The rapidly increasing field of surfaces under ambient conditions of temperature, and pressure in gas and liquid environments, reflects the importance of understanding surface properties in ...conditions closer to practical situations. This has been enabled by the emergence in the last two decades of a number of new techniques, both spectroscopy and microscopy, that can deliver atomic scale information with the required surface/interface sensitivity. Here we present a short review of recent advances to illustrate the novel understanding derived from the use of new techniques focusing on the gas–solid interface, where two barriers have been bridged: the pressure gap, and the temperature gap. The later gap is very important when dealing with weakly bound molecules, where only by the presence of gas at a suitable pressure can a measurable coverage of adsorbed molecules be achieved. The temperature gap manifests also in the removal of kinetic barriers. Future developments to continue extending the range of pressures are also mentioned. Finally, new challenges that appear, both from X-ray and electron-induced damage to the sample, and from contamination under high pressure of desired gases, while maintaining very low pressures of undesirable ones.
Pt-based alloy catalysts are promising candidates for fuel-cell applications, especially for cathodic oxygen reduction reaction (ORR) and anodic methanol oxidation reaction (MOR). The rational design ...of composition and morphology is crucial to promoting catalytic performances. Here, we report the synthesis of Pt–Co nanoframes via chemical etching of Co from solid rhombic dodecahedra. The obtained Pt–Co nanoframes exhibit excellent ORR mass activity in acidic electrolyte, which is as high as 0.40 A mgPt –1 initially and 0.34 A mgPt –1 after 10 000 potential cycles at 0.95 VRHE. Furthermore, their MOR mass activity in alkaline media is up to 4.28 A mgPt –1 and is 4-fold higher than that of commercial Pt/C catalyst. Experimental studies indicate that the weakened binding of intermediate carbonaceous poison contributes to the enhanced MOR behavior. More impressively, the Pt–Co nanoframes also demonstrate remarkable stability under long-term testing, which could be attributed to the negligible electrochemical Co dissolution.
The lubricating properties of graphite and graphene have been intensely studied by sliding a frictional force microscope tip against them to understand the origin of the observed low friction. In ...contrast, the relative motion of free graphene layers remains poorly understood. Here we report a study of the sliding behavior of graphene nanoflakes (GNFs) on a graphene surface. Using scanning tunneling microscopy, we found that the GNFs show facile translational and rotational motions between commensurate initial and final states at temperatures as low as 5 K. The motion is initiated by a tip-induced transition of the flakes from a commensurate to an incommensurate registry with the underlying graphene layer (the superlubric state), followed by rapid sliding until another commensurate position is reached. Counterintuitively, the average sliding distance of the flakes is larger at 5 K than at 77 K, indicating that thermal fluctuations are likely to trigger their transitions from superlubric back to commensurate ground states.
We provide direct evidence for the existence of isolated, one-dimensional charge density waves at mirror twin boundaries (MTBs) of single-layer semiconducting MoSe2. Such MTBs have been previously ...observed by transmission electron microscopy and have been predicted to be metallic in MoSe2 and MoS21, 2, 3, 4, 5, 6, 7. Our low-temperature scanning tunnelling microscopy/spectroscopy measurements revealed a substantial bandgap of 100 meV opening at the Fermi energy in the otherwise metallic one-dimensional structures. We found a periodic modulation in the density of states along the MTB, with a wavelength of approximately three lattice constants. In addition to mapping the energy-dependent density of states, we determined the atomic structure and bonding of the MTB through simultaneous high-resolution non-contact atomic force microscopy. Density functional theory calculations based on the observed structure reproduced both the gap opening and the spatially resolved density of states.
Most materials and devices typically operate under specific environmental conditions, many of them highly reactive. Heterogeneous catalysts, for example, work under high pressure of reactants or in ...acidic solutions. The relationship between surface structure and composition of materials during operation and their chemical properties needs to be established in order to understand the mechanisms at work and to enable the design of new and better materials. Although studies of the structure, composition, chemical state, and phase transformation under working conditions are challenging, progress has been made in recent years in the development of new techniques that operate under a variety of realistic environments. With them, new chemistry and new structures of materials that are only present under reaction conditions have been uncovered.
The adsorption and reactions of small molecules, such as water and oxygen, with graphene films is an area of active research, as graphene may hold the key to unique applications in electronics, ...batteries, and other technologies. Since the graphene films produced so far are typically polycrystalline, with point and line defects that can strongly affect gas adsorption, there is a need to understand their reactivity with environmentally abundant molecules that can adsorb and alter their properties. Here we report a study of the adsorption and reactions of water, oxygen, hydrogen, and ammonia on epitaxial graphene grown on Ru and Cu substrates using scanning tunneling microscopy (STM). We found that on Ru(0001) graphene line defects are extremely fragile toward chemical attack by water, which splits the graphene film into numerous fragments at temperatures as low as 90 K, followed by water intercalation under the graphene. On Cu(111) water can also split graphene but far less effectively, indicating that the chemical nature of the substrate strongly affects the reactivity of the C–C bonds in epitaxial graphene. Interestingly, no such effects were observed with other molecules, including oxygen, hydrogen, and ammonia also studied here.
Developing highly efficient and reversible hydrogenation-dehydrogenation catalysts shows great promise for hydrogen storage technologies with highly desirable economic and ecological benefits. ...Herein, we show that reaction sites consisting of single Pt atoms and neighboring oxygen vacancies (V
) can be prepared on CeO
(Pt
/CeO
) with unique catalytic properties for the reversible dehydrogenation and rehydrogenation of large molecules such as cyclohexane and methylcyclohexane. Specifically, we find that the dehydrogenation rate of cyclohexane and methylcyclohexane on such sites can reach values above 32,000 mol
mol
h
, which is 309 times higher than that of conventional supported Pt nanoparticles. Combining of DRIFTS, AP-XPS, EXAFS, and DFT calculations, we show that the Pt
/CeO
catalyst exhibits a super-synergistic effect between the catalytic Pt atom and its support, involving redox coupling between Pt and Ce ions, enabling adsorption, activation and reaction of large molecules with sufficient versatility to drive abstraction/addition of hydrogen without requiring multiple reaction sites.
Electrode/electrolyte interfaces play a vital role in various electrochemical systems, but
in situ
characterization of such buried interfaces remains a major challenge. Several efforts to develop ...techniques or to modify existing techniques to study such interfaces are showing great promise to overcome this challenge. Successful examples include electrochemical scanning tunneling microscopy (EC-STM), surface-sensitive vibrational spectroscopies, environmental transmission electron microscopy (E-TEM), and surface X-ray scattering. Other techniques such as X-ray core-level spectroscopies are element-specific and chemical-state-specific, and are being widely applied in materials science research. Herein we showcase four types of newly developed strategies to probe electrode/electrolyte interfaces
in situ
with X-ray core-level spectroscopies. These include the standing wave approach, the meniscus approach, and two liquid cell approaches based on X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy. These examples demonstrate that with proper modifications, many ultra-high-vacuum based techniques can be adapted to study buried electrode/electrolyte interfaces and provide interface-sensitive, element- and chemical-state-specific information, such as solute distribution, hydrogen-bonding network, and molecular reorientation. At present, each method has its own specific limitations, but all of them enable
in situ
and
operando
characterization of electrode/electrolyte interfaces that can provide important insights into a variety of electrochemical systems.
Emerging strategies make it possible to use soft and tender X-ray spectroscopies to probe electrode/electrolyte interfaces
in situ
.
The atoms at the surfaces of materials represent the frontier separating the bulk from the surrounding medium. Over the last decades, scientists have intensely studied the structure and properties of ...surfaces with the goal of understanding and improving the electronic and chemical properties of materials. The surface–medium interaction determines wetting, friction, chemical, biological, and electronic properties. The activity of catalysts, phenomena occurring in water droplets and particles in the atmosphere, and the electronic properties of semiconductor devices are direct consequences of surface-environment interactions. While the need to pursue studies in the normal environment that surrounds a material has always been recognized, the techniques used in the past have only partially fulfilled this need, as most of them work best under high vacuum conditions. My research over the last 10 years has focused on discovering the structure of a surface and its dynamics in real life—in everyday environments. This required the development of new techniques and methods. I present some of the new tools developed in my laboratory and new properties that were discovered by their application in the areas of environmental science, surface chemistry, and catalysis.
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