Structure and electrochemical characterization of carbon films formed by unbalanced magnetron (UBM) sputtering method

Abstract

We previously reported nanocarbon films formed by the electron cyclotron resonance (ECR) sputtering method. The films contain a nanocrystalline structure consisting of sp3 and sp2 bonds with an extremely flat surface (Ra=0.07nm). The film also has a wider potential window than glassy carbon and superior electrochemical activity to boron doped diamond for certain species. However, ECR sputtering equipment is much more expensive than that used for conventional sputtering and requires a ring-shaped target. Therefore, it is difficult to use this method to develop new electrode materials such as metal-carbon hybrid film. Here, we describe a nanocarbon film electrode that we developed with a potential window and electrochemical activity equivalent to those of ECR nanocarbon films by using unbalanced magnetron (UBM) sputtering equipment. Our approach uses conventional equipment and has widely controllable sputtering conditions including a high sputtering rate, a large sputtering area and the capacity for co-sputtering multiple materials. The film can contain a maximum of 53% sp3 bonds by increasing the substrate bias voltage between the target and substrate, and also exhibits a potential window equivalent to that of the ECR nanocarbon film. However, the electrode surface is about one order of magnitude rougher than that of the ECR nanocarbon film due to the effect of reflected Ar+ ions caused by the fact that the target surface is facing the substrate surface. By employing transmission electron microscopy, we could observe nanocrystalline graphene structures in the UBM nanocarbon film, which are difficult to observe in conventional diamond-like carbon film. The electron transfer rate at the UBM nanocarbon film is similar to those of ECR nanocarbon film for Ru(NH3)63+ and Fe(CN)64−, suggesting that the nanocrystalline structure could contribute to a relatively fast electron transfer rate. The UBM nanocarbon films were successfully used for detecting kynurenic acid, which has a high oxidation potential and is difficult to detect with a conventional glassy carbon electrode.