Article pubs.acs.org/cm
Unveiling the Role of Oxidation Debris on the Surface Chemistry of Graphene through the Anchoring of Ag Nanoparticles Andréia F. Faria,† Diego Stéfani T. Martinez,† Ana C. M. Moraes,† Marcelo E. H. Maia da Costa,‡ Eduardo B. Barros,§ Antonio G. Souza Filho,§ Amauri J. Paula,*,† and Oswaldo L. Alves*,† †
Laboratory of Solid State Chemistry, Institute of Chemistry, Universidade Estadual de Campinas, P.O. Box 6154, 13083-970 Campinas-SP, Brazil ‡ Department of Physics, Pontifícia Universidade Católica do Rio de Janeiro, P.O. Box 38071, 22451-900 Rio de Janeiro-RJ, Brazil § Department of Physics, Universidade Federal do Ceará, P.O. Box 6030, 60455-900 Fortaleza-CE, Brazil S Supporting Information *
ABSTRACT: The surface microchemical environment of graphene oxide (GO) has so far been oversimplified for understanding practical purposes. The amount as well as the accurate identification of each possible oxygenated group on the GO surface are difficult to describe not only due to the complex chemical nature of the oxidation reactions but also due to several intrinsic variables related to the production and chemical processing of GO-based materials. However, to advance toward a more realistic description of the GO chemical environment, it is necessary to distinguish the oxygenated fragments with very peculiar characteristics that have so far been treated as simply graphene oxide. In this way, small oxidized graphitic fragments adsorbed on the GO surface, named oxidation debris or carboxylated carbonaceous fragments (CCFs), have been here separated from commercially available GO. Spectroscopy and microscopy results indicated that the chemical nature of these fragments is different from that of GO. By using the decoration of GO with silver nanoparticles as a conceptual model, it was seen that the presence of oxidation debris on the GO surface greatly influences the associated kinetic processes, mainly due to the nucleation and stabilization capacity for silver nanoparticles provided by the oxidation debris fragments. Consequently, when CCFs are present, Ag nanoparticles are significantly smaller and less crystalline. Considering the GO microchemical environment pointed out here, these findings can be qualitatively extrapolated to all other covalent and noncovalent functionalizations of GO. KEYWORDS: graphene oxide, silver nanoparticles, nanocomposite, oxidation debris, carboxylated carbonaceous fragments
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exfoliation of graphite.6 This method has the advantage of producing water dispersible oxidized graphene single layers, a chemically functionalized graphene so-called graphene oxide (GO), that can be chemically reduced to generate graphene sheets.7−10 However, for several kinds of applications which do not demand high crystalline homogeneity of the graphene layers and demands a good chemical processability, such as in composites, sensors, molecular vehicles, and nanomedicine platforms, GO is the most promising system. In the context of the chemical functionalization of graphene, GO is the most used form of functionalized graphene mainly due to the reactivity of the oxygenated groups present as well as its very good water stability. GO has also been used as a starting platform for building nanocomposites and multifunctional systems,11−15 thus leading to specific properties and applications that result from
INTRODUCTION The rising of graphene,1 a single atomic layer of sp2-hybridized carbon with a honeycomb crystal lattice, may be considered a consequence of a natural tendency in the graphite science which had as a challenge the search and control of fewer graphitic layers. However, the thermodynamic stability observed for this truly two-dimensional (2D) carbon single crystal was totally unexpected,2 thus resulting in an impressive burst of interest in the science and technology routes that this material opened up after it was isolated. Its extraordinary electrical, thermal, and mechanical properties are associated with the 2D long-range π-bond conjugation, wherein electrons behave like massless relativistic particles, thus contributing to the emergence of striking physical phenomena.3,4 Top-down approaches by using mechanical exfoliation have generated graphene samples with very good quality, although this methodology does not present either high-yields or good scalability perspectives.5 Thus, the development of a highly efficient production method that could synthesize homogeneous graphene sheets led researchers to use the chemical © 2012 American Chemical Society
Received: June 20, 2012 Revised: September 24, 2012 Published: September 26, 2012 4080
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when silver nanoparticles are attached to as-purchased GO and GO after the removal of oxidation debris, which results in nanoparticles with totally different crystallographic and morphological characteristics. A part of the carboxylic reactive sites on graphene oxide used so far for a countless number of inorganic and organic functionalizations is, in fact, bonded to tiny oxidized carbonaceous fragments and not to graphene sheets.
synergistic effects from the different chemical constituents of the system. In the specific case of nanocomposites, GO and silver nanoparticles (AgNPs) have been reported to be a useful gas sensor, a surface intensity enhancer for Raman spectroscopy, and an antibacterial agent.16−19 The proposed chemical model for explaining the formation of AgNPs on the graphene oxide is based on the presence of carboxylic acids as well as hydroxyl and epoxy groups which are considered as possible nucleation sites for the formation and anchoring of the particles.20 Furthermore, the growth of AgNPs (and also nickel NPs) with high density and small sizes was observed to be directly related to the amount of oxygenated functional groups on GO.21,22 However, in contrast to the oversimplification used so far to elucidate the chemical reactions occurring on the GO surface, which is strictly based on interactions and bonds formed just between reactants and oxygenated groups present on the graphitic sheets, the microchemical environment appears to be much more complex. It involves the presence of adsorbed and reactive polyaromatic fragments with very peculiar features that greatly impact the GO chemistry, as we will show in this paper. Oxidation debris (or carboxylated carbonaceous fragments, CCFs) is a term that has been used to describe byproducts originated when sp2 carbon-based materials are submitted to oxidative treatments.23,24 These oxidized polyaromatic molecular fragments are strongly adsorbed on the graphitic sheets of the matrix through π−π stacking interactions (and also van der Waals interactions). The characteristics and quantity of CCFs strongly depend on the parameters used along the oxidation processes. Although the presence of this byproduct as well as its role in the resulting physical chemical properties of the whole system (debris + carbon matrix) have been already reported in carbon fiber and carbon nanotube sciences,23−27 it is surprising that their influence on graphene oxide chemistry has been minimally studied and such an important issue has not been mentioned in recently published reviews and perspectives on the field.22,28,29 To the best of our knowledge, this issue has been discussed just recently in one study that pointed out a loss of GO water dispersibility after the removal of oxidation debris.30 The authors predicted that CCFs would have strong implications on the GO functionalization chemistry. However, the presence of CCFs has been totally ignored so far in the majority of covalent and noncovalent chemical functionalization studies in GO by using its oxygenated groups as reaction or nucleation sites. Furthermore, the myriad of parameters used in the production of GO will naturally lead to inhomogeneous chemical characteristics of both GO and debris fragments, an issue that must be also considered in the chemistry of GO. By considering the broad perspectives of graphene as a revolutionary material in different science and technology areas such as semiconductors, sensors, composites, and energy storage devices,28,29,31−33 oxidation debris represents a key issue when the applications demand chemical purity of graphene-based materials. Here, we show that the so-called graphene oxidation debris indeed consists of adsorbed oxidized polyaromatic carbon fragments whose size clearly distinguishes from GO flakes once they stay suspended in water during the washing process. Although the chemical groups and size of oxidation debris would strongly depend on the many synthetic parameters used for obtaining GO, they are present in commercially available samples and they will necessarily contribute to the reactivity and physical chemical properties of GO. As we highlight in this paper, their influence is observed
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EXPERIMENTAL SECTION
The commercially available (CheapTubes, Brattleboro, VT) singlelayered graphene oxide (sample raw-GO: as-purchased graphene oxide) was synthesized by a modified Hummers method,34 which basically consists of an exfoliation method for graphite with a mixture of potassium permanganate (KMnO4) and concentrated sulfuric acid (H2SO4). To isolate oxidation debris fragments, a suspension of sample raw-GO (0.5 mg mL−1) was produced through sonication for 30 min in an ultrasound bath. The suspension was brought to reflux in a 0.1 mol L−1 NaOH solution for 90 min, thus resulting in a lightbrown supernatant and a black pelletized sediment. The black pellet was separated from the supernatant by filtration, reprotonated with a 1.0 mol L−1 HCl solution, dialyzed, and finally dried by lyophilization, thus yielding a black powder (sample df-GO: debris-free graphene oxide). This resultant black powder still possesses very good water stability. The supernatant was also reprotonated following the same method described above and dried through lyophilization to produce a brown powder (sample Debris: oxidation debris). All processes for the production of samples df-GO and Debris were repeated in triplicate to provide an adequate statistical confiability. The decoration of samples raw-GO and df-GO with AgNPs was performed through an in situ method by using sodium citrate as stabilizing and reducing agent. First, a raw-GO suspension in an aqueous solution containing 1.0 mmol L−1 of silver nitrate was brought to reflux, and as soon as it boiled, a 1.0 mmol L−1 solution of sodium citrate was added dropwise. The same procedure was carried out by using a df-GO dispersion. The resulting Ag-raw-GO (Ag decorated raw-GO) and Ag-df-GO (Ag decorated df-GO) nanocomposites were dialyzed and dried through lyophilization. The presence of AgNPs was readily confirmed by using UV−vis spectroscopy through the detection of the plasmon absorption band. A schematic diagram summarizing the steps used for removing the oxidation debris from the GO surface as well as the decoration of graphene oxide samples with AgNPs is shown in Figure 1. The percent weight of oxidation debris in the final recovered mass (after all washing procedures) was 16.2%. All details regarding synthetic procedures as well as the characterization techniques are described in the Supporting Information.
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RESULTS AND DISCUSSION After the removal of oxidation debris from commercially available GO through the washing process with a NaOH solution, a visual difference was observed in raw-GO and df-GO suspensions, the latter being darker (see Figure 1). However, their stability in water was not differentiated by time-dependent sedimentation experiments. Differences in the colloidal stability were indeed observed through centrifugation experiments. By increasing the rotations up to 18 000 rcf (relative centrifugal force) and measuring the light absorption of the supernatant of the resulting suspension at 450 nm (through UV−vis spectrophotometry), it was seen that just about 50% of the debris-free GO (df-GO) was still stable, in contrast to almost 90% of raw-GO (see Figure 1 in the Supporting Information). Furthermore, transmission electron microscopy (TEM) images of these two samples indicated a greater agglomeration of dfGO (see Figures 2a and 2b). This qualitative comparison was done when both samples were dried for TEM analysis on holey carbon grids. On these carbon films, raw graphene oxide has a 4081
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several spectra of samples raw-GO, df-GO, and Debris, we clearly observed random energy shifts for this peak that could be directly related to functionalization characteristics of each sample (presence of specific oxygen moieties). However, for this kind of material with such a complex chemical environment, it is hard to point out precisely through this technique a chemical structure variation among the samples that might be supported with a statistical confidence. Another morphological evaluation of graphene oxide and debris samples was carried out through atomic force microscopy (AFM, see Figure 2d−f). The measurements were performed by dropping and drying suspension samples on a mica surface, which was further analysized. The presence of single-layered sheets in sample raw-GO (Figure 2d) was confirmed through contrast measurements along the images, which are related to the sample height (around 1 nm for a single-layered graphitic sheet). Smaller oxidized fragments on single layered sheets were also visualized (see arrows in Figure 2d). After the elimination of these oxidation debris (sample dfGO), the material presented as agglomerates of graphitic sheets with up to four stacked layers, although aggregates of fewer layers were also identified (see arrows in Figure 2e). This result reveals the important role of debris fragments in the stabilization of single layered sheets of graphene oxide in sample raw-GO. Isolated oxidation debris fragments (sample Debris) consist mainly of small flakes of one-layered sheets (see arrows in Figure 2f) and also aggregates of up to two graphitic layers, with sizes significantly smaller as compared to the raw graphene oxide (raw-GO). Thermogravimetric analyses of samples raw-GO, df-GO, and Debris are presented in Figure 3a. Weight losses observed up to 250 °C in the thermogram of raw-GO are associated with the decomposition of oxygen moieties on graphene oxide as well as with the decomposition of oxidized polyaromatic fragments (oxidation debris). The curve stabilizes at around 4%-weight after 550 °C due to the presence of residues from the synthesis procedure of raw-GO. The pronounced decomposition at around 200 °C is no longer observed in the thermogram of sample df-GO, thus indicating the elimination of these fragments after the washing of raw-GO with NaOH. The relative weight-loss difference associated with debris fragments is approximately 14%, as calculated by using the weight loss phenomenon at about 225 °C. After isolated and lyophilized, the thermal decomposition profile of Debris was also obtained (see Figure 3a). The progressive weight loss along the overall temperature range without well-defined decomposition phenomena illustrates the chemical complexity of these residual structures. However, the presence of two characteristic weight losses around 420 and 500 °C in the thermogram indicates the polyaromatic nature of sample Debris; once at these temperatures there are the major weight losses of raw-GO and df-GO samples, both related to the breaking of the graphitic sheets. Weight losses at higher temperatures in the Debris thermogram (>550 °C) come from the formation of sodium-based compounds due to the washing process with NaOH, and the relative percentage of these impurities in weight is about 10%, a result related to specific parameters of the washing process used in this study. Fourier transform infrared spectra (FTIR) analyses of the samples were performed in order to differentiate chemical characteristics by probing their chemical bonds through the vibrational modes (see Figure 3b). The splitting of the CC stretching mode (υ(CC, aromatic), 1570 and 1630 cm−1) in
Figure 1. Diagram showing the protocol employed for (A) eliminating the oxidation debris (intrinsically generated from the oxidation process of graphite) and the (B) decoration process (through Ag+ reduction) with silver nanoparticles. Photographs of raw-GO (commercial graphene oxide) and df-GO (debris-free graphene oxide) suspensions are depicted.
natural tendency to unfold and reveal the sheets through strong interactions that occur mainly with the sheets edges and the holey supporting carbon. After the removal of oxidation debris, the sheets appear well packed forming agglomerates. The morphology of the smaller debris fragments was also observed through TEM (see Figure 2c). The ionization washing process with NaOH is able to separate oxidized graphitic entities of different sizes from the graphene sheets, which were previously indistinguishable (see arrows in Figure 2c). Another intrinsic characteristic phenomenon present in the bidimensional atomic sheets of graphene that was probed is the surface plasmon electronic transitions. We have accessed those transitions in the electron energy-loss spectrum (EELS), which was acquired using a spectrometer coupled to TEM. Due to a strong electron quantum confinement in graphene which results in discrete energy gaps, the π−π* transitions appear as well-defined electron energy-loss peaks in the low energy-loss region of the spectra (up to 10 eV, see top panels in Figure 2a−c). Peak intensities are mainly related to the number of graphene layers that interacted with the electron beam, and energy shifts are related to changes in the electronic characteristics of the material.35 In a recent theoretical study, it was shown that the amount and type of oxygenated groups (epoxy, hydroxyl, or carbonyl) bonded to graphene oxide could shift π−π* transition peaks in EELS spectra.36 Indeed, by comparing 4082
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Figure 2. TEM images and EELS spectra (upper panels) of samples (a) raw-GO, (b) df-GO, and (c) Debris. AFM images and height measurements (lower panels) of samples (d) raw-GO, (e) df-GO, and (f) Debris.
υ(C−O−C) + υ(C−O) modes. More precisely, the C−(CO)− C stretching and bending (∼1100−1300 cm−1 for ketones), C−O−C asymmetric and symmetric stretching (∼1040−1275 cm−1 for ethers) and the C−O stretching modes (∼1000−1200 cm−1 for alcohols and ∼1230 cm−1 for phenols) can be observed in the 950−1300 cm−1 range. C−H stretching modes (υ(CH2), υ(CH3), υas(CH2) and υas(CH3)) were indentified through small bands in the range from 2850 to 2980 cm−1 and the O−H stretching mode (υ(O−H)) appeared around 3416 cm−1. In this way, the debris-free graphene oxide (df-GO) still possesses oxygenated groups covalently bonded to the sheets that allow the formation of stable suspensions in aqueous medium (see Figure 1 in the Supporting Information), a result that contrasts recent findings regarding the water stability of the debris-free GO.30
the spectrum of the debris-free graphene oxide and also in spectrum of Debris suggests that their mutual interaction in sample raw-GO influences on the aromatic rings bond strengths of the graphitic sheets. Qualitatively, it is also observed that the integrated intensity ratio of carbonyl (υ(C O), 1720 cm−1) and CC (1570 and 1630 cm−1) bands decreases after the removal of CCFs, pointing out that the debris-free graphene oxide (df-GO) has less oxygen moieties than the raw sample (raw-GO). This observation is also supported by elemental analyses of carbon, oxygen, nitrogen, and hydrogen, which indicated an increase of C/H and C/O ratios after the removal of oxidation debris: from 26.9 and 0.8 to 33.7 and 1.2 (for samples raw-GO and df-GO, respectively). Broad bands below 1500 cm−1 in FTIR spectra can be mainly associated to δ(O−H, in-plane) + δ(CH2) + δas(CH3) + δ(CH3) + υ(C−CO−C) + δ(C−CO−C) + υas(C−O−C) + 4083
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Figure 4. C 1s X-ray photoelectron spectra (XPS) spectra of commercial graphene oxide (raw-GO), debris-free graphene oxide (df-GO), and oxidation debris fragments (Debris).
epoxy and hydroxyl groups as well as the integrated intensity ratio between these groups and carboxylic acids are significantly smaller in the debris-free sample (df-GO) compared to raw-GO and Debris. Therefore, the results suggest that the predominance of epoxy and hydroxyl groups in graphene oxide indicated by previous works36−38 are related to the oxygenated groups present in debris fragments, where these species are the major functional groups (see Figure 4). Furthermore, debris fragments possess graphitic sheets with a more preserved sp2 structure as compared to the other samples, once the sp3 carbon peak was not observed in the spectrum for sample Debris (see Figure 4). As previously reported in the literature,39 the reduction of graphene oxide was observed when GO suspensions were heated (up to 90 °C) under strong alkaline conditions (NaOH solution of 8 mol L−1). In the treatment we used to remove oxidation debris, the GO suspension was mixed with a much more diluted NaOH solution (0.1 mol L−1). XPS spectra (see Figure 4) indicated that samples raw-GO and Debris possess very similar CO/C sp2 and C−O/C sp2 intensity ratios, a fact that would not be evidenced if a pronounced reduction of the oxidized sheets occurred.39,40 Besides, the treatment with NaOH (0.1 mol L−1) has not promoted any variation in the ID/ IG ratio observed by Raman spectroscopy (see Figure 2 in the Supporting Information), thus indicating that a possible suppression of defects in the GO sheets through a reduction process has not occurred. In this way, it is not likely that this reduction effect would be induced by the washing process we performed on sample raw-GO, which is leading just to the separation of different chemical entities present in sample rawGO. It was shown that oxidation debris originating from carbon nanotubes are fragments that consist of highly unsaturated polyaromatic carbon structures containing a variety of oxy-
Figure 3. (a) Thermogravimetric curves and (b) infrared spectra (FTIR) of commercial graphene oxide (raw-GO), debris-free graphene oxide (df-GO), and oxidation debris fragments (Debris).
Complementarily, differences in the microchemical environment of samples raw-GO, df-GO, and Debris were analyzed by X-ray photoelectron spectroscopy (XPS). The spectra were deconvoluted (see Figure 4) by rising five voigt profile peaks assigned to C−C sp2 bonding (C sp2, 284.5 eV), C−C sp3 bonding (C sp3, 285.2 eV), epoxy and hydroxyl groups (C−O, ∼286.8 eV), carboxyl groups (CO, ∼288.6 eV), and the C− C shakeup (∼290.0 eV). The integrated intensity ratio between C−C and C−O groups for samples raw-GO, Debris and df-GO were 1.45, 1.73, and 4.74, respectively. These results indicate that the quantity of oxygen moieties in graphene oxide decreases after the removal of oxidation debris, which was already observed in FTIR spectra. The peak pattern observed for sample Debris is very similar to that seen for the unwashed GO (raw-GO). XPS spectra also indicate that the quantity of 4084
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Figure 5. TEM images and size distributions of silver nanoparticles deposited on (a) raw and (b) debris-free graphene oxide (Ag-raw-GO and Ag-dfGO samples, respectively).
genated groups.41 Their presence and influence on the CNTs surface chemistry greatly depend on the type of the nanotube. For multiwalled carbon nanotubes (MWCNTs), even though the generation of debris fragments also depends on the parameters used in the oxidation process, it was seen that much less of these byproducts are formed as compared to singlewalled carbon nanotubes (SWCNTs), by considering the same oxidation conditions.42 This low production of CCFs in the oxidation process of MWCNTs resulted in a weak influence when in situ covalent reactions were performed on the MWCNT surface with and without oxidation debris.42 However, for SWCNTs, the presence of oxidation debris greatly influences the surface chemical reactivity of these tubes once they are in greater quantity (percent weight) and practically decorate the overall carbon nanotube walls.42 When dealing with graphene oxide, an analogy from oxidized SWCNTs may be used. However, although they both have one atomic layer (SWCNT and GO), the concept of oxidation debris itself is more tricky here, since both a graphene oxide sheet and an oxidation debris fragment present oxidized carbon chemical structures with one atomic layer, but with different sizes. Consequently, oxidation debris could be considered just as smaller graphene oxide flakes. In this way, a natural question arises: how to distinguish a graphene oxide sheet from debris fragments? In a first consideration, size should be a proper parameter to distinguish these entities, since the separation phenomenon is based on the sedimentation of the debris-free graphene sheet and the dissolution of the debris fragment after
its ionization with NaOH. However, there is not a well-defined size threshold that would be able to promptly differentiate them, since the size dispersion is an intrinsic parameter from the graphene synthesis and follows graphene-based products along the necessary processes for their production. Ultracentrifugation and capillary electrophoresis may be useful tools to advance in this size-related issue involving debris fragments.43,44 Although it was confirmed that the df-GO sample still possesses oxygenated chemical groups that provide water dispersibility, the amount and/or chemical nature of these groups may lead to different results regarding the chemical functionalization of graphene oxide. An example of this behavior was observed here when AgNPs were nucleated on df-GO (see Figure 5). The accepted kinetic model for explaining this reaction involves the interaction of Ag+ with oxygenated radicals on GO. More precisely, there are multiple chemical events described that might lead to the formation of silver nanoparticles on GO: deprotonated phenolic groups can reduce Ag+;45 carboxylic acids can complex and reduce Ag+, and also stabilize the nanoparticles formed;45 and other carbonyl radicals and epoxy groups can interact with Ag+ through dipoleion interactions, thus promoting a preferential nucleation of AgNPs on the GO sheet.46 On the other hand, citrate anions present in the reaction medium promote the stabilization of colloidal silver nanoparticles.47 However, even with the confirmation of the presence of oxygenated groups in sample df-GO through FTIR and XPS spectroscopy (see Figure 3b and 4085
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Supporting Information), the larger nanoparticles of sample Agdf-GO presented a structure with a significantly greater spatial order in comparison to the smaller ones in sample Ag-raw-GO. By considering the promising perspectives of graphene oxide and metallic nanoparticles-based nanocomposites, a fine control of morphology as well as crystallinity of the nanoparticles are required, which will impact directly on the desired applications. In this way, recognizing the presence of oxidation debris fragments is necessary and, consequently, their effects on further steps of the graphene-based products engineering can be evaluated. Besides, in terms of purity, the existence of oxidation debris must also be considered for any fundamental study regarding the chemistry of graphene oxide.
4), the result of the kinetic process of nucleation and growth of AgNPs on the debris-free graphene oxide was significantly different from the sample which still had fragments on the surface, which presented smaller nanoparticles (see Figure 5). Furthermore, a visual effect of aggregation of the graphene oxide sheets is seen for the sample Ag-df-GO, in which was detected weak evidence that indicate the presence of graphene oxide single layers on the holey-carbon films used for TEM analyses. In order to demonstrate statistically the different morphologies of silver nanoparticles, size distribution histograms were obtained by counting 2,300 AgNPs for each sample in several TEM images. The fitting of the distribution peaks was obtained through Gaussian functions. As seen in the AgNPs size distribution for sample Ag-raw-GO (see Figure 5a), a clear size-dependent tendency toward smaller sizes and narrower distributions is associated with the presence of debris fragments on GO sheets, which indicates that these entities have a key role in the energetic stabilization process that allows the formation of very small nanoparticles (
