Engineering Sciences Reports, Kyushu University
Vol. 42, No. 1, pp. 9-18, SEP, 2020

九州大学大学院総合理工学府報告第 42 巻 第 1 号 9-18 頁 令和 2 年 9 月

Formation of Pt-rich AuPt Alloy Frame Nanorods with
Semi-spherical End Caps Using Au Nanorods as
Sacrificial Templates
Masaharu TSUJI*1,*2† Atsuhiko YAJIMA*3 Yukinori NAKASHIMA*3
Keiko UTO*1 Masashi HATTORI*2 Jun-Ichiro HAYASHI*1,*2
Takeshi TSUJI*4 and Hirofumi KAWAZUMI*5
E-mail of corresponding author: tsuji@cm.kyushu-u.ac.jp†(Received July 15, 2020, accepted July 24, 2020)
Pt-rich AuPt alloy frame nanorods (NRs) with semi-spherical end caps were synthesized using Au
NRs as sacrificial templates. Shape, size, and composition changes of product particles were
examined by using transmission electron microscopy (TEM), TEM energy dispersed X-ray
spectroscopy (EDS), and XRD patterns. Initially, Au-rich Au92Pt8 alloy NR cores with island-type
Pt-rich Au22Pt78 alloy shell particles were prepared by reducing Pt4+ over Au NRs using ascorbic acidin the presence of cetyl trimethyl ammonium bromide (CTAB). After the addition of HCl and heating
at 95 oC, site selective oxidative etching of AuPt alloy NRs by Cl- + O2 occurred. Then, Pt-rich
Au22Pt78 alloy frame NRs with four {100} bridges and semi-spherical Au90Pt10@Au22Pt78 end caps,
were formed. The formation of these AuPt alloy nanoframe structure was explained by faster etchingrates of side {2,5,0}, {5,12,0}, and {110} facets of AuNRs and AuPt island particles on these highsurface-energy facets than those of {100} and {111} facets with low surface energies. No AuPt frame
structure was formed from decahedral island-type Au@Au3Pt97 particles, because stable {111} facetsof decahedral Au cores and Au3Pt97 alloy shells were not etched by Cl- + O2. The results obtained fromthis study suggest a novel method for the formation of Pt-rich AuPt alloy frame structures andconstitute new information related to their formation mechanism.
Key words: AuPt alloy frame, Nanorod, Decahedral particles, Core-shell particles, Sacrificial

template, Site selective oxidative etching1.

Introduction

Metal nanoparticles have been used in wide
areas ranging from catalysis1) to electronics,2)
photonics,3)
sensing,4)imaging,5)
andmedicine.6) The merit of these applications
mainly arises from the expression of properties
with strong correlations to their individual or
combined physical and optical parameters,
including composition (mono-, bi-, tri-, or*1*2Institute for Materials Chemistry and Engineering
Department of Applied Science for Electronics andMaterials*3 Department of Applied Science for Electronics andMaterials, Graduate Student*4 Department of Materials Science, Shimane
University*5 Department of Biological and Environmental
Chemistry, Kindai University

polymetallic atoms), size, geometric shape(sphere, plates, rods and wires, or polygonal),
and structure (solid, hollow, convex, concave,or porous).7–9)
Among many structures, metallic hollow
nanostructures have received great attention
because they often exhibit exceptional optical,
electronic, and catalytic properties due to theincreased surface area, reduced density, andbroadly tunable localized surface plasmon
resonance (LSPR).10-16)
The most popular technique for generationof such metallic hollow nanostructures withscalable hollow interior and porous walls isgalvanic replacement (GR) reactions.14-17) GR
is a redox process, in which one metal getsoxidized by ions of another metal having ahigher reduction potential. The oxidized metal

10

Formation of Pt Frame Nanorods Using Au Nanorods

thus acts as a sacrificial template. We haverecently studied the syntheses of Ag–Au,
Ag–Pd,
and
Ag–Pt
alloy
triangular
nanoframes in aqueous solution using GR
reactions of Ag nanoprisms with HAuCl4,
Na2PdCl4, and H2PtCl6.18,19) GR reactions havealso been applied to the preparation of AuAg
alloy nanoframes using Ag nanorods (NRs)and nanowires as sacrificial templates.20-22)
Oxidative etching of the interior of solid
nanoparticles is an alternative method for the
preparation of metallic nanoparticles withhollow structures. The most outstanding
advantage of this method is that it isapplicable to template metals whose standard
electrode potentials (E o) are higher than thatof second metal deposited during GR. For
example, GR reactions for Rh–Au and Pt–Au
systems do not occur because the standard
electrode potentials of Rh3+/Rh, PtCl62-/PtCl42-,
and PtCl42-/Pt are lower than that of
AuCl4-/Au.
Rh3+ + 3e-

E o (V vs. SHE)

→ Rh
0.76 eV
PtCl62- + 2e- → PtCl42- +2Cl- 0.68 eV
PtCl42- + 2e- → Pt +4Cl0.76 eV
AuCl4- + 3e- → Au + 4Cl-.
1.00 eV

(1)
(2)
(3)
(4)

Therefore, the formation of hollow structuresof Rh and Pt by the addition of AuCl4- ion isdifficult. We have recently studied the
preparation of Rh frame NRs using Au@Rh
NRs as a sacrificial template in an aqueous
solution.23) We used a selective dissolution of
Au NRs and Rh to form such hollow structures.
The effects of each reagent on the formation of
Rh frame NRs were studied. To assess theeffects of the template shape, we also
attempted to prepare frame structures using
decahedral Au particles as a template. Basedon such data, the growth mechanisms of Rh
frame NRs were discussed.
In this study, the formation of Pt frame NRs
is studied by using the same oxidative etching
method as that used for preparation of Rh
frame NRs.23) We found that pure Pt
nanoframes are not formed but Pt-rich AuPt
alloy frame NRs are produced. The formation
mechanism of AuPt alloy frame NRs usingoxidative etching is discussed.

2.

Experiment

2.1

Materials and experimental procedures

For use in this study, H2PtCl6·6H2O(>98.5%), L(+)-ascorbic acid (>99.5%), cetyl

trimethylammoniumbromide(CTAB)(>99.0%), NaOH (>98%), distilled H2O (forHPLC level), 10% HCl in water, anddiethylene glycol (DEG: >99.5%) werepurchased from Kishida Chemical Co. Ltd.
Polyvinylpyrrolidone (PVP: Mw = 55 k interms of monomeric units) was obtained from
Aldrich Chemical Co. Inc. All reagents wereused without further purification. Gold NRs in
an aqueous solution involving CATB, whichwere prepared using a photochemical
method,24) were supplied by Dai Nippon Toryo
Co., Ltd. (W4 type).
Pt shells were overgrown on Au NRs in an
aqueous solution by reducing H2PtCl6·6H2O byascorbic acid in the presence of CTAB at 65 oC.
In a typical synthesis, 1.33 mL of Au NRs
involving 1.5 mM Au atoms in an aqueous
solution, 1 mL of 10 mM H2PtCl6·6H2O
solution were added to 13 mL of aqueous
solution involving 264 mM of CTAB. The
Pt/Au molar ratio was 5 for the preparation of
Au@Pt NRs. Then, 4 mL of 7.5 mM ascorbic
acid was injected into the solution describedabove using a syringe pump at an injection
rate of 33 μL/min. It took 121 min to inject allof the ascorbic acid solution. The reagent
solution was kept at 65 °C for 3–12 h undercontinuous stirring. The final concentrationsof Au atoms in Au NRs, H2PtCl6, CTAB, andascorbic acid were, respectively, 0.1, 0.5, 178,
and 1.55 mM.
After the formation of Au@Pt NRs, theproduct solution was heated at 95 oC. Then 6
mL of 5% HCl was added. The reagent solution
was kept at 95 oC for 12 h to prepare Pt-richframe structures by oxidative etching. When
we examine whether Au NRs used for thisstudy are dissolved by the addition of Cl- ionsor not, Au NRs, CTAB, and ascorbic acid withthe same concentrations as those describedabove were dissolved in 20 mL of aqueous
solution at 95 °C. Then 6 mL of 5% HCl wasadded. To examine whether Au NRs are
dissolved or not, extinction spectra of Au NRs
were observed after 1–6 h.
DEG was used as both a reductant andsolvent for the decahedral Au core seed
preparation.25,26) In the process, 2 g of PVP(Mw = 55 k) was dissolved in 25 mL DEG andheated to 230 °C in the oil bath. Then 20 mg of
HAuCl44H2O in 2 mL DEG was added to theabove solution, which was heated at 230 °C for
10 min. The final respective concentrations of
HAuCl44H2O and PVP in DEG were 1.8 and
670 mM. After centrifugal separation, Au

令和 2 年度

九州大学大学院総合理工学報告

seeds were redispersed in 24.3 mL aqueous
solution.
Decahedral Au@Pt particles were preparedusing a similar method to that used for the
preparation of Au@Pt NRs. 1 mL of 2 mM Au
seeds and 1 mL of 20 mM H2PtCl6·6H2O
solution were added to 13 mL of aqueous
solution involving 264 mM of CTAB at 65 °C. 7
mL of 40 mM NaOH was added to the reagent
solution to keep the pH value at 7. Then 4 mLof 7.5 mM ascorbic acid was injected into thereagent solution using a syringe pump. Finally,
6 mL of 5% HCl was added and heated at 95 oC
for 12 h to form frame structures.
When we examine whether decahedral Au
nanoparticles are dissolved by the addition of
HCl or not, decahedral Au seeds, CTAB, andascorbic acid with the same concentrations as
those described above were dissolved in 25 mL
of aqueous solution at 95 °C. Then 6 mL of 5%
HCl was added and heated at 95 oC for 6 h.
The extinction spectra of decahedral Au
particles after HCl addition was monitored at
16 h.
2.2

第 42 巻第1号

11

full width at half maximum (FWHM) of about
200 nm and a weak transverse SPR band at
about 510 nm.27,28) With increasing etchingtime from 0 h to 4 h, the main peak shifts
slightly from 900 nm to 880 nm because ofslight shortening of Au NRs and its intensity
decreases to about half. After 6 h, the peakalmost disappeared, and a weak broad band
was observed in the 6501200 nm region.
These results indicate that Au NRs are etchedoxidatively by the addition of HCl in ourconditions.
3.1.2

Formation of AuNR@AuPt particles

Figures 2a–2e portray TEM images of Au
NRs and the change in morphology that
occurred when Pt4+ was reduced over Au NRs
for 312 h. The average length and diameter of
Au NRs are 50 nm and 5 nm, respectively, so

Characterization of nanoparticles

Shape, size, and composition of productparticles were examined by using transmission
electron microscopy (TEM) and TEM energy
dispersed X-ray spectroscopy (EDS) (200 kV,
JEM-2100F; JEOL). Samples for TEM and
TEM-EDS observations were obtained bycentrifuging the colloidal solution three times
at 15,000 rpm for 30 min. XRD patterns of thesamples were measured using Cu K radiation
operating at 50 kV and 300 mA (RINT-TTR III,
Rigaku Corp.). Extinction spectra of the
product solutions were measured in the3001200 nm region using a spectrometer(UV-3600; Shimadzu Corp.).

3.

Results and discussion

3.1 Formation of Pt frame NRs from Au@Pt
NRs
3.1.1 Oxidative etching of Au NRs by the additionof HCl

It is known that Au metals are etched in an
aqueous solution by the addition of Cl-.14-16,23)
To ascertain whether Au NRs used in thisstudy are truly dissolved by the addition of Clions or not, we monitored changes in SPR bandof Au NRs after the addition HCl to the Au NR
solution (Fig. 1a). The SPR band of Au NRs isobserved in the 300–1200 nm region, with alongitudinal SPR band at about 900 nm with

Fig. 1. Time evolution of plasmom
resonance band of (a) Au NRs and (b)decahedral Au nanoparticles in an aqueous
solution in the presence of CTAB after the
addition of HCl.

12

Formation of Pt Frame Nanorods Using Au Nanorodsthat the aspect ratio is about 5. At a short
reaction time of 3 h, small sphericalisland-type particles with an average diameterof about 3 nm are grown over two top andbottom edges of Au NRs. At 6 h, the average
size of island particles on the top and bottom
edges increases to about 5 nm, and additionalisland particles are formed on side facets of Au
NRs. At 912 h, more island particles aregrown over all top and bottom and side facetsof Au NRs.

Fig. 2. TEM images of (a) Au NRs and(b)(e) Au@Pt NRs at reaction times of 312h.

Ma

To examine what metals contribute to thesemorphology changes, TEMEDS images andline analyses along each line are made forparticles obtained after HCl addition for 6 h
and 12 h (Figs. 3 and 4). Symbols AuMa, PtMa,
AuLa, and PtLa shown in Fig. 3f according to
the notation of JEOL are usually denoted by
AuM, PtM, AuL, and PtL, respectively.
In the EDS analysis of Au–Pt bimetallic
system, it was difficult to separate Au and Pt
components using the most intense AuMa and
PtMa peaks at 1.92.3 eV (Fig. 3f). Althoughintensities of the second AuLa and PtLa peaks
at 9.39.9 eV are weaker than those of the first
AuMa and PtMa ones, they are well separated.
Therefore, we used weaker AuLa and PtLa
EDS peaks to show Au and Pt components of
Au–Pt bimetallic particles in this work.
Figures 3a-3e show that Au NRs are
dominantly covered by island-type Pt shells
over the two top and bottom edges after 6 h,and only a small amount of Pt component isdeposited on side edges of Au NRs. Figs. 4a-4e
indicate that Au NRs are fully covered byisland-type Pt particles with shell thickness ofabout 20 nm after 12 h. It should be noted thata weak Au component exists outside the Au
NRs (see two blue circles in Fig. 4e). The Au
component extends from Au NR core to theouter edge of Pt shell component. These EDS
data imply that island-type spherical particles
are not composed of solely Pt component, but asmall amount of Au component (~10%) is

La

Fig. 3. TEM and TEM-EDS images of
Au@Pt NRs obtained at 6 h and line
analysis along a broad line (A) in Fig. 3a.

Fig. 4. TEM and TEM-EDS images of
Au@AuPt NRs obtained at 12 h and line
analysis along a broad line (A) in Fig. 4a.

令和 2 年度

九州大学大学院総合理工学報告

involved in the Pt shell particles. It isreasonable to assume that during Pt
island-shell formation, etching of Au NRs by
Br- + O2 pair15,23) and reduction of Au(I)/(AuIII)
to Au0 occur simultaneously. Therefore,
Pt-rich AuPt alloy shells are formed over Au
NRs. On the base of above TEM-EDS data,
reduction of Pt4+ over Au NRs by using
ascorbic acid in the presence of CTAB does notgive AuNR@Pt particles but AuNR core Pt-rich
AuPt alloy shell (AuNR@AuPt) particles.
3.1.3 Formation of AuPt frame NRs from
AuNR@AuPt particles
To form Pt or AuPt frame NRs from
AuNR@AuPt particles, we attempted selective
etching of Au component in AuNR@AuPt
particles by the addition of HCl. Fig. 5a showsa typical TEM image of AuPt products after
12 h etching, where frame structures withsemi-spherical end caps are observed. Fig. 5b
displays expanded TEM images of AuPt frame
NRs, which are composed of two or threebridge edges, depending on the view angle. It

Fig. 5. (a) TEM image of Pt rich-AuPt alloy
frame NRs obtained after HCl addition for
12 h. (b) Expanded TEM images of AuPt
frame NRs with two or three bride edges
and their frame structures observed fromtwo different view angles.

第 42 巻第1号

13

is concluded that the AuPt frame NRs reallyconsist of four bridges as shown in rightfigures in Fig. 5b. A similar four bridge
structure was observed for Rh frame NRs
formed from AuNR@Rh.23)
Figures 6a6f depict TEM and TEMEDS
images of AuPt products after 12 h etching
and their line analysis data. The formation of
AuPt frames from AuNR@AuPt particles was
confirmed from these EDS data. Frame
structures consist of AuPt alloy frame NRs
with semi-spherical Au core Pt-rich AuPt shell
end caps. No evidence of the formation of purePt frame NRs was observed from these EDS
data. It should be noted that Pt-rich AuPt
frame brides do not consist of a series of AuPt
island particles, but they are composed ofsmooth AuPt NRs (see also Fig. 5). The
average diameter of each AuPt bridge wasabout 5 nm. This indicates that duringoxidative etching, island-type small spherical
AuPt alloy particles over side facets of AuNRs
coalesce with each other, leading to rod-typeparticles with smooth surfaces. After oxidative
etching, Pt-rich AuPt shell end caps consist oflarger island-type particles than those ofisland-type shell particles in original
AuNR@AuPt particles. This suggests thatcoalescence of small island-type Pt-rich AuPt

Fig. 6. TEM and TEM-EDS images of
Pt-rich AuPt alloy frame NRs obtained after
HCl addition for 12 h and line analysis
along broad lines (A) and (B) in Fig. 6a.

14Formation of Pt Frame Nanorods Using Au Nanorods

particles also occurs for end cap shells, so thatlarger island-type particles are formed on theend caps.
The following oxidative etching processes
participate in the formation of AuPt
nanoframes from AuNR@AuPt.14-16,23)+ ClAu + 1/2O2  Au(I)/Au(III) + O2- (5)+ BrAu + 1/2O2  Au(I)/Au(III) + O2- (6)+ ClPt + 1/2O2  Pt(II)/Pt(IV) + O2-(7)+ BrPt + 1/2O2  Pt(II)/Pt(IV) + O2-(8)

AuPt alloy particles on specific Au NR surfaces
remained after oxidative etching, leading tofour bridge edges with semi-spherical end caps.
Not only dissolution of Au NR cores andPt-rich AuPt alloy island-type shells by above
oxidative etching, but also coalescence of small
island-type Pt-rich AuPt particles and atomic
rearrangement within them occur, resulting insmooth AuPt alloy bridge surfaces.

2 6075o range as a typical example. The
XRD patterns of Au NRs consist of single
component, indicating that the presence of thecrystalline gold with fcc structure (2 equals
38.19°,
44.39°,
64.59°,
and77.59°)
corresponding to {111}, {200}, {220}, and {311}planes, respectively. As demonstrated in Fig.
8a, XRD pattern of AuNR@AuPt consists of
two components. They are a major peak and aweak shoulder peak at smaller angles. The
major peaks were observed at 39.26°, 45.78°,
66.76°, and 80.64° in Fig. 7. These peaks shift
to smaller angles by 0.48o, 0.43o, 0.66o, and
0.57o from standard peaks of crystalline
platinum with fcc (39.74°, 46.21°, 67.42°, and81.21° for {111}, {200}, {220}, and {311} planes,
respectively). Thus, the major peaks can be
attributed to Pt-rich AuPt alloy peaks, whichis consistent with the TEM-EDS observation.
Weak shoulder peaks are observed at slightly
higher angles than those of pure gold peaks
due to intermixing of small amount of Pt
component with the major Au component.
Assuming complete mixing between Au and Pt
atoms in AuPt alloys, atomic ratios of Au and3.1.4 XRD analyses of AuNR, AuNR@AuPt
and AuPt nanoframes
To obtain information on crystal structuresof Au NRs, AuNR@AuPt, and AuPt alloy
nanoframes in more detail, powder XRD
patterns were measured. Fig. 7 shows their
XRD patterns in the 2= 3090o range, andFig. 8a displays expanded XRD patterns in theFig. 7. XRD patterns of Au NRs,
AuNR@AuPt, and Pt-rich AuPt nanoframes
before analysis using Vegard’s raw. Dottedlines show standard positions of fcc Pt
metals.

Fig. 8. (a) Expanded XRD patterns of Au
NRs, AuPtNR@AuPt, and AuPt nanoframes
after analysis using Vegard rule. Dottedlines show standard positions of fcc Au andPt metals. (b) deconvolution of XRD peak of
AuPtNR@AuPt using two Gaussians curves.

令和 2 年度

九州大学大学院総合理工学報告

Pt in AuPt alloys were estimated from {220}peaks using Vegard’s law (Fig. 8a).29) By
deconvoluting XRD peaks (Fig. 8b), peak
positions of Au-rich AuPt alloy and Pt-rich
AuPt alloy were estimated to be, respectively,
66.9o and 64.7o. From these values, thecompositions of Au-rich AuPt alloy and Pt-rich
AuPt alloy were evaluated to be Au90Pt10 and
Au22Pt78 alloys, respectively. On the basis ofabove facts, it is concluded that Au NR coresconsist of Au-rich Au90Pt10 alloys, whereas Pt
island-shells are composed of Pt-rich Au22Pt78
alloys.
After oxidative etching, Au-rich AuPt alloy
peak at about 64.7 eV becomes weak. However,deconvolution of XRD peaks of {220} facets of
AuPt alloy frames gave the same peak
positions for both weak Au-rich AuPt alloy
shoulder peak and strong main Pt-rich AuPt
alloy peak (not shown in Fig. 8b). These
results suggest that the composition of major
Pt-rich Au22Pt78 alloy component, whichconstitutes side bridge frame structure, areunchanged before and after oxidative etching.
At the same time the composition of minor
Au-rich Au90Pt10 alloy composition, whichconstitutes residual cores of semi-sphericalend caps, is also unchanged before and after
oxidative etching.
We have prepared AuPt alloy frame NRs
using Au NRs as a sacrificial template. Fig. 9ashows the most probable crystal structure of
Au NRs, which consist of eight equivalenthigh-index {2 5 0} or {5 12 0} facets andadditional eight facets of alternative {110} and{100} facets located between two {2 5 0} or {5 120} facets.23,30-33) The NR ends are terminatedby the {100} facet, and side facets and end
facets are combined with eight facets ofalternative {110} and {111} facets. The mostoutstanding feature of Au NRs is that they
have not only low-index facets but also
high-index facets.
The surface facets play an important role indetermining the shape of a nanocrystal. The
formation of surface facets is essentiallydriven by total interfacial free energy. Owing
to diﬀerent density of atomic steps, ledges, andkinks, fcc facets have diﬀerent surface
energies. In general, the surface energy of thethree typical low-index facets of an fcc metal
follows the trend of {111} < {100} <{110}.14,15) The surface energies of high-index{2 5 0} and {5 12 0} facets are expected to bemuch higher than those of above three
low-index facets.

第 42 巻第1号

15

Fig. 9. Crystal structure of Au NR andformation mechanism of Pt-rich Au22Pt78
frame NR from Au90Pt10NR@Au22Pt78 byoxidative etching.
Figures 9b9d show the formation
mechanism of Pt-rich Au22Pt78 alloy frame NR
with Au-rich Au90Pt10@Au22Pt78 alloy end caps.
A relatively large lattice mismatch (3.8%)
exists between fcc Au (lattice constant =0.4078 nm) and Pt (0.3924 nm). In addition,
Au NRs have unstable high-index facets suchas {2 5 0} or {5 12 0}. Therefore, layered growthof Pt shell over Au NR is difficult, so thatisland-type small spherical Pt particles
without distinct facets are grown over Au NRs(Fig. 9b). During island growth of Pt particles,
oxidative etching of Au NRs by Br- + O2 andreduction of Au+ and/or Au3+ and Pt4+ ions to
Au0 and Pt0 by ascorbic acid occur
simultaneously.
Therefore,
intermixing
between Au and Pt atoms takes place, leading
to Au-rich Au90Pt10NR alloy core and Pt-rich
Au22Pt78
alloy
shell
particles(Au90Pt10NR@Au22Pt78).
By the addition of HCl, Cl- ions and O2
dissolved in the reagent solution penetrate
into the interspace, and both Au90Pt10 NR
cores and Au22@Pt78 island shells are etched16

Formation of Pt Frame Nanorods Using Au Nanorods

through reactions (5) and (7). Then, a few
pores are formed (Fig. 9c), and finally fourbridge frame structure with semi-sphericalend caps are produced (Fig. 9d). Duringoxidative etching, surfaces of bridges becomesmooth and small island-type Pt-rich Au22Pt78
particles in the end caps become large bymutual coalescence.
Since the surface energy of metals depends
on not only facets but also metals, oxidative
etching is expected to exhibit specificselectivity toward diﬀerent facets and metals.
It is expected that etching rates of high-energy{2 5 0}, {5 12 0}, and {110} facets of Au arefaster than those of low-energy {100} and {111}facets of Au. Therefore, facet selective
oxidative etching occurs, leaving low-energy{100} and {111} facets. On the other hand, thefinal formation of Pt-rich Au22Pt78 frame
structures by the faster oxidative etching of
Au-rich Au90Pt10 alloy cores suggests thatoxidative etching rate of Au component isfaster than that of Pt component under thepresent experimental conditions, even though
Au-rich cores are covered by Pt-rich shells.
This means that passivation of Pt-rich
Au22Pt78 alloy shells is not so important for theoxidative etching of Au-rich Au90Pt10 alloy
cores, because Cl- and O2 etchant pairs caneasily penetrate into the interfaces of Pt-rich
AuPt alloy shells.
Although etching rate of Pt component isslower than that of Au one, most of Pt-rich
Au22@Pt78 island-shell particles are also etchedby Cl- + O2. However, the etching rate depends
strongly on the underlying facets of Au-rich
Au90Pt10 NR cores. The etching rates of Pt-rich
Au22Pt78 island-shell particles over side {100}facets of Au90Pt10 NRs are slower than thoseover {2 5 0}, {5 12 0}, and {110} facets.
Therefore, four Au22Pt78 frame bridges are left
(Fig. 9d). During oxidative etching, mutual
coalescence and atomic rearrangement of
Pt-rich Au22Pt78 island-shell particles occurs to
reduce total surface energies of products. As aresult, boundaries of island particles disappear
and rather smooth frame bridges are formed.
Stable {100} and {111} facets are major
facets of the top and bottom edges of
Au90Pt10@Au22Pt78 NRs. Therefore, etchingrates of Pt-rich Au22Pt78 island particles overthese stable facets are slow, so that theyremain as semi-spherical Au90Pt10@Au22Pt78
particles (Fig. 9d). During oxidative etching,coalescence of small island-type Pt-rich AuPt
particles occurs, so that larger island particles

are grown.
3.2 Attempt for the formation of Pt frames
from decahedral Au@Pt nanoparticles
3.2.1
Decahedral Au@AuPt nanoparticles
produced from Au decahedra as seeds

To examine the formation mechanism of
Pt-rich AuPt alloy nanoframes, we examinedthe effects of morphologies of Au cores for theformation of Pt-rich AuPt alloy nanoframes
using decahedral Au seeds. Pt4+ ions werereduced in the presence of decahedral Au cores
under the same experimental conditions as
those used for Pt-rich Au22Pt78 frames from
Au90Pt10@Au22Pt78 NRs. Figs. 10a and 10b
show typical TEM images of Au decahedral
particles and Au@Pt particles before HCladdition, respectively. Although Pt shells do
not form layered structures, Au decahedral
particles are covered by small sphericalisland-shell particles, as in the case of
Au90Pt10@Au22Pt78 NRs.

Fig. 10. TEM images of decahedral (a) Auand (b) Au@Pt particles.
Figures 11a-11e show TEM, TEM-EDS, andline analysis data of decahedral AuNR@Pt
particles. Results show that a small amount of
Au component (~3%) is involved in Pt
island-type spherical shell particles. Therefore,
small island-type spherical shell particles are
not composed of pure Pt particles but they
consist of Pt-rich Au3Pt97 alloy particles. A low
Au content in AuPt alloy arises from slower
etching rate of decahedral Au particles thanthat of Au NRs with unstable facets.
3.2.2 Attempt for the formation of Pt nanoframes
from decahedral Au@AuPt nanoparticles

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第 42 巻九州大学大学院総合理工学報告

Fig. 11. TEM and TEM-EDS images of
Au@AuPt and line analysis along a broadline (A) in Fig. 11a.
Upon examining oxidative etching of Au
decahedral particles by the addition of HCl, it
was found that they are dissolved as in thecase of Au NRs (Fig. 1b). This indicates that
decahedral Au nanoparticles can be etched asin the case of Au NRs by the addition of HCl.
We have examined whether frame
structures are formed after addition of thesame amount of HCl as that used for the
preparation Au22Pt78 frame NRs from
Au90Pt10@Au22Pt78 NRs. It is noteworthy thatno frame structure can be formed as presented
in Fig. 12a. Figs. 12b-12e show TEM-EDS
images and its line analysis data. Results
show that little shape and size changes occur
for decahedral Au@Au3Pt97 particles with anaverage diameter of ~80 nm after HCl addition.
The most probable reason for non-occurrenceofoxidative
etchingfordecahedral
Au@Au3Pt97 particles is that Au3Pt97 shells
over low index Au {111} facets are too stable tobe etched. Therefore, no frame structure isformedfrom
decahedral
Au@Au3Pt97
nanoparticles. This is consistent with theremaining semi-spherical Au90Pt10@Au22Pt78
end caps over top and bottom edges with {111}facets. Based on the effects of facts presented
above, surface energies of metallic facets play
important roles in frame structure formation
via facet selective oxidative etching.

第1号

17

Fig. 12. TEM and TEM-EDS images of
Au@AuPt obtained after HCl addition for
12 h and line analysis along a broad line(A) in Fig. 12a.

4.

Summary and Conclusion

Formation of Pt-rich AuPt alloy frame NRs
with semi-spherical end caps was examinedusing Au NRs as sacrificial templates. AuPt
alloy frame NRs were prepared through a
two-step process in which Au90Pt10@Au78Pt22
alloy particles were initially prepared using
ascorbic acid in the presence of CTAB. Thenthey were etched by the addition of HCl. As aresult, Au22Pt78 NR frame structures withsemi-spherical Au90Pt10@Au22Pt78 end caps
were formed. The formation of Au22Pt78 NR
frames over Au {100} facets was explained bytheir lower surface energies than those ofother facets with fast etching rates.
To obtain information related to effects ofshape of Au core for the formation of Pt-rich
AuPt frame NRs, decahedral Au@Au3Pt97
particles were prepared. No frame structure
was formed after the HCl addition. It was
explained by the slow etching rate of low-index{111} facets with the lowest surface energy.
Results obtained in this study suggest a new
synthesis method of unique Pt-rich AuPt alloy
frame structures at a low reaction
temperature of 95 °C in an aqueous solution
using Au NRs as sacrificial templates.
Acknowledgments
We thank Dai Nippon Toryo Co., Ltd. for18

Formation of Pt Frame Nanorods Using Au Nanorods

supplying Au NRs used in this study. This
work was supported by JSPS KAKENHI
Grant Numbers JP25286003, JP25550056.
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