Dynamic Soft Templating of Monolithic Au Thin Film Electrodeposited from Bicontinuous Microemulsion
Authors: Shiba Shunsuke , Hirabayashi Shoei , Niwa Osamu , Kato Dai , Kunitake Masashi , Matsuguchi Masanobu 
Published: December 2020

Electrochemical deposition has been the preferred method to use for the soft templating of nanoporous metal thin film because of its low-temperature metallization processes, which prevent damage to the structures not only of soft templates, including self-assembled surfactants or copolymers, but also of the formed nanoporous metals. Recently developed soft-templating methods are based on the replication of highly ordered micelle assemblies composed of surfactants or copolymers. However, it is still difficult to achieve wide size control over 100 nanometers by soft templating, though such control is necessary for electrocatalytic or biosensing applications.
We report on the fabrication of monolithic nanoporous gold (Au) thin films by electrodeposition in bicontinuous microemulsion (BME) as a dynamic soft template (Figure (a)). Au electrodeposition occurs only at the aqueous phase of BME, which has intertwined nanometer-ordered three-dimensional networks composed of aqueous and oil solution channels compartmentalized by surfactants and cosurfactants. The thermodynamically stable BME can be formed simply by “mixing” together of the components, and the solution/solution structure of BME could be controlled simply by changing the BME composition. We prepared three kinds of BME solutions, including water-rich BME (water/oil ratio: 66/34), oil-rich BME (33/67), and equally mixed BME (69/31), by mixing 5.00 mL of 1 M hydrochloric acid solution containing 50 mM tetrachloroaurate(III), 5.00 mL cyclohexane, 0.40 g sodium dodecyl sulfate (SDS), and different volumes of 2-methyl-2-butanol (470, 590, and 510 μL, respectively) as cosurfactants. Figure (b-d’) shows the top surface and cross-sectional FE-SEM images of the resultant gold thin film electrodeposited onto sputtered Au film at -0.30 V vs. Ag/AgCl for 300 sec in (a) oil-rich BME, (b) equally mixed BME, and (c) water-rich BME, respectively. The resultant Au films each had a unique monolithic nanostructure grown vertically to the electrode surface. The characteristic ligament diameters of these films increased from 40 to 200 nm as the aqueous solution content increased. These results clearly indicate that BME no doubt behaved as a soft template and successfully demonstrate that structural control can be easily achieved by changing the BME solution/solution structure. In the case of equally mixed BME, we also confirmed that the ligament axial length was controllable between 180 and 650 nm by changing the electrodeposition time from 200 to 400 s. The cross-sectional observation of all the films revealed that there were no side pores even though BME had a reticulated solution structure on the order of 100 nm or less. This indicates that monolithic BME structures changed continuously during electrodeposition depending on the film surface morphology and acted as dynamic soft templates.
We conducted voltammetric experiments to assess surface characteristics such as electrochemically active surface area (ECSA), exposed facets, and electrocatalytic activity. The apparent electrode area of each Au film was defined by putting insulating tape with a 2-mm-diameter hole on the electrode surface. The linear sweep voltammetry after forming a fully oxidized surface in 0.5 M H2SO4 solution revealed that the ECSA of the monolithic nanoporous Au film electrodeposited in equally mixed BME for 300 s was 4.2 times greater than that with the sputtered Au film (before electrodeposition). The cyclic voltammetry in the same solution revealed that the oxidation potential of the main peak current of the monolithic nanoporous Au thin film was lower (+1.08 V vs. Ag/AgCl) than that of sputtered Au thin film (1.29 V). These results indicate that the existence of well-exposed {100} facets and/or dominant step/kink sites at the monolithic nanoporous Au thin film probably derived from vertically grown, highly curved nanostructures. As a test reaction, the methanol oxidation reaction (MOR) was used to investigate the electrocatalytic performance of electrodeposited nanoporous Au films. The cyclic voltammetry obtained in the 0.5 M KOH aqueous solution in the absence and presence of 1 M methanol resulted in a 1.9 times larger peak top current (446 μA cm-2
app) at the monolithic nanoporous Au film than that of sputtered Au film due to high ECSA. Interestingly, the onset potential of MOR at the monolithic nanoporous Au film was -0.58 V, which is 0.20 V more negative than that at the sputtered Au film; this might derive from the {100} facets and/or the step/kink structure of the nanoporous Au film.

Figure 1