Impact of Grain-Dependent Boron Uptake on the Electrochemical and Electrical Properties of Polycrystalline Boron Doped Diamond Electrodes

Abstract

A combination of high-resolution electrical and electrochemical imaging techniques, in conjunction with cathodoluminescence (CL), is used to investigate the electrochemical behavior of oxygen-terminated highly doped polycrystalline boron doped diamond (BDD). The BDD has a dopant density approximately 5 x 10(20) atoms cm(-3), grain size ca. 5-40 microm, and thickness 500 microm. CL imaging demonstrates that boron uptake is nonuniform across the surface of BDD, and conducting atomic force microscopy (C-AFM) highlights how this impacts on the local conductivity. While C-AFM shows no evidence for enhanced grain boundary conductivity, two characteristic conductivity domains are found with resistances of ca. 100 kOmega and ca. 50 MOmega. With the use of scanning electrochemical microscopy (SECM), local heterogeneities are also observed in the electroactivity of the BDD surface, consistent with the two different types of conducting regions. Local currents of the magnitude expected for metal-like behavior are observed in some regions, suggesting degenerative doping of the grains (supported by CL studies). In other regions, slower electron transfer is apparent. However, even for the reduction of Ru(NH(3))(6)(3+), which occurs at potentials far negative of the flat-band potential for oxygen-terminated BDD, all areas of the surface show some electroactivity. This study highlights that the spatially heterogeneous conductivity and corresponding electroactivity of BDD are readily resolved using a combination of C-AFM, SECM, and CL.

Local Imaging of an Electrochemical Active/Inactive Region on a Conductive Carbon Surface by Using Scanning Electrochemical Microscopy

“…It has been reported that the current distribution is observed in SG-TC images for a BDD surface with a Ru(NH3)6 3+ mediator. [5][6][7] Our result is clearly different from the previously reported SECM images for a BDD substrate, because these current distributions for BDD substrates mainly reflect the bulk conductivity distribution, which is caused by the hole density difference derived from the boron concentration. Figure 5(C) indicates the results of line scanning, which results from changing the substrate potential by using 1 mM of the Fe 2+ mediator.…”

“…3,4 This SECM technique has recently been applied to the imaging of local electrochemical activity on carbon materials, such as borondoped diamond (BDD). [5][6][7] The Bard and Swain groups have reported the local electrochemical activity on low boron-doped diamond electrodes attributed to a crystal region and a grain boundary layer. 5 With BDD, the density of holes, which are major charge carriers in BDD, is different for each crystal facet because the boron uptake depends on the crystal growth facets.…”

“…5 With BDD, the density of holes, which are major charge carriers in BDD, is different for each crystal facet because the boron uptake depends on the crystal growth facets. 7,8 Since the hole density distribution affects the bulk conductivity in BDD, the current distribution is observed with a Ru(NH3)6 2+/3+ mediator, owing to a difference in electron transfer caused by the hole density. 5 9,10 An Fe(CN)6 3-/4-mediator exhibits high electron transfer rates for carbon surface conditions, such as the existence of an sp 2 edge plane.…”

Electrochemical oxidation of dihydronicotinamide adenine dinucleotide (NADH): comparison of highly oriented pyrolytic graphite (HOPG) and polycrystalline boron-doped diamond (pBDD) electrodes

“…Previous FE-SEM studies confirmed that lighter and darker areas correspond to less-doped (more charging) and moredoped (less charging) facets respectively. 56,59 Potentialresolved snap shots of electrochemical activity, from a series of images, at potentials of 0.5 V, 0.6 V and 0.7 V are shown in Figure 7 (b) (ii-iv). Close to the onset of the oxidation current (0.5 V), we begin to see the appearance of the more-doped facets on the electrochemical image, and as the working electrode potential is scanned positive, there is an increase in surface current, but particularly so in the more doped facets.…”

Intermittent‐contact Scanning Electrochemical Microscopy (IC‐SECM) as a Quantitative Probe of Defects in Single Crystal Boron Doped Diamond Electrodes

“…The thicker the film the larger the grain sizes [2]. This results in heterogeneous electron transfer (HET) rates that vary across the surface [5][6][7]. Moreover, non-diamond-like carbon (NDC) incorporation during synthesis can be exacerbated when grain boundaries are present, detrimentally affecting electrochemical performance; this is especially true of nc material [8].…”