Layer-by-layer assembly of multilayer films of polyoxometalates and tris(phenanthrolinedione) complexes of transition metals

Article abstract of DOI:10.1016/j.jssc.2006.01.069

A series of organic-inorganic composite films were prepared by the layer-by-layer self-assembly method containing the phendione complexes of transition metals [M(phendione)₃]²⁺ (M=Fe²⁺, Co²⁺, phendione=1,10-phenanthroline-5,6-dione) and the polyoxometalates (POMs). UV-vis spectroscopy was used to follow the fabrication process of (BW₁₂/[M(phendione)₃]²⁺)_n (BW₁₂=BW₁₂O₄₀^5-, M=Fe²⁺, Co²⁺) and (Co₄(PW₉)₂/[M(phendione)₃]²⁺)_n (Co₄(PW₉)₂=Co₄(H₂O)₂(PW₉O₃₄)₂^10-, M=Fe²⁺, Co²⁺) multilayer films. Electrochemical studies on the films illustrate that the POM species exhibit well-defined redox peaks and the phendione species show pH-dependent electrochemical behavior. The photoluminescent properties were investigated to show the (BW₁₂/[Fe(phendione)₃]²⁺)_n film with low-energey red photoluminescence at 672 nm.

Full text of DOI:10.1016/j.jssc.2006.01.069

ARTICLE IN PRESS
Journal of Solid State Chemistry 179 (2006) 1407–1414
www.elsevier.com/locate/jssc
Layer-by-layer assembly of multilayer films of polyoxometalates
and tris(phenanthrolinedione) complexes of transition metals
Ã
Shuiying Gaoa,b, Xing Lia, Chunpeng Yanga, Taohai Lia,b, Rong Caoa,
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, The Chinese Academy of Sciences,
Fuzhou 350002, China
bGraduate School of The Chinese Academy of Sciences, Beijing 100039, China
Received 8 December 2005; received in revised form 23 January 2006; accepted 29 January 2006
Available online 9 March 2006
Abstract
A series of organic–inorganic composite films were prepared by the layer-by-layer self-assembly method containing the phendione
complexes of transition metals [M(phendione)3]2+ (M ¼ Fe2+, Co2+, phendione ¼ 1,10-phenanthroline-5,6-dione) and the polyox-
ometalates (POMs). UV–vis spectroscopy was used to follow the fabrication process of (BW12/[M(phendione)3]2+)n (BW12 ¼ BW12O
,
5ꢀ
40
M ¼ Fe2+, Co2+) and (Co4(PW9)2/[M(phendione)3]2+
)
(Co4(PW9)2 ¼ Co4(H2O)2(PW9O34)120ꢀ, M ¼ Fe2+, Co2+) multilayer films.
n
Electrochemical studies on the films illustrate that the POM species exhibit well-defined redox peaks and the phendione species show pH-
dependent electrochemical behavior. The photoluminescent properties were investigated to show the (BW12/[Fe(phendione)3]2+)n film
with low-energey red photoluminescence at 672 nm.
r 2006 Elsevier Inc. All rights reserved.
Keywords: Polyoxometalates; Layer-by-layer self-assembly; 1,10-Phenanthroline-5,6-dione; Photoluminescence
1. Introduction
potential application in electrocatalytic [18], electrochromic
[19] and photoluminescent materials [20]. For example,
POMs containing rare earth metal have been used to
construct photoluminescent films with cationic polyelec-
trolytes [20]. However, the POMs functional multilayers
reported to date are mainly composed of negatively
charged POMs and positively charged polyelectrolytes,
the transition metal complexes as the charge-balancing
species in film fabrication are rarely reported. Transition
metal complexes as the charge-balancing species are
especially fascinating because of their unique properties
with well-defined surface functionality compared to con-
ventional polyelectrolytes [16,19,21–23]. Only a few exam-
The fabrication of functional ultrathin films has received
considerable interest in materials chemistry due to their
nanostructural and nanosized effect [1–4]. The layer-by-
layer self-assembly method proposed by Decher is based on
electrostatic interactions between cationic and anionic
polymers [5]. This method is a simple but powerful,
versatile, and economical technique, independent of sub-
strate size and topology. Therefore, the method has been
widely employed for the fabrication of ultrathin films of
charged materials such as charged polymers [6,7], organic
dyes [8–10], bimoleculars [11,12], nanoparticles [13,14], etc.
Polyoxometalates (POMs) as typical inorganic metal oxide
clusters [15], are very versatile inorganic building blocks for
the construction of functional thin films. POMs are used as
the anionic moiety to attract and bond multiply charged
cations in self-assembled multilayers [3,16,17]. On the other
hand, functional multilayer films containing POMs have
6ꢀ
ples such as multilayer films of Os(bpy)23+–P2Mo18O and
62
6ꢀ
Ru(bpy)23+–P2Mo18O
were reported [21,22], and the
62
studies mainly focused on the properties of transition metal
ions rather than ligands. In order to explore the potential
application of transition metal complexes in the nanos-
tructural materials, it is important to design, fabricate, and
study functional films containing functional ligands.
1,10-Phenanthroline-5,6-dione (phendione) is particu-
larly attractive because it carries an o-quinone moiety with
Ã
Corresponding author. Fax: +86 59183796710.
E-mail address: rcao@ms.fjirsm.ac.cn (R. Cao).
0022-4596/$ - see front matter r 2006 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2006.01.069

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S. Gao et al. / Journal of Solid State Chemistry 179 (2006) 1407–1414
1408
2+
water. Further purification was carried out by immersion
in a H2O/H2O2/NH3OH (5:1:1) (V/V/V) bath for 30 min at
70 1C.
O
O
N
O
O
N
N
2.2.1. Preparation of PEI–(Co4(PW9)2/
[Fe(phendione)3]2+)n multilayer films
N
X
The clean quartz slides were immersed in aqueous
solution of PEI (10 mg mlꢀ1) for 20 min, washing with
water and drying with nitrogen stream. The pre-coated
quartz slides were then alternately immersed in Co4(PW9)2
(5 mM) aqueous solution and [Fe(phendione)3](PF6)2
(12 mM) acetonitrile solution for 20 min. After each
immersion, the substrates were rinsed with water and dried
with nitrogen gas. Multilayer films on solid substrates
could be formed by repeating the above steps in a cyclic
fashion.
N
N
O
O
1 X = Fe2+
2 X = Co2+
Scheme 1. The structures of [Fe(phendione)3]2+ (1) and [Co(phen-
dione)3]2+ (2) complexes.
2.2.2. Preparation of PEI–(Co4(PW9)2/
[Co(phendione)3]2+)n multilayer films
pH-dependent electroactivity [24]. The ligand forms stable
complexes with transition metal ions and shows the
electrocatalytic activity toward the oxidation of NADH
(dihydronicotinamide adenine dinucleotide) [25]. More-
over, metal complexes of the ligand also change the
potentials through pH changes. Therefore, [Fe(phen-
dione)3]2+ and [Co(phendione)3]2+ complexes (Scheme 1),
which make them promising candidates, are used as large
and multiply charged cations to construct the multilayers
with anionic POMs in our study.
In this report, we present the fabrication of ultrathin
organic–inorganic composite films of phendione complexes
of transition metals and POMs by the layer-by-layer self-
assembly method. The functional films are characterized by
UV–vis spectroscopy. The electrochemical behavior of the
phendione complexes of the films was investigated in
different pH values by cyclic voltammogram. The photo-
luminescence properties of the films have also been
investigated by fluorescence spectroscopy.
The clean quartz slides were immersed in aqueous
solution of PEI (10 mg mlꢀ1) for 20 min, washing with
water and drying with nitrogen stream. The pre-coated
quartz slides were then alternately dipped into aqueous
solution of Co4(PW9)2 (5 mM) and [Co(phendione)3](PF6)2
(8 mM) acetonitrile solution for 20 min.After each immer-
sion, the substrates were washed with water and dried with
nitrogen gas.
2.2.3. Preparation of PEI–(BW12/[M(phendione)3]2+
(M ¼ Fe2+, Co2+) multilayer films
)
n
The clean quartz slides were immersed in aqueous
solution of PEI (10 mg mlꢀ1) for 20 min, washed with
water and dried with nitrogen stream. The pre-coated
quartz slides were then alternately immersed in aqueous
solution of 4 ꢁ 10ꢀ2 M BW12 and [M(phendione)3](PF6)2
(12 mM [Fe(phendione)3](PF6)2 or 8 mM [Co(phendio-
ne)3](PF6)2 for 20 min. After each immersion, the sub-
strates were washed with water and dried with nitrogen gas.
2. Experimental section
2.3. Characterization of the films
2.1. Materials
UV–vis absorption spectra were recorded on a quartz
slide using a Lambda35 spectrophotometer (Perkin-Elmer,
USA). AFM images were taken on a quartz slide using a
Nanoscope IIIa (Digital Instruments Inc., Veeco, USA)
operating in the tapping mode with silicon tips.
Electrochemical experiments were carried out on an
Epsilon Analyzer (BAS Inc., USA) in a three-electrode
cell: glassy carbon electrode (GCE, diameter 3 mm) as
working electrode, platinum wire as counter electrode, and
Ag/AgCl/KCl (3 M) as reference electrode. Prior to
modification, GCE was successively polished with 1.0 and
0.3 mm a-Al2O3 and ultrasonically washed with acetone and
water between each experiment. Then, the GCE was
modified with the multilayers. The procedure of electrode
modification was as follows: The clean electrode was
coated with PEI to introduce a positive charge onto its
Poly(ethylenimine) (PEI, 50 wt% aqueous solution) was
purchased from Aldrich Chemical Co. K5BW12O40 (BW12)
and K10Co4(H2O)2(PW9O34)2 (Co4(PW9)2) were prepared
by published methods [26,27]. Phendione and its complexes
[Fe(phendione)3](PF6)2 and [Co(phendione)3](PF6)2 were
synthesized according to Refs. [24,25,28]. All other
reagents were of analytical grade and used as received.
Buffer solutions were prepared from 0.2 M NaAc+0.3 M
HAc.
2.2. Preparation of the multilayers
The quartz slides were cleaned with a ‘‘piranha solution’’
at 80 1C for 40 min, and thoroughly rinsed with distilled

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S. Gao et al. / Journal of Solid State Chemistry 179 (2006) 1407–1414
1409
surface. Then the PEI-coated electrode was alternately
dipped into POMs (5 mM Co4(PW9)2 or 4 ꢁ 10ꢀ2 M BW12)
solution and [M(phendione)3](PF6)2 (12 mM [Fe(phen-
dione)3] (PF6)2 or 8 mM [Co(phendione)3] (PF6)2) solution
for 20 min each, followed by rinsing and drying. The
solutions were deaerated with prepurified nitrogen for at
least 15 min. Formal potentials Ep of the redox couples
were estimated and reported as the average values of the
anodic (Epa) and cathodic (Epc) peak potentials.
absorption band around 510 nm in the visible region is due
to metal-to-ligand charge transfer (MLCT) transition in the
[Fe(phendione)3]2+ film. The UV–vis spectrum of an
aqueous Co4(PW9)2 solution indicates one characteristic
absorption at 250 nm. Thus, the characteristic absorption at
250 nm can be employed to monitor the film growth. The
inset in Fig. 1a presents the plots of the absorbance values
for these multilayer films at 250 and 312 nm as a function of
the number of deposition cycles, indicating the regular
growth of the absorbance. For PEI–(Co4(PW9)2/[Co(phen-
dione)3]2+)n films (Fig. 1b), the absorption peak at 250 nm is
from the Co4(PW9)2, whereas the absorption peak at 312 nm
is due to the phendione ligand-centered p–p* transitions.
However, no absorption peak is observed in the visible
region, indicating there is no charge transition in MLCT
transition in [Co(phendione)3]2+ complex. The result is
consistent with the reported literature [24]. For PEI–(BW12/
Photoluminescence spectra were obtained at room
temperature with a FLS920 fluorescence spectrophot-
ometer (EDINbergh Instruments, British).
3. Results and discussion
UV–vis spectroscopy is used to monitor the layer-by-layer
self-assembly process of organic–inorganic composite films.
Fig. 1 shows the UV–vis absorption spectra of the multi-
layers assembled on quartz substrates. For PEI–(Co4(PW9)2/
[Fe(phendione)3]2+)n multilayer films (Fig. 1a), the absorp-
tion peak at 250nm is ascribed to Co4(PW9)2 polyanion,
whereas the absorption peak at 312 nm is from the
phendione ligand-centered p–p* transitions. The broad
[Fe(phendione)3]2+)n and PEI–(BW12/[Co(phendione)3]2+
)
n
multilayers (Fig. 1c and d), it was found that the absorption
peak at 259 nm was from the BW12 polyoxoanion and the
absorption peak at 312 nm was from phendione ligand.
Compared with (BW12/[Co(phendione)3]2+
)
multilayers,
multilayers have the broad
n
(BW12/[Fe(phendione)3]2+
)
n
0.6
0.3
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
250
312
250
312
0.4
0.3
0.2
0.1
0.0
0.25
0.2
0.5
0.4
0.3
0.2
0.1
0.0
0.15
0.1
0.05
0
0
1
2
3
4
5
6
7
8
number of bilayers
0
1 2 3 4 5 6 7 8 9 10
number of bilayers
200
300
400
500
600
700
800
200
300
400
500
600
700
800
(a)
Wavelength / nm
(b)
Wavelength / nm
0.3
0.35
0.3
0.5
0.4
0.3
0.2
0.1
0.0
259
0.4
0.25
312
0.2
259
0.25
312
0.2
0.15
0.1
0.05
0
0.3
0.2
0.1
0.0
0.15
0.1
0.05
0
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
number of bilayers
number of bilayers
200
300
400
500
600
700
800
200
300
400
500
600
700
800
(c)
Wavelength / nm
(d)
Wavelength / nm
Fig. 1. UV–vis absorption spectra of the multilayer films: (a) PEI–(Co4(PW9)2/[Fe(phendione)3]2+)n; (b) PEI–(Co4(PW9)2/[Co(phendione)3]2+)n;
(c) PEI–(BW12/[Fe(phendione)3]2+)n; (d) PEI–(BW12/[Co(phendione)3]2+)n.

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1410
absorption band around 510 nm in the visible region, due to
MLCT transition of [Fe(phendione)3]2+
that the RMS of the multilayer films with BW12
polyoxoanions is smaller than the films with Co4(PW9)2
polyoxoanions. This indicates large size of Co4(PW9)2
results in high RMS.
.
The absorption spectra exhibit the characteristic peaks
of the POMs and the metal complexes, suggesting
phendione metal complexes can be used as large, multiply
charged cations with anonic POMs to self-assembly multi-
layers. Moreover, for the films containing [Fe(phen-
dione)3]2+, the absorption spectra indicate the broad
band in the visible region, whereas the films with
[Co(phendione)3]2+show no absorption band in the visible
region. All the multilayer films were stable, because their
UV–vis spectra still remained unchanged after they were
placed in air for several months.
In order to obtain the surface morphology information
of the deposited films, atomic force microscopy (AFM)
images of the multilayer films were obtained (shown in
Fig. 2). As seen in Fig. 2, the composite film is relatively
uniform and smooth. Their surfaces exhibit a granular
texture. This reflects that the polyoxoanions aggregated to
a certain level. Furthermore, the root-mean-square (RMS)
of (BW12/[Fe(phendione)3]2+)4–BW12 and (Co4(PW9)2/
[Fe(phendione)3]2+)4–Co4(PW9)2 surfaces was determined
to be 2.510 and 4.899 nm calculated over an area of
1.0 ꢁ 1.0 mm2, respectively (Fig. 2a and b), while the RMS
of (BW12/[Co(phendione)3]2+)4–BW12 and (Co4(PW9)2/
[Co(phendione)3]2+)4–Co4(PW9)2 surfaces was determined
to be 0.866 and 2.033 nm calculated over an area of
1.0 ꢁ 1.0 mm2, respectively (Fig. 2c and d). It was found
Cyclic voltammetry (CV) was used to investigate the
electrochemical behavior of the multilayers. We investi-
gated two bilayers, because it is difficult for a substrate to
access the inner layers in the multilayer films with more
layer numbers. Fig. 3 indicates the electrochemical beha-
viors of the multilayers at different scan rates. For
PEI–(Co4(PW9)2/[Fe(phendione)3]2+
)
2
multilayers, three
redox peaks occur at potentials (Ep) of ꢀ0.744, ꢀ0.629
and ꢀ0.469 V in the negative potential due to the redox of
Co4(PW9)2 polyoxoanions, whereas the redox peak at
+0.300 V (Ep) in the positive potential is due to phendione
ligand (Fig. 3a). CV of the aqueous Co4(PW9)2 polyanion
was also investigated in buffer acetate solution (pH 4.4). It
was found that the CoII species do not show electroactivity
in the positive potential region in buffer acetate solution. It
was reported that the CoII species do not exhibit electro-
activity in the electrochemical studies of the aqueous
[Co4(H2O)2(P2W15O56)2]16ꢀ polyoxoanion by Ruhlmann
et al. [29], since the structure of [Co4(H2O)2(P2W15O56)2]16ꢀ
polyoxoanion is similar to that of tetranuclear sandwich
complexes Co4(PW9)2. Therefore, the redox peak at
+0.300 V (Ep) is ascribed to the phendione ligand. For
PEI–(Co4(PW9)2/[Co(phendione)3]2+)2, the electrochemical
behaviors of the multilayers are similar to (Co4(PW9)2/
Fig. 2. AFM images of the multilayers on quartz slides. (a) PEI–(BW12/[Fe(phendione)3]2+)4–BW12; (b) PEI–(Co4(PW9)2/[Fe(phendione)3]2+)4–Co4
(PW9)2; (c) PEI–(BW12/[Co(phendione)3]2+)4–BW12; (d) PEI–(Co4(PW9)2/[Co(phendione)3]2+)4–Co4(PW9)2 (1.0 ꢁ 1.0 mm2).

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1411
15
10
5
10
5
0
0
-5
-5
-10
-15
-20
-25
-30
-10
-15
-20
-25
-30
-35
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E / V vs. Ag|AgCl
-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E / V vs. Ag|AgCl
(a)
(b)
Fig. 3. Cyclic voltammograms of the multilayers in buffer solution (pH 4.4) at different scan rates. (a) PEI–(Co4(PW9)2/[Fe(phendione)3]2+)2;
(b) PEI–(Co4(PW9)2/[Co(phendione)3]2+)2. Scan rate: 50, 100, 150, 200, 250, 300, 350, 400 mV sꢀ1
.
10
5
10
5
0
0
-5
-5
-10
-15
-20
-10
-15
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E / V vs. Ag|AgCl
E / V vs. Ag|AgCl
(a)
(b)
Fig. 4. Cyclic voltammograms of the multilayers in NaCl solution at different scan rates. (a) PEI–(BW12/[Fe(phendione)3]2+)2; (b) PEI–(BW12
[Co(phendione)3]2+)2. Scan rate: 50, 100, 150, 200, 250, 300, 350, 400 mV sꢀ1
/
.
[Fe(phendione)3]2+
)
2
multilayers (Fig. 3b). Likewise, the
oanions (Fig. 4a and b), which occur at potentials (Ep) of
ꢀ0.603 and ꢀ0.377 V, respectively. Moreover, the electro-
redox peak at +0.379 V (Ep) in the positive potential is due
to phendione ligand. The redox peak of the phendione in
the positive potential is assigned to the two-electron/two-
proton reduction of the quinone moiety to the correspond-
ing hydro-quinone in acid media. The redox process of
metal ions in [Fe(phendione)3]2+ and [Co(phendione)3]2+
complexes was not observed for the multilayer films,
probably due to the fact that [Fe(phendione)3]2+ and
[Co(phendione)3]2+ are relatively stable compared to the
oxidized state. When the polyoxoanion Co4(PW9)2 is
replaced by BW12 polyoxoanion, the phendione ligand
shows one irreversible broad peak around 0.100 V (Epa) in
neutral media in the positive potential. Two redox peaks in
the negative potential are ascribed to the BW12 polyox-
chemical behaviors of PEI–(BW12/[Fe(phendione)3]2+
)
2
and PEI–(BW12/[Co(phendione)3]2+)2 multilayer films were
also studied in buffer acetate solution (pH 4.4) (as shown in
Fig. 5). In the negative potential region, the redox peaks are
ascribed to the BW12 polyoxoanions, whereas one irrever-
sible broad peak is due to the phendione ligand in the
positive potential region. The peak potentials of the
phendione ligand in PEI–(BW12/[Fe(phendione)3]2+)2 and
PEI–(BW12/[Co(phendione)3]2+)2 multilayer films occur at
+0.306 and +0.300 V, respectively. Apparently, when the
pH value is decreased to 4.4, the redox peak of the
phendione become positively shifted and the reodx peaks of
the BW12 polyoxoanion become irreversible. Therefore, the

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S. Gao et al. / Journal of Solid State Chemistry 179 (2006) 1407–1414
1412
20
0
20
0
-20
-40
-60
-80
-20
-40
-60
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0
E / V vs. Ag|AgCl
E / V vs. Ag|AgCl
(a)
(b)
Fig. 5. Cyclic voltammograms of the multilayers in buffer solution (pH 4.4) solution at different scan rates. (a) PEI–(BW12/[Fe(phendione)3]2+)2;
(b) PEI–(BW12/[Co(phendione)3]2+)2. Scan rate: 50, 100, 150, 200, 250, 300, 350, 400 mV sꢀ1
.
electrochemical behaviors of the BW12 polyoxoanion
and phendione in the multilayer films are pH dependent.
Cyclic voltammograms of [Fe(phendione)3]2+ and [Co
(phendione)3]2+ in the multilayers indicated that the
phendione ligands were electroactive in buffer acetate
solution at the potential of around +0.300 V. The small
variations in potentials were ascribed to transition metal
ions [25] and the polyoxoanions. It was found that the peak
potentials were independent of scan rate and that the peak
currents were directly proportional to scan rates of up to
0.4 V sꢀ1, indicating surface-controlled processes.
From the above result analyses, the electrochemical
behaviors of POMs and transition metal complexes remain
in the multilayer films and the behaviors is pH dependent.
Therefore, the multilayer films have potential application
in electrochemical sensors.
(a)
0
-20
-40
-60
-80
(b)
-0.8
-0.6
-0.4
-0.2
0.0
The electrocatalytic reduction of two substrates, NO2ꢀ
and H2O2, by the multilayer films was investigated. Nitrite
is a common pollutant from both agricultural and
industrial sources [16]. The direct reduction of nitrite at
bare carbon electrodes require a high overpotential. A
large variety of POMs was reported for the electrocatalytic
reduction of nitrite [3,18,30–32]. Fig. 6 shows the electro-
catalytic reduction toward NOꢀ2 . Fig. 6a exhibits the
cyclic voltammogram with PEI–(Co4(PW9)2/[Fe(phen-
dione)3]2+)2 in the absence of NOꢀ2 . The redox peaks of
Co4(PW9)2 polyoxoanion still remain. However, upon
the additions of nitrite, the cathodic peak current of
Co4(PW9)2 increased greatly and the corresponding anodic
peak current disappears completely. This clearly indicates
that the nitrite ion is electrocatalytically reduced by the
Co4(PW9)2 polyoxoanion in PEI–(Co4(PW9)2/[Fe(phen-
E / V vs. Ag|AgCl
Fig. 6. Cyclic voltammograms at GCE modified with PEI–(Co4(PW9)2/
[Fe(phendione)3]2+)2 multilayer film in pH 4.4 buffer solutions before (a)
and after (b) the addition of 2.5 mM NOꢀ2 . Scan rate: 100 mV sꢀ1
.
complexes [33]. Moreover, the PEI–(Co4(PW9)2/[Fe(phen-
dione)3]2+)2 film exhibits the electrocatalytic reduction of
H2O2 (Fig. 7). The electrochemical reduction of H2O2 into
water is of great interest due to its application in biosensors
and fuel cells [34,35]. By increasing the concentration of
H2O2, the cathodic peak current increased, while its anodic
peak current decreased. This shows a typical electrocata-
lytic reduction process. H2O2 is electrocatalytically reduced
by the Co4(PW9)2 polyoxoanion in PEI–(Co4(PW9)2/
[Fe(phendione)3]2+)2 film. The inset indicates the electro-
catalytic current increases linearly with the concentration
of H2O2 (Fig. 7). The possible mechanism of the catalytic
process is ascribed to the intermediate formation of a
peroxo complex between Co4(PW9)2 polyoxoanion and
hydrogen peroxide, which leads to a weakening of the
oxygen–oxygen bond [34]. Therefore, the multilayers
dione)3]2+
)
film. Under our experimental condition,
2
NOꢀ2 is the major species due to disproportionation
reaction HNO2 at pH value 4.4 [16]. The Co4(PW9)2
polyoxoanion reduces the NOꢀ2 to NO, which substitutes
water molecules in the labile site coordinated with
transition metal Co forming transition metal nitrosyl

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S. Gao et al. / Journal of Solid State Chemistry 179 (2006) 1407–1414
1413
the d–d states is above the MLCT state. In addition, BW12
component of the film increases spin–orbit coupling, which
enhances the allowedness of the normally forbidden
transition to the ground state. Thus, the complex
[Fe(phendione)3]2+ of the film is transited from the singlet
MLCT state to the triplet MLCT state through intersystem
crossing, which results in the longer wavelength emission
with (BW12/[Fe(phendione)3]2+)7. The multilayers contain-
ing [Co(phendione)3]2+ do not show photoluminescence
due to the absence of MLCT transition. Thus, it is
understandable that no photoluminescent properties were
observed for [Co(phendione)3]2+ films. Interestingly, the
0
-5
14
13
12
11
10
9
-10
-15
-20
multilayer film of (Co4(PW9)2/[Fe(phendione)3]2+
) also
8
8
0
5
10 15 20 25
has no emission at room temperature, indicating the
photoluminescent properties of the films are influenced
by the component of the films.
Concentration / mM
-0.8
-0.6
-0.4
-0.2
0.0
E / V vs. Ag|AgCl
4. Conclusion
Fig. 7. Cyclic voltammograms at GCE modified with PEI–(Co4(PW9)2/
[Fe (phendione)3]2+)2 multilayer film in pH 4.4 buffer solutions containing
H2O2 in various concentrations: 3.3, 9.9, 16.5, 22.1 mM. Scan rate:
In this paper, we have prepared ultrathin organic–
inorganic composite films that incorporate the phendione
complexes of transition metals and the POMs. CV
demonstrates [Fe(phendione)3]2+ and [Co(phendio-
ne)3]2+of the films exhibit different electrochemical beha-
viors in various pH values. The POMs of the multilayer
film exhibit electrocatalytic reduction of nitrite and
hydrogen peroxide. Therefore, the multilayer films have
potential applications as electrochemical sensors. The
photoluminescent properties of the multilayer films in-
dicate that the film with (BW12/[Fe(phendione)3]2+) emits
low-energy red photoluminesce at room temperature.
100 mV sꢀ1
.
14000
12000
10000
8000
6000
4000
2000
0
Acknowledgments
554.0 654.0 754.0 854.0
This work was financially supported by the financial
support from NNSF of China (90206040, 20325106,
50472021), NSF of Fujian Province (E0520003), and the
‘‘One Hundred Talent’’ project from CAS.
Wavelength / nm
Fig. 8. The emission spectroscopy of PEI–(BW12/[Fe(phendione)3]2+
)
7
multilayer film at room temperature (lex ¼ 270 nm).
exhibit the good electrocatalytic reduction of NOꢀ2 and
H2O2.
The photoluminescence of the multilayer films was also
References
[1] D.L. Feldheim, K.C. Grabar, M.J. Natan, T.E. Mallouk, J. Am.
Chem. Soc. 118 (1996) 7640.
investigated. It was found that (BW12/[Fe(phendione)3]2+
)
7
[2] H. Krass, E.A. Plummer, J.M. Haider, P.R. Barker, N.W. Alcock, Z.
Pikramenou, M.J. Hannon, D.G. Kurth, Angew. Chem. Int. Ed. 40
(2001) 3862.
films exhibits an emission maximum at 672 nm
(lex ¼ 270 nm) (Fig. 8), whereas the multilayers containing
[Co(phendione)3]2+ do not show detectable photolumines-
cence. It was investigated that the photoluminescent
emission of the solid phendione exhibited blue emission
at 490 nm. Hence the emission with (BW12/[Fe(phen-
dione)3]2+)7 is red-shifted compared to the phendione. It
is ascribed to MLCT absorption at 510 nm in [Fe(phen-
dione)3]2+ films. This is possibly because the strong
electrostatic interaction between BW12 polyoxoanion and
[Fe(phendione)3]2+ results in the change of the crystal field
of [Fe(phendione)3]2+ [23]. The change is that the energy of
[3] L. Bi, H. Wang, Y. Shen, E. Wang, S. Dong, Electrochem. Commun.
5 (2003) 913.
[4] J. Cho, F. Caruso, Chem. Mater. 17 (2005) 4547.
[5] G. Decher, Science 277 (1997) 1232.
[6] G. Decher, Y. Lvov, J. Schmitt, Thin Solid Films 244 (1994) 772.
[7] C.F.J. Faul, M. Antonietti, Adv. Mater. 15 (2003) 673.
[8] K. Ariga, Y. Lvov, T. Kunitake, J. Am. Chem. Soc. 119 (1997) 2224.
[9] J. Lang, M.H. Lin, J. Phys. Chem. B 103 (1999) 11393.
[10] S.L. Clark, E.S. Handy, M.F. Rubner, P.T. Hammond, Adv. Mater.
11 (1999) 1031.
[11] D.J. Zhou, A. Bruckbauer, L.M. Ying, C. Abell, D. Klenerman,
Nano Lett. 3 (2003) 1517.

ARTICLE IN PRESS
S. Gao et al. / Journal of Solid State Chemistry 179 (2006) 1407–1414
1414
´
[24] C.A. Goss, H.D. AbruNa, Inorg. Chem. 24 (1985) 4263.
[12] C.A. Constantine, S.V. Mello, A. Dupont, X.H. Cao, D. Santos,
O.N. Oliveira, F.T. Strixino, E.C. Pereira, T.C. Cheng, J.J. Defrank,
R.M. Leblanc, J. Am. Chem. Soc. 125 (2003) 1805.
´
ndez, H.D. AbruNa, E. Lorenzo,
[25] F. Tobalina, F. Pariente, L. Herna
Anal. Chim. Acta 395 (1999) 17.
´
[13] N.A. Kotov, I. De
13065.
´
´
kany, J.H. Fendler, J. Phys. Chem. 99 (1995)
[26] C. Rocchiccioli-Deltcheff, M. Fournier, R. Franck, R. Thouvenot,
Inorg. Chem. 22 (1983) 207.
[14] A.D. Pris, M.D. Porter, Nano Lett. 2 (2002) 1087.
[15] C.L. Hill (Ed.), Chem. Rev. 98 (1998) 1.
[16] N. Fay, E. Dempsey, T. McCrmac, J. Electroanal. Chem. 574 (2005)
359.
[27] R.G. Finke, M.W. Droege, P.J. Domaille, Inorg. Chem. 26 (1987)
3886.
[28] W. Paw, R. Eisenberg, Inorg. Chem. 36 (1997) 2287.
[29] L. Ruhlmann, L. Nadjo, J. Canny, R. Contant, R. Thouvenot, Eur.
J. Inorg. Chem. 4 (2002) 975.
[17] S.O. Liu, D.G. Kurth, H. Mohwald, D. Volkmer, Adv. Mater. 14
¨
(2002) 225.
[30] S. Liu, Z. Tang, Z. Wang, Z. Peng, E. Wang, S. Dong, J. Mater.
Chem. 10 (2000) 2727.
[18] L. Cheng, J. Liu, S. Dong, Anal. Chim. Acta 417 (2000) 133.
[19] G. Gao, L. Xu, W. Wang, W. An, Y. Qiu, Z. Wang, E. Wang,
J. Phys. Chem. B 109 (2005) 8948.
[31] W. Sun, F. Yang, H. Liu, J. Kong, S. Jin, G. Xie, J. Deng,
J. Electroanal. Chem. 451 (1998) 49.
[20] L. Xu, H. Zhang, E. Wang, D.G. Kurth, Z. Li, J. Mater. Chem. 12
(2002) 654.
[32] L. Ruhlmann, G. Genet, J. Electroanal. Chem. 568 (2004) 315.
[33] J.N. Younathan, K.S. Rhodes, T.J. Meyer, Inorg. Chem. 31 (1992)
3280.
[21] A. Kuhn, F.C. Anson, Langmuir 12 (1996) 5481.
[22] G.M. Kloster, F.C. Anson, Electrochim. Acta 44 (1999) 2271.
[23] H. Ma, J. Peng, Y. Chen, Y. Feng, E. Wang, J. Solid State Chem. 177
(2004) 3333.
[34] D. Martel, A. Kuhn, Electrochim. Acta 45 (2000) 1829.
[35] F. Matsumoto, S. Uesugi, N. Koura, T. Ohsaka, J. Electroanal.
Chem. 549 (2003) 71.