Multicolor luminescent material based on interaction between TiNbO ₅ ⁻ nanosheets and lanthanide ions for visualization of pH change in inorganic gel electrolyte

Layered nanosheet materials showing a drastic luminescence change in response to changes in proton concentration (pH) were prepared by sandwiching Eu³⁺ and Tb³⁺ cations with anionic TiNbO₅^- nanosheets using electrostatic interaction. Each trivalent lanthanide ion showed a different response to pH change: a strong red emission from Eu³⁺ was observed at low proton concentrations (pH: 13) and a green emission from Tb³⁺ was dominant at high proton concentrations (pH: 1). The photoluminescence intensity was determined by the balance between the photocatalytic activity of TiNbO₅^- nanosheets and energy transfer from the host layer to the guest lanthanide ions. Moreover, the trivalent lanthanide/TiNbO₅^- nanosheet hybrid formed a gel-like solid in aqueous solution, which functioned as an inorganic gel electrolyte when mixed with Na₂SO₄. The multicolor luminescence (red-yellow-green) of the lanthanide/TiNbO₅^- nanosheet hybrid enabled direct visualization of the diffusion of protons in an inorganic gel electrolyte during water electrolysis.