First-principles investigation of CuO decomposition and its transformation into Cu2O

This paper reports on the mechanisms of CuO decomposition and its associated phase transformation into ${\mathrm{Cu}}_{2}\mathrm{O}$, as a fundamental step of thermite materials reaction, where CuO serves as the oxidizer. The Frenkel pair defects in perfect bulk CuO show extremely high formation energy (>4 eV) indicating that its decomposition initiates at defects/interfaces/surfaces, the latter being sensitive to surface orientation. In contrast to a variety of CuO surfaces [(111), (110), ($10\overline{1}$)] exhibiting three-fold coordinated oxygen atoms, results show that oxygen vacancy formation requires higher disordered surfaces, such as the CuO(001), on which the vacancy formation activation energy is reduced to 1.31 eV. This leads to the exothermic formation of a chemisorbed O-O peroxy-bridge complex at the surface (\ensuremath{-}1.25 eV adsorption) that thermodynamically moderates the backreaction (vacancy annihilation). Further desorption of molecular oxygen necessitates an activation of 1.53 eV, compatible with CuO decomposition observed experimentally at 600 K (\ensuremath{\sim}second process duration). As a driving mechanism of oxygen release upon decomposition, migrations of the vacancy close to the surface and towards the bulk are determined for a number of crystalline directions and surface orientations and show considerable anisotropy, with two preferential directions: [110] and [001] with 1.35 and 1.77 eV activation, lowered to 1.08 and 1.03 eV activation when approaching the (111) and (001) surfaces, respectively. Finally, the CuO to ${\mathrm{Cu}}_{2}\mathrm{O}$ phase transformation follows a two-step process: a first structural modification from monoclinic to orthorhombic takes place when CuO has lost 12.5% of its oxygen atoms, followed by a barrierless transition to the cubic phase when 44--48% of the oxygen atoms have been removed, i.e., very close to stoichiometric ${\mathrm{Cu}}_{2}\mathrm{O}$.