Assessment of Acid and Thermal Oxidation Treatments for Removing Sp2 Bonded Carbon from the Surface of Boron Doped Diamond

Abstract

The presence of sp2 bonded carbon on a diamond or doped diamond surface, as a result of growth or processing, can affect material properties negatively, hence removal processes must be developed. Using boron doped diamond (BDD) we investigate the effectiveness of different removal methods via electrochemistry and transmission electron microscopy. We focus on two BDD surfaces, one processed by ns laser micromachining and the second which contains sp2 bonded carbon as a result of chemical vapour deposition (CVD) growth. After micromachining a layer of ordered graphite sits on the BDD surface, topped by fissured amorphous carbon (total thickness ~ m). Oxidative acid treatment at elevated temperature cannot remove all the sp2 bonded carbon and much smaller clusters of perpendicularly-orientated graphite (10’s nm), capped with a thinner layer of amorphous carbon – that we term “denatured graphite” – remain. In contrast, thermal oxidation in air at 600 oC is capable of all cluster removal, and can also be used to remove sp2 bonded carbon from CVD-grown BDD. Such understanding is important to any application where sp2 bonded carbon resulting from CVD growth or laser processing is detrimental for the intended application, e.g. in diamond quantum technology, photonics and electrochemistry.<br>

Design of Electrochemical Ozone Production Zero Gap Cells Based on Optimising sp2 Carbon Content in Boron Doped Diamond Electrodes

“…This process removes any weakly-attached sp 2 carbon introduced during laser machining and leaves behind a very robust form of sp 2 carbon. [25] Electrical contacts were added by sputtering a thin layer of Ti (10 nm) followed by a second layer of Au (400 nm) (Moorfield MiniLab 060 Platform Sputter system) onto either the contact tabs of the perforated electrodes or the back, lapped face of the patterned planar electrodes. The contact was annealed at 400 °C for 5 hrs in order to create an ohmic electrical connection.…”

“…As shown previously, the lasered BDD surface terminates in a very thin layer of graphite which itself is capped with an even thinner layer of amorphous sp 2 carbon. [25] To assess electrode stability and longevity the 70% sp 2 carbon EOP electrode was employed (vide supra). A calibration experiment was first run at a flow rate of 309 ± 4 mL min -1 and the cell was then continuously operated at a current of 0.3 A (current density of 0.297 A cm -2 ; voltage 9 -10 V) and a flow rate of 204 ± 2 mL min -1 for 20 hrs.…”

“…[43] The EELS spectra taken in the carbon layer (red box) identifies the carbon as being sp 2 bonded, due to the presence of a significant π* peak at ~285 eV (grey highlighted region in Figure 8d). [25] The broad shape of the σ* region, indicates it is amorphous carbon and not graphite. [43,44]; see comparison of In order to calculate the % of sp 2 carbon with respect to sp 3 carbon within the amorphous carbon layer, the "two windows" method was employed as described by Bruley et al [43] and described in SI.11.…”

“…It is well documented that ns laser micromachining of BDD (a popular method used to cut and structure freestanding diamond/BDD) results in a thin layer of sp 2 carbon introduced into the material surface where the laser has machined. [25,26] This remains integrated into the BDD surface even after extreme oxidative acid cleaning procedures. [25] In this way the ns laser can be used to both create perforated EOP electrodes and by changing the through-hole design, quantifiably control the amount of sp 2 carbon integrated into the BDD electrode.…”

“…[25,26] This remains integrated into the BDD surface even after extreme oxidative acid cleaning procedures. [25] In this way the ns laser can be used to both create perforated EOP electrodes and by changing the through-hole design, quantifiably control the amount of sp 2 carbon integrated into the BDD electrode. The lasering method also enables significantly higher fractional coverage levels of sp 2 carbon than using CVD alone.…”