Immobilization of TiO2 nanoparticles in polymeric substrates by chemical bonding for multi-cycle photodegradation of organic pollutants

Journal of Hazardous Materials 227–228 (2012) 185–194

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Immobilization of TiO2 nanoparticles in polymeric substrates by chemical bonding for multi-cycle photodegradation of organic pollutants Ping Lei a , Feng Wang a,∗ , Xiaowei Gao a , Yanfen Ding a , Shimin Zhang a , Jincai Zhao b , Shaoren Liu c , Mingshu Yang a,∗∗ a

Beijing National Laboratory for Molecular Science, Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China Key Lab of Photochemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China c Institute for the Control of Agrochemicals, Ministry of Agriculture, Beijing 100125, PR China b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 Chemical immobilization of TiO2 nanoparticles in PVA matrix.  Formation of the Ti O C bonds between TiO2 and PVA by heattreatment.  Increase in the available surface area of TiO2 nanoparticles.  Effect of heat-treatment on the photocatalytic activity of PVA/TiO2 hybrid film.  Excellent photocatalytic activity and recycle performance in multi-cycle use.

a r t i c l e

i n f o

Article history: Received 24 November 2011 Received in revised form 11 April 2012 Accepted 8 May 2012 Available online 15 May 2012 Keywords: Immobilization Polyvinyl alcohol Titanium dioxide Photodegradation Organic pollutant

a b s t r a c t Nano titanium dioxide (TiO2 ) photocatalyst is generally immobilized onto the matrix through the physical absorption, hydrogen bonding or chemical bonding, which is utilized for the application of wastewater treatment. In this research, TiO2 nanoparticles were immobilized in polyvinyl alcohol (PVA) matrix via solution-casting combined with heat-treatment method. Structure characterization indicated that Ti O C chemical bond formed via dehydration reaction between TiO2 and PVA during the heat treatment process, and TiO2 nanoparticles had been chemically immobilized in PVA matrix. Photodegradation results of methyl orange (MO) showed that the film with 10 wt% TiO2 and treated at 140 ◦ C for 2 h exhibited a remarkable ultraviolet (UV) photocatalytic activity, approximately close to the TiO2 slurry system. This was mainly attributed to the fixation effect by Ti O C chemical bonds, which was indirectly confirmed by the slight loss of TiO2 photocatalysts even after 25-cycle use. In addition, the good swelling ability of PVA matrix provided the MO molecules with more opportunities to fully contact with TiO2 , thus benefited the photocatalysis. This route to chemically immobilize TiO2 nanoparticles is simple and cheap to prepare polymer/TiO2 hybrid materials with high photocatalytic activity for multi-cycle use, which is of significance to the practical application of TiO2 catalysts. © 2012 Elsevier B.V. All rights reserved.

1. Introduction

∗ Corresponding author. Tel.: +86 10 82615665; fax: +86 10 82615665. ∗∗ Corresponding author. Tel.: +86 10 82615665; fax: +86 10 82615665. E-mail addresses: [email protected] (F. Wang), [email protected] (M. Yang). 0304-3894/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jhazmat.2012.05.029

Titanium dioxide (TiO2 ) is well known as a kind of semiconductor photocatalyst applied in air purification, solar energy conversion and wastewater treatment, owing to its high photocatalytic activity, low cost, nontoxicity and excellent stability under illumination [1–6]. In conventional process of wastewater treatment, TiO2 nanoparticles are generally used as a slurry

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system due to the large surface area of catalysts available for high photocatalytic efficiency [7,8]. Nevertheless, the filtration to eliminate and recycle the powdered TiO2 suspended in the treated water increases running cost and induces the secondary pollution, which has become a main limiting factor for practical application [9,10]. Alternatively, to solve the problem, many efforts have been devoted to immobilizing TiO2 nanoparticles on a variety of substrates [11–25], such as glass [17,18], stainless steel plate [19], carbon fiber film [20], TiO2 substrate [21,22] and polymers [23,24], by means of sol-gel method, thermal treatment, chemical vapor deposition (CVD) and electrophorectic deposition. And the adherences between substrates and TiO2 nanoparticles are often focused on the physical adsorption [20], electrostatic interaction [24], hydrogen bonding interaction [26] and chemical bonding interaction. For industrial applications, the photocatalysts are always expected to be recyclable for multi-cycle use. In consequence, the strong interactions between substrates and TiO2 nanoparticles are always required to avoid the loss of TiO2 nanoparticles during the long-term process of recycling [27,28]. Chemical bonding is considered to be the best way to anchor the TiO2 nanoparticles onto the substrates. Therefore, how to effectively immobilize TiO2 nanoparticles through chemical bonding and keep high photocatalytic activity in multi-cycle use has become an exciting and strategic research subject. Polymers have been widely used as the matrices to immobilize TiO2 photocatalyst by preparing polymer/TiO2 composite photocatalysts for their excellent properties of processibility and low-cost [23,29]. In particular, a series of polymers, which possess hydroxyl groups or carboxyl groups on the molecular chains, have been found to form chemical bonding with the hydroxyl groups on the surface of inorganic nanoparticles like nano SiO2 and nano TiO2 under given conditions. Among these polymers, polyvinyl alcohol (PVA), a hydrophilic polymer with good film-forming ability and low cost, could be attractive to immobilize TiO2 [30–32]. Generally, the TiO2 nanoparticles in polymer/TiO2 composites are usually completely or partly embedded in the matrices, leading to the reduction in the available specific surface areas of TiO2 catalysts [7,24,33]. As a result, the photocatalytic activity of the composite photocatalysts is generally lower than that of TiO2 suspended in aqueous solution. In comparison, the swelling ability of crosslinked PVA obtained by chemical [31,32,34] or physical [35,36] crosslinking methods, could provide the embedded TiO2 particles with more opportunities to fully contact with organic pollutant in solution, which is favorable to the photocatalysis. In this paper, we described a simple route to prepare a crosslinked PVA/TiO2 hybrid system with high photocatalytic efficiency during multi-cycle use, by a combination of solution-casting and heat-treatment method. The immobilization effect of TiO2 in PVA matrix by Ti O C chemical bonds was investigated. The recyclability of the PVA/TiO2 hybrid films and the loss of TiO2 were evaluated for MO degradation during 25 cycle times. In addition, the relationship among preparation condition, swelling degree and photocatalytic activity of the hybrid films was also discussed. The experimental results suggested that the prepared PVA/TiO2 hybrid films are considered as a promising candidate of recoverable and recyclable TiO2 hybrid photocatalysts with high photocatalytic activity.

2. Experimental 2.1. Chemicals and materials PVA (average polymerization degree of 1750 ± 50 and degree of hydrolysis of 98%) was purchased from Beijing Yili Fine Chemical

Co., China. TiO2 nanoparticles (P25, 20% rutile and 80% anatase) with a mean diameter of 21 nm were provided by Degussa, Germany. Methyl orange (MO), used as the model pollutant, was manufactured by Zhejiang Yongjia Fine Chemical Plant, China. All chemicals were used without further purification and deionized water was also used to prepare experimental solutions. 2.2. Preparation of the PVA/TiO2 hybrid films The PVA/TiO2 hybrid films were prepared by the following two steps: solution casting and heat-treatment process. 2.2.1. Solution casting TiO2 nanoparticles were initially dispersed in deionized water under sonication for 2 h. PVA was subsequently added into the TiO2 suspension, followed by mechanical stirring at 95 ◦ C for 1 h. Then the PVA/TiO2 /H2 O solution mixture was continuously stirred at 60 ◦ C for 3 h. After that, the flask containing the solution mixture rested in air to eliminate air bubbles and to cool the solution to room temperature. The resultant viscous bubble-free solution mixture was cast onto a clean glass plate to give a 1 cm-thick layer. The solvent was allowed to evaporate overnight in the atmosphere at room temperature. Finally, the dried hybrid films were collected with the thickness of ∼60 ␮m. The weight ratio of TiO2 to PVA was varied as 1, 3, 5, 7, 10, and 12 wt%, and the resulting hybrid films were designated as M1, M3, M5, M7, M10, and M12, respectively. 2.2.2. Heat-treatment process The regenerated hybrid films which were cut into the squared shape of 20 mm × 20 mm were heat-treated under vacuum at various temperatures for different time. The nomenclature adopted herein specifies each sample as follows: MX-Y-Z, where M indicates the PVA/TiO2 hybrid film, X represents the weight ratio (1, 3, 5, 7, 10, 12) of TiO2 to PVA in the initial solution system with the unit of wt%, Y indicates the treatment temperature (140, 160, 180, 200), in ◦ C; Z represents the treatment time (0.5, 2, 4, 6, 10) with the unit of h. 2.3. Characterization The morphology of the hybrid films was observed at a field emitting-scanning electron microscopy (FE-SEM, JSM-6700F) with an accelerating voltage of 5 kV. The samples were coated with platinum utilizing a GIKO IB-3 ion coater before observation. High-resolution transmission electron microscope (HRTEM, JEOL 2200FS) was carried out to survey the dispersion of TiO2 nanoparticles in the hybrid film at an accelerating voltage of 200 kV. The HRTEM sample of PVA/TiO2 hybrid film was prepared by epoxy resin embedding and ultrathin sectioning on a Leica Ultracut UCT ultramicrotome using a glass knife. The chemical structure of the samples and the interaction between PVA and TiO2 nanoparticles were confirmed using attenuated total reflectance/Fourier transform infrared spectrometry (ATR/FTIR). The spectra were recorded on a Thermo Nicolet 6700 FTIR equipped with an attenuated total reflectance device (Smart Orbit) with a diamond crystal. The resolution of the spectra was 4 cm−1 , and scans were repeated 32 times. The crystal structure of the hybrid films was investigated using a differential scanning calorimeter (DSC; PerkinElmer, DSC-7). The samples weighted ranged 2–3 mg and were heated from 20 ◦ C to the temperature when the sample began to degrade at a heating rate of 20 ◦ C/min under continuous nitrogen gas flow. The swelling ability of the hybrid films in pollutant solution was also measured during the photocatalysis. The masses of the dry films were first determined before photocatalysis. After every cycle of photocatalysis, the swollen sample films were taken out from

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Fig. 1. SEM images of the surfaces of (a) M10, (b) M10-140-2, (c) M10-160-2, (d) M10-180-2, (e) M10-200-2 and (f) TEM image of TiO2 immobilized in M10-140-2 hybrid film.

the pollutant solution and weighted as quickly as possible after the surface water was absorbed with filter paper. The percent degree of swelling (DS) was calculated below: DS =

Ms − Md × 100% Md

(1)

where Ms and Md donate the masses of the swollen and dry films, respectively. The residual amounts of TiO2 nanoparticles in PVA/TiO2 hybrid films after repeated photocatalysis were evaluated using thermal gravimetric analysis (TGA; PerkinElmer, TGA-7) under air atmosphere with airflow of 25 mL/min. The samples were first heated from 20 to 150 ◦ C, then retained at 150 ◦ C for 10 min in order to completely remove the absorbed water and finally continued to heat to 700 ◦ C at a heating rate of 20 ◦ C/min. 2.4. Photocatalytic degradation The photocatalytic activity of the sample films was evaluated from the degradation rate of MO in an aqueous solution with an initial concentration of 15 mg/L. The photocatalytic reaction was carried out in a UV analyzer (UV-3000, Jiapeng Technology Co. Ltd., Shanghai, China). An array of lamps (8 W × 6) locating below a

transparent plate acted as the UV light source with the wavelength of 300 nm. Prior to irradiation, the sample films (squared, each containing 10 mg TiO2 nanoparticles) were immersed into 10 mL MO solution inside a container, respectively. Subsequently, these containers were put onto the transparent plate in parallel, and the UV light illuminated to the bottom of containers. The average UV light intensity measured with a UV Radiometer was 90 ± 10 ␮W/cm2 . In order to avoid the concentration change caused by solution evaporation during irradiation, the solution containers were sealed with paraffin film. At given irradiation time intervals, the concentration of MO was monitored by detecting UV absorbance intensity at 465 nm with an ET99731 microcomputer COD/TOC determinator (Lovibond, Germany). In addition, for the purpose of comparison, 10 mg powdered TiO2 photocatalysts were also used to decompose MO. The powdered TiO2 was dispersed in 10 mL initial MO solution under stirring to form slurry. At regular intervals, the upper transparent solution obtained by centrifugation (10,000 rpm, 5 min) was collected for absorbance analysis. The recyclability of the PVA/TiO2 hybrid films was also investigated during multi-cycle photocatalysis. After irradiation, the PVA/TiO2 hybrid films were washed several times with deionized water to eliminate any residual organic matter and the surface water was absorbed using filter paper; for TiO2 slurry, the TiO2

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nanoparticles were repeatedly separated and washed by deionized water. Subsequently, a new MO solution (10 mL) was added and the irradiation was repeated for the next cycle. During the multi-cycle photocatalytic experiments, the effect of MO adsorption in the dark on photocatalytic efficiency could be negligible. The adsorption/desorption equilibrium tests indicated that, in the dark for 24 h, the percentage of MO adsorption were 2.6%, 1.4%, 0, and 0, respectively for M10-140-2, M10-160-2, M10-180-2, and M10200-2. The MO adsorption of these samples was low compared with the change of MO concentration under direct photocatalysis, which is consistent with Hosseini’s research [25]. Therefore, no adsorption balance in the dark before photoreaction was carried out. After each cycle, the rate constant k of MO degradation in aqueous solution was calculated and compared with the previous one. The procedure was repeated 25 times. 3. Results and discussion 3.1. Immobilization of TiO2 nanoparticles in PVA matrix 3.1.1. Morphology PVA/TiO2 hybrid films were prepared by the method described in the above experimental part. Fig. 1 shows the morphology of the hybrid films before and after heat-treatment. The SEM image of untreated hybrid film M10 displays that many small TiO2 aggregates are observed on the surface of the sample, indicating the poor dispersion of TiO2 nanoparticles in PVA matrix. Fig. 1b–d shows the SEM images of the hybrid films from M10-140-2 to M10-200-2 treated at 140, 160, 180 and 200 ◦ C, respectively. It is evident that these films display homogeneous microporous structure, which is mainly due to the shrinkage of the films at high temperature during heat-treatment. Moreover, the size and the number of these pores in films decrease with elevating the temperature, especially for M10-200-2, no microporous structure can be observed. It suggests that PVA molecule chains are inclined to rearrange to form crystalline regions under heating. With increasing temperature, the motion of PVA chains was accelerated and more crystalline regions were formed. The increased crystalline regions made the film dense, therefore, led to the decrease in the size and the number of these pores in treated hybrid films. The dispersion of TiO2 nanoparticles in PVA matrix was further clarified by HRTEM. Fig. 1f shows the HRTEM image of hybrid film M10-140-2. It can be observed that the pure nanosized TiO2 particles have poor dispersion in PVA matrix and formed agglomerates with the dimension from 60 nm to 250 nm, which is mainly attributed to the large superficial polarity of the TiO2 nanoparticles without any surface modification. 3.1.2. Chemical bonding immobilized TiO2 in PVA matrix To confirm the immobilization of TiO2 nanoparticles in PVA matrix, ATR/FTIR was performed in this study for the structure characterization. Fig. 2 shows the infrared spectra (in the region between 1500 cm−1 and 650 cm−1 ) of the hybrid films, M10-1402, M10-160-2, M10-180-2, and M10-200-2 obtained by heating the untreated hybrid film M10 at different temperature for 2 h. The spectrum of M10 contains all the absorption band of both the pure PVA film and TiO2 nanoparticles without any shift, suggesting the typical characteristics of a simple physical solution-blending between the two components. For the heat-treated hybrid films, interestingly, a new peak appears at 1261 cm−1 (spectra d–g in Fig. 2), which is assigned to the vibration of Ti O C bonds [37,38]. Meanwhile, the other two new peaks at 1028 cm−1 and 798 cm−1 are attributed to hypsochromic shift of O C C (1095 cm−1 ) and C C (817 cm−1 ), respectively, due to the change in the chemical environments caused by the formation of Ti O C bonds [39].

Fig. 2. ATR/FTIR spectra of (a) pure PVA, (b) pure TiO2 and the PVA/TiO2 hybrid films treated at different temperatures: (c) M10, (d) M10-140-2, (e) M10-160-2, (f) M10-180-2 and (g) M10-200-2.

These characteristic peaks become strong and more remarkable for the high-temperature-treated sample, M10-200-2. The above evidences demonstrated the formation of covalent Ti O C bonds between PVA and TiO2 nanoparticles, which led to the effective immobilization of TiO2 nanoparticles in PVA matrix. During the heat-treatment process, PVA chains absorbed heat and rearranged to form crystalline regions, which can be monitored by the peak intensity at 1143 cm−1 corresponding to the C C stretching vibration of the PVA crystals [40]. In the range of 140–180 ◦ C, the crystallinity of PVA increases as shown in Fig. 2d–f, the intensity of the peak at 1143 cm−1 increases with elevating heattreatment temperature. However, it is worth noting that this peak in M10-200-2 nearly disappears. It can be explained that, at high temperature, the OH groups in PVA molecular chains tend to react with the OH groups on TiO2 surfaces to form Ti O C bonds to a large extent, and that is why the peak at 1261 cm−1 (Ti O C) becomes strong and sharp at 200 ◦ C. On the basis of the results mentioned above, a schematic illustration of the reaction of TiO2 nanoparticles with PVA was proposed in Fig. 3. It is well known that there are always some active OH groups located on the surfaces of nano TiO2 [41]. Under heattreatment, these OH groups on TiO2 surfaces can form Ti O C bonds with the OH groups in PVA molecular chains through a dehydration reaction. These Ti O C bonds are covalent bonds, thus TiO2 nanoparticles can be firmly immobilized in PVA matrix. Meanwhile, massive crystalline regions are formed during heattreatment process. The crystalline regions and Ti O C chemical bonds could act as crosslinking sites to keep PVA stable in solution. 3.2. Photocatalytic activity of the PVA/TiO2 hybrid films Methyl orange (MO) is a typical kind of azo dyes. It has been found that MO could be efficiently degraded by the anatase TiO2 nanoparticles, and the concentration of MO in solution could be easily detected by the UV–vis spectroscopy. Therefore, in this research MO was employed as the target pollutant to evaluate and compare the photocatalytic activity between the TiO2 slurry system and the immobilized PVA/TiO2 hybrid films.

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Fig. 3. Schematic illustration of the immobilization of TiO2 in PVA matrix by Ti O C chemical bonding.

It is obvious that the effect of optical properties of the hybrid films is an important factor to the photocatalytic efficiency. Actually, many previous studies have focused on this issue. For example, Sudesh and co-workers [42] studied the effect of film thickness on the decolorization efficiency of methylene blue and found that the thinner film is better for photocatalysis. In our experiments, the pure PVA film with 60 ␮m thickness is transparent and the transmission is 98.0% to the 300 nm UV light. However, all the 10 wt% TiO2 content hybrid films with the same thickness were opaque with a near zero UV transmission, which indicated that the UV light irradiation had been almost completely absorbed by the TiO2 nanoparticles. Generally, the surface of a film could receive more UV irradiation than the middle of the film. To quantitatively study the surface effect, the films with different thickness and the same concentration of TiO2 were prepared. The results showed that the transmission was 28.4% to the 300 nm UV light for the 10 ␮m swollen hybrid film, indicating that the surface 10 ␮m of films absorbed 72.6% of the UV light. Meanwhile, the surface of films could contact with organic pollutants more easily. Therefore, the surface of films takes an important role in the photocatalytic efficiency. In order to minimize the influence of light emission

and surface effect, all the PVA/TiO2 hybrid film samples were prepared with the same thickness and under the same conditions, thus the difference of optical properties between different hybrid films could be neglected. 3.2.1. Effect of heat-treatment temperature on photocatalytic activity In order to investigate the effect of heat-treatment temperature on photocatalytic activity, the samples treated at different temperature were employed in MO degradation. As shown in Fig. 4a, relative concentration (Ct /C0 ) of MO remained in the solution is drawn as a function of time (t) to describe the degradation extent. The kinetics of MO degradation is further investigated using the data from Fig. 4a, which has been proposed to follow the apparent first-order reaction equation [6]: ln(C0 /Ct ) ≈ kt

(2)

where C0 is the initial concentration of MO, Ct is the concentration of MO at time t. For the apparent first-order reactions, the plot of ln (C0 /Ct ) versus t (not shown in this article) represents a straight line and the

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Fig. 5. The swelling degrees of the PVA/TiO2 hybrid films treated at different temperatures for 2 h.

Fig. 4. (a) Variations of MO concentration and (b) the reaction rate constant k for the photocatalytic degradation of MO during four cycles in the presence of the PVA/TiO2 hybrid films treated at different temperatures for 2 h.

slope is the rate constant k. The rate constant k for each sample was plotted as a function of cycle number (n) and shown in Fig. 4b. The high rate constant k represents high photocatalytic efficiency. It is observed that TiO2 powder shows high photocatalytic efficiency for the large surface areas when TiO2 nanoparticles were suspended in solution. Sample M10, with no heat-treatment, exhibits a gradual decrease in photocatalytic efficiency during repeated photocatalysis, mainly attributed to the loss of TiO2 catalysts caused by the absence of interaction between TiO2 particles and the polymer matrix. Interestingly, the photocatalytic efficiency becomes constant for all treated samples after Cycle 1. It confirms the existence of Ti O C chemical bonds between TiO2 and PVA matrix formed during heat-treatment process, which effectively prevents the loss of TiO2 . Meanwhile, for these samples, the photocatalytic activity decreases with increasing the treatment temperature from 140 to 200 ◦ C. The M10-140-2 sample treated at lowest temperature exhibits the highest photocatalytic efficiency, close to that of the pristine TiO2 powders in slurry system. 3.2.2. Relationship between the structure and the photocatalytic activity of PVA/TiO2 hybrid films In conventional methods by immobilizing TiO2 nanoparticles in polymer matrices, TiO2 particles are always enwrapped in matrix, thus results in the reduction in the available surface area of the

photocatalyst, leading to a sharp decrease in photocatalytic activity. In this research, interestingly, the PVA/TiO2 hybrid film M10-140-2 exhibits high photocatalytic activity, close to that of the TiO2 slurry system. This is mainly attributed to its excellent swelling ability of PVA matrix in MO solution. To clarify the effect of treatment temperature on photocatalytic activity of the PVA/TiO2 hybrid films, the swelling degree of sample films in MO solution during the process of photocatalysis is also examined and the results are shown in Fig. 5. It is obvious that the untreated sample M10 presents a remarkable decrease in the swelling degree during repeating cycles, owing to the weight loss of massive amorphous PVA dissolved in test solution. The heattreated hybrid films show relatively low photocatalytic efficiency compared with cycles 2–4. This could be explained as follows: for Cycle 1, the dry films were swelling from dry to swollen during photocatalytic reaction. Due to the incomplete swelling of the films, MO molecules were mainly degraded by the TiO2 on the film surfaces. Therefore, the films showed relatively low photocatalytic activity. For the following other cycles, the hybrid films had reached swelling equilibrium before photocatalytic reaction. The swelling degree remained almost constant for each individual treated sample during continuous use; therefore, the photocatalytic reactions were faster than Cycle 1. With elevating the treatment temperature from 140 to 200 ◦ C in each cycle, the swelling degree decreases. The similar tendency is observed in the photocatalysis curves in Fig. 4b. The swelling degree is strongly dependent on the structure of the hybrid films. In this research, heat-treatment is utilized to achieve physical crosslinking of PVA by increasing the crystallinity, for the purpose of endowing the films with good mechanical strength and stability in test solution. The degree of crystallinity (Xc ) of treated hybrid films is shown in Table 1. It can be noted that the crystallinity increases from M10 to M10-180-2 with elevating the temperature and slightly decreases for M10-200-2. The observation is in good agreement with the ATR/FTIR results. It is well known that PVA is a kind of semicrystalline polymer. The hydroxyl groups of PVA can form hydrogen bonds within chains or between chains, which leads to small crystallites scattered in an unordered and amorphous PVA matrix [43–45]. Under heattreatment, the random PVA chains in amorphous region rearranged to form ordered and denser crystalline region. At the same time, the OH groups on the surface of TiO2 reacted with the OH groups in PVA chains to form Ti O C bonds. The crystalline regions and Ti O C bonds served as physical and chemical crosslinking points, respectively, to form the three-dimensional network, endowing the hybrid film with excellent mechanical ability and swelling ability in water. The amorphous region in the network possesses large free

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Table 1 Crystallinity and weight loss of PVA/TiO2 hybrid films treated at the different temperatures. Samples

Hf

a

(J/g)

M10 M10-140-2 M10-160-2 M10-180-2 M10-200-2 a b c

Weight lossc

Crystallinity

52.8 63.8 69.7 73.8 72.7

Crystallinity (%) Xc = Hf /Hf

◦b

38.1 46.0 50.3 53.2 52.5

Weight (Cycle 4)

Weight (Cycle 25)

(mg)

(mg)

TiO2

PVA

TiO2

PVA

7.1 10.6 10.3 10.4 10.9

23.5 78.6 88.1 94.2 95.2

4.9 8.9 9.7 9.9 10.6

10.1 45.3 56.9 77.4 88.7

Calculated from DSC. Hf ◦ – The melting enthalpy for 100% crystalline PVA of 138.6 J/g [41]. Calculated from TGA. The initial weight for each sample is 110 mg containing 10 mg TiO2 nanoparticles and 100 mg PVA.

volume between the random PVA chains. When immersed into MO solution, the sample film was gradually swelling as the water and MO molecules were absorbed into the free volume of amorphous region and settled around the TiO2 catalysts immobilized in the film. As the photocatalysis progressed, the MO molecules around the TiO2 nanoparticles gradually degraded and other MO molecules outside the film gradually diffused into the film under the concentration gradient until all the MO molecules degraded. As a result, the concentration of MO in the solution decreased. It is clear that the higher swelling degree of PVA matrix can provide MO molecules with more opportunities to diffuse into the films and fully contact with TiO2 photocatalysts, thus increasing the effective surface areas of TiO2 catalysts. Therefore, better photocatalytic efficiency of the PVA/TiO2 system will be obtained. In heat-treatment process, the amorphous region reduced while the crystalline region and Ti O C bonds increased with elevating temperature, which led to a decrease in free volume and the swelling degree of the hybrid film. Under certain conditions of heat-treatment, both swelling property and chemical immobilization could be controlled efficiently, in order to obtain an excellent photocatalytic activity. As shown in Fig. 4, the hybrid film M10-14-2, treated at 140 ◦ C for 2 h, exhibits the highest photocatalytic activity during multi-cycle use. 3.2.3. Recyclability of the PVA/TiO2 hybrid films for multi-cycle use For practical application, photocatalyst is always expected to be useful for a relatively long time or recyclable (after regeneration if necessary) for multi-cycle use. Boule and co-workers [27] immobilized TiO2 on organic fibers, pumice stones and polymer film, and evaluated the photocatalytic efficiency during several weeks. The results suggested that the degradation of acid orange-7 became slower and slower during long-time use due to the elimination of TiO2 as long as the polymer film damaged after 7 weeks. Sudesh and co-workers [42] fabricated poly-3-hydroxybutyrate/TiO2 (P25) cast film which could completely degrade batik dye, but the film broken after six cycles. In the present research, the recyclability of the PVA/TiO2 hybrid films was also investigated by evaluating the rate constant k (calculated from using the function (2)) of MO degradation during continuous cycle use, and the results are presented in Fig. 6. A decreased rate constant for M10 sample during 25 cycles is obviously observed, owing to the great loss of TiO2 photocatalysts caused by the absence of chemical immobilization. Oppositely, it is evident that little change in photocatalytic efficiency of the samples from M10-140-2 to M10-200-2 within the accuracy of the experiment is observed even after 25 cycles. This phenomenon suggests that hardly any loss of TiO2 catalysts is occurring during multicycle use. It is because of the formation of Ti O C covalent bonds between TiO2 particles and PVA matrix, by which TiO2 particles are firmly immobilized in PVA matrix.

The loss of TiO2 nanoparticles during multi-cycle use is of great importance to the photocatalytic efficiency. For further confirming the effective immobilization of TiO2 , TGA measurement carried out from room temperature to 700 ◦ C was employed to evaluate the weight loss of TiO2 during multi-cycle use. PVA would be completely decomposed during the scan under air atmosphere, while TiO2 nanoparticles would remain. So the weight percentage at 700 ◦ C and the weight loss from 150 ◦ C to 700 ◦ C could be regarded as the percentage of the residual TiO2 and PVA in sample films, respectively. The resulting calculated data are listed in Table 1. The initial weight for each sample is 110 mg, containing 10 mg TiO2 and 100 mg PVA. After Cycle 4, TiO2 remained in M10 is 7.1 mg, that is to say, 29 wt% of TiO2 catalysts escaped, compared with its initial weight. Nevertheless, the heat-treated hybrid films show little loss of TiO2 , which is attributed to the strong Ti O C chemical bonds. After Cycle 25, there is only 4.9 mg TiO2 contained in sample M10 and 51 wt% of TiO2 lost. It is the reason why M10 exhibits a gradual reduction in photocatalytic activity during continuous use. In contrast, the weight loss of TiO2 is 1.1 wt%, 0.3 wt% and 0.1 wt% for sample M10-140-2, M10-160-2 and M10-180-2, respectively. Especially, M10-200-2 has almost no loss of TiO2 . This is ascribed to the massive formation of Ti O C bonds at the temperature of 200 ◦ C confirmed by FTIR result. In addition, the weight loss of PVA is attributed to the dissolution of the partial PVA amorphous region [46]. These results indicate that the TiO2 nanoparticles are effectively immobilized in PVA matrix by Ti O C chemical bonds and the hybrid films exhibit remarkable photocatalytic activity during multi-cycle use. This is a conclusion supported by the results from ATR/FTIR, swelling test, photocatalysis experiment and TGA.

Fig. 6. Recyclability of PVA/TiO2 hybrid films used for the degradation of MO.

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Fig. 7. The photographs of the hybrid films in deionized water after 25 cycle tests: (a) M10, b) M10-140-2, (c) M10-160-2, (d) M10-180-2, (e) M10-140-2 and (f) stretched M10-140-2 cut in a rectangle shape after 25 cycle tests.

Fig. 7 shows the photographs of the PVA/TiO2 hybrid film samples after 25 cycle tests. It is obvious that the film sample M10 became breakable and highly damaged. In fact, this sample had been broken even since Cycle 2 and the test solution became cloudy for the loss of TiO2 particles from the film into the test liquid. In contrast, the hybrid films undergoing heattreatment, shown in Fig. 7b–e respectively, still remained intact and maintained the shapes, even after 25 cycles (i.e. the missing small part in each sample was cut down for TGA analysis after Cycle 4). Fig. 7f shows the stretched wet film of M10140-2 after 25 cycles of photocatalysis. The sample remained elastic and was hard to break. To sum up, we confirmed that the preparation method of solution-casting followed by heattreatment used in this research could be effectively used to fabricate the multi-cycle used industrial wastewater film, not only

immobilizing TiO2 but also keeping high photocatalytic activity. 3.2.4. Effect of heat-treatment time on photocatalytic activity In order to investigate the effect of another factor of treated time in heat-treatment process on photocatalytic activity, the samples treated at 140 ◦ C for various time were also employed in MO degradation and the results are shown in Fig. 8. Compared with the untreated sample M10, the sample films from M10-140-0.5 to M10-140-10 show similar and stable photocatalytic activities as they reached swelling equilibrium after Cycle 1. This is because of the fact that the temperature is the main factor on crystallinity degree of the PVA/TiO2 hybrid films rather than time. It is thus concluded that the photocatalytic activity of the PVA/TiO2 hybrid films is mainly influenced by

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results in the reduction of photocatalytic activity. According to the above results, it can be concluded that the concentration of TiO2 in the hybrid films has little effect on the photocatalytic activity of the PVA/TiO2 hybrid films unless it is too low or too high. 4. Conclusions

Fig. 8. Variations of MO concentration during four cycles in the presence of the PVA/TiO2 hybrid films treated at 140 ◦ C for various time.

the heat-treatment temperature and less by the heat-treatment time. 3.2.5. Effect of TiO2 concentration in PVA/TiO2 hybrid films on photocatalytic activity The optimum dosage of TiO2 nanoparticles for efficient photocatalytic degradation of MO was also studied. Fig. 9 presents the results of photocatalysis using different concentrations of TiO2 in PVA/TiO2 hybrid films from M1 to M12 treated at 140 ◦ C for 2 h. After Cycle 1 when they reached swelling equilibrium, these PVA/TiO2 hybrid films exhibited stable photocatalytic activities. It is observed that the photocatalytic activity is enhanced with increasing the weight ratio of TiO2 to PVA from 1 wt% (M1) to 3 wt% (M3), and then keeps a similar result until 10 wt% (M10). The reason is that high TiO2 concentration, generally, is favorable for high photocatalytic efficiency, attributing to a higher yield of hydroxyl and superoxide radicals which can effectively degrade various organic dyes [47]. However, there was a significant reduction in the photocatalytic activity, as the weight ratio of TiO2 to PVA increased from 10 wt% (M12) to 12 wt% (M12). This can be attributed to the fact that TiO2 nanoparticles are more likely to aggregate in high concentration, which leads to the decrease in the available surface active sites and inhibition of MO from contacting TiO2 , therefore,

Fig. 9. Variations of MO concentration during four cycles in the presence of the PVA/TiO2 hybrid films with various TiO2 concentrations. These films all were treated at 140 ◦ C for 2 h.

TiO2 nanoparticles have been successfully immobilized in PVA matrix by Ti O C chemical bonds through heat-treating the solution-casting PVA/TiO2 hybrid films. The heat-treated films could keep high photocatalytic activity even after 25 cycles of photodegradation of MO in aqueous solution. The heat-treatment temperature plays a key role on photocatalytic activity of PVA/TiO2 hybrid films. Our results showed that the hybrid film with 10 wt% TiO2 and treated at 140 ◦ C for 2 h exhibited the highest photocatalytic activity, approximately close to the TiO2 slurry system. This can be attributed to the Ti O C chemical bonds between TiO2 nanoparticles and PVA matrix, which made the TiO2 nanoparticles difficult to lose from the hybrid films. This novel kind of chemical immobilization of TiO2 nanoparticles in PVA matrix is a promising method to prepare polymer/TiO2 hybrid material with high photocatalytic activity for multi-cycle use, which is of significance to the application of TiO2 catalysts in wastewater treatment industry. Acknowledgment This research was financially supported by National Basic Research Program of China (grant no. 2010CB933500) and National Natural Science Foundation of China (grant no. 50973115). References [1] D. Li, H. Haneda, S. Hishita, N. Ohashi, Visible-light-driven N-F-codoped TiO2 photocatalysts. 2. Optical characterization, photocatalysis, and potential application to air purification, Chem. Mater. 17 (2005) 2596–2602. [2] P. Pichat, J. Disdier, C. Hoang-Van, D. Mas, G. Goutailler, C. Gaysse, Purification/deodorization of indoor air and gaseous effluents by TiO2 photocatalysis, Catal. Today 63 (2000) 363–369. [3] X.Z. Li, F.B. Li, Study of Au/Au3+ -TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment, Environ. Sci. Technol. 35 (2001) 2381–2387. [4] A. Haarstrick, O.M. Kut, E. Heinzle, TiO2 -assisted degradation of environmentally relevant organic compounds in wastewater using a novel fluidized bed photoreactor, Environ. Sci. Technol. 30 (1996) 817–824. [5] J.C. Yu, J.G. Yu, J.C. Zhao, Enhanced photocatalytic activity of mesoporous and ordinary TiO2 thin films by sulfuric acid treatment, Appl. Catal., B 36 (2002) 31–43. [6] I.K. Konstantinou, T.A. Albanis, TiO2 -assisted photocatalytic degradation of azo dyes in aqueous solution: kinetic and mechanistic investigations: a review, Appl. Catal., B 49 (2004) 1–14. [7] R. Scotti, M. D’Arienzo, F. Morazzoni, I.R. Bellobono, Immobilization of hydrothermally produced TiO2 with different phase composition for photocatalytic degradation of phenol, Appl. Catal., B 88 (2009) 323–330. [8] T.A. McMurray, J.A. Byrne, P.S.M. Dunlop, J.G.M. Winkelman, B.R. Eggins, E.T. McAdams, Intrinsic kinetics of photocatalytic oxidation of formic and oxalic acid on immobilised TiO2 films, Appl. Catal., A 262 (2004) 105–110. [9] S. Kagaya, K. Shimizu, R. Arai, K. Hasegawa, Separation of titanium dioxide photocatalyst in its aqueous suspensions by coagulation with basic aluminium chloride, Water Res. 33 (1999) 1753–1755. [10] X.D. Xue, J.F. Fu, W.F. Zhu, X.C. Guo, Separation of ultrafine TiO2 from aqueous suspension and its reuse using cross-flow ultrafiltration (CFU), Desalination 225 (2008) 29–40. [11] X.F. Meng, Z.Z. Qian, H.T. Wang, X.W. Gao, S.M. Zhang, M.S. Yang, Sol–gel immobilization of SiO2 /TiO2 on hydrophobic clay and its removal of methyl orange from water, J. Sol-Gel Sci. Technol. 46 (2008) 195–200. [12] C.A. Coutinho, V.K. Gupta, Photocatalytic degradation of methyl orange using polymer–titania microcomposites, J. Colloid Interface Sci. 333 (2009) 457–464. [13] C.X. Wang, L.W. Yin, L.Y. Zhang, L. Kang, X.F. Wang, R. Gao, Magnetic ((-Fe2 O3 @SiO2 )n @TiO2 functional hybrid nanoparticles with actived photocatalytic ability, J. Phys. Chem. C 113 (2009) 4008–4011. [14] S.H. Xu, W.F. Shangguan, J. Yuan, M.X. Chen, J.W. Shi, Preparations and photocatalytic properties of magnetically separable nitrogen-doped TiO2 supported on nickel ferrite, Appl. Catal., B 71 (2007) 177–184. [15] H.C. Huang, G.L. Huang, H.L. Chen, Y.D. Lee, Immobilization of TiO2 nanoparticles on Fe-filled carbon nanocapsules for photocatalytic applications, Thin Solid Films 515 (2006) 1033–1037.

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