PHOTOINDUCED HOLE TRANSFER FROM TIO2 TO METHANOL MOLECULES IN AQUEOUS-SOLUTION STUDIED BY ELECTRON-PARAMAGNETIC-RESONANCE

An electron paramagnetic resonance (EPR) study is reported on the para magnetic species formed on irradiation of aqueous TiO2 colloids in the presence of methanol. In laser (308 nm) irradiated aqueous solution o f methanol at 1.9 K, the observed EPR signals are centered around g = 2 and have a line shape characteristic of the CH2OH radical. The appea rance of these radicals at the very low temperature of 1.9 K implicate s charge transfer from the oxygen lattice holes to methanol molecules within a few monolayers of the surface of the TiO2 particles. Differen t radicals are formed when CH3OH is chemisorbed on TiO2. The EPR signa ls detected in TiO2 colloidal solution prepared in methanol and then e vaporated and dissolved in water also show the formation of the CH2OH radical due to oxidation of chemisorbed methanol on the TiO2 surface. These particles, with high laser pulse intensities, oxidize the chemis orbed methanol further to give the CHO radical. In addition, the CH3 r adical is formed from reducing processes and observed at 6-50 K. In th e absence of methanol, holes are trapped by surface hydroxide groups f orming TI(IV)-O-Ti(IV)-O.. At higher temperature (150 K) in aqueous me thanol solution, the .CH2OH radical transfers an electron to TiO2, dou bling the yield of formation of surface Ti(III) ions which are stable in air-free solution. When an electron scavenger such as Hg2+ is prese nt in solution, the EPR signal of Ti(III) disappears since mercury ion s are reduced.