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

Juan-Jesus Velasco Vélez^1,2, Yi-Ying Chin³, Oliver James Burton⁴, Ruizhi Wang⁴, Stephan Hofmann⁴, Wei-Hao Hsu⁵, Takuji Ohigashi⁶, Meng-Hsua Tsai⁷, Way-Faung Pong⁷, Cheng-Hao Chuang^7*

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, email:chchuang@mail.tku.edu.tw

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 (Co^0/2+/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 Co^2+/3+ to Co^3+/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