Physical and electrochemical characterizations of erbium-doped tetraborate Li_1.9Er_0.1B₄O₇

Lithium tetraborate has received great attention due to its use in optical devices and solid batteries. The Er³⁺ doped lithium tetraborate (LTB-10%Er) was synthesized by solid-state reaction. The single-phase crystallizes in a tetragonal symmetry and a crystallite size of 66.4 nm was determined from the Williamson-Hall (W–H) plot. The Scanning Electron Microscopy shows irregularly shaped grains with sizes ranging from 0.4 to 2 μm. The presence of BO₃ and BO₄ units in the crystal structure and molecular associations were confirmed by attenuated total reflectance and Raman spectroscopy. X-ray Photoelectron Spectroscopy analysis allowed us to determine the chemical composition and valence states of LTB-10%Er, confirming the successful insertion of Er³⁺ into the Li⁺ site. A direct optical transition at 3.23 eV was determined by UV–visible spectrophotometry. The electrochemistry of the doped sample is studied for the first time; the n-type behavior is evidenced from the positive slope of the characteristic “Capacitance⁻²–Potential (C⁻²–E)” in the electrolytic solution (Na₂SO₄, 0.1 M). The flat band potential (E_fb = 0.29 V_SCE) is close to the photocurrent triggering potential (E_on = 0.22 V_SCE), indicating the absence of sub-band states in the band gap. The combined optical/electrochemical properties allowed to calculate the valence band (+ 2.56 V_SCE/7.31 eV_vacuum) potentials formed by the orbital O²⁻: 2p and conduction band (−0.67 V_SCE/4.08 eV_vacuum).