Influence of varying the nickel salt aqueous subphase on the formation of Ni3(hexaiminotriphenylene)2 metal–organic framework nanosheets at the air/liquid interface

Abstract

Metal–organic framework (MOF) nanosheets synthesized at the air/liquid interface exhibit properties, such as electrical conductivity, that are highly dependent on their structural attributes, including morphology, lateral dimensions, thickness, crystallinity, and orientation. Achieving precise control over these features, however, remains a significant challenge. Extending our previous works on the air/liquid interfacial synthesis of uniaxially oriented Ni₃(HITP)₂ nanosheets (HITP-Ni-NS), this study explores the profound influence of the metal salt counterion—a key parameter in MOF crystallization. We present a systematic investigation into how nickel acetate (Ni(OAc)₂), nickel chloride (NiCl₂), and nickel nitrate (Ni(NO₃)₂) precursors affect the resulting nanosheet morphology, thickness, crystallinity, and orientation. Our comparative interfacial syntheses demonstrate that variations in the counterion significantly impact crystal growth kinetics, leading to discernible differences in nanosheet architecture. Notably, while NiCl₂ and Ni(NO₃)₂ precursors result in the incorporation of unreacted HATP ligand stacks and subsequent nanosheet disorder, Ni(OAc)₂ consistently produces HITP-Ni-NS with the greatest thickness and maintains perfect alignment with a preferred ordered crystalline stacking orientation. These observed differences are attributed to variations in the HITP-Ni-NS crystal growth mechanism, likely mediated by the distinct pH of the nickel aqueous subphases. These findings highlight the critical role of the metal salt counterion in directing the growth and ultimately tailoring the functional properties of MOF nanosheets.