Contents lists available at ScienceDirect Materials Science & Engineering B journal homepage: www.elsevier.com/locate/mseb A new approach: Synthesis of cobalt aluminate nanoparticles using tamarind fruit extract Ioana Mindru a , Dana Gingasu a, ⁎ , Luminita Patron a , Adelina Ianculescu b , Vasile-Adrian Surdu b , Daniela C. Culita a , Silviu Preda a , Constantin-Daniel Negut c , Ovidiu Oprea b a “Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Splaiul Independentei 202, 060021 Bucharest, Romania b “Politehnica” University of Bucharest, Faculty of Applied Chemistry and Materials Science, Polizu Street 1–7, 011061 Bucharest, Romania c “Horia Hulubei” National Institute of Physics and Nuclear Engineering, P.O. Box MG-06, 077125 Magurele, Ilfov, Romania ARTICLE INFO Keywords: Cobalt aluminate Green synthesis X-ray diffraction Transmission electron microscopy Pigment ABSTRACT The nanostructured cobalt aluminate (CoAl 2 O 4 ) was obtained through an eco-friendly precursor method using aqueous extract of tamarind fruit. The use of plant extracts is a simple and effective approach for the preparation of CoAl 2 O 4 nanoparticles. The polynuclear complex precursor was characterized by Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible spectroscopy (UV–VIS) and thermal analysis. The cobalt aluminate was investigated by: X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), FTIR, UV–VIS and colour measurements by reflectance spectroscopy (RS). X-ray diffraction patterns confirmed the formation of cubic CoAl 2 O 4 spinel phase with good crystallinity. The average particle size from TEM is about 71 nm. Chromatic coordinates indicate that the bluest colour was obtained for the cobalt aluminate obtained by subsequent heat treatment. 1. Introduction The cobalt aluminate spinel (CoAl 2 O 4 ) known as Thenard’s blue has received significant attention due to its unique properties such as high refractive index, chemical reactivity and good thermal stability. Cobalt aluminate can be used as pigment for ceramic and enamel glazing, paper, paints [1–3], catalyst/photocatalyst [4,5], in sensing applications [6], etc. A large number of synthesis methods, such as coprecipitation method [1,7], hydrothermal route [8,9], sol-gel method [10,11], Pe- chini method [12,13], sonochemical method [14], precursor method [6,15], combustion/microwave process [16–19], have been developed for the obtaining cobalt aluminate. Some of these chemical methods are expensive and potentially hazardous to the environment. In this perspective, the development of new strategies for the synthesis of metal/ metal oxide nanoparticles using different sources like bacteria, fungus, algae, plants and others can be considered one of the main challenges of today These routes are environmentally friendly, using nontoxic, benign reagents. Green chemistry is an efficient option because not only leads to environmental benefits, but also to economic and social benefits. The nanocrystalline CoAl 2 O 4 spinel has been syn- thesized through sol-gel method, in the presence of lactose without adding external surfactant [5] and combustion reaction using egg white (ovalbumine) as a new fuel [20]. Among the green chemistry routes, the use of plant extracts from seeds, leaves, flowers, roots or fruits is of particular interest because the plants contain and therefore release a variety of chemical constituents, such as carbohydrates, aminoacids, flavonoids, vitamins, etc. which can act as capping/reducing/chelating agents. These metabolites can co- ordinate the metal ions to form polynuclear complex precursors, which lead to nanoparticles with high stability and narrow size distribution by thermal decomposition [21]. An overview of the literature shows only a few studies on the synthesis of cobalt aluminate nanoparticles using Aloe vera leaf extract [22], sesame (Sesame indicum L) leaves extract [4] and Opuntia dilleni extract [23]. In recent work, we have described the synthesis of CoAl 2 O 4 obtained through a green chemistry approach using cinnamon bark extract. We demonstrated that “classic” and microwave combus- tion methods are straightforward processes and can be used to syn- thesize nano-sized aluminate powders with a high degree of chemical homogeneity and an optimal particle size uniformity degree [24]. Tamarind (Tamarindus indica) belongs to the Fabaceae family, sub- family Caesalpinioideae. The tamarind fruit has a sweet acidic taste due to the high contents https://doi.org/10.1016/j.mseb.2019.05.031 Received 5 March 2018; Received in revised form 22 April 2019; Accepted 31 May 2019 ⁎ Corresponding author. E-mail address: d_gingasu@yahoo.com (D. Gingasu). Materials Science & Engineering B 246 (2019) 42–48 0921-5107/ © 2019 Elsevier B.V. All rights reserved. T