Tribo-Catalytic Degradation of Methyl Orange Solutions Enhanced by Silicon Single Crystals

Coating materials on the bottoms of reactors/beakers has emerged as an effective method to regulate tribo-catalytic reactions. In this study, silicon single crystals were coated on the bottoms of glass beakers, in which 30 mg/L methyl orange (MO) solutions suspended with alumina nanoparticles were subjected to magnetic stirring using Teflon magnetic rotary disks. With a gentle rotating speed of 400 rpm for the Teflon disks, the MO solutions were changed from yellow to colorless and the characteristic absorption peak of MO at 450 nm in the UV-Vis spectrum disappeared entirely within 120 min. Mass spectrometry tests were further performed to gain insights into the degradation process, which suggested that the degradation was initiated with the cleavage of the nitrogen-nitrogen double bond in ionized MO molecules by the attack of •OH radicals. Through comparison experiments, we established that the observed degradation was related to the friction between alumina and silicon during magnetic stirring, and hydroxyl and superoxide radicals were formed from the friction, according to electron paramagnetic resonance analysis. It is proposed that electron-hole pairs are excited in silicon single crystals through friction with alumina, which diffuse to the surface of the single crystals and result in the degradation.