Single-Orientation Nanoporous NiO Films: Spontaneous Evolution from Dense Low-Crystalline Ni(OH)_x Films

Nanoporous oxide semiconductor films with controlled crystal orientation are a challenge to form without the use of templates or epitaxial techniques. Here, we report novel nickel oxide (NiO) films with a [111] single-orientation nanoporous (111-SON) structure prepared from a nickel hydroxide (Ni­(OH)_x) precursor with a dense low-crystalline (DeLo) film structure. DeLo-Ni­(OH)_x precursor films were prepared on a substrate using a simple electrodeposition method from a Ni­(NO₃)₂ aqueous solution at room temperature. Scanning electron microscopy, X-ray diffraction, infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy were used to characterize the DeLo-Ni­(OH)_x. The obtained DeLo-Ni­(OH)_x precursor films were composed of Ni­(OH)_1.85–1.92(NO₃)_0.15–0.085·(0.80–0.98)­H₂O and had a [001]-oriented interstratified structure of α- and β-nickel hydroxide. Additionally, they exhibit a flat and amorphous-like film structure. Heat treatment of the DeLo-Ni­(OH)_x precursor films at 550 °C for 1 h in air led to the formation of SON-NiO films consisting of a myriad of [111]-oriented NiO grains (∼36 nm diameter) while maintaining the original film form. The decomposition process of the DeLo-Ni­(OH)_x and the growth process of [111]-NiO grains were elucidated by a series of ex situ measurements including thermogravimetric analysis. Optical and electrical characterization of the 111-SON-NiO films revealed the p-type energy band structure with a band gap energy of 3.7 eV and conductive paths in the vertical and horizontal directions.