Experimental and molecular dynamics study of graphene oxide quantum dots interaction with solvents and its aggregation mechanism
Graphical abstract
Introduction
Graphene quantum dots (GQDs), members of the graphene family materials, have shown interesting optical and electronic properties since their discovery. These properties, along with their high chemical stability and biocompatibility, make GQDs suitable for a broad spectrum of applications such as photocatalysis [1], electrocatalysis [2], bioimage [3], sensors [4], [5], photovoltaic solar cells [6] and so on. GQDs can be obtained from different methods, including top down [7] (chemical and electrochemical exfoliation, acidic oxidation [4], hydrotermal) and bottom-up [7] (microwave-assisted pyrolysis [3], [8], electrochemical carbonization [5], [9]) synthesis strategies.
Important features of GQDs, such as their behavior as metal-free semiconductor [9], [10] and their optical properties, are related to their size (quantum confinement), shape [11], surface chemical composition (edge effect), functionalization with oxygenated groups (OH, COOH, CO) [12], [13], [14], [15] or doping with N, S, Se or B [16], [17], [18]. When an oxidative method is used for GQDs synthesis, these particles present a higher degree of functionalization with oxygenated groups. These groups are mainly located at the edge of the particles, increasing the reactivity in these positions. Due to the number of oxygenated functional groups, these types of particles receive the name of graphene oxide quantum dots (GOQDs) or oxygenated GQDs [12], [13], [15].
Applications based on GOQDs include chemical sensors [19], electrocatalysis [20], biomedical [21], and others. However, despite the significant number of synthesis methods, these applications are limited because their purification and extraction demand a high cost in time and money [9]. Therefore, the identification of physical properties of GOQDs in different solvents, such as the adopted aggregation in them, could help identify solvents that can facilitate their extraction.
Several studies have reported that the optical properties of GOQDs vary depending on the solvent (negative solvatochromism) [22], and the aggregation state they adopt on it [23], [24], [25] As observed by Chinnusami et al [24] and Shixiong et al [26], GOQDs aggregates retard the charge transfer during photocatalytic processes, decreasing its efficiency. This effect gains interest under the consideration that GQDs and carbon quantum dots (CQDs) formed by pilled GQDs [27], generate very stable suspensions, and that these do not show signs of precipitation even after a year [27], [28]. Therefore, the comprehension of the aggregation process of GQDs and GOQDs could help choose the ideal solvent for preparing GOQDs or CQDs composites, depending on the application.
Forces like π-π stacking, hydrogen bonds, van der Waals interactions, electrostatic interactions, and collision of solvent molecules dominate the aggregation of graphenic materials (polyaromatic molecules) [29]. Molecular dynamics (MD) methods offer several advantages to study the behavior of graphenic materials in different solvents, such as graphene sheets [30], functionalized graphene sheets [31], fullerenes [32], [33], carbon nanotubes [34], graphene oxide sheets [29], carbon dots[27] and other polyaromatic compounds, such as asphaltenes [35], [36]. These methods allow the understanding of the adopted mechanism of GOQDs aggregation, which can be face-to-face (H aggregates), parallel-displaced (J aggregates), or T-shaped [36], [37], [38].
In this paper, we present results from MD simulations to understand the behavior of quantum dots of graphene oxide in polar (water, ethanol, and acetone), non-polar (chloroform and n-hexane), and aromatic (toluene) solvents. The aggregation state was evaluated by radial distribution functions (RDF), mean square displacement (MSD), and the number of hydrogen bonds (H-bond). These results are contrasted with experimental UV–Vis spectroscopy measurements of electrochemically synthesized GOQDs in different solvents.
Section snippets
GOQDs synthesis
GOQDs were synthesized by an electrochemical carbonization method [5], [9], [39], [40], using ethanol as a carbon source in an alkaline medium. Briefly, a mixture of ethanol (96%) and KOH 0.01 M was sonicated for 10 min. This was used as the electrolyte in a two-electrodes cell composed of two graphite rods (area of 2 cm2). A constant potential of 10 V was applied for 4 h to this system. After this time, the initially transparent solution acquired a brown color.
UV–Vis absorption and FT-IR measurements
The UV–Vis spectroscopic
Experimental and computational FT-IR and UV–Vis spectra
Calculated and experimental FT-IR spectra of the GOQDs are shown in Fig. 2.A. The FT-IR was computed at the B3LYP level using the triple split valence basis and the diffuse functions 6-311G(d, p). A scaling factor of 0.9661 was used on the calculated vibrational frequencies to compare it with experimental results. Stretching signals of OH from COOH groups appear as two bands (3820 cm−1 and 3180 cm−1) in the calculated spectrum and as one in the experimental (between 3743 and 3774 cm−1). The
Conclusions
GOQDs were synthesized by a simple, fast, highly scalable, and environmentally friendly electrochemical carbonization method. FT-IR spectroscopy was used for the characterization of this material. The optical properties of these molecules in different solvents were analyzed by UV–Vis spectroscopy, finding an effect of the solvent on the aggregation state and these properties.
To study the behavior of GOQDs in different solvents, MD methods were used. These simulations were carried out using
CRediT authorship contribution statement
V.R. Jauja-Ccana: Conceptualization, Visualization, Methodology, Investigation, Writing - original draft, Writing - review & editing, Software, Formal analysis, Validation. Allison V. Cordova Huaman: Visualization, Methodology, Investigation, Formal analysis, Writing - review & editing. Gustavo T. Feliciano: Conceptualization, Writing - review & editing, Supervision, Software, Validation. Adolfo La Rosa Toro: Conceptualization, Resources, Visualization, Writing - review & editing, Supervision,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was financially supported by the Fondo Nacional de Desarrollo Científico, Tecnológico y de Innovación Tecnológica of Peru (CONV-000208-2015-FONDECYT-DE). The authors thank to the Laboratory Investigation of Biopolymers and Metallopharmaceuticals (LIBIPMET) for the computational resources.
References (72)
- et al.
Interfacial charge transfer in oxygen deficient TiO2-graphene quantum dot hybrid and its influence on the enhanced visible light photocatalysis
Appl. Catal. B Environ.
(2018) - et al.
Graphene quantum dots functionalized gold nanoparticles for sensitive electrochemical detection of heavy metal ions
Electrochim. Acta.
(2015) - et al.
High performance electrochemical sensors for dopamine and epinephrine using nanocrystalline carbon quantum dots obtained under controlled chronoamperometric conditions
Electrochim. Acta.
(2016) - et al.
Enhanced photovoltaic performance of inverted polymer solar cells utilizing versatile chemically functionalized ZnO@graphene quantum dot monolayer
Nano Energy
(2016) - et al.
Graphene quantum dots in biomedical applications: recent advances and future challenges
Handb. Nanomater. Anal. Chem.
(2020) - et al.
Mass production of tunable multicolor graphene quantum dots from an energy resource of coke by a one-step electrochemical exfoliation
Carbon N. Y.
(2018) - et al.
Metal-free graphene quantum dots photosensitizer coupled with nickel phosphide cocatalyst for enhanced photocatalytic hydrogen production in water under visible light
Cuihua Xuebao/Chinese J. Catal.
(2018) - et al.
One-step synthesis of boron-doped graphene quantum dots for fluorescent sensors and biosensor
Talanta
(2019) - et al.
Carbon-based 0D/1D/2D assembly with desired structures and defect states as non-metal bifunctional electrocatalyst for zinc-air battery
J. Colloid Interface Sci.
(2021) - et al.
Graphene oxide quantum dots derived from coal for bioimaging facile and green approach
Sci. Rep.
(2019)