Journal of Alloys and Compounds 509 (2011) 8378–8381 Contents lists available at ScienceDirect Journal of Alloys and Compounds j our na l ho me p ag e: www.elsevier.com/locate/jallcom Effect of Co substitution on the structural and optical properties of ZnO nanoparticles synthesized by sol–gel route Mohd Arshad a , Ameer Azam a,b,∗ , Arham S. Ahmed a , S. Mollah c , Alim H. Naqvi a a Centre of Excellence in Materials Science (Nanomaterials), Department of Applied Physics, Aligarh Muslim University, Aligarh, India b Centre of Nanotechnology, King Abdul Aziz University, Jeddah, Saudi Arabia c Department of Physics, Aligarh Muslim University, Aligarh, India a r t i c l e i n f o Article history: Received 19 April 2011 Received in revised form 8 May 2011 Accepted 12 May 2011 Available online 7 June 2011 Keywords: ZnO Sol–gel XRD TEM EDAX FTIR a b s t r a c t Co doped ZnO nanoparticles were synthesized by sol–gel method and characterized by X-ray diffraction (XRD), Transmission electron microscopy (TEM), Energy dispersive X-ray analysis (EDAX), UV–Visible absorption spectroscopy and Fourier transform infrared spectroscopy (FTIR). XRD analysis revealed the formation of single phase structure of all samples which was further supported by FTIR data. With the increase in Co concentration from 0% to 5%, crystallite size was observed to vary from 27.1 to 21.3 nm. It suggests the prevention of crystal growth as a result of Co doping in ZnO. It was also evident from the absorption spectra that the absorbance tends to increase with the increase in dopant concentration. Optical band gap was found to increase slightly with the increase in Co content, confirming the size reduction as a result of Co doping. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Nanoscale materials are of great interest now-a-days for fun- damental as well as applied research point of view because the properties of the materials change drastically when the particle size reaches to nanometer range. The optical properties of nanocrys- talline semiconductors have been studied extensively in recent years. As the size of the material becomes smaller, the band gap becomes larger thereby changing the optical and electrical prop- erties of the material and making the material suitable for new applications and devices. Zinc oxide (ZnO) is an excellent n-type semiconductor with a wide band gap of 3.37 eV and a large exciton binding energy of 60 meV [1,2]. For these reasons, ZnO is used in a wide variety of applications, including opto-electronic devices [3–6], cataly- sis [7], light-emitting diodes (LEDs) [8], thermoelectric devices [9], varistors [10,11], flat panel displays [11] and surface acoustic wave devices [12]. Recent theoretical predictions [13–18] proposed transition metal (TM)-doped ZnO as one of the most promis- ing candidates for room-temperature ferromagnetism (RTFM). Additionally, the excellent optical transparency of ZnO and the ∗ Corresponding author at: Department of Physics, Aligarh Muslim University, Aligarh, India. E-mail addresses: azam222@rediffmail.com, ameerazam2009@gmail.com (A. Azam). possibility of band gap engineering through transition metal (TM) doping strongly encourages the exploration of the magneto-optical properties of the TM-doped ZnO system [19,20] which might lead to the development of novel magneto-optic electronic devices [21,22,19]. ZnO nanoparticles also have a variety of applications such as UV absorption, deodorization and antibacterial treatment [23–25]. Several methods are reported in literature for the synthesis of doped and undoped ZnO nanoparticles which can be categorized into either chemical or physical methods [26,27]. The chemical methods comprise thermal hydrolysis technique [28], hydrother- mal processing [29] and sol–gel method [30–32] while the Physical are vapor condensation method [33], spray pyrolysis [34–36] and thermo-chemical/flame decomposition of metal-organic pre- cursors [37,38]. Sol–gel technique is being extensively used for synthesis of advanced ceramics, production of nanocrystalline materials and for metallurgical treatment of ores and minerals to yield value-added materials. Sol–gel method is easy to produce relatively large quantities of nanoparticles at low cost. Most of the peoples are working over the magnetic proper- ties of the Transition Metal (TM) doped ZnO, like diluted magnetic semiconducting behavior, i.e. room temperature ferromagnetism (FM). Fukumura et al. [39] have reported that Mn doped ZnO show spin-glass behaviors having T C = 13 K and also imply a strong ferro- magnetic interaction. Jung et al. [40] reported Mn-doped ZnO films clearly showed ferromagnetic ordering and the T C as 30 and 45 K for the Zn 1-x Mn x O films with x = 0.1 and 0.3, respectively. Schwartz 0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2011.05.047