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 paramagnetic species formed on irradiation of aqueous TiO2 colloids in the presence of methanol. In laser (308 nm) irradiated aqueous solution of 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 appearance of these radicals at the very low temperature of 1.9 K implicates charge transfer from the oxygen lattice holes to methanol molecules within a few monolayers of the surface of the TiO2 particles. Different radicals are formed when CH3OH is chemisorbed on TiO2. The EPR signals detected in TiO2 colloidal solution prepared in methanol and then evaporated 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 chemisorbed methanol further to give the CHO radical. In addition, the CH3 radical is formed from reducing processes and observed at 6-50 K. In the absence of methanol, holes are trapped by surface hydroxide groups forming TI(IV)-O-Ti(IV)-O.. At higher temperature (150 K) in aqueous methanol solution, the .CH2OH radical transfers an electron to TiO2, doubling the yield of formation of surface Ti(III) ions which are stable in air-free solution. When an electron scavenger such as Hg2+ is present in solution, the EPR signal of Ti(III) disappears since mercury ions are reduced.