Accounting Carbonaceous Counterfeits in Graphene Materials Using the Thermogravimetric Analysis (TGA) Approach

Abstract

Counterfeits in the supply chain of high-value advanced materials such as graphene and their derivatives have become a concerning problem with a potential negative impact on this growing and emerging industry. Recent studies have revealed alarming facts that a large percentage of manufactured graphene materials on market are not graphene, raising considerable concerns for the end users. The common and recommended methods for the characterization of graphene materials, such as transmission electron microscopy (TEM), atomic force microscopy (AFM), and Raman spectroscopy based on spot analysis and probing properties of individual graphene particles, are limited to provide the determination of the properties of "bulk" graphene powders at a large scale and the identification of non-graphene components or purposely included additives. These limitations are creating counterfeit opportunities by adding low-cost black carbonaceous materials into manufactured graphene powders. To address this problem, it is critical to have reliable characterization methods, which can probe the specific properties of graphene powders at bulk scale, confirm their typical graphene signature, and detect the presence of unwanted additional compounds, where the thermogravimetric analysis (TGA) method is one of the most promising methods to perform this challenging task. This paper presents the evaluation of the TGA method and its ability to detect low-cost carbon additives such as graphite, carbon black, biochar, and activated carbon as potential counterfeiting materials to graphene materials and their derivatives such as graphene oxide (GO) and reduced GO. The superior performance of the TGA method is demonstrated here, showing its excellent capability to successfully detect these additives when mixed with graphene materials, which is not possible by two other comparative methods (Raman spectroscopy and powder X-ray diffraction (XRD)), which are used as the common characterization methods for graphene materials.

International Interlaboratory Comparison of Thermogravimetric Analysis of Graphene-Related Two-Dimensional Materials

“…The aim of assessing this parameter was to confirm the consistency of the TGA method to determine the thermal stability of GR2Ms, which is an important physical parameter defining their properties. 22,23 Results reported by participants (PP) and their LP analysis for all GR2Ms samples are summarized in Figures S6, S7 and Tables S3, S4. The results showed that 10 participants reported the thermal stability of GO in the expected range of 100−150 °C, while two participants (3 and 7) showed higher thermal stability between 180−250 °C.…”

“…These “bulk” or average properties of manufactured GR2Ms produced at large batches (e.g., 10–100 kg) are equally important for manufacturers and the end-users. Methods such as light microscopy (number of layers), gas adsorption analysis by the Brunauer–Emmett–Teller method (specific surface area), pH titration (acids and oxygen groups), TGA (qualitative and quantitative determination of components, thermal properties), and laser diffraction or dynamic light scattering (particle size distribution, PSD) have been successfully used and recommended for simple and low-cost characterizations of GR2Ms. − Some of these methods are currently under evaluation to assess their characterization capability, uncertainties, and limitations needed to developing reliable ISO standards.…”

“…The number of peaks, their peak shapes, and the peak or maximum mass change rate temperature ( T p ) are demonstrated to distinguish these different types of GR2Ms. , Representative TG graphs for GO, rGO, FLG, and graphite, powders are presented in Figure S1. By using these TG graphs, it was shown that it is possible to successfully discriminate FLG powders from “fake” graphene powders containing graphite, GO, or other carbon particles, confirming that TGA as a very useful complementary method for characterization and quality control of bulk GR2Ms. − However, to make TGA a standard method for GR2Ms, it is important to confirm its reliability and confidence.…”

“…These parameters are as follows: (1) the number of mass loss steps, (2) thermal stability, (3) temperature of maximum mass change rate ( T p ) for each decomposition step, (4) the mass contents (%) of moisture, oxygen groups, carbon, combustible organic impurities, and (5) the mass content (%) of noncombustible residues. These parameters have been selected to combine the measurement of physical and qualitative and quantitative chemical properties of GR2Ms based on their practical relevance, complementarity with other ISO characterization methods and the developed procedures from both the authors and the literature. − , …”

Microreactor-Based TG–TEM Synchronous Analysis

“…4 In a commercially available TGA instrument, a thermobalance with a submicrogram (sub-μg) mass resolution is used to measure material mass loss during a programmed heating procedure. Sample consumption of the mg level is usually needed to obtain a resulting TGA curve that contains quantitative information related to a temperature-dependent mass change, thereby analyzing the material structure, 5 composition, 6 thermal stability, 7 and reaction kinetics. 8 For a sample under TGA, the decomposition of the material may change the molecular formula or material phase of the sample, which is often accompanied by morphology change in the nanostructure.…”