DSpace at VNU: Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation

DSpace at VNU: Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhe...

ORIGINAL CONTRIBUTION

Effects of heat treatment and poly(vinylpyrrolidone)

(PVP) polymer on electrocatalytic activity of polyhedral

Pt nanoparticles towards their methanol oxidation

Nguyen Viet Long&Michitaka Ohtaki&

Masayuki Nogami&Tong Duy Hien

Received: 15 January 2011 / Revised: 27 February 2011 / Accepted: 3 April 2011 / Published online: 16 June 2011

# Springer-Verlag 2011

Abstract In this paper, the polyhedral Pt nanoparticles

under control were prepared by polyol method using

AgNO3and poly(vinylpyrrolidone) (PVP) in the reduction

of H2PtCl6 with ethylene glycol (EG) Transmission

electron microscopy (TEM) and high resolution (HR)

TEM measurements were used to investigate their

charac-terization In the case of the previous removal of PVP by

washing and heating at 300°C, the specific morphologies of polyhedral Pt nanoparticles were still observed However, the removal of PVP only by heat treatment at 300°C without washing causes the significant variation of their morphology The large Pt particles were observed in the self-aggregation and assembly of the as-prepared polyhe-dral Pt nanoparticles The pure Pt nanoparticles by washing and heat treatment showed the electrocatalytic property better than PVP-Pt nanoparticles by heat treatment due to the incomplete removal of PVP and by-products from AgNO3 Therefore, the removal modes of PVP without changing their characterization are required to obtain the good catalytic performance

Keywords Pt nanoparticles Polyhedral Pt nanoparticles

Pt nanocrystals Heat treatment Poly(vinylpyrrolidone) (PVP) polymer Fuel cells Pt-based catalysts AgNO3

Introduction

At present, the pure Pt nanoparticles-based catalysts are

of great interest owing to their specific features derived from size and morphology characterizations that are exploited as efficient catalysts in both homogeneous and heterogeneous catalytic technologies [1, 2] In addition, these have led to investigate the dependence of catalytic activity on the size and shape of Pt nanoparticles

as well as the surface-to-volume ratio and quantum effect because of their potential applications in electronics, catalysis, and biology [1–5] Therefore, many considerable works were focused on controlling their morphology and

N V Long

Department of Education and Training,

Posts and Telecommunications Institute of Technology,

Km 10 Nguyen Trai,

Hanoi, Vietnam

N V Long ( *):M Nogami

Department of Materials Science and Engineering,

Nagoya Institute of Technology,

Gokiso-cho, Showa-ku,

Nagoya 466-8555, Japan

e-mail: nguyenviet_long@yahoo.com

M Nogami

e-mail: mnogami@mtj.biglobe.ne.jp

M Nogami

e-mail: nogami@nitech.ac.jp

N V Long:M Ohtaki

Department of Molecular and Material Sciences, Interdisciplinary

Graduate School of Engineering Sciences, Kyushu University,

6-1 Kasugakouen, Kasuga,

Fukuoka 816-8580, Japan

T D Hien

Laboratory for Nanotechnology, Vietnam National University,

Ho Chi Minh, Vietnam

DOI 10.1007/s00396-011-2460-6

size for the catalytic and electrocatalytic reactions [4–6].

In catalysis, Pt nanoparticles and Pt-based catalysts

catalyze most of hydrogenation, and oxidation hydration

reactions for the synthesis of important compounds [1–6]

In addition, they exhibit the electrocatalytic activity for

CO, methanol oxidation reaction in fuel cells [1]

However, there are the significant difficulties in achieving

the good performance of Pt-based catalysts [1–7] Most

recently, Pt nanoparticles have been used as catalysts for

methanol and ethanol electrooxidation [4, 8–11]

depend-ing on their definite size and shape Nowadays, their uses

with supports, for example carbon nanotubes, oxide

matrices (CeO2, ZrO2, and CeO2–ZrO2 etc.), and

func-tional hybrid materials become very necessary [1–6] As a

result, the electrocatalytic behavior and ability of catalytic

enhancement are much higher and better than Pt

nano-particles only In addition, Pt-based catalysts are also the

core components of fuel cell technologies [1] Recently,

the role of metal-support interaction on the catalytic

activity of carbon-supported Pt nanoparticles towards

oxygen reduction and methanol oxidation has also been

shown [1–3] On the enormous demands of catalytic

materials of high quality and performance, researchers

have focused on the modifications and improvements of

the main factors that control both the size and shape of Pt

nanoparticles for the synthesis of Pt-based bimetallic and

supported nanoparticles [12,13] In this paper, our effort is

made in order to control the size and shape of Pt

nanoparticles using AgNO3 and achieve their desirable

catalytic characterizations Here, we present the

prepara-tion of Pt nanoparticles with the polyhedral shape using

the addition of AgNO3in the reduction of H2PtCl6in extra

EG at 160°C Especially, the important role of PVP

polymer as a ligand in the stabilization of Pt nanoparticles

is discussed in a comparison of the electrocatalytic activity

of PVP-protected Pt nanoparticles respective to the

different PVP removals on the surfaces of Pt nanoparticles

to the electrocatalytic enhancement of pure Pt

nano-particles On the other hand, the important role of heat

treatment is meaningfully considered in obtaining the

better catalytic activity of Pt nanoparticles Interestingly,

we find that the nucleation, growth, and formation

mechanisms of large and irregular Pt nanoparticles due to

their crystal overgrowth and assembly were observed by

the attachments of PVP-protected Pt nanoparticles in their

heat treatment at high various temperatures Furthermore,

their electrocatalytic behaviors for methanol oxidation

indicated that the pure Pt nanoparticle-based catalysts

exhibit the higher electrocatalytic activity than

PVP-protected Pt nanoparticles in the same conditions of heat

treatment We concluded that PVP must be removed from

the surface of Pt nanoparticles before the experiments of

their catalytic activity

Experimental methods Chemicals

In our typical preparation process, chemicals for the chemical synthesis of polyhedral Pt nanoparticles were poly(vinylpyrrolidone) (PVP, Mw= 55,000) as a stabilizer (Sigma-Aldrich), chloroplatinic acid hexahydrate as Pt precursor (Sigma-Aldrich), ACS reagent (Fw= 517.9,

H2PtCl6·6H2O), ethylene glycol (EG) (Mw= 62.07 g mol−1, 95.5 %) (Aldrich) as both solvent and reducing agent, silver nitrate (AgNO3) as a modifying agent, metals basis (Mw= 169.88 g mol−1, 99.9999%) (Aldrich) All the solvents of analytical grade and without further purification were ethanol, acetone, and hexane (Aldrich or Sigma-Aldrich)

In addition, ionized water and distilled water were also prepared

by a Narnstead nanopure H2O purification system

Synthesis of polyhedral Pt nanoparticles

In order to make polyhedral Pt nanoparticles, we used the stock solutions of 3 mL of EG and 0.5 mL of 0.04 M AgNO3, 2 mL of the solution of 0.0625 M H2PtCl6, and

4 mL of 0.375 M PVP polymer First, every a small volume

of 0.0625 M H2PtCl6of the total volume of 2 mL and every small volume of PVP of 0.375 M are simultaneously added into the volumetric flask in many times (first the addition of

20 μL of H2PtCl6 solution in the flask, and then the addition of 40 μL of PVP each time via a syringe), every

60 s per one time very quickly so that a volume of the solution of 0.375 M PVP was two times more than a volume of the solution of 0.0625 M H2PtCl6 [14] until

2 mL of H2PtCl6 and 4 mL of 0.375 M PVP were thoroughly used The reduction of [PtCl6]−2 by EG occurred and finished for 10–30 min, which was necessary for achieving their sharp and polyhedral morphologies The resultant mixture was heated and refluxed at 160°C for the chemical reduction of [PtCl6]−2, and the color of the resultant solution became dark-brown For the sharp Pt nanoparticles (cubic, octahedral, and tetrahedral main morphologies), the reduction of [PtCl6]−2by EG occurred and finished for 10–30 min, in contrast to their unsharp shapes, the reduction of [PtCl6]−2 by EG occurred for a long time Then, the product was centrifuged at 15,000 rpm for 15 min using Sigma 3K30C-Kubota centrifuge The supernatant was separated and precipitated by adding a triple volume of acetone to remove any impurities from outside and continuing to the centrifugation step at 12,000 rpm for 30 min again The precipitate was collected and diluted in 2 mL of ethanol with sonication for 15 min

to generate the solution of colloidal Pt nanoparticles by an ultrasound generator (200 W/37 kHz) Then, 6 mL volume

of hexane was added to wash and make the dispersion

adequately, and the solution was centrifuged at 3,000 rpm

for 10 min Noticeably, the precipitate was washed several

times with the same mixture of ethanol and hexane to

remove the impurities Finally, the precipitate of pure Pt

nanoparticles was redispersed in 3 mL of ethanol to obtain

colloidal Pt nanoparticles (sample 1) To study the effects of

heat treatment to PVP-protected Pt nanoparticles, we used

the resultant solution of the product of PVP-protected Pt

nanoparticles without the removal of PVP by the procedure

of centrifuging and washing with the mixture ethanol and

n-hexane (sample 2) In cyclic voltammetry experiment, we

prepared sample 3 in the same procedure as sample 1 but Pt

nanoparticles were dispersed in 3 mL of mili-Q water In

the case of PVP-protected Pt nanoparticles, we also

prepared sample 4 in order to investigate the effect of

PVP as well as the remaining AgNO3 reagent on the

electrocatalytic activity of Pt nanoparticle catalysts

Preparation of Pt nanoparticles catalyst

In order to obtain the pure Pt nanoparticles for

electro-chemical measurements, 1 mL of the resultant solution of

as-prepared nanoparticle was washed by using a triple

volume of acetone and followed by centrifugation at

5,000 rpm Next, the solid product was re-dispersed in

ethanol and a triple volume of hexane The resultant

mixture was centrifuged at 3,000 rpm The procedure of

washing Pt nanoparticles in the mixture of ethanol/

hexane was done three times After washing, the

nano-particles were dispersed in milli-Q water in order to

achieve the fixed density of colloidal Pt nanoparticles at

1 mg/mL with the aid of ICPS analyzer The working

electrode was a glassy carbon rod (RA5, Tokai Carbon

Co., Ltd.) with a diameter of 5.2 mm First, the electrode

surface was cleaned and activated by using a kind of

polishing-cloth (Buehler Textmet) with alumina slurry

(Aldrich, particle size about 50 nm), followed by copious

washing with milli-Q water This procedure was repeated

until the surface looked like a mirror Then, 3 μg of the

Pt loading was set onto the surface of the polished

electrode (samples 3 and 4) The loaded electrodes were

dried in air for 3 h at 25°C and heated with the heating

rate of 1°C/min up to 450°C in air and a keeping time of

2 h in order to remove organic species We used a very

slow heating rate to avoid the problem of sintering of the

nanoparticles [15–17] The electrodes were allowed to

cool normally and then exposed into the flow of the

mixture of H2/N2 gases (20%, 80%) at 100°C for 3 h to

reduce the existence of PtO and ensure a pristine catalyst

surface In order to improve the mechanical stability of

electrode surfaces, more 10μL of 5 wt.% Nafion®

solution was added onto the electrode and followed by drying in air

overnight before the electrochemical measurements

Material characterization UV–vis spectra and XRD methods The volumes of the reaction mixture were collected and used in an appropriate time during the synthesis They were investigated by UV–vis–NIR spectroscopy (Ubest 570 UV–vis–NIR spectrometer) for the com-parison of kinetics and mechanisms of the formation

of Pt nanoparticles Meanwhile, 3 mL of ethanol of Pt nanoparticles was set onto the slides of special glass for the XRD method with a small area ∼1 cm2

The drops of ethanol of Pt nanoparticles were poured on glass and were dried at 80°C and 100°C for 6 h prior to use Moreover, these glass slides were treated with ethanol to remove any impurities The X-ray diffraction patterns were recorded by a diffractometer (X'Pert-Phillips) operating at 45 kV/45 mA and using Cu-Kα radiation (1.54056 Å)

Transmission electron microscopy

In order to characterize Pt nanoparticles, 30 μL of ethanol

of Pt nanoparticles (sample 1) were prepared by using a copper grid At the same time, 30 μL of PVP-protected colloidal Pt nanoparticles (sample 2) was set onto a copper grid The solvent of ethanol was slowly dried and evaporated in air at the room temperature Then, two copper grids has heated and annealed at 573 K for 4 h to remove all solvents and PVP from the surfaces of Pt nanoparticles The images of Pt nanoparticles were obtained by a transmission electron microscopy (TEM) (JEOL-JEM-2010XII) operated at 200 kV The TEM data of particle shape and size was analyzed, and the HRTEM images were taken at 200 kV with a magnification up to 1,500,000 times

Electrochemical measurements Cyclic voltammetry experiment (samples 3 and 4) was performed at room temperature using a typical setup of three-electrode electrochemical system connected to a potentiostat (SI 1287 Electrochemical Interface, Solartron) The cell was a 50-mL glass vial, which was carefully treated with the mixture of H2SO4 and HNO3, and then washed generously with milli-Q water A leak-free AgCl/ Ag/NaCl electrode (RE-1B, ALS) served as the reference and all potentials were reported vs Ag/AgCl The counter electrode was a Pt coil (002234, ALS) The electrolyte solution was bubbled with N2gas for 30 min before every measurement and a N2blanket was kept during the actual course of potential sweeping For the base voltammetry, the electrolyte was the solution of 0.1 M HClO that was

diluted from 70% concentrated solution (Aldrich) using

milli-Q water The potential window between−0.2 to 1.0 V

with a sweep rate of 50 mV/s was used For the methanol

oxidation, the electrolyte was added with 1.0 M methanol

in milli-Q water The system measurements were cycled

until the stable voltammograms were achieved The

electrochemical surface areas (ECA) was estimated by

considering the area under the curve in the hydrogen

desorption region of the forward scan and using

0.21 mC/cm2 [17] for the monolayer of hydrogen

adsorbed on a Pt surface

Results and discussion

UV–vis spectra and XRD patterns of Pt nanoparticles

Figure1a shows the typical UV–vis absorption spectra of

the samples of Pt nanoparticle without using the

centrifu-gation, 3 mL of ethanol, 30 μL of 0.0265 M H2PtCl6

solution (also, 30μL of 0.375 M PVP, 30 μL of the product

of the solution containing Pt nanoparticles) The

compari-son is carried out in the UV–vis absorption spectra of PVP,

AgNO3, and H2PtCl6 in EG and the product of

PVP-protected Pt nanoparticles with AgNO3 There are the peaks

at the centered range ∼252–259 nm due to the

ligand-to-metal charge-transfer transition of [PtCl6]−2ions We found

that Pt nanoparticles were stabilized by PVP It was proven

that there are the sharp peaks at 251–259 nm because of the

fact that the ligand field splitting of Pt5d orbital expands to

the coordination of N and/or O atoms of PVP to Pt4+ions

and Pt nanocrytals [14] The sample with only PVP

exhibited the weak absorbance comparable to those of

other samples of Pt4+ ions and Pt nanocrytals It was

stated that the initial Pt seeds leading to Pt clusters, and

leading to that ultra-small size nanocrytals were formed in

this process for a short time of 1–3 min, which mainly

related to their nucleation from Pt4+ions into Pt0clusters

and nanoclusters [18–20] Then, the nucleation and growth

processes happened simultaneously Finally, we could

obtain our final product of PVP-protected Pt nanoparticles

in the extra EG solvent

As illustrated in Scheme1, three models of tetrahedral,

cubic, and octahedral Pt NPs are stabilized by chains or

monolayers or the layers of PVP polymer because of the

fact that PVP strongly, tightly binds to the surface of the

nanoparticles and sterically blocking them from

contact-ing each other [1] In this case, PVP can bind to a metal

surface through either the carbonyl or the tertiary amine of

the pyrrolidone ring due to the covalent interactions at the

surface At relative high PVP concentration, PVP

mono-layers will be formed by the PVP chains Consequently,

PVP monolayers will surround and cover the surfaces of

Pt nanoparticles The coverage of PVP monolayers or the thick or thin layer of PVP was explained in ref [49] Figure 1b shows two typical XRD patterns of Pt nanoparticles (pure Pt nanoparticles with heat treatment and PVP-protected Pt nanoparticles with heat treatment) The results represent the property of the crystalline Pt face centered cubic (fcc) phase The peaks are obviously characterized by a (111) peak, (200), (220), (311), and (222) peaks respective to 2θ values of about 39.52°, 45.96°, 67.40°, 81.10°, and 85.58°, respectively Two XRD patterns

of Pt nanoparticles are nearly similar in their appearance and shape The XRD peaks of Pt nanoparticles are broad and comparable to those of the corresponding bulk Pt material We can use one of XRD peaks and calculate the average size of Pt nanocrystallites on the basic of the width

of the reflection according to the Debye–Scherrer equation:

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

a

b

Wavelength(nm)

PVP in EG AgNO 3 in EG under stirring (2 h) at 160 o

C PVP-protected Pt nanoparticles with AgNO 3

AgNO 3 in EG

H 2 PtCl 6 in EG

2Theta(o)

(Sample 2) with PVP (Sample 1) without PVP

Fig 1 a UV –vis spectra of PVP, AgNO 3 , H 2 PtCl 6 in EG, and the product of PVP-protected Pt nanoparticles with AgNO 3 b Two XRD patterns of Pt nanoparticles: S1 for the pure Pt nanoparticles before the heat treatment (sample 1) and S2 for PVP-protected Pt nanoparticles containing AgNO 3 by heat treatment (sample 2)

D=0.9λ/(βcosθ), where β is the full width at half maximum

(FWHM) of the peak,θ is the angle of diffraction, and λ is

the wavelength of the X-ray radiation Here, the (220)

reflections of Pt nanoparticles can be used to calculate the

average size according to the above formula Thus, the

crystallite size of Pt particle was estimated about 8 nm

The effects of AgNO3and heat treatment on the size

and morphology of Pt nanoparticles

Figures2and3show the highly polyhedral Pt nanoparticles

synthesized with the well-controlled size of 7–15 nm

(sample 1) and very sharp shapes through the introduction

of the low contents of AgNO3as size and shape-controlling reagent and the gradual addition of the precursors of PVP and H2PtCl6in the 12:1 volume ratio [14] In addition, the appearances of the sharp cubic, octahedral, and tetrahedral shapes of Pt nanoparticles with short reduction time of

H2PtCl6due to their controlled growth of (100) and (111) selective surfaces are very good for their applications in catalytic activity and reactions, especially a lot of consid-eration of sharp tetrahedral Pt nanocrystals EG acts as a solvent and reducing agent at high temperature [9] around 160°C PVP uses as a particle stabilizer and controls the shape of the Pt particles, leading to cubic, tetrahedral, and octahedral Pt nanoparticles [1] It shows that every

(a)

(b)

(c)

(d)

(e)

Scheme 1 a The structure of

polyvinylpyrrolidone (PVP)

polymer The surface atoms of

Pt metal nanoparticle strongly

coordinate with O atoms of PVP

chain [ 49 ] b –d Models of

cubic, tetrahedral, and

octahe-dral Pt nanoparticles with the

protection of PVP polymer e A

description of one short chain of

PVP polymer

octahedral nanoparticle has the fixed very sharp corners and

the oppositely fixed truncated corners These remarks

directly related to the nucleation and growth of single

nanoparticles leading to determine the competitive

direc-tions for their sharply polyhedral morphologies between

cube and octahedra It was known that polyhedral Pt

nanoparticles have been formed in the homogenous growth

by modified polyol method using AgNO3 as

structure-controlling agent [9] Here, we observed that the

morphol-ogy and size of tetrahedral Pt nanoparticles were very good

for the catalytic activity Interestingly, the crystal

over-growths of Pt nanostructures after heat treatment were

observed at the same concentrations of H2PtCl6and AgNO3

precursors when we removed PVP without going through

the processes of washing and cleaning these nanoparticles

together with the sonication processes

The aggregation of Pt nanoparticles in Fig 2 was

observed in the face-to-face and one particle-by-one particle

behavior located at the surfaces of Pt nanoparticles by their

orientated attachment and random self-assembly leading to

the short chains of Pt nanoparticles in their orders at the

nanoscale degree The results showed their morphology

with the sharp surfaces, edges, and corners indicating the

perfections of Pt surfaces and their crystal structure for

their synthesis for a short time of 5–15 min These good

morphologies would be important for solid-catalysts

engineering, especially in the areas of industrial catalysts when the demands of Pt nanoparticles of nanocube and octahedra with their size-controlled synthesis have in-creased more

It has been proven that Ag species (Ag4

2+

or Ag0) can adsorb preferentially on the facets of Pt (100) planes, and Pt-based cations can replace Ag species by a favorable electrochemical reaction of 4Ag+H2PtCl6→4AgCl+Pt(0)+ 2HCl [4, 14] The role of Ag+ enhanced the (100) growth and/or suppresses (111) growth of Pt nanoparticles However,

Ag+ was not reduced by EG at high temperature at 160°C Thus, it is strongly believed that AgNO3plays the role of the structure-directing agent to Pt nanoparticles [14] In addition, they were possibly adsorbed on the surface of (100) facets, and cations based on surfaces of Pt nanoparticles were easily

be removed by organic solvents [23, 24] Most of Pt nanoparticles in our results are so-called single crystalline structures (such as polyhedrons of cubes, cuboctahedrons, octahedrons, tetrahedrons, and truncated polyhedrons of cubes, cuboctahedrons, octahedrons, tetrahedrons), so-called twinned nanostructures (such as decahedron and icosahedron), and various irregular nanostructures [21–28] Here, the distance between the adjacent lattice fringes was estimated, and the interplanar distance was about 0.190 nm, corresponding to the interplanar distance of the (111) plane (Fig.3a) The TEM image of the assembled Pt nanoparticles

(a) (b)

(c)

50 nm

50 nm

(d)

Fig 2 TEM and HRTEM

images of polyhedral Pt

nano-particles with scale bars: a–b

50 nm and c –d 10 nm The

existence of sharp and

tetrahe-dral Pt nanoparticles in our

preparation procedure Sample 1

was heated at 300°C after PVP

was removed by the mixture of

ethanol and n-hexane The

tem-perature was kept at 300°C for

4 h (sample 1)

leads to the formation of a porous and large particle

about 100 nm even without any control method when

removing PVP polymer in Fig 4 Therefore, their

assembly of Pt nanoparticles can be controlled in order

to use the specific templates (polymers or solvents) This

offers their potential applications in catalysis and biology

The morphology of large and porous particle looks like a

cubic Pt nanoparticle The phenomenon of self-assembly was illustrated in Scheme 2 Therefore, the self-assembled templates are employed for the orientation of colloidal nanocrystals obtained by self-assembly This approach is considered as the template-directed assembly The assembly techniques of particles around droplets (polymers or solvents) will open new routes to produce

2 nm

d 1

d 2

d 3

2 nm

{111}

{100}

(f) (e)

{111}

(g)

{111}

{111}

{100}

(i)

{100}

(h)

{111}

{100}

Fig 3 HRTEM images of polyhedral Pt nanoparticles with scale bars:

a –f 2 nm g–i Models of octahedral, cubic, cuboctahedral, and

tetrahedral Pt nanoparticles and the truncated octahedral and

tetrahe-dral Pt nanoparticle a Lattice fringe for d 1 =0.193 nm Sample 1 was heated at 300°C after PVP was removed by the mixture of ethanol and n-hexane This temperature was kept at 300°C for 4 h (sample 1)

functionalized ordered materials [48], potentially useful

in biological and catalytic applications

Figures 5 and 6 show the typical TEM and HRTEM

images of Pt nanoparticles by polyol method at 160°C with

0.5 mL of 0.04 M AgNO3in extra EG solvent (sample 2)

Most of Pt nanoparticles have their various morphologies

The overgrowths of polyhedral Pt nanoparticles annealed at

300°C for 4 h were observed Clearly, the morphology of Pt

nanoparticles was significantly changed by heat treatment

During their synthesis, PVP is used to stabilize and control

the size and morphology of Pt nanoparticles against their

aggregation In addition, the amount of PVP polymer can

bind the nanoparticle surface after the catalyst synthesis

Therefore, PVP should be removed in the minimal content

before the catalytic reactions Obviously, PVP polymer

plays an important role to stabilize their morphology and

size of these Pt nanoparticles Despite the fact that these Pt

nanoparticles were put on copper grids, they still have their

interfacial interactions to their surface attachments,

aggre-gation, and self-assembly leading to form the larger Pt

particles∼20 nm and up to 60 nm in our research through

their aggregation that was introduced in the final formation

of Pd nanostructures by the heat treatment [29] The

mechanism of their surface attachment, self-aggregation,

and assembly is illustrated in Scheme2

Electrocatalytic property of Pt nanoparticles

Figure7shows the cyclic voltammograms of polyhedral Pt

nanoparticles with the different removal of PVP polymer

acquired at 50 mV/s in 0.1 M HClO4 solution The

voltammograms display the typical shape for the base

voltammetry According to Volmer–Tafel mechanism for Pt

(110) sites, the current response in the potential range between−0.046 and −0.112 V denotes the weakly bounded state of hydrogen On the other hand, the manifestation for Pt(100) sites as governed by Volmer–Heyrovsky mechanism is observed at −0.05 V, which is referred as the strongly bounded hydrogen [30] Beyond 0.5 V in the forward sweep, the PtO formation is observed The negative peak in the reverse scan at around similar potential gives its reduction Moreover, a reduction current from−0.60 to 0.15 V can be attributed to a (100) terraces with long-range adsorption state [31] The relative peak height for the two aforementioned Pt sites can give some information to the shape and yield of the as-prepared nanoparticles [9] Typical polycrystalline Pt electrode has the peak height for (110) to be relatively higher than that for the (100) sites [32] For the polyhedral shape Pt nanoparticles, a similar case is noted where the (100) peaks show smaller height than that of (100) in the voltammetric response suggesting the morphologies of Pt nanoparticles that are mainly of (111) facets and (100) minor facets, i.e., polyhedral shapes of octahedrons, tetrahedrons, and cubes The catalytic activity of nano-particles of well-defined edges and large surface bound-aries are said to be greatly dependent on the bounding planes where catalytic activity follows the (110) > (100) > (111) order, as in the case of hydrogen-related reactions [32, 33] By calculating the charge transfer for the hydrogen adsorption and desorption of the catalysts in

(b) (a)

Scheme 2 The self-aggregation and assembly mechanism of various

Pt nanoparticles without using any methods of assembling is illustrated in the case of pure Pt nanoparticles after the PVP removal with ethanol and hexane

Fig 4 TEM of polyhedral Pt nanoparticles with scale bar of 50 nm

(the removal of PVP by the mixture of ethanol and hexane without

heat treatment) The phenomena of surface attachments,

self-aggregation, and assembly were confirmed

the voltammogram, the electrochemical surface area of Pt

nanoparticle can be determined The specific charge

transfer (QH) due to hydrogen adsorption and desorption

has the equation: QH= (QTotal−Qdl)/2, where QTotaldenotes

the total amount of charge during electro-adsorption and

desorption of hydrogen on Pt sites, and the QDLrepresents

the capacitive charge due to the double layer capacitance

[34] The area under the curve in the relevant region can

give the value of QTotal, and the value of Qdl can be

obtained by taking the area under the same region but with

upper and lower boundaries as horizontal lines, each passing

on a data point just outside the hydrogen desorption/

adsorption waves The electrochemical surface area (ECA)

is then calculated as follows: ECA=QH/(0.21×LPt), where 0.21 served as the conversion factor (in mC/cm2) for a monolayer of hydrogen and LPtis the catalyst loading on the glassy carbon surface (in milligrams per square centimeter) [35] The ECA was approximately 6.75 m2/g for washed-only Pt nanoparticles, 8.56 m2/g for directly heated only-Pt nanoparticles, and 10.53 m2/g for washed and heated Pt nanoparticles It can be deduced from the hydrogen desorption/adsorption region that there exists a significant difference in the effective surface area for catalytic reactions depending on the removal scheme implemented for PVP molecules When Pt nanoparticles are simply washed (with acetone, ethanol and hexane), the removal

50 nm

50 nm

10 nm

10 nm

2 nm

2 nm

d 4

d 5

[111]

[111]

[111]

Fig 5 TEM and HRTEM images

of polyhedral Pt nanoparticles with

scale bars: a –b 50 nm, c–d 10 nm,

and e –f 2 nm Insets in c

illus-trates the overgrowths of [111]

directions Lattice fringes: d 4 =

0.261 nm and d 5 =0.191 nm.

Sample 2 was heated at 300°C

without removing PVP by the

mixture of ethanol and n-hexane.

This temperature was kept at

300°C for 4 h (sample 2)

of PVP is said to be incomplete When the sample is

heated without washing, the hydrogen desorption

improves and is slightly better than washed samples

with an increase in ECA about 27% Even so, it is still

suspected that some residual PVP and carbonaceous

species are not completely removed from the Pt surface

It is considered that Pt nanoparticles subjected to both

washing and heating demonstrated the highest ECA value

with an increase about 56%, in contrast with

washed-only samples We have attempted to perform cyclic

voltammetry on unwashed and untreated samples, but

the corresponding voltammograms were extremely poor

due to severe blocking by EG and PVP The further

studies indicated that the onset of PVP decomposition by heat treatment is observed at a minimum of 200°C where

an improved catalytic reaction is confirmed for ethylene hydrogenation [36] Our heating temperature of 450°C was conducted at a very slow rate of 1°C/min in order to prevent particle sintering [37], thereby preserving the original shape and facets of the Pt nanoparticles The effect of PVP is said to be more of steric in nature, impeding the progress of catalytic reaction from occurring

by preventing incoming reactants from reaching on the

Pt nanoparticle surface and hindering the exit for reaction products and intermediates To demonstrate the effect of the three removal schemes for PVP to catalytic

2 nm

2 nm

2 nm

2 nm

2 nm

2 nm a structural variation from the corner

d6

Fig 6 HRTEM images of

polyhedral Pt nanoparticles

with scale bars: a –f 2 nm

(the removal of PVP only by the

heat treatment at 300°C for 4 h).

Lattice fringe: d 6 =0.191 nm

(sample 2)