Hyper-Rayleigh scattering (HRS) spectroscopy applied to nanoparticulate TiO2

We have probed the photophysics of nanoparticulate TiO2 using hyper-Rayleigh scattering (HRS) spectroscopy. We find that despite its Wurtzite structure, the anatase particles we prepared are only weakly hyperpolarizable, which suggests that second harmonic generation may couple transitions which are: (a) only weakly allowed, i.e. indirect; or (b) associated with states, probably surface states, of low density. Addition of a carboxy-functional spectral sensitizing dye further quenches second harmonic generation. We hypothesize that the dye competes efficiently with hyperpolarizable surface states for capture of photoholes. We also found that HRS could be used to follow formation of these photocatalyticallty active nanoparticles by acid-catalyzed oxopolymerization of Ti((OPr)-Pr-I)(4). The polymerization is a pseudo-first order process consistent with occurrence of an S(N)1 reaction at Ti(IV) in the rate determining step. (C) 2002 Elsevier Science B.V. All rights reserved.