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As stated in Scientific Reports, control the configuration of graphene oxide (GO) phases, or rather their smaller synthetic analogues, graphene (oxide) quantum dots (GOQD), for any of their Frequently used applications are essential. This includes highly ordered arrangements of nanomaterials for thin film or outer layer applications, distributed nanoparticles for matrix composites, and three-dimensional porous structures for hydrogels.
Research: The principle of controlling the aggregation and dispersion of water-based graphene oxide. Image Credit: Jarek_Sz/Shutterstock.com
The morphology of GO flakes in an aqueous environment is affected by external variables, such as pH and coexisting cations, as well as the chemical properties of the flakes themselves. Due to the unique hydrophilic, thermal and electrical properties of GO, people are interested in using the chemical properties of GO flakes to produce self-assembled GO structures with specific application geometries.
In order to improve the mechanical modulus and strength of the film and fiber, a high degree of alignment and strong contact between the sheets are required. On the other hand, the porous structure is more suitable for energy storage devices such as supercapacitors and hydrogels.
(a) A schematic diagram showing different morphologies and aggregation interaction modes formed by graphene (oxide) flakes. The flakes in the upper right corner (b) are examples of GO flakes oxidized to varying degrees. The first slice shows a GO slice with a C:O ratio of 2.5 at the atomic level. In this example, the carboxyl edge group is deprotonated by the calcium counter ion. The color scheme of the atomic system is: carbon is cyan, oxygen is red, hydrogen is white, calcium is yellow, and sodium is blue. The remaining flakes are displayed in coarse-grained levels. The red circles represent oxidized hydroxyl groups and edge carboxyl groups, and the gray represents graphite carbon atoms. The diameter of each flake is 10 nm. (c) shows the initial configuration of the dispersion (water not shown). There are five slices in each simulated cell. The blue solid line is the periodic boundary, and the lattice parameter is about 18nm. d) shows the different carboxyl edge groups considered in this study, (i) protonation\neutral (ie low pH), (ii) deprotonation with sodium counterion and (iii) deprotonation with calcium counterion . All atomic and coarse-grained visualizations in this figure and subsequent figures were created using VMD version 1.9.248,49. Image source: Suter, J. and Coveney, P., "Scientific Reports" magazine
For some purposes, dispersion is necessary; aggregation and the resulting loss of surface area have been described as the main factor that reduces GO protein binding capacity, which has an impact on the pharmaceutical industry.
GO is also considered as a possible adsorbent for the removal of environmental pollutants, such as radionuclides, by generating nanoparticle aggregation.
Graphene oxide quantum dots (GOQD), a reduced form of GO, have attracted a lot of attention recently. GOQD has shown multiple advantages in bioimaging, biological and metal sensors, photovoltaic cells, and photocatalysts related to the edge and quantum Hall effect. Since GOQD contains a higher proportion of edge sites than GO, they are more hydrophilic while maintaining the same amphiphilic properties as GO10. Photoluminescence is one of the most important applications of GOQD.
The aggregation interaction mode that exists in graphene oxide flakes in aqueous solution. (a) A snapshot of the coarse-grained simulation, illustrating the relevant binding mode. The red circle on the edge binding mode snapshot highlights how Ca2+ acts as an ion bridge and promotes the edge binding mode. The color scheme is the same as in Figure 1. In (b), we show the percentage of flakes in each combination mode of the different systems in our study. Different binding modes ("face-face", "part", "edge-edge" and "face-edge" are shown schematically in Figure 1). Unbonded flakes are classified as scattered. Image source: Suter, J. and Coveney, P., "Scientific Reports" magazine
In ultra-high resolution TEM or transmission electron microscope images, both GO and GOQD show areas of oxidation and graphite, instead of uniformly distributed oxidation sites on the GO surface.
These areas can be used as building blocks for self-assembled 3D designs or as catalysts for poorly polymerized structures. Therefore, it is essential to control secondary interactions such as static electricity and hydrogen bonds by changing the chemical structure of the flakes or by changing external parameters (such as pH or the presence of counterions).
Several variables that affect the accumulation of GO in water were discovered in the study. The solubility of GO in water is very large, in which a large number of oxygen-containing functional groups are deprotonated, which is well known.
Oxygen-containing functional groups are protonated at low pH, reducing the charge on the GO surface. When the electrostatic repulsive force is reduced, large-scale visible floc aggregation will form. By increasing the ionic strength and osmotic pressure of the aqueous solution to shield the negative charge, reduce the necessary coagulation concentration.
Molecular modeling is an important tool for studying GO flakes in polymer substrates because it allows researchers to change the structure and composition of GO, evaluate the thermodynamically stable morphology of GO, and observe the dispersion and aggregation kinetics in a highly realistic way.
Researchers may check whether the aggregation patterns seen in the simulation are the same as assumed, such as edge-to-edge and face-to-face binding. Users can check the balance between interactions with water molecules, ions, and other flakes by evaluating the interactions that exist in the atomic details of our aggregated GO flakes.
A snapshot of the 300K atom simulation after the simulated annealing run, illustrating the observed surface-to-surface binding modes of different parts of GO, where the C:O ratio is 2.5. (a)–(c) are snapshots of uncharged (ie, low pH GO), and (d) are snapshots of charged GO flakes with Na+ ions. The different binding modes are highlighted by dashed ellipses and circles. (a) Due to the π-π stacking interaction between sp2 carbon atoms, the flakes overlap. (b) The protonated carboxylic acid atom forms a hydrogen bond with the hydroxyl group on the overlapping sheet. The hydrogen bond is represented by a red dashed line and is defined by the distance between oxygen and hydrogen atoms <4.5 Å and the acceptor oxygen-donor oxygen-hydrogen atom angle <40°. (c) Blue circle: The hydroxyl group forms a hydrogen bond with the hydroxyl group on the overlapping sheet. Orange circle: The sp2 carbon atom interacts with the graphene oxide region on the adjacent sheet through van der Waals interaction. (d) Combination mode of two charged GO sheets with Na+ ions. The orange circle highlights the interaction between the sp2 carbon atoms and the hydrolyzed area on the adjacent flakes, and the blue circle shows the hydrogen bonding interaction between the flakes. The lower sheet has become wavy and twisted to separate the charged carboxylate groups. Image source: Suter, J. and Coveney, P., "Scientific Reports" magazine
Understanding the relationship between flake structure and composition is essential for the optimal working of water-based graphene oxide and graphene oxide quantum dot systems. Our simulations include real subdomains of oxidized and non-oxidized regions, showing that these domains are essential for GO and GOQD aggregation. We have seen many interactions between polymeric sheets, including high-density oxidized regions that establish hydrogen bonds with surrounding sheets and graphite-like accumulation in sp2 carbon regions.
The low pH allows the protonation of the carboxylic acid edge units, causing the flakes to aggregate in a face-to-face bonding mode, as shown in experiments, and confirms the reduction in electrostatic interactions reported in the DLVO study.
The degree of oxidation on the surface of the formed flakes determines the shape of the formed flakes at low pH; highly oxidized GO forms planar aggregates of overlapping flakes, which are combined through a variety of interactions. The hydrogen connection between hydroxyl groups on different flakes, the overlap of sp2 regions, and the connection between carboxylic acid groups and hydroxyl groups on various flakes are examples.
As evidenced by the experiment and expected from the DLVO study, the high pH with sodium counterion promotes the production of a stable dispersion of one or two flakes. Despite the electrostatic repulsion of the flakes, the hydrogen bonds between the high hydroxyl density regions and the sp2 or van der Waals contacts between the oxidized domains are sufficient to induce some highly oxidized GO flakes with the same charge to assemble into crystal-like clusters composed of two flakes .
Suter, J. and Coveney, P. (2021). The principle of controlling the aggregation and dispersion of water-based graphene oxide. Release time: November 17, 2021. https://www.nature.com/articles/s41598-021-01626-3
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Akhlaqul is passionate about engineering, renewable energy, science and business development.
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