ReviewDesigning and modification of bismuth oxyhalides BiOX (X = Cl, Br and I) photocatalysts for improved photocatalytic performance
Graphical abstract
Road map of the BiOX photocatalysts.
Introduction
There is a global threat of rapid depletion of non-renewable fossil fuels because of huge demand for energy to sustain the world's economy and ever-increasing population. The combustion of fossil fuels meets approximately 85 percent of the world's overall energy consumption. The excessive use of fossil fuels is adversely affecting our ecosystem. To address the energy and environmental concerns, it is critical to develop environmental friendly alternatives for these non-renewable fossil fuels [1]. Photocatalysis is an advanced oxidation process used to produce clean hydrogen energy. Hydrogen is a fuel with a high energy density (143,000 J/kg) that contributes zero to global warming [2]. Heterojunction photocatalysts have gotten a lot of attention because of their prospective use in green energy production, pollutant degradation, CO2 reduction, etc [2], [3]. The stability and recyclability of heterogeneous photocatalysts is also much better than homojunction photocatalysts. Thermal stability is another benefit of heterogeneous photocatalysts over homogeneous photocatalysts. The photocatalytic materials are categorized into metals with only UV light absorption, metals with visible light absorption, 2D metals and non-metals with distributed optical absorption in the visible region [3]. The primary limitation of the first class of photocatalytic materials is their low sensitivity to visible light. Since their optical band gap falls in UV region of solar spectrum, they are not efficient photocatalysts under sunlight for production of hydrogen from water and other solutions. This issue can be delt with second family of photocatalytic materials, however these materials did not show high hydrogen evolution efficiency due to high charge transfer impedance, rapid recombination rate of photoinduced electron-hole pairs and poor stability in the aqueous mediums [4]. The first two families mostly consist of oxides, sulfides and nitrides while 2D materials of third family include single layer or many layers materials with unique structural, electronic and optical properties.
The 2D layered materials, such as nitrides [5], halides [6], monochalcogenides [7], dichalcogenides [8], trichalcogenides [9] and phosphides have attracted immense attention due to their great potential as photocatalytic materials. Bismuth oxyhalides BiOX (X = Cl, Br and I) have emerged as potential candidate for photocatalytic applications. These materials consist of covalently bonded atomic layered planes separated by van der Waals gaps and belong to V-VI-VII family with layered tetragonal matlockite crystal structure consisting of in-built [Bi2O2] slabs with double halogen slab [10]. The multilayered structure of BiOX provides sufficient space to induce polarization of atoms and orbitals to build intrinsic static electric fields parallel to the crystal facet normal to [Bi2O2] and X slabs. The internal static electric field of these materials can prolong the lifetime of photoinduced charge carriers and thereby inhibiting their recombination. It is a crucial factor in the photocatalytic process to control the rate of photocatalysis [11]. BiOX based materials are chemically inert, non-toxic and anti-corrosive in aqueous medium. These materials contain several compounds sensitive to visible light due to their narrow optical band gaps. The valance band maxima of these compounds primarily consist of hybridized O 2p orbitals and Bi 6s2 while conduction maxima consist of Bi 6p orbitals [12]. Among BiOX (X = Cl, Br and I), BiOCl photocatalysts are effective under UV light irradiation due to their wide optical band (3.2 eV) [13], [14], [15]. On the other hand, BiOl photocatalysts can be activated under visible light due to their narrow optical band gap (1.77 eV). BiOBr with optical band gap of 2.64 eV reveals strong oxidation and reduction photoactivity below the visible region of the solar spectrum. The redox potential is used to estimate O2 conversion into superoxide radicals and H2 into H+ ions during a photocatalytic reaction. The previous studies confirm that redox potential of BiOCl and BiOBr materials is easily achievable due to their appropriate optical band gap. Contrarily, narrow band gap of BiOl make it difficult to achieve the desired redox potential [14], [15], [16], [17].
The BiOX materials are prepared using 1D nanofibers/wires, 2D nanoplates, nanosheets, nanoflakes and 3D hierarchical structures. The thickness and width of 1D materials fall in nanoscale range while length is of microscale range. The high aspect ratio of 1D material increases the lifetime of photoinduced charge carriers, which is a favorable condition for fast photocatalytic reactions. Liu et al. [18] reported boosted photocatalytic activity of BiOCl nanofibers under UV light irradiation. BiOCl photocatalyst showed three times higher photocatalytic activity than Bi2O3 nanofibre under identical experimental conditions. Similarly, the inherent electric field between Bi2O3 and X slabs for 2D nanomaterials could reduce the charge transport resistance to the surface of the photocatalyst and consequently accelerate the photocatalytic activity of BiOX materials. Jiang et al. [19] reported hydrothermally prepared 2D BiOCl nanosheets for improved photocatalytic activity under UV and visible light irradiation. The as-prepared nanosheets with {001} and {010} facets showed accelerated photocatalytic activity under UV and visible light irradiation. The photocurrent transient response of these nanosheets showed high photocatalytic response along {001} facets than {010} facets. The boosted photocatalytic response was attributed to high surface area of {001} facets than the counterpart facets. The 3D BiOX materials are even more effective than 1D and 2D materials due to their red shifted optical absorption, reduced diffusion length, low interfacial charge transfer resistance and abundant active sites [20], [21]. Zhang et al. [21] adopted solvothermal method to prepare 3D BiOBr microspheres using nanosheets. The prepared photocatalyst showed high photocatalytic activity under visible light illumination as compared to bulk BiOBr.
This review covers the methods of preparation of BiOX photocatalysts for hydrogen evolution and pollutant degradation. The criteria of photocatalytic hydrogen evolution and methods of enhancing photocatalytic activity of BiOX photocatalysts are also addressed in this review. Some doping strategies for BiOX, such as transition metals doping, rare earth ions doping, non-metal doping, and complex frameworks of binary composites, ternary composites, carbonaceous materials, graphene and plasmonic materials are also discussed in details [22].
Section snippets
Principle of photocatalytic hydrogen evolution
Catalysis is the process of changing the speed of a chemical reaction by adding a catalyst. The thermal catalysis and photocatalysis are two general classifications of catalysis. Photocatalysis is related to the process of photosynthesis that can be regarded as a chemical reaction induced by photon irradiation in the presence of a photocatalyst. When a photocatalytic material is exposed to light photons of energy equal to or greater than its band gap energy, the electrons and holes are produced
Crystallinity and crystallite size
The crystalline properties of the photocatalyst are important for increasing the photocatalytic activity. Usually, sharp XRD peaks indicate high crystallinity of the photocatalyst and vice versa. Similarly, width of XRD peaks of lattice planes translates the crystallite size. The wider XRD peaks suggest smaller crystallite sizes. The small crystallite is favorable for the photocatalytic activity as it reduces the distance of charge carriers for migration to the surface of the photocatalyst and
Hydrothermal method
The most common method, used for the synthesis of bismuth oxyhalides materials, is the hydrothermal method to produce materials with different morphologies, sizes and crystallinity. In hydrothermal method, water is used as solvent due to its environmental friendly nature, which can serve as a kind of catalyst for the production of preferred materials. This method has advantage over other methods due to its ability to control the uniform nucleation, growth, aging and aggregation of
Enrichment of bismuth content
Band edge potentials and oxidation/reduction potentials of the adsorbates strongly control the photocatalytic process. Theoretically, VB maxima of BiOX (X = Cl, Br and I) materials composes of mainly O2p orbits and Xnp (n = 3, 4 and 5) orbits while CB minima composed of primarily Bi 6p orbits. This indicates that VB maxima and CB minima of BiOX materials can be controlled with enhancing the Bi and O content and reducing X content. Therefore, the band edge potentials of BiOX materials are
Importance of composite photocatalysis
When light photon strikes the photocatalyst with appropriate energy usually greater than the band gap, electron-hole pairs are formed. These photo-radiative charge carriers tend to recombine and annihilate. However, the electrons which can survive the recombination with holes migrate to surface to initiate the redox reactions. The most desirable task during the photocatalytic process is avoiding the charge carriers to perish via recombination. A heterostructure possess the ability to main the
Hydrogen evolution
Photocatalytic hydrogen evolution takes place in water under sunlight irradiation. A suitable photocatalyst is used to convert solar energy into ecofriendly and renewable hydrogen fuel to minimize the dependence on non-renewable fossil fuels. BiOX materials posses layered structure and high potential to demonstrate effective photocatalytic hydrogen evolution. Several authors have reported efficient hydrogen evolution using BiOX as potential photocatalyst. Hao et al. [150] showed much better
Conclusions
This review summarizes the literature published on BiOX (X = Cl, Br and I) materials for diverse photocatalytic applications. The BiOX materials have been emerged as potential photocatalysts due to their exceptional layered structure and physiochemical characteristics. Despite these good properties, the photocatalytic activity of BiOX materials is not much promising due to rapid annihilation of photo-radiative charge carriers, limited optical absorption, low surface area and deficiency of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the efforts of King Khalid University, Saudi Arabia (Deanship of Scientific Research) for supporting this research through the Research Groups Project under the grant number (R.G.P.2/153/42).
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