Polymerizable Complex Synthesis of Pure Sr₂Nb_xTa_2-xO₇ Solid Solutions with High Photocatalytic Activities for Water Decomposition into H₂ and O₂

Phase-pure solid solutions with the composition of Sr 2 Nb x Ta 2 - x O 7 (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, Sr 2 Ta 2 O 7 ( x = 0) and Sr 2 Nb 2 O 7 ( x = 2), produced H 2 and O 2 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 The photocatalytic activity was dramatically decreased approximately by 1 order of magnitude once Ta has been replaced by Nb, even when the amount of Nb was small.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 H 2 and O 2 . 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 %)/Sr 2 Ta 2 O 7 prepared by the PC method at 800 °C for 48 h; the photocatalyst having a specific surface area of 10.4 m 2 ·g - 1 produced H 2 and O 2 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.