Electronic structure and the optical and photocatalytic properties of anatase doped with vanadium and carbon

The electronic structures of undoped anatase and anatase doped with carbon and vanadium have been calculated using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the LSDA + U approximation. It has been shown that the doping of TiO₂ leads to the formation of narrow bands of the C and Vimpurity states in the band gap. The calculations of the imaginary part of the dielectric function have made it possible to estimate the intensity of the optical absorption. It has been established that the doping with vanadium and carbon leads to optical absorption in the visible range and to an increase in the absorption in the ultraviolet range up to 4 eV. This should result in an increase in the photocatalytic activity on the surface of the doped anatase. The experimental determination of the photocatalytic activity of whiskers of the anatase doped with carbon and vanadium in the reaction of hydroquinone oxidation has confirmed the increase in the activity of the doped materials under exposure to ultraviolet, visible, and blue light. The phenomenon of dark catalysis in the anatase doped with carbon and vanadium has been interpreted within the concept of low-energy electronic excitatio ns between the impurity levels of carbon.     

Electronic band structure and optical absorption of nanotubular zinc oxide doped with Iron, Cobalt, or Copper

The absorption spectra of the precursor-derived solid solutions Zn_{1 ? x} M _x O (M = Fe, Co, Cu) with a tubular morphology of aggregates have been investigated in the ultraviolet and visible regions. The maximum metal concentration x in the Zn_{1 ? x} M _x O solid solutions is 0.075 for iron, 0.2 for cobalt, and 0.1 for copper. It has been found that the optical absorption and the band gap of the Zn_{1 ? x} M _x O compounds depend on the type of dopant. The obtained experimental data have been interpreted using the results of the performed ab initio calculations of the electronic band structure and optical absorption.     

Electronic structure and optical properties of anatase doped with bismuth and carbon

The calculations of the electronic structure of pure anatase and the anatase doped with carbon and/or bismuth have been carried out using the ab initio tight-binding linear muffin-tin orbital (TB-LMTO) method in the local spin density approximation with the inclusion of single-site Coulomb correlations (LSDA + U). The dielectric function, absorption coefficient, and refractive index have been calculated in the random phase approximation. It has been found that, upon doping, narrow bands of carbon and bismuth impurity states are formed in the band gap. The calculations of the optical absorption coefficient have demonstrated that the C,Bi-doping can lead to the absorption in the visible region and an enhancement of the absorption in the near-ultraviolet region. Therefore, the C,Bi-doping can increase the photocatalytic activity on the surface of doped anatase.     

Electronic band structure and distribution of excited electrons in the conduction band of anatase doped with boron, nitrogen, and carbon

A method for calculating the distribution of excited electrons in the conduction band of semiconductors has been proposed. This method takes into account both the excitation of electrons by means of an external light source and the transitions to the bottom of the conduction band due to the electron-phonon interaction. The interaction of electrons with the light field has been calculated from first principles in the dipole approximation using the linear muffin-tin orbital method. The electron-phonon interaction has been calculated in terms of the density functional perturbation theory. The method has been applied to the calculation of the quasi-steady-state distribution function of excited electrons in anatase doped with boron, nitrogen, and carbon. The correlations of the distribution function with the photocatalytic activity of doped anatase have been discussed.     

Electronic band structure and optical properties of silicon nanoporous pillar array

Silicon nanoporous pillar array (Si-NPA) is fabricated by hydrothermally etching single crystal silicon (c-Si) wafers in hydrofluoric acid containing ferric nitrate. Microstructure studies disclosed that it is a typical micron/nanometer structural composite system with clear hierarchical structures. The optical parameters of Si-NPA were calculated by general light-absorption theory and Kramers–Kronig relations based on the experimental data of reflectance and the variations compared with the counterparts of c-Si were analyzed. The features of the electronic band structure deduced from the optical measurements strongly indicate that Si-NPA material is a direct-band-gap semiconductor and possesses separated conduction sub-bands which accords with conduction band splitting caused by silicon nanocrystallites several nanometers in size. All these electronic and optical results are due to the quantum confinement effect of the carriers in silicon nanocrystallites.     

Concentration-dependent electronic structure and optical absorption properties of B-doped anatase TiO2

The concentration-dependent electronic structures and optical properties of B-doped anatase TiO₂ have been calculated using the density functional theory. The calculated results indicate that the electronic structures of B-doped TiO₂ have changed compared with those of pure TiO₂, which is mainly due to the new midgap states induced by B doping. As to the optical properties, we calculate the imaginary part of dielectric function ε₂(ω) and optical absorption spectra of pure and B-doped TiO₂. Two transitions E₁ and E₂ emerged after B doping. The intensity of absorption is enhanced by B doping both in the UV and visible regions. According to the results of imaginary part of dielectric function ε₂(ω) and DOS, it can be concluded that the two optical transitions correspond to the transitions from the O 2p states in the top of valence band to the midgap states and from the midgap states to the Ti 3d states in the bottom of conduction band, respectively. These results have important implications for the further development of photocatalytic materials.     

Magnetic,electronic, optical,and photocatalytic properties of nonmetal- and halogen-doped anatase TiO2 nanotubes

The structure stability, magnetic, electronic, optical, and photocatalytic properties of nonmetal (B, C, N, P, and S), and halogen (F, Cl, Br, and I)-doped anatase TiO₂ nanotubes (TNTs) have been investigated using spin polarized density functional theory. The N- and F-doped TNTs are the most stable among other doped TNTs. It is found that the magnetic moment of doped TNT is the difference between the number of the valence electrons of the dopant and host anion. All dopants decrease the band gap of TNT. The decrease in the band gap of nonmetal (C, N, P, and S)-doped TNTs, in particular N and P, is larger than that of halogen-doped TNTs due to the created states of the nonmetal dopant in the band gap. There is a good agreement between the calculation results and the experimental observations. Even though C-, N-, and P-doped TNTs have the lowest band gap, they cannot be used as a photocatalysis for water splitting. The B-, S-, and I-doped TiO₂ nanotubes are of great potential as candidates for water splitting in the visible light range.     

The effect of crystal lattice distortions on the electronic band structure and optical properties of the N,V- and N,Na-doped anatase

The crystal structure, energy band structure and optical absorption of the N,V-doped and N,Na-doped anatase are studied by means of the first-principle pseudo-potential plane wave and linear muffin-tin orbitals methods. We show that the nitrogen and vanadium atoms have a tendency to form covalently bonded pairs. The crystal lattice distortions associated with doping essentially affect the optical absorption. With doping the impurity bands emerge in the band gap of the host anatase, however, a noticeable increase of optical absorption takes place at the energy only above 3 eV. Possible impact of this effect on the photocatalytic activity of the doped anatase is outlined.     

Optical absorption and band structure of PbTe

It is shown that the rigid band model cannot be applied to the sodium tungsten bronzes outside of the cubic range. The origin of the band gap states observed in the semiconducting range is discussed.     

Electronic structure and optical absorption spectra of CdSe covered with ZnSe and ZnS epilayers

Using the first-principles methods we compute the electronic structure and the absorption spectra for a wurtzite CdSe (0001) slab covered with zincblende ZnSe and ZnS epilayers. For each structure we compute the DOS and the imaginary part of the dielectric function. We find that the semiconductor passivation shifts the ‘near Fermi-level’ states of the bare CdSe slab down to lower energy levels. The migration suggests the decrease of surface effects and energy loss. We observe the substantial reduction of the abnormal peaks in the absorption spectra of the bare CdSe slab, which seems to be a consequence of the DOS migration. This is consistent with the experimental results that a proper passivation enhance the luminescence efficiency. We also study the case that the epilayer surface is terminated with PH₃ and find the PH₃ passivation also reduces the surface state to some extent.     

Electronic structures and optical absorption of multilayer graphenes

We study the electronic structures and the optical absorption spectra of the multilayer graphenes in the effective mass approximation. We decompose the Hamiltonian of graphene with an arbitrary thickness into smaller subsystems effectively identical to monolayer or bilayer graphene, and express the optical spectrum as a summation over the subsystems. We include the full band parameters which compose the bulk graphite, and closely study their effects on the band structure. We found that the particular band parameters destroying the electron–hole symmetry can affect the optical spectrum through shift of the absorption edge.     

Hierarchical ZnO microrods: synthesis,structure, optical and photocatalytic properties

Hierarchical ZnO microrods have been prepared by cetyltrimethylammonium bromide (CTAB)-assisted hydrothermal process at low temperature. Sheetlike or fluffy ZnO nanostructures have been developed in a large density on the surface of the microrods. The ZnO cores grow preferentially along [001]; the nanosheets are single crystalline and grow in two dimensions along the [001] and [120] directions. The formation of hierarchical structures has been attributed to the presence of CTAB in the solution, which facilitates the secondary nucleation and growth. The nanosheet-covered hierarchical microrods have been analyzed by Raman and photoluminescence spectroscopy, and they exhibit a blue emission at 465 nm and a strong orange emission at 640 nm. Photocatalytic activities of the products have been examined, and the nanosheet-covered hierarchical microrods display the best activity in photodegradation of phenol due to their unique surface features and high surface area.     

Europium-doped mesoporous anatase with enhanced photocatalytic activity toward elimination of gaseous methanol

Mesoporous anatase doped with various amounts of Eu were synthesized via a sol-gel route using Pluronic P123 as template. The XRD and TEM results show that Eu doping favors the formation of anatase with smaller crystal size. The photocatalytic activity of the mesoporous anatase was tested in photocatalytic elimination of gaseous methanol. The photocatalysts exhibited higher activity than that of the commercial photocatalyst (Degussa, P25) under either ultraviolet or visible light irradiation. The superior activity of the mesoporous anatase may be attributed to the synergic effects of the surface area, crystal size and doped Eu species.     

Electronic optical,properties and widening band gap of graphene with Ge doping

The Density Functional Theory with full potential linearized augmented plane wave uses to study the pure graphene and doped with different amounts of Germanium (5.55, 8.33, and 12.5%). It uses also Generalized Gradient Approximation and Tran-Blaha modified Becke-Johnson formalism to investigate their electronic and optical properties. Furthermore, it utilizes Germanium is able to open band gap due to the p-states of Ge, which are in hybridization with p-states of Carbon. Germanium concentration decreasing or increasing can control the band gap opening of graphene system. The optical absorption increases in the ultraviolet range, due to the important absorption that contains germanium above 150 nm.     

H,F修饰单层GeTe的电子结构与光催化性质

采用基于密度泛函理论的第一性原理平面波赝势方法,计算了单层GeTe、表面氢化及氟化单层GeTe的晶体结构、稳定性、电子结构和光学性质.计算结果表明,经过修饰后, GeTe的晶格常数、键角、键长增大,且均具有较好的稳定性.电子结构分析表明,单层GeTe为间接带隙半导体,全氢化修饰、全氟化修饰以及氢氟共修饰(F, Ge同侧;H, Te同侧)则转变为直接带隙半导体,且修饰后的能隙均不同程度减小.载流子有效质量表明,全氢化、全氟化以及氢氟共修饰GeTe (F, Ge同侧;H, Te同侧)的有效质量减小,其载流子迁移率增强.带边势分析结果显示,单层GeTe能够光裂解水制氢和析氧,而修饰后的GeTe的价带带边势明显下移,其氧化性明显增强,能够光裂解水析O2, H2O2, O3以及OH·等产物.光学性质表明,修饰后的GeTe对可见光区和红、紫外区的光谱吸收效果明显增强,表明其在光催化领域有着广阔的应用前景.     

Quasiparticle band structure and optical spectrum of LiBr

The electronic structures of the Fe-doped perovskite ruthenates BaRu_1?x Fe _x O₃ with x = 0, 0.25, 0.5, 0.625, 0.75, and 1 are investigated through density-functional calculations. Large exchange splitting and small crystal field splitting are found in BaFeO₃, and a contrary scenario can take place on BaRuO₃ as expected since the Ru atom has a highly extended 4d orbital. The small exchange splitting and extended 4d states are the reasons why the obtained spin magnetic moment (0.628μ _B ) is significantly lower than the spin only value (2μ _B ) for the t _2g 3↑ t _2g 1↓ electronic configuration for Ru⁴⁺ ion. Further investigations suggest that Fe substitution at the Ru sites can suppress the bandwidths of Ru 4d orbital, leading to the half-metallic behaviour in BaRu_1?x Fe _x O₃ with x = 0.625 and 0.75. The different orbital feature of the Ru⁴⁺ ions in BaRu_0.375Fe_0.625O₃ is presented, which reflects the influence of Fe dopant on Ru 4d orbitals.     

Polar optical phonon scattering limited free carrier absorption in semiconductors with ellipsoidal energy band structure

A formula is derived for the calculation of free carrier absorption in semiconductors with ellipsoidal energy band structure, including the effects of electron screening, when the dominant scattering mechanism is due to polar modes. Computational results are also presented which show that the magnitude of the FCA is significantly affected by the mass anisotropy coefficient.     

Electronic band structure,stability, structural,and elastic properties of IrTi alloys

The structural properties and mechanical stabilities of B2-IrTi have been investigated using first-principle calculations. The elastic constants calculations indicate that the B2-IrTi is unstable to external strain and the softening of C₁₁−C₁₂ triggers the B2-IrTi (cubic) to L1₀-IrTi (tetragonal) phase transformation. Detailed electronic structure analysis revealed a Jahn–Teller-type band split that could be responsible for elastic softening and structure phase transition. The cubic–tetragonal transition is accompanied by a reduction in the density of states (DOS) at the Fermi level and the d-DOS of Ti at Fermi level plays a decisive role in destabilizing the B2-IrTi phase.     

铀的弹性、能带结构和光学常数的第一性原理计算

采用密度泛函理论,赝势平面波方法计算了金属铀a相的晶体结构,弹性常数,体模量,电子能带结构和光学常数(折射率n和消光系数k)等.其中,铀的晶格参数,弹性常数和体模量等与实验及其它第一性原理计算结果十分吻合.计算得到了铀的光学常数,与实验结果作了对比并进行了分析说明.