Morphology-controlled bandgap engineering and antifungal selectivity in Ca-doped CdFe2O4 nanoferrites synthesized by flash autocombustion
摘要
Cadmium ferrite (CdFe2O4) and calcium-doped (Cd0.5Ca0.5Fe2O4) nanoferrites were synthesized through a rapid flash autocombustion method followed by annealing at 600 °C. Their surface morphology, optical properties, and antifungal activities were systematically examined as components of a multifunctional nanomaterial platform. Two-dimensional atomic force microscopy revealed that calcium ion (Ca2+) substitution modifies a relatively coarse, highly agglomerated surface into a finer and more uniform nanostructure, decreasing the mean particle size derived from AFM from 51.7 nm to 41.06 nm and increasing the surface factor, S, from 1.943 × 105 to 2.392 × 105 cm2 g−1. UV–Vis–NIR diffuse reflectance spectroscopy, analyzed with Tauc plots, revealed a notable blue shift in the fundamental absorption edge, accompanied by an increase in the direct bandgap from 3.0 eV in CdFe2O4 to 3.3 eV in Cd0.5Ca0.5Fe2O4. This change aligns with the compositional tuning of the electronic structure in nanoferrites. Furthermore, substantial Urbach energies (1.36 eV for CdFe2O4 and 1.14 eV for Cd0.5Ca0.5Fe2O4) clearly indicate that low-energy absorption is predominantly governed by dense defect-related tail states rather than intrinsic band-edge transitions, highlighting an unusual defect landscape that significantly influences optical responses. The refractive index, derived from reflectance data, decreases slightly with Ca2+ doping, offering additional avenues to manipulate optical constants relevant to nanophotonic and optoelectronic applications. Antifungal assays conducted against Aspergillus flavus and Candida albicans demonstrated that undoped CdFe2O4 exhibits potent, broad-spectrum antifungal activity, with inhibition zones measuring 19 mm and 14 mm, respectively, and surpasses Amphotericin B against A. flavus. In contrast, Ca-doped Cd0.5Ca0.5Fe2O4 maintains moderate activity exclusively against A. flavus and shows no activity against C. albicans. This composition-dependent antifungal specificity, coupled with morphology and defect-controlled optical properties, underscores the potential of calcium-doped CdFe2O4 nanoferrites as a versatile nanoplatform. These findings hold promising implications for the development of nano-enabled antifungal surface coatings strictly for inanimate, high-risk industrial environments, contingent on minimizing cadmium leaching via polymeric encapsulation.