Interface-driven kinetic energy barrier modulation in Fe–Mo derivative-supported surface and heterointerface engineered MXene hybrids for alkaline hydrogen evolution

Abstract

Two-dimensional transition metal (TM) derivatives on conductive substrates such as MXenes can offer a promising avenue for accelerating the sluggish electron transfer kinetics associated with the multistep hydrogen evolution reaction (HER). Herein, we design a model electrocatalyst to study the role of the heterointerface in reducing the kinetic energy barrier, E_a, for alkaline HER. A facile and scalable surface-modification strategy for Ti₃C₂T_x (MX) by hydroxyl terminal functionalization followed by hybridizing with Fe–Mo-based derivatives (FeMo) is adopted for FeMo/f-MX hybrid synthesis. This enables the FeMo to hybridize effectively on the surface of Ti₃C₂(OH)_x by forming metal–O bonds. The electrochemical measurements as well as DFT studies reveal enhanced charge transfer kinetics for hybrids, due to the modulation of the electronic structure at the interface. Moreover, the hybrid coatings of FeMo/f-MX exhibit improved HER activity with a drastic decrease in overpotential from 218 mV to 22 mV, as compared to the FeMo coating, when the temperature increases from 20 to 50 °C. Thus, a 47.64% reduction in E_a (97.92 kJ mol⁻¹) is observed for hybrid coatings via the formation of a well-engineered heterointerface through the precise selection of TMs and MXenes. This study provides insights for constructing TM-decorated MXenes to synergistically enhance the electrochemical HER performance by reducing the E_avia surface and interface engineering.

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1 Introduction

In the world of rising energy concerns, electrochemical water splitting powered by renewable energy represents a promising approach for large-scale production of hydrogen to replace the subsiding carbon fuel economy.¹ However, the Hydrogen Evolution Reaction (HER) has a huge kinetic energy barrier (E_a) and thus requires a catalyst to accelerate the otherwise sluggish reaction kinetics. Furthermore, the HER in alkaline medium exhibits a significantly reduced rate compared to that in acidic solution,² primarily attributable to the added complexity in the processes of water adsorption–dissociation kinetics in environs predominated by OH⁻ ions. Currently, noble metal nanostructures exhibit considerable promise as high-performance HER electrocatalysts.³ But their extensive adoption is significantly hindered by the scarcity of these elements and their high cost. Hence, the design of a robust and highly active noble metal free electrocatalyst with low E_a for alkaline HER is a key concern. Developing such electrocatalysts with reduced E_a enables us to perform standard electrochemical experiments and thereby outperform the HER activity in alkaline media.⁴

In this scenario, it has been established that kinetically advantageous few-layered MXenes are regarded as highly promising substrates for the fabrication of electrocatalysts for the dispersion and loading of catalytically active materials.^5,6 Unlike carbonaceous substrates, the active site electronic characteristics of MXenes can be fine-tuned across a wide range through terminal functionalization and the alteration of transition metals (M₃C₂T_x, where M represents Ti, Nb, Mo, W, and others). When compared to other MXenes, such as Nb₂CT_x and Mo_1.33CT_x, Ti₃C₂T_x offers a higher specific surface area, superior electronic conductivity, and enhanced stability in alkaline electrolytes, making it an exceptional substrate.⁷ Furthermore, MXenes can regulate the electronic structure through electron orbital hybridization with active materials, facilitating charge transfer via synergistic effects, and have recently been widely reported as effective substrates.^8,9 However, the high surface energy and unsaturated electronic configurations in MXenes encourage the active materials to preferentially adsorb at phase boundaries rather than planar surfaces, resulting in poor HER performance and durability. To overcome existing challenges and optimize active material loading, various surface modification strategies, such as electronic structure modulation, heterointerface engineering, strain engineering, and defect engineering, have been explored. Among these, interface engineering stands out by simultaneously introducing abundant active sites and tuning the electronic structure at heterojunctions, which effectively reduce the activation energy (E_a) for the HER, further boosting the catalytic efficiency. Moreover, the hybridization of TMs with MXenes creates a more defect-enriched heterointerface, which favours the charge transfer kinetics at the interface during the targeted catalytic process.

Thus, a deeper understanding of defect-driven mechanisms at the catalytic interface is essential for the rational design of next-generation MXene-based electrocatalysts. Recently, many theoretical studies demonstrated that the interface of various mixed TM derivatives on MXenes can decrease the ΔG for hydrogen adsorption during alkaline HER to a large extent as compared to the single TM derivatives. Thus, the flexible modulation of HER kinetics via interface engineering with precise selection of TM derivatives has been recognized as a potential strategy for optimizing catalyst performance.¹⁰ A combination of Fe–Mo TM derivatives has gained recognition as a suitable alternative to Pt in the overall multi-step alkaline HER.¹¹ Their diverse attributes, such as ease of heteroatom doping, tunable phase transitions, and a wide range of stoichiometric compositions, offer the advantage of optimized ΔG_ad for intermediates, as supported by numerous studies. Additionally, the proper control and functionalization of the terminal groups of MXenes could effectively hybridize the TM derivatives, and thereby enhance the catalytic sites and HER performance.^12,13 For instance, the MXene–O functionalized nanosheets offer dual advantages by enhancing the exposure of active sites and facilitating interfacial charge transfer within the hybridized catalyst.

As inspired by the above discussion, the present study aims to employ the beneficial characteristics of MXenes and Fe–Mo-based TM derivatives (FeMo) to modulate the electronic structure at the interface for efficient alkaline HER. Accordingly, the properly functionalized MXene could serve as a scaffold for effective FeMo hybridization, which could synergistically enhance the multistep alkaline HER process. The flexible electronic modulation in the local environment at the interface as well as the defect-enriched lamellar structure of Fe–Mo derivatives could enhance the d-spacing of the resulting hybrid, which might lead to a high active site density. Hence, the decoration of the hydroxyl-terminal functionalized Ti₃C₂ MXene with Fe–Mo derivatives (FeMo/f-MX) could effectively yield superior charge transfer kinetics for the targeted HER process. Benefiting from the synergistic interactions, interfacial bonding, and electronic modulation at the interface, the FeMo/f-MX hybrids could exhibit a low overpotential with high consistency in 1 M NaOH. The significance of interfacial engineering in surface-interface modified MXene hybrids via experimental as well as DFT studies in reducing the kinetic energy barrier for the HER is also proposed. Herein, the present study envisions exploring the growth of TM derivatives of Fe and Mo on MXenes as a model catalyst to investigate the impact of a heterointerface on lowering the activation energy (E_a) for the HER.

2 Experimental section

2.1 Materials and chemicals used

Mild steel, sodium hydroxide (97%, Merck), hydrochloric acid (37%, Merck), stannous chloride (98%, Spectrochem), palladium chloride (99.9%, Sigma Aldrich), nickel sulfate hexahydrate (97%, Merck), succinic acid (98%, Nice), sodium hypophosphite (98%, Spectrochem), ferrous sulphate heptahydrate (98.5%, Merck), ammonium molybdate (Sigma Aldrich, 98.5%), titanium aluminium carbide (Sigma Aldrich), hydrogen fluoride (40%, Merck), potassium hydroxide (Merck, 85%), thiourea (Merck, 99%), ethanol (Merck 99.55), ferric chloride (Sigma Aldrich, 97%), and terephthalic acid (98%, Merck).

2.2 Synthesis of FeMo/f-MX structural hybrids

2.2.1 Synthesis of a hydroxyl terminal functionalized MXene, Ti₃C₂(OH)_x. 40 mL of 40% HF was prepared and placed in a Teflon bottle in a fume hood, and 4 g of Ti₃AlC₂ (MAX) was introduced pinch-wise and dissolved in it. The atomic Al layer was removed from MAX by etching for 24 h via magnetic stirring and centrifuging at 4000 rpm, followed by the addition of deionized water. This process was repeated until the pH of the supernatant solution reached 7. Subsequently, ultrasonic exfoliation was carried out for a duration of 0.5 h, and the resulting precipitate was vacuum-dried at 60 °C to yield Ti₃C₂T_x (MX). The as-obtained powder was surface functionalized to replace F with OH moieties via stirring with an excess of 10% KOH solution for 4 h. The solution was washed with DI and centrifuged 5 times until the pH reached 7. Then, it was dried to obtain OH-terminal functionalized Ti₃C₂(OH)_x. The as-prepared hydroxy terminal functionalized MXene (MXene-OH) is hereafter referred to as f-MX.

2.2.2 Synthesis of Fe–Mo transition metal derivatives. The MoS₂ nanoflowers were synthesized via a hydrothermal method. In a typical synthesis, 4.6344 g of ammonium molybdate and 0.7992 g of thiourea were dissolved in 60 mL of DI water and stirred for 1 h. Then, the above solution was hydrothermally treated at 200 °C for 18 h in an autoclave. The suspension obtained was filtered and washed with DI water followed by ethanol. The black solid obtained was dried at 60 °C overnight to obtain MoS₂ nanoflowers. The Fe₂O₃ nanoparticles were synthesized using 1.824 g FeCl₃ and 0.7476 g terephthalic acid in 60 mL DMF. The solution was stirred for 1 h followed by hydrothermal treatment at 120 °C for 15 h. The resulting yellow solution was filtered, washed with DI, centrifuged and dried at 60 °C overnight to obtain the Fe₂O₃-MOF. This powder was annealed at 500 °C for 30 min to obtain Fe₂O₃ nanoparticles. The Fe₂O₃–MoS₂ heterostructures were synthesized via the same procedure as for MoS₂ by introducing the as-prepared Fe₂O₃ nanoparticles. The as-prepared Fe₂O₃–MoS₂ heterostructure is hereafter referred to as FeMo. The loadings of Fe₂O₃ were varied as 0.1, 0.2, 0.3 and 0.4 g in this process to obtain 1FeMo, 2FeMo, 3FeMo and 4FeMo heterostructures, respectively.

2.2.3 Synthesis of FeMo/f-MX hybrids. The as-prepared f-MX and FeMo (1 : 1 wt%) were dispersed in 100 mL DI water and underwent hydrothermal treatment at 180 °C for 24 h. The presence of OH moieties in the functionalized MXene favours the coupling between them, and the FeMo are effectively anchored on the surface as well as in between the layers of the MXene. The solution was filtered, washed with DI, centrifuged and dried at 60 °C overnight to obtain Fe₂O₃–MoS₂/MXene-OH (FeMo/f-MX). The synthesis of FeMo/f-MX hybrids is depicted in Fig. 1.

Fig. 1 Schematic representation of fabrication of FeMo/f-MX hybrid coatings.

2.3 Synthesis of FeMo/f-MX@NiFeP hybrid coatings

Mild steel was used as the substrate for the electrode preparation. Using emery paper with grits ranging from P600 to P2000, the mild steel was mechanically polished to produce a uniform surface. After that, distilled water was used to thoroughly clean the electrodes. The substrate was chemically cleaned with 5% NaOH and then with 3% HCl, which was followed by treatment with a SnCl₂/HCl solution. Then, it was subjected to a PdCl₂/HCl nucleation bath. Following the aforementioned pre-treatments and activation procedures, the substrate became suitable for electroless coatings of prepared samples. The characteristics and stability of the coating were significantly influenced by the composition and conditions of the electroless bath. Sodium hypophosphite (20 g L⁻¹), succinic acid (20 g L⁻¹), a Ni ion source (NiSO₄ – 10 g L⁻¹) and an Fe ion source (FeSO₄ – 5 g L⁻¹) were the bath components for NiFeP alloy coating.¹⁴ To synthesize FeMo/f-MX@NiFeP, FeMo@NiFeP, MoS₂@NiFeP, f-MX@NiFeP, and MX@NiFeP, an ideal amount (2g L⁻¹) of as-prepared sample powders of FeMo/f-MX, FeMo, MoS₂, f-MX, and MX, respectively, was added separately to the above NiFeP bath. After two hours of bath stirring at a temperature of 80 ± 2 °C and a pH of 8, the corresponding coatings were obtained.

2.4 Physicochemical characterization studies

The enhanced physicochemical properties of the as-prepared samples, including phase purity, composition, particle size, crystallinity, morphology, surface area, and stability, were assessed using a variety of techniques. Using a scanning electron microscopy coupled with an energy dispersive spectroscopy (SEM-EDS) analyzer (Carl Zeiss EVO 18 Research) the morphology and elemental distribution of the samples were analyzed. A Fourier transform infrared analyzer (Thermo Scientific Nicolet iS50) was used to examine the functional group configurations of the samples in the 4000–650 nm range. A LabRAM HR Evolution Raman spectrometer was employed for Raman studies of the samples at a wavelength of 532 nm. For X-ray diffraction, a Bruker D8 ADVANCE with DAVINCI design with Cu-K radiation of 1.542 Å, a voltage and tube current of 40 kV and 30 mA, respectively, and a scan rate of 0.035° s⁻¹ was utilized to investigate the phase purity, nature, and crystallographic properties of samples. A high-resolution transmission electron microscope, Tecnai G2, FEI, The Netherlands, operating at 300 kV was used for acquiring high-resolution transmission electron microscopy (HRTEM) images and selected area electron diffraction (SAED) patterns to examine the topology and nature of the samples. Using a Thermo Scientific™ ESCALAB™ Xi+, X-ray photon electron spectroscopy (XPS) studies were carried out to examine the chemical composition as well as electronic environment of samples under vacuum conditions.

2.5 Electrochemical characterization studies

In order to assess the electrochemical activity of the as-prepared model catalyst, a range of characterization techniques, including linear sweep voltammetry (LSV), cyclic voltammetry (CV), Tafel polarization studies, chronopotentiometry (CP) and electrochemical impedance spectroscopy (EIS), were performed. The electrochemical HER performance evaluation of samples was investigated in 1 M NaOH solution using a three-electrode workstation with Ag/AgCl/KCl_{(1 M)}, Pt foil and sample coatings serving as the reference, counter and working electrodes, respectively. The potential in the Ag/AgCl scale was converted to RHE using the standard equation, E_RHE = E⁰_Ag/AgCl + 0.059pH + E_Ag/AgCl. A scan rate of 5 mV s⁻¹ was used to obtain the LSV curves, and Tafel plots were derived from them under 100% iR compensation. CV plots at various scan rates from 10 mV s⁻¹ to 100 mV s⁻¹ in a non-faradaic region of potential of −0.1 to −0.2 V vs. Ag/AgCl/KCl_{(1 M)} were used for calculating the double layer capacitance (C_dl) and electrochemically active surface area (ECSA). At frequencies between 100 kHz and 100 mHz, the electrochemical impedance spectroscopic investigations were assessed to obtain the Nyquist plot. The kinetic energy barrier (E_a) of the sample catalysts was estimated using the Arrhenius equation via electrochemical measurements by varying the reaction temperature from 20 to 50 °C. The durability and stability of the sample coatings were tested using CV cycling up to 2000 cycles. Furthermore, long term stability analysis of the coating was performed using chronopotentiometric analysis at 10 mA cm⁻² current density over a period of 20 h.

2.6 Density functional theory studies

In the present study, all density functional theory (DFT) calculations were performed using the GPAW code, employing a real-space grid-based approach. The Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional within the generalized gradient approximation (GGA) was used to describe electron exchange and correlation effects. The convergence criteria for the self-consistent field (SCF) calculations were set to 1 × 10⁻⁴ eV for both the total energy and electron density. A grid spacing of 0.18 Å was used. These computational settings were found to provide a good balance between accuracy and computational efficiency for the systems investigated.

3 Results and discussion

3.1 Confirmation of formation of FeMo/f-MX hybrids

3.1.1 Evaluation of morphology of hybrids. The morphological and structural evolutions of samples were evaluated by SEM analysis, as depicted in Fig. 2. The layered morphology of both the MXene (MX) and OH-functionalized MXene (f-MX) is almost similar, which authenticates that the surface functionalization does not alter the morphology of the MXene (Fig. S1). The successful etching of the MAX precursor during MX synthesis is confirmed by the transition in morphology from a dense stacked layered structure to an expanded exfoliated layer-by-layer structure due to the complete removal of atomic Al layers. The morphological studies of MoS₂ clearly reveal 3D nanoflower structures formed by the self-assembly of several thin nano-petals¹⁵ having a diameter varying from 50 to 100 nm. The MoS₂ nanoflowers have several folds and cavities which can conduct the electrolyte and enhance the targeted electrocatalytic process. Furthermore, the edge sites in the petals of MoS₂ act as active sites for Fe₂O₃ anchoring to form FeMo heterostructures, as evident from the SEM images. The existence of Fe₂O₃ on MoS₂ nanoflowers in FeMo heterostructures is confirmed by EDS elemental mapping (Fig. S2A).

Fig. 2 SEM images of (A) f-MX, (B) MoS₂ nanoflowers, (C) FeMo heterostructure and (D) FeMo/f-MX hybrids (inset showing the image illustration of the hybrid). (E) EDS spectrum and the corresponding elemental mapping of FeMo/f-MX hybrids.

The effective anchoring of FeMo heterostructures on f-MX is evidenced from the EDS spectrum (Fig. 2E). It consists of resolved peaks of elements such as Fe, Mo, O, S, Ti and C as major constituents of FeMo/f-MX, which are located at their respective binding energies. Moreover, the absence of any other major elemental peaks indicates the purity of the as-prepared hybrid. As for the FeMo/f-MX hybrids, the hydroxyl terminals of f-MXene offer the anchoring sites for these FeMo heterostructures during the synthesis, resulting in their uniform distribution on the layers and surfaces of f-MX. The elemental composition % of respective elements in FeMo/f-MX hybrids is provided in the SI as Fig. S3. The image illustration of the as-prepared FeMo/f-MX hybrid is provided in the SI along with the SEM image (Fig. S4).

3.1.2 Evaluation of topology and enhanced d-spacing in MXene layers. The as-prepared samples were further characterized by TEM analysis to investigate their detailed morphology (Fig. 3). The synthesized MoS₂ nanosheets formed a well-organized, petal-like structure, leading to a three-dimensional flower-like morphology with prominently exposed edges, as demonstrated by the TEM and HRTEM images.¹⁶ Additionally, the discontinuities in the petals indicate that the synthesized MoS₂ nanoflowers contain defects. The rings in the SAED pattern of layered MoS₂ suggest its amorphous nature, which display interlayer distances of 0.62 nm (d₀₀₂) and 0.27 nm (d₁₀₀) and are assigned to crystal planes of 2H-MoS₂.¹⁷ The as-prepared Fe₂O₃ nanoparticles showed quasi-spherical morphology with average particle dimensions of less than 100 nm range and smooth surface regions.¹⁸ Furthermore, the HRTEM image and SAED pattern reveal the hcp and highly crystalline nature of α-Fe₂O₃. The crystal lattice fringes corresponding to Fe₂O₃ are ∼0.27 nm, which are ascribed to (104) lattice planes. Meanwhile, the sharp spots in the SAED pattern of Fe₂O₃ nanoparticles again confirm the high crystallinity with interplanar spacings of 0.37 nm, 0.27 nm and 0.25 nm, corresponding to the (012) (104) and (110) planes, respectively, attributed to α-Fe₂O₃. It is evident from the HRTEM image of FeMo heterostructures that MoS₂ layers cap α-Fe₂O₃ nanoparticles,¹⁹ indicating the close proximity and direct contact between α-Fe₂O₃ and MoS₂. This intimate interface contact could help with the rapid electron transfer kinetics during the HER. Moreover, the bright spot along with the rings in SAED patterns of FeMo suggests a polycrystalline nature, and the coupling of both reduces the overall crystallinity, which might somehow be beneficial to achieving better electronic contact between the electrode–electrolyte interfaces during targeted catalytic reactions.

Fig. 3 (A) TEM image, (B) HRTEM image, and (C) SAED patterns of MoS₂ nanoflowers. (D) TEM image, (E) HRTEM image, and (F) SAED patterns of Fe₂O₃ nanoparticles. (G) TEM image, (H) HRTEM image, and (I) SAED patterns of the FeMo heterostructure. (J) TEM image, (K) HRTEM image, and (L) SAED patterns of FeMo/f-MX hybrids.

The TEM image of FeMo/f-MX hybrids (Fig. S5) demonstrates that FeMo heterostructures are firmly anchored both on the surface as well as the interlayer spacings of f-MX layers. Thus, the surface engineered f-MX functioned effectively as a substrate for the uniform dispersion of TM derivatives and provides a 3D hierarchical morphology to FeMo/f-MX hybrids. A lattice fringe width with the expanded interlayer spacing of 1.23 nm (d₀₀₂) for FeMo/f-MX hybrids can be related to the incorporation of FeMo particles in f-MX. The SAED patterns of hybrids also confirmed the coexistence of effectively hybridized FeMo heterostructure in f-MX substrates.

3.2 Probing the heterointerface and atomic interactions in defect-enriched hybrids

3.2.1 Crystalline and phase studies of hybrids. The crystallographic phases and orientations of as-prepared powders were determined using powder X-ray Diffraction (XRD) patterns (Fig. 4A). Compared to the MAX phase, the XRD patterns of MX show a notable decrease in peak intensities of 2θ between 35° and 43° corresponding to the (104) plane, indicating the effective chemical etching of the Al layer and authenticates the successful transformation of the MAX to MX phase (Fig. S6).²⁰ The peak shift of the (002) plane from 9.5° to 9° is ascribed to a considerable expansion in the layer spacing of the MXene due to the introduction of water molecules into these spaces previously occupied by aluminium (PDF #00-052-0875).²¹ As evidenced in XRD patterns, the hydroxyl functionalization has no significant impact on the crystalline structures of MX and f-MX. However, a significant increase in the interlayer spacing from 0.98 nm to 1.23 nm during functionalization is confirmed by shift of the (002) plane to a lower 2θ of 7.2°. This could effectively favour the growth of nanoparticles within f-MX interlayers. The XRD pattern of MoS₂ reveals characteristic broad peaks at 14.3°, 32.6°, 39.3°, and 58.9°, indexed to the (002), (100), (103), and (110) planes of 2H-MoS₂, respectively (PDF #01-075-1539).²² These broad and weak peaks authenticate the amorphous nature of MoS₂ sheets. As for Fe₂O₃, a series of sharp distinct characteristic peaks at 23.8°, 32.8°, 35.3°, 40.5°, 42.1°, 49.1°, 53.7°, 57.2°, 62.1° and 63.7° corresponding to (012), (104), (110), (113), (202), (024), (116), (018), (214) and (300) planes, respectively, can be ascribed to α-Fe₂O₃ (PDF #00-033-0664).²³ The d-spacing of as-prepared MoS₂ and Fe₂O₃ was calculated from the XRD peaks corresponding to (002) and (104) planes using the Bragg's equation to be 0.62 and 0.27 nm, respectively. It is worth noticing that the diminished peak intensity corresponding to MoS₂ in FeMo might be due to the poor crystalline nature of MoS₂. However, the presence of peaks corresponding to Fe₂O₃ in FeMo verifies the effective anchoring of α-Fe₂O₃ nanoparticles on MoS₂ sheets, confirming their strong integration. The diffraction peak of MoS₂ at 14.3° exhibits a blue shift in FeMo when compared to the bare 2H-MoS₂ phase.²⁴ The characteristic d-spacing d₀₀₂ of 2H-MoS₂ shifts from 0.62 nm to 0.66 nm in FeMo heterostructures, which is indicated by the shift of diffraction peaks to lower angles. This suggests the formation of a defect-rich lamellar structure with an increased lattice spacing, which might be probably due to the lattice distortion and oxygen insertion in MoS₂ layers via Fe₂O₃ incorporation. The d-spacing measurements obtained from the XRD studies align with the TEM results, reinforcing the evidence of lattice expansion due to Fe₂O₃ incorporation. This consistent finding across both techniques confirms the structural changes within the MoS₂ lattice. The investigation into the crystalline properties of FeMo hybrids, with varying Fe₂O₃ content (Fig. S2B), revealed that 2FeMo has higher crystallinity compared to other compositions. The diffraction patterns of FeMo/f-MX reveal distinct peaks corresponding to Fe₂O₃, MoS₂ and MXenes. The less prominent peaks of 2H-MoS₂ in FeMo/f-MX indicate that MoS₂ has poor crystallinity rather than an amorphous nature. While peaks for f-MX are not observed, this absence is likely due to its lower concentration and reduced crystallinity within the hybrid. However, an enhanced interlayer spacing, d₀₀₂, of 1.30 nm was observed for FeMo/f-MX, which authenticates the hybridization of FeMo heterostructures with f-MX substrates. Moreover, the noisy XRD background originates from minor secondary phases or poorly crystalline domains, which, instead of being detrimental, likely promote heterostructured interfaces, additional active sites, and improved charge transfer, thereby enhancing HER activity. To gain deeper insights into the characteristics of the FeMo/f-MX hybrid, further investigations were conducted using IR, Raman and XPS techniques.

Fig. 4 (A) XRD patterns, (B) FTIR spectra, and (C) Raman spectra of (a) MoS₂, (b) Fe₂O₃, (c) FeMo hybrid, (d) MX, (e) f-MX and (f) FeMo/f-MX hybrids. (D) XPS survey scan spectra and (E) O 1s, (F) Mo 3d and (G) S 2p high-resolution spectra of (a) FeMo heterostructures and (b) FeMo/f-MX hybrids.

3.2.2 Bonding characteristics and evidence of the defect-enriched heterointerface in hybrids. The FTIR spectra were analyzed to detect covalent bonding in the samples (Fig. 4B). The absorption bands at 3652 cm⁻¹ and 1616 cm⁻¹ indicate tensile vibrations of the –OH groups, which confirm the hydroxyl functionalization of the MXene, indicating the formation of f-MX. While a small amount of TiO₂ may also form due to surface oxidation, which is expected during MXene synthesis and functionalization. Peaks around 468 cm⁻¹ and 568 cm⁻¹ are associated with Mo–S and Mo–O bending vibrations, respectively.²⁵ Fe₂O₃ formation was confirmed by distinct peaks at 438 cm⁻¹ and 518 cm⁻¹, indicating stretching vibrations of Fe–O bonds.²⁶ The vibrational modes present in the spectra of α-Fe₂O₃ and MoS₂ are likewise observed in the FeMo spectrum. The sulfur complexes along with the active sites in MoS₂ formed bands at 1106 cm⁻¹ and 1436 cm⁻¹, and the merging of the Mo–S band at 568 cm⁻¹ with the Fe band produced a broad peak at 538 cm⁻¹. In addition, most of the absorption peaks of FeMo were also identified in FeMo/f-MX hybrids, which strongly suggests the effective hybridization of FeMo on f-MX substrates.

The Raman spectra of as-prepared samples, as presented in Fig. 4C, provide significant insights into the defect characteristics and structural changes that occurred during the synthesis. The bare MX and f-MX samples exhibit peaks related to the O–Ti vibrational modes, specifically at 150 cm⁻¹ (B_1g) and 620 cm⁻¹ (A_1g). The surface functionalization can be evident by the slight increase in the peak intensity at 405 cm⁻¹ in f-MX compared to MX.²⁷ MoS₂ shows peaks at approximately 380 cm⁻¹ and 405 cm⁻¹, attributed to the in-plane Mo–S phonon mode (E_2g1) associated with terrace-terminated MoS₂ and the out-of-plane Mo–S phonon mode (A_1g) linked to edge-terminated MoS₂, respectively. The ratio of E_2g1 and A_1g peak intensities serves as an indicator of the degree of edge exposure in MoS₂; a lower ratio suggests a higher fraction of exposed edge sites. Notably, the FeMo and FeMo/f-MX demonstrate a significantly lower E_2g1/A_1g ratio compared to bare MoS₂, indicating enhanced edge exposure. This greater edge exposure is attributed to the formation of a defect-rich structure, resulting in an increased number of coordinatively unsaturated edge sites. These defects are advantageous for electrocatalysis, as they offer more active sites for catalytic reactions via electronic modulation in MoS₂. These results authenticate the presence of a defect-rich surface for MoS₂ in FeMo/f-MX hybrids. Therefore, the combination of an optimized electronic structure and a higher density of exposed active edge sites in defect-enriched 2H-MoS₂ is anticipated to substantially improve the catalytic kinetics of the HER. Furthermore, a shift in the A_1g peak positions and appearance of a shoulder peak to a lower wavenumber for FeMo and FeMo/f-MX were observed, and these are ascribed to the crystal disorders due to structural modifications.²⁸ The E_2g peak of bare MoS₂ appears around 384 cm⁻¹ (ref. 24), and in FeMo heterostructures this peak exhibited a slight downshift, suggesting the influence of strain or changes in the local environment of the MoS₂ lattice caused by O insertion by Fe₂O₃ incorporation. Moreover, we noted an increase in the linewidth of the E_2g mode in the composite compared to pure MoS₂. A broadening of the line width of E_2g mode in the hybrid can be attributed to an increase in disorder within the lattice due to Fe₂O₃ incorporation and the resulting defects. Together, these findings underscore the structural modifications and enhanced catalytic potential associated with the FeMo/f-MX hybrid. On the other hand, peaks at binding energies of 128 cm⁻¹, 198 cm⁻¹, 350 cm⁻¹ and 293 cm⁻¹ correspond to J₁, J₂, J₃ and E_1g of 1T phase MoS₂, respectively. The bands at 668 cm⁻¹, 821 cm⁻¹, and 995 cm⁻¹ are associated with the vibrational energy states of Mo–O bonds.²⁹ Thus, the presence of 1T-phase of MoS₂ along with 2H MoS₂ is confirmed. The mixed phase is achieved during the hydrothermal synthesis route. The temperature during the synthesis process creates sulfur vacancies, which in turn alter the electron density and result in the movement of some Mo atoms to form the 1T phase along with the 2H form. The 2H phase of MoS₂, while stable, suffers from low electrical conductivity due to its semiconducting basal plane, limiting its catalytic efficiency. However, when combined with the 1T phase, mixed 1T/2H MoS₂ improves charge separation and offers distinct catalytic sites that are not found in the individual phases, compensating for the 2H phase's limitations and boosting its catalytic potential.³⁰ It is important to highlight that the signal associated with the lattice mode in FeMo/f-MX, FeMo and pure MoS₂ appears at 147.5 cm⁻¹ with enhanced definition and sharpness, indicating that this signal is provided exclusively by MoS₂. The FeMo/f-MX hybrids display characteristic peaks of MoS₂, along with weaker signals from Fe₂O₃ and f-MX, indicating the successful formation of FeMo/f-MX hybrids. Furthermore, the Raman elemental mapping also confirms the co-existence of Fe and Mo in FeMo/f-MX hybrids (Fig. S7).

A detailed insight about the electronic structure modulation via the incorporation of FeMo heterostructures onto f-MX was obtained by XPS studies (Fig. 4D–G). The peaks corresponding to the elements (Mo, O, S, Fe, Ti and C) at their respective binding energies in the survey spectrum of the hybrid indicate their presence in FeMo/fMX hybrids. The existence of multivalent Mo species Mo⁴⁺ and Mo⁶⁺ in hybrids is confirmed. The Mo 3d fine-scan spectra display two prominent peaks corresponding to Mo 3d_5/2 (Mo⁴⁺) and Mo 3d_3/2 (Mo⁴⁺) associated with Mo–S bonds,^31,32 confirming the presence of MoS₂ within the hybrids. The deconvoluted Mo 3d spectra provide a deep insight into the phases of MoS₂; the Mo 3d_5/2 peaks are deconvoluted into two peaks at 228.79 eV and 229.78 eV, whereas the Mo 3d_3/2 peak splits at 231.91 eV and 233.24 eV. In both cases, the initial peaks at lower binding energies correspond to 1T MoS₂ and the latter ones to 2H MoS₂. Thus, the present FeMo and FeMo/f-MX possess mixed phase MoS₂, and these results are consistent with the Raman studies. The % phase contribution of 1T and 2H in each samples is provided in Fig. 4F. This clearly depicts that the incorporation of FeMo onto f-MX induces a phase shift and the contribution of the 1T phase increases, and 1T : 2H is approximately 1 : 1 in the final FeMo/f-MX hybrid as compared to the FeMo heterostructures. Thus, forming a hybrid structure with the metallic 1T phase leads to a mixed 1T/2H MoS₂ configuration that enhances charge separation and introduces new active sites. These synergistic features help overcome the limitations of the 2H phase, significantly improving the overall catalytic activity. Notably, the binding energies of the Mo 3d_3/2 and Mo 3d_5/2 peaks in the hybrid coating exhibit a negative shift compared to the previously reported values for bare MoS₂. This shift indicates a strong electronic interaction between MoS₂ and Fe₂O₃ and f-MX layers, which suggests that charge transfer occurs more readily at the heterointerface in FeMo/f-MX hybrids, facilitating efficient charge carrier mobility and thereby promoting catalytic activity. In addition to the Mo–S bonds, the XPS spectra also reveal a peak at high binding energies, around 234–236 eV, which deconvolutes into two peaks at 234.94 eV and 236.31 eV, corresponding to the +6 oxidation state of Mo 3d_3/2 binding energies of Mo–O bonds, respectively. The presence of these peaks indicates the insertion of lattice oxygen into the hybrid structure. This oxygen insertion not only alters the electronic properties but also contributes to lattice strain and the introduction of defects within the FeMo system. The lattice strain and defects can play a significant role in enhancing catalytic performance, as they provide active sites for catalytic reactions and promote the adsorption of H_ad. Since the oxygen vacancies (O_v) play a significant role in the catalytic HER, the O_v of the samples are quantified. The O 1s spectrum of FeMo/f-MX shows three deconvoluted peaks at 530.58 eV, 531.69 eV and 532.90 eV corresponding to lattice O (O_L; metal–O bonds), O-vacancies (O_v) and adsorbed oxygen (O_ad), respectively. The O_v indicate the oxygen defective surface sites, which are catalytically active. The quantitative analysis of O_v can be performed from O 1s fine spectra of the samples via the area of peak intensity, as depicted in Fig. 4E. The quantitative analysis of O_v reveals that the final FeMo/f-MX hybrids exhibit a 28.08% increase in O_v as compared to bare FeMo samples, confirming that the incorporation of f-MX layers generates additional O_v in the hybrids. These enhanced O_v promote electron reshuffling by acting as electron capture sites, thereby influencing the coordination environment and the electronic states of the surface adsorbate during the multistep HER process. The existence of sulfur vacancies (S_v) in the samples is confirmed by the S 2p fine spectra. The quantitative S_v analysis provided in Fig. 4G and S8 reveals a 51.94% increase upon incorporation of FeMo onto f-MX layers. These results are consistent with the Raman results, as discussed earlier. Hence, the existence of mixed phases as well as the defect-enriched heterointerface modulates the electronic structure of the hybrids and could effectively promote the alkaline HER.

The TG analysis (Fig. S9 and S10A) along with DT analysis (Fig. S10B) was carried out to ensure the thermal stability of the as-synthesised FeMo/f-MX hybrids. The TGA profile showed an early weight loss below 100 °C due to the elimination of adsorbed moisture on the surface of the as-prepared materials. The DTA profile also indicates an endothermic process around 60 °C, ensuring the weight loss (3.01%) by surface moisture. The TGA profile clearly reveals a gradual weight loss as the temperature increased beyond 300 °C, likely due to the thermal degradation of MoS₂, possible release of sulfur and partial reduction of Fe₂O₃ by interaction with MoS₂. This can be verified by the sudden temperature change around 320 °C and 450 °C from the DTA profiles. The TGA reveals a lesser % weight loss for FeMo/f-MX (11.06% and 17.07%) as compared to the FeMo (15.52% and 18.21%) at 320 °C and 450 °C, respectively. Thus, the bare FeMo heterostructures show low thermal stability, while the FeMo/f-MX hybrids show more profound thermal stability. The TGA profile of f-MX reveals the higher thermal stability. Thus, as for the hybrid material, the incorporation of FeMo onto the f-MX matrix demonstrated a more gradual and suppressed weight loss profile compared to bare FeMo, indicating improved thermal stability. The introduction of the f-MX substrate increases the thermal stability of FeMo heterostructures. i.e., the terminal functionalization of MX effectively hybridized with the FeMo heterostructures and eventually increased the thermal stability of the overall catalytic system.

Thus, the enhanced physicochemical characteristics achieved via the formation of a heterointerface between TM derivatives (FeMo) and MXenes are evidenced by changes in the crystallographic phase, d-spacing shifts, evolution of mixed phases, presence of defects–vacancies, and electronic structure modulations. The crystallographic phase changes are authenticated by XRD studies, morphological details as well as enhanced d-spacing that support the formation of heterointerfaces, as revealed by the TEM studies. Raman spectroscopy highlights vibrational modes associated with both MoS₂, metal oxides and MXenes and authenticates the defect-enriched MoS₂ in hybrids. XPS reveals the formation of mixed phases as well as confirmed the defects as well as O_v and S_v at the heterointerface of the hybrids. Moreover, the enhancement in thermal stability was observed via TG-DTA profiles.

3.3 Fabrication of electroless coatings

The as-prepared samples were coated on mild steel substrates via well-established NiFeP electroless coatings with an optimal Fe content. The physicochemical characteristics of FeMo/f-MX@NiFeP were explored. Fig. 5A depicts the morphology analysis, Fig. 5B shows crystalline and phase analyses, and Fig. 5C depicts the electronic state evaluations of the sample coatings. These results indicate that the FeMo/f-MX@NiFeP coating possess enhanced physicochemical characteristics as compared to its counterparts. The SEM images of FeMo/f-MX@NiFeP coatings (Fig. 5A) verify the effective incorporation of FeMo/f-MX hybrids on the NiFeP matrix. A comparison of the SEM images of bare NiFeP, FeMo@NiFeP and FeMo/f-MX@NiFeP is given in the SI as Fig. S11. The effective incorporation of FeMo/f-MX hybrids on the NiFeP matrix was evidenced by the EDS mapping (Fig. S12). The XRD patterns of FeMo/f-MX@NiFeP were compared with those of bare NiFeP (Fig. 5B), revealing that both the electroless coatings possess a common peak of the (111) fcc plane of NiP at 2θ = 43.53° (JCPDS no. 34-0501),³³ and the peak at 2θ = 65° corresponds to the substrate. The peaks corresponding to MXene, Fe₂O₃ and MoS₂ were difficult to identify from the XRD patterns of FeMo/f-MX@NiFeP coatings, probably due to the very low loading of hybrids during NiFeP coating. However, the incorporation of FeMo/f-MX hybrids onto the NiFeP matrix can be verified by the decrease in peak intensity, which might be due to the less crystalline nature of hybrids. Moreover, the confirmation of hybrids on the NiFeP matrix was clearly evidenced from both SEM-EDS as well as XPS studies.

Fig. 5 (A) SEM image of FeMo/f-MX@NiFeP coating. (B) XRD patterns of (a) FeMo/f-MX@NiFeP and (b) bare NiFeP coating. (C) XPS survey scan spectra of FeMo/f-MX@NiFeP coatings and (C1–C6) the respective elemental scans.

An in-depth electronic insight about the FeMo/f-MX@NiFeP coatings was provided by XPS studies (Fig. 5C). The effective incorporation of FeMo/f-MX hybrids on the NiFeP matrix is confirmed by the presence of peaks corresponding to the elements (Mo, S, O, Fe, Ti, C, Ni and P) at their respective binding energies in the XPS survey spectra. As discussed earlier, the Mo 3d fine-scan spectra of FeMo/f-MX@NiFeP coating also display almost the same peak – Mo 3d_3/2 peaks – as that of hybrids. However, the peaks at 231.2 eV and 233.0 eV, corresponding to the +4 oxidation state of Mo 3d_3/2 associated with Mo–S bonds, and peaks at 234.5 eV and 236 eV, corresponding to the +6 oxidation state of Mo 3d_3/2 for Mo–O bonds,³⁴ exhibit a shift compared to the FeMo/f-MX hybrids, indicating a strong electronic interaction between the hybrids and the NiFeP matrix. The Ti 2p spectra show six peaks corresponding to Ti–C, Ti(II), Ti(III), and Ti(IV)–TiO₂, confirming their successful functionalization.³⁵ Furthermore, the XPS spectra of MX and f-MX are shown in Fig. S13, which reveal the functionalization of MXene. The C 1s spectra show a distinct peak around 282 eV corresponding to Ti–C bonds, confirming the formation of f-MX. Notably, the Ti 2p spectra indicate that a minor fraction of Ti is oxidized to TiO₂, which can be ascribed to partial surface oxidation occurring during the synthesis via etching and functionalization processes. Such surface oxidation is commonly observed in MXenes, and in this case reflects both the high reactivity of the Ti sites and the effective incorporation of surface functionalities. The coexistence of Ti–C bonds, OH functional groups, and a minor amount of TiO₂ suggests that the obtained f-MX possesses a rich interfacial chemistry, which is advantageous for subsequent heterointerface formation and catalytic activity.

The conversion from MX to f-MX involves replacing terminal groups with hydroxyl (–OH) groups, enhancing hydrophilicity and enabling strong interactions with TM derivatives. The Ti 2p binding energies of the FeMo/f-MX hybrids shift to higher values compared to MX in previous studies, indicating electron transfer from f-MX to FeMo, which suggests strong electronic coupling. The formation of Ti–O bonds provides sites for semiconductor growth and contributes to the stability of the hybrid. Moreover, the NiFeP matrix in the coating can be identified from the corresponding Ni 2p, Fe 2p and P 2p peaks.^36–38 These findings collectively highlight the successful anchoring of FeMo heterostructures onto f-MX in the NiFeP matrix, which facilitates favorable electronic structure modulation. Moreover, the incorporation of multivalent TM–MX hybrids introduces abundant redox-active sites, thereby enhancing the electrocatalytic performance during the alkaline HER.

The electrochemical HER performance and the influence of heterointerface in these coatings on HER activity were investigated, and the observations and results are shown in detail in the following section.

3.4 Electrochemical characterization of hybrid coatings

3.4.1 HER kinetic studies. The electrochemical HER activity of the FeMo/f-MX@NiFeP hybrid coating is compared with those of MX@NiFeP, f-MX@NiFeP and FeMo@NiFeP coatings, and the results are depicted in Fig. 6. The LSV analysis of all the electrode coatings was conducted at a scan rate of 5 mV s⁻¹ in 1 M NaOH electrolyte (Fig. 6A). Specifically, the hierarchical FeMo/f-MX@NiFeP hybrid coatings show enhanced HER performance, with an overpotential value (η₁₀) of 78.53 mV, considerably lower than the corresponding values of MX (172.24 mV), f-MX (152.54 mV) and FeMo (131.98 mV). This clearly reveals that the combination of FeMo on f-MX at NiFeP is more effective in reducing the overpotential values. Also, the effect of –OH functionalization on the hybrids was studied via conducting a control experiment with FeMo on an unfunctionalized MXene (FeMo/MX). The η₁₀ values of FeMo heterostructures on both the unfunctionalized and functionalized MXenes were compared. The η₁₀ results obtained for FeMo/f-MX (78.53 mV) and FeMo/MX (114.59 mV) suggest that the terminal functionalization enhances the HER activity (Table S1). The slight anodic feature in the LSV curves can be attributed to the hydrogen oxidation reaction (HOR), arising from partial re-oxidation of locally evolved H₂ at the electrode surface. This minor effect, commonly observed in alkaline HER studies, does not affect the overall assessment of HER activity. Furthermore, in order to optimize the initial FeMo composition, the FeMo heterostructures with varying Fe content were also prepared, and their electrochemical HER analyses, such as LSV and the corresponding Tafel slope analysis (Fig. S14) and 2C_dl plot (Fig. S15), were performed. Among the examined FeMo coatings, 2FeMo@NiFeP exhibits optimal HER activity with low η₁₀ as compared to the 1FeMo@NiFeP, 3FeMo@NiFeP and 4FeMo@NiFeP coatings.

Fig. 6 Comparative analysis of various electrochemical techniques such as (A) linear sweep voltammograms; (B) Tafel slopes from polarization studies; (C) η₁₀ and b values; (D) 2C_dl plots; (E) C_dl and ECSA values; (F) Nyquist plots from EIS studies of (a) MX@NiFeP, (b) f-MX@NiFeP, (c) FeMo@NiFeP, and (d) FeMo/f-MX@NiFeP coatings; (G) LSV plots at different reaction temperatures; (H) the corresponding Tafel slopes and (I) Arrhenius plot of FeMo/f-MX@NiFeP coatings; (J) comparison of electrochemical performance of FeMo/f-MX@NiFeP coatings with that of recently reported catalysts.

The Tafel curves were employed to unveil the HER kinetics and mechanism of the sample coatings. The Tafel plot (Fig. 6B) was obtained from the polarization curve as a function of overpotential (η) against the logarithm of current density (log j). Tafel parameters were elucidated from the Tafel equation,³⁹η = a + b log j, where a and b denote the intercept and Tafel slope, respectively. It is noteworthy that the b value holds significance, as it reflects the extent to which an electrocatalyst promotes charge transfer kinetics. A lower Tafel slope value indicates enhanced efficiency in facilitating charge transfer processes, thus providing valuable insights into the electrocatalytic activity of the material. Among various coatings, the FeMo/f-MX@NiFeP coating possesses the lowest Tafel slope of 91.15 mV dec⁻¹ as compared to MX@NiFeP (142.23 mV dec⁻¹), f-MX@NiFeP (137.54 mV dec⁻¹), FeMo@NiFeP (130.63 mV dec⁻¹) and FeMo/MX@NiFeP coatings (116.81 mV dec⁻¹). The Tafel slope shows an inverse relationship with the charge transfer coefficient,⁴⁰α, given by the equation b = (2.303RT)/αnF. The α values for MX@NiFeP, f-MX@NiFeP, FeMo@NiFeP, FeMo/MX@NiFeP and FeMo/f-MX@NiFeP coatings are 0.415, 0.430, 0.452, 0.506 and 0.648, respectively. The higher α value of hybrid coatings authenticates superior charge transfer at the electrode interface, leading to an improved performance in the HER. A bar diagram showing the η₁₀ and b values of coatings is illustrated in Fig. 6C. According to kinetic models for the HER, the b values are expected to be about 120, 40, and 30 mV dec⁻¹ for the rate-determining step (RDS) corresponding to Volmer, Heyrovsky, and Tafel steps, respectively.³⁴ The b values obtained for the as-prepared hybrid coatings suggest that the RDS in alkaline HER corresponds to the Volmer–Heyrovsky mechanism. The low b value of the hierarchical FeMo/f-MX hybrid coating indicates that the coupling of FeMo and f-MX is kinetically beneficial to the electrocatalytic HER. Thus, the η₁₀ and b values of the FeMo/f-MX@NiFeP coating confirm the superior HER kinetics, attributed to its beneficial electronic structure modulation achieved by heterointerface formation.

In order to study the gas bubble release from the as-prepared coatings during the hydrogen evolution process, CV studies up to 2000 cycles were performed. As for the bare NiFeP coatings, after 110 cycles of CV cycling, a portion of the NiFeP coating with extensive bubble coverage delaminated due to stretching pressures caused by the detachment of numerous big bubbles. On the other hand, even after 2000 continuous CV cycles, the FeMo/f-MX hybrid coatings held strong, probably because of the release of tiny H₂ bubbles from the surface. For faster reaction times, high internal pressure and in particular small bubbles should be beneficial.⁴¹ Delamination of coatings is a challenge during the HER which intensifies as the current density becomes apparent, leading to a critical significance that proper surface modifications are necessary for practical electrocatalysts towards gas evolution. In hydrogen evolution, the formation of H₂ bubbles results from the increased concentration of dissolved hydrogen near the catalyst surface, exceeding the supersaturation threshold, as described by Henry's law and nucleation theory. The aerophobicity of the catalyst surface, shaped by the material composition and three-dimensional morphology, governs the growth and release of hydrogen bubbles.⁴² Bare NiFeP coatings produce large, adherent bubbles that increase the overpotential, block active surface sites, hinder ion transport, and eventually lead to catalyst delamination. In contrast, the hybrid structural coating, having a hierarchical structure with high surface roughness, promotes hydrogen supersaturation while reducing the critical bubble size. The hydrogen bubble coverage clearly illustrates the strong link between catalyst morphology and bubble dynamics. Hybrid coatings exhibit a smaller critical bubble radius than bare NiFeP, which increases the frequency of hydrogen bubble release and subsequently enhances the HER rate while reducing the total overpotential. During the lowest bubble coverage, there is an intact contact between the electrolyte and the surface actives sites, which accelerates H₂ bubble release. Thus, the hierarchical morphology and surface characteristics offered by the FeMo/f-MX hybrid coatings effectively lower bubble adhesion, maintaining accessibility to active sites and enabling efficient ion transport during the HER.

3.4.2 DFT studies. The heterointerface interactions and the enhanced electrochemical activity from the electronic level of the sample coatings were studied via DFT simulations. The employed simulation models were MoS₂@NiFeP, FeMo@NiFeP, f-MX@NiFeP and FeMo/f-MX@NiFeP. Fig. 7 presents the optimized atomic interactions of the coatings, and these interfacial atomic bonds are consistent with the XPS analysis results depicted in Fig. 5C. In addition, the charge density difference plot (CDD) is used to further visualize the electron redistribution of the catalytic interface. The FeMo/f-MX@NiFeP hybrids exhibit a unique interfacial charge redistribution that enhances its electrocatalytic activity towards the HER. In alkaline media, the rate of the hydrogen evolution reaction (HER) is primarily limited by the water dissociation step, Volmer reaction (H₂O → H* + OH⁻). The CDD analysis reveals a sequential electron transfer from f-MX to MoS₂ nanoflowers and subsequently to Fe₂O₃. Electron donation from metallic Ti sites in f-MX to MoS₂ enhances the conductivity and charge mobility, while MoS₂ mediates further electron transfer to Fe₂O₃ particles, partially reducing the Fe centres. The interfacial charge redistribution induces an internal electric field and interfacial electronic polarization, thereby optimizing the local electronic environment around active sites. This redistribution facilitates water activation at the Fe sites and optimizes H* adsorption on Mo sites, synergistically enhancing the HER kinetics under alkaline conditions. The increased electron density at Fe centres and modified electronic structure of MoS₂ facilitate favourable hydrogen adsorption/desorption kinetics, as evidenced by the downshift in the MoS₂ d-band centre. Moreover, f-MX serves as a conductive scaffold, ensuring efficient charge transport and minimizing energy losses during the HER. These results are also consistent with previous studies, suggesting that the addition of an MXene host layer may enhance the HER activity.⁴³ The DFT studies by Jiang et al., 2023, on the effect of MoS₂ on the –OH functionalized carbonitride MXene reveal that the Ti₃CN(OH)₂@MoS₂ hybrids have the smallest ΔG_H*, as compared to the bare MoS₂ and MXene counterparts, suggesting a favourable RDS for HER kinetics via suitable H* adsorption characteristic due to the proper electronic structure modulation and charge distribution between TM derivatives and MXenes.²⁵ Nui et al., 2022, reveal that the effective incorporation of NiCoP on the Ti₃C₂T_x MXene favours the formation of the heterointerface, decreases the ΔG_H* and dramatically decreases the E_a for the HER.³⁶ Zhao et al., 2024, suggest that the mixed TM derivatives–MX shows higher HER kinetics than the single TM derivative–MX via exhibiting a lower ΔG_H*, suggesting the faster formation and release of hydrogen from CoNi–Ti₃C₂T_x (−0.05 eV) as compared to Ni–Ti₃C₂T_x (−0.21 eV) and Co–Ti₃C₂T_x (0.47 eV).⁴⁴ These recent theoretical studies demonstrate that the surface functionalization as well as interface engineering of MXenes with TM derivatives can decrease the ΔG_H* for hydrogen adsorption close to the value 0, during HER kinetics to a large extent, as compared to the bare TM derivatives.

Fig. 7 DFT studies showing the simulation models. (A) Optimized atomic structures and (B) charge distribution images of (a) MoS₂@NiFeP, (b) FeMo@NiFeP, (c) f-MX and (d) FeMo/f-MX@NiFeP electrocatalysts. (C) Optimized electronic arrangements in various catalysts. (D) Projected density of states in FeMo@NiFeP and (E) projected density of states in FeMo/f-MX@NiFeP hybrids.

The projected densities of states (PDOSs) of respective elements in FeMo@NiFeP and FeMo/f-MX@NiFeP are shown in Fig. 7D and E, respectively. FeMo/f-MX@NiFeP shows higher occupied states near the Fermi level, indicating higher electron transfer and thus higher electrical conductivity. The d band centre of Fe 3d in the optimized system shifts towards the Fermi level as compared to the FeMo, indicating augmented conductivity and enhanced H* binding during the multistep HER process. A high d state density close to the Fermi level indicates more active sites for H⁺ adsorption. Furthermore, redistribution of electronic states of Mo 4d and S 2p orbitals with enhanced intensity near Fermi levels is observed, indicating that the incorporation of FeMo heterostructures into f-MX not only enhances the electrical conductivity but also induces the charge density redistribution of MoS₂ to activate the basal-plane catalytic activity with more active sites. The interfacial electronic coupling in the FeMo/f-MX hybrid synergistically enhances the HER performance by redistributing the charge density and modulating the active site coordination, thereby improving the intrinsic catalytic activity.

3.4.3 Evaluation of the electrochemically active surface area. Evaluating the surface-active concentration is crucial to determining how well H adsorption performed catalytically. The electrochemically active surface area (ECSA) of the sample coatings is ascertained using the C_dl method via CV plots.⁴⁵ ECSA refers to the portions of electrode surface that are engaged in targeted electrochemical reactions, making it a crucial parameter in evaluating the performance of catalysts towards the HER. A larger ECSA typically signifies a higher number of active sites, which enhance both the activity and efficiency of the electrode. The double layer capacitance C_dl is closely linked to ECSA; a higher C_dl generally reflects a greater surface area available for charge accumulation during electrochemical processes. The C_dl can be obtained from CV curves by examining the current response in the non-faradaic region, where the current correlates with the scan rate (Fig. S16). Measuring the C_dl provides insights into the density of active sites and the overall efficiency of the electrode material. The slope of the linear fitting on the 2C_dl graph (Fig. 6D) was used to calculate the C_dl. Ultimately, the ratio of C_dl to C_s allowed for the evaluation of ECSA.⁴⁶ In this study, the ECSA values for MX@NiFeP, f-MX@NiFeP, MoS₂@NiFeP, FeMo@NiFeP and FeMo/f-MX@NiFeP were 2.38 cm², 5.99 cm², 1.70 cm², 7.28 cm² and 25.06 cm², respectively, with corresponding C_dl values of 0.0953, 0.2399, 0.0681, 0.2914 and 1.0025 mF cm⁻².

These findings suggest that FeMo/f-MX@NiFeP, with the highest ECSA and C_dl, likely exhibits superior catalytic performance for the HER compared to others. These results were consistent with LSV studies and Tafel values, as discussed earlier. Surface morphology plays a significant role in influencing both the ECSA and HER activity. As for the FeMo/f-MX@NiFeP, the specific hierarchical morphology enhances the surface roughness and porosity, which increase the contact between the electrolyte–electrode interface and can improve the charge transfer and mass transport, leading to greater catalytic efficiency in HER applications. Fig. 6E highlights the results, which indicate that the hybrid coatings possess a greater number of active sites. The number of actives sites (n) in the coatings was estimated by the voltammetric charge obtained from the area of CV plots (Fig. S17), and the results are tabulated in Table 1. The intrinsic activity of the hybrid coatings towards the HER was evaluated using the turnover frequency (TOF). The details of the TOF calculations are provided in the SI. The TOF value of the coatings at a fixed potential of 125 mV, 150 mV, 175 mV and 200 mV vs. RHE during the LSV measurements (Table S2) reveals the higher intrinsic activity of FeMo/f-MX@NiFeP coatings than their counterparts. The TOF values at a fixed potential of 200 mV of MX, f-MX, FeMo, FeMo/MX and FeMo/f-MX on NiFeP were 1.91 s⁻¹, 1.92 s⁻¹, 3.57 s⁻¹, 4.07 s⁻¹, and 5.6 s⁻¹, respectively. This authenticates that the as-prepared hybrid coating not only shows a higher ECSA value but also possesses enhanced intrinsic HER activity. Thus, the surface functionalization and interfacial engineering of MXenes with –OH moieties and FeMo heterostructures impart enhanced HER kinetics.

Table 1 Electrochemical HER characteristics of as-prepared electrodes

Catalysts	LSV and Tafel calculations				CV calculations
	η 10, mV	b, mV dec^−1	j 0, mA cm^−1	α	C dl, μF cm^−2	ECSA, cm^2	n, ×10^−4 mol
MX@NiFeP	172.24	142.23	0.615	0.415	95.2	2.38	49.02
f-MX@NiFeP	152.54	137.54	0.777	0.430	239.9	5.99	64.80
FeMo@NiFeP	131.98	130.63	0.976	0.452	291.4	7.28	52.81
FeMo/f-MX@NiFeP	78.53	91.15	1.375	0.648	1002.5	25.06	75.29

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3.4.4 Electrochemical impedance measurements. To investigate the intrinsic charge transfer, electrochemical impedance spectroscopy (EIS) was utilized to analyze interface characteristics without iR drop effects. The Nyquist plot (Fig. 6F) was fitted with an equivalent circuit model (inset: Fig. 6F), having the solution resistance (R_s), two charge transfer resistances (R_CT1 and R_CT2), and capacitances (C₁ and C₂). The EIS results indicate that fast charge transfer kinetics are observed in the lower frequency range, while R_s is reflected in the higher frequency range. The semicircle in the lower frequency region of the Nyquist plot suggests enhanced electrochemical reactions, signalling increased charge transfer.⁴⁷ Additionally, the diameter of the semicircle is directly correlated with the R_CT value.⁴⁸ From the Nyquist plot of hybrid coating, the bending of the Z_real value in the lower range results in a diminished semicircle, indicating the lowest R_CT value. This demonstrates that FeMo/f-MX@NiFeP systems exhibit faster charge transfer kinetics than all other systems. This was mainly attributed to the existence of more redox active sites at the heterointerface of FeMo/f-MX, as evidenced by XPS, which increases the charge transfer kinetics and reduces the R_CT values. Moreover, the hierarchical morphology offered by FeMo/f-MX@NiFeP hybrid coatings creates more surface roughness on the catalytic surface and allows for more active sites, revealing better contact with electrolyte. This in turn increases the H adsorption during the HER, and the results are in accordance with the ECSA value. All these results clearly indicate that the FeMo/f-MX@NiFeP system possesses enhanced charge transfer and a high C_dl owing to the unique electronic structure modulation by the formation of a heterointerface with enhanced surface area and multiple redox active sites, which facilitate the higher HER. The results of electrochemical measurements obtained from LSV, Tafel and CV analyses are summarized in Table 1 and the EIS results were summarized in Table S3.

3.4.5 Confirmation of the decrease in the kinetic energy barrier via heterointerface formation. A key strategy to enhance the reaction kinetics of the targeted electrochemical HER is the increase in the reaction temperature. Typically, industrial electrolyzers for water splitting operate within a temperature range of 70 to 90 °C. Following the principles of the Arrhenius equation, LSV studies (Fig. 6G) and Tafel slope analysis (Fig. 6H) of hybrid coatings were done in 1 M NaOH electrolyte at various temperatures to determine the kinetic energy barrier (E_a) values of the systems with and without heterointerfaces. To maintain consistency and to avoid alterations in the pre-exponential factor (A) due to high temperatures, a temperature range of 20 to 50 °C (20, 25, 30, 35, 40, 45, 50 °C) was selected in the present study. For comparative studies of hybrid coating, the LSV curves, Tafel plots and Arrhenius plots of MX@NiFeP, FeMo@NiFeP and FeMo/f-MX@NiFeP are provided as Fig. 8. There are notable enhancements in the HER activity observed on the catalysts with the increase in temperature, suggesting that higher temperatures positively influence the HER kinetics, leading to improved performance of the catalysts.⁴⁹ The η₁₀ value for FeMo/f-MX@NiFeP drops from 218 to 22 mV, as the temperature shoots up from 20 to 50 °C. Under the same conditions, the η₁₀ drop for FeMo@NiFeP is from 221 mV to 34 mV. The reaction temperature has a significance influence on the current density. At the equilibrium potential, where the anodic and cathodic currents are identical, the corresponding current density is termed the exchange current density (j₀). This value can be determined by extrapolating the linear regions of the Tafel plots to the x axis at η = 0. In addition, it can be evident that the j₀ values increase with the temperature. The j₀ value at 50 °C for FeMo/f-MX@MXene is approximately 10 times greater than its value at 20 °C. This enhancement of j₀ value with an increase in temperature is higher for FeMo/f-MX@NiFeP than for the bare FeMo@NiFeP. This could be probably due to the formation of a heterointerface between MXenes and FeMo. The η₁₀ and corresponding j₀ values for FeMo/f-MX@NiFeP, FeMo@NiFeP and MX@NiFeP at temperatures ranging from 20 to 50 °C are provided in Table 2.

Fig. 8 Comparative LSV profiles at varying temperatures ranging from 20 to 50 °C for (A) MX@NiFeP, (B) FeMo@NiFeP, and (C) FeMo/f-MX@NiFeP coatings. The corresponding Tafel slopes of (D) MX@NiFeP, (E) FeMo@NiFeP, and (F) FeMo/f-MX@NiFeP coatings. (G) Arrhenius plots of (a) MX@NiFeP, (b) FeMo@NiFeP and (c) FeMo/f-MX@NiFeP coatings. (H) Schematic illustration of surface modifications and interactions that lower the kinetic energy barrier in FeMo/f-MX@NiFeP coatings, optimizing the alkaline HER mechanism.

Table 2 Tafel parameters of electrode coatings at various temperatures

Temperature, °C	MX@NiFeP		FeMo@NiFeP		FeMo/f-MX@NiFeP
	η 10, mV	j 0, mA cm^−1	η 10, mV	j 0, mA cm^−1	η 10, mV	j 0, mA cm^−1
20 °C	221	0.054	221	0.008	218	0.062
25 °C	178	0.137	192	0.015	166	0.271
30 °C	171	0.225	142	0.056	117	0.947
35 °C	151	0.408	110	0.155	82	1.892
40 °C	129	0.503	85	0.571	60	3.110
45 °C	121	0.886	57	2.171	40	4.912
50 °C	111	1.102	34	4.510	22	6.455

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The determination of the kinetic energy barrier is crucial for electrocatalyst evaluation, as a lower barrier leads to faster reaction rates and better catalytic performance. Thus, minimizing the kinetic energy barrier is vital for developing effective HER catalysts. According to the Arrhenius equation, plotting log(j₀) against 1/T (Fig. 6I) results in a linear dependence, allowing the determination of the kinetic energy barrier (E_a). The LSV, Tafel plots, and Arrhenius plots for as-prepared sample coatings are presented in Fig. 8A–G. The calculated E_a values for FeMo@NiFeP and FeMo/f-MX@NiFeP are 187.02 and 97.92 kJ mol⁻¹, respectively. This indicates a 47.64% decrease in E_a for FeMo/f-MX@NiFeP compared to FeMo@NiFeP, suggesting that the interfaces introduced by FeMo on f-MXenes significantly reduce the E_a for the alkaline HER process. For a comparative analysis, the E_a for the bare MXene was investigated and found to be 65.84 kJ mol⁻¹. This authenticates that the decrease in E_a for the optimized hybrid coating was achieved only by introducing FeMo heterostructures to the MX@NiFeP matrix, and the heterointerface enhances charge transfer during catalytic activity.

The increased d-spacing, lattice strain via oxygen insertion and defects in MoS₂via Fe₂O₃ incorporation, as evident from XRD, TEM, Raman and XPS, modify the electronic environments in the resulting FeMo heterostructures. As for the FeMo/f-MX hybrids, the surface hydroxyl functionalization provided by f-MX effectively hybridizes FeMo heterostructures, and the resulting structural as well as electronic modulations synergistically enhance the overall catalytic activity of hybrids. This is due to the formation of a heterointerface between them, which leads to localized states that increase the charge mobility and active site availability and alter the reaction pathways, ultimately lowering the kinetic energy barrier for the HER. This strong interaction at the heterointerface enhances the electronic conductivity via forming new energy levels, which provides the fastest electron conduction route across the interface. The increased interplanar spacing, existence of mixed 1T/2H MoS₂ phases, and defect-enriched structure of FeMo provide more accessible active sites, while the oxygen incorporation causes a lattice strain effect, improving the electronic conductivity and charge transfer dynamics within the hybrid. Together, these characteristics enable the hybrid to exhibit significant reduction in the kinetic energy barrier, leading to an exceptional catalytic behavior, making it highly promising for energy-related applications. The electrochemical HER performance of the prepared hybrid coatings is comparable with that of recent electrodes (Fig. 6J). A schematic illustration of the adopted surface modifications and interactions in the FeMo/f-MX hybrids is depicted in Fig. 8H.

3.4.6 Stability of hybrid coatings. Chronoamperometric (CP) and cyclic voltammetric (CV) studies were employed for evaluating coating stability.⁵⁰ The FeMo/f-MX@NiFeP coatings were exposed to CP analysis (Fig. S18) continuously for 20 hours at a current density of 10 mA cm⁻², displaying outstanding stability with only minor surface degradation. Although longer CP analysis is often employed to evaluate durability more comprehensively, the 20 h CP operation used here is sufficient to capture the essential stability characteristics, as significant performance degradation, if any, typically manifests within the first 10–15 h of continuous CP testing. The nearly steady potential response obtained and minimal surface deterioration observed during the period confirm the robustness of the hybrid coatings. The CV cycling studies up to 2000 cycles have been utilized to additionally verify the stability (Fig. S19). The stability analysis reveals only a slight decrease in activity, which could be attributed to the electroactive materials leaching out of the electrodes, the high potential for catalyst oxidation, and the difficulty in timely diffusion of the released gas molecules from the electrode surface. The SEM images of FeMo/f-MX@NiFeP coatings following HER studies (Fig. S20) revealed this slight surface disintegration. H₂ evolution at the electrode surface was the cause of the tiny pits and cracks seen in the SEM images. The stability of the prepared electrode coatings was indicated by a small amount of visible active material deterioration in the SEM images.

3.5 Comparison of the electrochemical activity of FeMo/f-MX coatings with other reported systems

The appropriate electronic structure modulation via heterointerface formation is responsible for enhanced electrochemical HER performance of the as-prepared FeMo/f-MX@NiFeP coatings. Rapid electron transfer kinetics with exceptional stability during electrochemical processes are made possible by the proper selection of TM derivatives and functionalization of MXenes which favour numerous redox sites. The local electronic state during H adsorption and desorption, as well as the hybrid's binding strength, are crucial for the HER process because they significantly improve the interfacing of FeMo and f-MX. The electrochemical HER parameters obtained for FeMo/f-MX@NiFeP closely resemble the parameters of recently reported systems and industrial electrodes, as highlighted in Fig. 6J (Table 3). The functionalization of the MXene with terminal OH moieties facilitates the effective coupling with FeMo heterostructures during the synthesis. This further stimulated the hybrid material to respond effectively in electrochemical applications. Furthermore, unlike electroplating, the well-established electroless coating technique used here will function effectively with any substrate system that has irregular, non-uniform morphologies and a larger size. Although the current coating system is similar to those reported recently, there are differences in the electrolyte, substrate, and various other reaction conditions that make it challenging to compare the exact HER performance.

Table 3 Comparison of recently reported transition metal derivatives or MXene-based HER electrocatalysts (references for R1–R21 in the comparison table are provided in the SI)

Electrocatalyst	Electrolyte	η 10, mV	b, mV dec^−1	References
NiFe-LDH/MXene/NF^R1	1 M KOH	132	114	Nano Energy, 2019
CoP/NiCoP NTs^R2	1 M KOH	133	88	Adv. Energy Mater., 2019
NiFeP–NS^R3	1 M KOH	83	97	Appl. Catal., B, 2022
Pt/TiO2@TiC/Pt-5 min^R4	1 M KOH	48	54.03	ACS Nano, 2023
Pd/K^+/MXene (PdKMX)^R5	1 M KOH	72	69	ACS Nano, 2024
MoS2/CuS/MXene^R6	0.5 M H2SO4	115	—	Inorg. Chem., 2023
Ru/C3C4/C^R7	0.1 M KOH	79	—	J. Am. Chem. Soc., 2016
Cu2−xS@Ru^R8	1 M KOH	82	48	Small, 2017
Rh/SiNW^R9	0.5 M H2SO4	85	24	Nat. Commun., 2016
Ni NP/Ni–NC^R10	1 M KOH	147	114	Energy Environ. Sci., 2019
Ni/NiP^R11	1 M KOH	130	58	Adv. Funct. Mater., 2016
CoP/NCNHP^R12	1 M KOH	115	66	J. Am. Chem. Soc., 2018
NiCoN/C nanocages^R13	1 M KOH	103	—	Adv. Mater., 2019
Co-doped β-Mo2C^R14	1 M KOH	141	62	Adv. Funct. Mater., 2020
2D-MoS2/Co(OH)2^R15	1 M KOH	128	—	Adv. Mater., 2018
Mo2CTx/Co^R16	1 N H2SO4	180	90	J. Am. Chem. Soc., 2019
Mo2CTx^R16	1 N H2SO4	230	16	J. Am. Chem. Soc., 2019
MoS2/Ti3C2Tx@C^R17	0.5 M H2SO4	135	46	Adv. Mater., 2017
NiSe2/Ti3C2Tx^R18	0.5 M H2SO4	200	23.7	Nano-Micro Lett., 2019
MoS2/Ti3C2Tx nanoroll^R19	0.5 M H2SO4	152	70	Appl. Catal., B, 2019
BP QDs/Ti3C2Tx^R20	1 M KOH	190	83	J. Mater. Chem. A, 2018
NiCoP@MXene^R21	1 M KOH	75.2	79.6	Angew. Chem., Int. Ed., 2024
FeMo/f-MX@NiFeP	1 M NaOH	78.53	91.15	The present study

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4 Conclusions

A hydroxyl functionalized MXene layer (f-MX) with Fe–Mo-based transition metal derivatives (FeMo heterostructures) was employed as a model electrocatalyst to elucidate the influence of interfaces on alkaline HER. The surface modification of MXene with hydroxyl groups facilitated the efficient stabilization of FeMo via the formation of metal–oxygen bonds in the FeMo/f-MX hybrid. The surface decorations with Fe₂O₃ incorporated defect-enriched Mo heterostructures effectively adjust both the surface characteristics and the d-spacing in f-MX and result in an enhanced ECSA value and accessibility for surface-active centres. Electronic studies demonstrate that the Fe and Mo atoms in the FeMo/f-MX hybrid modulate the local charge distribution, exhibit variable oxidation states, and enhance HER kinetics by serving as redox-active centres that interact with the terminal oxygen atoms on the MXene substrate. The FeMo/f-MX shows much better electrochemical performance (η₁₀ = 78.53 mV) than its counterpart without interfaces (MX: η₁₀ = 172.24 mV, f-MX: η₁₀ = 152.54 mV, FeMo: η₁₀ = 131.98 mV, and FeMo/MX: η₁₀ = 114.59 mV) at room temperature. The electrochemical data reveal the enhanced electron transfer in FeMo/f-MX hybrid coatings, owing to the modulation of the electronic structure at the heterojunction and reduction of the E_a for the RDS in alkaline HER. The E_a of FeMo/f-MX@NiFeP hybrid coatings, determined to be 97.92 kJ mol⁻¹, represents a 47.64% reduction compared to FeMo (187.02 kJ mol⁻¹). Additionally, the η₁₀ values of FeMo/f-MX decrease significantly from 218 to 22 mV at 10 mA cm⁻², as compared to its counterparts, when the temperature increases from 20 °C to 50 °C. Thus, both the surface and interface engineering strategies adopted in the MXene facilitate the dispersion and anchoring of FeMo heterostructures, exposing more active sites, enhancing the conductivity of the hybrid, and forming Ti–O⋯Fe/Mo bonds at the interface. The synergistic effect, oxygen induced strain effect, interfacial bonding, and electronic modulations in FeMo/f-MX optimize the surface energy for improved water dissociation, and this catalytic system offers valuable insights for the development of advanced MXene-based materials with enhanced performance for diverse energy-related applications.

Author contributions

Sarika Sasidharan – methodology, investigation, formal analysis, writing – original draft, visualization, validation, review and editing; Sneha George – methodology, formal analysis, validation, review and editing; Anoop Ajayakumar Nair – DFT studies, software; Anjana Ratheesh – software, resources; Mohammed Aysha Shafna – software, resources; Sheik Muhammadhu Aboobakar Shibli – conceptualization, methodology, supervision, review and editing. All authors have given approval to the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: additional details on electrochemical measurements; calculations of the number of active sites, turnover frequency, ECSA, and E_a; SEM images of MX and f-MX; EDS elemental composition of FeMo/f-MX hybrids; SEM-EDS images and XRD patterns of various FeMo heterostructures; TEM image of FeMo/f-MX hybrids; comparison of XRD patterns of MAX, MX and f-MX; Raman elemental mapping of Fe and Mo in hybrids; TG-DTA profiles of f-MX, FeMo and FeMo/f-MX hybrids; SEM image of various electroless coatings; EDS mapping of FeMo/f-MX coatings; comparative XPS spectra of MX and f-MX; comparative analysis of LSV curves, Tafel slopes and 2C_dl plots of various FeMo@NiFeP coatings; 2C_dl plots of MoS₂@NiFeP, FeMo@NiFeP, MX@NiFeP, f-MX@NiFeP, and FeMo/f-MX@NiFeP coatings; CV plots of various electroless coatings; cycling studies up to 2000 cycles of FeMo/f-MX coatings; long-term CP analysis of FeMo/f-MX@NiFeP coatings up to 20 h; SEM image of coatings after HER studies. See DOI: https://doi.org/10.1039/d5ta06503h.

Acknowledgements

The authors acknowledge the Centre for Renewable Energy and Materials and Department of Chemistry for providing the facilities to carry out the work. The authors are thankful to CLIF, University of Kerala; Rajiv Gandhi Centre for Biotechnology (RGCB), Akkulam; CSIR-NCL, Pune and CSIR-NIIST, Thiruvananthapuram, for providing instrumental analysis facilities. Sarika Sasidharan acknowledges the financial support from Chief Minister's Nava Kerala Post-Doctoral Fellowship Scheme (G.O.(Rt)No. 606/2023/HEDN) of Kerala State Higher Education Council (KSHEC), Government of Kerala. Anjana Ratheesh acknowledges the support from the WOSA fellowship Scheme of DST (DST/WOS-A/LS-269/2019), Govt. of India.

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