Mandana Akia1, Lee Cremar1, Mircea Chipara2, Edgar Munoz1, Hilario Cortez1, Hector de Santiago3, Fernando J. Rodriguez"Macias4,5, Yadira I. Vega"Cantú4,5, Hamidreza Arandiyan6, Hongyu Sun7, Timothy P. Lodge8, Yuanbing Mao3, Karen Lozano1* 1 Department of Mechanical Engineering, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, Texas 78539, United States mandana.akia@utrgv.edu, lee.cremar@utrgv.edu, edgar.munoz01@utrgv.edu, hilario.cortez01@utrgv.edu, karen.lozano@utrgv.edu 2 Department of Physics, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, Texas, 78539, United States mircea.chipara@utrgv.edu 3 Department of Chemistry, University of Texas Rio Grande Valley, 1201 West University Drive, Edinburg, Texas, 78539, United States hector.desantiago01@utrgv.edu, yuanbing.mao@utrgv.edu 4 Tecnologico de Monterrey, Campus Monterrey, Ave. Eugenio Garza Sada 2501, Monterrey, N.L., México, 64849 yivega.work@gmail.com 5 Universidade Federal de Pernambuco, Pós"Graduação em Ciência de Materiais, Avenida Jornalista Aníbal Fernandes, Recife, PE, Brasil, 50740"560. dr.fernandojrm@gmail.com 6 Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia h.arandiyan@unsw.edu.au 7 Department of Micro" and Nanotechnology, Technical University of Denmark, 2800 Kongens, Lyngby, Denmark hsun@nanotech.dtu.dk 8 Department of Chemistry and Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, MN 55455, United States lodge@umn.edu Professor Karen Lozano at Karen.Lozano@UTRGV.edu Figure S1 Figure S2 Supporting information for Raman section Supporting information for XPS section Figure S3 References S"2 S"3 S"4 S"5 S"6 S"8 S-1 Figure S1. SEM images of the graphene sheets grown at different voltages show a clear decrease of the contrast between the fibers at higher voltage, which indicates the ultrathin nature of the graphene nanosheets: a) 3 kV, b) 7 kV, c) 15 kV, and d) 19 kV. Scale bars are 2µm. As the acceleration voltage is increased the penetration depth of the electron probe increases and the extremely thin graphene sheets become more transparent, with secondary electrons from the fibers below contributing more to the final image, until at 19 kV the graphene film almost seems to disappear. S-2 Figure S2(a,b). SEM images illustrating discontinuous irregular thick graphitized formations due to partial fiber dissolution, potentially contributing to the 2D′ Raman line as “defected graphene”. Image recorded at 7 kV; the scale bar is 1 µm. S-3 The Raman spectrum of the hybrid structure (see Fig. 4) is dominated by lines located at 1346 and 1589 cm–1, assigned to D and G bands, respectively. An additional small line was observed in the hybrid sample at 822 cm–1. The region of the spectrum assigned to the D and G bands, ranging from 1000 to 1700 cm–1, was simulated by a superposition of two Lorentzians, by using the expression: + = + + + (S"1) where I is the amplitude of the recorded spectrum at the Raman shift ω, Ik represents the amplitude of the Raman line at position ωk (here k=1, 2), and Wk is the width of the line. A, B, C are fitting constants allowing for a second order correction of the spectrum (mainly zero point and slope). The width of the Lorentzian is the distance between the inflection points, measured along the OX axis. The best curve"fitting has been obtained for the following parameters. D line: intensity 987,000, position 1346 cm–1, and width 136 cm–1; G line: intensity 549,000, position 1589 cm–1, and width 81 cm–1. The ratio between the amplitudes of the D and G lines (ID/IG=I1/I2) is about 1.8 and the ratio between the corresponding integrated intensities is about 3.0. This suggests that the sample has a characteristic length (average distance between defects) of the order of 2.0 nm1. The region extending from 2000 to 3500 cm–1 was fitted by a combination of three overlapping Raman lines, by using a similar equation. = + + + + + (S"2) The meaning of these parameters is as in Eqn S"1. The best fitting parameters for these lines are as follows. The first line, located at position 2664 cm–1, with an intensity of 220,000 and a width of 238 cm–1. This line was assigned to the 2D mode (historically, this band was labeled as G’). The line located at 2900 cm–1 has an intensity of 240,000 and a width of 250 cm–1. The last line is weak, with an intensity of 24,000, a width of 100 cm–1, and is located at 3186 cm–1. This line is close to the line observed at about 3250 cm–1 and identified as a 2D’ peak1. Actually, the ratio of these amplitudes is A2D/AG = 0.400 and the ratio of the integrated intensities (I2D/IG, i.e., the areas of these lines) is 1.18. This demonstrates the presence of graphene in the investigated samples. S-4 Figures S3a and b show the different bonding contributions for the carbon fiber (CF) / graphene"fiber hybrid structure (GFHS), and can be assigned to various types of oxygen" containing functional groups2"4. In Fig. S3b, the GFHS CIII peak found at 286.8 eV, C"O" R (R= C, H), can be assigned to epoxy (C"O), ether (C"O"C), or hydroxyl (C"OH) groups. The GFHS CIV peak at 287.8 eV (287.8 eV " CF ) corresponds to the carbonyl (" C=O) functionality in aldehydes or ketones, and the remaining oxygen containing groups, CV at 288.8 eV (288.6 eV " CF) and CVI at 289.9 eV (289.7 eV " CF), correspond to a carboxyl (O=C–OH) and carboxylate group (O"C=O")5,6. The GFHS CVII peak at 291.0 eV (291.6 eV for the carbon fibers without graphene) relates to the π"π* “shake"up” satellite due to photoionization of the 1s electron in the conjugated π"system found in graphite5"7. With inclusion of the oxygen"containing groups, the C=C percent hybridization for GFHS is 77.6% sp2 (with the shake"up contribution). This is greater than the CF produced herein, which has 73.1% sp2 character. Several reports have indicated that analysis of the X"ray induced CKLL Auger spectrum can provide a measure of the D"parameter, which gives an indication of the relative amount of sp2 and sp3 carbon. The measurement of the binding energy width (D), taken between maxima and minima of the differentiated Auger CKLL spectrum, therefore provides a fingerprint of the carbon atom arrangement8"10. The latter studies have shown highly oriented pyrolytic graphite with D~21.2 eV where the peak maximum has a binding energy of 284.4 eV. It was reported theoretically that when introducing structural changes (pentagons or Stone–Thrower–Wales (STW) defects) in graphene, the binding energy and full width at half maximum (FWHM) can change11. The addition of a STW defect to graphene was shown to the shift the binding energy upward by 0.2 eV and increase the FWHM; however, the opposite trend was seen when incorporating pentagon structures, as seen in fullerene11. S-5 Figure S3. Various functional groups for GFHS and CF as determined by peak deconvolution. The bonding percentages were determined from the peak area for each state (a & b). Traces of sulfur are attributed to the S 2s and 2p states in (c), which is indicated by the sodium bonding state at 1071.5 eV, as shown by the deconvoluted Na 1s spectrum in (d). Due to the sulfuric acid vapor treatment process, residual traces of sulfur were detected, as seen in Fig S3c, which shows the S 2s peak at 228 eV and the S 2p peak at 164.2 eV, respectively. The raw Na 1s state has a maximum peak at 1072 eV (Fig S3d), which is associated with elemental sodium. Spectral deconvolution of Na 1s further shows the elemental sodium chemical state at 1072 eV, and further revealed a bonding state associated with oxygen/sulfur containing compounds. The sub"band at lower binding energy (1071.5 eV) could be assigned to the other chemical states (Fig. S3d), which corresponded to various sodium"carbon/sulfur containing compounds (e.g., sodium thiosulfate (Na S O ), sodium carbonate (Na2CO3), and sodium sulfate (Na2SO4)). The O 1s spectrum may have states corresponding to the bonding of oxygen to sulfur, either in the form of organic sulfonates (C"SO3) or as inorganic sulfates. However, given the negligible sulfur content and the weight ratio of sulfur to oxygen in sulfate, this would suggest less than 2 wt.% oxygen. This would further corroborate that those other compounds, such as C"O or C"OH, fall within the O 1s spectrum. The latter is typically common due to surface oxidation from the air. Furthermore, the fitted deconvoluted O 1s spectrum showed a maximum peak for the one underlying state at 532.5 eV (532.7 eV" S-6 CF), which is broader than the raw O 1s peak. This peak has been reported to reflect contributions from surface oxidation or other oxides6,8,12. The broad O 1s spectrum and limited depth profile resolution can make peak fitting challenging, which can make chemical state assignments complex13,14. The latter occurs due to the overlapping peaks and therefore fitting constraints may be required, whereby overlapping components have the same FWHM. Given that sulfur and sodium are primarily found in the form of salts and present at less than 1 wt.%, this would indicate that the hybrid graphene"carbon fiber nanostructure is primarily graphitized carbon with slight oxidation on the surface. S-7 (1) Ferrari, A. C. Raman spectroscopy of graphene and graphite: disorder, electron–phonon coupling, doping and nonadiabatic effects 2007, 143, 47-57. (2) Bratt, A.; Barron, A. XPS of Carbon Nanomaterials; Openstax CNX, 2011. (3) Wanger, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of Xray Photoelectron Spectroscopy Perkin-Elmer Corp., 1979, 74-80. (4) Matsumoto, M.; Saito, Y.; Park, C.; Fukushima, T.; Aida, T. Ultrahigh-throughput exfoliation of graphite into pristine ‘single-layer’graphene using microwaves and molecularly engineered ionic liquids 2015, 7, 730-736. (5) Andreoli, E.; Barron, A. R. Correlating carbon dioxide capture and chemical changes in pyrolyzed polyethylenimine-C60 2015, 29, 4479-4487. (6) Lee, W.; Lee, J.; Reucroft, P. XPS study of carbon fiber surfaces treated by thermal oxidation in a gas mixture of O 2/(O 2+ N 2) 2001, 171, 136-142. (7) Ismagilov, Z. R.; Shalagina, A. E.; Podyacheva, O. Y.; Ischenko, A. V.; Kibis, L. S.; Boronin, A. I.; Chesalov, Y. A.; Kochubey, D. I.; Romanenko, A. I.; Anikeeva, O. B. Structure and electrical conductivity of nitrogen-doped carbon nanofibers Carbon 2009, 47, 1922-1929. (8) Fujimoto, A.; Yamada, Y.; Koinuma, M.; Sato, S. Origins of sp3C peaks in C1s X-ray 2016, 88, 6110-6114. Photoelectron Spectra of Carbon Materials (9) Jackson, S. T.; Nuzzo, R. G. Determining hybridization differences for amorphous carbon from the XPS C 1s envelope 1995, 90, 195-203. (10) Mezzi, A.; Kaciulis, S. Surface investigation of carbon films: from diamond to graphite 2010, 42, 1082-1084. (11) Kim, J. H.; Kim, C. H.; Yoon, H.; Youm, J. S.; Jung, Y. C.; Bunker, C. E.; Kim, Y. A.; Yang, K. S. Rationally engineered surface properties of carbon nanofibers for the enhanced supercapacitive performance of binary metal oxide nanosheets 2015, 3, 19867-19872. (12) Barr, T. L.; Yin, M. Concerted x-ray photoelectron spectroscopy study of the character 1992, 10, 2788-2795. of select carbonaceous materials (13) Taylor, A. Practical surface analysis, 2nd edn., vol I, auger and X-ray photoelectron spectroscopy. Edited by D. Briggs & M. P. Seah, John Wiley, New York, 1990 ! 1992, 53, 215-215. (14) Watts, J. F.; Wolstenholme, J. In An Introduction to Surface Analysis by XPS and AES; John Wiley & Sons, Ltd, 2005; pp 113-164. S-8