Theoretical and Experimental Study on M^IIM^III-Layered Double Hydroxides as Efficient Photocatalysts toward Oxygen Evolution from Water

Abstract

Recently, layered double hydroxides (LDHs) have attracted extensive attention in the field of energy storage and conversion, and an in-depth understanding of their semiconducting properties is rather limited. In this work, the electronic properties (band structure, density of states (DOS), and band edge placement) of M II M III -LDHs (M II = Mg, Co, Ni and Zn; M III = Al and Ga) were studied in detail. The thermodynamic mechanism toward oxygen evolution reaction (OER) was investigated by using the density functional theory plus U (DFT + U) method. The calculation results of band structure indicate that Mg and Zn-based LDHs (band gap energies larger than 3.1 eV) are ultraviolet responsive, while Co and Ni-based LDHs are responsive to visible light (band gap energies less than 3.1 eV). The DOS calculations reveal that the photogenerated hole localizes on the surface hydroxyl group of LDHs, facilitating the oxidization of a water molecule without a long transportation route. The band edge placements of M II M III -LDHs show that NiGa-, CoAl-, ZnAl-, and NiAl-LDHs have a driving force (0.965 eV, 0.836 eV, 0.667 eV, and 0.426 eV, respectively) toward oxygen evolution. However, the thermodynamic mechanism of these four LDHs reveal that only CoAl-LDH can overcome the reaction barrier (0.653 eV) via the driving force of photogenerated hole (0.836 eV). Experimental observations of MgAl-, CoAl-, and ZnAl-LDHs further prove that only CoAl-LDH is an efficient oxygen evolution photocatalyst (O 2 generation rate: 973 μmol h –1 g –1 ), agreeing well with the theoretical prediction. Therefore, this work provides an effective theoretical and experimental combined method for screening possible photocatalysts, which can be extended to other semiconductor materials in addition to LDHs.