Free-Radical-Assisted Rapid Synthesis of Graphene Quantum Dots

Subscriber access provided by CORNELL UNIVERSITY LIBRARY

Article

Free radical-assisted rapid synthesis of graphene quantum dots and their oxidizability studies Yan Li, Hui Liu, Xinqian Liu, Sen Li, Lifeng Wang, Ning Ma, and Dengli Qiu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b02422 • Publication Date (Web): 10 Aug 2016 Downloaded from http://pubs.acs.org on August 12, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 27

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

Langmuir

Free radical-assisted rapid synthesis of graphene quantum dots and their oxidizability studies Yan Lia*, Hui Liua, Xin-qian Liua, Sen Lia, Lifeng Wanga, Ning Maa,b* and Dengli Qiuc a

Department of Inorganic Nonmetallic Material, School of Materials Science and Engineering, University

of Science and Technology Beijing, Beijing 100083, China. b

College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin

150001, China. c

Bruker Nano Surface Business (Beijing Office), Beijing 100081, China.

ACS Paragon Plus Environment

Langmuir

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

Page 2 of 27

ABSTRACT: This work reports a modified electrochemical method for rapid and large-scale preparing graphene quantum dots (GQDs) by introduction of active free radicals, which were produced by hydrogen peroxide or ultraviolet radiation. These free radicals can deepen the oxidized or reduced level of working electrode in electrochemical process and thus lead to GQDs with high concentration and small size, but different surface oxidized degree. The improved oxidation and reduction mechanism were analyzed in this work. Meanwhile, the optical properties and oxidizability of GQDs with different surface oxidized degree were investigated. It is found that these GQDs can be used as an oxidizing agent and their oxidizability is related to the degree being oxidized. Keywords: graphene quantum dots; free radicals; rapid electrochemical synthesis; oxidizability


Page 3 of 27

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

Langmuir

Introduction Graphene oxide (GO) is a one-atom-thick, two-dimensional graphene macromolecule that is covalently decorated with numerous oxygen-containing hydroxyl and epoxy groups on the basal plane and a small quantity of carboxyl groups at the edges.1, 2 These unique structural features endow GO with excellent water solubility, stability, and surface paint ability via chemical reactions.3 Additionally, the presence of these oxygen-containing functional groups imbues GO with some oxidation ability. Indeed, GO can oxidize active metals such as Al, Fe, and Cu,4-6

non-metal ions such as iodic ions (I-),7 and ferrous iron

(Fe2+) at ambient temperature.8 Given that it also exhibits low toxicity, GO seems to be a promising and green oxidant that is appropriate for applications in organisms. However, because of its non-uniform size and shape and harsh preparation processes, which require the use of strong acid, GO is limited in the applications of

bio-oxidization and related nano-fields.9

Graphene quantum dots (GQDs), which are defined as the zero dimensional carbon nanostructures comprising single or few layer graphenes with a small size distribution mainly in a range of 3–20 nm ,10,11 have attracted significant interest because of their ultra-small and uniform sizes, unique photoluminescent properties, and low toxicity. These advantages make GQDs very promising for various applications, such as bioimaging, biosensing, and drug/gene delivery.12,

13

Similar to the molecular structure of GO,

GQDs not only retained graphene backbone in core region, but also had oxygen-containing functional groups in their surface, such as hydroxyl, epoxy, and carboxyl groups.14 The presence of surface oxygen groups made GQDs to be an efficient oxidizing agents and oxygen reduction reaction (ORR) catalysts.10 However, previously, little attention has been paid to investigating their oxidation ability. To date, many methods for the preparation of GQDs have been reported, such as acidic oxidation, hydrothermal or solvothermal synthesis,

16,17

physical cutting,18,

19

15

electrochemical oxidation,20 and

solution chemistry.21 These main synthesis strategies methods are summed up as bottom-up and top-down


Langmuir

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

Page 4 of 27

methods, in generally, the top-down approach is relatively easy and simple and therefore suitable for mass production. Until now, however, generating uniform diameter GQDs at the synthesis step by applying the top-down method has not been greatly successful,22 it is still highly desirable to develop a facile, high-yielding, and mass producing method for GQDs. Electrochemical approaches to the synthesis of GQDs have many advantages, such as mild preparation conditions, accessible precursors, and relatively simple operations.23 In our previous work, we successfully prepared GQDs using such method.20 The obtained products were high quality and small in size and exhibited both a narrow size distribution and a high degree of surface oxidization; thus, they were appropriate for bio-oxidization and related nano-field applications. However, the low yield and long preparation time limit further applications. If a simple improvement or minor amendment could be used in the prepared process for accelerating formation of GQDs, high-quality GQDs could be obtained in high yields. Typically, to electrochemical cyclic voltammogram (CV), an appropriate low redox potential (usually ranging from ±1.5 V to ±3 V) is applied to the working electrode. During oxidation process at the working electrode, this potential is sufficient to oxidize either C–C bonds or water and thereby generate hydroxyl and oxygen radicals, which play the role of electrochemical “scissors” in oxidative cleavage reactions.24 This potential cycling also drive the supporting electrolyte intercalate into the graphene layer of the electrode, promoting oxidation of the electrode and carbon exfoliation. In contrast, during reduction at the working electrode, the low redox potential provides enough energy to deoxidize the working electrode by breaking C-O bonds. Thus, accompanied by numerous oxidation and reduction process and electrolyte intercalation, the working electrode becomes exfoliated, and subsequently, GQDs are produced. 25 Based on the formation mechanism analyzed above, electrode oxidation and reduction play important roles in the formation of GQDs. Therefore, strengthening the degrees of electrode oxidation and reduction could enhance the production efficiency of GQDs. Therefore, in this paper, we first deepen the degrees of electrode oxidation and reduction by reacting the working electrode with introduced or generated active radicals, including HO·, H·, and electrons. These


Page 5 of 27

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

Langmuir

active radicals that were produced by hydrogen peroxide (H2O2) or ultraviolet (UV) irradiation acted to accelerate the generation of GQDs and regulate their degrees of surface oxidation. Then, we investigated GQDs’ optical properties and assessed the oxidation abilities of the obtained GQDs using Fe2+ and I‒ as reduced ions. We believe that our work provides a simple and green method for the efficient preparation of GQDs in high yields so that these materials’ applications can be extended to bio-oxidization and related nano-fields.

Experimental Section Preparation of GQDs. The GQDs were prepared by cyclic voltammograms (CV) using CHI 660D working station. Graphite rod, platinum electrode, and Ag/AgCl were used as the working, counter, and reference electrodes, respectively. 0.1M neutral phosphate buffer saline (PBS) solution was used as electrolyte. The CV window was set between -1.5 and 1.5 V at a scan rate of 0.1V/s, and the scanning cycle was 10800. The GQDs which were prepared by introducing UV irradiation by a handheld lamp (18W) were defined as GQDs-L. The UV wavelength was 254 nm, the distance of UV light and the breaker was 2 cm. The GQDs which made from PBS solution with addition of 1ml hydrogen peroxide (H2O2, 30 wt. %) were defined as GQDs-H. For comparison, we also prepared GQDs from PBS solution without H2O2 and UV irradiation. All the three prepared GQDs solutions were filtered with a cellulose filtration membrane whose pores are 0.22 µ m and dialyzed with a cellulose ester membrane bag (retained molecular weight: 3500Da) for 6 days to obliterate electrolyte. In order to calculate their concentration, three GQD solutions with a volume of 40 ml were evaporated into solid powder. Oxidative experiment of GQDs. For evaluating their oxidation ability, the concentration of three kind of GQDs were unified to 0.30 mg/ml, then two experiments (oxidizing Fe2+ to Fe3+, and I- to I2) were carried out. The first one is conducted as follows. Iron sulfate heptahydrate (FeSO4·7H2O) of 0.022 g was dissolved in 4ml GQDs, GQDs-L, GQDs-H aqueous solution, respectively. After complete reaction between three GQDs and Fe2+ for 18h, the pH values of three solutions were adjusted to 9. Fe2+ or Fe3+


Langmuir

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

Page 6 of 27

will precipitate with different colour in basic solution. The other oxidation ability assessment was conducted as follows．First, the pH values of three kinds of GQDs were adjusted to 5 using dilute hydrochloric acid. Then, potassium iodide (0.013 g) was dissolved in 4ml GQDs, GQDs-L and GQDs-H aqueous solution under argon atmosphere protection. The obtained concentration of Iwas 0.30mol/ml. After complete reaction between GQDs and I- for 18h, the starch indicator was dropped into the above solutions. In order to ensure the oxidation of I- entirely from the reaction with GQD, the argon atmosphere was continuously flowed into three GQD solutions to remove the dissolved oxygen till the experiment finished. Yields of GQDs. The yield of GQDs was studied by the reported method of Sun et al26. First, the prepared GQDs solution were oven-dried into GQDs powder (as shown in Fig.1), and their weights (“gross” weights: 4.15 mg for GQDs, 16.47 mg for GQDs-H

and 10.24 mg for GQDs-L) were obtained.

Then, we calculated the loss weight of graphite rods by weighing their respective weight before and after GQDs preparation. The loss weight of three electrodes were 22.35 mg, 41.98 mg and 36.35 mg. By dividing the weight of GQDs powder by the loss weight of graphite rods,

the yields of three GQDs were

determined to ca. 18.57 wt % (GQDs), 39.23.wt % (GQDs-H) and 28.17wt % (GQDs-L), respectively. Quantum yield (φ) of GQDs. The quantum yield (φ) of GQDs was calculated according to equation:

߮ = ߮ோ ×

‫ ܣ ܫ‬ோ ηଶ × × ‫ܫ‬ோ ‫ߟ ܣ‬ோଶ

where I is the measured integrated emission intensity, η is the refractive index of solvent, A refer to the optical density, and the subscript R means the reference standard with a known φ (Rhodamine B in ethanol solution, φR =0.68). Materials characterization. Transmission electron microscopy (TEM) images were recorded on an H-7650B electron microscope at 120 KV. AFM images in ambient air were taken with a Bruker Dimension Icon microscope. Fourier transform infrared(FT-IR) spectra were obtained using a


Page 7 of 27

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

Langmuir

NEXUS 670 spectrometer. Raman spectra characterization was carried out by RM 2000 Microscopic Confocal under He-Ne laser (excitation wavelength: 532.8 nm). X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB 2050Xi photoelectron spectrometer using monochromatized AlKα source; the binding energies were referenced to the C1s line at 284.5 eV. UV-vis absorption was recorded on a UNIC UV-2800 spectrophotometer. Photoluminescence (PL) spectra were acquired on an F-4500 FL Spectrophotometer.

Results and Discussion Figs.1a-c presented photographs of three aqueous solutions of GQDs and final product of GQDs powder. The solutions’ colors varied according to their different GQD concentrations: 0.10 mg/mL, 0.40 mg/mL, and 0.25 mg/mL, referred to as GQDs, GQDs-H, and GQDs-L, respectively. Further calculation by the Sun’s method

26

showed high yields of ca. 39.23.wt % and 28.17wt % for GQDs-H and GQDs-L

compared to 18.57 wt % (GQDs). This result indicated that the produced active radicals by H2O2 and UV irradiation during CV process, including HO·, H·, and electrons, favored for the cleavage of C-C bond from graphite electrodes and the generation of GQDs. The transmission electron microscopy (TEM) images shown in Fig. 2 revealed the morphologies and size distributions of the three types of GQDs. All of the GQDs were monodisperse and had spherical structures, but their size distributions differed. The GQDs’ sizes were distributed in the range of 1~5nm, with an average size of ca. 3.2 nm. GQDs-H, which were prepared with the addition of H2O2, exhibited a narrow size distribution (0.5~3 nm) and a small average particle size of ca.1.7 nm. Introducing UV irradiation during the preparation process exerted an effect similar to that of H2O2, reducing the size of the GQDs. Indeed, the GQDs-L had a narrow size distribution (1~4nm), with an average size of 2.4 nm. Although adjusting the electrochemical parameters was previously shown to control the size of the resulting GQDs,

27

the very small size and narrow size

distribution of the GQDs-H produced here have not been previously reported using an electrochemical approach. Further observation by high resolution TEM indicated the fine crystalline structures of the generated GQDs. The lattice parameter was measured to be 0.24 nm, matching with the (1120) crystal


Langmuir

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

Page 8 of 27

phase of graphite.28 Fig. 3 showed AFM images of the three GQDs and their height profiles. GQDs (Fig. 3a) and GQDs-L (Fig. 3c) have the thickness of ca. 0.8−1.6 nm, corresponding to 1−3 layers of graphene 28

. But, for GQDs-H (Fig. 3b),

they have an average thickness of ca. 0.9 nm, which is in accordance

with monolayered graphene.16 Fig.4a exhibited the Fourier transform infrared (FT-IR) spectra of the three GQDs. These spectra contained many stretching vibration peaks that corresponded to various oxygen-containing surface groups. The stretching vibrations at 1048 cm-1, 1420 cm-1, and 1608 cm-1 were ascribed to C-O-C,-COO, and C=O groups, respectively. The stretching vibrations at 2930 cm-1 and 3430 cm-1 were assigned to C-H and -OH groups. The stretching vibration at 2930 cm-1 was attributed to CO2 attached to the sample surface. Notably, all the stretching vibration peaks of the oxygen-containing functional groups of the GQDs-H were stronger than those of the GQDs and GQDs-L, especially the C=O stretching vibration peak. This finding indicated that the GQDs-H possessed the most oxygen-containing functional groups, as confirmed by the Raman spectra. The two characteristic peaks located at approximately 1360 and 1600 cm-1 in Fig. 3b corresponded to the D and G bands.29 The intensity ratio of these bands (ID/IG) could reflect the physical and structural changes in the graphene skeleton.30 The higher ID/IG of the GQDs-H and the obvious red-shift of the D band compared to that of the GQDs demonstrated that the GQDs-H possessed more defects.31 In contrast, the GQDs-L had a lower ID/IG value than GQDs, suggesting that the surface defects were reduced by UV irritation. X-ray photoelectron spectroscopy (XPS) results provided additional information for the analysis of surface oxygen. In the survey XPS spectra, only two strong C1s (284 eV) and O1s (532 eV) signals were observed in the three samples (Fig. 4c).The O/C atomic ratios of the GQDs, GQDs-H, and GQDs-L were determined to be ca. 0.48, 0.76, and 0.42, respectively. Thus, H2O2 increased the GQDs’ degree of surface oxidation, whereas UV irradiation played reduced role in GQDs formation. Based on the FT-IR results above, the C1s XPS spectra were deconvoluted into four Gaussian components at 284.5eV,286.1eV, 287.9eV, and 288.7eV, which corresponded to C=C/C-C, C-O/C-O-C, C=O, and COOH groups,


Page 9 of 27

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

Langmuir

respectively (Figs. 4e-g). Their detailed peak area ratios and O/C atomic ratios are summarized in Table 1. Compared with the GQDs, the GQDs-H had more intense C=O and COOH peaks, further suggested that the GQDs-H had a high degree of surface oxidation. In contrast, the GQDs-L exhibited relatively low intensity peaks corresponding to all oxygen-containing groups, indicating that UV irradiation hindered the formation of these groups or improved the deoxidization of GQDs. From GQDs-L had higher concentration and smaller size than GQDs, it was likely that UV irradiation improved deoxidization of GQDs during the formation process. Indeed, some reports had noted that removing COOH groups from GQDs was difficult, even when strong reducing agents were used.30 Our results demonstrated that introducing UV irradiation during the electrochemical preparation process could reduce the quantity of these groups, which was very important for controlling the oxygen-containing groups of prepared GQDs. However, the GQDs-L retained some oxygen-containing groups, possibly because of their small sizes and large edges, which lead to a large atomic ratio of edge and platelet carbons.32 Thus, the oxygen groups formed at the edges could not be completely reduced by UV irradiation. Based on the above results, H2O2 and UV irradiation clearly played important roles in accelerating the generation of GQDs: H2O2 increased the oxidation of GQDs, and UV radiation reduced the GQD to some extent. Detailed oxidation and reduction mechanisms were proposed as follows and are depicted in Scheme 1. During the GQDs-H-formation process, H2O2 readily dissociated to create HO2· radicals via anodic oxidation, and HO· radicals can be generated at the cathode under a low electric field.33 This reaction can be mathematically expressed as follows:

H 2O2 − e −  → HO2 ⋅ + H +

(1)

H 2O2 + e −  → OH − + HO ⋅

(2)

Both HO2· and HO· free radicals are strong oxidized radicals, when they are generated near an electrode, they can rapidly oxidize C–C bonds to form C-OH bonds.21 Then, the adjacent C-OH bonds can form C-O-C groups by losing a H2O molecule. If the C-O-C groups were arranged in lines on the surface of the


Langmuir

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

Page 10 of 27

graphite rod electrode, the graphene backbone would become fragile and be readily attacked.12 The regions contained graphene pieces with surrounded epoxy lines and/or edges in the working electrode may further cutting smaller by smaller during CV deoxidization by removing O atoms from a epoxy line.12 As a result, in addition to the continuous decomposition caused by H2O2, the formation of abundant C-O-C groups contributed to both accelerating electrode exfoliation and the generation of GQDs-H with ultra-small sizes. In addition, the effective intercalation of anions from phosphate-buffered saline (PBS) also helped to cut the graphene sheets into ultra-small pieces.23 Furthermore, HO2·and HO· radicals further oxidized the formed hydroxyl groups into carbonyl groups, some of which combined with OH· radicals to produce carboxyl

groups.13 Consequently, the GQDs-H contained abundant C=O and COOH

groups as shown by the FT-IR and XPS spectra, and had a higher concentration and smaller sizes than the GQDs. Among the various reduction methods available, UV irradiation is a feasible and “green” photo-reduction approach because it requires no hazardous or polluting reagents.34 In this work, exposing the reaction vessel to energetic radiation (254 nm) generated various types of free radicals in large quantities in the electrochemical cell, as described in equations 3 and 4.35

H 2 O UV → H ⋅ + HO ⋅

(3)

H 2 O UV → H + + HO ⋅ + e -aq (4)

Although the oxidant HO· radical was also generated by UV irradiation, the production of reduced H· radicals and electrons (eaq-) in aqueous solution favored the removal of bridging O atoms from the epoxy lines during deoxidization, thereby accelerating carbon exfoliation and the generation of GQDs. The co-existence of multiple free radicals is known to result in complicated reactions in the CV process. Here, the reducing agents appeared to occupy a dominant position in the competitive reactions. In addition, the high-energy UV irradiation (254 nm) may break C-OH bonds (358 kJ/mol) while leave C-C bonds intact


Page 11 of 27

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

Langmuir

because the bond energy of C-C bond (473 kJ/mol) is greater than that of C-O bond.36 Due to the dual role of UV irradiation in improving the degree of reduction, the GQDs-L exhibited less surface oxidation than the other materials generated here and were relatively smaller compared to the GQDs. Similar to other reports,37, 38 different degrees of surface oxidization influenced the optical properties of the GQDs, GQDs-H, and GQDs-L. In the UV-Vis spectra presented in Fig. 5a, the GQDs exhibited one typical absorption peak at 227 nm with a small absorption tail that extended to 300 nm. The main absorption peak was attributable to the π-π* transition of the aromatic sp2 domain, and the absorption tail could be ascribed to the n-π* transition of sp3 clusters.39, 40 Because of its relatively high degree of surface oxidization, the GQDs-H presented an obviously enhanced absorption tail. In contrast, the GQDs-L showed decreased n-π* electronic transitions. Similar to their UV-Vis spectra, the photoluminescence (PL) spectra of the three GQDs also exhibited obvious differences (Fig.5). GQDs presented a typical excitation-dependent feature, with the maximum emission peak occurring at ca.425 nm at an excitation wavelength (λex) of 260 nm. This feature is common in ﬂuorescent graphite dots.41 In contrast, for the GQDs-H, two fluorescence emission peaks—one at 430 nm and the other at 500 nm—were observed, as shown in Fig.5c. As the excitation wavelength was varied from 260 to 400 nm, the dominant emission in the PL spectra gradually shifted from 430 nm to 500 nm. And the emission peak gradually red-shifted to ca.525nm when the excitation wavelength was further changed from 420 nm to 480 nm, Thus, at least two types of surface hybridization structure dominated the PL emission of the GQDs-H because of their relatively high level of surface oxidization. The surface hybridization structure was composed of edge groups and a connected graphene core fragment.42, 43 Because of the partial removal of O atoms; the GQDs-L exhibited a PL excitation-dependent feature similar to that of the GQDs, as shown in Fig.5d. The similarity of the PL behaviors of the GQDs-L and GQDs and the difference between those of the GQDs-H and GQDs indicated that GQDs’ florescence emission results from their surface states rather than their sizes. Furthermore, the PL quantum yields of the GQDs, GQDs-H, and GQDs-L were 1.86%, 10.27%, and 2.57%, respectively. Thus, increasing the amount of surface defects improved the PL


Langmuir

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

Page 12 of 27

emission.44 Although the GQDs-L possessed fewer oxygen-containing functional groups, their reduced sizes increased the number of edges, which may increase the overall surface defect state.45 The emission wavelength of GQDs has been reported to be related to the band gap between the excited and ground state, which is correlated with the extent of the π-electron system and the surface chemistry; thus, this property is strongly dependent on the oxygen-containing groups possessed by the GQDs.32 Therefore, increasing the number of oxygen-containing groups will decrease the band gap of GQDs; for instance, according to density functional theory, the presence of carboxyl groups on sp2-hybridized carbons may induce significant local distortions, resulting in a narrower energy gap.46 Based on linear extrapolation of the absorption spectra of solutions of GQDs, the band gaps were calculated by plotting the square or square root of the absorption energy (A×E, where A is the absorbance, and E is the photon energy) against E, which enabled the determination of the direct or indirect band gap energy (Fig.6).The band gap energies of the GQDs, GQDs-H, and GQDs-L were calculated to be 2.90 eV, 2.58 eV, and 3.02 eV, respectively. These values were attributed to the fact that indirect gap transitions were likely to occur in GQDs because of their high O/C ratio.47 Converting these band gap energies into emission wavelengths, three wavelengths of 430 nm, 480 nm, and 410 nm were calculated.

They are similar to the observed

emission peak wavelengths in their corresponded PL spectra: ~430 nm in GQDs, ~430 and ~500 nm in GQDs-H, and ~420 nm in GQDs-L. The oxidation abilities of the three GQDs were investigated using Fe2+ and I‒ as reducing substances. Fig.7 presents the reactions. As shown in Fig.7a, a yellow precipitate of Fe(OH)3 appeared in the mixed GQDs-H solution. In contrast, the other two samples formed dark-green precipitates of Fe6(SO4)2(OH)4O3. Thus, because of their relatively high degree of surface oxidation, the GQDs-H possessed the strongest oxidation ability. They were able to oxidize Fe2+ to Fe3+, the redox potential for this reaction is 0.77 V (vs. standard hydrogen electrode, SHE).48 Therefore, although the exact redox potential was difficult to measure electrochemically, 49 this result confirmed that the redox potential of the GQDs-H is higher than 0.77 V. In contrast, those of the GQDs and GQDs-L were lower than 0.77 V.


Page 13 of 27

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

Langmuir

To differentiate the redox potentials of the GQDs and GQDs-L, I‒ was added to their aqueous solutions. A sample of the GQDs-H was also prepared to further confirm its oxidation ability. As expected, the I‒ that made full contact with the GQDs-H were oxidized to I2, and the GQDs-H solution turned blue. The GQDs solution turned a lighter blue colour, indicating that the GQDs could be reduced by I‒ (0.54 V vs. SHE for I‒/I2).48 The reaction process can be explained as follows. In acidic aqueous solution, hydrions readily interact with the oxygen-containing groups of GQDs, especially the hydroxyl and epoxy groups. This typical reduction process then induces a dehydration process. Subsequently, a positive charge is transferred to an anion via the GQDs through an electron-transfer process, and the anions are oxidized.6, 50 The GQDs-L possessed some remaining oxygen-containing functional groups, and the fact that their solution did not change color indicated that the GQDs-L had weak oxidation ability (lower that 0.54 V). Based on the results presented above, it can be concluded that GQDs with oxygen-containing groups have oxidation ability and that this ability is related to their degree of oxidation. The GQDs-H, which had the highest degree of oxidation, exhibited the strongest redox potential (higher than 0.77 V). In contrast, the redox potential of the GQDs was in the region of 0.54 to 0.77 V, and that of the GQDs-L was lower than 0.54 V. In addition, the fluorescent behavior of GQDs and GQDs-H after reacted with Fe2+ ions and iodide ions were investigated, as shown in Fig. 8. Because of no reaction between GQDs and Fe2+, the fluorescent behavior of GQDs almost did not change after the addition of Fe2+ (Fig. 8a). When GQDs-H oxidized Fe2+ ions into Fe3+, the fluorescence intensity of GQDs-H decreased greatly (Fig. 8b). This could be attributed to the strong interaction between GQDs-H and Fe3+ that facilitate charge transfer and thus restrain exciton recombination, leading to fluorescence quenching.51 This fluorescence decreased behavior was also observed in GQDs and GQDs-H after their reacted with I- (Fig. 8c and d), but the decreased extent is smaller than Fe3+. This phenomena may be caused by the weak interaction between GQDs and I-.

Conclusion


Langmuir

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

Page 14 of 27

In conclusion, high yields of GQDs with different surface oxygen contents were obtained by introduction of active free radicals in electrochemical process. The oxidant HO· radical produced by H2O2 could deepen the surface oxidized degree of GQDs and lead to their small size and high concentration. The reduced H· radical and eaq- could enhance deoxidization in the formation process of GQDs. The different optical properties of the three types of GQDs studied here indicated that the florescence emission of GQDs is associated with their surface states rather than their sizes. Furthermore, the obtained GQDs show certain oxidizability, which is related to their degree of surface oxidation.

Author information Corresponding Author Yan Li

E-mail: [email protected]

Ning Ma

E-mail: [email protected]

Author Contributions Yan Li and Ning Ma designed experiments and wrote the manuscript. Hui Liu, Xin-qian Liu and Sen Li performed the experiments. All authors have given approval to the final version of the manuscript. Funding Sources This work was supported by National Natural Science Foundation of China (Grant No. 51202011, 21374009), Beijing Organization department outstanding talented person project (2013D009006000001), Beijing Municipal Natural Science Foundation (2132049) and Fundamental Research Funds for the Central Universities (FRF-TP-15-005A3). Notes The authors declare no competing financial interest.

Acknowledgement Yan Li is thankful for the financial support from the program of the China Scholarship Council (CSC).


Page 15 of 27

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

Langmuir

References (1) Eda, G.; Chhowalla, M. Chemically Derived Graphene Oxide: Towards Large-Area Thin-Film Electronics and Optoelectronics. Adv. Mater. 2010, 22, 2392-2415. (2) Kozawa, D.; Miyauchi, Y.; Mouri, S.; Matsuda, K. Exploring the Origin of Blue and Ultraviolet Fluorescence in Graphene Oxide. J. Phys. Chem. Lett. 2013, 4, 2035-2040. (3) Srinivasan, S.; Je, S. H.; Back, S.; Barin, G.; Buyukcakir, O.; Guliyev, R.; Jung, Y.; Coskun, A. Ordered Supramolecular Gels Based on Graphene Oxide and Tetracationic Cyclophanes. Adv. Mater. 2014, 26, 2725-2729. (4) Fan, Z.; Wang, K.; Wei, T.; Yan, J.; Song, L.; Shao, B. An Environmentally Friendly and Efficient route for the Reduction of Graphene Oxide by Aluminum Powder. Carbon 2010, 48, 1686-1689. (5) Fan, Z.; Wang, K.; Yan, J.; Wei, T.; Zhi, L.; Feng, J.; Ren, Y.; Song, L.; Wei, F. Facile Synthesis of Graphene Nanosheets via Fe Reduction of Exfoliated Graphite Oxide. ACS Nano 2011, 5, 191-198. (6) Cao, X.; Qi, D.; Yin, S.; Bu, J.; Li, F.; Goh, C. F.; Zhang, S.; Chen, X. Ambient Fabrication of Large-Area Graphene Films via a Synchronous Reduction and Assembly Strategy. Adv. Mater. 2013, 25, 2957-2962. (7) Pei, S.; Zhao, J.; Du, J.; Ren, W.; Cheng, H. -M., Direct Reduction of Graphene Oxide Films into Highly Conductive and Flexible Graphene Films by Hydrohalic Acids. Carbon 2010, 48, 4466-4474. (8) Xue, Y.; Chen, H.; Yu, D.; Wang, S.; Yardeni, M.; Dai, Q.; Guo, M.; Liu, Y.; Lu, F.; Qu, J.; Dai, L. Oxidizing Metal Ions with Graphene Oxide: the In-situ Formation of Magnetic Nanoparticles on Self-reduced Graphene Sheets for Multifunctional Applications. Chem. Commun. 2011, 47, 11689-11691. (9) Yin, P. T.; Shah, S.; Chhowalla, M.; Lee, K., Design, Synthesis, and Characterization of Graphene-Nanoparticle Hybrid Materials for Bioapplications. Chem. Rev. 2015, 115, 2483-2531. (10) Wang, M.; Fang, Z.; Zhang, K.; Fang, J.; Qin, F.; Zhang, Z.; Li, J.; Liu, Y.;Lai, Y., Synergistically Enhanced Activity of Graphene Quantum Dots/Graphene Hydrogel Composites: A Novel All-carbon


Langmuir

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

Page 16 of 27

Hybrid Electrocatalyst for Metal/Air Batteries. Nanoscale 2016, 8, 11398-402. (11) Ran, X.; Sun, H.; Pu, F.; Ren, J.;Qu, X., Ag Nanoparticle-decorated Graphene Quantum Dots for Label-free, Rapid and Sensitive Detection of Ag+ and Biothiols. Chem. Commun. 2013, 49, 1079-1081. (12) Zheng, X. T.; Ananthanarayanan, A.; Luo, K. Q.; Chen, P. Glowing Graphene Quantum Dots and Carbon Dots: Properties, Syntheses, and Biological Applications. Small 2015, 11, 1620-1636. (13) Zhou, X.; Zhang, Y.; Wang, C.; Wu, X.; Yang, Y.; Zheng, B.; Wu, H.; Guo, S.; Zhang, J. Photo-Fenton Reaction of Graphene Oxide: A New Strategy to Prepare Graphene Quantum Dots for DNA Cleavage. ACS Nano 2012, 6, 6592-6599. (14) Bao, L.; Zhang, Z.; Tian, Z.; Zhang, L.; Liu, C.; Lin, Y.; Qi, B.; Pang, D. Electrochemical Tuning of Luminescent Carbon Nanodots: From Preparation to Luminescence Mechanism. Adv. Mater. 2011, 23, 5801-5806. (15) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L.; Song, L.; Alemany, L. B.; Zhan, X.; Gao, G. Vithayathil, S. A.; Kaipparettu, B. A.; Marti, A. A.; Hayashi, T.; Zhu, J.; Ajayan, P. M. Graphene Quantum Dots Derived from Carbon Fibers. Nano Lett. 2012, 12, 844-849. (16) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots. Adv. Mater. 2010, 22, 734-738. (17) Hu, C.; Liu, Y.; Yang, Y.; Cui, J.; Huang, Z.; Wang, Y.; Yang, L.; Wang, H.; Xiao, Y.;Rong, J., One-step Preparation of Nitrogen-doped Graphene Quantum Dots from Oxidized Debris of Graphene Oxide. J. Mater. Chem. B 2013, 1, 39-42. (18) Li, L.; Ji, J.; Fei, R.; Wang, C.; Lu, Q.; Zhang, J.; Jiang, L.; Zhu, J. A Facile Microwave Avenue to Electrochemiluminescent Two-Color Graphene Quantum Dots. Adv. Funct. Mater. 2012, 22, 2971-2979. (19) Zhuo, S.; Shao, M.; Lee, S. Upconversion and Downconversion Fluorescent Graphene Quantum Dots: Ultrasonic Preparation and Photocatalysis. ACS Nano 2012, 6, 1059-1064.


Page 17 of 27

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

Langmuir

(20) Li, Y.; Hu, Y.; Zhao, Y.; Shi, G.; Deng, L.; Hou, Y.; Qu, L. An Electrochemical Avenue to Green-Luminescent Graphene Quantum Dots as Potential Electron-Acceptors for Photovoltaics. Adv. Mater. 2011, 23, 776-780. (21) Dong, Y.; Chen, C.; Zheng, X.; Gao, L.; Cui, Z.; Yang, H.; Guo, C.; Chi, Y.; Li, C. M. One-step and High Yield Simultaneous Preparation of Single- and Multi-layer Graphene Quantum Dots from CX-72 Carbon Black. J. Mater. Chem. 2012, 22, 8764-8766. (22) Bak, S.; Kim, D.; Lee, H., Graphene Quantum Dots and Their Possible Energy Applications: A Review. Curr. Appl. Phys. 2016,. doi:10.1016/j.cap.2016.03.026. (23) Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots, and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355-381. (24) Li, L.; Wu, G.; Yang, G.; Peng, J.; Zhao, J.; Zhu, J. Focusing on Luminescent Graphene Quantum Dots: Current Status and Future Perspectives. Nanoscale 2013, 5, 4015-4039. (25) Zhang, M.; Bai, L.; Shang, W.; Xie, W.; Ma, H.; Fu, Y.; Fang, D.; Sun, H.; Fan, L.; Han, M.; Liu, C.; Yang, S. Facile Synthesis of Water-soluble, Highly Fluorescent Graphene Quantum Dots as a Robust Biological Label for Stem Cells. J. Mater. Chem. 2012, 22, 7461-7467. (26) Wang, Z.; Yu, J.; Zhang, X.; Li, N.; Liu, B.; Li, Y.; Wang, Y.; Wang, W.; Li, Y.; Zhang, L.; Dissanayake, S.; Suib, S. L.; Sun, L., Large-Scale and Controllable Synthesis of Graphene Quantum Dots from Rice Husk Biomass: A Comprehensive Utilization Strategy. ACS Appl. Mater. Interfaces 2016, 8, 1434-1439. (27) Liu, X.; Wang, J.; Li, Y.; Xue, W. Size Controllable Preparation of Graphitic Quantum Dots and Their Photoluminescence Behavior. Mater. Lett. 2016, 162, 56-59. (28) Sun, Y.; Wang, S.; Li, C.; Luo, P.; Tao, L.; Wei, Y.; Shi, G., Large Scale Preparation of Graphene Quantum Dots from Graphite with Tunable Fluorescence Properties. Phys. Chem. Chem. Phys. 2013, 15, 9907-9913.


Langmuir

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

Page 18 of 27

(29) Atienzar, P.; Primo, A.; Lavorato, C.; Molinari, R.; Garcia, H. Preparation of Graphene Quantum Dots from Pyrolyzed Alginate. Langmuir 2013, 29, 6141-6146. (30) Feng, Y.; Zhao, J.; Yan, X.; Tang, F.; Xue, Q. Enhancement in the Fluorescence of Graphene Quantum Dots by Hydrazine Hydrate Reduction. Carbon 2014, 66, 334-339. (31) Sun, S.; Wu, P. A One-step Strategy for Thermal- and pH-responsive Graphene Oxide Interpenetrating Polymer Hydrogel Networks. J. Mater. Chem. 2011, 21, 4095-4097. (32) Zhu, S.; Zhang, J.; Tang, S.; Qiao, C.; Wang, L.; Wang, H.; Liu, X.; Li, B.; Li, Y.; Yu, W.; Wang, X.; Sun, H.; Yang, B. Surface Chemistry Routes to Modulate the Photoluminescence of Graphene Quantum Dots: From Fluorescence Mechanism to Up-Conversion Bioimaging Applications. Adv. Funct. Mater. 2012, 22, 4732-4740. (33) Brillas, E.; Bastida, R. M.; Llosa, E.; Casado, J. Electrochemical Destruction of Aniline and 4-Chloroaniline for Wastewater Treatment Using a Carbon-PTFE 02-Fed Cathode. J. Electrochem. Soc. 1995, 142, 1733-1741. (34) Sun, H.; Wu, L.; Gao, N.; Ren, J.; Qu, X. Improvement of Photoluminescence of Graphene Quantum Dots with a Biocompatible Photochemical Reduction Pathway and Its Bioimaging Application. ACS Appl. Mater. Interfaces 2013, 5, 1174-1179. (35) Jin, S. H.; Kim, D. H.; Jun, G. H.; Hong, S. H.; Jeon, S. Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups. ACS Nano 2013, 7, 1239-1245. (36) Mulyana, Y.; Uenuma, M.; Ishikawa, Y.; Uraoka, Y. Reversible Oxidation of Graphene Through Ultraviolet/Ozone Treatment and Its Nonthermal Reduction through Ultraviolet Irradiation. J. Phys. Chem. C 2014, 118, 27372-27381. (37) Wang, S.; Cole, I. S.; Zhao, D.; Li, Q. The Dual Roles of Functional Groups in the Photoluminescence of Graphene Quantum Dots. Nanoscale 2016, 8, 7449-7458. (38) Hassan, M.; Haque, E.; Reddy, K. R.; Minett, A. I.; Chen, J.; Gomes, V. G. Edge-enriched Graphene


Page 19 of 27

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

Langmuir

Quantum Dots for Enhanced Photo-luminescence and Supercapacitance. Nanoscale 2014, 6, 11988-11994. (39) Tang, L.; Ji, R.; Cao, X.; Lin, J.; Jiang, H.; Li, X.; Teng, K. S.; Luk, C. M.; Zeng, S.; Hao, J.; Lau, S. P. Deep Ultraviolet Photoluminescence of Water-Soluble Self-Passivated Graphene Quantum Dots. ACS Nano 2012, 6, 5102-5110. (40) Shao, T.; Wang, G.; An, X.; Zhuo, S.; Xia, Y.; Zhu, C. A Reformative Oxidation Strategy Using High Concentration Nitric Acid for Enhancing the Emission Performance of Graphene Quantum Dots. RSC Adv. 2014, 4, 47977-47981. (41) Wang, L.; Zhu, S.; Wang, H.; Qu, S.; Zhang, Y.; Zhang, J.; Chen, Q.; Xu, H.; Han, W.; Yang, B.; Sun, H. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8, 2541-2547. (42) Das, S. K.; Liu, Y.; Yeom, S.; Kim, D. Y.; Richards, C. I. Single-Particle Fluorescence Intensity Fluctuations of Carbon Nanodots. Nano Lett. 2014, 14, 620-625. (43) Shang, J.; Ma, L.; Li, J.; Ai, W.; Yu, T.; Gurzadyan, G. G. The Origin of Fluorescence from Graphene Oxide. Sci. Rep. 2012, 2, 792. (44) Benitez-Martinez, S.; Valcarcel, M. Graphene Quantum Dots in Analytical Science. TrAC-Trend. Anal. Chem. 2015, 72, 93-113. (45) Lingam, K.; Podila, R.; Qian, H.; Serkiz, S.; Rao, A. M. Evidence for Edge-State Photoluminescence in Graphene Quantum Dots. Adv. Funct. Mater. 2013, 23, 5062-5065. (46) Ding, H.; Yu, S.; Wei, J.; Xiong, H. Full-Color Light-Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism. ACS Nano 2016, 10, 484-491. (47) Bao, L.; Liu, C.; Zhang, Z.; Pang, D. Photoluminescence-tunable Carbon Nanodots: Surface-state Energy-gap Tuning. Adv. Mater. 2015, 27, 1663-1667. (48) Hu, C.; Zhai, X.; Liu, L.; Zhao, Y.; Jiang, L.; Qu, L. Spontaneous Reduction and Assembly of Graphene Oxide into Three-Dimensional Graphene Network on Arbitrary Conductive Substrates. Sci.


Langmuir

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

Page 20 of 27

Rep. 2013, 3, 2065. (49) Roushani, M.; Abdi, Z. Novel Electrochemical Sensor Based on Graphene Quantum Dots/riboflavin Nanocomposite for the Detection of Persulfate. Sensor. Actuat. B-Chem. 2014, 201, 503-510. (50) Chen, I. P.; Chen, Y.; Chan, Y.; Saint Jhou, S.; Zhang, Y., One Pot Synthesis of Graphene Quantum Disks Derived from Single-layered Exfoliated Graphene Sheets and Their Application in Bioimaging. RSC Adv. 2014, 4, 25916-25919. (51) Zhang, Y. L.; Wang, L.; Zhang, H. C.; Liu, Y.; Wang, H. Y.; Kang, Z. H.; Lee, S. T., Graphitic Carbon Quantum Dots as a Fluorescent Sensing Platform for Highly Efficient Detection of Fe 3+ Ions. Rsc Adv. 2013, 3, 3733-3738.


Page 21 of 27

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

Langmuir

Figures and Tables Captions Fig. 1 Digital photographs of GQDs and the corresponding GQDs powder respectively, (a) GQDs (b) GQDs-H and (c) GQDs-L. Fig. 2 TEM image and the size distribution of (a) GQDs, (d) GQDs-H, (g) GQDs-L. Fig. 3 AFM image (up) and its height distribution (down) of (a) GQDs, (b) GQDs-H ,(c) GQDs-L. Fig. 4 (a) FT-IR spectra (b)Raman spectra of GQDs, GQD-H and GQD-L;(c) Survey XPS spectra and GQDs’C1s core region for (d) GQDs, (e) GQDs-H and (f) GQD-L. Fig. 5 (a) UV-vis absorption spectra of the GQDs; (b) (c) (d) PL spectra of GQDs recorded for the progressively

longer excitation wavelength of 20 nm increments (260–480nm).

Fig. 6 Plots of (A E)2 against photon energy (E) for the (a) GQDs, (b)GQDs-H and

(c)GQDs-L solution,

where A is the absorbance. Fig. 7 Images of oxidation colour reaction of (a) Fe2+, (b) I- by GQDs, GQDs-H and GQDs-L solution GQDs under protective atmosphere. Fig. 8 PL spectra of (a) GQDs after reacted with Fe2+ ions, (b) GQDs after reacted with I- iodide; (c) GQDs-H after reacted with Fe2+ ions (d) GQDs-H after reacted with I- iodide ions. Scheme 1 Schematic presentation of the the hydroxyl

and

oxygen

oxidative exfoliation

process

showing

the attack

of

radicals in GQDs-H synthesis.

Table 1 The peak area (A) ratios of oxygen-containing groups and O/C atomic ratio of GQDs，GQDs-L and GQDs-H. Table 2 Band gaps value of GQDs, GQDs-H and GQDs-L.


Langmuir

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

Page 22 of 27

Fig. 1 Digital photographs of GQDs and the corresponding GQDs powder respectively, (a) GQDs (b) GQDs-H and (c) GQDs-L.

Fig. 2 TEM images and the size distribution of (a) GQDs, (d) GQDs-H, (g) GQDs-L.

ACS Paragon Plus Environment 22

Page 23 of 27

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

Langmuir

Fig. 3 AFM image (up) and its height distribution (down) of (a) GQDs, (b) GQDs-H ,(c) GQDs-L.

Fig. 4

(a) FT-IR spectra (b)Raman spectra of GQDs, GQD-H and GQD-L;(c) Survey XPS

spectra and GQDs’C1s core region for (d) GQDs, (e) GQDs-H and (f) GQD-L.


Langmuir

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

Page 24 of 27

Fig. 5 (a) UV-vis absorption spectra of the GQDs; (b) (c) (d) PL spectra of GQDs recorded for the progressively

Fig.

6

longer excitation wavelength of 20 nm increments (260–480nm).

Plots of (A E)2 against photon energy (E) for the (a) GQDs, (b)GQDs-H and

(c)GQDs-L solution, where A is the absorbance.


Page 25 of 27

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

Langmuir

Fig. 7 Images of oxidation color reaction of (a) Fe2+, (b) I- by GQDs, GQDs-H and GQDs-L solution GQDs under protective atmosphere.

Fig. 8 PL spectra of (a) GQDs after reacted with Fe2+ ions, (b) GQDs after reacted with I- iodide; (c) GQDs-H after reacted with Fe2+ ions (d) GQDs-H after reacted with I- iodide ions.


Langmuir

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

Scheme 1 Schematic presentation of the attack

of

the hydroxyl

and

Page 26 of 27

oxidative exfoliation oxygen

process

showing

the

radicals in GQDs-H synthesis.

Table 1 The peak area (A) ratios of oxygen-containing groups and O/C atomic ratio of GQDs， GQDs-L and GQDs-H. Sample

AC-C/C=C(%)

AC-O/C-O-C(%)

AC=O (%)

ACOOH (%)

O/C

GQDs

68.35

22.89

3.92

4.64

0.48

GQDs-H

63.87

17.20

11.58

7.35

0.76

GQDs-L

86.52

13.48

1.54

2.42

0.42

Table 2 Band gaps value of GQDs, GQDs-H and GQDs-L. Sample

Eg(eV)

GQDs

2.90

GQDs-H

2.58

GQDs-L

3.02


Page 27 of 27

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

Langmuir

Table of Contents Graphic

High concentration GQDs with different surface oxidized degree were prepared by introduction of active free radicals, which were produced by hydrogen peroxide or ultraviolet radiation.


Free-Radical-Assisted Rapid Synthesis of Graphene Quantum Dots

Recommend Documents