It is reported that contaminants outgassed from these organic materials cause to degree the performance of optical systems mounted on spacecraft in orbit. We have been focusing on photocatalysts to solve this contamination problem. In this paper, photocatalytic activities of TiO2 in vacuum were measured from frequency change of QCMs. The QCM frequencies increased during UV irradiation to TiO2 on the QCM contaminated by oleamide and MR, which implied that TiO2 can decompose oleamide and MR and their mass on TiO2/QCM decreased in vacuum. We have succeeded to estimate photocatalytic activity from mass change. Moreover, this estimation method is effective to evaluate photocatalytic activity in vacuum.
Contamination outgassed from spacecraft's materials can degrade optical devices in orbit. Therefore, solving the contamination problem is important because it in uences the success of spacecraft missions. In this study, methyl red (MR) and oleamide were decomposed in vacuum by TiO2 photocatalyst. Absorbance spectra, mass decrease and GC/MS were measured after and before decomposition. In vacuum, TiO2 could decompose MR to intermediate products, whereas it could not decompose the intermediate products to volatile substances because TiO2 cannot open benzene rings of the intermediate products. On the other hand, TiO2 could decompose oleamide to volatile substances even in vacuum. However, decomposition by TiO2 stopped after a certain period of time in vacuum. The decrease in mass for oleamide by photocatalytic reaction in vacuum was enough compared to standard molecular contamination levels.
We have focused on photocatalytic materials to solve contamination problem for spacecraft. We have fabricated TiO2 thin films and measured decomposition rates of methyl orange (MO) and dioctyl phthalate (DOP) in vacuum by TiO2 thin films as a photocatalyst. From XRD results, fabricated TiO2 thin films have anatase-type crystal structure, which is known to have stronger decomposition activities than rutile-type TiO2. The TiO2 thin films we made were shown to decompose methylene blue (MB) solution, which means that the TiO2 thin films have general photocatalystic activity in atmosphere. In decomposition of MO in atmosphere and vacuum, TiO2 shows photocatalytic activity even in vacuum although the decomposition rate in vacuum is slower than that in atmosphere. In decomposition of DOP in vacuum, DOP was effused from an effusion cell in vacuum chamber and was deposited on a TiO2 thin film using the in-situ measurement apparatus at Tsukuba Space Center, JAXA. Transmission spectra of DOP on TiO2 thin films after UV irradiation were measured to estimate decomposition rate from absorbance of DOP. The results show that TiO2 thin films can decompose DOP even in vacuum. Moreover, H2O can promote the decomposition of DOP. In order to use photocatalyst materials in vacuum for long time, the studies on the durability of photocatalystic activity of TiO2 in vacuum and the effects of O2 and H2O are necessary in the future.
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