Rational design of dimensionally matched 2D/2D COF based photocatalysts for highly efficient noble-metal-free solar energy catalysis†

Haijun Hu^a, Xiaodong Sun*^a, Hui Li^b, Hongwei Huang^c and Tianyi Ma*^b
aInstitute of Clean Energy Chemistry, Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, People's Republic of China. E-mail: sunxiaodong@lnu.edu.cn
^bCentre for Atomaterials and Nanomanufacturing (CAN), School of Science, RMIT University, Melbourne, VIC 3000, Australia. E-mail: tianyi.ma@rmit.edu.au
^cBeijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences, Beijing, 100083, China

First published on 15th August 2024

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1. Introduction

Hydrogen energy and photocatalytic technology are important innovations in the field of clean energy, providing new solutions for sustainable development.^1–5 The application of this technology will drive the energy industry towards a more sustainable direction, laying the foundation for building a cleaner and greener energy system in the future. Hence, it becomes imperative to identify appropriate catalysts for harnessing solar energy to produce hydrogen fuel.^6,7 Covalent organic frameworks (COFs) stand out as the foremost category of porous organic substances distinguished by high porosity, robust thermochemical stability, and high crystallinity.^8–12 Due to their unique advantages, COFs have become a hot topic in the field of photocatalysis in recent years, in which two-dimensional (2D) COFs typically feature a substantial conjugated system and a stratified architecture characterized by robust π–π interactions among adjacent layers.^13–18 These properties contribute to an extensive light absorption capability of COF materials. The structural characteristics of COFs offer an optimal foundation for comprehensive exploration of their photocatalytic capabilities. However, pure COFs still struggle to achieve ultra-high photocatalytic efficiency due to the challenges of carrier recombination.^19–22 To overcome the problem of electron–hole recombination, methods such as metal doping, defect manufacturing, heterostructure construction, and ligand functionalization have been proposed.^23–26 Generally, adding a co-catalyst onto COFs is a simple and effective strategy to boost the charge transfer and separation. The precious metal Pt, as a co-catalyst, has been widely used to improve the photocatalytic performance of catalysts. Han and his team successfully constructed a Schottky heterojunction by uniformly distributing small Pt nanoparticles (NPs) on a triazine-derived COF.²⁷ The robust interaction between Pt NPs and CTF-1 led to rapid electron transfer. In addition, Pt NPs@CTF-1 showed a significantly lower hydrogen binding free energy (ΔG_H*) than that of unmodified CTF-1, thereby significantly enhancing photocatalytic activity. In Zhao's research, sodium citrate (SC) stabilized Pt NPs were incorporated into the pores of COFs.²⁸ The hydrogen bonding linkage between the SC molecules and COFs improved the structural stability. Accordingly, the composite achieved a visible light-driven photocatalytic hydrogen evolution rate of 1.13 mmol h⁻¹ with an AQY of 13.5% at 450 nm. Although precious metal co-catalysts can accelerate electron transfer and enhance the photocatalytic performance of materials, they are usually expensive, which raises the cost of catalyst preparation and is not conducive to practical applications. Thus, it is of great necessity to search for low-cost co-catalysts. Zhang et al. conducted photocatalytic hydrogen production experiments by loading NiS NPs onto COFs under visible light.²⁹ The presence of NiS on the TpBD-COF promoted the movement of photo-induced electrons to the cocatalyst. Consequently, the TpBD-COF with 3 wt% NiS loading (NiS3-BD) achieved a hydrogen evolution rate of 38.4 μmol h⁻¹, which outperformed that of TpBD-COF/Pt with an equivalent Pt loading (3 wt%). Besides, Yan and her colleagues used a simple solvothermal method to combine transition metal phosphides (TMPs) with 2D COFs to form TMP/TpPa-1-COF hybrid materials for photocatalytic applications.³⁰ As an electronic collector, Ni₁₂P₅ effectively improved the migration efficiency of photogenerated charges. Hence, the hydrogen evolution rate of Ni₁₂P₅/TpPa-1-COF can reach up to 31.6 μmol h⁻¹, which was nearly 19 times higher than that of the pristine COFs. However, many non-precious metal co-catalysts cannot fully show their optimal co-catalytic performance because they suffer from problems of dimensional mismatch with 2D COF materials. Compared with conventional co-catalysts, 2D co-catalysts have many advantages: first, tightly bound interfaces can be formed between two 2D materials, which is beneficial for charge transfer; second, more active sites are exposed for photocatalytic reactions due to the increased contact area; third, the distance of charge transfer is greatly shortened, and the efficiency of charge transfer is greatly improved; fourth, due to the reduction in charge transfer resistance on the same layer, photogenerated charges can be smoothly transferred along the material surface to the reaction site.

Herein, a 2D non-precious metal co-catalyst (WS₂) was combined with 2D COFs for photocatalytic hydrogen production. Due to the strong visible light response ability of WS₂, the light absorption range of WS₂/TP1C hybrid materials was significantly expanded. Moreover, WS₂ with an ultrathin structure formed a 2D/2D structure with TP1C, which not only provided a host of active sites for photocatalytic reactions, but also shortened the migration distance of photogenerated charges, leading to high carrier migration and separation efficiency. As a result, the synthesized 5% WS₂/TP1C exhibited excellent photocatalytic performance, with a hydrogen production rate of 4305 μmol g⁻¹ h⁻¹, which was approximately 18.74 times that of the pure COF and even exceeded the photocatalytic activity of Pt/TP1C.

2. Experiments

2.1. Experimental reagents

All reagents were not further purified after purchase from the manufacturer.

2.2. Material synthesis

WS₂ nanosheets were prepared via an easy hydrothermal process with tungsten chloride (WCl₆) and thioacetamide (CH₃CSNH₂) as raw materials on the basis of the previously cited article.³¹ 1,3,5-Triformylphloroglucinol (Tp) and paraphenylenediamine (Pa) were employed as starting materials for the synthesis of TpPa-1-COF via a solvothermal approach.³² x% WS₂/TpPa-1-COF hybrid materials were synthesized by incorporating WS₂ nanosheets into the process of COF preparation (x% represents the mass fraction of WS₂ in the composite).

3. Results and discussion

WS₂/TP1C hybrid materials were prepared by a solvothermal method, and the operation process is shown in Fig. 1a. First of all, WS₂ nanosheets were prepared via a simple hydrothermal reaction using WCl₆ and CH₃CSNH₂ as initial materials.³¹ During the preparation process of pure COFs, WS₂ nanosheets were added, and solvent heat treatment was performed to successfully prepare WS₂/TP1C hybrid materials. Then, scanning electron microscopy (SEM) and the transmission electron microscopy (TEM) analyses were carried out to characterize the morphology of the synthesized materials. As depicted in Fig. 1b, WS₂ displayed an aggregated nanosheet morphology, and TP1C showed a flower-like structure (Fig. 1c). The WS₂ nanosheets and flower-like COFs were tightly bound together in the composite material (Fig. 1d). In addition, in the high-resolution TEM image of WS₂/TP1C, the lattice spacing of WS₂ can be clearly observed, which was approximately 0.612 nm, corresponding to the (002) crystal plane of WS₂ (Fig. 1f).³³ Furthermore, energy dispersive spectrometry (EDS) mapping measurements were conducted on WS₂/TP1C composites. As shown in Fig. 1g–l, C, N, O, S, and W elements were densely distributed throughout the entire skeleton, indicating that these elements existed in hybrid materials. After that, the crystal structure of the prepared material was characterized through powder X-ray diffraction (PXRD) analysis. It can be seen from Fig. S4 (ESI†) that the peak positions of WS₂ were consistent with the JPCDS card (084-1398), which proved the successful reparation of WS₂ nanosheets.³⁴ As illustrated in Fig. S5 (ESI†), the diffraction peak position of TP1C matched well with the simulated peak position, and no other impurity peaks appeared, indicating its high crystallinity. In addition, the peaks appearing at 4.6° and 26.6° are attributed to the (100) and (001) crystal planes of pure COFs, respectively.³⁰ In the hybrid sample, the PXRD diffraction peaks of both TP1C and WS₂ can be observed, and the smaller peak intensity of WS₂ may be due to its low content. The above results indicated that the WS₂/TP1C composite materials were successfully prepared. As shown in Fig. S6 (ESI†), the weight of the prepared samples decreased as temperature increased. The weight loss before 200 °C primarily resulted from the evaporation of water or solvent on the surface or within the pores of COFs. The decomposition temperatures of bare COFs and the WS₂/TP1C hybrid material were about 450 and 430 °C, respectively, indicating their excellent thermal stability.

X-ray photoelectron spectroscopy (XPS) experiments were carried out to explore the chemical valence state and the elemental composition of synthesized products. As depicted in the XPS survey picture, bare TP1C had C, N and O elements, WS₂ was mainly composed of S and W, and the WS₂/TP1C hybrid sample had C, N, O, S and W elements (Fig. 2a). Additionally, more valence band information can be obtained through high-resolution XPS analyses. Fig. 2b shows high-resolution spectral information of C 1s. Evidently, the C 1s XPS spectrum of bare TP1C can be de-convoluted into three peaks at banding energies of 284.1, 285.3 and 288.2 eV, which are attributed to the CC, C–N and CO groups, respectively. The position of the peak of TP1C in the XPS shifted in the direction of reduced binding energy compared to that of WS₂/TP1C composites.³⁵

As depicted in Fig. 2c, the N 1s spectrum of WS₂/TP1C can be divided into two peaks (399.5 and 402.5 eV), in which the peak at a binding energy of 402.5 eV is a satellite peak.³⁶ Compared with pure COFs, the WS₂/TP1C hybrid material showed higher binding energy peaks in the N 1s XPS spectrum. Fig. 2d shows the high-resolution O 1s spectra of pure COFs, in which the XPS peaks at 530.3 and 532.1 eV can be evidently observed, corresponding to the CO and –OH groups.²⁹ The hybrid sample showed a minor upshift in comparison to TP1C in terms of the XPS peak position. As seen in Fig. 2e, the WS₂ nanosheets showed two binding energy peaks at 160.9 and 162.0 eV, which are attributed to S 2p_3/2 and S 2p_1/2, proving the presence of S²⁻.³⁷ The appearance of doublet XPS peaks at 31.6 and 33.7 eV for WS₂ was attributed to W 4f_7/2 and W 4f_5/2, which demonstrated an oxidation state of W⁴⁺ (Fig. 2f).^38–40 The binding energy peaks of W 4f and S 2p for WS₂/TP1C displayed a blue shift trend compared with those of pure WS₂, which illustrated electron transition from TP1C to WS₂, leading to an increased electron cloud density in WS₂. In conclusion, XPS tests further indicated that the WS₂/TP1C hybrid material was successfully prepared, and the change in the position of the binding energy peak suggested that WS₂ nanosheets were in close contact with COFs and a strong interaction occurred between the two samples.

Then, the UV-vis diffuse reflectance spectra (DRS) were obtained to study the light response capability of the prepared samples. As shown in Fig. 2g, the maximum light absorption edge of pure TP1C was approximately at 600 nm. Besides, WS₂ showed an expanded light absorption range due to its narrow bandgap. Therefore, when WS₂ was combined with COF materials, the hybridized sample exhibited a significantly red-shifted light absorption edge with a broadened light absorption range. Moreover, the bandgap energy of COFs can be calculated by the Kubelka–Munk equation:

αhv = A(hv − Eg)n/2	(1)

in which A, E_g, h, α and ν represent the proportional constant, band-gap energy, Planck constant, light efficiency and the absorption coefficient, respectively.⁴¹ Consequently, bare TP1C showed commendable light capture capability with band gaps about 2.1 eV (Fig. 2h). To further comprehend the material's band structure, Mott–Schottky (M–S) analysis was subsequently performed.

Obviously, TP1C can be considered as a typical n-type semiconductor material because the slope in Fig. 2i was positive. Besides, the bare COFs (TP1C) displayed flat-band potential (E_fb vs. Ag/AgCl) of about −0.69 V. Subsequently, the normal hydrogen electrode (NHE) potential of TP1C can be easily calculated through the equation E_fb (pH 0, vs. Ag/AgCl) + E_AgCl (E_AgCl ≈ 0.2 eV). As a result, E_fb vs. the NHE of pure COFs was −0.49 V. In general, the energy level of the lowest unoccupied molecular orbit (LUMO) was approximately 0.2 eV higher than E_fb vs. the NHE for n-type semiconductors.⁴² Therefore, the potential of the LUMO of TP1C was naturally calculated to be −0.69 V, which surpassed the H⁺/H₂ redox potential (Fig. S7, ESI†). According to the LUMO and bandgap of TP1C, the highest occupied molecular orbital (HOMO) of TP1C can be easily obtained using the equation E_g = HOMO–LUMO, which was about 1.41 V (Fig. S7, ESI†). The appropriate bandgap and aligned band positions of pure COFs were advantageous for advancing photocatalytic reactions.

Following that, the labsolar-6a all-glass automatic on-line trace gas analysis system (Beijing Perfectlight Technology Co., Ltd) was utilized to evaluate the photocatalytic hydrogen production efficiency of catalysts (Fig. S1, ESI†). The impact of various factors on the photocatalytic performance of the materials was explored by introducing different types of hole scavengers. As depicted in Fig. 3c, when L-ascorbic acid was utilized as hole scavenger, the WS₂/TP1C hybrid sample exhibited the highest photocatalytic activity. Accordingly, L-ascorbic acid was selected as a hole scavenger for subsequent photocatalytic experiments. As seen in Fig. 3a, pure COFs displayed relatively poor light-driven H₂ prodction performance because of the high recombination rate of the photogenerated carriers. Evidently, all WS₂/TP1C hybrid catalysts (1% WS₂/TP1C, 3% WS₂/TP1C, 5% WS₂/TP1C and 7% WS₂/TP1C) displayed improved photocatalytic activity, in which 5% WS₂/TP1C, with an appropriate mass ratio, exhibited the highest photocatalytic capability, with a H₂ evolution rate up to 4.31 mmol g⁻¹ h⁻¹, around 18.74 fold higher than that of bare COFs, even surpassing that of Pt/TP1C (Fig. S8 and S9, ESI†).

Additionally, the photocatalytic performance of the 5% WS₂/TP1C composite exceeded that of numerous previously reported photocatalysts (Fig. 3e). Of course, the activity of WS₂/TP1C composites was improved with the increase of the WS₂ content, while the photocatalytic activity of the composites started to decrease when the content of WS₂ was more than 5%, which may be due to the coverage of the active sites of photocatalysts. Besides, the apparent quantum efficiency (AQE) of 5% WS₂/TP1C can be acquired at a light wavelength of 420 nm, which was about 0.95% (Fig. 3b). After that, the cyclic tests were carried out to study the catalytic stability of synthesized products. As shown in Fig. 3d, after four cycle tests, the photocatalytic hydrogen production activity of the hybrid material decreased slightly, demonstrating its excellent recyclability. Moreover, Fourier-transform infrared (FT-IR) analysis was carried out for 5% WS₂/TP1C after the photocatalytic reaction, and the peak positions were almost consistent with those before the reaction, further proving that the prepared material possessed good durability (Fig. S11, ESI†).

The photoelectrochemical experiments served as a powerful method for investigating the intricate characteristics of charge transfer mechanisms within materials. Transient photocurrent tests were employed to gain direct insights into the dynamic processes involved in the separation of charge carriers. As depicted in Fig. 4a, the WS₂/TP1C hybrid materials exhibited a significantly enhanced photocurrent response in comparison with Pt/TP1C and bare TP1C, indicating their better carrier separation ability. Then, electrochemical impedance spectroscopy (EIS) experiments were carried out to assess the charge transfer resistance of the prepared samples. The 5% WS₂/TP1C has the smallest arc radius, indicating its lowest charge transfer resistance (Fig. 4b). It can be seen from time-resolved photoluminescence (TRPL) spectrum that 5% WS₂/TP1C has a longer lifetime over that of TP1C, proving its high carrier separation capacity (Fig. 4c). Then, the photoluminescence (PL) tests were carried out to further delve into the separation and dynamic behaviors of photo-induced charges. As displayed in Fig. S12 (ESI†), 5% WS₂/TP1C exhibited lower fluorescence intensity as compared to bare COFs and 5% Pt/TP1C due to its higher charge separation efficiency.^43–46 Besides, WS₂/TP1C showed a lower electrocatalytic H₂ production potential as compared to Pt/TP1C in the linear sweep voltammetry (LSV) experiments (Fig. 4d). On the basis of LSV plots, the Tafel slopes of the prepared materials can be obtained. As shown in Fig. 4e, TP1C exhibited lower photocatalytic activity with a larger Tafel slope. The carrier lifetime of the synthesized catalysts can be evaluated by the open circuit voltage decay (OCVD) experiments (Fig. 4f). Apparently, both TP1C and 5% Pt/TP1C displayed shorter carrier lifespan in compared with that of the 5% WS₂/TP1C hybrid sample, indicating the high electron–hole pair separation efficiency of the 5% WS₂/TP1C composite (Fig. 4g).⁴⁷ The results obtained from photoelectrochemical tests corresponded well with the photocatalytic hydrogen production performance of the materials, which demonstrated that WS₂ nanosheets accelerated the photo-induced charge transfer in COF materials and effectively solved the problem of carrier recombination. Subsequently, the hydrophilicity of the material was investigated via water contact angle (WCA) measurements. It was evident that the WS₂/TP1C hybrid sample with a smaller WCA demonstrated notably higher hydrophilicity, indicating the potential for achieving improved substrate enrichment and enhanced photocatalytic activity for water splitting (Fig. S13–S15, ESI†).

Through in-depth exploration of the energy band structure and a comprehensive array of H₂ production tests, the photocatalytic reaction mechanism of the WS₂/TP1C composite was explored (Fig. 4h). For WS₂/TP1C composites, TP1C, with a substantial specific surface area, not only offered abundant reactive sites for H₂ evolution reactions, but also facilitated the dispersion of WS₂ nanosheets. In addition, WS₂ not only enhanced the light absorption ability of hybrid materials but also acted as a co-catalyst to accelerate charge migration, thereby suppressing carrier recombination. When exposed to light, the photogenerated electrons in TP1C migrated from the HOMO to the LUMO, and then crossed the material interface to WS₂ nanosheets. After that, the hydrogen evolution reaction occurred as H⁺ in water was reduced to H₂ by the accumulated electrons on WS₂. Furthermore, the holes left on the HOMO were consumed by ascorbic acid, which served as a hole scavenger in solution. In short, WS₂, as a 2D co-catalyst, can be fully combined with 2D COF materials, effectively solving the problem of high carrier recombination rates in COF materials, resulting in a significant improvement in the photocatalytic activity.

4. Conclusions

To sum up, a sheet-like non-precious metal co-catalyst (WS₂) was integrated with 2D COF materials for photocatalytic hydrogen production applications via a simple solvothermal method. Compared with conventional co-catalysts, the combination of 2D WS₂ and 2D TP1C materials had many advantages: first, the strong interface interaction formed by WS₂ and TP1C was conducive to charge transfer; second, the two 2D materials can form a larger contact surface, exposing more active sites for the photocatalytic hydrogen production reaction; and lastly, the matched 2D/2D structure shortened the migration distance of photogenerated carriers compared with the reported nanoparticle co-catalysts, which was further favorable for carrier separation. As a consequence, the 5% WS₂/TP1C composite exhibited the most superior photocatalytic capacity, with a H₂ evolution rate of up to 4.31 mmol g⁻¹ h⁻¹. This rate was approximately 18.74 times greater than that of COFs alone, even exceeding the activity of TP1C with Pt as a co-catalyst. Additionally, the hydrogen production activity of the prepared catalysts showed almost no decrease after four cycles of experiments, demonstrating their excellent photocatalytic stability. This research provided groundbreaking insights into the synthesis of 2D precious metal co-catalysts to effectively improve the photocatalytic performance of COF materials.

Data availability

The data are available from the corresponding author on reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52071171, 52202248, 22101105), Open Project of State Key Laboratory of Inorganic Synthesis and Preparative Chemistry (2024-35), and Open Research Fund of Guangdong Advanced Carbon Materials Co., Ltd (Kargen-2024B1001). The authors thank Shiyanjia Lab (https://www.shiyanjia.com) for the support of XPS tests. The authors also thank SCI-GO (https://www.sci-go.com) for the support in the TRPL tests. T. M. acknowledged the Australian Research Council (ARC) through Future Fellowship (FT210100298), Discovery Project (DP220100603), Linkage Project (LP210200504, LP220100088, LP230200897) and Industrial Transformation Research Hub (IH240100009) schemes, the Australian Government through the Cooperative Research Centres Projects (CRCPXIII000077), the Australian Renewable Energy Agency (ARENA) as part of ARENA's Transformative Research Accelerating Commercialisation Program (TM021), and European Commission's Australia-Spain Network for Innovation and Research Excellence (AuSpire).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc02938k

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