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range of x. The photocatalytic activity was dramatically decreased approximately by 1 order of magnitude once Ta has been replaced by Nb, even when the amount of Nb was small.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.

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Highly crystallized polycrystalline film of single-phase CaWO4 has been prepared on a tungsten substrate at room temperature in an alkaline solution containing calcium ions by an electrochemical method with the current density of 1 mA/cm2. This film showed blue emission (456 nm wavelength) with excitation light (254 nm wavelength) at room temperature.

Crystallized luminescent calcium molybdate (CaMoO 4 )to dissolve. 8 These reactions (i.e., metal corrosion) occur sponfilm has been prepared on a molybdenum substrate in an taneously in the natural world with an accompanying negative alkaline solution containing calcium ions by active electrofree-energy change; the adage "rust never sleeps" is derived chemical dissolution of molybdenum at room temperature from this phenomenon. Much effort has been expended in (25؇C). The dissolution rate became faster with an increase developing artificially contrived coatings that isolate the metal from its environment or lower the corrosion rate to acceptable of pH value. A high concentration of calcium (0.02M) and a values. 9 Our attention will be rather concentrated on the prohigh pH value (13) favored the reaction of film formation.cesses whereby molybdenum ions in the solid crystalline lattice The film showed only a single green emission at 536 nm with may be induced to become solvated ions in an alkaline solution. the excitation of 285 nm at liquid-nitrogen temperatureWe will adapt the dissolution phenomenon for the preparation (؊196؇C), strongly suggesting that it consisted of wellof CaMoO 4 film on a molybdenum metal substrate in the solucrystallized defect-free crystals.tion containing calcium ions. In the following, we will consider what reactions happen under these conditions and identify their