Coupling the power of spinel nanoparticles: dual-function metal ferrite catalysts for advanced piezo-photocatalytic and electrocatalytic water splitting

Abstract

Hydrogen production through green methods is essential for achieving a green and sustainable energy goal. However, the slow reaction kinetics of the oxygen evolution reaction (OER) and the instability of catalysts for the hydrogen evolution reaction (HER) pose significant challenges to optimizing various water-splitting technologies. Herein, we present a comparative study encompassing mono- and binary metal–organic frameworks (MOFs) and mono- and binary ferrites, where ferrites outperform their MOF counterparts. Both binary MOFs and ferrites exhibited better performance than their mono variants. TEM analysis revealed the nanoscale morphology and crystalline features of the prepared ferrites. At the same time, EDS and BET analysis confirmed the chemical composition, specific surface area, and pore volume, highlighting their structural and electronic properties. Additionally, we compared piezo-photocatalysis and electrocatalysis, providing insights into the smart integration of both technologies. Gas chromatography demonstrated the synergistic coupling of piezoelectricity and photon-excited reactive species in Zn_0.5Co_0.5Fe₂O₄. Remarkably, the HER under both sonication and light achieved 484 μmol g⁻¹ h⁻¹, significantly higher than 232 μmol g⁻¹ h⁻¹ and 288 μmol g⁻¹ h⁻¹ for sonication and light alone, respectively. Electrochemical tests confirmed an OER and HER overpotential of 167 and 205 mV, respectively, along with 94% and 73% retention in chronoamperometry, validating its role as a multifunctional water-splitting electrolyzer. This work highlights the potential of designing innovative composites for stable and efficient water-splitting systems such as piezo-photocatalysis and electrocatalysis.

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Introduction

Hydrogen has emerged as a pivotal sustainable fuel, with the potential to replace conventional fossil fuels. Its versatility as a key precursor in the synthesis of commodity chemicals, such as green ammonia, and its role in CO₂ reduction within a circular economy underscore its significance across a diverse array of sustainable technologies.^1–3

The versatility of hydrogen and its high energy density, approximately 120 MJ kg⁻¹, positions it as a crucial sustainable fuel. However, around 90% of commercial hydrogen production currently depends on the steam methane reforming process, pointing out the significant issues of CO₂ emissions, facing green credentials and long-term viability concerns. Therefore, there is an urgent need to develop green technologies to address this crucial problem in hydrogen synthesis.^4–6

To resolve CO₂ emissions and environmental protection, catalysis plays a critical role in the efficient and sustainable advancement of green technology for hydrogen production. H₂ generation via the indigenous catalytic water-splitting approach has recently gained a lot of research interest.⁷ However, the catalytic approach is yet unable to generate H₂ on a feasible commercial scale.⁸ To improve the efficiency and effectiveness of catalytic techniques, recent research is investigating viable catalytic processes, such as electrocatalysis, photocatalysis, and piezo-photocatalysis.^9–12 Usually, proton exchange membranes are employed to electrolyze water at room temperature (RT) by using acidic or alkaline solutions. Despite the small-scale efficiency of PEMs being tremendous, the commercial scalability of PEM electrolyzers is still thwarted due to the high cost of catalysts and respective membranes.^13–15 Photocatalytic processes, including photocatalysis and using photoelectrochemical cells (PECs), hold significant promise for addressing the energy crisis. However, charge recombination of photogenerated charge carriers significantly hampers photocatalytic efficiency, thereby limiting the commercial potential of these technologies.^16–18

Piezo-catalysis harnesses mechanical vibrations to stimulate catalytic reactions and is emerging as a promising technology for efficient hydrogen production.¹⁹ Despite its potential to transform the renewable energy landscape, it faces limitations related to the underwater stability of piezo materials and the presence of stationary charge carriers, in contrast to the mobile carriers found in photocatalysis.^20,21 Piezo-photocatalysis represents an innovative approach that combines mechanical stress and light irradiation to enhance catalytic activity, offering a promising pathway for efficient energy conversion and environmental remediation.^22–24 Unlike standalone photocatalysis, where photogenerated charge carriers recombine before participating in redox reactions, piezo-photocatalysis introduces an internal electric field within the catalyst material due to piezoelectric polarization. This internal field facilitates charge separation by directing electrons and holes in opposite directions, thereby improving charge transport efficiency and reducing recombination losses.^25,26 As a result, piezo-photocatalytic systems demonstrate higher quantum efficiencies and superior hydrogen production rates compared to traditional photocatalysts.

Strain-induced piezoelectricity in nanomaterials often results in poor electrical conductivity and a reduced concentration of free charge carriers, which hinders their ability to drive electrochemical reactions.^17,27 Similarly, high charge recombination in photocatalysis limits the straightforward application of these materials in water splitting. However, combining piezoelectricity with photocatalytic nanomaterials can effectively address the inherent limitations of both processes. Forced polarization from piezo materials decreases charge recombination in photocatalysts, thereby enhancing overall catalytic efficiency.^28,29 Additionally, the higher conductivity of photocatalysts, when paired with piezo materials, increases the overall charge transport on the surface and bulk of photocatalysts, thus boosting the HER and OER rates.^30–34 The internal piezoelectric field synergistically interacts with photogenerated carriers, accelerating their transfer to reactive sites. This effect has been observed to boost charge separation efficiency by 1.5x to 3x compared to conventional photocatalysis. Additionally, this mechanism enables better utilization of photogenerated carriers, reducing energy losses and improving the overall catalytic performance.³⁵ Once the charge carriers reach the catalytic surface, they participate in redox reactions. Electrons reduce protons (H⁺) to generate H₂, while holes oxidize water molecules to produce O₂. The piezoelectric effect also contributes to modulating the band structure, further enhancing photon absorption and improving charge transport dynamics at the catalyst surface.³⁶

Recent studies have demonstrated that applying sound waves at specific frequencies (20–40 kHz) introduces additional mechanical stress in piezoelectric materials, significantly enhancing charge carrier separation efficiency. This effect is attributed to the increased piezoelectric polarization, which acts synergistically with light irradiation to suppress charge recombination by up to 40%.^37,38 In ZnO-based piezo-photocatalysts, ultrasound-assisted hydrogen evolution rates have been reported to improve by a factor of 2.3x compared to conventional photocatalysis. Similarly, BaTiO₃-based composites have achieved an HER rate of 6.1 mmol g⁻¹ h⁻¹ under simultaneous piezoelectric and photocatalytic activation.^39,40 These findings highlight the potential of integrating acoustic stimulation into piezo-photocatalytic systems to achieve higher efficiency in water splitting.

A variety of materials have been used in piezo-photocatalysis ranging from classical metal oxide, nitride, and sulfide semiconductors and titanates. Their piezoelectric features emerge from their non-centrosymmetric nature including ZnO, GaN, InN, CdS, MoS₂, BaTiO₃, BiTiO₃, BiNaTiO₃, PbZrTiO₃, and any composite of these materials.^41–44 Similar types of oxides, sulfides, and nitrides have also been used in electrocatalysis and photo-electrocatalysis in the quest of finding an economical yet efficient catalyst.^45,46 Materials with borderline piezoelectric properties like ferrites such as BiFeO₃ and titanates such as BiNaTiO₃ are ideal for water-splitting applications via both piezo-photocatalytic and electrocatalytic routes.^20,47,48 Similarly, metal–organic frameworks (MOF) and covalent organic frameworks (COF) nanomaterials are ideal not only for their hydrogen evolution reaction (HER) but also for their versatility to be used in both processes like borderline piezoelectric materials.^49–51

Metal–organic frameworks (MOFs) have gained attention as advanced materials in piezo-photocatalysis due to their extended polymeric chains, which enhance the stability and longevity of metal nanoparticles by preventing underwater oxidation.⁵² Similarly, perovskite halides have seen recent advancements through the incorporation of non-metal co-catalysts, although research on co-catalysts and promoters in piezo-photocatalysis is still in its infancy.⁵³ At the same time, hybrid techniques are revolutionizing water-splitting technologies. The integration of triboelectric and piezoelectric nanogenerators with photovoltaic systems in photoelectrochemical cells eliminates the need for external bias, a significant limitation of conventional methods.^54,55 These innovations not only improve efficiency but also create opportunities for coherence among methods such as photocatalysis, piezocatalysis, piezo-photocatalysis, and photoelectrochemical processes. The combination of novel materials and hybrid techniques underscores the growing potential to address challenges in renewable energy and catalysis.^56,57

Ferrites have been extensively studied in electro-catalysis, but their potential in piezo-photocatalysis remains largely unexplored. Among ferrites, BiFe₂O₃ is the only material investigated for piezo-photocatalytic applications. Given the borderline properties of ferrites that are akin to those of metals and piezoelectrics, a broader range of ferrite materials may be well-suited for piezo-photocatalytic water splitting.^52,56,57 In electro-catalysis, the metal component of ferrites serves as the active site for the hydrogen evolution reaction (HER) while oxygen vacancies act as active sites for the oxygen evolution reaction (OER). Similarly, in metal–organic frameworks (MOFs), metal ions contribute to HER sites and organic linkers serve as active sites for the OER. Due to the bifunctional potential of both MOFs and ferrites, we have explored their water-splitting capabilities in two distinct processes.^58,59

We have conducted an in-depth study comparing two distinct material types, i.e. metal ferrites and MOFs, to investigate water splitting in both piezo-photocatalysis and electrocatalysis. Our main goal in this research is to develop the zinc–cobalt ferrite for water splitting by using piezo-photocatalysis and electrocatalysis. According to our findings in both mono and binary variants of the MOFs and ferrites, the binary variants perform better in water splitting by using piezo-photocatalysis materials in both processes than their mono counterparts.

Experimental methods

Materials

All chemicals were purchased from Sigma-Aldrich and used without additional purification such as iron nitrate nonahydrate (Fe(NO₃)₃·9H₂O), zinc nitrate tetrahydrate (Zn(NO₃)₂·4H₂O), cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, sodium hydroxide (NaOH) and ethanol. Additionally, a Philips 635 nm bulb is employed in the reaction setup.

Synthesis of piezo-photocatalysts

As shown in Fig. 1, the synthesis of Zn_0.5Co_0.5Fe₂O₄ nanoparticles is carried out using the co-precipitation method, a widely recognized technique for producing nanoparticles with controlled properties. The process begins by dissolving 16.00 grams of iron(III) nitrate nonahydrate, 1.45 grams of cobalt(II) nitrate hexahydrate, and 1.45 grams of zinc(II) nitrate tetrahydrate in 400 milliliters of deionized water to prepare a precursor solution. The reaction is conducted in a glass beaker ensuring chemical stability and preventing contamination. Sodium hydroxide solution is added dropwise to the solution while stirring vigorously at a speed maintained between 1500 and 2500 rpm until the pH reaches 13. The reaction is maintained at 80 °C for an hour. After the reaction, the nanoparticles are cooled, washed with deionized water and ethanol through centrifugation, and then sintered at 800 °C for three hours to consolidate the material and improve its properties.

Fig. 1 Schematic of the co-precipitation synthesis of ZnCoFe₂O₄ nanoparticles.

A similar co-precipitation method is employed for synthesizing CoFe₂O₄ and ZnFe₂O₄, where 2.91 grams of cobalt(II) nitrate hexahydrate and zinc(II) nitrate tetrahydrate are incorporated into the precursor solution respectively. The remainder of the process, including purification and sintering, mirrors the procedure used for Zn_0.5Co_0.5Fe₂O₄ in Fig. S1.† All three synthesized materials, CoFe₂O_4, ZnFe₂O_4, and Zn_0.5Co_0.5Fe₂O₄, are essential for further characterization and potential applications in water splitting and hydrogen production.

Catalyst performance measurements

A Sigma-Aldrich Quickfit 24/29 flask with a rubber septum was used for the gas collection system, and a 10 mL gas-tight syringe was used to extract the H₂ sample to prevent leakage. To ensure accuracy, the system was pre-tested with N₂ gas to confirm no loss before actual measurements. Photo illumination was provided using a Philips Master HPI-T Plus 400W/645 E40 neutral stable white multi-gear bulb positioned at a fixed distance from the reaction flask to maintain consistent light intensity.⁶⁰ The sonicator, along with the reaction flask, was placed inside the photo chamber maintained at 25 °C. A Shimadzu GC-2010 Pro gas chromatograph, equipped with a thermal conductivity detector, measured the gas percentages three times from 10 mL samples of the reaction mixtures to improve precision. H₂ moles were calculated using the standard molar volume formula n = V/V_m.

Characterization

The characterization of the synthesized ferrite materials involves several key techniques. X-Ray Diffraction (XRD) is used for phase identification and crystallite size calculation by analyzing the diffraction patterns of finely ground ferrite powders. Structural parameters were analyzed using Rietveld refinement of the powder diffraction patterns with the help of CASAS software, and the refined crystal structures were visualized using VESTA software. Field Emission Transmission Electron Microscopy (FETEM) and Scanning Electron Microscopy (SEM) provide detailed images of the surface morphology and particle size after the ferrite powder is dispersed in ethanol, ultrasonicated, and drop-cast onto a conductive substrate. Fourier-Transform Infrared Spectroscopy (FTIR) is used to identify functional groups and bonding characteristics.

Electrochemical analysis

For electrochemical analysis, electrodes were fabricated by mixing 2 mg of the synthesized electrocatalyst with Nafion as a binder and carbon black to enhance conductivity. The resulting slurry was uniformly applied onto nickel foam (NF), which served as the working electrode. The electrocatalytic performance of CoFe₂O₄, ZnFe₂O₄, and ZnCoFe₂O₄ electrodes was evaluated for both the HER and OER in a 6 M potassium hydroxide (KOH) aqueous electrolyte. The electrochemical measurements were conducted at room temperature using a standard three-electrode cell configuration. A platinum wire was used as the counter electrode, while an Ag/AgCl electrode served as the reference. All potentials were converted to reversible hydrogen electrode (RHE) values, with the formula E_(RHE) = E (Ag/AgCl) + 0.059 × pH. The overpotentials were appropriately adjusted for IR compensation to account for ohmic losses.^61,62

Results and discussion

Fig. 2(a, b, and c), S2 and Table 1† provide a comprehensive structural analysis of the spinel ferrite nanoparticles ZnFe₂O₄, CoFe₂O₄, and CoZnFe₂O₄. The X-ray diffraction (XRD) patterns confirm the cubic spinel structure having a space group of Fd3m,⁶³ with lattice parameters derived as a = 8.429 Å for ZnFe₂O₄, a = 8.371 Å for CoFe₂O₄, and a = 8.412 Å for CoZnFe₂O₄ as shown in Fig. 3(b). These values align with the observed shifts in peak positions, reflecting the substitution effects of Co²⁺ and Zn²⁺ ions due to their ionic radii (Co²⁺: 0.72 Å and Zn²⁺: 0.74 Å). The slight lattice contraction in CoFe₂O₄ compared to ZnFe₂O₄ is consistent with the smaller ionic radius of Co²⁺, while CoZnFe₂O₄ exhibits intermediate behavior due to partial substitution. The refinement parameters (χ²) show improved fitting quality with CoZnFe₂O₄ (2.755) compared to ZnFe₂O₄ (4.412) and CoFe₂O₄ (3.517), indicating reliable crystallographic Rietveld modeling. The goodness-of-fit parameter (R_WP) follows a similar trend, with CoZnFe₂O₄ achieving the lowest value (2.114), further confirming the accuracy of its structural refinement. The full width at half-maximum (FWHM) values increase for CoZnFe₂O₄ (0.561) compared to ZnFe₂O₄ (0.481) and CoFe₂O₄ (0.407), indicating smaller crystallite size and higher micro-strain. The Rietveld refinement data were effectively correlated with crystallite size and micro-strain, as shown in Fig. 2 and Table 1. These structural parameters influence defect density, surface area, and charge separation efficiency. Such factors are crucial for enhancing the photocatalytic activity of ferrites in hydrogen production. Therefore, crystallographic analysis directly supports the material's performance validation.

Fig. 2 (a) X-ray diffraction (XRD) patterns and schematic crystal structures of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ nanoparticles. Rietveld analysis of the samples demonstrates precise peak fitting and quantitative phase composition, which confirms high crystallinity and structural refinement. (b) The extrapolation of the calculated lattice parameter from each diffraction peak versus the Nelson-Riley function estimates the precise lattice constant of the sample, for which the function is zero. The absence of impurity peaks indicates high phase purity, while slight shifts in peak positions in ZnCoFe₂O₄ reflect lattice distortion due to Co²⁺ and Zn²⁺ substitution. (c) The schematic representations illustrate the spinel crystal lattice with tetrahedral (A-site) and octahedral (B-site) cation arrangements, showcasing structural changes induced by ionic substitution.

Table 1 Summary of lattice parameters of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ nanoparticles

	ZnFe2O4	CoFe2O4	CoZnFe2O4
Lattice parameter, a (Å)	8.429	8.371	8.412
χ ^2	4.412	3.517	2.755
R WP	3.204	2.259	2.114
FWHM	0.481	0.407	0.561
Crystallite size (nm)	3.023	4.178	3.532
Dislocation density (nm^−2)	0.109	0.057	0.080

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Fig. 3 Morphological and elemental analyses of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ nanoparticles. SEM micrographs in (a), (b), and (c) represent the surface morphology of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄, respectively, showing agglomerated spherical nanoparticles.

The crystallite sizes are calculated to be 3.023 nm for ZnFe₂O₄, 4.178 nm for CoFe₂O₄, and 3.532 nm for CoZnFe₂O₄.⁶⁴ The dislocation density, inversely proportional to crystallite size, is highest for ZnFe₂O₄ (0.109 nm⁻²), followed by CoZnFe₂O₄ (0.080 nm⁻²) and CoFe₂O₄ (0.057 nm⁻²), indicating that CoFe₂O₄ possesses the most crystalline structure with fewer defects.⁶⁵

These structural results demonstrate the impact of ionic substitution on lattice parameters, crystallite size, and dislocation density, which directly influence the structural and functional properties of the spinel ferrite nanoparticles in piezo-photocatalysis and electrochemical properties. Also, the co-precipitation method has effectively controlled the synthesis parameters with uniform crystallite size, ensuring high phase purity and tunable crystallographic properties, essential for their potential applications in magnetic, catalytic, and energy storage technologies.

The morphology and composition of ZnCoFe₂O₄, CoFe₂O_4, and ZnFe₂O₄ nanoparticles, as illustrated in Fig. 3, were investigated using FE-SEM and EDS analyses. The SEM micrographs (a), (b), and (c) reveal the presence of spherical nanoparticles with an average size in the range of 20–30 nm. These nanoparticles exhibit slight agglomeration due to their high surface energy and dipole–dipole interactions among the particles, a characteristic commonly observed in nanoscale materials.⁶⁶

Fig. 3 shows the distinct crystalline grains despite porosity and agglomeration, and the distribution of grain size difference is observable. The existence of porosity and agglomeration could result from the synthesis process, resulting in material characteristic degradation, such as density and surface area.⁶⁷ The size and morphology of the grains were affected by the substitution of Zn²⁺ and Co²⁺ ions. The replacement/substitution of cations is known to change the nucleation and grain growth kinetics, resulting in the structural integrity and behavior of the particles. The SEM analysis highlights that all three ferrite samples share a similar morphology, consistent with their spinel structure. The ZnCoFe₂O₄ sample shows slightly more pronounced aggregation, which may be attributed to the mixed-ion substitution affecting the interparticle interactions and surface energy balance. This suggests that cation substitution not only alters the crystal structure but also significantly impacts particle packing and agglomeration.⁶⁷ Corresponding histograms have been included to quantify the particle size distribution for ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄, providing a clear statistical representation and distribution profile (ESI Fig. S4†). The histograms clearly illustrate the particle size uniformity and distribution range, thereby enhancing the clarity and reliability of morphological analyses.

The EDS spectra shown in Fig. S3(e), (f), and (g)† further confirm the elemental composition of the synthesized nanoparticles. The presence of oxygen (O), iron (Fe), cobalt (Co), and zinc (Zn) peaks validates the successful incorporation of these elements into the spinel lattice.⁶⁸ The quantitative analysis of the EDS data provides the weight and atomic percentages of the elements, confirming the stoichiometric composition of ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄. For ZnCoFe₂O₄, the simultaneous presence of both Zn and Co signals the effective doping of cobalt into the zinc ferrite structure, which is consistent with the observed lattice distortion and peak shifts in the XRD data in Fig. 2. These analyses confirm the high quality and purity of the sample and the effective integration of transition metal ions into the spinel lattice, resulting in a strong foundation for piezo-photocatalysis in water splitting and electrocatalysis processes.⁶⁹

Fig. 4 shows the TEM images of the synthesized samples and the discrete morphological characteristics of all the samples. The TEM images provide a deeper insight into particle morphology and uniform dispersion. ZnFe₂O₄ exhibits a uniform distribution of the particles, while others show large and more agglomerated particles due to the magnetic effect. The particles are well dispersed and uniformly distributed with mainly spherical and irregular shapes and range in the average particle size from 20 to 25 nm. Fig. 4(b) displays slightly larger particles compared to ZnFe₂O₄, with a mix of spherical and semi-cubic morphologies. Some degrees of particle agglomeration are observed in CoFe₂O₄ due to magnetic interactions. As shown in Fig. 4(a), ZnFe₂O₄ exhibits a more uniform particle size distribution with cubic and truncated morphologies, indicating that the integration of Zn and Co into the spinel structure improves particle uniformity.

Fig. 4 FETEM images of (a) ZnCoFe₂O₄, (b) CoFe₂O₄, and (c) ZnFe₂O₄ nanoparticles. The TEM micrographs illustrate uniform particle distributions and nanoscale grain sizes ranging from 20 to 25 nm. FETEM images reveal well-defined lattice fringes with inter-planar spacings of 0.26 nm and 0.25 nm corresponding to the (311) planes, 0.21 nm corresponding to the (400) plane, and 0.30 nm corresponding to the (220) plane, confirming the spinel crystal structure of the synthesized samples.

FETEM images further confirm the high crystallinity and well-defined lattice structures of all three samples. In ZnFe₂O₄, lattice fringes with spacings of 0.26 nm and 0.21 nm correspond to the (311) and (400) planes, respectively, while an additional spacing of 0.30 nm is attributed to the (220) plane. CoFe₂O₄ displays lattice fringes with a prominent spacing of 0.25 nm for the (311) planes, along with similar spacings of 0.21 nm and 0.30 nm for the (400) and (220) planes, consistent with its spinel structure. The ZnCoFe₂O₄ sample exhibits lattice fringes with spacings of 0.26 nm and 0.25 nm for the (311) planes and 0.21 nm and 0.30 nm for the (400) and (220) planes.^70,71 These observations confirm the successful incorporation of Zn and Co into the spinel matrix without disrupting the structural integrity of the lattice.⁷² The TEM and FETEM analyses collectively demonstrate the nanoscale size, uniformity, and crystallinity of the synthesized samples, highlighting their suitability for advanced applications.⁷³ These inter-planar spacings and their associated lattice planes in ZnCoFe₂O₃ align well with the known structural parameters of pristine ZnFe₂O₄ and CoFe₂O₄ nanospinels.⁷⁴

As shown in Fig. 5, TEM-EDS analysis and elemental mapping for ZnCoFe₂O₄ nanoparticles provide a comprehensive understanding of the material's composition and elemental distribution. The grayscale TEM image shows the nanoparticle morphology, while the overlaid elemental mapping displays the spatial distribution of Zn, Co, Fe, and O. The uniform and overlapping intensities of these maps confirm the homogeneous distribution of all elements, indicating successful integration of Zn and Co into the Fe-based spinel structure. The EDS spectrum highlights characteristic peaks for O, Fe, Co, and Zn, affirming the elemental presence in the sample. Quantitative analysis reveals that Fe accounts for the highest atomic percentage (52.05%), followed by Zn (24.53%) and Co (20.84%), with O making up 2.57%. This composition aligns closely with the theoretical stoichiometry of ZnCoFe₂O₄, indicating high-purity synthesis and structural consistency. The low oxygen content further suggests minimal surface oxidation, supporting the formation of a stable spinel phase. The TEM-EDS and elemental mapping data for CoFe₂O₄ and ZnFe₂O₄ are provided in the ESI file for reference in Fig. 5S(b and c)†.

Fig. 5 (a): FETEM-EDS and elemental mapping data for ZnCoFe₂O₄ nanoparticles. The FETEM-EDS elemental mapping confirms the uniform spatial distribution of Zn, Co, Fe, and O across the nanoparticle cluster.

Additionally, UV-vis analysis was performed and Tauc plots were obtained to determine the band gap of the ZnCoFe₂O₄ composite, which was found to be 2.4 eV in Fig. 6(a), which aligns with the existing literature.^75,76Fig. 6(b) shows the obtained FTIR spectra of the ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄ nanoparticle ferrite samples, revealing three primary peaks that provide insights into the vibrational behavior of chemical bonds within the ferrites. First, a notable peak positioned at approximately 3417 cm⁻¹ present in all samples signifies the stretching vibration of O–H bonds present in water molecules.^77,78 This common peak is observed consistently across all the samples, attributed to their exposure to ambient air before measurement. The peak at 1618–1632 cm⁻¹ corresponds to the stretching vibration of the C–O bond. Additionally, peaks appearing at 1101–1113 cm⁻¹ are assigned to the water molecules retained in the prepared nanoparticles.^79,80 The characteristic peak centered at around 572 cm⁻¹ corresponds to the stretching vibration of M–O bonds at the tetrahedral sites, a key feature of spinel ferrites. This peak's frequency subtly differs among the samples, being slightly higher in ZnCoFe₂O₄ compared to CoFe₂O₄ and ZnFe₂O₄. This disparity implies enhanced Fe–O bond strength in the former samples due to the greater electronegativity of cobalt and ions, resulting in a stronger attraction of oxygen atoms.⁸¹

Fig. 6 (a) UV-vis spectra and the Tauc plot of ZnCoFe₂O₄, (b) FTIR spectra and (c) BET surface area of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ nanoparticles showing characteristic O–H stretching (∼3417 cm⁻¹), metal-oxygen vibrations (∼579 cm⁻¹), and other functional group absorptions. The spectra confirm the spinel ferrite structure and reflect cation substitution effects. (c) BET surface area of ZnCoFe₂O₄.

The similarity in spectra across all three samples confirms their spinel ferrite structure, while the slight differences in peak positions highlight the impact of cation substitution. The main difference is in the tiny spaces within the structure where metal ions sit. In ZnFe₂O₄, zinc ions exclusively occupy tetrahedral and octahedral sites, while in ZnCoFe₂O₄, a mix of zinc and cobalt ions influences the Fe–O bond dynamics. These changes in cation composition and their effect on vibrational properties demonstrate how cation substitution affects the local bonding environment and structural behavior of the nanoparticles.⁸¹

This difference in the metal ions in these spaces affects the strong bonds between iron (Fe) and oxygen (O) atoms. This, in turn, affects the shifting of the peaks we see in the FTIR measurements. That's why we see slightly higher peaks in ZnCoFe₂O₄ spectra compared to CoFe₂O₄ and ZnFe₂O₄. It's like a small bump in the harmony of the spectra.⁸² This FTIR analysis confirms the synthesis and structural integrity of the ferrites, providing valuable insights into their chemical and vibrational characteristics.

The BET surface area analysis was conducted for ZnCoFe₂O₄ using a Micromeritics Gemini V11 2390 model as shown in Fig. 6(c). The samples were preheated at 200 °C for 12 hours to ensure proper degassing. The incorporation of Zn and Co into Fe₂O₄ resulted in a significant increase in surface area of 151.5 m² g⁻¹, making the surface nearly twice as active for catalytic applications. The increased surface area enhances accessibility to active sites, thereby improving catalytic efficiency, as catalytic reactions predominantly occur at the material's surface. This increase in surface area aligns with gas chromatography data, which showed a higher hydrogen yield for ZnCoFe₂O₄, compared to the individual CoFe₂O₄ and ZnFe₂O₄ samples.⁸³ The addition of Zn and Co in Fe₂O₃ increases the surface area by 151.5 m² g⁻¹i.e. making the overall surface more active for improved catalysis. The catalysis predominantly takes place at the surface, and an increased surface area facilitates greater accessibility to active sites for catalytic reactions.

Additionally, the BJH (Barrett–Joyner–Halenda) adsorption analysis summarized in Fig. 6(g) highlights the accumulative pore area, pore volume, and average pore diameter of ZnCoFe₂O₄. These parameters further confirm that the incorporation of Zn and Co enhances the textural properties of ZnCoFe₂O₄ making it more suitable for catalytic reactions by increasing the availability and accessibility of active sites.

To evaluate the piezo-photocatalytic effects on hydrogen production, the H₂ evolution performance of the synthesized samples was investigated under both light irradiation and sonication. A 100 mL reaction mixture was prepared with distilled water (DI), incorporating 25 mg of the catalyst.

Fig. 7 provides a comprehensive analysis of the hydrogen (H₂) production performance of ZnCoFe₂O₄, CoFe₂O_4, and ZnFe₂O₄ catalysts under various conditions, highlighting their efficiency, stability, and comparative advantages. Fig. 7(a) and (b) demonstrate the H₂ production rates of the three samples, with ZnCoFe₂O₄ achieving the highest rate of 434 μmol g⁻¹ h⁻¹, compared to 412 μmol g⁻¹ h⁻¹ for CoFe₂O₄ and 268 μmol g⁻¹ h⁻¹ for ZnFe₂O₄. The accumulated H₂ yield further emphasizes the superior catalytic efficiency of Zn_0.5Co_0.5Fe₂O₄, attributed to the synergistic role of cobalt and zinc ions in the spinel structure. Fig. 7(c) investigates the individual and combined effects of piezoelectric and photocatalytic mechanisms on ZnCoFe₂O₄. Sonication alone (piezoelectric effect) produced 232 μmol g⁻¹ h⁻¹ of H₂, while light irradiation (photocatalysis) resulted in a slightly higher yield of 288 μmol g⁻¹ h⁻¹. However, when sonication and light were applied simultaneously, synergistic interaction significantly enhanced the H₂ production rate to 494 μmol g⁻¹ h⁻¹. Fig. 7(d) examines the cycling stability of ZnCoFe₂O₄ over seven cycles of one-hour durations. The catalyst maintained stable performance with a slight increase in H₂ production from 418 μmol g⁻¹ h⁻¹ in the first cycle to 460 μmol g⁻¹ h⁻¹ in the seventh, demonstrating excellent durability for prolonged use. Fig. 7(e) compares the H₂ production efficiency of ZnCoFe₂O₄ with that of other materials, including Cu-MOF, Fe-MOF, and Mn-MOF. A ZnCoFe₂O₄ outperformed these materials, showcasing its superior catalytic activity and stability in hydrogen generation. Finally, Fig. 7(f) illustrates the cumulative hydrogen yield of Zn_0.5Co_0.5Fe₂O₄ under combined piezoelectric and photocatalytic conditions. The consistent increase in H₂ yield over time confirms the catalyst's high efficiency and sustainability for hydrogen evolution. Collectively, these results underline the exceptional performance of ZnCoFe₂O₄, highlighting its potential as a durable and efficient material for sustainable hydrogen production technologies. A summary of the hydrogen production rates for all tested samples is provided in Table 2. Fig. 7(f) illustrates the cumulative hydrogen yield of ZnCoFe₂O₄ under simultaneous piezoelectric and photocatalytic conditions, showing a steady increase in H₂ production over time. This enhanced yield is attributed to the synergistic interaction between Zn²⁺ and Co²⁺ ions within the spinel lattice, which optimizes charge separation and reduces recombination, as supported by recent studies. The dual activation via light and sonication promotes band edge modulation and polarization-induced charge generation, significantly improving catalytic activity.⁸³ This sustained performance also reflects the structural stability of ZnCoFe₂O₄ over extended cycles, as demonstrated in Fig. 7(d), aligning with literature reports on spinel ferrites exhibiting superior durability in hybrid catalysis systems. ZnCoFe₂O₄ is not only conductive and photo-piezoactive but also fully capable of generating charges under induced polarization and light irridiation. ZnCoFe₂O₄ directly splits water into its constituent gases i.e. H₂ and O₂ by following a series of reactions:

ZnCoFe₂O₄ ⇌ ZnCoFe₂O₄ + h⁺ + e⁻

H₂O + h⁺ ⇌ 2H⁺ + 1/2O₂

2H⁺ + 2e⁻ ⇌ H₂

Fig. 7 (a) Best compositions for H₂ yield among ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄, highlighting ZnCoFe₂O₄ as the most efficient. (b) Accumulated H₂ amount for each composition under identical conditions. (c) Contributions of photo, piezo, and their synergistic effects to H₂ evolution, demonstrating the combined enhancement. (d) Cycling stability of the catalysts, indicating the durability of ZnCoFe₂O₄ over multiple cycles. (e) Comparative analysis of H₂ yield for ZnCoFe₂O₄versus other reported materials, showcasing its superior performance.

Table 2 The hydrogen production of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ samples

Catalysts & solvents	H2 μmol g^−1 h^−1
Cu-MOF	103
Fe-MOF	107
Mn-MOF	282.7
Cu/Mn-MOF	280.86
Fe/Mn-MOF	308
ZnFe2O4	288
CoFe2O4	412.7
ZnCoFe2O4	434.5
Piezo-catalysis	232
Photo-catalysis	288
Piezo-photocatalysis	484

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Reactions were performed using the catalytic water mixture in the dark without sonication (Fig. 8(a)), in the dark with sonication (Fig. (b)), under light without sonication (Fig. (c)), and with both light and sonication (Fig. (d)) and samples collected were subject to GC and the results are shown in Fig. 7(c). The reaction in Fig. 8(a) results in zero HER; however, their synergistic effect (Fig. 8(d)) is evident i.e. 484 μmol g⁻¹ h⁻¹ much more than that of individual catalytic processes i.e. 232 μmol g⁻¹ h⁻¹ for piezocatalysis and 288 μmol g⁻¹ h⁻¹ for photocatalysis alone. Moreover, ZnCoFe₂O₄ turned out to be more photoactive as compared to its piezoactivity. Herein, forced polarization through piezoelectricity is responsible for reduced charge recombination and aiding photocatalysis for synergistically enhanced HER activity as illustrated in Fig. 8.

Fig. 8 Schematics of piezo- photocatalysis, (a) reaction performed in the dark without sonication results in no water splitting reaction demonstrated by zero HER, (b) & (c) piezo- and photocatalytic water splitting respectively, and (d) highest HER activity as a result of the synergistic effect between sonic and photonic energies.

The electrochemical performance of the synthesized ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄ electrocatalysts, deposited onto nickel foam, was evaluated using linear sweep voltammetry (LSV). The HER and OER were analyzed under identical conditions, employing a scan rate of 20 mV s⁻¹. The potential range was set from 0 to 1.4 V and 0 to −1.4 V for OER and HER measurements respectively. The kinetic performance of electrocatalysts was evaluated by calculating the Tafel slope using the equation (η = a + b  log  j), where η represents the overpotential and b is the Tafel slope. The study employed electrochemical impedance spectroscopy (EIS) to measure long-term stability at a constant applied voltage of 0.6 V. EIS was carried out in the frequency range of 20 kHz to 0.1 Hz. The Ag/AgCl reference electrode potentials were converted to reversible hydrogen electrode (RHE) potentials.⁸⁴

Fig. 9(a and b) presents comparative OER polarization curves for ZnCoFe₂O₄, CoFe₂O₄ and ZnFe₂O₄ electrocatalysts. The Tafel slope analysis in Fig. 9(a) further underscores the superior catalytic performance of ZnCoFe₂O₄, with a Tafel slope of 167 mV dec⁻¹, indicating faster electron transfer kinetics. By comparison, CoFe₂O₄ and ZnFe₂O₄ exhibit higher Tafel slopes of 260 mV dec⁻¹ and 300 mV dec⁻¹, respectively, which are indicative of slower reaction kinetics. These results suggest that the substitution of Zn and Co into the ferrite lattice improves the catalytic efficiency of ZnCoFe₂O₄ by lowering the energy barrier for the OER process. The fluctuations in current density observed in the region from approximately 1.4 to 1.6 V vs. RHE are attributed to transient phenomena, specifically bubble nucleation and release due to rapid oxygen evolution at the electrode–electrolyte interface. At elevated potentials, rapid gas evolution (O₂ during the OER) can lead to the accumulation and coalescence of gas bubbles on the electrode surface. These bubbles can temporarily block active sites, causing transient increases and decreases in current density due to fluctuating effective surface area and mass transport limitations.^85–88 As shown in Fig. 9(b), ZnFe₂O₄ exhibited higher overpotential and lower catalytic activity compared to ZnCoFe₂O₄ and CoFe₂O₄, both of which demonstrated a reduction in activation energy. In the polarization curves, CoFe₂O₄ and ZnFe₂O₄ exhibit higher overpotential and the lowest catalytic activity, highlighting their limited effectiveness as OER catalysts. Conversely, ZnCoFe₂O₄ and CoFe₂O₄ show significantly reduced overpotential, indicating a notable reduction in activation energy. Among these, ZnCoFe₂O₄ displayed enhanced OER activity with reduced overpotential outperforming the pristine ferrites, reaching a current density of 10 mA cm⁻² at an overpotential of 415 mV. Additionally, the nickel foam substrate exhibited negligible activity for the OER, indicating that its contribution to the overall water-splitting process is minimal. The corresponding Tafel slopes, calculated from the polarization data and presented in Fig. 9(c), are 205 mV dec⁻¹ for ZnCoFe₂O₄, 228 mV dec⁻¹ for CoFe₂O₄, and 307 mV dec⁻¹ for ZnFe₂O₄. A lower Tafel slope indicates more efficient electron transfer kinetics during the HER process, further confirming the superior catalytic properties of ZnCoFe₂O₄. The reduced Tafel slope for ZnCoFe₂O₄ suggests a facilitated reaction pathway and a lower energy barrier for hydrogen production. Similarly, the hydrogen evolution reaction (HER) polarization curves, as shown in Fig. 9(d), compare the catalytic performance of ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄. Among the samples, ZnCoFe₂O₄ exhibits superior catalytic activity at lower overpotentials compared to the other ferrites. The improved performance of ZnCoFe₂O₄ is attributed to the synergistic interaction of zinc and cobalt ions within the spinel lattice. Generally, the reaction pathway for the HER in alkaline media undergoes a three-step process, and metal ions in ferrites act as an active site for the HER. Therefore, optimizing metal sites has a direct impact on HER efficiency.⁸⁹ Water splitting undergoes a series of reactions.

Fig. 9 (a) Polarization curves of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄, showing the lowest overpotential for ZnCoFe₂O₄. (b) Tafel slopes highlight its superior kinetics. (c) Polarization curves under varying conditions confirm ZnCoFe₂O₄ higher efficiency (205 mV). (d) Tafel slopes further demonstrate enhanced kinetic performance. (e and f) Chronoamperometric curve of ZnCoFe₂O₄ for 14 hours at a constant applied voltage of 0.6 V, showing 94% and 73% retention of current density, indicating excellent long-term stability and durability for HER and OER performance.

Step 1: electron-sorption and proton-discharge

H₂O + e⁻ + ZnCoFe₂O₄ ⇌ ZnCo-H_ads·Fe₂O₄ +OH⁻ (Volmer step 120 mV dec⁻¹)

Step 2: electro-desorption

ZnCo-H_ads·Fe₂O₄ + H₂O + e⁻ ⇌ ZnCoFe₂O₄ + H₂ + OH⁻ (Heyrovsky step)

Step 3: chemical desorption

ZnCo-H_ads·Fe₂O₄ + ZnCo-H_ads Fe₂O₄ ⇌ 2ZnCoFe₂O₄+ H₂ (Tafel reaction 40 mV dec⁻¹)

The improved HER activity of ZnCoFe₂O₄, supported by its lower Tafel slope, indicates that the synergistic effect of zinc and cobalt enhances the availability of active sites and accelerates charge transfer, optimizing the reaction kinetics. Studies on ZnCoFe₂O₄ have demonstrated that the interaction between Zn and Co species facilitates the formation of unsaturated coordination sites and oxygen vacancies, significantly improving reactant adsorption and catalytic efficiency.⁹⁰ Similarly, in perovskite oxides such as CaFe_1−xCo_xO₃, the substitution of Co⁴⁺ into the Fe matrix has been reported to increase oxygen evolution reaction (OER) activity due to enhanced electronic interactions between Fe⁴⁺ and Co⁴⁺ ions.⁹¹ These findings align with our experimental observations, where the incorporation of Zn²⁺ and Co²⁺ into the spinel structure of ZnCoFe₂O₄ results in superior catalytic performance.

The integration of cobalt (Co) into zinc (Zn) ferrite remarkably enhances the HER kinetics by shrinking the energy barriers correlated with the reaction steps. This enhancement in catalytic performance is attributed to the synergistic effect of cobalt and zinc ions within the spinel lattice, which optimizes electron transfer and increases the availability of active sites. Its dual functionality can significantly reduce the overall cost of water-splitting systems, making it a promising candidate for sustainable hydrogen production.

Long-term stability is a critical parameter for the practical implementation of electrocatalysts in sustainable hydrogen production. To evaluate the durability of the ZnCoFe₂O₄ catalyst, a 14-hour chronoamperometric test was performed at a constant applied voltage of 0.6 V for the HER, as shown in Fig. 9(e). Remarkably, the catalyst retained 94% of its initial current density after 14 hours of continuous operation, demonstrating excellent stability and resilience under prolonged electrochemical conditions. This durability is primarily attributed to the enhanced ion penetration at the electrode–electrolyte interface, which occurs due to the reduction in ion diffusion resistance. The electrochemical activation of the ZnCoFe₂O₄ material during the testing process facilitates the generation of abundant active sites, thereby improving the interaction between the catalyst surface and reactant ions. These active sites significantly enhance the efficiency of the catalytic process by providing more accessible pathways for charge transfer.^92,93 In addition, the catalyst's stability (73%) toward the oxygen evolution reaction (OER) was examined using a separate 14-hr chronoamperometric test at an applied potential of 1.6 V, as shown in Fig. 9(f), highlighting the durability of ZnCoF₂O₄ for both HER and OER processes.

In addition to HER performance, the ZnCoFe₂O₄ catalyst was also evaluated for overall water-splitting in a 6 M KOH solution. The tests demonstrated the catalyst's superior performance in both HER and OER processes, further confirming its bifunctional catalytic capabilities. This high efficiency can be attributed to the synergy between cobalt and zinc ions within the spinel lattice, which optimizes the electronic structure, enhances charge transfer properties, and increases the density of active sites.⁸⁹

As shown in Fig. 10, cyclic voltammetry (CV) was conducted to evaluate the redox properties of mono- and binary ferrite electrocatalysts, ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄, over a voltage range of 0.00 V to 0.60 V at scan rates (SRs) varying from 5 mV s⁻¹ to 100 mV s⁻¹. Fig. 10(a–d) shows the CV curves of these ferrites, highlighting their distinct redox behavior, particularly at a scan rate of 20 mV s⁻¹. All samples exhibited redox peaks attributed to the OH⁻ and Fe³⁺/Fe⁴⁺ redox couple, essential for catalytic activity in HER and OER processes. Notably, ZnCoFe₂O₄ exhibits more pronounced redox peaks relative to CoFe₂O₄ and ZnFe₂O₄, suggesting enhancement in its catalytic efficiency. Such findings highlight the importance of faster electron transfers at the electrode interface for efficient hydrogen evolution and oxygen evolution processes.

Fig. 10 (a) Cyclic voltammogram (CV) of ZnCoFe₂O₄ at varying scan rates (SRs) (5–100 mV s⁻¹), showing distinct redox peaks associated with the OH⁻ and Fe³⁺/Fe⁴⁺ couple. (b) CV of CoFe₂O₄, under identical conditions, demonstrating lower redox activity compared to ZnCoFe₂O₄. (c) CV of ZnFe₂O₄, exhibiting weaker redox behavior relative to ZnCoFe₂O₄ and CoFe₂O₄. (d) Comparison of CV curves of ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄ at a scan rate of 20 mV s⁻¹, highlighting the superior redox performance of ZnCoFe₂O₄. (e) EIS Nyquist plots for ZnFe₂O₄, CoFe₂O₄, and ZnCoFe₂O₄, with the inset showing the high-frequency region. ZnCoFe₂O₄ exhibits the lowest charge transfer resistance, indicating superior charge transport and catalytic performance.

The ZnCoFe₂O₄ electrocatalyst displayed a well-defined oxidative peak at a higher potential compared to CoFe₂O₄ and ZnFe₂O₄, indicative of its superior redox reaction kinetics and higher electrochemical activity. This higher oxidative potential demonstrates the improved interaction of ZnCoFe₂O₄with the electrolyte, enhancing charge transfer at the electrode–electrolyte interface. The CV curves at different scan rates further reveal that ZnCoFe₂O₄ exhibits more pronounced redox features, suggesting a greater density of active sites available for catalytic reactions.

These findings underscore the superior performance of ZnCoFe₂O₄ as an electrocatalyst, demonstrating its effectiveness in facilitating both the HER and OER. The enhanced redox properties, combined with higher peak potentials and pronounced redox behavior, highlight its potential as a highly efficient and durable electrocatalyst for water-splitting applications. This performance improvement is attributed to the synergistic effect of cobalt and zinc substitution, which optimizes the electronic structure and increases active site availability, making ZnCoFe₂O₄ a promising candidate for sustainable energy applications.

Electrochemical impedance spectroscopy (EIS) was utilized to investigate the impedance response of ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄ electrocatalysts under an applied alternating current (AC) signal across a range of frequencies. The Nyquist plots in Fig. 10(e) display the real impedance (Z′) on the x-axis and the imaginary impedance (Z′′) on the y-axis, offering insights into the intrinsic resistance of the electrode material and the interfacial resistance between the electrode and the electrolyte. This analysis evaluates the conductivity of the samples and the electron transport mechanisms at the electrode–electrolyte interface.⁹⁴ A fitted equivalent circuit model was used to interpret the EIS data, incorporating elements such as the constant phase element (CPE) and Warburg impedance (W) to represent diffusion processes. The solution resistance (R_s) reflects the resistance offered by the electrolyte solution, which is critical for determining the system's overall impedance and electrochemical performance. Lower R_s values indicate improved ionic conductivity and reduced energy losses, facilitating better catalytic efficiency. The impedance behavior reflects minimal transfer limitation, and the dominant features are solution resistance (R_s) and Warburg diffusion (W). As summarized in Table 3, ZnCoFe₂O₄ exhibited a significantly lower R_s compared to CoFe₂O₄ and ZnFe₂O₄, indicating enhanced ionic transport and superior electrolyte conductivity.

Table 3 Solution resistance of ZnCoFe₂O₄, CoFe₂O₄, and ZnFe₂O₄^a

Materials	R s (Ω)	W (s × s^1/2)
a Furthermore, the Warburg impedance (W) decreased from 0.233 S s^1/2 for ZnFe2O4 to 0.158 S s^1/2 for ZnCoFe2O4, demonstrating more rapid diffusion of reactants and products within the electrolyte and improved mass transport to the electrode surface.
ZnCoFe2O4	1.58	158.9 × 10^−3
CoFe2O4	1.83	200.7 × 10^−3
ZnFe2O4	2.96	233.0 × 10^−3

---

These EIS results are consistent with the enhanced OER and HER activities previously reported for ZnCoFe₂O₄, confirming its superior electrochemical performance. The combination of reduced solution resistance, lower charge transfer impedance, and faster diffusion kinetics highlights ZnCoFe₂O₄ as a promising material for improving water-splitting efficiency in practical applications. These findings underscore its potential for use in energy-efficient hydrogen production systems.

Conclusion

In this comparative study, we have successfully optimized the synthesis of the binary ferrite composite ZnCoFe₂O₄ and thoroughly characterized its structural, morphological, and elemental properties. Our findings highlight the synergistic enhancement of photocatalytic water splitting with the assistance of sound waves, emphasizing the crucial role of piezoelectric elements in piezo-photocatalysis. The improved HER activity, supported by a lower Tafel slope and enhanced charge transfer efficiency, underscores the significance of cobalt and zinc ions in optimizing reaction kinetics and increasing active site availability. Metal ferrites prove to be versatile catalysts for various water-splitting processes, with process selection contingent on the specific application. While both piezo-photocatalytic and electro-catalytic systems offer distinct advantages and limitations, ZnCoFe₂O₄ is suitable for both processes. Furthermore, when comparing MOFs and ferrites, the latter outperforms the former, with binary ferrite composites demonstrating superior efficiency over their mono-ferrite counterparts. This work underscores the promise of cobalt-substituted zinc ferrites in outperforming traditional HER and OER electrode materials in terms of overpotential and underwater stability and cost-effectiveness. However, practical implementation challenges remain, particularly regarding large-scale synthesis, long-term durability under industrial conditions, and potential deactivation due to structural changes over prolonged operation. Future research should focus on optimizing material stability, investigating alternative dopants to further enhance catalytic activity, and integrating these catalysts into scalable water-splitting systems.

Data availability

The data that supports the findings of this study are available from the corresponding authors upon reasonable request.

Author contributions

S. J., F. M., M. Z. K., S. J., J. H. K., & M. S. conceptualized the study. S. J., F. M., S. J., M. A., & M. S. characterized and analyzed. S. J., F. M., M. S., M. A. & A. M. did the writing and characterization. M. Z. K. & J. H. K. carried out the measurement. S. J., F. M., M. S., M. A. & A. M. conducted the electrochemical experiment. S. J., F. M., M. S., A. M., A. H. B., & M. B. K. N. commented on the manuscript. All the authors discussed the results and contributed to the manuscript preparation.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

This work was jointly supported by the National Research Program of Universities (NRPU), Higher Education Commission (HEC), Pakistan research fund “Development of lab/pilot scale facilities for the production of piezoelectric material for multilayer energy devices” 20-15673/NRPU/R&D/HEC/2021 and by the MSIT (Ministry of Science and ICT), Korea, under the ITRC (Information Technology Research Centre) support program (IITP-2025-RS-2020-II201655, 50%) supervised by the IITP (Institute of Information and Communications Technology Planning And Evaluation).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01180a

‡ Both authors have contributed equally to this work.

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