2p Band Research Articles

Cu-alanine complex-derived CuO electrocatalysts with hierarchical nanostructures for efficient oxygen evolution

Nowadays, Cu-based materials have attracted extensive attention as electrocatalysts, while the inherent reason of the filling of high anti-bonding state of Cu d band (3d104s1) makes it difficult to hybridize with O 2p band of oxygen intermediates during the adsorption process of oxygen evolution reaction (OER). To increase the efficiency of Cu-based electrocatalysts, efforts have been made to optimize the electronic structures and to create surface defects and hierarchical nanostructures with more exposed accessible active sites. Herein, we report a facile method for preparing CuO electrocatalysts with hierarchical nanostructures using the Cu-alanine complex as a precursor through room-temperature chemical precipitation and subsequent calcination in air. Investigations of products obtained at different calcination temperatures reveal the relationship between OER activities and the material characteristics such as specific surface areas, crystal growth orientations, and element components. The product obtained at 500 °C exhibits the smallest overpotential of 290 mV in 1.0 mol/L KOH for electrocatalyzing OER. Combining with various characterizations of CuO electrocatalysts after OER activities, the possible catalytic mechanism and the influence factors of their OER performance are also discussed.

Rationalizing Effect of Metal Substitution in Ruthenium Pyrochlores on the Lattice Oxygen Binding and Oxygen Evolution Activity

Ruthenium pyrochlores, i.e. oxides of composition A2Ru2O7 − δ, are known as advanced catalysts for the oxygen evolution reaction (OER) in acidic conditions.1 In this work, we discuss a direct correlation between the OER activity of Y1.8M0.2Ru2O7 − δ pyrochlores (M = Y, Fe, Co, Ni, Cu), the lattice oxygen binding strength/lability and the formation enthalpy of the binary MOx oxide, i.e. metals M with highter enthalpies of formation for the respective oxide weaken the oxygen binding on the surface of Y1.8M0.2Ru2O7 − δ pyrochlores that results in enhanced OER activity. DFT studies captured changes in the electronic structure of the Y1.8M0.2Ru2O7 − δ oxides induced by M substitution and revealed trends in the electronic band structure that govern the OER performance of the Y1.8M0.2Ru2O7 − δ catalysts. Specifically, an increase in the density of oxygen vacancy (VO) sites and the OER activity in Y1.8M0.2Ru2O7 − δ is attributed to the shift of the O 2p band center closer to the Fermi level. Our work introduces Y1.8Cu0.2Ru2O7 − δ as a novel OER catalyst with enhanced OER activity and, more fundamentally, emphasizes the underexplored role that substituents can play in regulating the OER activity of complex oxides, i.e. by influencing the surface oxygen binding strength. Our results can be utilized for the rational design of new compositions for oxygen electrocatalysis and other applications that involve lattice oxygen sites.(1) Kim, J.; Shih, P.-C.; Tsao, K.-C.; Pan, Y.-T.; Yin, X.; Sun, C.-J.; Yang, H., J. Am. Chem. Soc. 2017, 139, 12076-12083.

Momentum Dependent Band Renormalization and Surface Aging Effect on a Zone Center Electron Pocket in NaSn2As2 Revealed by Angle-Resolved Photoemission Spectroscopy

We report an angle-resolved photoemission spectroscopy study on NaSn2As2 in which a Sn 5p band is expected to provide interesting Fermi surface geometry with hole-like and electron-like curvatures....

Deciphering interpenetrated interface of transition metal oxides/phosphates from atomic level for reliable Li/S electrocatalytic behavior

Abstract Lithium/sulfur (Li/S) batteries have superior advantages in their high energy-density and environmental benignity. However, the notorious shuttle effect of lithium polysulfides (LiPSs) and their sluggish redox reaction kinetics hinder their practical implementation. Herein, a electronic modulation strategy was developed to regulate the catalytic behavior of transition metal oxides (TMOs)/transition metal phosphates (TMPs) interface towards Li/S surface chemistry. Thus, the reduced energy gap between the energy centers of the cation 3d and anion 2p band in O-M-P bridging (M refers to the transition elements) effectively reduce the energy barrier of LiPSs conversion and improve the electron transfer owing to the interdoping effect, which further functions as catalytic centers to active the M sites for regulating LiPSs adsorption-diffusion-conversion process. Benefited from these structural advantages, the engineered TMOs/TMPs exhibits admirable electrocatalytic sulfur reaction kinetics based on theoretical calculations and electrochemical analyzations. As expected, the TMOs/TMPs sulfur cathode exhibits an ultralow capacity fading rate of 0.033% per cycle at 1C over 500 cycles. Even under raising sulfur loadings and lean electrolyte content, this cathode still exhibits outstanding electrochemical performance.

TEMPERATURE DEPENDENCE OF HOMOGENEOUS ANATASE-PHASED TiO2FILMS CHARACTERIZATION AND GAS-SENSING BEHAVIORS

Anatase-phased TiO2films were prepared at different temperatures (350, 400, 450 and 500∘C) using automated nebulizer spray pyrolysis (ANSP) method. The structural study (XRD) revealed the amorphous nature at 350∘C and remaining samples (400, 450 and 500∘C) show the tetragonal structure with 2[Formula: see text], 38.43, 48.49 and 55.54 corresponding to (101), (004), (200) and (105) reflected planes and it is well fitted with standard data. The compositional XPS analysis confirmed the core level primary element of Ti 2p, O 1s and valance band (VB) of Ti 3p, Ti 3s, O 2s peaks in the prepared samples. The 3D optical profilometer has shown that the thickness of the prepared films was decreased by increase in temperature. The AFM study exhibited average roughnesses (Ra) of the prepared films such as 0.058, 0.147, 0.176 and 0.194[Formula: see text]nm, respectively. The surface morphological study of FESEM has shown the cracked uneven distributed nature (350∘C) turn into evenly distributed closed packed agglomerated particles by the influence of temperature. The oscillating nature of transmittance (%) with redshift of the sharp absorption edge was observed in UV–Vis–NIR spectrophotometer and found the bandgap value about 3.58[Formula: see text]eV to 3.33[Formula: see text]eV through Tauc’s relation. The gas-sensing behavior has shown better response to C2H6O reducing gas at 300∘C operating temperature with 150 ppm gas concentration.

Decoupling the Voltage Hysteresis of Li‐Rich Cathodes: Electrochemical Monitoring, Modulation Anionic Redox Chemistry and Theoretical Verifying

AbstractCathodes in lithium‐ion batteries with anionic redox can deliver extraordinarily high specific capacities but also present many issues such as oxygen release, voltage hysteresis, and sluggish kinetics. Identifying problems and developing solutions for these materials are vital for creating high‐energy lithium‐ion batteries. Herein, the electrochemical and structural monitoring is conducted on lithium‐rich cathodes to directly probe the formation processes of larger voltage hysteresis. These results indicate that the charge‐compensation properties, structural evolution, and transition metal (TM) ions migration vary from oxidation to reduction process. This leads to huge differences in charge and discharge voltage profile. Meanwhile, the anionic redox processes display a slow kinetics process with large hysteresis (≈0.5 V), compared to fast cationic redox processes without any hysteresis. More importantly, a simple yet effective strategy has been proposed where fine‐modulating local oxygen environment by the lithium/oxygen (Li/O) ratio tunes the anionic redox chemistry. This effectively improves its electrochemical properties, including the operating voltage and kinetics. This is also verified by theoretical calculations that adjusting anionic redox chemistry by the Li/O ratio shifts the TM 3d—O 2p bands and the non‐bonding O 2p band to a lower energy level, resulting in a higher redox reaction potential.

Synthesis and enhanced electrocatalytic mechanism of mesoporous La0.8Sr0.2MnO3 nanowires as high-active electrocatalysts for Zn-air batteries

The one-dimensional mesoporous La0.8Sr0.2MnO3 catalysts with excellent electronic conduction were successfully prepared by molten salt template method to accelerate the kinetics of ORR for metal-air batteries. The as-synthesized mesoporous La0.8Sr0.2MnO3 nanowires with relatively high oxygen vacancy concentration and ratio of Mn4+/Mn3+ show high specific surface area of 31.03 m2 g−1, higher than the most of reported perovskite oxides. More importantly, more catalytic active sites can be exposed at the three-phase interface of ORR due to high specific surface area. The mesoporous La0.8Sr0.2MnO3 nanowires exhibit high half-wave potential and catalyze near 4 e− ORR process. Besides, the one-dimensional mesoporous La0.8Sr0.2MnO3-based Zn-air batteries show high peak power density of 113.86 mW cm−2. Simultaneously, density functional theory (DFT) calculation reveals that strengthened electrocatalytic mechanism of La1-xSrxMnO3 catalysts originates from lower formation energy of oxygen vacancy and more positive O 2p band center because of Sr dopants, which is consistent with the experimental results. Consequently, this investigation not only provide a new idea for developing mesoporous perovskite oxides nanowires but also reveals enhanced electrocatalytic mechanism of La1-xSrxMnO3 catalysts.

Improved Structural Properties in Homogeneously Doped Sm0.4Ce0.6O2-δ Epitaxial Thin Films: High Doping Effect on the Electronic Bands.

The study of ionic materials on nanometer scale is of great relevance for efficient miniaturized devices for energy applications. The epitaxial growth of thin films can be a valid route to tune the properties of the materials and thus obtain new degrees of freedom in materials design. High crystal quality SmxCe1-xO2-δ films are here reported at a high doping level up to x = 0.4, thanks to the good lattice matching with the (110) oriented NdGaO3 substrate. X-ray diffraction and transmission electron microscopy demonstrate the ordered structural quality and absence of Sm segregation at the macroscopic and atomic level, respectively. Therefore, in epitaxial thin films, the homogeneous doping can be obtained even with the high dopant content not always approachable in bulk form, getting even an improvement of the structural properties. In situ spectroscopic measurements by X-ray photoemission and X-ray absorption show the O 2p band shift toward the Fermi level, which can favor the oxygen exchange and vacancy formation on the surface when the Sm doping is increased to x = 0.4. X-ray absorption spectroscopy also confirms the absence of ordered oxygen vacancy clusters and further reveals that the 5d eg and t2g states are well separated by the crystal field in the undistorted local structure even in the case of a high doping level up to x = 0.4.

Unsaturated p-Metal-Based Metal–Organic Frameworks for Selective Nitrogen Reduction under Ambient Conditions

Electrochemical ammonia synthesis that utilizes renewable electricity in the nitrogen reduction reaction (NRR) has recently been remarkably considered. Of particular importance is to develop efficient electrocatalysts at low costs. Herein, highly selective nitrogen capture using porous aluminum-based metal-organic frameworks (MOFs) materials, MIL-100 (Al), is first designed for the electrochemical nitrogen fixation in alkaline media under ambient conditions. Owing to the unique structure, MIL-100 (Al) exhibits remarkable NRR properties (NH3 yield: 10.6 μg h-1 cm-2 mgcat.-1 and Faradaic efficiency: 22.6%) at a low overpotential (177 mV). Investigation indicates that the catalyst shows excellent N2-selective captures due to the unsaturated metal sites binding with N2. More specifically, as the Al 3p band can strongly interact with N 2p orbitals, Al as a main group metal presents a high and selective affinity to N2. The utilization of multifunctional MOF catalysts delivers both high N2 selectivity and abundant catalytic sites, resulting in remarkable efficiency for NH3 production.

Insights of the anionic redox in P2–Na0.67Ni0.33Mn0.67O2

The search for high-energy batteries has promoted intense attention to anionic redox in layered transition metal oxides because of their ability of delivering much higher capacity than traditional cathodes. P2–Na 0.67 Ni 0.33 Mn 0.67 O 2 electrode exhibits outstanding air-stability and high average potential. Very recently, the anionic redox in this promising sodium cathode has been evidenced by mapping of resonant inelastic X-ray scattering. However, the origin of this oxygen redox has not been recognized yet. Here, based on the combination of X-ray absorption spectroscopy and density functional theory (DFT) calculations, we demonstrate that with the remove of Na + , the Ni 2+ oxidized to Ni 3+ and followed by the oxidation of lattice oxygen. Our DFT calculation further confirms that the oxygen redox in Na 0.67 Ni 0.33 Mn 0.67 O 2 is rooted from Ni–O anti-bonding ( e g *) state rather than the non-bonding O 2p band and result in the highly reversible oxygen redox reactions without O 2 loss. Moreover, at very low sodium contents, it is highly possible that a charge redistribution process between the Ni and O ions occurs, which results in the inconsistent experimental observations in previous references. • Confirm the anionic redox reactions in P2–Na 0.67 Ni 0 . 33 Mn 0 . 67 O 2 at low sodium contents via h-XAS and DFT calculations. • No oxygen loss is observed during the first charge process of P2–Na 0.67 Ni 0.33 Mn 0.67 O 2 . • Clarify the insights of anionic redox in P2–Na 0.67 Ni 0.33 Mn 0.67 O 2 .

Photochemical activation of carbon dioxide in Mg+(CO2)(H2O)0,1

We combine multi-reference ab initio calculations with UV–VIS action spectroscopy to study photochemical activation of CO2 on a singly charged magnesium ion, [MgCO2(H2O)0,1]+, as a model system for the metal/ligand interactions relevant in CO2 photochemistry. For the non-hydrated species, two separated Mg+ 3s–3p bands are observed within 5.0 eV. The low-energy band splits upon hydration with one water molecule. [Mg(CO2)]+ decomposes highly state-selectively, predominantly via multiphoton processes. Within the low-energy band, CO2 is exclusively lost within the excited state manifold. For the high-energy band, an additional pathway becomes accessible: the CO2 ligand is activated via a charge transfer, with photochemistry taking place on the CO2– moiety eventually leading to a loss of CO after absorption of a second photon. Upon hydration, already excitation into the first and second excited state leads to CO2 activation in the excited state minimum; however, CO2 predominantly evaporates upon fluorescence or absorption of another photon.

Bismuth-Based Free-Standing Electrodes for Ambient-Condition Ammonia Production in Neutral Media

HighlightsThe Bi2O3 nanoplates homogeneously decorated free-standing exfoliated graphene (Bi2O3/FEG) was prepared by a facile electrochemical deposition method.The Bi2O3/FEG first used as nitrogen reduction electrocatalyst exhibits excellent electrocatalysis performance and stability for nitrogen reduction reaction in neutral media.The superior electrocatalytic nitrogen reduction activity is attributed to the strong interaction of the Bi 6p band with the N 2p orbitals, binder-free nature of the electrodes, and facile electron transfer through the graphene nanosheets.

Numerical investigation of the electronic and optical properties of LiLuF4 vacuum ultraviolet material

The electronic band structure and optical properties of a perfect LiLuF4 crystal were calculated using density functional theory within the projector-augmented wave method and a hybrid exchange-correlation functional. LiLuF4 is found to have a direct band gap of 10.55 eV, which is attributed to the transition between the F 2p and Li 3s bands. The dominant absorption peak originates from transitions between the F 2p to Lu 6d states and therefore, efficient emission in the vacuum ultraviolet (VUV) region is expected from LiLuF4 by exciting into the Lu 6d states using 15.21 eV (82 nm) of pump energy. The LiLuF4 crystal is highly transparent with a transmission edge of 10.37 eV (119 nm). Moreover, it is optically isotropic down to the transmission edge. Optical isotropy and transparency are desirable in the development of optical components for VUV applications, whereas the direct band gap transition is essential for efficient VUV light emission.

Anomalous magnetic behaviour of Bi based tetradymites

This work investigates the magnetic properties of the Bi-based bulk Bi2(Se1−xTex)3 tetradymites. The samples were prepared using the melting technique, and all composites are crystallised in the R3m Rhombohedral structure. The lattice cell volume increases monotonically with the Te introduction due to the larger size of Te in comparison with Se. The electrical resistivity data show a metal–semiconductor transition at a certain temperature (Tm) that arises from the overlapping between the 6p and 6s bands. The metal-like behaviour below Tm is in correspondence with the semiconducting degeneracy state, and the empirical analysis has confirmed the metal characteristic scattering mechanisms. The resistivity of the pristine Bi2Se3 compound decreases with the low Te doping level, then increases monotonically with the further Te amounts. This behaviour can be attributed to grain size effect and the change in carriers concentration and mobility. The magnetic measurements have shown the non-pure diamagnetism for the Bi2(Se1−xTex)3 composites, in disagreement with several studies, and show the coexistence of a paramagnetic component. This paramagnetic component arises from the orbital magnetisation, where, the electrons abundance (due to Te and the electron source defects) bends the electronic band, forming 2-dimensional electron gas state that leads to the spin-orbital magnetisation (the source of orbital magnetisation).

(Henry B. Linford Award Address for Distinguished Teaching) Intricacies of High-Energy Cathodes for Lithium-Ion Batteries

First, I take this occasion of receiving the Henry B. Linford Award for Distinguished Teaching to express my deepest gratitude to more than 250 students and postdoctoral fellows who have worked with me over the years, including the graduation of 60 Ph.D. students and 26 M.S. students. Through their hard work and dedication, my career at the University of Texas at Austin has become truly rewarding.The cathodes in lithium-ion batteries play a key role in terms of cost, energy density, cycle life, and safety. As a postdoctoral fellow working with the 2019 Chemistry Nobel Laureate John B. Goodenough in the 1980s, my career began with the identification of inductive effect in polyanion oxide cathodes LixFe2(XO)4 (X = Mo, W, and S) in drastically altering the operating voltage depending on the covalency of the X-O bond. For example, with the same structure and the same Fe2+/3+ redox couple, both LixFe2(MoO)4 and LixFe2(WO)4 operate at 3.0 V, while LixFe2(SO)4 operates at 3.6 V, compared to < 2 V for LixFe2O3. This finding served as the basis for the subsequent development of olivine LiFePO4 cathode by Goodenough’s group 10 years later in 1997. The polyanion family of materials have now become broad with a variety of compositions as electrode materials for both lithium-ion and sodium-ion batteries.The battery industry is now largely based on layered LiNi1-y-zMnyCozO2 (MNC) cathodes. My group (i) demonstrated the differences in the relative chemical instabilities of LiMO2 (M = Co, Ni, and Mn) cathodes through chemical analysis of oxygen content in chemically delithiated Li1-xMO2 in the early 2000s and (ii) correlated the results to a qualitative picture of the relative positions of the metal:3d band with respect to the top of the O2-:2p band. These results now exemplify the ability to increase the capacity by increasing the nickel content in NMC cathodes. This presentation will present the complexities and prospects of high-nickel layered oxide cathodes for next-generation lithium-ion batteries. Analogous to the layered oxide cathodes, the presentation will also focus on the intricacies involved with high-voltage spinel LiNi0.5Mn0.5O4 (LNMO) cathodes. Specifically, approaches to eliminate expensive cobalt from cathodes will be pointed out.Finally, sulfur offers a low-cost alternative to oxide cathodes with an order of magnitude higher capacity than the oxide cathodes. However, lithium-sulfur cells are hampered by poor ionic and electronic transport, polysulfide shuttling, and lithium-metal anode degradation. This presentation will focus on addressing these issues and fabricating pouch lithium-sulfur batteries with a high sulfur content and loading and low electrolyte amount, assisted by effective electrocatalysts, in order to be competitive with the current lithium-ion technology.The value of advanced analytical techniques, such as in-situ x-ray diffraction, x-ray photoelectron spectroscopy, time of flight secondary ion mass spectrometry, scanning electron microscopy, and high-resolution transmission electron microscopy in understanding the intricacies of the cathodes and mitigating the problems will be presented.

Characteristics of Cu-doped ZnO films prepared using magnetron co-sputtering

This work reports the fabrication of un-doped and cu-doped zinc oxide (ZnO) films using magnetron co-sputtering technique. The fabricated films were deposited on glass substrates followed by thermal annealing process in an ambient condition for 60 minutes at 400 °C. X-ray diffraction (XRD), ultraviolet-visible (UV-Vis) spectrophotometry, field emission scanning electron microscopy (FE-SEM) and Hall measurement with four-point Van der Pauw method were used to study the crystal orientation, optical properties, surface structure and the resistivity of the deposited films respectively. Polycrystalline films with hexagonal wurtzite structure were observed in the CZO films. The redshift and bandgap narrowing were discussed in optical analysis. The trend in bandgap narrowing was contributed by the hybridization of O 2p and Cu 3d band. Hall effect measurement showed that the resistance of deposited Cu-doped ZnO (CZO) films will increase at excessive-high Cu doping level.

Plasma Nitriding of TiO2 Nanotubes: N-Doping in Situ Investigations Using XPS.

The nitrogen doping of titanium dioxide nanotubes (TiO2 NTs) was investigated as a result of well-controlled plasma nitriding of TiO2 NTs at a low temperature. This way of nitrogen doping is proposed as an alternative to chemical/electrochemical methods. The plasma nitriding process was performed in a preparation chamber connected to an X-ray photoelectron spectroscopy (XPS) spectrometer, and the nitrogen-doped TiO2 NTs were next investigated in situ by XPS in the same ultrahigh vacuum (UHV) system. The collected high-resolution (HR) XPS spectra of N 1s, Ti 2p, O 1s, C 1s, and valence band (VB) revealed the formation of chemical bonds between titanium, nitrogen, and oxygen atoms as substitutional or interstitial species. Moreover, the results provided a characterization of the electronic states of N–TiO2 NTs generated by various plasma nitriding and annealing treatments. The VB XPS spectrum showed a reduction in the TiO2 band gap of about 0.6 eV for optimal nitriding and heat-treated conditions. The TiO2 NTs annealed at 450 or 650 °C in air (ex situ) and nitrided under UHV conditions were used as reference materials to check the formation of Ti–N bonds in the TiO2 lattice with a well-defined structure (anatase or a mixture of anatase and rutile). Scanning electron microscopy microscopic observations of the received materials were used to evaluate the morphology of the TiO2 NTs after each step of the nitriding and annealing treatments.

ZnO-CuxO heterostructure photocatalyst for efficient dye degradation

A simple aqueous solution process was used for the synthesis of Cu-loaded ZnO nanorod structures in which highly crystalline and hexagonal wurtzite ZnO, cubic CuO and Cu2O phases coexist. Absorption/emission characteristics identify band gap narrowing, which facilitates visible light absorption, as a consequence of hybridization of O 2p bands with Cu 3 d bands of the ZnO-CuxO heterostructure system. With use of the specific band energy position of the individual components (i.e., ZnO, Cu2O, and CuO), photogenerated electrons and holes could be made to migrate separately and thereby the carrier recombination could be delayed, making them amply available for photocatalytic reactions. The available electrons plausibly produce superoxide radical anion (O2·−), while holes generate hydroxyl radicals (·OH), which are reactive agents for dye degradation. Furthermore, the formation of a depletion region via Cu-induced charge separation within the ZnO-CuxO heterostructure and thermal-energy-mediated abstraction of the electric field barriers and subsequent migration of the positively charged defects from the subsurface region toward the interface lead to efficient photocatalytic performance.

Bismuth Substituted Strontium Cobalt Perovskites for Catalyzing Oxygen Evolution

In this study, we employ the concept of inductive effect through substitution with more electronegative/Lewis acidic A-site ions in the cobalt perovskites to alter O 2p band center and surface hydr...

Impact of calcination temperature on the structure, optical and photoluminescence properties of NanocrystallineCerium oxide thin films

Nanocrystalline cerium oxide films were effectively synthesized on glass substrates via hydrothermal and dip coating techniques at various calcination temperatures (CT) from 300 to 550 °C. Impact of CT on the structure, surface morphology, optical and photoluminescence (PL) properties of cerium oxide (CeOx) films has been investigated. The grazing incident in-plane X-ray diffraction (GIIXD) analysis revealed that thin films acquire cubic fluorite nanostructure of CeO2, with average crystallite size in the range of 3.77–7.63 nm for films of different CT. XPS results revealed the existence of amorphous Ce2O3 in nanostructure CeOx films with crystalline CeO2 phase that was identified by GIIXD results. AFM images manifestations of CeOx thin film at various temperatures with tinny nanoparticles which agglomerate as spherical grains appear randomly distributed with surface roughness ranging from 0.42 to 0.488 μm for films of different CT. Optical properties indicated a narrowing in direct and indirect band gaps of nanocrystalline films than bulk CeO2 to show some fluctuation with the influence of CT that affected with lowering in Ce3+/Ce4+ ratio. Other optical parameters were analysed with the influence of calcination temperatures CT. Photoluminescence (PL) studies indicate the presence of most intense PL emission peak appears at 419 nm revealed the fundamental transition of direct band gap of CeO2, in addition to some other PL emission peaks in the visible range attributed to structural and oxygen vacancies defects states among Ce4f and O 2p bands.