Polar
surfaces of ionic crystals are of growing technological importance,
with implications for the efficiency of photocatalysts, gas sensors,
and electronic devices. The creation of ionic nanocrystals with high
percentages of polar surfaces is an option for improving their efficiency
in the aforementioned applications but is hard to accomplish because
they are less thermodynamically stable and prone to vanish during
the growth process. Herein, we develop a strategy that is capable
of producing polar surface-dominated II–VI semiconductor nanocrystals,
including ZnS and CdS, from copper sulfide hexagonal nanoplates through
cation exchange reactions. The obtained wurtzite ZnS hexagonal nanoplates
have dominant {002} polar surfaces, occupying up to 97.8% of all surfaces.
Density functional theory calculations reveal the polar surfaces can
be stabilized by a charge transfer of 0.25 eV/formula from the anion-terminated
surface to the cation-terminated surface, which also explains the
presence of polar surfaces in the initial Cu1.75S hexagonal
nanoplates with cation deficiency prior to cation exchange reactions.
Experimental results showed that the HER activity could be boosted
by the surface polarization of polar surface-dominated ZnS hexagonal
nanoplates. We anticipate this strategy is general and could be used
with other systems to prepare nanocrystals with dominant polar surfaces.
Furthermore, the availability of colloidal semiconductor nanocrystals
with dominant polar surfaces produced through this strategy opens
a new avenue for improving their efficiency in catalysis, photocatalysis,
gas sensing, and other applications.