Microwave-induced plasma promoted dielectric heating: metrology and application to the photocatalytic activation of water

There is a clear and effective need for the synthesis of new materials that can sustain and develop future technologies and underpin modern society. To increase the diversity of materials accessible it is necessary to develop new synthetic techniques. The proposed research describes the application of microwave radiation to provide energy to drive chemical reactions between solids and/or between solids and gases. A confined gas when exposed to microwave radiation can ionise giving rise to a microwave-induced plasma (MIP) that can be used to provide heat to drive reaction between bulk solids and as a source of reactive gas species that can chemically modify a material. Many polar liquids (e.g. water) and some solids do interact directly with microwaves causing rapid heating and reaction, but many technologically important materials are transparent to microwaves at room temperature thus preventing use in reactions. However, direct microwave (dielectric) heating is temperature dependent and many materials will directly couple with microwaves at elevated temperatures. Unfortunately quantitative data enumerating the temperature dependence of dielectric heating for most solids is not currently available. We propose to synthesise new compounds and composite materials using a combination of MIP and dielectric heating that will be supported by measurements (metrology) of the temperature dependence of dielectric heating of precursor and product materials. The heat provided by the MIP will cause many solids that are microwave transparent at room temperature to exhibit significant dielectric heating at elevated temperatures. Initially MIP promoted dielectric heating will be identified for materials by monitoring the temperature of a reaction mixture in situ, where rapid temperature rises and sample temperatures in excess of the plasma temperature will indicate significant dielectric heating. Materials that exhibit strong temperature dependence will then be measured more rigorously and this information used to correlate the structure and morphology of reaction products and direct subsequent synthetic reactions. The temperature dependence of microwave heating will allow differential heating to be exploited, where in a heterogeneous mixture, different solids can be simultaneously heated to different temperatures. This is in direct contrast to traditional conduction/convection heating methods where a solid mixture is heated uniformly.The solids we will target are semiconducing catalysts relevant to the photocatalytic activation of water that generate hydrogen from solar energy. Hydrogen is a clean energy resource because the combustion product is water and therefore photocatalysis represents an opportunity to meet the increasing energy demands of society and also potentially replace limited fossil fuel resources that are detrimental to the environment. Photocatalysts typically comprise a heterogeneous composite of semiconducting metal oxide and metal particles/metal rich regions that will exhibit markedly different temperature dependence with respect to microwave heating giving rise to differential heating. A combination of differential heating and reactive MIPs therefore provides additional opportunity for novel materials synthesis by selective modification from reaction between a MIP and heated component. Furthermore, reactive MIP can be used to modify a solid to alter the dielectric properties to either increase or decrease the extent of dielectric heating.MIP promoted dielectric heating represents a distinct and adventurous synthetic method for the preparation of new materials. Metrology and rigorous characterisation of materials using a range of microscopy and other techniques will underpin exploratory synthetic work to realise the potential of this novel method.