Immobilization of TiO2 nanoparticles on polymeric substrates by using electrostatic interaction in the aqueous phase

https://doi.org/10.1016/j.apcatb.2008.01.036Get rights and content

Abstract

A new method for immobilization of TiO2 nanoparticles on polymeric substrates was developed to facilitate photocatalytic purification of contaminated air and water. TiO2 was immobilized by dipping a polymeric substrate, treated with polyvinyl chloride–polyvinyl acetate (PVC–PVA) copolymer and/or SiO2, into a TiO2/water suspension. The surface zeta potential measurement on TiO2 and treated polymers implies that this method is based on an electrostatic interaction between positively charged TiO2 and the negatively charged treated surface of the polymeric substrate. This method precludes the enwrapping of TiO2 particles in binding components, which has been a drawback of conventional methods, and enables bonding of the TiO2 nanoparticles on the substrate surface at high density. The TiO2-immobilized nonwoven polyester (PES) prepared using this method exhibited high photocatalytic activity for decomposing the air contaminant toluene. Furthermore, this method was applicable to polypropylene (PP) nonwoven, polyethylene (PE) nets and PE and PP films. TiO2 bonding was inhibited on PP by treatment with PVC–PVA copolymer and better TiO2 immobilization was observed on SiO2-treated PP.

Introduction

Photocatalytic oxidation reactions on TiO2 nanoparticles under UV-light irradiation have been used to decompose toxic substances in air [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] and water [15], [16], [17], [18], [19], [20] since the beginning of 1990. For practical use of these reactions, TiO2 nanoparticles must be immobilized on a suitable substrate, since the separation of nanoparticles from the reaction medium is difficult and costly. Hence, many techniques for immobilizing TiO2 nanoparticles have been developed mainly for inorganic substrates such as glass [3], [4], [6], [8], [16], [17], alumina [1], [21], clay [3], and stainless steel [17]. These substrates are stable against active oxygen species (e.g. radical dotOH and O2radical dot), which appear on TiO2 under UV-light irradiation. In contrast, much attention is required for immobilizing TiO2 on polymer substrates, since most polymers are degraded by such active oxygen species.

TiO2-bonded apatite and silica gel have been generally utilized to prevent degradation of textiles. Although detailed information is not available for these products, these materials have a role as both barrier layer and adsorbent. To immobilize on textiles, these products require a binder, but the latter frequently enwrap the TiO2 particles when mixed with photocatalyst, and this reduces the activity of photocatalytic materials [22]. Aqueous silica sols are often used to produce a SiO2 layer on inorganic substrates, but most of them are not applicable to polymers. Sol–gel reactions of alkoxysilane enable the formation of a layer of polysiloxane (SiO2 network) on polymers. To date, only a few authors reported the use of this technique to make a barrier on a polymeric substrate [23], [24], [25].

In this paper, we report a new method for immobilization of TiO2 nanoparticles on polymers. The sol–gel reaction of alkoxysilane was applied to produce a barrier layer of SiO2 network, followed by coating with a thin layer of polyvinyl chloride–polyvinyl acetate (PVC–PVA) copolymer. TiO2 was bonded in an aqueous phase using a physico-chemical interaction between TiO2 particles and a dried surface coated with PVC–PVA copolymer. This interaction was found when commercially available polyester (PES) filter with special treatment was soaked in TiO2/water suspension. Analysis revealed that the surface substance is PVC–PVA copolymer and this finding was applied to immobilization of TiO2 on polymers. Note that TiO2 bonding does not occur with PVA. TiO2-immobilized PES nonwoven, prepared using the aforementioned method, exhibited high photocatalytic activity in decomposing the air contaminant toluene, and could be used as a filter for existing photocatalytic air purifiers. Furthermore, this method for TiO2 immobilization could also be applicable to polypropylene (PP) nonwoven, polyethylene (PE) net, PE and PP films. TiO2 bonding was inhibited by treatment with PVC–PVA copolymer and better TiO2 immobilization was obtained on SiO2-treated PP.

Section snippets

Photocatalysts

We mainly used the TiO2 photocatalyst P25 (Japan Aerosil) with an average particle diameter of 21 nm and a specific surface area of 50 m2/g. To confirm the applicability of the new immobilization method to another type of TiO2, we also used ST-01 (Ishihara Sangyo) with average particle diameter of 7 nm and specific surface area of 300 m2/g. These photocatalysts were used as supplied without further treatments. An UV–vis photocatalyst BA-PW25 (Ecodevice) was used for elucidating the TiO2 bonding

Immobilized TiO2 particles on PES nonwoven

The immobilized TiO2 particles were uniformly distributed on the surface of the nonwoven PES fiber (Fig. 3A), and were densely packed (agglomerated) (Fig. 3B). Note that in conventional TiO2 immobilization methods, other mixed substances added as a binder or a barrier component, often enwrap TiO2 particles and this results in low photocatalytic activity [22]. In contrast, in the present method the TiO2 particles are unwrapped; consequently, we expect higher photocatalytic activity. Moreover,

Conclusions

A new method, which enables bonding TiO2 nanoparticles on the surface of the polymeric substrate at high density and gives high photocatalytic activity, was developed. TiO2 was immobilized on the substrate in aqueous phase by physico-chemical interaction between a primer substance coated on polymer (PVC–PVA copolymer and/or SiO2) and TiO2. The surface zeta potential measurement on TiO2 and treated polymers indicates that this method is principally based on an electrostatic interaction between

Acknowledgements

We thank Dr. A. Obuchi and Dr. J. Uchisawa for their invaluable help with the measurement of the surface zeta potential of photocatalysts and polymer samples.

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