Titanium dioxide (TiO 2 ) is a unique material that can be used as an excellent photocatalyst, hence it will be very useful to solve the current world pollution problem. For photocatalytic application, the cold spray process, which uses supersonic high velocity technique at moderate temperature is considered to be a suitable technique as it will inhibit the phase transformation of TiO 2 . It is known that obtaining a thick coating of TiO 2 is difficult as it is a hard ceramic material. However, we have successfully synthesised TiO 2 powders that can be cold sprayed. Our synthesis technique is a simple hydrolysis method of titanyl sulphate (TiOSO 4 ) in distilled water. It was discovered that the cold sprayability of TiO 2 powders is depended on the powder morphology and using our synthesis method, powder morphology can be easily altered. Addition of (NH 4 ) 2 SO 4 during hydrolysis and post synthesis hydrothermal treatment result in a unique oriented aggregated nanostructure of TiO 2 . Only TiO 2 with this nanostructure can be cold sprayed.

We have shown the fabrication of ceramics titanium dioxide (TiO 2 ) coating via cold spray process. Cold spraying of any hard ceramics coating is considered to be difficult, including TiO 2 . However, we could obtain a coating with thickness greater than 100 µm. The adhesion mechanism of this cold sprayed TiO 2 coating needs to be clarified. The effects of process gas parameters and substrate materials on coating's adhesion strength were studied. It was found that adhesion strength did not vary significantly with the change of process gas parameters, indicating that mechanical embedment is not the sole factor of coating¡¦s adhesion. The study of changing substrate materials reveals the degradation of adhesion strength due to surface oxide layer and that TiO 2 shows better adhesion on bare metal surface. Hence, by altering the surface chemistry, the adhesion of TiO 2 is varied, indicating the possibility of physical or chemical bonding between cold sprayed TiO 2 and substrate.

Cold spraying has been developed as a high-quality coating process. However, the deposition materials were limited as metallic materials. In this paper, titanium dioxide (TiO 2 ) ceramic particles are deposited on several substrate materials and formed thick coatings, making this approach suitable for a wide range of photocatalytic applications. In order to understand the adhesion mechanism of solid ceramic particles, the structures of feedstock particles are carefully observed with a scanning electron microscope (SEM) and a high-resolution transmission electron microscope (TEM). We find a porous structure which is agglomerated with nano-scaled primary particles. It is assumed that the break down phenomenon occurs due to the cold spray process and influences the adhesion of ceramic particles. The primary particles are bonded within a single crystal. This particular structure is the main factor to make adhesion between the particle and the substrate. SEM and TEM analysis clearly reveals adhesion mechanisms related to the impact of spraying ceramic particles toward the substrate.