Preparation And Modification Of Nanosized TiO₂ Materials With Photocathodic Protection Effect

TiO₂ is an n-type semiconductor with chemical and physical stability. In addition, if illuminated TiO₂ is in contact with a metal, electrons are injected from the semiconductor to the metal via the conduction band. As a result, the potential of the metal will be shifted in the negative direction to the flatland potential of TiO₂. If the potential is more negative than the potential at which the metal beings to oxidize, the metal can be protected from corrosion. The counterreaction by photogenerated holes (h+) is not the decomposition of TiO₂ but the oxidation of water. This behavior means that TiO₂ can act as a nonsacrificial anode.The photocathodic protection effect of TiO₂ is dependent on the photoelectrochemical efficiency, which has been improved in two ways: the first is to extend visible light response of TiO₂; the second is to decrease the undesirable hole-electron recombination. Nonmetal dopants, such as N, S, may be more appropriate for the improvement of photoelectrochemical activity of TiO₂ because of it extending the visible light response effectively with thermal stability and cost efficiency.In this work, TiO₂ and nonmetal (N, S mainly) doped TiO₂ materials were prepared by a simple sol-gel method, and their photocathodic protection effects were analyzed from experimental and theoretical approaches:The sol-gel process is an important step in the preparation of TiO₂ material. In our experiment, Tetrabutyl titanate was used as TiO₂ source. When the ratio of tetrabutyl titanate to water by volume is in the range of 2³, with less absolute ethanol and higher temperature, the system get gel point acceleratedly. The resulting crystal phases were determined by the structures of the precursor cation, which are affected by the exact pH of the precursor solutions. Through the analysis of the geometric parameters and Mulliken charge populations of the titanium complexes, theoretical correlation was set up between the precursor solution conditions and the resulting crystal structures.The interfacial structure of Ti/Si binary oxide, which will retard the crystal growth, is proved to be energetically feasible. When the binary clusters are small enough, the formation of edge-shared structures has the largest trend while corner-shared cluster can not form through the analysis of Gibbs energy. As the growth of the binary oxide clusters, the edge-shared structures are disrupted and ring-like clusters form.The structures and the stability of (TiO₂)n clusters with n = 1⁹ have been investigated using the density functional B3LYP/6-31G(d) method. The lowest-lying singlet clusters were put forward and some structure-stability correlation factors, such as coordination number and bond distance, were generalized. Infrared absorption spectra for the cluster structures are comparable with the spectra of rutile and anatase.In the solution with N, N-dimethylformamide, uniform and compact nanosized TiO₂ thin films were prepared on 303 stainless steels. The film shows good photocathodic protection effect as the electrode potential shifted in the negative direction and the weight loss of the steel decreased markedly. With excessively high thermal treating temperature, Fe atoms penetrated into the films and acted as the hole-electron recombination center, deceasing the photoelectrochemical activity and photocathodic protection effect of TiO₂ films.Triethylamine can introduce more nitrogen content into titania crystal than ammonia and ammonium chloride with small bonding energy of N-C bond. Our calculations gave smaller formation energies for substitutional N to O model which indicates the relatively easier synthesis. The doping of nitrogen atoms suppressed the growth of the titania crystal and the phase transformation. The substitutional-type doping was effective for the band gap narrowing of TiO₂ due to the mixing of N 2p with O 2p states, as a result, the photocathodic efficiency increased under the irradiation of white light.Less sulfur content can be introduced into TiO₂ as S=C in thiourea has a more bonding energy. Our calculations verified that S atoms to replace Ti atoms has the largest trend. The substitution results in the suppression of the growth of the titania crystal and the phase transformation. The doped TiO₂ materials have visible response due to the mixing of S 3p with O 2p states. Compared with N doped TiO₂, S doped TiO₂ has a narrower energy gap while a lower photocathodic protection efficiency. The impurity energy level introduced by S dopant act as the electron-hole recombination center, offset some charge carrier induced by the visible light response in a certain extent.