Prevention of surface recombination by electrochemical tuning of TiO2-passivated photocatalysts

Abstract

We present a systematic study of photoluminescence (PL) spectroscopy of TiO2-passivated GaAs as a function of electrochemical potential in an ionic liquid solution. We observe a 7X increase in the PL intensity as the GaAs transitions from accumulation to depletion due to the applied potential. We attribute this to the excellent control over the surface Fermi level enabled by the high capacitance of the electrochemical double layer and TiO2. This allows us to control the surface carrier concentration and corresponding non-radiative recombination rate. In addition to photoluminescence (PL) spectroscopy, we also measured the capacitance-potential (i.e., C-V) characteristics of these samples, which indicate flat band potentials that are consistent with these regimes of ion accumulation observed in the photoluminescence measurements. We have also performed electrostatic simulations of these C-V characteristics, which provide a detailed and quantitative picture of the conduction and valence band profiles and charge distribution at the surface of the semiconductor. These simulations also enable us to determine the range of potentials over which the semiconductor surface experiences depletion, inversion, and accumulation of free carriers. Based on these simulations, we can calculate the Shockley-Read-Hall recombination rate and model the PL intensity as a function of voltage. We show that this approach allows us to explain our experimental data well.

Mechanism of Laser-Induced Bulk and Surface Defect Generation in ZnO and TiO2 Nanoparticles: Effect on Photoelectrochemical Performance

“…46 Carrier concentrations and nonradiative recombination rates also mattered. 47 Meyer et al reported electrical and optical properties of porous nanocrystalline TiO 2 films and found that charge accumulated in Ti 3+ trap states. 48 Jaramillo et al described hierarchically branched TiO 2 nanorod structures with enhanced carrier collection efficiency.…”

“…Carrier diffusion path lengths may limit catalytic activity, highlighting the importance of nanostructuring. , Surface modifications were found to enhance PEC performance by passivation of surface states . Carrier concentrations and nonradiative recombination rates also mattered . Meyer et al reported electrical and optical properties of porous nanocrystalline TiO 2 films and found that charge accumulated in Ti 3+ trap states .…”