Insight into the origin of photoreactivity of various well-defined Bi₂WO₆ crystals: exposed heterojunction-like surface and oxygen defects

Abstract

An integrated examination of various influence factors (i.e., atomic, electronic and defect structure) is necessary to obtain an accurate structure–function relationship. In this work, we reported the synthesis of three kinds of well-defined Bi₂WO₆ photocatalysts with the aim of studying their atomic, electronic and defect-dependent photocatalytic properties. With examination of multiple characterization methods, it was found that nanosheets with a high percentage of exposed (020) facets were obtained. In addition, compared with single-crystalline nanosheets and nanoparticle-built microspheres, the nanosheet-assembled microspheres possess more surface oxygen defects. The visible light photoactivities of the prepared Bi₂WO₆ samples were carefully investigated by degradation of various pollutants (i.e., cationic Rhodamine B (RhB), anionic methyl orange (MO) and neutral ciprofloxacin (CIP)). The results revealed that the nanosheet-assembled Bi₂WO₆ microspheres show the best visible light photoactivities, which may be ascribed to the cooperative effect between the exposed facets and high surface oxygen defects. The results presented in this study clearly demonstrate the exposed facet and defect structure codependence of photoreactivity, which provides further insights into the orgin of activity in semiconductor photocatalysis.

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1. Introduction

Morphology and crystal facet engineering have attracted considerable research attention for both solar energy conversion and environmental applications.^1–4 In general, photocatalytic degradation reactions occur at the surface of photocatalysts. Thus, specific exposed facets with a higher percentage of under-coordinated atoms as active sites have been pursued.^5–7 For example, Wang et al. reported that the {111}-facet-bounded octahedra exhibit higher activity than the nanosheets with {101} facets.⁸ However, photocatalysis is the integration of adsorption of targeted molecules, light harvesting, separation, and surface transfer of photoexcited electrons and holes. Therefore, crystal facet engineering may not the exclusive factor in design of photocatalysts. An integrated examination of various influence factors (i.e., atomic, electronic and defect structure) is necessary before exploration of the relationship between the structure and function.

Bi₂WO₆, as one of the Aurivillius oxide families, has been used as an excellent solar-energy-conversion material and photocatalyst for water splitting and degradation of organic compounds under visible light irradiation.^9–12 However, the photocatalytic performance of Bi₂WO₆ remains limited owing to its low separation efficiency of photoinduced electron–hole pairs in photocatalytic process.^13,14 The shape-tailored synthesis of Bi₂WO₆ has recently attracted considerable attention. Various nanostructures of Bi₂WO₆, such as nanoparticles, nanorods, nanoplates and nanocages have been prepared by various routes.^15–17 Besides, Xie et al. synthesized the {001}-oriented Bi₂WO₆ nanosheet under acidic reaction conditions.¹⁸ Liu and co-workers investigated the photocatalytic activity of {010}-oriented flower-like hierarchical Bi₂WO₆.¹⁹ It was found that the photocatalytic properties can be effectively improved or optimized by controlling the surface atomic structures and large specific surface areas. However, it is less recognized that surface defects in real catalysts play essential roles in light absorption, electron capture and reactant activation.^20–22 Few studies are known about the co-effect of the exposed facet and surface oxygen defect on the photocatalytic performances of Bi₂WO₆.

In order to investigate the exposed facet- and surface defect-mediated photoactivity on Bi₂WO₆, a systematic exploration on multiple photocatalytic processes can be expected. Herein, we investigate the visible light photoreactivity of well-defined shaped Bi₂WO₆ on degradation of various organic pollutants, including cationic RhB, anionic MO and neutral CIP solutions. Surface exposed facet was clearly suggested to influence the separation efficiency and reactants activation. In addition, the photocatalytic results evidently proved the surface defect dependence of photocatalytic activities.

2. Experimental

2.1 Materials synthesis

The synthesis of the nanosheet-assembled Bi₂WO₆ microspheres was based on the previous reports with some modification.²³ Briefly, 4 mmol of Bi(NO₃)₃·5H₂O and 2 mmol of Na₂WO₄·2H₂O were added to 30 mL of distilled water under magnetic stirring, respectively. After that, the Na₂WO₄ solution was dropped to the Bi(NO₃)₃ suspension slowly. For the preparation of nanoparticle-assembled Bi₂WO₆ microspheres and single-crystalline nanosheets, the synthetic procedures were identical with that of nanosheet-assembled Bi₂WO₆ microspheres, except that a amount of sodium citrate (Cit-3Na) and Hexamethylenetetramine (HMT) was added to the Bi(NO₃)₃ suspension, respectively. The resulting precursor suspension was transferred into a Teflon-lined stainless steel autoclave (V = 100 mL), and then heated at 180 °C for 12 h. After the hydrothermal treatment, the product was filtered off, washed several times with absolute alcohol and distilled water, and finally dried at 80 °C overnight.

For the preparation of N-doped TiO₂ (N–TiO₂), 1 g of P25 was suspended in ethanol (10 mL). Then, urea (2 g) dissolved in 5 mL of ethanol and 1 mL of H₂O was added into the suspension. The mixture was stirred and heated to completely evaporate the solvent, followed by calcination in air at 400 °C for 4 h.²⁴

2.2 Materials characterization

The morphology and structure of the as-prepared product were characterized by field-emission scanning electron microscopy (SEM, S-4800, Hitachi) and transmission electron microscope (TEM, JOEL JEM 2001). The crystalline structure of the product was analyzed by an X-ray diffractometer (XRD) with a Bruker D8 diffractometer (Cu Kα radiation, λ = 1.5418 Å). X-Ray photoelectron spectroscopy (XPS) measurements were performed on an ESCALAB MKII X-ray photoelectron spectrometer by monochromated Al Kα X-rays. Ultraviolet visible (UV-vis) diffuse reflection spectra were measured by means of a UV-vis spectrophotometer (TU-1901, China), which involves an integrating sphere attachment in the range of 250 to 800 nm. BaSO₄ was used as the reflectance standard material. The N₂ adsorption–desorption isotherms was analyzed on a Quantachrome NOVA 2200e analyzer.

2.3 Photocatalytic test

The photocatalytic activities of the as-prepared Bi₂WO₆ products were evaluated by the degradation of Rhodamine B (RhB), methyl orange (MO) and ciprofloxacin (CIP) under visible light irradiation at room temperature. In a typical experiment, 30 mg of Bi₂WO₆ photocatalysts were added to an aqueous solution of RhB and MO (100 mL, 10⁻⁵ M), respectively. Besides, 50 mg of photocatalysts was added to CIP solution (100 mL, 10 mg L⁻¹). The photocatalytic reactor (PLS-SXE 300, Beijing Perfect light Co., Ltd.), consisting of a quartz glass with a circulating water jack and a 300 W Xe lamp with a 420 nm cutoff filter, was used as the visible light source. The light intensity striking the model pollutant solution was at ∼23 mW cm⁻², as measured by a FZ-A optical Radiometer (Photoelectric Instrument Factory of Beijing Norman University, Beijing, P. R. China). The optical spectrum of the 300 W Xe lamp with a 420 nm cutoff filter is given in Fig. S1.† Prior to irradiation, the suspensions were magnetically stirred in the dark for about 60 min to obtain a good adsorption–desorption equilibrium between the photocatalysts and model pollutants under ambient conditions. At certain time intervals, 5 mL of the solution was taken out and centrifuged to remove the photocatalyst. The concentrations of RhB, MO and CIP were analyzed by recording variations of the characteristic absorption band of 553, 463 and 272 nm using a TU-1901 spectrophotometer, respectively.

2.4 Photoelectrochemical measurements

The photoelectrochemical characteristics were measured in a CHI660D electrochemical working station using a standard three-compartment cell under Xe arc lamp irradiation with 300 W. In a typical procedure, a commercial ITO glass (1.0 × 1.0 cm²) was ultrasonicated in an ethanol and distilled water bath prior to use. The clean ITO glass substrate was then immersed in the slurry of the as-prepared photocatalyst (3 mg) and ethanol (3 mL) mixtures. The substrate was then vacuum-dried at 80 °C to eliminate ethanol and subsequently maintained at 100 °C overnight. ITO glass coated with the as-prepared samples, a piece of Pt sheet, a Ag/AgCl electrode and 0.01 M sodium sulfate were used as the working electrode, the counter electrode, the reference electrode and the electrolyte, respectively.

3. Results and discussion

3.1 Morphologies and structure of the prepared Bi₂WO₆ products

The morphologies of the as-prepared Bi₂WO₆ samples were examined by SEM analysis. As shown in Fig. 1a and b, the obtained Bi₂WO₆ sample has a sphere-like structure with diameters of about 3 μm, assembled from rectangular nanosheets. The assembly of the nanosheets results in numerous hierarchical pores. Besides, such nanosheet-assembled microspheres were very stable without breaking into individual nanosheet even upon ultrasonic treatment for long periods of time. In addition, in the presence of Cit-3Na in the reaction system, the prepared Bi₂WO₆ product also has a sphere-like structure, built by round-pill-like nanoparticles (Fig. 1c and d). Fig. 1e and f show the morphology of prepared samples in the presence of HMT in the reaction system. It can be clearly seen that the samples are composed of numerous nanosheets.

Fig. 1 SEM images of the prepared Bi₂WO₆ products: (a and b) nanosheet-assembled microspheres, (c and d) nanoparticle-assembled microspheres and (e and f) single-crystalline nanosheets.

To further obtain information about the structure of the as-prepared sample, the Bi₂WO₆ products with various shapes were characterized by TEM. As shown in Fig. S2a and b,† the sample shows spherical morphology and is comprised of sheet-like 2D nanostructures, which is consistent with the results of SEM analysis (Fig. 1a and b). Fig. S2c and d† shows the TEM image of products prepared in the presence of Cit-3Na. There is no significant contrast between the centre and edge, indicating the solid nature of the products. As shown in Fig. S2e,† the TEM image shows that the products possess a two-dimensional sheet-like structure.

As well characterized by the HRTEM (Fig. 2a and e), the distances between adjacent lattice fringes are measured as 2.73 and 1.98 nm, respectively. The values correspond to the interplanar distances of Bi₂WO₆ (200) and (202), respectively. The SAED patterns (Fig. 2b and f) show diffraction spots, confirming the single-crystal nature of nanosheets. Moreover, the set of diffraction spots can be indexed as the [010] zone axis of orthorhombic Bi₂WO₆. As a result, the nanosheets are dominantly terminated by (020) facets. The atomic structure of the (020) facets was shown in Fig. S3c and d.† As shown in Fig. 2c, the distances between adjacent lattice fringes are measured as 0.372 nm, corresponding to the interplanar distances of Bi₂WO₆ (111) facets. The well-defined diffraction rings (Fig. 2d) confirm the polycrystalline in nature of the nanoparticle-assembled Bi₂WO₆ microspheres.^25,26

Fig. 2 TEM images and the corresponding SAED pattern of the obtained Bi₂WO₆ products: (a and b) nanosheet-assembled microspheres (taking from the marked area in the Fig. S2b†); (c and d) nanoparticle-assembled microspheres and (e and f) single-crystalline nanosheets.

3.2 Crystal structure and chemical composition of the prepared Bi₂WO₆ products

The crystalline phase and purity of the as-prepared Bi₂WO₆ samples were investigated by XRD, as shown in Fig. 3. The XRD patterns of Bi₂WO₆ with various shapes show similar diffraction peaks, which can be well indexed into the orthorhombic phase Bi₂WO₆ (JCPDF no. 39-0256). No other crystalline phase can be found. Particularly, it can be clearly observed that the nanosheet-assembled Bi₂WO₆ microsphere has the best crystallinity, which mainly be due to the well-developed morphology of rectangular nanosheets.²⁷ Fig. S4† shows the enlarged profile of the XRD patterns of the prepared Bi₂WO₆ samples between angles 10–30°. The peak indicated by asterisks (★) in the XRD patterns can be indexed (020) diffraction peak. Fig. S3b† shows the crystal structure of Bi₂WO₆. It can be clearly observed that the Bi₂WO₆ has a layered structure, consisting of perovskite-like [WO₄]^2- layers sandwiched between layers of [Bi₂O₂]²⁺.^28,29 This may induce the electric field with the vector perpendicular to the [Bi₂O₂]²⁺ slabs and [WO₄]²⁻. The electric field may effectively separation of the photogenerated carriers along the [010] direction.

Fig. 3 XRD patterns of the prepared Bi₂WO₆ products: (a) nanosheet-assembled microspheres, (b) nanoparticle-assembled microspheres and (c) single-crystalline nanosheets.

The XPS measurements were performed to reveal the composition and chemical state of the prepared Bi₂WO₆ samples, as shown in Fig. 4. The three Bi₂WO₆ samples are composed of four elements, including Bi, O, W and adventitious carbon. As shown in Fig. 4b and c, the binding energy of Bi 4f and W 4f in prepared Bi₂WO₆ samples is nearly identical. Specially, the binding energies observed at 164.6 and 159.2 eV can be ascribed to Bi 4f_5/2 and Bi 4f_7/2, respectively.³⁰ The binding energies of W 4f_5/2 and W 4f_7/2 are 37.6 and 35.5 eV in the oxide form of Bi₂WO₆, which can attributed to the W atoms existing in a 6+ oxidation state.³¹

Fig. 4 XPS spectra of Bi₂WO₆ samples: (a) survey spectrum, (b) Bi 4f, (c) W 4f and (d) O 1s.

To gain more insight into the oxygen vacancies in the prepared Bi₂WO₆ samples, the chemical states of oxygen were investigated by XPS, as shown in Fig. 4d. The O1s spectra of the Bi₂WO₆ samples can be described as the superposition of three peaks by Gaussian distribution, located around 529.9, 531.7 and 533.3 eV, respectively. The O 1s peak at 533.3 eV is usually attributed to the presence of loosely bound oxygen on the surface of Bi₂WO₆.³² The low binding energy component located at 529.9 eV is attributed to the O²⁻ ions in the Bi₂WO₆ crystal lattice.³³ The medium binding energy component, centered at 531.7 eV, is associated with O²⁻ ions in the oxygen deficient regions. It is believed that the intensity of this peak is connected to the variations in the concentration of oxygen vacancies.³⁴ Therefore, changes in the intensity of this component may be connected in part to the variations in the concentration of oxygen vacancies. It can be found that the order of intensity of the peak at 531.8 eV is nanosheet-assembled microspheres > nanoparticle-assembled microspheres > single-crystalline nanosheets. As a result, the oxygen deficient content of nanosheet-assembled microspheres is higher than two other Bi₂WO₆ samples, which may be one origin of the superior photocatalytic activity of nanosheet-assembled Bi₂WO₆ microspheres.

3.3 Photocatalytic degradation of pollutes under visible light illumination

The visible light photocatalytic activities of different Bi₂WO₆ products were investigated by measuring the degradation of various organic contaminants (i.e. RhB, MO and CIP). Fig. 5a displays the results of RhB degradation. It can be seen that no significant degradation of RhB solution was observed in the absence of the Bi₂WO₆ photocatalysts. In addition, around 97% of RhB dye can be degraded by nanosheet-assembled products within 180 min irradiation, while nanoparticle-assembled microspheres can only decompose less than 74% of RhB. In addition, the single-crystalline nanosheets can degrade 78% of RhB in the same conditions. Apparently, nanosheet-assembled products have a higher photocatalytic activity than the other two samples. Moreover, in the presence of N–TiO₂, approximately 33% degradation of RhB was observed. Besides, it can be clearly observed that the characteristic absorption band of the RhB exhibits blue shift, which is resulting from the N-deethylation of RhB during irradiation (Fig. 5b).³⁵

Fig. 5 (a) Photodegradation of RhB dye solution with time over different photocatalysts under visible irradiation; (b) time-dependent optical absorption spectra of RhB degradation over nanosheet-assembled photocatalysts.

Bi₂WO₆ has been used as a photocatalyst not only for dye degradation but also for the photodegradation of colorless water pollutants. As a widely used antibiotic agent, CIP is difficult to be metabolized completely. Thus, continued emission of CIP into environments may accelerate antibiotic resistance with native bacterial populations.³⁶ The photocatalytic activities of various Bi₂WO₆ photocatalysts were evaluated by the degradation of CIP in solution under visible light irradiation. The results are shown in Fig. 6A. As for single-crystalline nanosheet samples, it shows poor photoactivity. About 30% of CIP is degraded after irradiation for 5 h. The photocatalytic activity of the nanosheet-assembled product is significantly improved compared with single-crystalline nanosheets. A photoactivity order of nanosheet-assembled microspheres > nanoparticle-assembled microspheres > single-crystalline nanosheets can be obtained. Fig. 6B shows the time-dependent absorption spectra of CIP solution in the presence of nanosheet-assembled products under visible light irradiation. It can be clearly observed that the absorption at λ = 272 nm of CIP evidently decreases with the increase of irradiation time. Furthermore, we investigated the degradation of MO over various Bi₂WO₆ samples, as shown in Fig. S5.† It can be seen that all three Bi₂WO₆ samples show inferior photoactivity on the degradation of MO dyes.

Fig. 6 (A) Photodegradation of CIP solution with time over different photocatalysts under visible irradiation: (a) single-crystalline nanosheets, (b) nanosheet-assembled microspheres and (c) nanoparticle-assembled microspheres; (B) time-dependent optical absorption spectra of CIP degradation over nanosheet-assembled Bi₂WO₆.

3.4 Plausible photocatalytic degradation mechanism

The photocatalysis on a semiconductor surface is mainly influenced by the following fundamental processes: pollutant adsorption, light absorption, separation and transfer of photogenerated carriers towards the surface, and eventual reduction/oxidation reactions. Fig. 7a shows the adsorption performances of various organic pollutants over the obtained Bi₂WO₆ samples. As an instance in adsorption of model pollutants, the order of adsorption ability over nanosheet-assembled samples is MO < CIP < RhB under the same conditions. MO dye is hardly adsorbed by the prepared Bi₂WO₆ samples. Besides, as for RhB, although the specific surface area of the nanosheet-assembled microspheres (26.97 ± 1 m² g⁻¹) and single-crystalline nanosheet (13.56 ± 1 m² g⁻¹) is less than that of the nanoparticle-assembled microspheres (40.72 ± 1 m² g⁻¹), nanosheet-assembled samples have a better adsorption ability than that of nanoparticle-assembled microspheres and single-crystalline nanosheets, respectively. However, the nanoparticle-assembled sample shows the best adsorption ability towards the CIP, which might be associated with its largest specific surface area. The corresponding specific surface area analysis is shown in Fig. S6.† The molecule structures of MO, CIP and RhB are shown in Fig. 7b–d, respectively. It can be clearly seen that MO, CIP and RhB are negatively charged, electrically neutral and positively charged, respectively. According the TEM analyses, the exposed facets of nanosheet-assembled Bi₂WO₆ and single-crystalline nanosheets are (020). In addition, the top of (020) facets is rich in negatively charged oxygen atoms (Fig. S3c†). Thus, positively charged RhB prefer to attach on the surface of (020) facets because of electrostatic interaction. All above results confirm that the surface of exposed facet can strongly interact with surrounding species, including H₂O and positively charged organics (Fig. 7e).

Fig. 7 (a) Adsorption various organic pollutants over the Bi₂WO₆ samples. (The amount of photocatalysts is 30 mg); molecular structures of (b) anionic MO, (c) neutral CIP and (d) cationic RhB; (e) schematic diagram showing the adsorption of RhB and H₂O on the surfaces of Bi₂WO₆ (020) facets.

The light absorption ability of various Bi₂WO₆ photocatalysts was investigated by the UV-vis spectra, as shown in Fig. 8. The nanosheet-assembled microspheres and single-crystalline nanosheets show similar absorption edge locating at around 430 nm. However, the light absorption edge of the nanoparticle-assembled sample was shifted to a longer wavelength. As a crystalline semiconductor, the optical absorption near the band edge is in accord with the equation αhν = A(hν − E_g)ⁿ, where α, h, n, E_g and A are the absorption coefficient, Plank constant, light frequency, band gap and a constant, respectively. Among them, n decides the characteristics of the transition in a semiconductor and this value for Bi₂WO₆ is 2.³⁷ As shown in Fig. 8A, the band gaps of nanosheet-assembled microspheres, single-crystalline nanosheets and nanoparticle-assembled microspheres can be determined to 2.51, 2.51 and 2.41 eV, respectively. It has been reported that the exposed facet has negligible effect on the absorption edge and band gap.^38,39 The VB position of nanosheet-assembled microspheres and single-crystalline nanosheets lies relatively positive than that of nanoparticle-assembled samples, as shown in Table S1.† This is, once excited the photoinduced holes, nanosheet-assembled samples and single-crystalline nanosheets will exhibit superior oxidizing ability to that of nanoparticle-assembled products. This difference in electronic structure should be one of the main reasons for the activities in photo-oxidation.⁴⁰ The sandwich structure of [Bi₂O₂]²⁺–[WO₄]²⁻–[Bi₂O₂]²⁺ of Bi₂WO₆ simulates the heterojunction interface with space charge that promotes separation of the photogenerated carriers in the interface, as shown in Fig. 8B. The bottom of conduction band is originated from W 5d orbital located in the middle layer, and the top of valence band is mainly composed of O 2p orbital from the surface [Bi₂O₂]²⁺.⁴¹ On irradiation, holes and electrons can be generated on the surface layer and in the middle layer and thus are separated directly. In brief, for the nanosheets with exposed (020) facets, both the sandwich structure and the upshift of the VB position are very favourable for photocatalysis.

Fig. 8 (A) UV-vis DRS spectra of various Bi₂WO₆ samples: (a) nanosheet-assembled microspheres, (b) nanoparticle-assembled microspheres and (c) single-crystalline nanosheets. Inset: corresponding plot of (αhv)^1/2 versus (hv) plot. (B) Band energy diagrams of various Bi₂WO₆ samples. W, O and Bi atoms are represented as blue, red and yarrow spheres, respectively.

Transfer of photogenerated electrons and holes to the catalyst surface is one of the most critical processes in a photocatalytic reaction. It has been found that anisotropically shaped particles have a lower charge-carrier recombination rate. For example, the TiO₂ nanobelts have less localized states near the band edge or in the band gap and possess an improved conductivity as compared to TiO₂ nanospheres.⁴² Thus, the long anisotropically shaped Bi₂WO₆ nanosheets are expected to inherently promote the efficient dissociation of hole–electron pairs and the transfer of charge carriers to different crystal facets. Fig. 9 displays the transient photocurrent responses of the as-prepared Bi₂WO₆ samples, further confirming this point of view. It can be clearly seen that the nanosheet-assembled sample shows the highest photocurrent response than that of two other Bi₂WO₆ products. The enhancement of photogenerated current response of nanosheets with exposed (020) facets can also be ascribed to the induced dipole between [Bi₂O₂]²⁺ layer and WO₄²⁻, introducing the self-built electric field parallelled to the [010] direction.^43,44

Fig. 9 Transient photocurrent responses of different Bi₂WO₆ nanostructures: (a) nanosheet-assembled microspheres, (b) single-crystalline nanosheets and (c) nanoparticle-assembled microspheres.

In particular, although the surface of the nanosheet-assembled microspheres and single-crystalline nanosheets exposed the same facets (Fig. 2b and f), the nanosheet-assembled microspheres electrode presents improved current compared to that of single-crystalline nanosheets one. Thus, there is other factor influencing on the charge separation process. It has been reported that the surface defect states, which occur naturally during the preparation process, usually serve as charge-carrier traps and impede the migration of charge carriers to the reactive sites at the catalyst surface.^33,45,46 The presence of oxygen vacancy in the nanosheet-assembled microspheres and single-crystalline nanosheets are different, which has been confirmed by XPS analysis (Fig. 4d). Thus, the difference of photocurrent intensity of nanosheet-assembled microspheres and single-crystalline nanosheet electrodes can be ascribed to the surface defect structures.

Based on the above analysis, the origin of photocatalytic activities of obtained Bi₂WO₆ samples can be ascribed to the following factor: atomic and surface defect structure. As an instance in photocatalytic degradation of RhB, the exposed (020) facets is beneficial for adsorption of cationic dye and improving the separation of photogenerated carrier. In addition, the presence of surface defect can served as charge carrier traps to further enhance its separation efficiency.

4. Conclusions

In summary, we studied the photocatalytic activity of well-defined Bi₂WO₆ crystals in degradation of various pollutants (i.e., cationic RhB, anionic MO and neutral CIP solution). Several surface factors were integrally examined to the origin of photoactivity, including surface facets terminations, electron structure and surface defect. As an instance in photocatalytic degradation of RhB, the exposed (020) facets were evidently prove to render nanosheet-assembled microspheres with superior activity than nanoparticle-assembled microspheres which has larger specific area. Furthermore, more surface defects existing in 3D architectures (i.e., nanosheet-assembled microspheres) could endow the Bi₂WO₆ semiconductors with high photocatalytic efficiencies compared to single-crystalline nanosheets. This work provides further insights into the complex surface effects in semiconductor photocatalysis.

Acknowledgements

This work is financially supported by Science and Technology Development Plan of Shandong Province, China (2014GNC110013) and the National Natural Science Foundation for Young (No. 2130696).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27295e

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