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Article

Embedding Thiophene-Amide into g-C3N4 Skeleton with Induction and Delocalization Effects for High Photocatalytic H2 Evolution

by
Shuang Tang
1,
Yang-Sen Xu
2 and
Wei-De Zhang
1,*
1
School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou 510640, China
2
Institute of Information Technology, Shenzhen Institute of Information Technology, Shenzhen 518172, China
*
Author to whom correspondence should be addressed.
Catalysts 2022, 12(9), 1043; https://doi.org/10.3390/catal12091043
Submission received: 13 August 2022 / Revised: 7 September 2022 / Accepted: 7 September 2022 / Published: 14 September 2022
(This article belongs to the Special Issue Advanced Catalysts for Achieving Hydrogen Economy from Liquids)
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Abstract

:
Molecular skeleton modification has become a recognized method that can effectively improve the photocatalytic performance of g-C3N4 because it not only effectively promotes charge separation, but also tunes the conjugated system of g-C3N4 to make it more conducive to photocatalytic reaction. Herein, thiophene-amide embedded g-C3N4 (TA-CN-x) was successfully prepared by simple one-step thermal polycondensation using urea as a precursor and ethyl-2-amino-4-phenylthiophene-3-carboxylate (EAPC) as an additive. After embedding with thiophene-amide, the induction and delocalization effects are formed in TA-CN-x, which significantly improves the migration efficiency of photogenerated charge carriers. Meanwhile, the conjugate structure is changed due to structural modification, resulting in significant enhancement of visible light absorption compared to the pure g-C3N4 (CN). Specifically, the optimized photocatalytic H2 evolution rate of TA-CN-2 reaches 245.4 μmol·h−1, which is 8.4 times that of CN (with Pt nanoparticles as a co-catalyst), and the apparent quantum efficiency (AQY) at 450 nm is 13.6%. This work opens up a new modification process for fully tapping the photocatalytic hydrogen absorption potential of g-C3N4-based materials.

Graphical Abstract

1. Introduction

Due to the depletion of traditional fossil energy and the increasingly serious global environmental problems, the development of renewable energy has become realistic and urgent. In this situation, photocatalytic technology has shown great advantages because of its wide range of applications, such as degradation [1,2,3] and CO2 reduction [4]. Apart from these, hydrogen energy, which possesses the advantages of being renewable, environmentally benign and high calorific value, is undoubtedly one of the ideal energy sources for the future [5]. Hydrogen production by photocatalytic water decomposition with solar energy is one of the effective ways to alleviate the global energy crisis and solve global environmental problems as well [6,7,8,9].
Since the first report using g-C3N4 as a catalyst for photocatalytic decomposition of water [10], this non-metallic semiconductor catalyst with high stability, visible light response and appropriate band gap has attracted extensive attention [11,12,13,14]. However, for g-C3N4, the utilization efficiency of visible light is insufficient, and the excited photogenerated electrons are easily captured by some highly electronegative nitrogen atoms in the g-C3N4 skeleton, resulting in recombination of photogenerated charges [15,16,17,18]. Therefore, reducing the band gap and promoting the separation of photogenerated charge carriers are the keys to improving the rate of photocatalytic decomposition of aquatic hydrogen by g-C3N4 [19,20,21].
Up to now, various strategies without using metal elements have been used to modify pure g-C3N4, in order to boost the photocatalytic performance of g-C3N4, such as doping with nonmetallic elements or molecules [22,23,24,25], regulation of micro morphologies [26,27,28,29], construction of heterojunctions with other semiconductors [30,31,32,33,34], and so on. Among all the above methods, molecular skeleton modification strategy based on conjugate structure regulation realized by small organic molecule embedding is undoubtedly an efficient modification approach, because it can not only controllably tune the band structure of g-C3N4, but also promotes separation and migration of photogenerated charge carriers, making it more conducive to photocatalytic hydrogen evolution [35,36]. It is reported that the conjugated aromatic rings embedded in the g-C3N4 molecular skeleton can induce electronic structure rearrangement and charge redistribution because of their ability of electron attracting and capturing [37,38]. However, in most previous studies, aromatic molecules are directly embedded in the heptazine ring of g-C3N4. Although it could promote the separation of photogenerated electron-hole pairs to some extent, in this case, the in-layer migration of photogenerated charges is random and nondirectional, which will also lead to recombination of photogenerated electrons and holes, thus reducing charge separation efficiency [39]. In contrast, embedding aromatic molecules between heptazine rings holds more advantages, because it can not only regulate the band structure but also promote the separation of photogenerated electrons and holes more effectively due to the induction and delocalization effects, so as to significantly improve the photocatalytic activity [40,41].
In this study, thiophene-amide is successfully embedded between heptazine rings in the g-C3N4 framework by simple and low-cost thermal condensation polymerization. The visible light absorption capacity and migration efficiency of the photogenerated charge carriers of modified catalysts (TA-CN-x) are significantly enhanced. As a result, the photocatalytic decomposition rate of aquatic hydrogen reaches 245.5 μmol·h−1 over the optimized catalyst TA-CN-2, which is 8.4 times that of pristine CN, and the hydrogen evolution rate of TA-CN-2 under monochromatic light at 450 nm reaches 72.3 μmol·h−1.

2. Results and Discussion

2.1. Morphology and Structure Analyses

In order to explore the formation process of TA-CN-x, firstly, TG was adopted to analyze urea, EAPC and their mixtures. As shown in Figure 1, both CN and TA-CN present similar weight loss curves. The two weight loss processes upon the increase in temperature correspond to the formation of triazine ring by urea polycondensation and further polycondensation of triazine ring to heptazine ring [42,43]. However, unlike pure urea, the mixture of urea and EAPC experiences an additional weight loss between 231 °C and 255 °C, corresponding to the reaction process of the amino and ester groups of EAPC with triazine. This reveals that EAPC is involved in the step-by-step polycondensation of urea, that is, the thiophene-amide structure is embedded successfully.
The crystal structure of the catalyst is characterized by XRD, as shown in Figure 2a. Both CN and TA-CN-x exhibit two obvious peaks at 12.9° and 27.7°, corresponding to the (100) and (002) crystal planes of g-C3N4, respectively [44,45]. However, compared to CN, as shown in Figure 2b, the (002) diffraction peak of TA-CN-x slightly shifts to a high angle, owing to the reduction in the interlayer stacking distance caused by the modification of EAPC [46], which is beneficial for the separation of photogenerated charge carriers between the layers of g-C3N4 [41].
Figure 3 shows the FT-IR spectra of CN, TA-CN-2 and EAPC. As can be seen in Figure 3a, TA-CN-2 exhibits similar characteristic absorption peaks to CN, locating at 3000–3400, 1100–1700 and 810 cm−1, indexing to the uncondensed terminal amine or hydroxyl groups [43], typical heterocyclic stretches of heptazine rings [47,48] and bending vibration absorption of heptazine units, respectively [49]. However, unlike CN, new peaks of TA-CN-2 appear at 1436, 1318 and 1281 cm−1, corresponding to the characteristic absorption peak of EAPC (Figure 3b). This demonstrates that the thiophene-amide molecule is embedded into the g-C3N4 skeleton.
The surface chemical compositions and valence states of CN and TA-CN-2 are characterized by XPS, and the results are shown in Figure S2 and Figure 4. As indicated in Figure S2, CN is mainly composed of C, N and O, while TA-CN-2 mainly contains C, N, O and a small amount of S. The elemental compositions of CN and TA-CN-2 obtained by XPS analysis are listed in Table 1. The atomic ratio of C/N in TA-CN-2 rises to 0.98 from 0.96 in CN, which is ascribed to the modification of EAPC.
High-resolution XPS spectra provide more details about the chemical valences of the elements and compositions of the samples. Figure 4a shows the high-resolution XPS spectra of C 1s of CN and TA-CN-2. One can see two strong peaks with a binding energy of 287.7 and 284.3 eV, corresponding to the sp2-hybridized N=C-N and C=C, respectively [35,50]. Meanwhile, a weak peak at 286.1 eV could be assigned to the C-N-C group of g-C3N4 [51]. The percentage of C species is summarized in Figure S3a. Since the existence of the thiophene-amide species and the benzene substituent on thiophene, the content of C=C in TA-CN-2 rises up to 39.7% from 38.8% in CN. Figure 4b presents high-resolution XPS spectra of N 1s of the samples, in which four types of N are identified at 403.9, 400.6, 399.5 and 398.2 eV, corresponding to π excitations, amino N (N-Hx), tertiary nitrogen (N-(C)3) and sp2-hybridized nitrogen (C=N-C), respectively [52,53,54]. As indicated in Figure S3b, the content of N-Hx in TA-CN-2 rises to 12.33% from 10.05% in CN, while the content of N-(C)3 decreases to 13.47% compared to that of CN (15.64%), proving that the thiophene-amide molecule is successfully embedded into the skeleton of g-C3N4. In C 1s and N 1s XPS spectra of CN and TA-CN-2, almost all of the peaks are not changed, revealing that the embedding of thiophene-amide does not destroy the basic skeleton of g-C3N4. However, the peak of amino N (N-Hx) in TA-CN-2 shifts to lower binding energy slightly by 0.1 eV compared to that in CN. This can be attributed to the increase in electron cloud density of amino N atoms caused by the strong limbic induction and delocalization effect of the embedded thiophene-amide, proving that the thiophene-amide structure can promote the migration of photogenerated charge carriers effectively for efficient photocatalytic hydrogen evolution reaction [55]. The XPS spectra of O 1s is shown in Figure 4c, in which surface −OH or adsorbed H2O (C-O/H2O), and carbonyl O (C=O) in TA-CN-2 are identified at 532.8 and 531.4 eV, respectively [56,57]. Similar to that of amino N (N-Hx), the peak of carbonyl O (C=O) in TA-CN-2 shifts to lower binding energy too, proving that not only the thiophene-amide is embedded but also the transport of photogenerated electrons is promoted [58]. Figure 4d presents the XPS spectrum of S 2p of TA-CN-2. The peak located at 163.3 eV can be identified as a sulfur-carbon single bond (C-S), demonstrating the successful embedding of thiophene-amide [59].
13C ssNMR spectra of CN and TA-CN-2 are shown in Figure 5, It can be seen that both CN and TA-CN-2 present two peaks at 165.3 and 156.9 ppm, which belong to C1 and C2, respectively [60,61,62]. However, unlike CN, a new peak of TA-CN-2 is detected at 163.1 ppm, which can be attributed to C3, proving the embedded thiophene-amide [63].
The morphologies of CN and TA-CN-2 are investigated by SEM and TEM. As shown in Figure 6a–f, both CN and TA-CN-2 present a typical ultra-thin layered structure. Figure 6g,h present the elemental distribution of TA-CN-2. It can be seen that TA-CN-2 contains C, N, O and S, in which a small amount of S comes from the embedded thiophene-amide. Moreover, the uniform distribution of the four elements proves that thiophene-amide is embedded in the g-C3N4 skeleton evenly. Figure S1 shows that the specific surface areas of CN and TA-CN-2 are similar.

2.2. Optical and Photoelectrochemical Properties Analyses

Figure 7a displays the optical absorption property of the samples. TA-CN-x shows a redshift of absorption edges compared to CN, and the visible light absorption capacity of TA-CN-x increases significantly, which is in favor of photocatalytic hydrogen evolution. The band gaps of CN and TA-CN-2 are calculated based on the Tauc method and shown in Figure 7b. CN presents a broad band gap of 2.96 eV while TA-CN-2 shows a much narrower band gap of 2.88 eV, suggesting that the embedded thiophene-amide is in favor of electron transition from valance band to conduction band to improve photocatalytic performance. The result of PL is shown in Figure 7c, in which CN presents a strong emission peak under 350 nm excitation light while the emission peak intensity of PCN is greatly reduced, and the peak position shows obvious redshift, proving that the recombination of photogenerated charges of PCN is inhibited [52,64,65]. To further study the charge transfer and recombination processes, TR-PL decay behavior is inspected, as shown in Figure 7d. The average fluorescence lifetime (τAve) of TA-CN-2 decreases to 4.46 ns, compared to 6.16 ns of CN, indicating that the recombination of photogenerated charge carriers is suppressed because of the induction and delocalization effects caused by the embedded thiophene-amide, which is beneficial for the photocatalytic reaction [66,67].
The transfer and separation processes of the photogenerated charge carriers are further validated by photoelectrochemical experiments. LSV curves of CN and TA-CN-2 without or under visible light irradiation are shown in Figure 8a,b, respectively, in both of which TA-CN-2 presents higher cathode current density and lower initial potential, proving that the existence of the Thiophene-Amide structure is more conducive to the hydrogen evolution reaction [39]. Figure 8c presents the transient photocurrent response curves of the catalysts. It is noted that TA-CN-2 exhibits a much higher photocurrent density, which is over 6 times that of CN. Furthermore, the EIS Nyquist plots shown in Figure 8d present a greatly reduced arc radius of TA-CN-2 than CN, suggesting a much higher photogenerated carrier migration efficiency of TA-CN-2 compared to CN due to the induction and delocalization effects [68]. All of the above analysis results indicate that the greatly improved photocatalytic hydrogen production performance of TA-CN-2 can be attributed to the high intrinsic reaction kinetics, effective separation and fast transfer of the photogenerated charge carriers.

2.3. Band Structure Analyses and DFT Calculation

In order to ascertain the band positions of the catalysts, XPS valence band spectra are analyzed and the result is shown in Figure 9a, which presents the difference between the valence band and Fermi level of CN (2.54 eV) and TA-CN-2 (2.27 eV). Furthermore, the flat band potential of a semiconductor in an electrolyte aqueous solution can be approximated to the surface Fermi level [69]. Therefore, the Fermi levels of the samples can be estimated by Mott-Schottky curves to be −0.73 V and −0.51 V vs. NHE (pH = 6.5) for CN and TA-CN-2, respectively (Figure 9b,c). Thus, the valence band (VB) and conduction band (CB) positions of CN and TA-CN-2 are obtained, as shown in Figure 9d. Compared with CN, the slight positive shift of the VB position of TA-CN does not reduce its reduction ability seriously. However, the band gap is reduced, which facilitates the photocatalytic hydrogen evolution reaction [70].
In addition, the energy band structure of the catalysts is further analyzed by DFT calculation. It can be seen from Figure 10 that the Homo level of CN is mainly due to the combination of nitrogen Pz orbitals, while the LUMO level is mainly from the C-N bond orbitals [43,71]. However, after embedding thiophene-amide, the HOMO and LUMO of TA-CN changed significantly, both of which are mainly derived from the embedded thiophene-amide, and the redistributed electron cloud of TA-CN-2 will accelerate the separation of the photogenerated charge carriers [72]. What is more, the HOMO of TA-CN shifts positively from −8.49 eV of CN to −8.02 eV, and the LUMO shifts negatively from −3.52 eV of CN to −5.26 eV, whose moving trend is consistent with the experimental results. It suggests that the induction and delocalization effects of the Thiophene-Amide structure are produced in the TA-CN skeleton, which can induce charge migration to promote the separation of photogenerated carriers to promote photocatalytic reaction performance.

2.4. Photocatalytic Hydrogen Production Performance Analyses

In order to study the role of EAPC on the improved photocatalytic hydrogen production performance of g-C3N4, the hydrogen production rates over the catalysts were examined. Figure 11a shows that the hydrogen evolution rate over TA-CN-x is much higher than that over CN, and the rate over TA-CN-2 is the highest, reaching up to 245.4 μmol·h−1, which is 8.4 times that over CN under the same conditions. As summarized in Table S1, the hydrogen production performance of the catalysts in this study is high. Figure 11b depicts the hydrogen production performance of TA-CN-2 excited by lights with different wavelengths. It is obvious that the hydrogen evolution rates match well with the UV-vis absorption curve, and the hydrogen evolution rate of TA-CN-2 excited by the light at a wavelength of 450 nm reaches 72.3 μmol·h−1 with AQY of 13.6%, proving that the light absorption ability is a key factor affecting the photocatalytic activity. The stability of the catalyst is also examined. Figure 11c shows that in the first 24 h, TA-CN maintains a stable hydrogen evolution rate, and the significant decrease in the hydrogen production rate in the 25th h is due to the depletion of TEOA. After supplementing TEOA, it returns to the initial hydrogen evolution, suggesting its outstanding stability. Moreover, the XRD pattern shown in Figure 11d as well as the SEM and HADDF figures shown in Figure S4 agree well with the stability test result.
Based on the experimental and theoretical calculation results, the possible photocatalytic H2 evolution reaction mechanism over TA-CN is proposed and shown in Figure 12. Under visible light irradiation, photogenerated charges are produced and separated in the TA-CN skeleton. Then, the photogenerated electrons migrate to the thiophene-amide structure due to the induction and delocalization effects. Then, the photogenerated electrons transfer to the Pt particles for the reduction in proton to H2, while the photogenerated holes are consumed by TEOA.

3. Materials and Methods

3.1. Materials

Urea (CH4N2O) was bought from Guangdong Guanghua Sci-Tech Co. Ltd. (Guangzhou, China). Ethyl-2-amino-4-phenylthiophene-3-carboxylate (C11H13NO2S, EAPC) and chloroplatinic acid hexahydrate (H2PtCl6·6H2O) were attained from Aladdin Industrial Corporation (Shanghai, China). All reagents were used without any further purification.

3.2. Synthesis of Photocatalysts

20 g urea was mixed with a certain amount of EAPC and thoroughly ground, and then the mixture was transferred to a 100 mL ceramic crucible with a cover. Afterwards, the mixture was calcined in a muffle furnace at 550 °C for 4 h at a heating rate of 2 °C min−1. The obtained thiophene-amide embedded g-C3N4 samples were named TA-CN-x (x = 1, 2, 3, 4 corresponding to EAPC of 5, 10, 15, 20 mg, respectively). Pure g-C3N4 (CN) was also synthesized under the same conditions without adding EAPC. The possible catalyst polymerization pathway is illustrated in Scheme 1.

3.3. Characterization

Thermogravimetric analysis (TG) was conducted on a TG-209-F1 analyzer (Netzsch, Selb, Bavaria, Germany) in an air atmosphere at a heating rate of 10 °C min−1. The phases and molecular structures of the samples were analyzed by Bruker-D8 X-ray diffractometer (XRD) (Bruker, Karlsruhe, Baden-Württemberg, Germany) using CuKα as a radiation source, and the relevant parameters were calculated according to the Bragg equation. Fourier transform infrared (FT-IR) spectroscopy was recorded on Nicolet iS10 (Thermo Fisher Scientific, Waltham, MA, USA), and the sample was prepared by tablet pressing method. X-ray photoelectron spectroscopy (XPS) was taken on an X-ray photoelectron spectrometer (K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA). 13C solid-state nuclear magnetic resonance spectra (13C ssNMR) of the samples were performed on Bruker Avance III HD 400WB (9.4 T) spectrometer (Bruker, Karlsruhe, Baden-Württemberg, Germany). Scanning electron microscopy (SEM) images were taken on Zeiss Merlin (Carl Zeiss AG, Jena, Baden-Württemberg, Germany) with an acceleration voltage of 5 kV. The microstructure and elemental distribution of the samples were tested by a transmission electron microscope (TEM), at a working voltage of 200 kV (JEM-2100F, JEOL, Tokyo, Japan). UV-vis absorption was assayed by a UV-vis diffuse reflectance spectrometer (U-3010, Hitachi, Tokyo, Japan), with BaSO4 as a reference. Photoluminescence (PL) spectra were recorded on a fluorescence spectrometer (F-4500, Hitachi, Tokyo, Japan). Time-resolved photoluminescence (TR-PL) spectra were obtained on a steady-state/transient fluorescence spectrometer (FLS-980, Edinburgh Instruments, Edinburgh, UK). Specific surface areas were measured on Micromeritics ASAP 2460 specific surface area analyzer (Micromeritics, Georgia, USA), using the Brunauer–Emmett–Teller calculation method (BET).

3.4. Photoelectrochemical Test

The electrochemical properties of the prepared samples were analyzed on an electrochemical workstation (Chenhua, Shanghai, China). The experiments were carried out on a three-electrode system, using a platinum plate as a counter electrode, saturated Ag/AgCl electrode as a reference electrode and FTO coated with a quantitative sample as a working electrode. The working electrode was prepared as follows: a certain amount of a sample and ethyl cellulose (mass ratio: 10:1) was weighted and dispersed in 2 mL ethanol. After being sonicated for 30 min and fully grounded, the mixture was evenly applied on the conductive surface of an FTO plate with an exposed area of 1 × 1 cm2. The working electrode was obtained after being dried at 60 °C for 2 h. Linear sweep voltammetry (LSV), current versus time (I-t) and Mott–Schottky tests were carried out in a phosphate buffer solution (pH = 6.5), while electrochemical impedance spectroscopy (EIS) was tested in 0.01 M K3[Fe(CN)6]/K4[Fe(CN)6] (mole ratio = 1:1) solution.

3.5. Photocatalytic Hydrogen Evolution Test

The photocatalytic performance of the sample was assayed on the LabSolar-IIIAG photocatalytic hydrogen production system (Beijing Perfect Light Technology Co. Ltd., Beijing, China). A xenon lamp of 300 W was used as the light source (λ < 420 nm ultraviolet was deducted by a filter), and the light intensity was tuned to 100 mW·cm−2 by an intensity meter. The as-prepared photocatalyst (50 mg) was uniformly suspended in 100 mL triethanolamine aqueous solution (10 vol%) with 3 mL H2PtCl6·H2O solution (0.5 mg·mL−1 Pt) as a cocatalyst. The hydrogen production test was carried out under vacuum, and the produced gas in the reaction was analyzed by a GC-7806 gas chromatograph (shiweipx, Beijing, China), using a 5A molecular sieve as a chromatographic column and N2 as a carrier gas. The hydrogen production rate was quantitatively analyzed by the external standard method. In addition, the quantum yield (AQE) of the catalyst was calculated according to equations 1 and 2 by measuring the hydrogen production activity of the catalyst at different wavelengths (λ = 380, 400, 450, 500, 550, and 600 nm).
N = E λ h c = E 0 A λ h c
A Q E % = 2   ×   n u m b e r   o f   H 2   m o l e c u l e s N
In Formula (1), N represents the number of incident photons, E0 represents the intensity of incident light power, A represents the illumination area, λ represents the wavelength of the incident light, h is the Planck constant, and c represents the speed of light.

3.6. Density Functional Theory (DFT) Calculation

Gaussian 09W (Gaussian 09W, Gaussian Inc., Wallingford, CT, USA, 1995–2013) B3LYP function, 6-31G d was used for DFT calculation. The energy levels of the sample were analyzed by simulating the highest occupied orbit (HOMO) and the lowest unoccupied orbit (LUMO).

4. Conclusions

In summary, a brand-new thiophene-amide embedded g-C3N4 photocatalyst is synthesized successfully based on a skeleton modification strategy using small organic molecules as modifiers. The optimized photocatalytic H2 evolution rate of TA-CN-2 reaches 245.4 μmol·h−1, which is 8.4 times that of CN (29.1 μmol·h−1), and the apparent quantum efficiency (AQY) at 450 nm is as high as 13.6%. The significant increase in photocatalytic hydrogen evolution rate mainly comes from the enhanced visible light absorption capacity and improved photogenerated-charge separation efficiency, caused by the induction and delocalization effects due to the embedded thiophene-amide. The work opens up the idea of using small organic molecules as modifiers to embed a binary aromatic-nonaromatic-groups conjugated system in the g-C3N4 framework by one-step thermal condensation polymerization to prepare stable and efficient g-C3N4-based photocatalysts.

Supplementary Materials

The following are available online at https://www.mdpi.com/article/10.3390/catal12091043/s1, Figure S1: N2 adsorption-desorption isotherms (a) and pore diameter distributions (b) of CN and TACN-2; Figure S2: XPS survey spectra of CN and TA-CN-2, Figure S3: Percentages of (a) carbon species and (b) nitrogen species of CN and TA-CN-2, Figure S4: SEM images (a,b) and HAADF image (c) of the fresh TA-CN-2 (before the stability test); SEM images (c,d) and HAADF image (e) of the used TA-CN-2 (after the stability test), Table S1: Comparison of photocatalytic performance over conjugated structure embedded g-C3N4 catalysts reported.

Author Contributions

Data curation, S.T.; funding acquisition, W.-D.Z.; supervision, W.-D.Z.; visualization, Y.-S.X.; writing–original draft, S.T.; writing–review & editing, W.-D.Z. 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 (NO. 21773074).

Data Availability Statement

Data are available on request.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 12 01043 g001 550
Figure 1. TG curves of CN, TA-CN-2 and EAPC.
Figure 1. TG curves of CN, TA-CN-2 and EAPC.
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Catalysts 12 01043 g002 550
Figure 2. (a) XRD patterns of CN and TA-CN-x, (b) enlarge view between 25° and 30°.
Figure 2. (a) XRD patterns of CN and TA-CN-x, (b) enlarge view between 25° and 30°.
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Figure 3. (a) FTIR spectra of CN, TA-CN-2 and EAPC, (b) enlarge view between 1800 and 1000 cm−1.
Figure 3. (a) FTIR spectra of CN, TA-CN-2 and EAPC, (b) enlarge view between 1800 and 1000 cm−1.
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Figure 4. High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s of CN and TA-CN-2. (d) High-resolution XPS spectrum of S 2p of TA-CN-2.
Figure 4. High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) O 1s of CN and TA-CN-2. (d) High-resolution XPS spectrum of S 2p of TA-CN-2.
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Figure 5. 13C ssNMR spectra of CN and TA-CN-2.
Figure 5. 13C ssNMR spectra of CN and TA-CN-2.
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Catalysts 12 01043 g006 550
Figure 6. (a,b) SEM images and (c) TEM image of CN; (d,e) SEM images and (f) TEM images of TA-CN-2; (g) HAADF image and elemental mapping images of (h) C, (i) N, (j) O and (k) S of TA-CN-2.
Figure 6. (a,b) SEM images and (c) TEM image of CN; (d,e) SEM images and (f) TEM images of TA-CN-2; (g) HAADF image and elemental mapping images of (h) C, (i) N, (j) O and (k) S of TA-CN-2.
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Figure 7. (a) UV-Vis diffuse reflection spectra of CN and TA-CN-x; (b) Tauc plots; (c) PL spectra and (d) TR-PL spectra of CN and TA-CN-2 (Inset: Double-exponential fitted lifetimes for CN and TA-CN-2).
Figure 7. (a) UV-Vis diffuse reflection spectra of CN and TA-CN-x; (b) Tauc plots; (c) PL spectra and (d) TR-PL spectra of CN and TA-CN-2 (Inset: Double-exponential fitted lifetimes for CN and TA-CN-2).
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Figure 8. (a) LSV curves without visible light irradiation; (b) LSV curves with visible light irradiation; (c) transient photocurrent response curves under visible light irradiation and (d) EIS Nyquist plots of CN and TA-CN-2.
Figure 8. (a) LSV curves without visible light irradiation; (b) LSV curves with visible light irradiation; (c) transient photocurrent response curves under visible light irradiation and (d) EIS Nyquist plots of CN and TA-CN-2.
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Figure 9. (a) XPS valance spectra, (b,c) Mott-Schottky plots and (d) band structure diagram of CN and TA-CN-2.
Figure 9. (a) XPS valance spectra, (b,c) Mott-Schottky plots and (d) band structure diagram of CN and TA-CN-2.
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Figure 10. Electronic structure of polymeric trimmer models and optimized HOMO-LUMO energy levels of CN and TA-CN-2.
Figure 10. Electronic structure of polymeric trimmer models and optimized HOMO-LUMO energy levels of CN and TA-CN-2.
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Figure 11. (a) Photocatalytic H2 evolution rates of CN and TA-CN-x (50 mg catalyst); (b) absorption spectrum and wavelength-dependent photocatalytic H2 evolution rates; (c) photocatalytic H2 evolution stability test in 30 h and (d) XRD patterns before and after stability test of TA-CN-2.
Figure 11. (a) Photocatalytic H2 evolution rates of CN and TA-CN-x (50 mg catalyst); (b) absorption spectrum and wavelength-dependent photocatalytic H2 evolution rates; (c) photocatalytic H2 evolution stability test in 30 h and (d) XRD patterns before and after stability test of TA-CN-2.
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Figure 12. Possible photocatalytic H2 evolution reaction mechanism of TA-CN.
Figure 12. Possible photocatalytic H2 evolution reaction mechanism of TA-CN.
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Scheme 1. Possible polymerization process of TA-CN-x.
Scheme 1. Possible polymerization process of TA-CN-x.
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Table
Table 1. The C, N, O, S elemental compositions and C/N molar ratios of the samples according to XPS.
Table 1. The C, N, O, S elemental compositions and C/N molar ratios of the samples according to XPS.
SampleAtomic Ratio (%)C/N
CNOS
CN46.7448.664.60—0.96
TA-CN-247.0747.934.570.430.98
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Tang, S.; Xu, Y.-S.; Zhang, W.-D. Embedding Thiophene-Amide into g-C3N4 Skeleton with Induction and Delocalization Effects for High Photocatalytic H2 Evolution. Catalysts 2022, 12, 1043. https://doi.org/10.3390/catal12091043

AMA Style

Tang S, Xu Y-S, Zhang W-D. Embedding Thiophene-Amide into g-C3N4 Skeleton with Induction and Delocalization Effects for High Photocatalytic H2 Evolution. Catalysts. 2022; 12(9):1043. https://doi.org/10.3390/catal12091043

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Tang, Shuang, Yang-Sen Xu, and Wei-De Zhang. 2022. "Embedding Thiophene-Amide into g-C3N4 Skeleton with Induction and Delocalization Effects for High Photocatalytic H2 Evolution" Catalysts 12, no. 9: 1043. https://doi.org/10.3390/catal12091043

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