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Nickel silica xerogel and synergistic effect of heavy metal additives on the thermal, structural, optical, and photoluminescence behaviors for photonic applications
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DOI：10.1016/j.ceramint.2025.11.306.png)
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Abstract
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In this study, high-performance NiO–SiO2 xerogels doped with 10 wt% Cd2+ or Bi3+ ions were successfully synthesized via the sol–gel route and comprehensively characterized using TGA, XRD, SEM–EDAX, UV–visible spectroscopy, and photoluminescence. The results reveal a remarkable synergistic impact of heavy-metal doping on the thermal stability, structural phases, and optical behavior of the xerogels. TGA analysis demonstrated that Bi doping significantly enhances high-temperature stability compared with Cd, with low-temperature weight loss attributed to the evaporation of surface-bound water and volatiles, and high-temperature loss linked to decomposition or phase transitions. XRD showed composition-dependent phase evolution: pure NiO crystallinity in 75SiO2–25NiO (wt.%), monoclinic CdSiO3 formation with Cd doping, and bismite (Bi2O3) emergence with Bi doping. SEM revealed well-defined square bipyramidal structures, and EDAX confirmed the targeted elemental composition. Optical characterization indicated that indirect and direct band gaps increased from 2.593 to 2.768 eV and from 3.307 to 3.972 eV, respectively, with Urbach energies in the 0.13–0.22 eV range and refractive indices between 2.31 and 2.46. PL spectra under 360 nm excitation displayed a sharp blue emission (∼442 nm) from intrinsic defects and a broad green emission (∼523 nm) associated with dopant effects. These emissions were assigned to Ni2+ d–d transitions: 1A1(G) → 3A2g(F) (cyan, ∼488 nm) and 1T2g(D) → 3A2g(F) (green, ∼533 nm). CIE coordinates confirmed intense green emission, positioning these xerogels as promising candidates for nanoscale photonic devices, including green LEDs and laser diodes.
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