Defect-Mediated Optical and Dielectric Behavior of NiAl 2 O 4 Spinel and Its Correlation with Visible-Light Rhodamine B Photodegradation

Nickel aluminate spinel (NiAl 2 O 4 ) was synthesized and investigated as a multifunctional oxide material with emphasis on its structural, optical, dielectric, and photocatalytic properties. X-ray diffraction confirmed the formation of a well-crystallized cubic spinel phase, while Raman spectroscopy revealed characteristic vibrational modes associated with the NiAl 2 O 4 framework and indicated the presence of local structural disorder. SEM observations showed an agglomerated morphology composed of irregular particles, and EDS analysis confirmed the presence of Ni, Al, and O as the main constituent elements. XPS analysis further evidenced the existence of Ni 2+ , Al 3+ , lattice oxygen, and oxygen-related surface defect species. Optical analysis revealed a band gap of about 3.36 eV, indicating that the material mainly absorbs in the near-UV region with limited visible-light response. Photoluminescence deconvolution showed broad defect-related emissions, suggesting the presence of localized states that can act as charge-trapping and recombination centers. The optical constants, including the refractive index, extinction coefficient, optically derived dielectric function, and optical conductivity, exhibited pronounced photon-energy dependence associated with the electronic response of the spinel. The real and imaginary components of the optical dielectric function showed dispersive behavior consistent with the wavelength-dependent optical response and the presence of localized electronic states. These optically derived quantities are interpreted within the investigated optical spectral range and are not considered equivalent to dielectric or electrical transport parameters obtained by conventional impedance spectroscopy. Dielectric analysis revealed significant dispersion of ε′ and ε″, together with non-linear variations in dissipation factor and relaxation time, showing a complex response that is consistent with contributions from localized electronic states and defect-related processes. The photocatalytic activity was evaluated through Rhodamine B degradation under visible-light irradiation. Compared with direct photolysis, NiAl 2 O 4 enhanced the degradation efficiency from 9.63% to 44.3% after 90 min, with an apparent rate constant of 3.31 × 10 −3 min −1 . The moderate photocatalytic response is discussed in relation to the wide optical band gap, defect-related electronic states, and agglomerated morphology, which may collectively limit visible-light utilization and effective charge transfer. This work highlights the relationships among crystal structure, defect-related spectroscopic features, optical response, and photocatalytic behavior, providing useful insight into the intrinsic properties of pristine NiAl 2 O 4 .