Optically Transparent Colloidal Dispersion of Titania Nanoparticles Storable for Longer than One Year Prepared by Sol/Gel Progressive Hydrolysis/Condensation

The molecular catalyst sensitized system (MCSS), where an excited molecular catalyst adsorbed on a semiconductor such as TiO₂ injects electrons to the conduction band of the semiconductor leading to hydrogen evolution/CO₂ reduction coupled with an oxidation of water on the molecular catalyst, has been one of the most probable candidates in the approach to artificial photosynthesis. For a full utilization of visible light, however, a serious light scattering of the aqueous suspension of TiO₂ in the visible region, which is generally experienced, should be avoided. Here, we report a preparation of optically transparent colloidal dispersion of TiO₂ by the sol/gel reaction of TiCl₄ through progressive hydrolysis/condensation under the basic condition without any calcination processes. The TiO₂ nanoparticles (TiO₂(NPs)) obtained were characterized as an amorphous particle (∼10-15 nm) having a microcrystal domain of anatase within several nm by XRD, Raman spectroscopies, XRF, XAFS, TG/DTA, and HRTEM, respectively. The energy-resolved distribution of carrier electron traps in TiO₂(NPs) as a fingerprint of TiO₂ was characterized through reversed double-beam photo-acoustic spectroscopy to have a close similarity to that of TiO₂(ST-01) as well as the observation of carrier traps by transient absorption spectroscopy. Though the powder TiO₂(NP) itself was not dispersed well in aqueous solution, the wet TiO₂(NPs) as prepared before being dried up provided a completely transparent aqueous dispersion under the acidic condition (1 M HCl). Addition of methanol enabled the colloidal dispersion (TiO₂(NPs, MeOH/H₂O, 0.1 M HCl)) to keep the optical transparency for longer than 1 year (550 days), which is the first example of TiO₂ dispersion storable for such a long period. TiO₂(NPs, MeOH/H₂O) exhibited a moderate photocatalytic reactivity of H₂ evolution with a quantum yield of ∼2.6% upon 365 nm light irradiation. An optically transparent thin film of TiO₂(NPs, MeOH/H₂O) was also successfully prepared on a glass plate to exhibit an enhanced hydrophilicity upon UV light irradiation.