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Letter to the Editor, supporting information

Electrochemical oxygen reduction activity of intermediate onion-like

carbon produced by the thermal transformation of nanodiamond

Naokatsu Kannari, Takayoshi Itakura and Jun-ichi Ozaki*

Graduate School of Science and Technology, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Gunma

376-8515, Japan

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1. Experimental procedure

1.1 Sample preparation

The nanodiamond (ND) material used was an aggregate of 5 nm primary

nanodiamond particles, which was purchased from Prosonic Inc. This was initially heat

treated at 1000°C (ND1000) in a nitrogen stream for 1 h, after which samples were

prepared with heat-treatment temperatures (HTTs) of between 1400°C (ND1400) and

2000°C (ND2000) by re-heating the ND1000 in a graphite furnace under dynamic

evacuation for 1 h. Finally, the samples were pulverized in a planetary ball mill at 750

rpm for 1.5 h in preparation for further investigation.

1.2 Sample characterization techniques

Specimens were prepared for analysis by transmission electron microscopy

(TEM) by first mixing with methanol, then dropping a small amount of this slurry onto

a copper grid and allowing the methanol to evaporate. Once dry, the grid was transferred

to a TEM apparatus (JEM 2010, JEOL) operating at an acceleration voltage of 200 kV.

X-ray diffraction (XRD) patterns were recorded for each specimen using an X-

ray diffractometer (RINT2100/PC, Rigaku Corp.) with Cu-K radiation at 20 mA and

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32 kV. Core-level X-ray photoelectron spectra (XPS) of C1s, O1s and N1s were

measured with a Kratos AXIS NOVA (SHIMADZU Corp.) spectrometer using Al-Kα

radiation (10 mA, 15 kV). Charge-up shift correction was performed by setting the sp2-

carbon peak at 284.5 eV.

A temperature-programmed desorption (TPD) apparatus (TPD-44, BEL Japan

Inc.) was used to evaluate the oxygen uptake of each sample. Prior to measurement, the

original surface functionality of each sample was removed by temperature sweeping up

to 1000°C in a helium stream. The samples were then exposed to 5 % O2/He for 20 min

at 150°C to allow oxygen to adsorb. Finally, after sweeping out the weakly bound

oxygen species by flowing helium at 40°C, the temperature was raised and any desorbed

oxygen species such as CO or CO2 were measured by mass spectrometry.

Rotating disk electrode (RDE) voltammetry was used to assess the ORR

activity of the samples. For this, a slurry was prepared by mixing 5 mg of sample with

50 L of Nafion solution (5% lower aliphatic alcohols, Aldrich), 150 L of ethanol

(99.5%, Wako Pure Chemicals, Co. Ltd.) and 150 L of deionized water in a plastic

conical vial (1.5 mL). Some of this slurry (4 μL) was then applied over the whole

surface of a working electrode, which consisted of a 6 mm diameter glass-like carbon

electrode embedded in a Teflon body (Nikko Keisoku Co. Ltd.), and a catalytic

electrode was obtained by evaporating away the solvents in air. This was then combined

with a Ag/AgCl (DKK-TOA Corp.) reference electrode and glass-like carbon rod

counter electrode in a 0.5 M H2SO4 electrolyte solution that was purged of any dissolved

oxygen by bubbling nitrogen. Cyclic potential sweeping treatment of this three-

electrode cell was performed between −0.2 V and 0.8 V vs. Ag/AgCl at 50 mV s-1 for

five cycles with a potentiostat (ALS 700A or ALS 700B, BAS Inc.). Reference linear-

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sweep voltammograms (N2-LSVs) were obtained by sweeping the potential from 0.8 V

vs. Ag/AgCl to −0.2 V vs. Ag/AgCl at 1 mV s-1 while rotating the electrode at 1500 rpm.

Next, the H2SO4 electrolyte was bubbled with oxygen to produce an O2-saturated acidic

media, and a second set of linear sweep voltammograms (O2-LSVs) were obtained. Net

voltammograms for the ORR were then calculated by subtracting N2-LSV from O2-LSV,

using a normal hydrogen electrode (NHE) standard to represent the potential.

2. Complementary data

2.1 Structural analysis

In the TEM images of the prepared samples shown in Fig. S-1, it can be seen

that the original ND was an aggregate of particles 2-5 nm in diameter; a structure that

remained unchanged in the ND1000 sample. The ND1400, on the other hand, shows

clear signs of transformation from ND to OLC. Furthermore, the OLC produced

contains numerous defects along with non-planar graphitic layers, which are particularly

evident near the open graphitic layers of the OLC structures. In the case of ND1600, the

lattice fringes of well-oriented graphitic layers can be quite clearly observed in the OLC

structures. At higher temperatures, namely ND1800 and ND2000, the appearance of

polygonization is indicative of the formation of linearly developed graphitic layers. All

of these structural changes are in agreement with a previous reports by Kuznetsov et al.

[1].

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Fig. S-1 TEM images of nanodiamonds (a) as received, and after heat treatment for 1 h

at (b) 1400, (c) 1600, (d) 1800 or (e) 2000°C. Arrows indicate defect sites.

Fig. S-2 shows the XRD profile of the ND before and after heat-treatment, with

the untreated ND and ND1000 samples both showing peaks at 42° and 75° that

correspond respectively to the (111) and (220) planes of diamond. However, the

ND1400 shows evidence of peaks attributable to the (002) and (11) reflections of

graphite, the development of which comes at the expense of the aforementioned

diffraction of the diamond structure. This agrees well with the synchrotron X-ray

diffraction studies of heat-treated nanodiamonds by Tomita et al., who also reported the

development of a graphitic structure at the expense of the original diamond structure

[2].

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Fig. S-2 XRD profile of heat-treated nanodiamonds.

2.2 XPS analysis of chemical composition

The chemical composition of the surface of each of the prepared samples, as

determined by XPS analysis, is given in Table S-1. The presence of nitrogen in only

those samples prepared at lower HTTs is considered to be the result of residual

precursors in the original ND material, which are subsequently removed at temperatures

in excess of 1600°C. The lack of any clear trend in the oxygen content is believed to

have been caused by fluctuation in either the oxygen of moisture content of the air

during treatment.

As shown in Fig. S-3, the C1s XPS spectrum of each sample was separated into

six sub-spectra: sp2-type carbon at 284.5 eV, sp3-type carbon at 285.1 eV, carbonyl-type

carbon at 286.1 eV, hydroxyl-type carbon at 287.1 eV, carboxyl-type carbon at 288.9 eV

and a plasmon satellite at 290.8 eV [3]. The relative fraction of each species is listed in

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Table S-2. Note that the last column of this table shows the bandwidth of the sub-spectra

corresponding to the sp2-type carbon, which provides an indicator as to the degree of

defects [4, 5].

N1s XPS spectra of ND1400 and ND1600 were deconvoluted into five

nitrogen species ((N-I (pyridine-type nitrogen, 398.5 0.2 eV) N-II (pyrrrole/prydone-

type nitrogen, 400.5 0.2 eV), N-III (quaternary-type nitrogen, 401.2 0.2 eV) and

N-IV (oxide-type nitrogen, 402.9 0.2 eV) [6] shown in Fig. S-5. Table S-3 shows

surface nitrogen distribution represented by N/C of ND1400 and ND1600 obtained from

the deconvolution results.

Table S-1 Surface chemical composition (atomic percent), as determined by XPS.

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Table S-2 Sub-spectra of the C1s XPS spectrum and bandwidth of sp2-type carbon.

sp2 sp3 C-OH C-O C-OOH Satelite

ND1400 37.5 21.4 20.3 11.6 4.5 4.8 0.86

ND1600 50.9 24.0 8.5 4.8 3.4 8.5 0.73

ND1800 50.8 22.3 8.9 4.8 5.2 7.9 0.68

ND2000 57.8 18.9 8.1 3.7 2.2 9.3 0.59

SampleDistribution of carbon species / % Bandwidth of

sp2-C / eV

285290295

Binding energy / eV

(c)

285290295

(d)

285290295

Nor

mal

ized

inte

nsity

(a)

285290295

(b)

Fig. S-3 C1s XPS spectra of each sample before and after peak separation analysis. (a)

ND1400, (b) ND1600, (c) ND1800 and (d) ND2000.

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1400 1600 1800 20000.5

0.6

0.7

0.8

0.9

Heat-treatment temperature / °C

Ban

dwid

th o

f sp2 -C

/ eV

Fig. S-4 Change in the bandwidth of sp2-type carbon with heat treatment temperature.

396398400402404406

(a)

Inte

nsity

/ cp

s

50 cps

396398400402404406

(b) 50 cps

396398400402404406

(c)

Binding energy / eV

50 cps

396398400402404406

(d) 50 cps

Fig. S-5 N1s XPS spectra of each sample before and after peak separation analysis.

(a) ND1400, (b) ND1600, (c) ND1800 and (d) ND2000.

Table S-3 Surface nitrogen distribution represented by N/C of ND1400 and ND1600

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Sample

N/C (x10-3)

Total

N-I N-II N-III N-IV398.50.2

eV400.50.2

eV401.20.2

eV402.90.2

eVND1400 10.4 0.5 6.2 1.2 2.4

ND1600 5.2 0.8 0.9 1.9 1.7

2.3 Oxygen uptake assessment by TPD

Fig. S-6 TPD spectra of the prepared samples. (a) CO2 and (d) CO spectra.

Table S-4 Desorbed oxygen species.

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2.4 BET surface area

The BET surface areas obtained for each sample by measuring its nitrogen

adsorption are listed in Table S-5. This shows that although the BET surface area does

initially increase with HTT, it remains essentially unchanged beyond 1400°C of HTT.

However, this would not appear to correspond to the change in the ORR activity seen in

Fig. 2 (b).

Table S-5 BET surface areas of the prepared samples.

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