Chemical Etching Evaluation of the Oxygen Reduction Reaction Activity of Graphene Sheets

A fuel cell utilizes a cathodic oxygen reduction reaction (ORR), in which a Pt catalyst is widely used. However, due to its high cost and scarceness of Pt,1 many researchers have devoted their efforts to finding an alternative catalyst, among which the promising candidates include nanocarbon materials such as graphene. The ORR activity of an electrode is usually evaluated by electrochemical measurement. However, electrochemical results are probably affected not only by the structure of nanocarbon at the molecular level but also by the conditions of the electrode loading a nanocarbon membrane, such as the effective surface area, the thickness of the carbon film, and the adhesive binding the nanocarbon. Such factors complicate the analysis of electrochemical results; this complication hampers the development of suitable nanocarbon materials to enhance ORR activity. In other words, demand is high for an experimental method for evaluating the catalytic performance of nanocarbon at the single-molecular or single-sheet level, i.e., the individual scale. We propose the use of catalyst-assisted chemical etching of a semiconductor surface to test the ORR activity of nanocarbon at the individual scale. In this scheme, a nanocarbon material is deposited individually on a semiconductor surface, which is immersed into a solution with dissolved O2 molecules. A nanocarbon whose catalytic activity enhances the ORR reaction oxidizes the semiconductor surface underneath it. A solution selected for etching and removing the resultant oxide produces an etched hollow under the nanocarbon. This etching process continues until the sample is removed from the solution. The depth of the etched hollow is easily measured by, e.g., atomic force microscopy (AFM), and is likely to be a good indicator of the ORR performance of the nanocarbon material used. Thus our method may serve as a “microscopic” technique to assess catalytic activity, in contrast with conventional “macroscopic” electrochemical measurement. The purpose of this study is to test the proposed concept. We prepared three different carbon materials, each dispersed in liquid: graphene oxide (GO), hydrazine-reduced GO (hyd-rGO), and ammonia-reduced GO (amm-rGO). First, cyclic voltammetry (CV) was carried out in an alkaline solution with an O2 concentration of approximately 9 ppm at 20 ℃. A working electrode was formed in advance by placing droplets of a graphene suspension on a commercial grassy-carbon electrode and then drying. The onset potentials for the different graphene membranes revealed higher ORR activity for both hyd-rGO and amm-rGO than for GO. In contrast, this difference in catalytic performance between hyd- and amm-rGO was difficult to detect using the CV curves. For the etching experiments, we used germanium (Ge) as a substrate because a Ge oxide (GeO2) is soluble in water; this is in contrast to the case for the more familiar SiO2. Droplets of the graphene suspension were placed onto Ge and then spin-coated. Then the Ge substrate, loaded with the graphene sheets (GO or hyd-rGO or amm-rGO), was immersed in water containing dissolved O2 molecules at a concentration of about 7 ppm. Water temperature was kept between 22 and 58 ℃. The resultant surface morphology was imaged by AFM. We found that for all sheet compositions, deeper hollows formed under the loaded graphene at higher water temperatures. Detailed analyses indicated that the rGO sheets (hyd-rGO and amm-rGO) possessed catalytic activities greater than that of GO; this finding agrees with findings from electrochemical measurement. In addition, amm-rGO exhibited the highest ORR performance among the three samples, for reasons discussed elsewhere.2 The present result demonstrates that catalyst-assisted chemical etching can be a useful tool for evaluating the ORR activity of graphene samples at the single-sheet level. (1) J. Dumont et al., ACS appl. Nano Mater. 2, 1675 (2019). (2) R. Mikurino, A. Ogasawara et al., J. Phys. Chem. C 124, 6121 (2020).