Non-Stoichiometry Induced Exsolution of Metal Oxide
Nanoparticles via Formation of Wavy Surfaces and their Enhanced Electrocatalytic
Activity: Case of Misfit Calcium Cobalt Oxide
posted on 2021-02-16, 15:33authored byKankona
Singha Roy, Chandramouli Subramaniam, Leela S. Panchakarla
Most
heterogeneous catalytic reactions demand high density and
yet spatially separated nanoparticles that are strongly anchored on
the oxide surfaces. Such nanoparticles can be deposited or synthesized
in situ via nonstoichiometric methods. To date, nanoparticles have
been exsolved from perovskite oxide surfaces using nonstoichiometric
processes. However, the density of the space-separated nanoparticles
on the oxide surfaces is still low. And less attention is paid toward
the changes that happen to the host during the nanoparticle exsolution
process. In this work, we demonstrated in situ exsolution of ultrafine
nanoparticles (∼5 nm) of either Co3O4 or Ca(OH)2 via judicious control of nonstoichiometry
in a misfit Ca3Co4O9 (CCO). The nanoparticle
density over the CCO surface reached as high as 8500/μm2, which is significantly higher than previously reported values.
High-resolution electron microscopy studies reveal the formation mechanism
of Co3O4 nanoparticles over CCO, and the formation
takes palace via the formation of wavy surfaces on the CCO. Defects
caused by the nonstoichiometric synthesis created microstrain within
the host CCO, resulting in making the new density of states near the
Fermi energy. Further, the exsolution process turned the inert host
(CCO) into electrocatalytically active toward water splitting. The
nonstoichiometric samples obtained by shorter annealing times showed
high electrocatalytic behavior for the hydrogen evolution (HER) and
oxygen evolution (OER) reactions. The catalytic activity is further
enhanced (reaching overpotential of 320 mV and 410 mV for HER and
OER respectively, for a current density of 10 mA/cm2) by
removing the surface nanoparticles. The observation indicates that
the active sites that are produced during the nonstoichiometric synthesis
also present in the bulk of the CCO (host). We believe that similar
nonstoichiometric synthesis can be applied to a wide variety of tricomponent
systems, and they could endow the hosts with novel properties for
applications such as catalysis and thermoelectrics.