The bonding of CO to metal surfaces

Au@AuPd Core-Alloyed Shell Nanoparticles for Enhanced Electrocatalytic Activity and Selectivity under Visible Light Excitation

Plasmonic catalysis has been employed to enhance molecular transformations under visible light excitation, leveraging the localized surface plasmon resonance (LSPR) in plasmonic nanoparticles. While plasmonic catalysis has been employed for accelerating reaction rates, achieving control over the reaction selectivity has remained a challenge. In addition, the incorporation of catalytic components into traditional plasmonic-catalytic antenna-reactor nanoparticles often leads to a decrease in optical absorption. To address these issues, this study focuses on the synthesis of bimetallic core@shell Au@AuPd nanoparticles (NPs) with ultralow loadings of palladium (Pd) into gold (Au) NPs. The goal is to achieve NPs with an Au core and a dilute alloyed shell containing both Au and Pd, with a low Pd content of around 10 atom %. By employing the (photo)electrocatalytic nitrite reduction reaction (NO2RR) as a model transformation, experimental and theoretical analyses show that this design enables enhanced catalytic activity and selectivity under visible light illumination. We found that the optimized Pd distribution in the alloyed shell allowed for stronger interaction with key adsorbed species, leading to improved catalytic activity and selectivity, both under no illumination and under visible light excitation conditions. The findings provide valuable insights for the rational design of antenna-reactor plasmonic-catalytic NPs with controlled activities and selectivity under visible light irradiation, addressing critical challenges to enable sustainable molecular transformations.

Visualizing Surface-Subsurface Cu Atom Exchange at the FeO/Pt(111) Surface Induced by CO Adsorption at 150 K

Structural evolution of solid catalyst surfaces induced by direct exposure to reaction gas has been extensively studied and is well understood. However, whether and how subsurface atomic structures are affected by the reaction atmosphere require further exploration. In this work, our results confirm that Cu clusters supported on FeO/Pt(111) (Cun/FeO/Pt) transform into surface CuCO complexes (CuCO/FeO/Pt) with exposure to CO at 78 K. Surprisingly, Cu clusters on Pt(111) buried under monolayer FeO film (FeO/Cun/Pt) can also transform into surface CuCO complexes on FeO/Pt(111) upon CO adsorption at 150 K. The place exchange of surface and subsurface Cu atoms at the FeO/Pt(111) surface can be mediated by exposing to CO at 150 K and keeping in ultrahigh vacuum at 300 K, alternatively. Calculation results reveal that CO adsorption induces restructuring of the FeO film above the Cu clusters, generating a diffusion channel for Cu atoms to pass through the FeO film and form surface CuCO, while Cu atoms remaining at the FeO-Pt interface are more thermodynamically favored without CO. Our work suggests that buried subsurface atoms may be involved in strong restructuring processes driven by reaction gas, which could strongly influence the catalytic performance.

Theoretical Investigation of the Potential-Dependent CO Adsorption on Copper Electrodes

Despite the importance of CO adsorption in many electrocatalytic reaction mechanisms, there has been little investigation of the dependence of the free energy of CO adsorption on the applied potential. Herein, we report on the potential-dependent adsorption of CO on Cu electrodes using a grand-canonical density functional theory approach. We demonstrate that, within the working potential range of electrocatalytic CO2 reduction on Cu(111) and Cu(100), the CO adsorption strength can change by over 0.1 eV. Our analyses explain the potential dependence through an interfacial capacitance loss upon CO adsorption as well as orbital relaxation induced by the electrode potential. Via sensitivity analysis with respect to two electrolyte model parameters (solvent dielectric constant and Debye screening length), we find that the surface excess charge density is a useful descriptor of the CO adsorption free energy.

Unravelling CO Activation on Flat and Stepped Co Surfaces: A Molecular Orbital Analysis

Structure sensitivity in heterogeneous catalysis dictates the overall activity and selectivity of a catalyst whose origins lie in the atomic configurations of the active sites. We explored the influence of the active site geometry on the dissociation activity of CO by investigating the electronic structure of CO adsorbed on 12 different Co sites and correlating its electronic structure features to the corresponding C-O dissociation barrier. By including the electronic structure analyses of CO adsorbed on step-edge sites, we expand upon the current models that primarily pertain to flat sites. The most important descriptors for activation of the C-O bond are the decrease in electron density in CO's 1π orbital , the occupation of 2π anti-bonding orbitals and the redistribution of electrons in the 3σ orbital. The enhanced weakening of the C-O bond that occurs when CO adsorbs on sites with a step-edge motif as compared to flat sites is caused by a distancing of the 1π orbital with respect to Co. This distancing reduces the electron-electron repulsion with the Co d-band. These results deepen our understanding of the electronic phenomena that enable the breaking of a molecular bond on a metal surface.

Charge-orbital synergistic engineering of TM@Ti3C2O1-xBx for highly selective CO2 electrochemical reduction

Inspired by MXene nanosheets and their regulation of surface functional groups, a series of Ti3C2-MXene-based single TM atom electrocatalysts with a doped boron (B) atom (TM@Ti3C2O2-xBx, TM is V, Cr, Mn, Fe, Co or Ni, x = 0.11) are proposed for achieving a high performance catalytic CO2 reduction reaction (CO2RR). The results reveal that the doped B atom involves in the adsorption reaction of CO2 molecules and CO intermediates in the CO2RR. The TM-to-C and B-to-C π-back bonding contribute to the activation of the CO2 molecules and CO intermediates in the CO2RR. Enough electrons from the single TM atom and B atom occupied orbitals can be injected into the CO2 molecules and *CO intermediates through direct bonding interactions, which effectively alleviates the difficulty of the first hydrogenation reaction step and further helps CO reduction towards CH4. The calculated values of ΔG for the first hydrogenation reaction and the formation of *CHO on Ti3C2O2-xBx are significantly smaller than those of other single-atom catalysts (SACs). Fe@Ti3C2O2-xBx is found to have the highest electrocatalytic activity with a limiting potential of ∼0.40 V and exhibits a high selectivity for obtaining CH4 through the CO2RR compared with the hydrogen evolution reaction. This work is expected to open a research path for engineering the charge-orbital state of the innate atoms of a substrate based on mechanistic insights, which guides the rational design of highly selective MXene-based CO2RR electrocatalysts.

Systematic DFT studies of CO-Tolerance and CO oxidation on Cu-doped Ni surfaces

Nickel-based surfaces have received significant attention as an efficient substrate for electrooxidation. This work studied doped nickel surfaces with Cu atoms to enhance the CO-Tolerance. A comparative study was performed for CO adsorption upon different cleavage facets of pristine and Cu-doped nickel surfaces, whereas the adsorption energy, charge transfer, and density of state for CO were estimated using GGA-RPBE calculation method. Several adsorption probabilities were considered, and the change in adsorption energy and bond lengths were used to explain the CO adsorption mechanism. Otherwise, the density of state was employed to study the 3σ and 1π orbital to demonstrate the adsorption of CO onto the different facets. According to our analysis, the Cu-doped nickel surface showed higher CO tolerance than the pristine nickel surface. Whereas the calculated CO adsorption energies of Cu-doped surfaces have more positive values than the non-doped counterparts. The catalytic ability of pristine and Cu-doped Ni(111) was studied to evaluate the ability of surface poisoning resistance. Thus, oxidation of CO to CO₂ was studied using the Eley-Rideal mechanism upon the pristine and Cu-doped surfaces of Ni(100) where the rate-determining step for CO oxidation upon the reported surfaces was estimated as CO + O₂* → CO₂* + O* by an energy barrier of 1.05 and 0.9 eV for pristine, and Cu-doped Ni (100).

Surface chemistry on a polarizable surface: Coupling of CO with KTaO3(001)

Polarizable materials attract attention in catalysis because they have a free parameter for tuning chemical reactivity. Their surfaces entangle the dielectric polarization with surface polarity, excess charge, and orbital hybridization. How this affects individual adsorbed molecules is shown for the incipient ferroelectric perovskite KTaO₃. This intrinsically polar material cleaves along (001) into KO- and TaO₂-terminated surface domains. At TaO₂ terraces, the polarity-compensating excess electrons form a two-dimensional electron gas and can also localize by coupling to ferroelectric distortions. TaO₂ terraces host two distinct types of CO molecules, adsorbed at equivalent lattice sites but charged differently as seen in atomic force microscopy/scanning tunneling microscopy. Temperature-programmed desorption shows substantially stronger binding of the charged CO; in density functional theory calculations, the excess charge favors a bipolaronic configuration coupled to the CO. These results pinpoint how adsorption states couple to ferroelectric polarization.

Data-Driven Investigation of Tellurium-Containing Semiconductors for CO2 Reduction: Trends in Adsorption and Scaling Relations

Light-assisted conversion of CO₂ into liquid fuels is one of several possible approaches to combating the rise of carbon dioxide emissions. Unfortunately, there are currently no known materials that are efficient, selective, or active enough to facilitate the photocatalytic CO₂ reduction reaction (CO₂RR) at an industrial scale. In this work, we employ density functional theory to explore potential tellurium-containing photocathodes for the CO₂RR by observing trends in adsorption properties arising from over 350 *H, 200 *CO, and 110 *CHO surface-adsorbate structures spanning 39 surfaces of 11 materials. Our results reveal a scaling relationship between *CHO and *H chemisorption energies and charge transfer values, while the scaling relation (typically found in transition metals) between *CO and *CHO adsorption energies is absent. We hypothesize the scaling relation between *H and *CHO to be related to the lone electron located on the bonding carbon atom, while the lack of scaling relation in *CO we attribute to the ability of the lone pair on the C atom to form multiple bonding modes. We compute two predominant orbital-level interactions in the *CO-surface bonds (either using s or p orbitals) in addition to bonding modes involving both σ and π interactions using the Crystal Orbital Hamiltonian Population analysis. We demonstrate that bonds involving the C s orbital are more chemisorptive than the C p orbitals of CO. In general, chemisorption trends demonstrate decreased *H competition with respect to *CO adsorption and enhanced *CHO stability. Finally, we uncover simple element-specific design rules with Te, Se, and Ga sites showing increased competition and Zn, Yb, Rb, Br, and Cl sites showing decreased competition for hydrogen adsorption. We anticipate that these trends will help further screen these materials for potential CO₂RR performance.

Compressive Stress and Charge Redistribution during CO Adsorption onto Pt

The change in surface stress associated with the adsorption and oxidative stripping of carbon monoxide (CO) on (111)-textured Pt is examined using the wafer curvature method in 0.1 mol/L KHCO3 electrolyte. The curvature of the Pt cantilever electrode was monitored as a function of potential in both CO-free and CO-saturated electrolytes. Although CO adsorbs as a neutral molecule, significant compressive stress, up to -1.3 N/m, is induced in the Pt. The magnitude of the stress change correlates directly with the CO coverage and, within the detection limits of the stress measurement, is elastically reversible. Density functional theory calculations of a CO-bound Pt surface indicate that charge redistribution from the first atomic layer of Pt to subsurface layers accounts for the observed compressive stress induced by the charge neutral adsorption of CO. A better understanding of adsorbate-induced surface stress is critical for the development of material platforms for sensing and catalysis.

Theoretical Roadmap for Best Oxygen Reduction Activity in Two-dimensional Transition Metal Tellurides

Developing highly active and cost-effective electrocatalysts to replace Pt-based catalysts for the sluggish oxygen reduction reaction (ORR) is a major challenge in the commercialization of fuel cells. Although two-dimensional (2D) transition-metal tellurides have recently been proposed as alternative low-cost ORR catalysts, a fundamental study on the origin of the activity is required to further optimize their composition and performance. Herein, we investigated the electronic properties and ORR catalytic performances of a series of exfoliable 2D transition-metal tellurides to uncover the underlying mechanisms by means of density functional theory simulations. Our in-depth analysis shows that the activation of the ORR mainly depends on the partially filled p_z state of active Te atoms, which can simultaneously accept and donate electrons behaving similarly to both the occupied and unoccupied d orbitals of Pt atoms. This results in a linear relationship between the p_z-band center and the adsorption free energies of O₂ and intermediates, indicating that the p_z-band center might be used as an effective descriptor to probe the performance of telluride catalysts. On this basis, we predicted several 2D transition-metal tellurides with promising catalytic performance and reduced precious-metal contents, where NbRhTe₄ reaches the top of the activity volcano with a limiting potential of 0.96 V. This study provides theoretical guidance to design high-performing 2D telluride ORR catalysts, and its principle might be applicable to other electrochemical reactions in 2D chalcogenides.

Using proposed theoretical guidelines, we designed several high-performing 2D telluride ORR catalysts, where NbRhTe₄ reaches the top of the activity volcano with a limiting potential of 0.96 V.

Ab initio investigation of the role of the d-states occupation on the adsorption properties of H2, CO, CH4 and CH3OH on the Fe13, Co13, Ni13 and Cu13 clusters

Here, we report a theoretical investigation, based on density functional theory calculations, into the role of the occupation d-states on the adsorption properties of CH₄, CO, H₂ and CH₃OH on 3d 13-atom transition-metal (TM₁₃) clusters (TM = Fe, Co, Ni, Cu). Except for Cu₁₃, a gradual increase in the occupation of the d-states, i.e., from Fe₁₃ to Ni₁₃, increases the magnitude of the adsorption energy almost linearly for the H₂/TM₁₃ and CO/TM₁₃ systems, which can be explained by the enhancement of the sp-d hybridization due to the shift of the d-states towards the highest occupied molecular orbital (HOMO). For Cu₁₃, the d-states are located well below the HOMO, which reduces the sp-d hybridization, and hence, a smaller adsorption energy is obtained. However, this picture does not hold for CH₄/TM₁₃ and CH₃OH/TM₁₃, where the adsorption energy has nearly the same value for all TM₁₃ clusters, which can be explained by electrostatic effects such as local polarization of the molecules and nearby TM atoms, and hence, the basic features of physisorption systems. Based on the electron density difference, the polarization effects are slightly larger for systems with empty d-states.

DFT insights into structural effects of Ni-Cu/CeO2 catalysts for CO selective reaction towards water-gas shift

The water-gas shift (WGS) reaction is a key step in hydrogen production, particularly to meet the high-purity H2 requirement of PEM fuel cells. The catalysts currently employed in large-scale WGS plants require a two-step process to overcome thermodynamic and kinetic limitations. Ni-Cu/CeO2 solids are promising catalysts for the one-step process required for small-scale applications, as the addition of Cu hinders undesired methanation reactions occurring on Ni/CeO2. In this work, we performed calculations on Ni4-xCux/CeO2(111) systems to evaluate the influence of cluster conformation on the selectivity towards water-gas shift. The structure and miscibility of CeO2-supported Ni4-xCux clusters were investigated and compared with those of gas-phase clusters to understand the effect of metal-support interactions. The adsorption of CO onto apical Ni and Cu atoms of Ni4-xCux/CeO2(111) systems was studied, and changes in the C-O bond strength were confirmed at the electronic level by investigating shifts in the 3σ and 1π orbitals. The selectivity towards WGS was evaluated using Brønsted-Evans-Polanyi relations for the C-O activation energy. Overall, a strengthening of the C-O bond and an increase in CO dissociation energy were verified on Cu-containing clusters, explaining the improvement in selectivity of Ni4-xCux/CeO2(111) systems.

Challenges for density functional theory: calculation of CO adsorption on electrocatalytically relevant metals

We assess four DFT functionals, RTPSS, RPBE, SCAN and B97M-rV, for surface interactions. We find that B97M-rV predicts the correct site preference for CO binding on Ag and Au while RTPSS performs well for surface relaxations and binding of CO on Pt and Cu.

A quantum chemical study of substituent effects on CN bonds in aryl isocyanide molecules adsorbed on the Pt surface

A periodicity implemented scheme of natural bond orbital (NBO) theory and normal mode analysis has been employed to investigate the tendency of the chemical bond strength of aryl isocyanide molecules with different para-substituted groups adsorbed on the Pt(111) surface. The NC bond order shows a clear correspondence with the NC stretching frequency; both of them exhibit a "volcano-like" profile as a function of the Hammett constant of the para-substituted groups for isolated molecules. When a molecule is adsorbed on the Pt(111) surface, the NC stretching frequency variations are determined by the resultant effect of σ donation and π back-donation between the molecule and the surface. The present comprehensive and systematic computations clarify the electron donating and withdrawing effects of the substituted groups on the interaction between the aryl isocyanide molecule and the transition metal substrate.

Electron density topological and adsorbate orbital analyses of water and carbon monoxide co-adsorption on platinum

The electron density topology of carbon monoxide (CO) on dry and hydrated platinum is evaluated under the quantum theory of atoms in molecules (QTAIM) and by adsorbate orbital approaches. The impact of water co-adsorbate on the electronic, structural, and vibrational properties of CO on Pt are modelled by periodic density functional theory (DFT). At low CO coverage, increased hydration weakens C-O bonds and strengthens C-Pt bonds, as verified by changes in bond lengths and stretching frequencies. These results are consistent with QTAIM, the 5σ donation-2π^* backdonation model, and our extended π-attraction σ-repulsion model (extended π-σ model). This work links changes in the non-zero eigenvalues of the electron density Hessian at QTAIM bond critical points to changes in the π and σ C-O bonds with systematic variation of CO/H₂O co-adsorbate scenarios. QTAIM invariably shows bond strengths and lengths as being negatively correlated. For atop CO on hydrated Pt, QTAIM and phenomenological models are consistent with a direct correlation between C-O bond strength and CO coverage. However, DFT modelling in the absence of hydration shows that C-O bond lengths are not negatively correlated to their stretching frequencies, in contrast to the Badger rule: When QTAIM and phenomenological models do not agree, the use of the non-zero eigenvalues of the electron density Hessian as inputs to the phenomenological models, aligns them with QTAIM. The C-O and C-Pt bond strengths of bridge and three-fold bound CO on dry and hydrated platinum are also evaluated by QTAIM and adsorbate orbital analyses.

First-principles descriptors of CO chemisorption on Ni and Cu surfaces

A comprehensive analysis of low coverage CO adsorption on Ni and Cu low-index miller surfaces – (100), (110), and (111) – over all the possible adsorption sites is presented.

Unveiling CO adsorption on Cu surfaces: new insights from molecular orbital principles

A holistic analysis of adsorption energies, charge transfer, and structural changes has been employed to highlight the variations in adsorption mechanisms upon changing the surface type and the adsorption site.

σ-Holes and σ-lumps direct the Lewis basic and acidic interactions of noble metal nanoparticles: introducing regium bonds

Using local DFT-based probes for electrostatic as well as charge transfer/polarization interactions, we are able to characterize Lewis basic and acidic sites on copper, silver and gold nanoparticles.

Subsurface Oxygen in Oxide-Derived Copper Electrocatalysts for Carbon Dioxide Reduction

Copper electrocatalysts derived from an oxide have shown extraordinary electrochemical properties for the carbon dioxide reduction reaction (CO₂RR). Using in situ ambient pressure X-ray photoelectron spectroscopy and quasi in situ electron energy-loss spectroscopy in a transmission electron microscope, we show that there is a substantial amount of residual oxygen in nanostructured, oxide-derived copper electrocatalysts but no residual copper oxide. On the basis of these findings in combination with density functional theory simulations, we propose that residual subsurface oxygen changes the electronic structure of the catalyst and creates sites with higher carbon monoxide binding energy. If such sites are stable under the strongly reducing conditions found in CO₂RR, these findings would explain the high efficiencies of oxide-derived copper in reducing carbon dioxide to multicarbon compounds such as ethylene.

Single-Molecule Dynamics in the Presence of Strong Intermolecular Interactions

In contrast to conventional spectroscopic studies of adsorbates at high coverage that provide only spatially averaged information, we have characterized the laterally confined shuttling dynamics of a single molecule under the influence of intermolecular interactions by vibrational spectroscopy using a scanning tunneling microscope. The bridge sites on Pt(111) are only occupied by a CO molecule that is surrounded by four other CO molecules at on-top sites. The bridge-site CO undergoes laterally confined shuttling toward an adjacent on-top site to transiently occupy a metastable site, which is slightly displaced from the center of an on-top site through repulsive interaction with adjacent on-top CO molecules. Analysis of action spectra for the shuttling events reveals the C-O stretch frequency of the metastable CO. We also constructed a modified potential energy surface incorporating the intermolecular interaction, which reveals the underlying mechanism and provides a new way to experimentally determine detailed information on the energetics of the metastable state.

How molecular is the chemisorptive bond?

Scaling rules differ for early and late transition metals. Their electronic structure and topological bond analysis are shown.

Size-dependent properties of transition metal clusters: from molecules to crystals and surfaces--computational studies with the program ParaGauss

In the so-called scalable regime the size-dependent behavior of the physical and chemical properties of transition metal clusters is described by scaling relationships. For most quantities this scalable regime is reached for cluster sizes between a few tens and a few hundreds of atoms, hence for systems for which an accurate treatment by density functional theory is still feasible. Thus, by invoking scaling relations one is able to obtain properties of very large nanoparticles and even the bulk limit from the results of a series of smaller cluster models. In this invited review we illustrate this strategy by exploiting results from computational studies that mostly were carried out with the density functional theory software ParaGauss. We address mainly the size-dependent behavior of the properties of transition metal clusters. To this end, we first present benchmark studies probing various approximations that are used in such density functional calculations. Subsequently we show how physical insight may be gained by exploring less understood types of systems. These applications range from bare clusters to nanoislands and nanoalloys to adsorption complexes.

Strong Influence of Coadsorbate Interaction on CO Desorption Dynamics on Ru(0001) Probed by Ultrafast X-Ray Spectroscopy and Ab Initio Simulations

We show that coadsorbed oxygen atoms have a dramatic influence on the CO desorption dynamics from Ru(0001). In contrast to the precursor-mediated desorption mechanism on Ru(0001), the presence of surface oxygen modifies the electronic structure of Ru atoms such that CO desorption occurs predominantly via the direct pathway. This phenomenon is directly observed in an ultrafast pump-probe experiment using a soft x-ray free-electron laser to monitor the dynamic evolution of the valence electronic structure of the surface species. This is supported with the potential of mean force along the CO desorption path obtained from density-functional theory calculations. Charge density distribution and frozen-orbital analysis suggest that the oxygen-induced reduction of the Pauli repulsion, and consequent increase of the dative interaction between the CO 5σ and the charged Ru atom, is the electronic origin of the distinct desorption dynamics. Ab initio molecular dynamics simulations of CO desorption from Ru(0001) and oxygen-coadsorbed Ru(0001) provide further insights into the surface bond-breaking process.

Hydrogen spillover on Rh/TiO2: the FTIR study of donated electrons, co-adsorbed CO and H/D exchange

Hydrogen spillover on Rh/TiO₂: molecular H₂ dissociates on nanocrystalline Rh; the produced H atoms spillover onto the titania thus protonating the semiconductor, while donating electrons to shallow trap (ST) states and the conduction band (CB) of TiO₂.

Spectral evidence for hydrogen-induced reversible segregation of CO adsorbed on titania-supported rhodium

Adsorption of hydrogen on titania-supported Rh nanoparticles partially covered by CO causes compression of the CO adlayer.

Selective ultrafast probing of transient hot chemisorbed and precursor states of CO on Ru(0001)

We have studied the femtosecond dynamics following optical laser excitation of CO adsorbed on a Ru surface by monitoring changes in the occupied and unoccupied electronic structure using ultrafast soft x-ray absorption and emission. We recently reported [M. Dell'Angela et al. Science 339, 1302 (2013)] a phonon-mediated transition into a weakly adsorbed precursor state occurring on a time scale of >2 ps prior to desorption. Here we focus on processes within the first picosecond after laser excitation and show that the metal-adsorbate coordination is initially increased due to hot-electron-driven vibrational excitations. This process is faster than, but occurs in parallel with, the transition into the precursor state. With resonant x-ray emission spectroscopy, we probe each of these states selectively and determine the respective transient populations depending on optical laser fluence. Ab initio molecular dynamics simulations of CO adsorbed on Ru(0001) were performed at 1500 and 3000 K providing insight into the desorption process.

CO-induced smoluchowski ripening of Pt cluster arrays on the graphene/Ir(111) moiré

Regular Pt cluster arrays grown on the moiré template formed by graphene on Ir(111) were tested for their stability with respect to CO gas exposure. Cluster stability and adsorption-induced processes were analyzed as a function of cluster size, with in situ scanning tunneling microscopy and X-ray photoelectron spectroscopy. Small clusters containing fewer than 10 atoms were unstable upon CO adsorption. They sintered through Smoluchowski ripening-cluster diffusion and coalescence-rather than the frequently reported Ostwald ripening mediated by metal-adsorbate complexes. Larger clusters remained immobile upon CO adsorption but became more three-dimensional. Careful analysis of the experimental data complemented by ab initio density functional theory calculations provides insight into the origin of the CO-induced Pt cluster ripening and shape transformations.

A setup for resonant inelastic soft x-ray scattering on liquids at free electron laser light sources

We present a flexible and compact experimental setup that combines an in vacuum liquid jet with an x-ray emission spectrometer to enable static and femtosecond time-resolved resonant inelastic soft x-ray scattering (RIXS) measurements from liquids at free electron laser (FEL) light sources. We demonstrate the feasibility of this type of experiments with the measurements performed at the Linac Coherent Light Source FEL facility. At the FEL we observed changes in the RIXS spectra at high peak fluences which currently sets a limit to maximum attainable count rate at FELs. The setup presented here opens up new possibilities to study the structure and dynamics in liquids.

Orthogonal Intermolecular Interactions of CO Molecules on a One-Dimensional Substrate

By low-temperature scanning tunneling microscopy, we study CO molecule chemisorption on a quasi–one-dimensional Cu(110)-(2×1)-O surface. Atom-resolved images reveal how the interaction of CO with the surface Cu-O- chains gives rise to orthogonal attractive and repulsive intermolecular interactions. First-principles calculations show that CO molecules induce unprecedented lifting of the host Cu atoms by 1 Å from the Cu-O- chains, enabling the Cu-CO unit to tilt by 45° from the surface normal. Contrary to the behavior of CO on metal surfaces, this structural distortion enables unprecedented, orthogonal, short-range intermolecular dipole-dipole attraction and long-range, surface-mediated repulsion. These interactions lead to self-assembly into molecular nanograting structures consisting of arrays of single-molecule-wide CO rows. The origin of the novel behavior of CO molecules in the electronic and geometrical properties of the quasi–one-dimensional substrate suggests that similar molecule-molecule and molecule-substrate interactions could play an important role at catalytic sites on reactive surfaces.

Orthogonal interactions of CO molecules on a one-dimensional substrate

We investigate the chemisorption structure of CO molecules on the quasi-one-dimensional Cu(110)-(2 × 1)-O surface by low-temperature scanning tunneling microscopy and density functional theory. Contrary to flat metal surfaces, where CO molecules adsorb in an upright geometry and interact through repulsive intermolecular interactions, we find the most stable adsorption structure of single CO molecules to be at Cu atoms of substrate Cu-O- chains with the Cu-CO unit bent by ~±45° in two equivalent structures at low coverages. At higher coverages, CO molecules combine in the same structure into highly ordered single-molecule-wide rows perpendicular to the substrate chains in an approximately 8 × 1 full monolayer structure. First-principles calculations attribute the unprecedented chemisorption behavior of CO molecules to lifting of the host Cu atoms by 1 Å from the surface Cu-O- chains, in order to optimize the bonding and reduce the repulsive interactions with the substrate. This structural distortion enables short-range intermolecular dipole-dipole attraction and creates orthogonal long-range surface-mediated repulsion leading to unusual self-assembly of CO molecules into coherent nanometer scale molecular grating structures.