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1 Effect of airborne contaminants on the wettability of supported graphene and graphite Zhiting Li 1,ǂ, Yongjin Wang 2, ǂ, Andrew Kozbial 2, Ganesh Shenoy 1, Feng Zhou 1, Rebecca McGinley 2, Patrick Ireland 2, Brittni Morganstein 2, Alyssa Kunkel 1, Sumedh P. Surwade 1, Lei Li 2 * and Haitao Liu 1 * 1 Department of Chemistry, and 2 Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, PA, 15260, U.S.A. ǂ These authors contributed equally to this work. *Correspondence to: hliu@pitt.edu (H.L.) and lel55@pitt.edu (L.L.) Supplementary Information 1. Additional Materials and Methods Growth of multilayer graphene on Ni. The CVD synthesis was conducted on a Ni film vacuum deposited on a silicon wafer. Briefly, on a silicon wafer was deposited a Ti layer at 0.4 nm/s for a total of 5 nm followed by a Ni layer at 0.1 nm/s for a total of 300 nm. The deposition was carried out on a Thermionics VE-180 e-beam evaporator at a pressure of 2 x 10-6 Torr. The CVD synthesis of multilayer graphene is almost the same as the case of single layer graphene, except the reaction temperature was 800 C. The Raman spectrum of the multilayer graphene sample is shown in Figure S8. Both the 2D/G ratio (0.8) and the line shape of the 2D peak indicate that the graphene sample was 2 3 layers thick. Graphene transferred onto different substrates. Gold substrate preparation: a Si wafer/ti (5 nm)/au (300 nm) substrate was prepared by first depositing a Ti layer on to a Si wafer at a rate of 0.4 nm/s for a total of 5 nm followed by depositing an Au layer at a rate of 0.1 nm/s for a total of 300 nm. The NATURE MATERIALS 1

2 deposition pressure was 2 x 10-6 Torr. Before its use, the Au substrate was rinsed with acetone and deionized (DI) water and blown dry with nitrogen. Si wafer with a 300 nm of SiO 2 (University Wafers) and glass substrate were soaked in piranha solution (70% concentrated sulphuric acid + 30% hydrogen peroxide (30%)) for 1 hr, followed by a thorough rinse of DI water and then blown dry with dry nitrogen before use. Caution: piranha solution reacts violently with organic compounds. Wear proper personal protection equipment and perform all experiments in a fume hood. The graphene film grown on copper foil was transferred to these substrates using poly-methyl-methacrylate (PMMA (Aldrich, M w )) following a reported procedure. S1 In a typical experiment, PMMA (50 mg/ml solution in anisole (Sigma-Aldrich, 99%)) was spin-coated (3,000 rpm, 1 min) onto one side of the copper foil. Then the PMMA/CVD-graphene layer was separated from the copper foil by etching in an aqueous solution of 1 M FeCl 3 (Sigma-Aldrich, 97%) and 3.5 M HCl (Fisher Scientific, 37.1%) for 20 min. The PMMA/CVD-graphene film was then transferred to a DI water bath to wash away any contaminations and then collected onto one of these three substrates and dried with a nitrogen gun. The PMMA film on the substrate was dissolved by immersion in acetone (8 hr), followed by a dichloromethane (Sigma-Aldrich, 99.5%) bath for another 8 hr. The CVD-graphene film was then annealed at 400 o C in an Argon atmosphere for 30 min in order to remove any remaining polymer residue. ATR-FTIR 2 NATURE MATERIALS

3 SUPPLEMENTARY INFORMATION The intensity of the ATR-FTIR signal could be affected by the contact between ATR crystal and the sample. We have carried out two control experiments to verify the reliability of our FTIR data. In one experiment, we left the ATR crystal in contact with an as-prepared graphene/copper sample for 100 min and observed that the CH 2 peaks remained unchanged. This experiment showed that contact between ATR crystal and graphene is consistent and that the increase of the CH 2 peak we observed in Figure 2b is indeed due to the hydrocarbon adsorption from air. In another experiment, we used a graphene/copper sample that has been stored in air for 2 days. We repeatedly released and pressed the ATR crystal to the sample surface and observed that the intensity of the IR peaks remained unchanged. This experiment again demonstrated that the contact between the ATR crystal and the graphene/copper sample is reproducible. UV/O 3 treatment of graphene/copper sample UV/O 3 treatment was carried out with a PSD Pro Series Digital UV Ozone System at room temperature. The treatment time was indicated in the main text. Thermal annealing of graphene/copper sample After CVD synthesis, a graphene/copper sample was taken out of the CVD chamber and exposed to air for 2 hr. The sample was then put back to the CVD chamber under flowing Ar (99.999%) at 500 mtorr for 30 min to flush out air in the chamber. The sample was then heated to 550 C for 1 hr and followed by a fast cooling under Ar to room temperature. The sample was taken out of the CVD NATURE MATERIALS 3

4 chamber to measure its WCA and Raman spectrum. Effect of organic vapor (1-octadecene) on the WCA of graphene In this experiment, we used carefully cleaned glass petri dishes (acetone wash followed by UV/Ozone cleaning for 30 min) to store the graphene/copper samples right after their synthesis. One piece of graphene/copper sample was kept in a glass petri dish without adding 1-octadecene, while the other piece was stored in a glass petri dish with an additional open container filled with ~ 1 ml of 1-octadecene (Aldrich, 90%). Care was taken to not directly contact the graphene/copper sample with liquid 1-octadecene. Both petri dishes were closed with a lid and heated to 50 o C using a hot plate inside a fume hood. During the measurement, samples were taken out of the petri dish every 15 min to measure WCA and then immediately put back (Figure S6). The possible role of copper oxidation on the change of WCA. In addition to the ones described in the main text, the following experimental results also argue against that copper oxidation is responsible for the observed change of WCA of a graphene/copper sample. (a) WCA of a graphene/copper sample stored in a glove box. A freshly prepared graphene/copper sample was immediately moved inside a N 2 glovebox (Mbraun UNIlab) that is oxygen and water free (ca. 1 ppm) but not hydrocarbon free due to the presence of plastic parts (e.g., rubber gloves). The graphene/copper sample was taken out of the glovebox at 30 min and 120 min to test its WCA and immediately returned to the glovebox after the measurement. The measured WCA at 30 min was 71.6º and 4 NATURE MATERIALS

5 SUPPLEMENTARY INFORMATION at 120 min was 76.0º. These values are within the range expected for air-exposed samples. Since the significantly reduced O 2 exposure does not affect the buildup of WCA, this experiment result does not support the idea that the oxidation of copper is responsible for the observed increase of WCA. (b) Copper XPS of an aged (3 day of air exposure) graphene/copper sample showed no sign of Cu 2+ on the surface (Figure S7). (c) The oxidation mechanism cannot explain the decrease of WCA upon annealing in an Ar atmosphere or mild UV/O 3 treatment. Both treatments, especially the 2 nd one, will results in, if any, an increase of copper oxidation. However, a sharp decrease of WCA was consistently observed after such treatments. Calculation of surface energy of graphene and graphite The surface energy of suspended graphene and HOPG is determined by the Girifalco-Good-Fowkes-Young equation: G [ w(1 cos )] 2 4 w d where G is the surface energy of graphene (or graphite), W is the surface energy of water (72.8 mj/m 2 ), d W is the dispersive component of the surface energy of water (21.8 mj/m 2 ) and θ is the water contact angle on suspended single layer graphene or freshly cleaved graphite. The assumptions we make are: (1) The interaction between water and graphene (or graphite) is dispersive in nature. To test the validity of this assumption, we determined the dispersive and polar components of the surface energy of freshly cleaved HOPG using Fowkes theory. S2 NATURE MATERIALS 5

6 Water and diiodomethane were the two testing liquids used in the contact angle measurement. The results showed that the dispersive components accounts for 86.2% of the total surface energy. (2) We neglect the spreading pressure ( e) in the calculation. A detailed discussion on the effect of the spreading pressure can be found in Ref S3. 2. Additional figures (a) (b) Contact Angle (Degree) Time (min) G/Cu Contact Angle (Degree) min 60min 1 Day Figure S1. (a) Temporal evolution of WCA for 6 graphene/copper samples upon their exposure to air. The samples were taken out of the CVD chamber at time 0. (b) WCA of 10 graphene/copper samples after ca. 1 min (black), 60 min (red), and 1 day (blue) of air exposure. The data points are shifted in the time axis for clarity. 6 NATURE MATERIALS

7 SUPPLEMENTARY INFORMATION (a) Contact Angle (Degree) Static Advancing Receding (b) Contact Angle Hysteresis Time (min) Time (min) Figure S2. (a) Temporal evolution of static, advancing, and receding WCA for the same graphene/copper sample upon its exposure to air. The samples were taken out of the CVD chamber at time 0. (b) WCA hysteresis as a function of time. The line is a guide to the eyes. NATURE MATERIALS 7

8 (a) (b) 1 day exposed in air 550 o C annealed Before thermal annealing After thermal annealing Transmittance (%) Wavenumber (cm -1 ) Raman Shift (cm -1 ) Figure S3. (a) ATR-FTIR spectrum of a graphene/copper sample before (black) and after (red) thermal annealing in Ar at 550ºC for 1h. (b) Raman spectra of a graphene/copper sample before and after thermal annealing in Ar at 550ºC. The D peak was very weak, indicating that the thermal annealing introduced minimal structural damage to graphene. The large background is typical for graphene/copper samples. 8 NATURE MATERIALS

9 SUPPLEMENTARY INFORMATION (a) Aged G/Cu UV/O 3 treated (b) New G/Cu Aged G/Cu After 2min UV/O BE(eV) BE(eV) Figure S4. (a) Carbon 1s XPS spectrum of an aged graphene sample (black) and the same sample after UV/O 3 treatment (red). The inset shows the difference of the two spectra. (b) Oxygen 1s XPS peak of the same graphene/copper sample taken within 10 min of air exposure (black), 3 days of air exposure (red), and after 2 min of UV/O 3 treatment (blue). The integrated peak intensities of the three spectra are: 852, 2092, and BE: binding energy. NATURE MATERIALS 9

10 Before UV/O 3 After UV/O Raman Shift (cm -1 ) Figure S5. Raman spectra of a graphene/copper sample before and after 4 min of UV/Ozone treatment. The D peak was weak, indicating that the UV/Ozone exposure introduced minimal structural damage to graphene. The large background is typical for graphene/copper samples.. 10 NATURE MATERIALS

11 SUPPLEMENTARY INFORMATION Contact Angle (Degree) in clean glass petri-dish in petri-dish with 1-octadecene Time (min) Figure S6. Temporal evolution of WCA for graphene/copper samples in the presence (red) and absence (black) of 1-octadecene vapor. NATURE MATERIALS 11

12 Cu(2p 3/2 ) Aged G/Cu Cu(2p 1/2 ) BE(eV) Figure S7. (a) Cu 2p XPS spectrum of an aged graphene/copper sample. Two Cu peaks at binding energies of ev and ev correspond to Cu2p 3/2 and Cu2p 1/2. The sample was exposed to ambient air for 3 days. BE: binding energy. 12 NATURE MATERIALS

13 SUPPLEMENTARY INFORMATION (a) G/SiO 2 before annealing After annealing (b) G/glass before annealing After annealing Intensity (a.u.) Raman Shift (cm -1 ) G/Au before annealing After annealing Intensity (a.u.) Intensity (a.u.) (c) (d) Raman Shift (cm -1 ) G/Ni before annealing After annealing Intensity (a.u.) Raman Shift (cm -1 ) Raman Shift (cm -1 ) Figure S8. Raman spectra of single layer graphene on Si/300 nm SiO 2, glass, and Au substrate as well as multilayer graphene on Ni substrate before (black) and after (red) thermal annealing in Ar at 600 o C. The D peaks were weak in all the cases, indicating that the thermal annealing introduced minimal structural damage to graphene. NATURE MATERIALS 13

14 Contact Angle (Degree) degree Annealing graphene/glass SiO 2 /Si Au Ni Time (min) Figure S9. Effect of thermal annealing (Ar, 600 o C, 60 min) on the WCA of single layer graphene deposited on Si/SiO 2, glass, and Au substrates as well as multi-layer graphene grown on a Ni substrate. The samples were taken out of the annealing chamber at time NATURE MATERIALS

15 SUPPLEMENTARY INFORMATION Table S1. Defect density of graphene samples. G/SiO 2 G/Au G/glass G/Ni G/Cu G/Cu I D /I G a defect density b (x10 10 cm -2 ) defect # per carbon atom(x10-6 ) before thermal annealing before UV/O ± ± ± ± ± ± ± ± ± ± ± ±2.3 after thermal annealing after UV/O 3 I D /I G a defect density b (x10 10 cm -2 ) defect # per carbon atom(x10-6 ) ± ± ± ± ± ± ± ± ± ± ± ±4.4 a : The intensity ratios (I D /I G ) were calculated by the integrated peak areas of the D peak to that of the G peak. The number density of carbon atoms for graphene used here is 3.85 x /cm 2. All calculations are based on the Raman spectra shown in Figure S3, S5, and S8. b : The defect density was calculated following the method reported in Cancado L. G., et al., Nano Lett. 2011, 11, NATURE MATERIALS 15

16 Table S2. Static WCA of graphene samples and substrates. This study Ref 10 Ref 14 Ref 5 Bare substrate Graphene area Graphene/copper 80 º a 80º 90º c 86º Graphene/SiO 2 39 º b 76º c 40º 93.8º Graphene/glass 15 º b 79º c 48.1º Graphene/gold 69 º b 78º c 78.8º Note: a : Measured on bare aged copper surface b : Measured on bare substrate next to graphene coating area after transferring c : Measured on samples after extended air exposure. Ref 10: Nature Mater. 2012, 11, 217 Ref 14: Phys. Rev. Lett. 2012, 109, Ref 5: ACS Nano 2011, 5, NATURE MATERIALS

17 SUPPLEMENTARY INFORMATION Table S3. Static WCA of graphene samples before and after ambient exposure. Substrate Single layer graphene on copper 2-3 layer graphene on Nickel Bare Substrates Graphene-Coated Substrates (before extended air exposure) Graphene-Coated Substrates (after 2 days air exposure) 0º a 46º 85º 0º b 59º 94º HOPG 64º 91º Note: a : Nano Lett. 2013, 13 (4), J. Phys. Chem. 1958, 62 (7), b : J. Phys. Chem. 1958, 62 (7), VCU Electronic Theses and Dissertations References S1. Li, X. S. et al. Transfer of Large-Area Graphene Films for High-Performance Transparent Conductive Electrodes. Nano Lett. 9, (2009). S2. Fowkes, F. M., Attractive Forces at Interfaces. Ind. Eng. Chem. 56, (1964). S3. Fowkes, F. M., Contact Angle, Wettability, and Adhesion. Advan. Chem. Ser. 43, 99 (1964). NATURE MATERIALS 17