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Kaolinite : defect states define material properties-a soft x-ray and first principles study of the band gap

In this paper, we have employed DFT and HSE06 methods to study the doping effects on the NaTaO3 photocatalyst. N, S, C, and P monodoping and N–N, C–S, P–P, and N–P codoping have been studied. The redopants’ formation energies have been calculated, and we find S monodoping is energetically more favorable than any other elemental doping. The mechanism of anion doping on the electronic properties of NaTaO3 is discussed. We find the band gap reduces significantly if we dope with anionic elements whose p orbital energy is higher than the O 2p orbitals. N and S can shift the valence band edge upward without losing the ability to split water into H2 and O2. Double-hole-mediated codoping can decrease the band gap significantly. On the basis of our calculations, codoping with N–N, C–S, and P–P could absorb visible light. However, they can only decompose water into H2 when the valence band edge is above the water oxidation level.

Anionic–anionic (N–N, P–P, N–P, and C–S) mediated coupling can be introduced in the layered perovskite La2Ti2O7 structure for visible light photocatalysis. The anionic–anionic codoped La2Ti2O7 systems lower the band gap much more than their respective monodoping systems. Moreover, the electronic band positions of the doped systems with respect to the water oxidation/reduction potentials show that codoped (N–N, N–P, and C–S) systems are more promising candidates for visible-light photocatalysis. The calculated defect formation energy shows that the codoped systems are more stable than their respective monodoped systems.

The layered perovskite Sr2Ta2O7 has been investigated for efficient visible light photocatalysis using the first principles study. The electronic structure of Sr2Ta2O7 is tuned by the anionic (N)/cationic (Mo, W) mono- and co-doping. Such doping creates impurity states in the band gap and therefore reduces the band gap significantly. The absolute band edge position of the doped Sr2Ta2O7 with respect to the water oxidation/reduction potential depends a lot on the p/d-orbital’s energies of anionic/cationic dopants, respectively. The stability of the co-doped system is governed by the Coulomb interactions and charge compensation effects.

To elucidate the usage of graphene oxide (GO) as a photocatalysis material, we have studied the effect of epoxy and hydroxyl functionalization on the electronic structure, work function, CBM/VBM position, and optical absorption spectra of GO using density functional theory calculations. By varying the coverage and relative ratio of the surface epoxy ( O ) and hydroxyl ( OH) groups, both band gap and work function of the GO materials can be tuned to meet the requirement of photocatalyst. Interestingly, the electronic structures of GO materials with 40–50% (33–67%) coverage and OH:O ratio of 2:1 (1:1) are suitable for both reduction and oxidation reactions for water splitting. Among of these systems, the GO composition with 50% coverage and OH:O (1:1) ratio can be very promising materials for visible-light-driven photocatalyst. Our results not only explain the recent experimental observations about 2-D graphene oxide as promising visible-light-driven photocatalyst materials but can also be very helpful in designing the optimal composition for higher performance.

A solid solution photocatalyst, Na1–xLaxFe1–xTaxO3 (x up to 0.06), was prepared by the conventional solid-state method. The photophysical properties of the samples were studied by various experimental techniques and the electronic structures were investigated by using screened hybrid density functional (HSE06) calculations. The solid solution photocatalyst showed absorption of visible light extending up to 450 nm. Upon loading of platinum nanoparticles cocatalyst, the photocatalytic hydrogen evolution of 0.81 μ·mol·h–1·g–1 was obtained for 2% doping of LaFeO3 in NaTaO3, under visible radiation (λ > 390 nm; 20% methanol solution). The photocatalytic properties of the solid solution were found to be better than Fe doped NaTaO3 compounds on account of the suitable band structure. The electronic structure analysis revealed that, in the case of Fe doping at the Ta site, unoccupied electronic states in between the band gap appear that are responsible for the visible-light absorption. However, in the case of La and Fe codoping (passivated doping) the mid-gap electronic states are completely filled, which makes the band structure suitable for the visible-light photocatalysis. The present solid solution of perovskites (LaFeO3 and NaTaO3) sheds light on the interesting photophysical properties and photocatalytic activities which could be beneficial for the photocatalysts derived from these compounds.

We present the optical absorption spectrum of BiTaO 4 using the photo acoustic spectroscopy (PAS) technique and first principles approach. Band gap have been estimated 2.65 and 2.45 eV using PAS me ...

Wide band gap magnetic semiconductors made of lanthanide compounds have a wide range of applications in opto-magneto-electronic industry and electro (photo) luminescence devices. We have carried out a systematic study of electronic, magnetic and optical properties of rare earth potassium sulfides KMS2 (M = Nd, Ho, Er, and Lu) using density functional theory (DFT) with full potential linearized augmented plane wave method (FP-LAPW). Different exchange and correlation approximations are employed such as generalized gradient approximation (GGA), GGA + U and TB-mBJ. It is inferred that the GGA + U approach correctly predicts the localized behavior of 4f electrons in lanthanide atoms for the calculation of band gaps, electronic, magnetic and optical properties. All compounds are stable in ferromagnetic phase except KLuS2, while KLuS2 is a non-magnetic semiconductor because there is no unpaired f-electron. Band gaps of KMS2 are estimated and these materials are found to be wide band gap semiconductors. These materials absorb mainly ultraviolet (UV) radiations, which make them good photoluminescent materials with a strong dependence on the direction of the polarization of incident photons.

The cationic–anionic mediated charge compensation effect was studied in the layered perovskite La2Ti2O7 for the visible light photocatalysis. Our screened hybrid density functional study shows that the electronic structure of La2Ti2O7 can be tuned by the cationic (V, Nb, Ta)/anionic (N) mono- and co-doping. Such mono-doping creates impurity states in the band gap which helps the electron–hole recombination. But if the charge compensation is made by the cationic–anionic mediated co-doping then such impurity states can be removed and can be a promising strategy for visible light photocatalysis. The absolute band edge position of the doped La2Ti2O7 has been aligned with respect to the water oxidation/reduction potential. The calculated defect formation energy shows the stability of the co-doping system is improved due to the coulomb interactions and charge compensations effect.

From first-principles calculations, we predict a planar stable graphene allotrope composed of a periodic array of tetragonal and octagonal (4, 8) carbon rings. The stability of this sheet is predicted from the room-temperature molecular dynamics study and the electronic structure is studied using state-of-the-art calculations such as the hybrid density functional and the GW approach. Moreover, the mechanical properties of (4, 8) carbon sheet are evaluated from the Young's modulus and intrinsic strength calculations. We find this is a stable planar semiconducting carbon sheet with a bandgap between 0.43 and 1.01 eV and whose mechanical properties are as good as graphene's.

The band gap reduction and effective utilization of visible solar light are possible by introducing the anionic hole-hole mediated coupling in Sr(2)Nb(2)O(7). By using the first principles calculations, we have investigated the mono- and co-anionic doping (S, N and C) in layered perovskite Sr(2)Nb(2)O(7) for the visible-light photocatalysis. Our electronic structure and optical absorption study shows that the mono- (N and S) and co-anionic doped (N-N and C-S) Sr(2)Nb(2)O(7) systems are promising materials for the visible light photocatalysis. The calculated binding energies show that if the hole-hole mediated coupling could be introduced, the co-doped systems would be more stable than their respective mono-doped systems. Optical absorption curves indicate that doping S, (N-N) and (C-S) in Sr(2)Nb(2)O(7) can harvest a longer wavelength of the visible light spectrum as compared to the pure Sr(2)Nb(2)O(7) for efficient photocatalysis.

The electronic structure of pure Sr2Nb2O7 and its electronic band position are being aligned with respect to the water oxidation/reduction potential level using hybrid functional (HSE06) theory. The experimental band gap (3.90 eV) of pure Sr2Nb2O7 can be reproduced (3.92 eV) using this level of theory. The cationic-anionic co-doping (Mo-N) in layered perovskite Sr2Nb2O7 structure reduces the band gap significantly, and its electronic band position is excellent for the visible-light photocatalysis. The respective cationic and anionic mono-doped systems create an occupied or unoccupied impurity states in the band gap, which can reduce the efficiency of the photocatalysis.

The electronic structure of pure BiNbO4 has been calculated and their electronic band positions have been aligned with respect to the water oxidation/reduction potential. The effect of cationic (Mo), anionic (N), and co-doping (Mo-N) on BiNbO4 has been studied and discussed with respect to the standard redox potential levels. Our results show that co-doping of Mo and N in BiNbO4 reduces the band gap up to 31.8%, thus making it a potential candidate for the photocatalysis of water for hydrogen production. The relative stability between the mono- and co-doped BiNbO4 materials show that co-doped material is more stable and feasible in comparison to the mono-doped materials.