Manufacturing a TiO₂-Based Semiconductor Film with Nanofluid Pool Boiling and Sintering Processes toward Solar-Cell...

For the first time, nanofluid boiling was applied as a process for the creation of a semiconductor TiO₂ nanoparticle film that can be deposited onto a conductive substrate (F-doped SnO₂ glass: FTO). A steel-base device designed for pool boiling was used to deposit a TiO₂-based nanofluid consisting of nanoparticles with an average size of about 20 nm. The boiling of the nanofluid directly on the FTO glass substrate allowed for the deposition of the nanoparticles onto the FTO surface. In principle, the surface responsible for transferring heat to the fluid can be covered with these nanoparticles when the nanofluid boils. Using the as-deposited films, crystal growth of the TiO₂ nanoparticle was controlled by varying the strategies of the post-sintering profile. The maximum temperatures, periods, and ramping rates for the obtained samples were systematically changed. Scanning electron microscopy (SEM) revealed that a densely packed TiO₂-nanoparticle layer was obtained for the as-deposited substrate via pool boiling. For the maximum temperature at 550 °C, the TiO₂ grain sizes became larger (~50 nm) and more round-shaped TiO₂ nanostructures were identified. Notably, we have demonstrated for the first time how the sintering of TiO₂ nanoparticles proceeds for the nanoporous TiO₂ films using high-resolution transmission electron microscopy (TEM) measurements. We found that the TiO₂ nanoparticles fused with each other and crystal growth occurred through neighboring 2-4 nanoparticles for the 550 °C sample, which was proved by the TEM analysis that continuous lattice fringes corresponding to the (101) anatase phase were clearly observed through the entire area of some nanoparticles aligned horizontally. In addition, the loss of the TiO₂ nanofluid (precursor solution) was completely avoided in our TiO₂ deposition. Unlike the commonly used spin-coating method, nanofluid pool boiling would provide an alternative cost-effective approach to manufacture semiconductor layers for various applications, such as solar cells.