Size Dependent Photocatalytic Activity of Mesoporous ZnIn2S4 Nanocrystal Networks

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 gcat–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.

The average lifetime (τ av ) was determined using the following equation:

Supporting Figures
Figure S1.TGA profiles (black lines) and the corresponding differential thermogravimetric (DTG) curve (red line) of (a) 4-ZIS, (b) 6-ZIS and (c) 12-ZIS NCFs, depicting a weight loss of 9.7-12.2%at a temperature range of 230 to 460 o C due to the decomposition of remaining organic residue.

Figure S2 .
Figure S2.Typical EDS spectra of the mesoporous n-ZIS NCFs and bulk polycrystalline ZIS.

Figure S3 .
Figure S3.(a) XPS survey scans and high-resolution XPS core-levels of (b) Zn 2p, (c) In 3d and (d) S 2p of the mesoporous n-ZIS NCFs.In panel d: the XPS S 2p deconvoluted spectra of S 2p 3/2 and S 2p 1/2 core-levels are represented as yellow and green curves.The red lines are fits to the experimental data.

Figure S5 .
Figure S5.The In … In and Zn … Zn interatomic distances in the hexagonal lattice of ZIS (a) without and (b) with sulfur vacancies (S v ).The presence of sulfur vacancies in the lattice of ZIS induces bond dislocations and thus alters the arrangement (interatomic distance) of neighboring Zn and In atoms.

Figure S6 .
Figure S6.Typical FESEM images of the bulk polycrystalline ZIS, showing micro-sized particles of 3-5 μm diameter.The bulk ZIS particles exhibit a flowerlike morphology composing by plenty of intersecting nanosheets with thickness of ~18-20 nm.

Figure S9 .
Figure S9.(a) SAXS pattens and (b)Guinier plots [I(q)  -q 2 R g 2 /3, where q is the scattering vector and R g is the radius of gyration] of the mesoporous n-ZIS NCFs, yielding an average particle size (d p ) from ~4.5 to ~11.3 nm.

Figure S14 .
Figure S14.(a) Typical EDS spectrum and XPS core-level spectra of the (a) Zn 2p, (b) In 3d and (c) S 2p of the reused 6-ZIS NCF catalyst.EDS analysis indicates Zn/In/S atomic rations ~1:1.9:3.9, very close to the composition of the fresh sample.

Figure S15 .
Figure S15.N 2 adsorption-desorption isotherms at -196 o C and the corresponding NLDFT pore size distribution plot (inset) of the 6-ZIS NCF catalyst retrieved after 15 hours of photocatalytic reaction.Analysis of the adsorption data with the BET method gives surface area of 163 m 2 g -1 , total pore volume of 0.15 cm 3 g -1 , and pore size of 5.4 nm.

Figure S17 .
Figure S17.VB-XPS spectra of the mesoporous n-ZIS NCFs.The half maximum of the step height of the VB spectra dictates a Fermi level offset (E VB -E f ) of 2.61, 2.51 and 2.45 eV for the 4-ZIS, 6-ZIS and 12-ZIS NCFs, respectively.

Table S1 .
Elemental composition of the mesoporous n-ZIS NCFs and polycrystalline ZIS materials.
a EDS Zn/In/S atomic ratio based on In atoms.

Table S4 .
Nyquist equivalent circuit fitted parameters of the mesoporous n-ZIS NCFs and polycrystalline ZIS materials.

Table S5 .
Time-resolved photoluminescence decay parameters of the mesoporous n-ZIS NCFs and polycrystalline ZIS materials.