The critical contribution of oxidation debris on the acidic properties of graphene oxide in an aqueous solution

Aggregation of graphene oxide upon the stripping of oxidized debris: An experimental and molecular dynamics simulation study

The most recent structural study of graphene oxide (GO) indicates that the oxidized debris (ODs) adhered to as-prepared GO will strip in certain aquatic settings. The impact of ODs stripping on the characteristics of GO has been widely reported, but its effects on GO aggregation have received less attention. Here, the influence of OD stripping on the GO aggregation property was identified, and the aggregation of as-prepared GO and GO upon OD stripping was compared. Upon ODs stripping, the pKa values of GO shifted from 3.91, 6.25, and 9.84 to 4.54, 6.65, and 10.21, respectively. Further analysis indicated the removal of ODs reduced the net negative charge and improved the hydrophobicity of GO, hence promoting the aggregation of GO. The acceleration of GO-Ca2+-OD aggregate formation was facilitated by the collective effects of ODs stripping, functional group deprotonation, double layer compression, OD bridging, and charge neutralization. The metal ions and stripped ODs attach to GO edges and link GO, which perform like bridges and contribute to further aggregation. In general, the existence of ODs adds complexity to the constructions and characteristics of GO, and it is important to take this into account while evaluating the aggregation characteristic of GO-based materials.

A Meta-Analysis to Revisit the Property-Aggregation Relationships of Carbon Nanomaterials: Experimental Observations versus Predictions of the DLVO Theory

Contradicting relationships between physicochemical properties of nanomaterials (e.g., size and ζ-potential) and their aggregation behavior have been constantly reported in previous literature, and such contradictions deviate from the predictions of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. To resolve such controversies, in this work, we employed a meta-analytic approach to synthesize the data from 46 individual studies reporting the critical coagulation concentration (CCC) of two carbon nanomaterials, namely, graphene oxide (GO) and carbon nanotube (CNT). The correlations between CCC and material physicochemical properties (i.e., size, ζ-potential, and surface functionalities) were examined and compared to the theoretical predictions. Results showed that the CCC of electrostatically stabilized carbon nanomaterials increased with decreasing nanomaterial size when their hydrodynamic sizes were smaller than ca. 200 nm. This is qualitatively consistent with the prediction of the DLVO theory but with a smaller threshold size than the predicted 2 μm. Above the threshold size, the material ζ-potential can be correlated to CCC for nanomaterials with moderate/low surface charge, in agreement with the DLVO theory. The correlation was not observed for highly charged nanomaterials because of their underestimated surface potential by the ζ-potential. Furthermore, a correlation between the C/O ratio and CCC was observed, where a lower C/O ratio resulted in a higher CCC. Overall, our findings rationalized the inconsistency between experimental observation and theoretical prediction and provided essential insights into the aggregation behavior of nanomaterials in water, which could facilitate their rational design.

Identifying the Stripping of Oxide Debris from Graphene Oxide: Evidence from Experimental Analysis and Molecular Simulation

In this study, characteristics of oxidation debris (OD) and its stripping mechanism from graphene oxide (GO) were explored. The results demonstrated that OD contains three components, namely, protein-, fulvic acid-, and humic acid-like substances; among these, protein-like substances with lower molecular weight and higher hydrophilicity were most liable to be stripped from GO and were the primary components stripped from GO at pH < 10, whereas humic acid- and fulvic acid-like substances were stripped from GO at pH > 10. During the stripping of OD, hydrogen bonds from carboxyl and carbonyl were the first to break, followed by hydrogen bonds from epoxy. Subsequently, π-π interactions were broken, and hydrogen bond interactions induced by hydroxyl groups were the hardest to break. After the stripping of OD, the recombination of OD on GO was observed, and regions containing relatively fewer oxygen-containing functional groups were favorable binding sites for the readsorbed OD. The stripping and recombination of OD on GO resulted in an uneven GO surface, which should be considered during the development of GO-based environmental materials and the evaluation of their environmental behavior.

The fate of aggregated graphene oxide upon the increasing of pH: An experimental and molecular dynamic study

Given the possible ecological dangers of graphene oxide (GO), a thorough understanding of its aggregation behavior is essential. During industrial applications, GOs may be used as multi-layered, and there is some possibility that GOs are released into the water environment in the aggregated state. Thus, elucidating the fate of aggregated GO is valuable for evaluating their environmental fate. In this work, the effect of pH on the fate of aggregated graphene oxide (GO) was explored using experimental measurements and molecular dynamic simulations and promoted aggregation of GO upon the increase of pH was observed. Additional investigations show that the presence of oxidation debris (ODs) on GO served as the primary driver of the unanticipated trend in aggregation behavior. GO consists of lightly oxidized functionalized graphene sheets and highly oxidized ODs. Upon the increase of pH and the deprotonation of functional groups, ODs are stripped from GO due to electrostatic repulsions and steric hindrance of water molecules. The stripping of ODs decreased the zeta potential and increased the hydrophobicity of GO, thus accelerating the aggregation. Additionally, the stripped ODs may recombine to GO edges and bridged GOs, which also contribute to further aggregation. Functional group deprotonation, ODs stripping, OD bridging, double layer compression, and charge neutralization all worked together to promote aggregation, resulting in the formation of FG-water-OD aggregates. Overall, the presence of ODs complicates the structures and properties of GO and should be considered during the development of GO-related nanomaterials and the evaluation of their environmental impact.

Improvement of Cs detection performance and formation of CsCl and Cs nanoparticles by tuning graphene oxide quantum dot-based nanocomposite

A new nanocomposite was developed using functionalized graphene oxide quantum dots (GOQDs) with cesium green molecules for the first time. Although the cesium green molecule works effectively only in the solid-state, without water, and in basic conditions, the functionalized GOQDs with cesium green made the nanocomposite work well as a cesium (Cs) detector in mixed solution (distilled water/THF). The nanocomposite can be employed as a Cs detector in both acidic and basic conditions. The present study revealed that the nanocomposite of GOQDs with cesium green showed an enhanced photoluminescence in basic conditions, while the intensity of the photoluminescence in acidic conditions is the superposition of the photoluminescence of the corresponding components. The photoluminescence of the nanocomposite was quenched (turned OFF) after Cs treatment in basic conditions. On the other hand, in the acidic conditions it was found that the photoluminescence intensity of this nanocomposite was enhanced (turned ON) by the Cs addition in two different Cs concentrations, 0.06 mmol L^-1 and 0.12 mmol L^-1. In addition, the movement of the nanocomposite (after Cs addition) under the electron beams through TEM measurement was observed. The formation of CsCl and Cs nanoparticles was identified. Specifically, the Cs cluster occurrence is discussed by taking into account the mobility effect of the adatoms on the composite layer under electron beam irradiation.

Mechanisms of the Aggregation of Graphene Oxide at High pH: Roles of Oxidation Debris and Metal Adsorption

In this work, aggregation of graphene oxide (GO) in synthetic surface water at high pH was elaborated, and experimental characterizations and molecular dynamics simulations were employed to uncover the mechanisms. According to previous studies, aggregation of GO is supposed to be impossible at high pH considering the deprotonation of functional groups on GO and the increased electrostatic repulsions. However, significant aggregations and a reversed trend in zeta potential at high pH were observed. One of the mechanisms was that the promoted metal adsorption at high pH can offset the negative charges generated by the deprotonation. Additionally, the stripping of oxidation debris (OD) on GO also contributes to the unexpected trend in the aggregation behavior and zeta potential. GO consists of lightly oxidized functionalized graphene (FG) sheets and highly oxidized OD. Upon the increase of pH and the deprotonation of functional groups on FG and OD, OD was stripped from FG, which decreased the electrostatic repulsions between FG sheets and accelerated the aggregation. The stripped ODs may recombine to FG edges and bridged FG sheets, which also contribute to the aggregation. Upon the stripping of OD and microstructure transformation of FG, FG-water-OD aggregates formed. According to this study, the aggregation of GO was accompanied by deprotonation of functional groups, metal adsorption, and surface property transformation triggered by the stripping of ODs and should be considered during the development of GO-related nanomaterials and the evaluation of its environmental impact.

Exploration of the Cs Trapping Phenomenon by Combining Graphene Oxide with α-K6P2W18O62 as Nanocomposite

A graphene oxide-based α-K₆P₂W₁₈O₆₂ (Dawson-type polyoxometalate) nanocomposite was formed by using two types of graphene oxide (GO) samples with different C/O compositions. Herein, based on the interaction of GO, polyoxometalates (POMs), and their nanocomposites with the Cs cation, quantitative data have been provided to explicate the morphology and Cs adsorption character. The morphology of the GO-POM nanocomposites was characterized by using TEM and SEM imaging. These results show that the POM particle successfully interacted above the surface of GO. The imaging also captured many small black spots on the surface of the nanocomposite after Cs adsorption. Furthermore, ICP-AES, the PXRD pattern, IR spectra, and Raman spectra all emphasized that the Cs adsorption occurred. The adsorption occurred by an aggregation process. Furthermore, the difference in the C/O ratio in each GO sample indicated that the ratio has significantly influenced the character of the GO-POM nanocomposite for the Cs adsorption. It was shown that the oxidized zone (sp²/sp³ hybrid carbon) of each nanocomposite sample was enlarged by forming the nanocomposite compared to the corresponding original GO sample. The Cs adsorption performance was also influenced after forming a composite. The present study also exhibited the fact that the sharp and intense diffractions in the PXRD were significantly reduced after the Cs adsorption. The result highlights that the interlayer distance was changed after Cs adsorption in all nanocomposite samples. This has a good correlation with the Raman spectra in which the second-order peaks changed after Cs adsorption.