Noncovalent interactions of surface adsorbed species control the self-assembly of calcinated nickel oxide nanoparticles

Utilizing state of the art diffraction, imaging and spectroscopic techniques in conjunction with two-dimensional correlation analysis, we provide novel in-depth insights into the physics and chemistry behind the different tendencies towards self-assembling in NiO nanoparticles as function of their surface facets. We demonstrate substantially different temperature dependence of the spectroscopic behavior of the two types of NiO NPs, polar versus non-polar faceted. Temperature-dependent spectroscopy data for NiO NPs obtained by the ammonia route are consistent with the process in which high amount of water molecules that take part in hydrogen-bonding interaction with the surface-adsorbed non-dissociated water molecules on the neutral (100) planes are lost during the thermal treatment and attached back upon cooling. Interactions between water molecules adsorbed on two vicinal NiO NPs are responsible for keeping the self-assembly of the Ni(OH)2 NPs upon heat treatment. In carbamide-based NiO NPs, the self-assembly of initially formed Ni3(OH)4(NO3)2 NPs is not preserved. These NPs are terminated with polar (111) atomic planes, on which water molecules dissociatively adsorb, giving surface hydroxyl groups. As the hydrogen bonding proton - donating and accepting abilities are negligible at OH-polar terminated NiO NPs, only unfavorable inter-NP interactions are possible which leads to disruption of the NP assembly.