Inorganic nanoparticles synthesized by the novel oil-in-water microemulsion reaction method and their potential applications

Abstract

[eng] The use of microemulsions as confined reaction media for nanoparticle synthesis presents various advantages compared to other methods. They offer a high control of nanoparticle size and often nanoparticles with crystalline structure and a high specific area are obtained directly during the synthesis, eliminating the need for calcination. Also, due to their thermodynamic stability, no energy input is needed for microemulsion formation; only a gentle agitation is sufficient to obtain microemulsions. In addition, simple equipment and mild conditions of temperature and pressure are employed. The traditional W/O microemulsion reaction method has been intensively studied. However, the use of W/O microemulsions has some drawbacks preventing their use in industrial applications. The most important drawback is the organic solvents used as the continuous phase, which are environmentally unfriendly. Also, they are cost ineffective. In order to overcome these drawbacks, our group has recently introduced the novel oil-in-water (O/W) microemulsion reaction method for inorganic nanoparticle synthesis. The advantage of using an aqueous continuous phase, the use of the organometallic precursors that can be loaded at comparable or higher concentration than the metallic salts in the traditional W/O microemulsions (promoting a higher nanoparticle yield), and the use of only one microemulsion (instead of two as in the traditional W/O method), indicate the promise of this novel method. This novel method consists in the use of organometallic precursors, dissolved in nanometer scale oil droplets of O/W microemulsions, and stabilized by a monolayer of hydrophilic surfactant. The precipitating agents, usually water-soluble, can be added directly or as aqueous solutions, without compromising microemulsion stability and droplet size; alternatively, if oil-soluble precipitating agents are available, then a two-microemulsions approach can be used. The main motivation for this development was that, to the best of our knowledge, there was a lack of methods in the literature employing O/W microemulsions in which the metallic precursors were dissolved (and confined) in the oil droplets. In addition, another strong motivation was that, from a practical and environmental point of view, the possibility of preparing inorganic nanoparticles using o/w microemulsions (as opposed to w/o microemulsions) could be highly advantageous, since the major (continuous) phase is water. Metal 2-ethylhexanoates have been used in the Oil-in-Water microemulsion reaction method as precursors of inorganic nanoparticles, mainly because these precursors were very soluble in hydrocarbons such as hexane or isooctane, which can be conveniently used as the oil phase in this approach. In addition, these type of precursors are available for a wide number of elements, they are inexpensive, air-stable (unlike alkoxide sol-gel precursors), and generally non-toxic. This type of precusors is often used for the synthesis of inorganic nanoparticles like metallic or metal oxides by thermal decomposition at temperatures as high as 200-300ºC, in polar organic solvents and generally under an inert atmosphere, or by pyrolysis at much higher temperatures. The various types of inorganic nanoparticles prepared in O/W microemulsion reaction method were selected on the basis of their specific characteristics for several potential applications. Oxide nanoparticles containing Cerium, Copper and Zinc were prepared for their interest in catalysis. In addition, CuO and CeO2 nanoparticles were also prepared as reference materials in order to compare properly the characteristics of the obtained materials. Semi-conductors such as Zinc oxide, Zinc peroxide and Zinc Selenide were synthesized and their specific characteristics such as fluorescent, electrical and photocatalytic properties were assessed. Finally, magnetic Mn-Zn ferrites nanoparticles with potential applications in fields such as biomedicine and thermoelectrics were prepared and their magnetic properties were assessed. Formation and stability of nanoparticle dispersions were also studied in order to assess their potential application in the biomedicine field.

[spa] Se ha introducido recientemente un nuevo método de reacción en microemulsiones aceite-en-agua para la síntesis de nanopartículas inorgánicas por parte de nuestro grupo de investigación (Sánchez-Domínguez et al. 2009). Es principalemente destacable el hecho de que con éste método se reduce el uso de solventes orgánicos ya que la fase continua es acuosa. Este método consiste en el uso de precursores organometálicos, disueltos en gotas nanómetricas de solvente orgánico (generalmente hidrocarburo) estabilizadas por una monocapa de tensioactivo hidrófilo, dispersas en la fase continua acuosa. Los agentes precipitantes por lo general solubles en agua se pueden añadir directamente o en forma de soluciones acuosas sin comprometer la estabilidad de la microemulsión, y el tamaño de gota. En el presente estudio, se ha estudiado la formación de microemulsiones en sistemas ternarios (o pseudoternarios) compuestos de agua, Synperonic® 10/6 (tensioactivo no-iónico) y hexano a varias temperaturas entre 25ºC y 35ºC. Además, se ha estudiado el efecto de la incorporación de precursor organometálico (Ce-2EH) en la formación de microemulsiones a varias temperaturas también. Después de definir las zonas de micromemulsión, se han elegido algunas composiciones para utilizarlas como medio de reacción para preparar nanopartículas inorgánicas. Además, se han sintetizado varios tipos de nanopartículas inorgánicas en microemulsiones aceite-en-agua; el tipo específico de nanopartículas ha sido seleccionado en base a sus propiedades específicas y diferentes aplicaciones potenciales. Se han preparado nanopartículas de óxidos mixtos de cerio, cobre y zinc por su alto interes en catálisis. También se han preparado nanopartículas de óxido de cobre y óxido de cerio para usarlos como materiales de referencia al comparar las características de las nanopartículas de los óxidos mixtos obtenidos. Asimismo, se han preparado semiconductores como óxido de zinc, peróxido de zinc y seleniuro de zinc y se han evaluado sus características específicas por ejemplo sus propiedades fluorescentes, eléctricas y fotocatalíticas. Por otra parte, se han sintetizado nanopartículas magnéticas tipo ferritas de zinc y manganeso con aplicaciones potenciales en biomedicina y materiales termoeléctricos. Se han estudiado sus propiedades magnéticas y se ha estudiado la formación y la estabilidad de varias dispersiones de dichas nanopartículas en diversos solventes, con el fin de ampliar el rango de aplicaciones potenciales.