formation mechanism of silver nanoparticle 1d microstructures and their hierarchical assembly into...

PAPER www.rsc.org/nanoscale | Nanoscale

Formation mechanism of silver nanoparticle 1D microstructures and theirhierarchical assembly into 3D superstructures†

Lorenza Suber*a and William. R. Plunkettb

Received 20th May 2009, Accepted 10th September 2009

First published as an Advance Article on the web 6th October 2009

DOI: 10.1039/b9nr00072k

Flower-like silver nanoparticle superstructures are prepared by the reaction of silver nitrate and

ascorbic acid in an acidic aqueous solution of a polynaphthalene system. The three-dimensional flower-

like structure has a purely hierarchic arrangement, wherein each petal is composed of bundles of silver

particle chains, each enclosed in a polymer sheath. The ordering arises from strong adsorption of silver

ions onto the polymer and by the interplay of the redox properties of nitric and ascorbic acid. As

a result, linear silver cyanide, formed on the polymer, probably due to intrinsic electric dipole fields,

organizes the silver particle chains in dumbbell-like structures, resembling buds and flower-like

structures. By dilution and heating of the mother liquors, it is also possible to obtain single petals, i.e.

micrometer sized bundles of linearly aggregated silver nanoparticle chains, each enclosed in a polymer

sheath.

The comprehension of the hierarchic assembly of silver nanoparticles, paves the way to a facile general

method to prepare polymer–metal nanoparticle chains and flower-like superstructures.

The results of this study improve both the understanding of the formation mechanism of hierarchic

structures at mild temperatures and our ability to tailor them to sizes and shapes appropriate for

technological purposes.

1. Introduction

Noble metal nanoparticles have attracted considerable attention

in the last few decades, due to their unique optical, electric,

catalytic and magnetic properties.1a–c Many metals can now be

processed into monodisperse particles with controllable compo-

sition and structure and can be produced in large quantities at

low cost through solution-phase methods.2a–c Particle size and

shape,3a–d as collective properties of assembled metal nano-

particles in 1-dimensional (1D), 2D or 3D structures, have been

shown to greatly affect the behaviour of nanomaterials.4–6

Fabrication of complex architectures with 3D or highly ordered

nanostructures is much desired in current materials synthesis,

holding promise for advanced applications in electronics and

optoelectronics.7 It is still a great challenge, however, to develop

simple and reliable methods for the synthesis of hierarchically self-

assembled architectures with controlled morphologies, which

strongly affect the nanomaterial properties.8a–f

The simplest route is probably self-assembly, in which ordered

aggregates are formed in a spontaneous process.9 Surfactant

molecules having amphiphilic properties, usually formed by an

ionic (cationic or anionic) head and a long carbon chain body,

have the unique ability to self-organize at interfaces or in solu-

tion and can form thermodynamically stable supramolecular

aCNR-Istituto di Struttura della Materia, P.O. Box 10, 00016Monterotondo St., Italy. E-mail: [email protected] for Advanced Materials Processing, Clarkson University,Potsdam, NY 13699-5814, USA

† Electronic supplementary information (ESI) available: AdditionalXRD pattern, FT-IR spectrum, TEM and HR-TEM images. See DOI:10.1039/b9nr00072k

128 | Nanoscale, 2010, 2, 128–133

assemblies such as micelles, microemulsions, lyotropic liquid

crystals and vesicles. Recently, a second class of aqueous

lyotropic mesophases, termed chromonic liquid crystals, has

come to be better recognized and understood.10a–c They are

formed by water-soluble molecules that contain rigid poly-

aromatic cores. They do not show a clear separation of hydro-

philic and hydrophobic parts, since the hydrophilic groups that

impart water solubility are distributed all around the periphery

of the hydrophobic aromatic rings. Consequently, they do not

form micelles, nor do they show any appreciable surface activity.

The driving force for self-association is a short-range inter-

molecular attraction involving both the s- and p-bonds of the

aromatic rings.11a–b These systems may also form relatively

concentrated meso-phases, though the detailed nature of the

molecular order within the aggregates has not been determined.

In a previous work, we used Daxad 19, a sodium salt of

polynaphthalene sulfonate formaldehyde condensate, as

a dispersing agent in the preparation of silver nanoparticles.12

The resulting formation of 2D structures, tabular hexagonal

particles, platelets and strips, then induced us to further

investigate its molecular organization. As a polyaromatic system

with possible chromonic-like behaviour, Daxad could form

lamellar structures and constitute casts for the 2D silver struct-

ures. The formation of the 2D meso-structures was explained by

a polymer-assisted aggregation of anisometric silver particles.13

A growing number of reports are being published on dendritic

and flower-like organic–inorganic structures, but there is little

discussion about their formation mechanism.

It is difficult to find studies of the interplay between the

different molecular components. Without this knowledge,

the synthesis of complex structures is left to trial and error.

This journal is ª The Royal Society of Chemistry 2010

Therefore, we have tried to study the formation mechanism of

these perfect copies of natural flowers, composed of polymer and

silver particles. A better understanding could help us to mimic

Nature, while developing novel applications. For instance, the

petals, formed by linearly aggregated silver nanoparticles inside

a polymer sheath, show potential for applications in sub-wave-

length optical guiding structures, i.e. in the transport of electro-

magnetic excitation along chains of non-contacting metal

nanoparticles.14a–b Among the metals, 1D silver nanostructures are

especially attractive because bulk silver shows the highest electrical

and thermal conductivities and nanoscale silver exhibits strong

surface plasmon resonance, dependent on its size and shape.15a–c

Herein we report the syntheses of micrometre-sized polymer–

silver particle flower-like structures and bundles of polymer–silver

particle chains by reduction of silver nitrate with ascorbic acid in

an aqueous acidic solution of Daxad 19 (0.4–0.2 wt%), and

discuss their formation mechanism, morphology and structure.

2. Experimental section

Materials

Silver nitrate and ascorbic acid, purchased from Aldrich, were of

the highest purity grade. HNO3 69.7 wt% was purchased from

Fischer. Daxad 19, henceforth referred to as Daxad, (sodium salt

of polynaphthalene sulfonate formaldehyde condensate, Mw

8000) was obtained from the Hampshire Chemical Company.

The method of preparation consists of reacting naphthalene with

sulfuric acid to form naphthalene sulfonic acid. The material is

then condensed with formaldehyde and the polymerised naph-

thalene sulfonic acid molecule is neutralized by sodium.

Elemental analyses (wt%): C: 42.42; H: 3.44; S: 6.20.

Analyses and instruments

Elemental analyses (C,H,N,S) were performed with a Perkin

Elmer CHNS/O Elemental Analyser in the CNR-Laboratorio di

Microanalisi, Area della Ricerca di Roma 1.

FT-IR spectra were obtained with a FT-IR Perkin-Elmer 16F

PC spectrometer. The samples were pressed in KBr pellets.

Powder X-ray diffraction (PXRD) measurements were carried

out in the 2q range 25–80� by means of an automated powder

diffractometer using Cu Ka radiation.

Scanning electron microscopy (SEM) measurements were

performed at 20 kV with a SEM-LEO1450VP unit, equipped

with an INCA300 EDS microanalysis facility. A few drops of the

sample suspension in water were filtered through a porous

polycarbonate membrane and the filtrate on the membrane was

left to dry in air. The membrane was fixed onto the SEM stub

with some drops of silver paste and sputtered with gold to ensure

electrical conductivity.

Table 1 Preparation conditions for samples reported in the text. All concen

Sample Morphology Temperature/�C Time/h Ag NO3

1 polyhedral particles 50 1 0.182 flower-like superstructure 50–100 20 0.183 bundle superstructure 60 16 0.10


Transmission electron microscopy (TEM) and X-ray

elemental analyses were performed with a JEOL 2010 TEM/

STEM (scanning transmission electron microscope), equipped

with an Oxford Instruments ‘‘Inca’’ EDS (energy dispersive

spectrometer) system, at an accelerating voltage of 200 kV. A

drop of a dilute sample suspension was placed on a carbon-

coated grid and allowed to dry at room temperature.

Preparation of samples

Polymer–polyhedral silver particles (Sample 1, Table 1). To 250

mL of a 0.22 M AgNO3 solution thermostated at 50 � 2 �C, 1.35 g

Daxad and 28 mL HNO3 were added within one minute under

mechanical stirring. Then a solution of 10 g of ascorbic acid in

30 mL H2O was added at the rate of 1.5 mL min�1. The resulting

slurry was stirred at 50 �C for 1 h. After cooling at room temper-

ature, the suspension was centrifuged and the precipitate was

washed three times with deionised water and dispersed in water.

Polymer–silver particle flower structures (Sample 2, Table 1).

To 250 mL of a 0.22 M AgNO3 solution thermostated at 50 �2 �C, 1.35 g Daxad 19 and 28 mL HNO3 were added within

a minute under mechanical stirring. Then a solution of 10 g of

ascorbic acid in 30 mL H2O was added at a rate of 1.5 mL min�1.

The resulting slurry was stirred at 50 �C for 1 h. The temperature

was then increased to 100 �C and the stirring switched off. After

4 h the silver particles dissolved, developing gas bubbles and,

after clouding of the orange solution, the initial formation of

a precipitate was observed. After 15 h at 60 �C, the temperature

was decreased to room temperature, the mother liquors were

siphoned and the precipitate was washed three times with

deionised water and dried under vacuum. 1.48 g of a beige

powder was obtained. Elemental analyses showed a sulfur weight

percentage below 0.3% (the instrument sensitivity) and detected

10.88 and 9.64 wt% for C and N, respectively. By difference then,

the Ag content in the sample was around 79.5 wt%. On the basis

of this result, the Ag yield, referred to the initial AgNO3 moles,

was 20%.

Polymer–silver particle bundle structures (Sample 3, Table 1).

Preparation as above. 200 mL mother liquors were diluted with

water to 350 mL and maintained at 60 � 2 �C overnight. After

settling of the precipitate, the mother liquors were siphoned, the

precipitate was washed three times by centrifugation with

deionised water and dispersed in water. The dispersion re-

precipitates within a few minutes.

3. Results and discussion

An easy chemical way to obtain nanostructured silver particles is

by reduction, in aqueous solution, of Ag NO3, the most common

silver salt, with ascorbic acid (C6H8O6) according to eqn (1).

trations are final

/mol L�1 HNO3/mol L�1 Daxad 19 (wt%) Ascorbic acid/mol L�1

1.00 0.44 0.181.00 0.44 0.180.57 0.24 0.10

Nanoscale, 2010, 2, 128–133 | 129

2Ag+ + C6H8O6 % 2Ag0 + C6H6O6 + 2H+ (1)

By mechanisms not yet completely understood, the silver

atoms assemble, forming particles. In order to avoid particle

agglomeration and maintain a good dispersion in water,

a dispersing agent is usually employed. Daxad, a polymer formed

by the condensation of naphthalene sulfonic acid with formal-

dehyde, is a good dispersing agent for silver particles, even in

very strong acidic conditions where the reduction of Ag+ is

slowed down due to the decrease of the double-deprotonated

ascorbate anion, ascorbate2�. Being a stronger reducing agent

than ascorbic acid, it is responsible for reduction in basic and

neutral conditions.16 In this way, by increasing the time to reach

the Ag0 supersaturation concentration, the particle formation

process can be tailored.

However, the acidic conditions can represent a drawback for

the reducing agent ascorbic acid. The nitrate group, at low pH, is

a stronger oxidant than Ag+ and oxidizes ascorbic acid

(eqns (2–4)).

NO3� + 4H3O+ + 3e� % NO[ + 6H2O E0 ¼ 0.96 V (2)

Fig. 1 a) SEM micrograph of silver particles formed after 1 h reaction at 50 �C

particle flower-like structures (Sample 2). The bar size is 10 mm. c): SEM micro

1 mm. d): TEM micrograph of the top of a petal. The bar size is 100 nm. Inset

according to the silver cubic structure (JCPDS-04-0783).

130 | Nanoscale, 2010, 2, 128–133

Ag+ + 1e� % Ag0 E0 ¼ 0.799 V (3)

NO3� + H2O +2e� % NO2

� + 2OH� E0 ¼ 0 V (4)

In fact, after heating the grey particle dispersion (Fig. 1a) at

100 �C for 3–4 h, the solution first turns clear and orange (the

colour of the Daxad solution) while the Ag0 is oxidized again to

Ag+, and then turns cloudy.

After 15 h at 60 �C, the solution containing a beige precipitate,

is cooled to room temperature. As shown in Figs. 1b–c, the

precipitate consists of micrometer-sized flower-like structures.

The silver structure is formed by linearly aggregated silver

particles (diameter 10–15 nm) enclosed in a polymer sheath

(Fig. 1d). Selected-area electron diffraction (SAED) of silver

particle flower-like structures (Fig.1d inset) showed rings indexed

according to the silver cubic structure.

High-resolution TEM investigations of the silver particle

structures revealed the presence of different particle shapes,

among them icosahedra (Figs. 2–3). The icosahedron shape,

resulting from 20 tetrahedra sharing an apex, is frequently

observed for face-centred cubic metallic nanocrystals;3c it denotes

(Sample 1). The bar size is 1.1 mm. b) SEM micrograph of polymer–silver

graph of a single polymer–silver particle flower (Sample 2). The bar size is

: SAED pattern of silver particle flower-like structures. Rings are indexed


Scheme 2

Scheme 1

multiple twinning, one of the most common defects in metal

nanocrystals.17

Scheme 1 summarizes in 3 steps the formation of the flower-

like polymer–silver particle structures.

Silver particles are thermally unstable in the reaction solution.

By increasing the temperature to 100 �C (step 2 in Scheme 1),

they dissolve because the nitrate groups, in strong acidic condi-

tions, oxidize Ag0 to Ag+ forming nitrogen oxides. The Ag+ ions

probably remain on the Daxad, due to the presence of the co-

ordinating sulfonic groups. Nitrogen oxide then reacts with

dehydroascorbate, probably forming as an intermediate

O-nitrosoascorbate. In turn, nitrosoascorbate decomposes into

erythro-ascorbate and cyanides, perceptible by the characteristic

smell (Scheme 2).18ab

The FT-IR spectrum of Sample 2 (Fig. 4) shows an absorption

at 2164 cm�1 assigned to the stretching vibration of the ChN

group of AgCN.19 XRD peaks from AgCN are also present in the

XRD spectrum of Sample 2 (see Fig. a of the ESI†).

The CN groups further increase the Ag+–polymer interactions,

forming –Ag–ChN–Ag–ChN- linear chains parallel to the

long direction of the polymer tubular sheath as shown, in the

Fig. 3 HR-TEM micrograph of an icosahedral silver particle inside the

polymer sheath (Sample 2). The bar size is 2 nm.

Fig. 2 HR-TEM micrograph of silver particles showing twinning defects

(Sample 2). The bar size is 2 nm.


HR-TEM micrograph of Sample 2 in Fig. 5, by the orientation of

the lattice fringes with a spacing of 0.301 nm corresponding to

the 110 crystalline planes of AgCN. The preferential orientation

is confirmed by the higher intensity of the [110] peak in the XRD

spectrum of Sample 2 with respect to the AgCN powder spec-

trum (Fig. a of the ESI†). Interestingly, Zhou et al.20 in the

reduction of KAu(CN)2 by ascorbic acid with poly-

vinylpyrrolidone as a dispersant agent, have obtained similar

linear polymer–particle structures, i.e. gold nanoparticle chains

enclosed in polymer sheaths (Fig. 3 of ref. 20). The reason for the

linear assembly, not investigated by the authors, could be

explained by AuCN chains present on the polymer.

Moreover, erythro-ascorbate is able to reduce Ag+ to Ag0

(Scheme 3).18b

Silver particles then start to form linearly along the polymer,

some forming contact areas of various configurations with

neighbouring particles: ordinary boundaries or twin boundaries

(one example is shown in Fig. 2, see also Figs. c–g of the ESI†). In

all cases, the assembly is driven by minimising the particle

interface energy and reducing the exposed surface area. A further

decrease of particle surface energy is obtained by interaction of

Fig. 4 FT-IR spectra of Daxad and of polymer–silver particle flower-

like structures (Sample 2) in a KBr pellet.19

Nanoscale, 2010, 2, 128–133 | 131

Fig. 5 HR-TEM micrograph showing the AgCN lattice fringes of 110

planes in Sample 2.

the particle surface with the polymer chains that, folding around

the particles, form the tubular polymer sheath.

As regards detection of possible flower-like water-soluble

precursors, after a reaction time of 20 h, SEM and EDS

investigations of the supernatant orange liquid (see Scheme 1,

step 3) show a composite material consisting of silver and poly-

mer. In Fig. 6, the elongated structures, probably due to the –Ag–

ChN–Ag–ChN– linear chains that are growing on the polymer,

are the precursors of the flower-like silver structures.

By further heating an aqueous solution of the supernatant

liquor in fact, structures of bundles of tiny polymer rods con-

taining linearly ordered silver nanoparticles precipitate (Sample

3). It is then sufficient to heat the diluted mother liquors of the

flower-like structures (Sample 2) to obtain bundles of aligned

silver particle chains enclosed in polymer sheaths (Fig. 7).

As regards the self-assembly of the polymer–silver particles

into flower-like structures, Fig. 1b shows that the ‘‘flowers’’ in

Sample 2 form at each half of a dumbbell. The dumbbells are

Fig. 6 SEM micrograph of the water-soluble precursor of the polymer–

silver flower-like structures. The bar size is 16 mm.

Scheme 3

132 | Nanoscale, 2010, 2, 128–133

observed either alone or superimposed at their midpoints into

more complex structures. The dumbbell shape may indicate that

intrinsic electric dipoles are present, as reported for example by

Kniep et al. for the morphogenesis of fluoroapatite–gelatin

composites.21 In the present case, they might be due to the

presence on the polymer of AgCN chains. In MCN linear

compounds (M ¼ metal elements of the groups I and II in the

periodic table) in fact, electric dipoles are formed because of

electron transfer from the metal atom to the CN group.22

Once the polymer becomes hydrophobic, because of the

presence of the water-insoluble AgCN on its surface, the poly-

mer–silver particle flower-like structures precipitate.

For the precipitation of calcium phosphate in colloidal

aggregates to form meso-skeletons of interconnected inorganic

needles, Mann suggested a dynamic interplay between the

organic and the inorganic parts and attributed the filament

formation to a compromise between the hydrophobic forces

tending to fold the polymer and the hydrophilic ones instead

drawing it towards the water solvent.23a–b In this case, the pres-

ence on the polymer surface of hydrophobic AgCN chains causes

the precipitation of the dumbbell-like structures. The dumbbell-

like structure probably results from the presence on the polymer

of AgCN electric dipole fields. Each half dumbbell constitutes

a bud (see for example Fig. 1b upper right) that, when open,

reveals its perfect flower-like structure. Moreover, during the

sedimentation process, two or more dumbbell structures may

join at their mid-points forming multi-bud flower structures

(Fig. 1b).

The perfection of the flower shape shown in Figs. 1b and c, is

probably the result of mutual strong and fine chemical and

structural interactions between the inorganic and organic agents

modulated by the interplay of the redox properties of nitric and

ascorbic acid towards silver (see Schemes 2 and 3).

Silver particles, rich in 111 facets with which the polymer

probably better interacts, start to grow, tending to assume

different shapes. The particles are driven to assemble linearly

because of two contributions: i) the presence of AgCN chains

aligned in the same direction and ii) an inter-particle aggregation

mechanism tending to minimize surface energy through contact

Fig. 7 SEM micrograph of polymer–silver bundles (Sample 3). Inset:

a polymer–silver bundle showing its sub-structure. Both bars are 2.2 mm.


areas (Fig. 2) as observed by Giersig et al. in the formation of

silver nanowires.24

Work is in progress to prepare, using as form-template MCN

chains, 1D and 3D organic–inorganic superstructures with other

metals (Cu, Au) and polymers (polyvinyl alcohol, chitosan, etc.).

Finally, we and other authors have recently observed perma-

nent magnetism in Ag0 thiol-capped nanoparticles.1c,25 Cyano-

bridged metal coordination polymers are known to show

unusual magnetic behaviours.26a–b For these reasons, we have

started to investigate the magnetic behaviour of the flower-like

structures. Preliminary results, showing a hysteretic magnetic

behaviour up to room temperature, are promising.

4. Conclusions

Morphological and structural analyses have shown the forma-

tion of perfect flower-like polymer–silver particle structures.

The formation mechanism has been studied by analysing the

reaction at different intermediate times. In 1 h reactions, by

reduction of Ag+ ions by ascorbic acid, Ag0 nanoparticles,

covered by the polymer, are formed. After further heating, they

dissolve because of the oxidation of Ag+ by nitrate. Ag+ ions

probably, due to coordination to the polymer sulfonic groups,

remain on the polymer.

Due to the interplay of the redox properties of ascorbic acid

and nitric acid and to the strong coordination of Ag+ ions, AgCN

linear chains grow on the polymer. They, together with an

oriented particle growth mechanism, contribute to the formation

of silver particle chain structures. Probably due to the presence of

the electric dipole fields of the AgCN linear structure, dumbbell-

like structures are then formed, each half constituting a bud- or

flower-like structure.

To our knowledge, this is the first example of the formation of

micrometre-sized hierarchic polymer–nanoparticle structures

resembling such perfect copies of natural flowers. It is also

remarkable that they are easily obtained in water at mild

temperatures.

This study has highlighted the important function of AgCN in

the formation both of linear and of flower-like polymer–nano-

particle structures.

Acknowledgements

The authors wish to thank Dr. Patrizia Imperatori and Dr.

Alessandra Mari for help with XRD measurements and in the

preparation of the samples respectively.

The work was supported by the NSF grant DMR-010244 and

by a CNR project termed: Ricerca Spontanea a Tema Libero

(RSTL.087.008).

Notes and references

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