CO₂ Laser Sintering of TiO₂ Nanoparticles Thin Films for Improved Transmittance

Photothermal CO₂ laser sintering of anatase titanium dioxide (TiO₂) nanoparticles (NPs) is reported in this paper for depositing transparent TiO₂ thin films on quartz substrates. Drops of TiO₂ NPs suspension are spread on the substrates using a spin-coating technique to prepare wet TiO₂ thin films of different thicknesses in a controlled manner. These wet films are subsequently heated with a CO₂ laser ( \(\lambda =10.6 \;\mu m\) $\lambda =10.6\mu m$ ) in two stages, corresponding to the drying process for evaporating the liquid of the nanoparticles suspension and the sintering process for bonding the nanoparticles to form a transparent TiO₂ film. The microstructures of the spin-coated (wet), dried, and sintered TiO₂ films have been studied using optical and scanning electron microscopy (SEM). The optical transmittance of the films is characterized by UV/Vis/NIR spectrophotometry and fourier-transform infrared (FTIR) spectroscopy. The transmittance of the sintered TiO₂ films increased nearly linearly over the ultraviolet (UV) to visible (Vis) range of wavelengths and reached an average transmittance above \(\sim 92\%\) $\sim 92\%$ in a certain infrared (IR) range, e.g. at \(2500 \;nm\) $2500nm$ wavelength, and their transparency is slightly affected by the film thickness. The X-ray diffraction (XRD) analysis revealed that the sintered TiO₂ films are polycrystalline with an anatase crystal structure, without the appearance of any traces of the rutile phase of TiO₂.