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Article

Regulating the Assembly of Precursors of Carbon Nitrides to Improve Photocatalytic Hydrogen Production

by
Xinying Liu
,
Chengxiao Zhao
,
Tahir Muhmood
* and
Xiaofei Yang
*
College of Science, Nanjing Forestry University, Nanjing 210037, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1634; https://doi.org/10.3390/catal12121634
Submission received: 9 November 2022 / Revised: 29 November 2022 / Accepted: 7 December 2022 / Published: 13 December 2022
(This article belongs to the Special Issue Electro/Photocatalyst for Degradation of Emerging Water Pollutants and Water Splitting)
Browse Figures
Versions Notes

Abstract

:
Two-dimensional graphitic carbon nitrides (2D g-C3N4) are promising photocatalysts for water splitting to hydrogen due to their non-toxicity and high stability. However, the bulk g-C3N4 has some intrinsic drawbacks, such as rapid electron–hole recombination and low charge-carrier mobility, resulting in poor photocatalytic activity. Here, 2,4-diamine-6-phenyl-1,3,5-triazine was employed as a precursor to regulating the assembly of melamine and cyanuric acid in water. The resulting g-C3N4 not only improved the visible light absorption and electron–hole separation but also provided more catalytic sites for enhanced photocatalytic hydrogen evolution. The modified g-C3N4 (CNP10-H) showed a hydrogen-releasing rate of 2184 μmol·g−1·h−1, much higher than the bulk g-C3N4.

1. Introduction

Taking inspiration from plant photosynthesis, hydrogen (H2) evolution from photocatalytic water splitting using solar energy has attracted worldwide interest, which provides a new strategy to resolve energy-shortage and environmental-pollution problems [1]. Among numerous semiconductor photocatalysts, two-dimensional (2D) photocatalytic materials have drawn significant attention due to their unique properties, such as appropriate band structure, ultrahigh specific surface area, and more exposed active sites [2]. Up to now, a large number of 2D photocatalysts have been used for photocatalytic water splitting, including transition metal dichalcogenides (TMDs) [3,4,5,6], oxides [7,8,9,10,11], black phosphorus (BP) [12,13], and graphitic carbon nitride (g-C3N4) [14,15,16,17,18,19].
Graphitic carbon nitride (g-C3N4) has attracted dramatic interest in the field of visible-light photocatalytic water-splitting. Hydrogen (H2) generation was first exploited in 2009 [20]. Owing to its appropriate bandgap (2.7 eV), it corresponds to visible-light response, controllable morphology, reasonable cost, and good stability [21,22,23]. However, although g-C3N4 is regarded as a promising candidate to implement hydrogen (H2) evolution, there are several hurdles that limit its photocatalytic performance, including fast recombination of photoinduced charge carriers and low surface area [24,25,26]. To overcome these problems, a variety of strategies have been developed to improve the g-C3N4 photocatalytic performance, which mainly includes doping with a metal or nonmetal [27,28,29,30,31], coupling with other semiconductors [32,33,34,35,36,37,38,39], and copolymerization with organic molecules [40,41,42,43].
Recently, the copolymerization of g-C3N4 precursor with organic monomers has been regarded as an efficient route, not only for modulating the electronic and band structure of g-C3N4 via control of its π-conjugate aromatic system to improve the photocatalytic activity and selectivity of pristine g-C3N4 but also to pave the way for further modification of g-C3N4 with sensitizers and cocatalysts [44,45,46,47,48,49]. For instance, Wang et al. obtained modified g-C3N4 by simple copolymerization with organic monomers like barbituric acid for efficient photocatalytic hydrogen production [41]. Wang et al. introduced thiophene in a g-C3N4 skeleton, which can effectively extend the aromatic π-conjugated system [50], promoting charge migration and separation and further enhancing photocatalytic hydrogen-evolution performance. It is worth noting that the copolymerization reactions mentioned above often involve the serious volatilization of one or more components before the end of the response due to the mismatch of physicochemical properties (e.g., sublimation temperature) between the components [51]. In our previous work [52], the pre-organization of asymmetric precursors in ethanol was investigated, and the significant enhancement of photocatalytic hydrogen production was observed in modified g-C3N4.
In this work, taking the effect of solvent polarity into consideration, we extend our previous work by continuously evaluating the photocatalytic hydrogen-evolution performance of a modified g-C3N4 nanostructures originating from the supramolecular assembly of asymmetric benzene-substituted melamine (2,4-diamine-6-phenyl-1,3,5-triazine), melamine, and cyanuric acid in water rather than ethanol. We further investigate the influence of water as well as the employed asymmetric precursor, benzene-substituted melamine, on the formation of pre-organized intermediates and the chemical and crystal structure, morphological feature, charge transfer and HER performance of the g-C3N4.

2. Materials and Methods

2.1. Materials

Melamine (M): C3H6N6, purity 99%; cyanuric acid (C): C3H3N3O3, purity ≥99.0% (HPLC); and 2,4-diamino-6-phenyl-1,3,5-triazine (Mp): C9H9N5, purity ≥ 98.0% (HPLC), were purchased from Aladdin.

2.2. Synthesis of CN-H and CNPx-H

The preparation of CN-H is shown in Figure 1a. Equimolar amounts (40 mmol) of cyanuric acid (C) and melamine (M) were stirred in deionized water (100 mL) for 8 h. Then, the white complex was collected by centrifugation and dried at 60 °C for 12 h. The white powder was put into the crucible with a cover and then heated to 550 °C at a rate of 2.3 °C/min in a muffle furnace and maintained at this temperature for 4 h. The yellow product was obtained and named CN-H.
Cyanuric acid (C), melamine (M), and 2,4-diamino-6-phenyl-1,3,5-triazine (Mp) with molar ratios of 1:1-x%:x% were stirred in 100 mL H2O for 8 h (the mole of C was held at 40 mmol; the x-value was 5, 10, 20, 30). Afterward, the aggregations (CMPx-H) were formed, collected by centrifugation, and dried at 60 °C for 12 h. Further thermal polymerization of CMPx-H yielded CNPx-H under the same conditions as CN-H.

2.3. Synthesis of CNP10

Cyanuric acid (C), melamine (M), and 2,4-diamino-6-phenyl-1,3,5-triazine (Mp) were directly submitted to thermal polymerization after thoroughly grinding under solvent-free conditions. The ground mixture with a molar ratio of 10:9:1 was heated up to 550 °C with a ramp rate of 2.3 °C/min in a muffle furnace and maintained at this temperature for 4 h; the production was named CNP10 (Figure 1b).

2.4. Characterizations

X-ray-diffraction (XRD) measurements were performed on a Rigaku Ultima IV diffractometer with Cu Kα radiation. Fourier-transform infrared (FTIR) spectra were recorded on a Perkin-Elmer spectrometer (SpectrumOneB), and all the samples were pressed into KBr pellets. UV-visible diffused reflectance spectra were performed on a UV-visible spectrophotometer (UV2450, Shimadzu, Japan), using BaSO4 as the reflectance standard. X-ray photoelectron spectroscopy (XPS) was obtained using a Perkin-Elmer PHI 5000C. Photoluminescence (P.L.) spectra were recorded on a QuantaMasterTM 40 with an excitation wavelength of 395 nm. The morphologies of the obtained samples were taken with field-emission scanning electron microscopy (FE-SEM, JSM-7001F) and TEM. The electron spin-resonance (ESR) measurements were also carried out on a JES FA200 spectrometer.

2.5. Photoelectrochemical Testing

Photoelectrochemical measurements of the samples were taken at a CHI-760E electrochemical workstation in a standard three-electrode cell with 1 M Na2SO4 solution as the electrodes. The 5 mg sample was dispersed in the mixture of 40 μL Nafion solution, 250 μL absolution ethanol, and 250 μL ethanediol; the slurry was coated onto cleaned 1.6 × 2.2 mm (photoactive area 1 cm2) FTO glasses and used as a working electrode. A Pt flake and an Ag/AgCl electrode were respectively used as counter electrode and reference electrode. Incident light was obtained from a 300 W Xenon lamp.

2.6. Photocatalytic-Activity Testing

Ten mg catalyst powder, 5 mL TEOA, and 80 μL H2PtCl6 aqueous solution (10 mg/mL) were added to 45 mL of ultrapure water and then treated by ultrasound for 30 min. A 300 W Xenon arc lamp (CEL-HXF300, CEAULight, Beijing, China) with a 420 nm cutoff filter was used as the light source. The photocatalytic reaction was carefully maintained at 6 °C. The generated gas was analyzed by gas chromatography (GC-7920) equipped with a thermal conductive detector (TCD) with high-purity N2 carrier gas.

3. Results and Discussion

XRD and FTIR were carried out to give insight into the crystal and chemical structures of the obtained samples. Figure 2a shows the diffraction patterns of CN-H and CNPx-H, which proves their graphite-like carbon-nitride (g-C3N4) structures due to the presence of (100) and (002) reflex of heptazine-based g-C3N4. However, as the amount of 4-diamino-6-phenyl-1,3,5-triazine (M.P.) increased, the characteristic diffraction peaks at 2θ = 13.0° and 27.5° gradually broadened and weakened, which might be due to the fact that Mp molecules were fused into the heptazine units in the plane and on the edge. FTIR spectra were further collected for analysis of the obtained samples. In Figure 2b, the broad peaks that appeared in the 3500–3000 cm−1 range were associated with the terminal amino groups. A series of peaks between 1700 cm−1 and 1200 cm−1 was assigned to the stretching vibration of heptazine heterocyclic in g-C3N4, and the peak located at 810 cm−1 belonged to the bending vibration of the heptazine units. These results again indicate the formation of g-C3N4. Additionally, the XRD pattern and FTIR spectrum of CNP10 were similar to those of CN and CNPx-H (Supplementary Materials Figures S1 and S2). Figure 3a shows the UV-visible diffuse reflectance spectra (DRS) of CN-H and CNPx-H. The absorption edge of CN-H was about 457 nm, which corresponded to the band gap of 2.70 eV (Figure 2b). With the increase in Mp content, the absorption in the visible region enhanced significantly, and the corresponding band gaps decreased from 2.70 to 2.26 eV.
The visible-light-driven hydrogen evolution of CN-H and CNPx-H was evaluated in the presence of TEOA and Pt. As shown in Figure 4a, compared with CN-H, the hydrogen evolution of CNP5-H and CNP10-H was enhanced, whereas that of CNP20-H and CNP30-H was the opposite. Obviously, CNP10-H showed the best performance with a hydrogen evolution rate of 2184 μmol·g−1·h−1 (Figure 4b). Furthermore, CNP10-H maintained high hydrogen production in five cycling sequences, indicating its good stability for photocatalysis (Figure 4d). CNP10 formed by calcining C, M, and Mp directly had poor photocatalytic activity (Figure 4c), indicating that the solvent’s preassembly was critical to improving the photocatalytic hydrogen evolution. In addition, the photocatalytic activity of CNP10-H was close to that of the CN-P10 prepared in our previous work [52], implying the change from ethanol to water had a slight effect on the performance improvement of modified g-C3N4.
The morphology of CN-H and CNP10-H were then characterized by SEM and TEM (Figure 5). The SEM image of CN-H revealed stacked-platelet morphology (Figure 5a). When the Mp content exceeded 10% (CNP10-H), the morphology changed to scattered shaving-like nanosheets (Figure 5b). The morphology of CNP10-H was further investigated by TEM. As shown in Figure 5c,d, CNP10-H consisted of several stacked layers of nanosheet structure with wrinkles, which was beneficial to exposing more activity sites. However, CNP10, by directly calcining, showed a massive irregular compact stacking-morphology structure (Figure 1b), which was different from that of CNP10-H. Compact stacking morphology is a disadvantage to exposing activity sites. This result is consistent with the previous photocatalytic hydrogen-evolution test.
To further investigate the effect of comonomers on the band structure of the obtained samples, the valence bands (VBs) of CN-H and CNP10-H were determined as 1.75 eV and 1.64 eV by XPS, respectively (Figure 6a). Next, the flat-band potentials of CN-H and CNP10-H were measured by Mott–Schottky plots (Figure 6b). By further converting, the conduction bands (CBs) of CN-H and CNP10-H were determined to be −0.95 eV and −0.96 eV, respectively (vs. standard hydrogen electrode). From the intersections of the straight lines (Figure 6c), band gaps of 2.70 eV and 2.60 eV were determined for CN-H and CNP10-H, respectively. The band structures of these samples are illustrated in Figure 6d. It is worth noting that the similar negative CB potentials of CN-H and CNP10-H satisfied the thermodynamic condition for the photocatalytic-splitting water, though the narrower bandgap of CNP10-H may have generated more photoexcited electrons to participate in proton reduction.
X-ray photoelectron spectroscopy (XPS) was used to investigate the chemical environment of surface elements further. As shown in Figure 7a, revealed signals corresponding to the elements C and N were observed in the spectrum survey. In the C 1 s spectra in Figure 7b, two peaks of these samples were located at about 288.2 eV and 284.8 eV. The peaks at 288.2 eV (288.1 eV) were related to the sp2-hybridized carbon in the N-containing aromatic (N-C=N) [53], which is considered the main aromatic-carbon species for the framework. The peak at 284.8 eV was attributed to impurity carbon. The high-resolution N 1s spectra contained four peaks (Figure 7c); the strongest peak, located about at 398.7 eV, was assigned to sp2-hybridized nitrogen bonded to carbon atoms (C=N-C), and the peak at 400.3 eV (400.4 eV) was related to the tertiary nitrogen groups (N-(C)3) [54,55]. The peaks at 401.2 eV (401.3 eV) were ascribed to amino functions (C-N-H) [19] due to the terminal amino groups on the surface. The weaker peaks at 404.3 eV (404.5 eV) originated from the positive charge localization in the heterocycles effect [56]. The C/N molar ratios of CN-H and CNP10-H were obtained from XPS. A gradual increase in the C/N molar ratio from 0.80 for CN-H to 0.81 for CNP10-H revealed the successful integration of the benzene ring in the g-C3N4 skeleton network on the Mp mole ratio of 10%. As shown in Figure 7d, the nitrogen adsorption–desorption isotherms can be further investigated by the surface area of the CN-H and CNP10-H; all of the samples exhibited typical type IV isotherms with H3-type hysteresis loops in the 0.8–1.0 high-relative-pressure region. The BET surface area of CN-H and CNP10-H was 52.8 and 67.0 m2/g, respectively. CNP10-H exhibited a higher surface area than others, which is consistent with photocatalytic hydrogen-evolution experimental results, indicating that the surface area was one of the most important reasons for the enhancement activity. In addition, the BET values were close to those of CN-P10 (63 m2/g) and CN (44.6 m2/g) reported in our previous work [52], indicating the slight influence of polar solvents on the structure of aggregations.
Photoelectrochemical and spectroscopic testing were performed to evaluate the charge-transfer properties further. Compared to CN-H and modified CNP10-H (Figure 8a), the modified CNP10-H showed a dramatic decrease in the PL emission intensity and red-shift of PL peaks. These results indicate that adding moderate amounts of Mp can provide a strong extension of the π-conjugated system, suggesting an accelerated separation and migration of photo-generated electrons and holes further inhibiting the electron–hole-pair recombination, due to the creation of surface heterostructures by copolymerization [57,58]. Furthermore, the photocatalytic performance was improved by inhibiting the recombination of photoinduced carriers, which is consistent with the photocatalytic hydrogen-evolution test. Figure 8b shows the transient photocurrent response of the obtained samples. The photocurrent density generated from the modified CNP10-H models was still higher than CN-H, further confirming that electron-hole-pair recombination is effectively inhibited when the copolymer monomer exists. Specifically, a strengthened photocurrent was displayed for CNP10-H, which further confirmed that moderate amounts of Mp effectively extend the π-conjugated system of g-C3N4 and significantly improve charge mobility. The charge-transfer properties of the sample CNP10 obtained by direct calcining were also tested. As shown in Figure 8c, electrochemical-impedance spectroscopy (EIS) was performed for the CN-H and modified CNP10-H was added to water in the synthetic process. Due to the presence of copolymer monomer, it was clear that CNP10-H revealed the slightest resistance. This obviously reduced diameter demonstrates the benefit of charge separation of photoemission electron–hole pairs. As shown in Figures S3 and S4, electrochemical-impedance spectroscopy and transient photocurrent response were measured in terms of the transfer efficiency of charge carriers; it is clear that CNP10 presented a slower charge transfer and faster photo-generated carrier recombination than CNP10-H, which is consistent with the results of the hydrogen-evolution activity.
As demonstrated in Figure 8d, the charge-carrier separation and recombination of the photoexcited carriers were further confirmed by room-temperature EPR spectra. According to the literature [32], the Lorentzian line originates from the unpaired electrons in the aromatic rings of carbon atoms. Furthermore, the peak intensity of CNP10-H was much higher than that of others under light irradiation, indicating that it can efficiently promote exciton dissociation to produce free electrons and holes for relevant photoredox reactions [59]. As expected, this phenomenon agrees with the previous experimental results.

4. Conclusions

In conclusion, by introducing organic monomers, 4-diamino-6-phenyl-1,3,5-triazine in the g-C3N4 skeleton can extend the aromatic π-conjugated system. CNP10-H exhibited 1.7 times more activity toward H2 evolution than when using CN-H (2184 μmol·g−1·h−1 vs. 1313 μmol·g−1·h−1, respectively). Compared with the traditional one-step calcining method, the supramolecular network structure could effectively inhibit the sublimation of the calcining process. The CNP10-H sample exhibited the highest photocatalytic activity for H2 evolution among these samples. The CNP10-H sample’s H2 evolution rate of 2184 μmol·g−1·h−1 was four times higher than that of CNP10 (558 μmol·g−1·h−1). The excellent photocatalytic activity was due to enlarged surface area, higher number of photocatalytic-reactive sites, and significantly improved charge-carrier transfer and separation. This work provides a method for the modification of carbon nitride by copolymerization. It overcomes the photocatalytic-activity reduction caused by mismatching sublimation temperature among components in copolymerization modification.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12121634/s1, Figure S1: XRD patterns of CNP10 and CNP10-H; Figure S2: FTIR spectra of CNP10 and CNP10-H; Figure S3: Nyquist plots EIS spectroscopy from CNP10 and CNP10-H; Figure S4: Photocurrent response of CNP10 and CNP10-H.

Author Contributions

X.L.: writing—original draft preparation, writing—review and editing, investigation, visualization; C.Z.: methodology, investigation, visualization, writing—original draft preparation; T.M.: writing—original draft preparation, writing—review and editing, validation, supervision, visualization; X.Y.: supervision, conceptualization, project administration, funding acquisition, data curation. All authors have read and agreed to the published version of the manuscript.

Funding

This work was funded by Natural Science Foundation of Jiangsu Province (BK20200777), and Science Fund for Distinguished Young Scholars of Nanjing Forestry University (JC2019002). This research was also supported by “Ministry of Science & Technolog China” (State funding/National project #: QN2022014008L), and Metasequoia Scientific Research Funding Nanjing Forestry University # 163101142 to Tahir Muhmood.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Advanced Analysis and Testing Center, Nanjing Forestry University.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Schematic illustration of the preparation of (a) CN-H and CNP10-H nanosheet. (b) CNP10 obtained by directly calcining.
Figure 1. Schematic illustration of the preparation of (a) CN-H and CNP10-H nanosheet. (b) CNP10 obtained by directly calcining.
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Figure 2. (a) XRD patterns and (b) FTIR spectra of CN-H and CNPx-H.
Figure 2. (a) XRD patterns and (b) FTIR spectra of CN-H and CNPx-H.
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Figure 3. (a) UV/Vis absorption spectra and (b) band-gap energies of CN-H and CNPx-H.
Figure 3. (a) UV/Vis absorption spectra and (b) band-gap energies of CN-H and CNPx-H.
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Figure 4. (a) Time-dependent photocatalytic H2 production of the samples; (b) photocatalytic H2 evolution rates of CNPx-H; (c) time-dependent photocatalytic H2 production of CNP10 and CNP10-H; (d) photocatalytic-stability test for H2 production of CNP10-H under visible-light (λ > 420 nm) irradiation using TEOA as the sacrificial agents and Pt as cocatalyst.
Figure 4. (a) Time-dependent photocatalytic H2 production of the samples; (b) photocatalytic H2 evolution rates of CNPx-H; (c) time-dependent photocatalytic H2 production of CNP10 and CNP10-H; (d) photocatalytic-stability test for H2 production of CNP10-H under visible-light (λ > 420 nm) irradiation using TEOA as the sacrificial agents and Pt as cocatalyst.
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Figure 5. SEM images of (a) CN-H and (b) CNP10-H; (c,d) TEM images of CNP10-H.
Figure 5. SEM images of (a) CN-H and (b) CNP10-H; (c,d) TEM images of CNP10-H.
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Figure 6. (a) VB XPS spectra of CN-H and CNP10-H; (b) Mott–Schottky plots of CN-H and CNP10-H; (c) band-gap energies of CN-H and CNP10-H; (d) band structure of CN-H and CNP10-H (1.75 eV and 1.64 eV are the valence band tops, and −0.95 eV and −0.96 eV are the conduction band bottoms).
Figure 6. (a) VB XPS spectra of CN-H and CNP10-H; (b) Mott–Schottky plots of CN-H and CNP10-H; (c) band-gap energies of CN-H and CNP10-H; (d) band structure of CN-H and CNP10-H (1.75 eV and 1.64 eV are the valence band tops, and −0.95 eV and −0.96 eV are the conduction band bottoms).
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Figure 7. (a) XPS-survey spectra, (b) C 1 s high-resolution spectra, and (c) N 1 s high-resolution spectra of CN-H and CNP10-H; (d) N2 adsorption-desorption isotherm of CN-H and CNP10-H.
Figure 7. (a) XPS-survey spectra, (b) C 1 s high-resolution spectra, and (c) N 1 s high-resolution spectra of CN-H and CNP10-H; (d) N2 adsorption-desorption isotherm of CN-H and CNP10-H.
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Figure 8. (a) Photoluminescence (PL) spectra, (b) photocurrent response, (c) Nyquist plots, and (d) room-temperature EPR spectra of CN-H and CNP10-H.
Figure 8. (a) Photoluminescence (PL) spectra, (b) photocurrent response, (c) Nyquist plots, and (d) room-temperature EPR spectra of CN-H and CNP10-H.
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Liu, X.; Zhao, C.; Muhmood, T.; Yang, X. Regulating the Assembly of Precursors of Carbon Nitrides to Improve Photocatalytic Hydrogen Production. Catalysts 2022, 12, 1634. https://doi.org/10.3390/catal12121634

AMA Style

Liu X, Zhao C, Muhmood T, Yang X. Regulating the Assembly of Precursors of Carbon Nitrides to Improve Photocatalytic Hydrogen Production. Catalysts. 2022; 12(12):1634. https://doi.org/10.3390/catal12121634

Chicago/Turabian Style

Liu, Xinying, Chengxiao Zhao, Tahir Muhmood, and Xiaofei Yang. 2022. "Regulating the Assembly of Precursors of Carbon Nitrides to Improve Photocatalytic Hydrogen Production" Catalysts 12, no. 12: 1634. https://doi.org/10.3390/catal12121634

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