Calculations of oxide formation on low-index Cu surfaces

Ambient Pressure X-ray Photoelectron Spectroscopy Study of Oxidation Phase Transitions on Cu(111) and Cu(110)

The surface structure effect on the oxidation of Cu has been investigated by performing ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Cu(111) and Cu(110) surfaces under oxygen pressures ranging from 10-8 to 1 mbar and temperatures from 300 to 750 K. The APXPS results show a subsequential phase transition from chemisorbed O/Cu overlayer to Cu2 O and then to CuO on both surfaces. For a given temperature, the oxygen pressure needed to induce initial formation of Cu2 O on Cu(110) is about two orders of magnitude greater than that on Cu(111), which is in contrast with the facile formation of O/Cu overlayer on clean Cu(110). The depth profile measurements during the initial stage of Cu2 O formation indicate the distinct growth modes of Cu2 O on the two surface orientations. We attribute these prominent effects of surface structure to the disparities in the kinetic processes, such as the dissociation and surface/bulk diffusion over O/Cu overlayers. Our findings provide new insights into the kinetics-controlled process of Cu oxidation by oxygen.

The Role of Grain Boundary Sites for the Oxidation of Copper Catalysts during the CO Oxidation Reaction

The oxidation of transition metal surfaces is a process that takes place readily at ambient conditions and that, depending on the specific catalytic reaction at hand, can either boost or hamper activity and selectivity. Cu catalysts are no exception in this respect since they exhibit different oxidation states for which contradicting activities have been reported, as, for example, in the catalytic oxidation of CO. Here, we investigate the impact of low-coordination sites on nanofabricated Cu nanoparticles with engineered grain boundaries on the oxidation of the Cu surface under CO oxidation reaction conditions. Combining multiplexed in situ single particle plasmonic nanoimaging, ex situ transmission electron microscopy imaging, and density functional theory calculations reveals a distinct dependence of particle oxidation rate on grain boundary density. Additionally, we found that the oxide predominantly nucleates at grain boundary-surface intersections, which leads to nonuniform oxide growth that suppresses Kirkendall-void formation. The oxide nucleation rate on Cu metal catalysts was revealed to be an interplay of surface coordination and CO oxidation behavior, with low coordination favoring Cu oxidation and high coordination favoring CO oxidation. These findings explain the observed single particle-specific onset of Cu oxidation as being the consequence of the individual particle grain structure and provide an explanation for widely distributed activity states of particles in catalyst bed ensembles.

Competing oxidation mechanisms in Cu nanoparticles and their plasmonic signatures

Chemical reactions involving nanoparticles often follow complex processes. In this respect, real-time probing of single nanoparticles under reactive conditions is crucial for uncovering the mechanisms driving the reaction pathway. Here, we have captured in situ the oxidation of single Cu nanoparticles to unravel a sequential competitive activation of different mechanisms at temperatures 50-200 °C. Using environmental scanning transmission electron microscopy, we monitor the evolution of oxide formation with sub-nanometre spatial resolution, and show how the prevalence of oxide island nucleation, Cabrera-Mott, Valensi-Carter and Kirkendall mechanisms under different conditions determines the morphology of the particles. Moreover, using in situ electron energy-loss spectroscopy, we probe the localised surface plasmons of individual particles during oxidation, and with the aid of finite-difference time-domain electrodynamic simulations investigate the signature of each mechanism in their plasmonic response. Our results shed light on the rich and intricate processes involved in the oxidation of nanoparticles, and provide in-depth insight into how these processes govern their morphology and optical response, beneficial for applications in catalysis, sensing, nanomedicine and plasmonics.

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.

Selectivity Control of Cu Nanocrystals in a Gas-Fed Flow Cell through CO2 Pulsed Electroreduction

In this study, we have taken advantage of a pulsed CO₂ electroreduction reaction (CO₂RR) approach to tune the product distribution at industrially relevant current densities in a gas-fed flow cell. We compared the CO₂RR selectivity of Cu catalysts subjected to either potentiostatic conditions (fixed applied potential of −0.7 V_RHE) or pulsed electrolysis conditions (1 s pulses at oxidative potentials ranging from E_an = 0.6 to 1.5 V_RHE, followed by 1 s pulses at −0.7 V_RHE) and identified the main parameters responsible for the enhanced product selectivity observed in the latter case. Herein, two distinct regimes were observed: (i) for E_an = 0.9 V_RHE we obtained 10% enhanced C₂ product selectivity (FE_C2H4 = 43.6% and FE_C2H5OH = 19.8%) in comparison to the potentiostatic CO₂RR at −0.7 V_RHE (FE_C2H4 = 40.9% and FE_C2H5OH = 11%), (ii) while for E_an = 1.2 V_RHE, high CH₄ selectivity (FE_CH4 = 48.3% vs 0.1% at constant −0.7 V_RHE) was observed. Operando spectroscopy (XAS, SERS) and ex situ microscopy (SEM and TEM) measurements revealed that these differences in catalyst selectivity can be ascribed to structural modifications and local pH effects. The morphological reconstruction of the catalyst observed after pulsed electrolysis with E_an = 0.9 V_RHE, including the presence of highly defective interfaces and grain boundaries, was found to play a key role in the enhancement of the C₂ product formation. In turn, pulsed electrolysis with E_an = 1.2 V_RHE caused the consumption of OH^– species near the catalyst surface, leading to an OH-poor environment favorable for CH₄ production.

Effects of the curvature gradient on the distribution and diffusion of colloids confined to surfaces

The properties and behavior of colloids confined to move on curved surfaces offer a fertile ground for analysis since the geometric constraints induce specific features that are not available in flat spaces. Given their pertinence for biological and physicochemical processes, both with potential useful applications, the development of the concepts and methodology necessary for a deeper understanding of these unconventional systems is indeed an essential pursuit. The present study discusses a general and rigorous algorithm for the implementation of Brownian dynamics simulations that solves underlying difficulties and shortcomings inherent to conventional first-order schemes. Still based on the Ermak-McCammon recipe, our approach complements it with the higher-order geodesical projections of the elementary jumps generated on the associated tangent plane. This strategy, which warrants the locally isotropic propagation of non-interacting particles, is tested with a model system of colloidal particles interacting through a screened Coulomb potential while confined to move on ellipsoidal surfaces. This allows us to measure the effects prompted by the curvature gradient on the static and dynamic properties of this system. The varying curvature thus induces energetically favorable configurations in which the particles maximize their Euclidean distancing by crowding the regions with the largest Gaussian curvature, while withdrawing from those with the lowest. In turn, these inhomogeneous distributions provoke the anisotropic self-diffusion of the confined colloids, which is examined by exploiting the pertinent geodesic radial coordinates. The proficient methods under consideration thus allows dealing with the rich and remarkable new phenomena generated by any distinctive surface geometry.

Highly reversible oxygen redox in layered compounds enabled by surface polyanions

Oxygen-anion redox in lithium-rich layered oxides can boost the capacity of lithium-ion battery cathodes. However, the over-oxidation of oxygen at highly charged states aggravates irreversible structure changes and deteriorates cycle performance. Here, we investigate the mechanism of surface degradation caused by oxygen oxidation and the kinetics of surface reconstruction. Considering Li₂MnO₃, we show through density functional theory calculations that a high energy orbital (lO_2p') at under-coordinated surface oxygen prefers over-oxidation over bulk oxygen, and that surface oxygen release is then kinetically favored during charging. We use a simple strategy of turning under-coordinated surface oxygen into polyanionic (SO₄)^2-, and show that these groups stabilize the surface of Li₂MnO₃ by depressing gas release and side reactions with the electrolyte. Experimental validation on Li_1.2Ni_0.2Mn_0.6O₂ shows that sulfur deposition enhances stability of the cathode with 99.0% capacity remaining (194 mA h g^-1) after 100 cycles at 1 C. Our work reveals a promising surface treatment to address the instability of highly charged layered cathode materials.

Fast Oxidation of Porous Cu Induced by Nano-Twinning

The fcc lattice of porous Cu prepared by dealloying Al₂Cu with HCl aqueous solution exhibits a high density of twinning defects with an average domain size of about 3 nm along the ⟨111⟩ directions. The high density of twinning was verified by X-ray diffraction and qualitatively interpreted by a structural model showing the 5% probability of twinning defect formation. Most of the twinning defects disappeared after annealing at 873 K for 24 h. Twinned Cu reveals much faster oxidation rate in comparison to that without (or with much fewer) twinning defects, as shown by X-ray diffraction and hydrogen differential scanning calorimetry. Using ab initio DFT calculations, we demonstrate that twinning defects in porous Cu are able to form nucleation centers for the growth of Cu₂O. The geometry of the V-shaped edges on the twinned {211} surfaces is favorable for formation of the basic structural elements of Cu₂O. The fast oxidation of porous Cu prepared by dealloying can thus be explained by the fast formation of the Cu₂O nucleation centers and their high density.

Adsorption of Carbon on Partially Oxidized Low-Index Cu Surfaces

We use first-principles calculations to study the carbon adsorption on copper slabs of (100) and (111) surfaces predosed by oxygen atoms. Our results show that on both surfaces, an incoming carbon atom has the ability to replace and completely desorb a previously surface-adsorbed oxygen atom producing CO and CO₂ molecules in the gas phase. By comparison, the (111) surface is better suited for oxygen desorption, and an incoming carbon atom can more easily bond to and desorb oxygen atoms even at low oxygen coverages. We examine this mechanism at two different temperatures for both surfaces at 0.5 ML oxygen coverage. An implication of this process is the experimentally proven cleaning effect of predosing copper surfaces with oxygen before graphene growth in the chemical vapor deposition process. Conversely, adsorption and diffusion of carbon atoms, both of which are necessary for the nucleation and growth of carbon nanotubes, may be hindered by the presence of the oxidized or partially oxidized surfaces.

Core electron binding energies of adsorbates on Cu(111) from first-principles calculations

C1s and O1s core level binding energy shifts have been calculated for various adsorbates on Cu(111) using the ΔSCF method.

Cu metal embedded in oxidized matrix catalyst to promote CO2 activation and CO dimerization for electrochemical reduction of CO2

We propose and validate with quantum mechanics methods a unique catalyst for electrochemical reduction of CO₂ (CO₂RR) in which selectivity and activity of CO and C₂ products are both enhanced at the borders of oxidized and metallic surface regions. This Cu metal embedded in oxidized matrix (MEOM) catalyst is consistent with observations that Cu₂O-based electrodes improve performance. However, we show that a fully oxidized matrix (FOM) model would not explain the experimentally observed performance boost, and we show that the FOM is not stable under CO₂ reduction conditions. This electrostatic tension between the Cu⁺ and Cu⁰ surface sites responsible for the MEOM mechanism suggests a unique strategy for designing more efficient and selective electrocatalysts for CO₂RR to valuable chemicals (HCO_x), a critical need for practical environmental and energy applications.