Physically and Chemically Induced Band Bending Changes at the Surfaces of TiO2 Nanoparticles Studied by Photoluminescence Spectroscopy

TiO2-based photocatalytic/photovoltaic applications have received enormous research interest. However, one key problem inhibiting practical application is the fast photoexcited electron/hole pair recombination rate, thus understanding photoexcited electron hole behavior is of critical importance. Band bending, which has been seldom studied but widely exist in semiconductor photochemistry, may play important roles in dictating electron and hole behavior. In this thesis, photoluminescence (PL) spectroscopy was employed to study the band bending changes in TiO2 nanoparticle powders caused by physical or chemical means.
We studied the effect of mixing singled-walled carbon nanotubes (SWNT) into TiO2 nanoparticle powders on charge transport improvement. It was found that SWNTs accept electrons from photoexcited TiO2 and quench the PL intensity from TiO2. However, charge transport through TiO2 nanoparticles is not improved by mixing SWNTs, which is inferred from similar time constant of the PL increase under continuous UV irradiation. This is due to the effect of charge immobilization caused by positively charged TiO2 particles which inhibit electrons transferred to SWNTs from being transported away through SWNT channels.
We also found that small coverages of 3 nm-Au nanoparticles deposited on TiO2 significantly diminish the 540 nm (2.3 eV) PL emission from TiO2 due to injection of photoexcited electrons into the Au nanoparticles. The lack of PL increase from Au/TiO2 during continuous UV irradiation is due to a short circuit established through Au nanoparticles where transferred electrons in Au recombine non-radiatively with holes in TiO2. The photoexcited electron transfer from TiO2 to the Au nanoparticles can occur from the Au particle perimeter over a distance of at least 4 nm.
Oxygen adsorption was found to change the band bending of the anatase phase TiO2 in P25 nanopowder in different ways. First, oxygen can adsorb through irreversible reaction with defects, reducing the intrinsic upward band bending at the TiO2 surface and resulting in increased PL emission. Second, oxygen exposure also leads to molecular chemisorption that yields an outermost negative charge at the surface which increases the upward band bending of TiO2 and decreases the PL emission. Since band bending plays an active role in directing charge carrier to the surfaces, the finding that oxygen adsorption can have two different, and quite opposite, effects on the band bending of TiO2 provides a new perspective on how oxygen may influence photocatalytic reaction efficiencies.
We also found that the band bending of TiO2 may change during the course of photoelectrochemical reactions. The PL intensity of TiO2 increases when exposed to hydrogen cations under continuous UV irradiation, signaling a decrease in the upward band bending and increased electron-hole radiative recombination rate, is caused by charge transfer during photoreduction of hydrogen cations. Residual photoexcited holes left in TiO2 due to the transfer of photoexcited electrons to hydrogen cations lowers the original upward band bending and increases the radiative charge recombination rate. This unusual observation that charge transfer during a photoelectrochemical reaction at a semiconductor interface alters the surface band bending and photoexcited electron/hole recombination rate suggests that photocatalytic activity may change during reactions.