Template-directed synthesis of highly ordered nanoporous graphitic carbon nitride through polymerization of cyanamide
Introduction
Recently significant progress has been made in the synthesis of ordered nanoporous carbons by using zeolites [1], mesoporous silica [2], [3], [4] and colloidal crystals [5], [6], [7] as solid templates. With divinyl benzene, sucrose, phenolic resin, furfurlyl alcohol and polyacrylonitrile as carbon precursors, the resulting carbons obtained in the relatively low temperature range of 700–1000 °C were mainly amorphous due to poor graphitization [2], [3], [4], [5], [6], [7]. There are only a few reports on ordered porous graphitized carbons, which can be synthesized usually by heating at high temperature exceeding 2000 °C [8], [9]. In our previous study, pitch-based graphitized carbon was synthesized by initially preparing a highly ordered porous carbon with colloidal crystals as a template and subsequently by further heating the template-free porous carbon at 2500 °C [10]. High graphitization has been achieved only at high temperature of 2300 °C or higher even in the case of mesophase pitch, a common graphitizable precursor.
There have been also great interests on carbon materials containing heteroatoms such as N, B and Si [11], [12], [13] due to the expectation of novel properties. The most exciting materials among them are carbon nitride phases due to their potential as a ceramic semiconductor and a precursor of super hard materials. Especially graphitic-C3N4 is a novel, rarely examined material [13]. Recently, melamine (2,4,6-triamine-s-triazine), C3N3(NH2)3 and other s-triazine (cyanuric; C3N3) ring compounds such as C3N3X3 (XCl, N, OH, NHCl) have been mainly employed as molecular precursor compounds for the synthesis of graphitic forms of C3N4 [14], [15], [16], [17], [18], [19]. More recently, Groenewolt and Antonietti [20] reported the synthesis of graphitic-C3N4 nanoparticles by simple polycondensation of cyanamide incorporated into porous silica template at 550 °C. The C3N4 structure was confirmed by X-ray photoelectron spectroscopy and infrared spectra. Highly ordered mesoporous structures made of carbon nitride were reported by Vinu et al. [21]. They synthesized hexagonally ordered mesoporous carbon nitride by inorganic templating through the polymerization reaction between ethylenediamide and carbontetrachloride. However, the carbon nitride revealed very low N content with the N/C atomic ratio of ∼0.2 and poor graphiticity despite the highly ordered mesopore array.
Despite great interest in large surface area of carbon nitride materials, it is generally difficult to produce extended nitrogen-rich carbon nitride with a highly ordered nanoporous structure. In this work, graphitic carbon nitride was prepared based on the Antonietti's procedure from condensation of cyanamide (CN–NH2) [20]. The process was used to fabricate highly ordered macroporous graphitic carbon nitride with C3N4 stoichiometry using colloidal silica as a template. The graphitic-C3N4 with highly ordered pore array as a new form of matter will provide a new opportunity in material science due to their potential applications.
Section snippets
Experimental
The monodisperse silica spheres were synthesized based on modification of the reported method [22], [23]. To fabricate the close-packed well ordered silica colloidal crystalline array, a suspension of the silica spheres was allowed to stand in absolute ethanol for slow separation into two phases, a silica layer and an ethanol layer through sedimentation. After decanting the ethanol solution, the silica layer was kept at room temperature and then dried in a vacuum at 100 °C overnight. The dried
Results and discussion
Fig. 1 shows the representative transmission electron microscope (TEM) (a) and the scanning electron microscope (SEM) (b) images of the resulting carbon nitride structure after etching the silica template. Successful morphological replication of colloidal crystals was proved by the TEM and SEM measurements. The presence of uniform spherical pores with about 260 nm in a diameter is clearly seen in both TEM and SEM images. The small black spots of about 60 nm shown in the SEM image of Fig. 1b are
Conclusions
Synthesis of well-ordered macroporous graphitic carbon nitride with C3N4 stoichiometry was demonstrated by polymerization of cyanamide using colloidal silica as a template at 550 °C. All of the results strongly supported the successful preparation of uniform porous graphitic carbon nitride compositionally with C3N4 stoichiometry and structurally with highly uniform ordered pore array of ∼260 nm in size. In particular, the extended C3N4 network structure probably consists of tri-s-triazine rings (C
Acknowledgements
The authors thank KRF (KRF-2004-201-C00048) and new BK-21 program for the financial support and KBSI at Jeonju, Chuncheon, Daejeon and Busan for SEM, TEM, XPS measurements.
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