Monitoring hole trapping in photoexcited TiO 2(110) using a surface photoreaction

Abstract

The hole-induced photodesorption of chemisorbed O 2 from a TiO 2(110) single crystal has been employed to monitor the kinetics of electron-hole pair (e-h) formation and hole trapping. Excitation is produced by 3.4 ± 0.05 eV photons at 110 K. Two separate O 2 desorption processes have been found which are characteristic of low photon fluxes and high photon fluxes. At a critical photon flux, F hv(crit), the slow O 2 photodesorption process suddenly converts to a fast process, signaling the saturation of hole traps in the TiO 2 crystal. Consequently, this allows photogenerated holes to more efficiently reach the surface, causing more rapid O 2 photodesorption. The estimated bulk concentration of hole traps is approximately 2.5 × 10 18 cm -3, involving a fraction of about 3 × 10 -5 of the atomic sites in the bulk. Both the slow and fast Ch photodesorption processes are described by a rate law that is proportional to F hv1/2, indicating that the steady-state concentration of holes, [h], is governed by second-order e-h pair recombination kinetics. Effective use is made of a hole scavenger molecule, adsorbed methanol (CH 3OH), to probe the role of added hole traps on the rate of the photodesorption of adsorbed O 2 molecules and on the magnitude of F hv(crit). © 2005 American Chemical Society.