Michael V. Mirkin , City University of New York (Queens College)
Electrocatalysis on the nanoscale involving metal nanoparticles and monolayer-protected clusters is a subject of considerable current interest. We are developing methodologies for electrodeposition of metals on the nanoscale aimed at preparing catalytic metal clusters and electrochemical probes with desired properties. Our first task was to demonstrate the feasibility and investigate the mechanism of electrodeposition of metals at nanoelectrodes. Major differences can be expected between nucleation/growth processes occurring on nanometer-sized electrodes and at larger interfaces. The reported values of the active site density on metal surfaces (N0) are in the range 104 < N0 < 1010 cm-2, and typically N0 does not exceed ~108 cm-2.1 Thus, the expected number of active sites on the surface of a ~30-nm-radius electrode is <<1.
Electrodeposition of a liquid metal (Hg) at Pt nanoelectrodes was used as a model system because Hg surface is uniform, and defect-free; its growth is not accompanied by the formation of dendrites and other complications.2 Pt nanoelectrodes were prepared by heat sealing/pulling an annealed 25 µm Pt wire into a borosilicate capillary under vacuum with the help of a laser pipet puller, as described earlier.3 To monitor the initial stages of metal deposition, we employed a new experimental setup based on the use of a patch clamp amplifier to enable measurements of pA-range currents on a microsecond/millisecond timescale (such experiments cannot be done using conventional electrochemical instrumentation). These experiments showed the possibility of a yet unknown mechanism. While at larger (e.g., micrometer-sized) electrodes, classical nucleation/growth pathway is always followed, at smaller (e.g., ~50 nm radius) electrodes two distinct types of behavior were observed. At lower overpotentials, the recorded potentiostatic transients fit well classical theory for a single growing nucleus. However, at higher overpotentials, no nucleation event was observed, and the deposition of Hg seemed to start instantaneously on the entire Pt surface. At even smaller electrodes, this highly unusual behavior was observed at both higher and lower overpotentials. These unprecedented observations may be attributed to a very low number of active nucleation sites on the nanoelectrode surface.
We also fabricated nanometer-sized electrodes by electrodeposition of different metals on disk-type, polished or recessed Pt nanoelectrodes.4 The deposition was monitored chronoamperometrically and by scanning electrochemical microscopy (SECM) to demonstrate feasibility of the shape and size control on the nanoscale. Well-shaped hemispherical nanoelectrodes were produced by depositing Hg at the surface of flat polished Pt tips. The size and geometry of such electrodes were verified independently by voltammetric, coulometric and SECM measurements. Although a nanoelectrode produced by deposition of a solid metal can yield well-shaped voltammograms, its geometry is hard to characterize and its surface is not polishable. These problems were overcome by etching a Pt nanoelectrode and then filling the resulting nanocavity with a different metal. By selecting suitable deposition time and keeping the metal ion concentration sufficiently low (µM), one can precisely control the amount of the deposited metal to obtain a flat polishable nanoelectrode. The possibility of electrodepositing nm-sized metal clusters was also demonstrated.
The PRF support enabled us to start this new project. A graduate student and a postdoctoral fellow involved in this research learned a great deal about nanoelectrochemistry and developed new methodologies, which are essential for the success of other projects underway in our laboratory.5
1. Heerman, L.; Tarallo, A. J. Electroanal. Chem. 1998, 451, 101.
2. Gunawardena, G.; Hills, G.; Scharifker, B. J. Electroanal. Chem. 1981, 130, 99.
3. Sun, P.; Mirkin, M. V. Anal. Chem. 2006, 78, 6526.
4. Velmurugan, J.; Mirkin, M. V. ChemPhysChem 2010, 11, 3011.
5. Nogala, W.; Velmurugan, J.; Mirkin, M. V., to be submitted to Nano Lett.