UV–Visible light induced photocatalytic studies of Cu doped ZnO nanoparticles prepared by co-precipitation method

Highlights

• Nano-sized undoped and Cu (1.0–5.0%) doped ZnO has been synthesized by co-precipitation route.

• PLE and PL spectra for doped ZnO NPs show large excitation and emission in visible region in comparison to undoped NPs.

• Degradation of Crystal Violet dye with Cu doping and at various pH is discussed in detail.

• Synthesized NPs at higher pH show remarkable modification in photocatalytic studies.

• Synthesized NPs can be use to degrade dyes of variable pH without adjusting pH of dye solution.

Abstract

Undoped and Copper (Cu) doped zinc oxide (ZnO) (Zn_1−xCu_xO, x = 0.01, 0.02, 0.03, 0.04 and 0.05) nanoparticles (NPs) capped with (1.0%) Thioglycerol (TG) have been successfully synthesized by co-precipitation method. The synthesized samples have been characterized by X-ray diffraction (XRD), Energy dispersive spectroscopy (EDS), Transmission electron microscopy (TEM), Photoluminescence (PL) and UV–Visible spectroscopy. Incorporation of impurity ions in pure ZnO NPs leads to a shift the spectra for both excitation and emission to visible region as compared to pure ZnO. The photocatalytic activity of undoped and Cu doped ZnO photocatalyst were investigated by the degradation of Crystal Violet (CV) dye in aqueous medium under exposure of UV–Visible light irradiations. Firstly, degradation of CV dye has been studied with TG capped, undoped and Cu doped ZnO NPs synthesized at pH-8.0. From the results, it has been found that Cu doping concentration of 3.0% is optimal for higher photophysical and photocatalytic properties. When pH of optimum doped NPs was varied from 8.0 to 10.0, 78.7% to 96.5% of CV dye has been degraded after 3.5 h. Further increasing the pH to 12.0 for optimum doped NPs, 100% dye degrades in 2.5 h. This shows that the doping and pH has a pronounced effect on the photocatalytic activity of ZnO NPs. Loading of catalyst shows maximum photodegradation of CV dye at 3.0 g/L of Cu (3.0%) doped ZnO at pH-12.0 for 10 mg/L of CV dye. Kinetic studies shows that photo degradation of CV follow a pseudo first-order kinetic law. The reason for enhanced degradation with Cu doped NPs in comparison to undoped NPs has been described and discussed in this work.

Introduction

The residual dyes from different sources like textile industries, pharmaceutical industries, bleaching industries, dyeing, paper and pulp industries etc. introduces a variety of organic pollutants into the natural resources of water. These pollutants generally contain hazardous organic compounds which can cause severe environmental problems. The discharge of dye containing effluents into the environmental water is undesirable because these colored dyes and their breakdown products are toxic in nature and even may cause cancer or mutagenic to life (Zaharia et al., 2009). These dyes remain in the environment for long time without adequate treatment. Both physical and chemical processes such as precipitation, flocculation, adsorption, ultra-filtration and reverse osmosis are applied to remove these toxic substances generated from dyes in wastewater. However, these techniques are non-destructive processes in which the toxic substances are only removed which in due course of time gets transformed to other toxic substances (Stock et al., 2000). Recently, photocatalytic technique has attracted more attention because it is able to oxidize low concentrations of organic pollutants into non toxic products (Kislov et al., 2009). Photocatalysis utilizes semiconductor photocatalysts to carry out a photo-induced oxidation process to break down organic contaminants and inactivate bacteria and viruses (Chandran et al., 2014, Rehman et al., 2009).

Till now, many kinds of semiconductors have been studied as photocatalysts including TiO₂, ZnO, ZnS, WO₃, CdS etc. (Parida and Parija, 2006, Sharma et al., 2012, Bhosale et al., 2014). TiO₂ is the most widely used effective photocatalyst for its high efficiency, non-toxic nature, photochemical stability and low cost. However, ZnO, a kind of semiconductor that has the similar band gap as TiO₂, is not thoroughly investigated. Since the contaminant molecules need to be adsorbed on the surface of photocatalyst before the reaction takes place, the surface area and crystal defects plays a significant role in the photocatalytic activity. Doping of metal oxide with transition metal ions increases the crystal defects and also affects the optical properties by shifting the optical absorption toward the visible region (Wang et al., 2013). Silver is one of the metal that influences the photocatalytic activity of ZnO particles by trapping the photoinduced charge carriers and inhibiting the charge recombination process (Zheng et al., 2007). As silver is very expensive metal so doping with copper (Cu) in ZnO is an alternative to modify absorption and emission in visible region. Cu can enter into the ZnO lattice substitutionally as deep acceptors in combination with neighboring oxygen (O) vacancy (Kanai, 1991). To the best of our knowledge, in most of the studies, pH of the dye solution has been varied by the addition of bases and acids to make it anionic or cationic without varying the pH of as synthesized semiconductor photocatalyst (Ji et al., 2009, Kong et al., 2010). In the present work, nanosized undoped and copper (Cu) doped with TG capped ZnO nanoparticles (NPs) were prepared by co-precipitation route at varying pH. These synthesized NPs were used as a catalyst to examine the photodegradation studies of crystal violet dye as a model of organic compound under UV–Visible irradiations. The effect of doping percentage, capping, pH and loading of catalyst has been investigated on photocatalytic degradation of CV dye. The efficiency of photocatalytic process was investigated in the double distilled water containing CV dye. The effect of pH and doping percentage of as prepared photocatalyst has been studied in detail. The results confirm that Cu doped and capped ZnO NPs synthesized at higher pH shows enhanced UV–Visible light induced degradation of CV dye. Also Cu doped NPs shows better catalytic efficiency than undoped counterparts which are contradictory to the others who have reported that Cu and Mn doped ZnO NPs show nearly same or lesser photocatalytic activity as compared to undoped ZnO NPs (Milenova et al., 2013, Donkova et al., 2010).