Synthesis of Mesoporous Silica Incorporated with Low Iron Concentration and Gelatin Co-Template via The Ultrasonication Method and Its Methylene Blue Photodegradation Performance

In this work, low iron concentration incorporated on mesoporous silica with gelatin co-template (Fe 2 O 3 /GSBA-15) has been successfully synthesized via the ultrasonication method. The physical, chemical, and structural properties of the samples were investigated with X-Ray Diffraction (XRD), Scanning Electron Microscope-Energy Dispersive X-Ray (SEM-EDX), Fourier Transform Infra-Red (FTIR), and N 2 adsorption-desorption. Results showed good distribution of low concentration of iron oxide on the gelatin mesoporous silica GSBA-15. Elemental and surface analysis presented that iron oxide incorporation with higher concentration exhibited lower surface area due to the blocking pore. The highest photocatalytic activity on the methylene blue dye degradation was achieved at 10% Fe 2 O 3 /GSBA-15 with ~80% efficiency. The results revealed that the photocatalytic activity of Fe 2 O 3 /GSBA-15 enhanced with the presence of iron oxide.


Introduction
Methylene blue (MB) is a liquid dye waste massively produced by the textile industry annually. MB waste enters the waters and poses many health risks [1]. MB waste is handled mainly through adsorption and photocatalysis [2,3]. The photocatalysis approach to reducing MB is famous as an environmentally friendly and high-efficiency method [4][5][6]. Many researchers have studied efficient photocatalysts to pro-duce an effective photodegradation process. One of the photocatalysts reviewed is iron oxide [7,8]. However, the easy agglomeration of iron oxide causes high cost and low photocatalytic efficiency [9,10]. Therefore, a suitable support material is considered effective in increasing iron oxide distribution. Besides, the support material is attempted to be synthesized from various materials that have high sustainability by minimizing synthetic materials, for example, with gelatin as the co-template [11][12][13][14]. The main problem is that the normal impregnation process still causes a large metal oxide agglomeration effect, reducing the photocatalytic efficien-cy. Hence, various impregnation modifications have been performed to deal with this problem so that metal oxides can be appropriately distributed, one of which is by the ultrasonication method [15][16][17][18].
The ultrasonication method is a simple synthesis pathway via a high-frequency sound field (>20 kHz). It is widely used in synthesizing organic and inorganic compounds because it is effective for homogenizing, mixing, and reducing particle size [16,19,20]. Ultrasonic waves have been used for the separation of agglomerates to make composites because they exhibit higher selectivity, reactivity, and improved product amount. Additionally, their energy savings are higher and cleaner than photochemical, hydrothermal, and pyrolysis [17,21,22].
However, the ultrasonication method has never been employed in the incorporation of iron oxide in mesoporous silica using gelatin as the co-template, especially at low concentrations of iron oxide. Thus, here, to prevent agglomeration and achieve better dispersion of iron oxide in silica, iron oxide precursors are impregnated on the silica surface via the ultrasonication method. Further, this research focuses on studying the effect of ultrasonication on iron oxide impregnation into mesopore silica with gelatin co-template. The synthesis results are characterized using XRD, SEM-EDS, FTIR, and Nitrogen (N2) adsorption-desorption. The synthesis results are applied in methylene blue photocatalysis to determine information related to degradation efficiency and an appropriate kinetic model.

Synthesis of Fe2O3/GSBA-15
Mesopore silica with gelatin as the cotemplate (GSBA-15) was prepared by following the previous study with a P123:gelatin ratio of 1:0.01 (%w/w) [23]. The incorporation of iron oxide into GSBA-15 was done by mixing H2O, Fe(Cl)6.6H2O, and GSBA-15 with a ratio of 2:10:100 (%w/v/w), respectively. It was then followed by ultrasonication at 100 rpm for 2 hours at a room temperature of 30 o C. The result was dried at 100 o C for 30 min and then activated with 0.5 M HCL, followed by calcination in a furnace at 600 °C for 6 hours. The resulting sample was named 1% Fe2O3/GSBA-15. The synthesis process was repeated for 5% and 10% concentration variations. Thus, there were three samples in this study, namely 1% Fe2O3/GSBA-15, 5% Fe2O3/GSBA-15, and 10% Fe2O3/GSBA-15.

Methylene Blue Photodegradation
The photocatalytic activity of Fe2O3/GSBA-15 was evaluated by methylene blue degradation. First, 200 mL of 5 ppm methylene blue solution was mixed into 50 mg Fe2O3/GSBA-15 photocatalyst. The solution was then irradiated under a UV lamp (300 W Xenon lamp at 365 nm). Samples were taken every 5 minutes of irradiation (until 90 minutes) and then measured by UV-Vis spectrophotometer at 665 nm (Shimadzu UV-3600) to determine the absorbance. Analysis of %Efficiency of methylene blue degradation was done using Equation (1).
where, C0 is the initial methylene blue concentration before irradiation, and Ct is the methylene blue concentration at t (min) after irradiation.

Characterization
Characterization was done using FTIR spectrophotometer (Nicolet 6700-Thermo Fisher Scientific), XRD (Philips X'pert, Cu-Ka 0. (2) where, D is the crystal size, λ is the wavelength used in the XRD test (1.54056 Å), B is the half-peak width in radians, and θ is the angular position of the peak formation. Besides, the crystallinity of the sample [23] can also be calculated using Equation 3.
(3)  [11]. The crystal size and crystallinity degree of gelatin mesoporous silica before and after 10% Fe2O3 impregnation change from 14% to 29% (Tabel 1). It indicates that the presence of iron oxide in the silica increases the crystal size and crystallinity. Figure 2 presents the N2 adsorptiondesorption isotherm of mesoporous silica GSBA-15 before and after iron oxide incorporation. It exhibits that all samples have isotherm type IV with a hysteresis loop of type H-1 at a wide relative pressure (P/P0) in the range of 0.5 to 0.8 and with almost vertical adsorption and desorption branches as a regular character of the mesoporous material. This type is in line with the previous report [24].

Results and Discussion
The surface area of the samples decreases from 481.7 m 2 /g to 385.0 m 2 /g after incorporating 10% iron oxide (Table 1). Besides, the pore volume also reduces from 0.62 cm 3 /g to 0.585 cm 3 /g due to pore clogging by iron oxide. The decrease in surface area after the incorporation of iron oxide in this study was only 18% which is much smaller than the previous study (43%). It is considered due to the ultrasonication effect homogenizing the distribution of iron oxide in the mesopore silica. Figure 3 shows the FTIR spectra of gelatin mesoporous silica (GSBA-15) before and after the incorporation of iron oxide. It demonstrates   Figure 4 depicts SEM images and particle size distribution of gelatin mesopores silica GSBA-15 before and after iron oxide incorporation. It reveals that the morphology of all mesopore silica samples is rod-shaped, with the particle size distribution between 354-757 nm. The morphology of all samples does not show significant aggregation of iron oxide. It is considered due to the ultrasonication effect, which helps distribute metal oxides on the silica surface. Nevertheless, the 10% iron oxide concentration decreases the particle size distribution. It might be because of silica shrinkage due to the intense attraction between Fe−O−Si on the surface.
Further, EDX analysis denotes that all samples experience an increase in Fe content after incorporating 1-10 % iron oxide. It has a Fe content of 0.97-5.98 % (Table 1). It indicates the success of the metal deposition process in mesopore silica by ultrasonication. Moreover, based on Figure 5, the increase in iron oxide content in mesoporous silica enhances the removal efficiency of methylene blue from 66% to 85% in 90 minutes of irradiation. It is considered due to the increase in well-dispersed catalytic sites, although in small levels, due to the effect of ultrasonication ( Figure 5, Equations (1)(2)(3)(4)(5).
Further, this result can be compared with several previous studies using non-ultrasonic methods (see Table 2), which revealed that the incorporation of low iron content (< 30%) into the support material showed an efficiency of less than 50%, while high iron content between 35-65 % achieved an efficiency of 50-99 %. High iron content is not economical for large-scale photodegradation applications, so this is an important consideration in saving the environment. The research results using ultrasonication with metal oxides, but not iron oxides, reached 99% efficiency with an average irradiation time of 150-700 minutes. From this result, the ultrasonication method on the incorporation of iron oxide into the gelatin mesoporous silica G-SBA-15 provides many advantages in increasing the dispersion of small levels of Fe particles to the silica surface, which enhances the degradation efficiency of methylene blue significantly in the first 90 minutes. For the issue of saving the environment on a large scale, this gives cost and time efficiency advantages.
The mechanism of the degradation reaction of methylene blue in the Fe2O3/GSBA-15 photocatalyst under UV light irradiation can be written as Equations (4 -8).
H2O + h + → OH * + H + (6)  Methylene Blue + e − → CO2 + H2O (8) Equations (4)(5)(6)(7)(8) indicate that when the Fe2O3/GSBA-15 photocatalyst absorbs photons with energy equal to or greater than the band gap energy of Fe2O3, the electrons in Fe2O3 are excited from the valence band to the conduction band. When iron is incorporated into the silica surface [31], holes and electron pairs can be generated. Then, e − and h + can migrate to the surface of Fe2O3/GSBA-15 and will enter into a redox reaction with organic pollutants, in this case, methylene blue, on the surface. Holes and electron pairs will react with H2O or OHto produce hydroxyl radicals. These radicals are powerful oxidizing agents and the main oxidizer in the photocatalytic oxidation of methylene blue to carbon dioxide, water, and other mineralized products. Meanwhile, electrons will react with methylene blue to produce reduced products, namely CO2 and H2O. In general, the iron oxide incorporated in the gelatin mesoporous silica G-SBA-15 reveals good photodegradation performance, making this material a future photocatalyst material.

Conclusions
The incorporation of iron oxide onto gelatin mesoporous silica GSBA-15 by the ultrasonication method has been successfully investigated in this work. The uniform dispersion of iron oxide on GSBA-15 as the supporting material was achieved with ultrasonic radiation. FTIR, XRD, and EDX analysis exhibited successful incorporation of 1-10% iron oxide onto GSBA-15 with the distribution particle size ranging from 30 to 70 nm and the iron content of 0.97 -5.98 %wt. The blocking pore of 1-10% iron oxide reduced surface area from 481 to 385 m 2 /g and pore volume by 5%. The study demonstrated an increase in the photocatalytic activity with enhancing methylene blue degradation as the increasing iron oxide concentration. The degradation efficiency reached optimum at ~ 80% on 10% Fe2O3/GSBA-15.