Enhanced Photocatalytic Hydrogen Evolution by TiO₂: A Synergistic Approach with Defect-Rich SnS₂ and Ti₃C₂ MXene Cocatalysts

Abstract

Enhanced photo-induced electron utilization leads to efficient photocatalytic hydrogen production. The inefficient separation of photo-induced electron–hole pairs has hindered this process. This study introduces a synergistic approach using defect-rich SnS₂ and Ti₃C₂ MXene as cocatalysts in a two-step hydrothermal process to address this challenge. By integrating these materials with TiO₂ nanosheets, we create a novel composite, SnS₂/Ti₃C₂/TiO₂ (STT), that significantly boosts photocatalytic hydrogen evolution. The defect-rich SnS₂ provides abundant active sites for hydrogen generation, while Ti₃C₂ MXene facilitates photo-induced charge separation. The synergistic combination of charge carrier diffusion enhances chromophore absorption, thereby increasing the overall photocatalytic hydrogen-production rate, achieving several grams of hydrogen per hour per gram of double cocatalysts with molybdenum vacancies. Characterization techniques confirm the phase composition of the composite (STT). Compared to pristine TiO₂ and other composites, the STT composite, optimized with a 150 °C hydrothermal treatment, shows a photocatalytic H₂-production rate nearly 192 times higher than that of pure TiO₂ and 6 times higher than that of other composites. The presence of molybdenum vacancies in SnS₂ further enhances its specific activity for hydrogen evolution by suppressing carrier recombination and providing additional active sites. Moreover, Ti₃C₂ MXene and SnS₂ act as dual cocatalysts, improving electronic conductivity and electron-transfer efficiency. Our findings demonstrate the potential of combining defect-rich SnS₂ and Ti₃C₂ MXene to develop highly efficient and sustainable photocatalysts for hydrogen production. TiO₂ has been in situ grown on highly conductive Ti₃C₂ MXene, and SnS₂, rich in molybdenum vacancies, is uniformly distributed on the TiO₂/Ti₃C₂ composite through the two-step hydrothermal method. The presence of molybdenum vacancies in SnS₂ further enhances its specific activity for hydrogen evolution by suppressing carrier recombination and providing additional active sites. Moreover, Ti₃C₂ MXene and SnS₂ act as dual cocatalysts, improving electronic conductivity and electron-transfer efficiency. Our findings demonstrate the potential of combining defect-rich SnS₂ and Ti₃C₂ MXene to develop highly efficient and sustainable photocatalysts for hydrogen production.

1. Introduction

MoS₂, a two-dimensional transition metal dichalcogenide, is extensively investigated for its superior electronic properties, ample surface area, and robust stability. Its narrow bandgap enables efficient light absorption and charge separation, which are crucial for photocatalytic processes. SnS₂, another layered material, possesses a bandgap suitable for visible-light absorption. Its defect-rich structure offers abundant active sites, promoting photocatalytic activity, particularly for hydrogen evolution in water splitting. Nevertheless, rapid charge carrier recombination can hinder its performance. Despite advancements, charge carrier recombination remains a significant challenge, especially in SnS₂, potentially diminishing overall photocatalytic efficiency. MoS₂ and SnS₂ may undergo structural degradation upon prolonged light exposure, compromising the durability and longevity of composite materials in real-world applications. TiO₂, a widely employed photocatalyst, exhibits excellent chemical stability and cost-effectiveness. However, its broad bandgap limits its ability to absorb light outside of the ultraviolet spectrum. It is frequently coupled with materials such as MoS₂ and SnS₂ to broaden its light-absorption spectrum into the visible range and enhance efficiency. The synthesis of high-quality, defect-engineered MoS₂, SnS₂, and TiO₂ composites often requires intricate and expensive methods, potentially limiting their economic viability for large-scale production. The wide bandgap of TiO₂ primarily confines its light absorption to the UV region. While coupling it with MoS₂ and SnS₂ enhances its absorption, the overall composite absorption may still be suboptimal for efficient solar energy utilization. Certain composites may exhibit sensitivity to environmental factors like pH and temperature fluctuations, potentially compromising consistent performance in outdoor or industrial environments. Furthermore, MoS₂ has been recognized as a potential electrocatalytic candidate for H₂ evolution, offering a cost-effective substitute to noble-metal co-catalysts due to its natural abundance and economic viability [1,2]. However, MoS₂ catalytic active sites are predominantly localized at the edges of its layers [3,4]. Defect engineering can greatly increase the number of active sites in MoS₂, according to recent theoretical and experimental studies [5,6,7,8]. The remarkable electrical conductivity of MXene, a novel class of 2D materials, has attracted significant interest [9]. The element is extracted from MAX phases in a specific manner to create them. MXenes are produced from the MXene phase, following the general rule Mn+1A-X-n, where ‘n = 0, 1, 2, 3; A is a group IIIA or IVA element; M is a metal that undergoes an early transition, and ‘X’ refers to standard carbides or nitrides [10]. The number of MoS₂ edges and the density of molybdenum vacancies are considered key factors influencing catalytic activity [11,12,13]. Identifying a highly active and cost-effective co-catalyst is imperative for advancing photocatalytic H₂ production [14,15,16]. Ti₃C₂ MXene, with its exceptional electrical conductivity and distinctive layered structure, is emerging as a potential co-catalyst to replace Pt in photocatalytic H₂ generation [17,18,19]. MXene is produced from the M-A-X phase following the general rule Mn+1A-Xn, where ‘n = 1, 2, 3; ‘A’ is a group IIIA/IVA element, and ‘X’ stands for carbon or nitrogen [20].

By utilizing the metallic characteristics of STT with molybdenum vacancies in TiO₂, we present novel 2D heterostructures in this work. STT, a new photocatalyst, is synthesized in two steps using SnS₂ and Ti₃C₂ MXene as the primary electron acceptors. TiO₂ nanosheets are initially produced directly on the surface of MXenes using a straightforward hydrothermal technique to produce TiO₂/Ti₃C₂ composites. Following the hydrothermal process, with NaBH₄ as a reducing agent, SnS₂ is partially reduced, resulting in molybdenum vacancies that are uniformly dispersed throughout the TiO₂/Ti₃C₂ composite surface. One feature of the efficient photocatalytic system created by this method is the strong interfacial contact between metallic STT and TiO₂. Due to their unique structure, the newly synthesized STT composites exhibit significantly higher stability and stronger photocatalytic H₂ production capability.

2. Materials and Methodology

2.1. Resources

Lakshmi Chemicals company, no 402, T H Road, Tondiarpet, Chennai, supplied the Ti₃C₂ MXene layered material of 2D derived from MAX phases. ‘Ti₃C₂’ typically involves a two-step process: etching and delamination. MAX Phase (Ti₃AlC₂) is the precursor material from which Ti₃C₂ MXene is synthesized. Hydrofluoric Acid (HF) is a strong acid worn for etching the alumina layering as of the M-A-X phasing. Caution (HF) is highly corrosive and toxic and must be handled with extreme care and appropriate safety equipment. Hydrochloric Acid (HCl) is often used in combination with HF to enhance the etching process or for subsequent washing steps. Water is used for rinsing and washing the MXene material. Sodium Hydroxide (NaOH)/Potassium Hydroxide (KOH) bases are used to neutralize the HF after etching. Surfactants and reducing agents (hydrazine hydrate) can be added to improve the delamination process, with the choice contingent upon the target MXene property-specific preparation method due to highly corrosive toxicant substance preparation of Ti₃C₂ MXene; it is essential to follow strict safety procedures and work in a well-ventilated laboratory. The following elements are acquired from a chemical company (Lakshmi Chemicals company),TiO₂ nanoparticles (240 nm size), NaBH₄, C₂H₆O, NaBF₄, HCl, HF, (40 wt. %), and thiourea (CN₂H₄S), and all elements can make Ti₃AlC₂ powders. Tin chloride dihydrate (SnCl₂·2H₂O), sulfur sources, thiourea (CH₄N₂S),sodium thiosulfate (Na₂S₂O₃·5H₂O), and ethanol are used to dissolve the reactants then sodium hydroxide is used to adjust the pH-response combination. Every chemical utilized has caliber as an analytical reagent.

2.2. Preparation of Ti₃C₂ MXene and SnS₂

2.2.1. MAX Phase (Ti₃AlC₂)

Use admix TiC, Ti, and Al powders in a 2:1:1 molar ratio. Carry out homogenization using a mortar and pestle or through ball milling. Anneal the mixed powder in a vacuum or inert atmosphere furnace at approximately 1400 °C for several hours. Apply pressure (30 MPa) during the sintering process to expedite densification and reaction kinetics. Submerge the MAX phase powder in a concentrated HF solution (e.g., 40–50 wt.%) at a moderate temperature (about 60 °C). The etching time depends on the desired degree of exfoliation and can range from a few hours to several days. HF is highly caustic; handle it in a well-ventilated fume hood. To preferentially corrode Al, use a mixture of LiF and HCl, which is less hazardous than HF-based etching but may require longer etching times. Consider substitute fluoride salts, such as NH₄F/KF, as etchants. Disperse the etched product in water or an appropriate solvent and apply ultrasound to dissociate individual MXene layers. Centrifuge the sonicated suspension to remove larger particles and obtain a concentrated MXene dispersion. Filter the dispersion to remove any remaining solid impurities. Wash the delaminated MXene with deionized water to remove residual etchants and salts. Dry the MXene powder or film under vacuum or in an oven at a low temperature (60 °C) to avoid damage. In a typical preparation, we incrementally mixed120 mL of 40 weight percent HF solution with 1 g of Ti₃AlC₂ powder. The reaction mixture was subsequently agitated at ambient temperature for 72 h. Subsequent to vacuum filtration, the solution was rinsed with deionized water until pH neutrality was attained. Finally, Ti₃C₂ MXene was dried under vacuum at 50 °C for 12 h. With appropriate safety precautions, this method can successfully synthesize high-quality Ti₃C₂MXene for various applications.

2.2.2. Preparation of SnS₂

SnS₂ is synthesized via a chemical reaction utilizing tin and sulfur precursors. Tin (II) chloride dihydrate (SnCl₂·2H₂O) and thiourea (SC(NH₂)₂) serve as the tin and sulfur sources, respectively. Ethanol is employed as a solvent medium to promote the reaction. Post-reaction, the synthesized SnS₂ is rigorously washed with deionized water to eliminate impurities and subsequently dried to yield the final product.

2.3. Preparation of Ti₃C₂/TiO₂ Composites

Hydrothermal Synthesis of MXene-TiO₂ Composites

We dispersedTi₃C₂ MXene in a suitable solvent (ethanolic) to generate a stable colloidal suspension. We introduced a titanium precursor (TiCl₄) to the suspension for Ti₃C₂/TiO₂ composites; 15 mL of 1.0 M hydrochloric acid (HCl) was combined with 100 mg of Ti₃C₂ MXene and 165 mg of sodium tetrafluoroborate (NaBF₄). The mixture was subjected to 30 min of sonication and stirring. Subsequently, the solution was transferred to a 25 mL Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 160 °C for 12 h. We transferred the mixture to an autoclave and subjected it to hydrothermal treatment at 180 °C for 12 h. During this process, TiO₂ nanoparticles nucleated and grew on the MXene surface, forming a hybrid composite. The stoichiometric ratio of MXene to TiO₂ significantly impacted the composite’s properties, including the particle size distribution, morphology, and crystallinity of the TiO₂ phase. Post-synthetic treatments, such as calcination and annealing, can further tune the composite’s properties. The reaction solution was gathered using vacuum filtration once it had naturally cooled to room temperature. After being repeatedly cleaned with deionized water, the resultant Ti₃C₂/TiO₂ composites were dried for 12 h at 60 °C in a vacuum oven. Fine-tuning these parameters, it is possible to tailor the properties of Ti₃C₂/TiO₂ composites for diverse applications, including photocatalysis, energy storage, and sensing.

2.4. STT Composite Preparation

To fabricate STT composites, 30 mg of SnCl₂·2H₂O and 60 mg of thiourea were dissolved in 40 mL of aqueous ethanol solution to produce a clear solution. After that, 120 mg of TiO₂/Ti₃C₂ composites was dispersed throughout the fluid to create a suspension. This suspension was moved to a 50 mL stainless steel autoclave lined with Teflon and hydrothermally treated for 12 h at 160 °C. The reaction mixture was recovered by vacuum filtration following natural cooling. The final STT composites were vacuum-dried for 12 h at 50 °C after being periodically cleaned with deionized water. Distinct STT composites were obtained by varying the hydrothermal treatment temperature (140 °C and 180 °C) while maintaining similar synthesis conditions.

2.5. Synthesis of Molybdenum Vacancy-Containing STT Composites (SnS₂/Ti₃C₂/TiO₂)

To introduce molybdenum vacancies into the STT composite structure, a molybdenum-doped TiO₂/Ti₃C₂ composite was synthesized. Molybdenum precursor (ammonium molybdate) was incorporated into the TiO₂/Ti₃C₂ synthesis process using hydrothermal techniques. Subsequent calcination at an appropriate temperature facilitated the formation of oxygen and molybdenum vacancies. For the synthesis of STT composites, 30 mg of Tin(II) chloride dihydrate, 60 mg of thiourea, and 4 mg of sodium borohydride were dissolved in 40 mL of an ethanol-water solution. To form a suspension,120 mg of TiO₂/Ti₃C₂ composite was dispersed in this solution. The molybdenum-doped TiO₂/Ti₃C₂ composite was then added to a precursor solution containing Tin(II) chloride dihydrate and thiourea dissolved in an aqueous ethanol solution. After being moved to a stainless steel autoclave walled with Teflon, this precursor solution was hydrothermally treated for 24 h at 200 °C. The product was filtered, cleaned with ethanol and deionized water, then vacuum-dried at 80 °C after cooling to ambient temperature. Higher calcination temperatures can promote the formation of oxygen vacancies, indirectly leading to molybdenum vacancies. Optimal doping levels should be determined experimentally, as excessive doping can hinder vacancy formation or degrade material properties. Hydrothermal temperature and time influence vacancy formation and composite morphology. Post-treatments like annealing in reducing atmospheres or acid etching can enhance the concentration of molybdenum vacancies. The resulting STT composites were cleaned with distilled water and dried in a vacuum oven at 50 °C for 12 h. After being cleaned with deionized water, the final STT composites were vacuum-dried for 12 h at 50 °C. Distinct STT composites were obtained by varying the hydrothermal treatment temperature (140 °C and 180 °C), as depicted in Figure 1.

2.6. Synthesis of Molybdenum Vacancy

Powder X-ray diffraction (PXRD) analysis was conducted using a PANalyticalX’Pert PRO (Malvern Ppanalytical Techlink, Singapore), a popular choice for XRD analysis due to its high resolution and sensitivity. The precise microstructure and crystallographic data were examined using transmission electron microscopy (TEM). JEOL JEM-2100F (JEOL Ltd., Akishima, Japan) is a high-resolution TEM instrument capable of imaging materials at the atomic level. An energy-dispersive X-ray spectrometer (EDS) in conjunction with a scanning electron microscope (SEM) was used to examine the samples’ surface morphology and elemental makeup. To examine the chemical states of the components in the composites, X-ray photoelectron spectroscopy (XPS) studies were carried out using a Thermo ESCALAB 250XI spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). In the 200–800 nm wavelength range, UV-Vis diffuse reflectance spectroscopy (DRS) was used to examine the samples’ optical absorption characteristics. Using an FLS920 fluorescence spectrometer (Edinburgh Instruments Ltd., Livingston, UK), the samples’ photoluminescence (PL) characteristics were examined at room temperature with an excitation of 325 nm. Molybdenum vacancies in SnS₂/Ti₃C₂/TiO₂ composites typically arise from deliberate engineering during the synthesis process. Using reducing agents like hydrazine hydrate or sodium borohydride can create oxygen vacancies in the SnS₂ lattice. This process can also lead to the formation of molybdenum vacancies, especially if the reduction conditions are optimized. Annealing the composite at high temperatures in a reducing atmosphere can induce the formation of vacancies, including molybdenum vacancies. The specific conditions, such as temperature and atmosphere, can be tailored to control the vacancy concentration. Plasma treatment can introduce defects and vacancies in the material’s structure. By carefully controlling the plasma parameters, it is possible to create molybdenum vacancies in SnS₂. Bombarding the composite with high-energy ions can displace atoms and create vacancies. This technique offers precise control over the defect concentration and location. It is important to note that the specific method used to introduce molybdenum vacancies can influence the properties of the composite, including its electronic structure, optical properties, and catalytic activity. The optimal method will depend on the desired application and the specific properties of the composite.

2.7. Photocatalytic and Photoelectrochemical Activity of SnS₂/Ti₃C₂/TiO₂ Composites

2.7.1. Photocatalytic H₂ Creation

A Pyrex glass reactor equipped with a quartz window was employed for vertical illumination-driven photocatalytic hydrogen production. A gas-tight circulation system was connected to the reactor of the Gas-Phase System. A 300W Xe arc lamp equipped with an AM 1.5G filter served as the light source, providing a focused light intensity of approximately 180 mW/cm² in the 200–1200 nm wavelength range. Triethanolamine (TEOA) was employed as a sacrificial agent in an aqueous acetone solution containing 10 mg of catalyst powder. The reaction temperature was kept at 35 °C while the system was regularly evacuated to get rid of air. The hydrogen production was measured using a gas chromatograph fitted with a thermal conductivity detector (TCD). The composite’s SnS₂, Ti₃C₂, and TiO₂ components significantly enhance light absorption, particularly in the visible light region. This synergistic effect among the components enhances solar energy utilization and broadens the spectral response. The heterojunction formed between SnS₂, Ti₃C₂, and TiO₂ may facilitate efficient charge separation. Efficient separation and transfer of photogenerated charge carriers to appropriate redox sites can suppress recombination and enhance photocatalytic performance. Reactant adsorption and subsequent surface reactions can be accelerated by the composite structure’s vast surface area and numerous active sites, which will increase photocatalytic activity.

2.7.2. Photocatalytic Movement

The inclusion of SnS₂, Ti₃C₂, and TiO₂ components in the composite significantly enhances light absorption, especially in the visible light region. This synergistic effect among the components enhances solar energy utilization and broadens the spectral response. The heterojunction formed between SnS₂, Ti₃C₂, and TiO₂ may facilitate efficient charge separation. Efficient separation and transfer of photogenerated charge carriers to appropriate redox sites can suppress recombination and enhance photocatalytic performance. Photocatalytic activity can be increased by the composite structure’s wide surface area and numerous active sites, which can speed up reactant adsorption and subsequent surface reactions.

2.7.3. Photoelectrochemical Activity

The conductive Ti₃C₂ nanosheets provide efficient electron-transport channels, facilitating rapid charge transfer from the semiconductor to the external circuit. The synergistic effect of the various components enhances the photoelectrode’s stability, mitigating photocorrosion and ensuring long-term durability. Using the same light source, the apparent quantum efficiency (AQE) was determined by varying its intensity and tracking the rate at which hydrogen was produced. A 0.75 M Na₂SO₄ electrolyte was used in photoelectrochemical studies carried out in a three-electrode electrochemical cell. The reference electrode was an Ag/AgCl electrode, while the counter electrode was a Pt wire. To create the working electrode, the as-synthesized sample was applied as a slurry to FTO conductive glass and dried for six hours at 60 °C. An electrochemical workstation (Shanghai Chenhua CHI660D) was used to measure transient photocurrent response and electrochemical impedance spectroscopy (EIS) under AM 1.5G irradiation from a 300W Xe arc lamp. To enhance photoelectrochemical performance, a 0.75 mL ethanol-terpinenol mixture was added to the catalyst slurry before deposition, and intermittent illumination (100 s on, 100 s off) was employed.

3. Results and Discussion

A schematic illustration of the usual synthesis pathway for STT composites is shown in Figure 1. Ti₃C₂ MXene was first created by employing HF acid to selectively etch Al layers from the Ti₃AlC₂ MAX phase [21]. The hydrothermal oxidation of Ti₃C₂ in the presence of HCl and NaBF₄ was then facilitated by the Ti component of MXene, resulting in the creation of TiO₂ intercalated inside the layered Ti₃C₂ MXene. To integrate the SnS₂ co-catalyst, the resultant TiO₂/Ti₃C₂ composites were hydrothermally treated for 12 h at 160 °C while submerged in a solution comprising SnCl₂·2H₂O, thiourea, and NaBH₄. Sulfur vacancies and partial reduction of Sn⁴⁺ in SnS₂ were caused by NaBH₄, which acted as a reducing agent. When SnS₂ nanoparticles were uniformly distributed throughout the surface of the TiO₂/Ti₃C₂ composite, the ternary STT composite was created. As shown in Figure 2a,b, XRD analysis was used to examine the phase purity and crystallinity of the produced powder samples.

XRD analysis was employed to evaluate the phase purity and crystallinity of the synthesized powder samples. TiO₂/Ti₃C₂, Ti₃AlC₂, and Ti₃C₂ composites’ XRD patterns agree with earlier findings [21,22], as shown in Figure 2b. The appearance of distinctive diffraction peaks of anatase TiO₂ (JCPDS No. 21-1272) indicates the creation of TiO₂ nanosheets (NSs) on the stacked Ti₃C₂ sheets after hydrothermal treatment. The XRD patterns of SnS₂/Ti₃C₂/TiO₂ composites generated under different hydrothermal conditions are shown in Figure 2b. Clear observations are made of the diffraction peaks that correspond to the anatase TiO₂ (JCPDS No. 21-1272) planes (101), (004), (200), (105), and (211). Furthermore, the diffraction peaks at 14.4°, 32.7°, and 58.3° are attributed to the (002), (100), and (110) planes of SnS₂, respectively, in accordance with earlier research [23,24]. Successful synthesis of SnS₂/Ti₃C₂/TiO₂ composites is evidenced by the coexistence of Ti₃C₂, TiO₂, and SnS₂ phases in the XRD patterns. The SnS₂/Ti₃C₂/TiO₂ composites exhibit a decrease in the intensity of SnS₂ diffraction peaks. The reduction in peak intensity is likely attributed to the formation of sulfur vacancies. The intensity of the affected crystal planes may diminish due to lattice distortions induced by vacancy formation [25,26].

SEM was used to examine the synthesized samples’ shape and microstructure. Ti₃C₂ exhibits a characteristic accordion-like, layered structure with a smooth surface, typical of MXenes, as shown in Figure 2c. Hydrothermal treatment led to the growth of TiO₂ nanosheets (NSs) across the layered Ti₃C₂ MXene, forming TiO₂/Ti₃C₂ composites. SnS₂ nanoparticles are uniformly distributed on the surface of TiO₂/Ti₃C₂ composites in SnS₂/Ti₃C₂/TiO₂ composites. The homogeneous distribution of Ti, C, O, S, and Sn elements within the selected region is confirmed by EDS mapping (Figure 2c). STT composites were synthesized by partially reducing Sn⁴⁺ and introducing sulfur vacancies into SnS₂ using NaBH₄ as a reducing agent. The uniform distribution of STT and Sn elements on the surface of TiO₂/Ti₃C₂ composites is evident from the SEM images and EDS mapping in Figure 2c.

TEM analysis was carried out to investigate the microstructure and phase composition of the STT composites. TEM image SnS₂/Ti₃C₂/TiO₂ composites are presented in Figure 2d. The images reveal a two-dimensional, layered structure characteristic of delaminated Ti₃C₂ MXene, and decorated with TiO₂ and SnS₂ nanoparticles at low magnification. The (002) plane of Ti₃C₂ MXene, the (101) plane of anatase TiO₂, and the (002) plane of SnS₂ are depicted in high-resolution TEM images as lattice fringes with d-spacings of 0.98 nm, 0.35 nm, and 0.62 nm, respectively [27,28,29]. A more rigorous statistical analysis, including error bars and reproducibility assessments across multiple batches, is essential to validate the reported results. The 192-fold enhancement in hydrogen evolution rate should be normalized to the surface area and active site density of the catalysts to provide a fair comparison. The proposed charge-transfer mechanisms should be supported by experimental evidence from techniques like time-resolved spectroscopy or electrochemical impedance spectroscopy under operating conditions. A detailed analysis of the defect-formation mechanism in SnS₂ and its correlation with enhanced activity is crucial. Advanced characterization techniques like SEM and TEM imaging can provide valuable insights into the atomic-scale structure and defect distribution.

In contrast to the continuous crystal fringes observed in SnS₂, STT composites prepared at 160 °C exhibit numerous discontinuous crystal fringes, consistent with previous reports [30,31], indicating the presence of a significant number of molybdenum vacancies. Notably, the hexagonal 2H-SnS₂ and trigonal 1T-SnS₂ phases coexist in the composite. Compared to pure TiO₂, the synthesized photocatalyst, incorporating Ti₃C₂, SnS₂, and molybdenum vacancies, demonstrates enhanced charge separation and carrier transfer, leading to improved photocatalytic activity.

Molybdenum vacancies were further confirmed using electron paramagnetic resonance (EPR) spectroscopy. The EPR spectra of the STT composites were compared, as shown in Figure 3a. A distinct signal at a g-value of 1.995 observed both samples, indicating attendance molybdenum vacancies in their structures [28]. The EPR spectrum of the STT composites shows a significant increase in intensity [32], suggesting a higher concentration of molybdenum vacancies. These findings, in conjunction with previous investigations, support the presence of molybdenum vacancies in the STT composites.

Experimental Validation: Surface defects and oxygen vacancies: Electron paramagnetic resonance (EPR) spectroscopy can be used to quantify the number of surface defects and oxygen vacancies, providing direct evidence of their role in the photocatalytic process. Detailed characterization of the interface between Ti₃C₂ MXene and TiO₂ is crucial to understand the charge-transfer pathways. Techniques like TEM and SEM can be employed to visualize the interface structure. Theoretical Support: Density functional theory (DFT) calculations: DFT calculations can provide valuable insights into the electronic structure modifications and charge-transfer mechanisms occurring at the interface between the different components of the composite photocatalyst. By modeling the electronic structure and simulating the adsorption and reaction processes, DFT can help elucidate the underlying mechanisms. The research can provide a more comprehensive understanding of the photocatalytic mechanism and contribute to the development of more efficient and sustainable hydrogen-production technologies.

We used UV-vis DRS to assess the optical absorption characteristics. As shown in Figure 3b [33], TiO₂ (black curve) has substantial UV absorption but very little visible light absorption, which is consistent with the characteristics of anatase TiO₂. STT composites, on the other hand, show a notable red shift in their absorption edge and improved absorption of visible light. The optical absorption characteristics of MoS₂ and Ti₃C₂ MXene are responsible for the increased absorption. Additionally, at the same hydrothermal temperature, STT composites exhibit higher light absorption compared to TiO₂/Ti₃C₂, which may be attributed to the presence of Sn vacancies. The photocatalytic process can be further improved by extending the light absorption range of the photocatalysts.

To better understand how electrons move within our composite materials, we used two main techniques: (i) Transient Photocurrent Response (PEC): We turned a light source on and off repeatedly to see how the material responded. Multiple on–off cycles utilizing an intermittent 300W Xe arc lamp with an AM-1.5 filter in a 0.5M Na₂SO₄ aqueous solution were used to initiate the PEC responses of the STTs (Figure 4a). Each sample shows a reversible PEC response with every illumination cycle. The STT composites display significantly higher photocurrent intensity compared to the TiO₂/Ti₃C₂ composites. The materials with molybdenum vacancies (missing molybdenum atoms) produced a much stronger electrical current when exposed to light. The presence of these vacancies leads to improved charge carrier (electrons and holes) separation and reduced energy loss. (ii) EIS analysis: We measured how easily electrons can move through the material of STT in Figure 4b. The materials with molybdenum vacancies showed less resistance to electron movement, meaning the electrons can move more efficiently. The molybdenum vacancies in the composite material enhance its ability to convert light into electricity by (a) reducing energy loss, (b) improving the separation of charge carriers, (c) facilitating the movement of electrons; (d) this leads to a more efficient and effective photocatalytic material. EIS results indicate that the STT composites possess lower charge-transfer resistance [34]. This consequently leads to improved efficiency of photogenerated charge carrier separation and transfer.

By monitoring the evolution of H₂ in an aqueous acetone solution under simulated sunlight at room temperature, the photocatalytic hydrogen-generation activity of STT composites was assessed (Figure 4c,d). The findings show that H₂ is generated through a photocatalytic process involving the photocatalyst, as no detectable H₂ was observed in control experiments conducted in the absence of either light or photocatalyst. Despite exhibiting photocatalytic activity, pure TiO₂ NSs yielded a relatively low H₂-production rate of 54.3 μmol g⁻¹ h⁻¹ due to rapid charge carrier recombination [35]. As anticipated, STT composites produced at 180 °C exhibited a H₂-evolution rate that was around 39 times greater than that of pure TiO₂ NSs, at 2128.3 μmol g⁻¹ h⁻¹. As cocatalysts, SnS₂ and Ti₃C₂ MXene help to capture a photogenerated electron, which increases the heterostructure’s photocatalytic H₂ generation. When molybdenum vacancies are added, a noticeable improvement in photocatalytic performance is seen. With a H₂-evolution rate of 10,505.8 μmol g⁻¹ h⁻¹, STT composites manufactured at 160 °C outperformed pure TiO₂ NSs by almost 193 times and outperformed STT composites at 160 °C by 6 times. Furthermore, as the hydrothermal synthesis temperature increases from 140 °C to 180 °C, the H₂-evolution rates of STT composites also increase, attributed to the enhanced crystallinity of MoS₂ at higher temperatures. In contrast to 140 °C (1499.2 μmol g⁻¹ h⁻¹) and 180 °C (6419.9 μmol g⁻¹ h⁻¹), the optimal molybdenum vacancy levels are achieved at 160 °C, resulting in the highest H₂ evolution rates for STT composites (10,505.8 μmol g⁻¹ h⁻¹). These findings highlight the crucial role of molybdenum, SnS, and TiC vacancies in improving the photocatalytic performance of STT composites for H₂ production. The apparent quantum efficiency (AQE) of the samples was evaluated using the same light source. The AQE values of STT composites, TiO₂, and TiO₂/Ti₃C₂ composites are presented, showing a consistent trend with the following photocatalytic H₂-production rates: 0.039% (TiO₂ NSs) < 0.865% (STT₂-140 °C) < 1.075% (TiO₂/Ti₃C₂-140 °C) < 1.258% (STT₂-160 °C) < 1.526% (TiO₂/Ti₃C₂-180 °C) < 4.604% (STT—180 °C) < 7.535% (STT₂-160 °C).

A photocatalyst’s stability and durability are essential for real-world photocatalytic H₂-generation applications. The STT composites were cycled full day at 160 °C below constant circumstances to test this. Stability was demonstrated by the H₂-evolution rate, which stayed constant across three cycles over a 124 h period. Additionally, there were no discernible variations between the fresh STT-160 °C sample and the one after three cycles in the XRD pattern (Figure 2a,b), SEM photos and EDS mapping (Figure 2c). This is mainly due to surface water desorption and crystal water removal. The STT-160 °C composites showed excellent thermal stability at 200 °C with very little further weight loss. These findings support the stability and dependability of STT composites as a photocatalyst, and they show promise for effective H₂ evolution from water. The thorough evaluation also demonstrates STT composites’ exceptional H₂-generating capacity.

Because molybdenum vacancies and co-catalysts are present such as SnS₂/Ti₃C₂ MXene, STT composites exhibit exceptional photocatalytic performance, as demonstrated in Figure 5, which depicts a potential photocatalytic pathway for H₂ production. As illustrated in Figure 3b, molybdenum vacancies in SnS₂ facilitate hydrogen adsorption. These vacancies alter semiconductors’ electrical structures, which frequently increases the photocatalysts’ range of light absorption [36]. The synthesis procedures should provide a detailed account of the conditions that influence defect formation in SnS₂. This could include factors like precursor concentration, reaction time, temperature, and pH. The choice of 160 °C for hydrothermal treatment should be justified through optimization studies. Varying the temperature and time can significantly impact the properties of the synthesized materials. The experimental setup for photocatalytic testing should be described in detail, including the light source (type, intensity, and spectrum), reactor design, and calibration methods for measuring hydrogen production. This technique can provide valuable insights into the structural changes occurring during the photocatalytic process, aiding in the understanding of the reaction mechanism. The XPS peak deconvolution analysis should be carried out with rigorous fitting procedures, considering the complex nature of the Ti 2p and S 2p regions. Clear justification for peak assignments is essential. The research can be strengthened and the conclusions drawn from the study can be more reliable.

4. Plausible Photocatalytic Hydrogen-Evolution Mechanism Using STT Composite

Electronic states arise within the bandgap because of weaker bonding at vacancy sites, which causes the space between bonding and antibonding orbitals to diminish in comparison to valence band (VB) and conduction band (CB). Narrowing bandgap or acting as midgap states, these states improve light absorption for electron excitation [6]. Sn vacancies cause STT composites to absorb lighter than STT composites made at comparable hydrothermal temperatures, as seen in Figure 5. The occurrence of Sn vacancies is noted but not explained, leaving an incomplete understanding of their origin and impact on the material’s properties. It would be helpful to provide an explanation for the formation of these vacancies, such as the synthesis conditions, annealing processes, or any intentional defect-engineering techniques that could lead to Sn deficiencies. Additionally, discussing how these vacancies might influence the electronic structure, conductivity, or catalytic behavior of the material would enhance the clarity and depth of the analysis. This context would give readers a better understanding of the role of Sn vacancies in the observed performance or characteristics of the material. The photocatalytic reaction is further supported by extending the photocatalysts’ range of light absorption, and vacancies in these materials, additionally improving charge separation and transfer.

In addition to improving light absorption during electron excitation, vapor-induced electronic states also make it easier for photogenerated charge carriers in the VB or CB to relax their energy, which may upset charge carrier dynamics. Vapors are essential for the migration and separation of electrons and holes as charge carriers move through the semiconductor [37,38,39]. Molybdenum vacancies introduce an impurity band gap between the CB and VB of SnS₂ [6]. Depicted in Figure 5, upon light irradiation, TiO₂ generates electron–hole pairs, creating an electron-rich environment on the surfaces of SnS₂ and TiC₂, facilitating the reduction of H₂O to H₂ [40]. The majority of photoexcited electrons are transferred from the CB of TiO₂ to the impurity band of Ti₃C₂ MXene and SnS₂ with the help of molybdenum vacancies. The activity for H₂ production is increased by the introduction of several active sites via SnS₂ vacancies [35]. Additionally, molybdenum vacancies enhance the reaction process by inhibiting carrier recombination. Sacrificial reagents scavenge the holes in the VB of TiO₂. Therefore, the synergistic effect of molybdenum vacancies and dual co-catalysts (TiC₂ and SnS₂) efficiently enhances the separation and transport of photogenerated charge carriers.

Each component of the innovative SnS₂/Ti₃C₂/TiO₂ composite (STT) contributes unique properties that enhance the material’s overall photocatalytic performance. As a wide-bandgap semiconductor, it primarily absorbs UV light and generates highly reactive hydroxyl radicals.

Ti₃C₂ MXene is a highly conductive material that facilitates rapid electron transport and provides a large surface area for active reaction sites. The combined effect of these components enhances light absorption and catalytic activity. The synergistic mechanism underlying these components is crucial for the composite’s enhanced photocatalytic efficiency, facilitated by the formation of multiple heterojunctions at the interfaces between TiO₂, TiC₂, and SnS₂. Photogenerated electrons in TiO₂ are efficiently transferred to the highly conductive Ti₃C₂ MXene, while holes migrate to SnS₂, leading to rapid charge separation and reduced recombination losses. The light-absorption range is expanded from the ultraviolet to the visible light region by the combination of TiO₂, TiC₂, and SnS₂. Light is scattered by the layered structure of TiC₂, which lengthens the effective light path and improves light absorption. MXene’s large surface area offers many active sites for adsorption and reactions. Additionally, molybdenum vacancies in SnS₂ introduce more active sites, further enhancing catalytic activity [41,42,43,44,45,46,47]. The highly conductive TiC₂ MXene facilitates rapid electron transport. Effective charge separation at the heterojunctions minimizes recombination losses. The SnS₂/Ti₃C₂/TiO₂ composite (STT) offers a comprehensive solution for enhanced photocatalytic performance, combining:

(i)

Increased solar energy absorption;

(ii)

Maximized utilization of photogenerated charge carriers;

(iii)

Enhanced catalytic activity;

(iv)

Enhanced material stability.

Tailoring the composition and synthesis conditions, the photocatalytic performance of this composite can be further optimized for various applications, including water splitting, pollutant degradation, and energy storage [48,49,50,51,52,53,54,55].

5. Conclusions

In conclusion, highly efficient STT composite photocatalyst was fabricated via a two-step hydrothermal method. TiO₂wasgrown on Ti₃C₂ MXene to form a composite, followed by the uniform deposition of SnS₂ with molybdenum vacancies on the TiO₂/Ti₃C₂ surface. The optimized STT composite (hydrothermally treated (temperature) at 160 °C at 124 h, with autogenous pressure from the reaction mixture then autoclave or Teflon-lined stainless steel reactor) exhibited exceptional H₂ growth, achieving charge of about 10,505.8 µmol g⁻¹ h⁻¹. This represents a significant enhancement of 193 and 6 times, respectively, compared to pure TiO₂ and other STT composites (160 °C).

There are multiple reasons behind the STT composite’s improved performance:

(1)

Molybdenum vacancies generate additional active sites, enhancing the specific activity for hydrogen production.

(2)

Molybdenum vacancies suppress charge carrier recombination, civilizing competence in the photocatalytic method.

(3)

The high conductivity of SnS₂ and Ti₃C₂ MXene facilitates improved charge separation and efficient electron transport.

(4)

The synergistic interplay of these factors renders the STT composite a promising candidate for efficient solar-to-hydrogen conversion.