A review of controllable synthesis and enhancement of performances of bismuth tungstate visible-light-driven photocatalysts

Abstract

Heterogeneous photocatalysts offer great potential for converting photon energy into chemical energy and for decomposing organic contaminants. Many efficient photocatalysts have been proposed, unfortunately, most of these photocatalysts work only in the UV light region. It is necessary to develop visible-light-active photocatalysts to effectively utilize solar energy. This paper reviews a novel non-TiO₂ photocatalyst, Bi₂WO₆, which shows high activity under visible light irradiation. The controllable synthesis of Bi₂WO₆ nanoplates and highly porous films of Bi₂WO₆, as well as several methods for improving the photocatalytic activity of Bi₂WO₆, specifically the efforts from the authors' group are reviewed.

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1. Introduction

Photocatalytic degradation of organic compounds for the purpose of purifying wastewater from industries and households has attracted much attention in recent years. Heterogeneous photocatalysts offer great potential for converting photon energy into chemical energy and for decomposing organic contaminants.^1–3 Typical examples are TiO₂-based photocatalytic detoxification of air and water for environmental remediation.^1–4 However, the band gap of TiO₂ is larger than 3.0 eV, which means it can only show activity under UV irradiation. So, the commercialization of this technology has been hindered.^5–7 Anions doping with N, C, and S or transition-metal cations doping is commonly used to function TiO₂ as a visible-light photocatalyst.^8–12 However, the fact that the states introduced can act as combining centers, as well as the thermal instability associated to doped materials, introduce some doubts about their performance. The development of efficient visible-light-active photocatalysts has been an urgent issue from the viewpoint of using solar energy.

In recent years, considerable attention has therefore been paid to development of novel visible-light-active materials for photocatalysis.^13–19 Binary metal oxides composed of transition metal cations with dⁿ (Fe₂O₃ (2.0 eV), Co₃O₄ (1.3 eV) etc.) or post-transition metal cations with ns² (PbO (2.1 eV), Bi₂O₃ (2.5 eV) etc.) electronic configurations possess narrow band gap energies. However, the former metal oxides suffer from high resistivities due to the small polaron dominated conductivity while the latter ones show unfavorable indirect band gap transitions resulting from the coupling between the filled cation s and anion p states. To overcome the intrinsic limitations of binary metal oxides, it is becoming necessary to combine multiple cations to form functionalized multiternary oxides.²⁰ Therefore, various new visible-light-active ternary metal oxides have been extensively investigated. The pioneering works done by Zou et al. displayed water splitting for H₂ and O₂ evolution in a stoichiometric amount over the NiO_x/In_0.9Ni_0.1TaO₄ photocatalyst under visible light irradiation.²¹ Following this work, many new visible-active catalysts have been reported, such as Bi₂WO₆,^22–28 BiVO₄,^29–33 CaBi₂O₄,³⁴ InVO₄,^35,36etc. These new ternary metal oxide semiconductors show great potential in the utilization of solar energy.

Bi₂WO₆ has been found to possess interesting physical properties such as ferroelectric piezoelectricity, pyroelectricity, catalytic behavior, and a nonlinear dielectric susceptibility.^37–44 Recently, Bi₂WO₆ has attracted extensive interest for its good performance in organic compound degradation and O₂ evolution under visible light irradiation.^22–26 Kudo and Hijii demonstrated photocatalytic O₂ evolution by Bi₂WO₆ from AgNO₃ solution.²⁶ More recently, Zou et al.²³ reported that Bi₂WO₆ showed not only the activity for photocatalytic O₂ evolution but also the activity of mineralizing both CHCl₃ and CH₃CHO contaminants under visible light irradiation. After these pioneering works, Bi₂WO₆ nanoparticles with different morphologies have been prepared via a hydrothermal method and enhanced activity was observed.^22,25,45,46 Bi₂WO₆ has been found to show great potential as a visible-light-active photocatalyst for organic contaminants decomposition and solar energy conversion.

Very recently, Bi₂WO₆ micro/nanostructures have been reviewed by Zhang et al.⁴⁷ Herein, we mainly review the work of the authors' group focusing on controllable synthesis and the enhancement of photocatalytic activity of Bi₂WO₆. The controllable syntheses of Bi₂WO₆ nanoplates and highly-ordered porous films of Bi₂WO₆ are addressed. Different methods on a purpose of improving the efficiency of Bi₂WO₆, such as doping, substitution, surface modification, electrochemical assisted photocatalysis etc., are also reviewed.

2. Controllable synthesis of Bi₂WO₆ nanoparticles

In the pioneering works of Bi₂WO₆ as a photocatalyst, Bi₂WO₆ was prepared via a solid state reaction, which results in big particle size and low surface area.^23,26 Many studies have been reported on the preparation and characterization of various nanosized semiconductors. The nanoparticles exhibit special photochemical characteristics. In particular, the band gap of nanoparticles increases with the decrease of the size, resulting in a stronger photocatalytic power. Other important properties such as optical and physical absorption and luminescence emission also undergo drastic changes.⁴⁸ We have for the first time reported a simple hydrothermal method to synthesis Bi₂WO₆ nanoplates with square laminar morphologies.^22,24,48

Bi₂WO₆ nanoplates were synthesized through a hydrothermal process. The start mixture was allowed to react in a Teflon-lined autoclave at different temperatures to obtain well-crystallized nanoplates. Crystal diffraction peaks were found when the temperature was no less than 120 °C.²⁴ Morphologies of the Bi₂WO₆ synthesized by the hydrothermal process were characterised by TEM, as shown in Fig. 1. The whole process of the nanoplate growth in detail was obtained by morphology observation of time series samples. Small nanoplates formed and further grew with the cost of the small irregular nanoparticles. For 20 h treatment, good-quality nanoplates could be obtained. The indexed select electron diffraction pattern for the (001) zone revealed the single-crystal nature of the nanoplates and confirmed that the nanoplates grow preferentially along the (001) plane, which were parallel to their instinct a × b layer plane.²⁴ The thickness of the nanoplates was about 5 nm estimated through HRTEM, corresponding to three repeating units, i.e., (3 × 1.6 nm) 4.8 nm. The schematic crystal structure of Bi₂WO₆ is presented in Fig. 2. The photocatalyst belongs to the orthorhombic system, space group Pca2₁. The Bi₂WO₆ crystal with a layered structure includes the corner-shared WO₆. Bi atom layers are sandwiched between WO₆ octahedral layers.^24,48

Fig. 1 Morphologies of time series samples treated at 160 °C: (a) 2, (b) 4, (c) 8, (d) 12, (e) 16, (f) 20, (g) 24, and (h) 36 h.²⁴

Fig. 2 Schematic crystal structure of Bi₂WO₆ photocatalyst.⁴⁸

Zhang and Zhu further discussed the formation mechanism of the Bi₂WO₆ nanoplates,²⁴ as displayed in Fig. 3. In the beginning, the formation of tiny crystalline nuclei in a supersaturated medium occurred. Then the crystal growth followed. The larger particles grew at the cost of the small ones, due to the energy difference in solubility between the large particles and the smaller particles, according to the well-known Gibbs–Thomson law. In early stages, an examination of the intermediate samples showed the coexistence of a small laminar structure and irregular crystalline nuclei. As the reaction continued, irregular nanoparticles vanished and larger nanoplates formed. The chains of octahedral-W usually play an important role in the high intrinsic anisotropic growth in various tungstates because of the facets which are perpendicular to these chains composed of highly distorted octahedral tungsten with dangling bonds. These facets usually have high chemical potential. It is believed that two-dimensional growth occurs only if the chemical potential of two surfaces is much higher than the others. The chains of octahedral-W equally existed along the a- and b-axes, which indicates that the (200) and (020) facets had much higher chemical potential compared to other facets. This structural feature will make the (200) and (020) facets very sensitive to the surrounding growth conditions. This intrinsic anisotropic growth habit could happen when enough foreign energy input overcomes the reaction barrier. The growth rates of the (200) and (020) facets are much higher. Finally the morphologies of the samples are platelike, as observed in TEM images.

Fig. 3 Growth mechanism of the nanoplates.²⁴

Wang and coworkers have prepared flower-like Bi₂WO₆ spherical superstructures by employing the similar hydrothermal method, but with an acidic (pH = 1) precursor.⁴⁹ The flower-like microstructures were found to be constructed from nanoplates with single crystal structure. The formation of this flower-like structure was proposed to follow a three-step process: self-aggregation, Ostwald ripening and self-organization (Fig. 4).

Fig. 4 SEM images of flower-like Bi₂WO₆ superstructures.⁴⁹

The Bi₂WO₆ nanoplates prepared at 160 °C for 20 hours were found to possess a surface area of 21.1 m² g⁻¹, which is much larger than the Bi₂WO₆ obtained from the conventional solid state reaction showing a surface area of 1.2 m² g⁻¹. Besides the hydrothermal process, Bi₂WO₆ nanoparticles have also been obtained by calcining an amorphous complex precursor at a calcination temperature of above 450 °C,⁵⁰ which is suitable for preparation of the Bi₂WO₆ photocatalytic films on glass by the dip-coating technique. The BET surface area of the Bi₂WO₆ nanoparticles prepared at 500 °C was about 8.6 m² g⁻¹, which is over 7 times that of the sample prepared by the traditional solid state reaction. Some other methods, such as low-temperature combustion, solvothermal, ultrasonic spray pyrolysis, and microwave assisted method, have also been developed to get nanosized Bi₂WO₆ particles with nanoplate, hollow sphere, flower-like or nest-like morphologies.^51–59

3. Photocatalytic activity of Bi₂WO₆ nanoparticles and mechanism study

Bi₂WO₆ nanostructures were found to show high activity in organic compound degradation under visible light irradiation.²⁴ Some factors involved in the Bi₂WO₆ photocatalysis process, including surface area, crystallization, hierarchical architecture etc., have been extensively studied.^24,60,61 Among Bi₂WO₆ nanoplate samples, only the sample prepared at 120 °C showed lower activities than the (solid state reaction) SSR sample. That sample is not highly crystallized, which is confirmed by the XRD result and TEM image. A lot of defects could act as an electron–hole recombination center. The activity increased with the hydrothermal process temperature. It has been reported by Amano et al.⁶² that the photocatalytic activity of amorphous Bi₂WO₆ was negligible due to the fast recombination of charge carriers and crystallization which provided a red shift of the photoabsorption edge and marked increase in the lifetime of photoexcited electrons, resulting in an increase in absorbed photons and photocatalytic reaction efficiency. The highest activity was obtained at 200 °C. When the temperature further increased, the activities decreased. It is mainly attributed to a worse crystalline phase and is consistent with the XRD and TEM results. The Bi₂WO₆ nanoplate prepared at 200 °C shows a photocatalytic activity 3 times higher than that of the SSR sample.²⁴

The photocatalytic activity for RhB degradation is found to be closely related with the pH value of the solution.²² As shown in Fig. 5, both the adsorption of RhB and the initial rate of RhB conversion decreased with an increase in the pH of the dispersion in the scope of pH 6.53–9.89, suggesting that high adsorption of RhB by Bi₂WO₆ promotes the transformed rate. An exception occurred at pH 5.03; another unknown factor may affect the greatly reduced photocatalytic activity. The solid particles were gained from the acidic suspension (pH 4.70) by a simple filtration and then observed by XRD. It was found that Bi₂WO₆ was unstable in acidic solution. It could completely transform to H₂WO₄ and Bi₂O₃, which is the reason that Bi₂WO₆ has a poor catalytic activity in acidic solution.

Fig. 5 Influence of pH value on the photocatalytic degradation kinetic rate constant of RhB over Bi₂WO₆ (A) and adsorption of RhB on the surface of Bi₂WO₆ (B).²²

Stability of a photocatalyst is very important from the viewpoint of practical application. The doped TiO₂ photocatalysts sometimes suffer from photocorrosion and instability. The stability of Bi₂WO₆ as a visible-light-driven photocatalyst is studied.²² After five recycles for the photodegradation of RhB, the catalyst did not exhibit any significant loss of activity, confirming Bi₂WO₆ is not photocorroded during the photocatalytic oxidation of the pollutant molecules. XRD analysis of the sample also showed that the crystal structure of the photocatalyst was not changed after the photocatalytic reaction.

Wang et al. and Zhang et al. have studied the photocatalytic properties of Bi₂WO₆ micro/nano-structures, including nanoplates, tyre/helix-like structure, disintegrated-flower-like and flower-like superstructures.^25,47,49 These Bi₂WO₆ micro/nano-structures are found to exhibit different photocatalytic activities under visible light irradiation, as shown in Fig. 6. Among these photocatalysts, the uncalcined flower-like Bi₂WO₆ superstructure possesses the highest photocatalytic performance, while the nanoplates structure shows the lowest activity in the photocatalytic degradation of RhB under visible light irradiation. The calcination process can further improve the photocatalytic performance of the flower-like Bi₂WO₆ superstructure.

Fig. 6 The photocatalytic activities of different Bi₂WO₆ micro/nano-structures: (A) uncalcined flower-like structure, (B) uncalcined disintegrated-flower-like structure, (C) calcined tyre/helix-like structure, (D) uncalcined nanoplates structure.⁴⁷

The visible light absorption of Bi₂WO₆ was proven by the UV-vis diffuse reflectance spectra. Bi₂WO₆ presented the photoabsorption properties from the UV light region to visible light shorter than 470 nm.²² The band gap of the photocatalyst was estimated to be 2.7 eV from the onset of the absorption edge. The color of the oxide was yellowish, as predicted from their photoabsorption spectrum. The electronic structure of Bi₂WO₆ was investigated by DFT calculations by several groups.^22,63,64Fig. 7 shows a typical electronic structure of Bi₂WO₆. The highest occupied and lowest unoccupied molecular orbital levels were composed of the hybrid orbitals of O 2p and Bi 6s and the W 5d orbitals, respectively. The band structure indicates that charge transfer upon photoexcitation occurs from the O 2p + Bi 6s hybrid orbitals to the empty W 5d orbitals. This transition which could be due to the 6s electrons usually occurs at a lower energy than the charge-transfer transition in WO₆⁶⁻. The valence band of Bi₂WO₆ was composed of O 2p and Bi 6s, which is similar to that of BiVO₄. The considerable visible-light absorption of Bi₂WO₆ is thus attributed to the transition from Bi 6s to the W 5d orbital.

Fig. 7 Energy band diagram (A) and density of states (B) for Bi₂WO₆ calculated by the DFT method.²²

The RhB/Bi₂WO₆ system was further examined by DMPO spin-trapped ESR spectroscopy to monitor the active radicals that form during the photodegraded process.²² There were no signals when the RhB/Bi₂WO₆ suspension was irradiated (Fig. 8). However, in the case of RhB/TiO₂, a spectrum displaying signals with characteristic intensity 1 : 2 : 2 : 1 for DMPO–˙OH adducts was obtained, indicating that the ˙OH radical was formed. In another experiment, it was found that the addition of 2-propanol, a well-known scavenger of ˙OH radicals, into the photoreaction system did not cause the apparent changes in the degradation rate of RhB. This indicates that the free ˙OH radicals could not be the main active oxygen species in this photochemical process.

Fig. 8 ESR signals of the DMPO–˙OH adducts as a function of light illumination time.²²

From the theoretical viewpoint, the production of ˙OH in the present system is almost impossible. Generally, in a valence band formed by Bi³⁺, holes formed by photoexcitation are regarded as Bi⁵⁺ (or Bi⁴⁺). Although the redox potential in an aqueous solution is different from that in solids, a standard redox potential of Bi₂O₄/BiO⁺ (Bi^V/Bi^III) (E° = +1.59 V at pH 0) could make sense for a rough estimation of the oxidation potential of the hole (Bi⁵⁺) photogenerated in the Bi₂WO₆ photocatalyst. However, the standard redox potential of Bi^V/Bi^III is more negative than that of OH˙/OH⁻ (+1.99 V), suggesting that the hole photogenerated on the surface of Bi₂WO₆ could not react with OH⁻/H₂O to form ˙OH. Therefore, the decomposition of RhB by Bi₂WO₆ could be due to the reaction with the photogenerated hole directly. Following the general information, we have reason to assume that the degradation of RhB on Bi₂WO₆ is not due to the involvement of ˙OH radicals. The direct hole transfer could play an important role. As a result, the photodegradation of RhB on Bi₂WO₆ is little affected by the presence of the ˙OH radical scavenger.

In Saison et al.'s work,⁶⁵ they found bismuth-based oxides such as Bi₂O₃, BiVO₄, and Bi₂WO₆ show a photocatalytic activity for the degradation of RhB in solution and decomposition of stearic acid under visible light irradiation. They demonstrated that the photocatalytic mechanism is tightly linked to the surface properties of the compounds. For weak acid solids, Bi₂O₃ and BiVO₄, the interaction with some pollutants such as RhB and stearic acid is somewhat weak and does not lead to an efficient degradation. On the contrary, for the solid Bi₂WO₆ bearing strong acid sites, the adsorption of these pollutants is very strong. The short distance between the pollutant and the photocatalyst surface allows the photogenerated electrons, holes, and radicals to reach the pollutant more easily, implying an efficient degradation under visible light. Other works show that Bi₂WO₆ is also, under visible light irradiation, active for NO, Bisphenol A degradation, bacteria inactivation etc.^66–69

4. Enhancement of the performance of Bi₂WO₆ by doping or substitution

A major limitation of achieving high photocatalytic efficiency in semiconductor systems is the quick recombination of charge carriers. Recombination, which has faster kinetics than surface redox reactions, is a major drawback as it reduces the quantum efficiency of photocatalysis. Therefore, ways to minimize the recombination rate are important if we are interested in maximizing the photocatalysis efficiency. For this purpose, fluorinated TiO₂ has been investigated in relation to doping (TiO_2−xF_x) or surface complexation (F-TiO₂).^70–78 Fluorinated Bi₂WO₆ and fluorine substituted Bi₂WO₆ have been studied by our group recently.^79,80

Fluorinated Bi₂WO₆ catalyst was synthesized by a simple hydrothermal process.⁸⁰ It was found that the interaction of F⁻ with the surface Bi³⁺ ion prohibited the enlargement of the crystal nuclei. The BET surface areas of Bi₂WO₆ products were also found to be highly dependent on R_F (represents the molar ratio of F⁻). Each sample showed a monotonic increase in the BET surface area with increasing R_F. The absorption onsets of the samples were red-shifted apparently, when R_F increased from 0.2 to 0.6. The difference of the absorbance edges could arise from the fluorination of the Bi₂WO₆ catalyst. The fluorinated Bi₂WO₆ has stronger acid sites, and enhanced acidity of the surface, which could contribute to higher photoactivity.

Fluorinated Bi₂WO₆ presented the enhanced photoactivity for the RhB degradation under simulative sunlight, which could be a synergetic effect of the surface fluorination and the doping of crystal lattice. To get a better handle on the mechanistic details of this photocatalytic system, the photodegradation process of RhB was examined.⁸⁰ As shown in Fig. 9, in the case of the Bi₂WO₆ catalyst, the absorption maximum of the suspension decreased by 92% in the presence of Bi₂WO₆ catalyst after irradiation for 300 min, and the band shifted from 553 to 515 nm. The color of the suspension changed directly from pink to light red. By contrast, the absorption maximum of the degraded solution exhibited concomitant, slight hypsochromic shifts in the presence of F-Bi₂WO₆. However, the hypsochromic shifts of the absorption maximum were more pronounced than those observed with the Bi₂WO₆ system. This hypsochromic shift of the absorption band was presumed to result from the formation of a series of N-deethylated intermediates in a stepwise manner. After irradiation for 210 min, ca. 98% of RhB was degraded in the F-Bi₂WO₆ system and the spectral maximum shifted somewhat from 553 to 500 nm. The color of the suspension changed gradually from pink to light green. Further irradiation caused the decrease of the absorption band at 500 nm, and the color of the suspension changed sequentially to colorless. This indicated that the fluorination of Bi₂WO₆ changed not only the photodegradation rate of the pollutant but also the mechanistic pathways of pollutant degradation. The fluorinated Bi₂WO₆ enhanced significantly the ratio of the deethylation process to the cleavage of the chromophore structure.

Fig. 9 UV-visible spectral changes of RhB over the catalysts (A) Bi₂WO₆ and (B) F-Bi₂WO₆. Variations in the distribution of the intermediate products in the corresponding suspensions, (C) Bi₂WO₆ and (D) F-Bi₂WO₆. The concentrations of the N-dealkylated intermediates were determined by the corresponding peak areas gained from the HPLC technique.⁸⁰

In the fluorinated Bi₂WO₆ system, five intermediates, namely, N,N-diethyl-N′-ethylrhodamine, N,N-diethylrhodamine, N-ethyl-N′-ethylrhodamine, N-ethylrhodamine, and rhodamine were thus identified, whereas the first three intermediates could only be identified in the case of the Bi₂WO₆ system. This result indicated that more RhB molecules were degraded via the deethylation process in the fluorinated Bi₂WO₆ system. It was proposed that the F⁻-containing function on the catalyst surface could serve as an electron-trapping site and enhance interfacial electron-transfer rates by tightly holding trapped electrons.

Fluorine substituted samples Bi₂WO_6−XF_2X were prepared by a two-step process.⁷⁹ It was found that F⁻ substitution could change the original coordination around the W and Bi atoms. Compared with Bi₂WO₆, the photocatalytic activity of Bi₂WO_6−XF_2X calcinated at 573 K increased about 2 times. The enhanced photocatalytic activity came from the following: (1) the mobility of photoexcited charge carriers in the valence and conduction bands was increased. Based on the DFT theoretical calculations,⁷⁹ the valence bandwidth of Bi₂WO_6−XF_2X was wider than that of Bi₂WO₆ because the F 2p orbitals contributed to the valence band formation. Consequently, the calculation results suggested that the wider and more dispersed bands of Bi₂WO_6−XF_2X would increase the mobility of photoexcited charge carriers in the valence and conduction bands. Impedance spectra of Bi₂WO_6−XF_2X showed a smaller arc radius of the EIS Nyquist plot, indicating that photogenerated electron–hole pairs were easily separated and transferred to the surface of the Bi₂WO_6−XF_2X sample. (2) Bi₂WO_6−XF_2X had a stronger oxidation power, which could induce the OH˙ radicals to take part in the photooxidation process. The ability of a semiconductor to photooxidate the adsorbed species on its surface is governed by the positions of its conduction band minimum (CBM) and valence band maximum (VBM) with respect to the redox potentials of the adsorbate, and thus, the photocatalytic ability of Bi₂WO₆ is determined to a great extent by the positions of its CBM and VBM. The CBM and VBM of Bi₂WO_6−XF_2X were lowered from those of Bi₂WO₆ by 0.90 and 0.79 eV, respectively. The lowering of the VBM indicated that Bi₂WO_6−XF_2X had a stronger oxidation power.

Recent studies have shown that substitution of the M site (A_xM_yO_z) with other metal ions could considerably improve the catalytic activity.⁸¹ The substitution of M sites might induce a slight modification of crystal structure due to the different ion radii, resulting in dramatic influence on the mobility of the charge carrier and change in the photocatalytic and photo-physical properties. Therefore, from the viewpoint of developing highly efficient Bi₂WO₆ photocatalysts with wider optical response in the visible spectral range, the substitution of W sites in Bi₂WO₆ with Mo has been further studied.⁸² Bi₂Mo_xW_1−xO₆ solid solutions with adjustable band gaps were prepared by Yu's group via hydrothermal treatments, the visible-light-induced photocatalytic activity of Bi₂WO₆ was found to be improved with certain extent of Mo substitution.⁸³

Phase-pure Bi₂Mo_xW_1−xO₆ (0 ≤ x ≤ 1) photocatalysts were also synthesized via a hydrothermal method.⁸² The as-prepared Bi₂Mo_xW_1−xO₆ photocatalysts had an Aurivillius crystal structure and showed special anisotropic growth. The optical absorption spectra of Bi₂Mo_xW_1−xO₆ were red-shifted monotonically as the value of x increased. Based on theoretical calculations, as shown in Fig. 10, the introduction of Mo atom into Bi₂WO₆ could reduce the conduction band level of Bi₂WO₆, so the band gap energy was reduced. The curvature of the conduction band became smaller with an increase of Mo content due to the different electronegativities of Mo 4d and W 5d. The mobility of the electronic carrier is reported to be proportional to the reciprocal effective mass of carrier that is in proportion to the curvature.⁸¹ The photocatalytic activities determined by rhodamine B degradation under visible light irradiation (λ > 420 nm) of Bi₂Mo_xW_1−xO₆ photocatalysts were significantly improved as compared with that of Bi₂MoO₆. The higher efficiency of Bi₂WO₆ was attributed to more effective photoelectron transfer in the conduction band with larger curvature. The photocatalytic activities under visible light irradiation (λ > 450 nm) of Bi₂Mo_xW_1−xO₆ photocatalysts were much higher than that of Bi₂WO₆.⁸²

Fig. 10 The evolution of electronic structure with the increased amount of Mo substitution.⁸²

5. Synthesis of Bi₂WO₆ porous films with enhanced activity

Novel visible-light-driven photocatalysts are primarily synthesized by high-temperature ceramic methods and usually utilized as suspended powders. The limitations of low photocatalytic efficiency and laborious recollection of powders restricted their applications considerably. The preparation of macroporous and mesoporous films has been proposed as a method to increase the effective surface area of photocatalysts, which would allow for increased light absorption and improve photocatalytic performance.^84–86 Macroporous and mesoporous structures would presumably have a photocatalytic activity similar to nanoparticles but would not require the high maintenance costs associated with nanoparticles.

Compared with single-metal oxide porous films, there are two difficulties in the preparation of ternary metal oxide porous films. One is the choice of the precursors of the ternary metal oxide. The cations of the starting sol solution must be homogeneously mixed on the molecular scale in order to obtain porous ternary metal oxides with pure phases, otherwise, the samples obtained will contain secondary phases. The other problem is the thermal stability of the porous structure. High calcination temperatures are usually necessary for the crystallization of ternary metal oxides, however, the porous structure may collapse upon high temperature treatment.⁸⁷

We have developed a method by combining the evaporation-induced self-assembly method and the amorphous complex precursor method to obtain the Bi₂WO₆ porous films (Fig. 11).⁸⁸ From the viewpoint of developing an economical, preferentially solution-based route to porous ternary metal oxide films, an amorphous complex precursor was prepared and utilized instead of metal alkoxides. The homogenous amorphous complex precursor is produced by complexation between diethylenetriaminepenta-acetic acid and the low cost metal oxides or salts. The high viscosity of the precursor makes it suitable for preparation of the complex oxide films by a dip-coating or spin-coating technique. Monodisperse carbon spheres of about 300 nm and 400 nm were synthesized according to the reported procedure,^89,90 and a packed carbon sphere monolayer was generated on an ITO glass by vertical deposition⁹¹ of the carbon spheres in ethanol. The spin-coating infiltration method was employed to achieve a homogeneous infiltration of the precursor. Finally, the carbon spheres were eliminated from the film by heating in air.

Fig. 11 Schematic illustration for the preparation of Bi₂WO₆ ordered porous films: (a) a packed carbon sphere monolayer on ITO glass; (b) infiltration of Bi₂WO₆ precursor by spin coating; (c) removal of carbon spheres by heating in air.⁸⁸

Fig. 12 shows the scanning electron microscope (SEM) images of the carbon sphere monolayer array and Bi₂WO₆ porous film synthesized with different sizes of carbon spheres and concentration of precursor. The average size of the open pores increased from 290 nm to 320 nm as the diameter of the carbon sphere template increased. All samples underwent shrinkage of the pore during calcination. As noted from the SEM images, the thickness of the Bi₂WO₆ walls could be adjusted by changing the concentration of the Bi₂WO₆ complex precursor. Furthermore, we can readily adjust the size of the pore by changing the diameter of carbon spheres, which could be easily realized by changing the hydrothermal temperature or time.

Fig. 12 SEM images of the carbon sphere template (a), and as-prepared Bi₂WO₆ porous films under different conditions (b, c, d).⁸⁸

The photocatalytic decomposition rate of methylene blue over porous films was found to be more than twice faster than nonporous samples under visible light. The photoelectrochemical measurements show that an enhanced photocurrent is obtained essentially over the entire potential range for the porous Bi₂WO₆ film (Fig. 13). It is interesting to note that there was a fivefold difference in the slope (photocurrent vs. potential) between the two films. The slope corresponds to the inverse resistance of contact between the working electrode material and the electrolyte.⁹² The slope is strongly related to the texture of the film. In the case of porous Bi₂WO₆ films, due to the much larger electrolyte exposed area, a much larger part of the structure is affected by the field of the Schottky junction at the semiconductor electrolyte interface. Thus the drift of photogenerated carriers within the Schottky space charge layer width becomes a key factor of the porous structure. Attributed to the much smaller distance from the place the charge carrier generated in Bi₂WO₆ to the interface between Bi₂WO₆ and electrolyte solution, the efficiency of the charge carrier transportation is much higher in porous Bi₂WO₆ films than in nonporous films, and an effective separation of the charge carriers could be anticipated.⁸⁸

Fig. 13 The potentiodynamic scans under chopped illumination for porous and nonporous Bi₂WO₆ films.⁸⁸ Electrolyte: 0.1 M Na₂SO₄, reference electrode: saturated calomel electrode (SCE).

Most recently, Wang and coworkers have successfully fabricated ordered mesoporous Bi₂WO₆via a hard-templated (SBA-15) nanocasting method (Fig. 14).⁹³ The mesoporous Bi₂WO₆ samples exhibit excellent photocatalytic decomposition of phenol under visible light irradiation. Compared with a reference nanoscale Bi₂WO₆ sample, the mesoporous sample shows much higher photocatalytic activity under the same conditions. The enhanced photocatalytic performance is attributed to the higher BET surface area, as well as a negative shift of conduction band and faster charge transfer in the mesoporous sample.

Fig. 14 TEM images of mesoporous Bi₂WO₆ prepared with SBA-15 as a hard-template.⁹³

6. Enhancement of the performance by surface modification

A major limitation to achieve high photocatalytic efficiency is the quick recombination of photo-generated charge carriers. Recombination has faster kinetics than surface redox reactions and greatly reduces the quantum efficiency of photocatalysis. Therefore, to enhance the photocatalytic efficiency, it is essential to retard the recombination of the charge carriers. Many works have been devoted to reduce the recombination of charge carriers by coupling the photocatalysts with other materials, such as noble metals,^94,95 semiconductors,^96,97 carbon nanotube,⁹⁸etc. Our group has developed conjugative π structure material hybridized semiconductors as efficient photocatalysts, such as C_60,^99–101 polyaniline,^102,103 graphite-like carbon,^104,105 and graphene.¹⁰⁶ The hybridization of photocatalyst with conjugative π structure material possessing good electrical conductivity could reduce the recombination of charge carriers, and increase the photocatalytic efficiency.

Concerning the high photocatalytic activity of nanosized Bi₂WO₆, it is expected that the visible photoactivity of Bi₂WO₆ for degradation of dye can be enhanced via synergetic effect of C₆₀ and Bi₂WO₆. C₆₀-modified Bi₂WO₆ photocatalyst is obtained by chemically adsorbing C₆₀ on the surface of Bi₂WO₆. The photodegradation results of dyes over C₆₀-modified Bi₂WO₆ under visible-light irradiation and solar (simulated by a xenon lamp) show that the photocatalytic activity can be significantly enhanced. It is postulated that the enhanced photoactivity of C₆₀-modified Bi₂WO₆ catalysts results from high migration efficiency of photoinduced electron–hole pairs.¹⁰⁷

The existence of C₆₀ was confirmed by TEM and the diffuse-reflection spectra (DRS) of C₆₀-modified Bi₂WO₆.¹⁰⁷ Bi₂WO₆ samples showed an absorption edge around 470 nm, which could be responsible for the visible-light induced photocatalytic activity. With the loading of C₆₀, C₆₀/Bi₂WO₆ displayed the same absorption edge as Bi₂WO₆. However, the samples exhibited a greater light attenuation throughout the visible wavelengths consistent with the gray color of the catalyst. The absorption intensity of the prepared samples changed with the increase of C₆₀/Bi₂WO₆ weight ratio. The absorption intensity increased rapidly with C₆₀/Bi₂WO₆ from 0.65 to 1.25%, but the increment was small from 1.25 to 3.0%. Considering the diameter of C₆₀ (0.71 nm) and the BET surface area of Bi₂WO₆ (8.68 m² g⁻¹), it can be estimated that the weight ratio is about 2.0% with a nearly compact C₆₀ monolayer coverage on Bi₂WO₆. The actual concentration of C₆₀ absorbed on the surface of Bi₂WO₆ could be even less than 2.0% because C₆₀ can only occupy the active absorption site. The absorption intensity remained almost unchanged when the ratio of C₆₀/Bi₂WO₆ increased from 1.25 to 2.0%, indicating that C₆₀ may aggregate to form a cluster on the surface of a Bi₂WO₆ nanosheet when the weight ratio of C₆₀/Bi₂WO₆ was above 2.0%.

The photocatalytic activity of the C₆₀/Bi₂WO₆ sample was evaluated by the degradation of MB in aqueous solution.¹⁰⁷Fig. 15 shows the photocatalytic degradation curve of normalized MB as a function of time. All the modified Bi₂WO₆ samples exhibited higher photocatalytic activities than pure Bi₂WO₆. Results showed that the loading amount of C₆₀ had a great influence on the photocatalytic activity of the as-prepared photocatalyst. When the loading amount was below 1.25%, the photocatalytic activities increased with the increase of loading amount of C₆₀. However, when the loading amount of C₆₀ exceeded 1.25%, the photocatalytic activities of samples decreased as the amount of C₆₀ increased. The optimal loading amount of C₆₀ on Bi₂WO₆ for increasing the photocatalytic activity was 1.25%. As mentioned in the result of diffuse reflection spectra, C₆₀ tended to aggregate on the surface of Bi₂WO₆ when the mass ratio of C₆₀ is above 1.25%, which resulted in the slower transmission of the photoinduced electrons. 1.25% C₆₀ and Bi₂WO₆ mechanical mixture as a reference was prepared by merely stirring. Its photocatalytic activity was similar to that of Bi₂WO₆ and much lower compared with 1.25% C₆₀-modified Bi₂WO₆ catalyst. The photocatalytic activity of Bi₂WO₆ modified by C₆₀ was enhanced by about 4 times compared with that of the Bi₂WO₆ sample. This result implied that the interaction between C₆₀ and Bi₂WO₆ photocatalyst took an important role in the enhancement of photoactivity.

Fig. 15 Photocatalytic degradation of MB by C₆₀-modified Bi₂WO₆ and Bi₂WO₆ under visible light irradiation (λ > 420 nm).¹⁰⁷

Electrochemical impedance spectroscopy (EIS) and photocurrent generation were used to investigate the photogenerated charge separation process on a ITO/Bi₂WO₆ film and a ITO/Bi₂WO₆/C₆₀ film.¹⁰⁷ The arc radius on the EIS Nyquist plot of the ITO/Bi₂WO₆/C₆₀ film was smaller than that of the ITO/Bi₂WO₆ film sample, which meant that an effective separation of photogenerated electron–hole pairs and fast interfacial charge transfer to the electron donor/electron acceptor occurred as suggested.¹⁰⁵ An obvious anodic photocurrent was obtained when anodic bias potential was present. The photocurrent increased about two times after being modified by C₆₀, suggesting the improvement of separation efficiency and inhibition of recombination of photoinduced electron–hole pairs.

The mechanism of the enhancement of photocatalytic activity and photocurrent generation via C₆₀ surface hybridization was thus proposed by Zhu and coworkers.¹⁰⁷ The schematic of photocatalytic mechanism is shown in Fig. 16. The reason should be closely attributed to the interaction between Bi₂WO₆ and C₆₀ which increased the photogenerated electron mobility in Bi₂WO₆. The C₆₀ molecule was mainly covered on the surface of Bi₂WO₆. C₆₀ acted as an electron shuttle that could effectively transfer the photoelectrons from the conduction band of Bi₂WO₆ after being illuminated under visible light irradiation. The delocalized conjugated π structure of C₆₀ made it easier to transfer the photoinduced electrons. Accordingly, the photogenerated electrons in the modified Bi₂WO₆ photocatalyst could easily migrate from the inner region to the surface to take part in the surface reaction.

Fig. 16 Possible pathway of the photoelectron transfer excited by visible light irradiation including the photocatalytic process for C₆₀-modified Bi₂WO₆.¹⁰⁷

Wang and coworkers recently studied the graphene/Bi₂WO₆ composite,¹⁰⁸ which was prepared via an in situ hydrothermal reaction in the presence of graphene oxide. The photocatalytic activity of the graphene/Bi₂WO₆ composite was greatly enhanced as compared with pristine Bi₂WO₆. The presence of graphene leads to the shift of the Fermi level and decrease in the conduction band potential. The enhanced photocatalytic performance was attributed to the negative shift of the Fermi level and the high migration efficiency of photoinduced electrons, resulting in suppressed charge recombination (Fig. 17).

Fig. 17 Photocatalytic scheme of Bi₂WO₆ (BWO) and graphene/Bi₂WO₆ (G-BWO) composite.¹⁰⁸

7. Enhanced degradation by combining electro-oxidation and photocatalysis

Photoelectrocatalytic (PEC) oxidation has been proven to be an efficient method in degrading organic contaminants in aqueous solutions.^109–119 In this process, the low electrical bias potential applied between the anode and cathode is used to prevent charge recombination; the electrochemical degradation of the target contaminants is usually avoided. Electrochemical techniques have been extensively studied in degrading organic contaminants in aqueous solution due to their easy applicability to automation, high efficiency, and environmental compatibility.^120–123 Although various types of electrodes with high O₂ evolution overpotential have been developed,¹²¹ the main problem hindering their large-scale application in wastewater treatment is the high electric energy consumption mainly caused by the side reaction of oxygen evolution. Moreover, electrode passivation is of susceptibility because of electropolymerization and fouling as summarized in the literature,^122,123 which decreases the electro-oxidation efficiency. It is known that O₂, acting as a scavenger of photogenerated electrons, is beneficial for the photocatalytic degradation of organic contaminants. Moreover, the photocatalytic reaction occurring on the photoanode surface could induce the formation of active radicals, which may activate the electrode. Since the strengths of electro-oxidation and photocatalysis are complementary, a system that employs them both seems a worthwhile endeavor.

We have prepared the Bi₂WO₆ nanoflake film electrode onto indium–tin oxide (ITO) glass via electrostatic self-assembly deposition (ESD) according to the method described in the literature.¹²⁴ The resultant Bi₂WO₆ nanoflake film exhibits photocatalytic activities towards degrading (4-chlorophenol) 4-CP under visible light irradiation. The removal efficiency of 4-CP was increased by applying a bias potential to the Bi₂WO₆ nanoflake film electrode. Moreover, a synergetic effect was observed in the degradation of 4-CP by the combined electro-oxidation and photocatalysis.¹²⁵

Photocatalysis, electrochemical degradation, and PEC degradation of 4-CP at the bias potential of 2.0 V were performed, respectively.¹²⁵ The variation of relative concentration of 4-CP as a function of reaction time is shown in Fig. 18. It is clear that 4-CP can be photocatalytically degraded using the Bi₂WO₆ nanoflake film electrode; it can also be degraded via the electro-oxidation process at the bias potential of 2 V. Clearly, the degradation rate of 4-CP was the largest under the PEC process with the same bias potential. The pseudo-first kinetic constant of 4-CP for the PEC process is larger than the sum of the electrochemical or photocatalytic process individually. A similar conclusion can be drawn by considering the reduction of TOC content. At the potential of 2 V, after reaction of 12 h, the individual electrochemical degradation and photocatalysis permit TOC reduction of 25% and 20%, respectively, while the combined PEC process leads to a TOC reduction of 65%. Thus, it is reasonable to conclude that a sort of synergetic effect occurs during the PEC degradation of 4-CP.

Fig. 18 Variation of 4-CP concentration under various conditions with initial concentration = 10 mg L⁻¹, applied bias potential = 2 V, and light intensity = 150 mA cm⁻².¹²⁵

The effect of the bias potential on the PEC degradation rate of 4-CP was further investigated.¹²⁵ As shown in Fig. 19, with the increase of external potential, the degradation rate of 4-CP increases gradually. The influence of applied bias potential on 4-CP degradation was more significant at higher potential than at lower potential. The application of potentials greater than the Bi₂WO₆ flat band potential across a photoelectrode increases the concentration of photogenerated holes or hydroxyl radicals on the surface by decreasing the rate of recombination of photogenerated holes and electrons. As a result, as the potential increases, the rate of 4-CP degradation increases, until most of the photogenerated electrons are removed either by the electric field or by reaction with dissolved oxygen. Further increase of the applied potential beyond the redox potential of 4-CP improves the degradation largely. In this case, the degradation of 4-CP was carried out by the combined electro-oxidation and photocatalysis simultaneously. However, at the bias potential greater than 2.0 V, the synergetic effect decreases gradually. A similar phenomenon was observed in our work on ZnWO₄,¹²⁶ and the explanation has been presented in detail. The overpotential for the oxygen evolution reaction increases with the current density. Thus, when the bias potential exceeds 2.0 V, most of the ˙OH radicals formed further react to form oxygen. To further explore the reason, the electrode surface was analyzed by XPS. It was found that the deposited film can be formed on the electrode surface during the electrochemical and PEC degradation. The faster original oxidation of organic compounds at higher potential leads to the formation of more intermediates, which were deposited on the electrode surface, blocking further access of organic compounds and slowing the degradation.

Fig. 19 The effect of the bias potential on 4-CP removal efficiency with initial concentration of 10 mg L⁻¹, reaction time = 8 h.¹²⁵

8. Conclusions

In this review, we first addressed the controllable synthesis of Bi₂WO₆ nanoparticles with different morphologies. The Bi₂WO₆ nanoplates showed high activity as photocatalysts under visible light irradiation. The photocatalytic mechanism during the photodegradation process with Bi₂WO₆ nanoplates was then revealed. Several methods were further developed to improve the photocatalytic efficiency of Bi₂WO₆:

(i) Fluorine doping and substitution were carried out on Bi₂WO₆ nanoplates, the resulting materials showed enhanced activity due to the structure modification and improved photogenerated charge carriers separation. Mo replacement of W in Bi₂WO₆ could extend the light absorption in the visible light range, and increase the light conversion.

(ii) A facile method was developed to prepare Bi₂WO₆ highly porous films. In comparison to the corresponding nonporous Bi₂WO₆ films, the porous films offer a highly enhanced photocatalytic activity under illumination with visible light. The porous film also showed much higher photocurrent conversion efficiency over a wide range of applied potentials.

(iii) The role of C₆₀ in storing and shuttling photoinduced electrons involved in a photocatalytic process is studied by coating Bi₂WO₆ with a monomolecular layer of C₆₀. The electronic contact between semiconductor and C₆₀ leads to efficient separation of electron–hole pairs to reduce electron–hole recombination.

(iv) The removal efficiency of 4-CP was increased by applying a bias potential to the Bi₂WO₆ nanoflake film electrode. A synergetic effect was observed in the degradation of 4-CP by the combined electro-oxidation and photocatalysis. The applied bias potential promoted the separation of electrons and holes.

Acknowledgements

The authors are grateful to many of the colleagues for constructive discussions, as well as the financial support from Chinese National Science Foundation (20925725 and 50972070) and National Basic Research Program of China (2007CB613303).

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