Jilin Wanga,
Lulu Zhanga,
Fei Long*a,
Weimin Wang*b,
Yunle Guc,
Shuyi Moa,
Zhengguang Zoua and
Zhengyi Fub
aSchool of Materials Science and Engineering, Key Laboratory of Nonferrous Materials and New Processing Technology of Ministry of Education, Guilin University of Technology, Guilin 541004, China. E-mail: longf@glut.edu.cn; jilinwang@glut.edu.cn; Fax: +86-773-5896436; Tel: +86-773-5896700
bThe State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, China
cNano and Ceramic Materials Research Center, Wuhan Institute of Technology, Wuhan 430073, China
First published on 22nd February 2016
High-quality ultrafine α/β-carbon nitride (α/β-C3N4) nanowires have been fabricated through a novel hot melt reduction synthetic method using polyvinylchloride ([–C2H3Cl–]n), ammonium chloride (NH4Cl) and ferric oxide (Fe2O3) as raw materials. The purity, structure, morphology, crystallinity and surface state of the as-prepared samples were investigated by FSEM, TEM, HRTEM, SAED, XRD, EDX, FTIR and XPS. The nanowires presented good crystallinity with a length range of 1–4 μm and an average diameter of about 10 nm. Every nanowire possessed a high specific surface area and rough surface with abundant exposed atoms/prominences, indicating that the surface structure will facilitate further surface modification, functionalization and related applications. In addition, UV-vis diffuse reflectance and the corresponding photoluminescence (PL) spectra indicated that the nanowires have a wide band gap (4.38 eV) and obvious ultraviolet luminescence properties at the maximum emission peak of about 340 nm. A catalytic reaction mechanism and the growth model were also proposed to explain the formation process of the C3N4 nanowires.
Several synthesis methods have been performed to fabricate these potentially super-hard phase carbon nitride materials, including physical vapor deposition,16–19 chemical vapor deposition,8,20,21 electrochemical deposition,22 solvothermal method,23–25 high temperature and pressure process26,27 and high energy ball milling reactions,28,29 etc. Among them, the former three deposition technologies were usually used to prepare super-hard phase carbon nitride films. However, most of the obtained films demonstrated low nitrogen content, changeful carbon to nitrogen ratios, amorphous and containing the other impurity elements. Even though previous research had reported the partially or wholly crystallized carbon nitride films, the results were only based on the XRD and TEM characterization, lacking reliability.30,31 On the other hand, as for the super-hard phase carbon nitride powders, high temperature and high pressure method required extreme conditions.27 High energy ball milling assisted with annealing processes was an effective approach to obtain α/β-C3N4 nano-materials, while the metal element (such as Fe) came from milling balls would react with C and formed difficultly removed compounds. Solvothermal method usually prepared graphite C3N4, α/β-C3N4 and/or the other multiple-phase carbon nitride structures.13,24,25,32
Generally speaking, on the basis of the literature analysis, it was obvious that the preparation and application research of super-hard phase carbon nitride materials mainly focused on the films. On the contrary, the corresponding research of super-hard phase carbon nitride powders processed at a slow pace up to now. It is due to the difficulty for effectively synthesis of high quality super-hard phase carbon nitride powders. Although few papers have reported the successful fabrication of different super-hard phase carbon nitride crystals or powders, the purity, yield and crystallinity of the products were unsatisfactory. Furthermore, the distinct and comprehensive chemical structure characterization was also difficult, limiting the related growth mechanism, physical and chemical properties, as well as applications of the super-hard phase carbon nitride. Therefore, it is urgent and significant to explore a practicable approach to achieve large amount of high quality super-hard phase carbon nitride powders.
In this paper, a novel kind of ultrafine α/β-C3N4 nanowires has been successfully fabricated through an effectively hot-melt reduction synthetic route for the first time. This method belongs to a solvent-free synthesis approach which is suitable for large amounts industrialized production. In addition, detailed characterizations have been carried out to determine the purity, structure, morphology, crystallinity and surface state of the as-prepared samples. On the base of the experimental results, the possible chemical reactions and growth mechanism were proposed to properly interpret the formation of the C3N4 nanowires. Besides, the band gap and optical property of the as-synthesized were also investigated.
The structure of the as-synthesized samples was analyzed using an X-ray diffraction (XRD, Rigaku D/MAX-LLIA X-ray diffractometer with Cu-Kα radiation). The element composition, valence, and chemical bond types were characterized by X-ray energy dispersive spectroscopy (EDX) attached to FSEM FEI Quanta FEG 250, Fourier transform infrared spectra (FTIR, Nicolet Nexus) and X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). The morphology and microstructure were studied with field scanning electron microscopy (FSEM, FEI Quanta FEG 250), transmission electron microscopy (TEM, JEOL JEM-2100F and Philip CM12), selected-area electron diffraction (SAED, Philip CM12). The UV-vis diffuse reflectance spectroscopy (DRS, Shimadzu, UV-3600) was performed using BaSO4 as the reference. The photoluminescence (PL) spectra were collected using a fluorescence spectrophotometer (Cary Eclipse VA, VARIAN).
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Fig. 1 Typical FSEM images (a and b) and EDX single nanowire line-scan spectra (c) of the as-synthesized C3N4 samples. Scale bars: (a) 2 μm and (b) 500 nm. |
Fig. 2 shows the typical TEM images of the as-synthesized C3N4 samples. It is obvious that the nanowires display solid internal structures with an average diameter of about 10 nm. Of course, some amorphous platelets and nanoparticles also be found in the high magnification TEM images (Fig. 2(b)–(d), marked by white dashed frames), which is well coincided with the above-mentioned FSEM analysis results. Moreover, it is interesting that the nanowires present a rough surface and continually growing tendency along the radial direction, as shown in the enlarged images of single nanowire (denoted by solid frames in Fig. 2(a) and (d)).
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Fig. 2 Typical TEM images of the as-synthesized C3N4 samples. Scale bars: (a) 100 nm, (b) 50 nm, (c) 50 nm and (d) 50 nm. |
Fig. 3 displays the HRTEM images of the as-synthesized C3N4 samples. It can be seen that the nanowires present rough surface and clear lattice fringe with different interlayer spacings of about 0.202 nm (Fig. 3(a) and (b)) and 0.242 nm (Fig. 3(c) and (d)), which corresponds well to the (210) plane of β-C3N4 and (201) plane of α-C3N4, respectively. In addition, both α and β phase C3N4 nanowires have a diameter of about 10 nm but different angles (0°, 12.5°, 25° and 60°) between crystal face array and axial directions of α and β phase C3N4 nanowires. Maybe the angle has been determined at the beginning of the growth process of the single C3N4 nanowire, which should be further studied in the future.
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Fig. 3 Typical HRTEM (a–d) and the corresponding enlarged images of the as-synthesized C3N4 samples. Scale bars: (a) 5 nm, (b) 5 nm, (c) 5 nm and (d) 5 nm. |
In order to make clear the surface condition of the C3N4 nanowires, the partial magnification HRTEM images have been investigated carefully (as shown in Fig. 3(A)–(C), pointed out by white arrows). The rough surface of nanowires is attributed to neither adhered other shaped byproduct particles nor surface breakage, but the different growth rates of the neighboring layers along the direction parallel to the crystal plane (marked with dashed lines). The crystal plane growth rate on one side is depended on the atom deposition arraying rate on this side, finally leading to the rough surface of the nanowires. Therefore, different growth angles and rough surface are both closely related with the growth mechanism of the different C3N4 nanowires. On the other hand, it is worth for special concern that every nanowire possesses a high specific surface area with abundant exposed atoms/prominences (marked with dashed lines). These exposed atoms/prominences will act as high active sites and facilitate the further surface modification, functionalization as well as the related applications.
The SAED pattern of the as-synthesized C3N4 nanowires is showed in Fig. 4. Five polycrystalline diffraction rings located at the d-spacing values of 2.39, 2.061, 1.473, 1.286 and 1.207 Å, which are corresponded to α-(201), β-(210), α-(311), β-(320) and β-(002) lattice planes of C3N4.30
The as-synthesized C3N4 nanowires are also studied by XRD (Fig. 5(a)), FTIR (Fig. 5(b)) and XPS (Fig. 5(c) and (d)). Five peaks at d-spacing of 2.389, 2.069, 1.465, 1.249 and 1.197 Å could be found in the typical XRD pattern of the samples, which were indexed as α-(201), β-(210), β-(301), α-(321) and β-(002) planes of the C3N4.30 Additionally, there is a broad peak at about 25.6° in the XRD pattern, indicating the existence of amorphous carbon in the as-synthesized C3N4 samples. The XRD results are in good agreement with that of FSEM, TEM, HRTEM and SAED characterizations. The FTIR spectrum reveals seven obvious absorption bands at about 3340, 1621, 1571, 1407, 1094, 936 and 802 cm−1. The broad absorption band near 3340 cm−1 can be ascribed to the stretching vibrations of O–H and/or N–H bonds. Three weak peaks at 1621, 1517 and 1407 cm−1 usually be assigned to the characteristic vibrations of CN, C
N and sp3 C–C bonds, respectively. And the strong peak at 1094 cm−1 could be resulted from the stretching vibrations of C–N bond. The other two shoulder peaks at 936 and 802 cm−1 could be attributed to the out-of-plane flexural vibration of sp2 graphite structure and C–N–C group respectively. These peaks have frequently emerged in the FTIR spectrum of carbon nitride compounds and/or films.8,13,24,29,33,34 Fig. 5(c) and (d) show the typical C1s and N1s XPS spectra respectively. The C1s spectrum could be deconvoluted into three peaks at 285.7, 286.6 and 289.1 eV, attributed to sp2C–N, sp3C–N and C–O, respectively. The deconvoluted N1s peak consists of two kinds of nitride atom centered at 398.4, 399.8 and 401.3 eV, respectively corresponding to N–sp3C, N–sp2C and N–O.25,35–40 The XPS results demonstrate a good bonding status between C and N atoms in the as-synthesized C3N4 samples.
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Fig. 5 Typical XRD pattern (a), FTIR (b) and XPS (c and d) spectra of the as-synthesized C3N4 nanowires. |
[–CH2–CHCl–]n → C* + H2 + HCl | (1) |
NH4Cl → N* + H2 + HCl | (2) |
C* + N* + H2 + Fe2O3 → [C*–N*–Fe*] + H2O | (3) |
[C*–N*–Fe*] → C3N4 + Fe | (4) |
HCl + Fe → FeCl2 + H2 | (5) |
Firstly, PVC ([–CH2–CHCl–]n) began to decompose at about 130 °C, followed produced active vapor C*, H2 and HCl (eqn (1)) at high temperature.41–43 Meanwhile, NH4Cl was also dissociated into active vapor N* and H2 and HCl (eqn (2)).44–46 Then new generated chemically active C*, N* and H2 reacted with catalyst Fe2O3 and formed carbon-nitride-catalyst middle product [C*–N*–Fe*] liquid drop and H2O vapor (eqn (3), Fig. 6(a)).44,47 According to the VLS growth mechanism, with the help of catalyst Fe, C3N4 nanowires began to grow on the surface of [C*–N*–Fe*] when the C*and N* were supersaturated (eqn (4), Fig. 6(b)).48,49 This growth process will not be stopped only in the case that the active C*/N* was depleted or the catalyst Fe was inactivated (Fig. 6(c)). Finally, HCl reacted with Fe and produced FeCl2 and H2 (eqn (5)). Because of the different growth rates of the of the neighboring layers along the direction parallel to the crystal plane, the obtained C3N4 nanowires will possess interesting rough surface (Fig. 6(d)).
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Fig. 7 Typical UV-vis diffuse reflectance spectrum (a) and the corresponding PL emission spectrum (b) of the as-synthesized C3N4 nanowires. |
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