The controlled fabrication of nanostructures has often used a substrate template to mediate and control the growth kinetics. Electronic substrate-mediated interactions have been demonstrated to guide the assembly of organic molecules or the nucleation of metal atoms but usually at cryogenic temperatures, where the diffusion has been limited. Combining STM, STS, and DFT studies, we report that the strong electronic interaction between transition metals and oxides could indeed govern the growth of low-dimensional oxide nanostructures. As a demonstration, a series of FeO triangles, which are of the same structure and electronic properties but with different sizes (side length >3 nm), are synthesized on Pt(111). The strong interfacial interaction confines the growth of FeO nanostructures, leading to a discrete size distribution and a uniform step structure. Given the same interfacial configuration, as-grown FeO nanostructures not only expose identical edge/surface structure but also exhibit the same electronic properties, as manifested by the local density of states and local work functions. We expect the interfacial confinement effect can be generally applied to control the growth of oxide nanostructures on transition metal surfaces. These oxide nanostructures of the same structure and electronic properties are excellent models for studies of nanoscale effects and applications.Keywords: FeO nanostructures; interfacial confinement; local density of states; local work function; scanning tunneling microscopy; strong metal−oxide interaction;

The interface between metal and reducible oxide has attracted increasing interest in catalysis. The FeOx–Pt interface has been a typical example, which showed remarkable activity for the preferential oxidation of CO (PROX) at low temperatures. However, model catalytic studies under vacuum conditions or in high-pressure O-rich environment at 450 K have reported two different active phases with iron in two different valence states, invoking a possible pressure gap. To identify the active phase for low-temperature CO oxidation and PROX, it is necessary to investigate the stability and activity of FeO/Pt(111) under the realistic reaction conditions. We thus conducted an in situ study on FeO/Pt(111) from ultrahigh vacuum to the atmospheric pressure of reactant gases. Our study shows FeO islands were easily oxidized in 1 Torr O2 to form the trilayer FeO2 islands. However, the presence of 2 Torr CO could prevent the oxidation of FeO islands and lead to CO oxidation at the FeO/Pt(111) interface. The FeO/Pt(111) surface exhibits an excellent activity for CO2 production with an initial reaction rate measured to be ∼1 × 1014 molecules·cm–2·s–1 at 300 K. FeO islands supported on Pt(111) were further investigated in the PROX gas, i.e., the mixture of 98.5% H2, 1% CO, and 0.5% O2, at elevated pressures up to 1 bar. Our results thus bridged the pressure gap and identified the bilayer FeO islands on Pt(111) as the active phase for PROX under the realistic reaction conditions.

The electrochemical CO2 reduction reaction (CO2RR) typically uses transition metals as the catalysts. To improve the efficiency, tremendous efforts have been dedicated to tuning the morphology, size, and structure of metal catalysts and employing electrolytes that enhance the adsorption of CO2. We report here a strategy to enhance CO2RR by constructing the metal–oxide interface. We demonstrate that Au–CeOx shows much higher activity and Faradaic efficiency than Au or CeOx alone for CO2RR. In situ scanning tunneling microscopy and synchrotron-radiation photoemission spectroscopy show that the Au–CeOx interface is dominant in enhancing CO2 adsorption and activation, which can be further promoted by the presence of hydroxyl groups. Density functional theory calculations indicate that the Au–CeOx interface is the active site for CO2 activation and the reduction to CO, where the synergy between Au and CeOx promotes the stability of key carboxyl intermediate (*COOH) and thus facilitates CO2RR. Similar interface-enhanced CO2RR is further observed on Ag–CeOx, demonstrating the generality of the strategy for enhancing CO2RR.

The adsorption of CO on Pt group metals, as a most fundamental elementary reaction step, has been widely studied in catalysis and electrocatalysis. Particularly, the structures of CO on Pt(111) have been extensively investigated, owing to its importance to both fundamental and applied catalysis. Yet, much less is known regarding CO adsorption on a Pt(111) surface modulated by supported oxide nanostructures, which is of more relevance to technical catalysis. We thus investigated the coverage-dependent adsorption of CO on a Pt(111) surface partially covered by FeOx nanostructures, which has been demonstrated as a remarkable catalyst for low-temperature CO oxidation. We found that, due to its strong chemisorption, the coverage-dependent structure of CO on bare Pt is not influenced by the presence of FeOx. But, oxygen-terminated FeOx nanostructures could modulate the diffusivity of CO at their vicinity, and thus affect the formation of ordered CO superstructures at low temperatures. Using scanning tunneling microscopy (STM), we inspected the diffusivity of CO, followed the phase transitions of CO domains, and resolved the molecular details of the coverage-dependent CO structures. Our results provide a full picture for CO adsorption on a Pt(111) surface modulated by oxide nanostructures and shed lights on the inter-adsorbate interaction on metal surfaces.Download high-res image (224KB)Download full-size imageCO adsorption on a Pt(111) surface partially covered by FeOx nanostructures displayed coverage-dependent structures on the bare Pt surface. Oxygen-terminated FeOx nanostructures could affect the formation of ordered CO superstructures at low temperatures.

•Local atomic and electronic structure of the Fe-N-C catalyst characterized by STM and STS.•The combination of air-AFM, UHV-STM and DFT calculations for the characterization of powder catalysts.•The selection of solvent is vital to the homogeneous dispersion of powder catalyst on a planar support.Atomic scale characterization of the surface structure of powder catalysts is essential to the identification of active sites, but remains a major challenge in catalysis research. We described here a procedure that combines atomic force microscopy (AFM), operated in air, and scanning tunneling microscopy (STM), operated in UHV, to obtain the atomic structure and local electronic properties of powder catalysts. The atomically dispersed Fe-N-C catalyst was used as an example, which was synthesized by low temperature ball milling methods. We discussed the effect of solvents in the dispersion of powder catalysts on a planar support, which is key to the subsequent atomic characterization. From the morphology, atomic structure and local electronic properties of the Fe-N-C catalyst, our combined measurements also provide an insight for the effect of ball milling in the preparation of atomically dispersed metal catalysts.