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

Abstract

Highlights: What are the main findings? Rietveld refinement analysis confirms the formation of the crystalline Hexagonal Phase of the monoclinic structure. The use of PVP influences the formation of the crystalline phases of Hap. The addition of PVP at low concentrations (0.1 and 0.2%wt) induces dimensional and morphological instability of Hap. At a PVP concentration of 0.3%wt, dimensional and morphological stability is restored, with nanorods being the predominant morphology. What are the implications of the main findings? The results indicate that PVP acts as an effective surfactant for achieving dimensional and morphological control of hydroxyapatite. The formation of well-defined nanorod morphologies at 0.3%wt PVP establishes a valuable reference for future studies on the controlled synthesis of Hap nanostructures. The proposed model provides insight into the interactions between PVP and Hap, contributing to the advancement of research on the dimensional and morphological control of nanomaterials. This understanding supports the design of materials with optimized properties for targeted applications, including biomaterials. 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.