Au Adatoms Research Articles

Theoretical study of CO adsorption on Au catalysts under environmental catalytic conditions

Density Functional Theory calculations with both standard GGA and hybrid functionals are performed on Au adatoms, steps, and low index surfaces with coordination numbers (CNs) varying from 3 to 9. The results are used to study adsorption thermodynamics and reactivity of CO on Au nanoparticles. We find that the hybrid functional improves calculated site preferences and predicts CO top site adsorption, regardless of the Au CN, in good agreement with experiments. The calculated adsorption energies vary monotonically with respect to Au CNs, and the results from the hybrid functional are around 20% smaller than the corresponding values from the GGA–PBE functional. A comparison with experimental adsorption energies suggests that these functionals may bound the true CO–Au interaction strength, and seven-coordinated Au atoms may be the active low-coordinated sites on many Au single crystal surfaces. However, thermodynamic analysis on Wulff-like Au particles at ambient temperatures shows that, even though the number of 6-coordinated corner Au atoms is much less than the number of 7-coordinated edge Au atoms and of higher-coordinated Au atoms, they are the dominant sites for CO adsorption on Au nanoparticles with sizes up to 10nm. In addition, we find that CO adsorption is not influenced by the shape of Au nanoparticles, but the CO oxidation reaction may be.

Nanoscale patterning of a self-assembled monolayer by modification of the molecule-substrate bond.

The intercalation of Cu at the interface of a self-assembled monolayer (SAM) and a Au(111)/mica substrate by underpotential deposition (UPD) is studied as a means of high resolution patterning. A SAM of 2-(4'-methylbiphenyl-4-yl)ethanethiol (BP2) prepared in a structural phase that renders the Au substrate completely passive against Cu-UPD, is patterned by modification with the tip of a scanning tunneling microscope. The tip-induced defects act as nucleation sites for Cu-UPD. The lateral diffusion of the metal at the SAM–substrate interface and, thus, the pattern dimensions are controlled by the deposition time. Patterning down to the sub-20 nm range is demonstrated. The difference in strength between the S–Au and S–Cu bond is harnessed to develop the latent Cu-UPD image into a patterned binary SAM. Demonstrated by the exchange of BP2 by adamantanethiol (AdSH) this is accomplished by a sequence of reductive desorption of BP2 in Cu free areas followed by adsorption of AdSH. The appearance of Au adatom islands upon the thiol exchange suggests that the interfacial structures of BP2 and AdSH SAMs are different.

Synthesis of nanoporous gold by free corrosion of an amorphous precursor

De-alloying of an amorphous precursor is attractive to get a fine microstructure after copious nucleation of crystalline hillocks. However, the noble metal atoms freed on the alloy surface gain high mobility causing coarsening of ligaments. This work describes the de-alloying by free corrosion of a Au-based amorphous alloy in HNO3 at different temperatures and concentrations. Gold ligaments ranging from 47 to 155nm have been obtained as a function of acid content. Scanning Electron Microscopy, Transmission Electron Microscopy and X-ray Diffraction was used to characterize the nanoporous structure of dealloyed samples. Moreover, an evaluation of the surface diffusion coefficient and of the activation energy and entropy for diffusion of Au adatoms have been performed to study the mechanism of dealloying under free corrosion conditions in comparison with previous studies.

When reconstruction comes around: Ni, Cu, and Au adatoms on δ-MoC(001)

The interaction of Au, Cu, and Ni metal atoms with the δ-MoC(001) surface was studied using periodic density functional (DF) calculations to analyze adsorption energies and equilibrium geometries, work functions, atomic charges, projected density of states (PDOS), and shifts of the transition metal d-band center. The atomic adsorption is found to cause an in-plane distortion of the surface, and, besides, the interaction strength turns to be coverage dependent. A lower coverage allows for a better accommodation of the adsorbate, alongside causing a d-band center shift to more negative energies, as shown by plots of the PDOS. Regardless of the coverage, interaction strength diminishes following the order Ni>Cu>Au. Adatom chemical activity can be inferred based on the calculated d-band center; Ni being the most active metal, followed by Cu, and then Au for every coverage studied. This result well correlates with experiments on other transition metal carbides. Atomic adsorption also diminishes surface work function.

A study on adatom transport through (√3 × √3)-R30°-CH3S self-assembled monolayers on Au(111) using first principles calculations.

Self-assembled monolayers on Au(111) have outstanding chemical, electrical, and optical properties, and Au adatoms seem to play a key role in these properties. Still, the fundamental understanding of adatom transport inside the self-assembled structure is very thin. In this paper we use first-principles calculations to reveal new details about the migration mechanism of Au adatoms in the presence of a CH3S self-assembled structure on Au(111). We study the inclusion of Au adatoms inside a well-packed (√3 × √3)-R30°-CH3S self-assembled lattice and present atomistic models supporting adatom migration by means of a hopping mechanism between pairs of CH3S species. Our calculations reveal that the transport of Au adatoms is slowed down inside the molecular network where the kinetic barrier for adatom migration is larger than on the clean Au surface. We attribute the hindered mobility of Au adatoms to the fact that adatom transport involves the breaking and making of Au-S bonds. Our results form a basis for further understanding the role played by defect transport in the properties of molecular assemblies.

Manipulating the Conformation of Single Organometallic Chains on Au(111)

The conformations of organometallic polymers formed via the bottom-up assembly of monomer units on a metal surface are investigated, and the relationship between the adsorption geometry of the individual monomer units, the conformational structure of the chain, and the overall shape of the polymer is explored. Iodine-functionalized monomer units deposited on a Au(111) substrate are found to form linear chain structures in which each monomer is linked to its neighbors via a Au adatom. Lateral manipulation of the linear chains using a scanning tunneling microscope allows the structure of the chain to be converted from a linear to a curved geometry, and it is shown that a transformation of the overall shape of the chain is coupled to a conformational rearrangement of the chain structure as well as a change in the adsorption geometry of the monomer units within the chain. The observed conformational structure of the curved chain is well-ordered and distinct from that of the linear chains. The structures of both the linear and curved chains are investigated by a combination of scanning tunneling microscopy measurements and theoretical calculations.

Kinetic Monte Carlo modeling of reaction-induced phase separation in Au/Ni(111) surface alloy

Kinetic Monte Carlo (KMC) simulations of Au–Ni phase separation in Au/Ni(111) surface alloy during nickel carbonyl formation reaction were performed at room temperature by taking into account realistic rates of Au and Ni adatom diffusion and CO adsorption and desorption, while keeping the rate of nickel carbonyl formation reaction as a free parameter. We also obtained pair and three-body interaction constants between Au and Ni adatoms using the ab initio calculations and demonstrated that their proper choice is crucial for understanding the Au–Ni separation process. Three regimes with qualitatively different Ni–CO reaction propagation kinetics in Au/Ni(111) surface alloy were found by varying the constants of trio-triangle interactions within the limits of their accuracy. The sensitivity of the proposed model to interaction parameters leads to the regimes that differ by step flow rate, Au islands formation mechanism (channel-type or homogenous flow), Au island contamination level by Ni impurities and reaction damping tendency at the reaction front. Nucleation of nickel-free Au islands was observed for a homogeneous step flow pattern, and the corresponding step flow rate is in a good quantitative agreement with existing experimental data.

Application of the site blocking method to the investigation of the kinetics of carbon monoxide electrooxidation on nanostructured Pt

In this work, we apply a recently developed selective site blocking with gold adatoms (Kuznetsov et al. Electrocatal 3:211–220, 2012) for studying the electrooxidation of adsorbed carbon monoxide on electrodeposited Pt. The reaction is studied using chronoamperometry, and the obtained current transients are fitted with a nucleation-and-growth model in order to determine the characteristic kinetic parameters on defect and terrace (facet) sites. We conclude that Au selectively blocks defect sites up to very high coverages of 0.42 MLs, strongly slowing down the reaction kinetics. Blocking of defect sites with Au adatoms also results in an increase of the packing density of CO on terrace sites. The rate constant of CO oxidation on defects is higher than that on terraces and exhibits higher Tafel slope.

Adsorbed CO induced change of the adsorption site and charge of Au adatoms on FeO(111)/Ru(0001)

A first principles study of the influence of CO adsorption on the charge state, adsorption site, and stability of Au adatoms on FeO/Ru(0001) was presented. Calculations were first carried out to explore the detailed interface structure of a bilayer FeO(111) film on Ru(0001). The HCP domain inside the Moire supercell has a rather small rumpling with both the Fe and O atoms directly bonded to the Ru substrate. The most stable adsorption of an Au atom on FeO/Ru(0001) is at the Fe-bridge site in the HCP domain, where Au binds with two flipped Fe atoms and is negatively charged. After exposure to CO, the Au anions at the Fe-bridge site changed their position to the O-top site by overcoming a small barrier of only 0.12 eV, where they formed stable Au + -CO species with a significant reduction in the formation energy. The results highlighted the importance of in situ characterization of supported catalysts under reaction conditions, and implications on catalyst stability were also discussed. The adsorption site and charge of Au adatoms on FeO(111)/Ru(0001) can be dramatically changed by CO adsorption, which highlights the importance of in situ characterization of supported metal catalyst under reaction conditions.

Modulating the electronic properties of graphdiyne nanoribbons

By ab initio calculation, Au, Cu, Fe, Ni, and Pt adatoms were proposed for modulating the electronic property of graphdiyne naoribbons (GDNRs). GDNRs of 1–4nm in width were found to be stable at room temperature, and the thermal rates of Au, Cu, Fe, Ni, and Pt adatoms escaping from GDNR are slower than 0.003atoms per hour even at 900K. According to the calculation, Au and Cu-decorated GDNRs are metallic with carrier concentrations close to that of graphene at room temperature, while Fe, Ni, and Pt-decorated GDNRs are n-type semiconductors with impurity states below Fermi energy. Heterojunction composed by doping Au, Cu, or Fe atom on one side of GDNR was proposed as metal–semiconductor rectifier with rectification ratio of 2.8, 1.5, or 2.5 at 1.0V, respectively.

Growth and Structure of Cu and Au on the Nonpolar ZnO(101̅0) Surface: STM, XPS, and DFT Studies

The morphology and electronic structure of Cu and Au clusters deposited via thermal evaporation onto ZnO(101̅0) substrates have been studied via scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). The initial stages of nucleation and growth (∼0.2 ML) of both Cu and Au are compared with density functional theory (DFT) calculations, which show an excellent agreement with the cluster morphologies observed by STM, with Cu nucleating three-dimensional (3D) islands even at small coverage while Au nucleates single-layer islands that grow layer by layer. DFT also gives insight into the diffusion behavior of Cu and Au adatoms on the ZnO substrate, showing strongly anisotropic diffusion barriers for Cu atoms which results in the experimentally observed preferential cluster nucleation along [0001] step edges, whereas Au shows no such anisotropy and Au clusters are observed to have no preferred nucleation sites. XPS results show a slight positive charging of the small Cu clusters at 0.2 ML coverage, which disappears at higher coverage. The single-layer Au islands formed at low coverage show some evidence of positive charging as well, which likewise disappears with increasing cluster size. Additionally, the Au clusters show a trend of increasing metallicity as the clusters grow and transition from single-layer islands to 3D structures, demonstrated by the increasing asymmetry in the Au 4f line shape as a function of Au coverage. In general, the observed charge transfer trends are supported by Bader charge analysis.

Theoretical Study of CO Adsorption on Perfect and Defective Gold Surfaces

The activities of Au(111), Au(100) and Au(110) surfaces, as well as the activity of an Au adatom on Au(111), Au(100) and Au(110) surfaces have been characterized by density functional theory using CO as a probe. The results clearly show that the activity of Au surfaces towards CO adsorption is attributable to the presence of low-coordinated gold atoms to which the molecule binds. The results may enrich the understanding of CO adsorption on Au surfaces.

Adsorption and diffusion of the Rh and Au adatom on graphene moiré/Ru(0001)

Detailed density functional theory calculations have been performed to investigate the adsorption and diffusion of the Rh and Au adatom on the graphene moiré superstructure on Ru(0001). The adsorption energies of each adatom in all of the non-equivalent C-top and C6 ring center sites on the graphene moiré have been calculated. The resulting potential energy surfaces encompass the entire graphene moiré unit cell and shows that the adsorption of both Rh1 and Au1 is most stable in the fcc region on the graphene moiré. The minimum-energy diffusion path between adjacent moiré cells is identified to run mostly directly between the fcc and hcp regions for Au1, but deviates toward the mound region for Rh1. The global diffusion barrier is estimated to be 0.53 eV for Rh1 and 0.71 eV for Au1, corresponding to a hopping rate between adjacent moiré cells of ~10(3) s(-1) and ~1 s(-1) at 298 K, respectively. The consequences of different hopping rates to cluster nucleation have been explored by performing Monte Carlo-based statistical analysis, which suggests that diffusing species other than adatoms need to be taken into account to develop an accurate description of cluster nucleation and growth on this surface.

Charging of Gold Atoms on Doped MgO and CaO: Identifying the Key Parameters by DFT Calculations

The use of transition metals (TM) as dopants is a versatile way to change the properties of oxides. One example is the possibility to induce charge transfer from the TM to species adsorbed on the oxide surface. In the case of wide-gap alkaline-earth oxides, such as MgO and CaO, the charge transfer is determined by the respective position of the energy levels of the impurity and the adsorbate and resides in the capability of the TM to be stabilized in various oxidation states. The structural and electronic properties of Cr- and Mo-doped MgO and CaO are investigated by density functional theory (DFT) calculations. Because of limitations of DFT in the description of band gaps and energy levels alignment, we compare the results obtained with standard GGA, GGA+U, and hybrid exchange-correlation functionals. The ability of the impurity ion to transfer one electron to the adsorbed species is considered for the case of electronegative Au adatoms. The study of the thermodynamic stability of the TM ions in combination with the formation of cation vacancies shows that Mo impurities are stable in the oxide lattice as Mo3+ or Mo4+ but can assume also higher oxidation states and are therefore able to transfer electrons to adsorbed gold. Conversely, Cr impurities are stabilized as Cr3+ and the high cost of further oxidation results in the absence of charge transfer to gold on both Cr-doped MgO and Cr-doped CaO.

In Situ Observation of a Deprotonation-Driven Phase Transformation: 4,4′-Biphenyldicarboxylic Acid on Au(111)

We have studied the growth of 4,4′-biphenyldicarboxylic acid (BDA) domains on Au(111) by means of low-energy electron microscopy (LEEM) and selective area low-energy electron diffraction (μLEED). Between 300 and 400 K, BDA forms three different 2-D crystalline phases, which are attributed to different degrees of deprotonation. At room temperature, BDA condenses in needle-shaped domains (α-phase) with the carboxylic acid–dimer as the main molecular bonding motif. Upon annealing from room temperature to above about 330 K, the needle-shaped domains change irreversibly into more compact ones. This remarkable feature is attributed to a partial deprotonation of the carboxylic end groups and thus breaking of the dimers, which allows the molecules to establish hydrogen bonds with adjacent molecules by a slight rotation (β-phase). A third phase is formed upon direct adsorption of BDA at substrate temperatures above 330 K. In this γ-phase the deprotonated BDA molecules form an open 2-D metal–organic coordination network, probably by incorporation of thermally excited Au adatoms.

Electrochemical dealloying of Al2(Au,X) (X = Pt, Pd, PtPd, Ni, Co and NiCo) alloys in NaCl aqueous solution

The electrochemical dealloying of rapidly solidified Al2(Au,X) (X = Pt, Pd, PtPd, Ni, Co and NiCo) precursors in a 1.0 M NaCl aqueous solution has been systematically investigated using electrochemical measurements including open-circuit measurement, potentiodynamic polarization and potentiostatic polarization, and microstructural analysis. The results show that the kind of alloying element(s) has a significant influence on the open-circuit and corrosion potentials of the rapidly solidified Al2(Au,X) precursors. The bulk dealloying of the Al2(Au,X) precursors is affected by the kind of alloying element(s) and also sensitive to the applied potential (or overpotential). The addition of Ni or/and Co easily leads to passivation on the surface of precursors during potentiostatic dealloying. The potentiostatic dealloying of Al2(Au,Pt), Al2(Au,Pd) and Al2(Au,Pt,Pd) results in the formation of ultrafine nanoporous AuPt, AuPd and AuPtPd alloys, owing to the pinning effect of Pt or/and Pd on surface diffusion of Au adatoms. In comparison, the potentiostatic dealloying of Al2(Au,Ni), Al2(Au,Co) and Al2(Au,Ni,Co) leads to the formation of nanoporous Au with a ligament/channel size of ~40 nm due to the simultaneous dissolution of Al and Ni/Co. Moreover, the addition of Pt, Pd or PtPd not only inhibits surface diffusion of Au adatoms (lower diffusivities), but also improves the activation energy for the diffusion process during potentiostatic dealloying. Based upon the present results, nanoporous metals or alloys can be greenly fabricated through electrochemical dealloying in NaCl solutions.

Charge competition with oxygen molecules determines the growth of gold particles on doped CaO films

The influence of gas-phase oxygen on the growth of Au nanoparticles on Mo-doped CaO films has been investigated by means of low temperature scanning tunnelling microscopy and X-ray photoelectron spectroscopy. Whereas at ideal vacuum conditions, only 2D Au islands develop on the oxide surface, the fraction of 3D deposits increases with increasing O2 pressure until they become the dominant species in 106 mbar oxygen. The morphology crossover arises from changes in the interfacial electron flow between Mo donors in the CaO lattice and different ad-species on the oxide surface. In the absence of 02 molecules, the donor electrons are predominately transferred to the Au ad-atoms, which consequently experience enhanced binding to the oxide surface and agglomerate into 2D islands. In an oxygen atmosphere, on the other hand, a substantial fraction of the excess electrons is trapped by adsorbed O2 molecules, while the Au atoms remain neutral and assemble into tall 3D particles that are typical for non-doped oxides. Our experiments demonstrate how the competition for charge between different adsorbates governs the physical and chemical properties of doped oxides, so widely used in heterogeneous catalysis.

Slow Gold Adatom Diffusion on Graphene: Effect of Silicon Dioxide and Hexagonal Boron Nitride Substrates

We examine the nucleation kinetics of Au clusters on graphene and explore the relationship with layer number and underlying supporting substrate of graphene. Using the mean field theory of diffusion-limited aggregation, morphology patterns are semiquantitatively analyzed to obtain Au adatom effective diffusion constants and activation energies. Under specified assumptions, the Au adatom diffusion constant for single-layer graphene supported on SiO2 is ∼50 times smaller than that for hexagonal boron nitride (h-BN)-supported graphene and on the order of 800 times smaller than that for multilayer graphite. Bilayer graphene on SiO2 shows a Au adatom diffusion constant similar to single-layer graphene on h-BN. Scanning probe data show that single-layer graphene is far flatter on h-BN than on SiO2. Two factors are proposed as contributing to the observed lower diffusion constants on single-layer graphene: local surface roughness and homogeneous loss of dispersion/van der Waals electronic stability in multilayers. Graphene Raman spectroscopy shows little charge transfer between Au nanoparticles and graphene.

Adsorption and diffusion of gold adatoms on boron nitride nanoribbons: A first-principles study

We have carried out first-principles calculations to explore the adsorption and diffusion of Au adatoms on boron nitride nanoribbons (BNNRs). We found that Au adatoms prefer to locate at the edge B site of the ribbons for both armchair (A-) and zigzag (Z-) BNNRs. Different diffusion paths, such as diffusion from central region to edge site, along the subedge sites or along the edge sites, are considered. The unique atomic arrangement and electronic structures of Z-BNNRs make the Au adatom tend to migrate only to B edge site rather than to the both edges. Different from the cases of graphene nanoribbons, the energy barriers for A-BNNRs are higher than those of the corresponding paths for Z-BNNRs. The electronic structure calculations indicate the wide-band-gap features are preserved in the Au-doped BNNRs as the Au concentration is low. With the increase of Au concentration, the Au adatoms form an atomic chain along the B zigzag edge, resulting in band gap closure. These results are expected to provide useful information for the development of nanoscaled electronic devices based on BNNRs.

Scanning Tunneling Microscopy and Theoretical Study of Water Adsorption on Fe3O4: Implications for Catalysis

The reduced surface of a natural Hematite single crystal α-Fe(2)O(3)(0001) sample has multiple surface domains with different terminations, Fe(2)O(3)(0001), FeO(111), and Fe(3)O(4)(111). The adsorption of water on this surface was investigated via Scanning Tunneling Microscopy (STM) and first-principle theoretical simulations. Water species are observed only on the Fe-terminated Fe(3)O(4)(111) surface at temperatures up to 235 K. Between 235 and 245 K we observed a change in the surface species from intact water molecules and hydroxyl groups bound to the surface to only hydroxyl groups atop the surface terminating Fe(III) cations. This indicates a low energy barrier for water dissociation on the surface of Fe(3)O(4) that is supported by our theoretical computations. Our first principles simulations confirm the identity of the surface species proposed from the STM images, finding that the most stable state of a water molecule is the dissociated one (OH + H), with OH atop surface terminating Fe(III) sites and H atop under-coordinated oxygen sites. Attempts to simulate reaction of the surface OH with coadsorbed CO fail because the only binding sites for CO are the surface Fe(III) atoms, which are blocked by the much more strongly bound OH. In order to promote this reaction we simulated a surface decorated with gold atoms. The Au adatoms are found to cap the under-coordinated oxygen sites and dosed CO is found to bind to the Au adatom. This newly created binding site for CO not only allows for coexistence of CO and OH on the surface of Fe(3)O(4) but also provides colocation between the two species. These two factors are likely promoters of catalytic activity on Au/Fe(3)O(4)(111) surfaces.