Method for producing luminescent nanoparticles

Abstract

The present invention relates to a method for producing luminescent nanoparticles wherein particle size of the nanoparticles is controlled. The method of the present invention includes admixing two or more rare earth metal salts in a first solvent and an organic oil to form a reaction mixture, and subjecting the reaction mixture to an inert gas so that flow rate of the inert gas is at least 2-5 L/h and pressure in the reaction vessel is at least 50 Pa over atmospheric pressure, preferably 50-80 Pa over atmospheric pressure.

Claims

1. A method for producing luminescent hexagonal non-aggregated upconverting nanoparticles selected from NaYF.sub.4:Yb, Er and NaYF.sub.4:Yb, Tm, the method comprising steps of: admixing two or more rare earth salts in a first solvent, wherein the first solvent is alcohol, and an organic oil, wherein the organic oil comprises a mixture of oleic acid and 1-octadecane to form a reaction mixture in a reaction vessel, subjecting the reaction mixture to a flow of an inert gas, wherein flow rate of the inert gas is 4-5 L/h and pressure in the reaction vessel 50-75 Pa over atmospheric pressure, heating the reaction mixture from a first temperature to a second temperature, wherein the first temperature is 15-25° C., and the second temperature is 150-200° C. cooling the reaction mixture from the second temperature to a third temperature, wherein the third temperature is 15-60° C., admixing to the reaction mixture fluoride ion and sodium ion in a second solvent, wherein the second solvent is alcohol, heating the reaction mixture from the third temperature to a fourth temperature, wherein the fourth temperature is 250-350° C.

2. The method according to claim 1, wherein the fluoride ion is admixed in the form of ammonium fluoride, and the sodium ion is admixed in the form of sodium hydroxide.

3. The method according to claim 1 further comprising collecting the upconverting nanoparticles.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIGS. 1 and 2 show the correlation between the particle size (length and width) and luminescence intensity of NaYF.sub.4: Yb.sup.3+, Er.sup.3+ particles, and

(2) FIGS. 3-9 show TEM images of NaYF.sub.4: Yb.sup.3+, Er.sup.3+ particles prepared according to Examples 1-7, respectively. Scale bar=500 nm (FIG. 3) or 200 nm (FIGS. 4-9).

DESCRIPTION

(3) The present disclosure concerns a method for producing upconverting nanoparticles (UCNPs), the method including steps of: admixing two or more rare earth salts in a first solvent and an organic oil, to form a reaction mixture in a reaction vessel, and subjecting the reaction mixture to a flow of an inert gas, wherein flow rate of the inert gas is 2-5 L/h, preferably 4-5 L/h, most preferably 4.8 L/h, and wherein pressure in the reaction vessel is at least 50 Pa over atmospheric pressure, preferably 50-80 Pa over atmospheric pressure.

(4) FIGS. 1 and 2 and Table 1 show the effect of flow rate in the method for producing an exemplary UCNP, namely NaYF.sub.4: Yb.sup.3+, Er.sup.3+, on the particle size and luminescence intensity. In the figures, the particles in the quarter space A were prepared by using flow rate 5.2 L/h, and the particles in the quarter space B were prepared by using identical reaction conditions but with flow rate of 4.8 L/h. Accordingly, the UCNPs prepared according to the present invention were larger in size and therefore more luminescent than the ones prepared by using a higher flow rate.

(5) FIGS. 3-9 show exemplary TEM images of NaYF.sub.4: Yb.sup.3+, Er.sup.3+ particles prepared by using various flow rates of the inert gas. The results are summarized in Table 2. Accordingly, in order to prevent or at least reduce aggregation, the flow rate of the inert gas should be at least 4 L/h, preferably 4.8 L/h.

(6) When the pressure in the reaction vessel is 50 Pa or more over the atmospheric pressure, the flow rate can be reduced, but it should be at least 2 L/h. The volume of the reaction vessel and especially the width of the exit port (outlet) affects the pressure in the reaction vessel while keeping the flow rate of the inert gas constant.

(7) In order to obtain optimal particles, i.e. to attain optimal particle size and intense luminescence, and to avoid aggregation and the formation of undesired cubic crystals, the flow rate of the inert gas should be 2-5 L/h, and the pressure in the reaction vessel should be at least 50 Pa, preferably 50-75 Pa over the atmospheric pressure 0.1 MPa.

(8) The first solvent is preferably a lower alcohol such as methanol, ethanol or propanol or their mixture, most preferably methanol. The organic oil is preferably a mixture of oleic acid and 1-octadecene or a mixture of 1-octadecene and oleylamine, or oleylamine alone. A preferable molar ratio of oleic acid and 1-octadecene is 0.3 and 0.7, respectively.

(9) The rare earths are preferably selected from a group consisting of yttrium, ytterbium, erbium, thulium, holmium, praseodymium, neodymium, cerium, samarium and terbium, preferably from thulium, yttrium, erbium and ytterbium, more preferably yttrium, ytterbium, erbium or thulium. A preferable salt is a halide salt, most preferably chloride salt. According to an exemplary embodiment the two or more rare earth salts are YCl.sub.3, YbCl.sub.3, and ErCl.sub.3. According to an exemplary embodiment the molar ratios of the above mentioned rare earth halides are 0.80, 0.18 and 0.02, respectively. According to another exemplary embodiment, the rare-earth salts are selected from YCl.sub.3, YbCl.sub.3 and TmCl.sub.3. According to an exemplary embodiment the molar ratios of these rare-earth halides are 0.747, 0.25 and 0.003, respectively.

(10) Exemplary inert gases are argon, nitrogen, helium, and mixtures thereof. A preferable inert gas is argon. The inert gas can be fed over the surface of the reaction mixture in the vessel, or it can be fed into the reaction mixture. The inert gas can be fed at ambient temperature, or it can be preheated to the temperature of the reaction mixture.

(11) According to another embodiment the method according to the present disclosure further includes heating the reaction mixture from a first temperature to a second temperature. The first temperature is preferably ambient temperature, typically 15-25° C., and the second temperature is typically 150-200° C., preferably about 160° C. According to this embodiment, the flow rate of inert gas is 2-5 L/h, preferably 4-5 L/h, most preferably 4.8 L/h.

(12) According to another embodiment the method according to the present disclosure further includes cooling the reaction mixture from the second temperature to a third temperature. The third temperature is typically 15-60° C. According to this embodiment, the flow rate of inert gas is 2-5 L/h, preferably 4-5 L/h, most preferably 4.8 L/h.

(13) According to another embodiment the method according to the present disclosure further includes admixing fluoride ion and sodium ion, in a second solvent, to the cooled reaction mixture and heating the mixture from the third temperature to a fourth temperature. The fourth temperature is preferably 250-350° C., more preferably about 300° C. An exemplary fourth temperature is 310° C. The fluoride ion is preferably admixed in the form of ammonium fluoride and the sodium ion as sodium hydroxide. Exemplary second solvents are lower alcohols such as methanol, ethanol and propanol and their mixtures. A preferable alcohol is methanol. The second solvent is preferably same as the first solvent. According to this embodiment, the flow rate of inert gas is 2-5 L/h, preferably 4-5 L/h, most preferably 4.8 L/h.

(14) According to a particular embodiment, the whole method is performed under the flow of inert gas, wherein the flow rate is 2-5 L/h, preferably 4-5 L/h, most preferably 4.8 L/h.

(15) According to another embodiment the method according to the present disclosure further includes collecting the upconverting nanoparticle. Particular upconverting nanoparticles prepared are NaYF.sub.4: Yb, Er and NaYF.sub.4:Yb, Tm.

(16) An exemplary apparatus suitable for preparation of the UCNPs includes a reaction vessel, such as a three neck flask equipped with inlet and exit ports for an inert gas, a thermometer and heating means such as an electric heating mantle.

Examples

(17) General Procedure

(18) 20 mL RECl.sub.3 (0.2 M, RE=Y, Yb, Er) in methanol were added to a 500 mL flask containing 30 mL oleic acid and 70 mL 1-octadecene and the solution was heated to 160° C. for 40 min and then cooled down to room temperature. Thereafter, 50 mL methanolic solution of NH.sub.4F (16 mmol) and NaOH (10 mmol) was added, and the solution was stirred for 30 min. After methanol was evaporated, the solution was heated to 310° C. for 1.5 h and cooled down to room temperature. The resulting nanoparticles were precipitated by the addition of ethanol and collected by centrifugation. The process was performed by subjecting a flow of argon to the reaction mixture. Results are shown in Tables 1 and 2.

(19) TABLE-US-00001 TABLE 1 Effect of the flow rate on the particle size and luminescence of the product. batch particle length particle width Flow rate L/h # luminescence.sup.a nm nm 5.2 1 83 280 28 33 5.2 2 71 263 27 32 5.2 3 76 064 27 30 5.2 4 70 076 25 31 5.2 5 85 977 25 30 5.2 6 99 980 28 34 5.2 7 107 511  29 34 5.2 8 79 421 27 32 5.2 9 92 648 27 32 5.2 10 69 426 29 34 4.8 11 121 658  31 35 4.8 12 123 703  32 37 4.8 13 131 464  33 37 4.8 14 132 104  31 38 .sup.acounts/μg of UCNP.

(20) TABLE-US-00002 TABLE 2 Effect of flow rate on the quality of the product. Argon flow Example (L/h) Quality of the product 1.sup.a 1.8.sup.c undesired cubic crystals, aggregates 2.sup.a 4.8.sup.c desired hexagonal crystals, some aggregates 3.sup.a 7.8.sup.c desired hexagonal crystals, no aggregates 4.sup.a 1.8.sup.c desired hexagonal crystals, no aggregates 5.sup.a 4.8.sup.d desired hexagonal crystals, no aggregates 6.sup.b 1.8.sup.c undesired cubic crystals, no aggregates 7.sup.b 4.8.sup.d desired hexagonal crystals, no aggregates .sup.areaction vessel 250 mL; .sup.breaction vessel 500 mL; .sup.cpressure in the reaction vessel < 50 Pa over 0.1 MPa; .sup.dpressure in the reaction vessel 50 Pa over 0.1 MPa; .sup.dpressure in the reaction vessel 75 Pa over 0.1 MPa.

(21) The non-limiting, specific examples provided in the description given above should not be construed as limiting the scope and/or the applicability of the appended claims.