Ambient Pressure Scanning Tunneling Microscopy Research Articles

Atomic-Scale Visualization of Heterolytic H2 Dissociation and COx Hydrogenation on ZnO under Ambient Conditions.

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2 dissociation and the hydrogenation of CO and CO2 on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2 on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2 on ZnO at 300 K does not impede the availability of surface sites for H2 dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2 on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2 on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2 at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2 on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.

Scanning tunneling microscopy under chemical reaction at solid–liquid and solid–gas interfaces

The task of bridging the pressure gap between ideal ultrahigh vacuum conditions and more realistic reaction conditions involving gas and liquid phases is crucial in surface and interfacial chemistry. Scanning tunneling microscopy (STM) has played a key role in addressing this challenge by enabling atomic-scale probing of the interface. STM enabled us to study surface structure, electronic structure, atomic manipulation, dynamics of molecules and atoms, and chemical properties of the surface at the atomic scale. Over the past four decades, the field of STM has undergone explosive growth. This review article focuses on recent advances in operando STM, specifically in the study of solid–liquid and solid–gas interfaces. It highlights the latest works in ambient-pressure STM, which has enabled the observation of atomic features under various gas and reaction conditions. This information sheds light on the surface mobility of adsorbates and atomic structures of reaction intermediates. The review also addresses research on electrochemical STM, which investigates the evolution of surface morphology under electrochemical processes and provides insights into atomic-scale reaction mechanisms. Finally, the article outlines future challenges and perspectives for operando STM techniques.

Revealing CO2 dissociation pathways at vicinal copper (997) interfaces

Size- and shape-tailored copper (Cu) nanocrystals can offer vicinal planes for facile carbon dioxide (CO2) activation. Despite extensive reactivity benchmarks, a correlation between CO2 conversion and morphology structure has not yet been established at vicinal Cu interfaces. Herein, ambient pressure scanning tunneling microscopy reveals step-broken Cu nanocluster evolutions on the Cu(997) surface under 1 mbar CO2(g). The CO2 dissociation reaction produces carbon monoxide (CO) adsorbate and atomic oxygen (O) at Cu step-edges, inducing complicated restructuring of the Cu atoms to compensate for increased surface chemical potential energy at ambient pressure. The CO molecules bound at under-coordinated Cu atoms contribute to the reversible Cu clustering with the pressure gap effect, whereas the dissociated oxygen leads to irreversible Cu faceting geometries. Synchrotron-based ambient pressure X-ray photoelectron spectroscopy identifies the chemical binding energy changes in CO-Cu complexes, which proves the characterized real-space evidence for the step-broken Cu nanoclusters under CO(g) environments. Our in situ surface observations provide a more realistic insight into Cu nanocatalyst designs for efficient CO2 conversion to renewable energy sources during C1 chemical reactions.

Enhanced Oxide Reduction by Hydrogen at Cuprous Oxide–Copper Interfaces near Ascending Step Edges

Understanding the dynamic processes involved in the interaction of hydrogen with oxides is of fundamental importance in catalysis. This paper probes the reduction of Cu2O-“29” surfaces by hydrogen at room temperature combining in situ ambient pressure scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Reduction of the atomic layer thin Cu2O film is observed to be preferentially initiated at step edges and terrace defects, where perfect Cu2O(111) terraces are found to be stable toward hydrogenation under the same conditions. After a long induction period, regions of partially reduced Cu2O-“29” and metallic Cu coexist before the surface is fully reduced to Cu(111). The reduction rate strongly depends on the nature of nearby Cu step edges. We propose a mechanism for the reduction of Cu2O-“29” by hydrogen where free copper atoms from ascending metallic step edges facilitate the formation of active ensembles for H2 dissociation and transfer H to the edges of Cu2O regions.

Enhanced dissociation activation of CO2 on the Bi/Cu(1 1 1) interface by the synergistic effect

Thermocatalytic CO2 reduction reaction (CO2RR) is one of the promising strategies to mitigate CO2 emissions. Meanwhile, understanding the reduction mechanism of CO2 on the catalyst surface is imperative for advancing catalyst design and the eventual industrialization under mild conditions. In this work, the catalytic role of bismuth/copper interface towards CO2RR is elaborately investigated via a combination of near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), ambient-pressure scanning tunneling microscopy (NAP-STM) and density function theory (DFT). It is demonstrated that the initial deposition of Bi on Cu results in the formation of Bi-Cu heterogenous structure, while the interface is buried at the thick coverage. Upon CO2 exposure, Bi is oxidized at interface due to the activation of CO2 on Cu sites and subsequent migration of oxygen to Bi, while Cu+ is induced afterwards in annealing acting as the further dissociation site. Compared with the pure Cu(111), the fraction of oxidation including both lattice and defective oxygen is significantly higher on the Bi/Cu interface, indicating the synergistic effect of Bi-Cu interface in the activation of CO2. Thus, our work clearly reveals the dissociation site evolution of the Bi-Cu bimetallic heterostructure and might promote the design of Cu-based catalysts for advancing thermocatalytic CO2RR.

Dynamic chemical processes on ZnO surfaces tuned by physisorption under ambient conditions

The catalytic properties of non-reducible metal oxides have intrigued continuous interest in the past decades. Often time, catalytic studies of bulk non-reducible oxides focused on their high-temperature applications owing to their weak interaction with small molecules. Hereby, combining ambient-pressure scanning tunneling microscopy (AP-STM), AP X-ray photoelectron spectroscopy (AP-XPS) and density functional theory (DFT) calculations, we studied the activation of CO and CO2 on ZnO, a typical non-reducible oxide and major catalytic material in the conversion of C1 molecules. By visualizing the chemical processes on ZnO surfaces at the atomic scale under AP conditions, we showed that new adsorbate structures induced by the enhanced physisorption and the concerted interaction of physisorbed molecules could facilitate the activation of CO and CO2 on ZnO. The reactivity of ZnO towards CO could be observed under AP conditions, where an ordered (2 × 1)–CO structure was observed on ZnO(101¯0). Meanwhile, chemisorption of CO2 on ZnO(101¯0) under AP conditions was also enhanced by physisorbed CO2, which minimizes the repulsion between surface dipoles and causes a (3 × 1)–CO2 structure. Our study has brought molecular insight into the fundamental chemistry and catalytic properties of ZnO surfaces under realistic reaction conditions.

Operando Surface Characterization on Catalytic and Energy Materials from Single Crystals to Nanoparticles.

Modern surface science faces two major challenges, a materials gap and a pressure gap. While studies on single crystal surface in ultrahigh vacuum have uncovered the atomic and electronic structures of the surface, the materials and environmental conditions of commercial catalysis are much more complicated, both in the structure of the materials and in the accessible pressure range of analysis instruments. Model systems and operando surface techniques have been developed to bridge these gaps. In this Review, we highlight the current trends in the development of the surface characterization techniques and methodologies in more realistic environments, with emphasis on recent research efforts at the Korea Advanced Institute of Science and Technology. We show principles and applications of the microscopic and spectroscopic surface techniques at ambient pressure that were used for the characterization of atomic structure, electronic structure, charge transport, and the mechanical properties of catalytic and energy materials. Ambient pressure scanning tunneling microscopy and X-ray photoelectron spectroscopy allow us to observe the surface restructuring that occurs during oxidation, reduction, and catalytic processes. In addition, we introduce the ambient pressure atomic force microscopy that revealed the morphological, mechanical, and charge transport properties that occur during the catalytic and energy conversion processes. Hot electron detection enables the monitoring of catalytic reactions and electronic excitations on the surface. Overall, the information on the nature of catalytic reactions obtained with operando spectroscopic and microscopic techniques may bring breakthroughs in some of the global energy and environmental problems the world is facing.

Reversible oxidation and reduction of gold-supported iron oxide islands at room temperature

Monolayer iron oxides grown on metal substrates have widely been used as model systems in heterogeneous catalysis. By means of ambient-pressure scanning tunneling microscopy (AP-STM), we studied the in situ oxidation and reduction of FeO(111) grown on Au(111) by oxygen (O2) and carbon monoxide (CO), respectively. Oxygen dislocation lines present on FeO islands are highly active for O2 dissociation. X-ray photoelectron spectroscopy measurements distinctly reveal the reversible oxidation and reduction of FeO islands after sequential exposure to O2 and CO. Our AP-STM results show that excess O atoms can be further incorporated on dislocation lines and react with CO, whereas the CO is not strong enough to reduce the FeO supported on Au(111) that is essential to retain the activity of oxygen dislocation lines.

Adsorbate-driven reactive interfacial Pt-NiO1-x nanostructure formation on the Pt3Ni(111) alloy surface.

The origin of the synergistic catalytic effect between metal catalysts and reducible oxides has been debated for decades. Clarification of this effect, namely, the strong metal-support interaction (SMSI), requires an understanding of the geometric and electronic structures of metal-metal oxide interfaces under operando conditions. We show that the inherent lattice mismatch of bimetallic materials selectively creates surface segregation of subsurface metal atoms. Interfacial metal-metal oxide nanostructures are then formed under chemical reaction environments at ambient pressure, which thus increases the catalytic activity for the CO oxidation reaction. Our in situ surface characterizations using ambient-pressure scanning tunneling microscopy and ambient-pressure x-ray photoelectron spectroscopy exhibit (i) a Pt-skin layer on the Pt-Ni alloyed surface under ultrahigh vacuum, (ii) selective Ni segregation followed by the formation of NiO1-x clusters under oxygen gas, and (iii) the coexistence of NiO1-x clusters on the Pt-skin during the CO oxidation reaction. The formation of interfacial Pt-NiO1-x nanostructures is responsible for a highly efficient step in the CO oxidation reaction. Density functional theory calculations of the Pt3Ni(111) surface demonstrate that a CO molecule adsorbed on an exposed Pt atom with an interfacial oxygen from a segregated NiO1-x cluster has a low surface energy barrier of 0.37 eV, compared with 0.86 eV for the Pt(111) surface.

In situ characterization of the liquid–solid interface by scanning tunneling microscopy

A chemical cell with sorption and turbomolecular pumps was attached to an ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) chamber to study the structure of surfaces etched or passivated by various chemicals. This system allows us to study various liquid–solid interfaces with little or no contamination that usually occurs in ambient pressure STM or during transportation of a sample in the air. Silicon surfaces treated by dilute HF, mixed solution of HF with NH4F and KOH were imaged by a UHV STM, revealing different surface structures. This system can be used as a new device for characterization of liquid–solid interfaces.

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite: T Tiedje et al, J Vac Sci Technol, A6, 1988, 372–375

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite: T Tiedje et al, J Vac Sci Technol, A6, 1988, 372–375

Tip contamination effects in ambient pressure scanning tunneling microscopy imaging of graphite

We have used electrical noise measurement and transmission electron microscopy to study the condition of metal tips used to obtain contrast current mode scanning tunneling microscope (STM) images of pyrolytic graphite in air. Electrochemically and mechanically sharpened tips of W, W0.97Re0.03, Pt, and Pt0.8Ir0.2, which gave giant corrugations in graphite images, were found to have various forms of carbonaceous contamination on the tip. Contamination in the form of liquid hydrocarbon, graphite clumps, and carbon fibers were observed. Tunneling current noise spectra from these tips exhibited a characteristic 1/f behavior from 10 Hz to 10 kHz. Clean atomic-resolution tips formed by sputtering thin layers of metal (Ni, Au, Pt) on prefabricated tips were found to exhibit lower noise in the tunneling current and ≤1-Å amplitude corrugations. Our interpretation of the present experiment, while similar to earlier explanations that associate the giant corrugations with contamination, differs in that we attribute the large corrugations to deformation of relatively soft carbonaceous material attached to the tip, rather than to elastic deformation of the graphite itself. In addition, we have found that several of the structural artifacts observed in atomic-resolution STM images of graphite can be explained by a moiré effect caused by a double tip.