Ce4+/Ce3+ redox-controlled luminescence ‘off/on’ switching of highly oriented Ce(OH)2Cl and Tb-doped Ce(OH)2Cl films

Abstract

The development of cerium (Ce) hosts and cerium-doped materials is required to meet the growing applications of cerium because cerium containing materials are attractive for many applications such as optical devices, fuel cells, gas sensors, oxygen adsorbents, and selective photocatalysts. In an effort to find new or little-known cerium compounds, Ce(OH)₂Cl, a member of the RE(OH)_3−xCl_x (RE = rare earths) family, was successfully synthesized under ambient pressure. Based on the Rietveld structure refinement of the X-ray diffraction pattern, Ce(OH)₂Cl crystallizes in the monoclinic space group P2₁/m with unit cell parameters of a = 6.2898(2) (Å), b = 3.9456(1) (Å), c = 6.8680(2) (Å), and β = 113.54(1)°. The effective 4f → 5d excitation of a Ce³⁺ ion in this compound caused a strong blue emission, which was attributed to electronic transitions from the 5d¹ excited state to the ²F_5/2 and ²F_7/2 levels of the 4f¹ ground state. Furthermore, Tb-doped Ce(OH)₂Cl showed a strong green ⁵D₄ → ⁷F₅ emission of Tb³⁺ upon excitation of Ce³⁺ ions due to the efficient energy transfer from Ce³⁺ to Tb³⁺ ions. In particular, the development of a synthetic route at ambient pressure allowed for simple deposition of Ce(OH)₂Cl and Tb-doped Ce(OH)₂Cl films. The growth direction of the film was significantly dependent on the substrate. The luminescence ‘turn-off/on’ switching was accomplished with highly oriented Ce(OH)₂Cl and Tb-doped Ce(OH)₂Cl films deposited on FTO glass. Both blue and green emissions could be reversibly quenched and restored by repeated oxidation and reduction in KMnO₄ and ascorbic acid solutions, respectively. This Ce⁴⁺/Ce³⁺ redox-controlled luminescence ‘off/on’ switching was operated with a brightness change enough to see visually but without a significant degradation of crystal structure and surface morphology.