Synthesis and structural characterizations of HAp–NaOH–Al2O3 composites for liquid petroleum gas sensing applications

Abstract

Abstract The main objective of present work was to synthesize sodium doped polycrystalline hydroxyapatite NaHAp (NaOH-HAp) powder by employing wet chemical precipitation method and its composites with alumina (Al2O3; 0, 10, 40, 70 wt%) using a scalable solid-state reaction method. Detailed investigations of NaHAp and its composites using X-ray powder diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy, UV–visible spectroscopy, scanning electron microscopy followed by energy-dispersive X-ray spectroscopy and sensing behaviour is carefully described. XRD results exhibited major and minor phase of HAp, Ca2P2O7 and NaOH for NaHAp sample while for all fabricated composites of HAp–NaOH–Al2O3 and revealed the major phase of hibonite CaO(Al2O3)6 along with secondary phases of Ca2P2O7, Na4[Al(PO4)2(OH)], Na3Al(OH)(HPO4)(PO4), NaAl3(PO4)2(OH)4 and NaOH. The crystallite size of NaHAp-based composites was also determined and lies in the range of 200–2800 nm, which is larger than that of Al2O3. FTIR and Raman spectroscopic studies reveal the bonding formation of P–O, O–P–O and Al–O due to intramolecular interaction of Na4(Al(PO4)2(OH)), Na3Al(OH)(HPO4)(PO4) and NaAl3(PO4)2(OH)4 in the HAp–NaOH–Al2O3 composite, while bonding formation of Al–O–H recognized to intermolecular interaction in between Al with H atoms of Na4(Al(PO4)2(OH)), Na3Al(OH)(HPO4)(PO4) and NaAl3(PO4)2(OH)4. The SEM and energy dispersive spectroscopy analysis revealed the presence of all constituent elements of used chemicals which also validate the purity of used materials. It is concluded that the fabricated sensor (60 NaHAp–40 Al2O3) shows lowest response and recovery time, 4 and 3 s for the 0.5 vol.% concentration of the LPG. Therefore, among all composites, this fabricated composite can be used for LPG gas sensing applications.