Direct observation of oxide formation on copper in alkaline solution by TEM

The formation and structure of the passive oxide layers on copper and it's electrochemical behaviors in aqueous solution have attracted considerable interest in the past several decades for this material plays an increasingly important role in many new industrial fields, such as sensor, fuel cell, microelectronics and nanoelectronics industry. The oxidation processes and the structure of oxide layers on bulky copper surface during anodic oxidation in the aqueous solution have been extensively studied using electrochemical and surface analytical techniques. However, there are still many different opinions with regard to the formation of oxide layers on the copper surface during the electrochemical oxidation and reduction in alkaline solution. In this paper, we demonstrate a controllable and reproducible method to fabricate electron transparent nanoneelde shape specimens for transmission electron microscopy (TEM) analysis. A perfect ultrasharp copper nanoneedle specimen with diameter of several tens nanometers in tip has been fabricated successfully utilizing an argon ion beam milling technique. This proposed sample preparation procedure is applicable to a wide variety of sharp needle shape samples for TEM observation and other associated characterization method such as scanning transmission electron microscopy (STEM), electron energy-loss spectrometry (eELS) and energy dispersive X-ray spectroscopy (eDX). Using the fabricated copper nanoneedle specimen, the structure of oxide layers formed on pure copper surface after electrochemical oxidation in different limited anodic potentials in 0.1 M NaOH solution was investigated by TEM observation directly. When anodic potential sweeping end to lower potential (-0.25V), a poorly crystallized and epitaxial Cu₂O layer was observed to form on the copper matrix. After electrochemical oxidation at higher anodic potential (0.40V), it is evident that only the stratification of Cu (iI) species were found on the copper surface. The Cu (iI) layers consist of a well crystallized and epitaxial CuO baselayer, a polycrystalline Cu(OH)₂ outer layer with random orientation, and an outermost scattered deposit of the finger-like crystalline Cu(OH)₂ nanoneedles, here, the copper hydroxide was presumably formed by a dissolution-precipitation mechanism.