Growth mechanism and formaldehyde sensing of mixed phase cobalt oxide nanowalls.

Surface morphology of transition metal oxide (TMO)-based semiconductor nanostructures plays a crucial role in determining their device performance, gas sensing abilities, and catalytic properties. Additionally, chemical stoichiometry of oxide materials significantly influences their structural, electronic, and optical characteristics. In this study, we present the successful fabrication of two-dimensional (2D) porous nanowalls (67 nm thick) of mixed cobalt oxide (CoO + Co3O4) phases using a controlled thermal oxidation process, which are later used for selective formaldehyde sensing. High-purity cobalt (Co) films were deposited on glass substrates via vacuum assisted electron beam evaporation and subsequently oxidized in air ambient at temperatures up to 800 °C. Below 400 °C, the CoO phase stabilizes the highly porous 2D nanowalls morphology, while at higher temperatures Co3O4 phase promotes the homogeneous nanograin morphology. EDX, XPS and HRTEM analyses collectively confirm the formation of a CoO (core) / Co3O4 (shell) nanowalls structure. These porous nanowalls of mixed oxide phase significantly impacted on the chemiresistive gas sensing ability of cobalt oxide thin films. Mixed phase nanowalls exhibited significantly higher sensitivity (~ 60%) to formaldehyde vapour, compared to the nanograin Co3O4 films. Furthermore, these unique mixed oxide nanowalls displayed an excellent selectivity towards low concentration formaldehyde vapour against other volatile organic compounds (VOCs). All these findings provide valuable insights into the phase stabilization and growth mechanisms of CoOx nanowalls, highlighting their potential for high-performance gas sensing applications.