ReviewBismuth oxyhalide layered materials for energy and environmental applications
Graphical abstract
Introduction
Photocatalysis is regarded as one of the most promising approaches for solving energy and environmental issues by the efficient utilization of solar energy [1], [2], [3], [4]. Hydrogen (H2) can be prepared through water splitting, and carbon-based fuels can be obtained by the photoreduction of CO2 with the consumption of only inexhaustible solar energy. The key requirement for achieving a high-efficiency photocatalytic process is the rational design of efficient photocatalysts [5], [6], [7], [8], [9]. Two-dimensional (2D) layered materials with suitable bandgaps may represent ideal photocatalytic materials to address the energy issue [10]. 2D layered materials such as nitrides (e.g. h-BN, g-C3N4) [11], [12], dichalcogenides (e.g. MoS2, WSe2) [13], trichalcogenides (e.g. TiS3, In2S3) [14], mono-chalcogenides (e.g. GeSe) [15], thiophosphates (e.g. FePS3, NiPS3) [16], and halides (e.g. InBr3, VI2) are of great interest owing to their extensive technological promise and potential applications ranging from electronics to catalysis [17], [18], [19], [20], [21]. These layered materials are composed of continuous layers of covalently bonded atomic-layer planes, separated successively by van der Waals gaps. Bismuth oxyhalides, BiOX (X = Cl, Br and I), a new class of promising layered materials for photocatalytic energy conversion and environmental remediation, have been intensively investigated [22], [23], [24]. They belong to the family of main-group multicomponent metal oxyhalides V-VI-VII, which crystallize in a tetragonal matlockite (PbFCl-type) structure. They are chemically stable, nontoxic, and corrosion-resistant. The crystal structure of BiOX is built by interlacing [Bi2O2] slabs with double halogen slabs to form a layered structure (Fig. 1). The intralayer atoms are connected by strong covalent bonding, and a weak van der Waals interaction exists among the interlayers. The open-layer crystalline structure enables enough space to polarize the related atoms and orbitals to form inherent internal static electric fields along the crystal orientation perpendicular to the [Bi2O2] and [X] slices [25]. The static electric fields that formed can promote effective separation of photogenerated electron-hole pairs, a key factor in photocatalysis that is responsible for the superior photocatalytic activity of BiOX [26]. Density functional theory (DFT) calculations show that the valence band (VB) maximum of BiOX primarily consists of hybrid O 2p orbitals and X np (n = 3, 4, and 5 for X = Cl, Br, and I) orbitals, whereas the conduction band (CB) minimum mainly consists of Bi 6p orbitals [27].
Unlike TiO2, which is active only in the ultraviolet (UV) region, BiOX possesses a variable bandgap of ~ 3.3 eV for BiOCl, 2.7 eV for BiOBr, and 1.8 eV for BiOI, enabling it to be a type of light-active photocatalyst. The unique characteristics of BiOX make it a very promising photocatalyst for various applications. Since Zhang's group [28] reported 3-dimensional (3D) hierarchical BiOX microspheres assembled with 2D nanoplates via the ethylene glycol (EG)–assisted solvothermal method for the effective degradation of methyl orange (MO), BiOX-based photocatalysis has become a hot research topic [29]. To date, various BiOX micro/nanostructures have been prepared, such as nanocrystals, nanowires, nanofibers, nanobelts, nanosheets, nanoplates, thin film, flower-like hierarchical structures, hollow microspheres, and porous nanospheres [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51]. To enable potential industrial applications, numerous attempts have been made to further improve the properties of BiOX by increasing the light harvesting, promoting the separation and transfer of photogenerated charge carriers, and boosting the catalytic efficiency between the active species and other substances in BiOX materials. The obtained BiOX materials display outstanding performance for photocatalytic applications such as pollutant removal [28], oxygen evolution [52], hydrogen evolution [53], CO2 reduction [54], nitrogen fixation [55], selective organic synthesis [35], and disinfection [56].
Herein, we present a comprehensive overview of recent advances in the design, regulation, and energy and environmental applications of BiOX-based materials. We start with a synthetic method of making new BiOX nanostructures. Then, regulating approaches for tuning the photocatalytic performance of BiOX-based materials are thoroughly discussed, ranging from a bismuth (Bi) -rich strategy, to elemental doping and defect control, interface engineering, solid solution and inner coupling, and heterojunction construction. This review also introduces different applications of BiOX-based materials in the fields of photocatalytic oxygen evolution, photocatalytic hydrogen evolution, CO2 photoreduction, photocatalytic nitrogen fixation, photocatalytic organic syntheses, photocatalytic disinfection, and photocatalytic removal of pollutants. Finally, concluding remarks and perspectives on the future exploration of BiOX-based materials are presented.
Section snippets
Synthetic methods for BiOX-based materials with engineered structure
The main approaches to making BiOX micro/nanostructures include the hydro/solvothermal [57], [58], precipitation [59], calcination [60], reverse microemulsion [29], microwave [61], template [30], and sonochemical methods [62]. Various BiOX micro/nanostructures such as 1D nanorods/wires, 2D nanoplates/sheets, and 3D hierarchical architectures have been prepared using these methods. Generally, BiOX materials are inclined to form a 2D nanosheet structure owing to the inherent layering
Bismuth-rich strategy
The photocatalytic ability of a semiconductor depends on the band edge potentials of the semiconductor and the oxidation-reduction potential of the adsorbed species. From the viewpoint of thermodynamic requirements for photocatalytic reactions, the equilibrium potential energy of the acceptor species should be lower (more positive) than that of the CB, and the equilibrium potential energy of donor species should be higher (more negative) than that of the VB. The redox potentials of the VB and
Photocatalytic oxygen evolution
Photocatalytic water splitting is regarded as a promising approach for addressing energy and environmental issues. In most cases, the oxygen evolution reaction is the bottleneck for photocatalytic overall water splitting owing to the complex four-electron redox process. The oxygen evolution reaction is an absolute hole-participating reaction, requiring that the VB edge be located at a more positive position than the potential of the oxidation half reaction. Recently, photocatalytic oxygen
Conclusions and perspective
BiOX materials are a class of promising photocatalysts for versatile energy and environmental-related applications because of their unique layered structure, tunable electronic structure, and excellent physicochemical properties. However, pristine BiOX typically has several inherent drawbacks, such as small specific surface area, low efficiency of light harvesting, lack of catalytic active sites, and high probability of recombination of photogenerated electron-hole pairs. To overcome these
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 21576123, 21476098, 21471069 and 51671003), the National Key Research and Development Program of China (No. 2016YFB0100201), the Doctoral Innovation Fund of Jiangsu Province (KYZZ16_0340) and the International Postdoctoral Exchange Fellowship Program of China Postdoctoral Council (No. 20150060). S. D. was sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic
Jun Di received his BS degree from Jiangsu University in 2012. He is a Ph.D. candidate under the supervision of Professor Huaming Li at Jiangsu University and currently a joint Ph.D. candidate under the guidance of Professor Zheng Liu at Nanyang Technological University. His research interests focus on design and synthesis of 2D materials for energy conversion.
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Jun Di received his BS degree from Jiangsu University in 2012. He is a Ph.D. candidate under the supervision of Professor Huaming Li at Jiangsu University and currently a joint Ph.D. candidate under the guidance of Professor Zheng Liu at Nanyang Technological University. His research interests focus on design and synthesis of 2D materials for energy conversion.
Jiexiang Xia received his BS and Ph.D. degrees in 2006 and 2011 (with Professor Huaming Li), both from the Jiangsu University, China. He is an associate professor at the Jiangsu University and carrying on postdoctoral research at the Oak Ridge National Laboratory (USA). He has published more than 110 papers with over 3900 citations (h-index: 35). His research interests include synthesis of ionic liquids, development of functional materials for energy and environmental applications.
Huaming Li received his BS degree from China West Normal University in 1985 and master degree from Chinese Academy of Sciences in 1992. He is a full professor at the Jiangsu University. His current research focuses on nanomaterials and ionic liquids for energy and environmental applications. He is the author and co-author of 310 original research papers published in SCI journals with 7600 citations and an h-index of 50.
Shaojun Guo is currently a Professor at Peking University. He received his Ph.D. in analytical chemistry from Chinese Academy of Sciences (2011) with Profs. Erkang Wang and Shaojun Dong, and joined Prof. Shouheng Sun's group as a postdoctoral research associate from Jan. 2011 to Jun. 2013 at Brown University. In 2014, 2015 and 2016, he was selected by Thomson Reuters into their prestigious list of World Most Highly Cited Researchers. He has published more than 190 papers with over 14,500 citations (h-index: 65). His research interests are in engineering nanocrystals and 2D materials for catalysis, renewable energy, optoelectronics and biosensors.
Sheng Dai obtained his Ph.D. (1990) in Chemistry at the University of Tennessee, Knoxville. He is currently a corporate fellow (the highest designation a researcher can receive at ORNL) and group leader at the Oak Ridge National Laboratory (ORNL) and a Professor of Chemistry at the University of Tennessee, Knoxville (UTK). His current research interests include ionic liquids, porous carbon and oxide materials, advanced materials and their applications for separation sciences and energy storage as well as catalysis by nanomaterials. He has published over 600 peer-reviewed journal papers with more than 26,000 total citations and an h-index of 82.