Semiconductor photocatalysts can rapidly degrade persistent organic pollutants in wastewater. Moreover, the photocatalytic process does not produce secondary pollution, and is relatively simple. Therefore, semiconductor photocatalysts show great potential for use in environmental management [1, 2, 3, 4, 5, 6]. Bismuth tungstate (Bi2WO6) is an n-type semiconductor with a small band gap of 2.7 eV that can absorb visible light and exhibit certain visible-light activity [7]. The activity of Bi2WO6 is closely related to its crystal properties and morphology.
Various Bi2WO6 nanomaterials with different shapes and morphologies have been reported, such as nanofilms [8], nanoflowers [9], nanobelts [10], microspheres [11], and microrods [12]. Zhu et al. [13] found that the photocatalytic activity and photoelectric conversion efficiency of porous Bi2WO6 films were far superior to those of solid Bi2WO6 films. Meanwhile, Zhang et al. [14] reported that under visible light irradiation, 3D Bi2WO6 microspheres assembled on nanofilms showed much higher activity toward degradation of rhodamine B (RhB) than Bi2WO6 nanofilms.
Microsphere photocatalysts assembled from units like nanoparticles, nanorods, or nanolayers possess advantages such as large surface area, easy separation, rich interfaces and good stability [15, 16, 17]. These microspheres not only inherit the characteristics of the structural units in them, but also have the synergistic effects from the interactions between the units, which benefit the adsorption of reactants and light harvesting. The construction of heterostructure is an effective strategy to improve photocatalytic performance. For example, Li et al. [18] found that the formation of an anatase/rutile heterojunction on a TiO2 surface and CdS/MoS2 heterojunction on a CdS surface can greatly increase hydrogen production. Additionally, a BiOI/Bi5O7I heterojunction in BiOI [19] and WO3/ZnO heterojunction in ZnO [20] obviously improved the photocatalytic performance of the main photocatalyst. We also found that the formation of well-defined junctions between Ag2O and Ag2CO3 effectively facilitated charge transfer between Ag2O and Ag2CO3 and suppressed the recombination of photogenerated electrons and holes, resulting in extremely high activity and stability toward photocatalytic degradation of pollutants. As a result, the activity and stability of structure of Ag2CO3 and Ag2O were 73 and 20 times, respectively, higher than those of Ag2CO3 alone [21].
In this paper, Bi2WO6 microspheres are first prepared by a hydrothermal route. Then, different contents of AgCl are deposited on the Bi2WO6 microspheres to produce a series of AgCl/Bi2WO6 composite microspheres. The influence of AgCl content on the texture and phtocatalytic activity of Bi2WO6 in the composites is then investigated.
Bi2WO6 microspheres were prepared by a hydrothermal route. Under vigorous stirring, 0.005 mol of Na2WO4·2H2O (AR grade, Sinopharm Chemical Reagent Co. Ltd, Shanghai, China) and 0.01 mol of Bi(NO3)3·5H2O (AR grade, Sinopharm Chemical Reagent Co. Ltd) were separately dissolved in deionized (DI) water (40 mL). The Na2WO4 solution was added to the Bi(NO3)3 solution. Then 0.01 g of hexadecyltrimethylammonium bromide (AR grade, Sinopharm Chemical Reagent Co. Ltd) was added to the above solution, which was subsequently stirred for 60 min. The suspension was transferred into a Teflon-lined stainless steel autoclave with a volume of 100 mL. The autoclave was sealed and maintained at 160 °C for 12 h under self-generated pressure and then allowed to cool to room temperature naturally. The product was filtered, washed several times with absolute alcohol and DI water, and finally dried at 60 °C for 5 h.
AgCl/Bi2WO6 composite microspheres were prepared by a precipitation method. Stoichiometric amounts of NaCl and AgNO3 were separately dissolved in DI water (20 mL). The Bi2WO6 microspheres were dispersed in the NaCl solution by ultrasonic irradiation for 10 min. The AgNO3 solution was then added dropwise to the stirred Bi2WO6 suspension. After stirring for a further 2 h, the produced composite was filtered, washed with DI water and absolute alcohol, and finally dried at 60 °C for 10 h. The final content of AgCl in the AgCl/Bi2WO6 microspheres was 5 wt%, 10 wt%, 20 wt% or 30 wt%.
X-ray diffraction (XRD) patterns were obtained on an X-ray diffractometer (Bruker D8 Advance, Germany) using Cu Kα radiation (λ = 0.15418 nm) at a scan rate of 0.05 °/s. The accelerating voltage and applied current were 40 kV and 40 mA, respectively. The Brunauer-Emmett-Teller (BET) surface areas of the samples were obtained from N2 adsorption-desorption isotherms measured at liquid N2 temperature using an automatic analyzer (Micromeritics, ASAP 2020). The samples were degassed for 2 h under vacuum at 120 °C prior to adsorption measurements. The microstructures of the samples were determined by a scanning electron microscope (SEM, XL30, Philips, the Netherlands). Transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDX) measurements were performed on an electron microscope (Tecnai 20, FEG) coupled with an energy-dispersive X-ray spectrometer (Oxford Instruments). Fourier transform infrared (FT-IR) spectra were recorded on a FT-IR spectrometer (Nicolet 470 USA) using KBr disk. Ultraviolet-visible (UV-Vis) diffuse reflectance spectra (DRS) were measured using a UV-Vis spectrophotometer (UV-2550, Shimadzu, Japan).
The photocatalytic activities of the samples were determined by measuring their ability to degrade RhB in aqueous solution. Each photocatalyst (50 mg) was suspended in an aqueous solution of RhB (10 mg/L, 100 mL). Before light irradiation, each suspension was stirred in the dark for 40 min to attain physical adsorption-desorption equilibrium between dye and photocatalyst. In visible-light activity tests, a 300-W iodine tungsten lamp was used as the light source, and in UV tests, a 7-W lamp with a wavelength of 254 nm was substituted for the visible lamp. Each suspension was magnetically stirred during the degradation process, and the reaction temperature was maintained at 20 °C by circulation of water. After fixed intervals of illumination, an aliquot of each suspension was taken out and centrifuged. The upper clear solution was analyzed by a spectrophotometer (UV-2550). The degradation percentage D = (C0-C)/C0 × 100%, where C0 is the initial dye concentration and C is the final dye concentration.
Figure 1 shows the XRD patterns of pure Bi2WO6 and AgCl/Bi2WO6 samples with different AgCl contents. Pure Bi2WO6 displays obvious diffraction peaks at 2θ = 28.3°, 32.9°, 47.2°, 55.9° and 58.6° that can be indexed to the (113), (200), (220), (313) and (226) planes, respectively, of orthorhombic-phase Bi2WO6 (JCPDS 73-2020). The lattice constants calculated for Bi2WO6 are a = 0.5457 nm, b = 0.5436 nm, and c = 1.6427 nm. For AgCl/Bi2WO6 samples with >5 wt% AgCl, new and weak diffraction peaks appeared at 2θ = 27.9°, 32.3°, and 46.3° that corresponded to the (111), (200), and (220) planes of AgCl (JCPDS 31-1238). The intensity of these new peaks increased with AgCl content.
We used the Scherrer equation, D = 0.89λ/(βcosθ), where β is the full width at half-maximum of the diffraction peak, λ is the wavelength of incident light (0.154 nm), and θ is the diffraction angle, to calculate the average crystallite size of the samples (Table 1). The average crystallite size of Bi2WO6 was around 16 nm, and the deposition of AgCl did not affect the crystalline properties of Bi2WO6. Therefore, the average crystallite size of AgCl/Bi2WO6 is similar to that of Bi2WO6.
The BET surface areas of the samples are listed in Table 1. The specific surface area of the AgCl/Bi2WO6 samples depends on AgCl content. Deposition of 5 wt%-10 wt% AgCl slightly increased the surface area of the catalysts. Further increasing the content of AgCl to 20 wt%-30 wt% decreased the specific surface area of the samples. A possible reason for this could be that a small amount of AgCl is well dispersed over the Bi2WO6 crystallites, which increases the BET surface area. However, when a large amount of AgCl is deposited on the Bi2WO6 crystallites, AgCl could aggregate into big particles, which decreases the surface area of the sample. A higher surface area should promote the adsorption of dye and increase the photocatalytic activity of the samples.
Figure 2 displays typical SEM images of the samples. Fig. 2(a) shows that the fabricated Bi2WO6 is composed of flower-like microspheres with a diameter of around 1.5-2 μm. The microspheres are composed of many small nanoplates. The surface of the nanoplates is smooth without defects or holes. The deposition of AgCl did not change the overall morphology of the Bi2WO6 microspheres. However, we can observe that numerous AgCl nanoparticles were deposited over the smooth surface of the Bi2WO6 nanoplates. Fig. 2(f) is an enlarged SEM image of the 30 wt% AgCl/Bi2WO6 sample. It reveals that although AgCl/Bi2WO6 retains the microspherical morphology, there are a large number of AgCl nanoparticles deposited on the Bi2WO6 nanoplates.
Figure 3 depicts low- and high-resolution TEM images of Bi2WO6 and 20 wt% AgCl/Bi2WO6 samples. The Bi2WO6 particles are nanoplates with square morphology. The particle size determined from the TEM image is 15-25 nm, which is consistent with the XRD results. The surface of each Bi2WO6 nanoplate is very smooth. The 20 wt% AgCl/Bi2WO6 sample consists of numerous spherical particles with a size of 2-5 nm deposited over the surface of the Bi2WO6 nanoplates. The high-resolution TEM image of the 20 wt% AgCl/Bi2WO6 sample (Fig. 3(c)) clearly shows the characteristic lattice fringes of AgCl and Bi2WO6, with a lattice spacing of AgCl of 0.32 nm, which corresponds to the (111) plane, and lattice spacing of Bi2WO6 of 0.31 nm, which corresponds to the (113) plane.
Selected-area elemental analysis of a spherical particle from the 20 wt% AgCl/Bi2WO6 sample was also carried out by EDX, as shown in Fig. 4. The particle contains O, W, Bi, Ag, and Cl, with contents of 11.34 wt%, 21.58 wt%, 48.00 wt%, 14.85 wt%, and 4.23 wt%, respectively. These values almost correspond to the composition of 20 wt% AgCl/Bi2WO6.
Figure 5 displays the FT-IR spectra of all of the samples. All spectra contain a peak at 3433 cm-1 that is assigned to the stretching and bending vibrations of surface -OH groups on the catalyst particles. The peak at 716 cm-1 is attributed to the stretching vibration of the W-O-W bond. The peaks at both 1110 and 440 cm-1are assigned to the stretching vibration of the Bi-O bond, while that at 578 cm-1 is attributed to the stretching vibration of the W-O bond. These peaks indicate the high crystallinity of Bi2WO6. The deposition of AgCl does not have a marked effect on the FT-IR spectrum of the Bi2WO6 microspheres.
UV-Vis DRS of the Bi2WO6, AgCl and AgCl/Bi2WO6 samples are shown in Fig. 6. Bi2WO6 strongly absorbs light from 200 to 360 nm, with weak absorption in the visible range. The absorption edge of Bi2WO6 is around 450 nm, while that of AgCl is about 490 nm, indicating that it has the ability to absorb visible light. With respect to Bi2WO6, the absorption edge of the AgCl/Bi2WO6 samples shifts to longer wavelength. The band-gap energy (Eg) for the catalysts was determined from the equation Eg = 1240/λg(eV) [22], where λg is the absorption edge, which was obtained from the intercept between the tangent of the absorption curve and abscissa. The calculated Eg for the samples are given in Table 2. Eg of Bi2WO6 and AgCl were 2.84 and 2.07 eV, respectively. The presence of AgCl did not change the band gap of Bi2WO6 because AgCl was only deposited on the surface of Bi2WO6, so it does not affect the crystal structure and energy level of Bi2WO6.
The photocatalytic activities of the samples were evaluated by measuring their ability to decompose RhB in aqueous solution under UV- or visible-light irradiation. Fig. 7 shows the change in concentration of RhB under UV-light irradiation in solutions containing different catalysts. Both AgCl and Bi2WO6 show low activity toward photocatalytic degradation of RhB. The deposition of 5 wt% AgCl on Bi2WO6 obviously increased its photocatalytic activity. As the content of AgCl was increased from 5 wt% to 20 wt%, the degradation rate of RhB gradually increased. When the content of AgCl was 20 wt%, the highest activity was obtained, and about 62% of RhB was degraded during 15 min of light irradiation. After 75 min of irradiation, the degradation percentages of RhB over AgCl, Bi2WO6, 5 wt% AgCl/Bi2WO6, 10 wt% AgCl/Bi2WO6, 20 wt% AgCl/Bi2WO6 and 30 wt% AgCl/Bi2WO6 were 48%, 60%, 63%, 82%, 98% and 92%, respectively. The stability of 20 wt% AgCl/Bi2WO6 was examined using a recycling test, which showed that the degradation percentage decreased as the number of cycles increased (data not shown).
Figure 8 illustrates the photocatalytic performance of the samples under visible-light irradiation. Under visible-light irradiation for 150 min, the degradation percentages of RhB over pure AgCl and Bi2WO6 were 52% and 47%, respectively. However, the deposition of AgCl markedly increased the visible-light photocatalytic activity of the catalysts. When the content of AgCl was 20 wt%, 99% of RB was degraded after 150 min of light irradiation.
The mechanism for the enhanced activity of AgCl/Bi2WO6 compared with those of AgCl and Bi2WO6 is now considered. The XRD and UV-Vis DRS analyses revealed that the deposition of AgCl had no marked effect on the surface area, crystal structure and light absorption of Bi2WO6. Therefore, the formation of an AgCl/Bi2WO6 heterojunction could be the main reason for the enhanced photocatalytic performance. When two semiconductors with suitable Eg are combined, a heterojunction can be produced. A potential difference is generated on the two sides of the heterojunction because of the different potential levels of the two conductors. Such an electric potential difference can promote the separation of photogenerated electrons (e-) and holes (h+), improving the photocatalytic activity of the semiconductor [23].
The positions of the valence band (VB) and conduction band (CB) for the samples were calculated by the electronegativity principle [24]. According to the empirical formulae EVB = X-Ee + 0.5 Eg and ECB = EVB - Eg (here, EVB, X, and Ee, are the energies of the VB edge potential, the absolute electronegativity, and free electrons on the hydrogen scale (4.5 eV), respectively) we calculated the potentials of the VB and CB of the samples; the results are shown in Table 3. The CB position of AgCl (0.54 eV) is more anodic than that of Bi2WO6 (0.17 eV). Therefore, an excited electron in the CB of Bi2WO6 can transfer to the CB of AgCl. As a result, the recombination of photogenerated e- and h+ over Bi2WO6 could be suppressed. Therefore, more e- and h+ could be available to produce active free radicals like •OH, and O2-• because e- can be captured by the surface-adsorbed O2 to produce O2-•, and the -OH groups can capture photogenerated h+ to form reactive •OH radicals. According to the literature [25, 26], in photodegradation of RhB over Bi2WO6, •OH racial oxidation is not the dominant photooxidation pathway, O2-• are the main radicals to decompose RhB. The proposed mechanism of the AgCl/Bi2WO6 photocatalyst heterojunction is outlined in Fig. 9.
Bi2WO6 microspheres with a diameter of 1.5-2 μm were fabricated using a hydrothermal method and then coated with AgCl. The effects of deposition of different contents of AgCl on the photocatalytic performance of the Bi2WO6 microspheres were investigated. Although the deposition of AgCl had no obvious effect on the crystal structure, surface area, and light absorption of Bi2WO6, the UV- and visible-light photocatalytic activity of the AgCl/Bi2WO6 samples was substantially promoted. The main reason for this activity increase was attributed to the formation of an AgCl/Bi2WO6 heterojunction that facilitates the separation of photogenerated e- and h+.
2015, Vol. 36 Issue (7): 987-993
PDF (1553 KB)
2015, Vol. 36 Issue (7): 987-993
PDF (1553 KB)