Synthesis and optical properties of colloidal CuO nanoparticles Sawsan Dagher a , Yousef Haik a , Ahmad I. Ayesh b,n , Nacir Tit b a Department of Mechanical Engineering, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates b Department of Physics, UAE University, P.O. Box 15551, Al-Ain, United Arab Emirates article info Article history: Received 7 March 2013 Received in revised form 8 January 2014 Accepted 7 February 2014 Available online 22 February 2014 Keywords: Optical properties of nanoparticles Photoluminescence CuO nanoparticles Excitation-dependent fluorescence shift abstract Copper oxide nanoparticles of sizes ranging from 1 to 25 nm were synthesized using a colloid microwave-thermal method, where the average size of CuO nanoparticles can be tailored by controlled microwave treatment time. The particle size was found to significantly decrease as the microwave processing time increases and can be controlled to have narrow size distributions. Both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the sizes of the prepared nanoparticles. The UV–visible absorption spectra of the nanoparticles are blue-shifted with the size reduction, and this is attributed to quantum-confinement (QC) effect. Furthermore, the photo- luminescence spectra showed UV and visible emissions, and were red shifted with increasing particles size and excitation wavelength. While the former observation confirms the QC effect and corroborates the results of UV–visible absorption spectra, the latter one is attributed to selective near band-edge excitonic transitions associated with defect states. & 2014 Elsevier B.V. All rights reserved. 1. Introduction CuO is an attractive p-type metal oxide semiconductor that has unique electrical, optical and catalytic properties [1]. CuO is widely used in electrochemical cells [2], gas sensors [3–5], magnetic storage media [6,7], photovoltaic cells [8], light emitters [9], thermoelectric materials [10,11], heat transfer nanofluids and for catalysis [12–14]. Using CuO nanoparticles with narrow size distribution for these applications would further promote the chemical reactivity of the nanoparticles because as the particle's size reduces the surface-to- volume ratio increases, and consequently the number of reactive sites increases [15–19]. Thus, CuO nanoparticles exhibit improved electronic and optical properties compared to their bulk equiva- lent [20–22]. As a result, many methods have been developed to synthesize CuO nanoparticles with different sizes as shown in Table 1. Microwave radiation generates heat by interacting with the solution molecules and causing friction between them due to the reorientation of their electric dipoles [37]. This leads to an increase in the reaction kinetics, by one to two orders of magnitude, and efficient volumetric heating compared to conventional heating. In addition, microwave-thermal method has many advantages such as: low synthesis temperatures, reduced reaction time, small particle size, narrow particle size distribution, high purity, low power consumption, and environment friendly [38–40]. In this paper CuO nanoparticles were synthesized using the colloid microwave-thermal process that allows the formation of nano- particles with an excellent size control. In addition, the variation of the optical behavior as a function of CuO nanoparticles' size has been investigated in this work. 2. Experimental CuO nanoparticle colloids were synthesized in dimethylformamide (CH 3 ) 2 NC(O)H (DMF) with purity of 99.0% (Sigma-Aldrich) using a microwave reactor (CEM, Discover-SP system, 909156, USA). The procedure is described as follows: 60 mg of copper(II) acetate mono- hydrate (Cu(CH 3 COO) 2 Á H 2 O) with purity of 99.8% (Sigma-Aldrich), was dissolved in 50 ml of DMF under rigorous magnetic stirring for about 3 h at room temperature. The produced solution was aged for 24 h before exposing it to microwave radiations at 45 1C with full power of 300 W and operating frequency of 2455 MHz. The solution was kept under rigorous magnetic stirring repeatedly to complete the reaction and the nucleation process. The working cycle of the microwave reactor was set as 30 s on and 15 s off. In order to monitor the particles size, the on/off heating procedure was repeated 36 times and the average particles size measurement was performed every 30 s. The particles size was found, in general, to decrease by increasing the reaction time. CuO nanoparticle formation can be described according to the following reaction [41,42]: Cu(CH 3 COO) 2 Á H 2 O þ (CH 3 ) 2 NC(O)H-CuO þ CH 3 COO 2 CHN (CH 3 ) 2 þ H 2 O Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jlumin Journal of Luminescence http://dx.doi.org/10.1016/j.jlumin.2014.02.015 0022-2313 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. E-mail address: ayesh@uaeu.ac.ae (A.I. Ayesh). Journal of Luminescence 151 (2014) 149–154