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.