Development of nanoporous gold films for electrochemical applications

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Development of nanoporous gold films for electrochemical applications
Lee, Keon-U
Shin, Hyung-Joon
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Graduate School of UNIST
Nanoporous gold thin films on various substrates were prepared by co-sputtering method and used for electrochemical applications. Binary gold alloy was made with less noble element which was selectively dissolved from the alloy to make nanoporous structures. The composition of alloy was easily controlled by adjusting each power of targets. The size of ligaments and holes were under ~100 nm. We used conventional three electrode system, nanoporous gold film, Ag/AgCl (3 M NaCl), and platinum wire were used as working electrode, reference electrode, and counter electrode, respectively. The cyclic voltammetry and chorono amperometry were employed. We made nanoporous gold for amperometric aniline sensor, polyaniline coated nanoporous gold for supercapacitor, and nanoporous gold palladium for ethanol oxidation reaction. First, we improved the fabrication process of nanoporous gold thin films. Previously, during the dealloying process, hydrofluoric acid (HF) solution was used for dissolving the Si the sacrificial element. Potassium hydroxide (KOH) solution were used instead of hydrofluoric acid solution due to its safety nature and more environmentally friendly method. For deposition, although silicon substrate is commonly used, glass substrate was chosen for availability and its lower cost. We conducted heat treatment for enhancing the adhesion between glass substrate and films. Furthermore, we used nanoporous gold as amperometric sensor for aniline. Constant voltage was applied to working electrode (nanoporous gold) and 10 μM of aniline were successively added. The current was linearly increased with the concentration of aniline and showed low detection limit (0.5 μM). The reaction was carried in both phosphate buffered saline (PBS) solution and tap water. Our nanoporous gold sensor exhibited high sensitivity, wide sensing range, and low detection limit. Secondly, we made polyaniline coated nanoporous gold for supercapacitor. For nanoporous gold, magnesium and acetic acid were used rather than silicon and hydrofluoric acid or potassium hydroxide. Then, polyaniline was uniformly coated on the nanoporous gold by electrochemical method. The electrolyte, concentration of aniline, and electrochemical techniques were optimized or uniform and compact coated polyaniline. The thickness of polyaniline was ~ 10 nm, which is confirmed by transmission electron microscopy (TEM). Our polyaniline-nanoporous gold supercapacitor showed high areal capacitance, fast charge-discharge rates, and good stability. Thirdly, nanoporous gold palladium was fabricated for ethanol oxidation reaction. The ternary alloy of gold, palladium, and magnesium were made and magnesium from the alloy were dissolved to make nanoporous gold palladium. The magnesium was easily dissolved by diluted acetic acid, an easy and environmentally friendly/safe method. Optimization of the composition and porosity of nanoporous gold palladium were done by adjusting each power of sputtering targets. TEM and energy dispersive x-ray spectroscopy (EDS) results showed gold and palladium are homogeneously distributed and magnesium is almost removed by diluted acetic acid. X-ray diffraction (XRD) and fast Fourier transformation (FFT) data showed that the lattice spacing of nanoporous gold palladium is ~2.3 Å, confirmed that gold and palladium are alloyed. Our nanoporous gold palladium exhibited high current density and great stability toward ethanol oxidation reaction in alkaline media. Nanoporous structures offer a number of advantages such as large surface area, high stability, and high catalytic activity. The nanoporous structures can be applied to not only gold but also other noble metals (platinum, palladium, etc.) or cheap metals (silver, copper, etc.) as well as bimetallic alloys. Using our co-sputtering method and nanoporous structures, versatile materials could be created for different research fields.
Department of Materials Science and Engineering
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