Preparation and Photocatalytic Activity of an Electrostatically self-assembled film made of [PMo12O40]3- and a bipolar hemicyanine cation

Article abstract of DOI:10.1166/jnn.2011.5313

An electrostatically self-assembled film of PMo₁₂/H⁶ has successfully been prepared on quartz substrates by alternating adsorption of [PMo₁₂O₄₀]³- (PMo₁₂) and a bipolar hemicyanine derivati

Full text of DOI:10.1166/jnn.2011.5313

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 9813–9817, 2011
Preparation and Photocatalytic Activity of an
Electrostatically Self-Assembled Film Made of
[PMo12O40]3− and a Bipolar Hemicyanine Cation
Li-Hua Gao1ꢀ∗, Wen-Chan Jiang1, and Ke-Zhi Wang2
1Department of Chemistry, School of Science, Beijing Technology and Business University, Beijing 100048, China
2College of Chemistry, Beijing Normal University, Beijing 100875, China
An electrostatically self-assembled film of PMo12/H6 has successfully been prepared on quartz sub-
3−
strates by alternating adsorption of [PMo12O40
]
(PMo12ꢀ and a bipolar hemicyanine derivative of
(E)-1,1ꢀ-(hexane-1,6-diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide (H6Br2ꢀ. The UV-visible
spectra showed that the film was uniformly deposited and the charge transfer between two film-
forming components might occur in the film. The photocatalytic performance of the film was studied
for the degradation of aqueous dye Methyl Red (MR) under UV irradiation. The degradation of MR
follows Langmuir-Hinshelwood first-order kinetics.
Keywords: Polyoxometalates, Hemicyanine, Self-Assembled Film, Photocatalysis, UV-Visible
Spectroscopy.
Delivered by Publishing Technology to: Chinese University of Hong Kong
IP: 41.189.254.81 On: Mon, 07 Dec 2015 10:26:41
1. INTRODUCTION
uses. Among the techniques for the fabrication of ultra-
Copyright: American Scientific Publishers
thin films, the electrostatic self assembly pioneered by
Decher14 has proved to be one of the most attractive tech-
niques, which could achieve multilayer assemblies by sim-
ple electrostatic attraction of oppositely charged species
on a layer-by-layer (LBL) basis with precisely controlled
thickness and layer sequences. To the best of our knowl-
edge, no reports have been made on the photocatalytic
application of POM-containing self-assembled films. Here,
a bipolar hemicyanine derivative of (E)-1,1ꢀ-(hexane-1,6-
diyl)bis(4-(4-(dimethylamino)styryl)pyridinium) bromide
(hereafter abbreviated as H6Br2ꢁ and a Keggin anion
of [PMo12O40]3− (hereafter abbreviated as PMo12ꢁ were
selected for use in the fabrication of an electrostatically
self-assembled film. The photocatalytic behavior of the
film for degradation of aqueous MR under near-ultraviolet
(UV) irradiation was studied.
Since the photocatalytic degradation of inorganic and
organic pollutants was introduced, the photocatalysts with
high catalytic performance have attracted much atten-
tion because of their relation to solar energy conver-
sion and environmental cleaning.1–3 In recent years, there
has been increasing interest in polyoxometalates (POMs),
a class of inorganic metal-oxide clusters, for catalysis4
and photocatalysis.5 Although they have been used as
an efficient photocatalyst for the removal of transition
metal ions or organic pollutants from water,6 which is
analogous to TiO2, a major drawback to the practical
applications of the POM system is its high water solu-
bility, which impedes recovery and reuse of the catalyst.
To conquer this shortcoming, much work has been cen-
tered on the preparation of insoluble POM catalysts, such
as support of POMs on high surface area solids such as
zeolite,7 SiO2,8 and TiO2;9 intercalation of POMs into lay-
ered double hydroxides,10 and precipitation of POMs with
large cations to form insoluble salts.11 Recently, Hu et al.
reported the composite films POM/SiO2 and POM/TiO2
prepared by the sol–gel method as well as the spin-
coating technique for photocatalytic degradation of organic
pollutants.12ꢀ13 Undoubtedly, POM-containing films as cat-
alysts have the advantage of easier handling for recycling
2. EXPERIMENTAL DETAILS
2.1. Materials
3-Aminopropyltrimethoxysilane (98%) was purchased
from Acros. Other chemicals were commercially available
and were used without further purification.
PMo12 was prepared according to the literature method.15
H6Br2 was synthesized by the route shown in Figure 1.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 11
1533-4880/2011/11/9813/005
doi:10.1166/jnn.2011.5313
9813

Preparation and Photocatalytic Activity of an Electrostatically Self-Assembled Film Made of [PMo12O40]3−
Gao et al.
in 1.0 mM H6Br2 aqueous solution. Repetition of the above
two steps yielded a (PMo12/H6ꢁn (n stands for the number
of bilayers) multilayer as shown in Figure 2.
2.3. Physical Measurements
C, H, and N elemental analyses were performed on a
Vario EL elemental analyzer. Infrared spectra were mea-
sured on a Nicolet Avatar 360 FT-IR (Fourier transform
Fig. 1. Synthetic route to dipolar hemicyanine H6Br2.
1
First, 1,1ꢀ-(hexane-1,6-diyl)bis(4-methylpyridinium) bro-
mide was prepared by the recrystallization of ethanol-
ethyl ether powder which was made by refluxing 20 mmol
of 1,6-dꢁibromohexane with 40 mmol of 4-methylpyridine
at 130 C. Then 5 mmol of the obtained 1,1ꢀ-(hexane-
1,6-diyl)bis(4-methylpyridinium) bromide was mixed with
10 mmol of 4-(NꢀN-dimethylamino)benzaldehyde and
1 mL of piperidine in 30 mL absolute ethanol and
refluxed for 7 h. On cooling the reaction mixture to the
room temperature, a red product formed, was filtered,
and was recrystallized three times from ethanol, giving
infrared) spectrometer as KBr disks. An H nuclear mag-
netic resonance (NMR) spectrum was obtained on a Bruker
DRX-500 spectrometer. UV-visible (UV-vis) spectra were
obtained on a GBC Cintra 10e UV-vis spectrophotometer.
2.4. Photocatalytic Procedure
An XPA-1 photoreactor made by Nanjing Xujiang Elec-
tromechanical Plant equipped with a water circulating
jacket and an opening for supply of air was used through-
out the experiment. The light source was a 300 W high-
pressure mercury lamp. For irradiation experiments, 4 mL
of MR solution (25 ꢃM) and a slide of the film (1 cm ×
2 cm) were placed into the photoreactor. All degradation
experiments were carried out at room temperature with the
photoreactor open to air. Decreases in the concentrations
of MR were analyzed by UV-vis spectroscopy. At given
ꢁ
2.6 g of H6Br2 (76%). M. p. 282∼283 C. Anal. Calcd.
for C36H44N4Br2:C, 62.43; H, 6.40; N, 8.09. Found:
C, 62.30; H, 6.20; N, 8.00. IR (KBr, cm−1ꢁ: 2996w,
2926w, 2860w, 1612w, 1586vs, 1528s, 1472w, 1452w,
1435w, 1365m, 1331m, 1211w, 1191w, 1167s, 1127w,
1
1068w, 1046w, 981w, 946w, 880w, 826w. H NMR (500
intervals of illumination, a sample of reaction solution was
Delivered by Publishing Technology to: Chinese University of Hong Kong
MHz, DMSO-d6): ꢂ ppm 8.80∼8.81(4H, d, –C5H4N),
IP: 41.189.254.81 On: Mon, 07 Dec 2015 10:26:41
taken out, and the changes in absorbance at ꢄmax 523 nm
8.08∼8.09(4H, d, –C5H4N), 7.93∼7.96(2H, d,
CH–
Copyright: American Scientific Publishers
of a peak maximum for MR were measured.
), 7.60∼7.61(4H, d, –Ar–H), 7.18∼7.21(2H, d, –CH ),
6.78∼6.80(4H, d, –Ar–H), 4.42∼4.45(4H, t, N–CH2),
3.03(12H, s, –N(CH3ꢁ2ꢁ, 1.89(4H, s, NCH2–CH2),
1.32(4H, s, NCH2CH2–CH2ꢁ.
3. RESULTS AND DISCUSSION
3.1. UV-Visible Absorption Characteristics of
(PMo12/H6ꢁn Films
2.2. Film Preparation
UV-visible spectroscopy was used to monitor the process
of fabricating the hybrid multilayer films of (PMo12/H6ꢁn
on quartz substrates. Figure 3 shows the comparison of
the absorption spectra for PMo12 in H2O, H6Br2 in H2O,
and a 10-layer (PMo12/H6ꢁ10 film. It is clearly seen that
the PMo12 aqueous solution exhibited a strong Od → Mo
The protonation of the 3-aminopropyltrimethoxysilaned
quartz substrate was performed as described before.16 The
substrate was then immersed for 30 min in an aqueous
solution of 1.0 mM PMo12 to adsorb a negatively charged
PMo12 monolayer. After washing with water and drying
with N2, the PMo12-coated substrate was placed for 30 min
Fig. 2. Schematic illustration of the multilayer assembly of (PMo12/H6ꢁn.
9814
J. Nanosci. Nanotechnol. 11, 9813–9817, 2011

Gao et al.
Preparation and Photocatalytic Activity of an Electrostatically Self-Assembled Film Made of [PMo12O40]3−
Fig. 3. The comparison of absorption spectra for PMo12 in (a) H2O,
(b) H6Br2 in H2O, and (c) (PMo12/H6ꢁn film.
Fig. 4. The absorption spectra of (PMo12/H6ꢁn multilayers (n = 1 to 10)
on double sides of quartz. The inset is the plot of absorbance at 492 nm
of the films versus layer number.
charge transfer (CT) absorption band at 220 nm. The
H6Br2 aqueous solution gave a ꢅ → ꢅ* absorption band
at 265 nm and a CT absorption band at 459 nm,16 the
multilayer film of (PMo12/H6ꢁ10 exhibited two absorption
bands at 275 and 492 nm. The former band was the
superposition of the PMo12-based and H6Br2-based CT
absorption bands because the intensity of the absorption
band at 275 nm is stronger than that of the absorption
approximately equal to 2.45 × 104 for H6Br2 in aqueous
solution, the surface concentration of H6Br2 in the film is
derived to be 6.8×10–10 mol cm−2. The occupied area per
molecule is derived to be 0.24 nm2.
3.2. Photocatalytic Activity
Delivered by Publishing Technology to: Chinese University of Hong Kong
band at 492 nm. The latter band has a characteristic of
In order to study the photocatalytic activity of films,
we chose azo-compound Methyl Red (Fig. 5)—a non-
biodegradable organic pollutant and represents an increas-
ing environmental danger—as the model pollutant. The
results of degradation of aqueous MR under irradiation in
the near-UV range are shown in Figure 6. When the film
was put into the aqueous MR solution, and was placed in
the dark for 220 min, MR was negligibly degraded. How-
ever, apparent decreases in the concentrations of MR were
observed by irradiating the MR solutions in the presence
of one slide of (PMo12/H6ꢁ14/PMo12 film, i.e., degradation
of MR was about 68% when irradiation time was 220 min.
These results suggest that degradation of MR mainly origi-
nated from photocatalysis of the (PMo12/H6ꢁ14/PMo12 film.
The photocatalytic activity of the (PMo12/H6ꢁ14/PMo12 film
is attributed to PMo12 component, which shares photo-
chemical characteristics similar to that of semiconductors
such as TiO2,21 that is, its highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) resemble the valence and conduction bands of
semiconductors, respectively. The charge transfer from
O2− to Mo6+ in the PMo12 occurs as a result of UV irra-
diation, resulting in the formation of a hole (O−ꢁ and a
6
IP: 41.189.254.81 On: Mon, 07 Dec 2015 10:26:41
H Br2-based CT absorption and is red shifted by 33 nm
Copyright: American Scientific Publishers
compared with the H6Br2-based CT band for the aque-
ous solution. This indicates that PMo12 and H6 were suc-
cessfully assembled into the film. The red-shift observed
for the CT transition between the film and the solu-
tion is most probably due to the charge transfer between
PMo12 and H6Br2, which has already been observed in pre-
viously reported hemicyanine-containing electrostatically
LBL self-assembled films.17–19
Figure 4 shows the UV-vis absorption spectra of the
LBL growth of the (PMo12/H6ꢁn self-assembled multilayer
film on a quartz substrate. As shown in Figure 4, the
absorption maxima are independent of the number of lay-
ers deposited, indicating that the interlayer molecular inter-
action is independent of layer number. The absorptions of
the films at 492 nm increased linearly with the number of
deposited (PMo12/H6ꢁn layers (inset of Fig. 4) with corre-
lation coefficients of 0.9970, demonstrating that the film
deposition is uniform and reproducible.
From the Lambert–Beer law of modification for two-
dimensional concentration—i.e., ꢆ = 10–3D/ꢇ,20 where
ꢆ is the surface concentration (mol cm−2ꢁ, D is the
absorbance per layer, and ꢇ is the molar extinction coeffi-
cient for the film-forming molecule in the film at a fixed
wavelength—the absorbance per layer at 492 nm, calcu-
lated from the slope in the linear relationship in the inset
of Figure 4, was found to be 1.67 × 10−2. If we sup-
pose that an ꢇ value at 492 nm for H6Br2 in the films is
Fig. 5. Structure of MR.
J. Nanosci. Nanotechnol. 11, 9813–9817, 2011
9815

Preparation and Photocatalytic Activity of an Electrostatically Self-Assembled Film Made of [PMo12O40]3−
Gao et al.
where C0 is the initial MR concentration in solution, and
t is the illumination time. The results of MR degrada-
tion kinetics in aqueous solution under UV light irradia-
tion are shown in Figure 7. It can be seen that a plot of
ln(C0/Cꢁ versus t represents a straight line, indicating the
MR degradations follow an apparent first order process.
The apparent first-order rate constant kapp was derived to
be 0.0053 min−1 by a slope of the linear regression.
4. CONCLUSIONS
We have successfully assembled an electrostatic self-
assembled film by alternating adsorption of a Keggin
anion of [PMo12O40]3− and a bipolar hemicyanine deriva-
tive of (E)-1,1ꢀ-(hexane-1,6-diyl)bis(4-(4-(dimethylamino)
styryl)pyridinium) bromide. The film exhibits interesting
photocatalytic activity for the degradation of aqueous
dye Methyl Red under UV irradiation. The degrada-
tion of MR follows Langmuir-Hinshelwood first-order
kinetics. The results indicate that this film photocata-
lyst for recycling uses has the advantage of easy separa-
tion from the reaction system over powdered or soluble
catalysts.
Fig. 6. Changes in MR concentrations as a function of UV irradiation
time. Conditions: C0 = 25 ꢃM, VMR = 4 ml.
trapped electron center (Mo5+ꢁ, i.e., the charge-transfer
excited state PMo∗12. O− has strong oxidation ability,22 and
is also responsible for the oxidation of the MR dye.
The degradation behavior of MR can be described by
the Langmuir-Hinshelwood rate Eq. (1),23 which is fur-
ther simplified to be a pseudo-first-order rate Eq. (2) with
respect to the MR concentration for experiments conducted
with low concentratons of MR:
Acknowledgments: The authors thank the National
Natural Science Foundation (Nos. 20971016, 90922004,
20871011,21171022), Beijing Natural Science Foundation
Delivered by Publishing Technology to: Chinese University of Hong Kong
dC
kKC
(1)
IP: 41.189.254.81 On: Mon, 07 Dec 2015 10:26:41
r = −
=
(2072011, 2092011), Funding Project for Innovation on
dt
1+KC
Copyright: American Scientific Publishers
Science, Technology and Graduate Education in Institu-
dC
tions of Higher Learning Under the Jurisdiction of Beijing
Municipality and Measurements Fund of Beijing Normal
University for financial support.
r = −
= kapp
C
(2)
dt
where C is the MR concentration in solution at illumina-
tion time t, k is the rate constant, K is the equilibrium
adsorption constant, and kapp is the the apparent pseudo-
first-order rate constant. Integrating Eq. (2) gives
References and Notes
ꢀ
ꢁ
C0
C
1. D. Y. Goswami, J. Solar Energy Eng. 119, 101 (1997).
2. M. R. Hoffmann, S. T. Martin, W. Chio, and D. W. Bahnemann,
Chem. Rev. 95, 69 (1995).
ln
= kapp
t
(3)
3. Y. H. Hsien, C. F. Chang, and Y. H. Chen, Appl. Catal. B 31, 241
(2001).
4. N. Mizuno and M. Misono, Chem. Rev. 98, 199 (1998).
5. A. Hiskia, A. Troupis, S. Antonaraki, E. Gkika, and
E. Papaconstantinou, Int. J. Environ. Anal. Chem. 86, 233 (2006).
6. E. Gkika, A. Troupis, A. Hiskia, and E. Papaconstantinou, Appl.
Catal., B: Environmental 62, 28 (2006).
7. R. R. Ozer and J. L. Ferry. J. Phys. Chem. B 106, 4336 (2002).
8. Y. H. Guo, Y. H. Wang, C. W. Hu, E. B. Wang, Y. C. Zhou, and
S. H. Feng, Chem. Mater. 12, 3501 (2000).
9. C. C. Chen, P. X. Lei, H. W. Ji, W. H. Ma, J. C. Zhao, H. Hidaka,
and N. Serpone, Environ. Sci. Tech. 38, 329 (2004).
10. Y. H. Guo, D. F. Li, C. W. Hu, Y. H. Wang, and E. B. Wang, Int.
J. Inorg. Mater. 3, 347 (2001).
11. Z. W. Xi, N. Zhou, Y. Sun, and K. L. Li, Science 292, 1139 (2001).
12. D. F. Li, Y. H. Guo, C. W. Hu, L. Mao, and E. B. Wang, Appl.
Catal. A: General 235, 11 (2002).
13. Y. Yang, Y. H. Guo, C. W. Hu, and E. B. Wang, Appl. Catal. A:
General 252, 305 (2003).
Fig. 7. First-order linear relation ln(C0/Ctꢁ = kappt. Conditions:
C0 = 25 ꢃM, VMR = 4 ml.
14. G. Decher, Science 277, 1232 (1997).
15. H. Wu, J. Biol. Chem. 43, 189 (1920).
9816
J. Nanosci. Nanotechnol. 11, 9813–9817, 2011

Gao et al.
Preparation and Photocatalytic Activity of an Electrostatically Self-Assembled Film Made of [PMo12O40]3−
16. L. Y. Wang, K. Z. Wang, and L. H. Gao, Acta Chim. Sinica 61, 1877
(2003).
17. L. H. Gao, X. J. Hu, D. S. Zhang, Y. Guo, and K. Z. Wang,
J. Nanosci. Nanotechnol. 8, 1355 (2008).
20. L. H. Gao, K. Z. Wang, L. Cai, H. X. Zhang, L. P. Jin, C. H. Huang,
and H. J. Gao, J. Phys. Chem. B. 110, 7402 (2006).
21. A. Hiskia, A. Mylonas, and E. Papaconstantinou, Chem. Soc. Rev.
30, 62 (2001).
18. L. H. Gao, K. Z. Wang, and L. Y. Wang, J. Nanosci. Nanotechnol.
10, 2108 (2010).
22. Y. Yang, Y. H. Guo, C. W. Hu, Y. H. Wang, and E. B. Wang, Appl.
Catal. A: General 273, 201 (2004).
19. L. H. Gao, L. Y. Wang, and K. Z. Wang, Solid State Phenomena
121–123, 457 (2007).
23. Y. Ku, W. H. Lee, and W. Y. Wang, J. Mol. Catal. A: Chem. 273, 191
(2004).
Received: 13 November 2010. Accepted: 18 April 2011.
Delivered by Publishing Technology to: Chinese University of Hong Kong
IP: 41.189.254.81 On: Mon, 07 Dec 2015 10:26:41
Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 11, 9813–9817, 2011
9817