TIME-RESOLVED MICROWAVE CONDUCTIVITY .2. QUANTUM-SIZED TIO2 AND THE EFFECT OF ADSORBATES AND LIGHT-INTENSITY ON CHARGE-CARRIER DYNAMICS

Charge-carrier recombination dynamics after a pulsed laser excitation are investigated by time-resolved microwave conductivity (TRMC) for quantum-sized (Q-) TiO2 and P25, a bulk-phase TiO2. Adsorbed scavengers such as HNO3, HCI, HCIO4, isopropyl alcohol, trans-decalin, tretranitromethane, and methyl viologen dichloride result in different charge-carrier recombination dynamics for Q-TiO2 and P25. The differences include a current doubling with isopropyl alcohol for which electron injection into Q-TiO2 is much slower than into P25 and relaxation of the selection rules of an indirect-bandgap semiconductor due to size quantization. However, the faster interfacial charge transfer predicted for Q-TiO2 due to a 0.2 eV gain in redox overpotentials is not observed. The effect of light intensity is also investigated. Above a critical injection level, fast recombination channels are opened, which may be a major factor resulting in the dependence of the steady-state photolysis quantum yields on l-1/2. The fast recombination channels are opened at lower injection levels for P25 than for Q-TiO2, and a model incorporating the heterogeneity of surface-hole traps is presented.