First-principles Research On Adsorption Of Silver, Platinum Clusters On Tio₂ Nanotube

TiO₂ nanotube with high surface area and unique functional properties has attracted extensively interest. In order to enhance the visible light (VIS) sensitivity of TiO₂ photocatalyst materials, people have made many efforts. The main modification methods are organic dyes photosensitive processs, noble metals deposition, metal ion doping, semiconductor coupling method and so on. Recently, noble metals deposition (Au, Ag, Pt and so on) on TiO₂ nanotube has attracted increasingly attention, and enhanced the visible light sensitivity. Nevertheless, related theoretical study is rare.In this thesis, the electronic structures of small Ag_n and Pt_n (n=1-4) clusters supported on the defect-free anatase TiO₂ NTs have been investigated by density functional theory (DFT) plane-wave pseudopotential method to address a further insight on the mechanism of the VIS sensitivity for Ag and Pt nanoparticles deposition on TiO₂ NT. Cambridge Serial Total Energy Package (CASTEP) with ultrasoft pseudopotentials is employed in our calculations to obtain the electronic structure of the TiO₂ nanotubes using a energy cut off of 380eV. The exchange-correlation interaction was treated within the generalized gradient approximation (GGA) with the functional parameterized by Perdew, Burke and Enzerh of (PBE). The main study contents and results of this paper are described in detail as follows:(1) TiO₂ NTs models were built by wrapping up one optimized (1 0 1) layer of the anatase structure along the [10 1?] direction. The (1 0 1) surface was chosen due to its thermodynamic stability. There are six different surface configurations. The most stable (101) surface is picked by energy calculation. To understand the interactive mechanism, structural and electronic properties of the adsorption systems, including Milliken charge, overlap population and partial density of states were analyzed. The results show there's better electronegativity on the 2-fold Oxygen. This is a potential adsorption site.(2) Structures and energies of adsorbed Ag monomers, dimmers, trimers and tetramers at bare anatase TiO₂ nanotube were systematically studied using GGA (PBE). The best optimized configurations are Ag(1-A), Ag(2-A), Ag(3-D) and Ag(4-A) by comparing the adsorption energies. Ag clusters prefer the bridging O_2f-O_2f site with formation of the Ag-O covalent bonds. In combination of Mulliken charge and overlap population analysis, the reaction between Ag clusters and TiO₂ nanotube is mainly through Ag-O_2f. Density of states is further considered. After adsorption of Ag clusters the anatase TiO₂ nanotube still retain semi-conductive property. O-2p state and Ti-3d state dominate the valence band (VB) and conduction band (CB). The impurity state at the top of valence band is contributed by Ag-4d state. This narrows the optical band gap and there's redshift in adsorption.(3) Structures and energies of adsorbed Pt monomers, dimmers, trimers and tetramers at bare anatase TiO₂ nanotube were systematically studied using GGA (PBE). The best optimized configurations are Pt (1-A), Pt (2-D), Pt (3-C) and Pt (4-A) by comparing the adsorption energies. Pt clusters also prefer the bridging O_2f-O_2f site with formation of the Pt-O and Pt-Ti covalent bonds. In combination of Mulliken charge and overlap population analysis, the reaction between Pt clusters and TiO₂ nanotube is mainly through Pt-O_2f and Pt-Ti. Density of states is further considered. After adsorption of Pt clusters the anatase TiO₂ nanotube still retain semi-conductive property. O-2p state and Ti-3d state dominate the valence band (VB) and conduction band (CB). The impurity state at the top of valence band is contributed by Pt-5d state. This narrows the optical band gap and there's redshift in adsorption.(4) In combination of adsorption energies, the adsorption energy of Pt clusters system is higher than Ag clusters adsorption. This means that the Pt clusters adsorption system is more stable than Ag clusters adsorption. Moreover, Pt clusters adsorption narrows the band gap.