Structural Design And Properties Study Of Effective Inorganic Semiconductor Photocatalysts Based On Energy Band Control

First principle method could be used to study the properties of functional materials from a sight of electronic level. It is set about the basic physical laws to understand and forecast the properties of materials by solving the Schrodinger equation. Recently, based on the development of theoretical methods and computer technique, first principle method has been used as a routine research mean in condensed matter physics, quantum chemistry and material science. Base on semiconductor photocatalytic technology and semiconductor band engineering, this thesis has developed the doping method to control the band structure to improve the visible light absorption and photocurrent density of semiconductor photocatalysts. The effect of different doping forms on electronic properties has been studied to reveal the mechanism of improving photocatalytic activity induced by doping. This work can supply a theoretical sight to understand photocatalysis and conduct the experiments to synthesize semiconductor photocatalysts with better activity. The main researches are listed as follow:The first chapter introduced the research background of this thesis, including the development and status quo of semiconductor photocatslytic technology and band engineering. In this chapter, the ideal of band control using doping method has been proposed to improve the activity of semiconductor photocatalysts.In the second chapter, we have introduced the basic theoretical methods of density functional theory. The main line is the development of exchange-correlation functional, including Local Density Approximation (LDA), Generalized Gradient Approximation (GGA) and self-consistent field theory. Then, several quantum chemistry software packages have been introduced.From the third chapter, this thesis has introduced different doping forms, including metal doping, nonmetal doping, and codping. The key points are the relation between geometry structure and band structure. The changes induced by different doping forms have been discussed in detailed. In the third chapter, we have systematically studied the effect of alkalis metals and alkaline earth metals on the visible light photocatalytic activity for h-WO3. The calculated results indicate the forms of alkalis metals and alkaline earth metals in the channel of h-WO3are related with both the ionic radius and the size of channel. For Li+, Be2+, Mg2+with a small ionic radius, the doping will adhere to the side of channel. The larger K+, Rb+, Cs+ionic will stabilize at the wide side of channel. The media ionic, such as Na+, Ca2+, Sr2+, and Ba2+, will be pinned at the narrow position of channel. The alkalis metals and alkaline earth metals doped in the lattice act as n-type impurity, which induce the conductor band side higher and the optical absorption edge blue shift. Importantly, the change of the band structure is benefit to improve the reduced ability of conductor electron. In addition, the WSd1reduced center is formed close to the bottom of conductor band, which will act as a trap of excited electrons to depress the recombination of electron-hole. The formation energy and charge population analysis suggest that Sr doping will be better for improving the photocatalytic activity of h-WO3.In the fourth chapter, the effect of transition metals doping on the distribution of active sites of CeO2(111) surface has been discussed. The present work indicates although the valence state of Ti and Mn doping is different, the coordination environment is similar—both are four coordination structures. And the coordination of surface lattice oxygen is changed from three to two, which improves the activity of surface oxygen. In the Zr doped CeO2(111) surface, the Zr-O bond length is contacted, and the lattice distortion promote the formation of surface oxygen vacancy. The doping form of Cu and Co is similar in CeO2(111) surface. Although the bond between one surface oxygen and doping is strengthened, there are two surface oxygen are activated. Due to the similar ionic radius of Tb and Ce, the activation of Tb doping for surface oxygen is slight. The formation energy of surface oxygen vacancy has been calculated and analysis. Based on the calculated results, we conclude the activation of surface oxygen induced by transition metals doping is linear correlation with the Pauling electronegativity of doping elements—the larger electronegativity, the better activated ability.In the fifth chapter, two different mechanisms for improving photo-catalytic activity in different types of F-doped ZnWO4are tentatively proposed, based on experiments and density function theory calculations. We have synthesize interstitial F doped ZnWO4, which showing a better photocatalytic activity than pure phase. And the optical absorption edge of interstitial F doped ZnWO4has about50nm red-shift. Theoretical calculations for the interstitial F-doped model indicate the red-shift is mainly attributed to the mixing states of F2p and O2p states localized above the top of the valence band. Electronic transitions from these localized states induce a band gap narrowing of about0.3eV. Furthermore, we have also given a theoretical modeling about F substituted doping. Our calculations show that a reduced W5+center adjacent to the doped F atom will act as a trap for the photo-induced electron, and will thus result in a reduction of electron-hole recombination and improvement of the photo-catalytic activity. This work shows that F-doped ZnWO4will be a promising photo-catalyst with favorable photo-catalytic activity in the UV region.In the sixth chapter, the origin of improved visible-light photocatalytic activity of N-doped ZnGa2O4is investigated using experimental and theoretical methods. Our experiments indicate N doped ZnGa2O4photocatalyst show a better photocatalytic activity of degrading methylene blue under visible light irradiation (λ>420nm). And the longer nitridation time of ZnGa2O4induces the better photocatalytic activity. This observation has been explained by theoretical calculations. In the case of one substituted N atom in the lattice, the optimized configuration shows an open-shell doublet state as a result of a paramagnetic N2-defect, thus introducing some partially occupied states in the band. Band gap narrowing is about0.3eV, which is not large enough to improve the visible-light response of ZnGa2O4. When two O atoms are substituted by two N atoms, two self-passivation mechanisms are found. One mechanism is induced by a N2dimer in the lattice. The two unpaired electrons are paired up together in the N-N bond, thus forming a nonmagnetic N24-defect with a closed-shell single state and passivating the defect level. The electron transition from the N-N π*states in the gap lead to a larger optical absorption red shift of about1.32eV. Another passivation mechanism originates from the synergistic effect of N-doping and oxygen vacancy. Through the charge transfer in the N-Vo-N structure, the two unpaired electrons are paired up and the two initial N2" defects changed to N3-defects, thus forming a continuous passivated defect band. Band gap narrowing is about0.75eV due to the mixing of N2p and O2p states in the VBM. Our work provides a solid basis for the origin of the enhanced visible-light photoactivity of N-ZnGa2O4The seventh chapter work is main about the effect of (N, C) codoping on the visible photo-activity of hexagonal wurtzite ZnS. For the mono N-or C-doped ZnS systems, the substituted doping just induces a slight band gap narrowing (less than0.2eV), while the interstitial doping leads to more significant optical absorption red shift. However, the calculated energy indicates the formation of the interstitial doping is difficult with the lower impurity concentration. For the N+C-codoped systems, the electron transition from the impurity states in the gap to the conduction band will induce a large red shift. However, the impurity states are partially occupied character, which may act as recombination center and reduce the photo-induced current density. The calculated results of2N+C codoped ZnS indicate the N-C-N trimer structure is formed in the ZnS lattice, and the trimer doping induces a larger red shift of photo-electron transition and passivates the partially occupied states by the charge compensation effect in acceptor-donor-acceptor pair. Our work shows that2N+C-codoping will be a promising way for improving the visible light activity of semiconductor photo-catalysts.In the eighth chapter, we have designed several ZnGa2O4models with2N+VA codoping, which not only significantly decrease the band gap but also depress the formation of electron-hole recombination center. The present calculations indicate the VA atoms substitute Ga atom acting as an n-type impurity, and the electron transition energy from the valence to the unoccupied levels has increased about0.4-0.7eV due to the B-M band shift. Although the higher N doping concentration in lattice of ZnGa2O4induces about0.5eV band gap narrowing, some partial impurity states are formed and localized above the top of valence band. For the codoped models, through the electron transfer from the acceptor-donor-acceptor pair of N-VA-N structure, the partial impurity states are passivated and form continuous impurity bands in the valence band maximum. And the band gap narrowing for2N+P, As, Sb, and Bi codoped models is1.205,1.056,0.888, and1.035eV, which are larger than that in only N doped model. In addition, the calculated formation energy for codoped models indicates the formation between N and VA doping is mutual promoted in the lattice of ZnGa2O4. Thus, we conclude the designed2N+VA copded models are benefit to improve the visible light photocatalytic activity of ZnGa2O4In the last chapter, we summarize the conclusions and innovative points of this dissertation, and preview the further studies.