The Photovoltage Properties Of A New Type Porous TiO₂ And Its Modified Samples

As a remewable, clean and green energy source, solar energy has a variety of potential applications, such as electricity generation, photocatalysis or electronic devices. Although the use of solar light by solar cells or in photocatalysis is very popular, the transfer mechanism of photogenerated charges is not clear so far. Therefore, for the use of solar energy, particularly for photocatalytic reaction, it is important to investigate the photoinduced charge transfer method and to control charge transfer direction.In this paper, the photovoltage properties of a new type of porous TiO₂ (P-TiO₂) are investigated through surface photovoltage spectroscopy (SPS) and field induced surface photovoltage spectroscopy (FISPS). The abundant surface states, which are related to the special porous structure, extent the SPS response band of P-TiO₂ to visible light region (500 nm), and equal the intensity of surface state transition to that of band-band transition. When an external electric field is added, the changes in the intensity of surface state transition are not so obvious compared with that of band-band transition. These results exhibit strong"localized effect", induced by the surface states.The porous TiO₂ is doped by Fe3+ with different proportions (2%, 5%). The photovoltage properties of Fe3+-doped P-TiO₂ are investigated. The Fe3+ doping can generated abundant of impurity levels and a large number of recombination centers in the P-TiO₂, reducing the number of electronic localized in the surface states and weaken the surface states photovoltaic response . As the doping ratio increased, the weakening phenomena becomes more pronounced. When the additional electric fields are applied, the photovoltage changes of Fe-P-TiO₂ are similar to those of P-TiO₂. To the 5%Fe-P-TiO₂ , the higher doping ratio induces more impurity levels, while more free electrons trapped by impurity levels can detrap due to excited photogenerated charges. These detrapped electrons make the "self-built electric field " direction reverse with the joint effect of concentration gradient and negative electric field. To the 2%Fe-P-TiO₂, the photovoltage is just weakened under the negative electric field without other changes.In addition, the photovoltage properties of sensitized P-TiO₂, which is sensitized by copper phthalocyanine (PcCu), are investigated. The interaction between the delocalized electronics of PcCu and surface states of P-TiO₂ that has "localized effect" results in that the surface states of P-TiO₂"inactive"and the related photo response disappears. When the light wavelength is 410 nm, the bulk phase transition disappears and dyes excited electrons keep injecting into the semiconductor. leading to band bending of sensitized P-TiO₂ reverses. So the transfer directions of photoinduced charge completely opposite at the wavelengths before and after 410 nm separately. When +0.5 V electric field is applied, the intensity of "self-built electric field" increased and the photogenerated charges can be separated more effectively to enhance the photovoltaic response. When -0.5 V electric field is applied, duing to the "self-built electric field" strength is greatly weakened and abundant of dyes excited electrons accumulate on the surface of P-TiO₂, the depletion layer easily transforms into cumulative layers, while the direction of "self-built electric field" also changes.