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

Abstract

This paper reports on the mechanisms of CuO decomposition and its associated phase transformation into Cu2O, as a fundamental step of thermite materials reaction, where CuO serves as the oxidizer. 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), (101 ̅ )) 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 (-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 (~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

Initial stage of titanium oxidation in Ti/CuO thermites: a molecular dynamics study using ReaxFF forcefields

“…Similarly, for Cu-Cu interatomic distance characteristic of bulk CuO at 200 K, a redistribution of copper atoms of the top CuO layer is taking place resulting in a significant reduction of the distance from ~3 to 2.6 Å. This process is relatively similar to the reconstruction of the CuO(001) free surface observed using DFT calculations, for which the well aligned Cu top surface layer splits in the direction perpendicular to the surface 43 . Careful visualization of the CuO/TiOx interface structure allows following this splitting process, which is partial at 500 K (Figure 2b), and continuously progresses to end up with the splitting of the full Cu monolayer into two Cu layers at 900 K (Figure 2d).…”

“…This process is relatively similar to the reconstruction of the CuO(001) free surface observed using DFT calculations, for which the well aligned Cu top surface layer splits in the direction perpendicular to the surface. 43 Careful visualization of the CuO/TiO x interface structure allows following this splitting process, which is partial at 500 K (Fig. 2b), and continuously progresses to end up with the splitting of the full Cu monolayer into two Cu layers at 900 K (Fig.…”

“…In this context, we created a defective CuO surface by distributing 2.4% of oxygen vacancies (removal of 16 oxygens) at the vicinity of the surface, in accordance with ref. 43 To obtain stable configurations despite the presence of oxygen vacancies, this defective CuO was subjected to a minimization step using the conjugate gradient (CG) algorithm 45 and then relaxed again at 300 K in the NVT ensemble over 0.1 ns. It has to be noted that a validation step of the CuO forcefield was operated making systematic comparison with published DFT results on bulk, subsurface and CuO (001) surface (Fig.…”

“…In this study, the Cu-rich surface is considered as the bottom surface slab with atomic positions frozen in their lattice sites to mimic a bulk material constraint. The top slab layer is therefore oxygen rich, as the model surface mostly used and documented in the literature, experimentally 42 , and theoretically 43,44 . The choice of the CuO(001) surface was strongly recommended by the recent DFT investigation by Jabraoui et al 43 , showing that the CuO(001) offers unique capability to release oxygen species issued from bulk CuO, which process requires oxygen vacancies to promote oxygen transport at an acceptable temperature (< 600 K, atmospheric pressure), in agreement with experimental findings.…”

“…The top slab layer is therefore oxygen rich, as the model surface mostly used and documented in the literature, experimentally 42 , and theoretically 43,44 . The choice of the CuO(001) surface was strongly recommended by the recent DFT investigation by Jabraoui et al 43 , showing that the CuO(001) offers unique capability to release oxygen species issued from bulk CuO, which process requires oxygen vacancies to promote oxygen transport at an acceptable temperature (< 600 K, atmospheric pressure), in agreement with experimental findings. In this context, we created a defective CuO surface by distributing 2.4% of oxygen vacancies (removal of 16 oxygens) at the vicinity of the surface, in accordance with ref.…”