Oxygen 2p Orbitals Research Articles

Application feasibility of palladium-decorated reduced graphene oxide as a CH4 gas nano-sensor from the perspective of the van der Waals corrected DFT computations

In this paper, ab initio simulation of structural, electronic and methane (CH 4 ) adsorption properties of palladium (Pd)-decorated reduced graphene oxide (rGO) has been performed using dispersion-corrected density functional theory. The stable adsorption geometry, adsorption energy, charge transfer, charge density difference, band structure, electronic density of states and work function are calculated to give further insight into the adsorption process. The stable rGO surface is composed of some local structural motifs from combinations of oxygen-containing functional groups (hydroxyl and epoxide groups). Our calculations indicate that, owing to the hybridization of palladium d-orbitals and oxygen p-orbitals, the Pd atom binds strongly to the epoxide group with the adsorption energies of −1.47 eV and −2.70 eV for the same side (GOOH(I)) and opposite sides (GOOH(II)) coadsorption of functional groups, respectively. Our results show that CH 4 molecules act as electron dopants for rGO and thus slightly change its conductivity. We also observe that, upon the introduction of Pd atoms, the CH 4 adsorption energies of GOOH(I) and GOOH(II) significantly modifies from −0.1 eV to −0.45 eV and −0.06 eV to −0.33 eV, respectively. Moreover, the charge transfer can be magnified from −0.01 e (−0.01 e) to −0.02 e (-0.03 e) by introducing Pd atoms into GOOH(I) (GOOH(II)) in the presence of CH 4 . Additionally, CH 4 adsorption changes the carrier mobility of Pd-decorated rGO. In light of these results, the Pd-decorated rGO can be a prominent candidate for CH 4 gas sensor application. • Using DFT calculations, adsorption of CH 4 on the Pd-decorated rGO is investigated. • CH 4 molecule is physisorbed on Pd-decorated rGO with reasonable adsorption energy. • Charge transfer is magnified by introducing Pd atom into rGO in the presence of CH 4 . • The direction of charge transfer is from the CH 4 molecule to the Pd-decorated rGO. • Carrier mobility and electron conductivity of Pd-decorated rGO changed after CH 4 adsorption.

Modeling electrochemical properties of LiMn_{1-x}Co_{x}BO_3 for cathode materials in lithium-ion rechargeable batteries

In this work, we report first-principle calculations of the electrochemical properties of lithitated and delithiated LiMn_{1-x}Co_{x}BO_3 (x = 0, 0.25, 0.5, 0.75, 1) crystals based on the density functional theory (DFT) with the generalized gradient approximation (GGA) and also considering the on-site Coulomb interaction, the so-called Hubbard correction. We found that the top of the valence band and the bottom of the conduction band of these crystals are mainly formed by the hybridization of the 3d orbitals of mixed Mn_{1-x}Co_{x} ions and oxygen 2p orbitals. We observed a band gap narrowing with an increase of cobalt concentration and that the Hubbard correction implies a better theoretical description of their electronic structures. When considering the delithiated materials, our calculations show a metallic behavior for intermediate cobalt concentrations (x = 0.25, 0.5, 0.75), which is a good quality for cathodic materials, as it improves the battery discharge process. We also obtained high (4.14 V vs. Li^+/Li^0 and 4.16 V vs. Li^+/Li^0) open circuit voltage (OCV) values at cobalt concentrations of x = 0.5 and 0.75, where we believe that if these high OCV values are accompanied by a high charge storage capacity, these compounds can become promising and useful cathode materials. Finally, our results are in accordance with previous calculations and also with experimental results.

Layered Perovskite Oxyiodide with Narrow Band Gap and Long Lifetime Carriers for Water Splitting Photocatalysis.

The development of semiconductors with narrow band gap and high stability is crucial for achieving solar to chemical energy conversion. Compounds with iodine, which has a high polarizability, have attracted attention because of their narrow band gap and long carrier lifetime, as typified by halide perovskite solar cells; however, they have been regarded as unsuitable for harsh photocatalytic water splitting because iodine is prone to self-oxidation. Here, we demonstrate that Ba2Bi3Nb2O11I, a layered Sillén-Aurivillius oxyiodide, not only has access to a wider range of visible light than its chloride and bromide counterparts, but also functions as a stable photocatalyst, efficiently oxidizing water. Density functional theory calculations reveal that the oxygen 2p orbitals in the perovskite block, rather than the fluorite Bi2O2 block as previously pointed out, anomalously push up the valence band maximum, which can be explained by a modified Madelung potential analysis that takes into account the high polarizability of iodine. In addition, the highly polarizable iodide contributes to longer carrier lifetime of Ba2Bi3Nb2O11I, allowing for a significantly higher quantum efficiency than its chloride and bromide counterparts. Visible-light-driven Z-scheme water splitting was achieved for the first time in an iodine-based system using Ba2Bi3Nb2O11I as an oxygen-evolution photocatalyst. The present study provides a novel approach for incorporating polarizable "soft" anions into building blocks of layered materials to manipulate the band structure and improve the carrier dynamics for visible-light responsive functions.

Lead adsorption onto Ni- and Pt-decorated nano γ-alumina: A first-principles study

An efficient computational procedure was proposed to study the adsorption of leadonto two surface decorated alumina nanostructures using density functional theory (DFT) calculations. Notably, lead removal of abundantly available low-price decorated γ-alumina compounds was superior over previously studied functionalized 2D graphene nanosheets. The binding energy for adsorption of Pb on the most stable configuration of γ-alumina was −2.96 eV. Evaluation of adsorption energy demonstrated a noticeable increase in the binding energy of Ni- and Pt-decorated nano γ-alumina as −3.40 eV and −4.29 eV, respectively. Enhanced adsorption energy was taken as a sign of the chemisorption process of Pb on decorated γ-alumina surfaces via covalent bond formation. The use of the projected density of states (PDOS) plot elucidated strong hybridization of Pb 6p with Ni 3d orbitals and 5d orbital of Pt atoms. Moreover, the hybridization of Pb with aluminum 3p and oxygen 2p orbitals proved the strong interaction between Pb and Pt-decorated γ-alumina surface. The results of this investigation can shed some light on decontamination of wastewater applying Ni- and Pt-decorated γ-alumina surfaces. The The proposed approach can also be applied in design and manufacture of iefficient solid adsorbents for the removal of lead contaminations.

Study of structural, electronic and optical properties of co-doped CeO2 using the density functional theory (DFT) method

The (B, C), (B, H), (B, N), (B, O), (B, P) and (B, S) co-doping effects on structural, electronic and optical properties of CeO2 systems have been studied to determine their potential for photo-catalytic activity using density functional theory and Hubbard U method. Both metallic and nonmetallic co-doping of elements show increase in cell volume and produce local lattice distortion. Due to this co-doping, energy gap between valence and conduction bands becomes narrower than that of pure CeO2, which provides easy way for the transfer of electrons from oxygen 2p orbitals to Ce 4f orbitals assisting in the process for photo-catalysis. Atomic orbitals of similar energy are mixed together except for the atomic s orbital of hydrogen atom. Further, optical absorption peaks for (B, C), (B, H) and ((B, N) co-doped CeO2 systems are found at higher energy range (blue shift) predicting to be better photo-catalytic material. Co-doped CeO2 materials are expected to speed up photoreaction and more electromagnetic waves are allowed to pass through them. Co-doping causes impurity levels to appear in the forbidden energy gap. Extended visible light absorption region has been observed leading to photo-catalytic activity enhancement for the visible light range (~1.5–6.2 eV).

Comparative Analysis of DFT+U, ACBN0, and Hybrid Functionals on the Spin Density of YTiO3 and SrRuO3

We present a quantitative analysis of the theoretical spin density map of two ferromagnetic perovskites, YTiO3 and SrRuO3. We calculated the spin density using the standard density functional theory (DFT)+U method, where the Hubbard U correction is applied to the Ti and Ru ions, and with the pseudo-hybrid ACBN0 method, where the Hubbard U parameters are determined self-consistently. The ACBN0 calculations yielded a large value of the Hubbard U of the oxygen 2p orbitals. We also used the screened hybrid HSE06 functional, which is widely used to describe the electronic structure of oxides. We used the Quantum Theory of Atoms in Molecules (QTAIM) theory and integrated the spin density in the atomic basins instead of projecting on atomic orbitals. This way, our results can be compared to experimental reports as well as to other DFT calculations.

(Invited) P-Type Thin Film Oxide Semiconductors for Thin Film Transistor-Based CMOS Logic

The development of products, and perhaps most notably displays, which rely on thin film transistors (TFTs) has been enabled by the availability of high quality, thin film semiconducting materials. Hydrogenated amorphous silicon (a-Si:H) was the first such material to meet the demands of the display industry. Other materials have since also gained importance including polycrystalline silicon, organic semiconductors and, most recently, oxide semiconductors. In this latter case, indium gallium zinc oxide (IGZO) has been successfully commercialised at scale. However, TFT circuits using any of these materials are based on NMOS or PMOS logic rather than CMOS preventing their use where low power consumption is essential such as display driving circuits, memories or logic as it has not proved to be possible to produce n-channel and p-channel TFTs with sufficiently similar performance characteristics in one material system. In the case of the thin film silicon-based materials, this is because the hole mobility is much lower than the electron mobility, whilst in organic semiconductors it is the p-type materials that are superior to the n-type. In this paper, the situation in oxide semiconductors is considered in detail.There are several oxide semiconductors which have been used to make excellent TFTs, perhaps most notably amorphous (a-)IGZO and amorphous zinc tin oxide (a-ZTO). Both of these materials are naturally n-type as their charge neutrality level is close to the conduction band edge. As a result, it has not proved to be possible to successfully introduce sufficient acceptor-type impurities in order to stably produce a p-type behaviour in these materials; completely different p-type thin film oxide semiconductors are therefore required. Cuprous oxide (Cu2O) is one such material, and this will be used as an exemplar in this work, although alternatives such as tin oxide (SnO) and nickel oxide (NiO) have also been reported.This move to alternative material systems presents other challenges. The conduction band minimum in n-type oxides is dominated by spatially extensive and spherically symmetric metal s-shell electron orbitals, and this allows an amorphous phase to be induced by stoichiometry control of oxide semiconductor which has no grain boundaries and is highly uniform but which retains a high electron mobility. However, the valence band maximum in the p-type materials is dominated by oxygen 2p-orbitals (sometimes with metal d-orbital hybridisation is found in Cu2O) which are highly directional. This means that any attempt to introduce an amorphous microstructure would result in a significant reduction in hole mobility compared with the crystalline state (it is this effect that is responsible for a-Si:H having an electron mobility that is 1,000 times lower than its crystalline counterpart).The types of deposition techniques (most commonly sputtering) that are used to produce thin film oxide semiconductors do not permit the production of single-crystalline thin film materials; nanocrystalline microstructures are created instead. This leads to the presence of grain boundaries, which are highly problematic as they contain a high density of defect states. As a result, they act as trap sites for holes and significantly reduce carrier mobility compared with the single crystal state. Some degree of mitigation is possible by controlling grain orientation during growth, and for Cu2O it is possible to select either the (100) or (111) orientation [1]. Grain size may also be increased and boundary defects decreased after growth by annealing [2]. The result is TFTs with a significantly lower field effect mobility (typically less than 1 cm2 V–1 s–1) than is found in the n-type oxide TFTs (typically over 10 cm2 V–1 s–1). Furthermore, these defective grain boundaries can allow the accumulation of electrons in the off-state leading to much higher off-state currents than would be expected [3]. As a result the switching ratio of p-type oxide TFTs is much lower (typically less than 103) than in equivalent n-type devices (>107).Despite these limitations, it is still possible to combine p-type and n-type oxide TFTs to make CMOS logic gates, but it is important to consider carefully the application requirements and the associated compromises that must be made in circuit design.The funding for this work by the Engineering and Physical Sciences Research Council through grant numbers EP/M013650/1, EP/P027032/1 and EP/L015455/1 is gratefully acknowledged.[1] Han, S. and Flewitt, A.J., Thin Solid Films, 704, 138000 (2020)[2] Han, S., et al., Appl. Phys. Lett., 109, 173502 (2016)[3] Han, S. and Flewitt, A.J., IEEE Electron Dev. Lett., 28, 1394 (2017)

Recent Progress in Mixed‐Anion Materials for Solar Fuel Production

Mixed‐anion materials, consisting of metal cation(s) and more than two anionic species in a single phase, are promising as photocatalysts for water splitting and CO2 fixation using visible light. Oxynitrides, oxysulfides, and oxyhalides that contain d0 transition‐metal or d10 typical‐metal cations have absorption bands in the visible‐light region and band‐edge potentials suitable for these reactions. In general, visible‐light absorption by these materials arises from the p orbitals of anions that are less electronegative than oxygen, forming a valence band above the level of oxygen 2p orbitals. Less electronegative anions in mixed‐anion compounds are essential for visible‐light absorption but are inherently less stable than oxygen, potentially causing oxidative decomposition of the materials during photoreaction. More importantly, mixed‐anion compounds are more difficult to synthesize than single‐anion compounds like oxides, and reducing the density of defects that act as recombination centers for photogenerated charge carriers in the material remains challenging. There is therefore much room for improving the activities of mixed‐anion photocatalysts. Herein, the development of mixed‐anion materials for solar‐to‐fuel conversion over the past 10 years is highlighted with a focus on key milestones in research developments.

P‐194: Late‐News‐Poster: Selenium 4p Orbital Enables High‐Mobility p‐Type Tin Oxyselenide Semiconductor for Thin‐Film Transistor Applications

Development of p‐type inorganic semiconductors for high performance thin‐film transistor (TFT) can be an essential ingredient for next‐generation display applications which requires a low power consumption. However, the conventional p‐type oxide semiconductors exhibit the poor electrical characteristics, which is mainly attributed to the inefficient conduction path formation from oxygen 2p orbitals. Here, we report a new p‐type material, tin oxyselenide (SnSeO) semiconductors for high performance p‐channel TFTs. The SnSeO was designed with the concept that valence band maximum (VBM), hole conduction path, becomes delocalized by hybridizing selenium 4p and tin 5s orbitals. The SnSe0.56O0.44 thin‐film exhibited the highest Hall mobility of 15.0 cm2 V‐1 s‐1 and hole carrier density of 1.2 x 1017 cm‐3. In addition, the SnSe0.56O0.44 TFT showed the excellent field‐effect mobility of 5.9 cm2 V‐1 s‐1 and current modulation ratio of 3.0 x 102. This high mobility is explained by the low hole effective mass through valence band engineering, which was confirmed by density functional theory calculation. Moreover, complementary metal oxide semiconductor (CMOS) inverter using the p‐type SnSe0.56O0.44 with n‐type IGZO showed excellent intrinsic gain of 15.7. This anion hybridization can provide a facile route to the realization of a high performance p‐channel TFT for next‐generation displays.

Efeito do Substituinte Sr na Transição de Fase do PT: Correlação Teórica e Experimental das Propriedades Eletrônicas e Estruturais

A transição de fase dos compostos PT, PST e ST, sintetizados pelo Método dos Precursores Poliméricos, foram estudados através da correlação teórica e experimental empregando DFT, utilizando o pacote computacional CRYSTAL14. Os modelos foram construídos em uma supercela 1x1x2 utilizando dados do refiamento de Rietveld. Os resultados foram interpretados em termos de Estrutura de Bandas, Densidade de Estados, evolução estrutural e valor de gap. Nestes modelos, foram possíveis a identifiação e a compreensão da transição de fase e redução na relação c/a concluindo que os modelos sofrem transição de fase conforme aumenta-se a concentração de substituinte Sr na matriz do PT.

Where Is the Fluoro Wall?: A Quantum Chemical Investigation.

Despite their isoelectronic properties, fluoro and oxo ligands exhibit completely different chemical behavior. Formally speaking, the first is known to exclusively form single bonds, while the latter is generally observed to form double (or even triple) bonds. The biggest difference, however, lies in what is known among inorganic chemists as the Oxo Wall: the fact that six-coordinate tetragonal transition metal oxo complexes are not observed beyond group 7 elements. While the Oxo Wall was explained a few decades ago, some questions regarding the nature of the Oxo Wall remain unanswered. For example, why do group 8 oxo complexes with high oxidation states not violate the Oxo Wall? Moreover, why are transition metal fluoro complexes observed through the whole transition metal series? In order to understand how the small difference between these two isoelectronic ligands can give rise to such different chemical behaviors, we conducted an extensive computational analysis of the geometric and electronic properties of model fluoro and oxo complexes with metals around the Oxo Wall. Among many insights into the details of the Oxo Wall, we mostly learned that the oxygen 2p orbitals are prone to meaningfully interact with transition metal d orbitals, because they match not only spatially but also energetically, while for fluorine the p orbital energies are lower to an extent that interaction with transition metal d orbitals is much reduced. This in turn implies that in those instances where the metal d orbitals principally accessible for interaction are occupied, the oxygen 2p orbitals are too exposed to be stable.

Multiorbital bond formation for stable oxygen-redox reaction in battery electrodes

Nonbonding oxygen 2p orbitals during oxygen-redox reaction are monitored using resonant inelastic X-ray scattering (RIXS).

Nanosize Cation-Disordered Rocksalt Oxides: Na2 TiO3 -NaMnO2 Binary System.

To realize the development of rechargeable sodium batteries, new positive electrode materials without less abundant elements are explored. Enrichment of sodium contents in host structures is required to increase the theoretical capacity as electrode materials, and therefore Na-excess compounds are systematically examined in a binary system of Na2 TiO3 -NaMnO2 . After several trials, synthesis of Na-excess compounds with a cation disordered rocksalt structure is successful by adapting a mechanical milling method. Among the tested electrode materials, Na1.14 Mn0.57 Ti0.29 O2 in this binary system delivers a large reversible capacity of ≈200 mA h g-1 , originating from reversible redox reactions of cationic Mn3+ /Mn4+ and anionic O2- /On - redox confirmed by X-ray absorption spectroscopy. Holes in oxygen 2p orbitals, which are formed by electrochemical oxidation, are energetically stabilized by electron donation from Mn ions. Moreover, reversibility of anionic redox is significantly improved compared with a former study on a binary system of Na3 NbO3 -NaMnO2 tested as model electrode materials.

Controlled Oxygen Redox for Excellent Power Capability in Layered Sodium-Based Compounds

Inducing anionic redox reaction of transition metal oxides in sodium ion batteries is one of the most reliable way to enhance capacity. Unfortunately, although the delivered capacity obtained from the transition-metal redox and oxygen redox is one of highest among sodium cathodes, the system suffered from not only serious capacity fading but also poor rate capability because of the sluggish kinetics of the oxygen redox. Given the overlapping in the density of states (DOS) between the oxygen 2p and cobalt 3d (Co4+/3+) orbitals in LiCoO2, which is one of the most representative materials showing good cyclability and acceptable rate performances, we applied this phenomenon to P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2). The P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) compounds were synthesized using a spray pyrolysis method. NaNO3, Mn(NO3)2·4H2O, Mg(NO3)2·6H2O, Co(NO3)2·6H2O, citric acid, and sucrose were used as starting materials for the spray pyrolysis. Calcination was conducted at 870 °C in air to obtain the P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2). The compounds were stored in an Ar-filled glove box to minimize the moisture contact during storage. Powder X-ray diffraction (XRD; PANalytical, Empyrean) using Cu Kα radiation was employed to identify the crystalline phases of the as-prepared P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) powders. The produced powders were examined using scanning electron microscopy (SEM; SU-8010, Hitachi) to analyze morphology of P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2) powders. Differential scanning calorimetry (DSC; Pyris 1, Perkin–Elmer) measurements were performed to analyze the thermal stability of desodiated electrodes. R2032 coin-type cells using Na metal as the negative electrode were assembled for electrochemical cell tests. The electrolyte solution was 0.5 M NaPF6 in a mixed solvent of propylene carbonate (PC) and fluoroethylene carbonate (FEC) in a volume ratio of 98:2. The fabricated cells were charged and then discharged between 1.5 and 4.6 V at a rate of 0.1C (26 mA g− 1), and the current was further raised to 5C (1.3 A g− 1) for a long-term cycle test at 25 °C. X-ray absorption near edge structure (XANES) spectroscopy was employed to understand the variation in the oxidation states of the transition metals and oxygen. Transmission electron microscopy (TEM; JEM-F200, JEOL) was used to analyze structural factors of the pristine P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode and the electrode after 1000 cycles. Density functional theory (DFT) calculations were performed using the projector augmented wave method implemented in the Vienna ab initio simulation package (VASP). Among studied compounds P2 Na0.6[Mg0.2Mn0.8 − xCox]O2 (x = 0.0–0.2), P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 electrode delivered high discharge capacity of about 214 mAh g− 1, which is attributed to the reactions of Co4+/3+, O1 − /2 −, and Mn4+/3+, with approximately 87% (186 mAh g− 1) of the capacity retention over 100 cycles at a rate of 0.1 C (26 mA g-1). More importantly, the present P2 Na0.6[Mg0.2Mn0.6Co0.2]O2 was able to deliver discharge capacities of approximately 108 mAh g− 1 at a rate of 5C (1.3 A g− 1), showing capacity retention of over 72% (78 mAh g− 1) after 1000 cycles (Figure 1). This outstanding electrode performance was rationalized by theoretical calculations; namely, the incorporation of Co in the crystal structure was shown to dramatically reduce the band-gap energies. Detailed information will be presented at the conference. Figure 1. Long–term cyclability at 5 C (1.3 A g− 1) for x = 0.2 in P2 Na0.6[Mg0.2Mn0.8 − xCox]O2. Figure 1

Theoretical studies of photocatalytic behaviors of isoelectronic C/Si/Ge/Sn-doped TiO2: DFT + U

The electronic structures and absorption spectra have been studied with density functional theory plus U for isoelectronic impurities C/Si/Ge/Sn-doped anatase TiO2. Our theoretical results show silicon atoms can attract electrons from the surrounding oxygen atoms due to their greater electronegativity than that of titanium atoms which leads to increasing the energy of 2p orbitals of the surrounding oxygen atoms and then forming new acceptor impurity level composed of oxygen 2p orbitals near the top of TiO2 valence band. This new acceptor level can easily bind the photoexcited holes for a period of time, thus effectively preventing the recombination of photo-excited electron-hole pairs, which play a key role in achieving higher light photocatalytic activity than pure TiO2 for degradation of contaminants in water. We also systematically described the electronic structure and optical properties of the germanium doped system and then explained experimental observations of absorption spectra with red-shift at a suitable Ge dopant concentration. Especially, compared with pure TiO2, Sn-doped TiO2 has better photocatalytic activity because of the synergistic effect of Sn atoms and oxygen vacancies. Our theoretical calculations not only explained the key role of isoelectronic defect in preventing the recombination of photo-excited electron-hole pairs but also provided a new way for enhance of photocatalytic activity.

(Invited) Defect Induced Oxygen Redox Chemistry

Lithium- and sodium-rich layered transition-metal oxides have recently been attracting significant interest because of their large capacity achieved by additional oxygen-redox reactions. However, layered transition-metal oxides exhibit structural degradation such as layer exfoliation or cracks upon deep charge, which is a major obstacle to achieve higher energy-density batteries. Defect are usually considered as a negative factor to realize stable operation of cathode materials. Here we show, by applying optimal design strategies, defect can not only effectively generate redox-active “orphaned” nonbonding oxygen 2p orbitals to “activate” additional oxygen-redox reactions, but also contribute to “stabilizing” reversible phase transformation with its effective negative point charge.

DFT Study on Oxygen-Vacancy Stability in Rutile/Anatase TiO2: Effect of Cationic Substitutions

In this study, a full-potential density functional theory was used to investigate the effects of Ti substitution by different cations. In both rutile and anatase, Ti atom was replaced by Ce, Au, Sn, Ag, Mo, Nb, Zr, and Y. Phase stability, electronic structure and formation energy of oxygen vacancy were compared for rutile and anatase. The results indicated that substitution of Ce and Zr increases anatase stability through which photocatalytic activity is enhanced. It seems that the cationic capacity and size play a critical role in anatase to rutile phase transition, where with an equi- or higher valence than Ti, larger cations increase the stability of anatase phase. Oxygen vacancy concentration, as a second factor of photocatalytic activity, was also studied by calculating its stability due to the cationic substitution. The data revealed that Au, Ag, Y, Ce and Sn effectively reduce oxygen-vacancy formation energy. Of the studied cations, Au and Ag had maximum reduction in band gap, by creating defect states in the middle of the band gap resulting from the overlaps of these elements d-orbitals and oxygen p-orbitals. Mo and Ce impurities did not have a significant effect on reducing gaps by creating defect states under the conduction band. Finally, Sn impurity also generated defect states in the middle of the gap merely with the lack of oxygen.

A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light Hydrogen Evolution and Carbon Dioxide Reduction.

Mixed anion compounds such as oxynitrides and oxychalcogenides are recognized as potential candidates of visible-light-driven photocatalysts since, as compared with oxygen 2p orbitals, p orbitals of less electronegative anion (e.g., N3-, S2-) can form a valence band that has more negative potential. In this regard, oxyfluorides appear unsuitable because of the higher electronegativity of fluorine. Here we show an exceptional case, an anion-ordered pyrochlore oxyfluoride Pb2Ti2O5.4F1.2 that has a small band gap (ca. 2.4 eV). With suitable modification of Pb2Ti2O5.4F1.2 by promoters such as platinum nanoparticles and a binuclear ruthenium(II) complex, Pb2Ti2O5.4F1.2 worked as a stable photocatalyst for visible-light-driven H2 evolution and CO2 reduction. Density functional theory calculations have revealed that the unprecedented visible-light-response of Pb2Ti2O5.4F1.2 arises from strong interaction between Pb-6s and O-2p orbitals, which is enabled by a short Pb-O bond in the pyrochlore lattice due to the fluorine substitution.

B-site cation order/disorder and their valence states in Ba3MnNb2O9 perovskite oxide

Polycrystalline samples Ba3MnNb2O9 synthesized by solid state reaction and single crystal samples grown by optical floating zone have been characterized using scanning transmission electron microscopy and electron energy loss spectroscopy. Three types of B-site Mn and Nb ordering phase are observed: fully ordered 1Mn:2Nb; fully disordered; nano-sized 1Mn:1Nb ordered. No electronic structure change for crystals with different ordering/disordering. The Mn valence is determined to be 2+, and Nb valence is 5+. Oxygen 2p orbitals hybridize with Mn 3d and Nb 4d orbitals. Factors that affect the electron energy loss near edge structures of transition metal white-lines in electron energy loss spectroscopy are explicitly illustrated and discussed.

Structurally modulated Li-rich cathode materials through cooperative cation doping and anion hybridization

High capacity Li-rich materials are mighty contenders for building rechargeable batteries that coincide with the demand in energy density. Fully realizing the extraordinary capacity involves oxygen evolution and related cation migration, resulting in phase transitions and deteriorations that would hinder their practical application. In an attempt to enhance the anodic redox participation and stabilize the structure at the same time, we proposed a structural modulation strategy with modification on anion hybridization intensifying and cation doping. Spectator ions with large ionic radius were introduced into the lattice during calcination with stannous chloride and the d-p hybridization between transition metal 3d and oxygen 2p orbitals was subsequently intensified along with expelling weakly bonded chloride species in the reheating process. Both of the reversible capacity and stability upon cycling were remarkably improved through the cooperation of bond alteration and dopant. This strategy might provide new insight into the modulation of the structure to truly fulfill the potential of Li-rich materials.