Polyoxometalate-based layered nano-tubular arrays: facile fabrication and superior performance for catalysis

Article abstract of DOI:10.1039/c5ra01247c

Keggin-type polyoxoanion PW₁₂O₄₀^3--containing one-dimensional nano-tubular arrays, with the structure polyethylenimine/polystyrenesulfonate/(poly(allylammonium)/polystyrenesulfonate)₃(poly(allylammonium)/PW₁₂O₄₀^3-)₈, were fabricated as a prototype using a layer-by-layer deposition technique in porous anodic aluminum oxide and polycarbonate templates with a pore diameter of 200 nm, and characterized by IR, UV-vis and SEM. The resulting nano-tubes have a uniform diameter of 180 ± 20 nm and a uniform wall thickness of 30 ± 5 nm. These arrays have shown superior performance in the UV light irradiated photo-degradation of Rhodamine B (selected as a representative dye) under mild conditions with respect to both the catalytic efficiency and operating convenience due to the confinement within the nano-tubes both in the latitudinal and radial direction. Remarkably, the catalytic activity of the nano-tubular arrays could be recovered by means of simple immersion in the polyoxoanion solution when the catalytic reactivity became reduced, which is vital in view of practical applications. The total organic carbon content changes and GC-MS measurements were conducted to identify the degradation products. The hydroxyl radical mechanism was found to be adopted by the photo-degradation reaction.

Full text of DOI:10.1039/c5ra01247c

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Polyoxometalate-based layered nano-tubular
arrays: facile fabrication and superior performance
Cite this: RSC Adv., 2015, 5, 24550
for catalysis†
ab
b
a
a
a
b
Hongpeng Zhen, Xiaolin Li, Lijuan Zhang,* Huan Lei, Chao Yu, Yunshan Zhou,*
a
a
a
Sadaf ul Hassan, Libo Qin and Hafiz Muhammad Asif
3
ꢀ
Keggin-type polyoxoanion PW12
O
40 -containing one-dimensional nano-tubular arrays, with the structure
(poly(allylammonium)/
, were fabricated as a prototype using a layer-by-layer deposition technique in porous anodic
polyethylenimine/polystyrenesulfonate/(poly(allylammonium)/polystyrenesulfonate)
3
3ꢀ
PW12O
40
)
8
aluminum oxide and polycarbonate templates with a pore diameter of 200 nm, and characterized by
IR, UV-vis and SEM. The resulting nano-tubes have a uniform diameter of 180 ꢁ 20 nm and a uniform
wall thickness of 30 ꢁ 5 nm. These arrays have shown superior performance in the UV light irradiated
photo-degradation of Rhodamine B (selected as a representative dye) under mild conditions with
respect to both the catalytic efficiency and operating convenience due to the confinement within the
nano-tubes both in the latitudinal and radial direction. Remarkably, the catalytic activity of the
nano-tubular arrays could be recovered by means of simple immersion in the polyoxoanion solution
when the catalytic reactivity became reduced, which is vital in view of practical applications. The total
organic carbon content changes and GC-MS measurements were conducted to identify the
degradation products. The hydroxyl radical mechanism was found to be adopted by the
photo-degradation reaction.
Received 21st January 2015
Accepted 23rd February 2015
DOI: 10.1039/c5ra01247c
www.rsc.org/advances
+
15–17
NH ,
or formation of nanoparticles and/or preparation of
4
Introduction

lms by coating POMs onto inorganic/organic composites such
18–21
Polyoxometalates (POMs) are a class of discrete transition metal as TiO
2
, SiO
2
, activated carbon or organic polymer.
However,
anionic clusters with abundant compositions, structures and the exploration of new approaches to synthesise POM catalysts
versatile properties leading to their successful application in that can be separated more conveniently from reaction systems
1–3
various elds. Among the important/potential applications, and have a more convenient recycling process as well as an
the catalytic activities of POMs are the most fascinating feature enhanced superior catalytic performance still remains
and they are outstanding for use in homo-/heterogeneous acid challenging.
4
–8
or redox catalyses with advantages such as photo-/thermal
stability,
Inspired by the continuing great success of nanotubular
9,10
11
22,23
non-toxicity,
low cost and environmentally materials in various elds,
incorporating polyoxometalates
12
friendly behavior. However, most POMs are highly-soluble in into nano-tubular arrays may be expected to overcome limita-
24
aqueous solution and form a homogeneous catalyst which is tions i.e., small surface area and difficulty in recycling and
oen inconvenient for industrial purposes.
13,14
13
In contrast, reuse due to water solubility, as well as to realize upgraded
heterogeneous catalysis is easier to run and separate, yielding a catalytic activity because of the specic characteristics such as
sustainable catalytic process. Many efforts have been made the nanotubular shape with controllable wall thickness and
towards the heterogenization of homogeneous POM-based aperture, high specic surface area exhibiting more active sites,
catalysts such as the syntheses of heterogeneous catalysts by connement both in the latitudinal and radial direction within
+
+
combining POMs with counter cations such as K , Cs and the nano-tubes and the possible synergistic and coupling effects
of arrays. In this work, we report the fabrication and charac-
terization of nano-tubular arrays based on the well-known a-
a
State Key Laboratory of Chemical Resource Engineering, Beijing University of Keggin-type H
PW12O40, which is selected as a representative
3
Chemical Technology, Beijing 100029, China. E-mail: zhouys@mail.buct.edu.cn;
15,25–33
due to its superior performance as a catalyst
(1882 refer-
ljzhang@mail.buct.edu.cn; Tel: +86-10-64414640
b
ences were found containing the concept “tungstophosphoric
acid catalysis” in the SciFinder database on Jan 16, 2015). The
nano-tubular arrays showed an excellent performance as a
photo-catalyst to degrade RhB (difficult to degrade completely
College of Materials Science and Engineering, Beijing University of Chemical
Technology, Beijing 100029, China
†
Electronic supplementary information (ESI) available: Fig. S1–S9 and Table S1.
DOI: 10.1039/c5ra01247c
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by conventional methods and also suspected to be carcino- equipped with a DB-5 MS capillary column (30 mm ꢂ 0.25 mm).
genic) under UV-vis light irradiation. Importantly, the catalytic The temperature program of the column was set as follows: at
ꢃ
ꢃ
ꢃ
ꢀ1
activity of the POM nano-tubular arrays can be renewed by 60 C, hold time ¼ 1 min; from 60 to 250 C, rate 20 C min
.
simple immersion in a H PW O -containing solution when The TOC measurement was performed on a Shimadzu TOC-
3
12 40
the catalytic reactivity is reduced aer recycling many times, 5000A analyser. Photoluminescence spectra were obtained on
which is very important in view of practical applications.
a Hitachi F-7000 uorescence spectrophotometer with an
emission slit of 5 nm using a 150 W xenon lamp as the light
source. The millipore lter setup for preparing the nano-tubular
array and carrying out the photo-catalysis reaction was
purchased from Auto science Co., Ltd (40 mL in volume and 25
mm in diameter) (Fig. S1†).
Experimental
Polyethylenimine (PEI, MW 50 000) and sodium poly-
styrenesulfonate (PSS, MW 70 000) were purchased from
Aldrich. Poly(allylammonium chloride) (PAH, MW 70 000) was
3
ꢀ)8 nano-
ꢀ
1
Preparation of PEI/PSS/(PAH/PSS) (PAH/PW O
purchased from Alfa Aesar. The solution of PEI (10 mg mL ),
3
12 40
ꢀ
1
ꢀ1
tubular arrays within porous templates
PSS (2 mg mL ) and PAH (1 mg mL ), each containing 0.1 mol
L
ꢀ
1
of NaCl to enhance the ionic strength, was prepared at pH
The AAO template was kept on the millipore lter (Fig. S1†) and
the pressure was adjusted to 0.020 MPa by a buffering device to
prevent it from breaking. Firstly, 20 mL deionized water was
1
.5 adjusted by dilute HCl. H 40$nH O and RhB were
3
PW12
O
2
purchased from Sinopharm Chemical Reagent Beijing Co., ltd.
All reagents used in this work were of analytical grade and used
as received. Deionized water was used throughout the experi-
ments. Porous anodic aluminium oxide membrane (AAO
template: diameter 25 mm, pore diameter 200 nm, average lm
thickness 60 mm) and nucleporetrack-etched polycarbonate
membranes (PC template: diameter 25 mm, pore diameter 200
nm, average lm thickness 20 mm) were purchased from
Whatman corp. The structures of the reactants used are shown
in Fig. 1.

owed through the template from each side under low pressure
conditions, then a 20 mL PEI solution was owed through the
template from each side. Subsequently, the template was
immersed into deionized water for 10 min and then washed
with 10 mL deionized water. Similarly, PSS and PAH were
alternately adsorbed onto the inner wall of the porous template
forming a precursor lm with the structure (PEI/PSS)/(PAH/
3
ꢀ
PSS)
3
. Then, PAH and PW12
O
40
were alternately adsorbed
40 solution
mol L ), respectively, resulting in the formation of
layered nano-tubular arrays with the structure PEI/PSS/(PAH/
onto the precursor lm from a PAH and a H
3
PW12O
ꢀ
3
ꢀ1
(10
Instruments and techniques of measurement
3ꢀ
PSS)
3
(PAH/PW12
O
40
) within the pores of the AAO template
8
The FT-IR spectra were recorded on a MAGNA-IR 750 (Nicolet)
(
denoted PW -AAO). All the preparation processes were per-
1
2
spectrophotometer with KBr pellets in the range of 400–4000
ꢀ
1
formed under ambient conditions. The same procedure was
performed when a PC template was used, resulting in the
formation of nano-tubular arrays with the structure PEI/PSS/
cm . Light intensity was recorded by a radiometer FZ-A (the
photoelectric instrument factory of Beijing Normal University,
China). Scanning electron microscopy (SEM) and energy
dispersive X-ray spectroscopy were carried out on a Zeiss Supra
3
ꢀ
(PAH/PSS)
3
(PAH/PW12
O
40
)
8
within the pore of the PC
template (denoted PW12-PC). The fabrication process of POM/
polyelectrolyte nano-tubular arrays through LBL coating and
the sequential release of nano-tubes are shown in Fig. 2. Release
and collection of the nano-tubes from the PC template for
characterization was carried out by dissolving the PC template
in chloroform followed by centrifugation of the solution. The
5
5 scanning electron microscope. The UV-vis spectra were
recorded on a SHIMADZU UV-2550 spectrometer. The photo-
reaction was conducted under a 250 W high-pressure mercury
lamp with a light intensity of 22 mW cm . GC/MS analyses
were carried out on a Shimadzu GC/MS QP2010 system
ꢀ2
Fig. 2 Schematic diagram illustrating the fabrication process of PEI/
3 8
PSS/(PAH/PSS) (PAH/PW12) nano-tubular arrays within the PC/AAO
Fig. 1 Structures of the polyelectrolyte PEI (a), PSS (b), PAH (c), dye template through the LBL coating method and release of the nano-
3
ꢀ
RhB (d), and the anion PW12
O
40 (e).
tubes from the template for characterization.
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3
ꢀ
n
method used for release and collection of the nano-tubes from the growth of a POM lm (PAH/PW12
O
40
)
on a quartz plate
35
the AAO template is given in the ESI.†
was monitored instead. It was found that while the charac-
35,36
teristic absorption
of the Keggin-type PW12 appearing at 266
3
ꢀ
n
nm attributed to the W ) Ob,c charge transition does not
change in position during the whole deposition process, the
absorbance intensity increases linearly with the increasing
Preparation of PEI/PSS/(PAH/PSS)
3
(PAH/PW12
O
40
) multi-
layered nanolms on a substrate
A quartz plate was immersed into a fresh piranha solution
3
40
ꢀ
number of bilayers, (PAH/PW12
O
)
n
(n ¼ 1–8) (Fig. 3). This
ꢃ
(
conc. H
2
SO
4
/30% H
2
O
2
¼ 7 : 3 (v/v)) at 80 C for 1 h, then rinsed
not only conrms the alternate deposition of negatively charged
with a copious amount of deionized water and dried in a
nitrogen atmosphere. Aer immersion in PEI and PSS, then
alternately in PAH and PSS for three cycles and nally alter-
3
ꢀ
PW12
O
40
anions and positively charged PAH but also
demonstrates that the thickness of the lms is precisely
controllable. Though this result is obtained from a at surface
and may not be directly extended to a cylindrical type substrate,
however, we expect that the same linear relationship exists for
ꢀ
3
ꢀ1
nately in PAH and H
3
PW12O40 (10 mol L ) each for 20 min,
the coated quartz was characterized by UV-vis in order to
3
ꢀ
monitor and determine the growth of (PAH/PW12
bilayers.
O
40
)
3
ꢀ
the growth of (PAH/PW12
O
40
) nano-tubes.
n
The SEM images of the PEI/PSS/(PAH/PSS)
3
(PAH/
nano-tubes released from the PC template (Fig. 4)
3
ꢀ
8
PW12
O
40
)
3
Photo-degradation of RhB by PEI/PSS/(PAH/PSS) (PAH/
show that the tubes have a uniform diameter of 180 ꢁ 20 nm
3
ꢀ
PW12
O
40
8
) nano-tubular arrays
and a uniform wall thickness which reaches 30 ꢁ 5 nm. The
3
ꢀ
For the photo-catalytic reaction, a high-pressure mercury lamp average thickness of each (PAH/PW O
2 40
) bilayer is roughly
1
was used as the light source. 40 mL of RhB solution (2 mg RhB calculated to be ca. 2 nm (Fig. 4b). The SEM images of nano-
per liter of solution) was ltered through the PW12-AAO and tubes released from AAO template (Fig. S2†) illustrates tubular
PW12-PC catalyst, respectively, which were placed on the milli- structures with uniform diameter of 180 nm ꢁ 20 nm and a wall
pore lter (Fig. S1†). The entire degradation process was carried thickness of 20 ꢁ 5 nm which is a little thinner than nano-tubes
out under ambient conditions. The characteristic absorbance released from PC template because of partial dissolution during
changes at lmax ¼ 554 nm of RhB were monitored and the release process in phosphoric acid (ESI†).
3
ꢀ
8
decoloration efficiency (DE) was calculated by the equation:
The FT-IR spectra of PEI/PSS/(PAH/PSS) (PAH/PW O
)
3
12 40
1
5,16,35,36
DE ¼ [(A
before the reaction, and A
ltration times.
0
ꢀ A
t
)/A
0
] ꢂ 100%, where A
0
stands for the absorbance nano-tubes (Fig. S3†) show the four characteristic peaks
stands for the absorbance at given of a-Keggin-type PW O
3
ꢀ
ꢀ1
t
1
2
40
: P–O bond in PO (n ¼ 1083 cm ),
4
as
ꢀ1
ꢀ1

W–O –W bond (n ¼ 890 cm ), W–O –W bond (n ¼ 800 cm
)
b
as
c
as
(O
b
is the oxygen atom between the two different W
3
O13 groups,
O
c
is the oxygen atom in the same W 13 group) and W]O
3
O
d
Results and discussion
Fabrication and characterization of PEI/PSS/(PAH/PSS)
ꢀ
1
bonds (nas ¼ 952 cm ). The red/blue shi of peaks compared to
3
(PAH/
3ꢀ
the parent PW12
O
40 can be attributed to the strong attraction
PW12
)
8
nano-tubular arrays
3ꢀ
between negatively charged PW O
2 40
and positively charged
1
Despite the so-called layer-by-layer (LBL) technique (viz., PAH. The semi-quantitative EDX analyses (Fig. S4†) show that
immersing a porous template such as AAO and PC alternately the P/W atom ratio of the nano-tubes released from the
into different reactant-containing solutions) having been templates is 1.13 : 10.52, which is relatively close to the ratio of
3
ꢀ
commonly adopted for preparing polyelectrolyte-involving PW12
O
40
(P/W ¼ 1 : 12). The above analyses further conrm
3ꢀ
nano-tubes, this method usually suffers from an unavoidable that the PW12
drawback, viz., the entrance of nano-/microsized pores of the arrays.
template are easily blocked due to the strong interaction
between the charged polyelectrolyte and the oppositely charged
template surface, which prevents the polyelectrolyte molecules
and other reactants from entering the pores and anchoring to
O
40
structure remains intact in the tubular
34
the inner template walls. Consequently, the desired nano-/
microtubes cannot be formed inside the pores of the template
in the process of alternate immersion.
In our present work, the blockage problem has been effec-
tively solved by using a reduced pressure ltration process,
where the pressurized ow of solutions of alternately charged
polyelectrolytes/polyoxoanions and the neutral washing steps
between them results in the LBL deposition of a PEI/PSS/(PAH/
PSS) (PAH/PW ) multilayer on the inner walls of the template.
3
12 8
UV-vis spectrum measurement is an effective method to
monitor the growth of a lm which is formed by the LBL tech-
nique. Since it is very difficult to directly monitor the growth of
each bilayer of (PAH/PW12) in the nanopores of the template, PW12
3ꢀ
Fig. 3 The UV-vis spectra of PEI/PSS/(PAH/PSS)
n ¼ 1–8) multilayered films. The inset shows the linear relationship
between absorbance intensity at 266 nm and the number of (PAH/
3
(PAH/PW12
O
40
)
n
(
3ꢀ
O
40 ) bilayers.
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3ꢀ
Fig.
4
The SEM images of PEI/PSS/(PAH/PSS)
3
(PAH/PW12
O
40
)
8
nano-tubes released from the PC template: (a) the side view of the
nano-tubes; (b) the cross-section view of the nano-tubes.
Photo-degradation of RhB solution by PEI/PSS/(PAH/
3
ꢀ
PSS) (PAH/PW O
)8 nano-tubular arrays
3
12 40
In the preliminary experiments, a PW12-AAO was rst
immersed into the dye solution for 0.5 h to ensure absorption–
desorption equilibrium between the dye and the catalyst
under dark conditions in order to avoid the effect of adsorp-
2
8
tion of the dye before all the measurements. Initially, the
inuence of the light source on the degradation efficiency was
discussed. Four types of experiments were conducted: (1)
under daylight irradiation with bare AAO, (2) under UV light
irradiation with bare AAO, (3) under daylight irradiation with
the PW -AAO and (4) under UV light irradiation with the
1
2
PW -AAO. Under each of the four conditions, 40 mL of RhB
1
2
solution was ltered 5 times circularly through a bare AAO
template and a PW12-AAO. The results showed that RhB was
more remarkably degraded (Fig. 5, Table S1†) under UV irra-
diation in the presence of the nano-tubular arrays as
Fig. 5 UV-vis absorption spectral changes of RhB vs. circular filtering
times at pH 5 under UV light with PW12-AAO (where the repeated
filtration process of filtered RhB solution through the catalyst is called
circular filtering). The arrow mark shows the degradation of the RhB
compared to the other conditions (Fig. S5†). The decoloration with increasing number of circular filtering (initial, original RhB solu-
tion; 1, first time filtering; 2, second time filtering; 3, third time filtering;
efficiency reached a maximum of 94.5% aer circular ltration
4, fourth time filtering; and 5, fifth time filtering) (a) and the degradation
5
times under UV light irradiation with the PW -AAO catalyst
12
effect of RhB under four different conditions: daylight with bare AAO
line a), UV light with bare AAO (line b), daylight with PW12-AAO (line c)
at pH 5. The results indicated that UV irradiation and the
nano-tubular array catalyst are compulsory for the degrada-
tion of RhB.
(
and UV light with PW12-AAO (line d) (b).
As shown in Fig. 6 and Fig. S6,† the photo-degradation effi-
ciency varies at different pH values of RhB solution (pH ¼ 3.0,
under the optimal conditions, viz., UV light irradiation and
pH ¼ 3. In addition, since the AAO template has better parallel
array morphology, larger pore density and greater thickness
(Fig. S2c and d†) than the PC template (Fig. S7a†), the total
4
.0, 5.0, and 6.0) under the optimal conditions, viz., UV light
irradiation with PW12-AAO (Table 1). The results show that PEI/
3ꢀ
PSS/(PAH/PSS) (PAH/PW O
)8 nano-tubular arrays have a
3
12 40
higher photo-catalytic activity at a lower pH value of the reaction
system (decoloration efficiency rises from 79.6% (pH ¼ 6.0) to
9
7.8% (pH ¼ 3.0)), which suggests that the acidic condition is
favorable to the degradation of RhB molecules in the presence
of the catalyst. It should be noted that the observation of effect
of the pH on photocatalytic RhB degradation is analogous to
31
those previously reported. However, when the pH value
decreases below 2.5, the acid–base equilibrium of RhB in
2
+
+
solution, H RhB 4 HRhB 4 RhB, will change and RhB may
2
combine with more protons to form the protonated structure
2
+
37
2
H RhB , which causes a blue shi of the peak from 554 nm
2
+
2
for RhB to 495 nm for H RhB and consequently it gets difficult
to monitor the catalytic efficiency at 554 nm using UV-vis
spectral measurements. Therefore, conditions at pH < 2.5 are
not considered in this study.
Fig. 6 Effect of pH on photo-degradation of RhB (line a, pH 6.0; line b,
pH 5.0; line c, pH 4.0 and line d, pH 3.0) vs. circular filtering times for
The above results clearly demonstrate that the new nano-
tubular array system shows high-efficient catalytic ability RhB solution with PW12-AAO under UV light.
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Table 1 The effect of pH values on RhB degradation under UV light
irradiation with PW12-AAO
Group
pH
Circulation times
Degradation efficiency (%)
1
2
3
4
6.0
5.0
4.0
3.0
5
5
5
5
79.6
94.5
95.0
97.8
number of nano-tubes and catalyst loading capacity within the
AAO template is subsequently larger than that within the PC
template, and the nano-tubular arrays obtained from AAO are
better than those from PC in terms of both quantity and regu-
larity. Consequently, much better photo-catalytic decoloration
Fig. 8 UV-vis absorption spectral changes of RhB by filtering once
through 5 overlapped PW12-PCs (line b) and 5 overlapped PW12-AAOs
(line c), and filtering 5 circular times through one PW12-AAO (line e)
and one PW12-PC (line d) at the optimal catalytic conditions. Line a is
results were obtained when PW -AAO was used (decoloration the UV-vis spectrum of the original RhB solution.
1
2
efficiency DE ¼ 98%) (Fig. 7 and S8†) as compared with PW -PC
1
2
(
decoloration efficiency DE ¼ 91%) under the same optimal
conditions.
Interestingly, it was observed that when 5 PW12-AAOs and 5
conditions. Prior to degradation, the PW12-AAO was immersed
in the dye solution (40 mL, 2 mg L ) for 0.5 h in the dark. The
maximum TOC removal of RhB was observed to be 42.6% (from
ꢀ
1
PW12-PCs were overlapped respectively and used once at the
optimal catalytic conditions, the photo-catalytic decoloration
efficiency (DE ¼ 91% for 5 PW12-AAOs, 77% for 5 PW12-PCs)
was a little bit weaker than that for ltering 5 times using a
single PW -AAO (DE ¼ 98%) and a single PW -PC (DE ¼
1.77 to 1.02 ppm) at the point when the solution was almost
decolorized aer 5 circular ltering times. Therefore, it can be
concluded that around half of the dye was mineralized into CO2
1
2
12
during the degradation process. However, CO was not the only
2
9
0%) (Fig. 8), respectively. This result could be tentatively
product as some other organic products were also generated.
Under the same conditions, the maximum TOC removal of RhB
was 37.7% with PW12-PC used as the photo-catalyst, which is a
little inferior to PW12-AAO.
A GC/MS technique was used to analyse the nal products of
RhB degradation. The listed products in Table 2 are the main parts
attributed to the fact that the middle nanotubular arrays
cannot be well irradiated by UV light when ve PW12-AAOs/
PW12-PCs overlap tightly together. Despite this moderate
decrease in photo-degradation efficiency, remarkable conve-
nience is achieved when 5 overlapped PW12-AAOs and 5
overlapped PW -PCs were used for one circular ltration as
1
2
of the RhB framework besides CO while the other products with
2
compared to using single PW -AAO and single PW -PC and
1
2
12
greatly lower intensities are neglected. From the above results, it
can be concluded that the conjugated xanthene structure of RhB

ltering 5 times.
25,26,28,29,31,37,38
was partially split up during the degradation reaction
in the presence of PEI/PSS/(PAH/PSS) (PAH/PW12
3
)
8
nano-
Total organic carbon (TOC) measurement in the degradation
of RhB and identication of the nal products
tubular arrays under the UV light. The relative amount of each
degradation product was calculated according to the TOC changes
Fig. 9 shows the TOC changes during photo-degradation of RhB
(Table 2).
with circular ltering times with a PW -AAO under the optimal
1
2
Fig. 7 The comparison of RhB decoloration efficiency with PW12-PC Fig. 9 TOC changes during photo-degradation of RhB versus circular
line a) and with PW12-AAO (line b) after 5 circular filtering times under filtering times with PW12-AAO as the catalyst under the optimal
the optimal conditions. conditions.
(
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Table 2 Final major products of the photo-degradation of RhB and
their respective amounts detected by GC/MS
Retention time (min)
Detected product
Relative amount
0.18
1
.0
1
5
.25
.2
0.06
1.00
Fig. 10 Photoluminescence spectral changes (lmax ¼ 425 nm) versus
3 2 6
CH (CH ) COOH
UV light irradiation time on the nano-tubes released from PC
ꢀ
4
templates in a 5 ꢂ 10 M basic solution of terephthalic acid (excitation
light l ¼ 315 nm, voltage ¼ 500 V, EX-Slit ¼ 10 nm, EM-Slit ¼ 10 nm).
Photo-catalysis mechanism study
3
ꢀ
Photo-degradation of RhB has been performed by PW12
O
40
the fact that the cOH radicals are produced at the catalyst/water
interface under UV light illumination, the density of cOH radi-
1
5,25,26
2 2
under various conditions, e.g., sunlight with H O ,
27–30
31,33
sunlight
and UV light.
Generally, it is accepted that cals becomes greater and greater in the direction from the
under UV irradiation, the excited POM molecule induces an center of the nano-tubes to the inner surface of the nano-tubes.
electron transition from the highest occupied molecular orbital Meanwhile, the connement both in the latitudinal and radial
(
HOMO) to the lowest unoccupied molecular orbital (LUMO). direction within the arrays forces the RhB molecules to ow
The excited state of the POM (POM*) can generate cOH radicals through the nano-tubes leading to more opportunities to access
in a water solution, which are extremely powerful oxidizing the produced reactive cOH radicals which can decompose RhB
agents to oxidize organic substrates. While the reduced POM into different species (Table 2) besides CO
2
. As a result, the
ꢀ
(
POM ) produced from the POM* by abstracting an electron nano-tubular arrays present unique capabilities.
from electron-donating substances can deliver the electron to
electron acceptors such as O , metal ions and re-change to
Nevertheless, it needs to be conrmed
whether or not the photo-catalytic processes in our work adopt a
2
33,39
Renewable catalytic activity of the PEI/PSS/(PAH/PSS)
nano-tubular arrays
3
(PAH/
oxidized POM.
PW12)
8
similar hydroxyl radical mechanism. It is known that hydroxyl Under the obtained optimal conditions, renewal and reuse of
radicals can be detected by the photoluminescence (PL) tech- the nano-tubular arrays was tested. In the experiment, 5 over-
nique using terephthalic acid (TA) as a probe molecule since the lapped PW12-AAOs and 5 overlapped PW12-PCs were used as
hydroxyl radicals can readily react with TA to produce a highly catalysts for ltering a RhB solution, respectively. It was
33

uorescent product 2-hydroxyterephthalic acid. This method observed that the catalytic activity of the nano-tubular arrays in
is based on the PL signal of 2-hydroxyterephthalic acid at 425 both AAO and PC faded aer successive reuse (Fig. S9†). This
nm. Moreover, the PL intensity of 2-hydroxyterephthalic acid is could be due to the loss of the POMs on the inner surface of the
proportional to the concentration of cOH radicals which are
produced from POM catalysis. So, a similar procedure for the
measurement of photo-catalytic activity was carried out whereby
ꢀ
4
just the RhB aqueous solution was replaced by the 5 ꢂ 10
M
ꢀ
3
TA aqueous solution (TA was dissolved in 2 ꢂ 10 M NaOH
solution). A gradual increase in PL intensity at about 425 nm
(
excited by 315 nm UV-light) is observed with an increasing
irradiation time for the nano-tubes released from PC templates
Fig. 10), while no PL increase is observed in the absence of UV
(
light or the nano-tubes. This suggests that the uorescence is
due to the chemical reactions between terephthalic acid and
cOH produced at the catalyst/water interface under UV illumi-
33
nation. Further observation indicates that the PL intensity
change is not linear with the irradiation time. This can be
ascribed to the fact that the PW12 catalyst is not stable in such
high basic conditions and can be decomposed gradually.
Though it is difficult to demonstrate quantitatively how the
microstructure has helped the catalysis, it can be said that due
Fig. 11 Catalytic activity of fresh catalyst (5 overlapped PW12-PCs)
cyan column) and the renewed catalysts (green column). Each
renewal process was conducted after the catalyst had been used to
(
to the high specic surface area exhibiting more active sites and photo-degrade fresh RhB solution three times.
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nano-tubes indicating that POMs leached into the solution can
not carry out catalysis as effectively as those within the nano-
tubular arrays because of the absence of connement both in
the latitudinal and radial direction. Importantly, the catalytic
activity of the POM nano-tubular arrays can be fully renewed by
5 C. L. Hill and C. M. Prosser-McCartha, Coord. Chem. Rev.,
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(
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Conclusion
1
5
In summary, we successfully prepared PEI/PSS/(PAH/PSS) -
3
3
ꢀ
PAH/PW O
2 40
(
)8 nano-tubular arrays in the porous templates,
1
1
AAO and PC, using the LBL method as a prototype, which not
only allowed the precise structural design (controllable wall
thickness and aperture) and control of the geometrical features
1
1
1
1
1
1
1
2
2
3 M. T. Pope, Heteropoly and Isopoly-Oxometalates, Springer-
Verlag, Berlin, 1983.
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the photo-degradation of RhB under mild conditions and the
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lent bonds were also undertaken to avoid leaching of POMs.
Given the large number of polyoxometalates of various struc-
tures and properties reported in the literature, this work may
provide new concepts for catalyst design and regeneration
based on polyoxometalates with a nano-connement effect,
array effect and coupling effect, which show extensive potential/
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The nancial support of the Natural Science Foundation of
China, PCSIRT (no. IRT1205), the Fundamental Research Funds
for the Central Universities (YS1406) and Beijing Engineering
Center for Hierarchical Catalysts is greatly acknowledged. Prof.
Xue Duan of Beijing University of Chemical Technology is
greatly acknowledged for his kind support.
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