  In this work, the instabilities at the nanoscale (i.e. nanoinstabilities) of
triangular pyramids-like Cu2O porous nanostructured films (PNFs) are studied by
heating treatments under different atmosphere and temperature. It is found that
the nanoscale building triangular-pyramids turn round preferentially at the
sharp angles and/or coalesce with their contacting ones by directional
diffusion and plastic flow of atoms, which are driven by the
nonuniformly-distributed surface nanocurvature. As a result, the triangular
pyramids become quasi-sphere shape and the PNF evolves into a big, dense
particles film. It is also observed that the heating or thermal activation
effect efficiently promotes the reduction or oxidation of Cu2O pyramids and the
crystallization or growth of the as-achieved Cu or CuO grains. The above
physical and chemical instabilities or changes at the nanoscale of Cu2O PNFs
can be well accounted for by the combined mechanism of nanocurvature effect and
thermal activation effect. The nanocurvature effect can lower the energy
barrier for the atom diffusion or plastic flow and lower the activation energy
for the chemical reactions, while the thermal activation effect can supply the
required kinetic energy or activation energy and make the atomic
transportations and reactions kinetically possible. The findings reveal the
evolution laws of morphology, crystal structure and composition of triangular
pyramids-like Cu2O PNF during heating treatments, which can further be extended
to other types of Cu2O PNFs. Also, the findings have important implications for
the nanoinstabilities of Cu2O PNFs-based devices, especially those working at a
high temperature.
