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Understanding hydrothermally reduced graphene oxide hydrogels: from reaction products to hydrogel properties Kaiwen Hu, Xingyi Xie, Thomas Szkopek, and Marta Cerruti Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04713 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 21, 2016
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Chemistry of Materials
Understanding hydrothermally reduced graphene oxide hydrogels: from reaction products to hydrogel properties 1‡
Kaiwen Hu , Xingyi Xie 1
1,2 ‡
3
, Thomas Szkopek and Marta Cerruti
1
Department of Mining and Materials Engineering, McGill University, Montreal, QC, H3A 2B2, Canada.
2
College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials and Engineering, Sichuan University, Chengdu, Sichuan, 610065, China.
3
Department of Electrical and Computer Engineering, McGill University, Montreal, QC, H3A 0E9, Canada.
‡
These authors equally contributed to the paper
ABSTRACT: We studied the chemical processes that take place during hydrothermal gelation of graphene oxide (GO), quantifying the reaction products generated during hydrothermal reduction. The gelation proceeds with disproportionation of GO yielding a large amount of CO2 (about a quarter of the original mass of GO), organic acidic fragments, and CO. The CO2 that is formed is trapped in the hydrogel creating macroscopic voids which can lead to cracking of the hydrogel during compression. We were able to quantify the amount of CO2 produced in-situ by adding ammonia during the synthesis, and converting CO2 into ionic carbonate species that we could easily quantify by titration. We used titration to evaluate the formation of organic acidic fragments too, and evaluated the amount of H2O and CO produced by thermogravimetric analysis and mass balance. The conversion of CO2 into ionic species allowed us to produce void-free hydrogels which remains structurally stable after extensive compression. However, such hydrogels on average showed lower mechanical strength and electrical conductivity than the hydrogels with voids. This is a result of the difference in chemistry and morphology between hydrogels reduced under acidic pH and basic pH. Our work provides for the first time a clear quantitative estimate of CO2 evolution and organic fragment formation during hydrothermal reduction of GO, an overall picture of the reaction products, and a deepened understanding of the conditions that can be used to prepare stronger and more conductive graphene hydrogels and aerogels.
Introduction
actions occur among the reduced graphene oxide (RGO) sheets 10, 12. These interactions cause phase separation of the RGO sheets from the solution and thus lead to final gelation.
Graphene aerogels are a new class of 3D carbon monoliths holding promise for applications as diverse as electrochemical energy storage 1, 2, pollutant adsorption 3-5 CO2 capture 6 and tissue engineering 7, 8. They consist of entangled single or few layers of graphene, and they partially retain the excellent properties of monolayer graphene such as high electrical conductivity, mechanical strength and surface area 4, 9, 10.
Two solution based reduction methods are often used to induce gelation of GO, namely chemical reduction 14, 18 and hydrothermal reduction 7, 8, 12, 13. While chemical reduction relies on the use of mild chemical reducing agents such as vitamin C 2, 14 and ethylenediamine 3, 18 to remove oxygen, hydrothermal reduction relies on the disproportionation of GO in water at elevated temperature, usually higher than 1500C 10, and thus requires the use of an autoclave. Compared with chemical reduction methods, hydrothermal generates RGO that is intrinsically more pure due to the “water only” process.
Sol-gel synthesis is the most common route to preparing graphene aerogels 10, 11. An aqueous dispersion of graphene oxide (GO) is often used as the starting material, and gelation occurs via either reduction of the GO solution 12-14, addition of polymer linkers 15, 16, or simply by increasing the concentration of GO 17. Solution based reduction of GO is an effective and facile way to induce solgel transition of GO. The hydrogels and aerogels produced by this method are intrinsically conductive. During the reduction, the number of oxygenated functional groups is largely reduced, and strong hydrophobic inter-
Although these reduction-based methods can produce graphene gels, the self-assembly and reduction mechanisms still need to be clarified, since they play an important role in determining the final structure and properties of the resulting gels 10. Several authors have worked
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on understanding the hydrothermal reduction and gelation of GO. The first report on hydrothermal reduction of GO prepared by Zhou et al. 19 emphasized the pH dependence of the reduction route. They found that the resulting RGO is much more dispersible in water when GO is reduced in basic pH rather than in acidic pH. This implies that the chemical process varies with solution pH. Xu et al.12 and Mungse et al.20 characterized the resulting RGO and studied the time and temperature dependence of the hydrothermal reduction. They found that the reduction process was completed in less than 6 hours with negligible change in C/O atomic ratio of the RGO. The authors also proposed several reduction pathways based on the Lerf-Klinowski model of GO, including various dehydration routes, decarboxylation and carbon removal from carbon skeleton. A recent report by Goldstein et al. 11 studied the chemical and structural transformations of GO by characterizing RGO at different times of reduction during the hydrothermal process. The results show that almost half of the mass of GO was lost during hydrothermal reduction and this “removed mass” had a carbon to oxygen atomic ratio of 0.45 as inferred from the difference in oxygen content measured by energy dispersive spectroscopy (EDS) before and after reduction. This implies that a large fraction of the reduction products is in the form of CO2.
Here, we present an in-depth analysis of the chemical processes that take place during hydrothermal reduction/gelation of GO and their effects on the chemistry, morphology, electrical and mechanical properties of the resulting hydrogels. Specifically, we identify and quantify different reaction products generated during the reduction. The reaction product is predominantly CO2 with minor organic fragments that remain in solution. The extensive CO2 evolution leads to the entrapment of CO2 bubbles inside the hydrogel, which form macroscopic voids in the gel. To quantify the amount of CO2 generation, we add a base (ammonia) to the precursor GO suspension to absorb the evolved CO2 and convert it into HCO3- and CO32- , which we then quantify by simple potentiometric titrations with acid. The addition of ammonia also allows us to prepare graphene hydrogels under basic pH. These gels do not have macroscopic voids and are better reduced, but turn out to have lower electrical conductivity and Young’s modulus than RGO hydrogels prepared in acidic pH. We explain these observations by considering the differences in chemical bonds and morphology of these hydrogels.
Despite these previous studies, the mechanisms of gelation and hydrothermal reduction are not yet fully understood. For example, there is a lack of consensus on the nature of the linkage (chemical vs. physical) that causes the formation of RGO hydrogels 10, 12, 21, 22. Also, hydrothermal reduction products are not characterized and quantified as in other reduction methods such as photochemical reduction23, 24 and thermal reduction25-27, because the process occurs inside an autoclave and is thus harder to probe. Common products including CO2, H2O, CO 25, 27, 28 and small organic fragments 23, 24 evolve from the thermal reduction or photochemical reduction of GO. Quantifying these byproducts and understanding how they form is crucial to understand how GO is reduced. Also, the evolution of gaseous products such as CO2 alters the organization and morphology of the resulting RGO structure, which self-assembles during reduction. For example, Eigler et al. 28 have shown that entrapment of CO2 during thermal reduction of GO can cause the formation of blisters on an RGO membrane at 160 °C; these blisters eventually blast at 210 °C, leading to the formation of nanometer sized holes in the membrane. Last but not least, the low molecular weight fragments released during the reduction are by themselves materials with unique properties such as photoluminescence 29, 30 and high electroactivity 31. Thus, these fragments can affect the chemistry of the resulting RGO assembly if they are retained during reduction. Thus, understanding the chemistry of the reduction process and the products generated allows for better control of the chemistry, structure and properties of the resulting RGO materials.
GO single layer aqueous dispersion synthesized by Hummer’s method with a concentration of 6.2 mg/ml was purchased from Graphene Supermarket (single layer>80%, lateral size: 0.5 μm to 5 μm). The nominal elemental composition of GO is given by the supplier as 79 at% carbon, 20 at% oxygen and 1 at% of other elements. In an ammonia-free synthesis, 6.45 ml of the suspension was taken and diluted to 10 ml (4 mg/ml) with distilled water in a glass vial. The glass vial was sealed in a Teflonlined autoclave at 1800C for 6 hours. After the hydrothermal reduction, a black hydrogel with very high water content (>98%) was formed. These graphene hydrogels (GHG) prepared in the absence of ammonia are referred to as GHG-N-0 (Table 1). In the syntheses including ammonia, different amounts (60, 170 or 290 µl) of concentrated ammonia aqueous solution (28-30 wt%) were added into the GO suspension, and the suspension was subjected to the previously described hydrothermal treatment. These GHG are referred to as GHG-N-60, -170 and 290, respectively (Table 1).
Experimental Hydrothermal reduction of GO
Analyte collection In addition to the GHGs, the hydrothermal reduction process also yielded a clear solution containing all the other reaction products. We refer to this solution as “analyte”, from which we determined the amount of carbonate species and the organic fragments formed during the reduction in different conditions. This analyte was collected simply by removing the GHGs from the vial. In addition, during the hydrothermal process, some water from the
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suspension evaporated and re-condensed on the Teflon wall. Thus, to collect all of the analyte, we rinsed the exterior of the glass vial and the interior of the Teflon liner with 1 ml of water and added the rinse water into the glass vial. Note that a small fraction of the analyte was trapped inside the GHGs and could not be collected. The amount of analyte loss was determined by weighing the analyte before and after GHG removal. As shown in Table 1, the analyte was named after its corresponding GHG. For example, GHG-N-170(A) is the analyte collected from the production of GHG-N-170.
source (Bruker, Germany) diffractometer. Mass spectrometry of the analytes was conducted on an Autoflex III (Bruker, Germany) mass spectrometer using Matrix (Dihydroxybenzoic acid)-assisted laser desorption/ionization (MALDI) in linear mode. The spectra of both positively charged and negatively charged species were recorded. UV-vis absorption spectra of the analytes were collected using UV-Carry 5000 NIR-UV-Vis spectrometer (Varians Inc. US). Attenuated total reflectance (ATR) Fourier transform infrared spectroscopy (FT-IR) spectra of the GHGs were taken using a Spectrum-Two IR spectrometer (Perkin Elmer, USA) with accumulation of 32 scans in the range of 400–4000 cm−1 at a resolution of 4 cm−1. . X-ray photoelectron spectroscopy (XPS) of the GHGs was performed on a monochromatic Al K-Alpha photoelectron spectrometer (Thermoscientific, USA). At least three survey scans were collected between 0 and 1200 eV with a step size of 1 eV for each sample. Elemental high resolution scans were performed with a step size of 0.1 eV and repeated at least 3 times. The peak fittings were completed with the software Thermo Avantage (version 4.6); spectral energies were calibrated by setting the C-C binding energy as 284.8 eV The microstructure of the GHGs was observed using an Inspect-50 field emission scanning electron microscope (FEI, Japan) at 5 kV with secondary electron imaging. Mechanical properties of the GHGs were measured on a Mach-1 mechanical tester (Biomomentum Inc., Canada) in compression mode with a constant compression rate of 0.03 mm/s (recommended by ASTM D1621 for cellular materials) using a multi-axis load cell (FT11358). The compression test stopped when the GHGs were compressed to a height of 5.5 mm. Four samples were tested for each GHG. The conductivity of each hydrogel was determined by current–voltage (I–V) measurement with bias voltage from −10 mV to +10 mV applied across the ends of each cylindrical hydrogel with a B1500A semiconductor parameter analyzer (Agilent Technologies, USA). The slope of each I–V curve gives the conductance (G), and the conductivity (κ) was calculated as κ=4LG/πD2, where L and D are the length and diameter of the hydrogel under test. The young’s moduli (E) of the GHGs were extracted from the slope of the linear elastic regime of the compressive stress-strain curves.
Titration for CO2 quantification We performed potentiometric titrations on each of the collected analytes to determine the amount of CO2 released during the hydrothermal process. In a typical titration experiment, approximately 5 g (accurate mass recorded) of the GHG analyte was diluted to 20 ml and titrated with 0.025 M HCl solution. The accurate concentration of the HCl solution was determined by titrating anhydrous Na2CO3 (99.999%, Sigma) with the acid (Figure S1). All titrations were performed manually using a burette and a digital pH meter (Thermo Fisher Scientific, USA). The analyte underwent constant stirring during the titration and a stable pH value (>5 seconds) was recorded after each titrant addition. Titration for organic fragments quantification The pH of the analyte GHG-N-0(A) was found to be acidic. To understand the composition of this analyte, we performed a potentiometric titration using as titrant a standard 0.0025 M NaOH solution, prepared by diluting 1 M NaOH standard solution (Acros Organics). Approximately 5 g of GHG-N-0(A) (accurate mass recorded) was diluted to 20 ml and titrated with the NaOH solution. The titration was performed using the same setup as described in the previous section. The same procedure was used to titrate a GO suspension (4 mg/ml) and the supernatant of the same GO suspension. The GO supernatant was collected in two steps. In the first step, the GO suspension was centrifuged at 14600 rpm for 1 hour to remove most of the GO from the supernatant. In the second step, the remaining GO was removed by filtering the supernatant through a 0.1 µm syringe filter. The accuracy of all titration experiments was evaluated by titrating three aliquots of GHG-N-0(A). The details were shown in SI (Figure S2 and Table S4)
Statistical analyses Statistical analyses on the elastic modulus, yield strengths and electrical conductivities of the GHGS were performed by two sample Student’s t-test using Origin 8.5. A p value less than 0.05 is considered significant.
Characterization
Results
Prior to characterization, the GHGs were frozen in liquid nitrogen and freeze dried at -1000C for 2 days at 20 mTorr to transform them into aerogels. Elemental analysis of the freeze-dried aerogels was performed using a dynamic flash combustion method (Fisons, UK). Two measurements were performed for each sample. Powder X-ray diffraction of calcite was performed on a D8 Discover Cu
We obtained two distinct products after the hydrothermal reduction of GO. On one hand, GO selfassembled and formed a GHG. This self-assembly process was reported extensively in literature 9, 13, 14, 20 and was proposed to be driven by the strong π-π and hydrophobic interaction of RGO above a critical concentration of the
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starting GO suspension 10, 12. On the other hand, the high efficiency of RGO self-assembly yielded a clear solution which contained the remainder of the reaction produts, which we refer to as the “analyte”. This system provided us with great convenience to easily separate the main product (GHG) from the reaction residues (analyte) and thus allowed us to study them independently.
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nation accounted for the consumption of protons in region II. H O + HCO ⇄ H O + H CO
(1)
In addition, a much less well-resolved inflection point started to evolve in the titration curve of GHG-N-60(A) at a pH of 7.68 and marked the beginning of region III; this inflection point was much more evident on the titration curves of GHG-N-170(A) and GHG-N-290(A), at pH of 7.45. The calculated pKb values relative to this inflection point for GHG-N-170(A) and GHG-N-290(A) were 5.50 and 5.29 respectively. The presence of such inflection points is indicative of another protonation process, occurring at higher pH than the process described in reaction (1). This could be caused by either the protonation of CO32-or the protonation of ammonia, as shown in reactions (2) and (3).
Table 1 shows the designation of all GHGs and their corresponding analytes, as well as the pH of the starting GO suspension and of the analytes. The pH of the initial GO solution was acidic with value around 3. Upon ammonia addition, the pH increased up to 11 depending on the quantity of ammonia. After hydrothermal reduction, GHG-N-0(A) is acidic, but its pH is greater than that of the precursor GO suspesion. On the contrary, all other analytes showed a decrease in pH compared to the respective precursor GO suspension, indicating the generation of acidic species during hydrothermal reduction process. Identification and quantification of CO2
H O + CO ⇄ H O + HCO
(2)
H O + NH OH ⇄ 2H O + NH
(3)
The calculated pKb values are higher than the pKb of CO32(3.67) and NH4OH (4.75). This discrepancy may be explained by the presence of the NH4OH/ NH4+ conjugate acid-base pair in these solutions. This weak basic buffer may cause a downshift in the pH of the first inflection point and thus an increase in the inferred pKb compared to the standard pKb values.
The effect of ammonia addition on the buoyancy of the GHGs is shown in Figure 1a. Without ammonia addition, GHG-N-0 floated. Relatively large bulges were visible on the surface of GHG-N-0 (Figure 1b), which were direct evidence of gas entrapment inside the GHG. These entrapped gas bubbles decrease the density of the GHG, thus allowing it to float in the analyte. When ammonia was added, the bulges disappeared, buoyancy was lost, and GHG-N-60, GHG-N-170 and GHG-N-290 sank.
When we added CaCl2 to GHG-N-290 (A), a white precipitate formed after gentle shaking (Figure 2b, inset). This material was CaCO3 in the form of trigonal calcite, as clearly shown by an XRD diffractogram (Figure 2b) 32, 33, thus confirming the presence of CO32- in the analyte.
Based on these observations, we hypothesized that the entrapped gas was CO2, which was converted to HCO3and CO32- upon reaction with ammonia. To prove this, we titrated all the analytes with 0.025 M HCl. The titration results are shown in Figure 2a. We divided the pH ranges into three different regions, corresponding to different processes.
To rule out any contribution related to reaction with atmospheric CO2 during hydrothermal reduction, we performed a control experiment in which we hydrothermally treated a solution containing 290 µl of concentrated ammonia in the absence of GO, and titrated the resulting solution. A typical strong acid into weak base titration curve was obtained (Figure S3), showing only one inflection point on the curve at a pH of 5.3 with a pKb value of 5.05, which corresponds to the complete protonation of ammonia as shown in reaction 3. This shows that atmospheric CO2 did not affect the titration results shown in Figure 2a.
In region I, no protonation occurred and the decrease in pH was solely due to the addition of acidic titrant to the analyte. As shown in Figure 2a, the titration curve of GHG-N-0(A) is completely located in this region. Therefore, there were no species capable of protonation in GHG-N-0(A), and thus no traces of HCO3- or CO32-. The titration curves of GHG-N-60(A), GHG-N-170(A) and GHG-N-290(A) extended into regions II and III. The titration curve of GHG-N-60(A) showed a clear inflection point at pH=4.83, which is the beginning of region II and corresponds to a pKb of 7.51, as shown in SI. We observed a similar inflection point for the other two analytes, GHGN-170(A) and GHG-N-290(A), with similar pKb values of 7.74 and 7.75 respectively. These pKb values are in agreement with that of HCO3- which is 7.7. This implies that this inflection point corresponds to the full protonation of HCO3-, i.e. the process shown in reaction (1). Such proto-
We calculated the amount of CO2 that was released during the reduction and converted into CO32- or HCO3- ions in GHG-N-60(A), GHG-N-170(A) and GHG-N-290(A) based on the acid consumption between the two inflection points in region II. The amount of CO2 evolution in terms of 1 gram of GO is summarized in Table 2. Since all CO32- ions were converted into HCO3- in region II, this acid consumption indicates the amount of HCO3- in the analyte and thus the amount of CO2 generated during
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reduction. Calculation details are shown in SI, and the acid consumptions are shown in Table S1. GHG-N-60(A) consumed the lowest amount of acid among the three analytes, while GHG-N-170(A) and GHG-N-290(A) showed similar acid consumptions. This indicates that the addition of only 60 µl of ammonia solution was not sufficient to absorb all the CO2 generated during the reduction and convert it into HCO3- and CO32-. If a sufficient amount of ammonia is added, all the CO2 generated should be converted into HCO3- and CO32-; adding more ammonia should not increase the amount of HCO3- and CO32- produced, and therefore the amount of acid consumed during analyte titration should remain constant. We saw this happening for GHG-N-170(A) and GHG-N290(A), which implies that the acid consumed to titrate these analytes actually corresponds to the total amount of CO2 generated. Thus, we used the CO2 amount quantified from GHG-N-170(A) and GHG-N-290(A) to account for the total CO2 generated during hydrothermal reduction. As shown in Table 2, on average, 5.6 mmol or 0.25 g of CO2 per gram of GO was generated during hydrothermal reduction based on the average of amount of CO2 for GHG-N-170(A) and GHG-N-290(A). Such mass was about a quarter of that of the original GO.
We found similar acidic fragments in the GO supernatant (Figure 3a, black curve). Although previous reports have shown the GO supernatant to be neutral 34, we found that the supernatant of our GO suspension was acidic, with a pH higher than that of GHG-N-0(A) (3.79 vs. 3.53, see Table 1). Such acidity was not caused by possible residues of GO—indeed, we effectively removed all of the GO content in the supernatant, as shown by UV-Vis spectroscopy (Figure S5). Upon addition of NaOH to the GO supernatant, the titration curve showed an inflection point at pH=6.98, which corresponds to a pKa of 4.20. This pKa is very similar to that measured for the titration of GHGN-0(A) (red curve), indicating that similar acidic fragments were present in these two solutions. However, a smaller amount of NaOH was consumed during the titration of GO supernatant than of GHG-N-0(A). This implies that fewer acidic species were present in the GO supernatant than in GHG-N-0(A), and therefore that the hydrothermal reduction generated additional acidic species in addition to those already existing in the original GO suspension.
Identification and quantification of organic fragments
Assuming one carboxylic acid per fragment molecule, we could quantify the molar concentration of fragments in GHG-N-0(A) and GO supernatant based on the acid consumption at their inflection points. Based on results from multiple titrations, the quantity of fragments in GHG-N0(A) (after subtraction of the contribution from dissolved CO2) ranged from 0.27 mmol/g (relative to 1 g of GO) to 0.35 mmol/g, while in the GO supernatant their production ranged from 0.12 mmol/g to 0.30 mmol/g. Therefore, more fragments were newly generated as a result of the hydrothermal treatment. The higher concentration of acidic fragments is consistent with the UV-Vis spectra of GHG-0(A) and GO supernatant (Figure S5) where a higher absorption in the UV-region was seen for GHG-N-0(A) as compared to that of GO supernatant.
Besides CO2, we also found traces of acidic organic fragments as oxidation products of the hydrothermal reduction of GO. The presence of these fragments was responsible for the acidity of GHG-N-0(A). Such observation agreed with previous observations of GO reduction in water under sunlight 23 and UV exposure 24 where organic fragments were released in addition to CO2 evolution. To prove this, we titrated GHG-N-0(A) with NaOH (Figure 3a, red curve). Upon NaOH addition, GHG-N-0(A) showed a short buffering region followed by a rather steep increase in pH, with an inflection point at pH= 6.86, corresponding to pKa of 4.19. Such behavior may well corresponded to the deprotonation of a monoprotic carboxylic acid; in fact, carboxylic acids commonly have pKa values between 3.5 and 5. To correctly evaluate the amount of these carboxylic acids, the presence of dissolved CO2 in the analyte solution needs to be taken into account; thus, we performed an additional titration on GHG-N-0(A) after removing CO2 by extensive boiling. As shown in Figure S4 and discussed in SI, we found that the presence of CO2 accounted for about 45% of the solution acidity, based on the difference in the NaOH titrant volumes at the inflection points measured on GHG-N-0(A) before and after CO2 outgassing.
We attempted to measure the mass of the organic fragments by freeze-drying GHG-N-0(A) and weighing the remaining mass. The organic fragment mass ranged from 2.5 mg to 4 mg for 40 mg of GO, which corresponds to 0.06 g to 0.1 g per gram of GO. We conducted mass spectrometry on the fragments using MALDI to gain information regarding their molecular weight. Figure 3b shows the negatively charged fragments found in the spectra of GHG-N-0(A) and GO supernatant, with most of the prominent species labeled with their m/z values. The positively charged fragments were dominated by the ionization products of the DHB matrix, as shown in SI (Figure S6). The MALDI spectra of GHG-N-0(A) and of GO supernatant were overall quite similar, showing most species in the m/z range of 280 to 360, and a few scattered around 480 and 650. This result
For comparison, we also titrated GO solution (Figure 3a, blue curve). In agreement with results from previous studies 34-36, GO showed no clear inflection because it generates a range of acidic species during the titration 34. This behavior was distinctly different from the acidic
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showed that the fragments present in the GO supernatant did not undergo further degradation during hydrothermal treatment, and that the newly generated fragments were very similar to those originally present.
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peared to saturate as more ammonia was added, as shown by the only marginal increase in nitrogen content of GHG-N-290 (4.7 at%) compared to GHG-N-170 (4.4 at%). The chemical nature of the nitrogen-containing species was revealed by high resolution XPS (Figure S9). The N1s spectra of GHG-N-60, GHG-N-170 and GHG-N-290 were very similar, including three components relative to quaternary N (402.5 eV), pyrrolic N (399.9 eV) and pyridinic N (398.5 eV) 8, 42. As shown in Table S6, pyrrolic and pyridinic species accounted for the majority of the nitrogen species (~75 at%) in these GHGs. The hydrogen content of the resulting GHGs also increased with increasing ammonia. Among all the GHGs, GHG-N-290 exhibited the highest hydrogen content, about 22 at%. Such increase in hydrogen content could be tied to the incorporation of hydrogen-containing nitrogen species such as pyrrolic N in the GHG.
Chemical characterization of GHGs To examine the reduction degree of the resulting GHGs, we conducted bulk elemental analysis using a dynamic flash combustion method. The elemental compositions (C, H, N, and S) of all GHGs and GO are summarized in Table 3. Before reduction, GO showed about 43 at% of C, which agreed with most of the previously reported values 37, 38. A high oxygen content (34 at%) was found in GO, which resulted in a C/O ratio of 1.3. The hydrogen content of GO was about 23 at% and could be attributed to the presence of intercalated water and hydroxyls. Although the presence of organosulfates as high as 5.6 wt% was reported in several studies 39, 40, we could not detect any sulfur by the dynamic combustion method, possibly due to its low content in our GO. The low sulfur content is likely a result of the low amount of impurity present in our original GO and the extensive washing used for final purification. As shown by Dimiev et al. 39, the sulfur content can be reduced by washing GO with nucleophilic solvents that can promote hydrolysis of organosulfate. Indeed, Eigler et al. 40 were able to remove almost all organosulfates by washing GO with dilute NaOH solution. This observation is dually confirmed by our FTIR and XPS results as shown in Figures S7 and S8. The two bands at 1413 cm-1 and 1218 cm-1, which were previously assigned to the symmetric and asymmetric νs=o of sulfates 39, 40, have very low intensity in our spectra. XPS analysis showed that the sulfur content is 0.76 ± 0.02 at% on GO, matching well with the nominal impurity content (
