Microsecond transient absorption spectra of suspended semiconducting metal oxide nanoparticles

Abstract

The time-resolved absorption spectra of opaque semiconductor powders have been typically studied by diffuse reflectance techniques. Herein, we show that time-resolved transmission spectroscopy of suspended nanoparticles is a suitable way to detect electrons and holes generated photochemically in these solids. Ultrasound dispersion of nanoparticulate metal oxide semiconductors, including TiO2, ZnO, NiO, and alpha-Fe2O3, in acetonitrile or water leads to the formation of indefinitely persistent suspensions in which metal oxide nanoparticles are dispersed in the liquid phase. Dynamic laser scattering reveals the presence of these nanoparticles of dynamic dimensions ranging from about 30 to 200 nm. Time-resolved transmission absorption spectroscopy of these suspensions allows detecting transients decaying in the microsecond time scale that are characterized by a broad absorption spanning the whole visible range and almost coincident temporal profiles in every wavelength that have been assigned to the state of charge separation. This assignment was supported by quenching experiments and by steady-state irradiation of the suspensions containing methyl viologen (electron acceptor) and tetramethyl-p-phenylenediarnine (hole acceptor) as probe molecules that lead to the generation of the visually observable corresponding radical cations.