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Size controllable preparation of graphitic quantum dots and their photoluminescence behavior
Author’s Accepted Manuscript Size controllable preparation of graphitic quantum dots and their photoluminescence behavior Xiaoguang Liu, Jun Wang, Yan Li, Wendong Xue
www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30617-0 http://dx.doi.org/10.1016/j.matlet.2015.09.105 MLBLUE19621
To appear in: Materials Letters Received date: 21 May 2015 Revised date: 11 September 2015 Accepted date: 26 September 2015 Cite this article as: Xiaoguang Liu, Jun Wang, Yan Li and Wendong Xue, Size controllable preparation of graphitic quantum dots and their photoluminescence behavior, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Size controllable preparation of graphitic quantum dots and their photoluminescence behavior Xiaoguang Liu, Jun Wang, Yan Li*, Wendong Xue School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract:
To control the size of graphitic quantum dots (GQDs), an electrochemical method was
successfully developed with applied potentials cycling between -5.0 and 5.0 V at various scan rates from 0.2-0.5 V/s. The average size of the resultant GQDs decreases from 12 to 3.4 nm with increasing scan rates, as verified by their transmission electron microstructure (TEM) images. The X-ray diffraction (XRD), optical absorption, photoluminescence (PL), Fourier Transform Infrared (FTIR) and photoluminescence excitation (PLE) behaviors of those GQDs were investigated. The result shows that the n to π* electronic transition resulting from the surface oxygen groups contributes to the PL emission, besides the π to π* electronic transition due to their sizes. Keywords: graphitic quantum dots; electrochemical method; photoluminescence; mechanism; carbon materials; nanomaterials
Corresponding author: Tel.: +86 10 62333140; E-mail: [email protected] (Yan Li)
1
1. Introduction The fluorescent carbon family, such as shortened carbon nanotubes, amorphous and crystalline carbon dots, and graphene quantum dots (GQDs) shows photoluminescence (PL) characteristics [1], opening their applications in optical/photovoltaic devices, energy efficient displays and lightning [2], and bioimaging [3]. GQDs have been considered as one of the most promising nano-materials due to their environmental-friendliness [4], high water solubility [5], high photostability, and resistance to metabolic degradation in bioapplications and excellent electrical/optical properties [6]. Numerous methods have been reported for GQDs preparation, such as nanolithography [7], hydrothermal process [8], chemical exfoliation [9], electrochemical oxidation method [10, 16], and UV irradiation and sonication assisted hummer method [11]. Nevertheless, due to the uncertainty on formation mechanisms precise control over of the sizes of GQDs still remains a challenge [12]. Herein, we propose a new strategy to achieve size-controllable GQDs by adjusting the scan rates within certain range of applied potentials in the electrochemical process. On the other hand, most of the experimental work has been focused on the preparation and the improvement of quantum yields [13]. The mechanism of PL is still under intensive discussion, varying from case to case [14]. For instance, the explanations of λex-dependent or independent PL of GQDs and their origin were widely discussed and generally classified into two catalogues including core related and surface state related emission [15]. In this work, the effects of size and surface oxygen groups on the PL of GQDs were studied and a surface oxygen group related luminescence mechanism was proposed. 2. Results and Discussion As-prepared GQDs show almost the same spherical shape for various scan rates from 0.2 (GQD0.2) to 0.3(GQD0.3), 0.4 (GQD0.4) to 0.5(GQD0.4) V/s, but they differ in their sizes (Fig. 1). Experimental details can be found in the online supplementary materials. GQD0.2 shows an average size of 12 nm, but with increasing scan rates, their average sizes decrease to 9, 5.3 and 3.4 nm for GQD0.3, GQD0.4 and GQD0.5, respectively. Particularly GQD0.2 demonstrates that >40% of them predominantly distributed at a diameter of 12 nm, which is apparently larger than the other GQDs.
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Fig. 1 X-ray diffraction (XRD) (Fig. 2) shows the characteristic (002) diffraction of the GQDs at 2 theta of ca. 23.5o, agreeing well with previous work[10]. Slight increases of full width at half maximum (FWHM) of GQDs with scan rates from 0.2 to 0.5 V/s confirm the reduction of GQDs sizes. Detailed mechanism of controlling the sizes of GQDs is in the electronic supplementary material. Graphene [41-1487]
Intensity (a.u.)
(002)
0.5 V/s
0.4 V/s
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0.2 V/s 15
20
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Size differences are further verified by their UV-vis absorption spectra (Fig. 3f), i.e. the GQD0.2 shows red-shift to ca. 221 nm whereas the absorption peaks of the other three GQDs-solutions located at ca. 206 nm identically. Generally, the peak below 270 nm is corresponding to the π-π* transition of the aromatic sp2 domains. Therefore, the aromatic sp2 domains of GQD0.2 are larger than that of the other three GQDs, agreeing well with the its TEM (Fig.1 a). In contrast, the other three smaller GQDs have different sizes (Fig.1 b-d), but they show identical absorption peaks, indicating the π-π* transition behaviors are similar when the sizes reduced to certain range, i.e 3.4 to 9 nm in this case. Usually, the UV-vis and PL spectra exhibit red shift as increasing the particle size of nanomaterials. Although the red-shift of UV-vis spectra was observed when the average size ≥12 nm, all the GQDs emitted the same blue-green luminescence on being excited by a 365 nm lamp (Fig. S1). This mismatch between sizes and luminescence was further investigated by PL spectra on the GQDs-solutions. These GQDs showed almost the same excitation-dependent PL emissions (Fig. 3a-d), as reported previously [17]. Further observation showed that the emission peak positions almost unchanged at 445 nm on being excited by the same wavelength at 360 nm ( Fig. 3e), reflecting their PL may originate from their similar surface oxygen groups[18]. This phenomenon at least confirms that the sizes of GQDs had little influence on the PL emission when reducing to such ranges as 3.4-12 nm. a
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To explain the mismatch between same PL emission and different sizes of the prepared GQDs, the PLE spectra of the GQD solutions were examined. According to the emission peaks in the PL spectra, the wavelengths of 440 nm were selected as the detection wavelength. As shown in Fig. 3e, GQD0.2 displayed one single broad shoulder peak at 340 nm, which might be the combination of the red-shifted π-π* and the locally existed n-π* electronic transitions[22]. GQD0.3 and GQD0.4 demonstrated their broad peaks at ca. 270 nm and the other shoulder peaks at ca. 340 nm. The broad peak of GQD0.5 slightly blue-shifted to 263nm, but its shoulder peak remained at ca. 340 nm. Same locations of the n-π* electronic transitions for the above samples clearly suggested the existence of surface oxygen groups. 0.2V/s 0.3V/s 0.4V/s 0.5V/s
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Fig. 4 To identify the surface oxygen groups of the GQDs, the FTIR spectra were measured (Fig. 4a). The four GQDs displayed almost identical stretching vibrations of –OH group at ca. 3420 cm-1 ( GQD0.5 at ca. 3408 cm-1), C=O group at ca. 1630 cm-1 and C-O-C group at ca.1045 cm-1, so the PL emission peaks due to these surface oxygen groups remain unchanged at 445nm. This, together with the PLE spectra (Fig. 3e), suggested the n-π*electron-transitions corresponding with these surface oxygen groups played dominant parts in the blue-green PL emission(Fig. 4b). This is supported by Zhang[15] that surface oxygen groups are important PL emission origins in addition to the sizes for graphene oxide quantum dots [20] and carbon nano-dots [21]. Particularly the C=O bonds are responsible for the blue-green PL emissions[19]. See the supplementary materials for detailed explanations. 3. Conclusion GQDs with controllable sizes in the range 3.4-12 nm had been prepared with the applied potential cycling from -5.0 and 5.0 V at various scan rates ranging from 0.2-0.5 V/s using the electrochemical method. The result showed that the higher the scan rates, the smaller the GQDs sizes. However, the
5
different size of GQDs did not lead to different PL color. Accompanied with the result of PLE and FTIR spectra, it is believed that when the GQDs reduce to certain sizes range, the PL emission tends to be dominated by the surface oxygen groups besides their sizes. Acknowledgement National Natural Science Foundation of China (Grant No. 51202011), Beijing Organization department outstanding talented person project (2013D009006000001) and the Fundamental Research Funds for the Central Universities (FRT-TP-14-010A2). Appendix Supplementary data can be found online. References [1] Baker SN, Baker GA. Luminescent carbon nanodots: emergent nanolights. Angew. Chem.-Int. Edit. 2010; 49 (38): 6726-44. [2] Sanderson K. Quantum dots go large. Nature 2009; 459: 760-1. [3] Sahu S, Behera B, Maitib TK, Mohapatra S. Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents. Chem. Commun. 2012; 48: 8835–7. [4] Ray SC, Saha A, Jana NR, Sarkar R. Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application. J. Phys. Chem. C 2009; 113: 18546-51. [5] Li H, Kang Z, Liu Y, Lee ST. Carbon nanodots: synthesis, properties and applications. J. Mater. Chem. 2012; 22(46): 24230-53. [6] Michalet X, Pinaud FF, Bentolila LA, Tsay JM, Doose S, Li JJ, et al. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 2005; 307:538-44. [7] Ritter KA, Lyding JW, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons. Nature Mater. 2009; 8: 235-42. [8] Jiang Y, Han Q, Jin C, Zhang J, Wang B. A fluorescence turn-off chemosensor based on N-doped carbon quantum dots for detection of Fe3+ in aqueous solution. Mater. lett. 2015; 144: 366-8. [9] Peng J, Gao W, Gupta BK, Liu Z, Romero-Aburto R, Ge L, et al. Graphene quantum dots derived from carbon fibers. Nano lett. 2012;12: 844-9. [10] Li Y, Hu Y, Zhao Y, Shi G, Deng L, Hou Y, et al. An electrochemical avenue to green-luminescent 6
graphene quantum dots as potential electron-acceptors for photovoltaics. Adv. Mater. 2011; 23: 776-80. [11] Swain AK, Li D, Bahadur D. UV-assisted production of ferromagnetic graphitic quantum dots from graphite. Carbon 2013; 57: 346-56. [12] Xu H, Zhou S, Xiao L, Li S, Song T, Wang Y, et al. Nanoreactor-confined synthesis and separation of Nanoreactor-confined synthesis and separation of recyclable SBA-15 template and their application for Fe(III) sensing. Carbon 2015; 87: 215-25. [13] Zhuo Y, Miao H, Zhong D, Zhu S, Yan X. One-step synthesis of high quantum-yield and excitation-independent emission carbon dots for cell imaging. Mater. lett. 2015; 139:197–200. [14] Yu P, Wen X, Toh, Y-R, Tang J. Temperature-dependent fluorescence in carbon dots. J. Phys. Chem. C 2012; 116:25552-7. [15] Zhang R, Liu Y, Yu L, Li Z, Sun S. Preparation of high-quality biocompatible carbon dots by extraction, with new thoughts on the luminescence mechanisms. Nanotechnology 2013; 24: 225601, 1-8. [16] Zang, Z, Nakamura A, Temmyo J. Single cuprous oxide films synthesized by radical oxidation at low temperature for PV application, Optics Express, 2013, 21(9): 11448-11456 [17] Liu R, Wu D, Feng X, Müllen K. Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology. J. Am. Chem. Soc. 2011; 133:15221-3. [18] Wang L, Zhu SJ, Wang H Y, Qu SN, Zhang YL, Zhang JH, et al. Common origin of green
luminescence in carbon nanodots and graphene quantum dots, ASC Nano, 2014, 8( 3): 2541–2547 [19] Loh KP, Bao Q, Eda G, Chhowalla M. Graphene oxide as a chemically tunable platform for optical applications. Nat. Chem. 2010, 2:1015–1024 [20] Liu F, Jang MH, Dong HH, Kim JH, Cho Y-H, Seo TS. Facile synthetic method for pristine graphene quantum dots and graphene oxide quantum dots: origin of blue and green luminescence. Adv. Mater. 2013; 25:3657-62. [21] Bao L, Zhang Z, Tian Z, Zhang L, Liu C, Lin Y, et al. Electrochemical tuning of luminescent carbon nanodots: from preparation to luminescence mechanism. Adv. Mater. 2011; 23: 5801-6.
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[22] Ge J, Li Y, Zhang B, Ma N, Wang J, Pu C, et al. Electrochemical tuning of optical properties of graphitic quantum dots, J. Lumin., 2015; 166:322–327.
Figure captions Fig.1 TEM images of GQDs prepared using applied potentials cycling between -5.0 and 5.0 V at various scan rates from 0.2 to 0.5 V/s: (a) GQD0.2, (b) GQD0.3, (c) GQD0.4 and (d) GQD0.5. Scale bars=20nm, insets are size distribution of each corresponding GQDs with calculated average sizes Fig. 2 XRD of GQDs prepared at different scan rates ranging from 0.2 to 0.5 V/s Fig. 3 PL spectra of GQDs prepared at different scan rates (a)0.2, (b)0.3, (c)0.4, and (d)0.5 V/s, the excitation wavelength ranges from 320 to 500 nm with an interval of 20nm; (e) PL of the above samples at the excitation wavelength 360 nm and its corresponding PLE with detection wavelength at 440nm; (f) UV-vis absorption spectra of the above samples Fig. 4 (a) FTIR spectra of GQDs prepared at different scan rates ranging from 0.2 to 0.5 V/s; (b) Scheme of n-π* electronic transition, illustrating the origins for blue-green PL emissions Graphical abstract
Highlights
Size controllable (3.4-12 nm) graphitic quantum dots (GQDs) were successfully 8
prepared using an electrochemical method
They show identical PL spectra at 445 nm on being excited by 360 nm
The PL emission originates from the surface oxygen groups besides sizes
9
www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30617-0 http://dx.doi.org/10.1016/j.matlet.2015.09.105 MLBLUE19621
To appear in: Materials Letters Received date: 21 May 2015 Revised date: 11 September 2015 Accepted date: 26 September 2015 Cite this article as: Xiaoguang Liu, Jun Wang, Yan Li and Wendong Xue, Size controllable preparation of graphitic quantum dots and their photoluminescence behavior, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.09.105 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Size controllable preparation of graphitic quantum dots and their photoluminescence behavior Xiaoguang Liu, Jun Wang, Yan Li*, Wendong Xue School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract:
To control the size of graphitic quantum dots (GQDs), an electrochemical method was
successfully developed with applied potentials cycling between -5.0 and 5.0 V at various scan rates from 0.2-0.5 V/s. The average size of the resultant GQDs decreases from 12 to 3.4 nm with increasing scan rates, as verified by their transmission electron microstructure (TEM) images. The X-ray diffraction (XRD), optical absorption, photoluminescence (PL), Fourier Transform Infrared (FTIR) and photoluminescence excitation (PLE) behaviors of those GQDs were investigated. The result shows that the n to π* electronic transition resulting from the surface oxygen groups contributes to the PL emission, besides the π to π* electronic transition due to their sizes. Keywords: graphitic quantum dots; electrochemical method; photoluminescence; mechanism; carbon materials; nanomaterials
Corresponding author: Tel.: +86 10 62333140; E-mail: [email protected] (Yan Li)
1
1. Introduction The fluorescent carbon family, such as shortened carbon nanotubes, amorphous and crystalline carbon dots, and graphene quantum dots (GQDs) shows photoluminescence (PL) characteristics [1], opening their applications in optical/photovoltaic devices, energy efficient displays and lightning [2], and bioimaging [3]. GQDs have been considered as one of the most promising nano-materials due to their environmental-friendliness [4], high water solubility [5], high photostability, and resistance to metabolic degradation in bioapplications and excellent electrical/optical properties [6]. Numerous methods have been reported for GQDs preparation, such as nanolithography [7], hydrothermal process [8], chemical exfoliation [9], electrochemical oxidation method [10, 16], and UV irradiation and sonication assisted hummer method [11]. Nevertheless, due to the uncertainty on formation mechanisms precise control over of the sizes of GQDs still remains a challenge [12]. Herein, we propose a new strategy to achieve size-controllable GQDs by adjusting the scan rates within certain range of applied potentials in the electrochemical process. On the other hand, most of the experimental work has been focused on the preparation and the improvement of quantum yields [13]. The mechanism of PL is still under intensive discussion, varying from case to case [14]. For instance, the explanations of λex-dependent or independent PL of GQDs and their origin were widely discussed and generally classified into two catalogues including core related and surface state related emission [15]. In this work, the effects of size and surface oxygen groups on the PL of GQDs were studied and a surface oxygen group related luminescence mechanism was proposed. 2. Results and Discussion As-prepared GQDs show almost the same spherical shape for various scan rates from 0.2 (GQD0.2) to 0.3(GQD0.3), 0.4 (GQD0.4) to 0.5(GQD0.4) V/s, but they differ in their sizes (Fig. 1). Experimental details can be found in the online supplementary materials. GQD0.2 shows an average size of 12 nm, but with increasing scan rates, their average sizes decrease to 9, 5.3 and 3.4 nm for GQD0.3, GQD0.4 and GQD0.5, respectively. Particularly GQD0.2 demonstrates that >40% of them predominantly distributed at a diameter of 12 nm, which is apparently larger than the other GQDs.
2
(b)
Fraction (%)
Fraction (%)
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Fig. 1 X-ray diffraction (XRD) (Fig. 2) shows the characteristic (002) diffraction of the GQDs at 2 theta of ca. 23.5o, agreeing well with previous work[10]. Slight increases of full width at half maximum (FWHM) of GQDs with scan rates from 0.2 to 0.5 V/s confirm the reduction of GQDs sizes. Detailed mechanism of controlling the sizes of GQDs is in the electronic supplementary material. Graphene [41-1487]
Intensity (a.u.)
(002)
0.5 V/s
0.4 V/s
0.3 V/s
0.2 V/s 15
20
25
30
2theta Fig. 2 3
35
40
Size differences are further verified by their UV-vis absorption spectra (Fig. 3f), i.e. the GQD0.2 shows red-shift to ca. 221 nm whereas the absorption peaks of the other three GQDs-solutions located at ca. 206 nm identically. Generally, the peak below 270 nm is corresponding to the π-π* transition of the aromatic sp2 domains. Therefore, the aromatic sp2 domains of GQD0.2 are larger than that of the other three GQDs, agreeing well with the its TEM (Fig.1 a). In contrast, the other three smaller GQDs have different sizes (Fig.1 b-d), but they show identical absorption peaks, indicating the π-π* transition behaviors are similar when the sizes reduced to certain range, i.e 3.4 to 9 nm in this case. Usually, the UV-vis and PL spectra exhibit red shift as increasing the particle size of nanomaterials. Although the red-shift of UV-vis spectra was observed when the average size ≥12 nm, all the GQDs emitted the same blue-green luminescence on being excited by a 365 nm lamp (Fig. S1). This mismatch between sizes and luminescence was further investigated by PL spectra on the GQDs-solutions. These GQDs showed almost the same excitation-dependent PL emissions (Fig. 3a-d), as reported previously [17]. Further observation showed that the emission peak positions almost unchanged at 445 nm on being excited by the same wavelength at 360 nm ( Fig. 3e), reflecting their PL may originate from their similar surface oxygen groups[18]. This phenomenon at least confirms that the sizes of GQDs had little influence on the PL emission when reducing to such ranges as 3.4-12 nm. a
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To explain the mismatch between same PL emission and different sizes of the prepared GQDs, the PLE spectra of the GQD solutions were examined. According to the emission peaks in the PL spectra, the wavelengths of 440 nm were selected as the detection wavelength. As shown in Fig. 3e, GQD0.2 displayed one single broad shoulder peak at 340 nm, which might be the combination of the red-shifted π-π* and the locally existed n-π* electronic transitions[22]. GQD0.3 and GQD0.4 demonstrated their broad peaks at ca. 270 nm and the other shoulder peaks at ca. 340 nm. The broad peak of GQD0.5 slightly blue-shifted to 263nm, but its shoulder peak remained at ca. 340 nm. Same locations of the n-π* electronic transitions for the above samples clearly suggested the existence of surface oxygen groups. 0.2V/s 0.3V/s 0.4V/s 0.5V/s
b
Green
π*
3420(O-H)
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Fig. 4 To identify the surface oxygen groups of the GQDs, the FTIR spectra were measured (Fig. 4a). The four GQDs displayed almost identical stretching vibrations of –OH group at ca. 3420 cm-1 ( GQD0.5 at ca. 3408 cm-1), C=O group at ca. 1630 cm-1 and C-O-C group at ca.1045 cm-1, so the PL emission peaks due to these surface oxygen groups remain unchanged at 445nm. This, together with the PLE spectra (Fig. 3e), suggested the n-π*electron-transitions corresponding with these surface oxygen groups played dominant parts in the blue-green PL emission(Fig. 4b). This is supported by Zhang[15] that surface oxygen groups are important PL emission origins in addition to the sizes for graphene oxide quantum dots [20] and carbon nano-dots [21]. Particularly the C=O bonds are responsible for the blue-green PL emissions[19]. See the supplementary materials for detailed explanations. 3. Conclusion GQDs with controllable sizes in the range 3.4-12 nm had been prepared with the applied potential cycling from -5.0 and 5.0 V at various scan rates ranging from 0.2-0.5 V/s using the electrochemical method. The result showed that the higher the scan rates, the smaller the GQDs sizes. However, the
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different size of GQDs did not lead to different PL color. Accompanied with the result of PLE and FTIR spectra, it is believed that when the GQDs reduce to certain sizes range, the PL emission tends to be dominated by the surface oxygen groups besides their sizes. Acknowledgement National Natural Science Foundation of China (Grant No. 51202011), Beijing Organization department outstanding talented person project (2013D009006000001) and the Fundamental Research Funds for the Central Universities (FRT-TP-14-010A2). Appendix Supplementary data can be found online. References [1] Baker SN, Baker GA. Luminescent carbon nanodots: emergent nanolights. Angew. Chem.-Int. Edit. 2010; 49 (38): 6726-44. [2] Sanderson K. Quantum dots go large. Nature 2009; 459: 760-1. [3] Sahu S, Behera B, Maitib TK, Mohapatra S. Simple one-step synthesis of highly luminescent carbon dots from orange juice: application as excellent bio-imaging agents. Chem. Commun. 2012; 48: 8835–7. [4] Ray SC, Saha A, Jana NR, Sarkar R. Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application. J. Phys. Chem. C 2009; 113: 18546-51. [5] Li H, Kang Z, Liu Y, Lee ST. Carbon nanodots: synthesis, properties and applications. J. Mater. Chem. 2012; 22(46): 24230-53. [6] Michalet X, Pinaud FF, Bentolila LA, Tsay JM, Doose S, Li JJ, et al. Quantum dots for live cells, in vivo imaging, and diagnostics. Science 2005; 307:538-44. [7] Ritter KA, Lyding JW, The influence of edge structure on the electronic properties of graphene quantum dots and nanoribbons. Nature Mater. 2009; 8: 235-42. [8] Jiang Y, Han Q, Jin C, Zhang J, Wang B. A fluorescence turn-off chemosensor based on N-doped carbon quantum dots for detection of Fe3+ in aqueous solution. Mater. lett. 2015; 144: 366-8. [9] Peng J, Gao W, Gupta BK, Liu Z, Romero-Aburto R, Ge L, et al. Graphene quantum dots derived from carbon fibers. Nano lett. 2012;12: 844-9. [10] Li Y, Hu Y, Zhao Y, Shi G, Deng L, Hou Y, et al. An electrochemical avenue to green-luminescent 6
graphene quantum dots as potential electron-acceptors for photovoltaics. Adv. Mater. 2011; 23: 776-80. [11] Swain AK, Li D, Bahadur D. UV-assisted production of ferromagnetic graphitic quantum dots from graphite. Carbon 2013; 57: 346-56. [12] Xu H, Zhou S, Xiao L, Li S, Song T, Wang Y, et al. Nanoreactor-confined synthesis and separation of Nanoreactor-confined synthesis and separation of recyclable SBA-15 template and their application for Fe(III) sensing. Carbon 2015; 87: 215-25. [13] Zhuo Y, Miao H, Zhong D, Zhu S, Yan X. One-step synthesis of high quantum-yield and excitation-independent emission carbon dots for cell imaging. Mater. lett. 2015; 139:197–200. [14] Yu P, Wen X, Toh, Y-R, Tang J. Temperature-dependent fluorescence in carbon dots. J. Phys. Chem. C 2012; 116:25552-7. [15] Zhang R, Liu Y, Yu L, Li Z, Sun S. Preparation of high-quality biocompatible carbon dots by extraction, with new thoughts on the luminescence mechanisms. Nanotechnology 2013; 24: 225601, 1-8. [16] Zang, Z, Nakamura A, Temmyo J. Single cuprous oxide films synthesized by radical oxidation at low temperature for PV application, Optics Express, 2013, 21(9): 11448-11456 [17] Liu R, Wu D, Feng X, Müllen K. Bottom-up fabrication of photoluminescent graphene quantum dots with uniform morphology. J. Am. Chem. Soc. 2011; 133:15221-3. [18] Wang L, Zhu SJ, Wang H Y, Qu SN, Zhang YL, Zhang JH, et al. Common origin of green
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Figure captions Fig.1 TEM images of GQDs prepared using applied potentials cycling between -5.0 and 5.0 V at various scan rates from 0.2 to 0.5 V/s: (a) GQD0.2, (b) GQD0.3, (c) GQD0.4 and (d) GQD0.5. Scale bars=20nm, insets are size distribution of each corresponding GQDs with calculated average sizes Fig. 2 XRD of GQDs prepared at different scan rates ranging from 0.2 to 0.5 V/s Fig. 3 PL spectra of GQDs prepared at different scan rates (a)0.2, (b)0.3, (c)0.4, and (d)0.5 V/s, the excitation wavelength ranges from 320 to 500 nm with an interval of 20nm; (e) PL of the above samples at the excitation wavelength 360 nm and its corresponding PLE with detection wavelength at 440nm; (f) UV-vis absorption spectra of the above samples Fig. 4 (a) FTIR spectra of GQDs prepared at different scan rates ranging from 0.2 to 0.5 V/s; (b) Scheme of n-π* electronic transition, illustrating the origins for blue-green PL emissions Graphical abstract
Highlights
Size controllable (3.4-12 nm) graphitic quantum dots (GQDs) were successfully 8
prepared using an electrochemical method
They show identical PL spectra at 445 nm on being excited by 360 nm
The PL emission originates from the surface oxygen groups besides sizes
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