Chemical analysis of graphene oxide films after heat and chemical treatments by X-ray photoelectron and Micro-Raman spectroscopy
Introduction
Thin films composed of graphene oxide or reduced graphene oxide platelets (such platelets are also referred to as “sheets”) have recently attracted attention due to their unique mechanical and optical properties [1], [2], [3], [4]. As a result of the extensive oxidation of the precursor graphite (resulting in graphite oxide which is then dispersed as layers in an aqueous colloidal suspension), ‘graphene oxide paper’[1] is considered not to be electrically conductive by electron or hole transport. However, it is possible to make reduced graphene oxide films that are electrically conductive [2], [3], [4], [5]. By tuning the chemistry of the platelets either prior to or after formation of the thin film, the film’s electrical conductivity, optical properties, and chemical character (hydrophobicity, etc.) can also be tuned [1], [2], [3], [4], [5], [6]. Such chemical tuning could also have a significant impact on the thermal conductivity, which to the best of our knowledge has not yet been studied.
The objective of this study is to evaluate any chemical changes of very thin films composed of graphene oxide platelets due to heat and/or chemical treatment by using X-ray photoelectron spectroscopy (XPS) and Micro-Raman spectroscopy. This study complements previous work conducted on other chemically modified graphene (CMG)-based materials, including ordered multilayer stacks of graphene oxide (thus, graphite oxide crystals, about which there is an extensive literature), thick ‘paper-like’ films of graphene oxide platelets, [1] and “chemically reduced graphene oxide” (CReGO) that is created by exposing an aqueous suspension of graphene oxide sheets to hydrazine, [7] as well as by the methods discussed in the aforementioned work [2], [3], [4], [5], [6]. Our group has also previously performed an XPS study of the impact of hydrazine treatment of a thin film that after curing consisted of separated graphene oxide platelets embedded in silica, deposited on a silica substrate [3]. These approximately 30 nm-thick silica/CMG composite films, derived from a sol–gel process, contained about 4 to 8 separated CMG platelets in any region in the through-thickness part of the silica film for the highest loading of the CMG platelets that was tested.
XPS is a surface sensitive technique used to analyze surface chemical composition and bonding. A sample is irradiated with an X-ray beam while the number of electrons that escape and their kinetic energy are simultaneously measured [8]. Raman spectroscopy is used to characterize the vibrational modes of molecules. Micro-Raman spectroscopy allows the probing of a small volume by combining a confocal microscope with the spectroscopy system [9]. The ability to collect Raman data from a very small area (∼350 nm in diameter) can in some situations reduce undesirable background signals and enhance chemical sensitivity. We have used micro-Raman spectroscopy in this study and refer to the spectra obtained as “Raman spectra”.
Section snippets
Experimental
Graphite oxide was prepared by the modified Hummers method [7], [10] and was then dispersed in water by sonication for 2 h. (GO was synthesized from natural graphite (SP-1, Bay Carbon, MI) by the modified Hummers method. Conc. H2SO4 (50 mL) was added into a 250 mL flask filled with graphite (2 g) at room temperature. The flask was then cooled to 0 °C in an ice bath, followed by slow addition of KMnO4 (7 g); the flask was then allowed to warm to room temperature. The temperature was then raised to 35
Results and discussion
An AFM image of a typical several-nanometer thick film of graphene oxide platelets deposited on the silicon nitride-on-silicon substrate is shown in Fig. 3. This film is made up of overlapping graphene oxide platelets so that a given region might consist of one layer (about 1 nm thick) or overlapped layers, with the overlapped regions typically having two overlapped platelets.
Conclusions
Several nanometer-thick films consisting of overlapping graphene oxide platelets deposited on silicon nitride-on-silicon substrates were studied using AFM, XPS, and micro-Raman spectroscopy. AFM was used to assess film thickness and variation of thickness, and it was found that a typical film has regions that are monolayer and other regions that are bilayer and trilayer. Nine different heat treatments (three under Argon, three under Argon and Hydrogen, and three under UHV) at different final
Acknowledgements
The authors thank P. Sherwood and Y. Sun for suggestions about deconvolution and assignment of XPS peaks. The work on this project was supported in part by DARPA iMINT, by SWAN-NRI, and by ONR. Part of the research of Gulay Bozoklu was supported by I2CAM NSF Grant No. DMR0645461. The XPS measurements presented here were performed in the Center for Nano and Molecular Science and Technology at the University of Texas-Austin and preliminary XPS data were obtained at the NUANCE Center at
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