Thin films including Methylene Blue (MB) have been prepared. They were dip-coated using the sol–gel reaction of tetraethyl orthosilicate (TEOS). The absorption spectra of the individual thin films have been observed as a function of time after preparation of the films. The relative contributions of the monomer (MB+), dimer ((MB+)2), and protonated species (MBH2+) existing in the individual thin films to the total absorption spectra were obtained. The acidity in each film was estimated from the relative abundance of the MBH2+. In the case of the system including a large amount of HCl as a catalyst, MB formed MBH2+ in addition to MB+ and (MB+)2 just after preparation of the films. However, MBH2+ in every film disappeared 5—10 min after the preparation. The formation of MBH2+ indicates the very high acidity around MB molecules. In each film just after the preparation, the rate of the hydrolysis became slower and the concentration of the released proton remarkably increased because the polycondensation rapidly proceeded with the contraction in volume. Although growing –SiOSi– networks prevent the evaporation of the acid together with water and ethanol, the decrease of MBH2+ indicates the lowering of acidity in the films due to the evaporation.
Dye-adsorbing ZnO thin films were prepared on ITO films by electroless deposition. The films were formed in an aqueous solution containing zinc nitrate, dimethylamine-borane, and eosin Y at 328 K. The film thickness was 1.2-2.0 μm. Thinner and larger-plane hexagonal columns were produced from the solution containing a higher concentration of eosin Y. A photocurrent was observed in the electrodes containing such ZnO films during light irradiation. The photoelectrochemical performance of the film was improved by increasing the concentration of eosin Y because of increases in the amount of absorbed photons and the electronic conductivity of ZnO.
Zinc oxide (ZnO) particles were deposited on a very thin titanium dioxide film by a photocatalytic reaction. The particles were formed in an aqueous solution of 0.10 mol dm−3 zinc nitrate at 343 K. The particles consisted of crystals with 0.20–1.0 µm-sized hexagonal columns. A photocurrent was observed in the electrodes containing such ZnO crystals during light irradiation.
Zinc oxide and layered zinc hydroxides were deposited from an aqueous solution of zinc nitrate at 323−358 K on a substrate plate with a very thin titanium dioxide film by a photocatalytic reaction. The amorphous or low crystalline zinc hydroxide aggregates were deposited at a low temperature. The zinc oxide crystals with about 1−2 μm-sized hexagonal columns and 10 nm-sized spheres were formed at 338−358 K. Nitrate ions in the solution were reduced to nitrite ions, and water was transformed into hydroxide ions by a photocatalytic reaction on the titanium dioxide film. The pH value increased on the substrate surface with the titanium dioxide film, which caused the zinc hydroxide formation on the film.The zinc hydroxides were then dehydrated and transformed into zinc oxide. The average crystallite size of the zinc oxide decreased with an increase in the reaction temperature because the reaction rates of the formation and dehydration of the zinc hydroxides increased which resulted in an increase in the formation rate of the crystal zinc oxide nuclei.
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