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ChemComm COMMUNICATION Ultra-rapid synthesis of 2D graphitic carbon nitride nanosheets via direct microwave heating for field emission a
Received 00th January 20xx, Accepted 00th January 20xx
a
ab
Yongzhi Yu, Qing Zhou and Jigang Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
The 2D g-C3N4 nanosheets were ultra-rapidly prepared via direct microwave heating approach. The as-synthesized g-C3N4 possessed a large surface area, few stacking layers, large aspect ratio and an enlarged bandgap. As a consequence, the excellent field emission property of 2D g-C3N4 nanosheets has been exhibited with extremely low turn-on fields. Being a hypothetical super-hard materials, β-C3N4, has attracted enormous attention from the scientific community. The graphitic carbon nitride (g-C3N4) is the stable allotrope of β-C3N4. The scientific community has found it is interesting due to its layered structure like graphite, where there are tri-striazine units connected with planar amino groups in each layer and weak van der Waals force between layers. Since the discovery of graphene and its unique properties,1 twodimensional (2D) nano-materials have attracted more and more attention owing to their extraordinary physicochemical properties.2-5 Excited by the achievement of graphene, various other 2D nano-materials are also successfully prepared.3 These 2D nano-materials exhibit vast promising applications in sensing,6,7 super capacitors,8 catalytic reactions,9,10 field emission source11 and so on. Among the functional properties, field emission has great commercial interest in displays and other electronic devices, and is one of the main features of nanomaterials and nanostructures.12 Recent researches indicate that 2D g-C3N4 nanosheets possess excellent electronic,13 optic properties,13-16 mechanical and thermal properties.17 Due to these unique features, extensively researches concerning on the novel fabrication methods for 2D g-C3N4 nanosheets characterized with simple, rapid and low cost have been carried out. Some methods
9
including thermal oxidation etching, liquid/ultrasonication16,18,19 assisted exfoliation in polar solvents and electrochemical methods have been applied to the preparation of g-C3N4 20 nanosheets. It is well known that the hydrogen-bond cohered strands of tri-s-trizaine units in the layers and combination force among the stacking layers is very weak. So, one of the popular methods for the preparation of g-C3N4 nanosheets is the exfoliation or “etching” of bulk g-C3N4. However, the exfoliation of the bulk g-C3N4 is a very timeconsuming process. Besides, the synthesis of bulk g-C3N4 from the polycondensation of nitrogen-rich precursors like 21 22 23 24 cyanamide, urea, thiourea and melamine is also a complex reactions involving multiple steps. At the same time, during the liquid-exfoliation process of bulk g-C3N4, some organic solvents such as N-Methyl pyrrolidone (NMP) and N,NDimethylformamide (DMF) used as exfoliation reagents might be harmful to human and environment. Finally, above methods always give a low yield of g-C3N4 nanosheets. Thus, developing a facile and reliable method is urgently required to produce the high-yield and high-quality g-C3N4 nanosheets. Herein, a novel method for the synthesis of 2D g-C3N4 nanosheets via ultra-rapid and simple microwave irradiation is reported. To the best of our knowledge, the 2D g-C3N4 nanosheets prepared by direct microwave treatment have not been reported previously. For the microwave preparation process, only melamine and carbon fibre were used as raw materials. Because the melamine cannot interact with microwave, carbon fibre is worked as microwave absorbent agent, namely absorb the microwave energy and convert it into heat. In comparison with exfoliation methods, microwave synthesis has many obvious advantages, especially the faster, simpler, and more environment-friendly route.25,26 The 2D g-C3N4 nanosheets were ultra-rapidly prepared via direct high-energy microwave heating approach using low-cost melamine and carbon fibre as precursors. Compared with bulk g-C3N4, the density of 2D g-C3 N4 nanosheets is quite low. Even a slight respiratory airflow can cause the drifting of these products in the air. As shown in Fig. S1, ESI†, one can find that the volume of 2D g-C3N4 nanosheets with the same weight is
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much larger than that of bulk g-C3N4. At the same time, it is easy to find the difference of the colours. The bulk g-C3N4 is exhibited in bright yellow, while the colour of 2D g-C3N4 nanosheets is characterized with light yellow. Some similar phenomenon concerning the appearance of 2D g-C3 N4 13 nanosheets can be found elsewhere. Besides, the occurrence of the Tyndall effect of the diluted dispersion of g-C3N4 nanosheets in water reveals the colloidal nature of dispersion (Fig. S2, ESI†). The morphology characterizations employed by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed the 2D nanosheets structure of the products. As shown in Fig. 1A that g-C3N4 nanosheets are characterized as lamina with slightly curved edges. In contrast, bulk g-C3N4, obtained by thermal-polycondensation of melamine at 560 °C for 4 h in the muffle furnace, is appeared as consisting of irregular particles and solid agglomerates with big size (Fig. S3, ESI†). N2 adsorption-desorption isotherms reveal that the specific surface area (SBET) of 2D g-C3N4 nanosheets is 239 m2 g−1, which are obviously higher than those (16 m2 g−1) of bulk g-C3N4. This shows good agreement to the SEM results. TEM images further demonstrated 2D structural nature of the obtained g-C3N4 nanosheets. One can find that these nanosheets are appeared in transparent, which indicates the thickness of these products is very thin (Fig. 1B).14 The enlarged morphology shown in Fig. 1C indicates that the amounts of the stacking laminar are approximately 5 layers or less. AFM image (Fig. 1E) and corresponding height profile (Fig. 1F) confirm that the thickness of 2D g-C3N4 nanosheets is about 1.6 nm, demonstrating that 2D g-C3N4 nanosheets comprise of only about 5 layers or less. It is well known that, in order to minimize the surface-energy, many wrinkles can be found on the surface of graphene. But for the 2D g-C3N4 nanosheets obtained via direct microwave irradiation, some bumps appear on the products’ surface (Fig. 1B and Fig. S4 of the ESI†). The corresponding high-resolution TEM (HRTEM)
Fig. 1 SEM image of g-C3N4 nanosheets (A). TEM image of 2D g-C3N4 nanosheets with low- (B) and high- (C) magnification ratio. The corresponding HRTEM image and SAED pattern (D). AFM image of 2D g-C3N4 nanosheets (E) and the corresponding thickness analysis (F).
image (Fig. 1D) displays that the lattice spacing is about 0.33 nm, such lattice spacing is consistent with the (002) facets of g27 C3N4. The corresponding selected area electron diffraction (SAED) pattern (Fig. 1D) also shows weak diffraction rings, 28 indexed as the (002) peak of g-C3N4. Besides, partial bumps appeared on the surface of 2D g-C3N4 nanosheets, namely the black spots shown in Fig. 1B and Fig. S4 of the ESI†, might gradually grow into pyramid-like arrays (Fig. S5, ESI†). Under the high-energy microwave irradiation process, some violent and complex synthesis reactions will be occurred, resulting in the production of 2D-g-C3N4 nanosheets with special morphology. Based on the observation of TEM and SEM, one can find that these bumps are the initial structure of the pyramid-like arrays grown on the surface of 2D-g-C3N4 nanosheets (Fig. S5, ESI†). The characterization result of X-ray diffraction (XRD) is agreement with that of TEM. As shown in Fig. 2A, in the case of bulk g-C3N4, a strong (002) peak around 27.6° is observed. Such peak should be attributed to the typical graphite-like periodic repeated stacking of the conjugated aromatic units.16 Obviously, the diffraction intensity of the corresponding peak of 2D g-C3N4 nanosheets decreases, indicating the drastic decreasing of the periodic stacking of the tri-s-triazine units. Such result is consistent with the characterization results of SEM, TEM and AFM. Therefore, one can deduce that 2D g-C3N4 nanosheets were successfully achieved. The similar investigation results can be found elsewhere.13,14,27 Besides, in comparison with bulk g-C3N4, the (002) peak of g-C3N4 nanosheets shifts from 27.6° to 27.1°, indicating the enlargement of interlayer spacing. Besides, the diffraction peak of (100) shown in the lower angle region, which corresponding to periodic array of the intraplanar tri-s-triazine motif stacking,16 almost disappears. Such phenomenon can be explained by the decrease of the planar size.13,14,29 The chemical structure of g-C3N4 was further investigated by FT-IR spectrum (Fig. 2B). The broad peaks between 3400 and 3000 cm−1 should be attributed to the stretching vibration of N–H bonds.30 This demonstrates that some residual NH or NH2 groups still exist in the 2D g-C3N4 nanosheets.31 The characteristic peak at 2167 cm−1 is ascribed to terminal cyano groups (C≡N).14,30 Similar phenomenon can be found elsewhere.20 Several peaks at 1331, 1433, and 1650 cm−1
Fig. 2 XRD pattern (A) and FT-IR spectrum (B) of: (a) bulk g-C3N4 and (b) 2D g-C3N4 nanosheets.
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correspond to the typical stretching vibration modes of C=N 32 −1 and C–N heterocycles. The sharp peak at 807 cm can be ascribed to the typical out-of-plane bending vibration 14,30,31 Therefore, the characteristic of tri-s-triazine unit. comprehensive characterizations demonstrate that 2D g-C3N4 nanosheets can be successfully synthesized via rapid and simple microwave heating method. And the formation of 2D gC3N4 nanosheets might be explained by the attendance of high active carbon atoms excited by the high-energy microwave irradiation, and some special polycondensation reactions of melamine occurred at a proper temperature (ESI†). Photoluminescence (PL) spectra and ultraviolet visible diffuse reflection spectra (UV-vis DRS) can be applied to investigate the electronic band structure of g-C3N4 nanosheets, and benefit in the understanding of the correlative field emission properties. Fig. 3A shows that PL peak shifts from 470 nm of bulk g-C3N4 to 429 nm of 2D g-C3N4 nanosheets, which can be explained by the quantum confinement effects.6,13 Correspondingly, the UV-vis spectra shown in Fig. 3B also display an obviously blue-shift of optical absorption. And the derived plots indicate bandgap changes from 2.69 to 2.88 eV, which is consistent with PL peak variation as shown in Fig. 3A. The reason for the enlarged bandgap can be explained by the giving play of quantum confinement effects and consequently resulting in shifting the conduction and valence band edges in opposite directions.6,13,33 Some research works concerning the quantum confinement effects of the nano-materials can be found elsewhere.5,34 The change of the bandgap also results in the color difference between bulk particles and 2D nanosheets. Traditional bulk g-C3N4 is bright yellow, while 2D nanosheets are appeared in light yellow (Fig. S1, ESI†). Being a conjugated semiconductor polymer, the electrons of 2D g-C3N4 nanosheets can be excited by a certain electric-field. It is well known that graphene exhibits outstanding field emission property due to its high aspect ratio, excellent electrical property, and stable physical and chemical performances.11 At the same time, the former researches have already demonstrated that carbon nitride films also have good field emission properties.35,36 2D g-C3N4 nanosheets is quite similar to graphene, so the field emission of 2D g-C3 N4 nanosheets is worthy of expectation. In this paper, based on
Fig. 3 PL spectra (A) and UV-vis (B) of: (a) bulk g-C3N4 and (b) 2D gC3N4 nanosheets. The wavelength of excitation light for PL spectra is 350 nm.
Fig. 4 The plot of the electron-emission current density (J) as a function of applied electric field (E) for 2D g-C3N4 nanosheets (A). Corresponding F-N plot (B).
the ultra-rapid synthesis of 2D g-C3N4 nanosheets via direct microwave heating, the field emission properties of the products were also investigated. The field emission behavior of 2D g-C3N4 nanosheets was estimated by measuring the I-V curves. Fig. 4A shows the field emission current density (J) of 2D g-C3N4 nanosheets versus applied electric field (E). Usually, the turn-on field is defined to the electric field required to produce a current density of 10 μA cm−2. The turn-on field of graphene is 5.2 V μm−1,11 and that of CNx film and carbon nanotube (CNTs) are 3.3~8.1835 and 6.51 V μm−1,37 respectively. In our work, it is worthy to be noticed that the turning point is appeared at approximately 0.5 V μm−1, with the electric current of 0.5 mA cm−2. In the electric field region of higher than 0.5 V μm−1, the current increase rapidly. So, the turn-on field is defined as 0.5 V μm−1. Compared with the above mentioned materials, the 2D g-C3N4 nanosheets exhibited excellent field emission property. Similar to graphene11 and CNTs,38 2D g-C3N4 nanosheets have large surface area, few layers’ thickness, high aspect ratio, excellent electrical conductivity, and good thermal properties, which satisfy all the requirements for a good field emitter. At the same time, correlative researches demonstrated that the incorporation of nitrogen into carbonbased materials can enhance the field emission property. The improvement should be attributed to the weak donor activity of nitrogen that make the Fermi level rise,35 work function lower and formation of more sp2 clusters.39 Furthermore, gC3N4 nanosheets have abundant edges (Fig. 1C), which may form a distorted sp3-hybridized geometry instead of a planar sp2-hybridized configuration. Correspondingly, there should be localized states at g-C3N4 nanosheets’ edges, and potential barriers of to the electron emission are probably decreased.11 Besides, due to the violent electromagnetic environment and strong energy importing, high-energy microwave synthesis is destined to be a non-equilibrium process. Therefore, a certain number of structure defects are easy formed in the g-C3N4 nanosheets during the ultra-rapid synthesis process. The defects can decrease electronic tunneling potential barriers, consequently increase the field emission properties of g-C3N4 nanosheets. The Fowler–Nordheim (F–N) theory is widely used to confirm the field emission properties of various materials
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11
37,40
36
including graphene, CNTs, carbon nitride films. Herein, the emission barrier or work function of 2D g-C3N4 nanosheets was also investigated using the F–N equation. 2
J=A
β E
2
−
e
3 BΦ 2
7 8
βE
(1)
Φ
Where, J is the emission current, A and B are constant with the −6 −2 3 −3/2 -1 values of 1.56×10 A·eV·V and B=6.83×10 V·eV ·μm , respectively. β is the field enhancement factor, Φ is the work function, and E is the applied field. The simplified equation is expressed as followed. 3
J Aβ 2 = e Φ E2
6
BΦ 2 − βE
9 10 11 12 13 14
(2)
Therefore, the linearized plot can be drawn according to the following equation.
15 16
3
J Aβ 2 BΦ 2 ln( 2 ) = ln − Φ βE E
(3)
18
The obtained F–N linear plot of 2D g-C3N4 nanosheets (Fig. 4B) is quite similar to that of carbon-based materials, e.g., 37,40 The fitted straight line at high electric carbon nanotube. fields demonstrates that 2D g-C3N4 nanosheets possess excellent field emission behavior. In summary, a simple, rapid and environment-friendly microwave heating method successfully employed to fabricate 2D g-C3N4 nanosheets using melamine and carbon fibres as raw materials. The obtained 2D g-C3N4 nanosheets possess the large surface area, few stacking layers, large aspect ratio and an enlarged bandgap. Meanwhile, the field emission measurements show that 2D g-C3N4 nanosheets exhibit excellent field emission property with extremely low turn-on −1 fields of around 0.5 V μm . And the field emission properties of g-C3N4 nanosheets might be further enhanced by optimizing the intrinsic structure, the morphology, defect concentration and thickness of g-C3N4 nanosheets. This work is supported by Program for New Century Excellent Talents in University (NECT-12-0119), Key Project of Science and Technology of Tibet Autonomous region (2015ZR14-14), Qing-lan Project of Jiangsu Province, Summit of the Six Top Talents Program of Jiangsu Province (2013-JY-007), and the Fundamental Research Funds for the Central Universities.
Notes and references 1 2 3 4 5
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19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35
K. S. Novoselov, A. K. Geim, S. V. Morozov, D. Jiang , Y. Zhang, S. V. Dubonos, I. V. Grigorieva and A. A. Firsov, Science, 2004, 306, 666. F. Xia, H. Wang, D. Xiao, M. Dubey and A. Ramasubramaniam, Nature Photonics, 2014, 8, 899. A. Gupta, T. Sakthivel and S. Seal, Prog. Mater. Sci., 2015, 73, 44. B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti and A. Kis, Nat. Nanotechnol., 2011, 6, 147. X. Zhang and Y. Xie, Chem. Soc. Rev., 2013, 42, 8187.
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F. Schedin, A. K. Geim, S. V. Morozov, E. W. Hill, P. Blake, M. I. Katsnelson and K. S. Novoselov, Nat. Mater., 2007, 6, 652. Z. Zhang, J. Huang, Q. Yuan and B. Dong, Nanoscale, 2014, 6, 9250. L. Peng, X. Peng, B. Liu, C. Z. Wu, Y. Xie and G. Yu, Nano Lett., 2013, 13, 2151. X. Huang, Z. Zeng, S. Bao, M. Wang, X. Qi, Z. Fan and H. Zhang, Nat. Commun., 2013, 4, 1444. Z. Zhang, J. Huang, M. Zhang, Q Yuan and B. Dong, Appl. Catal. B: Environ., 2015, 163, 298. Z. Wu, S. Pei, W. Ren, D. Tang, L. Gao, B. Liu, F. Li, C, Liu and H. Cheng, Adv. Mater., 2009, 21, 1756. X. Fang, Y. Bando, U. K. Gautam, C. Ye and D. Golberg, J. Mater. Chem., 2008, 18, 509. P. Niu, L. Zhang, G. Liu and H. Cheng, Adv. Funct., Mater., 2012, 22, 4763. S. Yang, Y. Gong, J. Zhang, L. Zhan, L. Ma, Z. Fang, V. Robert, X. Wang and P. M. Ajayan, Adv. Mater., 2013, 25, 2452. X. Zhang, X. Xie, H. Wang, J. Zhang, B. Pan and Y. Xie, J. Am. Chem. Soc., 2012, 135, 18. X. Wang, K. Maeda, A. Thomas, K. Takanabe, G. Xin, J. M. Carlsson, K. Domen and M. Antonietti, Nat. Mater., 2009, 8, 76. B. Mortazavi, G. Cuniberti and T. Rabczuk, Comp. Mater. Sci., 2015, 99, 285. X. She, H. Xu, Y. Xu, J. Yan, J. Xia, , L. Xu, Y. Song, Y. Jiang, Q. Zhang and H. Li, J. Mater. Chem. A, 2014, 2, 2563. T. Ma, Y. Tang, S. Dai and S. Qiao, Small, 2014, 10, 2382. Q. Lu, J. Deng, Y. Hou, H. Wang, H. Li and Y. Zhang, Chem. Commun., 2015, 51, 12251. F. Dong, M. Ou, Y. Jiang, S. Guo and Z. Wu, Ind. Eng. Chem. Res., 2014, 53, 2318. S. C. Lee, H. O. Lintang and L. Yuliati, Chem. Asian J., 2012, 7, 2139. G. Zhang, J. Zhang, M. Zhang and X. Wang, J. Mater. Chem., 2012, 22, 8083. S. Yan, Z. Li and Z. Zou, Langmuir, 2009, 25, 10397. M. Nüchter, B. Ondruschka, W. Bonrath, A. Gum, Green Chem., 2004, 6, 128. Y. Yuan, L. Yin, S. Cao, L. Gu, G. Xu, P. Du, H. Chai, Y. Liao and C. Xue, Green Chem., 2014, 16, 4663. J. Tian, Q, Liu, C. Ge, Z. Xing, M. A. Abdullah O. A. Y. Abdulrahman and X. Sun, Nanoscale, 2013, 5, 8921. X. Bai, J. Li, C. Cao and S. Hussain, Mater. Lett., 2011, 65, 1101. M. Groenewolt and M. Antonietti, Adv. Mater., 2005, 17, 1789. Y. Li, J. Zhang, Q. Wang, Y. Jin, D. Huang, Q. Cui and G. Zou, J. Phys. Chem. B, 2010, 114, 9429. B. V. Lotsch, M. Doblinger, J. Sehnert, L. Seyfarth, J. Senker, O. Oeckler, W. Schnick, Chem. Eur. J., 2007, 13, 4969. Y. Wang, X. Bai, C. Pan, J. He and Y. Zhu, J. Mater. Chem., 2012, 22, 11568. A. P. Alivisatos, Science, 1996, 271, 933. S. Ithurria, M. D. Tessier, B. Mahler, R. P. S. M. Lobo, B. Dubertret, Al. L. Efros, Nat. Mater., 2011, 10, 936. X. C. LeQuan, W. P. Kang, J. L. Davidson, M. Guo and B. K. Choi, Diamond Relat. Mater., 2009, 18, 191. J. Li, C. Gu, P. Xu, Q. Wang and W. Zheng, Mater. Sci. Eng., B, 2006, 126, 74. H. C. Chang, H. J. Tsai, W. Y. Lin, Y. C. Chu and W. K. Hsu, ACS Appl. Mat. Interfaces, 2015, 7, 14456. J. M. Bonard, H. Kind, T. Stockli and L. A. Nilsson, Solid-State Electr., 2001, 45, 893. P. Zhang, R. Li, H. Yang, Y. Feng and E. Xie, Solid State Sci., 2012, 14, 715. X. Yang, Z. Li, F. He, M. Liu, B. Bai, W. Liu, X. Qiu and H. Zhou, C. Li, Q. Dai, Small, 2015, 11, 3710.
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This article can be cited before page numbers have been issued, to do this please use: Y. Yu, Q. Zhou and J. Wang, Chem. Commun., 2016, DOI: 10.1039/C5CC10258H.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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ChemComm COMMUNICATION Ultra-rapid synthesis of 2D graphitic carbon nitride nanosheets via direct microwave heating for field emission a
Received 00th January 20xx, Accepted 00th January 20xx
a
ab
Yongzhi Yu, Qing Zhou and Jigang Wang*
DOI: 10.1039/x0xx00000x www.rsc.org/
The 2D g-C3N4 nanosheets were ultra-rapidly prepared via direct microwave heating approach. The as-synthesized g-C3N4 possessed a large surface area, few stacking layers, large aspect ratio and an enlarged bandgap. As a consequence, the excellent field emission property of 2D g-C3N4 nanosheets has been exhibited with extremely low turn-on fields. Being a hypothetical super-hard materials, β-C3N4, has attracted enormous attention from the scientific community. The graphitic carbon nitride (g-C3N4) is the stable allotrope of β-C3N4. The scientific community has found it is interesting due to its layered structure like graphite, where there are tri-striazine units connected with planar amino groups in each layer and weak van der Waals force between layers. Since the discovery of graphene and its unique properties,1 twodimensional (2D) nano-materials have attracted more and more attention owing to their extraordinary physicochemical properties.2-5 Excited by the achievement of graphene, various other 2D nano-materials are also successfully prepared.3 These 2D nano-materials exhibit vast promising applications in sensing,6,7 super capacitors,8 catalytic reactions,9,10 field emission source11 and so on. Among the functional properties, field emission has great commercial interest in displays and other electronic devices, and is one of the main features of nanomaterials and nanostructures.12 Recent researches indicate that 2D g-C3N4 nanosheets possess excellent electronic,13 optic properties,13-16 mechanical and thermal properties.17 Due to these unique features, extensively researches concerning on the novel fabrication methods for 2D g-C3N4 nanosheets characterized with simple, rapid and low cost have been carried out. Some methods
9
including thermal oxidation etching, liquid/ultrasonication16,18,19 assisted exfoliation in polar solvents and electrochemical methods have been applied to the preparation of g-C3N4 20 nanosheets. It is well known that the hydrogen-bond cohered strands of tri-s-trizaine units in the layers and combination force among the stacking layers is very weak. So, one of the popular methods for the preparation of g-C3N4 nanosheets is the exfoliation or “etching” of bulk g-C3N4. However, the exfoliation of the bulk g-C3N4 is a very timeconsuming process. Besides, the synthesis of bulk g-C3N4 from the polycondensation of nitrogen-rich precursors like 21 22 23 24 cyanamide, urea, thiourea and melamine is also a complex reactions involving multiple steps. At the same time, during the liquid-exfoliation process of bulk g-C3N4, some organic solvents such as N-Methyl pyrrolidone (NMP) and N,NDimethylformamide (DMF) used as exfoliation reagents might be harmful to human and environment. Finally, above methods always give a low yield of g-C3N4 nanosheets. Thus, developing a facile and reliable method is urgently required to produce the high-yield and high-quality g-C3N4 nanosheets. Herein, a novel method for the synthesis of 2D g-C3N4 nanosheets via ultra-rapid and simple microwave irradiation is reported. To the best of our knowledge, the 2D g-C3N4 nanosheets prepared by direct microwave treatment have not been reported previously. For the microwave preparation process, only melamine and carbon fibre were used as raw materials. Because the melamine cannot interact with microwave, carbon fibre is worked as microwave absorbent agent, namely absorb the microwave energy and convert it into heat. In comparison with exfoliation methods, microwave synthesis has many obvious advantages, especially the faster, simpler, and more environment-friendly route.25,26 The 2D g-C3N4 nanosheets were ultra-rapidly prepared via direct high-energy microwave heating approach using low-cost melamine and carbon fibre as precursors. Compared with bulk g-C3N4, the density of 2D g-C3 N4 nanosheets is quite low. Even a slight respiratory airflow can cause the drifting of these products in the air. As shown in Fig. S1, ESI†, one can find that the volume of 2D g-C3N4 nanosheets with the same weight is
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much larger than that of bulk g-C3N4. At the same time, it is easy to find the difference of the colours. The bulk g-C3N4 is exhibited in bright yellow, while the colour of 2D g-C3N4 nanosheets is characterized with light yellow. Some similar phenomenon concerning the appearance of 2D g-C3 N4 13 nanosheets can be found elsewhere. Besides, the occurrence of the Tyndall effect of the diluted dispersion of g-C3N4 nanosheets in water reveals the colloidal nature of dispersion (Fig. S2, ESI†). The morphology characterizations employed by using field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM) confirmed the 2D nanosheets structure of the products. As shown in Fig. 1A that g-C3N4 nanosheets are characterized as lamina with slightly curved edges. In contrast, bulk g-C3N4, obtained by thermal-polycondensation of melamine at 560 °C for 4 h in the muffle furnace, is appeared as consisting of irregular particles and solid agglomerates with big size (Fig. S3, ESI†). N2 adsorption-desorption isotherms reveal that the specific surface area (SBET) of 2D g-C3N4 nanosheets is 239 m2 g−1, which are obviously higher than those (16 m2 g−1) of bulk g-C3N4. This shows good agreement to the SEM results. TEM images further demonstrated 2D structural nature of the obtained g-C3N4 nanosheets. One can find that these nanosheets are appeared in transparent, which indicates the thickness of these products is very thin (Fig. 1B).14 The enlarged morphology shown in Fig. 1C indicates that the amounts of the stacking laminar are approximately 5 layers or less. AFM image (Fig. 1E) and corresponding height profile (Fig. 1F) confirm that the thickness of 2D g-C3N4 nanosheets is about 1.6 nm, demonstrating that 2D g-C3N4 nanosheets comprise of only about 5 layers or less. It is well known that, in order to minimize the surface-energy, many wrinkles can be found on the surface of graphene. But for the 2D g-C3N4 nanosheets obtained via direct microwave irradiation, some bumps appear on the products’ surface (Fig. 1B and Fig. S4 of the ESI†). The corresponding high-resolution TEM (HRTEM)
Fig. 1 SEM image of g-C3N4 nanosheets (A). TEM image of 2D g-C3N4 nanosheets with low- (B) and high- (C) magnification ratio. The corresponding HRTEM image and SAED pattern (D). AFM image of 2D g-C3N4 nanosheets (E) and the corresponding thickness analysis (F).
image (Fig. 1D) displays that the lattice spacing is about 0.33 nm, such lattice spacing is consistent with the (002) facets of g27 C3N4. The corresponding selected area electron diffraction (SAED) pattern (Fig. 1D) also shows weak diffraction rings, 28 indexed as the (002) peak of g-C3N4. Besides, partial bumps appeared on the surface of 2D g-C3N4 nanosheets, namely the black spots shown in Fig. 1B and Fig. S4 of the ESI†, might gradually grow into pyramid-like arrays (Fig. S5, ESI†). Under the high-energy microwave irradiation process, some violent and complex synthesis reactions will be occurred, resulting in the production of 2D-g-C3N4 nanosheets with special morphology. Based on the observation of TEM and SEM, one can find that these bumps are the initial structure of the pyramid-like arrays grown on the surface of 2D-g-C3N4 nanosheets (Fig. S5, ESI†). The characterization result of X-ray diffraction (XRD) is agreement with that of TEM. As shown in Fig. 2A, in the case of bulk g-C3N4, a strong (002) peak around 27.6° is observed. Such peak should be attributed to the typical graphite-like periodic repeated stacking of the conjugated aromatic units.16 Obviously, the diffraction intensity of the corresponding peak of 2D g-C3N4 nanosheets decreases, indicating the drastic decreasing of the periodic stacking of the tri-s-triazine units. Such result is consistent with the characterization results of SEM, TEM and AFM. Therefore, one can deduce that 2D g-C3N4 nanosheets were successfully achieved. The similar investigation results can be found elsewhere.13,14,27 Besides, in comparison with bulk g-C3N4, the (002) peak of g-C3N4 nanosheets shifts from 27.6° to 27.1°, indicating the enlargement of interlayer spacing. Besides, the diffraction peak of (100) shown in the lower angle region, which corresponding to periodic array of the intraplanar tri-s-triazine motif stacking,16 almost disappears. Such phenomenon can be explained by the decrease of the planar size.13,14,29 The chemical structure of g-C3N4 was further investigated by FT-IR spectrum (Fig. 2B). The broad peaks between 3400 and 3000 cm−1 should be attributed to the stretching vibration of N–H bonds.30 This demonstrates that some residual NH or NH2 groups still exist in the 2D g-C3N4 nanosheets.31 The characteristic peak at 2167 cm−1 is ascribed to terminal cyano groups (C≡N).14,30 Similar phenomenon can be found elsewhere.20 Several peaks at 1331, 1433, and 1650 cm−1
Fig. 2 XRD pattern (A) and FT-IR spectrum (B) of: (a) bulk g-C3N4 and (b) 2D g-C3N4 nanosheets.
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correspond to the typical stretching vibration modes of C=N 32 −1 and C–N heterocycles. The sharp peak at 807 cm can be ascribed to the typical out-of-plane bending vibration 14,30,31 Therefore, the characteristic of tri-s-triazine unit. comprehensive characterizations demonstrate that 2D g-C3N4 nanosheets can be successfully synthesized via rapid and simple microwave heating method. And the formation of 2D gC3N4 nanosheets might be explained by the attendance of high active carbon atoms excited by the high-energy microwave irradiation, and some special polycondensation reactions of melamine occurred at a proper temperature (ESI†). Photoluminescence (PL) spectra and ultraviolet visible diffuse reflection spectra (UV-vis DRS) can be applied to investigate the electronic band structure of g-C3N4 nanosheets, and benefit in the understanding of the correlative field emission properties. Fig. 3A shows that PL peak shifts from 470 nm of bulk g-C3N4 to 429 nm of 2D g-C3N4 nanosheets, which can be explained by the quantum confinement effects.6,13 Correspondingly, the UV-vis spectra shown in Fig. 3B also display an obviously blue-shift of optical absorption. And the derived plots indicate bandgap changes from 2.69 to 2.88 eV, which is consistent with PL peak variation as shown in Fig. 3A. The reason for the enlarged bandgap can be explained by the giving play of quantum confinement effects and consequently resulting in shifting the conduction and valence band edges in opposite directions.6,13,33 Some research works concerning the quantum confinement effects of the nano-materials can be found elsewhere.5,34 The change of the bandgap also results in the color difference between bulk particles and 2D nanosheets. Traditional bulk g-C3N4 is bright yellow, while 2D nanosheets are appeared in light yellow (Fig. S1, ESI†). Being a conjugated semiconductor polymer, the electrons of 2D g-C3N4 nanosheets can be excited by a certain electric-field. It is well known that graphene exhibits outstanding field emission property due to its high aspect ratio, excellent electrical property, and stable physical and chemical performances.11 At the same time, the former researches have already demonstrated that carbon nitride films also have good field emission properties.35,36 2D g-C3N4 nanosheets is quite similar to graphene, so the field emission of 2D g-C3 N4 nanosheets is worthy of expectation. In this paper, based on
Fig. 3 PL spectra (A) and UV-vis (B) of: (a) bulk g-C3N4 and (b) 2D gC3N4 nanosheets. The wavelength of excitation light for PL spectra is 350 nm.
Fig. 4 The plot of the electron-emission current density (J) as a function of applied electric field (E) for 2D g-C3N4 nanosheets (A). Corresponding F-N plot (B).
the ultra-rapid synthesis of 2D g-C3N4 nanosheets via direct microwave heating, the field emission properties of the products were also investigated. The field emission behavior of 2D g-C3N4 nanosheets was estimated by measuring the I-V curves. Fig. 4A shows the field emission current density (J) of 2D g-C3N4 nanosheets versus applied electric field (E). Usually, the turn-on field is defined to the electric field required to produce a current density of 10 μA cm−2. The turn-on field of graphene is 5.2 V μm−1,11 and that of CNx film and carbon nanotube (CNTs) are 3.3~8.1835 and 6.51 V μm−1,37 respectively. In our work, it is worthy to be noticed that the turning point is appeared at approximately 0.5 V μm−1, with the electric current of 0.5 mA cm−2. In the electric field region of higher than 0.5 V μm−1, the current increase rapidly. So, the turn-on field is defined as 0.5 V μm−1. Compared with the above mentioned materials, the 2D g-C3N4 nanosheets exhibited excellent field emission property. Similar to graphene11 and CNTs,38 2D g-C3N4 nanosheets have large surface area, few layers’ thickness, high aspect ratio, excellent electrical conductivity, and good thermal properties, which satisfy all the requirements for a good field emitter. At the same time, correlative researches demonstrated that the incorporation of nitrogen into carbonbased materials can enhance the field emission property. The improvement should be attributed to the weak donor activity of nitrogen that make the Fermi level rise,35 work function lower and formation of more sp2 clusters.39 Furthermore, gC3N4 nanosheets have abundant edges (Fig. 1C), which may form a distorted sp3-hybridized geometry instead of a planar sp2-hybridized configuration. Correspondingly, there should be localized states at g-C3N4 nanosheets’ edges, and potential barriers of to the electron emission are probably decreased.11 Besides, due to the violent electromagnetic environment and strong energy importing, high-energy microwave synthesis is destined to be a non-equilibrium process. Therefore, a certain number of structure defects are easy formed in the g-C3N4 nanosheets during the ultra-rapid synthesis process. The defects can decrease electronic tunneling potential barriers, consequently increase the field emission properties of g-C3N4 nanosheets. The Fowler–Nordheim (F–N) theory is widely used to confirm the field emission properties of various materials
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11
37,40
36
including graphene, CNTs, carbon nitride films. Herein, the emission barrier or work function of 2D g-C3N4 nanosheets was also investigated using the F–N equation. 2
J=A
β E
2
−
e
3 BΦ 2
7 8
βE
(1)
Φ
Where, J is the emission current, A and B are constant with the −6 −2 3 −3/2 -1 values of 1.56×10 A·eV·V and B=6.83×10 V·eV ·μm , respectively. β is the field enhancement factor, Φ is the work function, and E is the applied field. The simplified equation is expressed as followed. 3
J Aβ 2 = e Φ E2
6
BΦ 2 − βE
9 10 11 12 13 14
(2)
Therefore, the linearized plot can be drawn according to the following equation.
15 16
3
J Aβ 2 BΦ 2 ln( 2 ) = ln − Φ βE E
(3)
18
The obtained F–N linear plot of 2D g-C3N4 nanosheets (Fig. 4B) is quite similar to that of carbon-based materials, e.g., 37,40 The fitted straight line at high electric carbon nanotube. fields demonstrates that 2D g-C3N4 nanosheets possess excellent field emission behavior. In summary, a simple, rapid and environment-friendly microwave heating method successfully employed to fabricate 2D g-C3N4 nanosheets using melamine and carbon fibres as raw materials. The obtained 2D g-C3N4 nanosheets possess the large surface area, few stacking layers, large aspect ratio and an enlarged bandgap. Meanwhile, the field emission measurements show that 2D g-C3N4 nanosheets exhibit excellent field emission property with extremely low turn-on −1 fields of around 0.5 V μm . And the field emission properties of g-C3N4 nanosheets might be further enhanced by optimizing the intrinsic structure, the morphology, defect concentration and thickness of g-C3N4 nanosheets. This work is supported by Program for New Century Excellent Talents in University (NECT-12-0119), Key Project of Science and Technology of Tibet Autonomous region (2015ZR14-14), Qing-lan Project of Jiangsu Province, Summit of the Six Top Talents Program of Jiangsu Province (2013-JY-007), and the Fundamental Research Funds for the Central Universities.
Notes and references 1 2 3 4 5
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