Solid state phosphors in the form of nanosized particles attract a lot of attention. This interest is of both fundamental and practical character. Ultrafine and homogeneous fluorescence particles can be used as biological labels and in the various lightning or display equipment.
This paper explores the possibility to synthesize one of the most practically useful materials with strong red emission – YVO4 activated with Eu+3. The coprecipitation reaction was used in this work. The first synthesis by the similar approach was described in 1965, while this paper focuses on fine tuning of the solution chemistry in order to control precipitation process and to fabricate nanopowders of YVO4:Eu without secondary phases. The prepared materials were extensively characterized by XRD, TEM, and fluorescence spectroscopy.
The prepared materials were found to be single phase, and the lattice parameters of the Y1-xEuxVO4 solid solutions vs x follow Vegard’s law, which indicate systematic replacement of Y by Eu and absence of possible Eu containing amorphous phases. The particles size determined using Scherrer’s equation was 14 nm, while TEM data and light scattering measurements show average particle size to be 30-40nm, which can indicate presence of defects in the 40 nm crystallites. Unfortunately, the paper does not mention how the diffractometer contributions into the X-ray peak width were taken into account, and therefore it is difficult to conclude on the actual size of the coherent domains.
The emission spectra contain the intense peak at 615 nm corresponding to the 5D0-7F2 transition of the Eu3+ ion, while the excitation spectrum can be explained by the absorption of light by the VO43- species. For the range of Y1-xEuxVO4 (0 < x <0.5) composition, the maximum quantum yield was found at 15% for a europium content of x = 0.3. It should be mentioned that in the case of bulk materials a quantum efficiency of 70% is observed at x = 0.05. The difference in luminescence properties between the materials synthesized in the solution and the ones prepared by solid state reaction is explained by non-radiative recombination centers in the form of the -OH groups on the surface and defects in the crystal lattice. It was shown that replacement of usual water by deuterated water (which does not quench the fluorescence of Eu centers) as well as mild heating of the powders improves quantum efficiency and life time of the excited state.
The synthesis procedure in this work is very clearly described, and it can be reproduced easily. We used such synthesis in the introductory Materials Science laboratory course for freshmen students, and the same reaction can be used to synthesize Dy, Er, Tm and Sm activated YVO4 phosphors. The synthesis protocol can be described as follows. 2mmol of sodium metavanadate is dissolved in 40 ml of deionized water. The complete dissolution is achieved after 20 min of magnetic stirring. Then, the obtained solution if combined with 2 ml of 3M NaOH solution to yield Na3VO4 solution. The complete transformation of metavanadate into orthovanadate occurs at pH ~13.5. In parallel, 19 ml of 0.1 M solution of Y(NO3)3 should be combined with 4 ml of 0.025M Eu(NO3)3 solution. This ratio corresponds to 5 mol% of activator ions in the YVO4 phosphor. Attention: nitrites of rare-earth elements contain poorly defined amount of crystalline water, therefore, the exact concentration of metal ions in the solution should be analyzed to ensure right stoichiometry of the final material. The coprecipitation reaction between orthovanadate and RE nitrate solutions
Na3VO4 + Y(NO3)3 = YVO4 ↓+ 3NaNO3
is carried under vigorous magnetic stirring by adding drop wise (approximately 1 drop/s) the Y(NO3)3+Eu(NO3)3 solution INTO Na3VO4 solution using a titration burette. Formation of yellow powder indicates presence of polyvanadate ions. Presence of Y(OH)3 can not be identified visually, and it can be found only by the XRD characterization of the prepared materials.
The obtained colloid solution can be stabilized by addition of sodium hexametaphosphate (NaPO3)12-13*Na2O. In our synthesis long stability of colloid solution was not critical, and we aged the precipitate for 1h at 80OC. After aging, the powder material can be separated by centrifuge (also quite good yield can be achieved in a conventional filtration using Watman No.5C filtering paper). After washing, the powder should be dried at 105OC for 30-40 min.
To improve fluorescence intensity, we additionally heated up the synthesized powders at 900OC for 15 min in the alumina boat placed into the pre-heated tube furnace, followed by quenching into the air. SEM characterization of the annealed powders did not reveal any substantial agglomeration or crystal growth after the short annealing. The increase of emission intensity after heating is in agreement with the conclusion reached in the paper on the effect of crystal defects on luminescence quenching.