Size Dependent Photocatalytic Activity of Mesoporous ZnIn2S4 Nanocrystal Networks

Abstract

Understanding of the band-edge electronic structure and charge-transfer dynamics in size-confined nanostructures is vital in designing new materials for energy conversion applications, including green hydrogen production, decomposition of organic pollutants and solar cells. In this study, a series of mesoporous materials comprising continuous networks of linked zinc indium sulfide (ZnIn₂S₄) nanocrystals with a tunable diameter (ranging from 4 to 12 nm) is reported. These nanomaterials demonstrate intriguing size-dependent electronic properties, charge-transfer kinetics and photocatalytic behaviors. Our extensive characterizations uncover strong size effects on the catalytic activity of constituent ZnIn₂S₄ nanocrystals in the photochemical hydrogen evolution reaction. As an outcome, the optimized single-component ZnIn₂S₄ mesostructure produces hydrogen at a 7.8 mmol g_cat ^-1 h^-1 release rate under ultraviolet (UV)-visible light irradiation associated with an apparent quantum yield (AQY) of 17.2% at 420 ± 10 nm, far surpassing its microstructured counterpart by a factor of 10.7×. These findings provide a valuable perspective for the rational design of semiconductor nanostructures through synthetic engineering, aiming at the development of high-performance catalysts for zero-carbon energy-related applications.

Figure 1

(a) Schematic illustration of the synthetic procedure of mesoporous ZIS nanocrystal frameworks (NCFs) (step I: reflux synthesis of 3-MPA-capped ZIS NCs; step II: polymer-assisted chemical self-assembly; step III: template extraction toward open-pore structures). (b) XRD patterns of the as-prepared ZIS NCs and mesoporous n-ZIS NCFs. The standard diffraction pattern of the hexagonal ZnIn₂S₄ (JCPDS card no. 65–2023; red line) is also given. (c) Reduced atomic pair distribution functions G(r) of the mesoporous n-ZIS NCFs and polycrystalline bulk ZIS. (d) Crystal structure of hexagonal ZnIn₂S₄ (space group: P3̅m1). Representative (e) FESEM, (f) TEM, and (g) HRTEM images of 6-ZIS NCF. The insets of panels (e, g) show an enlarged region of the images. a.u., arbitrary units.

Figure 2

(a) N₂ adsorption (filled symbols) and desorption (empty symbols) isotherms at −196 °C for the mesoporous 6-ZIS NCF, random ZIS NC-aggregates (ZIS RNAs) and polycrystalline ZIS. The isotherms of ZIS RNAs are shifted by 15 cm³ g^–1 for clarity. Inset: the corresponding NLDFT pore-size distribution plots derived from the adsorption isotherms. (b) UV–vis absorption spectra and (inset) the corresponding Tauc plots of ZIS NCs, mesoporous n-ZIS NCFs and polycrystalline ZIS materials. (c) The energy bandgap as a function of the NC size for the as-prepared ZIS NCs and mesoporous n-ZIS NCFs. The size of constituent NCs was estimated from TEM analysis.

Figure 3

(a) Photocatalytic H₂ generation rates of different ZIS catalysts under nonoptimized conditions (1 mg mL^–1 catalyst in 0.35 M Na₂S/0.25 M Na₂SO₃ aqueous solution; λ ≥ 380 nm light irradiation; 20 ± 2 °C). (b) Time-dependent hydrogen evolutions (lines) and average H₂-production rates (column) at the course of the photocatalytic stability studies over 6-ZIS NCF catalyst. The H₂-production rates were averaged over 5 h of illumination. The stability test was conducted with 1.5 mg mL^–1 catalyst concentration in triethylamine (10% v/v) solution; 300 W Xe lamp irradiation (λ ≥ 380 nm).

Figure 4

(a) Mott–Schottky plots and (b) energy band diagrams (E_CB: conduction band energy, E_VB: valence band energy, E_F: Fermi level, H⁺/H₂ redox potential) of different ZIS catalysts. (c) Open circuit potential versus elapsed time for mesoporous n-ZIS NCFs under switching on/off AM 1.5G illumination (10 s light on). The inset shows the change of the V_OC at the catalyst/liquid interface under chopped illumination; when the light is switched on, a charge accumulation occurs at the interface of ZIS NCs, mitigating the surface band bending. (d) EIS Nyquist plots (Inset: equivalent Randles circuit model), (e) time-resolved PL decay spectra under 375 nm laser pulse excitation (Inset: magnified view of the PL decay spectra) and (f) transient photocurrent spectra under the applied bias of −1 V (100 W visible-light-emitting diode) of the mesoporous n-ZIS NCFs and polycrystalline bulk ZIS catalysts. In panels (a, d, e), the red lines are fit to the experimental data.