A Facile Green Synthesis and Optical Characterization of Nano Zinc Aluminate Phosphor

Abstract

Abstract Zinc based aluminate offer a wide energy band gap (3.8 eV) suitable for a phosphor applications. Therefore, in solid state lighting it is being widely used as a host material with rare earth and transition metal dopant ions. In addition, it has attracted considerable attention because of its excellent optical transparency, wide band-gap energy, high quantum yields and interesting thermal, mechanical and magnetic properties [1]. The typical applications of zinc aluminate are in optical and sensing devices, ceramic industries, as catalyst in industries, as paints in aerospace industry, dielectrics, and electronic applications [2-3]. The development of eco-friendly nano phosphors with high efficacy and stability is most important and urgent challenge for solid state lighting. We present facile green synthesis of nano zinc aluminate nanophosphor by microwave combustion method. Naturally available, piper nigrum seeds are used as a combustion agent. To study the effect of temperature on structural, compositional and optical properties of nano zinc aluminate, the as synthesized sample was further annealed at temperature 700⁰C and 900⁰C. The structural and optical characterizations were carried out using X-ray diffraction, diffuse reflectance spectroscopy and photoluminescence spectroscopy. Further the chemical compositions of the synthesized samples were confirmed by Fourier transform infra-red spectra. The X-ray diffraction patterns and Rietveld refinement analysis confirmed the cubic crystal system with space group Fd-3m. The average crystallite size calculated by Scherrer’s and Williamson Hall (W–H) plots are found to be in the 5-20 nm. The crystallinity index for the as prepared sample is found to be 60.17%. Samples annealed at temperature 700⁰C and 900⁰C reflect improved crystallinity. The optical behavior and band gap of prepared samples was determined by diffuse reflectance spectroscopy where Kubelka-Munk function is used to convert the reflectance data into linear absorption coefficient.The band gap energy of as prepared nano zinc aluminate  is found to be 4.48 eV which is larger than bulk band gap of zinc aluminate. The nano size formation prominent the effective band gap. The energy band gap is found to be modified at higher annealing temperatures. The obtained Fourier transform infra-red spectra explicate vibrational stretching frequencies corresponding to the cubic structure of nano zinc aluminate. The Fourier transform infra-red spectra also reveal distribution of metal ions in two different environments i.e. tetrahedral coordinated and octahedral coordinated. The vibrational frequency in the 400-900 cm^-1 are related to the vibration of Zn-O, Al-O and Zn-O-Al bonds of nano zinc aluminate. The luminescence characteristics of the nano zinc aluminate have been observed and analysed using photoluminescence spectroscopy. The most appealing aspect of the present study is the use of natural compound as raw material for the formation of nano zinc aluminate.