Harnessing Built-in Polarization in a 3D/2D ZnOS/NiO-g-C ₃ N ₄ Heterostructure for Piezo-Photocatalytic Hydrogen Evolution

Photocatalytic water splitting is regarded as a promising method for the production of green hydrogen. However, its practical efficiency is hampered by the quick recombination of the charge carriers. The integration of the piezoelectric effect with photocatalysis has been recognized as a powerful strategy to reduce charge recombination, promote charge separation, and ultimately reach high-efficiency hydrogen production. Herein, we report for the first time the application of zinc oxysulfide (ZnOS) as a piezoelectric photocatalyst for hydrogen evolution and its integration with NiO-modified g-C3N4 (NCN), forming a double type-II heterojunction. ZnOS/NCN heterostructures were constructed via a hydrothermal route and investigated by using piezoresponse force microscopy, spectroscopic, and electrochemical analyses. The incorporation of NCN into ZnOS results in an improved piezoelectric response and charge polarization, as confirmed by the significantly increased d33 coefficient from 64.46 to 200.59 pm/V. Consequently, the optimized ZnOS/NCN-7 wt % heterojunction achieves a synergistic piezo-photocatalytic hydrogen evolution rate of 21.9 mmol/g/h, which is 2.5- and 3-fold higher than pristine photocatalysis and piezocatalysis, respectively, accompanied by 47.8% PL quenching and a 28.5% longer carrier lifetime. Control and scavenger experiments, together with synergy factor (SF) analysis, reveal that under ultrasonication, the intrinsic piezoelectric polarization is the primary driving force for hydrogen evolution (SF = 2.51), while sonochemical reactions originating from acoustic cavitation act as a secondary amplification pathway, accounting for 4% of the total piezo-photocatalytic hydrogen production, as reflected by the reduced extended SF value of 2.27 in the ZnOS/NCN system. The proposed electron migration pathway within the type-II piezo-photocatalytic mechanism is further validated by the findings of XPS, work function analyses, and Mott–Schottky measurements. This work highlights the potential of piezoelectric field-driven charge regulation in ZnOS-based photocatalysts as an effective strategy to suppress charge recombination and enhance sustainable hydrogen production.