Synthesis of thermal-responsive photocatalysts by surface molecular imprinting for selective degradation of tetracycline

Article abstract of DOI:10.1039/c3ra44855j

A novel thermal-responsive surface molecular imprinted photocatalyst of poly(N-isopropylacrylamide) (PNIPAm) modified CdS/halloysite nanotubes (HNTs) was prepared using surface molecular imprinting technology and evaluated as a potential effective photocatalyst for selectively remove tetracycline (TC) existing in aquatic environments. Free radical polymerization of TC, NIPAm, N,N′-methylenebisacrylamide (MBA) and 2,2′-azobisisobutyronitrile (AIBN) was used as a molecular template, functional monomer, cross-linking agent and initiator, respectively. The imprinted photocatalysts were further characterized by SEM, EDS, TEM, UV-vis DRS, FT-IR, BET and TG, which showed that the CdS/HNTs were successfully imprinted with PNIPAm. In addition, benefiting from the common effect of the specific binding of imprinted polymers, thermo-responsiveness of PNIPAm shells, and high photocatalytic activity of CdS/HNTs cores, the obtained material showed high photocatalytic activity and excellent selectivity to remove molecular template TC and the photocatalytic activity could be tuned by changing the environmental temperature. According to our experimental and characterized results, the mechanism of the preparation of imprinted photocatalyst and its use in photocatalytic degradation was further discussed, respectively.

Full text of DOI:10.1039/c3ra44855j

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PAPER
Synthesis of thermal-responsive photocatalysts by
surface molecular imprinting for selective degradation
of tetracycline
Cite this: RSC Adv., 2013, 3, 26334
a
a
b
a
a
a
Weinan Xing, Liang Ni, Xinlin Liu, Yingying Luo, Ziyang Lu, Yongsheng Yan
and Pengwei Huo*a
A novel thermal-responsive surface molecular imprinted photocatalyst of poly(N-isopropylacrylamide)
PNIPAm) modified CdS/halloysite nanotubes (HNTs) was prepared using surface molecular imprinting
technology and evaluated as a potential effective photocatalyst for selectively remove tetracycline (TC)
(
0
existing in aquatic environments. Free radical polymerization of TC, NIPAm, N,N -methylenebisacrylamide
0
(
MBA) and 2,2 -azobisisobutyronitrile (AIBN) was used as a molecular template, functional monomer,
cross-linking agent and initiator, respectively. The imprinted photocatalysts were further characterized
by SEM, EDS, TEM, UV-vis DRS, FT-IR, BET and TG, which showed that the CdS/HNTs were successfully
imprinted with PNIPAm. In addition, benefiting from the common effect of the specific binding of
imprinted polymers, thermo-responsiveness of PNIPAm shells, and high photocatalytic activity of
CdS/HNTs cores, the obtained material showed high photocatalytic activity and excellent selectivity to
remove molecular template TC and the photocatalytic activity could be tuned by changing the
environmental temperature. According to our experimental and characterized results, the mechanism of
the preparation of imprinted photocatalyst and its use in photocatalytic degradation was further
discussed, respectively.
Received 3rd September 2013
Accepted 22nd October 2013
DOI: 10.1039/c3ra44855j
www.rsc.org/advances
5
and ne optical transmittance. Yet, it is well-known that pure
1
. Introduction
CdS is unstable upon light illumination, and photocorrosion
6,7
The increasing contamination of our environment with antibi-
may occur in photocatalytic reaction. In addition, the photo-
otics is certainly one of the greatest problems for the future of catalytic activity is not high enough for application. Several ways
humanity. Antibiotics are widely used for human and animal have been used to modify CdS to overcome its drawbacks, such
8
9
therapies, as well as in agricultural antimicrobials. However, as coupling CdS with other semiconductors or polymer,
10
the overuse and misuse of antibiotics has caused a serious doping with metal ion, depositing CdS onto the materials with
1
11
environmental problem, since such compounds can be found high surface area, and so on. In our previous study, CdS/HNTs
12
in municipal and industrial wastewater, wastewater sludge and
agricultural runoff. Thus, the way to effectively treat the anti-
biotics residues in the environment is urgent. The treatment
photocatalyst was synthesized, which have better photo-
catalytic activity. Halloysite nanotubes (HNTs) are a two-layered
aluminosilicate clay mineral, possessing a naturally occur-ring
2
methods have been investigated using microbial, chemical, and unique tubular structure on the nano-scale which has the
photochemical processes with varying success. Recent studies similar geometry to carbon nanotubes. Compared with carbon
indicate that photocatalytic technology is a green technology nanotubes, HNTs are readily obtainable, much cheaper and
which uses the solar energy and degrades pollutant to possessing of large reserves in China. On the other hand, HNTs
3
,4
non-toxic.
possess advantages of high stabile property, resistible against
13–15
Cadmium sulde (CdS) is an important II–VI semiconductor
and has received much attention due to its potential applica-
tions in electronics, optics, catalysis, and ceramics. CdS also has
organic solvents and ease of disposal or reusability.
There-
fore, the combination of CdS and HNTs effectively increase the
using rate of light and enhance the photocatalytic activity.
good photocatalytic activity due to its suitable band gap (2.4 eV) However, the modied photocatalysts have very poor selectivity
and cannot differentiate these polluted targets. As we all know,
the waste water usually includes different pollutants, it is
important that some high risk pollutant is removed specially.
a
Schools of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013,
PR China. E-mail: huopw@mail.ujs.edu.cn; Fax: +86 51188791108; Tel: +86
Thus, it is very urgent to improve the selectivity of photocatalyst.
5
1188790187
b
Molecular imprinting is a useful technique for the prepara-
tion of polymeric materials which provides articial
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013,
China
2
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receptor-like binding sites for template molecules. These selectivity and regeneration of photocatalyst were investigated.
potential binding sites are generated by copolymerization of a The results demonstrated the photocatalyst which we prepared
cross-linking agent with the complex formed from a template could efficiently and selectively photo-degradation of TC. The
and polymerizable monomers that have functional groups more worth noting is the tunable photocatalytic property of
specically interacting with the template through covalent or temperature responsive imprinted polymers by switch temper-
16
non-covalent bonds. Subsequently, the imprinted templates ature. At last, the mechanism of preparation and recognition
are removed from the polymer network and leaved binding sites were proposed.
with stereochemical complementarity to the template mole-
17,18
cules.
However, there are some defects for molecular
2
. Experiment section
imprinted photocatalyst, such as incomplete template removal,
slow mass transfer and low binding ability. The introduce of
surface molecular imprinting technology can reduce these
drawbacks, which the functional monomers and template
molecules are pre-graed on the surface of supported materials
and the template molecules are easily removed from the surface
2.1 Materials
HNTs were purchased from Zhengzhou Jinyang guang China-
ware Co. Ltd., Henan, China. Polyvinylpyrrolidone (PVP),
3-(methacryloyloxy) propyl trimethoxysilane (MPS), cadmium
chloride, thiourea, and salicylic acid (SA) were all purchased
from Shanghai Chemical Reagent Co., Ltd. and used as
received. N-Isopropylacrylamide (NIPAm), 2,2 -azobisisobutyr-
19–21
imprinted layer.
Thus, it is possible to use surface
imprinting technology to modify the surface of photocatalyst
and improve the selectivity photo-degradation of template
molecules.
0
0
onitrile (AIBN), N,N -methylenebisacrylamide (MBA), were
purchased from Aladdin Reagent Co., Ltd. Tetracycline (TC),
oxytetracycline HCl (OTC-HCl) and Ciprooxacin (CIP) were
purchased by Shanghai Shunbo Biological Engineering Co., Ltd.
Ethanol, methanol were purchased from Sinopharm Reagent
Co., Ltd. Deionized and doubly distilled water were used
throughout this work.
Recently, the development of smart imprinting systems has
attracted interests due to their remarkable responsive to
2
2
23
specic external stimuli such as light, temperature and pH
value.
24
Among them, temperature has attracted much
more attention because of its easily operability and widely
25
applicability. Zhao's group described a thermo-sensitive and
salt-sensitive molecularly imprinted hydrogel based on N-[3-
2.2 Sample preparation
(dimethylamino) propyl] methacrylamide (DMAPMA) and
N-isopropylacrylamide (NIPAm), and the imprinted polymer
exhibited positively thermo-sensitive molecular recognition of were fabricated according to our previously reported method.
2.2.1 Attaching CdS nanoparticles onto HNTs. CdS/HNTs
12
bovine serum albumin. A novel thermal-responsive magnetic Briey, 1.0 g HNTs were ultrasonically dispersed in 25.0 mL
molecularly imprinted polymer for selectively remove sulfame- NaOH solution. Then 20.0 mL CdCl solution was dropped in
2
thazine (SMZ) existed in aquatic environments has been 25.0 mL of HNTs suspension while stirring. Aer 2 h stirring,
reported, and NIPAm and acrylamide (AAm) were selected as added into thiourea solution dropping under stirring. Then, the
26
functional monomers. PNIPAM is a well-known thermo- solution was transferred to a teon-lined stainless-steel auto-
ꢀ
sensitive polymer, exhibiting a lower critical solution tempera- clave (100 mL capacity). The autoclave was heated at 160 C for
ꢀ
ture (LCST) at about 32 C in water solution. The hydrophilicity 12 h. Aer cooling to room temperature, the precipitates were
of PNIPAm increases along with the decreasing temperatures, rinsed several times with ethanol and deionized water and dried
ꢀ
which makes PNIPAm swell in water, and vice versa. Thus, the at 60 C for 12 h.
selective molecule recognition ability of the thermo-sensitive
2.2.2 Preparation of 3-(methacryloyloxy) propyl trimethoxy-
molecularly imprinted polymer varies when environmental silane (MPS) modied CdS/HNTs. A total of 2.0 g CdS/HNTs were
temperature changed. Thus, the preparation of stimuli- dispersed in a mixture of 10.0 mL MPS and 100.0 mL of ethanol.
ꢀ
responsive imprinted polymers based on nanosized materials is The mixture was stirred at 40 C for 12 h. The MPS-modied CdS/
of great scientic importance for the development of smart HNTs (MPS-CdS/HNTs) were then washed repeatedly with ethanol
ꢀ
surface molecular imprinted polymers.
and water, dried at 50 C for 12 h in vacuum.
It should be noted that, if three promising concepts
2.2.3 Preparation of surface imprinted photocatalysts. The
(a nanosized surface imprinting technique, high photocatalytic imprinted photocatalysts were prepared via surface molecular
activity CdS/HNTs and PNIPAm) were combined, the novel imprinted technology. The basic synthesis process as follows: in
thermal-responsive surface molecular imprinting photo- a three neck ask equipped with a magnetic stirring bar,
catalysts could be fabricated and exhibited intelligent sensi- 0.5 mmol molecular template of TC were completely dissolved
tivity and excellent selectivity remove antibiotics from waste into 100 mL solvent (v/v: methanol–H O ¼ 75/25). Then,
2
water. In this study, CdS/HNTs were prepared by hydrothermal 4.0 mmol NIPAm, 4.0 mmol MBA, 0.5 g MPS-CdS/HNTs and
synthesis method. The photocatalysts were further modied 0.2 g PVP were added into the mixed solution by ultrasonication
with thermo-responsive polymer PNIPAm. Free radical poly- for 10 minutes. Aer purging oxygen with nitrogen gas, 0.05 g of
0
merization of TC, NIPAm, N,N -methylenebisacrylamide (MBA) AIBN was added to the three-necked ask while the temperature
0
ꢀ
and 2,2 -azobisisobutyronitrile (AIBN) was used as molecular was increased to 60 C for 12 h under nitrogen protection. Aer
template, functional monomer, cross-linking agent and initi- the polymerization, the samples were ltered and washed with
ꢀ
ator, respectively. The characterization, photocatalytic activity, distilled water and dried at 50 C for 12 h.
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The template molecules were removed by washing with a source to provide visible light irradiation. Briey, photocatalyst
mixture of methanol–acetic acid (95 : 5, v/v) using Soxhlet (0.05 g) was added into 50 mL of TC solution. The mixed
extraction. When no template was detected in the extraction suspension was stirred for 20 min in the dark in order to reach
solution, the composites were further washed with distilled the adsorption–desorption equilibrium between the catalyst
water and ethanol. For comparison, the non-imprinted photo- and the TC. Then, the suspensions were exposed to visible-light
catalyst was also prepared using the same procedure, but irradiation. The temperature of the reactant solution was
without the addition of TC template.
maintained below 298 K by a ow of cooling water during the
reaction. The reaction continued for 1 h and conducted in
1
0 min interval. The reaction mixture was analyzed by UV-vis
2.3 Material characterization
spectrometer at wavelength of 357 nm. The photocatalytic
The morphology of photocatalysts was observed via the trans-
mission electron microscopy (TEM) on a JEOL IEM-200CX TEM.
The SEM image was examined with XL30-ESEM scanning elec-
tron microscopy (Philips-FEI Co., Netherlands). XRD patterns
were obtained with a D/max-RA X-ray diffractometer (Rigaku,
degradation rate (DR) was calculated by this equation: DR ¼
[
(1 ꢁ A /A )] ꢂ 100%. Where A is the initial absorbency of TC
i
0
0
i
solution which reached absorption equilibrium and A is the
absorbency of reaction solution.
Japan) equipped with Ni-ltrated Cu Ka radiation (40 kV,
ꢀ
2
00 mA). The 2q scanning angle range was 10–80 at a scanning
3
. Results and discussion
ꢀ
rate of 0.02 /0.2 s. The UV-vis diffuse reectance spectra (DRS)
were obtained with a Shimadzu 2450 UV-vis spectrophotometer
3.1 TEM, SEM and EDS analysis
The morphology and structure of the as-prepared photo-
catalysts were investigated by TEM SEM and EDS. As shown in
Fig. 1a, the majority of the samples were consisted of cylin-
drical tubes, and the nanotubes were open-ended. HNTs were
rather polydisperse in length. The empty lumen of HNTs is 15–
(
Shimazu, Japan) equipped with an integrating sphere at tach-
ment ISR 240A and BaSO powder as the internal standard. The
4
Fourier transform infrared (FT-IR) absorption spectra were
studied with a Nicolet Nexus 470 FT-IR (America thermo-elec-
tricity Company), using KBr pellets. The specic surface
area (BET) was estimated from the N
isotherms, measured by Quantachrome NOVA4000
surface area apparatus. TGA was performed for powder samples
using a Diamond TG/DTA Instruments (Perkin-Elmer, USA)
under a nitrogen atmosphere up to 800 C with a heating rate of
2
5 nm in diameter (see the inset of Fig. 1a at a higher
2
adsorption–desorption
magnication). As shown in Fig. 1b, large amounts of nano-
particles were deposited on the surface of HNTs. The surface of
a
CdS/HNTs-MPS was coated with
a non-uniform layer
ꢀ
compared with Fig. 1b, which may be the formation of
imprinting layer (Fig. 1(c)). From the SEM and EDS images in
Fig. 1d and e, which illustrated that C, N, O, Si, Al, Cd, S were
existed in the molecular imprinted photocatalyst. The peaks of
Al, Si and O were mainly generated by HNTs and the peaks of
ꢀ
ꢁ1
5
.0 C min .
2
.4 Measurement of photocatalytic activity
To test photocatalytic behavior of the as-synthesized samples, Cd and S were generated by CdS nanoparticles. Above all, the
photo-degradation of TC in water was chosen as a probe reac- CdS/HNTs was perfectly imprinted in the process of surface
tion. A 500 W Xe lamp with cutoff lter were used as the light imprinting.
Fig. 1 TEM images of (a) HNTs (b) CdS/HNTs-MPS (c) molecular imprinted, SEM (d) and EDS (e) image of molecular imprinted.
2
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CdS/HNTs, non-imprinted and molecular imprinted were also
increased. The reason may be caused from the imprinted
process which could introduce some organic parts, and the
organic parts can effectively enhance the absorbency of light.
According to the formula: E
g
¼ 1240/l (which E
g
is the band gap
energy and l is the wavelength of the absorption edge.) it was
estimated that the band gap of molecular imprinted photo-
catalyst was lower than CdS/HNTs. Therefore, there may be an
improved TC activity of molecular imprinted photocatalyst
under solar light irradiation. The main reason may be that the
molecular imprinted photocatalyst could produce more elec-
27
tronic-hole in visible light with a narrow band gap. Meanwhile
the surface defect was introduced aer the process of surface
imprinting and the elution. The surface defect can serve as
charge traps retarding electron–hole combination rate,
enhancing the interfacial charge transfer.
Fig. 2 XRD pattern of (a) CdS/HNTs (b) non-imprinted (c) molecular imprinted.
3.4 FT-IR spectra analysis
3.2 XRD analysis
In order to verify the expected products were obtained in
different steps of molecular imprinted fabrication, the FT-IR
spectra for HNTs, CdS/HNTs, CdS/HNTs-MPS and molecular
imprinted were recorded and showed in Fig. 4a–d, respectively.
The crystal structure of the imprinted photocatalyst was deter-
mined by XRD. Fig. 2 presented the XRD pattern of both types of
particles (non-imprinted and molecular imprinted) revealing
the similar XRD pattern as CdS/HNTs, indexed as belonging to a
hexagonal phase. Aer surface imprinting, the peak intensity
was weaker than those of CdS/HNTs, indicating that a layer of
the organic molecular layer was formed on the surface of
CdS/HNTs. The result was consistent with the corresponding
TEM image.
As shown in Fig. 4a, The characteristic bands in 3695 and
ꢁ1
3
627 cm were due to the stretching vibrations of inner-surface
ꢁ1
hydroxyl groups and the peak at 1635 cm was due to the
deformation vibration of interlayer water. The deformations of
28
Al–O–Si, Si–O–Si and O–H groups of the inner hydroxyl groups
ꢁ1
ꢁ1
ꢁ1
were at 536 cm , 1031 cm and 909 cm , respectively. In
ꢁ1
Fig. 2c, the peaks at 1713 cm (C]C bond stretching vibration)
indicated that CdS/HNTs were successfully modied by MPS. In
The optical absorption property and the band structure of Fig. 2d, the peaks at 1650 cm , 1528 cm (secondary amide
3.3 UV-vis DRS analysis
ꢁ
1
ꢁ1
ꢁ1
photocatalysts were determined by UV-vis DRS. As shown in C]O stretching), and 1390 cm
(deformation of methyl
Fig. 3, the absorption boundary of surface molecular imprinted groups on –C(CH ) could be attributed to the characteristic
3 2
)
26
photocatalyst red shied was seen obviously compared with peaks of PNIPAm, which implied that the fabrication of
CdS/HNTs and non-imprinted. And their absorption intensity of molecular imprinted photocatalyst had been successfully
performed.
Fig. 3 UV-vis diffuse reflectance spectra (DRS) of (a) CdS/HNTs (b) non-imprinted
c) molecular imprinted.
Fig. 4 FT-IR spectra of (a) HNTs (b) CdS/HNTs (c) CdS/HNTs-MPS (d) molecular
(
imprinted.
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different synthetic conditions, the reason mainly caused by the
imprinted process which can cover the surface holes and/or
increase the particle size.
3.7. Photocatalytic experiments
The photocatalysis experiments were conducted to research the
relationship between the photocatalytic efficiency and the
molar ratio of functional monomer, molecular template and
cross linking agent, the amount of imprinted molecular
template and polymerized time. Bearing this in mind, the effect
of the three factors was studied.
3.7.1 The impact of different molar ratio of molecular
template, functional monomer and cross linking agent on the
photocatalytic activity. As described previously, the interaction
among molecular template, functional monomer and cross
linking to polymerized extent on the surface of photocatalyst
were important. Fig. 6A showed photocatalytic effect on TC
using the as-prepared photocatalysts which prepared with the
different proportion of molecular template, functional mono-
mer and cross linking agent. It was clearly seen that the molar
rate of 1 : 8 : 8 showed the highest photocatalytic activity. The
reason may be as follows: when the functional monomer was
too little (1 : 4 : 8), template molecular may not be completely
bonding, which may affect the photocatalytic activity. However,
the high proportion of cross linking may produce the imprinted
photocatalyst with high cross-linking degree, which reduced the
photocatalytic activity. It indicated that appropriately prepared
proportion can effectively enhance the photocatalytic activity.
While the appropriately proportion may induced polymerized
the appropriately polymer layer which was exerted positive on
the photo-generated electrons transfers, and lead to the high
photocatalytic activity.
Fig. 5 TGA profiles of (a) CdS/HNTs (b) non-imprinted (c) molecular imprinted.
3.5 TG analysis
Fig. 5 showed the TGA curves of CdS/HNTs, non-imprinted and
molecular imprinted photocatalyst. As shown in Fig. 5a, the
weight loss processes at two periods, the loss of absorbed water
ꢀ
from 25 to 100 C, the dehydroxylation of structural aluminol
ꢀ
groups present in HNTs from 400 to 600 C. Compared with
CdS/HNTs, the signicantly different about weight losses of
non-imprinted and molecular imprinted photocatalysts were
assigned to the decomposition of the imprinted layer. Accord-
ing to the weight loss obtained from TGA analysis, the slight
difference of weight losses (non-imprinted and molecular
imprinted photocatalysts) may be due to the template
molecules, which made the graing density of polymer different
26
in polymerization. The remaining mass for non-imprinted
and molecular imprinted photocatalysts was assigned to the
thermal resistance of CdS/HNTs nanoparticles, and the quantity
of CdS/HNTs nanoparticles in the non-imprinted and molecular
imprinted photocatalysts were 84.77% and 86.14%, respec-
tively, indicating that the non-imprinted and molecular
imprinted possessed of similar morphological structure and
size distribution.
In order to study the inuence of temperature responsive-
ness on the photocatalytic activity, the other representative
ꢀ
temperature, i.e., 45 C (higher than LCST of polymers) was
selected as contrastive studies. As shown in Fig. 6B, the pho-
tocatalyst with different molar ratio performed a poor photo-
ꢀ
catalytic activity at 45 C. The results may be caused by the
thermo-sensitive dependence behaviors, the shrinking of the
polymer layer and the decreasing of hydrophilic properties
3.6 BET analysis
The specic surface area values were calculated using data of which can affect the access of template TC to polymers at high
liquid nitrogen physisorption experiments BET method. The temperatures. There was an unusual phenomenon in the later
BET analysis of the surface area, pore volume and pore size of stages of photocatalytic degradation, the DR of the photo-
molecular imprinted photocatalyst were shown in Table 1. From catalyst reduced with the increasing time. The main reason may
the table, we can see clearly that the surface areas changing with due to the instability of TC at illumination and high
Table 1 BET and pore analysis
Different molar ratio of molecular
template, functional monomer,
cross linking agent
Different amount imprinted
molecular template (mmol)
Different polymerized time
Photocatalyst
Surface area (m g
Pore size (nm)
1 : 4 : 8
1 : 8 : 8
14.97
10.73
0.04
1 : 8 : 12
13.41
10.18
0.03
0.5
0.25
15.23
10.22
0.05
0.1
0.05
22.23
10.68
0.07
0.01
24.72
11.29
0.07
3 h
6 h
12 h
24.72
11.29
0.07
24 h
26.04
11.49
0.07
2
ꢁ1
)
16.23
9.59
0.04
14.97
10.73
0.04
20.26
10.77
0.04
27.98
10.19
0.07
23.54
10.78
0.06
3
ꢁ1
Pore volume (cm g
)
2
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Fig. 7 The influence of different amount imprinted molecular template (TC) (a)
0
.5 mmol (b) 0.25 mmol (c) 0.1 mmol (d) 0.05 mmol (e) 0.01 mmol.
Fig. 6 (A) The influence of different molar ratio of molecular template, func-
tional monomer and cross linking agent (a) 1 : 4 : 8 (b) 1 : 8 : 8 (c) 1 : 8 : 12 at 25
ꢀ
C. (B) The influence of different molar ratio of molecular template, functional
ꢀ
monomer and cross linking agent (a) 1 : 4 : 8 (b) 1 : 8 : 8 (c) 1 : 8 : 12 at 45 C.
Fig. 8 The influence of different polymerized time (a) 24 h (b) 12 h (c) 6 h (d) 3 h.
temperatures, which the dimer of TC maybe formed, and lead to
the change of DR.
3.7.2 The impact of different amounts of molecular photocatalysts with different polymerized time were studied. As
template (TC) on the photocatalytic activity. In order to inves- shown in Fig. 8, 6 h was the best polymerized time for the
tigate the effect of different amounts of molecular template on photocatalytic activity. The cavity of template molecular could
the photocatalytic activity, the various amounts of TC template not be effectively formed for incomplete polymerization of
were used to prepare the surface imprinted photocatalyst. As imprinted layer of short polymerized time, which lead to lower
shown in Fig. 7, the amount of surface molecular imprinted photocatalytic activity. While the long polymerized time may
photocatalyst with 0.01 mmol showed the highest photo- induce the thick imprinted layer and lead to the coverage of
catalytic activity, while excessive molecular templates reduced active sites or the difficulty in transferring of electronic and
the photocatalytic activity. The reasons may be due to that the hole, which reduced the activity of photocatalytic materials.
more molecular templates could process more area of the Therefore, the imprinted thickness for the photocatalyst
photocatalysts surface which resulted incomplete imprinted degradation was important, the most suitable thin layer
polymerized layer and inuenced the formation of the effective exhibited better photocatalytic activity.
heterojunction for transfer of photoelectrons, and lead to low
photocatalytic activity.
At last, two control experiments were performed under
different conditions: (a) in the presence of the photocatalysts
3.7.3 The impact of different polymerized time on the but in the dark (b) with visible light irradiation but in the
photocatalytic activity. The thin and/or thick layer of imprinted absence of the photocatalysts. It was clearly seen that (Fig. 9) TC
polymer was an important factor for the photocatalytic activity molecules contacting with molecular imprinted photocatalyst
of surface imprinted photocatalyst. Thus, surface imprinted did not cause a sharp decline of the TC degradation rate in the
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and longevity maintained on the photocatalyst make it prom-
ising candidate in practical applications.
3.7.5 The selective degradation of molecular imprinted
photocatalyts. To study the selectivity of prepared surface
molecular imprinted photocatalyts on different targets, a series
of photo-degradation experiments with the same conditions
were conducted, and the result was obtained in Fig. 11. It was
clearly seen that the photo-degradation rate of non-target
molecular template were much lower than the photocatalyst of
target molecular template. It may be due to the specially sizes,
structures and functional groups of the template, different
binding forces could form between the TC and PNIPAm,
resulting in a different recognition effect. Moreover, OTC-HCl
has nearly the same structure as TC except the lack of hydroxyl
and chlorine functional group, but the photo-degradation
capacity was still lower than that for TC, suggesting that the
memory of specic functional groups also played an important
role in the preparation of molecular imprinted photocatalyts.
In the experiment, the measured apparent rate constants (k)
were used to evaluate the inuence of the molecular imprinted
photocatalysts on the photo-degradation rate of pollutants and
the selectivity of various photocatalysts, the results were shown
in Table 2. As anticipated, the rate constants for the target's
degradation over the surface molecular imprinted photo-
catalysts were larger than others. Especially, the apparent rate
Fig. 9 The degradation rate of TC in the presence of the photocatalysts but in
the dark (a) and with visible light irradiation but in the absence of the photo-
catalysts. (b) Degradation rate of TC over the samples: (c) CdS/HNTs (d) molecular
imprinted photocatalyst.
dark reaction condition. And, there was no appreciable degra-
dation of TC aer 60 min in the absence of photocatalysts.
Fig. 9c and d showed the degradation curves of TC on the
CdS/HNTs and molecular imprinted photocatalyst. It could be
seen that the two photocatalysts showed higher photocatalytic
activities. But, the DR of molecular imprinted photocatalyst was
decreased slightly compared with CdS/HNTs. The reason may
be that the active site of CdS was covered by polymer layer.
ꢁ
1
constant (k) of degradation of TC was 0.02809 min , which was
ꢁ
1
much higher than that of SA (0.00439 min ). This may due to
the maximum differentiation in structure between SA and TC.
Nevertheless, the structure of TC and OTC-HCl were very
similar, while the apparent rate constant (k) of degradation of
3.7.4 Stability and reusability of molecular imprinted
ꢁ
1
OTC-HCl (0.01199 min ) was still different from that of TC
photocatalyts. The most important factor for the practical utility
of a photocatalyst is its reusability and stability during the
course of illumination. As shown in Fig. 10, a slight decrease of
DR was found aer they were repeatedly performed and recy-
cled in TC photo-degradation for three times, indicating that
molecular imprinted photocatalyts had excellent photochem-
ical stability and reusability. The good recycling performance
ꢁ1
(
0.02809 min ). It suggested that the as-prepared molecular
imprinted photocatalyts had the strong ability to selective
recognition and photo-degradation of TC.
On the other hand, the R* (R* ¼ ktarget/Knon-target) was also
used to measure the selection of degradation. As shown in
Table 1, the calculated coefficients of selection R* relative to
Fig. 10 Photostability of molecular imprinted photocatalyst.
Fig. 11 The degradation rate of different targets.
2
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Table 2 Apparent rate constants and coefficients of selectivity for photocatalytic degradation
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
TC (min
)
CIP (min
)
SA (min
)
OTC-HCl (min
)
MIP (min
)
NIP (min
)
k
K
target
non-target
0.02809
—
—
—
—
0.00439
6.398633
—
0.02809
—
1.29447
0.0217
—
0.0143
1.964336
0.01199
2.342786
R*, R* ¼ ktarget/knon-target
Scheme 1 Schematic layout synthesis of (a) MPS-CdS/HNTs (b) imprinted photocatalyst and its use in photocatalytic degradation.
HCl-OTC, CIP and SA were 2.342786, 1.964336 and 6.398633,
The possible route for preparation imprinted photocatalysts
respectively. Suggesting that the coefficient of selectivity R* was shown in Scheme 1(b). In this procedure, NIPAm was used
mainly depended on the structure of target pollutant. In addi- as a functional monomer and combined with template mole-
tion, the coefficient selection of surface imprinted photo- cules TC. MBA as a cross-linking agent was chosen to participate
catalysts relative to non-imprinted was 1.29447, further in the polymerization reaction. Aer free radical solution poly-
conrmed that the surface molecular imprinted photocatalysts merization, the surface of CdS/HNTs-MPS was coated with
enhanced the photocatalytic selectivity toward the target temperature responsive molecularly imprinted layer. At last, the
contaminant.
template molecules were removed by extensive washing with a
3.7.6 The mechanism of preparation and application in mixture of methanol/acetic acid (95 : 5, v/v) using soxhlet
photo-degradation with imprinted photocatalysts. According to extraction. A large number of TC tailor-made vacancies were
our experimental and characterized results, the possible route for formed and these imprinted cavities had with stereo-chemical
preparation of surface molecular imprinted polymer modied complementarity to the template molecules (TC). In the process
photocatalyst and its use in photocatalytic degradation was of photocatalytic degradation, the TC molecules were rebound
shown in Scheme 1. MPS modied CdS/HNTs were prepared by to the adsorption sites at these imprinted cavities. Then, the
hydrothermal synthesis method and the main steps were shown adsorbed TC molecules were oxidized by the photo-generated
in Scheme 1(a). In the rst step, the surface of HNTs was modied oxidizing species such as_OH radicals. At last, the TC molecules
2
+
with Cd by interacting with cadmium chloride. Generally, the could be further degraded to CO , H O and other small inor-
2
2
30
surface of HNTs has negative charges under the alkaline condi- ganic molecular material. As surface molecular imprinted
tion. Thus, Cd was more likely tend to bind with HNTs. Then, polymers were thermoresponsive, the swelling/shrinking of the
hydrolysis of thiourea was promoted synchronously in alkaline polymer layer could be tunable by changing the environmental
29
2+
ꢁ
2ꢁ
2 2 2 2 2
conditions ((NH ) CS + 2OH / CH N +H O + S ). Finally, the temperature, which signicantly affected the photocatalytic
CdS nanocrystallines were formed on the surface of HNTs. Due to activity of imprinted photocatalysts.
the hydrothermal reaction system, the oriented self-aggregating
process of CdS nanocrystallines was promoted the growth of
nanocrystallines. The surface of CdS/HNTs modied with MPS
4. Conclusions
was through chemical modication, which the vinyl groups were In this work, new thermo-sensitive polymer used as shells and
introduced. FT-IR studies showed that the CdS/HNTs were CdS/HNTs as cores to fabricate a novel thermal-responsive
successful modied by MPS.
surface molecularly imprinted photocatalyst. The imprinted
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polymer layer provides the photocatalyst molecular recognition 10 M. Luo, Y. Liu, J. C. Hu, H. Liu and J. Li, ACS Appl. Mater.
ability to form the selective photo-degradation of the target Interfaces, 2012, 4, 1813–1821.
pollutant. Such selectivity is dependent mainly on the differ- 11 S. Y. Ryu, W. Balcerski, T. K. Lee and M. R. Hoffmann, J. Phys.
ence in the molecular size and shape between the target and Chem. C, 2007, 111, 18195–18203.
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surface imprinted photocatalysts could be tunable by changing Y. S. Yan, Appl. Surf. Sci., 2012, 259, 698–704.
the environmental temperature. Furthermore, the novel 13 D. Papoulis, S. Komarneni, A. Nikolopoulou and P. Tsolis-
thermal-responsive surface molecular imprinting photo- Katagas, Appl. Clay Sci., 2010, 50, 118–124.
catalysts retained comparable photocatalytic activity aer 14 Y. F. Xie, D. Y. Qian, D. L. Wu and X. F. Ma, Chem. Eng. J.,
repeated uses for three times. The excellent selective, photo- 2011, 168, 959–963.
catalytic activity and thermo-sensitivity will be more broad 15 V. Vergaro, E. Abdullayev, Y. M. Lvov, A. Zeitoun,
application prospects for the treatment and disposal the
residues of TC from waste water.
R.
Cingolani,
R.
Rinaldi
and
S.
Leporatti,
Biomacromolecules, 2010, 11, 820–826.
16 W. L. Zhang, Y. Li, Q. Wang, C. Wang, P. F. Wang and
K. Mao, Environ. Sci. Pollut. Res., 2013, 20, 1431–
Acknowledgements
1440.
We gratefully acknowledge the nancial support of the nancial 17 E. Caro, R. M. Marc ´e , F. Borrull, P. A. G. Cormack and
support of the Natural Science Foundation of China (no.
1207053), the Natural Science Foundation of Jiangsu Province
D. C. Sherrington, TrAC, Trends Anal. Chem., 2006, 25, 143–
154.
2
(
SBK2011460), the Specialized Research Fund for the Doctoral 18 Y. Li, X. Li, Y. Q. Li, J. Y. Qi, J. Bian and Y. X. Yuan, Environ.
Program of Higher Education (20113227110019), China Post-
Pollut., 2009, 157, 1879–1885.
doctoral Science Foundation (2011M500861, 2012M511219, 19 J. M. Pan, X. H. Zou, X. Wang, W. Guan, Y. S. Yan and J. Han,
011M500869) and the program for Postgraduate Research
Chem. Eng. J., 2010, 162, 910–918.
2
Innovation
in
University
of
Jiangsu
University 20 P. W. Huo, Z. Y. Lu, X. L. Liu, X. Gao, J. M. Pan, D. Wu,
(no. CXLX12_0634).
J. Ying, H. M. Li and Y. S. Yan, Chem. Eng. J., 2012, 189–
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2
1 H. Q. Tang, L. H. Zhu, C. Yu and X. T. Shen, Sep. Purif.
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