Diamond optically transparent electrodes: Demonstration of concept with ferri-ferrocyanide and methyl viologen

A new type of optically transparent electrode is reported on - an electrically conductive diamond thin film. The electrode was free-standing (0.38 mm thick and 8 mm in diameter), mechanically polished to a 7-nm rms roughness over a 10-μm linear distance, boron-doped (0.05% B/C in the reactant gas mixture), and mounted in a thin-layer transmission cell. The electrode has a short-wavelength cutoff of ∼225 nm, which is the indirect band gap of the material, and transmits light out to at least 1000 nm. In theory, the electrode has an optical window from 225 nm well out into the far-infrared, except for the boron acceptor band and the intrinsic multiphonon absorptions. The electrode was used to electrooxidize ferrocyanide to ferricyanide, and the absorbance change associated with the formation of the oxidized product (λmax = 420 nm) was spectroscopically monitored. The electrode was also used to electroreduce methyl viologen (MV2+) to the cation radical (MV+·) and the neutral (MV0). The depletion of MV2+ (λmax = 257 nm) and formation of MV+· (λmax = 398 and 605 nm) were spectroscopically monitored.