Porous defective carbon nitride obtained by a universal method for photocatalytic hydrogen production from water splitting

Journal of Colloid and Interface Science 566 (2020) 171–182

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Porous defective carbon nitride obtained by a universal method for photocatalytic hydrogen production from water splitting Liquan Jing a,b, Duidui Wang b, Yuanguo Xu b,⇑, Meng Xie c, Jia Yan d, Minqiang He b, Zhilong Song d, Hui Xu d, Huaming Li d,⇑ a

School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Pharmacy, Jiangsu University, Zhenjiang 212013, PR China d Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Urea nitrate is used as a gas template

for the first time in the synthesis of porous defective carbon nitride.  Photoactivity for hydrogen production has been increased that of U-C3N4 (or M-C3N4).  Porous defective carbon nitride can be more easily derivatized to form more H.

a r t i c l e

i n f o

Article history: Received 17 September 2019 Revised 10 January 2020 Accepted 14 January 2020 Available online 16 January 2020 Keywords: Urea nitrate UNU-C3N4 UNM-C3N4 Hydrogen production Degradation

a b s t r a c t For the first time, herein this work, we have developed an effective and adaptable method to introduce defects onto the polymeric carbon nitride by simply grinding urea with urea nitrate which resulting new carbon nitride composite (UNU-C3N4) and melamine with urea nitrate which resulting new carbon nitride composite (UNM-C3N4). The UNU-C3N4 reveals high performance towards photocatalytic hydrogen production and as well as photocatalytic removal of contaminants. The results confirm that the defects enhanced the specific surface area, and improved performance of adsorbed oxygen which beneficial to generate more active radicals and more conducive sties to improve d the overall photocatalytic performance. The high N, H, and O content-enhanced electron polarization effects, by introducing the additional N, H, and O atoms into the g-C3N4 matrix, which will increase the charge transfer rate and charge separation efficiency. At the same time, the results of ESR also expression that the new type of as-prepared carbon nitride samples exhibit abundant of hydrogen radical (H) formation, which is also assist to improve the photocatalytic hydrogen production performance. As expected, the H2 evolution rate of UNU-C3N4(or UNM-C3N4) underneath simulated solar light irradiation is 9.93 times (13.76 times) than that of U-C3N4 (urea as raw material) (or M-C3N4 (melamine as raw material)). The high hydrogen evolution rates of UNU-C3N4 and UNM-C3N4 are 830.94 and 556.79 lmol g 1 h 1 under the visible-light irradiation, respectively. Meanwhile, the synthesized UNU-C3N4 and UNM-C3N4 material are demonstrated an efficient ability to degrade pollutants. In general, this work provides a viable way to introduce defects and hydrogen bands into the structure of carbon nitride. Ó 2020 Elsevier Inc. All rights reserved.

1. Introduction ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Xu), [email protected] (H. Li). https://doi.org/10.1016/j.jcis.2020.01.044 0021-9797/Ó 2020 Elsevier Inc. All rights reserved.

The advantages of semiconductor photocatalysis technology in solving the increasingly serious environmental pollution and

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energy shortage problems have caused widespread concern worldwide [1–5]. In recent years, a large number of semiconductors materials have been reported for photocatalytic hydrogen production and degradation of organic pollutants [6–9]. Among them, graphitic carbon nitride (g-C3N4) has wide range of applications specifically in the field of photocatalytic water electrolysis and degradation of environmental pollutants due to its suitable bandgap (2.70 eV), low cost and easy availability, excellent physical and chemical stability, and good optical properties [10–13]. Bulk g-C3N4 can be facilely synthesized by calcining raw substances such as melamine [14], dicyandiamide [15,16], and urea [17,18]. Unfortunately, bulk g-C3N4 has shown some shortcomings in photocatalysis applications because of its low specific surface area and rapid recombination of electron and holes [19,20]. Two-dimensional (2D) nanosheet materials have significantly been attracted wide attention due to their unique atomic layer thickness, and the photoelectric response [21–25]. Since the bulk g-C3N4 has a weak van der Waals force between the CAN layers [26–30], it has been reported that a large number of methods existing to convert the bulk g-C3N4 into nanosheets in order to advance its photocatalytic activity. For example, Mao et al. gained g-C3N4 nanosheets through ultrasonic stripping, which had higher photocurrent response and upper photocatalytic activity than primary g-C3N4 [31]. Han et al. synthesized g-C3N4 nanosheets through liquid stripping in IPA-water and reported that the light absorption and photoresponsivity properties of the thin nanosheets are enhanced [32]. Bai et al. obtained a large amount of g-C3N4 nanosheets by cutting the insolent graphitic carbon nitride using a mixed solution of NH3 and H2O2. The synthesized structure leads to an increase in photocatalytic hydrogen production activity [33]. Niu et al. acquired g-C3N4 nanosheets by thermic oxidation in air. Underneath visible light, the hydrogen production rate of nanosheets was three times that of primary carbon nitride [34]. At the same time, a large number of defects were formed during the stripping of these carbon nitrides. The introduction of defect sites (vacancies, heteroatom doping and chemical defects) or the control of surface functions is considered to be another important parameter for modulating photocatalytic activity [35–37]. The C or N defect in g-C3N4 can usually adjust the band structure, broaden the light absorption range, and serve as the active site in the reaction system [38–43]. However, as best of my knowledge, only a few papers have reported on the method by adding urea nitrate to the carbon nitride polymerization process to synthesize defect exposure, porous large specific surface area, and more active sites of porous two-dimensional few-layered carbon nitride nanosheets. Based on the above research background, a universal and efficient method has developed to introduce new substances into the raw materials of carbon nitride, such as urea or melamine, to form defective two-dimensional thin-layer carbon nitride. The new carbon nitride was used for photocatalytic hydrogen production and photocatalytic removal of contaminants. We have found that a new gas template such as urea nitrate has not been used for the modification of two-dimensional materials such as carbon nitride. Therefore, a new type of carbon nitride (UNU-C3N4 or UNM-C3N4) can be obtained by simply grinding and calcining urea or melamine with urea nitrate. The degree of defects and hydrogen band enhanced the specific surface area, and adsorption ability of carbon dioxide of the new carbon nitride, and make it more conducive to the production of more active radicals and thus more favorable to the improvement of photocatalytic performance. The structure of this new carbon nitride was analyzed by optical, electrical, and fluorescence spectroscopy methods and their contribution to the photocatalytic performance was discussed. At the same time, the results of ESR also reveal that the new type of synthesized carbon nitride possess abundant hydrogen radical forma-

tion, which is also beneficial to the improvement of photocatalytic hydrogen production performance. 2. Experimental section All chemicals were purchased from Sinopharm Chemical Reagent Co., Ltd., all of which are analytical reagent grade and can be used without further purification. 2.1. Synthesis of g-C3N4 samples Briefly, 10 g of urea (or melamine) in a covered crucible was heated up to 550 °C for 1 h with a heating rate of 3 °C/min in a muffle furnace. Then the resulting yellow product is ground to obtain U-C3N4 (or M-C3M4). Urea nitrate was obtained by adding 42 mL of concentrated nitric acid to 40 g of urea, and then placed in an oven at 80 °C for 12 h. Urea (or melamine) and urea nitrate were ground together for 20 min and then the product in a covered crucible was heated up to 550 °C maintained for 1 h with a heating rate of 3 °C/min in a muffle furnace. The resulting yellow sample is directly labeled as UNU-C3N4 (or UNM-C3M4) without grinding. 2.2. Characterizations This part is in the supplementary information. 2.3. Photocatalytic activity This part is in the supplementary information. 3. Results and discussions 3.1. Structural characterization and morphological The UNU-C3N4 (or UNM-C3N4) was synthesized through a reaction using urea (or melamine) and urea nitrate as precursors. After gradual polycondensation and structural rearrangement, the precursors were copolymerized to synthesize our final sample due to the action of the urea nitrate foaming agent as illustrated in Scheme 1. The XRD crystal structures of urea and urea nitrate were analyzed, indicating that we synthesized a new substance urea nitrate and analyzed its morphology and structure (Fig. S1). The morphology and microstructure of U-C3N4 and UNU-C3N4 were investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The scanning electron microscopy images clearly show that the U-C3N4 with agglomerated morphology showed a large number of macroporous structures by the introduction of urea nitrate (Fig. 1a and 1c). TEM images (Fig. 1b, and 1d) further show that the structure of the UNU-C3N4 sample is more stretched and thin. For the SEM image, the UNM-C3N4 (Fig. 1f) showed a typical 2D sheet structure by revealing a wrinkled surface compared to the agglomerated M-C3N4 (Fig. 1e). For TEM images (Fig. 1g and h), it was apparent that UNM-C3N4 showed a sheet-like porous structure by comparing with M-C3N4 having a size of several micrometers, which corresponded well to their SEM results. The Atomic force microscopy (AFM) was used to observe analyze the thickness of the prepared sample. The thickness of the prepared UNU-C3N4 nanosheet (average thickness of about 1.2 nm (Fig. 2a and 2b)) became thinner and showed a lot of porous structure than U-C3N4 (average thickness of about 6.2 nm (Fig. S3a and S3b)). Further, the atomic force microscopy was performed to obtain the thickness of UNM-C3N4 nanosheets (Fig. 2c and 2d). It can be distinctly seen that the UNM-C3N4 also indicate a 2D

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Scheme 1. Illustration of the Reaction Route for the UNU-C3N4 or UNM-C3N4 nanosheet in this work.

Fig. 1. Typical SEM images of U-C3N4 (a), UNU-C3N4 (b), M-C3N4 (e) and UNM-C3N4 (f). Typical TEM images of U-C3N4 (c), UNU-C3N4 (d), M-C3N4 (g) and UNM-C3N4 (h).

nanosheet morphology with a length scale of plentiful microns, which is agreed with the TEM and SEM results. Compared to the thickness of M-C3N4 (average thickness of about 0.48 lm (Fig. S3c and S3d)), the thickness of the UNM-C3N4 can be directly measured to be approximately 1.3 nm. The above described results indicate that the entry of urea nitrate can enlarge the layer clearance of original g-C3N4 and generate the huge amounts of pore structures, and then synthesize UNU-C3N4 (UNM-C3N4) with a large porous specific surface area. More importantly, the generation of such structures is accompanied by the appearance of alteration in the group structure and the increase in active sites. The relationship of the above results to photocatalytic performance will be further discussed below. The chemical structures of the prepared catalysts were illustrated by using X-ray diffraction patterns (XRD) and FT-IR spectra. As shown in Fig. 3a, the XRD pattern of UNU-C3N4 (or UNM-C3N4) approximate to that of U-C3N4 (or M-C3N4), suggesting that the molecular skeleton of g-C3N4 has been marvelously maintained. Both samples showed two different diffraction peaks, one weaker

peak at approximately 13.0° corresponding to the in-plane repeating (1 0 0) crystal plane of the tris-triazine unit and the other stronger one at approximately 27.79° representing the layer spacing (0 0 2) crystal plane of the graphite structure [44]. Besides, we magnify the two XRD diffraction peaks (Fig. S2) and the diffraction peaks of the UNU-C3N4 (or UNM-C3N4) sample are weakened and enlarged by comparison with those of the primeval U-C3N4 (or M-C3N4), suggesting the casualty of regulated structures to a positive grade after urea nitrate activation. The interruption of the gC3N4 structure could ameliorate hole and dissociate electron to varying degrees [45]. Fig. 3b represents the FT-IR spectra of UC3N4, M-C3N4, UNU-C3N4 and UNM-C3N4samples. The sharp peak at about 812 cm 1 is distinctive of the out-of-plane deformation vibration modes of triazine units [46]. The various groups between 1200 and 1650 cm 1are equivalent to the stretching styles of aromatic C/N heterocycle [47,48]. The broad peaks at approximately 3100–3400 cm 1 can be certified to stretching the ANH2 group’s vibration modes and AOH band of the adsorbed H2O from the air. The N2 adsorption-desorption isotherm of the resulting sample

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Fig. 2. (a) AFM image and (b) corresponding height profile of UNU-C3N4; (c) AFM image and (d) corresponding height profile of UNM-C3N4.

was occupied to evaluate its texture parameters. A typical IV isotherm with a significant H3 hysteresis loop can be seen in Fig. 3c and 3d, which means that there are mesopores or macropores in the synthesized sample frame. The urea nitrate may cause macropores during the introduction process and is critical to the construction of the porous framework. The specific surface area of UNM-C3N4 (50.6 m2 g 1) possesses a somewhat higher specific surface area compared to M-C3N4 (9.1 m2 g 1) (Fig. 3c). Compared with U-C3N4 (37.75 m2/g), UNU-C3N4 (78.68 m2/g) has a significantly higher specific surface area and a larger pore volume (Fig. 3d). Therefore, a larger specific surface area and an improved pore structure contribute to the development of photocatalytic performance. The chemical states of U-C3N4 and UNU-C3N4 were tested by XPS to clarify the homologous elemental compositions. Both samples showed similar peak positions in the C 1s spectrum (Fig. 4a), two peaks can be fitted at 288.3 and 284.5 eV, and the peak at 288.3 eV is in tandem with the sp2 bond carbon in the heterocycle and the peak at 284.5 eV is credited to definite superficial carbon, respectively [49]. Similarly, the N 1s XPS spectrum of two samples (Fig. 4b) could be divided into three peaks at 400.2 (NA(C) 3) and 398.7 eV (CAN@C) [50]. A peak of the high-resolution O 1s peak (Fig. 4c) at about 532.2 eV, which may be due to CAOH bonds or surface-adsorbed O2 [51]. For the M-C3N4 and UNM-C3N4 catalysts, the C 1s spectra were almost similar (Fig. 4d). It could be validated that the C 1s spectrum of all samples can be divided into two peaks at 284.6 and 288.5 eV, which are attributed to graphite carbon, sp2 C atoms in tri-s-triazine units (NAC@N), respectively [52]. The high-resolution N 1s XPS spectra of the samples were divided into two peaks with different binding energies of 398.8 eV and 400.8 eV (Fig. 4e), which corresponded to the signal of sp2 pyridinic N involved in traizine ring (N@CAN group) and sp3 hybridized tertiary nitrogen of NA(C)3, respectively [39,53,54]. As shown in Fig. 4f, only an O 1s core level was observed for M-C3N4 at about

532.3 eV, assigned to surface adsorbed H2O [55]. Moreover, the higher area of the O 1s spectrum (about 532.3 eV) of the UNMC3N4 indicates that there is more H2O on the surface of UNMC3N4, thus facilitating the improvement of photocatalytic performance. As shown in Table 1, the N content in the prepared UNU-C3N4 catalyst was higher (contrast that of U-C3N4). The high N element content may lead to an increase in the absorbance of the visible light region [56]. In addition, the increased N element can also supply more active sites to enhance the photocatalytic activity of carbon nitride [57,58]. Further, the elemental analysis of UC3N4 and UNU-C3N4 is used to test the content of C, N, O, H (Table 2). The atomic N/C of U-C3N4 and UNU-C3N4 were 1.79 and 2.01, respectively. Thus, these results indicate that all of the synthesized UNU-C3N4 samples are nitrogen-rich. It is well known that abundant nitrogen is more often stored in materials in the form of NHx [59]. In addition, a portion of the N atom replaces the C atom bridged by the N atom [60,61], resulting in lower carbon content and a higher nitride content, which is consistent with XPS results. For M-C3N4 and UNM-C3N4, the surface atomic content of carbon, nitrogen and oxygen was 41.78%, 56.88% and 1.34% for bulk M-C3N4, and 40.56%, 55.64% and 3.80% for UNMC3N4, respectively. It can be seen that only the oxygen element shows a significant difference after the modification with urea nitrate. The elemental analysis (EA) of M-C3N4 and UNM-C3N4 nanosheet is likewise listed in Table 2. The atomic N/C of MC3N4 and UNM-C3N4 is 1.79 and 1.82, respectively. The higher atomic N/C in all samples is 1.82 for UNM-C3N4. The raised N/C atomic ratios of UNM-C3N4 from the EA analysis imply that more carbon vacancies have been produced in the UNM-C3N4 framework. Meanwhile, it also proves that the material has more oxygen groups in its skeleton. The above results indicate that both UNU-C3N4 and UNM-C3N4 can be rich in defective carbon nitride by the introduction of urea nitrate.

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Fig. 3. (a) XRD patterns and (b) FT-IR spectra of U-C3N4, M-C3N4, UNU-C3N4 and UNM-C3N4. (c) and (d) Nitrogen adsorption-desorption isotherms and of M-C3N4, UNM-C3N4, U-C3N4 and UNU-C3N4.

Fig. 4. High-resolution XPS spectra of (a) C1s, (b) N 1s and (c) O 1s of U-C3N4 and UNU-C3N4; High-resolution XPS spectra of (d) C1s, (e) N 1s and (f) O 1s of M-C3N4 and UNM-C3N4.

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Table 1 The element containing of prepared catalysts from XPS. Samples

N (at. %)

C (at. %)

O (at. %)

U-C3N4 UNU-C3N4 M-C3N4 UNM-C3N4

52.45 56.74 56.88 55.64

46.19 41.44 41.78 40.56

1.36 1.82 1.36 3.80

UV–vis optical absorption spectra were used to examine the light harvesting capability of the whole samples, as shown in Fig. 5. Both samples have a similar distribution and the light absorption capacity varies slightly. Relative to U-C3N4, the absorption of UNU-C3N4 at the visible region has been improved, which is consistent with the results of the above XPS analysis and elemental analysis. To further determine the bandgap of the catalyst, the Kubelka-Munk (K-M) function was used to convert the reflectance values to absorbance values. The band gap of two samples was calculated by plotting (ahm) 1/2 vs. photon energy (hm) in the inset of Fig. 5a. The obtained bandgap value of U-C3N4 and UNU-C3N4 is 2.65 eV and 2.56 eV, respectively. The above bandgap changes are also consistent with the previously reported bandgap of nitrogen-rich carbon nitride [62]. In the present study, the inserting of urea nitrate into the g-C3N4 synthesis resulted in an increased in nitrogen content to form N-rich UNU-C3N4, which brought about narrowing the bandgap of the sample. Relative to the original M-C3N4 (Fig. 5b), the absorption edges of UNM-C3N4 displayed a methodically slight blue-shift, which is caused by the quantum confinement effect in the layered nanosheets [63–66]. Furthermore, the band gap energy of bulk M-C3N4 and UNMC3N4 are respectively mensurated to be 2.56 and 2.63 eV from their Tauc plots (Fig. 5b). The mostly increased band gap is indeed salutary for improving the photocatalytic performance because of the expanded redox ability of charge carriers generated in the nanosheets.

3.2. Photocatalytic performances and recyclability test A visible-light-driven hydrogen evolution reaction was tested in the presence of a Pt cocatalyst and a triethanolamine sacrificial agent for the sake of evaluating the photocatalytic activity. As shown in Fig. 6a and 6b, the hydrogen evolution rate of UNUC3N4 as a photocatalyst was 830.94 lmol g 1 h 1, which was 9.93 times upper than that of U-C3N4 (83.65 lmol g 1 h 1). There are special concerns associated that the quantum efficiency at 400 nm is 3.61% (Fig. S4a). The hydrogen production rate of UNM-C3N4 (556.79 lmol g 1 h 1) is 13.76 times upper than that of primary M-C3N4 (40.48 lmol g 1 h 1), which indicates that the modification of urea nitrate is beneficial to improve the photocatalytic activity. The AQY reached 2.85% under illumination at 400 nm (Fig. S4b). Further, after normalizing our structure by specific surface area, we can still see that the activity of the two modified carbon nitride materials has been significantly improved (Fig. S5), which indicates that the increase in activity is not only related to the enhancement of specific surface area, but also to the increase of the specific surface area leads to the increase of active sites and the formation of defects. As shown in Fig. 6c, more

than 99.50% of RhB was degraded by the UNU-C3N4 photocatalyst within 30 min. Obviously, the catalytic capability of bulk U-C3N4 is drastically lower than that of UNU-C3N4 photocatalyst. After 30 min of exposure, the self-degradation of RhB is almost negligible. The degradation rate constant for UNU-C3N4 was about 2.86 times higher than that of the U-C3N4 (Fig. S6a). Compared to MC3N4, UNM-C3N4 degraded the concentration of RhB faster (Fig. 6d). After 60 min of light irradiation, M-C3N4 showed a degradation efficiency of 54.32%. For UNM-C3N4, the degradation efficiency increased from 54.32% to 99.85%. The degradation rate constant for UNM-C3N4 was about 10.66 times than that of the M-C3N4 (Fig. S6a). In addition, the obtained UNU-C3N4 and UNMC3N4 showed satisfactory stability even after four cycles (Fig. 6e, 6f and S7). The results of the cycle experiments show that the high photocatalytic hydrogen production activity and photocatalytic degradation activity of UNU-C3N4 and UNM-C3N4 are well maintained after four cycles, indicating that the catalyst has good stability. In order to reveal the photodegradation mechanism and the main radical species in the UNU-C3N4 system, 1.0 mmol of isopropanol (IPA), triethanolamine (TEOA) and benzoquinone (BQ) were used for the capture experiments, respectively, as OH, h+ and O2 scavengers. As shown in Figure S8a, the photocatalytic degradation efficiency of RhB after IPA addition is hardly reduced, which means that OH does not participate in the photodegradation process. However, the presence of TEOA and BQ greatly inhibited the decomposition of RhB (19.81% and 13.51% for TEOA and BQ, respectively), indicating that h+ and O2 are the master active groups in the photodegradation of RhB by UNU-C3N4. The effect of different free radical scavengers on the degradation of RhB by UNM-C3N4 was tested in Figure S8b. When isopropanol (IPA, OH) was added, no significant effect on photoactivity was observed, and only 29% of RhB was eliminated in 60 min after the addition of triethanolamine (TEOA, h+). Surprisingly, the removal of RhB was significantly inhibited after the addition of benzoquinone (BQ, O2 ), indicating that O2 plays a major role in this photocatalytic system. Based on free radical trapping, O2 free radicals and photogenerated h+ are chiefly responsible for the removal of organic contaminant RhB by UNM-C3N4.

3.3. Photochemical properties of different photocatalysts The photocurrent responses of all the samples show the efficiency of electron-hole pair separation rates (Fig. 7a and 7d). UNU-C3N4 (or UNM-C3N4) has a higher photocurrent response than U-C3N4 (or M-C3N4), indicating that the introduction of urea nitrate improves the photogenerated charge carrier separation of carbon nitride [67]. In order to better substantiate and comprehend the charge separation of the two samples, the luminescence (PL) spectrum was performed. As shown in Fig. 7b and 7e, compared to UC3N4 (or M-C3N4), UNU-C3N4 (or UNM-C3N4) exhibits a lower PL intensity at room temperature with an excitation wavelength of 370 nm, indicating that charge recombination is inhibited, promoting efficient charge separation [68], which is corresponding to the results of photocurrent. EPR measurements were executed to study the electronic ordonnances of the whole samples. The samples under visible light illumination showed only one Lorentz line centered at 3146.4 G (Fig. 7c and 7f). The EPR signal intensity of

Table 2 Elemental contents of the samples. Samples

N%

C%

H%

O%

N/C mass ratio

U-C3N4 UNU-C3N4 M-C3N4 UNM-C3N4

59.49 61.11 62.53 59.89

33.30 30.47 34.38 32.87

2.45 2.66 2.26 3.62

4.76 5.75 0.83 3.62

1.79 2.01 1.79 1.82

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Fig. 5. UV–vis DRS absorption spectra and Kubelka-Munk plots of prepared samples.

Fig. 6. (a) Photocatalytic H2 evolution over U-C3N4, M-C3N4 UNU-C3N4 and UNM-C3N4, (b) H2 generation rate of U-C3N4, M-C3N4 UNU-C3N4 and UNM-C3N4, (c) and (d) Photocatalytic degradation of RhB under solar light irradiation, (e) Stability test of UNU-C3N4 for H2 evolution under visible light irradiation, (f) RhB photodegradation cycle runs of the UNU-C3N4.

UNU-C3N4 (or UNM-C3N4) is upper as against that of original UC3N4 (or M-C3N4). The phenomenon indicates that UNU-C3N4 (UNM-C3N4) produces higher concentrations of unpaired electrons and more photochemical free radicals, and electron delocalization is enhanced [56,69]. More importantly, when U-C3N4 (or M-C3N4) and UNU-C3N4 (or UNM-C3N4) were irradiated with visible light, an enhanced EPR signal was observed, suggesting the efficient photochemical generation of free radical pairs in the semiconductor, further enhancing delocalized excitons and promoting the disjunction of electron-hole pairs, which is beneficial to advance the photocatalytic performance. In order to study the production of photoinduced electrons (e ), electron spin resonance (ESR) of photocatalysts was carried out using 2, 2, 6, 6-tetramethylpiperidin-1-oxyl (TEMPO) as a spin trapping reagent. The ESR silencing molecule TEMPOH is produced

by electron reduction of TEMPO and results in a decrease in the intensity and flattening of the ESR spectrum [70]. As shown in Fig. 8a, 8b, 8c and 8d, U-C3N4, M-C3N4 UNU-C3N4 and UNM-C3N4 caused a decrease in the TEMPO signal after irradiated for 6 min, indicating that electrons were transferred from photocatalysis to TEMPO. The consumption of TEMPO indicates the generation of reactive electrons during photoexcitation of the g-C3N4-based catalyst [71]. The decrease in TEMPO signal intensity caused by UNUC3N4 (or UNM-C3N4) is much greater than the decrease in TEMPO signal intensity caused by U-C3N4 (or M-C3N4), indicating that the photo-excited UNU-C3N4 (or UNM-C3N4) has a higher reduction ability as against U-C3N4 (or M-C3N4). The above upshots indicate that the introduction of urea nitrate into the synthesized UNU-C3N4 (or UNM-C3N4) promotes e production. Meanwhile, ESR measurements were taken to determine free radicals during

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Fig. 7. (a) Transient photocurrent response, (b) PL spectra, (c) Room temperature EPR spectra for U-C3N4 and UNU-C3N4. (d) Transient photocurrent response, (e) PL spectra, (f) Room temperature EPR spectra for M-C3N4 and UNM-C3N4.

Fig. 8. (a) and (b) TEMPO spin trapping ESR technique to investigate the generation of photoinduced electrons during the photoreaction over U-C3N4 and UNU-C3N4, (f) POBN spin-trapping (POBN-H) ESR spectra of U-C3N4 and UNU-C3N4 after visible light irradiation. (d) and (e) TEMPO spin trapping ESR technique to investigate the generation of photoinduced electrons during the photoreaction over M-C3N4 and UNM-C3N4, (f) POBN spin-trapping (POBN-H) ESR spectra of M-C3N4 and UNM-C3N4 after visible light irradiation.

photocatalytic hydrogen production. Fig. 8c and 8f displays the POBN spin capture ESR spectra of U-C3N4, M-C3N4 UNU-C3N4 and UNM-C3N4. As reported in the literature, POBN-H adducts can be observed in these two photocatalytic systems underneath visible-

light illumination61. The results display that H radical is an intermediate in the photocatalyst that water escapes from the photocatalyst. However, the rate of formation of H radicals on the two photocatalysts is different. As shown in Fig. 8c and 8f Fig. 9, at

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Fig. 9. (a) The VB XPS spectra of the U-C3N4 and UNU-C3N4. (b) The schematic band structures of U-C3N4 and UNU-C3N4. (c) The VB XPS spectra of the M-C3N4 and UNM-C3N4. (d) The schematic band structures of M-C3N4 and UNM-C3N4.

the 30th minute, the UNU-C3N4 (or UNM-C3N4) photocatalytic system displayed stronger peak intensities compared to U-C3N4 (or M-C3N4). These results suggest that the structure of UNU-C3N4 (or UNM-C3N4) promotes the formation of H radicals and enhances photocatalytic hydrogen production activity [72–74]. The valence band positions of the original U-C3N4 and UNUC3N4 samples were determined by VB-XPS to be 1.96 and 1.82 eV (Fig. 9a), and after that their conduction band (CB) were reckoned to be 0.69 and 0.74 eV, respectively. Consequently, the electronic band structure of the synthesized sample is schematically displayed in Fig. 9b. Obviously, it can be seen that

the catalysts obtained have comfortable band alignment to meet the water decomposition potential. Compared with U-C3N4, the prepared UNU-C3N4 nanosheets have a more negative CB position, indicating that the thermodynamic driving force of H+ reduction in photocatalytic hydrogen production is greater [65,75–78]. The XPS valence band was used to explore the valence band (VB) spectra of all the catalysts. As shown in Fig. 9c, the VB positions of M-C3N4 and UNM-C3N4 were evaluated to be 1.96 and 1.86 eV, respectively. Therefore, the conduction band (CB) levels of M-C3N4 and UNM-C3N4 were worked out to be 0.60 and 0.77 eV, respectively. A schematic diagram of the energy band

Fig. 10. Schematic illustration of the structural benefits of the UNU-C3N4 (or UNM-C3N4) nanosheets catalyst during hydrogen evolution process and degradation process.

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structure of the two catalysts prepared is exhibited in Fig. 9d, which relates to the redox potential of water splitting. Evidently, both M-C3N4 and UNM-C3N4 can satisfy the thermodynamic driving force of the water redox potential. In addition, the CB of UNMC3N4 has a higher negative level (D = 0.17 eV) than M-C3N4, suggesting that the UNM-C3N4 material has a greater thermodynamic driving force for hydrogen proton reduction during hydrogen evolution [79,80]. Based on the above results, a possible mechanism was proposed in Fig. 10.. For UNU-C3N4 (or UNM-C3N4), the photoexcited electrons are located at CB and leave holes in VB. Furthermore, the crimp and pore structure of UNU-C3N4 (or UNM-C3N4) also contributes to the separation efficiency of photogenerated charge pairs [81]. The photo-excited electrons on UNU-C3N4 (or UNM-C3N4) can be transferred from the inside to the surface of UNU-C3N4 (or UNM-C3N4) along with the porous structure [82]. For the degradation of RhB, superoxide radicals formed by the combination of electrons and oxygen and holes in the valence band are the principal reactive groups to decompose RhB. For H2 evolution, while holes are captured by TEOA, electrons on UNU-C3N4 (or UNM-C3N4) could be coupled to Pt and reacted with water. In the above process, the enhanced photocatalytic activity of the UNU-C3N4 (or UNM-C3N4) sample for RhB degradation and H2 precipitation can be assigned to the synergistic effect between good photoelectron properties, apparent wrinkles and porous structure. 4. Conclusions In summary, we have developed a universally effective method by introducing urea or melamine with urea nitrate to introduce defects on the carbon nitride, hydrogen bands, and also increase the specific surface, generate more active sites. In this way, defects can be fully utilized, the elemental composition can be adjusted, and the charge carrier separation and molecular oxygen adsorption of carbon nitride can be promoted, thereby enhancing the generation of active radicals and improving the photocatalytic activity. It is indicated by ESR that UNU-C3N4 (or UNM-C3N4) will form abundant hydrogen radicals (H), which is beneficial to the great improvement of photocatalytic hydrogen production performance. Moreover, when inorganic acids are mixed with carbon nitride (urea), this is due to the formation of new urea salts, not some simple acidity and alkalinity conversion factors. The formed urea salt will form a large number of defects and the thin layer structure during the polymerization of carbon nitride, thereby enhancing the photocatalytic performance. Compared with most current papers that use different concentrations of acid to adjust the pH value to adjust the carbon nitride structure of urea as a raw material to improve the photocatalytic performance [18,83]. This work clearly reveals the reaction mechanism of different acids in the synthesis of carbon nitride using urea. At the same time, it can also be used to synthesize ultra-thin and multi-defect new carbon nitride from other carbon nitride raw materials (melamine, etc.). This also provides a reliable theoretical and experimental basis for the synthesis of thin multi-defect carbon nitride with urea salts formed by other acids and urea. CRediT authorship contribution statement Liquan Jing: Formal analysis, Writing - original draft. Duidui Wang: Data curation. Yuanguo Xu: Writing - review & editing, Supervision. Meng Xie: Supervision. Jia Yan: Supervision. Minqiang He: Supervision. Zhilong Song: Formal analysis. Hui Xu: Supervision. Huaming Li: Writing - review & editing.

Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work is financially supported by the National Natural Science Foundation of China (No. 21777063, 21476098, 21506079). J. Y. acknowledges the China Postdoctoral Science Foundation (2017M621654) and Natural Science Foundation of Jiangsu Province (BK20180887). Jiangsu Planned Projects for Postdoctoral Research Funds (2018K007C). This study was supported by the high performance computing platform of Jiangsu University. Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2020.01.044. References [1] M.F. Kuehnel, K.L. Orchard, K.E. Dalle, E. Reisner, Selective photocatalytic CO2 reduction in water through anchoring of a molecular Ni catalyst on CdS nanocrystals, J. Am. Chem. Soc. 139 (2017) 7217–7223. [2] L. Xiao, Q. Zhang, P. Chen, L. Chen, F. Ding, J. Tang, Y.J. Li, C.T. Au, S.F. Yin, Copper-mediated metal–organic framework as efficient photocatalyst for partial oxidation of aromatic alcohols under visible light irradiation: synergism of plasmonic effect and schottky junction, Appl. Catal. B: Environ. 248 (2019) 380–387. [3] L. Chen, J. Tang, L.N. Song, P. Chen, J. He, C.T. Au, S.F. Yin, Heterogeneous photocatalysis for selective oxidation of alcohols and hydrocarbons, Appl. Catal. B: Environ. 242 (2019) 379–388. [4] P. Chen, F. Liu, H. Ding, S. Chen, L. Chen, Y.J. Li, C.T. Au, S.F. Yin, Porous doubleshell CdS@C3N4 octahedron derived by in situ supramolecular self-assembly for enhanced photocatalytic activity, Appl. Catal. B: Environ. 252 (2019) 33–40. [5] D.H. Wang, J.N. Pan, H.H. Li, J.J. Liu, Y.B. Wang, L.T. Kang, J.N. Yao, A pure organic heterostructure of l-oxo dimeric iron(iii) porphyrin and graphiticC3N4 for solar H2 reduction from water, J. Mater. Chem. A 4 (2016) 290–296. [6] L.Q. Jing, Y.G. Xu, M. Xie, J. Liu, J.J. Deng, L.Y. Huang, H. Xu, H.M. Li, Three dimensional polyaniline/MgIn2S4 nanoflower photocatalysts accelerated interfacial charge transfer for the photoreduction of Cr(VI), photodegradation of organic pollution and photocatalytic H2 production, Chem. Eng. J. 360 (2019) 1601–1612. [7] L.Q. Jing, Y.G. Xu, Z.G. Chen, M.Q. He, M. Xie, J. Liu, H. Xu, S.Q. Huang, H.M. Li, Different morphologies of SnS2 supported on 2D g-C3N4 for excellent and stable visible light photocatalytic hydrogen generation, ACS Sustainable Chem. Eng. 6 (2018) 5132–5141. [8] Y.G. Xu, J. Liu, M. Xie, L.Q. Jing, H. Xu, X.J. She, H.M. Li, J.M. Xie, Construction of novel CNT/LaVO4 nanostructures for efficient antibiotic photodegradation, Chem. Eng. J. 357 (2019) 487–497. [9] L.Q. Jing, Y.G. Xu, S.Q. Huang, M. Xie, M.Q. He, H. Xu, H.M. Li, Q. Zhang, Novel magnetic CoFe2O4/Ag/Ag3VO4 composites: Highly efficient visible light photocatalytic and antibacterial activity, Appl. Catal. B: Environ. 199 (2016) 11–22. [10] Y.H. Tian, L. Zhou, Q.H. Zhu, J.Y. Lei, L.Z. Wang, J.L. Zhang, Y.D. Liu, Hierarchical macro-mesoporous g-C3N4 with an inverse opal structure and vacancies for high-efficiency solar energy conversion and environmental remediation, Nanoscale 11 (2019) 20638–20647. [11] C.M. Li, S.Y. Yu, H.J. Dong, C.B. Liu, H.J. Wu, H.N. Che, G. Chen, Z-scheme mesoporous photocatalyst constructed by modification of Sn3O4 nanoclusters on g-C3N4 nanosheets with improved photocatalytic performance and mechanism insight, Appl. Catal. B: Environ. 238 (2018) 284–293. [12] Y.Z. Zheng, X.N. Dou, H.F. Li, J.-M. Lin, Bisulfite induced chemiluminescence of g-C3N4 nanosheets and enhanced by metal ions, Nanoscale 8 (2016) 4933– 4937. [13] C.C. Han, T. Zhang, Q.J. Cai, C.H. Ma, Z.F. Tong, Z.F. Liu, 0D CoP cocatalyst/2D gC3N4 nanosheets: An efficient photocatalyst for promoting photocatalytic hydrogen evolution, J. Am. Chem. Soc. 102 (2019) 5484–5493. [14] Z. Mo, H. Xu, Z.G. Chen, X.J. She, Y.H. Song, J.J. Wu, P.C. Yan, L. Xu, Y.C. Lei, S.Q. Yuan, H.M. Li, Self-assembled synthesis of defect-engineered graphitic carbon nitride nanotubes for efficient conversion of solar energy, Appl. Catal. B: Environ. 225 (2018) 154–161. [15] E.Z. Liu, J.B. Chen, Y.N. Ma, J. Feng, J. Jia, J. Fan, X.Y. Hu, Fabrication of 2D SnS2/gC3N4 heterojunction with enhanced H2 evolution during photocatalytic water splitting, J. Colloid Interface Sci. 524 (2018) 313–324.

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