Characterization of the Intrinsic Water Wettability ... - ACS Publications

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Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles Andrew Kozbial,† Charlie Trouba,† Haitao Liu,‡ and Lei Li*,†,§ †

Department of Chemical & Petroleum Engineering, Swanson School of Engineering, ‡Department of Chemistry, and §Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States S Supporting Information *

ABSTRACT: Elucidating the intrinsic water wettability of the graphitic surface has increasingly attracted research interests, triggered by the recent finding that the wellestablished hydrophobicity of graphitic surfaces actually results from airborne hydrocarbon contamination. Currently, static water contact angle (WCA) is often used to characterize the intrinsic water wettability of graphitic surfaces. In the current paper, we show that because of the existence of defects, static WCA does not necessarily characterize the intrinsic water wettability. Freshly exfoliated graphite of varying qualities, characterized using atomic force microscopy and Raman spectroscopy, was studied using static, advancing, and receding WCA measurements. The results showed that graphite of different qualities (i.e., defect density) always has a similar advancing WCA, but it could have very different static and receding WCAs. This finding indicates that defects play an important role in contact angle measurements, and the static contact angle does not always represent the intrinsic water wettability of pristine graphite. On the basis of the experimental results, a qualitative model is proposed to explain the effect of defects on static, advancing, and receding contact angles. The model suggests that the advancing WCA reflects the intrinsic water wettability of pristine (defect-free) graphite. Our results showed that the advancing WCA for pristine graphite is 68.6°, which indicates that graphitic carbon is intrinsically mildly hydrophilic.

1. INTRODUCTION Graphitic materials are critical to many important applications including adsorbents,1−3 electrodes,4−7 and solid lubricants.8−10 These applications rely on the precise control of graphite surface properties, for example, adhesion, friction, surface energy, and so forth, and water wettability is one of the most important parameters characterizing these surfaces.4,11−16 Since 1940, water wettability of graphite has been studied extensively, and it has been well accepted that graphite is hydrophobic. Fowkes and Harkins conducted wetting experiments on naturally mined Ceylon graphite and observed a water contact angle (WCA) of 85.3°−85.9°, using the tilting plate method.11 Using indirect meniscus height and advancing meniscus method, Morcos observed a WCA of 84.2° and 83.9°, respectively, on exfoliated graphite.17,18 Subsequent researchers typically used the sessile-drop method, that is, the static contact angle, and reported a similar hydrophobic behavior for exfoliated graphite and highly ordered pyrolytic graphite (HOPG).19−22 Adamson and Gast and Raj et al. reported the advancing WCA on graphite to be 86° and 91°, respectively.23,24 All of these preceding reports concluded that graphite is hydrophobic. However, there have been conflicting reports. Work by Tadros, Hu, and Adamson in 1974 suggested that pure isotropic carbon [polished, cleaned with toluene and acetone, © 2017 American Chemical Society

and then degassed for 10−15 h at room temperature (RT)] has an advancing WCA of 63° (53 °C) and 65° (1 °C).25 Schrader reported that oriented graphite has a static WCA of 50° when exfoliated in air and 35° when exfoliated under ultrahigh vacuum (UHV).15 Five years later, Schrader reported that HOPG (ZYB) has a static WCA of 65° when exfoliated under RT UHV and 38° when exfoliated under high-temperature UHV.26 Because of the relatively complex experimental procedure (i.e., condensing water with a cold finger, conducting experiments under UHV, high-temperature bakeout, and ion bombardment on some samples), Schrader’s observations that graphite is hydrophilic were not well-accepted by the community. For example, Garcia et al. questioned the validity of taking contact angle data under vacuum because water evaporation will artificially lower the WCA.13,27,28 In 2013, Li et al. reported temporally dependent WCA on both HOPG and graphene synthesized using chemical vapor deposition (CVD). The static WCA of freshly exfoliated HOPG (SPI-2) was 64.4° and increased to 91.0° after 2 days of aging in ambient air.29 They also proposed that the graphitic surface is intrinsically mildly hydrophilic and that adsorption of Received: November 21, 2016 Revised: December 26, 2016 Published: January 10, 2017 959

DOI: 10.1021/acs.langmuir.6b04193 Langmuir 2017, 33, 959−967

Article

Langmuir Table 1. Literature Data on the Intrinsic Hydrophilicity of Graphite* author Tadros 197425 Schrader 197515 Schrader 198026 Luna 199944 Cao 201145 Li 201329 Ashraf 201432 Kozbial 201430 Kozbial 201431 Amadei 201433 Wei 201534 Marbou 201535 Li 201636 Aria 201637 Ondarçuhu 201638

fresh WCA 63°a 65°b 50°−80°a 35°b 65°a 38° 30° 10° 64.4° 53°a 45°b 64.4° 64.6° 68.2°a 68.2°b 61.8° 50°a 35°b 65° 69° 62.4°

sample

method

pyrolytic carbon

advancing captive bubble

oriented graphite ZYB graphite

sessile dropa,b cold fingerb sessile drop by cold finger

graphite HOPG SPI-2 SPI-1

scanning force microscopy nanodrops sessile drop sessile drop

SPI-2 SPI-2 HOPG ZYH (Mikro Masch) ZYB (Ted Pella)

sessile drop sessile drop sessile dropa advancing WCAb sessile drop microscopic; ESEM; advancing WCAa,b

SPI-2 ZYH ZYA

sessile drop sessile drop advancing WCA

notes 53 °Ca 1 °Cb aira heated vacuumb RT vacuuma heated vacuumb nanoscale drop graphene templating ambient air aira UHP argonb ambient air ambient air ambient air edge surface is more hydrophilic than basal plane fresh; vacuuma heated vacuumb ambient air tested after 30 min in air tested within 5 min of exfoliation

*

Data published as of December 23, 2016.

considered macroscopically hydrophobic (WCA ≫ 90°) and repel water, whereas rose leaves are also considered macroscopically hydrophobic (WCA > 90°) but adhere to water.39−41 This illustrates that both water adhesion/repulsion and contact angle cannot be qualified by the terms “hydrophilic” and “hydrophobic”. For the purposes of our study, we focus not on water adhesion/repulsion but on the angle that water makes with the surface. We agree with the 90° demarcation to generally classify hydrophilic and hydrophobic behaviors; however, for this investigation, we consider contact angles closer to 90° to be relatively hydrophobic and lower contact angles to be relatively hydrophilic. This important designation is not the absolute definition of hydrophilic/hydrophobic but one contact angle value relative to another. All of these preceding studies since 2013 have taken into consideration the cleanliness of the graphite surface and sought to control sample contamination by either testing in air immediately after exfoliation or conducting both exfoliation and testing under vacuum. This is a critical aspect that differentiates recent studies from those conducted earlier (with the exception of experiments conducted by Tadros et al.25 and Schrader15,26). When all data on graphite wettability are compiled, there are two regimes of values: those with WCA ∼90° and those with a substantially lower WCA. The cause of these two regimes is clearly related to the cleanliness of the sample surface,29,30,33,42,43 and it is critical that future investigations ensure that contamination is taken into account. The WCAs reported in Table 1, ranging from 45° to 69°, are inconsistent even after the contamination issue is taken into account. One possible reason for the inconsistency is the quality of the graphite samples. A variety of samples were used in the previous studiesvarying in sample quality (i.e., defect density), vendor, and manufacturing process. Another factor is the contact angle testing methods, that is, static versus dynamic contact angle test. Dynamic contact angles provide information on wettability as the liquid drop advances onto an unwet surface (θa) and recedes from a wet surface (θr). The advancing

airborne hydrocarbons renders the surface hydrophobic, based on attenuated total reflection Fourier transform infrared (ATRFTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), ellipsometry, and theoretical analysis.29−31 Since 2014, this behavior has been confirmed by several groups, as summarized in Table 1. Ashraf et al. systematically investigated HOPG (SPI1) wettability and reported a static WCA of 53° ± 5° when the freshly exfoliated graphite was exposed to air for less than 5 s. The WCA increased temporally as the graphite was exposed to air and plateaued at 86° ± 4° after 2 days. Furthermore, they devised a method to exfoliate HOPG in an ultrahigh purity argon environment and observed a WCA of 45° ± 3°.32 Kozbial et al. reported the static WCA of freshly exfoliated HOPG (SPI2) to be approximately 64.5°,30,31 and they showed that surface energy is the highest on freshly exfoliated HOPG and decreases temporally to reflect the surface energy of adsorbed contaminants.31 Amadei et al. tested static and advancing WCAs on HOPG (type unspecified) and reported 68.2° ± 1.6° for both. They also observed an increase in the WCA temporally and attributed the change to the adsorption of both hydrocarbons and water molecules.33 Wei and Jia reported the static WCA of freshly exfoliated HOPG (ZYH) to be 61.8° ± 3.3° on the basal surface, when tested within 5 min.34 Microscopic water drops were condensed onto fresh HOPG (ZYB) in an environmental scanning electron microscopecontrolled environment by Marbou et al., and they observed an advancing WCA of 50° on freshly exfoliated HOPG under vacuum.35 Li et al. and Aria et al. tested fresh HOPG and reported the static WCA to be 65° (SPI-2) and 69° (ZYH), respectively.36,37 Most recently, Ondarçuhu has showed the advancing and receding WCAs of HOPG (ZYA) to be 62.4° ± 0.9° and 60.2° ± 1.1°, respectively.38 At this point, it is important to define the meaning of hydrophilic and hydrophobic. Traditionally, 90° has been considered the demarcation between hydrophilic and hydrophobic behaviors; however, this is certainly relative to the system under investigation. For example, lotus leaves are 960


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Langmuir contact angle (θa) is the upper limit of wettability and the receding contact angle (θr) is the lower limit of wettability of a given solid surface. Therefore, the static contact angle (θs) can lie anywhere between the advancing and receding contact angles (θa ≥ θs ≥ θr) and is a function of chemical heterogeneity at the water contact line.46−48 Essentially, θs approaches θa as chemical heterogeneity decreases, and θs approaches θr as chemical heterogeneity increases. Contact angle hysteresis (θH = θa − θr) has traditionally been used to describe the barrier to contact line movement due to surface roughness and chemical heterogeneity.46,48−50 A perfectly homogeneous and smooth surface is characterized by zero hysteresis, although this is often not observed on real surfaces. 51−55 The WCA of pristine graphite can be experimentally evaluated using graphite samples with known defect density: surface defects such as step edges and point defects create a chemically heterogeneous surface. The highest quality HOPG sample commercially available (ZYA) has a stepedge coverage of 0.3% measured using atomic force microscopy (AFM).4 Using step-edge coverage and the static WCA of freshly exfoliated HOPG, the intrinsic WCA of pristine graphite can potentially be modeled using Cassie−Baxter equation56 cos θCB = f1 cos θ1 + f2 cos θ2

Table 2. Details of Graphitic Samples* sample ZYA ZYH SPI-1 (ZYA) SPI-2 (ZYB) PG

vendor

size (mm)

mosaic spread

exfoliation

momentive momentive SPI

12 × 12 × 2 12 × 12 × 2 10 × 10 × 1

0.4° ± 0.1° 3.5° ± 1.5° 0.4° ± 0.1°

tape tape tape

SPI

20 × 20 × 2

0.8° ± 0.2°

tape

graphite store

25.4 × 25.4 × 3.2

N/A

razor

*

Momentive performance materials was previously GE advanced ceramics, and the samples are analogous.

(1)

Equation 1 allows for the relationship between surface defects and wetting to be quantified; thus, intrinsic WCA (θCB) can be calculated. Nanodroplets of water were reported to aggregate at step edges (defect sites) on graphite with a WCA of 10°;45 therefore, we take this value to be the contact angle of defects on graphite. θCB can be solved by taking f1 as the fraction of defects ( fd; 0.3%), θ1 as the WCA of defects (θd; 10°), f 2 as the fraction of pristine graphite ( fg where fg = 1 − fd), and θ2 as the experimental WCA of graphite (θg). Solving for θCB yields the contact angle on a representative homogeneous surface comprising defects and basal plane. This yields θg = 64.6° when θCB = 64.5° (taken as the average of our work, refs 30 and 31, in Table 1). This seems to indicate that defects have a negligible influence on wettability, and static contact angle characterizes the intrinsic water wettability of pristine graphite. However, two factors need to be considered here. First, the measured contact angle of HOPG is very close to the contact angle of pristine graphite only when the defect density is very low. Second, depending on the way defect density is determined, the effective defect density impacting the contact angle measurement could be very different. In the current paper, we systematically studied static, advancing, and receding WCAs of freshly exfoliated graphite with very different qualities. The results show that graphite of different qualities (i.e., defect density) always has similar advancing WCAs but could have very different static and receding WCAs. This finding indicates that defects play an important role in measuring static contact angles. As a result, the static contact angle does not necessarily represent the intrinsic water wettability of pristine graphite. We propose a qualitative model explaining the interaction dynamics that occur at the water contact line on the graphite surface. On the basis of this model, advancing WCA reflects the intrinsic water wettability of a pristine (defect-free) graphitic surface.

Figure 1. Optical and AFM images of the graphite samples. Optical images are taken at 5× magnification, and the scale bar is 500 μm. AFM images are 5 × 5 μm with a color scale of 10 nm for HOPG (ZYA, ZYH, SPI-1, and SPI-2) and 500 nm for PG. optical and AFM images of the five graphite samples. AFM images of ZYA, ZYH, SPI-1, and SPI-2 show that the surface is primarily a smooth basal plane with few line defects, that is, less edges or ridges between basal planes: essentially a graphene step edge. Contrarily, AFM images of PG show a bubbly surface without any well-defined basal plane. The optical and AFM results indicate that PG is poor in quality compared with HOPGs. This is verified using Raman spectroscopy (see Supporting Information), which shows that the D peak is present for PG but absent for ZYA, indicating that a significantly greater level of Raman detectable defects are present on PG. Furthermore, PG has a single, red-shifted 2D peak, whereas ZYA has a double peak, indicative of high quality of HOPG (see Figure S1).57−59 The defect densities of ZYA and PG were quantified by analyzing the Raman spectra of each sample and using empirically derived equations that correlate defect density with Raman signals.60 The distance between the defects (Ld) is substantially larger for ZYA than for PG: approximately 116 and 13 nm, respectively. The defect densities of ZYA and PG are approximately 24 and 1778 defects/μm2, respectively. See Supporting Information for a detailed discussion on Raman data analysis.

2. EXPERIMENTAL SECTION Table 2 lists the details of the five graphite samples used in this study. ZYA, ZYH, SPI-1, and SPI-2 are high-quality HOPGs, whereas pyrolytic graphite (PG) is a low-quality graphite. Figure 1 shows the 961


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Langmuir These graphite samples of varying qualities were exfoliated, and the static, advancing, and receding contact angle measurements were taken immediately using a VCA Optima XE contact angle tester. The procedure is analogous to that reported in our previous work.30 HOPG samples were exfoliated using 1 in. Scotch tape, and PG was exfoliated by carefully cleaving with a razor blade. The static WCA was evaluated using a 2 μL deionized (DI) water drop (Millipore Academic A10 with total organic carbon below 40 ppb). Advancing WCA measurements were taken by recording continuous addition of water to a sessile drop (2 μL) for 5−8 s. Receding WCA measurements were taken by withdrawing the liquid from the drop for 8 s. The moment when the contact angle is maximum (minimum) is taken as the advancing (receding) contact angle: θa is the moment right before the drop width increases, and θr is the moment right before the drop width decreases.

roughness is not the cause for the low static WCA on PG (see Supporting Information for detailed discussions). This leaves the possibility that the low static WCA for PG is caused by chemical heterogeneity (defects). The static WCA for all of the graphite samples plateaued at 85°−90° after aging in ambient air for 1−2 days. The freshly exfoliated graphite surface is mildly hydrophilic with a high surface energy that attracts airborne hydrocarbons that adsorb onto the fresh surface, “shield” the graphite, and cause the sample to appear hydrophobic, with a WCA reflecting that of hydrocarbons instead of the subjacent graphite. This mechanism has been described in previous work.29,30,32,33,43,61−63 3.2. Dynamic WCAs. In addition to the static WCA, advancing and receding WCAs were also taken on freshly exfoliated graphite to elucidate how wetting behavior changes as water advances onto the unwet surface (θa) and recedes from the wet surface (θr). Figure 3 shows dynamic WCA data for the graphite samples, and Figure 4 shows hysteresis. The data are tabulated in Table 4.

3. RESULTS AND DISCUSSION 3.1. Static WCA. Static WCA taken on freshly exfoliated graphite is shown in Figure 2 and Table 3. All four HOPG

Figure 2. Static WCA of freshly exfoliated graphite. Data are tabulated in Table 3 and presented as average ± standard deviation. Figure 3. Dynamic WCA of freshly exfoliated graphite. Data are tabulated in Table 4 and presented as average ± standard deviation.

Table 3. Static WCA of Fresh Graphitic Surface Exfoliated with Tape and Razor* tape exfoliated ZYA SPI-1 (ZYA) SPI-2 (ZYB) ZYH PG

WCA

N

WCA

N

± ± ± ±

22 11 21 17

66.4° ± 1.9°

20

47.4° ± 2.7°

45

65.6° 65.3° 65.1° 65.0°

1.4° 1.8° 1.1° 1.2°

Figure 3 depicts several important details about wettability that can be correlated with the defect density of the sample surface. Analysis of Raman spectroscopy data (see Supporting Information) quantitatively shows that PG has substantially more defects than the HOPG samples; thus, our conclusions will be based on comparisons between the four HOPG samples

razor exfoliated

*

N indicates number of tests.

samples (SPI-1, ZYA, SPI-2, and ZYH) have similar static WCA values with an average of 65.3° ± 1.3° (N = 71). This value matches most of the previously reported data.29−31,33,34,36−38 Furthermore, as shown in Table 3, ZYA exfoliated with both adhesive tape and razor yielded similar WCA values, indicating that the method of exfoliation does not impact wetting. By contrast, freshly exfoliated PG has a static WCA of 47.4° ± 2.7°, which is substantially lower than that of HOPGs. Optical and AFM images (Figure 1) show that PG has a significantly rougher surface, and Raman spectroscopy indicates that PG has many more surface defects than HOPG. Both of these findings are consistent with the fact that PG is the lowest quality graphite sample used in this investigation and has the greatest defect density, whereas all four HOPG samples have a similarly low defect density. Wenzel analysis showed that surface

Figure 4. Hysteresis of freshly exfoliated graphite. Data are tabulated in Table 4 and presented as average ± standard deviation. 962


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Langmuir Table 4. Dynamic WCA and Hysteresis of Graphite Samples* advancing WCA (θa) ZYA SPI-1 SPI-2 ZYH PG

66.3° 70.7° 65.2° 66.0° 75.6°

(±2.2°) (±1.8°) (±7.7°) (±3.0°) (±3.7°)

static WCA (θs) 65.7° 66.1° 62.7° 65.4° 46.0°

receding WCA (θr)

(±1.2°) (±1.9°) (±2.1°) (±1.6°) (±3.6°)

56.3° 46.2° 47.0° 50.2° 18.7°

(±2.2°) (±3.4°) (±6.2°) (±2.6°) (±6.6°)

hysteresis (θH)

N

10.1° 24.5° 17.4° 15.8° 53.5°

6 6 26 6 16

(±3.2°) (±3.4°) (±11.4°) (±3.8°) (±15.8°)

*

Data are presented as average (±standard deviation). N indicates the number of tests. Data are separate from and not inclusive of data in Table 3.

Figure 5. Advancing contact angle mechanism. Upper figures show the side view and lower figures show the top view of a water drop. Colored lines represent the water contact line. Black area represents the hydrophobic surface, and solid circles represent hydrophilic defects. (a) Static water drop placed on the sample surface. Contact line interacts with both the surface and defects. (b) Intermediate state: contact line becomes pinned by the hydrophobic surface while the drop begins to advance onto hydrophilic defects as the liquid is added. Contact angle increases. (c) Advancing contact angle: contact line remains pinned by the hydrophobic surface and does not advance as water is added to the drop. The drop advances onto the unwet surface once the energy barrier is overcome. Note: schematics are not to scale; defects are orders of magnitude larger to highlight their impact on wetting.

collectively against the PG sample. We did not seek to discriminate between the differences in the four HOPG samples because there is no significant difference observed between the static and advancing WCAs for these very high quality samples. Although there are likely differences in the defect density, they are not reflected in the changes in the advancing or static WCA. First, Figure 3 shows that the advancing WCA remains relatively constant for all five samples and suggests that it is independent of sample quality, that is, defect density. Second, the static WCA is similar to the advancing WCA for the highquality HOPG samples but is markedly lower for low-quality PG. This indicates that static WCA is dependent on the defect density. Because all four HOPG samples are of very high quality, there are likely not enough surface defects to depress static WCA, potentially suggesting a threshold where the static WCA reflects the advancing WCA below a certain defect density. We did not test this and, of course, it should be systematically investigated before any further inferences can be made. Third, the receding WCA is significantly lower than the static and advancing WCAs for all five samples. Strikingly, the surface with the highest defect density has the lowest receding WCA, suggesting that the two parameters are inversely correlated. Ondarçuhu et al. reported θa to be 62.4° ± 0.9° on ZYA (Scientech), which matches well with our value of 66.3° ± 2.2° on ZYA (Momentive).38 The discrepancy can be attributed to the experimental procedure and sample differences. In fact, our data show slight differences between Momentive (ZYA and ZYH) and SPI (SPI-1 and SPI-2) brands. Receding contact angles show a very different trend, giving insights into the role of defects on the surface. The HOPG samples all have receding

contact angles of 10°−20° below the static WCA and hysteresis of approximately 10°−25°. The receding WCA of PG is substantially lower than that of HOPG: approximately 25° and 50°, respectively. Hysteresis of PG is 2−5 times greater than the hysteresis of HOPG. This emphasizes the role of defects on receding WCA.24,46 Furthermore, our data are consistent with previous reports, showing that the advancing contact angle is more consistent than the receding contact angle.24,38,46,64 This behavior is not a consequence of surface roughness (see section S1 for a detailed discussion), and the only other difference between the samples is defect density; therefore, our data indicate that increasing the defect density (i.e., chemical heterogeneity) will concomitantly increase hysteresis. 3.3. Mechanisms of the Advancing and Receding Contact Angles. Daniel Pease first suggested that the air− liquid−solid interface is one-dimensional in regard to wetting, meaning that only the interactions at the water contact line influence the wetting behavior. He concluded that advancing WCA is related to the greatest amount of work required to wet the solid surface and receding WCA is related to the least amount of work required to dewet the solid surface.65 More recent investigations have conclusively demonstrated that chemical heterogeneity under the water drop does not affect wetting and that both static and dynamic WCAs are affected only by interactions at the contact line.47,48,66,67 Using a micropatterned array of uniform defects on glass (i.e., hydrophobic defects on a relatively hydrophilic substrate), Larsen and Taboryski experimentally showed that the water contact line does not remain circular but becomes pinned at defect sites. Moreover, their data showed that the Cassie− Baxter model is inadequate when the water contact line is not uniform. The Cassie−Baxter equation will underpredict WCA 963


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Figure 6. Receding contact angle mechanism. Upper figures show the side view and lower figures show the top view of a water drop. Colored lines represent the water contact line. Black area represents the hydrophobic surface, and solid circles represent hydrophilic defects. (a) Static water drop placed on the sample surface. Contact line interacts with both surface and defects. (b) Intermediate state: contact line becomes pinned on the defect while the drop begins to recede in the hydrophobic area as the liquid is withdrawn. Contact angle decreases. (c) Receding contact angle: contact line remains pinned by hydrophilic defects and does not recede as water is withdrawn from the drop. Pinning causes the unsymmetrical contact line. The drop recedes onto the wet surface once the energy barrier is overcome. Note: schematics are not to scale; defects are orders of magnitude larger to highlight their impact on wetting.

contact with the droplet. Figure 5b depicts this intermediate state where an arbitrarily small amount of liquid has been added to the static drop. Continued addition of liquid causes the contact line to advance until complete pinning by the substrate and WCA is at a maximum, which is defined as the advancing WCA and reflects the wettability of the substrate sans defects (Figure 5c). Receding contact angle is exactly the opposite mechanism, where high energy defects inhibit the water flow toward the middle of the droplet (Figure 6). The aforementioned model explains why wetting behavior on PG is drastically different from that on HOPG. The HOPG samples all have very low defect densities, as shown by Patel et al.4 and our defect density calculations (see Supporting Information for the detailed discussion). This means that θs will be near, or equivalent to, θa because of the small number of defects along the water contact line. PG has substantially more defects than HOPG, which will decrease the static WCA. Receding WCA is most affected by defects, and its value represents a composite surface of defects and graphite, meaning that θr will decrease as the defect density increases;46 thus, θr is significantly lower for PG than for HOPG. Advancing WCA probes only the pristine graphite surface and is independent of defects; thus, θa reflects the intrinsic wettability of graphite. In this investigation, the average θa for all 60 advancing WCA tests shown in Table 4 conducted on graphite of varying defect densities (ZYA, SPI-1, SPI-2, ZYH, and PG) is 68.6° ± 7.1°. Taking a weighted average, which is the average of each sample normalized by the number of tests, of θa in Table 4 yields 68.6° ± 5.0°. Furthermore, excluding PG from the calculation gives a θa of 66.1° ± 6.3° (N = 44). The similarity of these values further illustrates that advancing WCA is independent of defects and probes the pristine surface. Our empirically calculated value for the advancing WCA of pristine graphite, 68.6° ± 7.1°, agrees well with the majority of previous data in Table 1. Most striking is the similarity between our value and the original work performed by Tadros et al. and Malcolm Schrader a few decades ago. Recent data reported within the past several years range from 53° to 69°, with an average of 63.6° ± 4.6° when exfoliated in air and calculated using both advancing and static WCA measurements. Again, this matches our findings and illustrates the repeatability of the

when the defect density is high and overpredict WCA when the defect density is low. They rectified this discrepancy by replacing the Cassie−Baxter area fraction (f1 and f 2 in eq 1) with line fractions describing the fraction of the water contact line interacting with the defects and the substrate.68 Raj et al. derived a thermodynamic model to describe the dynamic contact angle behavior on a chemically heterogeneous surface. They also showed that contact line pinning caused the failure of the Cassie−Baxter model and that wettability of defects in relation to the substrate would dictate how advancing and receding WCAs behave. Their model shows that at any defect density, the Cassie−Baxter equation will underestimate the advancing WCA and overestimate the receding WCA because contact line pinning is not taken into account. On a hydrophobic substrate with hydrophilic defects, advancing WCA is independent of defect density up to the defect packing limit (point where defects become interconnected). Contrarily, receding WCA is a strong function of defect density up to the defect packing limit. This means that, below the defect packing limit, advancing WCA reflects the intrinsic wettability of the substrate and receding WCA is a combination of both the substrate and defect wettability.46 Using the theory of dynamic contact angle behavior presented above coupled with our empirical data on graphite samples with varying defect densities, we propose a qualitative model based on the Gibbs three-phase contact line69 that describes the role of defects on wettability for a relatively hydrophobic solid surface, that is, graphite, with relatively hydrophilic defects. Briefly, the solid surface is initially unwet (dry) with a random distribution of intrinsic defects. A water droplet placed on the sample surface indiscriminately interacts with both surface defects and the basal plane along its contact line, as shown in Figure 5a, which is reflected as the static WCA. Because static WCA is a function of placement on the sample along with the relative fraction of defects and substrate along the contact line (which itself changes with the specific location on the sample), static WCA is generally not reproducible when comparing samples with different defect densities. This is likely the cause of inconsistencies in the reported static WCA data throughout the literature.70−78 Adding liquid to the static water droplet will cause the contact line to spread locally over the hydrophilic defects already in 964


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■ ■

data and the accuracy of our method for determining the intrinsic WCA of a pristine surface with and without defects. 3.4. Surface Energy. The surface energy of freshly exfoliated graphite can be calculated using the Fowkes model.79,80 Static and advancing diiodomethane (99%; SigmaAldrich) contact angles (DCAs) taken on freshly exfoliated HOPG (SPI-2) are 16.3° ± 3.7° and 35.3° ± 2.1° (N = 10). The static DCA matches our previous data on the same HOPG sample.31 The total Fowkes surface energy is 50.9 ± 4.7 mJ/m2. The polar and dispersive surface energy components are 9.0 ± 3.7 and 41.9 ± 1.0 mJ/m2, respectively. Comparing these values to our previously reported data, the total surface energy is slightly less when calculated using advancing contact angles compared with static contact angles.31 This is not surprising because advancing contact angles are higher, which results in a lower surface energy. There is still a significant polar component, which accounts for approximately 18% of the total surface energy of freshly exfoliated graphite. These results are consistent with previous calculations using static contact angles, although the values vary widely depending on the model used.31,35,36,81,82

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.6b04193. Wenzel analysis, Raman spectra of graphite samples, Raman spectra fitting results of ZYA and PG, Raman spectra peak centers of ZYA and PG, and defect calculations for graphite samples (PDF)

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ACKNOWLEDGMENTS

H. Liu thanks ONR (N000141512520) for partial support.

4. CONCLUSIONS Water wettability of freshly exfoliated graphite of varying qualities was studied using static, advancing, and receding WCA measurements. The results show that graphite samples of different qualities (i.e., defect density) have similar advancing WCAs, but they could have very different static and receding WCAs. The finding indicates that defects play an important role when measuring the static contact angle. As a result, static contact angle cannot represent the intrinsic water wettability of pristine graphite. A qualitative model, supported by experimental findings, is proposed to explain the effect of defects on measuring the static, advancing, and receding contact angles. Furthermore, this approach is not limited to graphitic surface and can be applied to any solid surface with defects. Based on the model, advancing WCA reflects the intrinsic water wettability of pristine (defect-free) graphite. Our results show that the advancing WCA for pristine graphite is 68.6° ± 7.1°, and its surface energy is 50.9 ± 4.7 mJ/m2 based on the Fowkes model.

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrew Kozbial: 0000-0001-9301-2315 Haitao Liu: 0000-0003-3628-5688 Lei Li: 0000-0002-8679-9575 Notes

The authors declare no competing financial interest. 965


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