Samuel G. Booth

Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroimination and Iminohydroxylation Cyclization Reactions.

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et3N was proposed as a key step of this process.

Electrochemical Insight into the Brust–Schiffrin Synthesis of Au Nanoparticles

The mechanism of the Brust-Schiffrin gold nanoparticle synthesis has been investigated through the use of ion transfer voltammetry at the water/1,2-dichloroethane (DCE) solution interface, combined with X-ray absorption fine structure (XAFS) of the reaction between [AuCl4](-) and thiol (RSH) in homogeneous toluene (TL) solution. Ion transfer calculations indicate the formation of [AuCl2](-) at RSH/Au ratios from 0.2-2 with a time-dependent variation observed over several days. At RSH/Au ratios above 2 and after time periods greater than 24 h, the formation of Au(I)SR is also observed. The relative concentrations of reaction products observed at the liquid/liquid interface are in excellent agreement with those observed by XAFS for the corresponding reaction in a single homogeneous phase. BH4(-) ion transfer reactions between water and DCE indicate that the reduction of [AuCl4](-) or [AuCl2](-) to Au nanoparticles by BH4(-) proceeds in the bulk organic phase. On the other hand, BH4(-) was unable to reduce the insoluble [Au(I)SR]n species to Au nanoparticles. The number and size of the nanoparticles formed was dependent on the concentration ratio of RSH/Au, as well as the experimental duration because of the competing formation of the [Au(I)SR]n precipitate. Higher concentrations of nanoparticles, with diameters of 1.0-1.5 nm, were formed at RSH/Au ratios from 1 to 2.

Structure and bonding in Au( i ) chloride species: a critical examination of X-ray absorption spectroscopy (XAS) data

Au(I) chloride species are important reactants and intermediates in various processes across the chemical sciences and engineering. Structure and bonding in Au(I) species are often characterized by X-ray absorption spectroscopy (XAS), including measurements under reaction conditions. Previously reported XA spectra for Au(I) chloride species have varied significantly, likely as a result of radiation damage and/or partial disproportionation of [AuCl2]− ions, which are metastable under ambient conditions. By monitoring the decomposition of tetrabutylammonium dichloroaurate(I), TBA[AuCl2], in 1,2-dichlorobenzene we have obtained a reliable X-ray absorption spectrum of [AuCl2]− ions by combining the calculation of difference spectra with an extended X-ray absorption fine-structure (EXAFS) determination of the solution composition. The results show that the X-ray absorption near-edge structure (XANES) of [AuCl2]− is characterized by a weak Au 2p3/2 → 5d (‘white line’) transition, which agrees well with the spectrum predicted by electronic structure calculations using the FEFF8 code. Compared to [AuCl4]−, the determined [AuCl2]− spectrum has several distinctive features of diagnostic analytical value. A more detailed densities of states (DOS) analysis of the electronic structure suggests that the weak white line arises from a hybrid Au 6s/5d DOS band that is partially occupied, up to the level of the highest occupied molecular orbital (HOMO). Correlation of Cl coordination numbers determined from the EXAFS with the intensity of the white line in the XANES indicates that the decomposition is a primarily radiation-induced oxidation to Au(III) species with an average formula of [AuCl3OH]−.

Electrochemical modulation of SERS at the liquid/liquid interface.

A surface enhanced Raman scattering system to detect silver nanoparticle adsorption at the water|1,2-dichlorobenzene interface is reported. The Raman response as a function of distance on either side of the interface reveals a reproducible spatial variation, which is potential dependent for a number of adsorption and desorption cycles.

Metal Deposition at the Liquid–Liquid Interface

Metal nanoparticles are readily formed, with a reasonable degree of size and shape control, using solution-based reduction methods under ambient conditions. Despite the large number of reports in this field, much of our knowledge of nanoparticle growth is largely empirical, with the relationship between particle form and growth conditions, for example, still not well understood. Many nanoparticle preparation routes actually depend on not one, but two, solution phases, i.e. the syntheses involve reaction or transfer at the liquid-liquid (organic-water) interface. This interface can be polarised electrochemically, an approach that offers promise as a route to better understanding, and ultimately control, of nanoparticle growth.

The Offset Droplet: A new methodology for studying the solid/water interface using X-Ray Photoelectron Spectroscopy

The routine study of the solid-water interface by XPS is potentially revolutionary as this development opens up whole new areas of study for photoelectron spectroscopy. To date this has been realised by only a few groups worldwide and current techniques have significant restrictions on the type of samples which can be studied. Here we present a novel and uniquely flexible approach to the problem. By introducing a thin capillary into the NAP-XPS, a small droplet can be injected onto the sample surface, offset from the analysis area by several mm. By careful control of the droplet size a water layer of controllable thickness can be established in the analysis area-continuous with the bulk droplet. We present results from the solid-water interface on a vacuum prepared TiO2(110) single crystal and demonstrate that the solid/liquid interface is addressable.

Detection and Characterisation of Sub-Critical Nuclei during Reactive Pd Metal Nucleation by X-ray Absorption Spectroscopy

The interfacial reduction of aqueous [PdCl4]2− at the interface with an organic solution of ferrocene has been characterised by X-ray absorption fine structure (XAFS) spectroscopy. Use of a liquid–liquid interface as a model for homogeneous nucleation permits control of the thermodynamic driving force for nucleation, through variation of the [PdCl4]2− and ferrocene concentrations in the bulk of the adjacent phases. We demonstrate that this approach permits characterisation of the system under conditions of (i) no particle nucleation, (ii) fast spontaneous nucleation of stable nanoparticles and (iii) an intermediate state, in which formation of metastable Pd sub-critical nuclei takes place. Analysis of the XAFS spectra in the metastable state revealed a stochastically fluctuating equilibrium in which Pd nuclei are constantly formed and re-dissolved, as evident from oxidation state fluctuations detected by the Pd XAFS. Supersaturation was evidently sufficient to induce nanoparticle formation but insufficient for nuclei to grow beyond the critical cluster size. We were able to maintain a system in this metastable state for several hours. Such sub-critical clusters are predicted by classical nucleation theory, but have not been detected except in liquid-cell TEM imaging and scanning electrochemical microscopy studies.

Automated analysis of XANES: A feasibility study of Au reference compounds

With the advent of high-throughput and imaging core level spectroscopies (including X-ray absorption spectroscopy, XAS, as well as electron energy loss spectroscopy, EELS), automated data processing, visualisation and analytics will become a necessity. As a first step towards these objectives we examined the possibilities and limitations of a simple automated XANES peak fitting procedure written in MATLAB, for the parametrisation of XANES features, including ionisation potentials as well as the energies and intensities of electronic transitions. Using a series of Au L3-edge XANES reference spectra we show that most of the relevant information can be captured through a small number of rules applied to constrain the fits. Uncertainty in this strategy arises mostly when the ionisation potential (IP) overlaps with weak electronic transitions or features in the continuum beyond the IP, which can result in ambiguity through multiple equally good fits.

Electrodeposition of Prussian Blue/Carbon Nanotube Composites at a Liquid‑Liquid Interface

The iron complex hexacyanoferrate (Fe4[Fe(CN)6]3), known as Prussian Blue (PB), was electrodeposited over a free-standing carbon nanotube (CNT) film assembled at the interface between two immiscible liquids, water and 1,2-dichlorobenzene. Polarization of the interface achieved through a fixed potential or under potential variation enabled iron present inside CNTs to generate a stable CNT/PB composite. We report herein on the observation that the deposition of PB is dependent on both the pH and applied potential. It was found that aqueous phases containing K3[Fe(CN)6] can decompose under an applied potential, while those containing K4[Fe(CN)6] presented more stable behavior making it a suitable precursor for PB synthesis. The electrodeposition and modification of the interface was followed by in situ spectroelectrochemical Raman spectroscopy, which indicated that an increase in signal due to PB formation was acompanied by changes in the CNT bands due to modification of the CNT walls by decoration with PB, forming a composite structure.

Time and space resolved methods: General discussion

Jim De Yoreo presented some slides on in situ AFM, TEM, dynamic force spectroscopy (DFS) and optical spectroscopy investigations of nucleation in the calcium carbonate system: The free energy barrier to homogeneous nucleation of calcite calculated within the framework of classical nucleation theory (CNT) is prohibitive, even at concentrations exceeding the solubility limits of the amorphous phases. Consistent with this analysis, during nucleation in pure solutions, in our in situ TEM experiments we observed direct formation of all phases, including amorphous calcium carbonate (ACC), as well as the three predominant crystalline phases: calcite, vaterite, and aragonite, even under conditions in which ACC readily forms. In addition to direct formation pathways, we observed indirect pathways in which ACC transforms to aragonite and vaterite through nucleation within or on the precursors, rather than via dissolution and reprecipitation. We also observed aragonate transformation to calcite, but never recorded an instance in which ACC transforms into calcite, except via dissolution–reprecipitation reactions.