Hydrothermal synthesis of platinum-group-metal nanoparticles by using HEPES as a reductant and stabilizer

Article abstract of DOI:10.1002/asia.201000066

Platinum-group-metal (Ru, Os, Rh, Ir, Pd and Pt) nanoparticles are synthesized in an aqueous buffer solution of 4-(2-hydroxyethyl)-1-piper- azineethanesulfonic acid (HEPES) (200 mM, pH 7.4) under hydrothermal conditions (180 °C). Monodispersed (monodisper

Full text of DOI:10.1002/asia.201000066

FULL PAPERS
DOI: 10.1002/asia.201000066
Hydrothermal Synthesis of Platinum-Group-Metal Nanoparticles by Using
HEPES as a Reductant and Stabilizer**
Man-Ho So,[a] Chi-Ming Ho,[a] Rong Chen,[b] and Chi-Ming Che*[a]
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Chem. Asian J. 2010, 5, 1322 – 1331

Abstract: Platinum-group-metal (Ru,
Os, Rh, Ir, Pd and Pt) nanoparticles
are synthesized in an aqueous buffer
solution of 4-(2-hydroxyethyl)-1-piper-
azineethanesulfonic acid (HEPES)
(200 mm, pH 7.4) under hydrothermal
conditions (1808C). Monodispersed
(monodispersity: 11–15%) metal nano-
particles were obtained with an aver-
age particle size of less than 5 nm (Ru:
1.8Æ0.2, Os: 1.6Æ0.2, Rh: 4.5Æ0.5, Ir:
2.0Æ0.3, Pd: 3.8Æ0.4, Pt: 1.9Æ0.2 nm).
The size, monodispersity, and stability
of the as-obtained metal nanoparticles
were affected by the HEPES concen-
tration, pH of the HEPES buffer solu-
perazine groups) and terminal hydroxyl
groups can act as a reductant and sta-
bilizer. The HEPES molecules can
bind to the surface of metal nanoparti-
cles to prevent metal nanoparticles
from aggregation. These platinum-
group-metal nanoparticles could be de-
posited onto the surface of graphite,
which catalyzed the aerobic oxidation
of alcohols to aldehydes.
tion,
and
reaction
temperature.
HEPES with two tertiary amines (pi-
Keywords: HEPES buffer · hydro-
thermal synthesis · nanoparticles ·
platinum-group metals · supported
catalysts
Introduction
Over the past few years, the synthesis of metal nanoparti-
cles in aqueous medium without the generation of hazard-
ous substances has received increasing attention.[8] Wallen
and co-workers reported the “green” synthesis of Ag, Au,
and Au–Ag alloy nanoparticles, involving glucose and starch
as both a reductant and stabilizer.[9] Xia reported a general
synthesis of noble metal (Pd, Pt, Ag, and Au) nanoplates by
using PVP to act as both a reductant and stabilizer.[10] Re-
cently, we reported the use of biocompatible 4-(2-hydroxy-
The development of new synthetic methods for monodis-
persed nanomaterials is crucial for their subsequent applica-
tions in electronics[1] and biological and catalytic sciences.[2–4]
In particular, platinum-group-metal (Ru, Os, Ir, Rh, Pt, and
Pd) nanoparticles have demonstrated superior catalytic ac-
tivities relative to their bulk counterparts due to their large
surface area to volume ratios and the high density of active
sites on their surface.[3] A well-known method for the syn-
thesis of monodispersed platinum-group-metal nanoparticles
is through polyol (alcohol) reduction in the presence of cap-
ping agents (e.g. polyvinylpyrrolidone (PVP), polyethylene
glycol (PEG), and cetyltrimethylammonium bromide
(CTAB)).[5] Another method is the decomposition of organ-
ometallic precursors to generate metal nanoparticles under
carefully controlled conditions.[5,6] Although both methods
are able to produce metal nanoparticles with a narrow mon-
odispersity and well-defined morphology,[7] these methods
suffer from the drawbacks of using hazardous organic sol-
vents and chemicals, both of which are undesirable in the
context of green chemistry and biological application stud-
ies. In this regard, there has been considerable interest in
the development of methods for the synthesis of metal
nanoparticles in aqueous medium and without the use of
toxic reagents. In addition, a reagent playing dual roles as a
reductant of metal salts and a stabilizer of metal nanoparti-
cles would be appealing.
AHCTUNGTREGeNNUN thyl)-1-piperazineethanesulfonic acid (HEPES) for the syn-
thesis of Ag nanoparticles (average particle size about
10 nm), which was found to display cyto-protective activities
towards HIV-1 infected cells.[2c] The Ag nanoparticles could
also be synthesized by the reduction of Ag+ with HEPES
and branched polyethyleneimine.[11] Branched, flowered or
spherical Au nanoparticles could also be synthesized in
HEPES buffer solutions at ambient conditions without the
addition of surfactants or seeds.[12]
We envision that the mild reducing property of HEPES
allows the feasibility of controlling the kinetics of the reduc-
tion process by varying the reaction temperature or pH of
the buffer solution. Herein is described a general synthetic
route for both monodispersed (monodispersity: 11–15%)
and small sized (<5 nm) platinum-group-metal nanoparti-
cles in HEPES buffer (200 mm, pH 7.4) under hydrothermal
conditions. We show that HEPES acts as a reductant and a
stabilizer for the formation of metal nanoparticles. The pi-
perazine group of HEPES accounts for the reduction of
metal salts and the stabilization of metal nanoparticles. We
have also demonstrated that these metal nanoparticles could
be deposited onto the surface of graphite, which catalyzes
the aerobic oxidation of alcohols to aldehydes.
[a] Dr. M.-H. So, Dr. C.-M. Ho, Prof. C.-M. Che
Department of Chemistry and Open Laboratory
of Chemical Biology of the Institute of Molecular
Technology for Drug Discovery and Synthesis
The University of Hong Kong
Pokfulam Road, Hong Kong SAR (China)
Fax : (+852)2857-1586
E-mail: cmche@hku.hk
Results and Discussion
[b] Dr. R. Chen
Key Laboratory for Green Chemical Process of Ministry
of Education and School of Chemical Engineering & Pharmacy
Wuhan Institute of Technology
Synthesis and characterization of metal nanoparticles
Screening of buffer systems for nanoparticle synthesis: Re-
cently, we reported that HEPES can act as both a reductant
and a stabilizer for the synthesis of Ag and Au nanoparticles
in aqueous solutions.[2c,12d] In this work, the use of HEPES
Xiongchu Street, Wuhan, 430073 (China)
[**] HEPES=4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201000066.
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C.-M. Che et al.
FULL PAPERS
and other biocompatible buffers for the synthesis of metal
nanoparticles of platinum-group metals (Ru, Os, Rh, Ir, Pd,
and Pt) in aqueous medium has further been explored
under hydrothermal conditions. At the outset, the metal salt
(2 mm) was first dissolved into an aqueous HEPES buffer
solution (200 mm, pH 7.4) in a Teflon-lined stainless-steel
autoclave. The reactor was incubated at 1808C for an hour.
Examination of the HEPES buffer before and after the hy-
drothermal reaction by NMR spectroscopy did not show
any significant difference in the spectra, which revealed that
the HEPES molecules are stable under hydrothermal condi-
tions (see Figure S1 in the Supporting Information). The re-
action was considered to be completed when the UV/Vis ab-
sorption spectrum of the reaction mixture did not change
further (see Figure S2 in the Supporting Information), and/
or diffraction peaks due to the metal salts disappeared in
the powder XRD spectrum. All of these metal nanoparticles
suspended in water with no apparent aggregation or floccu-
lation of metal nanoparticles for weeks, which revealed that
HEPES is a good stabilizer, preventing the metal nanoparti-
cles from aggregation.
Examination of a suspension of metal nanoparticles by
transmission electron microscopy (TEM) revealed that
spherical metal nanoparticles with uniform sizes were ob-
tained (Figure 1). The average diameters of Ru, Os, Rh, Ir,
Pd, and Pt nanoparticles were 1.8Æ0.2, 1.6Æ0.2, 4.5Æ0.5,
2.0Æ0.3, 3.8Æ0.4, and 1.9Æ0.2 nm, respectively (Table 1).
The monodispersities of these metal nanoparticles were not
more than 15%, indicating that these nanoparticles were
monodispersed. When the reaction time was extended to
3 h, serious aggregation of metal nanoparticles was noted.
Figure 2 shows a typical TEM image of aggregated Os nano-
particles obtained from the treatment of Os metal salt
(2 mm) in HEPES buffer solution (200 mm, pH 7.4) for 3 h
and under hydrothermal conditions at 1808C.
Figure 1. TEM images of a) Ru, b) Os, c) Rh, d) Ir, e) Pd, and f) Pt nano-
particles prepared in HEPES buffer (200 mm, pH 7.4) under hydrother-
mal conditions at 1808C for 1 h.
Table 1. Average diameter and monodispersity of the metal nanoparti-
cles prepared in HEPES buffer (200 mm, pH 7.4) under hydrothermal
conditions at 1808C for 1 h.
High-resolution
transmission
electron
microscopy
(HRTEM) allows the imaging of metal nanoparticles at the
atomic level and can be used for the analysis of their crystal-
lographic structures. As depicted in Figure 3, these metal
nanoparticles displayed excellent crystallinity as revealed by
the clear lattice fringes and sharp diffraction spots in the
fast Fourier transform (FFT) images of the interested nano-
Metal nanoparticles
d [nm][a]
Monodispersity [%][b]
ruthenium
osmium
rhodium
iridium
palladium
platinum
1.8Æ0.2
1.6Æ0.2
4.5Æ0.5
2.0Æ0.3
3.8Æ0.4
1.9Æ0.2
11
13
11
15
11
11
[a] The average diameter (d) is measured from 300 randomly selected
nanoparticles. [b] Monodispersity of the diameter of nanoparticles is cal-
culated as follows: monodispersity=(standard deviation/average diame-
ter)ꢁ100%.
Abstract in Traditional Chinese:
particles (insets in Figure 3). The Rh nanoparticles revealed
a twinned crystal structure as shown by the HRTEM image
depicted in Figure 3c, whereas the HRTEM images of the
other metal (Ru, Os, Ir, Pt, and Pd) nanoparticles showed
that they were single crystals. Those lattice spacings (d-spac-
ing) could be indexed back to the crystal planes of their cor-
responding bulk metals further confirming the identities of
the metal nanoparticles at the microscopic level.
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Hydrothermal Synthesis of Platinum-Group-Metal Nanoparticles
Powder X-ray diffraction (XRD) analysis: Dry solid samples
have also been characterized by powder XRD for examining
the composition(s) and/or identity of the products. To im-
prove the crystallinity of the samples for better resolution of
the diffraction peaks, calcination (5508C, 6 h and argon at-
mosphere) was applied on some of the samples. As depicted
in Figure 4, all of the solid samples showed a diffraction pat-
tern that matches to the standard metal patterns (JCPDS
file of Ru: 06-0663, Os: 06-0662, Rh: 05-0685, Ir: 06-0598,
Pd: 46-1043, Pt: 04-0802). Broadening of the diffraction
peaks of the metal nanoparticles is attributed to the small
crystal size of the nanoparticles and we have calculated the
average particle size by using the Scherrer equation as tabu-
lated in Table S1 in the Supporting Information.[13]
Figure 2. TEM images of the Os nanoparticles prepared in HEPES
buffer (200 mm, pH 7.4) under hydrothermal conditions at 1808C for 3 h.
X-ray photoelectron spectroscopy (XPS) analysis: We have
examined the surface chemical oxidation state of the metal
nanoparticles by XPS analysis. As depicted in Table 2, all of
the binding energies of the interested peaks could be as-
signed to the corresponding metal by comparison with liter-
ature values.[14] The binding energy of the 3p3/2 peak of the
Ru nanoparticles obtained in this work is 462.7 eV, which is
slightly higher than the literature value (462.0 eV).[3h,14] This
reveals that the surface of the Ru nanoparticles contained
oxidized Ru species.
Role of HEPES buffer: HEPES contains three functional
groups: a sulfonic acid group, a piperazine group, and a ter-
minal hydroxyl group. To gain an insight into the role of
these groups, we examined three other zwitterionic N-substi-
tuted aminosulfonic acid buffers (3-[4-(2-hydroxyethyl)-1-pi-
perazinyl]propanesulfonic acid (EPPS), piperazine-N,N’-
bis(2-ethanesulfonic acid) (PIPES), and 2-(N-morpholino)e-
thanesulfonic acid (MES)). As an example, Ir nanoparticles
Figure 3. HRTEM images of a) Ru, b) Os, c) Rh, d) Ir, e) Pd, and f) Pt
nanoparticles prepared in HEPES buffer (200 mm, pH 7.4) under hydro-
thermal conditions at 1808C for 1 h. Insets are the FFT images of the in-
teresting nanoparticles. The scale bars in the insets are 10 nmÀ1
.
Table 2. Binding energy values of the metal nanoparticles at 3p3/2 (Ru),
3d5/2 (Rh and Pd), or 4f7/2 (Os, Ir, and Pt).
Metal nanoparticles[a]
ruthenium (3p3/2
osmium (4f7/2
rhodium (3d5/2
iridium (4f7/2
palladium (3d5/2
platinum (4f7/2
Binding energy [eV][b]
Assignment
)
462.7 (462.0)
51.0 (50.7)
307.3 (307.2)
61.0 (60.9)
335.3 (335.1)
71.0 (71.2)
Ru(0)
Os(0)
Rh(0)
Ir(0)
Pd(0)
Pt(0)
Energy dispersive X-ray (EDX) analysis was performed
to examine the elemental compositions of the metal nano-
particles (see Figure S3 in the Supporting Information).
Each EDX spectrum of nanoparticles reveals the presence
of the corresponding metal. The presence of both Cu and C
are attributed to the Formvar-coated copper grids. The
HEPES accounts for the presence of C, O, and S.
)
)
)
)
)
[a] Corresponding peak for XPS analysis is shown in parentheses.
[b] Standard binding energy values are shown in parentheses.
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C.-M. Che et al.
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were seriously aggregated (Fig-
ure 5b–c). This result indicates
that the tertiary amine in either
the piperazine (HEPES, EPPS,
and PIPES) or morpholine
group (MES) is capable of re-
ducing the metal salts. The ter-
minal hydroxyl group in
HEPES or EPPS might also act
as reductant. Importantly, the
terminal hydroxyl group is cru-
cial to the stabilization of metal
nanoparticles as illustrated by
serious aggregation of metal
nanoparticles in PIPES and
MES buffer (cf. Figure 1d with
5b–c).
Next, we examined the effect
of pH (5.4–9.4) on the forma-
tion of metal nanoparticles. In
the case of Ir nanoparticles, its
stability is sensitive to the pH
of
the
reaction
medium
(Figure 6). The Ir nanoparticles
in this work were found to seri-
ously aggregate at pH 5.4. Little
aggregation was noted for the
Ir nanoparticles prepared at
pH 6.4. Well-dispersed Ir nano-
particles could be successfully
obtained at pH 7.4–9.4. There is
no significant change in the
average particle size of Ir nano-
particles by altering the pH of
the reaction medium (Table 3).
At pH 5.4, the predominant
form of HEPES is a singly pro-
tonated amine. When the pH of
the buffer increases, the con-
centration of the deprotonated
form of HEPES increases. We
propose that the two free nitro-
gen atoms (piperazine group) in
the deprotonated form of
HEPES bind to the surface of
the metal nanoparticles, thereby
prohibiting the metal nanopar-
ticles from aggregation.
Figure 4. Powder XRD pattern of a) Ru, b) Os, c) Rh, d) Ir, e) Pd, and f) Pt nanoparticles. Plots A and B are
the XRD pattern of the nanoparticles after calcination at 5508C for 6 h under an argon atmosphere and
before calcination, respectively.
Figure 5. TEM images of the Ir nanoparticles prepared in a) EPPS (200 mm, pH 8.0), b) PIPES (200 mm,
pH 6.8), and c) MES buffer (200 mm, pH 6.4) under hydrothermal conditions at 1808C for 1 h.
The HEPES concentration is
were prepared by using aqueous buffer solutions of EPPS,
PIPES, and MES individually under hydrothermal condi-
tions at 1808C for one hour. No significant difference in
sizes and shapes of the Ir nanoparticles were noted when
EPPS buffer was used (Figure 5a). The average particle size
and monodispersity of the Ir nanoparticles were 2.1Æ0.3 nm
and 12.1%, respectively. When PIPES or MES buffer was
used, reduction of Ir salts took place but the Ir nanoparticles
another factor affecting the stability of metal nanoparticles.
As mentioned before, all metal nanoparticles prepared in
HEPES buffer (pH 7.4) at 200 mm under hydrothermal con-
ditions at 1808C were stable without aggregation. When the
concentration of HEPES was lowered to 50 mm, only Os
and Ir nanoparticles with a good stability were found,
whereas Pt nanoparticles were found to aggregate and pre-
cipitate at the bottom of the reaction vessel (Figure 7).
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Hydrothermal Synthesis of Platinum-Group-Metal Nanoparticles
Figure 8. TEM images of a) Os and b) Ir nanoparticles prepared in
HEPES buffer (5 mm, pH 7.4) under hydrothermal conditions at 1808C
for 1 h. Insets are the TEM images of the same samples at higher resolu-
tion.
HEPES (5 mm; pH 7.4) (TEM images of Os and Ir nanopar-
ticles are depicted in Figure 8 as examples).
Effect of reaction temperature: The effect of reaction tem-
perature on the synthesis of metal nanoparticles has been
examined. As an example, well-dispersed Ru nanoparticles
could be obtained at 1408C. However, when the reaction
temperature was increased to 2208C, a metallic black
powder containing seriously aggregated Ru nanoparticles
was found in the reaction vessel (Figure 9).
The synthesis of metal nanoparticles by refluxing the reac-
tion mixture under atmospheric conditions has also been ex-
amined. No reduction of Rh salts was noted after refluxing
Rh salts in HEPES solution (200 mm, pH 7.4) for one hour.
Figure 6. TEM images of the Ir nanoparticles prepared in HEPES solu-
tion (200 mm) of pH a) 5.4, b) 6.4, c) 8.4, and d) 9.4 under hydrothermal
conditions at 1808C for 1 h.
Table 3. Average diameter and monodispersity of the Ir nanoparticles
prepared in HEPES buffer (200 mm) under hydrothermal conditions at
1808C for 1 h at various pH values.
pH
d [nm][a]
Monodispersity [%][b]
5.4
6.4
7.4
8.4
9.4
2.2Æ0.3
1.9Æ0.3
2.0Æ0.3
2.0Æ0.2
2.0Æ0.2
14
16
15
10
10
[a] The average diameter (d) is measured from 300 randomly selected
nanoparticles. [b] Monodispersity of the diameter of nanoparticles is cal-
culated as follows: monodispersity=(standard deviation/average diame-
ter)ꢁ100%.
TEM analysis revealed that the Os and Ir nanoparticles
were approximately spherical in shape with an average size
of 2.3 and 2.2 nm, respectively. However, Pt nanoparticles
were aggregated and the particle size was about 3.1 nm
(Figure 7). Heavy precipitation was found for all of the
preparations conducted at an even lower concentration of
Figure 9. TEM images of the Ru nanoparticles prepared in HEPES
buffer (200 mm, pH 7.4) under hydrothermal conditions at a) 140 and
b) 2208C for 1 h.
In the case of Ru, Os, or Ir, a
black precipitate of aggregated
metal nanoparticles was ob-
tained (TEM images of Os and
Ir nanoparticles are shown as
examples; Figure 10). Well-seg-
regated Pt and Pd nanoparticles
could be prepared under reflux
conditions as revealed by TEM
images (Figure 10). The particle
sizes of Pt and Pd nanoparticles
were 2.9Æ0.3 and 5.1Æ0.8 nm,
Figure 7. TEM images of the a) Os, b) Ir, and c) Pt nanoparticles prepared in HEPES buffer (50 mm, pH 7.4)
under hydrothermal conditions at 1808C for 1 h.
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Figure 10. TEM images of the a) Os, b) Ir, c) Pt, and d) Pd nanoparticles
prepared in HEPES solution (200 mm, pH 7.4) under reflux at 1008C for
1 h.
respectively. It should be noted that the sizes of Pt and Pd
nanoparticles prepared under reflux conditions were signifi-
cantly larger than those prepared by the hydrothermal pro-
cess. The hydrothermal process provides an elevated reac-
tion temperature and pressure, which may probably increase
the reactivity of HEPES towards the reduction of metal
salts.[5a] The increased reactivity of HEPES results in a fast
nucleation step, thus the metal nanoparticles obtained by
the hydrothermal process are usually smaller and more uni-
form in size.
Figure 11. TEM images of the graphite-supported a) Ru, b) Os, c) Rh,
d) Ir, e) Pd, and f) Pt nanoparticle catalysts.
tion. These metal nanoparticles maintained their uniformity
after deposition onto the surface of graphite (monodispersi-
ty of Ru nanoparticles: 9.5, Os: 10.0, Rh: 13.0, Ir: 9.5, Pd:
11.0, Pt: 10.0%).
We further examined the catalytic activities of these
MNPs/C catalysts towards the aerobic oxidation of alco-
hols.[15,16] As depicted in Table 4, OsNPs/C can catalyze the
Graphite-supported platinum-group-metal nanoparticles:
One potential application of metal nanoparticles is in the
catalysis area.[3] Metal nanoparticles are usually deposited
onto the surface of the solid support, taking the advantage
of the easy separation of these solid supported catalysts
from reaction mixture. The solid support could affect the
electronic properties of nanoparticles (e.g. gold nanoparti-
cles supported on cerium(IV) oxide).[15]
In this work, the graphite-supported metal nanoparticles
(MNPs/C) catalysts were prepared by deposition of the
freshly prepared metal nanoparticles onto the surface of
graphite at a metal loading of 0.1 mmolgÀ1. As revealed by
the TEM images of the MNPs/C catalysts, these nanoparti-
cles were deposited onto the surface of graphite without se-
rious aggregation (Figure 11). The average particle sizes of
Ru, Os, Rh, Ir, Pd, and Pt nanoparticles were 2.1Æ0.2, 2.0Æ
0.2, 4.6Æ0.6, 2.1Æ0.2, 11.8Æ1.3, and 3.0Æ0.3 nm, respec-
tively. The size of the Rh and Ir nanoparticles did not signif-
icantly change, whereas the sizes of the Ru, Os, Pd, and Pt
nanoparticles increased after grafting onto the surface of
graphite. An increase in the size of the nanoparticles after
deposition onto the surface of graphite has previously been
reported[3i] and the mechanism for the enlargement of nano-
particles after deposition onto graphite is under investiga-
Table 4. Aerobic oxidation of 4-methylbenzyl alcohol 1a to 4-methylben-
zaldehyde 2a catalyzed by various MNPs/C catalysts.[a]
Entry
Catalyst
Conv. [%][b]
Yield [%][b,c]
1
2
3
4
RuNPs/C
OsNPs/C
RhNPs/C
IrNPs/C
PdNPs/C
PtNPs/C
graphite
89
92
10
90
93
70
73
65
68
0
38
5
<5
<5
0
6
7[d]
[a] Reaction conditions: 1a (0.4 mmol), MNPs/C (metal: 5 mol%), tolu-
ene (5 mL), O2 (1 atm.), 1108C, 24 h, unless otherwise stated. [b] Conver-
sion and yield were determined by 1H NMR spectroscopy by using
Ph2C=CH2 as the internal standard. [c] The yield was calculated based on
substrate conversion. [d] Graphite (200 mg) was used.
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Hydrothermal Synthesis of Platinum-Group-Metal Nanoparticles
aerobic oxidation of alcohol 1a with excellent substrate con-
version and product yield (Table 4, entry 2). By using the
“OsNPs/C+O2” protocol, other primary and secondary alco-
hols can also be effectively oxidized into aldehydes and ke-
tones, respectively, with product yields of up to 93%
(Table 5).
drothermal conditions in HEPES buffer solution (200 mm,
pH 7.4). No zero-valent metal nanoparticles were obtained
probably due to the large negative reduction potentials for
the reduction of these metal salts to elemental metal in the
zero oxidation state.[10] TEM images of the Mn3O4 nanoocta-
hedrons, Co3O4 nanocubes, and ZnO nanorods are depicted
in Figure 12 and the details of characterization are given in
Part IV of the Supporting Information.
Table 5. Aerobic oxidation of alcohol 1 by using the “OsNPs/C+O2” pro-
tocol.[a]
Conclusions
We have demonstrated the synthesis of a variety of plati-
num-group-metal nanoparticles (Ru, Os, Rh, Ir, Pt, and Pd)
and metal oxide nanocrystals (Mn3O4 nanooctahedrons,
Co3O4 nanocubes, and ZnO nanorods) by one-pot hydro-
thermal reactions in aqueous HEPES buffer solutions
(200 mm, pH 7.4). All these platinum-group-metal nanopar-
ticles are spherical and monodispersed with a size of less
than 5 nm (Ru: 1.8Æ0.2, Os: 1.6Æ0.2, Rh: 4.5Æ0.5, Ir:
2.0Æ0.3, Pd: 3.8Æ0.4, Pt: 1.9Æ0.2 nm). These metal nano-
particles were found to exhibit excellent crystallinity as con-
firmed by their HRTEM images. Based on the comparison
with other buffer solutions (EPPS, PIPES, and MES) and
the study on the effect of metal salt concentration and pH
of HEPES buffer, we propose that HEPES functions as
both a reductant and a stabilizer. The elevated temperature
and pressure provided by hydrothermal conditions are cru-
cial to increase the reactivity of HEPES towards the reduc-
tion of metal salts. Moreover, these metal nanoparticles can
be deposited onto the surface of graphite and act as poten-
tial heterogeneous catalysts for organic transformation reac-
tions (e.g. aerobic oxidation of alcohols to aldehydes). Con-
sidering its simplicity and the use of biocompatible HEPES
buffer, this synthetic protocol could be used to produce bio-
compatible metal nanoparticles for biological studies.
Entry Substrate
Product
t
Conv.
Yield
[d] [%][b]
[%][b,c]
1
2
1.5 100
93
1.5 100
75[d]
3
2
100
93
4
5
2
2
87
44
80
58
6
4
81
41
[a] Reaction conditions: 1 (0.4 mmol), OsNPs/C (Os: 5 mol%), toluene
(5 mL), O2 (1 atm.), 1108C, 24 h, unless otherwise stated. [b] Conversion
and yield were determined by H NMR spectroscopy by using Ph2C=CH2
as the internal standard. [c] Yield was calculated based on substrate con-
version. [d] A 15% yield of p-anisic acid was found.
1
We have also examined the reactions of first-row d-block
transition-metal salts in HEPES buffer solution under hy-
drothermal conditions. Metal oxide nanocrystals (Mn3O4
nanooctahedrons, Co3O4 nanocubes, and ZnO nanorods)
were obtained when their corresponding metal salts
(MnSO4·4H2O, CoCl2·6H2O, or ZnCl2) were used under hy-
Experimental Section
Chemicals: All the chemicals (analytical reagent grade) were purchased
from Aldrich and used as received without further purification unless
otherwise noted. HEPES, EPPS, PIPES, MES, manganese(II) sulfate tet-
rahydrate (MnSO4·4H2O), cobalt(II) chloride (CoCl2·6H2O), and zinc(II)
chloride (ZnCl2) were purchased from Aldrich. Ruthenium
hydrate (RuCl3·xH2O), rhodium
chloride hydrate (RhCl3·xH2O),
iridium(III) chloride hydrate
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
(IrCl3·xH2O), palladium(II) chloride
(PdCl2), and potassium tetrachloropla-
tinate(II) (K HCTUNGTRENNUNG
2A
Precious
OsmiumAHCNUTGTERG(NNUN III) chloride hydrate (Os-
from
Cl3·xH2O) was purchased from Strem
Chemicals. Distilled water was pur-
chased from Watsons Water.
Characterization and instrumentation:
The metal nanoparticles and metal
oxide nanocrystal samples were char-
acterized by powder XRD, XPS,
TEM, selected area electron diffrac-
Figure 12. TEM images of a) Mn3O4 nanooctahedrons, b) Co3O4 nanocubes, and c) ZnO nanorods prepared in
HEPES buffer (200 mm, pH 7.4) under hydrothermal conditions at 1808C for 1 h.
Chem. Asian J. 2010, 5, 1322 – 1331
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
1329

C.-M. Che et al.
FULL PAPERS
tion (SAED), energy-dispersive X-ray microanalysis (EDX), SEM, and
UV/Vis. The powder XRD measurement was performed on a Bruker D8
ADVANCE X-ray diffractometer with parallel CuKa radiation (l=
1.5406 ꢂ) and nickel filter. The scanning rate was 0.0058sÀ1 in the 2q
range from 30 to 908 and the step size is 0.058. Dry solid sample was
placed on a glass slide for XRD measurement. XPS analysis was per-
formed on a Physical Electronics Model 5600 with monochromatic AlKa
as the X-ray source. TEM and SAED were done on Philips Tecnai G2 20
S-TWIN with an accelerating voltage of 200 kV. TEM images were taken
by Gatan MultiScan Camera Model 794. The EDX analysis was per-
formed on Oxford Instruments Inca with a scanning range from 0 to
20 keV. SEM images were taken on LEO 1530 FEG operating at 5 kV.
The TEM sample was prepared by dropping a suspension of metal nano-
particles to the formvar-coated copper grids and then dried in a vacuum
desiccator. The average particle size and monodispersity of the nanopar-
ticles were measured against 300 nanoparticles by using DigitalMicrog-
raph(TM) Demo 3.6.5 software. The SEM sample was prepared by put-
ting solid sample onto silicon wafers, sticking onto the SEM specimen
mounts. The pH of the buffer solution was measured by digital pH meter
(HANNA Instruments), which was calibrated with pH standard buffer
solutions (pH 7.010; Aldrich) in advance. UV/Vis absorption measure-
ments were recorded on a Perkin–Elmer Lambda 900 UV/Vis spectro-
photometer. The organic products were characterized by NMR spectros-
copy (1H and 13C) and electron-impact mass spectrometry (EIMS). 1H
and 13C NMR spectra were recorded on Bruker DPX-300, Avance400 or
Bruker DPX-500 FTNMR spectrometers with chemical shifts (in ppm)
relative to tetramethylsilane. Mass spectra were obtained on a Finnigan
MAT 95 mass spectrometer.
tion under reflux conditions. In a typical procedure, metal salt (20 mmol)
was added to
a refluxing HEPES buffer solution (10 mL, 200 mm,
pH 7.4) at 1008C. The mixture was heated at 1008C for 1 h. A black pre-
cipitate was produced. This solid product was collected by centrifugation
and washed three times with distilled water to remove impurities. The
solid product was dried in a vacuum desiccator for characterization.
Synthesis of graphite-supported metal nanoparticles: Graphite (1 g) was
added to the freshly prepared metal nanoparticles solution (0.1 mmol)
and the mixture was stirred vigorously for 1 day. The MNPs/C catalyst
was collected by centrifugation and washed three times with distilled
water and absolute ethanol. The final product was oven-dried at 758C for
1 h and stored in a desiccator for characterization.
Synthesis of metal oxide nanocrystals in HEPES buffer solution by using
hydrothermal conditions: In a typical procedure, metal salt (20 mmol,
4.5 mg MnSO4·4H2O for Mn3O4 nanooctahedrons, 4.8 mg CoCl2·6H2O
for Co3O4 nanocubes, 2.7 mg ZnCl2 for ZnO nanorods) was dissolved in
HEPES buffer solution (10 mL; 200 mm, pH 7.4). The mixture was stirred
and ultrasonicated for 15 min to dissolve the metal salts. The mixture was
transferred to the Teflon-lined stainless steel autoclave and performed
the hydrothermal process at 1808C for 1 h. After the hydrothermal pro-
cess, the solid product was collected by centrifugation and washed three
times with distilled water to remove impurities. The solid product was
dried in a vacuum desiccator for characterization.
Catalyst screening for aerobic oxidation of alcohol 1a: Alcohols
(0.4 mmol) and MNPs/C catalyst (metal: 5 mol%) were added to toluene
(5 mL) in a glass tube connected with a condenser. The reaction flask
was connected to an O2 gas supply (99.7% min, Hong Kong Oxygen &
Acetylene). The mixture was stirred and heated at 1108C for 24 h. By fil-
tration against Celite, the MNPs/C catalyst was removed and the organic
product was collected in the filtrate. Solvent was evaporated away under
vacuum. The substrate conversion and product yield were determined by
1H NMR spectroscopy by using 1,1-diphenylethylene as the internal stan-
dard.
Preparation of HEPES buffer solution: The HEPES buffer solution
(200 mm, pH 7.4) was prepared according to the literature report.[2c] In a
typical experiment, HEPES (11.92 g, 50 mmol) was first dissolved in de-
ionized water (220 mL). The pH of this solution was about 5.4. Sodium
hydroxide solution (1m) was added slowly with vigorous stirring to adjust
to pH 7.4. The buffer solution was diluted to 250 mL with deionized
water to give HEPES buffer solution (200 mm, pH 7.4). For a HEPES
buffer solution with a concentration at 5 or 50 mm (pH 7.4), a desirable
amount of HEPES was used and sodium hydroxide (0.1m or 1m) was
used to adjust the pH of the buffer solution. The preparation of HEPES
solution (200 mm) at pH 5.4 was simply done by dissolving HEPES
(11.92 g, 50 mmol) in deionized water (250 mL). The preparation of
HEPES buffer solutions (200 mm) at pH 6.4, 8.4, and 9.4 were the same
as above by using sodium hydroxide (1m) for adjusting the pH of the so-
lution.
Aerobic oxidation of alcohols: Alcohols (0.4 mmol) and OsNPs/C cata-
lyst (Os: 5 mol%) were added to toluene (5 mL) in a glass tube connect-
ed with a condenser. The reaction flask was connected to an O2 gas
supply (99.7%min, Hong Kong Oxygen & Acetylene). The mixture was
stirred and heated at 1108C for a desired time. By filtration against
Celite, the OsNPs/C catalyst was removed and the organic product was
collected in the filtrate. Solvent was evaporated away under vacuum. The
substrate conversion and product yield were determined by 1H NMR
spectroscopy by using 1,1-diphenylethylene as the internal standard. Pure
product was isolated by flash chromatography and identified by 1H and
13C NMR spectroscopy and EIMS.
Synthesis of metal nanoparticles in HEPES buffer solution by using hy-
drothermal conditions: In
a typical procedure, metal salt (20 mmol,
5.2 mg RuCl3·nH2O for Ru nanoparticles, 7.0 mg OsCl3·nH2O for Os
nanoparticles, 5.3 mg RhCl3·nH2O for Rh nanoparticles, 7.0 mg
IrCl3·nH2O for Ir nanoparticles, 3.5 mg PdCl2 for Pd nanoparticles, and
8.3 mg K2PtCl4 for Pt nanoparticles) was dissolved in HEPES buffer solu-
tion (10 mL; 200 mm, pH 7.4). The mixture was stirred and ultrasonicated
for 15 min to dissolve the metal salts. The mixture was transferred to the
Teflon-lined stainless steel autoclave and underwent the hydrothermal
synthesis at 1808C for 1 h. Suspensions of metal nanoparticles were sub-
sequently produced. This synthetic protocol for metal nanoparticles was
the same through the entire work unless otherwise specified (e.g. HEPES
buffer of different concentration and pH, uses of EPPS, PIPES, and MES
buffer).
Acknowledgements
This work was supported by the Innovation and Technology Fund (ITS/
189/08), University Development Fund, and the Hong Kong Research
Grants Council (HKU 1/CRF/08 and CityU 2/06C). We thank Dr. Y. Liu
for his helpful discussions and translation of the abstract from English to
the traditional Chinese language. We offer our heartfelt thanks to F. Yu-
Fee Chan and W.-S. Lee of the Electron Microscope Unit of the Univer-
sity of Hong Kong for their technical assistance.
Solid samples of metal nanoparticles for XRD analysis were obtained by
the addition of a solvent mixture of acetone and ethanol (1:1) to induce
the precipitation. The solid was collected by centrifugation and washed
with a solvent mixture of water and ethanol (1:99) to remove the impuri-
ties and HEPES. The solid product was dried in a vacuum oven over-
night. Calcination was done for certain samples to improve the crystallin-
ity. The calcination was done at 5508C for 6 h under an argon atmos-
phere. The calcinated sample was kept under an argon atmosphere until
the entire system was cooled to room temperature (258C).
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Received: January 25, 2010
Chem. Asian J. 2010, 5, 1322 – 1331
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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