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 (TiO2) 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 TiO2 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 (BiVO4) has been proposed to overcome the drawback of photocatalytic system using TiO2 and together with enhance the photocatalytic activity during implementation. Since BiVO4 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 BiVO4 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 BiVO4 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 TiO2 photocatalytic activity was suggested by adding the mesoporous SiO2; the excellent efficiency of methyl orange (MO) removal was achieved by 2.5 h (Roldan et al., 2015). The increasing adsorption capacity of TiO2 catalyst was observed when the catalyst was combined with SiO2; the adsorption capacity was increased (Hu et al., 2012). The SiO2 addition also enhance the separation rate of electron–hole pairs under UV excitation. Further, the deposition of gold nanoparticles (Au) on the porous SiO2-WO3 composite can enhance the methylene blue (MB) adsorption capacity; the adsorption capacity of Au–SiO2-WO3 was greater than SiO2-WO3 and WO3 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–SiO2-WO3 catalyst, following by Au–WO3 and WO3 catalysts.

As all the above mentions, this study aimed to improve the surface morphology and photocatalytic activity of BiVO4 catalyst by coating SiO2. Various analytical techniques including X–ray 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 BiVO4/SiO2 composite rather than BiVO4 and SiO2. Further, the performance of BiVO4/SiO2 composites on wastewater treatment was preliminary studied in the batch test under visible light irradiation, and its performance was compared to the other two materials of BiVO4 and SiO2.

Experimental procedure

All chemicals used were of analytical grade and were used as received without any further purification. The chemicals including tetraethyl orthosilicate (TEOS), bismuth (III) nitrate pentahydrate [Bi(NO3)3·5H2O], ammonium metavanadate (NH4VO3), methylene blue powder, sodium hydroxide pellet (NaOH), ammonia solution (28%) and nitric acid (37% HNO3) were obtained from Sigma-Aldrich. All solutions were prepared with deionized water.

Preparation of SiO2 particles

SiO2 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 SiO2 were obtained as a white powder following heat treatment at 500°C for 1 h in ambient.

Preparation of monoclinic BiVO4 and SiO2-coated BiVO4 composites

Monoclinic BiVO4 were obtained by the co–precipitation method. Firstly, 12 mmol of bismuth (III) nitrate pentahydrate [Bi(NO3)3·5H2O] and the same volume of ammonium metavanadate (NH4VO3) were dissolved in 100 mL of 2 M nitric acid (HNO3) 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 BiVO4 was formed after calcination at 550°C for 4 h.

BiVO4-coated SiO2 composites were also prepared by the same method for comparison with an additional step of adding SiO2 powder to 100 mL of 2 M HNO3.

Photocatalytic reaction

Photocatalytic activities of the BiVO4, SiO2 and BiVO4/SiO2 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 C0 = 20 ppm) under magnetic stirring in darkness for 60 min to achieve adsorption–desorption 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 (Ct) 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 Kα (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 BiVO4 and BiVO4/SiO2 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 SiO2. The XRD pattern of BiVO4 without SiO2 was assigned to the standard monoclinic BiVO4 (JCPDS no. 14–0688) (Gotić et al., 2005). After coating BiVO4 with SiO2, the diffraction peaks matched well with the pure phase monoclinic BiVO4 and no peaks of any other phases or impurities were recorded. However, the diffraction intensity of BiVO4 decreased after coating SiO2, because the amorphous substance had the negative effect on crystallinity. Alternatively, self–doped Si4+ ions in the BiVO4 crystal structure might cause the decreasing crystallinity of BiVO4/SiO2 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 CeO2/SiO2 photocatalyts and Kumar et al. for TiO2/SiO2 nanocomposites in solar cell applications (Arun Kumar et al., 2012).

Figure 1

XRD patterns of as–prepared BiVO4, SiO2, and BiVO4/SiO2.

As shown in Figure 2a, the BiVO4 demonstrated the absorption edge of the visible region at 450 nm, corresponding to the optical band gap (Eg) of 2.60 eV which was calculated by the Kubelka–Munk function (see Figure 2b) (Sirita et al., 2007). Compared to BiVO4/SiO2 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 BiVO4 decreased from 2.60 to 2.30 eV in the composite materials, due to the influence of Si4+ ions doping into the lattice of BiVO4 which created the abundant doping energy levels. The estimated band gap values in this study was similar to those of BiVO4 reported by Jiang et al. (2012), who prepared the BiVO4 photocatalysts with different morphologies using the hydrothermal method. Liu et al. (2015) observed that the band gap energy of BiVO4/SiO2 catalyst estimated to be 2.32 eV, which was almost the same as that of calculate by this study.

Figure 2

(A) Diffuse reflectance UV–visible spectra and (B) the plot of adsorption function vs. photon energy for determination of band gap (Eg).

The TEM images of SiO2, BiVO4 and BiVO4/SiO2 composites are presented in Figure 3. The SiO2 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 BiVO4 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 BiVO4 and SiO2, which demonstrated that the rod–like BiVO4 core was covered by the SiO2 particles growing on the surface (Figure 3c).

Figure 3

TEM images of (a) SiO2, (b) BiVO4, and (c) SiO2-coated BiVO4.

The N2 adsorption-desorption isotherms (Figure 4a) show that the N2 adsorption of BiVO4/SiO2 composites were relatively higher than that of the pure BiVO4, however the value was much lower than that of the SiO2. The specific surface areas of SiO2, BiVO4/SiO2 composites, and BiVO4 were found to be 106.9959, 37.6851, and 19.4964 m2/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 SiO2, BiVO4/SiO2, and BiVO4 respectively (as summarized in Table 1). The surface area and pore size are positively related to the photocatalytic activity, therefore the photocatalytic activity of BiVO4/SiO2 composites were higher than that of pure BiVO4. Even though the surface area of SiO2 was higher than the BiVO4/SiO2 composite, the adsorption of pollutant by SiO2 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 4

(A), (B) N2 adsorption-desorption isotherms, and (C) BET linear plot of relative pressure.

Table 1

Surface properties of the prepared samples.

Sample	Specific surface area (m2/g)	Average pore size diameter (nm)
SiO2	106.9959	9.0316
BiVO4/SiO2	37.6851	11.0776
BiVO4	19.4964	11.8111

Figure 4b shows the N2 adsorption-desorption isotherms of BiVO4/SiO2 composites in the relative pressure (P/P0) 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)

where,

P0, Initial pressure of N2; P, Equilibrium pressure of N2 adsorption; Vm, Monolayer capacity; V, Amount of N2 adsorbed at standard temperature and pressure (STP).

where,

A, Cross-sectional area of the adsorbed N2; m, Adsorbate molecular weight; Na, 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 (Vm), it was 8.6569 cm3/g. The specific surface area was also calculated via the Vm value (see Equation 2). The result showed that the surface area of BiVO4/SiO2 composites was 37.6851 m2/g.

Figure 5a presents the degradation efficiency of MB as a function of Ct/C0 and visible irradiation time. The C0 was the initial concentration of MB before irradiation and Ct was the MB concentration at the interval irradiation time (t, min). For using the SiO2 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 BiVO4, the MB was removed around 10% under the dark adsorption, and 40% of MB was further degreased under the visible light irradiation. When the BiVO4/SiO2 composites was used, the MB removal efficiency reached 35 and 86% under the dark adsorption and visible light irradiation. As above explanation, the specific surface area of BiVO4/SiO2 composites were increased from BiVO4, due to the SiO2 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 BiVO4 was enhanced by coating the SiO2 particles.

Figure 5

MB concentration changes with irradiation time (A) Ct/C0 and (B) –ln Ct/C0.

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 Ct/C0 and t were positive with linear equation; the kinetic constant (k) were 0.0073 min−1 for BiVO4 and 0.0207 min−1 for BiVO4/SiO2 composites. The kinetic constant of MB degradation using BiVO4/SiO2 composites was approximately threefold higher than that using the single phase BiVO4.

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 BiVO4/SiO2. Furthermore, photocatalytic degradation occurs at or near the surface of the catalyst rather than in the bulk solution. Thus the higher photocatalytic activity of BiVO4/SiO2 is consistent with the higher adsorption of MB on the surface of BiVO4/SiO2 photocatalyst. Mesoporous SiO2 adsorbent enriches the MB molecules around the BiVO4 surface as shown in Figure 6 and the visible–light photocatalytic activity of the BiVO4 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 BiVO4 react with oxygen, water and hydroxide ions to produce free superoxide radicals (O) 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 CO2, H2O, CO2, NH, NO, and SO (Houas et al., 2001; Luan and Hu, 2012).

Figure 6

The proposed mechanism of photogenerated charge carriers of BiVO4 in BiVO4/SiO2 heterojunction.

Conclusions

BiVO4/SiO2 composites consisting of spherical SiO2 particles coated on BiVO4 nanorods were successfully prepared by co–precipitation. The composites exhibited higher photocatalytic activity compared to single monoclinic BiVO4 by degrading MB under visible–light irradiation due to the greater surface area of mesoporous SiO2. 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.

Conflict of interest statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.