Redox behavior of nanostructured molybdenum oxide-mesoporous silica hybrid materials

Encapsulation of molybdenum oxide MoO₃ into ordered mesoporous silica materials is demonstrated using different synthetic routes. Either supramolecular polyoxometalates of the "Müller type" or Na ₂MoO₄ were used as precursors. Molybdenum oxide (MoO ₃)-silica hybrid materials were characterized using nitrogen-sorption experiments, transmission electron microscopy, small-angle X-ray scattering, and infrared spectroscopy. Under reducing conditions, the hybrid materials undergo an electrochromic transition from colorless or slightly yellow to blue without any leaching of Mo species from the materials. The electrochromic response, which is fully reversible, is caused by a transition from Mo^VI to a mixed valence Mo^VI/Mo ^V molybdenum oxide encapsulated in the pores. The reduction process was studied using in situ electron spin resonance (ESR) and UV/vis absorption measurements. Materials of different pore sizes were used, and it was found that pore size effects the kinetics of the reaction.