Water Protects Graphitic Surface from Airborne Hydrocarbon

Water Protects Graphitic Surface from Airborne Hydrocarbon Contamination Zhiting Li,†,⊥ Andrew Kozbial,‡,⊥ Nikoloz Nioradze,† David Parobek,† Ganesh Jagadeesh Shenoy,† Muhammad Salim,† Shigeru Amemiya,† Lei Li,*,‡,§ and Haitao Liu*,† †

Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States Department of Chemical & Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States § Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States ‡

S Supporting Information *

ABSTRACT: The intrinsic wettability of graphitic materials, such as graphene and graphite, can be readily obscured by airborne hydrocarbon within 5−20 min of ambient air exposure. We report a convenient method to effectively preserve a freshly prepared graphitic surface simply through a water treatment technique. This approach significantly inhibits the hydrocarbon adsorption rate by a factor of ca. 20×, thus maintaining the intrinsic wetting behavior for many hours upon air exposure. Follow-up characterization shows that a nanometer-thick ice-like water forms on the graphitic surface, which remains stabilized at room temperature for at least 2−3 h and thus significantly decreases the adsorption of airborne hydrocarbon on the graphitic surface. This method has potential implications in minimizing hydrocarbon contamination during manufacturing, characterization, processing, and storage of graphene/graphite-based devices. As an example, we show that a water-treated graphite electrode maintains a high level of electrochemical activity in air for up to 1 day. KEYWORDS: graphite, graphene, cleaning, water adsorption, contamination, spectroscopy, cyclic voltammetry bic;21−23 even the debate about the intrinsic electrochemical activity of the graphite basal plane was related to the surface cleanliness of graphite.24−26 Therefore, understanding and controlling the airborne hydrocarbon contamination on graphitic materials is critical to their fundamental study and practical application, in general. A thorough and systematical investigation on the hydrocarbon-free graphitic surface has been restricted by rapid hydrocarbon adsorption onto the pristine surface. For example, our recent studies demonstrated the kinetics of airborne hydrocarbon adsorption on a highly oriented pyrolytic graphite (HOPG) surface, which becomes saturated within 10−15 min upon air exposure after exfoliation.1 Such a short time scale inevitably limits further characterization targeting the intrinsic properties of the graphitic surface. Maintaining a clean graphitic surface will require either a hydrocarbon-free environment,

I

t has been recently discovered that many graphitic surfaces, such as graphene and graphite, are mildly hydrophilic, and the previously observed hydrophobicity is actually due to contamination from airborne hydrocarbons.1−7 The airborne hydrocarbon contaminants are mostly composed of biogenic and anthropogenic alkanes, alkenes, alcohols, and aromatic species, which are associated with plant growth, fuel combustion, and the chemical industry.8−12 Although the concentration of such hydrocarbons usually ranges from parts per trillion to parts per million levels,13 they still demonstrate a profound impact on the surface wettability of graphitic materials because wettability is extremely surface sensitive. Given that wettability is one of the most fundamental surface properties, it directly impacts many other surface properties, such as adhesion,14 adsorption,15 carrier mobility,16 and charge doping.17 For example, there have been contradictory reports on the effect of substrate on the wettability of single-layer graphene, such as (non)wetting transparency;18−20 water has been shown to spontaneously wet the inside of carbon nanotubes, which were traditionally known as hydropho© XXXX American Chemical Society

Received: August 4, 2015 Accepted: December 1, 2015

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DOI: 10.1021/acsnano.5b04843 ACS Nano XXXX, XXX, XXX−XXX

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Figure 1. Effect of low-temperature storage on freshly exfoliated HOPG. (a) Temporal evolution of the WCA measured on freshly exfoliated HOPG stored at room temperature (rt) and low temperature. For the sample stored at low temperature, it was only exposed to rt during the WCA measurement. (b) Temporal evolution of the WCA on the low-temperature pretreated HOPG surface. The sample was stored at −15 °C for 30 min and then removed from the low-temperature environment at time 0. It was kept at rt throughout the measurement. Relative humidity = ∼40%.

surface characterization shows that water condenses on the graphitic surface, likely forming a 2D ice structure that persists even at room temperature. Our method provides a convenient strategy to fabricate graphitic materials with minimal hydrocarbon contamination, maintaining its intrinsic surface properties in device applications. As an example, we show that a watertreated graphite electrode maintained a high level of electrochemical activity in air for up to 1 day.

methods to clean the surface, or ways to protect the surface from contamination. It is extremely difficult to maintain a hydrocarbon-free environment because even a parts per trillion level of hydrocarbon is detrimental. Although it is possible to remove hydrocarbons from air by passing contaminated air through cryogenically cooled activated charcoal,27 such an approach requires the experimental setup to be isolated from ambient air, making it impractical for most experiments and large-scale applications. We also note that a glovebox and clean room do not provide a hydrocarbon-free environment; in fact, both contain high levels of hydrocarbon due to emission from plastics (e.g., gloves, wafer storage containers, etc.). Conventional techniques are not entirely effective at cleaning graphitic surfaces.28,29 For instance, solvent washing is able to remove surface contaminants, but solvent residue can easily become trapped on the graphitic surface due to the existence of step edges (i.e., defect sites);30,31 cleaning methods based on UV photo-oxidation removes adsorbed airborne hydrocarbons at the expense of producing additional defect sites on the basal plane;32−34 a high-speed air jet requires a pressure drop of about 105 Pa along the centerline of gas flow,35 which can easily blow off graphene/graphite layers from the underlying substrate.14 Additionally, CO2 cluster jet treatment of graphene creates point defects while removing contaminants.36 Tyler et al. showed that organic contaminates can be removed from graphene by irradiating with a low-energy Ar ion beam. Although some damage is incurred to the graphene surface, they claim that there is a threshold of no damage, and the process can be further improved to remove contaminants without causing surface damage.29 Thermal desorption through annealing has also been explored as a facile method for hydrocarbon removal; however, high temperature is needed if the sample is contaminated in air, and the process could also produce defects in graphene.37−39 All things considered, a strategy that simply protects a fresh graphitic surface from airborne hydrocarbon is expected to be suitable for both fundamental study and large-scale application of graphitic materials. Herein, we report a convenient method to significantly inhibit hydrocarbon contamination through water adsorption onto graphitic carbon. This approach significantly slows down (by a factor of ca. 20×) the airborne hydrocarbon contamination on a fresh graphitic surface, thus maintaining its intrinsic wetting behavior for many hours in air. Follow-up

RESULTS We and others have reported the kinetics of airborne hydrocarbon contamination on graphitic surfaces.1−7 Water contact angle (WCA) was shown to linearly correlate with the surface coverage of hydrocarbon. In one study, we observed that the WCA on freshly exfoliated HOPG is 64.4° and increases to ca. 90° within 15 min of air exposure, as a result of airborne hydrocarbon adsorption.1 A number of parallel reports have reproduced our results.4−6 Effect of Low-Temperature Storage on Wettability. In this work, we discovered that the intrinsic wettability of the HOPG surface can be maintained for at least 2 h when stored at low temperature. Figure 1a compares the time evolution of static WCA on two HOPG samples. Both samples are exfoliated at time zero and exposed to ambient air in a chemistry laboratory. One sample was stored at room temperature (rt); the other sample was stored at ca. −15 °C and only briefly exposed to rt air during WCA measurements (see Methods for details). Upon rt storage, the HOPG sample exhibited an initial WCA of ca. 65° and increased within 15 min to a constant value of ca. 90°, in agreement with our previous report.1 In comparison, HOPG stored at low temperature showed consistently lower WCA. The initial WCA was ca. 60° and reached ca. 70° within 15 min of air exposure but plateaued without further increase for 2 h. From the WCA data, we estimate that the surface coverage of hydrocarbon is less than 30% (see Supporting Information for detailed calculation). This inhibition of hydrocarbon contamination is highly reproducible regardless of testing time and location, although a small variation of WCA (60−65°) was observed on different HOPG samples right after exfoliation. HOPG also shows a much slower WCA increase even after removal from the low-temperature environment. In one experiment, an exfoliated HOPG was stored at low temperature (−15 °C) for 30 min, followed by exposure to lab air at rt (25 B


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Figure 2. ATR-FTIR spectra of freshly exfoliated HOPG stored at (a) room temperature and (b) low temperature for 2 days. For each sample, spectra were taken right after exfoliation (30 min low-temperature storage) and after 2 days storage in ambient air, respectively. The inset shows a magnified image of the dashed area in (a). Note that the spectra are plotted in absorbance mode and vertically shifted for clarity.

Figure 3. (a) C 1s and (b) O 1s XPS peaks of freshly exfoliated HOPG samples right after exfoliation (black), after 6 days ambient exposure at room temperature (red), and at low temperature (blue). The inset shows the fwhm of the C 1s peak using measurements from three different locations on the same sample surface. Note that the black and blue curves in (a) almost completely overlap.

which can be assigned to the −OH stretching vibrations of water molecules.41 Interestingly, unlike liquid-phase water that always shows an IR peak centered around 3400 cm−1, HOPG stored at low temperature gives rise to a water peak at 3250 cm−1, indicating an ice-like structure with a red-shifted −OH bond stretching at a lower frequency (vide inf ra).42 The impact of storage environment on hydrocarbon contamination of the HOPG surface was also observed with X-ray photoelectron spectroscopy (XPS). A freshly exfoliated HOPG sample with an air exposure time less than 10 s was first analyzed to provide a benchmark. As shown in Figure 3a, this sample displayed a single C 1s peak at 284.5 eV, corresponding to the sp2 structured C−C (surface and bulk).34 A small increase in peak width (0.040 eV, measured at full width at halfmaximum (fwhm)) was observed after exposing this sample to ambient air at rt for 6 days; this change in peak width was previously attributed to the adsorption of airborne hydrocarbon.2 In contrast, HOPG stored at low temperature, but otherwise the same conditions, showed a C 1s peak almost identical to that from the fresh surface, with a very small increase of peak width (fwhm) by 0.003 eV, only 7% of the change observed on the rt aged surface. Although the changes in peak width are small, the fwhm of all measurements are highly reproducible, as evidenced by the small standard deviation shown in Figure 3a, inset. We also note that the peak broadening we observed here is smaller than the case of the graphene/copper sample.2 We attribute this difference to the fact that the C 1s peak in graphite has significant

°C) when the WCA measurement started. As shown in Figure 1b, the initial WCA was 64° right after removal from low temperature, which slowly increased to ca. 72° within 1 h and eventually plateaued at ca. 85° after 20 h air exposure. Interestingly, although the kinetics of WCA evolution vary at different temperatures, the initial and final WCA on HOPG appear to be independent of storage methods, indicating only a temporary role played by low-temperature storage in affecting surface wetting behavior. In addition, when the aged (i.e., hydrocarbon-contaminated) HOPG surface was stored at low temperature, similar high WCA values (85−90°) were observed before and after storage, indicating that low-temperature treatment does not introduce additional defects on the HOPG surface and thus increases hydrophilicity. Effect of Low-Temperature Storage on Hydrocarbon Adsorption. The lower WCA observed on samples stored at low temperature is due to a decrease in the hydrocarbon adsorption. Shown in Figure 2 are attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) spectra of two freshly exfoliated HOPG samples, one stored at rt (Figure 2a) and the other at low temperature (Figure 2b) for 2 days. For the HOPG sample stored at rt, peaks at 2850 and 2930 cm−1 can be clearly observed after 2 days of air exposure, corresponding to symmetric and asymmetric stretching of the methylene group (−CH2−) of the adsorbed hydrocarbon, respectively.40 In comparison, such hydrocarbon-related peaks were barely detectable on HOPG stored at low temperature, while a broad peak around 3000−3500 cm−1 was observed, C


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Figure 4. Effect of water deposition on HOPG surface. (a) Temporal evolution of the WCA measured on the low-temperature pretreated HOPG surface. The sample was taken out of low temperature and surrounded by CaCl2 desiccant at time 0. (b) Temporal evolution of the WCA on freshly exfoliated (red) and aged (black) HOPG surface after 2 min steam treatment.

graphite may promote hydrocarbon adsorption. In another experiment, both freshly exfoliated and aged HOPG samples were placed in DI water steam for 2 min before storage in air at rt. The freshly exfoliated HOPG surface showed consistently lower WCA (65−70°) compared to that on the aged HOPG surface (88−95°) during the subsequent 2 h air exposure (Figure 4b). On aged HOPG, we observed similar high WCA (88−95°) before and after the steam treatment; this result shows that water in the environment, by itself, does not necessarily impact the wettability of graphite. Indeed, the effect of ambient humidity on wettability has been documented for a number of hydrocarbon-based materials, such as polyethylene terephthalate (PET), paraffin, etc. For all of these surfaces, the increase of relative humidity (RH) (25−100%) leads to a concurrent increase of WCA by only 2−5°,43 almost an order of magnitude smaller than the WCA change induced by hydrocarbon contamination. Collectively, these results suggest that the water adsorption on the fresh HOPG surface, but not on the aged, hydrocarbon-covered HOPG surface, inhibits hydrocarbon adsorption. In addition, the intrinsic WCA can be recovered so long as any secondary hydrocarbon contamination can be avoided during the water removal process. For the HOPG sample stored at low temperature, we annealed the sample at 150 °C in Ar for 1 h. This temperature is high enough to evaporate water from the graphitic surface yet not damage HOPG. The WCA of HOPG after annealing was 75°, about 10° higher than its intrinsic value. The high WCA suggests the presence of hydrocarbon contamination, which we attribute to high boiling point contaminants that adsorb during annealing or contamination already on the graphitic surface. In another experiment, we removed the surface-adsorbed water by keeping the HOPG sample in an XPS chamber for 30 min. The sample was pretreated at low temperature for 30 min and showed a WCA around 70°. The pressure inside the chamber was measured to be ∼1 × 10−10 Torr, so water is expected to readily desorb. After 30 min of vacuum treatment, the HOPG sample was taken out of the XPS chamber and showed a slight decrease of WCA to 65°. This experiment clearly indicates that the intrinsic wettability of the graphitic surface can be recovered after the removal of adsorbed water. We further illustrate the correlation between water and hydrocarbon adsorption on the HOPG surface through spectroscopic ellipsometry (SE), as shown in Figure 5a. In

contribution from the sublayers. Using the photoelectron penetration depth and takeoff angle, we estimated that the top graphene layer contributed only 18% of the total C 1s signal (see Supporting Information for detailed calculations). On the other hand, oxygen species on the HOPG surface displayed a much more remarkable change with different storage methods. We note that unlike the C 1s peak, which has contributions from both surface and subsurface layers,34 the O 1s peak presumably comes from only the HOPG surface, without interference from the bulk. As shown in Figure 3b, the fresh HOPG surface showed a negligible O 1s peak, which is expected for a freshly exfoliated graphite surface. Storage at low temperature resulted in a small increase of the O 1s peak that is only 25% of the change observed on the sample stored at rt. We have also analyzed the XPS data by plotting the difference between room temperature and low-temperature aged samples (Supplementary Figure 5). The C 1s differential spectrum clearly showed a peak at 286.6 eV with a shoulder at 288.7 eV. The integrated peak area indicated a C/O atomic ratio of about 3:1 for the adsorbed contamination, in accordance with our previous observation of airborne hydrocarbon accumulation on the Cu/graphene surface.2 We note that XPS measurements only provide a lower bound of the surface oxygen species, as some of the adsorbed molecules are expected to desorb under ultrahigh vacuum (UHV) conditions. Therefore, the peak area cannot be directly extrapolated to the surface coverage of hydrocarbon under ambient conditions. Nevertheless, both the ATR-FTIR and XPS results suggest that airborne hydrocarbon accumulation on the HOPG surface can be efficiently minimized during low-temperature storage. Role of Water in Reducing Hydrocarbon Contamination. We hypothesized that the reduced hydrocarbon contamination is due to the increased adsorption of water on graphite at low temperature. To test this hypothesis, we performed two control experiments to probe the role played by water during ambient air exposure. In one experiment, freshly exfoliated HOPG was pretreated at low temperature for 30 min, followed by storage in room temperature surrounded by anhydrous CaCl2. CaCl2 is a desiccant used to decrease the local vapor pressure of water. As shown in Figure 4a, this sample showed an increase of WCA from 65 to 80° within 90 min, in contrast to the much slower WCA increase when the sample was stored without desiccant but otherwise identical conditions (Figure 1b). This result suggests that less water on D


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Figure 5. (a) Spectroscpic ellipsometry monitoring of hydrocarbon thickness on an exfoliated HOPG surface exposed in ambient air with and without low-temperature treatment. (b) ATR-FTIR spectrum of a freshly exfoliated HOPG sample after low-temperature treatment. The sample was removed from low temperature at time 0. The inset shows the integrated peak area versus time for the peak at 3250 cm−1 (−OH stretching). Note that the spectra are plotted in absorbance mode and vertically shifted for clarity.

In the literature, spectroscopic ellipsometry is commonly employed to provide the average thickness and coverage of adsorbed water on surfaces.49,50 Generally, a film of less than monolayer coverage may demonstrate an inconsistent index of refraction depending on the surface density. However, Smith reported a similar correlation between the phase shift (see Supporting Information) and film thickness regardless of the surface coverage (θ = 0.05−1), which works very well for Hg, Au, Si, glass, and water films within 0−5 nm.51 In this regard, our approach to ellipsometrically measuring water thickness remains valid even at the submonolayer regime. Our hypothesis (i.e., that adsorbed water reduces hydrocarbon adsorption) implies that the adsorbed water stays on the HOPG surface for an extended period of time even at rt. To test this prediction, we conducted temporal ATR-FTIR experiments on freshly exfoliated HOPG that was stored at low temperature for 10 min. Unlike the data shown in Figure 2b, in this experiment, after the low-temperature treatment, the HOPG sample was left in air at rt throughout the entire experiment period. As shown in Figure 5b, instead of a substantial decrease in water peak (3100−3800 cm−1) as one would expect for liquid-phase water due to the desorption at rt (40% RH), the water adsorbed on the graphitic surface only showed a slight increase during the 3 h of ambient exposure, with a 15% monotonic increase of peak intensity. It is interesting that the main peak (center at 3250 cm−1) showed a red shift compared to normal liquid-like water (center at 3400 cm−1), suggesting an ice-like structure with a hydrogen-bonding network.42 Considering that this ATR-FTIR experiment has been conducted at rt for several hours, such orderly structure is most likely stabilized by the strong hydrogen bonding between chemisorbed water and the HOPG surface.44,48 Other minor peaks (center at 3650 and 3740 cm−1) refer to dissociated water monomers that imply a possible structural unit during the water adsorption process. For example, Wang et al. reported the IR spectra of a water hexamer, which shows a similar O−H stretching vibration at 3740 cm−1.52 More recently, a “square ice” structure has been observed between two graphene sheets, indicating another 2D ice structure on the graphitic surface.53 These results indicate that the stable “ice-like” structure of the adsorbed water on HOPG is the key to significantly inhibiting hydrocarbon adsorption in air, even at room temperature. Notably, our WCA (Figure 1) and ellipsometry (Figure 5) data showed very different kinetic behavior: saturated hydrocarbon coverage takes 15 min (WCA) and 50 min (SE). This

one experiment, we conducted temporal monitoring on the exfoliated HOPG surface exposed in ambient air at rt. During the first 50 min of air exposure, a linear growth of adsorbed hydrocarbon layers occurred, reaching a thickness of ca. 0.55 nm, after which the rate of increase drastically slowed down and plateaued at 0.60 nm after 5 h. Similar hydrocarbon adsorption kinetics have been observed in our previous studies.1 In another experiment, we stored freshly exfoliated HOPG in a cold glass Petri dish for 10 min, and the local temperature was measured to be −10 °C. The first ellipsometry data point was collected right after exfoliation of HOPG, and the surface was assumed to be free of hydrocarbon contamination. After 10 min of low-temperature treatment, the glass Petri dish was removed immediately and another ellipsometry measurement was conducted on the same location for 5 h. To analyze the data, we used a three-layer (HOPG/water/hydrocarbon) model in which the thicknesses of both water and hydrocarbon are determined by fitting (see Supporting Information and Supplementary Figure 3 for details). As shown in Figure 5a, the low-temperature treatment produced a water thickness of ca. 0.08 nm, indicative of submonolayer water on the HOPG surface (see Supporting Information for additional discussion). The presence of such adsorbed water significantly inhibited the follow-up hydrocarbon adsorption rate by 83%, which reached a hydrocarbon thickness of ca. 0.40 nm after 200 min ambient exposure. For clarification, monolayer coverage is defined as a uniform layer of water on the HOPG surface. Submonolayer coverage is defined as a nonuniform layer of water on the HOPG surface where there are potentially areas of monolayer coverage and areas of no coverage, such as water clusters/islands. Substantial research indicates that water physisorbs on graphite.44−46 Water at 85 K was shown to form 2D hydrogen-bonded submonolayer islands which aggregated as water coverage increased.47 The submonolayer to monolayer coverage of water shows no thermodynamically preferred orientation (2D), whereas the dipoles align parallel to the graphite surface as the water transitions to ice-like (3D).45,47,48 This creates a water monolayer thickness of about 0.35 nm above the graphite surface,45 which is similar to the 0.335 nm step edge of graphite where water nanodroplets nucleate.44 This notation can be confusing because the adsorbed water is not a uniform monolayer film on the entire surface but rather water islands that have a thickness of about a single water molecule. E


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Figure 6. Temporal evolution of WCA measured on (a) CVD-grown copper/graphene and (b) thermally annealed SiO2/graphene exposed in air at room temperature (red) and low temperature (black). The SiO2/graphene samples have been thermally annealed in vacuum at 500 °C for 1 h. Both copper/graphene and SiO2/graphene samples were taken out of CVD chamber at time 0.

Figure 7. CVs for the oxidation of 1 mM Fe(CN)64− in 1.0 M KCl at 0.1 V s−1 on freshly exfoliated HOPG exposed in air (a) at room temperature and (b) at low temperature for 0 h (black), 2 h (red), 8 h (blue), 1 day (green), and 3 days (pink). (c) ΔEp of HOPG stored in air at rt (red) and low temperature (black) as a function of ambient exposure time.

extended saturation time of about 200 min (Supplementary Figure 7). Extension to Other Graphitic Materials. We also discovered that the low-temperature storage method works well with graphene samples supported on different substrates. In Figure 6a, a chemical vapor deposition (CVD)-grown copper/graphene sample showed an initial WCA about 40− 45°, which drastically increased to 60° within 20 min of air exposure and plateaued to ca. 80° overnight. When stored at low temperature, intrinsic wettability can be effectively preserved with a constant WCA of ca. 50°, indicating minimal hydrocarbon deposition on the copper/graphene surface during the 2 h low-temperature storage. We also transferred graphene onto two SiO2 substrates and then thermally annealed both in vacuum at 500 °C to remove surface hydrocarbons. As shown in Figure 6b, we observed a decrease in their WCA immediately after thermal annealing, suggesting a substantial decrease of the surface hydrocarbon level. During the subsequent 1 h ambient air exposure, SiO2/graphene exposed in air at rt showed a rapid recovery of WCA (80−85°) due to airborne contamination. In contrast, SiO2/graphene stored at low temperature maintained a low WCA that varied between 59 and 65°, indicating reduced hydrocarbon contamination on the graphitic surface at low temperature. Applications in Electrochemistry. Finally, we show that our low-temperature storage method can be conveniently used

discrepancy can be explained by the fact that these tests have been conducted in different laboratories, which may contain different hydrocarbon content and concentrations.1 Unfortunately, a complete hydrocarbon inventory in air is still difficult to determine even today due to their complexity in species and sensitivity to local environment.43,54 In previous studies, Martinez-Martin et al. reported the use of mass spectrometry and Kelvin probe force microscopy to visualize the spatial distribution of the hydrocarbon species adsorbed on the HOPG surface.38 It is noted that they stored the HOPG sample in a vacuum for hours, resulting in only one dominant hydrocarbon species detected on the HOPG surface. In our experiments, the graphitic surface was exposed in ambient air to different types of hydrocarbons in much higher concentrations. As a result, a much higher thermal desorption temperature (500−600 °C) is required to remove those adsorbed hydrocarbon species.2 To clarify this location-dependent effect, we conducted both WCA and ellipsometry tests on a freshly exfoliated HOPG surface in the same environment (i.e., same room, same day). In these experiments, the fresh HOPG surface displayed a concurrent increase in WCA and hydrocarbon thickness within 50 min of air exposure (Supplementary Figures 6 and 7). This behavior is similar to that in our previous observations.1 Moreover, for the HOPG sample that was pretreated at low temperature, a similar concurrent increase in WCA and hydrocarbon thickness was also observed although with an F


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employing atomic force microscopy (AFM), lateral growth of water nanodroplets along the defect sites (step edges) was observed with 5−15 nm height while most of the basal plane area was still water-free. We note that the water contact angle on their HOPG sample was measured to be 90°, indicating the presence of hydrocarbon contamination. Interestingly, in their follow-up experiments conducted on a modified mica surface with a water contact angle of ca. 40°, a much more uniform distribution of water nanodroplets was observed with 0.5−1.5 nm height. In work reported by Chakarov et al.,47 they studied water adsorption on freshly exfoliated HOPG in UHV via a temperature-programmed desorption (TPD) process. Though still initiated from the defect sites, such water adsorption on fresh HOPG can grow two-dimensionally as the adsorption continues and eventually coalesce at a high coverage. In fact, the lattice parameter of sp2 C−C in graphite closely matches that of type I ice;47 that is, the graphite crystalline surface can serve as a good template for the epitaxial growth of ice-like water layers at low temperature. Indeed, a “square ice” structure was recently observed to exist between two graphene sheets.53 In this regard, a nanometer-thin layer of water film can be reasonably expected on a freshly exfoliated HOPG surface as a result of the low-temperature treatment. Given that water adsorption plays the central role in our proposed mechanism, one expects that the method developed here can protect other high-energy surfaces from airborne hydrocarbon contamination, as well. Indeed, it is known that on metal oxide surfaces hydrocarbon contamination slows down in a high-humidity environment.60 Compared to many other 2D materials (e.g., MoS2), graphene is unique in its extended π system. It has been proposed that water molecules adsorb on the graphitic surface, with hydrogen pointing to graphene surface as the preferred orientation.61 The presence of such H···π interaction is believed to stabilize the adsorbed water molecules. Due to the lack of π-conjugated system, other 2D materials such as MoS2 may show a different interaction with water molecules;62 the effectiveness of using water to prevent contamination on these surfaces needs to be further investigated. Our ATR-FTIR data indicate that little water was adsorbed on HOPG when the sample was only exposed to lab air at room temperature (Figure 2a). As such, one would expect that the adsorbed water rapidly desorbs once the HOPG sample is removed from the low-temperature environment. Surprisingly, this is not the case. According to our measurement (Figure 5b), the peak at 3250 cm−1, which corresponds to surface-adsorbed water, did not show any significant change in peak shape or intensity, indicating a relatively stable structure within at least a few hours. We speculate that there is a significant hysteresis in the formation and desorption of the 2D ice structure. Such structure is thermodynamically unstable at rt and ca. 40% RH; however, once several molecules adhere, the water molecules within the 2D ice structure are stabilized by multiple hydrogen bonds and therefore exhibit a high kinetic barrier for the desorption process. In our ellipsometry test (Supporting Information page 3), HOPG stored at low temperature over different times (10−30 min) resulted in a consistent thickness increase of 0.06−0.10 nm. This result indicates a sublayer of water. In the literature, the ice-like water structure has been intensively studied on many surfaces.58 Generally, once water molecules get adsorbed on a surface, they can either interact with the underlying substrate or collide with each other to form clusters (from

to preserve the intrinsic electroactivity of HOPG. It has been reported that the intrinsic electroactivity of many graphitic surfaces can be significantly lowered by adsorbed impurities upon ambient air exposure.25,55,56 Traditionally, minimal airborne contamination was achievable only for HOPG, which can be immersed into an electrolyte solution immediately after exfoliation.57 By contrast, our simple method will be useful for various graphitic materials when the prolonged exposure of a fresh surface to ambient air is inevitable before or during its characterization.26 In our experiments, the cyclic voltammogram (CV) measurements were conducted on five pieces of SPI-1 HOPG samples in 1.0 mM K4Fe(CN)6 in 1.0 M KCl (see Supporting Information for details). The HOPG samples were freshly exfoliated and then exposed in ambient air for different times before adding the solution. To confirm that the surface properties of the samples were identical, WCA and ellipsometry measurements were conducted on the freshly exfoliated surfaces and all five HOPG samples demonstrated similar WCA and spectroscpic ellipsometry trends upon air exposure. As shown in Figure 7a,b, our CV measurements on the freshly exfoliated HOPG surface show a peak separation (ΔEp) of ca. 59 mV, indicating a reversible behavior of the Fe(CN)64−/3− couple.25,57 For HOPG stored at rt (Figure 7a), we observed a rapid deterioration in the CV response, with the ΔEp increasing from 59 to 177 mV during 3 day ambient exposure. A similar trend has also been reported by Patel et al. under the same experimental condition.25 In contrast, HOPG stored at low temperature (Figure 7b) shows almost no change of electrochemistry (Δ(ΔEp) < 2 mV) for at least 24 h. As shown in Figure 7c, HOPG exposed at rt shows ca. 2.6× higher ΔEp after 24 h ambient exposure compared to HOPG stored at low temperature but otherwise the same condition. This result is consistent with the idea that hydrocarbon contamination blocks the heterogeneous electron transfer; the HOPG stored at low temperature has much less organic impurity contamination and therefore shows a higher electrochemical activity.25,26 Our results showed that low-temperature storage can be an effective and convenient method to minimize adsorption of organic impurities on the HOPG surface and preserve its intrinsic electrochemical reactivity for at least 24 h.

DISCUSSION Our data show that covering the graphitic surface with adsorbed water can substantially reduce the rate of hydrocarbon adsorption. The adsorption of water requires a high relative humidity environment (e.g., by lowering the temperature or using steam); surprisingly, once formed, the water persisted in low relative humidity environments (20−40%) for at least 2 h. Water Adsorption on Graphitic Surfaces. Generally speaking, water adsorption usually initiates from a chemisorption of 1−2 layers of water film that is tightly bound to the surface in a well-ordered structure, followed by subsequent physisorption of water layers with a more liquid-like character.58 The first 1−2 layers typically have much stronger bonding to the surface. The thickness of the physisorbed water layers ranges from nanometer to micrometer scale depending on the temperature, humidity, and substrate. Water adsorption on a graphitic surface (e.g., HOPG, graphene) is generally believed to initiate from the defect sites (i.e., step edges).44,47,59 For example, Cao et al. reported the use of graphene templating to visualize the microscopic structure of adsorbed water on the HOPG surface.44 By G


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water significantly slows down the rate of this process by a factor of ca. 4−6× according to our ellipsometry results. Comparison with Competing Cleaning Technologies. Although the protective role of water on the metallic surface has long been proposed,71 using this approach to protect the graphitic surface from hydrocarbon contamination has not been discussed in the literature. A significant advantage of the waterbased protection is that this technique potentially generates only water as waste product, which can be easily removed via low-temperature thermal annealing, UHV, or desiccant without affecting other surface properties of graphene/graphite. In contrast, other techniques targeting hydrocarbon removal (e.g., high-temperature thermal annealing, UV/O3, etc.) often introduce defects onto the graphene/graphite surface.2 We also note that direct soaking of the fresh graphitic surface in liquid water will inevitably lead to secondary contamination because the solubility of typical hydrocarbons in water is still high in the context of surface contamination.26 For example, the Nanopure water purification system (Thermo Scientific), one of the most popular purification systems used in research laboratories, produces purified water with a total organic carbon (TOC) level between 5 and 10 ppb, similar to that of the hydrocarbon in ambient air. The condensation of water at low temperature makes it possible to adsorb clean water onto the graphitic surface. Indeed, one way to ensure the cleanliness of the water droplets during contact angle measurement is to use a stainless steel coldfinger that was mounted vertically on the target surface for water-drop condensation.72 With regard to other lamellar materials (e.g., MoS2, WS2, or BN), this “water protection” method is also expected to be beneficial because all of these materials also adsorb hydrocarbons upon air exposure.

dimers to hexamers). For metallic substrates with multilayers of ice-like water film, a particular stable cluster (e.g., cyclic hexamer) is first formed as a basic motif of the first water layer. During the subsequent film growth, there is a competition between the water film lateral extension and the thickness increase via on-top stacking for newly adsorbed water molecules. As a result, a variable thickness of water film was observed by STM on the same metallic surface. In contrast, the graphitic surface only shows a sublayer of water lined up along the step edges.44 In previous studies, Teschke applied AFM to visualize such ice-like structures formed on HOPG at room temperature.48 Based on their observations, the ice-like water forms on the HOPG surface in such an arrangement that the distance between the second neighbors of carbon atoms from HOPG coincided with the distance of the first neighbors of water molecules in the ice-like structure. Mechanism of Reduced Hydrocarbon Adsorption. We attribute the inhibition of the hydrocarbon adsorption on the graphitic surface to adsorbed water. During the low-temperature storage, water is expected to adsorb (physisorption and/ or chemisorption) onto the fresh graphitic surface. We also note that 5 nm thick water islands have been reported on an HOPG surface that was exposed to ambient air at >90% RH.63,64 The presence of such water on a graphitic surface may significantly increase the van der Waals interaction distance between hydrocarbon and the graphitic surface. The adsorption of a hydrocarbon molecule on ice greatly depends on its partial pressure and the density of free surface −OH groups.65 For a thermodynamically stable ice phase, it has been observed that the saturated coverage is submonolayer to monolayer for a wide variety of small organic molecules (C2−C6 n-alcohols, acetone, acetic acid, etc.) between 200 and 240 K;66,67 the surface coverage further decreases for hydrocarbons with longer alkyl chains or aromatic groups due to the lower vapor pressure and water solubility.68,69 In all, the existence of ice-like water on the graphitic surface will result in a markedly suppressed hydrocarbon adsorption rate. However, it should be noted that the graphitic surface may still undergo slight hydrocarbon contamination during lowtemperature storage. Some airborne hydrocarbon species may contain hydrophilic groups (e.g., −OH, −COOH), which has a strong interaction with adsorbed water via hydrogen bonding.58,70 When a graphitic surface is covered with water, these hydrocarbon species can still be adsorbed onto the surface, resulting in a slight change of surface wettability depending on their structure and coverage. At much longer time scales, airborne hydrocarbons gradually replace adsorbed water on the surface. This is expected because some of the airborne hydrocarbons may have much higher molecular weight than water, and therefore, their interaction with the surface is stronger than water. In addition, a large hydrocarbon molecule could displace several water molecules, resulting in a positive change of entropy. It has been reported by Hayashi et al.60 that small molecules with a low boiling point rapidly adsorb on the water surface. However, as the exposure time increases, the original surface adsorbate will be gradually replaced by a competing adsorbate with a much higher molecular weight and/or boiling point, an effect termed “fruit-basket phenomenon”. Considering that airborne hydrocarbons have a boiling point higher than that of water, it is expected that the adsorbed water would eventually be replaced by airborne hydrocarbon. However, the existence of adsorbed

CONCLUSION Our study showed that storing a freshly prepared graphitic surface at low temperature can greatly minimize hydrocarbon contamination and thus preserve its intrinsic wetting behavior and electrochemical activity. Ellipsometry and ATR-FTIR data indicate that nanometer-thick water adsorbs on the graphitic surface during low-temperature treatment. This water remains a stable structure after a few hours of air exposure at room temperature and plays a vital role in slowing down the subsequent hydrocarbon adsorption. This new method offers a convenient solution to minimize hydrocarbon contaminants and may be useful in the surface modification of graphene and related device fabrication. METHODS Preparation of HOPG Samples. Both SPI-1 (10 × 10 × 1 mm) and SPI-2 (20 × 20 × 1 mm) HOPG samples were purchased from SPI Supplies and used for all experiments. Only SPI-1 HOPG samples were used in CV experiments due to size limitation. Exfoliation was performed by the well-established tape method, where a piece of adhesive tape was placed on the sample surface and gently rubbed to ensure contact between the tape and sample.73 The tape (Scotch brand 1 in.) was then carefully pulled back, removing the upper surface layer, thereby exposing a fresh HOPG surface. Care was taken to ensure that a complete layer of HOPG was removed in each exfoliation. Testing was performed on the pristine surface away from flakes and surface defects. Low-Temperature Storage. One piece of HOPG was kept in a glass Petri dish without a lid to ensure ambient air exposure. The glass Petri dish was carefully cleaned (acetone wash followed by UV/Ozone cleaning for 30 min) and then surrounded by a pile of dry ice particles during low-temperature storage. Care was taken to not directly contact H


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ACS Nano the HOPG sample with dry ice. The local temperature was mearured to be −15 to −20 °C using a noncontact infrared thermometer. During the water contact angle measurement, a sample was taken out of the Petri dish every 1−30 min and then immediately put back after measurement. An alternative low-tempertaure storage method was also applied by keeping the HOPG sample in a freezer with a local temperature around −15 °C. Simlar WCA evolution was also observed as shown in Supplementary Figure 4. Water Contact Angle Measurement. Contact angles were recorded on a VCA Optima XE at ambient conditions (22−25 °C and 20−40% RH) with DI water provided by a Millipore Academic A10 (total organic carbon below 40 ppb). A charge-coupled device camera was used to capture contact angles, and analysis was conducted on vendor-supplied software. Water droplets (2 μL) were used for static measurements. HOPG was exfoliated and the first test taken within 10 s of exfoliation. Spectroscopic Ellipsometry. A J.A. Woollam Co. Alpha-SE spectroscopic ellipsometer was used for measurements at a wavelength range of 380−900 nm and an incident angle of 70°. In situ data were collected continuously at 10 s intervals with a 10 s acquisition time. HOPG was exfoliated and the first test taken within 10 s of exfoliation. Data were collected and analyzed using vendor-supplied CompleteEASE. Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy. A Bruker VERTEX-70LS FTIR and a Bruker Hyperion 2000 FTIR microscope in reflectance mode and a MCT A detector (7000−600 cm−1) were cooled by liquid nitrogen. Initially, the system was purged for 20 min with nitrogen gas to remove water and ambient gases within the instrument. The nitrogen gas purge remained on throughout the entire experiment. Spectra collection was conducted for 300 scans at 4 cm−1 resolution using a germanium 20× ATR objective. Vendor-supplied software was used for data collection and analysis. For HOPG stored at rt, the sample was exfoliated and the first test taken within 10 s of exfoliation. For HOPG stored at low temperature, the first test was taken within 1 min after removal from the low-temperature environment. X-ray Photoelectron Spectroscopy. XPS measurements were carried out in a UHV chamber (base pressure ∼1 × 10−10 Torr) of an ESCALAB 250Xi XPS microprobe. Spectra were collected using the Al Kα X-ray line and a CAE analyzer. The characterization was operated with a band pass of 50 eV for both survey scans (1.0 eV/step) and detailed scans (0.1 eV/step). After raw data collection, the Thermo Scientific Advantage data system software was used in data analysis of background subtraction and peak fitting.

ACKNOWLEDGMENTS H.L. acknowledges partial support from AFOSR (FA9550-13-10083), NSF (CHE-1507629), and ONR (N000141310575 and N000141512520). L.L. acknowledge partial support for NSF (CMMI-1233161). S.A. is grateful for support by NSF (CHE1213452). We thank C. Amadei (Harvard) for helpful discussions. REFERENCES (1) Kozbial, A.; Li, Z.; Sun, J.; Gong, X.; Zhou, F.; Wang, Y.; Xu, H.; Liu, H.; Li, L. Understanding the Intrinsic Water Wettability of Graphite. Carbon 2014, 74, 218−225. (2) Li, Z.; Wang, Y.; Kozbial, A.; Shenoy, G.; Zhou, F.; McGinley, R.; Ireland, P.; Morganstein, B.; Kunkel, A.; Surwade, S. P.; et al. Effect of Airborne Contaminants on the Wettability of Supported Graphene and Graphite. Nat. Mater. 2013, 12, 925−931. (3) Kozbial, A.; Li, Z.; Conaway, C.; McGinley, R.; Dhingra, S.; Vahdat, V.; Zhou, F.; D’Urso, B.; Liu, H.; Li, L. Study on the Surface Energy of Graphene by Contact Angle Measurements. Langmuir 2014, 30, 8598−8606. (4) Ashraf, A.; Wu, Y.; Wang, M. C.; Aluru, N. R.; Dastgheib, S. A.; Nam, S. Spectroscopic Investigation of the Wettability of Multilayer Graphene Using Highly Ordered Pyrolytic Graphite as a Model Material. Langmuir 2014, 30, 12827−12836. (5) Wei, Y.; Jia, C. Q. Intrinsic Wettability of Graphitic Carbon. Carbon 2015, 87, 10−17. (6) Amadei, C. A.; Lai, C.-Y.; Heskes, D.; Chiesa, M. Time Dependent Wettability of Graphite Upon Ambient Exposure: The Role of Water Adsorption. J. Chem. Phys. 2014, 141, 084709. (7) Mücksch, C.; Rösch, C.; Müller-Renno, C.; Ziegler, C.; Urbassek, H. M. Consequences of Hydrocarbon Contamination for Wettability and Protein Adsorption on Graphite Surfaces. J. Phys. Chem. C 2015, 119, 12496−12501. (8) Sharkey, T. D. Emission of Low Molecular Mass Hydrocarbons from Plants. Trends Plant Sci. 1996, 1, 78−82. (9) Sharkey, T. D.; Wiberley, A. E.; Donohue, A. R. Isoprene Emission from Plants: Why and How. Ann. Bot. 2008, 101, 5−18. (10) Fraser, M. P.; Cass, G. R.; Simoneit, B. R. T. Gas-Phase and Particle-Phase Organic Compounds Emitted from Motor Vehicle Traffic in a Los Angeles Roadway Tunnel. Environ. Sci. Technol. 1998, 32, 2051−2060. (11) Goldan, P. D.; Kuster, W. C.; Fehsenfeld, F. C.; Montzka, S. A. Hydrocarbon Measurements in the Southeastern United States: The Rural Oxidants in the Southern Environment (ROSE) Program 1990. J. Geophys. Res. 1995, 100, 25945−25963. (12) Goldan, P. D.; Trainer, M.; Kuster, W. C.; Parrish, D. D.; Carpenter, J.; Roberts, J. M.; Yee, J. E.; Fehsenfeld, F. C. Measurements of Hydrocarbons, Oxygenated Hydrocarbons, Carbon Monoxide, and Nitrogen Oxides in an Urban Basin in Colorado: Implications for Emission Inventories. J. Geophys. Res. 1995, 100, 22771−22783. (13) Millet, D. B.; Donahue, N. M.; Pandis, S. N.; Polidori, A.; Stanier, C. O.; Turpin, B. J.; Goldstein, A. H. Atmospheric Volatile Organic Compound Measurements During the Pittsburgh Air Quality Study: Results, Interpretation, and Quantification of Primary and Secondary Contributions. J. Geophys. Res. 2005, 110, D07S07. (14) Koenig, S. P.; Boddeti, N. G.; Dunn, M. L.; Bunch, J. S. Ultrastrong Adhesion of Graphene Membranes. Nat. Nanotechnol. 2011, 6, 543−546. (15) Schedin, F.; Geim, A. K.; Morozov, S. V.; Hill, E. W.; Blake, P.; Katsnelson, M. I.; Novoselov, K. S. Detection of Individual Gas Molecules Adsorbed on Graphene. Nat. Mater. 2007, 6, 652−655. (16) Ponomarenko, L. A.; Yang, R.; Mohiuddin, T. M.; Katsnelson, M. I.; Novoselov, K. S.; Morozov, S. V.; Zhukov, A. A.; Schedin, F.; Hill, E. W.; Geim, A. K. Effect of a High-Kappa Environment on Charge Carrier Mobility in Graphene. Phys. Rev. Lett. 2009, 102, 206603.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.5b04843. Further information about ellipsometry, cyclic voltammetry, XPS, and data analysis (PDF)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions ⊥

Z.L. and A.K. contributed equally to this work.

Author Contributions

Z.L., H.L., and L.L. designed and directed the experiments. Z.L., A.K., N.N., D.P., G.S., and M.S. conducted the experiments. All authors discussed the results. Z.L. and H.L. co-wrote the manuscript with input from all authors. Notes

The authors declare no competing financial interest. I


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Water Protects Graphitic Surface from Airborne Hydrocarbon

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