Structure Se nsitivity of Photochemical Oxidation and Reduction Reactions on SrTiO₃ Surfaces

Abstract

The photochemical reduction of Ag+ and oxidation of Pb2+ from aqueous solution by SrTiO3 leave insoluble reaction products (silver and PbO2, respectively) on the surface. Microscopic analysis has been used to relate the rates of these two reactions to the structure and orientation of SrTiO3 surfaces. The nonpolar (100) surface is the most reactive for silver reduction and the composition of the termination layer does not influence this reaction. On the polar (111) surface, the reduction and oxidation reactions occur on terraces with different terminations and opposite charges; this leads to a nonuniform distribution of reaction products. The polar (110) surface is the least reactive, and the majority of the reaction products are observed at steps along <100> directions where the more reactive {100} surfaces are exposed. The distribution of oxidation products found on (110) terraces is also influenced by the composition and charge of the surface termination. The results show that the photochemical reactivity of SrTiO3 is anisotropic and that, on polar surfaces, dipolar fields arising from charged surface domains influence the transport of photogenerated charge carriers and promote spatially selective oxidation and reduction reactions.

The origin of photochemical anisotropy in SrTiO3

“…The relative reactivities of the SrTiO 3 surfaces bounding the faceted particles are consistent with earlier experiments on extended planar substrates: the {100} surfaces are the most reactive [7]. In the experiments on planar substrates, a uniform coverage of silver was observed on the (100) oriented surfaces while oxidized lead was found in irregularly spaced clusters.…”

“…However, polar surfaces with the appropriate charge have also been shown to strongly promote certain half reactions [4,7]. This suggests that there are other factors that might contribute to the observed anisotropic reactivity.…”

“…Therefore, we conclude that the majority of the electron-hole pairs used for the photochemical reactions are created by photons with energies greater than the band gap but less than 4. (110) and (111) orientations, but the amount of reaction product was much less than on the (100) oriented crystals [7].…”

“…We have previously reported the results of experiments on macroscopically planar ceramic and single crystal SrTiO 3 specimens [6,7]. The purpose of the present paper is to show that SrTiO 3 microcrystals also exhibit highly anisotropic reactivities and to propose an explanation for the observed anisotropy that is based upon the spectral distribution of the incident light and the electronic band structure of SrTiO 3 .…”

Photocatalytic Deposition of Metal Oxides on Semiconductor Particles: A Review

“…On the other hand, when a semiconductor is irradiated with UV light in presence of aqueous solutions containing dissolved Ag + or Pb 2+ cations a redox process is undertaken giving rise to the reduction of silver ions or the oxidation of lead ions, according to the following reactions [Giocondi et al 2003]:…”