Improving Photocatalytic Hydrogen Production with Sol–Gel Prepared NiTiO₃/TiO₂ Composite

Abstract

This study presents a comprehensive investigation into the synthesis, characterization, and photocatalytic performance of NiTiO₃/TiO₂ nanocomposites for solar hydrogen production. Through a carefully optimized sol–gel method, we synthesized a heterojunction photocatalyst comprising 99.2% NiTiO₃ and 0.8% anatase TiO₂. Extensive characterization using XRD, Raman spectroscopy, FTIR, UV–visible spectroscopy, photoluminescence spectroscopy, and TEM revealed the formation of an intimate heterojunction between rhombohedral NiTiO₃ and anatase TiO₂. The nanocomposite demonstrated remarkable improvements in optical and electronic properties, including enhanced UV–visible light absorption and an 85% reduction in charge carrier recombination compared to pristine NiTiO₃. Crystallite size analysis showed a reduction from 53.46 nm to 46.35 nm upon TiO₂ incorporation, leading to increased surface area and active sites. High-resolution TEM confirmed the formation of well-defined interfaces between NiTiO₃ and TiO₂, with lattice fringes of 0.349 nm and 0.249 nm corresponding to their respective crystallographic planes. Under UV irradiation, the NiTiO₃/TiO₂ nanocomposite exhibited superior photocatalytic performance, achieving a hydrogen evolution rate of 9.74 μmol min−1, representing a 17.1% improvement over pristine NiTiO₃. This enhancement is attributed to the synergistic effects of improved light absorption, reduced charge recombination, and efficient charge separation at the heterojunction interface. Our findings demonstrate the potential of NiTiO₃/TiO₂ nanocomposites as efficient photocatalysts for solar hydrogen production and contribute to the development of advanced materials for renewable energy applications.

1. Introduction

The global energy crisis and climate change concerns have intensified the need for sustainable energy solutions, driving research into renewable energy technologies [1]. The Intergovernmental Panel on Climate Change (IPCC) has emphasized the urgent need to address greenhouse gas (GHG) emissions, particularly those released through fossil fuel combustion [2]. In this context, solar-driven photocatalytic water splitting for hydrogen production has emerged as a promising approach for sustainable fuel generation, offering a potential pathway to reduce dependence on fossil fuels [3,4].

Photocatalytic water splitting mimics natural photosynthesis by storing solar energy in the chemical bonds of hydrogen fuel. This process involves the absorption of photons by a semiconductor material, generating electron–hole pairs that drive the water-splitting reaction [5]. The efficiency of this process is critically dependent on the properties of the semiconductor photocatalyst, including its bandgap, charge carrier dynamics, and surface characteristics [6].

Among the various semiconductors investigated for photocatalytic hydrogen generation [7,8], titanium dioxide (TiO₂) has been extensively studied due to its stability, non-toxicity, and appropriate band positions [9]. However, the TiO₂ wide bandgap of 3.2 eV limits its solar efficiency, as it can only utilize the ultraviolet portion of the solar spectrum [10]. To address this limitation, researchers have explored various strategies, including the development of composite materials that can harness a broader range of the solar spectrum while maintaining the advantageous properties of TiO₂ [11].

Recent advances in heterojunction photocatalysts have demonstrated that type-II band alignment provides a particularly effective strategy for charge separation [12]. In this configuration, the staggered band positions between two semiconductors create a built-in electric field that facilitates spatial separation of photogenerated electrons and holes, significantly reducing recombination rates while maintaining the redox potentials necessary for water splitting reactions [13].

In recent years, nickel titanate (NiTiO₃) has attracted significant attention as a promising material for photocatalytic applications. NiTiO₃, a perovskite-type oxide, possesses a narrower bandgap of 2.25–2.49 eV, allowing for improved visible light absorption [14].

This characteristic makes NiTiO₃ an attractive candidate for forming a type-II heterojunction with TiO₂, where the band positions (NiTiO₃: CB at −0.31 V vs. NHE; TiO₂: CB at −0.5 V vs. NHE) [15,16] create an energetically favorable pathway for charge separation. The integration of NiTiO₃ with TiO₂ presents several potential benefits for photocatalytic hydrogen production, including an extended light absorption range, enhanced charge separation through the type-II heterojunction structure, improved surface area and active sites, and tailored band alignment [12].

The development of NiTiO₃/TiO₂ type-II heterojunction composites represents an innovative approach to enhancing hydrogen production efficiency. By combining the complementary properties of these two materials, it is possible to create a synergistic system that addresses the limitations of each component. The NiTiO₃/TiO₂ composite can potentially offer improved utilization of the solar spectrum, facilitated spatial separation of photogenerated electrons and holes through the type-II band alignment, and a higher number of catalytically active sites for water splitting reactions [17].

The synthesis method plays a crucial role in determining the properties and performance of NiTiO₃/TiO₂ composites. Various techniques have been employed, including sol–gel [18], hydrothermal [19], and solid-state reactions [20]. Each method offers unique advantages in terms of controlling the composition, morphology, and interface between NiTiO₃ and TiO₂. The sol–gel method, in particular, has gained popularity due to its versatility in producing homogeneous composites with controllable stoichiometry and high purity [21,22].

The photocatalytic mechanism involved in hydrogen generation using NiTiO₃/TiO₂ type-II heterojunction composites is multifaceted. Upon irradiation with light, electrons in the valence band of both NiTiO₃ and TiO₂ are excited to their respective conduction bands, creating electron–hole pairs. The type-II band alignment facilitates the transfer of electrons from NiTiO₃ to TiO₂, while holes move in the opposite direction. This directional charge separation is critical for driving the reduction of protons to hydrogen gas at the catalyst surface while also oxidizing water to oxygen [23].

In this work, we investigate the structural, optical, and electronic properties of NiTiO₃/TiO₂ type-II heterojunction composites through comprehensive characterization techniques. We examine the relationship between structural features and photocatalytic performance, providing insights into the charge transfer mechanisms and surface reactions that govern hydrogen evolution. Our findings contribute to the broader understanding of heterojunction photocatalysts and their application in renewable energy systems.

2. Materials and Methods

2.1. Materials

All chemicals used in this study were of analytical grade and employed without further purification. 2-propanol, nickel (II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), titanium(IV) isopropoxide (Ti(OC₃H₇)₄), and citric acid were purchased from Merck Company (Darmstadt, Germany).

2.2. Pure NiTiO₃ and NiTiO₃/TiO₂ Composite Synthesis

The synthesis of nickel titanate (NiTiO₃) nanoparticles was accomplished using a modified sol–gel method at room temperature (25 °C), adapting the procedure previously reported by Quispe et al. [24]. The synthesis procedure began with the preparation of two precursor solutions: Solution A, consisting of 0.5 M nickel nitrate hexahydrate dissolved in 21.7 mL of 2-propanol, and Solution B, comprising 0.625 g of citric acid dissolved in 4 mL of 2-propanol. Both solutions were stirred separately for 30 min to ensure complete dissolution. Solution A was then gradually added to Solution B under constant stirring, and the resulting mixture was stirred for 2 h to ensure homogeneity. Subsequently, 0.5 M titanium isopropoxide was introduced to the mixed solution, and stirring continued until a homogeneous, viscous green gel formed. This gel was subjected to a drying process at 40 °C for 48 h to form a xerogel, which was then ground into a fine powder using an agate mortar. The obtained powder was calcined at 600 °C, obtaining the NiTiO₃ nanoparticles.

The NiTiO₃/TiO₂ composite was prepared by adding extra Ti(OC₃H₇)₄ to create samples with 3 mol% TiO₂. The subsequent steps followed the same procedure as for pure NiTiO₃ synthesis.

2.3. Characterization

Crystal structure and phase composition were analyzed by X-ray diffraction (AERIS Research, PANalitycal, Almelo, The Netherlands) using Cu-K_α radiation, with patterns collected from 20° to 80°, 2θ, at a 0.002° step size. Rietveld refinement of XRD data provided crystallite sizes and structural parameters.

Raman spectra were measured using a preconfigured spectrometer (MAYA 2000-Pro, Ocean Optics, Dunedin, FL, USA) with a 532 nm laser to study vibrational modes in the 100–900 cm⁻¹ range. Surface chemical bonding was examined using ATR-FTIR spectroscopy (Invenio R, Bruker, Saarbrucken, Germany) from 400 to 4000 cm⁻¹. Optical properties were investigated by UV-vis DRS (Evolution 220, Thermo Scientific, San Jose, CA, USA) from 200 to 1100 nm using an integrating sphere attachment.

Morphological analysis was conducted using Transmission Electron Microscope (Thermo Scientific Co., Eindhoven, The Netherlands), equipped with EDS for elemental analysis. Photoluminescence spectra were acquired using a FluoroMax Plus spectrometer (Horiba Scientific, Irvine, CA, USA) to assess charge carrier dynamics.

2.4. Hydrogen Evolution Test

Photocatalytic hydrogen production was evaluated using a two-port jacketed cell with methanol as a sacrificial electron donor. The reaction medium consisted of 0.5 L aqueous solution containing 10% methanol at pH 10, in which 250 mg of photocatalyst was dispersed under continuous stirring. The reactor temperature was maintained at 25 °C using a 1400 W chiller (Vevor, Cucamonga, CA, USA).

Prior to illumination, the cell was purged with ultra-high purity argon (99.999%) to create an inert atmosphere. Illumination was provided by a high-intensity UV lamp (Ultra-Vitalux UV-A, OSRAM, Garching, Germany), delivering 13.6 W UVA and 3.0 W UVB. Hydrogen evolution was monitored using a Clark-type amperometric sensor (H2-NP, Unisense, Denmark) connected to data acquisition software V3.3 (Sensor Trace Basic, Unisense, Denmark). The sensor transduced hydrogen oxidation current to a proportional voltage signal, enabling real-time quantification of hydrogen production.

3. Results

3.1. XRD Results

The X-ray diffraction patterns illustrated in Figure 1 indicate the successful synthesis of highly crystalline NiTiO₃ and NiTiO₃/TiO₂ nanocomposites. The pristine NiTiO₃ sample exhibits a monophasic hexagonal ilmenite structure, as indicated by the black-labeled hkl Miller indices. The diffraction peaks show excellent correspondence with the Crystallography Open Database (COD) reference pattern 96-153-5272, validating the high quality and phase purity of the sol–gel synthesized NiTiO₃.

Upon incorporation of TiO₂ in the composite, low-intensity diffraction peaks characteristic of anatase TiO₂ emerge, denoted by blue-labeled hkl Miller indices. These peaks align precisely with the COD reference pattern 96-900-9087, while the predominant NiTiO₃ phase is maintained. Quantitative phase analysis reveals a composition of 99.2% NiTiO₃ and 0.8% anatase TiO₂, indicating precise control over the nanocomposite stoichiometry.

A meticulous examination of the diffraction patterns reveals a subtle shift in the NiTiO₃ peaks towards higher 2θ angles in the composite sample, particularly noticeable for the high-intensity reflections. This peak displacement corresponds to a slight reduction in the NiTiO₃ lattice parameters (

Table 1. Structural parameters of NiTiO₃ and NiTiO₃/TiO₂ obtained from Rietveld refinement.

Structural	Sample
Parameter	NiTiO3	NiTiO3—TiO2
Phase Name	NiTiO3	NiTiO3	TiO2 (Anatase)
Phase (%)	100%	99.2	0.8
Crystal system	Hexagonal	Hexagonal	Tetragonal
Space group	R − 3	R − 3	I 41/a m d
a (Å)	5.02588	5.2576	3.78503
b (Å)	5.02588	5.2576	3.78503
c (Å)	13.80987	13.8102	9.50425
α (°)	90	90	90
β (°)	90	90	90
γ (°)	120	120	90
ρ (g/cm3)	5	5.03	3.59
D (nm)	53.46	46.35	68.94
Rexp (%)	6.322	6.13182
Rp (%)	4.197	3.7737
Rwp (%)	6.164	5.4464
GOF	0.97	0.88

), which is primarily attributed to the decrease in crystallite size from 53.46 nm in the pure phase to 46.35 nm in the composite. This observation aligns with the well-established relationship between crystallite size and lattice parameters, where smaller crystallites often exhibit lattice contraction due to increased surface tension and structural relaxation effects [25].

The structural parameters derived from Rietveld refinement of the XRD data (Table 1) provide further insights into the nanocomposite architecture. The decrease in NiTiO₃ crystallite size in the composite suggests a complex nanostructuring process during formation. The observed disparity in crystallite sizes between NiTiO₃ and TiO₂ phases may be attributed to Ostwald ripening during synthesis. Elevated temperatures could facilitate the growth of larger TiO₂ particles at the expense of smaller ones, explaining increased TiO₂ crystallite size in the composite. This process, driven by the inherent tendency of the system to minimize surface energy, likely contributes to the formation of complex nanostructures [26].

The observed decrease in NiTiO₃ crystallite size within the composite framework could engender an increased specific surface area and introduce beneficial defects, potentially leading to a higher density of catalytically active sites [27]. This phenomenon aligns with observations in analogous NiTiO₃-based nanocomposites and may significantly contribute to the enhanced photocatalytic performance of the composite material [27,28,29].

The preservation of the primary NiTiO₃ structure with the incorporation of a small fraction of anatase TiO₂ creates a heterojunction that is expected to facilitate efficient charge separation and enhance light absorption across a broader spectral range [30]. The structural modifications observed in the XRD analysis, including the lattice parameter contraction and crystallite size reduction of NiTiO₃, provide a foundation for understanding the enhanced photocatalytic performance of the NiTiO₃/TiO₂ nanocomposite.

3.2. Raman Spectroscopy Results

Raman spectroscopic analysis, shown in Figure 2, provides complementary insights into the vibrational modes and local atomic structure of the synthesized materials, corroborating and expanding upon the X-ray diffraction findings. The pristine NiTiO₃ sample exhibits characteristic Raman shifts at 245, 348, 397, 467, 520, 614, 710 and 772 cm⁻¹, which are in excellent concordance with reported values for rhombohedral NiTiO₃ [31]. These modes correspond to various Ti-O and Ni-O vibrational modes within the NiTiO₃ lattice. The emergence of additional Raman peaks at 148, 520, and around 630 cm⁻¹ in the NiTiO₃/TiO₂ nanocomposite spectrum can be attributed to the E_g, A_1g/B_1g, and E_g modes of anatase TiO₂ [32], respectively, confirming the presence of the TiO₂ phase and supporting the X-ray diffraction results.

The observed relative attenuation of NiTiO₃ peak intensities coupled with the prominence of anatase TiO₂ peaks in the composite sample suggests a strong interaction between the two phases. This interaction could potentially lead to the formation of interfacial regions, which have been demonstrated to be beneficial for charge carrier separation and transfer in analogous composite photocatalysts [17].

3.3. Fourier-Transform Infrared Spectroscopy Results

Fourier-transform infrared (FTIR) spectroscopy results show the chemical bonding and structural characteristics of the synthesized NiTiO₃ and NiTiO₃/TiO₂ samples (Figure 3). The full spectra (Figure 3a) span 4000–500 cm⁻¹, while Figure 3b focuses on the 1000–400 cm⁻¹ region, revealing distinctive metal-oxygen vibrations [33]. Both samples exhibit similar peak positions at 715, 610, 532, and 429 cm⁻¹, indicating preservation of the primary NiTiO₃ structure upon TiO₂ incorporation. These bands are assigned to Ni-O stretching, combined Ni-O and Ti-O stretching modes, Ti-O-Ti vibrations, and Ti-O stretching in the NiTiO₃ lattice, respectively [34]. The key difference lies in the relative peak intensities. The NiTiO₃/TiO₂ composite shows slightly higher transmittance across 4000–1000 cm⁻¹, suggesting minor changes in optical properties [18], possibly due to alterations in particle size, morphology, or surface characteristics. In the lower wavenumber region, the composite exhibits lower transmittance for peaks at 610 and 532 cm⁻¹, indicating reinforced metal–oxygen bonding. This preservation of peak positions with altered intensities suggests that TiO₂ incorporation primarily affects bond strength and abundance rather than introducing new structural features. Such subtle modifications could influence the electronic structure of the material and surface properties, potentially contributing to enhanced photocatalytic activity through more efficient light absorption, charge separation, and transfer between NiTiO₃ and TiO₂ phases, while maintaining the host material’s fundamental crystal structure [18,30].

3.4. UV–Visible Spectroscopy Results

The optical properties of the NiTiO₃ and NiTiO₃/TiO₂ nanocomposite were investigated using UV–visible spectroscopy and Tauc plot analysis, as shown in Figure 4. The absorption spectra (Figure 4a) reveal similar optical behavior for both samples across the 200–1000 nm range, with three distinct absorption regions. Of particular interest for photocatalytic hydrogen generation is the strong absorption band in the UV region (200–400 nm), attributed to O²⁻ → Ti⁴⁺ charge transfer transitions in the TiO₆ octahedra [35]. Notably, the NiTiO₃-TiO₂ nanocomposite exhibits enhanced absorption intensity in this UV range compared to pure NiTiO₃, suggesting a potentially higher photocatalytic activity under UV irradiation. This increased UV absorption in the composite may be due to the synergistic effect of NiTiO₃ and TiO₂, possibly arising from improved charge separation and transfer between the two phases. The visible region (400–650 nm) shows a broad absorption band likely corresponding to Ni²⁺ d-d transitions, while a near-infrared band (650–1000 nm) may be associated with Ni²⁺-Ti⁴⁺ intervalence charge transfer. Tauc plot analysis (Figure 4b) reveals indirect bandgap energies of 2.42 eV for pure NiTiO₃ (blue line projection) and 2.45 eV for the NiTiO₃-TiO₂ nanocomposite (purple line projection). The slight increase in bandgap energy for the nanocomposite indicates a minor modification of the electronic structure upon TiO₂ incorporation [18]. Despite this small bandgap widening, the overall enhanced absorption, particularly in the UV region critical for hydrogen generation, suggests that the NiTiO₃-TiO₂ nanocomposite may offer improved performance in UV-driven photocatalytic applications compared to pure NiTiO₃.

3.5. Photoluminiscence Spectroscopy Results

The photoluminescence (PL) spectra of the pristine NiTiO₃ and NiTiO₃/TiO₂ nanocomposite (Figure 5a) provide crucial insights into their optoelectronic properties and charge carrier dynamics. Both samples exhibit broad emission bands centered around 400 nm, attributed to radiative recombination of photogenerated excitons, likely originating from transitions between Ti 3d conduction band and O 2p valence band states [36]. Notably, the NiTiO₃/TiO₂ nanocomposite demonstrates an 85% reduction in PL intensity compared to pure NiTiO₃, indicating significantly suppressed radiative recombination and suggesting enhanced charge separation efficiency [23].

Deconvolution of the PL spectra (Figure 5b,c,

Table 2. Deconvolution values of PL results.

Sample	Peak	Area Intg	FWHM	Max Height	Center Grvty	Area IntgP
NiTiO3	1 (green line)	7,181,083.94	56.65583	119,077.172	404.1403	65.44915
	2 (blue line)	3,790,920.64	63.46499	56,329.4278	361.95239	34.55085
NiTiO3/TiO2	1 (green line)	1,107,529.89	59.48056	17,493.1386	400.0478	67.32776
	2 (blue line)	537,452.619	56.70732	8947.51769	352.23357	32.67224

) reveals two distinct emission peaks for both samples. In NiTiO₃, these peaks are centered at 404.14 nm (3.07 eV) and 361.95 nm (3.43 eV), while in NiTiO₃/TiO₂, they exhibit a hypsochromic shift to 400.05 nm (3.10 eV) and 352.23 nm (3.52 eV), respectively. This blue shift suggests a modification of the electronic structure upon heterojunction formation, possibly due to interfacial strain or quantum confinement effects at the nanoscale interface [37]. The lower energy peak likely corresponds to transitions involving the Ni²⁺ 3d⁸ band, which lies above the O 2p valence band, as suggested by previous studies on NiTiO₃ [38,39]. The higher energy peak may be associated with direct transitions between the O 2p valence band and the Ti 3d conduction band [18].

The relative contributions of these peaks remain consistent between samples, with the lower energy peak accounting for approximately 65–67% of the total emissions. This consistency indicates that the fundamental nature of the radiative recombination processes remains similar in both materials, despite the significant reduction in overall PL intensity in the composite.

The pronounced quenching of PL intensity in the NiTiO₃/TiO₂ nanocomposite signifies enhanced charge carrier separation efficiency, attributable to the formation of a type-II heterojunction. This interfacial electronic structure is expected to facilitate spatial segregation of photogenerated electrons and holes through band alignment, effectively suppressing radiative recombination and prolonging the lifetime of charge carriers available for surface redox reactions [19].

The band alignment between NiTiO₃ and TiO₂ can be inferred from literature values and our UV-Vis diffuse reflectance spectroscopy results. The conduction band of TiO₂ is generally known to be slightly lower than that of NiTiO₃, which would create a favorable energetic landscape for electron transfer from NiTiO₃ to TiO₂. This alignment would promote efficient charge separation, with electrons preferentially accumulating in TiO₂ and holes in NiTiO₃, explaining the observed PL quenching [40].

This synergistic effect of the heterojunction not only improves the photocatalytic activity but also potentially enhances the stability of the composite material by reducing the likelihood of photocorrosion [41].

3.6. Transmission Electron Mycroscopy Results

Transmission electron microscopy analyses (Figure 6) show the nanoscale morphology and crystalline structure of the synthesized materials. Pristine NiTiO₃ nanoparticles (Figure 6a) exhibit a quasi-spherical morphology with a mean diameter of 45.95 ± 19.36 nm, indicative of a relatively monodisperse size distribution. The observed particle agglomeration is characteristic of high surface energy nanoparticles synthesized via sol–gel methods [20,22]. High-resolution TEM (HRTEM) analysis of pristine NiTiO₃ (Figure 6b) reveals well-defined lattice fringes with an interplanar d-spacing of 0.368 nm, indexable to the (012) crystallographic plane of rhombohedral NiTiO₃, confirming the high degree of crystallinity in the synthesized material. The NiTiO₃/TiO₂ nanocomposite (Figure 6c) shows a slight increase in mean particle size to 49.39 ± 20.27 nm, consistent with the formation of a heterojunction nanostructure. HRTEM imaging of the nanocomposite (Figure 6d) reveals two distinct sets of lattice fringes, with d-spacings of 0.349 nm and 0.249 nm attributable to the (101) plane of anatase TiO₂ and the (110) plane of NiTiO₃, respectively. This direct visualization of the NiTiO₃/TiO₂ interface provides compelling evidence for the formation of an intimately contacted heterojunction structure, crucial for efficient interfacial charge transfer and enhanced photocatalytic performance [30].

3.7. Hydrogen Generation Results

Figure 7 reveals the photocatalytic hydrogen evolution performance of pristine NiTiO₃ and the NiTiO₃/TiO₂ nanocomposite under UV irradiation, demonstrating a clear enhancement in hydrogen production for the composite material. Both photocatalysts exhibit time-dependent hydrogen evolution, with the NiTiO₃/TiO₂ nanocomposite consistently outperforming the pristine NiTiO₃ throughout the 3 h reaction period. After 60 min of irradiation, the NiTiO₃/TiO₂ nanocomposite produces 635 μmol of H₂, compared to 551 μmol for pristine NiTiO₃, representing a 15.2% enhancement. This improvement becomes more pronounced as the reaction progresses, with the nanocomposite generating 1753 μmol of H₂ after 180 min, compared to 1500 μmol for pristine NiTiO₃—a 16.9% increase in hydrogen evolution. The hydrogen evolution rates, calculated from the slopes of the time-dependent production curves, further underscore the superior performance of the nanocomposite. The NiTiO₃/TiO₂ nanocomposite exhibits an average hydrogen evolution rate of 9.74 μmol min⁻¹, which is 17.1% higher than the 8.32 μmol min⁻¹ observed for pristine NiTiO₃. These results are consistent with previous studies on heterojunction photocatalysts, which have shown enhanced performance due to improved charge separation and extended light absorption [17,40].

The utilization of methanol as a sacrificial electron donor in this investigation corresponds with previously documented successful applications in titanate-based photocatalytic systems. As evidenced in

Table 3. Comparison of photocatalytic hydrogen production using different titanate-based materials and sacrificial agents.

N°	Catalyst	Cocatalyst	Light	Sacrificial Agent	H2 Production Rate	Reference
1	SrTiO3	Au-Al	Solar–Xenon (150 W)	Methanol	~285 µmol g−1 h−1	[44]
2	CaTiO3	Ag	Mercury lamp (450 W)	Glycerol (10%)	167 µmol g−1 h−1	[45]
3	SrTiO3	Bi-Fe	λ ≥ 250 nm	Sodium sulfite (0.05 M)	~180 µmol g−1 h−1	[46]
4	FeTiO3	-	Pen-Ray Ultraviolet lamp	Methanol	240.5 µmol g−1 h−1	[43]
5	NiTiO3	TiO2	Xe Lamp 250 W	Glycerol (5%)	87 µmol g−1 h−1	[40]
6	CoTiO3	-	UV light (365 ± 5 nm and 79.1 ± 0.5 mW cm−2)	Ethanol (10%)–water	0.3 mmol g−1 h−1	[42]
7	ZnTiO3	-	visible light emitted by a 250 W metal-halide Philips lamp MH/U	Methanol (10%)–water	31 µmol g−1 h−1	[47]
8	NiTiO3		UV Lamp (300 W)	500 mL methanol (10%)–water solution	499.2 µmol g−1 h−1	Present Work
9	NiTiO3	TiO2	UV Lamp (300 W)	500 mL methanol (10%)–water solution	584.4 µmol g−1 h−1	Present Work

, diverse sacrificial agents have been implemented for photocatalytic hydrogen evolution utilizing titanate-based materials, encompassing glycerol, sodium sulfite, and ethanol–water binary systems. The NiTiO₃/TiO₂ heterojunction system, employing 500 mL of 10% methanol–water solution, demonstrated a hydrogen evolution rate of 584.4 μmol g⁻¹ h⁻¹ under UV irradiation (300 W), representing a substantial enhancement compared to previously reported titanate photocatalysts. This photocatalytic activity substantially exceeds the performance metrics of NiTiO₃ modified with TiO₂ utilizing 5% glycerol (87 μmol g⁻¹ h⁻¹) [40] and CoTiO₃ in an ethanol–water medium (0.3 mmol h⁻¹ g⁻¹) [42]. Although certain systems, such as FeTiO₃ with methanol (240.5 μmol g⁻¹ h⁻¹) [43] and Au–Al-doped SrTiO₃ with methanol (285 μmol g⁻¹ h⁻¹) [44], exhibit notable activity, the NiTiO₃/TiO₂ system demonstrates superior photocatalytic performance. The enhanced efficacy of methanol as a sacrificial electron donor in this system may be attributed to its favorable oxidation potential and efficient hole-scavenging capabilities, particularly when coupled with the electronic band alignment of the NiTiO₃/TiO₂ heterojunction interface. Furthermore, the photocatalytic activity surpasses that of noble metal-modified titanates, such as Ag-doped CaTiO₃ (167 μmol g⁻¹ h⁻¹ with 10% glycerol) [45], emphasizing the exceptional efficiency of the methanol-mediated NiTiO₃/TiO₂ heterojunction architecture.

The improved photocatalytic performance of the NiTiO₃/TiO₂ nanocomposite can be explained by several mutually reinforcing factors that are linked to the energy band structures of the materials. Primarily, the creation of a type-II heterojunction between NiTiO₃ and TiO₂ enables effective spatial separation of the light-induced electron–hole pairs [12].

Based on the experimental results and existing knowledge about photocatalytic processes, we suggest an improved explanation for the hydrogen generation mechanism in the NiTiO₃/TiO₂ nanocomposite material (Figure 8):

Photoexcitation: upon UV light absorption, electron-hole pairs are generated in both NiTiO₃ and TiO₂:

$$\begin{gathered} {NiTiO}_3+hv\to {NiTiO}_3^{*}\left(e^{-}+h^{+}\right) \\ {TiO}_2+hv\to {TiO}_2^{*}(e^{-}+h^{+}) \end{gathered}$$

Charge separation: the type-II heterojunction facilitates electron transfer from NiTiO₃ to TiO₂, while holes migrate in the opposite direction:

$$\begin{gathered} {NiTiO}_3^{*}\left(e^{-}\right)+{TiO}_2\to {NiTiO}_3+{TiO}_2\left(e^{-}\right) \\ {NiTiO}_3+{TiO}_2^{*}\left(h^{+}\right)\to {NiTiO}_3\left(h^{+}\right)+{TiO}_2 \end{gathered}$$

Charge migration: the separated electrons and holes diffuse to the surface of the nanocomposite particles:

$$\begin{gathered} {TiO}_2\left(e^{-}\right) \to {TiO}_2\left(e^{-}surface\right) \\ {NiTiO}_3\left(h^{+}\right)\to {NiTiO}_3\left(h^{+} surface\right) \end{gathered}$$

Water reduction: photogenerated electrons in TiO₂ reduce water to hydrogen at the catalyst surface:

$$2H_{2}O+2e^{-}surface\to H_2+2O{H}^{-}$$

Methanol oxidation: holes in NiTiO₃ oxidize the methanol sacrificial agent by a sequence of steps:

$$\begin{gathered} C{H}_{3}OH+h^{+}surface\to C{H}_{2}OH+H^{+} \\ C{H}_{2}OH+H_{2}O\to HCHO+{2H}^{+}+2e^{-} \\ HCHO+H_{2}O\to HCOOH+2H^{+}+2e^{-} \\ HCOOH\to C{O}_2+2H^{+}+2e^{-} \end{gathered}$$

Charge transfer and recombination suppression: the heterojunction configuration facilitates charge separation and transfer, inhibiting electron-hole recombination:

$${NiTiO}_3\left(h^{+}\right)+{TiO}_2\left(e^{-}\right)\to {NiTiO}_3+{TiO}_2$$

This mechanism explains the observed enhancement in hydrogen evolution with the NiTiO₃/TiO₂ nanocomposite. The type-II band alignment introduces beneficial intermediate energy levels and catalytic sites at the interface [12]. The conduction band of TiO₂ (−0.5 V vs. NHE) is sufficiently negative to drive the hydrogen evolution reaction [16], while the conduction band of NiTiO₃ differs (−0.31 V vs. NHE) [15], which ensures efficient hole scavenging by methanol oxidation.

The overall methanol oxidation potential (E₀ = 0.02 V vs. NHE) provides a thermodynamic driving force for the reaction. The heterojunction structure facilitates spatial charge separation, reducing recombination and increasing the lifetime of charge carriers available for redox reactions at the surface.

This proposed mechanism accounts for the synergistic effects observed in the NiTiO₃/TiO₂ nanocomposite, explaining its superior photocatalytic performance compared to pristine NiTiO₃. The enhanced charge separation efficiency, as evidenced by the 85% reduction in PL intensity for the nanocomposite, correlates well with the improved hydrogen evolution rates. The blue shift observed in the PL spectra of the nanocomposite (peaks at 400.05 nm and 352.23 nm) compared to pristine NiTiO₃ (404.14 nm and 361.95 nm) suggests a modification of the electronic structure upon heterojunction formation, which may contribute to the improved charge separation and transfer processes.

The intimate contact between NiTiO₃ and TiO₂ phases, as observed in HRTEM, ensures efficient interfacial charge transfer, while the increased number of catalytically active sites resulting from the heterojunction structure enhances the overall photocatalytic hydrogen evolution performance. The prolonged lifetime of separated charge carriers, inferred from PL quenching, increases their availability for participation in surface redox reactions, such as the reduction of protons to hydrogen at the TiO₂ surface and the oxidation of methanol at the NiTiO₃ surface.

This comprehensive mechanism not only explains the observed enhancement in hydrogen evolution rates but also provides insights into the fundamental processes governing the photocatalytic activity of the NiTiO₃/TiO₂ nanocomposite, paving the way for further optimization of heterojunction photocatalysts for solar hydrogen production.

4. Conclusions

This study demonstrates the successful synthesis and optimization of NiTiO₃/TiO₂ nanocomposites via a modified sol–gel method, achieving precise control over phase composition (99.2% NiTiO₃, 0.8% TiO₂) and heterojunction formation. The nanocomposite exhibits significantly enhanced optical and electronic properties compared to pristine NiTiO₃, including a reduction in crystallite size from 53.46 nm to 46.35 nm, formation of intimate heterojunction interfaces, and preservation of phase purity with controlled TiO₂ incorporation. The material shows an 85% reduction in charge carrier recombination, enhanced UV–visible light absorption, and optimized band alignment for efficient charge transfer. Under photocatalytic conditions, the composite achieves a 17.1% higher average hydrogen evolution rate (9.74 μmol min⁻¹) and a 16.9% increase in total hydrogen production after 3 h, demonstrating improved stability and quantum efficiency.

The enhanced performance is attributed to several synergistic effects, including efficient spatial separation of photogenerated charges, reduced recombination rates through heterojunction formation, increased surface area and catalytic active sites, and optimized band alignment for photocatalytic reactions. Our findings provide crucial insights into the design principles of heterojunction photocatalysts for solar hydrogen production. The demonstrated performance improvements highlight the potential of NiTiO₃/TiO₂ nanocomposites as promising materials for efficient solar-to-hydrogen conversion technologies. Furthermore, this research contributes to the broader field of sustainable energy solutions by establishing a strategy for enhancing photocatalytic activity through nanostructure engineering. Future work should focus on optimization of TiO₂ content and morphology, investigation of co-catalyst addition, scaling up studies for practical applications, and integration with other renewable energy systems.