Temperature dependent microstructural defects and surface charge effects on antioxidant activity of green synthesized nanoceria

Abstract

This study reports a novel eco-friendly route for synthesizing cerium dioxide nanoparticles (nanoceria) that converts waste coffee husks into both reagent and process medium. Polyphenol rich phytochemicals chelate Ce³⁺, guide hydrolysis, and locally modulate redox conditions, imprinting abundant surface Ce³⁺ and oxygen vacancies that underpin activity. Reuse of the clarified supernatant in successive cycles boosts yield exponentially without added metal oxide precursor, highlighting intrinsic process efficiency. Subsequent calcination turns the bio templated precipitate into phase pure fluorite CeO₂ whose crystallite size, strain, and defect concentration can be tuned by temperature alone. Higher temperatures enlarge particles and improve crystallinity while removing vacancies and strain. Radical scavenging assays show the highest activity in uncalcined material and a steady decline with increasing temperature that parallels the loss of surface Ce³⁺ and vacancies. Statistical analysis confirms that antioxidant performance depends on defect density, quantum confinement, and surface charge, whereas external morphology and residual organics are negligible. The unique mechanism is phytochemical-directed defect engineering, which couples the use of agricultural waste with precise control of redox-active sites to deliver tuneable nanoceria for biomedical, agricultural, and environmental remediation applications.

Keywords: Zeta potential; Cerium dioxide; Green synthesis; Quantum confinement; Radical scavenging activity.

Fig. 1

Schematic representation of the synthesis of nanoceria from coffee husks.

Fig. 2

(a) Yield of uncalcined nanoceria across successive synthesis cycles, showing the effect of supernatant reuse, and (b) percentage yield of nanoceria as a function of calcination temperature, illustrating thermal loss and material recovery trends.

Fig. 3

XRD analysis and microstructural evolution of nanoceria: (a) Effect of adequate washing on samples calcined at 400 °C; (b) Crystallinity index calculation method; (c) XRD spectra under thermal treatments (i: 100 °C dried; ii–viii: 400–1000 °C calcined; ix: ICSD reference); (d) Peak broadening and crystallinity index; (e) Crystallite size and unit cell count; (f) Lattice strain and dislocation density.

Fig. 4

SEM micrographs of nanoceria synthesized at calcination temperatures from 400 to 1000 °C, labelled (a)–(g), respectively. Panel (h) shows the effect of calcination temperature on the polycrystallinity index, represented as the SEM/XRD crystallite size ratio.

Fig. 5

FTIR spectra of coffee husk-mediated nanoceria: (a) raw coffee husks, (b) dried at 100 °C, and (c) representative calcined sample (600 °C), showing functional group imprint during thermal transformation to ceria.

Fig. 6

Surface chemical properties of nanoceria as a function of calcination temperature: (a) FTIR peak broadening and absorbance maxima; (b) pH and zeta potential.

Fig. 7

Antioxidant activity of nanoceria: (a) dependence on calcination temperature; (b) key structural and surface properties governing the activity mechanism.