Reusable Three-Dimensional TiO₂@MoS₂ Core–Shell Photoreduction Material: Designed for High-Performance Seawater Uranium Extraction

Abstract

Photocatalysis offers a cost-effective and eco-friendly approach for environmental remediation, yet traditional powdered photocatalysts suffer from poor recyclability and separation challenges. To address these limitations, we developed a recyclable carbon fiber-supported composite photocatalyst (CC/TiO₂ NRs@MoS₂ NPs) featuring a three-dimensional hierarchical core–shell architecture. This structure comprises a TiO₂ seed layer, vertically aligned TiO₂ nanorod arrays as the core, and a MoS₂ nanoparticle shell, fabricated via sequential deposition. Under simulated solar irradiation, the TiO₂@MoS₂ heterojunction exhibited significantly enhanced uranium adsorption capacity, achieving a remarkable 97.3% photocatalytic removal efficiency within 2 h. At an initial uranium concentration of 200 ppm, the material demonstrated an exceptional extraction capacity of 976.7 mg g⁻¹, outperforming most reported photocatalysts. These findings highlight the potential of this 3D core–shell design for efficient uranium recovery and environmental purification applications.

1. Introduction

Nuclear energy has emerged as a prominent player in the development and utilization of clean energy resources, owing to its high energy density and minimal carbon emissions, as evidenced by numerous studies [1,2,3,4]. The rapid expansion of nuclear power in China is expected to lead to a significant depletion of on-land uranium reserves. This depletion poses a serious threat to the security and sustainable development of the country’s nuclear energy sector [5]. Photocatalytic technology, as an environmentally benign and sustainable approach characterized by operational safety and cost-effectiveness, can achieve the degradation of pollutants and sewage treatment. And it has enabled the emergence of photocatalytic uranium extraction from seawater—a novel strategy for uranium recovery [6,7,8,9]. Compared with conventional adsorption methods, the photocatalytic reduction of uranium demonstrates superior advantages including enhanced extraction capacity, exceptional selectivity, and minimized environmental impact, exhibiting significant application potential in seawater uranium extraction endeavors [10,11,12].

Titanium dioxide (TiO₂) photocatalysts have attracted considerable interest in the realm of photocatalytic water purification. This is primarily attributed to their non-toxicity to humans, stable physicochemical properties, and the ease of large-scale synthesis [13,14]. As an effective semiconductor photocatalyst, TiO₂ was first employed in photocatalytic uranium reduction reactions in 1990. However, the practical application of TiO₂ is impeded by two critical issues. Firstly, the excessive recombination of electron-hole pairs significantly undermines its photocatalytic efficiency, and its reduction potential is also insufficient. Secondly, the powder-based application format poses a major obstacle to the recyclability of the catalyst [15,16,17]. One-dimensional (1D) nanostructures (nanorods/nanowires) demonstrate exceptional optoelectronic performance through enhanced electron transport kinetics enabled by their high aspect ratio characteristics. The integration of 1D nanomaterials into three-dimensional (3D) array architectures represents an advanced photocatalytic strategy, as such configurations outperform their low-dimensional counterparts in photoelectric conversion efficiency [18,19,20,21]. To overcome the inherent limitations of single-component TiO₂ photocatalysts, particularly their weak visible-light absorption capacity and rapid electron-hole recombination, current research efforts are centered on constructing 3D core–shell heterostructures. This is achieved by combining TiO₂ nanorod arrays with complementary semiconductors [22]. This architectural design not only enhances the efficiency of solar energy utilization, but also improves the separation of charge carriers. As a result, it serves as a highly effective strategy for boosting the photocatalytic activity of titanium dioxide-based systems [23,24].

In the realm of photocatalytic research, molybdenum disulfide (MoS₂), a graphene-like two-dimensional material with unique structural and physicochemical attributes, has emerged as a focal point of investigation [25,26,27]. In recent years, the combination of metal oxide and metal sulfide compounds for photocatalysts has emerged as a burgeoning area of research, drawing significant attention from the scientific community [28]. As an efficient photocatalyst, MoS₂ and its derivatives have been widely utilized in photocatalytic heavy metal reduction. Its advantages, including a broad light-absorption range, strong reduction capability, and compatible band structure with TiO₂, make it an ideal candidate for constructing TiO₂-based heterojunctions [29]. A notable disadvantage that warrants mention is the susceptibility of its sulfur component to oxidation within a relatively short timeframe. Under specific environmental conditions, such as exposure to an oxygen-rich atmosphere or in the presence of strong oxidizing agents, the sulfur atoms in MoS₂ can readily undergo oxidation reactions.

Compared to conventional glass substrates, the three-dimensional fibrous architecture of carbon cloth offers distinct advantages. Its enhanced electrical conductivity significantly improves photoelectron transfer kinetics and electron mobility, while the hierarchical porous structure provides substantially greater active site density for improved UO₂²⁺ adsorption. Additionally, carbon cloth demonstrates superior chemical stability in corrosive nuclear waste environments compared to glass substrates, along with inherent flexibility that enables scalable reactor designs [30,31].

Therefore, this study developed a carbon cloth-supported three-dimensional core–shell structured TiO₂@MoS₂ heterojunction material via an impregnation–calcination/solvothermal–recalcination method for photocatalytic U(VI) reduction. The innovation of this study lies in the strategic layer-by-layer construction of core–shell structural units on three-dimensional (3D) porous supports. This approach not only creates expanded spatial domains for charge separation but also enables ordered carrier migration across the entire 3D matrix. By coupling the advantages of heterojunction engineering with array architecture modulation, this strategy provides a novel design paradigm for developing functional materials capable of photocatalytic uranium extraction from seawater.

2. Results and Discussions

Figure 1a shows the dip scorching–solvent hydrothermal–repeated dip scorching approach designed in this study for creating a TiO₂ NRs@MoS₂ NPs 3D core–shell heterostructure. First, a conformal TiO₂ crystal seed layer was uniformly immobilized on the surface of a carbon fiber by dip scorching. Next, a 3D TiO₂ nanorod array was created over the surface of the crystal seed layer by the solvent hydrothermal method. Then, MoS₂ nanoparticles were attached over the surface of the TiO₂ nanorod array as the shell by repeated dip scorching. This multi-step approach ensures precise control over hierarchical architecture and interfacial compatibility for enhanced photocatalytic performance. The series of samples involved in this method are designated as CC, CC/TiO₂, CC/TiO₂ NRs, and CC/TiO₂ NRs@MoS₂ NPs, with full component names provided: CC (carbon cloth), TiO₂ (titanium dioxide), TiO₂ NRs (titanium dioxide nanorods), and MoS₂ NPs (molybdenum disulfide nanoparticles).

Scanning electron microscopy (SEM) was used to observe the microstructure and morphological features of CC, CC/TiO₂, CC/TiO₂ NRs, and CC/TiO₂ NRs@MoS₂ NPs. The CC substrate consists of woven carbon fibers with a diameter of approximately 25 μm (Figure 1b,f). A three-dimensional photocatalytic unit structure was progressively constructed on the carbon fiber surface to enhance the separation of photogenerated charges. Carbon fibers, widely used in flexible electrode materials due to their excellent conductivity, serve as a recyclable carrier for photocatalytic materials, providing ample space for photogenerated charge diffusion. CC/TiO₂ features a TiO₂ seed layer coating approximately tens of nanometers thick adhered to the carbon fiber surface (Figure 1c,g), providing a platform for subsequent heterostructure growth. CC/TiO₂ NRs were synthesized via solvothermal growth, resulting in highly ordered, vertically aligned TiO₂ nanorod arrays densely covering the carbon fiber surface (Figure 1d,h). These nanorods measure approximately 1 μm in length and 150 nm in diameter, significantly increasing the material’s active surface area and providing stable support for subsequent shell coating. The interrod spacing facilitates effective encapsulation of the TiO₂ nanorod arrays by the MoS₂ shell during subsequent impregnation–calcination.

CC/TiO₂ NRs@MoS₂ NPs were obtained through secondary impregnation–calcination (Figure 1e,i). SEM images reveal complete coverage of TiO₂ nanorods by MoS₂, with the original TiO₂ arrays becoming barely observable, suggesting the formation of a TiO₂@MoS₂ core–shell structure. To clarify the interfacial morphology and crystalline structure of the TiO₂@MoS₂ core–shell heterostructure, high-resolution transmission electron microscopy (HRTEM) was performed. From the HRTEM image in Figure 1j,k, we can see that the TiO₂ nanorods have a complete crystal structure and their surfaces are tightly wrapped by numerous ellipsoidal MoS₂ nanoparticles, making up a coaxial heterointerface where the core and the shell are in close contact. The absence of an amorphous interfacial layer between highly crystalline TiO₂ and MoS₂ indicates minimal interfacial charge loss during carrier transport, significantly improving the migration of photogenerated charge carriers. Figure 1l displays two sets of lattice fringes with spacings of 0.630 nm and 0.351 nm, corresponding to the (002) plane of hexagonal MoS₂ (PDF#37-1492) and the (101) plane of tetragonal TiO₂ (PDF#77-0443), respectively. In conclusion, this study has successfully synthesized a well-defined carbon fiber-supported TiO₂@MoS₂ core–shell heterostructure material. It is hypothesized that this material will exhibit superior photoelectrochemical performance and thus holds great promise for photocatalytic uranium extraction from seawater.

In order to explore how heterojunctions are loaded layer-by-layer at the micro level, CC/TiO₂ NRs and CC/TiO₂ NRs@MoS₂ NPs were synthesized and analyzed by XRD (Figure 2a). In CC/TiO₂ NRs, the diffraction peak characteristic of rutile-type TiO₂ nanorods exhibits remarkable sharpness and intensity, signifying their high degree of crystallization.

In CC/TiO₂ NRs@MoS₂ NPs in the CC/TiO₂ NRs@MoS₂ NP composite, the characteristic diffraction peaks associated with both TiO₂ nanorods (NRs) and MoS₂ nanoparticles (NPs) are distinctly discernible, offering irrefutable evidence of the successful recombination of TiO₂ and MoS₂ into a core–shell heterojunction structure. Figure 2b presents the full-spectrum analysis results for CC/TiO₂ NRs. The spectrum unequivocally demonstrates that the sample predominantly comprises Ti and O elements. Similarly, Figure 2c illustrates the full-spectrum analysis outcomes for the CC/TiO₂ NRs@MoS₂ NP composite, where the spectrum clearly reveals the presence of Ti, O, Mo, and S elements as the principal constituents. The prominent peak at approximately 230 eV is attributable to Mo 3d, thereby directly substantiating the successful loading of MoS₂ onto the TiO₂ nanorods.

The X-ray photoelectron spectroscopy (XPS) spectrum in Figure 2d reveals two distinct peaks at 458.7 eV and 464.2 eV, corresponding to the Ti 2p_3/2 and Ti 2p_1/2 spin-orbit doublets, respectively, which are characteristic of Ti⁴⁺ in the anatase phase of TiO₂. These binding energies align well with previously reported values for bulk TiO₂, confirming the preservation of the TiO₂ nanorod (NR) structure after composite formation. Notably, when compared to the pristine CC/TiO₂ NRs, the central positions of both Ti 2p peaks in the CC/TiO₂ NRs@MoS₂ NP composite exhibit a measurable shift toward higher binding energies. This shift suggests a modification of the local electronic environment around Ti atoms, likely due to strong electronic interactions between the TiO₂ host and the MoS₂ nanoparticles (NPs). This electronic modulation is critical for enhancing photocatalytic performance, as it may facilitate more efficient separation of photogenerated electron-hole pairs and improve the reduction potential of conduction band electrons for U(VI) reduction.

To investigate the enhanced performance of the three-dimensional core–shell heterojunction, transient photocurrent measurements (Figure 3a) were conducted to assess the separation and migration efficiency of photogenerated charge carriers. The heterojunction exhibited periodic “on–off” photocurrent density response curves, and under repeated illumination, the photocatalysts in the electrolyte consistently demonstrated stable and reproducible signals. Figure 3a shows that CC/TiO₂, CC/TiO₂ NRs, and CC/TiO₂ NRs@MoS₂ NPs all exhibited stable photocurrent responses under illumination. The transient photocurrent density of CC/TiO₂ NRs was about twice that of CC/TiO₂ per unit area, and the photocurrent intensity of CC/TiO₂ NRs@MoS₂ NPs was 1.7 times that of CC/TiO₂ NRs. This indicates that the growth of TiO₂ nanorods improved the separation of photogenerated charge carriers. Compared to the two-dimensional TiO₂ seed layer on carbon cloth, the three-dimensional array structure facilitated rapid electron migration, further suppressing the recombination of photogenerated electrons and holes, thus producing a more stable and a stronger transient photocurrent. With the addition of MoS₂, a three-dimensional core–shell heterojunction formed between TiO₂ and MoS₂. The suitable bandgap and matched energy levels of this heterojunction enhanced the separation of photogenerated charge carriers and improved interfacial charge transfer, leading to the high transient photocurrent intensity.

The behavior of photogenerated charge carriers within the three-dimensional core–shell structure was analyzed using steady-state photoluminescence (PL) spectroscopy (Figure 3b). An excitation wavelength of 350 nm was used to measure the emission spectra of CC/TiO₂, CC/TiO₂ NRs, and CC/TiO₂ NRs@MoS₂ NPs in the wavelength range of 300–650 nm. The emission peak intensity of CC/TiO₂ NRs@MoS₂ NPs was significantly lower than that of CC/TiO₂ NRs, indicating a distinct quenching effect. This suggests that the recombination rate of photogenerated electrons and holes in CC/TiO₂ NRs@MoS₂ NPs was much lower than that in CC/TiO₂ NRs. This reduction in recombination is attributed to the three-dimensional core–shell structure formed by MoS₂ and TiO₂ nanorods, which enabled efficient separation and transfer of charges at their interface.

Considering the efficient charge separation and transfer efficiency of CC/TiO₂ NRs@MoS₂ NPs, their uranium extraction performance was further evaluated. Additional adsorption kinetic experiments were conducted under dark conditions, through which the sufficiency of a 15 min period for the system to reach adsorption equilibrium was confirmed (Figure S1). Under simulated sunlight irradiation, photocatalytic degradation experiments were conducted when the initial uranium concentration was 100 ppm (Figure 4a). After a dark reaction for 15 min, the uranium concentration decreased slightly, which was due to the weak adsorption of uranium on the sample surface. After 2 h of simulated sunlight irradiation, the photocatalytic removal rates of various samples were in the following order: CC/TiO₂ NRs@MoS₂ NPs (97.3%), CC/TiO₂ NRs (53.2%), and CC/TiO₂ (27.3%). The degradation performance of the CC/TiO₂ NRs@MoS₂ NP composite material was significantly higher than that of CC/TiO₂ and CC/TiO₂ NRs. This excellent performance is attributed to the construction of the three-dimensional core–shell heterostructure, which effectively promotes the separation and migration of photogenerated charge carriers, improves the photoquantum efficiency, constructs continuous and rapid charge separation and transfer channels, enables a large number of photogenerated charges to migrate to the surface of the photocatalyst to participate in the reaction, and thus significantly enhances the photocatalytic activity.

The influence of pH on the uranium extraction reaction was further studied in the pH range of 3–7. As shown in Figure 4b, when the solution was more acidic, the photocatalytic performance of CC/TiO₂ NRs@MoS₂ NPs for uranium was severely inhibited. As the pH value increased from 3 to 5, the catalytic performance of the catalyst improved significantly. This phenomenon can be attributed to the fact that at lower pH, the increased H⁺ in the solution and the positively charged surface of the catalyst generate electrostatic repulsion with the positively charged UO₂²⁺, hindering the capture of UO₂²⁺ by the catalyst and thus affecting the subsequent photoreduction performance.

To explore the saturation extraction capacity of CC/TiO₂ NRs@MoS₂ NPs for uranium, their photocatalytic uranium extraction ability was tested within the initial concentration range of 10–200 ppm (Figure 4c). After a reaction of 120 min, the removal rate of U(VI) by CC/TiO₂ NRs@MoS₂ NPs remained at a high level as the initial uranium concentration increased. As the initial uranium concentration increased, the uranium extraction also increased. When the initial uranium concentration was 200 ppm, the uranium extraction capacity of CC/TiO₂ NRs@MoS₂ NPs reached as high as 976.7 mg g⁻¹. It is worth noting that the uranium extraction level in the experiment did not fully reach the saturation extraction capacity of the photocatalyst. The abundant reaction sites of the three-dimensional core–shell structure ensured the rapid progress of the reaction without a sharp decline in the photoreduction rate due to the occupation of some active sites by the reduced uranium products. More importantly, the coexistence of multiple ions is also a major interference factor in seawater uranium extraction. Common metal ions in seawater, such as Na⁺, K⁺, V⁵⁺, Cu²⁺, and Fe³⁺, were introduced into the U(VI) reaction system to evaluate the anti-interference performance of the photocatalyst (Figure 4d). The influence of Na⁺ and K⁺ was slight. When V⁵⁺, Cu²⁺, and Fe³⁺ were present, the removal rate of uranium by the catalyst decreased slightly. The above results are attributed to the competitive adsorption of V⁵⁺, Cu²⁺, and Fe³⁺ with UO₂²⁺ during the reaction.

The recyclability of the photocatalyst is crucial for practical applications. To verify the stability and recycling effect of CC/TiO₂ NRs@MoS₂ NPs, four recycling experiments were carried out. After each cycle, the sample was soaked in deionized water for 60 min and dried at 100 °C for reuse. As shown in Figure 4e, even after four repeated uses, the material could still maintain a removal rate of 92.9% for uranium at 200 ppm, demonstrating its excellent reusability. By comparing the SEM images of the samples before and after four cycles (Figure 4f), it was clearly observed that after five cycles the MoS₂ nanoparticles were still tightly coated on the TiO₂ nanorods, maintaining a stable core–shell heterostructure, which confirmed the reusability of CC/TiO₂ NRs@MoS₂ NP removal efficiency and desorption rate of U(VI) and facilitated their practical applications (Table S1). By comparing with the literature [32,33,34,35,36,37], the advancement of this material is highlighted.

To gain a deeper understanding of how this material efficiently extracts uranium by examining charge transfer within its core–shell structure, we further utilized Kelvin probe force microscopy (KPFM). This technique enabled us to visually capture potential distribution images under illumination, thereby facilitating an in-depth study of charge migration patterns within the CC/TiO₂ NRs@MoS₂ NPs system. The AFM imaging results demonstrate that, when observed from top to bottom, the three-dimensional core–shell array units in CC/TiO₂ NRs@MoS₂ NPs are randomly distributed on the surface of the substrate (Figure 5a,c). Under dark conditions, the surface potential of the TiO₂ NRs@MoS₂ NPs units remains relatively low (Figure 5b). However, upon exposure to illumination, their surface potential increases significantly and exhibits a continuous distribution pattern (Figure 5d). This suggests that the three-dimensional core–shell structure can effectively facilitate the directional migration of charges within three-dimensional space, supporting the process of positive charge migration towards the apex of the TiO₂ NRs@MoS₂ NP units, away from the CC, under photocatalytic conditions.

Figure 6 summarizes the photocatalytic uranium extraction mechanism of the material in this study. In this study, a TiO₂ seed layer, TiO₂ nanorods, and MoS₂ nanoparticles were constructed layer-by-layer in situ on the surface of carbon fiber materials, ensuring the easy recyclability of the material. The MoS₂ nanoparticles coated on the TiO₂ nanorods formed a three-dimensional core–shell structure, increasing the catalyst loading and the degree of photocharge separation. Within the unit structure, under the joint regulation of the three-dimensional core–shell array and the heterojunction, positive charges are efficiently transported to the outer surface of the three-dimensional core–shell array units, while negative charges are transferred to the interior of the nanoarray. The photogenerated electrons within the unit are further conducted through carbon fibers, which serve as excellent conductors. Subsequently, uranium is extracted via reduction reactions occurring on the carbon fiber surface, with UO₂ identified as the primary reduction product. This method ingeniously achieves efficient charge separation in three-dimensional space, thereby effectively promoting the photocatalytic reduction process for uranium extraction. Notably, this approach significantly reduces carrier recombination efficiency, enhances the generation efficiency of active species, and fully exploits the immense potential of photocatalytic materials for uranium extraction from seawater.

3. Experimental

3.1. Materials and Reagents

Uranyl nitrate (Aladdin Chemical Reagent Co., Ltd., Shanghai, China), thiourea (Aladdin Chemical Reagent Co., Ltd.), ammonium molybdate (Tianjin Tianli Chemical Reagent Co., Ltd., Tianjin, China), titanium tetrabutoxide (>99%, Aladdin Chemical Reagent Co., Ltd.), HNO₃ (AR, Xi’an Sanpu Chemical Reagent Co., Ltd., Xi’an, China), sodium nitrate (Chengdu Jinshan Chemical Reagent Co., Ltd., Chengdu, China), potassium chloride (Chengdu Jinshan Chemical Reagent Co., Ltd.), sodium chloride (Chengdu Jinshan Chemical Reagent Co., Ltd.), calcium chloride (Chengdu Jinshan Chemical Reagent Co., Ltd.), magnesium chloride (Chengdu Jinshan Chemical Reagent Co., Ltd.), NaOH (AR, Xi’an Sanpu Chemical Reagent Co., Ltd.), absolute ethanol (AR, Tianjin Hengxing Chemical Reagent Co., Ltd., Tianjin, China), potassium hydroxide (>99.99%, Aladdin Chemical Reagent Co., Ltd.), and deionized water (laboratory-prepared). All reagents were of analytical grade and used without further purification.

3.2. Preparation of Photocatalysts

3.2.1. Preparation of CC/TiO₂ Seeds (Carbon Fiber-Supported TiO₂ Seed Layer)

Commercial carbon fiber cloth (CC) was used as the substrate. Prior to functionalization, the cloth was thoroughly rinsed with deionized water to eliminate surface contaminants and subsequently dried at 60 °C in an oven. A 1% (v/v) titanium tetrabutoxide (TTIP)-isopropanol solution was uniformly deposited onto the carbon fiber surface via dropwise addition using a micropipette until complete wetting was observed. The sample was then dried at 60 °C for 10 min, and this coating procedure was repeated three times to ensure uniform coverage. Following the coating steps, the sample underwent calcination in a muffle furnace at 400 °C for 4 h under ambient atmosphere, resulting in the formation of a carbon fiber-supported TiO₂ seed layer, denoted as CC/TiO₂ seeds.

3.2.2. Preparation of CC/TiO₂ NRs (Carbon Fiber-Supported TiO₂ Nanorod Arrays)

The hydrothermal synthesis was conducted by sequentially introducing 50 mL of 6 mol L⁻¹ hydrochloric acid (HCl) and 500 μL of titanium tetrabutoxide (TTIP) into a 200 mL Teflon-lined stainless-steel autoclave. The mixture was vigorously stirred for 30 min to ensure homogeneity. Subsequently, the pre-prepared CC/TiO₂ seeds were immersed in the reaction solution, and hydrothermal treatment was performed at 150 °C for 10 h in an oven. After natural cooling to room temperature, the resulting product was thoroughly rinsed three times with absolute ethanol followed by ultrapure water to remove residual reactants and byproducts. Finally, the sample was vacuum-dried at 100 °C for 8 h to yield carbon fiber-supported TiO₂ nanorods (CC/TiO₂ NRs).

3.2.3. Preparation of CC/TiO₂NRs@MoS₂ NPs (Carbon Fiber-Supported 3D Core–Shell Structure)

A precursor solution was formulated by dissolving 1 mmol of ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O) and 30 mmol of thiourea (CH₄N₂S) in 30 mL of deionized water under continuous magnetic stirring at room temperature for 2 h to ensure complete dissolution and homogeneity. The as-prepared CC/TiO₂ nanorods (NRs) were then immersed in the precursor solution for 30 min to facilitate adsorption of the molybdenum and sulfur precursors. Subsequently, the sample was calcined in a muffle furnace at 400 °C for 4 h under ambient atmosphere to induce thermal decomposition and formation of MoS₂ nanoparticles (NPs) on the TiO₂ nanorod surface, yielding the CC/TiO₂ NRs@MoS₂ NP composite.

For all three materials studied (CC, CC/TiO₂ NRs, and CC/TiO₂ NRs@MoS₂ NPs), the mass loadings were consistently controlled at approximately 350 ± 5 mg across multiple batches, as determined by gravimetric analysis.

3.3. Characterization

The phase composition and crystallinity of the samples were analyzed using X-ray diffraction (XRD) performed on a Bruker D8 Advanced instrument (Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15401 nm) at 40 kV. Scans were conducted from 5° to 90° at a rate of 5° min⁻¹. Surface elemental composition and chemical states were investigated via X-ray photoelectron spectroscopy (XPS) using an AXIS ULTRA DLD system (Cheshire, UK). Morphological features and elemental distribution were examined using a field-emission scanning electron microscope (FESEM, Gemini, Friedrichshafen, Germany) coupled with energy-dispersive X-ray spectroscopy (EDS) at an accelerating voltage of 15 kV. High-resolution transmission electron microscopy (HRTEM, JEM-2100F, JEOL Ltd., Tokyo, Japan) was employed to analyze microstructural details and lattice fringes. Optical absorption properties were assessed using ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS, Ocean Optics SB650, Orlando, FL, USA). Photoluminescence (PL) spectra and time-resolved fluorescence measurements were acquired with a fluorescence spectrometer (F-7000, Hitachi, Tokyo, Japan). Photoelectrochemical performance was evaluated via chronoamperometry using an electrochemical workstation (Princeton P4000, Trenton, NJ, USA).

3.4. Uranium Extraction Performance Evaluation

3.4.1. Preparation of Uranium-Containing Simulated Seawater

A base of simulated seawater was prepared by dissolving 23 g sodium chloride, 11 g magnesium chloride, 4 g sodium sulfate, 2 g calcium chloride, and 0.7 g potassium chloride in 300 mL deionized water within a 500 mL beaker. The solution was transferred to a 1000 mL volumetric flask and diluted to the mark with deionized water. To prepare a 1000 mg L⁻¹ uranium stock solution, 2.11 g uranyl nitrate was dissolved in 100 mL simulated seawater and diluted to 1000 mL. Serial dilutions of this stock solution (100, 200, 300, 400, 500, and 600 mL aliquots) were prepared in 1000 mL volumetric flasks by dilution with simulated seawater, yielding uranium concentrations of 100–600 mg L⁻¹. By adjusting to pH = 8 with a 0.2 M sodium bicarbonate solution and 0.1 M diluted hydrochloric acid, we obtained simulated seawater containing the standard uranium concentration, as required. This prepared simulated seawater was then applied to the experimental testing of the materials’ uranium extraction performance in the simulated marine conditions.

3.4.2. Uranium Concentration Detection Method

Standard solutions (100–600 mg L⁻¹ uranium) were analyzed by mixing 1 mL of each solution with arsenazo III reagent (1.0 × 10⁻⁴ M, prepared by diluting 5.0 mL of 2.0 × 10⁻³ M arsenazo III stock solution in 0.1 M perchloric acid to 100 mL with the same acid medium) and deionized water. Absorbance at 651 nm was measured using a UV–Vis spectrophotometer (SB650, USA) to establish a linear calibration curve relating uranium concentration to absorbance. Unknown concentrations were determined via this regression model.

3.4.3. Photocatalytic Uranium Extraction Experiment

A 100 mL uranium-containing seawater solution (1 mg L⁻¹ U) was irradiated in a glass reactor under a xenon lamp adjusted to simulate subsurface solar irradiance (20 mW cm⁻² at the reactor surface). We ensured that the carbon cloth-loaded catalyst (10 cm²) was immersed in the reaction solution and in full contact with it, using a nylon mesh to avoid mechanical contact with the magnetic stirrer (400 rpm). Samples (3 mL) were collected hourly from the mid-depth of the solution over 4 h using a hydrophilic syringe filter (Xi’an Sanpu Chemical Reagent Co., Ltd., China) (0.22 μm).

4. Conclusions

This study successfully developed a high-performance, recyclable photocatalytic system for uranium extraction by immobilizing TiO₂@MoS₂ core–shell heterojunction nanorods on carbon fiber (CC/TiO₂ NRs@MoS₂ NPs). Comprehensive physicochemical characterizations confirmed the successful fabrication of the 3D hierarchical core–shell architecture, which significantly enhanced catalyst loading density and interfacial charge separation efficiency. Photocatalytic experiments demonstrated exceptional uranium removal performance (97.3% efficiency within 2 h) and an unprecedented extraction capacity (976.7 mg g⁻¹ at 200 ppm U(VI)), outperforming most reported photocatalysts. Mechanistic investigations via surface potential mapping revealed that the stable core–shell structure facilitated sequential processes of photoexcitation, charge separation, internal transport, surface accumulation, and interfacial reduction, thereby achieving efficient spatial charge management. These findings provide critical insights into the design of advanced photocatalytic architectures for environmental remediation and resource recovery.