Heterogeneous CoS₂/MS₂ microspheres for an efficient oxygen evolution reaction

Abstract

Delicately fabricating efficient electro-catalysts to boost the catalytic kinetics of the oxygen evolution reaction (OER) is essential for realizing the efficient performance of water splitting into hydrogen to alleviate environmental issues and the energy crisis. Here, heterogeneous and hollow microspheres (denoted as CoS₂/MS₂-HPMS, M = Fe, FeCo) were obtained by the sulfurization of Fe³⁺-decorated ZIF-67 hollow microspheres (ZIF-67-HMS). At a current density of 10 mA cm⁻², the prepared CoS₂/MS₂-HPMS showed an overpotential of 217 mV for the OER, which is lower than that of commercial RuO₂ (336 mV). Density functional theory calculations demonstrated low reaction free energies of the intermediates of the OER and significant charge accumulation at the heterojunction interface between CoS₂ and FeCoS₂. This work establishes a method for the assembly of nanoparticles and the construction of heterojunctions.

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1. Introduction

Splitting water into hydrogen is a green and promising technology to alleviate the environmental issues and energy crises caused by the rapid consumption of non-renewable energy (natural gas, coal, and oil).^1–4 However, the sluggish catalytic kinetics of the oxygen evolution reaction (OER) hinders highly efficient water splitting.^5–8 Therefore, delicately fabricating efficient and stable electro-catalysts for the OER is essential for boosting water-splitting performance.^9–12 Highly ordered porous coordination polymers, MOFs, have been regarded as promising candidates for the OER due to their high surface areas and rich porous structures for the exposure of more active sites and facilitation of fast mass transport.^13–16 Increasing efforts have been focused on the synthesis of various MOF-based materials in recent years.^17–19 Nevertheless, proposing a rapid and environmentally friendly synthetic method for the fabrication of MOF-based materials with well-defined structures remains challenging. In conventional routes for the fabrication of morphology-controllable and structure-versatile MOF-based materials, either environmentally hazardous chemicals or complicated synthetic procedures are often required,^20–23 which hinders their practical applications. Therefore, developing a facile, green, and environmentally friendly approach for the fabrication of morphology-property-specific MOF-based materials is crucial for broadening their applicability.

Deep eutectic solvents (DESs) composed of Lewis/Brønsted acids and bases are a novel type of ionic liquid analog, possessing high thermal stability, tunable chemistry, etc.^24–26 Relative to traditional ionic liquids, DESs possess a unique combination of various properties such as easy synthesis, economical affordability, non-toxicity, and biodegradability.^27–29 As a mixture, the individual component of DESs endows the nanocrystals with a multi-self-assembly process due to its high ionic strength, making it possible to prepare functional materials with controllable morphologies and structures that are not accessible from other conventional fabrication routes.^30,31 However, the use of DESs for the preparation of metal–organic frameworks (MOFs) has rarely been reported. Previous reports indicate that MOF-derived bimetallic-based sulfides are potential catalysts for OER research because of their low band gaps, strong synergistic effect, and high conductivity.^32–36 The introduction of secondary metals, including Fe, Ni, V, Cu, Mn, and Mo, effectively tunes the electronic structure of the active sites,^37–39 thereby lowering their adsorption to *OOH intermediates and enhancing the intrinsic catalytic activity toward the OER.⁴⁰

Herein, we present a self-assembled method to prepare ZIF-67 hollow microspheres (ZIF-67-HMS) by using DES as a solvent. After vulcanizing the Fe³⁺-decorated ZIF-67-HMS, heterogeneous hollow and porous microspheres (denoted as CoS₂/MS₂-HPMS, M = Fe, FeCo) were generated. The prepared CoS₂/MS₂-HPMS exhibited high electrocatalytic activity and long-term durability for the OER owing to their unique heterogeneous structures and appropriate compositions. Density functional theory (DFT) calculations indicated that excellent catalytic performance originated from the lower reaction free energy of intermediates and electronic accumulation at the interface of CoS₂ and FeCoS₂. The current work presents a novel tactic for the assembly of nanoparticles and the construction of heterogeneous structures, which can be applied in catalytic chemistry.

2. Experimental

2.1 Materials

Ethanol (CH₃CH₂OH), methanol (CH₃OH), and ethylene glycol (EG) were ordered from Xilong Chemical Co., Ltd, China. 2-Methylimidazole (2-MeIm) was purchased from Alfa Aesar. Choline chloride (ChCl) and urea were bought from Sinopharm Chemical Reagent Co. Ltd, China. Cobalt(II) nitrate hexahydrate (Co(NO₃)₂·6H₂O, 99.99%), iron(III) chloride hexahydrate (FeCl₃·6H₂O, 99%), thioacetamide (TAA, 98%), and commercial ruthenium oxide (RuO₂, 99.9%) were supplied by Aladdin. Potassium hydroxide (KOH, 95%) was provided by Maclean Biochemical Technology Co., Ltd, and the 5 wt% Nafion solution was obtained from Sigma-Aldrich. All the purchased chemicals were used directly.

2.2 Synthesis of ZIF-67 polyhedra

In a conventional preparation route, Co(NO₃)₂·6H₂O (587.9 mg) and 2-MeIm (677.1 mg) were dissolved in 40 mL of ethanol. Then the mixture was transferred into a Teflon-lined stainless-steel autoclave (100 mL) and heated to 130 °C for 4 h. After being naturally cooled down, the obtained purple precipitate was washed with ethanol several times and then dried in a vacuum oven at 60 °C for 6 h. The resulting product was marked as ZIF-67 polyhedra.

2.3 Synthesis of ZIF-67-HMS

DESs were prepared based on the previously reported procedure.^41,42 In a typical synthesis, the eutectic mixture was obtained by stirring choline chloride and urea at 80 °C until the formation of a homogeneous colorless liquid, and then it was sealed and stored for the subsequent synthesis. For the synthesis of ZIF-67-HMS, 677.1 mg of 2-MeIm (8 mmol) was first dissolved in the as-prepared DESs (40 mL) to form a homogeneous colorless solution at 75 °C, followed by the addition of 587.9 mg of Co(NO₃)₂·6H₂O (2 mmol) and stirring for 5 min at 25 °C. Finally, the resultant mixture was poured into a Teflon-lined stainless-steel autoclave (100 mL) and heated at 130 °C for 4 h. After being naturally cooled down, the resulting purple precipitate was collected by centrifugation and washing with ethanol several times, and then it was dried in a vacuum oven at 60 °C for 6 h and finally, the obtained sample was labeled ZIF-67-HMS.

2.4 Synthesis of CoS₂/MS₂-HPMS and CoS/CoS₂-HMS

ZIF-67-HMS (20 mg) and FeCl₃·6H₂O (6.1 mg) were dissolved in 20 mL of EG solution containing 90 mg of TAA under ultrasonication. Later, the obtained solution was transferred into a Teflon-lined stainless-steel autoclave (25 mL) and heated for 2 h at 180 °C. After cooling down to 25 °C, the sample was gathered by centrifugation and washing repeatedly with ethanol and then dried in a vacuum oven at 60 °C for 6 h and the obtained sample was denoted as CoS₂/MS₂-HPMS. CoS/CoS₂-HMS was also synthesized by following the above-mentioned procedure in the absence of FeCl₃·6H₂O.

2.5 Electrochemical measurements

All electrochemical measurements were performed using the CHI 660E (Chenhua, Shanghai) electrochemical workstation equipped with a traditional three-electrode configuration, in which a saturated calomel electrode (SCE) was used as the reference electrode, a graphite rod was used as the counter electrode, and a carbon paper electrode coated with catalysts was used as the working electrode. All the applied potentials in OER studies were converted to the value of a reversible hydrogen electrode (RHE) by the Nernst equation of E_RHE = E_SCE + 0.05916pH + 0.241 (V). The overpotential (η) was obtained according to the formula, η = E_RHE − 1.23 (V). All the electrochemical experiments were conducted in an N₂-saturated 1.0 M KOH solution. Linear sweep voltammetry (LSV) measurements were performed at a scan rate of 5 mV s⁻¹ with 85% iR compensation. Electrochemical impedance spectroscopy (EIS) studies were conducted at 1.54 V vs. RHE. Chronoamperometry (CA) used in the OER was conducted through galvanostatic experiments at −10 mA cm⁻² for 20 h.

The working electrodes were prepared by pipetting 23 μL of catalyst ink onto pretreated carbon paper (load area: 0.5 cm²) with a mass loading of 0.275 mg cm⁻². The preparation of a homogeneous catalyst ink was as follows: first, a uniform mixture was obtained by ultrasonically dispersing 1.5 mL of H₂O, 1 mL of ethanol, and 15 μL of 5 wt% Nafion solution for 30 min, and then 3 mg of the as-synthesized catalyst was added into 500 μL of the above solution under ultrasonication to form the catalyst ink. The carbon paper was ultrasonically washed with a mixture of 10 wt% HNO₃ and 10 wt% H₂SO₄, tri-distilled water, and ethanol, respectively.

2.6 Theoretical calculations

Density functional theory (DFT) implemented in the Vienna ab initio simulation package (VASP) was used to conduct the theoretical calculations using ref. 43 and 44 as references. The electron exchange and correlation energy were used to calibrate the generalized gradient approximation in the Perdew–Burke–Ernzerhof functional (GGA-PBE).⁴⁵ The valence orbitals of S (3s, 3p), Fe (3d, 4s), Co (3d, 4s), O (2s, 2p), and H (1s) were described by plane-wave basis sets with a cutoff energy of 500 eV. The Monkhorst–Pack scheme with a (3 × 3 × 1) mesh for surface models with and without adsorbates was used for k-point sampling, and a (1 × 1 × 1) mesh was utilized for single-molecule adsorbates. For astringency, the values of electronic self-consistent iteration and force were set to 10⁻⁵ eV and 0.02 eV Å⁻¹, respectively.

3. Results and discussion

Fig. 1a schematically shows the synthesis of CoS₂/MS₂-HPMS. In brief, the ZIF-67-HMS were synthesized by the self-assembly of smaller ZIF-67 polyhedra by using DES as the structure-directing agent and solvent. Then, the obtained ZIF-67-HMS were subjected to sulfurization in the presence of a sulfurization agent (TAA) to produce MS₂ covered CoS₂/MS₂-HPMS heterostructures. During the synthesis, the selection of solvent is a critical factor. For instance, ZIF-67-HMS were obtained in DES medium, while ZIF-67 polyhedra were obtained in ethanol or methanol medium. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) techniques were employed to characterize the self-assembled ZIF-67-HMS. The SEM and TEM images showed that the ZIF-67 polyhedra were self-assembled into ZIF-67-HMS in DES medium (Fig. S1a and b†), which is different from the ZIF-67 polyhedra synthesized in methanol (Fig. S1c†) or ethanol medium (Fig. S1d†). This result displays the excellent self-assembly of ZIF-67 polyhedra in DESs. Notably, the self-assembled products of ZIF-67 polyhedra such as rhombic dodecahedra,⁴⁶ two-dimensional microplates, and ultrathin nanosheets^47,48 have been reported so far. Most importantly, the synthesized ZIF-67-HMS possess significant advantages such as good dispersibility and large specific surface areas, making them an ideal material for electrocatalysis research. The ZIF-67-HMS were subjected to sulfurization to increase the chemical/structural stability and electrical conductivity of the prepared sample⁴⁹ and to produce heterogeneous CoS₂/MS₂-HPMS. TEM images show the hollow porous microspheres of CoS₂/MS₂-HPMS (Fig. 1b and c). A comparison of the electron microscopy images of CoS₂/MS₂-HPMS (Fig. 1b and c) and ZIF-67-HMS (Fig. S1a and b†) reveals good morphology retention after the sulfurization of ZIF-67-HMS. The magnified TEM images of CoS₂/MS₂-HPMS reveal the crossed lattice fringes of MS₂ and CoS₂ nanoparticles (Fig. 1d–f), which is different from those of CoS/CoS₂-HMS (Fig. S2a–c†). This result suggests the formation of CoS₂/MS₂-HPMS heterostructures. From the magnified TEM images (Fig. 1g and h), the lattice spacings of 0.195 and 0.275 nm are assigned to the (220) and (200) crystal planes of CoS₂, respectively. From the enlarged TEM images (Fig. 1i–k), the lattice spacings for the (101) and (102) crystal planes of FeCoS₂ and the (200) crystal plane of FeS₂ were found to be 0.254, 0.196, and 0.270 nm, respectively. Although a low crystallinity of C composition was found in CoS₂/MS₂-HMS, rich active sites and heterostructures were fabricated, which aids in the promotion of OER performance. The high-angle annular dark-field scanning TEM images and the corresponding elemental mapping images of CoS₂/MS₂-HPMS are shown in Fig. 1l. Similar to CoS/CoS₂-HMS (Fig. S2c and d†), hollow structures are seen for CoS₂/MS₂-HPMS. Most importantly, the signals of Co, Fe, and S elements in CoS₂/MS₂-HPMS were evenly distributed in the whole selected areas, again indicating the heterostructures of CoS₂/MS₂-HPMS.

Fig. 1 (a) Schematic synthesis of CoS₂/MS₂-HPMS. (b–d) TEM images, (e–k) HRTEM images, and (l) HAADF-STEM, and the corresponding mapping images of CoS₂/MS₂-HPMS.

The crystal structures and phase compositions of the synthesized ZIF-67-HMS, CoS/CoS₂-HMS, and CoS₂/MS₂-HPMS were determined by the X-ray powder diffraction (XRD) technique. As shown in Fig. 2a, the XRD pattern verifies the successful fabrication of ZIF-67-HMS, which agrees with the previous report.³⁶ Compared with ZIF-67 polyhedra, ZIF-67-HMS exhibit much sharper diffraction peaks, indicating a much higher crystallinity of ZIF-67-HMS and a more excellent assembled medium of DESs. As shown in Fig. S3,† the XRD pattern of CoS/CoS₂-HMS well matches the standard cards of CoS (JCPDS no. 75-0605) and CoS₂ (JCPDS no. 89-1492). As for CoS₂/MS₂-HPMS (Fig. 2b), the diffraction peaks at 32.30, 36.24, 39.83, 46.33, and 54.94° correspond to CoS₂ (JCPDS no. 89-1492), and the diffraction peaks at 28.52, 33.04, 37.08, 40.77, 47.43, 56.28, 59.02, 61.69, and 64.29° are ascribed to FeS₂ (JCPDS no. 71-1680). In addition, the diffraction peaks at 30.70, 35.17, 46.35, and 54.58° are assigned to FeCoS₂ (JCPDS no. 75-0607). The X-ray photoelectron spectroscopy (XPS) spectra of CoS/CoS₂-HMS and CoS₂/MS₂-HPMS (Fig. 2c) exhibited the signals of Co 2p, S 2p, and Fe 2p, reconfirming the presence of Co, S, and Fe elements in CoS₂/MS₂-HPMS. The N 1s signal originated from the 2-methylimidazole (C₄H₆N₂) precursor and high-temperature decomposition of the urea component in DES. Fig. 2d shows the high-resolution XPS spectra of Co 2p in CoS/CoS₂-HMS and CoS₂/MS₂-HPMS, respectively. The fitted doublets at 777.8 (Co 2p_3/2) and 792.88 eV (Co 2p_1/2) are indexed to Co²⁺, while the other doublets located at 779.86 (Co 2p_3/2) and 795.67 eV (Co 2p_1/2) are matched to Co³⁺, which originated from the oxidation of Co²⁺ species exposed to air.⁵⁰ In addition, the peak of Co²⁺ in CoS₂/MS₂-HPMS located at 777.8 eV (Co 2p_3/2) shows a significant negative shift of ca. 0.41 eV compared to Co²⁺ in CoS/CoS₂-HMS (778.21 eV for Co 2p_3/2). This result demonstrates a strong electronic interaction at the interface of CoS₂ microspheres and the MS₂ layer, which is very advantageous for the enhancement of electrocatalytic performance towards the OER.^51–53 For the high-resolution spectrum of Fe 2p in CoS₂/MS₂-HPMS (Fig. 2e), two different valence states of Fe²⁺ (712.24 eV for Fe 2p_3/2 and 724.36 eV for Fe 2p_1/2) and Fe³⁺ (716.24 eV for Fe 2p_3/2 and 729.38 eV for Fe 2p_1/2) were verified in the prepared material. The presence of Fe³⁺ was ascribed to the easy oxidation of Fe²⁺ before the XPS characterization.⁵⁴ The S 2p high-resolution spectra of CoS/CoS₂-HMS and CoS₂/MS₂-HPMS indicate the existence of the S₂²⁻ (162.85 and 164.16 eV) and S²⁻ (160.92 and 162.03 eV) species in both materials (Fig. 2f).⁵⁵ Moreover, the peaks at 167.15 and 168.47 eV confirm that S₂²⁻ has been partly oxidized to S_xO^y− because of its inevitable contact with air. In addition, the existence of S₂²⁻, Co²⁺, and Fe²⁺ further confirms the formation of CoS₂ and FeS₂ phases, and the presence of S²⁻, Co²⁺, and Fe²⁺ also verifies the formation of CoS and FeCoS₂ phases.

Fig. 2 (a) XRD patterns of ZIF-67-HMS and ZIF-67 polyhedra, (b) XRD patterns of CoS₂/MS₂-HPMS, (c, d) XPS survey spectra and Co 2p spectra of CoS/CoS₂-HMS and CoS₂/MS₂-HPMS, (e) Fe 2p spectra of CoS₂/MS₂-HPMS, and (f) S 2p spectra of CoS/CoS₂-HMS and CoS₂/MS₂-HPMS.

To benchmark the electrocatalytic performance of the prepared CoS₂/MS₂-HPMS against ZIF-67-HMS, CoS/CoS₂-HMS, and commercial RuO₂, we conducted OER experiments in 1 M KOH solution. The LSV measurements with 85% iR compensation are shown in Fig. 3a. Specifically, CoS₂/MS₂-HPMS showed a small overpotential of 217 mV at a current density of 10 mA cm⁻², which is significantly lower than that of CoS/CoS₂-HMS (322 mV), ZIF-67-HMS (341 mV), commercial RuO₂ (336 mV), and other reported materials (Table S1†). In addition, the overpotential of ZIF-67-HMS is much lower than that of previously reported ZIF-67 polyhedra (530 mV) synthesized in a conventional solvent,⁵⁶ indicating that the unique hollow structure of ZIF-67-HMS facilitates the enhancement of OER performance. The higher OER activity on CoS₂/MS₂-HPMS compared to CoS/CoS₂-HMS can be due to the following features: first, the hollow porous heterostructure of CoS₂/MS₂-HPMS, which provides more exposed active centers and facilitates faster charge transfer; second, the synergistic effect of different components and the formation of heterojunctions. Most importantly, CoS₂/MS₂-HPMS displayed an overpotential of only 333 mV at a higher current density of 831 mA cm⁻², which is highly close to that of commercial RuO₂ at a current density of 10 mA cm⁻². This result implies that CoS₂/MS₂-HPMS possesses higher O₂ production efficiency compared to ZIF-67-HMS, CoS/CoS₂-HMS and commercial RuO₂. In Fig. 3b, the corresponding Tafel slope of CoS₂/MS₂-HPMS (65 mV dec⁻¹) is much smaller than that of ZIF-67-HMS (94 mV dec⁻¹), CoS/CoS₂-HMS (68 mV dec⁻¹), commercial RuO₂ (139 mV dec⁻¹), and most of the electrocatalysts reported earlier (Table S1†). These results indicate that CoS₂/MS₂-HPMS is more favorable for the promotion of OER kinetics. Notably, the catalytic activity of CoS/CoS₂-HMS is higher than that of ZIF-67-HMS, inferring that the presence of metal sulfide on the surface of the catalyst increases the number of unsaturated S atoms and enriches new active sites and, consequently, promotes its electrocatalytic ability towards the OER.⁵⁷ To further study the reason for the high OER catalytic activity on the synthesized CoS₂/MS₂-HPMS, cyclic voltammetry (CV) and EIS techniques were used for the estimation of the electrochemical surface areas (ECSAs) and the charge transfer kinetics, respectively. In the calculation formula of ECSA = C_dl/C_sp, the value of the electrochemical double-layer capacitance (C_dl) is half of the slope value of Δj versus the scan rate curve, whereas the specific capacitance (C_sp) is taken as 0.040 mF cm⁻² according to the relevant reports.^58,59 From the formula, the value of ECSA is positively correlated with C_dl. Thus, the C_dl obtained from CV at different scan rates from 10 to 60 mV s⁻¹ was used to evaluate the value of ECSAs.

Fig. 3 (a) LSV curves, (b) the corresponding Tafel slopes, (c) Nyquist plots, and (d) chronopotentiometric curves of ZIF-67-HMS, CoS/CoS₂-HMS, CoS₂/MS₂-HPMS, and commercial RuO₂. (e) The 1st and 5000th CV curves of CoS₂/MS₂-HPMS were recorded in 1 M KOH alkaline solution at a scan rate of 100 mV s⁻¹. (f) The 1st and 5000th LSV polarization curves of CoS₂/MS₂-HPMS were recorded in 1.0 M KOH with iR compensation of 85% at a scan rate of 5 mV s⁻¹.

Fig. S4a and b† show the CV curves of CoS₂/MS₂-HPMS and commercial RuO₂ in the potential range of the non-faradaic region (1.19–1.24 V vs. RHE), respectively, and Fig. S4c† displays the linear fitted line of Δj vs. the scan rate (ν) in the corresponding CV curves. As shown in Fig. S4c,† CoS₂/MS₂-HPMS has a significantly increased C_dl of 18.11 mF cm⁻² compared to that of commercial RuO₂ (1.73 mF cm⁻²), which indicates that CoS₂/MS₂-HPMS possesses larger ECSAs. The ECSA–normalized LSV curves are used to evaluate the intrinsic catalytic activity of CoS₂/MS₂-HPMS. As shown in Fig. S5,† CoS₂/MS₂-HPMS exhibits the lowest onset potentials for the OER, indicating a good specific activity of CoS₂/MS₂-HPMS. The EIS results of the prepared samples disclose a much smaller semicircle diameter of CoS₂/MS₂-HPMS compared to those of ZIF-67-HMS, CoS/CoS₂-HMS, and commercial RuO₂ (Fig. 3c). These results indicate that CoS₂/MS₂-HPMS has the smallest electrochemical impedance and the most favorable charge transfer. To evaluate the actual application of CoS₂/MS₂-HPMS, durability tests without iR compensation were performed for all the prepared catalysts at a constant current density of 10 mA cm⁻². As shown in Fig. 3d, CoS₂/MS₂-HPMS shows excellent stability compared to ZIF-67-HMS, CoS/CoS₂-HMS, and commercial RuO₂ catalysts, even after a continuous operation of 20 h. The overpotential of CoS₂/MS₂-HPMS hardly increased after 40 h of the CA test, further indicating the above conclusion (inset of Fig. 3d). Furthermore, the long-term durability of CoS₂/MS₂-HPMS was further studied by performing 5000 cycles of CV and LSV in the potential range of 0.94 to 1.69 V vs. RHE. As shown in Fig. 3e and f, the CV and LSV polarization curves almost overlap with their initial cycle after 5000 cycles, respectively, again confirming the good long-term stability of CoS₂/MS₂-HPMS. Notably, after 5000 cycles of CV tests, the morphology of CoS₂/MS₂-HPMS was well preserved without any noticeable change (Fig. S6†). The corresponding high-resolution TEM (HRTEM) images of CoS₂/MS₂-HPMS in Fig. S7† further reveal the appearance of FeOOH and CoOOH.^60–62 In addition, the corresponding peak ratios of Fe³⁺/Fe²⁺ and Co³⁺/Co²⁺ increase after 5000 cycles of CV measurements (Fig. S8†). These results indicate the true electrocatalytic active sites of Co and Fe species in CoS₂/MS₂-HPMS for the OER.

We used DFT to illustrate the mechanisms of the OER process on CoS₂/FeCoS₂ together with CoS₂ and FeCoS₂. Fig. 4a shows the calculated models of CoS₂, FeCoS₂, and CoS₂/MS₂-HPMS, where the CoS₂(001), FeCoS₂(001), and FeCoS₂@CoS₂ heterogeneous structures were used as the calculated faces, respectively.^63,64 As shown in Fig. 4b, the OER includes four elementary reaction steps with *OH, *O, *OOH, and *OO intermediates in the reaction pathway from *H₂O to O₂.⁶⁵ The elementary reaction step for the formation of O₂ (*OO → O₂) on CoS₂ is the potential-determining step (PDS) with a ΔG of 1.42 eV. As for FeCoS₂, the PDS is the formation of *OOH (*O → OOH*), in which the corresponding ΔG is 0.99 eV. Compared with CoS₂ and FeCoS₂, the ΔG for each elementary reaction step on CoS₂/FeCoS₂ is much lower. Our DFT calculations show that the two reactions (*O → OOH* and *OOH → OO*) are endothermic, but the energy barrier for *OOH → OO* (0.84 eV) is much higher, which can be regarded as the RDS of the OER and is accordant with the recent report.^66–68 These results indicate the heterojunction in the CoS₂/FeCoS₂ catalyst is conducive to promoting the OER activity. In addition, the density of states (DOS) of CoS₂/FeCoS₂ is much greater than that of CoS₂ and FeCoS₂ near the Fermi level (Fig. 4c). These results demonstrate that CoS₂/FeCoS₂ possesses a large carrier concentration and high intrinsic electrical conductivity originating from the electronic states near the Fermi level, which is favorable for the electron mobility in CoS₂/FeCoS₂.

Fig. 4 DFT calculations of CoS₂, FeCoS₂ and CoS₂/MS₂-HPMS. (a) Models for DFT calculations, (b) calculated Gibbs free energy, and (c) calculated DOS.

4. Conclusions

In summary, we demonstrated a green approach for the fabrication of CoS₂/MS₂-HPMS. By using DESs as a solvent, we successfully synthesized ZIF-67-HMS via the excellent self-assembly of ZIF-67 polyhedra. The obtained Fe³⁺-modified ZIF-67-HMS can be sulfurized into CoS₂/MS₂-HPMS with a unique heterojunction, besides the promoted conductivity, faster charge transfer, and enlarged ECSAs. All the results of electrochemical measurements and DFT calculations indicated that the heterojunction in CoS₂/MS₂-HPMS is key to the promotion of OER performance. Thus, the prepared CoS₂/MS₂-HPMS catalyst has a prospective application in overall water splitting. This work opens a novel approach for the self-assembly of MOF-based materials and the subsequent construction of heterostructures as well as their applications in the fields of energy, environment, and other catalytic reactions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22162006 and 22102035) and the Natural Science Foundation of Guangxi Province (2019GNSFGA245003, 2021GNSFBA220077, and GUIKE AD23026050).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3qi01878d

‡ These authors contributed equally to this work.

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