Self-activated superhydrophilic green ZnIn2S4 realizing solar-driven overall water splitting: close-to-unity stability for a full daytime

Abstract

Engineering an efficient semiconductor to sustainably produce green hydrogen via solar-driven water splitting is one of the cutting-edge strategies for carbon-neutral energy ecosystem. Herein, a superhydrophilic green hollow ZnIn₂S₄ (gZIS) was fabricated to realize unassisted photocatalytic overall water splitting. The hollow hierarchical framework benefits exposure of intrinsically active facets and activates inert basal planes. The superhydrophilic nature of gZIS promotes intense surface water molecule interactions. The presence of vacancies within gZIS facilitates photon energy utilization and charge transfer. Systematic theoretical computations signify the defect-induced charge redistribution of gZIS enhancing water activation and reducing surface kinetic barriers. Ultimately, the gZIS could drive photocatalytic pure water splitting by retaining close-to-unity stability for a full daytime reaction with performance comparable to other complex sulfide-based materials. This work reports a self-activated, single-component cocatalyst-free gZIS with great exploration value, potentially providing a state-of-the-art design and innovative aperture for efficient solar-driven hydrogen production to achieve carbon-neutrality.

Fig. 1. Catalyst synthesis and morphological characterizations.

a Schematic of the formation of ZIS and gZIS. The charges of the complexes are omitted in the figure for clarity. M denotes the metal ions, either Zn²⁺ or In³⁺, present in the solution. False-colored FESEM images for (b) ZIS and (c) gZIS. Magnified false-colored FESEM view for (d) ZIS and (e) gZIS, with the insets showing the original FESEM images. EDX elemental mappings for (f) ZIS and (g) gZIS.

Fig. 2. Structural characterizations and analysis.

TEM images for (a) ZIS and (b) gZIS. HRTEM images for (c) ZIS and (d) gZIS, with an inset showing the enlarged region with lattice distortion and defects in gZIS. Theoretical structural models for (e) pristine ZIS_T and (f) S-vacant gZIS_T. g Atomic-resolution spherical aberration-corrected BF-STEM imaging of gZIS with pre- and post-FFT. The magnified view shows the atomic arrangement with distorted hexagonal in concordance to the simulated result.

Fig. 3. Surface chemical and charge properties.

High-resolution XPS spectra of (a) Zn 2p and (b) S 2p for the as-synthesized samples. c EPR spectra for ZIS and gZIS indicating the presence of S_v. d Computed 3D charge density difference for gZIS_T¸ with the top showing the whole bilayer structure and the bottoms focus on the monolayer where S_v is present. Gray and green areas dictate the charge depletion and accumulation isosurfaces, respectively.

Fig. 4. Physical properties and water interaction study.

a XRD spectra for ZIS and gZIS. b Nitrogen adsorption-desorption isotherms of ZIS and gZIS with inset showing the respective pore size distribution. c Surface wettability static contact angle measurements for ZIS and gZIS; error bars represent the standard deviation from three independent runs. d Free water molecule with its respective O-H bond length and H-O-H bond angle. Theoretical modeling of water adsorption along the basal plane: (e) on Zn atom of ZIS_T, (f) on Zn atom of gZIS_T, and (g) in S_v position of gZIS_T.

Fig. 5. Photoelectrochemical and charge transfer characteristics.

a Transient photocurrent responses, (b) EIS Nyquist plot with the equivalent Randle circuit, (c) steady-state PL emission spectra and (d) transient TRPL decay spectra of ZIS and gZIS.

Fig. 6. Optoelectronic properties and band structure.

a UV-Vis diffuse reflectance spectra with inset showing the actual color of the samples, (b) KM function for band gap determination, and (c) MS plot for ZIS and gZIS. d Schematic of the electronic band structures of ZIS and gZIS with light absorption properties and photogeneration electron-holes pair formation mechanisms. e Theoretical calculated DOS and (f) respective ε_p for ZIS_T and gZIS_T.

Fig. 7. Water splitting mechanism and performance.

Gibbs free energy maps for (a) HER and (b) OER for ZIS_T and gZIS_T. c Photocatalytic HER and OER half-reaction under different sacrificial conditions. d Time-dependent solar-driven overall water splitting performance and (e) long-term photocatalytic stability performance of gZIS. Error bars represent the standard deviation from two independent runs.