X-ray responsiveness of sol–gel-derived glasses doped with rare-earth ions

Rare-earth doped glasses have garnered interest due to their potential applications in light-emitting devices. Although the sol–gel technique is useful in preparing them at moderately low temperatures, developing silicate glasses with excellent photoluminescence performance remains a formidable challenge due to their low solubility in the glass matrix and the difficulty in controlling valence states of rare-earth ions (RE). Here, we investigated whether these RE ions are reduced by heating in a hydrogen gas atmosphere or by irradiating with X-rays. We have succeeded in synthesizing Sm3+ and Eu3+ ion-doped Al2O3–SiO2 glasses with exceptionally strong photoluminescence. When heated in hydrogen gas, the Sm3+ and Eu3+ ions were reduced to their divalent states. However, when irradiated with X-rays, only Sm3+ ions were reduced to Sm2+; no reduction occurred in the Eu3+ ions. This was because when irradiated with X-rays, the hole centers become trapped in the oxygen ions bound to the Al3+ ions, and the electrons released from the oxygen ions are consequently captured by the nearest Sm3+ ions, resulting in the formation of Sm2+. In contrast, such a reduction does not occur in the Eu3+-doped glasses. It was further found that the reduced Sm2+ ions are easily oxidized to Sm3+ ions by heating at 250 °C in air. Thus, the Sm3+-doped Al2O3–SiO2 glasses could be used for X-ray therapy and sensor applications due to their fast redox reactions.