Tailoring the Electronic Properties of Graphene Quantum Dots by P

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Tailoring the Electronic Properties of Graphene Quantum Dots by P Doping and Its Enhanced Performance in Metal-Free Composite Photocatalyst Jiajia Qian, Chao Shen, Jing Yan, Fengna Xi, Xiaoping Dong, and Jiyang Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08702 • Publication Date (Web): 12 Dec 2017 Downloaded from http://pubs.acs.org on December 18, 2017

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The Journal of Physical Chemistry

Tailoring the Electronic Properties of Graphene Quantum Dots by P Doping and Its Enhanced Performance in Metal-free Composite Photocatalyst Jiajia Qian, Chao Shen, Jing Yan, Fengna Xi, Xiaoping Dong*, Jiyang Liu* Department of Chemistry, School of Sciences, Zhejiang Sci-Tech University, 928 Second Avenue, Xiasha Higher Education Zone, Hangzhou 310018, China. E-mail: [email protected]; [email protected]

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Abstract Graphene quantum dots (GQDs) as new 0 dimensional (0D) materials have exhibited promising applications in numerous fields, and these enormous potentials are largely dependent on its electronic properties. Herein we tailored the semiconductive behavior of GQDs and transformed the n-type hydrothermally synthesized GQDs to p-type via the modification of P=O groups with an electron-withdrawing ability. The obtained high-dispersed and stable P-doped GQDs (P-GQDs) had uniform size and thickness, high crystallinity and wide visible light absorption region. The subsequent assembly with the n-type graphitic carbon nitride (CN) formed a stable metal-free photocatalyst because of the π-π interaction between conjugated GQDs sheets and CN layers and the possible hydrogen bonds as well. Due to the formation of p-n junction on the P-GQDs/CN interface, the photo-generated charges were efficiently separated that resulted in an enhanced photocatalytic performance. This strategy illuminates the GQDs applications by controlling the electronic structure of GQDs based on the detailed application, not only for photocatalysis, but also for many fields, such as solar cells, catalysis, electrocatalysis and so on.

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1. Introduction Since the pioneer work at 2004 by K. S. Novoselov and A. K. Geim,1 graphene has received numerous attentions due to its fantastic properties, and exhibited applications in many fields, for instance electronics,2,3 energy conversion and storage,4,5 sensors,6,7 biomedicine8,9 and catalysis.10,11 As for photocatalysis, metallic graphene has been extensively employed to substitute noble metals as an electron acceptor, therefore promoting the photo-generated charge separation in semiconductor photocatalysts.12-16 However, graphene cannot absorb photon to produce electron-hole pairs because of lacking intrinsic band gap, which significantly limits its practical application in photocatalysis. It has been demonstrated that the band gap of graphene can be opened by size controlling from its native 2D morphology to 0D graphene quantum dots (GQDs).17-25 Unfortunately, the high recombination rate of photo-excited charges results in a poor photocatalytic activity of GQDs. That is meanwhile the reason why GQDs is mainly used in luminescence related fields, such as bioimaging26-29 and optical sensing.20,30,31 As a consequence, researchers usually employ GQDs as photo-sensitizers in photocatalysts like the dye-sensitization effect.32-36 Nevertheless, the lowest unoccupied molecular orbit (LUMO) of GQDs is not enough high and therefore the excited electrons cannot transfer to semiconductor photocatalysts with a high edge potential of conduction band (CB).37,38 Contrarily, GQDs would act as a recombination center to quench electron-hole pairs. 3 / 40



In various GQDs sensitized photocatalysts, GQDs/g-C3N4 composites present distinct advantages, including the low cost precursors, facile synthetic process, narrow band gap and high chemical stability of g-C3N4, wide spectral range, tightly coupled interface of GQDs/g-C3N4 due to the π-π interaction and the possible H-bond.38-45 Moreover, its metal-free nature avoids a secondary pollution by the released metal ions from the photo-corrosion effect. However, the high CB edge potential of g-C3N4 cannot accept excited electrons from GQDs (As shown in Figure S1). In this work, we employed a phosphor doping strategy to tailor the electronic property of GQDs, and transformed the n-type behavior of undoped GQDs to the p-type P-doped GQDs (P-GQDs). The subsequent assembly of P-GQDs with n-type g-C3N4 (CN) formed a p-n junction on the interface, which improved the separation of photo-produced chargers and accumulated electrons and holes on g-C3N4 and GQDs, respectively. To our knowledge, it is the first time to report a p-type GQDs material and the p-n style GQDs-based composite for photocatalysis. It will inspire the development of GQDs based photocatalysts with high charge separation efficiency, and meanwhile the tailoring of electronic property will also promote the wide application of GQDs in optics, sensor, energy and catalysis. 2. Experimental 2.1 material synthesis 2.1.1

Preparation of P-GQDs

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1,3,6-trinitropyrene was synthesized according to the literature.21,46 The 1,3,6-trinitropyrene (20 mg) and Na2HPO4•12H2O (600 mg) were dissolved in 10 mL ultrapure water containing NaOH (125 mM), following an intensive ultrasonic treatment for 0.5 h. The mixture was then transferred into Teflon-lined autoclave and heated for 4 h at 200 oC. After cooling down to room temperature, the obtained mixture was filtered through a 0.15 µm microporous membrane to remove insoluble carbon product. Then, un-reacted small molecules were removed by dialyzing in a dialysis bag (retained molecular weight of 1000 Da) against ultrapure water for 24 h. The obtained sample was further dialyzed in a dialysis bag (retained molecular weight: 3500 Da) against ultrapure water for 24 h to remove impurities with large size. The synthesis of GQDs was similar to that of P-GQDs, but without the addition of Na2HPO4•12H2O. 2.1.2

Preparation of P-GQDs/CN heterojunctions

The CN was obtained by thermal polycondensation of melamine, which was reported in our previous literatures.47,48 Then the metal-free P-GQDs/CN photocatalyst was prepared by one-step synthesis using ultrasonication. Typically, an amount of 1 g CN powder was added into 90 mL of P-GQDs solution (11.1 mg mL-1) and subsequently ultrasonically dispersed for 0.5 h at room temperature. After pouring off the clear supernatant, the sediments were rinsed with water several times and dried at 40 oC overnight. The resulting composite contains 1 wt % of P-GQDs. To determine the optimum loading content of P-GQDs, other doping contents (0.5 wt %,

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2 wt %, 4 wt %) were controlled by addition of different volumes of the P-GQDs solution. 2.2 Characterizations Transmission electron microscopy (TEM) observation was performed on a JEOL JEM-2100 electron microscope. The samples were characterized by X-ray powder diffraction (XRD) on a DX-2700 diffractometer (Dandong Haoyuan Instrument Co. Ltd. China). The thickness was assessed using a tapping mode atomic force microscope (AFM) (Multimode-8; Bruker; USA). The X-ray photoelectron spectra (XPS) were undertaken using a VG K-ALPHA instrument with an Al Kα monochromatic source. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet Avatar 370 spectrophotometer using the standard KBr disk method. UV-vis absorption spectra and diffuse reflectance spectra (DRS) were recorded on a Shimadzu 2450 UV-vis spectrometer with an integrating sphere using BaSO4 as the reference. Photoluminescence (PL) spectra were surveyed by Edinburgh FL/FS900 spectrophotometer. The electrochemical tests were performed on a CHI660E electrochemical workstation (Chenhua, Shanghai, China). 2.3 Photocatalytic tests The photocatalytic activities of photocatalysts were evaluated by the degradation of Rhodamine B (RhB) in aqueous solution under visible light irradiation using a 300 W xenon lamp (HSX-F300, Beijing NBet) with a 420 nm cutoff filter as the light source. The light intensity was evaluated by a light power meter (CEL-NP2000, China Education Au-light), and it is equal to 160 mW cm-2. The distribution of the 6 / 40


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irradiation light and the UV-vis spectrum of the 420 nm cutoff filter were shown in Figure S2. In each experiment, 100 mg of catalyst was added to an aqueous solution (100 mL) of RhB (10 mg L-1) in a beaker. Prior to irradiation, the catalyst was stirred in the dark for 60 min to ensure the establishment of adsorption-desorption equilibrium. At given time intervals, 3.5 mL of the solution was collected and subsequently centrifuged to remove the particles. The concentration of solution was analysed by measuring the maximum absorbance at 554 nm for RhB through a Shimadzu UV-2450 spectrophotometer. 2.4 The trapping and detection experiment of active species In order to detect the active species in the photocatalytic process, tert-butyl alcohol (TBA, 6 mM), benzoquinone (BQ, 0.5 mM) and ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 6 mM) were respectively added into the reaction solution as the scavengers for hydroxyl radicals (•OH), superoxide radicals (•O2-) and photo-generated holes (h+). The method was similar to the former degradation test. The •OH and •O2- radicals were directly detected by Electron paramagnetic resonance (EPR) technique (Bruker model A300-10/12 spectrometer). EPR signals of radicals trapped by 5,5-dimethyl-l-pyrroline-N-oxide (DMPO) were recorded at ambient temperature. The samples (10 mg) were measured in suspension dispersed in various solutions: purified methanol for detecting •O2- and deionized water for detecting •OH. The settings for the EPR spectrometer were as follows: center field, 3507 G; sweep width, 100 G; microwave frequency, 9.85 GHz; modulation frequency, 100 kHz; power, 20.00 mW. 7 / 40



2.5 Photo-electrochemical measurements The photo-electrochemical properties were studied on an electrochemical station (CHI660E, Chenhua, Shanghai, China) linked with computer. The Ag/AgCl electrode, platinum foil and Na2SO4 (0.2 M) were employed as reference electrode, counter electrode and electrolyte solution, respectively. The working electrode was fabricated as follow: 20 mg catalyst was ultrasonically dispersed in 2 mL distilled water for 0.5 h, and then the obtained suspension was coated on a 3 cm × 1 cm indium tin oxide (ITO) glass before drying at 40 oC for 12 h. 3. Results and discussion 3.1 Physicochemical properties of P-GQDs GQDs were synthesized via hydrothermal treating 1,3,6-trinitropyrene in NaOH solution (Scheme 1a). Because of the electron-donating effect of the modified hydroxyl groups, the obtained GQDs displayed an n type behavior. The P doping was achieved by the addition of Na2HPO4 in the synthetic process of GQDs (Scheme 1b). In general, P is used as an n type dopant in semiconductor Si or carbon materials due to its five valence electrons. However, the hydrothermal treatment with the presence of phosphate ions would bring P=O groups covalently bonded onto the GQDs layer. Furthermore, the electron-withdrawing effect of P=O groups would decrease the electron cloud density on GQDs sheet, consequently changing to p-type in P-GQDs from the n type in undoped GQDs. The as-prepared GQDs and P-GQDs showed the excellent dispersion and high stability in aqueous suspension even after storage for several months. Their UV-vis 8 / 40


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absorption spectra (Figure 1a and b) displayed strong absorption from visible light to UV region, and the absorption onsets were at ~ 600 nm and ~602 nm, meaning the optical band gaps of GQDs and P-GQDs were both 2.06 eV. The photo-generated carriers presented a strong recombination trend, resulting in intensive PL emission (Figure S3). Additionally, both the GQDs and P-GQDs displayed an excitation independent PL behavior (Figure S4), indicating single-emission fluorescence center and high crystallinity.21 It is well known that Mott-Schottky curve examination is an effective electrochemical technique to determine the semiconductor type and the flat band potential (Efb) of semiconductor. The curve of n-type semiconductors has a positive slope and p-type semiconductors have a negative slope. As shown in Figure 1c, the GQDs were evidently n-type. Nevertheless, the curve of P-GQDs (Figure 1d) presented a negative slope that implied its p-type nature. The flat band potential of semiconductor can be estimated by the intercept of the extrapolated straight line on the abscissa, and it is considered as their Fermi level (EF). Furthermore, the EF located near to the CB of n-type semiconductor and the VB of p-type semiconductor. Thus, the CB edge potential of GQDs was evaluated to be -1.34 eV vs. Ag/AgCl (namely -1.14 eV vs. NHE) and the VB maximum of P-GQDs was calculated to be 1.02 eV vs. Ag/AgCl (1.22 eV vs. NHE). Based on these above results, the schematic diagrams for the band structure of GQDs and P-GQDs were illustrated in Figure 1e and f. From the AFM image (Figure 2a), it could be seen that uniform quantum dots had been formed through the synthesis processes. The height profiles (Figure 2b) displayed a thickness of 1.5~2.0 nm, indicating the few-layer structure of P-GQDs 9 / 40



(5~6 layers). The size and morphology of the P-GQDs were examined by TEM and high-resolution TEM (HRTEM). The TEM image (Figure 2c) exhibited that the P-GQDs had relatively uniform particle distribution, giving an average lateral size of 2.2 ± 0.2 nm. As displayed in the HRTEM image (Figure 2d), the high crystallinity was revealed by the clear lattice fringes with an interplanar spacing of 0.245 nm, which was corresponding to the (1120) facet of sp2 graphitic carbon in graphene.23,49 Moreover, its fast Fourier transform (FFT) pattern (inset in Figure 2d) showed that the P-GQDs were crystalline hexagonal structures. To investigate the chemistry environment of surface elements, X-ray photoelectron spectroscopy was performed. Compared to the full-scale XPS spectrum of GQDs (Figure S5), the full-scale XPS spectrum of P-GQDs (Figure 3a) clearly showed the signals of C 1s, O 1s, P 2s and P 2p peaks, confirming the successful doping of phosphorus. Figure 3b assigned the well-fitted C1s spectrum, which could be divided into four different peaks, corresponding to the signals of C=O at 288.7 eV, C-O at 286.7 eV, C-P at 286.0 eV and sp2 hybridized carbon at 284.6 eV.50,51 The asymmetric O 1s spectrum (Figure 3c) was deconvoluted into four peaks, representing C-O at 534.7 eV, P-O at 533.5 eV, C=O at 532.4 eV and -OH at 531.5 eV.51,52 Furthermore, the P 2p XPS spectrum (Figure 3d) could be fitted into two parts, CPO3/C2PO2 and C3PO structures at 133.6 and 132.7 eV, respectively. It was because of the modification of P=O groups on the GQDs sheets that the P-GQDs displayed the p-type behavior. 3.2 Formation of P-GQDs/CN p-n junction photocatalyst 10 / 40


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Graphitic carbon nitride (g-C3N4), a graphite-like polymer, is composed of the parallel stacking of 2D conjugated layers that were formed by the polymerization of tri-s-triazine units. However, the incomplete condensation of tri-s-triazine rings results in the presence of hydrogen element in terminal –NH2 or –NH– forms. As coupling P-GQDs on the surface of CN, the π–π interaction between graphite layer in P-GQDs and polymeric tri-s-triazine plane in CN, as well as the possible H-bonding linking between the surface –OH groups on P-GQDs and terminal –NH2 or –NH– groups on CN would stabilize the composite of P-GQDs/CN (Figure 4a). The loading of P-GQDs on CN was directly demonstrated by TEM observation. Due to the similar contrast of CN and P-GQDs, it was difficult to clearly distinguish P-GQDs on CN surface. However, some dark zones with 2-3 nm size, which was in accordance with that of P-GQDs in Figure 2c, could still be seen on the CN surface (Figure 4b). By observing these dark zones via HRTEM technology (Figure 4c), clear lattice fringes were found, and the estimated intra-planar spacing was ~0.245 nm similar to that of individual P-GQDs in Figure 2d. These results indicated that these loaded dark zones on CN should be P-GQDs, and the formed P-GQDs/CN heterojunction with a closed interface between these compositions was favorable for the rapid transfer of photo-induced charges and therefore electrons and holes were collected on different sides of the heterojunction. Figure 4d compared the XRD patterns of CN, P-GQDs and P-GQDs/CN samples. Two pronounced diffractions at 13.1o and 27.6o in the pattern of CN corresponded to the in-planar (100) diffraction and the interlayer (002) diffraction, respectively.47 11 / 40



P-GQDs showed a weak broad interlayer (002) peak, which was attributed to the few-layer structure of P-GQDs. After coupling P-GQDs on CN, no evident difference in the patterns was found between CN and P-GQDs/CN, suggesting the modification of P-GQDs would not influence the crystal structure of CN. The surface groups of different samples were analyzed using FTIR technology in Figure 4e. The characteristic absorptions of CN at the region of 1200-1600 cm-1 and 810 cm-1 were observed in the spectrum of P-GQDs/CN.47 This result also demonstrated the CN structural preservation after the modification of P-GQDs. Compared with CN, the enhanced absorption of H-O vibration at 3600-3300 cm-1 was checked that was ascribed to the plentiful surface –OH of P-GQDs. The successful fabrication of P-GQDs/CN was further proved by XPS spectra (Figure 5a-d). The survey XPS spectra (Figure 5a) showed strong signals of C 1s, N 1s and O 1s, and the relative contents of C and O evidently increased after coupling P-GQDs on CN, despite the small loading percentage of P-GQDs (1%). This result implied that the surface modification of P-GQDs on CN. The C 1s spectra (Figure 5b) depicted two peaks located at the binding energies of 288.1 eV and 284.6 eV that were respectively attributed to the existence of the sp2-hybridized C (N-C=N) in the aromatic skeleton rings of CN and sp2 C-C=C bonds.53 The intensity enhancement of the peak at 284.6 eV in P-GQDs/CN should be from the carbon element in P-GQDs. Moreover, an additional peak at 286.7 eV appeared in the P-GQDs/CN heterojunction that resulted from the C-O groups in P-GQDs. The N 1s spectra of samples were plotted in Figure 5c, which could be fitted into three peaks. The signals located at 12 / 40


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401.1, 400.0 and 398.7 eV were respectively from the amino groups, the N-(C)3 and the N species in the conjugated CN rings.53 No obvious difference was observed between CN and P-GQDs/CN, implying the modification P-GQDs did not influence the CN matrix. The O 1s spectrum of P-GQDs/CN (Figure 5d) could be divided into two peaks. The peak of P-GQDs/CN at 532.3 eV was similar to the signal of CN, which was ascribed to absorbed water. Besides, the peak at 531.5 eV resulted from the surface -OH group. 53 Additionally, P element was not checked in the heterojunction because of the low P percentage in P-GQDs and the low P-GQDs content in P-GQDs/CN as well. The optical properties of the as-prepared CN and P-GQDs/CN samples were examined using UV-vis DRS technology. As shown in Figure 6a, CN held an absorption edge of ~450 nm that represented a band gap of 2.81 eV (inset of Figure 6a). With incorporation of P-GQDs, the absorption of P-GQDs/CN prolonged to > 500 nm, indicating that the composite can utilize much more solar light. The estimated band gap of CN in the composite was 2.81 eV that was similar to that of the individual CN, which revealed that the coupling of P-GQDs would not affect the electronic structure of CN. PL technique was used to study the capacity of effective charge transport and separation in photocatalysts. It is well known that the photocatalytic activity of CN is limited due to the high recombination ratio of photo-generated electron-hole pairs, which would produce strong PL emission (see in Figure 6b). The PL intensity decreased evidently after coupling P-GQDs on CN, demonstrating the electron-hole recombination had been suppressed in P-GQDs/CN. 13 / 40



However, the loading of GQDs on CN did not apparently change the PL intensity. These results suggested that the PL quenching should be attributed to the effective charge transport on the P-GQDs/CN interface but not from traps or defect states made by GQDs between the bandgap of the CN. 3.3 Enhanced photocatalytic performance of P-GQDs/CN p-n junction photocatalyst To determine an optimum loading content of P-GQDs, we compared the removal ratios of RhB using the P-GQDs/CN composites with various P-GQDs contents. As described in Figure 7a, a low proportion of P-GQDs (0.5 wt %) in the composite were insufficient for improving the migration and separation of charges between P-GQDs and CN (Figure 7b), which was demonstrated by the ~58% RhB removing efficiency after 120 min irradiation. However, the high loading percentage of P-GQDs would result in a thick aggregated layer on the CN surface and therefore suppressing the photo-absorption of CN (Figure 7b). As a consequence, the P-GQDs/CN heterojunction with 1 wt % P-GQDs displayed the highest photocatalytic activity. Under the visible light illumination, the characteristic peaks of RhB at 554 nm rapidly decreased with the optimized P-GQDs/CN, and the complete discolor was realized after 80 min (Figure 7c). We further used total organic carbon (TOC) examination to illustrate the mineralization level of organic dyes (Figure S6). The TOC value of RhB solution gradually decreased with the increase of irradiation time and ~45% TOC was removed after 120 min. As shown in Figure S7, the clear limewater gradually became turbid, indicating the CO2 evolution in the photocatalytic process. This result directly demonstrated the mineralization of RhB. To determine the intermediates, high 14 / 40


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performance liquid chromatography (HPLC) technology was employed to analyze the RhB solution in the photocatalytic process. As illustrated in Table S1, RhB was first degraded to p-hydroxybenzoic acid (p-HBA), and then was further oxidized to tartaric acid (TA) and oxalic acid (OA). Comparison of photocatalytic activity of various samples was presented in Figure 7d. Though a high photo-absorption of P-GQDs in the range of 410-520 nm, it exhibited very poor photocatalytic performance due to the high recombination rate of photo-generated charges. GQDs and P-GQDs might be formed to a p-n junction photocatalyst due to their different semiconductive behavior. However, the mixture of GQDs/P-GQDs also displayed a poor activity. This was ascribed to their high solubility in water, and the contacted interface could not be established. A similar photocatalytic behavior was observed by the undoped GQDs/CN and the individual CN samples, which was ascribed to the insufficient separation of photo-induced charges. Nevertheless, the photocatalytic performance was distinctly improved by the P-GQDs/CN samples. It might be related to the much more effective transfer of photo-generated carriers in P-GQDs/CN than those in CN and GQDs/CN samples. To better understand the photocatalytic behavior, the reaction kinetics was studied and the result was shown in Figure 7e. The RhB degradation over P-GQDs/CN obeyed the first-order kinetics: -ln(C/C0) = kt, where C0 and C are the concentration at the time zero and the concentration at time t for RhB, k is the degradation reaction rate constant, respectively. The k value of P-GQDs/CN was 0.03025 min-1, which was about 5.9, 126.5 and 3.5 times higher than those of CN, P-GQDs and GQDs/CN, 15 / 40



respectively. Moreover, the P-GQDs/CN showed significantly improved activity for degradation of RhB compared to other photocatalysts (Figure S8). To investigate the reusability, the reacted supernatant was removed after the sedimentation of P-GQDs/CN photocatalyst. The used catalyst was respectively washed with ethanol and water, and then a new RhB solution was added. The successive 4 runs (Figure 7f) implied that this P-GQDs/CN photocatalyst exhibited an extremely high stability and superior reusability, where no obvious decrease was observed at the fourth cycle. Other organic pollutants were also employed to evaluate the photocatalytic universality of P-GQDs/CN (Figure S9). Organic dyes (Methyl violet, Congo red and Reactive violet) and colorless organic pollutant (salicylic acid) could be removed by P-GQDs/CN under visible light irradiation. Besides, the photocatalytic H2 production by P-GQDs/CN was compared with CN, as described in Figure S10. After modification of P-GQDs on CN, the photocatalytic H2 generation activity increased from 80 µmol h-1 g-1 to 108 µmol h-1 g-1. 3.4 Possible photocatalytic mechanism of P-GQDs/CN p-n junction photocatalyst For well understanding the enhanced photocatalytic performance of P-GQDs/CN, we tried to plot the band alignment of the P-GQDs/CN heterojunction. As shown in Figure 8a, the CN was evident n-type that was in accordance with the reported results of graphitic carbon nitride.54 Thus, according to the band gap obtained from DRS result and the Efb value, the CB edge potential of CN was evaluated to be -1.28 eV vs. NHE and its VB maximum was determined to be 1.53 eV vs. NHE. The band alignments of these two heterojunctions were therefore described in Figure S11, 16 / 40


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which apparently presented a straddling style band structure. Due to the straddling style band structure the charge separation in GQDs/CN was insufficient (Figure 8b), which was responsible for the poor photocatalytic activity. The gigantic difference in photocatalytic behavior between GQDs/CN and P-GQDs/CN heterojunctions should result from their different junction style. As presented in Figure S12, the P-GQDs/CN showed a “V-shape” Mott-Schottky curve, demonstrating its p-n configuration.55 When a p-type semiconductor is joined to the n-type semiconductor, holes in the p region trend to diffuse to the n region and electrons likewise transfer from the n region to the p region, which results in the formation a depletion layer on the p-n interface. As a result, bands of the p-type semiconductor would be raised and bands of the n-type semiconductor would be lowered till their Fermi levels were equal. Following the p-n junction model, the band structure of P-GQDs/CN was illustrated in Figure 8c. It was evidently found that the band alignment changed from the straddling style to a staggered type, where the photo-induced electrons on the CB of the p-type P-GQDs would transport to the CB of the n-type CN, and likewise the photo-generated holes on the VB of CN diffused to the VB of P-GQDs. The separated electrons on the CB of CN could further reduce the adsorbed O2 to the strong oxidative ·O2- radical, and the holes accumulated on the VB of P-GQDs had powerful oxidation capability and could directly decompose organic pollutants. Furthermore, they could also oxidize water to produce oxidative hydroxide ·OH radical. Thus, three possible active species presented in the photocatalytic process. 17 / 40



To determine the main active species in photocatalytic process, we compared the photocatalytic activity of this P-GQDs/CN heterojunction with or without addition of various active species quenchers, including the hole scavenger EDTA-2Na, the ·O2quencher BQ and the ·OH scavenger TBA.47 As shown in Figure 9a, compared with the absence of any scavengers, the photocatalytic efficiency was markedly suppressed after the addition of TBA or BQ, but no apparent difference was observed with adding EDTA-2Na. These results suggested that ·OH and ·O2- radicals were the main active species in the photocatalytic process. Detection of hydroxyl radicals and superoxide radicals was carried out by EPR to further confirm the role of the above radicals in the P-GQDs/CN photocatalytic system. A quartet of signals with relative intensities of 1:2:2:1 were related to the DMPO-•OH adducts (Figure 9b), and four strong peaks with relatively similar height in Figure 9c were the characteristic signals of DMPO-•O2- adducts. Furthermore, the EPR signal intensity of DMPO-•OH adducts was much weaker than that of DMPO-•O2- adducts. This suggested that •O2- radicals were dominant in the photocatalytic process, coinciding with the conclusion of the active species trapping experiments. In order to study the transfer and separation of photo-excited carriers, electrochemical impedance spectroscopy (EIS, Nyquist plots) was performed in Figure 9d. The fitted values of Rct (charge transfer resistance) were 38 Ω and 45 Ω for CN and P-GQDs, respectively. Surprisingly, with the incorporation of P-GQDs, although in small amount, the P-GQDs/CN showed smaller arcradius about 28 Ω as 18 / 40


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compared with the above. This indicated a decrease in the solid state interface layer resistance and the charge transfer resistance on the surface. Further, the GQDs/CN also gave a bigger Rct than the P-GQDs/CN, which was consistent with their photocatalytic behavior. Moreover, as plotted in Figure 9e, CN, P-GQDs, P-GQDs/CN and GQDs/CN showed sensitive photocurrent responses during repeated on-off cycles under the visible-light irradiation. For the CN alone, the visible-light photocurrent response was stronger than the P-GQDs alone. After coupling P-GQDs with CN, the photocurrent response was enhanced by 3 times compared with CN alone. This suggested that the P-GQDs played an important role in the improved visible-light-driven photocatalytic performance of the P-GQDs/CN. The π conjugated P-GQDs may act as a photosensitizer, like organic dyes, and the photo-excited electrons transfer from the P-GQDs to CN. In addition, the lower photocurrent response of GQDs/CN than that of P-GQDs/CN demonstrated the higher charge separation rate in P-GQDs/CN. 4. Conclusions In summary, we tuned the electronic property of graphene quantum dots and obtained P-doped GQDs with a p-type semiconductive behaviour. A p-n junction was formed on the interface of P-GQDs and the n-type carbon nitride, which significant enhanced the separation rate of photo-generated electron-hole pairs in comparison to the published undoped GQDs based composites. This metal-free p-n P-GQDs/CN photocatalyst did not only exhibit improved photocatalytic performance, but also possessed high stability and excellent 19 / 40



reusability, good universality for various photo-oxidant reactions and photo-reduction reaction. This work inspires the development of metal-free composited photocatalysts with high charge separation ability. And, it also provides a strategy to control the electronic structure of GQDs by heteroatom doping and modification, which is of importance to GQDs for other optical, electrical, electrochemical and catalytic applications. Associated Content Section Supporting Information A straddling style band alignment of GQDs/CN composites (Figure S1); The light distribution of xenon lamp with a 420 nm cutoff filter and the UV-vis spectrum of the 420 nm cutoff filter (Figure S2); PL emission spectrum of P-GQDs (Figure S3); PL spectra of GQDs and P-GQDs with different excitation wavelengths (Figure S4); Survey XPS spectrum of GQDs (Figure S5); The TOC removal efficiency of P-GQDs/CN (Figure S6); Confirming the CO2 evolution (Figure S7); Comparison of photocatalytic activity of P-GQDs/CN with various photocatalysts (Figure S8); Photocatalytic performance of P-GQDs/CN for decomposition of organic pollutant (Figure S9) and photocatalytic H2 production (Figure S10); Band alignment for GQDs/CN and P-GQDs/CN (Figure S11); the Mott-Schottky curve of P-GQDs/CN. Conflict of interest The authors declare that they have no conflict of interest. Acknowledgements This work was financially supported from the financial support from the 20 / 40


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Scheme 1. The synthetic processes and structural illustrations for GQDs (a) and P-GQDs (b).

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Figure 1. UV-vis absorption spectra, Mott-Schottky curves and band alignments for GQDs (a,c,e) and P-GQDs (b,d,f).

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Figure 2. AFM image of P-GQDs (a) and the height profile (b) of red line in a; TEM image of P-GQDs (c) and the inset is the size distribution; HRTEM image of P-GQDs (d), and the inset is the FFT pattern.

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Figure 3. XPS spectra of P-GQDs: Survey (a), C 1s (b), O 1s (c) and P 2p (d) spectra.

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Figure 4. Structural illustration (a), TEM (b) and HRTEM images (c) of P-GQDs/CN; XRD patterns (d) and FTIR spectra (e) of CN, P-GQDs and P-GQDs/CN.

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Figure 5. XPS spectra of CN and P-GQDs/CN: Survey (a), C 1s (b), N 1s (c) and O 1s (d) spectra.

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Figure 6. (a) UV-vis DRS of CN, P-GQDs and P-GQDs/CN (1 wt %), and the inset of is the plots of (αhν)2 vs. hν; (b) PL spectra of CN, GQDs/CN and P-GQDs/CN (1 wt %).

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Figure 7. (a) Photocatalytic performance of P-GQDs/CN with various P-GQDs contents for removing RhB under visible light irradiation; (b) schematic illustration for the light absorption and charge transfer of P-GQDs/CN with various P-GQDs contents; (c) The spectral change of RhB solution under visible light illumination with the presence of the optimized P-GQDs/CN sample; (d) Comparison for photocatalytic activity and (e) the corresponding pseudo-first-order kinetics data of P-GQDs/CN with individual CN and P-GQDs as well as GQDs/CN; (f) the durability property of P-GQDs/CN for photo-degradation of RhB.

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Figure 8. (a) Mott-Schottky curve of CN; the possible photocatalytic mechanism of GQDs/CN (b) and P-GQDs/CN (c).

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Figure 9. (a) The photocatalytic activity of P-GQDs/CN (1wt %) with the addition of different scavenges; (b) DMPO spin-trapping EPR spectrum in deionized water in the presence of P-GQDs/CN (10 mg) and DMPO (10 mM). (c) DMPO spin-trapping EPR spectrum in purified methanol in the presence of P-GQDs/CN (10 mg) and DMPO (10 mM). (d) EIS curves and (e) transient photocurrent response spectra of CN, P-GQDs, GQDs/CN (1wt %) and P-GQDs/CN (1wt %).

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TOC graphic

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Scheme 1. The synthetic processes and structural illustrations for GQDs (a) and P-GQDs (b). 67x56mm (600 x 600 DPI)



Figure 1. UV-vis absorption spectra, Mott-Schottky curves and band alignments for GQDs (a,c,e) and PGQDs (b,d,f). 96x80mm (300 x 300 DPI)


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Figure 2. AFM image of P-GQDs (a) and the height profile (b) of red line in a; TEM image of P-GQDs (c) and the inset is the size distribution; HRTEM image of P-GQDs (d), and the inset is the FFT pattern. 101x83mm (220 x 220 DPI)



Figure 3. XPS spectra of P-GQDs: Survey (a), C 1s (b), O 1s (c) and P 2p (d) spectra. 94x75mm (300 x 300 DPI)


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Figure 4. Structural illustration (a), TEM (b) and HRTEM images (c) of P-GQDs/CN; XRD patterns (d) and FTIR spectra (e) of CN, P-GQDs and P-GQDs/CN. 81x53mm (300 x 300 DPI)



Figure 5. XPS spectra of CN and P-GQDs/CN: Survey (a), C 1s (b), N 1s (c) and O 1s (d) spectra. 94x72mm (300 x 300 DPI)


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Figure 6. (a) UV-vis DRS of CN, P-GQDs and P-GQDs/CN (1 wt %), and the inset of is the plots of (αhν)2 vs. hν; (b) PL spectra of CN, GQDs/CN and P-GQDs/CN (1 wt %). 46x18mm (300 x 300 DPI)



Figure 7. (a) Photocatalytic performance of P-GQDs/CN with various P-GQDs contents for removing RhB under visible light irradiation; (b) schematic illustration for the light absorption and charge transfer of PGQDs/CN with various P-GQDs contents; (c) The spectral change of RhB solution under visible light illumination with the presence of the optimized P-GQDs/CN sample; (d) Comparison for photocatalytic activity and (e) the corresponding pseudo-first-order kinetics data of P-GQDs/CN with individual CN and PGQDs as well as GQDs/CN; (f) the durability property of P-GQDs/CN for photo-degradation of RhB. 94x63mm (300 x 300 DPI)


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Figure 8. (a) Mott-Schottky curve of CN; the possible photocatalytic mechanism of GQDs/CN (b) and PGQDs/CN (c). 94x110mm (300 x 300 DPI)



Figure 9. (a) The photocatalytic activity of P-GQDs/CN (1wt %) with the addition of different scavenges; (b) DMPO spin-trapping EPR spectrum in deionized water in the presence of P-GQDs/CN (10 mg) and DMPO (10 mM). (c) DMPO spin-trapping EPR spectrum in purified methanol in the presence of P-GQDs/CN (10 mg) and DMPO (10 mM). (d) EIS curves and (e) transient photocurrent response spectra of CN, P-GQDs, GQDs/CN (1wt %) and P-GQDs/CN (1wt %). 92x71mm (300 x 300 DPI)


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Tailoring the Electronic Properties of Graphene Quantum Dots by P

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