J. of Supercritical Fluids 34 (2005) 91–97 Nanoparticle formation in rapid expansion of water-in-supercritical carbon dioxide microemulsion into liquid solution Mohammed J. Meziani a , Pankaj Pathak a , Frank Beacham a , Lawrence F. Allard b , Ya-Ping Sun a,∗ a Department of Chemistry, Howard L. Hunter Chemistry Laboratory, Clemson University, Clemson, South Carolina, SC 29634-0973, USA b High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee, TN 37831-6062, USA Received 17 April 2004; received in revised form 2 September 2004; accepted 16 October 2004 Abstract Silver nanoparticles were produced in the Rapid Expansion of a Supercritical Solution into a Liquid SOLVent (RESOLV). Perfluorinated surfactant-stabilized water-in-CO 2 microemulsion was used to dissolve silver salt for the rapid expansion. It was found that well-behaved nanocrystalline silver particles could be obtained when the reductive solution at the receiving end of the rapid expansion was adjusted to be highly basic. The results suggest that the pre-expansion reverse micelles in the microemulsion might be playing a significant role in the formation of the nanoparticles and that such a role could be affected by the basicity of the receiving solution in RESOLV. The apparent correlation between the parameters of the pre-expansion microemulsion and the properties of the produced nanoparticles is discussed. © 2004 Elsevier B.V. All rights reserved. Keywords: Silver nanoparticles; Water-in-CO 2 microemulsion; RESOLV 1. Introduction Supercritical fluid processing techniques, especially those based on the Rapid Expansion of Supercritical Solution (RESS), have been widely investigated for the production of fine inorganic and organic particles [1–5]. In traditional RESS, the solute is dissolved in a supercritical fluid to form a solution, followed by the rapid expansion of the solution across an orifice or a capillary nozzle into ambient air [6–9]. The high degree of supersaturation accompanying the rapid pressure reduction in the expansion results in homogenous nucleation and thereby the formation of well-dispersed parti- cles [5–8]. We have made a simple but significant modifica- tion to the traditional RESS process by expanding the super- critical solution into a liquid solvent instead of ambient air, or the Rapid Expansion of a Supercritical Solution into a Liquid SOLVent (RESOLV), to produce exclusively nanoscale par- ticles [1,10]. The RESOLV method has been applied to the ∗ Corresponding author. Tel.: +1 864 656 6701; fax: +1 864 656 5007. E-mail address: syaping@clemson.edu (Y.-P. Sun). preparation of nanoparticles with relatively narrow size dis- tributions from a variety of inorganic and polymeric materials [1,10–16]. Supercritical CO 2 is obviously a preferred solvent in RESS and RESOLV processing because of its excellent properties including nontoxic, nonflammable, inexpensive and environ- mentally benign [17]. However, the generally low solubil- ity of most solutes in neat supercritical CO 2 is a significant limitation. While co-solvents are often used to improve the solubility, for hydrophilic solutes a commonly employed ap- proach is to add surfactant to CO 2 to form water-in-CO 2 microemulsion [18–22]. In particular, perfluorinated surfac- tant has been used to stabilize water-in-CO 2 microemulsion in the preparation of metal and semiconductor nanoparti- cles via in situ chemical reduction and precipitation [19–22]. We have used similar water-in-CO 2 microemulsion to dis- solve metal salts for the rapid expansion in RESOLV, coupled with chemical reduction in the room-temperature receiving solution [13,14,23]. The metal nanoparticles thus obtained were found to have significantly poorer size distributions than those produced in RESOLV with neat supercritical solvents [12,13]. In an effort to improve the preparation of nanopar- 0896-8446/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2004.10.005