Controllable synthesis of 3D Ni(OH)₂ and NiO nanowalls on various substrates for high-performance nanosensors

Large-area and uniform three-dimensional (3D) β-Ni(OH)₂ and NiO nanowalls were synthesized on a variety of rigid and flexible substrates via a simple aqueous chemical deposition process. The β-Ni(OH)₂ nanowalls consist of single-crystal Ni(OH)₂ nanosheets that were vertically grown on different substrates. The height, crystallinity, and morphology of the Ni(OH)₂ nanowalls can be readily modified by adjusting the reaction time and concentration of the NiCl₂ solution. The synthesis mechanism of the Ni(OH)₂ nanowalls was determined through heterogeneous nucleation and subsequent oriented crystal growth. 3D NiO nanowalls were obtained by thermal decomposition of the Ni(OH)₂ nanowalls at 400°C in Ar atmosphere. Highly sensitive, selective gas sensors and electrochemical sensors based on these NiO nanowalls were developed. The chemiresistive gas sensors based on the NiO nanowalls grown on ceramic substrates exhibited an excellent performance with low detection limit for formaldehyde (8 ppb) and NO₂ (15 ppb). The electrochemical sensor based on the NiO nanowalls grown on an FTO glass substrate had a superior selectivity to non-enzymatic glucose with a detection limit of 200 nM.