Modulating Electrocatalysis on Graphene. Heterostructures: Physically Impermeable yet. Electronically Transparent Electrodes

Transcription

1 Supplementary Information Modulating Electrocatalysis on Graphene Heterostructures: Physically Impermeable yet Electronically Transparent Electrodes Jingshu Hui, 1,2 Srimanta Pakhira, 3,4,5,6, Richa Bhargava, 1 Zachary J. Barton, 1 Xuan Zhou, 1 Adam J. Chinderle, 1 Jose L. Mendoza-Cortes, 3,4,5,6* and Joaquín Rodríguez-López 1,7 * 1 Department of Chemistry, University of Illinois at Urbana Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, USA 2 Department of Materials Science and Engineering, University of Illinois at Urbana Champaign, 1304 West Green Street, Urbana, Illinois 61801, USA 3 Department of Chemical & Biomedical Engineering, Florida A&M Florida State University, Joint College of Engineering, 2525 Pottsdamer Street, Tallahassee, Florida, 32310, USA 4 Materials Science and Engineering Program, High Performance Materials Institute, Florida State University, 2005 Levy Avenue, Tallahassee, Florida, 32310, USA 5 Department of Scientific Computing, Florida State University, 110 N. Woodward Avenue, Tallahassee, Florida, 32304, USA 6 Condensed Matter Theory, National High Magnetic Field Laboratory (NHMFL), Florida State University, 1800 E. Paul Dirac Drive, Tallahassee, Florida, 32310, USA. 7 Beckman Institute for Advanced Science and Technology, University of Illinois at Urbana- Champaign, Urbana, Illinois 61801, USA. S1

2 Present Address: Discipline of Metallurgy Engineering and Materials Science, Indian Institute of Technology (IIT) Indore, Simrol Campus, Khandawa Road, Indore , MP, INDIA *To whom all correspondence should be addressed: Prof. Mendoza-Cortes: ( ) and (Phone) Prof. Rodríguez-López: ( ) and (Phone) S2

3 Materials All chemicals were purchased from commercial sources and used as received. Sodium sulfate (99.0%), sulfuric acid (99.999%), tetrahydrofuran (anhydrous, 99.9%), 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine iron(iii) chloride (FeOEP), tetrabutylammonium hexafluorophosphate (TBAPF 6, 99.0%), potassium cyanide (KCN, 96.0 %), sodium hydroxide (NaOH, 98.0 %), ethylenediaminetetraacetic acid disodium salt dihydrate (Na 2 EDTA 2H 2 O, 99.0%), acetone (99.5%), dichloromethane (DCM, 99.9%), isopropanol (99.5%) and glacial acetic acid (99.5%) were all purchased from Sigma-Aldrich. Si wafers were purchased from Monsanto. Si wafer with a 300 nm SiO 2 layer (Si/SiO 2 wafer) was purchased from University Wafer. 25 µm copper foil was purchased from Alfa Aesar. Two poly(methyl methacrylate) (PMMA) solutions were purchased from MicroChem: NanoTM 950K A4 (MW 950,000 PMMA, 4 w. % solution in anisole) and 495K A2 (MW 495,000 PMMA, 2 wt. % solution in anisole). Copper etchant (CE-100) was purchased from Transene Company. Deionized water (DI water) was filtered using a Millipore system. Ultra-high purity (UHP) argon was obtained from Airgas. Graphene growth and transfer Graphene was fabricated by chemical vapor deposition (CVD) on 25 µm thick copper foil. Copper foil was pre-treated by sequential exposure to acetone for 10 s, DI water for 10 s, acetic acid for 10 min, DI water for 10 s, acetone for 10 s, and lastly isopropanol for 10 s. 1 Following this cleaning procedure, the copper foil was placed in a tube furnace at 1000 and 40 mtorr with 100 sccm CH 4 and 50 sccm H 2 flowing for 25 min. 2 S3

4 A wet-etching procedure was applied to transfer graphene onto various substrates as described below. After CVD growth, the exposed surface of graphene supported on Cu foil was protected with 3 layers of spin-coated PMMA at 4000 rpm spin rate. The additions of the initial 495K A2 layer and the two subsequent 950K A4 layers were each followed by a 2 min curing process at 200 ºC. The combined PMMA/graphene/Cu substrates were placed in copper etchant solution and allowed to float there at 40 C for 4 h to dissolve the Cu foil. To eliminate any residual Cu metal, the floating PMMA/graphene sheets were subsequently rinsed with DI water 4 times, exposed to 0.1 M Na 2 EDTA aqueous solution for 1 h, and then rinsed again with DI water 4 times. Cleaned graphene/pmma sheets were transferred onto the desired substrates and then blow-dried with UHP argon. The protective PMMA layers were then dissolved by sequential exposure to anisole for 2 h, a DCM:acetone mixture (1:1 vol.) for 4 h, and lastly isopropanol for 2 h. S4

5 Figure S1. Raman evidence of full graphene coverage of p-au and p-pt substrates. a, Comparison between Raman spectra taken from the pure graphene area versus the center and edge of the Au-backed area in Figure 2e. b, Comparison between Raman spectra taken from the pure graphene area versus the center and edge of the Pt-backed area indicated in Figure 2h. Graphene supported on underlayer Au or Pt exhibits a larger fluorescence background but does not yield substantially different Raman peak shapes or 2D/G ratios from those observed for graphene on SiO 2. S5

6 Figure S2. SECM maps of ORR products collected over the graphene/p-pt heterostructures. a d, SECM SG/TC maps of H 2 O 2 production taken at a tip substrate gap of 8.8 µm over graphene/p- Pt where substrate activation towards ORR is decreased and summarized in e. The area between the two dashed circles indicates the approximate location of an asymmetric toroidal Pt underlayer. e, Lateral scans around the graphene/p-pt region with substrate bias from -0.1 V to 0.5 V, the line scanned location is indicated by dash line in panel a. All data were obtained with a Pt UME (r Pt = 12.5 µm, R G = 4) poised at 1.0 V vs. Ag/AgCl in a solution of 10 mm H 2 SO 4 and 0.1 M Na 2 SO 4 in O 2 -saturated water. S6

7 Figure S3. CV characterization of FeOEP adsorption behavior on graphene. a, CV at graphene in a solution consisting of 0.2 mm FeOEP and 0.1 M TBAPF 6 in THF. b, Comparison of CVs at graphene in THF containing 0.1 M TBAPF 6 as supporting electrolyte, both with (red trace) and without (black trace) 1 µm FeOEP. All CVs were taken at 0.1 V s -1. S7

8 Calculation of monolayer adsorption coverage (Γ s ) and the Gibbs free energy of adsorption ( G ads ). The adsorption coverage (Γ) as a function of concentration (C) with the equation: Γ = Γ s KC / (KC+1) Since graphene is a 2-D sheet with known exposed area in our electrochemical cell (cm 2 ), and we are characterizing the amount of FeOEP adsorption (moles) based on capacity of adsorption peaks, the units of Γ are pmol/cm 2. Fitting the data in Figure 5c with the above equation gives a monolayer adsorption coverage (Γ s ) of 157 pmol/cm 2 and an equilibrium constant (K) of L/mol. The red curve in Figure 5c indicates the fitting result: Γ = C / ( C) From K, the Gibbs free energy of adsorption ( G ads ) can be determined by the equation: G ads = - RTlnK For the data in Figure 5c, the G ads is kj/mol. S8

9 Figure S4. FeOEP adsorption behavior on various substrates. Small amounts of 1 mm FeOEP solution were spiked into blank electrolyte solution. Ferrocene was used as an internal standard. In the CV of FeOEP on graphene (top trace, black), the peaks around 0 V correspond to ferrocene, the peak at V is the reduction of adsorbed FeOEP on graphene, the peak at V relates to the reduction of FeOEP in solution, the peak at V is the oxidation of surfaceconfined FeOEP, and the peak around V can be attributed to the oxidation of FeOEP in solution. The other CV traces exhibit peaks at similar potentials for each species. A 1 ml S9

10 solution of 0.1 M LiBF 4 in acetonitrile was spiked with 8, 4, 8, 16, 16, 10 µl of 1 mm FeOEP in THF for graphene, HOPG, graphene/au, graphene/pt, Au, and Pt samples, respectively. S10

11 Figure S5. CVs of ORR at the various electrode combinations used in Figure 5d in the main text. All data were collected with a scan rate of 0.1 V s -1 in solution consisting of 10 mm H 2 SO 4 and 0.1 M Na 2 SO 4 in O 2 -saturated water. Gray zones indicate the standard deviation of at least 6 CVs. S11

12 Figure S6. ORR characterization of the E-beam deposited Pt and Au film on Si wafer. a, Full CV on Pt film, which including ORR region (0.7 V to 0.2 V), H adsorption region (0.1 V to -0.3 V), and surface oxide formation region (0.5 V to 1.2 V). 3 b, Comparison of CVs at ORR region on Pt film with O 2 saturated and O 2 fully removed via Ar bubbling solutions. c, Full CV on Au film, which including ORR region (0.2 V to -0.3 V), formation of Au oxide region (0.8 V to 1.5 V), and reduction of Au oxide region (1.0 V to 0.7 V). 4 d, Comparison of CVs at ORR region on Au film in solutions saturated with O 2 and fully deoxygenated via Ar bubbling. All data were collected with a scan rate of 0.1 V s -1 in solutions consisting of 10 mm H 2 SO 4 and 0.1 M Na 2 SO 4. O 2 or Ar bubbling methods were selected to create O 2 -saturation or O 2 -free environment, respectively. S12

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14 Table S1. Extrapolated CV peak ORR currents and positions for the substrates in Figure 5d. Substrate Potential/V Current/µA Pt FeOEP/Pt FeOEP/Graphene/Pt Graphene/Pt Au FeOEP/Au FeOEP/Graphene/Au Graphene/Au FeOEP/Graphene Graphene S14

15 Figure S7. Electronic structure and electronic properties (band structures, total density of states and projected DOSs for Au d-subshell electrons from left to right) of the FeOEP/graphene/Au heterostructure. S15

16 Figure S8. Schematic diagram of sample fabrication procedures. S16

17 REFERENCES 1. Cristarella, T. C.; Chinderle, A. J.; Hui, J.; Rodríguez-López, J. Single-Layer Graphene As a Stable and Transparent Electrode for Nonaqueous Radical Annihilation Electrogenerated Chemiluminescence. Langmuir 2015, 31, Wood, J. D.; Schmucker, S. W.; Lyons, A. S.; Pop, E.; Lyding, J. W. Effects of Polycrystalline Cu Substrate on Graphene Growth by Chemical Vapor Deposition. Nano Lett. 2011, 11, Sugawara, Y.; Okayasu, T.; Yadav, A. P.; Nishikata, A.; Tsuru, T. Dissolution Mechanism of Platinum in Sulfuric Acid Solution. J. Electrochem. Soc. 2012, 159, F779-F Diaz-Morales, O.; Calle-Vallejo, F.; de Munck, C.; Koper, M. T. M. Electrochemical Water Splitting by Gold: Evidence for an Oxide Decomposition Mechanism. Chem Sci 2013, 4, S17