Chemical identification of point defects and adsorbates on a metal oxide surface by atomic force microscopy.

Atomic force microscopy in the non-contact mode (nc-AFM) can provide atom-resolved images of the surface of, in principle, any material independent of its conductivity. Due to the complex mechanisms involved in the contrast formation in nc-AFM imaging, it is, however, far from trivial to identify individual surface atoms or adsorbates from AFM images. In this work, we successfully demonstrate how to extract detailed information about defects and the chemical identity of adsorbates on a metal oxide surface from nc-AFM images. We make use of the observation that the apex of the AFM tip can be altered to expose either a positive or negative tip termination. The complementary set of images recorded with the two tip terminations unambiguously define the ionic sub-lattices and reveal the exact positions of oxygen vacancies and hydroxyl (OH) defects on a TiO(2) surface. Chemical specificity is extracted by comparing the characteristic contrast patterns of the defects with results from comprehensive AFM simulations. Our methodology of analysis is generally applicable and may be pivotal for uncovering surface defects and adsorbates on other transition metal oxides designed for heterogeneous catalysis, photo-electrolysis or biocompatibility.

Methanation of CO over nickel: Mechanism and kinetics at high H2/CO ratios.

The CO methanation reaction over nickel was studied at low CO concentrations and at hydrogen pressures slightly above ambient pressure. The kinetics of this reaction is well described by a first-order expression with CO dissociation at the nickel surface as the rate-determining step. At very low CO concentrations, adsorption of CO molecules and H atoms compete for the sites at the surface, whereas the coverage of CO is close to unity at higher CO pressures. The ratio of the equilibrium constants for CO and H atom adsorption, K(CO)/K(H), was obtained from the rate of CO methanation at various CO concentrations. K(H) was determined independently from temperature programmed adsorption/desorption of hydrogen to be K(H) = 7.7 x 10(-4) (bar(-0.5)) exp[43 (kJ/mol)/RT] and hence the equilibrium constants for adsorption of CO molecules may be calculated to be K(CO) = 3 x 10(-7) (bar(-1)) exp[122 (kJ/mol)/RT]. Furthermore, the rate of dissociation of CO at the catalyst surface was determined to be 5 x 10(9) (s(-1)) exp[-96.7 (kJ/mol)/RT] assuming that 5% of the surface nickel atoms are active for CO dissociation. The results are compared to equilibrium and rate constants reported in the literature.

Atomic-scale imaging of carbon nanofibre growth.

The synthesis of carbon nanotubes with predefined structure and functionality plays a central role in the field of nanotechnology, whereas the inhibition of carbon growth is needed to prevent a breakdown of industrial catalysts for hydrogen and synthesis gas production. The growth of carbon nanotubes and nanofibres has therefore been widely studied. Recent advances in in situ techniques now open up the possibility of studying gas-solid interactions at the atomic level. Here we present time-resolved, high-resolution in situ transmission electron microscope observations of the formation of carbon nanofibres from methane decomposition over supported nickel nanocrystals. Carbon nanofibres are observed to develop through a reaction-induced reshaping of the nickel nanocrystals. Specifically, the nucleation and growth of graphene layers are found to be assisted by a dynamic formation and restructuring of mono-atomic step edges at the nickel surface. Density-functional theory calculations indicate that the observations are consistent with a growth mechanism involving surface diffusion of carbon and nickel atoms. The finding that metallic step edges act as spatiotemporal dynamic growth sites may be important for understanding other types of catalytic reactions and nanomaterial syntheses.

Atom-resolved imaging of dynamic shape changes in supported copper nanocrystals.

In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.