Adsorption and Photocatalytic Processes of Mesoporous SiO2-Coated Monoclinic BiVO4

The silicon dioxide (SiO2)–coated bismuth vanadate (BiVO4) composites as visible–driven–photocatalysts were successfully synthesized by the co–precipitation method. The effects of SiO2 coating on the structure, optical property, morphology and surface properties of the composites were investigated by X–ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), transmission electron microscopy (TEM) and Brunauer–Emmette–Teller (BET) measurements. The photocatalytic activity of monoclinic BiVO4 and BiVO4/SiO2 composites were evaluated according to the degradation of methylene blue (MB) dye aqueous solution under visible light irradiation. The SiO2−coated BiVO4 composites showed the enhancing photocatalytic activity approximately threefold in comparison with the single phase BiVO4.


INTRODUCTION
Nowadays, the advanced oxidation process is known as an effective method for water purification and wastewater treatment. One of the most famous advanced oxidation process is heterogeneous photocatalysis; the contaminant (i.e., organic compounds) containing in the water and wastewater is finally degraded to carbon dioxide (Legrini et al., 1993;Mukherjee and Ray, 1999). This process can remove the organic contaminant perfectly and does not generate the second contaminant (i.e., sludge and other organic compounds) which are required the further treatment and disposal. According to the heterogeneous photocatalysis, the titanium dioxide (TiO 2 ) has been played a role as the important catalyst to promote the photocatalytic activity. Due to its wide band gap of 3.2 eV, the photocatalyst of TiO 2 is typically activated under the UV light (the wavelength <390 nm is required), which accounts for 45-50% of solar radiation (Linsebigler et al., 1995;Bahnemann et al., 2007;Devipriya et al., 2012). This theoretical fact becomes the limitation and non-cost-effectiveness of actual photocatalytic system for purifying the water at the site.
Another catalyst of monoclinic bismuth vanadate (BiVO 4 ) has been proposed to overcome the drawback of photocatalytic system using TiO 2 and together with enhance the photocatalytic activity during implementation. Since BiVO 4 has narrow band gaps of 2.4 to 2.8 eV (Kudo et al., 2001;Xie et al., 2006;Li et al., 2008), this photocatalyst can be activated by the visible light and consequences the effective use of solar energy. However, the low specific surface area and poor surface textural property are the significant disadvantages of using BiVO 4 as the catalyst. Its low surface area and adsorption capacity cause the low efficiency of photocatalytic system for organic contaminant removal and also the long treatment period required. Therefore, the increase in specific surface area of BiVO 4 catalyst is necessary prior to imply the photocatalytic system to the actual wastewater.
Recently, alternative composite materials have been synthesized by combining metal oxide and porous materials (i.e., alumina, silica, zeolites, carbon black, charcoal) (Belessi et al., 2007;Wang et al., 2012;Xing et al., 2016) with the aim of improving the specific surface area, pore structure, and photocatalytic activity of catalysts (Gan et al., 2003;Kimura et al., 2003). For example, the enhancement of Ag-doped TiO 2 photocatalytic activity was suggested by adding the mesoporous SiO 2 ; the excellent efficiency of methyl orange (MO) removal was achieved by 2.5 h (Roldan et al., 2015). The increasing adsorption capacity of TiO 2 catalyst was observed when the catalyst was combined with SiO 2 ; the adsorption capacity was increased . The SiO 2 addition also enhance the separation rate of electron-hole pairs under UV excitation. Further, the deposition of gold nanoparticles (Au) on the porous SiO 2 -WO 3 composite can enhance the methylene blue (MB) adsorption capacity; the adsorption capacity of Au-SiO 2 -WO 3 was greater than SiO 2 -WO 3 and WO 3 respectively (DePuccio et al., 2015). The complete MB removal was achieved by 300 min under visible light, and the fast kinetic of MB removal was found in Au-SiO 2 -WO 3 catalyst, following by Au-WO 3 and WO 3 catalysts.
As all the above mentions, this study aimed to improve the surface morphology and photocatalytic activity of BiVO 4 catalyst by coating SiO 2 . Various analytical techniques including Xray diffraction (XRD), transmission electron microscopy (TEM), Brunauer-Emmett-Teller (BET) and UV-vis diffuse reflectance spectra (DRS) were used to clarify the better property of BiVO 4 /SiO 2 composite rather than BiVO 4 and SiO 2 . Further, the performance of BiVO 4 /SiO 2 composites on wastewater FIGURE 1 | XRD patterns of as-prepared BiVO 4 , SiO 2 , and BiVO 4 /SiO 2 . treatment was preliminary studied in the batch test under visible light irradiation, and its performance was compared to the other two materials of BiVO 4 and SiO 2 .

Preparation of SiO 2 Particles
SiO 2 particles were prepared by the sol-gel method. Ammonia solution (28%) was added in 100 mL of a mixed solution of absolute ethanol/DI water (80: 20 v/v) and stirred under ultrasonic dispersion for 60 min. Then, 20 mL of tetraethyl orthosilicate (TEOS) was added drop by drop to the mixed solution and stirred for 120 min at room temperature. After the reaction was homogenized, the fine particles were separated by centrifugation with typical rotating speed of 6,000 rpm for 15 min, washed by DI water and dried at 80 • C for 24 h in a hot air oven. Fine particles of SiO 2 were obtained as a white powder following heat treatment at 500 • C for 1 h in ambient.

Preparation of Monoclinic BiVO 4 and SiO 2 -Coated BiVO 4 Composites
Monoclinic BiVO 4 were obtained by the co-precipitation method. Firstly, 12 mmol of bismuth (III) nitrate pentahydrate [Bi(NO 3 ) 3 ·5H 2 O] and the same volume of ammonium metavanadate (NH 4 VO 3 ) were dissolved in 100 mL of 2 M nitric acid (HNO 3 ) under vigorous stirring. The pH of the mixed solution was adjusted to 9 by adding 3 M sodium hydroxide (NaOH). The yellow precipitate was then separated by centrifugation at 6,000 rpm for 15 min, washed thoroughly with distilled water and ethanol and finally dried in a hot air oven at 80 • C for 24 h. Crystalline monoclinic BiVO 4 was formed after calcination at 550 • C for 4 h.
BiVO 4 -coated SiO 2 composites were also prepared by the same method for comparison with an additional step of adding SiO 2 powder to 100 mL of 2 M HNO 3 .

Photocatalytic Reaction
Photocatalytic activities of the BiVO 4 , SiO 2 and BiVO 4 /SiO 2 composites were evaluated through degradation of methylene blue (MB) dye as a model organic pollutant under visible light.
A total of 0.20 g of photocatalyst was added to 100 mL MB aqueous solution (initial concentration C 0 = 20 ppm) under magnetic stirring in darkness for 60 min to achieve adsorptiondesorption equilibrium. The system was irradiated by three 18 W halogen lamps (Essential MR, Philips, Thailand) to investigate photocatalytic degradation. Reduction of MB concentration over time (C t ) was recorded every 15 min by measuring the intensity change of the characteristic absorption peak at 664 nm using UV-vis double beam spectroscopy (UV−6100, Mapada).

Characterisation
Crystal phase and structure of the prepared samples were characterized by powder X-ray diffraction (XRD, Philips X'Pert MPD) using Cu K α (λ = 1.54056 Å) radiation. Morphological changes in the composite materials were monitored by transmission electron microscopy (TEM, JSM−2010, JEOL). Brunauer-Emmett-Teller (BET) measurements (Adtosorb 1 MP, Quantachrome) were performed to compare the specific surface area of the BiVO 4 and BiVO 4 /SiO 2 composites. Measurement of UV-vis diffuse reflectance spectroscopy (DRS UV-vis, Shimadzu UV−3101PC) was carried out at room temperature to detect reflectance and absorbance spectra.

RESULTS AND DISCUSSION
In Figure 1, the broad XRD peak at 2θ = 22-23 • corresponded to the amorphous SiO 2 . The XRD pattern of BiVO 4 without SiO 2 was assigned to the standard monoclinic BiVO 4 (JCPDS no. 14-0688) (Gotić et al., 2005). After coating BiVO 4 with Frontiers in Chemistry | www.frontiersin.org SiO 2 , the diffraction peaks matched well with the pure phase monoclinic BiVO 4 and no peaks of any other phases or impurities were recorded. However, the diffraction intensity of BiVO 4 decreased after coating SiO 2 , because the amorphous substance had the negative effect on crystallinity. Alternatively, self-doped Si 4+ ions in the BiVO 4 crystal structure might cause the decreasing crystallinity of BiVO 4 /SiO 2 composites, and resulted in the broader peaks of the composite samples, which are similar to those reported by Phanichphant et al. (2016) for the binary composite CeO 2 /SiO 2 photocatalyts and Kumar et al. for TiO 2 /SiO 2 nanocomposites in solar cell applications (Arun Kumar et al., 2012).  Figure 2a, the BiVO 4 demonstrated the absorption edge of the visible region at 450 nm, corresponding to the optical band gap (E g ) of 2.60 eV which was calculated by the Kubelka-Munk function (see Figure 2b) (Sirita et al., 2007). Compared to BiVO 4 /SiO 2 composites, the value of the graph intercept was estimated at 2.30 eV, corresponding to the strong absorption edge in the visible region at 523 nm. The band gap energy of BiVO 4 decreased from 2.60 to 2.30 eV in the composite materials, due to the influence of Si 4+ ions doping into the lattice of BiVO 4 which created the abundant doping energy levels. The estimated band gap values in this study was similar to those of BiVO 4 reported by Jiang et al. (2012), who prepared the BiVO 4 photocatalysts with different morphologies using the hydrothermal method. Liu et al. (2015) observed that the band gap energy of BiVO 4 /SiO 2 catalyst estimated to be 2.32 eV, which was almost the same as that of calculate by this study.

As shown in
The TEM images of SiO 2 , BiVO 4 and BiVO 4 /SiO 2 composites are presented in Figure 3. The SiO 2 image shows the aggregation of spherical-shaped particles with diameters ranging of 20-30 nm (Figure 3a), while Figure 3b shows the rod-like nanostructures of monoclinic BiVO 4 with the diameter of 10 nm and the length of 60 nm. Typical TEM images are used for characterizing the composite materials and proving the heterojunction formation between BiVO 4 and SiO 2 , which demonstrated that the rod-like BiVO 4 core was covered by the SiO 2 particles growing on the surface (Figure 3c).
The N 2 adsorption-desorption isotherms (Figure 4a) show that the N 2 adsorption of BiVO 4 /SiO 2 composites were relatively higher than that of the pure BiVO 4 , however the value was much lower than that of the SiO 2 . The specific surface areas of SiO 2 , BiVO 4 /SiO 2 composites, and BiVO 4 were found to be 106.9959, 37.6851, and 19.4964 m 2 /g, respectively. In the meanwhile, the pore size was calculated by using the BJH method, and the results were 9.0316, 11.0776, and 11.8111 nm for SiO 2 , BiVO 4 /SiO 2 , and BiVO 4 respectively (as summarized in Table 1). The surface area and pore size are positively related to the photocatalytic activity, therefore the photocatalytic activity of BiVO 4 /SiO 2 composites were higher than that of pure BiVO 4 . Even though the surface area of SiO 2 was higher than the BiVO 4 /SiO 2 composite, the adsorption of pollutant by SiO 2 with high specific surface area have only the ability to transfer pollutants to alternative phases, but not completely get rid of them. Therefore, the photocatalytic process based on using the hydroxyl radicals is required in this study.  Figure 4b shows the N 2 adsorption-desorption isotherms of BiVO 4 /SiO 2 composites in the relative pressure (P/P 0 ) range 0.00-1.00. The curve exhibited Type IV isotherm characteristic with a small hysteresis loop at the relative pressure of 0.80-1.00. This indicated the existence of mesopores in the sample with the pore diameter ranging of 2-50 nm (Brunauer et al., 1940;Bae et al., 2010).
The information from the isotherm can be used to determine the specific surface area from the mathematical relations in Equation (1) and Equation (2) below (Itodo et al., 2010;Thommes et al., 2015) P/P 0 where, P 0 , Initial pressure of N 2 ; P, Equilibrium pressure of N 2 adsorption; V m , Monolayer capacity; V, Amount of N 2 adsorbed at standard temperature and pressure (STP).
where, A, Cross-sectional area of the adsorbed N 2 ; m, Adsorbate molecular weight; N a, Avogadro's number.
The intercept and slope of the plot in Figure 4c were used to calculate the maximum volume of gas adsorbed at the monolayer (V m ), it was 8.6569 cm 3 /g. The specific surface area was also calculated via the V m value (see Equation 2). The result showed that the surface area of BiVO 4 /SiO 2 composites was 37.6851 m 2 /g. Figure 5a presents the degradation efficiency of MB as a function of C t /C 0 and visible irradiation time. The C 0 was the initial concentration of MB before irradiation and C t was the MB concentration at the interval irradiation time (t, min). For using the SiO 2 as catalyst, the MB was removed of 83% under the dark adsorption, and only 5% of MB was further degraded under the visible light. For using the single phase monoclinic BiVO 4 , the MB was removed around 10% under the dark adsorption, and 40% of MB was further degreased under the visible light irradiation. When the BiVO 4 /SiO 2 composites was used, the MB removal efficiency reached 35 and 86% under Frontiers in Chemistry | www.frontiersin.org the dark adsorption and visible light irradiation. As above explanation, the specific surface area of BiVO 4 /SiO 2 composites were increased from BiVO 4 , due to the SiO 2 coating. The increasing specific surface area resulted in the high adsorption of MB molecules during 60 min of the darkness, and then the adsorbed MB was continuously degraded by photocatalytic activity during visible light. These results illustrated that the photocatalytic activity of BiVO 4 was enhanced by coating the SiO 2 particles.
The kinetics of MB degradation was analyzed using the pseudo-first order model, which was given in Equation (3) (Yetim and Tekin, 2017). In Figure 5b, the correlation of-ln C t /C 0 and t were positive with linear equation; the kinetic constant (k) were 0.0073 min −1 for BiVO 4 and 0.0207 min −1 for BiVO 4 /SiO 2 composites. The kinetic constant of MB degradation using BiVO 4 /SiO 2 composites was approximately threefold higher than that using the single phase BiVO 4 .
where k is the apparent rate constant of the pseudo-first order reaction (min −1 ).
Since the photocatalytic degradation of dyes is associated with dye adsorption onto the surface of BiVO 4 /SiO 2 . Furthermore, photocatalytic degradation occurs at or near the surface of the catalyst rather than in the bulk solution. Thus the higher photocatalytic activity of BiVO 4 /SiO 2 is consistent with the higher adsorption of MB on the surface of BiVO 4 /SiO 2 photocatalyst. Mesoporous SiO 2 adsorbent enriches the MB molecules around the BiVO 4 surface as shown in Figure 6 and the visible-light photocatalytic activity of the BiVO 4 interface in the composite materials is then excited to generate electrons (e-) and holes (h + ). Subsequently, photoexcited electrons in the valance band and hole in the conduction band of BiVO 4 react with oxygen, water and hydroxide ions to produce free superoxide radicals (O −• 2 ) and hydroxyl radicals (OH • ) as the main active oxidizing species, which then react with MB molecules during the photocatalytic process (Lin et al., 2014;Zhou et al., 2014). The final products of MB aqueous solution photocatalytic degradation are oxidized to CO 2 , H 2 O, CO 2 , NH + 4 , NO − 3 , and SO 2− 4 (Houas et al., 2001;Luan and Hu, 2012).

CONCLUSIONS
BiVO 4 /SiO 2 composites consisting of spherical SiO 2 particles coated on BiVO 4 nanorods were successfully prepared by coprecipitation. The composites exhibited higher photocatalytic activity compared to single monoclinic BiVO 4 by degrading MB under visible-light irradiation due to the greater surface area of mesoporous SiO 2 . Fabrication of heterogeneous semiconductors using mesoporous materials can produce promising alternative photocatalysts for wastewater treatment under light irradiation by combining adsorption and photocatalytic processes.

AUTHOR CONTRIBUTIONS
DC designed and performed the experiments and wrote the manuscript. SP, AN advised the data analysis and edited manuscript. PJ and WK advised the data analysis. All authors reviewed the approved the manuscript.