The effect of precipitants on developing Mn2O3 nanopowder for NIR reflective pigmentation study

Abstract

In this study, various precipitation experiments were conducted using MnCl₂⋅4H₂O salt along with different carbonate precipitants such as K₂CO₃, Na₂CO₃, and NH₄HCO₃ to produce Mn₂O₃ phase and to investigate whether these oxides could function as NIR reflective pigments. The products produced with three different precipitants show the formation of MnCO₃ as an intermediate phase. After being heated at 600 °C for 3 to 12 h, these intermediate products crystallize into pure cubic Mn₂O₃ phase materials. The TGA and DTA plots of the products made with K₂CO₃ and Na₂CO₃ as precipitating agents show a similar trend, indicating a weight loss of 24.5%–26.5%, while NH₄HCO₃ exhibits a relatively higher weight loss of ∼35%. Different morphologies are observed for K₂CO₃, Na₂CO₃, and NH₄HCO₃ as precipitants, including spherical/agglomerated shapes, a mix of spherical particles and nanowires, and porous-like nanospheres, respectively. The broad Raman peak observed between 640 and 650 cm⁻¹ is associated with the characteristic Mn-O-Mn stretching bridge of the Mn₂O₃ phase. The Tauc plots yielded E_g values in the range of 1.35–1.85 eV for the Mn₂O₃ samples obtained from different precipitants. NH₄HCO₃-derived Mn₂O₃ exhibits ∼60% NIR reflectance (1350–2500 nm), far higher than 15%–30% for K₂CO₃/Na₂CO₃ samples.