5.1 STATIC WATER CONTACT ANGLE
The water contact angle of freshly exfoliated graphite was evaluated using a 2 µL water drop. 1- inch scotch tape was used to exfoliate the HOPG samples as detailed in our previous work.62, 70, 75-77 PG was exfoliated by carefully cleaving with a razor blade. Section 5.4 (page 114)
discusses the effect that method of exfoliation – adhesive tape versus razor – has on the graphite surface. WCA on fresh ZYA is the same whether the sample is exfoliated by adhesive tape or razor; therefore, method of exfoliation has no influence on the intrinsic WCA and data obtained by either method can be directly compared.
WCA data on fresh HOPG and PG samples is shown in Figure 17 and Table 11. The first observation is that all four HOPG samples, regardless of quality, have similar WCA on the fresh surface: average WCA of the four HOPG samples is 65.3° ± 1.3° (N=71). This is analogous to WCA on freshly exfoliated SPI-2 HOPG previously reported by our group (see Section 2.4 on page 18).76, 77 Interestingly, wettability is not affected by HOPG quality even though the lower quality samples (ZYH and SPI-2) have more step edges, and likely more point defects, than the highest quality samples (ZYA and SPI-1). Figure 1 (page 9) shows optical and AFM images of the graphite samples. This supports the concept that all four HOPG samples are high quality and vary only in the number of step edges and size of crystallite domains. Furthermore, sample
quality is quantified by mosaicity, which is a bulk parameter, while wetting is only sensitive to the sample surface. This data shows that wetting is not a function of mosaicity and is affected by surface chemistry at the uppermost layers.
WCA of freshly exfoliated PG is 47.4° ± 2.7° which is substantially lower than HOPG. Wenzel and Cassie-Baxter analysis was performed to elucidate the effects of surface roughness and chemical heterogeneity on the observed WCA. The influence of surface defects was investigated to determine if the observed mild hydrophilicity of fresh graphite is intrinsic to sp2- hybridized carbon or if the hydrophilic behaviour is due to surface defects. The subsequent analysis seeks to clarify the relationship between defect density and water wettability.
WCA for all of the graphite samples was ca. 90° after aging in ambient air for 1-2 days. Increase of WCA is attributed to adsorption of adventitious hydrocarbons in the surrounding air. The graphite surface is initially mildly hydrophilic with high surface energy and attracts hydrocarbons which adsorb onto the fresh surface, “shield” the graphite, and cause the sample to appear hydrophobic with a WCA about 90°. Essentially, the WCA test is probing the hydrophobic hydrocarbon layer opposed to the subjacent graphite.71, 235 This mechanism is described in Section 2.2 (page 12) and in our previous work on graphene and HOPG.75-77
Mangolini et al. used near-edge x-ray absorption fine structure (NEXAFS) spectroscopy and XPS to investigate the thickness and chemical nature of the adsorbed contamination layer on diamond and amorphous carbon. They reported that the contaminant layer has a thickness of 0.6 ± 0.2 nm and consists of 19 ± 3% sp2 carbon. Moreover, they report the ratio of adsorbed oxygen-to-carbon to be 0.11 ± 0.02.236 These findings are significant because they probe the contaminant layer and begin to elucidate the type of hydrocarbons adsorbing onto the graphite
(Figure 3 on page 15) using ellipsometry which is similar to the 0.6 nm obtained by Mangolini.76,
236
This temporal dependence of WCA has been well documented since 2013 with numerous studies showing that fresh graphite, graphene, TMDC, and REO surfaces are intrinsically more hydrophilic than expected.62, 70, 71, 76 The concept of adventitious hydrocarbon contamination is well documented on metals35-40 but only recently has been applied to graphitic surfaces. Table 12 shows published data (as of March 31, 2016) which experimentally report that fresh graphite is mildly hydrophilic. No researchers have yet published data refuting the intrinsic hydrophilicity of these materials since 2013. The ongoing debate includes the wetting transparency effect for 2D materials,111, 123, 237-239 the cause of intrinsic mild hydrophilicity,76, 77 and the degree in which hydrocarbon and water adsorption influence wettability.63, 64
Importantly, this concept is becoming better understood as more researchers realize the importance of hydrocarbon contamination on graphitic surfaces and take into account exposure time in air when conducting experiments and fabricating devices. Work by Tadros, Hu, and Adamson in 1974 was the first to suggest that isotropic carbon (polished, cleaned with toluene and acetone, then degassed for 10-15 hours at room temperature) had a WCA of 63°.32 Subsequent studies reported by Malcolm Schrader published in 1975 and 1980 further show that the WCA of graphite may be 35°;33, 34 however, experimental concerns were presented as
discussed in Section 2.1.92 Nevertheless, until 2013, only two reports published in 1999 and 2011 mentioned the hydrophilicity of graphite although they did not explicitly study the wettability of the fresh surface.99, 100 Within the past two years, researchers have begun taking hydrocarbon contamination into consideration and the WCA of fresh surfaces is being used for computer simulations and modeling. Most recently, Ramos-Alvarado et al. used the water
contact angle of clean graphitic carbon for calibrating the water-carbon interaction potential for molecular dynamic simulations investigating wetting transparency on silicon.240
Figure 17. Static WCA of freshly exfoliated graphite.
Table 11. Static WCA of fresh graphitic surface exfoliated with tape and razor. N indicates number of tests.
Tape Exfoliated Razor Exfoliated
WCA N WCA N ZYA 65.6° ± 1.4° 22 66.4° ± 1.9° 20 SPI-1 65.3° ± 1.8° 11 -- -- SPI-2 65.1° ± 1.1° 21 -- -- SPI-2 Ref. 76 64.4° ± 2.9° 7 -- -- ZYH 65.0° ± 1.2° 17 -- -- 40 45 50 55 60 65 70 75 PG n=45 ZYH n=17 SPI-2 n=21 ZYA n=22 S ta tic W C A ( de g) SPI-1 n=11 ZYA PG
Table 12. Literature data on the intrinsic hydrophilicity of graphite. Data published as of March 31, 2016.
Author Fresh WCA Sample Method Notes
Tadros 197432 63° - 65° Pyrolytic carbon Captive bubble -- Schrader 197533 50°-80° a 35° b Oriented graphite a, b Sessile drop b Cold finger a Air b Heated vacuum Schrader 198034 65° a 38° ZYB graphite
Sessile drop by cold finger
a RT vacuum b Heated vacuum
Luna 1999100 30° Graphite Scanning force
microscopy Nanoscale drop
Cao 201199 10° HOPG Nanodrops Graphene
templating
Li 201377 64.4° SPI-2 Sessile drop --
Ashraf 2014230 53° SPI-1 Sessile drop 45° cleaved in UHP
argon
Kozbial 201476 64.4° SPI-2 Sessile drop --
Kozbial 201475 64.6° SPI-2 Sessile drop --
Amadei 201464 68° HOPG Sessile drop --
Wei 2015241 61° (Mikro Masch) ZYH Sessile drop
Edge surface is more hydrophilic
than basal plane Marbou 2015242 b a 50° 35° ZYB (Ted Pella) a, b Microscopic; ESEM
a Fresh; vacuum b Heated vacuum
Li 201562 65° SPI-2 Sessile drop --
Aria 2016243 69° HOPG Sessile drop Tested after 30 min