Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance

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1 Supplementary Materials Tuning the Double Layer of Graphene Oxide through Phosphorus Doping for Enhanced Supercapacitance Weixin Song, 1,3 Johannes Lischner, 1,2,3 Victoria Garcia Rocha1, 4 Heng Qin, 1,3 Jiahui Qi, 1,3 Joseph H.L. Hadden, 1,3 Cecilia Mattevi, 1,3 Fang Xie, 1,3 D. Jason Riley 1,3 * 1 Department of Materials, Imperial College London, London SW7 2AZ, UK. 2 Thomas Young Centre at Imperial College London, London SW7 2AZ, UK. 3 London Centre for Nanotechnology, London SW7 2AZ, UK. 4 Cardiff School of Engineering, Cardiff CF243AA, UK. * Corresponding author. jason.riley@imperial.ac.uk Experimental section Synthesis of P-doped graphene oxide. Tens of grams of graphene oxide (GO) were reproducibly and safely prepared by a modified synthesis 1 in a custom-built rig designed to manipulate up to 10 L of concentrated acids. In a typical synthesis, a 9:1 mixture of concentrated H2SO4/H3PO4 (3:0.3 L) was added to 24 g of natural graphite flakes ( µm sieved, Aldrich), followed by addition of 144 g of KMnO4 (6 wt equiv.). This reaction is slightly exothermic and temperature rose to C. The reacting suspension was then heated to 50 C and stirred at 400 rpm for 18 h. Next, it was cooled to room temperature and the oxidation stopped by dropwise addition of 1.72 L of 2%wt. aqueous H2O2. The graphene oxide suspension was washed by repeated centrifugations at 9000 rpm and redispersions in doubly distilled water (Thermo Scientific Sorvall LYNX 6000 Superspeed Centrifuge). The work-up was carried out until the supernatant water of the centrifuged GO was close to ph 6, typically occurring after 16 cycles of washing. A couple of low speed (<1000 rpm) centrifugation cycles were typically performed to remove unexfoliated graphite particles. To P dope the GO, the as-prepared material (10 cm 3, 6.7 g dm -3 ) was mixed with the P containing surfactant in 20 cm3 ethanol and then dried at 60 o C in the oven. The resultant mixture was transferred to a tube furnace with Argon flow and heated at 5 o C min -1 to a temperature of 500 o C, held at this temperature for 30 min, then heated at 3 o C min -1 to the target temperature and held at this temperature for 30 min. The product was collected after cooling to room temperature. For reducing GO, GO was heated in a tube furnace with Argon flow and heated at 5 o C min -1 to a temperature of 500 o C, held at this temperature for 30 min, then heated at 3 o C min -1 to 800 and 900 o C for another 30min. The amounts of P-surfactant and the annealing temperatures employed are shown in table S1. Material Characterization. Scanning electron microscopy (SEM) images were recorded on a LEO Gemini 1525 FEG. Transmission electron microscopy (TEM) images, 1

2 selected area electron diffraction (SAED), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) spectra were carried out on JEM 2100F. Atomic force microscopy (AFM) was carried out on a Bruker Innova Atomic Force Microscope. X-ray photoelectron spectroscopy (XPS) was measured through Thermo Scientific K-Alpha (hυ= ev). Raman spectroscopy and mapping were performed on Renishawin Via with 50X objective and 514nm laser with a power level of 50 mw as excitation source. X-ray diffraction (XRD) was carried out on a Bruker D8 diffractometer with monochromatic Cu K radiation (λ= Å). Electrochemical Tests. All the electrochemical tests were carried out on Autolab potentiostat at room temperature. Electrochemical Impedance Spectroscopy (EIS) was measured on solartron SI 1250 Frequency Response Analyzer from 60 KHz TO 10 mhz with an AC amplitude of 5 mv. Three-electrode system was employed composed of working electrode, mercury oxide electrode (MOE) as the reference and platinum mesh as counter electrode. The electrolyte was 6 M KOH (>85%, Sigma-Aldrich) solution in a sealed beaker. To prepare the working electrode the active material, PGO, was dispersed in ethanol and mixed with conductive carbon black (Sigma-Aldrich), PTFE binder (1%, Sigma-Aldrich) at a weight ratio of 8:1:1 to make an electrode slurry of concentration of 8 mg ml-1. Then 15~50 μl of slurry was pipetted on to a gold sheet and dried at room temperature. The electrode mass was measured before and after the slurry transfer to double check the mass of PGO. Simulation and models. Density-functional theory calculations using the planewave/pseudopotential approach were implemented in the Quantum Espresso software package 2. The ultrasoft GBRV pseudopotentials developed by Garrity et al 3., a wave function cut-off of 40 Ry, a charge density cut-off of 200 Ry and the PBE generalized gradient approximation 4 to the exchange-correlation energy functional were used. The graphene oxide sheets were separated by 20 Angstrom of vacuum. To further reduce spurious interactions between the sheets, the dipole correction developed by Bengtsson 5 was employed. Atomic positions were relaxed using the Broyden-Fletcher- Goldfarb-Shanno algorithm. As convergence criterion, the condition that all components of all forces must be smaller than 0.05 ev/angstrom was imposed. The unit cells contained up to 50 atoms and the Brillouin zone was sampled only at the Gamma point. Joint density-functional theory calculations were carried out using the JDFTx software package 6. In these calculations, the same convergence parameters as in the Quantum Espresso simulations were used. The water was described using a linear polarizable continuum model. The results were compared to nonlinear continuum models and only relatively small differences found. 2

3 Table S1 Experimental details for different reactions and products. The preparation method was the same despite the conditions listed. Label GO (6.7 g L -1 ) TPA TOP TBU Secondary temperature PGO 1TPA+1.3TOP, ml 1 mmol 1.3 mmol / 800 o C PGO 1TPA+1.3TOP, ml 1 mmol 1.3 mmol / 900 o C PGO 1TPA, ml 1 mmol / / 800 o C PGO 1TPA, ml 1 mmol / / 900 o C PGO 4TPA, ml 4 mmol / / 800 o C PGO 7TPA, ml 7 mmol / / 800 o C PGO 1.3TOP, ml / 1.3 mmol / 800 o C PGO 1.3TBU, ml / / 1.3 mmol 800 o C PGO 0.7TOP, ml / 0.7 mmol / 800 o C PGO 0.7TOP, ml / 0.7 mmol / 900 o C Fig. S1 XPS of PGO and GO to determine the atomic amount of C, O and P in each sample. (a) PGO 1TPA+1.3TOP,800 and PGO 1TPA+1.3TOP, 900. PGO 1TPA+1.3TOP,800 contains silicon impurities from the crucible. (b) PGO 1TPA, 800, PGO 1TPA, 900, PGO 4TPA, 800 and PGO 7TPA, 800.(c) PGO 1.3TOP, 800, PGO 1.3TBU, 800,PGO 0.7TOP, 800 and PGO 0.7TOP, 900.(d) GO. 3

4 Fig. S2 XPS of GO heated in Ar at 800 and 900 o C for 30min. Table S2 C, O content (at.%) and O:C ratio of GO 800 and GO 900 sample C O O:C GO % 18% 0.22 GO % 5.3% Fig. S3 XRD patterns of GO mixed with (a) TPA and TOP, (b) TPA and (c) TOP then heated at 60, 120 and 180 o C in an oven for 12h. 4

5 Fig. S4 N 2 adsorption/desorption isotherms and pore size distribution of (a) PGO 1TPA+1.3TOP,800,(b) PGO 1TPA, 800,(c) PGO 1.3TOP, 800 and (d) PGO 7TPA, 800. The specific surface area is calculated by BET theory and the total pore volume of pores is estimated until p/p 0 =0.98. Pore size distribution curve is obtained from BJH method. Table S3 Specific surface area (S BET ) from BET method, main pore size (D) and total pore volume (V) from BJH method of PGO 1TPA+1.3TOP,800, PGO 1TPA, 800, PGO 1.3TOP, 800 and PGO 7TPA, 800. Sample S BET /m 2 g -1 D/nm V/cm 3 g -1 PGO 1TPA+1.3TOP, PGO 1TPA, PGO 1.3TOP, PGO 7TPA,

6 Fig. S5 SEM images of (a) PGO 1TPA+1.3TOP,800, (b) PGO 1.3TOP,800 and (c) PGO 1TPA,800. Fig. S6 SEM images of PGO 1TPA+1.3TOP, 900 from basal and cross section after doping reaction. 6

7 Fig. S7 SEM images of (a, b) PGO1TPA, 900, (c, d) PGO4TPA, 800 and (e, f) PGO7TPA, 800 at different magnifications. Fig. S8 SEM images of (a, b) PGO0.73TOP, 800, (c, d) PGO0.73TOP, 900, and (e, f) PGO1.3TBU, 800, respectively. 7

8 Fig. S9 High-resolution XPS spectra of GO with fitted results. (a) C1s XPS fitted in 6 peaks, O1s XPS fitted in (b) 5 peaks and (c) 3 peaks, respectively. Table S4 Fitting results of high resolution C1s and O1s XPS spectra of GO samples. O1S spectra was fitted depending on five-peak model and three-peak model. C1s Bonds C-C C-OH C-O-C C=O HO-C=O CO 2 Position / ev Content 10.6% 35.8% 6.9% 31.8% 12.1% 2.8% O1s Bonds O=C-O-H C=O C-O-C H-O-C- H-O-C=O H 2 O Position / ev Content 13.3% 14.3% 19.7% 34.8% 14% 3.9% Bonds O=C- -O-C H 2 O FWHM / ev Position / ev Content 11.4% 84.7% 3.9% As listed in Table S4, C1s spectrum is deconvoluted into six components and they are assigned to sp 2 carbons (C-C) in aromatic rings (284.8 ev), C atoms bonded to hydroxyl (C-OH, 285.6), epoxide (C-O-C, ev), carbonyl ( C=O, 287.7eV), carboxyl (HO-C=O, 289eV) and CO 2 (O=C=O, 290.2eV) 7. Correspondingly, the O1s spectrum provides complementary information to support these oxygen groups from C1s analysis. The peaks at and 531.9eV are assigned to oxygen doubly bonded to aromatic carbon, and the aromatic carbon connecting hydroxyl group causes O=C OH to have a lower binding energy (531.3eV) because the electron donor of OH in comparison with carbon 8

9 linked with C atoms 8. For oxygen singly bonded to aliphatic carbon (-O-C-), the binding energy is 532.5eV for C-O-C and 533eV for C-O-H group, respectively. The phenolic oxygen singly bonded to aromatic carbon (-O-C=O) has a higher binding energy of 533.7eV because the electron-withdrawing -C=O group decreases the energy of core-level electrons of the O connecting -C=O in -O-C=O 9. The binding energy at 534.8eV is due to the chemisorbed/intercalated water molecules 10. For simplicity of the O1s spectrum, the deconvolution contains three components: O=C at 531.5eV with full width at half maximum (FWHM) of 1.66 ev, O-C involving H-O-C=O, H-O-C and C-O-C groups at ev with FWHM of 1.92eV and H 2 O at 534.8eV with FWHM of 1.62eV. Despite the groups linking carbon atoms in H-O-C and C-O-C, the epoxide group has a similar binding energy to C-O-H as the effects from C and H on the core-level electrons are approaching in 0.5eV 8. 9

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12 Fig. S10 High-resolution P2p and O1s XPS spectra of PGO. P2p and O1s spectra was fitted with three and four peaks, respectively. (a, b) PGO 1TPA, 800, (c, d) PGO 1.3TOP, 800, (e, f) PGO 1TPA, 900, (g, h) PGO 4TPA, 800, (i, j) PGO 7TPA, 800, (k, l) PGO 0.73TOP, 800, (m, n) PGO 0.73TOP, 900, (o, p) PGO 1.3TBU, 800 and (q, r) PGO 1TPA+1.3TOP,900. Figure S10 shows the high-resolution P2p and O1s XPS spectra and fitted components of PGO 1TPA, 800, PGO 1.3TOP, 800 and PGO 7TPA, 800, PGO 4TPA, 800, PGO 1TPA, 900, PGO 0.7TOP, 800, PGO 0.7TOP, 900, PGO 1.3TBU, 800 and PGO 1TPA+1.3TOP. The same fitted component from the deconvolution has the same FWHM for all the samples. High-resolution P2p XPS is deconvoluted into three components and every component is fitted in two peaks, namely 2P 1/2 and 2P 3/2 resulting from the spin-orbit splitting (the energy difference is 0.87eV). The 2P 3/2 peak located in the range of eV is the delegate of P-C bond and in the range of ev includes the characteristic contribution from P-O bond Specifically, the peak at around 132.5eV denotes the combined binding energy from P=O/P-O-P 14. The low content of P causes much noisy signal in the XPS spectra and results in peak shift to some degree. Correspondingly, O1s XPS spectra is deconvoluted into several components signifying the specific chemical bonds in consistence with the oxygen groups analysed from C1s and P2p spectra. However, some components comprise of the contribution from mixed categories of chemical bonds with similar binding energy. The range between 530 and 532 ev of O1s XPS spectra points to oxygen double bonded to carbon (C=O) and non-bridging oxygen in the phosphate group (P=O) 15, 17.The peak around 532.5eV indicates single-bonded O (-O-) in C-O-H and P-O-H and the one in the range of 533.2~533.5eV signifies the symmetric bridging O in P-O-P 13, 17 and O singly-bonded aromatic carbons (H-O-C=O). C=O and H-O-C=O groups are heritage from the precursor GO after P-doping, and P=O and P-O-P groups are from surface-absorbed pyrophosphate. The chemically absorbed O 2 and H 2 O on the surface also can be detected by XPS above 534 ev 13,

13 Fig. S11 Raman spectra of PGO in comparison with GO. (a, d) Spectra of GO, PGO 1TPA+1.3TOP,900 and PGO 1TPA+1.3TOP,800. (b) Spectra of GO, PGO 7TPA, 800,PGO 1TPA, 900 and PGO 1TPA, 800. (c) Spectra of GO,PGO 1.3TBU, 800, PGO 1.3TOP, 800, PGO 0.7TOP, 800 and PGO 0.7TOP, 900. Table S5 Raman analysis of PGO samples in comparison with GO. D and G band position and their intensity ratio. Label Re-label D(cm -1 ) G(cm -1 ) I D /I G GO PGO 1TPA+1.3TOP,800 PGO7.77% PGO 1TPA+1.3TOP, 900 PGO2.31% PGO 1TPA, 800 PGO1.76% PGO 1TPA, 900 PGO1.14% PGO 7TPA, 800 PGO10.19% PGO 1.3TOP, 800 PGO1.68% PGO 1.3TBU, 800 PGO1.4% PGO 0.7TOP, 800 PGO0.55% PGO 0.7TOP, 900 PGO2.2%

14 Fig. S12 (a, b) TEM images of PGO 1TPA+1.3TOP,800. (c) Selected area electron diffraction pattern (SAED) of the ellipse part in (a). (d) HRTEM of PGO 1TPA+1.3TOP,800. Fig. S13 (a, b) STEM images of PGO 1TPA+1.3TOP,800. (c-e) Elemental mapping images of (b). 14

15 Fig. S14 Cyclic voltammetry (CV) of PGO 1TPA+1.3TOP,800 in 6 M KOH. (a) CV curves of Au foil substrate and PGO7.77% loaded Au foil at 100 mv s -1.(b) CV curves of Au substrate and PGO 1TPA+1.3TOP,800 loaded to different amounts in a voltage range of -0.8~0.4V vs. MOE. (c) CV curves at different scan rates in a voltage range of -0.8~0.2V vs. MOE. (d) CV curves at different scan rates in a voltage range of - 0.8~0.4V vs. MOE. (e) CV curves in different voltage ranges at 100 mv s -1. (f) The charge/discharge curves of PGO 1TPA+1.3TOP,800 across different voltage ranges at 2 A g -1 in 6 M KOH. 15

16 Fig. S15 CV curves of (a) PGO 1.3TOP,800, (b) PGO 1TPA, 800 and (c) GO in 6 M KOH at different scan rates. (d) Charge/discharge curves of GO at 5 A g -1 and 10A g -1 in 6 M KOH between -0.8 and 0.2 V. 16

17 Fig. S16 Specific capacitance of PGO at different current densities in a voltage range of -0.5~0V vs. MOE in 6 M KOH. (a-j) PGO with P at.% of 7.77%, 2.31%, 1.76%, 1.14%, 2.49%, 10.19%, 0.55%, 2.2%, 1.68% and 1.4%. The using current densities are 2, 3, 5 and 10A g

18 Fig. S17 The relationship between the specific capacitance of PGO tested at 2 and 3A g -1 and their corresponding (a) P/C, (b) O/C and (c) P/O ratio. Alkyl P-acid is TPA used and alkyl P is the alkyl phosphine, TOP and TBP in this work. (d) Ragone plot of the as-prepared PGO samples. Fig. S18 Cycling performance of (a) PGO10.19% at 10 A g -1 and (b) PGO7.77% at 15 A g -1 in the voltage range of -0.5~0 V vs. MOE in 6 M KOH. Fig. S19 Specific capacitance of PGO10.19% under different current densities in a voltage range of - 0.5~0V vs. MOE after 5000 charge/discharge cycles. 18

19 Figure S20 (a) Nyquist plots and (b) bode plots of GO, PGO 1TPA, 800 and PGO 1TPA+1.3TOP, 800 after 5 CV cycles at 0.5 V s -1 and the corresponding fitted results. GO is fitted by the model in (d) and the other two are in (e). (c) Nyquist plots of GO, PGO 1TPA, 800 and PGO 1TPA+1.3TOP, 800 after 1000 CV cycles at 0.5 V s -1. Table S6 The parameters of components in the fitted equivalent circuit for the samples GO, PGO 1TPA, 800 and PGO 1TPA+1.3TOP, 800 after 5 CV cycles at 0.5 V s -1. Sample R R1 CPE1-T CPE1-P R2 CPE2-T CPE2-P GO E Error/% PGO1.76% E E Error/% PGO7.77% E E Error/% References 1. D. C. Marcano, D. V. Kosynkin, J. M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L. B. Alemany, W. Lu and J. M. Tour, ACS Nano, 2010, 4, P. Giannozzi, S. Baroni, N. Bonini, M. Calandra, R. Car, C. Cavazzoni, D. Ceresoli, G. L. Chiarotti, M. Cococcioni, I. Dabo, A. Dal Corso, S. de Gironcoli, S. Fabris, G. Fratesi, R. Gebauer, U. Gerstmann, C. Gougoussis, A. Kokalj, M. Lazzeri, L. Martin-Samos, N. Marzari, F. Mauri, R. Mazzarello, S. Paolini, A. Pasquarello, L. Paulatto, C. Sbraccia, S. Scandolo, G. Sclauzero, A. P. Seitsonen, A. Smogunov, P. Umari and R. M. Wentzcovitch, Journal of physics. Condensed matter : an Institute of Physics journal, 2009, 21, K. F. Garrity, J. W. Bennett, K. M. Rabe and D. Vanderbilt, Computational Materials Science, 2014, 81,

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