Preparation and characterization of AgI nanoparticles with controlled size, morphology and crystal structure Yu-Guo Guo, Jong-Sook Lee ⁎ , Joachim Maier Max Planck Institute for Solid State Research, 70569 Stuttgart, Germany Received 6 September 2005; received in revised form 19 January 2006; accepted 27 February 2006 Abstract AgI nanoparticles were prepared by solution-based routes using water-soluble anionic or cationic polyelectrolytes as capping agents. Depending on the polyelectrolytes, AgI nanoparticles with well-defined morphology, size, and phase compositions were obtained: the use of poly (sodium 4-styrenesulfonate) (PSS) resulted in AgI nano-rods of β-AgI in wurtzite structure (2H); with poly(acrylic acid sodium salt) (PAS) truncated-tetrahedron shaped γ-AgI nanoparticles (nanotetrahedra) in zinc-blende structure (3C) were obtained; by employing poly (diallyldimethylammonium chloride) (PDADMAC) plate-like AgI nanoparticles (nano-plates) consisting of unusual polytype phases of AgI (7H and 9R) were formed. Macroscopically unstable γ-AgI and 7H and 9R phases could be stabilized in the form of nanocrystalline powders. They transform reversibly into the high temperature α-AgI phase and exhibit unusually high ionic conductivity and substantially smaller transformation enthalpy values compared to the macroscopic β-AgI. © 2006 Elsevier B.V. All rights reserved. Keywords: Silver iodide; Nanocrystals; Phase stability; Ionic conductivity; Transformation enthalpy 1. Introduction Solution-phase synthesis (or colloidal synthesis) of monodis- perse nanometer-scale crystals or nanocrystals by ‘capping’ the particles with functionalized long-chain organic molecules (or surfactants) enables understanding and exploiting novel finite- size effects on electronic, magnetic, optical, catalytic properties for a variety of metals and semiconductors (see e.g. Refs. [1,2]). While size-effects on ionic transport properties are now highly sought after in the emerging field of nano-ionics for a wide range of applications in sensors, fuel cells, and batteries [3–6], prepa- ration of well-defined nanocrystals and nanocrystalline ceramics of ionically conducting materials has yet to be developed. Polymers such as block copolymers [7–9] and polyelec- trolytes [10,11] have been frequently used as capping agents. Among them, water-soluble anionic and cationic polyelectro- lytes were found to be extraordinarily effective in the preparation of nanoparticles with uniform shape and size for materials such as BaCrO 4 , BaSO 4 , [12] and ZnO [10,11]. We used water- soluble anionic or cationic polyelectrolytes as capping agents in the preparation of AgI nanoparticles precipitated from silver nitrate and potassium iodide solution. Different types of monodisperse AgI nanoparticles in large scale were obtained depending on the polyelectrolyte types. The characteristics of three different types of AgI nanoparticles will be presented. 2. Experimental AgI nanocrystals were prepared from the reagents AgNO 3 (99.8%, Merck KGaA) and KI (99.8%, Merck KGaA) using double-distilled water and/or ethanol (Carl Roth, 99.8%) as solvents and the following three polyelectrolytes (Sigma- Aldrich) as capping agents: 1) Poly(sodium 4-styrenesulfonate) (PSS, Mw ∼ 70,000 g mol - 1 ), 2) poly(acrylic acid sodium salt) (PAS, Mn ∼ 5100 g mol - 1 , Mw/Mn ≈ 3), and 3) poly (diallyldimethylammonium chloride) (PDADMAC, 20 wt.% in water, Mw ∼ 100,000–200,000 g mol - 1 ). Optimized preparation routes for three nanoparticles are as follows: 1) Stir 10 ml of 0.1 M AgNO 3 and 10 ml of 0.1 M KI (one after the other) into 280 ml of a 0.12 mM PSS solution. Solid State Ionics 177 (2006) 2467 – 2471 www.elsevier.com/locate/ssi ⁎ Corresponding author. Fax: +49 711 689 1722. E-mail address: jong-sook.lee@fkf.mpg.de (J.-S. Lee). 0167-2738/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ssi.2006.02.043