Heterojunctions of ZnS with Zn vacancies and hexagonal CdS pyramids for photocatalytic hydrogen production

Abstract

Metal sulfides with suitable morphology typically possess efficient solar light utilization and suitable band structures for photocatalytic hydrogen production. However, a significant challenge is faced in developing the optimal morphology of individual semiconductors and precisely modulating hybrid heterostructures. Herein, this work reports the superior photocatalytic hydrogen evolution performance of hexagonal CdS pyramids compared to other nanostructured CdS catalysts. Additionally, ZnS nanoparticles with zinc vacancies are grown in situ on the surface of the hexagonal CdS pyramids, meticulously constructing a Z-scheme heterojunction of CdS/ZnS. By elaborately adjusting the ratio of Cd to Zn sources, the interfacial structure of the heterojunction can be optimized to significantly reduce the obstruction of charge transfer. With the same concentrations of Cd and Zn sources, the CdS/ZnS heterojunction hexagonal pyramid–nanoparticle morphology exhibits a significantly enhanced photocatalytic hydrogen evolution rate of 4660 μmol h⁻¹ g⁻¹ under visible light, which is 4.6 and 84.72 times higher than those of pristine CdS pyramids and ZnS nanoparticles, respectively. The electron paramagnetic resonance (EPR) results comprehensively demonstrate the promoted interfacial charge separation in the Z-scheme CdS/ZnS. This work provides an effective strategy for the rational design of catalyst morphology and construction of Z-scheme heterojunction photocatalysts for efficient photocatalysis.

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1. Introduction

Energy transition and carbon emission reduction have been promoting large-scale development of new energy sources, including solar energy.^1–4 It is an ideal pathway for sustainable energy conversion that photocatalytic hydrogen production requires only sunlight irradiation and high photoconversion efficiency.^4–8 To archive high photoconversion efficiency, a vast array of semiconductor materials has been explored as photocatalysts for the hydrogen evolution reaction (HER). Metal sulfide (MS) photocatalysts with lower reduction potential are viewed as promising photocatalysts for H₂ production, owing to their advantageous response to visible light and adjustable band structures.^9–12 Among them, CdS is a narrow bandgap semiconductor with an energy gap (E_g) of approximately 2.4 eV and a suitable energy-band position, enabling it to absorb the visible spectrum while maintaining relatively strong redox abilities.^13–15 However, the photocatalytic activity of CdS itself toward water splitting remains very low due to high-rate charge recombination.^16–18 Two strategies have been proposed to address the low photocatalytic activity of CdS. One way involves optimizing the surface morphology of the photocatalysts to enhance their catalytic activity. Various morphologies of CdS nanostructures, such as nanorods,^18–20 nanoflowers,^21,22 and nanowires,^23,24 have been fabricated via different techniques. However, the instability issues of CdS limit its practical applications due to the photo-generated holes affecting the dissolution of Cd²⁺ and the self-oxidation of S²⁻. Another method is designing composite catalysts, which effectively promote charge transfer and separation, thereby enhancing the photocatalytic performance. It has been proved by combining with noble metals such as Pt and Au as co-catalysts, although its further application is limited by the high cost and scarcity of noble metals,^25,26 so the construction of CdS composites is a promising strategy. Anchoring suitable semiconductor materials on the surface of CdS can provide active surface sites and accelerate charge separation while maintaining strong electron reductivity. Various composite semiconductor photocatalyst systems incorporating CdS have been explored, such as CdS–TiO₂,²⁷ CdS/BiVO₄,^28,29 CdS/WO₃,^30,31 and CdS@CoS₂.^32,33 Simultaneously modulating the catalyst's nanostructure and integrating it with other materials can enhance hydrogen evolution performance. Combining specialized geometric topologies with heterojunction structures generates a greater number of catalytic active sites, thereby maximizing the surface chemical reactions of the catalyst.^34,35 Researchers have found that catalysts with abundant vacancy defects not only regulate the band structure but also exhibit excellent carrier diffusion properties.^36,37 The combination of ZnS with CdS has also been investigated to enhance stability and activity in solar energy conversion. For instance, Jin et al. reported that a ZnS shell grown on CdS formed a type I structure, enhancing the photocatalytic H₂ evolution activity of CdS.³⁸ By introducing defect engineering, the band structure of heterojunctions can be modified, thereby effectively enhancing the separation efficiency of photogenerated carriers.³⁹ Cheng et al. developed a type II structure by wrapping a vacancy-containing ZnS shell around CdS nanorods, which provides a new pathway for hole transport and significantly improves hydrogen production efficiency.⁴⁰ Guo et al. synthesized a zinc vacancy-mediated CdS/ZnS core/shell hybrid structure on CdS nanorods, forming a Z-scheme photocatalytic system.⁴¹ Additionally, it has been noted that ZnS deposited on the surface of CdS acts as a protective layer, aiding in the prevention of the photo-corrosion of CdS.⁴² However, the interaction between CdS in various morphologies and ZnS requires consideration of how to precisely regulate the site to enhance the separation of photogenerated carriers. Therefore, it is crucial to regulate the CdS configuration and adjust the Cd/Zn ratio during the sulfurization process to form intimate interfaces and optimize carrier transport.

Inspired by these studies, morphological control was coupled with interface engineering to reduce electron–hole recombination and increase the ability of photocatalytic hydrogen production. Modified multi-morphology nanocomposites with different molar ratios of CdS/ZnS were synthesized. It was found that the pyramid morphology of CdS significantly enhances the photocatalytic hydrogen production. Additionally, the optimal formulation for hydrogen production by the CdS/ZnS hexagonal pyramids–nanoparticles was explored under visible light irradiation (λ ≥  420 nm). The highest hydrogen production yield of 4660 μmol h⁻¹ g⁻¹ was achieved when the molar ratio of the Cd source to the Zn source was equal, and the stability study conducted over several cycles confirmed that the photocatalyst exhibits good stability. Moreover, this study found the Z-scheme structure between the interface of CdS and ZnS through carefully performed studies of light absorption, charge separation ability, and band alignment.

2. Experiments

2.1. Materials

Cadmium acetate (Cd(CH₃COO)₂·2H₂O), zinc acetate (Zn(CH₃COO)₂), thiourea (CH₄N₂S), thioacetamide (C₂H₅NS), lithium hydroxide (LiOH·H₂O), and anhydrous ethanol (C₂H₅OH) were supplied by Shanghai Macklin Biochemical Co, Ltd. Sodium sulfide (Na₂S·9H₂O) and sodium sulfite (Na₂SO₃) were obtained from Shanghai Aladdin Biochemical Technology Co., Ltd. All chemical reagents were of analytical grade (AR) and used as received without further purification. Deionized water was produced in-house in the laboratory.

2.1.1. Synthesis of CdS hexagonal pyramids. To prepare the CdS hexagonal pyramid samples, 2 g of CH₄N₂S, 1.33 g of Cd(CH₃COO)₂·2H₂O, and a certain mass of LiOH·H₂O were mixed thoroughly and stirred in 70 mL of deionized water within a polytetrafluoroethylene (PTFE) lining for 1 hour. The mixture was then transferred to a high-pressure autoclave and reacted at 200 °C for 12 hours in a homogeneous reactor. After cooling to room temperature, the resulting yellow precipitate was washed several times with deionized water and ethanol until the filtrate pH was neutral. The product was then dried in an oven at 60 °C for 8 hours to obtain the required sample. In addition, CdS nanoparticles and CdS nanorods were synthesized according to previously reported literature.⁴³

2.1.2. Synthesis of CdS/ZnS composites. CdS/ZnS hybrid heterojunction materials were synthesized via the hydrothermal method. The specific experimental procedures are illustrated in Fig. 1. A certain mass of CdS was first stirred in 40 mL of ethanol for 1 hour. Subsequently, a certain ratio of Zn(CH₃COO)₂ and C₂H₅NS was added to the above mixture and sonicated. The resultant dispersion was then transferred to a high-pressure autoclave and heated at 200 °C for 12 hours. After cooling to room temperature, the resulting yellow precipitate was thoroughly washed several times with water and ethanol. Simultaneously, the ratio of Cd(CH₃COO)₂ to C₂H₅NS was adjusted to vary the molar ratios of Cd/Zn as 1 : 1, 5 : 1, 1 : 3, 3 : 5, and 1 : 5, and the samples obtained were labeled as CZ1-1, CZ5-1, CZ1-3, CZ3-5, and CZ1-5. For comparison, ZnS nanoparticles were synthesized using the same method, but without the addition of pre-prepared CdS.

Fig. 1 Schematic illustration of the synthesis of CdS/ZnS hexagonal pyramid–nanoparticle composites.

2.2. Characterization methods

The crystalline structures of the samples were analyzed using X-ray powder diffraction (XRD, SmartLab SE). Their morphology and elemental composition were examined using field emission scanning electron microscopy (FESEM, SU8010) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G2 F20), which includes energy dispersive X-ray spectroscopy (EDS) for elemental analysis. The chemical composition of the samples was determined using a Thermo Scientific escalab 250 Xix X-ray photoelectron spectrophotometer (XPS) with an Al kα source, operated with an energy step size of 1 eV for survey scans and 0.1 eV for detailed scans. Diffuse reflection spectroscopy (DRS) was performed using UV-vis spectroscopy (Lambda 750s). Photoluminescence (PL) properties were investigated using a fluorescence spectrophotometer (Hitachi F-7000), and time-resolved photoluminescence (TRPL) spectra were obtained with an FLS1000 fluorescence spectrometer. Electron paramagnetic resonance (EPR) was conducted on an ESR spectrometer (MEX-nano, Bruker).

2.3. Photocatalytic test

Photocatalytic water splitting activity was measured in a top-irradiation-type reactor integrated with a closed gas circulation and evacuation system. A vacuum was applied to remove the air in the water. The photocatalysts (50 mg) were placed in 100 mL of deionized water with continuous stirring, which contained Na₂SO₃ (0.35 M) and Na₂S·9H₂O (0.25 M) as sacrifice agents. Before light irradiation, Ar was purged for 20 min to remove the air in the reactor. A 300-W xenon lamp with filter (λ ≥ 420 nm) was used as the light source to simulate visible light. The recirculating cooling water system was controlled at 6 °C. The generated gases were determined by gas chromatography equipped with thermal conductivity detector (TCD).

2.4. Photoelectrochemical characterization

Electrochemical analyses were conducted performed at room temperature using a CHI-660E electrochemical workstation configured with a three-electrode system. A platinum wire electrode was used as the counter electrode, and a saturated Ag/AgCl electrode served as the reference electrode. The electrolyte employed was a 0.5 M Na₂SO₄ solution (pH = 7). Illumination was provided by a 300-W xenon lamp equipped with a filter (λ ≥ 420 nm). For the preparation of the working electrode, 10 mg of the sample powder was suspended in a solution containing 0.5 mL of deionized water, 0.5 mL of ethanol, and 0.1 μL of Nafion. This mixture was sonicated for 10 minutes to ensure uniformity. The resultant suspension was then applied dropwise onto a 20 × 10 mm fluorine-doped tin oxide (FTO) conductive glass and subsequently dried at 60 °C for 2 hours.

3. Results and discussion

3.1. Photocatalyst characterization

The XRD patterns of bare CdS, ZnS, and CdS/ZnS nanopyramid–nanoparticle core–shell structures are shown in Fig. 2. The observed diffraction peaks of the ZnS sample align well with JCPDS No. 65-0566, with distinct diffraction characteristics at 28.9°, 48.1°, and 57.1° corresponding to the (111), (220), and (311) planes, respectively. For the prepared with various CdS morphologies, the peaks correspond to JCPDS No. 77-2306. However, the peak intensities vary, with those of the hexagonal CdS pyramids being sharper, indicating a higher crystallinity of the hexagonal CdS pyramids morphology (Fig. S1, ESI†). The XRD patterns of hexagonal pyramid-shaped CdS and ZnS with different molar ratios are shown in Fig. 2. The main crystal planes corresponding to the CdS nanopyramids are (100) at 24.8°, (002) at 26.5°, (101) at 28.2°, (110) at 43.7°, and (103) at 47.9°. When the ZnS ratio is low, as in the case of CZ5-1, the peaks primarily display as those of the CdS sample. Overlaps occur between the diffraction peaks of ZnS at 28.6° and 47.5°, corresponding to the (111) and (220) planes, and those of CdS at 24.8° and 47.9°, corresponding to the (101) and (103) planes. As the ratio of ZnS increases, the peak intensities of CdS exposed crystal planes such as (100), (002), and (110) gradually weaken, while the intensities of ZnS planes, particularly the (311) plane, gradually increase. Fig. 2 demonstrates the successful combination of CdS and ZnS into a heterojunction structure.

Fig. 2 XRD patterns of hexagonal CdS nanopyramids, ZnS nanoparticles, and CdS/ZnS hybrid heterojunction.

The microstructure of hexagonal CdS pyramids and the CdS/ZnS heterostructures are characterized using SEM and HRTEM. In Fig. 3a, the hexagonal CdS pyramid sample exhibits regular morphologies with smooth surfaces. However, due to the high hydrothermal temperatures, rapid crystal growth resulted in varied sizes of the hexagonal CdS pyramids. In addition, the hydrothermally synthesized CdS nanoparticles and nanorods tend to aggregate (Fig. S2, ESI†). Fig. 3 shows hexagonal CdS pyramids successfully combined with ZnS nanoparticles. For CZ5-1, the ratio of ZnS is low, with only a small mass of ZnS nanoparticles growing on the surface of the hexagonal CdS pyramids. As the ratio of ZnS increases, ZnS grows uniformly and tightly on the hexagonal CdS pyramids, but when the ZnS ratio is excessive, the ZnS particles tend to agglomerate. Fig. 3d shows the CZ1-1 heterostructures, where ZnS nanoparticles are tightly grown on the hexagonal CdS pyramids. The clear interfaces confirm the strong interfacial contact between them. Further investigation into the microstructure of the obtained nanocomposites is performed, as shown in Fig. 4. The hexagonal CdS pyramids, measuring 20 nm with clear lattice fringes spaced at 0.336 nm, correspond to the hexagonal CdS (002) planes, aligning with XRD results.³⁶ As shown in the SAED pattern in Fig. 4c, single-crystal diffraction spots for the hexagonal CdS pyramid are observed, corresponding to the (101), (102), and (103) planes. In the HRTEM images, lattice fringes with spacings of 0.323 nm and 0.278 nm correspond to the (111) and (200) planes of ZnS, respectively, located in the CZ1-1 hybrid heterostructure material (Fig. 4d and e). This indicates that a tightly contacted interface between CdS and ZnS has been established. Moreover, Fig. 4f presents the EDS elemental distribution maps for CZ1-1, clearly showing the presence and uniform distribution of Cd, Zn, and S elements within CZ5-1.

Fig. 3 SEM images of (a) hexagonal CdS pyramid and (b)–(f) CdS/ZnS heterojunction with different Cd and Zn sources.

Fig. 4 TEM images of (a) CdS and (b) CZ1-1 heterojunction, (c) SADE pattern of CZ1-1, (d) and (e) HRTEM images of CZ1-1, and (f) EDS mappings of CZ1-1.

XPS spectra were obtained to determine the chemical compositions and states of hexagonal CdS pyramid, ZnS, and CZ1-1. It can be clearly observed that Cd, S, and Zn signals appear in the full XPS survey scan (Fig. 5a). Characteristic peaks at 405.16 eV for (Cd 3d_3/2) and 405.16 eV (Cd 3d_5/2) correspond to the Cd²⁺ ions.⁷ The XPS of Zn 2p includes two characteristic peaks, located at 1044.83 eV (Zn 2p_1/2) and 1022.03 eV (Zn 2p_3/2), which are indicative of Zn²⁺ ions.⁴⁴ In the CZ1-1 hybrid heterostructure, the binding energy of Cd 3d shifts to lower energies compared to pure CdS, with a shift of approximately 0.32 eV for Cd 3d_3/2 and about 0.321 eV for Cd 3d_5/2. Conversely, the binding energy of Zn 2p in CZ1-1 moves to higher energies relative to pure ZnS (Fig. 5b and c). Furthermore, the high-resolution XPS region spectra of S 2p are provided in Fig. 5d. Compared to the original CdS pyramid, the binding energy of S 2p in CZ1-1 shifts to a lower energy, and compared to bare ZnS, the binding energy of S 2p in CZ1-1 shifts to a higher energy. High-resolution XPS analysis confirms the synthesis of the CdS/ZnS heterostructure.

Fig. 5 (a) XPS spectra for a survey scan and high-resolution XPS spectra of (b) Cd 3d, (c) Zn 2p, and (d) S 2p in hexagonal CdS pyramid, ZnS, and CZ1-1 samples.

3.2. Photocatalyst activity

The photocatalytic hydrogen evolution performance of the prepared samples is evaluated under the same experimental conditions without the addition of any co-catalysts. The hydrogen production yields for the original CdS nanoparticles were 203 μmol h⁻¹ g⁻¹, for CdS nanorods were 465 μmol h⁻¹ g⁻¹, both of which are lower than the 1010 μmol h⁻¹ g⁻¹ evolution rate achieved by the hexagonal CdS pyramid morphology (Fig. S1, ESI†). This performance discrepancy is possibly due to the high crystallinity of the hexagonal CdS pyramids, which enhances photocatalytic performance by efficiently facilitating electron transfer from the inner part to the surface through the crystal planes. Fig. 6a summarizes the average H₂ production yield of CdS/ZnS composites with different ZnS ratios. The CdS/ZnS hexagonal pyramid–nanoparticle hybrids show greatly enhanced H₂ generation yields compared to pure CdS and ZnS. The photocatalytic hydrogen evolution yield of CZ1-1 reached 4660 μmol h⁻¹ g⁻¹, the highest observed among all the photocatalysts. It is 4.6 times greater than that of pure hexagonal CdS pyramids, and up to 84.72 times higher than that of bare ZnS. This enhanced performance is attributed to the tight interfacial contact between CdS and ZnS, which facilitates the transfer of photogenerated carriers. The hydrogen production rate of CZ1-1 is nearly 23 times that of pure CdS nanoparticles and 10 times that of CdS nanorods (Fig. S1, ESI†). With an excess of ZnS or CdS, the limited contact area between the two phases hinders the effective interfacial transfer of photogenerated carriers, affecting photocatalytic efficiency. Additionally, when the Cd and Zn concentrations are equal, hydrogen production performance is enhanced for all morphologies of CdS (Fig. S1, ESI†). Fig. 6b shows the hydrogen production for 50 mg of different catalyst powders under the same reaction conditions. After 6 hours of irradiation, the hydrogen production of pure ZnS showed almost no change, whereas other catalysts exhibited a generally linear increase over time. To further confirm the driving force of the photocatalytic reaction, the apparent quantum yield (AQY) for photocatalytic hydrogen evolution of CZ1-1 was tested under different monochromatic wavelengths. The calculation method is described in the ESI.† The H₂ yields at different wavelengths are in accord with the light absorption, and the AQY declines as the wavelengths increase. CZ1-1 shows a clear wavelength dependency in hydrogen production efficiency, matching the trend displayed in its UV-vis spectrum. Among them, the highest AQY at 420 nm can reach an impressive 20.30%. This test confirms that light illumination is the primary driving force for the photocatalytic hydrogen production reaction. The stability of the photocatalyst is also important for its photocatalytic performance. The H₂ generation activity of CZ1-1 does not significantly drop during five cycles of testing (Fig. 6d), proving that the photocatalyst maintains good recyclability and continues to exhibit robust photocatalytic activity during the H₂ production process.

Fig. 6 (a) Photocatalytic hydrogen evolution rate and (b) hydrogen yields of samples. (c) Wavelength dependence of apparent quantum yield and UV-vis absorption spectrum of CZ1-1; (d) stability test of CZ1-1 in five cycles.

3.3. Light absorption and charge separation ability

The optical properties of the samples are analyzed using UV-vis absorption spectra. Fig. 7a shows that the absorption edge of pristine CdS is located at approximately 600 nm, while that of pure ZnS is around 380 nm, indicating that in the CdS/ZnS pyramid–nanoparticle heterostructures, as the proportion of ZnS increases, the absorption edge also gradually shifts. Notably, the CZ1-1 hybrid exhibits a significant redshift in its absorption edge to around 650 nm and enhances light absorption in the 550 nm to 800 nm range. This suggests that the hexagonal CdS/ZnS hexagonal pyramid–nanoparticle heterostructures extend the spectral absorption range compared to pure CdS and ZnS. However, with an excess of ZnS, the absorption edge gradually shifts, with CZ1-5 exhibiting an edge at 560 nm. This analysis suggests that doping ZnS with an optimal ratio of CdS can significantly increase spectral absorption, thereby improving photocatalytic activity by enhancing photo-induced electron transfer.

Fig. 7 (a) UV-vis DRS spectra of samples, (b) PL spectra, (c) EPR spectra of the V_zn in ZnS, (d) photocurrent response, (e) EIS plots, and (f) LSV curves of hexagonal CdS pyramids, ZnS, and CZ1-1.

For characterizing the recombination dynamics of photo-induced carriers in the photocatalysts, the steady-state photoluminescence (PL) was tested. Generally, the intensity of the PL peak is inversely proportional to the recombination rate of the excited electrons and holes, indicating that a weaker PL peak corresponds to a slower recombination rate.^45,46Fig. 7b displays the PL spectrum at an excitation wavelength of 280 nm, where ZnS exhibits the strongest emission peak, indicating severe recombination of photogenerated electrons and holes. Additionally, it also shows that CZ1-1 has the weakest emission peak intensity, suggesting that the formation of the hybrid heterostructure facilitates interfacial charge transfer while suppressing carrier recombination. Crystal defects can introduce intermediate energy levels within the band gap, influencing the luminescent properties of nanocrystals. PL spectroscopy can be employed to detect these defects, as point defects often serve as luminescent sites during the photoluminescence process.^37,41,47 ZnS contains four types of vacancies, zinc vacancies (V_Zn), interstitial zinc atoms (I_Zn), sulfur vacancies (V_S), and interstitial sulfur atoms (I_S).⁴⁸ No significant signal was detected in the 380–402 nm UV emission range under 280-nm excitation, indicating the absence of interstitial zinc (I_Zn).⁴⁹ In ZnS and CZ1-1, the main emission peaks at 451.60, 469, 484, and 493 nm are attributed to point defects caused by zinc vacancies, which arise from the radiation transition of conduction band (CB) electrons to the single negative charge state of a zinc vacancy (V_Zn).^50,51 The photoluminescence emission results clearly demonstrate the presence of zinc vacancies. It should be noted that the zinc vacancies in the ZnS samples create defect levels within its band gap, approximately 1.1 eV above the valence band of ZnS, acting as acceptor levels for photo-induced holes.^41,50 Electron paramagnetic resonance (EPR) spectroscopy was used to further demonstrate the presence of V_Zn in ZnS. In Fig. 7c, a distinct signal is observed in V_Zn-ZnS, generated by electrons captured by V_Zn, confirming the existence of V_Zn. While the exact g-value can vary depending on the nature of the defect and the local environment, the observed EPR signals are consistent with those reported for defects such as Zn vacancies and sulfur-related centers in ZnS.^52,53 In the photocatalytic response experiments, a 30 second on–off light cycle (λ ≥ 420 nm) over a period of 390 seconds was used to evaluate the photoresponse potential of the photocatalysts, as shown in Fig. 7d. The results revealed that the photocurrent density of CZ1-1 was significantly higher than that of CdS and ZnS, indicating that CZ1-1 exhibited superior charge transfer efficiency compared to single semiconductor materials, which is advantageous for enhancing photocatalytic hydrogen evolution reaction performance. To evaluate the effect of semiconductor coupling on charge separation and migration efficiency, electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) curves are also monitored. EIS analysis in Fig. 7e shows that the Nyquist plot of sample CZ1-1 exhibits the smallest semicircle radius, indicating it has the lowest charge transfer resistance (R_ct). This demonstrates that the in situ growth of ZnS on hexagonal CdS pyramids significantly promotes charge transport and separation. A lower R_ct implies that the recombination probability of photogenerated carriers before reaching the reaction interface is reduced, thereby enhancing the photocatalytic efficiency. Furthermore, the heterojunction formed between CdS and ZnS generates a synergistic effect, where ZnS acts as an electron donor transferring electrons to CdS, facilitating effective spatial charge separation and enabling the reduction reaction to occur on CdS. The smaller semicircle radius and reduced Warburg impedance in the EIS data further confirm the improved interfacial properties, enhancing the transport capabilities of ions and electrons at the interface. The LSV curves show that CZ1-1 has a lower hydrogen evolution potential compared to hexagonal CdS pyramids and ZnS nanoparticles (Fig. 7f). Combining these analyses, CZ1-1 demonstrates a broader spectral absorption range, faster charge migration rates, and more effective charge separation capabilities compared to the individual CdS and ZnS.

3.4. Photocatalytic mechanism

To elucidate the photocatalytic hydrogen production mechanism of the CdS/ZnS hexagonal pyramid–nanoparticle heterostructure, the flat-band potential positions of CdS and ZnS are determined by Mott–Schottky (M–S) curves (Fig. 8a and b). The flat-band potential (E_fb) of CdS is −0.63 V vs. Ag/AgCl, and for ZnS it is −0.42 V vs. Ag/AgCl. The positive slopes of the M–S plots indicate that both are typical n-type semiconductors.^54,55 For n-type semiconductors, the conduction band (E_CB) is approximately 0.2 V lower than E_fb, thus the E_CB of CdS and ZnS relative to the normal hydrogen electrode (NHE) are −0.63 V and −0.42 V, respectively (E_NHE = E_Ag/AgCl + 0.197 V).⁵⁶ Based on the results from UV-vis diffuse reflectance spectroscopy (DRS) and applying the Kubelka–Munk theory, Tauc plots are generated, revealing band gaps of 2.23 eV for CdS and 3.5 eV for ZnS (Fig. 8c).⁵⁷ The corresponding valence band potentials (E_VB) can be deduced to 1.72 V and 3.08 V vs. NHE, calculated using the formula E_VB = E_g + E_CB. It should be noted that the E_VB for ZnS at about 1.93 V could be attributed to zinc vacancies, which create induced defect energy levels within its band gap.⁵⁸

Fig. 8 Mott–Schottky plots of (a) CdS and (b) ZnS. (c) Plots of (αhν)²versus hν for CdS and ZnS. (d) DMPO spin-trapping ESR signals of ˙OH for CdS, ZnS, and CZ1-1. (e) Proposed Z-scheme photocatalytic mechanism of CZ1-1 hybrid heterojunction.

For further verifying the interaction mechanism between CdS and ZnS, EPR measurements of hydroxyl radicals (˙OH) are conducted using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a trapping agent. As shown in Fig. 8d, no significant DMPO–˙OH signal is detected for pure hexagonal CdS pyramids, as the E_VB of CdS (1.72 V) is more negative than the OH⁻/˙OH potential (1.99 V). This prevents the holes in the valence band (VB) of CdS from oxidizing OH⁻ to ˙OH.⁵⁹ In contrast, a clear DMPO–˙OH signal is observed for ZnS, primarily due to its suitable zinc vacancies. Additionally, the significantly stronger DMPO–˙OH signal in CZ1-1 suggests that photogenerated holes prefer to remain in the VB of ZnS rather than migrating to the VB of CdS, indicating effective charge separation in the heterostructure.

With the aid of band structure analysis, changes in signals can reflect the charge transfer paths between two semiconductors. As depicted in Fig. 8e, it is evident that photo-induced electrons and holes can remain on the conduction band (CB) of the hexagonal CdS pyramid and the VB of ZnS, respectively, only when Z-scheme interfacial charge transfer occurs. This retention of photoexcited carriers with high redox capabilities produces a stronger DMPO–˙OH signal. If in a type II heterojunction, both electrons and holes would migrate towards the weaker redox potentials, resulting in a reduced intensity of the DMPO–˙OH signal, which would contradict experimental outcomes. Fortunately, the Z-scheme mechanism enables better spatial charge separation and potent redox capabilities. Upon visible light irradiation, electrons in the VB of CdS and ZnS absorb photons and are excited to their respective CB. Subsequently, under the influence of the built-in electric field, the photogenerated electrons in ZnS and the photogenerated holes in the VB of CdS recombine, establishing a charge transfer pathway. Overall, through the regulation of the Cd and Zn source ratios, an enhancement in photocatalyst performance has been achieved, and experimental characterization has validated the Z-scheme interface charge transfer mechanism in the CdS/ZnS hexagonal pyramid–nanoparticle hybrid heterostructure.

4. Conclusion

This study effectively modulates photocatalytic hydrogen production by morphological control, interface optimization, and band alignment. CdS/ZnS composites with a hexagonal CdS-based pyramid, featuring a tightly interfaced Z-scheme heterojunction sharing sulfur atoms, were successfully fabricated via a straightforward hydrothermal method. Specifically, when the concentration of the Zn source is equal to that of the Cd source, the as-prepared CZ1-1 photocatalyst displays the highest photocatalytic hydrogen production yield of 4660 μmol h⁻¹ g⁻¹ under visible light irradiation. This yield is 4.6 times higher than that of pure CdS nanopyramids and 84.72 times higher than that of pure ZnS nanoparticles. Additionally, the apparent quantum efficiency for hydrogen production at 420 nm reaches 20.30%. This can be attributed to the presence of more active sites on the surface and the strong interfacial interaction within CZ1-1, which creates an inherent electric field that facilitates the efficient separation and migration of photogenerated charge carriers. Moreover, the Z-scheme mechanism is substantiated by EPR measurements tracking ˙OH radicals. This work offers a promising guide for the development of tightly interfaced all-sulfide heterostructures with desired properties to enhance photocatalytic hydrogen production.

Data availability

All data used in this study are internal data and have not been published in any public data repository. Data access can be provided upon request in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 52276062, 52076076).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4nj04075a

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