Influence of calcination temperature on phase, powder morphology and photoluminescence characteristics of Eu-doped ZnO nanophosphors prepared using sodium borohydride

Abstract

The prime focus of this work is to synthesize Eu-doped ZnO using sodium borohydride, and analyse the phase formation, functional groups, morphology, elemental mapping and photoluminescence characteristics including emission intensity, R/O ratio, luminescence lifetime, CIE, CCT and colour purity of Eu-doped ZnO phosphors, calcined in a wide range of temperature. XRD analysis confirmed the presence of only ZnO up to 800 °C. Minor phases such as EuBO3 and Eu(BO2)3 were also observed along with the major phase of ZnO at and above 1000 °C. FTIR results suggested that the presence of metal-oxygen, borate (BO3 and BO4) and hydroxyl (O–H/B–OH) groups in the as-synthesized sample may help to form EuBO3 and Eu(BO2)3 phases along with ZnO at higher temperature. Particles were found to be elliptical or elongated rod and polyhedral shape up to 800 °C, and it became nearly spherical and dumb-bell shape at 1000 °C. At high temperature of 1200 °C, the particles were consolidated in nature. Non-homogeneous distribution of Eu3+ ions were observed up to moderate temperature, and it became segregated/clustered in the ZnO matrix at 1200 °C, as seen from EDX mapping. The photoluminescence characteristics revealed an intense broad transition of 5D0 → 7F2 at 613 nm for samples calcined up to 800 °C. However, a splitting nature in 5D0 → 7F2 transition was observed for 1000 °C and 1200 °C samples, due to the non-homogenous distribution of Eu3+-ions along with induced secondary phases. The luminescence decay was analysed with the help of double exponential function, and the average lifetime of the calcined (400 °C) sample was found to be ∼1.43 ns. It was observed that the presence of secondary phases in the calcined (1000 °C and 1200 °C) samples do not hamper, rather moderately improve the photoluminescence behaviour of Eu-doped ZnO. Based on photoluminescence characteristics results, it may be suggested that the prepared samples might be a feasible material for use as a yellowish-orange, red or deep red-emitting phosphors.