Electrodeposition of palladium nanoparticles at the liquid�/liquidinterface using porous alumina templates

Mark Platt a, Robert A.W. Dryfe a,1,*, Edward P.L. Roberts b

a Department of Chemistry, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UKb Department of Chemical Engineering, University of Manchester Institute of Science and Technology, P.O. Box 88, Manchester M60 1QD, UK

Received 1 October 2002; received in revised form 10 April 2003; accepted 10 April 2003

Electrochimica Acta 48 (2003) 3037�/3046

www.elsevier.com/locate/electacta

Abstract

Alumina membranes, with mean pore diameters of 100 nm, have been used as templates to control the electrodeposition of

palladium. Deposition occurs at the polarised water�/organic interface, leading to the formation of nanoparticles. The particles are

formed at the mouth of the alumina pores, the locus of their formation being dictated by the position of the organic�/water interface.

It is shown that the relative position of the liquid phases with respect to the alumina is controlled by the surface wetting properties of

the liquids, rather than gravity. This in turn controls the interfacial position and hence the size of the particles deposited. The

presence of the alumina membrane prevents agglomeration. Electrochemical and electron microscopy data are presented in support

of this proposed deposition mechanism.

# 2003 Elsevier Ltd. All rights reserved.

Keywords: Liquidjliquid interface; Deposition; Nanoparticle; Template; Controlled growth

1. Introduction

The controlled production of materials on micrometer

and sub-micrometer length scales is currently the focus

of intense scientific interest [1,2]. The physical properties

and chemical reactivity of small metallic nanoparticles

can become functions of particle size [3], while the high

surface area to volume ratio leads to important applica-

tions in fields such as catalysis. An important issue is the

‘‘protection’’ of nanoparticles, to their prevent agglom-

eration to form larger structures [4]. Electrochemical

methods have been employed widely for the production

of micro and nanostructured materials, such as alumina

from the anodic oxidation of aluminium [5,6]. Electro-

chemical deposition has also been used to fabricate

controlled metallic micro and nanostructures by using

materials such as porous g-alumina as overlayers on

electrode surfaces, where the dimensions of the porous

overlayer control those of the resultant deposits [7]. This

so-called electrochemical ‘‘templating’’ method has been

applied to the production of various substrates, includ-

ing silver [8], gold [9], cadmium [10] and conducting

polymers [11,12], while the templates employed have

been extended to other porous materials such as ‘‘track-

etched’’ polycarbonate membranes [7]. Non-electroche-

mical methods, such as pulsed laser deposition, have

also been applied to the template synthesis of nanos-

tructured materials [13].

An advantage of the use of porous alumina as a

template is that the alumina may be dissolved readily

[10], to free the deposited nanoparticles. Rather than

depositing nanostructures consisting of a single mate-

rial, the template method has also been applied to the

sequential deposition of multi-metallic layers, thus

creating ‘‘barcodes’’ designed for use in biological

assaying [14]. The obvious technological applications

of nanostructured metals are enhanced by the ability to

deposit metals within alumina templates, since alumina

is widely used as a support for metal particles in

heterogeneous catalysis [15].

Beyond the technological applications of the electro-

deposition of nanostructures, there is also interest in the

fundamental aspects of this phenomenon since tem-

* Corresponding author. Fax: �/44-161-200-4559.

E-mail address: robert.dryfe@umist.ac.uk (R.A.W. Dryfe).1 ISE member.

0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/S0013-4686(03)00373-6

plated deposition allows the process to be studied under

‘‘high resolution’’ conditions, giving insights into me-

chanistic aspects of electrodeposition. Electrodeposition

presents a unique way to follow the nucleation andgrowth of structures as a function of driving force [16],

however, the initial stages of electrodeposition on solid

substrates are often controlled by strong interactions

between the nucleated nanoparticles and the substrate,

manifested as under-potential deposition and epitaxial

growth [17,18]. Electrochemical deposition can also be

performed at the interface between water and an

immiscible organic phase. Efrima and co-workers [19]have reported dendritic structures formed from silver

deposition, induced by bringing an electrode close to the

water�/dichloromethane interface.

A more readily controllable approach is the direct

polarisation of the water�/organic interface by applica-

tion of a variable potential difference using methodol-

ogy that has become well established [20], as shown

schematically in Fig. 1. Interfacial electron transfer canbe induced (see Fig. 1(a)) and, if one of the couples is a

metal precursor, the electrodeposition of metallic parti-

cles at the water�/organic interface can ensue. Deposi-

tion of copper [21], gold [22] and palladium [23,24]

particles using this approach has been reported. The

lack of a solid substrate and associated preferential

nucleation sites offers a unique way to observe the

relationship between the driving force for particlegrowth and the form of the resultant particles. However,

the lack of solid substrate also means that the particles

deposited at the water�/organic interface are mobile and

thus tend to agglomerate [24], which complicates the

analysis of the form-driving force relationship.

In a recent preliminary communication [25], we have

reported initial results from a novel strategy developed

for nanoparticle electrodeposition. Essentially, thisstrategy involves the combination of the template and

water�/organic deposition approaches discussed above:

we have applied this method to the technologically

important formation of Pd nanoparticles dispersed in a

porous g-alumina substrate, a system of relevance in

catalysis and gas separation [15,26]. We are unaware of

any previous reports of palladium nanoparticle electro-

deposition using the membrane-templated approach.The application of the template approach to the

water�/organic interface is depicted schematically in

Fig. 1. The locus of particle deposition, the water�/

organic interface, is retained at the mouth of the

alumina pores and deposition is induced by application

of an interfacial potential difference. The preliminary

report demonstrated that deposition of discrete Pd

nanoparticles, with dimensions controlled by the radiiof the membrane pores (mean value of 50 nm), could be

achieved [25]. Here, more detailed findings are pre-

sented, which fully demonstrate this route to electro-

deposition of nanosized metal particles, combining the

electrochemical data under various conditions with

SEM and TEM analysis of the deposited materials.

2. Experimental

Voltammetric experiments were performed using a

PC-controlled EG&G Model 273 potentiostat (Prince-

ton Applied Research Corp., Princeton, NJ, USA)

operating in four-electrode mode. The aqueous refer-

ence electrode was a silver�/sulfate wire and the organic

reference electrode was a silver�/silver chloride wire,

both produced in-house by oxidation of a 1 mm

diameter silver wire in the appropriate electrolyte. Allpotentials are reported with respect to these reference

electrodes. The counter electrodes were constructed in-

house by spot welding an approximately 1 cm2 area of

platinum gauze (Advent, Eynsham, UK) to a length of

platinum wire (Advent). The standard current conven-

tion for liquid�/liquid electrochemistry was followed;

namely positive currents correspond either to the

transfer of a positive charge from the aqueous to organicphase, or of a negative charge in the reverse direction.

Commercial g-alumina membranes, with nominal mean

pore diameter of 100 nm (Whatman International Ltd,

Maidstone, UK) were sealed to a 5 mm inner diameter

glass tube with a silicone sealant (RS components,

Corby, UK). Membranes were washed in water before

use. Under some conditions (see below) the membranes

were also pre-treated with trimethylchlorosilane (Al-drich Co., Gillingham, UK) by soaking in the pure

liquid for 12 h prior to experimentation, but were

otherwise used as-received. Relevant properties of the

membranes employed are summarised in Table 1. It

should be noted that the pore diameter of the mem-

branes is not uniform along the length of the pore: the

value quoted in Table 1 is the ‘‘active’’ diameter, which

determines the membrane’s use in filtration. Only thefinal 0.5 mm length of the pore is of this diameter, the

remaining ‘‘support’’ layer is two to three times larger

than the diameter of the active pore [27,28].

The cell was set up as shown in Fig. 1. The aqueous

phase comprised of ammonium tetrachloropalladate

Table 1

Data in column 1 obtained from manufacturer (Ref. [27]), data in

column 2 quoted in Ref. [28]

Property Column 1 Column 2

Mean pore diameter of ‘‘active’’ layer

(cm)

1.0�/10�5 8.3�/10�6

Standard deviation of pore diameter

(cm)

Not avail-

able

9/2.5�/

10�6

Pore density (cm�2) 1.0�/109 5.1�/109

Pore fraction 0.40 0.29

Total thickness (cm) 6.0�/10�3 6.0�/10�3

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/30463038

(supplied by Fluka, Gillingham, UK), at concentrations

between 1�/10�3 and 2�/10�3 mol dm�3, lithium

chloride (5�/10�2 mol dm�3, Lancaster Synthesis,

Morecambe, UK) and lithium sulfate (5�/10�2 mol

dm�3, Aldrich, Gillingham, UK), dissolved in water

with a resistivity of 18 MV cm obtained from a Milli-Q

purification system (Millipore, Watford, UK). The

liquid�/liquid interface was formed with water and 1,2-

dichloroethane (DCE; HPLC grade, obtained from

Lancaster Synthesis). The organic electrolyte, bis(tri-

phenylphosphoranylidene) ammonium tetrakis(penta-

fluoro)phenylborate (BTPPA TPBF20), was prepared

according to a reported metathesis procedure [29] from

equimolar amounts of bis(triphenylphosphoranylidene)

ammonium chloride (BTPPACl; Lancaster, 97%) and

lithium tetrakis(pentafluoro)phenyl borate (Boulder

Fig. 1. (a) Schematic of the deposition at the liquid�/liquid interface: (i) electron transfer at the liquid�/liquid interface; (ii) metal deposition via

electron transfer at the liquid�/liquid interface and (iii) templated metal deposition at the liquid�/liquid interface. (b) The cell configurations used for

metal deposition at the alumina-templated liquid�/liquid interface: the upper cell (stable arrangement) has the organic phase above the aqueous

phase; in the lower cell (unstable arrangement), the positions of the phases are reversed.

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/3046 3039

Scientific, Boulder, CO, 98% pure). The concentration

of BTPPA TPBF20 in the DCE phase of all cells was 5�/

10�3 mol dm�3. During deposition experiments butyl-

ferrocene (BuFc, Lancaster, 98%) was added in concen-

trations between 5�/10�4 and 1�/10�3 mol dm�3. The

reference electrode for the organic phase was formed by

placing the silver�/silver chloride wire in an aqueous

solution of BTPPACl and lithium chloride (1�/10�3

and 1�/10�2 mol dm�3, respectively). The generic

composition of the cell is summarised above (Cell 1),

where the double bar denotes the polarised interface,

and the location of the alumina membrane:

The cell configurations employed are shown schema-

tically in Fig. 1(b). The upper configuration involved

placement of the DCE phase on the upper side of the

alumina membrane, with the higher density aqueous

phase on the underside. Due to the hydrophilic nature of

the alumina surface, this arrangement was stable, with

the gravitational instability being countered by the

surface effects. The alternative configuration (the lower

one in Fig. 1(b)) inverted the placement of these phases:

the aqueous phase quickly traversed the alumina

membrane, making the establishment of a stable

liquid�/liquid interface difficult. Under these conditions,

the rate of ingress of the aqueous phase could be

minimised by the treatment of the alumina surface

with trimethylchlorosilane. Electrochemical experiments

were performed at ambient temperature (between 20 and

25 8C). Certain experiments (see below) were performed

using a Grant GD100 water bath (Grant Instruments,

Cambridgeshire, UK) at 25 8C, which was able to

control the cell temperature to 9/0.1 8C.

Following electrodeposition, the membranes were

separated from the cell and affixed to 12 mm diameter

imaging studs with Araldite adhesive (Ciba-Geigy,

Cambridge UK), for analysis via scanning electron

microscopy (SEM) and energy dispersive X-ray analysis

(EDX) using a Philips XL30 Field Emission Gun SEM

and a Philips SEM525 fitted with an EDX DX4 system.

SEM images were also taken of the freed Pd particles,

following dissolution of the alumina over a 72 h period,

by dropping a continuous stream (approximately 0.2

cm3 s�1) of 1 M HCl over the membrane. In this case,

the membranes were placed on top of a polyester filter

membrane (Poretics, Livermore, CA), also with nominal

pore diameters of 100 nm, to retain the liberated

nanoparticles. Transmission electron microscopy

(TEM) was performed using a JEOL model JEM 2000

FX on Pd-loaded membranes, which had been pre-

viously sliced transversely to give sub-micron cross-

sections using an Ultracut Microtome (Leica Micro-

systems GmbH, Wetzlar, Germany) [7].

3. Results and discussion

In the absence of a membrane at the water�/DCE

interface, cyclic voltammetry of Cell 1 displayed theresponse denoted ‘‘a’’ in Fig. 2(a), when no organic

phase BuFc was present. A capacitative current region is

delimited by the onset of current corresponding to

background electrolyte transfer at �/0.25 and �/0.90

V. Introduction of the ferrocene derivative, which

functions as an electron donor, led to the observation

of a charge transfer at approximately �/0.7 V. On the

basis of earlier reports [23,24], the current observed isattributed to reduction of the aqueous phase palladinate

salt by the ferrocene derivative. Metallic palladium is

thus formed by electron transfer at the water�/DCE

interface, according to Eq. (1):

PdCl2�4 (aq)�2BuFc(DCE)

0 Pd0�4Cl�(aq)�2BuFc�(DCE) (1)

The palladium deposits were visible, following pro-longed electrolysis, as dark irregular particles.

The experiment was repeated with the g-alumina

‘‘templating’’ the water�/DCE interface, using the upper

(stable) configuration of the cells illustrated in Fig. 1(b).

Under these conditions, the voltammetry labelled ‘‘b’’ in

Fig. 2(a) was observed, with a characteristic nucleation

loop seen during the first few voltammetric runs at

positive potentials [23]. On prolonged electrolysis, thenucleation loop was swamped by an irreversible process

(response ‘‘c’’ in Fig. 2(b)). The results are consistent

with an electrodeposition process, with palladium

growth occurring at or near the pores of the alumina

membrane. Chronoamperometry was also performed at

the alumina-templated water�/DCE interface. A typical

response is shown in Fig. 2(c), which once again shows a

characteristic peaked response, indicative of a nuclea-tion event limiting the current at short times.

SEM images were obtained of the Pd-loaded mem-

branes, and compared against their ‘‘blank’’ (i.e., not

exposed to Pd) counterparts under a variety of condi-

tions. Figs. 3(a) and (b) show SEM images of the blank

alumina membranes, both face on and side on, the latter

obtained by fracturing the membrane. The images show

that the alumina surface is rough on the approximately10 nm scale, and suggest that the pore diameter and

distribution are in reasonable agreement with previously

quoted values (listed in Table 1). Also evident are white

spots of between 100 and 300 nm diameter, which

previous reports have identified as aluminium oxide

precipitate [30]. EDX analysis (Table 2) confirms that

the blank sample contains only Al, O and relatively low

amounts of P and S, the latter from traces of the acidsused in the oxidative preparation of the membranes [6].

The side-on image also clearly shows the transition of

the pores between the support and narrower active

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/30463040

layers, although the damage to the pores induced by the

fracturing process is also evident.

Fig. 4(a) shows an SEM image of a membrane

following Pd deposition through an initial electrolysis

at the water�/DCE interface. By comparison with Fig.

3(a), which is on the same scale, grey shadows are visible

within approximately 5% of the pores, suggesting that

some Pd deposition has occurred. Importantly, the sizes

of the grey patches are of the order of the size of the

pores (mean diameter of 100 nm) and the underlying

structure of the alumina pores is always visible, indicat-

ing that deposition of approximately 100 nm particles

occurs at the mouth of the pores. The presence of

palladium within the membrane was confirmed by EDX

Fig. 2. (a) Initial cyclic voltammetric response obtained from Cell 1 at a scan rate of 0.1 V s�1. The organic phase was uppermost in the cell and x�/

1�/10�3 mol dm�3, where a: y�/0 and b: y�/5�/10�4 mol dm�3. (b) Cyclic voltammetric response of Cell 1 under the conditions of (a), but

following nine previous cycles with a: y�/0 and c: y�/5�/10�4 mol dm�3. (c) Initial chronoamperometric response of Cell 1, under the conditions of

(a), with applied potential difference stepped from �/0.45 to �/0.90 V.

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/3046 3041

(see Table 2). Prolonged electrolysis of the alumina-

modified water�/DCE interface, using a higher concen-

tration of organic phase electron donor, led to the

deposits such as those shown by SEM in Fig. 4(b). Most

of the pores appear to contain palladium, from theresolution of the SEM image (approximately 20 nm),

although close inspection shows that the underlying

structure of the alumina template is still apparent. A

side-on image of a fractured membrane would be

desirable to assess the depth of the particles deposited

within the pores. Attempts to obtain ‘‘side-on’’ SEM

images of the Pd-loaded membranes, comparable to that

shown in Fig. 3(b), were unsuccessful, in that the imageswere identical to those of the blank membranes. This is

presumably because the Pd nanoparticles were not

retained within the pores following the crude fracturing

process: this observation suggests that the interaction

between the alumina surface and the deposited particles

may be relatively weak.

An alternative approach to imaging was pursued by

using a microtoming method to prepare thin samples ofthe Pd-loaded membranes, to permit side-on imaging by

TEM. Higher resolution TEM images of Pd-loaded

alumina membranes are shown in Figs. 4(c) and (d).

Although the microtoming process has created more

local damage to the porous alumina structure, Pd

deposits are clearly visible in both images at the mouth

of the membrane pores. It is clear that the Pd present

within a given pore is discrete from metal withinneighbouring pores, as suggested by the SEM images.

Moreover, the depth of the Pd deposits (of the order of

100 nm) is also visible from TEM. Once more, EDX was

employed to confirm the presence of palladium. The

relative elemental abundances are given in Table 3. Note

that a higher percentage of palladium is observed when

compared against the EDX data of the SEM sample

(Table 2): the TEM sample has been subject to a moreprolonged electrolysis and the microtoming produces a

thinner sample, which is likely to contain a higher

percentage of palladium. It is difficult to conclude too

much about the form of the electrodeposited Pd, due to

Fig. 3. (a) Face-on SEM image of blank (i.e., prior to deposition) g-

alumina membrane. The scale bar is shown on the image. (b) Side-on

SEM image of fractured blank membrane.

Table 2

Composition, by weight percentage and by atom percentage of the

membranes shown in Fig. 4(b), as determined by EDX of the SEM

images

Sample Element Weight% Atom%

Blank membrane Oxygen 29.86 42.15

Aluminium 63.07 52.79

Phosphorus 3.04 2.22

Sulphur 4.04 2.84

Palladium-loaded membrane Oxygen 18.12 31.67

Aluminium 55.78 57.79

Phosphorus 2.56 2.31

Sulphur 3.37 2.94

Palladium 20.17 5.3

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/30463042

Fig. 4. (a) SEM image of alumina membrane following an initial deposition of palladium, under the conditions of Fig. 2(a), following five cyclic

voltammetric scans between �/0.30 and �/0.90 V at a scan rate of 0.1 V s�1. (b) SEM image of membrane following prolonged deposition of

palladium, with x�/1�/10�3 mol dm�3, y�/1�/10�3 mol dm�3, applying a potential difference of �/0.90 V continually for 1 h. (c) Side-on TEM

image of palladium-loaded sample, following exposure to conditions of (b). (d) Higher magnification of TEM image shown in (c).

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/3046 3043

the damage likely to have been induced by microtoming.

However, the appearance of the highest resolution TEM

image (Fig. 4(d)), suggests that the Pd deposits may not

be completely solid and, in fact, the Pd may show some

nanometre-scale porosity, particularly visible at the

outer edge of the particles.

SEM images were also taken of the Pd particles

obtained following chemical dissolution of the alumina

template. The Pd-loaded membrane was supported on a

polyester membrane (see Section 2) to retain the

liberated nanoparticles. Fig. 5 shows the resultant Pd

deposits, retained on the polyester membrane. Although

some agglomeration of the Pd nanoparticles is likely to

have occurred in solution following template dissolution

[24], it is clear from the magnification of the Pd deposits

shown in the inset, that some spherical structure is

retained in the particles, visible as ‘‘bulbs’’. The approx-

imate diameter of these bulbous intergrowths is 100 nm,

confirming that the dimensions of the alumina template

control those of the resultant nanoparticles, and hence

suggesting that growth does take place at the mouth of a

pore.A comparison was made between the form of the Pd

deposits obtained at the alumina-templated water�/DCE

interface, and those obtained at the bare water�/DCE

interface. A representative SEM image of Pd deposits

from the latter interface is shown in Fig. 6(a). Once

more, agglomerated particles are obtained, although

there is no single characteristic dimension of the

Table 3

Composition, by weight percentage and by atom percentage of the

membranes shown in Fig. 4(b), as determined by EDX of the TEM

images

Sample Element Weight% Atom%

Palladium-loaded membrane Oxygen 34.68 63.37

Aluminium 23.11 25.05

Palladium 42.21 11.60

Fig. 5. SEM image of palladium particles retained, using a polyester

membrane, following dissolution of alumina. The palladium was

prepared using the conditions of Fig. 4(b): the inset shows a

zoomed-in segment from the centre of the image.

Fig. 6. (a) SEM images of palladium particles obtained on electro-

deposition at the bare liquid�/liquid interface. Concentrations were

(see Cell 1) x�/1�/10�3 mol dm�3, y�/5�/10�4 mol dm�3. Twenty-

five cyclic voltammetric scans were performed between �/0.30 and �/

0.90 V at 0.1 V s�1. The deposited palladium was visible to the eye,

following this period. Palladium particles were collected by suction and

washed by filtration using the polyester membrane to retain the

particles. (b) SEM image of palladium deposited under thin layer

conditions, by inverting the order of the solution phases (see Fig. 1(b)).

The concentrations were (see Cell 1) x�/1�/10�3 mol dm�3, y�/5�/

10�4 mol dm�3. Fifteen cyclic voltammograms were performed

between �/0.30 and �/0.90 V at 0.1 V s�1.

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/30463044

spherical structures that compose each agglomerate.

Some agglomerates (e.g., in the far left of the image)

contain structures approximately 50 nm in diameter,

while those in the right of the image are closer to 200 nm

in diameter. The observations support the conclusion

previously inferred that agglomeration, following de-

position, affects particle growth at the liquid�/liquid

interface [24], and thus underline the utility of the

templated approach to deposition reported herein.

Moreover, these images are the first reported, to our

knowledge, of palladium nanoparticles grown at theliquid�/liquid interface: a previous report included a low

resolution SEM image of gold deposited at the water�/

DCE interface [22].

Palladium deposition was also performed in cells

where the positioning of the organic and aqueous phases

has been inverted (see Section 2 and the lower cell of

Fig. 1(b)). In such cases, a steady flow of the aqueous

phase through the alumina membrane was observed,making deposition experiments impossible. However,

prior treatment of the alumina with trimethylchlorosi-

lane rendered the membrane surface more hydrophobic

and slowed the rate at which the aqueous phase

traversed the membrane. Deposition experiments were

performed under these conditions: a typical SEM image

of deposited palladium is shown in Fig. 6(b). It is clear

that the palladium has ‘‘grown out’’ from the aluminasurface: the underlying structure of the alumina (and its

consequent influence on the deposited metal) has been

lost, hence there is no regularity in the dimensions of the

metallic growth. The similarity between the deposits

obtained in Fig. 6 suggests that the membrane has had

no influence on palladium growth in Fig. 6(b). The form

of the metal in the case of the inverted alumina-

templated interface implies that the aqueous phase hastraversed the membrane, albeit slowly, and formed a

‘‘thin layer’’ between the underside of the membrane

and the organic phase, from where deposition has

proceeded.

A final observation is that the intermediate stage of

templated deposition using the standard cell (upper cell

in Fig. 1(b)), between the initial growth pictured in Fig.

4(a) and the prolonged electrolysis of Fig. 4(b), led todeposits typified by the SEM image shown in Fig. 7(a).

The image shows that the palladium deposits, although

still separate, have tended not to grow in isolation.

Islands, consisting of regions of neighbouring pores that

have become blocked with palladium, are evident. The

diameters of these islands are between 500 and 700 nm.

Their appearance is surprising as nucleation of indivi-

dual nanoparticles ought to be a statistical event, andhence a random particle distribution would be expected.

Repetition of the experiment under thermostatic condi-

tions, by immersion of the cell within a water bath, gave

rise to the deposits shown in Fig. 7(b). Comparison of

Figs. 7(a) and (b) shows that a similar fraction of metal

has been deposited in each case, although the distribu-

tion of the metal is more even in the thermostatic case.

This observation may indicate that the palladium islandsresult from the presence of fluid instabilities, such as

local convection cells, within the solution phase. Particle

growth is dependent on the supply of reactants from

both the aqueous and organic phases, hence it is

Fig. 7. (a) SEM image of the islands of palladium deposited within the

alumina membrane from Cell 1 with x�/1�/10�3 mol dm�3, y�/5�/

10�4 mol dm�3, following a potential step from �/0.45 to �/0.90 V for

1 h at ambient temperature. (b) SEM image of palladium deposits

formed using the water bath to control the temperature of the system.

Cell 1 was employed with x�/1�/10�3 mol dm�3, y�/5�/10�4 mol

dm�3, following a potential step from �/0.45 to �/0.90 V for 20 min at

25.0 8C.

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/3046 3045

conceivable that natural convection in a given phase

may influence local transport and hence the growth of

palladium nanoparticles.

4. Conclusions

Electrodeposition at the template-modified liquid�/

liquid interface has been shown to be a viable route to

the controlled production of nanostructures. Dissolu-

tion of the alumina template, as well as SEM and TEM

imaging of the resultant palladium nanoparticles, con-

firm that particle growth occurs at the mouths of

template pores. Tuning of the pore size can be used to

tune the dimensions of the particles obtained. The

deposition voltage, electrolysis time or reactant concen-

tration (in either phase) can be used to control the

charge passed, and hence dimensions of the resultant

deposits. Many issues concerning the mechanism of this

interfacial deposition process remain to be clarified, and

such studies are in progress. For example, deposition

here has been performed under conditions reproduced

from previous works at the bare liquid�/liquid interface

where neither reactant was in excess (i.e., in terms of Cell

1, x :/y ) [23,24]. Quantitative analysis of the chron-

oamperometric response of the cell will be facilitated by

performing experiments under conditions where one

reactant is in excess, i.e., x �/y , or y �/x . We suggest

that the weak interaction observed between the pore

walls and the deposited particles is evidence that

nucleation is initiated at the liquid�/liquid interface,

rather than the surface of the alumina, although further

work is required to test this hypothesis. The TEM

images of the palladium deposits suggest that the

deposits themselves may display some porosity: we are

presently using other techniques (such as probe micro-

scopy methods) to investigate the structure of the

deposits further. Finally, we note that the deposition

process may effectively ‘‘freeze’’ the liquid�/liquid inter-

face and hence may be treated as a method by which

interfacial structure can be probed with nanometre

resolution.

Acknowledgements

The authors thank the EPRSC for financial support.

We are extremely grateful to Mr. I. Brough (Manchester

Materials Science Centre) and Dr. B. Bethune (UMIST

Corrosion & Protection Centre) for their assistance with

the SEM and TEM imaging, respectively.

References

[1] R.W.J. Scott, M.J. MacLachlan, G.A. Ozin, Curr. Opin. Solid

State Mat. Sci. 4 (1999) 113.

[2] R. Schuster, V. Kirchner, P. Allongue, G. Ertl, Science 289 (2000)

98.

[3] T. Dellwig, G. Rupprechter, H. Unterhalt, H.J. Freund, Phys.

Rev. Lett. 85 (2000) 776.

[4] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J.

Chem. Soc. Commun. (1994) 801.

[5] R.C. Funreaux, W.R. Rigby, A.P. Davidson, Nature 337 (1989)

147.

[6] F. Li, L. Zhang, R.M. Metzger, Chem. Mater. 10 (1998) 2470.

[7] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075.

[8] M.J. Tierney, C.R. Martin, J. Phys. Chem. 93 (1989) 2878.

[9] G.I. Hornyak, C.J. Patrissi, C.R. Martin, J. Phys. Chem. B 101

(1997) 1548.

[10] D. Al-Mawlawi, C.Z. Liu, M. Moskovits, J. Mater. Res. 9 (1994)

1014.

[11] L.S. Van Dyke, C.R. Martin, Langmuir 6 (1990) 1123.

[12] R.M. Penner, C.R. Martin, J. Electrochem. Soc. 133 (1986) 2206.

[13] W. Kautek, S. Reetz, S. Pentzien, Electrochim. Acta 40 (1995)

1461.

[14] S.R. Nicewarner-Pena, R.G. Freeman, B.D. Reiss, L. He, D.J.

Pena, I.D. Walton, R. Cromer, C.D. Keating, M.J. Natan,

Science 294 (2001) 137.

[15] S.I. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Hoji,

T. Namba, F. Mizukami, Science 295 (2002) 105.

[16] F. Glaguen, J.M. Leger, C. Lamy, A. Marmann, U. Stimming, R.

Vogel, Electrochim. Acta 44 (1999) 1805.

[17] G.A. Somorjai, Surf. Sci. 335 (1995) 10.

[18] M. Gierer, H. Over, H. Bludau, G. Ertl, Phys. Rev. B 52 (1995)

2927.

[19] L. Zeiri, O. Younes, S. Efrima, M. Deutsch, J. Phys. Chem. B 101

(1997) 9299.

[20] H.H.J. Girault, D.J. Schiffrin, in: A.J. Bard (Ed.), Electroanaly-

tical Chemistry, vol. 15, Marcel Dekker, New York, 1989, p. 1.

[21] M. Guainazzi, G. Silvestri, G. Serravalle, J. Chem. Soc., Chem.

Commun. (1975) 200.

[22] Y. Cheng, D.J. Schiffrin, J. Chem. Soc., Faraday Trans. 92 (1996)

3865.

[23] C. Johans, P. Liljeroth, K. Kontturi, Phys. Chem. Chem. Phys. 4

(2002) 1067.

[24] C. Johans, K. Kontturi, D.J. Schiffrin, J. Electroanal. Chem. 526

(2002) 29.

[25] M. Platt, R.A.W. Dryfe, E.P.L. Roberts, Chem. Commun. (2002)

2324.

[26] X.L. Pan, G.X. Xiong, S.S. Sheng, N. Stroh, H. Brunner, Chem.

Commun. (2001) 2536.

[27] Communication from manufacturer. Available from: http://

www.whatman.co.uk/products/analytical.

[28] A. Hernandez, J.I. Calvo, R. Pradanos, L. Palacio, M.L.

Rodrıguez, J.A. de Saja, J. Membr. Sci. 137 (1997) 89.

[29] D.J. Fermın, H.D. Duong, Z.F. Ding, P.F. Brevet, H.H. Girault,

Phys. Chem. Chem. Phys. 1 (1999) 1461.

[30] C.J. Miller, M. Majda, J. Electroanal. Chem. 207 (1986) 49.

M. Platt et al. / Electrochimica Acta 48 (2003) 3037�/30463046