Structural, morphological, optical and electrical properties of MnO2/NiO bilayer thin films synthesized by spin coating method

Abstract

Nickel oxide (NiO) and manganese dioxide (MnO2) are two technologically significant metal oxides showing p-type and n-type semiconducting behaviors, respectively. In this work, NiO thin films were deposited onto glass substrates using the sol-gel spin coating technique. NiO films were deposited at 3000, 3500, and 4000 rotation per minute (rpm). For the formation of MnO2/NiO bilayer composite films, MnO2 ¬films were deposited using the NiO thin films as the substrates. The as-deposited films were pre-heated at 250 ℃ for 10 min before annealing in the air at 600 ℃ for 1 hour. The film thickness was controlled via rpm variation during spin coating and the rpm was optimized observing the quality of the films. Field emission scanning electron microscopy analysis of NiO films revealed that the film surface contained agglomerated nanoparticles. The shape of the particles changed from spherical to uniformly distributed cubic particles in the MnO2/NiO film. Compositional analysis of NiO and MnO2/NiO was performed by the Energy dispersive X-ray analysis. From the X-ray photoelectron spectroscopy (XPS) investigation the oxidation state Ni2+ of NiO single-layer thin films and composite thin films Mn4+ and Ni2+ are identified by the survey analysis of XPS. The structure of the thin film was explored using X-ray diffraction (XRD) technique. The NiO film prepared at 3000 rpm showed XRD peaks for the (002) plane related to the hexagonal Ni2O3 structure. NiO films prepared at 3500 and 4000 rpm show peaks for (111), (200), and (220) planes related to face-centered cubic NiO structure. The MnO2/NiO film showed the existence of a (211) peak related to the α-MnO2 crystalline phase along with the (200) and (220) peaks for the NiO substrate. The crystallite size was found from 59 to 25 nm for NiO and from 60 to 15 nm for MnO2/NiO thin films, respectively. The least crystallite size was estimated for the films prepared at 4000 rpm. UV-vis spectroscopy analysis was carried out to record the transmittance data in the spectral range of 300 - 1100 nm. In the visible region of light, the maximum transmittance of the NiO films was found 98%, whereas the highest transmittance dropped to 54% for MnO2/NiO films in the NIR region. The optical band gap was estimated in the range of 3.73 to 3.77 eV for the NiO films and 2.56 to 2.70 eV for MnO2/NiO films. Photoluminescence spectra of NiO and MnO2/NiO films showed the peaks related to near-band-edge emission, green emission for oxygen vacancies or Ni interstitials, and yellow-orange emissions for the naturally deeper trap states. Room temperature electrical resistivity enhanced for the MnO2/NiO thin films in comparison with the NiO films. Activation energies were computed for NiO and MnO2/NiO thin films.