METHOD FOR SYNTHESISING COLLOIDAL SUSPENSIONS OF NANORODS

Abstract

A process for synthesizing La1-a-bAaBbPO4 nanorods, A being chosen from Y, Sc, Ce, Pr, Nd, Pm, Sm, Gd, Tb and mixtures thereof, B being a luminescence-activating dopant chosen from Eu, Yb, Er, Tm, Dy, Ho and mixtures thereof, with 0a0.5 and 0b0.2 the process involving: a. the preparation of an acidic mother liquor having a pH of between 1.0 and 3.0 by mixing at least, or even by mixing only: a solvent, a first constituent providing La3+ ions, a second constituent, in excess, providing PO43 ions, if a>0, a third constituent providing A3+ ions, if b>0, a constituent providing luminescence-activating dopant ions B3+, in amounts such that, in the mother liquor, the ratio of the number of moles of PO43 ions to the number of moles of La3+ ions and, where appropriate, A3+ and/or B3+ ions is between 1.10 and 1.50, and b. heating the mother liquor under hydrothermal conditions at a heating temperature above 120 C. until La1-a-bAaBbPO4 nanorods of monazite crystallographic structure are obtained.

Claims

1. A process for synthesizing La.sub.1-a-bA.sub.aB.sub.bPO.sub.4 nanorods, A being chosen from Y, Sc, Ce, Pr, Nd, Pm, Sm, Gd, Tb and mixtures thereof, B being a luminescence-activating dopant chosen from Eu, Yb, Er, Tm, Dy, Ho and mixtures thereof, with 0a0.5 and 0b0.2 the process involving: a. the preparation of an acidic mother liquor having a pH of between 1.0 and 3.0 by mixing at least: a solvent, a first constituent providing La.sup.3+ ions, a second constituent, in excess, providing PO.sub.4.sup.3 ions, if a>0, a third constituent providing A.sup.3+ ions, if b>0, a constituent providing luminescence-activating dopant ions B.sup.3+, in amounts such that, in the mother liquor, the ratio of the number of moles of PO.sub.4.sup.3 ions to the number of moles of La.sup.3+ ions and, where appropriate, A.sup.3+ ions and/or B.sup.3+ ions is between 1.10 and 1.50, b. heating the mother liquor under hydrothermal conditions at a heating temperature above 120 C. until La.sub.1-a-bA.sub.aB.sub.bPO.sub.4 nanorods of monazite crystallographic structure are obtained.

2. The process as claimed in claim 1, the amounts of the first and second constituents being such that the ratio of the number of moles of PO.sub.4.sup.3 ions to the number of moles of La.sup.3+ ions and, where appropriate, A.sup.3+ ions and/or B.sup.3+ ions is between 1.15 and 1.25, preferably equal to 1.20.

3. The process as claimed in claim 1, the first constituent being chosen from lanthanum nitrate, lanthanum chloride, lanthanum sulfate, lanthanum acetate, lanthanum oxide and mixtures thereof and/or the second constituent being chosen from (NH.sub.4).sub.2HPO.sub.4, Na.sub.2HPO.sub.4, NaH.sub.2PO.sub.4, Na.sub.3PO.sub.4 and mixtures thereof.

4. The process as claimed in claim 3, the second constituent being diammonium phosphate (NH.sub.4).sub.2HPO.sub.4 and/or the first constituent being lanthanum nitrate La(NO.sub.3).sub.3.

5. The process as claimed in claim 1, the element A being yttrium.

6. The process as claimed in claim 1, the coefficient a being equal to 0.

7. The process as claimed in claim 1, the luminescence-activating dopant B being chosen from Eu, Yb, Er and mixtures thereof.

8. The process as claimed in claim 1, the mother liquor being heated in step b) to a heating temperature of less than 200 C.

9. The process as claimed in claim 1, the solvent being polar.

10. The process as claimed in claim 1, including a step c), successive to step b) of washing and dissolving the nanorods to form a nanorod dispersion.

11. The process as claimed in claim 10, step c) being performed by dialysis.

12. The process as claimed in claim 1, the nanorods at the end of step b) having a length of less than 500 nm.

13. The process as claimed in claim 1, the nanorods having an aspect ratio, defined as the ratio of the length of a nanorod to the width of a nanorod, of less than 100.

14. The process as claimed in claim 1, the preparation of the mother liquor also involving the mixing of a complexing agent.

15. The process as claimed in claim 14, the complexing agent being ethylenediaminetetraacetic acid disodium salt dihydrate.

16. The process as claimed in claim 14, the complexing agent being added to the mixture in a proportion such that the ratio of the number of moles of La.sup.3+ ions, and where appropriate A.sup.3+ ions and/or B.sup.3+ ions, to the number of moles of complexing agent, is between 10 and 5000.

17. A colloidal dispersion of La.sub.1-a-bA.sub.aB.sub.bPO.sub.4 nanorods, A being chosen from Y, Sc, Ce, Pr, Nd, Pm, Sm, Gd, Tb and mixtures thereof, B being chosen from Eu, Yb, Er, Tm, Dy, Ho and mixtures thereof, with 0a0.5 and 0b0.2.

18. The dispersion as claimed in claim 17, the transmittance of the dispersion to radiation at a wavelength of 500 nm, measured at a nanorod concentration of 20 mg/ml, being greater than 50%.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0062] Other features and advantages of the invention will emerge more clearly on reading the following detailed description and on examining the appended drawing, in which:

[0063] FIG. 1 is a graph representing the evolution of the transmittance of the dispersion of Example 1 as a function of the wavelength of the incident radiation,

[0064] FIG. 2 represents the diffraction patterns of the powders obtained by Examples 1 and 2 after different heating times;

[0065] FIG. 3 represents the diffraction patterns of the powder obtained according to Example 1 and of the bulk material obtained according to Comparative Example 3;

[0066] FIG. 4 includes scanning microscopy images of nanorod powders;

[0067] FIG. 5 represents a) the evolution of the Zeta potential of a nanorod solution as a function of the pH of the solution and b) is a photograph of said solution;

[0068] FIG. 6 represents the luminescent emission spectrum of a nanorod under illumination by ultraviolet radiation of wavelength 394 nm as a function of the polarization angle of a filter relative to the axis of the nanorod; and

[0069] FIG. 7 are transmission electron microscopy photographs of nanorods obtained according to Examples 4 to 8 for different La:EDTA-Na.sub.2, ratios, the scale bar on each photograph corresponding to 50 nm.

EXAMPLES

[0070] The following starting materials from the company Sigma-Aldrich were used, without further purification, for the examples: [0071] lanthanum nitrate hexahydrate La(NO.sub.3).sub.3.Math.6H.sub.2O, purity greater than 99.99%, [0072] lanthanum oxide La.sub.2O.sub.3, purity greater than 99.9%, [0073] europium nitrate pentahydrate Eu(NO.sub.3).sub.3.Math.5H.sub.2O, purity greater than 99.99%, [0074] europium oxide Eu.sub.2O.sub.3, purity greater than 99.9%, [0075] diammonium phosphate ((NH.sub.4).sub.2HPO.sub.4, Analytical Reagent, A.R.), [0076] nitric acid HNO.sub.3, 70%, A.R.

Example 1 According to the Invention

[0077] 10 ml of an aqueous solution containing 0.0475 mol/1 La(NO.sub.3).sub.3 and 0.0025 mol/1 Eu(NO.sub.3).sub.3 were mixed in a tube with 12 ml of an aqueous solution containing 0.05 mol/1 (NH.sub.4).sub.2HPO.sub.4.

[0078] The diammonium phosphate was in excess in the mother liquor.

[0079] Its amount was such that the ratio of the number of PO.sub.4.sup.3 ions divided by the number of La.sup.3+ ions and Eu.sup.3+ ions was 1.2.

[0080] The amount of Eu(NO.sub.3).sub.3 was chosen so that the Eu.sup.3+ dopant concentration was 5%, expressed as a percentage of the total number of La.sup.3+ and Eu.sup.3+ ions.

[0081] Precipitation of rhabdophane-phase La.sub.0.95Eu.sub.0.05PO.sub.4 nanorods was observed as soon as the various constituents were placed in contact.

[0082] The mixture thus obtained, with a pH of 2, was then heated in a CEM brand Discover SP microwave reactor, to a heating temperature of 160 C. and maintained at this heating temperature for 2 hours.

[0083] After cooling, the La.sub.0.95Eu.sub.0.05PO.sub.4 nanorods were collected by centrifugation at 8000g for 20 minutes, and then dispersed in an aqueous nitric acid solution with a pH equal to 2. The suspension thus obtained was then dialyzed for 2 days through a membrane with a permeability of between 12 and 14 kDa against an aqueous nitric acid solution with a pH equal to 2.

[0084] The nanorods were redispersed in an aqueous nitric acid solution with a pH of 2. The volumetric concentration of nanorods in the solution was 0.4% (i.e. a content of 20 mg/ml). The transmittance of the dispersion at 500 nm, measured after 1 year, was greater than 55%, as shown in FIG. 1.

[0085] Powder samples were then obtained by drying the redispersed suspension at a drying temperature of 100 C. for 12 hours.

Comparative Example 2

[0086] 10 ml of an acidic aqueous solution containing 0.4 mol/l HNO.sub.3, 0.0475 mol/1 La(NO.sub.3).sub.3 and 0.0025 mol/1 Eu(NO.sub.3).sub.3 were mixed in a tube with 10 ml of an aqueous solution containing 0.05 mol/1 (NH.sub.4).sub.2HPO.sub.4.

[0087] The constituents La(NO.sub.3).sub.3 and (NH.sub.4).sub.2HPO.sub.4 were thus present in stoichiometric amounts.

[0088] The amount of Eu(NO.sub.3).sub.3 was chosen such that the Eu.sup.3+ dopant concentration was 5%, in percentages expressed on the basis of the total number of La.sup.3+ and Eu.sup.3+ ions.

[0089] No precipitation of La.sub.0.95Eu.sub.0.05PO.sub.4 was observed after mixing.

[0090] The mixture thus obtained, with a pH of 0.4, was then heated in the microwave reactor to a heating temperature of 160 C. and maintained at this temperature for 2 hours.

[0091] After cooling, La.sub.0.95Eu.sub.0.05PO.sub.4 nanorods were collected by centrifugation at 8000g for 20 minutes, and then dispersed in an aqueous nitric acid solution with a pH equal to 2. The suspension thus obtained was then dialyzed for 2 days through a membrane with a permeability of between 12 and 14 kDa against an aqueous nitric acid solution with a pH equal to 2.

[0092] Powder samples were then obtained by drying the dialyzed suspension at a drying temperature of 100 C. for 12 hours.

Comparative Example 3: Solid-State Synthesis

[0093] Lanthanum oxide, europium oxide and diammonium phosphate were mixed under stoichiometric conditions and ground in an agate mortar. The ground material was then heated at 800 C. for 1 hour, cooled and then maintained at 1100 C. for 12 hours. Lanthanum phosphate doped with 5% europium in bulk form, i.e. grains larger than several microns in size, was thus obtained.

Examples 4 to 8 According to the Invention

[0094] 12 ml of an aqueous solution containing 0.05 mol/1 of (NH.sub.4).sub.2HPO.sub.4 and one volume of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-Na.sub.2) were mixed to form a first mixture. After stirring for 30 minutes, 10 ml of an aqueous solution containing 0.04 mol/1 La(NO.sub.3).sub.3 and 0.01 mol/1 Eu(NO.sub.3).sub.3 were added to the first mixture.

[0095] The volume of ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-Na.sub.2) in the first mixture was chosen so that, in the mother liquor, the ratio La:EDTA-Na.sub.2 of the total number of moles of La.sup.3+ and Eu.sup.3+ ions to the number of moles of EDTA-Na.sub.2 was as indicated in Table 1.

TABLE-US-00001 TABLE 1 Example La:EDTA-Na.sub.2 4 10 5 50 6 100 7 1000 8 5000

[0096] The diammonium phosphate was in excess in the mother liquor. Its amount was such that the ratio of the number of PO.sub.4.sup.3 ions divided by the number of La.sup.3+ ions and Eu.sup.3+ ions was 1.2.

[0097] The amount of Eu(NO.sub.3).sub.3 was chosen so that the Eu.sup.3+ dopant concentration was 20%, expressed as a percentage of the total number of La.sup.3+ and Eu.sup.3+ ions.

[0098] The mother liquor thus prepared was stirred for 30 minutes.

[0099] Precipitation of rhabdophane-phase La.sub.0.8Eu.sub.0.2PO.sub.4 nanorods was observed as soon as the various constituents were placed in contact.

[0100] The mother liquor, with a pH of 2, was then heated in a CEM brand Discover SP microwave reactor, to a heating temperature of 160 C. and maintained at this heating temperature for 2 hours.

[0101] After cooling, the La.sub.0.8Eu.sub.0.2PO.sub.4 nanorods were collected by centrifugation at a speed of 11 000 rotations per minute for 30 minutes, and then washed twice in succession in a nitric acid solution with a pH equal to 2 to extract the excess EDTA-Na.sub.2.

[0102] Finally, the La.sub.0.8Eu.sub.0.2PO.sub.4 nanorods were dispersed in an aqueous nitric acid solution with a pH equal to 2. The suspension thus obtained was then dialyzed for 2 days through a membrane with a permeability of between 12 and 14 kDa against an aqueous nitric acid solution with a pH equal to 2.

Characterization Methods

[0103] In order to characterize the various products obtained for Examples 1 to 3, the following methods were used.

[0104] Crystal phase characterizations were performed by X-ray diffraction with a Bruker D8 Advance diffractometer using Cu K radiation of wavelength =1.5409 with a LynxEye XE-T detector.

[0105] Morphology observations were performed using a Hitachi S4800 field-effect scanning electron microscope under an electron acceleration voltage of 5 kV. The samples to be observed were prepared by depositing a drop of solution including the nanorods on a copper grid coated with a 3 nm thick carbon coating.

[0106] Zeta potential measurements were taken using a Malvern ZetaSizer Nano ZS device.

[0107] Emission spectra were measured using a superfluorometer (FluoroMax-4, Horiba) equipped with a 150 W xenon lamp and a Hamamatsu R928P photomultiplier tube.

[0108] FIG. 2 represents the evolution for Examples 1 and 2 of diffractograms measured by X-ray diffraction prior to the heating step (0 min) and for different heating periods (5 min and 60 min) at 160 C.

[0109] The theoretical diffractograms of the rhabdophane and monazite phases are represented on the lower and upper horizontal axes, respectively.

[0110] The powder according to the invention obtained via the process according to the invention in Example 1 has a rhabdophane phase as soon as the constituents are mixed (0 min). After 5 minutes of heating time, a gradual transition from the rhabdophane phase to the monazite phase is observed. However, for short heating times, the diffraction peaks are relatively broad, indicating that the crystallite sizes are small. Increasing the heating time results in peaks that become narrower and narrower, indicating that the particles are increasing in size or are improving in crystallinity.

[0111] Moreover, the diffractogram of the powder of Example 1 according to the invention is substantially identical to the diffractogram of the material obtained by the solid route of comparative Example 3, as observed in FIG. 3.

[0112] In the case of Comparative Example 2, no particles are formed during the mixing of the constituents. However, heating results directly in monocrystalline particles of monazite phase.

[0113] FIG. 4a) is a scanning microscopy photograph of the powder of Comparative Example 2. The nanorods are aggregated in clusters, most of them consisting of 5 to 30 nanorods. The powder of Example 2 has a mean length of 621 nm with a standard deviation of 135 nm. The nanorods have an aspect ratio of 100. The clusters have a mean length of about 620 nm and a width of about 60 nm.

[0114] FIG. 4b) is a scanning microscopy photograph of the powder of Example 1 according to the invention. The nanorods appear isolated and at a distance from each other. The nanorods of Example 1 have a mean length of 141 nm, with a standard deviation of 55 nm. The nanorods have an aspect ratio of 28 with a standard deviation of 11.

[0115] FIG. 5a) represents the evolution of the Zeta potential as a function of the pH in an aqueous solution including nanorods of Example 1 according to the invention.

[0116] The zero charge point is obtained at a pH of about 5.3. It is observed that for acidic pH values below the zero-point pH, the Zeta potential increases almost linearly with reducing pH and is 44 mV for a pH of 2, attesting to the excellent dispersibility of the nanorods.

[0117] This is confirmed by the observation in FIG. 4b), and also by the stability of the dispersion observed in FIG. 5b), which could be observed for over a year without any change in appearance, notably in terms of its light-scattering properties.

[0118] Finally, as shown in FIG. 6, the nanorods have a light emission spectrum that is polarized when illuminated by ultraviolet radiation with a wavelength equal to 394 nm. Changing the angle of a polarizing filter relative to the axis of a nanorod induces a variation in the intensity of certain radiation components.

[0119] This variation may notably be used to determine the orientation of the nanorods in a liquid, and to determine, for example, the local shear rate to which the liquid is subjected.

[0120] Table 2 indicates the characteristics of the nanorods obtained for Examples 4 to 8.

TABLE-US-00002 TABLE 2 Mean Mean length width Aspect Example La:EDTA-Na.sub.2 (nm) (nm) ratio 4 10 37 7 5.3 5 50 71 9 7.9 6 100 97 11 8.8 7 1000 129 10 12.9 8 5000 182 10 18.2

[0121] It is seen that in the range tested, an increase in the La:EDTA-Na.sub.2 ratio results in an increase in the mean length and aspect ratio of the nanorods, with the mean width of the nanorods varying only slightly with the La:EDTA-Na.sub.2 ratio.

[0122] The addition of EDTA-Na.sub.2 to the mother liquor thus allows simple modification of the length and aspect ratio of the nanorods, the ratio of the number of moles of La.sup.3+ ions and moles of Eu.sup.3+ to the number of moles of PO.sub.4.sup.3 being fixed.

[0123] The nanorods of Examples 4 to 8 are illustrated in the transmission microscopy photographs shown in FIG. 7, with the corresponding La:EDTA-Na.sub.2 ratio indicated on each photograph. Different sampling methods were used to acquire the photographs, which do not allow the individual dispersion of the nanorods to be conserved. Aggregates were formed during this sampling. However, the inventors verified that in each colloidal dispersion of Examples 4 to 8, the nanorods were well dispersed. This was confirmed notably by comparing the results of measurements of the lengths of the nanorods by dynamic light scattering with the measurements of these lengths in the photographs in FIG. 7. This was also confirmed by the observation of the limpidity and stream birefringence of each colloidal dispersion.

[0124] Finally, X-ray diffraction analysis of Examples 4 to 8 revealed that the La.sub.0.8Eu.sub.0.2PO.sub.4 nanorods have a monazite crystallographic structure.

[0125] Needless to say, the invention as claimed should not be understood as being limited to the examples of implementation of the process described by way of illustration.