Hybrid inorganic–polymer nanocomposites can be employed in diverse applications due to the potential combination of desired properties from both the organic and inorganic components. The use of novel bottom–up in situ synthesis methods for the fabrication of these nanocomposites is advantageous compared to top–down ex situ mixing methods, as it offers increased control over the structure and properties of the material. In this review, the focus will be on the application of the sol–gel process for the synthesis of inorganic oxide nanoparticles in epoxy and polysiloxane matrices. The effect of the synthesis conditions and the reactants used on the inorganic structures formed, the interactions between the polymer chains and the inorganic nanoparticles, and the resulting properties of the nanocomposites are appraised from several studies over the last two decades. Lastly, alternative in situ techniques and the applications of various polymer–inorganic oxide nanocomposites are briefly discussed.

Nanotechnology has motivated a tremendous effort in the synthesis approaches to grow free standing or hierarchical nanomaterials such as nanowires and nanorods. Bottom-up approaches based on chemistry are an important approach to produce nanomaterials, and here the concepts of growing oxide 1D nanostructures from chemical solutions are reviewed. The thermodynamic and kinetic aspects of the nucleation and growth of oxide compounds in solutions are presented with emphasis on hydrothermal and molten salt synthesis. The importance of solubility of precursors, the precursor chemistry, role of organic additives as well as the chemical complexity and dimensionality and symmetry of the crystal structure of the compound grown are highlighted.

Single-crystalline KNbO₃nanorods were prepared from Nb₂O₅ by hydrothermal synthesis at 180 °C in a KOH solution using sodium dodecyl sulfate surfactant. The morphology of the KNbO₃ product was strongly influenced by the addition of the surfactant, the concentration of the reactants, the reaction time and temperature, demonstrating that KN nanorods were only obtained in a narrow window of synthesis parameters. At the optimized conditions the orthorhombic KNbO₃nanorods were 100–300 nm in diameter, up to 5 µm in length and grew along the [001] direction. The proposed growth mechanism is based on self-assembly of cube-shaped or facetted KNbO₃nanocrystals into mesocrystals, which further ripen into nanorods. High temperature X-ray diffraction, calorimetry and Raman spectroscopy evidenced that the orthorhombic to tetragonal and tetragonal to cubic phase transition of KNbO₃nanorods occurred at significantly lower temperatures than for bulk KNbO₃, reflecting finite size or disorder effects.

The effect of the A-site cation ordering on the chemical stability, oxygen stoichiometry and electrical conductivity in layered LaBaCo₂O_5+δ double perovskite was studied as a function of temperature and partial pressure of oxygen. Tetragonal A-site cation ordered layered LaBaCo₂O_5+δ double perovskite was obtained by annealing cubic A-site cation disordered La_0.5Ba_0.5CoO_3-δ perovskite at 1100 °C in N₂. High temperature X-ray diffraction between room temperature (RT) and 800 °C revealed that LaBaCo₂O_5+δ remains tetragonal during heating in oxidizing atmosphere, but goes through two phase transitions in N₂ and between 450 °C and 675 °C from tetragonal P4/mmm to orthorhombic Pmmm and back to P4/mmm due to oxygen vacancy ordering followed by disordering of the oxygen vacancies. An anisotropic chemical and thermal expansion of LaBaCo₂O_5+δ was demonstrated. La_0.5Ba_0.5CoO_3-δ remained cubic at the studied temperature irrespective of partial pressure of oxygen. LaBaCo₂O_5+δ is metastable with respect to La_0.5Ba_0.5CoO_3-δ at oxidizing conditions inferred from the thermal evolution of the oxygen deficiency and oxidation state of Co in the two materials. The oxidation state of Co is higher in La_0.5Ba_0.5CoO_3-δ resulting in a higher electrical conductivity relative to LaBaCo₂O_5+δ. The conductivity in both materials was reduced with decreasing partial pressure of oxygen pointing to a p-type semiconducting behavior.