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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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References
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Onion-like carbon from ultra-disperse diamond. Chem Phys Lett
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