Study On The Adsorption And Catalytic Properties Of TiO₂Nanotube Arrays

The one-dimensional TiO2nanotube arrays (TNTAs) have highly-ordered and densely-packed structures. They lend numerous advantages such as increased surface-to-volume ratios, stronger adsorption ability, higher conductivity, tailored band gaps and better chemical durability. Therefore, TNTAs have been very promising in many fields such as photocatalysis, water photoelectrolysis, sensing and dye-sensitized solar cells. No doubt, it is significant to carry out related fundamental theoretical research. Up-to-date, study on the adsorption and reactions inside the confined hydrophilic metal oxide is still scarce. In this thesis, based on the TNTAs models previously built by our group, we conducted in-depth studies on relative stabilities, geometrical and electronic properties, adsorption and catalytic performances of TNTAs using density functional theory calculations. The current work is expected to provide reasonable explanation and prediction for experimental results.Firstly, we investigated the properties of anatase TNTAs constructed from (001) nanosheets. The results showed that (001) TNTAs are considerably more stable than the (001) nanosheet. Moreover, they are semiconductors with indirect band gaps. Similar to the bulk TiO2, the valence band and the conduction band of TNTAs predominantly comprises O2p and Ti3d, respectively.Secondly, we addressed the adsorption behaviors of gases in TNTAs. The common species like O2, H2, CO, NO, H2O, CO2, NO2, N2O, NH3and CH4in energy and environmental fields were chosen as the probe molecules. The results indicated that the adsorption of the same gas on (18,0) TNTAs is relatively stronger than that on (12,0) TNTAs. For both TNTAs, the adsorption energies of H2O and NH3are larger than-0.6eV, corresponding to strong chemisorption. In contrast, the adsorption energies of others are between-0.1and-0.3eV, indicative of moderate physisorption. Mulliken population analysis revealed that all molecules act as charge donors. The density of states confirmed that the electronic properties of TNTAs are sensitive to the adsorption of O2, NO and NO2.Finally, we further modulated the catalytic performance of TNTAs for water splitting by chemical doping methods. The results showed that Pt, N, Pt/N and Pt/F doping can significantly engineer the band gap of prestine TNTAs (p-TNTAs) while F doping has little effect but promotes the charge carriers diffusion. Proper tailored doping elements can reduce the overpotential of water splitting on TNTAs better. Particularly, the largest reduced values results from the codoping of metal and nonmetal elements due to their synergistic effects. It is also suggested that the required overpotentials can be divided into two groups:one is a high overpotential (～1.0V) on p-TNTAs, Pt/TNTAs and N-TNTAs; and the other is a low overpotential (-0.6V) on F-TNTAs, Pt/N-TNTAs and Pt/F-TNTAs. Furthermore, two kinds of linear relations between the binding energies of O*, HO*and HOO*intermediates are identified, and correspond to the two types of TNTAs with the high and low overpotential, respectively. The universal rule is helpful to promote the rational design of novel TiO2-based materials.