Surfactant-assisted synthesis of Keggin-type polyoxometalates nanorods

It was demonstrated that Keggin-type polyoxometalates nanorods (POMs-nanorods) were prepared from a solution containing a surfactant polyethylene glycol (PEG), when heteropolyanions [X ⁿ⁺M ₁₂O ₄₀] ^-(8-n) (X ⁿ⁺ = P ⁵⁺ and Si ⁴⁺; M = W and Mo) were precipitated by ammonia ion obtained by hydrolysis of urea at 373 K. In this method, PEG was used as a template and ammonia ion (NH ₄ ⁺) was a counterion. In an oil bath kept at 373 K without stirring, three kinds of ammonium salts of POMs, i.e., (NH ₄) ₃PW ₁₂O ₄₀, (NH ₄) ₄SiW ₁₂O ₄₀, and (NH ₄) ₃PMo ₁₂O ₄₀ nanorods were obtained. The POMs-nanorods were characterized by transmission electron microscopy (TEM), selected area electron diffraction (SAED), infrared spectra (IR), inductively coupled plasma atomic emission spectrometry (ICP-AES) and X-ray powder diffraction (XRD). TEM showed that nanorods were 0.5-1.5 μm in length, and 50-150 nm in diameter. The results of IR, ICP-AES and XRD proved the existence of Keggin structure. The formation process of POMs-nanorods, (NH ₄) _8-n [X ⁿ⁺M ₁₂O ₄₀] ^-(8-n) (X ⁿ⁺ + P ⁵⁺ and Si ⁴⁺; M = W and Mo) in a H ₂O/PEG system was discussed.