Tuning sp3/sp2 carbon ratio of heavily boron-doped diamond electrodes fabricated via hot-filament chemical vapour deposition with CO2 addition.

Boron-doped diamond (BDD) is an excellent electrode material for electrochemical sensors and both high sp3/sp2 carbon component ratios and high boron concentrations are significant factors for specific applications. Hot-filament chemical vapour deposition (HFCVD), which is a conventional technique for synthesising large and uniform BDD electrodes for industrial applications, presents challenges when synthesising films with the above characteristics. In this study, we demonstrate the stable long-term synthesis of heavily boron-doped polycrystalline diamond with a high sp3/sp2 ratio by adding CO2 gas and adjusting the H2/CH4/B(CH3)3/CO2 gas ratio during HFCVD. Adding an appropriate amount of CO2 gas not only reduces the sp2 carbon component but also improves crystallinity and increases growth rate while maintaining metallic conductivity despite a moderate decrease in boron concentration. Electrochemical measurements reveal that our BDD electrodes exhibit excellent characteristics comparable to those of high-standard BDD electrodes synthesised using microwave plasma-assisted CVD.