Nano-sized particles of MoO2 are studied for applications both as anode material for Li/Na batteries and in catalysis. The properties of the nanoparticles are intimately tied to the MoO2 structure. Therefore, knowledge of the structure is vital to improve the material performance in any application. Bulk MoO2 crystallizes in a distorted rutile structure. However, the structure of nano-sized MoO2 is currently not well characterized. The bulk structural model does not describe the atomic arrangement in the MoO2 nanoparticles, and the particle small size challenges conventional crystallographic techniques in determining the structure.1 The aim of this study is to elucidate the structure of nano-sized MoO2.
Several structural trends exist within the molybdenum oxide and rutile systems. The building blocks for molybdenum oxide structures are [MoO6]-octahedra, and the octahedra share corners and edges in various ways to form different crystal structures. Early studies of molybdenum oxides found a large homologous ‘Magneli’ series of molybdenum oxide phases2 (MonO3n-m) where the structures vary by the introduction of ordered crystallographic shear planes. A similar homologous series is also seen in rutile titanium oxide.
Here, we use X-ray total scattering with Pair Distribution Function (PDF) to investigate the structure of molybdenum oxide particles in sizes ranging from c. 2 nm to 100 nm synthesized via a simple one-step solvothermal synthesis. Taking advantage of the knowledge of the Magneli series, PDF analysis allows us to develop a new structural model for even the smallest nanoparticles of MoO2. We are able to show that the structural transformation of the nano-sized MoO2 is caused by a high concentration of crystallographic shear planes, which greatly alters the atomic arrangement on global and local scale compared to bulk MoO2. The 100 nm MoO2-particles also exhibit shear planes, but to a smaller degree resulting in unchanged long-range atomic correlations. In addition to determining the structure of nano-sized MoO2 this project showcases the strength of having both local and global structural information available by using total scattering techniques.
S. J. L. Billinge, I. Levin, Science, 2007, 316, 561-565
A. Magnéli, Acta Cryst, 1953, 6, 495