Multiscale structural modulation of layered double hydroxide nanosheets and their application in solar-driven catalysis

Abstract

Due to the high photocatalytic efficiency in activating chemical bonding under mild reaction conditions, photocatalysis has been acted as a green and promising technology. However, how to improve the quantum efficiency and solar energy utilization to further enhance the photocatalytic performance remains a challenge, and it is highly desirable to rational synthesize photocatalyst with suitable energy band structure, and efficient active sites. Layered double hydroxides (LDHs) is a kind of two-dimensional materials, with molecular formula [M12-+ x M x 3+(OH)2] x + [A x / n ] n -·mH2O. The layered structure of LDHs is similar to brucite Mg(OH)2, which is connected by the octahedral MO6 sites. The substitution of divalent metal cations (M2+) with trivalent metal cations (M3+) makes the layers positively charged, in which M2+, M3+ can be as one or more of the elements such as Mg, Al, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, In, etc. M2+, M3+ ions alternately arrange in the main layers at a high atomic level to form the planar structure ( a , b axis direction), and the alternation of the positively charged layers and the negatively charged anionic layers ( c axis direction) constitute the three-dimensional structure of LDHs. There are many unique structural characteristic of LDHs, such as the high dispersion of the metal elements in the layers, the controllable particle size and thickness, topological transformation and so on. The above properties have provide LDH as photocatalysts as the followings reason. Firstly, the metal elements of LDHs are arranged in an alternating order at the atomic level with flexible compositions (such as Fe, Co, Ni, Ti, etc.), and can be directly used as photocatalysts due to the tunable band gap. Secondly, the topological transformation property can turn LDH materials into heterogeneous mixed metal oxides or highly supported metal-containing catalysts, which are not easily agglomerated even under reaction condition. Thirdly, based on the tunable particle size and thickness of LDHs, it is possible to adjust the morphology of LDHs into nanoscale, which can fully expose the active sites and enhance the catalytic performance. Based on the abundant adjustable properties of LDHs, the following structures can be well optimized, such as their composition, particle sizes, defects, abundant interfaces, the high dispersion characteristics, and tunable energy band structures, etc. by using LDHs as catalysts for the potential enhanced catalytic efficiency. In this review, in order to activate H ˗ O, C = O, NN and C ˗ C bonds under mild condition, a series of LDH-based photocatalysts have been finely synthesized (like NiTi-LDH nanosheets, ZnAl-LDH, CoFe alloy derived from LDH, Co/Fe-hetrostructure derived from LDH, Cr-containing LDH, etc.), and widely applied in solar-driven water splitting, CO2 reduction, CO hydrogenation to high carbon hydrocarbons, ammonia synthesis, degradation and so on. Collectively, with the multiscale structural regulation of the microstructure, mesoscopic interfacial structure and highly dispersed metal-supported structure, the following properties such as the band gap, defect and the interface of LDH-based photocatalysts can be well modulated, and the solar-driven catalytic performance are demonstrated to be comprehensively facilitated. Furthermore, the relationship among the preparation, structure and catalysis has been well discussed, which will provide a strategy for the rational design of efficient photocatalysts.