Reports: ND549555-ND5: How Many Metal Atoms Can Act as a Catalyst?

Michael V. Mirkin, City University of New York (Queens College)

During the report year, we carried out fundamental studies of metal cluster deposition at nanometer-sized electrodes. The combination of nanoelectrode electrochemistry with atomic force microscopy (AFM) allowed us to investigate the mechanism of these processes that would not be accessible by larger electrochemical probes. The methodology was developed for AFM imaging of a nanoelectrodes in air and in solution can provide detailed information about its geometry and surface reactivity that would be hard to obtain by any other technique.1 This information is essential for reliable interpretation of nanoelectrochemical experimental data.2 The nanoelectrodes characterized by AFM are not damaged; they can be employed in electrochemical experiments and as SECM probes. In this way, one can monitor in situ the formation of nanoscale surface structures during electrochemical experiments, including nucleation/growth of metal nanoclusters.

Three-dimensional nucleation and growth on active surface sites are fundamentally important initial stages of electrodeposition of metals.3 Electrochemical studies of these processes are greatly complicated by the formation of multiple crystals interacting with each other.4 We investigated Ag electrodeposition on the surface of well-characterized, nanometer-sized Pt electrodes and measured the nucleation/growth kinetics of individual Ag crystals by combination of nanoelectrochemistry and AFM. Basic parameters, including the number of surface active sites, the kinetic time lag, and the number of growing nuclei, were directly accessed from current transients and in-situ AFM imaging.5

The existence of a single nucleation site on the surface of a 50 nm electrode persisting through several deposition/stripping cycles has been demonstrated. Two active sites can exist on the surface of a larger (e.g., 200 nm) electrode, so that metal crystals can form on either of them or on both of them in the same nucleation experiment. The kinetic time lag at nanoelectrodes was much shorter than could be expected from previous nucleation experiments at larger electrodes, indicating that the nucleation rate may be faster than the values reported in earlier studies. The growth of a nm-sized Ag nucleus was shown to be diffusion-controlled and to follow the classical theory over four orders of magnitude in time. The developed approach should also be useful for studying the effects of deposition conditions on the geometry of growing metal clusters and nanoparticles, monitoring the formation of dendrites, and addressing other practically important issues. The first practical application was fabrication of nanoelectrochemical sensors under the AFM control.6

The PRF support enabled us to start this new project. One undergraduate student, one PhD student and two postdoctoral fellows involved in this research learned a great deal about nanoelectrochemistry and scanning probe microscopy. We are currently applying for federal grants to further develop this new area of our research.

1.  Nogala, W.; Velmurugan, J.; Mirkin, M. V. Anal. Chem. 2012, 84, 5192.

2.  Murray, R. W. Chem. Rev. 2008, 108, 2688.

3.  Milchev, A. Electrocrystallization: fundamentals of nucleation and growth, Kluwer, Boston, 2002.

4.  Radisic, A.; Vereecken, P. M.; Hannon, J. B.; Searson, P. C.; Ross, F. M. Nano Lett. 2006, 6, 238.

5.  Velmurugan, J.; Noël, J.-M.; Nogala, W.; Mirkin, M. V. Chem. Sci. 2012,
DOI: 10.1039/C2SC21005C.

6.  Wang, Y.; Noël, J.-M.; Velmurugan, J.; Nogala, W.; Mirkin, M. V.; Lu, C.; Guille Collignon, M.; Lemaître, F.; Amatore, C. Proc. Nat. Acad. Sci. USA 2012, 109, 11534.