High efficiency energy transfer in Ce,Tb co-doped silica prepared by sol-gel method

Amorphous Ce,Tb co-doped silica (SiO₂) was prepared using the sol-gel method with the aim of studying the energy transfer from Ce to Tb ions. It was initially found that adding Ce reduced the Tb emission intensity, implying that energy transfer did not occur. In fact, the Ce single doped sample exhibited very poor emission. Ultraviolet-visible diffuse reflectance measurements displayed the signature of non-luminescent Ce⁴⁺ rather than Ce³⁺ ions. We therefore annealed the samples in a reducing atmosphere of 4% hydrogen in argon gas at 1000°C. The reduced Ce single doped samples exhibited bright luminescence and the diffuse reflectance measurements now showed the characteristic of Ce³⁺ ions. The reduced Ce,Tb co-doped samples excited at the Ce absorption wavelength gave characteristic Tb emission (with negligible Ce emission), indicating that very efficient energy transfer from Ce to Tb was achieved. For samples containing 1 mol% Tb, the maximum luminescence was found when co-doping with 0.5 mol% Ce. Using such co-doped silica samples exhibiting energy transfer, it is possible to obtain effective luminescence from the Tb³⁺ ions via excitation of Ce³⁺ ions at 325 nm, instead of exciting the Tb³⁺ ions directly which requires a shorter wavelength of about 227 nm.