Stepped Au Research Articles

Selective Oxidation of Methanol to Methyl Formate on Gold: The Role of Low-Coordinated Sites Revealed by Isothermal Pulsed Molecular Beam Experiments and AIMD Simulations.

To elucidate the role of low-coordinated sites in the partial methanol oxidation to methyl formate (MeFo), the isothermal reactivity of flat Au(111) and stepped Au(332) in pulsed molecular beam experiments was compared for a broad range of reaction conditions. Low-coordinated step sites were found to enhance MeFo selectivity, especially at low coverage conditions, as found at higher temperatures. The analysis of the transient kinetics provides evidence for the essential role of Au x O y phases for MeFo formation and the complex interplay of different oxygen species for the observed selectivity. Ab initio molecular dynamic simulations yielded microscopic insights in the formation of Au x O y phases on flat and stepped gold surfaces emphasizing the role of low-coordinated sites in their formation. Moreover, associated surface restructuring provides atomic-scale insights which align with the experimentally observed transient kinetics in MeFo formation.

Partial Oxidation of Methanol on Gold: How Selectivity Is Steered by Low-Coordinated Sites.

Partial methanol oxidation proceeds with high selectivity to methyl formate (MeFo) on nanoporous gold (npAu) catalysts. As low-coordinated sites on npAu were suggested to affect the selectivity, we experimentally investigated their role in the isothermal selectivity for flat Au(111) and stepped Au(332) model surfaces using a molecular beam approach under well-defined conditions. Direct comparison shows that steps enhance desired MeFo formation and lower undesired overoxidation. DFT calculations reveal differences in oxygen distribution that enhance the barriers to overoxidation at steps. Thus, these results provide an atomic-level understanding of factors controlling the complex reaction network on gold catalysts, such as npAu.

Au/Pt Bimetallic Nanowires with Stepped Pt Sites for Enhanced C-C Cleavage in C2+ Alcohol Electro-oxidation Reactions.

Efficient C-C bond cleavage and oxidation of alcohols to CO2 is the key to developing highly efficient alcohol fuel cells for renewable energy applications. In this work, we report the synthesis of core/shell Au/Pt nanowires (NWs) with stepped Pt clusters deposited along the ultrathin (2.3 nm) stepped Au NWs as an active catalyst to effectively oxidize alcohols to CO2. The catalytic oxidation reaction is dependent on the Au/Pt ratios, and the Au1.0/Pt0.2 NWs have the largest percentage (∼75%) of stepped Au/Pt sites and show the highest activity for ethanol electro-oxidation, reaching an unprecedented 196.9 A/mgPt (32.5 A/mgPt+Au). This NW catalyst is also active in catalyzing the oxidation of other primary alcohols, such as methanol, n-propanol, and ethylene glycol. In situ X-ray absorption spectroscopy and infrared spectroscopy are used to characterize the catalyst structure and to identify key reaction intermediates, providing concrete evidence that the synergy between the low-coordinated Pt sites and the stepped Au NWs is essential to catalyze the alcohol oxidation reaction, which is further supported by DFT calculations that the C-C bond cleavage is indeed enhanced on the undercoordinated Pt-Au surface. Our study provides important evidence that a core/shell structure with stepped core/shell sites is essential to enhance electrochemical oxidation of alcohols and will also be central to understanding electro-oxidation reactions and to the future development of highly efficient direct alcohol fuel cells for renewable energy applications.

Mechanism of Stoichiometrically Governed Titanium Oxide Brownian Tree Formation on Stepped Au(111)

Previously observed formation of substoichiometric titanium oxide dendritic structures across terraces of Au(111) is computationally studied and shown to follow the classical fractal formation mechanism of diffusion-limited aggregation (DLA). Globally optimized gas-phase oxide cluster structures are sampled in a variety of landing formations on gold surfaces and shown to favor isomers driving polymerization to Brownian tree fractal structures. Mobility of Ti3O5 monomers is shown to be extremely high, with diffusion barriers of 0.21 eV or less. Through bonding stabilization, polymerization of these monomers is energetically favorable and irreversible on the 111 terrace but geometrically impossible to propagate along the step edge. Simulated scanning tunneling microscopy (STM) images show strong similarity to experiment. By contrast, observation of Ti3O6 aggregating as wires along step edges is explained by the affinity of oxygen to step edges and statistical arguments for aggregation entropy at the step, in addition to low barriers for monomer diffusion and polymerization.

Enhanced photoluminescence of monolayer MoS2 on stepped gold structure

Different MoS2/Au heterostructures can play an important role in tuning the photoluminescence (PL) and optoelectrical properties of monolayer MoS2. Previous studies of PL of MoS2/Au heterostructures were mainly limited to the PL enhancement by using different Au nanostructures and PL quenching of monolayer MoS2 on flat Au surfaces. Here, we demonstrate the enhanced excitonic PL emissions of monolayer MoS2/Au heterostructures on Si/SiO2 substrates. By transferring the continuous monolayer MoS2 onto a stepped Au structure consisting of 60-nm and 100-nm Au films, the MoS2/Au-60 and MoS2/Au-100 heterostructures exhibit enhanced PL emissions, each with a blue-shifted PL peak in comparison with the MoS2/SiO2. Furthermore, the PL intensity of MoS2/Au-60 is about twice larger than that of MoS2/Au-100. The different enhanced excitonic PL emissions in MoS2/Au heterostructures can be attributed to the different charge transfer effects modified by the stepped Au structure. This work may provide an insight into the excitonic PL and charge transfer effect of MoS2 on Au film and yield novel phenomena in MoS2/Au heterostructures for further study of PL tuning and optoelectrical properties.

Bioactive Properties of Composites Based on Silicate Glasses and Different Silver and Gold Structures.

Using an ideal biomaterial to treat injured bones can accelerate the healing process and simultaneously exhibit antibacterial properties; thus protecting the patient from bacterial infections. Therefore, the aim of this work was to synthesize composites containing silicate-based bioactive glasses and different types of noble metal structures (i.e., AgI pyramids, AgIAu composites, Au nanocages, Au nanocages with added AgI). Bioactive glass was used as an osteoconductive bone substitute and Ag was used for its antibacterial character, while Au was included to accelerate the formation of new bone. To investigate the synergistic effects in these composites, two syntheses were carried out in two ways: AgIAu composites were added in either one step or AgI pyramids and Au nanocages were added separately. All composites showed good in vitro bioactivity. Transformation of AgI in bioactive glasses into Ag nanoparticles and other silver species resulted in good antibacterial behavior. It was observed that the Ag nanoparticles remained in the Au nanocages, which was also beneficial in terms of antibacterial properties. The presence of Au nanoparticles contributed to the composites achieving high cell viability. The most outstanding result was obtained by the consecutive addition of noble metals into the bioactive glasses, resulting in both a high antibacterial effect and good cell viability.

Desorption trends of small alcohols and the disruption of intermolecular interactions at defect sites on Au(111).

Gold-based catalysts have received tremendous attention as supports and nanoparticles for heterogeneous catalysis, in part due to the ability of nanoscale Au to catalyze reactions at low temperatures in oxidative environments. Surface defects are known active sites for low temperature Au chemistry, so a full understanding of the interplay between intermolecular interactions and surface morphology is essential to an advanced understanding of catalytic behavior and efficiency. In a systematic study to better understand the adsorption and intermolecular behavior of small alcohols (C1-C4) on Au(111) defect sites, coverage studies of methanol, ethanol, 1-propanol, 1-butanol, 2-butanol, and isobutanol have been conducted on Au(111) using ultrahigh vacuum temperature programmed desorption (UHV-TPD). These small alcohols molecularly adsorb on the Au(111) surface and high resolution experiments reveal distinct terrace, step edge, and kink adsorption features for each molecule. The hydrogen-bonded (H-bonded) networks of small alcohols on Au(111), except for 1-butanol and isobutanol, have been previously imaged on the molecular level at low temperatures by scanning tunneling microscopy. Primary C1-C3 alcohols exhibit planar H-bonded long extended zigzag chain networks while 2-butanol arranges in tetramer clusters of H-bonded molecules due to steric hindrance inhibiting the proximity of molecules on Au(111). Herein, the desorption energy of small primary alcohols was shown to trend linearly with increasing C1-C4 carbon chain length, indicating that the H-bonded molecular packing of 1-butanol resembles that of methanol, ethanol, and 1-propanol, while isobutanol and 2-butanol deviate from the trend. Butanol isomer studies allow the prediction of isobutanol long extended chains in contrast to tetramers. The distinction between the desorption of butanol isomers highlights the role of intermolecular interactions due to the difference in molecular packing structures on Au(111). Furthermore, by studying the energetics of terrace H-bonded networks in comparison with molecular adsorption at undercoordinated step edge and kink defect sites, it is shown that the contribution of stabilizing van der Waals forces to the overall adsorption energy is less for small alcohols adsorbed at kink sites (3.1 kJ mol-1 per CH2) and similar for those adsorbed at step edge (4.8 kJ mol-1 per CH2) and Au terrace sites (4.9 kJ mol-1 per CH2).

Confined Friedel oscillations on Au(111) terraces probed by thermovoltage scanning tunneling microscopy

The spatial dependence of the local density of states at the Fermi level of a stepped Au(111) surface is studied by thermovoltage scanning tunneling microscopy. The periodicity of the standing waves is not given by ${\ensuremath{\lambda}}_{\text{F}}/2=1.8$ nm, as expected based on the band diagram of Au(111), but rather varies between 1.5 and 2.1 nm depending on the exact width of the terraces. This counterintuitive result can be understood by considering the superposition of incident and reflected electron waves, which have a periodicity of ${\ensuremath{\lambda}}_{\text{F}}/2$. The change in periodicity is a direct consequence of the decaying nature of the Friedel oscillations.

Methanol oxidation on Au(332): an isothermal pulsed molecular beam study.

Isothermal molecular beam experiments on the methanol oxidation over the stepped Au(332) surface were conducted under well-defined ultra-high vacuum conditions. In the measurements, a continuous flux of methanol at excess in the gas phase and pulses of atomic oxygen were provided to the surface kept at 230 K. The formation of the partial oxidation product methyl formate under the applied conditions was evidenced by time-resolved mass spectrometry, and accumulation of formate species, which resulted in a deactivation of the surface for methyl formate formation, was followed by in situ Infrared Reflection Absorption Spectroscopy measurements. The results suggest a different reactivity of oxygen accumulated during the oxygen pulses and atomic oxygen for the competing reaction pathways in the oxidation of methanol to the desired partial and the unwanted overoxidation products.

(Invited) Electronic Structure and Raman Fingerprint of Atomically Precise Graphene Nanoribbons – a Key for Integration in Devices.

Graphene nanoribbons (GNRs) are a novel tunable material that combines the best attributes of graphene and carbon nanotube worlds. We present our recent results on the electronic, optical and vibrational properties of atomically precise GNRs, particularly, pristine and boron-doped armchair graphene nanoribbons of N = 7 carbon atoms width (7-AGNRs and B-7AGNRs). Spatially aligned GNRs are synthesized by bottom-up approach on stepped Au surface. Using angle-resolved photoemission spectroscopy (ARPES) we reveal energy band dispersion of these systems. ARPES data allow us to determine the energies of one-dimensional sub-bands and the effective masses of charge carriers, which are important for charge transport characterization. Using ultra-high vacuum Raman spectroscopy we probe phonons of air-sensitive B-7AGNRs in situ and reveal the existence of characteristic splittings and red-shifts of vibration modes due to the presence of boron atoms. We perform transfer of 7-AGNRs on to the insulating substrate, which allows tuning and probing their optical response. Particularly, bright photoluminescence in 7-AGNRs is controllably induced by defect-engineering. Finally, we partially transform N=7 to narrow-gap N=14 AGNRs via lateral edge fusion on large-area surface. The band structure of 14-AGNRs is visualized by ARPES. Raman fingerprint allows easy identification of type and orientation of GNRs on both metallic and insulating substrates. The fused GNRs are also integrated in the field-effect transistor.

CO Adsorption on Au(332): Combined Infrared Spectroscopy and Density Functional Theory Study

The interaction of molecules with low coordinated sites and possible modification of surfaces caused by adsorbates are of crucial importance for an understanding of chemical processes on surfaces. Herein, CO adsorption on a stepped Au(332) surface is investigated by combining infrared spectroscopy and density functional theory calculations. It is shown that infrared spectroscopic characterization of isotopically diluted mixtures allows distinguishing different components. These components exhibit significantly different adsorption energies as well as vibrational characteristics which can be understood with the help of a theoretical analysis based on density functional theory (DFT). In addition, it is shown that CO induces a restructuring of the surface, leading to a significantly more heterogeneous ensemble of adsorption sites and a significant reduction of sites with the highest CO adsorption energy.

Site- and surface species-dependent propylene oxidation with molecular oxygen on gold surface

Fundamental understandings of Au-catalyzed oxidation reactions with molecular oxygen are of great importance. Herein we report a successful preparation of molecularly-adsorbed O2 species on a stepped Au(997) single crystal model surface via a near-ambient pressure and low-temperature oxygen exposure and their oxidation reactivity with propylene via temperature programmed desorption spectra and polarization-modulated reflection-absorption infrared spectra. O2 molecularly adsorbs at the (111) step sites of Au(997) surface while C3H6 adsorbs at both (111) step sites and (111) terrace sites. C3H6(a) adsorbed at the (111) step sites is more stable than at the (111) terrace sites and its oxidation reactions are dominant; meanwhile, O2(a) is much more reactive than O(a) and prefers combustion reaction of C3H6(a). C3H6(a) adsorbed at the (111) step sites undergoes combustion reactions with O2(a) to produce CO2 and CO at low temperatures and partial oxidation reactions with O(a) to produce acrolein at high temperatures while C3H6(a) adsorbed at the (111) terrace sites undergoes combustion and partial oxidation reactions with O2(a) to respectively produce CO2/CO and acrolein at low temperatures. These results reveal site- and surface species-dependent reaction behaviors of propylene oxidation with molecular O2 on Au surfaces and provide a complete fundamental understanding of oxidation reactions with molecular O2 on Au surface.

Parameter dependent fabrication of Chromium nano-structures on Au(111) surface

Circular and rectangular Cr islands of nanometre size and monoatomic height exhibit an impromptu long range order on locally stepped Au(111) surface with narrow terraces. The circular islands are detached from the {100} microfaceted Au(111) step edges and nucleate between a pair of discommensuration lines of the herringbone reconstruction on ≈ 9.5 nm wide terraces. In contrast, the rectangular islands are attached to the {111} microfaceted step edges and are formed on even narrower terraces of ≈ 6 nm width. Both types of islands prefer the hcp region of the substrate as the nucleation centre. Cr deposition at elevated temperatures such as 470 K results in formation of triangular islands and spiral dislocations on wide substrate terraces. Interestingly however, at higher coverages of 3–4 ML, huge elongated ridges that are 90–260 nm in length, 10–20 nm in width and up to almost 3 nm height are observed. These ridges occur primarily along <11¯0> direction, with the rest forming at an angle of 60 or 120° with respect to this direction.

Switching from Reactant to Substrate Engineering in the Selective Synthesis of Graphene Nanoribbons.

The challenge of synthesizing graphene nanoribbons (GNRs) with atomic precision is currently being pursued along a one-way road, based on the synthesis of adequate molecular precursors that react in predefined ways through self-assembly processes. The synthetic options for GNR generation would multiply by adding a new direction to this readily successful approach, especially if both of them can be combined. We show here how GNR synthesis can be guided by an adequately nanotemplated substrate instead of by the traditionally designed reactants. The structural atomic precision, unachievable to date through top-down methods, is preserved by the self-assembly process. This new strategy's proof-of-concept compares experiments using 4,4''-dibromo-para-terphenyl as a molecular precursor on flat Au(111) and stepped Au(322) substrates. As opposed to the former, the periodic steps of the latter drive the selective synthesis of 6 atom-wide armchair GNRs, whose electronic properties have been further characterized in detail by scanning tunneling spectroscopy, angle resolved photoemission, and density functional theory calculations.

Oxygen-Driven Surface Evolution of Nanoporous Gold: Insights from Ab Initio Molecular Dynamics and Auger Electron Spectroscopy

Nanoporous gold (np-Au) has recently emerged as a highly selective environmentally friendly catalyst for low-temperature applications. Despite the seeming simplicity of this material, which consists of almost pure gold, its surface chemistry turns out to be more complex than anticipated. Interactions among gold, chemisorbed O atoms generated and consumed during catalysis, and trace amounts of Ag impurities present in np-Au lead to complex surface dynamics. In this work, theoretical modeling by means of ab initio molecular dynamics (AIMD) is combined with an Auger electron spectroscopic study to investigate oxygen-driven Ag surface diffusion on Au model surfaces exhibiting structural characteristics of np-Au. AIMD simulations reveal that surface O atoms dynamically form −(Au–O)– chain structures on the stepped Au(321) surface and lead to surface restructuring, but no chain formation is found on the flat Au(111). Ag impurities at low concentration lower the activation barrier for −(Au–O)– chain formation, w...

Excitonic States in Narrow Armchair Graphene Nanoribbons on Gold Surfaces

Narrow graphene nanoribbons (GNRs) exhibit electronic and optical properties that are not present in extended graphene. Most importantly, they possess band gaps in the order of a few electron volts, which has been subject to numerous studies. Here we report on the experimental observation of exctionic states in the band gap of N = 7 armchair GNRs (7-GNR) on Au(111) and Au(788) using energy- and angle-resolved two-photon photoemission spectroscopy. Thereby, an exciton binding energy in the 7-GNR on Au(111) of 160 ± 60 meV has been determined. On the stepped Au(788) surface, the exciton binding energy is in the same range.

Oxidation of formic acid on stepped Au(997) surface

The adsorption and reaction of formic acid (HCOOH) on clean and atomic oxygen-covered Au(997) surfaces were studied by temperature-programmed desorption/reaction spectroscopy (TPRS) and X-ray photoelectron spectroscopy (XPS). At 105 K, HCOOH molecularly adsorbs on clean Au(997) and interacts more strongly with low-coordinated Au atoms at (111) step sites than with those at (111) terrace sites. On an atomic oxygen-covered Au(997) surface, HCOOH reacts with oxygen atoms to form HCOO and OH at 105 K. Upon subsequent heating, surface reactions occur among adsorbed HCOO, OH, and atomic oxygen and produce CO2, H2O, and HCOOH between 250 and 400 K. The Au(111) steps bind surface adsorbates more strongly than the Au(111) terraces and exhibit larger barriers for HCOO(a) oxidation reactions. The surface reactions also depend on the relative coverages of co-existing surface species. Our results elucidate the elementary surface reactions between formic acid and oxygen adatoms on Au surfaces and highlight the effects of the coordination number of the Au atoms on the Au catalysis.

Numerical investigation of the submonolayer growth and relaxation of stepped Ag (110) and Au (110) surfaces

The terrace width distribution (TWD) as well as the adatom and island densities for some stepped fcc (110) surfaces with parallel and equidistant steps of equal stiffness are studied by means of kinetic Monte Carlo simulations. Reliable energy barriers for surface diffusion are used. They have been calculated by means of many-body potentials derived within the second moment approximation to the tight-binding model. The effects of the temperature, atom deposition flux and surface diffusion on quantities of interest in the early stage of the surface evolution process have been singled out. In most cases, linear, logarithmic and power-law behaviors are recovered for the evolution of the step width and terrace defects. The TWD is well described by the Gaussian distribution (GD) in the domain of temperature investigated but deviates from the Generalized Wigner Distribution (GWD).

How Does Nanoporous Gold Dissociate Molecular Oxygen?

Nanoporous Au and other dilute AgAu alloys are highly active and selective oxidation catalysts. Their ability to dissociate O2 is to a large extent unexplained, given that unsupported Au cannot generally dissociate O2, while large ensembles of Ag atoms (>4) are generally necessary to lower the O2 dissociation barrier significantly. Here, we identify a site on the surface of dilute AgAu alloys that is stable under reaction conditions and has a low O2 dissociation barrier, in agreement with experimental measurements. Although Ag generally prefers to disperse throughout Au, the presence of adsorbed O near surface steps creates sites of high local Ag concentration, where the Ag atoms sit in the rows next to the step Au atoms. O2 adsorbs on the Au step atoms, but the transition state involves significant Ag–O interaction, resulting in a barrier lower than expected from the adsorption energies of either the initial or final state.

Unusual ferromagnetic behaviour of embedded non-functionalized Au nanoparticles in Bi/Au bilayer films

Observation of ferromagnetic behavior of non-functionalized gold nanoparticles in contrast to the diamagnetic nature of bulk gold synthesized by single step Au ion irradiation.