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Graphitic materials: Intrinsic hydrophilicity and its implications
Accepted Manuscript Graphitic materials: Intrinsic hydrophilicity and its implications Haitao Liu, Lei Li PII: DOI: Reference:
S2352-4316(16)30205-X http://dx.doi.org/10.1016/j.eml.2017.01.010 EML 266
To appear in:
Extreme Mechanics Letters
Received date: 28 September 2016 Revised date: 31 January 2017 Accepted date: 31 January 2017 Please cite this article as: H. Liu, L. Li, Graphitic materials: Intrinsic hydrophilicity and its implications, Extreme Mechanics Letters (2017), http://dx.doi.org/10.1016/j.eml.2017.01.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphitic Materials: Intrinsic Hydrophilicity and Its Implications Haitao Liu a *, Lei Li b, c * a
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Department of Chemical & Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, 15261, USA c Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, 15261, USA b
*Corresponding authors: [email protected], [email protected]
Abstract Graphitic materials are long regarded as model hydrophobic materials. However, recent work has shown that graphite and graphene are much more hydrophilic than previously thought. It was revealed that the commonly observed hydrophobic nature of graphite is due to airborne hydrocarbon contamination that was not considered in previous studies. This perspective highlights these recent developments and discusses their implications to research on watercarbon interactions, wetting transparency, electrochemistry, adsorption and adhesion, and lubrication and wear.
Keywords: carbon; contact angle; surface contamination; electrochemistry; surface properties; wear.
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1. Background. Graphitic materials consist of primarily sp2 carbon and exist in forms such as graphite, carbon nanotubes, graphene, and activated carbon. They are used in numerous applications in modern society. An example of particular relevance to the field of mechanics is carbon fiber, which is a major component in light weight, high strength composites.1 Other applications of graphitic carbon materials include electrodes,2 lubricants,3 refractory materials,4 adsorbents,5 and bearings/seals.6 All these applications involve interfacial interaction between graphitic materials and their environments, such as bonding with polymer matrix, adsorption of molecules from the gas/liquid phase, electron transfer across the graphite-liquid interface, and friction against another solid surface. Fundamental surface properties, such as wettability and surface energy are extensively used to interpret and model these interfacial interactions. Wettability of graphitic materials has been studied for over 75 years. The early work extensively focused on natural and artificial graphite and more recent ones extended to carbon nanotubes and graphene.7-10 Many research groups around the world have carried out wetting measurements on various types of graphitic materials and based on these data, calculated surface energy by fitting the wetting data with models. Overall, this rich literature gave a consistent picture, that is, pristine graphitic materials predominately made of sp2 carbon are hydrophobic, with a water contact angle of ca. 85º, and have a low surface energy (Table 1). Surface modification, covalent or non-covalent, can increase or decrease the wettability by introducing polar or nonpolar functional groups, respectively.9,11
Table 1. Selected water contact values of graphitic materials. References Material Fowkes, 19407 Natural graphite Morcos, 197012 Exfoliated graphite Li, 200113 Carbon nanotube films Shin, 201011 HOPGa Ou, 201014 HOPG 15 Raj, 2013 Graphene a HOPG: highly oriented pyrolytic graphite.
Water Contact Angle 85.3° - 85.9° 84.2° 136.5° 91° 91° 90°
One question that caught little attention, however, is whether these wetting measurements reflect the intrinsic wetting behavior of the graphitic materials. This is a valid concern because wettability is highly surface sensitive. Even sub-monolayer amount of surface contamination can significantly impact the wettability. At the same time, surface contamination can occur quickly because only a small amount of material is needed. Little was done to verify that the graphitic surfaces used in previous studies were actually contamination-free. Schrader was among the first to challenge the conventional view that graphite is intrinsically hydrophobic. In a series of studies conducted in the 1970s and 1980s, he prepared clean graphite surfaces by high temperature annealing in an ultrahigh vacuum (UHV) chamber. 2
Water vapor was then introduced into the chamber and condensed onto a cold finger and used for contact angle measurement.16,17 Schrader reported water contact angle values around 35º using this method, significantly lower than the commonly accepted value of 85º. Measurements were also conducted on freshly cleaved graphite surfaces in air and contact angle values ranging from 40º to 50º were reported. Schrader pointed out that the result is consistent with surface contamination by airborne hydrocarbons but short of providing spectroscopic evidence to support this claim.16 Unfortunately, this work received little attention and acceptance by the carbon and tribology communities. A major concern of this work is the possibility of water evaporation under reduced pressure, which will result in a reduction in the measured water contact angle.
2. Intrinsic Hydrophilicity of Graphitic Materials. The rise of graphene renewed the interest in understanding the intrinsic wettability of graphitic materials. In the past 3 years, a number of studies have shown that both graphene and graphite can be rapidly contaminated by airborne hydrocarbons and such contamination is responsible for the hydrophobic behaviors that were consistently observed in the past 75 years.18-23 Notably, these recent work also addressed the concern of water evaporation in vacuum and provided convincing spectroscopic evidence that identifies the surface contamination as hydrocarbon and correlates its presence to the change of wettability. We briefly review these developments below. The Li and Liu groups studied the time evolution of the wettability of graphene and graphite upon their exposure to ambient air.18,21,24 They showed that the water contact angle of freshly prepared graphene and graphite samples are ca. 42º and 64º degrees, respectively. These values are significant lower than the commonly reported and accepted values of 80º to 90º for these materials. Upon exposure to ambient air, the contact angle values increased to ca. 90º, consistent with the previously accepted results. Using a combination of surface-sensitive spectroscopy techniques, including attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), and spectroscopic ellipsometry, it was shown that airborne hydrocarbons adsorb onto graphitic surfaces upon air exposure and the degree of surface contamination highly correlates with the change in the wettability (Figure 1). These data shows that graphitic materials are significantly more hydrophilic than previously believed. Note that the wettability of graphene depends on its supporting substrate. Recent studies suggest that a single layer graphene suspended in air is expected to be more hydrophobic than supported graphene, with a water contact angle of ca. 85 (see discussions on wetting transparency below).25 Several other research groups have reported parallel and follow-up studies that focus on this topic.19,20,22,26-28 Ashraf et al. reported the effect of exposure to supercritical water on the surface of several carbon materials.29 They showed that an as-received HOPG is hydrophobic with a water contact angle of 95º while the contact angle of freshly cleaved HOPG is 62º. The freshly cleaved HOPG also showed a much lower surface oxygen content than as-received isomolded graphite plate sample, suggesting the later was contaminated by airborne hydrocarbon. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), C x H y clusters ions were 3
(a)
(b)
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Figure 1. Surface contamination of graphene and graphite by airborne hydrocarbon and its impact on the wettability. (a) Time evolution of water contact angle of a copper-supported graphene sample measured in air. The inset shows photographs of water droplet of three measurements. (b) Time evolution of water contact angle (WCA) and thickness of hydrocarbon contaminant measured by ellipsometry of a freshly exfoliated HOPG sample in air. (c) ATR-FTIR spectra of a copper-supported graphene sample exposed to lab air. The inset shows the time evolution of the intensity of the symmetrical (s-CH2) and antisymmetrical stretching (as-CH2) peaks of -CH2-. (d) Carbon XPS peaks of a copper-supported graphene sample when it was fresh (black) and after several days of exposure in lab air (red). The inset shows the difference spectrum. Panels (a) (c) (d) are adapted with permission from Li et al. Nature Mater. 2013, 12, 925. Copyright Nature Publishing Group. Panel (b) is adapted with permission from Kozbial, et al. Carbon 2014, 74, 218. Copyright Elsevier. observed on both as-received iso-molded graphite plate and freshly cleaved HOPG. MartinezMartin et al. studied the surface contamination of graphite by Kelvin probe force microscopy and mass spectrometry. They observed adsorption of polyaromatic hydrocarbon onto a graphite sample placed inside a vacuum chamber. The adsorption is detected by both AFM topography imaging (a height increase of 0.8 nm) and change of contact potential. The contact angles for the freshly cleaved HOPG and the contaminated sample are 70º and 95º, respectively. Thermal 4
annealing at 337K and 1×10-5 mbar resulted in a complete desorption of the contaminant.30 Wei et al. reported water contact angle of edge plane of graphite as 60.8 ± 9º, depending on the orientation of the edge; thermal annealing in He gas reduced the contact angle value to 42.9 ± 6.7º, which the authors attributed to surface oxidation by the trace amount of O 2 in the He gas.22 Unlike the basal plane, there was no increase of the water contact angle upon air exposure for both as-prepared and annealed edge plane samples. The authors argued that the contamination of the edge plane is likely too fast to be resolved by contact angle measurement and the true water contact angle of the edge plane of graphite is likely <40º. The Chiesa group reported a series of studies combining both macroscopic (using water contact angle) and microscopic (using atomic force microscopy and scanning electron microscope) techniques to probe the wettability of graphitic carbon surface.19,26,31 They reported contact angle of 68.2 ± 1.6° for freshly cleaved HOPG that increased to 90° upon air exposure. Based on FTIR evidence, the authors attributed the increase in hydrophobicity to surface adsorption of both airborne hydrocarbon and water.19 Very recently, we reported the effect of defect on the wettability of graphite using dynamic contact angle measurements. It was shown that edge defect significantly impacts the static water contact value. We suggested that the advancing contact angle (68.6º ± 7.1°) reflects the intrinsic wettability of defect free basal plane graphite.32 One of the surprising findings from these studies is how easily and quickly the graphitic surfaces can be contaminated. Simply exposing the samples to lab air for as little as 10 min could result in a full monolayer coverage of hydrocarbon contaminant on graphite, completely changing its wettability.21 Indeed, although the concentration of hydrocarbon is extremely low, typically on the order of parts-per-billion (ppb) to parts-per-trillion (ppt) levels, diffusion in air is extremely fast and only minute amount of material is needed to form a monolayer coverage. This fast contamination kinetics may explain the consistent report of hydrophobic behavior of graphitic materials in the past. It is worth pointing out that while many researchers believe that clean rooms and gloveboxes are free of hydrocarbon contamination,33 this is not true. In fact, one of the major sources of airborne hydrocarbon is the plasticizer emission from plastics and rubbers used in flooring, furniture, and lab consumables.34 Therefore, while clean rooms and gloveboxes are particulate-free and H 2 O/O 2 -free, respectively, they are not hydrocarbon-free. For example, we have shown that a clean graphene sample stored in a polystyrene Petri dish showed a significantly faster increase of hydrophobicity than a similar sample stored in a glass Petri dish.18
3. Implications Given the unsuspected nature of the surface contamination process and the fact that many researchers has been unknowingly studying contaminated graphitic samples for over 75 years, one naturally wonders if there are far reaching implications and in particular, if research on other surface properties of graphitic materials have been similarly compromised. 3.1 Theoretical studies of water-carbon interactions. Although the theoretical study of water-graphitic carbon interaction has a long history, accurate modeling of such interaction is challenging due to the significant contribution from many-body effects. Jenness et al. modeled 5
the interaction between a water molecule and various acene molecules using density fittingdensity functional theory-symmetry-adapted perturbation theory (DF-DFT-SAPT) method. Extrapolating the data to infinite large acene allows them to predict a water-graphene interaction energy of -2.20 kcal/mol (-9.2 kJ/mol);35 in a later study, they updated their estimate to −3.0 ± 0.15 kcal/mol (-13.0 ± 0.63 kJ/mol).36 Bludský and coworkers used density-functional/coupledcluster (DFT/CC) level of theory to calculate the binding energy of water to graphene and graphite and found them to be ca. -13 kJ/mol and -15 kJ/mol, respectively.37,38 Ma et al. modeled water adsorption on graphene using quantum Monte Carlo and the random-phase approximation and obtained values in the range of -70 meV to -98 meV (-6.8 kJ/mol to 9.5 kJ/mol ), depending on the orientation of the water molecule.39 Voloshina et al. used CCSD(T) level of theory to model the water-graphene interaction and obtained binding energy of water of -135 meV (-13.0 kJ/mol) for the most favorable geometry.40 As can be seen, there is still a large variation in the calculated binding energy of water and the lack of reliable experimental data makes it difficult to assess the quality of the theoretical results. However, most of the computational studies showed that the preferred adsorption orientation for water is hydrogen pointing into graphene lattice. Such a geometry supports the idea that π-hydrogen bonding contributes to the intrinsic hydrophilicity of graphitic materials.41 Following the experimental report of the intrinsic hydrophilicity of graphitic materials, efforts have been made to understand this unexpected new result using molecular dynamics simulations. Prior to the discovery of intrinsic hydrophilicity of graphitic carbon, there are already many force fields developed for water-graphene/graphite interaction. Most of these force fields are empirical or derived by fitting to high level theory calculation.42-45 However, least one popular force field was developed by fitting to experimental data of water contact angle 86° and therefore underestimates the water-carbon interaction.46 As the recent experimental studies mostly reported water contact angles of ca. 60° for freshly cleaved HOPG, several groups reported updated force fields for water-carbon interactions. Wu et al. derived several sets of water-carbon interaction parameters by fitting to the recently reported high level theoretical calculations. They found that parameters obtained from random phase approximation (RPA) and DFT-SAPT calculation best reproduce the water contact angles of clean graphite. They also showed that contamination of graphite surface by ethane significantly increases the water contact angle.47 Ramos-Alvarado et al. also reported an updated water-carbon force field by fitting the calculated water contact angle to 64°.48,49 3.2 Wetting transparency. Due to the sub-nm thinness of graphene (and other 2D materials), liquid on graphene could interact with the substrate underneath graphene if their interactions are dominated by long range van der Waals forces. The single layer graphene should experience the most impact by the substrate; as the thickness of graphene increases, the effect of the substrate will diminish and for very thick graphene layers, its wettability should approach that of graphite. This effect was termed wetting transparency and could allow independent control of wetting and other material properties, such as electrical conductivity. Three early studies of wetting transparency concluded graphene as wetting transparent,50 partially wetting transparent,51 and non-wetting transparent.15 It is now understood that such conflicting result is due to, at least in part, airborne hydrocarbon contamination and when such effect was accounted 6
for, single layer graphene was shown to be partially wetting transparent and the degree of wetting transparency decreases with increasing thickness of graphene.18,52,53 However, there is still ongoing debate on the origin of the partial wetting transparency effect. Hong et al. and Ashraf et al. showed that charge doping could increase the hydrophilicity of graphene.54,55 Hong et al. further suggested that the wetting transparency effect arises from the charge doping of graphene by the substrate, instead of the long range van der Waals force.54 On the other hand, Annamalai et al. measured wettability of a number of 2D van der Waals heterostructures (e.g., graphene/BN, MoS 2 /BN).56 From surface energy calculations, they concluded that the interaction between liquid and graphene, MoS 2 , and WS 2 are dominated by van der Waals forces. However, when BN was used as a supporting substrate, there is a nonnegligible polar component, which the authors attributed to the polar nature of BN itself. Ondarcuhu et al. prepared partially suspended graphene and used thermal annealing to remove airborne contaminant before their wetting measurement.25 They concluded that ~20% of interaction experienced by the liquid comes from the supporting substrate underneath graphene. They also predicted water contact angles for graphene totally suspended in air and on water to be 85 ± 5° , and 61 ± 5° , respectively. In comparison, Li et al. reported a water contact angle of ca. 42° for copper supported graphene.18 3.3 Electrochemistry of graphitic carbons. Being one of the most important electrode materials, the electrochemical activity of graphitic carbons have been extensively studied.2 In particular, the basal plane of graphite is a good model electrode because its chemical structure is very well defined and chemically stable. One of the most fundamental properties of electrode
(a)
(b)
Figure 2. (a) Cyclic Voltammetry for the oxidation of 1 mM Fe(CN)64– in 0.1 M KCl after a freshly cleaved HOPG surface was left in air for 0 min (black), 1 h (red), and 3 h (green). Note that the air exposure increased the separation of the oxidation and reduction peaks, which indicates a decrease of the electrode performance. Reproduced with permission from Patel, A. N.; et al. J. Am. Chem. Soc. 2012, 134, 20117. Copyright American Chemical Society. (b) Nanogap voltammograms of 0.3 mM (ferrocenylmethyl)trimethylammonium in 50 mM KCl at an HOPG surface with reduced hydrocarbon contamination. Forward and reverse waves are represented by solid and dashed lines. Dots represent reversible voltammograms. Fitting of these data to models produced heterogeneous electron transfer rate constants of ≥12 cm/s. Reproduced with permission from Chen, R.; et al. Anal. Chem. 2016, 88, 8323. Copyright American Chemical Society. 7
performance is the heterogeneous electron transfer rate constant (k0), which describes the rate of electron transfer across the electrode-electrolyte interface. Work in the 1980’s has established that the k0 of basal plane of graphite is several orders of magnitude lower compared to that of noble metal electrode.57-59 This observation was attributed in part to the low density of state of the basal plane and the absence of specific adsorption of redox couples. Similar to the case of hydrophobicity of graphitic surfaces, the idea that pristine sp2 basal plane is of low electrochemical activity is generally accepted by the community. Recently, several groups have revisited the fundamental electrochemical properties of basal plane graphite using advanced electrochemical technique such as scanning electrochemical microscopy.60,61 To their surprise, they observed extremely fast electron transfer kinetics between basal plane graphite and solution redox couples (Figure 2a). The k0 values reported by these studies are several orders of magnitude higher than the previously reported values.62,63 The electrochemical activity is sensitive to the environment: the Amemiya group showed that by using ultraclean electrolyte solutions (ca. 1 - 2 ppb of total organic carbon (TOC)), extremely large k0 values (≥12 cm/s), comparable or even larger than those reported for Pt and Au electrodes, were obtained (Figure 2b).64,65 However, the electrode performance significantly degraded when using electrolyte having ca. 20 ppb TOC or after aging the electrode in air.27,66 These results strongly suggest that the previously observed low electrochemical activity of basal plane graphite is likely due to surface contamination.
4. Outlook. Given the precedence in the wetting and electrochemical studies, it is reasonable to suspect that studies of other interfacial properties, such as adhesion, adsorption, and friction, may not have properly accounted for the potential airborne contamination of the samples either. 4.1 Adhesion and adsorption are directly related to the surface energy of solid substrates. Traditionally, graphitic materials were viewed as non-polar in nature and therefore have low adsorption energy and adhesion energies. However, recent work has already showed that the surface energies of ‘clean’ graphite and graphene are noticeably larger (ca. 25%) than those of their ‘dirty’ counterparts. In addition, for clean graphitic materials, there is a non-negligible polar component in the surface energy that is absent when the surface got contaminated.24 These results suggests that the adhesion and adsorption behavior of clean graphitic materials may be very different from the dirty ones, both in term of their molecular mechanism and magnitude of the interfacial interaction. Efforts have already been made to update the molecular dynamics force field to better model graphitic surfaces.46,49 A clear understanding of the adsorption/adhesion behavior of clean graphitic surfaces may lead to improved adsorbents and stronger composite materials. 4.2 Lubrication and wear. Graphite is a well-known solid lubricant. Although the lubricity was commonly explained by the sliding action between the graphite layers, it is long known that the lubrication performance of graphite depends on certain components in air.67-70 Adsorption of H 2 O, O 2 , and various hydrocarbon vapors impacts the lubrication performance of graphite. The effect of hydrocarbon is especially strong. For example, even sub-ppm level of n8
heptane vapor can significantly reduce the wear of two sliding graphite rods and the effectiveness increase as the size of the hydrocarbon increases (Figure 3a).67 With this in mind, the airborne hydrocarbon contamination may very well play a role in the lubrication performance of graphite. Note that the airborne hydrocarbon is believed to physisorb onto graphitic surface and could be desorbed at elevated temperatures or under high vacuum.21,30 Coincidently, it is known that graphite-based lubricant does not work in ultra-high vacuum; in air, the friction coefficient and wear rate of graphite increase significantly at high temperature (ca. 300 ºC, Figure 3b).71 Understanding the role of airborne contamination on the lubrication of graphitic carbons may provide new insight into the mechanism of friction and wear of this important solid lubricant.
Wear Rate (mm/min)
(a)
(b)
0.1 0.08 0.06 0.04 0.02 0 0
10 30 40 20 Vapor Pressure (microns of Hg)
Figure 3. (a) Wear rate of graphite as a function of vapor pressure of n-heptane. The measurements started from 0 to 40 µmHg (top curve) then returned to 0 µmHg (bottom curve). Note the significant hysteresis in the measured wear rate. Figure replotted with permission using data from Savage, R. H.; et al. J. Appl. Phys. 1956, 27, 136. Copyright American Institute of Physics. (b) Wear of graphite by stainless steel as a function of temperature. Reproduced with permission from Luo, X.; et al. Nucl. Eng. Des. 2005, 235, 2261. Copyright Elsevier.
4.3 “Clean” manufacturing. The research has clearly demonstrated that ‘clean’ carbon shows superior properties compared to their ‘dirty’ counterpart, at least in the case of wetting and electrochemical activities. These properties could have significant implications to device performances given the wide spread use of graphitic materials in batteries, supercapacitors, and filters. An important question is whether it is possible to use clean carbon materials in these devices. Given how prevalence the airborne hydrocarbons are, is it even possible to produce clean carbon materials and maintain their cleanness in devices at an industrial scale? While realizing this goal will clearly need extensive work by scientists and engineers, the industry has many precedencies of manufacturing under highly-controlled environments. One such example is the assembly of Li-ion batteries from components that are extremely moisture sensitive. The work is done not in glove boxes but in dry rooms that maintain very low humidity levels.72
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It is conceivable that a hydrocarbon-free room can be constructed to process clean carbon materials. “Dirty” carbon materials could be cleaned by thermal annealing or other cleaning methods recently reported to remove hydrocarbon contaminant on graphene and graphite, e.g., ‘dry cleaning’, CO 2 jet, and UV/O 3 .18,21,73,74 Li et al. also showed that a thin-water layer coating could significantly decrease the rate of adsorption of airborne hydrocarbons on graphene and graphite by up to 20 times. Using this technique, they showed that a graphite electrode can maintain its high electrochemical activity for over a day.66 These ‘clean’ or less contaminated carbon materials could then be sealed (e.g., in a battery case) to prevent future contamination and maintain its high performance in the device. Certainly, the manufacturing processes need to be modified to eliminate hydrocarbon contamination sources, such as plastics parts that emit low molecular weight hydrocarbons.34 Demonstration of substantial improvement in the device performance is clearly needed to justify such efforts and we expect to see active research in these directions in the coming years.
Acknowledgement HL acknowledges partial support from ONR (N000141512520).
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(32) Kozbial, A.; Trouba, C.; Liu, H.; Li, L. Characterization of the Intrinsic Water Wettability of Graphite Using Contact Angle Measurements: Effect of Defects on Static and Dynamic Contact Angles Langmuir 2017. DOI: 10.1021/acs.langmuir.6b04193 (33) Güell, A. G.; Cuharuc, A. S.; Kim, Y.-R.; Zhang, G.; Tan, S.-y.; Ebejer, N.; Unwin, P. R. Redox-Dependent Spatially Resolved Electrochemistry at Graphene and Graphite Step Edges Acs Nano 2015, 9, 3558-3571. (34) Ho, D. X.; Kim, K. H.; Sohn, J. R.; Oh, Y. H.; Ahn, J. W. Emission Rates of Volatile Organic Compounds Released from Newly Produced Household Furniture Products Using a Large-Scale Chamber Testing Method Sci. World J. 2011, 11, 1597-1622. (35) Jenness, G. R.; Jordan, K. D. DF-DFT-SAPT Investigation of the Interaction of a Water Molecule to Coronene and Dodecabenzocoronene: Implications for the Water-Graphite Interaction Journal of Physical Chemistry C 2009, 113, 10242-10248. (36) Jenness, G. R.; Karalti, O.; Jordan, K. D. Benchmark calculations of water-acene interaction energies: Extrapolation to the water-graphene limit and assessment of dispersion-corrected DFT methods Physical chemistry chemical physics : PCCP 2010, 12, 6375-6381. (37) Rubes, M.; Kysilka, J.; Nachtigall, P.; Bludsky, O. DFT/CC investigation of physical adsorption on a graphite (0001) surface Physical chemistry chemical physics : PCCP 2010, 12, 64386444. (38) Kysilka, J.; Rubes, M.; Grajciar, L.; Nachtigall, P.; Bludsky, O. Accurate Description of Argon and Water Adsorption on Surfaces of Graphene-Based Carbon Allotropes Journal of Physical Chemistry A 2011, 115, 11387-11393. (39) Ma, J.; Michaelides, A.; Alfe, D.; Schimka, L.; Kresse, G.; Wang, E. G. Adsorption and diffusion of water on graphene from first principles Physical Review B 2011, 84. (40) Voloshina, E.; Usvyat, D.; Schutz, M.; Dedkov, Y.; Paulus, B. On the physisorption of water on graphene: a CCSD(T) study Physical chemistry chemical physics : PCCP 2011, 13, 1204112047. (41) Suzuki, S.; Green, P. G.; Bumgarner, R. E.; Dasgupta, S.; Goddard, W. A.; Blake, G. A. Benzene Forms Hydrogen-Bonds with Water Science 1992, 257, 942-944. (42) Hummer, G.; Rasaiah, J. C.; Noworyta, J. P. Water conduction through the hydrophobic channel of a carbon nanotube Nature 2001, 414, 188-190. (43) Gordillo, M. C.; Marti, J. Hydrogen bond structure of liquid water confined in nanotubes Chem Phys Lett 2000, 329, 341-345. (44) Pertsin, A.; Grunze, M. Water as a lubricant for graphite: A computer simulation study Journal of Chemical Physics 2006, 125. (45) Zhao, X. C.; Johnson, J. K. An effective potential for adsorption of polar molecules on graphite Mol Simulat 2005, 31, 1-10. (46) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. On the watercarbon interaction for use in molecular dynamics simulations of graphite and carbon nanotubes J Phys Chem B 2003, 107, 1345-1352. (47) Wu, Y. B.; Aluru, N. R. Graphitic Carbon-Water Nonbonded Interaction Parameters Journal of Physical Chemistry B 2013, 117, 8802-8813. (48) Ramos-Alvarado, B.; Kumar, S.; Peterson, G. P. Wettability of graphitic-carbon and silicon surfaces: MD modeling and theoretical analysis Journal of Chemical Physics 2015, 143, 044703 (49) Ramos-Alvarado, B.; Kumar, S.; Peterson, G. P. On the wettability transparency of graphene-coated silicon surfaces Journal of Chemical Physics 2016, 144, 014701. (50) Rafiee, J.; Mi, X.; Gullapalli, H.; Thomas, A. V.; Yavari, F.; Shi, Y.; Ajayan, P. M.; Koratkar, N. A. Wetting transparency of graphene Nat Mater 2012, 11, 217-222. (51) Shih, C.-J.; Wang, Q. H.; Lin, S.; Park, K.-C.; Jin, Z.; Strano, M. S.; Blankschtein, D. Breakdown in the Wetting Transparency of Graphene Physical Review Letters 2012, 109, 176101. 12
(52) Shih, C.-J.; Strano, M. S.; Blankschtein, D. Wetting translucency of graphene Nat Mater 2013, 12, 866-869. (53) Xu, K.; Heath, J. R. Wetting: Contact with what? Nat Mater 2013, 12, 872-873. (54) Hong, G.; Han, Y.; Schutzius, T. M.; Wang, Y. M.; Pan, Y.; Hu, M.; Jie, J. S.; Sharma, C. S.; Muller, U.; Poulikakos, D. On the Mechanism of Hydrophilicity of Graphene Nano Letters 2016, 16, 4447-4453. (55) Ashraf, A.; Wu, Y. B.; Wang, M. C.; Yong, K.; Sun, T.; Jing, Y. H.; Haasch, R. T.; Aluru, N. R.; Nam, S. Doping-Induced Tunable Wettability and Adhesion of Graphene Nano Letters 2016, 16, 4708-4712. (56) Annamalai, M.; Gopinadhan, K.; Han, S. A.; Saha, S.; Park, H. J.; Cho, E. B.; Kumar, B.; Patra, A.; Kim, S. W.; Venkatesan, T. Surface energy and wettability of van der Waals structures Nanoscale 2016, 8, 5764-5770. (57) Kneten, K. R.; Mccreery, R. L. Effects of Redox System Structure on Electron-Transfer Kinetics at Ordered Graphite and Glassy-Carbon Electrodes Analytical Chemistry 1992, 64, 2518-2524. (58) Bowling, R. J.; Packard, R. T.; Mccreery, R. L. Activation of Highly Ordered PyrolyticGraphite for Heterogeneous Electron-Transfer - Relationship between Electrochemical Performance and Carbon Microstructure Journal of the American Chemical Society 1989, 111, 1217-1223. (59) Wightman, R. M.; Deakin, M. R.; Kovach, P. M.; Kuhr, W. G.; Stutts, K. J. Methods to Improve Electrochemical Reversibility at Carbon Electrodes J Electrochem Soc 1984, 131, 1578-1583. (60) Unwin, P. R.; Güell, A. G.; Zhang, G. Nanoscale Electrochemistry of sp2 Carbon Materials: From Graphite and Graphene to Carbon Nanotubes Accounts of Chemical Research 2016. (61) Amemiya, S.; Chen, R.; Nioradze, N.; Kim, J. Scanning Electrochemical Microscopy of Carbon Nanomaterials and Graphite Accounts of Chemical Research 2016. (62) Patel, A. N.; Collignon, M. G.; O'Connell, M. A.; Hung, W. O. Y.; McKelvey, K.; Macpherson, J. V.; Unwin, P. R. A New View of Electrochemistry at Highly Oriented Pyrolytic Graphite Journal of the American Chemical Society 2012, 134, 20117-20130. (63) Chen, R.; Nioradze, N.; Santhosh, P.; Li, Z. T.; Surwade, S. P.; Shenoy, G. I.; Parobek, D. G.; Kim, M. A.; Liu, H. T.; Amemiya, S. Ultrafast Electron Transfer Kinetics of Graphene Grown by Chemical Vapor Deposition Angew Chem Int Edit 2015, 54, 15134-15137. (64) Chen, R.; Balla, R. J.; Li, Z.; Liu, H.; Amemiya, S. Origin of Asymmetry of Paired Nanogap Voltammograms Based on Scanning Electrochemical Microscopy: Contamination Not Adsorption Analytical Chemistry 2016, 88, 8323-8331. (65) Tan, S. Y.; Zhang, J.; Bond, A. M.; Macpherson, J. V.; Unwin, P. R. Impact of Adsorption on Scanning Electrochemical Microscopy Voltammetry and Implications for Nanogap Measurements Analytical Chemistry 2016, 88, 3272-3280. (66) Li, Z.; Kozbial, A.; Nioradze, N.; Parobek, D.; Shenoy, G. J.; Salim, M.; Amemiya, S.; Li, L.; Liu, H. Water Protects Graphitic Surface from Airborne Hydrocarbon Contamination Acs Nano 2016, 10, 349-359. (67) Savage, R. H.; Schaefer, D. L. Vapor Lubrication of Graphite Sliding Contacts Journal of Applied Physics 1956, 27, 136-138. (68) Savage, R. H. Physically and Chemically Adsorbed Films in the Lubrication of Graphite Sliding Contacts Ann Ny Acad Sci 1951, 53, 862-869. (69) Savage, R. H. Graphite Lubrication Journal of Applied Physics 1948, 19, 1-10. (70) Lancaster, J. K. A Review of the Influence of Environmental Humidity and Water on Friction, Lubrication and Wear Tribol Int 1990, 23, 371-389. (71) Luo, X. W.; Yu, S. Y.; Sheng, X. Y.; He, S. Y. Temperature effect on IG-11 graphite wear performance Nucl Eng Des 2005, 235, 2261-2274. (72) Li, J. L.; Daniel, C.; Wood, D. Materials processing for lithium-ion batteries J Power Sources 2011, 196, 2452-2460. (73) Algara-Siller, G.; Lehtinen, O.; Turchanin, A.; Kaiser, U. Dry-cleaning of graphene Applied Physics Letters 2014, 104. 13
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S2352-4316(16)30205-X http://dx.doi.org/10.1016/j.eml.2017.01.010 EML 266
To appear in:
Extreme Mechanics Letters
Received date: 28 September 2016 Revised date: 31 January 2017 Accepted date: 31 January 2017 Please cite this article as: H. Liu, L. Li, Graphitic materials: Intrinsic hydrophilicity and its implications, Extreme Mechanics Letters (2017), http://dx.doi.org/10.1016/j.eml.2017.01.010 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Graphitic Materials: Intrinsic Hydrophilicity and Its Implications Haitao Liu a *, Lei Li b, c * a
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA Department of Chemical & Petroleum Engineering, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, 15261, USA c Department of Mechanical Engineering & Materials Science, Swanson School of Engineering, University of Pittsburgh, Pittsburgh, PA, 15261, USA b
*Corresponding authors: [email protected], [email protected]
Abstract Graphitic materials are long regarded as model hydrophobic materials. However, recent work has shown that graphite and graphene are much more hydrophilic than previously thought. It was revealed that the commonly observed hydrophobic nature of graphite is due to airborne hydrocarbon contamination that was not considered in previous studies. This perspective highlights these recent developments and discusses their implications to research on watercarbon interactions, wetting transparency, electrochemistry, adsorption and adhesion, and lubrication and wear.
Keywords: carbon; contact angle; surface contamination; electrochemistry; surface properties; wear.
1
1. Background. Graphitic materials consist of primarily sp2 carbon and exist in forms such as graphite, carbon nanotubes, graphene, and activated carbon. They are used in numerous applications in modern society. An example of particular relevance to the field of mechanics is carbon fiber, which is a major component in light weight, high strength composites.1 Other applications of graphitic carbon materials include electrodes,2 lubricants,3 refractory materials,4 adsorbents,5 and bearings/seals.6 All these applications involve interfacial interaction between graphitic materials and their environments, such as bonding with polymer matrix, adsorption of molecules from the gas/liquid phase, electron transfer across the graphite-liquid interface, and friction against another solid surface. Fundamental surface properties, such as wettability and surface energy are extensively used to interpret and model these interfacial interactions. Wettability of graphitic materials has been studied for over 75 years. The early work extensively focused on natural and artificial graphite and more recent ones extended to carbon nanotubes and graphene.7-10 Many research groups around the world have carried out wetting measurements on various types of graphitic materials and based on these data, calculated surface energy by fitting the wetting data with models. Overall, this rich literature gave a consistent picture, that is, pristine graphitic materials predominately made of sp2 carbon are hydrophobic, with a water contact angle of ca. 85º, and have a low surface energy (Table 1). Surface modification, covalent or non-covalent, can increase or decrease the wettability by introducing polar or nonpolar functional groups, respectively.9,11
Table 1. Selected water contact values of graphitic materials. References Material Fowkes, 19407 Natural graphite Morcos, 197012 Exfoliated graphite Li, 200113 Carbon nanotube films Shin, 201011 HOPGa Ou, 201014 HOPG 15 Raj, 2013 Graphene a HOPG: highly oriented pyrolytic graphite.
Water Contact Angle 85.3° - 85.9° 84.2° 136.5° 91° 91° 90°
One question that caught little attention, however, is whether these wetting measurements reflect the intrinsic wetting behavior of the graphitic materials. This is a valid concern because wettability is highly surface sensitive. Even sub-monolayer amount of surface contamination can significantly impact the wettability. At the same time, surface contamination can occur quickly because only a small amount of material is needed. Little was done to verify that the graphitic surfaces used in previous studies were actually contamination-free. Schrader was among the first to challenge the conventional view that graphite is intrinsically hydrophobic. In a series of studies conducted in the 1970s and 1980s, he prepared clean graphite surfaces by high temperature annealing in an ultrahigh vacuum (UHV) chamber. 2
Water vapor was then introduced into the chamber and condensed onto a cold finger and used for contact angle measurement.16,17 Schrader reported water contact angle values around 35º using this method, significantly lower than the commonly accepted value of 85º. Measurements were also conducted on freshly cleaved graphite surfaces in air and contact angle values ranging from 40º to 50º were reported. Schrader pointed out that the result is consistent with surface contamination by airborne hydrocarbons but short of providing spectroscopic evidence to support this claim.16 Unfortunately, this work received little attention and acceptance by the carbon and tribology communities. A major concern of this work is the possibility of water evaporation under reduced pressure, which will result in a reduction in the measured water contact angle.
2. Intrinsic Hydrophilicity of Graphitic Materials. The rise of graphene renewed the interest in understanding the intrinsic wettability of graphitic materials. In the past 3 years, a number of studies have shown that both graphene and graphite can be rapidly contaminated by airborne hydrocarbons and such contamination is responsible for the hydrophobic behaviors that were consistently observed in the past 75 years.18-23 Notably, these recent work also addressed the concern of water evaporation in vacuum and provided convincing spectroscopic evidence that identifies the surface contamination as hydrocarbon and correlates its presence to the change of wettability. We briefly review these developments below. The Li and Liu groups studied the time evolution of the wettability of graphene and graphite upon their exposure to ambient air.18,21,24 They showed that the water contact angle of freshly prepared graphene and graphite samples are ca. 42º and 64º degrees, respectively. These values are significant lower than the commonly reported and accepted values of 80º to 90º for these materials. Upon exposure to ambient air, the contact angle values increased to ca. 90º, consistent with the previously accepted results. Using a combination of surface-sensitive spectroscopy techniques, including attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray Photoelectron Spectroscopy (XPS), and spectroscopic ellipsometry, it was shown that airborne hydrocarbons adsorb onto graphitic surfaces upon air exposure and the degree of surface contamination highly correlates with the change in the wettability (Figure 1). These data shows that graphitic materials are significantly more hydrophilic than previously believed. Note that the wettability of graphene depends on its supporting substrate. Recent studies suggest that a single layer graphene suspended in air is expected to be more hydrophobic than supported graphene, with a water contact angle of ca. 85 (see discussions on wetting transparency below).25 Several other research groups have reported parallel and follow-up studies that focus on this topic.19,20,22,26-28 Ashraf et al. reported the effect of exposure to supercritical water on the surface of several carbon materials.29 They showed that an as-received HOPG is hydrophobic with a water contact angle of 95º while the contact angle of freshly cleaved HOPG is 62º. The freshly cleaved HOPG also showed a much lower surface oxygen content than as-received isomolded graphite plate sample, suggesting the later was contaminated by airborne hydrocarbon. Using time-of-flight secondary ion mass spectrometry (ToF-SIMS), C x H y clusters ions were 3
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Figure 1. Surface contamination of graphene and graphite by airborne hydrocarbon and its impact on the wettability. (a) Time evolution of water contact angle of a copper-supported graphene sample measured in air. The inset shows photographs of water droplet of three measurements. (b) Time evolution of water contact angle (WCA) and thickness of hydrocarbon contaminant measured by ellipsometry of a freshly exfoliated HOPG sample in air. (c) ATR-FTIR spectra of a copper-supported graphene sample exposed to lab air. The inset shows the time evolution of the intensity of the symmetrical (s-CH2) and antisymmetrical stretching (as-CH2) peaks of -CH2-. (d) Carbon XPS peaks of a copper-supported graphene sample when it was fresh (black) and after several days of exposure in lab air (red). The inset shows the difference spectrum. Panels (a) (c) (d) are adapted with permission from Li et al. Nature Mater. 2013, 12, 925. Copyright Nature Publishing Group. Panel (b) is adapted with permission from Kozbial, et al. Carbon 2014, 74, 218. Copyright Elsevier. observed on both as-received iso-molded graphite plate and freshly cleaved HOPG. MartinezMartin et al. studied the surface contamination of graphite by Kelvin probe force microscopy and mass spectrometry. They observed adsorption of polyaromatic hydrocarbon onto a graphite sample placed inside a vacuum chamber. The adsorption is detected by both AFM topography imaging (a height increase of 0.8 nm) and change of contact potential. The contact angles for the freshly cleaved HOPG and the contaminated sample are 70º and 95º, respectively. Thermal 4
annealing at 337K and 1×10-5 mbar resulted in a complete desorption of the contaminant.30 Wei et al. reported water contact angle of edge plane of graphite as 60.8 ± 9º, depending on the orientation of the edge; thermal annealing in He gas reduced the contact angle value to 42.9 ± 6.7º, which the authors attributed to surface oxidation by the trace amount of O 2 in the He gas.22 Unlike the basal plane, there was no increase of the water contact angle upon air exposure for both as-prepared and annealed edge plane samples. The authors argued that the contamination of the edge plane is likely too fast to be resolved by contact angle measurement and the true water contact angle of the edge plane of graphite is likely <40º. The Chiesa group reported a series of studies combining both macroscopic (using water contact angle) and microscopic (using atomic force microscopy and scanning electron microscope) techniques to probe the wettability of graphitic carbon surface.19,26,31 They reported contact angle of 68.2 ± 1.6° for freshly cleaved HOPG that increased to 90° upon air exposure. Based on FTIR evidence, the authors attributed the increase in hydrophobicity to surface adsorption of both airborne hydrocarbon and water.19 Very recently, we reported the effect of defect on the wettability of graphite using dynamic contact angle measurements. It was shown that edge defect significantly impacts the static water contact value. We suggested that the advancing contact angle (68.6º ± 7.1°) reflects the intrinsic wettability of defect free basal plane graphite.32 One of the surprising findings from these studies is how easily and quickly the graphitic surfaces can be contaminated. Simply exposing the samples to lab air for as little as 10 min could result in a full monolayer coverage of hydrocarbon contaminant on graphite, completely changing its wettability.21 Indeed, although the concentration of hydrocarbon is extremely low, typically on the order of parts-per-billion (ppb) to parts-per-trillion (ppt) levels, diffusion in air is extremely fast and only minute amount of material is needed to form a monolayer coverage. This fast contamination kinetics may explain the consistent report of hydrophobic behavior of graphitic materials in the past. It is worth pointing out that while many researchers believe that clean rooms and gloveboxes are free of hydrocarbon contamination,33 this is not true. In fact, one of the major sources of airborne hydrocarbon is the plasticizer emission from plastics and rubbers used in flooring, furniture, and lab consumables.34 Therefore, while clean rooms and gloveboxes are particulate-free and H 2 O/O 2 -free, respectively, they are not hydrocarbon-free. For example, we have shown that a clean graphene sample stored in a polystyrene Petri dish showed a significantly faster increase of hydrophobicity than a similar sample stored in a glass Petri dish.18
3. Implications Given the unsuspected nature of the surface contamination process and the fact that many researchers has been unknowingly studying contaminated graphitic samples for over 75 years, one naturally wonders if there are far reaching implications and in particular, if research on other surface properties of graphitic materials have been similarly compromised. 3.1 Theoretical studies of water-carbon interactions. Although the theoretical study of water-graphitic carbon interaction has a long history, accurate modeling of such interaction is challenging due to the significant contribution from many-body effects. Jenness et al. modeled 5
the interaction between a water molecule and various acene molecules using density fittingdensity functional theory-symmetry-adapted perturbation theory (DF-DFT-SAPT) method. Extrapolating the data to infinite large acene allows them to predict a water-graphene interaction energy of -2.20 kcal/mol (-9.2 kJ/mol);35 in a later study, they updated their estimate to −3.0 ± 0.15 kcal/mol (-13.0 ± 0.63 kJ/mol).36 Bludský and coworkers used density-functional/coupledcluster (DFT/CC) level of theory to calculate the binding energy of water to graphene and graphite and found them to be ca. -13 kJ/mol and -15 kJ/mol, respectively.37,38 Ma et al. modeled water adsorption on graphene using quantum Monte Carlo and the random-phase approximation and obtained values in the range of -70 meV to -98 meV (-6.8 kJ/mol to 9.5 kJ/mol ), depending on the orientation of the water molecule.39 Voloshina et al. used CCSD(T) level of theory to model the water-graphene interaction and obtained binding energy of water of -135 meV (-13.0 kJ/mol) for the most favorable geometry.40 As can be seen, there is still a large variation in the calculated binding energy of water and the lack of reliable experimental data makes it difficult to assess the quality of the theoretical results. However, most of the computational studies showed that the preferred adsorption orientation for water is hydrogen pointing into graphene lattice. Such a geometry supports the idea that π-hydrogen bonding contributes to the intrinsic hydrophilicity of graphitic materials.41 Following the experimental report of the intrinsic hydrophilicity of graphitic materials, efforts have been made to understand this unexpected new result using molecular dynamics simulations. Prior to the discovery of intrinsic hydrophilicity of graphitic carbon, there are already many force fields developed for water-graphene/graphite interaction. Most of these force fields are empirical or derived by fitting to high level theory calculation.42-45 However, least one popular force field was developed by fitting to experimental data of water contact angle 86° and therefore underestimates the water-carbon interaction.46 As the recent experimental studies mostly reported water contact angles of ca. 60° for freshly cleaved HOPG, several groups reported updated force fields for water-carbon interactions. Wu et al. derived several sets of water-carbon interaction parameters by fitting to the recently reported high level theoretical calculations. They found that parameters obtained from random phase approximation (RPA) and DFT-SAPT calculation best reproduce the water contact angles of clean graphite. They also showed that contamination of graphite surface by ethane significantly increases the water contact angle.47 Ramos-Alvarado et al. also reported an updated water-carbon force field by fitting the calculated water contact angle to 64°.48,49 3.2 Wetting transparency. Due to the sub-nm thinness of graphene (and other 2D materials), liquid on graphene could interact with the substrate underneath graphene if their interactions are dominated by long range van der Waals forces. The single layer graphene should experience the most impact by the substrate; as the thickness of graphene increases, the effect of the substrate will diminish and for very thick graphene layers, its wettability should approach that of graphite. This effect was termed wetting transparency and could allow independent control of wetting and other material properties, such as electrical conductivity. Three early studies of wetting transparency concluded graphene as wetting transparent,50 partially wetting transparent,51 and non-wetting transparent.15 It is now understood that such conflicting result is due to, at least in part, airborne hydrocarbon contamination and when such effect was accounted 6
for, single layer graphene was shown to be partially wetting transparent and the degree of wetting transparency decreases with increasing thickness of graphene.18,52,53 However, there is still ongoing debate on the origin of the partial wetting transparency effect. Hong et al. and Ashraf et al. showed that charge doping could increase the hydrophilicity of graphene.54,55 Hong et al. further suggested that the wetting transparency effect arises from the charge doping of graphene by the substrate, instead of the long range van der Waals force.54 On the other hand, Annamalai et al. measured wettability of a number of 2D van der Waals heterostructures (e.g., graphene/BN, MoS 2 /BN).56 From surface energy calculations, they concluded that the interaction between liquid and graphene, MoS 2 , and WS 2 are dominated by van der Waals forces. However, when BN was used as a supporting substrate, there is a nonnegligible polar component, which the authors attributed to the polar nature of BN itself. Ondarcuhu et al. prepared partially suspended graphene and used thermal annealing to remove airborne contaminant before their wetting measurement.25 They concluded that ~20% of interaction experienced by the liquid comes from the supporting substrate underneath graphene. They also predicted water contact angles for graphene totally suspended in air and on water to be 85 ± 5° , and 61 ± 5° , respectively. In comparison, Li et al. reported a water contact angle of ca. 42° for copper supported graphene.18 3.3 Electrochemistry of graphitic carbons. Being one of the most important electrode materials, the electrochemical activity of graphitic carbons have been extensively studied.2 In particular, the basal plane of graphite is a good model electrode because its chemical structure is very well defined and chemically stable. One of the most fundamental properties of electrode
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(b)
Figure 2. (a) Cyclic Voltammetry for the oxidation of 1 mM Fe(CN)64– in 0.1 M KCl after a freshly cleaved HOPG surface was left in air for 0 min (black), 1 h (red), and 3 h (green). Note that the air exposure increased the separation of the oxidation and reduction peaks, which indicates a decrease of the electrode performance. Reproduced with permission from Patel, A. N.; et al. J. Am. Chem. Soc. 2012, 134, 20117. Copyright American Chemical Society. (b) Nanogap voltammograms of 0.3 mM (ferrocenylmethyl)trimethylammonium in 50 mM KCl at an HOPG surface with reduced hydrocarbon contamination. Forward and reverse waves are represented by solid and dashed lines. Dots represent reversible voltammograms. Fitting of these data to models produced heterogeneous electron transfer rate constants of ≥12 cm/s. Reproduced with permission from Chen, R.; et al. Anal. Chem. 2016, 88, 8323. Copyright American Chemical Society. 7
performance is the heterogeneous electron transfer rate constant (k0), which describes the rate of electron transfer across the electrode-electrolyte interface. Work in the 1980’s has established that the k0 of basal plane of graphite is several orders of magnitude lower compared to that of noble metal electrode.57-59 This observation was attributed in part to the low density of state of the basal plane and the absence of specific adsorption of redox couples. Similar to the case of hydrophobicity of graphitic surfaces, the idea that pristine sp2 basal plane is of low electrochemical activity is generally accepted by the community. Recently, several groups have revisited the fundamental electrochemical properties of basal plane graphite using advanced electrochemical technique such as scanning electrochemical microscopy.60,61 To their surprise, they observed extremely fast electron transfer kinetics between basal plane graphite and solution redox couples (Figure 2a). The k0 values reported by these studies are several orders of magnitude higher than the previously reported values.62,63 The electrochemical activity is sensitive to the environment: the Amemiya group showed that by using ultraclean electrolyte solutions (ca. 1 - 2 ppb of total organic carbon (TOC)), extremely large k0 values (≥12 cm/s), comparable or even larger than those reported for Pt and Au electrodes, were obtained (Figure 2b).64,65 However, the electrode performance significantly degraded when using electrolyte having ca. 20 ppb TOC or after aging the electrode in air.27,66 These results strongly suggest that the previously observed low electrochemical activity of basal plane graphite is likely due to surface contamination.
4. Outlook. Given the precedence in the wetting and electrochemical studies, it is reasonable to suspect that studies of other interfacial properties, such as adhesion, adsorption, and friction, may not have properly accounted for the potential airborne contamination of the samples either. 4.1 Adhesion and adsorption are directly related to the surface energy of solid substrates. Traditionally, graphitic materials were viewed as non-polar in nature and therefore have low adsorption energy and adhesion energies. However, recent work has already showed that the surface energies of ‘clean’ graphite and graphene are noticeably larger (ca. 25%) than those of their ‘dirty’ counterparts. In addition, for clean graphitic materials, there is a non-negligible polar component in the surface energy that is absent when the surface got contaminated.24 These results suggests that the adhesion and adsorption behavior of clean graphitic materials may be very different from the dirty ones, both in term of their molecular mechanism and magnitude of the interfacial interaction. Efforts have already been made to update the molecular dynamics force field to better model graphitic surfaces.46,49 A clear understanding of the adsorption/adhesion behavior of clean graphitic surfaces may lead to improved adsorbents and stronger composite materials. 4.2 Lubrication and wear. Graphite is a well-known solid lubricant. Although the lubricity was commonly explained by the sliding action between the graphite layers, it is long known that the lubrication performance of graphite depends on certain components in air.67-70 Adsorption of H 2 O, O 2 , and various hydrocarbon vapors impacts the lubrication performance of graphite. The effect of hydrocarbon is especially strong. For example, even sub-ppm level of n8
heptane vapor can significantly reduce the wear of two sliding graphite rods and the effectiveness increase as the size of the hydrocarbon increases (Figure 3a).67 With this in mind, the airborne hydrocarbon contamination may very well play a role in the lubrication performance of graphite. Note that the airborne hydrocarbon is believed to physisorb onto graphitic surface and could be desorbed at elevated temperatures or under high vacuum.21,30 Coincidently, it is known that graphite-based lubricant does not work in ultra-high vacuum; in air, the friction coefficient and wear rate of graphite increase significantly at high temperature (ca. 300 ºC, Figure 3b).71 Understanding the role of airborne contamination on the lubrication of graphitic carbons may provide new insight into the mechanism of friction and wear of this important solid lubricant.
Wear Rate (mm/min)
(a)
(b)
0.1 0.08 0.06 0.04 0.02 0 0
10 30 40 20 Vapor Pressure (microns of Hg)
Figure 3. (a) Wear rate of graphite as a function of vapor pressure of n-heptane. The measurements started from 0 to 40 µmHg (top curve) then returned to 0 µmHg (bottom curve). Note the significant hysteresis in the measured wear rate. Figure replotted with permission using data from Savage, R. H.; et al. J. Appl. Phys. 1956, 27, 136. Copyright American Institute of Physics. (b) Wear of graphite by stainless steel as a function of temperature. Reproduced with permission from Luo, X.; et al. Nucl. Eng. Des. 2005, 235, 2261. Copyright Elsevier.
4.3 “Clean” manufacturing. The research has clearly demonstrated that ‘clean’ carbon shows superior properties compared to their ‘dirty’ counterpart, at least in the case of wetting and electrochemical activities. These properties could have significant implications to device performances given the wide spread use of graphitic materials in batteries, supercapacitors, and filters. An important question is whether it is possible to use clean carbon materials in these devices. Given how prevalence the airborne hydrocarbons are, is it even possible to produce clean carbon materials and maintain their cleanness in devices at an industrial scale? While realizing this goal will clearly need extensive work by scientists and engineers, the industry has many precedencies of manufacturing under highly-controlled environments. One such example is the assembly of Li-ion batteries from components that are extremely moisture sensitive. The work is done not in glove boxes but in dry rooms that maintain very low humidity levels.72
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It is conceivable that a hydrocarbon-free room can be constructed to process clean carbon materials. “Dirty” carbon materials could be cleaned by thermal annealing or other cleaning methods recently reported to remove hydrocarbon contaminant on graphene and graphite, e.g., ‘dry cleaning’, CO 2 jet, and UV/O 3 .18,21,73,74 Li et al. also showed that a thin-water layer coating could significantly decrease the rate of adsorption of airborne hydrocarbons on graphene and graphite by up to 20 times. Using this technique, they showed that a graphite electrode can maintain its high electrochemical activity for over a day.66 These ‘clean’ or less contaminated carbon materials could then be sealed (e.g., in a battery case) to prevent future contamination and maintain its high performance in the device. Certainly, the manufacturing processes need to be modified to eliminate hydrocarbon contamination sources, such as plastics parts that emit low molecular weight hydrocarbons.34 Demonstration of substantial improvement in the device performance is clearly needed to justify such efforts and we expect to see active research in these directions in the coming years.
Acknowledgement HL acknowledges partial support from ONR (N000141512520).
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