Polymerizable Complex Synthesis of Pure Sr2Nb x Ta2 - x O7 Solid Solutions with High Photocatalytic Activities for Water Decomposition into H2 and O2

Phase-pure solid solutions with the composition of Sr2Nb x Ta2 - x O7 (SNT, x = 0−2) were prepared at 900 °C for 5 h by the Pechini-type polymerizable complex (PC) technique, based upon polymerization between citric acid and ethylene glycol. The two end compounds, Sr2Ta2O7 (x = 0) and Sr2Nb2O7 (x = 2), produced H2 and O2 in a stoichiometric ratio from pure water under UV light irradiation without a NiO cocatalyst. The photocatalytic activity of SNT for the water decomposition was greatly improved by loading NiO as a cocatalyst for a whole range of x. The photocatalytic activity was dramatically decreased approximately by 1 order of magnitude once Ta has been replaced by Nb, even when the amount of Nb was small. For all of the NiO-loaded SNT samples, water was stoichiometrically decomposed into H2 and O2. While samples prior to the complete crystallization showed very low activities despite their high surface area, the corresponding photocatalytic activities of well-crystallized samples depended primarily on their surface area. The low photocatalytic activities of such premature samples were interpreted as a consequence of the increased number of lattice defects acting as inactivation centers. The maximum photocatalytic activity was obtained for NiO (0.15 wt %)/Sr2Ta2O7 prepared by the PC method at 800 °C for 48 h; the photocatalyst having a specific surface area of 10.4 m2·g-1 produced H2 and O2 from pure water with specific rates of 3517 and 1733 μmol·h-1·g-1, respectively, 3.5 times larger than the best result for a sample prepared by the conventional solid-state reaction method.