Unveiling the Flexocatalytic Potential of Wide-Bandgap Spinel Oxides: Light-Free Hydrogen Evolution via Strain-Induced Polarization and Oxygen Vacancy Engineering

Abstract

Wide-bandgap spinel oxides, such as ZnAl2O4 (ZAO) and ZnCr2O4 (ZCO), are traditionally limited by poor electron activation under light irradiation, resulting in suboptimal photocatalytic performance. This study investigates a novel approach to overcoming these limitations by leveraging the flexoelectric effect in centrosymmetric porous nanoparticles with wrinkled surfaces. The inhomogeneous strain gradients generated under mechanical force induce flexoelectric polarization, offering a promising pathway to enhance photocatalytic activity. ZAO outperforms ZCO due to the smaller atomic radius of aluminum, allowing greater atomic displacement and higher polarization, which prolongs electron-hole recombination. Oxygen vacancies (OV) further enhance ZAO performance, with ZAO-200 (annealed at 200 °C) achieving the longest carrier lifetime (4.65 ns) and an exceptional hydrogen evolution rate of 3737 µmol g−1 h−1—206% higher than pristine ZAO—without light stimulation. Density functional theory (DFT) calculations confirm a lower hydrogen evolution reaction (HER) energy barrier for ZAO (ΔGH = −0.13 eV) compared to ZCO (1.34 eV) and reveal spontaneous water splitting at OV sites without energy input. A unique butterfly curve validates flexoelectric potential generation from strain gradients. These findings establish a novel framework for eco-friendly hydrogen production, demonstrating that flexoelectric polarization and OV engineering can surpass traditional photocatalytic methods in sustainability and efficiency.