Standard Guide for Characterization of Graphene Flakes
4.1 The remarkable structural, physical and chemical properties of graphene — particularly its mechanical strength, high electronic mobility, lightness, and transparency (single layer or a few layers) — have generated worldwide research and industrial production efforts aimed at developing practical applications. Various industrially scalable production methods have been developed, including bottom-up approaches that grow graphene from small molecules (with or without a substrate), and top-down methods that start with graphite and exfoliate it by mechanical, chemical or electrochemical methods to produce nanoscale product such as graphene flakes. Two common exfoliation methods are: (1) oxidation of graphite to graphene oxide (GO) followed by additional processing to form reduced graphene oxide (r-GO) (2) and, (2) liquid phase exfoliation of graphite (3). The exfoliation methods, as well as substrate-less bottom-up approaches, produce materials in the form of flakes that can be dispersed in various solvents, making them suitable for applications requiring solution processing. Although there are many commercial “graphene” materials available on the market, the quality of these products is highly variable (4). There are many challenges in assessing the physical properties of the materials. In this guide we discuss how Raman spectroscopy (Raman) and X-ray photoelectron spectroscopy (XPS), as well as atomic force microscopy (AFM) can be used to characterize materials consisting of flakes of graphene and related materials (that is, few layer graphene (FLG), GO, r-GO). Illustrative examples are provided showing how these methods can be used to identify the type of material present and to extract important parameters including lateral flake size, average flake thickness, ratio of intensities of the D and G modes (ID/IG) in the Raman spectrum and carbon to oxygen ratio. Specifically, when encountering an “unknown” material or product purporting to be “graphene,” it is essential to quantify the thickness and lateral flake size distributions by AFM, to assess the level of defects in the flakes using the ratio of intensities of the D and G bands in the Raman spectrum, and to determine the level of oxidation of the material (C/O ratio) using XPS. These measurands are important for qualitative assessment of the type of material present, as well as quantitative measures of the quality of the flakes which can be correlated with properties relevant to applications based on conductivity, optical transparency, and chemical reactivity.
4.2 It should be noted that these materials and products may exist in either a powder or dispersion (in liquid) form. Other techniques and measurements (ISO/TR 18196:2016) such as X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and surface area measurement, can also be used for characterization of graphene and related products but discussion of these methods is beyond the scope of this guide.
1.1 This standard will provide guidance on the measurement approaches for assessment of lateral flake size, average flake thickness, Raman intensity ratio of the D to G bands, and carbon/oxygen ratio for graphene and related products. The techniques included here are atomic force microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. Examples will be given for each type of measurement.
1.2 This guide is intended to serve as an example for manufacturers, producers, analysts, and others with an interest in graphene and related products such as graphene oxide and reduced graphene oxide. This Standard Guide is not intended to be a comprehensive overview of all possible characterization methods.
1.3 This guide does not include all sample preparation procedures for all possible materials and applications. The user must validate the appropriateness for their particular application.
1.4 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.
1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use.
1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
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