J. of Supercritical Fluids 87 (2014) 111–117 Contents lists available at ScienceDirect The Journal of Supercritical Fluids j our na l ho me page: www.elsevier.com/locate/supflu Coupling in situ synchrotron radiation with ex situ spectroscopy characterizations to study the formation of Ba 1-x Sr x TiO 3 nanoparticles in supercritical fluids Gilles Philippot a , Kirsten M.Ø. Jensen b , Mogens Christensen b , Catherine Elissalde a , Mario Maglione a , Bo B. Iversen b,∗∗ , Cyril Aymonier a,∗ a CNRS, Univ. Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France b Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Aarhus, Denmark a r t i c l e i n f o Article history: Received 21 October 2013 Received in revised form 16 December 2013 Accepted 16 December 2013 Keywords: Barium strontium titanate Supercritical fluids In situ Synchrotron WAXS Growth mechanism a b s t r a c t High quality barium strontium titanate (Ba x Sr 1 - x TiO 3 with 0 ≤ x ≤ 1–BST) nanoparticles can be syn- thesized using supercritical fluids technology. Well crystallized particles of 20 nm with a narrow size distribution were produced in a single step. The reaction is achieved at relatively low temperature (T < 400 ◦ C) and in tens of seconds. The combination of in situ synchrotron wide angle X-ray scatter- ing (WAXS) and ex situ analyses in the form of Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and high resolution trans- mission electron microscopy (HR-TEM) leads to an understanding of the influence of the substitution of barium cations with strontium ones on the BST nanoparticle growth. A correlation between particle size, density of –OH groups at the surface of the particles and BST composition is exhibited; the higher the –OH density and the lower the strontium concentration, the larger the particles. This confirms that the formation of BST nanoparticles in supercritical fluids is governed by a sol–gel mechanism. © 2014 Elsevier B.V. All rights reserved. 1. Introduction For several years, perovskite barium strontium titanate (BST) ceramics have attracted much attention due to their promis- ing properties. In addition to be a lead free material, it shows very interesting ferroelectric, piezoelectric, pyroelectric and dielec- tric properties which make it usable in common microelectronic devices like capacitors, dynamic random access memory, sensors and waveguides [1–4]. It can also be used as a photocatalyst [5] or in high k-hybrids nanocomposites for printed electronics [6]. Numerous synthesis routes have been developed to produce bar- ium titanate (BT); the oldest one being the solid state reaction of ball milled BaCO 3 (or BaO) and TiO 2 powders calcined several hours at high temperatures (>1000 ◦ C). The main drawbacks of this method are the high synthesis temperature, long synthesis dura- tion and the limited homogeneity/purity of the produced phase [1,3]. To overcome those issues some versatile approaches were ∗ Corresponding author. ICMCB-CNRS, 87 avenue du Dr Albert Schweitzer, 33608 Pessac Cedex, France. Tel.: +33540002672. ∗∗ Corresponding author. Center for Materials Crystallography, Department of Chemistry and iNANO, Aarhus University, Denmark. Tel.: +4587155982. E-mail addresses: bo@chem.au.dk (B.B. Iversen), aymonier@icmcb-bordeaux.cnrs.fr (C. Aymonier). developed as the coating of BaCO 3 crystals with an amorphous layer of titania enabling a solid state reaction at lower temperature (600–650 ◦ C). This led to formation of tetragonal BT nanocrystals of 100–200 nm with a narrow size distribution [7]. Another well- known synthesis method is the sol–gel process. This goes through a two-step reaction mechanism, where hydrolysis/condensation of the precursors are followed by an annealing step at 700–1000 ◦ C to produce nanocrystals [2,8–11]. Compared to the solid state method, the product is much more homogeneous, but the synthesis tem- perature and duration is still problematic. The coprecipitation (or oxalate) process can also be used. It consists in mixing barium and titanium salts and adjusting the pH value to achieve a simultaneous precipitation of the metal cations, followed by high temperature calcination. However, it can be difficult to reach the experimental conditions to get simultaneous precipitation of the species, and this often leads to poor stoichiometry of the final material [2,12–14]. A similar method based on BaCl 2 and TiCl 4 precursors has been optimized allowing the production of stoichiometric BT at low temperature (<100 ◦ C), however the synthesis duration is an issue [15,16]. Other possible techniques for BT production are the poly- meric precursor method and the mechanochemical synthesis, but they are both time and energy consuming [2]. A very promising method is the synthesis of not only BT but also the whole BST solid solution in supercritical fluids. This scalable technology allows a continuous production of metal oxides nanoparticles in a single 0896-8446/$ – see front matter © 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.supflu.2013.12.009