Effect of Surface Oxygen on the Wettability and Electrochemical Properties of Boron-Doped Nanocrystalline Diamond Electrodes in Room-Temperature Ionic Liquids

Abstract

This paper reports on how the surface chemistry of boron-doped nanocrystalline diamond (BDD) thin-film electrodes (H vs O) affects the wettability and electrochemical properties in two room-temperature ionic liquids (RTILs): [BMIM]­[PF6] and [HMIM]­[PF6]. Comparative measurements were made in 0.5 mol L–1 H2SO4. The BDD electrodes were modified by microwave or radio-frequency (RF) plasma treatment in H2 (H-BDD), Ar (Ar-BDD), or O2 (O-BDD). These modifications produced low-, medium-, and high-oxygen surface coverages. Atomic O/C ratios, as determined by X-ray photoelectron spectroscopy (XPS), were 0.01 for H-BDD, 0.08 for Ar-BDD, and 0.17 for O-BDD. The static contact angle of ultrapure water on the modified electrodes decreased from 110° (H-BDD) to 41° (O-BDD) with increasing surface oxygen coverage, as expected as the surface becomes more hydrophilic. Interestingly, the opposite trend was seen for both RTILs as the contact angle increased from 20° (H-BDD) to 50° (O-BDD) with increasing surface oxygen coverage. The cyclic voltammetric background current and potential-dependent capacitance in both RTILs were largest for BDD electrodes with the lowest O/C ratio (H-BDD) and smallest contact angle. Slightly larger voltammetric background currents and capacitance were observed in [HMIM]­[PF6] than in [BMIM]­[PF6]. Capacitance values ranged from 8 to 16 μF cm–2 over the potential range for H-BDD and from 4 to 6 μF cm–2 for O-BDD. The opposite trend was observed in H2SO4 as the voltammetric background current and capacitance were largest for BDD electrodes with the highest O/C ratio (O-BDD) and smallest contact angle. In summary, reducing the surface oxygen on BDD electrodes increases the wettability to two RTILs and this increases the voltammetric background current and capacitance.

Studies on tuning surface electronic properties of hydrogenated diamond by oxygen functionalization

“…Also, only a very weak van der Waals interaction exists between fully H-terminated diamond surface and water molecules that makes the surface to be hydrophobic [4]. However, when the surface consists of oxygen functional groups, the surface becomes polar and interacts with other polar medium like water [5].…”

“…One simple and effective way of modifying the band structure of diamond is to terminate the surface with functional groups which strongly controls the surface electronic properties such as work function, electron affinity (EA) and surface conductivity (SC). It is well known that the surface electronic properties of diamond are significantly influenced by different types of functionalization such as hydrogen, oxygen, fluorine, nitrogen and amine group [3][4][5].…”

Nanostructure Determines the Wettability of Gold Surfaces by Ionic Liquid Ultrathin Films

“…In Ref. Bhardwaj et al (2020). the authors highlight an increase in the contact angle of an imidazolium-based ionic liquid with increasing oxygen/carbon ratios of boron-doped nanocrystalline diamond (BDD) thin-film electrodes, as determined by XPS spectra.…”

“…the authors highlight an increase in the contact angle of an imidazolium-based ionic liquid with increasing oxygen/carbon ratios of boron-doped nanocrystalline diamond (BDD) thin-film electrodes, as determined by XPS spectra. They suggest that the alkyl side chain of the imidazolium ring of the cation might interact more strongly with the hydrogen rich surface due to hydrogen-bonding interactions, while the increased surface oxygen coverage, may orient the alkyl side chain away from the surface toward the bulk of the liquid (Bhardwaj et al, 2020). In our gold thin films the O/C ratio calculated by XPS spectra is quite similar: it is 0.4 for the evaporated and 0.5 for clusterassembled gold thin films.…”

“…The main features that determine ILs wettability, related to their chemical composition and to the properties of the solid surfaces, are barely studied. The cation interactions with the solid surface, in respect of its structure and functionalized group (Chang et al, 2017;Bordes et al, 2018), the anion nature (Pereira et al, 2015), the stoichiometry of the solid surface (Bhardwaj et al, 2020) and its polarity (Pereira et al, 2015), as also water content in the IL (Sedev, 2011;Delcheva et al, 2015) are just a few among the features that determine ILs wettability of a surface, and in particular the contact angle which derives from the minimization of the free energy connected to the interfacial tensions of the three interfaces: solid-vapor, solid-liquid, and liquid-vapor (Plechkova and Seddon, 2008).…”