Enhanced Photoelectrochemical Water Splitting Using a NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ Composite Photoanode: Synthesis, Characterization, and Performance

Abstract

NiFe₂O₄ and TiO₂ are widely studied for photoelectrochemical (PEC) applications due to their unique properties. Nitrogen-doped graphene (NG) and metal–organic frameworks (MOFs), such as MIL-100(Fe) (where MIL stands for Materials of Lavoisier Institute), are commonly incorporated to enhance PEC performance by offering a high surface area and facilitating efficient charge transport. Composite systems are commonly employed to overcome the limitations of individual PEC catalysts. In this study, a highly efficient NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ photoanode was developed to enhance photoelectrochemical water-splitting performance. The composite was synthesized via a hydrothermal method with a two-step heating process. X-ray diffraction confirmed the expected crystal structures, with peak broadening in NiFe₂O₄ indicating reduced crystallite size and increased lattice strain. X-ray photoelectron spectroscopy of the Ni 2p and Fe 2p regions validated the successful integration of NiFe₂O₄ into the composite. Electrochemical analysis demonstrated excellent performance, with linear sweep voltammetry achieving a peak photocurrent density of 3.5 mA cm⁻² at 1.23 V (vs RHE). Electrochemical impedance spectroscopy revealed a reduced charge-transfer resistance of 50 Ω, indicating improved charge transport. Optical and electronic properties were evaluated using UV-Vis spectroscopy and Tauc plots, revealing a direct bandgap of 2.1 eV. The composite exhibited stable photocurrent under amperometric J-t testing for 2000 s, demonstrating its durability. These findings underscore the potential of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ as a promising material for renewable energy applications, particularly in photoelectrochemical water splitting.

1. Introduction

Currently, researchers are primarily focused on converting solar energy into green energy storage solutions to address the ongoing energy crisis [1,2,3,4,5,6,7,8]. One promising approach is the production of hydrogen fuel through the conversion of solar energy into sustainable energy. In this context, photoelectrochemical (PEC) water splitting plays a pivotal role in harnessing sunlight to generate clean and renewable hydrogen, thereby reducing dependence on fossil fuels and helping to meet global energy demands [8,9,10,11,12]. The development of novel photocatalysts is crucial for enhancing the efficiency of hydrogen fuel production [13,14]. Among various semiconductors, metal oxide-based nano materials have developed as promising photocatalysts due to their unique structural features and favorable optoelectronic properties [15,16]. For example, NiFe₂O₄ is a spinel ferrite that is inexpensive, widely available, and exhibits favorable electrical properties that facilitate efficient charge transfer [17]. These characteristics make NiFe₂O₄ an attractive candidate for PEC applications, as it offers both the stability and catalytic activity essential for effective hydrogen production [18]. However, metal oxide-based photocatalysts also face challenges in PEC applications, such as wide bandgaps, fast charge recombination, low surface area, poor conductivity, and limited stability. These issues hinder their efficiency in visible light absorption and hydrogen production [19].

Recently, metal–organic frameworks (MOFs) have attracted significant attention as visible-light-absorbing materials and co-catalysts for semiconductor photoanodes, offering advantages over conventional water-splitting co-catalysts such as CoOOH and NiOOH. Visible-light-absorbing MOFs can enhance optical harvesting efficiency, while their porous structures do not hinder light penetration to the underlying semiconductor, facilitating accelerated charge transfer. Additionally, the high surface area and porosity of MOFs support effective diffusion of ions, electrolyte solutions, and gas bubbles, contributing to structural stability and sustained PEC performance [20,21,22]. Moreover, negatively charged coordinating groups and nonmetal atoms in MOFs can modulate the coordination environment of metal centers, thereby optimizing intermediate binding energies through d-band center tuning [23]. The well-ordered architecture of MOFs also provides remarkable synthetic tunability due to their flexible organic linkers and adjustable metal nodes, enabling tailored integration into photoelectrodes for enhanced PEC performance and transformation of traditional photoelectronic materials [24,25].

Nanocomposite catalysts significantly enhance PEC water-splitting performance by offering synergistic effects, enhanced light harvesting, improved charge separation and transport, increased active sites, optimized band alignment, and superior structural stability. For example, integrating the high surface area and excellent charge-transfer properties of carbon materials with the strong light absorption and catalytic capabilities of MOFs results in highly efficient photocatalysts [26,27]. This unique combination enhances solar energy harvesting and effectively drives the redox reactions required for hydrogen production [28]. MOFs offer solutions through their tunable bandgaps, high porosity, and large surface area, which enhance light harvesting and catalytic activity. MOFs also facilitate better charge separation and transport, reducing electron–hole recombination. When hybridized with metal oxides, MOFs improve conductivity and stability, making them promising materials for enhancing photoelectrochemical water-splitting efficiency. Their modular structure allows further optimization via functionalization or doping. Heterostructured composites play a crucial role in enhancing PEC water splitting by forming interfaces between semiconductors with different band structures [29,30,31,32]. These interfaces generate internal electric fields that promote efficient charge separation and directional transport of electrons and holes, minimizing recombination losses [33,34,35]. Additionally, heterojunctions allow for tailored band alignment, facilitating redox reactions and improving overall photocatalytic efficiency. By combining materials with varying bandgaps, heterostructures also broaden the light absorption range and improve solar energy utilization. Moreover, the interfacial regions often provide more active sites, further boosting the hydrogen and oxygen evolution reactions.

In this study, we developed an innovative NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite using a hydrothermal method with a two-step heating process. The integration of NiFe₂O₄ with nitrogen-doped graphene (NG), MIL-100(Fe), and TiO₂ aims to enhance three key aspects of PEC catalysts: charge separation, light absorption, and electronic transport. MIL-100(Fe), a MOF composed of Fe(III) ions and 1,3,5-benzenetricarboxylic acid linkers, offers a high surface area, mesoporosity, and structural stability, making it ideal for photocatalysis and PEC applications. Its iron centers facilitate light absorption, charge mediation, and efficient electron transport, improving catalytic performance. The combination of MIL-100(Fe) with the other components enhances visible light absorption and charge transport, optimizing water-splitting processes. The hydrothermal synthesis ensures proper integration of these components into a single structure, resulting in enhanced PEC efficiency and increased photocurrent density. Together, these elements offer a promising approach for sustainable hydrogen production through a breakthrough in photocatalytic materials.

2. Materials and Methods

2.1. Materials

The following materials were required for the hydrothermal synthesis of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂: The nickel source is Ni(II) Nitrate Hexahydrate (Ni(NO₃)₂·6H₂O); the iron source is Ferric Nitrate Nonahydrate (Fe(NO₃)₃·9H₂O); the carbonaceous support is Graphene Oxide (GO); and the extra component is Titanium Dioxide (TiO₂). Most of the chemicals used in this study were purchased from Aladdin Scientific (Shanghai, China). Deionized water was used as solvent, and solutions of sodium hydroxide (NaOH) and hydrochloric acid were used for pH adjustment.

2.2. Synthesis of NiFe₂O₄

A one-pot hydrothermal method was used for synthesizing NiFe₂O₄ nanoparticles by reacting sodium hydroxide (NaOH) with deionized water as the solvent, and nickel nitrate hexahydrate Ni(NO₃)₂·6H₂O, ferric nitrate nonahydrate Fe(NO₃)₃·9H₂O (both with 99.8% purity, Sigma-Aldrich, St. Louis, MO, USA), and sodium hydroxide (NaOH) were used as precursors. The ratio of nitrates (grams) to water (milliliters) was kept at 1:3, and the nitrates were dissolved in distilled water. Until nanoparticle production was seen, this solution was vigorously swirled. To get pure spinel phases, NaOH was then progressively added to the mixture at a ratio of 1:4 (NaOH: nitrates). The pH was then maintained at 11. After two hours of vigorous stirring, the resultant liquid was put into a 300-mL Teflon-lined stainless-steel autoclave. After being sealed, the autoclave was hydrothermally treated for 24 h at 180 °C. Following the procedure, the autoclave was allowed to gradually drop down to ambient temperature. The precursor material was filtered and repeatedly cleaned with distilled water and ethanol. The nickel ferrite nanoparticles were then separated and allowed to dry for 6 h at 60 °C [36].

2.3. Synthesis of Nitrogen-Doped Graphene

Graphene oxide (GO) was created from graphite powder to create nitrogen-doped graphene (NG). Generally, concentrated sulfuric acid was present when 1 g of graphite powder and 0.5 g of sodium nitrate (NaNO₃) were combined. After the mixture was frozen for t = 24 h, 3 g of KMnO₄ were added. The mixture was heated to 40 °C for 10 min. Then, 46 mL of water was added, and the temperature was gradually raised to 90 °C. After applying hydrogen peroxide to the whole mixture, the resultant product, or GO, was filtered, cleaned with a 5% HCl solution, and dried at 60 °C for 24 h. The nitrogen source (e.g., typically around urea 0.5 g) was dissolved in the graphene oxide dispersion. Stir the mixture using a sonicator (Tianjin Automatic Science Instrument Co., Tianjin, China) for 2 h and a magnetic stirrer for about 30 min to ensure complete dissolution, and then transfer the solution into a Teflon-lined autoclave (Tefic biotech, Xian, China) and keep it in the oven at 200 °C for 15 h. After that, it was cooled at room temperature, washed with water and ethanol, and dried at 60 °C for 8 h [37,38].

2.4. Synthesis of NiFe₂O₄/NG

The synthesis of the NiFe₂O₄/NG nanocomposite was carried out hydrothermally. In deionized water, nitrogen-doped graphene (NG) was continuously stirred while NiFe₂O₄ was dissolved in a 1:2 molar ratio. After bringing the mixture’s pH down to around 10–11, NG was added, and metal hydroxides started to develop. After that, sodium hydroxide, or NaOH, was progressively added. The liquid was stirred for 2 h before being put in a Teflon-lined stainless-steel autoclave (Tefic biotech, Xian, China). The sealed autoclave was heated to 180 °C for 24 h to facilitate the crystallization of NiFe₂O₄ and the incorporation of NG. After cooling to room temperature, the product was filtered and cleaned using ethanol and deionized water [39].

2.5. Synthesis of MIL-100(Fe)

The synthesis procedure was adapted from the work of Guesh et al. [32]. First, 2 g of Fe(NO₃)₃⋅9H₂O was dissolved in 30 mL of deionized water and stirred for 30 min to ensure complete dissolution. Separately, benzene-1,3,5-tricarboxylic acid was dissolved in 20 mL of ethanol and also stirred for 30 min. The two solutions were then gradually combined to form a homogeneous mixture. To control particle size and morphology, the TMA suspension was added dropwise to the mixture. The resulting solution was transferred into a Teflon-lined stainless steel autoclave and placed in a preheated oven at 180 °C for 16 h. After the reaction, the vessel was carefully removed and allowed to cool to room temperature. The product was collected by centrifugation and washed with ethanol and water to adjust the pH and remove any unreacted residues. Finally, the sample was dried in an oven at 60 °C for 8 h [40,41].

2.6. Synthesis of NiFe₂O₄/NG@MIL-100(Fe) Nanocomposite

The NiFe₂O₄/NG@MIL-100(Fe) nanocomposite was made by hydrothermal methods utilizing a mixture of metal nitrates and MIL-100(Fe). NiFe₂O₄/NG was dissolved in deionized water with the incorporation of nitrogen-doped graphene (NG) into the solution under continuous agitation. To confirm the precipitation of metal hydroxides and their deposition onto NG, sodium hydroxide (NaOH) was gradually added to the mixture, raising the pH to about 10–11. Before the final mixture was amalgamated with MIL-100(Fe), it was stirred for 2 h.

Following the addition of MIL-100(Fe) and the NiFe₂O₄/NG precursor, the entire mixture was placed in a stainless-steel autoclave lined with Teflon. To ensure proper crystallization of NiFeO₄ and uniform inclusion of NG and MIL-100(Fe), the autoclave was shut and heated to 180 °C for 24 h. After the product naturally cooled to room temperature, it was filtered and thoroughly cleaned with ethanol and deionized water to get rid of impurities. To improve crystallinity and ensure stable integration of the composite components, the NiFe₂O₄/NG@MIL-100(Fe) nanocomposite was optionally calcined at 400 °C for 2 h after being dried at 60 °C for 6–8 h.

2.7. Synthesis and Fabrication of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ Nanocomposite

A combination of metal nitrates, MIL-100(Fe), and TiO₂ was used in the hydrothermal production of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite (Figure 1). Deionized water was used to dissolve ferric nitrate nonahydrate (Fe(NO₃)₃·9H₂O) and nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) in a 1:2 molar ratio. The solution was continuously stirred while nitrogen-doped graphene (NG) was added. Sodium hydroxide (NaOH) was incrementally added to modify the pH to around 10–11, promoting the precipitation of metal hydroxides and their accumulation on NG. The resultant mixture was agitated for 2 h. MIL-100(Fe) was manufactured by dissolving 2 g of Fe(NO₃)₃·9H₂O in 30 mL of deionized water and combining it with benzene-1,3,5-tricarboxylic acid diluted in ethanol. The two solutions were combined to create a homogeneous suspension. Upon preparation of the NiFe₂O₄/NG precursor, it was amalgamated with the synthesized MIL-100(Fe) and commercially sourced TiO₂. The complete mixture was placed into a Teflon-lined stainless-steel autoclave, sealed, and subjected to heating at 180 °C for 24 h to facilitate the crystallization of NiFe₂O₄ and the uniform incorporation of NG, MIL-100(Fe), and TiO₂. Upon naturally cooling to ambient temperature, the product was subjected to filtration and extensively cleaned with deionized water and ethanol to eliminate contaminants. The NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite was dried at 60 °C for 24 h and optionally calcined at 600 °C for 2 h to enhance crystallinity and ensure stable integration of all components. To prepare the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ photoanode, 0.05 g of the nanocomposite was dispersed in distilled water and stirred for 30 min. The resulting suspension was then applied onto a 1 cm × 1 cm fluorine-doped tin oxide (FTO) glass substrate using the carbon paint method. The coated substrate was pre-dried on a hotplate at 60 °C for 10 min, followed by overnight drying at room temperature.

2.8. Characterization

D/MAX-2500 X-ray diffractometer (Ricagu, Tokyo, Japan) was used to confirm the X-ray diffraction (XRD) analysis with Cu Kα radiation while maintaining a scanning rate of 10 °C min⁻¹; the X-ray current was kept for 40 mA, and an X-ray voltage was conducted at 40 kV. For analyzing bond formations and functional groups, the Fourier transform infrared (FTIR) was conducted, and the spectra of the samples were recorded at 25 °C, covering the wavenumber range of 4000 to 400 cm⁻¹ (Perkin Elmer Spectrum GX FTIR Spectrometer, Waltham, MA, USA). The analysis of oxidation state and electron movement of the photoanode was conducted by using X-ray photoelectron spectroscopy (XPS) (ULVAC-PHI, Kanagawa, Japan). JEOL JSM-6701F (Tokyo, Japan) scanning electron microscope (SEM) was used to confirm surface morphology and structural analysis, and the morphological structures of the photoanodes was confirmed by transmission electron microscope (TEM, 2100F JEM, JEOL Ltd., Akishima, Japan). UV–vis diffuse reflectance spectra (UV–vis) were conducted to measure the bandgap of photoanodes at room temperature in the 200–800 nm range (Perkin Elmer, Waltham, MA, USA).

Using a conventional three-electrode configuration, the electrochemical performance was evaluated using linear sweep voltammetry (LSV). The working electrodes employed in the assessment were NiFe₂O₄, NiFe₂O₄/NG, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂. An Ag/AgCl electrode was used as the reference electrode, and a platinum electrode served as the counter electrode in this experiment. The electrodes were immersed in a 1 M NaOH solution. The LSV measurements were performed using a potentiostat (PGSTAT128N, Metrohm Autolab, Utrecht, The Netherlands), a piece of equipment frequently utilized in electrochemical research. The optical absorption spectra were obtained using a Perkin-Elmer Lambda 950 UV-vis spectrometer (PerkinElmer, Waltham, MA, USA), as previously mentioned. Electrochemical impedance spectroscopy (EIS) analysis was performed in the dark, with an amplitude of 10 mV, and in the frequency range of 1 MHz to 1 Hz.

One method frequently used in scientific study to ascertain the flat-band potential of photoelectrodes is the Mott–Schottky plot. Equation (1) can be used to compute the x-axis intercept and derive the flat-band potentials (V_fb):

$${\displaystyle \frac{1}{{\mathrm{C}}^2}}=\left({\displaystyle \frac{2}{{\mathrm{e}}_0\mathsf{\epsilon }{\mathsf{\epsilon }}_0{\mathrm{N}}_{\mathrm{d}}}}\right)\left[\mathrm{V}-{\mathrm{V}}_{\mathrm{f}\mathrm{b}}-{\displaystyle \frac{\mathrm{k}\mathrm{T}}{{\mathrm{e}}_0}}\right]$$

(1)

A semiconductor is made up of the following parts: In this study, the variable of interest is the applied voltage, as V. One of the most important parameters in this investigation is the charge carrier density, or N_d. An indicator of the capacitance connected to the space charge in a system is the space charge capacitance, as C. T stands for temperature. One of the fundamental constants in physics is the Boltzmann constant, represented by the letter k.

2.9. Photoelectrochemical Water Splitting

For the PEC water-splitting activity, 0.1 M Na₂SO₄ solution was used with a three-electrode configuration. This included Ag/AgCl reference electrode, a synthesized photoanode as the working electrode, and platinum as the counter electrode. The measurements of both dark and illuminated currents were taken using the PGSTAT128N potentiostat (Metrohm AG, Herisau, Switzerland) equipped with a 350 W xenon lamp and 1.5 AM filter. Calculations were performed to determine the charge carrier densities and conduction band using a Mott–Schottky analysis at 1 kHz.

3. Results and Discussions

3.1. Physiochemical Characterizations of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂

3.1.1. XRD Analysis

The XRD pattern of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite was obtained and is presented in Figure 2. The diffraction peaks observed at 2θ values of 30.3°, 35.6°, 37.3°, 43.3°, 53.7°, 57.2°, and 62.9° correspond to the cubic spinel structure of NiFe₂O₄ and match well with (JCPDS No. 01-074-2081) [42]. These peaks are indexed to the (220), (311), (222), (400), (105), (511), and (440) planes, confirming the presence of NiFe₂O₄ in the nanocomposite as shown in Figure S1. Peaks attributed to the anatase phase of TiO₂ were also identified at 2θ values of 25.3°, 37.8°, 48.0°, and 55.1°, which corresponded to match with JCPDS No. 01-070-7348 [43]. Additionally, the diffraction peaks at 2θ values of 15.9°, 19.6°, and 26.9° indicate the presence of the MIL-100(Fe) framework (COD: 7102029) [44]. The broad peaks around 26.0°, 43.5° and 45.0° are attributed to the graphitic (002), (400), and (101) planes of nitrogen-doped graphene (NG), indicating its exfoliated state in the composite. The combined analysis confirms the successful synthesis of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite with all constituent materials incorporated into the final structure.

The XRD pattern for the NiFe₂O₄/NG@MIL-100(Fe) nanocomposite shows characteristic peaks at 2θ values of 30.3°, 35.6°, 37.3°, 43.3°, 53.7°, 57.2°, and 62.9°, corresponding to the (220), (311), (222), (400), (105), (511), and (440) planes of cubic spinel NiFe₂O₄. These peaks match well with the standard JCPDS No. 01-074-2081 [42] confirming the formation of the NiFe₂O₄ phase in the composite. Additionally, peaks attributed to the MIL-100(Fe) framework (COD: 7102029) [44] are observed at 2θ values of 15.9°, 19.6°, and 26.9°, indicating the presence of MIL-100(Fe) consistent (COD: 7102029) [44] with the material shown in Figure S1. The XRD pattern for NiFe₂O₄/NG exhibits peaks at similar 2θ values to those of the NiFe₂O₄ phase, including notable peaks at 30.3°, 35.6°, 43.3°, and 57.2°, corresponding to the (220), (311), (400), and (511) planes, respectively. The broad peak around 26° can be attributed to the (002) plane of nitrogen-doped graphene (NG), indicating its graphitic structure. However, the NG signature may appear less intense due to the dominance of NiFe₂O₄ peaks.

Finally, the XRD pattern of pure NiFe₂O₄ reveals distinct peaks at 2θ values of 30.3°, 35.6°, 37.3°, 43.3°, 53.7°, and 62.9°, corresponding to the (220), (311), (222), (400), (511), and (440) planes. These peaks align well with the standard JCPDS No. 01-074-2081 [42] confirming the formation of pure cubic spinel NiFe₂O₄ shown in Figure S1.

3.1.2. Morphology Studies

Advanced characterization techniques were utilized to gain insights into the structural, morphological, and optoelectronic properties of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite. Moreover, transmission electron microscopy (TEM) and field emission scanning electron microscopy (FESEM) images were used to analyze the samples’ surface morphology and microstructures. At a lower magnification of 100 nm, TEM images reveal distinct features and particles with irregular, somewhat rounded shapes (Figure 3a). While the individual particles become more distinct at a greater magnification (50 nm, Figure 3b), the particles seem crowded together with different diameters, suggesting some degree of agglomeration. Although the general forms and edges are still rounded, the particle surface’s finer features are visible.

The HRTEM image in Figure 3c at 5 nm shows the information about the material’s internal crystal structure and matches the NiFe₂O₄ XRD crystals (311) by displaying a high-resolution lattice image with visible atomic-scale lattice fringes of 0.49 nm, confirming crystallinity and enhancing the structural stability of the composite. Figure 3d shows the selected area electron diffraction (SAED) pattern of the sample, proving the polycrystalline nature and well-distributed TiO₂ coating on the surface of NiFe₂O₄/NG@MIL-100(Fe). SEM images in Figure 3e–g display the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite. NiFe₂O₄ nanoparticles were uniformly distributed on the surface of the NG@MIL-100(Fe) matrix, as seen by the overall morphology of the nanocomposite at 500 µm in Figure 3e. A homogeneous structure is produced by the uniform distribution of TiO₂ nanoparticles on the composite, as shown by SEM. Figure 3f shows the surface morphology of the composite at a high magnification of 200 µm. It highlights the even distribution of TiO₂ particles on the NiFe₂O₄/NG@MIL-100(Fe) surface. This distribution is essential for improving photocatalytic performance.

Figure 3g at 1 µm shows the cross-section of the composite, illustrating the layered structure of the nanocomposite and demonstrating the uniform distribution of the various components (NiFe₂O₄,NG, MIL-100(Fe), and TiO₂), thereby confirming the successful synthesis of the composite material.

A scanning transmission electron microscope (STEM) was used to further probe the minute structural elements. A high-resolution view of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite at the atomic level is shown by the STEM picture in Figure 3h. The efficient integration of the NiFe₂O₄,NG, MIL-100(Fe), and TiO₂ components is demonstrated by the STEM investigation without any apparent phase separation noted, which guarantees the homogeneity and uniformity of the composite material at the nanoscale.

The nanocomposite’s elemental composition was ascertained by elemental mapping combined with energy dispersion X-ray spectroscopy (EDS). Figure 3i–n show the EDX mapping analysis results of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂, revealing the uniform dispersion of Ni, Fe, O, N, C, and Ti. The percentage distribution of these elements is presented in Table S1. Figure 4 shows the EDX spectrum of the sample, providing further evidence of the existence of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂.

3.1.3. FTIR Analysis

The presence of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂, NiFe₂O₄/NG@MIL-100(Fe), NiFe₂O₄/NG, and NiFe₂O₄ was confirmed and is presented by FTIR analysis as shown in Figure 5. The broad peaks around 3419 cm⁻¹ in all samples are attributed to the O–H stretching vibrations, indicating the presence of adsorbed water molecules on the surface of the materials. The peaks at 1650 cm⁻¹ in the NiFe₂O₄/NG and NiFe₂O₄/NG@MIL-100(Fe) spectra correspond to the bending vibrations of adsorbed water molecules. In the NiFe₂O₄ spectrum, distinct peaks are observed at 1697 cm⁻¹ and 1339 cm⁻¹, which are associated with the stretching vibrations of C=O and C–N bonds, respectively. The stretching vibrations of the organic linker in the MIL-100(Fe) structure are responsible for the bands at 1716 cm⁻¹ in the NiFe₂O₄/NG@MIL-100(Fe) spectra. The peaks observed in the range of 1069–1071 cm⁻¹ in the spectra of NiFe₂O₄, NiFe₂O₄/NG, and NiFe₂O₄/NG@MIL-100(Fe) indicate C–O stretching vibrations. Additionally, peaks at 790–769 cm⁻¹ in NiFe₂O₄/NG@MIL-100(Fe) are assigned to the Ti–O–Ti stretching vibrations, confirming the presence of TiO₂ in the composite. Lastly, the strong absorption bands at 737 cm⁻¹ and 1069 cm⁻¹ in the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite confirm the presence of both MIL-100(Fe) and TiO₂, suggesting a successful incorporation of both components in the final nanocomposite structure.

3.1.4. XPS Analysis

Figure 6 shows the existence of different components, chemical nature, and bonding in NiFe₂O₄/NG@MIL-100(Fe)/TiO₂, which was investigated using XPS. The figure illustrates the complexity of the composite by clearly characterizing the presence of each of the constituent elements, including Ni, Fe, N, C, Ti, and O. The high-resolution Ni XPS spectrum reveals two distinct Ni-related peaks (Figure 6a). The peaks at 855.95 eV and 873.51 eV correspond to Ni²⁺ 2p_3/2 and Ni²⁺ 2p_1/2, respectively, confirming the presence of Ni²⁺ oxidation states. Additionally, a satellite peak is observed, further validating the Ni²⁺ configuration in the composite structure [45,46]. XPS data for Fe 2p present two main Fe³⁺ peaks at 711.54 eV (Fe³⁺ 2p_3/2) and 724.84 eV (Fe³⁺ 2p_1/2) supported by satellite peaks at 719.85 eV and 733.92 eV, indicating the presence of Fe³⁺ oxidation states (Figure 6b). The spectrum displays two peaks at 709.5 eV (Fe²⁺ 2p_3/2) and 723.2 eV (Fe²⁺ 2p_1/2) that indicate trace amounts of divalent iron. The fitted spectral data reveal both Fe²⁺ and Fe³⁺ species in the material, yet Fe³⁺ predominates, as observed in NiFe₂O₄ samples. The observed satellite peaks, together with binding energies, validate that Fe³⁺ represents the main species in the sample moving forward, although weak Fe²⁺ signals could stem from synthesis-related reduction processes or fitting errors [47,48,49,50,51]. The N 1s XPS spectrum is deconvoluted into three individual peaks, each representing a different nitrogen species (Figure 6c). The peaks are attributed to pyridinic N at 398.72 eV, pyrrolic N at 399.92 eV, and graphitic N at 401.54 eV. This demonstrates the distribution of nitrogen species within the material [52,53]. Several peaks that correspond to different carbon bonds can be seen in the C 1s spectrum (Figure 6d). C-C at 284.8 eV, C-N at 285.9 eV, C-O at 286.32 eV, and C=O at 288.12 eV are among them; they verify that several carbon species are present in the composite and are interacting with other elements [54,55,56,57]. Two notable peaks in the Ti 2p spectrum are seen at 458.51 eV (Ti⁴⁺ 2p_3/2) and 464.52 eV (Ti⁴⁺ 2p_1/2) (Figure 6e). This validates titanium’s oxidation state in TiO₂, which is essential to the composition and characteristics of the material [58,59,60]. Several peaks that represent various oxygen species can be seen in the O 1s spectrum (Figure 6f). While additional peaks show the presence of metal–oxide (M-O) and metal–carbonate interactions, a prominent peak at 530.54 eV is linked to C=O bonds. This spectrum offers important information on the oxygen-bonding environment in the composite material. [61,62].

3.1.5. BET Measurement

BET curve for NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ is shown in Figure S2, and the inset shows the pore diameter distribution. Table S2 displays the BET surface area values of all the samples. NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ has a surface area value of 661 m²/g, which is significantly higher than that of pristine NiFe₂O₄ (60.5 m²/g). The increased surface area of the composite allows for better electron transport which results in improved PEC performance.

3.1.6. Williamson–Hall Plot

The Williamson–Hall (W-H) plot demonstrates how the lattice strain affects the nanocomposites (Figure 7). According to the W-H analysis, the slope of regression lines provides data about the lattice strain magnitude. According to the analysis, the three samples (NiFe₂O₄/NG@MIL-100(Fe)/TiO₂, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG) demonstrated tensile strain through positive slopes, but the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite showed the maximum tensile strain. NiFe₂O₄ without modifications undergoes compressive strain caused by its lower number of crystal defects. These experimental results demonstrate that introducing NG alongside MIL-100(Fe) and TiO₂ to the composites generates tensile strain effects. The tensile strain noted in these materials results in superior optoelectronic and photoelectrochemical properties that positively affect water-splitting performance.

3.2. Optoelectronic Analysis

The light absorption patterns together with Tauc plot data for NiFe₂O₄, NiFe₂O₄/NG, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposites are shown in Figure 8a. All samples exhibit significant absorbance throughout both visible and ultraviolet spectrums during measurements between 200–800 nm. Light-harvesting capabilities of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ reach their peak due to its highest absorbance within the visible range of the spectrum. The strong light absorption capability is vital for improving photocatalytic and photoelectrochemical efficiency during water-splitting reactions. The research shows that the structural integration of NG and MIL-100(Fe) together with TiO₂ generates substantial light absorption ratio in the visible spectrum of the composite material. NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ demonstrates maximum light absorption all throughout the visible wavelength region, making it highly efficient in terms of light harvesting. Two factors work together to boost photonic activity through nitrogen-doped graphene (NG) and MIL-100(Fe) and TiO₂ synergies. The optical properties of NiFe₂O₄ have improved after structural modifications and doping because the undoped NiFe₂O₄ sample demonstrates the lowest visual light absorbance ratio. The applied doping method shifts the absorption edge toward shorter wavelengths, resulting in a reduction of optical band gap energy, thus potentially enhancing photocatalytic efficiency under visible light conditions. These nanocomposites’ band gap energy (E_g) was analyzed using the Tauc plot, as illustrated in Figure 8b. The absorption coefficient α was calculated using the following equation:

$$\mathsf{\alpha }={\displaystyle \frac{1}{\mathrm{d}}}\ln \left({\displaystyle \frac{1}{\mathrm{T}}}\right)$$

(2)

where α is the optical absorption coefficient, d is the sample thickness, and T is the transmission. The Tauc plot relates the absorption coefficient to the incident photon energy by the equation

$$\mathsf{\alpha }{\mathrm{h}}_{\mathrm{v}}=\mathrm{A}{\left(\mathrm{h}\mathrm{v}-{\mathrm{E}}_{\mathrm{g}}\right)}^{\mathrm{n}}$$

(3)

where A is a constant, E_g is the band gap energy, and n is ½ for direct band gap transitions. From the Tauc plot, the band gap energy for the synthesized samples was found to decrease in the doped nanocomposites compared to pristine NiFe₂O₄, with the values approximately as follows: NiFe₂O₄ ~2.4 eV, NiFe₂O₄/NG ~2.3eV, NiFe₂O₄/NG@MIL-100(Fe) ~2.2 eV, and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ ~2.1 eV. Among the samples, NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ shows the highest absorbance in the visible region and the lowest band gap of ~2.1 eV, which enhances its light-harvesting capability. The reduction in the band gap energy across the doped samples, particularly NiFe₂O₄/NG@MIL-100(Fe)/TiO₂, confirms their superior light absorption. These improvements in light absorption and reduced band gap make the nanocomposite an efficient candidate for photoelectrochemical water-splitting applications.

3.3. Electrochemical Characterizations and Photoelectrochemical Water Splitting

Photocurrent densities were evaluated under illumination to assess the photoelectrochemical water-splitting performance of NiFe₂O₄, NiFe₂O₄/NG, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂. The NiFe₂O₄/NG@ MIL-100(Fe)/TiO₂ nanocomposite exhibits the greatest IPCE, peaking at around 80% in the visible spectrum, according to Figure 9a, which shows the incident photon-to-current efficiency (IPCE) (%) as a function of wavelength (nm). This high efficiency highlights its exceptional optoelectronic properties and enhanced ability to harvest light across a broad wavelength range. The remarkable performance is attributed to the synergistic interactions between the components of the nanocomposite, which facilitate efficient charge separation and transfer. These characteristics make NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ a promising candidate for water-splitting applications [63].

TRPL (Time-resolved photoluminescence) analysis was employed to analyze NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ in order to acquire valuable knowledge about carrier behavior, focusing on their recombination mechanisms (Figure S3). The TRPL spectra demonstrate time-dependent photoluminescence intensity decay behavior whose rates differ between samples, resulting in color-coded curves. Faster decay behavior in the TRPL spectra indicates elevated recombination rates for charge carriers, thus minimizing their efficient separation that usually affects photocatalytic performance negatively. The speed of decay in photoluminescence intensity provides insight into charge carrier separation capabilities because rapid decay signifies efficient charge separation. The observed outcomes carry substantial importance for water-splitting technology because extended charge-carrier durations both enhance photoelectrochemical reaction efficiency and decrease energy waste through improved charge separation.

The photoelectrochemical water-splitting performance of different samples was characterized by measuring the photocurrent densities under AM 1.5 G simulated sunlight (100 mW/cm²). Electrochemical impedance spectroscopy (EIS) was conducted to understand the charge-transfer kinetics at the electrode–electrolyte interface. The Nyquist plots, shown in the provided Figure 9b, reveal the electrochemical impedance of the pristine and doped samples. The charge-transfer resistance (RCT) is inferred from the radius of the arcs in the Nyquist plots; smaller arc radii indicate lower charge-transfer resistance, which facilitates more efficient electron transfer during the water-splitting process [64]. Among the nanocomposites, NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ shows the lowest charge-transfer resistance, both under light and dark conditions, indicating its superior ability to facilitate charge transfer at the semiconductor–electrolyte interface presented. A commonly used fitting model consisting of a resistive–capacitive (RC) circuit with two equivalent resistances, RS and RCT, and a constant phase element (CPE1) was employed for the doped samples to model the electrochemical behavior. The fitted values for charge-transfer resistance agree with the arc radii observed in the Nyquist plots. The significantly lower RCT for NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ confirms its superior charge-transfer capability, making it highly promising for water-splitting applications presented in Figure 9b.

Figure 9c shows the the half-cell applied bias photon-to-current efficiency (HC-ABPE%) of NiFe₂O₄, NiFe₂O₄/NG, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂. According to the data, the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite has the greatest HC-ABPE%, peaking at around 2.5% at about 0.7 V. NiFe₂O₄/NG@MIL-100(Fe) and NiFe₂O₄/NG come in second and third, respectively, with NiFe₂O₄ demonstrating the lowest efficiency. The structural synergy between NiFe₂O₄, NG, MIL-100(Fe), and TiO₂ is responsible for the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposite’s better HC-ABPE%, which highlights its improved optoelectronic capabilities and efficient charge separation. This improvement in performance highlights the promise of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ as an effective photoelectrochemical material for water-splitting applications, which is consistent with the structural and optoelectronic characteristics of the nanocomposite that were developed for the best possible solar energy consumption.

Mott–Schottky (MS) analysis was performed to study the effect of the composite structure on carrier density (N_d) and flat-band potential (V_fb) for the nanocomposites. As shown in the Mott–Schottky plots in Figure 9d, the positive slopes of all samples confirm the n-type semiconductor behavior, with electrons as the majority of charge carriers [65]. Among the samples, NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ demonstrated the highest carrier density, which was nearly double that of the pristine NiFe₂O₄ sample. The increased carrier density is attributed to the introduction of MIL-100(Fe) and TiO₂, which enhance the conductivity and mobility of charge carriers [66,67]. The flat-band potential (V_fb) for NiFe₂O₄, NiFe₂O₄/NG, NiFe₂O₄/NG@MIL-100(Fe), and NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ was found to be 0.45 V, 0.35 V, 0.50 V, and 0.58 V (vs. RHE), respectively. A lower flat-band potential implies easier charge separation under an applied potential, which is crucial for efficient water-splitting applications [68].

The impact of the composite structure on photocurrent density was examined in a 1 M NaOH solution using linear sweep voltammetry (LSV). Of the materials analyzed, NiFe₂O₄/NG@MIL-100(Fe)/TiO₂) had the highest photocurrent density, measuring over 3.5 mA cm⁻² at 1 V (vs. Ag/AgCl) (Figure 9e). Nitrogen-doped graphene (NG), MIL-100(Fe), and TiO₂ work in concert to enhance conductivity, light absorption, and charge separation, all of which lead to improved performance results. The enhanced photocurrent density highlights the potential applications of this nanocomposite in efficient water-splitting systems.

An amperometric current density versus time curve was used to evaluate the stability of the NiFe₂O₄/NG@MIL-100(Fe/TiO₂ nanocomposite), showing a constant photocurrent density of 0.5 mA cm⁻² maintained for 2000 s at 1 V (vs. Ag/AgCl). As illustrated in Figure 9f, this validates the endurance of the nanocomposite and emphasizes its potential for the long term. In order to evaluate the reusability and stability of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite during photocatalytic cycles, XRD measurements were conducted before and after its use for photocatalytic operations (Figure S4). XRD data of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite reveals that its crystalline structure persists unchanged following photocatalytic usage in the reaction system. The material kept its structural integrity along with phase stability throughout the photocatalytic process because no major changes or phase transitions occurred in XRD peak positions.

A detailed comparison of PEC performance parameters with similar heterostructured photoanodes is provided in Table S2. The NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite exhibits competitive performance, indicating its potential for practical PEC applications.

3.4. Photoelectrochemical Mechanism for Water Splitting

The schematic diagram of the NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite in a photoelectrochemical (PEC) water-splitting system is shown in Figure 10, highlighting its crucial role in sunlight-driven hydrogen and oxygen production. This composite, integrating NiFe₂O₄, nitrogen-doped graphene (NG), MIL-100(Fe), and TiO₂, combines unique properties that significantly enhance photocatalytic performance.

NiFe₂O₄ acts as the primary light absorber, generating electron–hole pairs upon sunlight exposure. The excited electrons transfer through NG, which serves as an excellent conductive pathway, and subsequently move from MIL-100(Fe) to TiO₂ (3.2 eV), where TiO₂ facilitates charge separation and provides additional photocatalytic sites. By reducing electron–hole recombination, the photogenerated electrons are able to travel through the external circuit to the Pt cathode, where they accumulate and reduce hydrogen ions (H⁺) to hydrogen atoms, which subsequently combine to form hydrogen gas (H₂). Meanwhile, the holes in the valence band are directed to the photoanode surface, promoting the oxidation of water molecules and producing O₂ gas, thus completing the water-splitting process.

The NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite extends light absorption into the visible range, whereas TiO₂ alone primarily responds to ultraviolet light. The composite architecture efficiently integrates NG with MIL-100(Fe) and NiFe₂O₄, enhancing visible-light harvesting and charge separation, and reducing electron–hole recombination. Optimized band alignment among the components further elevates photocatalytic activity. Additionally, the nanocomposite exhibits superior charge mobility and lower charge transfer resistance, optimizing its PEC water-splitting performance. The synergistic roles of NG (electron transfer), TiO₂ (structural stability), and NiFe₂O₄ and MIL-100(Fe) (light absorption) collectively contribute to the high photoactivity of the system.

4. Conclusions

In this study, we synthesized and characterized NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ nanocomposites using a hydrothermal method. The structural, optoelectronic, and photoelectrochemical properties were optimized for water-splitting applications. XRD and microscopy confirmed well-defined crystal structures, while UV-visible absorption spectra and Tauc plot analysis revealed enhanced light-harvesting and reduced band gap energy. The NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ composite demonstrated the highest photocurrent density and improved charge separation efficiency, as indicated by linear sweep voltammetry (LSV) and electrochemical impedance spectroscopy (EIS). Mott–Schottky analysis further showed an increase in carrier density, confirming the material’s suitability for long-term photoelectrochemical performance. These results highlight the potential of NiFe₂O₄/NG@MIL-100(Fe)/TiO₂ as a promising photocatalyst for efficient water-splitting applications.