Particularly developed transition from the 5D1 level of Eu3+ and its significant contribution to the improved photocatalysis of (Bi3Li)O4Cl2via prolonging the decay time of the excited state

Abstract

There is a close correlation between the photoluminescence and photocatalysis of a semiconductor because the two processes involve the same electronic transition. Eu³⁺ is one of the most popular rare-earth ions; it presents efficient luminescence and improved photocatalysis. Usually, the luminescence from the higher states (⁵D_1,2) of Eu³⁺ can be hardly observed due to cross-relaxation processes. This work demonstrates a particularly developed transition from the ⁵D₁ level of Eu³⁺ and its significant contribution to the improved photocatalysis of (Bi₃Li)O₄Cl₂via prolonging the decay time of the excited state. Single-phase (Bi_3−3xEu_3xLi)O₄Cl₂ (x = 0, 0.01, 0.03, 0.05) was synthesized by the sol–gel method in combination with a solid-state reaction. This is a bismuth layer structure with [(Bi/Li)O₂]²⁺ layers formed by alternative Bi³⁺ and Li⁺ stripes. The intrinsic luminescence of (Bi₃Li)O₄Cl₂ (λ_em = 500 nm) with a decay time of 0.26 μs was detected even at 300 K. Unusually, the phosphor shows a prominent transition from ⁵D₁ in (Bi_3−3xEu_3xLi)O₄Cl₂. Moreover, in addition to ⁵D₀ → ⁷F₃, ⁵D₀ → ⁷F_J (J = 0, 1, 2, 4) is characterized by intense transitions with comparable intensities. The intrinsic emission of (Bi₃Li)O₄Cl₂ has a decay time which involves the band transition of an electron from the valence band to the conduction band. In Eu³⁺-doped (Bi₃Li)O₄Cl₂, there are midgap states formed by the ⁵D₁ level with a longer lifetime of about 60 μs. The D₁ levels of Eu³⁺ significantly contribute to the separation of light-induced charges by prolonging the decay time of the excited states. This work demonstrates a simple strategy to develop optical materials with simultaneous luminescence and has improved the photocatalysis in bismuth oxychlorides featuring strong polarization and rigid phonons.