The WO3 nanostructures were synthesized by precipitation
technique from aqueous solutions of ammonium tungstate para
pentahydrate ((NH4)10W12O415H2O, Wako) and nitric acid
(HNO3, Merck). A pre-determined amount of the tungstate salt
was dissolved in de-ionized water and the resulting solution was
brought to 80 8C. With vigorous stirring, a warm, concentrated
nitric acidwas added dropwise. The amounts of the tungstate salt
and nitric acid were used such that the final concentrations of
both reactants were in the range of 5.3–16.0 mMand 1.5–6.0 M,
respectively, to study the effect of reactant concentrations on a
precipitation rate and yield of the precipitating product. With
continuous stirring, the mixed solution was kept at 80 8C for 30–
60 min, after which the precipitates were allowed to settle for 1
day at roomtemperature. The precipitate was washed by addition
of a large amount of de-ionized water into the precipitate
followed by stirring for about 10 min and allowing the
precipitates to settle down overnight before decanting the liquid.
This washing procedure was carried out twice. Finally, the
precipitates were separated by ultrafiltration using a polymer
membrane (pore size = 0.47 mm). After drying at 100 8C
overnight, the precipitates were calcined in air at temperatures
ranging from 150 to 800 8C for 2 h. In some experiments, the asprecipitates
were dried at room temperature under vacuum
before subjected to crystal structure analysis.