Rutile titanium dioxide nanoparticles and ordered acicular aggregates of same

Abstract

Ordered acicular aggregates of elongated TiO.sub.2 crystallites which resemble nano-sized flower bouquets and/or triangular funnels, and process for their preparation by thermally hydrolyzing a soluble TiO.sub.2 precursor compound in aqueous solution in the presence of a morphology controlling agent selected from carboxylic acids and amino acids.

Claims

1. A method for preparing calcined rutile TiO.sub.2 nanoparticles from rutile TiO.sub.2 nanoparticles which are aggregates of elongated TiO.sub.2 crystallites comprising: (a) forming an aqueous solution of a soluble titanium compound at a titanium concentration of from 0.5 to 1.0 moles per liter, optionally in the presence of a mineral acid; (b) introducing a morphology controlling agent or a mixture of said morphology controlling agents selected from (i) an a-hydroxy carboxylic acid of the formula RCH(OH)COOH, (ii) an a-hydroxy carboxamide of the formula RCH(OH)CONH.sub.2 or (iii) an -amino acid of the formula RCH(NH.sub.2)COOH, wherein said R group in each formula is an alkane, alkene, alkyne, arene, or cycloalkane group having 6 or more carbon atoms, into the solution at an acid- or carboxamide-to-titanium molar ratio of from 0.02 to 0.2 while simultaneously heating the solution to a temperature in the range of from 60 C. to 80 C. with constant stirring; (c) introducing TiO.sub.2 seeds into the stirred solution at a seed-to-TiO.sub.2 molar ratio of from 0.0005 to 0.0015 and maintaining the stirred solution including said TiO.sub.2 seeds at a temperature in the range of from 60 C. to 80 C. for a period of from one to 3 hours; (d) elevating the temperature of the stirred solution from step (d) to a value of from 100 C. to the refluxing temperature of the aqueous solution and maintaining said temperature for a period of from 2 hours to 4 hours to form a reaction product; (e) cooling the reaction mixture which results from step (e) to room or ambient temperature; (f) optionally neutralizing the reaction mixture; (g) separating and drying the reaction product; and (h) calcining the reaction product.

2. The method of claim 1 wherein said elongated TiO.sub.2 crystallites have a thickness of from 3 nm to 5 nm and a length of from 20 nm to 50 nm.

3. The method of claim 2 wherein said rutile TiO.sub.2 nanoparticles are aggregates wherein one set of ends of said elongated TiO.sub.2 crystallites of each aggregate are joined in a cluster and the opposite ends of said crystallites fan outwardly in the shape of a funnel wherein said funnel has a diameter at its widest part of about 50 nm and a height in the range of from 50 nm to 100 nm.

4. The method of claim 1 wherein said morphology controlling agent is selected from mandelic acid (C.sub.6H.sub.5CH(OH)COOH); 4-hydroxymandelic acid (C.sub.6H.sub.4(OH)CH(OH)COOH); benzilic acid ((C.sub.6H.sub.5).sub.2C(OH)COOH); 2-hydroxy-4-phenylbutyric acid (C.sub.6H.sub.5CH.sub.2CH.sub.2CH(OH)COOH); 2-hydroxy-2-phenylpropionic acid ((C.sub.6H.sub.5)(CH.sub.3)C(OH)COOH); 2-hydroxyoctanoic acid (CH.sub.3CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH(OH)COOH); mandelamide (C.sub.6H.sub.5CH(OH)CONH.sub.2); phenylalanine (C.sub.6H.sub.5CH.sub.2CH(NH.sub.2)COOH); tyrosine (C.sub.6H.sub.4(OH)CH.sub.2CH(NH.sub.2)COOH); ammonium (NH.sub.4+), sodium (Na+) and potassium (K+) salts of said morphology controlling agents; and mixtures of said morphology controlling agents.

5. The method of claim 1 wherein said soluble titanium compound is selected from titanium oxychloride (TiOCl.sub.2), titanium oxybromide (TiOBr.sub.2), titanium oxyiodide (TiOl.sub.2), titanium oxynitrate (TiO(NO.sub.3).sub.2), titanium trichloride (TiCl.sub.3), titanium tribromide(TiBr.sub.3), titanium oxalate (Ti.sub.2(C.sub.2O.sub.4).sub.3), potassium hexafluorotitanate(K.sub.2TiF.sub.6), ammonium hexafluorotitanate ((NH.sub.4).sub.2TiF.sub.6), potassium titanyloxolate (K.sub.2TiO(C.sub.2O.sub.4).sub.2), ammonium titanyloxolate ((NH.sub.4).sub.2TiO(C.sub.2O.sub.4).sub.2), titanium bis(ammonium lactate) dihydroxide ([CH.sub.3CH(O)COONH.sub.4].sub.2Ti(OH).sub.2) and mixtures thereof.

6. The method of claim 4 wherein said soluble titanium compound is selected from titanium oxychloride (TiOCl.sub.2), titanium oxybromide (TiOBr.sub.2), titanium oxyiodide (TiOl.sub.2), titanium oxynitrate (TiO(NO.sub.3).sub.2), titanium trichloride (TiCl.sub.3), titanium tribromide(TiBr.sub.3), titanium oxalate (Ti.sub.2(C.sub.2O.sub.4).sub.3), potassium hexafluorotitanate(K.sub.2TiF.sub.6), ammonium hexafluorotitanate ((NH.sub.4).sub.2TiF.sub.6), potassium titanyloxolate (K.sub.2TiO(C.sub.2O.sub.4).sub.2), ammonium titanyloxolate ((NH.sub.4).sub.2TiO(C.sub.2O.sub.4).sub.2), titanium bis(ammonium lactate) dihydroxide ([CH.sub.3CH(O)COONH.sub.4].sub.2Ti(OH).sub.2) and mixtures thereof.

7. The method of claim 6 wherein said morphology controlling agent is mandelic acid (C.sub.6H.sub.5CH(OH)COOH), said soluble titanium compound is titanium oxychloride (TiOCl.sub.2), and said TiO.sub.2 seeds comprise a slurry containing 0.2 g TiO.sub.2 in anatase phase.

8. The method of claim 7 wherein said morphology controlling agent is phenylalanine (C.sub.6H.sub.5CH.sub.2CH(NH.sub.2)COOH), said soluble titanium compound is titanium oxychloride (TiOCl.sub.2), and said TiO.sub.2 seeds comprise a slurry containing 0.2 g TiO.sub.2 in anatase phase.

9. Calcined rutile TiO.sub.2 nanoparticles which are produced by calcining rutile TiO.sub.2 nanoparticles which are ordered acicular aggregates of elongated TiO.sub.2 crystallites having a thickness in the range of from 3 nm to 5 nm in which one end of each of said elongated TiO.sub.2 crystallites are joined into a cluster such that the opposite ends of each of said elongated TiO.sub.2 crystallites extend outwardly in the shape of a nano-sized funnel structure having a diameter of about 50 nm and a height in the range of from 50 nm to 100 nm.

10. The calcined rutile TiO.sub.2 nanoparticles of claim 9 which are produced by the process of: (a) forming an aqueous solution of a soluble titanium compound at a titanium concentration of from 0.5 to 1.0 moles per liter; (b) introducing a morphology controlling agent or a mixture of said morphology controlling agents selected from (i) an -hydroxy carboxylic acid of the formula RCH(OH)COOH, (ii) an -hydroxy carboxamide of the formula RCH(OH)CONH.sub.2 or (iii) an -amino acid of the formula RCH(NH.sub.2)COOH, wherein said R group in each formula is an alkane, alkene, alkyne, arene, or cycloalkane group having 6 or more carbon atoms, into the solution at an acid- or carboxamide-to-titanium molar ratio of from 0.02 to 0.2 while simultaneously heating the solution to a temperature in the range of from 60 C. to 80 C. with constant stirring; (c) introducing TiO.sub.2 seeds into the stirred solution at a seed to TiO.sub.2 molar ratio of from 0.0005 to 0.0015 while maintaining the solution including said TiO.sub.2 seeds at a temperature in the range of from 60 C. to 80 C. for a period of from one to 3 hours; (d) elevating the temperature of the stirred solution from step (d) to a value of from 100 C. to the refluxing temperature of the aqueous solution and maintaining said temperature for a period of from 2 hours to 4 hours to form a reaction product; (e) cooling the reaction mixture which results from step (e) to room or ambient temperature; (f) optionally neutralizing the reaction mixture; (g) separating and drying the reaction product; and (h) calcining the reaction product.

11. The calcined rutile TiO.sub.2 nanoparticles of claim 10 wherein: (a) said morphology controlling agent is selected from mandelic acid (C.sub.6H.sub.5CH(OH)COOH); 4-hydroxymandelic acid (C.sub.6H.sub.4(OH)CH(OH)COOH); benzilic acid ((C.sub.6H.sub.5).sub.2C(OH)COOH); 2-hydroxy-4-phenylbutyric acid (C.sub.6H.sub.5CH.sub.2CH.sub.2CH(OH)COOH); 2-hydroxy-2-phenylpropionic acid ((C.sub.6H.sub.5)(CH.sub.3)C(OH)COOH); 2-hydroxyoctanoic acid (CH.sub.3CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH(OH)COOH); mandelamide (C.sub.6H.sub.5CH(OH)CONH.sub.2); phenylalanine (C.sub.6H.sub.5CH.sub.2CH(NH.sub.2)COOH); tyrosine (C.sub.6H.sub.4(OH)CH.sub.2CH(NH.sub.2)COOH); and ammonium (NH.sub.4+), sodium (Na+) and potassium (K+) salts thereof and mixtures thereof, and (b) said soluble titanium compound is selected from titanium oxychloride (TiOCl.sub.2), titanium oxybromide (TiOBr.sub.2), titanium oxyiodide (TiOI.sub.2), titanium oxynitrate (TiO(NO.sub.3).sub.2), titanium trichloride (TiCl.sub.3), titanium tribromide(TiBr.sub.3), titanium oxalate (Ti.sub.2(C.sub.2O.sub.4).sub.3), potassium hexafluorotitanate(K.sub.2TiF.sub.6), ammonium hexafluorotitanate ((NH.sub.4).sub.2TiF.sub.6), potassium titanyloxolate (K.sub.2TiO(C.sub.2O.sub.4).sub.2), ammonium titanyloxolate ((NH.sub.4).sub.2TiO(C.sub.2O.sub.4).sub.2), titanium bis(ammonium lactate) dihydroxide ([CH.sub.3CH(O)COONH.sub.4].sub.2Ti(OH).sub.2) and mixtures thereof.

12. The calcined rutile TiO.sub.2 nanoparticles of claim 10 wherein: (a) said morphology controlling agent is selected from phenylalanine (C.sub.6H.sub.5CH.sub.2CH(NH.sub.2)COOH) and mandelic acid (C.sub.6H.sub.5CH(OH)COOH) or a mixture thereof; and (b) said soluble titanium compound is titanium oxychloride (TiOCl.sub.2).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an SEM (Scanning Electron Microscopy) image of funnel-shaped rutile TiO.sub.2 nanoparticles according to the invention.

(2) FIG. 2 is an enlarged SEM image which illustrates in more detail ordered acicular aggregates of elongated TiO.sub.2 crystallites according to the invention.

(3) FIG. 3 is a TEM (Transmission Electron Microscopy) image of funnel-shaped rutile TiO.sub.2 nanoparticles according to the invention.

(4) FIG. 4 is an enlarged TEM image of funnel-shaped rutile TiO.sub.2 nanoparticles according to the invention.

(5) FIG. 5 is an X-ray diffraction (XRD) pattern of the funnel-shaped rutile TiO.sub.2 nanoparticles produced according to Example 1 and shown in FIG. 1.

(6) FIG. 6 is an SEM image of the shaped rutile TiO.sub.2 nanoparticles shown in FIG. 1 after calcining at 550 C. for 6 hours.

(7) FIG. 7 is an enlarged SEM image of the shaped rutile TiO.sub.2 nanoparticles shown in FIG. 6.

(8) FIG. 8 is an X-ray diffraction (XRD) pattern of the calcined rutile TiO.sub.2 nanoparticles shown in FIG. 6 which confirms that the rutile phase is present.

DETAILED DESCRIPTION OF THE INVENTION

(9) The novel rutile TiO.sub.2 nanoparticles, meaning the ordered acicular aggregates, are prepared by thermally hydrolyzing a soluble TiO.sub.2 precursor compound, or a mixture of such compounds, in aqueous solution in the presence of a morphology controlling agent, or a mixture of morphology controlling agents, under specific conditions. The term acicular as used herein refers to a crystal habit composed of a radiating mass of slender, needle-like crystals, and the term novel rutile TiO.sub.2 nanoparticles as used herein is intended to mean the ordered acicular aggregates of the needle-like TiO.sub.2 crystallites.

(10) The process is a wet chemical hydrolysis method in which the structure of the ordered acicular aggregates is controlled using a morphology controlling agent selected from (i) an -hydroxy carboxylic acid of the formula RCH(OH)COOH, (ii) an -hydroxy carboxamide of the formula RCH(OH)CONH.sub.2, or (iii) an -amino acid of the formula RCH(NH.sub.2)COOH, wherein R is an alkane, alkene, alkyne, arene, or cycloalkane group having 6 or more carbon atoms.

(11) The process begins by forming an aqueous solution of a soluble titanium compound at a titanium concentration of from 0.1 to 1.5 moles per liter, but preferably 0.5 to 1.0 moles per liter, optionally in the presence of a mineral acid. Distilled or deionized water can be used to form the aqueous solution, and a mineral acid, e.g., hydrochloric acid (HCl), can be introduced as needed for controlling the rate of hydrolysis.

(12) The morphology controlling agent, or a mixture thereof, is introduced into the solution at an acid- or carboxamide-to-titanium molar ratio of from 0.02 to 0.4, although best results have been observed when the ratio is from 0.02 to 0.2. The solution is simultaneously heated to a temperature in the range of from 60 C. to 80 C. with constant stirring. Thereafter, TiO.sub.2 seeds are introduced into the stirred solution at a seed-to-TiO.sub.2 molar ratio of from 0.0005 to 0.0015, and the stirred solution is maintained at a temperature in the range of from 60 C. to 80 C. for a period of from one to 3 hours. The TiO.sub.2 seeds can conveniently comprise a slurry of TiO.sub.2 in the anatase phase (available from Millennium Inorganic Chemicals), but other TiO.sub.2 nucleating agents can also be used with satisfactory results.

(13) The temperature of the stirred solution is next elevated to a value of from 100 C. to the refluxing temperature and maintained at that level for a period of from 2 hours to 4 hours during which time a reaction product is formed. The solution, i.e., reaction mixture which results, is then cooled to room or ambient temperature, and, optionally, it can be neutralized, e.g., pH of from 5 to 8, with introduction of a base, such as an ammonia solution or a sodium hydroxide solution. The reaction product is then separated by filtration and washed with dionized water to remove salts generated during hydrolysis. The resulting filter cake can be dried in an oven or re-slurried with water and spray dried.

(14) As noted above, the reaction product can then be calcined as desired over a wide range of time and temperature to enhance the properties of the resulting nanoparticles, such as by expanding or opening the pore structure and/or increasing the refractive index.

(15) For best results the soluble titanium precursor compound is selected from titanium oxychloride (TiOCl.sub.2), titanium oxybromide (TiOBr.sub.2), titanium oxyiodide (TiOI.sub.2), titanium oxynitrate (TiO(NO.sub.3).sub.2), titanium trichloride (TiCl.sub.3), titanium tribromide(TiBr.sub.3), titanium oxalate (Ti.sub.2(C.sub.2O.sub.4).sub.3), potassium hexafluorotitanate(K.sub.2TiF.sub.6), ammonium hexafluorotitanate ((NH.sub.4).sub.2TiF.sub.6), potassium titanyloxolate (K.sub.2TiO(C.sub.2O.sub.4).sub.2), ammonium titanyloxolate ((NH.sub.4).sub.2TiO(C.sub.2O.sub.4).sub.2), and titanium bis(ammonium lactate) dihydroxide ([CH.sub.3CH(O)COONH.sub.4].sub.2Ti(OH).sub.2). Other commercially available soluble titanium precursor compounds can be deployed in the process and produce satisfactory results and, although not specifically named herein, they are embraced within the described and claimed inventive concept(s).

(16) As noted above, morphology controlling agents, or mixtures thereof, for carrying out the inventive concept(s) include (i) -hydroxy carboxylic acids of the formula RCH(OH)COOH, (ii) -hydroxy carboxamides of the formula RCH(OH)CONH.sub.2, and (iii) -amino acids of the formula RCH(NH.sub.2)COOH, wherein R is an alkane, alkene, alkyne, arene, or cycloalkane group having 6 or more carbon atoms. Examples of such morphology controlling agents include, but are not limited to, mandelic acid (C.sub.6H.sub.5CH(OH)COOH); 4-hydroxymandelic acid (C.sub.6H.sub.4(OH)CH(OH)COOH); benzilic acid ((C.sub.6H.sub.5).sub.2C(OH)COOH); 2-hydroxy-4-phenylbutyric acid (C.sub.6H.sub.5CH.sub.2CH.sub.2CH(OH)COOH); 2-hydroxy-2-phenylpropionic acid ((C.sub.6H.sub.5)(CH.sub.3)C(OH)COOH); 2-hydroxyoctanoic acid (CH.sub.3CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH.sub.2CH(OH)COOH); mandelamide (C.sub.6H.sub.5CH(OH)CONH.sub.2); phenylalanine (C.sub.6H.sub.5CH.sub.2CH(NH.sub.2)COOH); and tyrosine (C.sub.6H.sub.4(OH)CH.sub.2CH(NH.sub.2)COOH). In addition, the ammonium (NH.sub.4+), sodium (Na+) and potassium (K+) salts of such acids and carboxamides can also be used.

(17) In a preferred embodiment of the invention, the morphology controlling agent is mandelic acid (C.sub.6H.sub.5CH(OH)COOH), and the soluble titanium compound is titanium oxychloride (TiOCl.sub.2).

(18) The process of the invention produces novel rutile TiO.sub.2 nanoparticles, i.e., the reaction product comprises ordered acicular aggregates of elongated, i.e., rod-like, TiO.sub.2 crystallites. The individual crystallites have a thickness in the range of from 3 nm to 5 nm, and one end of each of the rod-like crystallites are joined, i.e., assembled, into a cluster such that the opposite ends of each of the crystallites extend, or fan, outwardly in the general shape of a nano-sized flower bouquet or a funnel. The funnel-shaped structures have a diameter in the range of 50 nm and a height in the range of from 50 nm to 100 nm. The rutile TiO.sub.2 nanoparticles in powder form show a desirably high specific surface area and pore volume. It is preferred that specific surface area be in the range from 120 m.sup.2/g to 160 m.sup.2/g and that pore volume be in the range from 0.3 cm.sup.3/g to 0.5 cm.sup.3/g or higher.

EXAMPLES

(19) The present invention will be illustrated in further detail with reference to the working examples which follow and FIGS. 1-8. It should be noted, however, that these examples should not be construed to limit the scope of the described and claimed inventive concept(s).

Example 1

Preparation of Funnel-Shaped Nanoparticles Using Carboxylic Acids

(20) 1,255 g of deionized water, 9.5 g of mandelic acid (from Alfa Aesar), 97 g HCl solution (37% from Fisher Scientific), and 397 g of titanium oxychloride solution (25.2% in TiO.sub.2, from Millennium Inorganic Chemicals) were mixed together in a heated reactor equipped with a glass condenser and an overhead stirrer. While being constantly stirred, the mixture was heated to 65 C. A TiO.sub.2 seed slurry containing 0.2 g TiO.sub.2 in anatase phase (from Millennium Inorganic Chemicals) was added, and the hydrolysis reaction was maintained at 65 C. for 2 hours. During this period, TiO.sub.2 particles were formed and crystallized through hydrolysis of the titanium oxychloride precursor compound. The reaction temperature was then increased to 103 C., and that temperature was maintained for 4 hours. The hydrolysis was essentially complete at this stage.

(21) The resulting reaction mixture was then cooled to room temperature and transferred to a different container where the particles formed were allowed to settle for a few hours. After essentially all of the particles were observed to have settled to the bottom of the container, the mother liquor, i.e., liquid reaction medium, was removed and about the same volume of fresh deionized water was added to the container. The reaction mixture was then stirred to re-slurry the particles, and then the pH of the slurry was increased to a value of about 7 by slow addition of an ammonia solution (29%, Fisher Scientific). The particles comprising the reaction product were then separated from the liquid reaction mixture using a Buchner filter and washed with deionized water until the conductivity of the filtrate was lowered to about 500 S/cm. The wet filter cake sample was then stored as a slurry by re-slurring the filter cake with a small amount of deionized water. The powder form of the sample was obtained by drying the slurry sample in an oven overnight at 90 C. X-ray Diffraction (XRD) measurement on the powder sample, shown in FIG. 5, indicates that the sample contains 100% rutile with crystallite size about 8 nm. BET measurement on the powder sample shows that the powder has a specific surface area of 140 m.sup.2/g and a pore volume of 0.34 cm.sup.3/g.

(22) SEM images of the slurry sample are shown in FIG. 1 at a magnification of 10,000. Funnel-shaped nanoparticles can be seen more clearly in FIG. 2 at a magnification of 50,000. The TEM image of the slurry sample shown in FIG. 3 illustrates a funnel-shaped particle with a diameter in the range of 50 nm. The TEM image in FIG. 4. illustrates general alignment of individual nano-sized rutile TiO.sub.2 crystallites.

(23) The funnel-shaped nanoparticles shown in FIG. 2 were calcined at 550 C. for 6 hours. SEM images of the calcined nanoparticles can be seen in FIG. 6 (50,000 magnification) and in FIG. 7 (100,000 magnification). Calcining, which can be adjusted for time and temperature, operates to enhance the properties of the resulting nanoparticles by expanding or opening the pore structure and/or increasing the refractive index.

Example 2

Preparation of Funnel-Shaped Nanoparticles Using Amino Acids

(24) The same procedure was followed as in Example 1 except that 20.7 g phenylalanine (Alfa Aesar) was used as morphology controlling agent instead of mandelic acid. The SEM/TEM images of the reaction product were similar to those shown in FIGS. 1-4. XRD measurement on the powder sample indicates that the sample contains 100% rutile with crystallite size of about 9 nm. BET measurement on the powder sample shows that the powder has a specific surface area of 124 m.sup.2/g and a pore volume of 0.37 cm.sup.3/g.

Example 3

Preparation of Funnel-Shaped Nanoparticles Using Carboxylic Acids

(25) The same procedure was followed as in Example 1, except that 14.3 g of benzilic acid (Alfa Aesar) was used as the morphology controlling agent instead of mandelic acid. The SEM/TEM images of the reaction product are similar to those shown in FIGS. 1-4. The XRD measurement of the powder sample was similar to the measurement shown in FIG. 5 for the funnel-shaped nanoparticles produced using mandelic acid and confirms that the sample contains 100% rutile with crystallite size of about 8 nm. BET measurement on the powder sample shows that the powder has a specific surface area of 121 m.sup.2/g and a pore volume of 0.53 cm.sup.3/g.