First Principles Study Of Novel BiOCI Photocatalytic Materials

Photocatalytic technology is one of the effective methods of dealing with refractory and toxic organic pollutants in the environment. The TiO2photocatalytic degradation of organic pollutants has been an important topic and hotspot in this field, but TiO2photocatalyst can only absorb the ultraviolet light at wavelengths less than387nm and exhibits no photocatalytic activity under visible light irradiation. Therefore, the development of visible light responding novel photocatalysts becomes a key scientific problem in photocatalytic field.Recently, a new kind of photocatalytic materials based on bismuth oxyhalides BiOX(X=Cl, Br, I) have received much attention because of the inherent layer structure and particular electronic characteristics, and the BiOX has excellent photocatalytic activities and potential application prospects in photocatalysis. In order to thoroughly understand the microstructure, correlative properties and photocatalytic mechanism of the photocatalytic materials, this paper takes BiOCl as the research object, systematically study on the electronic and related properties of perfect BiOCl, BiOCl(110) surface, oxygen vacancy defects and3d transition metal doped BiOCl systems based on the first-principles calculation method combined with the experimental test and modern spectral analysis. The obtained results should provided the theoretical guidance for designing and synthesizing BiOCl photocatalyst materials with the excellent photocatalytic efficiency. The main research contents are as follows:(1) The microstructure, electronic and optical properties of perfect BiOCl were studied comparatively using the local density approximation LDA(CA-PZ) and generalized gradient approximation GGA(PBE, RPBE, PW91, WC, PBESOL) schemes, and the correlation between electronic structure and optical properties of perfect BiOCl were also discussed. The calculated results showed that there is no significant difference between LDA and GGA methods on the calculated intrinsic properties for perfect BiOCl. The Bi—O and Bi—Cl bonds display covalent and ionic bonding characteristics, respectively. In the Ella and E//c polarization directions for perfect BiOCl crystal, the static dielectric constant ε1(0) are5.59and4.61respectively, and the refractive index no are2.34and2.13respectively. The maximum energy loss peak is located at about19.0and19.5eV, corresponding to the sharp drop points of its reflection spectra, which are ascribed to electron transition between O2s states in valence band and Bi6p states at the minimum of conduction band. The optical absorption band edge of perfect BiOCl is3.40eV, corresponding to365nm, in good agreement with the reported experimental value of3.05-3.55eV.(2) In order to evaluate the effects of oxygen vacancy on the stability, electronic structure and optical absorption property of BiOCl, the structural relaxation, formation energies, electronic structures and absorption spectra of the perfect BiOCl and BiOCl containing oxygen vacancy(BiO15/16Cl) have been studied using first-principles method based on density functional theory(DFT). The simplistic models for the possible formation mechanism of oxygen vacancy and the efficient separation process of photo-induced electron-hole of BiO15/16Cl have been put forward. The calculated results show that the errors between lattice parameters and experiment values of BiOCl and Bi15/16Cl after optimization are less than7.5%. The calculated formation energy (-4.21eV) reveal that the existence of oxygen vacancy in the BiOCl crystal was very possible on the basis of thermodynamics. The density of states of BiO15/16Cl indicate that the existence of oxygen vacancy mainly affects the6p states of the neighboring Bi atoms and introduces a new electronic state within the forbidden band compared with perfect BiOCl, which acts as a capture center for excited electrons, and consequently improves the effective separation of electron-hole pairs, achieving the electronic transition of Cl2p or O2p states under the visible light irradiation. The absorption spectra show that BiO15/16Cl exhibites a new absorption band peak at about2.72eV (456nm) in the visible region, this is in good agreement with the experimental value of468nm.(3) The effects of Mn doping concentration on the electronic structure and optical absorption performance of MnxBi1-xOCl(x=0,0.0625,0.09375and0.125) using first-principles calculation based on DFT has been carried out. The macroscopical performance and experimental results cuold be explained from the view of microscopic properties, obtaining the appropriate Mn doping concentration, which provided the meaningful guidance for the preparation of Mn doped BiOCl photocatalyst. In order to further study the effects of oxygen vacancy on the photocatalytic properties of Mn doped BiOCl system, we adopted the DFT+U calculation method and thoroughly discussed the geometric structure, stability, Mulliken charge, band structure, electron density of states, optical absorption and effective mass of Mn doped BiOCl with the existence of oxygen vacancy. The results showed that Mn—O bond possesses a stronger covalent bonding strength than Bi—O bond. The modest Mn doping amount make optical absorption band edge of BiOCl to extend to visible region of550nm, consistent well with the experimental absorption range of400-600nm. With the increase of Mn doping concentration, the red-shift phenomenon of Bi1-xMnxOCI system is obvious and the intensity of visible light absorption increases gradually, but too much doping amount of Mn results in that the absorption of UV-light region greatly diminishes. The substitutional (or interval) Mn doped BiOCl exhibits p-type (or n-type) doping characteristic. The substitutional Mn doped BiOCl reveals the better feasible Mn doping behavior due to the beneficial p-type doping characteristic and p-d hybridization between substitutional Mn and adjacent Bi (or O) atoms in the CB (or VBM), forming the effective p-d hybridization and impurity level, consequently improving the separation of photo-induced electron-hole pairs. The occurrence of oxygen vacancy (Vo) in BiOCl crystal significantly influences the neighboring Bi6p states, forming a capture center of photo-excited electrons in the forbidden band, which enhances the efficient mobility of photo-generated carriers and improves the effective separation rate of electron-hole pairs. The better negative formation energy of substitutional Mn-doped BiOCl:VO1/VO2(-6.391/-6.299eV) demonstrates thermodynamically the favorable structural stability. Besides, the higher relative ratio values (4.50and4.43) for the effective masses of activated holes and electrons in the Mn-doped BiOCl:Vo system imply the lower recombination rate of electron-hole pairs.(4) The electronic structures and optical absorption properties of3d transition metal doped BiOCl system have been studied using first-principles method based on DFT+U. The calculated results indicated that the visible-light response of doped semiconductor BiOCl system is not only determined by its forbidden band width but also dominated by the contribution of the3d doping level as well as the electron configuration of3d up-and down-spin states. The3d transition metal doping can effectively ameliorate the electronic band structure of BiOCl, and make the doped BiOCl systems to produce the red-shift of optical absorption band edge, especially, Fe, Co, Ni doped BiOCl system appear the large absorption coefficient in the visible light region, which is associated with the electron configuration of3d orbitals to some extent. Our results should provide a theoretical basis for better understanding the relevance between microscopic electronic structures and macroscopic photocatalytic performances of3d transition metal doped BiOCl systems.(5) The electronic properties of BiOCl(110) surface have been calculated using first-principles method, and the calculated results revealed that BiOCl(110) surface exhibits the unique electronic characteristics and the Bi6p states of the outer layer Bi atoms for BiOCl(110) surface mainly dominate the energy range of0.66to0.89eV, resulting in that energy band structure of BiOCl move towards to the lower energy direction and enhancing the electron transition and the efficient separation of photo-induced electron-hole pairs, which would be benefit to improve the photocatalytic performance. In order to verify the reliability of the calculation, a highly exposed BiOCl(110) surface thin film with high crystallinity has been successfully prepared via a novel electrochemical method composed of a cathodic electrodeposition and an anodic oxidation at room temperature. The experimental results showed that the Bi film on the Ti substrate could be oxidized gradually to tetragonal BiOCl crystal with the increase of the anodic oxidation voltage. When the anodic oxidation voltage is2.0V, pure tetragonal BiOCl thin film with the highly exposed (110) surface (named BOC-2.0) can be obtained and the interlaced BiOCl nanosheets are arranged orderly and evenly distributed on the Ti substrate. BOC-2.0thin film could not only guarantee the intrinsic photochemical properties of BiOCl bulk but also exhibit additional electronic characteristics of BiOCl(110) surface, and consequently the wonderful synergistic effect between BiOCl bulk and BiOCl(110) surface accelerates the efficient separation of electron-hole pairs and produces the high reducing superoxide radicals O2-and strong oxidizing hydroxyl radicals*OH required for the degradation of organic compounds. For as-prepared BOC-2.0thin film, the degradation ratio of methyl orange (MO) reaches98%under2.5h UV irradiation and still remains90%at the fifth cycle, and the COD removal efficiency achieves73.47%after8h reaction time for50mg/L MO solution. Our experimental and theoretical results further reveal the guidance of first-principles calculations for the design and preparation of highly active photocatalyst, and this has laid a good foundation for deep understanding the electronic structure, morphology control, surface property and facet-dependent property of BiOCl photocatalytic materials.