Advanced chemical compositions and nanoarchitectures of bismuth based complex oxides for solar photocatalytic application

Abstract

Bismuth based complex oxides have attracted considerable interest due to their great potential to harvest solar light to solve the current environmental and energy crisis. Bismuth based complex oxides have excellent photo-oxidation ability for organic contaminant degradation and water oxidation via a photocatalytic process. Many efforts have been made to improve their photocatalytic performance, especially on the BiVO₄, Bi₂WO₆ and Bi₂MoO₆ materials, which have been mostly studied in the past few years. Significant progress in understanding the fundamentals and improving the photocatalytic performance has been made due to the various new developed concepts and approaches in recent years. In this review, we present a comprehensive overview on the fundamentals and recent advances of BiVO₄, Bi₂WO₆ and Bi₂MoO₆ photocatalysts. After the analysis of the structure–property relationships, the strategies that have been employed to enhance their photocatalytic performance are discussed in detail, including morphology control, surface modification, doping and construction of composite material. Furthermore, remarks on the challenges and perspectives of research directions are proposed for further development of the highly efficient bismuth based complex oxide photocatalysts.

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Songmei Sun

Songmei Sun received her BSc degree in Material Science and Engineering in 2005 at Shandong University of Technology and Ph.D. degree in Material Physics and Chemistry in 2010 from Shanghai Institute of Ceramics, Chinese Academy of Sciences. She has published more than 50 papers in the area of bismuth based oxide photocatalyst up to the present. She is currently an associate professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. Her current research is focused on the rational synthesis and mechanism exploration of bismuth based complex oxides for solar light driven photocatalysis.

Wenzhong Wang

Wenzhong Wang received his Ph.D. degree from University of Science and Technology of China in 1998. He worked as a postdoctoral fellow at Virginia Commonwealth University, Rutgers University and Colorado State University from 1999 to 2003. In 2004, he was supported by “Hundred Talents Program”, CAS. He currently is a full professor in Shanghai Institute of Ceramics, Chinese Academy of Sciences. His research interests include controlled fabrication, novel properties and photocatalytic applications of semiconductor nanostructures, especially with a focus on bismuth based semiconductors for environmental purification and solar light conversion.

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1. General background

With the rapid development of industry, environmental pollution and the energy crisis have been serious challenges for human beings. Photocatalysis, which could harvest energy directly from sunlight, offers a desirable approach to solving these two problems. Over the past few decades, photocatalysis has been widely investigated both theoretically and technologically after the Honda–Fujishima effect of water splitting using a TiO₂ electrode was reported in the early 1970s.^1–7 However, the typical photocatalyst TiO₂ is only active in the ultraviolet (UV) light range. Its wide band gap limits its practical applications under solar light. Although modification of TiO₂ by doping has been used to increase the visible light absorption,^8–11 doped materials often suffer from thermal instability and some dopants may function as recombination centers to decrease the photocatalytic performance.⁸

With this recognition, many recent studies have been focused on the development of non-TiO₂ based semiconductors that can serve as efficient solar light driven photocatalysts. Topics covered the chemical compositions, synthetic strategies, and structure–property relationships etc. Desirable photocatalysts should have a small band gap to increase the solar energy utilization efficiency and appropriate band edge positions to facilitate the light induced redox reactions. In other words, the valence band edge of an ideal photocatalyst is expected positive enough to provide sufficient overpotential for photo-oxidation reaction and the conduction band edge is negative enough for photo-reduction reaction. In the last several years, numerous studies on the visible light driven photocatalyst have proved it is difficult to find a stable semiconductor satisfying both cases. One viable option is the development of narrow band semiconductor with high photo-oxidation activity, for the light induced contaminant degradation and water oxidation mainly depend on the photo-oxidation ability of a photocatalyst. Noticeably, the major challenge for water splitting is the water oxidation process, which is an uphill reaction involved four-electron transfer with slow kinetics. Therefore, metal oxide semiconductors with intrinsic narrow band gap and deep valence bands attracted more and more attention on the development of visible light driven photocatalyst.^12–19 Although the deep valence bands endow these oxide photocatalyst high photo-oxidation ability, the photocatalytic efficiency is usually low because the excessive recombination of the photogenerated charge carriers. In the process of searching for efficient visible light driven oxide photocatalyst, bismuth based complex oxides were recognized as potential candidates. It was suggested the lone-pair distortion of Bi 6s orbital in these semiconductors may cause the pronounced overlap of O 2p and Bi 6s orbitals in the valence band, which is benefit to the mobility of photogenerated charge carriers for improving the photocatalytic activity.^20–24 Consequently, a great deal of effort was then devoted to the development of bismuth based complex oxide photocatalysts, such as BiVO₄,^25–31 Bi₂WO₆,^32–36 Bi₂MoO₆,^37–39 Bi₄Ti₃O₁₂,^40–42 BiFeO₃,^43–48 Bi₂Fe₄O₉,^49–52 Bi₅FeTi₃O₁₅,⁵³ BiOCl,^54–56 Bi₅O₇I (ref. 57 and 58) etc. Compared with their simple oxides without bismuth atoms, these complex oxides exhibited excellent photocatalytic activity by virtue of the improved charge transfer. However, the typical efficiencies of these bismuth based complex oxides were not satisfying because of their low conduction band levels and insufficient visible light absorption. There is still a great deal of effort to do for further improving their photocatalytic performance.

The critical process for semiconductor photocatalysis is the generation of electron–hole pairs, followed by the separation and transfer of the electrons and holes. Therefore, there are mainly two general approaches to improve the photocatalytic performance of bismuth based complex oxides. One way is to enhance the light absorption. Another way is to improve the utilization efficiency of the photo-generated carries, such as by decreasing the recombination rate and increasing the life time of the photo-generated carries. In the previous studies on the bismuth based complex oxide photocatalysts, it was found all of these processes are seriously affected by the composition and structure of the photocatalyst. Various strategies such as morphology control, doping, and construction of composite material, have been developed recently to improve their photocatalytic performance. Some basic principles of photocatalysis in bismuth based complex oxides may be concluded by systematically studying the obtained results up to the present. For this, Zhang et al.⁵⁹ and Zhu et al.⁶⁰ independently summarized a short review of Bi₂WO₆ photocatalyst on the controllable synthesis, microstructures and environmental application, mainly based on their contribution to this area. Choi et al. reviewed the BiVO₄ photoanodes for use in photoelectrochemical (PEC) water oxidation,⁶¹ but the studies on the suspension-type BiVO₄ used as photocatalysts for water oxidation or pollutant degradation were not included. Considering the great potential of bismuth based complex oxides, a systematic study on these materials is necessary to find out the fundamental principles for the construction of high-performance, commercial available solar light photocatalytic material.

Herein, we provide a comprehensive review of bismuth based complex oxides photocatalysts by focusing on the mostly studied BiVO₄, Bi₂WO₆ and Bi₂MoO₆ materials. It is the first complex work on bismuth based complex oxides covering the primary important factors for photocatalysis. In this review, the bases of bismuth based complex oxides will be first discussed, including the crystal structure, the electronic structure and the structure–performance relationship. Then the major factors affecting photocatalytic performances and strategies to improving the photocatalytic activity will be critically reviewed. For each section, we provide critical commentary based on our knowledge and related research experiences. This review contains the latest ideas and concepts in understanding the basis and improving the performance of bismuth based complex oxide photocatalyst. Representative works were selected from the most recent literature available. The intent of this review is to give the readers a critical discussion of the progress and perspective of bismuth based complex oxide photocatalysts, which could provide guidance for the designed synthesis of high-performance solar light driven photocatalyst.

2. Bases of Bi–M–O complex oxide (M = V, W, Mo) photocatalysts

Photocatalytic activity is closely related to the crystal and electronic structures. In this section, we present an overview on the crystal and electronic structures of Bi–M–O complex oxides and discuss how they affect their photocatalytic performance. This is indispensible for developing effective strategies to obtain highly efficient photocatalysts.

2.1 Crystal structure

BiVO₄ is known to exist in three crystalline phases: tetragonal zircon, monoclinic scheelite, and tetragonal scheelite structure.⁶² The process of phase transformation among them has been studied. When tetragonal zircon BiVO₄ is heat-treated above 670–770 K, monoclinic scheelite BiVO₄ was obtained. Among scheelite structures, the tetragonal phase is a high temperature phase and the phase transition between monoclinic scheelite BiVO₄ and tetragonal scheelite BiVO₄ was observed to occur reversibly at 528 K.⁶³ The tetragonal zircon BiVO₄ has a band gap of 2.9 eV. Its photocatalytic activity was low under visible light irradiation.²⁰ Therefore, the mostly studied BiVO₄ photocatalysts at present is the scheelite BiVO₄ with a band gap of 2.4 eV. Fig. 1a shows the schematic crystal structure of scheelite BiVO₄. It was found a layered structure of Bi–V–O units stacked parallel to the c axis. In this structure, each V ion is coordinated by four oxygen atoms forming a VO₄ tetrahedron. Each VO₄ tetrahedron does not make contact with a subsequent VO₄ tetrahedron, and each Bi ion is coordinated by eight oxygen atoms from eight different VO₄ tetrahedrons.⁶⁴

Fig. 1 Schematic crystal structure of BiVO₄, Bi₂WO₆ and Bi₂MoO₆ in the polyhedron mode: (a) scheelite BiVO₄, (b) Aurivillius orthorhombic Bi₂WO₆, (c) Aurivillius orthorhombic Bi₂MoO₆.

There are two types of O atom in this structure. O1 is coordinated to one Bi and V, while O2 is coordinated to two Bi and a single V. The only difference between the tetragonal and monoclinic scheelite structure is that the local environments of V and Bi ions are more significantly distorted in the monoclinic structure, which removes the four-fold symmetry necessary for a tetragonal system. For example, the lengths of the four V–O bond (1.72 Å) were equal in tetragonal scheelite, but two different V–O bond lengths exist in a monoclinic scheelite structure (1.77 Å and 1.69 Å). In the same manner, while only two very similar Bi–O distances exist in the tetragonal scheelite structure (2.453 Å and 2.499 Å), there are four different Bi–O distances (2.354 Å, 2.372 Å, 2.516 Å and 2.628 Å) in the monoclinic scheelite structure.^61,64

The band gap of BiVO₄ (s-m) (2.41 eV) was similar to that of BiVO₄ (s-t) (2.34 eV), suggesting the energy structures were similar to each other.⁶⁵ However, BiVO₄ (s-m) showed high activity. The disparate photocatalytic activity is originated from the variations of the crystal structure, which result in the modification of the electronic structure. Akihiko and co-workers considered that the distortion of a Bi–O polyhedron by a 6s² lone pair of Bi³⁺ play an important role in improving the photocatalytic performance of monoclinic BiVO₄ under visible light irradiation.⁶⁵ The further distortion of the Bi-polyhedra in the monoclinic scheelite structure endows its higher activity than tetragonal scheelite structure. These indicate that the photocatalytic performance of BiVO₄ largely depends on the crystal structure.

Different from BiVO₄, there are only two crystalline phases in Bi₂MO₆ (M = Mo, W) compounds: orthorhombic and monoclinic structure. Orthorhombic structures of Bi₂MO₆ (M = Mo, W) existed at low and intermediate temperatures (T < 960 °C) and monoclinic structure is a high temperature phase (T > 960 °C).^66,67 The presently studied Bi₂MO₆ (M = Mo, W) photocatalyst was usually orthorhombic phase. Fig. 1b and c show the schematic crystal structure of the orthorhombic Bi₂WO₆ and Bi₂MoO₆, respectively. It is obvious the Bi₂MO₆ (M = Mo, W) photocatalysts possess a layered structure which is composed of MO₆ octahedral layers and Bi–O–Bi layers. In this structure, each Bi atom is coordinated with four O and four M atoms. Each M atom is coordinated with six O atoms to form MO₆ octahedron. The MO₆ octahedrons are connected to each other by corner-sharing O atom. The (Bi₂O₂)²⁺ layers are sandwiched between MO₆ octahedral layers. The local environments of M and Bi ions are also distorted in the orthorhombic Bi₂MO₆ (M = Mo, W). For low temperature orthorhombic Bi₂WO₆ as an example, there are six different W–O bond lengths (1.85 Å, 1.87 Å, 1.728 Å, 1.720 Å, 2.240 Å and 2.215 Å) exist in the WO₆ octahedron⁶⁷ indicating a distorted octahedral structure. The off-centre octahedral distortions are a general feature of the structural chemistry of d⁰ metal cations. It also existed in the orthorhombic Bi₂MoO₆ photocatalyst.

Based on the above analysis, it was found both of the BiVO₄ and Bi₂MO₆ (M = Mo, W) photocatalysts possess a layer structure with local lattice distortions. Besides the reasons mentioned above for the crystal distortion, it is worth noting that the Bi 6s lone pair electrons is another driving force for asymmetric coordination environments in the Bi–M–O photocatalyst. The distortion of local crystal structure probably affects the electronic structure, which is responsible for the high visible light photocatalytic activity.

2.2 Electronic structure

The schematic band structure of the Bi-M-O compounds was shown in Fig. 2. All of the BiVO₄ and Bi₂MO₆ (M = Mo, W) photocatalysts have a direct band gap. Density functional theory (DFT) calculations by many researchers have show that the valence band of the BiVO₄ and Bi₂MO₆ (M = Mo, W) photocatalysts was mainly composed of O 2p and Bi 6s levels, while the conduction band was mainly composed of M nd (V 3d, Mo 4d, W 5d) levels.^23,68–71 The band structure indicates that charge transfer upon photoexcitation occurs from the O 2p + Bi 6s hybrid orbitals to the empty M nd orbitals. These authors also emphasized the importance of the sterically active Bi 6s² lone pair for both reducing the oxide band gaps and increasing the hole conductivity,⁶⁹ for the Bi 6s orbits are located upon the O 2p orbits and largely dispersed the valence band top. Combining the crystal structure and the electronic structure, M is expected to be a reduction site, whereas the oxidation could occur at either a Bi or an O site in the Bi–M–O complex photocatalysts.⁷² Then the photocatalytic performance of the Bi–M–O compounds would exhibit significant difference at different lattice planes, along with the varied density of the Bi, M and O atoms at different lattice planes.

Fig. 2 Schematic band structure and the calculated redox potentials of BiVO₄, Bi₂WO₆ and Bi₂MoO₆ at the point of zero charge.

Although the BiVO₄ and Bi₂MO₆ (M = Mo, W) photocatalysts have a similar configuration of the band structures, the band positions were different because of their different chemical composition and crystal structures. Knowledge of the band positions is useful in as much as they indicate the thermodynamic limitations for the photoreactions that can be carried out with the photogenerated charge carriers. Therefore, it is important to clarify the band edge potentials of the photocatalyst. For a M_aN_b compound, the conduction band edge can be predicted by the following equation:^73,74

where X_M and X_N are the absolute electronegativity of the atoms M and N respectively, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy; E^c is the energy of free electrons on the hydrogen scale (about 4.5 eV); E_g is the band gap of the semiconductor; and pH_ZPC is the point of zero charge. The estimated E_CB at pH_ZPC by the above equation for BiVO₄, Bi₂MoO₆ and Bi₂WO₆ is about 0.45 eV, 0.51 eV and 0.52 eV respectively (Fig. 2) using the Pearson absolute electronegativity.⁷⁵

The low conduction band level of the BiVO₄, Bi₂MoO₆ and Bi₂WO₆ photocatalyst indicates the insufficient reduction power of the photogenerated electrons with electron acceptors such as O₂. For the single-electron reduction of oxygen (O₂ + e⁻ → ˙O₂⁻ (aq.)), the standard redox potential is about −0.284 V vs. NHE which is much negative than the E_CB of BiVO₄, Bi₂MoO₆ and Bi₂WO₆ photocatalyst. This could increases the recombination rates of electron–hole pairs. Then the most effective approaches to improve the photocatalytic performance of the Bi–M–O photocatalysts are based on decreasing the recombination rate of photogenerated charge carriers. According to the predicted E_CB, the E_VB of the BiVO₄, Bi₂MoO₆ and Bi₂WO₆ is estimated at 2.85 eV, 3.1 eV and 3.27 eV respectively. It is clear the valence band position of Bi₂WO₆ is more positive than BiVO₄ and Bi₂MoO₆, indicating a higher photo-oxidation ability. For this predominance, the Bi₂WO₆ photocatalyst usually exhibited better performance than the BiVO₄ and Bi₂MoO₆ on the photocatalytic oxidation of various organic contaminants.

Besides the band edge position, the different effective mass of charge carriers is another important factor leading to the different photocatalytic efficiency. It is known that the electronic effective mass is inversely proportional to the curvature of conduction bands.⁷⁶ Dai's group compared the CB bottoms of Bi₂WO₆ and Bi₂MoO₆ by DFT calculations.⁷⁷ It was found the CB bottom of Bi₂WO₆ has larger curvature than Bi₂MoO₆, indicating a smaller electronic effective mass. The smaller effective mass of electrons for Bi₂WO₆ benefits to the separation and migration of photogenerated carriers and also resulted in better photocatalytic performance than Bi₂MoO₆.

3. Strategies to enhance photocatalytic properties

As described in Section 1, BiVO₄, Bi₂WO₆ and Bi₂MoO₆ are highly promising photocatalysts for use in environmental purification and solar water oxidation. The major limiting factor on improving their photocatalytic activity is the excessive electron–hole recombination. Recently, many efforts have been made to address this problem, which include morphology control, surface optimization, doping and construction of composite material. This section will give an overview of each of these strategies and discuss how these strategies affected the photocatalytic performance.

3.1 Structure and morphology control

The photocatalytic activity is closely related to the size, morphology, and crystallinity of the photocatalysts. Controlling the microstructure of the photocatalysts provides an effective way to enhance their photocatalytic properties.

Nanostructured photocatalyst usually exhibited excellent photocatalytic activity, because the photocatalytic behavior is closely related to the particle size. For randomly generated charge carriers, the average diffusion time from the bulk to the surface is given by τ = r²/π²D, where r is the grain radius and D is the diffusion coefficient of the carrier.⁷⁸ If the grain radius decreases, larger numbers of photogenerated charge carriers will transfer to the surface for photocatalytic reaction. It has been reported the nanoscale BiVO₄, Bi₂WO₆ and Bi₂MoO₆ exhibited much enhanced photocatalytic activity than that of their bulk samples obtained by solid state reaction.^79–82 In a recent study, Colón et al. prepared several BiVO₄ samples with different morphologies, such as peanut-like and needle-like architectures. It was found the photocatalytic activity of the prepared BiVO₄ is strongly affected by the crystallite size and morphology, and the needle-like BiVO₄ with smaller crystallite size exhibited better photocatalytic performance.⁸³

Because of the layered crystal structure, plate-like morphology is a primary nanostructure for the BiVO₄, Bi₂WO₆ and Bi₂MoO₆ photocatalyst. Yu et al. compared the photocatalytic activity of two dimensional (2D) nanostructures with other morphology.⁸⁴ In their studies, the 2D (disc-like and plate-like) BiVO₄ demonstrates better photocatalytic activity than 3D and bulk BiVO₄. Especially, the BiVO₄ nanoplate exhibited the highest photocatalytic activity on degradation of RhB. Zhu's group systematically studied the reasons for the excellent photocatalytic activity of Bi₂WO₆ nanoplates,^71,85,86 which is ascribed to the large BET surface area and the thin thickness of the laminar structure.

Though minimizing the particle size could effectively enhance the photocatalytic activities, the decreased particle size makes it harder to be separated and recycled from the treated waste water in industrial use. Nano-/micro-sized hierarchical structures integrate the virtues of nanoscale and micro-scale. Most of the recent studies on the morphological control of the BiVO₄, Bi₂WO₆ and Bi₂MoO₆ photocatalysts were focused on hierarchical structures. Hierarchical structured BiVO₄ has been reported exhibited dendrite,^87–89 flower-like⁹⁰ and olive-like morphology etc. (as shown in Fig. 3).^91–93 They exhibited excellent photocatalytic activity because of their unique morphology.^89,91,93 Flower-like Bi₂WO₆ and Bi₂MoO₆ microspheres are usually obtained when they are hydrothermally synthesized (as shown in Fig. 4).^94–107 The diameter of the flower-like microspheres is in the range of micrometers. Studies on the formation mechanism have indicated that these flower-like hierarchical structures are built from oriented aggregation of two-dimensional Bi₂WO₆ nanoplates with a thickness and average length in nanoscale.^95,97 Compared with bulk samples obtained by solid state reaction, these flower-like Bi₂WO₆ microspheres exhibited much higher photocatalytic activity on the degradation of various organic contaminants, such as RhB,^95,97 acetaldehyde,⁹⁹ and Orange-II.¹⁰¹ Recently, Amano et al. studied the effects of hierarchical architecture on the photocatalytic activity of Bi₂WO₆.¹⁰⁸ It was found the high photocatalytic activity did not depend on the assembling morphology of flakes. For Bi₂WO₆ samples of similar W/Bi ratio, the photocatalytic activity increased with an increase in their specific surface area. They hypothesize that both a small density of recombination centers and a large specific surface area are essential factors for a high photocatalytic activity.

Fig. 3 (a) Representative TEM image of dendrite BiVO₄ from ref. 87. (b) Representative TEM and SEM image of flower-like BiVO₄ from ref. 90. (c) Representative SEM image of olive-like BiVO₄ from ref. 91. Reprinted with permission. Copyright 2008, 2007 and 2009, Wiley-VCH, American Chemical Society and Royal Society of Chemistry.

Fig. 4 Typical SEM images of flower-like Bi₂WO₆ hierarchical structures from ref. 95. Reprinted with permission. Copyright 2007, Royal Society of Chemistry.

Because of the large specific surface area, porous nanostructure has been extensively studied recently. Mesoporous monoclinic BiVO₄ (Fig. 5a) prepared by Yu et al. with a high BET surface area of 59 m² g⁻¹ exhibited a superior visible light-driven photocatalytic activity for the methylene blue degradation and NO oxidation.¹⁰⁹ The enhanced activity is due to its physicochemical properties such as crystal size, BET surface area, and porous structure, which not only supplies more active sites for the degradation reaction but also effectively promotes the separation efficiency of the electron–hole pairs.¹⁰⁹ The advantages of ordered porous structure for photocatalysis were further proved by studying the photocatalytic performance of ordered macroporous Bi₂WO₆ (Fig. 5b and c).^110,111 It was believed the excellent photocatalytic activities were related to the improved light-harvesting properties, as well as the continuous porous structure.

Fig. 5 (a) TEM image of ordered mesoporous BiVO₄ using KIT-6 as a template from ref. 109. (b) SEM image of ordered macroporous Bi₂WO₆ film synthesized with 340 nm carbon spheres from ref. 110. (c) TEM image of three-dimensional ordered macroporous Bi₂WO₆ powder synthesized with 90 nm SiO₂ spheres from ref. 111. Reprinted with permission. Copyright 2008, 2009 and 2012, American Chemical Society, Wiley-VCH and Royal Society of Chemistry.

3.2 Surface optimization and modification

The photocatalytic degradation of a pollutant must proceed at the surface of the photocatalyst, and consequently the surface properties appear as an important parameter. It was reported when the monoclinic BiVO₄ photocatalyst (JCPDS file no. 14-0688, space group: I2/a, a = 5.195, b = 11.701, c = 5.092) exposes more facets of {010} (as shown in Fig. 6a and b), it exhibited greatly enhanced photocatalytic activity.^112–114 The high photocatalytic activity may be ascribed to the larger atom density of the {010} facets (as shown in Fig. 6c) on the exposed surfaces, which is beneficial to the adsorption and degradation of the pollutants.^112,113 Density functional theory calculations by Li et al. studied the surface properties of monoclinic BiVO₄.¹¹⁵ The calculated results indicated in all of the most stable terminated planes of BiVO₄, the V atoms are fully coordinated, and all of the dangling bonds are derived from the broken Bi–O bonds. A dangling bond plays an important role in the surface adsorption properties. The {010} facets is thermodynamically lowest-energy surface of monoclinic BiVO₄. The high density of cleaved Bi–O bonds in {010} facets may improve the adsorption property on the surface, which is advantageous for the photocatalytic reaction. On the other hand, considering that the photocatalytic oxidation of contaminant would occur at either a Bi or an O site in the BiVO₄ photocatalysts, the larger atom density of Bi and O in {010} facets could also improve the photocatalytic efficiency.

Fig. 6 (a) TEM image of {010} oriented BiVO₄ nanoplates from ref. 112. (b) TEM image of {010} oriented BiVO₄ nanosheets from ref. 113. (c) Schematic illustration of monoclinic BiVO₄ crystal structure viewed along different directions (1 × 1 × 1 cell) to show the structural feature of the (101), (100), (001) and (010) facets from ref. 112. Reprinted with permission. Copyright 2009 and 2006, American Chemical Society.

The photocatalytic activity of preferential oriented BiVO₄ was also investigated by Ye's group.¹¹⁶ They found the high photocatalytic activity of (001) plans resulted from their superior hydrophilic ability. The similar viewpoint was proposed by Skrabalak et al. in their studies on the structure-dependent photocatalytic properties of Bi₂WO₆.¹¹⁷ It was found that the enhanced performance of the Bi₂WO₆ microspheres is partially ascribed to bismuth-rich hydrophilic surface.

Saison et al. established a relationship between surface acidity and photocatalytic performance on RhB degradation in Bi₂O₃, BiVO₄ and Bi₂WO₆ materials.¹¹⁸ It was found the best photocatalytic performance for contaminants degradation was obtained with the Bi₂WO₆ sample that also exhibits the highest surface acidity. The most acid sites may promote a strong interaction with the pollutant, implying a short distance between the pollutant and the photocatalyst. Consequently, the photogenerated electrons, holes, and radicals can reach more easily to the pollutant, leading to an efficient degradation under visible light.

Surface fluorination is an effective method for modifying the surface property of oxide photocatalysts and then improving their photocatalytic performance. Zhu et al. synthesized fluorinated Bi₂WO₆ photocatalyst by a simple hydrothermal process.¹¹⁹ They found the fluorination of Bi₂WO₆ affected not only the reaction rate but also the mechanistic pathways of the RhB degradation. The fluorinated Bi₂WO₆ exhibited enhanced photocatalytic activity for the RhB degradation. More RhB molecules were degraded via the de-ethylation process by the fluorinated Bi₂WO₆ (as shown in Fig. 7). 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. As a result, more RhB molecules were degraded via the N-demethylation process.¹¹⁹ Yu et al. studied the surface fluorinated BiVO₄ photocatalyst synthesized by NaF-mediated hydrothermal processes.¹²⁰ It was found the surface fluorination favored the RhB adsorption and hole transfer between RhB molecules and BiVO₄ photocatalyst, thus progressively enhancing the initial direct hole transfer mediated de-ethylation process.

Fig. 7 UV-visible spectral changes of RhB (2 × 10⁻⁵ M, 50 mL) over Bi₂WO₆ and fluorinated Bi₂WO₆ photocatalyst from ref. 119. Reprinted with permission. Copyright 2008, American Chemical Society.

In a recent study by Luo et al., surface pretreatment by electrochemical cyclic voltammetry (CV) in the dark was found could remarkably enhance the photocurrent of Mo-doped BiVO₄ from the front side illumination.¹²¹ Fig. 8 illustrated the possible mechanism for the enhanced photocurrent. Some MoO_x segregation precipitated on the surface of the Mo-doped BiVO₄ electrode during the preparation of the photoelectrode. MoO_x acts as a recombination center and has a negative effect on the photocurrent. After the pretreatment, MoO_x on the surface was dissolved into the electrolyte, improving the separation and transfer of the photogenerated charge carriers.

Fig. 8 Possible sketch of the transfer of photogenerated electrons in Mo-doped BiVO₄ electrodes illuminated from the front side and the back side before and after the electrochemical pretreatment (reproduced from ref. 121 with permission from American Chemistry Society).

3.3 Doping

Previous studies have reported that doping impurities into a semiconductor can improve photocatalytic performance. However, the effect of doping may not always be advantageous. There are two kinds of dopant levels, including deep level and shallow level. A deep level usually acts as a recombination center, severely decrease the photocatalytic activity. Therefore, appropriate dopants and their concentrations are important to increase the performance of the photocatalysts. In this section, a few types of dopants that were reported to enhance the photocatalytic performances of BiVO₄, Bi₂WO₆ and Bi₂MoO₆ are reviewed and their roles are examined.

Since Ye et al. found the photocatalytic activity of BiVO₄ for O₂ evolution and methylene blue (MB) degradation under visible light irradiation was evidently improved by 2 atom% Mo doping,¹²² metal doped BiVO₄ has been obtained increasingly attention by many research groups. In the studies by Ye's group, they proposed the improved adsorption affinity towards electrolytes or organic molecules was an important reason for the enhanced activity of Mo doped BiVO₄, due to its higher surface acidity than pure BiVO₄. The surface acidity of BiVO₄ is possibly increased by Mo ion substitution, since Mo⁶⁺ ion has a larger electronegativity than that of V⁵⁺. Therefore, the role of Mo doping is thought as affecting the adsorption affinity rather than the photo-oxidation ability in determining the photocatalytic activity of the photocatalyst.

Zou's group further studied the Mo doped BiVO₄ for photoelectrochemical hydrogen generation from seawater.¹²³ The Mott–Schottky method was used to investigate the donor concentration of the Mo-doped BiVO₄, which can be calculated by the slope of the Mott–Schottky curves. It was found the carrier concentration increased about 80 times after doping with Mo (Fig. 9), indicating a high conductivity.

Fig. 9 Mott–Schottky plots for pure BiVO₄ and Mo-doped BiVO₄ in 0.5 M Na₂SO₄, the ac amplitude is 10 mV and the frequency is 1000 Hz (reproduced from ref. 123 with permission from The Royal Society of Chemistry).

In order to identify effective dopants for BiVO₄ more rapidly, Bard's group performed rapid screening of BiVO₄ photocatalysts doped with various metals (W, Fe, B, Cu, Zn, Ti, Nb, Sn, Co, Pb, Rb, Ru, Ag, Ga, Sr, and Ir) using a scanning electrochemical microscopy (SECM) technique.¹²⁴ Among the elements they have tried, only addition of 5–10% W showed noticeable enhancement of photocurrent. The Bi/V/W oxide with a 4.5 : 5 : 0.5 ratio exhibited the highest photocurrent under UV-visible and visible light irradiation. Addition at 10 to 20% levels of elements like Sn, Co, Pd, Rb, Ru, Ag, Ga, Sr, and Ir showed a negative effect and the others basically showed the same behavior as the Bi/V oxide. Mott–Schottky plots revealed that the majority carrier level in the W-doped sample increased approximately 2 times. The increase of the doping level with a decrease in the resistance of the material is probably a contributing factor in the enhanced photocurrent of the Bi/V/W (4.5/5/0.5) oxide. Using the same technique, Park et al. demonstrated that W/Mo co-doped BiVO₄ (Bi : V : W : Mo atomic ratio of 4.6 : 4.6 : 0.2 : 0.6) had 10 times higher photocurrent than undoped BiVO₄. No changes in band gap of W-doped BiVO₄ and W/Mo-doped BiVO₄ were observed, while the increase in carrier density was confirmed by the decrease in the slope of the Mott–Schottky plots. Furthermore, deformation of crystal structure of the scheelite BiVO₄ occurs with consecutive doping of W and Mo. It was found the crystal symmetry shifted from monoclinic to tetragonal. W and Mo in this study are revealed as excellent shallow dopants, which facilitate the separation of photogenerated electron–hole pairs and effectively increase the charge carrier density of BiVO₄ photocatalyst.¹²⁵

Mullins et al. also studied the Mo-, W- and Mo/W-doped BiVO₄ electrodes by ballistic deposition (BD).¹²⁶ They found the optimal levels for individual incorporation were 2.5% W and 5% Mo, which resulted in photocurrent densities that were 7 and 8 times higher than that of pure BiVO₄, respectively. Further improvement was observed for co-incorporation of Mo and W. The optimal level for co-incorporation was also found to be 6% Mo and 2% W, which exhibited a photocurrent density that was 10 times higher than that of pure BiVO₄.

Recently, Luo et al. systematically studied the effect of doping on BiVO₄ with higher valence metal ions (Mo⁶⁺, W⁶⁺ and Sn⁴⁺) at V⁵⁺ and Bi³⁺ sites by DFT calculation and photoelectrochemical measurements.¹²⁷ It was found Mo or W could substitute V sites, but Sn did not substitute Bi sites because of a higher formation energy and lower solubility of impurity ions, leading to serious SnO₂ segregation on the surface. Therefore, Mo⁶⁺ or W⁶⁺-doped BiVO₄ exhibited a much higher photocurrent while the photocurrent of Sn⁴⁺-doped BiVO₄ did not change obviously. Moreover, they found the surface segregation of the doping ions had a negative effect on the performance in the doped samples. Therefore, for an n-type semiconductor photoelectrode, doping with a metal ion with higher valence, lower formation energy and less surface segregation can improve the photoelectrochemical performance.

To integrate the advantages of both Bi₂WO₆ (high photocatalytic activity) and Bi₂MoO₆ (narrower band gap), many researchers have studied the W⁶⁺ doped Bi₂MoO₆ or Mo⁶⁺ doped Bi₂WO₆. From a structural viewpoint, Bi₂WO₆ is isostructural with Bi₂MoO₆. It could be expected that the substitution of W⁶⁺ and Mo⁶⁺ with each other may produce a stable Bi₂Mo_xW_1−xO₆ solid solution, because of the structural similarities between the cations, thus leading to improved properties. Actually, phase-pure Bi₂Mo_xW_1−xO₆ photocatalysts have been successfully synthesized by many research groups and their enhanced visible-light-driven photocatalytic activities were demonstrated.^128–131 For instance, Bi₂Mo_xW_1−xO₆ solid solutions with various compositions (x = 0, 0.25, 0.50, 0.75, and 1.00) have been prepared by Yu's group via hydrothermal treatments.¹²⁸ For the Bi₂Mo_xW_1−xO₆ solid solutions with x = 0.25, 0.50, and 0.75, valence band spectra from XPS indicate the valence band is widened and elevated when compared with the pure Bi₂WO₆ photocatalyst, leading to the narrower band gaps of the Bi₂Mo_xW_1−xO₆ solid solutions (x = 0.25, 0.50, and 0.75). The Bi₂Mo_0.25W_0.75O₆ sample with a relatively high W content exhibited the highest photocatalytic activity on the degradation of MB. Zhu's group reproduced the phase-pure Bi₂Mo_xW_1−xO₆ solid solutions (x = 0, 0.05, 0.25, 0.5, 0.75, 0.95, 1) and studied the effects of Mo replacement on the structure and visible-light-induced photocatalytic performances.¹²⁹ Theoretical calculations based on density functional theory revealed introduction of Mo atom into Bi₂WO₆ could reduce the conduction band level of Bi₂WO₆ and the curvature of the conduction band. Under visible light irradiation (λ > 420 nm), Bi₂WO₆ showed relatively higher photocatalytic activity than other samples. However, under visible light λ > 450 nm, Bi₂Mo_0.25W_0.75O₆ shows much higher activity than Bi₂WO₆. They assumed the higher efficiency of Bi₂WO₆ under visible light λ > 420 nm was attributed to more effective photoelectron transfer in the conduction band with larger curvature, while the higher performance of Bi₂Mo_0.25W_0.75O₆ under visible light λ > 450 nm was attributed to its lower band gap compared with Bi₂WO₆.

Different from the inter-replacement of W⁶⁺ and Mo⁶⁺, the substitution of M (M = W, Mo) sites with other different ions in Bi₂MO₆ (M = W, Mo) 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 then change the photocatalytic and photophysical properties. For instance, we prepared Zr⁴⁺ doped Bi₂WO₆ by substituting the W⁶⁺ ions in the crystal lattice.¹³² Because of the lower valence states of Zr, the substitution of W⁶⁺ by Zr⁴⁺ lead to an extrinsic oxygen vacancy (V_o) by charge compensation effect. The presence of V_o in the zirconium doped sample was confirmed by XPS analysis. The positively charged oxygen vacancy defects can improve oxygen adsorption and function as electron acceptors to reduce the recombination of charge carriers and then increase the photocatalytic performance. Lai et al. studied the influence of V_o on the electronic band structures of Bi₂MO₆ (M = W, Mo) with one V_o by means of density functional theory (DFT).⁷⁷ It was found V_o in Bi₂MO₆ (M = W, Mo) introduced localized W 5d or Mo 4d states in band gap, which may serve as a trapping center of photogenerated electrons and consequently improve photocatalytic oxidation performance (as shown in Fig. 10a).

Nonmetal elements doped samples were also studied to obtain a high-performance photocatalyst. For instance, F-substituted Bi₂WO₆ (Bi₂WO_6−xF_2x) photocatalysts were successfully synthesized by Zhu's group.¹³³ It was found that F-substitution could change the original coordination around the W and Bi atoms. Density functional calculations revealed that Bi₂WO_6−xF_2x possesses a wider valence bandwidth and lower valence band position, resulting in an increased mobility of photo-generated charge carriers and a stronger oxidation power. For this reason, the photocatalytic activity of Bi₂WO_6−xF_2x increased about 2 times compared with pure Bi₂WO₆ on the degradation of MB under visible-light (λ > 420 nm) irradiation. Another example for nonmetal doped Bi₂WO₆ by the substitution of O atoms is N doped Bi₂WO₆.¹³⁴ Our research group studied the effects of nitrogen doping on the crystal structure, optical properties, and photocatalytic performance experimentally. The results showed that the doped samples exhibit 2–3 times higher photocatalytic activities than the undoped one, which was strongly dependent on the N-doping level. The sample with the atomic ration of N to Bi of 0.5 exhibited the best photocatalytic activity. Close investigation revealed that doping Bi₂WO₆ with N could not only enhance the visible light adsorption, but also improve the charge separation and transfer, which may be the reason for the significantly enhanced photocatalytic activities. Dai's group studied the N doped Bi₂WO₆ by DFT calculations.⁷⁷ From the calculated results (as shown in Fig. 10b and 10c), it was found the substitution of N for O induces hybrid states of O 2p and N 2p near the top of valence band, leading to the increases of the VB width and the decreases of band gap. This study theoretically proved the increased mobility of photo-generated charge carriers, the enhanced visible-light absorption property, and the origin of the improved photocatalytic behavior of N doped Bi₂WO₆.

Fig. 10 The total and partial DOS of relaxed Bi₂WO₆: (a) with one O vacancy, (b) with one N atom, (c) with two N atoms. The navy dashed line represents the Fermi level from ref. 77. Reprinted with permission. Copyright 2012, Elsevier.

3.4 Construction of composite material

Construction of composite photocatalysts provides an interesting way to increase the photocatalytic performance by decreasing the charge recombination and extending the light response of photo-excitation. In this section, a few types of composite materials and the fundamentals for their improved photocatalytic activity will be reviewed.

3.4.1 Composite with semiconductor. Semiconductor-based composite material with a heterojunction structure has been extensively studied for obtaining a high-performance photocatalyst. When heterojunctions constituted by semiconductors with matching band potentials, photogenerated charge carriers can transport from one semiconductor to another driven by the contact electric field, leading to efficient separation of photogenerated electron–hole pairs. For a particulate BiVO₄ composite material, it has been reported the photocatalytic performance could be improved by compositing with Co₃O₄,^74,135 V₂O₅,^136,137 CeO₂,¹³⁸ Bi₂O₃,^139,140 Bi₂S₃,¹⁴¹ CuO (ref. 142) and Cu₂O (ref. 143) etc. Long et al. studied the mechanism for the improved photocatalytic activity of the Co₃O₄/BiVO₄ composite material with a p–n heterojunction structure.^74,135 The band edge position of the Co₃O₄ and BiVO₄ were estimated from the absolute electronegativity (eqn (1)). According to the band edge position, the excited electrons on the conduction band of the p-type Co₃O₄ could transfer to that of the n-type BiVO₄, and simultaneously holes on the valence band of n-type BiVO₄ can be transferred to that of p-type Co₃O₄ under the potential of the band energy difference. This process decreased the recombination rate of electron–hole pairs. The similar mechanism for the improved photocatalytic performance that the heterojunctions promote the separation of photogenerated electron–hole pairs was further confirmed in the V₂O₅–BiVO₄, Bi₂O₃–BiVO₄, Bi₂S₃–BiVO₄ and Cu₂O–BiVO₄ composites.^{137,139–141,143}

Su et al. employed surface photovoltage spectroscopy (SPS) and transient photovoltage (TPV) techniques to investigate the effect of heterojunction structure on the behavior of photogenerated charges in V₂O₅–BiVO₄ composite material.¹³⁷ Fig. 11a is the TPV spectra of bare BiVO₄, bare V₂O₅, and the 5.3 wt% V₂O₅–BiVO₄ composite. A positive TPV spectrum of V₂O₅ implies that positive charges accumulate on the V₂O₅ surface at the top electrode under irradiation, while the negative sign of BiVO₄ is attributed to negative charge accumulation on the surface. Different from bare V₂O₅ and BiVO₄, the V₂O₅–BiVO₄ composite semiconductor shows an initial negative response and next positive response reversed at 7 × 10⁻⁶ s in the TPV spectrum. For the V₂O₅–BiVO₄ composites, the intensities and lifetime of the TPV response are much improved compared with those of the bare BiVO₄ and V₂O₅. These TPV features of V₂O₅–BiVO₄ can be ascribed to the improved charge separation in the composite structure, as shown in Fig. 11b.

Fig. 11 (a) Transient photovoltage (TPV) responses of bare BiVO₄, bare V₂O₅, and 5.3 wt% V₂O₅–BiVO₄, (b) schematic diagram for electron–hole separation at the interface of the V₂O₅–BiVO₄ material from ref. 137. Reprinted with permission. Copyright 2011, American Chemistry Society.

BiVO₄/WO₃ composite photoanode films were widely studied for its excellent performance on water splitting.^144–146 The heterojunction structure provided enhanced photoconversion efficiency due to the improved charge separation in the composite structure.^144–146 The WO₃ layer played as barrier for the holes of BiVO₄ to reach FTO surface, which decreased the electron–hole recombination and caused the efficient water oxidation.¹⁴⁶

Saito and coworkers deposited a SnO₂ layer between WO₃ and BiVO₄ to further improve the photoconversion efficiency of the FTO/WO₃/BiVO₄ film.¹⁴⁷ The effect of a SnO₂ layer has been discussed in a FTO/SnO₂/BiVO₄ photoanode film by van de Krol's group before.¹⁴⁸ They found a SnO₂ layer could act as a hole mirror that prevents recombination via FTO-related defect states at the FTO/BiVO₄ interface (Fig. 12a). This approach promoted the charge collection and greatly improved the external quantum efficiency (IPCE) of the BiVO₄ films (as shown in Fig. 12b).

Fig. 12 (a) The schematic diagram illustrates the recombination at the defect state present at the FTO/BiVO₄ interface and the hole mirror effect of the SnO₂ layer, (b) comparison of the IPCE spectra of BiVO₄ on FTO with and without an ∼10 nm SnO₂ interfacial layer (reproduced from ref. 148 with permission from American Chemistry Society).

Bi₂WO₆/TiO₂ has been widely studied due to its much enhanced photocatalytic performances.^149–158 The VB position of TiO₂ is higher than that of Bi₂WO₆. When the TiO₂ coupled with Bi₂WO₆ to form a semiconductor heterojunction, photo-generated holes from the VB of Bi₂WO₆ would transfer to the VB of TiO₂, efficiently suppressing its recombination with photo-generated electrons from the Bi₂WO₆ photocatalyst. Therefore, Bi₂WO₆/TiO₂ composite material exhibited much enhanced photocatalytic activity on degradation of various contaminants, such as stearic acid,¹⁵⁸ acetaldehyde,¹⁴⁹ rhodamine B (RhB),^149,150,155 methylene blue (MB),^154,157 NH₄⁺/NH₃,¹⁵² and Brilliant Red X3B.¹⁵³

Besides the Bi₂WO₆/TiO₂ composite, it has been reported the photocatalytic performance of Bi₂WO₆ could be evidently improved by coupling with Bi₂O₃,^159–162 C₃N₄,^163,164 WO₃,¹⁶⁵ Co₃O₄,¹⁶⁶ CdS,¹⁶⁷ ZnO,¹⁶⁸ ZnWO₄,¹⁶⁹ and Bi₂S₃ (ref. 170) etc. There is some different from TiO₂/Bi₂WO₆ composite when the Bi₂WO₆ coupled with Bi₂O₃, C₃N₄, WO₃, Co₃O₄, CdS and Bi₂S₃. These semiconductors could be excited under visible light irradiation. Besides the effective charge separation from their heterostructure, the improved photocatalytic performance in these systems is inevitably associated with the extended energy response of photo-excitation, leading to enhanced charge carriers generation under visible light irradiation.

3.4.2 Composite with carbon material. Carbon materials, such as graphene oxide (GO), C₆₀, carbon quantum dots, carbon nanotube, carbon spheres and carbon nanofibers, have been used to composite with BiVO₄ and Bi₂MO₆ (M = W, Mo) photocatalysts to improve their photocatalytic performance.^171–185 Among these carbon materials, GO composite materials obtained the most attention recently, owing to its perfect conjugated sp²-hybridized carbon structure and large BET surface area. The conjugated sp²-hybridized structure provides abundance of delocalized electrons to enhance the transport of photo-generated electrons. The large BET surface area could increase the adsorption of target molecules. Both of these two predominance lead to an increase in the photo-conversion efficiency of GO composited photocatalyst. BiVO₄–RGO composite prepared by Ng et al. proved the predominance of RGO for efficient photoelectrochemical water splitting.¹⁷² A remarkable 10-fold enhancement in photoelectrochemical water splitting reaction is observed. A recent study for a composite material of RGO and Bi₂WO₆ quantum dots (QDs) (as shown in Fig. 13a) in our group also proved the advantage of RGO as a base material to improve the photocatalytic performance.¹⁷⁴ Electrochemical measurements indicated the electron conductivity is largely improved in the RGO–Bi₂WO₆ QDs composite. The electron lifetime in the RGO–Bi₂WO₆ QDs composites was increased 8-fold compared with that of the pure Bi₂WO₆ QDs. The tightly anchored interface facilitates the electron transfer from Bi₂WO₆ to RGO, leaving most of the photo-generated holes on the Bi₂WO₆ surface to take part in photo-oxidation reactions (Fig. 13b). Ma et al. investigated the influence of RGO amount on the photocatalytic activity in the RGO–Bi₂WO₆ composite material.¹⁷⁷ It was found the optimum amount of RGO is 20 mg for 1 mmol Bi₂WO₆, at which the RGO–Bi₂WO₆ sample displays the highest reactivity. Zhou et al. prepared RGO–Bi₂MoO₆ composites with a RGO content from 0.5 to 4.0 wt%.¹⁷⁸ Their studies indicate the photocatalytic activities of the RGO–Bi₂MoO₆ composites were dependent on the amount of graphene and it was found that the optimal hybridized amount of graphene was about 1.0 wt%. Similar with GO, the delocalized conjugated electrons in C₆₀ and carbon nanotubes could also increase the photogenerated electron transfer in their composite material to reduce electron–hole recombination and enhance the photoconversion efficiency, as reported in the C₆₀/Bi₂WO₆,¹⁸⁰ C₆₀/Bi₂MoO₆ (ref. 184) and carbon nanotube/BiVO₄ (ref. 173) composite materials. Besides the crystalline carbon material, amorphous carbon such as carbon spheres, carbon dots and carbon nanofibers composited Bi₂MO₆ (M = W, Mo) were also reported exhibited improved photocatalytic activity.^181–183 These studies revealed the enhanced specific surface area and the effective separation of the photogenerated charge carriers in the composite material is responsible for their improved photocatalytic activity.

Fig. 13 (a) TEM image of the RGO–Bi₂WO₆ QDs, (b) the schematic illustration of charge carrier transfer in RGO–Bi₂WO₆ QDs (reproduced from ref. 174 with permission from American Chemistry Society).

3.4.3 Metal loaded. Noble metals, such as Ag, Pt, Au and Pd, have been used as electron acceptors to increase the separation of the photo-generated hole–electron pairs and promote interfacial charge-transfer. It has been reported the noble metal loaded BiVO₄ and Bi₂MO₆ (M = W, Mo) photocatalysts exhibited enhanced photocatalytic activities on contaminants degradation and water oxidation.^186–192 Cao et al. studied the effect of noble metal on the photocatalytic process in detail in Au–BiVO₄ heterogeneous nanostructures.¹⁸⁶ Their studies revealed the gold-enhanced photocatalytic efficiencies are attributed to two reasons. First, the surface loaded gold nanoparticles may act as electron acceptors to decrease the recombination rate of the photogenerated electrons and holes. From the energy diagram of the Au–BiVO₄ junction (Fig. 14), it can be clearly seen the photo-generated electrons is prone to transfer into the Au nanoparticle because of its lower Fermi level when comparing with the CB potential of BiVO₄. Such a charge transfer process enhances the separation of photogenerated electrons and holes and thereby increases their lifetime. This is particularly important for photocatalytic water oxidation because it is a slow multi-electron process with kinetic restraints.¹⁹³ Second, the SPR effect of gold nanoparticles on BiVO₄ surfaces can also contribute to the improved visible light photocatalytic efficiency. To clarify this viewpoint, they compared the photocatalytic activity of Au–BiVO₄ with Pt–BiVO₄ nanosheets and pure BiVO₄ under irradiation of 500 ± 20 nm light. The results indicate the much higher photocatalytic activity of Au–BiVO₄ is arrived from the strong SPR at 500 ± 20 nm. It is believed that the gold SPR can enhance the electron–hole separation near the Au–BiVO₄ heterojunction, and thus improve the photocatalytic efficiency. Our recent studies indicated Ag-loaded Bi₂WO₆ photocatalysts exhibited significantly enhanced photocatalytic activity in inactivating E. coli, a Gram-negative bacterium, and S. epidermidis, a Gram-positive bacterium under visible light irradiation (λ > 420 nm).¹⁸⁷

Fig. 14 Schematic illustration of the detailed energy alignment in the Au–BiVO₄ heterogeneous structure and the proposed mechanism of photocatalytic water oxidation by Au–BiVO₄ nanosheets in the presence of sacrificial agent S₂O₈²⁻. The Fermi level (E_f) of Au shifts to more negative potentials (E^*_f) because of the transfer of photogenerated electrons from BiVO₄ to Au. The SPR excitation of the gold nanoparticle is believed to enhance electron–hole separation near the Au–BiVO₄ heterojunction (ref. 186 with permission from American Chemistry Society).

3.4.4 Co-catalyst loaded. A co-catalyst was usually coupled with a photocatalyst to increase the photo-conversion efficiency and to improve the stability of the photocatalyst, especially in a photocatalytic water splitting process. Oxygen evolution is kinetically the key step in the photocatalytic water splitting. Cocatalysts could lower the activation potential for O₂ evolution. Besides, lots of studies have revealed hole accumulation at the photoanode/electrolyte interface often results in photocorrosion,^194,195 and a cocatalyst could increase the stability for O₂ evolution. For BiVO₄ photoanodes as an instance, Sayama et al. found the surface V atoms dissolved from the bare BiVO₄ surface during the photoreaction,¹⁹⁴ inducing a significantly decreased photocurrent within a few minutes. To solve this problem, they attempt to modify the surface of BiVO₄ with Ag⁺ ions, which involved simply dipping the BiVO₄ electrode in an AgNO₃ solution. Although the exact form of the responsible species was not identified, the Ag-modified BiVO₄ electrode exhibited enhanced O₂ evolution and noticeably improved stability. They surmised the Ag⁺ ion in/on the BiVO₄ catalyzed the intrinsic photogeneration of oxygen with the holes. Recently, various cocatalyst have been combined with BiVO₄ to enhance the O₂ evolution while reducing photocorrosion.

Up to the present, the most commonly used cocatalyst to improve water oxidation kinetics of BiVO₄ or doped BiVO₄ photoanodes was Co–Pi.^196–202 Co–Pi have been demonstrated to work effectively as oxygen-evolving electrocatalyst (OEC) at pH 7 buffered with aqueous phosphate in electrolysis.²⁰³ When catalyzing water oxidation, Co–Pi undergoes proton-coupled electron transfers along with cyclic valency changes between Co(II/III) and Co(III/IV).²⁰⁴ In photoelectrolysis, it has been demonstrated the primary role of Co–Pi is to inhibit charge recombination by facilitating the hole transfer via the cobalt ion valency cycle.²⁰⁰ For Co–Pi modified W : BiVO₄ as an example, Zhong et al. found Co–Pi deposition yields a remarkable ∼440 mV cathodic shift in the onset potential for sustained PEC water oxidation at pH 8.¹⁹⁶ The much lower absolute onset potential is promising for solar water splitting in low-cost tandem PEC cells. Moreover, the PEC current of Co–Pi modified W : BiVO₄ is about three times higher than that of the parent W : BiVO₄ photoanode. Almost completely suppress of surface electron–hole recombination by Co–Pi addition was observed in their studies. They summarized the mechanism of Co–Pi on inhibition of the surface electron–hole recombination, as shown in Fig. 15. For bare W : BiVO₄, surface recombination (J_sr) is a major loss pathway which results in poor water oxidation (J_ox) and poor PEC photocurrent densities. After Co–Pi modification, almost all of the photogenerated holes that migrate to the W : BiVO₄ surfaces can be captured by the Co–Pi electrocatalyst to oxidize water with nearly quantum efficiency. Obviously, Co–Pi deposition provides a facile route by which photogenerated holes can be captured and used in the productive four-electron oxidation of water. Co–Pi modified Mo-doped BiVO₄ electrode prepared by Pilli et al. exhibited a similar superior PEC characteristics in terms of onset potential and photocurrent density.¹⁹⁷ Furthermore, it was found the Co–Pi/Mo-doped BiVO₄ photoelectrode exhibited better stability as compared to bare Mo-doped BiVO₄ electrode under photolysis conditions.

Fig. 15 Energy diagram showing the kinetic processes active in the Co–Pi/W : BiVO₄ PEC photoanodes. Electron–hole pairs are generated with a current density associated with photon-absorption (J_abs) and can recombine nonproductively with current densities associated with radiative or nonradiative bulk (J_br) and surface (J_sr) recombination. Electron collection at the back contact (J_PEC) and hole transfer to the oxidizable substrate (J_ox) are productive processes contributing to PEC device efficiency (ref. 196 with permission from American Chemistry Society).

For the same attempt to enhance O₂ evolution and reduce photocorrosion, Seabold et al. coupled the BiVO₄ photoanode with a different type of cocatalyst, FeOOH which is also a O₂ evolution electrocatalyst.¹⁹⁵ They found the BiVO₄/FeOOH films generated remarkably improved photocurrent for water oxidation, which is comparable to photocurrent generated by bare BiVO₄ using sulfite as the hole scavenger, verifying the ability of FeOOH to efficiently collect photogenerated holes from the BiVO₄ layer and facilitate water oxidation to O₂. The effect of FeOOH on the photostability of BiVO₄ was also examined in their study. It was found the photocurrent generated by the bare BiVO₄ film decreased significantly within a few minutes because of the anodic photocorrosion. When the FeOOH layer was added to the surface of the BiVO₄ electrode, a much enhanced photocurrent density was maintained for 6 h with only 2% of decay. This result demonstrates the exceptional performance of FeOOH for improving photostability as well as photocurrent of BiVO₄ in PEC water oxidation. The photocurrent-to-O₂ conversion efficiency of the prepared BiVO₄/FeOOH photoanode is up to 96%.

Comparing with water splitting, co-catalyst deposition was less focused on contaminant degradation. A typical study was recently reported by Lin et al. on Pt and RuO₂ co-loaded BiVO₄ (denoted as Pt–RuO₂/BiVO₄).²⁰⁵ The Pt–RuO₂/BiVO₄ could oxide thiophene to SO₃ in acetonitrile solution under visible light irradiation with molecular oxygen as oxidant. A high photocatalytic activity of thiophene oxidation was achieved by only loading 0.03 wt% of Pt and 0.01 wt% of RuO₂ as dual co-catalysts on BiVO₄. In Pt–RuO₂/BiVO₄, Pt acted as a reduction co-catalyst and RuO₂ acted as an oxidation co-catalyst. The co-existence of oxidation and reduction co-catalysts is beneficial for the efficient separation and transfer of the photo-excited electrons and holes, resulting in the high photo-catalytic activity of thiophene oxidation. Recently, Cu⁰ was found as an excellent co-catalyst for Bi₂WO₆ photocatalyst in our research group.²⁰⁶ The copper reagent not only served as a heterogeneous catalyst to generate hydroxyl radicals by Fenton-like reaction, but also acted as an electron shuttle that facilitated the circulation of the photogenerated carriers to improve the photocatalytic performance.

4. Summary and outlook

Over the past few years, the numerous efforts put on bismuth based complex oxide semiconductors have resulted in a rich database for their synthesis, properties, modifications, and photocatalytic applications. The mostly studied bismuth based complex oxide photocatalysts are BiVO₄, Bi₂WO₆ and Bi₂MoO₆ up to the present. This review summarized the structures and properties of these bismuth based photocatalysts for use in environmental purification and solar water oxidation, emphasizing the strategies that have been employed to enhance their photocatalytic properties. Although a significant improvement has been made for the development of highly efficient bismuth based complex oxides, there is still considerable problems for further investigated. The poor reduction power of the photogenerated electrons from the low conduction band edge potential resulted in the high recombination rates of photogenerated charge carriers, which is the main defect suppressing the photocatalytic performance of BiVO₄, Bi₂WO₆ and Bi₂MoO₆. Morphological control has significantly improved its photocatalytic performance by control of the crystallinity, BET surface area, grain size and microstructure et al. It is worth to note decreasing the grain size would significantly decrease the bulk recombination rate of the photogenerated charge carriers. Quantum sized photocatalysts should be paid special attention in the future. However, the current synthesis methods are not sufficient for large-scale production of quantum sized photocatalyst. For this reason, further efforts must be made on the development of robust and reliable strategies for fabricating various quantum sized photocatalysts. Surface control can be used to further improve the photocatalytic performance, considering the photocatalytic reactions proceed at the surface of the photocatalyst and only few of previous investigations were focused on the effect of crystal surface. This provides us with important research opportunities. We believe that the surface defects and configurations of surface atoms, but not limited to, should receive more attention in future studies. Preliminary investigations already show that photo-absorption and charge carrier separation could be improved by doping or construction of composite material. These investigations were mainly studied either experimentally or theoretically. More fundamental studies, combining the experiment, theory and calculation, are needed to help us to understand the photocatalytic principles. These research areas are ripe for growth. We will see an increasing number of new and novel progress of bismuth based complex oxides for application on environmental purification and solar water oxidation.

Acknowledgements

We acknowledge the financial support from the National Basic Research Program of China (2010CB933503, 2013CB933203), National Natural Science Foundation of China (51102262, 51272269), and the Science Foundation for Youth Scholar of State Key Laboratory of High Performance Ceramics and Superfine Microstructures (Grant no. SKL201204).

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