Structure and electrochemical properties of species formed as a result of Cu(II) ion adsorption onto TiO2 nanoparticles

Sol-gel-derived nanostructured TiO2 thin films and powders treated with Cu2+-containing solutions have been studied using electrochemical methods, ESR, XPS, and electroreflection spectroscopies. Analysis of the voltammograms in combination with ESR data has allowed us to reveal at least four types of copper species formed on the TiO2 surface after the adsorption of aqueous Cu2+ ions. These are monovalent copper ions, magnetically isolated Cu(II) ions, Cu(IT) ions forming specific areas ('"associates") with a high local concentration and strong interaction between the ions, and formally diamagnetic copper hydroxide species. The last type dominates when adsorbing Cu(II) ions from solutions with pH > 5 and its fraction increase with increasing the Cu2+ ion concentration in solution. The presence of Cu(I) ions at the TiO2 surface was proved independently by XPS measurements. The appearance of them can be associated with the partial reduction of adsorbed Cu'S ions by electrons of the TiO2 matrix. The Cu(TI) ions bound to the TiO2 surface give rise to the electroactive surface states within the band gap of the oxide. Their energy position has been determined by the electrolyte electroreflectance method. These surface states are located ca. 1.1 eV below the conduction band edge.