controlled deposition of iridium oxide nanoparticles on
TRANSCRIPT
Article Electrochemistry, 88(5), 392–396 (2020)
The 64th special issue "Frontiers of Carbon Materials"
Controlled Deposition of Iridium Oxide Nanoparticles on Graphene
Shuhei OGAWA,a Masanori HARA,a,* Seiya SUZUKI,a,b Prerna JOSHI,a and Masamichi YOSHIMURAa
a Toyota Technological Institute, 2-12-1 Hisakata, Tempaku, Nagoya 468-8511, Japanb International Center for Young Scientists (ICYS), National Institute for Materials Science (NIMS),1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
* Corresponding author: [email protected]
ABSTRACTFor hydrogen production by water electrolyzers, iridium dioxide (IrO2) works as a catalyst for oxygen evolution reaction (OER) at an anode.In this report, we aim to study the formation mechanism of IrO2 nanoparticles on graphene by inducing nanoscale defects artificially. Thedefects on graphene grown on a copper foil by chemical vapor deposition were created by UV-ozone treatment, and IrO2 nanoparticleswere deposited by hydrothermal synthesis method. We investigated the amount of defects and oxygen-functional groups on graphene byRaman spectroscopy and X-ray photoelectron spectroscopy (XPS). The size and distribution of defects and IrO2 nanoparticles on graphenewere analyzed by atomic force microscopy (AFM). Raman spectroscopy and XPS measurement showed that defects and oxygen-functionalgroups increased with the UV-ozone treatment time. The size of IrO2 nanoparticles was reduced to ca. 4.5 nm on defective graphene,whereas the nanoparticles deposited on pristine graphene is ca. 8.8 nm in diameter. It is found that the IrO2 nanoparticles were depositedand anchored on the edge of hole-like defects on graphene. In addition, the size of deposited nanoparticles can be controlled by the extentof modification in graphene.
© The Author(s) 2020. Published by ECSJ. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY,
http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium provided the original work is properly cited. [DOI:10.5796/electrochemistry.20-64075]. Uploading "PDF file created by publishers" to institutional repositories or public websites is not permitted by the copyrightlicense agreement.
Keywords : Water Electrolysis, Iridium Oxide Nanoparticles, Defect Induced Graphene, Atomic Force Microscope
1. Introduction
Recently, renewable energy sources has been introduced intosociety to handle environmental issues, especially global warming.However, in regard to renewable energy sources such as solar cellsand wind-powered electricity, the regulation of energy supply is notenough to meet commercial high demand. To overcome the issue,improving technology of energy conversion is necessary to storeand transport the energy effectively.1,2 A possible candidate ofthe energy storage techniques is hydrogen production by waterelectrolysis, including hydrogen evolution reaction (HER) andoxygen evolution reaction (OER).3,4 Water electrolysis is one of theprocesses for generating hydrogen molecules, where water splittingtakes place by application of a potential, and oxygen is generated atanode while hydrogen is produced at cathode. IrO2 and Pt arecommonly used as anode and cathode catalysts, respectively.5,6 Toimprove performance and cost of a commercially available waterelectrolyzer, high over-potential of OER and high cost of catalysts,in particular, for IrO2 anode, are critical issues in a cell with acidicelectrolyte or proton exchange membrane.7–10 A great amount ofefforts have been invested to develop catalysts for further improve-ment of water electrolysis efficiency.5,7–10
Recently, nanocarbon materials such as reduced graphene oxide(RGO) and carbon nanotubes (CNTs) have been focused assupporting materials to increase catalytic activity of IrO2 by theformation of nanoparticles.11–13 Nanocarbon materials are commonsupport materials for electrocatalysts, especially in fuel cells,14,15
because of their significant properties, such as high electricconductivity, high surface area, high chemical stability, andfacileness of structural modification on a nanoscale.16 The mostpopular preparation method to obtain RGO is thermal, hydro-
thermal, or chemical reduction of graphene oxide (GO), which isprepared from graphite by chemical exfoliation, for exampleHummers’ method.17–20 RGO possesses a lot of oxygen-functionalgroups, such as C-O, C=O, COOH, and C-OH.20 The oxygen-functional groups are expected to work as nucleation and anchoringsites for nanoparticles,21 resulting in homogeneous dispersion ofdeposited catalyst nanoparticles. As is well known for fuel cellcatalysts, high activity can be achieved by adjusting the catalystparticle size and loading amount of the homogeneously dispersedcatalysts.22–24 Inhomogeneous dispersion of catalysts acceleratesagglomeration of catalyst particles and suppresses efficient diffusionof reactant. A previous report on density functional theory (DFT)study claimed that the size of metal nanoparticles, related to itsspecific surface area, depends on the type or density of defects incarbon-based supporting materials.21 The behavior of metal (Pt andPd) nanoparticle deposition was also reported on an atomic scale onhighly oriented pyrolytic graphite or glassy carbon supports as amodel substrate.25,26 However, effect of nanocarbon or practicalcarbonaceous supporting materials on the formation of nanoparticlesis still unclear, because carbon supports contain a large amount ofdefects, and to identify the type of defects on a nanoscale is difficultfrom the point of view of controlling carbonaceous materialcharacteristics and limited observation methods with atomicresolution. In addition, effect of functional groups of carbons onthe deposition process of metal oxide such as IrO2 is not fullyunderstood.
In this study, in order to clarify the behavior of nano-sized IrO2
particle deposition on nanocarbon materials, we employed a largesize single crystal graphene prepared by chemical vapor deposition(CVD) as a model carbon substrate. The surface of the CVDgraphene was modified with defects or vacancies introduced by UV-
Electrochemistry Received: June 1, 2020
Accepted: June 20, 2020
Published online: July 31, 2020
The Electrochemical Society of Japan https://doi.org/10.5796/electrochemistry.20-64075
392
ozone treatment. We carefully investigated the relationship betweenthe morphology of monolayer graphene and IrO2 nanoparticlesdeposition by atomic force microscope (AFM) on a nano level.
2. Experimental
2.1 Preparation of grapheneThe preparation procedure of IrO2-deposited graphene is summa-
rized in Fig. 1. Graphene was synthesized by CVD on commerciallyavailable copper foils (99.96% in purity, 20 © 100mm2 in size,0.1mm in thickness, Nilaco Co.), which act as catalytic sub-strates.27,28 As-received Cu foils were introduced in a home-madeCVD chamber, and graphene was grown by using diluted CH4 in Ar(10 ppm) as the carbon source. The CVD growth of grapheneproceeded in 4 steps, heating, annealing, growth, and cooling(Fig. S1 of the Supporting Information, SI). First, the CVD chamberwas heated to 1035 °C in Ar, and the Cu foils were annealed underthe gas mixture of Ar (500 sccm) and H2 (100 sccm) for 45min.Then, graphene was grown at 1035 °C under mixture of diluted CH4
(1000 sccm) and H2 (10 sccm) for 6 h. Finally, the CVD chamberwas rapidly cooled down to room temperature under the mixture ofAr and H2 atmosphere. The graphene growth on the Cu foils wasconfirmed by an optical microscope (VHX-500, Keyence Co.).
The synthesized graphene on the Cu foils was transferred to SiO2
(300 nm)/Si substrate by a bubble transfer method.29,30 Briefly, apoly (methyl metacrylate) (PMMA) film was coated on thegraphene-grown Cu foil surface by spin coating,27–31 and thePMMA/graphene was exfoliated from the Cu foil by electrochemi-cally formed H2 bubbles between the graphene and the Cu surface.29
Then, the PMMA/graphene sheet was transferred onto SiO2/Sisubstrate. The PMMA film was dissolved in acetone, and thepristine graphene on SiO2/Si substrate (Fig. S2 of the SI) wasobtained.
Defects were introduced on the CVD graphene by oxidativeetching with ozone molecules induced by UV light in UV-ozonegenerator (TC-003, Meiwafosis Co., Ltd.) with a Hg lamp (185 nmand 254 nm). The ozone oxidation treatment was performed for 0–5min under atmospheric conditions.
2.2 Deposition of IrO2 particlesWe prepared Ir(OH)4 solution as iridium precursor solution to
deposit IrO2 nanoparticles by static retaining of 0.02M H2IrCl6nH2O(Wako Pure Chemical Industries, Ltd.) solution adjusted to pH =10 with 10% NaOH solution in a refrigerator overnight. The Ir
precursor solution was dropped on the graphene transferred on theSiO2/Si substrate, and then the sample was dried on a hot plate at80 °C. The Ir precursor-deposited graphene on the SiO2/Si substratewas moved into a Teflon lined autoclave with a mixture of ethanoland water (9:1) and heated at 150 °C for 4 h (hydrothermal method).During these processes, IrO2 nanoparticles were deposited ongraphene.12,13 The obtained sample was washed and dried overnightat room temperature. The deposited amount of Ir was controlled byadjusting the drop amount of precursor solution.
2.3 Characterization of samplesCrystallinity of graphene was confirmed by Raman spectroscopy
(lnVia Raman Microscope, Renishaw plc.) equipped with a 532 nmexcitation laser. A 50© magnification objective, a Peltier-cooledCCD camera, and an 1800-line mm¹1 grating were utilized forRaman spectroscopy. Graphene surface morphology and the size ofIrO2 nanoparticles were observed by atomic force microscopy(AFM, Multi-Mode 8, Bruker Co.) in a peak force tapping mode.The size of the nanoparticles is measured by AFM software.Elemental and chemical properties of the prepared samples wereanalyzed by scanning electron microscopy (SEM, SU3500, HitachiHigh Technologies Co.) with energy dispersive X-ray spectroscopy(EDX, EMAX Evolution X-Max, Horiba) and X-ray photoelectronspectroscopy (XPS, PHI5000 Versa Probe II, ULVAC-PHI) withAl KA radiation (1486.6 eV).
3. Results and Discussion
Figure 2(a) shows Raman spectra of the prepared graphene onthe SiO2/Si substrate (Fig. S3 of the SI) with various periods of UV-ozone treatment. After the UV-ozone treatment, three peaks of D, G,and GB were observed on the Raman spectra of graphene, whereasonly G and GB peaks were obtained on the pristine graphene. G peakderived from E2g vibration of sp2 C=C bond appears at 1585 cm¹1,and D-peak due to A1g breathing of sp2 C=C activated by defects isobserved at 1343 cm¹1. GB peak at 2685 cm¹1 is characteristic ofsecond-order Raman process involving two iTO phonon scatter-ing.27,32–34 The Raman spectra show increasing D peak intensitywith time of the UV-ozone treatment, indicating that defects wereintroduced in graphene by oxidation with ozone. Figure 2(b) showsthe plot of the relationship between UV-ozone treatment time (ozoneexposure time) and the intensity ratio of G and D bands (IG/ID ratio),which was calculated from the Raman spectra shown in Fig. 2(a).IG/ID ratio is used to represent the density of defects on graphene toknow the crystallinity of graphene. The value of the IG/ID ratiodecreased with the extending UV-ozone treatment period. The resultindicates that crystallinity of graphene was degraded by UV-ozonetreatment. The degradation could occur by reaction of carbon atomswith ozone molecules. Theoretically, ozone molecules produced inUV-ozone generator react with graphene to generate vacancies andoxygen-containing functional groups, which break weaker sp2 bondin graphene, whereas Ar ion bombardment forms vacancies withoutformation of additional oxygen-function groups around defects.35
Our results confirm that the amount of defects in the CVD graphenecan be controlled by the time of UV-ozone treatment.
Before deposition of IrO2 nanoparticles, we evaluated themorphology of the defects formed on graphene by AFM on a nanoscale. Figure 3 shows AFM images of graphene surface before andafter UV-ozone treatment with various exposure time, 1 to 5min. InFig. 3(a), a curved step line of 0.97 nm in height, corresponding tomonoatomic height of flat-laid graphene layer on SiO2 surface,36
was observed in the upper part of the AFM image. We conclude thatthe step line is the edge of the CVD graphene. The bright terracearea is CVD graphene, and the dark area in the upper part of theimage is SiO2 surface. The bright line-shaped structure on grapheneare wrinkles of graphene caused during the transfer process. The
IrO2
SiO2 /SiCu
graphene
(a) (b)
(c) (d)
Figure 1. Schematic images of processes for preparing IrO2
nanoparticles deposited on defective CVD graphene on SiO2/Sisubstrate. (a) Single layer graphene prepared on Cu foil by CVD.(b) CVD graphene transferred on SiO2/Si substrate. (c) Defects-introduced graphene by UV-ozone treatment. (d) IrO2 nanoparticlesdeposition on graphene prepared by hydrothermal reaction.
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small spots observed on an AFM image, which are 4 to 8 nm inheight, are impurities adsorbed on the surface during the transfer ofthe graphene from Cu foil to SiO2/Si substrate, such as the residuesof PMMA or small carbon particles. Figures 3(b)–(e) are enlargedimages of the white dot frame in Fig. 3(a) and were obtained atalmost the same area. After UV-ozone treatment, small holes wereformed on graphene as dark spots. During UV-ozone treatment,ozone molecules oxidize carbon atoms in the graphene lattice andremove them as a CO or CO2 gas molecules, creating holes ingraphene. An average depth of the holes estimated from cross-section analysis (Fig. S4 of the SI) of the AFM images is ca. 1.0 nm,similar to the height of graphene edge, and the value of holes depthwas unchanged even after UV-ozone treatment time, implying thatthe present CVD graphene is monolayer and the SiO2 substrate isexposed in the hole regions. Figure 4 shows the size and density ofthe holes as a function of UV-ozone treatment time. The hole sizebecame larger up to 800 nm2 with increasing UV-ozone treatmenttime, whereas the density decreases after 1min of the UV-ozonetreatment because of merging neighboring holes. In comparison withRaman spectroscopy shown in Fig. 2, the defects were clearlyobserved on the graphene, whereas the intensity of D band peak issmall for the graphene with 1min UV-ozone treatment. Although thecondition of UV-ozone treatment is same, it is difficult to correlate
them directly, because the sample for AFM measurement are notidentical to those for Raman spectroscopy. However, overalltendency is correct and reproducible. Although the D peak intensityis small after 1min treatment, D band intensity increases ascompared with the pristine graphene (Fig. 3(b)). In Fig. 3(c), it isclear that the number of bright particles (PMMA residues) are
(a) (b)
Figure 2. (a) Raman spectra of graphene exposed to ozone with various time. (b) IG/ID ratio as a function of exposure time estimated fromRaman spectra in Fig. 2(a).
SiO2
graphene
1.0 μm
1.0 μm 1.0 μm
Figure 3. (a) A large-scale AFM image of pristine graphene and magnified AFM images of the synthesized CVD graphene (dotted box)exposed to ozone with various time, (b) 0, (c) 1, (d) 3, and (e) 5min, obtained at the same area.
Are
a of
hol
e [n
m2 ]
Exposure time [min]
Den
sity
of h
ole
[μm
–2]
Figure 4. Plots of average area size of holes (blue circle) anddensity of defects on graphene formed by ozone treatment (redtriangle) as a function of exposure time.
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reduced and dark area in graphene is appeared. The latter may givethe evolution of D peak in Raman spectroscopy (Fig. 2). Thecontrast of dark area in Fig. 3 increases with ozone treatment time,and D peak grows accordingly in Fig. 2(b). The contrast change isaccompanied by the removal of PMMA residues adsorbed on thegraphene. The results confirm that ozone treatment has a role to etchthe graphene lattice and form defective graphene.
Figure 5 shows AFM images of graphene surface after IrO2
deposition by the hydrothermal synthesis method. No hole or defectstructure was observed on the pristine graphene, which is placedinside the dashed frame in Fig. 5(a). The result means that graphenewas not damaged physically during the hydrothermal treatmentprocess for IrO2 deposition. The white dots in the AFM imageare IrO2 nanoparticles formed on graphene, and the size of thenanoparticles is ³8.8 nm. Furthermore, we cannot observe theresidue of impurities on the graphene surface, because the impuritieswere removed during hydrothermal reaction of IrO2 deposition. Thecomposition and formation of IrO2 nanoparticles was also confirmedby SEM-EDX (Fig. S5 of the SI), where Ir elements were dispersedsparsely on the graphene surface, while IrO2 nanoparticles cannot beobserved clearly because of low resolution of SEM. Only a few IrO2
nanoparticles were observed on the pristine graphene, and thenumber of the IrO2 nanoparticles on pristine graphene is lesser thanthat on the UV-ozone treated graphene. The low density of the IrO2
nanoparticles is consistent with low defects on the pristine graphene.On the other hand, a large number of holes and nanoparticleswere distributed on the UV-treated graphene surface, as shown inFigs. 5(b)–(d), in which a whole area shows graphene surface. Theaverage size of deposited IrO2 nanoparticles was ³4.5 nm on alldefective graphene surfaces. We also confirm that the nanoparticleson the graphene are IrO2 because the size of the particles on theAFM images is smaller than that observed on defective graphene, 4to 8 nm in height, before IrO2 deposition. Furthermore, it is notedthat IrO2 nanoparticles were located along holes on the defectivegraphene, indicating that the edge of holes can anchor the particles37
as well as a nucleation site for the IrO2 nanoparticles. It isnoteworthy that we attempted to deposit IrO2 on SiO2/Si substratewithout graphene by the same procedure as described above, as acomparison. However, we did not obtain IrO2 nanoparticles on SiO2
surface because iridium precursor or iridium oxide diffused into ormigrated onto SiO2 surface, resulting in formation of agglomerates.Then, the agglomerated IrO2 particles were removed during washingthe SiO2/Si substrate with ultrapure water to remove residues.
Table 1 summarizes the size of defects, the density of defects,and deposited nanoparticles on graphene with various periods ofUV-ozone treatment. The values in Table 1 reveal that the density ofthe deposited IrO2 nanoparticles increases with the density ofdefects. The AFM observation suggests that the edge of the holesserves a critical role to form and distribute the IrO2 nanoparticles. Acomparison among the three graphene with the UV-ozone treatment
specifies that the distribution of IrO2 nanoparticles became worsewith increasing the UV-ozone treatment time, because the size of theholes increased and the number of holes decreased with the UV-ozone treatment period. In other words, the distribution of holesdecreased with the UV-ozone treatment period. Comparing IrO2
nanoparticles deposited on the graphene with and without the UV-ozone treatment, that size of the IrO2 nanoparticles became half afterthe UV-ozone treatment, and as a tendency, the size decreased withincreasing the time of UV-ozone treatment. However, after 1min ofUV-ozone treatment, the size of the IrO2 nanoparticle did not changesignificantly.
To discuss an essential factor deciding the size of the IrO2
nanoparticles, we calculated the total circumference of holes ongraphene from size and density of holes. Figure 6(a) shows therelationship between the size of the IrO2 nanoparticle and thecircumference of holes. Figure 6(b) also shows the relationshipbetween the size of the IrO2 nanoparticles and the concentration ofoxygen-functional groups such as ketone (C=O) and carboxyl(COOH) groups on graphene, estimated by XPS analysis33,38 of thegraphene surface (Fig. S6 of the SI) before the IrO2 particlesdeposition. Figure 6(a) indicates that the total edge length of holeson graphene has almost same value as on the defective graphene.The result implies that the ability to anchor nanoparticles alongholes is the same on all the defective graphene, resulting in similaraverage size of the IrO2 nanoparticles. On the other hand, theconcentration of oxygen-functional groups on graphene wasincreased with the increasing time of UV-ozone treatment, as shownin Fig. 6(b). In spite of the fact that oxygen-functional groups arecounted on anchoring and sticking the nanoparticles on carbonsupports, the additional effect on the IrO2 nanoparticles could not bedetermined in this research. We assume that the oxygen-functionalgroups on graphene were reduced under hydrothermal reactioncondition, and the amount of functional groups became similarunder IrO2 deposition process despite different UV-ozone treatmentperiod. In future, in situ analysis of reaction materials duringhydrothermal reaction is required to understand the effect of oxygen-functional groups on metal nanoparticle deposition.
(a) (b) (c) (d)
Figure 5. AFM images of IrO2 nanoparticles deposited on (a) pristine graphene and the defect-induced graphene with various ozoneexposure time, (b) 1, (c) 3, and (d) 5min.
Table 1. The area size and density of defects on CVD grapheneintroduced by various period of UV-ozone treatment, and the densityof deposited IrO2 nanoparticles on the defective graphene.
UV-ozonetreatment (min)
Average defectsize (nm2)
Density ofdefects (µm¹2)
Density ofparticles (µm¹2)
1 1.8 © 102 1.1 © 104 5.9 © 102
3 7.9 © 102 5.3 © 103 2.1 © 102
5 8.0 © 102 4.7 © 103 2.3 © 102
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4. Conclusion
In this study, we demonstrate CVD graphene as a model supportmaterial for IrO2 nanoparticles, and succeed to investigate the effectof defects in graphene sheets on formation of the IrO2 nanoparticles.The irradiation of UV light and photochemically-induced ozonemolecules oxidized the graphene surface and formed holy structureson graphene. The diligent AFM analysis reveals the morphologychange of graphene and deposited IrO2 particles on a nanoscale. It isfound that as the amount of defects increased, the size of the IrO2
nanoparticles decreased. In addition, the number of IrO2 nano-particles was proportional to the density of vacancy holes. Theresults suggest that the vacancies created by UV-ozone treatmentwork as nucleation sites for IrO2 nanoparticles. Here, we proposecontrolling the defects size and concentration, properly on nano-carbon supports such as graphene, by UV-ozone treatment toimprove the catalytic activity of metal nanoparticles deposited onthe nanocarbons. Though some phenomena such as the role ofoxygen-functional groups still remain obscure, appropriate mod-ification of support carbons on a nano level will encouragedevelopment of supported catalysts with exceptional activity.
Supporting Information
The supporting Information is available on the website at DOI:https://doi.org/10.5796/electrochemistry.20-64075.
Acknowledgments
This work was supported by Strategic Research Foundation
Grant-aided Project for Private Universities and JSPS KAKENHIGrant Number 17K05969 from the Ministry of Education, Culture,Sports, Science and Technology, Japan (MEXT). This work was alsosupported by JST-CREST Grant Number JPMJCR1875, Japan.
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(a)
(b)
Parti
cle
size
ofI
rO2
[nm
]Pa
rticl
e si
ze o
fIrO
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m]
Figure 6. Average particle size of IrO2 nanoparticles and (a)estimated total edge length of defect holes calculated from AFMimages of defective graphene, and (b) ratio of oxygen-functionalgroups formed on graphene obtained by XPS analysis.
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