Thermal Stability of Thiolate Self-Assembled Monolayers on Copper Surface

The formation of self-assembled monolayers (SAMs) of organothiol is one of the excellent methods for corrosion protection. This work studies the thermal stability of thiolate SAMs coating on a copper surface. Three types of thiolate SAMs including 1-octanethiol (OTT), 2-ethylhexanethiol (2-EHT), and 2-phenylethanethiol (2-PET) are investigated. These chemicals are similar in terms of the chemical formula but different in chemical structure. Contact angle, AFM, FT-IR, XPS, and potentiodynamic polarization are used to analyze hydrophilic and hydrophobic features, roughness, decomposition of SAMs, and corrosion inhibition efficiency, respectively. The optimum condition of oxygen plasma treatment is determined. The results show that the optimum time for the treatment is 15 s. The oxygen plasma increases roughness of the Cu surface and induces the hydrophilic feature, which is suitable for SAMs to form on the Cu surface. The Cu surfaces coated by each SAMs are annealed at the temperature ranging from 25 to 250°C. The OTT is decomposed at 80°C while the 2-EHT is decomposed at 140°C. The 2-PET is not decomposed at 140°C, because the 2-PET consists of aromatic rings that are more stable than other functional groups in OTT and 2-EHT structures. These results also refer to improvement of thiolate bond stability aided by aromatic ring in the 2-PET molecule. All SAMs are completely decomposed at 250°C. In conclusion, the 2-PET is the most favorable in terms of thermal stability.