Hervé Ménard

In situ growth of nanoparticles through control of non-stoichiometry

Surfaces decorated with uniformly dispersed catalytically active nanoparticles play a key role in many fields, including renewable energy and catalysis. Typically, these structures are prepared by deposition techniques, but alternatively they could be made by growing the nanoparticles in situ directly from the (porous) backbone support. Here we demonstrate that growing nano-size phases from perovskites can be controlled through judicious choice of composition, particularly by tuning deviations from the ideal ABO3 stoichiometry. This non-stoichiometry facilitates a change in equilibrium position to make particle exsolution much more dynamic, enabling the preparation of compositionally diverse nanoparticles (that is, metallic, oxides or mixtures) and seems to afford unprecedented control over particle size, distribution and surface anchorage. The phenomenon is also shown to be influenced strongly by surface reorganization characteristics. The concept exemplified here may serve in the design and development of more sophisticated oxide materials with advanced functionality across a range of possible domains of application.

Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution

Metal particles supported on oxide surfaces are used as catalysts for a wide variety of processes in the chemical and energy conversion industries. For catalytic applications, metal particles are generally formed on an oxide support by physical or chemical deposition, or less commonly by exsolution from it. Although fundamentally different, both methods might be assumed to produce morphologically and functionally similar particles. Here we show that unlike nickel particles deposited on perovskite oxides, exsolved analogues are socketed into the parent perovskite, leading to enhanced stability and a significant decrease in the propensity for hydrocarbon coking, indicative of a stronger metal–oxide interface. In addition, we reveal key surface effects and defect interactions critical for future design of exsolution-based perovskite materials for catalytic and other functionalities. This study provides a new dimension for tailoring particle–substrate interactions in the context of increasing interest for emergent interfacial phenomena.

An Investigation of crystal structure, surface area and surface chemistry of strontium niobate and their influence on photocatalytic performance

Sr(1-x)NbO3 is an unusual material that displays both metallic type conduction and photocatalytic activity, despite being an NbIV oxide, and it sustains photo-oxidation without degradation. The influence of crystal structure, surface area and surface chemistry on the photocatalytic activity of strontium niobate has been investigated. The crystal structure of strontium niobate depends on the Sr content of the A site, with cubic symmetry for Sr ≤ 0.92 and orthorhombic symmetry for 0.92 < Sr ≤ 0.97. The change of crystal structure from cubic to orthorhombic symmetry seems to have a negative effect on the photocatalytic activity, as the NbO6 octahedra become distorted and unfavourable for d-orbital overlapping. The photocatalytic activity increased significantly by enlarging the surface area through ball milling, nevertheless, a clear trend for the surface area effect on activity is not obtained among samples with different Sr content. An enrichment of Sr on the surface of strontium niobate was observed by XPS, which seems to be a governing factor for improving stability.

Improved metastable de-excitation spectrometer using laser-cooling techniques

Details of a new approach for performing metastable de-excitation spectroscopy are given. A beam of metastable (2S3) helium atoms, produced in a hollow cathode dc discharge, is collimated and subsequently focused using Doppler cooling of the 2S13–2P23 transition at 1083nm, forming an intense probe of up to 1×1012atomss−1cm−2. The large distance (2.5m) between source and sample means that the beam is relatively free of UV photons and 2S1 metastable atoms, removing the need for quench lamps and chopper wheels. As well as providing a clean high intensity source, the well defined nature of the beam is a necessary step towards using more sophisticated laser-cooling techniques with the ultimate aim of producing a metastable helium microscope. MDS and UPS spectra from Si(111) are shown.

Demonstration of chemistry at a point through restructuring and catalytic activation at anchored nanoparticles

Metal nanoparticles prepared by exsolution at the surface of perovskite oxides have been recently shown to enable new dimensions in catalysis and energy conversion and storage technologies owing to their socketed, well-anchored structure. Here we show that contrary to general belief, exsolved particles do not necessarily re-dissolve back into the underlying perovskite upon oxidation. Instead, they may remain pinned to their initial locations, allowing one to subject them to further chemical transformations to alter their composition, structure and functionality dramatically, while preserving their initial spatial arrangement. We refer to this concept as chemistry at a point and illustrate it by tracking individual nanoparticles throughout various chemical transformations. We demonstrate its remarkable practical utility by preparing a nanostructured earth abundant metal catalyst which rivals platinum on a weight basis over hundreds of hours of operation. Our concept enables the design of compositionally diverse confined oxide particles with superior stability and catalytic reactivity.Metal nanoparticles prepared by exsolution at the surface of perovskite oxides are key species in catalysis and energy fields. Here, the authors develop a chemistry at a point concept by tracking individual nanoparticles with excellent activity and stability throughout various chemical transformations.

A RAIRS study of the adsorption and decomposition of methylsilane on Cu(111)

The adsorption and decomposition of methylsilane gas onto Cu(111) has been investigated by reflection–absorption infrared spectroscopy (RAIRS). The initial adsorption of methylsilane at 15 K results in the formation of an initially ordered physisorbed monolayer, adsorbed with a small tilt angle from the plane of the surface. Further increase in exposure results in the formation of a more dense monolayer, with methylsilane lying nearly parallel to the surface of the crystal, before the growth of the disordered multilayer. Adsorption at 78 K appears to result in the formation of an SiH–CH3 species for which there is some evidence of further Si–Si coupling. At 295 K, methylsilane is observed to adsorb with the Si–C axis perpendicular to the surface. Adsorption at 395 K results in the decomposition of methylsilane, with both Si–H and Si–C bond scission. Adsorbing CO at 15 K on the Cu/Si surface structure thus formed indicates that CO adsorbs mostly in atop positions on Si atoms, suggesting that any metal atom sites are blocked by either adsorbed C or Si atoms.

Data underpinning:Nano-socketed nickel particles with enhanced coking resistance grown in situ by redox exsolution

DN thanks the European Project METSAPP (FCH JU-GA 278257) for funding. We also thank NSF and EPSRC for Materials World Network funding ref EP/J018414/1.