Spectroscopic Investigation of the Wettability of Multilayer Graphene Using Highly Ordered Pyrolytic Graphite as a Model Material

Abstract

We report the intrinsic water contact angle (WCA) of multilayer graphene, explore different methods of cleaning multilayer graphene, and evaluate the efficiency of those methods on the basis of spectroscopic analysis. Highly ordered pyrolytic graphite (HOPG) was used as a model material system to study the wettability of the multilayer graphene surface by WCA measurements. A WCA value of 45° ± 3° was measured for a clean HOPG surface, which can serve as the intrinsic WCA for multilayer graphene. A 1 min plasma treatment (100 W) decreased the WCA to 6°, owing to the creation of surface defects and functionalization by oxygen-containing groups. Molecular dynamics simulations of water droplets on the HOPG surface with or without the oxygen-containing defect sites confirmed the experimental results. Heat treatment at near atmospheric pressure and wet chemical cleaning methods using hydrofluoric acid and chloroform did not change the WCA significantly. Low-pressure, high-temperature annealing under argon and hydrogen reduced the WCA to 54°, close to the intrinsic WCA of HOPG. Raman spectroscopy and atomic force microscopy did not show any significant change for the HOPG surface after this treatment, confirming low-pressure, high-temperature annealing as an effective technique to clean multilayer graphene without damaging the surface. Time-of-flight secondary ion mass spectrometry indicated the existence of hydrocarbon species on the surface of the HOPG sample that was exposed to air for <5 min and the absence of these impurities in the bulk. X-ray photoelectron spectroscopy analyses of the sample surfaces after the different cleaning techniques were performed to correlate the WCA to the surface chemistry. X-ray photoelectron spectroscopy results revealed that the WCA value changed drastically, depending on the amounts of oxygen-containing and hydrocarbon-containing groups on the surface.

Low-Voltage Voltammetric Electrowetting of Graphite Surfaces by Ion Intercalation/Deintercalation

“…This result is in line with other studies that have shown graphite surfaces to be mildly hydrophilic with a contact angle of about 50ᵒ, 35,36,37 Although higher aqueous contact angle values have been reported in the literature, 38,39,40 variations are expected depending on the sample quality, measurement environment (including surface contamination which increases the contact angle) and droplet size. 35,36,41 Given the short timescale (<5 s) on which the droplet was deposited after cleavage of surface, 9 the value we report can be taken as the intrinsic CA of 1 mM NaClO4 at AM HOPG, and is in line with theoretical predictions. 11 % at +1.13 V, while the CA was reduced from 51ᵒ to 38ᵒ.…”

Understanding the intrinsic water wettability of graphite

“…53-65° and it adsorbs airborne hydrocarbons in the ambient air to minimize surface energy, i.e., appear more hydrophobic. 76,230,241 Similar results were also reported for monolayer graphene on copper and multilayer graphene on nickel 75,77 and the surface energy and wettability of freshly synthesized graphene on copper was found to be dependent upon exposure time to ambient air. 75 Lai et al attributed the change in graphene wettability to adsorption of both water molecules and hydrocarbons 63 and Nioradze et al demonstrated that HOPG electroactivity is significantly affected by organic impurities in water and air.…”

“…230 PG has a bubbly surface -corroborating Raman and XRD data that this sample is turbostratic with poor ABAB layer stacking -and step edges are not clearly defined. …”

“…Ashraf et al reported a similar oxygen value, 0.60 at%, for SPI-1 (comparable quality to ZYA) HOPG cleaved in air. 230 In comparison, carbon accounts for ca. 99.1 at% and oxygen accounts for ca.…”

“…The variability is unsurprising since oxygen is not intrinsic to graphite and adsorbs at both defect sites and on the basal plane. 230,241,317,341,342 Investigations on graphene doping show that molecular oxygen, organic molecules, and water vapor adsorb at edge sites, surface defects, and the basal plane in a manner which can also occur on graphite. [343][344][345] The amount of oxygen detected in the survey scan (Figure 32a) is ca.…”