TiO2 composite film by photopolymerization for the removal of Congo red

Applied Surface Science 445 (2018) 437–444

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One-step synthesis of bifunctional PEGDA/TiO2 composite film by photopolymerization for the removal of Congo red Yun-Yun Wei, Xiao-Ting Sun, Zhang-Run Xu ⇑ Research Center for Analytical Sciences, Northeastern University, Shenyang 110819, PR China

a r t i c l e

i n f o

Article history: Received 10 September 2017 Revised 18 March 2018 Accepted 20 March 2018 Available online 26 March 2018 Keywords: Wrinkles Poly-(ethylene glycol) double acrylate/ titanium dioxide film Adsorption Photodegradation Congo red

a b s t r a c t Wrinkled structures can provide enlarged surface areas for some living organisms to ingest nutrients. Imitating biological wrinkle structures offers an efficient way to enhance the adsorption surface for removing hazardous pollutants in wastewater. In this work, poly-(ethylene glycol) double acrylate (PEGDA)/TiO2 composite film with tunable surface wrinkles was synthesized. TiO2 nanoparticles were evenly immobilized in the PEGDA hydrogel simply by a facile photopolymerization method within 700 ms. Various wrinkle morphologies were obtained by precisely controlling UV exposure time. The composite film was characterized by X-ray diffraction, scanning electron microscopy, diffuse reflection spectroscopy, etc. Congo red was chosen as a model pollutant to demonstrate the adsorption and degradation capacity of the composite film. The experimental results showed that the introduction of wrinkled polymer improved the dispersibility of TiO2 nanoparticles. The removal efficiency reached 100% after 180-min adsorption in the darkness and 180-min UV irradiation. The composite film exhibited a much higher enrichment and photocatalysis capacity than the pure TiO2 powder, and could be developed as a reusable film for the removal of the organic pollutants in wastewater. Ó 2018 Elsevier B.V. All rights reserved.

1. Introduction It is widely acknowledged that most organic dyes in wastewater are poisonous, thus massive studies have been devoted to resolve the dye pollution issue effectively [1,2]. Photocatalysis is a sustainable, environment-friendly technological means for purifying wastewater [3,4]. Titanium dioxide, as a representative semiconductor photocatalyst, is widely applied to degrade the harmful organics [5–7]. To achieve high photocatalytic efficiency, TiO2 particles are commonly used as a slurry system to enlarge catalytic surface area [8]. However, the residual TiO2 nanoparticles in water are detrimental to living organisms, and they are costly to remove and likely to cause secondary pollution [9]. Besides, TiO2 nanoparticles tend to aggregate, which reduces the effective collision between photocatalysts and organic contaminants, declining photocatalytic efficiency and creating more difficulties in recycling [10,11]. The aforementioned issues greatly impede the practical applications of TiO2 nanoparticles in photocatalysis. Thus, recent researches were made to immobilize TiO2 nanoparticles on different substrates [12,13], such as quartz sand [14], carbon nanotubes [15] and polymers [16,17], for improving their dispersion and

⇑ Corresponding author. E-mail address: [email protected] (Z.-R. Xu). https://doi.org/10.1016/j.apsusc.2018.03.149 0169-4332/Ó 2018 Elsevier B.V. All rights reserved.

stability. Generally polymer materials are known as promising substrates to immobilize active catalysts, due to their oleophilic frameworks, toughness, flexibility and cheapness [18]. Most importantly, many water-insoluble gel polymers were regarded as absorbents to uptake or bind solute molecules [19,20], and they could be reused by utilizing appropriate organic solvents to strip the adsorbates [21,22]. The polymer composites could combine the functions of adsorption and photocatalysis in a controlled manner, which attracts great attention of researchers. There are many methods to incorporate inorganic nanoparticles into polymer substrates [23,24], such as sol-gel [25,26], dipcoating [27,28], hydrothermal [29] and spinning techniques [30]. These polymer composites could mostly improve photocatalytic capacities. However, there is still a strong need for bifunctional polymer materials with adsorption and degradation capacity. Rarely are low-cost and fast approaches for fabricating multifunctional composites reported. Photopolymerization is a convenient method to readily transform the liquid solution into cross-linked hydrogel network in the presence of optical excitation [31]. Compared with conventional chemical cross-linking and thermal polymerization methods, photopolymerization has many advantages such as saving time and gentle operation conditions [32,33]. More interestingly, a variety of 2D/3D microstructures, defined by different photomasks, could be formed facilely.

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Poly-(ethylene glycol) double acrylate (PEGDA) is a nonbiodegradable polymer, and has been widely investigated due to its non-toxicity, good hydrophilicity and biocompatibility [34,35]. It can be customized to encapsulate a variety of bioactive substances [36] by simply mixing them in precursor liquids and then rapid cross-link by photopolymerization [37,38]. More notably, environmentally-friendly PEGDA hydrogel can form different morphologies by changing the photocuring conditions or precursor composition [39,40]. Wrinkles, which are also defined as creases or folds, are ubiquitous in nature, such as plant leaves, animal intestinal tract and human skin. They are often used as efficient tools for energy intake or surface functionality enhancement [41,42]. PEGDA also can form abundant wrinkles on the surface. It is well recognized that surface area of adsorbent plays a significant role in adsorption of contaminants. PEGDA with wrinkles is likely to be an ideal substrate for adsorbing contaminants due to its enlarged surface area. Herein, we presented a facile and fast approach to immobilizing TiO2 nanoparticles on PEGDA hydrogel by photopolymerization. In this process, TiO2 nanoparticles were distributed evenly in prepolymer mixture by ultrasonic treatment and bound in 3D network structure of polymer under UV exposure, and no obvious leakage and agglomeration were observed in use. The PEGDA polymer with a self-wrinkling patterned surface was used as adsorbent and dispersant. The obtained PEGDA/TiO2 composite was used to remove Congo red (CR) in water, and displayed a favorable adsorptive properties and photocatalytic activities.

2. Experimental 2.1. Materials Anatase titanium dioxide (TiO2), with the mean diameter of 20 nm, was obtained from Xuancheng Jing Rui New Material Co., Ltd, (Xuancheng, China). Poly-(ethylene glycol) double acrylate (PEGDA) and 2-Hydroxy-2-methylpropiophenone (photoinitiator 1173) were purchased from Sigma-Aldrich (Shanghai) Trading Co., Ltd (Shanghai, China). CR was acquired from Yu Ming Industrial Co., Ltd (Shanghai, China). Anhydrous ethanol was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Poly(dimethylsiloxane) (PDMS) was received from Dow Corning (USA) Co., Ltd. All the reagents used were of analytical-reagent grade.

2.2. Synthesis of PEGDA/TiO2 composite film The PDMS pre-polymer and curing agent was mixed, degassed and heated to make two PDMS sheets. Then a cuboid was cut out in the middle of one PDMS sheet, and this sheet bonded to the other sheet by air plasma treatment in a plasma cleaner (PDC32G, Harrick Scientific Co., USA). PEGDA/TiO2 composite film was prepared by a simple UV polymerization process. As shown in Fig. 1, 60 lL of 1173 photoinitiator and 1.47 mL of PEGDA were added into 1.5 mL of ethanol under vigorous stir for 30 min, and then 50 mg of TiO2 nanoparticles was dispersed adequately in the prepolymer solution with ultrasonic treatment for 30 min to remove dissolved oxygen. After shaked thoroughly, the mixture was spread on the PDMS device by using 1 mL plastic injector, and the thickness of the PEGDA/ TiO2 film could be easily controlled by changing the volume of mixture. Subsequently, the mixture was exposed under a UV-LED lamp (240 mw/cm2, UVATA Precision Photoelectric Co., Ltd, Shanghai, China) for 700 ms to synthesize the composite film. The film was rinsed with ethanol, and then treated by plasma for 30 s to harden and swell the skin layer. Finally, the film was treated by

ultrasound for 30 min in ethanol and in water respectively, to remove any residual polymer monomer and photoinitiator. 2.3. Characterization Fourier transform infrared (FT-IR) spectra of TiO2, PEGDA and PEGDA/TiO2 were recorded by a Nicolet 6700 FT-IR spectrometer (Thermo Fisher, Waltham, MA, USA). The surface morphology of the composites was characterized by an Ultra Plus scanning electron microscope (SEM, Zeiss, Jena, Thuringia, Germany) with an accelerating voltage of 30 kV. The interior dispersion of TiO2 in the film was recorded by a transmission electron microscope (TEM, JEM2000EX, JEOL Ltd, Japan). The TEM sample of PEGDA/TiO2 composite film was prepared by epoxy resin (Epon812) embedding and ultrathin sectioning on a Leica Ultracut UCT ultramicrotome. X-ray diffraction (XRD) pattern was performed by an X’pert Pro MPD diffracto-meter (PW 3040/60, PANalytical B. V., Almelo, Holland), and the 2h scanning angle range was 10–80°. UV–vis diffuse reflectance spectra (DRS) of TiO2 and PEGDA/TiO2 were obtained by U-3900 UV–vis spectrophotometer (Hitachi, Japan). 2.4. Adsorption and degradation of Congo red Typically, a piece of PEGDA/TiO2 composite film (about 0.11 g) was added to 50 mL aqueous CR solution (100 mg L
1) in a reactor cell. Subsequently, the solution was stirred in complete darkness for 3 h to adsorb dye molecules. Then, the vessel was irradiated under a 10 mW/cm2 UV light at 365 nm (UV-LED, UVATA, shanghai, China). The concentration of CR was recorded by a UV–vis spectrophotometer (TU-1901, Persee, Beijing, China) at 498 nm every 10 min. The adsorption and catalytic properties of the composite film were evaluated by measuring the concentration of residual CR after adsorption in darkness and UV irradiation, respectively. 3. Results and discussion 3.1. Morphology characterization The morphology and microstructure of the composite film were presented by SEM and TEM. As shown in Fig. 2a and c, both the PEGDA and PEGDA/TiO2 composite films, formed with exposure time of 700 ms, exhibited homogeneous wrinkled structures. But there was no wrinkle on the surface of the composite when it was irradiated sufficiently for 2000 ms, as displayed in Fig. 2b. It is noteworthy that the white dots on the films (Fig. 2b and c) are TiO2 nanoparticles. The energy-dispersive X-ray (EDX) spectrum (Fig. 2d) confirms the presence of Ti element on the film, and the atomic ratio of oxygen to titanium is close to the theoretical value of 2. As shown in Fig. 3a, TiO2 powder displayed a good dispersion and compatibility with the prepolymer. TiO2 nanoparticles were bound in the 3D network structure of the polymer without obvious agglomeration after irradiated for 700 ms. Fig. 2b shows the even distribution of TiO2 nanoparticles on the composite surface. The internal distribution of TiO2 nanoparticles in the composite film was further clarified by TEM, as shown in Fig. 3b. In addition, some ravines existed on the composite film due to the evaporation of ethanol, further increasing the specific surface area. Comparing the microstructures of PEGDA film (Fig. 2a) and PEGDA/TiO2 (Fig. 2c) film, it can be seen that the existence of TiO2 slightly impacted the morphology of wrinkles. That’s because when ultraviolet excitation occurred across the band gap of TiO2 nanoparticles on the surface of polymer, electron-hole pairs were produced, which could react with oxygen and water in surroundings, and created some active oxygen free radicals. The oxygen free radicals accelerated the monomer polymerization, which lessened unconverted

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Fig. 1. Schematic diagram of preparation of the PEGDA/TiO2 film.

Fig. 2. SEM images of PEGDA film formed after exposure time of 700 ms (a), PEGDA/TiO2 composite formed after exposure time of 2000 ms (b), and PEGDA/TiO2 composite formed after exposure time of 700 ms (c), EDX spectrum of composite film (d). Scale bars are 30 mm (a and c) and 10 mm (b).

monomers in the film. Thus, the amplitude of wrinkles on the PEGDA/TiO2 film decreased compared with that on the PEGDA film. This phenomenon will be further discussed in the formation mechanism of wrinkles. 3.2. Formation mechanism of wrinkles In this work, wrinkles on the surface of the composite film were formed by the quenching of free radicals by oxygen (oxygen inhi-

bition) [43,44]. It is well-known that PDMS has excellent gas penetrability, and even can be used for cell culture [45]. The concentration gradient of oxygen can be acquired based on this property of PDMS. As shown in Fig. 1, oxygen from the surrounding environment gradually penetrated the PDMS walls, and consumed part of the free radicals on the surface region of the curing film [46]. Thus, a partially cured layer was formed close to the walls of the PDMS device, which was regarded as the skin layer. After rinse and plasma treatment, it could produce the cured wrinkles

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Fig. 3. TEM images of prepolymer mixture before photopolymerization (a) and PEGDA/TiO2 film after photopolymerization. Scale bars are 100 nm (a) and 400 nm (b).

on the film surface. The thickness of the skin layer is determined by the UV exposure time. The longer the UV exposure time is, the less unconverted monomer remains in the layer, and the thinner the skin layer is. The wavelength of wrinkles, k, can be expressed by the following equation


1=3 Es k  2p h 3Ef

ð1Þ

where h is the thickness of the skin layer, Es is Young’s modulus of the skin, and Ef is Young’s modulus of the substrate [47,48]. In the present work, UV exposure time is the only factor to decide the thickness of the skin layer, and thus decide the wavelength of the wrinkles. In brief, the wavelength will decrease as the exposure time increases. As shown in Fig. 4, when the UV exposure time was set respectively at 600 ms, 800 ms and 1000 ms, the folded structures gradually turned from 7.2 lm to 1.6 lm in wavelength and flattened. It is easy to draw the conclusion that the surface area of the composite film decreases with prolonged UV exposure time. 3.3. Composition and optical properties of PEGDA/TiO2 composite films Fig. 5 shows the FT-IR spectra of PEGDA/TiO2 composite, PEGDA and TiO2. In the spectrum of TiO2, the band at 400–800 cm
1 was the characteristic absorption peak of TiO2. The band of PEGDA/ TiO2 at 554.23 cm
1 proved the presence of TiO2 in the composite films. The XRD patterns of TiO2 powders, PEGDA and as-prepared composite films are shown in Fig. 6. The weak crystalline characteristic peaks of composite film at 25.4°, 37.9°, 48.0°, 53.8°, 54.9°, 62.8° and 70.45° agree with the characteristic peaks of TiO2 crystal powders (JCPDS no. 21-1272) [49]. The broad peak of the composite film at 22° is analogous with the only characteristic peak derived from PEGDA polymer. Meanwhile, the intensity of the characteristic peaks of the PEGDA/TiO2 include 1.6 mg TiO2 (marked as PEGDA/TiO2-1) reduces compared with those of the

Fig. 5. FT-IR spectra of PEGDA/TiO2 composite, PEGDA and TiO2.

PEGDA/TiO2 include 4.8 mg TiO2 (marked as PEGDA/TiO2-2) due to the decrease of TiO2 content. All of these suggest that the TiO2 nanoparticles were successfully immobilized in the polymer and the introduction of PEGDA did not change the physical property of TiO2. As shown in Fig. 7, the UV–vis spectra of PEGDA/TiO2-1, PEGDA/ TiO2-2 as well as the commercial TiO2 nanoparticles have strong absorption approximately in the range of 200–400 nm, which was attributable to the intrinsic band gap absorption of anatase TiO2 nanoparticles. The slight absorption bands of PEGDA/TiO2-1 and PEGDA/TiO2-2 between 400 and 550 nm were assigned to PEGDA. These results indicate that the existence of PEGDA prolonged the absorption range in visible region, and the absorption intensity strengthened gradually with the increase of PEGDA percentage in the composite film, implying that the composite film

Fig. 4. SEM images of wrinkles on PEGDA/TiO2 films formed after different exposure time of 600 ms (a), 800 ms (b) and 1000 ms (c). Scale bars are 4 lm.

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Fig. 6. XRD patterns of TiO2, PEGDA and PEGDA/TiO2 composite films. PEGDA/TiO2-1 includes 1.6 mg TiO2, and PEGDA/TiO2-2 includes 4.8 mg TiO2.

Fig. 7. UV–visible diffuses reflectance spectra of TiO2, PEGDA and PEGDA/TiO2 composite film. PEGDA/TiO2-1 includes 1.6 mg TiO2, and PEGDA/TiO2-2 includes 4.8 mg TiO2.

has a promising potential in photocatalytic degradation under visible illumination.

441

Fig. 8. Concentration change of CR solutions during adsorption (left of dotted line) and degradation (right of dotted line) over PEGDA/TiO2 composite film, PEGDA film and TiO2 nanoparticles, respectively.

degraded after 180-min UV irradiation. A slight decrease in the concentration of CR solution with the pure PEGDA membrane was observed due to the absence of photocatalytic activity. The influence of wrinkles on the adsorptive capability was also investigated. The PEGDA/TiO2 composite film with wrinkles exhibited markedly stronger absorption than the one without wrinkles. In conclusion, the composite film has better performance for the degradation of CR organic dyes in a short time. To demonstrate that the CR dye was degraded rather than absorbed, CR solution was used to further study the degradation capability of the composite film. About 0.11 g PEGDA/TiO2 film was put into 10 mL of 50 mg L
1 aqueous CR solution. Fig. 9 shows that CR in the solution was fully absorbed by the PEGDA/TiO2 film after 50 min in the darkness. The films loaded with CR were respectively irradiated for different time. And then the films were respectively placed in 75% ethanol (5 mL) to dissolve the nondegraded CR. The degree of degradation was estimated based on the CR concentrations in ethanol. The concentration of CR decreased dramatically in the first ten min, as shown in Fig. 9 (inset). Then, CR was stepwise decomposed by UV irradiation from 10 min to 70 min, and it was almost completely degraded after 70 min irradiation. And in the process of degradation, the aqueous

3.4. Adsorption and degradation capacity CR was chosen to evaluate the adsorptive properties and photocatalytic activities of the PEGDA/TiO2 composite film. Fig. 8 shows the adsorption and degradation of CR. Here, the composite film (0.11 g), PEGDA film (0.11 g) and TiO2 powder (0.01 g) were added to 50 mL of 100 mg L
1 CR solution under vigorous stirring. Both the PEGDA film and PEGDA/TiO2 composite film displayed considerably strong adsorptive capacities for CR. More than 65% of CR was absorbed by the composite film. The adsorption reached saturation after 180 min, and the adsorption capacity was 30.5 mg g
1. This phenomenon is possibly attributed to the large specific surface areas and network structure of the films. Conversely, the absorption of CR onto TiO2 nanoparticles was slight in the first 20 min, and then kept balance roughly. In the degradation step, the concentration of CR solution with the PEGDA/TiO2 film had a sharp decline when the film was exposed to UV irradiation, and CR was completely removed within 180 min. TiO2 nanoparticles also exhibited well photocatalytic activity, but only 46.2% CR was

Fig. 9. Concentration change of CR solutions during adsorption and degradation over PEGDA/TiO2 composite film. Inset: absorption spectra of extracted CR in ethanol.

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solution remained colorless and transparent, indicating no desorption of dye and no leakage of TiO2 nanoparticles. 3.5. Adsorption kinetics In this study, pseudo-first-order, pseudo-second-order, and intra-particle diffusion models were used to describe the adsorption behavior of CR on the prepared films. The pseudo-first-order model is defined as [50]

logðqe
qt Þ ¼ log qe


k1 t 2:303

ð2Þ

The pseudo-second-order is expressed as [51]

t 1 t ¼ þ qt k2 q2e qe

ð3Þ

where qe and qt represent respectively the amount of CR adsorbed at equilibrium and at time t, and k1 and k2 are the rate constants of pseudo-first-order and pseudo-second-order models respectively obtained from the slope of fit line. The kinetic curves are shown in Fig. 10. The parameters acquired from the models are displayed in Table 1. Compared with the pseudo-first-order model, pseudosecond-order model shows an excellent linear relation (R2 = 0.999), illustrating it is more suitable to describe the adsorption behavior of CR absorbed onto films. In addition, the normalized standard deviation (SD (%)) can be used to check whether the pseudo-second-order model can accurately describe CR adsorption process. The SD can be calculated according to the following equation [52]

SDð%Þ ¼ 100 

(    )1=2 X qt;exp
qt;cal qt;exp 2

ð4Þ

n
1

where qt,exp represents the experimental value, qt,cal is calculated by using the pseudo-second-order model, and n is the number of data points. The normalized SD of the pseudo-second-order kinetic model is less than 3%, as shown in Table 1, suggesting that the pseudo-second-order model was favorable to describe the CR adsorption behavior onto the prepared films. To understand the diffusion mechanism of adsorption process, further study of the kinetic data based on the intra-particle diffusion model is needed [53]. The equation of the intra-particle diffusion model can be expressed as follows

qt ¼ kd t 1=2 þ I

ð5Þ

where kd is the intra-particle diffusion rate constant, and I is a constant about the boundary layer thickness. The correlation coefficient of intra-particle diffusion (R2 = 0.974) is lower than the correlation coefficients of other kinetic models, and the curve of intra-particle diffusion model does not pass through the origin [54], illustrating that the adsorption involves intra-particle diffusion, which however is not the only rate-controlling step. 3.6. Adsorption isotherm To analyze the relationship between the adsorbent and the adsorbate at equilibrium, and estimate the maximum adsorption capacity of the adsorbent, the equilibrium adsorption data were analyzed by using two common adsorption isotherm models – Langmuir and Freundlich. Langmuir model is based on the assumptions of monolayer adsorption and no interaction between the adsorbed species. The equation is usually expressed as [50]

qe ¼

qm K L C e 1 þ K LCe

ð6Þ

where KL is a Langmuir adsorption constant, Ce is the equilibrium concentration of CR in solution, and qm represents the maximum adsorption capacity of the adsorbent corresponding to complete monolayer coverage on the surface. The essential characteristics of the Langmuir isotherm can be further expressed by an equilibrium parameter RL, which is defined as follows [55]

RL ¼

1 1 þ K LC0

ð7Þ

where C0 is the initial concentration of CR, and RL indicates the shape of Langmuir isotherm to be unfavorable (RL > 1), linear (RL = 1), favorable (1 > RL > 0) or irreversible (RL = 0). The Freundlich model describes a heterogeneous surface without saturation of active binding sites. The equation can be expressed in the following form [51]

qe ¼ K F C 1=n e

Fig. 10. Kinetic curves for CR adsorbed on the films. Initial concentration is 200 mg L
1.

ð8Þ

where KF represents the Freundlich isotherm constant, and 1/n is an empirical parameter related to the favorability of the adsorption process. If the value falls in the range of 0–1, it indicates that the adsorbate is favorably adsorbed onto the adsorbent.

Table 1 Kinetic parameters of CR adsorption onto the films. Initial concentration of CR is 200 mg L
1. Model

a

Parameter
1 a

Value

R2

SD (%)

48.9 0.0167

0.995

3.82

Pseudo-first-order

qe,cal (mg g k1 (min
1)

Pseudo-second-order

qe,cal (mg g
1) k2 (g mg
1 min
1)

62.4 0.000256

0.999

2.10

Intra-particle diffusion

I (mg g
1) kd (mg g
1 min
0.5)


0.996 3.65

0.974

8.64

)

qe,cal is the calculated value by the adsorption kinetic model.

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the maximum adsorption capacity calculated from the Langmuir model is 93.5 mg L
1 at 298 K. 3.7. Reusability In order to assess its reusability and cost effectiveness in practical application, the TiO2/PEGDA composite film (0.11 g) was tested for several cycles by using 10 mL of 100 mg L
1 aqueous CR solution. After each cycle, the film was treated by ultrasonic for 2 h using ethanol (75%) three times. Fig. 12 shows the stability of the TiO2/PEGDA composite film for adsorbing and degrading CR in the four cycles. The degradation activity of the recycled composite film had little change, and CR in solutions was removed almost completely even in the fourth cycle. It can be concluded that the TiO2/PEGDA composite film can be reused for organic dye degradation and the cost will be much lower in the future application. Fig. 11. Adsorption isotherms for CR adsorption on PEGDA/TiO2 film. Initial concentration of CR is 50–600 mg L
1, the volume of the CR solution is 50 mL, and the temperature is 298 K.

Table 2 Isotherm parameters for the adsorption of CR onto the films (T = 298 K). Isotherm

Parameter

Value

Langmuir

R2 qm (mg g
1) KL (L mg
1) RL

0.968 93.5 0.00129 0.564–0.939

Freundlich

R2 KF ((mg g
1) (L mg
1)1/n) 1/n

0.893 8.62 0.373

The Langmuir and Freundlich isotherms are shown in Fig. 11. Langmuir isotherm model is slightly better fitted than the Freundlich. The obtained model parameters are listed in Table 2. The correlation coefficient of Langmuir (R2 = 0.968) is higher than that of Freundlich model (R2 = 0.893), which is in agreement with the fitted curves. Furthermore, the factor RL, within the range of 0–1, proves that the adsorption of CR onto films is a favorable process. These results suggest that Langmuir isotherm model is appropriate to describe the adsorption process in this work. There is a monolayer adsorption of CR onto the adsorbent surface, and

Fig. 12. Repeated use of TiO2/PEGDA composite film to remove CR for four cycles. The initial concentration of CR solutions, 100 mg L
1; UV irradiance, 10 mw/cm2.

4. Conclusions The composite film of PEGDA and TiO2 was synthesized by photopolymerization, and wrinkles on the film were formed via partial curing, rinse and plasma treating, which is a simple, fast and environmentally-friendly preparation approach. Experimental results showed that the composite film, acting as adsorbent and catalyst, improved the removal efficiency of CR in water. The high adsorption capacity of the film is attributed to the controlled wrinkled surface, and the sorption kinetics of CR can be described by the pseudo-second-order model. The improved photocatalytic activity is attributed to the good dispersion of TiO2 nanoparticles in the film. The PEGDA/TiO2 composite film offered an enhanced adsorption and photocatalysis capacity compared with the TiO2 nanoparticles, and it could be reused for several times. The composite film with adsorption and degradation functions is promising in dyeing wastewater treatment, and the proposed photopolymerization method offers a simple pathway to synthesize inorganicorganic composite materials with enhanced surface. Acknowledgements This work was supported by the National Natural Science Foundation of China [21375012 and 21675020]. References [1] E. Filippo, C. Carlucci, A.L. Capodilupo, P. Perulli, F. Conciauro, G.A. Corrente, G. Gigli, G. Ciccarella, Facile preparation of TiO2–polyvinyl alcohol hybrid nanoparticles with improved visible light photocatalytic activity, Appl. Surf. Sci. 331 (2015) 292–298. [2] B. Dai, L. Zhang, H. Huang, C. Lu, J. Kou, Z. Xu, Photocatalysis of composite film PDMS-PMN-PT@TiO2 greatly improved via spatial electric field, Appl. Surf. Sci. 403 (2017) 9–14. [3] N.A.M. Nor, J. Jaafar, A.F. Ismail, M.A. Mohamed, M.A. Rahman, M.H.D. Othman, W.J. Lau, N. Yusof, Preparation and performance of PVDF-based nanocomposite membrane consisting of TiO2 nanofibers for organic pollutant decomposition in wastewater under UV irradiation, Desalination 391 (2016) 89–97. [4] R. Hao, G. Wang, C. Jiang, H. Tang, Q. Xu, In situ hydrothermal synthesis of gC3N4/TiO2 heterojunction photocatalysts with high specific surface area for Rhodamine B degradation, Appl. Surf. Sci. 411 (2017) 400–410. [5] F. Wu, X. Li, W. Liu, S. Zhang, Highly enhanced photocatalytic degradation of methylene blue over the indirect all-solid-state Z-scheme g-C3N4-RGO-TiO2 nanoheterojunctions, Appl. Surf. Sci. 405 (2017) 60–70. [6] H. Liu, J.B. Joo, M. Dahl, L. Fu, Z. Zeng, Y. Yin, Crystallinity control of TiO2 hollow shells through resin-protected calcination for enhanced photocatalytic activity, Energy Environ. Sci. 8 (2015) 286–296. [7] N. Justh, T. Firkala, K. László, J. Lábár, I.M. Szilágyi, Photocatalytic C60amorphous TiO2 composites prepared by atomic layer deposition, Appl. Surf. Sci. 419 (2017) 497–502. [8] K. Zhao, L. Feng, Z. Li, Y. Fu, X. Zhang, J. Wei, S. Wei, Preparation, characterization and photocatalytic degradation properties of a TiO2/calcium alginate composite film and the recovery of TiO2 nanoparticles, RSC Adv. 4 (2014) 51321–51329.

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