Selective deposition of Pt on Au nanoparticles using hydrogen presorbed into Au nanoparticles during NaBH4 treatment

Article abstract of DOI:10.1016/j.electacta.2009.01.022

A facile chemical method to prepare a Pt layer selectively on Au nanoparticles is developed. The hydrogen presorbed into Au nanoparticles during NaBH₄ treatment leads to the formation of a thin Pt layer on Au nanoparticles. The Pt-coated Au nan

Full text of DOI:10.1016/j.electacta.2009.01.022

Electrochimica Acta 54 (2009) 3441–3445
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Selective deposition of Pt on Au nanoparticles using hydrogen presorbed into Au
nanoparticles during NaBH4 treatment
Srikanta Patra, Jagotamoy Das, Haesik Yang∗
Department of Chemistry and Chemistry Institute of Functional Materials, Pusan National University, 30 Jangjeon-dong, Geumjeong-gu,
Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
A facile chemical method to prepare a Pt layer selectively on Au nanoparticles is developed. The hydrogen
presorbed into Au nanoparticles during NaBH4 treatment leads to the formation of a thin Pt layer on Au
nanoparticles. The Pt-coated Au nanoparticles exhibit significantly improved electrocatalytic activity for
Received 21 October 2008
Received in revised form 27 December 2008
Accepted 8 January 2009
the electrooxidation of formic acid.
Available online 19 January 2009
© 2009 Elsevier Ltd. All rights reserved.
Keywords:
Gold nanoparticle
Platinum
Hydrogen sorption
Electrocatalysis
Formic acid
1. Introduction
the adsorption of metal ions on a foreign metal followed by their
reduction [21–23]. However, nonspecific adsorption of metal ions
In recent years, bimetallic Pt-coated Au nanomaterials have gen-
erated enormous interest because of their unique catalytic [1–4],
electronic [5,6], and optical [7–11] properties compared to their
monometallic counterparts. Continuing efforts are being made to
develop new synthetic methods that can tailor nanomaterials to
have higher activities and lower costs. Au nanoparticles (AuNPs)
have been a basic starting material for this tailoring, because Au is
relatively cheap and AuNPs can be readily prepared in various sizes
and shapes [12–14]. Among noble metals, Pt is attractive because
of its superior electrocatalytic and catalytic activities [15,16], but
Pt-based applications have been limited by the inherent high con-
sumption of expensive Pt. As an attempt to reduce Pt demands,
the formation of a Pt layer on another relatively cheap metal has
been widely employed and such Pt layers exhibit better electrocat-
alytic activity than the bulk metal [4,17–20], and in the case of a
Pt (sub)monolayer, every Pt atom can contribute to the electrocat-
alytic reaction [19].
on other surfaces via electrostatic interaction or complex forma-
tion may be problematic. UPD has been commonly employed to
prepare a uniform metal (sub)monolayer on a foreign metal. Pt
UPD on Au metal being unfavorable, Cu UPD on Au metal followed
by Cu redox displacement with Pt is generally used to produce a
Pt (sub)monolayer [11,20,24–28]. Metal growth on a foreign seed
metal using mild reducing agents such as hydroxylamine is a sim-
ple chemical route to form a metal layer [17,18,29,31], however, it
is difficult to obtain an uniform thin layer.
Pt and Pd can sorb (adsorb and absorb) a large amount of hydro-
gen [32–40], and this hydrogen can be used to reduce metal ions.
Thus, reduction of metal ions by hydrogen presorbed into metal has
been applied to forming an Au layer on Pt nanoparticles [32–34], a
Ru layer on Pt nanoparticles [35], and a Pt layer on Pd nanoparticles
[36,37]. Unlike Pt and Pd, Au does not sorb hydrogen when the metal
is in contact with hydrogen gas [41]. Hence, it has not been feasible
to form a Pt layer on Au by using hydrogen presorbed during treat-
ment of hydrogen gas. In our recent study, however, it has been
shown that hydrogen is sorbed into AuNPs during NaBH4 treat-
ment [42]. Thus sorbed hydrogen might also be used for reduction
of metal ions.
The formation of a metal layer on a foreign metal has been
achieved by (i) spontaneous deposition [21–23], (ii) underpoten-
tial deposition (UPD) [11,20,24–28], (iii) metal growth on a foreign
seed metal [4,8–10,17–19,29–31], or (iv) reduction of metal ions by
presorbed hydrogen [32–37]. Spontaneous deposition is defined as
Herein, we present a simple but efficient chemical method to
prepare a Pt layer on AuNPs by using hydrogen presorbed into
AuNPs during NaBH4 treatment. The formation of a Pt layer was
confirmed by cyclic voltammetry and XPS, and the electrocatalytic
activities of Pt-coated and uncoated AuNPs for the electrooxidation
of formic acid were investigated and compared.
∗
Corresponding author. Tel.: +82 51 510 3681; fax: +82 51 516 7421.
E-mail address: hyang@pusan.ac.kr (H. Yang).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.01.022

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S. Patra et al. / Electrochimica Acta 54 (2009) 3441–3445
Scheme 1. Schematic representation for the preparation of a Pt layer on AuNPs using
sorbed hydrogen.
2. Experimental
Indium-tin oxide (ITO)-coated glasses were obtained from
Geomatec (www.geomatec.co.jp). An aqueous solution of citrate-
stabilized AuNPs was purchased from Sigma, and the diameter of
AuNPs was 20 3 nm. All buffer reagents and other inorganic chem-
icals were supplied by Sigma, Aldrich, or Fluka. All chemicals were
used as received. All aqueous solutions were prepared in doubly
distilled water.
Fig. 1. Cyclic voltammograms of AuNP/APTES/ITO electrodes obtained in a carbon-
ate buffer solution (pH 10) after (i, ii) and before (iii) NaBH4 treatment at a scan
rate of 10 mV s−1. Curves (i) and (iii) correspond to the first scan, whereas curve (ii)
corresponds to the second scan.
A carbonate buffer (pH 10.0) solution consisted of 0.1 M sodium
carbonate and sodium bicarbonate. A Tris buffer solution (pH 9.0)
consisted of 50 mM tromethamine (Tris) and 7 mM HCl.
ITO electrodes were pretreated according to the reported proce-
dure [42]. (3-aminopropyl)triethoxysilane-modified ITO electrode
was prepared by dipping the pretreated ITO into a methanolic
solution containing 2% (3-aminopropyl)triethoxysilane (APTES) for
12 h, followed by washing with methanol and dried at 50 ◦C. AuNP-
and APTES-modified ITO (AuNP/APTES/ITO) electrodes were pre-
pared by dropping 100 ␮L of 20-nm colloidal AuNPs solution on to
APTES-modified ITO electrodes for 2 h, followed by washing with
water.
Although the anodic peak potential in curve i of Fig. 1 (ca. 0.2 V)
is higher than the standard electrode potential of normal hydro-
gen electrode (−0.22 V), it is sufficiently lower than the potential
required for reducing [PtCl4]2− ions to elemental Pt (ca. 0.54 V).
Compared to Au electrodes, Pt electrodes require low overpo-
tentials for generating hydrogen cathodically. If a Pt layer is formed
on AuNPs, the electrochemical behavior for generating hydrogen
cathodically changes substantially. The formation of Pt on AuNPs
was verified by a cyclic voltammogram of AuNP/APTES/ITO elec-
trodes obtained in a condition suitable for generating hydrogen
cathodically (i.e., in 0.1 M H2SO4) after sequential NaBH4, Pt-ion,
and NH3 treatments (curve i of Fig. 2a). Large peaks related to
hydrogen generation and oxidation by Pt were observed around
−0.3 V, indicating that a Pt layer had formed on the AuNP/APTES/ITO
electrodes after the full sequential treatment. However, without
the NaBH4 treatment, such peaks were not observed after sequen-
tial Pt-ion and NH3 treatments (curve ii of Fig. 2a). Obviously, the
NaBH4 treatment played a crucial role in forming a Pt layer on
AuNP/APTES/ITO electrodes, in which hydrogen presorbed during
the NaBH4 treatment allowed Pt deposition. Cathodic as well as the
NaBH4 treatment allows hydrogen sorption into AuNPs [42]. Hence,
a cyclic voltammogram of AuNP/APTES/ITO electrodes obtained
after sequential cathodic, Pt-ion, and NH3 treatments (curve iii of
Fig. 2a) also showed Pt-related hydrogen peaks. This result con-
firms that sorbed hydrogen is involved in forming a Pt layer on the
AuNP/APTES/ITO electrodes. Although cathodic treatment can be
used for forming a Pt layer, it requires the application of high neg-
ative potentials to an electrode, which could damage the electrode
and its surface layer. Moreover, cathodic treatment is unfeasible
when AuNPs are not in contact with an electrode. Therefore, chem-
ical treatment such as with NaBH4 is more desirable for practical
use.
The electrochemical experiment was performed using a CHI
708C potentiostat (CH instruments). The electrochemical cell con-
sisted of the modified ITO working electrode, a Pt counter electrode,
and an Ag/AgCl reference electrode.
3. Results and discussion
A schematic representation for the formation of a Pt layer
on AuNPs is shown in Scheme 1. The AuNP/APTES/ITO elec-
trodes were immersed in a Tris buffer solution (50 mM Tris–HCl,
pH 9.0) containing 10 mM NaBH4 for 15 min, or the electrodes
were potentiostatically controlled at −0.6 V for 10 min in 0.1 M
H2SO4 (treatment i of Scheme 1: NaBH4 or cathodic treatment).
During the NaBH4 or cathodic treatment, a large amount of
hydrogen was sorbed into the AuNPs [42]. Next the hydrogen-
sorbed AuNP/APTES/ITO electrodes were washed with water and
immersed in an aqueous solution containing 0.1 mM K2PtCl4, which
formed a Pt layer on the AuNPs (treatment ii of Scheme 1: Pt-ion
treatment). Unreduced but adsorbed Pt ions were removed by then
immersing the electrodes in an ethanolic solution containing 2 M
NH3 for 10 min (treatment iii of Scheme 1: NH3 treatment), thusly,
producing the Pt/AuNP/APTES/ITO electrodes prepared via NaBH4
or cathodic treatment.
The sorption of hydrogen into AuNPs of AuNP/APTES/ITO elec-
trodes was checked using cyclic voltammetry. In the first anodic
scan recorded after NaBH4 treatment of AuNP/APTES/ITO electrodes
(curve i of Fig. 1), an anodic peak was observed at 0.2 V vs. Ag/AgCl,
which is due to the electrooxidation of the sorbed hydrogen [42].
This peak was not observed in the second anodic scan (curve ii
of Fig. 1), which also coincided well with the first scan recorded
without NaBH4 treatment (curve iii of Fig. 1). These results indi-
cate that sorbed hydrogen was easily oxidized at low potentials.
A Pt layer can be formed on AuNPs by the spontaneous deposi-
tion in which [PtCl4]2− ions are adsorbed onto AuNPs and reduced.
However, [PtCl4]2− ions can also be electrostatically adsorbed onto
+
the NH3 groups of APTES/ITO electrodes at neutral pHs [43]. If the
adsorbed Pt ions are reduced, small Pt nanoparticles is formed on
the APTES layer. Hence, the selective formation of Pt on AuNPs can-
not be achieved by spontaneous deposition. To form a Pt layer only
on the AuNPs and to reduce Pt ions via only hydrogen presorbed into
AuNPs, the Pt ions unreduced but adsorbed on the AuNP/APTES/ITO
electrodes should be completely removed. Repetitive water wash-
ing cannot remove adsorbed Pt ions from the AuNP/APTES/ITO
electrodes, whereas they can be efficiently removed by NH3 treat-

S. Patra et al. / Electrochimica Acta 54 (2009) 3441–3445
3443
Fig. 2. (a) Cyclic voltammograms of AuNP/APTES/ITO electrodes obtained in deaerated 0.1 M H2SO4 at a scan rate of 50 mV s−1 after (i) sequential NaBH4, Pt-ion, and
NH3 treatments, (ii) sequential Pt-ion and NH3 treatments, or (iii) sequential cathodic, Pt-ion, and NH3 treatments. (b) Cyclic voltammogram of (i) APTES/ITO and (ii)
AuNP/APTES/ITO electrodes obtained in deaerated 0.1 M H2SO4 at a scan rate of 50 mV s−1 after Pt-ion treatment followed by washing with water and reduction of Pt ions. Pt
ions were reduced by NaBH4.
from the electrodes by NH3 treatment. A cyclic voltammogram of
APTES/ITO electrodes recorded after sequential NaBH4, Pt-ion, and
NH3 treatments (curve ii of Fig. 3) showed no Pt-related peaks,
reconfirming removal of Pt ions from the electrodes by NH3 treat-
ment and also showing that the hydrogen was sorbed only into
AuNPs during the NaBH4 treatment. Therefore, a thin Pt layer was
formed only on hydrogen-sorbed AuNPs.
The presence of Pt on AuNPs was further confirmed by X-ray
photoelectron spectroscopy (XPS) (Fig. 4). The peaks at 86.8 and
83.2 eV in spectra of AuNP/APTES/ITO electrodes (curve i of Fig. 4a)
are associated with Au 4f5/2 and Au 4f7/2 core levels, respectively
[25]. Spectra of Pt/AuNP/APTES/ITO electrodes prepared via NaBH4
treatment (curve ii of Fig. 4b) contained two additional peaks cor-
responding to Pt 4f5/2 and Pt 4f7/2 at 75.1 and 71.6 eV, respectively,
indicating the presence of Pt0 states on the AuNPs [44]. The inten-
sity of Pt signals was much lower than that of Au, which implied
that the amount of Pt was quite small compared to that of Au
[19].
Pt-coated AuNPs are very active in the electrooxidation of formic
Fig. 3. Cyclic voltammograms of APTES/ITO electrodes obtained in deaerated 0.1 M
H2SO4 at a scan rate of 50 mV s−1 (i) without any treatment or (ii) after sequential
NaBH4, Pt-ion, and NH3 treatments.
acid [17,18]. The electrocatalytic activities of Pt/AuNP/APTES/ITO
and as-prepared AuNP/APTES/ITO electrodes for the electrooxida-
tion of formic acid were compared by their cyclic voltammograms
obtained in 0.1 M H2SO4 containing 1.0 M HCOOH. The activi-
ties of the AuNP/APTES/ITO electrodes were quite low (curve i of
Fig. 5a), but after NaBH4 treatment, the activities of these elec-
trodes increased significantly (curve ii of Fig. 5a). NaBH4 treatment
does enhance the electrocatalytic activity of AuNPs [42], however,
the improved activities of AuNP/APTES/ITO electrodes were signif-
icantly lower than the activities of Pt/AuNP/APTES/ITO electrodes
prepared via NaBH4 treatment (curve iii of Fig. 5b).
ment, possibly facilitated by Pt-ion complexation with NH3. A cyclic
voltammogram of APTES/ITO electrodes obtained in 0.1 M H2SO4
after Pt-ion treatment followed by washing with water and reduc-
ing Pt ions (curve i of Fig. 2b) was similar to that of AuNP/APTES/ITO
electrodes (curve ii of Fig. 2b). Even in the absence of AuNPs, Pt-
related peaks were observed, clearly showing that Pt ions were
adsorbed on the APTES layer as well as the AuNPs and, thus, it
was essential to remove adsorbed Pt ions for the selective forma-
tion of Pt on AuNPs. Notably, curve ii of Fig. 2a shows no Pt-related
hydrogen peaks, indicating complete removal of adsorbed Pt ions
Fig. 4. (a) Au and (b) Pt XPS spectra of (i) AuNP/APTES/ITO electrodes and (ii) Pt/AuNP/APTES/ITO electrodes.

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S. Patra et al. / Electrochimica Acta 54 (2009) 3441–3445
Fig. 5. Cyclic voltammograms obtained in 0.1 M H2SO4 and 1.0 M HCOOH at a scan rate of 50 mV s−1 on (a) AuNP/APTES/ITO electrodes (i) before and (ii) after NaBH4 treatment
and on (b) Pt/AuNP/APTES/ITO electrodes prepared via (iii) NaBH4 treatment and (iv) spontaneous deposition.
During HCOOH electrooxidation on Pt electrodes, CO-poisoning
effect due to HCOOH dehydration could be a serious problem
[17,45–47]. In this case, anodic current in the forward scan is much
lower than that in the reverse scan [17]. Cyclic voltammogram for
HCOOH electrooxidation on Pt/AuNP/APTES/ITO electrodes showed
one peak at 0.7 V in the forward scan and one peak at 0.56 V in the
reverse scan (curve iii of Fig. 5b). Two peak currents are not signifi-
cantly different. It indicates that CO-poisoning effect is not serious
on this electrode.
and Technology (MEST) and the Healthcare & Biotechnology
Development Program (A050426) of the Korea Health Industry
Development Institute. This study was also financially supported
by Pusan National University in the program, Post-Doc. 2007.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2009.01.022.
The amount of Pt loading was varied by changing the time of
NaBH4 treatment (Fig. S2 in Supplementary Materials). It has been
found that the peak current for HCOOH electrooxidation increased
with increasing the treatment time. However, the cyclic voltam-
mogram on Pt/AuNP/APTES/ITO electrodes obtained after 15-min
treatment (curve iii of Fig. 5b) is similar to that after 30-min treat-
ment (curve iii of Fig. S2 in Supplementary Materials). It shows that
there was no significant change in the peak current after NaBH4
treatment for more than 15 min. It seems that the hydrogen sorption
was saturated after 15-min NaBH4 treatment.
It is interesting to note that the electrocatalytic activi-
ties of Pt/AuNP/APTES/ITO electrodes prepared via spontaneous
deposition (curve iv of Fig. 5b) were lower than those of
Pt/AuNP/APTES/ITO electrodes prepared via NaBH4 treatment
(curve iii of Fig. 5b). Moreover, the anodic peak current associated
with hydrogen oxidation in the case of spontaneous deposition was
smaller than that of NaBH4 treatment (curve i of Fig. 2a), possibly
resulting from the effect that the amount of Pt formed via NaBH4
treatment was higher than that formed via spontaneous deposition.
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