Green upconversion luminescence of ZnO:Yb3+/Ho3+ nanophosphors prepared by various synthesis methods: Sol-gel and co-precipitation

Abstract

ZnO: Yb3+/Ho3+ nanophosphors were synthesized using sol-gel and co-precipitation methods. The effects of co-doping and synthesis methods were investigated on the nanopowder's crystal structure, morphology, and optical properties. X-ray diffraction (XRD) confirmed the hexagonal wurtzite structure of the ZnO powder. The calculated crystallite sizes of the nanopowders are 30.4 nm and 21.3 nm for sol-gel and co-precipitation methods, respectively. Scanning electron microscopy (SEM) revealed that different synthesis methods and doping influence the morphology of the prepared nanophosphors. Energy dispersive X-ray spectrometer (EDS) confirmed the elemental composition and phase purity. Diffuse reflectance spectra (DRS) exhibited several absorption bands at 448 and 541 nm from sol-gel synthesized nanopowders and 453, 483, 536, and 641 nm from co-precipitation synthesized nanopowders. The energy band gap (Eg) was affected by the various methods and doping. DRS with the aid of Kubelka-Munk demonstrated that Eg ranges from 3.23 to 3.27 eV and 3.28–3.33 eV for samples prepared with sol-gel and co-precipitation method, respectively. The upconversion (UC) photoluminescence (PL) emission spectra upon excitation wavelength of 980 nm revealed two emission peaks located at 545 and 661 nm from ZnO:Yb3+/Ho3+ nanophosphors prepared by sol-gel method. These emission peaks can be attributed to 5S2/5F4 → 4I8 and 5F5 → 5I8 transitions of Ho3+ ions. The power dependence of the UC emission intensity of ZnO:Yb3+/Ho3+ was also examined. Power dependence revealed that a two-photon process was involved in the UC emissions and the possible UC mechanisms were discussed in detail. To investigate possible degradation, the stability of the UC was examined. In fact, there was no indication of phosphor UC degradation. Green emission, on the other hand, increased by about 10 % during the first 8 h of the experiment before stabilizing.