Field Assisted Synthesis

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Field-assisted synthesis of SERS-active silver nanoparticles using conductingpolymers

Ping Xu,*ab Sea-Ho Jeon,a Nathan H. Mack,a Stephen K. Doorn,a Darrick J. Williams,a Xijiang Hanb

and Hsing-Lin Wang*a

Received 10th February 2010, Accepted 29th March 2010

DOI: 10.1039/c0nr00106f

A gradient of novel silver nanostructures with widely varying sizes and morphologies is fabricated

on a single conducting polyanilinegraphite (P-G) membrane with the assistance of an external

electric field. It is believed that the formation of such a silver gradient is a synergetic consequence

of the generation of a silver ion concentration gradient along with an electrokinetic flow of silver

ions in the field-assisted model, which greatly influences the nucleation and growth mechanism of

Ag particles on the P-G membrane. The produced silver dendrites, flowers and microspheres, with

sharp edges, intersections and bifurcations, all present strong surface enhanced Raman

spectroscopy (SERS) responses toward an organic target molecule, mercaptobenzoic acid (MBA).

This facile field-assisted synthesis of Ag nanoparticles via chemical reduction presents an

alternative approach to nanomaterial fabrication, which can yield a wide range of unique

structures with enhanced optical properties that were previously inaccessible by other syntheticroutes.

Introduction

Conducting polymers have been the subject of numerous inves-

tigations due to their potential in electronic and optical devices,

such as light-emitting diodes, sensors, batteries, and electro-

chemical supercapacitors.17 Specifically, polyaniline (PANI) has

emerged as the prime candidate for commercial applications due

to its lower price tag, ease of processing and environmental

stability.810 It has long been recognized that certain metal ions

having a reduction potential higher than that of a conducting

polymer can be reduced by the conducting polymer to form zero-valent metals.11,12 Recently, we have shown that a variety of

experimental parameters, such as dopants, metal ion solution

concentration, as well as the structure and composition of the

conducting polymer, can profoundly influence the overall shape

and size of the electrolessly deposited metal nanostructures.7,1315

It was also found that platinum and palladium nanostructures

chemically deposited on a PANI membrane were highly efficient

catalysts for regioselective hydrosilylation reactions and selective

hydrogenation of alkynes and cinnamaldehyde.16,17 The palla-

dium nanoparticles supported on polyaniline nanofibers have

been used as active catalysts for Suzuki coupling between aryl

chlorides and phenylboronic acid, and for phenol formation

from aryl halides and potassium hydroxide in water and air. 18 Itcan be imagined that nanocomposites consisting of metal

nanostructures and conducting polymers will have even broader

applicability in many other areas. One such application involves

generating materials with dramatically enhanced surface Raman

spectroscopy (SERS) sensitivities to form the basis of a rapid and

reliable biological and chemical sensing platform. SERS is widely

used to amplify Raman scattering signals of absorbed molecules

on a nanostructured metal surface. The origins of this enhance-

ment are primarily due to the strong light-induced electric field,

which strongly depends on the roughness/morphology of the

metal particles.19 Structures with sharp edges, intersections, and

bifurcations typically exhibit extremely strong signal enhance-

ments,2023 and therefore, construction of metal nanoparticles

into well-defined dimensionalities and shapes is of great interest

for SERS applications.2426

Here, we introduce the facile synthesis of nanostructured silver

gradients with varying morphologies on a PANI-graphite (P-G)

membrane with the assistance of an external electric field. In the

absence of an electric field, only silver microspheres assembled

from smaller silver nanosheets are obtained. A possible mecha-

nism for the generation of such silver nanostructures with

different morphologies is also proposed. The deposited silver

nanostructure gradient on the P-G membrane can be used as an

effective SERS substrate for detecting surface absorbed organic

molecules.

Experimental

Materials

Polyaniline (PANI) emeraldine base (EB) powder was obtained

from Aldrich. Graphite (99.9% Nanostructured & Amorphous

Materials Inc. Los Alamos, NM), N-methyl-2-pyrrolidone

(NMP, 99% Aldrich), heptamethyleneimine (HPMI, 98% Acros),

AgNO3 (99.9999% Aldrich), citric acid (99.9% Fisher) and

mercaptobenzoic acid (MBA, Aldrich 90%) were used as

received.

aC-PCS, Los Alamos National Laboratory, Los Alamos, NM 87545, USA.E-mail: [email protected] of Chemistry, Harbin Institute of Technology, Harbin150001, China. E-mail: [email protected]

Electronic supplementary information (ESI) available: EDAX, XRD,and SEM images. See DOI: 10.1039/c0nr00106f

1436 | Nanoscale, 2010, 2, 14361440 This journal is The Royal Society of Chemistry 2010

PAPER www.rsc.org/nanoscale | Nanoscale


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Fabrication of P-G membranes

The P-G membranes are produced by employing a phase inver-

sion method using water as the coagulation bath.16,17 Here a 5

wt% mixture of graphite and polyaniline is used as the active

material. In a typical experiment, 1.0925 g PANI (EB) powder

and 0.0575 g graphite powder were mixed in a 12 ml Teflon vial.

Then, 4.14 g NMP and 0.747 g HPMI were added. The mixture

was stirred for 0.51 h to form a homogeneous solution, followedby being poured onto a glass substrate and spread into a wet film

using a gardeners blade (Pompano Beach, FL) with a controlled

thickness. The wet film was then immersed into a water bath for

24 h, after which the resulting solid membrane was spontane-

ously delaminated from the glass substrate. The membrane was

then dried at room temperature for 12 h before doping in 0.25 M

citric acid for 3 days.

Preparation of Ag nanostructures on P-G membrane

For preparation of silver gradients on P-G membrane, a direct

current (DC) electric field was firstly applied to the membrane

(see the ESI). Then, one drop of 25 mM AgNO3 solution (1 mL)

was deposited on the membrane. One minute later, the

membrane was repeatedly rinsed with distilled water to remove

silver nitrate residue, followed by air drying for 2 h. To investi-

gate the silver structures produced on P-G membranes without

an electric field, a P-G membrane was immersed in an AgNO 3solution of varying concentrations for one minute, then rinsed

with distilled water and dried in air.

Characterization

Scanning electron microscopic (SEM) images were taken on an

FEI Inspect F SEM to study the morphologies of the silver

nanoparticles. The elemental composition was analyzed by

energy-dispersive X-ray spectroscopy (EDX). X-Ray diffraction(XRD) measurements were carried out on a Rigka Ultima III

diffractometer that uses fine line sealed Cu-Ka tube (l 1.5406A) X-rays. Transmission electron microscopic (TEM) and high

resolution TEM (HR-TEM) images were measured on a JEOL

3000F TEM. TEM samples were prepared by scratching the Ag

structures off of the PANI membrane onto a carbon-coated

copper grid. The metal-supported P-G membrane was immersed

in an MBA ethanol solution (3 mg/5 ml) for 15 min and then

rinsed in fresh ethanol prior to the surface-enhanced Raman

spectroscopy (SERS) measurements. The SERS spectra were

recorded on a Kaiser Raman spectrometer through a 20/0.50

microscope objective, coupled to a liquid nitrogen-cooled

charge-coupled device (CCD) detector (wavelength: 785 nm).

The incident laser power was kept at 1 mW and total accumu-

lation times of 10 s were employed.

Results and discussion

The P-G membrane was fabricated according to a slightly

modified literature procedure, where graphite was added to the

PANI powder mixture prior to membrane casting in order to

enhance its overall electrical conductivity. In a typical silver

nanostructure synthesis, a direct current (DC) voltage was first

applied to the P-G membrane. Then, 1 mL of 25 mM AgNO3

solution was dropped on to the P-G membrane that had previ-

ously been doped with citric acid, where a gradient of silver

particles spontaneously formed within 60 s (Fig. 1a). The

elemental composition of all the structures was confirmed by

energy-dispersive X-ray spectroscopic (EDX) analysis to be pure

silver and not silver salt. The XRD pattern of the structures on

the P-G membrane further verifies the presence of silver metals,

with the diffraction peaks at 2q 38.04, 44.20, 64.40, 77.32 and

81.52 corresponding to the (111), (200), (220), (311) and (222)crystal planes, respectively, of face-centered cubic silver (JCPDS

65-2871). The calculated intensity ratio between the (111) and

(200) is much higher than that of bulk silver, indicating that these

silver structures mainly grow along the [111] crystal plane (see the

ESI). The broad peak centered at 2q 24 in the XRD pattern

is ascribed to the amorphous polyaniline and graphite in the P-G

membrane substrate.15,27,28

The morphologies of the silver gradients produced on the P-G

membrane can be divided into five general areas (G1G5), where

each contains distinct Ag particle morphologies, as shown in

a series of scanning electron microscopic (SEM) images (Fig. 1b

f). Structures found on the P-G membrane near the negative

electrode (G1) exhibited highly branched, dendritic silvermorphologies up to 20 mm in length (Fig. 1b). The branches of

the dendritic structures are uniform with a diameter of approx-

imately 100 nm, a stark contrast from silver dendrites prepared

Fig. 1 (a) Macroscopic image of Ag gradient produced on a P-G

membrane under an electric field of 20 V for 1 min; and magnified SEM

images of Ag nanostructures from different parts on the P-G membrane:

(b)(f) correspond to the produced silver structures in regions G1G5

respectively.

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via chemical reduction of Ag+ ions by a zinc or copper plate,29,30

and electrochemical deposition on Ni/Cu electrodes at a poten-

tial of2.0 V,31 which have highly branched feather-like struc-

tures. Silver structures found slightly further from the negative

electrode (G2) consisted of a mixture of shorter dendrite

(compared to those at G1) and flower-like silver structures. The

silver flowers here have similar features to the dendrites in G1,

and presumably act as nucleation sites for their subsequent

longer dendrite growth. Moving further from the negative elec-trode (G3), flower-like structures become dominant, which

consist of larger three dimensional assemblies of Ag nanosheets

with thicknesses of approximately 70 nm, quite different from

those obtained by an electrochemical approach on a Pt film. 32

This trend continues with increasing distance from the negative

electrode, where uniformly smaller flower-like silver structures

are observed (G4). At the opposite side of the P-G membrane,

near the positive electrode, there is a region where no silver

deposition is observed (white frame in Fig. 1a), presumably due

to electrostatic repulsion of Ag ions by the electric field. Inter-

estingly, at the area between Ag deposition and bare PANI

surface (G5), silver microspheres are formed, which are

comprised of many densely packed 50 nm thick nanosheets. Itshould be noted that similar silver gradients are formed using

a variety of electric field potentials that range from 10 to 100 V

(see the ESI). At voltages higher than 200 V, the P-G membrane

breaks down, and voltages lower than 10 V cannot induce the

formation of silver gradients.

The transmission electron microscopic (TEM) images of the

Ag dendrites, flowers and microspheres are shown in Fig. 2. A

high-resolution TEM (HR-TEM) image of the Ag dendrites and

the corresponding selected area electron diffraction (SAED)

pattern are shown in Fig. 2b, which indicate that the marked area

is dominated by single crystalline growth along the [111] crystal

plane. Similarly, the HR-TEM image and SAED pattern of the

Ag flowers (Fig. 2d) also indicate single crystalline morphologiesalong the [111] crystal plane. From the TEM image of the Ag

microspheres (Fig. 2e), it is apparent that they are comprised of

Ag nanosheets, and HR-TEM and SAED analyses of the marked

area (Fig. 2f) show favorable growth along the [111] crystal

plane. A dihedral angle of 70.1 can be distinguished, which is

close to the theoretical value of 70.5.33 However, at the junction

of two Ag nanosheets, the d spacing was measured to be 0.209

nm, close to the theoretical value of [100] d spacing, 0.204 nm.

This may explain why even though Ag dendrites, flowers and

microspheres are almost exclusively single crystalline along the

[111] plane, a minute (200) diffraction peak can still be seen in the

XRD pattern.

The formation of such silver nanoparticle gradients on the P-Gmembranes is easily understood as an electrophoretic effect

resulting from field driven movement of the Ag+ ions toward the

negative electrode. The resulting Ag+ concentration gradient

proceeds to have direct impacts on the observed Ag nano-

structures. In the absence of graphite loading, no Ag gradients

can be formed, as the conductivity of the membrane is so low that

no differential electric field can be generated, thus motion of the

Ag+ ions cannot be induced. The concentration gradient formed

in the presence of an electric field is believed to be responsible for

the widely varying growth of nanostructured silver, whose

morphology is concentration dependent. However, it was found

that the concentration gradient alone does not produce all of the

varying silver structures seen in Fig. 1. Simply varying the Ag ion

concentrationin the absence of an electric fieldtends to yield

only Ag microspheres constructed of even finer Ag nano-

structures (Fig. 3). Yet, skeleton-like Ag structures were

produced when the P-G membrane was immersed in 5 mM

AgNO3 aqueous solution, indicating that to some extent,

morphological control of Ag structure can be realized by varying

the silver ion concentration. Increasing AgNO3 concentration

beyond 10 mM results in Ag microspheres composed of finer

sheet-like structures. With the increase in the concentration, thesize of the microspheres grew larger, with the sheet-like structures

becoming finer and more densely packed. Of note, however, is

that no concentration-related dendritic growth is observed in

these static (non-field driven) conditions. As previously demon-

strated, sheet-like structure growth of silver is an intrinsic char-

acteristic of citric acid-doped PANI.13

The above analyses demonstrate the effect of concentration

gradient and electrokinetic flow on the formation of nano-

structured Ag on a P-G membrane. As schematically illustrated

in Fig. 4, we hypothesize that the relative nucleation and growth

mechanisms of these Ag structures under static and field driven

Fig. 2 TEM and HR-TEM images of Ag dendrites (a, b), flowers (c, d)

and microspheres (e, f). HR-TEM images were taken from the marked

area shown in TEM images. Selected area electron diffraction (SAED)

patterns are inset in HR-TEM images.



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conditions are dramatically impacted by the movement of Ag+

ions. In the static case, Ag+ ions simply diffuse vertically from the

bulk solution down to the membrane surface. Upon interacting

with an active nucleation site on the PANI (an electron donor),

the Ag+ ion is reduced to form zero-valent Ag (Fig. 4a). Once

a metallic nucleation particle is formed, subsequent growth is

directed by the diffusion kinetics of the Ag+ ions in the bulk

solution. Static growth of Ag on the P-G membrane in this

regime results in microspheres with fine sheet-like substructures.

Conversely, when an electric field is applied to the P-G

membrane, the positively charged Ag+

ions are driven toward thenegative electrode, forming a Ag+ ion concentration gradient

(Fig. 4b). Here, the motion of Ag+ ions is a combination of

a vertical diffusion towards the surface and lateral electrokinetic

flow towards the cathode. When these Ag+ ions driven by elec-

trokinetic flow interact with the P-G membrane (or a nucleated

Ag particle on the membrane), the lateral movement of Ag ions is

believed to alter the growth mechanism of the Ag structures to

one that favors unidirectional dendritic growth as compared to

static (omnidirectional) growth. These effects are manifested as

dramatic changes in the Ag morphology as a function of distancefrom the cathode, showing a novel approach to Ag nano-

structure generation that was previously synthetically inacces-

sible.

The highly branched, dendritic silver structures promise to

have numerous SERS applications, as they contain ample

features consistent with electromagnetic hot spots necessary

for efficient SERS excitation. The relative SERS activities of

different silver gradient structures were measured from a self-

assembled monolayer of mercaptobenzoic acid (MBA) on the

metal surface using a 785 nm Raman apparatus in a normal

incidence, backscatter configuration. During the measurement,

the beam was focused on the Ag structures from different areas

through the microscope. MBA is suitable for SERS analysis as itreadily adsorbs to Au and Ag surfaces, and has intense benzene

ring stretches at 1075 cm1 and 1580 cm1, making it readily

identifiable in SERS spectra.34 The spectra taken on the various

metal morphologies are shown in Fig. 5. MBA has weak elec-

tronic interactions with nanostructured silver surfaces and does

not absorb at the Raman excitation frequency,21 thus the SERS

mechanism mainly results from the electromagnetic enhance-

ment associated with the Ag nanostructured surfaces. Notably,

one can see that all three structure regimes (dendrites, flowers,

and spheres) exhibit relatively strong SERS signals. Ag dendrites

and flowers have a relatively stronger SERS response as

compared to that of the microspheres, presumably due to the

differences in morphology and relative surface areas.35,36 Thesedata are consistent with a previous report where Au nanoflowers

exhibit strong SERS responses, attributed to the substantially

Fig. 4 Schematic illustration of the motion behaviors of silver ions

toward the P-G membrane surface and the evolution of the silver struc-

tures under different conditions: (a) without an external electric field; (b)

with a proper external electric field.

Fig. 5 SERS spectra of mercaptobenzoic acid (MBA) absorbed on (a)

silver dendrites, (b) silver flowers and (c) silver microspheres.

Fig. 3 SEM images of Ag structures prepared by immersing the P-G

membrane (without applying any electric field) in (a) 5 mM, (b) 10 mM,

(c) 25 mM and (d) 100 mM AgNO3 aqueous solution for 1 min. Scale bar:

1 mm.

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enhanced local electromagnetic fields generated by their unique

surface topography.37 We believe strongly overlapping electric

fields are present in these interstitial sites of Ag flowers and

dendrites, which result in hot spots along with corresponding

intense SERS activity.38 For the densely packed Ag micro-

spheres, the nanocavities formed between the neighboring

nanosheet structures may support electromagnetic enhance-

ments; however, the densely packed structure also reduces the

total surface area accessible to the adsorbed molecules, therebylimiting the number of SERS-active molecules (self-assembled

monolayer) on the metal surface, resulting in a relatively weaker

SERS response. Here, the Ag nanostructures fabricated on P-G

membranes by the field-assisted approach show comparable

SERS effect to some reported Ag films.39,40

Conclusions

In summary, we have demonstrated a field-assisted fabrication

method capable of preparing a wide range of nanostructured

silver on conducting polyanilinegraphite (P-G) membranes with

various sizes and morphologies. In the absence of an external

electric field, only silver microspheres that consist of denselypacked nanosheets are obtained. Our electric field-assisted model

suggests that the generation of a Ag+ concentration gradient

along with an electrokinetic flow of Ag+ ions dramatically

impacts the nucleation and growth of Ag particles on the P-G

membrane and leads to a wide range of Ag nanostructures that

are not readily available via other synthetic routes. Silver

dendrites, flowers and microspheres present morphologies

consistent with electromagnetic hot spots, resulting in strong

SERS responses. This work demonstrates the feasibility of using

Ag nanoparticle-decorated P-G membranes as highly efficient

SERS substrates via an extremely facile fabrication method-

ology. The field-assisted synthesis of Ag nanoparticles presents

an alternative approach to nanomaterial fabrication, whichyields a wide range of unique structures with enhanced optical

properties that were previous inaccessible.

Acknowledgements

PX thanks the support from the Joint Educational Ph.D.

Program of the Chinese Scholarship Council (CSC) and NSFC

(No. 20776032). HLW acknowledges the financial support from

Laboratory Directed Research and Development (LDRD) fund

under the auspices of DOE, BES Office of Science, and the

National Nanotechnology Enterprise Development Center

(NNEDC). This work was performed in part at the Center for

Integrated Nanotechnologies (CINT), at Los Alamos National

Laboratory (Contract DE-AC52-06NA25396) and Sandia

National Laboratories (Contract DE-AC04-94AL85000).

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