Tunable structure and intensive upconversion photoluminescence for Ho3+-Yb3+ codoped bismuth titanate composite synthesized by sol-gel-combustion (SGC) method

Abstract Ho3+ and Yb3+ codoped bismuth titanate (BTO) composite powders with infrared to visible upconversion luminescence (UCL) function were prepared by SGC method. The effects of Ho3+ and Yb3+ doping content on the structure and property were investigated for BTO: xHo, 0.2 Yb (x = 0, 0.02, 0.04, 0.06, 0.08, 0.1) and BTO: 0.02Ho, yYb (y = 0.1, 0.2, 0.3, 0.5, 0.7, 0.9) samples. All the samples include three bismuth titanate phases (Bi4Ti3O12, Bi2Ti2O7, and Bi20TiO32), and the phase proportion can be tuned by changing Ho3+ and Yb3+ doping content. These powders are well crystalized with honeycomb-like microscopic structure, and with good absorption for 233 nm, 310 nm and 975 nm wavelength. The band gap can be tuned from 3.53 eV to 4.03 eV when increasing Yb3+ content from y = 0 to y = 0.9. A strong 530–580 nm green emission band and a relative weak 630–690 nm red one corresponding to Ho3+: 5S2 → 5I8 and 5F5 → 5I8 transitions appear in the UCL spectra for all the BTO: Ho, Yb samples when pumped at 980 nm. The emission intensities can well be tuned with various Ho3+ and Yb3+ content. The optimal UCL was obtained in BTO: 0.02Ho, 0.5 Yb for all the prepared samples. The energy transfer mechanism is analyzed by building a two-photon energy transfer model, which is proved by the relationship between emission intensities and pumping power measurement. The concentration quenching of Ho3+ is caused by cross relaxation of CR1 and CR2 (Ho: 5F4, 5S2 + 5I8 → 5I4 + 5I7) and by CR3 (Ho: 5F4, 5S2 + Yb: 2F7/2 → Ho: 5I6 + Yb: 2F5/2) for Yb3+ quenching. The mean luminescence lifetime (τm) from Ho: 5S2 decreases monotonously with the increase of Ho3+ and Yb3+ content.