Epitaxial growth of graphene on Ru(0001) was investigated by photoemission electron microscopy (PEEM) and scanning tunneling microscopy (STM). By connecting the mesoscopic length scale of PEEM and the microscopic length scale of STM, we show that graphene overlayers with sizes ranging from nanometers to sub-millimeters have been prepared on Ru(0001) in a well-controllable fashion. From the systematic investigation of different methods to grow graphene on Ru(0001), the dominant factors in the graphene growth process have been revealed, which enables to grow graphene on transition metal surfaces in a controllable way. Additionally, the dynamic process of graphene etching by oxygen at temperatures between 600−1000 K was studied by in situ PEEM. The reaction kinetics results show that decrease in the graphene overlayers size is linearly dependent on the reaction time, indicating a reaction-controlled process. The catalytic effect of Ru substrate facilitates graphene oxidation, which shows relatively low activation energy of 27.2 kJ/mol.

Graphene layers are often exposed to gaseous environments in their synthesis and application processes, and interactions of graphene surfaces with molecules particularly H2 and O2 are of great importance in their physico-chemical properties. In this work, etching of graphene overlayers on Pt(111) in H2 and O2 atmospheres were investigated by in-situ low energy electron microscopy. Significant graphene etching was observed in 10-5 Torr H2 above 1023 K, which occurs simultaneously at graphene island edges and interiors with a determined reaction barrier at 5.7 eV. The similar etching phenomena were found in 10-7 Torr O2 above 973 K, while only island edges were reacted between 823 and 923 K. We suggest that etching of graphene edges is facilitated by Pt-aided hydrogenation or oxidation of edge carbon atoms while intercalation-etching is attributed to etching at the interiors at high temperatures. The different findings with etching in O2 and H2 depend on competitive adsorption, desorption, and diffusion processes of O and H atoms on Pt surface, as well as intercalation at the graphene/Pt interface.

Two-dimensional (2D) materials are characterised by their strong intraplanar bonding but weak interplanar interaction. Interfaces between neighboring 2D layers or between 2D overlayers and substrate surfaces provide intriguing confined spaces for chemical processes, which have stimulated a new area of “chemistry under 2D cover”. In particular, well-defined 2D material overlayers such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have been deposited on solid surfaces, which can be used as model systems to understand the new chemistry. In the present review, we first show that many atoms and molecules can intercalate ultrathin 2D materials supported on solid surfaces and the space under the 2D overlayers has been regarded as a 2D nanocontainer. Moreover, chemical reactions such as catalytic reactions, surface adlayer growth, chemical vapor deposition, and electrochemical reactions occur in the 2D confined spaces, which further act as 2D nanoreactors. It has been demonstrated that surface chemistry and catalysis are strongly modulated by the 2D covers, resulting in weakened molecule adsorption and enhanced surface reactions. Finally, we conclude that the confinement effect of the 2D cover leads to new chemistry in a small space, such as “catalysis under cover” and “electrochemistry under cover”. These new concepts enable us to design advanced nanocatalysts encapsulated with 2D material shells which may present improved performance in many important processes of heterogeneous catalysis, electrochemistry, and energy conversion.

The development of low-cost and high-performance electrocatalysts remains a challenge for the hydrogen oxidation reaction (HOR) in alkaline membrane fuel cells. Here, we have reported novel Ni@h-BN core–shell nanocatalysts consisting of nickel nanoparticles encapsulated in few-layer h-BN shells. The Ni@h-BN catalysts exhibit an improved HOR performance compared with the bare Ni nanoparticles. In situ characterization experiments and density functional theory calculations indicate that the interactions of the O, H, and OH species with the Ni surface under the h-BN shell are weakened, which helps to maintain the active metallic Ni phase both in air and in the electrolyte and strengthen the HOR processes occurring at the h-BN/Ni interfaces. These results suggest a new route for designing high-performance non-noble metal electrocatalysts with encapsulating two-dimensional material overlayers for HOR reactions.

Confined microenvironments formed in heterogeneous catalysts have recently been recognized as equally important as catalytically active sites. Understanding the fundamentals of confined catalysis has become an important topic in heterogeneous catalysis. Well-defined 2D space between a catalyst surface and a 2D material overlayer provides an ideal microenvironment to explore the confined catalysis experimentally and theoretically. Using density functional theory calculations, we reveal that adsorption of atoms and molecules on a Pt(111) surface always has been weakened under monolayer graphene, which is attributed to the geometric constraint and confinement field in the 2D space between the graphene overlayer and the Pt(111) surface. A similar result has been found on Pt(110) and Pt(100) surfaces covered with graphene. The microenvironment created by coating a catalyst surface with 2D material overlayer can be used to modulate surface reactivity, which has been illustrated by optimizing oxygen reduction reaction activity on Pt(111) covered by various 2D materials. We demonstrate a concept of confined catalysis under 2D cover based on a weak van der Waals interaction between 2D material overlayers and underlying catalyst surfaces.

•Graphene (Gr)/transition metal (TM: Fe, Co, Pt, and Au) interfaces form through TM intercalation at Gr/Ru(0001) surface.•Graphene-metal interaction strength follows the order of Ru ≈ Fe ≈ Co > Pt > Au.•Oxygen intercalation occurs at Gr/Fe, Gr/Co, Gr/Pt, and Gr/Ru interfaces but not at Gr/Au interface in air around 100 °C.Interaction between graphene (Gr) and metal plays an important role in physics and chemistry of graphene/metal interfaces. In this work, well-defined interfaces between graphene and transition metals (TMs) including Fe, Co, Pt, and Au were prepared through TM intercalation on Gr/Ru(0001) surface. The Gr-metal interaction was investigated using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy. We found that graphene interacts most strongly with Ru, Fe and Co and most weakly with Au, following the order of Ru ≈ Fe ≈ Co > Pt > Au. The Gr/Fe, Gr/Co, Gr/Pt, and Gr/Ru interfaces can be readily intercalated by oxygen when exposed to air and illuminated by an infrared lamp. In contrast, oxygen intercalation does not happen at the Gr/Au interface under the same condition. It is suggested that both Gr-metal interaction and oxygen adsorption on the underlying metal surface are critical in the oxygen intercalation and the Gr/metal interface stability.Download high-res image (135KB)Download full-size image

Encapsulation of metal nanoparticles with porous oxide shells is a successful strategy to design catalysts with high catalytic performance. We suggest an alternative route to cover metal nanoparticles with two-dimensional (2D) material shells such as hexagonal boron nitride (h-BN), in which active metal components are stabilized by the outer shells and meanwhile catalytic reactions occur at interfaces between cores and shells through feasible intercalation of the 2D material covers. As an illustration, Ni nanoparticles encapsulated with few-layer h-BN shells were constructed and applied in syngas methanation. Ni@h-BN core–shell nanocatalysts exhibit enhanced methanation activity, higher resistance to particle sintering, and suppressed carbon deposition and Ni loss in reactions. Surface science studies in h-BN/Ni(111) model systems and chemisorption data confirm the occurrence of methanation reactions on Ni surfaces under h-BN cover. The confinement effect of h-BN shells improves Ni-catalyzed reaction activity and Ni catalyst stability.Keywords: core−shell; hexagonal boron nitride (h-BN); intercalation; nickel; syngas methanation

It has been empirically established that graphitic carbon deposits often result in deactivation of metal catalysts due to the physical blockage of surface active sites. Our recent surface science works however demonstrate that molecules such as CO can adsorb on Pt(111) surface covered by graphene overlayers via an intercalation process, and surface reactions e.g. CO oxidation have been enhanced by the graphene covers. In this work, supported Pt nanocatalysts were coated by ultrathin graphitic carbon layers through chemical vapor deposition process forming Pt@C core–shell nanostructures, which were confirmed by characterizations of Raman spectroscopy, temperature-programmed oxidation and transmission electron microscopy. CO oxidation over the Pt@C catalysts shows a lower apparent activation energy compared with the pure Pt catalysts, and in-situ infrared studies indicate that the reactions occur under the graphitic shells. The present results suggest that coating metal nanocatalysts with ultrathin graphitic overlayers may be used to promote metal catalyzed reactions.

H2 atmosphere is often involved in growth and application of two-dimensional (2D) atomic crystals, and it is of great importance to understand interaction of the 2D materials with H2 molecules. Here, a full graphene layer and a full hexagonal boron nitride (h-BN) layer grown on Pt(111) were exposed to H2 atmosphere, which were investigated by in situ near ambient pressure X-ray photoelectron spectroscopy and quasi in situ ultraviolet photoelectron spectroscopy. We confirm the occurrence of hydrogen intercalation of the graphene and h-BN overlayers in ambient pressure H2. The hydrogen intercalation in 0.1 Torr H2 at room temperature and hydrogen desorption in 0.1 Torr H2 at 200 °C are fully reversible on the graphene/Pt(111) and h-BN/Pt(111) surfaces. Furthermore, hydrogen desorption on the graphene/Pt(111) and h-BN/Pt(111) surfaces was found to happen at lower temperature than that on the Pt(111) surface due to the graphene and h-BN cover effect.

The orientation control of graphene overlayers on metal surface is an important issue which remains as a challenge in graphene growth on Ni surface. Here we have demonstrated that epitaxial graphene overlayers can be obtained by annealing a nickel carbide covered Ni(111) surface using in situ surface imaging techniques. Epitaxial graphene islands nucleate and grow via segregation of dissolved carbon atoms to the top surface at about 400 °C. This is in contrast to a mixture of epitaxial and non-epitaxial graphene domains grown directly on Ni(111) at 540 °C. The different growth behaviors are related to the nucleation dynamics which is controlled by local carbon densities in the near surface region.在金属基底上制备石墨烯的过程中,石墨烯取向的控制是一个重要的科学问题。利用原位表面成像技术发现碳化镍覆盖的Ni(111)表面进行退火处理时可以得到外延石墨烯结构,在400°C以上溶解到体相和次表层中的碳原子偏析到表面,成核并长大成为外延石墨烯岛。与之形成鲜明对照的是,Ni(111)在540°C直接暴露乙烯得到的是外延和非外延石墨烯的混合结构。我们认为两种生长模式的区别在于近表面区域的局域碳浓度差异导致了不同的成核生长动力学。

In heterogeneous catalysis molecule–metal interaction is often modulated through structural modifications at the surface or under the surface of the metal catalyst. Here, we suggest an alternative way toward this modulation by placing a two-dimensional (2D) cover on the metal surface. As an illustration, CO adsorption on Pt(111) surface has been studied under 2D hexagonal boron nitride (h-BN) overlayer. Dynamic imaging data from surface electron microscopy and in situ surface spectroscopic results under near ambient pressure conditions confirm that CO molecules readily intercalate monolayer h-BN sheets on Pt(111) in CO atmosphere but desorb from the h-BN/Pt(111) interface even around room temperature in ultrahigh vacuum. The interaction of CO with Pt has been strongly weakened due to the confinement effect of the h-BN cover, and consequently, CO oxidation at the h-BN/Pt(111) interface was enhanced thanks to the alleviated CO poisoning effect.

•Intercalation under full graphene layer on Ru(0001) in 0.5 Torr O2 and at 150 °C•Intercalation under full graphene layer in air with an infrared lamp illumination•Free standing graphene overlayers after the O intercalation•Easier oxygen desorption under the graphene coverGraphene coatings have been widely considered as protection layers on metal surfaces to prevent surface oxidation and corrosion in gaseous atmospheres. Here, using in-situ ambient pressure X-ray photoelectron spectroscopy we demonstrate that oxygen intercalation readily occurs at full monolayer graphene/Ru(0001) interfaces in 0.5 Torr O2 around 150 °C, resulting in decoupling of the graphene overlayer from the Ru surface and oxidation of the metal surface. Moreover, oxygen intercalation has been observed even upon illumination of the graphene/Ru(0001) surface with an infrared lamp in air. These results indicate that the stability of graphene/metal interfaces under ambient conditions should be taken into consideration for future applications.

Metastable oxide phases containing coordinatively unsaturated metal sites are highly active in many catalytic reactions. The stabilization of these nanostructures during reactions remains a major challenge. Here, we show that metastable two-dimensional (2D) FeO structures can be grown on Pt(111) and Au(111), but not on the graphene surface. The well-defined 2D structure is achieved by an interface confinement effect originating from the strong interfacial bonding between Fe atoms and substrate surface atoms. The stabilization effect has been described by the interface confinement energy (Econfinement), which is the energy difference lowered by interfacing the 2D structure with a substrate and decreases in the sequence of Pt(111) > Au(111) > graphene. This interface effect is widely present in many metal–oxide composite catalysts and can be used to guide the rational design of catalytically active sites.

Nickel carbide and graphene overlayers were grown on Ni(111), which were in situ monitored by near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and low energy electron microscopy. CO adsorption and desorption on the formed carbon-modified Ni(111) surfaces were further investigated by NAP-XPS. We found that the carbidic carbon weakens CO adsorption on Ni, resulting in quick CO desorption around room temperature. A full graphene layer on Ni(111) blocks CO adsorption in 10–6 Torr CO, while CO intercalates the graphene overlayers in 0.1 Torr CO at room temperature. On the graphene/CO/Ni(111) surface, the major part of intercalated CO molecules desorbs extensively around 90 °C from the graphene/Ni interface and the remaining part gets trapped under the graphene even at 200 °C. These results suggest that the surface reactivity of a metal catalyst can be strongly modulated by surface carbon structures.

The interface between a two-dimensional (2D) atomic crystal and a metal surface can be regarded as a nanoreactor, in which molecule adsorption and catalytic reactions may occur. In this work, we demonstrate that oxygen intercalation and desorption occur at the interface between hexagonal boron nitride (h-BN) overlayer and Pt(111) surface by using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS), photoemission electron microscopy, and low-energy electron microscopy. Furthermore, CO oxidation under the h-BN cover was also observed by NAP-XPS. The present results indicate that the nanospace under the 2D cover can be used for surface reactions, in which novel surface chemistry may be induced by the nanoconfinement effect.二维层状材料与金属表面所形成的界面可被视为一个微型的纳米反应器, 为气体的扩散及催化反应的发生提供了空间。六方氮化硼(h-BN)是一类典型的二维层状材料, 本文Pt(111)表面生长h-BN单层结构, 利用低能电子显微镜(LEEM)、光电发射电子显微镜(PEEM)和近常压X射线光电子能谱(NAP-XPS)等表面技术原位研究h-BN/Pt(111)表面暴露O2气氛的界面结构变化, 证实氧(O)在h-BN结构下的插层和脱附过程; 发现在相对较高的温度及较低的h-BN覆盖度更有利于气体在界面处的扩散. 此外, 我们还观察到在h-BN“盖子”限域下的Pt表面催化CO氧化反应.

Heterostructures of two-dimensional (2D) atomic crystals have attracted increasing attention, while fabrication of the 2D stacking structures remains a challenge. In this work, we present a route toward formation of 2D heterostructures via confined growth of a 2D adlayer underneath the other 2D overlayer. Taking a hexagonal boron nitride (h-BN) monolayer on Ni(111) as a model system, both epitaxial and nonepitaxial h-BN islands have been identified on the Ni surface. Surface science studies combined with density functional theory calculations reveal that the nonepitaxial h-BN islands interact weakly with the Ni(111) surface, which creates a 2D nanospace underneath the h-BN islands. An additional h-BN or graphene layer can be grown in the space between the nonepitaxial h-BN islands and Ni(111) surface, forming h-BN/h-BN bilayer structures and h-BN/graphene heterostructures. These results suggest that confined growth under 2D covers may provide an effective route to obtain stacks of 2D atomic crystals.Keywords: graphene; heterostructures; hexagonal boron nitride; LEEM; Ni(111);

Oxide nanostructures grown on noble metal surfaces are often highly active in many reactions, in which the oxide/metal interfaces play an important role. In the present work, we studied the surface structures of FeOx-on-Pt and NiOx-on-Pt catalysts and their activity to CO oxidation reactions using both model catalysts and supported nanocatalysts. Although the active FeO1−x structure is stabilized on the Pt surface in a reductive reaction atmosphere, it is prone to change to an FeO2−x structure in oxidative reaction gases and becomes deactivated. In contrast, a NiO1−x surface structure supported on Pt is stable in both reductive and oxidative CO oxidation atmospheres. Consequently, CO oxidation over the NiO1−x-on-Pt catalyst is further enhanced in the CO oxidation atmosphere with an excess of O2. The present results demonstrate that the stability of the active oxide surface phases depends on the stabilization effect of the substrate surface and is also related to whether the oxide exhibits a variable oxidation state.

Silica supported Pt–Co and Au–Co nanoparticles (NPs) were subjected to various redox processes and characterized by X-ray diffraction, X-ray absorption near edge structure, and X-ray photoelectron spectroscopy. We found that most of the Co oxide (CoOx) species on Pt NPs can be reduced at 100 °C forming an alloy structure with Pt at elevated temperatures. Oxidation of Co in the reduced sample takes place gradually with increasing temperatures. In contrast, temperatures higher than 400 °C are needed to reduce CoOx on Au NPs and Co atoms hardly form an alloy with Au even at 600 °C. The Co species in the reduced Au–Co/SiO2 sample were quickly oxidized in an O2 atmosphere at room temperature. High CO oxidation activity was observed in the Pt–Co/SiO2 catalyst reduced below 300 °C; however this necessitated reduction at 600 °C of the Au–Co/SiO2 catalyst. The results illustrate a stronger interaction of Co (CoOx) with Pt than with Au. In both systems, the optimum treatment conditions are to produce a similar CoO-on-noble metal (NM) active structure and maximize the density of interface sites between the surface CoO structure and the NM support.

In heterogeneous catalysis, graphitic carbon formed on metal often poisons metal-catalyzed reactions through physical blockage of surface active sites. In materials science, recent works show that graphene overlayers can passivate metal surfaces acting as gas-impermeable protection coatings. However, here we show using in situ surface electron microscopy and photoemission spectroscopy that CO can be readily trapped inside the two-dimensional space between the graphene overlayer and Ru(0001) surface under near-ambient conditions. The intercalated CO molecules effectively decouple the graphene overlayer from the Ru substrate. Meanwhile, the graphene cover exerts a strong confinement effect on the surface chemistry of CO on Ru(0001), showing that a high-coverage CO adlayer can be kept at the graphene/Ru interface at room temperature which desorbs intensively and completely around 390 K. This finding challenges the traditional concept of graphene films as passivation layers, indicating that the surface graphitic carbon can be used to modify the surface chemistry of metals.

Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.

Structural changes of FeOx nanostructures supported on Pt(111) and Pt foil with response to oxidation and reduction treatments in O2 and H2 atmospheres upto 1.0 bar have been investigated by using X-ray photoelectron spectroscopy and scanning tunneling microscopy. We show that submonolayer O–Fe bilayer (FeO) structure on Pt(111) can be transformed to O–Fe–O trilayer (FeO2) upon oxidation in 5.0 × 10−6 mbar O2, while the FeO to FeO2 transformation happens over the full FeO film only with the O2 partial pressure above 1.0 × 10−3 mbar. Reduction of the submonolayer FeO2 structure back to the FeO structure occurs when exposed to 1.0 mbar H2 at room temperature (RT). In contrast, the full FeO2 structure can be kept even under 1.0 bar H2 exposure condition. The FeOx coverage and FeOx/Pt boundary play a critical role in the redox behavior of the supported FeOx nanostructures. Furthermore, we show that the FeOx nanostructures supported on Pt foil can be oxidized in a similar way as those on the Pt(111) surface. However, the Pt foil supported FeO2 nanostructures can be more deeply reduced to the state close to metallic Fe in 1.0 mbar H2 at RT. The close-packed Pt(111) surface exhibits a stronger confinement effect on the FeO overlayer than the open polycrystalline Pt surface.

Heterogeneous catalysts, often consisting of metal nanoparticles supported on high-surface-area oxide solids, are common in industrial chemical reactions. Researchers have increasingly recognized the importance of oxides in heterogeneous catalysts: that they are not just a support to help the dispersion of supported metal nanoparticles, but rather interact with supported metal nanoparticles and affect the catalysis. The critical role of oxides in catalytic reactions can become very prominent when oxides cover metal surfaces forming the inverse catalysts.The source of the catalytic activity in homogeneous catalysts and metalloenzymes is often coordinatively unsaturated (CUS) transition metal (TM) cations, which can undergo facile electron transfer and promote catalytic reactions. Organic ligands and proteins confine these CUS cations, making them highly active and stable. In heterogeneous catalysis, however, confining these highly active CUS centers on an inorganic solid so that they are robust enough to endure the reaction environment while staying flexible enough to perform their catalysis remains a challenge.In this Account, we describe a strategy to confine the active CUS centers on the solid surface at the interface between a TM oxide (TMO) and a noble metal (NM). Among metals, NMs have high electron negativity and low oxygen affinity. This means that TM cations of the oxide bind strongly to NM atoms at the interface, forming oxygen-terminated-bilayer TMO nanostructures. The resulting CUS sites at the edges of the TMO nanostructure are highly active for catalytic oxidation reactions. Meanwhile, the strong interactions between TMOs and NMs prevent further oxidation of the bilayer TMO phases, which would otherwise result in the saturation of oxygen coordination and the deactivation of the CUS cations. We report that we can also tune the oxide–metal interactions to modulate the bonding of reactants with CUS centers, optimizing their catalytic performance.We review our recent progress on oxide-on-metal inverse catalysts, mainly the TMO-on-Pt (TM = Fe, Co, and Ni) systems and discuss the interface-confinement effect, an important factor in the behavior of these catalytic systems. We have studied both model catalyst systems and real supported nanocatalysts. Surface science studies and density functional theory calculations in model systems illustrate the importance of the oxide–metal interfaces in the creation and stabilization of surface active centers, and reveal the reaction mechanism at these active sites. In real catalysts, we describe facile preparation processes for fabricating the oxide-on-metal nanostructures. We have demonstrated excellent performance of the inverse catalysts in oxidation reactions such as CO oxidation. We believe that the interface confinement effect can be employed to design highly efficient novel catalysts and that the inverse oxide-on-metal catalysts may find wide applications in heterogeneous catalysis.

Supported Au–Ni nanocatalysts consisting of Au nanoparticles decorated with Ni/NiO nanostructures were synthesized using a two-step method and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. The structural characterization indicates that a small part of surface Ni atoms can diffuse into Au cores upon reduction at 600 °C, while the alloyed Ni atoms segregate onto the Au nanoparticle surfaces when oxidizing at a similar temperature. The inward and outward diffusion of Ni atoms at Au surfaces are reversible with response to cycled reduction and oxidation treatments. CO oxidation reactions over the Au–Ni catalysts suggest that Au nanoparticles decorated with highly dispersed NiO nanopatches are the active surface architecture. Optimum activity has been observed by maximizing the density of boundaries between NiO patches and Au surfaces, which can be achieved through suitable redox pretreatments of the Au–Ni catalysts. NiO surface structures in the “NiO-on-Au” inverse catalysts not only enhance Au-catalyzed CO oxidation performance but also prevent sintering of Au nanoparticles at elevated temperatures. Similar high activity for the CO oxidation has been observed in the “CoOx-on-Au” and “FeOx-on-Au” inverse catalysts.Keywords: Au catalysis; CO oxidation; inverse catalysts; NiO; redox treatment

Formation of wrinkles at graphene/Pt(111) surface was investigated by low energy electron microscopy (LEEM). Reversible wrinkling and unwrinkling of graphene sheets were observed upon cycled heating and cooling treatments, exhibiting a hysteresis effect with the temperature. In situ LEEM studies of graphene oxidation show preferential oxidation of the wrinkles than flat graphene sheets and graphene edges. The function of the wrinkles as one-dimensional (1D) nanosized gas inlets for oxygen and the strain at the distorted sp2-hybridized carbon atoms of the wrinkle sites can be attributed to the enhanced reactivity of wrinkles to the oxidation. Meanwhile, wrinkles also served as nanosized gas inlets for oxidation of CO intercalated between graphene and Pt(111). Considering that wrinkles are frequently present in graphene structures, the role of wrinkles as 1D reaction channels and their enhanced reactivity to reactions may have an important effect on graphene chemistry.

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 (FeOx) 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.

Highlights•Ni penetrates through graphene via exchange intercalation mechanism.•Pb intercalates through extended defect sites of graphene, such as edges.•Strong interaction of intercalant with carbon favors the exchange mechanism.•Intercalant interacting with carbon weakly intercalates through extended defects.Both Ni and Pb intercalation reactions at graphene/Ru(0001) interface were studied by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It is suggested that the Ni intercalation is dominated by an exchange intercalation mechanism, in which Ni adatoms produce transient atomic-scale defects in the graphene lattice and penetrate through the carbon monolayer. In contrast, the Pb intercalation process needs to be facilitated by the diffusion of Pb atoms through extended defect sites of graphene, such as open edges and domain boundaries. The two contrast intercalation mechanisms originate from the different interaction strength of the intercalated elements with carbon. Different responses of the graphene electronic structure to the Ni and Pb intercalation reactions were observed by PEEM.

Surface Fe ensembles, surface alloyed Fe atoms, and subsurface Fe species have been identified at Pt surfaces on the basis of studies in Fe–Pt(111) model systems and supported Pt–Fe nanoparticles (NPs). The surface Fe ensemble changes to ferrous oxide and forms a highly active and stable “FeO-on-Pt” structure in preferential oxidation of CO in the presence of H2 (PROX), which, however, gets fully oxidized in CO oxidation in the absence of H2 (COOX) and becomes inactive in the reaction. The surface alloyed Fe remains stable under the H2-rich and O2-rich reaction conditions, which are active for both PROX and COOX reactions. Accordingly, highly efficient Pt–Fe catalysts for the PROX and COOX reactions can be prepared via mild reduction and/or acid leaching.

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.

The nucleation and thermal stability of Au, Ni, and Au–Ni nanoclusters on 6H-SiC(0001) carbon nanomesh as well as the interaction between Au–Ni bimetallic clusters and reactive gases have been studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Both Au and Ni atoms grow as three-dimensional (3D) clusters. Annealing the Au/carbon nanomesh surface up to 1150 °C leads to complete desorption of the Au clusters, while interfacial reaction occurs between Ni clusters and the substrate surface when the Ni clusters are subjected to the same annealing process. The nucleation of Au–Ni clusters depends critically on the deposition sequence. Au atoms preferentially nucleate on the existing Ni clusters, leading to the formation of bimetallic clusters with Au enriched on the surface. If the deposition sequence is reversed, a part of Ni atoms nucleate between the Au clusters. The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh surface, irrespective of the deposition sequence. XPS characterization reveals that Ni atoms in Au–Ni bimetallic clusters are oxidized upon exposure to 5.0 × 10− 7 mbar O2 for 5 min at room temperature while negligible structure change can be detected when the bimetallic clusters are exposed to CO gas under the similar conditions.Highlights► Au clusters completely desorb from SiC(0001)-carbon nanomesh surface upon annealing up to 1150 °C. ► Interfacial reaction occurs between Ni and the substrate during thermal annealing via an intercalation process. ► Au–Ni bimetallic clusters with Au enriched on the surface form when depositing Ni first followed by Au growth. ► The thermal stability of the Au–Ni clusters resembles that of the Ni/carbon nanomesh. ► Ni atoms in Au–Ni bimetallic clusters get oxidized by exposure to 5.0 × 10− 7 mbar O2.

In situ low-energy electron microscopy (LEEM) studies in the epitaxial growth of graphene on Ru(0001) show that the graphene growth can be tailored via surface treatments of the substrate. Downhill growth of graphene was observed over the clean Ru(0001) surface with well-defined steps, forming sector-shaped graphene sheets. When the substrate surface was treated by Ar+ sputtering to produce subsurface Ar gas bubbles, round-shape graphene sheets were obtained by growing in both uphill and downhill directions. Correspondingly, anisotropic intercalation of oxygen occurs at the graphene/normal Ru(0001) interface, whereas isotropic intercalation of oxygen occurs at the graphene/Ar-sputtered Ru(0001) interface. The subsurface gas bubbles affect C–Ru interaction, which is attributed to the observed different behaviors of the graphene growth and oxygen intercalation.

Various well-defined Ni−Pt(111) model catalysts are constructed at atomic-level precision under ultra-high-vacuum conditions and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. Subsequent studies of CO oxidation over the surfaces show that a sandwich surface (NiO1−x/Pt/Ni/Pt(111)) consisting of both surface Ni oxide nanoislands and subsurface Ni atoms at a Pt(111) surface presents the highest reactivity. A similar sandwich structure has been obtained in supported Pt−Ni nanoparticles via activation in H2 at an intermediate temperature and established by techniques including acid leaching, inductively coupled plasma, and X-ray adsorption near-edge structure. Among the supported Pt−Ni catalysts studied, the sandwich bimetallic catalysts demonstrate the highest activity to CO oxidation, where 100% CO conversion occurs near room temperature. Both surface science studies of model catalysts and catalytic reaction experiments on supported catalysts illustrate the synergetic effect of the surface and subsurface Ni species on the CO oxidation, in which the surface Ni oxide nanoislands activate O2, producing atomic O species, while the subsurface Ni atoms further enhance the elementary reaction of CO oxidation with O.

Identical-size graphene nanoclusters (GNCs) form on Ru(0001) mediated by the substrate-induced clustering effect. The two kinds of uniform GNCs were identified as the seven C6-ring (noted as 7-C6) and three C6-ring (3-C6) structures with a dome-shape by using scanning tunneling microscopy.

Over Ni–Pt(111) model catalysts, changes in the surface structure were observed during alternating reduction and oxidation (redox) treatments at variable temperatures (VTs). Pt skin with subsurface Ni and NiO on Pt(111) form upon high-temperature reduction and high-temperature oxidation, respectively. Both Ni and Pt atoms are present on the surface with the low-temperature redox treatments. The similar surface structures can be constructed at supported Pt–Ni nanoparticles through the VT redox treatments. Both the surface structure and the CO oxidation performance of the supported Pt–Ni catalysts showed well-defined oscillations with the treatment temperature and the redox potential. The demonstrated treatment–structure–reactivity relationship at the PtNi catalysts aids in the design of advanced bimetallic catalysts.

Graphene growth and dissolution on Ru(0001) was dynamically imaged by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It was found that multilayer graphene grows on the metal surface in a layer-by-layer mode and the removal of graphene multilayers also occurs one layer after another. The topmost surface of the formed multilayer graphene is physically continuous as indicated by scanning tunneling microscope (STM) images. Accordingly, a bottom-up growth mechanism of multilayer graphene on Ru(0001) was proposed, which would help to prepare graphene overlayers with controlled thickness.

Two-dimensional (2D) FeO nanoislands with a well-controlled size, density, and surface structure have been grown on Pt(111) by a two-step preparation process, which consists of Fe deposition at low temperatures, such as 150 K, in an O2 atmosphere and subsequent annealing at elevated temperatures in ultrahigh vacuum. The atomic structure, chemical composition, and electronic state of the formed FeO nanoislands were investigated by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy, and high-resolution electron energy loss spectroscopy. The formation of the metastable 2D FeO surface phase can be attributed to confinement effects at interfaces between nanostructured oxides and metal substrates, which originate from the strong interaction between FeO and Pt(111). Furthermore, the STM and scanning tunneling spectroscopic data indicate that the formed Pt−FeO boundaries or edges of the FeO nanoislands present distinct chemical and electronic characteristics, which could be highly active in many catalytic processes.

Three reconstructed 6H-SiC(0001) surfaces, including a Si-rich 3 × 3 surface, a C-rich 6√3 × 6√3 surface, and a graphitized SiC surface, were used as substrates for the deposition of Pt overlayers. The interaction between Pt and the SiC(0001) surfaces was studied by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Pt reacts readily with the 3 × 3 surface to form platinum silicide even at room temperature. On the graphitized SiC surface, metal particles with low lateral dispersion form and keep on aggregating upon annealing. In contrast, homogeneously distributed small Pt nanoclusters were grown on the C-rich 6√3 × 6√3 surface. The unique nanomesh surface structure helps to stabilize the Pt nanoclusters until 800 °C. Above 1000 °C, Pt tends to diffuse into the subsurface region, forming the C/Pt silicide/SiC(0001) interface structure. The different surface electronic structures of the three Pt/SiC(0001) systems were discussed as well. The present data show that surface reconstruction provides an effective route to control the growth of metal overlayers and the formation of metal/substrate interfaces.

The growth mechanism of monolayer (ML) graphene on Ru(0001) via pyrolysis of C2H4 was studied by scanning tunneling microscopy (STM), high-resolution electron energy loss spectroscopy (HREELS), and ultraviolet photoelectron spectroscopy (UPS). On the basis of the mechanistic understanding, graphene overlayers ranging from nanographene clusters to graphene film with 1 ML coverage were prepared in a well-controlled way. O2 adsorption on the graphene/Ru(0001) surface was investigated by STM, UPS, and X-ray photoelectron spectroscopy (XPS). It is revealed that the Ru(0001) surface fully covered by graphene becomes passivated to O2 adsorption at room temperature and only activated again at elevated temperatures (>500 K). The adsorbed oxygen intercalates between the topmost graphene overlayer and the Ru(0001) substrate surface. These intercalated oxygen atoms decouple the graphene layer from the Ru(0001) substrate, forming quasi-freestanding monolayer graphene atomic crystals floating on the O−Ru(0001) surface.

Monolayer graphene was epitaxially grown on Ru(0001) through exposure of the Ru(0001) to ethylene at room temperature followed by annealing in ultrahigh vacuum at elevated temperatures. The resulting graphene structures were studied by scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS). The graphene/Ru(0001) surface was used as a periodic template for growth of metal nanoclusters. Highly dispersed Pt clusters with well controlled size and spatial distribution were fabricated on the surface.

Adsorption of carbon tetrachloride (CCl4) on Si(1 1 1)-7 × 7 at room temperature (RT) and low temperature (LT) was investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). It was demonstrated that at RT CCl4 dissociates on the Si(1 1 1)-7 × 7 surface leaving the surface extensively adsorbed with atomic Cl species. The dissociated Cl shows site preference to Si restatoms resulting in quick extinction of dangling bonds at the Si restatoms. At LT (around 120 K), both molecular and dissociative adsorption of CCl4 occurs, which produces Cl, CClx (x ⩽ 3), and CCl4 on the surface. The dangling bonds at the restatoms and adatoms were simultaneously quenched upon the LT CCl4 adsorption. The site selectivity of restatoms to adatoms for molecule adsorption on the Si(1 1 1)-7 × 7 surface is discussed.

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

Interactions between metals and oxides are key factors to determine the performance of metal/oxide heterojunctions, particularly in nanotechnology, where the miniaturization of devices down to the nanoregime leads to an enormous increase in the density of interfaces. One central issue of concern in engineering metal/oxide interfaces is to understand and control the interactions which consist of two fundamental aspects: (i) interfacial charge redistribution — electronic interaction, and (ii) interfacial atom transport — chemical interaction. The present paper focuses on recent advances in both electronic and atomic level understanding of the metal–oxide interactions at temperatures below 1000 ∘C, with special emphasis on model systems like ultrathin metal overlayers or metal nanoclusters supported on well-defined oxide surfaces. The important factors determining the metal–oxide interactions are provided. Guidelines are given in order to predict the interactions in such systems, and methods to desirably tune them are suggested.The review starts with a brief summary of the physics and chemistry of heterophase interface contacts. Basic concepts for quantifying the electronic interaction at metal/oxide interfaces are compared to well-developed contact theories and calculation methods. The chemical interaction between metals and oxides, i.e., the interface chemical reaction, is described in terms of its thermodynamics and kinetics. We review the different chemical driving forces and the influence of kinetics on interface reactions, proposing a strong interplay between the chemical interaction and electronic interaction, which is decisive for the final interfacial reactivity. In addition, a brief review of solid–gas interface reactions (oxidation of metal surfaces and etching of semiconductor surfaces) is given, in addition to a comparison of a similar mechanism dominating in solid–solid and solid–gas interface reactions.The main body of the paper reviews experimental and theoretical results from the literature concerning the interactions between metals and oxides (TiO2, SrTiO3, Al2O3, MgO, SiO2, etc.). Chemical reactions, e.g., redox reactions, encapsulation reactions, and alloy formation reactions, are highlighted for metals in contact with mixed conducting oxides of TiO2 and SrTiO3. The dependence of the chemical interactions on the electronic structure of the contacting metal and oxide phases is demonstrated. This dependence originates from the interplay between interfacial space charge transfer and diffusion of ionic defects across interfaces. Interactions between metals and insulating oxides, such as Al2O3, MgO, and SiO2, are strongly confined to the interfaces. Literature results are cited which discuss how the metal/oxide interactions vary with oxide surface properties (surface defects, surface termination, surface hydroxylation, etc.). However, on the surfaces of thin oxide films grown on conducting supports, the effect of the conducting substrates on metal–oxide interactions should be carefully considered.In the summary, we conclude how variations in the electronic structure of the metal/oxide junctions enable one to tune the interfacial reactivity and, furthermore, control the macroscopic properties of the interfaces. This includes strong metal–support interactions (SMSI), catalytic performance, electrical, and mechanical properties.

The growth of ultrathin Cr overlayers on SrTiO3(1 0 0) was studied by X-ray photoelectron spectroscopy, scanning tunneling microscopy, and transmission electron microscopy. It is found that the metal–oxide interaction strongly depends on the deposition temperature. At T < 600 °C, the interfaces are atomically sharp. Local charge transfer happens between the interfacial Cr adatoms and the topmost substrate atoms. The binding energy shift of Cr 2p is dominated by the final state effects. In case of T > 600 °C, bulk diffusion of oxygen in the oxide substrate may occur, which results in a redox reaction and the formation of new reaction phases at the interfaces. In this temperature regime, the binding energy shift of Cr 2p is mainly controlled by the initial state effects.

The orientation control of graphene overlayers on metal surface is an important issue which remains as a challenge in graphene growth on Ni surface. Here we have demonstrated that epitaxial graphene overlayers can be obtained by annealing a nickel carbide covered Ni(111) surface using in situ surface imaging techniques. Epitaxial graphene islands nucleate and grow via segregation of dissolved carbon atoms to the top surface at about 400 °C. This is in contrast to a mixture of epitaxial and non-epitaxial graphene domains grown directly on Ni(111) at 540 °C. The different growth behaviors are related to the nucleation dynamics which is controlled by local carbon densities in the near surface region.

The space between a two-dimensional (2D) material overlayer and a metal surface can be regarded as a nanoreactor, in which molecule adsorption and surface reaction may occur. In this work, we present CO intercalation under a hexagonal boron nitride (h-BN) overlayer on Ru(0001) at room temperature, observed using X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and scanning tunneling microscopy. Critical factors influencing the interfacial process have been investigated, including CO partial pressure, h-BN coverage, and oxygen pre-adsorption on the Ru surface. It has been identified that CO adsorption on the bare Ru surface region plays an important role in CO intercalation. Comparative studies of CO intercalation at h-BN/Ru(0001) and graphene/Ru(0001) interfaces indicate that CO starts to intercalate h-BN overlayers more easily than graphene. Temperature-programmed CO desorption experiments from h-BN/CO/Ru(0001) and graphene/CO/Ru(0001) surfaces reveal a similar confinement effect of the 2D cover on CO adsorption, which results in a more abrupt and quick CO desorption in comparison with the CO/Ru(0001) surface.

Graphene growth and dissolution on Ru(0001) was dynamically imaged by low energy electron microscopy (LEEM) and photoemission electron microscopy (PEEM). It was found that multilayer graphene grows on the metal surface in a layer-by-layer mode and the removal of graphene multilayers also occurs one layer after another. The topmost surface of the formed multilayer graphene is physically continuous as indicated by scanning tunneling microscope (STM) images. Accordingly, a bottom-up growth mechanism of multilayer graphene on Ru(0001) was proposed, which would help to prepare graphene overlayers with controlled thickness.

Formation of wrinkles at graphene/Pt(111) surface was investigated by low energy electron microscopy (LEEM). Reversible wrinkling and unwrinkling of graphene sheets were observed upon cycled heating and cooling treatments, exhibiting a hysteresis effect with the temperature. In situ LEEM studies of graphene oxidation show preferential oxidation of the wrinkles than flat graphene sheets and graphene edges. The function of the wrinkles as one-dimensional (1D) nanosized gas inlets for oxygen and the strain at the distorted sp2-hybridized carbon atoms of the wrinkle sites can be attributed to the enhanced reactivity of wrinkles to the oxidation. Meanwhile, wrinkles also served as nanosized gas inlets for oxidation of CO intercalated between graphene and Pt(111). Considering that wrinkles are frequently present in graphene structures, the role of wrinkles as 1D reaction channels and their enhanced reactivity to reactions may have an important effect on graphene chemistry.

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 (FeOx) 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.

Identical-size graphene nanoclusters (GNCs) form on Ru(0001) mediated by the substrate-induced clustering effect. The two kinds of uniform GNCs were identified as the seven C6-ring (noted as 7-C6) and three C6-ring (3-C6) structures with a dome-shape by using scanning tunneling microscopy.

Silica supported Pt–Co and Au–Co nanoparticles (NPs) were subjected to various redox processes and characterized by X-ray diffraction, X-ray absorption near edge structure, and X-ray photoelectron spectroscopy. We found that most of the Co oxide (CoOx) species on Pt NPs can be reduced at 100 °C forming an alloy structure with Pt at elevated temperatures. Oxidation of Co in the reduced sample takes place gradually with increasing temperatures. In contrast, temperatures higher than 400 °C are needed to reduce CoOx on Au NPs and Co atoms hardly form an alloy with Au even at 600 °C. The Co species in the reduced Au–Co/SiO2 sample were quickly oxidized in an O2 atmosphere at room temperature. High CO oxidation activity was observed in the Pt–Co/SiO2 catalyst reduced below 300 °C; however this necessitated reduction at 600 °C of the Au–Co/SiO2 catalyst. The results illustrate a stronger interaction of Co (CoOx) with Pt than with Au. In both systems, the optimum treatment conditions are to produce a similar CoO-on-noble metal (NM) active structure and maximize the density of interface sites between the surface CoO structure and the NM support.

Two-dimensional (2D) materials are characterised by their strong intraplanar bonding but weak interplanar interaction. Interfaces between neighboring 2D layers or between 2D overlayers and substrate surfaces provide intriguing confined spaces for chemical processes, which have stimulated a new area of “chemistry under 2D cover”. In particular, well-defined 2D material overlayers such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have been deposited on solid surfaces, which can be used as model systems to understand the new chemistry. In the present review, we first show that many atoms and molecules can intercalate ultrathin 2D materials supported on solid surfaces and the space under the 2D overlayers has been regarded as a 2D nanocontainer. Moreover, chemical reactions such as catalytic reactions, surface adlayer growth, chemical vapor deposition, and electrochemical reactions occur in the 2D confined spaces, which further act as 2D nanoreactors. It has been demonstrated that surface chemistry and catalysis are strongly modulated by the 2D covers, resulting in weakened molecule adsorption and enhanced surface reactions. Finally, we conclude that the confinement effect of the 2D cover leads to new chemistry in a small space, such as “catalysis under cover” and “electrochemistry under cover”. These new concepts enable us to design advanced nanocatalysts encapsulated with 2D material shells which may present improved performance in many important processes of heterogeneous catalysis, electrochemistry, and energy conversion.