Detection of Single Crystal Facets of Pt Ultramicroelectrode: Application of Ohmic Microscopy

Analyses of spatially-resolved ohmic microscopy measurements,^1,2,3 using a polycrystalline Pt disk electrode embedded in an insulating surface were found to yield quantitative agreement between theory and experiment.  Our efforts are currently being focused toward assessing the spatial resolution of this technique by employing facetted Pt single crystal facets grown on ultra-small platinum electrodes. The ohmic response,  which represents a direct measure of  the current flowing through a small area of the electrode, has confirmed  the presence of  individual facets on the Pt single crystal bead with clear Pt (111) and (100) orientation.

EXPERIMENTAL

The facetted Pt single crystal microelectrode (ca. 150 μm in diameter) used for these measurements was grown following the method reported by Komanicky and Fawcett.⁴  Double barrel capillaries were pulled to a diameter of ca. 5µm for each barrel, and filled with an aqueous 0.1 M H₂SO₄ solution. Microreference (μ-ref) Ag/AgCl electrodes were made by inserting two Ag wires coated with a layer of AgCl (0.125 mm in diameter) formed by dipping the wire  in a saturated KCl aqueous solution for at least 12 h. The μ-ref Ag/AgCl electrodes and Pt UME were mounted on individual holders connected in turn to individual micromanipulators. Once the tips of μ-ref electrodes were placed in close vicinity of a Pt single crystal facet, cyclic voltammetric curves were recorded using a potentiostat (Autolab, PGSTAT302N, Nova 1.6, Metrohm). The time response of the potential difference between the two μ-ref  electrodes, denoted as, Δφ_sol, amplified 100 times (Ina131, Texas instrument),  was recorded  on a digital multimeter (Keithley 3706 System Switch/Multimeter) using the Labview platform.

  RESULTS AND DISCUSSION

Shown in Figure 1 (upper panel) is the cyclic voltammogram of the facetted Pt single crystal in 0.1M H₂SO₄ recorded at a scan rate ν = 10V/s displaying a hydrogen adsorption|desorption region characteristic of  polycrystalline Pt in this electrolyte. Close inspection of the voltammogram at more positive potentials revealed the presence of two small peaks centered at about  0.45 V vs RHE which are  unique to the (111) surface. Significant differences were observed in the Δφ_sol vs E curves recorded simultaneously (see black curve in lower panel, Fig. 1) where the features associated with the (111) face can be clearly discerned based on a comparison with the voltammogram of a genuine Pt(111) electrode (see red curve in lower panel, Fig1).. Similar conclusions could be drawn from data collected with the μ-ref electrodes placed in the center of the Pt(100) facet. In that case, however, the Δφ_sol vs E curves displayed in addition to a peak unique to Pt(100),  a prominent feature associated with Pt(100) steps. Efforts are now underway to solve Laplace’s equation for this type of facetted electrode to gain insight into the actual spatial resolution of ohmic microscopy in its present state of development and in particular the effect of neighboring regions on the recorded signal.

Acknowledgements

This work was supported by a Grant from NSF.

Reference

1. C. A. Cartier, D. Kumsa, Z. Feng, H. Zhu, D. A. Scherson, Anal.Chem. 84 (2012), 7080
2. Y. Chen , A. Belianinov, D. A. Scherson, J. phy. Chem. C, 112 (2008), 8754
3. M. Ignatowitz, E. Oesterschulze, Applied physics letters, 101 (2012), 251601
4. V. Komanicky, W. Ronald Fawcett, J. Electroanal. Chemistry, 556  (2003) 109
5. J.M. Feliu, J.M. Orts, R Gómez, A. Aldaz, J. Clavilier J. Electroanal. Chem 372(1994),265