Thus, the core/shell ratio of 1:0.25 was chosen for the following experimental steps. In this work, it was found when the core/shell ratio was 1:0.25, the YVO 4:Eu 4 core–shell nanoparticles had the highest photoluminescence intensity. The molar ratios of the core to the shell (core/shell ratio) were 1:0.2, 1:0.25, 1:0.5, and 1:1. The resultant YVO 4:Eu 4 core–shell nanoparticles were collected by centrifugation, washed with anhydrous ethanol for 3 times, and dried at 80 ☌ in air. The mixture solution was poured into a 50 mL Teflon-lined stainless steel autoclave and heated at 200 ☌ for 2 h, and then naturally cooled down to room temperature. Third, 17 mL of de-ionized water was added to the above mixture solution under magnetic stirring for another 5 min. The above two solutions were mixed, and appropriate amount of Y(NO 3) 3♶H 2O was added to the mixture solution with vigorous magnetic stirring for 5 min. Second, 1 mmol of the annealed YVO 4:Eu 3+ nanoparticles were re-dispersed into 9 mL ethylene glycol under continuous ultrasonication for 30 min. First, 0.75 mmol Na 3VO 4♱2H 2O and 0.05 g PVP were dissolved in 8 mL ethylene glycol and 1 mL deionized water. The pristine YVO 4 shell, which is homo-nanoparticled to the core, is expected to passivate the surface non-radiative traps or defects of the annealed YVO 4:Eu 3+ nanoparticles. The annealed YVO 4:Eu 3+ nanoparticles ( Section 2.2) were used as a core to grow a pristine YVO 4 shell for forming the YVO 4:Eu 4 core–shell nanoparticles ( Figure 1). Previous work on LaF 3:Eu 3+ has shown that its excitation spectral bandwidth can be effectively broadened by using a mixed photoluminescence sensitization from different types of organic ligands. Up to now, however, the excitation bandwidth of YVO 4:Ln 3+ is still not wide enough to cover the UV spectrum of the solar irradiance. It was found that the peaking wavelength of its excitation spectrum is shifted to the longer wavelength at 342 nm and the band edge is extended to about 360 nm. To extend its excitation spectrum, Bi 3+ ions have been co-doped into YVO 4:Ln 3+. This, however, is too narrow to cover the UV spectrum (250–400 nm) of the solar lights on earth surface for applications in solar cells. The excitation spectrum of VO 4 3− peaks at about 325 nm, and has a spectral range from 260 nm to 330 nm at the half maximum and a band edge at 350 nm. Such an energy transfer process effectively overcomes the drawback of low absorption cross sections of parity forbidden 4f–4f transitions of Ln 3+ ions. The strong photoluminescence of YVO 4:Ln 3+ relies on the efficient energy transfer process from the host VO 4 3- complexes to the Ln 3+, in which the VO 4 3- is well excited by the UV lights.

Yvo4 yvo4 core shell