Laurence D. Marks

Experimental studies of small particle structures

Data on the experimental structure of small particles is reviewed, the emphasis being an attempt to correlate experimental information with theoretical models. First, a general discussion of some of the controlling factors is presented, primarily equilibrium shapes of small particles, the effect of surface stresses, kinetics, and the role of chemisorption and the substrate. Experimental techniques for obtaining information about small particles are then described, primarily electron microscopy approaches. Experimental data on the static structure of small particles is then reviewed, both single crystals and the many, complicated twinned structures in face-centred cubic materials. An overview is then given of some of the more recent results on dynamic phenomena in small particles. Finally, a general model merging thermodynamic and kinetic factors is presented to attempt to rationalize the available data, followed by a brief discussion.

Surface structure and energetics of multiply twinned particles

Abstract Experimental results from thermally annealed particles of silver and gold in the size range 10–50 nm show the presence of re-entrant surfaces on the decahedral multiply twinned particles (MTPs). A theoretical model is developed to explain these results based on the Wulff construction modified to include twin-boundaries. In addition to reproducing the re-entrant surfaces, the theory suggests that the total surface energy of the decahedral form is sandwiched between those of a single crystal and an icosahedral MTP. Consequently the stability of the decahedral particles (in a certain size range) relative to single crystals can now be explained. A useful perturbation modification is also outlined which enables qualitative comparisons of the surface energies to be made, and shows that the twin boundaries in MTPs limit the possible surface facets. Explicit shapes and energies are presented for two extreme models of the faceting. Finally, a model is outlined to justify the difference between the structu...

The structure and chemistry of the TiO2-rich surface of SrTiO3 (001)

Oxide surfaces are important for applications in catalysis and thin film growth. An important frontier in solid-state inorganic chemistry is the prediction of the surface structure of an oxide. Comparatively little is known about atomic arrangements at oxide surfaces at present, and there has been considerable discussion concerning the forces that control such arrangements. For instance, one model suggests that the dominant factor is a reduction of Coulomb forces; another favours minimization of ‘dangling bonds’ by charge transfer to states below the Fermi energy. The surface structure and properties of SrTiO3—a standard model for oxides with a perovskite structure—have been studied extensively. Here we report a solution of the 2 × 1 SrTiO3 (001) surface structure obtained through a combination of high-resolution electron microscopy and theoretical direct methods. Our results indicate that surface rearrangement of TiO6-x units into edge-sharing blocks determines the SrO-deficient surface structure of SrTiO3. We suggest that this structural concept can be extended to perovskite surfaces in general.

On the potential role of hydroxyl groups in CO oxidation over Au/Al2O3

The deuterium isotope effect in the steady state CO oxidation rate over Au/-Al2O3 in the presence of H2 or H2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O2, and 40.5% H2 at room temperature, CO oxidation exhibited a deuterium isotope effect (kH/kD )o f 1.4 ± 0.2. The rate of D2 oxidation was also slower than the oxidation of H2, such that the selectivity for CO oxidation was 86% in the presence of D2 versus 77% in the presence of H2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O2, and 1.5% H2O. H2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D 2, whereas H2O and D2O were equally effective. The difference was attributed to the different mechanisms with which H 2 or H2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/-Al2O3 was rather inactive. It could be activated by treatment with a mixture of H2 and H2 Oa t 100 ◦ C, although treatment by either H2 or H2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.

Correlated structure and optical property studies of plasmonic nanoparticles

This article provides a review of our recent studies of single metal nanoparticles and single nanoparticle clusters aimed at correlating the structural and plasmonic properties of the same entity. The correlation between the structure and the optical properties arising from the localized surface plasmon resonance (LSPR) on single nanoparticles from various samples is described. Nanoparticles of different materials (Ag and Au) and shapes (spheres, cubes, triangles) are considered. Experiments were carried out using transmission electron microscopy (TEM), dark-field spectroscopy, and surface-enhanced Raman spectroscopy (SERS). Results of those measurements were compared with electrodynamics calculations to provide insight into the interpretation and physical meaning of the experimental results. We examine correlated studies of triangular nanoparticle arrays to highlight the significance of single entity measurements over ensemble-averaged measurements. Furthermore, we show how an examination of statistics on large data sets helps draw quantitative structure―LSPR relationships. We also show that implementing SERS in correlated measurements improves the understanding of factors important in determining SERS enhancements. Finally, we extend the scope of correlated measurements to the tracking and controlled manipulation of single nanoparticles, thus paving the way for in vivo diagnostics using nanomaterials.

Elastic strains and the energy balance for multiply twinned particles

Abstract The energetics of multiply twinned particles (MTPs) are investigated using elasticity theory. This allows the homogeneous strain models to be critically compared with the disclination model for the strains in decahedral particles and with a new model for the strains in icosahedral particles based on inhomogeneous elasticity. The overall energy balance between MTPs and single crystals is then evaluated, including the significant cost of elastially distorting the surface and using two extreme models of the faceting. The results of this analysis indicate that icosahedral MTPs will be more stable than single crystals for small sizes only for strong faceting conditions, decahedral MTPs being true intermediaries between the two. Experimentally observed stress-relief mechanisms provide indirect evidence for the inhomogeneous strain models.

Identification of active sites in CO oxidation and water-gas shift over supported Pt catalysts

Comparing active site reactivity Noble metal nanoparticles often exhibit behaviors distinct from atomic and bulk versions of the same material. Gold and platinum dispersed on metal oxide supports, for example, show remarkable low-temperature reactivity for carbon monoxide (CO) oxidation by oxygen or water. Ding et al. used infrared spectroscopy to identify CO adsorbed on isolated platinum atoms or nanoparticles dispersed on zeolite and oxide supports. Temperature-programmed desorption studies showed that CO reacted at much lower temperatures when adsorbed on nanoparticles versus on isolated metal atoms. Science, this issue p. 189 Infrared spectroscopy reveals that carbon monoxide oxidizes more readily on supported noble metal nanoparticles than on isolated atoms. [Also see Perspective by Stephens et al.] Identification and characterization of catalytic active sites are the prerequisites for an atomic-level understanding of the catalytic mechanism and rational design of high-performance heterogeneous catalysts. Indirect evidence in recent reports suggests that platinum (Pt) single atoms are exceptionally active catalytic sites. We demonstrate that infrared spectroscopy can be a fast and convenient characterization method with which to directly distinguish and quantify Pt single atoms from nanoparticles. In addition, we directly observe that only Pt nanoparticles show activity for carbon monoxide (CO) oxidation and water-gas shift at low temperatures, whereas Pt single atoms behave as spectators. The lack of catalytic activity of Pt single atoms can be partly attributed to the strong binding of CO molecules.

Three-dimensional imaging of dislocations in a nanoparticle at atomic resolution

Dislocations and their interactions strongly influence many material properties, ranging from the strength of metals and alloys to the efficiency of light-emitting diodes and laser diodes. Several experimental methods can be used to visualize dislocations. Transmission electron microscopy (TEM) has long been used to image dislocations in materials, and high-resolution electron microscopy can reveal dislocation core structures in high detail, particularly in annular dark-field mode. A TEM image, however, represents a two-dimensional projection of a three-dimensional (3D) object (although stereo TEM provides limited information about 3D dislocations). X-ray topography can image dislocations in three dimensions, but with reduced resolution. Using weak-beam dark-field TEM and scanning TEM, electron tomography has been used to image 3D dislocations at a resolution of about five nanometres (refs 15, 16). Atom probe tomography can offer higher-resolution 3D characterization of dislocations, but requires needle-shaped samples and can detect only about 60 per cent of the atoms in a sample. Here we report 3D imaging of dislocations in materials at atomic resolution by electron tomography. By applying 3D Fourier filtering together with equal-slope tomographic reconstruction, we observe nearly all the atoms in a multiply twinned platinum nanoparticle. We observed atomic steps at 3D twin boundaries and imaged the 3D core structure of edge and screw dislocations at atomic resolution. These dislocations and the atomic steps at the twin boundaries, which appear to be stress-relief mechanisms, are not visible in conventional two-dimensional projections. The ability to image 3D disordered structures such as dislocations at atomic resolution is expected to find applications in materials science, nanoscience, solid-state physics and chemistry.

High resolution studies of small particles of gold and silver. I. Multiply-twinned particles

Abstract The structure of multiply-twinned particles of gold and silver found in the early stages of particulate growth has been studied using direct lattice imaging methods with the Cambridge University 600 kV high resolution electron microscope. There was widespread evidence for the presence of strain-relieving partial dislocations in icosahedral particles. However, the possibility of imaging artefacts arising from double diffraction due to overlapping tetrahedral projections needed to be excluded, for example by slight particle tilting, before the actual presence of the dislocations was substantiated. An analysis of particle geometry, also including the effects of inhomogeneous strain, showed that straight Moirefringes did not necessarily indicate the absence of strain within a particle. Moreover, it has been shown that, in some particle orientations, two different, though very similar, Moire fringes can arise with their interaction and final appearance being very dependent on particle orientation.

Modified Wulff constructions for twinned particles

Abstract This paper reports the application of a modified form of the Wulff construction to derive theoretical shapes and total surface energies for twinned particles. It is proposed that the incorporation of twin boundaries in faceted particles leads to a number of constrained local minimum in the total surface energy (at constant volume), in addition to the minimum of a single crystal Wulff polyhedron. A procedure for generating these constrained local minima is described. The model is then applied to determine the structures of twinned particles in face-centred materials. Two possible configurations are examined, an arrangement where the crystals are symmetric about the twin boundaries, and an asymmetric configuration. Shapes and total surface energies for a large number of twinned structures are given using a simple model which includes only (111) and (100) surface facets. Excellent agreement is obtained between the theoretical model and experimental results.