Tunable surfaces : using polyoxoniobates and -tantalates as molecular building blocks

Abstract: This thesis describes and explains the use of aqueous polyoxometalates (POMs), as molecular building blocks in the fabrication of tunable solid-state materials, such as thin films.  Microwave irradiation in the synthesis of Nb10, Nb6 and Ta6 is a rapid and efficient alternative to conventional hydrothermal methods, while offering equal – if not greater yields – of product. Through microwave irradiation, the concept of an activation pH of anhydrous Nb2O5, niobic and tantalic acids has been devised to explain what products are accessible from each oxide and under what conditions.The controlled deposition of metal oxide thin films is possible via an iterative spin coating and annealing process of POMs. This approach facilitates deposition of alkali-free, metal oxide thin films with varying crystallinity, thickness and roughness. The ability to deposit successive layers of these polyoxometalate films was a new approach to create thicker and layered metal oxide films. This approach offers an efficient and reproducible means of creating tunable metal oxide thin films or other solid-state structures, which exemplifies the use of POMs as molecular building blocks.Transition metal niobate thin films can be deposited onto silicon wafers via the aqueous deposition of transition metal-hexaniobate mixtures. This facilitates assessment of the electrochemical properties of these materials, without a need for coin cells or inert conditions. This allows for characterisation of the pseudocapacitive properties of these materials, which abodes well for developing electrochemical energy storage devices.The viability of using aqueous polyoxoniobate-alkali nitrate mixtures was leveraged as a means of depositing alkali niobate (LiNbO3, NaNbO3, KNbO3) thin films, plus Rb and Cs doped Nb2O5 films and powders. Synthesising solid-state niobates that can incorporate all the alkali cations, entirely from aqueous polyoxoniobate solutions, confers the ability to develop lead-free piezoelectric, ferroelectric and antiferroelectric materials without the use of expensive or energy intensive methods.