Design and development of a visible light harvesting Ni–Zn/Cr–CO 3 2 LDH system for hydrogen evolution† Niranjan Baliarsingh,‡ Lagnamayee Mohapatra‡ and Kulamani Parida‡ * In this work we have prepared a series of visible-light active Ni–Zn/Cr–CO 3 2 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 Ni 2+ substitution in the brucite layer of Zn/Cr–CO 3 2 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– CO 3 2 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. Introduction Driven by increasing energy needs, decreasing fossil fuel resources and environmental concerns, the search for clean and renewable energy is attracting massive research interest. Hydrogen is an ideal clean energy, as well as an important raw material, in many chemical industries. At present, hydrogen is mainly produced from fossil fuels, such as natural gas by steam reforming. Many methods such as physical, chemical 1 and biological processes 2,3 have been applied to produce hydrogen, but visible light induced photochemical water splitting has been regarded as the cleanest route of H 2 production. The photochemical cleavage of water to generate H 2 and O 2 has been studied since the early 1970s by Fujishima and Honda 4 and still attracts considerable interest to researchers because of its potential low cost solar energy conversion. 5 H 2 O / ½O 2 (D)+H 2 (g); DG ¼ +237 kJ mol 1 ,(E ¼ 2.13 eV, l min ¼ 1100 nm) (1) To date, more than 100 photocatalytic systems based on metal oxides have been reported to be active for overall water splitting according to eqn (1). 6–8 However, the large band gaps of these metal oxides restricts the utilization of visible photons, which are the major part of the solar spectrum (i.e. UV light occupies only about 4–5% of the whole solar radiation spectrum while the visible light accounts for 43%). For the economical use of water and solar energy, the catalyst should be inexpensive, have suit- able thermodynamic potential for water splitting, a sufficiently narrow band gap to harvest visible photons and be stable against photo corrosion. Therefore it is necessary to develop visible light responsive photocatalysts for efficient solar energy conversion. Some titanates, such as TiO 2 9–10 and SrTiO 3 11 , have been known as promising photocatalysts in the last three decades. Several exfoliated layered metal oxide nanosheets are applicable as photocatalysts for the photooxidation of organic pollutant molecules and also for the photo production of H 2 gas. 12–21 In comparison with layered metal oxides, layered metal hydroxides, such as layered double hydroxides, are less investigated as pho- tocatalysts. A recent report about the high photocatalytic activity of the Zn–M-LDHs (M ¼ Cr, Ti, Al) material evokes research interest on the application of LDHs as photocatalysts. 22–26 Layered double hydroxides (LDHs) are also known as hydrotalcites (HT), with general formula [M II 1x M III x (OH) 2 ] [A n ] x/n $mH 2 O, where M II includes divalent cations like Mg 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ etc.,M III may be Al 3+ , Fe 3+ , Cr 3+ etc. and ‘A’ might be an organic and/or inorganic anion. The layered structure consists of two-dimensional (2D) monolayers of edge-shared octahedral coordinated units, M(OH) 6 , which stack Colloids & Materials Chemistry Department, CSIR-Institute of Minerals & Materials Technology, Bhubaneswar-751013, Orissa, India. E-mail: paridakulamani@yahoo. com; Fax: +91-674-2379237; Tel: +91-674-2379425 † Electronic supplementary information (ESI) available: Fig. S1–S3 show the volume of hydrogen evolution obtained for LDH4 with different catalyst doses for 1 h, reusability study over LDH4 material and the XRD pattern of recycle LDH4 material respectively. See DOI: 10.1039/c2ta00933a ‡ All authors contributed equally to this work and declare no competing nancial interest. Cite this: J. Mater. Chem. A, 2013, 1, 4236 Received 1st November 2012 Accepted 7th January 2013 DOI: 10.1039/c2ta00933a www.rsc.org/MaterialsA 4236 | J. Mater. Chem. A, 2013, 1, 4236–4243 This journal is ª The Royal Society of Chemistry 2013 Journal of Materials Chemistry A PAPER Downloaded by National Chiao Tung University on 11 March 2013 Published on 15 February 2013 on http://pubs.rsc.org | doi:10.1039/C2TA00933A View Article Online View Journal | View Issue