Review of Sol-Gel Derived Mixed Metal Oxide Thin Film Coatings with the Addition of Carbon Materials for Selective Surface Applications

With dwindling fossil fuels (and the inherent environmental consequences of their use), it has become essential to seek a renewable energy supply for a species with an exponential energy thirst. Solar radiation can be converted into a usable form of energy such as heat or electricity via photothermal conversion or by photovoltaic conversion, respectively. Selective solar surface is the key portion of a solar thermal collector determining strongly the efficiency in solar-thermal conversion. Selective surfaces consist of solar absorbing thin films made of novel metallic nanoparticles embedded in metallic oxides are used as thermal solar collectors that absorb and convert solar radiation into heat. Since the selective surfaces are the easiest and most direct way of harvesting the solar energy, in recent years, they have received significant perception for mid and high temperature applications. However, traditional thermal collector based solar devices are costly to manufacture and in that very process also incurs environmental cost. To address that issue, mixed metal oxides and carbon materials are paving the way into the future for the potential development of selective solar devices. The utilisation of carbon structures such as graphene, graphene oxide and carbon nanotubes, in conjunction with mixed metal oxides in thin film based devices, shows a lot of promise. This review will discuss the use of carbon materials in the mixed metal oxides thin films via the sol-gel method of synthesis, and will analyse some recent results in the field. The structural, optical, electronic and mechanical properties of Mn-Co, Cu-Co and Ni-Co oxide sol-gel derived thin films are discussed through XRD, SEM, FESEM, AFM, XPS, UV-Vis, FTIR, NEXAFS and nanoindentation techniques. The effects of varying concentrations of Co and Cu, in mixed metal oxide films as well as varying the dip speed in the sol-gel process with the resultant effect on solar absorptance were examined for the CuCoO thin films coated on an aluminium substrate. This report also characterises the structural and optical properties of CuCoO-based thin films with different absorbing properties, together with the addition of graphene oxide. At the same time, the selective behaviours of these coatings are examined. Throughout the investigations, these films demonstrate technical features which are promising for a good selective solar surface.