Spinel Co3O4, comprising two types of cobalt ions: one Co(2+) in the tetrahedral site (Co(2+)(Td)) and the other two Co(3+) in the octahedral site (Co(3+)(Oh)), has been widely explored as a promising oxygen evolution reaction (OER) catalyst for water electrolysis. However, the roles of two geometrical cobalt ions toward the OER have remained elusive. Here, we investigated the geometrical-site-dependent OER activity of Co3O4 catalyst by substituting Co(2+)(Td) and Co(3+)(Oh) with inactive Zn(2+) and Al(3+), respectively. Following a thorough in operando analysis by electrochemical impedance spectroscopy and X-ray absorption spectroscopy, it was revealed that Co(2+)Td site is responsible for the formation of cobalt oxyhydroxide (CoOOH), which acted as the active site for water oxidation.

Artificial photosynthesis using semiconductors has been investigated for more than three decades for the purpose of transferring solar energy into chemical fuels. Numerous studies have revealed that the introduction of plasmonic materials into photochemical reaction can substantially enhance the photo response to the solar splitting of water. Until recently, few systematic studies have provided clear evidence concerning how plasmon excitation and which factor dominates the solar splitting of water in photovoltaic devices. This work demonstrates the effects of plasmons upon an Au nanostructure-ZnO nanorods array as a photoanode. Several strategies have been successfully adopted to reveal the mutually independent contributions of various plasmonic effects under solar irradiation. These have clarified that the coupling of hot electrons that are formed by plasmons and the electromagnetic field can effectively increase the probability of a photochemical reaction in the splitting of water. These findings support a new approach to investigating localized plasmon-induced effects and charge separation in photoelectrochemical processes, and solar water splitting was used herein as platform to explore mechanisms of enhancement of surface plasmon resonance.

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The formation of μ-OO peroxide (Co-OO-Co) moieties on spinel CoO electrocatalyst prior to the rise of the electrochemical oxygen evolution reaction (OER) current was identified by in situ spectroscopic methods. Through a combination of independent in situ X-ray absorption, grazing-angle X-ray diffraction, and Raman analysis, we observed a clear coincidence between the formation of μ-OO peroxide moieties and the rise of the anodic peak during OER. This finding implies that a chemical reaction step could be generally ignored before the onset of OER current. More importantly, the tetrahedral Co ions in the spinel CoO could be the vital species to initiate the formation of the μ-OO peroxide moieties.

The orthophosphate host family, A(I)B(II)PO(4) (A(I) = monovalent cation, B(II) = divalent cation), has recently been made available as phosphors that combine with near-UV lighting chips for use in solid-state white light-emitting diodes (LEDs). This study elucidates the crystalline structure and lattice parameters of the products via a solid-state reaction, using powder X-ray diffraction (XRD) and GSAS refinement. The versatility of the phosphor host A(I)B(II)PO(4) is established by examining isovalent substitutions of four cations in the structure-Li or K for A(I), Sr or Ba for B(II)-and three doped activators, RE = Eu(2+), Tb(3+), and Sm(3+). The luminescence properties, decay time, and Commission Internationale de l'Eclairage (CIE) chromaticity index are determined for various concentrations of these activators and metal constituents of the host. The thermal stabilities of all of these compounds are determined for the first time from the crystal structure and the coordination environment of the rare-earth metal. The morphology, composition, and particle size were measured in detail. Finally, density functional calculations were performed using the generalized gradient approximation plus an on-site Coulombic interaction correction (GGA+U) scheme to investigate the electronic structures of the KSrPO(4) system. A concise model was proposed to explain the luminescence mechanism.

Tuning and optimizing luminescent properties of oxonitridosilicates phosphors are important for white light-emitting diode (WLED) applications. To improve the color rendering index, correlated color temperature and thermal stability of layer-structured MSi2O2N2:Eu (M = Sr, Ba) phosphors, cation substitutions have been used to adjust their luminescent properties. However, the underlying mechanisms are still unclear. In this research, a series of (Sr1–x Ba x )Si2O2N2:Eu (0 ≤ x ≤ 1) compounds were prepared by solid-state reaction, after which systematic emission variations were investigated. The crystal structures of (Sr1–x Ba x )Si2O2N2:Eu (0 ≤ x ≤ 1) are nominally divided into three sections, namely, Phase 1 (0 ≤ x ≤ 0.65), Phase 2 (0.65 < x < 0.80), and Phase 3 (0.80 ≤ x ≤ 1) based on the X-ray diffraction measurements. These experimental results are further confirmed by optimizing the crystal structure data with first-principle calculations. Continuous luminescence adjustments from green to yellow are observed in Phase 1 with gradual replacement of Sr2+ with Ba2+, and the abnormal redshift is clarified through extended X-ray absorption fine structure analysis. Sr(Eu)–O/N bond length shrinkage in local structure causes the redshift emission, and the corresponding luminescence mechanism is proposed. Controllable luminescence in Phase 2 (from blue to white) and Phase 3 (from cyan to yellowish green) are observed. Based on the high-resolution transmission electron microscopy and selected area electron diffraction analysis, the two kinds of luminescence tuning are attributed to phase segregation. This study serves as a guide in developing oxonitride luminescent materials with controllable optical properties based on variations in local coordination environments through cation substitutions.

to develop OER electrocatalysts with a low overpotential in order to scale down the energy expenditure of water electrolysis.To date, noble metal oxides, such as iridium oxide and ruthenium oxide, are regarded as the benchmarked OER electrocatalysts, [10][11][12][13] but their scarcity and high cost hinder them from the wide utilization. [14] For these reasons, it is of the essence to explore earth-abundant substances showing comparable performance as costly noble-metal electrocatalysts. Recently, earth-abundant transition-metalbased materials exhibited high performance and superb stability toward OER, especially for the cobalt and nickel-based oxides/nonoxides. [15][16][17][18][19][20][21][22] Furthermore, various metal dopants would greatly affect the activities and intrinsic attributes. For example, tungsten, chromium, iron, and zinc-doping have been discovered to improve the activities in contrast to pristine metal oxides owing to the optimization of adsorption energy for surface intermediates or the increment of roughness factor. [23][24][25][26][27][28][29] Interestingly, among these elements, iron can commonly perform significant enhancement in catalytic activities toward oxygen evolution reaction as compared to other elements. [30,31] Boettcher's group proposed that Fe ions granted the catalytic activities while Co ions acted as the conductive oxides to transport the charge carriers in an Fe-Co metal oxide system. [32] Friebel et al. conducted a series of operando experiments and calculations to demonstrate that Fe ions in Ni 1-x Fe x OOH system would alter the adsorption energies of OER intermediates over the electrocatalytic surface and thus reduce the overpotential of OER, whereas the formation of low-activity FeOOH declined the resulting activities in the cases of higher Fe content. [33] Howbeit, the behavior of iron based on the material insight was not elaborated. Toward this end, it is crucial to establish the direct relationship between the material characteristics and the catalytic activities.Recently, we reported that the geometrical sites in spinel cobalt oxide served distinct functions. In the case of Co 3 O 4 , the cobalt ions in octahedral site (Co 3+ (Oh) ) contributed to surface double layer capacitance while those in tetrahedral site (Co 2+ (Td) ) were able to adsorb oxygen ions onto the surface for being the active species. [34,35] It suggested that even for the identical Introduction of iron in various catalytic systems has served a crucial function to significantly enhance the catalytic activity toward oxygen evolution reaction (OER), but the relationship between material properties and catalysis is still elusive. In this study, by regulating the distinctive geometric sites in spinel, Fe occupies the octahedral sites (Fe 3+ (Oh) ) and confines Co to the tetrahedral site (Co 2+ (Td) ), resulting in a strikingly high activity (η j = 10 mA cm −2 = 229 mV and η j = 100 mA cm −2 = 281 mV). Further enrichment of Fe ions would occupy the tetrahedral sites to decline the amount of Co 2+ (Td) a...

The state-of-the-art active HER catalysts in acid media (e.g., Pt) generally lose considerable catalytic performance in alkaline media mainly due to the additional water dissociation step. To address this issue, synergistic hybrid catalysts are always designed by coupling them with metal (hydro)oxides. However, such hybrid systems usually suffer from long reaction path, high cost and complex preparation methods. Here, we discover a single-phase HER catalyst, SrTi0.7Ru0.3O3-δ (STRO) perovskite oxide highlighted with an unusual super-exchange effect, which exhibits excellent HER performance in alkaline media via atomic-scale synergistic active centers. With insights from first-principles calculations, the intrinsically synergistic interplays between multiple active centers in STRO are uncovered to accurately catalyze different elementary steps of alkaline HER; namely, the Ti sites facilitates nearly-barrierless water dissociation, Ru sites function favorably for OH* desorption, and non-metal oxygen sites (i.e., oxygen vacancies/lattice oxygen) promotes optimal H* adsorption and H2 desorption.