Electrochemical properties of UNCD thin films

Scientific Achievement

Boron-doped microcrystalline diamond film electrochemical electrodes have been known for some time to have superior properties because they span a potential window of ~ 4 ev in aqueous solutions compared, for example, to platinum electrodes with a potential span of ~ 1.5 ev. The reason is that diamond electrodes have a high overpotential for the evolution of both H2 and O2. The morphology of microcrystalline diamond films, however, is such that thicknesses of several microns are required to obtain pinhole-free films, which in any event do not conformally coat microtip electrode arrays required for certain biomedical and sensor applications. The recent development at Argonne of n-type-doped ultrananocrystalline diamond films displaying conductivities that rival those of boron-doped microcrystalline films suggests the possibility of using this unique material as electrochemical electrodes, which can overcome the limitations enumerated above.

We have recently demonstrated that nitrogen-doped ultrananocrystalline diamond (UNCD) thin films can in fact function as excellent electrodes. Electrically conductive UNCD films (~1 mm thick) were deposited on conducting Si and W substrates from CH4/N2/Ar gas mixtures using microwave plasma CVD. Such films are continuous even at thicknesses of ~100 nm. The grain size is 3 to 10 nm, and the grain boundaries are 0.2 to 0.5 nm wide (two carbon atoms). Nitrogen appears to substitutionally insert into the grain boundaries and the film concentration (~1020 atoms/cm3) scales with the N2 added to the source gas mixture up to about the 5% level. The nitrogen-doped films are void of pinholes and cracks, and electrically conducting due in part to the high concentration of nitrogen impurities and or the nitrogen-related defects (sp2-bonding). The films possess semi-metallic electronic properties over a potential range from at least -1.5 to 1.0 V vs. SCE. The electrodes, like boron-doped microcrystalline diamond, exhibit a wide working potential window, a low background current, and high degree of electrochemical activity for redox systems such as Fe(CN)6-3/-4, Ru(NH3)6+3/+2, IrCl6-2/-3, and methyl viologen (MV+2/+). More sluggish electrode kinetics are observed for 4-methylcatechol, presumably due to weak adsoption on the surface. Apparent heterogeneous electron transfer rate constants of 10-2 to 10-1 cm/s are observed for Fe(CN)6-3/-4, Ru(NH3)6+3/+2, IrCl6-2/-3, and MV+2/+ at films without any pretreatment.

Significance

The full significance of this result is realized when one considers that diamond is a highly biocompatible material. This result thus opens the door for a number of potential applications of UNCD as a biochemical electrode. UNCD should show several advantages over platinum electrodes (which lead to redox reactions) in a number of biological contexts, such as nerve stimulation. Work just starting in our group is a broad collaborative project funded by DOE biomedical engineering program to develop an artificial retina, with the key challenge being the development of biocompatible electrodes to stimulate ganglion and bipolar nerve cells in the retina in people suffering from retinitis pigmentosa without leading to deleterious redox reactions (UNCD has a high overpotential for O2, H2 evolution), degradation of the electrode, or damage to the nerve cells. UNCD may also be used as a hermetic coating for the retinal implant developed in this project.

Performers

D.M. Gruen, J.A. Carlisle, L.A. Curtiss, G. M. Swain (Michigan State University)

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