COPPER PHOTODEPOSITION ON TIO2 STUDIED WITH HREM AND EXAFS

The possibility of direct selective surface activation by photodeposition of autocatalytic Cu particles on photosensitive TiO2 substrates in an alkaline electroless copper solution has been investigated. The nucleation and growth of particles were characterized by transmission electron microscopy (TEM). Ex situ high resolution TEM and in situ extended x-ray absorption fine structure (EXAFS) spectroscopy were used to determine the oxidation state of photodeposited particles on thin polycrystalline TiO2 films and suspended TiO2 powders, respectively. It is shown that in the initial stages of photodeposition small (2O particles are formed. Electroactive Cu particles are only observed at relatively long illumination times and/or high light intensities. These results are explained with a model of the illuminated TiO2 crystallite/electrolyte interface, which considers simple redox reactions involving transfer of TiO2 photoelectrons to energy levels of the couples between complexed Cu2+ ions, Cu2O and Cu. From this it is concluded that under illumination Cu particles are only formed if deposition of Cu2O is limited by either mass transport of complexed Cu2+ ions or kinetic factors.