Synthesis and structural characterisation of CdS nanoparticles prepared in a four-components “water-in-oil” microemulsion A. Agostiano a, * , M. Catalano b , M.L. Curri c , M. Della Monica a , L. Manna a , L. Vasanelli b a Dipartimento di Chimica, Universita ` degli Studi di Bari, Campus Universitario, Via Orabona 4, I-70126 Bari, Italy b CNR IME, Via Arnesano, I-73100 Lecce, Italy c CNR CS CFILM, c/o Dipartimento di Chimica, Universita ` degli Studi di Bari, Campus Universitario, Via Orabona 4, I-70126 Bari, Italy Abstract Nanostructured semiconductor particles are currently under intense investigation because of their enhanced photoreactivity and photo- catalytic properties due to the quantum-size effect and the dependence of the photophysical and photochemical properties on their size as it approaches the exciton diameter. This increasing interest has led to the development of several synthetic procedures to prepare and stabilise uniform crystallites. In this paper, we report a novel synthetic pathway to obtain cadmium sulphide (CdS) nanoparticles in a quaternary “water-in-oil” microemulsion formed by a cationic surfactant cetyltrimethylammonium bromide (CTAB), pentanol, n-hexane and water. The synthesis of CdS in this system is achieved by mixing two microemulsions containing Cd(NO 3 ) 2 and Na 2 S, respectively. The nanocrystals have been characterised by using UV–visible spectroscopy and Transmission Electron Microscopy to investigate the influence of various parameters of the particles’ formation and stability in solution. Capping of nanoparticles with suitable organic molecules has been performed in order to increase their stability and afford solubility in a wide range of solvents.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Nanoparticles; Semiconductor; Elecron microscopy; UV–vis spectroscopy; Reverse micelles; Surface modification 1. Introduction Small, monodisperse particles of semiconductors have received impressive attention in recent years because of the expectation of novel physical properties (optics and electronics) related to the use of such new materials (Henglein, 1989; Alivisatos, 1996a). This increasing interest has led to a wide range of preparative approaches to nanos- tructures. The key point to any synthetic investigation must be a careful control of semiconductors’ size and, even more important, their size-distribution. Amongst the several investigated methods (Alivisatos, 1996b), the use of microemulsions to obtain ultrafine parti- cles represents an effective pathway (Kortan et al., 1990; Eastoe and Warne, 1996). “Water-in-oil” microemulsions consist of nanometer sized water droplets that are dispersed in a continuous oil medium and stabilised by surfactants molecules localised at water/oil interface (Ko-no, 1997). This reverse micellar systems are heterogeneous on a molecular scale, but thermodynamically stable. Reverse micelles are suitable reaction media for the synthesis of nanoparticles, since water droplets represent nanoreactors which favour the formation of small crystallites with a narrow size distribution (Petit et al., 1994; Sato et al., 1995; Tanori et al., 1995; Cizeron and Pileni, 1995, 1997; Pileni, 1997). This method offers several advantages: it is a soft tech- nique, does not demand for extreme temperature or pressure conditions, can be used to perform several chemical reac- tions and does not require any special equipment. Several studies have been carried out on the factors affecting the size, shape and crystalline forms of particles prepared in microemulsions, such as the droplet dimension, surfactant concentration and so on. However, the role played by the various factors governing the formation of nanoparticles is still to be elucidated and the precise mechanism of their formation has to be resolved (Modes and Lianos, 1989; Herron et al., 1990; Towey et al., 1990; Rivas and Lopez-Quintela, 1993; Tojo et al., 1997). The use of particles obtained in this way, as functional materials in chemical processes, (see photocatalytic appli- cations) (Meyer et al., 1984; Tricot and Fendler, 1984a,b; Hagfeldt and Gratzel, 1995) or for optoelectronic devices, such as a light emitting diode (Colvin et al., 1994), requires their efficient recovery and stabilisation. One of the problems in the recovery procedure is caused by high surface energy of ultrafine particles, which makes them coagulate irreversibly when reverse micelles are destroyed without any protection treatment. To cope with this problem, surface modification of the particles with Micron 31 (2000) 253–258 PERGAMON 0968-4328/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0968-4328(99)00091-8 www.elsevier.com/locate/micron * Corresponding author.