Photocatalytic efficiency of ZnO plates in degradation of azo dye solutions

Abstract

Tape casting method was used to prepare ZnO plates for photocatalytic degradation of Reactive Orange 16 (RO 16) and Reactive Red 180 (RR 180) textile dyes in aqueous solutions. The plates were sintered at 700 °C and 1050 °C and characterized by using TG-DTA, BET and SEM. The sintering process at 700 °C yields relatively high surface area ZnO plates comparing with the plates sintered at 1050 °C. The photocatalytic activity was enhanced when the high surface area plates were used in photocatalytic degradation processes. Up to 95.7% and 88.6% color removal was obtained in 90 min for the RR 180 and RO 16 aqueous solutions, respectively, when the plates were sintered at 700 °C. Total organic carbon (TOC) removal was 43% at 180 min for RR 180 solution. Photocorrosion tendency was observed to some extend during photocatalysis when multiple tests were performed on the same plate. This corrosive effect was recovered by the memory effect when the plates were subjected to dark interval overnight.

Introduction

In recent years, novel heterogeneous metal oxide semiconductor materials such as TiO₂ and ZnO have been developed and attracted considerable attention owing to their photocatalytic ability in the degradation of various environmental pollutants such as dyes, pesticides, detergents, and volatile organic compounds under UV-light irradiation [1], [2], [3], [4], [5]. Heterogeneous photocatalysis is a promising new alternative method among advanced oxidation processes (AOPs) which generally includes UV/H₂O₂, UV/O₃ or UV/Fenton's reagent for oxidative removal of organic chemicals [6], [7]. Heterogeneous semiconductors in the field of photocatalysis were investigated deeply because of its high efficiency, commercial availability and high chemical stability. When the semiconductor particles are illuminated with UV-light, an electron promotes from the valence band to the conduction band due to photoexcitation, thus leaving an electron deficiency or hole in the valence band; in this way, electron/hole pairs are generated. These electron hole pairs can either recombine or can interact separately with other molecules. Both reductive and oxidative processes can occur at/or near the surface of the photoexcited semiconductor particle [8]. In aerated aqueous suspensions, oxygen adsorbed on the surface of the catalyst acts as an electron trap on the conduction band and electron/hole recombination can be effectively prevented and lifetime of holes is prolonged.

TiO₂ as a semiconductor oxide has been investigated extensively since 1980s and found as very efficient photocatalyst due to its abundant availability, cost-effectiveness and chemical stability. However, current researches show that ZnO can also be used as a very efficient semiconductor photocatalyst when compared with TiO₂ [9], [10]. ZnO and TiO₂ have similar band gap energies around 3.2 eV [11], [12]. Lower cost and better performance of ZnO in the degradation of organic molecules in both acidic and basic medium have stimulated many researchers to further explore the properties of this oxide in many photocatalytic reactions [13], [14]. Therefore, ZnO is very effective as TiO₂ in photocatalytic degradation processes under UV radiation. Some studies have also confirmed that ZnO exhibits a better efficiency than TiO₂ in photocatalytic degradation of some dyes even in aqueous solutions [15], [16], [17]. Other advantages of ZnO have also been claimed that it had absorption capacity with larger fraction of solar spectrum than TiO₂ [18]. In some studies it is mentioned that ZnO loses its efficiency because of photocorrosion tendency when it is illuminated with UV-light after test runs more than once [19]. On the other hand, this corrosion tendency can be removed by keeping the catalyst in aqueous solutions in the dark, thereby increasing the efficiency of semiconductor photocatalyst [20]. This process is called memory effect of recovering the catalyst surface which also depends on the time interval.

In many applications, ZnO has been used in the form of fine powders suspended in water. Despite the simplicity of this technique, recovery of the ZnO powders after photocatalytic water treatment is necessary. Due to the cost of recovery operations and the possible powder loss, researchers have used other methods to immobilize ZnO powders on a substrate material [21]. High quality ZnO thin films have been prepared by many different techniques such as chemical vapor deposition (CVD) [22], physical vapor deposition (PVD) [23], molecular beam epitaxy (MBE) [24], anodizing [25], sputtering [26] and spray pyrolysis [27]. Considering the simplicity and the costs, these techniques still have disadvantages comparing with the use of powders.

In this study, ZnO powders are shaped into the self-supported plates as an alternative to both powder and costly thin film coating techniques. Tape casting is one of the ceramic processing techniques and is applied to prepare ZnO plates in relatively simple, fast, cost effective and environmentally friendly ways [28], [29], [30]. To evaluate the efficiency of the plates in the degradation of model azo dye solutions, commercial azo dyes (namely, Remazol Brilliant Orange 3R (C.I. Reactive Orange 16) and Remazol Brilliant Red F3B (C.I. Reactive Red 180)) were used.