Kévin Croué

The total surface free energy of nanometric particles in contact with a liquid electrolyte can be easily accessed through potentiometric measurements. This requires a special though simple designed set-up, and a careful chemical and textural characterization of the powders. This is here first exemplified for non-stoichiometric NiO, in contact with Ni²⁺ aqueous solutions, which total surface free energy was evaluated to 0.34–0.36 J/m² for monolithic particles, independently of the level of non-stoichiometry. Much larger values are systematically observed for mosaic powders; hence indicating a large contribution of the solid-solid inter-crystallites interfacial energy or strains.

The determination of the specifically surface of thin films is crucial in many cases, but it can be problematic for complex surface (no geometric shapes). This work presents a method employed for the calculation of the specifically surface of thin films using atomic force microscope data. After the presentation of the mathematical model and how the program performs, we use simple geometric surfaces to discuss the coherence and the efficiency of our method. In the second part, we have experimentally measured the specifically surface of two samples. The specifically surface of the first sample (consisting in easily geometric shapes) has been calculated by our program and results have been confirmed by another mathematical approximation calculation. For the second sample its important roughness was not suitable with the approximation whereas coherent results were only obtained with our method.

The isothermal air heating (320 °C) of monolithic nanometric Ni_(1−x)O (~4 nm, 7% Ni³⁺) particles results in their sintering and concomitant loss of excess oxygen, leading to almost stoichiometric monolithic larger particles (~7 nm, 0.5% Ni³⁺). By coupling crystallographic, thermal, textural and chemical analyses of the powders along heating, we could demonstrate that their evolutions in structure, texture and chemical composition are intimately linked and non-monotonous. First, while the particles/crystallites progressively coalesce and increase in size, their level of crystallographic strains increases until a saturation step in oxygen is reached (8 h). With time, this excess in oxygen is then progressively released, as well as the associated strains, and then the particles size and specific surface area stabilize (13 h). Beyond 13 h, the level of strains still monotonously decreases while a transient gain in oxygen/weight and a temporary decrease in crystallite size are observed before the final monolithic powder is produced (34 h).