CA2780831A1 - Method for fabrication of metal oxide nanoparticles - Google Patents

Abstract

A method for fabricating indium tin oxide nanoparticles wherein a precursor is precipitated from an aqueous solution and dried by freeze drying. The resulting nanopowder can be used to produce an indium tin oxide coating for hard surfaces such as window glass. The resulting window coating is useful for control of temperature in buildings.

Description

Method for Fabrication of Metal Oxide Nanoparticles Field of the Invention [0001] The invention relates, in general, to the field of nanotechnology and, in particular, to a method of fabricating easily dispersable indium tin oxide nanoparticles which are suitable for use in ITO coating products.
Background of the Invention [0002] Indium tin oxide (ITO) is one of the most sought-after transparent conductive oxides (TOO). In the form of thin transparent layers, ITO is able to reflect infrared light and, at the same time, has a relatively high electronic conductivity. As a result, there are a very large number of possible uses for ITO
including liquid crystal displays, low emissive windows, anti-static coatings, solar cells, biomolecular microarrays and gas sensing devices.

[0003] ITO is particularly suitable for use in solar control because of its high transparency in the visible range (400-800nm) and also its high infrared absorption/reflection. Accordingly, ITO window coatings provide an economical and environmentally safe means of controlling temperature within buildings.

[0004] ITO nanoparticles are of particular commercial interest. Nanometer sized particles have unusual mechanical, optical, magnetic and electrical properties that make them suitable for novel applications. There is no universally agreed upon definition of a nanoparticle, however, most research groups consider that particles with at least one dimension smaller than 100 nm are nanoparticles and those with a dimension in the range 100 nm < d < ¨10 pm are microparticles.
The physical properties of nanoparticles can vary significantly as the particle size changes. The specific properties of nanoparticles include: dispersability in an immiscible phase, uniformity and fine grain size, extremely high specific surface area, control of the scattering of light, enhanced chemical activity of atoms and molecules at the interface, absorptive capabilities, microstructure control, transport properties of small domains and, in pores, controlled electronic states of atoms. Nanoparticles do not scatter visible light because of their very small size (2 times smaller than the wavelength of the visible light). As a result, nanoparticles are particularly well suited for optical coatings. However, the use of ITO nanoparticles in coating products necessitates the dispersion of the nanoparticles in a medium. Excellent dispersion is required in order to produce a product which can be used to coat a surface evenly.

[0005]
Fabrication techniques for nanoparticles are of commercial importance because small variations in the synthetic method can result in changes in the materials' properties, such as infrared reflection and resistivity. The preferred fabrication technique for ITO thin films and nanoparticles involve sputtering, plasma spraying and high temperature solvent-based synthesis. Most often, ITO
nanoparticles are precipitated and then dried in a vacuum oven at temperatures above 180 C for several hours, resulting in a dense, yellow material. For example, Song et al, Colloids and Surfaces, Physicochem. Eng. Aspects 2005, 257-258, 539-542, describes drying a precipitate under reduced pressure in a vacuum oven for 24 hours. This material is later thermally crystallized at temperatures above 500 C for various hours with further annealing at 300 C to induce doping. US Patent 6,533,966 to Nonninger describes a method of preparing an indium tin oxide suspension or powder which includes the steps of precipitating from a solvent, and then heating to 190 C to remove the solvent.

These methods require expensive capital investments which increase the cost of consumer applications. Furthermore, heat-based drying methods tend to produce agglomerated nanoparticles which are difficult to disperse and are therefore poorly adapted for use in liquid coating products such as paints. As a result, it has been difficult to formulate liquid coating products which can be easily applied to hard surfaces such as window glass.

[0006]
Accordingly, a need exists for an efficient method of fabricating easily disperable ITO nanoparticles. Furthermore, there is a need for an improved method of producing liquid coating products containing ITO nanoparticles to produce commercially useful coatings and paints.

= CA 02780831 2012-06-20 Summary of the Invention [0007] A method is provided for fabrication of ITO nanoparticles and a means of preparing a liquid coating comprising ITO nanoparticles.

[0008] In a first aspect the invention provides a method of producing easily disperable ITO nanoparticles, which method comprises precipitating an ITO
precursor from an aqueous solution and then drying the precipitate by freeze drying.

[0009] In a further aspect, the invention provides a method of preparing a liquid coating comprising ITO nanoparticles, which method includes precipitating an ITO
precursor from an aqueous solution containing at least one indium compound and at least one tin compound in the presence of at least one surface-modifying component, then separating the resulting precipitate from the water by filtration or centrifugation and drying the precipitate by freeze drying to form a nanopowder.
The nanopowder is sintered at 250-500 C under reducing conditions. The nanopowder is combined with a solvent and a dispersant to form a paste and the paste is dissolved in water or an organic solvent to form a liquid coating.
Brief Description of the Drawings [0010] In order that the invention may be more clearly understood, embodiments thereof will now be described in detail by way of example, with reference to the accompanying drawings, in which:
Figure 1 is a graph illustrating the UV-Vis-NIR spectrum for a film of approximately 6 microns thickness. This glass shows an IR transmission of 50%; and Figure 2 is a graph illustrating the UV-Vis-NIR spectrum of an inorganic thin film produced by spin coating an ethanol dispersion of ITO. Inorganic matrix is MPTS.

Description of Preferred Embodiments [0011] The invention comprises the synthesis of inorganic nanoparticles of ITO
and their dispersion into a suitable matrix. The matrices can include an inorganic based matrix and polymer dispersion (clear coat) for glass applications.

[0012] The ITO nanoparticles can be treated with surface modifiers to control their dispersion in commercial matrices such as paint latex and inorganic glassy materials.

[0013] The synthetic method for fabricating re-dispersable ITO powders is based on a surface modification of the particles performed during the precipitation and the growth of the particles from a liquid phase. By the control of their free surface energy, their growth and crystallite size can be adjusted and the aggregation process can be prevented.

[0014] ITO nanoparticles are typically synthesized using a co-precipitation method. In previously described coprecipitation methods, inorganic salts were precipitated from aqueous solution and dried by heating. The dried precipitate is then sintered at temperatures above 300 C and ball milled to sizes below 100 nm.
In the present invention, the heating at high temperature to dry and crystallize the precursor mixture is avoided by using a freeze drying protocol, which creates an easily dispersible ITO powder that can be doped at low temperatures under forming gas conditions. The inventors of the present application have discovered that freeze drying results surprising and beneficial property in the resulting nanopowder which was not previously known or predicted. Specifically, the resulting nanopowder has very little agglomeration, and is therefore easily dispersed into a liquid. This makes the resulting ITO nanopowder particularly well adapted for use in liquid paint products which can be easily applied to hard surfaces such as window glass.
In the present invention, an ITO precursor is precipitated from an aqueous solution containing at least one indium compound, at least one tin compound and a surface-modifying compound. Examples of suitable indium compounds include indium(III) chloride, indium(III) iodide, indium(I) chloride, indium(I) iodide, indium(III)nitrate, indium(III) acetate, indium(III) sulphate, an indium(III) alkoxide, and mixtures thereof. A particularly preferred indium compound is indium(III) chloride. Examples of suitable tin compounds include tin (IV) chloride, tin (II) chloride, tin(IV) sulphate, tin(II) sulphate, a tin(IV) alkoxide, a tin(I1)alkoxides. A
particularly preferred tin compound is tin (IV) chloride.

[0015] The surface-modifying compound comprises an organic compound having a carboxylic acid functional group or precursor. Suitable compounds include saturated and unsaturated monocarboxylic and polycarboxylic acids, diketones, amino acids, polyethylene exide derivatives, lactones and lactams, acid amides and monoamines and polyamines. A
particularly suitable surface modifying compound is caprolactam.

[0016] The indium compounds and tin compounds are preferably completely in solution in distilled (pure) water prior to precipitation. The use of water as the solvent reduces cost and minimizes the use of organic solvents.

[0017] The resulting precipitate is separated from the liquid by filtration or centrifugation and then dried by freeze drying. Surprisingly, it has been observed that freeze drying produces a nanopowder, which can be easily dispersed.As well, it produces a final product with excellent heat blocking properties.

[0018]
Following freeze drying, the ITO powder is white in colour. It shows little agglomeration, far less than observed following heat drying. The nanopowder is doped by sintering under reducing conditions.
Sintering is conducted under forming gas to form a blue powder. Sintering takes place at 550 C, preferably, 300-400 C, most preferably at 350 C in the presence of forming gas. Forming gas can be 95`)/01\12/5`)/0H2. Surprisingly, the inventors have found that freeze drying improves the thermal reduction of the ITO under reducing conditions.

[0019] The ITO powder is used to prepare coating solutions by dispersing the powder in a solvent such as water or an organic solvent (e.g. ethanol, isopropanol, ethylene glycol).
Preferred solvents include glycols and glycol ethers. The solvent is selected based on the final solvent matrix for the ITO
nanopowder. For use with glass, the ITO nanoparticles may be dispersed in 042-(2-Methoxyethoxy)ethyl]glycolic acid. The concentration of the dispersing agent ranges between 5% to 16 wt.%. The concentration of the ITO powder ranges between 75% to 80 wt.%. These ranges result in an ITO paste which is easy to handle and demonstrates excellent performance. The wetted ITO powder is dispersed mechanically to break up agglomerated powder and ensure that the particles are evenly wetted and dispersed down to the primary size. The result is a thick, highly viscous paste that is the raw material for a coating solution.

[0020] The ITO nanoparticle paste can be easily dispersed in water or an organic solvent. Suitable organic solvents include a C1-6 alcohol, preferably ethanol or isopropanol, or an acrylic/urethane blend. Other suitable solvents include oligomeric polysilanes, commercial polyurethane, commercial acrylic paints and mixtures thereof. The inventors have discovered that nanopowders produced by freeze drying of the precursor precipitate, as described herein, produce dispersions which are particularly well suited for commercial acrylic paints. The resulting nano-liquid can be used to coat a hard surface such as glass, a solar cell or a liquid crystal display using standard techniques such as spin, dip and draw bar coating. This coating is particularly suitable for use on windows so as to provide a means of controlling temperature within buildings.
The coating of the invention can be applied to windows by various techniques including drawdown, liquid spraying and electronic spraying directly on the surface. There is no need for removal or replacement of the glass from the window since no treatment of the glass is required. An externally applied coating is superior to an ITO film covering because the coating does not delaminate.

[0021] Example 1: Preparation of ITO Nanoparticles [0022] All the reagents and solvents are commercially available from Aldrich, Strem, or Fisher Scientific, and were used without further purification.
Distilled deionized water (ddH20) was obtained from a MilliQ water purification unit.
Freeze drying was achieved using a Labconco FreeZone 4.5 freeze drier. Prior to freeze drying, the samples were frozen in a Labconco Shell Freezer at -42 C.
Centrifugation was performed using a Beckman Coulter Avanti J-26 XPI
Ultracentrifuge. Nanopowder materials were characterized by X-Ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). Transparent films were deposited by spin and draw bar coating and analysed by UV-Vis-NR spectroscopy.

[0023] To a 4L Erlenmeyer flask equipped with a magnetic stirrer is added ddH20 and the liquid is heated and allowed to stabilize at 50 C. To the heated liquid is added InC13-420 (185g, 0.631mol, FW=293.26g/mol), Sna4-5H20 (18.0g, 0.0514 mol, FW=350.50g/mol) and E-caprolactam (5.70g, 0.0504mol, FW=113.16g/mol). The solids were allowed to dissolve over 5 minutes with stirring held at 500 rpm throughout the reaction. To this heated solution was added NH4OH (aqueous, 28-30%, 105 ml). A white precipitate immediately formed and the mixture was allowed to stir for 24 hours.

[0024] After allowing the reaction mixture to cool to room temperature, (aqueous, 28-30%, 280 ml) was added and the mixture was stirred for 5 minutes to ensure proper mixing. The stir bar was then removed and the mixture was allowed to set for 1 hour. The resulting white precipitate was isolated by centrifugation (2000 rpm, 10 minutes, -700G), the supernatant was discarded and the white solid was redispersed in a total of 1.5L ddH20 over 1 hour using a mechanical stirrer (900 rpm). The resulting suspension was separated by centrifugation (2000 rpm, 10 minutes, -700G) and the redispersion/centrifugation steps were repeated 2 more times. The final white precipitate was then dispersed in 700 ml ddH20 using a mechanical stirrer (500 rpm, 15 minutes), transferred to freeze drying flasks, put in the shell freezer (30 minutes) and freeze dried for 16 hours to yield a light and airy white nanopowder (110 g).

[0025] Occasionally, a grainy solid was obtained, which could be converted to the airy white nanopowder by adding milling balls to the freeze drying flasks, followed by shaking and another round of freeze drying.

[0026] It was possible to obtain a finer white nanopowder by using double the volume of water and dispersing more sparingly during the shell freezing/freeze drying stage.

[0027] The freeze dried material was then sintered at 350 C under a forming gas (95% N2, 5% H2) atmosphere in a tube furnace. It was first purged with forming gas for 30 minutes at a flow rate of 200 l/h. When the desired temperature was achieved, the furnace was left for 3 hours under the flow of forming gas at a flow rate of 100 l/h and then cooled to room temperature. The obtained annealed powder shows a green/blue colour which turns bluer after exposure to air. The powder is then characterized to determine the phase structure and the crystallite size using XRD.

[0028] The XRD patterns of ITO powders after annealing show clearly that the particles are crystalline with the In203 cubic bixbyite crystal structure without the existence of any SnOx phases. There is a slight shift of the peaks toward small angles by the doping with Sn indicating an expansion of the unit cell caused by the substitution of In atoms by Sn atoms. This is a common observation in ITO
powders. The crystallite size calculated using the Scherrer equation from the broadening of the [222] peak was ¨15 nm. SEM shows that the particle size is consistent with the XRD result, but some polydispersity in the particle size is observed. Some agglomeration of the nanoparticles was observed.

[0029] Example 2: Preparation of ITO coat dispersions with ethylene glycol [0030] 10 g of ITO nanopowder was wetted with 9.7 g of ethylene glycol/di-ethyleneglycol-monobutyl ether (EG/DEGMBE; 1:1 w/w) and 1.9 g of TEGO
Dispers 750 W dispersion agent. The wetted ITO powder was dispersed mechanically using a planetary ball mill and a crucible charged with 0.5 mm ytrium oxide-stabilized ZrO2 balls (-100mL). The milling process was 1 hour at 200 rpm =
followed by 30 minutes at 400 rpm with resting intervals of 1 minute. The milling process broke up the agglomerated powder and ensured that the particles were evenly wetted and dispersed down to the primary size. A thick and highly viscous dark blue ITO paste was obtained. This paste was dispersed in ¨180 g of commercial acrylic-urethane clear coat paint. The blue dispersion was coated on glass using a #12 bar on commercial window glass. Performance of the glass was determined by UV-Vis-NIR spectroscopy. Figure 1 illustrates the UV-VIS-NIR
spectrum for a film approximately 6 microns in thickness.

[0031] Example 3: Production of a 5% Dispersion of ITO Particles [0032] For an inorganic based 5% dispersion, a similar approach was followed using 0.57 g of 042-(2-Methoxyethoxy)ethyl]glycolic acid as the dispersing agent and 20 g of hydrolyzed silane as a polymeric matrix. The total sample was diluted with ethanol (160 g). The dispersion was spin coated or bar draw coated on a commercial window glass. The inorganic film was cured after coated in a vacuum oven at 150 C for 3 hours. The hydrolyzed silane was used as both an addition promoter and a surface modification agent in order to bind the particles to the matrix while ensuring good adhesion to the glass surface. As pre-hydrolysed silane, 3-glycidyloxypropyltrimethoxysilane (GPTS) and mercaptopropyltrimethoxysilane (MPTS) were used but other inorganic matrices can also be used. The UV-VIS-NIR spectrum is shown in Figure 2.

[0033] The present invention has been described hereinabove by way of specific embodiments thereof. However, it can be modified, to the extent of the subject invention as defined in the appended claims.

Claims (6)

1. A method of preparing a liquid coating comprising indium tin oxide nanoparticles, said method comprising:
(a) precipitating an indium tin oxide precursor from an aqueous solution containing at least one indium compound and at least one tin compound in the presence of at least one surface-modifying component;
(b) separating the precipitate from the water by filtration or centrifugation;
(c) drying the precipitate by freeze drying to form a nanopowder;
(d) heating the nanopowder at 250-500°C under reducing conditions;
(e) combining the nanopowder with a solvent and a dispersant to form a paste; and (d) dissolving the paste in water or an organic solvent to form the liquid coating.

2. The method of claim 1 wherein the surface modifying component is a saturated and unsaturated monocarboxylic or polycarboxylic acid, a diketone, an amino acid, a polyethylene exide derivative, a lactone, a lactam, an acid amide, a monoamine or a polyamines.

3. The method of claim 2 wherein the surface modifying component is caprolactam.

4. The method of any one of claims 1 to 3 wherein the organic solvent is a C1-6 alcohol or an acrylic/urethane blend.

5. The method of claim 4 wherein the alcohol is ethanol or isopropanol.

6. The method of any one of claims 1 to 5 wherein the liquid coating is an acrylic paint.