Large scale production of graphene quantum dots through the reaction of graphene oxide with sodium hypochlorite

Abstract

Graphene quantum dots (GQDs) assume unique chemical/physical properties owing to their single atomic layered structure and quantum confinement. A facile and effective approach to the large scale production of GQDs is highly demanded to facilitate their wide applications. In this work, we demonstrated that graphene oxide (GO) reacted with sodium hypochlorite (NaClO) under UV irradiation and the reaction could be applied to generate GQDs. The reaction proceeded very quickly and GQDs could be obtained in a few minutes. The as-generated GQDs possess high crystallinity. More strikingly, the yield of GQDs prepared through the reaction of GO with NaClO is the highest in terms of all the methods developed so far, up to 78% in weight. The results of fluorescence behavior studies revealed that the as-obtained GQDs exhibit stronger fluorescence emission, compared with that prepared though the photo-Fenton reaction of GO we proposed in our previous work. Moreover, it was clarified that the oxygen-containing groups on the plane of GQDs played a key role on the fluorescence behavior.

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Graphene has aroused tremendous research interest due to its unique one atomic layered structure and fantastic properties, such as large surface area, high carrier transport mobility, excellent mechanical strength and superior thermal stability.^1,2 However, graphene assumes a zero-band electronic structure, which severely limits its applications in electronic and optoelectronic devices.³ Therefore, it is highly demanded to open up the band gap of graphene to facilitate its applications in such devices. Up to now, converting two dimensional (2D) graphene sheets into 0D graphene quantum dots (GQDs), with lateral dimensions less than one hundred nanometers, is an effective strategy to tune the band gap of graphene. Apart from the intrinsic properties of graphene, GQDs have discrete band-gaps and show typical semiconducting properties associated with quantum confinement and edge effects.^4–6 Additionally, GQDs assume low toxicity, excellent solubility, stable photoluminescence and chemical inertia,^7–13 generating great promise for GQDs in various applications, for instance, in photovoltaics, light emitting diodes, electrochemical/chemical sensors, catalysis, and biological systems.^14–20 To achieve their various applications, the prerequisite is the preparation of GQDs in bulk scale and several approaches have been developed so far. Among them, the commonly used strategy is cutting micrometer sized graphene or other carbon materials into nanometer sized sheets under harsh conditions, such as strong acid oxidation,^21,22 hydrothermal reaction,^23–25 and electrochemical oxidation.^26,27 However, these methods suffer from some unfavorable disadvantages, such as complicated procedure, damage on the aromatic carbon framework, time consuming, low yield and so on. Another strategy to synthesize GQDs is based on condensation reaction of small organic molecules.^28,29 While, the as-generated GQDs have strong tendency to form aggregation which hinder their practical applications. Some other approaches have also been developed, such as ionic liquid-assisted grinding of natural graphite,³⁰ ruthenium-catalyzed cage-opening of C₆₀,³¹ they are not suitable for massive production of GQDs due to the complicated procedure and low yield.

In our previous work, we obtained GQDs through the photo-Fenton reaction of graphene oxide (GO) for the first time.³² The GO can react with Fenton reagent under UV irradiation, and GO sheets experienced different morphology changing gradually and could be converted into GQDs by controlling the reaction time. It is a convenient and fast way to prepare GQDs in water because no harsh and complicated treatments were involved in the whole procedure. More importantly, the yield of as-generated GQDs can reach up to 45%, which is the highest reported in the literature. However, the drawback of this strategy is that the photo-Fenton reaction of GO is preferable to carry out in acid condition, because the reaction rate is dramatically decreased at high pH values. More significantly, the photoluminescence emission of as-obtained GQDs is too weak to exert their applications in cellular imaging and certain optoelectronic devices which require strong fluorescence emission. Above all, it is highly demanded to explore a facile and effective approach to prepare GQDs with high fluorescence emission in bulk scale.

The reactions involving free radical accomplish very rapidly and are widely used in practical production. For the photo-Fenton reaction of GO, the reactive species is hydroxyl radical (OH˙), which has powerful oxidation ability. However, OH˙ is not stable enough in alkaline solution. Here, in this work, we explore another radical oxidation reaction to prepare GQDs in a wide range of pH values. More remarkably, the as-obtained GQDs possess higher fluorescence emission strength compared with that by photo-Fenton reaction.

Sodium hypochlorite (NaClO) is a commonly used oxidant in wastewater treatment under alkaline condition to decompose many kinds of polluted aromatic organic compounds. It can generate oxygen radicals [O] through self-decomposition, which also has strong oxidation ability.^33,34 The UV light can dramatically accelerate the decomposition of NaClO. GO, as a derivative of graphene, could be considered as superb aromatic molecule with oxygen-containing functional groups on its surface, it should have the potential to react with NaClO.^35,36 In this work, the reaction of GO with NaClO under UV irradiation was demonstrated. By simply controlling the reaction time, GQDs can be obtained within a short time. The morphology and chemical composite of GQDs have been characterized. The as-generated GQDs possess uniform size distribution and fine crystallinity. Fluorescence spectrum showed that GQDs exhibit strong photoluminescence (PL) emission. We demonstrated that the reaction of GO with NaClO was a facile and effective strategy to prepare GQDs in large scale.

The reaction experiment of GO with NaClO was carried out in a photoreactor equipped with an irradiation lamp. The reaction was performed under stirring in a quartz tube which was installed 6 cm away from the irradiation lamp. In a typical experiment, 750 μL NaClO aqueous solution (the content of effective chlorine is 5.2%) was added in 30 mL, 0.5 mg mL⁻¹ aqueous suspension of GO in a 40 mL quartz tube. The mixture was exposed to a mercury lamp (365 nm, 1000 W) under continuously stirring. With the UV radiation, NaClO can easily decompose to generate [O] and reacted with GO. The obtained GQDs were dialyzed in ultrapure water for 2 days to remove sodium ion and other small molecules. The solid GQDs could be obtained through evaporating the aqueous suspension of GQDs to remove water for further analysis.

Atomic force microscope (AFM) is an effective technology to characterize the surface morphology of 2D materials. The morphology of GO during the reaction progress was changing with the increase of time, which was monitored by AFM. The aqueous solution of GO was prepared following our previous work.³⁵ The GO sheets have lateral sizes from several micrometers to hundreds of nanometers, as seen in Fig. 1a. After reaction for 8 min, the surface of GO became rough and small holes were observed on the basal plane of GO, as shown in Fig. 1b. With the reaction time increasing, there were much more holes formed and the holes became larger. The flat GO sheets became the porous structure (Fig. 1c). What is exciting that the micrometer sized GO sheets were cut into GQDs finally, and the whole progress was just accomplished within 13 min. As depicted in Fig. 1d, the as-generated GQDs had uniform size distribution with diameter ca. 20–40 nm. The topographic height of GQDs was ∼1.2 nm, measured from the height profile of the AFM images (Fig. 1f), comparable with the height of GO (Fig. 1e), suggesting that GQDs also possess single atomic layered structure. Viewing the whole reaction progress, we notice that for the reaction of GO with NaClO, it has the same morphology changing as Fenton reaction of GO that the flat sheets were converted to nanoporous structure and finally were cut into GQDs.³² While, the reaction of GO with NaClO performed rapidly and GQDs can be obtained in fewer minutes. For the same concentration of GO aqueous suspension, it took at least 35 min to cut GO sheets into GQDs by Fenton reagent under the same condition, even if the usage amount of H₂O₂ was increased enough. For NaClO, it can react with GO quickly and GQDs can be generated in a shorter time (13 min). More strikingly, the yield of as-generated GQDs improved a lot, reaching up to 78% in weight, much higher than that by Fenton reaction of GO (45% in weight).

Fig. 1 AFM images of GO sheets before (a), and after being reacted with NaClO under the UV irradiation for 8 (b), 11 (c), and 13 (d) min, respectively; height profile of GO sheets (e) and GQDs (f). All images were acquired under the tapping mode. All scale bars equal to 500 nm.

Moreover, for the photo-Fenton reaction of GO, it was preferable to conduct in acid condition at pH 4. The reaction rate suppressed dramatically with the increasing of pH values. When the pH reached to 12, there was almost no reaction happening and the GO sheets retained their sheets motif, because OH˙ derived from H₂O₂ was not stable in alkaline condition. NaClO was commonly used in wastewater treatment under alkaline condition. It was speculated that the reaction of GO and NaClO should be carried out at high pH values. The reaction of GO with NaClO was conduct at different pH values of 3, 7, and 12. As shown in Fig. 2, after reaction for 8 min, a few small holes appeared on the surface of GO sheets at pH 3, 7 and 12. With time increasing, the GO sheets were cut into GQDs at the same time, and the obtained GQDs possessed the similar morphology, illustrating that the pH values have no obvious effect on the formation of GQDs. It concluded that the preparation of GQDs though the reaction of GO with NaClO can be accomplished in a wide range of pH value, both in acidic and alkaline condition, which is useful for the application of GQDs in biological system and catalysis.

Fig. 2 AFM images of the GO after being reacted with NaClO at different pH values, 3, 7 and 12 under the UV irradiation for 8 and 13 min. All scale bars equal 500 nm.

Transmission electron microscopy (TEM) imaging was also obtained. As displayed in Fig. 3a, the GQDs obtained at pH 12 for 13 min have lateral size ca. 20–40 nm, which was consistent with GQDs generated by Fenton reaction. From the contrast imaging, it can induce that GQDs was single atomic layered like GO. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 3b) showed conspicuous fringes with a lattice constant of 0.245 nm, similar to graphite (0.246 nm).³⁷ The well defined selected area electron diffraction (SAED) pattern (inset in Fig. 3b) indicted that the as-obtained GQDs possess high crystallinity, illustrating there were fewer defects on the surface of GQDs. This was further verified by Raman spectrum. The lower ratio of D to G bands in Raman spectra reflects their electronic conjugate statues and 2D crystallinity of the basal plane. Comparing with GO, the G band intensity is stronger than D band (Fig. 3c). This result confirms that the as-generated GQDs have a defect-free basal plane.

Fig. 3 TEM (a) and HRTEM (b) images of GQDs, inset in panel (b) presents a typical SAED of GQDs; Raman (c) and UV-Vis (d) spectrum of GO and GQDs.

From the UV-Vis absorption spectrum of GO solution before and after reaction (Fig. 3d), we see that a shoulder peak at ∼290–300 nm, corresponding to n–π* transition of the C=O band,³⁸ almost vanished after reaction, which illustrated that the oxygen-containing groups had a great change. To determine the chemical composition variation of GO and the resultant GQDs, X-photoelectron spectroscopy (XPS) measurements were carried out. For both of GO and GQDs, there were four peaks at 284.8, 286.7, 287.2 and 288.5 eV in the C1s spectrum, assigning to C=C/C–C in aromatic rings, C–O (epoxy and alkoxy), C=O and COOH groups, respectively. As depicted in Fig. 4, after GO sheets were converted into GQDs, the C–O peak intensity decreased dramatically, indicating epoxy and alkoxy groups were removed, while, COOH increased obviously in GQDs. The carbon to oxygen atomic ratio increased after reaction (from 65.05 : 34.95 in GO to 56.67 : 43.33 in GQDs), illustrating more COOH was generated in GQDs. Taking photo-Fenton reaction of GO in mind, we speculate that the C–O (epoxy and alkoxy) on the basal plane of GO served as the reaction sites, which were easily attacked by [O] derived from the self-decomposition of NaClO and C–C/C=C bonds were broken subsequently, and peripheral carbon was oxidized into COOH during the formation of GQDs. The more COOH group made GQDs possess excellent solubility and could be stable in water for several months without any agglomeration.

Fig. 4 XPS spectra of C1s of GO (a) and as-generated GQDs (b). The peaks 1, 2, 3, and 4 correspond to C=C/C–C in aromatic rings, C–O (epoxy and alkoxy), C=O, and COOH groups, respectively.

The photoluminescence (PL) properties of as-generated GQDs were explored as well. The PL spectrum of GQDs showed excitation-dependent behavior. When the excitation wavelength changed from 260 to 360 nm, as shown in Fig. 5a, a prominent peak exhibited at 450 nm with a shoulder peak at around 520 nm, which ascribe to intrinsic state emission and defect state emission, respectively.^39,40 It showed the strongest PL emission when excited at 260 and 320 nm, which was consistent with PL excitation (PLE) spectrum (Fig. 5b). With the increasing of excitation wavelength from 400 nm to 540 nm, the peak at 450 nm almost vanished, while the other PL peak became dominant and red-shifted (Fig. 5c). The reason for red-shifted is that the defect state emission is derived from the functional groups on the surface of GQDs, which have different energy level, leading to a series of emissive trap. Significantly, as seen in Fig. 5d, when excited in the range from 580 to 660 nm, the PL behavior was independent on excitation wavelength with the strongest emission located at 450 nm. It has been proved that the second-order diffraction light of wavelength λ/2 coexisting in the excitation monochromatic light of wavelength λ from a xenon lamp light is responsible for this phenomenon.⁴¹

Fig. 5 Photoluminescence (PL) spectra of GQDs under various range of excitation wavelength, (a) 260–380 nm, (c) 400–540 nm, (d) 580–660 nm; (b) the PL excitation (PLE) spectrum of as-generated GQDs recorded with the strongest emission at 450 nm.

Excitingly, the fluorescence intensity of as-generated GQDs by NaClO is much stronger, comparing with that prepared by photo-Fenton reaction, as shown in Fig. 6a. The quantum yield of GQDs by NaClO is 0.35 (rhodamine B as reference), about 3 times higher than GQDs prepared by photo-Fenton. It was speculated that the chemical composition play a key role on the PL emission, which was verified by FT-IR spectroscopy. From Fig. 6b we see that for GQDs prepared by NaClO, the epoxy C–O stretching vibration peak at 1226 cm⁻¹ almost vanished and alkoxy C–O stretching peak at 1052 cm⁻¹ was weak, while, there was some residue of epoxy C–O and alkoxy C–O in that by Fenton reagent. It was demonstrated that C–O and COOH always induced non-irradiative recombination of localized electron–hole pairs and suppressed the intrinsic emission.⁴² In the case of NaClO, because of the hydrolysis of NaClO, amount of OH– was generated in the solution. The reaction was conducted in alkaline solution, which facilitated converting carboxyl into carboxylate. In our previous work, we knew that the PL intensity of GQDs was stronger at high pH, because the negatively charged carboxylate groups, as electron donors, benefit on the PL emission. But even if the pH values of GQDs suspension are adjusted to the same, it was found that the GQDs by NaClO still possess the stronger PL emission. In alkaline condition, the UV irradiation can remove the oxygen-containing groups on the GO. For the reaction of GO with NaClO, oxygen radicals [O] was dissociated from the decomposition of NaClO and attacked the oxygen-containing groups on GO sheets. Meanwhile, amount of epoxy C–O and alkoxy C–O were removed in alkaline condition under UV irradiation, as depicted in Fig. 6b. The decrease of C–O and conversion of COOH in alkaline condition result in the increase of PL emission.

Fig. 6 The PL emission (a) and FT-IR spectra (b) of GQDs by NaClO (red line) and that though photo-Fenton reaction of GO (black line).

In conclusion, we demonstrated the reaction of GO with NaClO under UV irradiation. Oxygen radicals [O] dissociated from NaClO attacked the oxygen-containing groups, and GO sheets were oxidized into porous structure and GQDs gradually. The GQDs can be easily obtained in massive scale simply by controlling the reaction time. The preparation can be carried out in a wide range of pH values, both in acid and alkaline conditions. The obtained GQDs possess high stability in water. Comparing with photo-Fenton reaction of GO, the as-generated GQDs possess stronger PL emission and higher yield. Through the exploration of the fluorescence emission, we verify that the oxygen-containing groups play a key role on the PL behavior. It is a facile and effective way to prepare GQDs in bulk scale and it is beneficial for the practical application of GQDs.

Acknowledgements

We thank the National Science foundation of China (no. 51502231) and the Fundamental Research Funds for the Central Universities (no. JB151405) for the financial support of this work.

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