Hydrolytic Modification of SiO2 Microspheres with Na2SiO3 and the Performance of Supported Nano-TiO2 Composite Photocatalyst

Modified microspheres (SiO2-M) were obtained by the hydrolytic modification of silicon dioxide (SiO2) microspheres with Na2SiO3, and then, SiO2-M was used as a carrier to prepare a composite photocatalyst (SiO2-M/TiO2) using the sol-gel method; i.e., nano-TiO2 was loaded on the surface of SiO2-M. The structure, morphology, and photocatalytic properties of SiO2-M/TiO2 were investigated. Besides, the mechanism of the effect of SiO2-M was also explored. The results show that the hydrolytic modification of Na2SiO3 coated the surface of SiO2 microspheres with an amorphous SiO2 shell layer and increased the quantity of hydroxyl groups. The photocatalytic performance of the composite photocatalyst was slightly better than that of pure nano-TiO2 and significantly better than that of the composite photocatalyst supported by unmodified SiO2. Thus, increasing the loading capacity of nano-TiO2, improving the dispersion of TiO2, and increasing the active surface sites are essential factors for improving the functional efficiency of nano-TiO2. This work provides a new concept for the design of composite photocatalysts by optimizing the performance of the carrier.


Introduction
With social developments and the increasing scale of industrial production, water pollution, including the discharge of organic pollutants such as organic dyes and antibiotics, has become increasingly severe, posing a significant threat to human health and ecosystems [1,2]. At present, industries mainly purify wastewater by biodegradation, chemical precipitation, and adsorption, but generally suffer from high costs, low efficiency, and susceptibility to secondary pollution [3,4]. Inspired by photosynthesis, which uses light energy to drive chemical reactions, photocatalysis can degrade pollutants by using the strong oxidation of electron-hole pairs generated from semiconductors to purify wastewater and remediate the environment. Photocatalytic technology has become a research hotspot in the field of environmental purification [4][5][6] and has demonstrated broad application prospects.
Titanium dioxide nanoparticles (TiO 2 ) are the photocatalytic materials most studied in recent years in preparation and applications, due to their non-toxicity, chemical stability, and superior performance [7,8]. However, TiO 2 also suffers from an easy combination of photogenerated electrons and holes [9], low utilization of sunlight [10], reduced photocatalytic effect due to particle agglomeration [11], and difficult recycling in wastewater [12]. Elemental doping [13,14], noble metal deposition [15], heterostructure construction [11,14], Materials 2021, 14, 2553 2 of 18 and morphology control [16] have been adopted to improve TiO 2 photocatalytic performance. It was found that loading TiO 2 onto carriers enhances photocatalytic performance and enables the recycling and reuse of TiO 2 , resulting in lower application costs [17]. Krios et al. [18] strengthened the photocatalytic activity of TiO 2 and enabled recycling owing to the effective interactions between TiO 2 and zeolite by loading TiO 2 onto the pretreated zeolite's surface. Huang et al. [19] prepared TiO 2 /graphene composites using a solvothermal method. The interfacial chemical bonds in TiO 2 /graphene significantly reduced the recombination of electron-hole pairs and increased the quantity of holes involved in the photooxidation process. Zuo et al. [20] improved the photocatalytic efficiency of TiO 2 by binding nano-TiO 2 on the surface of diatomite with adsorption properties. Some studies synthesized core-shell structured TiO 2 nanomaterials using noble metals as cores [21], and others prepared thin films by immobilizing TiO 2 on solid support substrates (stainless steel, titanium alloys, titanium metal, and tin oxide-coated glasses) [22] to enhance TiO 2 performance. However, the surface of theses carriers generally only has a small number of groups binding to TiO 2 , which is inconvenient for regulating the morphology of nano-TiO 2 . The uneven particle size of these carriers also reduces the performance of composite photocatalysts. Furthermore, solid support substrates, noble metals, and graphene as carriers are expensive, with a hydrophobic surface and difficult compatibility with the hybrid system. Therefore, it is of positive significance to select carriers with a homogeneous size, low price, and ability to improve the performance of loaded TiO 2 nanoparticles through modification.
Silicon dioxide (SiO 2 ) is commonly applied as a photocatalyst carrier, since it is rich in surface hydroxyl groups that can promote the dispersion and stability of the photocatalyst [23,24]. Many studies have confirmed the improved photocatalytic performance of composite material (SiO 2 /TiO 2 ). Wang et al. [25] prepared TiO 2 -SiO 2 core-shell nanocomposites with different coating thicknesses. The silica coating with a high specific surface area accelerated the removal of rhodamine B to some extent. Cizmar et al. [26] found that recombination between electrons and holes at the composite photocatalyst surface was reduced after copper modification, which improved the photocatalytic performance of the composite materials. SiO 2 microspheres, a byproduct of the electrofusion process to produce zirconium oxide (ZrO 2 ), mainly consist of spherical amorphous SiO 2 particles. Such a byproduct can meet the requirements as a carrier to load TiO 2 nanoparticles for its high purity, low price, high chemical, and high-temperature thermal stability [27,28]. SiO 2 microspheres produced under high temperature and dry conditions have fewer surface hydroxyl groups, lower reactivity, and a smoother surface than that of SiO 2 prepared in a liquid solution, resulting in its unsatisfying ability to be a substrate. Furthermore, through inorganic surface modification, the SiO 2 surface can be coated with active ingredients to increase the number of surface hydroxyl groups, leading to the improvement of the surface activity of SiO 2 microspheres. It is critical to keep the surface properties of silica under control because of the various silanol bases (≡Si-OH) reactions (such as chlorination, ammonification, and esterification) [29]. Wang et al. [17] treated SiO 2 microspheres with sodium hydroxide solution to change the surface of microspheres from smooth to rough and increase the quantity of hydroxyl groups. It is important for improving the functionality of SiO 2 microspheres loaded with nano-TiO 2 and CdS photocatalysts. However, the modification by sodium hydroxide treatment conditions is harsh and corrosive to the equipment because of the reaction under a very high pH.
In this paper, the sodium silicate (Na 2 SiO 3 ) hydrolysis method was used to modify SiO 2 microspheres. Modified SiO 2 (SiO 2 -M) was obtained by deposition of Na 2 SiO 3 hydrolysate at the surface of SiO 2 microspheres. Then, an SiO 2 -M/TiO 2 composite photocatalyst was prepared through the sol-gel method. Furthermore, its structure, performance, and influence mechanism of SiO 2 -M were studied.

Raw Materials and Reagents
The raw material of SiO 2 microspheres was the byproduct of fused zirconia production, which was provided by Henan Jiaozuo Weina Powder Technology Co., Ltd., Jiazuo, China. It mainly comprised spherical particles with an amorphous SiO 2 phase, and the main chemical composition (wt.%) was SiO 2 97.31 and ZrO 2 0.81. The test results show that the coarse end diameter (D90) was 4.973 µm, and the median diameter (D50) was 1.804 µm. The pure nano-TiO 2 used in this study was prepared using the sol-gel method, and it was an anatase phase with a size of about 20-30 nm.
Tetra-butyl titanate ((C 16 H 36 O 4 )Ti) was used as a Ti 4+ source and was a product of Beijing Yili Fine Chemicals Co., Ltd, Beijing, China. Acetyl acetone (C 5 H 8 O 2 ) was a hydrolysis inhibitor, which was produced by Xilong Chemical Co., Ltd. (Guangzhou, China). Sodium silicate (Na 2 SiO 3 ) from Beijing Yili Fine Chemicals Co., Ltd. was used as a modifier. Methyl orange was used as the target pollutant for photocatalytic degradation produced by Sinopharm Chemical Reagent Co., Ltd, Shanghai, China. Anhydrous ethanol (CH 3 CH 2 OH) deionized water was used as solvent.

Modification of SiO 2 Microspheres
First, 2 g of SiO 2 microspheres were decentralized in 100 mL of distilled water to form a dispersed suspension. Then, a certain amount of Na 2 SiO 3 was put into the suspension to obtain a mixed solution. After stirring evenly, the mixed solution was moved to an oil bath at 80 • C for preheating for 20 min. In the meantime, 1 mol/L H 2 SO 4 was added to the mixture dropwise to adjust the preset pH values. Then, the modified product was washed with deionized water five times after stirring for 3 h. Finally, the modified SiO 2 microspheres (SiO 2 -M) were obtained after drying and grinding. For convenience of comparison, SiO 2 -M obtained at a different dosage of Na 2 SiO 3 (0 g, 0.2 g, 0.4 g, 0.6 g, 1.0 g) and different pH values (pH = 9.0, 8.5, 8.0, 7.5, 7.0, 6.5) were denoted as X-SiO 2 -M-Y, where X represents the dosage of Na 2 SiO 3 (g), and Y represents the pH value of solution during hydrolysis. The hydrolysis equation of Na 2 SiO 3 was Thus, the surface modifiers of SiO 2 microspheres in SiO 2 -M should be amorphous SiO 2 and SiO 2 •nH 2 O.

Preparation of SiO 2 -M/TiO 2 Composite Photocatalyst
First, 15 g of SiO 2 -M was put into a 500 mL conical flask with 140 mL of ethanol. After being evenly dispersed, 20 g of Tetra-Butyl Titanate (TBOT) was added dropwise to obtain a white suspension recorded as liquid A. Then, a mixture of 60 mL of anhydrous ethanol, 60 mL of deionized water, and 3 mL of acetylacetone was added to liquid A dropwise to obtain the mixed solution B. After stirring for a while, the mixed solution B was centrifuged to obtain the precipitate. The white sediments were washed with distilled water and dried to obtain the precursor of SiO 2 -M/TiO 2 . Finally, the precursor was calcined in a muffle furnace at 700 • C for 2 h to acquire composite photocatalysts.

Preparation of Pure TiO 2
A total of 20 g of tetrabutyl titanate (TBOT) was added dropwise to 140 mL of anhydrous ethanol to obtain a yellow transparent solution. Then, a mixture of 60 mL of anhydrous ethanol, 60 mL of deionized water, and 3 mL of acetylacetone was added to the yellow transparent solution dropwise to form a mixed solution. Next, the TiO 2 gel was obtained after stirring for a while. Finally, the gel was dried and calcined in a muffle furnace at 400 • C for 2 h to acquire pure nano-TiO 2 .

Testing and Characterization
Field emission scanning electron microscopy (SEM) (Gemini SEM, Carl Zeiss AG, Braunschweig, Germany) and high-resolution field emission transmission electron microscopy (TEM) (JEM-2100F, JEOL, Tokyo, Japan) were used to observe the morphology, structure, and size of SiO 2 microspheres, and SiO 2 -M and SiO 2 -M/TiO 2 particles. The elemental composition and content of the sample were measured by using a German Bruker S4-Explorer fluorescence spectrometer (XRF). The X-ray diffraction patterns were measured with a D8 advance X-ray diffractometer (Bruker, Karlsruhe, Germany) using Cu K α radiation. The specific surface area of samples was tested by a nitrogen adsorption facility (Autosorb IQ, Quantachrome, Boynton Beach, FL, USA). The surface functional groups were examined with an infrared spectroscope (Spectrum 100, PerkinElmer, Shanghai, China) using KBr as background. The point of zero charge (PZC) was measured by using a Zetasizer Nano ZS90 (Malvern, Malvern City, UK). The amount of hydroxyl group on the surface of SiO 2 microspheres was measured before and after the hydrolytic modification of Na 2 SiO 3 by the acid-base titration method. Firstly, 2 g of the sample to be tested was added to 75 mL of NaCl solution (20 wt.%) and 25 mL of anhydrous ethanol to form a suspension. After the suspension was evenly stirred, the suspension's pH was first reduced to 4 with 0.1 M HCl solution, and then, the pH of the suspension was increased to 9 by slowly adding a 0.1 M NaOH solution dropwise. The volume of NaOH solution used in the process was recorded, and the number of hydroxyl groups on the surface of the sample could be calculated using Equation (3).
where N is the number of hydroxyl groups on the surface of the sample (/nm 2 ), C is the concentration of NaOH solution (mol/L), V is the volume of NaOH solution used in the titration process (L), N A is Avogadro constant (6.022 × 10 23 mol −1 ), m is the sample mass (g), and S is the specific surface area of the sample (m 2 /g). The degradation of methyl orange evaluated the photocatalytic performance of SiO 2 -M/TiO 2 under ultraviolet light (the 300 W mercury lamp was used as the ultraviolet light source). The photocatalytic degradation tests involved in the experiments were carried out using a PhchemIII photoreactor from Beijing Newbit Technology Co., Ltd, Beijing, China. The reactor contains a light-source controller, stirrer, and circulation cooler, and can perform 12 sets of tests simultaneously. First, 50 mg SiO 2 -M/TiO 2 was added to 50 mL of a certain concentration (C 0 ) of methyl orange solution to obtain the mixture of the solid content of 1 g/L of suspension. Then, the suspension in dark conditions was stirred for 60 min to obtain SiO 2 -M/TiO 2 on the adsorption equilibrium of methyl orange solution. After opening the ultraviolet light source mercury lamp (300 w), 5 mL of the sample was collected every 10 min. Next, the samples were centrifuged at 8000 r/min for 5 min to obtain the supernatant. Finally, the absorbance to the wavelength of 464 nm of the supernatant was measured by Cary 5000 UV-vis spectrophotometer (Agilent Technologies Inc, Santa Clara, CA, USA). The relationship between the absorbance and concentration was used to determine the concentration of methyl orange in the solution (C). The degradation effect of the photocatalyst was assessed by the change in C/C 0 . At the same time, the degradation extent D was obtained by C and C 0 , D = 100%(C 0 − C)/C 0 .  Figure 1 shows the SEM images of SiO 2 microspheres modified with different Na 2 SiO 3 dosages (the pH of hydrolysis was 8.5). As can be seen from this figure, the unmodified SiO 2 microspheres (SiO 2 ) are spherical with good dispersion. The diameter of the microspheres was about 0.5-2 µm, and the surface was smooth. However, a small amount of debris was attached to the unmodified SiO 2 microspheres, which could be impurities in the raw material. Compared with the unmodified SiO 2 microspheres, the spherical morphology of the modified SiO 2 -M remained unchanged, while the surface debris disappeared. It indicated that the hydrolytic modification with Na 2 SiO 3 first led to the cleaning of impurities on the surface of the microspheres, which would reduce the interference of impurities and facilitate the loading of nano-TiO 2 onto the surface of the SiO 2 microspheres. By observing the surface of the fractured microspheres, it was found that a uniform shell coating formed on the surface of SiO 2 -M was amorphous SiO 2 or SiO 2 •nH 2 O from the hydrolysate of Na 2 SiO 3 ( Figure 1d). As can be seen from Figure 1b-e, the hydrolysis products increased with the increase of Na 2 SiO 3 dosage, so it can be presumed that the shell layer on the surface of SiO 2 microspheres was continuously thickening and the coverage area increasing. As the hydrolyzed products of Na 2 SiO 3 (SiO 2 and SiO 2 •nH 2 O) were produced in solution, the quantity of hydroxyl groups on the microspheres' surface was more than that on the surface of the unmodified microspheres. Therefore, it is speculated that the quantity of hydroxyl groups on the surface of the 0.6-SiO 2 -M microspheres was more than that on the surface of the unmodified microspheres, which undoubtedly becomes a critical factor in improving the level of nano-TiO 2 supported and the properties of the composite photocatalyst. Besides, compared with 0.6-SiO 2 -M, the surface of 1.0-SiO 2 -M was smoother. Therefore, 0.6-SiO 2 -M was more suitable as a carrier.

Characterization of SiO2 Microspheres
3.1.1. Morphology Figure 1 shows the SEM images of SiO2 microspheres modified with different Na2SiO3 dosages (the pH of hydrolysis was 8.5). As can be seen from this figure, the unmodified SiO2 microspheres (SiO2) are spherical with good dispersion. The diameter of the microspheres was about 0.5-2 μm, and the surface was smooth. However, a small amount of debris was attached to the unmodified SiO2 microspheres, which could be impurities in the raw material. Compared with the unmodified SiO2 microspheres, the spherical morphology of the modified SiO2-M remained unchanged, while the surface debris disappeared. It indicated that the hydrolytic modification with Na2SiO3 first led to the cleaning of impurities on the surface of the microspheres, which would reduce the interference of impurities and facilitate the loading of nano-TiO2 onto the surface of the SiO2 microspheres. By observing the surface of the fractured microspheres, it was found that a uniform shell coating formed on the surface of SiO2-M was amorphous SiO2 or SiO2•nH2O from the hydrolysate of Na2SiO3 ( Figure 1d). As can be seen from Figure 1b-e, the hydrolysis products increased with the increase of Na2SiO3 dosage, so it can be presumed that the shell layer on the surface of SiO2 microspheres was continuously thickening and the coverage area increasing. As the hydrolyzed products of Na2SiO3 (SiO2 and SiO2•nH2O) were produced in solution, the quantity of hydroxyl groups on the microspheres' surface was more than that on the surface of the unmodified microspheres. Therefore, it is speculated that the quantity of hydroxyl groups on the surface of the 0.6-SiO2-M microspheres was more than that on the surface of the unmodified microspheres, which undoubtedly becomes a critical factor in improving the level of nano-TiO2 supported and the properties of the composite photocatalyst. Besides, compared with 0.6-SiO2-M, the surface of 1.0-SiO2-M was smoother. Therefore, 0.6-SiO2-M was more suitable as a carrier.  Figure 2 shows SEM images of a hydrolytic modification of SiO2 microspheres by Na2SiO3 (dosage 0.6 g) at a different pH of the suspension. A certain amount of acceptable debris can be seen on the whole surface of 0.6-SiO2-M-9 microspheres with uniform distribution. Due to the low H + concentration in the system and weak hydrolysis reaction of Na2SiO3 when the pH value was 9, it is not enough to produce more hydrolysates to attach to the surface of SiO2 microspheres, nor could the impurity debris on the surface be cleaned off. As the pH value dropped to 8.5 and 8, H + concentration and the hydrolysis  Figure 2 shows SEM images of a hydrolytic modification of SiO 2 microspheres by Na 2 SiO 3 (dosage 0.6 g) at a different pH of the suspension. A certain amount of acceptable debris can be seen on the whole surface of 0.6-SiO 2 -M-9 microspheres with uniform distribution. Due to the low H + concentration in the system and weak hydrolysis reaction of Na 2 SiO 3 when the pH value was 9, it is not enough to produce more hydrolysates to attach to the surface of SiO 2 microspheres, nor could the impurity debris on the surface be cleaned off. As the pH value dropped to 8.5 and 8, H + concentration and the hydrolysis degree of Na 2 SiO 3 increased. Therefore, more hydrolysates were produced to attach to the surface of SiO 2 microspheres, and impurities were removed, showing a gradually smooth surface (Figure 2d-f). When the pH value continued to drop to 7.5 and 7, the hydrolysis reaction of Na 2 SiO 3 became more intense, and a large number of products could be produced in a short time. The excessive local concentration caused the hydrolysis products to nucleate spontaneously and created sedimentation on the surface of SiO 2 microspheres in the form of particles, leading to a rough surface (Figure 2b,c). The pH value continued to decrease to 6.5, and the hydrolysis rate further accelerated. At this time, the hydrolysates rapidly nucleated, and a large number of particles formed, wrapping the surface of SiO 2 microspheres (Figure 2a). These hydrolysates caused the agglomeration among SiO 2 microspheres, which was not beneficial for loading nano-TiO 2 . According to the above results, Na 2 SiO 3 hydrolytic modification was carried out when the pH value was 7.5. Figure 2g,h shows lower-magnification SEM images of 0.6-SiO 2 -M-7.5. It can be seen that the majority of the SiO 2 originating from the hydrolysis of Na 2 SiO 3 was not deposited separately from the surface of the microspheres, indicating that the microstructure of the sample was homogeneous.
surface of SiO2 microspheres, and impurities were removed, showing a gradually smooth surface (Figure 2d-f). When the pH value continued to drop to 7.5 and 7, the hydrolysis reaction of Na2SiO3 became more intense, and a large number of products could be produced in a short time. The excessive local concentration caused the hydrolysis products to nucleate spontaneously and created sedimentation on the surface of SiO2 microspheres in the form of particles, leading to a rough surface (Figure 2b,c). The pH value continued to decrease to 6.5, and the hydrolysis rate further accelerated. At this time, the hydrolysates rapidly nucleated, and a large number of particles formed, wrapping the surface of SiO2 microspheres (Figure 2a). These hydrolysates caused the agglomeration among SiO2 microspheres, which was not beneficial for loading nano-TiO2. According to the above results, Na2SiO3 hydrolytic modification was carried out when the pH value was 7.5. Figure 2g,h shows lower-magnification SEM images of 0.6-SiO2-M-7.5. It can be seen that the majority of the SiO2 originating from the hydrolysis of Na2SiO3 was not deposited separately from the surface of the microspheres, indicating that the microstructure of the sample was homogeneous.  Table 1 listed the quantity of hydroxyl groups on the surface of 0.6-SiO2-M-7.5 and unmodified SiO2 microspheres. Furthermore, 1.64 hydroxyl groups/nm 2 for 0.6-SiO2-M-7.5 was 4.3 times higher than that of unmodified SiO2 microspheres (0.38 groups/nm 2 ), indicating that the hydrolysis modification of Na2SiO3 substantially increased the quantity of hydroxyl groups on the surface of SiO2 microspheres. It became a critical factor in enhancing the carrier effect of SiO2 microspheres.  Table 1 listed the quantity of hydroxyl groups on the surface of 0.6-SiO 2 -M-7.5 and unmodified SiO 2 microspheres. Furthermore, 1.64 hydroxyl groups/nm 2 for 0.6-SiO 2 -M-7.5 was 4.3 times higher than that of unmodified SiO 2 microspheres (0.38 groups/nm 2 ), indicating that the hydrolysis modification of Na 2 SiO 3 substantially increased the quantity of hydroxyl groups on the surface of SiO 2 microspheres. It became a critical factor in enhancing the carrier effect of SiO 2 microspheres. To explore the effect of Na 2 SiO 3 , an SiO 2 -M/TiO 2 composite photocatalyst was prepared via the sol-gel method using SiO 2 -M/TiO 2 composite photocatalyst with Na 2 SiO 3 content of 0.6 g and pH values of 9.0, 8.5, 8.0, and 7.5 as carriers. The degradation behavior of SiO 2 -M/TiO 2 on methyl orange solution is shown in Figure 3. As can be seen from this figure, with the pH value of Na 2 SiO 3 modification increasing from 6.5 to 7.5, the degradation efficiency of methyl orange by SiO 2 -M/TiO 2 increased. The degradation efficiency was represented by a decrease in the C/C 0 value of methyl orange solution at the same illumination time. With the continuous increase of the pH value to 9.0, the degradation efficiency of SiO 2 -M/TiO 2 gradually decreased. By contrast, the degradation of SiO 2 -M/TiO 2 was strongest at pH 7.5 and 8.5. The C/C 0 values of MO with 0.6-SiO 2 -M-7.5/TiO 2 and 0.6-SiO 2 -M-8.5/TiO 2 were reduced to 0.01 and 0.02 after UV irradiation for 30 min, respectively. The degradation extent was 99% and 98%, respectively. The degradation extent reached 100% after 40 min of UV irradiation. It can also be seen from Figure 3 that the degradation extent of 0.6-SiO 2 -M-7.5/TiO 2 and 0.6-SiO 2 -M-8.5/TiO 2 was noticeably greater than that of the composite photocatalyst (SiO 2 /TiO 2 ) with unmodified SiO 2 microspheres as the carrier, indicating that the hydrolysis modification of Na 2 SiO 3 significantly promotes the quality of nano-TiO 2 supported by SiO 2 microspheres.  To explore the effect of Na2SiO3, an SiO2-M/TiO2 composite photocatalyst was prepared via the sol-gel method using SiO2-M/TiO2 composite photocatalyst with Na2SiO3 content of 0.6 g and pH values of 9.0, 8.5, 8.0, and 7.5 as carriers. The degradation behavior of SiO2-M/TiO2 on methyl orange solution is shown in Figure 3. As can be seen from this figure, with the pH value of Na2SiO3 modification increasing from 6.5 to 7.5, the degradation efficiency of methyl orange by SiO2-M/TiO2 increased. The degradation efficiency was represented by a decrease in the C/C0 value of methyl orange solution at the same illumination time. With the continuous increase of the pH value to 9.0, the degradation efficiency of SiO2-M/TiO2 gradually decreased. By contrast, the degradation of SiO2-M/TiO2 was strongest at pH 7.5 and 8.5. The C/C0 values of MO with 0.6-SiO2-M-7.5/TiO2 and 0.6-SiO2-M-8.5/TiO2 were reduced to 0.01 and 0.02 after UV irradiation for 30 min, respectively. The degradation extent was 99% and 98%, respectively. The degradation extent reached 100% after 40 min of UV irradiation. It can also be seen from Figure 3 that the degradation extent of 0.6-SiO2-M-7.5/TiO2 and 0.6-SiO2-M-8.5/TiO2 was noticeably greater than that of the composite photocatalyst (SiO2/TiO2) with unmodified SiO2 microspheres as the carrier, indicating that the hydrolysis modification of Na2SiO3 significantly promotes the quality of nano-TiO2 supported by SiO2 microspheres. Based on the results of SEM (Figure 2), it was also found that the photocatalytic effect of SiO2-M/TiO2 is closely related to the modification effect of SiO2 microspheres. Accord- Based on the results of SEM (Figure 2), it was also found that the photocatalytic effect of SiO 2 -M/TiO 2 is closely related to the modification effect of SiO 2 microspheres. According to Equations (1) and (2), low H + concentration led to a decrease in hydrolysates, resulting in a decrease in the number of hydroxyl groups on the 0.6-SiO 2 -M-9.0 surface. It is detrimental to the loading of nano-TiO 2 , which reduced the photocatalytic performance of the samples. Large quantities of hydrolysates were formed due to the high H + concentration, which results in the severe agglomeration of 0.6-SiO 2 -M-6.5. Therefore, the photocatalytic degradation ability of SiO 2 -M/TiO 2 prepared by using 0.6-SiO 2 -M-6.5 as carriers was relatively weak. On the other hand, the surface of 0.6-SiO 2 -M-7.5 was coated with more hydrolysates with a large number of hydroxyl groups, while at the same time the dispersion of SiO 2 microspheres was good. Thus, it exhibited a strong photocatalytic degradation effect.

TGA and DSC of SiO 2 -M
To estimate the coating amount of SiO 2 -M, TGA-DSC curves of SiO 2 and 0.6-SiO 2 -M-7.5 were measured. The results are shown in Figure 4. It can be seen that the whole TGA process of 0.6-SiO 2 -M-7.5 was divided into three stages: 25 • C to 80 • C, 80 • C to 180 • C, and 180 • C to 800 • C. The first stage was mainly caused by the evaporation of adsorbed water molecules and the second stage can be attributed to the removal of water from the SiO 2 •nH 2 O. The third stage was produced by condensation between adjacent hydroxyl groups [30]. In comparison, SiO 2 had no significant weight loss until 180 • C; this is because the raw material production of unmodified SiO 2 microspheres was generated under high temperature solid phase conditions. The TGA curves suggested that the weight fraction of hydrolysate (amorphous SiO 2 and SiO 2 •nH 2 O) in 0.6-SiO 2 -M-7.5 was approximately 7.7%.
tration, which results in the severe agglomeration of 0.6-SiO2-M-6.5. Therefore, the pho catalytic degradation ability of SiO2-M/TiO2 prepared by using 0.6-SiO2-M-6.5 as carri was relatively weak. On the other hand, the surface of 0.6-SiO2-M-7.5 was coated w more hydrolysates with a large number of hydroxyl groups, while at the same time dispersion of SiO2 microspheres was good. Thus, it exhibited a strong photocatalytic d radation effect.

TGA and DSC of SiO2-M
To estimate the coating amount of SiO2-M, TGA-DSC curves of SiO2 and 0.6-SiO2-7.5 were measured. The results are shown in Figure 4. It can be seen that the whole TG process of 0.6-SiO2-M-7.5 was divided into three stages: 25 °C to 80 °C, 80 °C to 180 and 180 °C to 800 °C. The first stage was mainly caused by the evaporation of adsorb water molecules and the second stage can be attributed to the removal of water from SiO2•nH2O. The third stage was produced by condensation between adjacent hydro groups [30]. In comparison, SiO2 had no significant weight loss until 180°C; this is beca the raw material production of unmodified SiO2 microspheres was generated under h temperature solid phase conditions. The TGA curves suggested that the weight fract of hydrolysate (amorphous SiO2 and SiO2•nH2O) in 0.6-SiO2-M-7.5 was approximat 7.7%.

Photocatalytic Performance
To investigate the effect of methyl orange concentration on degradation. The 1 0.6-SiO2-M-7.5/TiO2 was added to methyl orange solutions with different initial conc trations for UV light degradation, and the changes in C/C0 and −ln(C/C0) in methyl oran solutions after degradation are shown in Figure 5a,b, respectively. It is evident from F ure 5a that the degradation effect of 0.6-SiO2-M-7.5/TiO2 on the methyl orange solution various original concentrations was gradually enhanced and finally reached a higher d radation extent with the increase of illumination time, indicating that it showed a stro degradation effect on the methyl orange solution of various concentrations. It was no that the degradation extent of MO at different initial concentrations of 0.6-SiO2-M-7.5/T was significantly different. The degradation extent of 10 mg/L and 15 mg/L solution w the highest. When the illumination time was 40 min, the degradation extent reached 10 and 99%, respectively (C/C0 value was 0 and 0.01). As the initial concentration of met orange continued to increase to 20, 25, and 30 mg/L, the degradation rate of methyl oran gradually decreased, which was obviously caused by the high concentration of polluta

Photocatalytic Performance
To investigate the effect of methyl orange concentration on degradation. The 1 g/L 0.6-SiO 2 -M-7.5/TiO 2 was added to methyl orange solutions with different initial concentrations for UV light degradation, and the changes in C/C 0 and −ln(C/C 0 ) in methyl orange solutions after degradation are shown in Figure 5a,b, respectively. It is evident from Figure 5a that the degradation effect of 0.6-SiO 2 -M-7.5/TiO 2 on the methyl orange solution of various original concentrations was gradually enhanced and finally reached a higher degradation extent with the increase of illumination time, indicating that it showed a strong degradation effect on the methyl orange solution of various concentrations. It was noted that the degradation extent of MO at different initial concentrations of 0.6-SiO 2 -M-7.5/TiO 2 was significantly different. The degradation extent of 10 mg/L and 15 mg/L solution was the highest. When the illumination time was 40 min, the degradation extent reached 100% and 99%, respectively (C/C 0 value was 0 and 0.01). As the initial concentration of methyl orange continued to increase to 20, 25, and 30 mg/L, the degradation rate of methyl orange gradually decreased, which was obviously caused by the high concentration of pollutants and the long reaction time. It can also be observed from Figure 5a that the extent of methyl orange degradation by 0.6-SiO 2 -M-7.5/TiO 2 could still reach 80% (C/C 0 was 0.2) when the illumination time was 60 min, indicating that a strong degradation effect could yet be achieved when the illumination time was adequately extended. The above results indicated that 0.6-SiO 2 -M-7.5/TiO 2 could effectively degrade methyl orange solutions of different concentrations. and the long reaction time. It can also be observed from Figure 5a that the extent of methyl orange degradation by 0.6-SiO2-M-7.5/TiO2 could still reach 80% (C/C0 was 0.2) when the illumination time was 60 min, indicating that a strong degradation effect could yet be achieved when the illumination time was adequately extended. The above results indicated that 0.6-SiO2-M-7.5/TiO2 could effectively degrade methyl orange solutions of different concentrations. In addition, the degradation kinetics of MO by SiO2-M-/TiO2 were studied by using the Langmuir-Hinshelwood model to fit the experimental data. Due to the low concentration of reactants, the degradation process was considered to confirm the pseudo-firstorder kinetic equation [31], as shown in Equation (3) −ln(C/C0) = kappt (4) where Kapp is a pseudo-first-order degradation rate constant, reflecting the rate of degradation. It can be seen from Figure 5b that the R 2 values of −ln(C/C0) -t fitting equation obtained by degradation of methyl orange at various initial concentrations were all greater than 0.98, indicating an excellent linear correlation. Thus, it conformed to the characteristics of a quasi-first-order reaction. Among them, Kapp decreased with the growth of MO concentration, indicating the decrease of the degradation reaction rate, which was consistent with Figure 5a. Furthermore, when the initial concentration was 10 ppm, the photocatalyst degradation rate for pollutants decreased after 20 min of illumination. This was attributed to the fact that the pollutants in a lower initial concentration solution were mostly degraded after 20 min of light exposure, resulting in a decline in the number of degradable pollutants in the same amount of time. Therefore, the rate exhibited a decrease. Qin et al. [32] found that higher initial concentrations had higher photocatalytic true concentration ratios by listing the apparent reaction rate constants and the photocatalytic actual concentration ratios at different initial concentrations of pollutants, and higher initial concentrations could improve the efficiency of UV light utilization.
To explore the effect of hydrolytic modification with Na2SiO3 on the photocatalytic performance of TiO2, the photocatalytic degradation behavior of 0.6-SiO2-M-7.5/TiO2 and its comparison samples was tested using methyl orange (MO) as the target pollutant (shown in Figure 6). Figure 6a shows the amount of TiO2 in SiO2, SiO2/TiO2, and 0.6-SiO2-M-7.5/TiO2. There was almost no TiO2 in SiO2 microspheres. In contrast, the amount of TiO2 in 0.6-SiO2-M-7.5/TiO2 was higher than that in SiO2/TiO2, indicating that the hydrolysis modification with Na2SiO3 increased the loading of nano-TiO2 onto the surface of SiO2. Figure 6b presents a comparison of the results for the degradation of MO (10 mg/L) by SiO2/TiO2, 0.6-SiO2-M-7.5/TiO2, and a physical mixture of TiO2 and SiO2-M (14%-TiO2: In addition, the degradation kinetics of MO by SiO 2 -M-/TiO 2 were studied by using the Langmuir-Hinshelwood model to fit the experimental data. Due to the low concentration of reactants, the degradation process was considered to confirm the pseudo-first-order kinetic equation [31], as shown in Equation (3) −ln(C/C 0 ) = k app t (4) where K app is a pseudo-first-order degradation rate constant, reflecting the rate of degradation. It can be seen from Figure 5b that the R 2 values of −ln(C/C 0 ) -t fitting equation obtained by degradation of methyl orange at various initial concentrations were all greater than 0.98, indicating an excellent linear correlation. Thus, it conformed to the characteristics of a quasi-first-order reaction. Among them, K app decreased with the growth of MO concentration, indicating the decrease of the degradation reaction rate, which was consistent with Figure 5a. Furthermore, when the initial concentration was 10 ppm, the photocatalyst degradation rate for pollutants decreased after 20 min of illumination. This was attributed to the fact that the pollutants in a lower initial concentration solution were mostly degraded after 20 min of light exposure, resulting in a decline in the number of degradable pollutants in the same amount of time. Therefore, the rate exhibited a decrease. Qin et al. [32] found that higher initial concentrations had higher photocatalytic true concentration ratios by listing the apparent reaction rate constants and the photocatalytic actual concentration ratios at different initial concentrations of pollutants, and higher initial concentrations could improve the efficiency of UV light utilization.
14% TiO2 + 86% SiO2; 20%-TiO2: 20% TiO2 + 80% 0.6-SiO2-M-7.5; TiO2: 100% TiO2). It can be seen that SiO2/TiO2 and 0.6-SiO2-M-7.5/TiO2 exhibited a better degradation efficiency than 14%-TiO2 and 20%-TiO2, suggesting that both SiO2 and hydrolysis modification with Na2SiO3 have a certain enhancement on the photocatalytic reactivity of TiO2. Among all the samples, 0.6-SiO2-M-7.5/TiO2 showed an excellent degradation effect. The degradation extent of 0.6-SiO2-M-7.5/TiO2 was 93.4% under 20 min, and 100% under 40 min, which was much better than 20%-TiO2. The degradation rate constant of 0.6-SiO2-M-7.5/TiO2 was 0.14534 min −1 , which was about 1.4, 4.0, and 2.0 times higher than that of SiO2/TiO2, 14%-TiO2, and 20%-TiO2, respectively (shown in Figure 6d). Besides, total organic carbon (TOC) was measured to evaluate the photocatalytic activity more accurately. The TOC content of methyl orange solution added with 0.6-SiO2-M-7.5/TiO2 was 15.8% of the original concentration, and that with nano-TiO2 was 15.1%. The degradation extent converted from the TOC was 84.2% and 84.9%, respectively. Methyl orange solution added with composite material and nano-TiO2 was nearly completely mineralized. Although the degradation of methyl orange by SiO2-M-TiO2 composites was not significantly improved compared to that of nano-TiO2, considering that the proportion of nano-TiO2 in the composites was lower at the same catalyst dosage, the performance-enhancement effect due to loading was considered to be obvious. By using low-priced SiO2 microspheres as a carrier, the composite material was prepared by loading or coating Although the degradation of methyl orange by SiO 2 -M-TiO 2 composites was not significantly improved compared to that of nano-TiO 2 , considering that the proportion of nano-TiO 2 in the composites was lower at the same catalyst dosage, the performanceenhancement effect due to loading was considered to be obvious. By using low-priced SiO 2 microspheres as a carrier, the composite material was prepared by loading or coating nano-TiO 2 particles with photocatalytic properties on the carrier surface, which not only reduced the cost by reducing the amount of nano-TiO 2 particles but also maximized the function of nano-TiO 2 particles for enhancing the utilization rate of TiO 2 . Furthermore, SiO 2 microspheres as an inert carrier had a larger particle size than TiO 2 on a macroscopic scale, making it possible to realize the recyclability of SiO 2 -M/TiO 2 and further reduce the cost.

Morphology
In order to understand the influence of Na 2 SiO 3 hydrolytic modification on supported nano-TiO 2 , the morphologies of unmodified SiO 2 , 0.6-SiO 2 -M-7.5, SiO 2 /TiO 2, and 0.6-SiO 2 -M-7.5/TiO 2 were characterized by SEM, and the results are shown in Figure 7a-f. As can be seen from Figure 6a,d, the surfaces of unmodified SiO 2 and 0.6-SiO 2 -M-7.5 were relatively clean, and a small number of attached particles could be caused by impurities brought in by SiO 2 raw materials and coating modification, respectively. By contrast, the surface morphology of SiO 2 /TiO 2 and 0.6-SiO 2 -M-7.5/TiO 2 showed a large number of attached particles, which was obviously caused by the loading of nano-TiO 2 . In particular, a large quantity of loadings on the surface of 0.6-SiO 2 -M-7.5/TiO 2 basically formed a complete coverage of the SiO 2 surface. As for SiO 2 /TiO 2 , loading TiO 2 onto the surface created a smaller amount and only scattered on the surface of SiO 2 microspheres (Figure 7b-f), suggesting that the hydrolysis modification of Na 2 SiO 3 increased the loading of nano-TiO 2 onto the surface of SiO 2 microspheres. It is undoubtedly caused by the coverage of hydrolysis products around the SiO 2 -M surface, increased roughness, and the number of hydroxyl groups. It is consistent with the result that the photocatalytic degradation performance of 0.6-SiO 2 -M-7.5/TiO 2 is better than that of SiO 2 /TiO 2 ( Figure 3). As seen from the SEM area enlargements of SiO 2 /TiO 2 and 0.6-SiO 2 -M-7.5/TiO 2 (Figure 7c,f), the particle sizes of TiO 2 loaded onto the surfaces of both were 45-60 nm and 12-20 nm, respectively, and the latter was significantly smaller than the former. nano-TiO2 particles with photocatalytic properties on the carrier surface, which not only reduced the cost by reducing the amount of nano-TiO2 particles but also maximized the function of nano-TiO2 particles for enhancing the utilization rate of TiO2. Furthermore, SiO2 microspheres as an inert carrier had a larger particle size than TiO2 on a macroscopic scale, making it possible to realize the recyclability of SiO2-M/TiO2 and further reduce the cost.

Morphology
In order to understand the influence of Na2SiO3 hydrolytic modification on supported nano-TiO2, the morphologies of unmodified SiO2, 0.6-SiO2-M-7.5, SiO2/TiO2, and 0.6-SiO2-M-7.5/TiO2 were characterized by SEM, and the results are shown in Figure 7a-f. As can be seen from Figure 6a,d, the surfaces of unmodified SiO2 and 0.6-SiO2-M-7.5 were relatively clean, and a small number of attached particles could be caused by impurities brought in by SiO2 raw materials and coating modification, respectively. By contrast, the surface morphology of SiO2/TiO2 and 0.6-SiO2-M-7.5/TiO2 showed a large number of attached particles, which was obviously caused by the loading of nano-TiO2. In particular, a large quantity of loadings on the surface of 0.6-SiO2-M-7.5/TiO2 basically formed a complete coverage of the SiO2 surface. As for SiO2/TiO2, loading TiO2 onto the surface created a smaller amount and only scattered on the surface of SiO2 microspheres (Figure 7b-f), suggesting that the hydrolysis modification of Na2SiO3 increased the loading of nano-TiO2 onto the surface of SiO2 microspheres. It is undoubtedly caused by the coverage of hydrolysis products around the SiO2-M surface, increased roughness, and the number of hydroxyl groups. It is consistent with the result that the photocatalytic degradation performance of 0.6-SiO2-M-7.5/TiO2 is better than that of SiO2/TiO2 ( Figure 3). As seen from the SEM area enlargements of SiO2/TiO2 and 0.6-SiO2-M-7.5/TiO2 (Figure 7c,f), the particle sizes of TiO2 loaded onto the surfaces of both were 45-60 nm and 12-20 nm, respectively, and the latter was significantly smaller than the former. The TEM image of 0.6-SiO 2 -M-7.5/TiO 2 in Figure 7g shows that fine nano-TiO 2 particles are evenly coated on the surface of SiO 2 microspheres. It confirmed the results of SEM. Figure 7 shows an HRTEM image of the surface area of 0.6-SiO 2 -M-7.5/TiO 2 particles at h, which could be divided into three parts. The black site was the part close to the surface of the SiO 2 microsphere, with a relatively regular boundary. The red dotted line near the outer surface of the microsphere shows strong electron transmission and no lattice fringes. It is speculated that it is amorphous SiO 2 or SiO 2 •nH 2 O hydrolysates of Na 2 SiO 3 . The outermost yellow dotted line is the crystalline phase with fringes. The measured fringe spacing was 0.35 nm, corresponding to the (101) surface spacing of anatase (ICDDcard#21-1272), indicating that this crystalline phase was supported by anatase-type nano-TiO 2 . Moreover, the grain size was about 10-20 nm, which is in agreement with the observation results of SEM. The results in Figure 7h show that the amorphous SiO 2 or SiO 2 •nH 2 O coating layer was around the surface of SiO 2 microspheres after hydrolytic modification by Na 2 SiO 3 , and then nano-TiO 2 was supported on the surface of the coating layer to form a ternary composite structure.  The TEM image of 0.6-SiO2-M-7.5/TiO2 in Figure 7g shows that fine nano-TiO2 particles are evenly coated on the surface of SiO2 microspheres. It confirmed the results of SEM. Figure 7 shows an HRTEM image of the surface area of 0.6-SiO2-M-7.5/TiO2 particles at h, which could be divided into three parts. The black site was the part close to the surface of the SiO2 microsphere, with a relatively regular boundary. The red dotted line near the outer surface of the microsphere shows strong electron transmission and no lattice fringes. It is speculated that it is amorphous SiO2 or SiO2•nH2O hydrolysates of Na2SiO3. The outermost yellow dotted line is the crystalline phase with fringes. The measured fringe spacing was 0.35 nm, corresponding to the (101) surface spacing of anatase (ICDDcard#21-1272), indicating that this crystalline phase was supported by anatase-type nano-TiO2. Moreover, the grain size was about 10-20 nm, which is in agreement with the observation results of SEM. The results in Figure 7h show that the amorphous SiO2 or SiO2•nH2O coating layer was around the surface of SiO2 microspheres after hydrolytic modification by Na2SiO3, and then nano-TiO2 was supported on the surface of the coating layer to form a ternary composite structure. Figure 8 shows the element mapping results of O, Si, Ti, Na, and S of 0.6-SiO2-M-7.5/TiO2. O and Si elements were uniformly distributed at the corresponding positions of 0.6-SiO2-M-7.5/TiO2 composite particles. Moreover, Ti elements were also uniformly distributed. It was further indicated that the small debris on 0.6-SiO2-M-7.5 could be nano-TiO2 particles.  Figure 9a shows the XRD patterns of SiO2, 0.6-SiO2-M-7.5, SiO2/TiO2, 0.6-SiO2-M-7/TiO2, 0.6-SiO2-M-7.5/TiO2, and nano-TiO2. In the XRD pattern of SiO2, a weak and broad characteristic peak appeared between the diffraction angle (2θ) of 15 and 28°, indicating that the unmodified SiO2 had an amorphous structure. The XRD pattern of 0.6-SiO2-M-7.5 showed a stronger diffraction peak between 15 and 28° than SiO2, indicating that the modification of Na2SiO3 has no effect on the phase composition of SiO2. In the XRD patterns of the composite photocatalysts, diffraction peaks of anatase were present at 25.  In the XRD pattern of SiO 2 , a weak and broad characteristic peak appeared between the diffraction angle (2θ) of 15 and 28 • , indicating that the unmodified SiO 2 had an amorphous structure. The XRD pattern of 0.6-SiO 2 -M-7.5 showed a stronger diffraction peak between 15 and 28 • than SiO 2 , indicating that the modification of Na 2 SiO 3 has no effect on the phase composition of SiO 2 . In the XRD patterns of the composite photocatalysts, diffraction peaks of anatase were present at 25.  (024), respectively. This might be due to oxygen vacancy defects entering the TiO 2 lattice, resulting in the formation of the Magnéli phase. Calcined at low temperatures, Ti 6 O 11 fails to completely transform into anatase [33].

Crystalline and Structure
SiO2/TiO2, suggesting that different pH modifications did not affect the physical phase nano-TiO2 in the composite samples. However, the characteristic diffraction peaks of atase in both 0.6-SiO2-M-7/TiO2 and 0.6-SiO2-M-7.5/TiO2 were stronger than those SiO2/TiO2, presumably due to the hydrolytic modification of Na2SiO3 to increase amount of loaded TiO2 nanoparticles on the surface of SiO2. In addition, the XRD d were taken into the Scherrer formula, and the average size of the anatase phase TiO2 gra in SiO2/TiO2 and 0.6-SiO2-M-7.5/TiO2 was calculated to be 20 nm and 15 nm, respective It was hypothesized that there is less aggregation of TiO2 particles as a result of the f mation of more Ti-O-Si bonds after the modification. Besides, in the XRD pattern of Ti diffraction peaks of Ti6O11 were present at 22.8° and 27.7°, corresponding to the anat (101) and (024), respectively. This might be due to oxygen vacancy defects entering TiO2 lattice, resulting in the formation of the Magnéli phase. Calcined at low temperatur Ti6O11 fails to completely transform into anatase [33]. To investigate the change in electrical charges on the surface of SiO2 microsphe before and after modification, the points of zero charge (PZC) of 0.6-SiO2-M-7.5 and S were tested (Figure 9b). When the pH was lower than 1, the potential of 0.6-SiO2-M-(ζ1) was negative, indicating that the surface was negatively charged, while the surface SiO2 was positively charged at this time (ζ2 was higher than 0). After the pH was grea than 1, the potentials of both became negative. However, the absolute value of ζ1 w consistently increased compared to the absolute value of ζ2, illustrating that the treatm modification of Na2SiO3 strengthened the negative charge on the surface of SiO2 mic spheres. It was obviously due to the increase of the number of hydroxyl groups on surface of SiO2 microspheres after modification, which was consistent with the results Table 1. It also suggested that the modification was more favorable for the bonding tween SiO2 and nano-TiO2. To investigate the change in electrical charges on the surface of SiO 2 microspheres before and after modification, the points of zero charge (PZC) of 0.6-SiO 2 -M-7.5 and SiO 2 were tested (Figure 9b). When the pH was lower than 1, the potential of 0.6-SiO 2 -M-7.5 (ζ 1 ) was negative, indicating that the surface was negatively charged, while the surface of SiO 2 was positively charged at this time (ζ 2 was higher than 0). After the pH was greater than 1, the potentials of both became negative. However, the absolute value of ζ 1 was consistently increased compared to the absolute value of ζ 2 , illustrating that the treatment modification of Na 2 SiO 3 strengthened the negative charge on the surface of SiO 2 microspheres. It was obviously due to the increase of the number of hydroxyl groups on the surface of SiO 2 microspheres after modification, which was consistent with the results in Table 1. It also suggested that the modification was more favorable for the bonding between SiO 2 and nano-TiO 2 .
The N 2 adsorption-desorption isotherms and pore-size distribution of SiO 2 , 0.6-SiO 2 -M-7.5, 0.6-SiO 2 -M-7.5/TiO 2 , and nano-TiO 2 are displayed in Table 2 and Figure 9c,d to explore the specific surface area and pore structure features. In Table 2, it can be noticed that SiO 2 had a certain total pore volume, a small average pore size, and a mesoporous structure located at 10-15 nm (Figure 9d), which may be caused by the accumulation of microspheres. The decrease in specific surface area after modification was due to the coating of the modifier. In addition, the total pore volume and specific surface area of nano-TiO 2 were significantly higher than the other samples and exhibited a broad pore distribution in the range of 20-50 nm (Figure 9d). This is reflected in the porous structural characteristics of the agglomerated TiO 2 particle aggregates, which are in agreement with the SEM result of nano-TiO 2 (Figure 7i). The total pore volume of the composite material was dramatically reduced, signaling the improved dispersion of TiO 2 after loading. As seen in Figure 9c, the adsorption isotherms of nano-TiO 2 belong to type IV of the adsorption isotherm. At pressures of P/P 0 = 0.4 to 1.0, the adsorption volume increased substantially with increasing pressure. It suggested the presence of a certain amount of porous distribution apparently attributable to agglomeration. The adsorption isotherms of SiO 2 , 0.6-SiO 2 -M-7.5, and 0.6-SiO 2 -M-7.5/TiO 2 revealed that they have almost no pore structure. By contrast, the unmodified SiO 2 microspheres showed a moderate increase in adsorption with increasing pressure at high pressures of P/P 0 = 0.8 to 1.0 compared with the modified microspheres, further confirming that the modification was beneficial in promoting the dispersibility of the microspheres. To study the effect of modification on the surface properties of the material, the functional groups of SiO 2 , 0.6-SiO 2 -M-7.5, 0.6-SiO 2 -M-7.5/TiO 2 , and nano-TiO 2 were tested by infrared spectroscopy (shown in Figure 10). The absorption peak at 1101 cm −1 in the SiO 2 spectrum corresponds to the asymmetric stretching vibration of Si-O-Si [34]. The absorption peaks at 808 cm −1 and 477 cm −1 were attributed to the symmetric contraction and deformation vibrations of Si-O, and the absorption peak at 3252 cm −1 was the stretching and bending vibration of Si-OH or adsorbed water [35,36]. All of the above absorption peaks have been observed in the 0.6-SiO 2 -M-7.5 spectrum with an increase in peak intensity compared with SiO 2 . Moreover, a feature peak at 3650 cm −1 corresponded to the structural hydroxyl group, which was in agreement with the measured hydroxyl number results and supported the SEM and TEM results. In the 0.6-SiO 2 -M-7.5/TiO 2 spectrum, the signature peak of the hydroxyl group was noticeably weaker. The absorption peaks at 1101 cm −1 , 808 cm −1 , and 477 cm −1 were also weaker and offset to a certain extent, but the peak's intensity was stronger than SiO 2 . It suggested that nano-TiO 2 and 0.6-SiO 2 -M-7.5 form a Ti-O-Si bond through the interaction of hydroxyl groups on the surface. The above results not only demonstrated that the modification increased the number of hydroxyl groups on the surface of SiO 2 microspheres, but also suggested that TiO 2 was stably coupled with the amorphous SiO 2 and SiO 2 •nH 2 O by forming Ti-O-Si bonds.

Mechanism of SiO2-M to Enhance the Performance of Nano-TiO2
According to the above results, the mechanism of improvement of the photocat performance of nano-TiO2 by SiO2-M supporting can be summarized. First, the raw rial production of unmodified SiO2 microspheres was in the high-temperature solidcondition, resulting in extremely small amounts of hydroxyl groups on the surface. difficult for the colloidal particles of TiO2 precursors generated by the sol-gel meth combine with SiO2 microspheres through their respective surface hydroxyl groups ing (forming Ti-OH---OH-Si and Ti-O-Si bonds). On the one hand, it led to loading o small amount of nano-TiO2 on the surface of SiO2 microspheres. On the other hand, to an increased apparent granularity due to the agglomeration of the nano-TiO2. Aft hydrolysis modification of SiO2 microspheres by Na2SiO3, the quantity of hydroxyl g on the surface of the microspheres rose significantly due to the attachment of hydrol Therefore, the loading amount of nano-TiO2 eventually increased. Second, in the pre tion process of SiO2-M/TiO2, the increase of the quantity of hydroxyl groups on the su of SiO2-M and the expansion of the surface binding with the nano-TiO2 precursor reduced the size and dispersion of TiO2, leading to the rise of the exposed degree active spot. Thirdly, as the TiO2 scale loaded onto the surface of SiO2-M was reduc was beneficial to enhance the quantum effect of nano-TiO2. A schematic diagram re ing the above mechanism and the preparation process of SiO2-M/TiO2 is shown in F 11.

Mechanism of SiO 2 -M to Enhance the Performance of Nano-TiO 2
According to the above results, the mechanism of improvement of the photocatalytic performance of nano-TiO 2 by SiO 2 -M supporting can be summarized. First, the raw material production of unmodified SiO 2 microspheres was in the high-temperature solidphase condition, resulting in extremely small amounts of hydroxyl groups on the surface. It was difficult for the colloidal particles of TiO 2 precursors generated by the sol-gel method to combine with SiO 2 microspheres through their respective surface hydroxyl groups bonding (forming Ti-OH-OH-Si and Ti-O-Si bonds). On the one hand, it led to loading only a small amount of nano-TiO 2 on the surface of SiO 2 microspheres. On the other hand, it led to an increased apparent granularity due to the agglomeration of the nano-TiO 2 . After the hydrolysis modification of SiO 2 microspheres by Na 2 SiO 3 , the quantity of hydroxyl groups on the surface of the microspheres rose significantly due to the attachment of hydrolysate. Therefore, the loading amount of nano-TiO 2 eventually increased. Second, in the preparation process of SiO 2 -M/TiO 2 , the increase of the quantity of hydroxyl groups on the surface of SiO 2 -M and the expansion of the surface binding with the nano-TiO 2 precursor both reduced the size and dispersion of TiO 2 , leading to the rise of the exposed degree of the active spot. Thirdly, as the TiO 2 scale loaded onto the surface of SiO 2 -M was reduced, it was beneficial to enhance the quantum effect of nano-TiO 2 . A schematic diagram reflecting the above mechanism and the preparation process of SiO 2 -M/TiO 2 is shown in Figure 11. Materials 2021, 14, x FOR PEER REVIEW 16 of 18 . Figure 11. Schematic diagram of loading nano-TiO2 on SiO2 microspheres (a-c).

Conclusions
By the hydrolytic modification of industrial by-product SiO2 microspheres with Na2SiO3, modified SiO2 microspheres (SiO2-M) coated with a certain amount of Na2SiO3 hydrolyzed products (amorphous SiO2 or SiO2•nH2O) were obtained. Compared with unmodified SiO2 microspheres, the quantity of hydroxyl groups on the surface of SiO2-M was significantly increased.
SiO2-M supporting nano-TiO2 composite photocatalyst (SiO2-M/TiO2) was prepared through the sol-gel method using SiO2-M as the carrier. SiO2-M/TiO2 had acceptable photocatalytic degradation of MO, comparable with that of pure nano-TiO2. The degradation performance of MO was noticeably better than that of unmodified SiO2 microspheres supporting nano-TiO2 products and slightly better than that of pure nano-TiO2. SiO2-M/TiO2 was formed by nano-TiO2 uniformly supported on the surface of SiO2-M; the size of TiO2 particles was 10-20 nm with good dispersion.
The influence mechanism of nano-TiO2 supported by SiO2-M to improve the photocatalytic performance and the hydrolysis modification of Na2SiO3 was as follows: The quantity of hydroxyl groups on the surface of SiO2-M increased, which enhanced the degree of binding with TiO2 precursors. As a result, the loading capacity of nano-TiO2 on the surface of SiO2-M was remarkably increased compared with that of unmodified SiO2 microspheres. The size of nano-TiO2 supported on the surface of SiO2-M was smaller than that on the surface of unmodified SiO2 microspheres, and the dispersion of nano-TiO2 was higher, which leads to increased exposure to the active site of nano-TiO2.

Conclusions
By the hydrolytic modification of industrial by-product SiO 2 microspheres with Na 2 SiO 3 , modified SiO 2 microspheres (SiO 2 -M) coated with a certain amount of Na 2 SiO 3 hydrolyzed products (amorphous SiO 2 or SiO 2 •nH 2 O) were obtained. Compared with unmodified SiO 2 microspheres, the quantity of hydroxyl groups on the surface of SiO 2 -M was significantly increased.
SiO 2 -M supporting nano-TiO 2 composite photocatalyst (SiO 2 -M/TiO 2 ) was prepared through the sol-gel method using SiO 2 -M as the carrier. SiO 2 -M/TiO 2 had acceptable photocatalytic degradation of MO, comparable with that of pure nano-TiO 2 . The degradation performance of MO was noticeably better than that of unmodified SiO 2 microspheres supporting nano-TiO 2 products and slightly better than that of pure nano-TiO 2 . SiO 2 -M/TiO 2 was formed by nano-TiO 2 uniformly supported on the surface of SiO 2 -M; the size of TiO 2 particles was 10-20 nm with good dispersion.
The influence mechanism of nano-TiO 2 supported by SiO 2 -M to improve the photocatalytic performance and the hydrolysis modification of Na 2 SiO 3 was as follows: The quantity of hydroxyl groups on the surface of SiO 2 -M increased, which enhanced the degree of binding with TiO 2 precursors. As a result, the loading capacity of nano-TiO 2 on the surface of SiO 2 -M was remarkably increased compared with that of unmodified SiO 2 microspheres. The size of nano-TiO 2 supported on the surface of SiO 2 -M was smaller than that on the surface of unmodified SiO 2 microspheres, and the dispersion of nano-TiO 2 was higher, which leads to increased exposure to the active site of nano-TiO 2 .
This study played a positive role in improving the property of supported nano-TiO 2 and reducing the manufacturing and application photocatalyst cost by optimizing the carrier's performance.