White-Light Emission from Unmodified Graphene Oxide Quantum Dots

Article pubs.acs.org/JPCC

White-Light Emission from Unmodified Graphene Oxide Quantum Dots Tufan Ghosh and Edamana Prasad* Department of Chemistry, Indian Institute of Technology Madras, Chennai 600 036, India S Supporting Information *

ABSTRACT: We report herein the synthesis and characterization of unmodified graphene oxide quantum dots (GOQDs) with white-light-emitting properties, upon photoexcitation at 340 nm. The Commission International de l’Éclairage (CIE) 1931 chromaticity coordinates for GOQDs (x = 0.29, y = 0.34) suggest that highly pure white-light emission was achieved. A detailed mechanistic study was carried out utilizing UV−visible absorption, steady-state and time-resolved fluorescence spectroscopy, and dynamic light scattering (DLS) techniques to understand the origin of the white-light emission. The results taken together suggest that GOQDs could self-assemble in solution and thus transform the luminescence behavior. Furthermore, the results indicate that the pH of the medium also plays a crucial role in assisting the aggregation to generate the white-light emission. The concentrationdependent DLS measurements support a cooperative mechanism for the aggregation kinetics in the system. More importantly, the study suggests that white-light emission can be generated from unmodified graphene oxide quantum dots by tuning their nanoscopic aggregation properties.

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INTRODUCTION Graphene quantum dots (GQDs) have emerged as a relatively new member in the graphene family.1−3 GQDs, with zero dimension and unique electronic structure, exhibit interesting luminescence properties compared to those of two-dimensional zero-bandgap pristine graphene.4,5 Recent research in this field has generated enormous interest because of the potential utility of GQDs in various fields including optoelectronics, biomedicine, sensors, and photovoltaics.6−16 There has been sustained interest in understanding the emission properties of GQDs such as tunable luminescence, two-photon-induced luminescence, and photostability.16−18 The luminescence properties of GQDs can be tuned from blue to yellow, and even to red.19 Tunable luminescence from GQDs has been attributed to various causes, such as effects of size, edge structure, surface functionalization, and doping.16,17,20 For example, Peng et al. treated carbon fibers with a mixture of concentrated HNO3 and H2SO4 and showed that tunable luminescence from blue to yellow could be observed as the reaction temperature was varied from 80 to 120 °C.8 In another attempt, Tetsuka et al. prepared amino-functionalized GQDs and showed that their photoluminescence changed from violet to yellow with increasing degree of functionalization.17 Also, Jin et al. showed that the luminescence of GQDs can be red-shifted by functionalization with amine groups.21 Whereas the tunable luminescence of GQDs has been reported in the literature in limited numbers, understanding of the origin and mechanism of this phenomenon requires further investigation.22 Materials with tunable emission properties are important because they can be effectively utilized in generating white-light emissions. Various materials are used for the generation of © 2015 American Chemical Society

white-light emission. For example, small organic molecules, quantum dots, polymers, and inorganic hybrid materials have been typically used for white-light generation.23−27 Whereas multicolor luminescence from GQDs has been reported in the literature, only a handful of reports are available regarding white-light generation from GQDs.20,28−30 However, in most of the reported cases, the white-light emission was achieved by chemical modification of the GQDs. For example, Li et al. synthesized chlorine-doped GQDs by the hydrothermal treatment of fructose and hydrochloric acid and showed that white light can be generated when samples are photoexcited at 350 nm.20 In another attempt, Kwon et al. prepared octylamine-functionalized GQDs of various sizes (i.e., 2, 4, 7, and 10 nm) and showed that 10-nm GQDs exhibit white-light emission.28 Also, Luk et al. succeeded in producing white-lightemitting diodes (LEDs) from GQD−agar composite materials.29 Most recently, Sekiya et al. functionalized the edges of GQDs with poly(aryl ether) dendron, which led to white-light emission.30 However, white-light emission from unmodified GQDs has not been reported. In the present study, we have synthesized graphene oxide quantum dots (GOQDs) by acid treatment of graphene oxide (GO) at 70 °C and observed a bright white-light emission from the as-synthesized, unmodified GOQDs upon photoexcitation at 350 nm. The effects of pH and concentration on the whitelight emission from GOQDs were investigated. We also investigated the origin of this white luminescence with the Received: October 1, 2014 Revised: December 29, 2014 Published: January 14, 2015 2733

DOI: 10.1021/jp511787a J. Phys. Chem. C 2015, 119, 2733−2742

Article

The Journal of Physical Chemistry C Scheme 1. Schematic Representation of the Possible Synthesis of GOQDs from GO

ysis, at a particular pH. The pH-dependent steady-state luminescence spectra were recorded from a series of GOQD solutions having various pH values at the same concentration. Time-Correlated Single-Photon-Counting (TCSPC) Measurements. Time-resolved luminescence decay experiments were recorded on a Horiba Jobin Yvon FluoroCube instrument in a time-correlated single-photon-counting (TCSPC) arrangement. A 340-nm nano-LED with a pulse repetition rate of 1 MHz was used as the light source. The instrument response function (IRF) was collected by using a scatterer (Ludox AS40 colloidal silica, Sigma-Aldrich). For the 340-nm LED light source, the instrumental full width at halfmaximum including detector response was ∼1.2 ns. The excited-state decay of the samples was collected by fixing the emission wavelength at a particular wavelength in the range of 450−610 nm. The decay was fitted using IBH software (DAS6) according to the following triexponential decay equation

help of steady-state and time-resolved luminescence spectroscopies. Experiments using dynamic light scattering techniques provided valuable information regarding the role of aggregation in controlling the white-light emission from the GOQDs. This is the first report of white-light emission from unmodified GOQDs with CIE (Commission International de l’Éclairage) 1931 chromaticity coordinate (x, y) values of (0.29, 0.34), which are very close to those of pure white-light emission (0.33, 0.33).

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EXPERIMENTAL SECTION Materials. Natural graphite powder (300 mesh) was purchased from Alfa Aesar and used without further purification. GO was synthesized according the Hummers’ method and characterized as described previously.31 Concentrated nitric acid (98%), sodium hydroxide, and hydrochloride acid were purchased from Merck Specialties Pvt. Ltd., Mumbai, India, and utilized without further purification. All experiments were carried out in doubly distilled water. Transmission Electron Microscopy (TEM) Study. Transmission electron microscopy (TEM) images were recorded on a JEOL 3010 instrument operating at 200 kV. A diluted sample solution was drop-casted onto a carbon-coated copper grid and then dried at room temperature overnight. The size distribution graph was plotted manually from the TEM image. Dynamic Light Scattering (DLS) Experiments. Dynamic light scattering (DLS) experiments were carried out on a Malvern Zetasizer Nano ZS-90 instrument. A red laser having a wavelength of 632.8 nm was utilized, and the scattering was measured at a 90° angle with respect to the light source. A series of solutions with different concentrations were made for use in concentration-dependent DLS measurements. All DLS measurements were carried out at 298 K. UV−Visible Absorption Experiments. UV−visible absorption measurements were carried out on a Jasco V-660 spectrophotometer at 298 K. The samples were placed in a quartz cuvette with a path length of 1 cm. Concentrationdependent UV−visible absorption studies were carried out with a series of GOQD solutions having different concentrations at a particular pH. Similarly, pH-dependent UV−visible measurements were carried out from a series of GOQD solutions with various pH values. The pH of the solutions was adjusted by adding dilute HCl or NaOH solution. Steady-State Luminescence Experiments. Steady-state luminescence spectra were recorded on a Horiba Jobin Yvon Fluoromax-4 spectrofluorimeter. A four-face transparent quartz cuvette was used for the experiments. The emissions of the sample were monitored at a 90° angle with respect to the excitation light source. The excitation and emission slit widths were both kept as 3 nm. A series of GOQD solutions having different concentrations were made by diluting a stock solution for concentration-dependent steady-state luminescence anal-

3

I (t ) =

∑ A i e −t / τ

i

i=1

(1)

where τi is the decay time and Ai represents the amplitude of the corresponding decay.32 A χ2 value in the range 1.01 ≤ χ2 ≤ 1.07 was assumed to provide a good fit. The experiments were carried out at acidic pH (2) as well as alkaline pH (12). Synthesis of GO and GOQDs. GO was synthesized by oxidation of natural graphite using a H2SO4/KMnO4/NaNO3 mixture according to Hummers’ method. The details of the synthesis and characterization of GO were reported in our previous article.31 GOQDs were synthesized by the oxidative cutting of GO following a literature method with slight modifications.33 The typical synthesis of GOQDs involves mixing of 0.15 g of GO with concentrated HNO3 (98%) and stirring the mixture for 30 min at room temperature. Then, the temperature of the reaction bath was increased to 70 °C, and the reaction mixture is kept at this temperature for 8 h. Subsequently, the highly acidic reaction mixture was diluted with 100 mL of doubly distilled water. Sodium hydroxide pellets were used to make an alkaline solution of GOQDs. A stock solution of GOQDs was prepared and utilized for further experiments (details of the synthesis are given in the Supporting Information).

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RESULTS AND DISCUSSION Synthesis and Characterization of GOQDs. The chemical synthesis of GOQDs typically involves a rich carbon source (graphene oxide, graphite, etc.), an oxidant (a mixture of concentrated H2SO4 and HNO3 or concentrated HNO3 in some cases), and a stabilizing agent.3 Herein, we followed a literature method with slight modifications, where GO was treated with concentrated HNO3 at 70 °C for 8 h (Scheme 1).33 Then, the mixture was neutralized, ultrasonicated, and 2734

DOI: 10.1021/jp511787a J. Phys. Chem. C 2015, 119, 2733−2742

Article

The Journal of Physical Chemistry C

Figure 1. (a,b) TEM images of GOQDs at various magnifications. Scale bar for each image is 20 nm. Inset in panel b: Size distribution plot obtained for GOQDs showing an average size of 3 nm. (c) UV−visible absorption spectra of GO (black) and GOQDs (red). Inset in panel c: Expanded region of the absorption spectrum of GOQDs; the shoulder band for GOQDs is situated around 385 nm. The peak at 300 nm for both spectra is assigned to the n−π* transition of carbonyl moieties. The band below 250 nm is assigned to the π−π* transition of the aromatic network. (d) Excitation spectra with emission collected at 470 nm (black) and 600 nm (red) and the steady-state luminescence spectra of GOQDs (0.96 mg/mL) upon excitation at 350 nm (blue) and 540 nm (magenta).

filtered through a 0.22-μm membrane filter paper to remove large and unreacted GO sheets. An aqueous solution of the assynthesized GOQDs was deep brown in color. Because the GOQDs were synthesized by a concentrated acid treatment, huge numbers of oxygen functional groups (hydroxyl, epoxy, ketone, carboxylic acid, etc.) are expected to be present, as suggested by previous reports.8 The GOQDs were characterized by various experimental techniques, including transmission electron microscopy and UV−visible absorption and steady-state luminescence spectroscopies. Figure 1a,b show TEM images of GOQDs. As can be seen in the figure, the GOQDs are a few nanometers in size. The size distribution plots obtained from the TEM image show that most of the GOQDs are 3 nm in size (Figure 1b, inset). Figure 1c compares the UV−visible absorption spectra of GO and GOQDs in aqueous medium. GO and GOQDs both exhibit two similar absorption bands: one at 300 nm (n−π* transition of the carbonyl moieties) and the other below 250 nm (π−π* transition of the aromatic network).34 However, GOQDs exhibit a sharp n−π* band in the absorption spectrum, suggesting the presence of a large number of carbonyl functional groups, compared to GO. Interestingly, the UV− visible absorption spectrum of GOQDs also displays a redshifted shoulder band around 385 nm, which is assigned to the formation of J-type self-assemblies.35 We corroborated this

hypothesis through a detailed UV−visible absorption and luminescence spectroscopic study along with DLS measurements. The results indicate that GOQDs can self-assemble in solution. Next, we investigated the luminescence properties of the GOQDs. A typical broad emission around 470 nm, along with a shoulder band around 600 nm, was obtained when the sample was photoexcited at 350 nm (Figure 1d). The emission at 460 nm originates from exciton recombination at the free zigzag edges of GOQDs.22 Nonetheless, the emission peak around 600 nm is unusual for aqueous solutions of GOQDs. For further characterization of the GOQDs, we recorded their excitation spectrum. The excitation spectrum (emission collected at 470 nm) typically shows two sharp peaks, one at 265 nm (4.68 eV) and the other at 340 nm (3.65 eV), along with a broad peak around 375 nm (3.3 eV). Pan et al. showed that the luminescence of GOQDs arises from the radiative relaxation of the excited electrons from the highest occupied molecular orbital (HOMO) to the triplet lowest unoccupied molecular orbital (LUMO).22 According to Pan et al., GOQDs have a triplet ground state (σ1π1) when the difference in energy (δE) between the σ1 and π1 states is