Micro- and NanocrystallineNiO Synthesized by JouleHeating 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.