Spray pyrolysis as an alternative route to produce thermoelectric layers of NaxCoO2

Abstract

This study investigates polycrystalline Na_xCoO₂ thin films synthesized via spray pyrolysis on glass substrates over a wide compositional range of x. The films were grown using cobalt acetate tetrahydrate and sodium hydroxide as precursors, employing different precursor solution molarities to control the growth dynamics. The characterization of the system was conducted using X-ray Diffraction, Scanning Electron Microscopy, Optical Transmittance, and Raman spectroscopy. Raman spectroscopy revealed oxygen in-plane motion modes under specific excitation wavelengths due to optical resonance conditions and identified the films as γ-Na_xCoO₂. The growth of γ-Na_xCoO₂ phase is favored by a high-molarity synthesis mode followed by an oxygen thermal annealing. At room temperature, films with higher sodium content exhibited decreased Seebeck coefficients and electrical resistivity. Notably, a maximum power factor of approximately ∼3.3 µW/K²m was achieved at x = 0.93. Conversely, films prepared at low-molarities presents Na-content Co₃O₄ phases and Seebeck coefficients up to 500 μV/K yet suffer from poor electrical conductivity. These findings underscore the viability of spray pyrolysis as a scalable method for producing p-type thermoelectric layers such as NaCoO₂.