Nitride-based solid-state materials have attracted a great deal of attention in many fields. For example, numerous studies have focused on the use of multifarious metal and non-metal nitrides as semiconductor photocatalysts to achieve efficient photon energy conversion ^{[1, 2]}. The reported studies have clearly shown that the valence band of nitride semiconductors commonly consists of N 2p orbitals or hybridized N 2p and O 2p orbitals (such as Ta₃N₅ and TaON) above the O 2p valence band of the corresponding oxides (e.g., Ta₂O₅). This serves to extend the photoactive region to the visible spectrum for efficient use of sunlight ^[3]. Therefore, many nitride semiconductors activated by visible light have been exploited and are considered as important candidates for artificial energy conversion, although the stability of these nitride-based inorganic semiconductors is a concern ^{[4, 5]}. It is reported that surface kinetic controls using co-catalysts can significantly prevent metal nitride photocatalysts from photo-corrosion, increasing the potential of a number of nitride semiconductors for use in artificial photosynthesis.

The graphitic carbon nitride (CN) polymer is a semiconductor regarded as the most stable crystal phase of the binary CN allotropes under ambient conditions. Since the report of CN redox photocatalysis in 2009, this robust conjugated polymer, with a band gap of ~2.7 eV, has stimulated a great deal of scientific interest owing to its unique and tunable optical and electronic properties ^{[6, 7, 8]}. According to previous studies, CN polymers are usually synthesized by a bulk self-polymerization reaction of N-rich precursors at high temperature (500-650 °C) ^{[9, 10, 11]}. The starting materials are generally the compounds containing or engendering triazine motifs in these reactions. The bulk condensation is typically limited by reaction kinetics, and the synthetic materials are not well condensed/crystalline and exhibit low specific surface areas. It has been shown that the crystallinity of a semiconductor greatly influences its photoredox function because the crystal structure imperfections can hamper the fast migration and separation of light-induced charge carriers ^{[12, 13]}. Recently, it has been shown that the crystallinity of the CN polymer can be observably enhanced by heating melamine in an airtight system under an autogenic pressure of NH₃, compared with that of CN synthesized by pyrolyzing melamine in ambient air ^[14]. The enhancement in crystallinity stems from the condensation reactions. However, the materials thus produced in the closed system are generally triazine-based CN polymers with a much reduced photocatalytic activity. Therefore, a new synthetic procedure for carbon nitride polymers is called for.

It is well known that the nitrogen in NH₃ gas, generally a nucleophilic reagent, is very reactive at high temperature, reacting with metals or metal chlorides/oxides to produce metal nitrides/oxynitrides ^{[15, 16, 17]}. The nitrogen can also react with non-metal elements, such as B, P, or Si, to yield the corresponding non-metallic nitride materials ^{[18, 19]}. Indeed, the synthesis of nitrogen-rich carbon-based materials has been demonstrated by thermal annealing of amorphous carbon powders under NH₃ atmosphere at high temperature (> 1000 °C), revealing that the nitrogen can combine with the stable carbon structure to form covalent C-N bonds at high temperature ^[20]. However, the nitridation of organic molecules with NH₃ at high temperature, to produce nitride semiconductors for heterogeneous photocatalysis, has not been widely reported, and thus far, the nitridation of triazine trichloride by the acid-base polymerization reaction to form covalent carbon nitride frameworks is yet to be investigated.

Herein, we introduce a thermal nitridation route for the synthesis of well-defined heptazine-based covalent carbon nitride frameworks by using NH₃ and solid triazine trichloride as starting materials, and explore their photocatalytic reactivities. Our experiments show that the synthetic route allows one to nitridize the solid cyanuric trichloride into heptazine-based CN nanostructures at elevated temperature, as shown in Scheme 1. In the first stage of the synthesis, the gas-phase nitridation of organic molecules can produce a carbon nitride polymer composed of s-triazine rings from a nucleophilic substitution reaction of solid triazine trichloride with NH₃. Chlorine can easily depart from the triazine trichloride to produce NH₄Cl after the reaction of the formed HCl with NH₃, and the amino group is strongly bound with carbon because the amino group is a much poorer leaving-group than chlorine ^[21]. Polymeric CN materials consisting of triazine motifs are kinetically labile with regard to their further conversion into heptazine-based carbon nitride frameworks at high temperature ^[22]. This nitridation process might yield well-condensed polymelon by fusion of triazine units.

Scheme 1. Thermal nitridation induced polymerization of cyanuric trichloride (CC) to g-C3N4 with flowing NH3.

In our experiments, thermal-nitridation-derived carbon nitride samples were synthesized by heating cyanuric chloride (1.38 g) at a certain temperature for 2 h with a ramp rate of 5.0 °C/min under flowing NH₃ (25 mL/min). The as-synthesized products are denoted as CNC-X, where X is an arbitrary number that represents the pyrolyzing temperature (Table 1). As a reference, melamine-derived carbon nitride (g-CN), synthesized from the traditional thermal-condensation process, was also obtained ^[9]. We then investigated the properties of well- defined carbon nitride polymers prepared at different degrees of layer condensation, and studied the visible light-activated catalytic performance of the resultant CNC samples for hydrogen photosynthesis from an aqueous solution of triethanolamine.

Table 1 Physicochemical properties of g-CN and CNC samples. Catalyst ABET (m2/g) C/N molar ratio H content (wt%) HER a (µmol/h) g-CN b 10 0.67 1.91 12 CNC-1 c 26 0.66 2.04 68 CNC-2 c 54 0.67 1.76 101 CNC-3 c 69 0.68 1.61 120 a H2 evolution rate. b Synthesized by heating melamine at 550 °C in air. c The numbers 1, 2, and 3 correspond to reaction temperatures of 500, 550, and 600 °C, respectively.

Table 1 Physicochemical properties of g-CN and CNC samples.

First, we analyzed the chemical structure and composition of the products by X-ray photoelectron spectroscopy (XPS) characterization, which is sensitive to the chemical environment of lightweight elements. Signals of C, N, and O in the survey spectrum in Fig. 1(a) were recorded, but no peak assigned to Cl could be observed. The absence of Cl in the final sample was confirmed by the high-resolution spectra of Cl 2p, as shown in Fig. 1(b). The absence of Cl species in the as-prepared CNC solid is further confirmed by elemental analysis, and the values of the C/N ratios are in accordance with that of the melamine-derived g-CN (Table 1). Additionally, the presence of O is probably the result of the H₂O and O₂ adsorbed on the surface of the resulting samples. The structure details relating to the framework C and N elements in the CNC-3 sample were measured by their corresponding high-resolution spectra. The C 1s XPS spectrum in Fig. 1(c) shows one main peak at 287.9 eV, which is related to an sp²-bonded carbon (N-C=N). The other weak C 1s peak at 284.6 eV is attributed to carbon impurities ^[10]. In Fig. 1(d), the N 1s spectrum can be deconvoluted into four peaks. The strongest N1s peak at 398.4 eV is identified as an sp²-bonded nitrogen (C-N=C). The N 1s peaks at 399.5 and 400.7 eV are attributed to N atoms that are bound to three C atoms. The last peak at 404.1 eV is assigned to π-excitations. This result reveals that the as-obtained CNC-3 sample is dominated by tri-s-triazine structures that are basic units of the g-CN polymer, as confirmed by the solid-state ¹³C nuclear magnetic resonance (NMR) analysis (Fig. 2), in which there are two groups of resonances at δ = 162.5/164.8 and 156.4, separately assigned to CN₂(NH_x) and CN₃ motifs ^[14].

Fig. 1. XPS survey spectrum of the CNC-3 sample (a) and the corresponding high-resolution XPS spectra of Cl 2p (b), C 1s (c), and N 1s (d).

Fig. 2. Solid-state 13C NMR spectrum of the CNC-3 sample.

The X-ray diffraction (XRD) patterns of CNC materials are analogous to that of bulk g-CN in Fig. 3(a), which is governed by the characteristic interlayer stacking (002) peak and in-plane repeating motifs (100) peak ^[9]. This result indicates a graphitic layer structure for CNC samples, similar to that of g-CN. A careful analysis of Fig. 3(b) revealed a clear shift of the (002) peak from g-CN at 27.5° to CNC-3 at 27.8°, revealing a reduction in the interlayer stacking distance from 0.325 to 0.322 nm ^[23]. Additionally, the CNC materials in Fig. 3(c) exhibited narrower (002) peaks than those of the melamine-derived g-CN, indicating a higher degree of crystalline order for the nitridation products compared with that of the reference. It should be noted that all CNC products feature some residual amount of hydrogen that is also characterized in normal g-CN, which decreases with increased condensation temperature (Table 1).

The well-resolved and strong fourier transformed infrared (FT-IR) absorption bands further support the formation of the carbon nitride structure. In Fig. 3(d), all CNC solids in the 1200-1600 cm⁻¹ region feature several stretching vibrations of aromatic CN heterocycles, together with a typical breathing mode of triazine units at 800 cm⁻¹. The results are consistent with the IR observations for the melamine-derived sample. These results demonstrate that we have successfully prepared the heptazine-based carbon nitride frameworks through this modified thermal condensation approach.

Fig. 3. XRD patterns (a, b, c) and FT-IR spectra (d) of the g-CN and CNC-X samples.

The morphology and microstructure of g-CN and CNC-3 samples were investigated by transmission electron microscopy (TEM), as shown in Fig. 4. The thin-slice-like CNC-3 sample is quite different from the reference sample composed of bulky platelet-like solid agglomerates in Fig. 4(a), probably owing to the use of the active precursors that ameliorates the kinetic hindrance of the solid-state reaction. In some regions, curved layers were also recorded for the CNC-3 sample in Fig. 4(b) and (c), which is conducive to the reduction of surface tension. This is somewhat similar to the case of the presence of carbon scrolls in graphene. The formation of nanosheet morphology was further confirmed by atomic force microscopy (AFM). In the cross-sectional AFM image in Fig. 4(d), we can observe several nanosheets present on the surface of a mica wafer. The nanosheets have a uniform thickness of approximately 3-6 nm.

Fig. 4. TEM images of g-CN (a) and CNC-3 (b, c), and AFM image of CNC-3 (d).

The data obtained from N₂ adsorption-desorption measurements (Table 1) reveal that the specific surface area of CNC samples is larger than that of the reference sample. This opened-up nanosheet structure with enlarged surface area would provide multiple active sites, facilitate surface chemical species transfer, and promote charge separation, which are all characteristics that are desirable for surface-dependent photochemical reactions.

Changing the thermal conditions enables the fine adjustment of the electronic and optical features, as revealed by the UV-vis diffuse reflectance spectra for CNC samples shown in Fig. 5. Typical semiconductor absorption with a band gap of 2.95 eV is observed for CNC-3 and 2.69 eV for g-CN. We propose that the obvious blue shift of the absorption edge arises from the structure disorder and enlarged surface area. Indeed, a similar hypsochromic-shift has been observed for the reported carbon nitride-based samples with a denser packing and enlarged surface area ^{[24, 25, 26]}.

Fig. 5. UV-vis diffuse reflectance spectra of CNC-X, with g-CN as reference and the corresponding Tauc plot (inset).

Photo-assisted hydrogen production was then performed under visible light excitation of greater than 410 nm, using a water-triethanolamine solution. Pt was used as a cocatalyst, owing to its well-known low over-potential for water reduction. Both CNC samples and reference g-CN are active photocatalysts for H₂ generation, as recorded in Table 1. The performance of g-CN, in terms of hydrogen evolution rate (HER), was 12 µmol/h, which is clearly lower than those of CNC materials (HER = 68, 101, 120 µmol/h for CNC-1, CNC-2, and CNC-3, respectively). Generally, the activity of the photocatalysts relates to a series of parameters, such as optical absorption, surface area, absorption capacity, charge carrier recombination rate and morphology. In our case, with increasing temperature, the absorption of the CNC samples in the visible region decreased, indicating that the optical properties are not the main factor that affects the catalytic performance. Hence, the observed improvement of photoactivity, relative to the g-CN, may be associated with an increase in surface area, an alternative nanosheet morphology that facilitates surface species transfer and charge-carrier separation, as well as the reduced protonation status ^[11]. We also measured their photocatalytic stability, which is acknowledged as an important parameter for evaluating a photocatalyst. Figure 6 shows a typical time course of H₂ evolution. A stable H₂ release rate of approximately 120 µmol/h was observed for CNC-3. Moreover, there was only a slight reduction of the photoactivity after illumination for four continuous runs, indicating a good stability of the sample.

Fig. 6. Stability test of H2 photosynthesis (evacuation every 4 h) for the CNC-3 sample with visible light irradiation (λ > 410 nm).

In summary, we have used a thermal nitridation approach for the reaction of solid triazine trichloride with NH₃ to achieve conjugated heptazine-based CN framewoks. The visible light-activated CNC samples all photocatalyze H₂ generation from an aqueous triethanolamine solution, and exhibit a much higher reactivity compared with the g-CN catalyst. The developed nanostructures exhibit reduced dimension, greater surface areas, good crystallinity, and suitable band structure. These characteristics are proposed to all contribute to the efficient photoreactivity. This thermal nitridation route, using reactive NH₃ as a starting material for the preparation of CN polymers combined with efficient visible light activity, is also of significant interest for yielding different modifications of CN-based catalysts for solar energy application ^[27].