Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon

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Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon Atoms Growth via Energetic Material Deflagration Yousong Liu, Bing Gao, Zhiqiang Qiao, Yingjie Hu, Wenfang Zheng, Long Zhang, Yong Zhou, Guangbin Ji, and Guangcheng Yang Chem. Mater., Just Accepted Manuscript • Publication Date (Web): 04 Jun 2015 Downloaded from http://pubs.acs.org on June 5, 2015

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Chemistry of Materials

Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon Atoms Growth via Energetic Material Deflagration Yousong Liu,1,2 Bing Gao,1 Zhiqiang Qiao,1 Yingjie Hu,3 Wenfang Zheng,3 Long Zhang,1 Yong Zhou,4 Guangbin Ji,2 and Guangcheng Yang1* 1Institute

of Chemical Materials, China Academy of Engineering Physics, Mianyang, Sichuan, 621900, China. of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China. 3Nanjing University of Science and Technology, Nanjing 210094, China. 4 National Laboratory of Solid State Microstructures, School of Physics, Eco-materials and Renewable Energy Research Center (ERERC) Nanjing University, Nanjing 210093, China. 2College

ABSTRACT: Graphene quantum dots (GQDs) with quantum confine and size effect is proposed to be applicable in photovoltaic, nano devices and so on, due to extraordinary electronic and optical properties. Here we report a facile approach to synthesize gram-scale GQDs from active carbon atoms, which are obtained via the deflagration reaction of Polytetrafluoroethylene (PTFE) and Si, growing from highto low-temperature zones when travelling through the deflagration flame in a short time with releasing gas as the carrier medium. The prepared GQDs were aggregated into carbon nanospheres, thus Hummer’s method was utilized to exfoliate the GQDs aggregations into individual GQDs. We show that the length of GQDs is ~10 nm and the exfoliated GQDs solution presents an obvious fluorescence effect with a strong emission peak at 570 nm at 460 nm excitation. And these GQDs are demonstrated to be excellent probes for cellular imaging. Furthermore, we propose a growth mechanism based on computer simulation, which is well verified by experimental reproduction. Our study opens up a promising route for high-yield and high-quality GQDs, as well as other various quantum dots.

as the initial reactants to generate active single carbon atoms, as well as a deflagration flame to limit their growing into small-sized few-layered GQDs with ultrashort growth time and sharply decreased growth rate from the high-temperature inner flame to the low-temperature outer flame. The prepared GQDs were aggregated into carbon nanospheres (~70 nm in size, ~12 g per time), in which the individual GQD exhibits average diameter of ~10 nm and thickness of ~1 nm. The carbon nanosphere output can be facilely scaled-up if enough Si and PTFE reactants are supplied. The individual GQDs were exfoliated from the carbon nanospheres by using the simple Hummer’s method, which exhibit strong fluorescence property and excellent cell imaging performance. As the production yield of monodisperse GQDs is as high as 26 wt%, gram-scale exfoliated GQDs (~3 g per time) may be easily prepared from the present deflagration flame technique, advantageously over the precedent methods. The growth mechanism of GQDs is proposed and well verified by experimental reproduction and theoretical calculations. The significance and novelty of the work is to provide a promising route for high-yield and high-quality preparation of not only for GQDs, but also for other quantum dots.

INTRODUCTION Graphene quantum dots (GQDs), which are graphene nanosheets of less than 100 nm in size,1 exhibit unique optical and electronic properties due to their quantum confinement and edge effects, and are proposed to be applicable in cell imaging, photovoltaic devices, optical and electronic devices.2-6 Therefore, many research works have been devoted to both theoretical prediction and experimental synthesis of GQDs.7,8 Up to now, various methods have been demonstrated in preparation of GQDs, which can be classified into the top-down and bottom-up method. The top-down methods can be described as cutting or cleavage processes of carbonaceous materials (such as carbon fibers, graphite or graphene nanosheets), including electron beam lithography,1 hydrothermal or solvothermal cutting strategies,9,10 chemical exfoliation,11-13 and electrochemical oxidation processes.14,15 These routes get the advantages of abundant raw materials, large scale production and simple operation. Conversely, bottom-up methods are often used to synthesize GQDs via growth and assembly of small carbonaceous molecules including stepwise solution chemistry methods,16,17 cyclodehydrogenation of polyphenylene precursors,18,19 carbonizing some special organic precursors,20 and cage opening of fullerenes.21 The bottom-up methods are capable to control the size and shape of GQDs. Although great progress has been made in developing various strategies for preparation of GQDs, it is still a challenge to obtain GQDs with high rate, high yield and simplicity in operation. In this work, large-scale synthesis of GQDs is realized by deflagration reaction of energetic materials based on precise control of growth process of single carbon atoms. Polytetrafluoroethylene (PTFE) and Si, the carbon-rich energetic materials are employed

EXPERIMENTAL SECTION Synthetic Procedures. GQDs-aggreagated nanospheres synthesis: In a typical process, 50 g PTFE micropowder (2 µm) and 15 g Si nanoparticles (100 nm) were added into 200 mL cyclohexane and stirred for 2 h. The device is composed of a cavity of holding reactant and electrical detonator; all components are intergrated in a stainless steel setup with a diameter of 15 cm and 20 cm height at ambient pressure. After drying at 60 °C in a vacuum oven for 12 h, the mixture powder of Si and PTFE was placed in 1

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pseudopotentials (ECP) by Dolg 25 and Bergner 26, which explicitly treat scalar relativistic corrections. A spin-unrestricted calculation was performed. The real space cutoff radius was 0.37 nm, and the convergence tolerance for SCF was 1.0×10-6 Ha. Those of geometrical optimization for energy and maximum force were 1.0×10-5 Ha and 0.002 Ha/Å, respectively. In this study, the silicon surface is represented by the silicon (111)-(3×3) surface model. The surface was constructed by a periodically repeated twolayer slab of silicon atoms with a vacuum region of approximately 20 Å. A model of C10F20 over silicon surface is selected for the fluorine divorce from PTFE to combine with Si. A transition state (TS) between two immediate stable structures was first identified by linear synchronous transit 27 and then cyclically refined by quadratic synchronous transit and conjugate gradient methods. Each TS was converged within 0.002 Ha/Å. In this study, the armchair surface of graphite is represented by the graphite (100) surface model with 36 carbon atoms. The zigzag surface of graphite is represented by the graphite (111) surface model with 32 carbon atoms. These surfaces were constructed by periodically repeated single-layer slab of carbon atoms with a vacuum region of approximately 10 Å. The k-points set for such a supercell was 3×3×1 during all the calculations. All computed stationary points and transition states on the potential energy surface were confirmed by frequency analysis and minimum energy pathway (MEP) search based on Nudged-Elastic Band (NEB) algorithm 28 in Dmol3 package.

an enclosed stainless steel setup with an electrical match on top for ignition. The enclosed operation is utilized to collect the produced carbon materials, avoiding the loss which can be caused by the flow of SiF4 flue gas. After the combustion of Si and PTFE was completed, the black products were collected and transferred to a beaker containing 200 mL of 1 M NaOH. The product was stirred in the alkaline solution at 60 °C overnight to remove the excess Si powder. The product was then obtained after the precipitate was centrifuged, after which it was washed with deionized water several times to remove NaOH and Na2SiO3 and then dried in a vacuum oven for 12 h. The graphene quantum dots were oxidized and exfoliated from the product via a modified Hummer’s method. In a typical process, the as-prepared carbon nanospheres (1 g) were added to a solution of 34 mL of cold H2SO4 (0 °C) and 0.75 g NaNO3. KMnO4 (5 g) was then gradually added under continuous stirring in an ice bath. The solution was further stirred at 35 °C for 2 h, and then MilliQ-H2O (200 mL) was added. After stirring for 15 min, the reaction was terminated by the addition of 4 mL of 30% H2O2. After removing the insoluble by centrifugation at 10000 rpm, the exfoliated GQDs solution was dialyzed in a dialysis bag (retained molecular weight: 3500 Da) overnight. The concentration of GQDs is measured to be ~80 µg/mL. Characterization Methods. The morphology and particle size were determined through scanning electron microscopy (SEM, JEOL S4800) and transmission electron microscopy (TEM, JEOL JSM-2010) with an accelerating voltage of 200 kV. UV-Vis absorption spectra were obtained using a UV-Vis spectrometer (Shimadzu UV-3600), whereas photoluminescence (PL) spectra were obtained using a Varian Cary Eclipse spectrophotometer. The excitation wavelength λex varied from 400 nm to 580 nm, and the bandwidths of excitation and emission were both 5 nm. The AFM images were obtained through SPM-9600 atomic force microscopy. The Raman spectra of the samples were recorded by a Renishaw Raman RE01 scope using a 785 nm infrared laser. Cell Toxicity Assay. Cell toxicity was examined by AlamarBlue assays. HeLa cells were cultured and maintained in Dulbecco’s modiﬁed Eagle’s medium (DMEM, Invitrogen, Carlsbad, CA, USA) supplemented with 10% (v/v) fetal calf serum (FCS, Gibco-Invitrogen), 1% (v/v) penicillin/streptomycin (Invitrogen) at 37 °C in a humidiﬁed 5% CO2 atmosphere. Cells were seeded into 96-well plates (0.1 × 105 cells/well) overnight and treated with GQDs. After 24 h, 10% (v/v) Alamar Blue reagent (Sunbio Medical Biotechnology, Shanghai, China) was added, and the cells were incubated for another hour until the medium color changed from indigo blue to pink. Fluorescence intensities were measured using a Verioskan Flash Multimode Reader (Thermo Scientific, Waltham, MA, USA) with excitation at 560 nm and emission at 590 nm. Cell imaging. HeLa cells were seeded into 30 mm cell culture dishes with glass bottom at a density of 1 x103 cells/cm2. After overnight incubation, cells were treated with 100 µl GQDs for 4 hours, and then washed 3-5 times with PBS to remove medium with GQDs. Nucleus were stained with 2-(4-Amidinophenyl)-6indolecarbamidine dihydrochloride (DAPI, Beyotime, Shanghai, China) for 10-15 min. After 3-5 times washing with PBS, the fluorescence images were acquired by a Leica confocal laser scanning microscopy system TCS SP8 (Leica, Mannheim, Germany) at 364 nm excitation and 454 nm emission for DAPI, 460 nm excitation and 570 nm emission for GQDs. Theoretical Calculation Details. DFT calculations of the GGA-PBE 22 functions were performed by Materials Studio Dmo3 23 from Accelrys. The wave function was expanded in terms of numerical basis sets of double numerical quality (DNP 24 ) with d-type polarization functions on each atom. The core electrons for the palladium atoms were modeled using effective core

RESULTS AND DISCUSSION Our design strategy, described in experimental section, used a deflagration reaction between Si and PTFE to synthesize GQDs in gram-scale. Figure 1 illustrates the morphological observation of the GQDs-aggregated nanospheres. The Field emission scanning electron (FE-SEM) image in Figure 1a reveals that the nanospheres are nearly uniform in size and morphology with a diameter of ca. 70 nm. The colour of the corresponding products is black (inset of Figure 1a), which is remarkably different from the Si and PTFE energetic materials (Figure S1a). Typical TEM image (Figure 1b) of the as-prepared carbon nanoparticles shows regular nanospheres with a diameter of ca. 70 nm, which is also in agreement with the SEM image. Furthermore, the prepared carbon materials show polycrystalline behaviour and the diffraction rings can be characterized as (002) and (101) diffractions from inner to outer by calculating the ratio of the corresponding radii based on the inset SAED pattern in Figure 1b. Figure 1c shows the porous and stripe morphology of the GQDs-aggregated nanospheres, which may be resulted from the rapid aggregation rate of GQDs and spatial block effect of SiF4 gas. The HRTEM images (Figure 1d) clearly exhibit the typical image of the few-layered GQD nanospheres, with the number of layers ranging from 2 to 5. The measured lattice fringe space of this material is about 0.35 nm, which is in accordance with the layer distance between the fewlayered graphene.29 The lengths of the graphitic structure range from 5 nm to 15 nm, indicating that the average size of the GQDs is about 10 nm. Moreover, some monolayered GQDs can be observed in the inset of Figure 1d. To confirm the GQDs aggregation morphologies, we magnified some other regions of the nanospheres. Evidently, some individual GQDs can be identified (Figures. 1e and f). As shown in Figure 1e, the observed lattice spacing of 0.24 nm agrees with that of the in-plane lattice spacing of graphene (100) facet, indicating that the obtained nanospheres was composed of aggregation of graphene quantum dots. In addition, the interface between the GQDs also can be observed in Figure 1f to support the GQDs-aggregation nanostructure.

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Figure 1. SEM and TEM images of GQDs-aggregated nanospheres. a) A typical SEM image of the GQDs-aggregated nanospheres, with the inset digital photograph showing the large-scale production of the obtained GQDs-aggregated nanospheres. b) TEM images. The obtained carbon materials are in the size of ~70 nm. And the inset SAED image shows its polycrystalline property. c) TEM image of individual GQDs-aggregated nanospheres. d) High-resolution TEM images of the samples. The marked red areas showing the produced carbon nanoparticles are composed of 1 to 5 graphitic layered GQDs with average size of ~10 nm. (e, f) High-resolution TEM images of different sample regions that demonstrate GQDs aggregation morphology. graphene (100) facet. The inset corresponding fast Fourier transform (FFT) pattern shows a high crystalline structure of the GQD. The atomic force microscope (AFM) image (Figure 2d) of the GQDs shows that the resultant GQDs have the diameters ranging from 5 nm to 30 nm (average 10 nm), which is in agreement with the TEM results. The inset presents on the topographic height of the oxidized GQDs， which exhibits that their heights are mostly less than 2 nm (nearly 100%). Statistical analysis of oxidized GQDs height in Figure 2e obtained from more than 200 samples shows that about 85% of the oxidized GQDs have a thickness lower than 1 nm, suggesting that GQDs are mostly single layered, as the height of graphene sheets increases with decorated oxygencontaining functional groups in the oxidation process.

Modified Hummer’s method that has been widely applied for mass production of large-area GO sheets from graphite was employed to exfoliate the prepared GQDs from their aggregated nanospheres.30 And individual dispersed GQDs solution was obtained after the exfoliation process. It should be pointed out that about 26 wt% of GQDs can be exfoliated from the carbon nanospheres, which is much higher than the yield of previous works (5 wt% for the hydrothermal cutting of oxide graphene,10 10 wt% for the dissolve of thermal plasma jet soot,12 20 wt% for the chemical exfoliation of graphite nanoparticles 11 and coal 13). The TEM image (Figure 2a) of the exfoliated GQDs reveals that the average size of the GQDs is ~10 nm. High-resolution TEM images (Figure 2b) reveal the high crystallinity of the GQDs. The lattice fringe space of 0.24 nm (Figure 2c) agrees well with that of the

Figure 2. Characteristic results of the exfoliated GQDs. a) TEM image of the exfoliated GQDs; b) and c) HRTEM images of GQDs and the corresponding 2D FFT image. d) AFM image of the GQDs deposited on the mica substrate. Inset shows the height profile along the grey line. e) Height distribution of GQDs. Height ≤1 nm, one layer; Ca. 1.5 nm, two layers; Ca. 2 nm, three layers. Nearly all the exfoliated dots: 1 to 3 layers. f) Raman spectrum of the raw GQD nanospheres (red) and exfoliated GQDs (black). g) UV–Vis absorption (Abs, black) and PL (at 400 nm excitation, red) spectra of the exfoliated GQDs. h) PL spectra of exfoliated GQDs at different excitation wavelengths. 3



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Raman spectrum was carried out to provide the quality of the samples. Figure 2f red curve shows the two major components of the spectrum consist of D-band and G-band peaks at 1335 and 1587 cm-1, respectively. The D-band is considered as a breathing mode of k-point phonons of A1g symmetry, attributed to local defects and disorders, particularly the defects located at the edges of graphene and the G-band is generally assigned to the E2g phonon of sp2 bonds of carbon atoms which used to reflect degree of graphitization. The relatively high D-band intensity, which is due to the increasing fraction of disordered edge carbons, serves as defects by breaking the translational symmetry of the lattice,31 and is resulted from the small crystallite size of GQDs in their aggregation nanospheres. Thus, the intensity ratio of the D-band and Gband (ID/IG) of a Raman spectrum can reflect the degree of defects and disorders of the graphitic structures. For the GQDsaggregated nanospheres here, the ID/IG intensity ratio is as large as 1.02 for the high defects and disorders assigned to the size effect of its composition of small-sized few-layered GQDs. The Raman spectrum of the oxide GQDs after exfoliation (marked in black in Figure 2f), in which two characteristic peaks at 1325 (D band) and 1595 cm-1 (G band) can be observed. The ID/IG ratio increases from 1.02 to 1.75, which is assigned to the increasing fraction of edge carbons and disordered areas because of the decreased crystalline size of graphene quantum dots and formation of the oxygen-containing carbonaceous band (such as C-OH, C=O and O=C-OH) defects after the oxidation and exfoliation processes via Hummer’s method. The UV–Vis absorption spectrum of oxidized GQD aqueous solution is shown in Figure 2g for the oxidized GQDs, a typical absorption peak at ca. 220 nm is observed, which is assigned to the π → π* transition of aromatic sp2 domains.32 The PL spectrum of exfoliated GQDs in Figure 2g shows a strong emission peak at 570 nm with a Stokes shift of 170 nm (0.92 eV) upon excitation at 400 nm. Like other luminescent carbon nanoparticles,33 the exfoliated GQDs also exhibit an excitation-dependent behaviour, a red shift, and an intensity decline from 570 nm to 650 nm when the excitation wavelengths increase from 460 nm to 580 nm (Figure 2h). The emission wavelength of exfoliated GQDs is excitation

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independent when the excitation wavelength is lower than 460 nm (Figure S2).

Figure 3. Cell imaging of human cervical cancer HeLa cells after incubation without (a-d) and with (e-h) GQDs. a) and e) Bright-field image of HeLa cells. b) and f) Fluorescent images under 460 nm excitation show the agglomerated green GQDs surrounding each nucleus. c) and g) Fluorescent images under 364 nm excitation show the individual nucleus stained blue with DAPL. d) and h) The merged images. Further experiments were performed using human cervical cancer HeLa cells to demonstrate the potential application of exfoliated GQDs for cell imaging. The cells were cultured and maintained in DMEM medium described in the experimental section. Cell viability assay in Figure S3 shows that GQDs demonstrate very low cytotoxicity on human cervical cancer HeLa cells, indicating that they are suitable for biomedical applications. Figure 3e to 3h shows the images of HeLa cells treated with GQDs for 4 h. The HeLa cells treated with GQDs exhibit bright green colour in cytoplasm when imaged on the confocal laser scanning microscope with excitation at 460 nm as shown in Figure 3f. Compared with the HeLa cells treated without GQDs in Figure 3a to 3d, it can be clearly observed that the nucleus stained blue with DAPL and were surrounded with agglomerated green GQDs in high contrast, indicating GQDs’ promising applications as excellent bioimaging medium, as well as drug and gene carriers.

Figure 4. Schematic strategy proposed to synthesize GQDs. 2 µm PTFE and 100 nm Si was used as precursors to supply active carbon atoms and deflagration flame for the carbon growth process into GQDs. a) Temperature distribution image of the reaction flame measured via an IR thermometer. b) The deflagration reaction between Si and PTFE was ready to take place through breaking C-F bond and forming Si-F bond before ignition. c) The obtained active carbon atoms and SiF4 molecules. d) Active carbon atoms grow into GQDs. e) The GQDs aggregated into carbon nanospheres. f) The GQDs aggregation structure of the nanospheres. g) TEM image of the GQDs exfoliated from the obtained GQDs-aggregated nanospheres. h) TEM of the GQDs aggregation structure corresponding to f). We propose an alternative mechanism based on a bottom-up approach for experimentally efficient reproducing GQDs through a simple deflagration reaction of Si/PTFE energetic materials. The evenly mixed reactant of Si (100 nm) and PTFE (2 µm) were

chosen because of their inexpensive, easy-obtained and huge energy release. nSi + (C2F4)n → 2nC + nSiF4↑ △H = -5500 kJ/kg 34 The formation processes of GQDs are shown in Figure 4. First, after ignited by an electrical match, the Si atoms will capture F 4


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atoms from PTFE, generating active carbon atoms (as well as carbon clusters, e.g. C2 and C3 species etc.) and a deflagration flame with liberation of heat, as shown in Figure 4b and 4c. The temperature distribution of the deflagration flame (Figure 4a) detected by an IR thermometer reveals that it is higher than 1300 K in the inner flame. Subsequently, the active carbon atoms will grow into graphene nanosheets (GQDs) via a strong and stable sp2 bond, as shown in Figure 4d. In the growth process, the deflagration flame with rapid temperature decrease will intensely decline growth rate of carbon atoms. Moreover, the gas product SiF4 can serve as a carrier medium to transfer growing carbon clusters through the whole deflagration flame, resulting in a sharply shortened carbon growth time. The deflagration flame with appropriate carbon atoms growth rate and growth time plays a key role in the

formation of GQDs. The growth process of carbon atoms into GQDs is rapid and uniform because energetic materials possess high dynamic rate, and the deflagration flame is radially uniform in temperature distribution and chemical species concentrations. As shown in Figure 4e, due to the existence of residual active dangling bonds at the end of the GQDs, the graphene nanosheets structures would crimp and aggregate with each other into nanospheres to lower their surface energy and remain thermally stable. The gathering rate is so fast that the surrounding SiF4 molecules are unable to escape from the nanospheres, resulting in the GQDsaggregated nanospheres presenting porous morphologies (Figure 4f and Figure 4h). Figure 4g shows the typical TEM image of the GQDs exfoliated from the GQDs-aggregated nanospheres, which is in uniform with the size of ~10 nm.

Figure 5. Flame emission spectra (a) of radicals in the deflagration flame, Mass spectra (b) of the GQDs-aggregated nanospheres toluene solution and 13C NMR spectra (c) of the GQDs-aggregated nanospheres deuterochloroform solution. To identify the carbon atoms in the growth process, we first utilize Prlon infrared spectrometer to detect the radicals in the deflagration flame. As shown in Figure 5a, the stong peaks at 588.76 and 589.33 nm can be assigned to the CF2 redicals, which gives us a direct proof of the single carbon atoms, for that the CF2 redicals will be transformed to single carbon atoms after the F atoms captured by the Si atoms. we dissolve the small carbon molecules in the GQDs-aggregated nanospheres into methylbenzene under ultrasonic treatment for the subsequent mass spectra test. As shown in Figure S4b, the obtained methylbenzene solution presents reddish color, indicating the existence of the small carbon molecules in the GQDs-aggregated nanospheres. As shown in Figure 5b, many mass spectra peaks can be observed, indicating the diversity of the small carbon molecules as the intermediate products in the GQDs growth process. The m/z of 720 can be observed and assigned to the C60 signal, which is in agreement with the HPLC results in Figure S4a. In order to further analyze the structural information of small carbon molecules, NMR measurements were also conducted on the GQDs-aggregated nanospheres dissolved in the deuterochioroform. As shown in Figure 5c, signals between 100-160 ppm represent aromatic carbons (B1 area), signals between 60-100 ppm represent aliphatic carbon with an adjacent oxygen atom (B2 area), whereas signals at 0-60 ppm represent aliphatic carbons (B3 area).35 The aromatic carbon signals in B1 area demonstrate the formation of 6-membered rings in the small carbon molecules, which corroborates our growth mechanism of GQDs. The low signal intensity of aliphatic carbons in B3 area reveals their low concentration in the small carbon molecules, indicating the transformation of aliphatic carbon to aromatic carbon in the deflagration flame. The aliphatic carbon with an adjacent oxygen atom may result from the partial oxidation of GQD nucleus by the O2 or H2O species in the open air environment. To further clarify the GQDs growth mechanism, theoretical calculations, IR temperature measurement and high-speed photography were conducted. A theoretical reaction model (Supplementary Figure S5) of one F atom separated from PTFE to combine

with Si is set up by Materials Studio. The resultant potential energy curve of the C-F bond through the whole reaction process, including the breaking of C-F bond and the formation of Si-F bond, shows a monotonic decreasing trend, indicating that the designed reaction is spontaneous with no reaction energy barrier to overcome. After a whole decomposition of PTFE, exothermic reaction of Si/PTFE occurred.36 The deflagration reaction is so fast that we can consider it as an adiabatic model. Under these circumstances, a temperature of the reaction is calculated which reaches as high as 3659 K.37 However, the measured temperature in this work is far lower (~1300 K, Supplementary Figure S6a), which is resulted from the lower release of heat from relatively lower reaction rate with large reactants size, as well as the measure error caused by signal acquisition shortage with a long distance between the IR thermometer and deflagration flame to protect the instrument. High-speed photography is employed to analyze the rapid growth of GQDs from carbon atoms. As shown in Figure S7, the high-speed photography of the deflagration process is ca. 40 ms, indicating the actual growth time of the carbon atoms is even shorter. Due to the high flow rate of the produced species in the deflagration reaction, together with low contact opportunities caused by the blocking effect of SiF4, the ultrashort growth time of the carbon atoms plays an important role in the formation of small-sized fewer-layered GQDs, assisted with the growth rate decline in the deflagration flame. Theoretical simulations were performed to verify the growth mechanism of carbon atoms in equilibrium conditions. The active energy of carbon atoms incorporating at the Zigzag edge and armchair edge of graphene quantum dots has been calculated to investigate the probably edge growing mode. Figure 6a shows the graphene growth with carbon atoms incorporated at the Zigzag (ZZ) edge. The active energy of one 5-membered ring formation was found to be 155.89 kcal/mol higher than that of one 6membered ring. Similarly, in the graphene armchair edge growth shown in Figure 6b, the active energy of one 5-membered ring and one 6-membered ring formation was 68.00 kcal/mol higher than that of two 6-membered rings formation with 5 carbon atoms 5



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incorporated, and 407.13 kcal/mol higher than that of three 6membered rings formation with two more carbon atoms incorporated. Therefore, it can be concluded that the 6-membered ring is more thermodynamic stability in the carbon atoms growth process at either ZZ edge or AC edge, indicating that carbon atoms may trend to grow into graphene nanostructure with 6-membered rings incorporation in a high temperature approximate equilibrium condition. 38 However, for the 5-membered rings, they are found to be more stable in lower temperature than the higher and therefore are more frequently incorporated into the growing structures, which hinder further growth as incorporation of 5-membered rings creates portions of edge that are unable to grow. 39 In addition, it’s found that 5-membered rings make the graphene sheet curved for that their carbon atoms not in the same plane with 6-membered

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rings. It gives a theoretical explanation of the producing of spherical C60 in our deflagration flame, which may resulted from the formation of 5-membered rings when the carbon atoms travelling through the low temperature outer flame. Furthermore, it can be inferred that GQDs with larger size, more flat morphology will be obtained in a higher temperature deflagration flame, as it enhanced the growth rate of carbon atoms in forms of 6-membered rings incorporation in an improved high temperature zone and produced relatively low content of 5-membered rings in the nearly constant low temperature environment. Conversely, if in deflagration flame with lower temperature, GQDs with smaller size or even amorphous carbon will be obtained from lowered carbon growth rate in corporation of 6-membered rings and more 5membered rings formation to hinder the edge growth.

Figure 6. Theoretical calculations for the growth mechanism of GQDs from carbon atoms in equilibrium conditions. a, The graphene growth with carbon atoms incorporated at the Zigzag (ZZ) edge with the active energy of one 5-membered ring formation was found to be 155.89 kcal/mol higher than that of one 6-membered ring. b, The active energy of one 5-membered ring and one 6-membered ring formation was 68.00 kcal/mol higher than that of two 6-membered rings formation with 5 carbon atoms incorporated, and 407.13 kcal/mol higher than that of three 6-membered rings formation with two more carbon atoms incorporated. gy. (b) High-resolution TEM image of marked red square area of the samples, from which GQDs with few layers (2 to 15 layers) and size about 10-25 nm can be observed. (c) and (d) TEM images of carbon nanosheets obtained from annealing of reactant mixture of Si (100 nm) and PTFE (2 µm) at 1000 K. Insets are the high-resolution images of magnified red square areas, showing the amorphous morphology of the carbon material without growing process in the flame. In order to verify our inferences on the growth mechanism of GQDs, we investigate the effect of flame temperature on the morphology of GQDs. The mixture reactant of Si and PTFE with smaller particle size generates a higher temperature environment (Supplementary Figure S6b). It was supposed to be resulted from the scale effect of the reactant, containing a more rapid reaction rate with more heat liberation, prolonged carbon growth time with increased flame size, etc. Figure 7a shows the morphology of carbon nanoparticles obtained from flame synthesis of 100 nm Si and 300 nm PTFE, which is very different from the GQDsaggregated nanospheres with 2 µm PTFE. As expected, with reactant particle size reduced, the obtained graphitic nanostructure became larger in size, silk-like and continuous in morphology. The magnified red square area (Figure 7b) from Figure 7a shows a typical HRTEM photograph of the morphology. Quite significantly, some larger GQDs with few layers (from 2 to 15 layers), lengths ranging from 10-25 nm and flat morphology can be identified. The increased layers and lengths of the GQDs reveal the formation of a larger crystalline size and higher graphitization

Figure 7. TEM characterization and comparison of the carbon species obtained from different size of reactants and reaction mode. (a) TEM image of the GQD nanoparticles obtained from the deflagration reaction of Si (100 nm) and PTFE (300 nm). The obtained carbon materials present silk-like, continuous morpholo6


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degree, which may be resulted from the promoted carbon atoms growth process in higher reaction temperature up to ~1800 K (a melt of crucible melt point: ~2000 K, as shown in Figure S8), proving that the promoted carbon atoms growth rate in corporation of 6-membered rings and low content of 5-membered rings formation are realized in a higher temperature. From the Raman spectra of the GQDs in Figure S9, two characteristic peaks at 1330 and 1586 cm-1, corresponding to the D and G band of graphene can be observed, respectively. The ID/IG ratio decreases from 1.02 to 0.73, indicating a higher graphitization degree of the GQDs. Additionally, a contrast experiment was conducted by only heating the mixture reactant of Si (100 nm) and PTFE (2 µm) at 1000 K in a closed container by temperature programming instead of ignition to investigate the effect of flame on carbon atoms growing into GQDs. Figure 7c and 6d show that without a flame with rapid temperature decrease, carbon nanosheets of ca. 500 nm in size were obtained differently. As shown in the insert paragraph in Figure 7c and 7d, only amorphous morphology can be observed, which is possibly resulted from the slow function of Si atoms capturing F atoms from PTFE with peaceful liberation of heat, generating a uniform, low temperature environment which generates more 5-membered rings and hinders carbon atoms growth. This gives us a significant clue that the high temperature for carbon growth and rapid temperature decrease for appropriate growth time are the key points of the deflagration synthesis of GQDs. Furthermore, it can be inferred that the lengths of GQDs and number of their layers can be controlled by tuning the carbon growth degree through varying reactant particle size or reactant species, etc.

PTFE. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Correspondence and requests for materials should be addressed to Guangcheng Yang (E-mail: [email protected]).

Author Contributions Yang G.C. and Ji G.B. designed the research; Liu Y.S. performed the experiments; Qiao Z.Q., Zheng W.F. and Hu Y.J. performed theoretical calculations, as well as TEM and AFM characterizations; Yang G.C. and Hu H.L. were involved in discussions, critical assessment, and analysis of the experimental results; and Liu Y.S. and Gao B. wrote the manuscript. All authors contributed to the writing and revision of the manuscript.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (No. 11272292, 11372288, 51306093, and 51172109), National High Technology Research and Development Program of China (863 Program) (No. 2013AA050905), Development Foundation of CAEP (No. 2014B0302041), the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (No. 14zxfk08), the Funding of Jiangsu Innovation Program for Graduate Education (No. CXLX12_0148), and the Fundamental Research Funds for the Central Universities.

CONCLUSIONS In summary, we have proposed an alternative approach to synthesize gram-scale GQDs via a bottom-up deflagration reaction ignited by energetic materials combined with a Hummer’s mothed exfoliation process. The prepared GQDs exhibit obvious fluorescence effect and biocampatibility thus can be used as an ecofriendly material in biolabeling and bioimaging. The high temperature resulting from the exothermic reaction between Si and PTFE in the inner flame leads the generation of carbon atoms at a highly active atomic state. The size and layer number of the obtained GQDs are strongly depend on the flame temperature. Furthermore, we also disclose a GQDs growth mechanism, which is well verified by the experiment reproduction. Our work on the energetic materials deflagration reaction opens up a promising route for high-yield and high-quality GQDs, as well as other new various quantum dots.

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ASSOCIATED CONTENT Supporting Information Optical photographs of the Si and PTFE energetic materials and obtained GQDs-aggregated nanospheres. PL spectra of GQDs at different excitation wavelengths from 340 nm to 440 nm. Cell viability assay with human cervical cancer HeLa cells treated with different amount of GQDs. High performance liquid chromatography (HPLC) spectrum of the GQDs-aggregated nanospheres toluene solution after ultrasonic treatment. Theoretical calculation model of C-F bond breakage, formation of Si-F bond and the potential energy curve of the C-F bond by MS. Temperature–time curve of the deflagration reaction measured by a non-contact IR thermometer. High-speed photography of the deflagration reaction. Theoretical calculations for the growth mechanism of GQDs from carbon atoms in equilibrium conditions. Imagine of the Al2O3 crucible after deflagration reaction. Raman spectrum of the GQDs obtained from the deflagration of 100 nm Si and 300 nm 7



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The table of contents Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon Atoms Growth via Energetic Material Deflagration Yousong Liu,1,2 Bing Gao,1 Zhiqiang Qiao,1 Yingjie Hu,3 Wenfang Zheng,3 Long Zhang,1 Yong Zhou,4 Guangbin Ji,2 and Guangcheng Yang1* Gram-scale synthesis of graphene quantum dots from single carbon atoms has been achieved via a simple deflagration reaction of energetic materials (Si and polytetrafluoroethylene), which provides a high-temperature environment to produce carbon atoms and creates a rapid decrease in temperature to limit their growth process into small-sized few-layered graphitic structures by sharply lowering their growth rate and time. The prepared GQDs exhibit obvious fluorescence effect and excellent cellular imaging properties. The growth mechanism of GQDs was proposed and well verified by the experiment and theoretical calculation. Keywords: deflagration reaction •carbon atoms•growth rate • graphene quantum dots •cell fluorescence imaging

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Gram-Scale Synthesis of Graphene Quantum Dots from Single Carbon

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