Hierarchical mesoporous titania nanoshell encapsulated on polyimide nanofiber as flexible, highly reactive, energy saving and recyclable photocatalyst for water purification
Graphical abstract
A novel hierarchical mesoporous titania nanoshell@polyimide nanofiber is constructed via the in-situ complexation-hydrolysis strategy. This unique core-shell hybrid nanofiber manifests super high reactive surface area (213 m2/g), smaller pore size (2.7 nm), great photocatalytic activity and admirable photocatalytic efficiency retention (88%) after 10 cycles, which makes it a flexible, energy saving and recyclable photocatalyst.
Introduction
Since the discovery of photoinduced water-splitting under the catalysis of titanium dioxide (TiO2) and Pt by Fujishima and Honda in 1972 (Fujishima and Honda, 1972; Pelaez et al., 2012), the development of TiO2 photocatalysis has been triggered and drawn more and more attention in recent years due to the urgent demands of the high catalytic efficiency of TiO2 on contaminants removal, air purification and water disinfection. Actually, for decades, TiO2 has always been the most widely investigated photocatalyst among various metal oxide semiconductors, because of its low cost, high photo-activity, suitable electronic and optical properties, excellent chemical and thermal stability, high catalytic activity, strong oxidization capability and the availability of immobilization on various substrates (Wang et al., 2009; Ullah et al., 2015). Nowadays, the work mechanism of the photocatalytic function of TiO2 has been generally disclosed and could be divided into four steps as illustrated in Scheme 1 (Gaya and Abdullah, 2008; Ahmed et al., 2010; Shan et al., 2010; Akpan and Hameed, 2009; Özkan et al., 2004; Houas et al., 2001; Li Puma et al., 2008).
Conventionally, a suspended slurry of TiO2 powders or nanoparticles was usually adopted in order to maximize light absorption and mass transfer in the traditional way. Although it shows great contact surface and efficiency, some drawbacks also appear, such as low light utilization efficiency, harmful to human’s health and the requirement of costly post-treatment separation process for catalyst recovery (Dong et al., 2015). Therefore, the idea of immobilizing TiO2 on different substrates has been put forward to address the above-mentioned issues. This approach can minimize the catalyst cost and generate longer contact time of the photocatalyst with pollutants (Singh et al., 2013). A variety of materials have been reported to serve as the supports for photocatalyst immobilization, including glass mats (Dhananjeyan et al., 2001), synthetic fabrics (Singh et al., 2013), polymers (Yang et al., 2006), hollow glass spheres (Jiang et al., 2013), reactor walls (Mahmoodi et al., 2006), glass fiber/plate (Hofstadler et al., 1994; Jawad et al., 2015), silica gel (Ding et al., 2000), fabric or wool (Bozzi et al., 2005), micro-porous cellulose membranes (Gebru and Das, 2017), quartz optical fibers (Choi et al., 2001), alumina clays (Fabiyi and Skelton, 2000), ceramic membranes and monoliths (Gieselmann et al., 1988), stainless steel (Fernández et al., 1995), zeolites (Huang et al., 2008), anodized iron (Velásquez et al., 2012), glass plates (Khataee et al., 2009), etc. The above pioneering works indicate that ideal support materials for TiO2 immobilization are supposed to have the following characteristics: (1) The substrate must have strong affinities with the photocatalyst in order to anchor the catalysts steadily; (2) The photocatalyst activity should not be affected significantly after immobilization; (3) The support should provide high specific contact area and high absorption for pollutants to be degraded; (4) Avoiding the leaching of photocatalyst; (5) The support should be able to withstand the strong oxidization generated by the photocatalyst and UV irradiation.
Yet scholars tend to choose polymers as supports due to their hydrophobic nature, readily accessibility, surface modifiability, diversity of selectivity, low density for developing of buoyant photocatalysts. In 1995, Tennakone published the first study of developing a polyethylene (PE) film supported TiO2 photocatalyst via a simple thermal treatment method (Tennakone et al., 1995). Since then, plenty of polymer supports have been studied, including foamed PE sheet (Naskar et al., 1998), polystyrene (PS) (Choi et al., 2001; Ata et al., 2017), poly(vinylidene difluoride)-co-trifluoroethylene (P(VDF-TrFE)) (Teixeira et al., 2016), cellulosic fibers (Jouali et al., 2019), high density polyethylene (HDPE) beads (Singh et al., 2013), polyvinyl alcohol (PVA) film (Singh et al., 2013), polyaniline (PANI) film (Singh et al., 2013), polyethylene terephthalate (PET) bottles (Fostier et al., 2008), rubber latex (Sriwong et al., 2008), etc. In recent years, more and more attentions have been paid to using nanofiber mats as the support of photocatalyst because of their high surface area. Paula T. Hammond reported a novel method to layer-by-layer (LBL) assemble TiO2 nanoparticles and polyhedral oligomeric silsesquioxane (POSS) molecules on various electrospun fibers for different potential applications, such as a protective clothing system, photocatalysis, sensors, and electrodes (Lee et al., 2009). After that, more and more polymer-based nanofiber mats were used to anchor the TiO2, and they all exhibited comparable photocatalytic activity, such as polyacrylonitrile (PAN)-based electrospun nanofiber webs (Im et al., 2008), PAN/multi-walled carbon nanotube composite nanofibers (Mohamed et al., 2018), and nylon-6 electrospun nanocomposite mat containing silver nanoparticles (Pant et al., 2011).
As a class of high-performance polymeric material, polyimide (PI) is well-known for its outstanding high and low temperature stability, high strength, excellent chemical resistance, UV and nuclear resistance, and dimensional stability (Dong et al., 2019a). PI could be easily electrospun into ultrafine nanofibers with controlled diameter varying from tens to hundreds of nanometers (Dong et al., 2019b). This endows the prepared PI nanofiber nonwoven mat with large specific surface area, and makes it extremely suitable support candidate material for TiO2 immobilization. Furthermore, the organic contaminants tend to gather on the PI surface where the photocatalyst immobilized because of the strong adsorption capability of PI nanofiber mat, which would bring longer contact time between contaminants and photocatalyst, and higher photocatalytic efficiency (i.e., “bait-hook and destroy” strategy) (Lee et al., 2018). Therefore, PI nanofiber nonwoven is selected as the support of TiO2 immobilization in this work, which has never reported before, to our knowledge. A novel hierarchical mesoporous anatase TiO2 nanoshell@PI core conformation is constructed via an in-situ complexation-hydrolysis strategy without agglomeration of TiO2 nanoparticles, realizing the extremely uniform distribution and high loading of TiO2, and consequently achieving high surface area for reaction.
Section snippets
Preparation of hierarchical mesoporous TiO2 nanoshell@PI nanofiber membranes
The 4,4′-oxydianiline (ODA) and pyromellitic dianhydride (PMDA) were used for synthesis of the precursor of PI, i.e., poly (amic acid) (PAA) resin with inherent viscosity of 2.81 dL g−1, via condensation polymerization in dimethyl formamide (DMF) at about 0 °C. Dianhydride had a 1% (mol/mol) offset during the synthesis of PAA and the solid content of PAA in DMF was 12 wt %. The as-prepared PAA resin was then electrospun into PAA nanofiber mats by an electrospinning apparatus operated at about
Morphologies of the hierarchical mesoporous TiO2 nanoshell@PI nanofibers
Fig. 1 gives the photocatalysis diagram, digital picture, flame-resistance and morphologies of the hierarchical mesoporous TiO2 nanoshell@PI nanofiber membranes prepared via the in-situ complexation-hydrolysis technique. As manifested in Fig. 1(b), the TiO2@PI nanofiber membrane is a yellow flat mat with excellent flexibility and no breakage occurs after being bended, which is extremely beneficial for practical applications and recovery. The outstanding flexibility can be mainly ascribed to the
Conclusion
A novel hierarchical mesoporous TiO2 nanoshell@PI configuration has been built up via the in-situ complexation-hydrolysis strategy by utilizing the TiOSO4 as the source of TiO2 and H2SO4 as the crystalline regulator. Compared with the conventional TiO2 slurry and polymer supports, this flexible core-shell nanoarchitecture gives several advantages: (1) The PI nanofiber supported TiO2 with great UV resistance and thermal stability can be the long-term reusable photocatalyst; (2) The formation of
Notes
There are no conflicts to declare.
Author contributions section
Guoqing Dong contributed the central idea, analyzed most of the data, and wrote the initial draft of the paper. The remaining authors contributed to refining the ideas, carrying out additional analyses and finalizing this paper. All authors read and approved the manuscript.
Declaration of competing interest
No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors and tacitly or explicitly by the responsible authorities where the work was carried out for publication, and if accepted, this manuscript will not be published elsewhere in the same form, in English or in any other language, without the written consent of the Publisher. I would like to declare on behalf of my co-authors that the work described was original research that has not been
Acknowledgment
This work was supported by the National Key Basic Research Program of China [973 Program, 2014CB643604]; the National Natural Science Foundation of China [51673017, 21404005]; the Natural Science Foundation of Jiangsu Province [BK20140006, BK20150273] and Changzhou Sci & Tech Program [CZ20150001].
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