Comparison of the Electrical, Optical, and Electrochemical Properties of Diamond and Indium Tin Oxide Thin-Film Electrodes

The electrical, optical, and electrochemical properties of an optically transparent diamond electrode are reported on and compared with those of a commonly used optically transparent electrode (OTE), indium tin oxide (ITO). The OTE was formed by coating a thin film (ca. 0.5−1 μm) of boron-doped diamond on quartz. The electrode possesses an electrical resistivity of 10 - 2 Ω cm, or less, and an optical transparency of ca. 55% between 300 and 900 nm. The properties were evaluated before and after 48 h exposure tests to different organic solvents (hexane, toluene, methanol, and dichloromethane) and aggressive aqueous solutions (1 M HNO 3 and 1 M NaOH) and 2 h electrochemical polarizations in the same aqueous media. In contrast to ITO, the electrical and optical properties of the diamond OTE were stable during all the aqueous solution and organic solvent soak tests and during 2 h electrochemical polarizations at maximum anodic and cathodic current densities of ≥5 mA/cm 2 . The electrochemical properties of the diamond OTE were also unaffected by soak tests or electrochemical polarization. For example, cyclic voltammetric Δ E p values of 86 mV for Fe(CN) 6 3-/4- and 67 mV for Ru(NH 3 ) 6 3+/2+ were observed prior to and remained unchanged after 1 h polarizations in strong acid and base. Electrochemical atomic force microscopy (ECAFM) was used to explore the diamond and ITO surface morphology during anodic and cathodic polarization. No structural changes were observed for diamond while extensive surface roughening was seen for ITO, consistent with microstructural alterations. Finally, the homogeneity of the diamond OTE's electrical properties was probed by conductivity-probe atomic force microscopy (CPAFM). It was observed that the film consists of isolated regions of high electrical conductivity separated by zones of lower conductivity.