High photocatalytic degradation activity of the polyvinyl chloride (PVC)–vitamin C (VC)–TiO₂ nano-composite film

Abstract

A novel photodegradable polyvinyl chloride (PVC)–vitamin C (VC)–TiO₂ nano-composite film was prepared by embedding VC modified nano-TiO₂ photocatalyst into the commercial PVC plastic. The solid-phase photocatalytic degradation behavior of PVC–VC–TiO₂ nano-composite film under UV light irradiation was investigated and compared with those of the PVC–TiO₂ film and the pure PVC film, with the aid of UV–Vis spectroscopy, scanning electron microscopy (SEM), weight loss monitoring, and X-ray diffraction spectra (XRD). The results show that PVC–VC–TiO₂ nano-composite film has a high photocatalytic activity; the photocatalytic degradation rate of it is two times higher than that of PVC–TiO₂ film and fifteen times higher than that of pure PVC film. The optimal mass ratio of VC to TiO₂ is found to be 0.5. The mechanism of enhancing photocatalytic activity is attributed to the formation of a Ti^IV–VC charge-transfer complex with five-member chelate ring structure and a rapid photogenerated charge separation is thus achieved.

Introduction

Polyvinyl chloride (PVC) and related plastic products are non-biodegradable in natural environment because of their chemical inertness. The waste PVC plastics become one of the main sources of “white pollution”. Traditional processing methods, such as garbage deposit or incineration, cause a serious secondary pollution [1], [2], [3], [4]. Therefore, the development of degradable PVC plastics becomes an important issue. Several biodegradable PVC plastics have been reported elsewhere [5], [6], [7]. There are some disadvantages in these PVC plastics, mainly the long degradation cycle and the incompleteness of the degradation, which limit their practical applications. On the other hand, the photodegradable PVC by embedding photocatalysts into the plastics demonstrate a high potential in rapid decomposition of the plastics. Particularly, the photocatalytic degradation of PVC generates no dioxins and the decomposition process is easily to occur under natural environmental conditions [8], [9], [10], [11], [12], [13]. The photocatalytic degradation of polymers by means of TiO₂ has been proved an attractive and efficient decomposition technique for treatment of waste polymers, in open-air under UV light irradiation or solar exposure [11], [12], [13], [14], [15], [16], [17], [18], [19], [20].

However, slow reaction rate and poor solar efficiency have already hindered the practical application of this technology. To eliminate these drawbacks, many attempts have been carried out to modify TiO₂ characteristics by surface modifiers through three ways: (a) by inhibiting charge recombination; (b) by expanding the wavelength response range; and (c) by changing the selectivity or yield of a particular product [21]. Recent studies indicate that the surface modification of TiO₂ by metallophthalocyanine shows a high photocatalytic activity for degradation of polymers [22], [23], [24]. Vitamin C (VC), a naturally available compound, has been used as a modifier in a solid-state cell based on TiO₂ and CuI, where the photo-excited dye molecules inject electrons into the conduction band of TiO₂ and holes into the valence band of CuI. It is proved that VC can satisfy the charge separation requirements and increase the photo-effects of the cell [21], [25]. Ou et al. [26] reported that the surface modification of TiO₂ with VC greatly enhanced the methyl orange photocatalytic decolorization rate due to the formation of a charge-transfer bidentate complex. To our best knowledge, there is no research on waste plastics treatment by VC modification of nano-TiO₂.

In this paper, a new kind of photodegradable PVC–VC–TiO₂ nano-composite film was synthesized using VC modified nano-TiO₂ as the photocatalyst. The photodegradation performance has been investigated. The PVC–VC–TiO₂ nano-composite film shows much higher photodegradation efficiency under UV light irradiation than the PVC–TiO₂ composite film and the pure PVC film.