Electron accumulation effects on photocatalytic activity of energy storage MoO3 photocatalysts in vacuum for spacecraft contamination control

Abstract

This study investigated the photocatalytic activity of MoO${}_3$ films under vacuum conditions to elucidate the role of photoexcited electron accumulation in the deactivation of photocatalysis, particularly in the field of spacecraft contamination control. The decomposition of citric acid under UV irradiation in vacuum was monitored using mass-loss measurements obtained with a quartz crystal microbalance. Results show that only $13.95\mu g$/cm${}^2$ of the initial $50\mu g$/cm${}^2$ contaminant was decomposed, indicating the limited photocatalytic activity of MoO${}_3$ in vacuum. X-ray photoelectron spectroscopy and photographic observations confirmed the reduction of Mo${}^{6+}$ to Mo${}^{5+}$ and the occurrence of photochromism in MoO${}_3$ after UV exposure. Notably, photocatalytic activity was restored following prolonged exposure to ambient air. These findings demonstrate that, in vacuum, the absence of atmospheric oxygen leads to electron accumulation, which deactivates the photocatalyst. This insight is crucial for the development of photocatalysts capable of long-term operation in spacecraft applications.