Photoexcited Electron Hopping between TiO2 Particles: Effect of Single-Walled Carbon Nanotubes

Photoluminescence (PL) spectroscopy was employed to study the effect of embedded single-walled carbon nanotubes (SWNTs) on charge transport in powdered TiO2. It was found that 15 wt % SWNTs mixed with TiO2 accept electrons from photoexcited TiO2 and quench the PL intensity from TiO2. The PL quenching efficiency by SWNTs is proportional to the fractional occupancy of TiO2 particles which experience electrical contact with SWNTs. Ultraviolet light was used to cause surface charging of the SWNT/TiO2 sample, and the charging/discharging rate was measured using PL. The PL charging/discharging rate of all SWNT/TiO2 mixtures is identical to that of pure TiO2, indicating that SWNTs only accept photoexcited electron from TiO2 but do not transport electrons under conditions of the experiment. This is due to the effect of positively charged TiO2 particles which immobilize the photoexcited electrons transferred to SWNTs at the interface between first-layer TiO2 and SWNTs, inhibiting charge transport through SWNT channels. The inhibition of charge transport through SWNTs may be removed by applying an external voltage to overcome the built-in electric field which prevents current flow in the SWNTs.