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Article

Design and Architecture of P-O Co-Doped Porous g-C3N4 by Supramolecular Self-Assembly for Enhanced Hydrogen Evolution

by
Ximiao Zhu
1,
Fan Yang
1,
Jinhua Liu
2,
Guangying Zhou
1,
Dongdong Chen
1,
Zhang Liu
3,* and
Jianzhang Fang
1,4,*
1
School of Environment, South China Normal University, Guangzhou 510006, China
2
Jiangmen Solid Waste Treatment Co., Ltd., Jiangmen 529000, China
3
Division of Environment and Sustainability, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China
4
Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety and MOE Key Laboratory of Theoretical Chemistry of Environment, South China Normal University, Guangzhou 510006, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(12), 1583; https://doi.org/10.3390/catal12121583
Submission received: 12 October 2022 / Revised: 24 November 2022 / Accepted: 30 November 2022 / Published: 5 December 2022
(This article belongs to the Special Issue Visible-Light Photocatalysis for Sustainable Environmental Remediation)
Browse Figures
Versions Notes

Abstract

:
A novel phosphorus and oxygen co-doped graphitic carbon nitride (sheetP-O-CNSSA) photocatalyst was successfully synthesized and applied for H2 evolution under visible light. In the synthesis process of sheetP-O-CNSSA, the supramolecular complex was developed by the self-assembly and copolymerization reaction among melamine, cyanuric acid (CA) and trithiocyanuric acid (TCA) to act as g-C3N4 precursors, while (NH4)2HPO4 was applied as P and O precursors for element doping. The chemical structures, morphologies, and optical properties of the sheetP-O-CNSSA were characterized by a series of measurements, i.e., XRD, FT-IR, SEM, TEM, UV-vis DRS, and PL. The results suggested that the introduction of P and O elements could enhance the separation and migration efficiency of photogenerated electrons and holes in the energy band of g-C3N4. The photocatalytic tests over Erythrosin B (EB) sensitized sheetP-O-CNSSA indicated that the hydrogen evolution was greatly enhanced compared with other catalysts and non-sensitized sheetP-O-CNSSA under visible light irradiation. Finally, a possible dye-sensitized photocatalysis mechanism was also proposed on the basis of the as-obtained results.

1. Introduction

In recent years, the excessive consumption of fossil fuel has brought a series of severe environmental problems, thus it is very urgent to develop and utilize environmentally friendly and clean energy at present [1,2]. In the past decades, photocatalytic hydrogen production utilizing solar energy was considered to be a huge potential strategy for hydrogen generation [3,4,5]. It was reported that metal-free polymeric graphitic carbon nitride (i.e., g-C3N4) possessed good visible light response activity, suitable position, and chemical stability, and thus can be a candidate for substituting the traditional expensive metal-based photocatalysts [6,7]. Since 2009, Wang and co-researchers found that g-C3N4 photocatalyst could split water to produce hydrogen under visible light [8]. Research about g-C3N4-based catalysts for photocatalytic hydrogen production is emerging and will continue to be attractive [3,4,9,10] in the coming years. Nevertheless, the bulk g-C3N4 still has the following defects [9,10]: (1) small specific surface area; (2) scarce reaction active site; (3) insufficient visible light absorption (λ < 460 nm) ability and low quantum efficiency; and (4) high recombination rate of photo-generated electrons and holes. It is therefore highly desirable to develop novel synthetic strategies and methods to modify the g-C3N4 structure and texture to enhance its photocatalytic performance.
Morphology modification is one of the most efficient strategies to improve the photocatalytic performance of g-C3N4 material [11,12]. It is reported that the porous sheet-like structure can provide much more active sites, larger SBET (i.e., specific surface area), and multiple channels for electron exchange, thus it can significantly promote electron-holes separation [13,14]. In addition, the porous sheet-like samples always possess a narrower bandgap and fewer useless functional groups, such as the –NH group and –NH2. The structure and morphology of the supramolecular precursor complex can be easily tuned via modulating the hydrogen bonds, while the morphology of the as-prepared products can be well retained with the supramolecular precursors after calcining at high temperature [15]. Specifically, under the high-temperature treatment, the supramolecular complex will decompose into small molecules and release gases, these processes will significantly affect the morphology and structure of g-C3N4 [16,17]. The supramolecular complex was often fabricated by the mix of melamine and other s-triazine heterocycle, and in the structure of the supramolecular complex, melamine was closely connected with other precursors via hydrogen bonds. Menny et al. synthesized g-C3N4 using a cyanuric acid–melamine self-assembly complex as a precursor could significantly improve the photocatalytic activity compared with that prepared by thermal decomposition of single organics (i.e., cyanuric acid or melamine) [17]. On the whole, physicochemical properties (such as morphology, edge band structure, and optical properties) can be easily controlled by reasonably modifying the reaction conditions of the self-assembly processes.
In addition, in the past twenty years, several other strategies were developed to optimize the properties of g-C3N4, such as metal (such as Cu [18,19], Fe [20,21], Au [22], and Ru [23]) doping, nonmetal (such as Cl [24], B [25,26], P [27,28,29], S [30,31], and I [32]) doping, surface modification (alkalinized [33], protonation [34]), exfoliating [35], or coupling with other photocatalytic semiconductor constructing heterojunctions (such as g-C3N4/Bi2WO6 [36], g-C3N4/Cu2O [37], and g-C3N4/BiPO4 [38]). Among the above approaches, non-metal doping was considered to be one of the most convenient and economical methods [28,39,40]. Specifically, non-metal doping could not only modulate the band structure but also promote the charge transfer process [41]. Zhang et al. [32] synthesized I-doped g-C3N4 and used it as a photocatalyst to split water into hydrogen and oxygen. They found that I doping could improve the light absorption ability and charge transfer efficiency of g-C3N4, thus dramatically increasing the amount of hydrogen prodution. Ran et al. used 2-aminoethylphosphonic acid (AEP) and melamine as raw materials preparing porous P-doped g-C3N4, and confirmed that the as-obtained sample showed an improved H2 production ability [27]. P doping could be greatly extended to the light-responsive region up to 557 nm, promoting the mass-transfer process and enhancing the light-harvesting ability.
To further improve the photocatalytic activity, photosensitization of a photocatalyst with an organic dye is a favorable approach for converting long wavelengths of light to energy for photocatalysts [42,43]. Currently, Erythrosin B(EB) sensitization has attracted much attention in H2 evolution. Lei et al. reported on a novel solid–liquid photocatalytic system that compliments tungsten carbide (WC) photocatalysts with liquid-phase photosensitizing EB; the hydrogen evolution rate reached 66 μmol/h, while pure WC showed poor activity (almost no H2 generation) [44]. Zhang et al. used EB as a sensitizer for Cu/g-C3N4 photocatalytic hydrogen evolution and reached a rate that was 26 times higher than that of the non-sensitized system [45]. Yin et al. discovered that MoS2/C exhibits an excellent photocatalytic hydrogen evolution activity (872.3 μmol/h) sensitized by EB under LED light irradiation [46]. Therefore, EB dye sensitization for promoting H2 evolution has been demonstrated to be an effective approach.
In this paper, a novel crystalline P and O co-doped g-C3N4 (i.e., sheetP-O-CNSSA) was successfully developed. In the synthesized process of sheetP-O-CNSSA, self-assembly supramolecular complex fabricated by copolymerization of melamine–cyanuric acid and trithiocyanuric acid was applied as g-C3N4 precursor, and (NH4)2HPO4 served as the P and O source. The physicochemical properties of the sheetP-O-CNSSA were characterized by a series of measurements. The photocatalytic performance of the as-obtained photocatalysts sensitized with water-soluble Erythrosin B dye were evaluated by photocatalytic hydrogen production experiments under visible light irradiation.

2. Results and Discussion

2.1. Characterization of the As-Prepared Catalysts

The crystal structures of the obtained samples were characterized by X-ray diffraction as shown in Figure 1. It can be observed that there are two characteristic peaks located at 13.1° and 27.2° in the XRD curve of the bulk g-C3N4 (i.e., CN) , which are corresponding to the typical in-planar (100) peak and the interlayer stacking of the aromatic segments indexed as (002) peak, respectively [6,10]. However, in the XRD spectra of P-O co-doped g-C3N4, the characteristic peaks located at 27.2° all shifted to 27.5°. It was reported that element doping could significantly alter the topology of the condensed g-C3N4 sample, thus resulting in the shift of the interlayer peak [17,47]. Therefore, it is very reasonable to deduce that the heteroatoms (i.e., O and P) are successfully doped into the lattices of g-C3N4. In addition, no other impurity peaks except these two peaks are observed, indicating that all the as-prepared samples are the typical unique graphitic structure.
FT-IR measurements were conducted to further illustrate the chemical composition of the as-prepared samples, the results are shown in Figure 2. In the spectrum of the CN, there are a series of peaks in the range of 1200~1600 cm-1, which are related to the stretching modes of CN heterocycles [3,48]. In addition, the sharp peak appeared at 810 cm−1 correspond to the bending vibrations of heptazine rings [3,48]. In the FT-IR spectra of the other three samples, the characteristic vibrational peak of CN heterocycles and heptazine rings could also be observed, suggesting that element doping will not destroy the structural integrity of g-C3N4. Furthermore, one weak peak appears at 1080 cm-1, which could be resulted by the stretching vibration of C-O-C [49], suggesting that extra O was successfully doped into the skeleton of g-C3N4. Unfortunately, the phosphorus-related groups were undetected in all the FT-IR spectra, which may be due to the low content of phosphorus in these catalysts.
The N2 adsorption-desorption measurements were conducted to investigated the pore structure and Brunauer–Emmett–Teller (BET) surface area. As shown in Figure 3, the isotherms of the four samples were coincided with type IV with a H3 hysteresis loop, indicating the presence of a mesoporous structure [48]. The BET surface areas of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA were 13.06 m2 g−1, 36,57 m2 g−1, 46,73 m2 g−1, and 55.56 m2 g−1, respectively. The pore-size distribution curves were acquired by using the Barret–Joyner–Halender (BJH) method (inset in Figure 3). The result further confirmed that there were much more mesoporous structures in the matrix ofsheetP-O-CNSSA, and its average pore size was about 16.8 nm, which is smaller than CN (about 18.5 nm). It is well known that larger BET surface areas and good pore structure could provide more catalytic sites, thus dramatically improving the catalytic efficiency.
The morphology of the catalysts was observed by SEM and displayed in Figure 4A-D. It can be seen that CN exhibited the typical slate-like and stacked lamellar texture (Figure 4A). Compared with CN, the morphology of P-O-CN was apparently changed after the element doping. When TCA was added in the synthesis process, the morphology of rodP-O-CNSSA changed from bulk to rod. In contrast, in the SEM image of sheetP-O-CNSSA, it can be clearly observed that the disordered porous structure was composed of small sheets. The morphology of CN and sheetP-O-CNSSA was further investigated by TEM (Figure 4E,F). The image in Figure 4E indicated that CN exhibited the typical layered sheet-like surface, while the image of sheetP-O-CNSSA (Figure 4F) revealed that some white void spaces formed on the surface of the sample. In addition, the thickness of rodP-O-CNSSA was much smaller than that of CN. The smaller thickness and higher void spaces could afford more reaction active sites.
The chemical compositions of the sheetP-O-CNSSA and CN were explored by XPS, and the results are exhibited in Figure 5. The XPS survey spectra indicate that three main elements of C, N and O exist in CN, while four main elements of C, N, O and P could be observed in sheetP-O-CNSSA. Additionally, the intensity of O 1s peak of sheetP-O-CNSSA was much stronger than that of CN, all these indicate that a certain amount P and O were successfully doped into the matrix of CN heterocycles. Figure 5B showed the high resolution XPS spectra of C 1s. It can be observed that the spectrum of CN can be deconvoluted into three kinds of C species. Specifically, the peaks located at 288.3 eV and 284.8 eV are attributed to the sp2 hybridized C-NH2 group and the graphitic C-C, respectively, and the peak located at 286.2 eV could be ascribed to sp2 C atoms bonded to N (i.e., C=N or C≡N) in an aromatic ring [40,48]. Compared with CN, a new peak located at ca 288.9 eV corresponding to N-C-O bonds can be observed in the C 1s spectrum of sheetP-O-CNSSA, indicating O was doped into the matrix of g-C3N4 [40]. Figure 5C showed the high resolution XPS spectra of N 1s of CN; the peak could be deconvoluted to three different peaks, which were located at 398.6 eV, 399.1 eV, and 400.9 eV, respectively [40,48]. The former one was corresponding to the sp2 hybridized aromatic nitrogen atoms bonded to carbon atoms (C-N=C), the middle one was ascribed to tertiary nitrogen (N-(C)3), and the last one was attributed to amino groups carrying hydrogen ((C)2-N-H) [40,50,51]. It can be observed that the peak located at ~398.6 eV corresponding to C-N=C in sheetP-O-CNSSA shifted to a lower binding energy. According to Fu et al. [40], the lower binding energy was attributed to the introduction of O atoms with higher electronegativity in the aromatic CN heterocycles (electronegativity: O = 3.5, N = 3.0, C = 2.5). It could be concluded that O atoms were preferentially bonded to C atoms by substituting dicoordinated N atoms and was successfully doped into the matrix of sheetP-O-CNSSA.Figure 5D exhibited the high resolution XPS spectra of O 1s; there was only one peak located at 532.6 eV in the spectrum of CN, which was assigned to adsorb O2. The peak in the spectrum of sheetP-O-CNSSA could be deconvoluted to two distinct peaks located at 530.9 eV and 532.6 eV, which could be assigned to N-C-O (or C-O) species and adsorbed O2 [41,52], respectively. In the high resolution XPS spectra of P 2p (Figure 5E), it can be observed that the P element (ca 0.75 atom%) only exists in the sheetP-O-CNSSA sample. In addition, the peak can be deconvoluted to two peaks located at 134.4 eV and 133.5 eV, which were corresponding to the P-N and P=N bond, respectively [39]. In addition, the high resolution XPS spectra of S 2p (Figure 5F) excluded the possible doping of S-containing species.

2.2. Formation Mechanism of the Doped Catalysts

Based on the above-mentioned results, a possible synthetic route of the sheetP-O-CNSSA was proposed and was shown in Scheme 1. Melamine combined with trithiocyanuric acid and cyanuric acid through the hydrogen bond, resulting in the formation of a supramolecular complex. In the calcining process, TCA decomposes into small molecules and CA decomposes into gases; these processes will dramatically affect the transformation of melamine to g-C3N4, thus changing the morphology and structure of g-C3N4. It was reported that the radius of sulfur atoms was much larger than carbon and nitrogen atoms and thus it is difficult for the sulfur element to enter the matrix of g-C3N4 [31]. Moreover, the high temperature of the air in the atmosphere results in the oxidation of sulfur to sulfur trioxide or sulfur dioxide. Therefore, it was very reasonable that sulfur was not detected by the measurements. When (NH4)2HPO4 was introduced to the system, the P and O elements would enter the framework of g-C3N4 at an elevated temperature. It is well known that the valence electron of the P atom is five, which is higher than that of the C atom (about four). When four electrons of P were connected with the adjacent nitrogen through a covalent bond, the remaining one electron of the P atom delocalized into the π conjugated triazine ring [29]. The amount of electron for the O element was also higher than that of the N atom; the substitution of N by O could also change the electronic property of the g-C3N4 system.

2.3. Band Structure of the Catalysts

The optical adsorption properties of the samples were tested by UV-vis diffuse reflectance spectra. As shown in Figure 6A, the absorption band edge of all the doped g-C3N4 samples showed a slight blue shift from 460 nm to 450 nm compared with the bulk CN [53]. It was reported that the blue shift might be caused by the reduction in the size of g-C3N4. Combining the results of TEM, it was very reasonable to attribute the blue shift to the smaller size of g-C3N4. Then, sheetP-O-CNSSA and CN were selected as representatives to study the band gap energy (Eg), which was calculated through the Tauc equation; the results were shown in Figure 6B [48,54]. It can be observed that the Eg values of sheetP-O-CNSSA and CN were 2.85 eV and 2.76 eV, respectively. The reason for the increase in the band gap was ascribed to the quantum confinement effect resulting from the shift of the valence and conduction band edges in opposite directions [30]. Thus, it was concluded that P and O doping can modify the optical and electronic properties of g-C3N4.
In order to further investigate the effect of the molecular self-assembly and P-O co doping on the electronic structure, valence band X-ray photoelectron spectroscopy (VB-XPS) was put forward to determine the potential of the conduction band (CB) and valance band (VB). As is shown in Figure 7, the optical VB potential of CN and sheetP-O-CNSSA was calculated to be 2.45 eV and 2.15 eV, respectively. Combined with the above UV-vis DRS result, the optical CB potential can also be calculated, namely, −0.31 eV and −0.70 eV, respectively. These results indicated that the supramolecular assembly and P-O doping could change electronic structure and elevate the position of conduction band for g-C3N4. The more negativeconduction band for the catalysts is conducive to the photocatalytic hydrogen evolution.
The photoluminescence (PL) is a useful technology for studying the efficiency of photogenerated electrons and hole migration and separation. Here, PL was also used and the results were exhibited in Figure 8. In general, the low intensity of the luminescence peak indicates the high separation rate of photogenerated charge [48]. It can observed that the broad PL band of CN was located at approximately 460 nm (the wavelength of excitation light was 367 nm), and the luminescence of CN showed a strong emission, indicating the extraordinarily high photogenerated charge carrier recombination [55]. The results showed that sheetP-O-CNSSA exhibited the lowest PL peak intensity, suggesting the highest separation efficiency of photogenerated electrons and holes. According to the calculation results in the previous literature, the phosphorus doping could promote the dispersion of the contour distribution of HOMO and LUMO; thus, it could enhance the mobility of photogenerated carriers [56]. On the basis of PL, it can be concluded that the phosphorus and oxygen doping were beneficial for enhancing the separation rate of the photogenerated charge.

2.4. Hydrogen Evolution Performance

Here, photocatalytic hydrogen reduction tests on EB-sensitized catalysts were carried out by using TEOA as a sacrificial regent under visible light irradiation; the results were shown in Figure 9. Control experiments showed that no H2 evolution could be detected in the absence of a photocatalyst or without irradiation. For CN, the rate of the photogenerated H2 could reach 86 μmol g−1 h−1 during the reaction (ca 5 h). The rates of the photogenerated H2 over the other three samples were higher than that of CN due to the higher separate efficiency of photogenerated electrons and holes. Among them, the sheetP-O-CNSSA showed the highest H2 evolution rate of 234 μmol g−1 h−1, which was about three times higher than g-C3N4. The experimental result indicated that supramolecular self-assembly and P-O co-doping could modify the electronic properties and structure of g-C3N4, thus promoting the photocatalytic efficiency of the catalysts. Figure 10 shows the H2 evolution rate over the sheetP-O-CNSSA with various amounts of EB dye. The H2 evolution rates with the addition of variable amounts of EB were all remarkably higher than that with no sensitization. This can be attributed to the heteroatom effect of the substitute in the xanthene ring of EB, which improves the quantum yield of the triplet lifetime compared with other dye analogs [57]. Reducing or increasing the EB amount are both detrimental to H2 production, which results from either insufficient sensitizing agent (diluted system) or pronounced light shielding effects. The H2 evolution rate reaches as high as 234 μmol g−1 h−1, which is about 4.1 times higher than that of a non-sensitized system.
In addition, the reusability of sheetP-O-CNSSA for photocatalytic H2 evolution under visible light irradiation was tested, the results are shown in Figure 11. In each cycle (about 5 h), the EB dye and sacrificial regent (i.e., TEOA) were renewed. The data suggested that the photocatalytic H2 evolution ability of the sheetP-O-CNSSA decreased slightly after five cycles. The main reasons for this phenomenon could be ascribed to a small loss of sheetP-O-CNSSA in the centrifugation separation process when replacing EB and TEOA. The results indicated that sheetP-O-CNSSA was a stable and active catalyst in the photocatalytic H2 production reaction in the course of 20 h under visible light irradiation.

2.5. Mechanism of Photocatalytic Hydrogen Production

Based on the above result, a possible mechanism of EB-sensitized sheetP-O-CNSSA for the photocatalytic hydrogen evolution reaction was proposed (Scheme 2). According to the previous publication, the LUMO and HOMO potentials of EB are −0.9 and 1.4 eV (vs. NHE), respectively [42], while the conduction band and valence band edge of sheetP-O-CNSSA were calculated to be -0.70 and 2.15 eV (vs. NHE), respectively. When the sheetP-O-CNSSA is exposed to the visible light, the photogenerated electrons and holes in the catalyst are separated and then electrons will migrateto the CB of the sheetP-O-CNSSA, initiating hydrogen reduction reactions (Equations (1) and (2)). The EB molecule is excited and subsequently *EB is formed in solution (Equation (3)). The LUMO of EB is more negative than the CB of the sheetP-O-CNSSA, which is thermodynamically favorable for electron transfer from the photoexcited EB dye to sheetP-O-CNSSA. Therefore, electrons are released from adsorbed *EB and then rapidly injected into the conduction band of sheetP-O-CNSSA (Equation (4)) resulting in intensive reduction in the reaction of H+ to H2 (Equation (2)). Simultaneously, *EB was oxidized to form EB+ in the HOMO of EB (Equation (4)), and then the EB+ could be reduced back to EB by the electron donor TEOA ((Equation (5)), indicating that EB dye can be recycled in photocatalytic H2 evolution. The photogenerated holes insheetP-O-CNSSA are also eliminated by TEOA ((Equation (6)). Overall,the related reactions were as follows:
sheetP-O-CNSSA + hν → sheetP-O-CNSSA + h+(sheetP-O-CNSSA) + e−(sheetP-O-CNSSA)
e−(sheetP-O-CNSSA) + H+ → H2
EB + hν → *EB
*EB + sheetP-O-CNSSA → e−(sheetP-O-CNSSA) + EB+ + sheetP-O-CNSSA
EB+ + TEOA → EB + TEOA+
h+(sheetP-O-CNSSA)+ TEOA →TEOA+
On a whole, the photocatalytic activities of catalysts for hydrogen production were mainly influenced by the effective separation of electron–hole pairs. On the one hand, O doping could prevent the combination of photoinduced electrons and holes and thus improve the migration and separation efficiency of photoinduced charge carries [49,52]. On the other hand, P doping could significantly improve the charge mobility and shorten the electron diffusion path from the interior to the surface [27]. In addition, the porous structure with larger BET surface areas after the copolymerization of the supramolecular complex could offer more active sites for mass transfer [58].

3. Materials and Methods

3.1. Materials

Melamine and EB were purchased from ShangHai Mackin biochemical reagents Co., Ltd. Cyanuric acid (CA), trithiocyanuric acid, and (NH4)2HPO4 were bought from Aladdin chemical reagents Co., Ltd. (Shanghai, China). All chemical reagents were chemical grade purity and used without further purification.

3.2. Catalyst Preparation

3.2.1. Synthesize of Supramolecular Complex

Firstly, 1 mmol melamine, 1 mmol CA, and 1 mmol TCA were added into an 100 mL alumina crucible. Then, 30 mL of distilled water was added and the solution was stirred for 4 h at room temperature. Afterwards, 0.2 g of (NH4)2HPO4 was added into the suspension and the solution was stirred for another 10 min. After stirring, the crucible was put into an oven set as 100 °C. After the water in crucible was completely drained out, the supramolecular complex was successfully synthesized.

3.2.2. Synthesis of g-C3N4 Catalysts

The above as-prepared supramolecular complex was transferred to a muffle furnace. Then, the temperature of the furnace was increased to 530 °C at a rate of 5 °C/min and kept for 2 h. The materials fabricated through this strategy were denoted as sheetP-O-CNSSA. Furthermore, for the purpose of comparison, the other catalyst (denoted as rodP-O-CNSSA) was also prepared by changing the composition of the supramolecular complex precursor. Specifically, rodP-O-CNSSA means there is no CA in the synthesis process of the supramolecular complex precursors. The synthesis process of P-O-CN was identical to the above except that melamine served as the only precursor. In addition, the bulk g-C3N4 was also prepared by calcining the melamine under the identical condition and was denoted as CN.

3.3. Catalyst Characterization

Power X-ray diffraction (XRD) measurements were conducted on a Bruker D8 advance X-ray diffractometer (Bruker AXS, Karlsruhe, Germany) with a Cu Kα radiation source. Fourier-transformed infrared (FT-IR) analyses were carried out on a FT-IR spectroscopy (FT-IR, Nicolet 6700, Waltham, MA, USA) using the KBr pellet technique. The surface composition information of the as-prepared material was tested by X-ray electron spectroscopy (XPS, ESCALAB250, THERMO VG, Waltham, MA, USA) with monochromatized Al Kα as the X-ray source. The morphologies of the samples were investigated by field-emission scanning electron microscope (FE-SEM, ZEISS Ultra55, Zeiss, Oberkochen, Germany) and transmission electron microscope (TEM, JEM-2100HR, JEOL, Tokyo, Japan). The UV-Vis diffuse reflection spectra (DRS) were acquired by using a UV-Vis spectrophotometer (UV-3010, Hiachi, Tokyo, Japan) and all the obtained data were converted to absorbance through the Kubelka–Munk function. The photoluminescence (PL) emission spectra were recorded on a fluorescence spectrophotometer meter (PL, F-2700, Hiachi, Japan) at room temperature.

3.4. Photocatalytic Experiments

The photocatalytic experiments were carried out in a 350 mL reactor at ambient temperature. In the photocatalytic tests, a 300 W xenon lamp (PLS-SXE 300, Beijing, Trustech) equipped with UV filters (λ < 420 nm) was applied as a visible light source. In each test, 5 mg of the as-prepared catalyst and 5 mg of EB were added into a bottle containing 60 mL of deionized water and 10 mL of triethanolamine under constant stirring. Before the visible light irradiation, nitrogen gas (N2) was added into the reaction device to remove air and then stirred for about another 30 min. The amount of photo-generated hydrogen in the reaction was analyzed via gas chromatography (GC-9560, Huaai Chromatography Technology Co., Ltd. Shanghai, China) with a thermal conductivity detector (TCD).

4. Conclusions

In summary, a novel method was proposed for the preparation of phosphorus and oxygen co-doped graphitic carbon nitride (sheetP-O-CNSSA) using self-assembly supramolecular complex as the precursor. The results of a series of measurements confirmed that phosphorus and oxygen were successfully introduced into the matrix of g-C3N4. The results of the photocatalytic hydrogen reduction reaction indicated that an Erythrosin-B-sensitized sheetP-O-CNSSA system exhibited superior activity compared with the bulk g-C3N4 (i.e., CN). It was confirmed that the supramolecular self-assembly and co-doping of O and P atoms modified the surface chemical structure and significantly suppressed the recombination of photogenerated electrons and holes. This work provides a new synthesis approach for developing an efficient g-C3N4-based photocatalyst for photocatalytic hydrogen evolution under dye sensitization.

Author Contributions

X.Z. wrote the draft and improved the manuscript; the manuscript was reviewed and edited by F.Y., J.L., G.Z., D.C., Z.L., and J.F. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to acknowledge Guangdong Basic and Applied Basic Research Foundation (No. 2022A1515011014), China.

Data Availability Statement

The data that support the findings in the work are available from the corresponding author upon reasonable request.

Conflicts of Interest

The authors have no known conflicts of interest to declare.

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Figure 1. XRD patterns of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
Figure 1. XRD patterns of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
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Figure 2. FT-IR spectra of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
Figure 2. FT-IR spectra of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
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Figure 3. N2 adsorption–desorption isotherms of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA. The inset shows the corresponding pore distribution curves.
Figure 3. N2 adsorption–desorption isotherms of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA. The inset shows the corresponding pore distribution curves.
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Figure 4. SEM images of (A) CN, (B) P-O-CN, (C) rodP-O-CNSSA, and (D) sheetP-O-CNSSA. TEM images (E) CN and (F) sheetP-O-CNSSA.
Figure 4. SEM images of (A) CN, (B) P-O-CN, (C) rodP-O-CNSSA, and (D) sheetP-O-CNSSA. TEM images (E) CN and (F) sheetP-O-CNSSA.
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Figure 5. Survey and high-resolution XPS spectra of CN and sheetP-O-CNSSA (A) survey, (B) C 1s, (C) N 1s, (D) O 1s, (E) P 2p, and (F) S 2p.
Figure 5. Survey and high-resolution XPS spectra of CN and sheetP-O-CNSSA (A) survey, (B) C 1s, (C) N 1s, (D) O 1s, (E) P 2p, and (F) S 2p.
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Scheme 1. Schematic diagram of the formation of the porous sheetP-O-CNSSA catalyst.
Scheme 1. Schematic diagram of the formation of the porous sheetP-O-CNSSA catalyst.
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Figure 6. (A) UV-vis diffuse reflection spectra of as-obtained samples and (B) comparison in optical absorption of CN and sheetP-O-CNSSA.
Figure 6. (A) UV-vis diffuse reflection spectra of as-obtained samples and (B) comparison in optical absorption of CN and sheetP-O-CNSSA.
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Figure 7. XPS valence band spectra of CN and sheetP-O-CNSSA.
Figure 7. XPS valence band spectra of CN and sheetP-O-CNSSA.
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Figure 8. Photoluminescence spectra of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
Figure 8. Photoluminescence spectra of CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA.
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Figure 9. Photocatalytic H2 evolution over the four catalysts (CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA) with 5 mg EB dye under visible light irradiation.
Figure 9. Photocatalytic H2 evolution over the four catalysts (CN, P-O-CN, rodP-O-CNSSA, and sheetP-O-CNSSA) with 5 mg EB dye under visible light irradiation.
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Figure 10. sheetP-O-CNSSA photocatalytic H2 evolution over the systems with various amount ofEB dye.
Figure 10. sheetP-O-CNSSA photocatalytic H2 evolution over the systems with various amount ofEB dye.
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Figure 11. Stability test over sheetP-O-CNSSA for photocatalytic H2 evolution reaction.
Figure 11. Stability test over sheetP-O-CNSSA for photocatalytic H2 evolution reaction.
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Scheme 2. Schematic of photocatalytic hydrogen production over the EB-sensitized sheetP-O-CNSSA system.
Scheme 2. Schematic of photocatalytic hydrogen production over the EB-sensitized sheetP-O-CNSSA system.
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Zhu, X.; Yang, F.; Liu, J.; Zhou, G.; Chen, D.; Liu, Z.; Fang, J. Design and Architecture of P-O Co-Doped Porous g-C3N4 by Supramolecular Self-Assembly for Enhanced Hydrogen Evolution. Catalysts 2022, 12, 1583. https://doi.org/10.3390/catal12121583

AMA Style

Zhu X, Yang F, Liu J, Zhou G, Chen D, Liu Z, Fang J. Design and Architecture of P-O Co-Doped Porous g-C3N4 by Supramolecular Self-Assembly for Enhanced Hydrogen Evolution. Catalysts. 2022; 12(12):1583. https://doi.org/10.3390/catal12121583

Chicago/Turabian Style

Zhu, Ximiao, Fan Yang, Jinhua Liu, Guangying Zhou, Dongdong Chen, Zhang Liu, and Jianzhang Fang. 2022. "Design and Architecture of P-O Co-Doped Porous g-C3N4 by Supramolecular Self-Assembly for Enhanced Hydrogen Evolution" Catalysts 12, no. 12: 1583. https://doi.org/10.3390/catal12121583

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