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Microwave synthesis to change nanoparticles’ material properties

Introduction

The material properties of nanomaterials are dependent on their size. Depending on the field of application, it is therefore essential to have the narrowest and most exact size distribution possible in order to obtain nanoparticles with defined properties. In recent years, microwave-assisted synthesis has established itself as a modern way of producing different nanomaterials in order to effectively influence both size distribution and material properties.

Using microwave irradiation it is possible to synthesize nanoparticles with exact parameter control in a short time and also change particle properties and particle size as required. The fields of application for these produced nanoparticles range from medical uses (drug delivery systems, formulations) to use in many industries, i.e. vehicle manufacturing (coatings, windshields, energy storage), cosmetics (sun protection, shampoo, toothpaste), textile production (outdoor clothing, shoes) and electronics (circuit boards, solar cells, LEDs, touchscreens). In this last field of application, quantum dots (QD) are often the focus of research.

Quantum dots

The group led by Professor Kappe at the Christian Doppler Laboratory for Microwave Chemistry at the Karl-Franzens University of Graz investigated the synthesis of “tailor-made” monodisperse photoluminescent CdSe quantum dots made of selenium dioxide and different cadmium complexes. During synthesis at 240 °C for five minutes in Monowave 300 the exact time of the addition of oleic acid was varied in order to influence the spherical expansion and agglomeration of the nanoparticles. (Agglomeration is the clustering of several particles to form a larger unit which can be broken up by dispersion.) Subsequently the size distribution of the particles was determined using small-angle X-ray scattering (SAXSess mc²).

It was possible to influence the size distribution of the quantum dots in the range from 0.5 nm to 4 nm via the chosen cadmium complexes and the time at which the oleic acid was added. This also results in a shift in the color spectrum, which means the photoluminescence of the produced particles varies from green-yellow to orange-red. This therefore enables the production of individually shining quantum dots (e.g. for the manufacture of LEDs).

Quantum Dots with gradually stepping emission from violet to deep red Source

As shown by other working groups in Germany and the USA, these QD synthesis protocols can be easily scaled, which means that microwave-assisted production is also possible in larger scales (up to several dozen grams). This makes the method interesting for industry and not only limited to research laboratories.

Both microwave synthesis instruments, Monowave 300 and Masterwave BTR from Anton Paar, make it possible to easily scale up the yield from grams to kilograms. A big benefit is that the same synthesis protocol can be used in both instruments as one of the most important parameters – the temperature – is exactly measured in both instruments. Just the weight of the sample changes, depending on whether the yield will be in grams or kilograms per day.

Advantages of microwave synthesis

The short reaction times which are possible using microwave irradiation considerably simplify the production of nanomaterials. Microwave reactors allow easy access to high temperatures and pressures which are only obtained in special, difficult to handle autoclaves over a long period of time. Special hydrothermal syntheses at temperatures well above 200 °C can be carried out in a fraction of the time.

The size and morphology of the nanoparticles can be significantly affected by small changes in the reaction conditions. For this reason, temperature measurements and pressure control which are as exact as possible are important properties of a modern microwave reactor so that it delivers reproducible results. This is particularly important for industrial applications. Besides, an exact reaction temperature is the decisive parameter required for transfer of the optimized reaction protocol into a larger scale.

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