Synthesis, physical and electrochemical characterizations of organically templated cobalt-aluminophosphate. Application to oxygen evolution

The hybrid semiconductor CYCHA-CoAl₂P₃O₁₂·mH₂O was elaborated by hydrothermal route at 453 K in presence of cyclohexylamine (CYCHA) as organic template. The X-ray diffraction pattern of the blue product indicates a zeolite type CAP-CHA 5 topology which crystallizes in a hexagonal system (SG:R$\overline{3}$) with the lattice constants: a = 13.5494 (3) Å, c = 15.2795 (5) Å. The optical, physical and photo-electrochemical characterizations of the material were investigated. The diffuse reflectance indicates an optical transition at 1.84 eV, directly allowed due to the crystal field splitting of Co ³⁺ in tetrahedral coordination. The electrical conduction occurs by low polaron hopping with activation energy of 37 meV. The conduction band is formed by Co³⁺: e.g. orbital (5.15 eV) below vacuum. The valence band, deriving mostly from O²⁻: 2p orbital with a potential (6.85 eV), is adequately located with respect to O₂/H₂O level (~ 1.40 V_SCE), thus leading to an oxidation of water under illumination. The best photoactivity was found at neutral pH (~ 7) with an oxygen evolution rate of 38 µmol min⁻¹ g⁻¹ and a quantum yield of 0.40%. The as-synthesized material also showed adsorption capacity of 90% of the methyl violet dye in aqueous solution with a half life of 43 min.