Fabrication And Photoelectrical Properties Of TiO₂ Nanoparticles Capped With Carboxylic Acid

Titanium dioxide（TiO₂） has the advantages of low cost, good stability, and environment friendliness. As a kind of broad-band semiconductor, Ti O₂ has excellent performance in optoelectronics, photocatalysis, solar cell and so on. The direct energy band gap of anatase TiO₂ is 3.2eV, which causes the ordinary TiO₂ can only absorb UV light. UV light only accounts for less than 7% of the total energy of the sun, therefore the low utilization of solar light is an important reason that hinders the development of TiO₂ applications. It is important to study how to extend the spectral response range of TiO₂ to visible light, for making it more efficient to use solar light. Researchers have proposed a lot of methods for improving the performance of TiO₂, such as compositing with other semiconductors, dye sensitization, doping of metal or nonmetal elements, etc. However, these methods have brought some new problems, such as surface instability. In order to solve this problem, we attempted to seek solutions from TiO₂ itself, by changing the structure of the TiO₂ to adjust the band gap of TiO₂. In this paper, Ti O₂ nano-clusters capped with stearic acid（SA） and and methacrylic acid（MAA） were prepared by hydrolysis of TiCl4, and the optical properties of the TiO₂ were studied by absorption spectra and fluorescence spectra.In order to study the influence of carboxyl acid capped on the photoelectric properties of TiO₂, we changed the ratio of carboxyl acid and titanium source, and found the absorption spectrum of TiO₂ was red shifted. When the content of carboxylic acid reached a certain value, the absorption spectrum of TiO₂ was no longer red shifted with the saturation of the carboxyl group on the surface of TiO₂. Thus it was proved that the carboxyl group which was bounded to the TiO₂ surface played an important role in promoting interfacial polarization, enhancing the electron phonon coupling, and producing the self trapped exciton states. The self trapped exciton states can be used as a more effective energy level, which resulted in the red shift of absorption spectra and the luminescence behavior at room temperature. In addition, by studying the fluorescence spectra of TiO₂ capped with the carboxylic acid with different chain length, it was found that the chain length of carboxyl acid had influence on change of the surface structure and the generation of the surface trapped states, and could change the stability of electronic energy state and the ability of the carrier escaping from TiO₂. Light-emitting of TiO₂ capped with longer chain carboxylic acid（SA） is stronger, and TiO₂ capped with short chain carboxylic acid（MAA） had a small light-emitting peak in the infrared region. At the same time, X-ray diffraction（XRD） and energy dispersive spectrometer（EDS） were used to characterize TiO₂ structure characteristics, and it was found that TiO₂ was nano clusters and long-range disorder. After calcinations, carboxylic acid on surface was removes, and then we could get crystallized anatase TiO₂. The band gap of TiO₂ was characterized by cyclic voltammetry. It was proved that the carboxyl group on the surface of TiO₂ could reduce the band gap of TiO₂ to about 1.7eV. Therefore, we can get TiO₂ with narrower bandgap by capping carboxyl acid, and the short chain carboxylic acid capping is beneficial for the optical carrier to escape to the solvent. We can expect it to play a greater role in the catalyst and photoelectric converter.