1. Introduction Room temperature ionic liquids (ILs) are a class of organic salts that are liquids at or near room temperature in their pure state, which exhibit interesting properties like negligible vapour pressure, high ionic conductivity, good dissolving ability, high thermal and chemical stability, wide range of viscosities, large electrochemical window, non-fammability, etc. Some of these properties make them potential green solvents or reaction media or templates for many applications, like inorganic synthesis of different metal oxide nanostructures [1-4]. On the other hand, ZnO is a wide band gap semiconductor oxide (around 3.3 eV), with a large excitation binding energy (60 meV), which became one of the most studied electronic and photonic materials [5,6]. Nonlinear optical properties have also been reported for ZnO thin flms [7]. ZnO plays an important role as part of various composites [8-11]. In addition, ZnO is one of the most tested nanomaterials in photocatalysis, especially when obtained as nano-size particles [12-15]. Recently, efforts have been made for the synthesis of ZnO nanoparticles with controlled morphologies and improved properties [16-21]. The literature indicates that different ZnO nanostructures with different one dimensional morphologies (from nanoparticles to nanorods to nanowires) can be successfully synthesized in ionic liquids based on imidazolium at low temperature [22-28]. In the present work we have investigated whether adding an organic solvent to the ionic liquid, and using Central European Journal of Chemistry Synthesis and characterization of ZnO nanostructures obtained in mixtures of ionic liquids with organic solvents + * E-mail: ciocirlan_o@yahoo.com + The article has been presented at the 18th Romanian International Conference on Chemistry and Chemical Engineering - RICCCE18 - held in New Montana, Sinaia, Romania on 4-7 September, 2013. Received 6 October 2013; Accepted 25 November 2013 Abstract: © Versita Sp. z o.o. Keywords: Zinc oxide • Ionic liquids • Ethylene glycol • Dimethyl sulfoxide 1 Department of Inorganic Chemistry, Physical Chemistry and Electrochemistry, Politehnica University of Bucharest, 011061 Bucharest, Romania 2 SIMONa Department, University Politehnica Bucharest, P.O. Box 12-134, Bucharest, Romania 3 METAV-CD, 020015 Bucharest, Romania Ovidiu Oprea 1 , Oana Ciocirlan 1* , Alina Badanoiu 2 , Eugeniu Vasile 3 RICCCE 18 ZnO nanoparticles were synthesized in mixtures of ionic liquids based on imidazolium cation with organic solvents (dimethyl sulfoxide and ethylene glycol) by a simple, one-step solution route at low temperature. The effect of these mixtures on the morphology, size and properties of as obtained ZnO nanopowders was investigated. The obtained nanopowders have been characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), UV-Vis absorption spectroscopy (UV-Vis) and photoluminescence (PL). The effect of the ionic liquid mixture on the photocatalytic degradation of methylene blue has been analysed. The XRD studies confrmed the hexagonal wurtzite structure of the obtained ZnO powder. The UV-Vis absorption spectra present the typical shape for ZnO, with a broad band situated in the UV region, with the maximum around 360 nm. The calculated band-gap energy is in interval 3.25-3.28 eV. The synthesized ZnO nanopowders have high photocatalytic activity against methylene blue, the best results being obtained when 1-ethyl-3- methylimidazolium tetrafuoroborate was used as the solvent. Cent. Eur. J. Chem. • 12(7) • 2014 • 749–756 DOI: 10.2478/s11532-014-0507-5 749