Solvothermal Synthesis of Ultrasmall Tungsten Oxide Nanoparticles

The synthesis of catalytically useful, ultrasmall oxide nanoparticles (NPs) of group 5 and 6 metals is not readily achievable through reported methods. In this work, we introduce a one-pot, two-precursor synthesis route to <2 nm MOx NPs in which a polyoxometalate salt is decomposed thermally in a high-boiling organic solvent oleylamine. The use of ammonium metatungstate resulted in oleylamine-coated, crystalline WOx NPs at consistently high yields of 92 +/- 5%. The semicrystalline NPs contained 20-36 WOx structural units per particle, as determined from aberration-corrected high-resolution scanning transmission electron microscopy, and an organic coating of 16-20 oleylamine molecules, as determined by thermogravimetric analysis. The NPs had a mean size of 1.6 +/- 0.3 nm, as estimated from atomic force microscopy and small-angle X-ray scattering measurements. Carrying out the synthesis in the presence of organic oxidant trimethylamine N-oxide led to smaller WOx NPs (1.0 +/- 0.4 nm), whereas the reductant 1,12-dodecanediol led to WOx nanorods (4 +/- 1 nm x 20 +/- 5 nm). These findings provide a new method to control the size and shape of transition metal oxide NPs, which will be especially useful in catalysis.