Enhanced Oxygen Evolution Reaction Performance on NiS<i>_x</i>@Co₃O₄/Nickel Foam Electrocatalysts with Their Photothermal Property

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiSx-coupled nanourchin-like Co3O4 was prepared on nickel foam (NiSx@Co3O4/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiSx@Co3O4/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co3O4/NF and NiSx/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiSx@Co3O4/NF in 1.0 M KOH owing to the NiSx/Co3O4 synergetic effect. In addition, the oxygen evolution reaction performance of NiSx@Co3O4/NF can be obviously enhanced under near-infrared light irradiation, since NiSx@Co3O4 can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiSx@Co3O4/NF at 50 mA cm–2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy

Enhanced Oxygen Evolution Reaction Performance on NiS<i>_x</i>@Co₃O₄/Nickel Foam Electrocatalysts with Their Photothermal Property

Based on the principle of heterogeneous catalysis for water electrolysis, electrocatalysts with appropriate electronic structure and photothermal property are expected to drive the oxygen evolution reaction effectively. Herein, amorphous NiSx-coupled nanourchin-like Co3O4 was prepared on nickel foam (NiSx@Co3O4/NF) and investigated as a electrocatalyst for photothermal-assisted oxygen evolution reaction. The experimental investigations and simulant calculations jointly revealed NiSx@Co3O4/NF to be of suitable electronic structure and high near-infrared photothermal conversion capability to achieve the oxygen evolution reaction advantageously both in thermodynamics and in kinetics. Relative to Co3O4/NF and NiSx/NF, better oxygen evolution reaction activity, kinetics, and stability were achieved on NiSx@Co3O4/NF in 1.0 M KOH owing to the NiSx/Co3O4 synergetic effect. In addition, the oxygen evolution reaction performance of NiSx@Co3O4/NF can be obviously enhanced under near-infrared light irradiation, since NiSx@Co3O4 can absorb the near-infrared light to produce electric and thermal field. For the photothermal-mediated oxygen evolution reaction, the overpotential and Tafel slope of NiSx@Co3O4/NF at 50 mA cm–2 were reduced by 23 mV and 13 mV/dec, respectively. The present work provides an inspiring reference to design and develop photothermal-assisted water electrolysis using abundant solar energy

Fabrication and Properties of a Free-Standing Two-Dimensional Titania

The synthesis of free-standing two-dimensional titania (2-D TiO₂) with a reduced band gap presents complex challenges to synthetic chemists. Here, we report a free-standing 2-D TiO₂ sheet synthesized via a one-step solvothermal methodology, with a measured optical onset at ∼1.84 eV. Using first-principles calculations in combination with experiment, we propose that the as-formed 2-D TiO₂ sheets are layers of the lepidocrocite TiO₂ structure, but with large nonuniform strains consistent with its crumpled morphology. These strains cause a significant change in the quasiparticle band structure and optical absorption spectra, resulting in large absorption in the visible-light region. This narrow band gap 2-D TiO₂ can catalyze the formation of singlet oxygen and the degradation of dye pollutants with low-energy photons of solar light. Our work demonstrates that lattice strains intrinsic to 2-D materials, especially its crumpled, free-standing forms, can result in new and useful properties

Stable Pt Single Atoms and Nanoclusters on Ultrathin CuO Film and Their Performances in CO Oxidation

A series of model catalysts consisting of Pt single atoms and nanoclusters supported by monolayered CuO film grown at Cu(110) were successfully prepared, which could be stabilized well above room temperature and which exhibited a high performance in CO oxidation at temperatures as low as ∼360 K. Combined scanning tunneling microscopy and temperature-programmed desorption measurements directly evidenced that at the initial CO oxidation stage, oxygen vacancy in the CuO lattice was generated at the nearest neighbor of the Pt nanoclusters. The experimental measurements showed that the oxidation activity was inversely proportional to the Pt nanocluster size. In contrast, the Pt single atoms possessed no surface reactivity for the CO oxidation because of the early and complete desorption of CO before its oxidation on the model catalysts commenced

Iron Carbidization on Thin-Film Silica and Silicon: A Near-Ambient-Pressure X‑ray Photoelectron Spectroscopy and Scanning Tunneling Microscopy Study

Model catalysts consisting of iron particles with similar size deposited on thin-film silica (Fe/SiO₂) and on silicon (Fe/Si) were used to study iron carbidization in a CO atmosphere using in situ near-ambient-pressure X-ray photoelectron spectroscopy. Significant differences were observed for CO adsorption, CO dissociation, and iron carbidization when the support was changed from thin-film silica to silicon. Stronger adsorption of CO on Fe/Si than that on Fe/SiO₂ was evident from the higher CO equilibrium coverage found at a given temperature in the presence of 1 mbar of CO gas. On thin-film silica, iron starts to carbidize at 150 °C, while the onset of carbidization is at 100 °C on the silicon support. The main reason for the different onset temperature for carbidization is the efficiency of removal of oxygen species after CO dissociation. On thin-film silica, oxygen species formed by CO dissociation block the iron surface until ∼150 °C, when CO₂ formation removes surface oxygen. Instead, on the silicon support, oxygen species readily spill over to the silicon. As a consequence, oxygen removal is not rate-limiting anymore and carbidization of iron can proceed at a lower temperature

Dynamic Oxygen on Surface: Catalytic Intermediate and Coking Barrier in the Modeled CO₂ Reforming of CH₄ on Ni (111)

We identify Ni–O phases as important intermediates in a modeled dry (CO₂) reforming of methane catalyzed by Ni (111), based on results from <i>in operando</i> near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) experiments, corroborated by low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) measurements. We find that, under a CO₂ or CO₂–CH₄ atmosphere, the Ni–O phases exist in the forms of p(2 × 2)-structured chemisorbed oxygen (Chem-O), epitaxial NiO (111), or oxygen-rich Ni_<i>x</i>O_<i>y</i> (x < y, typically Ni₂O₃), depending on the chemical potential. The growth rates of the Ni–O phases have a negative correlation with temperature from 600 to 900 K, proving that their dynamic concentrations in the reaction are not limited by CO₂ activation, but by their thermal stability. Between 300 and 800 K (1:1 CH₄ and CO₂ mixture), oxidation by CO₂ is dominant, resulting in a fully Ni–O covered surface. Between 800 and 900 K, a partially oxidized Ni (111) exists which could greatly facilitate the effective conversion of CH₄. As CH₄ is activation-limited and dissociates mainly on metallic nickel, the released carbon species can quickly react with the adjacent oxygen (Ni–O phases) to form CO. After combining with carbon and releasing CO molecules, the Ni–O phases can be further regenerated through oxidation by CO₂. In this way, the Ni–O phases participate in the catalytic process, acting as an intermediate in addition to the previously reported Ni–C phases. We also reveal the carbon phobic property of the Ni–O phases, which links to the intrinsic coking resistance of the catalysts. The low dynamic coverage of surface oxygen at higher temperatures (>900 K) is inferred to be an underlying factor causing carbon aggregation. Therefore, solutions based on Ni–O stabilization are proposed in developing coking resisting catalysts

Steering Surface Reaction at Specific Sites with Self-Assembly Strategy

To discern the catalytic activity of different active sites, a self-assembly strategy is applied to confine the involved species that are “attached” to specific surface sites. The employed probe reaction system is the Ullmann coupling of 4-bromobiphenyl, C₆H₅C₆H₄Br, on an atomically flat Ag(111) surface, which is explored by combined scanning tunneling microscopy, synchrotron X-ray photoelectron spectroscopy, and density functional theory calculations. The catalytic cycle involves the detachment of the Br atom from the initial reactant to form an organometallic intermediate, C₆H₅C₆H₄AgC₆H₄C₆H₅, which subsequently self-assembles with its central Ag atom residing either on 2-fold bridge or 3-fold hollow sites at full coverage. The hollow site turns out to be catalytically more active than the bridge one, allowing us to achieve site-steered reaction control from the intermediate to the final coupling product, <i>p</i>-quaterphenyl, at 390 and 410 K, respectively

Unraveling Charge State of Supported Au Single-Atoms during CO Oxidation

Thermally stable Au single-atoms supported by monolayered CuO grown at Cu(110) have been successfully prepared. The charge transfer from the CuO support to single Au atoms is confirmed to play a key role in tuning the activity for CO oxidation. Initially, the negatively charged Au single-atom is active for CO oxidation with its adjacent lattice O atom depleted to generate an O vacancy in the CuO monolayer. Afterward, the Au single-atom is neutralized, preventing further CO reaction. The produced O vacancy can be healed by exposure to O₂ at 400 K and accordingly the reaction activity is restored

Lattice-Directed Formation of Covalent and Organometallic Molecular Wires by Terminal Alkynes on Ag Surfaces

Surface reactions of 2,5-diethynyl-1,4-bis(phenylethynyl)benzene on Ag(111), Ag(110), and Ag(100) were systematically explored and scrutinized by scanning tunneling microscopy, molecular mechanics simulations, and density functional theory calculations. On Ag(111), Glaser coupling reaction became dominant, yielding one-dimensional molecular wires formed by covalent bonds. On Ag(110) and Ag(100), however, the terminal alkynes reacted with surface metal atoms, leading to one-dimensional organometallic nanostructures. Detailed experimental and theoretical analyses revealed that such a lattice dependence of the terminal alkyne reaction at surfaces originated from the matching degree between the periodicities of the produced molecular wires and the substrate lattice structures

Epitaxial Growth of Single Layer Blue Phosphorus: A New Phase of Two-Dimensional Phosphorus

Blue phosphorus, a previously unknown phase of phosphorus, has been recently predicted by theoretical calculations and shares its layered structure and high stability with black phosphorus, a rapidly rising two-dimensional material. Here, we report a molecular beam epitaxial growth of single layer blue phosphorus on Au(111) by using black phosphorus as precursor, through the combination of <i>in situ</i> low temperature scanning tunneling microscopy and density functional theory calculation. The structure of the as-grown single layer blue phosphorus on Au(111) is explained with a (4 × 4) blue phosphorus unit cell coinciding with a (5 × 5) Au(111) unit cell, and this is verified by the theoretical calculations. The electronic bandgap of single layer blue phosphorus on Au(111) is determined to be 1.10 eV by scanning tunneling spectroscopy measurement. The realization of epitaxial growth of large-scale and high quality atomic-layered blue phosphorus can enable the rapid development of novel electronic and optoelectronic devices based on this emerging two-dimensional material