Preparation of Self-Organized Nanoporous Anodic Gallium Oxide and Its Potentiometric Behavior
Metallic gallium has been utilized to fabricate µm-scale electrodes within polydimethylsiloxane (PDMS)-based microchannels [1,2] and glass capillaries  by taking advantage of its low melting point (ca. 30 °C). These electrodes can be prepared by freezing liquid gallium introduced into the space of interest at low temperature. The resulting gallium electrodes have been applied for conductivity  and amperometric  detection.
Recently, we have found the formation of self-organized nanoporous structures by anodizing solid gallium metal , as with aluminum and other valve metals (e.g., Ti) . The nanoporous structures could be obtained by anodizing solid gallium metal in ice-cooled 4 and 6 M aqueous H2SO4 at 10 V and 15 V. The diameters of the resulting nanopores have a narrow diameter distribution (relative standard deviation: ca. 10-20%), and could be controlled in the range of 18-40 nm by varying the H2SO4 concentration and anodization voltage. The EDX data of the materials indicate that the nanoporous materials were based on gallium oxide (Ga2O3) with sulfate as an impurity introduced during the anodization. Such nanoporous structures could be formed on the surface of solid gallium metal introduced at the end of a glass capillary, indicating the capability to integrate nanoporous monoliths with microfluidic channels.
More recently, we have investigated the potentiometric pH responses of the nanoporous anodic oxide-coated gallium electrodes . This project was initiated under the hypothesis that, due to the presence of the impurity sulfate, the physicochemical properties of the anodic gallium oxide is different from that of common gallium oxide that are known as a wide bandgap semiconductor (~ 4.8 eV). The oxide-coated gallium electrodes exhibited potentiometric pH responses (i.e., a potential increase at lower pH) only after potentiometric measurements in a solution of pH ≤ 3. This observation suggests that the anodic oxide layer is not conductive enough for reliable potentiometric measurements, and thus needs to be dissolved in an acidic solution to measure a potential defined by redox reactions involving the underlying gallium electrode. The response slope was smaller than that measured at gallium electrodes coated with much thinner native oxide layers, reflecting the sluggish redox reaction at the anodic-oxide coated electrodes. The corrosion of the gallium oxide and gallium metal limits their applicability for analytical devices. These results provide fundamental knowledge required to develop miniaturized electrodes based on nanoporous anodic oxide-coated gallium.
This work was primarily funded by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy (DE-FG02-12ER16095). The authors also acknowledge Terry C. Johnson Center for Basic Cancer Research and Targeted Excellence Funds of Kansas State University for partial financial support of this work.
1. So, J.-H.; Dickey, M. D. Lab Chip 2011, 11, 905-911.
2. Thredgold, L. D.; Khodakov, D. A.; Ellis, A. V.; Lenehan, C. E. Analyst 2013, 138, 4275-4279.
3. Wei, C.; Bard, A. J.; Kapui, I.; Nagy, G.; Toth, K. Anal. Chem. 1996, 68, 2651-2655.
4. Pandey, B.; Thapa, P. S.; Higgins, D. A.; Ito, T. Langmuir 2012, 28, 13705-13711.
5. Ghicov, A.; Schmuki, P. Chem. Commun. 2009, 2791-2808.
6. Pandey, B.; Cox, C.; Thapa, P. S.; Ito, T. submitted.