Nano-chemical mapping of cobalt nanoparticles on the graphene-related membranes
Juan-Jesus Velasco Vélez1,2, Yi-Ying Chin3, Oliver James Burton4, Ruizhi Wang4, Stephan Hofmann4, Wei-Hao Hsu5, Takuji Ohigashi6, Meng-Hsua Tsai7, Way-Faung Pong7, Cheng-Hao Chuang7*
1Chemical Energy Conversion, Max Planck Institute, Mlheim, Germany
2Fritz-Haber-Institut, Max-Planck-Gesellschaft, Berlin, Germany
3Department of Physics, National Chung Cheng University, Chiayi, Taiwan
4Engineering Department, University of Cambridge, Cambridge, UK
5Institute of Physics, Academia Sinica, Nankang, Taipei,, Taiwan
6UVsor, Institute for Molecular Science, Okazaki, Japan
7Department of Physics, Tamkang University, New Taipei City, Taiwan
* Presenter:Cheng-Hao Chuang,
Free-standing graphene and graphene oxide membranes are a promising candidate for use as in-situ environmental windows in X-ray (electron) microscopy. In this study, the membranes were used as the working electrode, and cobalt nanoparticles (Co-NPs) were grown directly on top of the graphene through electrochemical deposition for interfacial variation. The electronic structure and the chemical bonding states of the Co-NPs and the membrane materials were examined by using the high spatial resolution and element-specific properties of scanning transmission X-ray microscopy. X-ray absorption spectra of C, O, and Co revealed that Co-NP size increased in accordance with oxidation state (Co0/2+/3+), depending on the configuration of carbon bonding (C–C/C–OH/HO–C=O/O–C(O)–O/C=O–like state). We conducted a spectral comparison of the dipped graphene and the electrodeposited Co–graphene sample, which revealed an increase in C–OH formation before Co-NPs growth. In addition to electron transfer and electrochemical reduction, the oxidation evolution from C–OH to HO–C=O (or defect) and the O–C(O)–O (or /C=O) state paralleled the increase in Co-NPs size. We curve-fitted the results to demonstrate the reduction in chemical structure from mixing Co2+/3+ to Co3+/2+/0, and to explain the interfacial modulation and the unique metal Co0 layer on the surface of the Co-NPs. Our results provide information for the design of a reliable graphene window and offer an example for the interpretation of experimental X-ray (electron) microscopy.

Keywords: Nanocatalysts, Scanning transmission X-ray microscopy, Nanoscale characterization, Two-dimensional membrane, Energy Materials