Electrochemical Performance of Carbon-Nanotube-Supported Tubular Diamond

Tubular diamond structures with high surface areas are very desirable for various potential electrochemical applications. Here, we report a simple and cost-effective two-step method for the synthesis of a diamond tube with a porous tube wall from carbon nanotube (CNT) hollow fibers via pulsed laser annealing (PLA) and hot filament chemical vapor deposition (HFCVD). These diamond tubes exhibit high double-layer capacitances of 11.65-18.07 mF cm(-2), three orders of magnitudes higher than the equivalent flat diamond films. Scanning electron microscopy (SEM) shows the presence of diamond microspheres composed of both micro- and nanocrystallites on the entire tube after 3-6 h HFCVD. The number density of the diamond, the average size of diamond microspheres, and the nanocrystallite content on the microspheres can be controlled by HFCVD time and laser annealing parameters of CNT hollow fibers. The electron back-scattered diffraction analysis shows the crystallographic orientation of the prepared diamond along the < 101 > plane. Raman spectra show a sharp characteristic/signature diamond peak at similar to 1332 cm(-1), corresponding to an unstrained high-quality diamond. The magnificent electrochemical performances of these CNT-supported diamond tubes are explained by their significantly enhanced electroactive surface area and the presence of a very small fraction (0.73-1.03%) of sp(2) carbon in diamond tubes for electron conduction. The density of states, band gaps, and outmost quantum capacitance (similar to 200 mu F/cm(2) at -2.2 V electrode potential) of the tubular diamond are calculated by the density functional theory calculations, which support our experimental findings and suggest its future potentiality as an efficient supercapacitor electrode material.