Hydrogen energy presents a promising solution to alleviate the energy crisis and environmental pollution owing to its zero-carbon emission and high calorific value [1,2]. Electrochemical water splitting, consisting of hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode, is an eco-friendly and efficient technology for hydrogen production [3]. OER is a four-electron transfer process that suffers from a higher energy barrier and sluggish kinetics, so it is regarded as the bottleneck step of the whole water splitting [4,5]. Therefore, finding an efficient and stable OER catalyst to reduce the anode reaction overpotential is vital to improve the efficiency of hydrogen production. So far, precious metal-based catalysts (such as RuO₂, and IrO₂) show optimum OER activity, however, their large-scale practical applications are still hindered by high cost and low storage capacity [6,7]. Consequently, the development of highly efficient non-precious metal catalysts is crucial for the future applications of OER.

To date, a great deal of non-precious metal catalysts has been reported to boost OER performance. Thereinto, transition metal hydroxides (TMHs) have attracted increasing attention because of their large specific surface area, abundant metal active sites and low cost [8-11]. Nevertheless, inferior electrical conductivity and strong aggregation tendency of TMHs lead to low catalytic activity and unideal stability in OER process [12-14]. Metal-organic frameworks (MOFs) have become promising materials in various applications due to their unique structural advantages [15,16]. In recent years, some studies have shown that TMHs derived from MOFs precursors by means of hydrolysis, chemical etching, ion-exchange, etc. can effectively restrain their agglomeration [17-21]. For instance, Chen et al. used 2D Hofmann-MOFs as self-sacrificial templates to synthesize FeNi-LDH nanosheets via a hydrolysis transformation assisted by NaBH₄, which effectually inhibited the self-stacking of LDHs, resulting in excellent OER activity [19]. Che group reported the self-transforming Co(OH)₂ nanosheet arrays from ZIF-67, which bring high exposure of active sites for OER [20]. Although the unique structure of TMHs obtained from the MOFs template endows improved OER performance, it is still necessary to take other measures to further boost their OER conductivity and reactivity.

Recently reported that integrating TMHs with other functional materials (such as MXene, graphene, and NiP_x) to construct hetero-structured composites is a promising way to solve the aforementioned problems [22-24]. This is mainly attributed to the fact that interfacial coupling can induce the redistribution of local electrons and regulate the electronic structure of atoms, thereby improving electrochemical reactivity, conductivity and stability [25,26]. Among the functional materials, MXenes are deemed as ideal substrate material to fabricate heterostructures for effectively modulating the electronic structures of target catalysts. Since the abundant surface negative terminations produced during the synthesis process, MXenes can easily anchor the functional units to form versatile hybrids via electrostatic absorption, hydrogen bonding, or covalent bonding [27-29], which is important for inhibiting MXene self-restacking and generating strong interface interactions at the molecular level. They are usually written as M_n+1X_nT_x, where M is an early transition metal, X is C and/or N, T_x represents surface functional groups such as -O, -OH and -F [30]. As the most studied material, Ti₃C₂T_x has been widely used as a substrate material to design extended heterostructures for energy storage and conversion applications [31,32]. In 2015, Anasori et al. discovered an ordered double-transition bimetallic Mo₂TiC₂T_x MXene with two layers of Mo atoms located on the outer side, while Ti atoms occupy the middle metal layer [33,34]. Unlike the conventional Ti₃C₂T_x, Mo atoms are present on the surface of Mo₂TiC₂T_x, thus the type of interface bonds can be tuned when it is combined with other components [35-38]. Moreover, related research has shown that Mo₂TiC₂T_x possesses a relatively stronger Ti-C chemical bond than that of Ti₃C₂T_x [39]; however, compared with Ti₃C₂T_x, there is relatively less research on coupling Mo₂TiC₂T_x with other functional materials to regulate the OER performance [40].

Herein, Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets were prepared using ZIF-67 as a raw material through simple topochemical transformations at room temperature. Detailed XPS and XANES analyses reveal that the interfacial coupling between Co(OH)₂ and Mo₂TiC₂T_x enables the electron transfer from Co(OH)₂ to Mo₂TiC₂T_x, thereby increasing the valence state of Co ions and the electron density around the Mo species, which contributes to the high OER reactivity. Moreover, in the process of topological structure conversion, ZIF-67 was in situ converted into Co(OH)₂ nanosheets, which strongly combined with the Mo₂TiC₂T_x nanosheets. Such hetero-nanosheets endow the composites with abundant active sites and well-developed electron transport channels as well as robust structural stability. As a result, the Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets exhibit excellent OER performances with low overpotentials of 283 mV on glass-carbon electrode, 227 mV on nickel foam at 10 mA/cm² and can be stable for 100 h in alkaline solution, holding great promise for practical applications.

As depicted in Fig. S1 (Supporting information), Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets were synthesized through a convenient room temperature topochemical transformation strategy, involving the successive preparation of Mo₂TiC₂T_x and ZIF-67/Mo₂TiC₂T_x composites. The details of the procedure are as follows: Mo₂TiC₂T_x nanosheets were first obtained by a combined method of NaF/HCl etching and liquid exfoliation. Notably, during the etching procedure, Al atoms were removed from between layers, while fluorine and oxygen-containing functional groups were adsorbed on the surface of Mo atoms. These terminated Mo₂TiC₂T_x nanosheets can easily adsorb the Co²⁺ ions in cobalt salts by the electrostatic interactions, which greatly facilitates the direct growth of ZIF-67 on Mo₂TiC₂T_x nanosheets. Subsequently, under the continued ultrasound process, the ZIF-67 in ZIF-67/Mo₂TiC₂T_x composites undergoes rapid hydrolysis and self-transforms into Co(OH)₂ nanosheets, thereby leading to the in-situ formation of tightly contacted Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets. The Co(OH)₂/Mo₂TiC₂T_x (i.e., Co(OH)₂/Mo₂TiC₂T_x-3 in the performance section) with the 40 mg Mo₂TiC₂T_x loading is discussed in the following characterizations because it shows the highest electrocatalytic activity, see below. And the inductively coupled plasma emission spectrometer (ICP-OES) reveals the content of Co, Mo and Ti in this composite is 42.22 wt%, 8.5 wt% and 2.17 wt%, respectively, so the hybrid atomic ratio of Co:Mo:Ti is approximately 8:1:0.5.

X-ray diffraction (XRD) technique was performed to study the structural information of the as-prepared samples, as shown in Fig. 1a. Compared with Mo₂TiAlC₂ precursor, the characteristic peak at 39° disappears of Mo₂TiC₂T_x and the (002) peak appears at 6.9°, indicating the successful transformation from the Mo₂TiAlC₂ phase to the Mo₂TiC₂T_x structure via NaF/HCl etching. Meanwhile, the XRD pattern of the obtained ZIF-67 precursor is in good match with the previous report [20]. Under continuous ultrasound treatment at room temperature, ZIF-67 experiences a topochemical transformation and its diffraction peaks completely disappear. Moreover, a set of diffraction peaks corresponding to α-Co(OH)₂ appear (JCPDS card No. 46–0605), which confirms that ZIF-67 is completely converted to Co(OH)₂ at room temperature. When Mo₂TiC₂T_x is added into the above system, the XRD pattern of the product shows the characteristic diffraction peaks of Co(OH)₂ and Mo₂TiC₂T_x, demonstrating the successful preparation of Co(OH)₂/Mo₂TiC₂T_x composite. Additionally, compared with pure Mo₂TiC₂T_x, the (002) peak of Co(OH)₂/Mo₂TiC₂T_x shifts towards a lower angle direction (from 6.9° to 6.3°), indicating that coupling Co(OH)₂ can increase the layer spacing of Mo₂TiC₂T_x nanosheets, thereby preventing them from stacking. The microstructure information of these samples was detected by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As displayed in Fig. S2a (Supporting information), the etched Mo₂TiC₂T_x displays a typical accordion-like structure. After ultrasonic treatment in NMP, the layered Mo₂TiC₂T_x was exfoliated into ultrathin nanosheets with a lateral size of about several hundreds of nanometers, as is confirmed by the TEM image (Fig. 1b). In the HRTEM image (Fig. 1c), the lattice fringe of 0.255 nm corresponds to the diffraction peak at 2θ = 35° of Mo₂TiC₂T_x. Furthermore, from the inserted fast Fourier transform (FFT) image, the typical hexagonal symmetry of Mo₂TiC₂T_x nanosheets can be seen. The SEM image in Fig. S2c (Supporting information) shows that Co(OH)₂ derived from rhombohedral dodecahedron ZIF-67 (Fig. S2b in Supporting information) also possesses nanosheets structure, which ensures intimate contact with Mo₂TiC₂T_x nanosheets, providing abundant active sites and facilitating electron transport channels. The SEM (Fig. S2d in Supporting information) and TEM (Fig. 1d) images confirm that the hetero-nanosheets morphology of as-obtained Co(OH)₂/Mo₂TiC₂T_x with a lateral size of several hundred nanometers, and the high transparency of TEM indicates their ultrathin feature. Moreover, the corresponding HRTEM image of the Co(OH)₂/Mo₂TiC₂T_x (Fig. 1e) demonstrates the formation of a heterogeneous interface between Co(OH)₂ and Mo₂TiC₂T_x. Wherein, the lattice fringe of 0.213 nm belongs to the (200) crystal plane of CoO, since the ultrathin Co(OH)₂ nanosheets are extremely sensitive to electron beam irradiation and can be quickly converted to CoO under electron beam [41], which is further confirmed by TEM and HRTEM images of the single Co(OH)₂ nanosheets (Figs. S3a-c in Supporting information). Meanwhile, the lattice fringe of 0.255 nm matches well with the diffraction peak at 35° of Mo₂TiC₂T_x. These two crystal planes are consistent with that of individual Co(OH)₂ and Mo₂TiC₂T_x, respectively, further proving the existence of heterogeneous interfaces in Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets. The heterostructure is believed to play an important role in boosting electron transfer and stability as well as tuning electron structure [42]. In addition, the high-angle annular dark-field (HAADF) image of Co(OH)₂/Mo₂TiC₂T_x and the corresponding element mapping images show a uniform distribution of Co, C, O, Mo and Ti (Figs. 1f and g) across the nanosheet. These results confirm the achievement of Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets.

X-ray photoelectron spectroscopy (XPS) was used to study the chemical valence state of Co(OH)₂/Mo₂TiC₂T_x and the electronic interaction between Co(OH)₂ and Mo₂TiC₂T_x. As shown in Fig. 2a, the survey XPS spectra further confirm the elemental composition of the samples. The high-resolution Co 2p spectrum of the Co(OH)₂ demonstrates the Co 2p_3/2 (781.0 eV) and Co 2p_1/2 (796.8 eV) peaks as well as two shakeup satellite peaks, respectively, indicating the valence state of Co species is +2 (Fig. 2b). Interestingly, compared with the Co(OH)₂, the Co 2p peaks of Co(OH)₂/Mo₂TiC₂T_x are shifted towards higher binding energies, implying that a decrease in local charge density of Co ions accompanied with higher valence state [32]. The Mo 3d spectrum of Mo₂TiC₂T_x (Fig. 2c) can be divided into four peaks, of which the peak located at 229.5 and 232.7 eV correspond to the Mo 3d_5/2 and 3d_3/2 levels for the Mo-C bonds, which shows that 4+ is the dominant oxidation state. Two small peaks at 232.4 and 235.7 eV belong to Mo 3d_5/2 and 3d_3/2 levels of surface Mo-O bonds (Mo⁶⁺), which can be attributed to the surface oxide-containing terminations [36,40]. Compared with Mo₂TiC₂T_x, an obvious shift of Mo-C bond to the lower binding energy is observed for the Co(OH)₂/Mo₂TiC₂T_x, showing an increase in the charge density around the Mo atoms. Moreover, in the XPS O 1s spectra (Fig. S4 in Supporting information), except for the Co-OH peak (531.5 eV) and adsorbed water peak (532.9 eV), a new peak at 530.5 eV attributed to the Co-O-Mo bonds appears in the Co(OH)₂/Mo₂TiC₂T_x, which are favourable for the electron transfer between Co(OH)₂ and Mo₂TiC₂T_x [43,44]. To go further, the interface electronic interaction between Co(OH)₂ and Mo₂TiC₂T_x in Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets was also examined by soft X-ray adsorption near-edge structure (XANES). As shown in Fig. 2d, the Co L-edge splits into lower energy L₃-edge and higher energy L₂-edge due to spin-orbit splitting. Although the peak positions of Co L-edge for Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets have almost no change by comparison with that of Co(OH)₂, their peak area is higher than that of Co(OH)₂. As is known, the normalized peak area of the sample can explain the occupied state of electrons. The larger peak area shows that there are more empty orbits with higher valence state [45]. Compared with Co(OH)₂, the Co ions in Co(OH)₂/Mo₂TiC₂T_x have more empty orbitals, indicating that higher Co ions are generated, which is consistent with the results of Co 2p XPS. High valence Co ions have been proven to be beneficial for improving OER reactivity [46,47].

To shed light on the role of interfacial coupling play in the OER, the electrocatalytic performances of the obtained Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets, pure Co(OH)₂ and Mo₂TiC₂T_x were examined using a three-electrode system in 1.0 mol/L KOH solution at room temperature. Moreover, to achieve optimal OER performance, Co(OH)₂/Mo₂TiC₂T_x composites with different amounts of Mo₂TiC₂T_x were synthesized and tested. As shown in Fig. 3, the OER performance of the sample with 0.285 mg/cm² loaded on the glass-carbon (GC) electrode was first evaluated. Fig. 3a shows the polarization curves of different samples, in which Co(OH)₂/Mo₂TiC₂T_x-3 has the best OER activity. The overpotentials that correspond to Co(OH)₂ and four Co(OH)₂/Mo₂TiC₂T_x composites at 10 mA/cm² are 323, 309, 298, 283 and 358 mV, respectively (Fig. 3b). From the above values, it can be seen that the amount of Mo₂TiC₂T_x has a significant impact on the OER activity of the composites, which may be attributed to the strength of interface coupling. The Co(OH)₂/Mo₂TiC₂T_x-3 achieves an optimized interface coupling effect. Moreover, to reveal the effect of the surface Mo atom on the OER activity of Mo₂TiC₂T_x, Co(OH)₂/Ti₃C₂T_x composites were also prepared (Fig. S5 in Supporting information) and tested. Compared with the Co(OH)₂/Mo₂TiC₂T_x-3, the Co(OH)₂/Ti₃C₂T_x has a larger Ƞ₁₀ of 334 mV, which directly proves the beneficial role of surface Mo atoms on Mo₂TiC₂T_x OER process. The corresponding Tafel slope in Fig. 3c further unravel the best OER kinetics of the Co(OH)₂/Mo₂TiC₂T_x-3 (80 mV/dec) by comparison with the Co(OH)₂/Mo₂TiC₂T_x-1 (116 mV/dec), Co(OH)₂/Mo₂TiC₂T_x-2 (95 mV/dec), Co(OH)₂/Mo₂TiC₂T_x-4 (108 mV/dec) and Co(OH)₂/Ti₃C₂T_x (93 mV/dec), which could be attributed to the moderate interfacial coupling between Mo₂TiC₂T_x and Co(OH)₂.

The charge transfer kinetics of as-prepared catalysts during the OER process were further estimated by electrochemical impedance spectroscopy (EIS). As shown in Fig. 3d, compared with other samples, the Co(OH)₂/Mo₂TiC₂T_x-3 has the smallest semicircular diameter with charge-transfer resistance of 44 Ω, suggesting its fastest charge transfer kinetics, which is consistent with the result of the Tafel slope. The double-layer capacitance (C_dl) was measured in the non-Faraday range (Fig. S6 in Supporting information) to assess the electrochemical surface area (ECSA) of the samples. As displayed in Fig. 3e, among all the catalysts, Co(OH)₂/Mo₂TiC₂T_x-3 has the highest C_dl value of 3.40 mF/cm², indicating that more active sites are exposed for OER, which contributes to the excellent OER activity. Besides, stability is an important indicator for the practical application of catalysts. Therefore, a long-term CV cycling test was carried out to assess the stability of the Co(OH)₂/Mo₂TiC₂T_x-3. As shown in Fig. 3f, slight differences between LSV curves can be observed before and after 1000 CV cycles, demonstrating the good durability of the Co(OH)₂/Mo₂TiC₂T_x-3 hybrid nanosheets.

To explore whether the performance of the Co(OH)₂/Mo₂TiC₂T_x remains unchanged when the amount of catalyst is increased by dozens of times. The obtained samples were coated on the common nickel foam (NF) collector with a high loading amount of 10 mg/cm² to further investigate the OER activity, which is conducive to the industrial application. As shown in Figs. 4a and b, Figs. S7 and S8 (Supporting information), the comparative trend for OER performance of these samples coated on NF is similar to that of the samples with GC electrode as the substrate, where Co(OH)₂/Mo₂TiC₂T_x-3 still has the best OER activity of 227 mV at 10 mA/cm², demonstrating the outstanding substrate-independent OER activity. This overpotential is also comparable to previously reported advanced OER electrocatalysts, as displayed in Table S1 (Supporting information). Moreover, the chronopotentiometry curve at 10 mA/cm² (Fig. 4c) shows no significant change in OER activity over 100 h, indicating that the Co(OH)₂/Mo₂TiC₂T_x-3 has excellent long-term OER stability. Besides, the XRD and TEM were conducted to confirm the structure and morphology of Co(OH)₂/Mo₂TiC₂T_x-3 after a long-term chronopotentiometry test (Fig. S9 in Supporting information). By the analysis of post-OER characterization, Co(OH)₂/Mo₂TiC₂T_x still presents the nanosheet morphology, while partially Co(OH)₂ is oxidized to CoOOH during OER process. To reveal the OER intrinsic activity of the obtained catalysts, density functional theory (DFT) was performed to study the free energy of OER intermediates. In the calculation, the structural models of the reconstructed CoOOH, Mo₂TiC₂T_x and CoOOH/Mo₂TiC₂T_x heterostructure were established for DFT calculations. The OER process in alkaline media involves the *, OH*, O* and OOH* intermediates, where * stands for the active sites. Moreover, the step with the largest change in Gibbs free energy (ΔG) is considered to be the rate-determining step (RDS). As displayed in Fig. 4d, the RDS of CoOOH is the formation of OOH* intermediate from the O*, with the largest free energy barrier of 1.63 eV. For Mo₂TiC₂T_x, the OOH* intermediate reacts with OH⁻ and further converts into O₂ is the RDS, with the largest free energy barrier of 2.76 eV. As for the CoOOH/Mo₂TiC₂T_x, the generation of O* from OH* is the RDS, and the free energy barrier is 1.41 eV. The reduction of RDS energy barrier reveals that the coupling of Mo₂TiC₂T_x onto the CoOOH can optimize the intermediate adsorption and provide favourable OER kinetics, thereby enhancing the OER catalytic activity.

In summary, we have successfully prepared Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets as efficient catalysts for OER via a simple MOF-derivative method at room temperature. The introduction of Mo₂TiC₂T_x effectively optimizes the electronic structures of Co(OH)₂, leading to improved reactivity and conductivity. Moreover, the hetero-nanosheets offer rich active sites and ensure structural stability. With these merits, the optimal Co(OH)₂/Mo₂TiC₂T_x hetero-nanosheets deliver excellent OER performance and good stability. Besides, the OER activity of Co(OH)₂/Mo₂TiC₂T_x is superior to that of Co(OH)₂/Ti₃C₂T_x, indicating the advantage of double transition metal carbide as a substrate material. Therefore, this work not only provides an economical and convenient strategy for designing advanced electrocatalysts for OER but also expands the breadth of study in double transition metal MXenes.

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.

This work was supported by the National Natural Science Foundation of China (Nos. 22371165, 21971143, 22209098), the Natural Science Foundation of Hubei Province (No. 2022CFB326), the 111 Project (No. D20015), ITOYMR in the Higher Education Institutions of Hubei Province (No. T201904), the Key Project Foundation of Hubei Three Gorges Laboratory (No. Z2022078) and the Opening Foundation of Hubei Three Gorges Laboratory (No. SK213002). The authors also thank beamlines MCD-A and MCD-B (Soochow Beamline for Energy Materials) of the National Synchrotron Radiation Laboratory (Hefei, China) for providing soft X-ray absorption spectrometry testing.

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109774.