self-assembly of lacunary dawson type polyoxometalates and poly(allylamine hydrochloride) multilayer...

Self-assembly of lacunary Dawson type polyoxometalates

and poly(allylamine hydrochloride) multilayer films:

photoluminescent and electrochemical behavior

Min Jianga, Enbo Wanga,*, Xiuli Wanga, Aiguo Wub, Zhenhui Kanga,Suoyuan Liana, Lin Xua, Zhuang Lib

aInstitute of Polyoxometalate Chemistry, Department of Chemistry, Northeast Normal University,

Changchun, Jilin 130024, PR ChinabState Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,

Chinese Academy of Sciences, Jilin 130022, PR China

Received in revised form 15 August 2004; accepted 15 August 2004

Available online 22 October 2004

Abstract

Ultrathin multilayer films containing a lacunary Dawson type polyoxometalate (POM) cluster K17[Ln(P2Mo17O61)2]

(LnPMo, Ln = Eu, Tb) and poly(allylamine hydrochloride) have been prepared by electrostatic layer-by-layer self-assembly

(ELSA) method. The multilayer films have been characterized by UV–vis absorption spectroscopy, ellipsometry and atomic

force microscopy (AFM). The photoluminescent behavior of films at room temperature was investigated to show the Ln3+

characteristic emission pattern. The occurrence of photoluminescent activity confirms the potential applications in creating

luminescent materials. Additionally, the electrochemical behavior of multilayer films was studied and the growth of multilayer

films can be observed.

# 2004 Elsevier B.V. All rights reserved.

PACS: 78.55; 82.80.F

Keywords: Polyoxometalates; Multilayers; Photoluminescence; Electrochemistry

www.elsevier.com/locate/apsusc

Applied Surface Science 242 (2005) 199–206

* Corresponding author. Present address: Department of Chemistry, Chinese Academy of Science, Renming Road 138, Changchun, PR

China. Tel.: +86 431 5098787; fax: +86 431 5098787.

E-mail address: [email protected] (E. Wang).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2004.08.012

M. Jiang et al. / Applied Surface Science 242 (2005) 199–206200

Fig. 1. (a) Chemical structures of the polyelectrolytes and

[Ln(P2Mo17O61)2]17� (Ln = Eu, Tb) clusters used in the work.

(b) Schematic diagram of the builtup of the multilayer assemblies

via alternate adsorption of cationic polyelectrolytes and anionic

[Ln(P2Mo17O61)2]17� clusters on substrates.

1. Introduction

Recently, the design and preparation of functional

thin-film materials and coatings with effective

control at nanoscale level have attracted considerable

interest. Some common methods have been adopted

to prepare such composite film materials with

potential applications, including electrochemical

method [1], Langmuir–Blodgett technique [2] and

electrostatic layer-by-layer self-assembly (ELSA)

[3]. Since Decher reported that the multilayer films

could be fabricated through the electrostatic inter-

action between polyelectrolytes, the ELSA technique

has been widely utilized [4]. Due to many advantages

of this method, such as easiness in fabrication,

independence of substrate size and topology, good

mechanical and chemical stability of the prepared

film, it has been recognized that the ELSA technique

is a powerful and versatile means for assembling

multicomposite supramolecular structures with a

good control over the layer composition and thick-

ness [5]. Up to now, many different multilayer

films, with nanocrystallites [6], colloidal particles

[7], metal [8], semiconductor [9], clay [10], protein

[11], metallosupramolecules [12] and other functional

components incorporated into polymer matrixes, have

been fabricated.

Polyoxometalates (POMs) are a still rapidly

growing class of compounds with various applications

ranging from catalysis to analytical chemistry and

medicine [13], which can be regarded as one of

extremely versatile inorganic entities for construction

of functionally active solids and the favorite candi-

dates to be transformed into nanometer-sized materi-

als. And that practical application of POMs in these

areas depends on the preparation of thin polyoxome-

talate-containing films. Hence, Ichinose et al. have

prepared ELSA films containing isopolymolybdate

{(NH4)4[Mo8O26]} [14]. Kurth and co-workers

[15,16] have also prepared polyelectrolyte multilayer

films containing novel molybdenum (VI) POM

clusters. Their results display that the ELSA technique

can be applied in fabricating multilayer films of

anionic POMs and cationic polyelectrolyte. However,

these reports focus mainly on the assembly process

and absorption mechanism of the composite films,

while the properties of these films as materials are

rarely reported [17].

In the recent years, since an intermolecular energy

transfer from the oxygen-to-metal charge-transfer

(O ! M CT) to a lanthanide cation can occur in the

POMs lattice, the photoluminescent behavior of

some POMs containing lanthanide cations was

investigated by Yamase and other groups [18–20].

By virtue of combining POMs with lanthanide

cations, it is possible to convert POMs into functional

components for practical applications [21]. There-

fore, we intentionally incorporated POMs containing

rare earth ion into multilayer films and investigated

their photoluminescent behavior. Lately, we have

fabricated photoluminescent multilayer films con-

sisting of the polyoxotungstoeuropate cluster K12-

[EuP5W30O110] and poly(allylamine hydrochloride)

by the ELSA technique [22]. Among such POMs,

rare-earth-containing lacunary Dawson type POMs

[Ln(P2Mo17O61)2]17� own a peculiar structure, in

M. Jiang et al. / Applied Surface Science 242 (2005) 199–206 201

which the lanthanide cation is coordinated with two

P2Mo17O61 groups, whose axial lines are at about

908 with respect to each other. The Eu3+ ion lies in

the center of a slightly distorted square antiprism (see

Fig. 1a).

In this article, we report the preparation of ultrathin

multilayer films of POMs and polyelectrolyte by the

consecutive stepwise adsorption of sandwiched

K17[Ln(P2Mo17O61)2] (LnPMo, Ln = Eu, Tb) and

poly(allylamine hydrochloride). The multilayer films

have been characterized by UV–vis absorption

spectroscopy, ellipsometry and atomic force micro-

scopy (AFM). And we also investigated photolumi-

nescent properties and electrochemical behavior of

the multilayer films and found it would be applied in

material science.

2. Experimental details

2.1. Materials

Lacunary Dawson type polyoxometalates,

K17[Ln(P2Mo17O61)2]�nH2O (LnPMo, Ln = Eu, Tb),

were synthesized according to reference [23]. Poly(-

ethylenimine) (PEI; MW = 50,000), poly(styrenesul-

fonate) (PSS; MW = 70,000) and poly(allylamine

hydrochloride) (PAH; MW = 70,000) were purchased

from Aldrich and were used without further treatment

(chemical structures of three polyelectrolytes shown

in Fig. 1a). Sodium chloride (AR grade) was obtained

commercially and used. The pH was adjusted by

adding 0.1 M HCl aqueous solution. The water used in

all experiments was deionized with a resistivity of 17–

18 MV cm�1. The glassy carbon electrode (GCE) was

polished with 1.0- and 0.3-mm a-Al2O3 powders

successively and sonicated in water for about 3 min

after each polishing step. Finally, the electrodes were

sonicated in ethanol, washed with ethanol, and dried

with high-purity nitrogen stream immediately before

use.

2.2. Characterization of the films

UV–vis absorption spectra were recorded on a

quartz slide using a 756 CRT UV–vis spectro-

photometer. Ellipsometric data were acquired by

using an AUEL-III automated laser ellipsometer with

2 mW HeNe laser (l = 632.8 nm) light source and an

angle of incidence of 70 for silicon substrates. Atomic

force microscopy images were taken on silicon slides

using a Nanoscope III, an instrument (Digital

Instruments) operating in the contact mode with

silicon nitride tips. Typically, the surface was scanned

at 2 Hz with 512 lines per image resolution and �0.5

to 0 V set point. No filter technique was applied to the

images present. Photoluminescence spectra were

measured using FL-2T2 instrument (SPEX Corp;

USA) with a 450 W xenon lamp monochromatized by

a double-grating (1200 gratings mm�1). Cyclic vol-

tammetry was performed with a CHI 600 voltam-

metric analyzer in a conventional three-electrode

electrochemical cell using GCE as the working

electrode, twisted platinum wire as the auxiliary

electrode and Ag/AgCl as reference electrode.

2.3. Preparation of self-assembly films

The substrates (quartz and silicon) were cleaned by

immersion in the ‘‘piranha solution’’ containing three

parts H2O2 (30% aqueous solution) and seven parts oil

of vitriol (H2SO4) at 80 8C for 40 min and rinsed with

copious deionized water. Then, the substrate was

immersed in a solution of NH4OH (29% aqueous

solution), H2O2 (30% aqueous solution) and pure

water with a volume ratio 1:1:5 at 70 8C for 20 min,

followed by extensively washing with water. The

cleaned substrates were immersed in 10�2 M PEI

solution (based on the molecular weight of the

monomer unit, pH � 9.0) for 20 min, 10�2 M PSS

solution for 20 min and 10�2 M PAH solution (pH �4.0, containing 1 M NaCl) for 20 min, and then

alternate immersion in LnPMo (Ln = Eu, Tb) (10�3 M,

pH = 4–5) solution and PAH solution for 20 min,

respectively. Water rinsing and N2 drying steps were

performed after each adsorption cycle. This procedure

resulted in the builtup of multilayer films. According

to the literature [24], the electrochemical modification

of a glassy carbon electrode was performed in an

anhydrous ethanol solution containing 3 mM 4-

aminobenzoic acid (4-ABA) and 0.1 M LiClO4 by

scanning between 0 and +0.9 V (versus Ag/Ag+).

After the modification, the electrode was successively

rinsed with ethanol and deionized water and sonicated

for 10 min in water to remove the physically adsorbed

materials. Through the attachment of 4-ABA to GCE,


Fig. 3. Effect of PAH solutions containing different NaCl concen-

a stable and negatively charged surface can be

achieved at pH > 3.1. The electrode was then

immersed alternately in an aqueous solution of

10�2 M PAH and 10�3 M TbPMo for 20 min,

respectively. Between each modification, the result

electrode was thoroughly washed with water and dried

with N2. All adsorption procedures were performed at

room temperature. Fig. 1b also shows schematically

the assembly process flow. Repetition of steps 4 and 5

produces the polyelectrolyte multilayer films contain-

ing LnPMo. The multilayer architectures can be

expressed as PEI/PSS/PAH/(LnPMo/PAH)n, where n

is the number of bilayers.
tration (0, 0.1, 0.5 and 1.0 M) on the growth of (LnPMo/PAH)12
multilayer films.

3. Results and discussion

3.1. Assembly and characterization of multilayer

films

The buildup of multilayer films is influenced by

many factors, such as solution concentration, pH,

ionic strength and adsorption time. From the former

results, it can be concluded that higher solution

concentration benefits more adsorption quantity [25].

Therefore, we chose 10�3 M LnPMo solution and

10�2 M PAH solution. Since LnPMo is stable in the

pH = 3–5 [23], the pH of polyelectrolyte PAH is

important for the fabrication of (LnPMo/PAH)n

multilayer films. So the pH of PAH was adjusted to

ca. 4.0. With respect to the influence of adsorption

time, it can be seen that 20 min is the optimum

adsorption time from Fig. 2, the adsorption of

Fig. 2. Absorbance variation of an LnPMo monolayer on the PEI/

PSS/PAH substrates, with immersion periods of 5, 10, 15, 20, 25 and

30 min.

monolayer was saturated at this stage. In order to

investigate the effect of ionic strength on the

multilayer growth, we chose four 10�2 M PAH

solutions, containing 0, 0.1, 0.5 and 1.0 M NaCl,

respectively. Fig. 3 gives the dependence of growth

step on ionic strength. Obviously, more LnPMo is

adsorbed on the polyelectrolyte at higher ionic

strength after every dipping step. Regular film growth

was found for all cases, but the magnitude of the

growth step increased with increasing ionic strength.

We proposed that, at high ionic strength, the PAH

adsorbs at the interface in a more condensed and

coiled conformation, and parts of LnPMo anions both

adsorb at the top of the PAH interface and also diffuse

into the PAH layer. However, at low ionic strength,

LnPMo anions only adsorb at the top of the PAH

interface [25]. In addition, it is necessary to rinse the

films with water and then dry with nitrogen after each

adsorption step. Based on the condition mentioned

above, highly reproducible and homogeneous films

can be obtained.

UV–vis spectroscopy is a useful technique and has

been used to monitor the growth process of multi-

layers. Fig. 4 shows the absorption spectra of (TbPMo/

PAH)n (with n = 0–12) multilayer films deposited on a

precursor PEI/PSS/PAH film on quartz substrates. The

absorption band at 225 nm for the precursor film is due

to the benzene chromophores in PSS. PAH does not

absorb above 200 nm, and therefore, its presence in

the film is not reflected in the absorption spectra [14].

The UV–vis spectrum of an aqueous LnPMo solution

indicates two characteristic absorption bands at 200


Fig. 4. UV–vis spectra of (TbPMo/PAH)n multilayers films with n =

0–12 on PEI/PSS/PAH-modified quartz substrates (both sides).

These curves, from bottom to top, correspond to n = 0–12, respec-

tively. The dash curve presents the spectrum of TbPMo solution. The

inset displays the absorbance growth at 200 and 260 nm as a function

of the number of TbPMo/PAH bilayers.

and 260 nm, which can be applied to monitor the film

growth. However, we cannot see the absorption peak

at 260 nm in the multilayer films from Fig. 4. It can be

explained that the content of LnPMo in the film is very

low and absorption peak is very weak. However, if

LnPMo was kept as outermost layer (PAH is outermost

layer in the article), the absorption peak at 260 nm can

be seen distinctly. It indicated that some loss of

LnPMo occurred at each step of PAH deposition. As

shown in the inset of Fig. 4, the absorbance values, for

(TbPMo/PAH)n multilayer films with n = 0–12, varied

linearly with n at two wavelengths (200 and 260 nm).

In the case of the (EuPMo/PAH)n multilayer films,

similar absorption spectra and linear variation of

values with layer numbers have also been observed

(not shown). The above discussion shows that an

approximately equal amount of LnPMo is deposited

for each adsorption procedure, and that (LnPMo/

PAH)n multilayer films grow uniformly with each

deposition cycle. Therefore, it is concluded that the

deposition process is very consistent and highly

reproducible. Additionally, it is found that the

absorbance values for the first LnPMo/PAH bilayer

are larger than the following bilayer, indicating a

greater amount of LnPMo deposited onto the

precursor PEI/PSS/PAH at the beginning. It has been

explained that the polyanions can penetrate into

polyelectrolyte layers. So there is no obvious interface

between the POMs and polyelectrolyte.

The surface density, G, of LnPMo on PAH surface

in the (LnPMo/PAH)n films can be calculated using

G ¼ ½ðAl=2Þe�1l NA 10�3, where Al is the absor-

bance of LnPMo in the film at given wavelength (l), elis the extinction coefficient of LnPMo in solution

(M�1 cm�1) at l, and NA is Avogadro’s number

[16,26]. Based on the absorbance values in the

wavelength range of 190–300 nm and the correspond-

ing molar extinction coefficients calculated from

the absorption spectrum of the aqueous LnPMo

solution, the average surface concentration of LnPMo

per bilayer is 3.2 1013 clusters per cm2. This

corresponds to an average area per LnPMo of 3.1 nm2.

The number of clusters in unit area is affected by many

factors, including the processing and type of the

substrate, the concentration of the solution, the

nature of the polyanion and the adsorption time

[27]. Since the difference between the radius of Eu3+

and radius of Tb3+ is very small, under the same

experimental conditions, their surface densities are

almost equal.

From ellipsometry, it can be seen that the thickness

of the multilayered structure increases linearly with

the number of bilayers, and deposition is very

reproducible and film growth is essentially uniform.

The average film thickness for the LnPMo/PAH

bilayer is about 3.0 nm.

AFM can provide further detailed information

involving the surface morphology and the homo-

geneity of the deposited films down to nanometer

scale. So the topography of silicon-supported

(LnPMo/PAH)n films was also investigated by AFM

in tapping mode, which provided insight into the

internal structure of the multilayers. Fig. 5a shows

typical AFM image taken from the precursor PEI/PSS/

PAH film on silicon slide. From this image, it can be

observed that the outermost PAH surface layer on the

precursor film is uniform and smooth, with a mean

roughness of 0.5 nm from an area of 2.0 mm 2.0 mm. But after adsorption of TbPMo, followed by

adsorption of a PAH surface layer, the mean roughness

of surface has increased to 2.3 nm (as shown in

Fig. 5b), which is three times larger than that of the

precursor film (the mean roughness of EuPMo surface

reached 2.3 nm). In addition, AFM images of the

sample show a multitude of small domains. The

domains display a round shape with the diameter along

the horizontal axis of ca. 20–30 nm. Furthermore, both


Fig. 5. AFM images of (a) a PEI/PSS/PAH layer on silicon wafer

and (b) a bilayer TbPMo/PAH on PEI/PSS/PAH substrates.

Fig. 6. Photoluminescent spectra of (TbPMo/PAH)20 multilayer

films on quartz.

Fig. 7. Photoluminescent spectra of (EuPMo/PAH)20 multilayer

films on quartz.

topography and packing of LnPMo nanoclusters can

repeat in subsequent layers. Particularly, one needs to

point out that the multilayer containing LnPMo

nanoclusters can be considered as a very homoge-

neous composite material, which does not have

obvious interfaces between the polyelectrolyte and

inorganic component.

3.2. Photoluminescent properties of multilayer films

The excitation spectrum of the solution of the

K17[Tb(P2Mo17O61)2] at 586 nm shows an obvious

peak at 280 nm. So we chose 280 nm as excitation

wavelength to get the emission spectrum of the

K17[Tb(P2Mo17O61)2] solution. Then, we investi-

gated photoluminescent spectrum of the (TbPMo/

PAH)20 multilayer films on quartz substrate which

was excited at the wavelength of 280 nm at room

temperature (as shown in Fig. 6). As shown in the

spectrum, the most intense bands at 547 nm

correspond to the 5D4 ! 7F5. Bands at 492 nm

ascribed to 5D4 ! 7F6 emission transition. The other

two bands at 585 and 622 nm are assigned to the5D4 ! 7F4 and 5D4 ! 7F3, respectively. These bands

correspond to the characteristic 5D4 ! 7FJ (J = 3–6)

transition of Tb3+ ions [21]. For (TbPMo/PAH)20

multilayer films, photoluminescent spectrum exhibits

multiple peak structure, which is very similar to that

found in the TbPMo solution. It again confirms that

the structure of TbPMo is not changed in the film and

TbPMo can exist in the film stably. On account of the

low content and low fluorescence efficiency of

TbPMo, the finer structure is not revealed in the

multilayer films spectrum. In the same way, we

investigated photoluminescent spectrum of the

(EuPMo/PAH)20 multilayer films on quartz substrate

which was excited at the wavelength of 394 nm at

room temperature (as shown in Fig. 7). The Eu3+

characteristic emission pattern can be observed,

which consists of two 5D0 ! 7F1 (591 and 597nm),


two 5D0 ! 7F2 (617 and 621 nm), one 5D0 ! 7F3

(652 nm) and one 5D0 ! 7F4 (701 nm) [28,29].

According to Fig. 1, in the structure of EuPMo, the

Eu3+ ion lies in the center of a slightly distorted

square antiprism. Because the 5D0 ! 7F2 emission is

very sensitive to deviations from D4d symmetry, so

the 5D0 ! 7F2 intensity is stronger than others in the

EuPMo with its angled configuration [28]. The

occurrence of photoluminescent activity confirms

the potential for creating luminescent multilayer films

with POMs and their use as functional components

may be achieved by adjusting the thickness, composi-

tion and structure of the multilayer films.

3.3. Electrochemical behavior of multilayer films

We built up POM-containing multilayer films on the

electrodes in order to study their electrochemical

behavior. By means of electrochemical measurements,

we can also see the growth of the (LnPMo/PAH)n

multilayer films. Here, we only give the results of

(TbPMo/PAH)n multilayer films due to comparability

with (EuPMo/PAH)n multilayer films. Fig. 8 displays

cyclic voltammograms of (TbPMo/PAH)n multilayers

(with n = 0.5, 2.5, 3.5, 4.5, 5.5, 6.5) assembled on glassy

carbon electrode coated by 4-aminobenzoic acid. It is

obvious that the peak currents increase with increasing

number of bilayers and multilayer films exhibit three

reversible redox waves at 0.239, 0.127 and �0.060 V,

Fig. 8. Cyclic voltammograms of (TbPMo/PAH)n multilayers (with

n = 0.5, 2.5, 3.5, 4.5, 5.5, 6.5 from inside to outside, that is to say, the

outermost layer is TbPMo) assembled on glassy carbon electrode

(GCE) in aqueous 0.5 mM NaHSO4. Scan rate: 100 mV s�1. The

dash curve shows the cyclic voltammograms of TbPMo solution in

0.5 mM NaHSO4.

withDEpvaluesof56,53and60 mV,respectively,which

are similar to the result of the TbPMo solution [23]. The

three reversible redoxpeakscorrespondtoreductionand

oxidationthroughtwo-,four-andsix-electronprocesses,

respectively. Moreover, each additional deposition of

PAH/TbPMostill results in the reductionpeakpotentials

for the three redox waves shifting to more negative

values while the corresponding oxidation peak poten-

tials shift to more positive values. The above experi-

mental results show that LnPMo anions and PAH can be

fabricated on the surface of a glassy carbon electrode by

ELSAmethod basedonelectrostatic interaction. Inview

of POMs having excellent electrocatalysis, many

researchers have studied the electrocatalytic properties

of POMs-containing multilayer films [30,31]. As for the

electrocatalytic function of (LnPMo/PAH)n multilayer

films, we are in further research.

4. Conclusions

In this paper, we have demonstrated the assembly

process of (LnPMo/PAH)n multilayer films on the

solid substrates via the ELSA method. The photo-

luminescent properties of the multilayer films are of

potential importance to the fabrication of luminescent

multilayer films containing lanthanide-cation POMs.

The electrochemical behavior of the multilayer films

further proves the feasibility of multilayer films

forming. By means of investigating of photolumines-

cent properties and electrochemical behavior of

multilayer films, it is found that the films would be

applied in material sciences.

Acknowledgements

This project was financially supported by the

National Natural Science Foundation of China (No.

20171010). We thank Mr. Hongyu Zhang for his

helpful and constructive discussions during prepara-

tion of the manuscript.

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