Rare α-MoO₃-K₂Mo₄O₁₃ hybrid structures obtained via a single molecular precursor approach

Rare K/Mo-based hybrid structures were obtained through a new approach, employing the 4d cyanometallate complex, K₄[Mo^IV(CN)₈], as a new single molecular precursor. The dissociation of the K₄[Mo^IV(CN)₈] complex, under hydrothermal conditions, led to the dark-brown amorphous material P. The subsequent calcination of P gave rise to the light-yellow crystalline products 1 (at 400 °C) and 2 (at 500 °C). The XRD data and Rietveld analysis revealed that 1 and 2 consist of a mixture of two crystalline phases: orthorhombic α-MoO₃ and triclinic K₂Mo₄O₁₃, in close percentages, with a prevalence of the molybdenum oxide phase. In situ XRD experiments up to 630 °C showed that the thermal conversion of P results in the formation of the α-MoO₃-K₂Mo₄O₁₃ mixture, in which α-MoO₃ grows along the (0k0) direction, as the temperature increases, whereas the triclinic K₂Mo₄O₁₃ phase melts at 520° C. These results were confirmed by the in situ Raman experiments on P. The FE-SEM analysis revealed a temperature-dependent morphology: the fused quasi-spherical aggregates of very small nanoparticles of P (up to 10 nm) were converted into large conglomerates comprising a mixture of fused rods/prisms of α-MoO₃ and nano- and sub-micron sized polyhedral-type K₂Mo₄O₁₃ particles for 1, and, for 2, into well-separated phases of α-MoO₃ micro-particles with a 1D morphology and sub-micron polyhedral particles of potassium tetramolybdate. For 2, HR-TEM investigations revealed details of a complex self-assembly mechanism of the elongated particles of molybdenum oxide into 1D hierarchical structures (belts, rods and thick prisms). The capacitive features of the graphite-electrodes modified with P, 1 and 2 materials, as well as with a mixture of graphene and 2 were investigated.