Graphene and its analogues are potential candidates in various applications, such as photovoltaics, catalysis, fuel cells, sensors, and batteries. But a detailed understanding of graphene needs accurate surface characterization. And this can only be provided through advancements in X-ray photoelectron spectroscopy (XPS) instrumentation.
Introduction: Graphene and its Applications
Graphene – a two-dimensional allotrope of carbon - consists of a tightly packed layer of carbon atoms, bonded together in a hexagonal lattice. And it’s this hexagonal (or honeycomb) structure that gives graphene its extraordinary properties. For example, it’s the strongest material in the world, as well as one of the lightest. Also, pristine graphene has the highest known electron conductivity (200 000 cm2 V-1 s-1), excellent thermal conductivity (∼5000 W m-1 K-1), and a high Young’s modulus .
But synthesizing graphene isn’t easy, and most methods are complicated and expensive. So there’s a need for cheaper, simpler, and more efficient ways of producing it. One such method for making graphene on a large scale is the chemical reduction of graphene oxide . Applications of the latter include transparent conductive films, a tin oxide (ITO) replacement in batteries and touch screens, as well as electrode materials for batteries and solar cells. Most importantly, graphene oxide is easier and less expensive to make than graphene, and so may enter mass production sooner.
The range of applications for graphene can be expanded through interfacing with other materials, thus fundamentally changing its properties. This is done through functionalization of the graphene surface, with either soft matters or solid inorganic materials . And this functionalization holds the key to enabling the fabrication of high performance composites.
Understanding the surface of graphene
A lot of research is aimed at comprehending the unique characteristics of graphene and graphene oxide. These studies lay the foundation for the applications of graphene in areas such as photovoltaics, catalysis, fuel cells, sensors, and batteries .
And because functionalization of graphene and graphene oxide is so important for these applications, a thorough understanding of the material surface, and how it can be engineered, is critical.
The demand for high performance materials is only going to increase, and so the importance of surface engineering cannot be understated. Why? Because the surface is the point of interaction with the external environment. So many of the problems with materials and devices can be solved through a detailed understanding of the interactions – both physical and chemical – that take place on graphene’s surface.
XPS: The Ideal Technique for Analyzing Graphene’s Surface
X-ray photoelectron spectroscopy (XPS) is an ideal technique for characterizing a material’s surface chemistry with extreme selectivity. It can also identify chemical states on a surface, as well as measure the elemental composition, empirical formula, and electronic state of the elements present.
A solid surface is irradiated with X-rays, and the kinetic energy and electrons emitted from the top 10 nm are analyzed. A photoelectron spectrum is recorded by counting ejected electrons over a range of kinetic energies. The spectrum consists of peaks from the atoms that emit electrons of a specific characteristic energy. The energies and intensities of these peaks allow users to identify and quantify all surface elements present (except hydrogen) .
This information is critical for many industrial and research applications where surface or thin film composition plays a critical role in performance. And it goes without saying that XPS is an extremely powerful technique for characterizing graphene and its analogues.
Case Study: Using XPS to Characterize Graphene Oxide
There are plenty of examples in the literature of using XPS to study the surface of graphene and graphene oxide [5-7]. For example, the technique was used to study the composition of graphene quantum dots , and athermally photoreduced graphene oxides for three-dimensional holographic images . It was also used alongside TEM, Raman spectroscopy, AFM, and cyclic voltammetry to analyze porphyrin functionalized graphene nanosheets .
One particular study used XPS (and other techniques) to study MnO2-graphene oxide nanocomposites . MnO2 on graphene oxide is a potential electrode material for supercapacitors, and its structure and morphology are key elements that need to be understood.
Through XPS analysis of the chemical structure, electrochemical characteristics, and morphology, the authors showed that the properties of graphene oxide influence the morphology and electrochemical activity of the nanocomposites. They also compared graphene oxide fabricated from commercial expanded graphite, with that fabricated from commercial graphite powder. From this comparison, they showed that graphene oxide fabricated from the former had more functional groups and a larger interplane gap. For more detail on this study, see reference .
The Next Step: Choosing the Right XPS Model for Your Graphene Research
Interest in graphene and graphene oxide, as well as the demand for graphene-based devices, is only going to increase. To keep pace with research needs, development of high-performance XPS instrumentation is essential.
And as the end-user, the burden falls on you to choose the right XPS instrument for your research.
The Thermo Scientific™ K-Alpha™ spectrometer is designed to achieve research-grade results with minimal effort. The multiple-technique ESCALAB Xi+ boasts a full range of XPS techniques and capabilities. And our Theta Probe instrument specializes in ultra thin-film applications with optimized angle-resolved XPS. Each instrument assures peak performance for your next surface analysis experiment.
Learn more about how Thermo Scientific instrumentation can help you with your XPS needs
"Functionalized-Graphene Composites: Fabrication and Applications in Sustainable Energy and Environment" - X. Gong et al, Chem. Mater. 2016. DOI: 10.1021/acs.chemmater.6b01447
Graphene Oxide - Graphenea
"Chemical functionalization of graphene and its applications" - T. Kuila et al, P. Mat. Sci. 2012. DOI: 10.1016/j.pmatsci.2012.03.002
What is X-Ray Photoelectron Spectroscopy - XPS Simplified, Thermo Scientific
"Analysis of Reduced Graphene Oxides by X-ray Photoelectron Spectroscopy and Electrochemical Capacitance" - K. Michio et al, Chem. Lett. 2013. DOI: 10.1246/cl.130152
"In Situ X-ray Photoelectron Spectroscopy Study of Lithium Interaction with Graphene and Nitrogen-Doped Graphene Films Produced by Chemical Vapor Deposition" - L. G. Bulusheva et al, J. Phys. Chem. 2017. DOI: 10.1021/acs.jpcc.6b12687
"Porphyrin functionalized graphene nanosheets-based electrochemical aptasensor for label-free ATP detection" - H. Zhang et al, J. Mater. Chem 2012. DOI: 10.1039/C2JM35379B
"Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots" - D. Qu et al, Scientific Reports 2014. DOI: 10.1038/srep05294
"Athermally photoreduced graphene oxides for three-dimensional holographic images" -X. Li et al, Nature Communications 2015. DOI: 10.1038/ncomms7984
"Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites" - H. Yang et al, Nano Express 2011, DOI: 10.1186/1556-276X-6-531
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