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Adsorption transparency of supported graphene
Journal Pre-proof Adsorption transparency of supported graphene Morteza H. Bagheri, Rebecca T. Loibl, J. Anibal Boscoboinik, Scott N. Schiffres PII:
S0008-6223(19)30893-0
DOI:
https://doi.org/10.1016/j.carbon.2019.08.083
Reference:
CARBON 14564
To appear in:
Carbon
Received Date: 17 June 2019 Revised Date:
22 August 2019
Accepted Date: 28 August 2019
Please cite this article as: M.H. Bagheri, R.T. Loibl, J.A. Boscoboinik, S.N. Schiffres, Adsorption transparency of supported graphene, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.08.083. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
CARBON (Elsevier Publications) Title: Adsorption Transparency of Supported Graphene Authors: Morteza H. Bagheri†, Rebecca T. Loibl†, J. Anibal Boscoboinik‡, Scott N. Schiffres†* Affiliations: †
Department of Mechanical Engineering, State University of New York, Binghamton, NY
13902, United States ‡Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 11973, United States * Corresponding Author. E-mail: [email protected] (Scott Schiffres) Abstract: It remains controversial whether the substrate affects the wettability of a single graphene sheet. In this manuscript, we investigate if adsorption transparency exists for graphene, analogous to the contact angle transparency previously reported. The adsorption transparency of graphene supported by different metals is characterized by experimentally measuring the adsorption behavior of water on graphene. By comparing the adsorption isotherm, the heat of adsorption, and the excess interfacial free energy, we find that physisorption on graphene is largely controlled by the supporting substrate. Adding graphene to Pt and Al decreased the interfacial free energy by about 10% when the samples were exposed to water vapor from high vacuum to 5 Torr. This value was larger for graphene on Au (ca. 30%). While we chose to use graphene and water for this study, the results have broader implications to 2D materials in general. This adsorption transparency phenomena has applications in a range of technologies where adsorption and wetting are important: molecular electronics, heat transfer, catalysis, ice-nucleation, additive manufacturing, and corrosion control.
1. Introduction While graphene is well known for its exceptional mechanical, electronic, optical, and thermal properties [1–4], it also has the potential to modify bulk surface phenomena like wetting [5], adsorption [6], ice-nucleation [7], and oxidation of a substrate [8,9]. Its ability to protect metal surfaces from reactive environments makes single-layer graphene an attractive oxidation barrier for metals [10–13]. Specifically, a graphene coated copper surface has shown a 30 to 400% increase in condensation heat transfer due to the suppression of copper oxidation, which delays the onset of film condensation [5,14]. Graphene coated nanochannels showed an increase in the slip length, which enables faster flow through the nanochannels for applications like water desalination and nano-filtration [15,16]. As a low cost catalyst support, graphene can assist in making more selective and active heterogeneous catalyst systems [17]. The effect of the substrate on graphene wetting remains controversial. Several groups have conducted experiments that both support and refute the graphene transparency hypothesis [5,14,18–22]. Most experimental studies on graphene wetting transparency used water contact angle measurements, which has repeatability challenges due to the hysteresis of advancing and receding contact angles in addition to sensitivity to airborne contaminants. Airborne contaminants, such as hydrocarbons and water vapors present in the atmosphere, significantly alter the wettability of graphene by decreasing the surface energy of the solid [23–28]. Studying wetting energetics through the measurement of the energy of adsorption has benefits in that the material is in a controlled vacuum environment, as opposed to ambient air, like typical contact angle experiments. Contamination will contribute less to the adsorption area than contact angle, as the adsorption energy of the first monolayer will be a weighted average over the surface area. Consequently, small areal concentrations of contaminants only contribute to an error in
adsorption properties proportional to their coverage area [29], unlike contact angle experiments [30]. Moreover, adsorption experiments remove the uncertainties due to advancing and receding hysteresis. In this work, we measure the adsorption isotherms of graphene supported by gold, platinum and aluminum substrates. From the adsorption isotherms measured at two temperatures, the heat of adsorption is extracted versus uptake and substrate. Additionally, the excess interfacial free energy for each substrate is calculated to compare the wettability of the surfaces. Similar control experiments were also performed on the metal substrates. 2. Theory 2.1. Heat of Adsorption Adsorption energy and contact angle are related, as both are functions of the energetics of molecules adhering to the surface. The vapor adsorbing onto the surface at high enough pressures will eventually form a multi-layer film. As the number of adsorbed layers increases, the heat of adsorption asymptotes to the enthalpy of condensation, ℎ . Although the enthalpy of adsorption, ℎ , is released as heat, most of this heat goes towards the mechanical work stretching the liquid-vapor surface [31]. The more exothermic the adsorption of vapor is onto the surface, the better the wetting and the smaller the contact angles will tend to be. The degree of wettability of a surface is mainly determined by its surface free energy. However, since wetting occurs on a surface with an unknown quantity of adsorption, and the entropy change during wetting is not well known [31,32], the change of surface free energy during wetting is not a straightforward function of the heat of adsorption. From the Gibbs-Helmholtz equation ( = ℎ +
( / ) ), the heat of adsorption can be related to the change of surface free energy [33,34],
where, and ℎ are the Gibbs free energy and the enthalpy contents of the system per unit area, respectively. The heat of adsorption can be calculated using the Clausius-Clapeyron equation [35]. Since the adsorption of water on a surface is a function of both the attraction of the water molecules with the surface and the water molecules with themselves in the first few monolayers [36], the heat of adsorption is a function of the surface coverage, namely the adsorption uptake (Γ). Thus, from the Clausius-Clapeyron equation, the isosteric heat of adsorption can be written as [37], ℎ = ((ln )⁄(−1⁄ ))
(1)
Where, is the gas constant of the vapor, is the temperature during the adsorption process, and Γ is the amount of adsorbed vapors (uptake per unit area) at the corresponding pressure (). 2.2. Interfacial Free Energy To compare the adsorption energies with literature wetting experiments, we developed a thermodynamic formulation. The liquid-solid, solid-vapor, and liquid-vapor surface energies determine the wettability of the surface. From a thermodynamics point of view, the reversible work of separating two phases (e.g. liquid and solid) is equal to the change of the free energy of the system [38]. According to the Gibbs adsorption equation, the excess interfacial free energy (change of surface free energy), ∆, of a solid surface exposed to a gas approximated as an ideal gas is equal to [39], ∆ = Γ ln
(2)
For a clean surface with no adsorbed species, ∆ = − , where and are the surface free energy of the solid surface with and without adsorbed species, respectively. By comparing the value of ∆ for a surface with and without a graphene coating, the interaction of the two
surfaces with water molecules can be distinguished. Under the same condition of vapor exposure, a surface with a higher absolute value of ∆ represents as a more hydrophilic surface, and vice versa. 3. Experimental Adsorption isotherms were measured in an environmental vacuum chamber using a 5MHz ATcut quartz crystal microbalance (QCM). A Stanford Research Systems QCM200 measured the frequency change of different metal coated QCMs as the pressure of water vapor was swept. The resonant frequency of a QCM depends on the magnitude of the adsorbed mass, the effect of the hydrostatic pressure of the gas on the elastic modulus of quartz, and the visco-elastic coupling to the gas [40,41]. For the range of pressure studied here, the effect of the hydrostatic pressure on the frequency shift was negligible (ca. 1%) (See Supporting Information), thus we did not consider it in our calculations for the adsorption isotherms. We used Sauerbrey equation [42] to convert the frequency shift to the adsorbed mass. This relation gives identical results to the Zmatch method which considers the viscoelastic coupling that is neglected by the Sauerbrey equation (See Supporting Information). Pt, Au and Al were selected as the metal coatings on the QCM (FIL-TECH CRYSTALS), as they represent a wide range of surface energies [43]. All the QCM crystals were cleaned prior to each experiment by sonicating (BRANSON 2800) in acetone and isopropanol (VWR International) for 15 minutes each, followed by 5 minutes Oxygen and 10 minutes Argon plasma cleaning on the high level setting (HARRICK PLASMA PDC-32G). To minimize airborne contamination, each sample was transferred to the environmental vacuum chamber immediately after the cleaning. A dry pumping station (PFEIFFER HiCube 80 Eco) was used to eliminate the possibility of hydrocarbon contamination from conventional oil pumps. A liquid Nitrogen trap before the pumping station freezes
condensable vapors prior to the turbo-molecular pump stage. The schematic of the experimental adsorption measurement setup is shown in the Supporting Information. The chamber was instrumented with multiple thermistor elements (OMEGA Precision Thermistor 44013), positioned close to the QCM and the vapor source to accurately measure the temperature during the tests. A DI water (18.2 MΩ-cm) reservoir connected to the chamber supplied the vapor for the adsorption measurements. Before each experiment, the vapor source was degassed by opening it multiple times to a low vacuum (PFEIFFER MVP 015-4) for a couple seconds. This process was repeated until the vapor pressure (MKS 910 DualTrans) of the source matched the saturation pressure of water at the corresponding temperature. Before each adsorption measurement, the sample inside the vacuum chamber was pumped down for about 12 hours until the pressure reached 3x10-6 Torr (MKS 390 Micro-Ion® ATM). To keep the QCM and the vapor source at the same temperature, the whole chamber was wrapped with ¼” copper tubes connected to a recirculating bath (HAAKE K20). PMMA-backed CVD graphene (Monolayer Graphene on Polymer Film, Graphenea) was transferred onto the QCMs in a class 100 cleanroom [44,45]. We used the same QCMs for adsorption on graphene as we did for bare metals. Each QCM was cleaned before deposition by the aforementioned cleaning process. After the deposition, the sample was dried in air for 1 hour followed by another hour of annealing on a hotplate at 150˚C. Then, the sample was kept under low vacuum for 24 hours to remove the trapped water between the substrate and graphene caused by the wet transfer process. This not only minimized the effect of trapped water by altering the interaction of adsorbed molecules with graphene, but also allowed for a better bond between graphene and the substrate. CVD grown graphene, an effective method to grow graphene on a large scale [46], requires a sacrificial layer usually consisting of PMMA. As a result, a
transferred CVD-grown film normally leaves some PMMA residue on the graphene, which affects the quality. To remove the sacrificial backing (PMMA), the sample was soaked in acetone for an hour, soaked in IPA for another hour and then dried with Nitrogen. To further remove the PMMA, the sample was heated for 2 hours under high vacuum at 300˚C with a ramping temperature of 5˚C/min. 4. Results and Discussions 4.1. Graphene Quality Validation Although pristine Graphene is made up of sp2-hybridized carbons, defects and amorphous graphitic carbon result in sp3-hybridization [47]. To determine the level of organic residue left on graphene, we performed XPS on the graphene coated samples after annealing at 300˚C under vacuum. The AP-XPS instrument used is located at the Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory (BNL). The XPS was performed under ultra-high vacuum using a SPECS PHOIBOS NAP 150 hemispherical analyzer. A monochromatic Al Kα X-ray source was used to acquire the spectra for 300 µm × 300 µm sample areas. Figure 1 shows the deconvoluted C 1s core level using the Gaussian-Lorentzian method to fit the data. The Shirley algorithm was used to subtract the background and the spectra is normalized based on the area of the metal peak (Au 4f). The peaks at 284.6 eV (±0.1 eV) and 285.3 eV (±0.1 eV) are associated with the sp2 and sp3 hybridization of C-C bonding, respectively. The presence of PMMA illustrates peaks at 285.0 eV (±0.2 eV), 285.7 eV (±0.1 eV), 286.8 eV (±0.2 eV), and 288.9 eV (±0.2 eV), which are associated with bonds of carbon atoms from the contributions of pendant of the PMMA residue (C1), the polymer backbone (C2), the methoxy functional group (C3), and the carboxyl functional group (C4), respectively [48,49]. After the chemical and thermal treatment, only about 15% of the PMMA residue (mostly associated with the carbon
atoms in the polymer back bone) is left and more than 3/4 of the surface is covered with sp2hybridized graphene. Furthermore, our O1s core level spectra after annealing (in-situ) shows no peaks associated with water molecules which could be trapped under graphene during the wet transfer process (see Supporting Information) [50].
Figure 1 C 1s core level XPS spectra of graphene coated Au annealed at 300 ˚C. The binding energy of sp2 and sp3 hybridized carbons are 284.6 eV (±0.1 eV) and 285.3 eV (±0.1 eV), respectively.
We also characterize the quality of graphene with Raman spectroscopy (Renishaw inVia confocal Raman microscope). The spectra (Figure 2) was acquired over several lines using a 532 nm, 50 mW laser and a 100X microscope objective (NA=0.85). To avoid heating the graphene, the laser power was kept under 5mW with 3 accumulations of 10 seconds each. The high 2D peak to G peak ratio (I2D/IG > 2) shows the presence of a single layer graphene [51]. Moreover, the small D peak to G peak ratio (ID/IG ~ 5%) indicates an insignificant amount of defects in the graphene [52].
a)
b)
Figure 2 a) Raman spectra of the graphene coated Au annealed at 300 ˚C (Inset: SEM of the same sample after the water adsorption and desorption cycle; scale bar is equal to 5 µm). b) D to G and 2D to G peak ratio over a mapped line. The large 2D to G peak ratio denotes the presence of a single layer graphene
As the surface roughness plays an important role on water adsorption [53], we performed AFM on a sample half covered with graphene (XE-70, Park Systems). The roughness of the aluminum substrate and aluminum with graphene are comparable within the uncertainty, indicating that graphene adheres to all asperities of the surface (Figure S2) [44]. 4.2. Adsorption Isotherms and The Heat of Adsorption The adsorption isotherms of water at two different temperatures for three different metals (Pt, Au, and Al) with and without a graphene conformal coating are shown in Figure 3. The graphene coated surfaces follow similar adsorption isotherm shapes as the corresponding bare metal. However, they show slightly lower uptakes compared to the metal surface which denotes a more hydrophobic surface for graphene. We repeated the experiment for three different Au coated QCMs to demonstrate the repeatability of the results (Figure S5).
a)
a)
b)
b)
c)
c)
Figure 3 Adsorption uptake of water molecules on a) Pt, b) Au, and c) Al vs. the metals coated with graphene at two different temperatures. Surfaces with graphene coating have a similar isotherm shape to the metal but slightly lower uptakes.
Figure 4 Heat of adsorption of water molecules on a) Pt, b) Au, and c) Al vs the metals coated with graphene as a function of the adsorption uptake. Surfaces with a graphene coating are less exothermic in comparison to the bare metals.
As mentioned before, by measuring the uptake at two relatively close temperatures for a range of pressures, the heat of adsorption can be calculated as a function of the adsorption uptake via the Clausius-Clapeyron relation [54]. The heat of adsorption corresponding to the isotherms in Figure 3 are shown in Figure 4 as a function of the adsorption uptake. The graphene coated surfaces exhibit a less exothermic heat of adsorption than the bare metal substrate. This less energetic adsorption demonstrates that graphene weakens the interaction of the surface with the surrounding molecules in the atmosphere to some extent and makes the surface more hydrophobic. Note that the uncertainty in the heat of adsorption was calculated by propagating the uncertainty due to the temperature and pressure sensors, and the QCM as independent noise sources. 4.3. Excess Interfacial Free Energy To distinguish the wettability of a surface with and without graphene coating, we also calculated the excess interfacial free energy of the surface when exposed to water vapor using Gibbs adsorption equation. Figure 5a compares the excess interfacial free energy of the three metals with and without graphene coating when exposed to water vapor. Under the same condition of vapor exposure, each metal, depending on its surface energy, showed a different behavior interacting with water molecules. Likewise, the metals coated with graphene followed the same trend as the supporting substrate, showing that the supporting substrate in graphene manipulates graphene’s interaction with water molecules. Moreover, the surfaces coated with graphene show a smaller excess interfacial free energy when exposed to water vapor in comparison to the corresponding bare metal. This resembles a lower surface energy and consequently more hydrophobicity for the surface with the graphene coating. A comparison of our experimental work for the percent change of free energy of the Au surface
(with and without graphene coating) vs the percent change of the work of adhesion calculated based on Young-Dupre equation [32] using the CA measured by MD simulation and experimental droplet visualization is shown in Figure 5b. Our data for Au matches better with the MD simulation work, possibly because our surface is cleaner due conducting the experiment in a vacuum environment. The CA measured experimentally in a lab setting already has a layer of vapors on the surface. This adsorbed layer of vapor makes experimental CA measurements in atmospheric conditions difficult to reproduce due to changing surface properties caused by contamination and variable humidity levels.
a)
b)
Figure 5 a) Excess interfacial free energy of the metal surfaces exposed to water vapor with and without graphene coating (0-5 Torr, 22˚C). The graphene coated samples have a lower surface free energy compared to the bare metal surfaces, indicating less hydrophilicity for graphene coated metal substrates. b) A comparison of the change of free energy of the surface for our experimental measurements (0-5 Torr) and the work of adhesion calculated from literature contact angle measurements and MD simulations [14,20,24,55–57].
5. Conclusions
By studying the adsorption of water molecules on supported graphene, we have shown that the interaction of graphene with water molecules is affected by the supporting substrate in the three metals we studied. Our adsorption isotherms of graphene supported by Pt, Au, and Al had relatively close uptakes to the bare metals (slightly lower uptake with a similar isotherm shape). We did find that the graphene coated surfaces have a slightly less energetic adsorption than the bare metal, which agrees with the trend in wetting studies that observe a slight increase in the contact angle of the graphene coated surface compared to the bare substrates. This technique can be used for studying the adsorption of other 2D materials and supporting substrates.
Acknowledgements We greatly appreciate the helpful conversations with Shawn Wagoner, Eric Borguet, Manuel Smeu, and Jeffrey Mativetsky. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Abbreviations QCM, quartz crystal microbalance; PMMA, Poly(methyl methacrylate); CVD, chemical vapor deposition; XPS, X-ray photoelectron spectroscopy; AP-XPS, ambient pressure XPS; AFM, atomic force microscopy; CA, contact angle; MD, molecular dynamics; Gr/Pt, graphene on platinum; Gr/Au, graphene on gold; Gr/Al, graphene on aluminum. ORCiD Morteza H. Bagheri, ORCiD: 0000-0002-8676-0777 Rebecca T. Loibl, ORCiD:0000-0002-9560-8742 J. Anibal Boscoboinik, ORCiD: 0000-0002-5090-7079 Scott N. Schiffres, ORCiD: 0000-0003-1847-7715 Author contributions SNS conceived the research and provided guidance on the experiments. MHB designed the experimental setup, annealing chamber, and test procedure. MHB and RTL prepared QCM samples and performed adsorption uptake measurements. JAB, RTL and SNS performed XPS measurements under the guidance of JAB. MHB post processed the adsorption and XPS data. MHB performed Raman and SEM. All authors discussed the results and contributed to the paper.
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S0008-6223(19)30893-0
DOI:
https://doi.org/10.1016/j.carbon.2019.08.083
Reference:
CARBON 14564
To appear in:
Carbon
Received Date: 17 June 2019 Revised Date:
22 August 2019
Accepted Date: 28 August 2019
Please cite this article as: M.H. Bagheri, R.T. Loibl, J.A. Boscoboinik, S.N. Schiffres, Adsorption transparency of supported graphene, Carbon (2019), doi: https://doi.org/10.1016/j.carbon.2019.08.083. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2019 Published by Elsevier Ltd.
CARBON (Elsevier Publications) Title: Adsorption Transparency of Supported Graphene Authors: Morteza H. Bagheri†, Rebecca T. Loibl†, J. Anibal Boscoboinik‡, Scott N. Schiffres†* Affiliations: †
Department of Mechanical Engineering, State University of New York, Binghamton, NY
13902, United States ‡Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, NY, 11973, United States * Corresponding Author. E-mail: [email protected] (Scott Schiffres) Abstract: It remains controversial whether the substrate affects the wettability of a single graphene sheet. In this manuscript, we investigate if adsorption transparency exists for graphene, analogous to the contact angle transparency previously reported. The adsorption transparency of graphene supported by different metals is characterized by experimentally measuring the adsorption behavior of water on graphene. By comparing the adsorption isotherm, the heat of adsorption, and the excess interfacial free energy, we find that physisorption on graphene is largely controlled by the supporting substrate. Adding graphene to Pt and Al decreased the interfacial free energy by about 10% when the samples were exposed to water vapor from high vacuum to 5 Torr. This value was larger for graphene on Au (ca. 30%). While we chose to use graphene and water for this study, the results have broader implications to 2D materials in general. This adsorption transparency phenomena has applications in a range of technologies where adsorption and wetting are important: molecular electronics, heat transfer, catalysis, ice-nucleation, additive manufacturing, and corrosion control.
1. Introduction While graphene is well known for its exceptional mechanical, electronic, optical, and thermal properties [1–4], it also has the potential to modify bulk surface phenomena like wetting [5], adsorption [6], ice-nucleation [7], and oxidation of a substrate [8,9]. Its ability to protect metal surfaces from reactive environments makes single-layer graphene an attractive oxidation barrier for metals [10–13]. Specifically, a graphene coated copper surface has shown a 30 to 400% increase in condensation heat transfer due to the suppression of copper oxidation, which delays the onset of film condensation [5,14]. Graphene coated nanochannels showed an increase in the slip length, which enables faster flow through the nanochannels for applications like water desalination and nano-filtration [15,16]. As a low cost catalyst support, graphene can assist in making more selective and active heterogeneous catalyst systems [17]. The effect of the substrate on graphene wetting remains controversial. Several groups have conducted experiments that both support and refute the graphene transparency hypothesis [5,14,18–22]. Most experimental studies on graphene wetting transparency used water contact angle measurements, which has repeatability challenges due to the hysteresis of advancing and receding contact angles in addition to sensitivity to airborne contaminants. Airborne contaminants, such as hydrocarbons and water vapors present in the atmosphere, significantly alter the wettability of graphene by decreasing the surface energy of the solid [23–28]. Studying wetting energetics through the measurement of the energy of adsorption has benefits in that the material is in a controlled vacuum environment, as opposed to ambient air, like typical contact angle experiments. Contamination will contribute less to the adsorption area than contact angle, as the adsorption energy of the first monolayer will be a weighted average over the surface area. Consequently, small areal concentrations of contaminants only contribute to an error in
adsorption properties proportional to their coverage area [29], unlike contact angle experiments [30]. Moreover, adsorption experiments remove the uncertainties due to advancing and receding hysteresis. In this work, we measure the adsorption isotherms of graphene supported by gold, platinum and aluminum substrates. From the adsorption isotherms measured at two temperatures, the heat of adsorption is extracted versus uptake and substrate. Additionally, the excess interfacial free energy for each substrate is calculated to compare the wettability of the surfaces. Similar control experiments were also performed on the metal substrates. 2. Theory 2.1. Heat of Adsorption Adsorption energy and contact angle are related, as both are functions of the energetics of molecules adhering to the surface. The vapor adsorbing onto the surface at high enough pressures will eventually form a multi-layer film. As the number of adsorbed layers increases, the heat of adsorption asymptotes to the enthalpy of condensation, ℎ . Although the enthalpy of adsorption, ℎ , is released as heat, most of this heat goes towards the mechanical work stretching the liquid-vapor surface [31]. The more exothermic the adsorption of vapor is onto the surface, the better the wetting and the smaller the contact angles will tend to be. The degree of wettability of a surface is mainly determined by its surface free energy. However, since wetting occurs on a surface with an unknown quantity of adsorption, and the entropy change during wetting is not well known [31,32], the change of surface free energy during wetting is not a straightforward function of the heat of adsorption. From the Gibbs-Helmholtz equation ( = ℎ +
( / ) ), the heat of adsorption can be related to the change of surface free energy [33,34],
where, and ℎ are the Gibbs free energy and the enthalpy contents of the system per unit area, respectively. The heat of adsorption can be calculated using the Clausius-Clapeyron equation [35]. Since the adsorption of water on a surface is a function of both the attraction of the water molecules with the surface and the water molecules with themselves in the first few monolayers [36], the heat of adsorption is a function of the surface coverage, namely the adsorption uptake (Γ). Thus, from the Clausius-Clapeyron equation, the isosteric heat of adsorption can be written as [37], ℎ = ((ln )⁄(−1⁄ ))
(1)
Where, is the gas constant of the vapor, is the temperature during the adsorption process, and Γ is the amount of adsorbed vapors (uptake per unit area) at the corresponding pressure (). 2.2. Interfacial Free Energy To compare the adsorption energies with literature wetting experiments, we developed a thermodynamic formulation. The liquid-solid, solid-vapor, and liquid-vapor surface energies determine the wettability of the surface. From a thermodynamics point of view, the reversible work of separating two phases (e.g. liquid and solid) is equal to the change of the free energy of the system [38]. According to the Gibbs adsorption equation, the excess interfacial free energy (change of surface free energy), ∆, of a solid surface exposed to a gas approximated as an ideal gas is equal to [39], ∆ = Γ ln
(2)
For a clean surface with no adsorbed species, ∆ = − , where and are the surface free energy of the solid surface with and without adsorbed species, respectively. By comparing the value of ∆ for a surface with and without a graphene coating, the interaction of the two
surfaces with water molecules can be distinguished. Under the same condition of vapor exposure, a surface with a higher absolute value of ∆ represents as a more hydrophilic surface, and vice versa. 3. Experimental Adsorption isotherms were measured in an environmental vacuum chamber using a 5MHz ATcut quartz crystal microbalance (QCM). A Stanford Research Systems QCM200 measured the frequency change of different metal coated QCMs as the pressure of water vapor was swept. The resonant frequency of a QCM depends on the magnitude of the adsorbed mass, the effect of the hydrostatic pressure of the gas on the elastic modulus of quartz, and the visco-elastic coupling to the gas [40,41]. For the range of pressure studied here, the effect of the hydrostatic pressure on the frequency shift was negligible (ca. 1%) (See Supporting Information), thus we did not consider it in our calculations for the adsorption isotherms. We used Sauerbrey equation [42] to convert the frequency shift to the adsorbed mass. This relation gives identical results to the Zmatch method which considers the viscoelastic coupling that is neglected by the Sauerbrey equation (See Supporting Information). Pt, Au and Al were selected as the metal coatings on the QCM (FIL-TECH CRYSTALS), as they represent a wide range of surface energies [43]. All the QCM crystals were cleaned prior to each experiment by sonicating (BRANSON 2800) in acetone and isopropanol (VWR International) for 15 minutes each, followed by 5 minutes Oxygen and 10 minutes Argon plasma cleaning on the high level setting (HARRICK PLASMA PDC-32G). To minimize airborne contamination, each sample was transferred to the environmental vacuum chamber immediately after the cleaning. A dry pumping station (PFEIFFER HiCube 80 Eco) was used to eliminate the possibility of hydrocarbon contamination from conventional oil pumps. A liquid Nitrogen trap before the pumping station freezes
condensable vapors prior to the turbo-molecular pump stage. The schematic of the experimental adsorption measurement setup is shown in the Supporting Information. The chamber was instrumented with multiple thermistor elements (OMEGA Precision Thermistor 44013), positioned close to the QCM and the vapor source to accurately measure the temperature during the tests. A DI water (18.2 MΩ-cm) reservoir connected to the chamber supplied the vapor for the adsorption measurements. Before each experiment, the vapor source was degassed by opening it multiple times to a low vacuum (PFEIFFER MVP 015-4) for a couple seconds. This process was repeated until the vapor pressure (MKS 910 DualTrans) of the source matched the saturation pressure of water at the corresponding temperature. Before each adsorption measurement, the sample inside the vacuum chamber was pumped down for about 12 hours until the pressure reached 3x10-6 Torr (MKS 390 Micro-Ion® ATM). To keep the QCM and the vapor source at the same temperature, the whole chamber was wrapped with ¼” copper tubes connected to a recirculating bath (HAAKE K20). PMMA-backed CVD graphene (Monolayer Graphene on Polymer Film, Graphenea) was transferred onto the QCMs in a class 100 cleanroom [44,45]. We used the same QCMs for adsorption on graphene as we did for bare metals. Each QCM was cleaned before deposition by the aforementioned cleaning process. After the deposition, the sample was dried in air for 1 hour followed by another hour of annealing on a hotplate at 150˚C. Then, the sample was kept under low vacuum for 24 hours to remove the trapped water between the substrate and graphene caused by the wet transfer process. This not only minimized the effect of trapped water by altering the interaction of adsorbed molecules with graphene, but also allowed for a better bond between graphene and the substrate. CVD grown graphene, an effective method to grow graphene on a large scale [46], requires a sacrificial layer usually consisting of PMMA. As a result, a
transferred CVD-grown film normally leaves some PMMA residue on the graphene, which affects the quality. To remove the sacrificial backing (PMMA), the sample was soaked in acetone for an hour, soaked in IPA for another hour and then dried with Nitrogen. To further remove the PMMA, the sample was heated for 2 hours under high vacuum at 300˚C with a ramping temperature of 5˚C/min. 4. Results and Discussions 4.1. Graphene Quality Validation Although pristine Graphene is made up of sp2-hybridized carbons, defects and amorphous graphitic carbon result in sp3-hybridization [47]. To determine the level of organic residue left on graphene, we performed XPS on the graphene coated samples after annealing at 300˚C under vacuum. The AP-XPS instrument used is located at the Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory (BNL). The XPS was performed under ultra-high vacuum using a SPECS PHOIBOS NAP 150 hemispherical analyzer. A monochromatic Al Kα X-ray source was used to acquire the spectra for 300 µm × 300 µm sample areas. Figure 1 shows the deconvoluted C 1s core level using the Gaussian-Lorentzian method to fit the data. The Shirley algorithm was used to subtract the background and the spectra is normalized based on the area of the metal peak (Au 4f). The peaks at 284.6 eV (±0.1 eV) and 285.3 eV (±0.1 eV) are associated with the sp2 and sp3 hybridization of C-C bonding, respectively. The presence of PMMA illustrates peaks at 285.0 eV (±0.2 eV), 285.7 eV (±0.1 eV), 286.8 eV (±0.2 eV), and 288.9 eV (±0.2 eV), which are associated with bonds of carbon atoms from the contributions of pendant of the PMMA residue (C1), the polymer backbone (C2), the methoxy functional group (C3), and the carboxyl functional group (C4), respectively [48,49]. After the chemical and thermal treatment, only about 15% of the PMMA residue (mostly associated with the carbon
atoms in the polymer back bone) is left and more than 3/4 of the surface is covered with sp2hybridized graphene. Furthermore, our O1s core level spectra after annealing (in-situ) shows no peaks associated with water molecules which could be trapped under graphene during the wet transfer process (see Supporting Information) [50].
Figure 1 C 1s core level XPS spectra of graphene coated Au annealed at 300 ˚C. The binding energy of sp2 and sp3 hybridized carbons are 284.6 eV (±0.1 eV) and 285.3 eV (±0.1 eV), respectively.
We also characterize the quality of graphene with Raman spectroscopy (Renishaw inVia confocal Raman microscope). The spectra (Figure 2) was acquired over several lines using a 532 nm, 50 mW laser and a 100X microscope objective (NA=0.85). To avoid heating the graphene, the laser power was kept under 5mW with 3 accumulations of 10 seconds each. The high 2D peak to G peak ratio (I2D/IG > 2) shows the presence of a single layer graphene [51]. Moreover, the small D peak to G peak ratio (ID/IG ~ 5%) indicates an insignificant amount of defects in the graphene [52].
a)
b)
Figure 2 a) Raman spectra of the graphene coated Au annealed at 300 ˚C (Inset: SEM of the same sample after the water adsorption and desorption cycle; scale bar is equal to 5 µm). b) D to G and 2D to G peak ratio over a mapped line. The large 2D to G peak ratio denotes the presence of a single layer graphene
As the surface roughness plays an important role on water adsorption [53], we performed AFM on a sample half covered with graphene (XE-70, Park Systems). The roughness of the aluminum substrate and aluminum with graphene are comparable within the uncertainty, indicating that graphene adheres to all asperities of the surface (Figure S2) [44]. 4.2. Adsorption Isotherms and The Heat of Adsorption The adsorption isotherms of water at two different temperatures for three different metals (Pt, Au, and Al) with and without a graphene conformal coating are shown in Figure 3. The graphene coated surfaces follow similar adsorption isotherm shapes as the corresponding bare metal. However, they show slightly lower uptakes compared to the metal surface which denotes a more hydrophobic surface for graphene. We repeated the experiment for three different Au coated QCMs to demonstrate the repeatability of the results (Figure S5).
a)
a)
b)
b)
c)
c)
Figure 3 Adsorption uptake of water molecules on a) Pt, b) Au, and c) Al vs. the metals coated with graphene at two different temperatures. Surfaces with graphene coating have a similar isotherm shape to the metal but slightly lower uptakes.
Figure 4 Heat of adsorption of water molecules on a) Pt, b) Au, and c) Al vs the metals coated with graphene as a function of the adsorption uptake. Surfaces with a graphene coating are less exothermic in comparison to the bare metals.
As mentioned before, by measuring the uptake at two relatively close temperatures for a range of pressures, the heat of adsorption can be calculated as a function of the adsorption uptake via the Clausius-Clapeyron relation [54]. The heat of adsorption corresponding to the isotherms in Figure 3 are shown in Figure 4 as a function of the adsorption uptake. The graphene coated surfaces exhibit a less exothermic heat of adsorption than the bare metal substrate. This less energetic adsorption demonstrates that graphene weakens the interaction of the surface with the surrounding molecules in the atmosphere to some extent and makes the surface more hydrophobic. Note that the uncertainty in the heat of adsorption was calculated by propagating the uncertainty due to the temperature and pressure sensors, and the QCM as independent noise sources. 4.3. Excess Interfacial Free Energy To distinguish the wettability of a surface with and without graphene coating, we also calculated the excess interfacial free energy of the surface when exposed to water vapor using Gibbs adsorption equation. Figure 5a compares the excess interfacial free energy of the three metals with and without graphene coating when exposed to water vapor. Under the same condition of vapor exposure, each metal, depending on its surface energy, showed a different behavior interacting with water molecules. Likewise, the metals coated with graphene followed the same trend as the supporting substrate, showing that the supporting substrate in graphene manipulates graphene’s interaction with water molecules. Moreover, the surfaces coated with graphene show a smaller excess interfacial free energy when exposed to water vapor in comparison to the corresponding bare metal. This resembles a lower surface energy and consequently more hydrophobicity for the surface with the graphene coating. A comparison of our experimental work for the percent change of free energy of the Au surface
(with and without graphene coating) vs the percent change of the work of adhesion calculated based on Young-Dupre equation [32] using the CA measured by MD simulation and experimental droplet visualization is shown in Figure 5b. Our data for Au matches better with the MD simulation work, possibly because our surface is cleaner due conducting the experiment in a vacuum environment. The CA measured experimentally in a lab setting already has a layer of vapors on the surface. This adsorbed layer of vapor makes experimental CA measurements in atmospheric conditions difficult to reproduce due to changing surface properties caused by contamination and variable humidity levels.
a)
b)
Figure 5 a) Excess interfacial free energy of the metal surfaces exposed to water vapor with and without graphene coating (0-5 Torr, 22˚C). The graphene coated samples have a lower surface free energy compared to the bare metal surfaces, indicating less hydrophilicity for graphene coated metal substrates. b) A comparison of the change of free energy of the surface for our experimental measurements (0-5 Torr) and the work of adhesion calculated from literature contact angle measurements and MD simulations [14,20,24,55–57].
5. Conclusions
By studying the adsorption of water molecules on supported graphene, we have shown that the interaction of graphene with water molecules is affected by the supporting substrate in the three metals we studied. Our adsorption isotherms of graphene supported by Pt, Au, and Al had relatively close uptakes to the bare metals (slightly lower uptake with a similar isotherm shape). We did find that the graphene coated surfaces have a slightly less energetic adsorption than the bare metal, which agrees with the trend in wetting studies that observe a slight increase in the contact angle of the graphene coated surface compared to the bare substrates. This technique can be used for studying the adsorption of other 2D materials and supporting substrates.
Acknowledgements We greatly appreciate the helpful conversations with Shawn Wagoner, Eric Borguet, Manuel Smeu, and Jeffrey Mativetsky. This research used resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. Abbreviations QCM, quartz crystal microbalance; PMMA, Poly(methyl methacrylate); CVD, chemical vapor deposition; XPS, X-ray photoelectron spectroscopy; AP-XPS, ambient pressure XPS; AFM, atomic force microscopy; CA, contact angle; MD, molecular dynamics; Gr/Pt, graphene on platinum; Gr/Au, graphene on gold; Gr/Al, graphene on aluminum. ORCiD Morteza H. Bagheri, ORCiD: 0000-0002-8676-0777 Rebecca T. Loibl, ORCiD:0000-0002-9560-8742 J. Anibal Boscoboinik, ORCiD: 0000-0002-5090-7079 Scott N. Schiffres, ORCiD: 0000-0003-1847-7715 Author contributions SNS conceived the research and provided guidance on the experiments. MHB designed the experimental setup, annealing chamber, and test procedure. MHB and RTL prepared QCM samples and performed adsorption uptake measurements. JAB, RTL and SNS performed XPS measurements under the guidance of JAB. MHB post processed the adsorption and XPS data. MHB performed Raman and SEM. All authors discussed the results and contributed to the paper.
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