The yield and mobility of charge carriers in smooth and nanoporous TiO2 films

The product of the quantum yield, eta, and the mobility, Sigma mu, of charge carriers formed on flash-photolysis (7 ns, 308 nm) of thin films of TiO2 has been measured as a function of intensity from ca. 0.5 to 2000 mu J/cm(2) using the time-resolved microwave conductivity technique (TRMC). For a smooth, 78 nm film formed by electron-beam evaporation (EBE) eta Sigma mu was 1.9 cm(2)/Vs for the lowest intensity used and remained close to constant up to approximately 10 mu J/cm(2). Above this intensity eta Sigma mu decreased gradually to 0.35 m(2)/Vs at 2000 mu J/cm(2). For nanoporous films of Degussa P25 and Solaronix TiO2 eta Sigma mu increased with increasing intensity up to approximately 10 mu J/cm(2) above which it decreased gradually. The maximum values attained (0.08 and 0.3 cm(2)/Vs respectively) were more than an order of magnitude lower than for the EBE film.