Synthesis and characterization of new environmentally benign tantalum-doped Ce 0.8 Zr 0.2 O 2 yellow pigments: Applications in coloring of plastics V.S. Vishnu, Giable George, V. Divya, M.L.P. Reddy * National Institute for Interdisciplinary Science and Technology (NIIST), CSIR, Thiruvananthapuram 695 019, Kerala, India article info Article history: Received 28 October 2008 Received in revised form 1 November 2008 Accepted 3 November 2008 Available online 12 November 2008 Keywords: Novel ecological yellow pigments CeO 2 –ZrO 2 –Ta 2 O 5 Solid solution Coloring of plastics abstract New inorganic pigments having the general formula Ce 1(xþy) Zr x Ta y O 2þd (x ranges from 0.15 to 0.2 and y ranges from 0 to 0.05) displaying colors ranging from white to yellow have been synthesized by a traditional solid-state route, as viable alternatives to lead, cadmium and chromium based yellow toxic inorganic colorants. The products were characterized by X-ray powder diffraction, UV–vis diffuse reflectance spectroscopy and CIE-L*a*b* 1976 color scales. The coloring mechanism is based on the strong absorptions of the pigments in the visible region under 500 nm, which could originate from the addi- tional energy level between O 2p valence band and the Ce 4f conduction band by forming a hybrid orbital of Ta 5d and O 2p . The designed yellow pigments consist of non-toxic elements and further found to be thermally and chemically stable. The pigments were also found to be suitable candidates for the color- ation of plastics. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction In recent years there is a great demand for the development of new yellow colored inorganic materials to substitute for industrial pigments that are based on heavy metals hazardous to health and environment [1]. Inorganic pigments are widely used in various applications such as paints, inks, plastics, rubbers, ceramics, enamels and glasses [2]. Although PbCrO 4 , PbMoO 4 , Pb 2 Sb 2 O 7 and CdS have been widely employed as conventional yellow inorganic pigments, the use of these pigments has been restricted because they contain toxic elements [3]. Praseodymium yellow (ZrSiO 4 :Pr) has been known as one of the environmentally benign yellow pigments, which has been applied for many surface coating applications [4–6]. However, this pigment requires high temperature calcinations during preparation, which tends to induce particle growth of the pigment. Hence it is difficult to apply the praseodymium yellow to paints and inks in which fine dispersions of the pigment are essential. Recently, several rare earth-based yellow inorganic pigments have been proposed by many researchers including our group as viable alternatives to the existing traditional toxic pigments [7–10]. Among several non-toxic yellow pigments, CeO 2 and related materials have attracted much attention because of their opacity, low toxicity and high thermal stability [11–14]. The coloring mechanism is based on the charge-transfer band from O 2p to Ce 4f in the semi-conducting CeO 2 . The band gap between the anionic O 2p valence band and the cationic Ce 4f conduction band can be modi- fied by the formation of solid solutions, which introduces an additional electronic level between the valence and conduction bands. As a result a shift of the charge-transfer band is observed. Crystalline cerium molybdenum oxides have been reported as novel yellow pigments as alternatives to lead, cadmium and chro- mium based toxic pigments [11]. The reflectance spectrum of the cerium double molybdates indicates strong absorption in both visible and ultraviolet regions, which could originate from the O 2p –Ce 4f and the O 2p –Mo 3d double charge transitions and as a result the pigments show yellow color. Amorphous cerium tungstate, Ce 1x M x W 2 O 8 (M ¼ Zr or Ti, 0  x  0.6) has been reported as a possible ecological inorganic yellow pigment. This pigment exhibits brilliant yellow color due the effective absorp- tions in the visible and ultraviolet regions (under 500 nm effi- ciently), which is originated from the O 2p –Ce 4f and the O 2p –W 5d double charge transitions [12]. Earlier investigations reveal that the classical toxic inorganic pigments can be replaced by solid solutions of perovskites CaTaO 2 N and LaTaON 2 , which gives colors ranging from yellow to deep red [9]. Although these pigments are non-toxic and show excellent color hue, it is necessary to heat the starting materials in a flow of toxic and inflammable ammonia gas for a long time (20–60 h) to synthesize them. Therefore, research needs to be performed in developing novel yellow inorganic pigments with various advantages over traditional toxic pigment formulations. Thus, the present paper is focused on the development of a novel class of yellow pigments based on tantalum-doped Ce 0.8 Zr 0.2 O 2 systems as alternatives to the existing toxic inorganic pigments. The new pigments of the general formula Ce 1(xþy) Zr x Ta y O 2þd (x ranges * Corresponding author. Tel.: þ91 471 2515360; fax: þ91 471 2491712. E-mail address: mlpreddy55@gmail.com (M.L.P. Reddy). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.dyepig.2008.11.001 Dyes and Pigments 82 (2009) 53–57