Spinel manganese-cobalt-nickel oxide nanowire arrays with enhanced absorption of light: hydrothermal growth and optical properties

Manganese-cobalt-nickel transition metal oxide (Mn-Co-Ni-O, MCNO) nanowire arrays (NWAs) with spinel structure were synthesized on Al₂O₃ substrates via a facile hydrothermal process followed by heat treatment. The present work focuses on studying the variation of optical properties of MCNO NWAs as a function of their morphologies tuned by the reaction temperature. The reflection of tapered MCNO NWAs is a combination of antireflection due to their gradient-refractive-index profile and diffuse reflection arising from multiple scatting, depending on the nanowire size and density. Specifically, MCNO NWAs prepared at 100°C exhibited a remarkable enhancement in absorption (above 80%) over a wide wavelength range (300-1650 nm) due to their low average reflectance and transmittance. By using Tauc’s relation, two band gaps of MCNO NWAs were determined, which increased with increasing reaction temperature. Consequently, MCNO NWAs serve as both antireflection layers and absorbers, offering a promising approach to improve the performance of thermistor detectors.