  Context. Carbonaceous nano-grains play a fundamental role in the
physico-chemistry of the interstellar medium (ISM) and especially of
photon-dominated regions (PDRs). Their properties vary with the local physical
conditions and affect the local chemistry and dynamics. Aims. We aim to
highlight the evolution of carbonaceous nano-grains in three different PDRs and
propose a scenario of dust evolution as a response to the physical conditions.
Methods. We used Spitzer/IRAC (3.6, 4.5, 5.8, and 8 $\mu$m) and Spitzer/MIPS
(24 $\mu$m) together with Herschel/PACS (70 $\mu$m) to map dust emission in
IC63 and the Orion Bar. To assess the dust properties, we modelled the dust
emission in these regions using the radiative transfer code SOC together with
the THEMIS dust model. Results. Regardless of the PDR, we find that nano-grains
are depleted and that their minimum size is larger than in the diffuse ISM
(DISM), which suggests that the mechanisms that lead nano-grains to be
photo-destroyed are very efficient below a given critical size limit. The
evolution of the nano-grain dust-to-gas mass ratio with both G0 and the
effective temperature of the illuminating star indicates a competition between
the nano-grain formation through the fragmentation of larger grains and
nano-grain photo-destruction. We modelled dust collisions driven by radiative
pressure with a classical 1D approach to show that this is a viable scenario
for explaining nano-grain formation through fragmentation and, thus, the
variations observed in nano-grain dust-to-gas mass ratios from one PDR to
another. Conclusions. We find a broad variation in the nano-grain dust
properties from one PDR to another, along with a general trend of nano-grain
depletion in these regions. We propose a viable scenario of nano-grain
formation through fragmentation of large grains due to radiative
pressure-induced collisions.
