Functional pigments from chromium(III) oxide nanoparticles Selvam Sangeetha a , Rizwan Basha a , Kalarical Janardhanan Sreeram a, * , Sriman Narayanan Sangilimuthu b , Balachandran Unni Nair a a Chemical Laboratory, Central Leather Research Institute, Council of Scientific and Industrial Research, Adyar, Chennai 600020, India b National Center for Nanoscience and Nanotechnology, University of Madras, Guindy Campus, Chennai 600025, India article info Article history: Received 30 January 2012 Received in revised form 15 March 2012 Accepted 18 March 2012 Available online 24 March 2012 Keywords: Nano-Cr 2 O 3 Cool pigments Environmentally benign Reflectance Near-infrared Rare earth doping abstract Pigments with properties such as weather stability, thermal resistance and having functional properties such as NIR reflectance are desirable. In this, the inorganic pigments serve better than the organic pigments. However, the release of metal ions such as Pb and Co on exposure to the environment is raising alarms. One of the approaches considered ideal is to use naturally occurring oxides or their synthetic analogs as pigments after suitable modifications. One such classic example is that of chromium(III) oxide. We report a predominant solid state route, wherein a chromiumeurea complex prepared under solvent free conditions was calcined at high temperature to obtain chromium(III) oxide nanoparticles. Intro- ducing rare earth metal ions such as La and Pr, which have in recent years been reported to be envi- ronmentally benign and available in sufficient quantities resulted in nano-oxides with enhanced near-infrared reflectance, thus qualifying them as cool colorants. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Exponential growth of nanomaterials possessing unique and enhanced properties when compared to the bulk has been reported during the last decade. Nanotechnology seems to have penetrated into almost all fields of science, with major breakthroughs reported from time to time. Cr 2 O 3 is known for its increased pigment opacity, high UV attenuation, poor deterioration and transparency to visible light. Several noteworthy applications in paints, plastic, construc- tion materials, refractories, enamels, etc. have been possible with Cr 2 O 3 [1,2]. Some of these applications are due to its high oxidative resistance and melting temperature [3]. A permanent green coloring in cement or lime-bonded construction material is reported to be achieved only by using Cr 2 O 3 [2]. By employing nano-sized chromium(III) oxide, newer or enhanced applications in surface coatings have been possible [4]. Colorant researchers are recently focusing on the development of cool pigments which have high near-infrared (NIR) solar reflec- tance to control heat buildup. A high solar reflectance from roofs, automobiles, upholstery, etc. can reduce heat gain [5,6]. Higher NIR reflectance depends on factors such as particle size, particle size distribution and refractive index of the material. Electronic structure of several metal ions is ideal for obtaining the desired refractive index [7]. Mixed metal oxides of Cr 2 O 3 with TiO 2 , Al 2 O 3 and V 2 O 5 [8] as well as some nano-crystalline oxides have been reported as NIR reflective pigments. The doping with rare earths which have right electronic structure for reflective properties has been reported recently [9e13]. Coupling this information with the toxicity reports on vanadium compounds [14] and the environ- mentally benign characteristics of rare earth elements such as low cytotoxicity [18,19], it is possible to develop rare earth doped Cr 2 O 3 nanoparticles for high NIR reflective coating. Many research groups have reported the synthesis of Cr 2 O 3 nanoparticles by complexing chromium(III) salts with urea under aqueous conditions [15e17]. Herein, we report a green synthesis protocol for nano-Cr 2 O 3 utilizing a solvent free approach employ- ing urea decomposition strategy. The nano-Cr 2 O 3 thus obtained has been characterized using XRD and SEM and has been effectively modified to function as a NIR reflective cool green colorant by doping with rare earth elements for surface coating applications. 2. Materials and methods 2.1. Materials Chromium nitrate nonahydrate and praseodymium nitrate hexahydrate were purchased from SigmaeAldrich, urea was * Corresponding author. Tel.: þ91 44 24411630; fax: þ91 44 24911589. E-mail address: kjsreeram@clri.res.in (K.J. Sreeram). Contents lists available at SciVerse ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2012.03.019 Dyes and Pigments 94 (2012) 548e552