Preparation of Nano-TiO2-Coated SiO2 Microsphere Composite Material and Evaluation of Its Self-Cleaning Property

In order to improve the dispersion of nano-TiO2 particles and enhance its self-cleaning properties, including photocatalytic degradation of pollutants and surface hydrophilicity, we prepared nano-TiO2-coated SiO2 microsphere composite self-cleaning materials (SiO2–TiO2) by co-grinding SiO2 microspheres and TiO2 soliquid and calcining the ground product. The structure, morphology, and self-cleaning properties of the SiO2–TiO2 were characterized. The characterization results showed that the degradation efficiency of methyl orange by SiO2–TiO2 was 97%, which was significantly higher than that obtained by pure nano-TiO2. The minimum water contact angle of SiO2–TiO2 was 8°, indicating strong hydrophilicity and the good self-cleaning effect. The as-prepared SiO2–TiO2 was characterized by the nano-TiO2 particles uniformly coated on the SiO2 microspheres and distributed in the gap among the microspheres. The nano-TiO2 particles were in an anatase phase with the particle size of 15–20 nm. The nano-TiO2 particles were combined with SiO2 microspheres via the dehydroxylation of hydroxyl groups on their surfaces.


Introduction
Nano-titanium dioxide (TiO 2 ) is a typical semiconductor material with excellent properties. Moreover, it is stable, cheap, and non-toxic [1,2]. Therefore, it has been widely applied in the environmental protection [3], energy [4], and other fields [5,6]. In addition to the photocatalytic activity of TiO 2 under ultraviolet (UV) irradiation, the self-cleaning effect due to photoinduced hydrophilic properties of TiO 2 has always been one of the hotspots [7,8]. Its self-cleaning mechanism is generally ascribed to two effects [9,10]. Firstly, under the irradiation of ultraviolet light or ultraviolet in sunlight, the active components induced by the photocatalytic action of TiO 2 on the TiO 2 self-cleaning film can react with the pollutants adhering to the surface, thus achieving the decomposition of pollutants. Secondly, due to the super-hydrophilicity of the self-cleaning film, the decomposed products can be washed away by rain, so as to maintain the clean material surface [11]. In China and other developing countries, the contents of dust and oily dirt are high in the urban atmosphere and dust and oily dirt tend to adhere to building walls and glass surface to make the surface dirty. Nano-TiO 2 self-cleaning materials may be used to coat such surfaces [12,13].
However, some factors restrict the application scope of nano-TiO 2 self-cleaning materials. For example, the agglomeration phenomenon and poor dispersivity of TiO 2 particles in the application system significantly reduces its self-cleaning effect [14,15]. Coating TiO 2 particles on the matrix surface of SiO 2 microspheres were 93.78% SiO 2 and 4.96% ZrO 2 . SiO 2 is mainly composed of amorphous phase, exhibiting the microsphere morphology with the particle size of 1-3 µm. The SiO 2 particles are aggregated to form the aggregates with the larger particle size. After depolymerizing the aggregates, the SiO 2 microspheres exist in a dispersed state.
Tetrabutyl titanate (C 16 H 36 O 4 Ti) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) was used as the titanium source. Acetylacetone (C 5 H 8 O 2 ) supplied by Xi Long Chemical Co., Ltd. (Guangzhou, China) was used as a hydrolysis control agent. Methyl orange (C 14 H 14 N 3 SO 3 Na) from Beijing Chemical Industry Group Co., Ltd. (Beijing, China) was used as a target pollution for photocatalytic degradation. Ethanol and deionized water are also used as solvents throughout the preparation process.

Depolymerization of SiO 2 Microspheres
Considering the agglomeration effect of particles in the raw SiO 2 microspheres, SiO 2 microspheres need to be depolymerized and dispersed before compositing with nano-TiO 2 . The depolymerization method was described as follows: The SiO 2 microsphere materials were added into the ethanol solution to form a suspension. After adding ceramic grinding balls (the ratio of ball to material, 3:1), the suspension was then ground in the mixing mill (CSDM-S3, Beijing Paleozoic Powder Technology Co., Ltd., Beijing, China) for 60 min. Finally, the dispersed SiO 2 microspheres were obtained after ball-material separation, filtration, and desiccation.

Preparation of Nano-TiO 2 Soliquid
Firstly, 8.5 mL of tetrabutyl titanate was dissolved into 10 mL of ethanol solution. The mixed solution was stirred evenly and marked as Solution A. Then, 1.3 mL of acetylacetone was dissolved into 10 mL of ethanol solution, and the obtained solution was marked as Solution B. Then, Solution B was slowly added into Solution A and 19.35 mL of the mixture of ethanol and water (water 0.85 mL) was also added into Solution A. Afterwards, the mixture was stirred vigorously at room temperature for 12 h and the stirred mixture was aged for 48 h to obtain the nano-TiO 2 soliquid. The viscosity of the nano-TiO 2 soliquid obtained after 48-h aging was measured to be 2 × 10 −3 Pa·s by a digital display viscometer (NDJ-8S, Shanghai Precision Instrument and Meter Co., Ltd., Shanghai, China). For comparison, partial nano-TiO 2 soliquid was dried and calcined to prepare TiO 2 nanoparticles. According to the X-ray diffraction (XRD) data and the Scherrer Equation, the grain size of nano-TiO 2 was calculated to be 15-20 nm.

Preparation of SiO 2 -TiO 2
Firstly, the dispersed SiO 2 microspheres were added into the ethanol solution, which was stirred to form a suspension. Secondly, the suspension was added into the aged nano-TiO 2 soliquid to form the SiO 2 /TiO 2 mixture. Thirdly, the SiO 2 /TiO 2 mixture were stirred by a CSDM-S3 mixing mill (Beijing Gosdel Powder&Technology Co., Ltd., Beijing, China) for 90 min after the addition of a certain amount of grinding balls to obtain the SiO 2 /TiO 2 soliquid composites. Then, the SiO 2 /TiO 2 soliquid composites were put in a SRJX-5-13 chamber electric furnace (Tianjin Taisite Instrument Co., LTD, Tianjin, China) and calcined at 500 • C for 2 h. Finally, the SiO 2 -TiO 2 was prepared.

Evaluation of Self-Cleaning Performance
Photocatalytic Activity The photocatalytic degradation performance of SiO 2 -TiO 2 was tested with the methyl orange as the target degradation pollutant. The system was irradiated by a mercury lamp (100 W, the main wavelength of 254 nm). Then, 40 mg of SiO 2 -TiO 2 was added to 50 mL of prepared methyl orange dilution (concentration 10 mg/L). In order to reduce the measurement error caused by sample adsorption, the dark reaction was carried out for 0.5 h and then the concentration of methyl orange (C 0 ) in the solution was measured. After turning on the light source, the concentration of methyl orange (C) in solution was measured every 20 min. The photocatalytic degradation performance of the samples was characterized and evaluated based on the change of C/C 0 .
The concentration of methyl orange was measured according to the following procedure. Firstly, the solution was centrifuged and the absorbance of the supernatant was measured with a Cary 5000 UV-VIS spectrophotometer (USA Varian, Palo Alto, CA, USA). The concentration of methyl orange in the solution was calculated according to the relationship between absorbance and concentration.

Hydrophilicity
The hydrophilicity of the SiO 2 -TiO 2 particles was characterized based on the wetting degree of water on its surface. The wetting degree was reflected by the measured water contact angle on its surface. The SiO 2 -TiO 2 composite powder was pressed into a sheet-like sample by a tableting machine and then the water contact angle was measured by a contact angle meter (JC2000D, Shanghai Zhongchen Digital Technic Apparatus Co. Ltd., Shanghai, China) three times. The measurement results were averaged.

Characterization of Structure and Morphology
We observed the morphology of SiO 2 -TiO 2 by scanning electron microscope (SEM) (S-3500N, Hitachi, Ltd., Tokyo, Japan) and transmission electron microscope (TEM) (FEI Tecnai G2 F20, Portland, OR, USA). The surface functional groups were examined by an infrared spectroscope (Spectrum 100, PerkinElmer Instruments (Shanghai) Co., Ltd., Shanghai, China) with KBr as the medium, and the weights of each sample and KBr were, respectively, 1 and 200 mg. The phase analysis was carried out with an X-ray diffractometer (D/MAX2000, Rigaku Corporation, Tokyo, Japan).The specific surface areas of SiO 2 and SiO 2 -TiO 2 were tested by the QuadraSorb SI specific surface area analyzer (Quantachrome Instrument Company, Boynton Beach, FL, USA). In addition, the surface roughness of SiO 2 microspheres and SiO 2 -TiO 2 were evaluated using a Mutimode VIII atomic force microscope (Bruke, Fremont, CA, USA).  Figure 1a represents the degradation behaviors of methyl orange dye during irradiation as a function of time (min) in the presence of SiO 2 -TiO 2 with different TiO 2 ratios (the mass ratio of TiO 2 to SiO 2 -TiO 2 ). As shown in Figure 1a, the SiO 2 microspheres exhibit no degradation effect on methyl orange, whereas pure TiO 2 has a certain degradation effect on methyl orange. All of the prepared SiO 2 -TiO 2 materials exhibit the significantly higher photocatalytic degradation efficiency on methyl orange dye than that of pure nano-TiO 2 . Among these SiO 2 -TiO 2 samples, with SiO 2 -TiO 2 -40 (TiO 2 ratio is 40%) as the photocatalyst, after the solution was irradiated for 40 min, the C/Co was reduced to about 0.1 and the degradation efficiency reached 90%. After the 120 min irradiation, the degradation efficiency reached 97%. With the pure nano-TiO 2 as the photocatalyst, the degradation efficiencies after 40 and 120 min respectively reached 50% and 90%. The abovementioned results indicated that the photocatalytic activity of nano-TiO 2 had been greatly improved when TiO 2 coated the surface of SiO 2 microspheres. In addition, the TiO 2 ratio had a significant effect on the degradation efficiency of SiO 2 -TiO 2 . With the increase in the TiO 2 ratio from 20% to 40%, the photocatalytic degradation efficiency gradually increased and finally reached its maximum value. When the mass ratio of TiO 2 increased to 50%, the degradation efficiency decreased. However, the degradation efficiency of SiO 2 -TiO 2 with different TiO 2 ratios was always higher than that of pure nano-TiO 2 . The phenomenon might be interpreted in two aspects: Firstly, the coating of nano-TiO 2 on SiO 2 microsphere surface could improve the dispersibility of nano-TiO 2 , thus resulting in an increase in the number of reactive groups under irradiation and increasing the quantum efficiency. Secondly, SiO 2 had a high reflection efficiency on ultraviolet radiation, and the light reflected by SiO 2 could be absorbed by TiO 2 , thus improving the absorption of ultraviolet light by SiO 2 -TiO 2 . The specific surface area analysis results showed that the surface area of SiO 2 had been significantly incresed from its original value of 5.698 to 44.410 m 2 /g after TiO 2 coating. This result also comfirmed that the SiO 2 microspheres had been coated by nano TiO 2 effectively. Figure 1b shows the influence of the ratio of grinding ball to materials (B-M) in the grinding process on the photocatalytic activity of SiO 2 -TiO 2 . The degradation efficiency of SiO 2 -TiO 2 samples prepared with grinding balls was significantly higher than that of the SiO 2 -TiO 2 prepared without grinding balls (B-M is 0). The degradation effect was the best when the B-M ratio was 5. After 120 min irradiation, the highest degradation efficiency was 95% (C/C 0 = 0.05) at the B-M ratio of 5% and 65% at the B-M of 0. The above results showed that the grinding process had an important effect on the performance of SiO 2 -TiO 2 . Therefore, the proper B-M ratio should be selected. As shown in Figure 1b, the degradation effect of SiO 2 -TiO 2 is stronger than that of pure nano-TiO 2 . The result is consistent with the results shown in Figure 1a. microsphere surface could improve the dispersibility of nano-TiO2, thus resulting in an increase in the number of reactive groups under irradiation and increasing the quantum efficiency. Secondly, SiO2 had a high reflection efficiency on ultraviolet radiation, and the light reflected by SiO2 could be absorbed by TiO2, thus improving the absorption of ultraviolet light by SiO2-TiO2. The specific surface area analysis results showed that the surface area of SiO2 had been significantly incresed from its original value of 5.698 to 44.410 m 2 /g after TiO2 coating. This result also comfirmed that the SiO2 microspheres had been coated by nano TiO2 effectively. Figure 1b shows the influence of the ratio of grinding ball to materials (B-M) in the grinding process on the photocatalytic activity of SiO2-TiO2. The degradation efficiency of SiO2-TiO2 samples prepared with grinding balls was significantly higher than that of the SiO2-TiO2 prepared without grinding balls (B-M is 0). The degradation effect was the best when the B-M ratio was 5. After 120 min irradiation, the highest degradation efficiency was 95% (C/C0 = 0.05) at the B-M ratio of 5% and 65% at the B-M of 0. The above results showed that the grinding process had an important effect on the performance of SiO2-TiO2. Therefore, the proper B-M ratio should be selected. As shown in Figure 1b, the degradation effect of SiO2-TiO2 is stronger than that of pure nano-TiO2. The result is consistent with the results shown in Figure 1a. The UV-VIS absorption spectra of bare SiO2 microspheres, nano-TiO2, and SiO2-TiO2-50 were obtained for comparison ( Figure 2). The light absorption of SiO2 in a wavelength range between 300 and 400 nm was insignificant, whereas TiO2 absorbed light with the wavelength below 400 nm. The SiO2-TiO2 exhibited the higher light absorption in a wavelength range from 200 to 400 nm than that of pure nano-TiO2, which was completely different from bare SiO2 microspheres. The results indicated that the SiO2-TiO2 had the higher UV absorption due to the high reflection efficiency on ultraviolet radiation by SiO2 microspheres, confirming that SiO2 microspheres were coated by nano-TiO2 particles with similar light absorption properties to TiO2. Meanwhile, this results contribute to the good photocatalytic activity of SiO2-TiO2. The UV-VIS absorption spectra of bare SiO 2 microspheres, nano-TiO 2 , and SiO 2 -TiO 2 -50 were obtained for comparison ( Figure 2). The light absorption of SiO 2 in a wavelength range between 300 and 400 nm was insignificant, whereas TiO 2 absorbed light with the wavelength below 400 nm. The SiO 2 -TiO 2 exhibited the higher light absorption in a wavelength range from 200 to 400 nm than that of pure nano-TiO 2 , which was completely different from bare SiO 2 microspheres. The results indicated that the SiO 2 -TiO 2 had the higher UV absorption due to the high reflection efficiency on ultraviolet radiation by SiO 2 microspheres, confirming that SiO 2 microspheres were coated by nano-TiO 2 particles with similar light absorption properties to TiO 2 . Meanwhile, this results contribute to the good photocatalytic activity of SiO 2 -TiO 2 .

Results and Discussion
SiO2-TiO2 exhibited the higher light absorption in a wavelength range from 200 to 400 nm than that of pure nano-TiO2, which was completely different from bare SiO2 microspheres. The results indicated that the SiO2-TiO2 had the higher UV absorption due to the high reflection efficiency on ultraviolet radiation by SiO2 microspheres, confirming that SiO2 microspheres were coated by nano-TiO2 particles with similar light absorption properties to TiO2. Meanwhile, this results contribute to the good photocatalytic activity of SiO2-TiO2.   Figure 3 shows the change of water contact angle of SiO 2 -TiO 2 particles with different TiO 2 ratios after irradiation by ultraviolet light for 2 h. For the SiO 2 microsphere materials, the contact angle was maintained to be 28 • after UV irradiation, indicating that the UV light had no effect on its hydrophilicity. The water contact angle of pure TiO 2 is 26 • before UV irradiation, which is higher than that of SiO 2 -TiO 2 , indicating that the coating of TiO 2 on SiO 2 surface can improve the hydrophilicity of TiO 2 . The improvement effect may be interpreted as follows. The dispersion of nano-TiO 2 was improved and then more active hydroxyl groups on TiO 2 surface were exposed. Meanwhile, the water contact angle of pure TiO 2 decreased from 26 • to 10 • after UV irradiation, indicating the photoinduced hydrophilicity of TiO 2 . The water contact angle of SiO 2 -TiO 2 was 15-18 • and decreased to 8-13 • after UV irradiation, showing the strong hydrophilicity. The SiO 2 -TiO 2 -40 (TiO 2 ratio is 40) showed the strongest hydrophilicity and its water contact angles were 17 • and 8 • before and after UV irradiation respectively. The strong photo-induced hydrophilicity and photocatalytic activity of SiO 2 -TiO 2 indicate its good self-cleaning performance.  Figure 3 shows the change of water contact angle of SiO2-TiO2 particles with different TiO2 ratios after irradiation by ultraviolet light for 2 h. For the SiO2 microsphere materials, the contact angle was maintained to be 28° after UV irradiation, indicating that the UV light had no effect on its hydrophilicity. The water contact angle of pure TiO2 is 26° before UV irradiation, which is higher than that of SiO2-TiO2, indicating that the coating of TiO2 on SiO2 surface can improve the hydrophilicity of TiO2. The improvement effect may be interpreted as follows. The dispersion of nano-TiO2 was improved and then more active hydroxyl groups on TiO2 surface were exposed. Meanwhile, the water contact angle of pure TiO2 decreased from 26° to 10° after UV irradiation, indicating the photoinduced hydrophilicity of TiO2. The water contact angle of SiO2-TiO2 was 15-18° and decreased to 8-13° after UV irradiation, showing the strong hydrophilicity. The SiO2-TiO2-40 (TiO2 ratio is 40) showed the strongest hydrophilicity and its water contact angles were 17° and 8° before and after UV irradiation respectively. The strong photo-induced hydrophilicity and photocatalytic activity of SiO2-TiO2 indicate its good self-cleaning performance. To investigate the mechanism of the photoinduced hydrophilicity of SiO2-TiO2, the infrared spectral analysis was carried out. Figure 4 shows the Fourier transform infrared spectroscopy (FT-IR) spectra of SiO2-TiO2-20 and SiO2-TiO2-30 before and after UV irradiation. The characteristic absorption peaks in the range of 2800-3800 cm −1 and 1620 cm −1 in all the samples were ascribed to the vibration of the hydroxyl groups on the SiO2-TiO2 surface. When the TiO2 ratio was 30%, after the UV irradiation (b2 in Figure 4), the intensity of the absorption peak in the range of 2800-3800 cm −1 in the FTIR spectrum of SiO2-TiO2 was higher than that in the spectrum b1 (before the UV irradiation) and the peak was shifted to the higher wavenumber. Meanwhile, the absorption peak at 1620 cm −1 in b2 was sharper than that in b1. The abovementioned results indicated that the number of hydroxyl groups on the surface of SiO2-TiO2 increased after UV irradiation and that the SiO2-TiO2 exhibited the reaction activity with water. We believed that the production of hydroxyl groups was induced by the photoinduced action of TiO2. The change was consistent with the remarkable enhancement of the surface hydrophilicity of SiO2-TiO2 after UV irradiation in Figure 3. To investigate the mechanism of the photoinduced hydrophilicity of SiO 2 -TiO 2 , the infrared spectral analysis was carried out. Figure 4 shows the Fourier transform infrared spectroscopy (FT-IR) spectra of SiO 2 -TiO 2 -20 and SiO 2 -TiO 2 -30 before and after UV irradiation. The characteristic absorption peaks in the range of 2800-3800 cm −1 and 1620 cm −1 in all the samples were ascribed to the vibration of the hydroxyl groups on the SiO 2 -TiO 2 surface. When the TiO 2 ratio was 30%, after the UV irradiation (b2 in Figure 4), the intensity of the absorption peak in the range of 2800-3800 cm −1 in the FTIR spectrum of SiO 2 -TiO 2 was higher than that in the spectrum b1 (before the UV irradiation) and the peak was shifted to the higher wavenumber. Meanwhile, the absorption peak at 1620 cm −1 in b2 was sharper than that in b1. The abovementioned results indicated that the number of hydroxyl groups on the surface of SiO 2 -TiO 2 increased after UV irradiation and that the SiO 2 -TiO 2 exhibited the reaction activity with water. We believed that the production of hydroxyl groups was induced by

Structure and Morphology of SiO2-TiO2
3.2.1. XRD Analysis Figure 5 shows the XRD patterns of SiO2-TiO2 with different TiO2 ratios. In addition to the diffraction peak of amorphous SiO2 microspheres, the diffraction peaks of the anatase phase also appeared in the XRD patterns of all SiO2-TiO2 samples, and the intensity of diffraction peaks of the anatase phase increased with the increase in the TiO2 ratio. Especially, when the TiO2 ratio was 50%, the complete anatase diffraction peak (JCPDS 21-1272) appeared in the XRD pattern of SiO2-TiO2-50 ( Figure 5c) [32]. The abovementioned results indicated that nano-TiO2 existed as an anatase phase. Among all the TiO2 crystal phases, the anatase exhibited the highest photocatalytic activity, which was consistent with the results of photocatalytic activity and photoinduced hydrophilicity of SiO2-TiO2.  Figure 6 shows the SEM images of SiO2-TiO2 with different TiO2 ratios. In Figure 6a, the exposed surfaces of SiO2 microspheres are smooth without covering. However, the micron-submicron hierarchical structure morphology can be observed in Figure 6b-d. The surface of the SiO2 microspheres became rough and was covered with a certain amount of irregular particles. Meanwhile, with the increase in the TiO2 ratio, the roughness and coverage area of the SiO2 microsphere surface increased accordingly. According to the preparation process, it was presumed that the coating on the surface of the microspheres should be nano-TiO2 particles. The surface  Figure 5 shows the XRD patterns of SiO 2 -TiO 2 with different TiO 2 ratios. In addition to the diffraction peak of amorphous SiO 2 microspheres, the diffraction peaks of the anatase phase also appeared in the XRD patterns of all SiO 2 -TiO 2 samples, and the intensity of diffraction peaks of the anatase phase increased with the increase in the TiO 2 ratio. Especially, when the TiO 2 ratio was 50%, the complete anatase diffraction peak (JCPDS 21-1272) appeared in the XRD pattern of SiO 2 -TiO 2 -50 ( Figure 5c) [32]. The abovementioned results indicated that nano-TiO 2 existed as an anatase phase. Among all the TiO 2 crystal phases, the anatase exhibited the highest photocatalytic activity, which was consistent with the results of photocatalytic activity and photoinduced hydrophilicity of SiO 2 -TiO 2 .  Figure 5 shows the XRD patterns of SiO2-TiO2 with different TiO2 ratios. In addition to the diffraction peak of amorphous SiO2 microspheres, the diffraction peaks of the anatase phase also appeared in the XRD patterns of all SiO2-TiO2 samples, and the intensity of diffraction peaks of the anatase phase increased with the increase in the TiO2 ratio. Especially, when the TiO2 ratio was 50%, the complete anatase diffraction peak (JCPDS 21-1272) appeared in the XRD pattern of SiO2-TiO2-50 (Figure 5c) [32]. The abovementioned results indicated that nano-TiO2 existed as an anatase phase. Among all the TiO2 crystal phases, the anatase exhibited the highest photocatalytic activity, which was consistent with the results of photocatalytic activity and photoinduced hydrophilicity of SiO2-TiO2.  Figure 6 shows the SEM images of SiO2-TiO2 with different TiO2 ratios. In Figure 6a, the exposed surfaces of SiO2 microspheres are smooth without covering. However, the micron-submicron hierarchical structure morphology can be observed in Figure 6b-d. The surface of the SiO2 microspheres became rough and was covered with a certain amount of irregular particles. Meanwhile, with the increase in the TiO2 ratio, the roughness and coverage area of the SiO2 microsphere surface increased accordingly. According to the preparation process, it was presumed that the coating on the surface of the microspheres should be nano-TiO2 particles. The surface  Figure 6 shows the SEM images of SiO 2 -TiO 2 with different TiO 2 ratios. In Figure 6a, the exposed surfaces of SiO 2 microspheres are smooth without covering. However, the micron-submicron hierarchical structure morphology can be observed in Figure 6b-d. The surface of the SiO 2 microspheres became rough and was covered with a certain amount of irregular particles. Meanwhile, with the increase in the TiO 2 ratio, the roughness and coverage area of the SiO 2 microsphere surface increased accordingly. According to the preparation process, it was presumed that the coating on the surface of the microspheres should be nano-TiO 2 particles. The surface roughness of SiO 2 microspheres and SiO 2 -TiO 2 -50 were evaluated using an atomic force microscope, and the corresponding atomic force microscope (AFM) images were shown in Figure 6a,d (see the built-in images). The tested surface roughness of SiO 2 microspheres and SiO 2 -TiO 2 were 1.63 and 18.4 nm, respectively. These results show that the surface roughness of SiO 2 increased significantly after it was coated by nano-TiO 2 , indicating that the surface structure of SiO 2 has changed. Additionally, in the magnification image of SiO 2 -TiO 2 shown in Figure 6b, the nano-TiO 2 particles not only uniformly coated the surface of the SiO 2 microspheres, but also exist in the gap among SiO 2 microspheres. In this way, several microspheres were connected together as a whole. roughness of SiO2 microspheres and SiO2-TiO2-50 were evaluated using an atomic force microscope, and the corresponding atomic force microscope (AFM) images were shown in Figure 6a,d (see the built-in images). The tested surface roughness of SiO2 microspheres and SiO2-TiO2 were 1.63 and 18.4 nm, respectively. These results show that the surface roughness of SiO2 increased significantly after it was coated by nano-TiO2, indicating that the surface structure of SiO2 has changed. Additionally, in the magnification image of SiO2-TiO2 shown in Figure 6b, the nano-TiO2 particles not only uniformly coated the surface of the SiO2 microspheres, but also exist in the gap among SiO2 microspheres. In this way, several microspheres were connected together as a whole.

Morphology and Element Analysis
To confirm the composition of the coating on the surface of SiO2 microsphere, a surface scanning analysis of the main elements in the selected part of the SiO2-TiO2 SEM was carried out (Figure 7). The Ti element was almost distributed throughout the scan area, like the distribution of Si element. The distribution density of Ti element is proportional to the TiO2 ratio. This confirmed that the nano-TiO2 particles had coated the surface and were distributed in the gap among SiO2 microsphere. The results were consistent with SEM results (Figure 6).  To confirm the composition of the coating on the surface of SiO 2 microsphere, a surface scanning analysis of the main elements in the selected part of the SiO 2 -TiO 2 SEM was carried out (Figure 7). The Ti element was almost distributed throughout the scan area, like the distribution of Si element. The distribution density of Ti element is proportional to the TiO 2 ratio. This confirmed that the nano-TiO 2 particles had coated the surface and were distributed in the gap among SiO 2 microsphere. The results were consistent with SEM results (Figure 6).  Figure 8 shows the TEM and high resolution transmission electron microscopy (HRTEM) images of the SiO2-TiO2 samples (TiO2 ratio is 40%). Circular SiO2 microspheres and irregular nano-TiO2 particles surrounding the SiO2 microspheres are observed in Figure 8a, confirming that the nano-TiO2 particles has coated the surface of SiO2 microspheres. In the HRTEM (Figure 8c), the interplanar spacing of the three major facets were measured to be d = 0.352 nm [33], which was consistent with the (101) crystal face of anatase (JCPDS 21-1272). The above results indicated that the nano-TiO2 coating on the surface of SiO2 microspheres was anatase and that the mainly exposed crystal face was (101).  Figure 9 shows the FT-IR spectra of SiO2 and SiO2-TiO2 with different TiO2 ratios. The absorption bands at 1115, 808, and 477 cm −1 are typical absorption bands of Si-O bonds, indicating that the main component of the composite is SiO2 [34].With the increase in the TiO2 ratio, the intensity of absorption bands corresponding to SiO2 decreased, indicating that the nano-TiO2 coated the SiO2 surface. In addition, the absorption bands (3200-3550 cm −1 ) derived from Si-OH and Ti-OH showed the significant displacement and broadening phenomena when the SiO2 was coated by the nano-TiO2, indicating that the chemical environment had been changed and the association degree of hydroxyl groups on particles surface had increased. It was obviously caused by the formation of hydrogen bonds between Si-OH and Ti-OH or the further dehydroxylation reaction. It should be inferred that  Figure 8 shows the TEM and high resolution transmission electron microscopy (HRTEM) images of the SiO 2 -TiO 2 samples (TiO 2 ratio is 40%). Circular SiO 2 microspheres and irregular nano-TiO 2 particles surrounding the SiO 2 microspheres are observed in Figure 8a, confirming that the nano-TiO 2 particles has coated the surface of SiO 2 microspheres. In the HRTEM (Figure 8c), the interplanar spacing of the three major facets were measured to be d = 0.352 nm [33], which was consistent with the (101) crystal face of anatase (JCPDS 21-1272). The above results indicated that the nano-TiO 2 coating on the surface of SiO 2 microspheres was anatase and that the mainly exposed crystal face was (101).  Figure 8 shows the TEM and high resolution transmission electron microscopy (HRTEM) images of the SiO2-TiO2 samples (TiO2 ratio is 40%). Circular SiO2 microspheres and irregular nano-TiO2 particles surrounding the SiO2 microspheres are observed in Figure 8a, confirming that the nano-TiO2 particles has coated the surface of SiO2 microspheres. In the HRTEM (Figure 8c), the interplanar spacing of the three major facets were measured to be d = 0.352 nm [33], which was consistent with the (101) crystal face of anatase (JCPDS 21-1272). The above results indicated that the nano-TiO2 coating on the surface of SiO2 microspheres was anatase and that the mainly exposed crystal face was (101).  Figure 9 shows the FT-IR spectra of SiO2 and SiO2-TiO2 with different TiO2 ratios. The absorption bands at 1115, 808, and 477 cm −1 are typical absorption bands of Si-O bonds, indicating that the main component of the composite is SiO2 [34].With the increase in the TiO2 ratio, the intensity of absorption bands corresponding to SiO2 decreased, indicating that the nano-TiO2 coated the SiO2 surface. In addition, the absorption bands (3200-3550 cm −1 ) derived from Si-OH and Ti-OH showed the significant displacement and broadening phenomena when the SiO2 was coated by the nano-TiO2, indicating that the chemical environment had been changed and the association degree of hydroxyl groups on particles surface had increased. It was obviously caused by the formation of hydrogen bonds between Si-OH and Ti-OH or the further dehydroxylation reaction. It should be inferred that  Figure 9 shows the FT-IR spectra of SiO 2 and SiO 2 -TiO 2 with different TiO 2 ratios. The absorption bands at 1115, 808, and 477 cm −1 are typical absorption bands of Si-O bonds, indicating that the main component of the composite is SiO 2 [34].With the increase in the TiO 2 ratio, the intensity of absorption bands corresponding to SiO 2 decreased, indicating that the nano-TiO 2 coated the SiO 2 surface. In addition, the absorption bands (3200-3550 cm −1 ) derived from Si-OH and Ti-OH showed the significant displacement and broadening phenomena when the SiO 2 was coated by the nano-TiO 2 , indicating that the chemical environment had been changed and the association degree of hydroxyl groups on particles surface had increased. It was obviously caused by the formation of hydrogen bonds between Si-OH and Ti-OH or the further dehydroxylation reaction. It should be inferred that the chemical combination between SiO 2 microspheres and nano-TiO 2 particles was formed through the interaction of hydroxyl groups on their surfaces. Nanomaterials 2017, 7, 367 10 of 12 the chemical combination between SiO2 microspheres and nano-TiO2 particles was formed through the interaction of hydroxyl groups on their surfaces. Figure 9. FT-IR of SiO2-TiO2 with different TiO2 ratios.SiO2-TiO2-20, 30, 40, 50 represent the mass ratio of TiO2 to SiO2-TiO2 is 20%, 30%, 40% and 50%; The black rectangle region represents the absorption peak caused by the vibration of the hydroxyl radical. Figure 10 shows the schematic diagram of the bonding mechanism of SiO2-TiO2. Based on the above results, the bonding mechanism can be described as follows: firstly, the SiO2 microspheres were ground in the ethanol medium with grinding balls. The strong grinding force made SiO2 microspheres depolymerization and exposed more hydroxyl groups, thus displaying the higher reactivity. Secondly, the prepared nano-TiO2 soliquid was ground with the activated SiO2 violently, so that the collision probability between particles increased and lead to the contact and reactions between the hydroxyl groups on the SiO2 and TiO2 surfaces. Finally, water produced by the dehydroxylation of the particles was further removed by calcination. The SiO2 and TiO2 particles were bounded by -Si-O-Ti-bonds. The strength of the chemical bond was stronger than that of van der Waals forces and other physical forces, so the coating of nano-TiO2 on SiO2 surface was firm.

Conclusions
In the study, with the by-product SiO2 microspheres produced during the industry production of fused-zirconia as the substrates, SiO2-TiO2 particles were prepared by the wet-grinding of SiO2 microspheres and nano-TiO2 and calcination of the ground product. The degradation efficiency of SiO2-TiO2 on methyl orange reached 97%, which was significantly higher than that of pure nano-TiO2. The water contact angle of SiO2-TiO2 was 8°, indicating the strong photoinduced hydrophilicity and the good self-cleaning effect.  Figure 10 shows the schematic diagram of the bonding mechanism of SiO 2 -TiO 2 . Based on the above results, the bonding mechanism can be described as follows: firstly, the SiO 2 microspheres were ground in the ethanol medium with grinding balls. The strong grinding force made SiO 2 microspheres depolymerization and exposed more hydroxyl groups, thus displaying the higher reactivity. Secondly, the prepared nano-TiO 2 soliquid was ground with the activated SiO 2 violently, so that the collision probability between particles increased and lead to the contact and reactions between the hydroxyl groups on the SiO 2 and TiO 2 surfaces. Finally, water produced by the dehydroxylation of the particles was further removed by calcination. The SiO 2 and TiO 2 particles were bounded by -Si-O-Ti-bonds. The strength of the chemical bond was stronger than that of van der Waals forces and other physical forces, so the coating of nano-TiO 2 on SiO 2 surface was firm.
Nanomaterials 2017, 7, 367 10 of 12 the chemical combination between SiO2 microspheres and nano-TiO2 particles was formed through the interaction of hydroxyl groups on their surfaces. Figure 9. FT-IR of SiO2-TiO2 with different TiO2 ratios.SiO2-TiO2-20, 30, 40, 50 represent the mass ratio of TiO2 to SiO2-TiO2 is 20%, 30%, 40% and 50%; The black rectangle region represents the absorption peak caused by the vibration of the hydroxyl radical. Figure 10 shows the schematic diagram of the bonding mechanism of SiO2-TiO2. Based on the above results, the bonding mechanism can be described as follows: firstly, the SiO2 microspheres were ground in the ethanol medium with grinding balls. The strong grinding force made SiO2 microspheres depolymerization and exposed more hydroxyl groups, thus displaying the higher reactivity. Secondly, the prepared nano-TiO2 soliquid was ground with the activated SiO2 violently, so that the collision probability between particles increased and lead to the contact and reactions between the hydroxyl groups on the SiO2 and TiO2 surfaces. Finally, water produced by the dehydroxylation of the particles was further removed by calcination. The SiO2 and TiO2 particles were bounded by -Si-O-Ti-bonds. The strength of the chemical bond was stronger than that of van der Waals forces and other physical forces, so the coating of nano-TiO2 on SiO2 surface was firm.

Conclusions
In the study, with the by-product SiO2 microspheres produced during the industry production of fused-zirconia as the substrates, SiO2-TiO2 particles were prepared by the wet-grinding of SiO2 microspheres and nano-TiO2 and calcination of the ground product. The degradation efficiency of SiO2-TiO2 on methyl orange reached 97%, which was significantly higher than that of pure nano-TiO2. The water contact angle of SiO2-TiO2 was 8°, indicating the strong photoinduced hydrophilicity and the good self-cleaning effect.

Conclusions
In the study, with the by-product SiO 2 microspheres produced during the industry production of fused-zirconia as the substrates, SiO 2 -TiO 2 particles were prepared by the wet-grinding of SiO 2 microspheres and nano-TiO 2 and calcination of the ground product. The degradation efficiency of SiO 2 -TiO 2 on methyl orange reached 97%, which was significantly higher than that of pure nano-TiO 2 .
The water contact angle of SiO 2 -TiO 2 was 8 • , indicating the strong photoinduced hydrophilicity and the good self-cleaning effect.
The SiO 2 -TiO 2 particles were characterized by the nano-TiO 2 uniformly coated on the SiO 2 microspheres and distributed in the microsphere gap. The nano-TiO 2 particles existed in an anatase phase with the particle size of 15-20 nm and are combined with SiO 2 microspheres by the dehydration of hydroxyl groups on particle surfaces.