Micro- and Nanocrystalline NiO Synthesized by Joule Heating and Thermal Oxidation Methods: A Comparative Study.

The high stability and intrinsic p-type nature of nickel oxide (NiO) make it an interesting material for modern oxide-based technology microdevices. Nowadays, the industry demands more sustainable, highly efficient, and low-energy-consumption synthesis routes as an alternative to conventional methods that commonly involve high temperatures for long times or complex chemical routes. In this work, a fast, low-cost, and energy-saving synthesis based on the Joule heating (JH) process has been employed for the fabrication of micro- and nanocrystalline NiO. The as-grown NiO samples have been investigated as a function of the growth parameters, and special attention has been paid to the differences and similarities between microcrystals grown by JH or vapor-solid (VS) thermal treatments. In particular, Raman spectroscopy reveals that the JH process results in a very reproducible and controllable NiO microcrystalline structure as compared to VS regardless of the fast oxidation process. Cross-sectional analysis of the NiO grown by JH confirms the presence of inner/outer regions with variable microstructure, composition, and physical properties as a function of the different oxidation conditions promoted during the JH process. The mechanisms underlying the JH process have been discussed and compared with those related to the VS method.