Interfacial Engineering of CdS/ReS₂ Nanocomposites for Enhanced Charge Separation and Photocatalytic Hydrogen Production

Abstract

CdS is a promising photocatalyst for solar-driven hydrogen production due to its favorable optical properties and electronic structure. However, rapid recombination of photogenerated carriers and photocorrosion significantly limit its practical application. In this study, we developed a sustainable strategy by constructing CdS/ReS₂ nanocomposites through hydrothermal interfacial engineering. On this basis, ReS₂ nanosheets were intercalated on the surface of CdS by the hydrothermal method for catalyst modification. The introduction of ReS₂ can effectively enhance the photoelectrochemical performance of CdS and accelerate the transfer of photogenerated carriers. The effects of different ReS₂ loadings on the photocatalytic activity of CdS were explored experimentally, and the data revealed that the photocatalytic hydrogen evolution efficiency reached 50 mmol g⁻¹ h⁻¹ when the loading amount of ReS₂ was 7 wt% and did not show any obvious attenuation during four cycles. This study provides a robust surface engineering strategy to enhance the catalytic efficiency of CdS photocatalysts and provides a theoretical basis for its application in photocatalytic hydrogen precipitation. This study also emphasizes the potential of abundant, non-precious metal materials for promoting scalable, environmentally friendly hydrogen production technologies that align with the principles of green chemistry and sustainable energy systems.

1. Introduction

The utilization of solar energy for chemical energy storage stands as a pivotal strategy in advancing renewable energy technologies. Photocatalytic water dissociation enables the generation of molecular hydrogen, positioning this carbon-neutral fuel as a sustainable successor to traditional hydrocarbon-based energy systems [1,2,3]. Hydrogen is considered an ideal material for addressing global energy and environmental issues because of its high calorific value and pollution-free nature. The scalable generation of hydrogen continues to pose a fundamental challenge in advancing hydrogen-based energy systems. Conventional approaches for hydrogen generation—including fossil fuel reforming, biomass processing, and electrochemical water splitting—are inherently constrained by their dependence on finite carbon resources and concomitant greenhouse gas emissions. Solar-driven photocatalytic water splitting, conversely, represents an environmentally benign alternative characterized by inherent safety and enhanced energy conversion efficiency, thereby emerging as a pivotal research frontier in sustainable energy technologies. Therefore, the design and development of efficient photocatalysts are particularly important.

CdS, as a photocatalytic material, has garnered considerable attention in the domain of aqueous remediation, carbon dioxide reduction, and water splitting due to its excellent photocatalytic performance and chemical stability. As an important semiconductor material, CdS possesses an appropriate bandgap (2.4 eV) and a reducing conduction band edge, along with superior light absorption properties and photoelectric conversion characteristics. In photocatalytic reactions, CdS generates electron–hole pairs by absorbing light energy and participates in the processes of photogenerated charge separation and transfer, thereby promoting the progression of catalytic reactions [4,5,6,7]. Junnan Tao et al. developed a novel CoP@AAH co-catalyst and loaded it on CdS nanorods via an in situ phosphorylation process, and the optimized CdS/CoP@AAH hybrid catalyst presented a photocatalytic H₂ precipitation rate of 54.9 mmol·g⁻¹·h⁻¹, an apparent quantum efficiency of 40.62%, and good cycling stability [8]. Sheng Liu et al [9] synthesized a CdS-Cu1.81 S-type heterojunction nanorods (HNRs) for photocatalytic hydrogen production driven by the NIR to UV region. However, it was found that CdS photocatalytic materials suffer from many limitations in the application process, such as the low application efficiency caused by the rapid photogenerated electron-hole complexation and the poor stability affected by the photo corrosion phenomenon induced by the severe photo corrosion of CdS by the photoexcited holes [10,11,12]. To overcome these problems, a number of inhibition strategies have emerged in the current field of research, including loading co-catalysts, constructing heterojunctions, building defects, noble metal doping, and interfacial engineering [13,14,15,16]. Dongting Yue et al. synthesized FeNiS-CdTe/CdS quantum dots with FeNiS nanoparticles as co-catalysts, and the material increased its photocatalytic performance by about 32-fold over the pristine system [17]. Xiangdong Xue et al. introduced CoP₂ as a co-catalyst into the CdS photocatalytic system for the first time, and constructed a new interfacial transversal electron migration pathway, and the peak hydrogen evolution rate CoP₂/CdS was as high as 1071 mmol·g⁻¹·h⁻¹ [18]. This system ranks among the most durable solar-driven H₂-evolving architectures reported to date, exhibiting unparalleled operational stability under continuous illumination. In addition, Huang et al. reported a two-dimensional/two-dimensional heterojunction system based on Ti₃C₂T_x MXene and CdS, which significantly improved carrier transport efficiency through photothermal effects [19]. Huang et al. utilized ultra-thin two-dimensional NiS/Ni-CdS photocatalysts to achieve the photocatalytic conversion of biomass alcohols while simultaneously producing hydrogen, further demonstrating a new strategy for optimizing the performance of CdS-based photocatalysts through interface engineering [20]. This highlights the importance of designing interface charge transfer pathways in enhancing stability and catalytic efficiency. These research findings provide a solid theoretical foundation and technical support for the practical application of CdS-based photocatalysts.

In the research on cocatalysts, noble metal cocatalysts (Au, Pt, Pd), nickel-based cocatalysts (Ni, NiS, and NiO), and two-dimensional transition metal dichalcogenides (MoS₂, WS₂, and ReS₂) have been widely applied in photocatalysis. Among these catalysts, ReS₂ is considered a promising cocatalyst due to its stable phase structure, excellent charge transfer capability, and abundance of unsaturated sites. To date, several reports have demonstrated that ReS₂ materials, when used as cocatalysts, can effectively enhance photocatalytic performance under visible light irradiation. For example, Na Su et al. [21] loaded ReS₂ onto a CdS/ZnS catalyst, and CdS/ZnS-ReS₂ heterostructure demonstrated exceptional photocatalytic proficiency, achieving a peak hydrogen evolution rate of 10,722 μmol·g⁻¹·h⁻¹. This performance represents a 178-fold enhancement over pristine CdS and a fivefold increase relative to the CdS/ZnS binary composite, while maintaining remarkable operational stability over prolonged illumination cycles [21]. Jiachao Xu et al. designed a 2D/2D ReS₂/ZnIn₂S₄ heterostructured nanolamellar photocatalyst, and ReS₂/ZnIn₂S₄ (3 wt%) showed excellent photocatalytic activity [22]. X. Wang et al. loaded rhenium disulfide nanosheets as co-catalysts on tri-s-triazine-based semiconductor nanotubes for photocatalytic hydrogen production, and the prepared 1T’-ReS₂/g-C₃N₄ composites exhibited significantly enhanced photocatalytic activity, and the ReS₂ nanosheets as co-catalysts can promote the generation of more photoexcited charges from g-C₃N₄ nanotubes, facilitate carrier migration and separation, and provide abundant active sites for photocatalysis [23].

While ReS₂ as a cocatalyst has been explored in prior studies involving planar or particulate CdS substrates [21,22,23], this work leverages a unique dendritic CdS architecture to amplify interfacial interactions. The three-dimensional branched morphology offers significantly increased surface area and abundant anchoring sites compared to conventional structures, facilitating higher ReS₂ loading density and optimizing interfacial charge transfer pathways. This structural engineering distinguishes our approach by enhancing both cocatalyst dispersion and electron migration efficiency.

A two-step synthesis strategy was developed to fabricate hierarchical CdS nanostructures decorated with ReS₂ nanosheets. Initially, CdS with branched morphology was prepared via solvothermal treatment. Subsequently, ReS₂ was in situ anchored onto the CdS surface through a secondary hydrothermal process, where the molar ratio of ammonium perrhenate precursor was precisely modulated to regulate ReS₂ loading density. Structural optimization demonstrated that the composite with 7 wt% ReS₂ issued higher photocatalytic activity, achieving a hydrogen evolution rate of 50 mmol·g⁻¹·h⁻¹ under visible light irradiation. This value represents a 230-fold enhancement compared to unmodified CdS. Furthermore, accelerated durability tests demonstrated remarkable stability, with negligible activity loss observed over four consecutive reaction cycles, confirming effective mitigation of photo corrosion phenomena.

2. Experimental Section

A dendritic CdS architecture was fabricated through a straightforward hydrothermal protocol. In the standard procedure, cadmium chloride (0.1 mol) and thiourea (0.1 mol) were homogenously dispersed in 60 mL of ultrapure water under continuous magnetic agitation for 120 min to achieve a transparent homogeneous mixture. The solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave and kept at 180 °C for 24 h. Upon reaching ambient temperature, the resultant, orange-colored material was isolated via vacuum filtration. To ensure purity, sequential rinsing cycles with ethanol and ultrapure water were performed to remove unreacted precursors. The rinsed samples were then dehydrated in a controlled drying oven maintained at 60 °C for 12 h, resulting in a uniformly orange-hued final product.

Synthesis of dendritic CdS/ReS₂: 300 mg of CdS, 22.59 mg of NH₄ReO₄, 25.62 mg of CH₄N₂S, and 17.55 mg of HONH₃Cl were dissolved in 60 mL of deionized water, and the solution was subsequently dispensed into 100 mL of a Teflon-lined stainless-steel autoclave and subjected to 180 °C for 20 h. After cooling to room temperature, the products were collected by filtration. products were collected by filtration. Then, the precipitates underwent multiple rinse cycles in distilled water and ethanol (3 cycles each). Final dehydration was achieved via vacuum drying at 60 °C until mass stabilization. That is, the obtained CdS/ReS₂ material (Figure 1a) was used for the next characterization step. To evaluate the effect of ReS₂ content, four distinct composites with varying ReS₂ mass ratios (3%, 5%, 7%, and 9%) were fabricated, labeled as C-R-3, C-R-5, C-R-7, and C-R-9. The precise regulation of ReS₂ incorporation was achieved by modulating the ammonium perrhenate precursor concentration during synthesis. The numerical suffixes in sample labels directly correlate to the ReS₂ weight percentage within each composite.

Powder XRD characterization utilized a Bruker D8 Advance system (Cu Kα, λ = 1.5406 Å) in Bragg–Brentano geometry, scanning 2θ = 10–80° at 7°·min⁻¹. The structural and surface features of the synthesized materials were analyzed using scanning electron microscopy (Sigma300 Zeiss, Zeiss, Oberkochen, Germany) transmission electron microscopy (JEM 2100F, JEOL, Tokyo, Japan) and high-resolution TEM (JEOL, JEM 2100F, Japan). To validate the elemental composition and chemical states, X-ray photoelectron spectroscopy (Thermo Scientific K-Alpha spectrometer, Thermo Fisher Scientific, Waltham, MA, USA) was employed under Al Kα monochromatic X-ray irradiation. The light absorption properties of the samples were measured by a UV-vis spectrophotometer (TU-1950, Prodigy, Beijing, China) using BaSO₄ as a reflectance standard. The emission properties of the synthesized photocatalysts were characterized via photoluminescence spectroscopy (Duetta, Horiba, Kyoto, Japan). Measurements were conducted under monochromatic excitation at 360 nm to probe charge carrier dynamics. Time-resolved photoluminescence spectra were collected on a fluorescence lifetime spectrometer (Duetta, Horiba, Kyoto, Japan).

The photocatalytic hydrogen evolution experiment was conducted in a closed gas circulation and vacuum system, using a 250 mL Pyrex cell as the photoreactor. In brief, 10 mg of catalyst was dispersed in 50 mL of a mixed aqueous solution containing 0.35 M Na₂S and 0.25 M Na₂SO₃ as sacrificial agents (pH = 13.2). The solution was then thoroughly degassed and irradiated with a 300 W xenon lamp (Zhongjiao Jinyuan, CEL-HXF300-T3, Beijing, China) equipped with a simulated sunlight filter (AM 1.5) placed vertically at the top of the reactor. The reactor was maintained at low temperature using a cold hydrazine recirculating water system, with the cooling water recirculation system maintaining a temperature of 6 °C. Throughout the reaction process, the system fan was kept running to maintain gas concentration equilibrium. A certain amount of generated gas was collected every half hour and analyzed for hydrogen content using a gas chromatograph (GC-7902, Zhongjiao Jinyuan, Beijing, China, TCD) with argon as the carrier gas. The photocatalytic activity of the sample was evaluated based on the average H₂ release rate every 4 h.

3. Results and Discussion

In this paper, we rationally designed and constructed a method to prepare dendritic CdS/ReS₂ materials by using dendritic structured CdS as a substrate. The two-step synthesis process of dendritic CdS/ReS₂ materials is shown in Figure 1a. First, the dendritic CdS precursor material was prepared by hydrothermal method. Then CdS was ReS₂ doped using different amounts of NH₄ReO₄ and C₂H₅NS under hydrothermal conditions to obtain CdS/ReS₂ materials. As shown in Figure 1b, the XRD patterns of CdS and C-R-5 are consistent with the standard pattern of CdS (JCPDS no. 77-2306), and CdS, C-R-3, C-R-5, C-R-7, and C-R-9 all show the same XRD peaks (Figure 1c), and the peaks are all very sharp without any obvious peaks of impurities, which indicates that the materials have been well-crystallized, and all of them have hexagonal square crystals are present. In addition, the phase structure of CdS remains unchanged after the introduction of ReS₂. No obvious characteristic peaks are found for ReS₂ in CdS/ReS₂, which is mainly attributed to the low loading of ReS₂, and the characteristic peaks cannot be detected. The collective experimental data conclusively demonstrate the effective fabrication of CdS/ReS₂ heterostructures, corroborating the phase-pure incorporation of ReS₂ into the heterostructured scaffold.

Meanwhile, the SEM and TEM images showed that the CdS material showed a dendritic shape (Figure S1a,b), and the CdS/ReS₂ material consisted of ReS₂ nanosheets and CdS nanomaterials with a dendritic structure (Figure S1c,d). Structural analysis revealed that pristine CdS exhibits a well-defined branch-like morphology with homogeneous dimensional uniformity. Notably, following ReS₂ incorporation, the initially smooth surfaces transitioned to a textured topography. Furthermore, lamellar nanostructures attributed to ReS₂ were uniformly distributed across the CdS substrate, as evidenced by combined microscopic observations. This interfacial modification aligns with XRD patterns, substantiating the effective heterostructure formation between CdS and ReS₂. Elemental composition analysis via energy-dispersive X-ray spectroscopy (EDS) was conducted on the CdS/ReS₂ composite. Spectral profiles of C-R-7 (Figure S1e,f) unequivocally validated the coexistence of Cd, S, and Re within the heterostructure. Spatial mapping further revealed homogeneous dispersion of Re, S, and Cd across the leaf-like morphology, confirming the absence of localized elemental aggregation and reinforcing the efficacy of ReS₂ incorporation.

To systematically characterize the crystallographic features and surface topographies of the interface-engineered CdS/ReS₂ heteronanostructures, they were investigated by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), and the results are shown in Figure 2a. Transmission electron microscopy (TEM) analysis of CdS and C-R-7 heterostructures (Figure 2e) revealed intimate interfacial interactions between the components. High-resolution imaging demonstrated that ReS₂ nanosheets were firmly anchored onto the CdS substrate, suggesting strong chemical coupling at the heterojunction interface. Figure 2b presents a high-resolution TEM (HRTEM) micrograph of the CdS nanostructure, and its lattice stripes can be clearly detected. Figure 2c,d shows the clear lattice streaks observed by the Gatan microscope set with grid spacing of 0.35 nm, which corresponds to the (100)-crystalline surface of the CdS material [24]. High-resolution TEM analysis of the composite (Figure 2f) provided atomic-level evidence for the incorporation of rhenium disulfide (ReS₂), with distinct lattice fringes corresponding to its crystallographic planes. These observations unequivocally validate the structural integration of ReS₂ within the heterostructure. Figure 2g,h shows clear lattice streaks of ReS₂ observed by the Gatan microscope group with a lattice spacing of 0.31 nm, which corresponds to the (100) crystallographic plane of the ReS₂ material [25]. Meanwhile, we performed an EDS energy spectrum line scan of CdS/ReS₂, as shown in Figure 2i–k, on the surface of the material, we selected two different cross sections for line scanning, and the results showed that Cd, S, and Re existed in the material, and the content of Re was much smaller than Cd and S, which is in agreement with the ideal situation, proving that the material was successfully prepared. Critically, the dendritic framework of CdS plays a pivotal role in augmenting the ReS₂/CdS synergy. As evidenced by TEM and EDS mapping (Figure 2e–k), the hierarchical branches provide radially distributed conductive channels that shorten carrier diffusion distances to ReS₂ sites. Moreover, the interpenetrating network creates microreactor-like domains, concentrating reactive species and maximizing cocatalyst utilization—advantages unattainable in non-dendritic systems.

X-ray photoelectron spectroscopy (XPS) was employed to probe the elemental composition and oxidation states of the CdS/ReS₂ heterostructure. Representative survey spectra (Figure 3a) of the dendritic composite confirmed the uniform distribution of cadmium (Cd), rhenium (Re), and sulfur (S) within the material. To further elucidate the bonding configurations, high-resolution XPS spectra were systematically analyzed, focusing on the core-level signals of Cd, Re, and S. As shown in Figure 3b, for CdS/ReS₂, the two peaks located near 405.05 eV and 411.84 eV are attributed to Cd 3d_5/2 and Cd 3d_3/2, indicating that Cd in CdS/ReS₂ exists in the Cd²⁺ state. Compared with dendritic CdS, the Cd 3d core-level spectra in the CdS/ReS₂ heterostructure exhibited a distinct positive binding energy shift compared to pristine CdS. This phenomenon is indicative of directional charge migration from the CdS framework to ReS₂, corroborating the formation of interfacial electronic coupling. For the high-resolution XPS spectra of Re (Figure 3c), the two peaks appearing at 41.8 eV and 44.2 eV were attributed to Re 4f_5/2 and Re 4f_7/2, respectively, and the above results confirmed the successful deposition of ReS₂ on the CdS surface. Compared with the dendritic CdS, the peak of Re 4f in CdS/ReS₂ is shifted to the direction of low binding energy, indicating that the electrons are clustered on ReS₂. The S 2p peak in Figure 3d can be deconvoluted into two peaks by the XPS peak fitting procedure, where the peaks located around 162.07 eV and 163.29 eV can be attributed to S 2p_3/2 and S 2p_1/2, respectively. Meanwhile, the peak of S 2p in CdS/ReS₂ moves toward the high binding energy direction, and the above results further demonstrate that electrons in the loaded CdS/ReS₂ material move from the CdS to ReS₂ and complete the hydrogen precipitation reaction on the surface of ReS₂.

UV-vis diffuse reflectance spectroscopy was used to evaluate the light-harvesting properties of CdS and CdS/ReS₂ composites (Figure 4a). Notably, CdS/ReS₂ retained visible absorption below 530 nm while displaying enhanced absorption at longer wavelengths (>530 nm), attributable to ReS₂-induced light scattering effects. To quantify the bandgap, the Kubelka-Munk function (αhν)² = α (hν − E_g)² was applied, where n = 4 aligns with the n-type semiconductor nature of CdS [26,27]. Tauc plots derived from (αhν) versus hν (Figure 4b) yielded bandgap values of 2.33 eV for pristine CdS and 2.25 eV for the C-R-7 composite, indicating a narrowed bandgap post-ReS₂ incorporation. The narrowing of the bandgap after loading indicates that the photogenerated electron holes of the loaded material can be separated more easily during the catalytic reaction, and it also proves that the material is successfully loaded.

The relative energy levels of the conduction band (CB) and the valence band (VB) are critical for the photocatalytic activity of semiconductor materials. Research has demonstrated that materials with elevated CB and VB potentials exhibit superior photocatalytic performance, as these energy configurations enhance charge carrier separation and redox capability. The Mott-Schottky spectra of CdS and C-R-7 (Figure 4d) show that the flat-band potentials of CdS and C-R-7 with respect to the electrode with Ag/AgCl are −0.69 eV and −0.85 eV; therefore, the Fermi energy levels (E_f) of CdS and C-R-7 relative to the Ag/AgCl electrode are −0.69 eV and −0.85 eV, and then the Fermi energy levels of the CdS and C-R-7 materials relative to the standard hydrogen electrode (SHE) are calculated by the formula: E_SHE = E_Ag/AgCl + 0.197 + 0.059 × pH, which are −0.08 eV and −0.24 eV (Table S1), respectively. XPS valence band spectra (Figure 4c) revealed the valence band maximum (VBM) positions of CdS and C-R-7 relative to the Fermi level (E_f) as 1.74 eV and 0.33 eV, respectively. Through conversion to the standard hydrogen electrode (SHE) scale, the VBM potentials were calculated as 1.66 eV (CdS) and 0.09 eV (C-R-7). Utilizing the relationship E_VB = E_g + E_CB, the conduction band minimum (CBM) potentials were derived, yielding values of −0.67 eV for CdS and −2.16 eV for C-R-7 versus SHE (Table S2).

Figure 4e illustrates the energy band configurations of CdS and C-R-7. Comparative analysis reveals that the conduction band minimum (CBM) of C-R-7 is positioned at a more negative potential relative to CdS. This downward shift in CBM energy level enhances the thermodynamic favorability of reduction reactions, signifying superior reductive capacity in the C-R-7 composite. For efficient solar-driven water splitting, the semiconductor photocatalyst must satisfy two fundamental electronic criteria: Bandgap energy (E_g) ≥ 1.23 eV (corresponding to an absorption edge wavelength ≤1000 nm), ensuring sufficient thermodynamic driving force for water dissociation. Meanwhile, alignment of water redox potentials within the band structure: The conduction band minimum (CBM) must be positioned at a more negative potential than the H⁺/H₂ reduction level (0 V vs. SHE, pH 0). The valence band maximum (VBM) should exceed the O₂/H₂O oxidation potential (1.23 V vs. SHE, pH 0), enabling hole-mediated water oxidation. The CBM energy of C-R-7 was much more than 0 V, which further proved that C-R-7 could be well applied in photocatalytic decomposition. 7 can be well applied in photocatalytic water decomposition for hydrogen production [28].

The migration, separation and capture processes of photoinduced charge carriers were further investigated using steady-state photoluminescence (PL) spectroscopy. Broadly speaking, the lower the PL intensity, the higher the photoinduced charge carrier separation efficiency and therefore the higher the photocurrent density [29]. As shown in Figure 5a, it can be found that C-R-7 exhibits the lowest PL intensity among all the samples, which indicates that the electron-hole pair complexation rate is the lowest at this time, exhibiting enhanced electron-hole separation efficiency while exhibiting the highest photocurrent density. Time-resolved photoluminescence spectroscopy further elucidated the photogenerated charge behavior in the composites. As depicted in Figure 5b, the carrier lifetime of C-R-7 (0.122364 ns) exhibited a marked reduction compared to pristine CdS (0.929633 ns). This pronounced decrease in lifetime underscores the role of ReS₂ as an efficient electron mediator, effectively suppressing charge carrier recombination. Enhanced separation of photogenerated charge carriers in CdS/ReS₂ facilitates greater electron availability for participation in superior charge dissociation efficiency reactions (HER), thereby augmenting the overall photocatalytic activity.

The transfer and separation rate of electron–hole pairs during the photocatalytic reaction will greatly affect the performance of the photocatalysts; therefore, we performed transient photocurrent tests to analyze separation efficacy of photogenerated electron–hole pairs. As shown in Figure 5c, applying seven photo switching cycles, the photocurrent response can be observed quickly, indicating that the material has good charge transfer capability. Comparative analysis of transient photocurrent responses revealed a significant enhancement in the CdS/ReS₂ composite relative to pristine CdS, validating improved charge carrier separation efficiency in the heterostructured material. Notably, among the synthesized samples, C-R-7 displayed the most pronounced photocurrent density, underscoring its optimal charge transport kinetics [30]. EIS analysis was also performed (Figure 5d) to confirm the above results. CdS/ReS₂ showed smaller semicircular diameters than pristine CdS, indicating favorable interfacial electron transfer capability on CdS/ReS₂.C-R-7 showed the smallest semicircular radius, with smaller semicircular radii indicating lower electrical resistance and faster electron transfer during the catalytic reaction [31].

For the comprehensive analysis of surface potential engineering, we analyzed the charge transport and recombination kinetics of the charge transport layer using Kelvin probe force microscopy (KPFM). Surface potentials of C-R-7 composites were detected under dark and 365 nm-UV light irradiation conditions, as shown in Figure 6. The surface potential maps obtained under dark conditions show a slight potential difference (ΔE = 25 mV), indicating a relatively low rate of electron transfer. However, a subsequent scan at the same position after illumination increased the potential difference to 35 mV, demonstrating interfacial potential gradient generation with concomitant charge transport enhancement. KPFM measurements revealed a pronounced surface potential disparity between C-R-7 and pristine CdS under illumination. This enhanced potential gradient in C-R-7 unequivocally indicates facilitated interfacial charge migration upon light irradiation, a critical factor contributing to its augmented photocatalytic efficiency. Furthermore, the observed trend aligns with electrochemical impedance spectroscopy (EIS) data [32,33], which corroborates the superior charge transport kinetics in the composite material.

The photocatalytic behavior of the composites was systematically assessed, motivated by the dual role of ReS₂ in augmenting visible light harvesting efficiency and optimizing charge carrier separation dynamics. This investigation aimed to elucidate the structure-activity relationship between ReS₂ incorporation and enhanced redox performance. The hydrogen production performance of the CdS and CdS/ReS₂ samples under visible light irradiation with NaS₂-NaSO₃ as a sacrificial reagent is shown in Figure 7a, which displays the photocatalytic rate of hydrogen production for each of the five different samples, and as we can see, the loaded material exhibits enhanced photocatalytic hydrogen production performance, and the CdS/ReS₂ material with 7 wt% ReS₂ content shows maximum quantum yield for photocatalytic water reduction, with the H₂ release rate reaching 50 mmol·g⁻¹·h⁻¹, which is about 230 times higher than the unmodified CdS pristine material. This indicates that the photocatalytic performance can be greatly improved by using ReS₂ as a co-catalyst. In addition, C-R-7 presents a much higher H₂ precipitation efficiency than other reported noble metal-free photocatalysts, and even higher than the hydrogen evolution efficiency of noble-metal-containing photocatalysts (Figure 7b). These results indicate that CdS/ReS₂ photocatalysts with a loading of 7 wt% have low cost as well as high hydrogen production performance.

Extensive studies have identified photocorrosion as a critical bottleneck limiting the hydrogen evolution efficiency and operational durability of CdS-driven photocatalytic processes. This is also demonstrated in Figure 7c. Therefore, cycling tests were performed on the CdS/ReS₂ material. As shown in Figure 7d, the performance of the material did not decrease after 16 h of irradiation, and it still has good photocatalytic hydrogen production performance. The results show that CdS/ReS₂ has superior photocatalytic efficacy and robust operational durability, and it still maintains good stability after 4 cycles, and it can effectively resist photo corrosion.

The polarized C-R-7 material improved the adsorption of H₂O and showed good wettability [34] as revealed by the hydrophilic test (Figure 7e). Initial water contact angle measurements for CdS and C-R-7 surfaces yielded values of 129° and 120°, respectively. Following a 5 min equilibrium period, these angles decreased to 114° (CdS) and 58° (C-R-7). The observed reduction in contact angles demonstrates progressive surface hydration, where lower contact angle values correlate with enhanced surface hydrophilicity. According to the results of the contact angle test, the contact angle of C-R-7 was smaller than that of CdS; meanwhile, in 5 min, compared with the CdS material, there was a significant change in the contact angle of C-R-7, which indicated that C-R-7 had strong hydrophilicity and further proved that C-R-7 was a suitable catalyst for hydrogen production. Structural characterization demonstrated a remarkable enhancement in BET surface area (Figures S2 and S3) and mesoporous volume after ReS₂ incorporation, aligning with theoretical predictions for heterostructure formation. This structural optimization in C-R-7 not only confirms the successful synthesis protocol but also highlights its superior textural attributes (Table S3). The augmented surface characteristics directly correlate with an elevated density of accessible active sites, thereby promoting enhanced catalytic reactivity.

To gain mechanistic insights into the photocatalytic hydrogen evolution behavior, the apparent quantum yield (AQY) and solar-to-hydrogen (STH) conversion efficiency of the CdS/ReS₂ composite were systematically evaluated under monochromatic illumination across varying wavelengths (Table S4). The AQYs of CdS/ReS₂ at the wavelengths of 365 nm, 420 nm, 520 nm, 600 nm, 650 nm, and 700 nm, respectively, were 2.027%, 2.417%, 0.712%, 0.023%, 0.028%, and 0.029% (Figure 8a). The hydrogen evolution rate exhibited peak activity under 420 nm illumination, demonstrating wavelength-specific optimization. Notably, the apparent quantum yield (AQY) trend displayed a direct proportionality to the spectral absorption characteristics of the material, confirming incident photon absorption as the dominant driving force for photocatalytic H₂ production. The STH of CdS/ReS₂ at 365 nm, 420 nm, 520 nm, 600 nm, 650 nm, and 700 nm were 7.332%, 10.060%, 3.670%, respectively, 0.139%, 0.182%, and 0.198% (Figure 8d). Under 420 nm illumination, the C-R-7 composite achieved a solar-to-hydrogen (STH) conversion efficiency of 10.060%, surpassing the majority of benchmark photocatalysts documented in the literature. Complementary incident photon-to-current efficiency (IPCE) measurements (Figure 8b,c) demonstrated exceptional photoconversion capabilities in C-R-7, with spectral responses closely aligned to its UV-vis absorption profile. These findings collectively underscore the synergistic enhancement of electronic conductivity and charge separation kinetics in C-R-7, directly contributing to its superior photocatalytic performance.

Figure 8e,f show the in situ FT-IR spectra of the reaction of H2O on CdS and C-R-7. In the IR spectra, the H-O-H bending vibration (δH-O-H) is at ~1650 cm⁻¹ and the O-H stretching vibration (νO-H) is at ~3400 cm⁻¹. The νO-H mode corresponds to hydroxyl bond cleavage, which facilitates proton release and influences hydrogen evolution kinetics, while δH-O-H reflects water molecular adsorption on the catalyst surface. Prolonged illumination induced a progressive increase in the intensity of both δH-O-H and νO-H signals, signifying continuous water consumption during photocatalytic hydrogen generation. Notably, CdS displayed minimal spectral changes until 30 min of light exposure, whereas C-R-7 exhibited rapid intensity modulation immediately upon irradiation. This distinct response further corroborates the superior hydrogen evolution capability of the C-R-7 composite.

XPS characterization of the materials showed that the peak of Cd 3d in CdS/ReS₂ shifted toward high binding energy with respect to CdS (Figure 3b), indicating electron transfer from CdS to ReS₂. High-resolution Re 4f spectra (Figure 3c) revealed a negative binding energy shift in CdS/ReS₂ compared to pristine CdS, suggesting electron enrichment on the rhenium disulfide component. This electronic redistribution phenomenon confirms directional charge transfer from CdS to ReS₂, where the hydrogen evolution reaction preferentially occurs at ReS₂ active sites.

As illustrated in Figure 9, ReS₂ nanosheets function as auxiliary catalysts anchored on dendritic CdS architectures. Upon visible-light irradiation, CdS undergoes photoexcitation, generating electron–hole pairs. Following photoexcitation, electrons undergo interband transitions from the valence band to the conduction band, leaving photogenerated holes in the valence band that are rapidly neutralized by sacrificial electron donors. These conduction band electrons subsequently migrate to ReS₂ catalytic sites, where they facilitate the reduction in protons through the reaction: 2H⁺ + 2e⁻ → H₂. Notably, band structure modulation via heterojunction formation (Figure 4b) resulted in an enlarged bandgap, which effectively suppresses charge carrier recombination. This dual optimization of interfacial charge separation and reaction kinetics collectively enhances the photocatalytic performance.

4. Conclusions

In this work, a thin layer of ReS₂ was grown on the dendritic CdS material by the hydrothermal method. The resulting CdS/ReS₂ photocatalysts were morphologically uniform and structurally stable with good interfacial contacts, which could significantly promote charge separation, inhibit charge complexation, and provide numerous catalytic centers for hydrogen evolution. The loading of ReS₂ nanosheets was conveniently controlled by varying the concentration of ammonium perrhenate. Furthermore, CdS/ReS₂ with a ReS₂ content of 7 wt% exhibited optimal hydrogen evolution rate (50 mmol·g⁻¹·h⁻¹) under visible-light-driven conditions with robust stability. The enhanced catalytic performance of ReS₂ arises from its dual role as an electron sink and cocatalyst, synergistically coupled with CdS through intimate interfacial contact. This configuration accelerates surface hydrogen evolution kinetics by optimizing charge transfer pathways. The rational design of dendritic CdS/ReS₂ heterostructures establishes a paradigm for developing noble metal-free photocatalysts with high solar-driven water splitting efficiency, offering a scalable and cost-effective route for sustainable hydrogen production.