Hydrothermal synthesis of α-MoO3 nanorods via acidification of ammonium heptamolybdate tetrahydrate

Abstract

One-dimensional nanostructures of orthorhombic molybdenum trioxide (α-MoO3) have been synthesized in the forms of ribbons or rods via acidification under hydrothermal conditions at 140-200 °C. The reaction path has been revealed with our kinetic investigations, which shows the following sequence: (i) from the starting compound (NH4)6Mo7O24 ·4H2O to (ii) formation of intermediate compound ((NH4)2O)0.0866 ·MoO3·0.231H2O, and then to (iii) final α-MoO3 nanoribbons or nanorods in 100% phase purity. The optimal growth temperature is in the range of 170-180 °C under the current experimental settings. At higher reaction temperatures, this transformation can be accelerated, but with poorer crystal morphological homogeneity. It has been found that the dimensions of these rectangular nanorods are about 50 nm in thickness, 150-300 nm (mean value at 200 nm) in width, and a few tens of micrometers in length. The crystal morphology can be further altered with inorganic salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3. Using an H2S/H2 stream, the above-prepared α-MoO3 nanorods can be converted completely to 2H-MoS2 nanorods at 600 °C. The original rodlike morphology is well-retained, although the aspect ratio of the oxide template is reduced upon the sulfidation treatment.