Molecular Engineering of g-C₃N₄ with Dibenzothiophene Groups as Electron Donor for Enhanced Photocatalytic H₂-Production

INTRODUCTION

The over-consumption of traditional fossil energy has slowed down the sustainable development of human society. Photocatalytic hydrogen evolution (PHE) has shown great potential in the field of solar energy utilization and transformation and is believed to be a promising solution to the world energy crisis.^[1-5] High stability and efficiency are the two most important indicators of a photocatalyst. Over the past decade, graphitic carbon nitride (g-C₃N₄) has gradually attracted widespread attention^[6-8] because of its superior properties comprising of non-toxicity, "earth-abundant" nature, suitable band gap, stable physical and chemical properties, etc.^[9-12] However, the practical application of pristine g-C₃N₄ has encountered several obstacles, e.g., low conversion efficiency of solar energy and slow mobility of photoexcited electron-hole pairs. To solve these problems, a number of strategies have been developed, including building heterojunctions with WO₃, ^[13] CdS, ^[14] ZnIn₂S₄, ^[15] etc., and loading co-catalysts with sulfide, ^[16] selenide, ^[17] single metal atoms, ^{[18, 19]} and so on as well as the morphology engineering to form nanosheets, ^{[20, 21]}, nanocages, ^[22] etc. Besides, element doping^[23-25] and crystallinity improvement^[26] are also beneficial to the enhancement of photocatalytic performance.

Apart from the above-mentioned strategies, molecular engineering is proposed more advantageous for the modification of g-C₃N₄, because it is convenient to distinguish the role of grafted molecular groups as electron donor (D) or acceptor (A) based on the difference of charge distribution. For instance, a series of studies^{[27, 28]} reported that acceptor blocks were introduced into the skeleton of g-C₃N₄ to establish D-A type copolymers. Sun et al.^[29] prepared benzenesulfonyl chloride incorporated g-C₃N₄ for enhanced H₂ production. Besides, Fan et al.^[30] selected dibromo aromatics as the electron acceptor comonomers to react with urea and constructed successfully novel intramolecular D-A copolymers with enhanced photocatalytic H₂ evolution. Li et al.^[31] incorporated 4, 4'-(benzoc 1, 2, 5 thiadiazole-4, 7-diyl)diphenylamine (as an electron acceptor) into the g-C₃N₄ networks to establish donor-π-acceptor-π-donor polymers for a high degree of intramolecular charge transfer.

However, by introducing electron acceptor into the structural network of g-C₃N₄, the reduction ability of photocatalyst is weakened. This is because the reduction ability of g-C₃N₄ depends on the energy level of its lowest unoccupied molecular orbital (LUMO). The more negative LUMO energy level indicates that g-C₃N₄ possesses stronger reduction ability. When g-C₃N₄ is modified with small molecules, the LUMO for electron accumulation is determined by the low energy-level units. Usually, the LUMO of the acceptor unit is lower than that of g-C₃N₄. Thus, when g-C₃N₄ is modified with acceptor units, its reduction ability will be weakened. Hence, introducing electron donor units which possess higher LUMO levels into the g-C₃N₄ frameworks where triazine rings serve as electron acceptor is a rational way to promote the photocatalytic performance while maintaining the strong reduction ability of photoinduced electrons.

Herein, dibenzothiophene (DBT)-incorporated g-C₃N₄ compounds were successfully synthesized by heat treatment of g-C₃N₄ and dibenzothiophene-4-formaldehyde. DBT as an electron-rich unit plays the role of electron donor and delivers electrons to the tri-s-triazine rings (as electron acceptor), to establish a g-C₃N₄-based intramolecular D-A copolymer (TCN-DBT_x, in which x (mg) represents the quantity of dibenzothiophene-4-formaldehyde added). This D-A copolymer can be employed as a photocatalyst with much better charge migration and separation to reduce protons, as compared to pristine g-C₃N₄. Under visible-light irradiation, the D-A type g-C₃N₄ shows a steady and excellent activity of hydrogen evolution, with an optimal rate of 3334 μmol g^-1 h^-1, which is 2.5 times that of pristine g-C₃N₄. Such performance enhancement could be owing to the reinforced optical response, the boosted transfer and separation of excited charge carriers caused by the D-A structure modification.

RESULTS AND DISCUSSION

Structure Analysis. TCN-DBT_x was fabricated by the copolymerization of a mixture of g-C₃N₄ and DBT (Figure 1). X-ray diffraction (XRD) patterns (Figure 2a) show two typical diffraction peaks at 13.0° and 27.3° for all samples, which are assigned to (100) and (002) planes, respectively.^{[32, 33]} The former is indexed to the in-plane repeated structural packing of tri-s-triazine motifs, while the latter stems from the regular graphite-like interlayer stacking of conjugated aromatic systems.^[34] The introduction of a small amount of DBT does not change the primary chemical skeleton of g-C₃N₄. Fourier transform infrared spectra (FTIR) of DBT-modified samples are also similar with that of TCN (Figure 2b). The sharp signal at 810 cm^-1 is caused by the representative breathing of heterocycles involving N in g-C₃N₄.^[19] The peaks ranging from 1200 to 1600 cm^-1 are the characteristics of stretching vibration modes of tri-s-triazine (C₆N₇) rings.^[35] The extensive absorption band emerging at the scope of 3000-3400 cm^-1 is related to the vibrations of N–H bonds owing to uncondensed amino groups or O-H bonds resulting from adsorbed H₂O.^[36] When the DBT content is increased to 16 mg, a characteristic peak corresponding to the aromatic C=C^[30] appears at 1508 cm^-1 (Figure S1), which is also the evidence of successful incorporation of DBT into g-C₃N₄ networks.

Figure 1

Figure 1.  (a) Schematic illustration of formation route for TCN-DBT_x and (b) experimental operation for the preparation of TCN-DBT_x.

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Figure 2

Figure 2.  (a) XRD patterns and (b) FTIR spectra of the as-obtained samples. (c) Full survey and high-resolution X-ray photoelectron spectra (XPS) (d) C 1s, (e) N 1s of TCN and TCN-DBT₄, and (f) S 2p of TCN-DBT₄.

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To acquire more detailed information about the influence of DBT on the chemical states, the XPS analyses of TCN and TCN-DBT₄ were conducted. The constituent elements of TCN basically comprise C, N and O (Figure 2c and Table S1). Compared with TCN, TCN-DBT₄ contains a tiny amount of S. According to the estimated results listed in Table S1, the atomic ratio of C to N in TCN is 0.74, while TCN-DBT₄ has a larger value due to the incorporation of DBT. Two peaks at 284.8 and 288.1 eV are deconvoluted for the high-resolution C 1s signals of the two samples (Figure 2d). The former of TCN results from adventitious carbon on the surface while the latter corresponds to the sp² hybridized carbon (N-C=N).^[37-39] Note that the peak area at 284.8 eV of TCN-DBT₄ (8.06%) is increased in comparison with that of TCN (7.61%) due to the presence of aromatic rings in DBT. The deconvoluted spectra of N 1 s are mainly comprised of three peaks (Figure 2e). The signal at 398.5 eV is characteristic of the sp² hybridized aromatic N bonded to C, (C=N-C)^[40] and the peak located at 400.2 eV originates from the tertiary N bonded to three carbon (N-C₃).^{[41, 42]} Besides, the peak of 401.1 eV is the feature of the amino groups (-NH_x).^[43] Importantly, the existence of sulfur species in TCN-DBT₄ is readily demonstrated by the obvious signals appearing at 163.8 (S 2p3/2) and 164.9 eV (S 2p1/2) in Figure 2f, which are related to thiophene-based molecules.^{[36, 44]} Another distinct peak centered at 168.4 eV is indexed to S-O of sulfur oxides (SO_x) from the partial decomposition of DBT during heat treatment in air.^{[45, 46]}

The morphology and microstructure of various samples were investigated by field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). As depicted in Figure 3 and Figure S2, both TCN and TCN-DBT₄ exhibit obvious thin nanosheet structure. Note that apart from C and N, S is also uniformly distributed in the elemental mapping of TCN-DBT₄. These results again prove the successful incorporation of DBT into g-C₃N₄ frameworks without changing its main molecular structure.

Figure 3

Figure 3.  (a, b) FESEM images of TCN and TCN-DBT₄. (c) TEM image and (d) corresponding elemental mapping of TCN-DBT₄.

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The nitrogen adsorption-desorption isothermal curves and Barrett-Joyner-Halenda (BJH) pore plots are illustrated in Figure S3. Both TCN and TCN-DBT₄ present typical type-IV isotherms with high adsorption capacity in high relative pressure region (p/p₀ > 0.65).^[47] Such curves indicate that there are plentiful mesopores and macropores, which is consistent with the BJH pore plots. In addition, both samples possess H3-hysteresis loops, revealing the formation of slit-like pores caused by aggregation and stacking of nanosheets.^[48-51] The similar structure feature also leads to the similarly high specific surface area for of TCN (193 m² g^-1) and TCN-DBT₄ (189 m² g^-1), as well as similar pore volume and size (Table S2), for providing abundant reaction sites.

Analysis of the Optical and Electronic Properties. UV-vis diffuse reflectance spectra (DRS) were employed to explore the optical properties of TCN and TCN-DBT_x. In contrast with TCN, the absorption intensity of DBT-modified samples in visible-light region was enhanced (Figure 4a). Correspondingly, the band-gaps of TCN and TCN-DBT₄ were determined to be 2.80 and 2.76 eV, respectively (Figure 4b).^{[52, 53]} Mott-Schottky plots were utilized to explore the flat-band potentials (E_FB) of TCN and TCN-DBT₄. The curves of TCN and TCN-DBT₄ (Figure 4c and 4d) are characteristic of positive slope, corresponding to typical n-type semiconductor feature.^[54] Generally, for n-type semiconductors, the position of the conduction band (CB) bottom is consistent with the flat band potential.^{[27, 55]} Therefore, the CB of TCN and TCN-DBT₄ can be roughly estimated as -1.18 and -1.30 eV, respectively. Combined with the band gap values obtained from UV-vis DRS, the band alignments are thus illustrated in Figure 4f. The result reveals that the incorporation of dibenthioohene groups could narrow the band gap of carbon nitride and cause an upshift of the CB. These alterations not only permit more electrons to participate in the reduction of protons, but also raise the reduction ability of photoinduced electrons.

Figure 4

Figure 4.  (a) UV-vis DRS and (b) plots of (αhv)^1/2 versus (hv) for TCN and TCN-DBT₄. (c, d) Mott-Schottky plots collected at 1000, 1500, and 2000 Hz and (e) band alignments of TCN and TCN-DBT₄.

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PHE Activity of As-obtained Samples. The PHE performance of TCN and TCN-DBT_x was evaluated under visible-light irradiation. As displayed in Figure 5a, TCN presents a relatively low PHE activity due to the rapid recombination of photogenerated electron-hole pairs. After the introduction of DBT into the g-C₃N₄ frameworks, all samples show much better PHE performance than that of TCN. Particularly, TCN-DBT₄ shows the highest PHE rate of 3334 μmol h^-1 g^-1, which is 2.5 times that of TCN (1332 μmol h^-1 g^-1).

Figure 5

Figure 5.  (a) PHE rates of TCN and TCN-DBT_x. (b) Stability test of PHE over TCN-DBT₄. (c) XRD patterns of TCN-DBT₄ before and after reaction. (d) FESEM image of TCN-DBT₄ after reaction.

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Note that after four cycles in 12 h, TCN-DBT₄ still remains excellent photocatalytic activity (Figure 5b). The structure and morphology of TCN-DBT₄ are also retained after the 4-cycle reactions, as evidenced by the XRD patterns (Figure 5c) and FESEM image (Figure 5d). These results demonstrate the superior stability of TCN-DBT₄ catalyst. The apparent quantum yield (AQY) of TCN and TCN-DBT₄ at monochromatic light irradiation of 420 nm is 0.57% and 0.82%, respectively.

Mechanism for the Enhanced PHE Activity. Quenching photoluminescence (PL) emission as an indicator of retarded radiation recombination implies the increased charge separation possibility.^[56] PL spectra were thus measured to monitor the activity of excited charge carriers within the catalysts. As shown in Figure 6a, TCN exhibits a strong PL emission at ~430 nm. After modifying g-C₃N₄ with DBT, the peak intensity of TCN-DBT_x decreases significantly with a slight red shift as the amount of DBT increases, revealing the suppression of electron-hole recombination. Time-resolved photoluminescence (TRPL) spectra were then recorded and shown in Figure 6b, where a three-exponential fitting is employed to investigate the luminescence decay curves.^[19] The calculated average PL lifetime (τ_ave) of TCN and TCN-DBT₄ is 11.65 and 10.30 ns, respectively (Table S3). The reduced PL lifetime demonstrates the improved exciton separation.^{[12, 57]}

Figure 6

Figure 6.  (a) PL spectra of TCN and TCN-DBT_x. (b) TRPL spectra, (c) electrochemical impedance spectroscopy (EIS) plots and (d) photocurrent curves for TCN and TCN-DBT₄.

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The EIS Nyquist plots of TCN and TCN-DBT₄ were also observed in dark conditions (Figure 6c). When DBT was used as electron donor to modify g-C₃N₄, the semicircular radius becomes significantly smaller as compared to that of TCN, revealing the greatly reduced charge transfer resistance of TCN-DBT₄.^[58-60] Moreover, the photocurrent intensity of TCN-DBT₄ is much higher than that of TCN (Figure 6d), and does not decrease noticeably within five cycles, indicating the faster dissociation efficiency of electron-hole pairs with high stability.^[61-63] Further electron paramagnetic resonance (EPR) analysis (Figure 7) shows an obvious g value of ~2.0051, the typical feature of π-conjugated aromatic delocalization within TCN and TCN-DBT_x.^{[10, 64, 65]} After DBT modification, the EPR signals of the sample are remarkably enhanced, revealing the existence of more unpaired electrons of the extended π-conjugated systems which are beneficial to the dissociation and transfer of photogenerated charge carriers.

Figure 7

Figure 7.  EPR spectra of TCN and TCN-DBT_x.

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DFT calculations were employed to investigate the electronic structures of the catalysts. Figure 8a and S4 present the spatial structure, electronic structure, optimized highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of TCN and TCN-DBT₄. Compared to TCN, the HOMO change of TCN-DBT₄ is not obvious. But its LUMO moves from the tri-s-triazine ring to DBT and the neighboring tri-s-triazine ring. This demonstrates the incorporation of DBT contributes to the spatial separation of HOMO and LUMO, which could induce the intramolecular electron transfer under light irradiation. Besides, Figure 8b and 8c reveal the projected density of states (PDOS) of TCN and TCN-DBT₄. As for TCN-DBT₄, the combination of nitrogen P_z orbitals generally devotes mainly to HOMO and C-N hybrid orbitals are the main contributor to LUMO, which is similar to TCN. And the TCN-DBT₄ effectually narrows the HOMO-LUMO gap from 2.424 eV (TCN) to 2.119 eV, consistent with the results of UV-vis DRS analysis.

Figure 8

Figure 8.  (a) Electronic structure and optimized HOMO and LUMO energy levels of TCN and TCN-DBT₄. (b-c) PDOS profiles of TCN and TCN-DBT₄. (d) H₂ adsorption free Gibbs energies

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As shown in Figure S5, when DBT is connected to the tri-s-triazine ring, one unit of DBT loses 0.28 e, and correspondingly, one unit of tri-s-triazine ring gains 0.28 e. This result demonstrates that DBT acts as an electron donor in the entire network of the polymer. Therefore, a built-in electric field from DBT to tri-s-triazine ring is formed, which can further promote the transfer of photogenerated electrons from HOMO to LUMO and inhibit the recombination of photogenerated electron-hole pairs. Moreover, H₂ adsorption free Gibbs energies (ΔG_H*) were calculated to elucidate the kinetic mechanism of the catalytic process. In Figure 8d, the ΔG_H* value of TCN-DBT_x (-0.17 eV) is significantly reduced in comparison with that of TCN (0.23 eV), indicating the easier thermodynamic process of hydrogen evolution over TCN-DBT₄ catalyst.^{[10, 66]}

CONCLUSION

In summary, intramolecular D-A modification of g-C₃N_4­ is readily achieved by the copolymerization of g-C₃N₄ and DBT. A series of characterization and DFT calculation demonstrate that the DBT unit could act as an electron donor after incorporating into the g-C₃N₄ frameworks. Such D-A modification of g-C₃N₄ helps to adjust the electronic structure with more negative CB for stronger reduction ability. More importantly, the introduction of DBT into g-C₃N₄ not only increases the electron delocalization, but also promotes the intramolecular charge transfer via inducing the internal electric field, as well as accelerating the surface catalytic process via reducing the reaction barrier. As a result, the optimal g-C₃N₄-based D-A copolymer shows the best hydrogen production performance (3334 μmol h^-1 g^-1), which is 2.5 times that of pristine g-C₃N₄. This work introduces a facile but effective route for constructing g-C₃N₄-based D-A copolymer with high charge separation efficiency and excellent photocatalytic activity.

EXPERIMENTAL

Synthesis of Graphitic Carbon Nitride (g-C₃N₄). g-C₃N₄ was synthesized as follows. In a typical process, 10 g of urea was placed in a crucible with a cover, then heated to 550 ℃ with a ramping rate of 5 ℃ per minute in a muffle furnace and held for 2 h. The obtained solid was cooled to room temperature and ground into fine powder for next investigation.

Synthesis of Dibenzothiophene-incorporated Graphitic Carbon Nitride. The samples were prepared by thermal polymerization of a mixture of dibenzothiophene-4-formaldehyde and g-C₃N₄. In a typical procedure, 360 mg of g-C₃N₄ and a certain amount of dibenzothiophene-4-formaldehyde were dispersed in a beaker containing 40 mL of deionized water and stirred vigorously for 3 hours. Then the mixture was centrifuged, and washed with deionized water and alcohol for 3 times, respectively. After that, the resulting powders were placed in an alumina porcelain boat and heated to 450 ℃ for 2 h in a muffle furnace at a heating rate of 5 ℃ per minute. The as-obtained samples were recorded as TCN-DBT_x (x = 2, 4, 6), in which x (mg) represents the quantity of dibenzothiophene-4-formaldehyde added. The sample used for comparison was prepared by the same synthetic procedure using g-C₃N₄ as precursor without the addition of dibenzothiophene-4-formaldehyde, and denoted as TCN.

Photocatalytic Hydrogen Evolution Tests. At room temperature, a 100 mL three-necked flask was used as the photocatalytic hydrogen production reactor to conduct the activity test of the catalyst. 20 mg of photocatalyst was placed in 72 mL of deioni-zed water and dispersed by ultrasonic for 10 min, and then a certain amount of H₂PtCl₆·6H₂O aqueous solution was used as the precursor to load 1 wt% Pt on the sample as a cocatalyst by photodeposition method. Then 8 mL of triethanolamine (TEOA) was added to the above solution as a hole trapping agent, and the reactor was made a closed system using rubber stoppers and tape. Before light irradiation, the mixed aqueous solution was subjected to N₂ for 30 minutes under magnetic stirring to remove oxygen in the closed system. Then it was illuminated under a Xe lamp (400-800 nm) and kept magnetic stirring. One hour later, 400 μL of gas was extracted from the reactor with a sampler and injected into a gas chromatograph (GC-14C, Shimadzu, Japan, TCD, 5 A molecular sieve column) with N₂ as the carrier gas.

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