Enhanced electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond through oxygen treatment

The electrochemical capacitance of nitrogen-doped ultrananocrystalline diamond (N-UNCD) can be dramatically increased by treating the surface with an RF-oxygen plasma. Such treated surfaces display excellent properties for use as electrodes in neural stimulation and recording. In the present work, we elucidate the origins of this phenomenon by investigating the effects of different methods of oxygen termination. We found that the increase in electrochemical capacitance is dependent on the details of the method used for oxygen termination. Whilst N-UNCD subjected to UV/ozone treatment, oxygen plasma treatment, and furnace annealing in oxygen gas all displayed increased surface capacitance, the highest capacitance was exhibited by the oxygen annealed sample, with which we achieved ~ 3 orders of magnitude increase in the electrochemical capacitance as compared to the as-grown sample. The maximum recorded capacitance was 3746 ± 132 µF cm−2, which is substantially greater than previously reported N-UNCD electrodes’ electrochemical capacitance (1070 µF cm−2, W. Tong. et al, 2016). Our findings point to the presence of sub-surface solid state capacitance which contributes significantly to the observed electrochemical capacitance of the oxygen terminated N-UNCD electrodes. When combined with the favourable biocompatibility and inertness of the N-UNCD, our approach may provide a route towards the development of advanced neural sensing and stimulating electrodes.