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Article

Amorphous Carbon and Cyano-Group Self-Modified P-Doped g-C3N4 for Boosting Photocatalytic H2 Evolution

by
Hang Gao
1,
Minghao Zhang
1,
Huixin Li
1,
Yiran Zhang
1,
Caixia Song
1,2,* and
Debao Wang
1,*
1
College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
2
College of Materials Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042, China
*
Authors to whom correspondence should be addressed.
Catalysts 2024, 14(8), 523; https://doi.org/10.3390/catal14080523
Submission received: 27 June 2024 / Revised: 5 August 2024 / Accepted: 10 August 2024 / Published: 13 August 2024
(This article belongs to the Special Issue Featured Papers in “Environmental Catalysis” Section)
Browse Figures
Versions Notes

Abstract

:
Designing g-C3N4-based nanostructured photocatalysts is crucial to boosting their application in advancing clean energy and sustainable environmental solutions. In this study, cyano groups and amorphous carbon self-modified P-doped g-C3N4 (PCNx) photocatalysts were designed and prepared by one-pot calcination. Melamine phosphate was employed as a multifunctional precursor to simultaneously achieve P-doping and amorphous carbon/cyano group self-modification in the g-C3N4 photocatalyst. The molar ratio of urea to melamine phosphate regulates the content of amorphous carbon and cyano groups, which further enhances the conductivity of g-C3N4. Due to the high conductivity of amorphous carbon and cyano groups, the charge transfer process was further accelerated. As a result, the optimized P-doping and amorphous carbon/cyano-group in PCN2 photocatalyst led to an excellent H2 production rate of 157.86 µmol·g−1·h−1 under visible light, which is approximately 2.4 times and 3 times higher than those of CN and PCN. The work developed an alternative strategy for the construction of highly efficient g-C3N4-based photocatalysts.

1. Introduction

Hydrogen (H2) has been widely accepted to be an environmentally friendly and renewable energy source because merely water and energy are the oxidation products of H2, which makes it a promising, clean, and efficient fuel option to reduce environmental issues and fulfill the global energy demand [1]. Of the various options available, H2 energy sources can be generated from water and sunlight, which offers one of the most promising pathways to achieve carbon neutrality by consuming two of the most abundant natural resources on earth [2]. Photocatalytic hydrogen evolution uses free solar energy to excite a semiconductor, generating photogenerated charges and catalyzing the H+ reduction. The key factor is to design a photocatalyst with excellent visible light response, cost-effective, and stable in practical application. Various semiconductor materials have been extensively researched to develop stable and high-performing photocatalysts for the production of hydrogen energy, including metal oxides, sulfides, and their composites, as well as non-metal compounds [3,4,5,6,7,8]. Among them, graphitic carbon nitride (g-C3N4)-based photocatalysts have gained intensive investigation due to their low cost, high stability, non-metallic properties, and suitable band gap (~2.7 eV) [9,10,11]. Unfortunately, single-component g-C3N4 shows lower photocatalytic H2 production efficiency due to the rapid photocarrier recombination associated with its symmetry structure, which leads to the overlap of photogenerated electrons and holes [12]. Therefore, the preparation of g-C3N4-based photocatalysts with high photocatalytic hydrogen production performance still faces a tremendous challenge.
Until now, numerous approaches have been employed to overcome the intrinsic drawbacks, such as element doping and carbon material modification [13,14,15,16]. Element doping, especially P element doping, can change the electrical structure of g-C3N4 due to unique electronegativity and valence electron structure, which improves photocatalytic performance. For example, Hammoud et al. prepared the P-doped g-C3N4 photocatalyst using NaH2PO4 dopant precursor, which exhibited increased visible light absorption and promoted charge-carrier separation [17]. Yu et al. prepared the P-doped photocatalyst by pre-hydrothermal and calcination processes. The introduction of P element effectively adjusted the band gap and accelerated the charge transfer separation. Furthermore, the surface defects were formed because of the P element doping, which provided more active sites for the photocatalytic hydrogen production reaction [18]. Sun et al. revealed that tuning the content of P element doping could modulate the electronic donor concentration, which regulated the Fermi level from below to above the doping level. Therefore, the photocarrier lifetime was prolonged when the doping state neared the Fermi level [19]. P element is preferred to replace a single C element to form a P-N bond. The formed P-N bond can act as a charge transfer bridge, which results in enhanced visible light absorption, more active sites, and efficient separation of photocarriers [20]. Recently, some research revealed that the photocatalytic performance is significantly influenced by the positions of P-element doped in the g-C3N4 network. For example, Yu et al. prepared dual P-doped site modified g-C3N4 for H2O2 production using two phosphorus dopants. The results indicated that dual P-site doping g-C3N4 exhibited higher photocatalytic performance than its single P-site counterpart because of its large surface area and more carrier transport channels [21].
Carbon materials with excellent conductivity have generally been used to strengthen the photocatalytic activity of the photocatalysts. For instance, Tian et al. found that amorphous carbon (a-C)-decorated g-C3N4 exhibited promoted performance for photocatalytic H2 production, which was caused by the facilitated charge transfer efficiency due to the excellent conductivity of a-C [22]. Li et al. synthesized a carbon particle-decorated photocatalyst, which significantly accelerated the storage and transfer of electrons. The photocatalytic hydrogen production performance of the modified photocatalyst was remarkably enhanced [23]. Cyano groups can trap photogenerated electrons, which suppress charge recombination. For instance, Chang et al. reported that cyano groups, as stronger electron-withdrawing groups, could prevent the recombination of photocatalytic carriers, enhance the electron excitation from n→π*, and accelerate the H+ adsorption, which caused improved photocatalytic hydrogen production activity [24]. Other researchers successfully introduced the cyano groups on the backbone edge of g-C3N4. The cyano groups played important roles in the photocatalytic hydrogen reaction, which worked as electron capture centers to alter the band gap of g-C3N4, reducing the recombination rate of electron-hole pairs and accelerating the charge separation. Compared with single component g-C3N4, the photocatalytic hydrogen production activity was enhanced 13.5 times [25]. The modification of amorphous carbon and cyano groups generally requires supernumerary precursors. For instance, Xu et al. synthesized amorphous carbon-decorated g-C3N4 by calcining urea with glucose. The amorphous carbon in the interlayer was grown in situ, which contributed to more efficient charge transfer [26]. Huang et al. prepared cyano group-modified g-C3N4 using trithiocyanuric acid and melamine by calcination. Cyano groups enhanced the formation of medium basic sites and assisted the H2 activation of Ru co-catalyst, leading to higher photocatalytic performance [27]. It is expected that the regulation of amorphous carbon and cyano groups could further enhance the photocatalytic activity of g-C3N4, and a detailed investigation is urgent.
In this study, urea and melamine phosphate were used as precursors to synthesize a self-modified g-C3N4 photocatalyst, which is schematically illustrated in Scheme 1. Melamine phosphate achieves P-site doping and self-modification of amorphous carbon/cyano groups. The in situ synthesized amorphous carbon and cyano groups promote the bulk-to-surface charge transfer. The molar ratio of urea modulates the concentration of amorphous carbon and cyano groups, which further promotes the conductivity of g-C3N4. Therefore, the g-C3N4 prepared from the dual precursors shows excellent photocatalytic hydrogen production activity.

2. Results and Discussions

2.1. Photocatalyst Characterization

To investigate the crystal structures of photocatalysts, the prepared samples were subjected to powder X-ray diffraction (XRD). As presented in Figure 1A, the distinct peak at around 27.9° observed in the CN, PCN, and PCN2 samples corresponded to the typical (002) crystal plane of g-C3N4 [28]. In addition to the (002) diffraction of g-C3N4, a broad and weak peak around 22.5° in the PCN and PCN2 samples was observed more clearly than in the CN sample, which may be caused by the P element doping and the formation of amorphous structure carbon [23,26]. CN exhibited a stronger (002) peak intensity, which represents the interfacial stacking reflection of the conjugated aromatic structure. For PCN and PCN2, the (002) peak remained the dominant diffraction, indicating the phase structure remained. However, the (002) peak intensity became weaker, which corresponded to the broken interlayer arrangement in the photocatalysts. At the same time, the peak intensity of amorphous carbon of PCN2 was slightly weaker than that of PCN, suggesting that the concentration of amorphous carbon decreased. Meanwhile, the characteristic peak of amorphous carbon in PCN2 was much broader than in PCN, which assigned a smaller particle size. The results indicate that the molar ratio of urea to melamine phosphate regulates the content of amorphous structures in the samples, which benefits photocatalytic performance.
Fourier transform infrared (FT-IR) spectra were applied to estimate the molecular structures of CN, PCN, and PCN2. As reported in Figure 1B, a sharp peak in 802 cm−1 originated from the typical vibration of triazine units of g-C3N4 [29]. The absorption at 3400–3000 cm−1 was assigned to the stretching vibration of N-H. Additionally, the peak at about 1700–1200 cm−1 was ascribed to the typical C-N and C=N heterocycles characteristic stretching vibration pattern [30,31]. These characteristic peaks illustrated the formation of a typical g-C3N4 structure [32]. Impressively, a new peak at 2200 cm−1 was discovered in PCN and PCN2, which corresponded to the cyano groups [33]. The peak intensities of cyano groups were reduced in PCN2, assessing that the tunable concentration could be achieved by adding urea. Furthermore, the vibration mode at 950 cm−1 discovered in PCN and PCN2 corresponded to the P-N bond, which proved that P replaced C atoms in the structure [34].
The morphology and nanostructure of PCN2 were determined by SEM, TEM, and HRTEM images. As shown in Figure 2A, PCN2 showed a typical nano-sheet stacking morphology. For the morphology comparison with PCN2, SEM images of both CN and PCN have been provided in Figure S1. Figure S1a shows the sheet-like structure of bulk g-C3N4, while PCN in Figure S1b possesses ultra-thin sheet structures as that of PCN2. TEM and HRTEM images further revealed the layered structure of PCN2, as shown in Figure 2B,C, which was consistent with the SEM spectrum. The inset of Figure 2C clearly presents the lattice fringe of g-C3N4 with a spacing of 0.33 nm [22,35]. The AFD image and element mappings in Figure 2D indicate the uniform distribution of C, N, and P elements in the PCN2 sample.
In many instances, the catalytic activity of a photocatalyst is related to its specific surface area and pore structure. Therefore, the N2 adsorption/desorption isotherms of PCN and PCN2 photocatalysts have been recorded. The specific surface areas were determined using the BET (Brunauer–Emmet–Teller) method and the pore diameters were analyzed through BJH (Barrett–Joyner–Halenda) method. As displayed in Figure S2, both photocatalysts present type-IV isotherms with a hysteresis loop, which implies the existence of mesopores in PCN and PCN2 [24]. The insets of Figure S2 confirm that both of them have a pore size distribution ranging from 2 nm to 10 nm. Calculated from N2 adsorption/desorption isotherms, PCN possesses a surface area of 29.6 m2 g−1 and pore diameters centered at 3.8 nm, while PCN2 has a surface area of 52.4 m2 g−1 and pore diameters centered at 6.8 nm. It is evident that PCN possesses a higher BET surface area and a larger dominant pore diameter. This type of mesoporous structure allows for more effective penetration and absorption of light and offers more active sites for enhanced photocatalytic H2 evolution activity of photocatalysts.
XPS measurements were applied to quantify the surface chemical composition of photocatalysts. Adventitious carbon (284.8 eV) was conducted to calibrate the binding energy scale. As shown in Figure 3A, the elements P, C, N, and O were detected in PCN and PCN2. The surface element contents of PCN and PCN2 photocatalysts are listed in Table S1, further confirming the addition of urea could regulate the content of carbon and P-doping in the samples. Three peaks at 284.8 eV, 286.3 eV, and 288.5 eV were observed in C 1s spectra of PCN2 from Figure 3B, which could be assigned to amorphous carbon (C-C groups) and -C≡N and N-C=N bonds [36,37,38,39], indicating the formation of cyano groups. The N 1s spectra could be deconvoluted into two diffraction peaks at 397.8 eV and 399.4 eV, as reported in Figure 3C, which belong to the C=N-C bond and sp2 N in N-(C)3 bond [40,41]. The distinct characteristic peaks at around 133 eV and 134 eV in Figure 3D were attributed to the P-N and P=N coordination in the PCN and PCN2 samples, indicating that the P atoms have dual doping sites in aromatic rings [42,43]. The O1s spectra in Figure S3 have been fitted into two components at around 533.8 and 532.0 eV, which could be assigned to surface adsorbed oxygen species (such as water and oxygen) because the relative intensity of these peaks is constant [44]. All the results confirm that cyano-group self-modified P-doped g-C3N4 has been successfully prepared by employing urea and melamine phosphate as multifunctional precursors. The XPS data are in agreement with the above-discussed FT-IR results.

2.2. Photocatalytic Performance of Catalysts

The photocatalytic hydrogen activities of PCN, CN, and PCNx were carried out with 10% TEOA as the sacrificial agent. 2 wt% Pt, as co-catalyst, was loaded onto the surface of the photocatalysts using a photo deposition method before the reaction. As shown in Figure 4A, the hydrogen production rate of PCN was 313.3 µmol g−1, while that of CN was 395.1 µmol g−1. The optimized PCNx exhibited the highest photocatalytic hydrogen production rate compared to both CN and PCN after urea regulation, which were 755.04 µmol g−1 for PCN1, 947.16 µmol g−1 for PCN2, and 874.74 µmol g−1 for PCN3. PCN2 demonstrated the best photocatalytic hydrogen production rate of 157.86 µmol g−1 h−1, as shown in Figure 4B, which was about 2.4 times and 3 times higher than those of CN and PCN, respectively. For comparison, the PCN2 without Pt co-catalyst can achieve a photocatalytic hydrogen production of 616.8 µmol g−1 and a hydrogen production rate of 102.8 µmol g−1 h−1, which are even much higher than those of the PCN and CN with Pt co-catalyst.
The stability of the photocatalyst was an important factor for industrial production and application. PCN2 was used as the photocatalyst to conduct the five cyclic experiments. As shown in Figure 4C, the photocatalytic hydrogen production just shows a slight drop and remains more than 95% after five-cycle experiments, indicating the good stability and reusability of the PCN2 photocatalyst. The stability of PCN2 was further identified using XRD, FTIR, and SEM. The characteristic peaks of g-C3N4 and amorphous carbon were still clearly detected in the XRD spectra of PCN2 after the reaction (Figure 4D). The peaks at 39.7° and 46.2° corresponded to the (111) and (200) crystal faces of the Pt co-catalyst, indicating the successful deposition of the Pt co-catalyst. Cyano groups and characteristic P-N bonds were also detected in the FTIR spectra, as shown in Figure 4E. PCN2 still retained the nanosheet morphology after the reaction, as shown in Figure 4F, demonstrating the high stability of the prepared photocatalyst. In order to further reveal the excellent performance of the prepared PCN2 photocatalyst, a comparison with previous results was outlined in Table 1. Obviously, the photocatalytic hydrogen production activity of PCN2 was superior to that of most CN-based photocatalysts, indicating the outstanding performance of PCN2.

2.3. Mechanism of Photocatalytic H2 Production

UV-vis diffuse reflectance spectra (UV-vis DRS) were recorded to evaluate the light absorption properties of the photocatalysts. According to Figure 5A, CN, PCN, and PCN2 had intrinsic absorption edges around 450 nm. PCN2 showed more progressive visible light absorption ability than CN and PCN, indicating more effective use of solar energy. The band gap energies (Eg) of PCN, CN, and PCN2 were calculated by the following formula [55]:
(αhν)n = A(hν − Eg)
where α denotes the absorption coefficient, and h and ν represent Planck’s constant and incident light frequency, respectively. A is the proportionality constant, and Eg represents band gap energy. n takes the value of 1/2 for indirect band gap semiconductor. Then, the band gaps of CN, PCN, and PCN2 were estimated to be 2.28 eV, 2.23 eV, and 2.18 eV, respectively, as shown in the inset of Figure 5A.
Mott–Schottky spectra were conducted to determine the semiconductor type and flat band potentials of the photocatalysts. As depicted in Figure 5B–D, CN, PCN, and PCN2 all showed an n-type slope, and the flat band potentials were estimated to be −1.33 eV, −1.38 eV, and −1.3 eV (V vs. Ag/AgCl, pH = 6.8), respectively. According to the equation [56]
E(vs. NHE, pH=0) = E(vs. Ag/AgCl) + 0.059 × pH + E(Ag/AgCl)
The flat band potentials of CN, PCN, and PCN2 were converted to normal hydrogen electrodes (vs. NHE) for −1.13 eV, −1.18 eV, and −1.1 eV, respectively. The lower flat band potential also represents the better electrical conductivity of PCN2 [57]. The CB potentials were generally more negative than the flat band by about −0.2 eV [58]. Therefore, the CB potentials of CN, PCN, and PCN2 were determined to be −1.33 eV, −1.38 eV, and −1.3 eV (vs. NHE), respectively. The bandgap alignments of the photocatalysts are illustrated in Figure 5E, which provided evidence for charge transfer. The charge carrier density (ND) reflects the electron density of the photocatalysts, and a higher ND value indicates a greater electron density in the photocatalyst. ND could be calculated by applying the following equation [59]:
N D = 2 ε ε 0 e 0 d 1 / C 2 d V − 1
which ε represents the dielectric constant (εg-C3N4 = 2) [60], ε0 corresponds to the vacuum permittivity (8.86 × 10−12 F m−1), e0 is the electronic charge unit (1.6 × 10−19 C), and V is the potential. As shown in Figure 5F, the values of ND were calculated to be 7.7 × 1019 cm−3 for PCN2, 5.8 × 1019 cm−3 for PCN, and 3.7 × 1019 cm−3 for CN, indicating the highest electron density of PCN2. According to the above analysis, the photocatalyst prepared from the dual precursors exhibited a narrower band gap and a higher electron density.
The photocarrier separation performances of the fabricated photocatalysts were investigated by transient photocurrent response measurement (I-t) and electrochemical impedance spectra (EIS) tests. Higher photocurrent density of photocatalysts indicated faster charge transfer capability. As shown in Figure 6A, PCN2 showed higher photocurrent intensity than CN and PCN, demonstrating an improved photocarrier separation efficiency. A smaller arc radius in EIS spectra usually suggests a lower electron transfer resistance. As shown in Figure 6B, the resistivity of PCN2 was 1.7 × 105 Ω, which was much smaller than those of PCN (12 × 105 Ω) and CN (75 × 105 Ω). The resistivities of the prepared samples decreased in the order PCN2 < CN < PCN, demonstrating the reduced charge transfer resistance of PCN2. Photoluminescence (PL) was analyzed to determine the recombination rate of photocarriers. Lower fluorescence intensity indicated an enhanced separation rate of the photocarriers. Obviously, PCN2 showed the lowest emission intensity, as shown in Figure 6C, proving the inhibited photocarrier recombination ability. Time-resolved photoluminescence (TRPL) was implemented to investigate the photocarrier separation process and the lifetime of the photocatalysts. It was determined from Figure 6D that PCN2 exhibited the longest lifetime compared to PCN and CN, which demonstrated the promoted photocarrier separation rate [61].
According to the above discussion, a possible mechanism of the enhanced photocatalytic hydrogen production mechanism is proposed, as shown in Figure 7. Photoelectrons are promoted from the valence band of PCN to the conduction band under visible light. Then the photoelectrons are quickly transferred from the bulk to the surface because of the high conductivity of amorphous carbon and cyano groups. The molar ratio of urea to melamine phosphate regulates the content of amorphous carbon and cyano groups for the optimum property. The photoelectrons at the surface are then captured by Pt for photocatalytic hydrogen production. The synergetic effect of the ultra-thin structure and P-doping could supply a large quantity of active sites for photocatalytic hydrogen production [62]. As a result, PCN2 shows enhanced photocatalytic hydrogen production activity.

3. Materials and Methods

3.1. Materials

Melamine phosphate (C3H6N6(H3PO4)n, 99%) was purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China); Urea (CH4N2O, 99%), ethanol (CH3CH2OH, 99%), triethanolamine (TEOA, 99%), and H2PtCl6 (99%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All chemical reagents in this report were of analytical grade and employed without further treatment.

3.2. Fabrication of Photocatalysts

Preparation of PCNx: Amorphous carbon and cyano-group self-modified g-C3N4 photocatalysts were prepared using the one-pot thermal polymerization method, which was schematically illustrated in Scheme 1. Typically, 5 mmol melamine phosphate and different amounts of urea were adequately ground in a mortar for 30 min. The resultant powders were transferred to a porcelain boat covered with aluminum foil and heated at 600 °C for 4 h with a heating rate of 10 °C/min under air. The obtained yellow products were washed with deionized water and ethanol and then dried at 60 °C. The final photocatalysts obtained with different amounts of urea (6.6 mmol, 10.8 mmol, and 15 mmol) were labeled as PCN1, PCN2, and PCN3.
For comparison, 5 mmol melamine phosphate or 83.3 mmol urea was subjected to fabricate photocatalysts from a single precursor with the same procedure. The obtained samples were expressed as PCN and CN, respectively. The yield of prepared samples was outlined in Table S2 (supporting information). All the samples have a yield higher than 74%.

3.3. Characterizations of Photocatalysts

A D-MAX 2500/PC powder X-ray diffraction diffractometer (XRD) (Rigaku Corporation, Tokyo, Japan) was used to identify the crystal structures of synthesized carbon nitride composite photocatalysts with Cu Kα radiation at 40 kV and 150 mA. A Nicolet IS50 (Thermofisher Scientific, Waltham, MA, USA) spectrometer was used to record Fourier-transform infrared (FTIR) spectra in the form of KBr pellets. Scanning electron microscopy (SEM, JSM-6700F microscope, JEOL Ltd., Tokyo, Japan) was carried out to determine the morphologies of photocatalysts. A JEM-F200 transmission electron microscope (TEM) (JEOL Ltd., Tokyo, Japan) was used to obtain TEM and high-resolution TEM (HRTEM) images and element mappings. An ESCALAB 250XI X-ray photoelectron spectrometer (XPS) (Thermo Fisher Scientific Inc., Waltham, MA USA) was applied to analyze the surface chemical states of the photocatalysts using Al-Kα radiation. Photoluminescence (PL) measurements were conducted on an LS-55 fluorescence spectrophotometer (Perkin Elmer, Waltham, MA, USA) under the excitation wavelength of 375 nm. A Perkin-Elmer Lambda 750 spectrophotometer (Perkin Elmer, Waltham, MA, USA) was employed to collect the UV-Vis Diffuse reflectance spectra (DRS) of the prepared composites. Time-resolved photoluminescence (TRPL) was measured by a fluorescence lifetime spectrophotometer (Edinburgh Instruments FS5, Edinburgh Instruments Ltd., Livingston, UK) with an excitation wavelength of 375 nm. N2 desorption adsorption isotherms were recorded on a N2 adsorption analyzer (Micromeritics ASAP 2460 Version 3.01).
Detailed photoelectrochemical measurements were supplied as the electronic supporting information.

3.4. Photocatalytic Hydrogen Production Measurement

The photocatalytic hydrogen production experiments were performed in a sealable quartz vessel with the irradiation of a CEL-HXF 300 Xe lamp (CEAULIGHT, Beijing, China, λ > 420 nm). Typically, 100 mg photocatalysts were dispersed in 100 mL aqueous solution, which contained 10% vol (10 mL) TEOA as the sacrificial agent. 2 wt% Pt was photo-deposition on the photocatalyst. 0.266 mL H2PtCl6 (1 g/50 mL) was added to the system and irradiated for 1 h before the reaction. The reaction temperature was maintained at 7 °C through cyclical cooling water. The quartz vessel was vacuumed thoroughly. The H2 content was determined using an online gas chromatograph (Agilent 7890A, Hong Kong, China) equipped with a thermal conductivity detector and a 4 m 5 Å molecular sieve column. High-purity N2 was used as the carrier gas. The produced H2 was analyzed at an interval of every 30 min.
The photocatalytic cyclic stability measurement was performed by batch experiments. After each reaction, the photocatalyst was centrifugated and dried at 60 °C and then reused in 100 mL fresh aqueous solution containing 10% TEOA (10 mL) for the next 6 h reaction.

4. Conclusions

In summary, P-doped g-C3N4 with self-modified amorphous carbon and cyano groups was prepared using a one-pot method. Melamine phosphate was used as an important precursor to prepare P-doped g-C3N4, which simultaneously caused the self-modification of amorphous carbon and cyano groups. Both amorphous carbon and cyano groups could enhance the photocarrier transfer from the bulk to the surface because of their high conductivity. Urea regulated the concentration and structural properties of amorphous carbon and cyano groups, which further enhanced the conductivity and visible light absorption of the photocatalyst. Therefore, PCN2 exhibited excellent photocatalytic hydrogen production activity compared to PCN and CN, which was about 157.86 µmol·g−1·h−1. The work developed an alternative strategy for the construction of high-performance g-C3N4 photocatalysts.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/catal14080523/s1: photoelectrochemical measurements of the catalysts and some experimental results (Figures S1–S3, Tables S1 and S2).

Author Contributions

Conceptualization, H.G. and C.S.; methodology, H.G.; validation, M.Z., H.L. and Y.Z.; investigation, H.G. and M.Z.; writing—original draft preparation, H.G.; writing—review and editing, C.S. and D.W; supervision, C.S. and D.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 52072194).

Data Availability Statement

The data presented in this study are available upon request from the authors.

Conflicts of Interest

The authors declare no conflicts of interest.

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Catalysts 14 00523 sch001
Scheme 1. Schematic illustration for the preparation of PCNx photocatalysts.
Scheme 1. Schematic illustration for the preparation of PCNx photocatalysts.
Catalysts 14 00523 sch001
Catalysts 14 00523 g001
Figure 1. (A) XRD pattern and (B) FT-IR spectra of CN, PCN, and PCN2.
Figure 1. (A) XRD pattern and (B) FT-IR spectra of CN, PCN, and PCN2.
Catalysts 14 00523 g001
Catalysts 14 00523 g002
Figure 2. (A) SEM, (B) TEM, and (C) HRTEM images of PCN2; (D) AFD image and element mappings of PCN2.
Figure 2. (A) SEM, (B) TEM, and (C) HRTEM images of PCN2; (D) AFD image and element mappings of PCN2.
Catalysts 14 00523 g002
Catalysts 14 00523 g003
Figure 3. (A) XPS survey spectra, (B) C 1s, (C) N 1s, and (D) P 2p spectra of PCN and PCN2.
Figure 3. (A) XPS survey spectra, (B) C 1s, (C) N 1s, and (D) P 2p spectra of PCN and PCN2.
Catalysts 14 00523 g003
Catalysts 14 00523 g004
Figure 4. (A,B) Photocatalytic hydrogen production performances of PCN, CN, and PCNx; (C) recycling hydrogen evolution of PCN2; (D) XRD spectra, (E) FTIR, and (F) SEM of PCN2 after the reaction.
Figure 4. (A,B) Photocatalytic hydrogen production performances of PCN, CN, and PCNx; (C) recycling hydrogen evolution of PCN2; (D) XRD spectra, (E) FTIR, and (F) SEM of PCN2 after the reaction.
Catalysts 14 00523 g004
Catalysts 14 00523 g005
Figure 5. (A) UV-vis DRS spectra and band gap (inset) of PCN, CN, and PCN2; (B) Mott–Schottky spectra of PCN2; (C) Mott–Schottky spectra of CN; (D) Mott–Schottky spectra of PCN; (E) energy band alignments of PCN, CN, and PCN2; (F) ND of PCN, CN, and PCN2.
Figure 5. (A) UV-vis DRS spectra and band gap (inset) of PCN, CN, and PCN2; (B) Mott–Schottky spectra of PCN2; (C) Mott–Schottky spectra of CN; (D) Mott–Schottky spectra of PCN; (E) energy band alignments of PCN, CN, and PCN2; (F) ND of PCN, CN, and PCN2.
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Catalysts 14 00523 g006
Figure 6. (A) I-t curves, (B) EIS spectra, (C) PL, and (D) TRPL of PCN, CN, and PCN2.
Figure 6. (A) I-t curves, (B) EIS spectra, (C) PL, and (D) TRPL of PCN, CN, and PCN2.
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Catalysts 14 00523 g007
Figure 7. Possible mechanism for the photocatalytic hydrogen production performance of PCN2.
Figure 7. Possible mechanism for the photocatalytic hydrogen production performance of PCN2.
Catalysts 14 00523 g007
Table 1. A comparison of the photocatalytic hydrogen production activity of PCN2 with that of other reported photocatalysts.
Table 1. A comparison of the photocatalytic hydrogen production activity of PCN2 with that of other reported photocatalysts.
CatalystsLight SourceScavengersDosage of PhotocatalystH2 Evolution Rate (µmol g−1 h−1)Ref.
PCN2300 W Xe lampTEOA100 mg157.86This work
g-C3N4-Zn−1@Pt250 W Xe lampTEOA75 mg78.7[45]
g-C3N4/Ni2P300 W Xe lampTEOA20 mg82.5[46]
LaFeO3/g-C3N4/NiS300 W Xe lampTEOA100 mg121[47]
Eu/CN300 W Xe lampTEOA50 mg128.8[48]
CuO/CN300 W LED lampTEOA50 mg130.1[49]
WC/g-C3N4300 W Xe lampTEOA50 mg146.1[50]
g-C3N4/MeTMC-COP300 W Xe lampTEOA10 mg11.8[51]
CuO/pCN400 W Xe lampTEOA50 mg30[52]
p-CNGT300 W Xe lampTEOA30 mg33.1[53]
Co3O4@g-C3N4/CNFs300 W Xe lampTEOA5 mg67.17[54]
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Gao, H.; Zhang, M.; Li, H.; Zhang, Y.; Song, C.; Wang, D. Amorphous Carbon and Cyano-Group Self-Modified P-Doped g-C3N4 for Boosting Photocatalytic H2 Evolution. Catalysts 2024, 14, 523. https://doi.org/10.3390/catal14080523

AMA Style

Gao H, Zhang M, Li H, Zhang Y, Song C, Wang D. Amorphous Carbon and Cyano-Group Self-Modified P-Doped g-C3N4 for Boosting Photocatalytic H2 Evolution. Catalysts. 2024; 14(8):523. https://doi.org/10.3390/catal14080523

Chicago/Turabian Style

Gao, Hang, Minghao Zhang, Huixin Li, Yiran Zhang, Caixia Song, and Debao Wang. 2024. "Amorphous Carbon and Cyano-Group Self-Modified P-Doped g-C3N4 for Boosting Photocatalytic H2 Evolution" Catalysts 14, no. 8: 523. https://doi.org/10.3390/catal14080523

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