Enhancing Electrical and Optical Properties in Layered NaxCoO2 through Sodium Non-Stoichiometry: Insights into Surface Band Bending and Oxygen Vacancies

The concentration of sodium ions (Na+) in layered sodium cobaltite (NaxCoO2) is a pivotal determinant influencing its physical and chemical properties. This study focused on NaxCoO2 with varying sodium content (x = 1, 0.9, 0.7, 0.5, 0.3, 0.1) to establish a correlation between the sodium concentration and the substantial modulation of the structural and electrical properties. X-ray diffraction and Raman studies confirmed the formation of hexagonal structured NaxCoO2 prepared by the sol-gel method, while a reduction in Na content increased the percentage of the Co3O4 phase. Notably, as the Na content was lowered, the Raman peak shifted towards a lower wavenumber due to the lattice alignment along the c-axis of the crystal plane. Morphological analysis displayed the growth of distinct plate-like structures, with a thickness ranging from 10-15 μm, which gradually decreased on lowering Na content. X-ray photoelectron spectroscopy studies revealed an increase in the relative concentration of Co4+/Co3+ with a decrease in sodium content implying a lattice distortion in CoO6 octahedra which led to the creation of additional oxygen vacancies in Na0.7CoO2 (NCO7). Optical studies highlighted a band bending phenomenon in the NCO7, while photoluminescence studies indicated an increase in emission intensity, suggesting the presence of oxygen vacancies in NCO7. Electrical measurements demonstrated the highest conductivity for NCO7 emphasizing the significance of the optimal lattice distortion, generation of oxygen vacancies, and surface band bending effect for efficient charge carrier transportation. This study highlights the crucial role of the sodium ion content between the CoO2 layers in precise tuning of the structural, electronic structure/distortion, optical, and electrical properties of NaxCoO2. The insights gained particularly in optimizing the optical and electrical properties through sodium non-stoichiometry, may hold significant implications for diverse energy conversion and storage devices, catalysis, and even optoelectronics devices.