Full Length ArticleOne-step synthesis of bifunctional PEGDA/TiO2 composite film by photopolymerization for the removal of Congo red
Graphical abstract
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], [6], [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 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.
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.
Section snippets
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).
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)
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
Acknowledgements
This work was supported by the National Natural Science Foundation of China [21375012 and 21675020].
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