Dynamic transformation between bilayer islands and dinuclear clusters of Cr oxide on Au(111) through environment and interface effects.

SignificanceFor oxide catalysts, it is important to elucidate and further control their atomic structures. In this work, well-defined CrO2 bilayer islands and Cr2O7 dinuclear clusters have been grown on Au(111) and unambiguously identified by scanning tunneling microscopy and theoretical calculations. Upon cycled redox treatments, the two kinds of oxide nanostructures can be reversibly transformed. It is interesting to note that both Cr oxides do not exist in bulk but need to be stabilized by the metal surface and the specific environment. Our results suggest that both redox atmosphere and interface confinement effects can be used to construct an oxide nanostructure with the specific chemical state and structure.

Confinement-Induced Indium Oxide Nanolayers Formed on Oxide Support for Enhanced CO2 Hydrogenation Reaction.

An enclosed nanospace often shows a significant confinement effect on chemistry within its inner cavity, while whether an open space can have this effect remains elusive. Here, we show that the open surface of TiO2 creates a confined environment for In2O3 which drives spontaneous transformation of free In2O3 nanoparticles in physical contact with TiO2 nanoparticles into In oxide (InOx) nanolayers covering onto the TiO2 surface during CO2 hydrogenation to CO. The formed InOx nanolayers are easy to create surface oxygen vacancies but are against over-reduction to metallic In in the H2-rich atmospheres, which thus show significantly enhanced activity and stability in comparison with the pure In2O3 catalyst. The formation of interfacial In-O-Ti bonding is identified to drive the In2O3 dispersion and stabilize the metastable InOx layers. The InOx overlayers with distinct chemistry from their free counterpart can be confined on various oxide surfaces, demonstrating the important confinement effect at oxide/oxide interfaces.

Enhanced Methanol Synthesis over Self-Limited ZnOx Overlayers on Cu Nanoparticles Formed via Gas-Phase Migration Route.

Supported metal catalysts are widely used for chemical conversion, in which construction of high density metal-oxide or oxide-metal interface is an important means to improve their reaction performance. Here, Cu@ZnOx encapsulation structure has been in situ constructed through gas-phase migration of Zn species from ZnO particles onto surface of Cu nanoparticles under CO2 hydrogenation atmosphere at 450 °C. The gas-phase deposition of Zn species onto the Cu surface and growth of ZnOx overlayer is self-limited under the high temperature and redox gas (CO2 /H2 ) conditions. Accordingly, high density ZnOx -Cu interface sites can be effectively tailored to have an enhanced activity in CO2 hydrogenation to methanol. This work reveals a new route for the construction of active oxide-metal interface and classic strong metal-support interaction state through gas-phase migration of support species induced by high temperature redox reaction atmosphere.

Deciphering the effects of electrolyte concentration on the performance of lithium batteries by correlative surface characterization.

The formation of solid electrolyte interphase (SEI) and lithium ion intercalation are two crucial processes in lithium ion batteries. Given the complexity and challenges involved in investigating real batteries, a combination of model batteries comprising well-defined ultrathin graphite electrodes with surface-sensitive techniques can provide valuable information on these processes. Here, a comparative study of the performance of batteries using regular- and high-concentration electrolytes (RCE and HCE) is presented, aided by a myriad of correlative surface science techniques. It is found that the SEI formed in HCE possesses a thinner organic layer and more inorganic substances. Such an effective SEI protects the graphite electrode and facilitates the processes of Li+ intercalation/deintercalation, thereby improving the reversibility and cycling stability of the battery, superior to those with RCE. Moreover, the battery performance can be readily tuned by switching the electrolytes. The good cycling stability of the battery can still be maintained in RCE after the preformation of a robust SEI on the ultrathin graphite electrode in HCE. This work highlights the importance of electrolyte-modulated SEI formation to battery performance and provides powerful platforms to comprehensively study the SEI formation and ion intercalation processes.

In Situ Visualization of Atmosphere-Dependent Relaxation and Failure in Energy Storage Electrodes.

Ambient atmosphere is critical for the surface/interface chemistry of electrodes that governs the operation and failure in energy storage devices (ESDs). Here, taking an Al/graphite battery as an example, both the relaxation and failure processes in the working graphite electrodes have been dynamically monitored by multiple in situ surface and interface characterization methods within various well-controlled atmospheres. Relaxation effects are manifested by recoverable stage-structure change and electronic relaxation occurring in anhydrous inert atmospheres and O2, which are induced by the anion/cation redistribution within the neighboring graphene layers and have slight influence on the long-term cycling. In contrast, rapid and unrecoverable failure behaviors happen in hydrous atmospheres as shown by the stage-structure degradation and electronic decoupling between guest ions and host graphite, which are caused by the hydrolysis between newly intercalated H2O molecules and intercalants. Consistent with the characterization results, exposure to H2O can cause nearly 100% capacity loss. The methodology and concept adopted in this work to unravel the battery mechanism under ambient conditions are universal and significant to investigate many ESDs.

Structural transformation of h-BN overlayers on Pt(111) in oxidative atmospheres.

Interaction of hexagonal boron nitride (h-BN) with gases is of great importance for its properties and applications. In the present work, the structural changes of h-BN overlayers on Pt(111) in oxidative atmospheres including O2 and NO2 have been investigated by using low energy electron microscopy, Auger electron spectroscopy, X-ray photoelectron spectroscopy (XPS), and near ambient pressure XPS. We find that h-BN islands can be intercalated by oxygen in 10-6 Torr O2 at 200 °C, while oxygen intercalation of full layer h-BN around 200 °C requires near ambient pressure O2 (0.1 Torr) or such a strong oxidant as NO2 (10-6 Torr). h-BN overlayers can be etched away in the gases at much high temperatures, e.g. 800 °C. Upon mild oxidation in O2 or NO2 at temperatures of 400-450 °C, h-BN is transformed to boron oxide (BOx) overlayers, which can be converted back to h-BN by heating in NH3 at 800 °C.

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.

Influence of CH···N Interaction in the Self-Assembly of an Oligo(isoquinolyne-ethynylyne) Molecule with Distinct Conformational States.

Molecular conformational flexibility can play an important role in supramolecular self-assembly on surfaces, affecting not least chiral molecular assemblies. To explicitly and systematically investigate the role of molecular conformational flexibility in surface self-assembly, we synthesized a three-bit conformational switch where each of three switching units on the molecules can assume one of two distinct binary positions on the surface. The molecules are designed to promote C-H···N type hydrogen bonds between the switching units. While supramolecular self-assembly based on strong hydrogen-bonding interactions has been widely explored, less is known about the role of such weaker directional interactions for surface self-assembly. The synthesized molecules consist of three nitrogen-containing isoquinoline (IQ) bits connected by ethynylene spokes and terminated by tert-butyl (tBu) groups. Using high-resolution scanning tunnelling microscopy, we investigate the self-assembly of the IQ-tBu molecules on a Au(111) surface under ultrahigh-vacuum conditions. The molecules form extended domains of brick-wall structure where the molecular backbones are packed regularly but without selection of specific molecular conformations. However, statistical analysis of the extended network demonstrates alignment/correlation for the orientations of the switching units indicating specific interactions. The primary interaction motifs in the structure are quantified from DFT calculations, showing that the brick-wall structure is indeed stabilized by two types of weak C-H···N bonds, involving either aromatic hydrogens on the IQ groups or nonaromatic hydrogens on the tBu groups. Analysis of the C-H···N interactions in the brick-wall structure explains the observed distribution and alignment of molecular conformations as well as the overall organization of the molecular surface structures.

External Voltage-Induced Restructuring of the Solid-State Electrode/Electrolyte Interface Revealed by X-ray Photoelectron Spectroscopy Depth Profiling Analysis.

Solid/solid interfaces between electrodes and electrolytes play an important role in all-solid-state energy devices, while microscopic investigations of the buried interfaces remain challenging. Here, we construct metal|yttria-stabilized zirconia (YSZ)|Au model cells consisting of a metal film cathode (metal (M) = Au, Ni, and Ag), a single crystalline YSZ electrolyte, and a Au film anode, and use quasi in situ X-ray photoelectron spectroscopy depth profiling analysis to investigate the restructuring of buried interfaces between metal cathodes and YSZ. After applying 2.9 V at 500 °C, interfacial Zr4+ ions in the electrolyte are reduced and then interdiffuse with metal cathode overlayers, forming a miscible ZrM alloy interlayer. The interface restructuring degree follows the sequence of Au|YSZ|Au > Ni|YSZ|Au > Ag|YSZ|Au. Meanwhile, surface segregation of Zr on the cathode surface is also observed, whose degree follows the sequence of Ag|YSZ|Au > Ni|YSZ|Au > Au|YSZ|Au. Notably, the strong ZrM alloy formation enhances the interface restructuring but suppresses the Zr surface segregation. This work provides a fundamental understanding of the interfacial reaction at the buried electrode/electrolyte interface.

Water-assisted oxidative redispersion of Cu particles through formation of Cu hydroxide at room temperature.

Sintering of active metal species often happens during catalytic reactions, which requires redispersion in a reactive atmosphere at elevated temperatures to recover the activity. Herein, we report a simple method to redisperse sintered Cu catalysts via O2-H2O treatment at room temperature. In-situ spectroscopic characterizations reveal that H2O induces the formation of hydroxylated Cu species in humid O2, pushing surface diffusion of Cu atoms at room temperature. Further, surface OH groups formed on most hydroxylable support surfaces such as γ-Al2O3, SiO2, and CeO2 in the humid atmosphere help to pull the mobile Cu species and enhance Cu redispersion. Both pushing and pulling effects of gaseous H2O promote the structural transformation of Cu aggregates into highly dispersed Cu species at room temperature, which exhibit enhanced activity in reverse water gas shift and preferential oxidation of carbon monoxide reactions. These findings highlight the important role of H2O in the dynamic structure evolution of supported metal nanocatalysts and lay the foundation for the regeneration of sintered catalysts under mild conditions.

Reversible structural transformation of FeO(x) 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.

Oxide-Metal Interaction Probed by Scanning Tunneling Microscope Manipulation of Cr2O7 Clusters on Au(111).

Interfacial interaction plays a crucial rule in catalysis over supported catalysts, and the catalyst-support interaction needs to be explored at microscopic scale. Here, we use the scanning tunneling microscope (STM) tip to manipulate Cr2O7 dinuclear clusters on Au(111) and find that the Cr2O7-Au interaction can be weakened by an electric field in the STM junction, facilitating rotation and translation of the individual clusters at the imaging temperature (78 K). Surface alloying with Cu makes the manipulation of the Cr2O7 clusters hard due to the enhanced Cr2O7-substrate interaction. Density functional theory calculations reveal that barrier for translation of a Cr2O7 cluster on the surface can be increased by surface alloying, influencing the tip manipulation. Our study demonstrates that the oxide-metal interfacial interaction can be probed by STM tip manipulation of supported oxide clusters, which provides a new method to investigate the interfacial interaction.

Periodic Arrays of Metal Nanoclusters on Ultrathin Fe-Oxide Films Modulated by Metal-Oxide Interactions.

Rational design of highly stable and active metal catalysts requires a deep understanding of metal-support interactions at the atomic scale. Here, ultrathin films of FeO and FeO2-x grown on Pt(111) are used as templates for the construction of well-defined metal nanoclusters. Periodic arrays of Cu clusters in the form of monomers and trimers are preferentially located at FCC domains of FeO/Pt(111) surface, while the selective location of Cu clusters at FeO2 domains is observed on FeO2-x /Pt(111) surface. The preferential nucleation and formation of well-ordered Cu clusters are driven by different interactions of Cu with the Fe oxide domains in the sequence of FeO2-FCC > FeO-FCC > FeO-HCP > FeO-TOP, which is further validated by density functional theory calculations. It has been revealed that the p-band center as a reactivity descriptor of surface O atoms determines the interaction between metal adatoms and Fe oxides. The modulated metal-oxide interaction provides guidance for the rational design of supported single-atom and nanocluster catalysts.

Ferrous centers confined on core-shell nanostructures for low-temperature CO oxidation.

A noble metal (NM) can stabilize monolayer-dispersed surface oxide phases with metastable nature. The formed "oxide-on-metal" inverse catalyst presents better catalytic performance than the NM because of the introduction of coordinatively unsaturated cations at the oxide-metal boundaries. Here we demonstrate that an ultrathin NM layer grown on a non-NM core can impose the same constraint on the supported oxide as the bulk NM. Cu@Pt core-shell nanoparticles (NPs) decorated with FeO patches use much less Pt but exhibit performance similar to that of Pt NPs covered with surface FeO patches in the catalytic oxidation of CO. The "oxide-on-core@shell" inverse catalyst system may open a new avenue for the design of advanced nanocatalysts with decreased usage of noble metals.

Self-assembly of manganese phthalocyanine on Pb(111) surface: a scanning tunneling microscopy study.

The self-assembled structure of submonolayer manganese phthalocyanine (MnPc) on Pb(111) surface is investigated by using low-temperature scanning tunneling microscopy (STM). A "holelike" superlattice, which is superimposed on the self-assembled nearly quadratic network, is observed. High resolution STM images reveal that there are two distinct azimuthal orientations of MnPc molecules. It is found that by taking the two different orientations the self-assembly can further be optimized energetically by maximizing intermolecular orbital overlapping. It is this intralayer energy minimization process that leads to the characteristic holelike superlattice.

Operando surface science methodology reveals surface effect in charge storage electrodes.

Surface and interface play critical roles in energy storage devices, calling for operando characterization techniques to probe the electrified surfaces/interfaces. In this work, surface science methodology, including electron spectroscopy and scanning probe microscopy, has been successfully applied to visualize electrochemical processes at operating electrode surfaces in an Al/graphite model battery. Intercalation of anions together with cations is directly observed in the surface region of a graphite electrode with tens of nanometers thickness, the concentration of which is one order higher than that in bulk. An intercalation pseudocapacitance mechanism and a double specific capacity in the electrode surface region are expected based on the super-dense intercalants and anion/cation co-intercalation, which are in sharp contrast to the battery-like mechanism in the electrode bulk. The distinct electrochemical mechanism at the electrode surface is verified by performance tests of real battery devices, showing that a surface-dominant, nanometer-thick graphite cathode outperforms a bulk-dominant, micrometer-thick graphite cathode. Our findings highlight the important surface effect of working electrodes in charge storage systems.

In situ identification of the metallic state of Ag nanoclusters in oxidative dispersion.

Oxidative dispersion has been widely used in regeneration of sintered metal catalysts and fabrication of single atom catalysts, which is attributed to an oxidation-induced dispersion mechanism. However, the interplay of gas-metal-support interaction in the dispersion processes, especially the gas-metal interaction has not been well illustrated. Here, we show dynamic dispersion of silver nanostructures on silicon nitride surface under reducing/oxidizing conditions and during carbon monoxide oxidation reaction. Utilizing environmental scanning (transmission) electron microscopy and near-ambient pressure photoelectron spectroscopy/photoemission electron microscopy, we unravel a new adsorption-induced dispersion mechanism in such a typical oxidative dispersion process. The strong gas-metal interaction achieved by chemisorption of oxygen on nearly-metallic silver nanoclusters is the internal driving force for dispersion. In situ observations show that the dispersed nearly-metallic silver nanoclusters are oxidized upon cooling in oxygen atmosphere, which could mislead to the understanding of oxidation-induced dispersion. We further understand the oxidative dispersion mechanism from the view of dynamic equilibrium taking temperature and gas pressure into account, which should be applied to many other metals such as gold, copper, palladium, etc. and other reaction conditions.

Tailoring phthalocyanine metalation reaction by quantum size effect.

We here report our experimental study of the quantum size effect modulated metalation reaction of phthalocyanine (H(2)Pc) by low temperature scanning tunneling microscopy. When iron atoms were deposited onto Pb(111) thin films (2-5 nm thick) precovered by a self-assembled H(2)Pc monolayer, a surface metalation reaction to iron phthalocyanine (FePc) was observed. The amount of the FePc products was found to change prominently whenever the film thickness varies by one atomic layer and exhibits thickness-dependent oscillatory behavior. We show that the oscillation can be well-understood by the quantum size effect in the Pb thin films. The present study gives direct proof for tailoring a surface chemical reaction by quantum confinement.

Global and local expression of chirality in serine on the Cu{110} surface.

Establishing a molecular-level understanding of enantioselectivity and chiral resolution at the organic-inorganic interfaces is a key challenge in the field of heterogeneous catalysis. As a model system, we investigate the adsorption geometry of serine on Cu{110} using a combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The chirality of enantiopure chemisorbed layers, where serine is in its deprotonated (anionic) state, is expressed at three levels: (i) the molecules form dimers whose orientation with respect to the substrate depends on the molecular chirality, (ii) dimers of L- and D-enantiomers aggregate into superstructures with chiral (-1 ∓2; 4 0) lattices, respectively, which are mirror images of each other, and (iii) small islands have elongated shapes with the dominant direction depending on the chirality of the molecules. Dimer and superlattice formation can be explained in terms of intra- and interdimer bonds involving carboxylate, amino, and ß-OH groups. The stability of the layers increases with the size of ordered islands. In racemic mixtures, we observe chiral resolution into small ordered enantiopure islands, which appears to be driven by the formation of homochiral dimer subunits and the directionality of interdimer hydrogen bonds. These islands show the same enantiospecific elongated shapes those as in low-coverage enantiopure layers.

Tunable deep ultraviolet laser based near ambient pressure photoemission electron microscope for surface imaging in the millibar regime.

A newly developed instrument comprising a near ambient pressure (NAP) photoemission electron microscope (PEEM) and a tunable deep ultraviolet (DUV) laser source is described. This NAP-PEEM instrument enables dynamic imaging of solid surfaces in gases at pressures up to 1 mbar. A diode laser (976 nm) can illuminate a sample from the backside for in situ heating in gases up to 1200 K in minutes. The DUV laser with a tunable wavelength between 175 nm and 210 nm is perpendicularly incident onto the sample surface for PEEM imaging of a wide spectrum of solids with different surface work functions. Using this setup, we have first demonstrated spatiotemporal oscillation patterns of CO oxidation reaction on Pt(110) from high vacuum to NAPs and gas-induced restructuring of metal nanostructures in millibar gases. The new facility promises important applications in heterogeneous catalysis, electrochemical devices, and other surface processes under nearly working conditions.