CERAMICS INTERNATIONAL Available online at www.sciencedirect.com Ceramics International 40 (2014) 6561–6568 Phase tuning of zirconia nanocrystals by varying the surfactant and alkaline mineralizer A. Uma Maheswari a,n , Sreedevi R. Mohan a , S. Saravana Kumar b , M. Sivakumar a a Department of Sciences, Amrita School of Engineering, Amrita Vishwa Vidyapeetham, Amritanagar, Coimbatore 641 112, India b Department of Physics, NSS College, Pandalam, Pathanamthitta District 689 501, India Received 17 August 2013; received in revised form 22 November 2013; accepted 22 November 2013 Available online 8 December 2013 Abstract The influence of cationic (CTAB)/neutral polymeric (PVP) surfactants and strong (NaOH)/weak (NH 4 OH) alkaline mineralizers on phase stabilization of zirconia nanocrystals synthesized by chemical precipitation is investigated. X-ray diffraction and micro-Raman analysis of the as- prepared samples show that tetragonal zirconia is predominant as compared to monoclinic using PVP with NH 4 OH. The phases are also evident from lattice fringes of TEM images and the corresponding SAED pattern. Photoluminescence spectra of samples reveal oxygen vacancies present in the zirconia nanocrystals. The group H Raman vibration modes identified are attributed to surface defects and quantum size effects of nanocrystals. The phase stabilization of zirconia nanocrystals is explained using the polymerization rate of tetramers during synthesis. The rate can be varied by proper selection of the surfactant and the mineralizer. A slow polymerization rate with PVP and NH 4 OH favors the formation of tetragonal zirconia. Thus, a simple method for phase stabilization of zirconia nanocrystals at room temperature using chemical precipitation by varying the surfactant and the mineralizer is demonstrated. & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: Chemical precipitation; Tetragonal Zirconia; Phase stabilization; Cationic and Neutral Polymeric Surfactants; Mineralizer 1. Introduction Recently there has been a renewed interest in exploring phase dependent properties of zirconia nanocrystals due to their technological importance. Generally zirconia exists in three temperature dependent polymorphs namely thermodynamically stable monoclinic (below 1170 1C), metastable tetragonal (between 1170 and 2370 1C) and cubic (above 2370 1C) [1]. Among these, tetragonal zirconia has high strength and fracture toughness which makes it suitable for the fabrication of structural ceramics [2] and functional materials [3]. Its excellent mechan- ical properties are due to stress induced martensitic tetragonal-to- monoclinic transformation which results in toughening [4]. In addition, tetragonal zirconia has high chemical and dimensional stability and hence used as a ceramic biomaterial for dental restorations [5]. Tetragonal zirconia coatings enhance stainless steel's resistance to corrosion and wear [6]. It is also used in thermal barrier coatings of gas turbine parts owing to its low thermal conductivity/high thermal expansion coefficient when compared to monoclinic zirconia [1]. Thin films of tetragonal zirconia are used as gate for microelectronic devices due to their high dielectric constant [7]. The ionic conductivity of tetragonal/ cubic zirconia is high due to the presence of oxygen vacancies at grain boundaries and hence they are used as solid electrolytes for oxygen sensors [8] and oxide fuel cells [9]. Zirconia modified with sulphate anions is well suited for catalytic reactions like hydrogenation and isomerisation [10]. The presence of acidic, basic hydroxyl groups and coordinatively unsaturated Lewis acidic-base Zr 4 þ O 2  pairs can enhance its phase dependent catalytic reactions. For instance, sulfated tetragonal zirconia can act as an active catalyst for n-butane isomerization [11] whereas monoclinic zirconia is used for selective hydrogenation of CO 2 to produce methanol [12]. Zirconia nanocrystals exhibit phase dependent luminescence and used for the fabrication of eco- friendly photonic systems [13]. www.elsevier.com/locate/ceramint 0272-8842/$ - see front matter & 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2013.11.109 n Corresponding author. Tel.: +91 422 2685000×5617; fax: +91 422 2656274. E-mail address: a_umamaheswari@cb.amrita.edu (A. Uma Maheswari).