| Recently, metal oxide semiconductors with wide band gap have been attracting more and more attention due to their diverse applications, which include photocatalyst, diluted magnetic semiconductor (DMS) and gas sensor. Perovskite-type SrTiO3 is one of the promising photocatalyst candidates due to its good performance in degenerating organic pollutants and over all splitting H2O for H2/O2 evolution. However, because of the wide band gap of SrTiO3 of 3.2 eV, it presents photocatalytic activity under ultraviolet light with wave length< 387 nm. The ultraviolet light corresponds to about 5% of solar energy while visible light accounts for 43%. The wide band gap restricts the solar energy convertion efficiency of SrTiO3. In order to use the visible light and improve the energy convertion efficiency, doping foreign elements is one of the most important approaches. For SrTiO3, effects of metal and nonmetal doping on the photocatalytic performance have been investigated in experiments. Doping introduces new levels in the band gap or narrows the band gap, which makes the SrTiO3 visible light-driven. As we know, a conventional photocatalytic process dominantly contains three steps:(1) excitation of electron-hole pairs under light irradiation; (2) separation of photogenerated electron-hole pairs, and subsequent migration to the photocatalytst surface; (3) redox reaction over the surface involving electron-hole pairs. However, electron-hole pairs tend to recombine in bulk material and on the surface. As a result, reduction in recombination of photogenerated electron-hole pairs is important for the photocatalytic efficiency. It has been identified that metal deposition on photocatalyst surface can greatly improve the activity because metal can reduce the recombination of photogenerated electron-hole pairs. For SrTiO3, it has been verified that loading some metals, such as Cu, Ag, Au and Pt, can obviously enhance the photocatalytic activity. Except for SrTiO3, some other new-type materials also have been developed as promising photocatalysts with excellent performance due to the unique structure and electronic properties, such as GaN/ZnO solid solution, Bi-based oxides Bi12TiO20, Bi2Ti2O7and Bi4Ti3O12.In addition to the application of photocatalyst, SnO2 is the most studied semiconductors as gas sensors. However, the application of SnO2 as gas sensor is greatly limited by poor sensitivity, low respond speed and lack of selectivity. It has been found that transition metals doping can promote the sensing performance. For instance, Cu-doping in SnO2 can obviously improve the sensitivity toward H2S. Several mechanisms have been proposed to explain the enhancement in sensitivity due to Cu-doping.In recent years, robust ferromagnetism at room temperature derived from doing in metal oxide semiconductors has been extensively studied experimentally as well as theoretical calculations. SnO2 is one of the most investigated materials.In this dissertation, we have studied the influences of metal, nonmetal doping or co-doping and metal adsorption on the electronic structures of the above-mentioned metal oxide semiconductors and the relation between electronic structure and the application as photocatalyst, DMS and gas sensor. This dissertation was organized in six chapters:in the first chapter, we introduce the background and research progress of the above-mentioned metal oxide semiconductors in corresponding applications; in the second chapter, the theoretical background of the methodology employed in this dissertation is concisely represented, i.e., the first-principles calculation based on the density functional theory (DFT) and the codes employed are introduced; in the third chapter, effects of doping or co-doping and metals adsorbed on the surface on SrTiO3 as a photocatalyst are studied. Besides, the structure and electronic properties of Bi-based oxides Bi12TiO20, Bi2Ti2O7 and Bi4Ti3O12, solid solution of GaN/ZnO are studied. The effects of nonmetals C and N doping are also discussed; in the fourth chapter, the properties of H2S adsorption on SnO2 and the role Cu played in enhancing the sensing performance of SnO2 toward H2S are discussed; in the fifth chapter, the origin of the ferromagnetism and the coupling interaction in nonmagnetic transition metals doped SnO2 are studied. Effects of oxygen vacancy on the ferromagnetism are explored; in the sixth chapter; we give a summarization of the contents in this dissertation, and points out several open problems to be resolved and the subsequent research plan. The studied contents and main conclusions are listed as follows:(1) The effects of Cr-doping on the electronic and optical adsorption properties of SrTiO3 have been studied. The results indicate that Cr dopant prefers to replace the Sr atom rather than the Ti atom. In fact, the doped Cr atoms prefer to partially replace Sr and partially replace Ti in the doped SrTiO3. For substitutional Cr for Sr doping, some Cr 3d gap states are introduced near the bottom of the conduction band, while the induced Cr 3d states are more close to the middle of the band gap in the substitutional Cr for Ti doped SrTiO3. Cr 3d states closed to the middle of the band gap serve as recombination center of photogenerated electron-hole pairs, which gives a reasonable explanation for the difference in the photocatalytic activity of water splitting of doped SrTiO3 with Cr at different cation sites.(2) The electronic properties of N-doped, La-doped and N/La-codoped SrTiO3 and the effects of doing on photocatalytic activity have been studied. The results indicate that N-doping introduces some accepter levels above the top of the valence band. These acceptor levels are unstable and thus oxygen vacancies tend to appear to keep charge balance. Oxygen vacancy is assumed as recombination center of photoexcited electron-hole pairs. Substituting N for O and simultaneously substituting La for Sr is the optimal N/La-codoping structure. Introduction of La increases the N solubility and then improves the N concentration in SrTiO3. More importantly, N 2p defect states are passivated by La doping due to the donor-acceptor pair (DAP) recombination and thus charge balance is kept without the formation of oxygen vacancy. Compared with that of undoped SrTiO3, band gap of co-doped SrTiO3 is narrowed, which is the reason for the photocatalytic activity under visible light.(3) The co-doping synergistic effects in N/nonmetal-(H, F, Cl, Br and I) and N/metal-(V, Nb, Ta, Sc, Y and La) codoped SrTiO3 and the effects of co-doping on the electronic structure have been studied. The results indicate that H, F, V, Nb, Sc, Y and La can improve the solubility of N in SrTiO3 and thus increase N concentration. Higher N concentration facilitates the band gap narrowing. Relatively larger distortion occurs in the octahedron structure and internal field is introduced, which can promote the separation of electron-hole pairs and improve the photocatalytic activity. Because of the co-dopingsynergistic effect, partially occupied N 2p states are passivated and changed into completely occupied. There no accept levels appear above the top of the valence band and thus suppresses the formation of oxygen vacancy, which plays as recombination center of electron-hole pairs. Except N/Sc-codoped SrTiO3, narrowed band gap can be realized for other codoped systems. (4) Ag adsorption properties on the two terminations of SrTiO3 (001) surface, SrO-and TiO2-termination, have been studied. Grand thermodynamic potentials illustrate that SrO-and TiO2-termination have a comparable range of thermodynamic stability indicating that either termination can be formed depending on whether growth occurs under Sr-rich or Ti-rich conditions. Because of the difference in coordination number with respect to surface O atoms, adsorption energy of Ag on TiO2-termination is lower than that of the Ag adsorption on SrO-termination. With metal coverage increasing, it shows an attractive interaction between ad-atoms on the SrO-termination, while a relatively weak repulsive interaction on the TiO2-termination. On the SrO-termination, Ag 4d states are more localized and Ag 5s states are partially occupied indicating that there is no obvious charge transfer between Ag and surface. On the TiO2-termination, interaction between Ag and surface O atom is characterized in covalent.(5) The atomic Cu adsorption properties on the SrTiO3 (001) surface with TiO2-and SrO-termination and its effects on electronic structure and photocatalytic activity have been studied. For Cu adsorption on the TiO2-termination, Cu atom donates electron to the surface and thus lowers the surface work function. Induced dipole moment tends to depolarize with metal coverage increasing accompaning with repulsive interaction between the ad-atoms. Some hybridized states are introduced in band gap, which are responsible for exprimentally observed photocatalytic activity under the visible light. Evaluation of Fukui functions indicates that Cu plays as electon trapping center and mediates the charge transfer from electronically excited SrTiO3 to target species. For Cu on the SrO-termination, Cu atom prefers to be adsorbed over a surface O atom. Because of lower coordination with respect to surface O compared with Cu atom on the TiO2-termination, adsorption energy increases. At Cu coverage of 0.25 ML, charge transfer across interface is negligible presenting approximate zerovalence of Cu atom. With the Cu coverage increasing, there is attractive interaction between ad-atoms and charge transfer direction reverses. At higher Cu converage of 0.5 ML and 1 ML, work function change are positive values suggesting negatively charged Cu, namely, charge transcfers from SrTiO3 to Cu. (6) The Ag-mediated charge transfer between electron-doped SrTiO3 and chemical probe molecules CO and NO has been studied. The results show that charge transfers from electron-rich SrTiO3 (Nb-doping) to Ag, which indicates the Ag-played electron trapping center role of photogenerated electron-hole pairs. Fukui functions show that Ag atom on SrTiO3 (001) surface is a strongly active site to adsorb acceptor or donor species. The mediating role Ag played in charge transfer can also be identified. The SrO-terminated (001) surface has weak activity to CO and NO and the electrostatic attraction predominantly contributes to the interaction between SrTiO3 and molecules rather than a chemical bonding. CO and NO molecule bonds are weakened (activated) because of the charge accommodation of?2P* orbital. It confirms the mediating role Ag played in the charge transfer from electron-rich SrTiO3 to targets suggesting that photogenerated electron-hole pairs can be efficiently separated via metal depositions on photocatalyst surface.(7) The atomic Au adsorption properties on the SrO-termination of SrTiO3 (001) surface have been studied. Furthermore, the mediating role Au played in storing and shutting charge from SrTiO3:Nb to NO was inspected. Au atom prefers to be adsorbed uprightly above a surface O atom on SrTiO3, while on SrTiO3:Nb it converges to a bridge site over two surface oxygen atoms. Induced surface dipole moment decreases with Au coverage increasing, in other words, it appears to depolarize. Evaluation of Fukui functions suggests that Au atom on SrTiO3 (001) surface is an active site to adsorb acceptor or donor species. Evidence that Au prefers to transit electron from electron-rich SrTiO3 to target species has been represented. SrO-termination has weak activity to NO while molecule can be strongly adsorbed on negatively charged Au. It was confirmed that charge transfer from electron-rich SrTiO3 to NO is mediated by Au. Transferred charge occupies the NO?2p* antibonding states and the molecular bond is activated.(8) The adsorption and dissociation of H2O, atomic Pt adsorption on the SrTiO3 (001) surface and the mediating role Pt played in the charge transfer have been studied. The results indicate that H2O is molecularly absorbed on the SrO-termination with two H atoms bonded to two surface O atoms and the H-O-H plane being perpendicular to the surface. The dissociative adsorption of H2O is 0.363 eV in energy more stable than molecular adsorption. The transition state (TS) for H2O dissociation implies that molecule H2O appears to rotate with one H atom abstracted from H2O. The potential energy of the TS (-0.68 eV) and the energy barrier (0.221 eV) show that the dissociation occurs spontaneously, which indicates that intact H2O molecule can hardly exist on the SrO-termination. In addition, further study indicates that H and O are readily to recombine as OH on the (001) surface with SrO-termination. Pt prefers to bond a surface O atom and intraunit polarization suggests important contribution to the Pt-O bonding. Single Pt adsorption introduces surface dipole moment and changes the surface work function. Adsorption energy decreases with the adsorbate coverage decreasing, which indicates repulsive interaction between ad-atoms. Induced surface dipole moment appears to depolarize. Comparing with Pt adsorption on SrTiO3, more Pt states are occupied illustrating the charge further transfers from SrTiO3:Nb to Pt. The role of electron trapping center Pt played on photocatalyst surface can be addressed. Pt-mediated charge transfer from electron-rich SrTiO3 to molecule H2O has been identified. The transferred charge is predominantly localized on the O atom of H2O and H-O bonds in H2O are somewhat activated due to the charge transfer.(9) The electronic structure of three BTO structures Bi12TiO20, Bi2Ti207, Bi4Ti3O12 and the effects of C and N doping on the electronic structures have been studied. For the undoped BTO structures, the electronic characters of O and Ti are similar to that of TiO2 structures. Bi 6s and Bi 6p states mainly contribute to the valance band and overlap with the O 2p states, which is in favor of the migration of electron-hole pairs. According to the defect formation energies, C and N elements can be incorporated into the BTO structures very easily during the sample preparation process, except for the Bi12TiO20. C 2p and N 2p states result in the narrowed band gap and some C 2p or N 2p impurity states appear within the band gap, both of which are responsible for the photocatalytic activity of BTO structures under visible light.(10) The electronic structure of GaN-rich GaN/ZnO solid solution and the origin of photocatalytic activity under visible light have been studied. In order to examine the co-effect of Zn and O on the structural and electronic properties, the substitutional Zn for Ga doped and substitutional O to N doped GaN have also been studied. Results indicate that the substitution of Zn atom for Ga atom causes the formation of acceptor levels above the valence band and the substitution of O atom for N atom introduces donor level under the conduction band. The charge balance can be kept when Zn and O are doped simultaneously. The strong p-d repulsion between N 2p and Zn 3d states raises the valence band maximum resulting in the narrowed band gap. Photocatalytic activity of the solid solution under visible light is attributable to the narrowed band gap. The relative position of the O atom to the Zn atom also influences the band gap.(11) The H2S adsorption properties on SnO2 (110) surface and the influences of Cu-doping on the sensing performance of SnO2 toward H2S have been studied. H2S is dissociatively adsorbed on the SnO2(110) surface with one ruptured H atom located at a bridging oxygen atom, while the HS complex bonded to a surface 5-coordinated Sn atom. H2S adsorption does not inject electronic states into the band gap of and has no remarkable change on the electrical conductivity of the SnO2 surface. Because the electronic transfer is less, the sensitivity of the stoichiometric SnO2 to H2S is low. Cu-doping in SnO2 evidently improves the formation of surface oxygen vacancy, which is important for the adsorption of oxygen species on the SnO2 surface. Reaction between adsorbed oxygen species and H2S underlies the sensing mechanism of Cu-doped SnO2:H2S donates electrons back to SnO2surface and thus decreases the potential barrier in depletion region and increases the conductance of the surface. The origin of the enhancement in sensing properties of Cu-doped SnO2 toward H2S can be addressed.(12) The magnetic properties in Zn-doped SnO2 have been studied. The results reveal that Zn-doping introduces a total magnetic moment of 1.47?B per supercell. Holes mainly localize on the first coordination shell of O atoms surrounding the Zn atom and are polarized with the same spin orientation as that of the dopant. Magnetic moments dominantly come from the spin polarization of O 2p states while the spin polarization of Zn 3d states is relatively weak. Ferromagnetism in Zn-doped SnO2 can be ascribed to the hole-mediated p-d exchange coupling interaction between the local magnetic moments of Zn and O.(13) The electronic and magnetic properties of Cr-doped SnO2 and the effects of O vacancy on the ferromagnetism have been studied. The results indicate that Cr doping obviously introduces ferromagnetism in SnO2. The formation of O oxygen obviously weakens the ferromagnetic coupling in Cr-doped SnO2, which is similar with that in Cr-doped TiO2. Exchange constants for Cr-doped SnO2 without O vacancy suggest a similar behavior with the superexchange in Co-doped SnO2. For Cr-doped SnO2 in the presence of O vacancy, exchange constants indicate a similar oscillated behavior with SnO2 with tin vacancy and Fe-doped SnO2.The results above-mentioned reveal the modification effects of doping, co-doping and metal adsorption on surface on the photocatalytic efficiency; the origin of robust ferromagnetism derived from transition metals doping; and the role metal played in enhancing the sensing performance. The conclusions are helpful to understand the properties of doing in semiconductor and the contacting behavior between metal and semiconductor. The current results are also helpful to the synthesis of functional materials with high-performance. |
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