Switching from antiferromagnetism to ferromagnetism in nanocrystalline La1−xSr1+xCoO4 (x = 0.0, 0.2, 0.4 and 0.6) layered perovskite oxides: effects of mixed valence states of Co on their magnetic, optical and photocatalytic properties

Abstract

Although extensive work has already been done on bulk La_1−xSr_1+xCoO₄ layered oxides, there is very limited studies on their nano-crystalline nature, particularly for those with a higher Sr level (x ≥ 0.4), which may exhibit interesting magnetic properties and in particular photocatalytic properties because of their enhanced surface area. Moreover, the phases with higher Sr doping levels can be formed only at higher temperatures, which otherwise contain significant amounts of impurities. In order to synthesize them in a nanocrystalline form in the pure state at the lowest possible temperature, we applied a glycine nitrate combustion method using excess fuels for the synthesis of nanocrystalline La_1−xSr_1+xCoO₄ (x = 0.0, 0.2, 0.4 and 0.6) layered perovskite oxides. Efforts were also made to synthesize phases with x > 0.6 but remained unsuccessful. The prepared nanophases were thoroughly characterized using PXRD, XPS, SEM, SQUID VSM and UV-DRS to examine the structural, morphological, optical, magnetic and photocatalytic properties. All the phases exhibit mixed valence states of +3 and +4 of Co, as confirmed by XPS analysis, which has a remarkable effect on various physical properties, especially magnetic and photocatalytic properties. An abrupt transition in magnetic behaviour from antiferromagnetic for x = 0 and 0.2 phases to ferromagnetic in case of x = 0.4 and 0.6 was observed. The optical band gap values (E_g) of all the synthesized nanophases obtained from DRS are in the visible range (1.57 to 1.47 eV), which make them suitable and reliable photocatalysts for dye degradation. Moreover, the synthesized nanophases were assessed for their photocatalytic activity by employing rhodamine B (RhB) dye as a model contaminant. The results revealed that the La_0.4Sr_1.6CoO₄ photocatalyst efficiently and swiftly degraded RhB within 60 min. Considering the performance of the La_0.4Sr_1.6CoO₄ photocatalyst, the study was expanded to include the degradation of other organic dye pollutants, namely, methyl orange (MO), methylene blue (MB) and a ternary combination of these contaminants (RhB + MB + MO).