Effects of the stirring time in TiO_2 sol chemical on the optical transmittance of the spin-coated films over the visible-to-near-infrared region were studied. It was observed that the longer stirring time, from 12 hours to 48 hours could provide films with either observably higher or lower transmittance, depending on the solvent. In Isopropanol, or IPA, the longer stirring time of the sol chemical leaded to a lower optical transmittance. In hydrochloric acid, or HCl, the trend was reversed; the longer stirring time of the sol chemical would cause a higher optical transmittance. Effects in both solvent were more pronounced at lower wavelengths, i.e. the increasing or decreasing of the transmittance with the extended stirring time could be seen more noticeably at low wavelengths. We believed that these phenomena were caused by the change in dispersion of the TiO_2 particle in the solvent over the stirring time. In HCl, which is an acid-based solvent, the particle could be better dispersed than that in IPA, resulting in a proportional relation between the stirring time and the transmittance of the spin-coated films. This assumption was supported when the transmittance of the films in IPA and HCL were compared as it could be seen that the transmittance of the former was lower. The results from this study could be used to help select an appropriate preparation time of the TiO_2 sol chemicals in different solvents to obtain a spin-coated film with a required level of optical transmittance.
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