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Chemistry Letters 2002
Preparation of Pt Nanoparticles Assembled in Multilayer Films
Yan Shen, Jianyun Liu, Aiguo Wu, Junguang Jiang, Lihua Bi, Baifeng Liu, Zhuang Li, and Shaojun Dongꢀ
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Science,
Changchun, Jilin 130022, P. R. China
(Received January 7, 2002; CL-020025)
PtCl62ꢁ anions were assembled on a glassy carbon electrode
with [tetrakis(N-methylpyridyl)porphyrinato]cobalt cations
through layer-by-layer method, then electrochemically reduced
to yield zero valent Pt nanoparticles. Regular growth and surface
morphology of themultilayer films were characterized by UV/vis,
XPS and AFM.
In recent years, the study of nanoparticles of noble metals has
been an active area because the metal particles at nanoscale are
useful as catalysts and materials with special electronic and
optical properties.1{3 Owing to their unique properties different
from bulk materials, many preparation methods of metal
nanoparticles have been developed, such as wet-chemistry4 and
electrochemical method.5 In all these preparation methods, metal
nanoparticles dispersed within a polymeric matrix offer attractive
routes for combining properties stemming from the inorganic
nanoparticle and the polymer.6;7 However, the metal nanoparti-
cles must be presynthesized by the reduction of a metal salt with a
reducing agent in solution in most cases.8
Figure 1. UV/vis absorption spectra of multilayer
films (PtCl62ꢁ/CoTMPyP)n on ITO with n ¼
1; 2; 3; ꢂ ꢂ ꢂ, 8 and 9 (curves from lower to upper),
respectively. The inset shows the relationship of
absorbance at 448 nm vs the number of bilayers.
In this article, a direct synthetic method of platinum
nanoparticles with three dimensional order on a glassy carbon
electrode (GCE) based on layer-by-layer (LBL) formation is
presented. The preparation route is similar to the strategy used to
prepare Pd nanoparticles reported recently.9 A glassy carbon
electrode was first derivatized with 4-aminobenzoic acid (4-
ABA) according to the published procedures,10 and [tetrakis(N-
indicate that after reduction, most of the chlorine is released from
the metal-ligand, and the rest balances the positive charge of
CoTMPyP in the multilayer films.
Atomic force microscopy (AFM) imaging provides more
detailed information involving the formation of Pt nanoparticles
after electrochemical reduction of PtCl6 anions in multilayer
methylpyridyl)porphyrinato]cobalt (CoTMPyP) cations and
2ꢁ
PtCl6
interaction. Then, PtCl6
anions were alternately assembled by electrostatic
2ꢁ
2ꢁ
anions sandwiched between the
films, and the surface morphology and homogeneity of the films.
Figure 2 presents AFM images of the multilayer films on
(PtCl62ꢁ/CoTMPyP)6/4-ABA/GCE. Before electrochemical re-
duction (Figure 2A), the modified film exhibites a poor surface
morphology because of the relatively rough surface of the glassy
carbon. After the film was electrochemically reduced at ꢁ0:7 V
for 30 minutes (Figure 2B), Pt nanoparticles can be seen clearly.
They are homogeneously distributed on the electrode surface.
The average diameter of Pt nanoparticles is 6–10 nm. Such Pt
nanoparticles in multilayer films exhibit excellent catalytic
activity for the reduction of dioxygen. Rotating disk electrode
(RDE) voltammetry and rotating ring-disk electrode (RRDE)
voltammetry have been carried out, and confirmed that these Pt
nanoparticles can catalyze four electron reduction of O2 to water
in 0.5 M H2SO4 solution. Thus-prepared noble metal Pt
nanoparticles may be used as a promising electrode materials in
fuel cell.
CoTMPyP layers were electrochemically reduced applying
potential step negatively to yield zero valent Pt particles.
Figure 1 shows the UV/vis absorption spectra of 1; 2; 3; ꢂ ꢂ ꢂ, 8
and 9 bilayers of the PtCl62ꢁ/CoTMPyP film on ITO surface,
respectively. A characteristic absorption band of CoTMPyP at
448 nm appears. The linear increase in absorbance with the
number of layers suggests a regular deposition of the film (Figure
1 inset). After the deposition, the PtCl26ꢁ anions sandwiched
between two CoTMPyP layers of the multilayer films were
reduced by electrolysis under constant potential at ꢁ0:7 V in an
N2-saturated 0.1 M KCl solution. X-ray photoelectron spectrum
(XPS) measurements were made to confirm the complete
2ꢁ
reduction of PtCl6 anions and the formation of Pt (0) on
multilayer-modified GCE. (The figure not shown.) The Pt (4f7=2
)
and Pt (4f5=2) peaks are present at 74.6 and 77.9 eV, respectively,
prior to reduction. After electrochemical reduction, both of the
peaks shift to 71.4 and 74.7 eV, respectively, which is consistent
with the change in oxidation state of platinum from þ4 to 0.11
Importantly, the peak strength of chlorine element weakens
greatly after reduction, while the peak strength of other elements
such as N and Co has negligible changes. These observations
Copyright Ó 2002 The Chemical Society of Japan
Shen, Yan
