Hierarchical mesoporous titania nanoshell encapsulated on polyimide nanofiber as flexible, highly reactive, energy saving and recyclable photocatalyst for water purification

Highlights

• A novel hierarchical mesoporous TiO₂ nanoshell@polyimide nanofiber is constructed.

• Superior high reactive surface area is achieved from hierarchical mesoporous TiO₂ nanoshell and ultrafine PI nanofiber.

• The PI nanofiber supported photocatalyst exhibits superb flexibility, reactivity, energy saving and recyclability.

Abstract

Titania is well known for its outstanding photocatalytic performance and widely applied in contaminants removal, air purification and water disinfection. However, directly using of titania powders in these applications will bring several drawbacks, such as low light utilization, costly separation recovery and human health concerns. Hence, numerous efforts have been tried for immobilization of titania on various substrates in order to overcome the aforementioned issues. Moreover, traditional methods for immobilization of titania on polymer substrates also have the disadvantages of surface area reduction, titania leaching and polymer degradation by oxidative radicals. Thus, in this work, a novel hierarchical mesoporous titania nanoshell@polyimide nanofiber is constructed via the in-situ complexation-hydrolysis strategy by utilizing titanium oxysulfate as the precursor of titania and sulfuric acid as the crystal structure regulator. Benefited from the hierarchical mesoporous titania nanoshell and ultrafine electrospun polyimide nanofiber, a superior high reactive surface area is achieved (213 m²/g) via the stacking of nanopores (pore size distribution: 2–4 nm, 41.4%; 4–10 nm, 36.7%; 10–20 nm, 21.9%), leading to the excellent photocatalytic activity. Additionally, this buoyant PI nanofiber supported photocatalyst exhibits superb flexibility, and is highly reactive and energy saving since it can be easily recovered from the contaminated water without costly separation compared to the traditional titania slurry. More importantly, this unique photocatalyst demonstrates admirable photocatalytic efficiency retention (88%) after 10 cycles, which makes it a recyclable and economical photocatalytic material.

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.