A simple process to prepare few-layer g-C3N4 nanosheets with enhanced photocatalytic activities

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A simple process to prepare few-layer g-C3 N4 nanosheets with enhanced photocatalytic activities Yongning Ma a , Enzhou Liu a , Xiaoyun Hu b , Chunni Tang a , Jun Wan a , Juan Li a , Jun Fan a,∗ a b

School of Chemical Engineering, Northwest University, Xi’an 710069, PR China School of Physics, Northwest University, Xi’an 710069, PR China

a r t i c l e

i n f o

Article history: Received 20 June 2015 Received in revised form 20 August 2015 Accepted 21 August 2015 Available online xxx Keywords: Layer materials Nanosheets Carbon nitride Chemical exfoliation Photocatalysis

a b s t r a c t Graphitic carbon nitride (g-C3 N4 ) nanosheets with few-layer and single-layer structures were prepared by a simple chemical exfoliation method. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier-transform infrared spectroscopy, UV–vis absorption spectroscopy and photoluminescence spectroscopy, respectively. The investigations indicated that the bulk g-C3 N4 can be easily exfoliated into g-C3 N4 nanosheets with few-layer structure under the assistance of HNO3 . The specific surface area of few-layer g-C3 N4 reached 179.5 m2 g−1 , which was about 10.3 times higher than that of bulk g-C3 N4 (17.4 m2 g−1 ). The absorption edge of few-layer g-C3 N4 nanosheets showed a blue shift from 486 nm to 458 nm, corresponding to an increase in the band gap from 2.55 eV to 2.70 eV compared with that of bulk g-C3 N4 . In addition, the position of the emission peak of few-layer g-C3 N4 was blue shifted from 461 nm to 439 nm. The photocatalytic performance of samples was evaluated by the degradation of methylene blue and the photocatalytic water splitting under visible light irradiation. The experimental results indicated that the degradation rate of MB on few-layer g-C3 N4 nanosheets was about 1.3 times higher than that of bulk g-C3 N4 , the H2 production rate reached 110.68 ␮mol g−1 h−1 over g-C3 N4 nanosheets, which was 1.22 times higher than that of bulk g-C3 N4 . © 2015 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen has been identified as a renewable clean energy carrier due to its high-energy capacity and environmental friendliness, which can effectively address the problems of growing energy needs and environmental pollution from fossil energy utilization [1–3]. Photocatalytic water splitting is a promising way to obtain the hydrogen energy, and the preparation of high-efficiency photocatalyst is a challenge for the most of the photocatalyst researchers [4–7]. Recently, two-dimensional (2D) nanosheets showed a very good performance in photocatalytic water splitting to product hydrogen under visible light irradiation. 2D nanosheets, made of single atomic layers, have attracted tremendous attention owing to their unique properties and potential applications in the areas of electronics, sensors, catalysts and energy storage [8–13]. Layer g-C3 N4 nanosheets with high specific surface area exhibit many intriguing properties different from bulk g-C3 N4 for a wide range of potential applications, especially the applications in photocatalytic water splitting and photoelectric conversion

∗ Corresponding author. E-mail address: [email protected] (J. Fan).

[14–17]. Many strategies have been developed to fabricate g-C3 N4 nanosheets with few-layer structures, especially nanosheet with a single atomic layer [18,19]. Typically, few-layer g-C3 N4 nanosheets can be synthesized via various methods [20], including Hummers method [21], mechanical exfoliation method [22], thermal exfoliation method [23], electrochemical exfoliation method [24] and supercritical fluid exfoliation method [25]. In this work, we have successfully prepared g-C3 N4 nanosheets with few-layer and single layer structures by a simple chemical exfoliation process employing bulk g-C3 N4 as precursor. The photocatalytic performance of samples was evaluated by degradation of methylene blue (MB) and photocatalytic water splitting under visible light irradiation, the mechanism of chemical exfoliation process of bulk g-C3 N4 to few-layer g-C3 N4 and the mechanism of photocatalytic splitting water were proposed. 2. Experimental 2.1. Preparation of bulk g-C3 N4 Bulk g-C3 N4 was prepared by directly heating 3.0 g melamine powder (Tianjin Fuchen Chemical Reagents factory, China) in a muffle furnace for 3 h at 600 ◦ C [28].

http://dx.doi.org/10.1016/j.apsusc.2015.08.174 0169-4332/© 2015 Elsevier B.V. All rights reserved.

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Fig. 1. (a) SEM image of prepared bulk g-C3 N4 . (b) SEM image of prepared few-layer g-C3 N4 . (c) HRTEM image of the prepared few-layer g-C3 N4 . (d) SAED image of the corresponding few-layer g-C3 N4 .

2.2. Preparation of few-layer g-C3 N4

2.3. Characterization of the samples

The as prepared bulk g-C3 N4 (0.5 g) was mixed with 30 mL HNO3 (65 wt%) in a 100 mL flask and stirred for 8 h at room temperature, Then the mixture was slowly poured into 200 mL of deionized water and sonicated for exfoliation and control the temperature of water between 60 and 80 ◦ C. The suspension was centrifugated at 8000 rpm to remove any unexfoliated g-C3 N4 , and then the suspension was centrifuged and washed with deionized water and ethanol to remove any residual acid. Finally, the suspension was dried at 80 ◦ C to obtain few-layer g-C3 N4 nanosheets.

The crystalline phases and morphologies of the samples were characterized by Shimadzu XRD-6000 powder diffractometer and scanning electron microscopy (SEM, JEOL JSM-6390A) and transmission electron microscopy (TEM) images were obtained by Tecnai G2 F20S-TWIN electron microscope operated at an accelerating voltage of 100 kV. The UV–Vis diffuse reflectance spectra were obtained on a Shimadzu UV-3600 UV/vis/NIR spectrophotometer with an integrating sphere detector, and BaSO4 was used as the reflectance standard material. Photoluminescence (PL) spectra were investigated on Hitachi F-7000 florescence

Bulk g-C3N4

Intensity

Few-layer g-C3N4

4000

Fig. 2. XRD diffraction of g-C3 N4 .

3500

3000

2500

2000

1500

Wavenumber (nm-1)

1000

500

Fig. 3. FTIR spectrum of g-C3 N4 .

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a

1.4

a

1.00

Bulk g-C3N4

3

Few-layer g-C3N4

1.2

Catalyst-free

0.95 Percentage of MB

Intensity

1.0 0.8 2.7eV

0.6 2.55eV

0.4 0.2 0.0 200

400

600

0.90 0.85

Bulk g-C3N4

0.80 Few-layer g-C3N4

0.75 0.70

800

Wavelength (nm)

0

b

1

2 3 Irradition time (h)

4

5

Bulk g-C3N4 Few-layer g-C3N4

450

500

550

600

Wavelength (nm) 180

Volume Adsorbted (cm3/g)

160

C

80 60

Few-layer g-C3N4

40 Bulk g-C3N4

20 0 0

140 120

Few-layer g-C3N4

100

Bulk g-C3N4

2

4

6 8 10 12 Irradition time (0.5h)

14

16

Fig. 5. (a) Photocatalytic degrade MB of g-C3 N4 , (b) Photocatalytic water splitting for hydrogen production of g-C3 N4 .

80

2.5. Photocatalytic water splitting

60 40 20 0 0.0

Amount of Hydrogen (umol/0.1g)

Intensity 400

b

461nm

439nm

0.2

0.4 0.6 Relative Pressure (p/p0)

0.8

1.0

Fig. 4. (a) UV–vis absorption spectrum of g-C3 N4 . (b) PL spectrum of g-C3 N4 . (c) N2 adsorption–desorption isotherms of the samples.

spectrophotometer. Fourier transformed infrared (FTIR) spectra were recorded on a Bruker VERTEX 700 spectrometer. The Brunauer–Emmett–Teller (BET) specific surface area characterized by NOVA 2000e.

Photocatalytic water splitting to generate hydrogen was carried out in photoreactor. The visible light source was A Xenon lamp equipped with a 400-nm cut-off optical filter, while the UV light source was a high pressure Hg lamp with the wavelength of 254 nm. Both of the lamps were positioned at the same side out of the reactor. The 0.1 g sample was fixed in the reactor containing 100 mL 10 vol% ethanol/H2 O solution and faced to the lamps. The system was deaerated by bubbling high-purity nitrogen for 30 min, and then illuminated for 7 h under magnetic stirring. A needle-type probe was inserted into the reactor to withdraw generated gas in 30 min intervals. An external standard in the same concentration range was used to determine the photocatalytic activity of the samples [29–31]. 3. Results and discussion

2.4. Photodegradation of methylene blue

3.1. Morphology and microstructure of the few-layer g-C3 N4

Photocatalytic activity of the sample was evaluated by the degradation of methylene blue (MB) under 300 W Xe lamp with a 400 nm cutoff filter. 0.1 g of sample was added into 250 mL MB (5 mg L−1 ) in a Pyrex photocatalytic reactor. The sample was kept in the dark for 3 h prior to irradiation for establishing adsorption desorption equilibrium. At different times, the absorbance of MB solution was determined using a Shimadzu UV-3600 spectrophotometer.

Fig. 1 showed the morphology and microstructure of bulk gC3 N4 and few-layer g-C3 N4 . The SEM image of bulk g-C3 N4 in Fig. 1a and few-layer g-C3 N4 in Fig. 1b, It is clear that bulk g-C3 N4 can be exfoliated to nanosheets with ultrathin structure, and the size of few-layer g-C3 N4 was about 4–10 ␮m. Fig. 1c and d showed the representative HRTEM and SAED images of the few-layer g-C3 N4 , It can be observed from the HRTEM that the distance of lattice fringe with diameters of 0.34–0.35 nm are uniformly distributed on the

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Fig. 6. Preparation mechanism of few-layer g-C3 N4 .

surface of few-layer g-C3 N4 . The darker part in the TEM image can be attributed to the overlap of several few-layer g-C3 N4 [21]. In addition, selected area electron diffraction (SAED) of the few-layer g-C3 N4 indicated that the structure of few-layer g-C3 N4 was six atoms distributes in each angle arranged. 3.2. Structural and spectroscopy characterization Fig. 2 showed the typical (0 0 2) interlayer-stacking peak at 27.5◦ corresponded to an interlayer distance of d = 0.326 nm for g-C3 N4 and the other is the week peak at 13.1◦ corresponding to the (1 0 0) plane, which is related to the in-plane structural packing motif of tri-s-triazine units [32–34]. Obviously, after the exfoliation of the bulk g-C3 N4 , the (0 0 2) peak intensity of few-layer g-C3 N4 was decreased significantly which indicated the interlayer structure was destroyed after exfoliation [35,36]. Meanwhile, the chemical structures of the samples were further analyzed by FTIR and the results were showed in Fig. 3. The strong FTIR absorption (Fig. 3) bands of few-layer g-C3 N4 revealed a typically molecular structure of g-C3 N4 . The FT-IR spectrum of the few-layer g-C3 N4 was similar to the bulk g-C3 N4 , indicated that the few-layer g-C3 N4 keeps the same chemical structure with the bulk g-C3 N4 [37]. The broad peaks between 3750 and 3000 cm−1 are caused by the N–H stretches [38], the absorption bands of few-layer g-C3 N4 was broaden slightly than bulk g-C3 N4 , which can be owned to the interlayer spacing broaden. The absorption bands near 1572 and 1632 cm−1 are attributed to C N stretching, while the three bands at 1253, 1320 and 1425 cm−1 correspond to aromatic C N stretching. The peak at 807 cm−1 belongs to triazine ring mode [39], which corresponds to condensed C,N

heterocycles, that showed the plane structure of few-layer g-C3 N4 was similar to bulk g-C3 N4 . The UV–Vis absorption (Fig. 4a) edge of few-layer g-C3 N4 nanosheets showed a blue shift from 486 nm to 458 nm, corresponding to an increased band gap from 2.55 eV to 2.70 eV compared with that of bulk g-C3 N4 . In addition, the PL spectrum (Fig. 4b) of the few-layer g-C3 N4 appeared slightly blue shifted by about 20 nm compared with the bulk g-C3 N4 , which was shifted from 461 nm to 439 nm, this blue shift performance can be presumably ascribed to the decrease of conjugation length and the strong quantum confinement effect due to the ultrathin structure of few-layer g-C3 N4 [40]. The shape of the curve for few-layer g-C3 N4 was similar to that of bulk g-C3 N4 , whereas the peak intensity decreased, considering that the PL emission results from the free charge carrier recombination, the decreased peak intensity indicates that few-layer g-C3 N4 is helpful for the separation of photogenerated electron–hole pairs [24]. The N2 adsorption-desorption equilibrium curve (Fig. 4c) indicated that the specific surface area of few-layer g-C3 N4 reached 179.5 m2 g−1 , which was about 10.3 times higher than that of bulk g-C3 N4 (17.4 m2 g−1 ). The quantum confinement effect and the increased specific surface area of few-layer g-C3 N4 proved the interlayer distance of bulk g-C3 N4 was expanded [41], in other words, the bulk g-C3 N4 was exfoliated into few-layer g-C3 N4 as we expected. 3.3. Improved photocatalytic activities of few-layer g-C3 N4 nanosheets The photocatalytic activities of few-layer g-C3 N4 nanosheets were estimated by degradation of methylene blue (MB) (Fig. 5a) and

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Fig. 7. Hydrogen production mechanism of few-layer g-C3 N4 .

photocatalytic water splitting to product hydrogen (Fig. 5b) under visible light irradiation. The experimental results indicate that the degradation rate of MB on few-layer g-C3 N4 nanosheets was about 1.3 times higher than that on bulk g-C3 N4 . The H2 production rate of few-layer g-C3 N4 nanosheets reached 110.68 ␮mol g−1 h−1 over, which was 1.22 times higher than that of bulk g-C3 N4 [42]. Based on what has been observed and discussed above, it is reasonable to conclude that the enhanced photocatalytic activity could be attributed to the improving charge transfer property and higher special surface area of few-layer g-C3 N4 . 3.4. Preparation mechanism of few-layer g-C3 N4 nanosheets Fig. 6 illustrated the synthetic strategy of few-layer g-C3 N4 nanosheets. Layer molecular sheets of C, N materials have many similar properties with graphene [37]. Firstly, the polymers are linked by N or NH species as in melon, however, it may also contain triazine rings derived from the melamine precursor [43]. The NH2 was separated from melamine through the heat treatment, and then the separated component condensation to a polymerization monomer structure [26]. At the same time, the polymerization monomer was heap up to bulk g-C3 N4 because the temperature of muffle furnace cannot be cold down immediately. HNO3 was the best candidate to exfoliate the bulk g-C3 N4 to fewlayer g-C3 N4 , which can be owned to the proton of HNO3 reduced the electron cloud density between the interlayer of C and N atoms [44], that lead to the Van der Waals’ force decrease of the interlayer. Accordingly, the bulk g-C3 N4 exfoliated to few-layer g-C3 N4 through ultrasonic easily [45–47]. 3.5. Mechanism of few-layer g-C3 N4 photocatalytic water splitting to product hydrogen The visible light response of g-C3 N4 originates from electron transition from the valence band (VB) populated by N2p orbitals to the conduction band (CB) formed by C2p orbitals [48]. Moreover, besides photogenerated holes, the presence of hydrogen protons (H+ ), hydroxyl radical (• OH) and acetic acid (CH3 COOH) has been evidenced in the visible light-responsive catalytic with g-C3 N4 [49,50]. Thus, the photocatalytic water splitting to product hydrogen mechanism over g-C3 N4 is shown in Fig. 7, the electrons can

transfer from the VB to the CB by absorbing photons, and then the CB electrons (e− ) and VB holes (h+ ) emerge after transitions, they can migrate toward the g-C3 N4 surface. The holes can be consumed by reacting with water molecules to form H+ and • OH. H+ can receive an electron from the conduction band to form an H atom. Finally, two H atoms combine with each other to form a H2 molecule. To facilitate both the reduction and oxidation by photoexcited electrons and holes, ethanol is used as the sacrificial agent in our work. • OH can be consumed through a chemical reaction with CH3 CH2 OH. At last, we accelerated the decomposition of H2 O to H+ and • OH, increasing the rate of H2 evolution [1,27,50,51]. Hydrogen production rate of few-layer g-C3 N4 was higher than that of bulk g-C3 N4 can be explained as the few-layer g-C3 N4 have bigger specific surface area and stronger adsorption ability. g − C3 N4 + hv → e− + h+

(1)

h+ + H2 O → OH + H+

(2)

• OH + CH CH OH 3 2

→ CH3 COOH + H+

(3)

H+ + e− → H

(4)

2H → H2

(5)

4. Conclusion In summary, this work successfully developed a general method for environment friendly to exfoliate bulk g-C3 N4 to few-layer gC3 N4 . The characterization revealed that the obtained few-layer g-C3 N4 was ultrathin and the distance of lattice fringe with diameters of 0.34–0.35 nm are uniformly distributed on the surface of few-layer g-C3 N4 , moreover, some of them was arranged in single atomic. Photocatalytic activities showed an increased performance in photocatalytic degradation MB and water decomposition hydrogen production, The experimental results indicated that the degradation rate of MB on few-layer g-C3 N4 nanosheets was about 1.3 times higher than that of bulk g-C3 N4 , the H2 production rate reached 110.68 ␮mol g−1 h−1 over g-C3 N4 nanosheets, which was 1.22 times higher than that of bulk g-C3 N4 .

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Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21476183, 21306150 and 51372201), the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20136101110009), the Shaanxi Provincial Research Foundation for Basic Research, China (No. 2015JM5159). References [1] T. Sun, E.Z. Liu, J. Fan, X.Y. Hu, F. Wu, W.Q. Hou, Y.H. Yang, L.M. Kang, High photocatalytic activity of hydrogen production from water over Fe doped and Ag deposited anatase TiO2 catalyst synthesized by solvothermal method, Chem. Eng. J. 228 (2013) 896–906. [2] T. Chen, L. Qiu, Z. Yang, An integrated “Energy Wire” for both photoelectric conversion and energy storage, Angew. Chem. 124 (2012) 12143–12146. [3] P. Luo, H. Niu, G. Zheng, Z.B. Cai, J. Ren, H.P. Li, H.J. Lin, X.M. Sun, H.S. Peng, Enhancement of photoelectric conversion by high-voltage electric field assisted crystallization of a novel ternary-encapsulated spherical TiO2 aggregate for solar cells, Electrochim. Acta 55 (2010) 2697–2705. [4] T.X. Wei, Y.R. Shi, J. Zhai, L.B. Gan, C.H. Huang, T.T. Liu, L.M. Ying, G.B. Luo, X.S. Zhao, Synthesis monolayer fabrication and photoelectric conversion property of two pyrrolidine fullerene carboxylic acid derivatives, Chem. Phys. Lett. 319 (2000) 7–12. [5] Z.K. Zhao, Y.T. Dai, G.F. Ge, Q. Mao, Z.M. Rong, G.R. Wang, A facile approach to fabricate an N-doped mesoporous graphene/nanodiamond hybrid nanocomposite with synergistically enhanced catalysis, ChemCatChem 7 (2015) 1070–1077. [6] A. Mathew, T. Pradeep, Noble metal clusters: applications in energy, environment, and biology, Part. Part. Syst. Char. 31 (2014) 1017–1053. [7] K.J. Naidoo, S.J. Hughes, J.R. Moss, Computational investigations into the potential use of poly(benzyl phenyl ether) dendrimers as supports for organometallic catalysts, Macromolecules 32 (1999) 331–341. [8] Y.H. Yang, E.Z. Liu, H.Z. Dai, L.M. Kang, H.T. Wu, J. Fan, X.Y. Hu, H.C. Liu, Photocatalytic activity of Ag–TiO2 -graphene ternary nanocomposites and application in hydrogen evolution by water splitting, Int. J. Hydrogen Energy 39 (2014) 7664–7671. [9] W.L. Zhang, B.J. Park, H.J. Choi, Colloidal graphene oxide/polyaniline nanocomposite and its electrorheology, Chem. Commun. 30 (2010) 5596–5598. [10] H.T. Wu, J. Fan, E.Z. Liu, X.Y. Hu, Y.N. Ma, X. Fan, Y.Y. Li, C.N. Tang, Facile hydrothermal synthesis of TiO2 nanospindles-reduced graphene oxide composite with a enhanced photocatalytic activity, J. Alloys Compd. 623 (2015) 298–303. [11] M. Guglielmi, P. Colombo, V. Rigato, A. Boscolo-Boscoletto, A. Debattisti, Compositional and microstructural characterization of RuO2 –TiO2 catalysts synthesized by the sol–gel method, J. Electrochem. Soc. 139 (1992) 1655–1661. [12] X. Fan, J. Wan, E.Z. Liu, L. Sun, Y. Hu, H. Li, X.Y. Hu, J. Fan, High-efficiency photoelectrocatalytic hydrogen generation enabled by Ag deposited and Ce doped TiO2 nanotube arrays, Ceram. Int. 41 (2015) 5107–5116. [13] S. Tonda, S. Kumar, S. Kandula, V. Shanker, Fe-doped and mediated graphitic carbon nitride nanosheets for enhanced photocatalytic performance under natural sunlight, J. Mater. Chem. A 2 (2014) 6772–6780. [14] J. Xu, L. Zhang, R. Shi, Y.F. Zhou, Chemical exfoliation of graphitic carbon nitride for efficient heterogeneous photocatalysis, J. Mater. Chem. A 1 (2013) 14766–14772. [15] X. She, H. Xu, Y. Xu, J. Yan, J.X. Xia, L. Xu, Y.H. Song, Y. Jiang, Q. Zhang, H.M. Li, Exfoliated graphene-like carbon nitride in organic solvents: enhanced photocatalytic activity and highly selective and sensitive sensor for the detection of trace amounts of Cu2+ , J. Mater. Chem. A 2 (2014) 2563–2570. [16] P. Zhou, J.H. Wu, W.L. Yu, G.H. Zhao, G.J. Fang, S.W. Cao, Vectorial doping-promoting charge transfer in anatase TiO2 {0 0 1} surface, Appl. Surf. Sci. 319 (2014) 167–172. [17] S.A.A. Kamal, R. Ritikos, S.A. Rahman, Wetting behaviour of carbon nitride nanostructures grown by plasma enhanced chemical vapour deposition technique, Appl. Surf. Sci. (2015) 146–153. [18] P. Zhang, Y. Wang, J. Yao, C.M. Wang, C. Yan, M. Antonietti, H.R. Li, Visible-light-induced metal-free allylic oxidation utilizing a coupled photocatalytic system of g-C3 N4 and N-hydroxy compounds, Adv. Synth. Catal. 353 (2011) 1447–1451. [19] Y. Shiraishi, H. Hirakawa, Y. Togawa, H. Takayuki, Noble-metal-free deoxygenation of epoxides titanium dioxide as a photocatalytically regenerable electron-transfer catalyst, ACS Catal. 4 (2014) 1642–1649. [20] Y. Xu, W. Hong, H. Bai, C. Li, G.Q. Shi, Strong and ductile poly(vinyl alcohol)/graphene oxide composite films with a layered structure, Carbon 47 (2009) 3538–3543. [21] Y.J. Zhong, Z.Q. Wang, J.Y. Feng, S.C. Yan, H.T. Zhang, Z.S. Li, Z.G. Zou, Improvement in photocatalytic H2 evolution over g-C3 N4 prepared from protonated melamine, Appl. Surf. Sci. 295 (2014) 253–259. [22] P.K. Ang, W. Shuai, Q.L. Bao, T.L. John, K.P. Loh, High-throughput synthesis of graphene by intercalation-exfoliation of graphite oxide and study of ionic screening in graphene transistor, ACS Nano 3 (2009) 3587–3594.

[23] S.Z. Hu, M. Lin, J.G. You, F.Y. Li, Z.P. Fan, G. Lu, D. Liu, J.Z. Gui, Enhanced visible light photocatalytic performance of g-C3 N4 photocatalysts co-doped with iron and phosphorus, Appl. Surf. Sci. 311 (2014) 164–171. [24] S. Pang, J.M. Englert, H.N. Tsao, Y. Hernandez, A. Hirsch, X.L. Feng, K. Mullen, Extrinsic corrugation-assisted mechanical exfoliation of monolayer graphene, Adv. Mater. 22 (2010) 5374–5377. [25] S. Balendhran, Z.O. Jian, M. Bhaskaran, S. Sriram, S. Ippolito, Z. Vasic, E. Kats, S. Bhargava, S. Zhuiykov, K.Z. Kalantar, Atomically thin layers of MoS2 via a two step thermal evaporation-exfoliation method, Nanoscale 4 (2012) 461–466. [26] C.Y. Su, A.Y. Lu, Y.P. Xu, F.R. Chen, A.N. Khlobystov, L.J. Li, High-quality thin graphene films from fast electrochemical exfoliation, ACS Nano 5 (2011) 2332–2339. [27] J.H. Lim, D. Monllor-Satoca, J.S. Jang, S. Lee, W.Y. Choi, Visible light photocatalysis of fullerol-complexed TiO2 enhanced by Nb doping, Appl. Catal. B: Environ. 152/153 (2014) 233–240. [28] Z. Zhao, Y. Sun, F. Dong, Graphitic carbon nitride based nanocomposites: a review, Nanoscale 7 (2015) 15–37. [29] D. Rangappa, Rapid and direct conversion of graphite crystals into high-yielding good-quality graphene by supercritical fluid exfoliation, Chemistry 16 (2010) 6488–6494. [30] R. Gago, I. Jiménez, D. Cáceres, F.A. Rueda, T. Sajavaara, J.M. Albella, A.C. Font, I. Vergara, J. Raisanen, E. Rauhala, Hardening mechanisms in graphitic carbon nitride films grown with N2 /Ar ion assistance, Chem. Mater. 13 (2000) 129–135. [31] G. Liao, S. Chen, X. Quan, H.T. Yu, H.M. Zhao, Graphene oxide modified g-C3 N4 hybrid with enhanced photocatalytic capability under visible light irradiation, J. Mater. Chem. 22 (2012) 2721–2726. [32] S.C. Yan, Photodegradation performance of g-C3 N4 fabricated by directly heating melamine, Langmuir 25 (2009) 10397–10401. [33] T.S. Wang, D.L. Yu, Y.J. Tian, F.R. Xiao, J.L. He, D.C. Li, W.K. Wang, L. Li, Cubic-C3 N4 nanoparticles synthesized in CNx/TiNx multilayer films, Chem. Phys. Lett. 334 (2001) 7–11. [34] J.G. Yu, Y. Wang, W. Xiao, Enhanced photoelectrocatalytic performance of SnO2 /TiO2 rutile composite films, J. Mater. Chem. A 1 (2013) 10727–10735. [35] S.W. Cao, J.X. Low, J.G. Yu, M. Jaroniec, Polymeric photocatalysts based on graphitic carbon nitride, Adv. Mater. 27 (2015) 2150–2176. [36] L. Ge, C. Han, Synthesis of MWNTs/g-C3 N4 composite photocatalysts with efficient visible light photocatalytic hydrogen evolution activity, Appl. Catal. B: Environ. 117 (2012) 268–274. [37] S.W. Cao, Y.P. Yuan, J. Fang, M.M. Shahjamali, F.Y.C. Boey, J. Barber, S.C.J. Loo, C. Xue, In-situ growth of CdS quantum dots on g-C3 N4 nanosheets for highly efficient photocatalytic hydrogen generation under visible light irradiation, Int. J. Hydrogen Energy 38 (2013) 1258–1266. [38] H.G. Yu, J.G. Yu, B. Cheng, M.H. Zhou, Effects of hydrothermal post-treatment on microstructures and morphology of titanate nanoribbons, J. Solid State Chem. 179 (2006) 349–354. [39] X.J. Bai, J. Li, C.B. Cao, Synthesis of hollow carbon nitride microspheres by an electrodeposition method, Appl. Surf. Sci. 256 (2010) 2327–2331. [40] J.Y. Zhang, X.A. Fan, L. Lu, X.M. Hu, G.Q. Li, Ferrites based infrared radiation coatings with high emissivity and high thermal shock resistance and their application on energy-saving kettle, Appl. Surf. Sci. 344 (2015) 223–229. [41] J. Zhu, P. Xiao, H. Li, S.A. Carabineiro, Graphitic carbon nitride: synthesis, properties, and applications in catalysis, ACS Appl. Mater. Interfaces 6 (2014) 16449–16465. [42] O. Zandi, T.W. Hamann, Enhanced water splitting efficiency through selective surface state removal, J. Phys. Chem. Lett. 5 (2014) 1522–1526. [43] K. Sreejith, J. Nuwad, C.G.S. Pillai, Low voltage electrodeposition of CNx films and study of the effect of the deposition voltage on bonding configurations, Appl. Surf. Sci. 252 (2005) 321–329. [44] H.P. Corporation, Synthesis and morphological transformation of conjugated amphiphilic diblock copolymers in mixed solvents, J. Nanomater. 2013 (2013) 749–756. [45] J.H. Yang, G. Kim, K. Domen, J.H. Choy, Tailoring the mesoporous texture of graphitic carbon nitride, J. Nanosci. Nanotechnol. 13 (2013) 7487–7492. [46] Q.Y. Lin, L. Li, S.J. Liang, M.H. Liu, J.H. Bi, L. Wu, Efficient synthesis of monolayer carbon nitride 2D nanosheet with tunable concentration and enhanced visible-light photocatalytic activities, Appl. Catal. B: Environ. 163 (2015) 135–142. [47] J.G. Yu, S.H. Wang, B. Cheng, F. Huang, Noble metal-free Ni(OH)2 –g-C3 N4 composite photocatalyst with enhanced visible-light photocatalytic H2 -production activity, Catal. Sci. Technol. 3 (2013) 1782–1789. [48] S.K. Mohapatra, K.S. Raja, M. Misra, V.K. Mahajan, M. Ahmadian, Synthesis of self-organized mixed oxide nanotubes by sonoelectrochemical anodization of Ti–8Mn alloy, Electrochim. Acta 53 (2007) 590–597. [49] V. Pfeifer, P. Erhart, S. Li, K. Rachut, J. Morasch, J. Brotz, P. Reckers, Energy band alignment between anatase and rutile TiO2 , J. Phys. Chem. Lett. 5 (2013) 205–207. [50] S. Cao, J. Yu, g-C3 N4 -based photocatalysts for hydrogen generation, J. Phys. Chem. Lett. 5 (2014) 2101–2107. [51] A. Kasahara, K. Nukumizu, G. Hitoki, T. Takata, J.N. Kondo, M. Hara, H. Kobaayashi, K. Domen, Photoreactions on LaTiO2 N under visible light irradiation, J. Phys. Chem. A 106 (2002) 6750–6753.

Please cite this article in press as: Y. Ma, et al., A simple process to prepare few-layer g-C3 N4 nanosheets with enhanced photocatalytic activities, Appl. Surf. Sci. (2015), http://dx.doi.org/10.1016/j.apsusc.2015.08.174