The photo-generated charge property of TiO 2 nanoparticles annealed at different temperatures was studied by the field induced surface photovoltage spectrum (FISPS) technique. The powder of TiO 2 was prepared and annealed at 400, 600 and 800 ℃, respectively. The crystalline of TiO 2 transformed from anatase to rutile with the annealing temperature increasesing. And the photo-generated charges property of TiO 2 behaves differently after it was annealed at different temperatures. When the powder was annealed at 400 ℃, the particle size is so small that the energy band can not be formed completely, so the bound exciton state was formed when it was illuminated by the proper incident light, and it inverse antisymmetrically under external field for locatization effect. While, the powder was annealed at 600 ℃, the surface phtovoltaic response presents the characteristics of homojunction for different crystalline interfaces. After being annealed at 800 ℃, the energy band was formed completely and the larger particle size(36 nm) was got. Consequently, a band for the dissociation of free exciton emerged in the band edge of the surface photovotaic response distinguished from the band-band transition, and the direction of the photovoltaic response resulted from the free excition is consistent with the external field, that is positive under positive field, and it inverse symmetrically under external field.
Transient photovoltage(PV) technique was applied to investigate the separation and the transport mechanism of the photo-induced charge carriers on nano-TiO_ 2 film electrode. The positive PV transients were observed whenever the light was incident from the gauze platinum(top illumination) or the ITO electrode(bottom illumination). This implies that the photo-induced electrons always accumulate near the ITO. Simultaneously, it is found that under the singe pulse illumination, PV transient at bottom illumination needs a shorter time to reach its maximum than that at top illumination. This indicates that the photo-induced carriers are separated faster on TiO_ 2/ITO interface than that in the bulk of the TiO_ 2 film. These demonstrate the existence of the contact potential on the TiO_ 2/ITO interface, with the downward band bending from the TiO_ 2 to ITO, which may cause the excess carriers to be separated by drift. Under the repeated pulses illumination, the PV transients at top illumination remained unchanged, while those at bottom illumination changed significantly. This results from the trapping of the excess electrons on the TiO_ 2/ITO interface.
The ZnO quantum dot particles with an average particle size 3 nm and the ZnO nanorods with an average length of 80 nm and width about 14 nm were synthesized by the sol-gel method. The surface photovoltaic properties of ZnO nanoparticles were investigated by means of FISPS technique and the bands in FISPS were identified. The results show that the surface photovoltaic properties of ZnO nanoparticles make a great difference for the structures or sizes due to the quantum confinement effect. The photovoltaic response intensity of ZnO nanorods is two-orders of magnitude stronger than that of quantum dot particles. And it is found that the ZnO quantum dot particles exhibit significant quantum confinement properties of photogenerated charges. The photovoltaic response bands of ZnO nanorods are related to band-band transition and bound exciton transition, respectively.
