# Hydrazine-Induced Sulfur Vacancies Promote Interfacial Charge Redistribution in ZnS/Gel-Derived TiO2 for Enhanced CO2 Activation and Methanation

## TL;DR

This study shows how creating sulfur vacancies in a ZnS/TiO2 photocatalyst improves CO2 conversion into methane by enhancing charge separation and CO2 activation.

## Contribution

The novel use of hydrazine-induced sulfur vacancies to synergistically enhance interfacial charge redistribution and CO2 methanation selectivity.

## Key findings

- The ZnS/gel-derived TiO2-0.48 composite achieved 6.76 μmol·g−1·h−1 CH4 and 14.47 μmol·g−1·h−1 CO yields with 31.8% CH4 selectivity.
- Sulfur vacancies reduced the energy barrier for *COOH formation from +0.51 eV to +0.21 eV, promoting CO2 activation.
- A Z-scheme-compatible charge migration model was proposed to explain the enhanced redox potentials and CO2 methanation.

## Abstract

Defect engineering in semiconductor heterojunctions offers a promising route for enhancing the selectivity of photocatalytic CO2 conversion. In this work, a ZnS/gel-derived TiO2 photocatalyst featuring sulfur vacancies introduced via hydrazine hydrate (N2H4) treatment is developed. XRD, HRTEM, and XPS analyses confirm the formation of a crystalline heterointerface and a defect-rich ZnS surface, enabling effective interfacial electronic modulation. The optimized ZnS/gel-derived TiO2-0.48 composite achieves CH4 and CO yields of 6.76 and 14.47 μmol·g−1·h−1, respectively, with a CH4 selectivity of 31.8% and an electron selectivity of 65.1%, clearly outperforming pristine TiO2 and the corresponding single-component catalysts under identical conditions. Photoluminescence quenching, enhanced photocurrent response, and reduced charge-transfer resistance indicate significantly improved interfacial charge separation. Mott–Schottky analysis combined with optical bandgap measurements reveals pronounced interfacial charge redistribution in the composite system. Considering the intrinsic band structure of ZnS and gel-derived TiO2, a Z-scheme-compatible interfacial charge migration model is proposed, in which photogenerated electrons with strong reductive power are preferentially retained on ZnS, while holes with strong oxidative capability remain on gel-derived TiO2. This charge migration pathway preserves high redox potentials, facilitating multi-electron CO2 methanation and water oxidation. Density functional theory calculations further demonstrate that sulfur vacancies stabilize *COOH and *CO intermediates and reduce the energy barrier for *COOH formation from +0.51 eV to +0.21 eV, thereby promoting CO2 activation and CH4 formation. These results reveal that sulfur vacancies not only activate CO2 molecules but also regulate interfacial charge migration behavior. This work provides a synergistic strategy combining defect engineering and interfacial electronic modulation to enhance selectivity and mechanistic understanding in CO2-to-CH4 photoconversion.