Tailoring the electrochemical properties of 2D-hBN via physical linear defects: physicochemical, computational and electrochemical characterisation

Monolayer hexagonal-boron nitride films (2D-hBN) are typically reported within the literature to be electrochemically inactive due to their considerable band gap (ca. 5.2–5.8 eV). It is demonstrated herein that introducing physical linear defects (PLDs) upon the basal plane surface of 2D-hBN gives rise to electrochemically useful signatures. The reason for this transformation from insulator to semiconductor (inferred from physicochemical and computational characterisation) is likely due to full hydrogenation and oxygen passivation of the boron and/or nitrogen at edge sites. This results in a decrease in the band gap (from ca. 6.11 to 2.36/2.84 eV; theoretical calculated values, for the fully hydrogenated oxygen passivation at the N or B respectively). The 2D-hBN films are shown to be tailored through the introduction of PLDs, with the electrochemical behaviour dependent upon the surface coverage of edge plane-sites/defects, which is correlated with electrochemical performance towards redox probes (hexaammineruthenium(iii) chloride and Fe2+/3+) and the hydrogen evolution reaction. This manuscript de-convolutes, for the first time, the fundamental electron transfer properties of 2D-hBN, demonstrating that through implementation of PLDs, one can beneficially tailor the electrochemical properties of this nanomaterial.


Methods:
All chemicals used were of analytical grade and were used as received from the manufacturer without any further purification. All solutions were prepared with deionised water of resistivity no less than 18.2 MΩ cm and were vigorously degassed prior to electrochemical measurements with high purity, oxygen free nitrogen. The tested solutions were 0. 5  Electrochemical measurements were carried out using an Autolab PGSTAT204 potentiostat. All measurements were conducted using a three-electrode system with a Pt-wire counter electrode, a silver chloride (Ag/AgCl) reference electrode and a 1 x 1 mm CVD-grown mono-layer 2D-hBN (hBN) on a SiO 2 wafer from Graphene Supermarket completing the circuit. The 2D-hBN samples were in contact with the electrolyte with an area of ca. 0.8 x 0.8 cm.
Raman mapping spectroscopic analysis was performed using a Thermo Scientific DXR Raman Microscope fitted with a 532 nm excitation laser at a low power of 6 mW to avoid any heating effects. Spectra were recorded using a 10-seconds exposure time for 10 accumulations in each point. To collect the map we used a step size of 10 × 10 µm, to collect a Raman profile between the region of 1100 and 2000 cm -1 . Scanning electron microscope (SEM) images were obtained using a JSM-5600LV (JEOL, Japan) model.
The purposeful modification or deliberate creation of defects upon the mono-layer films consisted of drawing/etching ca. 1 mm long-line across the surface of the electrode using a fine diamond scriber from Lattice Gear. Figure S1C and S1D shows SEM images of the modification of PLDs in SiO 2 /Si and 2D-hBN on SiO 2 wafer. Figure S1E shows the no-presence of 2D-hBN Raman peak at the newly created edges on a 2D-hBN electrode, and Figure S1F depicts the Raman spectra of the diamond scriber tip (diamond Raman peak 4 at 1332 cm -1 ), showing that there is no contamination on the samples from it.
The X-ray photoelectron spectroscopy (XPS) data was acquired using an AXIS Supra (Kratos, UK), equipped with an Al X-ray source (1486.6 eV) operating at 300 W in order to perform survey scans and 450 W for narrow scans. All X-rays were mono-chromated using a 500 mm Rowland circle quartz crystal X-ray mirror. The angle between X-ray source and The molecular geometry of 2D-hBN-NRs with fully-hydrogenated edge planes (fh-hBN-NRs) were optimized using the B3LYP/LANL2DZ functional and the results were compared with the mono-hydrogenated structure (mh-hBN-NR) ( Figure S2). As oxygen presence was detected in the XPS results, oxygen-passivation was also explored, bonding an oxygen atom to either an edge-plane nitrogen or boron atom in order to study both possibilities.
It is worth to mention that, in this study the effect of only one atom of oxygen is investigated which means that the obtained results can be maximized by increasing the number of oxygen atoms used for passivating the edge plane nitrogen and/or boron atoms of the nanoribbon. (S1A and S1B respectively) (Scan rate: 50 mVs -1 , vs. Ag/AgCl) using a SiO 2 wafer (no 2D-hBN) as an electrode. SEM image of a PLD-SiO 2 wafer (S1C) and PLD-2D-hBN electrode (S1D) being utilised towards voltammetric methods. Raman profiles of a PLD-2D-hBN electrode at its newly physical defects (after utilised towards voltammetric) (S1E) and diamond scriber tip with a typical diamond Raman peak 4 at 1332 cm -1 (F) showing the lack of 2D-hBN or diamond Raman peak in Figure S1E.