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Electronic structure modulation among multiple metal sites is key to the design of efficient catalysts. Most studies have focused on regulating 3d transition-metal active ions through other d-block metals, while few have utilized f-block metals. Herein, we report a new class of catalyst, namely, UCoO4 with alternative CoO6 and 5f-related UO6 octahedra, as a unique example of a 5f-covalent compound that exhibits enhanced electrocatalytic oxygen evolution reaction (OER) activity because of the presence of the U 5f–O 2p–Co 3d network. UCoO4 exhibits a low overpotential of 250 mV at 10 mA cm–2, surpassing other unitary cobalt-based catalysts ever reported. X-ray absorption spectroscopy revealed that the Co2+ ion in pristine UCoO4 was converted to high-valence Co3+/4+, while U6+ remained unchanged during the OER, indicating that only Co was the active site. Density functional theory calculations demonstrated that the OER activity of Co3+/4+ was synergistically enhanced by the covalent bonding of U6+-5f in the U 5f–O 2p–Co 3d network. This study opens new avenues for the realization of electronic structure manipulation via unique 5f involvement.

Synchrotron-based X-ray spectroscopic and microscopic techniques are used to identify the origin of enhancement of the photoelectrochemical (PEC) properties of BiVO4 (BVO) that is coated on ZnO nanodendrites (hereafter referred to as BVO/ZnO). The atomic and electronic structures of core-shell BVO/ZnO nanodendrites have been well-characterized, and the heterojunction has been determined to favor the migration of charge carriers under the PEC condition. The variation of charge density between ZnO and BVO in core-shell BVO/ZnO nanodendrites with many unpaired O 2p-derived states at the interface forms interfacial oxygen defects and yields a band gap of approximately 2.6 eV in BVO/ZnO nanocomposites. Atomic structural distortions at the interface of BVO/ZnO nanodendrites, which support the fact that there are many interfacial oxygen defects, affect the O 2p-V 3d hybridization and reduce the crystal field energy 10Dq ∼2.1 eV. Such an interfacial atomic/electronic structure and band gap modulation increase the efficiency of absorption of solar light and electron-hole separation. This study provides evidence that the interfacial oxygen defects act as a trapping center and are critical for the charge transfer, retarding electron-hole recombination, and high absorption of visible light, which can result in favorable PEC properties of a nanostructured core-shell BVO/ZnO heterojunction. Insights into the local atomic and electronic structures of the BVO/ZnO heterojunction support the fabrication of semiconductor heterojunctions with optimal compositions and an optimal interface, which are sought to maximize solar light utilization and the transportation of charge carriers for PEC water splitting and related applications.

Modulation of bimetallic nanocatalysts with atomic precision would allow for significant increases in catalyst activity through the optimization of heteroatomic interplay. In practice, this level o...

Li-CO2 batteries have recently attracted attention as promising candidates for next-generation energy storage devices due to their extremely high theoretical energy density. The real application of Li-CO2 cells involves addressing several drawbacks, including high charging potential, poor coulombic efficiency, and low rechargeability. Molybdenum disulfide supported on carbon nanotubes (MoS2/CNT) with various ratios functioned as a cathode catalyst for Li-CO2 batteries. The optimal MoS2/CNT composite achieved a maximum discharge capacity of 8551 mAh g-1 with a coulombic efficiency of 96.7%. This hybrid also obtained an initial charging plateau of 3.87 V at a current density of 100 mA g-1 with a cutoff capacity of 500 mAh g-1. It provided ideal electrochemical stability of 142 cycles at the current densities of 100 mA g-1, which was comparable with that of some precious metal catalysts. This optimized MoS2/CNT was also cycled at 200 and 400 mA g-1 for 112 and 55 times, respectively. Density functional theory calculations demonstrated that the sulfided Mo-edge (s-Mo-edge) on MoS2 materials showed appropriate adsorption strengths of Li, CO2, and Li2CO3. Moreover, joint results of Raman profiles and extended X-ray absorption fine structure spectra elucidated that the catalytic efficiencies of MoS2/CNT hybrids were proportional to the quantities of exposed s-Mo-edge active sites.

The exact role of a defect structure on transition metal compounds for electrocatalytic oxygen evolution reaction (OER), which is a very dynamic process, remains unclear. Studying the structure-activity relationship of defective electrocatalysts under operando conditions is crucial for understanding their intrinsic reaction mechanism and dynamic behavior of defect sites. Co3O4 with rich oxygen vacancy (VO) has been reported to efficiently catalyze OER. Herein, we constructed pure spinel Co3O4 and VO-rich Co3O4 as catalyst models to study the defect mechanism and investigate the dynamic behavior of defect sites during the electrocatalytic OER process by various operando characterizations. Operando electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) implied that the VO could facilitate the pre-oxidation of the low-valence Co (Co2+, part of which was induced by the VO to balance the charge) at a relatively lower applied potential. This observation confirmed that the VO could initialize the surface reconstruction of VO-Co3O4 prior to the occurrence of the OER process. The quasi-operando X-ray photoelectron spectroscopy (XPS) and operando X-ray absorption fine structure (XAFS) results further demonstrated the oxygen vacancies were filled with OH• first for VO-Co3O4 and facilitated pre-oxidation of low-valence Co and promoted reconstruction/deprotonation of intermediate Co-OOH•. This work provides insight into the defect mechanism in Co3O4 for OER in a dynamic way by observing the surface dynamic evolution process of defective electrocatalysts and identifying the real active sites during the electrocatalysis process. The current finding would motivate the community to focus more on the dynamic behavior of defect electrocatalysts.

Superoxide (LiO2) formation on charging is one of the major causes of poor cycle life of Li–O2 batteries. The underlying mechanism controlling LiO2 formation remains elusive. Here, we reveal that t...

In-depth understanding of electrocatalytically active species transformation during oxygen evolution reaction (OER) is highly guidable to design effective electrocatalysts for water splitting. Here...

The electronic structure of layered double hydroxides (LDHs) can be modulated by heteroatom doping and creating vacancies. The number of exposed active sites can be enriched by exfoliating the bulk structure into fewer layers. Herein, we successfully achieved multielement doping and exfoliation for Co3Fe LDHs by one SF6-plasma etching step at room temperature (named as Co3Fe LDHs-SF6). The obtained Co3Fe LDHs-SF6 ultrathin nanosheets display outstanding oxygen evolution reaction (OER) activity, which only needs 268 mV overpotential to reach 10 mA cm-2. Tafel slope and charge transfer resistance are dramatically decreased indicating a faster reaction kinetic rate. The excellent OER activity can be attributed to an increased number of active sites and an optimized electronic structure modulated by the incorporation of electron-withdrawing F, electron-donating S, and abundant vacancies resulting in proper adsorption energy to oxygen species.

We have revealed that cubelike and hexarhombic docadehedron-like Cu single crystals showed an enhancement of C2 products (ethylene and ethanol) while octahedron-like Cu nanoscale single crystals pr...

Advances in new systems of organic–inorganic hybrid nanocomposites are less prevalent, owing to a lack of facile strategies for precise control of their structures, compositions, and, hence, their properties. In this work, Au-BINOL hybrid nanocomposites with eccentric and concentric nanostructures were produced. The hybrid nanocomposites containing two distinct moieties of inorganic Au nanocrystals and organic BINOL nanospheres were applied to the catalytic hydrogenation of 4-nitrophenol with NaBH4 in the aqueous phase with and without the illumination of visible light. Here, we demonstrate that the existence of Au–BINOL interfaces offers benefits to their performance. The eccentric nanostructures made with CTAC show the superior activity from large Au–BINOL interfaces formed between the BINOL nanospheres and the faces of Au nanoplates. They further exhibit a high Au localized surface plasmon resonance (LSPR)-enhancement effect on the photoreduction of 4-nitrophenol, which is attributed to the strong LSPR...

The present study reports the effect of Ni ion implantation on the structural, compositional, electrical, and thermoelectric properties and electronic structures of CoSb3 skutterudite thin films deposited on Si substrate by pulsed laser deposition. Ni ions were implanted at 200 keV in CoSb3 thin films at three different fluences: 3 × 1015, 6 × 1015, and 1.5 × 1016 ions/cm2. X-ray diffraction of Ni-implanted films shows an additional phase of Co0.75Ni0.25Sb3. The electrical measurement of pristine films exhibits typical semiconductor behavior, while the Ni-implanted films show an abrupt increase in resistivity, which may be attributed to the formation of Co0.75Ni0.25Sb3 and material modification by the energetic ion beam. The Seebeck coefficient measurements imply that all the films are n-type with maximum Seebeck coefficient of ∼75 μV/K at ∼410 K for 2% Ni-implanted film which is about 7 times higher than the pristine CoSb3 film. The X-ray absorption study of Ni, Co, and Sb spectrum confirms that Ni ions ...

We report the contribution of oxygen vacancies for enhancing the optical and visible photocatalytic properties of Cu-doped CeO2 nanoparticles (NPs) synthesized through a low-temperature coprecipitation method. Doping Cu ions in the ceria lattice in different mole percentages, 0, 3, 5, 7, 9, and 15 wt %, results in enhancement of visible photocatalytic properties even under natural sunlight. Transmission electron microscopy and X-ray diffraction studies showcase the monodispersive nature of Cu-doped CeO2 NPs in the size range of 3–7 nm with face-centered cubic structure. The Cu-based defect states induce a narrow band function in ceria nanostructures and influence the red shift in absorption with the Cu concentrations. Visible photocatalytic degradation of methylene blue was investigated in the presence of pure CeO2 NPs, CuO NPs, and Cu-doped CeO2 NPs. These studies revealed that the 7 wt % of Cu-doped CeO2 NPs exhibit the degradation rates of 1.41 × 10–2 and 1.12 × 10–2 min–1 under exposure to natural sun...

Plasmonic photocatalyst of Au nanoparticle-decorated hollow mesoporous TiO2 with 0, 0.1, 0.25, 0.5, and 1% Au content was successfully synthesized by a hydrothermal method. Controlling the particle size of Au coated on TiO2 hollow microspheres (AuTHMSs) is expected to improve the photocatalytic ability. Our results of X-ray absorption spectroscopy (XAS) indicated that the coated Au ions are nulvalent and cause a lattice distortion as well as a variation in Ti 3d orbital orientation. It is also inferred that TiO6 octahedral symmetry is significantly affected by the Au incorporation, giving rise to an increase in the Ti 3d t2g unoccupied state. UV–visible absorption spectra and I–V measurements were performed to examine localized surface plasmon resonance (LSPR) effect and photoelectrocatalytic (PEC) ability. We present the first in situ XAS measurements on AuTHMS system, which enabled us to correlate the electronic structure and photocatalytic property of the material. An analysis of the results showed an ...

The electrodeposition nature of copper on a gold electrode in a 4.8 pH CuSO4 solution was inquired using X-ray absorption spectroscopy, electrochemical quartz crystal microbalance, and thermal desorption spectroscopy techniques. Our results point out that the electrodeposition of copper prompts the formation of stable oxi-hydroxide species with a formal oxidation state Cu+ without the evidence of metallic copper formation (Cu0). Moreover, the subsequent anodic polarization of Cu2Oaq yields the formation of CuO, in the formal oxidation state Cu2+, which is dissolved at higher anodic potential. It was found that the dissolution process needs less charge than that required for the electrodeposition indicating a nonreversible process most likely due to concomitant water splitting and formation of protons during the electrodeposition.

The effects of growth time and interface between the iron oxyhydroxide (FeOOH) and carbon materials (carbon nanotubes (CNT) and reduced graphene oxide (RGO)) to form an asymmetric supercapacitor was studied by X-ray absorption spectroscopy (XAS) and electrochemical measurements. FeOOH/CNT (FCNT) and FeOOH/RGO (FRGO) were successfully synthesized by a simple spontaneous redox reaction with FeCl3. The RGO functions as an ideal substrate, providing rich growth sites for FeOOH, and it is believed to facilitate the transport of electrons/ions across the electrode/electrolyte interface. FRGO has been identified as a supercapacitor and found to exhibit significantly greater capacitance than FCNT. To gain further insight into the effects of growth times and the interface of FeOOH for FCNT and FRGO, the electronic structures of FCNT and FRGO with various FeOOH growth times were elucidated by XAS. The difference between the surface electronic structures of CNT and RGO yields different nucleation and growth rates of...

In anode, electrocatalytic water splitting involves oxygen evolution reaction (OER), which is a complex and sluggish reaction, and thus the efficiency to produce hydrogen is seriously limited by OER. We report that CoTe2 exhibits optimized OER activity for the first time. Multiwalled carbon nanotube (MWCNT) is utilized to support CoTe2 in generating a synergistic effect to enhance OER activity and improve stability by tuning different loading amounts of CoTe2 on CNT. In 1.0 M KOH, bare CoTe2 needed overpotential of 323 mV to produce 10 mA/cm2 with Tafel slope of 85.1 mV/dec, but CoTe2/carbon nanotube (CNT) with optimized loading amount of CoTe2 required only 291 mV to produce10 mA/cm2 with Tafel slope of 44.2 mV/dec. X-ray absorption near edge structure (XANES) was applied to prove that an electron transfer from eg band of CoTe2 to CNT caused a synergistic effect. This electron transfer modulated the bond strength of oxygen-related intermediate species on the surface of catalyst and optimized OER performa...

A solid-state symmetric supercapacitor is successfully fabricated by assembling the sulfuric acid–poly(vinyl alcohol) (H2SO4–PVA) gel electrolyte between the two pieces of electrochemically activated reduced graphene oxide (aRGO) electrodes. The electrochemical and electronic properties of reduced graphene oxide (RGO) and aRGO were characterized by ex situ X-ray absorption spectroscopy (XAS). The aRGO exhibits better electrochemical supercapacitive performances than RGO, owing to the conjunction of the electrochemical double-layer capacitance (EDLC) and the pseudocapacitance. The specific capacitance increases with the oxygen-containing functional groups content in the electrochemically activated aRGO than RGO, increasing the pseudocapacitive contribution. The aRGO20//aRGO20 solid-state symmetric supercapacitors (SSC) exhibit an energy density of 4.7 Wh kg–1 at a power density of 402 W kg–1 and 4 Wh kg–1 at a power density of 1989 W kg–1, which is competitive with the commercially available supercapacitor...

One-dimensional (1D) TiO2 nanorod arrays as photoelectrode have great potential for solar photoelectrochemical (PEC) hydrogen generation. However, the large band gap and Ti-growth unit preference of rutile TiO2 limit its solar light utilizing and multijunction nanostructure photoelectrode design. This paper presents a double-sided tandem structure for quantum dot cosensitized photoelectrodes with excellent solar PEC hydrogen generation. TiO2 nanorod arrays were grown directly on transparent and conductive glass substrates by hydrothermal method and then coated with CdS or CdSe as photosensitizer to extend successfully their photoresponse to visible light. Given the transparent substrate, TiO2 nanorod arrays could be grown on both sides, allowing the formation of the tandem structure of cosensitized CdS and CdSe with high reactivity under visible light. The double-sided CdS and CdSe cosensitized 1D TiO2 photoelectrode exhibited the highest solar-to-hydrogen conversion efficiency of 2.78% and pronounced enh...

The present study reports the effect of Co doping on the structural, optical, magnetic, and electronic properties of CeO2 nanoparticles (NPs) synthesized by a simple low-temperature co-precipitation method. Co doping was introduced by adding CoCl3 with different mole percentages (0%, 2%, 4%, and 6%) to cerium nitrate, which resulted in room-temperature ferromagnetism (RTFM). TEM and XRD analysis showed that the Co-doped CeO2 NPs are monodispersed with face centered cubic structure. The 6% Co-doped CeO2 NPs showed a coercivity value of 155 Oe and saturation magnetization of 0.028 emu/g at room temperature. The electronic structures of the as-prepared CeO2 and Co-doped CeO2 NPs were investigated by X-ray absorption near-edge structure (XANES) spectroscopy. The XANES spectra at Ce M- and L-edges clearly indicated a decrease in the valency state of Ce ions from Ce4+ to Ce3+ upon Co doping. This causes redistribution of oxygen ions and Co–Co bonding. The XANES study revealed that Co doping plays a prominent ro...

In this study, the defect structure of Y doped CeO2 nanoparticles (NPs) was investigated systematically by using spectroscopy and microscopy. The doping level of Y ranges from 0% to 15%. It is demonstrated that Y3+ substitutes Ce and governs the formation of oxygen vacancy. At low doping level, Y3+ randomly distributed throughout the particle. However, as doping level increased above 9%, Y3+ aggregates at the surface and forms Y-rich clusters. Room temperature ferromagnetism (FM) was observed in these Y-doped CeO2 NPs. It is found that the value of saturation magnetization (Ms) increases until Y reaches 9%, then it decreases. Raman, X-ray absorption near edge spectroscopy and X-ray magnetic circular dichroism (XMCD) analysis has provided several aspects on the electronic properties of theses nanoparticles. A charge delocalization occurs upon Y doping on the Ce(Y)-O(VO)-Ce(Y) orbitals. The magnetism is evidenced by XMCD spectroscopy only on Ce orbitals, and the magnetism intensity is mainly related to the ...

This study investigates the effect of Fe(3+) on the electronic structure of nanocrystalline ceria. Systematic synchrotron X-ray absorption spectroscopy coupled with scanning transmission electron microscopy/electron energy loss spectroscopy was utilized. The oxygen vacancies can be engineered and their number varied with the degree of iron doping. Comparing the local electronic structure around Ce sites with that around Fe sites reveals two stages of defect engineering. The concentration of Ce(3+) and the distribution of defects differ between lower and higher degrees of doping. Charge is transferred between Ce and Fe when the doping level is less than 5%, but this effect is not significant at a doping level of over 5%. This transfer of charge is verified by energy loss spectroscopy. These Fe-modified ceria nanoparticles exhibit core-shell-like structures at low doping levels and this finding is consistent with the results of scanning transmission electron microscopy/electron energy loss spectroscopy. More Fe is distributed at the surface for doping levels less than 5%, whereas the homogeneity of Fe in the system increases for doping levels higher than 5%. X-ray magnetic circular dichroism spectroscopy reveals that Ce, rather than Fe, is responsible for the ferromagnetism. Interestingly, Ce(3+) is not essential for producing the ferromagnetism. The oxygen vacancies and the defect structure are suggested to be the main causes of the ferromagnetism. The charge transfer and defect structure Fe(3+)-Vo-Ce(3+) and Fe(3+)-Vo-Fe(3+) are critical for the magnetism, and the change in saturated magnetization can be understood as being caused by the competition between interactions that originate from magnetic polarons and from paired ions.

This study presents the effect of Cr3+ on the formation of ferromagnetism in Cr-doped CeO2 nanoparticles (NPs). Systematic synchrotron X-ray spectroscopy analysis was utilized to investigate the defects in CeO2 NPs. X-ray absorption spectrum (XAS) revealed that the magnetic properties are correlated to the type and structure of defects. Both concentration of Ce3+ and the oxygen vacancy increased with increasing the content of Cr3+. The magnetism raises as the Cr increases and reaches maximum at Cr3+ ≈ 11% and, in turn, diminishes beyond this value. Interestingly, X-ray magnetic circular dichroism (XMCD) results indicate that ferromagnetism was contributed mainly by Ce3+ ions but not Cr3+. With comparing to the concentration dependence of Ce3+ in reduced undoped CeO2 nanoparticles, the relationships between the defects and magnetism were unraveled. The magnetism of CeO2 NPs was closely related to the oxygen deficiency. The major effect of doping Cr3+ on the formation of ferromagnetism in CeO2 nanoparticles...

This investigation demonstrates a strong dependence of the magnetism of CeO2 nanoparticles on the concentration of oxygen vacancies. A strong magnetic signal is over for a narrow range (0.1 < Ce3+/Ce < 0.2) of Ce3+ concentrations in the nanoparticles. Previous studies have determined that most of the vacancies are at the surface of the particle and, given particles of size 3–10 nm, the strong magnetic signal corresponds to an even narrower range of Ce3+ content at the surface (0.40 < Ce3+surface/Cesurface < 0.45). X-ray magnetic circular dichroism (XMCD) measurement reveals that electrons in Ce bear magnetic moments while oxygen atoms do not respond magnetically indicating that the bounded magnetic polarons are more important to magnetism than is oxygen-mediated exchange.

This study reports that highly oxygen-deficient CeO2 nanoparticles (NPs) can be obtained without reduction treatment by using thermal decomposition method. Different amounts of surfactants are used...

We report the electronic structure study of intermetallic CeCo2 nanothin films of various thicknesses by X-ray absorption near-edge structure (XANES) spectroscopy at Ce L3-, Co K-, and L2,3-edges. The Ce L3-edge absorption spectra reveal that the contribution of tetravalent Ce component increases with the film thickness, and all investigated nanothin films exhibit intermediate valence nature. Variation of the spectral intensities observed at the Co K-edge threshold implies modification in the Co 3d states and the enhancement of 3d-4f-5d hybridization. The Co 3d and Ce 4f occupation numbers were estimated from these spectroscopic results. The present study brings out how the surface-to-bulk ratio and the charge transfer between Ce and Co ions affect the electronic structure of nanothin films.