English

• 0   Introduction

  Titanium dioxide (TiO₂) has attracted a great deal of research attention because of their potential appli-cations in the photodegradation of organic pollutants, photocatalytic water splitting for hydrogen generation, dye-sensitized solar cells, and even gas sensors and biosensors, due to its low cost and abundant elements (Ti and O), long-term stability, and environmental-friendly characteristics^[1-3]. However, its wide band gap and fast recombination of the photogenerated electron-hole (e^--h⁺) are two limitations for its contemporary applications^[4].

  To overcome the above limitations, some measures have been taken. At present, one effective approach is to adjust TiO₂ morphology. One-dimensional (1D) TiO₂ nanomaterials have been receiving extensive interests^[5-8]. Moreover, compared with other forms of TiO₂, titania nanotubes possess the distinguishing features of nanotubes including large specific surface area, good electron/proton conductivity, and high aspect ratio. In addition, the open mesoporous morphology of TiO₂ nanotubes can efficiently transfer the electrons along the 1D path without grain boundaries and junctions, while hollow space can capture scattered light to increase light harvesting as well as easier separation and recovery than TiO₂ nanoparticles due to the length in the micrometer range^[9].

  Another strategy is to construct the heterostru-ctures by the wide band-gap semiconductor with a narrow band-gap semiconductor (with the proper band positions)^{[1, 10]}. Yu and Li fabricated and reported Ag-based heterojunction^[11-16], Au NPs loaded onto the α-Bi₂O₃/Bi₂O₂CO₃^[17], F-Bi₂MoO₆^[18], anatase/rutile TiO₂ particles^[19], and MoS₂/CdS composite^[20], which are more efficient than individual component in photocatalytic properties. In particular, Qian and Ma groups successfully fabricated UCNPs/semiconductors for NIR-driven photocatalysis, such as UCNPs/TiO₂ nano-fiber^[21], UCNPs/TiO₂/CdS nanofibers^[22], NYF@TiO₂-Au core@shell microspheres^[23]. They show unique optical properties with wide absorption and enhanced photocatalytic abilities towards to organic dye removal efficiency under irradiation with NIR. Such synergistic interactions of heterojunction between two kinds of semiconductors are fairly powerful not only in improving the visible light harvesting ability but also in extending the lifetime of photoinduced electrons and holes via an internal charge transfer, facilitating the separation of electron-hole pairs and reducing the chance of recombination^[24-26].

  Among these, bismuth tungstate (Bi₂WO₆), as a typical Aurivillius oxide, has a layered structure with perovskite-like slabs of WO₆ and [Bi₂O₂]²⁺ layer and has important physical and chemical properties such as ferroelectric piezoelectricity, catalytic behavior and nonlinear dielectric susceptibility^[27-28]. More importan-tly, Bi₂WO₆ is a promising visible light-driven photo-catalyst with high photocatalytic activity^[29-30]. However, the photocatalytic activity of pure Bi₂WO₆ is limited by difficult migration and high recombination proba-bility of photogenerated e^--h⁺ pairs. Therefore, the combination of tubular morphology and heterostructure construction is a useful approach for designing hetero-structure photocatalysts with high charge separation efficiency.

  In order to improve the photocatalytic activity, the construction of TiO₂-Bi₂WO₆ heterostructures has become a hot research, and some achievement has been obtained in recent years. For instance, Wang et al. successfully synthesized TiO₂-Bi₂WO₆ nanofibers by electrospinning technique^[31]. Colón et al. and other groups have reported TiO₂ modified flower^[32]/sphere^[33]/hollow tube-like Bi₂WO₆^[34]. Wu and Luo et al. reported the preparation TiO₂ nanobelts^[35]/TiO₂ nanotubes^[36] grown on titanium (Ti) foil decorated with Bi₂WO₆ nanocrystals, respectively. These results indicate that the photocatalytic activities of TiO₂-Bi₂WO₆ hetero-junctions show enhanced photocatalytic performance in comparison with individual components of Bi₂WO₆ or TiO₂. To the best of our knowledge, much less notice has been taken of the preparation of TiO₂ nanotubes synthesized by alkali hydrothermal treat-ment modified with Bi₂WO₆. Moreover, few investiga-tions were carried on the comparative mechanism of the enhanced photocatalytic activity for organic pollu-tants under multiple modes including UV, visible, and microwave-assisted photocatalysis. What is more, they lack direct evidence to explain photocatalytic mechan-ism under multiple modes that serve as background data for the environmental behavior of organic pollutants.

  In this work, Bi₂WO₆/TiO₂ nanotubes (Bi₂WO₆/TiO₂-NTs) heterostructures were fabricated by multi-component assembly approach combined with hydro-thermal treatment, which is free from the usage of additives or surfactants. Subsequently, the photocatal-ytic activities of Bi₂WO₆/TiO₂ nanotubes under multiple modes including UV, visible, and microwave-assisted photocatalysis were also studied in this work. Direct evidence to explain comparatively photocatalytic mechanism under multiple modes was supplied by free radical and hole trapping experiments. The relationship between the morphology, structure, optical properties and the photocatalytic activities of Bi₂WO₆/TiO₂ heterostructures under multiple modes was investigated in detail.

  1   Experimental

  1.1   Preparation of Bi₂WO₆/TiO₂ nanotubes

  In a typical procedure^[37], TiO₂ nanotubes were dispersed in H₂O (5 mL) under vigorously stirring for 0.5 h. Meanwhile, Bi(NO₃)₃·5H₂O (0.972 g) and Na₂WO₄ ·2H₂O (0.329 g) were dissolved in glacial acetic acid (HAc, 10 mL) and H₂O (5 mL), respectively. Subse-quently, the above solutions were added into TiO₂ nanotubes suspension to form a white suspension. After stirring for 2 h, the resulting mixture was suffered from hydrothermal treatment at 150 ℃ for 4 h, and the resulting precipitate was dried and washed with deionized water for three times. The obtained powder was further dried at 80 ℃ for 24 h. The final product was denoted as Bi₂WO₆/TiO₂-NTs-x, where x represents the doping of TiO₂ nanotubes (mass percentage).

  1.2   Characterization of the catalyst

  X-ray diffraction patterns were obtained on a Bruker-AXS (D8) X-ray diffractometer with Cu Kα radiation (λ=0.154 06 nm) at 40 kV and 40 mA in 2θ ranging from 20° to 80°. X-ray photoelectron spectros-copy (XPS) characterization was carried out on an ESCALAB 250Xi spectrometer equipped with Al Kα radiation at 300 W. N₂ adsorption-desorption isotherm analyses of samples were obtained at 77 K using Micromeritics 3H-2000PS2. The morphologies of synthesized samples were analyzed using a scanning electron microscope (SEM) (HitachiS-4300) and trans-mission electron microscope (TEM) and high resolu-tion transmission electron microscope (HRTEM) (JEM-2100F). UV-Vis diffused absorption spectra (UV-Vis DRS) were recorded using a UV-Vis spectrophoto-meter (TU-1901) over the wavelength range of 200~800 nm and BaSO₄ as a reference material.

  1.3   Photocatalytic tests

  Photocatalytic activities of the Bi₂WO₆/TiO₂-NTs composite were studied by monitoring the degradation behaviors of rhodamine B (RhB) under multimode (including UV, visible, and microwave-assisted photo-catalysis mode). The 125 W high pressure mercury lamp (λ=313.2 nm), 400 W Xe lamp (λ=410.0 nm; moreover, the inner sleeve was made of No. 11 glass to filter out ultraviolet from the Xe lamp), and 15 W microwave electrodeless lamp (MEL, UV emission wavelength mainly located at 278 nm, U shape, 100 W output power of microwave reactor), were used as UV, visible light, and microwave-assisted photocata-lysis mode light source, respectively. The concetration of RhB was 50 mg·L^-1. Moreover, the amounts of the catalyst (liquid volume) for the three modes (UV, visible, and microwave-assisted photocatalysis) were 100 mg (100 mL), 200 mg (220 mL), and 300 mg (500 mL), respectively.

  The photocatalytic reaction was carried out in a quartz photoreactor. Prior to irradiation, the suspen-sion containing the solid catalyst and an aqueous solution of the contaminant was ultrasonicated for 10 min and then stirred for 1.5 h in the dark to ensure adsorption-desorption equilibrium. The reaction temp-erature was maintained at (30±2) ℃ by circulation of water through an external cooling jacket or by circulating solution to a cooler with the peristaltic pump. At certain time intervals, suspensions (5 mL) were sampled and centrifuged to remove the photo-catalyst particles. Decreases in the concentrations of RhB, methyl orange (MO), crystal violet (CV), and methylene blue (MB) were analyzed by TU-1901 UV-Vis spectrophotometer at λ=553, 464, 582, and 664 nm, respectively.

  2   Results and discussion

  2.1   Compositional and structural information

  XRD was used to characterize the crystal structure of the as-prepared Bi₂WO₆/TiO₂-NTs, as well as pure TiO₂-NTs and Bi₂WO₆ (Fig. 1). The diffraction peaks of pure TiO₂-NTs and Bi₂WO₆ are well matched with the standard patterns of anatase phase of TiO₂ (PDF No.21-1272)^[37] and orthorhombic phase of Bi₂WO₆ (PDF No.39-0256), respectively. After the coupling of Bi₂WO₆ and TiO₂-NTs, when the TiO₂-NTs loading increases from 25% to 50%, the diffraction peaks of TiO₂ intensify gradually, whereas the peak intensities of Bi₂WO₆ decrease. No impurity peak is found in Bi₂WO₆/TiO₂-NTs composites, suggesting that the composites exhibit a coexistence of both Bi₂WO₆ and TiO₂ phases.

  图1 XRD patterns of the samples Figure1. XRD patterns of the samples

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  Valence states and the surface chemical comp-osition of the as-prepared samples were investigated by XPS technique. As shown in Fig. 2a, the peaks at 458.68 and 464.48 eV are attributed to Ti2p_3/2 and Ti2p_1/2, respectively, confirming the titanium species in the composite is Ti⁴⁺. After introduction of the Bi₂WO₆ into the TiO₂ nanotubes, the binding energies of Ti2p_3/2 and Ti2p_1/2 shift to higher values (458.78 and 465.28 eV, respectively), which is attributed to diffusion of W⁶⁺ ions into the TiO₂ lattice and further generation of WOTi bond linkage^{[35, 37]}. As displayed in Fig. 2b and c, for pure Bi₂WO₆, the characteristic peaks at 164.58 and 159.28 eV are ascribed to Bi4f_5/2 and Bi4f_7/2 from Bi³⁺ in the lattice and the binding energy of W4f_5/2 and W4f_7/2 at 37.88 and 35.78 eV, respectively, are corresponded to W^{6+ [35]}. In the XPS spectrum of Bi₂WO₆/TiO₂-NTs, in contrast with Bi₂WO₆, the binding energy of Bi4f_5/2 (164.38 eV) and Bi4f_7/2 (159.08 eV) decreases by 0.2 eV while that of W4f_5/2 (37.58 eV)and W4f_7/2 (35.58 eV) decreases by 0.3 eV. The results suggest that the chemical environment surrounding Bi and W has changed, which is possibly influenced by TiO₂-NTs. Thus, we can confirm that the TiO₂-NTs successfully modified by Bi₂WO₆.

  图2 XPS spectra of Ti2p (a), Bi4f (b), and W4f (c) regions for TiO₂-NTs, Bi₂WO₆, and Bi₂WO₆/TiO₂-NTs-35 Figure2. XPS spectra of Ti2p (a), Bi4f (b), and W4f (c) regions for TiO₂-NTs, Bi₂WO₆, and Bi₂WO₆/TiO₂-NTs-35

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  The porosity of the Bi₂WO₆/TiO₂-NTs heterostru-ctures is investigated by N₂ adsorption-desorption isotherms and the corresponding BJH pore size distri-bution. As shown in Fig. 3a, the isotherms exhibit type Ⅳ with an H3 hysteresis loop characteristic of meso-porous material^[37], which is confirmed by the pore size distribution (Fig. 3b). Moreover, the formation of such mesoporous materials is attributed to the aggregation of the Bi₂WO₆ nanoparticles adhering to the surface of the TiO₂ nanotubes. More importantly, as shown in Table 1, the measured BET surface areas of Bi₂WO₆/TiO₂-NTs-25 (80 m²·g^-1), Bi₂WO₆/TiO₂-NTs-35 (88 m²·g^-1) and Bi₂WO₆/TiO₂-NTs-50 (101 m²·g^-1) are greatly enhanced compared with that of Bi₂WO₆ (44 m²·g^-1). Meanwhile, the specific surface areas of composite materials increase indeed together with the increase of TiO₂-NTs contents from 25% to 50%.

  图3 Nitrogen adsorption-desorption isotherms (a) and BJH pore size distribution curves (b) of samples Figure3. Nitrogen adsorption-desorption isotherms (a) and BJH pore size distribution curves (b) of samples

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  Table 1.  Textural parameters of various TiO₂-based materials

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  Sample	SBET/ (M2·g-1)	Vp / (cm3·g-1)	Dp / nm
  TiO2-NTs	151	0.44	8.50
  Bi2WO6	44	0.16	10.44
  Bi2WO6/TiO2-NTs-25	80	0.26	9.12
  Bi2WO6/TiO2-NTs-35	88	0.33	10.20
  Bi2WO6/TiO2-NTs-50	101	0.35	9.84

  2.2   Morphology

  The morphology and microstructure of the photocatalysts were also investigated. As shown in the SEM image (Fig. 4a), TiO₂-NTs show the nanotubular morphology with an average diameter of 30 nm and length of 1 μm. While Bi₂WO₆ exhibits a typical structure of nanosheet consist of nanoparticles with the side length of 50~250 nm and thickness of 20~40 nm (Fig. 4b). As displayed in Fig. 4c~e, morphologies of TiO₂ and Bi₂WO₆ change obviously after the combination by TiO₂-NTs and Bi₂WO₆ through hydro-thermal treatment. The typical morphology structure of Bi₂WO₆/TiO₂-NTs-25 consists of smooth TiO₂ nano-tubes and curled Bi₂WO₆ flakes, which link mutually to each other. Moreover, the surface of TiO₂ nanotubes becomes rough obviously after Bi₂WO₆ modification when TiO₂ nanotubes loading increases from 35% to 50%. While Bi₂WO₆ changes from flakes to smaller nanoparticles. Furthermore, smaller Bi₂WO₆ nanoparti-cles homogeneously disperse on the surface of TiO₂ nanotubes in-situ growth process. Compared with TiO₂ -NTs and Bi₂WO₆, aggregation of Bi₂WO₆/TiO₂-NTs has intensively alleviated with the loading of TiO₂ nanotubes increasing from 0 to 50%.

  图4 SEM images of the samples Figure4. SEM images of the samples

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  In order to further confirm the Bi₂WO₆/TiO₂-NTs heterostructures, HRTEM was used to investigate the detailed structure information of the Bi₂WO₆/TiO₂-NTs. The corresponding HRTEM image displays two types of clear lattice fringes, as shown in Fig. 4f. The interplanar spacing of 0.35 and 0.315 nm corresponds to the (101) crystal plane of TiO₂-NTs and the (131) crystal plane of the orthorhombic phase of Bi₂WO₆, respectively^[18-19]. According to the results of XRD, XPS, SEM and HRTEM, we assume that Bi₂WO₆/TiO₂-NTs heterostructures with Bi₂WO₆ nanoparticles on the surface of TiO₂ nanotubes have been prepared successfully.

  Based on the above results and discussion, we put forward the plausible formation of Bi₂WO₆/TiO₂-NTs heterojunction. Considering Bi(NO₃)₃ with crystal water, Bi₂O₂(OH)NO₃ is formed through the following hydrolysis and condensation reaction in the glacial acetic acid-water system (Eq.1~2). When Na₂WO₄ solution is added to the above reaction solution, Bi₂WO₆ nanoparticles are obtained (Eq.3)^[38]. Then the intro-duction of TiO₂-NTs into Bi₂WO₆ suspension, Bi₂WO₆ nanoparticles aggregate around TiO₂-NTs. Subsequ-ently, at high temperature and high pressure, Bi₂WO₆ nanoparticles grow into curled flakes or smaller nanoparticles and homogeneously dispersed on the surface of TiO₂ nanotubes in-situ growth process, resu-lting in the formation of Bi₂WO₆/TiO₂-NTs heterojunc-tion^[31].

  2.3   Optical property

  UV-Vis diffused absorption spectra (UV-Vis DRS) were carried out to investigate the optical properties of the photocatalysts. As shown in Fig. 5a, the pure TiO₂-NTs and Bi₂WO₆ exhibit a fundamental absorption edge at around 388 and 450 nm, which originate from the charge transfer response of TiO₂-NTs and Bi₂WO₆ from the valence band to the cond-uction band, respectively^[39]. Compared with pure TiO₂-NTs, the absorption edges of Bi₂WO₆/TiO₂-NTs showed obvious red-shift to the longer wavelength within the range of visible light.

  图5 UV-Vis DRS (a) and plot of (αhν)^1/2 versus hν (b) for Bi₂WO₆, TiO₂-NTs and the Bi₂WO₆/TiO₂-NTs materials Figure5. UV-Vis DRS (a) and plot of (αhν)^1/2 versus hν (b) for Bi₂WO₆, TiO₂-NTs and the Bi₂WO₆/TiO₂-NTs materials

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  It is known that the optical absorption near the band edge of prepared samples obeys the following equation: (αhν)ⁿ=K(hν-E_g). In this equation, K, α, h, hν, E_g are constant, absorption coefficient, Planck constant, energy of the incident photon, band gap, respectively, and n is 0.5 and 1 for a direct and indirect band gap semi-conductor^[38]. According to the formula, the calculated band gaps (E_g) of samples are 2.75 eV (Bi₂WO₆), 2.87 eV (Bi₂WO₆/TiO₂-NTs-25), 2.94 eV (Bi₂WO₆/TiO₂-NTs-35), 3.00 eV (Bi₂WO₆/TiO₂-NTs-50), and 3.20 eV (TiO₂-NTs), respectively.

  The conduction band (CB) and valence band (VB) positions of the Bi₂WO₆ and TiO₂ samples are estimated by the following equations: E_VB=X-E_e+0.5E_g; E_CB=E_VB-E_g, where E_VB and E_CB are the VB and CB edge potentials, E_e is the energy of free electrons on the hydrogen scale (about 4.5 eV vs NHE). The X values for the Bi₂WO₆ and TiO₂ materials are 6.21 and 5.81 eV, respectively^[40-41]. The E_g of Bi₂WO₆ and TiO₂-NTs are estimated to be 2.75 and 3.20 eV, respectively. Herein, the CB and VB edge potentials of Bi₂WO₆ and TiO₂-NTs are calculated at 0.34 and 3.09 eV, and -0.29 and 2.91 eV, respectively.

  2.4   Photocatalytic activity

  The photocatalytic performance of the Bi₂WO₆/TiO₂-NTs heterostructures in terms of photodegrada-tion of RhB molecules under multiple modes inclu-ding UV, visible, and microwave-assisted photocataly-sis was investigated.

  Fig. 6a shows the photocatalytic activities of photocatalysts. Under UV light irradiation alone (without catalyst), only 3% RhB is degraded, which means the RhB can remain stability under long time irradiation. However, apparent changes in the concen-tration of RhB are observed in the existence of both light and catalyst. After irradiation for 90 min, 46.8%, 61.5%, 70.0%, 88.9%, 82.4% and 74.1% of the RhB is degraded by using the TiO₂-NTs, Bi₂WO₆, Bi₂WO₆/TiO₂-NTs-25, Bi₂WO₆/TiO₂-NTs-35, Bi₂WO₆/TiO₂-NTs-50, and P25, respectively.

  图6 Normalized decrease concentration of C_t /C₀ of RhB solution containing different photocatalysts under UV (a) and visible (b) light irradiation; (c) -ln(C_t /C₀) as a function of irradiation time for RhB degradation over photocatalysts; (d) Photocatalytic degradation RhB profiles obtained using different photocatalysts under microwave-assisted photocatalysis mode for 15 min; (e) Photocatalytic degradation RhB profiles by Bi₂WO₆/TiO₂-NTs-35 obtained under multimode for 15 min; (f) Normalized decrease concentrations of C_t /C₀ of different dyes using Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation Figure6. Normalized decrease concentration of C_t /C₀ of RhB solution containing different photocatalysts under UV (a) and visible (b) light irradiation; (c) -ln(C_t /C₀) as a function of irradiation time for RhB degradation over photocatalysts; (d) Photocatalytic degradation RhB profiles obtained using different photocatalysts under microwave-assisted photocatalysis mode for 15 min; (e) Photocatalytic degradation RhB profiles by Bi₂WO₆/TiO₂-NTs-35 obtained under multimode for 15 min; (f) Normalized decrease concentrations of C_t /C₀ of different dyes using Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation

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  Fig. 6b displays the photocatalytic activity of prepared samples under the visible light irradiation. It is found that the photocatalytic performance of Bi₂WO₆/TiO₂-NTs-35 to degrade RhB under visible light irradiation surpasses that of its individual counterparts.

  At the same time, the kinetics of photocatalytic degradation of RhB is investigated by simplified Langmuir-Hinshelwood model. The pseudo-first-order rate constant (k_app) is calculated using the formula -ln(C_t/C₀)=k_appt, where C₀ and C_t are the initial conce-ntration and concentration at reaction time t of RhB, respectively. From Fig. 6c, under visible irradiation, the rate constant over Bi₂WO₆/TiO₂-NTs-35, Bi₂WO₆, P25, and TiO₂-NTs is 1.10×10^-2, 8.45×10^-3, 3.71×10^-3, and 1.27×10^-3 min^-1, respectively. Moreover, Bi₂WO₆/TiO₂-NTs-35 shows the highest first-order rate constant, which is about 1.2 and 8.7 times greater than that of pure Bi₂WO₆ and TiO₂-NTs, respectively.

  Fig. 6d also exhibits the photocatalytic activity of different photocatalysts under microwave-assisted photocatalysis mode with electrodeless discharge lamp activated by microwaves as the light source. Bi₂WO₆/TiO₂-NTs-35 shows highest photocatalytic activity towards RhB degradation under microwave-assisted photocatalysis mode. Moreover, Fig. 6e displays photo-catalytic activities of Bi₂WO₆/TiO₂-NTs-35 under diff-erent modes after irradiation for 15 min. In contrast with UV and visible mode, the Bi₂WO₆/TiO₂-NTs-35 shows higher activity under microwave-assisted photo-catalytic mode. In addition, different kinds of dyes were selected to evaluate the photocatalytic activity under UV light irradiation (Fig. 6f). The cationic dyes (CV, MB, and RhB) are effectively degraded, while the degradation of anionic dye (MO) is poor, which is attributed to the different structure and adsorption of dyes.

  To evaluate the stability and reusability of Bi₂WO₆/TiO₂-NTs-35 heterostructures for practical application, the photocatalytic degradation of RhB with the same photocatalyst is carried out for several times. As displayed in Fig. 7, degradation curve has no obvious decline after four cycles of RhB degradation reaction under UV light irradiation, which indicates Bi₂WO₆/TiO₂-NTs-35 heterostructures maintain high stability.

  图7 Recycling for the photodegradation of RhB in the presence of Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation Figure7. Recycling for the photodegradation of RhB in the presence of Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation

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  2.5   Possible pathway of RhB degradation in Bi₂WO₆/TiO₂-NTs system

  The above photocatalytic tests indicate that: (ⅰ) the photocatalytic activity of pure TiO₂-NTs can be further increased by introduction proper Bi₂WO₆ loading under multimode; (ⅱ) in contrast to UV and visible mode, Bi₂WO₆/TiO₂-NTs showed higher photo-catalytic activity under microwave-assisted photo-catalysis mode. The influence factors towards the excellent photocatalytic activity of Bi₂WO₆/TiO₂-NTs are discussed.

  Firstly, Bi₂WO₆ modified TiO₂ nanotubes play a major role in improving the photocatalytic activity of TiO₂ nanotubes. On one hand, according to UV-Vis DRS analysis, Bi₂WO₆/TiO₂-NTs heterostructures have a narrow band gap and exhibit enhanced UV and visible light absorption, consequently increases the utilization of light. On the other hand, the formed heterostructures between Bi₂WO₆ and TiO₂-NTs photocatalysts can extend the lifetime of photoinduced electrons and holes via an internal charge transfer, further facilitate the separation of e^--h⁺ pairs and reduce the chance of recombination. These well-separated electrons and holes can participate in the overall photocatalysis process.

  Secondly, the open mesoporous morphology of nanotubes can enhance the contact between the substance and photocatalysts during the photocatalytic reaction. Meanwhile, the nanotubes provide an efficient transport channel for photogenerated electrons.

  Thirdly, degradation mode influences the photo-catalytic activity of Bi₂WO₆/TiO₂-NTs heterostructures. Compared with UV and visible mode, Bi₂WO₆/TiO₂-NTs heterostructures display highest photocatalytic activity under microwave-assisted photocatalysis mode. Microwave enhances the reactants mobility and diffus-ion leading to increased exchange of reactants between catalyst surface and solution^[42]. Moreover, more ·OH and ·O₂^- radicals are generated by photocatalysis with microwave irradiation than photocatalysis alone to enhance the separation of e^--h⁺ pairs^[43-45], which will be confirmed by the following trapping experiments.

  As shown in Fig. 8, the RhB degradation rate under UV degraded mode decreases obviously with the addition of disodium ethylenediaminetetraacetate (EDTA-2Na, 1 mmol·L^-1) as scavenger for h⁺ (from 88.9% to 9.3%), is moderately reduced with the addi-tion of benzoquinone (BQ, 1 mmol·L^-1) as scavenger for ·O₂^- (from 88.9% to 58.6%) and tert-butyl alcohol (t-BuOH, 1 mmol·L^-1) as scavenger for ·OH (from 88.9% to 73.7%)^[46-48]. Similar results are found in RhB photodegradation over Bi₂WO₆/TiO₂-NTs-35 under visible mode. Compared with UV and visible mode, under microwave-assisted mode, there is a little difference. The degradation rate toward RhB exhibits a significant decrease when EDTA-2Na, BQ, and t-BuOH are introduced. Furthermore, RhB degradation rate is reduced from the original 77.4% to 23.9%, 30.7%, and 37.5%, respectively. These results suggest that: (ⅰ) under the three modes, the degradation of RhB is primarily driven by h⁺, ·OH, and ·O₂^-; (ⅱ) under the UV and visible mode, h⁺ is the dominant reactive oxidants; (ⅲ) under microwave-assisted mode, h⁺, ·OH and ·O₂^- make nearly equal contribution to RhB degradation. That is to say, more ·OH and ·O₂^- are generated under microwave-assisted photocatalysis mode compared with UV and visible mode.

  图8 Controlled experiments using different radical scavengers for the degradation of RhB by Bi₂WO₆/TiO₂-NTs-35 under different modes Figure8. Controlled experiments using different radical scavengers for the degradation of RhB by Bi₂WO₆/TiO₂-NTs-35 under different modes

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  Based on the above results, the photocatalytic mechanism for Bi₂WO₆/TiO₂-NTs heterostructures photocatalyst is tentatively proposed and schematically illustrated in Scheme 1. The conduction bands (CB) (the valence band (VB), band gap) of TiO₂-NTs and Bi₂WO₆ are at -0.29 and 0.34 eV, respectively. Hence, under UV or MEL irradiation, both TiO₂-NTs and Bi₂WO₆ are excited, and photogenerated electrons and holes are in their CB and VB, respectively. Subsequently, the photoexcited electrons in the CB of TiO₂-NTs transfer to the CB of Bi₂WO₆, which is due to that E_CB of TiO₂-NTs is more negative than that of Bi₂WO₆. Simultaneously, the holes in the E_VB of Bi₂WO₆ move to TiO₂-NTs due to that the E_VB of Bi₂WO₆ is more positive than that of TiO₂-NTs. The h_VB⁺ reacts with the absorbed H₂O molecules or deoxidizes dioxygen dissolved in the aqueous solution to form ·OH radicals. In addition, the E_CB can be easily oxidized by dioxygen to produce ·O₂^- radicals. With the help of ·OH, h_VB⁺ and ·O₂^- species, RhB is degraded and then mineralized. The photocatalytic process under visible light irradiation is similar with UV (microwave-assisted photocatalytic mode) except TiO₂-NTs are not excited.

  图Scheme1 Photodegradation mechanism of Bi₂WO₆/TiO₂-NTs-35 heterostructures photocatalyst under UV mode Scheme1. Photodegradation mechanism of Bi₂WO₆/TiO₂-NTs-35 heterostructures photocatalyst under UV mode

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  3   Conclusions

  In summary, Bi₂WO₆/TiO₂-NTs heterostructures were fabricated by multicomponent assembly approach combined with hydrothermal treatment. Bi₂WO₆ flakes or nanoparticles dispersed on the surface of TiO₂ nanotubes to form heterostructures. The prepared Bi₂WO₆/TiO₂-NTs heterostructures exhibit considerably high photocatalytic activity towards the degradation of RhB under multimode including UV, visible and microwave-assisted photocatalysis. This enhanced photocatalytic activity is due to more efficient separa-tion of the e^--h⁺ pairs, originating from the introduc-tion of Bi₂WO₆ modified TiO₂-NTs, the nanotubular geometries, and the degradation mode. The h⁺, ·OH, and ·O₂^- radicals are the main active species during the photocatalysis process under multimode. Moreover, more ·OH and ·O₂^- radicals are generated by photo-catalyst with microwave-assisted irradiation. This work can provide important inspirations in developing the photocatalytic heterostructures materials.

  Acknowledgments: This work is supported by the Natural Science Foundation of China (Grants No.21376126, 81403067), the Program for Young Teachers Scientific Research in Qiqihar University (Grant No.2014K-M03), and the Basic Business Special Scientific Research of Heilongjiang Province Education Department (Grant No.135109204).

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  Figure 1  XRD patterns of the samples

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  Figure 2  XPS spectra of Ti2p (a), Bi4f (b), and W4f (c) regions for TiO₂-NTs, Bi₂WO₆, and Bi₂WO₆/TiO₂-NTs-35

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  Figure 3  Nitrogen adsorption-desorption isotherms (a) and BJH pore size distribution curves (b) of samples

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  Figure 4  SEM images of the samples

  (a) TiO₂-NTs, (b) Bi₂WO₆, (c) Bi₂WO₆/TiO₂-NTs-25, (d) Bi₂WO₆/TiO₂-NTs-35, (e) Bi₂WO₆/TiO₂-NTs-50, (f) HRTEM image of Bi₂WO₆/TiO₂-NTs-35

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  Figure 5  UV-Vis DRS (a) and plot of (αhν)^1/2 versus hν (b) for Bi₂WO₆, TiO₂-NTs and the Bi₂WO₆/TiO₂-NTs materials

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  Figure 6  Normalized decrease concentration of C_t /C₀ of RhB solution containing different photocatalysts under UV (a) and visible (b) light irradiation; (c) -ln(C_t /C₀) as a function of irradiation time for RhB degradation over photocatalysts; (d) Photocatalytic degradation RhB profiles obtained using different photocatalysts under microwave-assisted photocatalysis mode for 15 min; (e) Photocatalytic degradation RhB profiles by Bi₂WO₆/TiO₂-NTs-35 obtained under multimode for 15 min; (f) Normalized decrease concentrations of C_t /C₀ of different dyes using Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation

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  Figure 7  Recycling for the photodegradation of RhB in the presence of Bi₂WO₆/TiO₂-NTs-35 under UV light irradiation

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  Figure 8  Controlled experiments using different radical scavengers for the degradation of RhB by Bi₂WO₆/TiO₂-NTs-35 under different modes

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  Scheme1  Photodegradation mechanism of Bi₂WO₆/TiO₂-NTs-35 heterostructures photocatalyst under UV mode

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  Table 1.  Textural parameters of various TiO₂-based materials

  Sample	SBET/ (M2·g-1)	Vp / (cm3·g-1)	Dp / nm
  TiO2-NTs	151	0.44	8.50
  Bi2WO6	44	0.16	10.44
  Bi2WO6/TiO2-NTs-25	80	0.26	9.12
  Bi2WO6/TiO2-NTs-35	88	0.33	10.20
  Bi2WO6/TiO2-NTs-50	101	0.35	9.84

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