Preparation of graphite phase carbon nitride (g-C3N4) micro-nano bouquet by thermal polymerization

A novel kind of g-C3N4 micro-nano bouquets were successfully prepared via a simple method using melamine and ammonium chloride as raw materials. X-ray diffractometer (XRD), field emission scanning electron microscope (FESEM), x-ray energy spectrometer (EDX), transmission electron microscope (TEM), high resolution transmission electron microscope (HRTEM),fourier transform infrared spectrometer (FT-IR) and x-ray photoelectron spectroscopy (XPS) were used to characterize the as-synthesized samples. The results indicated that the samples presented graphitic C3N4 micro-nano bouquets. Every microstructure was composed of many petals cross gathered along with the different directions. And the tip of every single petal contained quantities of nano bouquet structures with smaller diameters. In addition, abundant nanoparticles/nanorods distributed and intertwined together on the surface of the nano bouquet structure, and then formed cocoon-like porous morphology. Besides, based on the experimental results, the reasonable chemical reactions and the corresponding growth mechanism during the preparation process of g-C3N4 micro-nano bouquets were proposed. Finally, the UV–vis results showed that the sample was a wide band gap (about 3.11 eV) semiconductor.

Previous studies have reported a variety of synthesis methods of graphite-phase g-C 3 N 4 materials, including template method [11][12][13][14][15], thermal polymerization method [16][17][18][19][20], mechanical ball mill method [21] and solvent thermal method [22][23][24][25][26][27]. For example, Liu et al took porous alumina as a template to prepare g-C 3 N 4 nanotubes, but there were many aluminum oxide impurities in the final products, and the experimental process was relatively complex and inconvenient to operate [12]. Wang et al [13] prepared mesoporous g-C 3 N 4 by taking three-dimensional cubic mesoporous silica as template and melamine as precursor. Wang [11] et al used calcium carbonate particles as hard templates and calcined melamine to prepare blocky g-C 3 N 4 . However, a certain amount of carbon dopant remained in the samples. Lqbal, W et al [16] used one-step method to prepare g-C 3 N 4 nanosheets using melamine and ammonium sulphate as the bubble template in air. Lu [17] et al took urea as raw material and polymerized it into g-C 3 N 4 tablets in muffle furnace. For mechanical ball mill method [21], it generally requires long time ball mill under the protection of high-pressure nitrogen. Carbon, iron and other elements in ball mill existed in various forms of impurities and compounds and were difficult to remove. Solvent-thermal method could ensure the reasonable proportion of products and the impurities were easier to remove. This method generally requires the solvent as a liquid phase reaction medium, which is suitable for the preparation of g-C 3 N 4 [25,26].
Herein, a novel high-pressure thermal polymerization method was reported to prepare g-C 3 N 4 micro-nano bouquets in a high-temperature and high-pressure reactor. XRD, FTIR, EDS, XPS, FESEM, TEM and HRTEM were used to characterize the phase, element composition, valence and bonding characteristics, macroscopic morphology and microstructure of the samples. In addition, UV-vis was used to study the ultraviolet luminescence characteristics and band gap of the samples. Finally, on the basis of the experimental results, the chemical reaction process and growth mechanism of g-C 3 N 4 micro-nano bouquets in the preparation process were also discussed.

Experimental
2.1. Materials synthesis Melamine (C 3 H 6 N 6 99%) and Ammonium Chloride (NH 4 Cl 99%) (China Pharmaceutical Group Co., Ltd) were of analytically pure grade and used without further purification. In a typical experiment process, the reagents were put into the reaction kettle. The crude product was collected after high pressure thermal polymerization for 5 h at 450°C (heating rate of 5°C min −1 ) in the tubular furnace. The obtained sample was dispersed in 50ml deionized water. Then, 40ml hydrochloric acid (12 mol l −1 ) and 2ml nitric acid (15 mol l −1 ) were added into suspension and stirred for 12 h at 80°C. After that, g-C 3 N 4 micro-nano bouquets were obtained with a yield of about 5% by centrifugation, washing and drying for 24 h at 80°C.

Characterization
Phase analysis of the sample was performed by x-ray powder diffractometer (XRD, X, Pert PRO MRD, Cu-k radiation x-ray source, lambda=1.5406A, voltage 40 kV, 40 mA). Element composition valence state and bonding properties of the samples were through x-ray energy (EDS) along with FESEM instruments, Fourier transform infrared spectrometer (FTIR Nicolet Nexus) and x-ray photoelectron spectroscopy analysis (XPS, ESCALAB 250xi spectrometer). Secondary structure and microstructure of samples were observed on the field emission scanning electron microscope (FESEM, Hitachi S4800) and high resolution transmission electron microscopy (TEM/HRTEM, JEM-2100F); The band gap of the sample can be measured by ultraviolet and visible spectrophotometer (UV-vis, UV-3600). . The sample displayed a novel micro-nano bouquets, which was composed of multiple petals intersecting and gathering together in different directions (figures 1(a), (b)), while the end of every single petal contained many nano-bouquet with smaller diameters (indicated by black arrow). The micro-nano bouquets presents the phenomenon of agglomeration (figure 1(a)), which was because that the sample powders were directly sprinkled on the conductive adhesive during the sample preparation process. Each petal was about 5 m in length and 1 m in diameter. As shown in the enlarged images of the petals (figures 1(b)-(d)), the top of the petals was a nano-bouquet with a smaller diameter of less than 500 nm. Besides, some ends of these micro-nano bouquets presented hollow depressed structures (dotted circles). The outer layer of serrate thin wall had a thickness of less than 30nm. And a large number of solid nanoparticles could be observed at the other ends of these micro-nano bouquets (pointed by white arrows). The appeared two different ends might be attributed to the different etching degree during the growth process of the micro-nano bouquet. In addition, it is worth noting that the surface of the bouquet samples were relatively rough, which was attributed to the appearance of a large number of nanoparticles/nanorods existed on the surface. These nanoparticles/nanorods were randomly distributed and interwoven together, forming a cocoon-like pore structure on the surface of the bouquet. These nanoparticles/nanorods could increase the specific surface area of the micro-nano bouquets and enhance the chemical reactivity of the product. Of course, it could also be found that some of the micro-nano bouquet presented a smooth surface with only a few holes (as shown in figure b). This phenomenon could also be caused by the different etching degree of the nano-bouquet during the growth process. The higher the etching degree, the more nanoparticles/nanorods were formed, the rougher of the surface. More holes would be formed finally. Figure 2 shows the EDS map scanning images of g-C 3 N 4 micro-nano bouquets. The structure of micro-nano bouquets contains C, N and O elements. The obvious sample morphology could be observed by scanning the N element scanning, while it was not obvious by scanning the C element surface. This was due to the use of carbon conductive adhesive in EDS sample preparation process. The existence of O element was due to the absorption of water vapor in the air on the large surface area of this special structure. Therefore, EDS mapping scan proved that the sample was carbon nitride. Figure 3 shows the TEM (3(a) and 3(b)) and HRTEM (3(c) and 3(d)) images of g-C 3 N 4 micro-nano bouquets. As shown in figures 3(a) and (b), the top part of the bouquet presented a transparent sawtooth shape (marked by dotted box in figure 3(a)), indicating that the outer wall of the bouquet was relatively thin and could  be easily penetrated by the electron beam. In addition, a small amount of solid conical shape appeared at the top of the bouquet (pointed by dotted box in figure 3(b)), which might be caused by different etching degrees. Figures 3(c) and (b) displayed the HRTEM images of a single serrated structure and a solid tapered end respectively. The results showed that the surface of the two structures was relatively rough, the atomic arrangement and lattice fringe were not clear, and the crystallinity was poor. The crystal plane spacing of the sample was measured to be about 0.303nm, corresponded to be the (002) crystal plane of g-C 3 N 4 . Therefore, the TEM and HRTEM images analysis results of the samples were consistent with that of the FSEM results mentioned above. Figure 4 shows the typical XRD (a), FTIR (b) and XPS (c), (d) spectra of the samples. From the XRD spectrum figure 4(a), the diffraction peak located at 26.8°was corresponded to (002) crystal plane of g-C 3 N 4 (when the d value was 0.322 nm) [28]. Four obvious absorption peaks ( figure 4(b)) located at 3445, 1631, 1316 and 834 cm −1 in the FTIR spectra. The peaks at 834 cm −1 was ascribed to the deformation vibration mode of triazine ring, and the 1316 cm −1 and 1631 cm −1 diffraction peaks were attributed to the contraction vibration mode of C-N and C=N respectively [29,30]. The wide absorption peak at 3445m −1 was generally due to surface hydrolysis and oxidation. In order to further study the chemical bond characteristics of the sample, XPS tests were also performed , as shown in figures 4(c) and (d). The binding energy of C1s spectrum fitting was located at 283.60 eV, 285.31 eV and 287.24 eV (A, B and C peak), corresponded to sp 3 hybridization of C-C, C=N and C-N respectively. N1s spectrum could be decomposed into two peaks of 397.55 eV and 399.20 eV (D and E peak), attributed to sp 3 hybridization of N-C bond and sp 2 hybridization of N=C bond, respectively [31][32][33]. Therefore, FTIR and XPS analysis results also proved that the prepared sample was g-C 3 N 4 from different perspectives.

Chemical reaction process and growth mechanism of micro-nano bouquets
In the synthesis process of g-C 3 N 4 micro-nano bouquets, the following chemical reactions might occur:

* *
Firstly, the decomposition of melamine began at about 350°C. With the increase of temperature, C 3 H 6 N 6 was resolved into the active C * and N * (equation (1)) finally. At the same time, NH 4 Cl will be reduced to NH 3 and HCl at about 170°C [16,[34][35][36] and further decomposition to active N * and H 2 (equation (2)). The decomposition of C 3 H 6 N 6 and NH 4 Cl in the reaction process will produce a large number of gaseous substances (such as C * , N * and H 2 ), and then create a high-pressure growth environment. According to the growth mechanism of Vapor-solid (VS), g-C 3 N 4 micro-nano bouquets began to grow under special high temperature and pressure conditions (equation (4), figure 5(a)). Attributed to generated excessive activity gaseous substances N * and H 2 , micro-nano structure surface layer of the new generation of g-C 3 N 4 bouquets would be etched by these gas under the condition of high temperature and high pressure. Thus, nanoparticles and nano short stick appeared on the surface of the sample. These nanoparticles and nano short rods intertwined together followed formed a kind of interesting silkworm cocoon hole structure. And the exact formation mechanism still needs further research. This supposition could tentatively be supported by the observation of the HRTEM and SEM images. Figure 5 shows the reasonable growth mechanism and FSEM images of g-C 3 N 4 at different growth stages. As can be seen from figure 5(a), the surface and end of g-C 3 N 4 micro-nano bouquets without be etched were relatively smooth. With the beginning of the etching process, the nanoparticle and nanorod structure first appeared on the surface (pointed by dotted box in figure 5(b)). However, the end began to crack without obvious etching (pointed by dotted box in figure 5(c)). With the prolongation of etching time, obvious nanoparticles and splitting phenomenon appeared at the end of the single micro-nano bouquet (framed by dotted line in figure 5(d)). In the later stage of etching, surface and end etching were obvious, and the nanoparticles grew together to form interwoven cocoon-shaped structures ( figure 5(e)). At the same time, the end was splited into several one-dimensional structures with smaller diameters ( figure 5(f)). Figure 6 shows the UV-visible absorption spectrum and the calculated band gap value of g-C 3 N 4 micro-nano bouquets. According to the literature, g-C 3 N 4 is a direct bandgap semiconductor [33]. The corresponding bandgap value (illustrated in figure 6) was calculated to be 3.11 eV, which was different from the value of g-C 3 N 4 (about 2.7 eV) prepared in the literature [37]. This might be due to structural differences and internal defects that lead to changes of the band gap of graphite phase carbon nitride (2.88-2.97 eV). In addition, calculated based on the first-principles of density functional theory, Xu et al [38] calculated that the band gap was about 3.1 eV, which was consistent with that measured in this paper. Therefore, the prepared g-C 3 N 4 micro-nano flower bundle structure would have a good application prospect in the high-temperature wideband gap semiconductor field, such as ultraviolet light emitting devices, high-power electronic devices with highfrequency which could be used at high temperature.

Conclusion
A g-C 3 N 4 micro-nano bouquets was prepared by high pressure thermal polymerization method using traditional materials. Single micro-nano bouquets was composed of multiple petals intersecting and gathering together in different directions, and the end of single petal contained many nano-bouquets with smaller diameter. In addition, the surface of single micro-nano bouquets contains a large number of nanoparticles and nanorods, which was randomly distributed and interwoven together, followed formed cocoon-liked pore  structure on the surface of the micro-nano bouquet. These unique morphological and structural features can further improve the surface roughness and specific surface area of g-C 3 N 4 . It could also serve as surface active sites and enhance chemical reactivity of the sample. Besides, on the basis of the experimental results, the main chemical reaction process, reasonable growth mechanism and corresponding growth model involved in the preparation of g-C 3 N 4 micro-nano bouquets were also proposed. Finally, Uv-vis results showed that g-C 3 N 4 micro-nano bouquets was a wideband gap semiconductor (band gap value was 3.11 eV). This interesting material will have good application prospects in high-temperature wideband gap semiconductor field, such as high-frequency high-temperature, high-power electronic devices and ultraviolet light emitting devices.
The authors declare no conflicts of interest.