Generation of plasmas in supercritical xenon inside microcapillaries for synthesis of diamondoid*

Diamondoids are series of \textit{sp}$^{3}$ hybridized carbon nanomaterials that could be applied in various fields such as pharmacy and optoelectronics. In our previous studies, higher order diamondoids were synthesized in supercritical fluid (SCF) plasmas in a batch-type reactor using adamantane (C$_{10}$H$_{16})$, the smallest diamondoid, as a precursor and seed. However the yield was low and the selectivity was difficult to control. We have developed a continuous flow SCF microplasma reactor that allows discharge volume and residence time to be adjusted. The electrodes consist of a tungsten wire inserted into a fused silica capillary and a sputtered silver outside of the capillary. We dissolved adamantane in supercritical xenon near critical point, and then generated DBDs inside the capillary using a nominal constant xenon flow rate of 0$\sim $2.3 mL min$^{-1}$. Micro-Raman spectra of the synthesized products show peaks that are characteristic of hydrocarbons possessing \textit{sp}$^{3}$ hybridized bonds while gas-chromatography/mass spectrometry spectra indicate the synthesis of diamantane (C$_{14}$H$_{20})$ and possibly isomers of diamondoids consisting of up to nine cages, nonamantane. It is suggested that this type of SCF microplasma reactor might be effective not only for synthesis of diamondoids, but also other nanomaterials.