The effect of electrode size and surface heterogeneity on electrochemical properties of ultrananocrystalline diamond microelectrode

Abstract

We report here the effect of electrode size on electrochemical properties of boron-doped ultrananocrystalline diamond (UNCD) microelectrodes using electrochemical impedance spectroscopy (EIS). By reducing microelectrode size from 250-μm to 10-μm diameter (D), the shape of impedance spectra changes from linear line to two-arcs. The fitting of experimental data to electrochemical circuit model suggests that each arc likely corresponds to UNCD grains and grain boundary phases. The two phases become separable as a result of microelectrode size reduction. In addition, for D ≤ 100-μm, microstructural and morphological defects/heterogeneities of grain boundaries and the presence of surface oxygen are also revealed in the spectra. The microelectrode size reduction specifically affect the impedance of the grain boundaries, e.g. for ultramicroelectrodes, UMEs (D ≤ 25-μm), as the grain boundary impedance increases by ~30-fold. Thus, at UMEs, the grain-grain boundary properties are revealed more sensitively in the spectra. Atomic force microscopy, scanning electron microscopy, Raman spectroscopy and surface profilometry measurements were performed to study the influence of microfabrication on surface properties. A significant increase in surface roughness after microfabrication shows that heterogeneities as observed in the spectra are not only due to intrinsic UNCD properties but also arises from microfabrication.

Keywords: Diamond; Electrochemical; Impedance spectroscopy; Kinetics; Nanocrystalline; icroelectrode.

Fig. 1.

(a) SEM image of one of the chips showing the nine individually addressable 200-μm microelectrodes in 3 × 3 array format. The inset SEM image shows the UNCD surface morphology. (b) Cyclic voltammograms of UNCD microelectrodes of varying diameters (10, 25, 50, 100, 150, 200 and 250 μm). The inset shows the voltammograms of smaller size microelectrodes (c) The ΔE_p values for different sized microelectrodes. The solution is 5 mM $\text{Fe}{(\text{CN})}_6^{3-/4-}$ in 1 M KCl electrolyte. Scan rate is 100 mV/s. The scale bar in (a) is 200-μm and 1-μm (inset).

Fig. 2.

Effect of buffered oxide etchant (BOE) on UNCD surface morphology. UNCD film was exposed to freshly prepared 10:1 BOE. AFM images of surfaces with no treatment, (a,c) and 10 min treatment (b,d). The scan size is 25 μm². (e) Raman spectra of UNCD microsurface before (black curve) and after 10 min BOE treatment (blue curve).

Fig. 3.

Nyquist plot of UNCD microelectrodes: (a) EIS spectra 1 for 250-μm (black dotted), 200-μm (green dotted), 150-μm (brown dotted). (b) EIS spectra 2 for 100-μm (orange dotted), 50-μm (red dotted). (c) EIS spectra 3 for 25-μm (gray dotted) and 10-μm (purple dotted). The inset in (c) shows the expanded view of 25-μm plot. The solid curves are fitted to experimental data. The electrolyte is 5 mM $\text{Fe}{(\text{CN})}_6^{3-/4-}$ in 1 M KCl. 10 mV amplitude, 0.1 Hz–100 kHz.

Fig. 4.

Overlay of Nyquist plots of 25-μm UNCD microelectrodes. The eight spectra were collected from 4 different UNCD chips. The electrolyte is 5 mM $\text{Fe}{(\text{CN})}_6^{3-/4-}$ in 1 M KCl. 10 mV amplitude, 0.1 Hz–100 kHz.

Fig. 5.

Nyquist plots of 10-μm UNCD (blue), MCD (brown) and platinum (black) microelectrodes. The electrolyte is 5 mM $\text{Fe}{(\text{CN})}_6^{3-/4-}$ in 1 M KCl. 10 mV amplitude, 0.1 Hz–100 kHz.

Fig. 6.

Bode and Bode phase plots of UNCD microelectrodes: 250-μm (black), 200-μm (green), 150-μm (brown), 100-μm (orange), 50-μm (red), 25-μm (gray) and 10-μm (purple). The solid curves are fitted to the experimental data. The electrolyte is 5 mM $\text{Fe}{(\text{CN})}_6^{3-/4-}$ in 1 M KCl. The inset shows the SEM image of MCD microelectrode surface. The scale bar is 1-μm.