25172 | Phys. Chem. Chem. Phys., 2015, 17, 25172--25181 This journal is © the Owner Societies 2015 Cite this: Phys. Chem. Chem. Phys., 2015, 17, 25172 Highly efficient photocatalytic degradation of organic dyes by Cu doped ZnO nanostructures Sini Kuriakose, a Biswarup Satpati b and Satyabrata Mohapatra* a Copper doped ZnO nanostructures have been synthesized by a facile wet chemical method. Structural properties of as-synthesized nanomaterials have been studied by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) with energy dispersive X-ray spectroscopy, while UV-visible absorption spectroscopy and Raman spectroscopy have been used to study their optical properties. Sunlight driven photocatalytic degradation of methylene blue (MB) and methyl orange (MO) dyes in water was used to evaluate the photocatalytic activities of Cu doped ZnO nanostructures using UV-visible absorption spectroscopy. The results showed that there is an optimum Cu doping level which leads to the highly enhanced photocatalytic activity of Cu doped ZnO nanostructures, as compared to pure ZnO nanostructures. A mechanism for the enhanced photocatalytic activity of Cu–ZnO nanostructures is tentatively proposed. The enhanced photocatalytic activity of Cu–ZnO nanostructures is attributed to the combined effects of improved separation of photogenerated charge carriers due to optimal Cu doping in ZnO nanostructures and the formation of ZnO–CuO nanoheterojunctions. 1. Introduction Waste water containing hazardous organic pollutants from various industries causes severe environmental pollution. The organic dyes in the effluents coming from the industries are difficult to degrade due to their stability and chemical structure. One of the top priorities today is to find an efficient solution for environ- mental remediation. 1 Photocatalytic degradation of organic dyes and pollutants is one of the most efficient and low cost methods used for this purpose. Photocatalysis provides a promising technique for the removal of toxic organic pollutants from industrial waste water and may solve some of the major environmental issues originating from the contaminated water. 2–16 Semiconductor photocatalysis has emerged as a viable technology due to various advantages including low cost, low energy consumption, non-toxicity and the ability to utilize UV and/or visible light effectively. 17 ZnO is an n-type wide band gap semiconductor with a large excitonic binding energy, which finds a wide range of technological applications. ZnO nanostructures such as nano- flowers, 18 nanodisks, 19 nanospindles, 20 nanoplants 21 and nanocages 22 have been synthesised by different routes and have found applications in diverse fields such as gas sensors, 23,24 solar cells, 25,26 photodetection, 27 medicine 28 and photocatalytic degradation of organic dyes. 8,17–20,29–31 ZnO has been widely used as a photocatalyst for UV light or sunlight driven photo- catalytic degradation of organic dyes due to its low cost, efficient UV light utilization capability and non-toxicity. 32–34 However, the use of ZnO as a photocatalyst for commercial purpose is limited due to its drawbacks such as quick recom- bination of photogenerated charge carriers and low stability due to photocorrosion and inefficient utilization of sunlight particularly in the visible region. To overcome these drawbacks, various strategies viz. decoration with noble metals, making nanocomposites with other semiconductors and doping with metals have been developed. Doping is an efficient technique to enhance the photogenerated charge separation in ZnO and thus improves its photocatalytic efficiency. 35,36 Many transition metals such as Co, 21 Cu, 37 and Mn 38 have been used to enhance the photocatalytic activity of ZnO. Among these Cu is consid- ered the most efficient one for improving the photocatalytic efficiency of ZnO. Doping of Cu into ZnO leads to significant changes in the electrical, optical and magnetic properties of ZnO. 39–42 Cu doping into ZnO creates defects in the lattice, which increases the optical absorption in the visible region and hence improves the photocatalytic activity. Pawar et al. 43 reported the synthesis of Cu doped ZnO microstructures and showed that 7% Cu doped ZnO takes 50 minutes for photo- catalytic degradation of methylene blue (MB) and rhodamine B (RhB) dyes. The enhanced photocatalytic activity was attributed to the existence of (001) polar surfaces, oxygen vacancies and increased absorption of visible light. Mohan et al. 44 synthesized Cu doped ZnO nanorods through the vapour transport method, studied UV light driven photocatalytic degradation of the resazurin a School of Basic and Applied Sciences, Guru Gobind Singh Indraprastha University, Dwarka, New Delhi 110078, India. E-mail: smiuac@gmail.com b Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata 700064, India Received 23rd March 2015, Accepted 27th August 2015 DOI: 10.1039/c5cp01681a www.rsc.org/pccp PCCP PAPER Published on 02 September 2015. Downloaded by Guru Gobind Singh Indraprastha University on 09/10/2015 08:51:48. View Article Online View Journal | View Issue