PHOTOELECTROCHEMICAL STUDY OF THE TIO2-CR SYSTEM - OBSERVATION OF STRONG(001) RUTILE PHOTOETCHING IN THE PRESENCE OF CR(VI)

The photoassisted electroreduction of Cr(VI) to Cr(III) at (001) TiO2 rutile with UV light has been studied by using conventional photoelect rochemical techniques. Specific adsorption of Cr(VI) on the TiO2 surfa ce is only possible at high chromium concentrations (almost-equal-to 0 .2 M K2Cr2O7) and very acidic electrolyte (pH almost-equal-to 1). With diluted solutions a weaker interaction seems to dominate Cr(VI) adsor ption. This explains the 1/2 order kinetics found in the literature fo r the photocatalytic reduction of Cr(VI) at suspended TiO2 powders. Cr (VI) electroreduction was found to be higher at acidic than at neutral or basic electrolytes, and attributed to the interaction of dissolved Cr(VI) anions with the pH dependent electric charge of the TiO2 surfa ce. Strong anisotropic TiO2 photocorrosion has been observed for the f irst time in Cr(VI) containing electrolytes, at pH almost-equal-to 1.0 . TiO2 photoetching is believed to be a catalytic process associated w ith the photo-oxidation of oxygen coordinated Cr(i)3+ interstitial ion s, directly generated on the semiconductor surface by reduction of dis solved dichromate ions. Key points in rutile photocorrosion are the we akness of surface Ti-O bonds, owing to hole trapping at O:2p valence b and orbitals, and the further surface bonding reorganization, involvin g the generation and simultaneous breaking of Cr(i)-O and Ti-O bonds ( TiO2 dissolution) respectively.