the number and size of the pores.

In Fig. 1-d and 1-e, we note that the morphology of the surfaces of the glasses etched at 150°C for 4 hours and 6 hours show a more ramified structure and the formation of network nanoflakes. In addition, the attack is homogeneous over the entire surface. We also note the creation of other deep pores for the surface of the glass attacked for 6 hours. The obtained results confirm that increasing the attack time and temperature affect the morphology of glass surfaces at a fixed concentration of NH4OH and lead to better structuring according to what was obtained in the literature [16]. 3.2 Infrared characterization 3.2.1 Etching step The spectra relating to the surface of the glass before structuring (white glass) and after structuring are illustrated in Fig. 2. All the spectra were given for the spectral zone between 600 and 1300 cm-1. We notice the presence (Fig. 2–a) of bands between 760 cm-1 and 1060 cm-1. The band at 760 cm-1 corresponds to the non-bridging oxygen (Si-O-) elongation vibration in the glass. The wide band at 900 cm-1 is attributed to the symmetrical elongation vibration of the Si-O-Si siloxane groups, the shoulder at 1060 cm-1 being assigned to the antisymmetric stretching vibrations of the Si-O bonds [13]. After structuring the glass, we observe a significant decrease in the intensity of these absorbance bands (spectrum in red). Indeed, we note the decrease in intensity of the bands at 900 cm-1 and 1060 cm-1, this attests the attack of the Si-O-Si bond by the alkaline solution (ammonium hydroxide). The decrease of the Si-O-Si bond indicates the dissolution of the vitreous network and the formation of a porous structure. This result joins the result obtained by the team of J. Xiong [13], where they found that the porous structure of glass promotes the diffusion of hydroxyl ions. In our study, we confirmed this by the appearance of a new broad band located around 3500 cm-1 corresponding to the Si-OH bonds (Fig. 2-b) [13, 22]. The decrease of the band located at 760 cm-1 was explained by the formation of a new bond between the non-bridging oxygen Si-O- and OH hydroxyl ions to form the new Si-OH bond, located at 670 cm-1 (deformation vibration). The comparison of the FTIR spectra (Fig.3) shows that the absorbance bands attributed to the hydroxyl groups Si-OH around 3500 cm-1 and at 670 cm-1, strongly depend on the hydrothermal attack conditions (temperature and treatment time). We observe that the intensity of the bands reaches its maximum for a treatment temperature equal to 150 ° C for 6 hours and decreases to reach its minimum for 100 ° C for 4 hours, this is due essentially to a better

diffusion of the hydroxyl ions in increasing the attack time and temperature, which further favors the formation of new Si-OH bonds”. Thus, we notice that the dissolution of the vitreous network is less accentuated compared to that observed when the attack is carried out at 150 ° C for 6 hours, which is confirmed by a slight decrease in the intensity of the characteristic bands of the glass surface. 3.2.2 Silanization step Silanization allows, after hydrolysis reactions, the formation of siloxane groups. This mechanism has been confirmed in Fig. 4-a by increasing the intensities of the bands between760 cm-1 and 1060 cm-1, relative to Si-O-Si groups [13]. Other changes appear in the FTIR spectrum. We observe the presence of new bands between 1565 and 1642 cm-1 (Fig. 4b). We can attribute these to the deformation vibration mode of –NH3 and NH2 groups [23]. Another new band located at 1460 cm-1 is assigned to the shear mode deformation vibration of the CH2 group [24]. The Silanization of glass surfaces after structuring has been confirmed by the appearance of new bands (Fig. 4-c). These are the two bands at 2865 and 2927 cm-1, which we attributed respectively to the symmetric and asymmetric vibrations of the CH2 group [24, 25]. We also noticed the appearance of another new band at 3620 cm-1, which we linked to the asymmetric elongation vibration of the NH bond [24]. The decrease in the wide band at 3500 cm-1 that we attributed to the Si-OH groups can be explained by the consumption of these latter during the Silanization reaction (Fig. 4-d). 3.2.3 Octadecanoic acid grafting step We wish to recall that the grafting of fatty acids on the surface of the glass is carried out between the carboxylic groups of the acid and the NH2 amine groups of the surface of the silanized glass, in the presence of a mixture of EDC/NHS. Indeed, this was confirmed by the appearance of a new band located at 1522 cm-1 and the band located at 1638 cm-1 (Fig.5-a), these bands, respectively called amide I and amide II, they correspond to the modes of vibration of elongation of the C=O bond and deformation of the N-H bond [26]. The grafting of octadecanoic acid was confirmed by the increase of symmetric and asymmetric CH2 bands at 2855 and 2927 cm-1 respectively (Fig.5-b). Thus, we notice the appearance of a new band located at 2960cm-1, attributed to the asymmetric vibration of the CH3 bond [27]. 3.3 Contact angle measurements

3.3.1 After nanotexturing The contact angle measurement was performed on a white and nanotextured glass surface. The results obtained show a significant decrease in the contact angle value after nanotexturing. This decrease after texturing is explained by the formation of new polar groupings which are the Si-OH. The comparison between the contact angle values between textured glass surfaces with different processing conditions (temperature and time) is illustrated in Fig. 6 We note that nanotexturing of glasses was performed with the same molar concentration of ammonium hydroxide (2.5 M). The results show that the value of the contact angle increases from 52.4° for a glass surface without texturing to reach a value of 5.2° for a textured glass surface for 6 hours at a temperature of 150°C; nevertheless, the value of the contact angle exceeds 19° for a structured glass surface at a temperature of 100°C for 4 hours. Indeed, these nanotexturing conditions correspond to the smallest temperature value and the shortest duration of attack. We can explain the evolution of the contact angle value in this way, by the fact that the increase of the temperature for a longer treatment time would cause the rupture of the most covalent bonds in the glass network [28]. Under these conditions, the porous layer becomes thicker, thus increasing its surface area, which subsequently allows more diffusion of the hydroxyl ions making the surface of the glass more hydrophilic [15, 16]. 3.3.2 After silanization Fig. 7-a, shows that the value of the contact angle undergoes significant modifications, after silanization with APTES of a textured sample at 150° C for 6 hours, we notice that the value of the contact angle increases from a value of 5.2° to a value of 57.5°. This increase is explained by the formation of new apolar groups (CH2 groups), making the surface of the glass more hydrophobic. This result has been confirmed by FTIR analysis in ATR geometry. Furthermore, we remark that the value of the contact angle essentially depends on the conditions of the nanotexturing where the value of the contact angle reaches its minimum which equals 45.5° (Fig.7-d), after the silanization of a textured sample in a 100°C for 4 hours. This can be explained by the fact that the augmentation of the duration and the temperature during the hydrothermal attack, increases the formation of the hydroxyl ions and thus favorizing the salinization reaction. 3.3.3 After octadecanoic acid grafting In this part, we are interested in comparing the effect of octadecanoic acid grafting on the wettability for different texturing conditions, for a concentration C2=10-2 M; the results obtained

show that the value of the contact angle reaches its maximum, which is equal to 124.5° (Fig. 8a), for a textured glass surface at 150°C. For 6 hours grafted with octadecanoic acid. This is explained by the attachment of a long carbon chain with a saturated terminal (the CH2 and CH3 groups) of octadecanoic acid (18 carbons) [21]. Whereas for the same grafting conditions and for texturing carried out at 100°C, for 4 hours, the value of the contact angle does not exceed 90.0° (Fig. 8-d). The explanation for this phenomenon is the effect of temperature and attack time on the characteristics of the porous layer. As already explained above, as the temperature and the attack time increase, the thickness of the porous layer increases, this gives it a larger surface area, which allows it to hang a greater number of hydrophobic molecules during grafting [15]. 3.4 UV-Visible characterization It should be noted that the transmission measurements were made, with respect to a reference sample; it is a flat glass without nanotexcturing, this is why we did not present the spectrum for this sample. For a good presentation of the evolution of transmission, we expressed the transmission T(λ), in weighted values in a spectral interval [λ1λ2]. This quantity is determined by integration in [λ1λ2] of the experimental values with respect to a Sp (λ) solar spectral irradiance of reference. The latter varies from one place to another (outside or inside the Earth's atmosphere and the angle of incidence of the solar spectrum). λ2 𝜆1

∫ 𝑆P (𝜆) 𝑇 (𝜆) 𝑑𝜆

𝑇P =

𝜆

∫𝜆2𝑆P (𝜆) 𝑑𝜆

(1)

1

For a terrestrial photovoltaic application, the reference solar irradiance used, corresponds to the global solar spectrum AM 1.5G [29]. It is described by the American Standard for Tests and Measurements (ASTM G-173-03), which corresponds to solar radiation arriving at an angle of 48.2° on the surface of the earth with a power of illumination of 100 mW.cm-2 (or 1000 Wm2).

As

part

of

this

study,

we

are

interested

in

silicon-based

photovoltaic

cells.

In this case the absorption threshold is around λ= 1107 nm, the useful part of solar irradiance is between λ1= 400 nm and λ2= 1000 nm, which represent the limits of the integration interval of all our calculations in this work.

3.4.1 Effect of glass nanotexturing

In Fig. 9-a, we trace the evolution of glass transmission after texturing, for different conditions. We observe a decrease in the transmission for all the spectra recorded after the hydrothermal attack in an ammonium hydroxide solution. As shown in Fig. 9-b, the weighted transmission decreases with increasing temperature and treatment time, for a fixed molar concentration of the NH4OH solution (2.5M); indeed, we find that the weighted transmission reaches its maximum (95.90%) for an attack carried out at 100°C for 4 hours, and then it begins to decrease to reach a value of 77.55% for 6 hours at 150°C. This decrease may be due to the thickness of the porous layer formed, thus affecting the refractive index of the nanotextured layer [16]. To better explain this phenomenon, it is first necessary to make a small reminder on some notions of optics; indeed, when a beam of light passes through a piece of glass, it is divided into three parts. One part is reflected, the second part is absorbed, and the last part, which represents most of the light, goes through the glass. They can be represented by R, A and T, respectively. Since the light absorbed in a glass can be ignored the incident light (I) could be summarized as follows [30]: 𝐼 = 𝑇 + 𝑅 = 100% (2) Thus, the increase in light transmission is the result of reducing the reflection or the antireflective effect. The ideal antireflection coating of two parameters [9, 31]: 1- 𝑑 = λ⁄4𝑛𝑐, where 𝑑 is the thickness of the coating, λ is the wavelength of the incident light. 1

2- The refractive index 𝑛c = (𝑛𝑎𝑛𝑠)

2

, where 𝑛𝑎 and 𝑛𝑠 are the refractive indices of air

(1.0) and substrate (glass, 1.5), respectively [32]. Thus, the refractive index is related to the porosity of the nanotextured layer, according to the Lorentz-Lorenz relation which is written in the general case, in the following form [33]. 𝑛2𝑐 ― 1 𝑛2𝑐

𝑛2𝑠 ― 1

= (1 ― 𝑝) 𝑛2 + 2 (3) +2 𝑠

Where, 𝑛𝑐 is the refractive index of the nanotextured layer (or porous layer), 𝑝 is the porosity (it is the volume fraction of the pores or the filling factor) and 𝑛𝑠 is the refractive index of the skeleton solid nanostructures (refractive index very close to the glass index, 1.5). For this reason, we can conclude that the transmittance or antireflection depends directly on the porosity and the thickness of the nanotextured layer. Therefore, it is recommended to modify the roughness or the porosity by modifying the attack conditions, in order to improve the transmission and to minimize the value of the refractive index.

3.4.2 Effect of nanotexturing on transmission after acid grafting In order to study, the effect of nanotexturing on the transmission of the glasses, after grafting, the transmission measurements were carried out on a series of textured samples with different conditions of time and temperature and on a glass surface plane. We would like to note that this study was carried out for graft samples at a C2=10-2 M concentration of octadecanoic acid. The result obtained (Fig. 10) shows that the transmission reaches its maximum, for a flat glass sample, where the value of the weighted transmission reaches 100% (Table II). This indicates that the graft layer of octadecanoic acid with a concentration of 10-2 M is quite transparent and does not reduce the transmission of glass. On the other hand, we see a sharp increase in the values of the weighted transmission of structured surfaces after grafting. 3.5 Effect of concentration of grafted acid 3.5.1 Contact angle measurements This part aims to reveal the effect of the concentration of grafted acid on the wettability of the glass surface. In order to achieve this objective, we have presented the contact angle values of octadecanoic acid, grafted on glass surfaces in the nanotexturing conditions: 150°C for 6 hours. The results collected (Fig. 11), show that the value of the contact angle increases from 118.8° to 124.5°, when the concentration varies between C1 = 10-3 M to C2 = 10-2 M, then we notice a decrease in the contact angle value to reach its minimum (118.4°) when the molar concentration increases to C4 = 10-1 M. We explained this result by the condensation of the carbon chains of the grafted acid on the surface of the glass (steric hindrance).

3.5.2 UV-Visible measurements Fig. 12-a illustrates the different transmission spectra of the structured glass surface for 6 hours at 150°C, before and after grafting with different concentrations of octadecanoic acid. The results obtained show an increase in transmission after grafting along the visible range. We can explain this result by the fact that the modification of the glass surfaces can minimize the reflected light at the interface between the surface of the nanotextured glass and the grafted layer [34]. The plot of the variation in weighted transmission as a function of grafting concentration (Fig. 12-b) shows a marked increase in transmission.

3.5.3 Infrared characterization

Fig. 13 shows the normalized FTIR spectra relating to the samples of structured glass in an ammonium hydroxide solution with a molar concentration of 2.5 M, for 6 hours at 150°C, after grafting of the acid at different concentrations at a temperature of 80°C. The FTIR results presented in Fig. 13 shows, for all the spectra, the presence of all the absorption bands at the same wave number but with different intensities for concentrations between 10-3 M and 10-2 M. From the spectra, we can observe an increase in the intensity of the bands relating to the vibrations of the CH2 and CH3 groups (Fig. 13-b), when the concentration of the octadecanoic acid varies from C1=10-3 M to C2=10-2 M. This increase is followed by a decrease in the intensities of the same bands for concentrations of octadecanoic acid at C2=10-2 M (C3= 510-2 M, C4=10-1 M).This result can be explained by the condensation of the carbon chains of the grafted acid on the surface of the glass (steric hindrance), which prevents the attachment of the octadecanoic acid via its carboxylic function. Indeed, this result is in agreement with the ATR spectra, which we collected in the spectral range between 1300 cm-1 and 1900 cm-1 (Fig. 13-a), where, we have noticed the increase of the amide I and amide II band intensities, when the concentration of octadecanoic acid varies from C1=10-3 M to C2=10-2 M, then we noticed a shift of these two bands towards the great energies. 4. Conclusion The results obtained show a decrease in the wettability after the grafting of octadecanoic acid on nanotextured glass surfaces. This decrease in wettability was accompanied with an increase in transmission. This attests the success of the acid grafting process established in this study to elaborate hydrophobic and anti-reflective layers. The analysis of the state of wettability of the modified glass surface has shown a considerable reduction in the value of the contact angle, after nanotexturing, due to the formation of new more polar groups. Nevertheless, we have noted a remarkable increase in the value of the contact angle of the grafted surfaces by octadecanoic acid. This confirms the formation of new groups with apolar endings. Moreover, the transmission spectra showed a decrease after nanotexturing of the glass. However, after grafting the octadecanoic acid, we found an improvement in the transmission. This result is probably due to reducing the rate of the reflected light at the interface between the glass surface and the grafted acid layer. The study of the effect of the molar concentration of the grafted acid, showed coherence between the different analysis techniques exploited, indicating a steric hindrance effect when the concentration exceeds 10-2M. These primary results will prompt to a complete optimizing of the transmission of glasses. This involves modifying the nanotexturing conditions by varying

other parameters namely the molar concentration of ammonium hydroxide and even applying other ranges of time and attack temperatures. Also, we are projecting for the rest of this work, to increase the hydrophobicity of the surfaces, while preserving their transmission. This can be achieved by changing the grafting conditions (concentration, time and temperature).

Acknowledgments We are grateful to the financial support of the National Research Fund-Algeria (DGRSDT/MESRS).

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Fig. 1. SEM images of surfaces treated in the condition of (a) untreated; (b)100°C, 4 h and 2.5 M; (c) 100°C, 6 h and 2.5 M; (d) 150°C, 4 h and 2.5 M; (e) 150°C, 6 h and 2.5 M. Fig. 2. ATR-FTIR spectra of the glass surface before and after texturing. a) Spectral area 6001300cm-1, b) Spectral area 3000-3600cm-1 Fig. 3. ATR-FTIR spectra of the nanotextured glass surface in the condition of, a) 4 hours, 100°C, b) 6 hours, 100°C, c) 4 hours, 150°C, d) 6 hours, 150°C. Fig. 4. ATR-FTIR spectra of the textured glass surface before and after silanization a) Spectral area 600-1300 cm-1, b) Spectral area 1300-1800 cm-1, c) Spectral area 2750-3050 cm1, d) Spectral area 3000-3700 cm-1. Fig. 5. ATR-FTIR spectra of the silanized glass surface before and after grafting of octadecanoic acid, a) Spectral area1300-1850 cm-1, b) Spectral area 2750-3100 cm-1.

Fig. 6. Photographs and values of the contact angle of 3μL of a drop deposited on glass surface, before and after etching in the condition of a) 6h, 150°C, b) 4h, 150°C, c )6h, 100°C, d) 4h, 100°C Fig. 7. Photographs and values of the contact angle of 3μL of a drop deposited on glass surface before and after silanization for different etching conditions: a) 6 hours, 150°C, b) 4 hours, 150°C, c) 6 hours, 100°C, d) 4 hours, 100°C Fig. 8. Photographs and values of the contact angle of 3μL of a drop deposited on glass surface after grafting of octadecanoic acid after etching in the condition, a) 6h, 150°C, b) 4h,150°C, c) 6h, 100°C, d) 4h, 100°C Fig. 9. a) Transmission Spectra after nanotexturing in the condition of, a) 6 hours, 150°C, b) 4 hours, 150°C, c) 6 hours, 100°C, d) 4 hours, 100°C, B) Variation of weighted transmission according to nanotexturing conditions Fig. 10. Transmission Spectra after grafting of octadecanoic acid, a) on flat, and textured glass surfaces, in the condition of b) 4 hours, 100°C, c) 6 hours, 100°C, d) 4 hours, 150°C, e) 6 hours, 150°C. Fig. 11. Photographs and values of the contact angle of 3μL of a drop of water deposited on the surface of the nanotextured glass after grafting of octadecanoic acid at different concentrations Fig. 12. Transmission spectra of the nanotextured glass at 150°C for 6 hours, A) before and after grafting for different concentrations of octadecanoic acid: a) C1=10-3 M, b) C2=10-2 M, c) C3=510-2 M, d) C4=10-1 M, B) Variation of the weighted transmission for the various grafting concentrations Fig. 13. ATR-FTIR spectra of the grafted glass surface for different concentrations of octadecanoic acid, a) Spectral area1300-1800 cm-1, b) Spectral area 2800-3000 cm-1. Scheme 1. Different steps of functionalization of nanotextured glass layer.

Table 1. Temperature time ranges of sample exposure

Temperature (°C)

Time (hours)

Concentration (mol/l)

100

4

2.5

100

6

2.5

150

4

2.5

150

6

2.5

Table 2. The values of the weighted transmission before and after grafting of octadecanoic acid for different nanotexturing conditions.

Nano-texcturing conditions

Before grafting

After grafting

6 hours, 150°C

77,55%

85,63%

4 hours, 150°C

87,97%

85,32%

6 hours, 100°C

95,79%

97,79%

4 hours, 100°C

95,88%

97,91%