Characterization of Dark-Colored Nanoporous Anodic Films on Zinc

Ryoya Masuda,Damian Kowalski,Taisuke Nozawa,Yoshitaka Aoki,Hiroki Habazaki,Sho Kitano

doi:10.3390/coatings10111014

Abstract

In this study, zinc is anodized at different voltages in 0.1 mol·dm−3 KOH electrolyte to form nanoporous anodic films. Dark-colored anodic films are formed at anodizing voltages ≤6 V, whereas colorless anodic films are developed at voltages ≥7 V. The anodic films formed at all voltages consist of crystalline ZnO, which was identified by X-ray diffraction and Raman spectroscopy. The Raman spectra of the dark-colored anodic films show the enhanced intensity of the LO phonon mode due to electric-field-induced Raman scattering, which may be associated with the presence of metallic Zn nanoparticles in the anodic films. Scanning electron micrographs and transmission electron micrographs of the cross-section of the dark-colored anodized zinc reveal the formation of two-layer porous anodic films with a highly rough metal/film interface. In contrast, nanoporous anodic films of uniform thickness with a relatively flat metal/film interface are formed for the colorless anodized zinc. The transmission electron microscopy (TEM)/energy dispersive X-ray spectroscopy (EDS) analysis suggested the presence of zinc nanoparticles in the dark-colored anodic films. The non-uniform anodizing and the formation of metal-nanoparticle-dispersed porous anodic films cause the formation of dark-colored anodic films on zinc.

Highlights

Anodizing is an electrochemical process that forms nanostructured oxide surface films on a range of metals
The steady-state current density increased with increasing anodizing voltage up to 6 V; for anodizing voltages ≥7 V, the final current densities after anodizing for 30 min were similar
The similarity of the (002) lattice spacing at all anodizing voltages in this study suggests the little introduction of oxygen vacancies in the anodic films on zinc

Summary

Introduction

Anodizing is an electrochemical process that forms nanostructured oxide surface films on a range of metals. Anodizing techniques to form nanostructured surfaces have been extended to a range of metals in the last two decades, such as iron [8,9,10], cobalt [11], nickel [12,13], zinc [14], zirconium [15], niobium [16], and tantalum [17] Utilizing organic electrolytes such as ethylene glycol and glycerol, containing fluoride and a small amount of water is one of the keys to form thick self-ordered nanoporous or nanotubular anodic films on a wide range of metals.

Methods

Results

Conclusion