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Carbon 99 (2016) 111e117

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Carbon

journal homepage: www.elsevier .com/locate/carbon

N-doped graphitic carbon-incorporated g-C3N4 for remarkablyenhanced photocatalytic H2 evolution under visible light

Yajun Zhou, Lingxia Zhang*, Weimin Huang, Qinglu Kong, Xiangqian Fan, Min Wang,Jianlin Shi**

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, 1295 Ding-xiRoad, Shanghai 200050, China

a r t i c l e i n f o

Article history:Received 8 October 2015Received in revised form1 December 2015Accepted 7 December 2015Available online 10 December 2015

* Corresponding author.** Corresponding author.

E-mail addresses: zhlingxia@mail.sic.ac.cn (L. Zhan

http://dx.doi.org/10.1016/j.carbon.2015.12.0080008-6223/© 2015 Elsevier Ltd. All rights reserved.

a b s t r a c t

Described herein is a facile one-pot strategy to synthesize N-doped graphitic carbon-incorporated g-C3N4

by adding slight amount of citric acid into urea as the precursor during thermal polymerization. Theobtained materials retained the original framework of g-C3N4 and show remarkably enhanced visiblelight harvesting and promoted photo-excited charge carrier separation and transfer. The high-resolutionN 1s spectrum of XPS showed a graphitic N peak, which could be attributed to N-doped graphitic carbon.In addition to the common-recognized light harvesting enhancement and charge carrier recombinationinhibition, the incorporation of N-doped graphitic carbon into the planar framework of g-C3N4 is sug-gested to result in extended and delocalized p-conjugated system of this copolymer, thus greatlyelevating the photocatalytic performance for H2 evolution by water splitting under visible light. The H2

evolution rate on N-doped graphitic carbon-incorporated g-C3N4 reached 64 mmol h�1, which is almost4.3 times the rate on pure g-C3N4. This approach may provide a promising route for rational design ofhigh performance, cost-effective and metal-free photocatalysts.

© 2015 Elsevier Ltd. All rights reserved.

1. Introduction

The ever-increasing demand for clean and renewable energy hasstimulated extensive researches in the photosynthesis of cleanfuels [1]. Semiconductor-based photocatalysis and photo-electrocatalysis hold promise as alternative approaches for solarenergy harvesting and storage [2]. As a novel and promising pho-tocatalyst, graphitic carbon nitride (g-C3N4) has sparked significantexcitement in energy applications ranging from fuel cells [3,4],supercapacitors [5], electrocatalytic water splitting (HER [6], OER[7]), to photocatalysis for solar water splitting and pollutantsphotodegradation [8], etc. Besides, g-C3N4 can be applied as achemosensor for selective optical sensing of metal ions, because itssurface exposed terminal amino groups (-NH2 or ¼ NH groups) canact as Lewis basic sites [9]. Among its various applications, photo-catalysis using g-C3N4 has attracted researchers' great attentiondue to its unique physicochemical properties. As an organic

g), jlshi@mail.sic.ac.cn (J. Shi).

semiconductor consisting of carbon and nitrogen, which are amongthe most abundant elements on the Earth, g-C3N4 is metal-free andsustainable, and can be cheaply obtained from simple precursorslike urea, cyanamide, dicyandiamide, melamine, etc. It is featuredwith 2D conjugated planes, in which tri-s-triazine units periodi-cally repeat themselves, and stacked together through van derWaals interactions. The advantages of g-C3N4, such as high thermaland chemical stability, suitable band gap (~2.7 eV) and simplepreparations, are extremely favorable for practical applications inphotocatalysis [10,11]. However, its known poor visible-light utili-zation and fast charge carrier recombination are the serious limi-tations. Significant progresses have recently been made in thediscovery and design of optimized geC3N4ebased photocatalystsby band engineering [12,13], micro-/nano-structure construction[14,15], bionic synthesis [16], co-catalyst combination [17], surface/interface modification [18], etc.

Recently, researchers aimed to extend the light absorption andelevate the charge separation efficiency by copolymerization withorganic monomers or constructing polymerepolymer composites[19e22]. Manas et al. incorporated a subtle amount of the pyrim-idine moiety into the g-C3N4 layer structure using a strategy ofcombining super molecular aggregation with melted ionic

Y. Zhou et al. / Carbon 99 (2016) 111e117112

polycondensation, and the apparent quantum efficiency (AQE) ofthis photocatalyst reached approximately 7% at 420 nm [19]. Chenet al. adopted a molecular doping strategy to incorporate p-defi-cient pyridine ring entities into the conjugated matrix of g-C3N4 torelocate its p-electrons [20]. Zhang et al. grafted a variety of aro-matic groups on g-C3N4 to extend the delocalization of p-electronsand optimize its photocatalytic performance [21]. Yan constructedcomposite catalysts of g-C3N4 and poly(3-hexylthiophene) andelevated the H2 evolution rate by up to 300 times [22]. Mostrecently, the construction of g-C3N4-based intramolecular donor-acceptor conjugated copolymers has been reported by our group,which showed elevated activity of g-C3N4 for hydrogen evolution[23]. It is obvious that copolymerization modification and con-struction of composite polymer is a smart and efficient way tomodulate the intrinsic electronic property and optimize the cata-lytic performance of g-C3N4. Whereas, we note that all theemployed chemicals containing aromatic heterocycles in abovereports are not environmentally friendly.

Carbon or graphene composited photocatalysts have become ahot interest recently, especially carbon quantum dots (CQDs) orgraphene quantum dots (GQDs) sensitized semiconductors [24,25].Recently, Kang and his coworkers reported a very interesting result.They synthesized carbon nanodots/g-C3N4 photocatalyst withimpressive performance for solar water splitting [26]. Citric acid, aweak organic acid with three carboxyl groups, is one of thecommonly used precursors to synthesize carbon materials throughdehydration and carbonization [27,28]. Qu and his coworkers pre-pared N-doped GQDs using citric acid and urea as carbon precursorand N-containing base, respectively [29]. They believed that citricacid had self-assembled into sheet structure through an intermo-lecular dehydrolysis reaction, and amide formed between eNH2andeCOOHunder the presence of amine, thus N-doped GQDswereobtained. In an earlier report by Schaber et al., urea was found tomelt at about 133 �C during the pyrolysis, accompanying thedecomposition and vaporization [30].

As discussed above, only N-doped GQDs were obtained by hy-drothermal treatments using urea-added citric acid as the carbonprecursors in the above reports, however, we deduce that urea, themost commonly used precursor for g-C3N4 fabrication, would offera liquid and amine-rich environment to react or copolymerize withcitric acid during the process of high temperature treatment, thusoffering an opportunity to yield g-C3N4 -based composite.

Here we report, for the first time as far as we know, a facile one-pot approach of calcining the mixture of urea and small amount ofcitric acid for the synthesis of N-doped graphitic carbon-incorporated g-C3N4 composite, in which citric acid played therole of carbon source and reacted with urea to form N-dopedgraphitic carbon units in g-C3N4 matrix. The as-obtained compos-ites showed remarkably enhanced photocatalytic performance forH2 evolution by water-splitting under visible light.

2. Experimental section

2.1. Chemicals

Urea (AR) was purchased from Sinopharm Chemical ReagentCo.Ltd. (Shanghai, China), and citric acid monohydrate (ACS) wasobtained from Alfa Aesar. All chemicals were used as receivedwithout further treatment.

2.2. Synthesis of photocatalysts

The pure g-C3N4 was prepared by directly heating urea (20 g) at550 �C for 4 h, with a ramp rate of 2 �C min�1 in air. The productdenoted as CN. The modified samples were prepared by mixing

urea (20 g) with different amounts (15 mg, 20 mg, or 25 mg) ofcitric acid monohydrate, subsequently calcining the mixture in acruciblewith a cover at 550 �C for 4 hwith a ramp rate of 2 �Cmin�1

in air. The obtained samples were denoted as CN-x (x ¼ 15, 20, 25,respectively).

2.3. Characterization

X-ray diffraction measurements were collected on a RigakuUltima IV diffractometer (Cu Ka radiation). Fourier transformedinfrared (FTIR) spectra were recorded with a Nicolet iS10 FTIRspectrometer. Transmission electron microscopic (TEM) imagingand selected area electron diffraction (SAED) were performed on aJEOL 200CX electron microscope operated at 200 kV. Nitrogenadsorptionedesorption isotherms at 77 K were measured on aMicromeritics TriStar 3000 instrument. All the samples weredegassed at 150 �C for 8 h under flowing N2 before the measure-ment. Elemental analysis (C, H, N) was performed on a VARIO EL IIImicroanalyzer. X-ray photoelectron spectroscopy (XPS) measure-ment was carried out on a Thermo Scientific ESCALAB 250 spec-trometer with Al Ka radiation as the excitation source. Bindingenergies for the high resolution spectrawere calibrated by setting C1s to 284.6 eV. The UVevis absorption spectra were recorded on aUV-3600 Shimadzu spectroscope. Photoluminescence spectra (PL)of the samples were obtained at room temperature excited byincident light of 370 nm on FluoroMax®4 fluorescence spectrom-eter. PL spectra of CN and CN-20 dispersed in deionized water weremeasured on RF-5301 PC (Shimadzu).

2.4. Photoelectrochemical measurements

Electrochemical measurements were carried out on a CHI 660Delectrochemical workstation (CH Instruments, Shanghai, China)with a standard three-electrode cell, which employed an FTOelectrode deposited with samples as the working electrode, aplatinum sheet as the counter electrode and saturated Ag/AgCl asthe reference electrode. Using a 300W Xe lamp (PLS-SEX300C,Perfectlight Limited, Beijing) with a 420 nm cut-off filter as visiblelight source and 0.2 M Na2SO4 aqueous solution (50 mL) as theelectrolyte. The photocurrent was measured at a bias voltage of0.1 V. The working electrodes were prepared as follows: First, FTOglasses were cleaned by sonication successively with distilled wa-ter, acetone and ethanol for 30 min. Then, 4 mg of sample powderCN, CN-15, CN-20, CN-25 was ultrasonically dispersed in 500uLethanol with 20 uL Nafion solution (5%). One solution withoutsample powder is used for blank experiment. The resultingdispersion was drop-coated onto the FTO side. Then the electrodeswere sealed with epoxy resin except for the 0.25 cm2 sample arealeft for photoexcitation experiments, and then dried in air. EISNyquist plots were obtained with an amplitude of 5 mV over thefrequency range from 105 to 0.01 Hz at a bias voltage of 0 V.

2.5. Photocatalytic water splitting test

The visible light-induced H2 evolutionwas carried out in a Pyrextopeirradiation reaction vessel connected to a closed glass gas-circulation system (Lab-Solar-III AG, Perfectlight Limited, Beijing).A 300 W xenon lamp (CEL-HXF300, Ceaulight, Beijing) with a420 nm cut-off filter was chosen as a visible light source, and thelight intensity was 230 mW cm�2 (tested by FieldMaxII-TO,Coherent). Photocatalyst (50 mg) was suspended in an aqueoussolution (100 mL) containing triethanolamine (10 vol%), and 3 wt%Pt was loaded on the surface of the catalyst by the in situ photo-deposition method using H2PtCl6 as the starting material. Thereactant solutionwas evacuated several times to remove air prior to

Y. Zhou et al. / Carbon 99 (2016) 111e117 113

the irradiation experiment. The temperature of the reaction solu-tion was maintained at 10 �C by a flow of cooling water during thephotocatalytic reaction. The evolved gases were analyzed by gaschromatography (GC7900, Techcomp) equipped with a thermalconductive detector (TCD) and a 5 Å molecular sieve column, usingnitrogen as the carrier gas.

3. Results and discussion

3.1. Structure and morphology characterization

As shown in Fig. 1a of the XRD patterns of as-synthesizedmaterials, two distinct diffraction peaks are found for all thesamples, which can be ascribed to the typical diffraction peaks ofg-C3N4 (JCPDS 87e1526). The stronger peak at around 27.6� rep-resents the (002) inter-planar graphitic stacking with an interlayerdistance of d ¼ 0.322 nm [31]. The other peak at around 12.8�

corresponds to the (100) reflection presenting in-plane structuralpacking motif of tri-s-triazine [32]. The small amount of citric acidaddition led to slight change in the location, intensity and shape ofthese two diffraction peaks, indicating that the samples obtainedby citric acid-assisted calcination well retain the molecularframework of pristine g-C3N4. Furthermore, the FT-IR spectra inFig. S1 confirm this conclusion, all of these samples exhibit severaltypical absorption bands reveal the characteristic structure of g-C3N4. The absorption bands located at 1200-1600 cm�1 relate tothe stretching modes of aromatic CeN heterocycles; The peak atca. 807 cm�1 represents the breathing mode of the triazine units;While the broad band at 3000-3500 cm�1 belongs to the NeHvibration due to the surface uncondensed amine groups [33].However, for these samples CN-x in Fig. 1b, there are two extrapeaks sited at 1557 cm�1 and 1635 cm�1, respectively, enhancedwith increasing addition of citric acid, suggesting the presence of aC]C skeletal vibration band of aromatic domains [34]. Thus, it canbe concluded that this additional carbon species has been copo-lymerized into the molecular structure of g-C3N4, without chang-ing its original framework because of the ultra-low dosage of citricacid.

Fig. 1. (a) XRD patterns and (b) FT-IR spectra (1400-1700 cm�1) of CN and CN-x (x ¼ 15, 2corresponding selected area electron diffraction (SAED) patterns. (A color version of this fig

In Fig. 1c and d of TEM images, both samples exhibit typicalnanosheet structure and CN-20 is more curly at the edge than CN.Their corresponding selected area electron diffraction patterninserted in Fig. 1c and d also confirms their polycrystalline nature.As mentioned above, citric acid can be used to synthesize CQDs orGQDs through a “bottom-up” approach [35], but in the presentstudy no CQDs or GQDs can be found even under high resolutionTEM imaging. Table S1 summarizes the BET specific surface areasand elemental compositions of the obtained samples. It can beinferred that the small amount of citric acid addition can slightlyincrease the specific surface area of g-C3N4, which may be causedby the increased gaseous by-product like CO2, H2O. Elementalanalysis (C, H, N) was performed to investigate the elementalcomposition of the samples. As listed in Table S1, the four samplesexhibit similar C, H, N contents. Based on the above results,including XRD, FT-IR, TEM and elemental analysis, we can concludethat in the resultant CN-x samples no isolated carbon particles havebeen formed and the additional carbon species must have beenincorporated into the network of g-C3N4, without changing itsoriginal planar triazine molecular structure.

In order to further reveal the surface chemical compositions ofthe obtained samples, X-ray photoelectron spectra (XPS) wererecorded. The survey spectra in Fig. S2 demonstrate that the twosamples CN and CN-20 are mainly composed of C and N elements,in line with the results of elemental analysis. The high-resolution C1s spectra are shown in Fig. 2a. CN and CN-20 have similar C 1sspectra with two peaks located at about 284.6 eV and 288.1 eV,respectively. In a previous report, the peak at 284.6 eV has beenattributed to the signal of CeC bonds of graphitic carbon impuritiesin g-C3N4 [36], while where and how these carbon species wereincorporated in g-C3N4 were not clarified. The peak with a bindingenergy of 288.1 eV can be further deconvoluted into two Gaussian-Lorenzian peaks. The main contribution peak at 288.1 eV is attrib-uted to the sp2-hybridized carbon bonded to N atom inside thetriazine rings, while the minor peak at 288.8 eV is assigned to thesp2-hybridized carbon in the triazine ring bonded to the aminogroup [37]. It is worth noting that the two peaks at 284.6 eV (5.89%)and 288.8 eV (13.09%) of sample CN-20 are much stronger

0, 25) samples. Typical TEM images of (c) CN-20 and (d) CN. The inset images are theure can be viewed online.)

Fig. 2. XPS spectra of CN and CN-20, (a) high-resolution C 1s spectra, (b) high-resolution N 1s spectra. (A color version of this figure can be viewed online.)

Y. Zhou et al. / Carbon 99 (2016) 111e117114

compared to those (284.6 eV: 4.38%, 288.8 eV: 5.71%) of CN. Thehigh-resolution N 1s spectrum of CN-20 is a little different fromthat of CN as shown in Fig. 2b. The N 1s spectrum of CN can bemainly deconvoluted into four peaks with binding energies atabout 398.6 eV, 399.6 eV, 400.7 eV and 404.4 eV. The dominantpeak at 398.6 eV corresponds to the sp2-hybridized nitrogen in C-containing triazine rings (CeN]C), whereas the peak located at399.6 eV is usually attributed to the bridging N atoms in N-(C)3groups. The peak at 400.7 eV indicates the amino groups (NeH),and the peak at 404.4 eV is attributed to charging effects [38]. Insample CN-20, an extra peak located at 401 eV, which correspondsto the graphitic N (Fig. S3) [39], can be observed, indicating that N-doped graphitic carbon has been formed and introduced into the g-C3N4 matrix. This can be further confirmed by the enhancedgraphitic CeC signal at 284.6 eV (5.89%) in C 1s spectrum of CN-20.TG curves obtained during the thermal polymerization of urea andurea/citric acid mixture are shown in Fig. S4. It can be observed thatthe addition of citric acid leads to the less weight loss above themelting point (~132.7 �C) of urea, implying that reaction/conden-sation has taken place between citric acid and urea. Consideringthat carboxyl groups of citric acid can react with amine groups ofurea, the copolymerization between citric acid and urea will takeplace during the calcination process to form N-doped graphiticcarbon, which could be dangled onto the surface or inserted in tri-s-triazine network of g-C3N4 matrix (Fig. 3). However, because ofthe ultra-low dosage of citric acid and the similar properties of g-C3N4 and N-doped graphitic carbon, it's hard to distinguish N-doped graphitic carbon from the g-C3N4 matrix based on XRD,Raman spectra (Fig. S5) and even NMR [26,40].

Fig. 3. Scheme of two probable existence forms of N-doped graphitic carbon in g-C3N4: (a) dangled on the layer surface of g-C3N4, (b) inserted into the g-C3N4 network.(A color version of this figure can be viewed online.)

3.2. Optical properties and electronic band structures

The optical properties of the as-prepared samples were quali-tatively probed by UVevis diffuse reflectance absorption andphotoluminescence (PL) spectroscopy, as shown in Fig. 4. TheUVevis diffuse reflectance absorption spectra in Fig. 4a indicatethat the intrinsic absorption edge (that is, band gap) of CN-x red-

shifts a little in comparison to that of CN. At the same time, theabsorption spectra of CN-x samples extend to the whole visible

Fig. 4. (a) UVevis diffuse reflectance absorption spectra (inserted photograph), (b) PL spectra of CN and CN-x (x ¼ 15, 20, 25) samples. PL spectra of (c) CN-20 and (d) CN dispersedin deionized water at different excitation wavelengths. (A color version of this figure can be viewed online.)

Y. Zhou et al. / Carbon 99 (2016) 111e117 115

light region, namely, their light absorbance has been remarkablyenhanced. The photograph inserted in Fig. 4a depicts the colorchange of the photocatalysts from light yellow to brown yellow (inthe web version) with the increase of citric acid addition, corre-sponding to their gradually expanded absorption band andenhanced visible light harvesting. For CN-x samples, N-dopedgraphitic carbon was formed in g-C3N4 matrix, and the lone pairelectron on graphitic N may trigger the p-electron delocalization inthis conjugated system, which is most probably responsible fortheir enhanced photoabsorption and photoresponsivity [41].

The photoluminescence (PL) spectra were obtained by moni-toring at an excitation wavelength of 370 nm, and are shown inFig. 4b. The pure sample CN shows a strong emission peak at ca.460 nm, while an apparent fluorescence quenching is observed forCN-x, indicating the suppressed recombination among photo-generated charge carriers in CN-x. Simultaneously, the emissionpeaks of CN-x shifts toward longer wavelength compared to that ofCN (from 450 nm to 470 nm). The red shift of both the band gap andPL peak of CN-x (x¼ 15, 20, 25) may be ascribed to the incorporatedgraphitic N, which could promote the p-electron delocalization ofg-C3N4 network.

We further measured the PL spectra of CN and CN-20 dispersedin deionized water at several different excitation wavelengths ac-cording to the UVevis absorption spectra. In Fig. S7, the typicalabsorption peak located at 325 nm can be attributed to the intrinsicabsorption of g-C3N4 [42], and the absorption shoulder peak atabout 250 nm can be assigned to the p-p* transition [43]. Fig. 4cand d shows the PL emission spectra of CN and CN-20. The emissionpeaks of the two samples are excitation-independent. Comparedwith the PL peak at 440 nm of CN in Fig. 4d, the emission peak at470 nm of CN-20 (Fig. 4c), which corresponds to the bandebandtransition emission (HOMOeLUMO transition), shows a large redshift, which is in accordance with the above characterizations. Two

peaks at 360 nm and 400 nm become standout when the excitationwavelength moves to deep ultraviolet light (240e270 nm), indi-cating that the more electrons must have been excited to higherorbital or inner shell electrons have been excited [44]. Associatedwith the absorption peak at about 250 nm, which is attributed tothe p-p* transition, excitation of 250 nm can result in the strongestemission peak at 360 nm, so we infer that 250 nm is the bestexcitationwavelength. Meanwhile, considering that the energy gapof p-p* transition is larger than that of n-p* transition, we concludethat the PL peaks at 360 nm and 400 nm can be ascribed to theradiative recombination of p-p* and n-p* transition, respectively.Furthermore, the emission peak at 360 nm of CN-20 is largelyenhanced compared to that of CN especially under the excitation ataround 250 nm, which can be attributed to the extended delocal-ization of p-electrons by the incorporation of N-doped graphiticcarbon, and consequently the increased probability of p-p* tran-sition and radiative recombination as well. Such a phenomenon isin consistence with the increased transient photocurrent of thecomposite compared to pristine CN.

3.3. Photocatalytic activity and catalytic mechanism

H2-evolution by visible light-induced water splitting was car-ried out to evaluate the photocatalytic performance of these sam-ples. 3 wt% Pt and 10 vol% triethanolamine were used as co-catalystand hole sacrificial agent, respectively, under visible light irradia-tion (l > 420 nm). As illustrated in Fig. 5a, CN-x samples showremarkably enhanced hydrogen evolution performance. Theaverage H2 evolution rate of CN-20 reaches 64 umol h�1, which isalmost 4.3 times that of CN (15 umol h�1). The stability of CN-20was also performed by repeating the photocatalytic experimentunder the same condition for four cycles (24 h). As shown in Fig. 5b,there is no significant decrease in the H2 evolution rate after four

Fig. 5. (a) Photocatalytic H2 evolution performances on CN and CN-x, (x ¼ 15, 20, 25). (b) Recycling behavior of H2 evolution on CN-20. (c) Transient photocurrents of CN and CN-x,(x ¼ 15, 20, 25) under cycling illuminations. (d) EIS Nyquist plots of CN and CN-20 in the dark. (A color version of this figure can be viewed online.)

Y. Zhou et al. / Carbon 99 (2016) 111e117116

cycles, confirming the high stability of this g-C3N4modified with N-doped graphitic carbon in photocatalytic reaction. CN-20 beforeand after the recycling experiment of H2 evolutionwere checked byFT-IR spectra (Fig. S8). No spectroscopic change can be detected,indicating its excellent chemical stability during the photocatalysisprocess.

The transient photocurrent responses of the as-obtained sam-ples were measured to study the excitation and transfer of photo-generated charge carriers for several oneoff cycles under visible-light irradiation, which are plotted in Fig. 5c. All the samplesexhibit prompt photocurrent responses on each illumination. Asexpected, CN-x samples show much enhanced photocurrentresponse, and CN-20 possesses the highest photocurrent intensity,whereas CN displays the lowest value among all the samples,indicating that CN-20 is the most effective in charge separation,which is in line with its highest photocatalytic activity. The elec-trochemical impedance spectra (EIS) were recorded to investigatethe (photo)electrochemical properties of CN and CN-20. The EISNyquist plots of CN and CN-20 electrodes in a 0.2 M Na2SO4aqueous solution in the dark are shown in Fig. 5d. Both of theNyquist plots exhibit semicircle-like curve and the diameters areused to assess the impedance. Much smaller semicircle can beobserved for CN-20, reflecting that the incorporation of N-dopedgraphitic carbon can efficiently reduce the charge transfer resis-tance at the material surface/interface [21]. This suggests a stronglyimproved electronic conductivity in the non-photoexcited stateand would be very beneficial for photoexcited charge separationduring photocatalysis. Both photocurrent and EIS experimentsprove the incorporation of N-doped graphitic carbon can obviouslyfacilitate charge carrier separation and transfer in thesephotocatalysts.

Based on the above characterizations and discussions, this N-doped graphitic carbon incorporated with g-C3N4 demonstrate

similar surface area and morphology to the pristine. More impor-tantly, the lone pair electron on the graphitic N atom leads toeffectively extended and delocalized aromatic p-conjugated sys-tem, which has been confirmed by the strengthened PL emissionpeak at 360 nm and 400 nm due to the enhanced radiativerecombination of p-p* and n-p* transition by the incorporation ofthe N-doped graphitic carbon.

4. Conclusions

In conclusion, N-doped graphitic carbon-incorporated g-C3N4

photocatalysts have been successfully obtained through a facileone-pot thermo-induced polymerization strategy by introducingslight amount of citric acid into urea. The N atom doping ingraphitic carbon of the composite is believed to be responsible forextended and delocalized aromatic p-conjugated system of g-C3N4and consequently much promoted separation and transfer ofcharge carriers, which, together with the improved visible lightharvesting and inhibited charge carrier recombination, resulted inremarkably enhanced photocatalytic H2 evolution activity by watersplitting under visible light. The H2 evolution rate on an optimizedN-doped graphitic carbon-incorporated g-C3N4 photocatalyst (CN-20) reached 64 mmol h�1, which is almost 3.3 times higher than thatof the pristine g-C3N4. This work may offer a new promising routefor the rational design and synthesis of high performance metal-free photocatalysts from simple, inexpensive and environmentalfriendly precursors.

Acknowledgments

This work was financially supported by the National Key BasicResearch Program of China (2013CB933200), National 863 plansprojects (2012AA062703), National Natural Science Foundation of

Y. Zhou et al. / Carbon 99 (2016) 111e117 117

China (21177137), and Youth Innovation Promotion Association ofCAS (2012200).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.carbon.2015.12.008.

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