Surface photovoltage spectrum(SPS), field-induced surface photovoltage spectrum(FISPS) and gas probe techniques are used together to study surface photovoltage(SPV) property of ZnO nanoparticles. The SPS of ZnO nanoparticles consists of two parts, band-to-band transition(P1,P2) and surface states transition(P3). The result shows that the SPV response is mostly attributed to the built-in field formed by active adsorbates, most of which is O 2. Competitive adsorption of O 2 and H 2O exists on ZnO surface. They play different roles in the influence on SPS. The Adsorption of O 2 weakens P3, whereas H 2O enhances P3. And the velocity of adsorption and desorption of O 2 on the surface is significantly faster than H 2O. The adsorption and desorption of H 2O may be correlated with the adjustment of surface structure.
The effect of built-in field on the surface photovoltage(SPV) response of ZnO nanoparticles was investigated by means of surface photovoltage spectroscopy(SPS). From the results of in situ SPS in atmosphere and in vacuum, we suggest that the built-in field should be a main condition for producing SPV response. By comparison of SPS with PL in vacuum as well as in atmosphere, we deduce that by changing the ambience of ZnO, its functional properties can be modulated.
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.
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.
