Design and development of a visible light harvesting Ni–Zn/Cr–CO32− LDH system for hydrogen evolution

Abstract

In this work we have prepared a series of visible-light active Ni–Zn/Cr–CO₃²⁻ LDHs with (Ni + Zn)/Cr ratio 2.0, while varying the Ni/Zn atomic ratio to 0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0, and tested them for visible light photocatalytic hydrogen evolution. The photophysical and photocatalytic properties of the samples were evaluated thoroughly by PXRD, TEM and FESEM, UV-vis DRS, FTIR, PL and BET-Surface area. The PXRD measurement demonstrates a characteristic of hydrotalcite with long range order structure. The shifting of the d₁₁₀ plane towards a lower angle clearly indicates that there is Ni²⁺ substitution in the brucite layer of Zn/Cr–CO₃²⁻ LDH. Different diffuse reflectance spectra revealed that the absorption in the visible region is attributed to the metal-to-metal charge-transfer (MMCT) excitation of Zn^II/Ni^II–O–Cr^III to Zn^I/Ni^I–O–Cr^IV. This oxo-bridged hetero-bimetallic assembly acts as a photo-induced centre for hydrogen evolution. The photocatalytic studies suggested that the Ni–Zn/Cr–CO₃²⁻ LDH with Ni : Zn ratio of 75 : 25 exhibits the best photocatalytic activity under visible light radiation compared with the others. The detailed mechanism for the photocatalytic decomposition of water to hydrogen has also been discussed.