Transparent nanoporous P-type NiO films grown directly on non-native substrates by anodization

While electrochemical anodization has been used to form a number of nanostructured n-type semiconducting metal oxides for optoelectronic device applications, there exists a dearth of p-type metal oxide films that are solution processable. Herein, we formed p-type semiconducting NiO films by vacuum depositing Ni thin films on non-native substrates (transparent conductive oxide (TCO)-coated glass substrates and silicon wafers) using magnetron sputtering, and subsequently anodizing and annealing the Ni films. The Ni films were subjected to electrochemical anodization in diethylene glycol based organic electrolytes and subsequently annealed at 600 °C to form nanoporous NiO films with a pore size of ~ 20 nm. Runaway etching is a key issue in Ni anodization which was mitigated through the use of ice bath cooling and galvanostatic anodization. The choice of substrate is found to be critical to the resulting morphology owing to the differing surface roughness. Crystalline NiO is found to have formed from Ni(OH)₂ and NiOOH during annealing, and an additional NiSi layer is noted for NiO films on Si wafers. The bandgap of the NiO was estimated to be 3.5 eV. Electrochemical impedance spectroscopy and Mott–Schottky analysis confirmed p-type semiconducting behaviour, and enabled measurement of an acceptor density (N_A) of 2.85 × 10¹⁸ cm⁻³ and a flatband potential (V_FB) of 0.687 V versus Ag/AgCl.