First-Principles Study Of Molecular Adsorption On TiO₂Surfaces

Among metal oxide semiconductor photocatalytic materials, TiO2has attracted widely attention and is considered to be the most promising materials in the fields of renewable energy and environmental protection due to its stable properties, high activity, low cost and non-toxicity. In order to investigate the interaction mechanisms and influence factors of TiO2material in photocatalytic reactions, scientists have done a great deal of research on it and made great progress. Researches show that there exist many factors influencing the photocatalysis process. Besides the wide band gap (3.2eV for anatase) which inhibits the efficiency of optical adorption, some other factors such as crystal structure, constituents of grains, grain surfaces, dopants, carrier trapping agent and external environment would have various degrees of influence on the photocatalytic process. The excellent photocatalytic performance mainly derives from anatase and rutile polymorph. Especially, the anatase polymorph which has relatively high activity contributes significantly to the photocatalytic performance of TiO2Generally, the high activity of anatase polymorph is mainly resulted from the higher oxidizing ability and larger capability for adsorbing molecules or hydroxyl groups, the smaller grain size and high specific surface area. Whileas the larger grains and lower adsorption capability is the principal origin of lower activity of rutile polymorph. Researches indicate that the activity of TiO2is closely related to the highly active surface of grains. The highly active surfaces of TiO2have high capability of oxidation and degradation in the interaction with molecules. This has raised awareness of important role of highly active surfaces in the photocatalytic reactions. Both experimental and theoretical works show that (001) surface has the highest activity among the facets of anatase polymorph. The interaction between molecules and (001) surface is quite strong. Thus, molecule is relatively easy to dissociate on this surface. In addition, TiO2-B(100) was found to have similar high activity. Obviously, increasing the proportion of highly active surface among the facets of grains is very meaningful to enhance the photocatalytic performance of TiO2grains. As an important research tool in the material study fields, the frist-prinples method is becoming an essential complement to experimental investigations and widely used in the studies of properties of TiO2materials. Through investigating the properties of TiO2via first-principles method, we would have an insight into the detailed process and mechanisms of TiO2in the interaction with molecules from the microcosmic perspective thoroughly and profoundly. In this article, we systematically investigated the some properties such as geomtries parameters, electronic structures, activities and adsorption capability of molecules of TiO2anatase (101), rutile (110), anatase (001) and TiO2-B(100) surfaces. Furthermore, the mechanisms of interactions between surfaces and molecules have also been investigated and explained. The main outcomes of this work conclude as follows.(i) Using DFT method, we investigated formaldehyde (HCHO) adsorption on rutile (110), anatase (101), anatase (001) unreconstructed surface as well as anatase (001)-(1×4) reconstructed surface. The results show that the activities of these surface have the order of anatase (101)<rutile (110)<anatase (001). Though the anatase (001) surface has the highest activity, it is unstable and has the trend of reconstruction. Interestingly, we found that the reconstruction of anatase (001) surface has not only kept the high activity but also raised the surface stability. HCHO molecule can be stably adsorbed chemically on all of these surfaces. In the most stable adsorption configurations, the molecule forms into a dioxymethylene species (CH2O2) through the bonding with a surface2fold coordinated O atom (O2C). Dioxymethylene species was found to be the important product of HCHO adsorption and has been usually observed experimentally. The carbonyl of dioxymethylene in the adsorbed HCHO is longer14-17%than that of free HCHO molecule indicating that intramolecular interaction between C and O atoms has been weakened and it is easy to be decomposed, which may be an important factor in the degradation of HCHO. Additionally, involving the adsorption on anatase (001) surface,(1×4) reconstructed surface has more capability of adsorption than (1×1) unreconstructed surface due to the higher adsorption energy. How eVer, the (1×4) reconstructed surface has a surface energy of0.52J/m2, which is much lower than that of unreconstructed surface, implying that the reconstruction would improve not only the reactivity but also the stability. Therefore, increase of the proportion of (1×4) reconstructed surface among the grain surfaces would improve the performance such as activity and stability. These results indicate that a careful preparation of novel anatase TiO2crystals with a large amount of (001) facets may be an important way for further improvement of properties of titania-based photocatalyst and gas sensors.(ii) The adsorptions of HCHO without and with hydroxyl group co-adsorption on TiO2-B (100) surface are systematically investigated using the first principle method. The results show that the adsorption of HCHO with chemical adsorption structures in the form of dioxymethylene species (CH2O2) occurs on both unhydroxylated and hydroxylated surfaces. On the hydroxylated surface, effects of two kinds of hydroxyl groups, one is bridging OH (BH) group, another is terminal OH (TH) group, which are products of dissociation of water on TiO2-B (100) surface, are found.The adsorption of HCHO was weakened by the existence of BH, while enhanced by the co-adsorption of TH. Through the electronic structure analysis, we found that as compared to the adsorption on the unhydroxylated surface, HCHO gains more electrons on the hydroxylated surfaces; and HCHO gain more electrons with TH co-adsorption among the two hydroxylated surfaces. This difference originates from the different effects of hydroxyl groups upon the surface. That is, BH raises the Fermi1eVel and TH lowers the Fermi1eVel. This induces the surfaces different chemical activities. Additionally, the existence of H2O was also found to have promotive effect on the adsorption energies of HCHO on the larger periodicity, indicating low density of H2O molecule on the surface can strengthen the interaction between HCHO and TiO2-B surfaces. These results indicate that H2O molecule and hydroxyl groups, which are the fragments of dissociated HoO, play a completely different role when co-adsorption with HCHO on TiO2-B (100) surface:and a humid environment may have an important effect on the TiO2-B (100) surface in photocatalysis reaction processes.(iii) Using the DFT+U method, we investigated the influence of F-dopants, which substitute the O atoms near the Ti5C atom in the anatase (001) surface, on the surface activities. Three kinds of F-dopants of FⅠ. FⅡ and FⅢ, which are the F-dopants substituting a surface O2C atom, or a surface O3C atom. or an O3C atom below a surface Ti5C atom, representing F-dopants of different depths, were considered. The results of interaction between F-doped surfaces and molecules show that FⅠ dopant raises the stability of surface, while FⅢ dopant raises the activity of surface. The surface doped with FⅡ has an activity similar to the pristine one. Additionally, the surface energy was found to not be the only factor which can influence the surface properties. In the F doped surface, another important factor is the excess electron induced by F-dopant and fully located in the3d orbital of T3+. This electron pushes the Fermi1eVel to the bottom of conduction band and makes the slab an n-type semiconductor. As a result. the activity of surface was greatly enhanced. In most cases, the adsorption of molecules on F-doped surfaces is stronger than on the pristine one. The study shows that Ti3+has some unique effects on not only electronic and magnetic but also structural properties of adsorbed gases. The bridge role of Ti3+induced by F-dopants in electron transferring and enhancing effects on adsorptions suggests the great potential of this species for future applications of titania-based catalyst.