Mesoporous Dual-Semiconductor ZnS/CdS Nanocomposites as Efficient Visible Light Photocatalysts for Hydrogen Generation

Abstract

The development of functional catalysts for the photogeneration of hydrogen (H₂) via water-splitting is crucial in the pursuit of sustainable energy solutions. To that end, metal-sulfide semiconductors, such as CdS and ZnS, can play a significant role in the process due to their interesting optoelectronic and catalytic properties. However, inefficient charge-carrier dissociation and poor photochemical stability remain significant limitations to photocatalytic efficiency. Herein, dual-semiconductor nanocomposites of ZnS/CdS nanocrystal assemblies (NCAs) are developed as efficient visible light photocatalysts for H₂ generation. The resultant materials, synthesized via a polymer-templated self-polymerization method, comprise a unique combination of ~5-7 nm-sized metal-sulfide nanoparticles that are interlinked to form a 3D open-pore structure with large internal surface area (up to 285 m² g^-1) and uniform pores (circa 6-7 nm). By adjusting the ratio of constituent nanoparticles, the optimized ZnS/CdS catalyst with 50 wt.% ZnS content demonstrates a remarkable stability and visible light H₂-evolution activity (~29 mmol g^-1 h^-1 mass activity) with an apparent quantum yield (AQY) of 60% at 420 nm. Photocatalytic evaluation experiments combined with electrochemical and spectroscopic studies suggest that the superior photocatalytic performance of these materials stems from the accessible 3D open-pore structure and the efficient defect-mediated charge transfer mechanism at the ZnS/CdS nanointerfaces. Overall, this work provides a new perspective for designing functional and stable photocatalytic materials for sustainable H₂ production.

Keywords: ZnS/CdS heterojunctions; cadmium sulfide; mesoporous materials; metal chalcogenides; nanocomposites; nanoparticles; photocatalytic hydrogen production; water splitting; zinc sulfide.

Figure 1

(a) Schematic representation of the synthetic procedure of mesoporous ZnS/CdS NCAs. (b) Typical XRD patterns of mesoporous CdS and ZnS NCAs and the as-prepared ZnS/CdS nanocomposites with different ZnS loadings.

Figure 2

(a) Typical FE-SEM image, (b) EDS mappings of Cd, Zn and S elements, (c) TEM image and high magnification TEM image (inset) showing CdS (yellow circles) and ZnS (white circles) nanoparticles in intimate contact, and (d) HRTEM image for the mesoporous 50-ZnS/CdS nanocomposite. In HRTEM, the lattice fringes with d-spacings of 3.1 and 3.4 Å are indexed to the (111) planes of the cubic structures of ZnS (white lines) and CdS (yellow lines), respectively.

Figure 3

(a) Cd 3d, (b) Zn 2p and (c) S 2p XPS core-level spectra of mesoporous CdS and ZnS NCAs and the 50-ZnS/CdS nanocomposite.

Figure 4

(a) Comparative N₂ adsorption (filled cycles) and desorption (open cycles) isotherms at −196 °C and (inset) the corresponding NLDFT pore size distribution plots calculated from the adsorption branch of isotherms for the mesoporous CdS, ZnS and 50-ZnS/CdS NCAs. For clarity, the isotherms of 50-ZnS/CdS and ZnS NCAs are offset by 50 and 100 cm³ g⁻¹, respectively. (b) UV-vis/NIR diffuse reflectance spectra and (c) Tauc plots (i.e., the curves of (αhν)² versus photon energy (hν), where α, h and ν are the absorption coefficient, Planck’s constant and light frequency, respectively) of mesoporous CdS, ZnS and ZnS/CdS NCAs with different wt.% ZnS content. Inset: magnification of the Tauc plot in the energy range of 2.2–3.0 eV for clarity.

Figure 5

(a) Photocatalytic H₂ evolution activities for mesoporous CdS, ZnS and the different ZnS/CdS NCAs, together with the ZnS/CdS bulk and ZnS/CdS RNAs reference catalysts with 50 wt.% ZnS content. The photocatalytic reactions were carried out in an airtight reactor, using 1 mg mL⁻¹ catalyst concentration in a 0.35 M Na₂S and 0.25 M Na₂SO₃ aqueous electrolyte. (b) Photocatalytic H₂ evolution rates for the mesoporous 50-ZnS/CdS catalyst (1 mg mL⁻¹ catalyst concentration) using different concentrations of Na₂S/Na₂SO₃ reagents and (c) different mass loadings of the catalyst in a 1.4 M Na₂S and 1.0 M Na₂SO₃ aqueous electrolyte. (d) Photocatalytic recycling tests of the 50-ZnS/CdS catalyst (1 mg mL⁻¹) in a 1.4 M Na₂S and 1.0 M Na₂SO₃ aqueous electrolyte. All the H₂-evolution rates obtained as an average over the initial 3-h reaction period. All photocatalytic tests were conducted under visible light irradiation using a 300 W Xenon light source with a UV cutoff filter (λ ≥ 420 nm).

Figure 6

(a) Mott–Schottky plots where the E_FB values are obtained from the intercepts of the extrapolated linear fits of the 1/C² vs. potential curves, (b) energy band diagrams (the band-edge diagram of the ZnS/CdS bulk sample with 50 wt.% ZnS is also given) and (c) EIS Nyquist plots and equivalent circuit model (inset) for the mesoporous CdS, ZnS and ZnS/CdS NCAs. (d) Comparative time-resolved photoluminescence (TR-PL) decay curves of mesoporous CdS and 50-ZnS/CdS NCAs and ZnS/CdS bulk reference with 50 wt.% ZnS content.

Figure 7

(a) Schematic energy diagram for the point defect states in ZnS (V_S = sulfur vacancy, I_S = interstitial sulfur, V_Zn = zinc vacancy, and I_Zn = interstitial zinc). The defect-state energy levels are estimated according to the peak positions of the ZnS NCAs PL spectrum. (b) Schematic illustration of the proposed quasi-type-II charge-transfer and visible light photocatalytic H₂-generation mechanism for the ZnS/CdS nanocomposites. The presented energy levels correspond to the band diagram of the 50-ZnS/CdS catalyst (E_CB = conduction band level, E_VB = valence band level, E_F = Fermi level and DS = donor states).