Nano-chemical mapping and structure identification of cobalt nanoparticles on the graphene-related membranes

Cheng-Hao Chuang^1*, Takuji Ohigashi², Russell Clemente¹, Jan Sebastian Dominic Rodriguez¹, Wei-Hao Hsu³, Way-Faung Pong¹

¹Department of Physics, Tamkang University, Tamsui, Taiwan
²UVSOR, Institute for Molecular Science, Okazaki, Japan
³Institute of Physics, Academia Sinica, Nankang, Taiwan

* Presenter:Cheng-Hao Chuang, email:chchuang@mail.tku.edu.tw

With the increasing demand in the sustainable energy, any possibility to substitute the fossil fuel becomes a key melody in the next generation technologies. Hydrogen or oxygen evolution reaction (HER or OER) over metal oxides are thought to own the promising power capability. It becomes crucial in the advanced electrode materials for the good catalytic performance, which is actually unclear in the reaction mechanism and catalytic optimization. Non-noble materials, such as transition metal oxide, have been heightened because of some reasons: (a) abundant hydroxyl groups on the surface, (b) size-diminished metal crystal, (c) anti-corrosion resistance in the alkaline solution. Cobalt oxides as one of active electrocatalysts has been proven to be one class of HER/OER in alkaline environment. On the other hand, graphene sheet is considered as one of important 2D materials due to the unique electric transfer property and thinnest thickness. The ability to grow the functional nanomaterials on the top of graphene substrate has been proposed due to the ideal single-atom thickness, excellent electrical conductivity, transparent electrode, flexible nanosheet, and stable structure of graphene-based materials. The facial functional groups (hydroxyl, carboxyl, and epoxide) of nanocomposite synthesis are characteristic for the interesting interaction with metal site. Our previous result indicates the strong covalent bonding between O and Co through the electrochemical deposition method. Since the nano-scaled microscopic probing is still absent for the detail and direct observation, we will demonstrate the element-specific chemical images to promote the graphene-related electrode for the future liquid-cell applications. Scanning Transmission X-ray Microscopy (STXM) equipped with the optical lens (Zone Plate and Order Sorting Aperture) can visualize the same interest of areas by the transmission mode of X-ray absorption spectroscopy. The interaction between grown Co₃O₄ and graphene is characteristic of Co anchoring on the graphene, while carbon atom of graphene replaces oxygen ligands of Co₃O₄ as Co(CO)_x formation at the solid-liquid interface. Besides, our work will also provide the reduced graphene oxide (GO) and nitrogen-doped GO membrane as the electrochemical window materials, in order to demonstrate the modification of interface property of Co and its HER performance.

Keywords: graphene, scanning transmission X-ray microscopy, hydrogen evolution reaction, reduced graphene oxide, nitrogen-doped graphene oxide