Influence of water gas on surface chemistry and physics of partially-reduced graphene oxide membranes

Jan Sebastian Dominic Rodriguez^1,2, Takuji Ohigashi^3,4, Chi-Cheng Lee¹, Meng-Hsuan Tsai^1,5, Chueh-Cheng Yang^5,6, Chia-Hsin Wang⁵, Hsiang-Chih Chiu⁷, Cheng-Hao Chuang^1*

¹Department of Physics, Tamkang University, New Taipei City, Taiwan
²Institute of Chemistry, Leiden University, Leiden, Netherlands
³Institute for Molecular Science, Okazak, Japan
⁴Photon Factory, Institute of Materials Structure Sciences, Tsukuba, Japan
⁵National Synchrotron Radiation Research Center, Hsinchu, Taiwan
⁶Department of Materials Science and Engineering, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
⁷Department of Physics, National Taiwan Normal University, Taipei, Taiwan

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

Oxidizing surface in reduced graphene oxide (rGO) is characterized as epoxide, hydroxyl, carboxyl, and ether functional groups, particularly mixing the individual sheets and complex structures. Through the electrochemical reduction method, the porous space and stacking structure can result in gas transportation, rapid kinetics, and even selectivity for the extra modulation of oxygenated groups. Our objectives in studying the polar coupling at the solid-liquid interface are the chemical capability and physical structure concerning water gas and rGO. Prior efforts to investigate the mechanism of water adsorption on graphene oxide (GO) have led to understanding the role of the oxygen functional groups in the process. However, the phenomena of water adsorption on the surface have yet to be unraveled. This work investigated reduced GO surfaces' behavior and chemical characteristics when exposed to different humidity conditions. The surface potential (SP) and morphology of rGO and GO are examined with an in-situ humidity environment (5%, 30%, 50%, 85%, and back to 5%), and the work function decreased from 5.65 eV (5% RH) to 5.38 eV (85% RH). However, some areas of the surface potential maps followed different trends, with this phenomenon being induced by the variation in the concentration of oxygen functional groups, causing other sites to behave differently. Ambient-pressure XPS has shown increased adsorbed H₂O (535.5 eV) with increased H₂O pressure. However, the observation of a reduced FWHM accompanying the increased peak intensity is notable. Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS) observes the core-level state of rGO at C and O K-edge with a function of the water pressure. X-ray absorption spectroscopy (XAS) during controlled humidity conditions indicated increased C-O-C signals at elevated RH, indicative of changes in the chemical composition of GO in the presence of adsorbed water. The signal of water gas has a border width in the spectral range of binding energy, compared with that of further desorption in the thermal rGO membrane (T-rGO). The gas coupling stems from an extra existence of oxidation in the high binding energy, which disappears in T-rGO. Since the nano-scaled identification with the absorption spectrum is essential to study the water interaction in rGO and T-rGO, the Scanning Transmission X-ray Microscopy (STXM) is conducted to seek the specific oxidation groups and chemical convection of water gas. The oxidation coordination on the rGO displays an extreme increase of diverse oxygenated groups from low to high humidity. With decreasing humidity, only the C-O-C bond at the C site and water adsorption at the O site can be diminished by the dry gas flow. The C=C reconstructure, new C-OH bonding, and decreasing C-O-C/C-OOH bonding are recognized in T-rGO, accompanied by reduced liquid water at the O site. These findings deepen our understanding and offer insights into enhancing GO-based humidity sensors and dehumidification materials.

Keywords: Reduced graphene oxide, Ambient pressure X-ray photoelectron spectroscopy, Scanning transmission X-ray microscopy, In-situ humidity environment, Surface potential