March's Paper of the Month

 Exposure to the environment can cause metallic materials to deteriorate but a 2D material coating may help protect against corrosion and oxidation. 

Exposure to the environment can cause metallic materials to deteriorate but a 2D material coating may help protect against corrosion and oxidation. 

Metals are a significantly important material for a range of different industries including oil, chemical, aerospace, pharmaceutical and medical. However, metallic components exposed to the environment are prone to corrosion and oxidation. 

Various methods have been implemented to protect metals from corrosion, including galvanising, painting and electroplating. Recent studies have been looking at the potential benefits of using 2D materials as protective coatings for metals.  For example, graphene, the first 2D material to be discovered, is highly impermeable to liquids, gases and chemicals. It is also only one atomic layer thick and therefore would not affect the morphology of the metal. Such qualities make it an attractive candidate for coating metals. 

The potential problem with graphene is the high electronic conductivity and the direct contact with the metal could create a galvanic cell which over time would cause degradation of the metal. 

As such other 2D materials have been investigated. 

Hexagonal Boron nitride (hBN) has been studied as a potential alternative as it has the same permeability as graphene and does not form a galvanic cell. 

This month’s paper of the month comes from the Technical University of Denmark from the Micro & Nanotechnology department. They compared the protective properties of graphene and hBN under two oxidation environments, one simulating an acute and one a long-term. 

The group grew both materials on copper through chemical vapour deposition. They then conducted a variety of experiments to analyse their capability as barriers to corrosion and oxidation. This included Raman spectroscopy, x-ray photoemission spectroscopy and X-ray induced auger electron spectroscopy. 

To simulate the short acute oxidative conditions, the samples were first heated from room temperature to 400°C for 45 minutes. The second experiment was an isothermal experiment where samples were held at 50°C for 60 hours to simulate long-term oxidative conditions. 
They conducted the heating of their samples inside a custom Linkam LTS600, which was used in conjunction with a Raman microscope. 

The results from the Raman spectroscopy indicated graphene to be an effective oxidation barrier in the acute oxidative environment. Between temperatures 150°C to 300°C hBN was less effective which is assumed to be due to the higher density of grain boundaries and wrinkles, which are known to induce faster oxidation of the copper substrate.

However above 300°C the oxidation of the graphene coat increased as measured by the increase in the Raman intensity of the copper oxide peaks that were larger than that for hBN. 

Results from the isothermal experiment showed the barrier properties of graphene were effective only in short periods. After being held for 9 hours in 50°C, the oxidation of graphene resulted in an increase in copper oxide peaks. The failings of the graphene coat were due to the galvanic cell formation.  

The x-ray photoemission spectroscopy and x-ray induced Auger electron spectroscopy results showed hBN to be a better coating under the long term oxidative conditions. At 9 hours, the material showed little to no oxidation. After 40 hours there was a detectable increase in Cu(OH)2 but this was negligible compared to the graphene coated sample. 

They also showed the main peak on the surface of the graphene sample was copper oxide and copper for the hBN sample. The lack of a measurable oxide peak in the hBN sample demonstrates its superior ability as a protective barrier under long term oxidative conditions. 

When discussing the role of the LTS600, Dr Luca Camilli explained, “The system ‘window + heater’ enables the experiment to be possible. The heater allowed us to reach the desired temperatures for the oxidation experiments, while the window allowed us to study the oxidation process through Raman spectroscopy. The laser used for Raman passes through the window, impinges on the sample and is reflected back to the detector, through the window. “

Their research highlights another great potential application for 2D materials which would be greatly beneficial for many different industries. 

By Tabassum Mujtaba

Galbiati, M. et al. Real-time oxide evolution of copper protected by graphene and boron nitride barriers. Sci. Rep. 7, 39770; doi: 10.1038/srep39770 (2017).