Preparation and characterization of boron-doped diamond powder - A possible dimensionally stable electrocatalyst support material

Electrically conducting diamond powder was prepared by coating insulating diamond powder (8-12 mu m diam, similar to 2 m(2)/g) with a thin boron-doped layer using microwave plasma-assisted chemical vapor deposition. Deposition times from 1 to 6 h were evaluated. Scanning electron microscopy (SEM) revealed that the diamond powder particles become more faceted and more secondary growths form with increasing deposition time. Fusion of neighboring particles was also observed with increasing growth time. The first-order diamond phonon line appeared in the Raman spectrum at ca. 1331 cm(-1) for deposition times up to 4 h, and was downshifted to as low as 1317 cm-1 for some particles after the 6-h growth. Electrical resistance measurements of the bulk powder (no binder) confirmed that a conductive diamond overlayer formed, as the conductivity increased from near zero (insulating, < 10(-5) S/cm) for the uncoated powder to 1.5 S/cm after the 6-h growth. Ohmic behavior was seen in current-voltage curves recorded for the 4-h powder between +/- 10 V. Cyclic voltammetric i-E curves for Fe(CN)(6)(3-/4-) and Ru(NH3)(6)(3+/2+) were recorded to evaluate the electrochemical properties of the conductive powder when mixed with a polytetrafluoroethylene binder. At scan rates between 10 and 500 mV/s, Delta Ep for both redox systems was high, ranging from 140 to 350 mV, consistent with significant ohmic resistance within the powder/binder electrode. Our results at this point suggest that the resistance is mainly due to poor particle-particle connectivity. Anodic polarization at 1.6 V vs Ag/AgCl for 1 h (25 degrees C) was performed to evaluate the morphological and microstructural stability of the conductive diamond in comparison with graphite and glassy carbon (GC) powders. The total charge passed during polarization was largest for the GC powder (0.88 C/cm(2)) and smallest for conductive diamond powder (0.18 C/cm(2)). SEM images taken of conductive diamond powder after polarization showed no evidence of microstructural degradation, while significant morphological and microstructural changes were seen for the GC powder. (c) 2005 The Electrochemical Society.