Gareth S. Parkinson

Carbon monoxide-induced adatom sintering in a Pd–Fe3O4 model catalyst

The coarsening of catalytically-active metal clusters is often accelerated by the presence of gases through the formation of mobile intermediates, though the exact mechanism through which this happens is often subject to debate. We use scanning tunneling microscopy (STM) to follow the CO induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature. We show that highly-mobile Pd-carbonyl species, formed via the so-called skyhook effect, are temporarily trapped at other Pd adatoms. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. While CO increases the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to the skyhook effect and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport.The coarsening of catalytically active metal clusters is often accelerated by the presence of gases, but the role played by gas molecules is difficult to ascertain and varies from system to system. We use scanning tunnelling microscopy to follow the CO-induced coalescence of Pd adatoms supported on the Fe3O4(001) surface at room temperature, and find Pd-carbonyl species to be responsible for mobility in this system. Once these reach a critical density, clusters nucleate; subsequent coarsening occurs through cluster diffusion and coalescence. Whereas CO induces the mobility in the Pd/Fe3O4 system, surface hydroxyls have the opposite effect. Pd atoms transported to surface OH groups are no longer susceptible to carbonyl formation and remain isolated. Following the evolution from well-dispersed metal adatoms into clusters, atom-by-atom, allows identification of the key processes that underlie gas-induced mass transport.

Subsurface cation vacancy stabilization of the magnetite (001) surface

Iron oxides play an increasingly prominent role in heterogeneous catalysis, hydrogen production, spintronics, and drug delivery. The surface or material interface can be performance-limiting in these applications, so it is vital to determine accurate atomic-scale structures for iron oxides and understand why they form. Using a combination of quantitative low-energy electron diffraction, scanning tunneling microscopy, and density functional theory calculations, we show that an ordered array of subsurface iron vacancies and interstitials underlies the well-known (2×2)R45° reconstruction of Fe3O4(001). This hitherto unobserved stabilization mechanism occurs because the iron oxides prefer to redistribute cations in the lattice in response to oxidizing or reducing environments. Many other metal oxides also achieve stoichiometry variation in this way, so such surface structures are likely commonplace. The surface reconstruction of magnetite is explained more accurately with the inclusion of subsurface cation vacancies. [Also see Perspective by Chambers] Stabilization of the surfaces of magnetite Accurate structures of iron oxide surfaces are important for understanding their role in catalysis, and, for oxides such as magnetite, applications in magnetism and spin physics. The accepted low-energy electron diffraction (LEED) structure for the surface of magnetite, in which the bulk surface termination undergoes an undulating distortion, has a relatively poor agreement with experiment. Bliem et al. show that the LEED structure is much more accurately described by a structure that includes subsurface cation vacancies and occupation of interstitial sites (see the Perspective by Chambers). Such cation redistribution occurs in many metal oxides and may play a role in their surface structures. Science, this issue p. 1215; see also p. 1186

Room temperature water splitting at the surface of magnetite.

An array of surface science measurements has revealed novel water adsorption behavior at the Fe(3)O(4)(001) surface. Following room temperature exposure to water, a low coverage of hydrogen atoms is observed, with no associated water hydroxyl group. Mild annealing of the hydrogenated surface leads to desorption of water via abstraction of surface oxygen atoms, leading to a reduction of the surface. These results point to an irreversible splitting of the water molecule. The observed phenomena are discussed in the context of recent DFT calculations (Mulakaluri, N.; Pentcheva, R.; Scheffler, M. J. Phys. Chem. C 2010, 114, 11148), which show that the Jahn-Teller distorted surface isolates adsorbed H in a geometry that could kinetically hinder recombinative desorption. In contrast, the adsorption geometry facilitates interaction between water hydroxyl species, which are concluded to leave the surface following a reactive desorption process, possibly via the creation of O(2).

Nickel carbide as a source of grain rotation in epitaxial graphene.

Graphene has a close lattice match to the Ni(111) surface, resulting in a preference for 1 × 1 configurations. We have investigated graphene grown by chemical vapor deposition (CVD) on the nickel carbide (Ni(2)C) reconstruction of Ni(111) with scanning tunneling microscopy (STM). The presence of excess carbon, in the form of Ni(2)C, prevents graphene from adopting the preferred 1 × 1 configuration and leads to grain rotation. STM measurements show that residual Ni(2)C domains are present under rotated graphene. Nickel vacancy islands are observed at the periphery of rotated grains and indicate Ni(2)C dissolution after graphene growth. Density functional theory (DFT) calculations predict a very weak (van der Waals type) interaction of graphene with the underlying Ni(2)C, which should facilitate a phase separation of the carbide into metal-supported graphene. These results demonstrate that surface phases such as Ni(2)C can play a major role in the quality of epitaxial graphene.

Charge Trapping at the Step Edges of TiO2 Anatase (101)

A combination of photoemission, atomic force, and scanning tunneling microscopy/spectroscopy measurements shows that excess electrons in the TiO2 anatase (101) surface are trapped at step edges. Consequently, steps act as preferred adsorption sites for O2 . In density functional theory calculations electrons localize at clean step edges, this tendency is enhanced by O vacancies and hydroxylation. The results show the importance of defects for the wide-ranging applications of titania.

Dual role of CO in the stability of subnano Pt clusters at the Fe3O4(001) surface

Significance The catalytic activity of metal particles is highly size-dependent in the subnanometer regime, which makes understanding how and why particle sizes change in reactive atmospheres particularly important. Here, we show that carbon monoxide plays a dual role in the coarsening of otherwise highly stable Pt atoms on an Fe3O4(001) support: CO adsorption weakens the adatom–support interaction inducing mobility, and stabilizes the Pt dimer against decay into two adatoms. Our results illustrate how molecules modify the clustering dynamics on surfaces, provide much-needed insight into how deactivation and redispersion can occur in single-atom catalyst systems, and demonstrate an approach to prepare size-distinguished clusters for studies of the size effect. Interactions between catalytically active metal particles and reactant gases depend strongly on the particle size, particularly in the subnanometer regime where the addition of just one atom can induce substantial changes in stability, morphology, and reactivity. Here, time-lapse scanning tunneling microscopy (STM) and density functional theory (DFT)-based calculations are used to study how CO exposure affects the stability of Pt adatoms and subnano clusters at the Fe3O4(001) surface, a model CO oxidation catalyst. The results reveal that CO plays a dual role: first, it induces mobility among otherwise stable Pt adatoms through the formation of Pt carbonyls (Pt1–CO), leading to agglomeration into subnano clusters. Second, the presence of the CO stabilizes the smallest clusters against decay at room temperature, significantly modifying the growth kinetics. At elevated temperatures, CO desorption results in a partial redispersion and recovery of the Pt adatom phase.

Cluster Nucleation and Growth from a Highly Supersaturated Adatom Phase: Silver on Magnetite

The atomic-scale mechanisms underlying the growth of Ag on the (√2×√2)R45°-Fe3O4(001) surface were studied using scanning tunneling microscopy and density functional theory based calculations. For coverages up to 0.5 ML, Ag adatoms populate the surface exclusively; agglomeration into nanoparticles occurs only with the lifting of the reconstruction at 720 K. Above 0.5 ML, Ag clusters nucleate spontaneously and grow at the expense of the surrounding material with mild annealing. This unusual behavior results from a kinetic barrier associated with the (√2×√2)R45° reconstruction, which prevents adatoms from transitioning to the thermodynamically favorable 3D phase. The barrier is identified as the large separation between stable adsorption sites, which prevents homogeneous cluster nucleation and the instability of the Ag dimer against decay to two adatoms. Since the system is dominated by kinetics as long as the (√2×√2)R45° reconstruction exists, the growth is not well described by the traditional growth modes. It can be understood, however, as the result of supersaturation within an adsorption template system.

Adsorption of water at the SrO surface of ruthenates

While perovskite oxides hold promise in applications ranging from solid oxide fuel cells to catalysts, their surface chemistry is poorly understood at the molecular level. Here we follow the formation of the first monolayer of water at the (001) surfaces of Srn+1RunO3n+1 (n = 1, 2) using low-temperature scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory. These layered perovskites cleave between neighboring SrO planes, yielding almost ideal, rocksalt-like surfaces. An adsorbed monomer dissociates and forms a pair of hydroxide ions. The OH stemming from the original molecule stays trapped at Sr-Sr bridge positions, circling the surface OH with a measured activation energy of 187±10 meV. At higher coverage dimers of dissociated water assemble into one-dimensional chains and form a percolating network where water adsorbs molecularly in the gaps. Our work shows the limitations of applying surface chemistry concepts derived for binary rocksalt oxides to perovskites.

An Atomic-Scale View of CO and H-2 Oxidation on a Pt/Fe3O4 Model Catalyst

Metal-support interactions are frequently invoked to explain the enhanced catalytic activity of metal nanoparticles dispersed over reducible metal oxide supports, yet the atomic-scale mechanisms are rarely known. In this report, scanning tunneling microscopy was used to study a Pt1-6/Fe3O4 model catalyst exposed to CO, H2, O2, and mixtures thereof at 550 K. CO extracts lattice oxygen atoms at the cluster perimeter to form CO2, creating large holes in the metal oxide surface. H2 and O2 dissociate on the metal clusters and spill over onto the support. The former creates surface hydroxy groups, which react with the support, ultimately leading to the desorption of water, while oxygen atoms react with Fe from the bulk to create new Fe3O4(001) islands. The presence of the Pt is crucial because it catalyzes reactions that already occur on the bare iron oxide surface, but only at higher temperatures.

Electron transfer between anatase TiO2 and an O2 molecule directly observed by atomic force microscopy

Significance Molecular oxygen is an inert species, unable to enter chemical reactions. Activation occurs through the acceptance of an extra electron; this catalytic step plays a major role in applications such as heterogeneous catalysis and fuel cells. It is also used by all living organisms. We show that the two different charge states of O2 can be easily distinguished by atomic force microscopy (AFM). We directly injected or removed electrons into/from the O2 molecule by the AFM tip, switching the O2 reactivity. These results open new possibilities for studying catalytic and photocatalytic processes. Activation of molecular oxygen is a key step in converting fuels into energy, but there is precious little experimental insight into how the process proceeds at the atomic scale. Here, we show that a combined atomic force microscopy/scanning tunneling microscopy (AFM/STM) experiment can both distinguish neutral O2 molecules in the triplet state from negatively charged (O2)− radicals and charge and discharge the molecules at will. By measuring the chemical forces above the different species adsorbed on an anatase TiO2 surface, we show that the tip-generated (O2)− radicals are identical to those created when (i) an O2 molecule accepts an electron from a near-surface dopant or (ii) when a photo-generated electron is transferred following irradiation of the anatase sample with UV light. Kelvin probe spectroscopy measurements indicate that electron transfer between the TiO2 and the adsorbed molecules is governed by competition between electron affinity of the physisorbed (triplet) O2 and band bending induced by the (O2)− radicals. Temperature–programmed desorption and X-ray photoelectron spectroscopy data provide information about thermal stability of the species, and confirm the chemical identification inferred from AFM/STM.