PAPER www.rsc.org/materials | Journal of Materials Chemistry

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online / Journal Homepage / Table of Contents for this issue

Synthesis of various crystalline gold nanostructures in water: Thepolyoxometalate b-[H4PMo12O40]3� as the reducing and stabilizing agent†‡

Guangjin Zhang,xa Bineta Keita,a Rosa Ngo Biboum,a Fr�ed�eric Miserque,b Patrick Berthet,c Anne Dolbecq,d

Pierre Mialane,d Laure Catalae and Louis Nadjo*a

Received 20th February 2009, Accepted 15th April 2009

First published as an Advance Article on the web 26th May 2009

DOI: 10.1039/b903599k

This paper reports a facile, one-pot, room-temperature synthesis, in water, of various

Au nanostructures using, as reducing agent, the mixed-valence polyoxometalate

b-H3[H4P(MoV)4(MoVI)8O40]3�. By modifying the initial concentrations of the polyoxometalate

and chloroauric acid, the morphology of the synthesized Au nanostructure can be tuned.

The whole process is a ‘‘green-chemistry-type’’ synthesis of Au0 nanostructures.

Introduction

The current strong interest in gold nanostructures is due to their

promising properties, including optical, electronic, catalytic, and

medicinal properties in various biotechnology and materials

science areas.1–5 As a consequence, intense research has been

devoted to the morphological control of these nanostructures in

recent years. The difficulty of the issues to be mastered has

generated a plethora of recipes. Even so, control of shape, in

addition to size, has enabled tuning of the optical, optoelec-

tronic, magnetic, and catalytic properties associated with these

nanostructures.6–8 To date, solution-based wet chemical synthesis

is believed to be the best route to new nanostructures.9–13

Most procedures have the use of an organic environment and

a relatively high temperature in common. Specifically for gold,

a well-documented reduction procedure involves only citrate,

metallic salt, and water in the system. The size of Au nano-

particles can be simply tuned by varying the molar ratio of citrate

and chloroauric acid, which was clearly demonstrated by Frens

and developed by Yang et al.14,15 However, this simple system

still needs to be operated at high temperature (in boiling water),

and only polycrystalline nanoparticles or nanowires can be

aLaboratoire de Chimie Physique, Groupe d’Electrochimie et dePhoto�electrochimie, UMR 8000, CNRS, Universit�e Paris-Sud, Batiment350, 91405 Orsay Cedex, France. E-mail: nadjo@lcp.u-psud.fr; Fax: +331 69 15 61 88; Tel: +33 1 69 15 77 51bLaboratoire de R�eactivit�e des Surfaces et Interfaces/CEA-Saclay DEN/DANS/DPC/SCP, Bat 391, 91191 Gif-sur-Yvette Cedex, FrancecLaboratoire de Physico-Chimie de l’Etat Solide, ICMMO, UMR 8182,CNRS, Universit�e Paris-Sud, Batiment 410, 91405 Orsay Cedex, FrancedInstitut Lavoisier de Versailles, UMR 8180, Universit�e de Versailles StQuentin 45, avenue des Etats-Unis, 78035 Versailles Cedex, FranceeLaboratoire de Chimie Inorganique, ICMMO, UMR 8182, CNRS,Universit�e Paris-Sud, Batiment 410, 91405 Orsay Cedex, France

† This paper is part of a Journal of Materials Chemistry theme issue onGreen Materials. Guest editors: James Clark and Duncan Macquarrie.

‡ Electronic supplementary information (ESI) available: General mothersolution work-up, XPS analysis of the various Au nanostructures andelectrochemical characterization of the Au0 NPs. See DOI:10.1039/b903599k

x Present address: Beijing National Laboratory for Molecular Sciences,Key Laboratory of Photochemistry, Institute of Chemistry, ChineseAcademy of Sciences, Beijing, 100190, China

This journal is ª The Royal Society of Chemistry 2009

obtained by this method. Finally, there are numerous reports on

the synthesis of monocrystalline Au nanostructures by wet

chemical methods. In most cases, selected organic capping agents

are added to get monocrystalline Au nanostructures, making

these systems complicated and non-environmentally friendly.

Except for a few examples using natural carbonaceous products

as reductants and/or capping agents in the synthesis of Au

nanostructures,16–23 usual conditions are far from those desirable

for green-chemistry-type processes, which would require simple

systems at room temperature, with energy saving and atom

economy, in an environmentally-friendly solvent such as water.24

Thus, the challenge remains to reach full morphological control

of gold nanostructures under environmentally-friendly condi-

tions. In the search for green-chemistry-type conditions for the

synthesis of metal nanostructures, it was recently demonstrated

that appropriately selected polyoxometalates (POMs), with

built-in reduction capabilities, could serve as both reducing and

capping agents in water at room temperature.25 In other words,

efforts are directed toward the synthesis and/or selection of

POMs in which one or several addenda atoms or substituent

centers could participate in electron-donating events.

POMs are anionic structures containing early transition metal

elements in their highest oxidation state, which display enormous

structural variety and exciting properties.26–30 Of particular

relevance to the present issue, most POMs display reversible

electron-transfer behavior. Their oxidized forms may only accept

electrons; in contrast, their reduced forms, owing to their elec-

tron and proton transfer and/or storage abilities, may behave as

donors or acceptors of several electrons without any significant

structural change. Such reversible charge-transfer ability makes

POMs ideal candidates for homogeneous-phase electron-

exchange reactions. Based on these considerations, a new room-

temperature synthesis method for metal nanoparticles was

introduced, using partially reduced POMs both as reductants

and stabilizers in water at room temperature. Pt, Pd, Ag and Au

nanoparticles, and even 1D Ag nanowires were successfully

synthesized by this method.25,30–35 These results were obtained

through the careful selection of appropriate POMs. This

synthesis method was reviewed recently and discussed within the

framework of metal nanoparticle preparation from metallic salts

using POMs simultaneously as reducing and capping agents.35

J. Mater. Chem., 2009, 19, 8639–8644 | 8639

Fig. 1 SPR spectra of Au nanostructures synthesized with C0POM¼ 1 mM

and g ¼ 1, 0.4 and 0.1.

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online

In particular, it was compared with the photochemical approach

with regard to energy-saving possibilities.36 An interesting survey

of the techniques used for synthesizing metal nanoparticles with

POMs as additives/stabilizers can be found in a recent paper by

Finke et al.37

We now report on the variety of gold nanoarchitectures that

can be generated using only one POM. Instead of concentrating

efforts on the synthesis of exclusively uniform gold nano-

structures, this paper explores the possibility of generating

various nano-objects with a single POM, in the absence of any

surfactant or seed, in an attempt to qualitatively understand the

mechanistic pathways that govern such systems. This approach is

expected to open the way toward procedures aimed at fully

controlling the morphology of Au nanostructures during ‘‘green-

chemistry-type’’ synthesis using POMs. To our knowledge, such

an approach has never been considered. Basically, it is hypoth-

esized that it might be possible to manipulate the overall

formation kinetics of nanostructures through the mere variation

of some operational parameters of the system, and, as a conse-

quence, that both the size and shape of these nanostructures can

be tuned, thus establishing the versatility of POMs in this type of

synthesis.

Fig. 2 TEM images of synthesized Au NPs at different molar ratios with

C0POM ¼ 1 mM and a) and b) g ¼ 1, c) g ¼ 0.4, and d) g ¼ 0.1. Note the

scale bars which show clearly the difference in size of the nanostructures

prepared using various g values.

Results and discussion

The POM selected for this work, b-H3[H4PMo12O40],36 was

synthesized electrochemically. It belongs to the b-Keggin series

and is most conveniently reformulated as b-H3[H4PMo12O40] to

highlight its reduced centers. The protonation state is valid for

pH ¼ 2.38 Preliminary assays demonstrated that this POM can

easily react with chloroauric acid at room temperature to form

Au nanostructures and that the reaction can be followed using its

distinct color changes. Routine pH measurements for all the

experiments give values that range from 2 to 3, depending on the

initial concentration of the POM. The operational parameters

used are essentially the initial concentration of the POM (C 0POM)

and the molar ratio (g) defined as an excess parameter with the

following formulation: g ¼ [metallic salt]/[POM]. Typically, the

POM concentration was between 0.1 and 2 mM, and g values

ranged from 0.1 to 10. After mixing the two mother solutions to

obtain a chosen initial molar ratio, the reaction was found to run

fast, with the solution turning to pink-red or light blue in roughly

1 min or less. The exact color nuance depends both on the initial

concentration of metallic salt, and on g, and indicates the

formation of different Au nanostructures, which are character-

ized below. Provisionally, it is worth noting that the shortness or

even the absence of an induction period, during which

the absorbance remains low and constant, stands in sharp

contrast to observations made with a mild reductant such as

aspartic acid.23

Solids were separated from the reaction mixture after centri-

fugation, washed with water, and re-dispersed in water before

analysis.

In a first series of experiments, C 0POM was kept at 1 mM, and

g values were varied from 0.1 to 5. Fig. 1 shows three Surface

Plasmon Resonance (SPR) spectra of the synthesized Au nano-

structures. For g ¼ 1, a broad SPR band was observed centered

at 590 nm, featuring a typical dipole resonance associated

with spherical or quasi-spherical Au nanoparticles (NPs).39,40

8640 | J. Mater. Chem., 2009, 19, 8639–8644

Figs. 2a and b are TEM images associated with this spectrum and

recorded at two different magnifications. In full agreement with

the SPR spectrum, the NPs are spherically shaped with a diam-

eter of around 110 nm and a scattered size distribution. A few

anisotropic and irregularly shaped structures are also visible.

Further characterization of these nanoparticles by XPS is rele-

gated to the ESI (Fig. S1a)‡ and confirms that Au0 NPs are

indeed obtained. Furthermore, this technique also revealed the

presence of phosphorus and molybdenum atoms, indicating that

POM species are deposited on the surface of the nanoparticles.

The relative atomic composition of the analyzed deposit is

58% Mo and 42% Au, taking into account the Scofield sensitivity

factors. It turns out that 10–12% of the initial POM is engaged in

the capping process of the Au NPs. The presence of POM species

capping the Au NPs is also confirmed by electrochemistry

(Fig. S2).‡

This journal is ª The Royal Society of Chemistry 2009

Fig. 4 TEM images of synthesized Au NPs, with g¼ 1, at (a) C0POM¼ 0.5 mM

and (b) C0POM ¼ 0.1 mM.

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online

Increasing g from 1 to 5 has no significant influence on either

the shape or the size of the nanoparticles. New and interesting

features occur when g is below 1. When g reaches 0.4, the color of

the solution turned from blue to violet, the SPR spectrum

observed for g ¼ 1 splits into two components, with a narrow

band blue shifted to 560 nm and a new broad band extending

over the domain from 700–1100 nm. Correlatively, the corre-

sponding TEM image (Fig. 2c) displays quasi-monodisperse

nanoparticles with a diameter decreased to 70 nm, accompanied

by numerous triangular and hexagonal nanoplates. The lower

contrast observed for the polygons suggests that they are flat,

unlike the other nanostructures around them. In turn, the SPR

band peaking at 560 nm should be associated with spherical

nanoparticles, while the broad band covering the domain

from 700–1100 nm features the in-plane dipole resonance of

nanoplates.

With a further decrease of g to 0.1, the solution turned blue.

One of the SPR bands continues blue-shifting to 530 nm, whereas

another in-plane dipole band displays an important red shift,

becomes better separated from the former band, and is better

defined with a large intensity. For g ¼ 0.1, the TEM image

(Fig. 2d) is dominated by polygon-shaped nanostructures,

including triangles and hexagons, with some nanoplates.

XRD analysis was performed to identify the crystallinity of the

nanostructures. Two typical XRD patterns for samples with

g¼ 1 and 0.1, respectively, are shown in Fig. 3. All peaks comply

with fcc gold.41 The ratio of the (111) and (200) diffraction

intensities shows a strong dependence on g. At g ¼ 1, the

intensity ratio (1.91) is almost the same as that of bulk

polycrystalline gold (1.89), indicating no predominant surface in

the nanoparticles and their polycrystalline nature. For g ¼ 0.1,

the intensity ratio between the (111) and (200) diffractions is

much larger, reaching 5.15, which indicates that the

nanoparticles are dominated by the (111) surface. These obser-

vations are in good agreement with those reported concerning

the predominance of crystalline surfaces.42

A second series of experiments was performed by changing the

initial concentration at a given molar ratio. Keeping g¼ 1, C 0POM

was varied from 0.1 to 2 mM. At an initial POM concentration of

2 mM, the reaction is too fast for useful visual observation.

Fig. 3 XRD patterns of the synthesized Au NPs at two different values

of g.

This journal is ª The Royal Society of Chemistry 2009

A large amount of black precipitate formed less than 10 s after

mixing the reactants, and the solution turned colorless. TEM

analysis showed that sub-micron-sized particles (larger than

600 nm) had formed in the solution. After several days’ aging,

some golden-colored flakes were found on the surface of the

reaction mixture.

Slower kinetics were observed when C 0POM was decreased.

Typically, with g ¼ 1, it took about 5 min for reaction of the

C 0POM ¼ 0.5 mM solution and nearly 10 min for reaction of the

C 0POM¼ 0.1 mM solution to progress to completion. Fig. 4 shows

the TEM images for these two initial concentrations with g ¼ 1

and the corresponding SPR spectra (Fig. 5). The spectrum

obtained for C 0POM ¼ 1 mM and g ¼ 1 is added for comparison.

The size of the observed nanoparticles decreased dramatically

with the decrease in C 0POM as confirmed by the following size

measurements (for g ¼ 1): 110 nm (C 0POM ¼ 1 mM), 60 nm

(C 0POM ¼ 0.5 mM) and less than 10 nm (C 0

POM ¼ 0.1 mM). The

corresponding SPR bands experience a continuous blue shift

when the C 0POM decreases: the peaks are observed respectively at

590 nm for C 0POM ¼ 1 mM, 540 nm for C 0

POM ¼ 0.5 mM and

520 nm for C 0POM ¼ 0.1 mM. These observations completely

agree with theoretical expectations based on the evolution of

nanoparticle sizes,39,40 and with experimental results on related

systems.42,43 It is worth noting that Au NPs with a diameter

ca. 10 nm or less should display quantum size effects, analysis of

which is beyond the scope of this work.

Fig. 5 SPR spectra of the Au NPs at values of C0POM with a given g of 1.

The spectra absorbances are normalized to unity.

J. Mater. Chem., 2009, 19, 8639–8644 | 8641

Fig. 6 Au nanostructures prepared with C0POM ¼ 0.1 mM and g ¼ 4

for a) and b), and g ¼ 0.2 for c) and d).

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online

At C 0POM ¼ 0.1 mM, a striking molar ratio dependence of the

synthesized gold nanostructures was observed. In addition to the

quasi-spherical nanoparticles routinely obtained for higher

initial concentrations with concentration-dependent sizes,

2D-type nanostructures appear and develop both for small and

large g values. Typically, with C 0POM ¼ 0.1 mM and g ¼ 4, the

solution turned light blue, roughly 5 min after mixing the reac-

tants. Importantly, TEM analysis revealed the presence of

twisted nanowires. These nanowires were interconnected to form

a network structure as shown in Fig. 6a. This 2D gold nanowire

network extends over a surface of several square micrometres.

The average diameter of the nanowires was approximately

10 nm. Their electron diffraction pattern consists of scattered

points corresponding to random and independent nanoparticles,

an indication of the polycrystalline nature of the nanowires

(Fig. 6b). The SPR spectrum of the nanowires is shown in Fig. 7.

It displays an almost flat absorbance curve with a broad band

extending roughly from 500 nm to 1200 nm.

Alternatively, with C 0POM ¼ 0.1 mM, another significant effect

was observed for g ¼ 0.2. The color of the colloidal solution

Fig. 7 SPR spectra of the Au nanostructures synthesized with

C0POM ¼ 0.1 mM, g ¼ 4 (Au nanowires) and C0

POM ¼ 0.1 mM, g ¼ 0.2

(Au nanoplates).

8642 | J. Mater. Chem., 2009, 19, 8639–8644

became deep blue and its spectrum showed a very large and wide

SPR band extending from the near-IR into the whole visible

range (Fig. 7). Note that the absorbance keeps increasing from

roughly 500 nm to 1200 nm, which is the upper limit of the

spectral domain that could be safely explored with our UV–vis

spectrometer in water. The associated TEM analysis images in

Figs. 6c and d show some irregular oblates, nanoplates and

nanobelts without any visible Au NPs. The electronic diffraction

patterns indicate the polycrystalline nature of these nano-

structures. The XPS analysis of these two subtly morphologically

different materials confirms that they indeed consist of

Au0 nanostructures (see the ESI, Figs. S1b and c).

It is worth noting that re-dispersion of the centrifuged solids in

water, whether they contained gold nanoparticles, nanowires or

nanoplates, gave clear colloidal solutions which remained stable

(without precipitation) over several months.

Finally, several preliminary experiments toward the charac-

terization and reactivity of the synthesized Au0 NPs were carried

out by cyclic voltammetry. For this purpose, a few mL of the

centrifuged and washed aqueous Au0 nanoparticle suspension

was deposited on a polished glassy carbon (GC) surface, and left

to dry in air at room temperature. The surface was then covered

with 3 mL of 5 wt% Nafion solution and again left to dry in air at

room temperature.33 Detailed cyclic voltammetry results are

described in the ESI. In short, the cyclic voltammogram obtained

in pure 0.5 M H2SO4 (pH ¼ 0.3) shows simultaneously

the characteristic features of bulk gold electrodes and also of

POM redox systems. This observation constitutes a new

confirmation25,30–35 that POMs remain attached to the surface of

nanostructures synthesized in their presence (Fig. S2).

The catalytic activity for oxygen reduction was studied. The

synthesized Au NPs showed a typical two-wave reduction of

oxygen (Fig. S3) in 0.4 M PBS solution (pH ¼ 7), thus indicating

the participation of two scarcely separated two-electron

steps.44,45 Also, the current for H2O2 reduction in the PBS

solution (Fig. S4) shows a linear increase in the concentration

range of 5 mM to 10 mM.

Scheme 1 sums up the morphological possibilities during the

synthesis of Au0 nanostructures by b-[H4P(MoV)4(MoVI)8O40]3�

and suggests some recyclability of the POM.

Analogous smooth and steady evolution in gold nanostructure

shape with experimental operational parameters, has been

observed previously, in an organic environment.1,42,43,46 Using

aspartic acid as the reducing, particle-stabilizing, and shape-

directing agents, Lee et al. realized the synthesis of Au

Scheme 1

This journal is ª The Royal Society of Chemistry 2009

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online

nanoplates, nanoribbons and nanowires in aqueous solution.23 Other

groups used different experimental conditions. Viswanath et al.

used potassium oxalate as the reducing and capping agent along

with polyvinylpyrrolidone (PVP) as the co-capping agent at

room temperature. However, it is worth noting that no Au

colloid was formed in the absence of PVP.42 Xia et al. show that

PVP alone can be used at 100 �C in aqueous solution to prepare

anisotropic gold nanostructures without any additional capping

agent or reductant.43 Kreiter et al. used 2-mercaptosuccinic acid

as both reducing and capping agent to prepare gold nanowires

from [AuCl4]� in boiling water.46 Adachi and coworkers47 initi-

ated the reaction at 80 �C in a slight modification of the

conventional citrate reduction method of [AuCl4]� and selected

the g conditions suitable for fabricating Au nanowires. The

criteria for achieving this goal offer the opportunity for an

interesting comparison of the influence of operational synthesis

parameters between the [AuCl4]�/b-[H4P(MoV)4(MoVI)8O40]3�

and the [AuCl4]�/citrate systems.

In addition to the relatively slow reaction kinetics, favored by

the small initial concentration of metallic salt necessary in the

two systems, the three factors considered to critically impact the

formation of nanowires can be considered and compared with

the observations made with the present POM:

(i) An insufficient amount of capping agent (citrate) was

considered to favor the tendency of primarily synthesized tiny Au

NPs to undergo fusion into wire-like structures; indeed, relatively

flat nanowires are obtained when this condition is applied to the

POM (C 0POM ¼ 0.1 mM and g ¼ 4). However, the conclusion

cannot be generalized, as flat nanoribbons are also observed

when an excess of POM (C 0POM¼ 0.1 mM and g¼ 0.2) is present.

We note, as a general observation, that flat nanostructures tend

to prevail when the excess parameter g decreases, whatever the

concentration of POM, in the low-concentration regime; such

remarks underscore the difference between the citrate-based and

the POM-based systems. In short, the presence of excess POM,

acting both as reductant and capping agent, does not necessarily

induce the exclusive formation of spherical nanostructures.

(ii) The competitive adsorption of [AuCl4]� and citrate ions on

the surface of the initially synthesized Au NPs was found to

preferentially favor [AuCl4]� even in the presence of excess citrate

ions.47 The overall outcome is an attractive interaction between

the Au NPs, and this preferential adsorption lowers the surface

charge of nanoparticles, with an increase of the van der Waals

attractive forces. In short, the interesting feature in this issue is

the demonstration that gold ions themselves play an important

role in forming and stabilizing the shape of nanowires.

(iii) Based on AFM measurements, the gold surfaces were

observed to jump toward contact when approaching around 10 nm.

As concerns these last two criteria, no such clear-cut conclu-

sions can be drawn for the present POM with the available

experiments. Actually, these criteria do not seem essential

for explaining the observations made with the POM. The

difference between the evolutions of the [AuCl4]�/citrate and the

[AuCl4]�/b-[H4P(MoV)4(MoVI)8O40]3� systems must be ascribed

to the difference in adsorbability of citrate ions, of [AuCl4]� and

of the POM on Au NPs. As a matter of fact, Mo-containing

oxometalates are known for their propensity to self-assemble on

metal and other solid surfaces.48,49 TEM observation of at least

a monolayer-thick POM shell on several metal nanostructures

This journal is ª The Royal Society of Chemistry 2009

synthesized from Mo-based POMs provides a further confir-

mation of this property.30–35 Cyclic voltammetry and XPS char-

acterizations of the Au0 nanostructures synthesized in the present

work also support this behavior. The adsorbability of the POM is

likely to exceed that of citrate or [AuCl4]�, thus minimizing the

effect of other influences. Finally, preferential adsorption of the

POM on particular planes will inhibit crystal growth in these

directions, relative to other directions.

Experimental

Mostly, standard procedures were used in these experiments, and

these are described briefly, when necessary, as part of the Results

and discussion section. The UV–vis spectra were recorded on

a Perkin Elmer Lambda 19 spectrophotometer. Transmission

electron microscopy (TEM) observations were performed with

a JEOL 100CXII transmission electron microscope at an accel-

erating voltage of 100 kV. Complete experimental details for

mother solution work-up and for XPS analysis are given in the

ESI.‡

Electrochemistry equipment, apparatus and procedures

For electrochemical studies, the source, mounting and polishing

of the glassy carbon (GC, Tokai, Japan) electrodes have been

described previously.50 The glassy carbon samples had a diameter

of 3 mm. The electrochemical set-up was an EG & G 273 A

driven by a PC with the M270 software. Potentials are quoted

against a saturated calomel electrode (SCE). The counter elec-

trode was a platinum gauze of large surface area.

Pure water was used throughout. It was obtained by passing

through a RiOs 8 unit followed by a Millipore-Q Academic

purification set. The solutions were de-aerated thoroughly for at

least 30 min with pure argon and kept under a positive pressure

of this gas during the experiments.

Conclusion

In summary, the synthesis of uniform Au nano-objects is a goal

toward which much effort is directed. The first step in this issue is

to determine and control the operational parameters that govern

the size, shape, and geometry of the various nanostructures. This

necessary endeavor has led to an abundance of reducing agents

and capping agents from which to choose. The present work

establishes the versatility of a POM in the synthesis of gold

nanostructures without any need for an organic environment. It

is demonstrated that b-[H4P(MoV)4(MoVI)8O40]3� can be used to

reduce [AuCl4]� and to generate different nanostructures via

a continuous smooth variation of shapes, the POM acting both

as a reducing and a protecting agent. These results are achieved

in water at room temperature. In addition, no side-products are

expected in this synthesis. Also, except for the amount seques-

tered as capping agent of nanostructures, the POM is recoverable

and the transformations do not prevent its recyclability, thus

making the whole process chemically efficient. It is worth

emphasizing the ability of POMs to fully reproduce nano-

structure shapes, as usually encountered with organic or complex

systems. The behavior of the present system fulfils most of the

requirements of a fully green chemistry process. Work in prog-

ress with POMs will study the kinetics and mechanisms of the

J. Mater. Chem., 2009, 19, 8639–8644 | 8643

Publ

ishe

d on

26

May

200

9. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

13/

07/2

014

22:4

5:57

. View Article Online

nanostructure transformations observed in the present paper, in

search for still better control over nanostructure shape.

Acknowledgements

This work was supported by the CNRS (UMR 8000, 8180 and

8182), the Universit�e Paris-Sud 11, the Universit�e Versailles

Saint-Quentin and the CEA-Saclay (Laboratoire de R�eactivit�e

des Surfaces et Interfaces). G.Z. thanks the European Commu-

nity for a Marie Curie International Incoming Fellowship

(contract no. 040487). The authors thank P. Beaunier, Universit�e

Paris VI, for TEM analyses.

References

1 M.-C. Daniel and D. Astruc, Chem. Rev., 2004, 104, 293.2 G. Zhang, B. Keita, C. T. Craescu, S. Miron, P. de Oliveira and

L. Nadjo, Biomacromolecules, 2008, 9, 812.3 Y. J. Xiong, J. M. Mclellan, J. Y. Chen, Y. D. Yin, Z. Y. Li and

Y. N. Xia, J. Am. Chem. Soc., 2005, 127, 17118.4 R. Bri~nas, M. H. Hu, L. P. Qian, E. S. Lymar and J. F. Hainfeld,

J. Am. Chem. Soc., 2008, 130, 975.5 L. He, M. D. Musick, S. R. Nicewarner, F. G. Salinas, S. J. Benkovic,

M. J. Natan and C. D. Keating, J. Am. Chem. Soc., 2000, 122, 9071.6 Y. Cui, Q. Wei, H. Park and C. M. Lieber, Science, 2001, 293, 1289.7 M. Bockrath, W. Liang, D. Bozovic, J. H. Hafner, C. M. Lieber,

M. Tinkham and H. Park, Science, 2001, 291, 283.8 C. Kan, X. G. Zhu and G. H. Wang, J. Phys. Chem. B, 2006, 110,

4651.9 F. Kim, S. Connor, H. J. Song, T. Kuykenda and P. D. Yang, Angew.

Chem., Int. Ed., 2004, 43, 3673.10 Y. Sun and Y. Xia, Adv. Mater., 2002, 14, 833.11 Y. Sun, B. Gates, B. Mayers and Y. Xia, Nano Lett., 2002, 2, 165.12 B. Nikoobakht and M. A. El-Sayed, Chem. Mater., 2003, 15, 1957.13 N. R. Jana, L. Gearheart and C. J. Murphy, Adv. Mater., 2001, 13,

1389.14 G. Frens, Nature Phys. Sci., 1973, 241, 20.15 X. H. Ji, X. G. Song, J. Li, Y. B. Bai, W. S. Yang and X. G. Peng,

J. Am. Chem. Soc., 2007, 129, 13939.16 S. S. Shankar, A. Rai, B. Ankamwar, A. A. Amit Singh and

M. Sastry, Nat. Mater., 2004, 3, 482.17 J. Xie, J. Y. Lee, D. I. C. Wang and Y. P. Ting, Small, 2007, 3, 672.18 S. K. Bhargava, J. M. Booth, S. Agrawal, P. Coloe and G. Kar,

Langmuir, 2005, 21, 5949.19 P. Selvakannan, S. Mandal, S. Phadtare, R. Pasricha and M. Sastry,

Langmuir, 2003, 19, 3545.20 P. Selvakannan, S. Mandal, S. Phadtare, A. Gole, R. Pasricha,

S. D. Adyanthaya and M. Sastry, J. Colloid Interface Sci., 2004,269, 97.

21 Y. Shao, Y. Jin and S. Dong, Chem. Commun., 2004, 1104.

8644 | J. Mater. Chem., 2009, 19, 8639–8644

22 F. X. Zhang, L. Han, L. B. Israel, J. G. Daras, M. M. Maye, N. K. Liand C. J. Zhong, Analyst, 2002, 127, 462.

23 Y. N. Tan, J. Y. Lee and D. I. C. Wang, J. Phys. Chem. C, 2008, 112,5463.

24 J. A. Dahl, B. L. S. Maddux and J. E. Hutchison, Chem. Rev., 2007,107, 2228.

25 B. Keita, I. M. Mbomekalle, L. Nadjo and C. Haut, Electrochem.Commun., 2004, 6, 978.

26 C. L. Hill (Guest Editor), Chem. Rev., 1998, 98, 1–389.27 M. T. Pope, in Comprehensive Coordination Chemistry II: Transition

Metal Groups 3–6, ed. A. G. Wedd, Elsevier, New York, 2004, vol.4(ch. 4.10), pp. 635–678.

28 C. L. Hill, in Comprehensive Coordination Chemistry II: TransitionMetal Groups 3–6, ed. A. G. Wedd, Elsevier, New York, 2004, vol.4(ch. 4.11), pp. 679–786.

29 B. Keita, and L. Nadjo, in Encyclopedia of Electrochemistry,ed. A. J. Bard and M. Stratmann, Wiley-VCH, Weinheim, 2006,vol. 7, pp. 607–700.

30 B. Keita, G. Zhang, A. Dolbecq, P. Mialane, F. S�echeresse,F. Miserque and L. Nadjo, J. Phys. Chem. C, 2007, 111, 8145.

31 G. Zhang, B. Keita, A. Dolbecq, P. Mialane, F. S�echeresse,F. Miserque and L. Nadjo, Chem. Mater., 2007, 19, 5821.

32 B. Keita, R. N. Biboum, I. M. Mbomekalle, S. Floquet, C. Simonnet-J�egat, E. Cadot, F. Miserque, P. Berthet and L. Nadjo, J. Mater.Chem., 2008, 18, 3196.

33 J. Zhang, B. Keita, L. Nadjo, I. M. Mbomekalle and T. Liu,Langmuir, 2008, 24, 5277.

34 A. Dolbecq, J.-D. Compain, P. Mialane, J. Marrot, F. S�echeresse,B. Keita, L. R. B. Holzle, F. Miserque and L. Nadjo, Chem.–Eur.J., 2009, 15, 733.

35 B. Keita, T. Liu and L. Nadjo, J. Mater. Chem., 2009, 19, 19.36 A. Troupis, E. Gkika, A. Hiskia and E. Papaconstantinou,

C. R. Chim., 2006, 9, 851.37 C. R. Graham, L. S. Ott and R. G. Finke, Langmuir, 2009, 25, 1327.38 E. Ishikawa and T. Yamase, Bull. Chem. Soc. Jpn., 2000, 73, 641.39 S. Link and M. A. El-Sayed, J. Phys. Chem. B, 1999, 103, 4212.40 K. Lance Kelly, E. Coronado, L. Lin Zhao and G. C. Schatz, J. Phys.

Chem. B, 2003, 107, 668.41 JCPDS file no. 04-0487.42 S. Navaladian, C. M. Janet, B. Viswanathan, T. K. Varadarajan and

R. P. Viswanath, J. Phys. Chem. C, 2007, 111, 14150.43 B. Lim, P. H. C. Camargo and Y. Xia, Langmuir, 2008, 24, 10437.44 M. S. El-Deab and T. Ohsaka, Electrochem. Commun., 2002, 4, 288.45 M. Mirdamadi-Esfahani, M. Mostafavi, B. Keita, L. Nadjo,

P. Kooyman, A. Etcheberry, M. Imperor and H. Remita, GoldBull., 2008, 41, 98.

46 K. Vasilev, T. Zhu, M. Wilms, G. Gillies, I. Lieberwirth, S. Mitler,W. Knoll and M. Kreiter, Langmuir, 2005, 21, 12399.

47 L. H. Pei, K. Mori and M. Adachi, Langmuir, 2004, 20, 7837.48 N. T. Flynn and A. A. Gewirth, Phys. Chem. Chem. Phys., 2004, 6,

1310.49 W. G. Klemperer and C. G. Wall, Chem. Rev., 1998, 98, 297.50 B. Keita and L. Nadjo, J. Electroanal. Chem., 1988, 243, 87.

This journal is ª The Royal Society of Chemistry 2009