Abstract
The heterostructured TiO2/N-Bi2WO6 composites were prepared by a facile sol-gel-hydrothermal method. The phase structures, morphologies, and optical properties of the samples were characterized by using X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), energy dispersive spectroscopy (EDS), and UV-vis diffuse reflectance spectroscopy. The photocatalytic activities for rhodamine B of the as-prepared products were measured under visible and ultraviolet light irradiation at room temperature. The TiO2/N-Bi2WO6 composites exhibited much higher photocatalytic performances than TiO2 as well as Bi2WO6. The enhancement in the visible light photocatalytic performance of the TiO2/N-Bi2WO6 composites could be attributed to the effective electron-hole separations at the interfaces of the two semiconductors, which facilitate the transfer of the photoinduced carriers.
1. Introduction
In the past decades, TiO2, as an effective photocatalyst with a band gap of 3.2 eV, has been widely investigated in environmental remediation and solar utilization, owing to its effectiveness, cheapness, and chemical stability. However, its low visible light response and high photoinduced charge combination limited the utilization of solar energy. To date, most of research on photocatalysis was focused on the development of novel photocatalysts with high visible light responsivity. Very recently, Bi2WO6 with high visible light responsivity was widely investigated to degrade organic dye waste water [1–3] and decompose volatile gaseous pollutant [4]. Bi2WO6 is regarded as a promising visible light photocatalyst for dealing with the environmental problems in water and air due to its narrow band gap. Unfortunately, the low photocatalytic performance of Bi2WO6 was caused by its poor adsorption property and weak migration of photoinduced charge carriers.
To improve the photoactivity of Bi2WO6, hierarchical nest-like structure [5] and hierarchical flowers [6] with a hollow structure were constructed by solution self-assembly of nanoplates, providing high surface area and porous structure for adsorption of organic molecules. As is well known, high photoinduced charge recombination is a harmful factor to photocatalytic activity of Bi2WO6. The “Schottky” barrier between the metal and the semiconducting photocatalyst was considered to be an efficient path to improve the interfacial charge transfer process and accelerate the charge carrier separation. Bi2WO6/Cu0 [7] with Fenton-like synergistic effect and Bi2WO6/Ag0 [8] heterojunctions were designed to prevent the electron-hole recombination and improve the photocatalytic performance. In addition, carbon-modified Bi2WO6 [9, 10] was also considered to enhance the photoactivity due to high specific surface area and high conductivity of carbon, accelerating the charge transfer from photocatalyst to the liquid-solid interface contacted with organic pollutants by making use of carbon’s unique electron transport properties. Heterostructured Bi2WO6-TiO2 composite [11, 12] was discovered to possess the synergetic effect between TiO2 and Bi2WO6 which leads to an effective charge carrier separation, exhibiting the outstanding photocatalytic performance under sunlike irradiation.
In our experiments, nitrogen-doped Bi2WO6 was synthesized by a hydrothermal method in the first step. And then nanosized TiO2 particles were coated on the surface of N-doped Bi2WO6 nanoplates by a facile sol-gel-hydrothermal process. The influences of nitrogen doping on the structure, optical properties, and morphologies of Bi2WO6 were investigated. And the mechanism of enhanced photocatalytic activities of TiO2/N-Bi2WO6 was explained by nitrogen doping and the synergetic effect between TiO2 and N-doped Bi2WO6.
2. Experimental
2.1. Synthesis of N-Doped Bi2WO6
All the reagents were of analytical purity and were used as received from Shanghai Chemical Company. Similar to the reported experiments [13], in a typical procedure, aqueous solutions of 10 mmol Bi(NO3)3·5H2O and 5 mmol Na2WO4·2H2O were mixed together, then the mixture was magnetic stirred for two hours at room temperature. Afterward, aqueous solution containing desired amounts of urea (CO(NH2)2) was added for N-doped Bi2WO6 with the atomic ratio of N to Bi in 0.5 in the precursor. Then, the suspension was added into a 50 mL Teflon-lined autoclave up to 80% of the total volume. The suspension in the autoclave was heated at 160°C for 24 h. Subsequently, the autoclave was cooled to room temperature. The products were collected by filtration and then were washed by deionized water. The samples were then dried at 80°C for several hours and were denoted as N-BWO.
2.2. Preparation of the Heterostructured TiO2/N-Bi2WO6 Composites
The heterostructured TiO2/N-Bi2WO6 composite was synthesized via a sol-gel-hydrothermal process. Briefly, a mixed solution of 20 mmol of Ti(OC4H9)4, 183 mmol of ethanol (C2H5OH), and 1 g of the as-prepared N-doped Bi2WO6 was magnetic stirred for 30 minutes, forming a homogeneous mixed suspension. Subsequently, a mixed solution of 183 mmol of ethanol (C2H5OH), 75 mmol of H2O, and 2.8 mmol HNO3 was dropped into the above suspension and magnetic stirred for another 30 minutes to get a highly dispersed gelatin. As-obtained gelatin was aged for a whole night and was transferred into the autoclave of 50 mL with 15 mL H2O at 160°C for 24 h, denoted as TiO2/N-BWO. The resultant samples were calcined at 400°C for 1 h, 2 h (5°C/min) to remove solvents and other organic species, which were denoted as TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h, respectively. For comparison, the pristine Bi2WO6 was synthesized by a hydrothermal process at 160°C for 24 h, denoted as BWO. And the pure TiO2 was fabricated via a sol-gel-hydrothermal method at 160°C for 24 h, denoted as H-TiO2.
2.3. Catalyst Characterization
The crystalline phases of the as-prepared catalysts were confirmed by powder X-ray diffraction (XRD). The XRD patterns were obtained for the heterostructured TiO2/N-Bi2WO6 samples by using a Rigaku Multiflex diffractometer at 40 kV and 200 mA with monochromated high-intensity CuKα radiation. The surface morphology of the as-synthesized samples was observed by a field emission scanning electron microscope (FE-SEM). High-resolution transmission electron microscopy (HRTEM) was performed on JEOL-2010F operated at 200 kV. To prepare the transmission electron microscopy (TEM) sample, a small amount of samples was ultrasonically dispersed in ethanol. A drop of such suspension was placed on a 200-mesh Cu grid with holey carbon film and dried completely in air. The UV-vis diffuse reflectance spectra (DRS) were acquired on a Shimadzu UV-2450 spectrophotometer with ISR-240A integrating sphere assembly in the range of 200–800 nm. BaSO4 was used as a reflectance standard.
2.4. Evaluation of Photocatalytic Activity
The photocatalytic performance of the heterostructured TiO2/N-Bi2WO6 samples was evaluated by decomposing rhodamine B (RhB) under visible and ultraviolet light irradiation at room temperature. A 300 W Xe lamp (CEL-HXB UV300, Beijing China Education Au-light Co., Ltd.) equipped with an ultraviolet cutoff filter to provide visible light ( nm) was used as the visible light source, and which equipped with an ultraviolet reflected filter to provide ultraviolet light ( nm) was regarded as the ultraviolet light source. And the distance between the liquid surface of the suspension and the light source was set about 7 cm. The photodegradation experiments were carried out with the sample powder (50 mg) suspended in RhB aqueous solution (100 mL, 4 × 10−5 mol L−1) with constant magnetic stirring. Prior to the irradiation, the suspensions were magnetically stirred in the dark for 2 h to establish the adsorption/desorption equilibrium. At the given time intervals, about 4 mL of the suspension was taken for further analysis after centrifugation. RhB photodegradation was analyzed by recording the absorbance at the characteristic band at 553 nm as a function of irradiation time on a UV-vis spectrophotometer (Shimadzu UV 2450).
3. Results and Discussion
3.1. XRD Patterns of TiO2/N-Bi2WO6 Composites
The phase structures of as-prepared samples were investigated using powder X-ray diffraction. The XRD patterns for the heterostructured anatase TiO2/N-Bi2WO6 composites are shown in Figure 1(a). For comparison, the XRD patterns of the pure Bi2WO6, N-doped Bi2WO6, and the pure TiO2 are also given. It is observed that the as-prepared pure Bi2WO6 powder was in good agreement with the standard orthorhombic phase of Bi2WO6 (JCPDS no. 39–0256), while diffraction peaks of H-TiO2, corresponding to the standard tetragonal phase of anatase (JCPDS no. 21–1272), were clearly observed. No new crystal orientations or changes in preferential orientations of N-doped Bi2WO6 are observed comparing to pure Bi2WO6, despite of the presence of doping with nitrogen. However, the diffracted intensity of (131) crystallographic plane displays an obvious decrease owing to the doping of nitrogen. And the widening diffraction peak of (131) crystallographic plane of N-doped Bi2WO6 is induced by the doping of nitrogen in Figure 1(b). In addition, the peak position of N-doped Bi2WO6 shifts slightly toward a lower 2θ value. In the XRD patterns for the heterostructured TiO2/N-Bi2WO6 composites, the anatase phase TiO2 peak of (101) crystallographic plane gradually appeared and the diffracted intensity of N-doped Bi2WO6 drastically decreased in Figure 1(a). After the heat treatment, the phase compositions of the as-prepared composites do not change any, indicating that N-Bi2WO6 plays a suppressive role in the phase transformation from anatase to rutile phase of TiO2. Highly dispersed TiO2 coated on the surface of N-Bi2WO6 resulted in the weak peaks corresponding to N-Bi2WO6 and anatase TiO2 which were observed in the heterostructured TiO2/N-Bi2WO6 composites. The average crystal grain sizes of the as-prepared products are calculated from (131) crystallographic plane in Figure 1(b) according to the Scherrer formula and are summarized in Table 1. It is found that, for the pure Bi2WO6, the average crystal grain size is about 20.67 nm, while the average crystal grain size in N-doped Bi2WO6 and TiO2/N-Bi2WO6 decreased due to the doping with nitrogen and the suppressive effect of TiO2 on the growth of N-Bi2WO6. The average crystal grain size of TiO2 increased owing to hydrothermal reaction and postcalcination. Since the ionic radius of N3- (0.171 nm) is larger than that of O2- (0.140 nm) and the high electron density of N3- ions, the most of N3- ions were doped in the crystal lattice of Bi2WO6 [14]. Thus, the crystal lattice cell parameter of Bi2WO6 along b-axis direction has a slightly increase whereas that along c-axis direction has a slightly decrease owing to the doping with nitrogen (in Table 2).
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3.2. SEM
The morphologies of the as-prepared products were characterized by SEM. As shown in Figure 2, the pure Bi2WO6 product is composed of the large and highly dispersed nanoplates. The platelike morphology is maintained after doping with nitrogen though the sheet sizes become much lower. Moreover, the small nanoplates become more and more aggregated. For comparison, the as-prepared TiO2 is composed of the small anatase spherical aggregates via a hydrothermal process. The significant morphology modification is induced by the incorporation of TiO2 into the N-doped Bi2WO6 structure. No distinct platelike structure in the TiO2/N-Bi2WO6 is observed owing to the coating of the spherical TiO2 particles on the surface of nanoplates. After calcination, the TiO2 nanoparticles occur the crystal growth and the heterostructured TiO2/N-Bi2WO6 displays an aggregate trend.
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3.3. TEM
To confirm the microstructure of the heterostructured TiO2/N-Bi2WO6, the TEM photographs of the as-synthesized N-BWO and TiO2/N-BWO are shown in Figure 3. These results show that the nitrogen-doped Bi2WO6 sample is composed of highly dispersed and homogeneous nanoplates, indicating that doping with nitrogen does not result in the change of the morphologies of Bi2WO6 (in Figure 3(a)). Figure 3(b) reveals that the weak crystallinity of N-BWO is obtained as a result of doping with nitrogen. The microstructure of the TiO2/N-BWO composite was further studied by TEM and HRTEM. Figure 3(c) shows the typical TEM image of the TiO2/N-BWO heterostructure, in which many small spherical TiO2 nanoparticles of approximately 5–10 nm with an anatase phase are coated on the surface of the platelike N-doped Bi2WO6 particles. The nanoplates of N-doped Bi2WO6 were hardly observed in Figure 3(c), indicating that the well-coated anatase TiO2 layer was obtained on the surface of N-doped Bi2WO6 nanoplates. The representative HRTEM image of the magnified view of the top right corner area is given in Figure 3(d). As can be seen, three sets of different lattice fringes were clearly observed. The lattice fringes of 0.211 nm and 0.270 nm correspond to the (241) and (002) crystallographic planes of nitrogen-doped γ-Bi2WO6, while the lattice fringe of 0.243 nm matches well with the (103) crystallographic plane of anatase TiO2. The well-defined fringes and the high crystallinity of the heterostructured TiO2/N-Bi2WO6 composites facilitate the separation of the photoinduced carriers, improving the corresponding photocatalytic activities [15]. The distinct interface was observed between segregated units of TiO2 and N-doped Bi2WO6 nanoparticles, revealing that the heterostructure indeed formed from these two materials.
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To further confirm the formation of the TiO2/N-BWO heterostructure, the energy dispersive spectra were measured to analyze the species and contents of the as-prepared N-BWO and TiO2/N-BWO samples. Figure 4 shows that there are some peaks of Bi, W, and O elements in the spectrum of N-BWO, while the peak of N element is not observed owing to the slightly doping with nitrogen. For comparison, the peak of Ti element is discovered except the peaks of Bi, W, and O elements. The content of each element is listed in Table 3. The atomic ratio of Bi : W : O is 2.67 : 1 : 2.31 which is different from that of pure Bi2WO6, indicating that the stoichiometric ratio of Bi2WO6 is changed by the doping with nitrogen. Moreover, small amount of residual carbon is also observed in the N-doped Bi2WO6 sample. Different from the situation of N-BWO, the atomic ratio of Bi : W : Ti : O of the heterostructured TiO2/N-BWO is 3.61 : 1: 102.87 : 97.74 due to the incorporation of anatase TiO2. The discrepancy of the atomic ratio was induced by the thick TiO2 coating layer which influenced the detection signal of the energy dispersive spectroscopy. This result was in accordance with the TEM observation of the existence of TiO2 coating layer on the surface of N-doped Bi2WO6 nanoplates.
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3.4. Optical Properties
To evaluate the photoresponsivity, the UV-vis diffuse reflectance spectra of the as-prepared samples are investigated. As can be seen in Figure 5(a), H-TiO2 exhibits remarkably strong absorption in the ultraviolet light region blow the wavelength of 400 nm. In contrast, the absorption edge of the pure BWO sample has an obvious red shift to approximate 460 nm in the visible light region and its ultraviolet absorption is weak. In comparison with the pure Bi2WO6, the N-BWO sample has a slightly red shift of the absorption edge. However, it is interesting that the absorption for the ultraviolet light greatly increased as a result of doping with nitrogen, revealing that the N-BWO sample possesses the excellent ultraviolet and visible light photoresponsivities. The absorption plots show all TiO2/N-BWO samples exhibit a slightly blue shift of the absorption edge and have strong light response in the ultraviolet region with the addition of TiO2 composition. In addition, with prolonging the calcination time, the absorption intensities in the visible light region increase significantly, which indicates that there is a synergetic effect on the light absorption between TiO2 and N-doped Bi2WO6. Based on the equation [16–18], the relationship between and photon energy is shown in Figure 5(b). And the band gaps of the as-prepared samples were estimated to be 2.92, 2.67, 2.60, 2.75, 2.73, and 2.71 eV in Table 4, corresponding to H-TiO2, BWO, N-BWO, TiO2/N-BWO, TiO2/N-BWO-400/1 h, and TiO2/N-BWO-400/2 h, respectively. From this result, it is deduced that the band gap of N-doped Bi2WO6 becomes much narrower owing to the influence of doping with nitrogen, whereas that of TiO2/N-BWO composites become much wider due to the introduction of TiO2.
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3.5. Photocatalytic Activities
Figure 6 shows the photocatalytic activities of the as-prepared samples in the degradation of RhB under both ultraviolet and visible light irradiation. As can be seen in Figure 6(a), when the system was illuminated with visible light, the pure Bi2WO6 samples exhibited the lowest photocatalytic activity and the lowest degradation ratio of RhB. It was interesting that as-prepared H-TiO2 exhibited a higher photocatalytic activity for the degradation of RhB, which was different from the traditional viewpoint that TiO2 was an effective ultraviolet-light-driven photocatalyst [19, 20]. However, the nitrogen-doped Bi2WO6 displayed a slightly higher photocatalytic activity than both H-TiO2, and pure BWO. This result proved that the enhanced visible light photocatalytic performance was caused by the doping with nitrogen. In the case of the heterostructured TiO2/N-BWO composite without calcination, the highest photocatalytic activity for the degradation of RhB was obtained, which was probably caused by the effective separation of photoinduced charge carriers owing to the synergetic effect between TiO2 and N-doped Bi2WO6 [21, 22]. The visible-light-induced photocatalytic activity of the heterostructured TiO2/N-BWO composite was higher than that of the commercial P25. This indicated that the formation of the new heterostructure was beneficial to accelerating the separation of the photoinduced charges, improving the visible-light-induced photocatalytic activity. A negative influence on photocatalytic activity was induced by prolonging the calcination time. It was a probable cause that the crystal growth of TiO2 and N-BWO resulted in the decrease of photocatalytic reactive sites. As all we know, decreased grain radius would be beneficial to reducing the recombination opportunities of the photoinduced electron-hole pairs.
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The photocatalytic activity was also evaluated under ultraviolet light irradiation. As shown in Figure 6(b), N-doped Bi2WO6 exhibited the optimal photocatalytic activity for the degradation of RhB among the as-prepared samples. The residual concentration of RhB in the solution was close to zero after photocatalytic reaction for 40 minutes. It proved that the doping with nitrogen into the crystal lattice of Bi2WO6 was beneficial to improving the ultraviolet-light-induced photocatalytic performance. However, it was lower than that of the commercial P25 owing to its weak ultraviolet light photoresponsivity and its large crystal size. And H-TiO2 was found to exhibit much lower photocatalytic activity than N-doped Bi2WO6. However, the photocatalytic activity of H-TiO2 under ultraviolet light irradiation was slightly higher than that of TiO2/N-BWO. In the case of ultraviolet light illumination, the photocatalytic activity of the heterostructured TiO2/N-Bi2WO6 samples was much lower than that of N-BWO. Furthermore, the photocatalytic activity of TiO2/N-Bi2WO6 heterostructure become lower and lower with prolonging the calcination time under ultraviolet light illumination. This was contrary with the results of UV-vis diffuse reflectance spectra in Figure 5. These results showed that the ultraviolet-light-driven photocatalytic activities of N-BWO, H-TiO2, and TiO2/N-BWO decrease with the enhanced ultraviolet light photoresponsivity as shown in Figure 5. Herein, the probable cause of the decreased photocatalytic activities was relative to the increased recombination opportunities of the photoinduced charges. From SEM images in Figure 2, the obvious aggregation and the crystal growth of TiO2 particles on the surfaces of N-doped Bi2WO6 particles were observed, which was adverse to the effective photoinduced charge separation. The photocatalytic activity was influenced greatly by the dispersivity and the crystal size of catalyst’s particles. A better dispersivity and a smaller crystal size of the catalyst’s particles were beneficial to the separation of the photoinduced electrons and holes. In the case of ultraviolet light irradiation, the TiO2 component in the heterostructured TiO2/N-BWO composite played the main function during the photodegradation of RhB as shown in Figure 9. The increased crystal size of TiO2 would result in the increased recombination opportunities of the photogenerated charges consequentially. A thicker TiO2 layer coated on the surfaces of N-doped Bi2WO6 particles was adverse to the utilization of the photoactivity of the N-doped Bi2WO6 component. Moreover, the photodegradation for RhB using N-doped Bi2WO6 as catalyst was incomplete, producing a large number of intermediates. The above-mentioned causes were responsible for lower photocatalytic activity of the heterostructured TiO2/N-BWO composite without calcination under ultraviolet light irradiation, comparing with H-TiO2 and N-doped Bi2WO6. Nevertheless, the pure BWO exhibited the lowest photocatalytic activity due to its weak ultraviolet light photoresponsivity. By comparison, all of the as-prepared samples exhibited higher photocatalytic activities under ultraviolet light irradiation than that under visible light irradiation.
To distinguish the photodegradation rate for RhB over the as-prepared catalysts, the kinetic-fitted curves plots of photodegradation of RhB under different light source with various wavelength ranges are displayed in Figure 7 and the corresponding values of the photodegradation rate constant k for RhB and linearly dependent coefficient are listed in Table 5. The–Ln() linearly increased, which meant the photodegradation of RhB under both ultraviolet and visible light irradiation over as-prepared samples could be described as first-order reaction [16]. The photocatalytic activity of the as-prepared samples can be also evaluated by the values of k, that is, the higher the value of k, the better the photocatalytic activity [23]. Moreover, the larger the coefficients , the better the linear dependence relation. From the fitted results in Table 5, under ultraviolet light illumination, the order of the values of k was 1.55 × 10−1 > 7.53 × 10−2 > 6.69 × 10−2 > 6.58 × 10−2 > 5.71 × 10−2 > 3.78 × 10−2 > 6.20 × 10−3, corresponding to the slopes of kinetic fitted curves of P25, N-BWO, TiO2/N-BWO, H-TiO2, TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h and pure BWO, respectively. The ultraviolet-light-driven photodegradation rate for RhB using the heterostructured TiO2/N-BWO composite without calcination is much lower than that of the commercial P25. As we know, P25 was an excellent ultraviolet-light-responsive photocatalyst. Hence, the photocatalytic activity of P25 under ultraviolet light irradiation should be much higher than that under visible light irradiation. This viewpoint was proved by the comparison of Figures 6(a) and 6(b). By contrast, under visible light irradiation, the order of the values of k was 3.66 × 10−2 > 2.61 × 10−2 > 2.29 × 10−2 > 2.02 × 10−2 > 1.48 × 10−2 > 1.41 × 10−2 > 5.97 × 10−3, corresponding to the slopes of kinetic-fitted curves of TiO2/N-BWO, P25, N-BWO, TiO2/N-BWO-400/1 h, TiO2/N-BWO-400/2 h, H-TiO2, and pure BWO, respectively. By comparison, the photodegradation rate for RhB under ultraviolet light irradiation was higher than that under visible light irradiation. N-doped Bi2WO6 exhibited the highest photodegradation rate under ultraviolet light irradiation, whereas TiO2/N-BWO exhibited the highest photodegradation rate under visible light irradiation. Furthermore, the photodegradation rate of the heterostructured TiO2/N-BWO composite without calcination under visible light irradiation was much higher than that of the commercial P25. This result indicated that the formation of the heterostructure facilitated the effective separation of the photoinduced charge carriers on the interface between TiO2 and N-doped Bi2WO6, improving visible-light-driven photocatalytic performance.
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3.6. Photocatalytic Mechanism
The temporal evolutions of the spectral changes during the photodegradation of RhB over N-BWO, TiO2/N-BWO, and H-TiO2 under ultraviolet light irradiation are shown in Figure 8. As all we know, RhB possesses a maximal absorption band at 553 nm. If degraded over N-doped Bi2WO6 under ultraviolet light irradiation for 32 minutes (in Figure 8(a)), the spectral maximum of RhB shifted from 553 to 531 nm, indicating that the mechanistic pathways of RhB degradation over N-doped Bi2WO6 was multisteps process accompanying with the deethylation process of the cleavage of the chromophore structure [16]. In the case of TiO2/N-BWO (in Figure 8(b)), the deethylation function was weaken owing to the incorporation of TiO2 with an anatase phase under ultraviolet light irradiation. Thus, the photodegradation of RhB occurred mainly by decomposing the pollutants to small molecules of CO2 and H2O. As for H-TiO2 in Figure 8(c), the deethylation phenomenon was hardly seen during the degradation of RhB under ultraviolet light irradiation. By comparison, the photodegradation rate of RhB was accelerated by the doping with nitrogen into the crystal lattice of Bi2WO6, because the number of photoinduced electrons and holes to participate in the photocatalytic reaction was increased remarkably, resulting in the enhanced photocatalytic activity. Under ultraviolet light irradiation, the heterostructure of TiO2/N-BWO does not significantly influence the photodegradation rate of RhB comparing to both N-BWO and H-TiO2 due to the strong oxidation performance of H-TiO2 with an intense ultraviolet photoresponsivity.
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Based on the above discussions, the as-prepared H-TiO2 and N-BWO were not only used as ultraviolet-light-driven photocatalysts but also regarded as visible-light-driven photocatalysts. Therefore, the heterostructure formed between H-TiO2 and N-doped Bi2WO6 was expected to possess the excellent photocatalytic performances as a bifunctional photocatalyst. Thus the heterostructured TiO2/N-BWO composite exhibited the excellent photocatalytic activities under both ultraviolet light and visible light irradiation as shown in Figure 9. Furthermore, the mechanisms of photodegradation of RhB over TiO2/N-BWO were completely distinct under different conditions of light irradiation. In our experiments, the composition of TiO2 in the heterostructure played an important role in the photodegradation of RhB under ultraviolet light irradiation (Figure 9(a)), while the composition of N-doped Bi2WO6 in the heterostructure played an important role in the photodegradation of RhB under visible light irradiation (Figure 9(b)). In addition, the photodegradation pathways of RhB were also disparate in the two cases. As for ultraviolet-light-driven photodegradation, the molecules of RhB over TiO2/N-BWO were degraded into small inorganic molecules of CO2 and H2O directly. Different from the case of ultraviolet photodegradation, the decomposition of RhB over TiO2/N-BWO was carried out step by step via a deethylation process under visible light illumination, resulting in the obvious blue shift of the maximal absorption band of RhB. The interface formed between H-TiO2 and N-doped Bi2WO6 can facilitate the transportation of photoinduced charge carriers and suppress the recombination of photogenerated electrons and holes pairs, inducing the enhanced photocatalytic activity under visible light illumination.
The enhanced visible-light-driven photocatalytic performance is related to the corresponding energy band structure. The doping with nitrogen resulted in the formation of a dopant energy level at the bottom of conduction band of Bi2WO6. The narrowing band gap of N-BWO was beneficial to increasing the transfer rate of electrons to the photocatalyst surface, promoting photocatalytic reaction. According to the estimated Eg values of the TiO2 and N-doped Bi2WO6 in Figure 5(b) and the conduction band (CB) and valence band (VB) levels of Bi2WO6 are lower than that of TiO2 [11, 12], the energy band structure of TiO2/N-BWO composite was schematically shown in Figure 10. The strong interaction between H-TiO2 and N-BWO resulted in a synergetic effect on the photocatalytic activity of the heterostructure. The photoinduced electron-hole pairs were produced by the excitation of TiO2/N-BWO under ultraviolet and visible light irradiation. The photoinduced electrons transferred from the conduction band of H-TiO2 to that of N-doped Bi2WO6 due to the joint of the internal electric fields between two materials. However, the photoinduced holes transferred from the valence band of N-doped Bi2WO6 to that of H-TiO2. Thus, the effective photoinduced charge separation provided more and more active free radicals to participate in the photocatalytic degradation of RhB. The novel design of the bifunctional TiO2/N-BWO heterostructure provided a new approach to develop the visible-light-driven photocatalysts. But N-doped Bi2WO6 was more likely to be excitated by visible light, being regarded as the main functional component under visible light irradiation. However, TiO2 was more likely to be excitated by ultraviolet light, being regarded as the main functional component under ultraviolet light irradiation. Therefore, TiO2/N-BWO composite exhibits diverse photodegradation mechanisms for RhB under different light irradiation conditions.
4. Conclusion
Heterojuncted TiO2/N-Bi2WO6 composites were synthesized by a facile sol-gel-hydrothermal process. The as-prepared TiO2/N-Bi2WO6 composites display a wide range of light absorption due to the introduction of TiO2, and the corresponding photocatalytic activities against rhodamine B are slightly improved in comparison with pristine Bi2WO6 and TiO2. It is also found that the existence of TiO2 in the heterojuncted TiO2/N-Bi2WO6 composites plays an important role in the photocatalytic property. The enhancement in the visible light photocatalytic performance of TiO2/Bi2WO6 composites can be attributed to the effective electron-hole separations at the interfaces between TiO2 and N-Bi2WO6, which facilitate the transfer of the photoinduced carriers. The experiments prove that the heterojuncted TiO2/Bi2WO6 composites are promising photocatalysts under visible and ultraviolet light irradiation.
Acknowledgment
The authors are grateful for the financial support by the National High Technology Research and Development Program of China (863 Program no. 2010AA064907).