Excellent UV-Light Triggered Photocatalytic Performance of ZnO.SiO2 Nanocomposite for Water Pollutant Compound Methyl Orange Dye

The photocatalytic activity of eco-friendly zinc oxide doped silica nanocomposites, synthesized via a co-precipitation method followed by heat-treatment at 300, 600, and 900 °C is investigated. The samples have been characterized by employing X-ray diffraction method, and further analyzed using the Rietveld Refinement method. The samples show a space group P63mc with hexagonal structure. The prepared composites are tested for their photocatalytic activities for the degradation of methyl orange-based water pollutants under ultra-violet (UV) irradiation using a 125 W mercury lamp. A systematic analysis of parameters such as the irradiation time, pH value, annealing temperatures, and the concentration of sodium hydroxide impacting the degradation of the methyl orange (MO) is carried out using UV-visible spectroscopy. The ZnO.SiO2 nanocomposite annealed at 300 °C at a pH value of seven shows a maximum photo-degradation ability (~98.1%) towards methyl orange, while the photo-degradation ability of ZnO.SiO2 nanocomposites decreases with annealing temperature (i.e., for 600 and 900 °C) due to the aspect ratio. Moreover, it is seen that with increment in the concentration of the NaOH (i.e., from 1 to 3 g), the photo-degradation of the dye component is enhanced from 20.9 to 53.8%, whereas a reverse trend of degradation ability is observed for higher concentrations.


Introduction
To meet the demand and supply of developing countries, numerous small and largescale industries are flourishing. The fast-growing industries produce a large number of toxic chemical wastes due to a lack of proper management and knowledge. The chemical wastes contain several non-biodegradable elements and other water pollutants, which are harmful to both aquatic and human life. Major water pollutants involve dye molecules used in pharmaceuticals, paper, leather, plastic, cosmetics, food, and textile industries [1,2]. It is reported that the yearly generation of the synthetic dye is greater than 7 × 10 5 tons, out of which around 2 × 10 3 tons of dyes are discharged in water bodies only as textile waste [3]. Among many dyes, azo dyes like methyl orange (MO) are used in laboratories as pH indicators, water-soluble dye products in textiles, printing, paper, pharmaceuticals, and food industries [4]. MO displays an orange colour in the basic medium and a red colour in the acidic medium [5]. Due to its high solubility, it is difficult to remove MO from solvents using a conventional treatment system [6]. Drinking MO contaminated piezoelectric nozzle of 0.05 mm and drop rate of 0.02 mL/s) to 50 mL 0.6 M HCl, which resulted in white precipitates of SiO 2 . The obtained suspension was stirred at 8000 rpm for 3 h at 50 • C. Afterward, NaCl was removed from the suspension by washing it several times using distilled water [28]. For the synthesis of the ZnO.SiO 2 nanocomposite, both suspensions were mixed and stirred [28]. The white precipitates were filtered with grade 5 Whatman filter paper and then washed and dried in air. By following the procedure (Scheme 1a) two samples (0.1 M)ZnO.(0.3M)SiO 2 (ZS1) and (0.05 M)ZnO.(0.3 M)SiO 2 (ZS2) were prepared; furthermore, these samples were annealed at the different temperatures of 300, 600, and 900 • C for 2 h. For ease of understanding, the samples annealed at 300, 600, and 900 • C are named as ZS1 300 , ZS1 600 , and ZS1 900 , respectively. Nanomaterials 2021, 11,2548 3 of 17 mL ethanol, following the procedure reported in our previous work [27]. Secondly, the preparation of SiO2 nano-suspension was prepared by mixing 0.3 M SiO2 in 50 mL 0.6 M NaOH, which resulted in Na2SiO3. The prepared solution was drop-wise added (using the piezoelectric nozzle of 0.05 mm and drop rate of 0.02 mL/s) to 50 mL 0.6 M HCl, which resulted in white precipitates of SiO2. The obtained suspension was stirred at 8000 rpm for 3 h at 50 °C. Afterward, NaCl was removed from the suspension by washing it several times using distilled water [28]. For the synthesis of the ZnO.SiO2 nanocomposite, both suspensions were mixed and stirred [28]. The white precipitates were filtered with grade 5 Whatman filter paper and then washed and dried in air. By following the procedure (Scheme 1a) two samples (0.1 M)ZnO.(0.3M)SiO2 (ZS1) and (0.05 M)ZnO.(0.3 M)SiO2 (ZS2) were prepared; furthermore, these samples were annealed at the different temperatures of 300, 600, and 900 °C for 2 h. For ease of understanding, the samples annealed at 300, 600, and 900 °C are named as ZS1300, ZS1600, and ZS1900, respectively. Scheme 1. Schematic diagram of (a) preparation of SiO2.ZnO nanoparticles with the expected morphology (ZnO nanoparticles embedded into SiO2 porous structure) and, (b) photocatalytic activity set-up.

Characterization
For the determination of the crystallite size and lattice constant, the X-ray diffraction (XRD) patterns of the samples were acquired with an X-ray diffractometer (Philips PW/1710), operated at 50 kV and 40 mA in range of 10 to 70 degrees. The XRD data was analyzed through Rietveld refinement for a perfect matching of phases and to determine various lattice parameters, dislocation density, microstrain, Wyckoff positions, bond length, and bond angle, etc. For morphological analysis, transmission electron microscopy was performed on a Hitachi (H-7500) instrument. Fourier transform infrared spectrometer (Bruker compact alpha-II) was used for recording the spectra of nanocomposites in a range of 4000-400 cm −1 using Kbr pallets. The UV-Vis absorption spectra of the prepared samples were acquired with a Lambda 750 spectrophotometer (Perkin) in a wavelength from 300 to 700 nm. The surface area (SBET) was evaluated using N2-adsorption measurements performed on a Quantachrome Instruments at 77.3500 K. Nitrogen and carbon were used as adsorbate and adsorbent, respectively. Scheme 1. Schematic diagram of (a) preparation of SiO 2 .ZnO nanoparticles with the expected morphology (ZnO nanoparticles embedded into SiO 2 porous structure) and, (b) photocatalytic activity set-up.

Characterization
For the determination of the crystallite size and lattice constant, the X-ray diffraction (XRD) patterns of the samples were acquired with an X-ray diffractometer (Philips PW/1710), operated at 50 kV and 40 mA in range of 10 to 70 degrees. The XRD data was analyzed through Rietveld refinement for a perfect matching of phases and to determine various lattice parameters, dislocation density, microstrain, Wyckoff positions, bond length, and bond angle, etc. For morphological analysis, transmission electron microscopy was performed on a Hitachi (H-7500) instrument. Fourier transform infrared spectrometer (Bruker compact alpha-II) was used for recording the spectra of nanocomposites in a range of 4000-400 cm −1 using Kbr pallets. The UV-Vis absorption spectra of the prepared samples were acquired with a Lambda 750 spectrophotometer (Perkin) in a wavelength from 300 to 700 nm. The surface area (S BET ) was evaluated using N 2 -adsorption measurements performed on a Quantachrome Instruments at 77.3500 K. Nitrogen and carbon were used as adsorbate and adsorbent, respectively.

Photocatalytic Activity
The photocatalytic properties of ZnO.SiO 2 nanocomposite were evaluated by testing the photo-degradation of methyl orange (MO) in water using UV irradiation (125 WHg lamp). In a typical reaction, 30 mg ZS1 composite and 10 mL of 30 mg/80 mL of methyl orange dye aqueous solution were stirred using a magnetic stirrer for an hour to establish the adsorption/desorption equilibrium of MO on the surface of catalyst before the irradiated process. For this experiment, two mercury lamps were placed 5 cm away from the magnetic stirrer on which the solution of catalyst, as well as dye, were stirred, as shown in Scheme 1b. The effect of pH (at 7 and 12) on photocatalytic activity was analyzed. The pH of methyl orange dye solution (MODS) was controlled by adding diluted Nitric acid and NaOH. For this analysis, solutions of ZS1 (ZS1 300 , ZS1 600 and ZS1 900 ) and MODS were prepared with pH values of 7 and 12. At consequent intervals of time (i.e., 0 min, 20 min, 40 min, 60 min, 100 min) 2 mL of samples were extracted from each solution and analyzed using UV-Vis spectroscopy. The effect of NaOH conc. on the photo-degradation of dye was also evaluated. For this analysis, solutions of ZS1 300 and MODS were prepared with different conc. of NaOH (1 g, 2 g, 3 g, and 4 g). Afterward, the slurry was not stirred, and the resulting dye content was determined at the end of the experiment. The photo-degradation percentage of methyl orange is calculated using Equation (1) where, A o is the initial concentration of MO and A t is the concentration of MO after time t. The experiment has been repeated again to ensure the reproducibility of the results.

Zero-Point Charge
The value of zero-point charge (ZPC) was calculated with the help of salt titration method. The ZPC value of the catalyst was determined using a 10 mol solution of NaClO 4 . In this method, 0.2 g of catalyst was taken in 10 mL deionized water, and 0.01 mol/L NaOH was added to get the required pH. The suspension was taken in ten different beakers and the pH of the suspensions was initially set to different values from two to 11. This pH value is denoted as pH initial . The sample of each beaker was stirred briskly for 24 h at room temperature, and the value of pH was recorded. This pH value is denoted as pH final . The ZPC is obtained from the plot, between ∆pH (pH final -pH initial ) and pH initial as depicted in Figure 1. For ZPC, ∆pH needs to be zero. The ZPC of ZS1 300 is investigated and its value is found as 8 [30].

Photocatalytic Activity
The photocatalytic properties of ZnO.SiO2 nanocomposite were evaluated by testing the photo-degradation of methyl orange (MO) in water using UV irradiation (125 WHg lamp). In a typical reaction, 30 mg ZS1 composite and 10 mL of 30 mg/80 mL of methyl orange dye aqueous solution were stirred using a magnetic stirrer for an hour to establish the adsorption/desorption equilibrium of MO on the surface of catalyst before the irradiated process. For this experiment, two mercury lamps were placed 5 cm away from the magnetic stirrer on which the solution of catalyst, as well as dye, were stirred, as shown in Scheme 1b. The effect of pH (at 7 and 12) on photocatalytic activity was analyzed. The pH of methyl orange dye solution (MODS) was controlled by adding diluted Nitric acid and NaOH. For this analysis, solutions of ZS1 (ZS1300, ZS1600 and ZS1900) and MODS were prepared with pH values of 7 and 12. At consequent intervals of time (i.e., 0 min, 20 min, 40 min, 60 min, 100 min) 2 mL of samples were extracted from each solution and analyzed using UV-Vis spectroscopy. The effect of NaOH conc. on the photo-degradation of dye was also evaluated. For this analysis, solutions of ZS1300 and MODS were prepared with different conc. of NaOH (1 g, 2 g, 3 g, and 4 g). Afterward, the slurry was not stirred, and the resulting dye content was determined at the end of the experiment. The photo-degradation percentage of methyl orange is calculated using Equation (1) [29] Photo − degradation (removal efficiency) = A − A A × 100 (1) where, AO is the initial concentration of MO and At is the concentration of MO after time t. The experiment has been repeated again to ensure the reproducibility of the results.

Zero-Point Charge
The value of zero-point charge (ZPC) was calculated with the help of salt titration method. The ZPC value of the catalyst was determined using a 10 mol solution of NaClO4. In this method, 0.2 g of catalyst was taken in 10 mL deionized water, and 0.01 mol/L NaOH was added to get the required pH. The suspension was taken in ten different beakers and the pH of the suspensions was initially set to different values from two to 11. This pH value is denoted as pHinitial. The sample of each beaker was stirred briskly for 24 h at room temperature, and the value of pH was recorded. This pH value is denoted as pHfinal. The ZPC is obtained from the plot, between ΔpH (pHfinal-pHinitial) and pHinitial as depicted in Figure 1. For ZPC, ΔpH needs to be zero. The ZPC of ZS1300 is investigated and its value is found as 8 [30].  The value of ZPC implies that surface of the catalyst is positively charged at pH < pH ZPC , and for pH > pH ZPC will be negatively charged. Therefore, ZPC is considered an important feature for photocatalytic activity as it affects the surface property of catalyst. Under basic pH conditions, the negatively charged pollutant compounds will be repelled from the negatively charged catalyst surface, which results in to decrease in absorption of the pollutant compound. As a result, the degradation efficiency of the sample falls at a higher pH [31].

XRD Analysis
The XRD patterns of ZS1 300 , ZS1 600 and ZS1 900 samples are shown in Figure 2A. It can be observed from Figure 2A [32][33][34]. In both diffraction patterns, a broad hump is present at 2θ~21.98 • , which reveals the presence of amorphous silica. The observed peaks in in the diffraction pattern reveal the formation of the hexagonal structure of ZnO embedded in silica matrix, with the space group P63mc (JCPDS card no. 790205). The value of ZPC implies that surface of the catalyst is positively charged at pH < pHZPC, and for pH > pHZPC will be negatively charged. Therefore, ZPC is considered an important feature for photocatalytic activity as it affects the surface property of catalyst. Under basic pH conditions, the negatively charged pollutant compounds will be repelled from the negatively charged catalyst surface, which results in to decrease in absorption of the pollutant compound. As a result, the degradation efficiency of the sample falls at a higher pH [31].

XRD Analysis
The XRD patterns of ZS1300, ZS1600 and ZS1900 samples are shown in Figure 2A. It can be observed from Figure 2A [32][33][34]. In both diffraction patterns, a broad hump is present at 2θ~21.98°, which reveals the presence of amorphous silica. The observed peaks in in the diffraction pattern reveal the formation of the hexagonal structure of ZnO embedded in silica matrix, with the space group P63mc (JCPDS card no. 790205). The value of structural parameters like plane (hkl), crystallite size (D), d-spacing, full width at half maximum (β), intensity, microstrain (ε) and the dislocation density (δ) of ZnO.SiO2 composite are obtained from XRD analysis and are shown in Table S1. The crystallite size (D) is calculated by using Scherrer's formula as: The average crystallite size is calculated from the three peaks of highest intensity in the pattern, which was characterized by lattice plane (100), (002) and (101) at 2θ ~ 31.85, 34.55 and 36.35°, respectively inserted in Table S1. Dislocation density (δ) is calculated as [35]: However, the XRD pattern of the sample ZS1 900 also contained some new peaks (Figure 2A The value of structural parameters like plane (hkl), crystallite size (D), d-spacing, full width at half maximum (β), intensity, microstrain (ε) and the dislocation density (δ) of ZnO.SiO 2 composite are obtained from XRD analysis and are shown in Table S1. The crystallite size (D) is calculated by using Scherrer's formula as: The average crystallite size is calculated from the three peaks of highest intensity in the pattern, which was characterized by lattice plane (100), (002) and (101) at 2θ~31.85, Nanomaterials 2021, 11, 2548 6 of 17 34.55 and 36.35 • , respectively inserted in Table S1. Dislocation density (δ) is calculated as [35]: and microstrain (ε) value of the nanocomposite ZnO.SiO 2 is evaluated as, The Rietveld refined X-ray diffractogram of ZS1 300 nanocomposite is shown in Figure 2B. For the quantitative analysis of the phase present in the sample, Rietveld refinement was carried out of the XRD data. The atomic position and isothermal parameters for Zinc and Oxygen are fixed and the other parameters like lattice constants, shape parameters, and scale factors are considered as free parameters during the time of fitting. The parameters, such as background factors and scale factors, are refined in the first step of the Rietveld refinement. The background is fitted using a linear interpolation method and pseudovoigt is chosen for peak profile. In the succeeding stage, the structural parameters (lattice parameters, width parameter, preferred orientation, asymmetry, and atomic coordinates, etc.) are refined. Figure 2B approves the hexagonal structure of ZnO embedded in SiO 2 . The Wyckoff positions parameters x a , y b , z c of atoms obtained from refinement are tabulated in Table S2. The norm's factor for the best fit, such as the Reliability factors (R factors), Profile R factor (R B = 13.6), Weighted R Factor (R wp = 14.3), Expected values (R exp = 7.17), Bragg R Factor (R B = 6.02), R f -Factor (R B = 2.78) and Goodness fit factor (χ 2 = 3.9) value obtained from Rietveld refinement, confirm the precision of the Rietveld refinement analysis. The value of lattice parameters obtained from the refinement are as follows: a = b = 3.249 Å ; c = 5.2058 Å ; α = 90 • ; γ = 120 o , and ratio of c/a is 1.6022. The bond lengths between different atoms and bond angles between them are evaluated and presented in Table S3.
The type of the grain boundaries may play an important role in the photocatalytic mechanism. The grain boundary specific area (S a ) i.e., the ratio of the area enclosed by the grain boundary and volume of grain, is estimated as [36] S a = 1.65 D where, D is the crystallite size estimated from XRD studies using Equation (2). The Equation (5) is valid for both single and polycrystalline forms of ZnO system having a random distribution of shapes and grains. The values of S a are tabulated in Table 1. It is observed that the grain boundary-specific area is decreased with an increase in temperature.
The higher value i.e., 4.445 × 10 7 m 2 m −3 of S a is calculated for the ZS1 sample annealed at 300 • C. With a change in the precursor's concentration i.e., sample ZS2, it is observed from the XRD patterns of ZS2 300 and ZS2 600 for the ZnO phase that the crystallite size decreased compared to ZS1 300 and ZS1 600 , respectively. The XRD data for ZS2 300 and ZS2 600 are provided in the Supplementary File. The diffractogram of ZS2 900 was found to have marginal changes compare to ZS1 900 . The phase structure observed for ZS2 samples is the same as observed in the case of ZS1 samples.

TEM Analysis
Transmission electron microscopy (TEM) is a utile technique for the analysis of particle size and the morphology of materials. The microscopic images of ZnO.SiO 2 nanocomposites annealed at 300 • C are depicted in Figure 3. The average particles size is found to vary between 20 to 30 nm, which is nearly analogous to the crystallite size calculated from the XRD results. From the figure, a uniform morphology has been observed in which the particles are appearing roughly as spherical shapes. The micrograph shows the weak aggregation of ZnO particles that confirm the presence of particles in the silica matrix. Some particles are observed to possess sharp grain boundaries and a clear morphology in the silica matrix.

TEM Analysis
Transmission electron microscopy (TEM) is a utile technique for the analysis of particle size and the morphology of materials. The microscopic images of ZnO.SiO2 nanocomposites annealed at 300 °C are depicted in Figure 3. The average particles size is found to vary between 20 to 30 nm, which is nearly analogous to the crystallite size calculated from the XRD results. From the figure, a uniform morphology has been observed in which the particles are appearing roughly as spherical shapes. The micrograph shows the weak aggregation of ZnO particles that confirm the presence of particles in the silica matrix. Some particles are observed to possess sharp grain boundaries and a clear morphology in the silica matrix.

FTIR Analysis
For the further investigation of the molecular structures of ZS1300, ZS1600 and ZS1900 nanocomposites, Fourier-transform infrared (FTIR) spectroscopy studies are carried out and the FTIR spectra of these samples are shown in Figure 3.
Characteristic peaks of these samples are observed at 1081 and 799 cm −1 , which represent the stretching vibration mode and symmetric stretching of the Si-O-Si group, respectively [26]. The peak at around 475 cm −1 in the spectra is due to the vibration of the Zn, and the O bond confirms the presence of ZnO in the samples [37]. The presence of bonds at 1500 cm −1 and 1632 cm −1 are attributed to the stretching vibration of the (−OH) group attached with Si and ZnO, respectively [38]. The hydroxyl group results from the hygroscopic nature of ZnO and Si that come during the synthesis. It is also observed that the content of hydroxyl group in the sample decreased with an increase in temperature, suggesting the presence of impurities mostly near the surfaces of ZnO and Si. Moreover, it can be found that the peak at 1081.98 of ZS1300 sample had a strong intensity. However, for ZS1600 and ZS1900, the peak intensity decreased due to the decomposition of hydroxyl group Figure 4a

FTIR Analysis
For the further investigation of the molecular structures of ZS1 300 , ZS1 600 and ZS1 900 nanocomposites, Fourier-transform infrared (FTIR) spectroscopy studies are carried out and the FTIR spectra of these samples are shown in Figure 3.
Characteristic peaks of these samples are observed at 1081 and 799 cm −1 , which represent the stretching vibration mode and symmetric stretching of the Si-O-Si group, respectively [26]. The peak at around 475 cm −1 in the spectra is due to the vibration of the Zn, and the O bond confirms the presence of ZnO in the samples [37]. The presence of bonds at 1500 cm −1 and 1632 cm −1 are attributed to the stretching vibration of the (−OH) group attached with Si and ZnO, respectively [38]. The hydroxyl group results from the hygroscopic nature of ZnO and Si that come during the synthesis. It is also observed that the content of hydroxyl group in the sample decreased with an increase in temperature, suggesting the presence of impurities mostly near the surfaces of ZnO and Si. Moreover, it can be found that the peak at 1081.98 of ZS1 300 sample had a strong intensity. However, for ZS1 600 and ZS1 900 , the peak intensity decreased due to the decomposition of hydroxyl group Figure 4a

UV-Visible Analysis
For the examination of the optical band gap of the ZS1300, ZS1600 and ZS1900 samples, UV-Visible spectroscopy is performed, as shown in Figure 5. It is seen that band edge shifted to a lower wavelength edge (i.e., blue shift), suggesting an alteration in the optical band gap. Thereby, to estimate the energy band gap (Eg) of the catalyst samples, Tauc's relation the spectra (250-700 nm) is used, which is given by ( ℎ ) 1/ = (ℎ − ) (6) where , ℎ, and are the absorption coefficients, Planks constant, power factor for the transition mode, and constant, respectively. For the allowed direct transitions, is taken as 1/2. The band gap values for the nanocomposites samples are determined by plotting ( ℎ ) 1/ versus ℎ , as shown in Figure 5. The estimated values of Eg for the ZS1300, ZS1600, and ZS1900 nanocomposites are 3.26 eV, 3.25 eV, and 3.36 eV, respectively. The optical band gap of ZS1300 is found to be smaller than the characteristic value of ZnO, i.e., 3.37 eV at room temperature. This may be due to the occurrence of intrinsic defects like oxygen vacancies and zinc interstitials in the ZnO

UV-Visible Analysis
For the examination of the optical band gap of the ZS1 300 , ZS1 600 and ZS1 900 samples, UV-Visible spectroscopy is performed, as shown in Figure 5. It is seen that band edge shifted to a lower wavelength edge (i.e., blue shift), suggesting an alteration in the optical band gap. Thereby, to estimate the energy band gap (E g ) of the catalyst samples, Tauc's relation the spectra (250-700 nm) is used, which is given by where α, h, n and A are the absorption coefficients, Planks constant, power factor for the transition mode, and constant, respectively. For the allowed direct transitions, n is taken as 1/2. The band gap values for the nanocomposites samples are determined by plotting (αhν) 1/n versus hν, as shown in Figure 5.

UV-Visible Analysis
For the examination of the optical band gap of the ZS1300, ZS1600 and ZS1900 samples, UV-Visible spectroscopy is performed, as shown in Figure 5. It is seen that band edge shifted to a lower wavelength edge (i.e., blue shift), suggesting an alteration in the optical band gap. Thereby, to estimate the energy band gap (Eg) of the catalyst samples, Tauc's relation the spectra (250-700 nm) is used, which is given by ( ℎ ) 1/ = (ℎ − ) (6) where , ℎ, and are the absorption coefficients, Planks constant, power factor for the transition mode, and constant, respectively. For the allowed direct transitions, is taken as 1/2. The band gap values for the nanocomposites samples are determined by plotting ( ℎ ) 1/ versus ℎ , as shown in Figure 5. The estimated values of Eg for the ZS1300, ZS1600, and ZS1900 nanocomposites are 3.26 eV, 3.25 eV, and 3.36 eV, respectively. The optical band gap of ZS1300 is found to be smaller than the characteristic value of ZnO, i.e., 3.37 eV at room temperature. This may be due to the occurrence of intrinsic defects like oxygen vacancies and zinc interstitials in the ZnO The estimated values of E g for the ZS1 300 , ZS1 600 , and ZS1 900 nanocomposites are 3.26 eV, 3.25 eV, and 3.36 eV, respectively. The optical band gap of ZS1 300 is found to be Nanomaterials 2021, 11, 2548 9 of 17 smaller than the characteristic value of ZnO, i.e., 3.37 eV at room temperature. This may be due to the occurrence of intrinsic defects like oxygen vacancies and zinc interstitials in the ZnO [36,39]. The ZS1 900 sample has a higher optical band gap (very near to 3.37 eV) which attributes to an increase in crystallite size, as can be seen from XRD studies. The observed values of E g w.r.t. crystallite size are tabulated in Table 1.

BET Analysis
Surface area is an important parameter that affects photocatalytic activity due to the surface dependency of the process. With a larger surface area, the active sites are larger and with better photocatalytic activities. Therefore, to measure the surface and porosity of the ZS1 300 and ZS1 900 nanocomposite samples, the Brunauer-Emmett-Teller (BET) measurements are performed in a nitrogen environment. The nitrogen (N 2 ) adsorptiondesorption isotherm curves of the ZS1 300 sample are shown in Figure 6a. The shape of adsorption-desorption curves depicts the features of isotherm of type IV (as per IUPAC classification) with H4 hysteresis, which reflects the mesoporous kind nature of the samples under discussion. The BET curve having three main segments, i.e., (i) concave-shaped for relative pressures from 0.05 to 0.1, (ii) linear-shaped up to 0.3, and (iii) convex-shaped for above 0.3, the (ii) or linear-shaped segment, as shown in inset of Figure 6a, denotes a realization of the first monolayer of the gas molecules and commencement of the second layer. The linear segment of the BET curve is known as the 'B' point, which is further used to calculate the specific surface area. From the BET equation: whereas, w denotes the volume of adsorbed gas at normal temperature (T) and pressure (P); w m represents the volume of adsorbed gas for monolayer per gram of adsorbent; P • denotes saturated vapor pressure of N 2 at normal temperature; C refers to a constant depending on the H a (heat absorbed by the monolayer of N 2 ); and H L (liquefaction parameter of the gas) in the range of 0.05-0.3 of relative pressure, given by flowing relation can be seen that the composite contains a total pore volume of 0.01667 cm 3 /g and an average pore diameter of 3.838 nm. The ZS1300 nanocomposite shows a higher specific surface area and it is reported that a higher surface area is advantageous for superior photocatalytic performance [40].

Photocatalytic Studies
Absorption of UV light with energy greater than the band gap of the catalyst material causes generation of electron-hole pairs in ZnO particles i.e., electrons (e − ) in conduction band and holes (h + ) in the valence band holes. The holes react with H2O or hydroxide ions, which are adsorbed on the surface of the ZnO particles and produce • OH. Conversely, the The Equation (7) represents an equation of a straight line in between p/P 0 w(1 -P/P 0 ) vs. P P 0 with slope C−1 w m C and intercept of 1 w m C having the values of 206.87578 g/cm 3 and 0.0088 g/cm 3 , respectively. The volume of the gas molecules monolayer (w m ) can be obtained using the values of slope and intercept of the curve in the straight-line portion of the isotherm. The specific surface area can be calculated using the relation, S BET = w m A m N, where N is the Avogadro number and A m is the molecular cross-sectional area (for nitrogen; A m = 0.162 nm 2 ). The measured values of the specific surface areas (S BET ) for the samples ZS1 300 & ZS1 900 are found to be 16.8 m 2 /g and 0.001 m 2 /g, respectively. The decrease in specific surface area with annealing temperature can be attributed to the penetration of ZnO nanocrystallites into the porous structure of SiO 2 . The filling of the closed pores of SiO 2 with ZnO resulted in the decrease in porosity of the nanocomposite samples. This can easily be predicted and corroborated with the change in XRD pattern from ZS1 300 to ZS1 900 , wherein the XRD pattern of ZS1 300 is vastly matched with the pure ZnO hexagonal structure. On the flip side, the XRD pattern of ZS1 900 consists of the sharps reflections of ZnO along with the well-visualized reflection of SiO 2 , which points to the mixed phase of the nanocomposite.
The respective pore size distribution curve for the sample under discussion is obtained with the adsorption particulars employing the Barrett-Joyner-Halenda (BJH) method and depicted in Figure 6b. This figure confirms the mesoporous nature of the prepared composite. The insets depict the change in quantity adsorbed as a function of the deposited layer thickness and the change in layer thickness with relative pressure. It can be seen that the composite contains a total pore volume of 0.01667 cm 3 /g and an average pore diameter of 3.838 nm. The ZS1 300 nanocomposite shows a higher specific surface area and it is reported that a higher surface area is advantageous for superior photocatalytic performance [40].

Photocatalytic Studies
Absorption of UV light with energy greater than the band gap of the catalyst material causes generation of electron-hole pairs in ZnO particles i.e., electrons (e − ) in conduction band and holes (h + ) in the valence band holes. The holes react with H 2 O or hydroxide ions, which are adsorbed on the surface of the ZnO particles and produce • OH. Conversely, the electron reduces O 2 and produces O 2 •− as well as other oxygen species (H 2 O 2 and • OH) [32,33]. Both the holes and the • OH are super reactive to organic compounds that are in contact with these radicals [33]. The oxidizing tendency of the • OH radicals is high enough to split C-H as well as C-C bonds of MO sitting on the outer surface of ZnO.SiO 2 nanocomposite leading to CO 2 and H 2 O production as shown in Scheme 2.
The ability of photocatalyst to decompose the pollutant compound MO, depends upon the different factors, such as the annealing temperature of the photocatalyst, the pH value of solution containing MO dye, and the time of irradiation and different conc. of NaOH. The effect of these factors on the catalytic performance of the material is examined. For an investigation of the effect of pH conc. and annealing temperatures of catalytic material on MO dye, samples of MO and ZS1 (annealed at 300, 600 & 900 • C) were prepared distinctly with different pH concentrations. The samples' names are abbreviated as S1 (ZS1 300 , pH7), S2 (ZS1 300 , pH12), S3 (ZS1 600 , pH7), S4 (ZS1 600 , pH12), S5 (ZS1 900 , pH7), and S6 (ZS1 900 , pH12). The UV-visible spectra of each sample showing degradation of MO w.r.t. irradiation time, depicted in Figure 7.
which are adsorbed on the surface of the ZnO particles and produce • OH. Conversely, the electron reduces O2 and produces O2 •-as well as other oxygen species (H2O2 and • OH) [32,33]. Both the holes and the • OH are super reactive to organic compounds that are in contact with these radicals [33]. The oxidizing tendency of the • OH radicals is high enough to split C-H as well as C-C bonds of MO sitting on the outer surface of ZnO.SiO2 nanocomposite leading to CO2 and H2O production as shown in Scheme 2. Scheme 2. Schematic illustration of the photocatalytic mechanism of ZnO.SiO2 photocatalyst for degradation of MO.  Figure 8a). Interestingly, it is observed that the photocatalytic activity of the S1 sample is relatively higher than the other samples. As the value of pH is increased, the degradation response is decreased and only 39.7 % of MO degradation response is observed for sample S2 (i.e., at pH = 12). The photo-degradation activity depends on the surface properties of the catalyst. Generally, pH changes the surface properties of catalysts like surface charge properties that oversee the absorption mechanism of MO on catalytic surface. At a pH 7 (below ZPC pH i.e., 8), the surface of ZnO is positively charged, which can absorb negatively charged MO due to electrostatic attraction, facilitating the degradation reaction. In contrast, at a higher pH (=12), absorption is decreased due to electrostatic repulsive forces among the catalytic surface and the dyed surface. As a result, photo-degradation response becomes minuscule; this is firstly because of the decrease in adsorption of the dye molecules on surface of the photocatalyst surface at higher pH [41,42], and secondly due to rapid scavenging of the hydroxyl radicals produced via reaction between holes and OH − , which causes a decrease in free radicals [42,43]. It is reported that pH affects the production of these free radicals that, in turn, lower the degradation rate of dye, or the termination of degradation [31].
It is observed that on increasing the irradiation time under light irradiating lamps, the photo-degradation percentage of methyl orange also increased, as shown in Figure 8b. It is observed from Figure 8b that all the samples attained maximum photo-degradation (%) in about 100 min. This may be due to the production of more and more e-h pairs and the corresponding excitation of an electron from the valence band to the conduction band [44]. Accordingly, the production of hydroxyl radicals is rose, which resulted in an enhanced photodegradation (%). The higher degradation efficiency of the ZS1 300 nanocomposite sample, as compared to other samples, can be attributed to its small band gap and larger surface area.
NaOH. The effect of these factors on the catalytic performance of the material is examined. For an investigation of the effect of pH conc. and annealing temperatures of catalytic material on MO dye, samples of MO and ZS1 (annealed at 300, 600 & 900 °C) were prepared distinctly with different pH concentrations. The samples' names are abbreviated as S1 (ZS1300, pH7), S2 (ZS1300, pH12), S3 (ZS1600, pH7), S4 (ZS1600, pH12), S5 (ZS1900, pH7), and S6 (ZS1900, pH12). The UV-visible spectra of each sample showing degradation of MO w.r.t. irradiation time, depicted in Figure 7.  contrast, at a higher pH (=12), absorption is decreased due to electrostatic repulsive forces among the catalytic surface and the dyed surface. As a result, photo-degradation response becomes minuscule; this is firstly because of the decrease in adsorption of the dye molecules on surface of the photocatalyst surface at higher pH [41,42], and secondly due to rapid scavenging of the hydroxyl radicals produced via reaction between holes and OH − , which causes a decrease in free radicals [42,43]. It is reported that pH affects the production of these free radicals that, in turn, lower the degradation rate of dye, or the termination of degradation [31]. It is observed that on increasing the irradiation time under light irradiating lamps, the photo-degradation percentage of methyl orange also increased, as shown in Figure 8b. It is observed from Figure 8b that all the samples attained maximum photo-degradation (%) in about 100 min. This may be due to the production of more and more e-h pairs and the corresponding excitation of an electron from the valence band to the conduction band [44]. Accordingly, the production of hydroxyl radicals is rose, which resulted in an enhanced photodegradation (%). The higher degradation efficiency of the ZS1300 nanocomposite sample, as compared to other samples, can be attributed to its small band gap and larger surface area.

Effect of Annealing Temperatures
The photocatalytic efficiency of the ZS1 nanocomposite at different annealing temperatures has also been assessed for the photo-degradation of MO. Figure 9 shows the correlation between photo-degradation (%) of MO with annealing temperatures of ZS1 nanocomposite. It is observed that the photocatalytic degradation of MO decreases from

Effect of Annealing Temperatures
The photocatalytic efficiency of the ZS1 nanocomposite at different annealing temperatures has also been assessed for the photo-degradation of MO. Figure 9 shows the correlation between photo-degradation (%) of MO with annealing temperatures of ZS1 nanocomposite. It is observed that the photocatalytic degradation of MO decreases from 98.1% to about 39% with an increase in the annealing temperature of the sample ZS1 (i.e., sample ZS1 300 to ZS1 900 . The highest activity observed for the ZS1 300 nanocomposite may be ascribed to the higher specific surface area, lower band gap value, and efficient charge separation ability of the sample through the charge exchange process in the existence of UV light. The lowest photo-degradation ability of the sample ZS1 900 can be due to the decrease in specific surface area due to the filling of the pore by the penetration of ZnO into the SiO 2 porous matrix, as observed from the XRD and BET studies. The lower reactivity of SiO 2 towards photo-degradation and the presence of higher content of SiO 2 on the upper surface as expected from XRD pattern could be another reason for the lower efficiency of ZS1 900 sample. 98.1% to about 39% with an increase in the annealing temperature of the sample ZS1 (i.e., sample ZS1300 to ZS1900. The highest activity observed for the ZS1300 nanocomposite may be ascribed to the higher specific surface area, lower band gap value, and efficient charge separation ability of the sample through the charge exchange process in the existence of UV light. The lowest photo-degradation ability of the sample ZS1900 can be due to the decrease in specific surface area due to the filling of the pore by the penetration of ZnO into the SiO2 porous matrix, as observed from the XRD and BET studies. The lower reactivity of SiO2 towards photo-degradation and the presence of higher content of SiO2 on the upper surface as expected from XRD pattern could be another reason for the lower efficiency of ZS1900 sample. It is reported that in the photocatalytic mechanism, the production of • OH is expected to occur mainly on an internal surface i.e., the ZnO surface [45,46]. In addition, MO approaches the ZnO surface via the pores present in the surface area of sample ZS1. Thus, the photonic degradation efficiency is being limited by the absorption or diffusion rate of It is reported that in the photocatalytic mechanism, the production of • OH is expected to occur mainly on an internal surface i.e., the ZnO surface [45,46]. In addition, MO approaches the ZnO surface via the pores present in the surface area of sample ZS1. Thus, the photonic degradation efficiency is being limited by the absorption or diffusion rate of the MO molecule into the photo-catalytically active surface. As a result of the difference in the size and the number of pores of the ZS1 samples (ZS1 300 , ZS1 600 & ZS1 900 ,), the photo-degradation (%) is changed accordingly. Therefore, it is concluded that the ZS1 300 assists the transport properties of all reactants involved in the photocatalytic process, which enhanced the overall photocatalytic activity.

Effect of NaOH Concentration
To investigate the effect of NaOH concentration on the photo-degradation ability of the nanocomposite samples, four different concentrations (1, 2, 3 and 4 g) of NaOH are mixed with the photocatalyst and MO solution. In a typical process, 0.38 g of catalyst powder was dispersed in 100 mL deionized water and then 10 mL of methyl orange dye solution was added to catalyst solution and stirred to adsorb the dye molecules on surface of the photocatalyst. After that, 1 g of NaOH powder was mixed with the above solution containing methyl orange and photocatalyst, then the solution was stirred in absence of light to achieve equilibrium between adsorption and desorption and then irradiated with visible irradiation light with continuous stirring [47]. After the decoloration of methyl orange, a part of the solution was used for recording the absorbance spectrum using a UV-Visible spectrophotometer and photo-degradation percentage was calculated using Equation (1). The same steps are followed for the degradation of MO with different weights (2, 3 and 4 g) of NaOH and the recorded absorption spectra are shown in Figure 10a. Visible spectrophotometer and photo-degradation percentage was calculated using Equation (1). The same steps are followed for the degradation of MO with different weights (2, 3 and 4 g) of NaOH and the recorded absorption spectra are shown in Figure 10a. It is observed that the decoloration time of methyl orange decreases from 275 through 95 to 40 min with an increase in NaOH concentration from 1 g through 2 g to 3 g. However, for the 4 g of NaOH, again the decoloration time increases to 80 min. It is observed that the sample containing 3 g concentration of NaOH shows a higher photo-degradation of 53.8 %, and beyond 3 g NaOH concentration again the photo-degradation (%) decreases to 27.5%, with 4 g of NaOH (Figure 10b). The results showed that the photo-degradation rate increased with an increase in NaOH concentration up to 3 g of NaOH, which may be due to OH − ions produced by NaOH acting as a strong hole scavenger and generating hydroxyl radicals, thus reducing the e-h pair recombination [45]. Moreover, with a further increase in concentration (>3 g) of NaOH, the degradation rate became poor, since a greater amount of hydroxyl radicals may oxidize the catalyst [45].
Compared to other metal oxide composites tabulated in Table 2, the present ZnO.SiO2 prepared through the co-precipitation method shows enhanced photocatalytic efficiency for the degradation of MO.  It is observed that the decoloration time of methyl orange decreases from 275 through 95 to 40 min with an increase in NaOH concentration from 1 g through 2 g to 3 g. However, for the 4 g of NaOH, again the decoloration time increases to 80 min. It is observed that the sample containing 3 g concentration of NaOH shows a higher photo-degradation of 53.8 %, and beyond 3 g NaOH concentration again the photo-degradation (%) decreases to 27.5%, with 4 g of NaOH (Figure 10b). The results showed that the photo-degradation rate increased with an increase in NaOH concentration up to 3 g of NaOH, which may be due to OH − ions produced by NaOH acting as a strong hole scavenger and generating hydroxyl radicals, thus reducing the e-h pair recombination [45]. Moreover, with a further increase in concentration (>3 g) of NaOH, the degradation rate became poor, since a greater amount of hydroxyl radicals may oxidize the catalyst [45].
Compared to other metal oxide composites tabulated in Table 2, the present ZnO.SiO 2 prepared through the co-precipitation method shows enhanced photocatalytic efficiency for the degradation of MO.

Conclusions
ZnO.SiO 2 nanocomposites are successfully synthesized via the co-precipitation method. The specific surface area decreases from 16.8 m 2 /g and 0.001 m 2 /g on increasing the annealing temperature from 300 to 900 • C due to blocking of SiO 2 pores via the penetration of ZnO into these pores. It is found that the specific surface area of the nanocomposite plays an important role in photocatalytic activities and the degradation of methyl orange dye. The effect of various parameters, such as the pH value of the solution, irradiation time, annealing temperature, and NaOH concentration on photocatalytic performance of ZS1 nanocomposite have been investigated. The sample ZS1 300 shows a higher photodegradation efficiency of 98.1% at pH 7 for 100 min irradiation times. The high efficiency of ZS1 300 can be governed by the following mechanisms: (i) a higher specific area that assures the maximum interaction with UV irradiation; (ii) an optimized bandgap resulting in enhanced absorption efficiency of UV irradiation where more electron-hole pairs are produced; (iii) more irradiation time proportional to more generation electron-hole pairs; and (iv) at lower pH below the ZPC pH, which increases the absorption of pollutant dye that resulting higher efficiency. OH − radicals act as active species for the degradation of the pollutant. For the degradation, the formation of hole and hydroxyl radicals is proven to be the crucial factor. The results revealed that ZnO.SiO 2 nanocomposite is an effective, inexpensive, and eco-friendly catalyst for the degradation of methyl organic pollutants.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/nano11102548/s1, Table S1: Structural parameters of ZS1300 achieved from X-ray Diffraction Pattern, Table S2: Fractional atomic coordinates and isothermal parameter of different atoms obtained from the Rietveld anal-ysis of XRD patterns for the sample ZS1300, Table S3: Bond Length and bond angles of sample ZS1300, Table S4: Structural parameters of ZS1600 achieved from X-ray Diffraction Pattern, Table S5: Structural parameters of ZS1900 achieved from X-ray Diffraction Pattern, Table S6: Structural parameters of ZS2300 achieved from X-ray Diffraction, Table S7: Structural parameters of ZS2600 achieved from X-ray Diffraction Pattern.