Hydrothermal synthesis of defect-induced pristine α-NaCe(WO4)2: a novel material for solid state lighting and gas sensing

Abstract

Triclinic NaCe(WO₄)₂ with oxygen monovacancies and divacancies has been successfully prepared via a facile cetyltrimethyl ammonium bromide (CTAB)-assisted hydrothermal technique. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been employed to determine the unit cell and microstructure of the NaCe(WO₄)₂. The oxygen vacancies, structural distortion etc. have been investigated using Fourier-transform infrared, Raman and X-ray photoelectron spectroscopies. The synthesized samples exhibit an intense blue emission at 434 nm due to the 5d–4f transition of Ce³⁺ within the CeO₈ dodecahedra, while the emission at 485 nm is ascribed to the 5d–4f transition within CeO₇. It has also been identified that two emissions at 451 and 520 nm come from CeO₆. Additionally, we find that the temperature of the hydrothermal reaction guides the formation of CeO₇ and CeO₆. In contrast to a previous ethylenediamine tetraacetic acid (EDTA)-assisted synthesis of NaCe(WO₄)₂ that results in a predominant green emission, our samples exhibit strong violet emissions indicating that less CeO₇ and CeO₆ is formed when using CTAB. We have also conducted ab initio calculations using density-functional theory, which reveals that the valence and conduction bands comprise of the O 2p orbitals and a O 2p–Ce 5d hybridization, respectively. The Ce 5d_z², 5d_yz and 5d_xz orbitals mostly facilitate the 5d–4f transition within the CeO₇ and CeO₆ polyhedra. Commission Internationale de I'Eclairage coordinates are found in the blue region with a correlated color temperature (CCT) of ∼7715 K indicating the potential for α-NaCe(WO₄)₂ to be used in cold solid state lighting applications. Finally, we also observe that the oxygen vacancies can act as active centers for the adsorption of molecular oxygen, which in consequence leads NaCe(WO₄)₂ to have gas sensing properties.