Mechanism of enhanced photocatalytic activity of Cr-doped ZnO nanoparticles revealed by photoluminescence emission and electron spin resonance

In this work, we successfully synthesized Cr³⁺ doped ZnO nanoparticles using a sol-gel method, and elucidated how Cr³⁺ dopant is critical for enhancing photocatalytic activity. The nature of the point defect analyzed by electron spin resonance (ESR), and photoluminescence (PL) emission revealed the role of the Cr³⁺ dopant. When introducing Cr³⁺ ions in ZnO, the PL emission intensity decreased, indicating a reduction of the radiative recombination rate due to the heterojunction formation between the dopant and the host. The Cr³⁺ doped ZnO nanostructures showed that the typical ESR signal with g-factor value ∼1.96 was completely passivated, indicating the diffusion of electrons near the conduction band into the dopant ions. The doped Cr³⁺ ion acts as an electron trap in the ZnO crystal described as Cr³⁺ + e⁻ → Cr²⁺. The mechanism for enhancing the photocatalytic activity of heterogeneous ZnO:Cr³⁺ was proposed in respect of point defect evolution through the manner of Cr³⁺ doping. As a result, the photocatalytic efficiency investigated by measuring methylene blue degradation under 210 min of direct sunlight irradiation reached 93.5% for 1 at % Cr³⁺ doped ZnO, which was significantly improved compared to 59.8% of the pure ZnO.