The oxygen evolution reaction (OER) is a crucial reaction in different advanced energy devices referring the renewable energy storage and conversion. Among them, the electrochemical water splitting devices are regarded as one of the most promising routes to effectively produce the high-purity hydrogen with zero-pollutions and zero emissions [1–4]. However, the intrinsic sluggish reaction kinetics and complexed 4e⁻ electron transfer during OER process required much energy to overcome the energy barriers to obtain favorable catalytic performance [5–8]. The state-of-the-art Ru/Ir-based electrocatalysts suffered from the resource shortage and fancy price, which is difficult for large-scale industrial applications [9–12]. Herein, the design on efficient OER catalytic materials with both high catalytic property and low cost is in urgent need.

The transition-metal-based (TM-based) catalysts possess abundant reserves and adjustable electronic structures, which emerge as the most potential OER catalysts for promising catalytic properties [13–15]. It has been confirmed that the robust control on geometric construction, phase structure, and electronic structure could effectively improve the OER catalytic properties. Among different morphologies in TM-based catalysts, core@shell structures feature the merits of favorable resistance to corrosion with enhanced stability, newly-formed active heterointerface with well synergistic effect, full interaction with fast reaction kinetics [16–18]. TM-based catalysts with core@shell structures are expected to exhibit improved catalytic performance.

Except for the design on geometric construction, the phase engineering is an in-depth strategy to enhance the OER performance at the micro-interfacial level. Especially for the introduction of amorphous phase [19,20]. The amorphization as a simple yet efficient methods could construct short-range atomic defects in crystalline catalysts. Compared with the pure crystalline catalysts, the amorphous structures possessed the following merits [21,22]: (1) The chemical and structural disarrangement, which endowed amorphous structures with complete activity, rather than surficial activity in crystalline counterparts; (2) The rich defects and dangling bonds, which assist to the catalytic performance enhancement; (3) Flexible structure, which helps convert the sluggish species into in-situ active phase or species. Benefitting from the advantages, the amorphization offer a potent route to achieve superior catalytic performance by circumventing the slow OER kinetics in rigid crystalline counterparts for constructing heterogeneous amorphous/crystalline catalysts [23,24].

As for the electronic structure tuning in TM-based catalysts family, S element with a high abundance could not only couple with O species to form the active intermediates for activity improvement, but also tend to generate the oxysulfide layer for stability enhancement [25]. It has been confirmed that TM-based sulfides (TMSs) with the favorable adjustable self-reconstruction effect could serve as robust pre-catalysts in OER catalysis. However, the monometallic M_xS_y sulfides usually suffer from the unsatisfied stability and insufficient active sites, which resulted in the unfavorable reaction kinetics for degraded OER performance [26–28]. It has been confirmed that the integration of heterogeneous structures with different multi-metallic sulfides components could help to form the built-in field at heterointerfaces, in the meanwhile regulate the chemisorption of O intermediates species [29–31]. It is helpful to obtain well-tuned electronic structures with effective electron transfer and well-distributed electron distribution for improved OER properties [32].

In this perspective, we combined the superiorities of mixed-dimensional core@shell construction, a/c heterophase, and hierarchical sulfides with well-tuned electronic structures thus to prepare the a/c AgS@CoS catalysts by controllable vulcanization. Due to the favorable synergistic effect, abundant unsaturated metal atoms, and well-distributed electron distribution, the optimized AgS@CoS-2 catalysts exhibited remarkable OER catalytic properties in alkaline media. Specifically, the optimized AgS@CoS-2 catalysts possessed the lowest overpotential and Tafel slope than that of other AgS@CoS, Ag@LDHs, Ag NWs, and commercial RuO₂ catalysts. The AgS@CoS-2 catalysts also displayed superior catalytic durability, as well as the remarkable overall water splitting performance. This study could construct the hierarchical core@shell sulfide catalysts, as well as help to understand the relationship between electron structure and catalytic activity, which offer new thinking for the performance enhancement.

The synthetic process of AgS@CoS catalysts was shown in Scheme 1. The one-dimensional (1D) Ag NWs were prepared. Then, the ZIF-67 crystals were obtained under the existence of Ag NWs to fabricate the Ag@ZIF-67 catalysts. Soon afterwards, the Ag@ZIF-67 catalysts were converted into the Ag@Co LDHs by in-situ ion etching process. In specific, the protons oriented from the Co(NO₃)₂ in ethanol solution could etch the ZIF-67 crystals to release both Co²⁺ and Co³⁺, which could be oxidized into the Co³⁺ by the NO₃⁻ and the dissolved O species [33]. The coprecipitation could promote the nucleation and growth of Co LDHs on the surface of Ag@ZIF-67 catalysts to form the Ag@Co LDHs catalysts. Finally, the AgS@CoS catalysts were obtained by a wet-chemical vulcanization [34]. Notably, both the inner Ag NWs and outer Co LDHs have been converted into the AgS and CoS species with coarse surfaces, respectively.

The physical characterizations of AgS@CoS catalysts were shown in Fig. 1. The evolution process was shown in Figs. 1a–c. The Ag NWs were firstly obtained (Fig. 1a), which displayed a distinct 1D structure with the mean diameter of about 55 nm (Fig. S1 in Supporting information). The obtained Ag NWs possessed smooth surfaces and clear boundary, which emerged as potential template materials to guide the subsequent synthesis. After coating with ZIF-67 nanocrystals, the Ag@ZIF-67 catalysts could reserve the basic 1D structures of Ag NWs with the ZIF-67 crystals attached on them (Fig. S2 in Supporting information). After the Co etching, the Ag@Co LDHs catalysts were obtained (Fig. 1b). The Ag NWs were wrapped by ultrathin Co LDHs nanosheets, which grew in different directions (Fig. S3 in Supporting information). After vulcanization by TAA, both the inner Ag NWs and the outer Co LDHs were converted into AgS core and CoS shells under the role of S²⁻ from TAA (Fig. 1c). More figures could be seen in Fig. S4 (Supporting information) as well. The original Ag NWs were destroyed and reconstructed into the AgS species owing the low K_sp(AgS) value. Meanwhile, the CoS shells featured the rough surface, which still wrapped up the inner AgS to generate the hierarchical AgS@CoS core@shell catalysts (Fig. 1d). In specific, the uneven surfaces of catalysts could add the interaction areas for faster mass transmission and reactant diffusion, which could boost the catalytic reactions for higher catalytic activity [35–37]. For controlling the sulfuration levels as comparison, the AgS@CoS-1 and AgS@CoS-3 catalysts were prepared by adjusting the vulcanizing time. As shown in Fig. S5 (Supporting information), both the AgS@CoS-1 and AgS@CoS-3 catalysts could retain the 1D structures. Meanwhile, the diameter was increasing as the sulfuration time adding. The massive outer Co LDHs was also converted to CoS species in AgS@CoS-3 catalysts.

HRTEM images were collected to characterize the detailed phase structures in core and shell region. As shown in Fig. 1e, the outer shell structures were mainly amorphous phase, which confirmed that sulfuration could introduce the amorphization in catalysts. The inner core regions exhibited high-density amorphous phase as well with a small amount of crystalline phase in the dotted line areas. In specific, the local amplified HRTEM images (area Ⅰ) and relative diffraction rings also confirmed the existence of amorphous phase. The magnified HRTEM images (area Ⅰ and Ⅱ) displayed two lattice fringes with different lattice spacing of 0.256 and 0.281 nm, which could be ascribed to the (311) plane in Co₉S₈ phases and (112) plane in Ag₂S phases, respectively. This amorphous/crystalline heterophase could induce rich defects and improve the structural stability, which could also adjust the reaction intermediate adsorption/desorption energy along with the S introduction for superior catalytic behaviors. This unique hierarchical core@shell construction was confirmed by the line-scan and elemental mapping analysis (Figs. 1f and g). The Ag, Co, and S elements were all detected across the blue arrow (Fig. 1f). The Ag elements mainly distributed in the core region of AgS@CoS catalysts while Co and S mainly existed in the shell region of catalysts, implying the formation of core@shell architecture as well. Similar in the mapping images referring to various elements (Fig. 1g), the distribution of three elements confirmed the conclusion derived in the results of line-scan profiles.

The physical characterizations of different catalysts were furthered conducted in Fig. 2. The EDS spectrum of AgS@CoS and Ag@Co LDHs catalysts was recorded to compare their chemical composition variation. As exhibited in Fig. 2a, the element atomic ratio of Ag, Co and S in AgS@CoS catalysts were 5.5/42.3/52.2, respectively. Meanwhile, the Ag and Co in Ag@Co LDHs were measured as 88.7/11.3. The elemental composition of Ag@ZIF-67 was also displayed in Fig. S6 (Supporting information). The results indicated that the S atoms had been successfully introduced to the former Ag@Co LDHs by wet-chemical sulfuration method. For clarify the formation process, the FT-IR spectrum of products in different process referring to the AgS@CoS, Ag@Co LDH, and Ag@ZIF-67 catalysts were recorded in Fig. 2b. For Ag@ZIF-67 catalysts, the peaks between 600 cm⁻¹ and 1500 cm⁻¹ were corresponded to the stretching and bending modes of imidazole rings, which confirmed the existence of 2-MIm species. The peaks on 1581 cm⁻¹ were attributed to the C-N stretching mode in 2-MIm. Meanwhile, the peaks on ~2950 cm⁻¹ were ascribed to the C-H stretching mode of the aromatic ring and the aliphatic chain in 2-MIm [38]. The results indicated the formation of ZIF-67 crystals. After Co etching, the outer ZIF-67 crystals had converted into the Co LDHs, which were evidenced by the FT-IR results as well. The peaks on 500–1000 cm⁻¹ became ambiguous, which is difficult to observe the sharp peaks. In addition, the peaks on 1581 and 2930 cm⁻¹ had changed compared with that in Ag@ZIF-67 crystals, which implied the collapse of ZIF-67 structures [39]. After sulfuration, the peaks on 1150 cm⁻¹ were weaken, which were corresponded to the S-O bending vibration in CoS species and confirmed the generation of metal sulfides [40]. In addition, XRD technique was conducted to analyze the crystal structure. The XRD patterns of Ag NWs, Ag@ZIF-67, Ag@Co LDHs, and AgS@CoS catalysts were shown in Fig. 2c. The Ag NWs exhibited the distinct diffraction peaks, which demonstrated the existence of crystalline phase. After the ZIF-67 coating and subsequent LDHs forming, the crystallinity decreased and the possible amorphous phase gradually generated. Finally, the AgS@CoS catalysts exhibited no obvious diffraction peaks, which confirmed the formation of high-density amorphous structures. The results were consistent with that of HRTEM characterizations.

For studying the chemical states and surrounding variation, the XPS spectra of Ag@Co LDHs, and AgS@CoS catalysts were recorded in Figs. 2d-i. In Fig. 2d, the Ag, Co and S elements could be all detected in AgS@CoS catalysts compared with the Ag and Co elements in Ag@Co LDHs catalysts, which still confirmed the successful sulfuration. As shown in Fig. 2e, the Co 2p spectra displayed three group peaks. The peaks at 780.7 eV and 796.7 eV were corresponded to the formation of Co²⁺ species. Meanwhile, the peaks at 782.6 eV and 798.6 eV could be ascribed to the Co³⁺ species. Compared to the peaks position of Co 2p in Ag@Co LDHs catalysts, the binding energy of Co 2p in AgS@CoS have shifted to a higher position (about 0.4 eV), which demonstrated that the electrons have transfer from the Ag@Co LDHs to AgS@CoS by the covalent bonds between S and anion ions [41]. In Fig. 2f, two peaks could be observed in both Ag@Co LDHs and AgS@CoS catalysts, which were attributed to the Ag 3d_3/2 and 3d_5/2 orbits. The peaks were related to the formation of Ag⁺ species in oxidation state in two samples. Meanwhile, the group peaks shifted slightly after sulfuration. Herein, the electron coupling and binding energy could be well tuned for favorable adsorption. Owing to the edge sites of Ag⁺ species on heterointerface, the unique local electrostatic field were simultaneously formed for high energy sites with better catalytic behaviors as well. In the S 2p spectra (Fig. 2g), the peaks on 168.7 eV could be ascribed to the sulfides species, while the peaks on 169.8 eV and 162.4 eV were related to the residual SO₄²⁻ species and unsaturated S atoms at Co-S sites.

Then, the O 1s spectra was analyzed (Fig. 2h). For Ag@Co LDHs, the three peaks were located at 532.3, 531.6, and 531 eV, respectively, which related to the hydroxyl oxygen (O_OH⁻), oxygen vacancy (O_vac), and adsorbed oxygen species (O_ads). After sulfuration, the peaks at 533.1, 532.1, and 531.4 eV could be ascribed to the O_H2O, O_OH⁻, and O_vac, respectively [42,43]. In Fig. 2i, we also analyzed the ratio of different Co species and O species. Before sulfuration, the Ag@Co LDHs could generate lots of oxygen vacancies. The appropriate O_vac level still existed in the AgS@CoS catalysts, which could effectively lower the O 2p band and Fermi energy for catalytic activity promotion. After sulfuration, the peak area ratio of Co²⁺/Co³⁺ decreased from 0.368 to 0.325, indicating that the higher metal ions increase due to electron transfer. The increasement of high-valence ions could enhance the chemisorption of OH⁻ for faster reaction kinetics [44].

The electrocatalytic OER performance of AgS@CoS-2, Ag@Co LDHs, Ag@ZIF-67, CoS, Co LDHs, ZIF-67, and RuO₂ catalysts were investigated in Fig. 3. For comparison, the ZIF-67, Co LDHs and CoS counterparts were prepared. The structures and elemental contents of those catalysts were investigated in Figs. S7–S10 (Supporting information). We firstly recorded CV curves of those catalysts and the C_dl obtained by fitting were displayed in Figs. S11 and S12 (Supporting information). The C_dl of AgS@CoS catalysts was 12.4 mF/cm², which was higher than other tested catalysts. The high C_dl value indicated that the high active surface area for potentially superior performance. Then, the LSV curves were recorded to illustrate the OER activity in alkaline media. The AgS@CoS catalysts displayed the lowest onset potential than other catalysts (Fig. S13 in Supporting information), implying the active reaction kinetics. As observed in Fig. 3a, the AgS@CoS catalysts showed a low overpotential of 260 mV at 10 mA/cm², which is lower than Ag@Co LDHs (296 mV), Ag@ZIF-67 (394 mV), CoS (381 mV), Co LDHs (414 mV), ZIF-67 (475 mV), and RuO₂ (430 mV) catalysts (Fig. 3b). The AgS@CoS catalysts also possessed the lowest overpotential when the current density increased. The Tafel slopes of the tested catalysts were 64.4 mV/dec for AgS@CoS, 86.3 mV/dec for Ag@Co LDHs, 91.9 mV/dec for Ag@ZIF-67, and 106.7 mV/dec for RuO₂ (Fig. 3c). Obviously, the AgS@CoS displayed the smallest Tafel slope, which indicated the superior catalytic kinetics owing to the rich catalytic active sites and optimized synergistic effect. Besides, the EIS tests of different catalysts were conducted (Fig. 3d). The AgS@CoS catalysts exhibited the smallest radius of the semicircle than other tested catalysts, including the favorable electron transfer kinetics for improved catalytic performance. In addition, the long-term stability of AgS@CoS catalysts was evaluated. In Fig. 3e, the current density could maintain about 10 mA/cm² for about 60 h, implying the remarkable durability of AgS@CoS catalysts. For comparison, we analyzed many reported catalysts including the sulfides, amorphous materials, Co-based catalysts and noble metal catalysts and concluded their catalytic properties in Fig. 3f. The AgS@CoS catalysts still exhibited high catalytic performance than the above reported catalysts. The reasons for performance enhancement could be concluded as follows (Fig. 3g): (1) The core@shell structures could add the active areas; (2) The AgS/CoS heterointerface could offer rich phase boundary for well-tuned synergistic effect; (3) The hybrid a/c structures could enhance the reaction kinetics for faster mass and charge transfer [14,45,46].

In addition, we coupled the AgS@CoS catalysts with the commercial Pt/C catalysts to establish the two-couple system to analyze the overall water splitting (OWS) performance. As depicted in Fig. 4a, the AgS@CoS and Pt/C catalysts were utilized as anode and cathode, respectively. The electrode couples required a low cell voltage of 1.54 V to afford the current density of 10 mA/cm², respectively (Fig. 4b). The OWS catalytic activity also surpassed other reported electron couples. Furthermore, the AgS@CoS//Pt/C couples still displayed superb stability with the only negligible voltage augment is observed about 50 h consecutive electrolysis at the fixed current density of 10 mA/cm². As shown in Fig. 4c, the inserted two figures reflected the structural variation of AgS@CoS catalysts before and after OER catalysis. After OER catalysis, the morphology of AgS@CoS catalysts had slightly changed while keeping most of the morphologies, further indicating the favorable structural stability.

In addition, we estimated the variation on the elemental composition and chemical environments of the AgS@CoS catalysts before and after OER. As shown in Fig. 4d, the atomic ratio of Ag/Co/S was 6.1/76.6/17.3, the Co elemental constituents have decreased a lot compared with the original ratio of Ag/Co/S (5.5/42.3/52.2), indicating the dissolution and loss of Co species in solution and the possible formation of new phase. The chemical states of other elementals were investigated (Figs. 4e-i). After OER catalysis, the Ag, Co, S, and O elements could still be detected in the samples (Fig. 4e). The peak intensity with the Co²⁺/Co³⁺ of Co 2p in the catalysts had decreased after OER tests compared to that before OER tests, implying the generation of CoOOH phase. For the comparison of Ag 3d spectra in AgS@CoS catalysts before and after OER, the peaks sill located at about 367.8 eV and 373.8 eV, which implied the existence of Ag⁺ species. The AgS species still existed in the AgS@CoS catalysts after OER without much loss, which further confirmed that the original could be largely inherited after OER. In Fig. 4h, the intensity of S species decreased, which implied that the oxidation of sulfur species. The results were consistence with that of EDS results. For the O 1s spectrum (Fig. 4i), the peak at 530.6 eV could be ascribed to the lattice oxygen species (O_latt), which further confirmed the generation of active CoOOH species. In addition, the O_vac ratio could keep at an appropriate level. The HRTEM image of AgS@CoS catalysts after OER were obtained (Fig. S14 in Supporting information). It could be observed that some crystalline phase formed, which might be ascribed to the active CoOOH phase [47,48]. Herein, the favorable electronic properties could be attributed to these reasons: (1) The existence of AgS species ensured the structural stability without excess morphology collapse and damage. (2) The newly-formed CoOOH phase could coordinate with amorphous phase to generate the robust amorphous/crystalline hybrid phase with rich unsaturated sites and high conductivity for balanced activity and stability. (3) The stable O_vac levels assisted to provide abundant defects with favorable electronic transfer for active reaction kinetics [49–51].

In conclusion, we synthesized the hierarchical a/c AgS@CoS core@shell catalysts by using the Ag NWs and ZIF-67 as the starting materials, following the Co LDH coating and final wet-chemical vulcanization. The AgS@CoS catalysts were obtained and featured the following merits: (1) The well-defined 1D core@shell construction improved the synergistic interaction with boosted electron and mass transfer; (2) The AgS@CoS a/c heterophase provide more unsaturated sites with abundant active centers; (3) The formed oxysulfides layer after OER enhanced the structural stability with less metal leaching. Based on the rational optimizations, the as-prepared AgS@CoS-2 catalysts displayed the superior OER catalytic performance, whose overpotential is 260 mV at 10 mA/cm², 34, 132, and 168 mV lower than Ag@Co LDHs, Ag@ZIF-67, and RuO₂ catalysts. More importantly, the AgS@CoS-2 catalysts could maintain the OER durability for 60 h. The AgS@CoS-2//Pt/C electrode couples only need an ultralow cell voltage of 1.54 V to achieve the OWS current density of 10 mA/cm².

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Yangping Zhang: Conceptualization. Tianpeng Liu: Conceptualization. Jun Yu: Conceptualization. Zhengying Wu: Conceptualization. Dongqiong Wang: Conceptualization. Yukou Du: Conceptualization.

Acknowledgment

This work was supported by National Natural Science Foundation of China (Nos. 52073199, 52274304).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110275.