Phase Transformations of Cobalt Oxides in Co_xO_y-ZnO Multipod Nanostructures via Combustion from Thermopower Waves

The study of combustion at the interfaces of materials and chemical fuels has led to developments in diverse fields such as materials chemistry and energy conversion. Recently, it has been suggested that thermopower waves can utilize chemical-thermal-electrical-energy conversion in hybrid structures comprising nanomaterials and combustible fuels to produce enhanced combustion waves with concomitant voltage generation. In this study, this is the first time that the direct phase transformation of Co-doped ZnO via instant combustion waves and its applications to thermopower waves is presented. It is demonstrated that the chemical combustion waves at the surfaces of Co₃O₄-ZnO multipod nanostructures (deep brown in color) enable direct phase transformations to newly formed CoO-ZnO_1-x nanoparticles (olive green in color). The oxygen molecules are released from Co₃O₄-ZnO to CoO-ZnO_1-x under high-temperature conditions in the reaction front regime in combustion, whereas the CoO-ZnO multipod nanoparticles do not undergo any transformations and thus do not experience any color change. This oxygen-release mechanism is applicable to thermopower waves, enhances the self-propagating combustion velocity, and forms lattice defects that interrupt the charge-carrier movements inside the nanostructures. The chemical transformation and corresponding energy transport observed in this study can contribute to diverse potential applications, including direct-combustion synthesis and energy conversion.