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SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant
Accepted Manuscript Title: Sol-hydrothermal synthesis of inorganic-framework molecularly imprinted TiO2 /SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant Author: Fang Deng Yin Liu Xubiao Luo Shaolin Wu Shenglian Luo Chaktong Au Ruoxi Qi PII: DOI: Reference:
S0304-3894(14)00443-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.05.088 HAZMAT 15995
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
20-3-2014 28-5-2014 29-5-2014
Please cite this article as: F. Deng, Y. Liu, X. Luo, S. Wu, S. Luo, C. Au, R. Qi, Sol-hydrothermal synthesis of inorganic-framework molecularly imprinted TiO2 /SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.05.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sol-hydrothermal synthesis of inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant
ip t
Fang Denga,b, Yin Liua,b, Xubiao Luoa,b*, Shaolin Wua,b, Shenglian Luoa,b∗, Chaktong
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and
Resources Recycle, Nanchang 330063, PR China
College of Environmental and Chemical Engineering, Nanchang Hangkong
University, Nanchang 330063, PR China
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong
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c
an
b
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a
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Auc, Ruoxi Qia,b
Kong, PR China
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ABSTRACT: Inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite (MIP-TiO2/SiO2) was successfully prepared by sol-hydrothermal method using
Ac ce p
4-nitrophenol as template. The morphology, structure, optical property, zeta-potential and photocurrent of MIP-TiO2/SiO2 were characterized. The adsorption performance and photocatalytic selectivity were also studied. MIP-TiO2/SiO2 shows higher
adsorption capacity and selectivity than the non-imprinted TiO2/SiO2 (NIP-TiO2/SiO2). Kinetics results show that the adsorption equilibrium of 4-nitrophenol on
MIP-TiO2/SiO2 is established within 20 min, and the adsorption process obeys the pseudo-second-order model. Moreover, MIP-TiO2/SiO2 can completely degrade
∗
Corresponding author. Tel.: +86 791 83953372; Fax: +86 791 8395373. E-mail addresses: [email protected] (S. Luo), [email protected] (X. Luo). 1
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4-nitrophenol within 30 min, while NIP-TiO2/SiO2 takes 110 min. It was found that the MIP-TiO2/SiO2 photocatalyst shows molecular recognition ability, leading to selective adsorption and molecular recognitive photocatalytic degradation of
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4-nitrophenol. Furthermore, because of its inorganic framework, MIP-TiO2/SiO2
molecularly
imprinted
photocatalyst;
TiO2/SiO2
nanocomposite;
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Keywords:
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shows excellent reusability.
sol-hydrothermal method; molecular recognition; photocatalytic ability
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1. Introduction
Environmental pollution with ill effects on human and animal health has
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attracted much attention in the last decade [1,2]. Heterogeneous semiconductor photocatalysis is an effective and promising technique to tackle pollution issues [3-8].
te
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Nano-sized TiO2 has emerged as a versatile functional material and effective photocatalyst in environmental remediation due to its inexpensive, non-toxic and
Ac ce p
excellent photoelectric properties [9-13]. However, in many cases the TiO2 photocatalyst can not meet the requirements of pollution control [14,15]. For example, it is difficult to selectively degrade highly toxic pollutants in a complicated aqueous system since photo-oxidation of organic compounds over TiO2 is dominated by free radicals and the related mechanism is known to be non-selective [16-20]. Therefore, the development of photocatalysts that show photocatalytic selectivity is urgently required. Up to now, much efforts have been devoted to enhance the photocatalytic selectivity of TiO2 for removal of target organic pollutants. It was reported that
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molecularly imprinted polymers (MIPs) with specific binding sites complementary to the template molecules in terms of size, shape and functional group exhibit specific recognition ability for the target molecules [21-23]. Applying the molecularly
ip t
imprinting technique in the synthesis of TiO2, one can generate molecularly imprinted
cr
TiO2 with recognition ability towards a target pollutant. This kind of molecularly
us
imprinted photocatalysts have a promising application in selective and preferential degradation of a highly toxic pollutant that exists in low level together with other less
an
hazardous pollutants in a water system [15-20]. Moreover, molecularly imprinted photocatalysts could selectively remove the organic pollutants with the nutrients being
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remained in the wastewater, resolving serious shortage of water agricultural irrigation [24]. Molecularly imprinted TiO2 photocatalysts can be fabricated by coating an
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d
organic layer of molecularly imprinted polymers on the surface of TiO2 nanoparticles. Although the layer of organic MIPs enables the TiO2 to exhibit high selectivity for
Ac ce p
UV-photodegradation of a target pollutant, the selectivity of these organic-inorganic hybrids decreases due to the degradation of organic MIPs upon extended UV illumination. To overcome this drawback, Han et al [25] developed the Cu2+-doped
2,4-dichlorophenol molecularly imprinted TiO2-SiO2 while Shen et al [19]
synthesized
Al3+-doped
diethyl
phthalate
molecularly
imprinted
TiO2-SiO2
nanocomposites, both without the use of organic MIPs [19,25]. However, the population of imprinted cavities on the surface is low and Lewis acid-base interaction between the nanocomposites and target molecules is weak. Recently, our group prepared inorganic-framework molecularly imprinted TiO2/WO3 nanocomposites by a
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facile one-step sol-gel method using tetrabutyl orthotitanate as titanium source as well as the precursor of functional monomer. Nonetheless, it was pointed out that the process is energy-consuming and some imprinted cavities are destroyed upon
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high-temperature calcination [24]. Therefore, it is desirable to develop mild methods
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to prepare molecularly imprinted photocatalysts that are rich in imprinted cavities and
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can perform excellently.
In the present study, inorganic-framework molecularly imprinted TiO2/SiO2
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(MIP-TiO2/SiO2) nanocomposites were prepared by sol-hydrothermal method using tetrabutyl orthotitanate and tetraethyl orthosilicate as the precursor of functional
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monomer, and 4-nitrophenol (4NP) as template (also the target contaminant). The morphology, structure and photocatalytic activity of the MIP-TiO2/SiO2 were studied.
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Compared with the non-imprinted TiO2/SiO2 (NIP-TiO2/SiO2), MIP-TiO2/SiO2 shows much higher adsorption capacity, adsorption selectivity and molecular recognitive
Ac ce p
photocatalytic activity for 4NP. 2. Experimental 2.1 Materials
Tetrabutyl orthotitanate was purchased from Shanghai Kefeng Chemistry Co.,
Ltd. (Shanghai, China). 4-nitrophenol (4NP) and 2-nitrophenol (2NP) were obtained from Shanghai Jingxi Chemical Technology Co., Ltd. (Shanghai, China). Tetraethyl orthosilicate was acquired from Shanghai Chemical Reagent Purchasing and Supplying Chemical Plant (Shanghai, China). Ethanol was supplied by Shantou Xilong Chemical Co., Ltd. (Shantou, China). Acetic acid was provided by Hubei
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University Chemical Factory (Wuhan, China). Methanol was obtained from Shanghai Zhenxing Chemistry Co., Ltd. (Shanghai, China). Ammonia was obtained from Nanchang Xinxing Chemical Plant (Nanchang, China). All the reagents were of
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analytical grade and used as received. The water used in this study was purified using
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a Milli-Q water system (Bedford, USA).
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2.2 Sol-hydrothermal synthesis of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Briefly, 20 mL tetrabutyl orthotitanate, 0.635 mL tetraethyl orthosilicate, 0.195 g
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4NP and 4 mL acetic acid (as hydrolysis inhibitor) were dissolved in 26 mL ethanol. With the solution stirred vigorously for 1 h, a mixed solution of 8 mL water, 12 mL
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acetic acid and 12 mL ethanol was added dropwise to it. The as-resulted solution was stirred for 6 h at 10 oC for preparation of pre-assembly solution, and then transferred
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to 150 mL teflon-lined autoclave. The autoclave was sealed and heated in electric
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oven at 140 oC for 12 hours, then cooled to room temperature (RT). The obtained
Ac ce p
precipitate was washed with methanol: ammonia solution (1: 1, v/v) in a Soxhlet extraction system until 4NP (as template) was not detected by a LC-20A high-performance liquid chromatography (HPLC, Shimadzu, Japan) equipped with a
C18 ODS column and an ultraviolet detector. Finally, the precipitate was washed with water several times and freezed-dried at low temperature to afford MIP-TiO2/SiO2.
For comparison, the non-imprinted TiO2/SiO2 photocatalyst (NIP-TiO2/SiO2) was prepared following the same procedure but without the addition of 4NP. 2.3 Characterization The morphology of photocatalysts was observed using transmission electron
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microscopy (TEM, JEOL, Japan) and scanning electron microscopy (SEM, Shimadzu, Japan). The X-ray diffraction (XRD) patterns of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 obtained using an automatic X-ray diffractometer with Cu Kα radiation (Rigaku
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D/max 2200PC, operated at 40 kV and 40 mA). A UV-2000 scan UV-vis
cr
spectrophotometer (Unico, USA) equipped with Labsphere diffuse reflectance
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accessory was used to obtain the reflectance spectra of the photocatalysts over a range of 200-600 nm (BaSO4 as reflectance reference). Zeta potential of MIP-TiO2/SiO2 and
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NIP-TiO2/SiO2 was measured using a Zeta potential analyzer (Malvern Nano ZS90). The specific surface area and pore structure of samples were determined over a NOVA
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2000e surface area & pore size analyzer (Quantachrome, USA). 2.4 Study of adsorption performance in the dark
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2.4.1 Adsorption kinetics of 4NP in the dark We mixed 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 with 100 mL of 10 mg/L
Ac ce p
4NP solution. The mixture was shaken at 20 °C using a thermostat oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at a speed of 165 r/min for 2 h in the absence of light and sampled at different time intervals. After immediate filtration, the concentrations of free 4NP in the samples were measured by HPLC. The mobile phase was acetonitrile-water and flow rate of mobile phase was maintained at 0.8 mL/min. The detection wavelength for 4NP was set at 315 nm. 2.4.2 Adsorption isotherm in the dark We mixed 30 mg MIP-TiO2/SiO2 or NIP-TiO2/SiO2 photocatalyst with 20 mL
4NP solutions with specific initial concentrations ranging from 100 to 1200 µmol/L.
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In the dark, the samples were shaken at 20 oC for 2 h, and then subject to filtration for the removal of photocatalysts. The concentration of 4NP in the filtrates were
qe =
(C o - C e ) • V m
(1)
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measured by HPLC. The adsorption capacity (qe) is calculated as follows:
cr
where C0 (mg/L) is the initial concentration of 4NP, Ce (mg/L) is the 4NP
m (g) is the mass of the photocatalysts.
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2.4.3 Competitive batch rebinding in the dark
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concentration after adsorption equilibrium, V (L) is the volume of 4NP solution, and
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In order to evaluate the adsorption selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark, competitive adsorption of 4NP with respect to 2NP was
d
studied. The binary solution of 4NP and 2NP was prepared with the individual initial
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concentration being 10 mg/L. Then 30 mg MIP-TiO2/SiO2 or NIP-TiO2/SiO2 was
Ac ce p
added to 20 mL binary solution. The mixture was oscillated in the dark for 2 h and analyzed in a way similar to that described for steady-state binding studies. The concentrations of 4NP and 2NP in the filtrate were measured by LC-20AD HPLC equipped with a C18 ODS column and an ultraviolet detector. The detection wavelengths were set at 315 nm for 4NP and 279 nm for 2NP. 2.5 Photoelectrochemical measurements All the electrochemical experiments were performed on a CHI660C Electrochemical Workstation (Shanghai Chenhua Instrument Corporation, China). A three-electrode system was employed using a KCl-saturated calomel electrode (SCE) as reference electrode, a carbon electrode as counter electrode, and an indium tin 7
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oxide (ITO) conducting glass coated with a film of MIP-TiO2/SiO2 or NIP-TiO2/SiO2 as working electrode. The MIP-TiO2/SiO2 or NIP-TiO2/SiO2 film was deposited on ITO glass by the dip-coating method. Briefly, a piece of ITO glass was washed in turn
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with acetone, ethanol, and distilled water in an ultrasonic bath for 10 min, and dried at
cr
RT. In the meantime, MIP-TiO2/SiO2 or NIP-TiO2/SiO2 (0.015 g) was dispersed in
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distilled water (10 mL) and ultrasonically treated for 30 min. The as-resulted MIP-TiO2/SiO2 or NIP-TiO2/SiO2 suspension (30 μL) was deposited onto the surface
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of ITO using a microsyringe and allowed to dry in an oven at 60 °C. We used 0.1 mol/L KOH as supporting electrolyte. The applied voltage was 0 V for the ITO
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working electrode. All measurements were carried out at RT with the potentials reported with respect to SCE.
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2.6 Measurements of photocatalytic activity 300 W xenon lamp (PLS-SXE300, Beijing Trusttech Co., Ltd., China) (320
Ac ce p
nm<λ<780 nm) was used as the light source of simulated solar light (with spectrum closely matching that of solar light). Photocatalytic activity of MIP-TiO2/SiO2 or
NIP-TiO2/SiO2 was evaluated by monitoring the photodegradation of 4NP. First, 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 was suspended in 100 mL 10 mg/L 4NP solution, and stirred in the dark for half an hour to reach adsorption equilibrium. Then the mixed solution was irradiated by 300 W xenon lamp under continuous stirring. During the adsorption stage, the mixture was sampled at the 5th, 15th, 20th and 30th minutes, and in the irradiation stage, samples were taken every 10 min. The samples were filtered immediately to remove photocatalysts, and the filtrate was analyzed by HPLC.
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2.7 Molecular recognitive photocatalytic activity of MIP-TiO2/SiO2 In order to evaluate the photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2, competitive photodegradation of 4NP with respect to 2NP was
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studied. We added 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 to a 100 mL binary
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solution of 4NP and 2NP (with individual initial concentration being 10 mg/L). The
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mixture was stirred in the dark for half an hour to reach adsorption equilibrium. Then the mixed solution was irradiated by a 300 W xenon lamp under continuous stirring.
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The mixture was sampled at different time intervals and after immediate filtration, the concentrations of 4NP and 2NP in the filtrate were measured by HPLC.
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2.8 The effect of initial pH on the photocatalytic activity of MIP-TiO2/SiO2 We adjusted the initial pH of solution by adding KOH or HCl to evaluate the
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d
effect of solution pH on the photocatalytic activity of MIP-TiO2/SiO2. The experiment was conducted under condition similar to that adopted for the study of photocatalytic
Ac ce p
activity as illustrated above.
2.9 Reusability of MIP-TiO2/SiO2 For the economic purpose of practical application, it is essential to evaluate the
reusability of photocatalysts. We evaluated the reusability of MIP-TiO2/SiO2 in a test of four cycles. The spent MIP-TiO2/SiO2 photocatalyst was recovered by filtration and
regenerated through washing with a large amount of deionized water before being dried in an oven at 60 oC. The regenerated MIP-TiO2/SiO2 was used for the next cycle of photocatalytic degradation under the same conditions. 3. Results and discussion
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3.1 XRD analysis It has been reported that TiO2 has three main crystalline structures: anatase, rutile and brookite, and anatase TiO2 shows high photocatalytic activity in the
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photodegradation of most pollutants due to its low recombination rate of
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photo-generated electrons and holes [26,27]. The X-ray diffraction (XRD) patterns of
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MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were shown in Figure 1. The XRD data for MIP-TiO2/SiO2 and NIP-TiO2/SiO2 matched well with the standard anatase pattern
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(PDF#04-0477). The characteristic peaks at 2θ = 25.488◦, 38.556◦, 48.135◦, 54.095◦, 55.17◦ and 62.71◦ are ascribed to (101), (004), (200), (105), (211) and (204)
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reflections of anatase TiO2, respectively. The results confirm the presence of anatase TiO2 in MIP-TiO2/SiO2 and NIP-TiO2/SiO2. Moreover, the crystallinity of
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MIP-TiO2/SiO2 is better than that of NIP-TiO2/SiO2, suggesting that the addition of 4NP as template results in enhancement of TiO2 crystallinity. We do not detect any
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signals that are ascribable to SiO2 over MIP-TiO2/SiO2 and NIP-TiO2/SiO2, and such a
phenomenon is in accord with the observation reported by Zhong et al [28] and Mahesh et al [29] over SiO2-TiO2 samples of different kinds [28,29]. It is plausible
that the SiO2 is amorphous, or Si as an interstitial atom enters into the crystal structure
of TiO2 and occupies some of the lattice sites of TiO2. The average crystallite sizes of anatase in the samples were calculated by applying the Debye-Scherrer formula: DScherrer = Kλ / βcosθ
(2)
where DScherrer is the average crystallite size, λ was the wavelength of the X-ray
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radiation (λ=0.154056 nm), K is the Scherrer constant (K=0.89), β is the corrected band broadening (full width at half-maximum (FWHM)), and θ is the diffraction angle. Based on the XRD results, the average size of MIP-TiO2/SiO2 and
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NIP-TiO2/SiO2 was found to be 12.5 and 13.7 nm, respectively.
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3.2 Morphologies of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
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The morphologies of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were investigated by transmission electron microscopy (TEM). Figure 2 shows representativeTEM images
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of MIP-TiO2/SiO2 and NIP-TiO2/SiO2. Most of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 particles are in the form of well-dispersed nanospheres that are regular in shape and
consistent with the XRD results.
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narrow in diameter distribution, showing a mean diameter of about 15 nm, in
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3.3 BET surface areas and pore structures
Figure 3 shows the nitrogen adsorption-desorption isotherms and the
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corresponding pore size distribution curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2. The results of BET surface area, pore volume, and average pore size of MIP-TiO2/SiO2 and
NIP-TiO2/SiO2
are
listed
in
Table
1.
The
nitrogen
adsorption-desorption curves of the two samples are rather similar; both showing type-IV isotherms that indicate the presence of mesopores. The H2 hysteresis loops suggest the presence of pores with narrow necks and wider bodies (ink-bottle pores) formed through the agglomeration of primary crystallites. 3.4 UV-vis diffuse reflectance spectra The optical properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 samples were
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investigated by the UV-vis spectroscopy, as shown in Figure 4. The absorption edges of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 are roughly equal, indicating that the band gap energy (Eg) of the two are close. However, compared with NIP-TiO2/SiO2,
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MIP-TiO2/SiO2 exhibits stronger light absorption in both ultraviolet and visible light
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regions, plausibly due to higher crystallization degree of anatase phase of
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MIP-TiO2/SiO2. It is known that a photocatalyst with higher light-absorption ability usually shows better photocatalytic efficiency. It is hence envisaged that
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MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in photocatalytic activity. 3.5 Zeta potential
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The Zeta potential of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 is shown in Figure 5. With increase of pH value there is a general decrease in surface charge density, and
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the pHzpc (point of zero charge) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 is 6.55 and 3.55, respectively. At pH>pHzpc, the surface is negatively charged, suggesting higher
Ac ce p
affinity for cations. At pH < pHzpc, the surface charge is positive, suggesting higher affinity for anions. At the near neutral environment, MIP-TiO2/SiO2 and
NIP-TiO2/SiO2 are negatively charged with a zeta potential of about -20.98 and -31.97 mV, indicating the existence of the surface hydroxyl groups. 3.6 Photocurrent test
Figure 6 shows the photocurrent-time curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 ITO electrodes under xenon lamp irradiation. It can be seen from Figure 6 that the photocurrents increase when the irradiation of UV-visible light is turned on, and then decrease when the irradiation is turned off. The current values of
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MIP-TiO2/SiO2 and NIP-TiO2/SiO2 ITO electrodes are 1.86 and 0.75 µA/cm2, respectively, indicating that MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in terms of the generation, separation, and transfer efficiency of electron/hole pairs. The result is
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consistent with the order of photocatalytic activity.
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3.7 The adsorption properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark
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3.7.1 The adsorption kinetics of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark
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Adsorption kinetics, demonstrating the solute uptake rate, is one of the most important characters which could represent the adsorption efficiency and mass
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transportation, and adsorption kinetics have obvious influence on the subsequent photodegradation of organic pollutants. Figure 7 shows the adsorption kinetics of 4NP
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d
on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark. It could be seen from Figure 7 that the adsorption amounts of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 increase
Ac ce p
rapidly at the beginning, then adsorption rate gradually becomes slow until equilibrium is established. The time required to achieve the adsorption equilibrium is only 20 min, and there is no obvious change of adsorption amount from 20 to 50 min. Moreover, the adsorption rate of MIP-TiO2/SiO2 is twice that of MIP-TiO2/SiO2. High adsorption rate of MIP-TiO2/SiO2 can be attributed to the abundance of adsorption sites, good affinity of MIP-TiO2/SiO2 for 4NP, and geometric matching between 4NP and imprinted cavities. To investigate the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, the pseudo-first-order and pseudo-second-order
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kinetics models were used to fit the adsorption kinetics of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2. The pseudo-first-order kinetics equation is expressed as ln(qe − qt ) = ln qe − k1t
(3)
ip t
where qe is the amount of adsorbate at equilibrium (mg/g), qt is the amount of
cr
adsorbate at any time t (mg/g), and k1 is adsorption constant (min-1). The values of k1,
correlation coefficients (R2) and the theoretical qe value estimated from
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pseudo-first-order kinetics equation are listed in Table 2. The R2 values are relatively
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low, indicating that the adsorption of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 do not follow pseudo-first-order kinetics equation.
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The linear form of pseudo-second-order model is expressed as followed:
(4)
d
t 1 1 = + •t 2 qt k2 • qe qe
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k2 is the constant rate of pseudo-second-order (g·mg-1min-1). The regression curve of
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t/qt versus t was rather linear (Figure 8), and qe and k2 can be obtained from the slope and intercept of the plot, respectively. The values of k2, correlation coefficients (R2)
and the theoretical qe value estimated from pseudo-second-order model are also listed
in Table 2. The R2 values are close to 1, indicating that the adsorption kinetics of 4NP
on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 follow the pseudo-second-order model. The theoretical qe value estimated from the pseudo-second-order kinetic model is also very
close to the experimental value. 3.7.2 Adsorption isotherm Figure 9 shows the adsorption isotherms of 4NP over MIP-TiO2/SiO2 and NIP-TiO2/SiO2 photocatalysts. It can be seen from Figure 9 that the equilibrium 14
Page 14 of 32
adsorption capacity (qe) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 for 4NP increased sharply with increasing concentration of 4NP in the low concentration range, then increased slightly, and finally reached saturated adsorption. Once reaching saturated
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adsorption, the maximum adsorption capacities do not change with increasing
cr
concentration of 4NP. Moreover, the adsorption capacity of MIP-TiO2/SiO2 for 4NP is
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much higher than that of NIP-TiO2/SiO2. It is well known that adsorption capability has significant effect on the degradation of organic pollutants. Therefore, it can be
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inferred that MIP-TiO2/SiO2 would exhibit higher degradation ability towards 4NP in comparison with NIP-TiO2/SiO2.
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3.7.3 Adsorption selectivity
Being similar in chemical properties as well as equal in molecular weight and
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composition, 2-NP is chosen as a competitor of 4NP for adsorption on MIP-TiO2/SiO2 and NIP-TiO2/SiO2, and the results are shown in Fig. 10. The adsorption capacity of
Ac ce p
MIP-TiO2/SiO2 for 4NP is much higher than that for 2NP, while NIP-TiO2/SiO2 shows similar adsorption capacity towards 4NP and 2NP. The intrinsic molecular recognitive adsorption of MIP-TiO2/SiO2 towards 4NP is hence attributed to the imprinted
cavities and specific binding sites of MIP-TiO2/SiO2. 3.8 Photocatalytic activity and photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 The photocatalytic abilities of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were evaluated
in the degradation of 4NP. Figure 11a shows the adsorption process in the dark and photocatalytic degradation of 4NP over the MIP-TiO2/SiO2 and NIP-TiO2/SiO2
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photocatalysts under xenon lamp irradiation. In the dark adsorption stage, the MIP-TiO2/SiO2 showed superior adsorption performance, and this result is consistent with that of adsorption kinetics above. Moreover, the degradation of 4NP over the
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MIP-TiO2/SiO2 photocatalyst comes to completion within 30 min, while it takes 110
cr
min over NIP-TiO2/SiO2.
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In order to evaluate the photocatalytic selectivity, the photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 was evaluated by degrading 4NP in binary
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solutions containing 4NP (10 mg/L) and a co-existing pollutant 2NP (10 mg/L). The experimental results are shown in Figure 11b. MIP-TiO2/SiO2 showed much higher
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photocatalytic activity for 4NP than 2NP, while NIP-TiO2/SiO2 exhibited almost the same photocatalytic activity for 4NP as 2NP. The above results indicate that
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MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in photocatalytic activity and selectivity. We attribute the high performance of MIP-TiO2/SiO2 to the enhanced light absorption
Ac ce p
ability in both ultraviolet and visible light regions, the high separation and transfer efficiency of electron/hole pairs, and the good affinity of MIP-TiO2/SiO2 for 4NP. 3.9 The effect of initial pH on the photocatalytic activity of MIP-TiO2/SiO2 Solution pH is an important parameter that influences the photocatalytic
degradation since it influences the surface charge properties of the MIP-TiO2/SiO2, the charge of 4NP molecules, the adsorption of 4NP on the surface of MIP-TiO2/SiO2 and the concentration of the hydroxyl radicals. Figure 12 shows the removal efficiency of 4NP on MIP-TiO2/SiO2 in the pH value range from 3.0 to 9.0. The photocatalytic
degradation efficiency of 4NP increases remarkably with increasing pH from 3.0 to
16
Page 16 of 32
4.0. When the pH was 4.0, it reached the maximum degradation efficiency, and then decreases obviously at higher pH, which is similar to the results reported by Baran and Muneer [30,31]. The highest degradation efficiency at pH 4.0 is explained as
ip t
follows. When pH increases from 3.3 to 4.0, 4NP is primarily in its nonionic form,
cr
and its solubility in water is minimized and the adsorption of 4NP on the catalyst is
us
maximized due to the involvement of surface hydroxyl groups [32]. Moreover, at a pH of 4.0, there is the formation of HO2• radicals known to be primary oxidizing
an
species responsible for pollutant degradation [33]. With further rise of solution pH (i.e. when pH>pHzpc), the surface of MIP-TiO2/SiO2 is negatively charged while 4NP tends
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to exist in its anionic form [32]. The electrostatic repulsion between the catalyst surface and 4NP hinders 4NP adsorption. Consequently, there is decrease in
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d
degradation efficiency. 3.10 Reusability of MIP-TiO2/SiO2 The performance of a catalyst in the recycling experiments is of great significance
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for its application to environmental technology. Therefore, the stability and reusability of MIP-TiO2/SiO2 were investigated. The photocatalytic degradation-regeneration
cycles were repeated for four times. The photocatalytic degradation ability of the regenerated MIP-TiO2/SiO2 is shown in Figure 13. It can be seen that the
photocatalytic degradation ability of MIP-TiO2/SiO2 is stable for four photocatalytic degradation-regeneration cycles without obvious decrease in the photocatalytic degradation of target molecules, indicating that MIP-TiO2/SiO2 has excellent regeneration, and can be used repeatedly.
4. Conclusions
17
Page 17 of 32
Inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite with molecular
recognitive
photocatalytic
ability
was
successfully
prepared
under mild condition. The molecular imprinted cavities on the surface of TiO2/SiO2
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nanocomposite provided MIP-TiO2/SiO2 with selective affinity and specific molecular
cr
recognition ability, leading to preferential photocatalytic degradation towards target
us
contaminant. Because of the inorganic framework and the facile release of active adsorption sites upon regeneration, MIP-TiO2/SiO2 is highly reusable.
an
Acknowledgements
This work was financially supported by Natural Science Foundation of China
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(50978132, 51178213, 51238002, 51272099, 51308278, 51368045), Program for New Century Excellent Talents in University (NCET-11-1004), Cultivating Program for
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Young Scientists of Jiangxi Province of China (20112BCB23016) Natural Science Foundation of Jiangxi Province (20122BAB213014) and Department of Education
Ac ce p
Fund of Jiangxi Province (GJJ13506). References
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ip t
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ip t
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cr
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molecular sol–gel imprinting technology for selective solid-phase micro extraction of caffeine from human serum and determination by gas chromatography/mass spectrometry, Anal. Chim. Acta 727 (2012) 20-25. [24] X. Luo, F. Deng, L. Min, S. Luo, B. Guo, G. Zeng, C. Au, Facile one-step synthesis of inorganic-framework molecularly imprinted TiO2/WO3 nanocomposite and its molecular recognitive photocatalytic degradation of target contaminant, Environ. Sci. Technol. 47 (2013) 7404-7412. [25] D. M. Han, G. L. Dai, W. P. Jia, H. D. Liang, Preparation and photocatalytic activity of Cu2+-doped 2,4-dichlorophenol molecularly imprinted SiO2-TiO2
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ip t
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cr
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[33] F. Deng, X. Luo, K. Li, X. Tu, S. Luo, L. Yang, N. Zhou, H. Shu, The effect of vinyl-containing ionic liquid on the photocatalytic activity of iron-doped TiO2, J. Mol.
Ac ce p
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d
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an
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cr
ip t
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Figure caption Figure 1 XRD patterns of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 2 TEM images of: (a) MIP-TiO2/SiO2 and (b) NIP-TiO2/SiO2
cr
size distributions curves (b) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
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Figure 3 Nitrogen adsorption-desorption isotherms (a) and the corresponding pore
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Figure 4 UV-vis diffuse reflectance spectra of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 5 Zeta-potential of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 samples suspended in
an
20 mL 4NP solution as a function of pH
Figure 6 Transient photocurrent-time curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
M
ITO electrodes under xenon lamp irradiation
te
NIP-TiO2/SiO2
d
Figure 7 Adsorption kinetics for 4NP adsorption on MIP-TiO2/SiO2 and
Figure 8 Linear plot of t/q vs t for MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Ac ce p
Figure 9 Adsorption isotherm for 4NP adsorption on MIP-TiO2/SiO2 and
NIP-TiO2/SiO2
Figure 10 Adsorption selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 11 (a) Time profile of 4NP removal by the MIP-TiO2/SiO2 and NIP-TiO2/SiO2, (b) photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 12 The effect of initial pH on removal efficiency of 4NP by MIP-TiO2/SiO2 Figure 13 Stability and potential regeneration of MIP-TiO2/SiO2
24
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ip t cr us
Ac ce p
te
d
M
an
Figure 1
Figure 2
Figure 3
25
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ip t cr us an
Ac ce p
te
d
M
Figure 4
Figure 5
26
Page 26 of 32
ip t cr us an
Ac ce p
te
d
M
Figure 6
Figure 7
27
Page 27 of 32
ip t cr us an
Ac ce p
te
d
M
Figure 8
Figure 9
28
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ip t cr us an M
Figure 10
(a)
Ac ce p
te
d
(b)
Figure 11
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ip t cr us
Ac ce p
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Figure 12
Figure 13
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Table 1 Physical properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Average pore 3
SBET (m /g)
Pore volume (m /g)
ip t
Samples
2
MIP-TiO2/SiO2
186.49
0.36
7.66
NIP-TiO2/SiO2
184.49
0.33
cr
size (nm)
us
7.20
an
Table 2 Adsorption kinetic parameters for 4NP adsorption on MIP-TiO2/SiO2 and
Pseudo-first-order
Pseudo-second-order
qe(mg/g)
K1(min-1)
R2
qe(mg/g)
K2(g·m-1·min-1)
R2
4NP-TiO2/SiO2
2.575
d
Photocatalyst
M
NIP-TiO2/SiO2
0.941
2.671
0.085
0.986
NIP-TiO2/SiO2
0.074
-0.238
0.777
0.373
3.064
0.998
Ac ce p
te
0.102
31
Page 31 of 32
Ac ce p
te
d
M
an
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cr
ip t
Highlights MIP-TiO2/SiO2 nanocomposite was prepared by sol-hydrothermal method. MIP-TiO2/SiO2 shows molecular recognition ability. The structure-activity relationship of MIP-TiO2/SiO2 was established. MIP-TiO2/SiO2 shows higher adsorption capacity and selectivity than NIP-TiO2/SiO2. MIP-TiO2/SiO2 exhibits photocatalytic selectivity towards target contaminant.
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S0304-3894(14)00443-9 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.05.088 HAZMAT 15995
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
20-3-2014 28-5-2014 29-5-2014
Please cite this article as: F. Deng, Y. Liu, X. Luo, S. Wu, S. Luo, C. Au, R. Qi, Sol-hydrothermal synthesis of inorganic-framework molecularly imprinted TiO2 /SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.05.088 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Sol-hydrothermal synthesis of inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite and its preferential photocatalytic degradation towards target contaminant
ip t
Fang Denga,b, Yin Liua,b, Xubiao Luoa,b*, Shaolin Wua,b, Shenglian Luoa,b∗, Chaktong
Key Laboratory of Jiangxi Province for Persistent Pollutants Control and
Resources Recycle, Nanchang 330063, PR China
College of Environmental and Chemical Engineering, Nanchang Hangkong
University, Nanchang 330063, PR China
Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong
M
c
an
b
us
a
cr
Auc, Ruoxi Qia,b
Kong, PR China
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d
ABSTRACT: Inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite (MIP-TiO2/SiO2) was successfully prepared by sol-hydrothermal method using
Ac ce p
4-nitrophenol as template. The morphology, structure, optical property, zeta-potential and photocurrent of MIP-TiO2/SiO2 were characterized. The adsorption performance and photocatalytic selectivity were also studied. MIP-TiO2/SiO2 shows higher
adsorption capacity and selectivity than the non-imprinted TiO2/SiO2 (NIP-TiO2/SiO2). Kinetics results show that the adsorption equilibrium of 4-nitrophenol on
MIP-TiO2/SiO2 is established within 20 min, and the adsorption process obeys the pseudo-second-order model. Moreover, MIP-TiO2/SiO2 can completely degrade
∗
Corresponding author. Tel.: +86 791 83953372; Fax: +86 791 8395373. E-mail addresses: [email protected] (S. Luo), [email protected] (X. Luo). 1
Page 1 of 32
4-nitrophenol within 30 min, while NIP-TiO2/SiO2 takes 110 min. It was found that the MIP-TiO2/SiO2 photocatalyst shows molecular recognition ability, leading to selective adsorption and molecular recognitive photocatalytic degradation of
ip t
4-nitrophenol. Furthermore, because of its inorganic framework, MIP-TiO2/SiO2
molecularly
imprinted
photocatalyst;
TiO2/SiO2
nanocomposite;
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Keywords:
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shows excellent reusability.
sol-hydrothermal method; molecular recognition; photocatalytic ability
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1. Introduction
Environmental pollution with ill effects on human and animal health has
M
attracted much attention in the last decade [1,2]. Heterogeneous semiconductor photocatalysis is an effective and promising technique to tackle pollution issues [3-8].
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d
Nano-sized TiO2 has emerged as a versatile functional material and effective photocatalyst in environmental remediation due to its inexpensive, non-toxic and
Ac ce p
excellent photoelectric properties [9-13]. However, in many cases the TiO2 photocatalyst can not meet the requirements of pollution control [14,15]. For example, it is difficult to selectively degrade highly toxic pollutants in a complicated aqueous system since photo-oxidation of organic compounds over TiO2 is dominated by free radicals and the related mechanism is known to be non-selective [16-20]. Therefore, the development of photocatalysts that show photocatalytic selectivity is urgently required. Up to now, much efforts have been devoted to enhance the photocatalytic selectivity of TiO2 for removal of target organic pollutants. It was reported that
2
Page 2 of 32
molecularly imprinted polymers (MIPs) with specific binding sites complementary to the template molecules in terms of size, shape and functional group exhibit specific recognition ability for the target molecules [21-23]. Applying the molecularly
ip t
imprinting technique in the synthesis of TiO2, one can generate molecularly imprinted
cr
TiO2 with recognition ability towards a target pollutant. This kind of molecularly
us
imprinted photocatalysts have a promising application in selective and preferential degradation of a highly toxic pollutant that exists in low level together with other less
an
hazardous pollutants in a water system [15-20]. Moreover, molecularly imprinted photocatalysts could selectively remove the organic pollutants with the nutrients being
M
remained in the wastewater, resolving serious shortage of water agricultural irrigation [24]. Molecularly imprinted TiO2 photocatalysts can be fabricated by coating an
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organic layer of molecularly imprinted polymers on the surface of TiO2 nanoparticles. Although the layer of organic MIPs enables the TiO2 to exhibit high selectivity for
Ac ce p
UV-photodegradation of a target pollutant, the selectivity of these organic-inorganic hybrids decreases due to the degradation of organic MIPs upon extended UV illumination. To overcome this drawback, Han et al [25] developed the Cu2+-doped
2,4-dichlorophenol molecularly imprinted TiO2-SiO2 while Shen et al [19]
synthesized
Al3+-doped
diethyl
phthalate
molecularly
imprinted
TiO2-SiO2
nanocomposites, both without the use of organic MIPs [19,25]. However, the population of imprinted cavities on the surface is low and Lewis acid-base interaction between the nanocomposites and target molecules is weak. Recently, our group prepared inorganic-framework molecularly imprinted TiO2/WO3 nanocomposites by a
3
Page 3 of 32
facile one-step sol-gel method using tetrabutyl orthotitanate as titanium source as well as the precursor of functional monomer. Nonetheless, it was pointed out that the process is energy-consuming and some imprinted cavities are destroyed upon
ip t
high-temperature calcination [24]. Therefore, it is desirable to develop mild methods
cr
to prepare molecularly imprinted photocatalysts that are rich in imprinted cavities and
us
can perform excellently.
In the present study, inorganic-framework molecularly imprinted TiO2/SiO2
an
(MIP-TiO2/SiO2) nanocomposites were prepared by sol-hydrothermal method using tetrabutyl orthotitanate and tetraethyl orthosilicate as the precursor of functional
M
monomer, and 4-nitrophenol (4NP) as template (also the target contaminant). The morphology, structure and photocatalytic activity of the MIP-TiO2/SiO2 were studied.
te
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Compared with the non-imprinted TiO2/SiO2 (NIP-TiO2/SiO2), MIP-TiO2/SiO2 shows much higher adsorption capacity, adsorption selectivity and molecular recognitive
Ac ce p
photocatalytic activity for 4NP. 2. Experimental 2.1 Materials
Tetrabutyl orthotitanate was purchased from Shanghai Kefeng Chemistry Co.,
Ltd. (Shanghai, China). 4-nitrophenol (4NP) and 2-nitrophenol (2NP) were obtained from Shanghai Jingxi Chemical Technology Co., Ltd. (Shanghai, China). Tetraethyl orthosilicate was acquired from Shanghai Chemical Reagent Purchasing and Supplying Chemical Plant (Shanghai, China). Ethanol was supplied by Shantou Xilong Chemical Co., Ltd. (Shantou, China). Acetic acid was provided by Hubei
4
Page 4 of 32
University Chemical Factory (Wuhan, China). Methanol was obtained from Shanghai Zhenxing Chemistry Co., Ltd. (Shanghai, China). Ammonia was obtained from Nanchang Xinxing Chemical Plant (Nanchang, China). All the reagents were of
ip t
analytical grade and used as received. The water used in this study was purified using
cr
a Milli-Q water system (Bedford, USA).
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2.2 Sol-hydrothermal synthesis of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Briefly, 20 mL tetrabutyl orthotitanate, 0.635 mL tetraethyl orthosilicate, 0.195 g
an
4NP and 4 mL acetic acid (as hydrolysis inhibitor) were dissolved in 26 mL ethanol. With the solution stirred vigorously for 1 h, a mixed solution of 8 mL water, 12 mL
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acetic acid and 12 mL ethanol was added dropwise to it. The as-resulted solution was stirred for 6 h at 10 oC for preparation of pre-assembly solution, and then transferred
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to 150 mL teflon-lined autoclave. The autoclave was sealed and heated in electric
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oven at 140 oC for 12 hours, then cooled to room temperature (RT). The obtained
Ac ce p
precipitate was washed with methanol: ammonia solution (1: 1, v/v) in a Soxhlet extraction system until 4NP (as template) was not detected by a LC-20A high-performance liquid chromatography (HPLC, Shimadzu, Japan) equipped with a
C18 ODS column and an ultraviolet detector. Finally, the precipitate was washed with water several times and freezed-dried at low temperature to afford MIP-TiO2/SiO2.
For comparison, the non-imprinted TiO2/SiO2 photocatalyst (NIP-TiO2/SiO2) was prepared following the same procedure but without the addition of 4NP. 2.3 Characterization The morphology of photocatalysts was observed using transmission electron
5
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microscopy (TEM, JEOL, Japan) and scanning electron microscopy (SEM, Shimadzu, Japan). The X-ray diffraction (XRD) patterns of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 obtained using an automatic X-ray diffractometer with Cu Kα radiation (Rigaku
ip t
D/max 2200PC, operated at 40 kV and 40 mA). A UV-2000 scan UV-vis
cr
spectrophotometer (Unico, USA) equipped with Labsphere diffuse reflectance
us
accessory was used to obtain the reflectance spectra of the photocatalysts over a range of 200-600 nm (BaSO4 as reflectance reference). Zeta potential of MIP-TiO2/SiO2 and
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NIP-TiO2/SiO2 was measured using a Zeta potential analyzer (Malvern Nano ZS90). The specific surface area and pore structure of samples were determined over a NOVA
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2000e surface area & pore size analyzer (Quantachrome, USA). 2.4 Study of adsorption performance in the dark
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2.4.1 Adsorption kinetics of 4NP in the dark We mixed 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 with 100 mL of 10 mg/L
Ac ce p
4NP solution. The mixture was shaken at 20 °C using a thermostat oscillator (Changzhou Guohua Instrument Company, Changzhou, China) at a speed of 165 r/min for 2 h in the absence of light and sampled at different time intervals. After immediate filtration, the concentrations of free 4NP in the samples were measured by HPLC. The mobile phase was acetonitrile-water and flow rate of mobile phase was maintained at 0.8 mL/min. The detection wavelength for 4NP was set at 315 nm. 2.4.2 Adsorption isotherm in the dark We mixed 30 mg MIP-TiO2/SiO2 or NIP-TiO2/SiO2 photocatalyst with 20 mL
4NP solutions with specific initial concentrations ranging from 100 to 1200 µmol/L.
6
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In the dark, the samples were shaken at 20 oC for 2 h, and then subject to filtration for the removal of photocatalysts. The concentration of 4NP in the filtrates were
qe =
(C o - C e ) • V m
(1)
ip t
measured by HPLC. The adsorption capacity (qe) is calculated as follows:
cr
where C0 (mg/L) is the initial concentration of 4NP, Ce (mg/L) is the 4NP
m (g) is the mass of the photocatalysts.
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2.4.3 Competitive batch rebinding in the dark
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concentration after adsorption equilibrium, V (L) is the volume of 4NP solution, and
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In order to evaluate the adsorption selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark, competitive adsorption of 4NP with respect to 2NP was
d
studied. The binary solution of 4NP and 2NP was prepared with the individual initial
te
concentration being 10 mg/L. Then 30 mg MIP-TiO2/SiO2 or NIP-TiO2/SiO2 was
Ac ce p
added to 20 mL binary solution. The mixture was oscillated in the dark for 2 h and analyzed in a way similar to that described for steady-state binding studies. The concentrations of 4NP and 2NP in the filtrate were measured by LC-20AD HPLC equipped with a C18 ODS column and an ultraviolet detector. The detection wavelengths were set at 315 nm for 4NP and 279 nm for 2NP. 2.5 Photoelectrochemical measurements All the electrochemical experiments were performed on a CHI660C Electrochemical Workstation (Shanghai Chenhua Instrument Corporation, China). A three-electrode system was employed using a KCl-saturated calomel electrode (SCE) as reference electrode, a carbon electrode as counter electrode, and an indium tin 7
Page 7 of 32
oxide (ITO) conducting glass coated with a film of MIP-TiO2/SiO2 or NIP-TiO2/SiO2 as working electrode. The MIP-TiO2/SiO2 or NIP-TiO2/SiO2 film was deposited on ITO glass by the dip-coating method. Briefly, a piece of ITO glass was washed in turn
ip t
with acetone, ethanol, and distilled water in an ultrasonic bath for 10 min, and dried at
cr
RT. In the meantime, MIP-TiO2/SiO2 or NIP-TiO2/SiO2 (0.015 g) was dispersed in
us
distilled water (10 mL) and ultrasonically treated for 30 min. The as-resulted MIP-TiO2/SiO2 or NIP-TiO2/SiO2 suspension (30 μL) was deposited onto the surface
an
of ITO using a microsyringe and allowed to dry in an oven at 60 °C. We used 0.1 mol/L KOH as supporting electrolyte. The applied voltage was 0 V for the ITO
M
working electrode. All measurements were carried out at RT with the potentials reported with respect to SCE.
te
d
2.6 Measurements of photocatalytic activity 300 W xenon lamp (PLS-SXE300, Beijing Trusttech Co., Ltd., China) (320
Ac ce p
nm<λ<780 nm) was used as the light source of simulated solar light (with spectrum closely matching that of solar light). Photocatalytic activity of MIP-TiO2/SiO2 or
NIP-TiO2/SiO2 was evaluated by monitoring the photodegradation of 4NP. First, 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 was suspended in 100 mL 10 mg/L 4NP solution, and stirred in the dark for half an hour to reach adsorption equilibrium. Then the mixed solution was irradiated by 300 W xenon lamp under continuous stirring. During the adsorption stage, the mixture was sampled at the 5th, 15th, 20th and 30th minutes, and in the irradiation stage, samples were taken every 10 min. The samples were filtered immediately to remove photocatalysts, and the filtrate was analyzed by HPLC.
8
Page 8 of 32
2.7 Molecular recognitive photocatalytic activity of MIP-TiO2/SiO2 In order to evaluate the photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2, competitive photodegradation of 4NP with respect to 2NP was
ip t
studied. We added 0.15 g MIP-TiO2/SiO2 or NIP-TiO2/SiO2 to a 100 mL binary
cr
solution of 4NP and 2NP (with individual initial concentration being 10 mg/L). The
us
mixture was stirred in the dark for half an hour to reach adsorption equilibrium. Then the mixed solution was irradiated by a 300 W xenon lamp under continuous stirring.
an
The mixture was sampled at different time intervals and after immediate filtration, the concentrations of 4NP and 2NP in the filtrate were measured by HPLC.
M
2.8 The effect of initial pH on the photocatalytic activity of MIP-TiO2/SiO2 We adjusted the initial pH of solution by adding KOH or HCl to evaluate the
te
d
effect of solution pH on the photocatalytic activity of MIP-TiO2/SiO2. The experiment was conducted under condition similar to that adopted for the study of photocatalytic
Ac ce p
activity as illustrated above.
2.9 Reusability of MIP-TiO2/SiO2 For the economic purpose of practical application, it is essential to evaluate the
reusability of photocatalysts. We evaluated the reusability of MIP-TiO2/SiO2 in a test of four cycles. The spent MIP-TiO2/SiO2 photocatalyst was recovered by filtration and
regenerated through washing with a large amount of deionized water before being dried in an oven at 60 oC. The regenerated MIP-TiO2/SiO2 was used for the next cycle of photocatalytic degradation under the same conditions. 3. Results and discussion
9
Page 9 of 32
3.1 XRD analysis It has been reported that TiO2 has three main crystalline structures: anatase, rutile and brookite, and anatase TiO2 shows high photocatalytic activity in the
ip t
photodegradation of most pollutants due to its low recombination rate of
cr
photo-generated electrons and holes [26,27]. The X-ray diffraction (XRD) patterns of
us
MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were shown in Figure 1. The XRD data for MIP-TiO2/SiO2 and NIP-TiO2/SiO2 matched well with the standard anatase pattern
an
(PDF#04-0477). The characteristic peaks at 2θ = 25.488◦, 38.556◦, 48.135◦, 54.095◦, 55.17◦ and 62.71◦ are ascribed to (101), (004), (200), (105), (211) and (204)
M
reflections of anatase TiO2, respectively. The results confirm the presence of anatase TiO2 in MIP-TiO2/SiO2 and NIP-TiO2/SiO2. Moreover, the crystallinity of
te
d
MIP-TiO2/SiO2 is better than that of NIP-TiO2/SiO2, suggesting that the addition of 4NP as template results in enhancement of TiO2 crystallinity. We do not detect any
Ac ce p
signals that are ascribable to SiO2 over MIP-TiO2/SiO2 and NIP-TiO2/SiO2, and such a
phenomenon is in accord with the observation reported by Zhong et al [28] and Mahesh et al [29] over SiO2-TiO2 samples of different kinds [28,29]. It is plausible
that the SiO2 is amorphous, or Si as an interstitial atom enters into the crystal structure
of TiO2 and occupies some of the lattice sites of TiO2. The average crystallite sizes of anatase in the samples were calculated by applying the Debye-Scherrer formula: DScherrer = Kλ / βcosθ
(2)
where DScherrer is the average crystallite size, λ was the wavelength of the X-ray
10
Page 10 of 32
radiation (λ=0.154056 nm), K is the Scherrer constant (K=0.89), β is the corrected band broadening (full width at half-maximum (FWHM)), and θ is the diffraction angle. Based on the XRD results, the average size of MIP-TiO2/SiO2 and
ip t
NIP-TiO2/SiO2 was found to be 12.5 and 13.7 nm, respectively.
cr
3.2 Morphologies of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
us
The morphologies of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were investigated by transmission electron microscopy (TEM). Figure 2 shows representativeTEM images
an
of MIP-TiO2/SiO2 and NIP-TiO2/SiO2. Most of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 particles are in the form of well-dispersed nanospheres that are regular in shape and
consistent with the XRD results.
M
narrow in diameter distribution, showing a mean diameter of about 15 nm, in
te
d
3.3 BET surface areas and pore structures
Figure 3 shows the nitrogen adsorption-desorption isotherms and the
Ac ce p
corresponding pore size distribution curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2. The results of BET surface area, pore volume, and average pore size of MIP-TiO2/SiO2 and
NIP-TiO2/SiO2
are
listed
in
Table
1.
The
nitrogen
adsorption-desorption curves of the two samples are rather similar; both showing type-IV isotherms that indicate the presence of mesopores. The H2 hysteresis loops suggest the presence of pores with narrow necks and wider bodies (ink-bottle pores) formed through the agglomeration of primary crystallites. 3.4 UV-vis diffuse reflectance spectra The optical properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 samples were
11
Page 11 of 32
investigated by the UV-vis spectroscopy, as shown in Figure 4. The absorption edges of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 are roughly equal, indicating that the band gap energy (Eg) of the two are close. However, compared with NIP-TiO2/SiO2,
ip t
MIP-TiO2/SiO2 exhibits stronger light absorption in both ultraviolet and visible light
cr
regions, plausibly due to higher crystallization degree of anatase phase of
us
MIP-TiO2/SiO2. It is known that a photocatalyst with higher light-absorption ability usually shows better photocatalytic efficiency. It is hence envisaged that
an
MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in photocatalytic activity. 3.5 Zeta potential
M
The Zeta potential of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 is shown in Figure 5. With increase of pH value there is a general decrease in surface charge density, and
te
d
the pHzpc (point of zero charge) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 is 6.55 and 3.55, respectively. At pH>pHzpc, the surface is negatively charged, suggesting higher
Ac ce p
affinity for cations. At pH < pHzpc, the surface charge is positive, suggesting higher affinity for anions. At the near neutral environment, MIP-TiO2/SiO2 and
NIP-TiO2/SiO2 are negatively charged with a zeta potential of about -20.98 and -31.97 mV, indicating the existence of the surface hydroxyl groups. 3.6 Photocurrent test
Figure 6 shows the photocurrent-time curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 ITO electrodes under xenon lamp irradiation. It can be seen from Figure 6 that the photocurrents increase when the irradiation of UV-visible light is turned on, and then decrease when the irradiation is turned off. The current values of
12
Page 12 of 32
MIP-TiO2/SiO2 and NIP-TiO2/SiO2 ITO electrodes are 1.86 and 0.75 µA/cm2, respectively, indicating that MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in terms of the generation, separation, and transfer efficiency of electron/hole pairs. The result is
ip t
consistent with the order of photocatalytic activity.
cr
3.7 The adsorption properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark
us
3.7.1 The adsorption kinetics of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark
an
Adsorption kinetics, demonstrating the solute uptake rate, is one of the most important characters which could represent the adsorption efficiency and mass
M
transportation, and adsorption kinetics have obvious influence on the subsequent photodegradation of organic pollutants. Figure 7 shows the adsorption kinetics of 4NP
te
d
on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 in the dark. It could be seen from Figure 7 that the adsorption amounts of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 increase
Ac ce p
rapidly at the beginning, then adsorption rate gradually becomes slow until equilibrium is established. The time required to achieve the adsorption equilibrium is only 20 min, and there is no obvious change of adsorption amount from 20 to 50 min. Moreover, the adsorption rate of MIP-TiO2/SiO2 is twice that of MIP-TiO2/SiO2. High adsorption rate of MIP-TiO2/SiO2 can be attributed to the abundance of adsorption sites, good affinity of MIP-TiO2/SiO2 for 4NP, and geometric matching between 4NP and imprinted cavities. To investigate the controlling mechanism of adsorption processes such as mass transfer and chemical reaction, the pseudo-first-order and pseudo-second-order
13
Page 13 of 32
kinetics models were used to fit the adsorption kinetics of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2. The pseudo-first-order kinetics equation is expressed as ln(qe − qt ) = ln qe − k1t
(3)
ip t
where qe is the amount of adsorbate at equilibrium (mg/g), qt is the amount of
cr
adsorbate at any time t (mg/g), and k1 is adsorption constant (min-1). The values of k1,
correlation coefficients (R2) and the theoretical qe value estimated from
us
pseudo-first-order kinetics equation are listed in Table 2. The R2 values are relatively
an
low, indicating that the adsorption of 4NP on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 do not follow pseudo-first-order kinetics equation.
M
The linear form of pseudo-second-order model is expressed as followed:
(4)
d
t 1 1 = + •t 2 qt k2 • qe qe
te
k2 is the constant rate of pseudo-second-order (g·mg-1min-1). The regression curve of
Ac ce p
t/qt versus t was rather linear (Figure 8), and qe and k2 can be obtained from the slope and intercept of the plot, respectively. The values of k2, correlation coefficients (R2)
and the theoretical qe value estimated from pseudo-second-order model are also listed
in Table 2. The R2 values are close to 1, indicating that the adsorption kinetics of 4NP
on MIP-TiO2/SiO2 and NIP-TiO2/SiO2 follow the pseudo-second-order model. The theoretical qe value estimated from the pseudo-second-order kinetic model is also very
close to the experimental value. 3.7.2 Adsorption isotherm Figure 9 shows the adsorption isotherms of 4NP over MIP-TiO2/SiO2 and NIP-TiO2/SiO2 photocatalysts. It can be seen from Figure 9 that the equilibrium 14
Page 14 of 32
adsorption capacity (qe) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 for 4NP increased sharply with increasing concentration of 4NP in the low concentration range, then increased slightly, and finally reached saturated adsorption. Once reaching saturated
ip t
adsorption, the maximum adsorption capacities do not change with increasing
cr
concentration of 4NP. Moreover, the adsorption capacity of MIP-TiO2/SiO2 for 4NP is
us
much higher than that of NIP-TiO2/SiO2. It is well known that adsorption capability has significant effect on the degradation of organic pollutants. Therefore, it can be
an
inferred that MIP-TiO2/SiO2 would exhibit higher degradation ability towards 4NP in comparison with NIP-TiO2/SiO2.
M
3.7.3 Adsorption selectivity
Being similar in chemical properties as well as equal in molecular weight and
te
d
composition, 2-NP is chosen as a competitor of 4NP for adsorption on MIP-TiO2/SiO2 and NIP-TiO2/SiO2, and the results are shown in Fig. 10. The adsorption capacity of
Ac ce p
MIP-TiO2/SiO2 for 4NP is much higher than that for 2NP, while NIP-TiO2/SiO2 shows similar adsorption capacity towards 4NP and 2NP. The intrinsic molecular recognitive adsorption of MIP-TiO2/SiO2 towards 4NP is hence attributed to the imprinted
cavities and specific binding sites of MIP-TiO2/SiO2. 3.8 Photocatalytic activity and photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 The photocatalytic abilities of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 were evaluated
in the degradation of 4NP. Figure 11a shows the adsorption process in the dark and photocatalytic degradation of 4NP over the MIP-TiO2/SiO2 and NIP-TiO2/SiO2
15
Page 15 of 32
photocatalysts under xenon lamp irradiation. In the dark adsorption stage, the MIP-TiO2/SiO2 showed superior adsorption performance, and this result is consistent with that of adsorption kinetics above. Moreover, the degradation of 4NP over the
ip t
MIP-TiO2/SiO2 photocatalyst comes to completion within 30 min, while it takes 110
cr
min over NIP-TiO2/SiO2.
us
In order to evaluate the photocatalytic selectivity, the photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 was evaluated by degrading 4NP in binary
an
solutions containing 4NP (10 mg/L) and a co-existing pollutant 2NP (10 mg/L). The experimental results are shown in Figure 11b. MIP-TiO2/SiO2 showed much higher
M
photocatalytic activity for 4NP than 2NP, while NIP-TiO2/SiO2 exhibited almost the same photocatalytic activity for 4NP as 2NP. The above results indicate that
te
d
MIP-TiO2/SiO2 is superior to NIP-TiO2/SiO2 in photocatalytic activity and selectivity. We attribute the high performance of MIP-TiO2/SiO2 to the enhanced light absorption
Ac ce p
ability in both ultraviolet and visible light regions, the high separation and transfer efficiency of electron/hole pairs, and the good affinity of MIP-TiO2/SiO2 for 4NP. 3.9 The effect of initial pH on the photocatalytic activity of MIP-TiO2/SiO2 Solution pH is an important parameter that influences the photocatalytic
degradation since it influences the surface charge properties of the MIP-TiO2/SiO2, the charge of 4NP molecules, the adsorption of 4NP on the surface of MIP-TiO2/SiO2 and the concentration of the hydroxyl radicals. Figure 12 shows the removal efficiency of 4NP on MIP-TiO2/SiO2 in the pH value range from 3.0 to 9.0. The photocatalytic
degradation efficiency of 4NP increases remarkably with increasing pH from 3.0 to
16
Page 16 of 32
4.0. When the pH was 4.0, it reached the maximum degradation efficiency, and then decreases obviously at higher pH, which is similar to the results reported by Baran and Muneer [30,31]. The highest degradation efficiency at pH 4.0 is explained as
ip t
follows. When pH increases from 3.3 to 4.0, 4NP is primarily in its nonionic form,
cr
and its solubility in water is minimized and the adsorption of 4NP on the catalyst is
us
maximized due to the involvement of surface hydroxyl groups [32]. Moreover, at a pH of 4.0, there is the formation of HO2• radicals known to be primary oxidizing
an
species responsible for pollutant degradation [33]. With further rise of solution pH (i.e. when pH>pHzpc), the surface of MIP-TiO2/SiO2 is negatively charged while 4NP tends
M
to exist in its anionic form [32]. The electrostatic repulsion between the catalyst surface and 4NP hinders 4NP adsorption. Consequently, there is decrease in
te
d
degradation efficiency. 3.10 Reusability of MIP-TiO2/SiO2 The performance of a catalyst in the recycling experiments is of great significance
Ac ce p
for its application to environmental technology. Therefore, the stability and reusability of MIP-TiO2/SiO2 were investigated. The photocatalytic degradation-regeneration
cycles were repeated for four times. The photocatalytic degradation ability of the regenerated MIP-TiO2/SiO2 is shown in Figure 13. It can be seen that the
photocatalytic degradation ability of MIP-TiO2/SiO2 is stable for four photocatalytic degradation-regeneration cycles without obvious decrease in the photocatalytic degradation of target molecules, indicating that MIP-TiO2/SiO2 has excellent regeneration, and can be used repeatedly.
4. Conclusions
17
Page 17 of 32
Inorganic-framework molecularly imprinted TiO2/SiO2 nanocomposite with molecular
recognitive
photocatalytic
ability
was
successfully
prepared
under mild condition. The molecular imprinted cavities on the surface of TiO2/SiO2
ip t
nanocomposite provided MIP-TiO2/SiO2 with selective affinity and specific molecular
cr
recognition ability, leading to preferential photocatalytic degradation towards target
us
contaminant. Because of the inorganic framework and the facile release of active adsorption sites upon regeneration, MIP-TiO2/SiO2 is highly reusable.
an
Acknowledgements
This work was financially supported by Natural Science Foundation of China
M
(50978132, 51178213, 51238002, 51272099, 51308278, 51368045), Program for New Century Excellent Talents in University (NCET-11-1004), Cultivating Program for
te
d
Young Scientists of Jiangxi Province of China (20112BCB23016) Natural Science Foundation of Jiangxi Province (20122BAB213014) and Department of Education
Ac ce p
Fund of Jiangxi Province (GJJ13506). References
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ip t
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ip t
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Figure caption Figure 1 XRD patterns of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 2 TEM images of: (a) MIP-TiO2/SiO2 and (b) NIP-TiO2/SiO2
cr
size distributions curves (b) of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
ip t
Figure 3 Nitrogen adsorption-desorption isotherms (a) and the corresponding pore
us
Figure 4 UV-vis diffuse reflectance spectra of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 5 Zeta-potential of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 samples suspended in
an
20 mL 4NP solution as a function of pH
Figure 6 Transient photocurrent-time curves of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
M
ITO electrodes under xenon lamp irradiation
te
NIP-TiO2/SiO2
d
Figure 7 Adsorption kinetics for 4NP adsorption on MIP-TiO2/SiO2 and
Figure 8 Linear plot of t/q vs t for MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Ac ce p
Figure 9 Adsorption isotherm for 4NP adsorption on MIP-TiO2/SiO2 and
NIP-TiO2/SiO2
Figure 10 Adsorption selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 11 (a) Time profile of 4NP removal by the MIP-TiO2/SiO2 and NIP-TiO2/SiO2, (b) photocatalytic selectivity of MIP-TiO2/SiO2 and NIP-TiO2/SiO2 Figure 12 The effect of initial pH on removal efficiency of 4NP by MIP-TiO2/SiO2 Figure 13 Stability and potential regeneration of MIP-TiO2/SiO2
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ip t cr us
Ac ce p
te
d
M
an
Figure 1
Figure 2
Figure 3
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ip t cr us an
Ac ce p
te
d
M
Figure 4
Figure 5
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ip t cr us an
Ac ce p
te
d
M
Figure 6
Figure 7
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ip t cr us an
Ac ce p
te
d
M
Figure 8
Figure 9
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ip t cr us an M
Figure 10
(a)
Ac ce p
te
d
(b)
Figure 11
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ip t cr us
Ac ce p
te
d
M
an
Figure 12
Figure 13
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Table 1 Physical properties of MIP-TiO2/SiO2 and NIP-TiO2/SiO2
Average pore 3
SBET (m /g)
Pore volume (m /g)
ip t
Samples
2
MIP-TiO2/SiO2
186.49
0.36
7.66
NIP-TiO2/SiO2
184.49
0.33
cr
size (nm)
us
7.20
an
Table 2 Adsorption kinetic parameters for 4NP adsorption on MIP-TiO2/SiO2 and
Pseudo-first-order
Pseudo-second-order
qe(mg/g)
K1(min-1)
R2
qe(mg/g)
K2(g·m-1·min-1)
R2
4NP-TiO2/SiO2
2.575
d
Photocatalyst
M
NIP-TiO2/SiO2
0.941
2.671
0.085
0.986
NIP-TiO2/SiO2
0.074
-0.238
0.777
0.373
3.064
0.998
Ac ce p
te
0.102
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Ac ce p
te
d
M
an
us
cr
ip t
Highlights MIP-TiO2/SiO2 nanocomposite was prepared by sol-hydrothermal method. MIP-TiO2/SiO2 shows molecular recognition ability. The structure-activity relationship of MIP-TiO2/SiO2 was established. MIP-TiO2/SiO2 shows higher adsorption capacity and selectivity than NIP-TiO2/SiO2. MIP-TiO2/SiO2 exhibits photocatalytic selectivity towards target contaminant.
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