Synthesis and characterization of Co/Ti layered double hydroxide and its application as a photocatalyst for degradation of aqueous Congo Red.

2 : 1 Co/Ti layered double hydroxide (LDH) was synthesized hydrothermally using commercially available Co(NO3)2·6H2O and TiCl4, on a urea template. The high surface area material (~180 m2 g-1) had a narrow band gap (2.67 eV) and shallow and deep trap defect sites. The layered nanomaterial exhibited remarkable semiconductor properties and demonstrated excellent visible light decolourisation efficiency for the anionic dye Congo Red in aqueous medium. The photocatalytic efficiency of the LDH was better than common commercial materials in use such as ZnO, ZnS, NiO, CoO, TiO2 and Degussa P25. The presence of different surface states of defect sites in the LDH was confirmed by PL, EIS and XPS measurements. XRD, DRS, FT-IR, AFM, TEM, SEM/EDX and TG/DTG analyses yielded information about the structural, morphological properties and thermal stability of the LDH. BET N2 adsorption-desorption measurements at 77 K gave surface area and porosity data for the LDH. The surface charge characteristics of the LDH were evaluated with ξ-potential measurements over a wide pH-range in aqueous medium. The photocatalytic behaviour towards decolourisation of the dye was evaluated depending on the reaction variables of pH, LDH amount, initial dye concentration and effects of quenchers, and variation of molar ratios of Co/Ti LDH. The pseudo-first order model satisfactorily described the degradation kinetics of the anionic dye. The photocatalytic mechanistic pathways of the LDH were explained on the basis of an electron-hole (e--h+) hopping conduction model and also photosensitization of the dye. The maximum catalytic efficiency was observed with 15.0 mg of LDH at pH 4 for the anionic Congo Red dye at a concentration 1 × 10-5 M. The LDH was stable even after the fifth catalytic cycle, indicating its remarkable efficiency in potential decolourisation treatments. The dye degradation products were analysed with GC-MS and a reaction mechanism was proposed for the breakdown of the dye to simple and less toxic components.