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Electronic supplementary information

Enhancement of CO2 adsorption on

oxygen-functionalized epitaxial graphene surface

at near-ambient conditions

Susumu Yamamoto,a, * Kaori Takeuchi,a Yuji Hamamoto,b Ro-Ya Liu,a, 1

Yuichiro Shiozawa,a Takanori Koitaya,a, 2, 3 Takashi Someya,a Keiichiro Tashima,c

Hirokazu Fukidome,c Kozo Mukai,a Shinya Yoshimoto,a Maki Suemitsu,c

Yoshitada Morikawa,b Jun Yoshinobu,a Iwao Matsudaa

a The Institute for Solid State Physics, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,

Chiba 277-8581, Japan b Department of Precision Science and Technology, Graduate School of Engineering,

Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan c Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira,

Aobaku-ku, Sendai, Miyagi 980-8577, Japan

1 Present address: Institute of Physics, Academia Sinica, Nankang, Taipei 11529, Taiwan 2 Present address: Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1,

Komaba, Meguro-ku, Tokyo 153-8902, Japan 3 Present address: Japan Science and Technology Agency (JST), Precursory Research for

Embryonic Science and Technology (PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012,

Japan

Table of Contents 1. Coverage calibration

2. DFT calculations of CO2 on graphene with epoxy dimers

Electronic Supplementary Material (ESI) for Physical Chemistry Chemical Physics.This journal is © the Owner Societies 2018

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1. Coverage calibration

The coverages of CO2 and epoxy groups on graphene were determined by comparing

the XPS peak intensity of CO2 molecules and epoxy groups to that of the monolayer

graphene in C 1s XPS spectra in UHV and under near-ambient pressure gas atmosphere.

Note that the coverage of CO2 and epoxy groups on graphene is given by the fractional

coverage as determined by the number of CO2 molecules and epoxy groups per surface

carbon atom on graphene (3.82 × 1015 cm-2 at = 1).

The details of the coverage calibration procedure are explained by XPS spectra

measured under near-ambient pressure gas. Figure S1 shows C 1s XPS spectra of (a) the

pristine epitaxial graphene and (b) the oxygen-functionalized epitaxial graphene

measured in 1.6 mbar CO2 at 175 K. The incident photon energy was 740 eV. C 1s XPS

spectra were deconvoluted by mixed Gaussian-Lorentzian functions after a Shirley

background subtraction. The C 1s XPS spectrum of the pristine epitaxial graphene in

Figure S1 (a) was fit with five peaks: Graphene (G), SiC, surface components due to

buffer layer (S1 and S2), gas-phase CO2 (CO2(g)). The C 1s XPS spectrum of the

oxygen-functionalized graphene in Figure S1 (b) was fit with seven peaks. Two peaks,

epoxy and adsorbed CO2 (CO2(a)), were added to the five peaks for the pristine

graphene. Energy separations between the substrate peaks (G, SiC, S1, and S2) were set

following the previous report.1 The results of the peak fitting parameters are

summarized in Table S1: peak position, full width at half maximum (FWHM),

Lorentzian-Gaussian (L/G) mixing ratio. In the coverage calibration, the peak intensities

of adsorbed CO2 and epoxy groups on graphene in Figure S1(b) are compared with that

of the pristine monolayer graphene in Figure S1(a). In this procedure, the attenuation of

photoelectrons by gas-phase molecules can be cancelled out because both the sample

spectrum (Figure S1(b)) and the reference spectrum (Figure S1(a)) are measured in the

same gas pressure and with the same kinetic energy (photon energy).

2. DFT calculations of CO2 on graphene with epoxy dimers The DFT calculations presented in the main text have revealed that the enhancement of

the interaction energy of CO2 Eint on the oxygen-functionalized graphene can be

quantitatively explained by the interaction between a CO2 molecule and an epoxy group.

The structural model, however, includes a single epoxy group in the 4×4 unit cell,

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whose coverage (= 0.031) is approximately half that of the experimental estimation,

0.07. Here we provide some insights into how the coverage of epoxy groups influences

Eint and why the single-epoxy model is enough to explain the enhancement of Eint.

To this end, we also investigate CO2 adsorption on the oxygen-functionalized

graphene with two epoxy groups in the same unit cell. So far, the structures of epoxy

groups on graphene have been studied extensively using DFT calculations,2-13 which

have demonstrated that epoxy groups prefer to come close to each other. Thus, we here

focus on four typical dimer structures of epoxy groups shown in Fig. S2, where two O

atoms are adsorbed on next nearest bridge sites. Our calculated results show that epoxy

groups in a1, a2, a3, and a4 configurations are more stable than the single-epoxy model

by 18.1, 14.6, 28.1, and 28.9 kJ/mol per O atom. Oxygen adsorption on the same side

(a1 and a2) is less stable than on the both sides because the former (latter) increases

(decreases) the distortion in the freestanding graphene sheet.2-13 Still, we consider the

possibility of the former because the latter could be destabilized by the influence of the

buffer layer on the SiC surface, which is ignored in the present calculations.

By examining several adsorption sites of CO2 on the oxygen-functionalized graphene

sheets, we find that for every system the most stable adsorption site is a bridge site next

to one of the epoxy groups as shown in Fig. S2 (b1)-(b4). Note that the relative

configurations between the CO2 and the nearest epoxy group is analogous to that in the

single-epoxy model, which results in similar interaction energy curves as shown in Fig.

S3 (see Fig. 3 position 2).

To understand the analogy, we plot in Fig. S2 (c1)-(c4) the electron density

difference ∆ between before and after the adsorption of oxygen atoms on graphene.

For comparison, we also show the result of for the single-epoxy model in Fig. S2

(c5), where electric polarization induces an anisotropic electron density near the epoxy

group. The interactions between the anisotropic charge and the intrinsic quadrupole

moment of CO2 stabilize the adsorption of CO2 on the oxygen-functionalized graphene.

In Fig. S2 (c1)-(c4), the anisotropic structure of is retained near respective epoxy

groups, hence the behaviour of CO2 adsorption is essentially unchanged from the

single-epoxy model.

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Figure S1. C 1s XPS spectra of (a) the pristine epitaxial graphene and (b) the

oxygen-functionalized epitaxial graphene measured in 1.6 mbar CO2 at 175 K. The

incident photon energy is 740 eV. The total energy resolution is approximately 600

meV. The photon flux densities are 1.0×1016 photons/s∙cm2 for (a) and 7.3×1016

photons/s∙cm2 for (b).

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Figure S2. (a1-4) The structural model of epoxy dimers on graphene. (b1-4) The most

stable adsorption site of CO2 on each oxygen-functionalized graphene. (c1-4) Electron

density difference between before and after the adsorption of oxygen atoms in each

dimer configuration. Increase (decrease) in electron density is represented by red (blue)

isosurfaces. For comparison, the results for the single-epoxy model are shown in (a5),

(b5), and (c5).

Figure S3. Interaction energy as a function of distance d between CO2 and the

oxygen-functionalized graphene with epoxy dimers.

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Table S1. Summary of peak fitting parameters of C 1s XPS spectrum on the

oxygen-functionalized epitaxial graphene measured in 1.6 mbar CO2 at 175 K (Figure

S1(b)). Seven peaks were used for the peak fit: SiC, Graphene (G), surface components

due to buffer layer (S1 and S2), epoxy, adsorbed CO2, and gas-phase CO2. The

following fitting parameters are reported: Binding energy (BE), energy shift relative to

SiC peakE, full width at half maximum (FWHM), Lorentzian-Gaussian (L/G) mixing

ratio. L/G ratio is a parameter of linear combination of Lorentzian and Gaussian; L/G=

0 for 100% Gaussian and L/G= 1 for 100% Lorentzian.

BE (eV) E (eV) FWHM (eV) L/G (%)

SiC 283.74 0 0.86 0

Graphene 284.71 0.97 1.11 9

S1 285.00 1.26 1.77 0

S2 285.61 1.87 1.77 24

Epoxy 286.70 2.96 1.50 0

Adsorbed CO2 291.24 7.50 1.18 6

Gas-phase CO2 292.97 9.23 0.85 0