A Density Functional Theory and Microkinetic Study of Acetylene Partial Oxidation on the Perfect and Defective Cu2O (111) Surface Models

The catalytic removal of C2H2 by Cu2O was studied by investigating the adsorption and partial oxidation mechanism of C2H2 on both perfect (stoichiometric) and CuCUS-defective Cu2O (111) surface models using density functional theory calculations. The chemisorption of C2H2 on perfect and defective surface models needs to overcome the energy barrier of 0.70 and 0.81 eV at 0 K. The direct decomposition of C2H2 on both surface models is energy demanding with the energy barrier of 1.92 and 1.62 eV for the perfect and defective surface models, respectively. The H-abstractions of the chemisorbed C2H2 by a series of radicals including H, OH, HO2, CH3, O, and O2 following the Langmuir−Hinshelwood mechanism have been compared. On the perfect Cu2O (111) surface model, the activity order of the adsorbed radicals toward H-abstraction of C2H2 is: OH > O2 > HO2 > O > CH3 > H, while on the defective Cu2O (111) surface model, the activity follows the sequence: O > OH > O2 > HO2 > H > CH3. The CuCUS defect could remarkably facilitate the H-abstraction of C2H2 by O2. The partial oxidation of C2H2 on the Cu2O (111) surface model tends to proceed with the chemisorption process and the following H-abstraction process rather than the direct decomposition process. The reaction of C2H2 H-abstraction by O2 dictates the C2H2 overall reaction rate on the perfect Cu2O (111) surface model and the chemisorption of C2H2 is the rate-determining step on the defective Cu2O (111) surface model. The results of this work could benefit the understanding of the C2H2 reaction on the Cu2O (111) surface and future heterogeneous modeling.

Ingrained: An Automated Framework for Fusing Atomic-Scale Image Simulations into Experiments

To fully leverage the power of image simulation to corroborate and explain patterns and structures in atomic resolution microscopy, an initial correspondence between the simulation and experimental image must be established at the outset of further high accuracy simulations or calculations. Furthermore, if simulation is to be used in context of highly automated processes or high-throughput optimization, the process of finding this correspondence itself must be automated. In this work, "ingrained," an open-source automation framework which solves for this correspondence and fuses atomic resolution image simulations into the experimental images to which they correspond, is introduced. Herein, the overall "ingrained" workflow, focusing on its application to interface structure approximations, and the development of an experimentally rationalized forward model for scanning tunneling microscopy simulation are described.

Plasma-assisted oxidation of Cu(100) and Cu(111)

Oxidized copper surfaces have attracted significant attention in recent years due to their unique catalytic properties, including their enhanced hydrocarbon selectivity during the electrochemical reduction of CO₂. Although oxygen plasma has been used to create highly active copper oxide electrodes for CO₂RR, how such treatment alters the copper surface is still poorly understood. Here, we study the oxidation of Cu(100) and Cu(111) surfaces by sequential exposure to a low-pressure oxygen plasma at room temperature. We used scanning tunnelling microscopy (STM), low energy electron microscopy (LEEM), X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure spectroscopy (NEXAFS) and low energy electron diffraction (LEED) for the comprehensive characterization of the resulting oxide films. O₂-plasma exposure initially induces the growth of 3-dimensional oxide islands surrounded by an O-covered Cu surface. With ongoing plasma exposure, the islands coalesce and form a closed oxide film. Utilizing spectroscopy, we traced the evolution of metallic Cu, Cu₂O and CuO species upon oxygen plasma exposure and found a dependence of the surface structure and chemical state on the substrate's orientation. On Cu(100) the oxide islands grow with a lower rate than on the (111) surface. Furthermore, while on Cu(100) only Cu₂O is formed during the initial growth phase, both Cu₂O and CuO species are simultaneously generated on Cu(111). Finally, prolonged oxygen plasma exposure results in a sandwiched film structure with CuO at the surface and Cu₂O at the interface to the metallic support. A stable CuO(111) surface orientation is identified in both cases, aligned to the Cu(111) support, but with two coexisting rotational domains on Cu(100). These findings illustrate the possibility of tailoring the oxidation state, structure and morphology of metallic surfaces for a wide range of applications through oxygen plasma treatments.

A low-pressure oxygen plasma oxidized Cu(100) and Cu(111) surfaces at room temperature. The time-dependent evolution of surface structure and chemical composition is reported in detail for a range of exposure times up to 30 min.

Termination-dependent electronic structure and atomic-scale screening behavior of the Cu2O(111) surface

By combining differential conductance (dI/dV) spectroscopy with a scanning tunneling microscope and hybrid density functional theory simulations we explore the electronic characteristics of the (1 × 1) and (√3 × √3)R30° terminations of the Cu₂O(111) surface close to thermodynamic equilibrium. Although frequently observed experimentally, the composition and atomic structure of these two terminations remain controversial. Our results show that their measured electronic signatures, such as the conduction band onset deduced from dI/dVmeasurements, the bias-dependent appearance of surface topographic features, as well as the work function retrieved from field emission resonances unambiguously confirm their recent assignment to a (1 × 1) Cu-deficient (CuD) and a (√3 × √3)R30° nano-pyramidal reconstruction. Moreover, we demonstrate that due to a different localization of the screening charges at these Cu-deficient terminations, their electronic characteristics qualitatively differ from those of the stoichiometric (1 × 1) and O-deficient (√3 × √3) terminations often assumed in the literature. As a consequence, aside from the topographic differences recently pointed out, also their electronic characteristics may contribute to a radical change in the common perception of the Cu₂O(111) surface reactivity.

The effects of substitutional doping on Cu vacancy formation in Cu2O(111): a density functional theory study

Using density functional theory (DFT) calculations, we examined the effects of substitutional doping on the formation of Cu vacancies in Cu₂O(111). Upon replacing coordinatively unsaturated O with other elements (N, F, P, S, and Cl) and calculating the formation energies, we found that compared to the undoped surface, Cu vacancy formation is most favorable in the F-doped surface and least favorable in the N-doped Cu₂O(111) surface. In addition, we found that in most cases, vacancy formation of the coordinatively saturated Cu has higher vacancy formation energy than coordinatively unsaturated Cu atoms. In the electron localization function plots and the projected charge distributions of the local density of states, we found bonding enhancement between Cu and N in N-Cu₂O(111) but no corresponding enhancement was observed in the F-doped surface. Our results showed that the interaction between the substitutional dopants and the Cu atoms affects the formation of the doped system and the Cu vacancy formation in the surface.

Unraveling CO adsorption on model single-atom catalysts

Understanding how the local environment of a "single-atom" catalyst affects stability and reactivity remains a challenge. We present an in-depth study of copper₁, silver₁, gold₁, nickel₁, palladium₁, platinum₁, rhodium₁, and iridium₁ species on Fe₃O₄(001), a model support in which all metals occupy the same twofold-coordinated adsorption site upon deposition at room temperature. Surface science techniques revealed that CO adsorption strength at single metal sites differs from the respective metal surfaces and supported clusters. Charge transfer into the support modifies the d-states of the metal atom and the strength of the metal-CO bond. These effects could strengthen the bond (as for Ag₁-CO) or weaken it (as for Ni₁-CO), but CO-induced structural distortions reduce adsorption energies from those expected on the basis of electronic structure alone. The extent of the relaxations depends on the local geometry and could be predicted by analogy to coordination chemistry.

Potential-Dependent Morphology of Copper Catalysts During CO2 Electroreduction Revealed by In Situ Atomic Force Microscopy

Electrochemical AFM is a powerful tool for the real-space characterization of catalysts under realistic electrochemical CO2 reduction (CO2 RR) conditions. The evolution of structural features ranging from the micrometer to the atomic scale could be resolved during CO2 RR. Using Cu(100) as model surface, distinct nanoscale surface morphologies and their potential-dependent transformations from granular to smoothly curved mound-pit surfaces or structures with rectangular terraces are revealed during CO2 RR in 0.1 m KHCO3 . The density of undercoordinated copper sites during CO2 RR is shown to increase with decreasing potential. In situ atomic-scale imaging reveals specific adsorption occurring at distinct cathodic potentials impacting the observed catalyst structure. These results show the complex interrelation of the morphology, structure, defect density, applied potential, and electrolyte in copper CO2 RR catalysts.

Influence of surface defect density on the ultrafast hot carrier relaxation and transport in [Formula: see text] photoelectrodes

Cuprous oxide ([Formula: see text]) is a promising material for photoelectrochemical energy conversion due to its small direct band gap, high absorbance, and its Earth-abundant constituents. High conversion efficiencies require transport of photoexcited charges to the interface without energy loss. We studied the electron dynamics in [Formula: see text](111) by time-resolved two-photon photoemission for different surface defect densities in order to elucidate the influence on charge carrier transport. On the pristine bulk terminated surface, the principal conduction bands could be resolved, and ultrafast, elastic transport of electrons to the surface was observed. On a reconstructed surface the carrier transport is strongly suppressed and defect states dominate the spectra. Evidence for surface oxygen vacancies acting as efficient carrier traps is provided, what is important for further engineering of [Formula: see text] based photoelectrodes.

Facet-Specific Photocatalytic Activity Enhancement of Cu2O Polyhedra Functionalized with 4-Ethynylanaline Resulting from Band Structure Tuning

Cu₂O rhombic dodecahedra, octahedra, and cubes were densely modified with conjugated 4-ethynylaniline (4-EA) for facet-dependent photocatalytic activity examination. Infrared spectroscopy affirms bonding of the acetylenic group of 4-EA onto the surface copper atoms. The photocatalytically inactive Cu₂O cubes showed surprisingly high activity toward methyl orange photodegradation after 4-EA modification, while the already active Cu₂O rhombic dodecahedra and octahedra exhibited a photocatalytic activity enhancement. Electron, hole, and radical scavenger experiments prove that the photocatalytic charge transport processes have occurred in the functionalized Cu₂O cubes. Electrochemical impedance spectroscopy also indicates reduced charge transfer resistance of the functionalized Cu₂O crystals. A band diagram constructed from UV-vis spectral and Mott-Schottky measurements reveals significant band energy shifts in all Cu₂O samples after decorating with 4-EA. From density functional theory (DFT) calculations, a new band has emerged slightly above the valence band maximum within the band gap of Cu₂O, which has been found to originate from 4-EA through band-decomposed charge density analysis. The increased charge density localized on the 4-EA molecule and the smallest electron transition energy to reach the 4-EA-generated band are factors making {100}-bound Cu₂O cubes photocatalytically active. Proper molecular decoration represents a powerful approach to improving the photocatalytic efficiency of semiconductors.

A study on the acidity of sulfated CuO layers grown by surface reconstruction of Cu2O with specific exposed facets

Surface reconstruction and sulfation improve the acidity of Cu₂O, and moderate Lewis acid sites are the active sites in Pechmann condensation.