International Journal of PhotoenergyVolume 2012 (2012), Article ID 469178, 12 pagesdoi:10.1155/2012/469178

Research Article

Sol-Gel-Hydrothermal Synthesis of the Heterostructured - Composite with High-Visible-Light- and Ultraviolet-Light-Induced Photocatalytic Performances

Jiang Zhang,¹ Zheng-Hong Huang,¹ Yong Xu,² and Feiyu Kang¹

¹Laboratory of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China²Department of Technology, Beijing Tongfang Puri-Tech Co., Ltd., Beijing 100083, China

Received 15 July 2011; Accepted 22 August 2011

Academic Editor: Jinlong Zhang

Copyright © 2012 Jiang Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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).

Table 1: Crystal sizes of the as-synthesized samples.

Sample
D(BWO) (131)/nm
D(TiO₂) (101)/nm

H-TiO₂
—
6.92

BWO
20.67
—

N-BWO
12.82
—

TiO₂/N-BWO
12.00
14.24

TiO₂/N-BWO-400/1 h
15.86
12.58

TiO₂/N-BWO-400/2 h
10.09
21.60

Table 2: Crystal lattice cell parameters of Bi₂WO₆ in the as-synthesized samples.

Sample
a/nm
b/nm
c/nm

BWO
5.45
16.43
5.44

N-BWO
5.45
16.44
5.44

TiO₂/N-BWO
5.44
16.48
5.43

TiO₂/N-BWO-400/1 h
5.45
16.51
5.41

TiO₂/N-BWO-400/2 h
5.45
16.44
5.41

(a)

(b)

Figure 1: XRD patterns of TiO₂/N-BWO, TiO₂/N-BWO-400/1 h, TiO₂/N-BWO-400/2 h, BWO, N-BWO, and H-TiO₂.

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.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2: Selected SEM micrographs for the as-prepared samples.

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.

(a)

(b)

(c)

(d)

Figure 3: TEM photographs for the as-prepared N-BWO (a, b) and TiO₂/N-BWO (c, d).

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.

Table 3: Characterization data of EDS for N-BWO and TiO₂/N-BWO samples.

N-BWO

Element
Weight ratio (%)
Atomic ratio (%)

C K
0.43
2.47

O K
0.95
4.11

Al K
0.29
0.74

Cu K
79.23
86.15

W M
4.74
1.78

Bi M
14.36
4.75

Total
100.00

TiO₂/N-BWO

Element
Weight ratio (%)
Atomic ratio (%)

O K
10.61
30.30

Si K
0.63
1.02

Ca K
0.37
0.42

Ti K
33.44
31.89

Cu K
48.59
34.94

W M
1.23
0.31

Bi M
5.14
1.12

Total
100.00

(a)

(b)

Figure 4: Energy dispersive spectra for the as-prepared N-BWO and TiO₂/N-BWO.

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.

Table 4: Estimation of the band gap energy of the as-prepared samples.

Sample
Band gap energy (eV)

H-TiO₂
2.92

BWO
2.67

N-BWO
2.60

TiO₂/N-BWO
2.75

TiO₂/N-BWO-400/1 h
2.73

TiO₂/N-BWO-400/2 h
2.71

(a)

(b)

Figure 5: (a) UV-vis diffuse reflectance spectra for the as-synthesized samples. (b) The relationship between and photon energy.

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.

(a)

(b)

Figure 6: Photocatalytic activity for decomposition of rhodamine B (4 × 10−5 mol L⁻¹) under (a) visible light (>420 nm) and (b) ultraviolet light (<400 nm) irradiation at room temperature in air over the as-prepared catalysts. : the concentration of RhB after dark adsorption, : the concentration of RhB during the photocatalytic reaction.

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.

Table 5: The values of the photodegradation rate constant k for RhB and linearly dependent coefficient .

Sample
Ultraviolet light
Visible light

k (mol L−1 min⁻¹)
R ²
k (mol L−1 min⁻¹)

H-TiO₂
6.58 × 10⁻²
0.927
1.41 × 10⁻²
0.982

BWO
6.20 × 10⁻³
0.929
5.97 × 10⁻³
0.931

N-BWO
7.53 × 10⁻²
0.929
2.29 × 10⁻²
0.964

TiO₂/N-BWO
6.69 × 10⁻²
0.928
3.66 × 10⁻²
0.946

TiO₂/N-BWO-400/1 h
5.71 × 10⁻²
0.899
2.02 × 10⁻²
0.959

TiO₂/N-BWO-400/2 h
3.78 × 10⁻²
0.905
1.48 × 10⁻²
0.991

P25
1.55 × 10⁻¹
0.945
2.61 × 10⁻²
0.982

(a)

(b)

Figure 7: Kinetic fitted curves plots of photocatalytic degradation of RhB (4 × 10−5 mol L⁻¹) under ultraviolet light (<400 nm) (a) and visible light (>420 nm) (b) irradiation at room temperature in air over the as-prepared catalysts.

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.

(a)

(b)

(c)

Figure 8: Temporal evolution of the spectral changes during the photodegradation of RhB over N-BWO, TiO₂/N-BWO, and H-TiO₂ under ultraviolet light irradiation.

(a)

(b)

Figure 9: Temporal evolution of the spectral changes during the photodegradation of RhB over TiO₂/N-BWO under ultraviolet light irradiation (a) and visible light irradiation (b).

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.

Figure 10: Schematic diagram for energy band matching and electron-hole separation.

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).

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