Monitoring hole trapping in photoexcited TiO2(110) using a surface photoreaction.

The hole-induced photodesorption of chemisorbed O2 from a TiO2(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 O2 desorption processes have been found which are characteristic of low photon fluxes and high photon fluxes. At a critical photon flux, Fhnu(crit), the slow O2 photodesorption process suddenly converts to a fast process, signaling the saturation of hole traps in the TiO2 crystal. Consequently, this allows photogenerated holes to more efficiently reach the surface, causing more rapid O2 photodesorption. The estimated bulk concentration of hole traps is approximately 2.5 x 10(18) cm(-3), involving a fraction of about 3 x 10(-5) of the atomic sites in the bulk. Both the slow and fast O2 photodesorption processes are described by a rate law that is proportional to Fhnu(1/2), 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 (CH3OH), to probe the role of added hole traps on the rate of the photodesorption of adsorbed O2 molecules and on the magnitude of Fhnu(crit).