Coal-derived Graphene Quantum Dots Produced by Ultrasonic

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Coal-derived Graphene Quantum Dots Produced by Ultrasonic Physical Tailoring and Their Capacity for Cu(II) Detection Yating Zhang, Keke Li, Shaozhao Ren, Yongqiang Dang, Guoyang Liu, Ruizhe Zhang, Kaibo Zhang, Xueying Long, and Kaili Jia ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06792 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 9, 2019

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ACS Sustainable Chemistry & Engineering

Coal-derived Graphene Quantum Dots Produced by Ultrasonic Physical Tailoring and Their Capacity for Cu(II) Detection Yating Zhang, *,

,‡

Keke Li,

,‡

Shaozhao Ren, Yongqiang Dang, Guoyang Liu,

,‡

Ruizhe Zhang

Kaibo Zhang, Xueying Long , and Kaili Jia College of Chemistry & Chemical Engineering, Xi’an University of Science and Technology, Xi’an, Shaanxi 710054, China ‡ Key

Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land

and Resources, Xi’an 710021, Shaanxi, China

ABSTRACT: Anthracite is a plentiful and affordable natural resource with a high coalification degree and many graphene-like sp2 carbon crystallites, which is fascinating for the development of novel coal-based carbon materials to achieve the value-added utilization of coal resources. In this work, a facile one-step ultrasonic physical tailoring procedure for the fabrication of blue luminescent coal-derived graphene quantum dots (C-GQDs) was exploited using Taixi Anthracite as carbon source. The as-prepared C-GQDs possess uniformly distributed sizes and diameters of 3.2±1.0 nm, and its aqueous solution can remain in stable homogeneous phase even after 2 months at room temperature. Moreover, we found that the C-GQDs exhibit two different distinctive emission modes. The evolution of the surface states and the electronic structure analysis revealed that two different types of fluorescence centers could be ascribed to nanosized sp2 carbon domains and oxygen functional group defects. Meanwhile, the unique electronic and chemical properties endowed the C-GQDs with a sensitive response to Cu2+, it was demonstrated as a potential fluorescent material for reliable, labelfree and selective detection of Cu2+, showing a great promise for real-world sensor applications. KEYWORDS: Coal, Graphene quantum dots, Synthesis, Ion detection



Corresponding author. Tel: 0086-29-85583183. E-mail: [email protected] (Y.T. Zhang) Address: No. 58, Yanta Middle Road, Xi’an 710054, China

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INTRODUCTION Graphene quantum dots (GQDs) are a novel fluorescent carbon nanomaterial composed of sp 2carbon domain structure. They possess strong edge effects and quantum confinement due to their unique particle sizes (below 60 nm).1-4 GQDs are promising alternatives for conventional semiconductor-based quantum dots because of their better biocompatibility, high photo stability and minimal toxicity.5-8 Many different ways have been used to synthesize diverse types of GQDs, which can be mainly divided into “top-to-down” and “bottom-to-up” strategies. These methods include acid-treating petroleum coke or coal,9-12 carbon fibers,13-15 graphene oxide,16 or multi-walled carbon nanotubes17 in the “top-to-down” approaches. The “bottom-to-up” approaches include oxidative condensation of aryl groups of polyphenylene dendritic precursors,18 nitration of pyrene,19 and pyrolysis or carbonization of citric acid.20, 21 In the above feedstocks, anthracite coal is a cheap and ubiquitous energy resource with a high coalification degree. It contains many small sp 2 carbon crystallites that can be easily exfoliated.22, 23 In view of the aforementioned unique properties, the preparation of GQDs from coal is feasible and effective. So far, the common method to prepare coalderived GQDs is chemical oxidation etching.11, 12, 24, 25 However, this is limited by the corresponding rigorous reaction conditions, such as higher temperature and longer time, in concentrated sulfuric and nitric acids. What's more, acid oxidation will induce considerable oxygen-containing groups, which will sharply decrease the size of the sp 2 conjugated carbons in the coal matrix resulting in weaker fluorescence intensity of GQDs.25 Herein, we describe a facile and fast method to prepare blue luminescent coal-derived graphene quantum dots (C-GQDs) via the straightforward physical process. We employ an ultrasonic cell crusher to fabricate C-GQDs without any acid treatment. As illustrated in Figure 1, TaiXi anthracite coal (TX-coal) precursor was dispersed homogeneously in N,N-dimethylformamide (DMF) to create hydrogen bonds rapidly between DMF molecules and the oxygen-containing groups (such as hydroxyl and carboxyl) of the coal molecules. Subsequently, the suspension is ultrasonicated in an ultrasonic cell crusher, and acoustic cavitation (oscillation, expansion, contraction, bursting or


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collapse of micro bubbles in fluid) generated by ultrasonic wave in liquid causes intense local heating (5000 °C), high pressure (in excess of 500 bar) and cooling rates that are higher than 1010 K s-1.26, 27 Thus, this high-energy circumstance triggers a series of chemical reactions (such as thermal decomposition and dissociation) to break bridge bonds and create the new oxygen-containing groups at the edges of the coal molecules. These can quickly combine with the DMF molecules and form a hydrogen bond to avoid restoration of bridge bonds. As a result, the pulverized TX-coal with micrometer size (Figure S1, Supporting Information, the micrograph of TX-coal) is finally fragmented into nanosized C-GQDs with the stronger fluorescence intensity. The C-GQDs exhibited a good quantum yield (QY) of 5.98% (Figure S2, Supporting Information, QY calculation of the CGQDs using quinine sulfate as the reference).

Figure 1. Schematic illustration of C-GQDs synthesis. EXPERIMENTAL SECTION Materials. Taixi anthracite coal from Ningxia Autonomous Region of China was pulverized and sieved to 74 μm before use(Table S1, Supporting Information, Analysis data of the TX-coal sample). Dialysis bags (retained molecular weight of 1000 Da) were ordered from the Sinopharm Chemical Reagent Co., Ltd. All chemicals (analytical reagent) were commercially available and used as received. Aqueous solutions of Cu2+ were prepared from their nitrate salts. The solutions of Na+, Mg2+, Ca2+, Al3+, Pb2+, Fe3+, Hg2+, Mn2+, Co2+ and Ba2+ were prepared from their chloride salts, the solutions ACS Paragon Plus Environment


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of K+ and Cd2+ were prepared from their sulfuric acid salt, and freshly prepared before use. The deionized water (DI-water) was used throughout this experiments. Preparation of C-GQDs. In a synthesis procedure, 200 mg of anthracite coal was put into a 50 mL N,N-dimethylformamide (DMF), and the suspension was irradiated by 21 kHz ultrasonic waves (Ultrasonic cell crusher, HANUO, HN-650Y, power output of 650 W, China) for 2 h. Subsequently, the suspension was filtered using a 0.22 μm Millipore polytetrafluoroethylene membrane. Next, the filtrate underwent dialysis for 3 days to completely remove the DMF. After purification, the solid CGQDs was obtained by using vacuum freeze-drying. Preparation of reduced coal-derived graphene quantum dots (RC-GQDs). C-GQDs aqueous (0.13 mg mL-1) were added to 100 mg of NaBH4. The obtained mixture was stirred lightly at ambient temperature for 6 h. Afterwards, unreacted impurities were completely removed by dialyzing in a 3500 Da dialysis bag against DI-water for 3 days. The result product was referred to as RC-GQDs. Cu2+ Detection. The 10 mM Cu2+ were prepared by dissolving 14.496 mg of Cu(NO3)2·3H2O in water and adjusting the solution volume to 6 mL. Detection of Cu2+ was carried out at ambient temperature in Tris buffer (10 mmol, pH=7.11). After Cu2+ ions were added to C-GQDs (0.02 mg mL1)

and gently shaken for 5 min, the fluorescence emission spectra of the solution was measured from

350 to 560 nm under the excitation wavelength at 300 nm. A similar process was used for different concentrations of Cu2+ ions and other metal ions. Characterization. Transmission electron microscopy (TEM) and high resolution analytical TEM (HRTEM) micrographs were obtained with a JEOL JEM 2100F operated at 200 kV. Scanning electron microscopy (SEM) images were taken by Hitachi S-4800. Photoluminescence (PL) spectra of CGQDs and RC-GQDs dispersions in quartz cuvettes were measured by fluorescence spectrophotometer (Perkin-Elmer LS-55). UV-visible spectra of C-GQDs and RC-GQDs dispersions in quartz cuvettes were obtained using a UV-visible spectrophotometer (Thermo-Scientific Evolution-220). X-ray photoelectron spectroscopy (XPS) measurements were made using a Thermo ESCALAB 250Xi spectrometer. Fourier transform infrared (FTIR) spectra were recorded with a


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Perkin-Elmer Spectrum GX FTIR spectrometer. RESULTS AND DISCUSSION Characterization of Materials. The as-obtained C-GQDs solution still keeps homogeneous phase at ambient temperature even after 2 months without any noticeable precipitation (Figure S3, Supporting Information, the C-GQDs solution for 2 months under ambient conditions). Figure 2a manifests the C-GQDs with uniformly distributed sizes and diameters of 3.2 ± 1.0 nm. Further HRTEM observation in Figure 2b reveals that the C-GQDs is about 8-10 stacked monatomic graphene layers and the distinct interplanar distance are measured to be about 0.34 nm, which corresponds to the (002) lattice fringe of graphene. The Fast Fourier transform (FFT) images (Figure 2b) reveals hexagonal lattices suggesting that the C-GQDs are crystalline hexagonal structures.

a)

b) d002=0.34 nm

25

Percentage (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20 15 10 5 0

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3

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5

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Particle size /nm

Figure 2. a) Transmission electron microscopy (TEM) image of C-GQDs, and b) high-resolution TEM (HRTEM) image of C-GQDs. Inset is the FFT pattern of C-GQDs. To further explore the optical absorption and photoluminescence (PL) properties of the C-GQDs, an intensive study of UV-vis and PL was conducted respectively. Figure 3a depicts that the C-GQDs exhibited an absorbance peak at about 220 nm on account of π-π* transitions of C=C, and a small shoulder peak at around 308 nm in connection with n-π* transition of the C-O or C=O. 24, 28-30 However, this shoulder shifts to about 320 nm after the C-GQDs was reduced by sodium borohydride (denoted as RC-GQDs, see Figure 3b). This can be attributed to the decrease in oxygen-containing groups at the edges of C-GQDs. The PL spectrum of C-GQDs exhibits a main emission peak situated at about 469 nm with a shoulder peak around 429 nm. It is noteworthy that the PL peak was varied after C-GQDs were reduced by sodium borohydride (Figure 3c). These changes in UV-vis and PL ACS Paragon Plus Environment


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spectra reflect emission at 429 nm and 469 nm due to the different electronically excited states in the heterogeneous electronic construction of the C-GQDs.31,

32

Interestingly, when the excitation

wavelength changed from 270 to 340 nm, the wavelength of PL peak remained constant, but its intensity decreased rapidly. This indicates that the PL spectra of C-GQDs possess an excitation wavelength that is independent in contrast to GQDs made from graphite (Figure 3d, inside: the photograph of C-GQDs solution was taken under UV hand lamp).29,

33

Correspondingly, the

photoluminescence excitation (PLE) spectra of C-GQDs were investigated at various emission wavelengths (λem) and had a maximum intensity at the λex/λem = 238/410 nm (Figure S4, Supporting Information, the PLE spectra for C-GQDs at different emission wavelengths). These optical properties of C-GQDs are similar to previously reported values for C-GQDs.24, 25

Figure 3. a) UV–vis absorbance for C-GQDs, b) UV–vis absorbance for RC-GQDs, c) PL spectra of the C-GQDs and RC-GQDs (0.02 mg mL-1 in water) at 300 nm excitation wavelength, and d) the PL spectra of C-GQDs (0.02 mg mL-1 in water) at varying excitation wavelengths. It is well known that the radiative recombination of electron-hole pairs mediates the bright fluorescence in C-GQDs.34 In a previous discussion, we concluded that the two fluorescence emissions of C-GQDs at 429 and 469 nm are probably associated with different excited states. During


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the preparation of C-GQDs, the bridge bond in the coal matrix has been broken under the ultrasound irradiation resulting in a nano-sized sp2 carbon crystalline structure with many edge defects. In addition, the long-wavelength emission of RC-GQDs was positioned at ~456 nm, which is between the short-wavelength and long-wavelength emission of C-GQDs. This is ascribed to the partial removal of the oxygen functional group on surface of C-GQDs.31 With that in mind, we hypothesize that one emission peak at ~429 nm may originate from the sp2 carbon domains, and another longwavelength emission at ~469 nm likely stems from defect state emission due to edge defects of CGQDs.31, 35

Figure 4. a) XPS spectra of TX-coal, C-GQDs and RC-GQDs; b,c) High-resolution XPS C1s spectra of C-GQDs and RC-GQDs; and d) FTIR spectra of C-GQDs and RC-GQDs. The XPS spectra shows an obvious graphitic C1s peak at 284.17 eV and an O1s peak at 531.17 eV for TX-coal, C-GQDs and RC-GQDs (Figure 4a). Compared with Tx-coal, the C1s peak and O1s peak of the C-GQDs are weaker. It confirms that the oxygen-containing groups are successfully created at the edges of coal due to the breaking of bridge bonds. High-resolution spectra for TX-coal, C-GQDs and RC-GQDs (Figure 4b, c and Figure S5, Supporting Information, high-resolution XPS C1s spectra of TX-coal) show the change in graphite carbon content (C=C) and oxygenated carbon ACS Paragon Plus Environment


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(C-OH, C=O and C-O) (Table S2, Supporting Information, XPS analysis of C-GQDs and RC-GQDs). The result proves that the percentage of graphitic carbon increased, and the percentage of oxygenated carbon decreased in RC-GQDs. The FT-IR spectra of C-GQDs and RC-GQDs (Figure 4d) both show stretching vibrations of O-H (hydroxy) at 3410 cm-1, C=C (polycyclic aromatic hydrocarbons) skeletal vibration peaks at 1450 cm -1, and C-H (aromatic rings) at 860 cm-1. In addition, the FT-IR spectrum of as-prepared C-GQDs has a vibrational absorption band of C=O at 1635 cm−1 and a stretching vibration of C-O in C-O-C (epoxy) groups at 1391-1076 cm−1. The results of XPS and FTIR spectra of C-GQDs and RC-GQDs implied that the oxygen-containing groups in the as-prepared C-GQDs are partially reduced by sodium borohydride (Figure 5). The results further confirms that short-wavelength emission and long-wavelength emission in C-GQDs is mainly ascribed to the π-π* transition of sp2 conjugated carbons as well as the n-π* transitions of oxygenous groups. More details on the PL spectra of RC-GQDs are offered in Figure S6 (Supporting Information, the PL spectra for RC-GQDs at different excitation wavelengths).

Figure 5. Scheme of the preparation route of RC-GQDs (suggested structures) via chemical reduction of C-GQDs with NaBH4. Capacity of C-GQDs for detection of Cu2+. To further explore the C-GQDs as a potential probe for ion detection and environmental monitoring, facile and straightforward fluorescence probes for the detection of metal ions in Tris buffer (pH=7.11) were investigated. The metal ions include Al3+, Ba2+, Ca2+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, K+, Mg2+, Mn2+, Na+, and Pb4+, and the concentration of each metal ion is 20 μmol L-1. These consequences demonstrated in Figure 6, where F1 and F2 are the fluorescence intensities of C-GQDs at 429 nm and 469 nm. Remarkably, the Fluorescence (FL) intensity ratio (F1/F2) of the C-GQDs for Cu2+ is higher than other metal ions, which indicates that the Cu2+ ions have a prominent quenching effect on the long-wavelength fluorescence of C-GQDs. ACS Paragon Plus Environment

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Figure 6. a) Emission spectra of C-GQDs in the existence of various metal ions (the content of the C-GQDs, 0.02 mg mL-1; the concentration of metal ions, 20 μmol L-1), b) Fluorescence intensity ratio (F1/F2) response of C-GQDs to the various metal ions. To further assess the potential of C-GQDs to detect Cu2+ in water, the variations of fluorescence intensities for C-GQDs in different concentrations of Cu2+ were studied. The Cu2+ obviously quench the fluorescence of C-GQDs, but they have no influence on the FL wavelength (Figure 7a). While Cu2+ ions have no absorbance peak near 300 nm,36 the C-GQDs with Cu2+ caused a blue-shift (∆λ=33 nm for 6.6 μmol L-1) of the absorbance peak at 308 nm with obvious absorption enhancement (Figure S7, Supporting Information, the UV–vis spectra of C-GQDs in the absence and presence of Cu 2+). These results are similar to that previously reported for graphene quantum dots. 37, 38 Furthermore, the quenched fluorescence of the C-GQDs by Cu2+ can be gradually retrieved by adding ethylene diamine tetraacetic acid (EDTA, a strong metal ion chelator, Figure S8, Supporting Information, the fluorescence spectra of C-GQDs, C-GQDs+Cu2+ and C-GQDs+Cu2++EDTA) with same molar amount. This suggested that the EDTA chelates copper more than C-GQDs.37

Figure 7. a) Emission spectra of C-GQDs on the concentration of Cu2+ in a range of 0–110 μM, b) ACS Paragon Plus Environment


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the fluorescence intensity ratio (F1/F2) of C-GQDs to the various concentration of Cu2+ (inset: fluorescence intensity ratio of C-GQDs versus the Cu2+concentrations from 0 to 8 μM). The fluorescence quenching of C-GQDs by Cu2+ is attributed to multi-site coordination between Cu2+ and oxygen-containing groups of C-GQDs. The C-GQDs possess a great number of oxygencontaining groups, which can serve as the electron donors and combine with Cu2+ to generate nonfluorescence complexes in this process.37, 39 In addition, the FL intensity ratio (F1/F2) gradually increases to around 27% of its original value and plateaus when the concentration of Cu 2+ is 11.4 μmol L-1. The quenching of C-GQDs’ FL intensity ratio by Cu2+ has a linear dependence with the concentration of Cu2+ from 0 to 8 μmol L-1 (Figure 7b). The regression equation is F0/F=0.0296C+0.9408 with R2 = 0.9716, where F0 and F are respectively the fluorescence intensities of the C-GQDs with and without Cu2+ and C is the concentration of Cu2+. The detection limit was calculated to be 0.29 μM at a signal-to-noise ratio of 3 (3σ/m, σ - the standard deviation of the blank signal, m - the slope of the linear calibration plot). These results indicate that fluorescent C-CQDs have great promise for the detection of Cu2+. CONCLUSIONS In summary, we developed a purely mechanical fabrication method for cutting anthracite coal into nanosized C-GQDs with strong blue emission. These as-made C-GQDs show two PL emission peaks at 429 nm and 469 nm. After reduction with sodium borohydride, the PL peak (469 nm) had blueshifted, which can be correlated to decreased oxygen functional groups and increased sp 2-carbon domain structures in C-GQDs. We also proved that the coal-derived graphene quantum dots can serve as a potential fluorescent probe to detect Cu2+ in water. This approach offers low cost, good selectivity, high sensitivity and wide linear response range with promising future sensor applications.

ASSOCIATED CONTENT Supporting Information Additional experimental data and discussion. The Supporting Information is available free of charge


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on the ACS Publications website at DOI:

AUTHOR INFORMATION Corresponding Author Tel.: 0086-29-85583183. E-mail: [email protected] (Y.T.Z.) ORCID Yating Zhang: Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support from the Natural Science Foundation of China (No. U1703251, U1810113), the Key R&D Program of Shaanxi Province (No. 2017ZDCXL-GY-10-0102), and the Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Land and Resources Open Research Topic (No. KF2016-4).

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Synopsis We develop a straightforward strategy to generate graphene quantum dots using anthracite as carbon sources, which possesses a high sensitive response to Cu(II) ions.

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Coal-derived Graphene Quantum Dots Produced by Ultrasonic

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