Yanxiao Ning

Reversible structural transformation of FeOx nanostructures on Pt under cycling redox conditions and its effect on oxidation catalysis

Understanding dynamic changes of catalytically active nanostructures under reaction conditions is a pivotal challenge in catalysis research, which has been extensively addressed in metal nanoparticles but is less explored in supported oxide nanocatalysts. Here, structural changes of iron oxide (FeO(x)) nanostructures supported on Pt in a gaseous environment were examined by scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and in situ X-ray absorption spectroscopy using both model systems and real catalysts. O-Fe (FeO) bilayer nanostructures can be stabilized on Pt surfaces in reductive environments such as vacuum conditions and H2-rich reaction gas, which are highly active for low temperature CO oxidation. In contrast, exposure to H2-free oxidative gases produces a less active O-Fe-O (FeO2) trilayer structure. Reversible transformation between the FeO bilayer and FeO2 trilayer structures can be achieved under alternating reduction and oxidation conditions, leading to oscillation in the catalytic oxidation performance.

Enhanced oxidation resistance of active nanostructures via dynamic size effect

A major challenge limiting the practical applications of nanomaterials is that the activities of nanostructures (NSs) increase with reduced size, often sacrificing their stability in the chemical environment. Under oxidative conditions, NSs with smaller sizes and higher defect densities are commonly expected to oxidize more easily, since high-concentration defects can facilitate oxidation by enhancing the reactivity with O2 and providing a fast channel for oxygen incorporation. Here, using FeO NSs as an example, we show to the contrary, that reducing the size of active NSs can drastically increase their oxidation resistance. A maximum oxidation resistance is found for FeO NSs with dimensions below 3.2 nm. Rather than being determined by the structure or electronic properties of active sites, the enhanced oxidation resistance originates from the size-dependent structural dynamics of FeO NSs in O2. We find this dynamic size effect to govern the chemical properties of active NSs.

Unique Transformation from Graphene to Carbide on Re(0001) Induced by Strong Carbon–Metal Interaction

During graphene growth on various transition metals in the periodic table, metal carbides always emerge to behave as complex intermediates. On VIII metals, metastable carbides usually evolve and then transform into graphene along the phase interfaces, and even no metal carbides can form on IB-IIB metals. In contrast, during graphene growth on group IVB-VIB metals, carbides are usually generated even before the evolution of graphene and stably exist throughout the whole growth process. However, for the remaining transition metals, e.g., group VIIB, located in between IVB-VIB and VIII, the interplay between graphene and carbide is still vague. Herein, on Re(0001) (VIIB), we have revealed a novel transition from graphene to metal carbide (reverse to that on VIII metals) for the first time. This transition experienced graphene decomposition, dissolution, and carbon segregation processes, as evidenced by scanning tunneling microscopy (STM) and on-site, variable-temperature low electron energy diffraction (LEED) characterizations. This work thus completes the picture about the interplay between graphene and carbide on/in transition metals in the periodic table, as well as discloses a new territory for the growth of carbon-related materials, especially the metal carbide.

Structure and Electronic Properties of Interface-Confined Oxide Nanostructures

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.

Abnormal growth kinetics of h-BN epitaxial monolayer on Ru(0001) enhanced by subsurface Ar species

Growth kinetics of epitaxial films often follows the diffusion-limited aggregation mechanism, which shows a “fractal-to-compact” morphological transition with increasing growth temperature or decreasing deposition flux. Here, we observe an abnormal “compact-to-fractal” morphological transition with increasing growth temperature for hexagonal boron nitride growth on the Ru(0001) surface. The unusual growth process can be explained by a reaction-limited aggregation (RLA) mechanism. Moreover, introduction of the subsurface Ar atoms has enhanced this RLA growth behavior by decreasing both reaction and diffusion barriers. Our work may shed light on the epitaxial growth of two-dimensional atomic crystals and help to control their morphology.Growth kinetics of epitaxial films often follows the diffusion-limited aggregation mechanism, which shows a “fractal-to-compact” morphological transition with increasing growth temperature or decreasing deposition flux. Here, we observe an abnormal “compact-to-fractal” morphological transition with increasing growth temperature for hexagonal boron nitride growth on the Ru(0001) surface. The unusual growth process can be explained by a reaction-limited aggregation (RLA) mechanism. Moreover, introduction of the subsurface Ar atoms has enhanced this RLA growth behavior by decreasing both reaction and diffusion barriers. Our work may shed light on the epitaxial growth of two-dimensional atomic crystals and help to control their morphology.