Iron-based nanoparticles and methods of processing

Abstract

To provide methods and compositions including iron-based nanoparticles that are free of drawbacks of the prior art techniques.SOLUTION: There are provided methods and compositions including iron-based nanoparticles. A method of processing an iron-based nanoparticle to form a capped iron-based nanoparticle may include introducing a cation and introducing an anion. The cation may bond to a portion of an outer region of the iron-based nanoparticle to define an intermediate nanoparticle. The anion may react with the intermediate nanoparticle to form a cation-anion coating, or a shell, on one or more portions of the outer region of the intermediate nanoparticle to form an iron-based capped nanoparticle. The cation-anion coating may cover only a portion of the outer region.SELECTED DRAWING: None

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 637,500, filed April 23, 2024, the disclosure of which is incorporated herein by reference in its entirety. [Background technology]

[0002] The present disclosure generally relates to compositions comprising iron-based nanoparticles, systems comprising iron-based nanoparticles, and methods comprising iron-based nanoparticles. Summary of the Invention [Problem to be solved by the invention]

[0003] Nanoparticles may tend to coalesce to form agglomerates, reducing the exposed surface area and resulting in a lower energy form. In forming agglomerates, the nanoparticles are attracted to each other by relatively weak forces (e.g., van der Waals forces) and can form relatively weak bonds with each other. This tendency to form agglomerates can result in an increase in the average size of the nanoparticle system. [Means for solving the problem]

[0004] Iron-based nanoparticles can be treated or processed to reduce the formation of agglomerates. Iron-based nanoparticles can include iron oxide compounds such as FeO3 and FeO4. In some embodiments, a method for treating iron-based nanoparticles includes introducing cations to the outer region of the iron-based nanoparticles and then introducing anions to the outer region of the iron-based nanoparticles. The cations can bind to the outer region (e.g., via weak attractive forces) to define intermediate nanoparticles. The anions can then react with the cations of the intermediate nanoparticles to form a cation-anion coating, or shell, on one or more portions of the outer region of the intermediate nanoparticles, thereby forming iron-based capped nanoparticles. The shell may cover only a portion of the outer region, leaving another portion of the outer region exposed to the environment. The presence of the shell can reduce the occurrence of nanoparticle aggregation, resulting in the nanoparticles remaining as discrete individual nanoparticles. The shell can be water-insoluble and heat-resistant, such that the shell can remain attached to the outer region of the nanoparticles after additional processing. In some embodiments, the shell can keep the nanoparticles stable upon exposure to high heat, such that further coarsening of the nanoparticles is reduced and the average size of the nanoparticles remains small.

[0005] According to one example ("Example 1"), a method includes introducing cations to an iron-based nanoparticle having an outer region, where the cations bind to a portion of the outer region to define an intermediate nanoparticle, and introducing anions to the intermediate nanoparticle, where the anions react with the intermediate nanoparticle to form a cation-anion coating on one or more portions of the outer region of the intermediate nanoparticle, thereby forming an iron-based capped nanoparticle.

[0006] According to another example ("Example 2") related to Example 1, the iron-based capped nanoparticles include iron oxide.

[0007] According to another example ("Example 3") related to Example 2, the iron oxide includes either Fe2O3 or Fe3O4.

[0008] According to another example ("Example 4") related to Example 1, cations are introduced at a target molar ratio, which is 0.1% to 5% relative to the iron-based nanoparticles.

[0009] According to another example ("Example 5") related to Example 4, the anion is introduced in excess of the target molar ratio.

[0010] According to another example ("Example 6") related to Example 1, the step of introducing the anion further includes the step of introducing the anion at a substantially uniform rate of 1 to 100 mol / hour.

[0011] According to another example ("Example 7") related to Example 1, the method further includes suspending iron-based nanoparticles in a fluid prior to the step of introducing the cations.

[0012] According to another example ("Example 8") related to Example 7, the method further includes mixing the iron-based nanoparticles and the cations together in a fluid.

[0013] According to another example ("Example 9") related to Example 8, the step of introducing the anions further comprises immersion feeding the anions during mixing of the iron-based nanoparticles and the metal cations.

[0014] According to another example ("Example 10") related to Example 1, the method further includes milling the iron-based nanoparticles before the step of introducing the cations.

[0015] According to one example ("Example 11"), a method includes suspending iron-based nanoparticles in a fluid; exposing the iron-based nanoparticles to cations, thereby forming a mixture; mixing the mixture; and exposing the mixture to anions, wherein the anions react with the cations to form a precipitate on a portion of the outer region of the iron-based nanoparticles, thereby forming capped nanoparticles.

[0016] According to another example ("Example 12") related to Example 11, the method further comprises milling the iron-based nanoparticles.

[0017] According to another example related to Example 11 ("Example 13"), exposing the iron-based nanoparticles to cations further includes introducing the cations at a target molar ratio, the target molar ratio being 0.1% to 5% relative to the iron-based nanoparticles.

[0018] According to another example ("Example 14") related to Example 13, exposing the mixture to anions further includes introducing the anions in excess of a target molar ratio.

[0019] According to one example ("Example 15"), a method includes doping iron-based nanoparticles with an insoluble precipitate, wherein the iron-based nanoparticles include iron oxide.

[0020] According to another example ("Example 16") related to Example 15, doping the iron-based nanoparticles further includes introducing cations into the iron-based nanoparticles, where the cations bind to one or more portions of the outer region of the iron-based nanoparticles, and introducing anions into the iron-based nanoparticles, where the anions react with the cations in the portions of the outer region of the iron-based nanoparticles to form an insoluble precipitate.

[0021] According to one example ("Example 17"), a composition includes iron-based nanoparticles having an outer region and a shell covering at least a portion of the outer region, the shell including a cation-anion pair.

[0022] According to another example ("Example 18") related to Example 17, the iron-based nanoparticles have an average aggregate size of less than 2 microns.

[0023] According to another example ("Example 19") related to Example 17, the iron-based nanoparticles have a size of about 5 nanometers or larger.

[0024] According to another example ("Example 20") related to Example 17, the iron-based nanoparticles have a size of about 40 nanometers or less.

[0025] According to another example ("Example 21") related to Example 17, the cation-anion pair includes a metal cation.

[0026] According to another example ("Example 22") related to Example 17, the metal cation is Ca 2+ , Mg 2+ , Sr 2+ , Ba 2+ , Mn 2+ , or Cr 2+ Contains one of the following:

[0027] According to another example ("Example 23") related to Example 17, the cation-anion pair includes an anion, and optionally, the anion includes at least one of a hydroxide and a carbonate.

[0028] According to another example ("Example 24") related to Example 17, the cation-anion pair includes a divalent compound, and the divalent compound optionally includes at least one of ammonium carbonate, sodium carbonate, ammonium hydroxide, sodium hydroxide, trisodium phosphate, and ammonium phosphate.

[0029] According to another example ("Example 25") related to Example 17, the cation-anion pair includes a divalent compound containing chloride.

[0030] According to another example ("Example 26") related to Example 17, the shell covers 99% or less of the outer area of ​​the iron-based nanoparticles.

[0031] According to another example related to Example 17 ("Example 27"), the iron-based nanoparticles include iron oxide.

[0032] According to another example ("Example 28") related to Example 27, the iron oxide includes either Fe3O4 or Fe2O3.

[0033] According to one example ("Example 29"), a composition includes iron-based nanoparticles having an outer region that includes a grain boundary, and insoluble precipitates connected to at least a portion of the grain boundary, wherein 99% or less of the grain boundary is covered by the insoluble precipitate.

[0034] According to another example ("Example 30") related to Example 29, the insoluble precipitate includes at least one of an insoluble hydroxide or an insoluble carbonate.

[0035] According to another example ("Example 31") related to Example 29, the iron-based nanoparticles include iron oxide.

[0036] According to another example ("Example 32") related to Example 31, the iron oxide includes either Fe3O4 or Fe2O3.

[0037] According to another example ("Example 33") related to Example 29, the insoluble precipitate comprises a cation-anion pair.

[0038] According to one example ("Example 34"), the mixture includes at least one iron-based nanoparticle having an outer region and an anion-cation pair bound to a first portion of the outer region, and a second portion of the outer region is not covered by the anion-cation pair.

[0039] According to another example ("Example 35") related to Example 34, the iron-based nanoparticles have a size of about 5 nanometers or larger.

[0040] According to another example ("Example 36") related to Example 34, the iron-based nanoparticles have a size of about 40 nanometers or less.

[0041] According to another example ("Example 37") related to Example 34, the anion-cation pairs cover 99% or less of the outer area of ​​the iron-based nanoparticles.

[0042] The above examples are merely examples and should not be construed as limiting or otherwise narrowing the scope of any inventive concepts otherwise provided in this disclosure. While several examples are disclosed, still other embodiments will become apparent to those skilled in the art from the following detailed description. The following detailed description shows and describes illustrative examples. Accordingly, the drawings and detailed description should be regarded as illustrative and not restrictive. [Brief explanation of the drawings]

[0043] The accompanying drawings are included to provide a further understanding of the present disclosure, are incorporated in and constitute a part of this specification, illustrate embodiments, and together with the description serve to explain the principles of the present disclosure.

[0044] [Figure 1] 1 is an SEM image of a nanoparticle agglomerate according to one embodiment.

[0045] [Figure 2] 2 is an SEM image of FIG. 1 at a higher magnification according to one embodiment.

[0046] [Figure 3] FIG. 1 is a process flow diagram for processing nanoparticles according to one embodiment.

[0047] [Figure 4] FIG. 1 is a schematic diagram of an intermediate nanoparticle according to one embodiment.

[0048] [Figure 5] FIG. 2 is a schematic diagram of a shell present in the region of a nanoparticle according to one embodiment.

[0049] Detailed Description Definitions and Nomenclature The present disclosure is not intended to be limiting. For example, the terminology used in this application should be interpreted broadly in accordance with the meaning that one of ordinary skill in the art would ascribe to that terminology.

[0050] With respect to terminology that includes imprecision, the terms "approximately" and "about" can be used interchangeably to describe a measurement that includes the stated measurement as well as any measurement that is reasonably close to the stated measurement. A measurement that is reasonably close to the stated measurement refers to a measurement that deviates from the stated measurement by a reasonably small amount, to the extent that one of ordinary skill in the art would understand and easily identify. Such deviations can result from, for example, measurement error, differences in calibration of measuring and / or manufacturing equipment, human error in reading and / or setting measurements, fine-tuning made to optimize performance and / or structural parameters to account for differences in measurements associated with other components, specific implementation scenarios, imprecise adjustment and / or manipulation of objects by humans or machines, and the like. If it is determined that one of ordinary skill in the art would not be able to easily identify such a reasonably small difference, the terms "approximately" and "about" can be interpreted to mean ±10% of the stated value.

[0051] As used herein, the term "cation" or "metal cation" generally refers to an atom that has lost an electron such that the atom has an overall positive charge to form a cation. A metal cation can refer to the elemental metal that forms the cation.

[0052] As used herein, the term "anion" generally refers to an atom that gains electrons during a reaction such that the atom has an overall positive or negative charge and forms an anion.

[0053] With respect to the "capped nanoparticles," "cation-anion coatings," "precipitates," or "shells" described herein, the terms "coating," "bonded," "impregnated," "doped," and "connected" can be used interchangeably to refer to the covering of one or more regions of the iron-based nanoparticles. Description of Various Embodiments

[0054] Those skilled in the art will readily appreciate that the various aspects of the present disclosure may be implemented by numerous methods and apparatuses configured to perform the intended functions. It should be noted that the accompanying drawings referred to herein are not necessarily drawn to scale and may be exaggerated to illustrate various aspects of the present disclosure, and in that regard the drawings should not be construed as limiting.

[0055] Furthermore, it will be understood that certain features of the present disclosure that are described herein in the context of separate examples for clarity's sake, may also be provided in combination in a single example. Conversely, various features of the present disclosure that are, for brevity's sake, described in the context of a single example, may also be provided separately or in any subcombination.

[0056] 1 and 2 show SEM images of a nanoparticle agglomerate 10 according to some embodiments. The nanoparticle agglomerate 10 may be comprised of a plurality of individual nanoparticles. The plurality of individual nanoparticles may include iron-based nanoparticles 12 having an outer region 13. The iron-based nanoparticles 12 described herein may include iron oxide, including one of Fe3O4 or Fe2O3. However, other forms of iron are also contemplated, including, but not limited to, rust, ferrous chloride (FeCl2), and ferric chloride (FeCl3).

[0057] The iron-based nanoparticles 12 can include a first nanoparticle 14, a second nanoparticle 16, and a third nanoparticle 18. The nanoparticle agglomerate 10 can include one or more boundaries 20, such as interfaces, voids, inclusions of other phases (magnetic or non-magnetic), or some other discontinuity, such as a strain gradient, between each nanoparticle of the nanoparticle agglomerate 10. Although three nanoparticles are shown, in other embodiments, more or less than three nanoparticles can form the nanoparticle agglomerate 10.

[0058] As shown in Figure 1, each of the first, second, and third nanoparticles 14, 16, and 18 can have different shapes and sizes. In some embodiments, as shown in Figure 1, each of the first, second, and third nanoparticles 14, 16, and 18 comprises non-spherical or irregular shapes, and the irregular shapes are all different from each other. Thus, the corresponding outer regions 13 and peripheries of each of the first, second, and third nanoparticles 14, 16, and 18 can include various topographies, including both smooth and non-smooth outer regions.

[0059] Additional processing steps may be required to reduce the coalescence and formation of nanoparticle agglomerates 10. Figure 3 shows a process 22 for treating iron-based nanoparticles, according to one embodiment. Process 22 can reduce the formation of nanoparticle agglomerates 10 and increase the amount of exposed surface area of ​​individual nanoparticles. This process has been described with respect to a plurality of iron-based nanoparticles 12, which may include nanoparticles substantially similar to first, second, and third nanoparticles 14, 16, 18.

[0060] Process 22 of FIG. 3 can include multiple steps, optionally including step 24 of milling the nanoparticle agglomerates, step 26 of suspending the iron-based nanoparticles in a fluid, step 28 of agitating the iron-based nanoparticles when suspended, step 30 of introducing a cation, step 32 of introducing an anion, and optionally step 34 of post-treating the capped iron-based nanoparticles.

[0061] In some embodiments, step 24 of milling the plurality of nanoparticle agglomerates is optionally performed. Milling can aid in the fragmentation of the nanoparticle agglomerates 10 to generate a plurality of iron-based nanoparticles 12 prior to further processing steps and can aid in the potential dispersion of the plurality of iron-based nanoparticles 12 within a fluid. In processes that perform milling, step 24 of milling the plurality of nanoparticle agglomerates can include a wet ball milling process in a fluid medium. The fluid medium can include at least one of deionized (DI) water or a solvent. Prior to milling, the nanoparticle agglomerates 10 can have a size greater than about 100 microns. The plurality of nanoparticle agglomerates 10 can be milled until the median size of the nanoparticle agglomerates 10 is about 50 microns or smaller, optionally about 10 microns or smaller, and optionally about 2 microns or smaller. The size of the nanoparticle agglomerates 10 can be about 50 microns to 40 microns, about 40 microns to 30 microns, about 30 microns to 20 microns, about 20 microns to 10 microns, about 10 microns to 5 microns, or about 5 microns to 2 microns. The median size (e.g., D50) can be measured by laser diffraction. In other embodiments, the plurality of iron-based nanoparticles 12 can be pre-milled, or a milling step can be omitted, in which case the plurality of iron-based nanoparticles 12 can be of a predetermined size. In some embodiments, the predetermined size of the iron-based nanoparticles 12 is about 30 nanometers or smaller, or about 30 nanometers to 25 nanometers, about 25 nanometers to 20 nanometers, about 20 nanometers to 15 nanometers, about 15 nanometers to 10 nanometers, about 10 nanometers to 5 nanometers, or about 5 nanometers to 1 nanometer.

[0062] Optionally, step 24 of milling the plurality of nanoparticle agglomerates can be followed by step 26 of suspending the plurality of iron-based nanoparticles in a fluid. The fluid can include at least one of deionized (DI) water, a solvent (e.g., a polar solvent), or a hydroxide, such as lithium hydroxide, sodium hydroxide, potassium hydroxide, rubidium hydroxide, and cesium hydroxide (or other strong basic hydroxide). In some embodiments, the fluid medium of the milling process is used to suspend the plurality of iron-based nanoparticles 12.

[0063] After step 26 of suspending the plurality of iron-based nanoparticles in a fluid, step 28 of agitating the plurality of iron-based nanoparticles during suspension can be performed. The step of agitating the plurality of iron-based nanoparticles 12 can include, but is not limited to, mechanical agitation, including at least one of stirring and mixing. Mechanical agitation can create an environment in which the plurality of iron-based nanoparticles 12 are uniformly suspended in the fluid medium. The plurality of iron-based nanoparticles 12 can be agitated in a vessel, including, but not limited to, a reaction vessel. In some embodiments, the step of agitating the plurality of iron-based nanoparticles includes the use of a stir bar connected to a stirrer, such as a stir bar for an overhead stirrer. The agitator can include a multi-blade axial stirrer with three or four blades. Optionally, the stir bar diameter can be about one-third the width of the vessel containing the fluid and the plurality of iron-based nanoparticles. In some embodiments, the stir bar agitates the fluid and the plurality of iron-based nanoparticles at about 400 RPM for about 15 minutes, or until the fluid is determined to be relatively uniform and / or at equilibrium. In some embodiments, a color indicator may be added to the fluid to visually indicate a relatively uniform mixture.

[0064] The step of introducing cations 30 can be performed while the plurality of iron-based nanoparticles 12 are being stirred. The step of introducing cations 30 can be performed after it is determined that the fluid and the plurality of iron-based nanoparticles are relatively uniformly mixed. The cations 36 themselves can be suspended in a fluid prior to the introduction. The fluid can be the same or different from the fluid used to suspend the nanoparticles. The introduction of cations 36 into the container can form a mixture including the cations 36, the plurality of iron-based nanoparticles 12, and the fluid. The mixture can be continuously stirred after the introduction of cations 36, which facilitates exposure of the cations to the outer regions 13 of the plurality of iron-based nanoparticles 12. The continuous stirring can also help facilitate relatively uniform mixing of the mixture. As shown in FIG. 4 , due to their positive charge, the cations 36 can associate or form relatively weak bonds with one or more portions 46a of the outer regions 13 of the plurality of iron-based nanoparticles 12, forming a first layer on at least a portion of the outer regions 13, thereby defining intermediate nanoparticles 40.

[0065] Step 30 of introducing cations can be followed by step 32 of introducing anions. The anions 38 themselves can be suspended in the same fluid of the mixture prior to introduction. Continuous stirring of the anions 38, fluid, and intermediate nanoparticles 40 (FIG. 4) during introduction can promote access and reaction of the anions 38 with the cations 36 and promote relatively uniform contact with the iron-based nanoparticles 12. The anions 38 can react with the cations 36 of the intermediate nanoparticles 40 due to the attraction between their respective negative and positive charges. This results in the formation of a cation-anion coating, or shell 44, on one or more portions 48a of the outer region 13 of the iron-based nanoparticles 12, as shown in FIG. 5, thereby defining the iron-based capped nanoparticles 46. The shell 44 may not cover the entire outer region 13 and can optionally cover about 99% or less of the outer region 13, as described below. Another portion 48b of the iron-based capped nanoparticles 46 is not covered by the shell 44 and can be exposed to the environment.

[0066] The coverage of the outer region 13 of the iron-based nanoparticles 12 by the shell 44 can be affected, at least in part, by the volume or mass of the cations 36 and anions 38 present in the mixture and / or by the rate of introduction of the cations 36 and anions 38. The cations 36 can be introduced at a target molar ratio of cations 36 to iron-based nanoparticles 12. The target molar ratio of cations 36 can be less than about 5%, optionally about 2.5%, about 0.1% to about 0.5%, about 0.5% to about 1%, about 1% to about 1.5%, about 1.5% to about 2%, about 2% to about 2.5%, about 2.5% to about 3%, about 3% to about 3.5%, about 3.5% to about 4%, about 4% to about 4.5%, or about 4.5% to about 5%. In other embodiments, the cations 36 can be introduced as a target weight percent of cations 36 relative to the iron-based nanoparticles 12. The target weight percent can vary depending on the type of cation 36 selected, as described further below. The target weight percent of cation 36 can be about 16.5 wt % or lower, and can be about 0.04 wt % to 16.5 wt %, about 0.04 wt % to 4 wt %, about 4 wt % to 8 wt %, about 8 wt % to 12 wt %, or about 12 wt % to 16.5 wt %. As such, cation 36 is a limiting agent in the formation of shell 44. Continuous agitation of cation 36 and iron-based nanoparticles 12 promotes relatively uniform mixing, resulting in cation 36 accessing and reacting with outer regions 13 of each of the plurality of iron-based nanoparticles 12.

[0067] Introducing anions 38 in excess of a target molar ratio of cations 36 can help ensure complete reaction of cations 36. Anions 38 can be introduced via a dip-feed process. In some embodiments, anions 38 can be introduced at a substantially uniform rate of about 1 to 100 mol / hour, optionally about 19 mol / hour, about 17 to 18 mol / hour, about 1 mol / hour to 10 mol / hour, about 10 to 20 mol / hour, about 20 to 30 mol / hour, about 30 to 40 mol / hour, about 40 to 50 mol / hour, about 50 to 60 mol / hour, about 60 to 70 mol / hour, about 70 to 80 mol / hour, about 80 to 90 mol / hour, or about 90 to 100 mol / hour. In other embodiments, anions 38 can be introduced as a target weight percent relative to iron-based nanoparticles 12. The target weight percent can vary depending on the type of anion 38 selected, as described further below. The target weight percent of anion 38 can be about 9500 g / h or lower, and can be between about 17 g / h and 9500 g / h, between about 17 g / h and 1500 g / h, between about 1500 g / h and 3000 g / h, between about 3000 g / h and 4500 g / h, between about 4500 g / h and 6000 g / h, between about 6000 g / h and 7500 g / h, or between about 7500 g / h and 9500 g / h.

[0068] The rate of introduction of anions 38 can be selected so that the final molar ratio of anions 38 to iron-based nanoparticles 12 is about 0.3-0.4, optionally 0.34, about 0.3-0.32, about 0.32-0.34, about 0.34-0.36, about 0.36-0.38, or about 0.38-0.4. In other embodiments, the total volume of anions 38 can be introduced at once rather than at a substantially uniform rate.

[0069] In other embodiments, the reverse is also contemplated, i.e., anion 38 is the limiting agent, and anion 38 is first introduced to iron-based nanoparticles 12 at a target concentration (e.g., a target molar ratio), and second, cation 36 is introduced to iron-based nanoparticles 12 in excess of the target concentration.

[0070] The coverage of the outer region 13 with the shell 44 can also be affected by reaction temperature and continuous agitation. The process of forming the shell 44 can be carried out at room temperature (e.g., about 20°C to 30°C) without additional heating. In other embodiments, the formation of the shell 44 can be carried out at a temperature above room temperature, such as about 95°C or lower. The process can also be carried out at standard atmospheric pressure (e.g., approximately 1 atm). Continuous agitation can be employed to promote the reaction between the cations 36 and the anions 38 to completion through relatively uniform mixing of the cations 36, anions 38, and the plurality of iron-based nanoparticles 12.

[0071] Once the iron-based capped nanoparticles 46 are formed, step 34 can be optionally performed to post-process the plurality of capped iron-based nanoparticles. The fluid can be removed from the container, leaving the iron-based capped nanoparticles 46 in a separated state. The iron-based capped nanoparticles 46 can then be optionally rinsed one or more times with deionized (DI) water or distilled water. The cation-anion precipitates of the shell 44 are insoluble precipitates with low solubility in water and other polar solvents and therefore do not dissolve during subsequent processing steps, such as the rinsing step. Therefore, the shell 44 is not removed from the iron-based capped nanoparticles 46 during the rinsing step, but remains attached to one or more outer regions 13. After rinsing, the iron-based capped nanoparticles 46 can be dried to remove excess fluid.

[0072] The presence of shell 44 on iron-based capped nanoparticles 46 can be verified using X-ray photoelectron spectroscopy (XPS) measurements. The components of shell 44 may be present in lower total concentrations than iron or iron oxide, but were found to remain present after the steps of process 22 as described above.

[0073] 5 is a schematic diagram of a shell 44 present in the outer region 13 of an iron-based nanoparticle 12, according to one embodiment. While the iron-based capped nanoparticle 46 is shown as spherical, other non-spherical or irregular shapes are contemplated, such as those shapes described above with respect to FIGS. 1-2.

[0074] The cations 36 of the shell 44 can be selected from halides of the periodic table. The cations 36 can be metal cations selected from the group including alkali metals, alkaline earth metals, or transition metals. The cations 36 can be introduced as compounds or in ionic form. In some examples, the cations 36 are introduced to the plurality of iron-based nanoparticles as compounds such as chlorides (e.g., MCl2, where M collectively represents a metal), e.g., CaCl2, where M is a divalent compound (e.g., having a 2+ charge in ionic form). MCl2 (including CaCl2) is soluble in water, such that MCl2 is converted to M during the cation introducing step 30. 2+ ions and Cl - ions, where M 2+ The ions are weakly bound to the outer region 13 of the iron-based nanoparticles 12. Metal cations (M) include, but are not limited to, calcium (Ca 2+ ), magnesium (Mg 2+ ), strontium (Sr 2+ ), barium (Ba 2+ ), manganese (Mn 2+ ), or chromium (Cr 2+ ) can be mentioned.

[0075] The anions 38 of the shell 44 include, but are not limited to, oxides (O), hydroxides (OH), and the like. - ), bicarbonate (HCO3 - ), or carbonate (CO3 2-). The anion 38 can be introduced as a compound, including, but not limited to, ammonium carbonate ((NH4)2CO3), sodium carbonate (Na2CO3), ammonium hydroxide (NH4OH), or sodium hydroxide (NaOH). Like the cationic compounds, ((NH4)2CO3) (and any of the other compounds mentioned above) are soluble in water, so that ((NH4)2CO3) can be converted to ammonium (NH4 + ) ions and carbonate (CO3 2- ) ions, and carbonate (CO3 2- ) ions are present in the outer region 13 of the metal M 2+ The anion 38 can optionally include phosphates, such as, but not limited to, monobasic phosphate (HPO 2- ), dibasic phosphate (H2PO4 -), trisodium phosphate (NaPO), and ammonium phosphate ((NH)PO). Precipitates of cations 36 and anions 38 can then nucleate and grow on the outer region 13 of the iron-based nanoparticle 12 to form a shell 44. Upon formation, the shell 44 can form a monolayer or stacks on the outer region 13 of the iron-based nanoparticle 12, or a combination of both growths. The shell 44 is attached to the outer region 13 of the iron-based nanoparticle, covering at least a portion 46a of the iron-based nanoparticle. The shell 44 comprises an insoluble cation-ion precipitate that is not soluble in water or other polar solvents, and can include, but is not limited to, manganese hydroxide (Mn(OH)), manganese dioxide (MnO), chromium hydroxide (Cr(OH)), calcium carbonate (CaCO), magnesium carbonate (MgCO), strontium carbonate (SrCO), or barium carbonate (BaCO). In addition to being insoluble, the cation-anion precipitate of shell 44 can also be thermally stable, such that iron-based capped nanoparticles 46 can be exposed to high temperatures in subsequent processes and still retain shell 44 in outer region 13. The high temperatures can be between about 220°C and 350°C, between about 220°C and 250°C, between about 250°C and 300°C, or between about 300°C and 350°C.

[0076] In some embodiments, using CaCO or BaCO as anion 38 resulted in smaller nanoparticle size and a narrower size distribution (smaller standard deviation) in iron-based capped nanoparticles 46 compared to using either MgCO or SrCO as anion 38. This may be due to the lower solubility or greater resistance to removal during the rinsing process of CaCO or BaCO compared to MgCO or SrCO. Thus, the use of CaCO or BaCO can result in greater resistance to removal from outer region 13 of iron-based nanoparticles 12, which may help reduce instances of nanoparticle aggregation occurring after formation of shell 44.

[0077] After the formation of the shell 44, excess chloride ions (Cl - ), ammonium ion (NH4 + ) or sodium ion (Na + ), and any reacted NH4Cl, NaCl, or ClOH compounds, can be disposed of with the fluid in the post-treatment steps described above. The ions, compounds, and fluids of process 22 are considered harmless for disposal.

[0078] The shell 44 helps prevent the iron-based capped nanoparticles 46 from coalescing to form nanoparticle agglomerates 10, as shown in FIGS. 1-2. This allows the iron-based capped nanoparticles 46 to be separated from one another and maintain a relatively small size. This also prevents the iron-based capped nanoparticles 46 from growing larger when exposed to high heat in subsequent processes, thereby allowing the iron-based capped nanoparticles 46 to maintain their size. In some embodiments, the average aggregate size (e.g., D50 size) of the plurality of iron-based nanoparticles 12 is about 10 microns or less, or 2 microns or less. In other words, the average size of the plurality of iron-based nanoparticles 12 can be greater than about 5 microns. The iron-based nanoparticles 12 may be about 5 nanometers or larger and about 40 nanometers or smaller in size, and may be about 40 nanometers to 35 nanometers, about 35 nanometers to 30 nanometers, about 30 nanometers to 25 nanometers, about 25 nanometers to 20 nanometers, about 20 nanometers to 15 nanometers, about 15 nanometers to 10 nanometers, or about 10 nanometers to 5 nanometers in size.

[0079] The shell 44 can cover one or more portions 48a of the outer region 13 of the iron-based capped nanoparticle 46, such that one or more portions 48b are not covered by the shell 44. The shell 44 can cover one or more boundaries 20 (e.g., grain boundaries as shown in FIG. 2) of the iron-based nanoparticle 12. In some embodiments, the shell 44 covers about 99% to 95% of the outer region 13, about 95% to 90% of the outer region 13, about 90% to 85% of the outer region 13, about 85% to 80% of the outer region 13, about 80% to 75% of the outer region 13, about 75% to 70% of the outer region 13, about 70% to 65% of the outer region 13, about 65% to 60% of the outer region 13, about 60% to 55% of the outer region 13, or about 5% to 60% of the outer region 13. It covers 5% to 50%, about 50% to 45% of the outer region 13, about 45% to 40% of the outer region 13, about 40% to 35% of the outer region 13, about 35% to 30% of the outer region 13, about 30% to 25% of the outer region 13, about 25% to 20% of the outer region 13, about 20% to 15% of the outer region 13, about 15% to 10% of the outer region 13, about 10% to 5% of the outer region 13, or about 5% to 1% of the outer region 13.

[0080] In some embodiments, the thickness of shell 44 can be about 1 nm to 10 nm, or optionally about 1 nm to 5 nm. The thickness of shell 44 can optionally be about 1 nm to 2 nm, about 2 nm to 3 nm, about 3 nm to 4 nm, about 4 nm to 5 nm, about 5 nm to 6 nm, about 6 nm to 7 nm, about 7 nm to 8 nm, about 8 nm to 9 nm, or about 9 nm to 10 nm.

[0081] The invention of this application has been described above both generically and with reference to specific embodiments. It will be apparent to those skilled in the art that various changes and modifications may be made to the embodiments without departing from the scope of the present disclosure. Thus, it is intended that the embodiments cover the modifications and variations of this invention provided that they come within the scope of the appended claims and their equivalents.

Claims

1. 1. A method comprising: introducing cations into an iron-based nanoparticle having an outer region, the cations binding to a portion of the outer region to define an intermediate nanoparticle; introducing anions to the intermediate nanoparticles, wherein the anions react with the intermediate nanoparticles to form a cation-anion coating on one or more portions of the outer region of the intermediate nanoparticles, thereby forming iron-based capped nanoparticles; A method comprising:

2. The method of claim 1 , wherein the iron-based capped nanoparticles comprise iron oxide.

3. The iron oxide is Fe ２ O ３ and Fe ３ O ４ The method of claim 2, comprising any one of:

4. 10. The method of claim 1, wherein the cations are introduced at a target molar ratio, the target molar ratio being between 0.1% and 5% relative to the iron-based nanoparticles.

5. 5. The method of claim 4, wherein the anion is introduced in excess of the target molar ratio.

6. 10. The method of claim 1, wherein the step of introducing the anion further comprises the step of introducing the anion at a substantially uniform rate of 1 to 100 mol / hour.

7. 10. The method of claim 1, further comprising suspending the iron-based nanoparticles in a fluid prior to the step of introducing the cations.

8. 8. The method of claim 7, further comprising mixing the iron-based nanoparticles and cations together in the fluid.

9. 9. The method of claim 8, wherein the step of introducing the anions further comprises immersion feeding the anions during mixing of the iron-based nanoparticles with the metal cations.

10. 10. The method of claim 1, further comprising the step of milling the iron-based nanoparticles prior to the step of introducing the cations.

11. 1. A method comprising: suspending iron-based nanoparticles in a fluid; exposing the iron-based nanoparticles to cations, thereby forming a mixture; mixing the mixture; exposing the mixture to anions such that the anions react with the cations to form precipitates on a portion of the outer region of the iron-based nanoparticles, thereby forming capped nanoparticles; A method comprising:

12. The method of claim 11 further comprising the step of milling the iron-based nanoparticles.

13. 12. The method of claim 11, wherein exposing the iron-based nanoparticles to cations further comprises introducing the cations at a target molar ratio, the target molar ratio being between 0.1% and 5% relative to the iron-based nanoparticles.

14. 14. The method of claim 13, wherein exposing the mixture to anions further comprises introducing the anions in excess of the target molar ratio.

15. 1. A method comprising: doping iron-based nanoparticles with an insoluble precipitate, wherein the iron-based nanoparticles comprise iron oxide; A method comprising:

16. doping the iron-based nanoparticles, introducing cations into the iron-based nanoparticles, wherein the cations bind to one or more portions of the outer region of the iron-based nanoparticles; introducing anions into the iron-based nanoparticles, wherein the anions react with the cations in the portion of the outer region of the iron-based nanoparticles to form an insoluble precipitate; 16. The method of claim 15, further comprising:

17. 1. A composition comprising: iron-based nanoparticles having an outer region; a shell covering at least a portion of said outer region, said shell comprising a cation-anion pair; A composition comprising:

18. 18. The composition of claim 17, wherein the iron-based nanoparticles have an average aggregate size of less than 2 microns.

19. 18. The composition of claim 17, wherein the iron-based nanoparticles have a size of about 5 nanometers or greater.

20. 18. The composition of claim 17, wherein the iron-based nanoparticles have a size of about 40 nanometers or less.

21. 18. The composition of claim 17, wherein the cation-anion pair comprises a metal cation comprising at least one of an alkali metal, an alkaline earth metal, or a transition metal.

22. 18. The composition of claim 17, wherein the cation-anion pair comprises an anion comprising at least one of an oxide, hydroxide, and carbonate.

23. The cation-anion pair may be ammonium carbonate, sodium carbonate, ammonium hydroxide, sodium hydroxide, trisodium phosphate, manganese hydroxide (Mn(OH) ２ ), manganese dioxide (MnO ２ 20. The composition of claim 17, comprising at least one of: ammonium phosphate;

24. 18. The composition of claim 17, wherein the cation-anion pair comprises chloride.

25. 18. The composition of claim 17, wherein the shell covers 99% or less of the outer region of the iron-based nanoparticle.

26. 18. The composition of claim 17, wherein the iron-based nanoparticles comprise iron oxide.

27. The iron oxide is Fe ３ O ４ or Fe ２ O ３ 27. The composition of claim 26, comprising any one of:

28. 1. A composition comprising: iron-based nanoparticles having an outer region that includes a grain boundary; insoluble precipitates connected to at least a portion of the grain boundaries, wherein 99% or less of the grain boundaries are covered by the insoluble precipitates; A composition comprising:

29. 30. The composition of claim 28, wherein the insoluble precipitate comprises at least one of an insoluble hydroxide or an insoluble carbonate.

30. 30. The composition of claim 28, wherein the iron-based nanoparticles comprise iron oxide.

31. The iron oxide is Fe ３ O ４ or Fe ２ O ３ 31. The composition of claim 30, comprising any one of:

32. 29. The composition of claim 28, wherein the insoluble precipitate comprises a cation-anion pair.

33. A mixture, the mixture comprising: at least one iron-based nanoparticle having an outer region; an anion-cation pair bound to a first portion of the outer region; Including, a second portion of said outer region not covered with said anion-cation pair;

34. 34. The mixture of claim 33, wherein the iron-based nanoparticles have a size of about 5 nanometers or greater.

35. 34. The mixture of claim 33, wherein the iron-based nanoparticles have a size of about 40 nanometers or less.

36. 34. The mixture of claim 33, wherein the anion-cation pairs cover 99% or less of the outer region of the iron-based nanoparticles.

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