Abstract
In this investigation, the solid-state reaction method was used to synthesize Ce3+, Eu3+, Eu2+ and SO42- doped ZnO nanoparticle powder phosphors that were prepared included ZnO-SO4:xCe3+, ZnO-SO4:xEu3+, and ZnO-SO4:Ce3+:xEu3+ with varying molar concentrations (where x = 0.5, 1.0, 1.5, 2.0 and 2.5 mol %). Furthermore, ZnO-SO4: xEu2+ and ZnO-SO4:Ce3+:xEu2+ (where x = 0.5, 1.0, 1.5, 2.0, 2.5 and 3.0 mol %) were also prepared under reducing atmosphere [argon (Ar) and hydrogen (H2) gas]. The technique of X-ray diffraction (XRD) was utilised to identify crystalline phase, the particle size distribution and crystalline degree of the prepared phosphor materials. Scanning Electron Microscopy with Energy Dispersion Spectroscopy (SEM-EDS) measurements were used to evalute the surface morphology (sizes and shapes) of the phosphor materials. The elemental composition of the phosphor material (EDS) was evaluated by utilizing SEM-EDS. Fourier Transformation Infrared (FT-IR) utilized to identify the stretching/bending mode frequencies for different structural groups. Additionally, the chemical valence states of the compound atoms were investigated using X-ray Photoelectron spectroscopy. The absorption properties of the phosphors were studied to use the ultraviolet-visible-near infrared (UV-Vis) optical spectroscopy. The energy band gap values were estimated from the reflectance data by means of the Kubelka-Munk function.
The XRD results confirmed that our materials crystallized into hexagonal wurzite structures of ZnO. This was consistent with the standard JCPDS crystallographic data file no. 01-089-7102. The addition of SO42- anionic group and Ce3+, Eu3+ and Eu2+ dopants had no influence in the main crystal structure of ZnO. Using the Scherrer’s equation, the crystal sizes of the materials were calculated and that the average particles sizes in the range of 36 to 78 nm for the ZnO-SO4: Ce3+, ZnO-SO4: x Eu3+ and ZnO-SO4: Ce3+, xEu3+ while for ZnO-SO4: x Eu2+ and ZnO-SO4: Ce3+, Eu2+ from the average crystallite size ranged from 11 to 26 nanometers. SEM results displayed that our powders were composed of irregular particles with different shapes and sizes. The EDS analysis confirmed the existence of all the chemical elements from which our materials were made. For further structural ilucidation, electronic states and chemical composition analyses, the FTIR and XPS were used, and the data confirmed successful crystallization of the hexagonal wurtzite ZnO incorporating all the dopants. The photoluminescene spectra of ZnO-SO4: Ce3+ were dominated by photoemission attributed with native defects in ZnO. An intense red photoluminescence was obtained from PL properties confirms an intense red color emission for a 2 mol% of Eu3+ doped ZnO-SO4 phosphor under excitation of 466 nm. This emission was ascribed to the Eu3+ transition 5D0 →7F2. In contrast, the PL emission for the ZnO-SO4: x Eu2+ lies in the blue region, in accordance with 4f 65d1- 4f7 transition in the Eu2+ ion, with a 351 nm excitation wavelength. From all the PL results we noticed that ZnO defect emissions such as zinc interstitials (Zni), zinc vacancies (VZn) and oxygen vacancy (Vo) could be influenced by different rare earth ions in producing different color emissions. Apart from singly doped ZnO-SO4, doubly doped ZnO-SO4 matrices also influenced the defect emissions of these materials. For example, defects emission from ZnO-SO4: Ce3+, x Eu3+ was influenced by the dopant ions. In this compound, energy was transferred from Ce3+ to Eu3+ ions, which accounts for the enhanced red emissions observed at 618 nm. Similarly, transfer of energy from Ce3+ to Eu2+ was observed in ZnO-SO4: Ce3+, xEu2+ compound, it accounts for enhanced blue emission around the 459- 495 nm region. From the above PL results, emissions in the visible region could be enhanced in two possible ways, viz through defect emission or through the energy transfer phemomena. Findings from PL results suggested that these phosphor materials could become a good candidate for use in color display technologies. KEYWORDS
Solid-state method, ZnO, ZnO-SO4, Rare earth (Eu3+, Eu2+, Ce3+), photoluminescence, energy transfer.KEYWORDS
Solid-state method, ZnO, ZnO-SO4, Rare earth (Eu3+, Eu2+, Ce3+), photoluminescence, energy transfer.