Oxide-based thermoelectric nanomaterials have gained significant attention due to their exceptional stability in high-temperature oxidative environments, making them suitable for applications above 300°C. Unlike traditional thermoelectric materials such as bismuth telluride or lead chalcogenides, oxides exhibit robust chemical and thermal stability, which is critical for waste heat recovery in industrial processes or power generation in harsh conditions. Key oxide materials under investigation include zinc oxide (ZnO), strontium titanate (SrTiO3), and calcium cobaltate (Ca3Co4O9), each offering unique advantages in terms of electrical and thermal transport properties.

The synthesis of oxide thermoelectric nanomaterials involves several techniques tailored to achieve precise stoichiometry, crystallinity, and nanostructuring. Sol-gel methods are widely used due to their ability to produce homogeneous precursors with controlled composition. For instance, ZnO nanoparticles synthesized via sol-gel processes exhibit tunable defect concentrations, which directly influence electrical conductivity. Pulsed laser deposition (PLD) is another prominent technique, particularly for layered oxides like Ca3Co4O9, as it allows for epitaxial growth of thin films with well-defined crystallographic orientations. Hydrothermal synthesis is also employed for SrTiO3, yielding nanocrystals with high phase purity and controlled morphology. These methods enable the engineering of microstructural features that are crucial for optimizing thermoelectric performance.

A major challenge in oxide thermoelectrics is their inherently low electrical conductivity compared to conventional semiconductors. Oxygen vacancy engineering has emerged as a key strategy to enhance charge carrier concentration and mobility. In ZnO, intentional creation of oxygen vacancies through doping (e.g., Al, Ga) or reduction annealing introduces shallow donor states, significantly improving n-type conductivity. For SrTiO3, oxygen deficiency can be controlled during synthesis or post-annealing, leading to increased electron density. Similarly, in Ca3Co4O9, a p-type oxide, cation substitution (e.g., with Na or Ag) optimizes hole transport while maintaining structural integrity. The balance between oxygen vacancy concentration and carrier scattering must be carefully managed to avoid detrimental effects on carrier mobility.

Thermal conductivity reduction is another critical factor in improving the thermoelectric figure of merit (ZT) of oxide materials. Layered structures, such as those found in Ca3Co4O9, inherently exhibit low thermal conductivity due to strong phonon scattering at interlayer interfaces. Nanostructuring further enhances this effect by introducing grain boundaries and defects that impede heat-carrying phonons. For example, SrTiO3 nanocomposites with embedded secondary phases (e.g., SrO or TiO2 nanoparticles) demonstrate reduced lattice thermal conductivity without compromising electrical properties. Similarly, ZnO-based materials with nanoscale porosity or superlattice structures achieve significant phonon scattering while maintaining reasonable electrical transport.

The stability of oxide thermoelectrics under oxidative conditions is a defining advantage. Unlike sulfides or tellurides, oxides do not decompose or oxidize further at high temperatures, ensuring long-term operational reliability. For instance, Ca3Co4O9 retains its structural and thermoelectric properties even after prolonged exposure to air at 800°C, making it suitable for high-temperature applications. ZnO and SrTiO3 also exhibit excellent thermal stability, though their performance may degrade if excessive oxygen loss occurs under reducing atmospheres.

Recent advances in oxide thermoelectrics focus on synergistic optimization of electrical and thermal properties through advanced synthesis and defect engineering. For example, dual doping strategies in ZnO—combining aliovalent dopants for carrier concentration tuning with isovalent dopants for lattice strain—have shown promise in decoupling electrical and thermal transport. In SrTiO3, interface engineering in thin-film heterostructures has led to enhanced power factors while suppressing thermal conductivity. Layered cobaltates continue to benefit from texturing techniques that align crystallographic planes to favor anisotropic charge transport.

Despite progress, challenges remain in achieving ZT values comparable to traditional thermoelectrics. The inherently wide bandgaps of oxides limit carrier concentrations, while strong phonon-electron coupling in some materials reduces mobility. Future research directions may explore novel oxide compositions with intrinsically low thermal conductivity, advanced nanostructuring approaches, and hybrid materials combining oxides with conductive phases.

In summary, oxide-based thermoelectric nanomaterials represent a promising class of materials for high-temperature energy conversion. Their stability in oxidative environments, coupled with advances in synthesis and property engineering, positions them as viable candidates for sustainable thermoelectric applications. Continued refinement of defect chemistry, nanostructuring, and layered design will be essential to unlock their full potential.