The concentration of sodium ions (Na) in layered sodium cobaltite (NaCoO) is a pivotal determinant influencing its physical and chemical properties. This study focused on NaCoO with varying sodium content ( = 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 NaCoO prepared by the sol-gel method, while a reduction in Na content increased the percentage of the CoO 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 to 15 μm, which gradually decreased on lowering Na content. X-ray photoelectron spectroscopy studies revealed an increase in the relative concentration of Co/Co with a decrease in sodium content implying a lattice distortion in CoO octahedra which led to the creation of additional oxygen vacancies in NaCoO (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 CoO layers in precise tuning of the structural, electronic structure/distortion, optical, and electrical properties of NaCoO. 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.

中文翻译：

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通过钠非化学计量增强层状 NaxCoO2 的电学和光学性质：深入了解表面能带弯曲和氧空位

层状钴酸钠（NaCoO）中钠离子（Na）的浓度是影响其物理和化学性质的关键决定因素。本研究重点关注不同钠含量（= 1、0.9、0.7、0.5、0.3、0.1）的 NaCoO，以建立钠浓度与结构和电性能的显着调节之间的相关性。 X射线衍射和拉曼研究证实了溶胶-凝胶法制备的六方结构NaCoO的形成，而Na含量的减少增加了CoO相的百分比。值得注意的是，随着Na含量降低，由于沿晶面c轴的晶格排列，拉曼峰向较低波数移动。形态分析显示明显的板状结构生长，厚度范围为10至15μm，随着Na含量的降低，板状结构逐渐减小。 X 射线光电子能谱研究表明，随着钠含量的降低，Co/Co 的相对浓度增加，这意味着 CoO 八面体发生晶格畸变，从而导致 NaCoO (NCO7) 中产生额外的氧空位。光学研究强调了 NCO7 中的能带弯曲现象，而光致发光研究表明发射强度增加，表明 NCO7 中存在氧空位。电学测量表明 NCO7 具有最高的电导率，强调了最佳晶格畸变、氧空位的产生以及表面能带弯曲效应对于有效载流子传输的重要性。这项研究强调了 CoO 层之间的钠离子含量在精确调节 NaCoO 的结构、电子结构/畸变、光学和电学性质方面的关键作用。特别是通过钠非化学计量优化光学和电学特性所获得的见解，可能对各种能量转换和存储设备、催化甚至光电子设备具有重大影响。