L. M. Darabian¹*; T. L. G. Costa¹, D. F. Cipriano¹, C. W. Cremasco¹, M. A. Schettino Jr¹, J. C. C. Freitas¹
1 Laboratory of Carbon and Ceramic Materials, Department of Physics, Federal University of Espírito Santo, Vitória, ES, Brazil
Abstract
Graphene quantum dots (GQDs) are nanosized systems that combine beneficial properties typical of graphenic materials (such as chemical stability, biocompatibility and ease of preparation from low-cost precursors) with remarkable photoluminescent features. GQDs are well-known for their low cytotoxicity and for being promising candidates as fluorescent probes for bioimaging. In this work, we present a simple, low-cost and acid-free synthesis of GQDs, starting from an alcoholic aqueous suspension of graphene oxide (GO) and using a hydrothermal route.
The hydrothermal treatments were conducted using a homemade hydrothermal reactor that allows the control of the heating program and the assessment of the internal pressure generated in the reaction. The synthesized GQDs exhibited bright blue/green luminescence under UV light, showing the success of the chosen route and opening the way for future applications of these materials in the field of optoelectronic devices.
Keywords: Graphene Quantum Dots, Hydrothermal Synthesis,
1. Introduction
Graphene quantum dots (GQDs) are nanosized systems that combine beneficial properties typical of graphenic materials with remarkable photoluminescent features [1,2]. Due to the large amount of their possible applications, GQDs have been widely studied and several methods for their synthesis have been developed [1,3]. These methods can be divided into two main approaches: bottom-up and top-down [2,3] In the bottom-up strategy, solution-based chemical routes allow the synthesis of GQDs starting from several types of organic precursors [3]. The purification of the produced GQDs is complicated and the purified GQDs cannot be dispersed in water easily, which limit their field of application. Comparing to bottom-up strategies, the top-down approach is a fairly simple process. The critical aspect of the top-down methods is how to cut large graphene-based nanosheets into small aggregates (nanodots) [1-3].
In this work, we propose a simple, low-cost and acid-free synthesis of GQDs, starting from an alcoholic aqueous suspension of GO and using a hydrothermal route. The hydrothermal treatments were conducted using a home designed hydrothermal system that allows the control of the heating program while assessing the internal pressure generated in the reaction. The synthesized GQDs exhibited bright blue/green luminescence under UV light, showing the success of the chosen route and opening the way for future applications of these materials in the field of optoelectronic devices [1-3].
2. Materials and methods
2.1. Sample Preparation
The GO sample was prepared by a modified Hummers method [4]. After drying, the GO thick film was manually cut into small flakes and then a suspension was prepared using 100 mg of GO added to 50 mL of 99.8% ethanol and 50 mL of distilled water. Afterwards, this suspension was sonicated during 40 min to obtain a homogeneous dispersion.
The GQDs production followed a hydrothermal route similar to that described at Tian et al.
and Xie et al. works [5,6]. The dispersion was transferred to a teflon-coated stainless steel homemade hydrothermal reactor and thermally treated at programmed temperatures of 125 and 175 °C during 2 h. The work of this device is controlled by a homemade program written using
the Labview platform; with this program, it is possible not only to set the operational temperature and the residence time, but also to record in real time the pressure and the temperature of the reactional environment inside the reactor.
After the conclusion of the reaction, the samples were left to cool down to room temperature naturally for 12 h. Afterwards, the obtained suspensions were centrifuged at 4000 rpm during 3 h and then the supernatant was carefully collected and left to dry and at a stove at 50 °C during 24 h. The dry material was then redispersed in 50 mL of distilled water using an ultrasonic bath for 20 min. The dispersion was double filtered to remove bigger particles and contamination and finally the filtered dispersion was characterized by the methods below. These samples are labeled by their temperature of synthesis, GQD_125 and GQD_175. For comparison, a reference sample was also prepared adding 2 mg of GO to 100 mL of distilled water; this suspension was sonicated for 40 min and filtered afterwards to remove the undispersed material. It is labelled as GO_Ref. The idea is to compare the synthesized dispersions to a diluted dispersion of the precursor (GO) in distilled water
2.2. Characterization
The produced samples were first analyzed by recording a picture of the aqueous suspensions in the dark under a 5mW ultraviolet (UV) light with a wavelength of 365 nm produced by a commercial UV LED flashlight (Nitecore, model GEM 10 UV). When GQDs suspensions are tested with UV light they usually produce a visible glow, usually green or blue [2,5,6]. There are also cases where other visible colors are observed, depending on the GQDs composition [1,2].
The UV-visible optical absorption of the samples was analyzed using a Globlal Analyzer GTA-97 spectrophotometer. Fluorescence spectra were recorded in a Perkin Elmer LS 55 Fluorescence Spectrometer. Finally, transmission electron microscopy (TEM) images were recorded using a JEOL microscope, model JEM1400.
3. Results and Discussion
The hydrothermal reactor used in this work revealed interesting findings on how the temperature and pressure of the reactional environment evolves over time, as illustrated in Figure 1. Regarding the temperature profile, it is possible to observe an overshoot of ~15-20 °C in the beginning of the temperature plateau (Figs. 1a and 1c), even after careful adjustment of the temperature control parameters. A similar pattern was observed for the internal pressure (Figs. 1b and 1d), which showed a significant increase in the beginning of the temperature plateau, before reaching a nearly stable value. The maximum pressure reached values around 3 and 23 bar for the thermal treatments performed at 125 and 175 ºC, respectively.
The pictures taken under 5mW, 365 nm UV light showed a bright bluish-green glow for both GQD samples (Figs. 2b and 2d). For comparison, Figure 2f also shows the corresponding picture obtained for the reference dispersion (GO_Ref), where no glow is observed. On the other hand, all samples (GO_Ref, GQD_125 and GQD_175) look perfectly transparent under white light (Fig. 2a, 2c, 2e). These results thus indicate that both produced samples exhibit photoluminescent behavior when irradiated under UV light, which is a typical feature of aqueous GQD suspensions [1,2].
Fig. 2 – Samples under white light: a) GQD_125, c) GQD_175, e) GO_Ref. Samples under 5mW power, 365nm wavelength light: b) GQD_125, d) GQD_175, f) GO_Ref.
The UV-vis absorption spectra obtained for the produced samples are shown in Fig.4a; these spectra show a strong absorption band centered close to 225 nm, as usually observed in GO and undoped GQDs dispersions [3,5,6]. Even though the GO-water reference sample also exhibits an absorption band in this region, it is important to highlight that it is not as intense as for the GQDs dispersions. And fluorescence spectra recorded in the visible spectral region under 365 nm excitation for the produced samples are shown in Fig. 4b, confirming the photoluminescent behavior qualitatively observed in the images shown in Fig. 2. As expected, no fluorescence was detected for the GO_Ref sample, while the fluorescence signal is clearly observed with maximum intensity around 450 nm for both GQDs dispersions [5,6].
Fig. 4 – a) UV-Vis absorption spectra: blue) GQD_125, red) GQD_175, black) GO_Ref. b) Fluorescence spectra under 365 nm excitation: blue) GQD_125, red) GQD_175, black)
GO_Ref.
Finally, the TEM images recorded for the produced samples (Fig. 7) reveal that the GQD_125 and the GQD_175 samples are composed of particles with average sizes around 60
and 30 nm, respectively This result is the clearest evidence of the success of the acid-free hydrothermal synthesis with the homemade hydrothermal reactor used in this work.
Fig. 7 – TEM images of the GQDs dispersions: a) GQD_125 and b) GQD_175.
4. Conclusion
A simple, low-cost and acid-free route for the synthesis of GQDs, starting from an alcoholic aqueous suspension of graphene oxide and using a homemade hydrothermal reactor has been described in this work. The developed device allowed the control of the heating program and the assessment of the internal pressure generated during the reaction at high temperatures. The synthesized GQDs were fully characterized, showing the success of the chosen route and opening the way for future applications of these materials in the field of optoelectronic devices.
Acknowledgments
The authors would like to thank Laboratory of Cellular Ultrastructure Carlos Alberto Redins (LUCCAR – UFES), grant MCT/FINEP/CT- INFRA – PROINFRA 01/2006, for performing the TEM analyses and LabPetro (UFES, Brazil) for recording the FTIR and fluorescence spectra (Technical Cooperation Agreements n. 0050.0022844.06.4). The authors also thank FAPES (grant 345/2019), CAPES and CNPq for their financial support.
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