Gopi Undavalli Venkata, Reddi Uday Sai, Karthik M, Smaran Kumar Sai
Department of Chemistry, Prasanthi Nilayam Campus, Sri Sathya Sai Institute of Higher Learning, Prasanthi Nilayam- 515134, Andhra Pradesh, India.
Department of Chemistry, Brindavan Campus, Sri Sathya Sai Institute of Higher Learning, Kadugodi, Bengaluru-560067, Karnataka, India.
Nanotechnology. 2023 Dec 8;35(8). doi: 10.1088/1361-6528/ad0f54.
Annealing step is a critical step in the hydrothermal assisted synthesis of La/Ni oxides such as LaNiO(LNOA) and LaNiO(LNON). In the current study, we have discovered the interlink between the atmosphere and temperature conditions which dictate the product formed. La/Ni nitrate precursors were subjected to a hydrothermal synthesis followed by an annealing step at appropriate temperature and varying atmosphere resulting in the synthesis of the corresponding oxides. The annealing temperature was varied in the range between 650 °C and 800 °C and also the annealing was carried out either in pure Natmosphere or air. From the x-ray diffraction analysis, it was inferred that annealing in air invariably resulted in the rhombohedral LaNiO(LNOA) perovskite phase, while annealing in Natmosphere resulted in an orthorhombic Ruddlesden-Popper phase LaNiO(LNON), a layered oxide containing traces of NiO phase. Typically, iodometric titrations substantiate the presence of Ni (III) which further can be correlated to the presence of oxygen vacancies (). Iodometric test results demarcated the difference between the two phases with absolutely minimal Iliberated from the LNON samples proving that negligible amount of Ni (III) was present in LNON Scanning electron microscopy (SEM) images showed an agglomeration of particles annealed at higher temperatures irrespective of the atmosphere. Temperature dependent oxygen non-stoichiometry (Δ) was analyzed through thermogravimetric analysis, wherein Δwas inversely proportional to the annealing temperature for all of the LNOA samples. Considering that largevalues favor pseudocapacitive behaviour, it was observed LNOA oxides showed excellent pseudocapacitive behaviour compared to the LNON oxides. Dunn deconvolution of the cyclic voltammograms of LNOA 800 °C at 5 m Vsindicated that diffusive contribution (66%) was predominant over capacitive contributions. The LNOA sample annealed at 800 °C displayed the highest specific capacitance of 100.3 F gat 1 A gcurrent density.
退火步骤是水热辅助合成La/Ni氧化物（如LaNiO（LNOA）和LaNiO（LNON））中的关键步骤。在当前研究中，我们发现了决定所形成产物的气氛和温度条件之间的相互联系。将La/Ni硝酸盐前驱体进行水热合成，然后在适当温度和不同气氛下进行退火步骤，从而合成相应的氧化物。退火温度在650℃至800℃之间变化，并且退火在纯N气氛或空气中进行。通过X射线衍射分析推断，在空气中退火总是会得到菱面体LaNiO（LNOA）钙钛矿相，而在N气氛中退火则会得到正交晶系的Ruddlesden-Popper相LaNiO（LNON），这是一种含有微量NiO相的层状氧化物。通常，碘量滴定法证实了Ni（III）的存在，这进一步可与氧空位的存在相关联。碘量测试结果区分了这两个相，从LNON样品中释放出的I绝对最少，证明LNON中存在的Ni（III）量可忽略不计。扫描电子显微镜（SEM）图像显示，无论气氛如何，在较高温度下退火的颗粒都会发生团聚。通过热重分析对温度依赖性氧非化学计量比（Δ）进行了分析，其中对于所有LNOA样品，Δ与退火温度成反比。考虑到较大的值有利于赝电容行为，观察到与LNON氧化物相比，LNOA氧化物表现出优异的赝电容行为。在5 m Vs下对800℃的LNOA的循环伏安图进行Dunn去卷积表明，扩散贡献（66%）比电容贡献占主导。在800℃退火的LNOA样品在1 A g电流密度下显示出最高比电容为100.3 F g。
