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

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 d(110) 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.