  Tailoring nanoparticles composition and morphology is of particular interest
for improving their performance for catalysis. A challenge of this approach is
that the nanoparticles optimized initial structure often changes during use.
Visualizing the three dimensional (3D) structural transformation in situ is
therefore critical, but often prohibitively difficult experimentally. Although
electron tomography provides opportunities for 3D imaging, restrictions in the
tilt range of in situ holders together with electron dose considerations limit
the possibilities for in situ electron tomography studies. Here, we present an
in situ 3D imaging methodology using single particle reconstruction (SPR) that
allows 3D reconstruction of nanoparticles with controlled electron dose and
without tilting the microscope stage. This in situ SPR methodology was employed
to investigate the restructuring and elemental redistribution within a
population of PtNi nanoparticles at elevated temperatures. We further examined
the atomic structure of PtNi and found a heat induced transition from a
disordered to an ordered phase. Changes in structure and elemental distribution
were linked to a loss of catalytic activity in the oxygen reduction reaction.
The in situ SPR methodology employed here could be extended to a wide range of
in situ studies employing not only heating, but gaseous, aqueous or
electrochemical environments to reveal in operando nanoparticle evolution in
3D.
