Smart decoration of mesoporous TiO2 nanospheres with noble metal alloy nanoparticles into core-shell, yolk-core-shell, and surface-dispersion morphologies

Article abstract of DOI:10.1002/ejic.201402646

Noble metal alloy-decorated spherical mesoporous TiO₂ nanoparticles were synthesized by the one-pot, single-step reaction of Ti(OiPr)₄, Au³⁺, Pt⁴⁺, and Pd²⁺ or by the one-pot reaction of hollow mesopo

Full text of DOI:10.1002/ejic.201402646

SHORT COMMUNICATION
DOI:10.1002/ejic.201402646
Smart Decoration of Mesoporous TiO2 Nanospheres with
Noble Metal Alloy Nanoparticles into Core–Shell,
Yolk–Core–Shell, and Surface-Dispersion Morphologies
Pengyu Wang,*[a] Hiroko Tooriyama,[a] Kazuya Yokoyama,[a]
Masataka Ohtani,[a] Haruyasu Asahara,[a] Tomoya Konishi,[b]
Nagatoshi Nishiwaki,[a] Masahiko Shimoda,[c]
Yoshiyuki Yamashita,[d] Hideki Yoshikawa,[d] and Kazuya Kobiro*[a]
Keywords: Mesoporous materials / Nanoparticles / Solvothermal synthesis / Titanium / Alloys
Noble metal alloy-decorated spherical mesoporous TiO2
nanoparticles were synthesized by the one-pot, single-step
reaction of Ti(OiPr)4, Au3+, Pt4+, and Pd2+ or by the one-pot
porous nanoparticles with core–shell, yolk–core–shell, and
surface-dispersion morphologies were easily constructed.
The noble metal alloy cores of the Au–Pt–Pd@TiO2 and Au–
Pt@TiO2 yolk–core–shell nanoparticles have Au@Pt–Pd and
Au@Pt embedded sub-core–shell structures, respectively.
reaction of hollow mesoporous TiO2 nanospheres with Au3+
,
Pt4+, and Pd2+ by means of a solvothermal method in meth-
anol. Au–Pt–Pd@TiO2, Au–Pt@TiO2, and Au@TiO2 meso-
intensively studied as a new approach to improve the cata-
lytic activity of TiO2 with the expectation that the syner-
Introduction
[4]
Much attention has been paid to noble metal (NM)
multimetallic nanoparticles (NPs) because of their unique
properties that make them attractive for applications in the
fields of catalysis, electronic devices, optical devices, opto-
electronics, and microelectronics.[1] To exhibit high perform-
ance in catalysis, aggregation among NPs should be avo-
ided. For this purpose, NM NPs are embedded into ma-
trixes, such as polymers, dendrimers, graphene, mesoporous
silica, mesoporous alumina, zeolites, and metal–organic
frameworks.[2] On the other hand, TiO2 NPs have attracted
extensive attention as one of the most promising functional
materials from the viewpoints of their chemical stability,
thermostability, lack of toxicity, photo-semiconductor
properties, and relatively low cost.[3] Recently, the loading
of metallic NPs onto/inside mesoporous TiO2 NPs has been
gistic interactions between the metallic NPs and TiO2 will
strongly enhance its catalytic activity.[5] In this context, sev-
eral metallic NP@TiO2 NPs have been reported;[6] however,
the literature contains few reports concerning the smart
decoration of spherical mesoporous TiO2 NPs with core–
shell alloy NPs.
Recently, the solvothermal approach has been used in the
synthesis of NM NPs, serving not only as a reaction me-
dium but also as a reducing reagent, to yield metal NPs
from the corresponding metal ions.[7] We reported an ex-
tremely simple synthetic approach for the preparation of
solid and hollow spherical mesoporous metal oxide NPs,
referred to as mesoporously architected roundly integrated
metal oxide (MARIMO) NPs, in high-temperature meth-
anol.[8] Given the unique behavior of high-temperature
methanol to cause the formation of both NM NPs and
TiO2 NPs, the preparation of multimetallic NPs@TiO2 NPs
with different morphologies should be possible by con-
trolling the rate of formation of the NM NPs and TiO2
NPs. In this paper, we describe a smart, simple, and versa-
tile method for the decoration of solid-MARIMO (S-MA-
RIMO) and hollow-MARIMO (H-MARIMO) TiO2 NPs
with metallic NM NPs, such as Au–Pt–Pd alloy, Au–Pt al-
[a] School of Environmental Science and Engineering, Kochi
University of Technology,
185 Miyanokuchi, Tosayamada, Kami, Kochi 782-8502, Japan
E-mail: wang.pengyu@kochi-tech.ac.jp
kobiro.kazuya@kochi-tech.ac.jp
http://www.env.kochi-tech.ac.jp/kobiro/external/index.html
[b] Center for Collaborative Research, Anan National College of
Technology,
265 Aoki Minobayashi, Anan, Tokushima 774-0017, Japan
[c] Surface Physics Group, Surface Physics and Structure Unit,
Advanced Key Technologies Division, National Institute for loy, and Au, to form core–shell, yolk–core–shell, and sur-
Materials Science,
face-dispersion MARIMO TiO2 NPs.
1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
[d] Synchrotron X-ray Station at SPring-8, National Institute for
Results and Discussion
First, a single-step, one-pot preparation of trimetallic
Au-Pt–Pd@TiO2 was executed. A mixture of Ti(OiPr)4,
Materials Science
1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402646.
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HAuCl4·4H2O, PtCl4, Pd(NO3)2, phthalic acid, and meth- shell during a total 100° rotation in the TEM images (Fig-
anol [Ti(OiPr)4/metal salt/phthalic acid = 10:1:50 (molar ure 3, a).
ratio)] was heated to 300 °C in a sealed SUS-316 tubing.[9]
Transmission electron microscopy (TEM) and energy-dis-
persive X-ray spectroscopy (EDX) mapping images clearly
reveal that the particles belong to the category of MAR-
IMO NPs. The metal NPs were covered by mesoporous
TiO2 shells to afford core–shell NPs (Figure 1, a). The EDX
mapping images and EDX line-scan plots clearly indicate
that the MARIMO NPs consist of Au, Pt, Pd, Ti, and O
atoms. Interestingly, the central-core NPs consist of Au–Pt–
Pd trimetallic alloy and are covered homogeneously by Ti
and O atoms (Figure 1, a). The XRD pattern of the NPs
was very broad, which can be ascribed to a mixture of ana-
tase TiO2, cubic Au, cubic Pt, and cubic Pd diffraction
peaks (Figure 2, a). The possibility of the formation of Au–
Pt–Pd alloy NPs attaching to the outer surface of the MA-
RIMO TiO2 shells was completely excluded on the basis of
Figure 2. XRD spectra of Au–Pt–Pd alloy@TiO2 NPs. (a) Au–Pt–
TEM images taken at different tilt angles, for which the
Pd alloy@TiO2 core–shell NPs, (b) Au–Pt–Pd alloy@TiO2 yolk–
core Au–Pt–Pd alloy particles never stick out of the TiO2
core–shell NPs, and (c) Au–Pt–Pd alloy@TiO2 surface-dispersion
NPs.
Figure 3. TEM images of the synthesized trimetallic alloy@TiO2
NPs at different angles: (a) Au–Pt–Pd alloy@TiO2 core–shell NPs
and (b) Au@Pt–Pd alloy@TiO2 yolk–core–shell NPs.
A “ship-in-a-bottle” technique was applied to form yolk–
core–shell NPs.[10] High-temperature methanol, especially
so-called supercritical methanol (Tc = 239.5 °C, Pc
=
8.09 MPa), is well known for its high permeability in nar-
row pores and its ability to reduce NM ions to yield the
corresponding zero-valent metals.[11] Thus, homogeneously
dispersed Au3+, Pt4+, and Pd2+ ions in methanol under high
pressure would penetrate into the interiors of the MAR-
IMO TiO2 pores and be reduced by high-temperature meth-
anol to afford Au0, Pt0, and Pd0 metals, respectively, with a
yolk–core–shell structure. In fact, upon treating a mixture
of H-MARIMO TiO2 NPs (Figure S1, Supporting Infor-
mation),[9] HAuCl4·4H2O, PtCl4, Pd(NO3)2, and methanol
Figure 1. EDX mapping images and line-scan EDX plots of the
synthesized trimetallic alloy@TiO2 NPs. (a) Au–Pt–Pd alloy@TiO2
core-shell NPs, (b) Au@Pt–Pd alloy@TiO2 yolk–core–shell NPs,
and (c) Au–Pt–Pd alloy@TiO2 surface-dispersion NPs.
[H-MARIMO TiO2/HAuCl4·4H2O/PtCl4/Pd(NO3)2
=
10:1:1:1 (molar ratio)] in the sealed SUS-316 tubing at
300 °C for 10 min, yolk–core–shell NPs were formed, as
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Scheme 1. Plausible mechanism for the formation of the core–shell NPs in high-temperature methanol: (a) Au–Pt–Pd@TiO2 core–shell
NPs and (b) Au@Pt–Pd@TiO2 yolk–core–shell NPs.
indicated by TEM analysis (Figure 1, b). Interestingly, the
A proposed mechanism for the formation of MARIMO
EDX mapping images and line-scan plots clearly demon- multimetallic alloy@TiO2 NPs in high-temperature meth-
strate that Au–Pt–Pd alloy NPs are situated inside the H- anol is presented in Scheme 1. In the case of the one-pot
MARIMO TiO2 NPs (Figure 1, b). Surprisingly, EDX line- synthesis, Au3+, Pt4+, and Pd2+ are readily and rapidly re-
scan plots revealed that the central NPs consist of Au@Pt– duced by high-temperature methanol to form a Au–Pt–Pd
Pd sub-core–shell structures, in which the core Au NP is alloy.[7] Then, mesoporous TiO2 is generated relatively
covered by a Pt–Pd shell (Figure 1, b). Furthermore, TEM slowly and covers the surface of the alloy NPs, which leads
images at different tilt angles also clearly demonstrate the to the core–shell structure (Scheme 1, a). In the case of H-
yolk–core–shell structure of the NPs (Figure 3, b). In ad- MARIMO TiO2 affording Au@Pt–Pd@TiO2 yolk sub–
dition, the XRD patterns of the obtained NPs indicate the core–shell NPs, a conclusive discussion is not possible; how-
presence of anatase TiO2, cubic Au, cubic Pt, and cubic Pd. ever, the results could be explained by different permeation
Upon performing the reaction with S-MARIMO TiO2 NPs speeds and/or different reduction potentials of Au3+, Pt4+
,
(Figure S2),[9] Au–Pt–Pd alloy NPs were attached onto the and Pd2+. Thus, the Au3+ ions might penetrate the shell
surface and/or inside the mesopores of the S-MARIMO and/or might be reduced faster than Pt4+ and Pd2+ in the
TiO2 NPs, as expected (see parts c in Figures 1 and 2).
limited amounts of the ions in the nanoscale inside hole of
In the case of Au–Pt–Pd alloy@TiO2 surface dispersion, the H-MARIMO TiO2 NPs to form Au@Pt–Pd@TiO2
Au 4f, Pt 4f, and Pd 3d core-level photoemission peaks were yolk–core–shell NPs (Scheme 1, b).
observed by common low-energy X-ray photoelectron spec-
As an application, Au@H-MARIMO TiO2 surface dis-
troscopy (XPS) by using Mg-Kα radiation (hν = 1254 eV, persion NPs were successfully applied as a catalyst, in which
Figure S3), whereas no photoemission signal from Au was 4-nitrophenol, 4-nitroanisole, and 4-bromonitrobenzene
detected in the cases of Au–Pt–Pd alloy@TiO2 core–shell were reduced to the corresponding aniline derivatives in
and Au@Pt–Pd alloy@TiO2 yolk–core–shell NPs. However, quantitative yields.[9] In addition, the catalyst could be used
we succeeded in observing Au 4f, Pt 4f, and Pd 3d core-level repeatedly at least 12 times with a turnover number over
photoemission peaks by high-energy X-ray photoelectron 60000.
spectroscopy (HEX-PES) by using synchrotron radiation
(hν = 5950 eV) at SPring-8 (Figures S4 and S5). These re-
sults clearly indicate that Au, Pt, and Pd atoms are embed-
ded deep inside the MARIMO TiO2 NPs and that photo-
Conclusions
electrons with higher kinetic energies released by high-en-
ergy X-ray excitation can escape through the thick TiO2 Au–Pt@TiO2, and monometallic Au@TiO2 mesoporous
shell. nanoparticles (NPs) with different morphologies, such as
In conclusion, trimetallic Au–Pt–Pd@TiO2, bimetallic
In addition, the bimetallic Au–Pt alloy@TiO2 NPs (Fig- core–shell, yolk–core–shell, and surface dispersion, were
ures S6–S8) and monometallic Au@TiO2 NPs (Figures S9– synthesized by using a simple solvothermal method by
S11) with different morphologies, such as core–shell, yolk– using high-temperature methanol. The alloy cores of the
core–shell, and surface-dispersion NPs, were synthesized by Au–Pt–Pd@TiO2 and Au–Pt@TiO2 yolk–core–shell NPs
using similar solvothermal methods in methanol. In the exhibit Au@Pt–Pd and Au@Pt embedded sub-core–shell
case of bimetallic Au–Pt alloy@TiO2 yolk–core–shell NPs, structures, respectively. Thus, the solvothermal approach in
Au@Pt sub-core–shell structure was also afforded inside the methanol is a versatile simple method for the synthesis of
MARIMO TiO2 NPs, as shown in the EDX mapping noble metal (alloy)@metal oxide mesoporous NPs. These
images and EDX line-scan plot (Figure S6, b).
noble metal decorated core–shell TiO2 NPs can be applied
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Experimental Section
Typical Procedure for the Synthesis of NM@TiO2 MARIMO NPs
in Methanol: To
a solution of phthalic acid in methanol
(0.5 molL–1, 20 mL) was added the noble metal salts (0.2 mmol)
and Ti(OiPr)4 (0.66 mL 2 mmol) with vigorous stirring. The solu-
tion was stirred vigorously overnight. The solution (3.5 mL) was
transferred into an SUS-316 batch-type reactor (10 mL volume).
The reactor was sealed with a screw cap. The reactor was then
placed in a molten salt bath at 300 °C for 10 min. The reaction was
quenched by placing the reactor into an ice-water bath. The screw
cap was opened after the reactor was cooled completely. The crude
product was sonicated in methanol (30 min) and centrifuged
(6600 rpm, 10 °C, 30 min). The upper layer was decanted. This pro-
cedure was repeated three times. The obtained powdery product
was then dried in vacuo at 30 °C for 24 h.
[3]
[4]
[5]
[6]
Supporting Information (see footnote on the first page of this arti-
cle): TEM and SEM images, XRD spectra, and HEX-PES spectra
of the nanoparticles.
Acknowledgments
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This work was partially supported by the Kochi Prefectural Indus-
try–Academia–Government Collaboration Research Promotion
Operation (Creation of New Business and Industry Program). The
HEX-PES measurements were performed under the approval of
the NIMS Beamline Station (proposal numbers 2012B4603 and
2013B4900). This work was also partially supported by Nanotechn-
ology Platform Project (grant number A-13-NM-0099) and the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT), Japan. The authors would like to thank Dr. S. Ueda and
Mr. T. Ishimaru for improving HEX-PES in BL15XU at SPring-8.
The authors are grateful to HiSOR, Hiroshima University, and
JAEA/SPring-8 for the development of HEX-PES in BL15XU at
SPring-8.
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Published Online: August 15, 2014
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