Template-free synthesis of molybdenum oxide-based hierarchical microstructures at low temperatures

Molybdenum oxide-based hierarchical microstructures assembled radially by anisotropic nanostructures (nanoplates/microprisms/nanorods) have been synthesized at low temperatures for the first time via a facile template-free hydrothermal approach. The influence of hydrothermal temperature, time, concentration of the peroxomolybdic acid solution, and additives on the morphology of products has been investigated. It has been found that the morphology of urchin-shaped microstructures of peroxo-modified molybdenum oxide hydrates can be controlled in the temperature range of 65-75 °C. The as-synthesized uniform microstructures, which are collected from the 0.9 mol/L molybdenum solutions after 14 h at 75 °C, are 35-40 μm in diameter, and they are constructed by nanoscale blocks of 300-400 nm in width and ∼190 nm in thickness. The dimensions of microstructures and their building blocks are dramatically magnified when the molybdenum concentration is reduced, while those dimensions can be efficiently decreased by adding small quantities of HNO3 or LiNO3 into the precursor solution. By moderate calcination, α-MoO3 with a hierarchical microstructure can be achieved. The possible formation mechanism of urchin-shaped microstructures is discussed using the results of UV-visible spectroscopy and electron microscopy.