Bond length contraction in gold nanoparticles (original) (raw)
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Structure of catalytically active gold nanoparticles by XAFS spectroscopy
Journal of Physics: Conference Series, 2009
A series of Au/Al 2 O 3 catalysts prepared by the different procedures and series of model system Au/Si(100), Au/Si(111) differing in the mean Au particle sizes (from 2 nm up to 30 nm) were studied by EXAFS and XANES techniques. No visible changes in electron states of Au for all samples prepared by different methods in comparison with bulk material were detected. Oxygen atoms were not detected around Au atoms within the detection limits of our study (a few per cents of surrounding atoms). A gradual reduction of the Au-Au bond length and first shell Au-Au coordination number and an increase in the Debye-Waller factor are observed as the size of supported Au particles decreases. The significant increase of structural disorder for the smallest Au particles comparatively to the bulk Au metal and sizeable particles was detected. These variations in micro-structural parameters of Au nanoparticles are in line with an increase in their catalytic activity in CO oxidation. For model systems some coordination of Au-atoms from islands and Si-atoms from crystal surface was detected for thin Au "films".
Journal of the American Chemical Society, 2018
Here we report on the very low size stability of electrocatalytically-active 1.5 to 2.0 nm diameter tetrakis(hydroxymethyl)phosphonium chloride-stabilized Au nanoparticles (THPC Au2nm NPs) chemically attached to glass/indium tin oxide electrodes. The potential for oxidative dissolution of THPC Au2nm NPs in the presence of bromide is about 250 mV negative of 4 nm diameter citrate-stabilized Au NPs (Cit Au4nm NPs) and 450 mV negative of bulk Au, which provides us with an easy method to assess the size stability using anodic stripping voltammetry (ASV). The THPC Au2nm NPs show a strong CO2 reduction wave at about-0.40 V (vs. RHE), which is non-existent for the Cit Au4nm NPs or bulk Au. The THPC Au2nm NPs are also comparatively more electroactive for the hydrogen evolution reaction (HER). In acid electrolyte, however, the potential for surface Au2O3 formation on THPC Au2nm NPs is significantly negative relative to bulk Au and a single cycle through the surface oxide and reduction waves leads to an increase in the NP size to about 4 nm. Similarly, the THPC Au2nm NPs undergo Ostwald ripening in the presence of bromide within 5 minutes at potentials well before oxidation, which increases
Journal of Catalysis, 2006
Au catalysts with different metallic particle sizes and supported on silica, alumina, titania, zirconia, ceria, and niobia were prepared, and the reduced catalysts were characterized by EXAFS spectroscopy. As the Au-Au coordination number decreased, the interatomic bond length decreased. The Au-Au bond length contraction appears to be independent of the support type. A correlation between the dispersion of Pt catalysts determined by hydrogen chemisorption and the EXAFS Pt-Pt coordination number was established and used to determine the dispersion of fully reduced Au catalysts. In addition, the Au particle size was estimated using a literature correlation of the EXAFS coordination number. For particles larger than about 40 Å, there was little change in the metallic bond length, whereas in catalysts with gold particles smaller than 30 Å, the Au-Au distance decreased with decreasing particle size, with a maximum contraction of about 0.15 Å. Decreasing particle size also brought a decrease in the intensity of the white line of the XANES spectrum. Both the decrease in bond distance and white line intensity were consistent with an increase in the d-electron density of Au atoms in very small particles. Au particles smaller than about 30 Å were also reactive to air, leading to oxidation of up to 15% of the atoms of the gold particles, depending on the size; larger particles were not oxidized. These oxidized Au atoms in small particles are suggested to be active for CO oxidation.
EXAFS studies on gold nanoparticles over novel catalytic materials
Radiation Physics and Chemistry, 2006
Novel nanogold catalytic systems made up of gold nanoparticles ($2-6 nm) supported on niobium, ytterbium, lanthanum and cerium oxide materials were synthesized. XAS is uniquely suited for studying catalytic systems with low metal and high metal dispersion. Au L 3 edge X-ray absorption spectroscopic measurements were carried out over a series of supported gold nanoparticles. The interesting results obtained from EXAFS and XANES confirms the typical characteristics and structure of gold nanoparticles in these materials. r
Synthesis of gold nanoparticles with different atomistic structural characteristics
Materials Characterization, 2007
A chemical reduction method was used to produce nanometric gold particles. Depending on the concentration of the main reactant compound different nanometric sizes and consequently different atomic structural configurations of the particles are obtained. Insights on the structural nature of the gold nanoparticles are obtained through a comparison between digitally-processed experimental high-resolution electron microscopy images and theoretically-simulated images obtained with a multislice approach of the dynamical theory of electron diffraction. Quantum molecular mechanical calculations, based on density functional theory, are carried out to explain the relationships between the stability of the gold nanoparticles, the atomic structural configurations and the size of nanoparticles.
Gold nanoparticles a renaissance in gold chemistry
Gold Bulletin, 1996
Vapour synthesis techniques have been used to prepare nanoparticulate dispersions of gold and other precious metals in non-aqueous solvents. The dimensions of these solvent-stabilised particles, which can be controlled within the 1-3nm size regime, effectively encompass the areas of molecular chemistry (as typified by high-nuclearity metal clusters) and the smaller colloidal metals. Gold nanoparticles differ from those of the other metals in exhibiting unusual time-and concentration-dependent behaviour. A regime of preparative conditions under which 1-3nm size gold particles, which are stable with respect to aggregation as a function of time, is defined. Some implications for these new developments are indicated.
The Journal of Physical Chemistry C, 2011
The origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO 2 (110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O 2 , Ar, or a mixture of O 2 þ CO and of Ar þ CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle's diameter of 2.1 ( 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both "model" and "real" catalysts.