Exploring halide perovskite nanocrystal decomposition: Insight by <i>in-situ</i> electron paramagnetic resonance spectroscopy

Zero-dimensional (0D) and three-dimensional (3D) halide perovskite nanocrystals (HP-NCs), owing to their unique optoelectronic properties, are extensively studied for photocatalytic activity. However, HPs are highly sensitive to light, humidity, and other environmental factors, which accelerate their decomposition. Understanding the decomposition process is crucial for gaining insights into how to stabilize HP-NCs. Here, we investigate the radical-driven decomposition process and dynamics of the 0D Cs₄PbBr₆ and 3D CsPbBr₃ NCs under the influence of visible light and a polar solvent by electron paramagnetic resonance (EPR) spectroscopy. Our findings indicate that light accelerates radical formation over time, making the decomposition of HP-NCs a self-sustaining process. Upon illumination of the NCs, hydroperoxyl radicals are formed first, followed by unconventional Br, Cs, and Pb-related radicals, indicating the initiation of NC decomposition. The decomposition of CsPbBr₃ NCs starts after 3 min of light exposure, while Cs₄PbBr₆ NCs take 18 min, indicating the greater stability of the latter. Additionally, we evaluated the photocatalytic activity of the HPs toward degrading organic dyes. The 3D CsPbBr₃ NCs performed as superior photocatalysts compared to their 0D Cs₄PbBr₆ NCs counterparts. Yet, linking the results of EPR measurements with the photocatalytic efficacy suggests that the CsPbBr₃ NCs undergo degradation during the photocatalytic process, thereby serving as a sacrificial agent to enhance photocatalytic activity. The understanding derived from EPR spectroscopy in tracking radical formation and dynamics can be extended to enhance the stability and efficiency of various nanomaterials in optoelectronic and photocatalytic applications, thus contributing to advancements beyond the HP family.