The Effect of Zinc Oxide Supported on Gelatin Mesoporous Silica (GSBA-15) on Structural Character and Their Methylene Blue Photodegradation Performance

Gelatin mesoporous silica SBA-15 (GSBA-15) with rod-like morphology has been successfully synthesized by hydrothermal method using P-123:gelatin, then aged at 90 °C for 24 h and calcined at 550 °C for 5 h. GSBA-15 was impregnated with ZnO amounts of 1; 5; and 10 wt% to obtain Zn/GSBA-15. Samples were characterized by X-ray Diffraction (XRD), Fourier Transform Infra Red (FTIR), Scanning Electron Microscopy (SEM), and Brunauer-Emmett-Teller (BET). The efficiency of methylene blue photodegradation was determined by a UV-Vis spectrophotometer. The FTIR result is functional groups of ZnO/GSBA- 15, those were Si−O−Si, −OH, Zn−OH, and Zn−O. The morphology of ZnO/GSBA-15 was rod-like, and it consisted of silica, oxygen, and Zn. The surface area and pore volume of GSBA-15 declined (surface area from 520.8 to 351.9 m 2 /g and pore volume from 0.707 to 0.564 cm 3 /g) after ZnO impregnation due to pore blocking. At the same time, increasing pore diameter (from 2.82 nm to 3.19 nm) and crystallite size (from 5.1 nm to 12.6 nm) were observed due to the overlapping of ZnO-Silica particles. The increasing incorporation of ZnO on the silica GSBA-15 framework increases the photodegradation performance from 88.76% to 94.90% due to the high surface area, functional group rich, and dispersion of ZnO active sites. Effect on and Their Blue Photodegradation


Introduction
One of the textile industry wastes is dye waste, usually containing organic compounds that are difficult to degrade biologically, such as methylene blue [1,2]. The methylene blue is a toxic aromatic hydrocarbon compound (C16H18ClN3S), with the permissible concentration threshold in waters being 5-10 mg/L [3,4] sive to manufacture, environmentally friendly, and can be made under standard reaction conditions with a band gap of 3.17 eV [12]. Previous research explained that ZnO catalysts have degradation performance for methylene blue [13][14][15]. However, metal oxide as the catalytic center has undergone agglomeration, reducing the catalytic performance [16]. To overcome this matter, the current research focus involves host or supporting materials for metal catalysts such as carbon, silica, and polymers [17][18][19].
One of the supporting materials widely employed as a ZnO host is mesoporous silica. Its pore size can be adjusted from 2 to 50 nm by template engineering and optimization of reaction conditions [3,4,16]. The other uniqueness of mesoporous silica is that it is inert and has a highly ordered structure, a large pore size, a large specific surface area, thick frame walls, high thermal stability. Moreover, its size can be controlled easily [20]. The use of mesoporous silica as a host of ZnO in the degradation process of methylene blue dyes has been widely studied, such as ZnO/MCM-41 and ZnO/mesoporous silica, which have higher degradation activity than ZnO [3,4]. However, the synthesis of mesoporous silica requires synthetic soft templates, such as CTAB, P123, or F127 [21 -23], which have high prices with low levels of sustainability, and their production is complicated and involves many chemicals, some of which require advanced handling at high costs. Several researchers overcame the problem by replacing the use of synthetic templates with environmentally friendly organic materials such as natural gelatin, starch, and gum arabic [23][24][25].
The previous research has combined gelatin with a synthetic template to produce carbon foam worm-holes, mesoporous carbon microspheres, and hexagonal flake-like hematite with a high surface area of 90 to 220 m 2 /g with pore size up to 5-10 nm [11,26,27]. The use of gelatin which contains amine (−NH2) groups that have a high affinity to interact strongly with hydroxyl (OH) by hydrogen bonds, gives advantages as a pore directing agent to produce the unique material in the previous reports [28][29][30][31]. So, in this study, the mesoporous silica was synthesized using gelatin. To minimize the synthetic organic template, ZnO metal oxide was impregnated into resulted mesoporous silica from gelatin template (GSSBA-15), and the photodegradation of methylene blue performance was investigated.

Synthesis of mesoporous silica GSBA-15
Synthesis of mesoporous silica applied gelatin as a template labeled as GSBA-15. It was prepared by adding 4 g of P123 and 0.04 g of gelatin to 19.5 mL of 37% HCl in 127 mL of distilled water. It was then stirred at 40 °C (500 rpm) for 3 h. The 8.62 g of TEOS was added to the mixture, and the stirring was continued for 24 h. The resulting mixture was carried out for hydrothermal process at 90 °C for 24 h in autoclave with Teflon liner (Figure 1). The resulting sample was filtered, washed, and dried for 24 h at 70 °C and followed by a second drying step at 100 °C for 24 h. Finally, the dry sample was calcined in a furnace at 550 °C for 5 h and then labeled as GSBA-15. For activation, 1.5 g of GSBA-15 was mixed with 50 mL of 0.1 M HCl for 24 h. It was then filtered, washed, and dried for 24 h at 100 °C.

Impregnation of ZnO/GSBA-15
1% of Zn(CH3COO)2.2H2O (0.033716 g) was dissolved in 42 mL of ethanol solvent while stirred with a magnetic stirrer at 70 °C for 2 h. The solution obtained was cooled to room temperature. It was then mixed with a Zinc Oxide (ZnO) solution with 1 gram of activated GSBA-15 while vigorously stirring. Next, it was dried at 80 °C for 24 h for eliminating solvent. It was then calcined in a furnace at a temperature of 550 °C for 4 h to form 1% ZnO/GSBA-15. The number of ZnO ratios was increased to obtain the ZnO:G-SBA ratios of 5% and 10%.

Photocatalytic Activity of ZnO/GSBA-15
The photocatalytic activity of ZnO/GSBA-15 was evaluated by measuring the degradation of methylene blue in wastewater solution under UV radiation. The experiment was carried out in a photocatalytic reactor in an Erlenmeyer containing 200 mL of the methylene blue solution with an initial concentration of 5 mg.L −1 . The 50 mg of ZnO/GSBA-15 samples from each ZnO mass variation were dispersed into the methylene blue solution by shaking for 30 min in the dark to achieve adsorption-desorption equilibrium. The photocatalytic reactor was set up to degrade methylene blue with the black box, which was used to prevent light from surrounding for dark adsorption reaction. Afterward, the mixture was placed vertically under a 300 W Xenon lamp equipped with an optical transmission filter (λ = 365 nm). A total of 10 mL of the suspension was collected at a fixed reaction time and centrifuged to remove the catalyst. The concentration of methylene blue supernatant was determined by measuring the absorbance of the solution at a wavelength of 665 nm with a UV-Vis spectrophotometer (Shimadzu UV-3600). The percentage of methylene blue photodegradation was calculated using the equation as follows: (1) where, C0 and Ct represent methylene blue concentration in the solution before and after the UV irradiation, respectively.  (2)).
(2) D is the crystal size in Å, λ is the wavelength source used in the XRD test (1.54056 Å), and β is the half-peak width in radians. θ is the angular position of the peak formation. The crystallinity of the sample could also be computed by Equation (3).
(3) Figure 2 shows the X-ray diffractogram of ZnO/GSBA-15 with ZnO mass ratios of 1, 5, and 10% w/w. The prominent peaks for mesoporous silica (JCPDS-850897) were observed in the amorphous phase at 21.08°, with the miller field (100), which is in accordance with the literature that there is no crystal phase of mesoporous silica GSBA-15. It also exhibits the successfully mesoporous silica synthesized by gelatin addition in the synthetic organic template [20]. When ZnO was increased from 1% to 10%,

X-Ray Diffraction (XRD) Characterization
cos D (%) 100 =  + Crystallinity crystalline peak area crystalline peak area amorphous peak area there were no significant ZnO peaks (JCPDS 361451) and no differences in mesoporous silica peaks' position. This fact suggests that the low amount of ZnO (according to EDX that Zn is detected close to 1-6%) does not change the stability of mesoporous silica GSBA-15. This opinion is also based on several previous studies reporting that the peak of ZnO in the supporting material was detected by XRD when the mass ratio of ZnO to the supporting material was more than 30-40% w/w [3,14,15,17,32,33]. Further, XRD does not give information regarding the existence of ZnO due to the low content of ZnO, which may be thoroughly dispersed in the mesoporous silica GSBA-15 matrix. Hence, the amorphous structure of mesoporous silica GSBA-15 has a quenched crystal-line structure of ZnO. However, the detection of ZnO can be done by EDX and FTIR, but the structure of mesoporous silica GSBA-15 has been confirmed from nitrogen adsorption-desorption isotherms. Table 1 shows the effect of the ZnO addition on the crystallite size of the mesoporous silica of GSBA-15, which exhibits an enlargement with the increasing ZnO amounts. It reveals that ZnO has fully dispersed onto the surface of the mesoporous silica and has a significant effect on the crystallite size because ZnO and the resulting silica particles overlap.
During ZnO precursor infiltration, the overlapping particle has been done to the mesopore structure. The overlapping step has a big opportunity to produce a big crystallite size and a larger space as a new pore ( Figure 5). The other factor that influences the enlarging crystallite size after ZnO incorporation is the growth of ZnO, which is affected by precipitation rate, Zn-supporting material, and the calcination temperature [40]. The growth of ZnO in GSBA-15 is detected by the change of the crystallite size by XRD before and after impregnation data. In conclusion, the growth of ZnO particles in GBA-15 is governed by increasing the amount of Zn precursor, which has a significant impact on the ZnO distribution, crystallite size, and new space in the composite Figure 3 displays the FTIR spectra of GSBA-15, which exhibited some significant differences between silica samples before and after ZnO impregnation. The intensity of the peaks at 3381.52-3453.69 cm −1 significantly increases for the sample at higher ZnO concentration, suggesting ZnO's influence in increasing hydroxyl region (−OH) of mesoporous silica that is associated with stretching vibrations of the Si−OH group. In the ZnO/GSBA-15 1% and GSBA-15 samples, the absorption bands are observed at 2108.71 cm −1 wavenumber, which indicates a stretch of CO2 due to the partial decomposition of gelatin. The peak at 1630 cm −1 corresponds to the bending modes of adsorbed water [16]. The high intensity of mesoporous ribbon character at 1042.84 cm -1 is for symmetric modes stretching of lattice vibrations  Analysis of surface area using the BET method determines the surface area, pore volume, and pore diameter of GSBA-15 and ZnO/GSBA-15. Based on the BET analysis, the adsorption-desorption curves of GSBA-15, ZnO/GSBA-15 5 and 10 wt% are exposed in Figure 4.

Fourier Transform Infrared Spectrophotometer (FTIR) Characterization
Based on Figure 4, all samples in the adsorption-desorption match with the type IV isotherm of mesoporous materials with hysteresis loop type of H-1, which is related to porous materials with a narrow pore size distribution [16]. Thus, it is apparent that there is an inflection position located at a relative pressure (P/Po) in the range of 0.4-0.9, which indicates that ZnO loading in GSBA-15 causes a change in the shape of the hysteresis loop. Still, there is no significant difference in the isotherm form between GSBA -15 and ZnO/GSBA-15. Table 3 presents experimental data based on the analysis of the surface area, pore volume, and pore diameter. Positive correlation results are found in Table 3 where a higher surface area would result in lower pore diameter. It can be seen in the declining surface area from 500.8 to 351.9 m 2 /g and pore volume from 0.71 to 0.56 cm 3 /g, suggesting that ZnO is confined inside the pores of mesoporous silica GSBA-15. However, the decreasing surface area after impregnation significantly influences the increasing pore size from 2.82 nm to 3.19 nm, which is close to 11.5%. Typically, after impregnation, the pore diameter of silica would be smaller than before due to the pore blocking of ZnO to the silica. However, the increasing pore size after ZnO impregnation is logical due   Table 3. Results of sample porosity analysis using BET characterization.    Table 4. Analysis of elemental composition and average particle diameter of GSBA-15, ZnO/GSBA-15 1, 5, and 10 wt%.
to the overlapping between ZnO-ZnO and ZnOsilica particles to form a larger space as a new pore size ( Figure 5).

Scanning Electron Microscopy-Energy Dispersive X-Ray (SEM-EDX)
SEM characterization was employed to examine the morphology, distribution, and particle shape of GSBA-15, ZnO/GSBA-15 1, 5, and 10 wt% powders. The morphology of all samples is almost the same, giving general information that increasing ZnO incorporated to silica surface amount does not significantly affect the stability of the silica GSBA-15 framework. Figure 5 demonstrates the surface GSBA-15 as the number of white granules pipe as rod-like morphology. According to histogram measurements, the average size of the particles formed is between 345 nm to 525 nm (equal to 0.35-0.53 µm), so it can be concluded that the particles formed are micro-sized. Histogram by SEM reveals that increasing ZnO has a significant effect on decreasing the sharp distribution particle due to the ZnO growth, the overlapping between ZnO and silica particles as the same explanation by XRD. Silica GSBA-15 and ZnO/GSBA-15 are mesoporous samples that are categorized as bulk. The particle size is not identical to the crystallite size in the bulk scale. Crystallite is part of the grain, while the grain is part of the particle, so that the crystallite size < grain (grain) < particle. Therefore, the crystallite sizes of GSBA-15 by XRD are smaller than particle sizes by SEM.
From Table 4, it is apparent that each photocatalyst has the highest percentage of elemental O content, which is more than 45%. In the GSBA-15 photocatalyst, which had been impregnated with ZnO, it was identified the presence of Zn in it so that the Zn impregnation process into GSBA-15 silica was successfully carried out. The percentage of Zn increased significantly with the mass % of Zn contained in GSBA-15, but the content of the Zn10% in ZnO/GSBA-15 photocatalyst was different from the percentage of Zn added, which was only 6.12%. The difference in the percentage of Zn between the experiment and EDX detection is predicted due to the leaching of ZnO during the washing and filtrating step from the GSBA-15 surface. In addition, the incorporation of ZnO onto GSB-15 decreases %C due to the high ZnO insertion during interaction onto silica surface. So, the high ZnO on GSBA-15 can improve the stability of silica during organic decomposition at high temperatures and have a significant influence on reducing carbon.

Photocatalytic ZnO/GSBA-15
Testing the ZnO's effect onto GSBA-15 as a photocatalyst on the degradation process of methylene blue is to determine which type of photocatalyst is most effective in producing •OH radicals. Figure 6 describes the duration of irradiation in the photodegradation process, which illustrates the effect of the interaction (contact) between photocatalyst and light (hv) in producing •OH radicals and contact between •OH radicals and methylene blue in the degradation process. In dark conditions or when the photocatalyst is at first 30 min, high MB removal values were obtained due to the high surface area of GSBA-15, not only via physical interaction on surface and pore but also chemistry interaction between MB-Silica and MB-Silica-Zn. The second step at min 30 to 90 was the photodegradation reaction in which a total MB removal improved due to the light source as the photon energy to produce •OH radicals. This interaction was strong that the methylene blue was not only adsorbed on the silica surface but also degraded optimally [15,33] due to the excitation of electrons from the conduction band to the valence band by photon, so that any •OH radicals were formed [22]. The total efficiency value of the degradation product using ZnO/GSBA-15 photocatalyst was higher than in the dark condition, concluding the significant effect of ZnO incorporation.
According to Table 5, the value of the degradation efficiency improves with the increase in the ZnO mass. It is in accordance with the pore diameter value in the BET analysis, which also enlarges, so that the absorption of methylene blue is maximized. The mechanism of the degradation reaction of methylene blue in the photocatalyst ZnO and ZnO/GSBA-15 under UV light irradiation can be written as follows:  Table 5. Efficiency of degradation methylene blue by photocatalysts GSBA-15, ZnO/GSBA-15 1, 5, 10 wt%, and ZnO at 90 min of irradiation time.
•OH + MB → CO2 + H2O (4) MB + e − → 16CO2 + 6H2O + HCl + HNO3 + H2SO4 (5) The activity of ZnO oxide on GSBA-15 as a photocatalyst significantly enhanced MB removal. Before irradiation, the initial adsorption has optimized the interaction between MB and the ZnO-silica surface. The photodegradation process can occur through the direct reaction of hydroxyl radicals with MB adsorbed on the sur-face of the photocatalyst. The high silica surface area supports the interaction of MB with ZnO/GSBA-15 higher than direct photocatalytic degradation without adsorption. When the ZnO and ZnO/GSBA-15 photocatalysts absorb photons with an energy equal to or greater than the energy width of the ZnO bandgap, then the electrons in ZnO are excited from the valence band to the conduction band. Then, electron-hole pairs are generated. Where, e − and h + are electrons in the conduction band (e − ) and electron vacancies in the valence band (h + ), respectively. These two entities can migrate to the surface of ZnO and ZnO/GSBA-15 and enter into redox reactions with organic pollutants; in this case, it is methylene blue on the surface. Hole electrons will react with H2O or OH − to produce a hydroxyl radical (•OH). The active radical degrade N−C3 bond then −CH3 is oxidized to HCHO or HCOOH, then active radical break the molecule with C−S and C−N bonds and produce unstable smaller organic as side product. This radical is a powerful oxidizing agent and the primary oxidizer in the photocatalytic oxidation of methylene blue to carbon dioxide, water, and other mineralized products, such as hydrochloric, sulfuric, and nitric acid. Meanwhile, the electrons (e − ) will react with methylene blue to produce reduced products as smaller organic, namely CO2 and H2O as primary prod-    Table 6. The effect of dark adsorption and photodegradation on total MB removal.

Bulletin of Chemical Reaction Engineering & Catalysis, 17 (2), 2022, 371
Copyright © 2022, ISSN 1978-2993 ucts, and other ion, such as Cl − , SO4 −2 and NO3, as side product [37,38]. Based on Figures 7 and 8 and Table 6, the degradation of methylene blue (MB) advances with the increase in UV irradiation time. At 0-30 minutes of dark adsorption, there was a reasonably significant increase in degradation, while at 30-90 min, the degradation advanced from 2.96 to 5.09, along with increasing ZnO from 1-5%. The average MB decolorization via dark adsorption and photodegradation using ZnO/SBA-15 achieved 85% and 4%, respectively. This high decolorization during dark adsorption is predicted to come from a high surface area and large pore, which facilitates MB adsorption. However, the % MB removal using GSBA-15 after ZnO impregnation was higher than before due to the inert character of GSBA-15. The inert (neutral character) of GSBA-15 facilitates adsorption of MB species by the pore and interaction between MB and Si−O−Si only, which explains the lower MB removal during dark adsorption than after ZnO incorporation. This result is in line with the previous report [15]. According to the previous research, mesoporous silica had a very small amount of acidic site compared to the silica after ZnO incorporation, in which the acidity increased 8-9 times due to the dispersion of zinc oxide [15]. It clarifies the higher % MB removal of GSBA-15 after incorporating ZnO during dark adsorption, which ZnO on silica surface facilitates the adsorption via strong electrostatic attraction between MB, Si−O−Si, and the active site of ZnO. Thus, it can be concluded that the electrostatic interaction between catalyst surface and MB as organic dyes plays a major role in the dark adsorption step.
From the literature, we found that the band gap of the ZnO bulk is 3.3 eV [32] to 3.3 eV [15]. However, after impregnating onto silicabased material, the band gap of ZnO improved by 3.38 to 4.01 eV from 5-30% ZnO weight [15,32]. This information demonstrates that ZnO/GSBA-15 in this work has the same probability of absorbing light in the UV region. For that reason, we correlated the bandgap information from previous research with the photodegradation performance of ZnO/GSBA15. Figure 6 exhibits that the degradation curve can be divided into two stages. One is a rapid adsorption process from zero min to 30 min. The other is a photocatalytic degradation process from 40 min to 90 min. There is an increasing degradation from 2.9% to 5% due to an increasing ZnO content from 1-5%, as presented in Table 7.
This phenomenon confirms two factors: first, the low content of ZnO in GSBA-15. Previous studies report that 17%TiO2/SBA-15 achieved 4% photodegradation for 350 min and 58% MB removal via 60 min dark adsorption [39]. Other literature mentioned that ZnO/Silica with amount ZnO 5-20% achieved 85 to 99% of total MB removal from dark adsorption (60-120 min) and photodegradation (120-350 min) which the real ZnO element by EDX is 4.9 to 20.1 %w/w [15,32,33]. However, the photodegradation in our manuscript was higher than in previous studies with higher ZnO content as an active site due to the high surface area of silica GSBA-15 as a supporting catalyst Another factor is time for dark adsorption and irradiation time. The previous research reported a longer time for both steps and reached  Hence, the second reason concludes that the lower irradiation time significantly affects the photodegradation performance. However, the higher total MB removal of ZnO/GSBA-15 is not only from the surface area but also from the site active distribution factor. This result is in line with previous research [15,32,33,39].

Conclusion
Mesoporous silica (GSBA-15) with rod-like morphology has been successfully synthesized using gelatin and P123 with the template method. Increasing crystallite size and pore diameter of GSBA-15 after ZnO incorporation was due to variation of weight ratios between ZnO and GSBA-15 from 1% to 10%, but the surface area decreased from 520 m 2 /g to 345 m 2 /g due to the pore blocking and overlapped silica-ZnO particle. The FTIR data of GSBA-15 after impregnation show the formation of Si−O−Si and Zn−OH bonds, and the main element of Si, Zn, and O were observed in EDX. A combination of dark adsorption and photocatalytic degradation of methylene blue using ZnO/GSBA-15 achieved 94%, implying the appropriate catalyst for dye molecule degradation.