Formation and characterization of ZnO films on zinc substrate by plasma electrolytic oxidation

Abstract

This paper presents the results of the investigation of plasma electrolytic oxidation (PEO) of zinc foil at constant potential of 480 V in a mixed solution of Na2SiO3 ⋅ 5H2O and KOH. Optical emission spectroscopy technique was used to obtain electron temperature and electron number density in microdischarges. The spectral line shape analysis of hydrogen Hβ line indicates the presence of two types of microdischarges during PEO characterized by electron number densities of around 1.6 × 1021 m− 3 and 2.2 × 1022 m− 3. The electron temperature of (3600 ± 300) K was estimated by measuring the relative line intensities of zinc atomic lines at 334.50 nm and 472.21 nm. Structural and optical properties of films formed by PEO were probed by scanning electron microscopy with energy dispersive X-ray spectroscopy, X-ray diffraction, diffuse reflectance spectroscopy and photoluminescence. The main constituents of ZnO films are Zn, Si and O, with clearly observed presence of wurtzite type ZnO structure. Photoluminescence measurements show that ZnO films have broad emission band in the visible region. With increasing excitation wavelength the maximum of PL emission spectra shifts to longer wavelengths, indicating the existence of various defects in obtained coatings (single and double ionized oxygen vacancies, zinc vacancies and oxygen interstitial defects).