Experimental and molecular dynamics study of graphene oxide quantum dots interaction with solvents and its aggregation mechanism

https://doi.org/10.1016/j.molliq.2021.116136Get rights and content

Highlights

  • MD simulation was used to study GOQDs aggregation in solvents of different nature.

  • The aggregation process of GOQDs in solvents is determined by RDF analysis.

  • The optical behavior of aggregates was studied by UV–Vis spectroscopy and DFT.

  • Intermolecular forces (GOQDs-GOQDs, GOQDs-solvent) dominate GOQDs aggregation.

Abstract

This work describes the aggregation process and explains the optical behavior of graphene oxide quantum dots (GOQDs) in different solvents using molecular dynamics, DFT, and experimental observations. The optical behavior of electrochemically synthesized GOQDs in different solvents was analyzed by UV–Vis spectroscopy, and dependence between the spectra and the solvents (water, ethanol, acetone, chloroform, toluene, and n-hexane) was found. Molecular dynamics methods were used to determine that the local structure of the solvent molecules and the nature of intermolecular forces between GOQDs dominate their aggregation state and their optical properties in each solvent. These computational studies based on liquid-liquid systems provide a fast and straightforward approach to develop synthesis and purification methods that allow tailored advanced optical properties of GOQDs.

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 (single bondOH, single bondCOOH, Cdouble bondO) [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 single bondOH from single bondCOOH 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.

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