Au Surface Research Articles

Electrochemical Deterioration of a Gold Thin Film in Bicarbonate Solution: Correlative Optical, Nano-Mechanical, and Chemical Considerations.

The electrochemical dissolution of metals in liquid electrolytes is of great concern for various electrochemical technologies. However, it is also the focus for boosting metal recovery processes, e.g., from electronic wastes, one of which is the extraction of gold (Au) using eco-friendly electrolytes. To shed more light on the possibility and improvement of such metal leaching, this work presents the investigation of electrochemical deterioration of a Au nanofilm coated on a silicon (Si) support─ubiquitous materials in electronic components─in aqueous potassium bicarbonate (KHCO3) electrolyte, a solution widely used in food products. In addition to the time-dependent in situ Raman spectra revealing the reduced reflectivity of Au associated with its molecular degradation, the significant role of the electric double layer (EDL) at the metal/electrolyte interface is also indicated. Analyzing surface adhesion maps and force-distance spectra acquired using atomic force microscopy and spectroscopy (AFM/AFS), metal degradation is related to nanoscale worm-shaped reconstructed surface features that act as local sites of reduced refractive index. Complemented by the non-negligible fraction of K observed on the treated Au surfaces using scanning electron microscopy and energy dispersive spectroscopy (SEM/EDS), an electrochemical framework of metal degradation is proposed, depicting the electric-field-induced transport of K+ and H+ ions toward the Au surface regardless of bias polarity and implying the formation of ternary gold hydrides. The consideration of faradic current in the EDL circuit also suggests the occurrence of ternary gold oxides. Such determination is reinforced by the attributability of the polar and electrostatic characteristics of the worm-like features to residual negatively charged anionic clusters of the hydrides and the oxides. The analysis process and the new understanding of metal degradation provide a potential route for inspecting other metal/solution systems.

Unraveling the Synergy of Au and Polar MgO(111) Surface in Photothermal Catalytic Oxidative Coupling of Methane

AbstractDirect and selective catalytic oxidative coupling of methane (OCM) into high‐carbon products is a great challenge in C1 chemistry. Herein, the successful fabrication of a series of Au clusters loaded on different MgO facets of (111), (110), and (100) is reported. This work demonstrates the Au‐loaded MgO(111) (denoted as Au/MgO(111)) shows the highest C2H6 yield of 12733.4 µmol g−1 h−1 and selectivity of 90.4%, which is 2.46 times higher than that of Au/MgO(110) (5171.4 µmol g−1 h−1) and 25.14 times higher than that of Au/MgO(100) (506.4 µmol g−1 h−1). Moreover, the high activity of Au/MgO(111) can be well maintained over 100 h. Detailed in situ experiments and theoretical calculation reveal such great performance can be attributed to 1) the strong electronic affinity of the surface oxygen species on polar MgO(111) and CH4with the lowest adsorption energy of −0.82 eV; 2) the Au/MgO(111) shows the lowest ΔERDS of 0.53 eV in the rate‐deterime step of OCM that comes in activating the second CH4 molecule; 3) the Au can act as a hole acceptor under light irradiation and adsorb *CH3 with a strong d‐σ hybridization, resulting in an increased C2H6 selectivity.

Single-Particle Spectroelectrochemistry: Revealing the Electrochemical Tuning Mechanism of Chemical Interface Damping in 1,2-Benzenedithiol-Adsorbed Single Gold Nanorods.

Chemical interface damping (CID) is a newly proposed plasmon damping pathway based on interfacial hot-electron transfer from metal to adsorbate molecules. However, achieving in situ tunability of CID in single gold nanorods (AuNRs) remains a considerable challenge. Here, we present the CID effect induced by benzene 1,2-dithiol (BDT) molecule adsorption on single AuNRs and the effective electrochemical tunability of CID in BDT-adsorbed AuNRs immobilized on an indium tin oxide (ITO) surface. Manipulations of the electrochemical potential alter the electron density of AuNRs, thereby influencing and tuning the localized surface plasmon resonance (LSPR) spectrum, with cathodic potential blueshifting and anodic potential redshifting. The strong adsorption of BDT on Au induced CID in single AuNRs. The potential-induced LSPR scattering spectra of BDT-adsorbed AuNRs for linear potential sweep showed a stable LSPR spectral response, irrespective of the concentrations of BDT molecules. Due to the involvement of two Au-S bonds, BDT molecules have a higher free adsorption energy and a lower desorption rate on the Au surface. This resulted in a stable LSPR spectral response for a linear electrochemical potential sweep. Furthermore, a constant anodic and cathodic potential application showed the tunability of the CID at the BDT-Au interface.

Molecular Evidence for the Axial Coordination Effect of Atomic Iodine on Fe‐N4 Sites in Oxygen Reduction Reaction

AbstractWe present a molecular‐scale investigation of the axial coordination effect of atomic iodine on Fe‐N4 sites in the oxygen reduction reaction (ORR) by electrochemical scanning tunneling microscopy (ECSTM). A well‐defined model catalytic system with explicit and uniform iodine‐coordinated Fe‐N4 sites was constructed facilely by the self‐assembly of iron(II) phthalocyanine (FePc) on an I‐modified Au(111) surface. The electrocatalytic activity of FePc for the ORR shows notable enhancement with axial iodine ligands. The modulation of the electronic structure of Fe sites to evoke a higher spin configuration by axial iodine was evidenced. The interaction strength between oxygen‐containing species and active centers becomes weaker due to the presence of iodine ligands, and the reaction is thermodynamically preferable. Furthermore, the reaction dynamics of FePc on I/Au(111) were explicitly determined via in situ ECSTM potential pulse experiments. In contrast, axial atomic iodine was found inefficacious for improving the activity of Co‐N4 sites, and electron rearrangement was found to be marginal, demonstrating that adequate interactions between axial ligands and metal sites for optimizing electronic structures and catalytic behaviors are prerequisites for the impactful role of axial ligands.

Tailoring Carrier Dynamics of BiVO4 Photoanode via Dual Incorporation of Au and Co(OH)x Cooperative Modification for Photoelectrochemical Water Splitting

AbstractPhotoelectrochemical solar to hydrogen production is a promising way to achieve carbon neutrality, but severe charge recombination in photoanodes limits the conversion efficiency. Herein, Au nanoparticles and Co(OH)x co‐sensitized bismuth vanadate (BiVO4) to construct AuCo(OH)x/BiVO4 photoanode for significantly enhancing the performance of photoelectrochemical water splitting. This process significantly improves the bulk charge carrier separation efficiency, the surface kinetics of water oxidation, and the electron density of BiVO4 photoanode through Au surface plasmon resonance (SPR) and Co(OH)x oxygen evolution catalysts effect. Additionally, the enhancement of the *O and the *OOH generation accelerate the oxygen evolution reaction kinetics. Consequently, the constructed AuCo(OH)x/BiVO4 photoanode demonstrates an excellent photocurrent of 6.2 mA cm−2 at 1.23 V versus reversible hydrogen electrode and a stable continuous output within 42 h. This work contributes to developing high‐efficiency and high‐stability photoanodes for solar H2 production through SPR effect and oxygen evolution catalysts.

Solution Versus On‐Surface Synthesis of Peripherally Oxygen‐Annulated Porphyrins through C−O Bond Formation

AbstractThis study investigates the synthesis of tetra‐ and octa‐O‐fused porphyrinoids employing an oxidative O‐annulation approach through C−H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra‐high vacuum (UHV) conditions. X‐ray photoelectron spectroscopy, scanning tunneling microscopy, and non‐contact atomic force microscopy elucidated the preferential formation of pyran moieties via C−O bond formation and subsequent self‐assembly driven by C−H⋅⋅⋅O interactions. Furthermore, the O‐annulation process was found to reduce the HOMO–LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O‐atoms.

Solution Versus On-Surface Synthesis of Peripherally Oxygen-Annulated Porphyrins through C-O Bond Formation.

This study investigates the synthesis of tetra- and octa-O-fused porphyrinoids employing an oxidative O-annulation approach through C-H activation. Despite encountering challenges such as overoxidation and instability in conventional solution protocols, successful synthesis was achieved on Au(111) surfaces under ultra-high vacuum (UHV) conditions. X-ray photoelectron spectroscopy, scanning tunneling microscopy, and non-contact atomic force microscopy elucidated the preferential formation of pyran moieties via C-O bond formation and subsequent self-assembly driven by C-H⋅⋅⋅O interactions. Furthermore, the O-annulation process was found to reduce the HOMO-LUMO gap by lifting the HOMO energy level, with the effect rising upon increasing the number of embedded O-atoms.

Zr-MOF-based substrate with inherent internal standards facilitates reliable SERS analysis of sulfonamide antibiotics

A multifunctional structure comprising gold nanoparticles immobilized within a zirconium-based metal-organic framework (AuNPs/Zr-MOF) was systematically designed. With the capabilities of analyte enrichment, Raman signal enhancement and internal standard calibration, the as-prepared AuNPs/Zr-MOF substrates demonstrate the outstanding SERS sensing performance toward the representative sulfonamide antibiotics (SAs), sulfadiazine, sulfamerazine, sulfamethazine and sulfaquinoxaline. For the first time, these four SAs are detected by SERS in real honey samples at levels of 2.8 ∼ 5.2 ng·g−1 within 10 min. The extensive π-bond conjugated system of Zr-MOF significantly enhances the concentration of SAs through π-π interactions. The entrapped SAs are adsorbed perpendicularly onto the surface of Au via coordinated interactions between Au and two oxygen atoms, or between Au and the terminal amino group. To achieve reliable quantitative analysis, the inherent vibration of the Zr-MOF structure was used as an internal standard label. Combined with a simple clean-up protocol, the established SERE sensing method is superior to conventional methods in terms of time, while achieving the impressive sensitivity. This study presents a robust SERS-based approach for the rapid detection and monitoring of multiple analytes.

In air STM observation of Au(111) surface disturbance including Au magic fingers as modified by solvent choice

A widely studied surface phenomena on Au(111) is the formation of Au magic fingers, which were first discovered nearly 20 years ago. A variety of experimental conditions have been used to observe the formation of Au magic fingers with a slight preference to ultra-high vacuum and low temperature studies. With the advances in scanning probe techniques, it is possible to study these unique structures under more relevant conditions including in air and at room temperature. After exposure to a 0.1 M solvent solution, Au(111) displayed three types of surface disturbances, including the formation of Au magic fingers, based on the identity of the solvent. The type of disturbance was dependent on the solvent molecule's characteristics, specifically its total charge and its electrolytic behavior in aqueous environments. The mechanism of disturbance relied on a strong tip-surface interaction and the mass transport of Au atoms, which was modified by the solvent selected. Overall, the ability to form organized nanostructures, like Au magic fingers, in a repeated way in environments outside of UHV and without a protective liquid layer increases the utility of these structures into a wider array of fields and applied areas.

Enhancing Stability of Surface Au under Oxidizing Conditions through Reduced Bulk Au Content.

Contrary to the common assumption that a higher bulk content of precious metals facilitates the preservation of more surface noble metal by serving as a reservoir for surface enrichment, we demonstrate that a lower bulk content of Au results in a more stable arrangement of Au atoms at the surface of Cu-Au nanoparticles when exposed to an O2 atmosphere. Using ambient pressure X-ray photoelectron spectroscopy, we investigate the surface segregation and oxidation behavior of Cu-Au nanoparticles across various compositions. Our results reveal that in Au-rich nanoparticles exposed to an H2 atmosphere, surface segregation prompts the formation of a continuous Au-enriched shell, which subsequently oxidizes into a complete CuOx shell upon transitioning to an O2 atmosphere. Conversely, in Au-poor nanoparticles during H2 treatment, segregation results in the emergence of Au clusters embedded within the surface layer, persisting upon exposure to O2. This unexpected phenomenon shows that reducing the bulk content of precious metals can enhance the surface stability of noble atoms under oxidizing conditions, as further demonstrated by comparing the catalytic performance of Cu-Au nanoparticles with varying Au bulk contents in CO oxidation.

Formation of positionally ordered but orientationally disordered molecular organization on surface

Positionally ordered but orientationally disordered molecular structures are commonly found in materials like liquid crystals and molecular glasses. Understanding these structures and their phase transitions helps in designing materials with a wide range of applications. Herein, we report the formation of positionally ordered but orientationally disordered structures via adsorption and organization of 2,4,6-tri([1,1'-biphenyl]-2-yl)-1,3,5-triazine (TBTA) molecules on different coin metal surfaces. It is found that deposition of TBTA molecules on Au(111), Ag(111), and Cu(111) surfaces leads to similar hexagonal lattices, differing in molecular orientation. The molecules have two orientations on Au(111) and Ag(111) surfaces, giving birth to positionally ordered but orientationally disordered molecular structures. The regularity of the structures on Ag(111) is slightly better than that on Au(111). On Cu(111) surface, however, all molecules exhibit the same orientation, resulting in a long-range ordered hexagonal assembly. The density functional theory calculations demonstrate that the matching between the substrate lattice and the hexagonal lattice of molecular structure is responsible for the different molecular organizations.

Salivary Cortisol Detection with a Fully Inkjet-Printed Paper-Based Electrochemical Sensor

Electrochemical paper-based analytical devices (ePADs) offer an innovative, low-cost, and environmentally friendly approach for real-time diagnostics. In this study, we developed a functional all-inkjet paper-based electrochemical immunosensor using gold (Au) printed ink to detect salivary cortisol. Covalent binding of the cortisol monoclonal antibody onto the printed Au surface was achieved through electrodeposition of 4-carboxymethylaniline (CMA), with ethanolamine passivation to prevent non-specific binding. The ePAD exhibited a linear response within the physiological cortisol range (5–20 ng/mL), with sensitivities of 25, 23, and 19 Ω·ng/mL and R2 values of 0.995, 0.979, and 0.99, respectively. Additionally, interference studies against tumor necrosis factor-α (TNF-α) and N-terminal pro-B-type natriuretic peptide (NT-proBNP) yielded excellent results. This novel ePAD, fabricated using inkjet printing technology on paper, simplifies the process, reduces environmental impact, and lowers fabrication costs.

Enhancing vertical assembly of Dot-LEDs for display application using metal chelate coordination chemical linkers

The chemical linker assembly technique for dot-LEDs described in this study offers practical solutions to critical challenges in micro-LED display production, such as high costs, low yield in mass transfer processes, and high cost of micro-LED chips. Dot-LEDs feature a cylindrical structure with a diameter ranging from 0.75 to 1.35 μm and an aspect ratio of 1.1∼1.2 for height to diameter. We introduce a viable approach using N-(carboxymethyl)-N-(11-mercaptoundecyl)glycine (gly-thiol) chemical linkers for the face-selective vertical assembly of dot-LEDs. The size scale of dot-LED enables solution assembly processes when manufacturing pixels. By bonding the Au surface of the p-GaN face to the bottom Au electrodes using the chelate bond of Au-gly-thiol-Zn2+-gly-thiol-Au, we intend to enhance the face-selective vertical assembly. The process involves forming two chelate coordination bonds of Zn2+ ion surrounded by a gly-thiol-treated dot and bottom electrodes, achieving over 60 % efficiency in face-selective vertical assembly. We achieved bright and uniform dot-LED electroluminescence devices, with a peak external quantum efficiency (EQE) of 8.1 % at 3.4 V and a luminance of 22,387 cd/m2 at 9.0 V. Our findings suggest substantial improvements in the assembly and efficiency of micro-nano-scale dot-LED displays, providing a display platform technology for their application in future display systems.

Hierarchical Equations of Motion for Quantum Chemical Dynamics: Recent Methodology Developments and Applications.

ConspectusQuantum effects are critical to understanding many chemical dynamical processes in condensed phases, where interactions between molecules and their environment are usually strong and non-Markovian. In this Account, we review recent progress from our group in development and application of the hierarchical equations of motion (HEOM) method, highlighting its ability to address some challenging problems in quantum chemical dynamics.In the HEOM method, the bath degrees of freedom are represented using effective modes from exponential decomposition of the bath correlation function. Complex spectral densities and low temperature simulations often require a larger number of modes, making the simulations very expensive. Recent advances, such as the barycentric spectral decomposition (BSD) technique, can significantly reduce the number of effective modes, allowing to handle complex spectral densities and enabling simulations at very low temperatures, including near-zero temperature dynamics.Another key improvement in the computational efficiency is the use of tensor network methods like matrix product states and hierarchical tensor networks. These techniques allow for efficient HEOM propagation with thousands of effective modes, crucial for simulating large molecular systems interacting with multiple baths. This combination enables simulations of excitation energy transfer (EET) in systems like the Fenna-Matthews-Olson (FMO) complex and even larger systems with experimentally determined spectral densities.The versatility of the HEOM method is demonstrated through applications to a wide range of chemical dynamics problems. Simulations of EET and related ultrafast spectroscopy are first briefly covered. Applications of the HEOM to quantum tunneling effects in proton transfer reactions are then presented. Early works have studied the non-Kramers dependence of the rate constant as a function of bath friction due to deep tunneling and revealed vibrationally nonadiabatic dynamics within the so-called nontraditional view of proton transfer reactions. A recent work on the large kinetic isotope effects in soybean lipoxygenase also indicated that many quantum correction approximations to classical transition-state theory may fall short in describing deep tunneling effects.Charge transport and separation dynamics in organic semiconductors are another area where the HEOM method has been instrumental. We first demonstrate that the HEOM provides a unified description of both band-like and thermally assisted charge carrier transport in organic materials. The effect of non-nearest neighbor transitions is then investigated by combining generalized master equations with exact memory kernels. The HEOM method also enables simulation of charge separation in organic photovoltaics (OPVs) and reveals how factors such as external electric fields, entropy, and charge delocalization influence the charge separation barrier and dynamics.Moreover, HEOM has been applied to investigate hydrogen atom scattering on the Au(111) surface and vibrational energy relaxation at molecule-metal interfaces. These studies provide deeper insights into how electron-hole pair excitations and temporary charge transfer states influence the nuclear motion, offering a new framework for simulating nonadiabatic dynamics on metal surfaces.In summary, the HEOM method has developed into a robust tool for simulating quantum effects in condensed phases. Future developments in algorithm efficiency and computational power will likely expand its applicability to even more complex systems.

The Performance of Ethylene Partial Oxidation on Group IB Metal Stepped Surfaces

Group IB metal catalysts, particularly Ag, Au, and Cu, exhibit particular selectivity for ethylene oxide (EO) formation, while Ag demonstrating the highest performance so far. Previous studies have explored the EO formation mechanism on (100) and (111) surfaces of group IB metals, but the reaction mechanism on the (211) facet (analogous to the edge sites of a catalyst particle) remains poorly understood. Herein, we fill in the knowledge gap by analyzing ethylene partial oxidation to EO on the (211) surfaces of Ag, Au, and Cu through density functional theory (DFT) calculations, scaling relationship analysis, and microkinetic modeling. Our study demonstrates that the (211) surface decreases the energy barrier for the dissociation of oxygen molecules into oxygen atoms, while unfavorable for the production of EO. Therefore, we should preserve an appropriate concentration of (211) surface on the nanoparticles when designing catalysts for the ethylene epoxidation reaction.

A Cascade Reaction Triggered by H‐H Steric Hindrance: Dimeric Covalent Organic Frameworks on Au(111) and Dimeric Nanoribbons on Ag(111)

Comprehensive SummaryIn on‐surface synthesis, dimers are typically utilized to explore reaction mechanisms or as intermediates in the formation of final products. However, constructing the innovative nanostructures with dimers as building blocks remains challenging. Here, using non‐planar 2,2′,7,7′‐tetrabromo‐9,9′‐biflurenyliden molecules, we have successfully synthesized dimeric covalent organic frameworks (COFs) on the Au(111) surface through a temperature‐controlled cascade reaction. Notably, the H‐H steric hindrance within precursors caused by double bonds leads to selective stepwise debromination during the thermal annealing, which promotes the dimerization through intermolecular Ullmann coupling and cyclodehydrogenation reaction to form COFs primarily constituted by dimer building blocks. Combining scanning tunneling microscopy/spectroscopy and density functional theory calculations, we have precisely confirmed the structural evolution and reaction mechanism. Furthermore, by introducing Ag adatoms to form C−Ag−C intermediates, we have successfully regulated the reaction path and synthesized one‐dimensional nanoribbons with dimers as building blocks. This work not only validates the strategy of synthesizing dimeric nanostructures on different surfaces through cascade reactions induced by precursor design, but also enriches the research field of surface synthesis of COFs and nanoribbons.

Probing Deuteration-Induced Phase Separation in Supported Lipid Monolayers using Hyperspectral TERS Imaging.

In this study, we investigate the impact of deuteration on the formation of phase-separated domains in supported lipid monolayers using hyperspectral Tip-Enhanced Raman Spectroscopy (TERS) imaging. The intricate organization of biological membranes plays a crucial role in cellular functions. Various factors that influence domain formation have been identified in previous studies such as lipid tail length and cholesterol concentration. Deuterium labeling of lipids has proven useful for probing cellular structures and dynamics, but its impact on lipid phase separation remains underexplored. By examining 1:1 mixed monolayers of dipalmitoylphosphatidylcholine (DPPC) and deuterated DPPC on Au(111) surfaces, we reveal partial segregation of domains rich in deuterated and nondeuterated lipids. This study addresses a gap in knowledge by examining the impact of deuteration on lipid tail behavior, offering new insights into how even subtle structural modifications can influence phase behavior. Furthermore, it demonstrates that TERS can be a powerful, nondestructive, and label-free nanoanalytical tool for analyzing lipid membranes and advance the field of membrane biophysics.

Tip-induced local Fermi level alignment: A Stark shift in vacuum level in scanning tunneling microscope configurations

Electric fields in the junction of a scanning tunneling microscope (STM) are generally considered to have a negligible impact on the vacuum level (VL) of materials. We employed field emission resonance (FER) in the STM, combined with the triangular potential model, to measure the VL of the Ag(100) surface under varying electric currents. Unexpectedly, our results reveal that the VL exhibits a linear positive energy shift with increasing electric field strength. We suggest that this Stark shift in the VL arises from local Fermi level alignment induced by the STM tip. Additionally, we examined the VL of Ag islands grown on Cu(111) and Au(111) surfaces under different currents. Despite the Ag island having a lower work function than the Cu(111) and Au(111) surfaces, the energy shift in the VL with respect to the electric field on the Ag island is almost identical to that on the substrate under the same tip structure. This suggests that the Stark shift of the VL is insensitive to the work function. These findings are crucial for utilizing FER to measure local work function variations on surfaces, as the measured value is not influenced by the STM tip structure or the tunneling current, both of which can alter the electric field.

Surface chemo-mechanics coupling of MnO2/Au layered composites under electrochemical environment

The intersection of surface mechanics and electrochemistry has gradually become a prominent topic in the realm of new energy research. To explore the influence of the electrochemical process on the stress of energy storage electrode composite, supercapacitor material manganese dioxide (MnO2) was selected as the coating and the MnO2/Au composites were prepared by the electrochemical deposition. The surface stress of MnO2/Au composites under different electrochemical modes and the stress variation law of MnO2 with varying thicknesses were determined. The addition of MnO2 could alter the stress response and the strain magnitude of the gold electrode with varied electrochemical potential. The reversible tensile strain within the Au surface during 0.3 ∼ 0.8 V charging was induced by the absorption of SO42−, while the compressive strain of the MnO2/Au composites was related to the extraction of Na+ ions resulting from the redox pseudocapacitance of MnO2. Notably, the produced stress exhibited a linear correlation with the capacitance. And the long-cycle tests revealed that as the specific capacitance of electrodes decreased, the induced tensile stress progressively diminished. This study emphasized the surface chemo-mechanics coupling of both the Au layer and MnO2/Au composites, which would provide a new approach to regulating surface strain in the solid–liquid interface.

On-Surface Stepwise Double Dehydrogenation for the Formation of a para-Quinodimethane-Containing Undecacene Isomer.

The on-surface synthesis of an isomer of undecacene, bearing two four-membered rings and two para-quinodimethane moieties, starting from a tetramethyl-substituted diepoxy precursor, is presented. The transformation implies a thermal double deoxygenation followed by a stepwise double dehydrogenation reaction on the Au(111) surface, locally induced by inelastic tunneling electrons. This results in the transformation of para-dimethylbenzene moieties into non-aromatic para-quinodimethanes. The structures and electronic properties of the intermediate and final products are investigated at the single molecule level with high spatial resolution, using both scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy. The experimental results are supported by density functional theory calculations.