Phosphate-induced enhancement of thermal and optical properties in Cu–pyrazole systems: Influence on NIR absorption and thermal behaviour of coating composites

A phosphate-incorporated copper–pyrazole coordination solid was developed to investigate thermal robustness and heat modulation behavior. Four related copper–pyrazole systems were prepared through a simple co-precipitation route: a phosphate-free thermally treated sample (CuPZ1), a phosphate-free untreated sample (CuPZ2), a phosphate-containing thermally treated sample (CuPZP1), and a phosphate-containing untreated sample (CuPZP2). The synthesized samples were characterized using ICP-OES, CHNS, XRD, FTIR, TG–DTG–DTA, DSC, SEM, and UV–Vis–NIR techniques. Structural analysis revealed that phosphate incorporation improves morphology with smoother surface, more grain connectivity and crystallinity. Optical properties showed strong absorption in the NIR region with a band gap of ∼3.0 eV and refractive index ∼2.35. Among the four samples CuPZP1 exhibited the highest band gap 3.07 eV and refractive index 2.37, indicating enhanced optical density. Thermal analyses demonstrated improved stability and controlled heat modulation in phosphate-incorporated systems. When added into a paint matrix, the CuPZP1 composite displayed a distinct heat storage feature (110–140 °C) in Cp profiles, enabling effective thermal buffering. This thermal response falls within the temperature range commonly associated with solar-driven water evaporation systems (≈100–120 °C), indicating possible relevance for future solar thermal studies. Overall, this result informs the role of phosphate incorporation towards the tuning of structural, morphological, optical, and thermal properties of Cu–pyrazole materials for energy-responsive coating applications.