Dominant Polar Surfaces of Colloidal II–VI Wurtzite Semiconductor Nanocrystals Enabled by Cation Exchange

Abstract

Polar surfaces of ionic crystals are of growing technological importance, with implications for the efficiency of photocatalysts, gas sensors and electronic devices. Creation of ionic nanocrystals with large percentages of polar surfaces is an option to improve their efficiency in aforementioned applications but is hard to be accomplished because they are less thermodynamically stable and prone to vanish during the growth process. Herein we developed 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 hexagonal prism-shaped wurtzite ZnS hexagonal nanoplates have dominant {002} polar surfaces, occupying up to 97.8% of all surfaces. Density functional 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 to other systems to prepare nanocrystals with dominant polar surfaces. Furthermore, the availability of colloidal semiconductor nanocrystals with dominant polar surfaces produced through this strategy open a new avenue for improving their efficiency in catalysis, photocatalysis, gas sensing and other applications.