Tailoring Structural Distortions and Ionic Defects as Alternative Strategy to Modulate Reactive Oxygen Species and Photocatalytic Activity in SnO₂ Nanoparticles

This study presents annealing time variation as an alternative approach for tuning structural and ionic defects in SnO2 nanoparticles (NPs) synthesized by a modified Pechini method, to modulate charge transfer and the formation of reactive oxygen species (ROS), enabling a correlation of these with photocatalytic activity. Structural refinements of the X-ray diffraction (XRD) data, combined with Raman and infrared (IR) spectroscopy, revealed that the samples adopted a tetragonal P42/mnm symmetry, with a distinct short-range structural order (associated with intraoctahedral [SnO6] distortions), which strongly depends on the annealing time. Field emission scanning electron microscopy (FE-SEM), scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM/EDX), high-resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller (BET) analyses showed the formation of homogeneous NPs smaller than 50 nm in the SnO2 samples, with a surface area between 12.072 and 14.102 m2 g–1. The presence of unusual reduced Sn3+ species associated with oxygen vacancies (VO) was evidenced by electron paramagnetic resonance (EPR) spectroscopy, and Sn2+ was characterized by Sn MNN Auger electron emission. The amount of these defects also depends on the annealing time. The synergistic effect and the photocatalytic mechanism were then elucidated. The degree of intraoctahedral [SnO6] distortions and the amounts of Sn3+:VO defects play a fundamental role in the charge transfer mechanism to modulate ROS generation during photoexcitation, as indicated by the photohydroxylation of terephthalic acid (TA) and in situ spin-trapping EPR measurements. Specifically, we show the formation of two main ROS, •OOH and •OH, which leads to a different photocatalytic pathway in SnO2 NPs. In conclusion, our study enlightens and uncovers the importance of choosing appropriate conditions for materials processing to fine-tune structural and electronic properties to design other functional materials.