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Chemical Nature of Redox-Controlled Photoluminescence of Graphene Quantum Dots by Post-Synthesis Treatment Yan Li, Xinqian Liu, Jun Wang, Hui Liu, Sen Li, Yanbing Hou, Wan Wan, Wen-Dong Xue, Ning Ma, and Jin Z. Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07935 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 26, 2016

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The Journal of Physical Chemistry

Chemical Nature of Redox-Controlled Photoluminescence of Graphene Quantum Dots by Post-Synthesis Treatment Yan Lia*, Xinqian Liua, Jun Wanga, Hui Liua, Sen Lia, Yanbing Houc, Wan Wana, Wendong Xuea, Ning Maa,b* and Jin Zhong Zhangd a

School of Materials Science and Engineering, University of Science and Technology Beijing,

Beijing 100083, PR China b

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

150001, PR China c

Key Laboratory of Luminescence and Optical Information, Institute of Optoelectronic

Technology, Beijing Jiaotong University, Beijing 100044, PR China

d

Department of Chemistry and Biochemistry, University of California, Santa Cruz, CA 95064,

USA

ACS Paragon Plus Environment

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Abstract: Graphene quantum dots (GQDs) have attracted considerable attention because of their unique photoluminescence (PL) properties. Nowadays, several approaches have been reported to improve PL quantum yield (PLQY) and regulate PL colors for their different applications. However, most reports show that higher oxygen content leads to lower PLQY. Here, we report a novel approach to enhance PLQY at high oxygen content. Both oxidation and reduction of GQDs have been demonstrated to improve the PLQY of GQDs and control their PL colors after they were first preparation. The oxidation treatment using hydrogen peroxide (H2O2) and ultraviolet (UV) light, enhanced the PLQY of GQDs from 1.51±0.02% to 3.99±0.02%, and the PL color could also be tuned from green to yellow-green under UV irradiation. When the UV light was removed, reduction reaction occurred immediately, which further improved the PLQY to 10.37±0.01% and changed the PL color to blue. Since there was no heteroatoms introduced and the GQDs maintained their original size and concentration, these treated GQDs allow us to combine the detailed structural and optical studies to testify the chemical nature of the observed PL: the PL originated from different surface states and the specific hybridization of states from the surface functional groups and the connected graphene core is responsible to specific PL colors.


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Introduction Graphene quantum dots (GQDs), a new type of carbon nanomaterials, have drawn increasing attention in recent years,1-3 because of their unique photoluminescence (PL) properties, excellent stability in water and other organic solvents, as well as low toxicity, making them promising for applications such as bioimaging, sensing, and optoelectronics.4 To date, several synthesis methods have been demonstrated for the preparation of GQDs, such as acidic oxidation,5 hydrothermal6 or solvothermal synthesis,7 electrochemical oxidation,8 carbonization of organic precursors.9-10 Two issues with most of the prepared GQDs are their low PL quantum yield (PLQY) and lack of a good understanding of the chemical nature of the observed PL, which limit their applications. Furthermore, there is a need for effective strategies to control the PL at will. To address these issues, a number of modification methods have been proposed to improve their PLQY and regulate PL colors, including reducing non-radiation groups, increasing the πconjugated structure of GQDs, and introducing heteroatoms to GQDs.11-17 Most of these studies found that higher oxygen content leads to lower PLQY of GQDs and red-shifted PL.4 The PL control was always achieved through modification during the GQD preparation process, which can vary substantially from synthesis to synthesis and result in large variations in size and concentration of the GQDs. Therefore, it is highly desired to develop a general and nondestructive post-synthesis approach to optimize the PL properties of GQDs and further cover their PL origin. In this work, we have designed and demonstrated a versatile post-synthesis approach through a UV-assisted photochemical oxidation reaction with H2O2 and an additional reduction reaction between oxidized GQDs and H2O2. Following the oxidization and reduction processes, not only the PLQY of GQDs was improved, but also the PL color can be tuned in a controllable manner. The simultaneous improvement of both PLQY and oxygen content was different with previous


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report. However, since H2O2 only changed the degree of surface oxidation of the GQDs and the usage amount of H2O2 was small, their size and concentration was essentially unaltered and no other undesired heteroatoms were introduced in the process. This approach allowed us to investigate in detail the origin and chemical nature of the observed PL. Experiment Synthesis of GQDs. GQDs were prepared by electrochemical cyclic voltammetry technology using a CHI660D electrochemical workstation, as described in our previous reports.8 Graphite rod, platinum electrode, and Ag/AgCl were used as the working, counter, and reference electrode, respectively. 0.1 M phosphate buffer saline (PBS) solution with a pH value of 7.0 was used as the electrolyte. The applied potential window was set to −5.0–5.0 V, the scan rate was 0.5 V/s, and continuous conducted 129,600 cycles. After filtering by a 220 nm filter membrane and dialyzing with a cellulose ester membrane bag (retained molecular weight: 3500 Da) for 6 days, GQDs aqueous solutions were obtained, which were used for further redox treatments. Post-synthesis treatment of GQDs. An aqueous solution of GQDs (6.0 mL) was poured into a 10 mL glass vial, and then 240 µL (1 wt%) of H2O2 was dropped into the solution. A small amount of H2O2 (ca. 4 vol% of total volume) ensured that the concentration of GQDs maintained the same (0.05 mg/ml) during the post-treatment process. After stirring for 1 min, the mixture solution was exposed to UV irradiation with a wavelength of 365 nm, which was provided by an 18 W UV lamp. The distance between the mixture solution and the UV light was 0.5 cm and the exposure time was 24 h. After this oxidation process, the PL color of GQDs changed from green to yellow-green, and thus these GQDs were denoted as Y-GQDs. Next, 3 mL of Y-GQDs were taken out and set aside for 120 h without UV light. The PL color of Y-GQDs changed from yellow-green to blue, and these GQDs were denoted as B-GQDs accordingly. In order to remove


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the excessive H2O2, once Y-GQDs or B-GQDs were obtained, a dialysis process was carried out immediately. Preparation of the GQDs in different electrolytes. The electrolysis of the graphite rod was performed on CHI660E with a constant voltage 5V. The graphite rod was inserted as an anode into 20mL 0.1M NaOH and KCl aqueous solution, respectively, paralleled to a Pt foil used as counter electrode. The electrolysis lasted for 3h, resulting in the electrolyte forming homogeneous black solution. And then, the aqueous GQDs were collected by filtering the resulting solution using a 220nm filter membrane and dialyzing in deionized water in a dialysis bag (nominal 3500Da) for 6 days. The as-prepared GQDs in different electrolytes had been named as GQDNaOH, GQDKCl, correspondingly. Characterization. Transmission electron microscopy (TEM) images and high resolution TEM (HRTEM) were obtained using a JEM-2010 and F-20 transmission electron microscope, respectively. Atomic force microscopy (AFM) images were conducted by a Bruker Dimension Icon microscope. FT-IR spectra were recorded on a NEXUS spectrometer 670. X-ray photoelectron spectra (XPS) were measured by an ESCALAB250 Xi photoelectron spectrometer (Thermo Fisher). Raman spectra were recorded on a LabRAM HR Evolution with a 514.5 nm laser. The UV-vis absorption spectra of GQDs solutions were obtained by a UNIC UV-2800 spectrophotometer. The PL and PLE spectra were conducted by an F-4500 FL Spectrophotometer with a slit width of 5 nm. The excitation wavelength started from 300 nm and progressively increased to longer excitation wavelengths in 20 nm increments. The detection wavelength of the PLE spectrum was 450 and 500 nm. The time-resolved PL results were obtained using a TRIAX 320 Spectrograph and Tektronix TDS 540D Digital Phosphor


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Oscilloscope excited with a 355 nm laser. The PLQY (φ) of GQDs, Y-GQDs, and B-GQDs were calculated according to equation (1)18: I

AR

IR

A

φ=φR × ×

×

η2 ηR 2

(1)

where I is the measured integrated emission intensity, η is the refractive index of the solvent, A is the optical density, and the subscript R refers to the reference standard with a known φ (quinine sulfate in 0.1 M H2SO4, φR=0.55).18 The refractive index of H2O2 and H2SO4 are 1.33 and 1.42, respectively. The absorbance of GQDs, Y-GQDs, and B-GQDs was kept under 0.1 at 320 nm, 320 nm and 340nm excitation wavelength in a 10×10 mm fluorescence cuvette, respectively. Results GQDs prepared by electrochemical methods usually have narrow size distributions. As shown in Figure 1a, the TEM image of the as-prepared GQDs exhibited spherical shapes and excellent monodispersity. By measuring hundreds of GQDs, the size was determined to be approximately 1.5–3 nm, with an average diameter of 2.2±0.27 nm. In addition, the lattice parameter shown in HRTEM was 0.22 nm, matching with the (1120) crystal phase of graphite.4 The AFM image (Figures 1b-c) revealed a typical topographic height of 0.5 and 1.5 nm, suggesting that most of the GQDs consist of 1-3 graphene layers. After oxidization and reduction treatments, Y-GQDs and B-GQDs maintained their original morphology, size, and height distribution, as seen in the TEM and AFM images (Figures 1d-i). This indicated that the post-synthesis approach is a nonintrusive strategy for GQD modification, which leaves the original structure of GQDs unaltered. The XPS spectra of the three types of GQDs in Figure 2a showed two typical C1s and O1s peaks at 284.5 eV and 532 eV,14 but with different relative peak intensities. The O:C atomic ratio was 0.42 for the pristine GQDs while the ratio was increased to 0.54 for the Y-GQDs, consistent with


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an oxidation reaction expected between GQDs and H2O2 under UV irradiation. Correspondingly, the O:C atomic ratio of B-GQDs was decreased back to 0.42, suggesting that the Y-GQDs were transformed to B-GQDs through a reduction process after removing the UV light. The oxidation and reduction processes were further confirmed by Raman spectroscopy. As shown in Figure 2b, the Raman spectra of the three GQDs showed two main peaks at approximately 1345 cm-1 (Dband) and 1595 cm-1 (G-band). The G-band is related to the E2g phonon of sp2 carbon atoms, whereas the D-band is associated with sp3-hybridized carbon.19-21 The intensity ratio of the Dand G-band (ID/IG) is an indicator of surface defects of GQDs. The ID/IG values of the three GQDs first increased from 0.63 for the pristine GQDs to 0.83 for Y-GQDs, and then decreased back down to 0.63 for B-GQDs, same as the original value of GQDs. This is consistent with the introduction and removal of oxygen atoms, respectively. To determine the chemical nature of surface oxygen atoms, FT-IR spectra were measured for all samples, as shown in Figure 2c. Besides the typical C=C and C–H stretching vibrations around 1603, 2857 and 2919 cm-1, the spectra of all samples showed several peaks around 1101(1047), 1635, and 3422 cm-1, corresponding to epoxy groups (C–O–C), carbonyl groups (C=O), and hydroxyl groups (-OH), respectively.17,

22

Based on these FT-IR results, detailed C1s XPS

spectra were deconvoluted into four bands (Figures 2d–f) to better determine the changes of surface oxygen groups in each GQD sample. These deconvoluted spectra included a C=C/C–C peak with a binding energy of 284.5 eV, a C-OH peak with a binding energy of 285.8 eV, a C– O–C peak at 286.5 eV,23 and a C=O peak at 287.9 eV.24 As compared to the pristine GQDs, YGQDs had higher peak intensity for the C–O–C bond but lower peak intensity for the C=O bond, whereas B-GQDs showed higher peak intensity for the C–OH bond. These changes in the peak intensities indicated that the post-treatment process only altered the surface functional groups of


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the GQDs. Considering that the surface defects in GQDs may be the origin of PL,25 our postsynthesis treatment approach should be useful in determining the nature of the PL. Figures 3a–c showed representative PL spectra of the three GQDs under different excitation wavelengths. These PL spectra had several distinction features. First, the PL spectra of Y-GQDs and B-GQDs were narrower than that of the pristine GQDs. Second, the PL maximum of GQDs was ca. 480 nm with an excitation of 300 nm. However, the PL maximum of Y-GQDs and BGQDs was ca. 490 nm and 450 nm with excitation of 320 nm and 340 nm, respectively. Third, the pristine GQDs emitted green light when excited by a hand-held UV light source with a wavelength of 365 nm, whereas Y-GQDs and B-GQDs emitted bright yellow-green and blue PL, respectively (shown in the insets of Figures 3a–c). As can be seen from Figure 2d-f and 3, the PL spectra of Y-GQDs red shifted compared with that of the pristine GQDs, which correlates with the increase of C-O-C groups. However, along with the increase of C-OH groups, the PL spectra of B-GQDs blue shifted. It is thus clear that different hybridization of states from the surface functional groups and the connected graphene core leads to different PL band shift. Finally, different from some reported results on GQDs that the PL intensity decreased with increasing degree of surface oxidation,4 both the Y-GQDs with higher degree of oxidation and B-GQDs with the same degree of oxidation as the pristine GQDs exhibited improved PL intensity at different excitation wavelengths. The maximum enhancement factors for the intensity ratios of IY-GQDs/IGQDs and IB-GQDs/IGQDs were 6.6 and 9.5 with excitation at 340 nm. Furthermore, the PLQY of GQDs, Y-GQDs and B-GQDs were calculated to be 1.51±0.02%, 3.99±0.02% and 10.37±0.01%, respectively, using quinine sulfate as a reference.18 Because the added volume of the H2O2 solution was small compared to the GQDs solution, the three GQDs could be considered with the same concentration of 0.05 mg/ml.


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UV-Vis absorption and PL excitation (PLE) spectra of the three GQD samples were shown in Figure 4. The UV-Vis spectra of the pristine GQDs showed a strong absorption band peaked at 224 nm and several shoulder peaks at ca. 260 nm and 300 nm. The 224 nm band could be assigned to the electron transition from π orbital to π* orbital of the C=C bond in the sp2 cluster, while the peaks near 260 nm and 300 nm usually originate from the electronic transition from n orbital to π* orbital of the surface oxygen groups.4, 26 Figure 4a showed that the π–π* plasmon peak for both Y-GQDs and B-GQDs was red-shifted to 234 nm. The evolution of the π–π* plasmon peak depends on two kinds of conjugation effects: one related to nanometer-scale sp2 clusters, and the other from the linking of chromophore units such as C=C and C–O.23 For our results, the red shift of the π–π* plasmon peak of Y-GQDs and B-GQDs likely resulted from a change of surface chromophore units rather than variation of sp2 clusters. However, the strong absorption due to π to π* transition did not result in PL emission. In the PLE spectra with the detection wavelengths of 450 nm and 500 nm (Figure.4b-c), a strong peak at ca. 320 nm and a weak peak at ca. 220 or 230 nm were observed for all GQD samples. The strong PLE peaks were likely due to n to π* transitions. Moreover, the TRPL decay curves of the three samples as shown in Figure S1 exhibited different lifetimes τ: 14.16 ±0.01 ns for GQDs, 16.10±0.01 ns for YGQDs, and 13.33±0.03 ns for B-GQDs. The difference in lifetime is not significant but does indicate that the PL lifetime of three GQD samples are affected by defect states due to differences in the nature and number of their functional groups. Discussion It is known that H2O2 can dissociate quickly into •OH radicals under UV light as shown in equation (2): UV

H2 O2 ሱሮ 2 ∙OH

(2)


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The •OH radicals are very reactive and exhibit strong oxidative properties in many reactions.27-28 In the GQDs solution, the •OH radicals could attack single carbon–oxygen (C–O) bonds and transform them into double carbon–oxygen (C=O) bonds. They could also attack carbon atoms with positive charge in π-conjugated systems, such as defected carbon atoms with unsaturated bonds, to form C–OH bonds. The adjacent C-OH bonds can form C-O-C groups by losing H2O molecules. If the C-O-C groups were arranged in line on the surface of GQDs, the graphene backbone would become fragile and break down into smaller sizes. However, the C-O-C groups on the GQD surface are difficult to arrange in line because of the low concentration of •OH radicals (just 0.038% of H2O2 by weight in GQD solution) and small size of GQDs. Considering the unchanged size and morphology of GQDs and the increased number of C-O-C groups of YGQDs based on the XPS spectra, we speculated that a limited number of •OH radicals oxidized the GQDs rather than breaking them down into smaller sizes. Therefore, following the oxidation reaction between GQDs and •OH radicals, the total oxygen content was increased in Y-GQDs as found in the XPS results. The corresponding transformation process was illustrated in Figure 5. Accompanied with the oxidation process, Y-GQDs gradually reached a high oxidized degree. A final O:C atomic ratio of 0.54 was obtained, which was similar to that of graphene oxide (GO).29 Several reports have suggested that GO is a strong oxidant and has the ability to oxidize Fe2+ to Fe3+.30 Considering the similar conjugated structure and oxygen content of Y-GQDs and GO, along with similar experimental result of oxidization Fe2+ to Fe3+ as shown in Figure S2, it is believed that Y-GQDs had strong oxidation ability, perhaps stronger than H2O2. When the UV light was turned off, H2O2, rather than •OH radicals, mainly existed in the Y-GQD solution, was then oxidized by Y-GQDs and transformed into O2. The possible reaction was shown in equation (3).


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H2 O2 +Y-GQDs→B-GQDs+H2 O+O2

(3)

To demonstrate the above oxidization and reduction mechanism, two comparative experiments were conducted. First, isopropanol (IPA), reported a scavenger of •OH,31 was added to the GQDs solution before the oxidization process. The almost unchanged PL behavior of the solution with IPA addition in Figures S3a–b revealed that •OH radicals played important role on the oxidation reaction of GQDs. In order to make clear that H2O2 participated in the reduce reaction with YGQDs, once Y-GQDs were formed, a dialysis process was immediately carried out to remove the excessive H2O2. The relevant PL emissions were shown in Figure S3. There was little difference between Y-GQDs and dialyzed Y-GQDs except for a small decrease in the PL intensity of dialyzed Y-GQDs. It demonstrated that the transform reaction from Y-GQDs to B-GQDs could be terminated. The PL state of Y-GQDs could be fixed by removing the redox stimuli. Additional experiments were conducted to demonstrate that H2O2 and UV light played a synergistic role in the oxidation process of GQDs. As shown in Figure S4, introducing only UV light (24 h), the PL spectra of GQDs exhibited slight red shift but a decreased intensity compared with the pristine sample. But when only H2O2 was introduced (1%) without UV light, the PL spectra of GQDs, when excited from 320 nm to 360 nm, blue shift rather than red shift with an enhanced PL intensity was shown. Similar phenomenon was observed in the Y-GQDs to BGQDs conversion process, which confirmed that the GQDs had stronger oxidizing ability than H2O2. The degree of oxidation of the GQDs depended on the concentration of H2O2. From Figure S5, it was clear that the GQDs treated with 5% H2O2 changed its color from green to yellow-green in 1h. However, the GQDs treated with 10% H2O2 almost lost their PL emission after post-treatment for 10 min. These results indicated that increasing concentration of H2O2 accelerated the oxidization reaction but too high a concentration of H2O2 could damage the


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conjugated structure of GQDs. Similar to high concentration of H2O2, longer UV irradiation time could not result in further improved PL property, but could actually break the conjugated structures of the GQDs as shown in Figure S6. On the basis of the PL properties and surface oxidation analysis as well as the surface morphology of the GQDs, we propose a model in terms of oxidation and reduction to explain the origin of the observed PL. On the PL origin, some earlier reports suggested that the PL of GQDs may originate from intrinsic state emission, due to quantum size effect or conjugated π domains and zigzag sites, while other studies considered surface state emission, determined by hybridization of the carbon backbone and the connected chemical groups, to dominate the PL emission.

[6,32-33]

Considering that the three GQDs have similar size and concentration but

different surface oxidized degree, it is reasonable that the different PL behaviors are related to their surface states, which are due to mixing or hybridization of electronic states (shown in Figure 6) from the edge functional groups and the connected partial graphene core.34 Furthermore, as shown in the detailed C1s spectra (Figures 2e-f), Y-GQDs had higher peak intensity for the C–O–C bond but lower peak intensity for the C=O bond, whereas B-GQDs showed higher peak intensity for the C–OH bond compared to that of GQDs. The varied relative ratios of each surface oxygen group guided us to reach that different surface state response to different PL emission. The hybridization of states from C=O bond and the connected graphene core (labeled as G-C=O) are likely responsible for the PL emission at 490 nm, while the hybridization of state from C-O-C bond and the connected graphene core (labeled as G-C-O-C) results in the PL band at 540 nm. The –OH-related hybridized states (labeled as G-OH) mainly contribute to the PL emission at 440 nm.


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To date, several reports have discussed the origin of the observed PL in relation to G-C=O and G-OH hybridization state. For example, Wang and coworkers had stated that the common green luminescence emission centers in these GQDs synthesized by bottom-up and top-down methods can be unambiguously assigned to special edge states consisting of several carbon atoms on the edge of carbon backbone and functional groups with C=O (carbonyl and carboxyl groups).35 Meanwhile, based on other studies (shown as Figure S7-S10),36-39 a similar conclusion can be drawn. For the G-OH hybridized states, Yang et.al pointed out that they mainly contribute to the blue emission and increase the electron density of the π structure in GQDs.34 But for the G-C-OC hybridized states, Maiti et al. found that the yellow-red emission of graphene oxide (GO) originated from the epoxy/hydroxyl-related defect-assisted localized states,23 while other studies indicated that C–O–C groups in GQDs induce non-radiative recombination of localized electronhole pairs and suppress intrinsic state emission in GQDs.22 In this work, in order to determine the role of G-C-O-C hybridized states in the PL of the GQDs, additional control experiment was conducted. NaOH and KCl solutions were used as electrolytes to prepare GQDs. Because C–O–C groups in alkaline solution would lead to ring-opening reactions, the resulting GQDs, labeled as, GQDNaOH and GQDKCl, should have different amount of C–O–C groups. As shown in Figure 7a, the FT-IR spectrum of the GQDKCl displayed stronger absorption in the region of stretching vibration of C–O–C (1048 cm-1) than GQDNaOH. Meanwhile, their detailed C1s XPS spectra (Figure 7b-c) and O1s spectra (Figure S11) confirmed high content of C–O–C (25.61% in C1s spectra) in GQDKCl and low content (3.73% in C1s spectra) in GQDNaOH. When irradiated by 365 nm UV light, GQDNaOH exhibited strong PL emission at 480 nm, while GQDKCl showed its PL band between 480~550 nm in Figure 8a. Their corresponding PL colors


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were shown in the inset of Figure 8a. These PL behaviors were consistent with our proposed mechanism for the origin of the PL and suggested that surface defect state resulting from the GC-O-C hybridized states in GQDs can capture excited electrons, producing fluorescence at ca. 540 nm. In addition, the PL spectra of both GQDs exhibited excitation wavelength-dependent behaviors (Figure 8c-d). The PL emission of GQDNaOH showed an almost line relationship with the excitation wavelength in terms of the PL maximum, besides a few shoulder peaks were observed in Figure 8b. In contrast, GQDKCl exhibited a much broader PL spectrum and obviously dominated by different PL peaks. These PL spectra can be deconvoluted into three Gaussian-like bands, centered at ~440 nm (peak 1), ~490 nm (peak 2), and ~540 nm (peak 3), respectively, as shown in Figure 9a-b. As increasing the excitation wavelength, the three PL peaks almost did not change their position but had varied the integrated areas (shown in Figures 9b,d and Figure S12b). When three PL positions were applied to GQDNaOH, a similar phenomenon was observed in Figure 9a, c and Figure S12a. This result indicated that deconvolution was a good approach to uncover the different PL bands. Moreover, comparing the deconvoluted PL spectra (Figures 9ab) with corresponding C1s XPS spectra of both GQDNaOH and GQDKCl (Figure 7b-c), it was found that the high content of G-C=O structure corresponded to a large peak area of peak 2 while a high content of G-C-O-C structure corresponded to a large peak area for peak 3. This result again supported our proposed explanation for the chemical origin of the observed PL in that specific hybridized states can be associated with a particular PL band. Such correlation is critical to understanding the origin of the observed PL in the GQDs. Conclusions A simple post-synthesis method has been developed to simultaneously improve the PLQY and control the PL color of GQDs. Because this approach only alters surface oxygen groups without


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changing their original sizes and introducing other heteroatoms, it is a general method for modifying GQDs after they are first prepared. Importantly, the GQDs prepared by this approach allow us to conduct a detailed study of the origin or chemical nature of the observed PL. On the basis of their surface morphology and surface oxidation, as well as their PL properties, a new model based on oxidation and reduction is proposed to explain the chemical origin of the PL of these GQDs. Hybridized states derived from surface defect states and graphene core states are the primary PL centers of the GQDs, and different kinds of hybridized states are responsible for the different PL emission colors observed. Specifically, G-C=O, G-C–O–C, and G–OH hybridized states are identified to be responsible to the green PL at 490 nm, yellow-green PL at 540 nm and blue emission at 440 nm, respectively. Understanding of the PL mechanism is important for controlling the PL properties of GQDs and their potential applications in different fields such as photonics, sensing, and imaging. Associated Content Section Supporting Information The TRPL decay curves analysis, oxidized experiment from Fe2+ to Fe3+ by GO, several comparative and additional experiments of GQDs oxidization and reduction, some reports discussed the PL origin of GQDs related to G-C=O and G-OH hybridization state and the detailed O1s XPS spectra and deconvoluted PL spectra of the GQDNaOH and GQDKCl can be found in supporting information (PDF). This material is available free of charge via the Internet at http://pubs.acs.org/. Corresponding Author *Email: [email protected] (Yan Li), Tel: +86 1062333140;


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*Email: *[email protected] (Ning Ma); Tel: +86 45182568276; The authors declare no competing financial interest. Acknowledgement The authors are grateful to Dr. Dengli Qiu in the Bruker Nano Surface Business (Beijing Office) for his help on the AFM measurements. This work was supported by National Natural Science Foundation of China (Grant No. 21674011, 21374009), Beijing Municipal Natural Science Foundation (2132049), Beijing Organization department outstanding talented person project (2013D009006000001) and Fundamental Research Funds for the Central Universities (FRF-TP-15-005A3). Yan Li is thankful for financial support from the program of the China Scholarship Council (CSC). JZZ is grateful to NASA for financial support through the MACES Center at UC Merced (NNX15AQ01A). Reference 1.

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Figure 1. TEM images of the GQDs (a), Y-GQDs (d) and B-GQDs (g). The histograms of size distributions and HRTEM images for the three samples are shown in the inserts of (a), (d) and (g), respectively. The standard deviations of the three samples are 0.27, 0.21, 0.24 nm. AFM images of the GQDs (b), Y-GQDs (e) and B-GQDs (h). The corresponding height profiles are shown in (c), (f) and (i), respectively.


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Figure 2. The XPS survey spectra (a) and the Raman spectra (b) as well as the FT-IR spectra (c) of the pristine GQDs, Y-GQDs and B-GQDs. The C1s XPS spectra of GQDs (d), Y-GQDs (e) and B-GQDs (f).

Figure 3. The PL spectra of the pristine GQDs (a), Y-GQDs (b) and B-GQDs (c). The photographs of three GQDs samples with excitation at 365 nm are shown in the insets of (a), (b), (c), respectively.


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Figure 4. UV-Vis absorption spectra of the pristine GQDs, Y-GQDs and B-GQDs (a), PLE spectra with a emission wavelength of 450 nm (b) and 500 nm (c).

Figure 5. A proposed reaction mechanism for the post-oxidation treatment process.

Figure 6. A schematic mechanism for the hybridized states. MOs present molecular orbitals.


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Figure 7. The FT-IR spectrum (a) and the high-resolution C1s XPS spectra of thus-prepared GQDNaOH (b) and GQDKCl (c).

Figure 8. The UV-Vis spectrum (a) (insert: PL emission under 360nm excitement), the PL positions at different excitation wavelengths for GQDNaOH and GQDKCl (b) and the normalized PL spectrum of the GQDNaOH (c) and GQDKCl (d).


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Figure 9. The deconvoluted PL spectrum of GQDNaOH (a) and GQDKCl (b) under 360nm excitation and the area changes of peak 1, peak 2, and peak 3 (c)-(d) when the excitation wavelength increased from 320 nm to 380 nm.


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