Synthesis of highly dispersed platinum nanoparticles on multiwalled carbon nanotubes and their electrocatalytic activity toward hydrogen peroxide

Article abstract of DOI:10.1016/j.jallcom.2010.06.085

Multiwalled carbon nanotubes were successfully coated with Pt nanoparticles through NH₃ gas pretreatment. Pt with particle size distribution of 1-4 nm was highly dispersed on carbon nanotubes. Cyclic voltammetry of ferrocyanide system was used

Full text of DOI:10.1016/j.jallcom.2010.06.085

Journal of Alloys and Compounds 505 (2010) 604–608
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Synthesis of highly dispersed platinum nanoparticles on multiwalled carbon
nanotubes and their electrocatalytic activity toward hydrogen peroxide
Jing Zhang, Lian Gao∗
State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Multiwalled carbon nanotubes were successfully coated with Pt nanoparticles through NH3 gas pre-
treatment. Pt with particle size distribution of 1–4 nm was highly dispersed on carbon nanotubes. Cyclic
voltammetry of ferrocyanide system was used to characterize the real electroactive surface area of plat-
inum nanoparticle–multiwalled carbon nanotube (PtNP–MWCNT) coated glassy carbon electrode, and
cyclic voltammetry for the PtNP–MWCNT coated electrode toward detecting H2O2 was measured to
characterize the electrocatalytic property of the composite. Our results showed that the electroactive
surface area of PtNP–MWCNT was higher than those of pristine multiwalled carbon nanotubes and Pt
nanoparticles, and the composite exhibited the best electrocatalytic property for H2O2.
© 2010 Elsevier B.V. All rights reserved.
Received 4 February 2010
Received in revised form 12 June 2010
Accepted 14 June 2010
Available online 25 June 2010
Keywords:
Carbon nanotube
Platinum
Composite materials
Electrochemical reactions
1. Introduction
Therefore, compositing CNTs and PtNPs have gained grow-
ing interest for utilization in amperometric enzyme biosensors,
Carbon nanotubes (CNTs) exhibit unique mechanical, elec-
trical and electrochemical properties, which have gained great
interest for the application of CNTs in biosensors. Due to their
good electronic conductivity and biocompatibility, CNTs have been
applied in many amperometric enzyme biosensors, such as glu-
cose [1], cholesterol [2], and myoglobin [3] biosensors. In addition,
owing to their fast electron-transfer ability, CNTs demonstrate
electrocatalytic activities toward many molecules like H2O2 [4], ␤-
nicotinamide adenine dinucleotide [5], and dopamine [6]. The good
electrocatalytic properties of CNTs and their good biocompatibil-
ity provide great potential for fabricating excellent amperometric
biosensors.
On the other hand, transition metal nanoparticles like gold [7],
platinum [8], palladium [9], copper [10], nickel [11] and their binary
[12–18] or ternary metals [14,19–21] are widely used to enhance
the performances of electrodes because of their high catalytic
activity. Particularly, Pt nanoparticles (PtNPs) are demonstrated
to lower the H2O2 oxidation/reduction overvoltage efficiently
[22–25], which is very important for biosensors to avoid inter-
ference from other co-oxidable substances like ascorbic acid and
acetaminophen. However, aggregation of PtNPs often makes such
electrodes prone to poisoning or corrosion, and prohibits their
applications [26].
because H2O2 is released during the oxidase-based enzyme reac-
tion [27]. CNTs, not only act as a support material to avoid the
aggregation of PtNPs, but also combine both of their advantages
to possess special properties (huge surface area, enhanced cat-
alytic activity, and good biocompatibility) [28]. As a result, the
platinum nanoparticle–carbon nanotube (PtNP–CNT) composite
modified electrodes are usually used to fabricate amperometric
enzyme biosensors.
According to previous literatures, some researchers fabricate
PtNP–CNT composite modified electrodes via physisorbing pre-
synthesized PtNPs on CNTs [29], or via electrodeposition of PtNPs
from Pt salt solution on CNTs [27,28]. However, physisorbed PtNPs
are easily removed by agitating the composite in liquid and then
formed Pt agglomeration [30–32], and electrodeposition of PtNPs
is possibly concurrent reduction of H+ [33]. More researchers prone
to synthesize PtNP–CNT composite via chemical reduction, and
then modify the electrode to investigate their biosensing proper-
ties [2,34,35]. Acid treatment is a common pretreating method to
generate acid groups on CNTs so that PtNPs are easy to be linked
onto CNTs. For example, Shi et al. [2] prepared PtNP–CNT by pre-
treating CNTs in HNO3 and applied the composite in cholesterol
biosensors. Xie et al. [35] synthesized PtNP–CNT by pretreating
CNTs in the mixture of nitric acid and sulfuric and applied in an
amperometric glucose biosensor. However, it is found to be diffi-
cult to control the particle size and dispersion of PtNPs on CNTs
surface by acid pretreatment [36]. Besides, acid treatment usually
leads to structural damage of CNTs and loss of electron conductivity
[37]. Yang et al. [38] demonstrated a new method (ultrasonic oxi-
∗
Corresponding author. Tel.: +86 21 52412718; fax: +86 21 52413122.
E-mail address: liangao@mail.sic.ac.cn (L. Gao).
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.06.085

J. Zhang, L. Gao / Journal of Alloys and Compounds 505 (2010) 604–608
605
dation) to pretreat CNTs, however, ultrasonic can make MWCNTs
short out, which may influence the electrocatalytic properties of the
composite. Other approaches like functionalizing CNTs with surfac-
tants [39] or polymers, dispersing CNTs in dimethylformamide [40],
and depositing Pt using water-in-supercritical CO2 microemulsion
[41] are also reported. However, preparing well-dispersed PtNPs on
CNTs is still a challenge. Therefore, it is necessary to explore new
ways for pretreating CNTs so that PtNPs coat CNTs with uniform
size and good dispersion.
In our previous work, the titania-carbon nanotube [42] and
gold-carbon nanotube composites were successfully fabricated
through pretreating CNTs in NH3 gas. Compared with wet chemical
treatments to CNTs, NH3 treatment may be more convenient and
introduce no impurities [43]. In this paper, we report a success-
ful method for fabricating the Pt nanoparticle–multiwalled carbon
nanotube (PtNP–MWCNT) composite through pretreating MWC-
NTs in NH3 gas, and modify the composite on the glassy carbon
(GC) electrode to detect H2O2.
size distribution of PtNPs on MWCNTs was obtained by measuring about 300
nanoparticles. The Pt content in the composite was determined by energy dispersive
spectrometer (EDS, INCA Energy, Oxford Instruments, UK) and electron probe X-
ray microanalyser (EPMA-8705QH2, Shimadzu, Japan). Images of the PtNP–MWCNT
composite were taken on a field emission scanning electron microscope (FESEM,
S-4800, Hitachi, Japan). The electrochemical experiments were conducted using the
advanced electrochemical system (PARSTAT 2273, Princeton Applied Research Co.,
Ltd., USA). A three electrode configuration was employed with a cell volume of 15 ml,
PtNP–MWCNT, pristine MWCNTs or PtNPs modified GC electrode (3 mm diameter,
CHI104) as a working electrode, an Ag/AgCl reference electrode (CHI111), and a plat-
inum wire counter electrode (CHI115). The GC electrode was polished with 1, 0.3,
and 0.05 ␮m alumina polishing slurry each time before use.
3. Results and discussion
Fig. 1(A) shows the TEM image of pristine MWCNTs. As claimed
by the producer, MWCNTs have lengths of 5–15 ␮m, diameters of
20–40 nm, and a purity of more than 95%. The TEM image of the
synthesized PtNPs is shown in Fig. 1(B), and it is observed that
every small Pt nanoparticle is highly crystalline. However, the lat-
tice fringes of all the PtNPs are random, and the dispersibility of
PtNPs is not good.
2. Experimental
Fig. 2(A) and (B) displays the TEM images of PtNP–MWCNT.
PtNPs coat MWCNTs uniformly, and almost no Pt agglomerates are
observed. From EDS measurement (not shown), the Pt content of
the PtNP–MWCNT is about 26 wt.%, which approximately agrees
with the composition in the starting mixture (28 wt.%). It means
that almost all the PtNPs deposit on MWCNTs. TEM images in high
resolution (Fig. 2(C) and (D)) reveal that every small Pt nanoparticle
coating on MWCNTs is highly crystalline and the lattice fringes of
all the PtNPs are also random. However, the aggregation of PtNPs
is avoided because MWCNTs act as a support material. Fig. 2(E)
shows that the PtNP–MWCNT has narrow PtNPs size distributions
with average diameter of about 1.9 nm. Most of the PtNPs are in
the range of 1–3 nm, with a few particles larger than 4 nm. To ver-
ify the uniformity of PtNPs deposited on MWCNTs, SEM images of
PtNP–MWCNT are provided in Fig. 3. It is observed that almost all
MWCNTs are coated with small PtNPs, and the dispersion of PtNPs
on each MWCNT is uniform.
Such small PtNPs and good dispersion on MWCNTs are
attributed to the following. Firstly, sodium citrate acts as a sta-
bilizer to prevent PtNPs from aggregating, according to Lin et al.
[44]. Negatively charged citrate ions adsorb onto Pt via electrostatic
interaction, resulting in the electrostatic repulsion between Pt par-
ticles [44]. Secondly, according to many fundamental researches
on the surface chemistry of activated carbon [45,46] and our pre-
vious studies [42,43], NH3 treatment is able to remove part of
the negatively charged acid oxygen-containing functional groups,
2.1. Chemicals
MWCNTs, prepared by chemical vapor deposition (CVD), were obtained from
Shenzhen Nanotech Port Co., Ltd. (Shenzhen, China). Hexachloroplatinic acid
hexahydrate (H2PtCl6·6H2O), sodium citrate (Na3C6H5O7·2H2O), potassium hexa-
cyanoferrate (II) trihydrate (K4[Fe(CN)6]·3H2O) and potassium chloride (KCl) with
analytical grade were used. A phosphate buffer (0.1 M, pH 7.0) was made by mixing
K2HPO4·3H2O and KH2PO4 aqueous solution in an appropriate proportion.
2.2. Preparation of PtNP–MWCNT and the modified electrodes
NH3 gas pretreated MWCNTs (denoted as NH3-MWCNTs) were obtained by
heating pristine MWCNTs at 600 ◦C for 3 h under NH3 atmosphere (>99%, 1 l/min).
Then 0.01 g NH3-MWCNTs was immersed into 100 ml of 0.6 mM sodium citrate
aqueous solution by ultrasonication. After filtration and drying, MWCNTs coated
with sodium citrate were obtained. Ten milliliters of 2 mM H2PtCl6, 90 ml of CH3OH,
and the sodium citrate-coated MWCNTs were refluxed in an oil bath at 80 ◦C for
90 min to get the PtNP–MWCNT composite. PtNPs were also synthesized by reflux-
ing the mixture of 100 ml of 0.6 mM sodium citrate aqueous solution, 10 ml of 2 mM
H2PtCl6 and 90 ml of CH3OH in an oil bath at 80 ◦C for 90 min.
For preparing modified electrodes, a 0.5 wt.% Nafion solution was firstly pre-
pared by diluting the 5 wt.% Nafion solution with phosphate buffer (0.1 M, pH 7.0).
Then, 2 mg of PtNP–MWCNT, pristine MWCNTs and PtNPs were dispersed in 1 ml
of 0.5 wt.% Nafion solutions by ultrasonication for 4 h. Finally, a 20 ␮L aliquot of
the above solution was dropped onto the GC electrode, and allowed to dry at room
temperature.
2.3. Characterization
The morphology of MWCNTs, PtNPs, and PtNP–MWCNT was characterized by
the transmission electron microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan). The
Fig. 1. TEM images of (A) pristine MWCNTs and (B) PtNPs.

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J. Zhang, L. Gao / Journal of Alloys and Compounds 505 (2010) 604–608
Fig. 2. (A and B) TEM images of PtNP–MWCNT, (C) high resolution TEM image of PtNP–MWCNT, (D) magnification of the square area in (C), and (E) PtNPs size distribution
of PtNP–MWCNT.
and introduce positively charged nitrogen-containing groups onto
MWCNTs. When sodium citrate is mixed with NH3-MWCNTs,
the negatively charged citrate ions firstly adsorb onto the pos-
itively charged nitrogen-containing groups on NH3-MWCNTs by
electrostatic attraction [43]. Then H2PtCl6 and CH3OH react with
citrate-coated MWCNTs. Since NH3 pretreatment makes most
part of NH3-MWCNTs have positively charged groups, negatively
charged citrate which acts as a stabilizer is easy to coat on the
sidewall of MWCNTs uniformly. As a result, PtNPs with small
sizes are uniformly reduced by CH3OH onto the sidewall of MWC-
NTs. For MWCNTs pretreated by acid method, there are lots of
negatively charged oxygen-containing groups, which are easily
repulsed with negatively charged citrate ions. Thus, the citrate
coatings on MWCNTs are not as uniform as those on NH3-
MWCNTs. As a result, the PtNPs are easily aggregated on the
side wall of MWCNTs. Therefore, NH3 pretreatment to MWC-
NTs is an effective way for fabricating highly dispersed PtNPs on
MWCNTs.
Cyclic voltammetry (CV) of ferrocyanide system is a common
method to characterize the real electroactive surface area of the
modified electrode [30,31,35]. Fig. 4 shows the steady-state CV
4−
(third cycle recorded) in 20 mM Fe(CN)6
and 0.2 M KCl for
PtNP–MWCNT, pristine MWCNTs and PtNPs modified electrodes
at 20 mV s−1. The well-defined oxidation and reduction peaks are
due to the Fe3+/Fe2+ redox couple. The electroactive surface area
can be estimated according to the Randles-Sevcik equation [47].
Ip = 2.69 × 105AD1/2n3/2ꢀ1/2
C
In our case, D (the diffusion coefficient of the molecule in solution),
n (the number of electrons participating in the redox reaction), ꢀ
(the potential scan rate), C (the concentration of the probe molecule
in the bulk solution) are the same. The electroactive surface area A
is directly proportional to the peak current value Ip. From Fig. 4, we
find that the ratio of the peak current of PtNP–MWCNT and MWC-
NTs modified electrode is around 2.2, which means 120% more
Fig. 3. (A and B) SEM images of PtNP–MWCNT.

J. Zhang, L. Gao / Journal of Alloys and Compounds 505 (2010) 604–608
607
important for the electrocatalytic properties of PtNP–MWCNT
toward H2O2. Therefore, the PtNP–MWCNT composite displays bet-
ter electrocatalytic activity toward H2O2 than those of pristine
MWCNTs and PtNPs.
4. Conclusions
We have demonstrated an easy strategy to attach PtNPs onto
MWCNTs through pretreating MWCNTs in NH3 gas. Highly dis-
persed PtNPs on MWCNTs with small particle size (average size
is 1.9 nm) and higher electroactive surface area was successfully
fabricated. The composite exhibited better electrocatalytic activity
toward H2O2 than those of pristine MWCNTs and PtNPs. This kind
of PtNP–MWCNT composite is potentially useful in amperometric
enzyme biosensors.
Acknowledgments
Fig. 4. Cyclic voltammograms (20 mV s−1) in 20 mM K4[Fe(CN)6] and 0.2 M KCl at
GC electrodes modified with (a) PtNP–MWCNT, (b) pristine MWCNTs, (c) PtNPs
dispersed in 0.5 wt.% Nafion solution.
This work was supported by the National Key Basic Research
Development Program of China (2005CB623605), the Shanghai
Rising-Star Program (08QA14073), the Shanghai Talents Program
Foundation, and the Shanghai Institute of Ceramics (SCX200709).
electroactive surface area are achieved after PtNPs decorating on
MWCNTs. For PtNPs modified electrode, the electroactive surface
area is small, which may be due to the aggregation of PtNPs.
According to Yang et al. [30], excellent electrocatalytic activ-
ity of the electrode toward H2O2 implies that the electrode is
able to provide a good signal transduction in fabricating amper-
ometric enzyme biosensors, because H2O2 is the product of
oxidase-based enzyme reactions. Therefore, the electrocatalytic
activity of PtNP–MWCNT, pristine MWCNTs and PtNPs modi-
fied GC electrodes for detecting H2O2 is investigated and the CV
(third cycle) is shown in Fig. 5. For the pristine MWCNTs and
PtNPs modified electrodes, current responses are small with higher
oxidation/reduction potentials. For the PtNP–MWCNT composite
modified electrode, largely increased current signals with lower
oxidation/reduction overvoltage are observed, which means that
PtNP–MWCNT exhibits the best electrocatalytic activity toward
H2O2. The good electrocatalytic property of PtNP–MWCNT is
attributed to three reasons. Firstly, the excellent electron-transfer
ability of MWCNTs improves the electrocatalytic activity of PtNPs.
Secondly, the composite with small PtNPs highly dispersed on
MWCNTs avoids the aggregation of PtNPs and furthermore avoids
the electrode poisoning. Thirdly, the PtNP–MWCNT modified elec-
trode exhibits the highest electroactive surface area, which is
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