TiO₂ Coating on Inner Wall of a Narrow Glass Tube by an Atmospheric-Pressure RF Plasma

Abstract

  TiO₂ has a photocatalytic function when irradiated with ultraviolet (UV) light. One of photochemical reaction on a TiO₂ surface is the photo-induced redox reactions of adsorbed substances, and the other is the photo-induced high hydrophilicity. It is expected, therefore, that a UV transparent glass tube having a TiO₂ coated inner wall shows the self-cleaning and hydrophilic properties. TiO₂ coating on an inner wall of a narrow (φ2.5 mm) quartz glass tube by an atmospheric-pressure RF (13.56 MHz, 12 W) plasma using titanium tetraisopropoxide (TTIP) as a precursor and a carrier gas He is studied. RF driven parallel-plate electrodes are closely attached on the outer surface of a glass tube to generate He glow plasma inside the tube. It revealed that O₂ addition of 2% or more to the TTIP/He gas is key to prepare a pure TiO₂, otherwise TiC is mixed among the deposited TiO₂ films. The tube inner wall was covered with TiO₂ films of 500-900 nm in a thickness after a TTIP/O₂(2%)/He plasma treatment for 20 min. TiO₂ microparticles with 1-4 μm in a diameter also deposited on an inner wall under the electrodes and the number of microparticles increased with increasing a gas flow rate. It seems that these microparticles are charged negatively and, therefore, are attracted toward the electrodes.