| TiO2 continues to remain a major concern by us which has good activity under UV light, is cheap, non-toxic, and with high chemical stability. But, because of the forbidden band width of titanium dioxide, it can not fully utilize the solar energy. After modification that N be mixed into TiO2, it is more easily to produce catalysis in visible light. The utilization of solar energy by TiO2 is low, and the ultraviolet light of solar energy less than 380 nm wavelength only could be absorbed by TiO2-rutile commonly used whose forbidden band width is only 3.0 eV. In order to better use of solar energy, it is a feasible scheme that doping of TiO2 on making the TiO2 absorption band red shift in the direction of visible light. Due to the introduction of impurity atoms, it is likely producing the middle level in TiO2 forbidden band, also possibly the forbidden band width is narrowed through a mixture of band. Under the above two cases, the energy of the electron transition in the valence band to the conduction band both could be effectively reduced, so that they can absorb the redshift. And N doping TiO2 can obtain obvious narrow-band effect, which can effectively improve the photo catalytic effect. But now, there has been not comparative study on different minerals doped N into TiO2. Therefore, this article mainly study of physical properties from the different mineral phases after TiO2 doped crystal electronic structure, geometric structure and related mineral.According to the research object, the 2×2×2 and 2×2×1 superlattice TiO2 model doped with rutile and anatase has been respectively setted up. Based on first principles, this paper optimizes the doping structure using generalized gradient approximation and calculates the doped TiO2 band structure, density of states using local density approximation. According to physical properties in the calculation module CASTEP, this paper calculates the optical properties of the mixed model. The purpose of comparative analysis of n/p samples doped with anatase actual preparation and optical spectrum detection, is to verify the accuracy of theoretical calculation and feasibility of the practical production. Finally, a comparative analysis of the single N doping rutile and anatase have been done.Pure anatase and rutile were carried out simulations to obtain rutile and anatase band structure and electronic structure. Rutile combination calculation results compared with the previous results, found that the calculated band gap of 2.048eV, less than the experimental value (3.0eV), while others use different methods to calculate the band gap (for 1.9eV) are in good agreement. Because of the density functional theory itself there is a certain bias in the calculation of the energy band structure, for the comparative analysis of the whole system, the error does not affect the qualitative analysis we have done, and thus also shows that it is feasible to use methods based on first principles simulation. Through comparative analysis found that pure rutile relative to pure anatase, due to the characteristics geometries, electronic structures there are some differences in the density of states of rutile each atom reflecting its conduction band is significantly lower than the anatase when mineral structure, and the position of the top of the valence band substantially the same, compared to the anatase-rutile for which the band gap is significantly smaller, which helps the response to visible light of rutile. Electronic structure of rutile, Reflecting the bound outer electrons, and therefore have also the interaction force between atoms is far higher than the anatase, which also happens to prove the stability of the geometric structure of the reasons, therefore, the mineral surface thereof, but also difficult to form a large number of empty points to improve oxidative capacity, also because of its small band gap, it is easy to let the excited electrons back to the valence band, which although better visible response shift, but there is also the problem of low catalytic efficiency.Although the higher activity of anatase, but at a high temperature occurs easily converted to rutile phase transformation. Therefore, for better stability of many of the rutile modification should also pay attention to. Based on actual cases, rutile TiO2 photocatalytic degradation of certain contaminants in equally high photocatalytic activity, under certain conditions, rutile and anatase particles synergies. Therefore, the doping of rutile there are some practical needs. Therefore, this article has specialized for the same period of the elements N O elements, B, C, F four kinds of anion-doped rutile situations were simulated calculation and analysis showed that the N atom impurity-doped rutile can effectively reduce the band gap, and forming a P-type semiconductor, so that the crystal doped with a strong electron adsorption and oxidation resistance, and the C, B dopant atoms, P atoms in the orbital electrons near the Fermi level is formed at the transition level, and the material has the strong conductivity, loss characteristics of the semiconductor material. The Fermi level in the conduction band bottom by doped of F to form an n-type semiconductor. The results show that, N atom can significantly decrease the doping rutile bandgap near the Fermi level in the impurity level is formed, the visible light can effectively improve the response with respect to several other single anionic dopant, it has a good modified advantage.Compared with double N atoms doped TiO2 rutile phase of geometric structure and electronic structure. The results show that two N atoms occupy two adjacent O impurity atom position. The lowest energy of the system, showing that the state of the most stable geometry is sufficient, in the actual production and the doping process is most likely to occur. From the energy band structure and density analysis can get dual N atoms doped TiO2 resulting in the emergence of the impurity levels in the band gap, and when taken to the nearest neighbor impurity atoms occupying bandgap minimum, indicating that it can help increase light catalytic activity, as the distance between two impurity atom is increased, the band gap gradually becomes larger. Obtained by a single N atom in the incorporation of the band gap width of the rutile 1.553eV, and N atoms incorporated into the double band gap width as rutile spacing between the two impurity atom changing. When the impurity atoms in the nearest neighbor sited, the smallest band gap, the band gap width with increasing distance from the two impurity atom is increased, will be gradually increased. When the distance between the two impurity atoms is 0.3377nm, the pair of N atoms doped rutile bandgap larger than a single N atom doping. In general, by introducing an impurity N, the band gap of the energy band showing significantly reduced as a whole. When taken to the nearest neighbor N atoms double occupancy, the band gap is smaller than single-doped N, N-doped double can increase the photocatalytic effect in theoretically.Since the most common type of defect in the form of three-dimensional periodic crystal is vacancy point defects, it can also affect the physical properties of the material, combined with the actual situation under neutral principles of electricity, in a dual N position selected based on the combination oxygen vacancy situation was simulated. Due to the incorporation of the double N atoms ortho rutile TiO2 doped supercell model has six possible when (2 × 2 × 2), respectively, for different models geometry optimization, compare their total energy found in the structure of the system, when the two when nitrogen atoms occupy the nearest neighbor impurity position, minimum energy system, the structure of the most stable. Analysis can be seen from the band structure and density of states, when dual N is the nearest neighbor position, though not the smallest band gap, but the architecture of the total energy minimum, also shows that during the experiment, more prone to double N atoms will follow the most Nearby appear. The dual N atoms in the equilibrium position is relatively difficult to achieve but, when the dual N atom in position angle compared to other models for the larger band gap and change the geometry is not obvious, and therefore did not make the crystal structure of a large deformation, which due to the N atom one less than the oxygen atom O electrons when in the nearest position, the lack of electrons, and the electron must share, close to each other as it will cause distortion. When the position in the symmetrical double-N, each N atom is surrounded by other atoms just bound to the N atom, and forms a symmetrical position relatively stable equilibrium, but the distortion is small, forcing the outer electron further away, but also led further away from the conduction band position of the Fermi level, which is difficult to be excited. Smaller band gap crystal material is mainly due to the emergence of the impurity level, but the absolute bandwidth between the conduction band and valence band still exists. N in selected ortho-doped double structure is more stable, based on the analysis and comparison of the eight possible electrically neutral oxygen-vacancy-doped model, starting from its geometric structure to structure the lowest total energy system as a starting point, found when the oxygen atom vacancies occur in the vicinity of position and two nitrogen atoms in his back position, the lowest energy structure of the most stable. From the energy band structure and density analysis can get, when vacancy doping, the migration of impurity elements on the Fermi level is not large, but due to oxygen vacancies, leading to the conduction band of metal atoms move to the Fermi level, the formation of the N-type semiconductor material, but not as responsive to visible light bis N-doped case, but due to the existence of the atomic vacancy of oxygen, will make it easier to capture the mineral free electrons, and thus the intensity of the oxidation reaction will be improved.Explore whether the actual cause changes in the doping system, optical properties, this paper studies the calculation of nitrogen-doped, nitrogen/phosphorus geometry anatase TiO2 co-doped with the first principles of plane wave ultrasoft pseudopotential method, electronic structural and optical properties. Electronic structure calculations show that, although a single N atom in the band gap TiO2 doped with the impurity level occurs, the energy band gap is reduced, although the visible light to improve the response; however, the position of the conduction band and the valence band did not change, and can not produce good photocatalytic properties; while anatase N/P co-doped only after the band gap is reduced, improving the utilization of visible light, the conduction band and valence band are moving to a lower energy level, and the bottom of the conduction band through a fee m able and formation of the N-type semiconductor, is conducive to the generation of photo-generated electrons to enhance oxidation and photocatalytic properties. Dielectric function, absorption spectrum and reflection spectrum display, N/P co-doped anatase new peak interval appears in 0-3eV, and the peak relative to the single N-doped high, showing a significant red shift, will increase the visible response. Sample XRD, IR, RRS also confirmed N/P co-doping, without substantial change in the original crystal structure, the crystal surface can be reduced clearance, to reduce the vibration frequency of a covalent bond, such that the sharp doping titanium redshift effect will occur, which is consistent with the theoretical analysis conclusions.In this paper, rutile and anatase single N-doped and also do a comparative analysis. Through comparative analysis of single N-doped electronic structure and band structure of rutile and anatase after supercell model, found the electronic structure of rutile anatase doped with respect to doping also exists after a certain the case of differences in the density of states of each atom rutile Figure reflects its conduction band is significantly lower than the anatase structure, while the position of the top of the valence band is basically the same, the rutile compared to anatase band gap in terms of its is significantly smaller, which contributes to the visible light response of rutile. Since the introduction of impurity atoms will result in rutile Ti atomic electronic structure changes, due to the more O atoms, N atoms, the number of outermost electrons to itself less, indicating that the impurity atoms in bound outer electrons would be larger, and thus between each atom force is also much higher than the anatase, which also happens to prove the stability of the geometric structure of the reasons, therefore, the mineral surface thereof, it is difficult to form a large number of known points air to increase the oxidation capacity, but also due to the smaller band gap, it is easy to let the excited electrons back to the valence band, which although better visible response shift, but there is also the problem of low catalytic efficiency.Finally, for the anion doping, the average would be rutile and anatase photocatalytic and light response have beneficial effects, particularly N-doped elements to be significantly better than the same period the other three O atoms B, C, F. But the single N-doped anatase and rutile band gap reduction is far better than the double N-doped effect. While in the oxygen-vacancy situation, although due to the larger space will increase to characterize the catalytic performance, but did not reduce the band gap too much, but relative to the increase in single-doped, visible responsiveness decreased. By comparing the simulation and experimental results analysis after N /P co-doped anatase, can prove benign improve optical properties, further evidence of the study of the physical properties of the mineral-based First Principles correctness. Therefore, the actual production, the better to control the number of N atoms in the doping concentration and oxygen vacancies, both to meet but also to enhance the photocatalytic properties of visible light to meet the response, followed by analysis of the results showed that the co-doping is effective in improving effective means of optical properties of TiO2, and finally, because of anatase and rutile can be modified by doping, should be combined with the advantages and disadvantages of each of the two phases, in the actual production environment for different usage ratio of each other to form synergistic function. |
PDF Full Text Request