3D self-assembled polar vs non-polar NiO nanoparticles nanoengineered from turbostratic Ni3(OH)4(NO3)2 and ordered β-Ni(OH)2 intermediates

A surfactant-free ammonia and carbamide precursor-modulated engineering of self-assembled flower-like 3D NiO nanostructures based on ordered β-Ni(OH)2 and turbostratic Ni3(OH)4(NO3)2 nanoplate-structured intermediates is reported. Employing complementary structural and spectroscopic techniques, fundamental insight into structural and chemical transformation from intermediates to NiO nanoparticles (NPs) is provided. FTIR, Raman and DSC show that the transformation of intermediates to NiO NPs goes through subsequent loss of NO3− and OH− species, through double step phase transformation at 306 and 326 °C, corresponding to free interlayers ions and H2O species loss, followed by loss of chemically bonded OH− and NO3− ions. Transformation to NiO NPs via ammonia route proceeds as a single phase-transition, accompanied with loss of OH− species at 298 °C. The full transformation to NiO NPs of both intermediates is achieved at 350 °C by annealing in the air atmosphere. Ammonia derived NPs keep the nanoflower morphology by self-assembly into nanoplates, enabled by H2O mediated adhesion on the NiO NPs {100} neutral surfaces. Structural transformations of turbostratic Ni3(OH)4(NO3)2 nanoplates result in formation of NiO NPs dominantly shaped by inert polar OH terminated (111) atomic planes, leading to loss of initial self-assembled 3D structure. DFT calculations support these observations, confirming that H2O adsorbs dissociatively on polar {111} surfaces, while only physisorption is energetically feasible on {100} surfaces. NiO NPs obtained by two different routes have overall different properties: carbamide derived NPs are 3 times larger (15.5 vs 5.4 nm), possess larger band gap (3.6 vs 3.2 eV), and more Ni deficient. The intensity ratio of the surface optical (SO) modes to the transversal and longitudinal optical modes is ~ 40 times higher in the NiO NPs obtained from β-Ni(OH)2 compared to Ni3(OH)4(NO3)2-derived NPs. SO phonon lifetime is an order of magnitude shorter in NiO obtained from β-Ni(OH)2, reflecting the much smaller NP size. The choice of the precursor defines the size, morphology, crystallographic surface orientations and band gap of the NiO NPs, with Ni deficiency providing pathways of utilizing them as p-type material, and allows precise nanoengineering of polar and neutral surfaces dominated NiO NPs, of exceptional importance to the use in catalysis.