Boosted Photoelectrochemical Water Oxidation Performance with a Quaternary Heterostructure: CoFe₂O₄/MWCNT-Doped MIL-100(Fe)/TiO₂

Abstract

Cobalt ferrite (CoFe₂O₄) combined with multi-walled carbon nanotubes (MWCNTs) is an outstanding material regarding photoelectrochemical water oxidation (PEC-WO) because of its excellent catalytic properties and stability. On the other hand, surface imperfections in CoFe₂O₄ can cause band bending and surface Fermi level pinning, significantly reducing its PEC conversion efficiency. Heterostructure engineering is essential for achieving increased light-gathering capacity and charge separation efficiency for PEC-WO. In this study, a quaternary heterostructure of CoFe₂O₄/MWCNT-doped Metal–Organic Framework-100 (Iron), MIL-100(Fe)/Titanium Oxide (TiO₂) was synthesized by using a combination of hydrothermal, solvothermal, and “dip and dry” techniques. Characterization results confirmed the formation of a structural network of MIL-100(Fe) on TiO₂ surfaces, enhanced by the incorporation of MWCNTs during the hydrothermal reaction. Under 1 sun irradiation, the resultant quaternary heterostructure displayed a photocurrent density (J_ph) of 3.70 mA cm⁻² under free bias voltage, which is around 3.08 times more than that of pristine TiO₂ photoanodes (J_ph = 1.20 mA cm⁻²). This investigation highlights the advantages of the MIL-100(Fe) network in improving the solar PEC-WO performance of TiO₂ photoanodes.

1. Introduction

With the depletion of fossil fuels and growing environmental concerns, it is critical to seek out clean and renewable energy sources that can replace traditional fossil energy. In the natural world, green plants use a process called photosynthesis to transform carbon dioxide and water into food by harnessing solar energy. Artificial photosynthesis—which imitates the process of photosynthesis found in plants—is a viable method of storing solar power and is being researched to create solar fuel [1,2,3]. The water oxidation half-reaction is more difficult than the reductive ones and is crucial to both natural and artificial photosynthesis. It necessitates the transfer of 4e⁻ and 4H⁺ together with the simultaneous production of an O=O bond, which results in slow kinetics, large energy barriers, and uphill reaction thermodynamics. Photoelectrochemical water splitting (PEC-WS) is a promising method for producing clean, renewable energy [4]. The primary goal of photoelectrochemical (PEC) systems is to produce high-energy, zero-carbon hydrogen (H₂), which is essential for advancing sustainable energy solutions. Hydrogen is considered a clean fuel that can be utilized in various sectors, including transportation, industrial processes, and energy storage. It plays a crucial role in decarbonizing the global energy system, making it a key player in achieving a low-carbon future. PEC systems, by harnessing solar energy, offer an effective method for generating hydrogen through water splitting, which is a critical solution to meet the world’s growing energy demands while reducing carbon emissions. The importance of hydrogen in future energy systems is highlighted in recent studies, such as the one by Guan et al. [5], which discusses the transformative potential of hydrogen in the context of energy storage and decarbonization. This study underscores hydrogen’s pivotal role in building a sustainable energy future, thereby emphasizing the need for efficient PEC technologies to enhance hydrogen production [6]. However, the efficiency of PEC-WS is often limited by sluggish kinetics, charge recombination, and inadequate light absorption [7,8]. To overcome these challenges, various strategies have been explored, including the design and optimization of semiconductor materials, heterostructure engineering, and the incorporation of dopants to enhance charge transfer and catalytic activity [8,9].

Oxide metals have emerged as a highly studied photoanode material for PEC-WS due to their cost-effectiveness, environmental friendliness, and durability in aqueous electrolytes [8,9,10]. However, they cannot effectively capture the solar spectrum’s vast range since most have high band gaps. The quick recombination of photoinduced charge carriers (e⁻/h⁺) was prevented from achieving higher efficiency [11]. Titanium dioxide (TiO₂) has long been regarded as a benchmark photocatalyst due to its stability, abundance, and favorable band gap for water oxidation [12]. However, its PEC performance is also restricted by rapid charge recombination and insufficient visible light absorption. To address these drawbacks, heterojunctions combining oxide metals such as TiO₂ with other materials, such as conductive multi-wall carbon nanotubes (MWCNTs) and metal–organic frameworks (MOFs), have shown great promise in enhancing charge separation and expanding the light absorption spectrum [13]. Although CoFe₂O₄ and TiO₂ are effective PEC-WS catalysts, they have moderate electrical conductivity, which can slow down charge transfer [14,15,16]. MWCNTs have excellent electrical conductivity, acting as conductive networks that enhance electron transport. They can facilitate faster and more efficient charge transfer between the catalyst and the electrode, reducing resistance and improving overall current density during water splitting. Interestingly, recent studies have shown that including MWCNTs may significantly influence metal oxides’ shape, arrangement, composition, and catalytic efficiency [13,17]. Li et al. [18] have demonstrated the synthesis of CoMnAl double-layer oxides using homogeneous urea precipitation. The oxides obtained from the experiment demonstrate a notable alteration in surface morphology, shifting from a plate-like structure to a more uniform shape. This structural transformation enhances the catalytic activity, allowing for the efficient breakdown of organic bisphenols. The double oxides were examined in the presence and absence of MWCNTs. The efficiency of photocatalytic organic dye degradation was found to be enhanced by the creation of hollow MnO₂ nanotubes with highly porous walls through the impact of MWCNTs. In addition, it is worth noting that MWCNTs possess exceptional electrical and thermal conductivity, along with a large surface area. These unique properties make MWCNTs a promising candidate for enhancing the efficiency of photocatalytic water splitting [19]. Metal–organic frameworks (MOFs) are made up of metal ions or clusters and polytopic organic linkers, having high porosity, huge surface areas, and ease of structural and functional adjustment. They have been extensively studied for applications such as drug delivery, gas storage, and separation, sensors, and catalysis. Recent advancements in rational design and post-synthetic modification have given functionalized MOFs possible uses in artificial photosynthesis [20,21].

Previous studies have shown that heterostructures have the potential to enhance photoconversion efficiency, improve carrier collection, facilitate electron-hole separation, and enhance solar absorption [20,21]. The CoFe₂O₄/TiO₂ heterostructures have demonstrated a significant improvement in the efficiency of photoelectrochemical water splitting (PEC-WS) when compared to similar structures [14,15,16], TiO₂/MWCNT [13], TiO₂/nox-MWCNTs/Si [17], MWCNTs-CoFe₂O₄ [22,23,24], TiO₂/NH₂-MIL-125 nanocomposite [25], Cu-MOF/BCZ [26], ZnO/Co@Zn-ZIFs [27], MIL-100(Fe)/TiO₂ [28], and others. Several theoretical investigations have also been conducted to determine the optimal mix of different materials for producing heterostructures while being considered a strong candidate for PEC-WS, which falls short in terms of its photocurrent densities [29]. CoFe₂O₄ and MWCNTs possess comparable crystal structures and exhibit a modest lattice misfit of 1.8%. Consequently, they serve as suitable materials to create a heterostructure photocatalyst for PEC-WS devices [11,29]. However, most of these heterostructures are binary or ternary composites, and their overall PEC performance has not been remarkable [30,31]. In addition, many of these studies do not adequately address unassisted PEC-WS, which restricts the breadth of their applications.

In this study, we present a novel quaternary heterostructure composed of CoFe₂O₄, multi-walled carbon nanotubes (MWCNTs), MIL-100(Fe), and TiO₂ to boost PEC water oxidation performance. CoFe₂O₄, a spinel ferrite, offers excellent magnetic and catalytic properties, while MWCNTs provide enhanced electrical conductivity and a high surface area for charge transport. By doping MIL-100(Fe) into the heterostructure, we aim to leverage its active sites for water oxidation, while the TiO₂ matrix ensures long-term stability and light absorption. This multi-component system is designed to facilitate efficient charge separation and transfer, reduce recombination losses, and ultimately enhance water oxidation efficiency. This study introduces a novel quaternary heterostructure made up of CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂, a combination not widely explored in previous research [32]. In contrast to earlier studies that primarily focus on binary or ternary composites, our approach integrates four distinct materials to boost charge separation, enhance light absorption, and improve photocurrent density. The findings will offer a fresh pathway for developing more efficient and sustainable PEC systems aimed at hydrogen production. A combination of these materials is expected to synergistically improve the PEC performance by optimizing both the photoelectrochemical and electrochemical pathways, leading to a significant boost in water oxidation activity.

2. Results and Discussions

2.1. Physiochemical Characterizations of CoFe₂O₄/MWCNts@MIL-100(Fe)/TiO₂

CoFe₂O₄/MWCNts@MIL-100(Fe)/TiO₂ exhibits stronger light absorption, and a redshifted absorption edge compared to its binary and ternary counterparts (Figure S1), indicating the modifications broaden the light absorption range and enhance the material’s light-harvesting capabilities. The XRD analysis of CoFe₂O₄/MWCNTs@MIL-100(Fe) showed characteristic crystal planes such as (220), (311), (400), (511), (440), (002), and (101), consistent with a corresponding standard pattern of JCPDS card No. 01-080-6487 (CoFe₂O₄) and JCPDS card No. 00-01-0646 (MWCNTs). Similarly, the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ composite displayed peaks matching crystal planes like (101), (004), (200), (105), and others, as per JCPDS card No. 01-070-7348 (TiO₂). This confirms the successful synthesis of the multi-component composite. For detailed XRD peaks, refer to Figure 1 and Figure S2, indicating the successful integration of all components. Figure S3 shows the XRD characterization results before and after the reaction of composition in order to investigate changes in phase composition and crystal structure. The XRD patterns after the reaction show a reduction in peak intensity and slight broadening compared to the pre-reaction state, indicating surface modifications or partial amorphization [33]. However, the characteristic peaks of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ remain intact, confirming the structural stability of the heterostructure, which is crucial for its durability in photoelectrochemical applications.

Furthermore, the surface morphology and microstructures of the samples were analyzed using transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images, as shown in Figure 2a–f, where we can observe that TEM images show distinct features and particles with irregular, somewhat rounded shapes at a lower magnification at 100 nm Figure 2a. The particles appear clustered together with varying sizes, suggesting some degree of agglomeration, while in a higher magnification (50 nm, Figure 2b), the individual particles become more clearly defined. The edges are smooth, and the overall shapes remain rounded, but finer details of the particle surface can be observed. The HRTEM in Figure 2c shows a high-resolution lattice image with visible atomic-scale lattice fringes of 0.46 nm indicating crystallinity, providing information about the internal crystal structure of the material, which matches with the (311) of XRD crystal.

The SEM images illustrate a densely interconnected network of particles that possess a porous, sponge-like architecture. At a magnification of 10 μm in Figure 2d, the particles appear to form loose, irregular aggregates with rough surfaces and noticeable voids, suggesting robust interparticle interactions likely driven by sintering. This leads to clusters that are irregularly shaped and highly textured, lacking defined geometrical forms.

When observing at 5 μm in Figure 2e, attention is directed to a smaller area, offering a closer look at the porous structure and individual clusters. The surface morphology continues to be irregular, underscoring the amorphous qualities of the aggregates. Finally, at 3 μm in Figure 2f, the highest magnification reveals the complex details of these clusters, further confirming the observations of a convoluted and irregular arrangement of particles.

EDX mapping analysis results (Figure 2g–l) of CoFe₂O₄/MWCNTs@MIL-100(Fe)/reveal the dispersion of C, O, Ti, Fe, and Co. The molar contents of these elements are approximately 14.11%, 42.20%, 41.28%, 1.82%, and 0.58%, respectively. The elemental proportions can be found in

Table 1. Elemental percentage of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂.

Elements	Atomic Number	Normalized Mass %	Atomic %	Abs. Error Mass% (3σ)
C	6	14.11	24.91	1.99
O	8	42.20	55.91	6.39
Ti	22	41.28	18.28	2.17
Fe	26	1.82	0.69	0.14
Co	27	0.58	0.21	0.09

. These results further provide evidence of the existence of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ (Figure 3).

FTIR spectroscopic techniques were used to examine the chemical composition and functional groups of CoFe₂O₄, a compound consisting of cobalt ferrite. The spinel structure had significant absorption bands around 590 cm⁻¹, which corresponded to the vibrational stretching of metal–oxygen connections. Crystal field absorption bands at a frequency of 1064 cm⁻¹ were used to detect electronic transitions between cobalt and iron ions. The presence of medium-intensity bands at 1370 cm⁻¹ and 1627 cm⁻¹ suggests the existence of additional vibrational modes. The presence of a peak at 3400 cm⁻¹ indicates the occurrence of O-H vibrational stretching, which may be ascribed to the presence of hydroxyl groups on the surface that may come from water that has been adsorbed onto the surface or from surface functionalization. The CoFe₂O₄/MWCNTs exhibit prominent absorption peaks at 580 cm⁻¹, indicating stretching vibrations associated with metal–oxygen bonds in the spinel structure of cobalt ferrite. There is an extra peak seen at 1627 cm⁻¹, which corresponds to the stretching vibration of the C=C bonds in the graphitic structure of the nanotubes. The presence of hydroxyl groups is believed to cause another broad band around 3427 cm⁻¹, which is attributed to vibrational stretching. The CoFe₂O₄/MWCNTs@MIL-100(Fe) composite’s spectrum displays distinct peaks that correspond to its 3-components. CoFe₂O₄ exhibits metal–oxygen (M-O) bonds, MWCNTs display stretching vibrations of carbon–carbon (C=C), carbon–hydrogen (C-H), and oxygen–hydrogen (O-H) bonds, and MIL-100(Fe) includes carboxylate functional groups. The double bonds (C=C) in multi-walled carbon nanotubes (MWCNTs) may be identified at a frequency of 582 cm⁻¹. Additionally, the symmetric and asymmetric vibrations of carboxylate groups can be recognized at frequencies of 1026 cm⁻¹, 1396 cm⁻¹, 1481 cm⁻¹, and 1635 cm⁻¹. The CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ composite exhibits significant structural characteristics, including a wider band at 3400 cm⁻¹, which suggests the existence of hydroxyl groups. The presence of hydroxyl groups and adsorbed water is indicated by a peak detected at 3400 cm⁻¹.

Additionally, water bending vibrations can be observed at 1617 cm⁻¹. A peak at 1470 cm⁻¹ indicates the occurrence of C-H bending from MWCNTs. The peaks observed at 1026 cm⁻¹ and 1120 cm⁻¹ also provide evidence of Ti-O-Ti stretching from TiO₂. The 582 cm⁻¹ peak corresponds to Fe-O stretching vibrations, commonly observed in CoFe₂O₄. These features confirmed the CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂ in the composite, which also offers valuable insights into their interactions [34,35]. For details, see Figure 4.

Further Raman spectroscopy analysis was conducted on the photoelectrodes, revealing the presence of multiple peaks associated with various emission modes (Figures S4–S6). The Raman spectroscopy of CoFe₂O₄ reveals distinct peaks that correspond to specific vibrational modes. The peaks observed in this study provide valuable insights into the structure and composition of the material under investigation. The peak observed at 698 cm⁻¹ suggests that there is symmetric stretching happening in the tetrahedral sites of the metal–oxygen bonds. It is hypothesized that the peak observed at 470 cm⁻¹ is associated with the bending or deformation modes that take place in the octahedral sites. The observed peaks in the samples suggest the presence of cobalt ferrite based on the data obtained. The presence of MWCNTs alongside CoFe₂O₄ is confirmed by the additional peaks observed in Figure S5 for CoFe₂O₄/MWCNTs. The presence of peaks at 1309 cm⁻¹ (D-band) and 1582 cm⁻¹ (G-band) suggests the presence of carbon nanotubes. The D-band is a reliable indicator of defects and disorder present in the carbon lattice. It is important to mention that the presence of the G-band in the sample suggests the potential presence of multi-walled carbon nanotubes. This band is linked to the in-plane vibration of carbon atoms bonded with sp2 hybridization.

Concerning Figure S6, the spectrum exhibits peaks that are indicative of the CoFe₂O₄/MWCNTs and MIL-100(Fe) substances. The peak observed at 2187 cm⁻¹ is indicative of stretching vibrations in MIL-100(Fe), which is a possible explanation for its presence. Moreover, the observed peaks at 1310 cm⁻¹ and 572 cm⁻¹ suggest the existence of carbon nanotubes and the CoFe₂O₄ phase, respectively. The presence of a peak at a wavenumber of 387 cm⁻¹ indicates the potential existence of the MIL-100(Fe) framework structure. The spectrum of the composite in Figure 5 exhibits discernible peaks that correspond to its various components, including CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂. Based on the observed peak at 624 cm⁻¹, it can be inferred that the presence of the TiO₂ phase is indicated. It is well-known among researchers that this frequency range is associated with Raman-active modes. The presence of peaks at 2187 cm⁻¹, 1309 cm⁻¹, and 563 cm⁻¹ indicates the possible presence of MWCNTs and CoFe₂O₄.

In addition, the presence of peaks at 270 cm⁻¹ and 385 cm⁻¹ suggests the possibility of interactions among the different components in the composite material. The presence of additional peaks in the spectra suggests an interaction and the coexistence of MIL-100(Fe) and TiO₂ within the nanocomposite materials [36]. The findings indicate that the incorporation of these elements led to positive results, suggesting that certain structural changes may have been implemented [6,35,36,37,38].

The examination employed X-ray photoelectron spectroscopy (XPS) to investigate the elements, chemical structures, and bonding present in CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂. XPS is a widely used technique in materials science and surface chemistry research. It can provide valuable insights into the chemical composition and electronic structures of elements present on the surface of a material. Figure 6a displays the XPS full spectrum of the sample. The survey peak in Figure 6a indicates the presence of several elements, namely Co 2p, Fe 2p, Ti 2p, C 1s, and O 1s.

In Figure 6b, we present the Co 2p spectrum. The identification of Co²⁺ and Co³⁺ ions within the spinel structure of CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂ can be achieved by analyzing the distinct peaks in the XPS spectrum for Co 2p. The peaks observed at around 785.92 and 802.13 eV in the data are recognized as satellite peaks and are labeled as “Sat.” The presence of two spin–orbit doublets in the Co 2p orbitals suggests a unique oxidation state, which is an interesting finding for researchers in the field. The presence of dual peaks at 780.21 eV in the Co 2p_3/2 spectrum and 795.4 eV in the Co 2p_1/2 spectrum suggests the possible presence of Co³⁺ ions. Furthermore, the presence of distinct peaks at 782.5 eV in Co 2p_3/2 and 797.8 eV in Co 2p_1/2 suggests the potential existence of Co²⁺ ions [39,40]. The Co²⁺ peaks observed in the data may potentially be linked to catalytic activity, as they can readily transform into metallic Co⁰.

The second and third samples, MWCNTs and MIL-100(Fe), display four distinct Co 2p_3/2 and Co 2p_1/2 peaks with reduced binding energies in comparison to CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂. The satellite peaks at 785.92 eV and 803.13 eV in both samples suggest the existence of cobalt in the Co²⁺ and Co³⁺ oxidation states. The materials, namely CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂, display distinct peaks in their respective spectra. Notably, the addition of MWCNTs and MIL-100(Fe) leads to the observation of four discernible peaks. Upon analyzing the CoFe₂O₄/MWCNTs/MIL-100(Fe)/TiO₂ composite, we have detected an additional peak, resulting in a total of five peaks. It is worth mentioning that the binding energies of CoFe₂O₄, MWCNTs, MIL-100(Fe), and TiO₂ exhibit higher values in comparison to the MWCNTs and MIL-100(Fe) samples.

Regarding the Fe 2p spectrum, the data presented in Figure 6c offer evidence that supports the existence of Fe³⁺ and Fe²⁺, thus confirming the presence of iron oxides within CoFe₂O₄ and MIL-100(Fe). The XPS spectra of Fe 2p_1/2 and 2p_3/2 peaks, following previous research, exhibit the Fe 2p_3/2 peak at 711.54 eV, which represents a distinct energy level transition. The peak observed at 724.59 eV indicates the potential existence of various iron oxidation states, including Fe₂O₃ and FeO. The Fe 2p spectrum suggests that the peak at 711.54 eV corresponds to the Fe³⁺ oxidation state in CoFe₂O₄. The combination of CoFe₂O₄ with MIL-100(Fe) yields a unique peak pattern, exhibiting a higher binding energy for the CoFe₂O₄/MWCNTs/MIL-100(Fe)/TiO₂ composite. The binding energies of each sample were found to be lower when compared to the binding energies of CoFe₂O₄ and MIL-100(Fe) individually. This finding is consistent with previous studies on Fe 2p [41,42,43].

In Figure 6d, the presence of Ti⁴⁺ in TiO₂ is illustrated. The observed peaks at approximately 458.5 eV and 464.5 eV correspond to the Ti 2p_3/2 and Ti 2p_1/2 states, respectively. Conducting XPS analysis allows for a comprehensive examination of the Ti 2p peaks in titania composite materials, providing an additional understanding of the properties of TiO₂. The central peak at 285.91 eV corresponds to the C 1s orbital, suggesting the presence of carbon, as observed in Figure 6e. Furthermore, the presence of a peak at 284.8 eV indicates the potential formation of carbon bonds with metal oxides [44,45,46].

The O 1s spectrum in Figure 6f exhibits a prominent peak at 530.54 eV, indicating the presence of metal carbonates. Moreover, the observed peaks at 531.67 eV and 533.26 eV indicate the potential existence of C–O and C=O bonds, respectively [47,48].

Overall, the XPS analysis offers a comprehensive insight into the elemental composition, chemical structures, and bonding characteristics of the CoFe₂O₄/MWCNTs/MIL-100(Fe)/TiO₂ composite. The spectrum data reveals various oxidation states and bonding interactions that involve cobalt, iron, titanium, carbon, and oxygen. The observed peaks and binding energies provide evidence supporting the successful synthesis and integration of these components. The surface chemistry of the composite, as well as its potential interactions, provides valuable insights for future research.

BET analysis, as shown in Figure 7, presents two key components: a larger graph illustrating the overall surface area comparison between CoFe₂O₄, MWCNTs, MIL-100(Fe), TiO₂, also the composite CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂, with a smaller inset focusing on specific surface area details or adsorption isotherms. The prominent figure highlights the substantially increased surface area of CoFe₂O₄/MWCNTs@ MIL-100(Fe)/TiO₂ composite, which reaches 1240 m²/g, surpassing the individual contributions of CoFe₂O₄ (71 m²/g), MWCNTs (260 m²/g), MIL-100(Fe) (768.5 m²/g), and TiO₂ (121 m²/g) (Table S1). The smaller inset provides a more detailed view, possibly showcasing the adsorption–desorption isotherms, which corroborate the enhanced surface area and confirm the improved material properties. This significant increase in surface area, facilitated by the composite structure, enhances electron transport and improves PEC activity [49,50].

2.2. Electrochemical Characterizations and Photoelectrochemical Water Splitting

To assess and evaluate the impact of our research, we conducted linear sweep voltammetry (LSV) measurements on the CoFe₂O₄ photoanode combined with MWCNTs@MIL-100(Fe) and TiO₂ (Figure 8a). The LSV profiles for different samples, namely CoFe₂O₄, CoFe₂O₄/MWCNTs, CoFe₂O₄/MWCNTs@MIL-100(Fe), and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂, are presented in this plot. Additionally, a baseline condition is included where no light is present (dark conditions). The measurements were conducted in a 0.5 M NaOH electrolyte solution. The results presented in this study offer a thorough analysis of the electrochemical performance of various configurations of composite photoanodes. All of the photoanodes that were fabricated exhibited a negligible dark current throughout our experiment. Furthermore, a noteworthy observation was made regarding the substantial rise in current density when exposed to light, suggesting the presence of remarkable photoactive characteristics. The photocurrents were observed to be generated at an applied bias of approximately −0.49 V. Subsequently, a linear increase was observed, and eventually a saturation point was reached at 0.9 V. The photoanode made of CoFe₂O₄ demonstrated a noteworthy photocurrent density (J_ph) of 1.70 mA cm⁻² when no bias was applied. According to our research, it has been discovered that CoFe₂O₄ exhibits photoactivity upon exposure to light and possesses active sites that facilitate water splitting process. The reactions to the evolution of oxygen (OER) and hydrogen (HER) are crucial in numerous research fields. The photocurrent density observed for the CoFe₂O₄/MWCNTs composite material is approximately 2.20 mA cm⁻².

During the experimentation with CoFe₂O₄/MWCNTs@MIL-100(Fe), it was observed that the catalyst exhibited a photocurrent density of approximately 2.95 mA cm⁻². The effects of TiO₂ deposition were compared on CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂. Our research has revealed that the latter demonstrated a noteworthy enhancement in photocurrent density, measuring at 3.50 mA cm⁻². This value is approximately 2.08 times greater than the photocurrent density observed in CoFe₂O₄. It is important to highlight that the performance of bare CoFe₂O₄ in terms of PEC is relatively low. It is possible that the improved performance of the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ photoanode in terms of PEC performance can be attributed to various factors in the creation of a larger number of porous TiO₂ structures with favorable mechanical properties, increased active sites, and open spaces between the nanosheets. These factors contribute to better dissipation and the generation of gaseous products [51].

Figure 8b illustrates the wavelength-dependent function of the Incident Photon to Current Conversion Efficiency (IPCE) for different samples, namely CoFe₂O₄/ MWCNTs@MIL-100(Fe)/TiO₂, CoFe₂O₄/MWCNTs@MIL-100(Fe), CoFe₂O₄/MWCNTs, and CoFe₂O₄. Our research findings indicate that the IPCE of the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ catalyst starts at 13% at a wavelength of 350 nm and gradually decreases to 3% at 550 nm when compared to other catalysts. It can be inferred that the catalyst efficiently transforms high-energy photons into current, especially at shorter wavelengths. The initial IPCE of the CoFe₂O₄/MWCNTs material is approximately 12% at a wavelength of 350 nm, which subsequently decreases to around 2.5% at 550 nm. The IPCE values start at around 11% when measured at a wavelength of 350 nm and decrease to about 2.5% at a wavelength of 550 nm. The IPCE of CoFe₂O₄ starts at approximately 9% at a wavelength of 350 nm and decreases by 2% at a wavelength of 550 nm. The researcher observed that materials with the highest IPCE achieve the highest efficiency in converting high-energy photons to current at shorter wavelengths. Regarding CoFe₂O₄, despite starting with the lowest IPCE, it exhibits a consistent decline similar to the other samples. The synthesized materials of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ demonstrate remarkable efficiency in absorbing, converting, and harnessing high-energy photons for photoelectrochemical applications.

To provide a clearer understanding of the high PEC efficiency, we determined the applied bias photon-to-current efficiency (ABPE) for various photoanodes, including CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂, CoFe₂O₄/MWCNTs@MIL-100(Fe), CoFe₂O₄/MWCNTs, and CoFe₂O₄/hybrid photoanodes (Figure 8c). The reversible potential of water’s standard state is 1.23 V. The photocurrent density is denoted as J_ph, and the power density of the illuminating light source is represented as P_light. The photoelectrode, CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂, shows a noticeable 1.0% increase in the ABPE value. At approximately 0.5 V, the ABPE% reaches its maximum value of around 0.85% and starts at approximately 0%. The MWCNT ABPE% exhibits a starting value of approximately 0% and gradually increases to a peak value of approximately 0.75% at around 0.5 V. When comparing different materials, it is observed that the CoFe₂O₄ ABPE% also starts at approximately 0% and reaches its peak value of around 0.55% at a voltage of 0.5 V.

Nyquist plots are used to assess the charge transfer properties of photoelectrodes (Figure 8d). These plots are commonly used to gain insights into the ongoing processes. The diameters of the semicircles are directly related to the charge transfer resistances at the interface between the semiconductor and the electrode. The charge transfer resistance (Rct) values for the CoFe₂O₄, CoFe₂O₄/MWCNTs, CoFe₂O₄/MWCNTs@MIL-100(Fe), and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ photoelectrodes were measured to be 22912, 20465, 18628, and 3911 Ω cm⁻², respectively. The photoanode CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ demonstrates the lowest Rct values, suggesting that charge transfer within the composite heterostructure photoanodes is efficient. In addition, the researcher determined the values of solution resistance (Rs) for the various photoanodes. The Rs values for the photoanodes CoFe₂O₄, CoFe₂O₄/MWCNTs, CoFe₂O₄/MWCNTs@MIL-100(Fe), and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ were determined to be 100.20, 10.24, 9.14, and 7.39 Ω cm⁻², respectively. The alteration of TiO₂ has resulted in a decrease in resistance values, indicating an improvement in the conductivity of the heterostructure photoanode.

The band structures of the samples were also studied by Mott–Schottky plot analysis (Figure 8e). Based on theoretical considerations, it is expected that the flat band potential values will align with the onset potential of the measured photocurrent [19]. The values observed of the flat band potential for the CoFe₂O₄ and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ photoanodes were −1.29 V and −0.57 V, respectively. The hybridization of TiO₂ results in an alteration of the flat band potential towards higher positive values, which is in line with the findings of previous research [19,52]. Thus, the enhanced performance of the PEC system can be attributed to the beneficial effects of the heterostructure, as observed in our research. The heterostructure enhances conductivity and promotes effective interfacial charge transfer, leading to enhanced PEC activity of the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ electrode.

The electrochemically active surface area (ECSA) was quantified through calculations. The catalytic activity of the electrode is known to increase with an increase in the ECSA [52,53]. Based on previous research [51], it has been discovered that there is a direct relationship between the electrochemical double-layer capacitance (Cdl) and the ECSA. The determination of Cdl was conducted by analyzing the cyclic voltammetry (CV) curves. The larger ECSA (0.0092 cm²) and higher Cdl (0.313 μF cm⁻²) observed in the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ electrode suggest that the electrode possesses a larger electrochemical active surface area in contact with the electrolyte. The increased catalytic activity for water oxidation is attributed to the enhanced surface area.

Assessing the long-term stability of ternary heterostructure photoelectrodes is of utmost importance for their practical application, as photocorrosion issues are commonly observed in CoFe₂O₄ photoelectrodes. The chronoamperometric J–T curves of the CoFe₂O₄ and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ photoanodes were measured throughout 12,000 s, as depicted in Figure 8f. According to the obtained photocurrent retention value of approximately 2.95 mA cm⁻², we can conclude that the ternary heterostructure CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ exhibits improved stability. This finding holds great potential for various practical applications, as it can be utilized in a wide range of real-world scenarios. Nevertheless, it is important to mention that the J–T curves of the pure CoFe₂O₄ photoanode displayed periodic oscillations in the photocurrent and a relatively low retention of photocurrent density, measuring approximately 1.20 mA cm⁻². According to our research findings, the addition of TiO₂ alone does not show a significant improvement in PEC performance. When combined with other semiconductors to form hybrids, TiO₂ can improve the stability of PEC water oxidation. This is mainly attributed to its structural integrity and chemical inertness. The findings presented in this study have direct implications for researchers working on the development of stable and efficient photoelectrodes for water oxidation [22].

Table S3 lists the comparative analysis results of the performance of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ against similar catalysts in photoelectrochemical water splitting. The CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ heterostructure developed in this study exhibits a photocurrent density of 3.70 mA cm⁻² at an applied bias of 1.23 V vs. RHE, with an IPCE of 13% at 350 nm. This performance is competitive with, and in some cases superior to, other reported catalysts. The stability of the synthesized heterostructure, retaining 95% of its photocurrent over 12,000 s, underscores its potential for practical applications in PEC water splitting.

2.3. Proposed Photoelectrochemical Mechanism

Figure 9 depicts a schematic illustration of the photoelectrochemical mechanism, which aids in understanding the PEC-WS process. Our research focuses on investigating a band alignment formed by CoFe₂O₄/MWCNTs@ MIL-100(Fe)/TiO₂ heterojunction. Particular band alignment has shown promising results in separating photogenerated charge carriers (e⁻/h⁺) across the interface [11,29]. Instead of solely acting as an adsorbent and dispersion reagent, TiO₂ also has the potential to act as a photosensitizer [31]. Therefore, heterojunction structures combined with TiO₂ and other wide bandgap semiconductors can achieve visible light photoactivity in the composite photoanode. Under visible light, the photoexcitation of TiO₂ in the composite heterostructure involves the transition of TiO₂ from its ground state to an excited state, known as TiO₂*. As a result of this photoexcitation, photogenerated electrons are produced [54,55]. Subsequently, the excited state of TiO₂* injects electrons into the conduction band of CoFe₂O₄, which then migrate to the conduction band of MWCNTs within the MIL-100(Fe) framework [56,57]. Ultimately, the electrons make their way to the Pt counter electrode, where they facilitate water reduction and trigger hydrogen production (H₂). During this process, photoinduced holes transfer from the valence band of MWCNTs/ CoFe₂O₄@MIL-100(Fe)/TiO₂ to the valence band of CoFe₂O₄/MWCNTs@MIL-100(Fe). This transfer enables the oxidation of water and the subsequent production of O₂. It is believed the impressive photoelectrochemical performance of the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ heterostructure stems from the synergistic interplay between its components. Under illumination, TiO₂ generates electron–hole pairs, where the photogenerated electrons are efficiently transferred via CoFe₂O₄ to MWCNTs, which serve as a highly conductive pathway to reduce recombination losses. At the same time, MIL-100(Fe) enhances light absorption through its high porosity and offers additional active sites for water oxidation. This coordinated mechanism ensures effective charge separation and transport, leading to improved hydrogen evolution at the counter electrode and oxygen production at the photoanode. These combined effects account for the heterostructure’s high photocurrent density and excellent stability.

Past studies have explored different photosensitizers, like MWCNTs, graphene, and C60, to develop comparable photocatalytic devices and reaction processes. According to the research conducted by Kamat et al. [58], it has been observed that the C60/TiO₂ composite photocatalyst can be activated by exposing it to a laser beam of visible light. This activation process generates electrons that can move through the TiO₂ conduction band. To accurately study the photosensitizer role of composites, Yang et al. [54] have put forth three fundamental prerequisites: (i) The semiconductor should not be capable of being photoexcited by visible light on its own in graphene-semiconductor composites. (ii) The semiconductor and graphene should interact to enhance charge carrier lifetime and transfer efficiency. (iii) To avoid the self-induced photosensitization effect, it is important to utilize suitable probe reactions, such as organic dye photosensitization for semiconductors. Wang et al. [50] suggest that MWCNTs can act as a photosensitizer for TiO₂ under visible light, facilitating the transfer of photoinduced electrons from the MWCNTs to the TiO₂/MIL-100(Fe) photocatalyst. The close interfacial contact between CoFe₂O₄ and MWCNTs prevents undesired recombination of e⁻/h⁺ couples. Additionally, the TiO₂ in the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ heterostructure enables photosensitizer-driven charge excitation. Additional support for this claim can be found by evaluating the photoanodes through photoluminescence research. This method is widely acknowledged for its ability to provide insights into the effectiveness of charge carrier separation. The main reason for photoluminescence emission is the recombination of excited electrons and holes. A decrease in photoluminescence intensity indicates a reduction in the efficiency of charge carrier recombination [59,60]. MIL-100(Fe) and TiO₂ significantly reduce the photoluminescence intensity of the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ electrode. This decrease in intensity can be attributed to interfacial band bending, which improves the performance of the PEC system and enhances the efficiency of charge carrier separation [4,53].

Heterostructures similar to CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ can be developed to further improve performance of photoanodes. These kind of heterojunction structures can promote the efficient separation and prolonged lifetime of photoexcited electrons and holes by the cocktail effect, meaning that the different factors that make up the structure work in synergy with each other to deliver superior performance. Firstly, the TiO₂ modifies the surface texture and acts as channels for sensitizing visible light and transporting electrons. Secondly, the nanostructures of MWCNTs and MIL-100 have unique and porous characteristics, allowing for a large contact area between the electrode and electrolyte. Additionally, they contain voids that facilitate light transfer to the CoFe₂O₄ thin film underneath. Furthermore, the MWCNTs in these composites are hybridized, meaning they have been combined with other materials. This study highlights the potential of heterostructures in improving PEC water splitting efficiency, which is critical for sustainable hydrogen production. The novel approach of combining TiO₂ with MIL-100(Fe), CoFe₂O₄, and MWCNTs demonstrates the importance of synergistic interactions in creating efficient photoanodes. This work lays a strong foundation for developing efficient, versatile, and scalable heterostructures for PEC water splitting and beyond, while offering pathways for innovation in material science and sustainable energy.

3. Materials and Methods

3.1. Materials

CoCl₂·6H₂O, FeCl₃·6H₂O, NaOH, C₂H₅OH, Benzene-1,3,5-tricarboxylic acid (H₃BTC) and N, N-Dimethylformamide (DMF) are commonly used in research studies, and hydrofluoric acid (H.F.) were all purchased from Sigma-Aldrich in Shanghai, China. MWCNTs and commercial titania (TiO₂) were purchased from Sigma-Aldrich in China.

3.2. Preparation of CoFe₂O₄

The solution was prepared by dissolving small quantities of urea, cobalt (II) chloride hexahydrate, and iron (III) chloride hexahydrate in a carefully measured volume of distilled water. After combining the components for ten minutes, the mixture’s pH experienced a notable increase to 12.56 due to the release of ammonia resulting from the breakdown of urea. The solution was subsequently transferred to a 100 mL autoclave made of stainless steel and coated with Teflon. The sample was subjected to a temperature of 200 °C for a duration of 24 h. The autoclave’s high temperature and pressure facilitated the formation and growth of CoFe₂O₄ nanoparticles. Following the cooling process, the CoFe₂O₄ nanoparticles sample was subjected to multiple washes using distilled water and ethanol to eliminate any potential contaminants. The cobalt iron oxide nanoparticles were subjected to annealing in a controlled muffle furnace for a period of 2 h. The annealing process involved heating the nanoparticles at temperatures ranging from 600 °C to 800 °C. The heating process maintained a constant rate of 5 °C/min [61].

3.3. MWCNTs Treatment

To obtain a consistent suspension, a quantity of 1 g of MWCNTs was subjected to sonication in 100 mL of ethanol for 30 min. The solution was subjected to a series of steps, which included washing, filtering, and drying at 80 °C for 10 h. The sample was collected as multi-walled carbon nanotubes (MWCNTs).

3.4. Synthesis of MIL-100(Fe)

The synthesis technique was based on the research conducted by He et al. [62]. A solution was prepared by dissolving 2 g of FeCl₃·6H₂O in 30 mL of deionized water in a beaker. The mixture was stirred for 30 min until it completely dissolved. The benzene-1,3,5-tricarboxylic acid was dissolved in 20 mL of ethanol in a separate beaker and stirred for 30 min. The two solutions were stirred together gently until they achieved a uniform mixture. The TMA suspension was added meticulously, drop by drop, to carefully control the morphology and size of the particles. The suspension was then subsequently transferred to a Teflon-lined stainless-steel autoclave. The reaction vessel was carefully sealed and positioned inside an oven that was adjusted to a temperature of 180 °C. After 16 h, the vessel was carefully removed from its position and left to cool down to the temperature of its surroundings. The powder was centrifuged and subsequently treated with ethanol and water to adjust the pH and remove any remaining unreacted components. The sample was subjected to a drying process in an oven maintained at a temperature of 60 °C for a duration of 8 h.

3.5. Synthesis of CoFe₂O₄/MWCNTs

To synthesize CoFe₂O₄/MWCNTs, a dispersion of 15 mg of MWCNTs was prepared by sonicating them in 20 mL of deionized water for 1 h. The synthesized CoFe₂O₄ was added into the MWCNTs dispersion at a concentration of 50 mg. The mixture was then subjected to sonication for 10 min using a sonicator operating at 20 kHz, 30% amplitude, and 750 W. The mixture was stirred for 12 h at room temperature after sonication. The obtained precipitates were centrifuged and washed multiple times with deionized water, and then with ethanol. The CoFe₂O₄/MWCNTs nanohybrid was obtained by drying the washed precipitates at 70 °C for 10 h [63].

3.6. Synthesis of CoFe₂O₄/MWCNTs@MIL-100(Fe)

The hydrothermal synthesis of CoFe₂O₄/MWCNTs@Mil-100(Fe) involved precise control of reaction conditions and parameters to ensure the desired product formation. The CoFe₂O₄ nanoparticles were successfully incorporated onto the surface of MWCNTs, and a mixture consisting of 2 g of CoFe₂O₄/MWCNTs and 4 g of MIL-100(Fe) was prepared in a small beaker. The solution was subsequently dissolved in 50 mL of deionized water. The mixture was subjected to sonication for 10 min, after which it was stirred for 30 min. The mixture was subsequently transferred to a Teflon-lined stainless-steel autoclave, then placed inside a heating oven that was adjusted to a temperature of 150 °C for a duration of 15 h. The reacted product was subjected to a washing and filtration procedure using deionized water and ethanol. The sample was subjected to a drying process at 80 °C for a duration of 10 h. The sample obtained after drying was CoFe₂O₄/MWCNTs@MIL-100(Fe) [34].

3.7. Synthesis of CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂

To synthesize CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂, a mixture of CoFe₂O₄/MWCNTs @MIL-100(Fe) and TiO₂ was prepared in a beaker containing 100 mL of deionized water. The mixture was subsequently agitated for 30 min. Subsequently, the concoctions were transferred to a stainless-steel autoclave that was lined with Teflon. The oven was preheated to 120 °C and maintained at that temperature for a duration of 15 h. Following that, the autoclave was allowed to cool down to the surrounding temperature. The TiO₂ substrate was obtained from the CoFe₂O₄/MWCNTs@MIL-100(Fe) composite. After the filtering process, the composite was rinsed with deionized water and ethanol. It was then dried at 60 °C for 10 h. The substance that was collected for this study is CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ powder [6]. The synthesis route is illustrated in Figure 10.

3.8. Characterization

X-ray diffraction (XRD) analysis was conducted on a D/MAX-2500 X-ray diffractometer with Cu Kα radiation for the research used with an X-ray current of 40 mA and an X-ray voltage of 40 kV while maintaining a scanning rate of 10 °C /min⁻¹. The FTIR spectra of the samples were recorded at 25 °C, covering the wavenumber range of 4000 to 400 cm⁻¹, for analyzing bond formations and functional groups. The researchers at Tianjin University used a Raman microscope to perform Raman spectroscopy, using an excitation wavelength of 514 nm. Surface morphology and structural analysis were conducted using a scanning electron microscope (SEM, S-4800 Hitachi, Hitachi, Chiyoda City, Tokyo, Japan) and a transmission electron microscope (TEM, 2100F JEM, JEOL Ltd., Akishima, Tokyo, Japan), respectively. An analysis was conducted using X-ray photoelectron spectroscopy (XPS, PHI-5000, Ulvac PHI, Kanagawa, Japan) with Al Kα radiation to determine the chemical state and elemental composition. The surface area analysis was performed using the Brunauer–Emmett–Teller (BET) method. The measurements were performed at a temperature of 200 °C using a 3H-2000PS2 instrument (BSD Instrucment, Beijing, China).

Cyclic voltammetry (CV) was used to assess the electrochemical performance with a traditional three-electrode setup. CoFe₂O₄, CoFe₂O₄/MWCNTs, CoFe₂O₄/MWCNTs@MIL-100(Fe), and CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ samples were the working electrodes used in the evaluation. In this experiment, a platinum counter electrode was employed, while a saturated calomel electrode (SCE) served as the reference electrode. We submerged both electrodes in a 1 M NaOH solution. An equipment that is often used in electrochemical research, the PARSTAT 3000 potentiostat (AMETEK Scientific Instruments, Oak Ridge, TN, USA), was employed to do the CV measurements. A Perkin-Elmer Lambda 950 UV-vis spectrometer (PerkinElmer, Waltham, MA, USA) was used to acquire the optical absorption spectra, as previously described [33]. A diode-pumped solid-state laser (Ekspla, Vilnius, Lithuania) and an Enhanced CCD (P1-Max3) from Princeton Instruments were used to get the photoluminescence spectra. The excitation laser used was 270 nm in wavelength. Electrochemical impedance spectroscopy (EIS) analysis was conducted within the frequency range of 1 MHz to 1 Hz, with an amplitude of 10 mV, under dark conditions.

The Mott–Schottky plot is a widely employed technique in scientific research to determine the flat band potential of photoelectrodes. The flat band potentials (V_fb) can be determined by calculating the x-axis intercept using Equation (1).

$${\displaystyle \frac{1}{C^2}}=\left({\displaystyle \frac{2}{e_0\epsilon {\epsilon }_0{N}_d}}\right)\left[V-V_{\mathrm{fb}}-{\displaystyle \frac{kT}{e_0}}\right]$$

(1)

The components of a semiconductor are as follows: The applied voltage, V, is the variable of interest in this study. The charge carrier density, denoted as N_d, is a crucial parameter in this study. The space charge capacitance, denoted as C, is a parameter that characterizes the capacitance associated with the space charge in a system. T represents the temperature. The Boltzmann constant, denoted by k, is a fundamental constant in physics.

3.9. Photoelectrochemical Water Splitting

The 2-electrode cell assembly was employed in the PEC investigation to evaluate the water-splitting properties of the prepared samples. The assembly, which was connected to the PARSTAT 3000 high-polarization potentiostat, was used in combination with a specific 0.5 M NaOH aqueous electrolyte to research the properties of PEC-WS. The photoelectrodes utilized in this study were the CoFe₂O₄/MWCNTs@MIL-100(Fe)/TiO₂ thin film samples that were prepared in advance. The counter electrode, however, consisted of a Pt wire. The photoelectrodes, with an exposed O-ring surface area of about 0.52 cm², were directly connected to the electrolyte. To enhance the transmission of light emitted by a 300 W Xenon lamp into the Teflon chamber, a quartz window was installed. According to our research findings, it has been determined that the light intensity of the lamp is approximately 100 mW cm⁻².

4. Conclusions

This study explores a cutting-edge heterojunction construction strategy to significantly boost the photoexcitation water oxidation performance of CoFe₂O₄/MWCNTs@ MIL-100(Fe)/TiO₂ photoanodes. Utilizing an in situ growth method eliminates the need for preformed semiconductor particles, streamlining the fabrication process and enhancing efficiency. The pivotal role of multi-walled carbon nanotubes (MWCNTs) in fine-tuning the porous MIL-100(Fe)/TiO₂ heterostructure is thoroughly examined, revealing a self-assembled architecture that integrates porous Mil-100 with MWCNTs and TiO₂. This innovative quaternary heterostructure demonstrates remarkable potential for unassisted photoexcitation water splitting, achieving an impressive photocurrent density of 3.70 mA/cm² under 1 sun illumination. Additionally, the structure delivers superior incident photon-to-current efficiency and applied bias photon-to-current efficiency at a wavelength of 350 nm and an applied bias of 0.5 V, underscoring its advanced functionality. The findings highlight the powerful synergy of physicochemical synthesis techniques in crafting hierarchical, functional materials, paving the way for the next generation of high-performance solar energy conversion devices.