(Hydrolysis reaction) MFx(x-2n)- + H2O → MOn + xF- + 2nH+
(F- scavenging reaction) Al3+ + 6HF → H3AlF6 + 3/2H2
Previously, TiO2 was deposited on the surface of AAO mesopores by the LPD within 250 nm, and in this study, the TiO2 thin film onto AAO mesopore wall which has the extreme small diameter, i.e., ca. 20-40 nm, was prepared by the LPD. In this research, the achievement of the LPD reaction at 0 °C or less by the use of the H2O/EtOH mixed solvent and the deposition of oxide thin film to mesopore inner wall owing to the decrease of the LPD reaction activity under low temperature were executed. Furthermore, the specific ion conductivity of the electrolyte solution in the mesopore channels in the prepared TiO2/AAO nanocomposites was evaluated.
AAO membranes were prepared by two-step anodization as previous report [1,2]. AAO template whose thichness is ca. 500 m (i.e. the aspect ratio of a AAO membrane thichness to a mesopore diameter is ca. 5000) was dipped in (NH4)2SiF6 or (NH4)2TiF6 solution of various concentration for 0-60 h at -20-40 °C. Prepared SiO2 or TiO2/AAO nanocomposite was used as a separator of a four-electrode cell and an impedance measurement was carried out. LiClO4/EC:DEC = 1:1 (v/v) of 0.5 to 2.5 mol L-1was used as an electrolyte solution.
The results of LPD reaction using (NH4)2TiF6 solution of 10 mol L-1 in 1 : 1 (v/v) H2O/EtOH at -20-5 °C (The LPD reaction solution was still liquid-state at -20 °C) for 10 h were shown in Fig. 1. The TiO2 thin film deposition within the deep mesopores without collapse of the mesopore structure was achieved at -13.5 °C. The AAO mesopore structure collapsed because the LPD reaction reactivity was too high at 5 °C, and the TiO2 thin film was not generated because the reactivity was too low at -20 °C. It is suggested that the moderate decrease in the LPD reaction activity by using an aqueous/organic solvent mixture was necessary to achieve oxide thin films deposition on the AAO mesopore wall, while retaining the AAO mesoporous structure. The Nyquist plots for four-electrode cells using SiO2 or TiO2/AAO nanocomposites show two semicircles for each nanocomposite. The lower frequency region of the semi-circle was assigned as ion transport resistance in the SiO2 and TiO2 modified mesopore walls, and the fitting of the semicircle in the low frequency region, the specific conductivity in the pore was calculated. For a LiClO4 solution of 0.5-2.5 mol L-1, the specific Li+ ion conductivities after SiO2 or TiO2 thin film coating by LPD in the mesopore of the AAO nanocomposites was compared with those without thin film coating (i.e. as-prepared AAO). The conductivity for the TiO2/AAO nanocomposite is 3 times that for as-prepared AAO, whereas that the conductivity for the SiO2/AAO nanocomposite is similar for as-prepared AAO. Even if the concentration was changed, the ratios of the specific Li+ ion conductivity in the SiO2 or TiO2 modified mesopore to the electrolyte solution and as-prepared AAO were almost constant. These activation energies of Li+ ion conduction in mesopores for each sample are 14.6 (as-prepared AAO), 13.8 (SiO2/AAO), 13.2 kJ mol-1 (TiO2/AAO), respectively. It is suggested that the ionic conduction are influenced by surface electric potential of oxides.
This work was supported by CREST, JST.  T. Fukutsuka, et al., Electrochim. Acta., 199, 380 (2016)