Characterization of photocatalytic, wetting and optical properties of TiO2 thin films and demonstration of uniform coating on a 3-D surface in the mass transport controlled regime

Titania is a well-known self-cleaning coating for window glass, but it could also be an important industrial coating and active surface if a thicker coating with tunable properties could be applied to stainless steel objects. In this work the tunable properties of TiO2 were studied and a coating on a 3D stainless steel object with the selected microstructure was demonstrated. TiO2 thin films were deposited onto fused silica slides over a range of temperature using the direct liquid injection pulsed-pressure MOCVD technique. All depositions used the same concentration and injection rate of titanium isopropoxide in dilute toluene solution in a cold-wall reactor. The number of pulses was adjusted so that films deposited at different temperatures were the same thickness of around 2 μm. Deposition below 400 °C was kinetically controlled and above 400 °C was mass transport controlled. XRD showed all films were anatase phase TiO2 with highly textured columnar structure in the mass transport regime. SEM, TEM, and AFM analysis showed a marked difference between films deposited in the different regimes. Columnar grain diameter and surface roughness were much smaller and growth rate much higher in the mass transport regime. Surface wettability and MB dye degradation rate under UV radiation were both markedly higher for mass transport controlled deposition. An interesting finding was hydrophilic behavior of the high temperature thin films without UV irradiation. Coating conformality of 0.91 was measured around the circumference of a stainless steel ferrule with coating thickness of 8.7 μm deposited in the mass transport controlled regime.