Synthesis and characterization of CdS nanoparticles embedded in a polymethylmethacrylate matrix

https://doi.org/10.1016/j.jcis.2004.10.019Get rights and content

Abstract

CdS nanopowder capped with sodium bis(2-ethylhexyl)sulfosuccinate was synthesized by using water-in-oil microemulsions. The CdS nanoparticles of about 5 nm obtained were embedded in polymethylmethacrylate matrix by a photocuring process. The transparent yellow solid compound was characterized by optical absorption and emission spectroscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The properties of this compound were compared with those of the nanopowder dispersed in heptane and in methylmethacrylate. The results obtained indicate that the nanoparticles are homogeneously dispersed in the matrix and do not change in size during the embedding process. Even if the surface slightly changes its luminescence properties, as a consequence of the different new chemical environment, the final product seems to be suitable for practical applications.

Introduction

In the last few years, the production of semiconductor nanoparticles was the focus of much research because of their potential applications in optoelectronics [1], photocatalysis [2], solar energy conversion [3], and water pollutant photodegradation [4]. Devices using the properties of nanoparticles are promising due to the possibility of tailoring a number of optical, electrophysical, and magnetic properties by changing nanoparticle size, which can be controlled during synthesis. Although a wide range of synthetic methods are now available, it seems clear that there is still a major problem associated with the reproducible preparation of materials with good mechanical properties for technological applications. A promising way to prepare these kinds of materials uses the interplay between polymer chemistry and quantum dot synthesis [5].

In contrast to microelectronic circuits, operating via ordered functional elements such as logical gates, polymer nanocomposites use the properties of disordered systems, as the nanoparticles are stochastically dispersed in the system. These properties can be used to manufacture LEDs, photodiodes, solar cells, and sensors and for other technological applications [6].

Two main methods are used to obtain nanoparticles embedded in a matrix [7]: one carries out the nanoparticle synthesis inside the polymeric matrix [8]; the other consists of allowing monomer polymerization after nanoparticle dispersion has occurred [9].

The goal of the present article was to incorporate CdS nanoparticles into a solid matrix where the growth of the particles and their coalescence processes are inhibited. To use the CdS optical properties the matrix must be transparent. CdS nanoparticles, capped with sodium bis(2-ethylhexyl)sulfosuccinate, were embedded in a polymethylmethacrylate (PMMA) matrix by means of a photocuring process, which is photoinduced hardening of a monomeric substrate. PMMA, a vinyl polymer obtained by free radical polymerization of the methylmethacrylate monomer (hereafter MMA), is a thermoplastic transparent material used as a shatterproof replacement for glass. Its refractive index is equal to 1.49, in the range of the typical refractive index of glass (1.3–1.7); whereas its haze (1–3%) is higher than that of glass, which ranges from 0 to 0.17%.

CdS nanoparticles were synthesized in water-in-oil microemulsions, and to stop the growth process an amine was added before extraction. Nanoparticle growth was followed in situ by UV–vis spectroscopy; at the end of the growth process and after the embedding process, the size was confirmed by high-resolution transmission electron microscopy (HRTEM). Before the embedding process, nanoparticle elemental composition was determined using energy-dispersive X-ray (EDX) spectroscopy. Optical properties of the nanoparticles embedded in the polymer matrix were revealed by photoluminescence spectroscopy and compared with those of the nanopowder dispersed in heptane and in MMA.

Section snippets

Materials

Sodium bis(2-ethylhexyl) sulfosuccinate (AOT, Aldrich 98%), cadmium sulfate (Aldrich, 99%), tetrabutylammonium hydrogen sulfide (TBAS, Fluka, 99%), bis(2-ethylhexyl)-amine (BEA, Aldrich, 99%), and n-heptane (Sigma, 99%) were used as received. Water was bidistilled. Methylmethacrylate (99.0%, Aldrich) was purified by using a disposable column, to eliminate the polymerization inhibitor.

Methods

UV–vis absorption spectra were obtained in the range 300–600 nm by using a double-beam Beckman DU-640

CdS nanoparticle synthesis

Among the various methods employed to produce size-controlled nanoparticles, a promising one is based on the use of water-in-oil (w/o) microemulsions. Nanoparticles of many metal sulfides were synthesized in this way by mixing two w/o microemulsions containing the suitable water-soluble metal salt and sodium sulfide, respectively [10], [11]. In this procedure nanoparticles need to be separated from the reaction medium. Bare nanoparticles are thermodynamically unstable with respect to growth and

Conclusions

It has been shown that CdS nanoparticles can be dispersed in MMA monomers and that CdS nanoparticles embedded in a PMMA matrix can be obtained as a transparent yellow solid by a photocuring process, initiated by activating a photoinitiator with 365-nm radiation. Such a material could find interesting optical applications.

The proposed procedure does not damage the CdS optical properties, is fast, and has the advantage of working at room temperature. Before the extraction process, HRTEM

Acknowledgments

The authors are most grateful to Dr. Casinelli (Basel) for preparation of the PMMA ultrathin sections for HRTEM. Financial support from the Consiglio Nazionale delle Ricerche (Progetto Finalizzato Materiali Speciali per tecnologie avanzate II) and from MURST (PRIN-COFIN 2001 “Sintesi di nanoparticelle assistita da microonde”) is gratefully acknowledged.

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