Structure Tuning of Bi2MoO6 and Their Enhanced

Critical Reviews in Solid State and Materials Sciences

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Structure Tuning of Bi2MoO6 and Their Enhanced Visible Light Photocatalytic Performances Yanhua Peng, Yan Zhang, Fenghui Tian, Jinqiu Zhang & Jianqiang Yu To cite this article: Yanhua Peng, Yan Zhang, Fenghui Tian, Jinqiu Zhang & Jianqiang Yu (2017): Structure Tuning of Bi2MoO6 and Their Enhanced Visible Light Photocatalytic Performances, Critical Reviews in Solid State and Materials Sciences, DOI: 10.1080/10408436.2016.1200009 To link to this article: http://dx.doi.org/10.1080/10408436.2016.1200009

Published online: 29 Jun 2017.

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Date: 21 September 2017, At: 18:07

CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 2017, VOL. 0, NO. 0, 1–26 https://doi.org/10.1080/10408436.2016.1200009

Structure Tuning of Bi2MoO6 and Their Enhanced Visible Light Photocatalytic Performances Yanhua Penga, Yan Zhanga, Fenghui Tiana, Jinqiu Zhanga, and Jianqiang Yua,b

Downloaded by [Qingdao University] at 18:07 21 September 2017

a Collaborative Innovation Centre for Marine Biomass Fibers, Materials and Textiles of Shandong Province, College of Chemistry and Chemical Engineering, Qingdao University, Qingdao, China; bLaboratory of Clean Energy Chemistry and Materials, Lanzhou Institute of Chemical Physics, Chinese Academy of Science, Lanzhou, China

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Structure-tuning strategies for synthesis and modification of Bi2MoO6, a novel visible light photocatalyst, have progressed at a quick pace. The enhancement of photocatalytic performances has been obtained through several morphology controls including hierarchical structures and heterojunctional nanocomposites. In this article, various structure modifications and their structural advantages in photocatalysis will be reviewed. In the first section, the structures of Bi2MoO6 such as crystal structures, electronic structures, and band structures will be presented. In the second section, many controllable synthesis approaches for modification of Bi2MoO6, including solid-state reaction, co-precipitation, solvothermal, and hydrothermal methods will be introduced. In the last section, the enhancement of photocatalytic activity for Bi2MoO6 due to the structure tuning will be discussed. The comprehensive review will provide perspectives on the research of efficient photocatalysts under visible light irradiation.

Bismuth molybdate; photocatalysis; visible-light photocatalyst; structural modifications; highly efficient photocatalyst

Table of Contents Introduction ................................................................................................................................................................................................2 Structure information of Bi2MoO6 .......................................................................................................................................................3 Controllable synthesis methods .............................................................................................................................................................4 3.1. Solid-state reaction .............................................................................................................................................................................4 3.2. Co-precipitation method...................................................................................................................................................................4 3.3. Hydrothermal method.......................................................................................................................................................................5 3.3.1. Reaction time ...........................................................................................................................................................................5 3.3.2. Reaction temperature .............................................................................................................................................................5 3.3.3. pH values ..................................................................................................................................................................................6 3.3.4. Surfactant..................................................................................................................................................................................7 3.4. Solvothermal method.........................................................................................................................................................................7 Structure tuning for enhancement of visible-light activity ............................................................................................................8 4.1. Hierarchical structure ........................................................................................................................................................................8 4.1.1. Nanosheets ...............................................................................................................................................................................9 4.1.2. Hollow spheres ......................................................................................................................................................................10 4.1.3. Flower-like structures...........................................................................................................................................................11 4.1.4. Inter-crossed structures .......................................................................................................................................................11 4.2. Heterojunctional nanocomposites ................................................................................................................................................14 4.2.1. Semiconductor/Bi2MoO6 heterojunctional composites................................................................................................14 4.2.2. Carbon or graphene-like/Bi2MoO6 heterojunctional composites...............................................................................15 4.2.3. Multiple functional components .......................................................................................................................................16 Conclusion and outlook .........................................................................................................................................................................17

CONTACT Jianqiang Yu [email protected] Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/bsms. © 2017 Taylor & Francis Group, LLC

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Acknowledgments ....................................................................................................................................................................................20 Funding.......................................................................................................................................................................................................20 References...................................................................................................................................................................................................20


1. Introduction Semiconductor photocatalysis, as the renewable technology, offers a greatly potential for environmental remediation and energy production.1–4 Among various semiconductors, TiO2 hasbeenextensivelyproventobethemostpromisingandsuitablematerialduetoitssuperiorphotocatalyticactivity,chemical stability, low cost, and nontoxicity.5–10 However, its large band gap (3.2 eV) inhibits the effective absorption of visible light, which greatly restricts the efficiency for the utilization of solar energy.11,12 To overcome this issue, much effort has been devoted to the exploration and fabrication of novel semiconductor materials with appropriate band-gap for improving thephotocatlyticperformances. In recent years, considerable attention has been paid to the metal oxides semiconductors with narrow band gap.13– 19 Among numerous potential binary metal oxides, such as Fe2O3 (2.0 eV), WO3 (2.6 eV) transition metals with dn or PbO (2.1 eV), and Bi2O3 (2.5 eV) post-transition metals with ns2, possess suitable band gap for visible light excitation. However, very low efficiency is often obtained by using the binary metal oxides due to their intrinsic limitations.20 The reason for the transition metal oxides is the high resistivity due to the small polaron dominated conductivity while low visible light absorption for the post-transition metal oxides is due to the indirect band gap. Therefore, it is necessary to form functionalized multiternary oxides to overcome the disadvantages of binary photocatalysts. Recently, many novel ternary metal oxides, such as BiVO4,21–28 Bi2WO6,29– 39 Bi2MoO6,40–45 CaBi2O4,46 and InVO4,47,48 have been emerging owing to their excellent visible-light-driven photocatalytic performance. Among them, Bi2MoO6, as one of the important Aurivillius oxides possessing special perovekite-like layer structures, has attracted great attention for its excellent photocatalytic activity under visible light irradiation.41,49–51 Tailoring the morphology and surface structure of inorganic materials at nanoscale by varying the synthesis methods and conditions has long been employed for endowing the distinctive properties.52–54 Recently, the Bi2MoO6 nanosheets have been successfully fabricated by the hydrothermal and solvothermal method,41,55 which have not only higher surface areas than the particles prepared by coprecipitation method56,57 and solid-state reaction,58,59 but also increase the active sites on the surface for photocatalytic reactions with enhanced performance. More recently, Bi2MoO6 materials have been synthesized the formation of hierarchical structures on the basis of nanosheet-building, including

fibers,51,60–61 hollow spheres,62–65 flower-like structures,66 and inter-crossed nanosheets,67 which make the surface areas dramatically increase than the plate structures and improve the effective separation of photoexcited electronhole pairs and decrease the probability of electron-hole recombination. Meanwhile, various heterojunctional nanocomposites based on Bi2MoO6, such as Bi2MoO6/TiO2,61,68 Bi2MoO6/BiOX (X D Cl, Br),69,70 Bi2MoO6/C3N4,71 Bi2MoO6/Grenphen,44,72,73 and multiple functional components (Ag/AgBr/Bi2MoO6),74,75 etc., have been designed and extensively investigated, which not only increase the response range of solar spectrum but prevent the recombination of photoexcited electron-hole pairs. Owing to those results, several surface tuning strategies for Bi2MoO6 as efficient photocatalysts have been successfully developed and some expected results, i.e., the increased optical absorption efficiency and the decreased photo-generated carriers recombination efficiency, have also been obtained. Since the photocatalytic performances of semiconductor photocatalyst are mainly dependent on three kinds of efficiencies, including optical absorption, photogenerated charge separation, and carrier-induced surface reaction, it is clear that the surface-tuning strategies are well effective approach for the efficient phtocatalytic processes. Hence, it is of significance to review recent advances in the development of structure modification for Bi2MoO6. In this article, we focus on the recent advances on structure-tuning strategies of Bi2MoO6 as efficient photocatalysts for utilization of visible-light absorption, including our group results. The structures of Bi2MoO6 including crystal structures, electronic structures, and band structures will be presented in the first section of this article. In the second section, various synthesis methods for morphology controls will be summarized. In the last section, the enhancement of photocatalytic activity of Bi2MoO6 due to the structure tuning for hierarchical structures and heterojunctional nanocomposites will be discussed.

2. Structure information of Bi2MoO6 Bismuth molybdates have the general chemical formula Bi2O3¢ nMoO3, where n D 3, 2, or 1, corresponding to a-Bi2Mo3O12, b-Bi2Mo2O9, and g-Bi2MoO6 phases, respectively. The phases showed catalytic activities for the selective oxidation or ammoxidation of lower olefins,76,77 are generally a-Bi2Mo3O12 and b-Bi2Mo2O9, while g-Bi2MoO6, only with an Aurivillius structure, has photocatalytic activity for


CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES

O2 evolution under visible-light irradiation.41 To understand the relationship between structure and catalytic properties, the structure differences of bismuth molybdates are presented in Figure 1. The structure of a-Bi2Mo3O12 is a defective scheelite in which one out of every three Bi sites is vacant, and the structure of b-Bi2Mo2O9 is a fluorite related super structure with metal site vacancies,76,78 which can be considered as defective fluorite structures. The structure of g-Bi2MoO6 is a typical Aurivillius structure consisting of (Bi2O2)2C sheets alternating with MoO42¡ perovskite layers.77 In addition to the fine structure differences of phases, it is obvious that the local coordination around molybdenum is tetrahedral in both a and b phases and distorted octahedral in the g phase. These may be the origin of the observed different catalytic activities for selective oxidation of lower olefins or photocatalysis. Similarly, the structural differences were also observed for g-Bi2MoO6 phase in three polymorphic forms, including g (the low-temperature phase), I (the

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intermediate-temperature phase), and g’ (the high-temperature phase).57,59,79 The transition from g to I is reversible while the transition from I to g’ is not, which result in both g- and g’-Bi2MoO6 are stable whereas IBi2MoO6 is metastable. It is the main structure difference between the low-temperature phase and the high-temperature phase is that the cation distribution forms a flurite-related supercell with infinite channels of Bi polyhedral surrounded by Mo tetrahedra rather than octahedra.80 Obviously, the visible-light-driven photocatlytic performances for the low-temperature g-Bi2MoO6 are mainly contributed to the distorted MoO6 octahedral structures as seen in the well-known WO3 photocatalyst.81,82 In another words, only these materials containing MoO6 octahedra in the structure showed the photocatalytic activity whereas those containing MoO4 tetrahedra showed negligible photocatalytic activities among the bismuth molybdates. It may be because that the distorted MoO6 octahedral structures could affect the

Figure 1. Schematic crystal structures of (a) a-Bi2Mo3O12, (b) b-Bi2Mo2O9, and (c) g-Bi2MoO6, and the electronic structures of (d) a-Bi2Mo3O12, (e) b-Bi2Mo2O9, and (f) g-Bi2MoO6. (© American Chemical Society. Reprinted with permission from Yoshiki et al.81 Permission to reuse must be obtained from the rightsholder.) (© Elsevier. Reprinted with permission from Li et al.116 Permission to reuse must be obtained from the rightsholder.)


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energy gap between HOMO and LUMO, the width of the conduction band, and the delocalization of excitation energy. The band structures and electronic properties of bismuth molybdates, including a-Bi2Mo3O12, b-Bi2Mo2O9, and g-Bi2MoO6 (low-temperature and high-temperature phases), were studied by density functional theory (DFT) calculations to clarify which orbitals contribute to valence band (VB) and conduction band (CB).81,83–84 Kudo et al.81 suggested that VB of the low-temperature Bi2MoO6 was mainly contributed by O 2p orbitals and CB was derived from the Mo 4d orbitals and Bi 6p orbitals. The contribution of the Bi 6s orbitals to the valence band formation, as seen in BiVO4 and Bi2WO6,85,86 was not observed. Visible-light absorption was revealed to be due to the transition from the VB consisting of O 2p orbitals to CB derived from the primary Mo 4d orbitals in MoO6 octahedra and the secondary Bi 6p orbitals. The valence bands of a, b, and g (high-temperature phase) mainly consisted O 2p orbitals as well as occurred for the g low-temperature phase. However, the tops of VB consisted not only of the O 2p orbitals but also of Bi 6s orbitals partly, which result in a narrowing of their band gaps. All conduction bands of the three materials consisted of Mo 4d orbitals as well as the low-temperature Bi2MoO6 phase. In contrast, Long and co-workers83 revealed that the top of valence band was mainly formed by O 2p orbitals, but hybridized with some Bi 6s orbitals by CASTEP mode as Kudo used. Lai also reported that the VB of Aurivillius Bi2MoO6 contributed by O 2p orbitals, and Mo 4d, Bi 6s states located at VB edge by Vienna ab initio Simulation Package (VASP).84 Unfortunately, the microscopic insight into the electronic structures of the Aurivillius Bi2MoO6 is not completely performed, which do not allow for an accurate analysis of photocatalytic activity of Bi2MoO6. Consequently, a number of theoretical DFT calculations would be asked for to model the structure and provide the more accurate information.

3. Controllable synthesis methods The relationship between structure and property is one of the central issues in material chemistry.87 Bi2MoO6, as one of the heterogeneous catalytic materials, has been studied for decades. Different synthesis routes have been proposed in order to get a material with better textural structures and, consequently, higher photocatalytic performances. In the previous, the preparation methods which were usually used to synthesize Bi2MoO6, are coprecipitation method56,57,88,89 and solid-state reaction.58,59,81,90,91 However, these techniques needed a high temperature set at ca. 400–700 C, meanwhile, the structure properties of the catalyst, such as purity,

morphology, surface texture and grain shape, were not easily controlled.92 In recent years, many advanced synthesis technologies have fast developed, including hydrothermal method, solvothermal method, ultrasound, etc. Among the various routes, it is demonstrated that the hydrothermal method and the solvothermal method are the suitable methods for promoting anisotropic crystal growth and producing various morphologies with high purity and narrow particle size at low temperatures.93,94 Different synthesis routes for preparation of Bi2MoO6 with good textural properties and high photocatalytic activity are discussed in details below. 3.1 Solid-state reaction The solid-state reaction was usually used in preparation of heterogeneous catalysts with good crystal structures.59,81,90–91 Since Bi2MoO6 has the general chemical formula of Bi2O3¢nMoO3, the mixtures of the precursor materials (usually Bi2O3 and MoO3 with some ratios) were calcined at 673–973 K for 5 h in air using an alumina crucible. Generally, the products prepared by this method were often particles with larger sizes (about 1– 3 mm), smaller surface areas (about 0.9–1.2m2g¡1), and larger surface resistance (about 20 kohm cm (–1)),59 and it was very difficult to control different morphology of materials. In addition, the sintering temperature also strongly influenced the phase transformation, so the activities of Bi2MoO6 prepared by the solid-state reaction were very dependent on the sintering temperature.81 The enhancement of photocatalytic performance through controlling the temperature was limited because of the small surface areas of particles and high recombination of photoexcited electron-hole pairs on the surface. 3.2 Co-precipitation method In recent years, many advanced preparation methods in homogenous aqueous solution have been fast developed.95,96 Cruz et al.89 have a tried to control the morphology of Bi2MoO6 by co-precipitation technology which was easy, inexpensive, and low temperature than the solid-state reaction. The preparation process was that the bismuth nitrate solution was added to the molybdate solution then stirred vigorously. The intermediates could be easily decomposed at the temperatures of 300–400 C. Generally, the products by this route showed an important heterogeneity in the shape of particles with the size range of 40–800 nm, which are very different from the samples obtained by solid-state reaction. In addition, the surface area is about 2.3–4.5 m2g¡1, larger 4–10 times compared to the products by solid-state reaction.56,57,88 However, this method is still difficult to tuning the


morphology and structure of Bi2MoO6 by varying the reaction conditions.


3.3 Hydrothermal method As it is well known, control of the particle size, shape, crystal structure, and other structural parameters are necessary to achieve the rational design for photocatalytic application.87,97,98 Among the solution-based growth routes, hydrothermal and solvothermal methods have received a large amount of interest for various morphologies, different dimensional levels, high surface-tovolume ratios and permeability.99 In the reaction process, the initial nuclei were often used as building units under suitable solution conditions such as acidic or alkali solution system, so the influence of precursor dosage and reaction condition (time, pH value) on the growth of materials had to be considered.97,100 In addition, the surface of the building units could be first modified by the additional surfactants.41,50,55,101–105 The effect of different types of surfactant (non-ionic and ionic surfactants) could also be considered due to the facet-controlled fabrication or orient growth. Then, subsequent self-assembly, or “growth-then-assembly” process could induce the formation of various morphologies of nanostructures at different reaction time. To reduce the synthesis time, facile microwave-assisted routes were explored since microwave heating could supply an economical and energyefficient way to achieve rapid heating, faster kinetics, higher yield, and better reproducibility.106–108 Consequently, the photocatalytic performances are primarily dependent on the physicochemical properties and morphologies while the physicochemical properties of Bi2MoO6 are strongly dependent on the fabricated conditions. Therefore, we will discuss the influence of hydrothermal reaction parameters on the physicochemical properties of Bi2MoO6 in the flowing sections, in order to supply total guidance to achieve controllable fabrication from atom levels. 3.3.1 Reaction time The influences of reaction times were systematically explored by Kongmark and co-workers during hydrothermal process by in situ combined high-resolution powder diffraction, X-ray absorption spectroscopy, and Raman scattering experiments.109–111 They confirmed that the crystal formation occurred in two steps through the formation of an intermediate fluorite structure, but the nature and structure of the intermediary close to Bi2O3 could not be precisely determined.109 Then they were carried out on particles obtained at 180 C while varying the synthesis time.110 The formation of spherical particles was evidenced after 30 min, and plate-like

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particles of Bi2MoO6 were occurring for a longer duration. Furthermore, the kinetic analysis was carried out using data from the above three different characterization techniques. Consequently, a tentative scenario of Bi2MoO6 formation was proposed (see Figure 2). A distorted fluorite structure (Bi2O3), in which [MoO4] species are randomly distributed, forms in the initial stage of the reaction, in agreement with the need of [MoO4]2¡ ions in the solution. These species may then self-assemble, in a similar way as in L-Bi2nC4MonO6(nC1), before explosion into small strips that will serve as germs for the growth of g-Bi2MoO6 particles.111 Bi2MoO6 is built up by the intergrowth of fluorite-like [Bi2O2]2C and pseudo-perovskite layers [MoO4]2¡. It is inferred that the results could be of first importance for the catalyst activity. The similar growth mechanism of anisotropy, a characteristic lamellar-like growth habit, has also been observed in Bi2WO6 nanoplates,38,39 which is the same Aurivillius structure as well as Bi2MoO6. Nevertheless, Beale et al. revealed that Bi2MoO6 was formed directly from the amorphous gel mixture without forming any intermediate during the hydrothermal process.112–114 A common understanding of growth mechanism for Bi2MoO6 under hydrothermal process is very difficult to reach, consequently, a further investigation would be asked for in the future. 3.3.2 Reaction temperature Hydrothermal temperature is another predominant factor which affects the nuclei growth, crystalline, and morphology of Bi2MoO6.111–115 Generally, the temperature of fabrication for Bi2MoO6 under hydrothermal routes is below 200 C in order to obtain suitable sizes, good crystalline, and various morphologies. Beale et al. investigated the kinetic process of Bi2MoO6 under hydrothermal routes in the range of 140–160 C.112–114 They confirmed that the nuclei growth process at the initial reaction was dependent on the hydrothermal temperature due to the precursor diffusion. The temperature is lower, the nucleation rate is slower, and the crystalline is better. Moreover, Li and co-workers systematically investigated a wide temperature range from 100–240 C on the structures and photocatalytic activity of photocatalyst.115 They observed that the density of diffraction peaks increased with the hydrothermal temperature increasing from 100 C to 180 C, but the density decreased when the temperature continued to increase to 240 C. According to the “Ostwald ripening process” which is usually used to explain how a crystal is form, it could be concluded that the precursor initially formed spherical particles, and assemble nanobuilding blocks into the bigger particles and even larger nanoplates. If the temperature was too low, it is very difficult to form


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Figure 2. The characterization of Bi2MoO6 during the hydrothermal reaction at 180 C (a) HRPD patterns, (b) in situ Raman spectra, (c) Mo K-edge XANES spectra, and (d) proposed atomic-level models for different times. (© Royal Society of Chemistry. Reprinted with permission from Chanapa et al.109 Permission to reuse must be obtained from the rightsholder.) (© Elsevier. Reprinted with permission from Kongmark et al.110 Permission to reuse must be obtained from the rightsholder.) (© American Chemical Society. Reprinted with permission from Kongmark et al.111 Permission to reuse must be obtained from the rightsholder.)

the nuclei, but if the temperature was too high, the nuclei growth was so fast that the particles hardly assemble to bigger particles or nanoplates. It is the reason why the temperature is set at the range of 140–180 C under hydrothermal process. 3.3.3 pH values It is well known that the pH values is a very crucial parameter in the control of structures and morphologies of products prepared by the hydrothermal process.116–120 Zhou117 and Zhang118 reported that the Bi2MoO6 prepared by hydrothermal procedure at wide pH range (from 1–13) are found to be virtually pure g-Bi2MoO6. Nevertheless, the crystals formed nanosheets with exposed {010} facets at acidic conditions but grew along the [010] direction and formed 1D microrods at basic condition. Similar results were also observed in the synthesis of some tungstates.121 The formation process was described as follow: at

acidic solutions, there would be much more HC in the system to be adsorbed on the {010} facet with high density of oxygen atoms. Therefore, the surface energy of this facet would be markedly reduced. Thus, it resulted in the crystal growth rate along {010} orientation was slow and formed nanosheets with exposed {010} facet finally. Under the alkaline condition, the dominant anions OH¡ would prefer to adsorb on the {100} or {001} facet because of the low density of oxygen atoms, and lower the surface energy of these facets, subsequently prohibited the enlargement of {010} planes and drived the growth of nuclei along the [010] direction forming microrods.122,123 Moreover, they also found that the photocatalytic performances of nanosheets prepared at acidic condition were higher than microrods prepared at basic condition. They considered that the high photocatalytic activity could be related to the higher surface area and the large distortion of the Mo-O polyhedron in



crystal structure combing the results of BET, Raman spectrum, and FTIR spectra.118 It is taken for granted that photocatalysts with higher surface areas can absorb more organic molecules and tend to perform admirable photocatalytic activity. However, the Bi2MoO6 nanosheets prepared under different pH values with close surface area show distinctly different photocatalytic activity.117 Zhou et al. believed that the exposed {010} facets may contribute a lot to the enhanced photocatalytic activity. Considering amounts of photocatalysts with exposed high-reactive facets indeed exhibiting enhanced photocatalytic performance, e.g., {001}-facet-exposed anatase TiO2,124– 127 we also keep consistent with the enhanced photocatalytic performance mainly due to the high-reactive facets exposure. Unfortunately, theoretical DFT calculations have hardly been performed to model the structure as well as reactivity of high reactive facets of Bi2MoO6. The reactivity of high-reactive facets which are either Bi-terminated or O-terminated depends on their termination. The lack of theoretical works about Bi2MoO6 surfaces does not allow for correlating the enhanced photocatalytic performance with surface structure of Bi2MoO6. Hence, a number of theoretical DFT calculations would be asked for to be performed to model the high reactive facets of Bi2MoO6 and further study the mechanism for the high photocatalytic performance of the exposed highreactive facets. In addition, the pH value is also a key factor in the control of different compositions.128,129 Xie et al. reported that the samples prepared at pH value ranging from 0.4–9 are assigned to three compositions, Bi2MoO6 and Bi2O3 (pH lower 0.4), pure Bi2MoO6 (pH from 2–7), and Bi2MoO6 and MoO3 (pH excess to 9).49 Ren and coworkers observed that the samples prepared at pH 5 and 6 were pure Bi2MoO6 while the mix phases of Bi2MoO6 and Bi3.64Mo0.36O6.55 under pH from 6.6–7.0 and the pure Bi3.64Mo0.36O6.55 as the pH values increased to 9.128 The products synthesized by A. Phiruangrat at pH range from 2–12 were also classified into three main compositions: Bi2MoO6, Bi2MoO6/Bi4MoO9 composites, and Bi4MoO9.129 On the basis of these results, it can conclude that the acidic condition is favorable for the formation of Bi2MoO6, while the alkaline medium favors the formation of Bi3.64Mo0.36O6.55 or Bi4MoO9, which were formed after alkali etching process, similarly to the formation of Bi2O3/Bi2MoO6 heterojunction.130 The main effect of tuning pH value was to modulate the kinetics of nucleation and growth of the crystal by controlling experimentally the surface free energy. The preferential adsorption of molecules and ions in solution onto different crystal facets directed the growth of

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particles into various shapes and compositions by controlling the growth rates along different crystal axes. Which facet is the high reactive surface and how to control the high reactive surface exposure with high ratios, and how to tune the favorable ratios of different phases for the highest efficiency? These problems are complex technical issues to obtain the materials by tuning of the pH value of the reaction system to satisfy the needs as an excellent photocatalyst. Consequently, systematical and further investigations are urgent to do. 3.3.4 Surfactant As usual, the externally added surfactants or capping agents can stabilize particular crystal facets by surface induction so as to alter the growth rates in different crystal planes, resulting in the formation of anisotropic Bi2MoO6 nanostructures.41,50,55,101–103 According to the charge character of surfactant, two types surfactants are usually used, including non-ionic surfactant and ionic surfactant (cationic surfactant and anionic surfactant), respectively. Ying50 and Dong103 fabricated the Bi2MoO6 nanoplates in the presence of polyvinyl pyrrolidone (PVP). PVP, as a dispersant and size inhibitor, played an important role in the synthesis process. The inhibiting effect from PVP for the growth of crystallites promoted the generation of ultrathin shells and limited the size of the product in the Ostwald ripening process. For anionic surfactants, such as sodium dodecyl benzene sulfonate (SDBS) and sodium oleate usually used, the group of organic anion would react with Bi3C to for Bi3C-organic anion coordination complexes, make the energy of surfaces containing amount of Bi3C decrease, and form the nanoplates with exposure some facets.55,102 The similar mechanism of cationic surfactants that our group often used cetyltrimethylammonium bromide (CTAB), could also explain the formation of nanoplates.41,101 The cation would react with Mo4O132¡ to for Mo4O132¡-organic anion coordination complexes in order to minimize surface energy. The free polymer molecules preferentially absorbed on the primary nanoplates and functioned as potential crystal face inhibitors in the system, which formed polymer interlayers on the nanoplates and benefited the formation of oriented nucleation. Therefore, the face-inhibitor function of surfactants played important roles in the formation of sheet structures. 3.4 Solvothermal method Recently, the solvothermal route using alcohol or the mixture of alcohol and water as reaction medium, has been strongly developed. Hierarchical structures such as hierarchical flowers and hollow nanospheres have been extensively fabricated via hydrothermal solvothermal


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routes.62–66,131–134 In this process, large crystallites are essentially immobile while the smaller ones undergo mass relation through dissolving and regrowing by surface induction, which creates the interior space within the original aggregates. Recently, Tian et al. prepared TiO2 hierarchical flowers and TiO2 thorny tubes in the solvothermal system containing ethanol and glycerol using different titanium sources.133,134 Moreover, they also fabricated the hierarchical flower-like Bi2MoO6 hollow spheres via a solvothermal process in the presence of ethylene glycol (EG).63 Gou et al. also prepared the microspheres of mesoporous Bi2MoO6 by a templatefree solothermal process in the presence of ethylene glycol.64 In these reaction system, EG acted as a coordination agent to induce these aggregated nanoparticles on the surfaces of microspheres as seeds to grow into 2D plate-like structures for highly intrinsic anisotropic properties of Bi2MoO6. Meanwhile, the different morphologies such as nanosheets and nanorods have been also fabricated by the microwave-assisted solvothermal route.34,51,62 Consequently, Ostwald ripening via hydrothermal solvothermal routes, is thought as an effective strategy to prepare various hierarchical structures. We summarized the structures and morphologies of Bi2MoO6 through various fabricated preparations varying reaction conditions (see Figure 3).

4. Structure tuning for enhancement of visiblelight activity It is well-known that there are three typical processes mainly influencing the photocatalytic performances of

semiconductor photocatalysts including the generation of electron-hole pairs, the separation of the charge carriers, and the diffusion to the surface where they take part in the photocatalytic redox reactions. Since surface properties would greatly affect the above three processes, it is obvious that the properties of semiconductor surfaces and/or interfaces are very important for efficient photocatalytic reactions. Hence, it is of great importance to carry out investigations on structure tuning for improving the activity of photocatalyst and the utilization of solar energy. The quick recombination of photo-generated electrons and holes is a dominant limitation of achieving high photocatalytic efficiency, although Bi2MoO6 has extended the absorption to the visible light region (> 420 nm). It is demonstrated that the photocatalytic performances are greatly dependent on the properties of Bi2MoO6 surfaces. Therefore, effective approaches to retard the recombination of electrons and holes are taken to improve the photocatalysis efficiency of oxide-base semiconductors. Owing to this, several surface tuning strategies for Bi2MoO6 as efficient photocatalysts have been extensively studied. In this section, we will review two important surface tuning strategies of Bi2MoO6 including the hierarchical architecture and heterojunctional nanocomposites. 4.1 Hierarchical structure Because hierarchical structures possess large surface area, porous structure and are benefit to separate the photogenerated electrons and holes, the structures have drawn great attention to improve the photocatalytic activity of photocatalysts.131–134 Control of the dimension, porosity,

Figure 3. Summary of the structures and morphologies of Bi2MoO6 through various fabricated preparations varying reaction conditions. (© Elsevier. Reprinted with permission from Yin et al.62 Permission to reuse must be obtained from the rightsholder.) (© Royal Society of Chemistry. Reprinted with permission from Tian et al.63 Permission to reuse must be obtained from the rightsholder.) (© Elsevier. Reprinted with permission from Cruz & Alfaro89 Permission to reuse must be obtained from the rightsholder.) (© Elsevier. Reprinted with permission from Thang et al.92 Permission to reuse must be obtained from the rightsholder.) (© Royal Society of Chemistry. Reprinted with permission from Li et al.66 Permission to reuse must be obtained from the rightsholder.) (© Royal Society of Chemistry. Reprinted with permission from Ma et al.67 Permission to reuse must be obtained from the rightsholder.) (© Royal Society of Chemistry. Reprinted with permission from Zhang et al.72 Permission to reuse must be obtained from the rightsholder.)


and the pore size, distribution, shape and organization, are necessary to achieve the rational design for photocatalytic application. Up until now, the hierarchical architectures of solution-based Bi2MoO6 into various dimensional nanostructures with excellent properties including nanosheets, hollow spheres, flower-like structures and intercrossed nanosheets, has been reported in recent years.41– 67 In the section, we will introduce the advantages of different hierarchical structures for improving the photocatalytic activity and the related mechanisms.

properties.135–137 Taking anatase TiO2 as an example, its low-index facets, such as {001}, {101}, and {010} with well-defined geometric and electronic structures, were well exploited as the active spots for various types of reactions with enhanced photocatalytic activity.138,139 Besides TiO2, WO3, BiOCl, BiVO4, and Ag3PO4 have also been studied the facet-dependent photocatalytic properties.140–143 Bi2MoO6 nanoplates have been easily obtained through the hydrothermal process with or without surfactant.41,55,101,118,144 Yu et al. revealed that the nanoplate-structured Bi2MoO6 fabricated by hydrothermal method was more active than the bulk sample prepared by the solid-state method.41,50,101 Zhang and co-workers reported that


4.1.1 Nanosheets The nanoplate structures have the advantages of facetdependent photocatalytic and other surface-related

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Figure 4. The morphology of nanosheets and the atomic structures of exposed {001} and {010} facets. (© John Wiley and Sons. Reprinted with permission from Long et al.55 Permission to reuse must be obtained from the rightsholder.)

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carriers in the nanoplates can be more efficiently utilized. Undoubtedly, crystal facet engineering of semiconductors is an exciting direction with great potential to achieve highly active new-generation catalysts, especially the photocatalysts. However, because of the lack of sufficient theoretical and experimental investigations, it is difficult to obtain the structure-performance relationship of Bi2MoO6 from the atomic-level insight. Therefore, further theoretical and experimental studies in order to discover the facet-dependent photocatalytic properties are needed in the future.


the Bi2MoO6 nanosheets was 12 times higher photocatalytic activity than the microrod.118 Following, it was found that the Bi2MoO6 sheets with dominant {010} facets exhibited higher photocatalytic activity than the microrod-based microsphere under visible light irradiation (see Figure 4).55,144 The enhanced performance may be contributed to the advantages of facetenhanced photocatalytic characteristics we have discussed in the section of pH values, which is because that the direct exposure of more photoactive sites to the reaction substrates, and sequentially the charge

Figure 5. The morphology and photocatalytic activity of the hierarchical Bi2MoO6 hollow spheres. (© Royal Society of Chemistry. Reprinted with permission from Tian et al.63 Permission to reuse must be obtained from the rightsholder.)



4.1.2 Hollow spheres Compare to the nanosheets, the hollow spheres have exhibited higher visible light photocatalytic activity due to their porous structure, good permeability and large surface area.62–65,145 Recently, Bi2MoO6 hierarchical hollow structures have also been successfully synthesized (see Figure 5). Tian et al. reported that the hierarchical Bi2MoO6 hollow spheres exhibited excellent visible light photocatalytic efficiency for RhB degradation, which was much higher than that of Bi2MoO6 prepared by solid-state reaction and TiO2.63 The Bi2MoO6 microspheres prepared by Guo et al. have a larger surface area (about 50.86 m2g¡1) and a special porous structures and also shown superior performance for dye pollutant degradation in comparison to the P25 in the H2O2-containing solution.64 In addition, cagelike Bi2MoO6 hollow spheres have also exhibited much higher efficiency than Bi2MoO6 synthesized by SSR in the degradation of phenol under visible light irradiation.62 It has been demonstrated that the enhancement of photocatalytic activity could be attributed to the increase of the surface area providing more active catalytic sites and the hollow interior cavity enhancing the light-absorption and increasing the mobility of charge. 4.1.3 Flower-like structures As the similar Aurivillius oxides, Bi2WO6 has been fabricated to flower-like or nest-like structures and exhibited the enhanced photocatalytic activity.100,146,147 In recent two years, hierarchical flower-like Bi2MoO6 microstructures have also been fabricated by solvothermal process in the presence of ethylenddiamine.66 The flower-like microstructure was built from thin flakes with a thickness of about 20 nm and the nanoflakes were intercrossed with each other and aggregated together. These flake-like building blocks were constructed by many smaller spherical particles with a diameter of »15 nm. Through controlling experimental factors including reaction temperature, time and volume of ethylenediamine (EN), different morphologies were fabricated and obvious differences were obtained (see Figure 6). It was clear that the flower-like Bi2MoO6 microstructures have higher efficient visible light photocatalytic activity than the sphere and cage-like Bi2MoO6. The enhancement of photocatalytic performances might be attributed to the intercrossed hierarchical structures promoting the interfacial electron transfer process and facilitating catalytic reactions, the larger surface areas providing more active catalytic sites and the wider band gap enhancing its reductive and oxidative properties. 4.1.4 Inter-crossed structures Following the great development of synthetic technique, a template method, especially for the hard template synthesis, was an effective route to prepare hierarchical structures.

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At the beginning of the year, Ma et al. fabricated 1D Bi2MoO6 intercrossed frameworks by using MoO3 nanobelts as the growth templates and molybdate source.67 Especially, the novel Bi2MoO6 intercrossed frameworks show remarkably enhanced photocatalytic activity toward the organic dyes photocatalytic degradation, which far exceeded that of conventional Bi2MoO6 nanoparticles and nanoplates. From the photoelectrochemical study, the enhancement of photocatalytic performance was contributed to the intercrossed frameworks which can facilitate the photoinduced charge separation and transfer. The schematic illustration was shown in Figure 7. Comparison of structures and performances with different hierarchical structures, it can be concluded that no matter what morphology of Bi2MoO6 is, the enhancement of photocatalytic activity would be obtained if only the hierarchical structures of Bi2MoO6 fabricated have larger surface area for more active sites formation and more reactant adsorption and inter-crossed or porous structures for preventing the recombination of photo-excited electron-hole pairs. Certainly, no matter which morphology Bi2MoO6 was fabricated, the photocatalyst reacted with organic dyes is still only Bi2MoO6 material. So the photocatalytic mechanism for Bi2MoO6 is identical for various hierarchical structures. Based on the results of photocatalytic experiments, the mechanism for the photo-degradation of the dye pollutant was proposed. The flat-band potential of Bi2MoO6 has been determined to be ¡0.32 eV vs. NHE at pH D 7.0,83 which is more negative than the redox potential of ¢O2¡/O2 (0.28 eV).148 So the electrons can react with the adsorbed O2 to form active oxygen species ¢O2¡ radicals. According to the equation of EVB D ECB C Eg, the edge of the valence band of Bi2MoO6 is estimated to be C2.33 eV. However, for different understanding to the oxidation of ¢OH/H2O, there are three different explanations for dyes degradation of photocatalysis. Some researchers consider the redox potential of ¢OH/H2O is to be C1.23 eV (it is the potential of O2/H2O in fact), which is more negative than the edge of the valence band of Bi2MoO6. So the holes can react with the adsorbed H2O to form active oxygen species ¢OH radicals.55,144 Thus, the oxidative ¢O2¡ and ¢OH radicals are predominantly responsible for the dye pollutant degradation. The responsible/ reactions were shown below: hv

Bi2 MoO6 ! e ¡ C h C C

h C H2 O ! OH C H e ¡ C O2 ! O2¡

(1) C

OH 6 O2¡ C RhB ! CO2 C H2 O

(2) (3) (4)

However, others reported that the potential of ¢OH/H2O (C2.68 eV in theory) is higher than the

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Figure 6. The comparison of flower-like Bi2MoO6 microstructures on the morphology (A-C), absorption (D), surface areas (E), and photocatalytic activity (F–G). (© Royal Society of Chemistry. Reprinted with permission from Li et al.66 Permission to reuse must be obtained from the rightsholder.)

edge of the valence band of Bi2MoO6, so the adsorbed H2O cannot be oxidized directly into ¢OH radicals by holes.64,71 Nevertheless, the ¢OH radicals have been detected on the Bi2MoO6 surfaces under visible light irradiation.64 The yield of ¢OH from ¢O2¡ with the assistance of photo generated electrons in the presence of H2O2 was also reported.149,150 Consequently, they considered that the photocatalytic performances

were attributed to the presence of H2O2 which could suppress the recombination of electron-hole pairs and increase the concentration of ¢OH or ¢OOH (see Figure 8).64,71 Another one thought the degradation of RhB may be directly oxidized by the photogenerated holes just like Bi2WO6.86 Therefore, it is concluded that the widely accepted mechanisms are still further to come to agreement.

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Figure 7. The schematic illustration of photodegradation of Bi2MoO6 intercrossed frameworks. (© Royal Society of Chemistry. Reprinted with permission from Ma et al.67 Permission to reuse must be obtained from the rightsholder.)

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Figure 8. Schematic representations of one mechanism of the dye pollutant (RhB) on Bi2MoO6 surfaces. (© Elsevier. Reprinted with permission from Zhang et al.51 Permission to reuse must be obtained from the rightsholder.)

Although great successes have been achieved in controls of hierarchical architectures from one-dimension to multidimensions, from bulk structures to hollow architectures with various morphologies including sphere, flower-likes, and intercrossed framworks, and the enhanced photocatalytic performances have obtained, there are still much challenge to face in the controllable modification of the hierarchical structure. One of major challenge for hierarchical architectures synthesis process is the lack of in situ metrology to characterize the kinetics and structures. Meanwhile, a fine structure-tuning of hierarchical properties related with the photocatalytic performance, including the morphology, porosity, and active sites, is hardly difficult to achieve, especially, how to synthesize the desired hierarchical architectures according to the demand of photocatalytic application is the most important issue.

4.2 Heterojunctional nanocomposites In general, a valuable photocatalyst should have a wide enough band gap to provide energetic electrons, highefficiency separation of the photogenerated electrons and holes, and also substantial inhibition towards photocorrosion. Furthermore, a small enough band gap to allow for efficient absorption overlap with the solar spectrum is also necessary for the photocatalyst to effectively utilize solar energy. However, any kind of single component is unlikely to satisfy all the critical requirements. Fortunately, heterojunctional structures with multiple integrated functional components could combine the advantages of different components to overcome drawbacks of single component. Therefore, various heterojunctional structures of Bi2MoO6 have vigorously developed.68–75

In comparison with hierarchical structures, heterojunctional structure is another efficient approach to improve the charge separation in photocatalysis. The presence of another component can not only extend the absorption of solar energy from UV to visible light, but also improve the migration rate of electron on the surface. Moreover, the contacts between two components could contribute to an effective separation of electronholes pairs, minimizing the energy dissipating electronhole recombination so as to be favorable for high photocatalytic activity. According to the reported literatures, along with our works, we will discuss some heterojunctional structures of Bi2MoO6, and the different mechanisms will be presented. 4.2.1 Semiconductor/Bi2MoO6 heterojunctional composites Semiconductor/Bi2MoO6 heterojunctions are becoming a popular strategy to enhance the photocatalytic activity because of their tunable light absorption and the effective inhibition of the recombination of photogenerated electron and hole pairs.87,151 Up until now, various semiconductor/Bi2MoO6 heterojunctions, including TiO2,61,68 BiOX,69,70 Bi2O2CO3,152 Ag3PO4,153,154 and 130,155,156 have been reported to be successfully fabBi2O3, ricated. The photoactivity of BiOCl/Bi2MoO6 heterojunctions were two times higher for the RhB degradation and 1.5 times higher for the phenol photodegradation compared to pure Bi2MoO6.69 Heterostructured Bi2O2CO3/Bi2MoO6 nanocomposites were reported with higher activity and stability for the photocatalytic reactions.152 Xu et al. reported that Bi2O3/Bi2MoO6 microspheres shown good properties of antimicrobial effect and the decomposition of RhB under visible light excitation.150 Combining the p-type Bi2O3 together with the n-type Bi2MoO6 can greatly inhibited the



15

Figure 9. Schematic illustration of photocatalytic mechanism for Ag3PO4/Bi2MoO6 heterojunctions. (© Royal Society of Chemistry. Reprinted with permission from Xu & Zhang153. Permission to reuse must be obtained from the rightsholder.)

recombination of photoinduced carriers by the formation of inner electric field in the p-n junction. In addition, hierarchical heterostructures have also received attention. One-dimensional TiO2/Bi2MoO6 hierarchical heterostructures possessed a much higher degradation rate of RhB than the unmodified TiO2 and Bi2MoO6 under UV and visible light.61,68 From the above numerous publications, they consistently considered that the observed improvement in photocatalytic performance was contributed to the extended absorption in the UV and visible light region and the effective separation of photogerated carriers at the interfaces. A common mechanism was proposed as presented below. Upon light excitation, the photogenerated electrons from the component with more negative conduction band will flow into the other one with the lower conduction band. At the same time, the photogenerated holes from the semiconductor with a lower valence band will jump to the one with a higher valence band (Figure 9). The band offset can efficiently promote the charge separation, inhibit the charge recombination and thus enhance the photocatalytic performance.87 Take Ag3PO4/Bi2MoO6 heterojunctions as an example.153 According to the publications, the valence band of Ag3PO4 is located at C2.9 eV vs NHE,157 which is larger than the potential of ¢OH/H2O (C2.68 eV). So the holes can directly oxidize the adsorbed H2O to ¢OH in Ag3PO4. The CB of Bi2MoO6 is lower than the potential

of ¢O2¡/O2 (0.28 eV),150 so the electrons from Bi2MoO6 can react with dissolved O2 to ¢O2¡. The results of radical scavengers revealed that the hole and radicals (¢OH and/or ¢O2¡) were the main reactive species for the degradation of dye pollutant. 4.2.2 Carbon or graphene-like/Bi2MoO6 heterojunctional composites Various carbon allotropes, including 0D fullerenes, 1D carbon nanotubes, and 2D graphene, have been widely used in photocatalysis and heterogenous catalysis because of large electron-storage capacity and good electrical conductivity.158 Due to the special electronic properties of carbon materials, carbon/Bi2MoO6 heterojunctional composites have been drawn more attention recently.60,159,160 The fascination with carbon/Bi2MoO6 heterojunctional composites originates from their unique structural and the above electronic properties, which could promote the mobility of the photo-generated electrons and holes and retard the recombination of the photo-generated carriers so that result in the photocatalytic efficiency enhancing obviously. For example, C60 and carbon nanofibers modified Bi2MoO6 shown the superior photocatalytic properties than that of the pure component under the visible light excitation.60,160 The enhanced photocatalytic activity may be attributed to the extend absorption in the visible light region because of the Bi2MoO6 nanosheet, and the effective separation of the photogenerated electrons and hole pairs at the heterojunction interface.


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Figure 10. The morphology, structure, and photocatalytic activity of graphene/Bi2MoO6 heterojunctional composite. (© Royal Society of Chemistry. Reprinted with permission from Zhang et al.72 Permission to reuse must be obtained from the rightsholder.)

Recently, graphene has also been used in the photocatalytic area due to its special 2D sp2-hybridized carbon structure, including large specific surface area, high intrinsic electron mobility and good mechanical strength.87,158 Therefore, it is an excellent candidate for fabricating graphene/Bi2MoO6 heterojunctional composite photocatalysts. Our group prepared graphene/Bi2MoO6 heterojunctional composite by a facile

hydrothermal method with good uniformity and highly oriented growth (see Figure 10).72 Reduced graphene oxide was observed to be formed on the surface of Bi2MoO6 nanoplates. Compared to the pure Bi2MoO6 nanoplates, a remarkable enhanced visible light photocatalytic destruction of bacteria was demonstrated over the graphene/Bi2MoO6 heterojunctional composite. In addition, the graphitic carbon nitride (g-C3N4) has also

Figure 11. The postulated mechanism of the visible light induced photodegradation of RhB with carbon/Bi2MoO6 heterostructures. (© Royal Society of Chemistry. Reprinted with permission from Zhang et al.60 Permission to reuse must be obtained from the rightsholder.)


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Figure 12. The morphology and the possible mechanism of Ag–AgBr–Bi2MoO6 heterogenous composites. (© John Wiley and Sons. Reprinted with permission from Tian et al.75 Permission to reuse must be obtained from the rightsholder.)

attracted a greatly scientific interest in photocatalysis due to the similar properties to graphene. Yan et al. reported the C3N4/Bi2MoO6 heterojunctions exhibited higher photocatalytic activity than pure C3N4 and Bi2MoO6 towards the degradation of dye pollutants (RhB and MB) in the visible light region.71 From the above results, it can be concluded that the enhanced photocatalytic performance of carbon or graphene-like/Bi2MoO6 heterojunctional composites is ascribed to the mainly three factors. First, the formation of the heterojunction enhanced charge separation and transfer efficiency. Second, Bi2MoO6 component extended the absorption to the visible light region. The last one, the large surface area increased absorption of reactants and products. The postulated mechanism of carbon or graphene-like/Bi2MoO6 heterojunctional composites was shown in Figure 11 (taking carbon nanofibers modified Bi2MoO6 as an example). According to the values of potential, the photogenerated holes obviously

Figure 13. Schematic diagram of gradient charge transfer at Bi2MoO6/Bi2WO6 ternary heterojunction. (© Elsevier. Reprinted with permission from Zhang et al.74 Permission to reuse must be obtained from the rightsholder.)

cannot oxidized the absorbed H2O, so the degradation of RhB was attributed to the hole (hC) and ¢O2¡ species. 4.2.3 Multiple functional components Compared to single or two component heterojunctional photocatalysts, multiple functional components could offer more benefits for promoting the separation of photoinduced electrons and holes, and extending the light absorption range, and also other special properties. Design of multiple heterojunctional composites with suitable structures would provide more chance than single Bi2MoO6 to attain high-efficiency photocatalysts.74,75,161–163 For example, Tian and co-workers prepared hierarchical Ag/AgBr/ Bi2MoO6 heterogenous composites by using the prepared hierarchical Bi2MoO6 hollow spheres as the supporting material.75 The hierarchical Ag/AgBr/Bi2MoO6 composite exhibited higher photocatalytic activity than both photocatalysts containing single visible light-active components and crushed Ag/AgBr/Bi2MoO6 particles. The reason for the enhanced activity was attributed to the formation of effective nanojunctions, which led to low recombination rates of the electrons and holes generated by light irradiation. Ag in the Ag/AgBr/Bi2MoO6 heterogenous composites acted as storage and/or recombination centers for conductor band electrons of Bi2MoO6 and valence band holes of AgBr, and contributed to the enhancement of interfacial charge transfer and separation. The possible mechanism maybe is the Z-scheme bridge (see Figure 12), which also has been proved in Au/CdS/TiO2,161 Ag/AgBr/ Bi2WO6,162 and Ag/AgBr/TiO2 systems.163 In addition, Zhang et al. synthesized the Bi2MoO6/ Bi2WO6 heterojunction, which exhibited desirable combinations of their photocatalytic activities and much higher than the individual component.75 Surprisingly, it was found that the heterojunction photocatalytic performance could

Photodegradation of RhB61 Photodegradation of MO and phtocatalytic oxygen evolution68

Bi2MoO6 heterojunctional composites

Bi2S3/Bi2MoO6

Bi2O3/Bi2MoO6 Photodegradation of RhB130,155,156

Photodegradation of phenol69

Photodegradation of RhB

69,70

TiO2/Bi2MoO6

Semiconductor/

BiOX/Bi2MoO6

Photodegradation of RhB67

inter-crossed structures

Irregular multi-plate structure (100 nm–1 mm), thickness (20 nm) Reaction rate (0.022 min¡1) Photocurrent (0.3–0.4 mA/cm2) Photodegradation efficiency (95% within 120 min) Reaction rate (0.0035 min¡1) Band gap (2.62–2.71 eV) Nanosheets morphology Surface area (17.72 m2g¡1) Photocurrent (0.055 mA/cm2) Photodegradation efficiency (98% within 100 min) Band gap (2.90 eV)

Band gap (2.38–2.93 eV) Nanoplates thickness (10 nm) and widths (40–60 nm) Surface area (10–40 m2g¡1) Reaction rate (0.022 min¡1) Photodegradation efficiency (92% within 5 h) Photodegradation efficiency (90.4% within 80 min) Photocurrent (0.7 mA/cm2) Oxygen evolution rate (0.668 mmol h¡1 g¡1) Band gap (2.58–2.95 eV)

Photodegradation of RhB64,66

Nanoplates thickness (15 nm) and widths and lengths (100–200 nm) Surface area (40 m2g¡1) Reaction rate (0.0218 min¡1) Photodegradation efficiency (95–100% within 120 min) Reaction rate (0.0049 min¡1) Photodegradation efficiency (40% within 120 min) Band gap (2.50–2.60 eV) Nanoplates thickness (20 nm) and widths and lengths (100–200 nm) Surface area (60 m2g¡1) Reaction rate (0.09 min¡1) Photodegradation efficiency (100% within 20 min) Nanobelts widths and lengths (120 nm, tens of micrometers, respectively) Surface area (44.64 m2g¡1) Photodegradation efficiency (100% within 30 min)

Oxygen evolution rate (15 mmol h¡1) Average size (50 nm) Surface area (18.75 m2g¡1) Band gap (2.60 eV)

Surface area (3.5 m2g¡1)

Band gap (2.70 eV)

Performance or parameter

Photodegradation of phenol62

Photodegradation of Rhodamine B (RhB)63–65

flower-like structures

hollow spheres

Oxygen evolution reaction (OER) in AgNO3 solution41,101 Photodegradation of Methylene Blue (MB)55,

nanosheets 144

Photocatalytic system

Structures

Table 1. Summary of Bi2MoO6 with various hierarchical structures and their heterostructures for photocatalytic application.


Heterojunction structures facilitate the separation of photogenerated electrons and holes.

Heterojunctions promote the separation of the electron–hole pairs. The .OH radical is the main active species during degradation..

Unique inter-crossed structures enhance the visible light absorption, facilitate the rapid transfer of organic molecules, improve the effective separation of photoexcited electron–hole pairs and decrease the probability of electron-hole recombination. Bi2MoO6 act as a sensitizer to absorb the visible light. The heterostructure can prevent the recombination. Meanwhile, the radical anions (O2¡ and HO) form.

A larger surface area, and better intercrossed hierarchical structures and the ¢OH radicals play a critical role

Hierarchical flower-like hollow structure, good permeability, large surface area and better crystallization the ¢OH radicals play a critical role

Large surface area and nanosheets structure with more reactive sites High reactive facet {010} exposure

Activity origin or mechanism

18 Y. PENG ET AL.

Multiple functional components

Carbon or graphenelike/Bi2MoO6 heterojunctional composites

Photodegradation of alizarin red S (ARS) and phenol75

Ag/AgBr/Bi2MoO6

C3N4/ Bi2MoO6 Photodegradation of RhB and MB71

Photocatalytic destruction of bacteria

Photocatalytic inactivation efficiencies (100% 3 h) Microspheres composed of nanosheets Surface area (30–35 m2g¡1) Reaction rate (0.012–0.065 min¡1) Photodegradation efficiency (98% within 60 min) Hierarchical floriated hollow spherical superstructure Wide adsorption peak around 490–700 nm Reaction rate (0.04908 min¡1) Photodegradation efficiency (89% within 60 min)

Thickness (20 nm) Surface area (43.18 m2g¡1) Photodegradation efficiency (87% within 6 h) A platelet particle morphology with sizes ranging from 0.2–1.0 mm Charge transfer resistance decreased

Photodegradation of RhB60,160 Graphene/Bi2MoO6

Carbon/Bi2MoO6

72

Surface area (30 m2g¡1) Reaction rate (0.0592 min¡1) Photodegradation efficiency (100% within 70 min) Nanofibers with hierarchical heterostructures

Photodegradation of RhB and MB153,154

Ap3PO4/Bi2MoO6

Flower-like superstructures (1–3 mm), thickness (15 nm) Surface area (74.9 m2g¡1) Reaction rate (0.0643 min¡1) Photodegradation efficiency (100% within 60 min) Individual flowerlike sphere Thickness (10 nm)

Photodegradation of parachlorophenol156


Their strong visible light absorption, the high migration efficiency of photo-induced carriers, and the interfacial electronic interaction. Migration efficiency of the photoinduced carriers at the interface of the composite and the enhanced efficiency of photoharvesting owing to the stable deposition of Ag/AgBr nanoparticles and the formation of effective nanojunctions.

Bi2MoO6 has a higher charge carrier mobility and provided the pathway for the transport of charge carriers. And graphene has a superior electrical conductivity.

The extended absorption in the visible light region and the effective separation of the photogenerated carriers at the Bi2MoO6– CNF interface.

The extended absorption in the visible light region and the effective separation of photogenerated carriers at the Ag3PO4/ Bi2MoO6 interfaces. The radical of hC and _OH were the main reactive species for the degradation.

CRITICAL REVIEWS IN SOLID STATE AND MATERIALS SCIENCES 19


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be remarkable enhanced after being annealed at 500 C, which was ascribed to form a novel Bi2WxMo1-xO6 thin layer at the Bi2MoO6/Bi2WO6 interface. Then, the heterojunction was converted to a ternary heterojunction. A highefficient gradient charge transfer was observed from Bi2MoO6 to Bi2WxMo1-xO6 to Bi2WO6 (see Figure 13). Generally, the fabrication of binary or multiple functional heterojunctions includes the following step. The main materials were firstly prepared to act as supporters, and then the other components were controlled to grow on the surfaces of host materials to fabricate the heterojunctions. Thus, the most important point for the preparation of heterojunctional composites are the surface controlled growth of the functional component since the controlled growth can guarantee the good distribution on the main solid body and then induce the formation of effective heterojunctional photocatalysts. We summarized the various hierarchical structures and heterostrcutures of Bi2MoO6 for photocatalytic application and clear comparison would be reflected (see Table 1).

5. Conclusion and outlook It can be seen that an increase in research activity on the enhancement of photocatalytic performance of Bi2MoO6 under visible-light irradiation over the past decades. This review has covered several effective and important structure tuning strategies for Bi2MoO6-based photocatalysts, especially for hierarchical structures and heterjunctional nanocomposites. As discussed above, the structure-tuning approaches have been shown to significantly improve the photocatalytic performance by prolonging the charge carrier lifetime and retarding the photogenerated charge separation. For many metal oxide semiconductors, the quick recombination of charge carriers is still an important question that limits greatly the photocatalytic performance. We first addressed the controllable synthesis and structure modification of Bi2MoO6 with different strategies. This is of great significance not only for Bi2MoO6based photocatalysts for enhancement of photoactivity, but also for other metal oxide photocatalysts for structure modification from the practical application view. However, the studies on Bi2MoO6-based visible-light-active photocatalysts with controllable synthesis and structure tuning are still in the early stages and a number of challenges need to be overcome in the years ahead. (i) Proper control in the physicochemical properties by the different synthetic methodology and reaction factors, including reaction time, reaction temperature, pH values, the surfactant, etc., has provided opportunities to controllably fabricate Bi2MoO6-based visible-light-active photocatalysts. However, non-agglomerated state, controlled

crystal size and tunable phase content, and largescale and low-cost synthetic processes are still the challenges for controllable synthesis to be overcome. (ii) The enhanced photocatalytic activity of Bi2MoO6 has been achieved in hierarchical architectures with various dimensional morphologies, such as nanofibers, nanorods, and nanosheets, even complex hollow sphere, flower-like, and intercrossed frameworks. Consequently, it can be concluded that the enhancement of photocatalytic activity would be obtained if the hierarchical structures greatly increased the surface area and prevented the recombination of photo-excited electron-hole pairs. Therefore, the hierarchical structures with high reactive facets exposure and larger surface area would be the favorable in the photocatalytic application. Nevertheless, the facet-dependent photocatalytic properties possessing greatly attractive potential are still a challenge due to the lack of sufficient theoretical and experimental results. In addition, how to synthesize the desired hierarchical architectures according to the demand of photocatalytic application is the most important issue. (iii) The heterojunction composites of Bi2MoO6 with semiconductors, carbon and graphene-like materials, and multiple functional components have solved the recombination of photogenerated electrons and holes in some degree and enhanced the photocatalytic activity. However, simple fabrication of effective-contacted heterojunctional nanocomposites to guarantee the good distribution on the main solid body for photocatalytic application in practice is still a challenge for practical applications.

Acknowledgment The authors are grateful to many of their colleagues for constructive discussion.

Funding The authors acknowledge the financial support from China Postdoctoral Science Foundation (No. 2014M551869), Shandong Excellent Young Scientist Research Award Fund (No. BS2015CL002), and Qingdao Postdoctoral Application Research Project Fund.

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Structure Tuning of Bi2MoO6 and Their Enhanced

Structure Tuning of Bi2MoO6 and Their Enhanced

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