Photon Factory Activity Report 2016 #34 (2017)

BL-27A, BL-27B/2015G611

Activity Enhancement of Platinum Nanoparticle Catalysts
on Ion-Beam-Irradiated Carbon Support
Kenta Kakitani1,2, Tetsuya Kimata1, Tetsuya Yamaki2*, Shunya Yamamoto2, Iwao Shimoyama3,Daiju Matsumura3, Akihiro Iwase4, Tomitsugu Taguchi2, Tomohiro Kobayashi5, Takayuki Terai1
1
The University of Tokyo, Bunkyo-ku, Tokyo113-8656, Japan
2
National Institutes for Quantum and Radiological Science and Technology,Takasaki, Gunma 370-1292, Japan
3
Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan
4Osaka Prefecture University, Sakai, Osaka 599-8531, Japan
5RIKEN, Wako, Saitama 351-0198, Japan
Platinum (Pt) nanoparticle catalysts were prepared on the glassy carbon substrate pre-irradiated with energetic argon ions.
The X-ray photoelectron spectra in the Pt4f and C1s regions demonstrated that Pt-C bonding was formed between Pt
nanoparticles and the irradiated substrate. From the electrochemical measurements, the ion-irradiation of the carbon
support was found to enhance the catalytic activity of the Pt nanoparticles for the oxygen reduction reaction. X-rayabsorption studies at the Pt M3 edge indicated that the Pt oxidation was suppressed on the ion-irradiated carbon support;
this chemical-state change would result in the observed high catalytic activity.
1 Introduction
Polymer electrolyte fuel cell (PEFC) is one of theimportant candidates of the clean power sources forautomotive and residential applications. In PEFC, carbonsupported platinum nanoparticles (Pt NPs) are commonlyemployed as cathode catalysts for the oxygen reduction
reaction (ORR). However, insufficient ORR activity ofthe Pt NPs is the serious challenge to overcome before
wide commercialization of PEFCs, and many studies
have been carried out to achieve activity enhancement of
the Pt catalysts.
One promising strategy for improving the activity is toutilize the Pt-carbon support interaction. Recent
theoretical investigations by first-principle calculations
have suggested that the lattice vacancy in the graphitic
structure of the carbon support could enhance the ORR
activity of the supported Pt NPs [1], but no experimental
evidence has been reported. This could be because theconventional methods of support-surface modification
using chemical treatments [2] cannot introduce latticevacancy.
Ion-beam irradiation is an excellent technology forcreating lattice vacancies in a material; their density canbe well controlled. In this study, therefore, we used theion beam to induce vacancy formation in the carbon
support, expecting that a high ORR activity would berealized by the Pt-carbon interfacial electronic interaction.
2 Experiment
Unpolished 1-mm-thick glassy carbon (GC) substrates
were irradiated with 380 keV Ar + at the fluence of 1.0 ×
1016 ions/cm2 using the ion implanter at the Takasaki Ion
Accelerators for Advanced Radiation Application(TIARA) facility of Takasaki Advanced Radiation
Research Institute, National Institutes for Quantum andFig. 1: TEM image of the deposited Pt NPs on a gridmesh for electron microscopy.
Radiological Science and Technology. The irradiated GC
substrate was then heated at 400°C in an N2 atmospherefor 1 h. Pt NPs were prepared on the substrate by RFmagnetron sputtering. The sputtering time and the plasma
output were 60 s and 20 W, respectively. As thecomparative sample, the Pt NPs were also sputtered on
the non-irradiated GC substrate. These prepared samples
were called “irradiated sample” and “non-irradiatedsample” respectively later in this report. Figure 1 shows atransmission electron microscope (TEM) image of the Pt
NPs deposited directly on a TEM grid under the sameconditions. The Pt NPs observed as black circles had anaverage diameter of about 5 nm.
The X-ray photoelectron spectroscopy (XPS)
measurements were performed using PHI 5000 Versa
Probe (ULVAC-PHI, Inc., Japan) with an Al Kα (1486.6eV) X-ray as the light source. The Pt 4f and C 1s spectraPhoton Factory Activity Report 2016 #34 (2017)

were taken with a pass energy of 23.5 eV with a 0.1 eVstep and averaged for 20 and 40 scans, respectively. A 2
keV Ar+ sputtering gun was used for surface cleaning.
The duration of the sputtering was set to 5 s, whichcorresponded to a sputter depth of less than 1 nm.
The ORR activity of the irradiated and non-irradiatedsamples was analyzed by electrochemical measurements
using HZ-5000 Potentiostat (HOKUTO Denko Corp.,
Japan). All measurements were carried out in a threeelectrode cell with an Ag/AgCl reference electrode and aPt wire counter electrode. All the potentials in this report
are given with respect to the reversible hydrogenelectrode (RHE). Cyclic voltammogram (CV) wasrecorded at 25°C in an N2-saturated 0.1 M HClO4solution. The potential was swept between 0.05 and 1.25
V at a scan rate of 50 mV/s. The electrochemical surface
area (ECSA) was evaluated from the hydrogen adsorption
region of the CV [3]. After the CV measurements, therotating disk
electrode (RDE) measurements wereconducted in the same potential range in an O2-saturated
0.1 M HClO4 solution at a scan rate of 20 mV/s. The
speeds of the rotation were set to 400, 900, 1600, and2500 rpm.
The X-ray absorption data at the Pt M3 edge were
measured in the total electron yield mode by recording
the sample current. The measurements were performed
using the synchrotron radiation on the BL-27A at Photon
Factory in the high energy accelerator researchorganization. The synchrotron radiation beam wasgenerated from the bending magnet and it was linearlypolarized in the horizontal direction. In the BL-27A,
double-crystals of InSb (111) were used as amonochromator. The details of the BL-27A were reported
elsewhere [4].
3 Results and Discussion
Figure 2 shows the Pt 4f XPS spectra of (a) theirradiated sample and (b) the standard Pt plate. The Pt
4f7/2 peak was broader on the high energy side in (a) thanin (b), suggesting the presence of Pt-C bonding between
the GC substrate and the deposited Pt NPs. Figure 3compares the C 1s XPS spectra between (a) the irradiatedsample and (b) a pristine GC substrate. Spectrum (a) in
Figure 3 exhibited a slightly wider peak; interestingly, it
had a small bump on the low energy side in addition to a
tail on the high energy side. This result can be attributed
to carbide formation, making it possible to confirm theexistence of Pt-C bonding at the interface. In other words,
the usage of the ion-irradiated GC as a support of Pt
catalysts induced the interfacial Pt-C bonding [5].

Fig. 2: Pt 4f XPS spectra of (a) the Pt NPs on the Ar+irradiated GC substrate and (b) standard Pt plate.

Fig. 3: C 1s XPS spectra of the Pt NPs on the (a) preirradiated and (b) pristine GC substrates.
The mass-transport-corrected kinetic current, Ik wascalculated by applying the measured current, I and themeasured diffusion limited current, I∞ to the following
equation [6].
𝐼 ∙ 𝐼∞𝐼𝑘 =𝐼∞ − 𝐼
Then, the specific activity, ik was derived bystandardizing Ik by the ECSA. Figure 4 shows the Tafel
slopes for the irradiated and non-irradiated samples. Thespecific activity was higher for the irradiated sample thanfor the non-irradiated one at any voltage. Strikingly, onlyby the ion-irradiation of the carbon support, the specificactivity at 0.85 V was increased by about 2.5 times [7].
In order to clarify the mechanism of the observed highactivity, X-ray absorption data were acquired. Figure 5
shows the X-ray absorption near edge structure (XANES)
spectra of the irradiated and non-irradiated samples after
normalization and background subtraction. The intensityof the peak in the vicinity of the threshold in the XANES(also called white-line) was lower for the irradiatedsample. The white-line intensity generally gets lower as
the Pt 5d orbital is more filled. Thus the result indicatesthat more electrons were populated in the Pt 5d orbital inPhoton Factory Activity Report 2016 #34 (2017)

Fig. 4: Tafel plots for the ORR of the irradiated and nonirradiated samples.

Fig. 5: Pt M3 edge XANES spectra of the irradiated andnon-irradiated samples.

the irradiated sample than in the non-irradiated one. We
consider here that the Pt NPs on the irradiated substrate
could be less oxidized [8]; in other words, the Pt-carbon
support interaction formed by the pre-irradiated GC
substrate possibly suppresses the formation of Pt oxide.
Stamenkovic et al. [9] reported that this chemical-statechange of the Pt NPs increased the kinetics of the ratelimiting step of the ORR (the removal of adsorbed O andOH species from the Pt NPs). Therefore, the suppressionof the Pt oxidation resulting from the Ar +-irradiation ofthe carbon support would account for the observed high
ORR activity of the Pt NPs.
In conclusion, the Pt-C bonding formed between theion-beam-irradiated carbon support and the Pt NPssuppressed the Pt oxidation, thereby improving thecatalytic activity of the Pt NPs for the ORR. Theoreticalcomputations based on the density functional theory are
now in progress to study how the ion-beam-induced
vacancies in the carbon support resulted in thesuppression of the Pt oxidation [10].

Acknowledgement
This research study was partly performed under theNIMS-RIKEN-JAEA-QST
Cooperative
Research
Program on Quantum Beam Science and Technology.
XPS measurements were conducted at Advanced
Characterization Nanotechnology Platform of theUniversity of Tokyo, supported by “Nanotechnology
Platform” of the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan. A part of thiswork was financially supported by JSPS KAKENHI.
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[7] K. Kakitani et al., Housyasen To Sangyou 141, 29(2016) (Japanese).
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[9] V. Stamenkovic et al., Angew. Chem. 118, 2963
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* yamaki.tetsuya@qst.go.jp