Density Functional Study Of The Doped Anatase Crystal And The Adsorption Of Amino Acids On The Anatase (101) Surface

Due to the outstanding photocatalytic property, chemical stability, nontoxicity,and low-cost, TiO₂is considered as one of the most promising semiconductorphotocatalysts. However, the wide intrinsic band gap of TiO₂（anatase:3.2eV, rutile:3.0eV） only allows the absorption of the ultraviolet light in the solar irradiation. Itthus attracts much attention of researchers to expand its optical absorption to thevisible light. One feasible solution is doping other elements into the TiO₂crystal toimprove its photocatalytic performance.Based on density functional theory （DFT） with the generalized gradientapproximation （GGA） and the ultrasoft pseudopotential, we studied the geometriesand electronic properties of the anatase TiO₂crystal doped by N, V, Cr, Mn, Fe, or Cosingle atom and by N-V, N-Cr, N-Mn, N-Fe, N-Co double atoms. We found that themonodoped N atom generated N2p impure orbitals on the top of the valence bandand forced the conduction band to lower energy levels. The narrowed band gapimproved the photocatalytic activities in the visible light range. After replacing one Tiatom of the anatase crystal, the V, Cr, Mn, Fe, Co transition metal atoms had anobvious influence on the forbidden band gap through their3d impure orbitals. V andCo directly decreased the band gap. Differently, Mn, Fe, and Cr extended the bandgap but generated intermediate energy levels. Therefore, V, Cr, Mn, Fe, Co could allmake the optical adsorption edge have a red-shift. Analyzing the band structures anddensity of states, we surmised that the photocatalytic activities of the anatase crystalcould be effectively promoted by doping V, Mn or Fe, while a smaller improvementby Cr or Co.As N-V，N-Cr，N-Mn，N-Fe or N-Co codoped into the anatase crystal, thephotocatalytic behavior was more obviously moved to the visible light range throughthe synergistic interaction between the N and transition metals. In the N-V codoped anatase, the band gap was significantly decreased since both conduction band andvalence band are extended. There was an obvious adsorption edge red-shift. While inthe other four codoped models, N2p orbitals are localized on the top of the valenceband and Ti3d orbitals generated intermediate energy levels, which reduced theexcitation energies for electrons transition and made the adsorption edge red-shifted.We surmised that the codped anatase would have better photocatalytic activity thanmonodoped ones because of the synergy.On the other hand, the application of TiO₂nanomaterial in biomedical fields hasbeen rapidly developed as its attractive photocatalytic effects in the biological systems.With regard to the biological effects, we investigated the adsorption of the simplestbiological molecules—amino acids on the anatase （101） surface by usingSCC-DFTB method. One group included the serine and threonine, which both containa hydroxyl side chain. Another group was the aspartate acid and glutamic acid, whichboth have a carboxyl side chain. Our results demonstrated an automatic deprotonationof the neutral carboxyl group for all amino acids on the surface. The adsorptionenergy between the carboxyl moiety （–COOH） and the surface was estimated of about-30kcal/mol. The hydroxyl moiety （–OH） contributed about-10kcal/mol adsorptionenergy via the hydrogen bond or the electrostatic interaction. The amino moiety （-NH2）on the surface was repulsive and unstable. Thus, the activity order of three groupsfrom high to low is–COOH>>–OH>–NH2. The serine and threonine both preferredmonodendate configurations derived by the carboxyl oxygen bound with the surfacetitanium. While the aspartate acid and glutamic acid preferred bidentate-bridgeconfigurations through both α-COOH and β-COOH oxygen bound with two surfacetitanium atoms. Analyzing the attachment results, adsorption energy and Mullikencharge, we reputed that the interaction between amino acids and anatase （101） surfacewas mainly attributed to the Ti-O bond betweem the carboxyl and the surface,additionally slighter electrostatic interaction and hydrogen bonds between thehydroxyl and the surface.