Donald Bethell

Synthesis of thiol-derivatised gold nanoparticles in a two-phase Liquid–Liquid system

Using two-phase (water–toluene) reduction of AuCl4– by sodium borohydride in the presence of an alkanethiol, solutions of 1–3 nm gold particles bearing a surface coating of thiol have been prepared and characterised; this novel material can be handled as a simple chemical compound.

A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups.

So-called bottom-up fabrication methods aim to assemble and integrate molecular components exhibiting specific functions into electronic devices that are orders of magnitude smaller than can be fabricated by lithographic techniques. Fundamental to the success of the bottom-up approach is the ability to control electron transport across molecular components. Organic molecules containing redox centres—chemical species whose oxidation number, and hence electronic structure, can be changed reversibly—support resonant tunnelling and display promising functional behaviour when sandwiched as molecular layers between electrical contacts, but their integration into more complex assemblies remains challenging. For this reason, functionalized metal nanoparticles have attracted much interest: they exhibit single-electron characteristics (such as quantized capacitance charging) and can be organized through simple self-assembly methods into well ordered structures, with the nanoparticles at controlled locations. Here we report scanning tunnelling microscopy measurements showing that organic molecules containing redox centres can be used to attach metal nanoparticles to electrode surfaces and so control the electron transport between them. Our system consists of gold nanoclusters a few nanometres across and functionalized with polymethylene chains that carry a central, reversibly reducible bipyridinium moiety. We expect that the ability to electronically contact metal nanoparticles via redox-active molecules, and to alter profoundly their tunnelling properties by charge injection into these molecules, can form the basis for a range of nanoscale electronic switches.

Spontaneous ordering of bimodal ensembles of nanoscopic gold clusters

The controlled fabrication of very small structures at scales beyond the current limits of lithographic techniques is a technological goal of great practical and fundamental interest. Important progress has been made over the past few years in the preparation of ordered ensembles of metal and semiconductor nanocrystals. For example, monodisperse fractions of thiol-stabilized gold nanoparticles have been crystallized into two- and three-dimensional superlattices. Metal particles stabilized by quaternary ammonium salts can also self-assemble into superlattice structures,. Gold particle preparations with quite broad (polydisperse) size distributions also show some tendency to form ordered structures by a process involving spontaneous size segregation,. Here we report that alkanethiol-derivatized gold nanocrystals of different, well defined sizes organize themselves spontaneously into complex, ordered two-dimensional arrays that are structurally related to both colloidal crystals and alloys between metals of different atomic radii. We observe three types of organization: first, different-sized particles intimately mixed, forming an ordered bimodal array (Fig. 1); second, size-segregated regions, each containing hexagonal-close-packed monodisperse particles (Fig. 2); and third, a structure in which particles of several different sizes occupy random positions in a pseudo-hexagonal lattice (Fig. 3).

Synthesis and reactions of functionalised gold nanoparticles

Stable functionalised gold nanoparticles are prepared by simultaneous reduction of tetrachloroaurate ions and attachment of bifunctional organic thiol molecules to the growing gold nuclei leading to a material whose chemical behaviour is characterised by the vacant functionality of the bifunctional thiol ligand.

From monolayers to nanostructured materials : an organic chemist's view of self-assembly

Abstract Simple methods are described for the production of gold nanoparticles with narrow size distributions by reduction of tetrachloroaurate solutions in the presence of thiol-containing organic compounds which self-assemble on the gold surface. Stable solutions of somewhat larger particles can be produced if the thiol is absent. The thiol-derivatized materials are stable in air over long periods and can be handled in much the same way as simple organic compounds. Using dithiols as the derivatizing spacer units, ways have been developed for the preparation of materials in three dimensional form and as thin films attached to a solid substrate. Such materials show conductivities that mimic the behaviour of semiconductors and that depend markedly on the structure of the dithiol used to link the gold particles together. The increase in conductivity with increasing temperature probably involves activated electron hopping from particle to particle. Surfaces treated with a coating of the materials show electroreflectance changes with applied potential that also differ according to the structure of the dithiol spacer. Unusual effects have been observed on heterogeneous electron transfer from electrode surfaces treated with layers of the gold nanoparticles and dithiol spacers. Applications for these nanostructured materials can be envisaged, which range from submicroelectronic devices and circuitry to electrical modification of the reflectance of glass. Such applications will require a multidisciplinary approach with a substantial organic chemical research input.

Long-range electron tunnelling in oligo-porphyrin molecular wires

Short chains of porphyrin molecules can mediate electron transport over distances as long as 5–10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction. A combination of calculations and electrical measurements on oligo-porphyrin wires in single-molecule junctions strongly suggest that the mechanism of long-range charge transport is phase-coherent electron tunnelling.

Measurement of single molecule conductivity using the spontaneous formation of molecular wires

A technique to measure the electrical conductivity of single molecules has been demonstrated. The method is based on trapping molecules between an STM tip and a substrate. The spontaneous attachment and detachment of α,ω-alkanedithiol molecular wires was easily monitored in the time domain. Electrical contact between the target molecule and the gold probes was achieved by the use of thiol groups present at each end of the molecule. Characteristic jumps in the tunnelling current were observed when the tip was positioned at a constant height and the STM feedback loop was disabled. Histograms of the measured current jump values were used to calculate the molecular conductivity as a function of bias and chain length. In addition, it is demonstrated that these measurements can be carried out in a variety of environments, including aqueous electrolytes. The changes in conductivity with chain length obtained are in agreement with previous results obtained using a conducting AFM and the origin of some discrepancies in the literature is analysed.

Oxidation of alcohols using supported gold and gold–palladium nanoparticles

A key discovery in the last two decades has been the realisation that gold, when prepared as supported nanoparticles, is exceptionally effective as an oxidation catalyst, particularly for the oxidation of carbon monoxide at sub-ambient temperature but also for a number of organic reactions of synthetic significance. To some extent this observation is counterintuitive since extended gold surfaces do not chemisorb oxygen, nor do they corrode. For some oxidation reactions, the catalytic activity is markedly enhanced by the addition of palladium. This paper is concerned with recent advances in understanding the mechanism of catalysis by gold–palladium alloy nanoparticles of one such organic reaction, the oxidation of alcohols to the corresponding carbonyl compounds by molecular oxygen. We report detailed reaction studies using a high activity catalyst prepared by sol-immobilisation on a titania support. Using solvent-free conditions, benzyl alcohol is oxidised primarily to benzaldehyde but small amounts of toluene are also formed. The origin of these products is explored using initial rate measurements, deuterium labelling and kinetic isotope effects, and by the study of substituent effects. The effect of changing the nature of the catalyst support is also briefly examined. On the basis of all the results, we consider that we have evidence for multiple reaction pathways in this heterogeneous system. We put forward general mechanisms for the overall processes and describe confirmatory experiments in support of these, and we suggest possible reaction intermediates involved in the heterogeneously catalysed reaction.

Solvent-free oxidation of benzyl alcohol using Au–Pd catalysts prepared by sol immobilisation

We report the preparation of Au-Pd nanocrystalline catalysts supported on TiO(2) and carbon prepared via a sol-immobilisation technique using three different preparation strategies; namely, simultaneous formation of the sols for both metals or initial formation of a seed sol of one of the metals followed by a separate step in which a coating sol of the second metal is added. The catalysts have been structurally characterised using a combination of transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts have been evaluated for the oxidation of benzyl alcohol under solvent-free conditions. The catalysts prepared using the sol immobilisation technique show higher activity when compared with catalysts prepared by impregnation, particularly as lower metal concentrations can be used. The Au-Pd catalysts were all more active than the corresponding monometallic supported Au or Pd catalysts. For 1 wt% Au-Pd/TiO(2) the order of metal addition in the preparation was not observed to be significant with respect to selectivity or activity. However, the 1 wt% Au-Pd/carbon catalysts are more active but less selective to benzaldehyde than the TiO(2)-supported catalysts when compared at iso-conversion. Furthermore, for the carbon-supported catalyst the order of metal addition has a very marked affect on activity. The carbon-supported catalysts are also more significantly affected by heat treatment, e.g. calcination at 400 degrees C leads to the activity being decreased by an order of magnitude, whereas the TiO(2)-supported catalysts show a 50% decrease in activity. Toluene is observed as a by-product of the reaction and conditions have been identified that minimise its formation. It is proposed that toluene and benzaldehyde are formed by competing parallel reactions of the initial benzyl intermediate via an adsorbed benzylidene species that can either be hydrogenated or oxidised. Hence, conditions that maximise the availability of oxygen on the catalyst surface favour the synthesis of benzaldehyde.

Controlling the Duality of the Mechanism in Liquid‐Phase Oxidation of Benzyl Alcohol Catalysed by Supported Au–Pd Nanoparticles

In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.