Nitrogen-Rich Carbon Nitride Hollow Vessels: Synthesis

Jul 6, 2010 - Yingai Li, Jian Zhang,* Qiushi Wang, Yunxia Jin, Dahai Huang, Qiliang Cui, and. Guangtian Zou. State Key Laboratory of Superhard Materia...
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J. Phys. Chem. B 2010, 114, 9429–9434

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Nitrogen-Rich Carbon Nitride Hollow Vessels: Synthesis, Characterization, and Their Properties Yingai Li, Jian Zhang,* Qiushi Wang, Yunxia Jin, Dahai Huang, Qiliang Cui, and Guangtian Zou State Key Laboratory of Superhard Materials, Jilin UniVersity, Changchun 130012, P. R. China ReceiVed: April 26, 2010; ReVised Manuscript ReceiVed: June 16, 2010

Bulk quantities of nitrogen-rich graphitic carbon nitride are synthesized via a facile reactive pyrolysis process with a mixture of melamine and cyanuric chloride as the molecular precursors. Scanning electron microscopy and transmission electron microscopy studies show that micrometer-sized hollow vessels with high aspect ratios have been successfully elaborated without the designed addition of any catalyst or template. The composition of the prepared carbon nitride determined by combustion method is C3N4.91H1.00O0.22, with the N/C ratio to be 1.64, indicating a high nitrogen content. X-ray diffraction pattern reveals the regular stacking of graphene CNx monolayers along the (002) direction with the presence of turbostratic ordering of C and N atoms in the a-b basal planes. X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy investigations provide further evidence for graphite-like sp2-bonded building blocks based on both triazine and heptazine ring units bridged by 3-fold coordinated nitrogen atoms. The optical properties of the sample are investigated by UV-vis absorption and photoluminescence spectroscopy, which show features characteristic of π-π* and n-π* electronic transitions involving lone pairs of nitrogen atoms. Thermogravimetric analysis curves of the synthesized graphitic carbon nitride hollow vessels show typical weight loss steps related to the volatilization of triazine and heptazine structural units. Introduction Carbon nitride materials are regarded as a valuable extension to complement carbon in material applications and have experienced a renaissance of research activity in recent years.1 Carbon nitrides and related compounds are of great engineering importance due to their unique properties such as high hardness, low friction coefficient, reliable chemical inertness, stable electron field emission, wide-band optical transparency, biocompatibility, and high oxygen and/or water resistivity,1-6 which promise a variety of technological and biological applications. Considering the nature of bonding and the arrangement of atoms in the lattice network of a solid, candidate carbon nitride architectures can be distinguished as 3-D (3-dimensional), 2-D, and 1-D structures, depending on the degree of oligomerization. Dense 3-D polymorphs with C3N4 composition were predicted to possess very high bulk modulus comparable to that of diamond and hold promise as a new family of superhard materials.1,7,8 Great efforts have been focused on the elaboration of those dense hard carbon nitride materials in the form of bulk ceramics or thin films. However, despite the large numbers of hints from different groups that crystalline C3N4 phases are obtainable, no unambiguous evidence for the existence of such a superhard phase is available.1,9 Meanwhile, the layered 2-D polymorphs, graphitic carbon nitride (g-C3N4), and the 1-D solid or tubular nanostructured polymorphs of carbon nitride have recently garnered much attention for their versatile properties apt to being exploited in various applications. For example, carbon nitride nanotubes produced through template-assisted polymerization between (CH2NH2)2 and CCl4 have been proven to be excellent catalyst supports for noble metal nanoparticles which shows high activity in cyclohexene hydrogenation.10 It is recently demonstrated that g-C3N4 exhibits photocatalytic * Corresponding author: Tel +86-431-8516 8252; fax +86-431-8516 8346; e-mail [email protected].

activities for water reduction into H2 or oxidation into O2 under visible lights in the presence of a proper sacrificial electron donor or acceptor, respectively, even without the need for precious metal cocatalysts.11,12 As introduction of N atoms endows the carbon nitride materials polar nature, an extensive effort has been moved to the development of 2-D porous carbon nitride materials with high surface area and well-defined pore size, in pursuit of novel applications such as catalysis, molecular separations, low dielectric devices, and sorption of bulky molecules.13 Therefore, novel procedures to defined graphitic carbon nitride materials and a better understanding of the reaction sequence are ongoing topics of research. As for the synthesis of g-C3N4-related materials, the most postulated network structures used to be based on the triazine ring units (C3N3) cross-linked by trigonal planar coordinated N atoms,8,14 2-fold coordinated N atoms,15 or carbodiimide groups16 as building blocks. However, theoretical studies via ab initio calculations also indicated that g-C3N4 networks based on the heptazine heterocyclic ring (C6N7) structure units linked in the similar manners should exist and be thermodynamically more stable than the corresponding triazine arrangements.17 Results from recent studies, in which the quest for carbon nitride materials using larger conjugated heptazine precursors and rigorous structural investigations of the triazine to heptazine thermal conversion processes were carried out,18 have prompted a reexamination of likely local structures present in graphitic carbon nitride networks. Detailed examinations of the local structure of an amorphous nitrogen-rich carbon nitride material formed via the rapid moderate-temperature self-sustaining exothermic decomposition of a reactive triazine precursor, C3N3(NHCl)3, clearly testify to the ring fragmentation and reorganization into predominantly larger heptazine-like structural building blocks.19 Sketch maps of the structural models proposed

10.1021/jp103729c  2010 American Chemical Society Published on Web 07/06/2010

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Figure 1. Sketch maps of the two adopted molecular precursors: (a) melamine and (b) cyanuric chloride and the postulated structural models of the graphitic carbon nitride based on (c) triazine and (d) heptazine heterocyclic ring units cross-linked by trigonal planar coordinated N atoms.

for layered g-C3N4 materials based on the nitrogen-bridged triazine or heptazine ring units as building blocks are illustrated in Figure 1. Hydrogen is most often encountered in synthetic 2-D carbon nitride materials, irrespective of what process is involved, which makes these materials more appropriately described as CNxHy (x g 1) materials. Moreover, soft conditions (solvothermal process or polycondensation in moderate thermal conditions) appear to be particularly attractive in the sense that various useful morphologies are allowed to be obtainable.20 In a previous work, we have reported the synthesis of graphitic carbon nitride via a two-step pyrolysis of melamine (C3N6H6) at 800 °C for 2 h under vacuum conditions.21 Chemical elemental analysis showed that additional H and O contaminations (H, 3.3 wt %; O, 6.5 wt %) were present in the final products. In order to fulfill the quest for the hydrogen-free prototype g-C3N4, which seems essential both for the understanding of the backbone structure of carbon nitride and for the elaboration of dense hard C3N4 phases using modern high-pressure high-temperature (HPHT) techniques, we present in this paper large-scale synthesis of nitrogen-rich graphitic carbon nitride with remarkably reduced H and O contents, via the reactive thermolysis of C3N6H6 and C3N3Cl3. What is more, the morphology of the final product in this study is found to be micrometer-sized hollow vessels, promising a variety of porosity-related applications. Experimental Section Synthesis. In a typical synthesis process, a quartz tube, 25 mm in inner diameter and 1000 mm in length, was used as the reaction chamber. Fine powders of cyanuric chloride (2,4,6trichloro-1,3,5-triazine, C3N3Cl3, analytical reagent, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan) and melamine (2,4,6triamino-1,3,5-triazine, C3N6H6, chemical pure, Sinopharm Chemical Reagent Co. Ltd., Shanghai, China) were mixed with a molar ratio of 1:2, serving as the precursors. The mixed powders were subjected to ball milling until a homogeneous mixture was obtained. 3 g of the mixture was pressed into a

Li et al. cylindrical column and placed in the central region of the quartz tube thereafter. First, the reaction chamber was evacuated to less than 1 Pa (about 10-2 Torr) by a pump and heated to 150 °C in an electric furnace. Such condition was maintained for about 24 h to eliminate the adsorbed moisture and oxygen completely. Then the pump was turned off, and the reaction chamber was heated to 800 °C within 30 min. The reaction chamber was maintained at this temperature for 2 h under the autogenous pressure due to the pyrolysis of the precursors. The quartz tube was cooled to ambient temperature naturally, and the final pressure inside the reaction chamber reached about 40 kPa. Floccular powders with yellow to brown colors were collected from the inner wall of the quartz tube where the temperature ranged from about 400 to 600 °C during the pyrolysis. No product could be found at the central hightemperature region. Generally, C3N3Cl3 boiled at a temperature a little higher than 190 °C. During the synthesis, it would evaporate and be transported to the low-temperature region of the quartz tube at a faster rate than C3N6H6, which would sublime at about 300 °C. Thus, an incomplete reaction between C3N3Cl3 and C3N6H6 could be anticipated. In order to further eliminate hydrogen, the collected powders were mixed with an extra amount of C3N3Cl3 and treated again in processes described above. Then the powders were collected and washed with acetone, ethanol, and distilled water repeatedly to remove the residual reactants and the byproducts. Finally, samples with a yellowish-brown color were obtained after the product was dried in a hot air flow. Characterization. The characterizations of the samples were carried out via elemental analysis (EA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) in parallel. The composition of the sample was examined by elemental analysis on a CHNS-O analyzer (FLASH EA 1112 series) through the conventional combustion method. The bonding states of the elements were studied via XPS on an EASY ESCA spectrometer (VG ESCA LAB MKII). Structural analysis of the products was carried out by powder X-ray diffractometry on a Rigaku D/max γA diffractometer working with Cu KR radiation (λ ) 0.154 178 nm). The FTIR studies were carried out on a Nicolet Avatar 360 FTIR spectrometer with the prepared powders diluted in KBr pellets. The SEM images of the sample were taken on a HITACHI S4800 microscope working at 20.0 kV, which is equipped with an energy dispersive spectrometer (EDS). The TEM and high-resolution TEM (HRTEM) micrographs of the sample, as well as the selected area electron diffraction (SAED) pattern, were obtained via a JEM-2100F transmission electron microscope using an accelerating voltage of 200 kV. The ultraviolet-visible (UV-vis) absorption spectra of the synthesized product were taken on a Shimadzu UV-3150 spectrometer, and the photoluminescence (PL) measurement was conducted under 250 nm UV fluorescent light excitation by a Shimadzu RF-5301PC spectrophotometer at room temperature. Thermogravimetric analyses (TGA) were performed at a heating rate of 10 °C min-1 under flowing argon on a Perkin-Elmer Pyris Diamond TGA/DTA system. Results and Discussion The elemental analysis results (C, 32.97 wt %; N, 62.93 wt %; H, 0.92 wt %; O, 3.18 wt %) of the prepared sample indicated that the average composition was C3N4.91H1.00O0.22, with the N/C ratio to be 1.64, which is higher than the expected one (1.33) of the theoretically predicted C3N4 empirical stoichiometry. Comparing with the results reported in our previous work,21 it can be seen that H content has been considerably reduced by about 3 times and O content has been more than

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Figure 2. X-ray diffraction patterns of (a) the as-prepared raw product and (b) the final sample washed thoroughly with acetone, ethanol, and distilled water.

halved. The composition of our product is similar to those of the amorphous nitrogen-rich carbon nitride materials (C3N4+x, where 0.5 < x < 0.8) elaborated by rapid decomposition of a chloric solo precursor trichloromelamine ((C3N3)(NHCl)3).19 It may be indicated that presence of Cl (and presumably other halogen elements) may be effective for the reduction of H in the growth of CNx materials. As a concomitant result, loss of nitrogen at elevated temperature may be partly prevented since a considerable amount of nitrogen volatilize in the form of hydrogenous gases such as NH3. An ideal reaction in this work may be formulated hypothetically as

C3N3Cl3 + 2C3N6H6 f 3C3N4 + 3NH3 + 3HCl The typical XRD patterns of the as-prepared raw product and the sample washed thoroughly with acetone, ethanol, and distilled water are shown in Figure 2. Most diffraction peaks of the raw product can be assigned to the byproduct NH4Cl (PDF No. 73-1491), as shown in Figure 2a, which suggests that a chemical reaction has taken place between the precursors C3N3Cl3 and C3N6H6. It can be seen that the raw materials and the byproducts have been removed almost completely after washing (Figure 2b). As for the final product, only a strong sharp peak at the position of 2θ ) 27.1° is observed in the pattern, with the corresponding d-spacing to be 0.329 nm, which is a characteristic interplanar stacking peak of aromatic systems, indexed as the (002) peak for graphitic materials. The peak position is in good accordance with recent reports on graphitic carbon nitride made by polymerization of cyanamide, dicyandiamide, or melamine22 and our previous study.21 This (002) stacking is found to be significantly more tight than the packing in pure carbon with graphene units (d ) 0.353 nm) and in crystalline graphite (d ) 0.335 nm).23 The higher packing density perpendicular to the layers can be attributed to the localization of the electrons and stronger binding between the layers due to heteroatom (N) substitution in aromatic systems. The absence of the (hk) peaks (intraplanar reflections) indicates the presence of turbostratic ordering of C and N atoms in the graphene layers.13,24 It may be speculated that there is regular stacking of the graphene CNx monolayers along the (002) direction, while some degree of disorder may exist in the a-b basal planes. XPS were used to measure the core level chemical shifts and thus characterize the bonding nature of the prepared sample. The survey scan XPS spectrum and the deconvoluted C 1s and N 1s spectra are shown in Figure 3. The survey scan spectrum (Figure 3a) for the overall composition of the product shows

Figure 3. XPS spectra of nitrogen-rich graphitic carbon nitride hollow vessels synthesized via reactive pyrolysis of triazine and cyanuric chloride: (a) survey scan, (b) deconvoluted C 1s, and (c) deconvoluted N 1s spectra.

only strong peaks for carbon and nitrogen as expected. No peaks due to other elements may be discerned within detectability. It is indicated that the synthesized material is composed primarily of carbon and nitrogen. Figure 3b shows that C 1s spectrum can be deconvoluted into two Gaussian peaks. As shown in Figure 1, either in a triazine-based or in a heptazine-based graphitic structure, the bonding states of C atoms are similar to those of the C atoms in melamine molecules (C3N6H6). In all three cases, carbon atoms have one double and two single bonds with three N neighbors. Thus, the interpretation of the C 1s XPS spectra may follow the results on C3N6H6.25 The peak at 284.6 eV, corresponding to C-C coordination, is identified as originating from pure graphitic carbon presumably formed during minor decomposition of the carbon nitride sample under X-ray irradiation or due to carbon-containing contaminations.25,26 The peak at 288.2 eV, corresponding to NdC-N2 coordination, is identified as originating from carbon atoms that have one double and two single bonds with three N neighbors.22,25,26 Figure 3c shows that N 1s spectrum can be deconvoluted into several Gaussian peaks. The predominant component with a binding energy of 398.7 eV may be attributed to electrons originating from nitrogen atoms sp2-bonded to carbon26 or, more precisely, to nitrogen atoms in CdN-C groups.22,27 Deconvolution also reveals a weak additional signal at 401.1 eV, indicative of amino groups carrying hydrogen (-NHx).22 Another weaker signal is found at 396.5 eV and may be attributed to N-O related species, since in these groups nitrogen atoms have less negative charge than those mentioned above. However, integration of the lines shows that nitrogen atoms in -NHx and N-O related species amount to less than 10% of the total nitrogen content. The typical FTIR spectrum of the final carbon nitride sample is displayed in Figure 4. A broad but structured absorption band in the region between 900 and 1800 cm-1 dominates the spectrum, with distinguishable maxima at 1325, 1400, and 1627 cm-1 and shoulders at about 1172, 1254, and 1478 cm-1. A single sharp peak at about 808 cm-1 is also present. These results are in fairly good accordance with recent reports of graphitic carbon nitride materials prepared by polycondensation19,28,29 and polymerization reactions.10 The signals at 808, 1172, 1254, 1325, 1478, and 1627 cm-1 may be attributed to the skeletal vibrations of heptazine heterocyclic ring (C6N7) units, comparing with the similar absorptions of heptazine compounds such as C6N7Cl3,

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Figure 4. FTIR spectrum of the prepared graphitic carbon nitride powder diluted in a KBr pellet.

Figure 6. (a) TEM micrograph of the representative graphitic carbon nitride hollow vessels. The examples of the closed and the open ends of the hollow vessels are shown in (b) and (c), respectively. (d) HRTEM micrograph of the prepared nitrogen-rich graphitic carbon nitride. Encircled are tiny crystalline regions. The scale bars in (a), (b), and (c) represent 1 µm, and the one in (d) represents 5 nm.

Figure 5. (a) Typical SEM image of the synthesized graphitic carbon nitride hollow vessels. The scale bar represents 50 µm. (b) Corresponding EDX spectrum of the synthesized sample.

C6N7(N3)3, and C6N7(NH2)3.17,28,30,31 However, the signals at 1400 cm-1 may originate from the stretching vibrations of triazine ring (C3N3) units,28 whereas the sharp peak at about 808 cm-1 can be attributed to the ring-sextant out-of-plane bending vibration characteristic of both triazine and heptazine ring systems.30 In this sense, the final product seems not fully condensed, with both triazine and heptazine ring units present. As shown in Figure 4, there are two broad bands centered at about 3135 and 3425 cm-1, which can be assigned to the symmetric and asymmetric stretching modes of amino groups, respectively. Terminal -NHx (x ) 1, 2) groups at the defect sites or the surfaces of the sample (and their intermolecular hydrogen bonding) may contribute to the signals.10,30 However, the influence of adsorbed moisture on the frequency region above 3000 cm-1 of the FTIR spectrum may not be completely ruled out. In the SEM image of the prepared carbon nitride sample, as shown in Figure 5a, large numbers of long, thin, columniform structures may be found. It can be seen that these cylindraceous columns are randomly oriented, with their diameters ranging

from 0.5 to 5 µm and the lengths of them to be up to tens of (and some even hundreds of) micrometers. Several bundles composed of these thin columns are observable. However, lamellar sheets and bulky bumps with irregular shapes are also present. The final sample seems rather insulative under electron beam irradiation in SEM studies; sparkling spots occur even after Pt has been sputtered to enhance conductivity. The typical EDX spectrum of the prepared nitrogen-rich carbon nitride is shown in Figure 5b. It can be seen that the sample contains mainly C and N elements. Pt element coming from the sputtered Pt-coating is also detected. In addition, the sample also contains a small amount of O element, which may be due to adsorption or surface oxidation. In a wide spectrum range, no signals for other elements can be observed. The quantitative analysis demonstrated the composition of the sample should be: C, 37.03 at %; N, 60.19 at %; O, 2.54 at %; Pt, 0.24 at %. Thus, an average structural formula may be indicated to be C3N4.88O0.21, with the N/C ratio to be 1.63, which is in accordance with the elemental composition obtained via combustion method. In the TEM images of the prepared carbon nitride sample, as shown in Figure 6a, the columniform structures are proven to be tubular hollow vessels. It can be seen that the wall thickness is uniform throughout the vessels and can be estimated to be in the range of 200-250 nm. It seems that the walls of the vessels are crinkled in that the contrast of the microvessels exhibits complicated stripelike features. Both closed ends (Figure 6b) and open ends (Figure 6c) are observed, which give further evidence that the columniform structures are microtube-like architectures. What is more, spheroid-shaped nanoparticles, with the diameters to be about 200-300 nm, are found filling the interior of the vessels. The HRTEM image of the prepared carbon nitride sample is shown in Figure 6d, together with the selected area electron beam diffraction (SAED) pattern (inset). The HRTEM results, which reveal the final product has a weakly

Nitrogen-Rich Carbon Nitride Hollow Vessels ordered graphitic structure, are similar to those reported for the carbon nitride microfibers synthesized via HPHT technique by using graphitic carbon nitride as precursor.32 Long-range order seems absent in the product, whereas short-range order within 3-5 nm may be observed. Many small crystalline regions, characteristic of 3-7 parallel fringes, can be distinguished in the HRTEM image (encircled regions, see also Figure S1 in the Supporting Information). Based on these observations, a reasonable hypothesis can be made that the microstructure of the product may be composed of a 3-D arrangement of extended, bent, and cross-linked graphitic basal planes, which is similar to the circumstances in the substoichiometric fullerene-like carbon nitride (FL-CNx) compounds.4,27,33,34 Through systematic first-principles DFT calculations, it has been concluded that for FL-CNx the C-N bond rotation due to incorporated N atoms not only results in buckling of the graphene sheet but also can promote a cross-linkage between layers, and thus curvature and cross-linkage of the graphene sheets may be favored energetically.35 Despite the difference in the nitrogen concentrations, these experimental and theoretical findings may be readily adopted to understand the microstructures and local coordination of atoms of the nitrogen-rich carbon nitride prepared in our case. The SAED pattern presents two dispersive diffraction rings, corresponding to the (002) and (004) planes of the graphitic structure. These results are in good agreement with our XRD studies. In a recent work, micro- and nanotubes of an amorphous carbon nitride material were reported to be synthesized by metathesis reactions between cyanuric chloride (C3N3Cl3) and various solid nitrogen sources such as alkali carbodiimides (Li2(CN2)), dicyanamide (Na[N(CN)2]), dinitridoborate (Li3(BN2)), and nitride (Li3N).36 Needle-shaped crystals of N(C3N3Cl2)3 were identified as an intermediate which served as the template that controlled the growth of the carbon nitride tubes. However, the situation is quite different in our case. There is no obvious evidence of such an intermediate as N(C3N3Cl2)3 or other in the XRD pattern of the raw product (Figure 2). In fact, only the targeted carbon nitride and well-crystallized NH4Cl are found to coexist. On the other hand, nanometer-sized spheroid-shaped nanoparticles, rather than crystallites with enough sizes, are observed inside the prepared hollow vessels (Figure 6). Thus, a template-controlled growth mechanism as proposed in the above-mentioned interesting work seems invalid for our product. In our previous study,21 nanovessels and curled nanosheets were formed by a two-step pyrolysis of a solo precursor melamine (C3N6H6) under vacuum conditions, without the aid of any catalyst or template. It can be indicated that spontaneous curling of carbon nitride lamella and nucleation of hollow vessels under our experimental conditions are possible. The optical properties of the synthesized nitrogen-rich graphitic carbon nitride sample are investigated by UV-vis absorption and photoluminescence (PL) spectroscopy. Two strong absorption peaks may be observed at about 250 and 400 nm, respectively, as shown in Figure 7a. The weaker peak at 250 nm has been attributed to π-π* electronic transition in the aromatic 1,3,5-triazine compounds in the literature, while the stronger and dominant peak at 400 nm may be attributed to the n-π* electronic transitions involving lone pairs of nitrogen atoms.26,37 However, it should be pointed out that networks composed of conjugated heterocyclic ring systems such as both triazine and heptazine may have absorptions due to π-π* and n-π* electronic transition in this range as well as molecular nitrogen-containing aromatic systems.38 The longer wavelengths region (λ > 300 nm) of our UV-vis absorption spectrum resembles to a large extent the UV-vis diffuse reflectance spectrum of the polymeric graphitic carbon nitride material, which is recently reported to present highly desirable photoca-

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Figure 7. (a) UV-vis absorption and (b) PL spectra of the synthesized graphitic carbon nitride hollow vessels.

talysis in water reduction into hydrogen.11 A sharp absorption edge can be observed in the 400-450 nm region. The corresponding bandgap of the synthesized nitrogen-rich graphitic carbon nitride sample is estimated to be 2.7-2.8 eV, showing an intrinsic semiconductor-like absorption in the blue to violet region of the visible spectrum. This bandgap has been shown to be sufficiently large to overcome the endothermic character of the water-splitting reaction (requiring 1.23 eV theoretically). In our previous study,21 only the π-π* absorption band at about 235-250 nm could be observed for the graphitic carbon nitride prepared through two-step pyrolysis of the solo precursor melamine. The n-π* absorption band, however, was hardly discernible in that case. It seems that the contents of O and especially H strongly affect the UV-vis absorption behaviors of carbon nitride materials. A reasonable postulation can be made that the H atoms introduced into the graphitic carbon nitride network preferentially bond with N atoms, wherever in the manner of σ bonds or H bonds. These proton-like H atoms have strong affinity for the lone pairs of the N atoms; thus, H incorporation into the carbon nitride network may depress the distribution of the nonbonding electrons on the N sites. As a result, higher H contents lead to weakening and eventually diminishing of the n-π* absorption band, and vice versa. As O is usually introduced into the graphitic carbon nitride network in the form of -OH groups, O content may affect the intensity of the n-π* absorption band in much the similar way as H. From a viewpoint of the energy band theory, the incorporation of H and O may introduce new functional bands, which will attract electrons from the nonbonding band derived from the lone pairs of N atoms. Photoluminescence properties have been reported for graphitic carbon nitride by several groups.37,38 Strong photoluminescence at room temperature with a maximum at about 435 nm has also been reported in our previous study.21 The typical room temperature photoluminescence spectrum of the synthesized product in this work is shown in Figure 7b. A broad PL emission band can be clearly observed in the 400-700 nm spectrum range with two maxima at about 450 and 502 nm. A shoulder at about 518 nm may be discerned as fine structure. The emission maximum at 450 nm shows quite precise correspondence with the absorption edge due to n-π* electronic transition discussed above. However, the exact mechanism for this broad and strong green-blue PL is still indecisive at this stage. Thermogravimetric analysis (TGA) curve of the synthesized graphitic carbon nitride hollow vessels is shown in Figure 8. It can be seen that during calcination under argon atmosphere the sample shows a sharp mass loss (about 25% of the total) at about 307 °C and then gradually loses the remaining weight entirely until the temperature rises to 725 °C. The TGA

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Figure 8. TGA curve of the obtained graphitic carbon nitride hollow vessels.

characteristics of our sample bear much resemblance to those reported for the carbon nitride microfibers synthesized via HPHT technique by using graphitic carbon nitride as precursor.32 The former mass loss can be attributed to volatilization of triazinebased structure units, and the latter step of the TGA curve is related to the loss of heptazine-based units or other advanced condensates. Since the sample looses 100% of its weight with no remnant at last, sublimation of such material may be proposed. An initial weight loss is also observed below 200 °C, which may arise from the evaporation of adsorbed moisture in the pores and/or on the surfaces of the hollow vessels. Conclusions This article describes the powder synthesis of high-nitrogencontent graphitic carbon nitride with a unique tubular morphology in gram quantities via the reactive thermolysis of two molecular precursors, C3N6H6 and C3N3Cl3. Structural characterizations based on XRD and SAED indicate the presence of turbostratic ordering of C and N atoms in the graphene layers of the product. Spectroscopic analyses via XPS and FTIR techniques are consistent with the layered structure containing both triazine and heptazine structural units and sp2-hybridized bonding feature. SEM and TEM morphologies investigations reveal the successful elaboration of micrometer-sized tubular hollow vessels. HRTEM image justifies weak ordering localized within several nanometers with the absence of long-range ordering. A broad and strong photoluminescence in the greenblue region is observed, which is possibly related to π-π* and n-π* electronic transitions involving lone pairs of nitrogen atoms. Potential application of the synthesized nitrogen-rich graphitic carbon nitride hollow vessels in photocatalysts and catalytic supports, drug delivery, and optical and electronic fields may be anticipated. Acknowledgment. This work was supported by the Research Fund for the Doctoral Program of Higher Education of China (grant 20070183175), the Natural Science Foundation of China (grant 50772043), the National Basic Research Program of China (grants 2005CB724400 and 2001CB711201), and the ResearchStartupFoundationofJilinUniversity(grant419080103460). Supporting Information Available: HRTEM micrograph of a selected region of the prepared nitrogen-rich graphitic carbon nitride. This material is available free of charge via the Internet at http://pubs.acs.org.

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