Effect of PVP Concentration on the Crystalline Structure and Morphology of Hydroxyapatite via Microwave-Assisted Hydrothermal Synthesis

In this study, hydroxyapatite was synthesized using a microwave-assisted hydrothermal method. Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) and ammonium phosphate ((NH₄)₂HPO₄) served as precursors in a pH 10 ammonium hydroxide (NH₄OH) solution. Polyvinylpyrrolidone (PVP) was employed as a surfactant at varying concentrations of 0 (M0), 0.1% (M1), 0.2% (M2), and 0.3%wt (M3) to control particle size and morphology. The synthesized samples were characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). The addition of PVP during synthesis resulted in Ca/P ratios ranging from 0.93 to 1.37, and promoted predominantly rod-like morphologies. Samples M1 and M3 exhibited average diameters of 11.23-104.24 nm and lengths of 47.21-222.32 nm. XRD analysis confirmed the presence of both hexagonal and monoclinic phases, with crystallite sizes varying from 18.66 to 22.49 nm. FTIR spectra of sample M1 revealed an elongation at 3432 cm^-1 corresponding to OH^- groups, indicative of water absorption within the material structure. Vibrational bands at 2950-2300, 1090, and 975 cm^-1, attributed to C-H bonds in PVP were also identified. These findings highlight the influence of PVP concentration on the structural and morphological properties of hydroxyapatite, providing insights into its potential applications in various fields.