Electrochemical cathodic treatment: A non-destructive way to hydrogenate conductive ultrananocrystalline diamond films

Abstract

The excellent conductivity of nitrogen-doped ultrananocrystalline diamond (UNCD) films mainly originates from sp 2 -bonded carbon located in the grain boundaries. But conventional hydrogen plasma treatment to achieve surface hydrogenation would preferentially etch sp 2 -bonded carbon and deteriorate the conductivity. In this article, an electrochemical cathodic treatment (ECT) was developed as an alternative method to hydrogenate the surface of nitrogen-doped conductive UNCD films. The treatment was done in acidic solution at À30 V for 10 min. X-ray photoelectron spectroscopy revealed that, after the ECT, the photoelectron subpeaks related to carbon-oxygen bonding significantly decreased and there was no obvious variation in the content of sp 2 -bonded carbon. Compared to pristine UNCD (P-UNCD) films, the hydrogenated UNCD (H-UNCD) films exhibited a marked change in electrochemical responses such as cyclic voltammetry, capacitance-voltage and electrochemical impedance spectroscopy, suggesting the different surface termination between two UNCD films. More importantly, Raman spectra, scanning electron microscopy and Hall-effect measurement, revealed that no obvious variations in atomic bonds and surface morphology as well as electrical properties existed before and after the ECT. All of these results demonstrated that ECT can be a non-destructive surface hydrogenation method for nitrogen-doped conductive UNCD films with a high content of sp 2 -bonded carbon atoms.

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“…Consequently, there are almost no C–O bonds observable in the CPVG C 1s data. The two peaks located at 284.5 and 285.5 eV may be attributed to C sp 2 and sp 3 bonds, respectively . By contrast, the C 1s of CP indicates that the CP is mainly composed of C sp 2 bonds (peak at 284.5 eV) and C sp 3 bonds (peak at 285.5 eV), with peaks at higher binding energies resulting from C–OH, C–OH, and COOH (as shown in Table S3).…”

“…The two peaks located at 284.5 and 285.5 eV may be attributed to C sp 2 and sp 3 bonds, respectively. 38 By contrast, the C 1s of CP indicates that the CP is mainly composed of C sp 2 bonds (peak at 284.5 eV) and C sp 3 bonds (peak at 285.5 eV), with peaks at higher binding energies resulting from C−OH, C−OH, and COOH (as shown in Table S3). 39 Examination of the C 1s data for CP and CPVG after 50 cycles, shown in Figures 3i and 3j, reveals peaks at 286.7 and 286.5 eV, which may be attributed to C−O or C−N functional groups; the peaks at 289 and 288 eV were assigned to the C O based groups; and the peaks at 290.2 and 289.6 eV represent the −COO− or CO 3 2− based groups.…”