Structural and Thermal Insights into One-Step Synthesized Phosphate-Based Mn–Pyrazole–Oxyhydroxide Nanocomposites

Abstract

With the industrial sector accounting for 42% of India’s total electricity consumption, there is a pressing need to develop energy-responsive materials that can enhance furnace efficiency and reduce overall energy usage. The present study reports the synthesis of a novel Mn–pyrazole–oxyhydroxide nanocomposite designed to combine the redox activity and thermal stability of manganese, the strong coordination ability and heat resistance of pyrazole, and the high thermal conductivity of manganese oxyhydroxide. The composite was synthesized via a simple one-step precipitation method. Following the precipitation, the resulting material was filtered and subjected to two different drying conditions: one at an elevated temperature of 80°C–90°C, referred to as the calcined material (SS-1), with the composition [Mn₂(µ-C₃H₄N₂)(OH)₄(µ-HPO₄)₂·(NaHPO₄)₂]·(Mn-O-OH)·2H₂O; and the other at room temperature, referred to as the non-calcined material (SS-2), with the composition Mn(C₃H₄N₂)(HPO₄)(OH)₄·(MnHPO₄)·(Mn-O-OH)₂·(NaH₂PO₄)₂·2.5H₂O. Their compositions were confirmed through ICP-OES, CHNS, and TGA analyses. Powder X-ray diffraction (PXRD) was performed to determine the crystallite sizes, revealing an average of 83.5 nm for SS-1 and a comparatively smaller size of 60.8 nm for SS-2. FTIR and TGA analyses verified the presence of various atomic binding modes and distinct molecular components, while UV-Vis-NIR spectroscopy provided insights into their band gaps and refractive indices. SS-1 exhibited a band gap of 1.46 eV with a refractive index of 2.84, whereas SS-2 showed a lower band gap of 0.50 eV and a higher refractive index of 3.71. Importantly, DSC analysis revealed low specific heat capacities for both composites—0.148 J/g·K for SS-1 and 0.167 J/g·K for SS-2. These low specific heat values classify them as low thermal inertia materials. Consequently, both SS-1 and SS-2 demonstrate strong potential as energy-responsive thermal coatings.