XPS Valence Band Spectra Research Articles

Electronic structure of Au-Sn compounds grown on Au(111)

The electronic structure of Au-Sn intermetallic layers of different compositions grown on Au(111) to the thickness of several nanometers has been studied in this work. The layer, interface and the substrate related components in the Au 4$f$ and Sn 4$d$ core-level spectra obtained using x-ray photoelectron spectroscopy (XPS) vary with deposition parameters to reveal the details of the Au-Sn formation. While AuSn is grown by deposition at room temperature, Au rich compounds form as a result of heat treatment through inter diffusion of Au and Sn. Deposition at high temperature forms more Au rich compositions compared to post annealing at the same temperature due to the kinetic energy of the impinging Sn atoms in the former case. Post annealing, on the other hand, stabilizes the bulk phases such as AuSn and Au$_5$Sn and exhibits an activated behavior for transition from the former to the latter with increasing temperature. The XPS valence band spectra of AuSn and Au$_{5}$Sn layers show good agreement with the density functional theory calculation, indicating that these have the bulk structure reported in literature. However, the influence of anti-site defects is observed in Au$_5$Sn. Low energy electron diffraction study reveals that although the AuSn layer is ordered, its top surface is disordered at room temperature. Surface order is obtained by annealing or deposition at elevated temperatures and dispersing bands are observed by angle resolved photoemission spectroscopy. Both electron-like and hole-like bands are evident for the ($\sqrt{3}$$\times$$\sqrt{3}$)R30$^{\circ}$ phase, while a nearly free electron-like parabolic surface state is observed for the p(3$\times$3)R15$^{\circ}$ phase.

XPS evidence of degradation mechanism in CH3NH3PbI3 hybrid perovskite

In this study, we investigate the photo-/thermal degradation mechanism of hybrid perovskites by using x-ray photoelectron (XPS) valence band (VB) spectra coupling with density functional theory (DFT) calculations. Herein, CH3NH3PbI3 is respectively subjected to irradiation with visible light and annealing at an exposure of 0–1000 h. It is found from XPS survey spectra that, in both cases (irradiation and annealing), a decrease in the I:Pb ratio is observed with aging time, which unambiguously indicates the formation of PbI2 as the product of photo/thermal degradation. The comparison of the XPS VB spectra of irradiated and annealed perovskites with the DFT calculations of CH3NH3PbI3 and PbI2 compounds have showed a systematic decrease in the contribution of I-5p states, which allows us to determine the respective threshold for degradation, which is 500 h for light irradiation and 200 h for annealing. This discrepancy might be due to the fact that the relaxation of thermal excitations of the system is carried out only by the phonons (which are non-radiative physical processes) while the radiative processes occurred during the photoexcitation will elastically or inelastically divert part of the external energy from the system to reduce its impact on perovskite degradation.

Simulation within a DFT framework and experimental study of the valence-band electronic structure and optical properties of quaternary selenide Cu2HgSnSe4

We report data of simulation within a density functional theory (DFT) framework, employing different approaches for exchange correlation (XC) potential, of the electronic structure and experimental verification of these band-structure calculations using X-ray photoelectron spectroscopy (XPS) of Cu2HgSnSe4 crystal. The present studies indicate that fair good agreement of total density of states (DOS) with the valence band XPS spectrum of the Cu2HgSnSe4 crystal is derived when DFT simulation is performed within modified Becke-Johnson (mBJ) potential and taking into consideration Hubbard correction parameter U and spin-orbit (SO) coupling (mBJ–U–SO approach). The mBJ–U–SO calculation reveals the best fit of total DOS to the experimental valence band XPS spectrum as well as a fairly good energy gap value for this compound. The present theoretical data indicate that the principal contributors to the Cu2HgSnSe4 valence band are Se 4p states with their main input in upper and central portions of the band, while Sn 5 s and Hg 6 s states make prevailed contributions in its lower portion and the bottom is formed by Hg 5d states. The quaternary Cu2HgSnSe4 selenide is a direct gap semiconductor. Based on the present mBJ–U–SO band-structure data, the main optical constants are calculated indicating that the compound under consideration is a very promising optoelectronic material.

Multi-valued charge trapping memory based on amorphous-indium-gallium-zinc oxide thin film transistor with designed Ga2O3/Al2O3 stack insulator

Multi-valued charge trapping memory is demonstrated based on an amorphous-indium-gallium-zinc oxide thin-film transistor (a-IGZO-TFT) with a Ga2O3/Al2O3 stack gate insulator. The memory device shows a positive shift of threshold voltage as large as 12.6 V after +20 V/1 s programming. The erasing process can be divided into two stages. Vertical electrical field erases the trapped electrons partially, resulting in negative threshold voltage shift with hump current in sub-threshold region. It is defined as a first erasing stage. Furthermore, the memory can be erased by UV-light effectively due to the generated electron-hole pairs in a-IGZO channel. It is defined as second erasing stage. The good retention characteristics are confirmed for various storage states. The storage mechanism is analyzed according to the energy band alignment obtained by XPS narrow scans and valence band spectra.

Highly efficient Z-scheme g-C3N4/ZnO photocatalysts constructed by co-melting-recrystallizing mixed precursors for wastewater treatment

Due to the powerful functions of photocatalytic technology in degrading organic pollutants by using sunlight, the development of photocatalytic materials with visible light response has received great attention in order to meet the requirements of energy saving and emission reduction. Herein, we synthesize a new type of composite photocatalyst composed of graphite-like C3N4 and ZnO by co-melting-recrystallizing the precursors of two pure catalysts at 600 °C under air atmosphere. The composites present an enhanced photocatalytic activity under visible light irradiation, the kinetic constant of Methyl Orange (MO) degradation with g-C3N4/ZnO is 2.89 and 8.23 times those of the pure g-C3N4 and ZnO, respectively. Moreover, the high stability of the photocatalyst was reflected in the photodegradation of MO. The ZnO was successfully incorporated into the lattice of g-C3N4, and a Zn–N bond was formed, which resulted in an enhanced charge transfer efficiency in composites. Therefore, the low separation efficiency of photogenerated electrons–hole pairs existed in a single catalyst was well solved after g-C3N4 was combined with ZnO. The adjustment of raw material ratio shows great influence on the photocatalytic activity, and the optimum synergetic effect of the composite catalyst was found at a weight ratio of 1/30 of ZnO precursor to g-C3N4 precursor. We concluded that the photocatalytic system formed by g-C3N4 and ZnO should be the Z-scheme heterojunction with an improved transfer efficiency of photogenerated electron–hole pairs and strong redox capacity, which can be confirmed by radical trapping experiments, DMPO-ESR technique and valence band XPS spectra measurements.

Study of PdO species on surface of TiO2 for photoreduction of CO2 into CH4

Different percentage of PdO composited TiO2 photocatalysts (TiO2-PdOX%, X ≥ 1.5) are prepared using the sol-gel way. It can be revealed that the inducted Pd ions play a role of PdO surface species instead of entering into the TiO2 crystal lattice determined by the characterization measurements of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and High-resolution transmission electron microscopy (HRTEM). The behaviors of photogenerated electron-hole pairs and band structure of the synthesized TiO2-PdOX% (X ≥ 1.5) photocatalysts are investigated by Photoluminescence (PL), PL lifetime decay curves, XPS valence band (VB) spectra, the density-functional theory (DFT) calculation and UV–vis diffuse reflectance spectra. The TiO2-PdOX% (X ≥ 1.5) photocatalysts exhibit higher activity for conversion of CO2 to CH4 than pristine TiO2 due to the presence of PdO. The inactivation mechanism for TiO2-PdOX% photoreduction is also discussed. This work can further explain the influence of surface species on photocatalytic performance.

Structural and X-ray spectroscopy studies of Pb1-xBax(NO3)2 solid solutions

Pb0.75(3)Ba0.25(3)(NO3)2, Pb0.68(3)Ba0.32(3)(NO3)2, Ba0.58(3)Pb0.42(3)(NO3)2, Ba0.81(3)Pb0.19(3)(NO3)2 crystals were grown for the first time when studying the Pb(NO3)2–Ba(NO3)2–H2O system. The crystals were grown in water solution with different relation of Pb(NO3)2 and Ba(NO3)2. It was found that series of solid solutions were formed by interaction of simple Pb(NO3)2 and Ba(NO3)2 compounds with a slow increase of volume and cell parameters from pure lead nitrate to pure barium nitrate. The crystal structure of all obtained Pb1-xBax(NO3)2 crystals belong to a cubic space group Pa-3. In these structures, Pb and Ba cations have the same coordination number 12, but different ionic radii (1.49 and 1.61 Å, respectively), anionic group is an equilateral triangle with O–O–O angle of 60° and O–O bond lengths ranging from 2.1305(8) to 2.2585(7) Å. The Pb2+ ion has a lone pair of electrons that are likely to be stereochemically active in these solid solutions leading to a significant amount of local disorder which explains the spread in the lattice parameters. The Pb1-xBax(NO3)2 solid solutions were characterized by X-ray photoelectron spectroscopy (XPS) by measuring the binding energies of the core-levels of constituting atoms and the shapes of the valence band. The XPS measurements indicate that the substitution of Pb for Ba does not cause changes in the charge states of the atoms constituting the Pb1-xBax(NO3)2 crystals. Matching the XPS valence-band spectra and the X-ray emission O Kα bands on a common energy scale indicates that the principal contributions of O 2p states occur at the top of the valence band of the crystals under study with also their substantial input in the upper and central portions of the band.

Photocatalytic performance of Bi2VO5.5/Bi2O3 laminated composite films under simulated sunlight irradiation

Bi2VO5.5/Bi2O3 laminated composite thin films with each different layer numbers were prepared on fused silica substrates by chemical solution deposition method with a following rapid annealing process. The phase structure, surface morphology, absorption spectrum and photocatalytic degradation characteristics of the thin films were characterized. X-ray diffraction patterns show that the laminated thin films are consist of polycrystalline Bi2O3 films and c-axis oriented Bi2VO5.5 films. With the increasing of the number of Bi2VO5.5 layers as well as the decrease of the number of Bi2O3 layers, the absorption spectrum of the composite film shifted towards the longer wavelength side. Meanwhile, under simulated sunlight, the degradation ability of the thin films to Methylene Blue gradually increased and reach a maximum at appropriate layer ratio. The optimal laminated composite thin film with the highest degradation rate are 6 layers of Bi2VO5.5 films superposed upon 2 layers of Bi2O3 films. The degradation rate decreases only by 2% after the 5 cycles of degradation. The reasons for the enhancement of the photocatalytic properties of the thin film can be attribute to the laminated structure and the matching energy level, which are benefit for the transferring of photo-induced electrons. According to fitting results of the band gap and the XPS valence band spectrum, the mechanism of photocatalytic degradation of MB for Bi2VO5.5/Bi2O3 laminated composite films irradiation are analyzed. Under simulated sunlight irradiation, the photogenerated electrons on the conduction band of the Bi2O3 can transfer to the conduction band of Bi2VO5.5. Meanwhile, the photogenerated holes on the valence band of Bi2VO5.5 can also be migrated to the valence band of Bi2O3, thus suppress the recombination, prolong the lifetime of photo-generated carriers, and increase the photocatalytic degradation efficiency.

Calculations within DFT framework of the electronic and optical properties of quaternary sulfide Tl2PbSiS4, a prospective optoelectronic semiconductor

We report results of computation within framework of density functional theory (DFT) of the electronic band-structure of quaternary thallium lead silicon sulfide, Tl2PbSiS4. Especially, we employ different approaches for exchange correlation (XC) potential to find the best fit of the theoretical total density of states (TDOS) curve to the experimental valence band (VB) spectrum as elucidated employing the method of X-ray photoelectron spectroscopy (XPS). The present findings indicate that the best fit of the experimental and theoretical data regarding the shape of the VB XPS spectrum and TDOS is derived when employing in the computation procedure for XC potential the modified Becke-Johnson (MBJ) approach, which includes also spin-orbit (SO) effect and Hubbard parameter U for strongly correlated electrons of d symmetry (MBJ + U + SO technique). Based on these findings, we have performed computation of the electronic band-structure and primary optical constants of Tl2PbSiS4. The MBJ + U + SO calculations present that the main contributors at the top of the VB in this quaternary sulfide are S 3p states, the central portion is prevailed by the input of S 3p and Si 3p states, while the bottom is determined by contributions of the electronic states of s symmetry associated with lead and silicon.

Probing the impact of surface reactivity on charge transport in dimensional phase changed tungsten films

A clear understanding of the surface chemical reactivity of tungsten (W) films is indispensable for photocatalytic, sensing and memory applications, especially in the presence of WOx (0 ≤ x ≤ 3) for low thicknesses. Here, surface reactivity of the film through diffusion of oxygen and its ability to make bonds with W is identified by X-ray photoelectron spectroscopy (XPS). Further inspection of XPS valence band spectra confirms the possible hybridization of W 5d and O 2p electrons in the presence of defect states near Fermi level. Exploration of surface morphology by scanning electron microscopy (SEM) reveals agglomeration of grains with increasing film thickness. Detailed microstructural and grazing-incidence X-ray diffraction (GIXRD) studies suggest the formation of β W nanocrystallites in amorphous matrix, and establish a knowledge of thickness dependent phase transformation of W beside surface oxidation. The nonlinear surface current–voltage characteristics at low thickness further indicates dimensional phase change owing to the involvement of point defects. We also report a detailed study of interstitial and vacancy mediated diffusion probability of oxygen in W films, where the estimated diffusion constant is found to be relatively higher than that of other body-centered cubic transition metals.

DC plasma electrolytic oxidation treatment of gum metal for dental implants

Vanadium- and aluminium-free biomedical Ti alloys are getting more and more attention because of the harmful character of these alloying elements. For this reason, it was proposed to substitute them with more biocompatible elements such as Ta, Nb and Zr. So-called gum metal alloys (Ti-Nb-Zr-Ta systems) belong to β-type Ti alloys which are characterized by excellent biocompatibility and possess the modulus of elasticity more closely related to that of a human bone. The present study aims at elucidating the process stages during DC plasma electrolytic oxidation (PEO) of gum metal (Ti-36Nb-3Zr-2Ta alloy) in calcium hypophosphite-based electrolyte system. The effect of the stages on the surface characteristics of the obtained oxide coatings was also determined. The coatings were characterized using SEM/EDS, XPS and contact angle measurements. It was found that the process comprised at least five different stages. The initiation of relatively strong B-type sparks in the third stage of the treatment was necessary to induce significant incorporation of electrolyte components into the oxide coatings. As the treatment continued beyond that point, the composition of the oxide films was getting more similar to that of hydroxyapatite as it was determined from the valence band XPS spectra. The surface morphology of the films was also closely related to the process stage at which the further growth of the oxide was halted. The composition of the electrolyte (regarding the salts used and their concentration) had a tremendous influence on the duration of the process stages and the surface characteristics of the obtained films. The process was also tested for its usefulness in coating gum metal dental implants in order to form bioactive interfaces. Keeping the voltage below a certain limit (before the onset of more powerful surface sparks) was crucial to obtaining good-quality coatings.

Surface plasma Ag-decorated single-crystalline TiO2−x(B) nanorod/defect-rich g-C3N4 nanosheet ternary superstructure 3D heterojunctions as enhanced visible-light-driven photocatalyst

Ag-TiO2−x(B)/g-C3N4 ternary heterojunctions photocatalysts are fabricated by hydrothermal-calcination, photo-deposition procedure, and followed by in-situ solid-state chemical reduction procedure. As-obtained photocatalysts are consisted with heterojunctions between 2D g-C3N4 sheets and 1D TiO2(B) single-crystalline nanorods. The band gap of Ag-TiO2−x(B)/g-C3N4 ternary heterojunctions photocatalysts is reduced to ∼2.23 eV due to plasma Ag and surface engineering. Under visible light irradiation, it has an optimal photocatalytic property for the reduction of Cr6+ (95%) and degradation of NH4+ (93%). The apparent reaction rate constants (k) of ternary heterojunctions photocatalysts for NH4+ and Cr6+ are 25 and 12 folds higher than that of original TiO2(B). Furthermore, Ag-TiO2−x(B)/g-C3N4 also has excellent hydrogen production efficiency, which is up to 410 µmol h−1 g−1. This enhancement can be attributed to the unique heterojunction formed by 1D single-crystalline TiO2(B) nanorods and 2D g-C3N4 sheets, surface plasma resonance effect of plasma Ag nanoparticle, and surface engineering. A possible photocatalytic mechanism is also proposed by analysizing the XPS valence-band spectra and the Mott-Schottky.

Study the photocatalytic mechanism of the novel Ag/p-Ag2O/n-BiVO4 plasmonic photocatalyst for the simultaneous removal of BPA and chromium(VI)

Herein, we demonstrate the successful construction of a novel flowerlike Ag/p-Ag2O/n-BiVO4 plasmonic photocatalyst for the photocatalytic oxidation of BPA and reduction of chromium(VI) simultaneously under visible light irradiation. Among these samples, 2mM-Ag/p-Ag2O/n-BiVO4 exhibits the highest photocatalytic performances. The photocatalytic reduction and oxidation efficiency can achieve 69.8 and 91.9%, respectively, after 100 min visible-light irradiation. The enhancement mechanism for the plasmonic photocatalyst is explored, which can be attributed to the enhanced absorbance in the visible light region, and the facilitated charge transfer and the suppressed recombination of electron-hole pairs in the Ag/p-Ag2O/n-BiVO4. The results of the electrochemical impedance spectroscopy, the fluorescence emission spectra and the time-resolved fluorescence emission decay spectra indicate the enhanced charge separation in the Ag/p-Ag2O/n-BiVO4 plasmonic photocatalyst. In addition, the free radical scavenging test shows that h+ and OH radicals play crucial roles in the photocatalytic oxidation reaction. The photogenerated electrons in the photocatalytic reduction process, are confirmed by the DMPO spin-trapping technology. Moreover, the band structures of various photocatalysts are revealed via the valence band XPS spectra and the Fermi level obtained by CASTEP code.

P,Se‐Codoped MoS2 Nanosheets as Accelerated Electrocatalysts for Hydrogen Evolution

AbstractAs the half reaction of water splitting, hydrogen evolution reaction (HER) is a prospective way to generate clean fuel of hydrogen. Molybdenum disulfide (MoS2), as a member of the transition‐metal dichalcogenides, has attracted much research attention since its potential HER activity is predicted to be even comparable to Pt. However, the HER activity of MoS2 is still far from desirable, owing to the high resistance and limited intrinsic active sites. Herein, we have successfully engineered MoS2 by doping P and Se to regulate the electron density. The optimized P,Se‐MoS2/CNTs exhibit a low overpotential of 110 mV at 10 mA cm−2 and a small Tafel slope of 49 mV dec−1, much better than P‐MoS2/CNTs, Se‐MoS2/CNTs, MoS2/CNTs, and closing to commercial Pt/C. The P,Se‐MoS2/CNTs also display excellent electrochemical stability over 15 days. Raman, XPS and surface valence band spectra confirm that P and Se codoping modulate the electron densities of the catalytic sites, which can largely improve the HER activity.

Yellow Emission from Low Coordination Site of Sr2 SiO4 :Eu2+ , Ce3+ : Influence of Lanthanide Dopants on the Electron Density and Crystallinity in Crystal Site Engineering Approach.

Lanthanide doping through a crystal site engineering approach tunes the emission wavelength suitable for LED applications, but weak emission from low coordination sites remains a huge challenge. Herein the use of a sensitizer is reported to enhance the emission strength and unravel the crystallinity and phase, as this approach demands a large amount of dopants. Doping of Eu2+ ions at SrO10 and SrO9 sites of Sr2 SiO4 (S2 S), respectively, tunes the emission from green to yellow and controlled doping of a Ce3+ sensitizer quadruples the quantum efficiency of yellow emission. Remarkably, doping of Eu2+ at the SrO9 site produces polycrystals, whereas co-doping of Ce3+ and Eu2+ at the same site produces single crystals. DFT calculations further delineate the underlying changes wherein strong interaction of dopant with its neighbours determines the electron density, and thus the crystallinity and phase, rather than usual explanation of aliovalent conditions, which is further substantiated by TEM results. Irrespective of dopant valence, use of large amounts of dopants and their interaction with the host is responsible for the crystallinity and phase change (α'-S2 S to β-S2 S). The XPS valence band spectra experimentally evidences the changes in bonding nature of O 2p and O 2s orbitals of silicate and its electron density, due to doping at the two sites. In short, the outcomes resulting from this work could be extended for the development of other two-coordination site lanthanide-doped materials and crystallization of inorganic materials.

XPS on Gd1−xCexCo2 Intermetallic Compounds

The electronic structure of Gd1−xCexCo2 compounds with x = 0.2, 0.4, 0.6, 0.75, and 0.9 is investigated by means of X‐ray photoelectron spectroscopy (XPS). All investigated samples are single phase and possess a cubic MgCu2 Laves type structure (C15). XPS core level spectra point out the intermediate valence state of Ce ions and that the Co atoms carry magnetic moments of similar magnitude in all compounds but smaller than in pure Co metal. The XPS valence band spectra of Gd1−xCexCo2 compounds are mainly the result of the Co 3d, Ce 5d, and Ce 4f states superposition, influenced by the 3d–5d and 3d–4f hybridizations.

Electronic band-structure and optical constants of Pb2GeS4: Ab initio calculations and X-ray spectroscopy experiments

The electronic band-structure of the ternary sulfide Pb2GeS4 was investigated by combining experimental and theoretical methods. Binding energy (BE) values of core electrons of Pb2GeS4 are measured employing X-ray photoelectron spectroscopy (XPS) for as-synthesized and treated with Ar+-ions crystal surfaces. The XPS measurements indicate that Ar+-ion treatment does not change the BE values of the core-level electrons of atoms constituting the Pb2GeS4 single crystal as well as peculiarities of the XPS valence band (VB) spectrum. The treatment does not cause changes in the crystal surface stoichiometry. The band-structure calculations based on density functional theory (DFT) reveal total density of states and partial densities of states of Pb2GeS4 within different exchange–correlation approximations. The best fit with the experiment is derived when the DFT calculations of Pb2GeS4 employ modified Becke-Johnson potential with Hubbard-corrected functional and taking into account spin–orbit (SO) interaction. The calculations indicate that top and upper portion of the VB is composed mainly by S 3p states, its central portion is formed by Ge 4p and S 3p states, while contributions of Pb 6s states dominate at its bottom with slightly smaller contributions of Ge 4s states as well. Contributions of unoccupied Pb 6p states dominate at the conduction band (CB) bottom. Regarding the occupation of the VB by Ge 4p and S 3p states, the theoretical data are confirmed experimentally by matching the XPS VB spectrum on a common energy scale with the X-ray emission spectra representing the valence S p and Ge p states. The present calculations yield that the VB maximum is positioned at the Y point, while the CB minimum at the Г point; this fact indicates that Pb2GeS4 sulfide is an indirect-gap material. The principal optical constants are also elucidated using the ab initio DFT calculations.

Electronic structure and optical properties of (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals, novel light-converting materials

Electronic structure and optical properties were studied for novel (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals synthesized by solution-melt technique. In particular, X-ray photoelectron spectroscopy (XPS) was used to measure core-level binding energies and valence-band spectra for as-synthesized and Ar+ ion-irradiated surfaces of these crystals. Presented XPS measurements show that the (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 single crystals are rather stable in relation to Ar+ ion-irradiation. X-ray emission (XE) S Kβ1,3 and Ga Kβ2 bands were measured for the (Ga70La30)2S300 crystal giving information on the energy distribution of the S 3p and Ga 4p states, respectively. A comparison of these XE bands on a common energy scale with the XPS valence-band spectrum of (Ga70La30)2S300 indicates that the principal contribution of the S 3p and Ga 4p states occurs mainly at the top and in the central part of the valence band, respectively. In addition, optical absorption and photoluminescence spectra of the crystals were explored. Energy band gap values are estimated as 2.01 and 1.99 eV at room temperature for the (Ga70La30)2S300 and (Ga69.75La29.75Er0.5)2S300 crystals, respectively. Observed high-intensity green photoluminescence band when excited by a laser emitting at 810 nm suggests that the (Ga69.75La29.75Er0.5)2S300 crystal is a very attractive material for infrared to visible light conversion.

Correlating Voltage Profile to Molecular Transformations in Ramsdellite MnO2 and Its Implication for Polymorph Engineering of Lithium Ion Battery Cathodes

Transition metal oxides are the primary choice for cathode materials in lithium ion batteries. These oxides occur as different polymorphs that have varied structures, stabilities, and affinities for Li+, and consequently, it is desirable to develop heuristics for the choice of the optimal polymorph. MnO2 is the most extensively used cathode material for lithium ion battery, and it exists in more than ten polymorphic forms. Among them, ϒ-MnO2, the most electrochemically active polymorph, is a mixture of pyrolusite and ramsdellite polymorphs. Here we have focused on the less explored ramsdellite MnO2 (R-MnO2). Highly crystalline R-MnO2 has been synthesized, and the experimentally obtained discharge features are compared with the calculations based on density functional theory followed by cluster expansion to obtain molecular insights into the intercalation process. R-MnO2 shows voltage fading beyond x = 0.5 (in LixMnO2). Computational results suggest that change in Li environment from tetrahedral to octahedral beyond x = 0.5 (in LixMnO2) causes the voltage fading. This change in Li environment is also correlated with experimentally obtained Mn 2p, O 1s, and valence band XPS spectra. The shift in the d-band center, plotted for valence band spectra at different lithium concentrations, is adduced for the migration of lithium ion from the tetrahedral site to the less favorable octahedral site beyond x = 0.5. Volume expansion of about 20% at full lithiation is accompanied by Jahn–Teller distortion of MnO6. Further, computations reveal the diffusional energy barrier to be 200 meV in the limit of dilute Li concentration (x = 0.125) and 481 meV in the limit of dilute vacancy concentration (x = 0.875). A structural rationale for the energy barrier is developed. The experimental and computational studies provide insight into the lithiation mechanism in R-MnO2 and are relevant to rationalize the high performance of the intergrowth structure of ϒ-MnO2. We discuss a wider implication of the study by correlating the voltage profile with structural changes in MnO2 polymorphs, which gives general design principles to make better cathodes through polymorphic engineering.

Bi5+ , Bi(3-x)+ , and Oxygen Vacancy Induced BiOClx I1-x Solid Solution toward Promoting Visible-Light Driven Photocatalytic Activity.

BiOClx I1-x solid solutions with different band gaps were synthesized by adjusting the initial Cl to I molar ratios through a chemical precipitation method at room temperature. The structures, morphologies and optical properties of the samples were characterized by XRD, XPS, Raman, SEM, TEM and UV/Vis, respectively. The photocatalytic experiments showed that the BiOCl0.9 I0.1 sample totally decomposed a large concentration of 50 mg L-1 aqueous Rhodamine B (RhB) solution within 12 minutes under visible light irradiation (λ>420 nm), which is 11 times higher than that of pure BiOI. Furthermore, the electron band structure and density of states of BiOCl, BiOI and BiOClx I1-x have been investigated using the DFT (density functional theory) calculation method and electrochemical methods. It was found that there are multiple crystal defects of Bi5+ , Bi(3-x)+ , and oxygen vacancies in the BiOClx I1-x samples. The results for Mott-Schottky plots and valence-band XPS spectra showed the position of conduction band (CB) for BiOCl0.9 I0.1 was up-shifted, which is favourable to the redox capacity for the photocatalysts. It could be elucidated that the synergistic effects of multiple crystal defects and unique band structure are critical to improving solar driven photocatalytic activity. This work provides a new highlight toward the construction of high property photocatalysts by tuning the crystal defect and band structure in a simple and efficient way.