NH4Cl-induced low-temperature formation of nitrogen-rich g-C3N4 nanosheets with improved photocatalytic hydrogen evolution

Carbon 153 (2019) 757e766

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Carbon journal homepage: www.elsevier.com/locate/carbon

NH4Cl-induced low-temperature formation of nitrogen-rich g-C3N4 nanosheets with improved photocatalytic hydrogen evolution Xinhe Wu a, Duoduo Gao b, Ping Wang b, Huogen Yu a, b, *, Jiaguo Yu c a

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, People's Republic of China Department of Chemistry, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan, 430070, People's Republic of China c State Key Laboratory of Advanced Technology for Material Synthesis and Processing, Wuhan University of Technology, Wuhan, 430070, People's Republic of China b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 June 2019 Received in revised form 12 July 2019 Accepted 24 July 2019 Available online 25 July 2019

The high-temperature secondary calcination (>500
C) of bulk g-C3N4 usually suffers from a very low yield of g-C3N4 nanosheets owing to its serious and massive depolymerization. In this study, a NH4Clinduced low-temperature second-calcination approach has been used to synthesize nitrogen-rich g-C3N4 nanosheets with a high yield (ca. 32 wt%), which includes the initial intercalation of NH4Cl into the interlayers of bulk g-C3N4 and the following direct low-temperature calcination at 400
C. It is found that during the calcination process, the thermal gas flow (HCl and NH3) from NH4Cl decomposition not only can efficiently facilitate the delamination and depolymerization of the g-C3N4 structure, but also can introduce many amino groups on the g-C3N4 surface, resulting in the successful synthesis of nitrogenrich g-C3N4 nanosheets at such a low temperature. Experimental data suggests that the resulting nitrogen-rich g-C3N4 nanosheets show a distinct enhancement for the H2-evolution performance mainly owing to the introduction of amino groups, which can efficiently enrich Hþ from water to facilitate the rapid generation of H2. This study may open up a fire-new insight for the preparation of high-efficiency nanometer materials. © 2019 Elsevier Ltd. All rights reserved.

Keywords: Photocatalysis H2 evolution g-C3N4 nanosheets High yield Low temperature

1. Introduction Sparked by the growing energy crisis and environmental pollution issues, semiconductor photocatalysis has been widely investigated to be a promising technique for dealing with global energy and environment crisis [1e5]. Among numerous photocatalysts [6e10], graphitic carbon nitride (g-C3N4) has been applied in the various fields of dye degradation [11,12], H2 production [13,14] and CO2 reduction [15,16] owing to its outstanding electronic properties and unique band-gap structure [17,18]. However, the well-known g-C3N4 always displays a very small surface area due to its serious polymerization during high-temperature calcination, causing its weak photocatalytic activity [19,20]. Therefore, the g-C3N4 nanosheets with large surface areas have been extensively investigated to improve its photocatalytic activity via the

* Corresponding author. State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan, 430070, People's Republic of China. E-mail address: [email protected] (H. Yu). https://doi.org/10.1016/j.carbon.2019.07.083 0008-6223/© 2019 Elsevier Ltd. All rights reserved.

chemical exfoliation method, which is first originated from the synthesis of graphene oxide by the Hummers' method [21]. In this case, the usual acids such as sulphuric acid (H2SO4) [22,23], hydrochloric acid (HCl) [24,25] and nitric acid (HNO3) [26,27] have been widely used to effectively promote the delamination of conventional bulk g-C3N4 for the preparation of g-C3N4 nanosheets. As a consequnence, the prepared g-C3N4 nanosheets exhibit a superior performance due to their highly increased surface area. Unfortunately, the above strategies always include the use of high concentrations of strong acids, which are harmful to environment. Hence, it is in urgent need to exploit simple and green routes to synthesize the g-C3N4 nanosheets with excellent photocatalytic activity. Excepting for the above chemical exfoliation of conventional bulk g-C3N4, recently, the secondary calcination of bulk g-C3N4 has been demonstrated to be a green and efficient technique for the preparation of g-C3N4 nanosheets. In this case, various g-C3N4 nanosheets with high photocatalytic rate have been widely synthesized. For instance, Liu et al. exploited a facile thermal exfoliation route to synthesize g-C3N4 nanosheets from bulk g-C3N4 at a

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high temperature (500
C) [28]. Yang et al. also exfoliated the bulk g-C3N4 to fabricate ultrathin g-C3N4 nanosheets under an argon atmosphere at a high temperature (600
C) [29]. In addition, She et al. synthesized the monolayer g-C3N4 nanosheets via a three calcination method at a high temperature (550
C) [30]. Consequently, the as-prepared g-C3N4 usually display an obvious nanosheet-like architecture with excellent photocatalytic performance. However, the secondary high-temperature calcination of bulk g-C3N4 for the preparation of g-C3N4 nanosheets always exhibits a very low yield. In fact, the secondary high-temperature calcination usually includes the interlayer delamination and inplane depolymerization of g-C3N4 structure. However, most of the bulk g-C3N4 could be effectively depolymerized into small units, which can easily escape from g-C3N4 surface, resulting in its low yield of g-C3N4 nanosheets. Therefore, it is expected that the yield of g-C3N4 nanosheets can be greatly increased if the bulk g-C3N4 can be effectively delaminated to form g-C3N4 nanosheets at a lower temperature calcination, which would not cause the remarkable depolymerization of bulk g-C3N4. In this study, the nitrogen-rich g-C3N4 nanosheets with a highly improved yield (32 wt%) were successfully synthesized via a NH4Clinduced low-temperature second-calcination approach, including the initial intercalation of NH4Cl into the interlayers of bulk g-C3N4 and the following direct low-temperature calcination at 400
C. During the calcination process, it is interesting to find that the thermal gas flow (HCl and NH3) from NH4Cl decomposition not only can efficiently facilitate the delamination and depolymerization of the g-C3N4 structure, but also can introduce many amino groups on the g-C3N4 surface, causing the formation of nitrogenrich g-C3N4 (N-CN) nanosheets at a low temperature. Experimental results reveal that the obtained nitrogen-rich g-C3N4 nanosheets show a prominent enhancement for the H2-evolution activity in contrast to the bulk g-C3N4. Moreover, except for the NH4Cl, the ammonium sulfate ((NH4)2SO4) can also be used to synthesize the similar g-C3N4 nanosheets by the identical induced method. As far as we know, the simple and efficient preparation of nitrogen-rich g-C3N4 nanosheets with a high yield is firstly reported by the NH4Cl-induced low-temperature formation approach. This work provides a novel and simple route to synthesize g-C3N4 nanosheets, which may open up a fire-new insight for synthesis of effective photocatalysts. 2. Experimental section

amounts of NH4Cl were varied and the corresponding products were denoted as N-CNX, where the X (0, 3, 5, 10 and 15) was the mass ratio of NH4Cl to bulk g-C3N4. 2.3. Characterization The morphological structure was observed by FESEM (JSM7500, Japan), TEM (JEM-2100F, Japan) and AFM (AR, MFP-3D). XRD (Japan) was applied to acquire the crystallographic features of resultant samples. TG-DSC (STA-449F3, Germany) was used to detect the thermal stability of the resulting samples. The FTIR spectra (Thermo Nicolet, America) were carried to get the variation of various groups. The BET results were obtained from nitrogen adsorption-desorption apparatus (ASAP 2020 from USA). UVevis DRS can be obtained by UV-2450 instrument of Shimadzu from Japan. The Mg Ka source radiation was applied to get the XPS results (KRATOA XSAM800). The PL spectra (FLS920, UK) were conducted to analyze the lifetime of the photogenerated charges. 2.4. Photocatalytic hydrogen-generation activity The hydrogen-generation activity of bulk g-C3N4 and N-CN samples was tested on the basis of our reported routes [33,34]. Specifically, the as-prepared photocatalysts (50 mg) were dispersed into 80 mL of lactic acid (10 vol%) solution (Pt (1 wt%) as a cocatalyst). After that, the above system is purified by purging with N2 for 20 min. The irradiation source was four 420-nm LEDs (50.0 mW cm2). The gas chromatograph (GC-2014C, Japan) is used to detect the contents of generated hydrogen. 2.5. Photoelectrochemical measurements Photoelectrochemical tests are conducted on an electrochemical analyzer (CHI660E) with Na2SO4 solution (0.5 M) as the electrolyte. The test system includes Ag/AgCl (reference electrode), sampleloaded FTO glass (working electrode) and Pt wire (counter electrode). An anaerobic environment was created by purging with N2 and the light source was a LED (420-nm, 3 W). The sample-loaded FTO glasses can be prepared as follows: 10 mg sample is uniformly mixed with a solution of D-520 Nafion and ethanol (1:1) by ultrasonic for 30 min. The obtained mixture was loaded on the prepurged FTO glass surface, and was dried at 50
C for 12 h. The i-t curve was conducted on the open circuit voltage and the EIS was performed in a range of 103-106 Hz.

2.1. Preparation of bulk g-C3N4 3. Results and discussion The bulk g-C3N4 was prepared via a pyrolysis route, similar with previous reports [31,32]. Typically, melamine powder (4 g) was put into a crucible and kept at 550
C for 4 h, and the resultant yellow bulk was bulk g-C3N4. 2.2. Preparation of nitrogen-rich g-C3N4 nanosheets The nitrogen-rich g-C3N4 nanosheets were synthesized via a NH4Cl-induced low-temperature formation strategy. Specifically, 1.0 g of the above bulk g-C3N4 and 5.0 g of NH4Cl powders were added into 80 mL of deionized water and stirred at 80
C for 2 h. After that, the above solution was kept at 90
C overnight to dry the water, and the resultant pale yellow power was ascribed to the NH4Cl-intercalated bulk g-C3N4 (named as NH4Cl-CN-X, where the X (0, 3, 5, 10 and 15) was the mass ratio of NH4Cl to bulk g-C3N4). Finally, the above NH4Cl-CN precursor was directly calcined at 400
C for 2 h to produce the nitrogen-rich g-C3N4 (N-CN) nanosheets. To further demonstrate the influence of NH4Cl contents on the formation of nitrogen-rich g-C3N4 nanosheets, the adding

3.1. Synthetic strategy of nitrogen-rich g-C3N4 (N-CN) nanosheets According to the wide reports [35], it is believed that the traditional secondary calcination of bulk g-C3N4 for the synthesis of g-C3N4 nanosheets is usually at a temperature higher than 500
C. However, in this work, it is interesting to find that at a lower calcination temperature (400
C), the bulk g-C3N4 can be efficiently delaminated to form nitrogen-rich g-C3N4 (N-CN) nanosheets via a NH4Cl-induced low-temperature approach, as illustrated in Fig. 1. Firstly, NH4Cl can be well intercalated into the interlayers of bulk gC3N4 to form the NH4Cl-intercalated bulk g-C3N4 (NH4Cl-CN) intermediates by a facile impregnation method at 80
C (Fig. S1). Secondly, the obtained NH4Cl-CN intermediates were directly calcined at 400
C for the formation of N-CN nanosheets (Fig. 1A). In this case, the NH4Cl can be completely decomposed to produce the thermal shock of gas flow (HCl and NH3), which can efficiently promote the delamination of bulk g-C3N4 by destroying the weak van der Waals forces among g-C3N4 interlayers (Fig. 1B) [36].

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Fig. 1. Graphical illustration for the synthesis of nitrogen-rich g-C3N4 nanosheets via a NH4Cl intercalation and subsequent calcination route. (A colour version of this figure can be viewed online.)

Moreover, the HCl molecules released from the NH4Cl decomposition can work as a scissor to tailor the g-C3N4 framework into small nanosheet units [37]. Simultaneously, the above tailoring process of HCl molecules cause the exposure of sp2-hybridized carbon, which could be further connected by the NH3 molecules (released from NH4Cl) to form many terminal amino groups on the g-C3N4 surface [38], causing the formation of nitrogen-rich g-C3N4 (N-CN) nanosheets (Fig. 1B and C). As a consequence, during the above calcination process, the thermal gas flow (HCl and NH3 from NH4Cl decomposition) not only can efficiently facilitate the delamination and depolymerization of the g-C3N4 structure, but also can introduce many amino groups on the g-C3N4 surface, resulting in the successful synthesis of nitrogen-rich g-C3N4 (N-CN) nanosheets at such a low temperature. To demonstrate that the NH4Cl can be well intercalated into the interlayers of bulk g-C3N4 to form the NH4Cl-CN intermediates, the resulting NH4Cl-CN intermediates (before calcination process) are first analyzed by XRD patterns (Fig. 2A), FT-IR spectra (Fig. 2B) and XPS results (Fig. S2 and Table S1). Obviously, the NH4Cl-CN intermediates display the typical diffraction peaks of NH4Cl and gC3N4 (13.1
and 27.4
) [39,40], strongly revealing the formation of NH4Cl-CN intermediates (Fig. 2A). Remarkably, the (100) diffraction peak corresponding to the uniform tri-s-triazine structural unit of NH4Cl-CN intermediates becomes weaker than that of bulk g-C3N4, which can be owing to the disordered stacking of tri-s-triazine caused by the introduction of NH4Cl into the interlayers of g-C3N4 [41]. After low-temperature (400
C) calcination of NH4Cl-CN intermediates, the resultant g-C3N4 nanosheet (N-CN5) sample displays the same XRD pattern as bulk g-C3N4, firmly revealing that the NH4Cl has been completely decomposed and does not break the intrinsic structure of g-C3N4 nanosheets. The above results can

further be well demonstrated by their corresponding FT-IR spectra (Fig. 2B). Compared with bulk g-C3N4, the absorption peak at 810 cm1 (assigned to the uniform tri-s-triazine unit in-plane) for NH4Cl-CN intermediates becomes obviously weaker, while that of N-H (3000-3500 cm1) displays a significant enhancement, further identifying the successful intercalation of NH4Cl into the interlayers of g-C3N4. In addition, the corresponding XPS results (Fig. S2) indicate that compared with the N-CN3 sample, the NH4Cl-CN-3 exhibits new XPS peaks of Cl element and the N-H bonding (400.8 eV) in NH4Cl-CN-3 sample exhibits a significantly enhancement. According to Table S1, it is found that the N-H amount can be calculated to be 29.21% for the NH4Cl-CN-3 samples, while that of N-CN3 sample is 7.45%, further confirming the successful introduction of NH4Cl. To demonstrate that the intercalation of NH4Cl into the interlayers of bulk g-C3N4 can effectively promote the formation of gC3N4 nanosheets, the DSC results are displayed in Fig. 2C. For the bulk g-C3N4, a maximum endothermic peak at 721
C can be clearly observed in the range of 500e750
C, which belongs to the entire decomposition of bulk g-C3N4, similar with the previous results [42]. For the NH4Cl-CN intermediates, an endothermic peak at 190.6
C can be ascribed to the desorption of H2O or -OH [43], while that at 299.5
C can be attributed to the decomposition of NH4Cl in the NH4Cl-CN intermediates. Additionally, the decomposition peak (690.2
C) of g-C3N4 structure is much lower than that of bulk gC3N4 (721
C), obviously suggesting the formation of g-C3N4 nanosheets with a weaker thermostability. Therefore, it is believed that NH4Cl can be intercalated into the interlayers of bulk g-C3N4 and efficiently promote the formation of g-C3N4 nanosheets. It is commonly known that the high yield is one of the prerequisites for industrial application of photocatalysts.

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Fig. 2. (A) XRD patterns and (B) FT-IR spectra of (a) bulk g-C3N4, (b) NH4Cl, (c) NH4Cl-CN-3 and (d) N-CN5; (C) DSC and (D) TG curves for the (a) bulk g-C3N4 and (b) NH4Cl-CN-3. (A colour version of this figure can be viewed online.)

Unfortunately, traditional secondary calcination of bulk g-C3N4 for the synthesis of g-C3N4 nanosheets usually suffers from an extremely low yield [35]. In this work, the yield of g-C3N4 nanosheets from melamine precursor by the traditional secondary calcination strategy (secondary calcination at 550
C for 2 h) can be calculated to be ca. 7.8 wt% (Table 1), which is similar with the widely reported results (lower than 10 wt%) [28,44,45]. However, the exact yield of present g-C3N4 nanosheets by the present NH4Clinduced low-temperature approach can be calculated to be ca. 32.3 wt% (Table 1), which is over 4 times higher than that of the traditional secondary calcination method. To further investigate the possible reasons for the present high yield of g-C3N4 nanosheets induced by NH4Cl, the TG curves are exhibited in Fig. 2D. Evidently, the bulk g-C3N4 is very stable at 400
C and it cannot be depolymerized until the temperature is higher than 500
C. Therefore, the g-C3N4 nanosheets cannot be produced by directly calcining bulk gC3N4 at 400
C (Table 1). For the NH4Cl-CN intermediates, a mass loss of ca. 73.30 wt% (before 400
C) can be distinctly found, which can be attributed to the decomposition of NH4Cl. In fact, the above decomposition of NH4Cl in the NH4Cl-CN intermediates is

accompanied by the production of g-C3N4 nanosheets, which can cause an obviously decreased decomposition temperature (ca. 30
) for the following complete decomposition (Fig. 2D). Therefore, it is clear that the NH4Cl can be completely decomposed and release the gases (HCl and NH3) to facilitate the efficient delamination of bulk g-C3N4 at 400
C (without the serious depolymerization which usually occurs at a temperature higher than 500
C), causing the high-efficiency transformation of bulk g-C3N4 to g-C3N4 nanosheets. On the basis of above analyses, it is believed that the intercalation of NH4Cl into the interlayers of bulk g-C3N4 not only can induce the formation of g-C3N4 nanosheets at a low temperature, but also can greatly improve their yields. 3.2. Microstructures of nitrogen-rich g-C3N4 (N-CN) nanosheets The formation of nitrogen-rich g-C3N4 (N-CN) nanosheets can be firstly proved by the XRD patterns, FESEM and TEM images. Fig. 3 shows the XRD patterns of bulk g-C3N4 and N-CN nanosheets. In contrast to the bulk g-C3N4 and NH4Cl (Fig. 2A), all the N-CN nanosheet samples exhibit the typical diffraction peaks (13.1
and

Table 1 The masses of the N-CN nanosheet samples, together with those of MA and NH4Cl used for preparing them. Samples

Melamine Precursors (g)

Bulk g-C3N4 products (g)

Secondary calcination Bulk g-C3N4 (g)

NH4Cl (g)

(a) Traditional calcination (b) N-CN0 (c) N-CN3 (d) N-CN5 (e) N-CN10

4.0 4.0 4.0 4.0 4.0

1.6023 1.6035 1.6041 1.6030 1.6016

1.0 1.0 1.0 1.0 1.0

~ 0.0 3.0 5.0 10.0

a b c

The yield is the transformation ratio from melamine precursor to g-C3N4 nanosheets. The yield is obtained from a traditional secondary calcination strategy (secondary calcination at 550
C for 2 h). The yield is obtained from the present NH4Cl-induced formation route (secondary calcination at 400
C for 2 h).

Products (g)

0.1953 0.8502 0.8358 0.8072 0.8245

a

Nanosheet yield (wt %)

b

7.8%

~ c c c

33.4% 32.3% 33.0%

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Fig. 3. XRD patterns of various samples: (a) bulk g-C3N4, (b) N-CN0, (c) N-CN3, (d) NCN5 and (e) N-CN10. (A colour version of this figure can be viewed online.)

27.4
) of g-C3N4 [46], further suggesting that the NH4Cl molecules cannot break the characteristic structure of g-C3N4 and have been completely decomposed during the formation of N-CN nanosheets. The FESEM and TEM images distinctly show that the conventional g-C3N4 displays a bulk structure (Fig. 4A and Fig. S3), similar with the wildly reported results [47e49]. With the intercalation of NH4Cl, the resultant N-CN5 sample displays a typical nanosheetlike architecture (Fig. 4BeD and Fig. S3), obviously thinner and smaller than the bulk g-C3N4 sample. Moreover, its corresponding typical AFM images and thickness analyses in Fig. 4E reveal that the N-CN5 sample displays ultrathin nanosheet architecture within a thickness of 8 nm. The formation of N-CN nanosheets can further be illustrated by the nitrogen adsorption-desorption isotherm (Fig. S4 and Table S2). It is distinct that the bulk g-C3N4 exhibits a very small specific surface area (3.61 m2 g1), consistent with the wide reports [42]. With the intercalation of NH4Cl, the surface areas of the resultant N-CN samples have an obvious enhancement. Specifically, the surface area of the N-CN5 is calculated to be about 29.93 m2 g1, obviously higher than that of the bulk g-C3N4 by a factor of 8. Consequently, the N-CN nanosheets can be successfully prepared via the NH4Cl-induced low-temperature approach. On basis of the above analyses, it is believed that NH4Cl can efficiently induce the formation of N-CN nanosheets. Therefore, it is highly required to explore the influence of NH4Cl on the microstructures and electronic structures of N-CN nanosheets. The FTIR spectra were conducted and shown in Fig. 5. Obviously, all the N-CN samples exhibit the characterized absorption peaks of bulk g-C3N4, including the peak at 810 cm1 assigning to the tri-s-triazine units, the peaks in the region of 1200e1650 cm1 corresponding to the CN hererocycles and the absorption band (3000-3500 cm1) originating from modes of N-H bond [50,51]. In fact, further observation (inset in Fig. 5) indicates that the transmissivity of N-H band for NCN0, N-CN3 and N-CN5 are 74, 70 and 64%, respectively, revealing that the contents of amino groups on the surface of N-CN samples are gradually increased with the intercalation of more NH4Cl, which further certify the proposed formation mechanism of nitrogen-rich g-C3N4 nanosheets in Fig. 1C. Therefore, it is clear that many amino groups have been introduced on the surface of N-CN nanosheets by the NH4Cl-induced low-temperature approach. The XPS and ESR spectra were employed to further investigate the variation of surface microstructures and electronic structures for resultant N-CN nanosheet samples. For the XPS survey spectra in Fig. 6, it is found that in addition to the carbon and nitrogen elements from the g-C3N4, all N-CN photocatalysts display the peaks of O 1s originating from the adsorbed H2O or -OH [52]. Moreover, it is obvious that with the increase of NH4Cl contents, the N1s peaks of the resultant N-CN samples show a slightly

Fig. 4. TEM images of various samples: (A) N-CN0 and (BeD) N-CN5; (E) AFM images and thickness analyses of N-CN5 sample. (A colour version of this figure can be viewed online.)

Fig. 5. FTIR spectra of various samples: (a) N-CN0, (b) N-CN3, (c) N-CN5 and (d) NCN10. (A colour version of this figure can be viewed online.)

strengthened (Fig. 6A). Specifically, the amounts of nitrogen element in the N-CN0, N-CN3 and N-CN5 samples are 34.44, 37.96 and 46.70 at %, respectively (Table 2). To exactly investigate the

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Fig. 6. (A) XPS survey spectra and (B) N1s high-resolution XPS spectra of various samples: (a) N-CN0, (b) N-CN3, (c) N-CN5 and (d) N-CN10. (A colour version of this figure can be viewed online.)

Table 2 The element components of various samples according to XPS results. Samples

C

O

N

r(N-H/N)

(a) N-CN0 (b) N-CN3 (c) N-CN5 (d) N-CN10

56.14 54.29 48.41 49.91

9.42 7.75 4.89 6.13

34.44 37.96 46.70 43.96

6.55% 8.67% 12.33% 11.53%

variation of nitrogen state, the N 1s high-resolution XPS spectra of various samples are exhibited in Fig. 6B. The peaks at binding energies of 398.6, 399.7 and 400.8 eV are assigned to sp2-hybridized nitrogen (C¼N-C), bridging nitrogen (N-C3) and N-H bonding, respectively [53]. Notably, the peak at 400.8 eV (N-H bonding) exhibits a significant enhancement with the increase of NH4Cl contents. Concretely, the amounts of N-H are calculated to be 6.55, 8.67 and 12.33% for the N-CN0, N-CN3 and N-CN5 samples (Table 2), respectively, strongly indicating the introduction of many amino groups on the surface of resultant N-CN nanosheets, which can further be certificated by their ESR spectra (Fig. 7). All samples show an ESR signal with a g-factor of 2.003, which is owing to the unpaired electron of p-bonded aromatic rings [54]. In contrast to bulk g-C3N4, the ESR intensity of N-CN0 sample is slightly enhanced due to its increased defects sites and surface area. Moreover, the NCN5 exhibits a significantly enhanced ESR intensity among three samples, suggesting the introduction of more N element induced by NH4Cl [55]. Therefore, the above XPS and ESR results further indicated that nitrogen-rich N-CN nanosheets can be successfully

Fig. 7. Room temperature ESR spectra of various samples: (a) bulk g-C3N4, (b) N-CN0 and (c) N-CN5. (A colour version of this figure can be viewed online.)

synthesized via the present NH4Cl-induced low-temperature approach. The optical-absorption property of N-CN samples is characterized by UVevis spectra (Fig. 8A). It can be clearly observed that with the intercalation of NH4Cl, the resultant N-CN samples exhibit an enhanced light-absorption ability, which can be attributed to the increased contents of N in N-CN samples [56]. Moreover, the photographs (inset) of N-CN photocatalysts exhibit that the yellow color of g-C3N4 gradually darkens with more addition of NH4Cl, consistent with their optical-absorption property. To investigate the bandgap variation of N-CN samples, their UVevis spectra can be transformed into Tauc plots [57] (Fig. 8B). It is obvious that compared with the bulk g-C3N4 (2.53 eV), the bandgap of N-CN5 sample exhibits a slightly decrease (2.46 eV) due to the increasing contents of N in N-CN samples. 3.3. Photocatalytic activity and mechanism Hydrogen-evolution activity of various photocatalysts was tested under visible light. Fig. 9A reveals that the bulk g-C3N4 photocatalyst exhibits an obvious H2-evolution rate (7.5 mmol h1). With the intercalation of NH4Cl, the hydrogen-evolution rates of resultant N-CN photocatalysts show a significant increase. Particularly, the N-CN5 sample achieves the best performance for a hydrogen-evolution activity of 15.5 mmol h1 (AQE ¼ 0.48%, Support information), about 2 times larger than that of bulk g-C3N4. However, further increasing the content of NH4Cl would cause the declined hydrogen-evolution rate of resultant g-C3N4 samples. To evaluate the stability of N-CN5 for H2 generation, its cycle experiments are carried out (Fig. 9B). Evidently, the N-CN5 photocatalyst can maintain its initial efficiency during repeating tests. Moreover, to further demonstrate the structure stability of N-CN5 photocatalyst, the XRD patterns and FT-IR spectra of N-CN5 sample before and after circulating tests were conducted and the results are shown in Fig. S5. It is clear that compared with the fresh N-CN5, the used N-CN5 sample exhibits no detectable differences, further indicating the excellent chemical stability of N-CN5 photocatalyst. The above experimental data clearly reveals that the N-CN nanosheets synthesized by the NH4Cl-induced low-temperature approach exhibit an obviously enhanced H2-production activity. Considering the simple and efficient synthesis route, it is quite necessary to identify the rational mechanism of the resulting N-CN nanosheets. Obviously, according to the TEM, AFM, BET and UVevis results, the thin nanosheet structures and reduced band gap of NCN samples play one of the most significant roles for the high hydrogen-evolution activity. Moreover, in accordance with the FT-

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Fig. 8. (A) UVevis diffused reflectance, (inset) corresponding photographs and (B) Tauc plots of various samples: (a) bulk g-C3N4, (b) N-CN0, (c) N-CN3, (d) N-CN5 and (e) N-CN10. (A colour version of this figure can be viewed online.)

Fig. 9. (A) The photocatalytic H2-production activities of various samples (a) bulk g-C3N4, (b) N-CN0, (c) N-CN3, (d) N-CN5, (e) N-CN10 and (f) N-CN15; (B) Photocatalytic cycling test of typical samples: (a) bulk g-C3N4 and (d) N-CN5; (C) The schematic diagram illustrating the photocatalytic H2-evolution mechanism of N-CN nanosheet photocatalysts. (A colour version of this figure can be viewed online.)

IR, XPS and ESR results, the amino groups on the surface of resultant N-CN nanosheets could be the key role for the greatly enhanced hydrogen-evolution rate. Herein, a rational mechanism of N-CN nanosheets is exhibited in Fig. 9C. On one hand, the thin nanosheet structures of N-CN samples can shorten the transfer distance of photoexcited carriers to promote the rapid production of H2 [41]. On the other hand, the amino groups on the surface of resultant N-CN can efficiently enrich Hþ from water, and then serve as the interfacial active sites to facilitate H2 generation, which is consistent with the wide reports on the functions of amino groups [58,59]. Therefore, the N-CN nanosheets display an obviously larger rate than the bulk g-C3N4.

To verify the above mechanism, the i-t curve and EIS of N-CN photocatalysts were conducted. Fig. 10A indicates that in contrast to the bulk g-C3N4 and N-CN0 photocatalysts, the N-CN5 sample displays a strengthened photocurrent density, suggesting that the photoexcited electrons can be effectively separated from the N-CN5 sample, resulting in a rapider oxidizing reaction of photoexcited holes. Furthermore, for the EIS plots (Fig. 10B), the N-CN5 sample exhibits a smaller semicircle than the bulk g-C3N4 and N-CN0, revealing a higher transmission speed of photoexcited carriers in the N-CN5 sample. Consequently, it is believed that the higher transport efficiency and faster interfacial catalytic reactions of photoexcited carriers facilitate the improved H2-generation activity

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Fig. 10. (A) Transient photocurrent responses, (B) electrochemical impedance spectra and (C) Time-resolved PL spectra for various samples: (a) bulk g-C3N4, (b) N-CN0 and (c) NCN5. (A colour version of this figure can be viewed online.)

of the resulting N-CN sample. To further investigate the above results, the time-resolved PL spectra were carried out (Fig. 10C), and the fitted results were presented in Table 3. The short lifetime (t1) can be ascribed to the radiative process, and the t2 and t3 primarily represent the lifetimes of nonradiative and energy transfer process [60,61]. According to Table 3, it is clear that t2 and t3 of the N-CN5 are larger than that of bulk g-C3N4 and N-CN0. Moreover, the percentage of charge carriers for N-CN5 is significantly larger than that of bulk g-C3N4 and N-CN0. Therefore, it can be deduced that the NCN5 can provide more photoexcited carriers to participate the photocatalytic reactions, resulting in its highly improved H2-production activity. On the basis of above results, it is obvious that with the intercalation of NH4Cl, the g-C3N4 nanosheets with excellent hydrogenproduction rate can be facilely synthesized. Therefore, except for the NH4Cl, it is significant to demonstrate that whether the formation of g-C3N4 nanosheets can also be induced by other materials via a similar synthesis route. In this case, the ammonium sulfate ((NH4)2SO4) is used to replace NH4Cl to promote the formation of g-C3N4 nanosheets via an identical prepared strategy (Supporting Information). Fig. 11 reveals that the resulting sample induced by (NH4)2SO4 displays an obvious nanosheet-like architecture with a curly surface morphology, clearly indicating the formation of thin g-C3N4 nanosheets (Fig. 11 A and B). Experimental data suggests that the resulting g-C3N4 nanosheets induced by

(NH4)2SO4 have an obviously larger hydrogen-evolution activity than the conventional bulk g-C3N4, and even larger than the N-CN5 nanosheet sample. Hence, the above results firmly suggest the universality and versatility of present ammonium chloride (NH4Cl)induced low-temperature approach for the synthesization of gC3N4 nanosheets, which may open up a novel insight for synthesizing various nanometer materials. 4. Conclusions A facile and efficient NH4Cl-induced low-temperature approach has been exploited to synthesize the nitrogen-rich g-C3N4 nanosheets with a highly improved yield (ca. 32 wt%). Specifically, the NH4Cl can be intercalated into the interlayer of bulk g-C3N4 by a facile impregnation method and promote the formation of nitrogen-rich g-C3N4 nanosheets in the subsequent lowtemperature calcination. Consequently, during the calcination process, the thermal gas flow (HCl and NH3) from NH4Cl decomposition not only can efficiently promote the delamination and depolymerization of the g-C3N4 structure, but also can introduce many amino groups on the g-C3N4 surface, resulting in the successful synthesis of nitrogen-rich g-C3N4 (N-CN) nanosheets at such a low temperature. Experimental data reveals that the obtained nitrogen-rich g-C3N4 nanosheets show an obvious enhancement for the H2-evolution performance, and the N-CN5 nanosheets

Table 3 Fluorescence emission lifetime and relevant percentage data fitted by a three-exponential function. Samples

t1 (ns)

A1 (%)

t2 (ns)

A2 (%)

t3 (ns)

A3 (%)

Average lifetime (ns)

(a) Bulk g-C3N4 (b) N-CN0 (c) N-CN5

4.57 4.58 5.10

33.91 36.28 25.57

1.34 1.40 1.57

62.98 60.48 72.87

22.55 21.86 25.37

3.11 3.24 1.56

7.75 7.55 6.50

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Fig. 11. (A, B) Typical TEM images of the g-C3N4 nanosheets ((NH4)2SO4); (C) The photocatalytic H2-production activities of various samples (a) bulk g-C3N4, (b) N-CN0, (c) N-CN5 (NH4Cl) and (d) g-C3N4 nanosheets ((NH4)2SO4). (A colour version of this figure can be viewed online.)

displays the highest activity, which is 2 times higher than the bulk g-C3N4. Moreover, it is meaningful to find that except for the NH4Cl, the ammonium sulfate ((NH4)2SO4) can also be used to synthesize the g-C3N4 nanosheets by the similar synthetic method, firmly indicating the universality for the present synthesis strategy. This work may open up a fire-new insight for the synthesis of highefficiency nanostructured materials. Acknowledgements

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This work was supported by the National Natural Science Foundation of China (51872221 and 21771142), the Fundamental Research Funds for the Central Universities (WUT 2019IB002) and the 111 Project (No. B18038).

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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.carbon.2019.07.083.

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