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41667-AC5
Controlling Electroless Deposition within Microporous Hosts

Robert A.W. Dryfe, University of Manchester

The work has continued to explore the main themes of the project, as set out in the end of year 1 and year 2 reports. The overall goal of the work has been (i) to understand the process controlling/affecting the metal deposition process at the liquid/liquid (L/L) interface and (ii) to obtain information on the structure of the resultant deposits. An improved in situ spectroelectrochemical cell has been developed to monitor the deposition process at the water/organic interface, building on work reported in the year 1 report. This work was performed in collaboration with Drs Ruiz and Collina (University of Burgos, Spain), and made use of the improved instrumental set-up available in their laboratory. The findings confirmed our earlier conclusions, that a mixed kinetic-diffusion model adequately described the metal deposition process at the L/L interface: a general conclusion from this work is that the interfacial potential can be used to control the deposition rate. The interfacial potential can be controlled through the common-ion ratio, for a spontaneous reduction, the ratio of aqueous: organic perchlorate ion concentration has been varied to achieve this, keeping the ionic strength of each phase constant. We have also continued the work reported in the year 2 report, namely the use of zeolite membranes (specifically of silicalite, pore diameter ca. 0.6 nm, synthesised in house) as templates to control the deposition process. This work has shown (see nugget) that deposit morphology can be readily controlled through the use of such templates, for deposition at the liquid/liquid interface and also for conventional electrodeposition (the latter is a new departure, which has been investigated over the past year). In summary, this work presents a simple solution phase route to the deposition of metal particles of ca. 1 nm diameter. In the last couple of months we have been studying the voltammetric response of the deposits, to correlate the observed responses with those seen for metal deposits of conventional dimensions.

We have also investigated two new aspects of this project during the past 12 months. The first of these is the application of classical electrochemical models to the electrochemically-driven deposition process at the liquid/liquid interface. In the liquid/liquid case, both the transport of the metal precursor and of the reducing agent (i.e. diffusion in each phase) must be considered. Previous attempts by other workers to develop current-time models (as a function of nucleation rate constant) to describe this situation have led to poor agreement with experiment. A simplified experimental approach was undertaken here, where an excess of the electron donor was used experimentally. This permitted us to make the approximation that transport was solely controlled by one phase, and thereby conventional (single phase) models for electrodeposition could be applied. These were found to give reasonable fits to the experimental data, moreover the nucleation parameters so obtained were physically reasonable. The second new aspect is the use of in situ optical microscopy to observe the structures formed on metal deposition. As the available microscopy data indicates, nanometre scale structures are formed. However, in the absence of a specific template (such as the zeolites, see above), the particles tend to aggregate to form larger, ill-defined clusters – as described in the year 1 report. Rather than trying to restrict this process, we have sought to take advantage of it, given that the aggregates can be observed with optical microscopy once they reach the micron scale. This allows us to apply image analysis techniques to the aggregates in situ: an essential element which is missing from our other analyses of morphology at the L/L interface. We have thereby been able to extract parameters such as the radii of gyration and number of aggregates as functions of time, and also to determine the fractal dimensions of the aggregates. The analysis has shown that standard 2-dimensional cluster aggregation models appear to apply, at least for the system studied (gold deposition at the L/L interface), although the aggregation rate is slowed by an increase in the aqueous perchlorate ion ratio – contrary to the effect of this ion on the metal deposition rate (see above).

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