Preparation and Characterization of Graphitic Carbon Nitride Through Pyrolysis of Melamine

Appl Phys A (2009) 94: 387–392 DOI 10.1007/s00339-008-4816-4

Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine

Xuefei Li · Jian Zhang · Longhai Shen · Yanmei Ma · Weiwei Lei · Qiliang Cui · Guangtian Zou

Received: 28 January 2008 / Accepted: 7 July 2008 / Published online: 5 August 2008 © Springer-Verlag 2008

Abstract Graphitic carbon nitride (g-C3N4) has been syn- 1 Introduction thesized via a two-step pyrolysis of melamine (C3H6N6) at 800°C for 2 h under vacuum conditions. X-ray diffraction Since it was theoretically predicted that the hardness of (XRD) patterns strongly indicate that the synthesized sam- C3N4 phases (α-, β-, and cubic) might be comparable to or higher than that of diamond [1–3], great efforts have been ple is g-C3N4. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) morphologies in- made to synthesize these novel materials. Their hardness, dicate that the product is mainly composed of graphitic car- wear resistance, and low coefficient of friction are very at- tractive properties for industrial applications. Up to the mo- bon nitride. The stoichiometric ratio of C:N is determined ment, several methods have been employed to synthesize to be 0.72 by elemental analysis (EA). Chemical bond- C N films [4–23], including laser ablation of graphite tar- ing of the sample has been investigated by X-ray photo- 3 4 get [4–6], magnetic sputtering [7–9], radio frequency re- electron spectroscopy (XPS) and Fourier transform infrared active sputtering [10, 11], ion-beam evaporation deposi- spectroscopy (FTIR). Electron energy loss spectroscopy tion [12, 13], shock-wave compression [14], hot filament (EELS) verifies the bonding state between carbon and ni- CVD (chemical vapor deposition) [15], pyrolysis of high- trogen atoms. Optical properties of the g-C3N4 were inves- nitrogen content precursors [16, 17], MPCVD (microwave tigated by PL (photoluminescence) measurements and UV– plasma chemical vapor deposition) [18], and magnetic fil- Vis (ultraviolet–visible) absorption spectra. We suppose its tered plasma stream deposition [19–21]. So far only a few luminescent properties may have potential application as groups have claimed the observation of pure crystallized component of optical nanoscale devices. Thermogravimet- g-C3N4, β-C3N4 and c-C3N4 [22–29]. Demazeau et al. re- ric analysis (TGA) and differential thermal analysis (DTA) ported on the pyrolysis of melamine in the presence of a were also performed. nitriding solvent (NH2NH2) and the preparation of crystallized g-C3N4 at 3 GPa and 800–850°C [22]. Bai et al. synthesized crystallized g-C N by the reaction of CCl PACS 81.05.Uw · 81.05.Zx · 81.16.Be · 62.23.Kn · 3 4 4 and NH Cl at 400°C, but did not provide any XPS or · 4 77.84.Bw 82.30.Lp EDAX (energy dispersive spectroscopy) data to determine the stoichiometric N:C ratio [23]. Liu et al. synthesized g-C3N4 nanocrystallites via benzene–thermal reaction be- X. Li · J. Zhang · L. Shen · Y. Ma · W. Lei · Q. Cui · G. Zou () tween C3N3Cl3 and NaNH2 at 180–220°C for 8–12 h [24]. National Laboratory of Superhard Materials, Jilin University, Zou’s group synthesized bulk g-C3N4 via high-temperature Changchun 130012, China high-pressure pyrolysis of melamine [25]. Zhang et al. syn- e-mail: [email protected] thesized g-C3N4 by a solid-state reaction of C6N9H4Cl J. Zhang at 1.0–1.5 GPa and 500–550°C [26]. Yin et al. synthe- e-mail: [email protected] sized β-C3N4 nanorods through mechanochemical reaction X. Li and subsequent thermal annealing under a streaming flow Jilin Normal University, Siping 136000, China of NH3 gas [27]. Tang et al. synthesized nitrogen-doped 388 X. Li et al. carbon nanotubular fibers through the aerosol-assisted de- 2 Experimental details composition of dimethylformamide in the presence of cat- alyst [28]. Ming et al. discovered c-C3N4 at high-pressure A quartz tube, 25 mm in inner diameter and 1000 mm and high-temperature conditions using g-C3N4 as initial ma- in length, was used as a reaction chamber. Analytical terials [29]. grade melamine powder (3 g) was pressed into a cylin- In this work, we report the synthesis of graphitic carbon drical column and then placed in the middle region of the quartz tube. The reaction chamber was evacuated to nitride (g-C N ) through direct pyrolysis of melamine under 3 4 less than 1 Pa (about 10−2 Torr). In order to react fully, vacuum conditions. Under medium temperatures, pyrolysis the initial materials were heated in an electric furnace of the melamine powders proceeds gradually. The triazine at a rate of 1°C/min up to 800°C, and this temperature rings of melamine molecules would not be destroyed com- was maintained for 2 h under vacuum conditions. The pletely. Thus active groups such as triazine-ring-based radi- quartz tube was cooled to ambient temperature naturally. cals and -NHx are formed as a result of the decomposition of Then the yellow powder was collected from the inner wall melamine. Active groups escape to the low temperature re- of the quartz tube. In order to obtain well-crystallized gion, where the triazine rings are interlinked with N atoms. samples, the obtained powders were treated repeatedly Eventually, nucleation of g-C3N4 nanocrystals occurs as the in processes described above. Finally, g-C3N4 had been stream of active groups reaches supersaturation under tem- prepared. The chemical reaction could be written as perature gradient. Meanwhile NH3 is formed as byproducts.

The crystal structure of the sample was determined by X- The TEM and SEM images are shown in Figs. 2 and 3, ray diffraction (XRD). The morphology and electron dif- respectively. It can be seen that the sample is composed of a fraction of the sample were carried out using SEM and large number of nanosheets. Some nanovessels and particles TEM. Chemical composition was analyzed by chemical el- can also be found. It can be suggested that some nanosheets ement analysis. X-ray photoelectron spectroscopy (XPS) re- have been curled to form the nanovessels and some tiny sults confirmed the corresponding binding energy of the nanosheets have been curled to form closed structures like sample and the infrared characteristic peaks were character- particles. The inhomogeneity in the morphology of the sam- ized by Fourier transform infrared (FTIR) spectroscopy. The ples is supposed to be due to uneven temperature distrib- electron energy-loss spectroscopy (EELS) was also used ution. In the corresponding electron diffraction (ED) pat- to verify the bonding state between carbon and nitrogen tern shown in the inset of Fig. 2, the diffraction ring can be atoms. Optical properties were studied by PL measurements and UV–Vis absorption spectra. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) profiles were obtained under flowing air.

3 Results and discussion

XRD was performed on a Rigaku-Dmax-Ra powder X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). A typical XRD pattern is shown in Fig. 1. The strongest sharp re- ﬂection peak at the position of 27.5° (d = 3.22 Å) matches the predicted (002) diffraction of g-C3N4 [30], which is very similar to the XRD patterns of g-C3N4 reported in the pre- vious works [22–24]. Fig. 1 A typical XRD pattern of the prepared g-C3N4 sample Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine 389 Table 1 Elemental composition of the samples obtained from CHN analysis

Element C N H Other elements (O) (weight %) (weight %) (weight %) (weight %)

Content 34.50 55.66 3.307 6.533

Table 2 XPS analytic data of the sample

Name Start BE Peak BE End BE Height FWHM (eV) (eV) (eV) Counts (eV)

Fig. 2 TEM images and electron diffraction (ED) pattern of g-C3N4 C1s 290.45 284.75 281.95 20561.79 1.85 O1s 535.55 531.96 529.65 10966.11 1.78 N1s 402.5 398.38 396.15 10455.05 1.88

Fig. 3 SEM images of g-C3N4

Fig. 4 The survey XPS spectrum of g-C3N4 considered as the (002) reﬂection of the graphitic structure of polycrystalline carbon nitride, which further supports the The structure of g-C3N4 can be regarded as graphitic car- XRD results. bon nitride (g-C3N4) single layers aligned relatively to each Chemical element analysis was performed on Perkin– other along the hexagonal c-axis [32–34]. Figure 5 shows Elmer 240C CHNS-O analyzer. Table 1 shows the result of an atomic distribution structure of a graphitic carbon nitride the elemental analysis. An average value of 0.72 was deter- (g-C3N4) single layer [32]. It is shown that the graphitic mined for the C:N ratio (comparing to the theoretical value single layer of g-C3N4 may be deemed as s-triazine rings of C3N4 being 0.75). Additional small amounts of hydrogen (C3N3) bridged by three-fold coordinated N atoms on the and oxygen may come from amino groups and thus O2 or C sites. This kind of structure may be corroborated by our H2O is adsorbed to the surface of the sample. Because the XPS and FTIR studies. sample is a nanoscaled material, the surface is densely ter- Figure 6 shows the XPS spectrum of C1s. Two peaks can minated with amino groups because of the connectivity [31]. be distinguished to be centered at 284.6 and 288.3 eV. The Chemical bonding of the prepared g-C3N4 was further major peak at 284.6 eV is exclusively assigned to carbon investigated by X-ray photoelectron spectroscopy (XPS). atoms (C–C bonding) in a pure carbon environment, i.e., The results are listed in Table 2, which shows the positions graphitic or amorphous carbons either in our sample or ad- of the spectral lines and their full width at half-maximum sorbed to the surface. The peak at 288.3 eV is identiﬁed as (FWHM). Figure 4 shows the survey scan XPS spectrum. originating from carbon atoms bonded with three N neigh- It indicates the presence of only C, N, and a small amount bors, as shown in Fig. 5. of O, which may be due to surface absorption and oxida- Figure 7 shows the XPS spectrum of N1s. It could be de- tion. Therefore, it may be concluded that the as-prepared convoluted into two peaks with binding energies of 398.2 bulk sample is pure graphitic CN phase. The C:N ratio esti- and 399.1 eV. The peak at 399.1 eV corresponds to N atoms mated from XPS analysis is 0.70, which is very close to the trigonally bonded to three sp2 carbon atoms in the C–N result of the elemental analysis obtained from CHN analysis. network. The peak at 398.2 eV is attributed to N atoms 390 X. Li et al.

Fig. 5 Structure model of single layer of g-C3N4 Fig. 8 The FTIR spectrum of g-C3N4

Fig. 6 The XPS spectrum of C1s of g-C3N4 Fig. 9 The EELS spectrum of g-C3N4

tal analysis (C:N = 0.72, a little lower than the ideal 0.75 stoichiometry). The peaks at 801 and 1470 cm−1 belong to the s-triazine ring modes [35, 36]. The absorption peak at 1613 cm−1 is attributed to C=N[23, 37, 38] and the one at 1335 cm−1 to C–N [39]. The sharp peak at 1613 cm−1 can be deemed as an indication of good crystallinity of g-C3N4 [23]. Figure 9 shows the EELS (electron energy loss spectroscopy) spectrum. The spectrum comes from an individ- ual nanofiber of the g-C3N4. This spectrum indicates that the first near-edge structure peak of C and N can be attributed to 1s → π ∗ electronic transitions. The fine struc- Fig. 7 The XPS spectrum of N1s of g-C3N4 ture of the edges is indicative of the sp2 bonding [30]. In the C K-edge of this spectrum, the peak at 283.8 eV can be ∗ sp2-bonded to two carbon atoms. The XPS data gives an evi- attributed to the 1s → π electronic transition and another ∗ dence for the existence of graphite-like sp2-bonded structure located at 293.3 eV to the 1s → σ transition. The corre- ∗ ∗ in graphitic carbon nitride (Fig. 5). sponding π and σ transitions in the N K-edge appear at The FTIR spectrum (Fig. 8) also suggests the existence 398.5 and 403.5 eV, respectively. Close similarities between of graphite-like sp2 bonded structure [35]. The spectrum of the carbon and nitrogen K edges are indicative of strong co- the product shows broad bands of the stretching and defor- valent bonding between C and N atoms. An average value −1 mation modes of -NH2 groups at 3342 and 3469 cm .The of 0.70 was determined for the C:N ratio by EELS which is existence of amino groups is in agreement with the elemen- again very close to the result of the elemental analysis. Preparation and characterization of graphitic carbon nitride through pyrolysis of melamine 391

Fig. 10 The UV–Vis spectrum of g-C3N4 Fig. 12 The TG-DTA spectrum of g-C3N4

the graphite to form CO2. An initial weight loss is also observed below 200°C, which may arise from the evaporation of adsorbed water to the sample surface.

4 Conclusion

In short, all of the characterization results strongly indicate that we have successfully prepared well-crystallized graphitic carbon nitride (g-C3N4) via a novel and facile route. This is a signiﬁcant step towards future synthesis of superhard cubic carbon nitride and other carbon nitride phases from g-C3N4.

Fig. 11 The PL spectrum of g-C3N4 Acknowledgements This work was supported ﬁnancially by Natural Science Foundation of China (No. 10304005, 50372023, 50334030) Figure 10 shows the UV–Vis absorption spectra of the and National Basic Research Program of China (No. 2005CB724400, prepared g-C N sample. The observed strong absorption 2001CB711201), and Research Fund for the Doctoral Program of 3 4 Higher Education of China (20070183175). band is centered at 235–250 nm. The band is attributed in the absorption range characteristic to π → π ∗ electronic transition in the aromatic 1,3,5-triazine compounds [15, 19]. The References result also shows the presence of structure based on the s- triazine rings, which is in accord with the EELS and FTIR 1. M.L. Cohen, Phys. Rev. B 32, 7988 (1985) analysis. 2. A.Y. Liu, M.L. Cohen, Science 245, 841 (1989) Figure 11 shows a typical photoluminescent spectrum of 3. M.L. Cohen, Science 261, 307 (1993) 4. C. Niu, Y.Z. Lu, C.M. Lieber, Science 261, 334 (1993) the prepared g-C3N4 sample. The sample exhibits strong 5. I. Bertoti, T. Szorenyi, F. Antoni, E. Fogarassy, Diam. Relat. photoluminescence at the room temperature. The lumines- Mater. 11, 1157 (2002) cence has a maximum at about 435 nm. A blue color can be 6. A.A. Khawwam, C. Jama, P. Goudmand, O. Dessaux, A.E. Achari, seen by the naked eye. The luminescent characteristics prob- P. Dhamelincourt, G. Patrat, Thin Solid Films 408, 15 (2002) → ∗ 7. J. Xu, X. Deng, J. Zhang, W. Lu, T. Ma, Thin Solid Films 390, 107 ably depend on the π π transitions of the s-triazine ring (2001) systems. Such properties may be applied to light-emitting 8. D. Li, X. Chu, S.C. Cheng, X.W. Lin, V.P. Dravid, Y.W. Chung, diodes [19]. M.S. Wong, W.D. Sproul, Appl. Phys. Lett. 67, 203 (1995) Figure 12 shows the results of thermogravimetric analy- 9. H. Sjöström, L. Hultman, J.-E. Sundgren, S.V. Hainsworth, T.F. Page, G.S.A.M. Theunissen, J. Vac. Sci. Technol. A 14, 56 (1996) sis (TGA) and differential thermal analysis (DTA) for the 10. K.G. Kreider, M.J. Tarlov, G.J. Gillen, G.E. Poirier, L.H. Robins, sample tested in air. The sample is stable at temperatures L.K. Ives, W.D. Bowers, R.B. Marinenko, D.T. Smith, J. Mater. under 500°C. A sharp weight loss step occurs in the temper- Res. 10, 3079 (1995) ature range of about 550 to 700°C, which may be attributed 11. K.M. Yu, M.L. Cohen, E.E. Haller, W.L. Hansen, A.Y. Liu, I.C. Wu, Phys. Rev. B 49, 5034 (1994) to the oxidation of the g-C3N4 to form graphite and N2.The 12. Z.B. Zhou, R.Q. Cui, Q.J. Pang, G.M. Hadi, Z.M. Ding, W.Y. Li, exothermic peak at 740°C may be caused by the oxidation of Sol. Energy Mater. Sol. C 70, 487 (2001) 392 X. Li et al.

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