Morphological Analysis of Graphene Oxide by Scanning Electron Microscopy and Correlative Field-flow Fractionation Coupled with Multi-angle Light Scattering

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Since the first free-standing monolayer graphene sheet was successfully prepared in 2004 [1], graphene and graphene oxide materials achieved the necessary technical readiness level to be considered for use in commercial products. Moreover, the focus has shifted from fundamental research towards expanding the applicability of graphene-related 2D materials (GR2M) and to improve their competitiveness with established materials [2]. Significant advances have been made especially in applications regarding optoelectronics, energy storage materials, chemical additives, sensor applications etc. [3]. Composite products containing graphene and functionalized graphene such as inks and dyes have shown enhanced performance regarding longevity, wettability, and can be tailored for specific purposes through dedicated functionalization. For accurate physico-chemical characterization, GR2M products in their raw form or as part of composites present challenges in terms of sample preparation, choice of analytical method and evaluation of data. For instance, in the context of imaging, these challenges encompass: (a) the selection of images magnifications being representative for all the flakes ranging in size from hundreds of nanometers to micrometers; (b) the selection of representative flakes for adequate statistics, which may involve the separation of overlapping/agglomerating flakes by segmentation; and (c) the classification of diverse morphologies such as irregularly shaped/crumpled flakes, porous flakes and particulate features present in the sample. The complexity of the analytical task has needed the introduction of specific ontology for 2D materials to identify the proper descriptors characterizing confidently the morphological features of interest. Regarding light scattering techniques such as Dynamic Light Scattering (DLS) and Multi-Angle Light Scattering (MALS) commonly used for process control in industry as a first measure, an alternative approach would be necessary. This is in part due to the use of the standard sphere-model for 2D materials as appearing to be inappropriate, whilst a disc-shape model potentially yields more suitable results. Standardization efforts are underway to establish a baseline for accurate characterization of aimed measurands with sufficient statistics. To date, the measurement methods recommended by standardization bodies for the morphological-structural characterization of GR2M’s are AFM, Raman Spectroscopy and SEM and/or TEM. The acquisition of statistically relevant numbers of flakes for a thorough characterization using TEM and AFM is particularly time-consuming. The size distribution of graphene oxide- and graphene-containing inks was investigated by using a correlative approach coupling Centrifugal Field-Flow Fractionation (CF3) [4] with MALS. Up to now, promising results for Field-Flow Fractionation have been achieved only with respect to the separation into size classes of GO samples as well as of graphene oxide mixed with graphene by Asymmetrical Field-Flow Fractionation (AF4) [5]. Besides the online characterization by MALS, the eluting size fractions obtained by CF3 were also collected and subsequently measured by SEM. Successful separation into size fractions allows us to apply ensemble techniques such as MALS to samples that were previously not measurable according to best-practices. In this study, the following material sub-classes have been observed with SEM: (i) nano-graphite mixed with graphene flakes, (ii) graphene oxide few- and multi-layer flakes with diverse and highly complex morphology, and (iii) graphene oxide of well-defined size and shape with >95% single- and bilayer content were investigated. Data on the class size ranges was obtained by MALS after separation with CF3 and consideration of a disc-shape model. Significant effort was invested into the sample preparation for CF3 measurements to achieve a recovery rate of >80%, well above the recommended 70% by ISO/TS 21362:2018 for validation purposes. The material fractions collected after the CF3 measurement were separately deposited on a silicon wafer and the size results of the SEM analysis were correlated with the corresponding mean sizes obtained with MALS. CF3 analysis can, with appropriate downstream detection systems, provide information on size, size distribution and concentration over the time-resolution. Figure 1 shows three distinctive peaks. Two of the three measurements runs are in good agreement whereas the first run is showing a substantial shift of the concentration signal for times >20 min. For the modelling of the disc radii the measurement runs 02 and 03 were considered only. The SEM image of the GO fraction FR1 represents the smallest flakes, i.e. well below 1 µm. This finding is confirmed by batch DLS analysis of FR1 indicating a mean size of 200 nm. The second fraction, FR2, reveals deformed, rather round flakes of approximately 1 µm size, which appear to be agglomerated. Batch DLS of FR2 provides a diameter of 425 nm. The third fraction, FR3, barely contained any flakes visible on the substrate. Their size is above 1 µm. The DLS result of FR3 is 915 nm in diameter, which is in good agreement with the diameter of 900 nm obtained by MALS. The last fraction, FR4, appears to show the closest matches to the original untreated sample with flake sizes ranging from sub-micrometer to tens of micrometer. The diameter found with DLS for FR4 is 1.5 µm. We can conclude that CF3 results suggest successful size separation of the GR2M selected in this study. Further, these obtained fractions of size sub-classes can be analyzed by SEM for validation purpose or more detailed morphological analysis. Further investigations are in progress [6]. a) Graphene oxide flakes as deposited from water suspension on a silicon wafer before CF3 treatment. b) CF3 UV trace results of a GO-flakes sample (BA002) showing the concentration (mg·L-1) plotted over time (min) detected by UV, repeated 2 times. The collected fractions (FR) are identified, see FR1 (with the smallest size), FR2, FR3 and FR4 (with the largest size), and corresponding SEM micrographs of these fractions were taken.

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