Spectroscopic Characterization and Luminescence Study of Zinc Aluminate Nanocrystals Prepared by Microwave Combustion Method using Natural Black Pepper as Combustion Agent

Undoped znc aluminate (ZnAl 2 O 4 ) nanocrystals were prepared via microwave combustion using natural black pepper extract as fuel. We present the effect of annealing temperatures on the luminescence, structural, and optical properties of zinc aluminate nanocrystals. The samples were chemically and structurally characterized by different techniques including x-ray diffraction, energy dispersive x-ray spectroscopy, Fourier-transform infra-red, and Raman spectroscopy. Further, the optical and luminescence behavior was studied by diffuse reflectance and photoluminescence measurements. The results indicated the formation of normal spinel, single phase ZnAl 2 O 4 with the size in the range 8–22 nm changed by the annealing temperature. No structural transformation was observed after annealing up to 900°C. The presence of Zn-O, Al-O, and Al-Zn bonds and the change in the bond length of the Zn-O bond after annealing the zinc aluminate spinel revealed the existence of certain antisite defects. The annealed zinc aluminate nanocrystals presented an improvement in optical reflectivity (86%) in the visible region and a strong absorption band near 220 nm. In the photoluminescence study, emission bands in the violet-blue regions with weak emission in the green region were observed under UV excitation wavelengths. It was found that the annealing temperature influences a major impact on defects formation and defect-related luminescence characteristics. A significantly enhanced broad blue emission band centered at ~ 450 nm was dominant after thermal treatment, confirmed by the CIE diagram. The possible electronic transitions are presented and discussed by an energy-level diagram. The average decay time of the ZnAl 2 O 4 nanocrystals annealed at 900°C measured by photoluminescence decay was 0.21 ms. Thus, prepared ZnAl 2 O 4 nanocrystals can be potentially applied for solid-state lighting as blue-emitting luminescent material and for biosensing applications.