13/4/2023
19:43 | Graphene oxide classification and standardizationDownload PDFDownload PDFArticleOpen AccessPublished: 13 April 2023Graphene oxide classification and standardizationKatarzyna Z. Donato, Hui Li Tan, ...A. H. Castro Neto Show authorsScientific Reports volume 13, Article number: 6064 (2023) Cite this articleMetricsdetailsAbstractThere is a need to classify and standardize graphene-related materials giving the growing use of this materials industrially. One of the most used and more difficult to classify is graphene oxide (GO). Inconsistent definitions of GO, closely relating it to graphene, are found in the literature and industrial brochures. Hence, although they have very different physicochemical properties and industrial applications, commonly used classifications of graphene and GO definitions are not substantial. Consequently, the lack of regulation and standardization create trust issues among sellers and buyers that impede industrial development and progress. With that in mind, this study offers a critical assessment of 34 commercially available GOs, characterized using a systematic and reliable protocol for accessing their quality. We establish correlations between GO physicochemical properties and its applications leading to rationale for its classification.IntroductionGraphene oxide (GO) is a member of a family of two-dimensional (2D) materials, derived from the oxidation of 2D graphitic structures (sp2 to sp3 carbon conversion). As any other 2D material, in powder form GO presents a statistical nature in terms of its thickness and lateral size distribution. However, GO is an amorphous and non-stoichiometric 2D material, bearing a blend of different functional oxygen groups1. In fact, there is no consensus on how to represent the structural model for GO. Hence, important structural details are often neglected, including metallic impurities, functional groups of other heteroatoms, carbon vacancies and radicals, and CH bonds, which directly depend on the oxidation method used2.There is a plethora of methods available to obtain GO, including the chemical, electrochemical and microbial oxidation of a variety of carbon-based materials3,4,5,6. The most common results found in the literature involve GO obtained via chemical oxidation of graphite or, most recently, the direct fast oxidation of graphene7. However, when considering commercially available GO, it narrows down to the almost exclusive use of chemically oxidized graphite.Graphite oxidation dates back to 1859 when Benjamin Brodie developed the so-called graphic acid8. Later many different approaches were developed aiming to improve Brodie's method2,9. These modifications always involve the addition of a new reactant that leaves its physical footprint in terms of residues and defects that consequently affects GO's applicability10. For instance, the methods developed by Staudenmaier11 and Hofmann12 methods optimize Brodie's approach with the use of potassium chlorate (KClO3) and nitric acid (HNO3). On the other hand, the vastly used Hummers method13 applies a mixture of sulfuric acid, sodium nitrate, and potassium permanganate (KMnO4) to graphite, yielding materials with very different electrochemical properties than the ones obtained by the other aforementioned methods14. This issue is further aggravated by the fact that graphene and GO, in low dose, are not toxic per se, but their cytotoxicity emerges from defects and contaminants15.In this article we survey the overall reliability and consistency of commercially available GO from producers around the world. The closest standardized characterization protocol available for GO is the ISO/TS 21356-1:202116 that was developed for graphene and, with few exceptions17,18,19, no adaptation was tried for GO. Moreover, most of the literature focuses on identifying specific phenomena for small sets of samples. Therefore, we propose a guideline for sample preparation and characterization, including a flowchart that combines up-to-date laboratory common practices for GO characterization (Supplementary Information, SI, Figure S1). Detailed experimental descriptions for individual analyses are extensively discussed in individual sections, focusing on the challenges related to a large number of heterogeneous samples.A total of 34 different commercially available GO samples were acquired from 25 companies located in 11 countries, 23 of them were obtained as powders and 11 as dispersions in water (5 of these from the same producer as a powder sample), with a price range varying from USD$ 0.40 to USD$ 2300 per gram. This colossal price variation and the lack of information about the synthetic method adopted by most GO producers are exemplary results of the lack of standardization. A blind analysis process was defined, where all the analysts performed the characterizations without knowing any information about the samples. Furthermore, 2 of the 34 products were acquired under a different name than GO, i.e., hydroxyl- and carboxy-functionalized graphene, to act as decoys for the analysts. This was made to evaluate if these samples clearly behave as outliers or stay undetectable among the GO samples, helping to define how broad the GO term can be used, and how similar GO and modified graphene can be within the current product standards.Flakes' lateral size and thicknessConcomitantly with any other member of the 2D materials family, the GO dimensions imply dramatic effects on its properties, to such an extent that it can define the formation of liquid crystal phases in water20. Consequently, variations in the dimensions of GO flakes yield different products for different applications, especially in areas such as polymer composites where interphase effects are critical21,22.Optical microscopy (OM) is the simplest method to estimate the lateral size of GO's flakes. However, it can only be used to observe sample appearance prior to more quantitative characterizations, as defined by the ISO norm related to graphene16. Although this approach is often found in the GO literature, we observe a large discrepancy among samples and their optical definition when using the ISO-recommended substrate composition (Si/SiO2) for non-purified commercial GOs. Samples with large flakes and fewer impurities are clearly observed with OM, whereas smaller and more contaminated flakes are barely visible due to artifacts created by the interaction between the Si/SiO2 substrate and organic contaminants. This can be partially resolved by using Si substrates, although losing the qualitative information of flake thickness23 (Fig. 1a). For this reason, all samples were analyzed using both Si and Si/SiO2 substrates, and the one presenting the most defined flake morphology and distribution was used for further characterizations (Figure S2).Figure 1?Thickness and lateral size determination. (a) Examples of OM images of samples characterized using Si/SiO2 or Si substrates. (b) Examples of thickness and lateral size collection by AFM, followed by the distributions of average (c) thickness and (d) lateral size, and (e) correlation between thickness and lateral size for each sample with the inset representing 3 different Tiers (T1?=?h???10 nm and l???1 ?m; T2?=?h???10 nm and 1 ?m?>?l???0.5 ?m; T3?=?15 nm???h?>?10 nm and l???0.5 ?m). (f) Example of SEM image for lateral size collection, highlighting the smaller often neglected flake fragments, followed by (g) the lateral size distribution using a software interface. (h) Finally, the difference between the average lateral size obtained from AFM (manually) and SEM (software).Full size imageThe flake thickness distribution of the samples was assessed by AFM, considering the height profile (in triplicate) of at least 30 individual flakes per GO sample (Fig. 1b and details in the SI, Sect. 4, Figure S3). In total, 31 of the originally acquired samples could be analyzed by AFM and 3 of them were above the thickness range allowed for the measurement. From the measured samples, 20 presented an average thickness??0.5 form a plateau at?~?45%?>?O/C-O in their composition, as witnessed in Fig. 2d. Although on a much smaller scale, COH functionalities also vary largely among GO samples (Fig. 2d), which is a clear fingerprint of the presence of water during the chemical oxidation process29,30. A clear correlation emerges when we interpolate the information about the GOs' defects by Raman (ID/IG), their degree of oxidation by EA (O/C), and their amount of sp3 carbon by XPS (C sp3) (Fig. 2e). Relatively small distribution regions could be attributed to these values, where 27 out of 34 GO samples presented 0.9???ID/IG???1.2, 0.8???O/C???1.2 and 25%???% C sp3???42%. Among the 7 outliers are the 2 decoy GO samples we introduced, presenting a clear differentiation from the highly defective and low oxidation "GOs".During the oxidation process, O-related functionalities are added among the graphite layers and increase the interlayer spacing (d), which can be followed by XRD. Ideally, the characteristic diffraction (002) peak of graphite (2??~?26.3°, d?~?3.4 Å) should completely disappear, and all the remaining diffraction structure at this (°) is residual unoxidized graphite31. At least 5 out of 34 samples analyzed present intense residual (002) peaks, and several others presented it in lower intensities (SI, Sect. 10, Figure S13). Moreover, the (001) peak is formed as a result of oxidation and d varies related to the type of modification introduced to the basal plane of graphite. The characteristic GO (001) peak in 2??=?9.312.1°, corresponding to the d?=?9.57.3 Å, was observed for 30 out of 34 samples (Figure S13). Although it is known32 that larger groups and/or more oxygen produce a larger d, more variables seem to be at play as this pattern did not emerge for our set of samples. Both FTIR and ATR-FTIR were performed to supplement the XPS information about the functional groups, but the overall heterogeneity of the samples only allowed for qualitative functional group identification (SI, Sect. 11). Importantly, we detected fingerprint bands for sulfate/sulfonate groups that confirm that part of the sulfur residue detected by EA is not only adsorbed on the GOs but covalently bonded (SI, Table S6). On the other hand, we correlate the interlayer spacing and O/C ratios, indicating the carbonaceous group within which the analyzed materials can be associated to. Figure 2g displays this correlation to all tested materials and compares them to values found in the literature for graphite, reduced GO, disordered graphite, intercalated graphite, and highly oxidized GO31,33,34. We must highlight that even though these values are estimations without discounting eventual water (or other residual solvents) contributions35, the XRD peak segregations were defined enough for interpretation after the sample drying process (SI, Figure S13). Most of the samples fit into the highly oxidized GO region, however, four samples present characteristics of graphite, and one sample resembled more intercalated graphite. Curiously, only our 2 decoy samples were expected to appear in the graphite region (due to the expected low oxidation), however, another 2 samples are also displayed there, and the 4 samples are indistinguishable. The same 4 GOs presented very weak and noisy FTIR spectra and did not display one or more of the GO's fingerprint vibrational bands (details in the SI, Sect. 11, Figure S14, and Tables S6 and S7). In fact, the decoy samples are only clearly distinguishable when Raman spectroscopy is included to the analytical protocol (Fig. 2e).Finally, we also investigate the content of residual metallic impurities present in the commercial GOs, using ICP-OES (details in SI, Sect. 12, Table S8). There are two main sources of the metallic residues in GO, the graphite used for oxidation and the major components of the reactants used in the different synthetic steps. However, indirect contamination with extraneous metals coming as trace residues of reactants have also been reported10. Consequently, purification steps are essential for the GO syntheses due to their large chemical footprint, but they considerably increase the production cost of GO. Thus, impurities become a point of concern when acquiring GO from a seller. In Fig. 2f, we summarize the total amount (by weight), the major (present in thousands of ppm) and the minor (present in hundreds of ppm) metallic impurities present in the commercial GOs. They were organized in ascending order of the major component of the figure for better visualization. Astonishingly, 8 of the samples presented?>?1wt.% of metallic residues, including an extreme case with?>?4wt%. The majority of the largely contaminated GOs are powder samples, whilst, with 3 exceptions (S-004, S-006 and S-011), dispersion samples presented orders of magnitude less impurities. The major contaminants observed are Mn and Na, followed by K, Mg, Ca and Fe, present in thousands of ppm. These contaminants are easily traceable since they are part of reaction components and/or cations largely present in water36, tracing it back to the water in the washing process. However, Al and Cr were also present in surprisingly large amounts in some GOs (hundreds of ppm), while Pt, Zn, V, Cu, Co, Se and Ba were lesser (but still in tens of ppm in some samples).Concentration and water-stabilityAmong the most unexpected conclusions we have reached from this study is that the unreliability of commercially available GO starts from the products' contents. The GOs acquired as powders were tested for their dispersibility and apparent stability in water and compared to the ones acquired already as dispersions (SI, Sect. 13 and Figure S16). From the 23 powder samples, 9 of them presented very poor stability in water, not forming homogeneous suspensions and precipitating shortly after sonication. Moreover, among the initially stable powder samples, 70% of them precipitated in???±?0.5 mg/mL, and 1 sample presented a concentration 5?×?lower than the value described in the product label. This is particularly concerning because many researchers rely on UVVis analysis of GO to establish its concentration. However, such a heterogeneous group of samples is not comparable via this technique. Since the optical absorption of GO is dominated by the ??* plasmon that is dependent on the linking chromophore units (e.g., C=C, C=O, and CO bonds), variations on those units will strongly affect the concentration determination37. This can be clearly evidenced by the large variation in absorbance among the different commercial GOs when characterized under the same concentration (0.05 mg/mL), including 9 of them that do not even present a typical GO UVVis absorption curve (SI, Sect. 15 and Figure S17). These differences in content and type of O-groups also lead to large differences in water stability as previously described (SI, Sect. 13, Figure S15). The stability in water, as opposed to the dispersion concentration, can be reliably determined by UVVis using a simple method we propose (Fig. 3a, and details in SI, Sect. 15). Briefly, we classify the GOs into 5 different solution stability groups, presenting also different features that can be observed by UVVis, which accurately match with the apparent water stability (Fig. 3b, and details in SI, Sect. 12). Indeed, only 21 of the 34 commercial GOs presented good stability in water, which is a highly regarded property that should be expected from any GO product.Figure 3?Samples with different levels of stability, with image insets showing examples of water dispersions just after sonication (top), after 24 h (middle), and after 30 days of storing (bottom) (a). Correlation between O/C ratio, lateral size, flake thickness, and the GOs' water stability, grouping the sample by stability from 1 to 5 (details in SI, Sect. 15) (b). Correlation between O/C ratio, total residue, and sheet resistivity, grouping the samples by film formation ability where group 1 gather samples that form films with high resistivity, group 2 form films with decreasing resistivity and group 3 does not form films (c).Full size imageThere is a complex interplay between flake sizes and oxidation controlling water stability, thus, the correlation between O/C ratio, lateral size, and flake thickness of the GOs sets a good guideline to understand their water stability. Not surprisingly, the water stability of the commercial GOs decreases with increasing average flake thickness and decreasing O/C (Fig. 3b). However, interestingly, the influence of flake lateral sizes on the GOs' water stability seems to be little to none, as observed by the scattered results when the samples are organized in order of their group of stability (Fig. 3b). In fact, the most unstable samples as per UVVis determination (Group 5, SI, Sect. 15) are exactly the samples with extremely low O/C ratio and/or extremely thick flakes, including the 2 (non-GO) decoy samples. There is also a major influence of the metallic residue of the samples which will be detailed in the film formation discussion.Although surface charge density usually confers a large influence on the stability of particles in solution31, we could not observe any major influence of it in this set of samples as the difference in GO stability was not directly assessable by characterizing their electrophoretic properties. The dispersion of commercial GOs presented a large difference in pH (pH 4.48.6 at 0.05 mg/mL, SI, Sect. 16 and Table S10). This is an indicator of large differences in synthesis protocols adopted and, consequently, residual additives, which we clearly observed (Fig. 2f). In fact, for many of the samples the amount of contamination is so high that it seems to be the dominant effect on the pH variation and instability. For a fair comparison among the samples, zeta potential (?) analysis was performed both in the original pH and after having the pH adjusted to 67, to ensure a stable pH range for GO38. The very unstable samples are not measured, as they do not present stable or reliable ? values. All the other GOs https://www.nature.com/articles/s41598-023-33350-5 | 
fireball xl5 | |
Customisable watchlists with full streaming quotes from leading exchanges, such as LSE, NASDAQ, NYSE, AMEX, Bovespa, BIT and more.
Hot Features
Premium
