Metal oxide magnetic nanoparticles and method for producing the same

Abstract

This invention provides magnetic nanoparticles that exhibit ferromagnetism even at wavelengths of 10 nm or less. [Solution] The magnetic nanoparticles of the present invention have an average primary particle diameter and a crystallite diameter of 3 nm or less, at least one of which is 3 nm or less. The metal constituting the oxygen-excess / deficient metal oxide nanoparticles is preferably one or more selected from alkali metals, alkaline earth metals, post-transition metals, metalloids, lanthanides, and actinides. The nanoparticles are mixed using a mixing step of mixing a supercritical, subcritical, or gas-phase aqueous material with a solution of a metal complex, with a mixing rate k using Kolmogorov's theory. mix Determine the mixing rate k mix and apparent reaction rate k app Using this, the true reaction rate k is 1 / k = 1 / k app -1 / k mix From this, we can derive the Damkeller number Da = k / k mix By setting it to <<1, the mixing time can be controlled within a range of 0.015 seconds to 1 second.

Description

[Technical Field]

[0001] This invention relates to metal oxide magnetic nanoparticles and a method for producing the same. [Background technology]

[0002] The applications of nanoparticles have been expanding rapidly in recent years. Composite materials of polymers and nanoparticles are expected to exhibit the contrasting properties of polymers (moldability) and inorganic materials (refractive index, dielectric constant, luminescence, electrical conductivity, thermal conductivity, magnetism, etc.). Nanofluids, in which nanoparticles are dispersed in a solvent, possess not only these properties but also fluidity, and are expected to have a wide range of applications, such as heat transport in air conditioners, optical and thermal conductivity required for immersion lithography, and electromagnetic fluids with viscous switching functions, and development is progressing accordingly.

[0003] Currently, memory materials such as magnetic films are being manufactured by compositing polymers and magnetic nanomaterials. The number of memory bits is ultimately determined by the size of the memory element. In the case of needle-shaped ferrite nanoparticles, it is possible to reduce the nanoparticle size to less than a few tens of nanometers, but because an S / N magnetic field is applied from the magnetic head in a direction parallel to the film in the particle domain, and because of S / N magnetic field interference, the size of the magnetic memory domain is much larger than the particle size, becoming sub-micrometer size. In contrast, with magnetic memory materials such as barium ferrite, plate-like particles are arranged in the plane of the film, and S / N magnetism is generated perpendicular to the plate-like particles, so in principle, it is possible to make one particle one bit. Although they can be synthesized by glass crystallization or hydrothermal synthesis, the size of nanoparticles actually used is several tens of nanometers or larger. Smaller magnetic nanoparticles are needed.

[0004] In magnetic nanofluids, a clutch function can be achieved by aligning nanoparticles with a magnetic field to increase viscosity. While an electric field response to a dielectric is also possible, a magnetic field response is expected to yield higher responsiveness. In this case, superparamagnetism (ferromagnetism disappearing at nanoscale) is assumed to manifest at tens of nanometers or less (mostly 10 nm or less). This eliminates residual magnetism in the particles by removing the magnetic field, allowing for repeated use. Furthermore, since ferromagnetism is manifested when magnetically applied particles are aligned, a higher level of functionality is achieved than with an electric field response. However, there are limits to this magnetic manifestation, and stronger magnetic manifestation has been sought.

[0005] The simultaneous manifestation of magnetic and optical properties is exemplified by response functions such as the Faraday effect and the Kerr effect. When a magnetic field is applied to nanofluids or transparent composite materials in which nanoparticles are transparently dispersed, their optical activity changes in accordance with the magnetic field. If two materials with optical activity differing by 90 degrees are superimposed, light transmission will cease, while materials with the same optical activity will transmit light. This makes it possible to create smart windows that control light transmission by switching the magnetic field on and off. These can also be used as switches for opening and closing windows. If wavelength dependence is possible, applying this to infrared radiation (heat rays) could significantly reduce energy consumption by introducing light in cold weather and reflecting it in hot weather.

[0006] In many cases, for both hybrid materials and nanofluids, the properties exhibited, such as refractive index, dielectric constant, thermal conductivity, and electrical conductivity, are proportional to the density of the nanoparticles, requiring high-concentration dispersion of nanoparticles. To achieve this, it is necessary to maximize the affinity between the nanoparticles and the medium (polymer, solvent), and thus modify the surface of the nanoparticles with organic molecules. Furthermore, even if good high-concentration dispersion is possible, low viscosity is desirable considering the moldability of the composite material and the flowability of the nanofluid, and optimal organic modification is required to achieve this. Patent document 1 proposes good dispersion of organically modified nanoparticles. Patent document 2 proposes controlling the viscosity when dispersing organically modified nanoparticles in a solvent. [Prior art documents] [License]

[0007] [License 1] Special Announcement No. 2024-165976 [License 2] Special Announcement No. 2024-042775 [License 3] Special Announcement No. 2022-141963 [Non-licensed literature]

[0008] [Non-licensed Document 1] Sasaki Ra, Modulation of New Inorganic Nano Particles for Gd Neutral Particle Capture Therapy Using Supercritical Hydrothermal Method, 2006, Vol. 43, No. 6, pp. 440-444, Journal of the Chinese Society for Powder Engineering, DOI https: / / doi.org / 10.4164 / sptj.43.440 [Non-licensed Document 2] Singh, V; Naka, T; Adschiri, T, Hydrothermal synthesis of inorganic-organic hybrid gadolinium hydroxide nanoclusters with controlled size and morphology, DALTON TRANSACTIONS, 2013, 42 (45), pp.16176-16184 [Non-licensed Document 3] A. Yoko, Y. Tanaka, G. Seong, D. Hojo, T. Tomai, and T. Adschiri, Mixing and Solvent Effects on Kinetics of Supercritical Hydrothermal synthesis: Reaction of Nickel Nitrate to Nickel Oxide, J. Phys. Chem. C, 2020, 124, 4772-4780. [Non-licensed Document 4] N. Aoki, A. Sato, H. Sasaki, A. Litwinowicz, G. Seong, T. Aida, D. Hojo, S. Takami, and T. Adschiri, Kinetics study to identify reaction-controlled conditions for supercritical hydrothermal nanoparticle synthesis with flow-type reactors, J. of Supercritical Fluids 2016, 110, 161-166. [Overview of the project] [Problems that the invention aims to solve]

[0009] Nanoparticle sizes are generally considered to be 100 nm or less. From the perspective of controlling the viscosity of nanofluids (Patent Document 2), high viscosity is observed when the size is tens of nm or less, requiring different handling. In the case of optical materials, the size of dispersed particles (including aggregates) can suppress light scattering by making it 1 / 10 or less of the wavelength of light used, and the required particle size is 40 nm or less in the visible light region. Here, we focus on nanoparticles of 100 nm, and especially 40 nm or less, depending on the properties required in the application area. While there is a need to enhance this effect, magnetic nanomaterial synthesis technology for particles smaller than 10 nm has not yet been established.

[0010] Generally, neodymium magnets are used as strong magnetic materials, but strontium ferrite is sometimes used as a rare metal equivalent. Ferrite materials are mainly used as memory materials. Examples of perpendicular magnetic memory materials include barium ferrite and strontium ferrite. When dealing with nano-sized particles, it is difficult to synthesize large quantities of metal nanoparticles with a size of several tens of nanometers or less, so metal oxides are used instead. Patent document 2 describes a method for synthesizing these nanoparticles while also achieving organic modification. However, generally, when the size is several tens of nanometers or less, superparamagnetic effects occur, and the properties as a memory magnetic material are lost, which is a problem.

[0011] In the medical field, magnetic materials are used in MRI diagnosis. This utilizes the magnetic field response of Gd. Gd complexes are used, but high concentrations are difficult to achieve. As an alternative, (hydroxylated) Gd nanoparticles can also be used (Non-Patent Literature 1, 2). Magnetite particles can also be used. Furthermore, there is thermotherapy as a cancer treatment method. This treatment method utilizes the fact that cancer cells are more sensitive to heat than normal cells. Electric field-responsive and magnetic field-responsive particles are sometimes used, but applying a strong electric field may cause the destruction of normal cells, so a magnetic field response is preferable. However, in all cases, the disposal of particles after use is a problem. Generally, particles larger than 10 nm may be trapped by some macrophages, but even if not, there are limits to their dissolution and removal in the body. In these cancer diagnoses and treatments, treatment takes precedence over these risks, so these issues have not been given sufficient consideration, but they are important issues that need to be solved in the future, and even smaller magnetic nanoparticles, similar to molecules, are needed.

[0012] As described above, many nanomagnetic materials require smaller particles. However, from a few tens of nanometers, and especially below 10 nm, the emergence of superparamagnetic effects has made it fundamentally difficult to achieve ferromagnetism. Furthermore, the types of metals that can be expected to exhibit magnetism have been limited to transition metals, but expanding the range of magnetic metal oxides is also an important challenge.

[0013] This invention has been made in view of the above problems, and aims to provide magnetic nanoparticles that exhibit ferromagnetism even at a wavelength of 10 nm or less. [Means for solving the problem]

[0014] Under these circumstances, the inventors have been diligently investigating nano-sizing technology for metal oxides and studying the emergence of magnetism. As a result, they have discovered that ferromagnetism can be induced in metal oxides, which normally do not exhibit ferromagnetism, by reducing their particle size to several nanometers, thus completing the present invention. Specifically, the present invention provides the following:

[0015] The invention relating to the first feature provides metal oxide magnetic nanoparticles that are oxygen-rich or oxygen-deficient, in which at least one of the mean primary particle diameter and crystallite diameter is several nm, preferably 3 nm or less.

[0016] According to the invention relating to the first feature, when the particle size is several nanometers or less, the proportion of metal atoms on the surface to the total particle size increases, and depending on the number of oxygen atoms at the end of the surface, it is possible to synthesize a metal oxide in which the charge balance deviates from the original balance of the metal oxide, and due to the influence of the surface, ferromagnetism can be exhibited even in metal oxides that do not normally exhibit magnetism. Furthermore, because it is due to the influence of the surface, ferromagnetism can be exhibited even in nanoparticles of several nanometers, which generally exhibit superparamagnetism as seen in magnetic nanoparticles. In other words, regardless of whether it is magnetic or non-magnetic, it is possible to exhibit magnetism approximately 10 times or more compared to metal oxide nanoparticles with a diameter of 7 nm. Moreover, ferromagnetism can be similarly exhibited not only when the metal constituting the metal oxide is a magnetic transition element, but also in metal elements other than transition elements that are originally low-magnetic or non-magnetic.

[0017] The principle was investigated using computational science. When dealing with nano-sized particles of a few nanometers, the proportion of surface atoms to the total particle becomes extremely large, and the influence of the surface becomes significant. For example, in the case of CeO2 of slightly less than 5 nm, the number of layers is 19 as seen in the HR-TEM image, so 19 3 This is the number of Ce atoms. The number of surface atoms is 19. 2The calculation is multiplied by 6. In other words, 6 / 19 (32%) of the total number of surface atoms in the particle is 6 / 38, or slightly less than 14%, in the case of 10 nm, there are 38 layers, and in the case of 20 nm, it is only 6 / 76 (8%), but in the case of 3 nm, there are 11 layers, and the number of surface atoms becomes as high as 6 / 11 (55%). In other words, the effect of surface atoms, which could be almost ignored in large particles, becomes apparent. In the calculation, we performed the calculation for the case where the surface Ce is terminated with oxygen. In CeO2, the ratio of Ce to O is 1:2 in bulk, but the proportion of oxygen becomes very large, and the electronic state of Ce changes significantly.

[0018] In fact, when the oxygen atom termination of Ce was reduced, the magnetic effect disappeared. This clearly indicates that the effect is due to the surface. Naturally, a similar effect would appear in the reduced state as well.

[0019] In other words, the key to the emergence of new nanoparticle magnetism lies in reducing the particle size to a few nanometers or less and controlling the oxidation and reduction state of the surface. Regarding the possibility of oxidation and reduction of minute nanoparticles, it has already been shown that the oxygen ion conductivity of minute nanoparticles synthesized by the supercritical method is two to three orders of magnitude larger, and that oxidation and reduction are actually more likely to occur and easier to control (Patent Document 3). That is, because oxidation and reduction can be freely controlled, the amount of oxygen bound to the surface of nanoparticles synthesized by the supercritical method can be controlled. Furthermore, the emergence of oxidation and reduction has been observed even at around room temperature. This indicates the possibility of synthesizing magnetic nanoparticles and suggests that they can also be used as oxygen and hydrogen sensors (magnetic monitors).

[0020] Conventional approaches to the manifestation of nanoscale effects, including the emergence of superparamagnetism, have not considered the influence of such surfaces. This is because, conventionally, it was not possible to synthesize such ultrafine nanoparticles, and therefore, their influence could not be sufficiently investigated. In this invention, in principle, this surface effect appears for all metal oxides. In particular, we have newly discovered that the effect becomes larger when the nanoparticle size is several nanometers or less, and that it manifests through the control of the oxidation-reduction state of the surface. In other words, we have found that the manifestation of magnetism due to surface effects of several nanometers or less manifests to a greater or lesser degree for all metals in principle. Because complete dispersion of high-concentration nanoparticles and viscosity control are possible through organic modification and affinity improvement (Patent Documents 2 and 3), this will lead to solving problems in magnetic nanoparticle composite materials and magnetic nanofluidics.

[0021] The invention relating to the second feature is the invention relating to the first feature, providing nanoparticles in which the metal constituting the metal oxide magnetic nanoparticles is selected from one or more types, including transition metals, alkaline earth metals, post-transition metals, metalloids, lanthanides, and actinides. In the case of composite oxides composed of multiple metals, a similar effect can be obtained if the number of bonds between one or more types of metals and oxygen on the exposed surface is controlled to be different from that of the bulk.

[0022] Until now, the metal species that could be expected to exhibit magnetism had been limited to transition metals. According to the invention relating to the second feature, the metal species that can be expected to exhibit magnetism can be expanded to include metals other than transition metals, thereby broadening the range of applications for different types of magnetic metal oxides. Even with transition metals such as Fe, Co, and Ni, which are generally known to exhibit magnetism, it is known that superparamagnetic effects appear at the nanoscale. However, even in such cases, the surface effect becomes larger than the bulk properties due to the use of minute nanoparticles, and ferromagnetism originating from the surface can be generated by controlling the degree of oxidation and reduction.

[0023] The invention relating to the third feature provides nanoparticles relating to the first or second feature, wherein the surface is modified with organic molecules.

[0024] According to the invention relating to the third feature, since the surface is modified with organic molecules, nanoparticles can be dispersed well at high concentrations in organic materials (polymers) or solvents, thus increasing the degree of freedom in applications such as magnetic resonance imaging contrast agents, drug delivery systems, biosensors, magnetic adsorbents, recording media, spintronic devices, and negative electrode materials for secondary batteries.

[0025] The invention relating to the fourth feature provides undoped nanoparticles, which are an invention relating to any of the first to third features.

[0026] According to the invention relating to the fourth feature, nanoparticles can be manufactured stably because they are a single component of metal oxide. In the case of doped nanoparticles, instability may increase depending on the doping state. In addition, there are advantages such as being able to avoid using harmful elements such as chromium and being able to reduce manufacturing costs.

[0027] The invention relating to the fifth feature provides nanoparticles relating to any of the first to fourth features, wherein the type of metal, the presence or absence of doping, and the dopant element are the same, and the mass magnetization and volume magnetization are five times or more compared to metal oxide nanoparticles with a diameter of 7 nm.

[0028] According to the invention relating to the fifth feature, it is possible to provide nanoparticles having higher magnetism.

[0029] The invention relating to the sixth feature provides a magnetic resonance imaging contrast agent containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the seventh feature provides a drug delivery system containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the eighth feature provides a biosensor containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the ninth feature provides a magnetic adsorbent containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the tenth feature provides a recording medium containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the eleventh feature provides a spintronics device containing nanoparticles of the invention relating to any one of the first to fifth features. The invention relating to the twelfth feature provides a negative electrode material for a secondary battery containing nanoparticles of the invention relating to any one of the first to fifth features.

[0030] The invention relating to the 13th feature includes a mixing step of mixing a supercritical, subcritical, or gas-phase aqueous material with a solution of a metal complex, using Kolmogorov's theory to achieve a mixing rate k mix Determine the mixing rate k mix and apparent reaction rate k app Using this, the true reaction rate k is 1 / k = 1 / k app -1 / k mix From this, we can derive the Damkeller number Da = k / k mix By setting <<1, the mixing time is controlled within a range of 0.015 seconds to 1 second. In addition to controlling the mixing time, the method for producing metal oxide magnetic nanoparticles is provided, which controls particle growth by coalescence of initial nuclei (i.e., non-classical nucleation) to control at least one of the average primary particle diameter and crystallite diameter of the nanoparticles to a range of several nm, preferably 3 nm or less.

[0031] When a supercritical, subcritical, or gas-phase aqueous material is mixed with a solution of a metal complex, the metal complex hydrolyzes, and the salt of the hydrolyzed metal complex is instantly dehydrated without oxidation within a short time of 0.015 seconds to 1 second inclusive to produce nanocrystals of organically modified metal oxide. Initial nanonuclei grow in particle size while undergoing collision and coalescence; however, in the case of organically modified nanocrystals, the surface energy is reduced, so they are more stable in terms of equilibrium theory, the probability of coalescence due to collision is low, and it becomes possible to synthesize smaller fine nanoparticles.

[0032] Since the surface of the particles is basically oxygen-terminated, when the particle size is made extremely small, the crystal formation effect of the metal oxide increases, and oxygen becomes excessive relative to the stoichiometric amount. For example, in the case of cerium oxide, the chemical formula is CeO₂, so O / Ce should be 2. However, in extremely fine fine particles with a diameter of 3 nm or less as in the present invention, when the surface Ce is terminated with oxygen atoms, oxygen becomes excessive relative to the stoichiometric amount, and the actually measured value of O / Ce is about 2.2 to 2.4. Conversely, when the surface has oxygen vacancies (dangling bonds) or shares oxygen with an adjacent Ce atom, the overall O / Ce becomes much less than 2. Therefore, when the diameter is 3 nm or less as in the present invention, unlike bulk metal oxides, oxygen-excess / oxygen-deficient CeO 2+±δ (δ is 0.1 to 0.4) is obtained. [Effects of the Invention]

[0033] According to the present invention, magnetic nanoparticles that exhibit ferromagnetism even when having a size of 10 nm or less can be provided. This ferromagnetism is not limited to cases where the element is a transition metal element, and can also be exhibited even when the element is a metal element other than a transition metal element. [Brief Description of Drawings]

[0034] [Figure 1] FIG. 1 is a schematic diagram showing an outline when the method described in the present embodiment is implemented in a continuous reaction system. [Figure 2] FIG. 2 is a schematic diagram showing a preparation example of an organometallic complex. [Figure 3]Figure 3 is a schematic diagram showing the contact area where a raw material liquid containing an organometallic complex comes into contact with an aqueous material in a supercritical state or the like. [Figure 4] Figure 4 is a schematic diagram showing the recovery of the product after the completion of the hydrothermal synthesis reaction in this test example. [Figure 5] Figure 5 shows the magnetization curve of metal oxide nanoparticles at a temperature of 100K. [Figure 6] Figure 6 shows the magnetic susceptibility and temperature dependence of magnetization when using CeO2±δ with an average particle diameter of 1 nm. [Figure 7] Figure 7 shows the magnetic susceptibility and temperature dependence of magnetization when using CeO2±δ with an average particle diameter of 1 nm. [Figure 8] Figure 8 shows the analysis results of the density of states (DOS) of nanoparticles. [Modes for carrying out the invention]

[0035] The following describes specific embodiments of the present invention in detail. However, the present invention is not limited in any way to the following embodiments, and can be implemented with appropriate modifications within the scope of the object of the present invention.

[0036] <Metal oxide magnetic nanoparticles> The nanoparticles described in this embodiment are oxides, and at least one of the mean primary particle diameter and crystallite diameter is 3 nm or less.

[0037] While transition metal oxide particles are widely known as particles that exhibit magnetism, when their diameter is less than 10 nm, even at extremely small sizes, they exhibit superparamagnetism, and ferromagnetism disappears due to nanoscale reduction. The nanoparticles described in this embodiment are oxides, and because the balance of their valencies deviates from the original balance of metal oxides, they can exhibit ferromagnetism rather than superparamagnetism even at extremely small sizes of 3 nm or less.

[0038] 〔metal〕 Furthermore, ferromagnetism can be exhibited not only when the metal is a transition element, but also with other metal elements that are inherently low-magnetic or non-magnetic. Therefore, the metal constituting the metal oxide nanoparticles is not particularly limited and may be a transition metal or a non-transition metal.

[0039] Examples of metals other than transition metals include one or more selected from alkaline earth metals, post-transition metals, metalloids, lanthanides, and actinides. According to the invention described in this embodiment, the range of metal species that can be expected to exhibit magnetism can be expanded to include metals other than transition metals, thereby broadening the range of applications for different types of magnetic metal oxides.

[0040] In the case of composite oxides composed of two or more metal ions, the above-mentioned metals contribute to the emergence of nanomagnetism, and alkali metals and alkaline earth metals may be included as crystalline constituent ions in the composite metal.

[0041] Even with composite oxides, if the number of oxygen atoms bonded to the surface by one or more of the metals is controlled to differ from that of the bulk material, then in the case of fine nanoparticles, this effect will be greater than that of the bulk material, potentially leading to the emergence of magnetism.

[0042] Alkaline earth metals are elements belonging to Group 2 of the periodic table, and include beryllium, magnesium, calcium, strontium, barium, and radium. However, since beryllium has the characteristic of readily forming covalent bonds with other elements, like nonmetallic elements, in this embodiment it is preferable to use one or more elements selected from magnesium, calcium, strontium, barium, and radium among the alkaline earth metals.

[0043] Post-transition metals, also known as poor metals or P-block metals, are metallic elements located within the P-block elements of the periodic table. These include aluminum, gallium, indium, thallium, tin, lead, bismuth, and polonium.

[0044] Metalloids are a general term for substances that exhibit properties intermediate between metals and nonmetals in the classification of elements, and include boron, silicon, germanium, arsenic, antimony, and tellurium.

[0045] Lanthanides are a collective term for 15 elements with atomic numbers from 57 to 71, i.e., from lanthanum to lutetium, and include lanthanum, cerium, praseodymium, neodymium, promethium, samarium, europium, gadolinium, terbium, dysprodium, holmium, erbium, thulium, ytterbium, and lutetium.

[0046] Actinides are a collective term for 15 elements with atomic numbers from 89 to 103, i.e., from actinium to lawrencium, and include actinium, thorium, protactinium, uranium, neptunium, plutonium, americium, curium, bercklium, californium, einsteinium, fermium, mendelevium, nobelium, and lawrencium.

[0047] This principle stems from the termination of oxygen atoms on the surface, and not from whether the metal is prone to having multiple valencies. However, among these metals, particles that are commonly used but are not known to exhibit magnetism, and which are prone to having multiple valencies, such as cerium and vanadium, exhibit this effect more readily.

[0048] [Diameter] The metal oxide nanoparticles have a mean primary particle diameter and a crystallite diameter, at least one of which is 7 nm or less, preferably 3 nm or less.

[0049] While there is no particular lower limit to the diameter, it is preferable that the lower limit be 1 nm or greater, as this allows for the stable production of nanoparticles with a coefficient of variation of 0.5 or less.

[0050] The average primary particle size of nanoparticles is defined as the arithmetic mean obtained by capturing images of the particles using a TEM (transmission electron microscope), analyzing the TEM images of 50 particles using image analysis and image measurement software, and measuring the diagonal of a square. In this case, if the particle size distribution is wide, such as when the coefficient of variation exceeds 2.0 within this particle size range, care must be taken to determine whether the particles within the field of view are representative of all particles.

[0051] Furthermore, in this embodiment, the crystallite size of the nanoparticles is determined using X-ray diffraction (XRD). CuKα rays (λ=1.5418 Å) are used as the X-ray source. The 2θ scanning speed is 3° / min. The following Scherrer equation is used as the method for calculating the crystallite size using the full width at half maximum (FWHM) of the X-ray diffraction peak.

number

[0052] On the other hand, the broadening of diffraction peaks is caused by both size effects and strain effects. The size effect is separated and calculated using the Halder-Wagner method shown below.

number

number

number

[0053] β' represents the peak integral width, and ε represents the microstrain. From equations (3) and (4), equation (2) can be expressed as equation (5).

number

[0054] From equation (5), for β' / (tanθsinθ), (β' / tanθ) 2 The slope of the plotted graph is Kλ / D, and the crystallite size D is determined from this slope.

[0055] A smaller coefficient of variation in diameter is preferable. In this embodiment, the coefficient of variation is 0.5 or less, preferably 0.3 or less, and more preferably 0.2 or less.

[0056] The coefficient of variation of the mean primary particle diameter can be calculated by dividing the standard deviation of the mean primary particle diameter by the mean primary particle diameter. That is, coefficient of variation = standard deviation of mean primary particle diameter / mean primary particle diameter.

[0057] Similarly, the coefficient of variation of the crystallite size can be obtained by dividing the standard deviation of the crystallite size by the crystallite size. That is, coefficient of variation = standard deviation of crystallite size / crystallite size.

[0058] [Organic modification] It is preferable that the surface of the nanoparticles is modified with organic molecules. Because the surface is modified with organic molecules, the nanoparticles can be dispersed in organic materials (polymers), which increases the flexibility of applications such as magnetic resonance imaging contrast agents, drug delivery systems, biosensors, magnetic adsorbents, recording media, spintronic devices, and negative electrode materials for secondary batteries.

[0059] Examples of organic modifying groups include linear or branched alkyl groups which may be substituted with substituents, cyclic alkyl groups which may be substituted with substituents, substituted aryl groups which may be substituted, aralkyl groups which may be substituted with substituents, and saturated or unsaturated heterocyclic groups which may be substituted with substituents. Examples of substituents include carboxyl groups, cyano groups, nitro groups, halogens, ester groups, amide groups, ketone groups, formyl groups, ether groups, hydroxyl groups, amino groups, sulfonyl groups, -O-, -NH-, -S-, etc.

[0060] [Presence or absence of doping] In this embodiment, the presence or absence of doping is not particularly limited, but it is preferable that the nanoparticles are undoped in order to stably produce nanoparticles. When nanoparticles are undoped, they are a single component of metal oxide, so they can be stably produced. When nanoparticles are doped, instability may increase depending on the doping state. In addition, there are advantages such as being able to avoid using harmful elements such as chromium and being able to reduce manufacturing costs.

[0061] [Mass magnetization and volume magnetization] The nanoparticles described in this embodiment preferably have the same type of metal, presence or absence of doping, and dopant element, and preferably have a mass magnetization and volume magnetization that are 5 times or more than metal oxide nanoparticles with a diameter of 7 nm.

[0062] <Method for manufacturing nanoparticles> Figure 1 is a schematic diagram showing the general configuration of the method according to this embodiment when implemented in a continuous reaction system. The method according to this embodiment includes a mixing step of mixing a supercritical, subcritical, or gas-phase aqueous material with a solution of an organometallic complex to obtain metal oxide nanoparticles.

[0063] [Water-based materials] The water-based material is not particularly limited. A water-based material refers to water, polar organic solvents, or mixed solvents of water and polar organic solvents. Examples of water-based materials include water, alcohols, carboxylic acids, ketones, ethers, esters, amides, amines, sulfur compounds, and mixtures thereof.

[0064] Examples of alcohols include methanol, ethanol, isopropyl alcohol, t-butyl alcohol, propylene glycol, and phenol.

[0065] Examples of carboxylic acids include lower carboxylic acids such as formic acid, acetic acid, propionic acid, butyric acid, valeric acid, and caproic acid.

[0066] Examples of ketones include acetone, methyl ethyl ketone, and methyl isobutyl ketone.

[0067] Examples of ethers include ethylene glycol monobutyl ether, ethylene glycol monoethyl ether, tetrahydrofuran, dioxane, and methyl cellosolve.

[0068] Examples of esters include ethyl acetate and butyl acetate.

[0069] Examples of amides include formamide, dimethylformamide, acetamide, dimethylacetamide, nitromethane, and acetonitrile.

[0070] Examples of amines include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine.

[0071] Examples of sulfur compounds include dimethyl sulfoxide.

[0072] Here, treatment under supercritical conditions when the aqueous material is water is called hydrothermal treatment, and treatment under supercritical conditions when the aqueous material is alcohols, etc., is called solvothermal treatment. Hydrothermal treatment and solvothermal treatment are based on different principles. Hydrothermal treatment is achieved by hydrolysis, while solvothermal treatment, when alcohols, etc. are used, is achieved by solvolysis (alcolysis) reaction. In solvothermal treatment using a nonpolar organic solvent, particle synthesis occurs through the thermal decomposition of the raw materials.

[0073] Even if the liquid medium (in this embodiment, an organometallic complex) has a hydrated structure, hydrolysis cannot occur without a large amount of water. Moreover, the reactivity of hydrothermal treatment is determined by the dielectric constant at the site, and a large amount of bulk H2O is required.

[0074] Therefore, even if the liquid medium is an alcohol, it cannot undergo hydrolysis because alcohols are different from water.

[0075] In this embodiment, the reaction may be carried out by either hydrolysis or solvorization, but the aqueous material is more preferably water that can be subjected to hydrothermal treatment because it is easier to handle.

[0076] However, under high temperature and high pressure conditions, water and organic solvents form a homogeneous phase, so a mixed solvent can be used as the reaction field. In this case, hydrolysis also occurs, and this reaction can be controlled by controlling the polarity and dielectric constant of the mixed solvent.

[0077] In the following sections, since the principle is similar, we will explain the case of water, including water-organic mixed solvents.

[0078] [Pressurization of aqueous materials] The aqueous material is pressurized in a pressurized section. An example of a pressurized section is a pressurizing pump. By pressurizing the aqueous material and then preheating it, supercritical, subcritical, or gaseous aqueous material can be continuously supplied.

[0079] The pressure of the aqueous material after pressurization should be above the saturated vapor pressure. A pressure below the saturated vapor pressure is undesirable because it prevents the nucleation of metal oxide nanoparticles even when in contact with a raw material liquid containing organometallic complexes.

[0080] To effectively achieve nucleation of metal oxide nanoparticles, the pressure of the water-based material after pressurization is preferably 0.5 MPa or higher, more preferably 10 MPa or higher, and even more preferably 20 MPa or higher.

[0081] Furthermore, the pressure of the aqueous material after pressurization should be 40 MPa or less. Exceeding 40 MPa is undesirable because it significantly increases the cost of improving the pressure resistance of the continuous manufacturing equipment and also makes the contact area between the aqueous material and the organometallic complex more susceptible to deterioration.

[0082] From the viewpoint of reducing costs related to the pressure resistance of continuous manufacturing equipment, it is preferable that the pressure of the water-based material after pressurization be 37 MPa or less. Furthermore, from the viewpoint of suppressing deterioration of contact parts, it is even more preferable that the pressure of the water-based material after pressurization be 35 MPa or less. This reduces costs related to the pressure resistance of continuous manufacturing equipment and also suppresses deterioration of contact parts.

[0083] [Preheating of water-based materials] The preheating section is not particularly limited as long as it heats the aqueous material. Examples of preheating sections include heating devices that irradiate the aqueous material with microwaves, and heating devices that heat the aqueous material by heat conduction from heating elements such as heaters. Preheating heats the aqueous material, bringing it to a subcritical state.

[0084] The temperature of the aqueous material after preheating should be 300°C or higher. Below 300°C is undesirable because, in most cases, the aqueous material and the organometallic complex cannot form a homogeneous phase. Furthermore, secondary changes such as aggregation and collision occur after particle formation, and stable bond formation through dehydration reactions cannot be expected, which is also undesirable.

[0085] Furthermore, the temperature of the aqueous material after preheating should be 450°C or lower. If the temperature of the aqueous material is too high, it may hinder the uniform control of particle size within a range of 3 nm or less, which is undesirable.

[0086] The preheating section is, for example, made in a downward sloping shape. If the flow velocity of the aqueous material is low, the preheated, low-density aqueous material will try to rise, causing high-temperature, high-pressure water to flow back into the preheating section, which may make temperature control impossible. To prevent such backflow, it is preferable to provide a throttling section between multiple heating sections. The throttling section is not particularly limited as long as it prevents backflow. Examples of throttling sections include configurations using thin tubes or spiral tubes.

[0087] [Solutions of organometallic complexes] The raw material solution in this embodiment contains an organometallic complex. Even if the raw material solution contains a salt of the same type of metal (including an inorganic metal complex), it is not possible to keep the diameter of the generated metal oxide nanoparticles (at least one of the mean primary particle diameter and the crystallite diameter; hereafter, unless otherwise specified, the term "diameter" refers to "at least one of the mean primary particle diameter and the crystallite diameter") within the range of 3 nm or less, much less to control the coefficient of variation of said diameter to 1.0 or less.

[0088] The form of the raw material liquid is not particularly limited as long as it is fluid, and examples include aqueous solutions, slurries, pastes, or suspensions containing the raw material components.

[0089] If it is difficult to prepare an aqueous slurry, the raw materials can be dispersed in an aqueous material such as ethanol to create a slurry.

[0090] Organometallic complexes can be prepared by mixing a metal salt with an organically modified salt. Figure 2 is a schematic diagram of the process of obtaining cerium(IV) octanoate by mixing cerium(IV) ammonium nitrate ((NH4)2Ce(NO3)6) with sodium octanoate, as an example. First, an aqueous solution of cerium(IV) ammonium nitrate is added to an aqueous solution of sodium octanoate and stirred. Then, the solid phase is recovered by filtration, washed with water and ethanol, and dried. This yields cerium(IV) octanoate.

[0091] It is preferable to control the molar ratio of the organometallic complex to the organic material relative to the metal constituting the organometallic complex. Furthermore, if the metal element constituting the organometallic complex is a metal element that can exist in multiple valencies, it is preferable that the valency of the metal element constituting the organometallic complex contained in the solution is controlled to be a relatively large valency within the same metal element. This allows for more precise control of the nanoparticle diameter and coefficient of variation.

[0092] The following explains the effect of cerium valency in cerium-carboxylate complex precursors in batch synthesis. When octanoic acid Ce(III) is used as the precursor, Ce(OH)3 rod-shaped particles are obtained as an intermediate, and oxidation proceeds over a long period of time, resulting in CeO 2±δ Ce(OH)3 is generated. Under these conditions, a high concentration of octanoic acid Ce(III) was used as the raw material, so Ce(OH)3 was rapidly generated, and because the carboxylic acid concentration relative to Ce(OH)3 was low, the redissolution rate was also low. Therefore, the formation and growth of Ce(OH)3 crystals proceeded preferentially over the Ce oxidation reaction.

[0093] In contrast, when octanoic acid Ce(IV) is used as a precursor, a more uniform organic modification of CeO is obtained than conventional methods. 2±δ Nanoparticles are obtained. By using a tetravalent Ce complex as a precursor, oxidation reactions are unnecessary, and hydrolysis and dehydration occur immediately, resulting in homogeneous CeO 2±δ It is thought to form crystal nuclei.

[0094] Even in flow-through reactions using Ce(IV) octanoate precursors, more uniform nanoparticles can be obtained compared to Ce(III) octanoate or cerium salt precursors. This is thought to be because surface modification occurs during nucleation.

[0095] Conventional free-flow synthesis methods have had the challenge of not being able to obtain uniform nanoparticles. However, the synthesis method of the present invention makes it possible to obtain uniform nanoparticles with a controlled size of 3 nm or less in a short time, even using a free-flow synthesis method.

[0096] [Degassing and pressurizing] The raw material degassing unit is not particularly limited as long as it degasses the raw material liquid containing organometallic complexes. Examples of raw material degassing units include existing degassing devices such as ultrasonic degassing devices, degassing devices that reduce pressure, degassing devices that supply noble gases into the raw material liquid, and degassing devices that use permeable membranes, as well as degassing devices that combine these existing degassing devices. By degassing the raw material liquid, fluctuations in the supply amount of the raw material liquid caused by bubbles generated by dissolved gases can be suppressed. In addition, corrosion of various components due to dissolved oxygen can be avoided.

[0097] The raw material pressurizing unit is not particularly limited as long as it pressurizes the raw material liquid containing organometallic complexes. For example, a pressurizing pump can be used as the raw material pressurizing unit. By pressurizing the raw material liquid, it can be brought into contact with aqueous materials under high pressure.

[0098] [Contact area] The contact area is not particularly limited as long as it mixes a raw material liquid containing an organometallic complex with an aqueous material in a supercritical state or the like.

[0099] Figure 3 is a schematic diagram showing an example of a contact area. As shown in Figure 3(A), the contact area preferably includes a structure in which the tip of a nozzle formed at the end of a raw material supply means that supplies a raw material liquid containing an organometallic complex faces a high-temperature, high-pressure aqueous material supplied from an aqueous material supply means that supplies an aqueous material, and the raw material liquid is ejected from the tip of this nozzle. By ejecting the raw material liquid from the tip of the nozzle, the raw material liquid and the aqueous material are mixed at high speed, promoting the reaction efficiency. Furthermore, in the case of a phase-separating raw material liquid, by ejecting the raw material liquid from the tip of the nozzle, the raw material liquid is atomized and dispersed in the aqueous material, and the raw material in the raw material liquid is formed into a fine emulsion in the high-temperature, high-pressure aqueous material, further promoting the reaction efficiency.

[0100] The contact portion is not particularly limited as long as it mixes the aqueous material and the raw material liquid, and other configurations are possible. For example, as shown in Figure 3(B), it may be a structure in which an aqueous material guided from a substantially horizontal direction mixes with a raw material liquid guided from a substantially horizontal direction, as shown in Figure 3(C), it may be a structure in which an aqueous material guided from a substantially horizontal direction mixes with a raw material liquid guided from a substantially vertical direction, as shown in Figure 3(D), and it may be a structure in which an aqueous material guided from an oblique upward direction mixes with a raw material liquid guided from an oblique upward direction.

[0101] At the contact point, the raw material liquid is instantaneously heated to a subcritical temperature by the heat contained in the aqueous material supplied from the aqueous material supply means, and the reaction between the raw material liquid and the aqueous material begins. This reaction initiates the surface treatment reaction of the organometallic complex.

[0102] Note that the contact section is not a mandatory component. Rapid mixing can be achieved by providing a contact section, but it is not required. In this case, as shown in Figure 3, the raw material slurry passes directly through the preheating section shown in Figure 1.

[0103] [Reaction Processing Area] Return to Figure 1. The reaction section (reactor) is not particularly limited as long as it maintains supercritical, subcritical, or gas phase conditions for a predetermined time. Examples of reaction sections include reactors covered with a constant temperature layer such as a spiral tube wound multiple times inside a heating cylinder, a molten salt bath jacket, or a fluidized sand bath.

[0104] By making the reaction section a reactor covered with a spiral tube wound multiple times inside a heating cylinder or a constant-temperature layer, temperature changes and temperature unevenness in the mixture of raw material liquid and aqueous material due to heat conduction through the wall from the contact area can be prevented, thereby achieving the precise temperature control required for particle synthesis under supercritical, subcritical, or gas phase conditions.

[0105] Furthermore, a hydrocyclone can be provided downstream of the reaction section. By providing a hydrocyclone, the reaction product and the fluid can be separated, and the reactor temperature can be controlled by the pressure of this fluid. The configuration with a hydrocyclone is preferable because it can improve the recovery yield of the reaction product and also improve the controllability of the reactor temperature. A configuration in which the reaction section is formed in a substantially vertical direction, and the mixture of the raw material liquid and the aqueous material flows from the upper contact section to the lower hydrocyclone, is preferable because it makes it easier to maintain a uniform temperature in the reaction section.

[0106] The reaction temperature is preferably controllable within the range of 300°C to 450°C. By controlling the reaction temperature, the diameter and coefficient of variation of the generated metal oxide nanoparticles can be controlled.

[0107] When the reaction product of the raw material liquid and the aqueous material passes through the reaction section, a high-temperature, high-pressure fluid containing metal oxide nanoparticles is discharged from the outlet of the reaction section.

[0108] The reaction time is preferably controllable within a range of 0.015 seconds to 1 second. When a supercritical, subcritical, or gas-phase aqueous material is mixed with a solution of an organometallic complex, the organometallic complex undergoes hydrolysis, and the resulting salt of the organometallic complex is instantly dehydrated without oxidation within a short time of 0.015 seconds to 1 second, generating metal oxide crystals. Initial nanonuclei increase in particle size through collision and coalescence, but in the case of organically modified nanocrystals, the surface energy is lower, making them more stable from an equilibrium perspective, the probability of coalescence due to collisions is lower, and it becomes possible to synthesize even smaller nanoparticles.

[0109] Since the surface of the particles is basically oxygen-terminated, when the particle size is made very small, the crystal formation effect of the metal oxide becomes large, and oxygen becomes excess beyond the stoichiometric value. For example, in the case of cerium oxide, the chemical formula is CeO2, so O / Ce should be 2. However, in the case of extremely small nanoparticles with a diameter of 3 nm or less, as in the present invention, if the Ce on the surface is terminated by an oxygen atom, oxygen becomes excess beyond the stoichiometric value, and the measured value is approximately O / Ce = 2.2 to 2.4. Conversely, if the surface is oxygen-deficient (a ring bond) or shares oxygen with an adjacent Ce atom, then the overall O / Ce << 2. Therefore, in the case of particles with a diameter of 3 nm or less, as in the present invention, unlike bulk metal oxides, oxygen excess / deficient CeO2 2±δ (δ is 0.1 to 0.4) is obtained.

[0110] When the reactor is a continuous reaction system, the reaction time can be controlled by changing the volume of the continuous reactor or by changing the flow rate of the mixed raw materials (aqueous material and organometallic complex solution) supplied to the continuous reactor. This allows for synthesis concentrations of 0.1 g / l or higher and reaction rates of 0.8 or higher. This technology enables the simultaneous achievement of further miniaturization of metal oxide nanoparticles and uniform particle size distribution, not only in batch production but also in continuous manufacturing systems.

[0111] When the reactor is a continuous reaction system, it is more preferable to control the mixing time to a range of 1 second or less by setting the Reynolds number (Re number) to 3000 or higher. A Reynolds number of 6000 or higher is more preferable, and 10000 or higher is even more preferable. By changing the volume of the continuous reactor or by changing the flow rate of the mixing raw materials of the aqueous material and organometallic complex solution supplied to the continuous reactor, the Re number of the mixing section, i.e., the mixing time, can be controlled to a range of 1 second or less. This makes it possible to achieve both further miniaturization of metal oxide nanoparticles and uniform particle size distribution not only in batches but also in continuous production systems.

[0112] The higher the Re number in the reaction section, the greater the mixing rate, which becomes the rate-limiting factor in the reaction, thereby enabling the synthesis of uniform particles with smaller particles (Non-Patent Documents 3, 4). Furthermore, the mixing rate can be evaluated using Kolmogorov's theory (Non-Patent Document 3). In other words, the reaction rate can be evaluated from a small number of experimental samples, and a reactor that becomes the rate-limiting factor can be designed.

[0113] Let's explain the mixing rate in more detail. Mixing rate k mix This can be evaluated as a function of the Re number from Kolmogorov's theory. In that case, the apparent reaction rate k obtained from a small number of experimental points, such as one or two points, can be evaluated. app Using this, the true reaction rate k is 1 / k = 1 / k app -1 / k mix It can be evaluated from that.

[0114] To obtain uniform and smaller particles, synthesis under reaction-controlled conditions is necessary. That is, 1 / k >> 1 / k mix That is, Damkeller number Da = k / k mix It is necessary that it be << 1.

[0115] The Damkeller number is preferably 1 / 5 or less, more preferably 1 / 10 or less, and even more preferably 1 / 100 or less.

[0116] Compared to the hydrolysis rate of metal salts when metal salts are used as raw materials, the hydrolysis rate of metal complexes is slower, and therefore this condition can be met even with relatively slow mixing, i.e., with a relatively low Re number. However, even so, in order to control the mixing time in a continuous reaction system to a range of 1 second or less, it is preferable to set the Re number to 3000 or more, more preferably to 6000 or more, and even more preferably to 10000 or more.

[0117] The above describes setting the reaction rate-limiting conditions by making the mixing rate sufficiently faster than the reaction rate, which is a requirement for uniform nucleation. Then, the generated initial nuclei repeatedly coalesce and merge to grow into particles. This is known as non-classical nucleation. This process occurs in a very short time, less than a few seconds, but it can be controlled by controlling the residence time. This allows for the synthesis of metal oxide nanoparticles of a few nanometers, which are normally impossible to synthesize.

[0118] [Recovery of metal oxide nanoparticles] The recovery of metal oxide nanoparticles can be achieved by cooling the high-temperature, high-pressure fluid containing particles discharged from the reaction section, and then recovering the particles from the cooled fluid using an in-line filter or the like.

[0119] In this process, a washing step may be included to wash the mixed product and remove the organometallic complex. This makes it possible to provide metal oxide nanoparticles free of residual organometallic complexes (unreacted impurities), with a particle size of 9.0 nm or less and a uniform particle shape distribution.

[0120] <Applications of nanoparticles> It is possible to use material systems that do not normally exhibit magnetism, which is expected to offer advantages in terms of resource constraints and reduced manufacturing costs. In particular, for applications of nanoparticles such as magnetic resonance imaging contrast agents, drug delivery systems, biosensors, magnetic adsorbents, recording media, spintronic devices and negative electrode materials for secondary batteries, and optical devices, it is expected that in bio-pharmaceutical applications, capture by macrophages can be avoided, and adverse effects on the body after use can be reduced. The size of antibodies used as target molecules is much larger, which actually leads to the conferring of magnetic recognition function to antibody drugs, etc. In sensors, the surface reaction activity becomes one or two orders of magnitude larger than before, and since this can be confirmed by magnetic response, the accuracy is further improved. In memory media and spintronics, further miniaturization of domain and dot sizes is required. The manifestation of superparamagnetic effects was a problem, but this can be overcome. Also, in optical materials such as smart windows whose opening and closing can be controlled by magnetic response, high concentration and transparency are prerequisites. The wavelength of light in the visible light range is several hundred nm, and the suppression of Rayleigh scattering of light needs to be at most several tens of nm or less, but scattering still occurs at high concentrations. In contrast, if that can be reduced to a few nanometers or less, the effect will be almost eliminated.

[0121] Generally, magnetic nanoparticles have the property of strongly scattering light. Specifically, the following problems can be cited.

[0122] (1) Decreased light transmittance Magnetic nanoparticles have the property of strongly scattering light. In particular, Mie scattering occurs for visible light and near-infrared wavelengths, reducing light transmittance. For example, in magnetic nanoparticles used for bioimaging (MRI contrast agents, photothermal therapy materials), if the light scattering is too strong, it becomes difficult for light to penetrate deep into the tissue.

[0123] (2) Decreased accuracy of optical measurements In samples containing magnetic nanoparticles, significant light scattering can reduce the accuracy of absorption spectra and fluorescence measurements. For example, in UV-Vis absorption measurements, the original absorption peak is distorted by light scattering. Furthermore, in analyses using fluorescently labeled nanoparticles, background noise due to light scattering increases, reducing measurement sensitivity.

[0124] (3) Degradation of image contrast In optical microscopy and bioimaging using magnetic nanoparticles, problems arise such as image blurring or reduced contrast due to light scattering.

[0125] (4) Effects of photoinduced heating One challenge is that the scattering of light by magnetic nanoparticles can cause localized heating, potentially leading to unwanted localized heating. For example, in photothermal therapy (PTT), nanoparticles are heated by irradiating them with light of a specific wavelength, but if the scattering is too great, uniform heating becomes difficult.

[0126] According to the invention described in this embodiment, not only can the problem of light scattering be solved by the small diameter, but the nanoparticles can also be made approximately 10 times more magnetic than metal oxide nanoparticles with a diameter of 10 nm. In this respect, the nanoparticles described in this embodiment are suitable for the above-mentioned applications.

[0127] Furthermore, the number of memory bits is ultimately determined by the size of the memory element. Therefore, by using the nanoparticles described in this embodiment as a storage medium, it is possible to further miniaturize the storage medium.

[0128] In addition, using the nanoparticles described in this embodiment as a magnetic resonance imaging contrast agent, a drug delivery system, or a biosensor has the advantage of potentially enabling dissolution and removal within the body. [Examples]

[0129] The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. There isn't one.

[0130] <Metal oxide nanoparticles (oxygen-rich / deficient CeO 2±δ ) synthesis > [Synthesis of cerium(IV) octanoate] Cerium(IV) octanoate was synthesized by metathesis. First, a 0.1 M aqueous solution of ammonium cerium(IV) nitrate and a 0.3 M aqueous solution of sodium octanoate were prepared separately. Next, 500 mL of the ammonium cerium(IV) nitrate solution was added to 500 mL of the sodium octanoate solution and stirred at 25°C for 60 minutes. The cerium(IV) octanoate product was recovered by pressure filtration using a polytetrafluoroethylene (PTFE) filter with a pore size of 0.1 μm, washed with water and ethanol, and then dried overnight at 70°C.

[0131] [Supercritical hydrothermal synthesis] Supercritical hydrothermal synthesis was performed using a flow-through hydrothermal apparatus. A predetermined amount of cerium(IV) octanoate and octanoic acid were dissolved in benzene and mixed with heated water to carry out the reaction. The cerium concentration at this time was 0.03 mol / L. The water flow rate was 24 mL / min, and the benzene solution flow rate was 8 mL / min. Water was supplied using a plunger-type pump (manufactured by Nippon Seimitsu), and the benzene solution was supplied using a syringe pump (manufactured by ISCO). The reaction was carried out at 340°C and 30 MPa using a SUS (stainless steel) reaction tube. By changing the tube volume, the reaction was carried out for predetermined times (0.04 seconds, 95 seconds).

[0132] [Recovery of the product] Figure 4 is a schematic diagram showing the recovery of the product after the reaction is complete. First, after the reaction was complete, the container was rapidly cooled in a water bath at room temperature, and the product was recovered using benzene. After removing the water, methanol was added to the benzene solution. The volume ratio of benzene to methanol was 1:1. The obtained solid product was washed twice with ethanol, then centrifuged and decanted. The solid was redispersed in cyclohexane and then freeze-dried.

[0133] 〔evaluation〕 When the cerium oxide nanoparticles obtained were observed using a transmission electron microscope (TEM, H-7650, Hitachi High-Technologies Corporation), the average primary particle size of the nanoparticles was 1 nm (reaction time 0.04 seconds) and 7 nm (reaction time 95 seconds), and the coefficients of variation were 0.17 and 0.23, respectively.

[0134] <Magnetization curve> A magnetization curve was created for the obtained nanoparticles at a temperature of 100K. The results are shown in Figure 5.

[0135] The horizontal axis in Figure 5 represents the magnetic field strength, and the unit is 10⁻¹⁰. 3 It is Oe. Also, the vertical axis in Figure 5 represents the mass magnetization per unit weight of cerium oxide, and the unit is 10⁻¹⁰. -3 It's emu / g.

[0136] In Figure 5, the intercept is the ferromagnetic component m0, and the slope ΔM g / ΔH is the paramagnetic component (magnetic susceptibility).

[0137] CeO with an average particle size of 1 nm 2+δ Regarding this, both paramagnetism and ferromagnetism were observed. On the other hand, CeO with an average particle size of 7 nm 2+δ Regarding this, only paramagnetism was observed.

[0138] <Magnetic susceptibility and temperature dependence of magnetization> Figure 6 shows CeO with an average particle size of 1 nm. 2+δ The magnetic susceptibility and temperature dependence of magnetization when using [the specified method] are shown. Figure 7 shows CeO2 with an average particle size of 1 nm. 2+δ The magnetic susceptibility and temperature dependence of magnetization when using [the specified method] are shown. The horizontal axis in Figures 6 and 7 is temperature, in Kelvin. The vertical axis in Figures 6 and 7 is the paramagnetic component (magnetic susceptibility) ΔM. g / ΔH or ferromagnetic component m0. In each figure, ○ represents temperature and paramagnetic component (magnetic susceptibility) ΔM g This plot shows the relationship with / ΔH, and △ represents the relationship between temperature and the ferromagnetic component m0.

[0139] The temperature dependence of the paramagnetic susceptibility follows the Curie-Weiss law shown in equation (6).

number

[0140] In equation (6), χ0 is the temperature-independent magnetic susceptibility, C is the Curie constant, and θ is the Weiss temperature.

[0141] 1nm CeO 2+δ From the experimental results, the Curie constant C = 0.05 is obtained. From this C, CeO 2+δ Ce 3+ The proportion R is given by R = Ce 3+ / (Ce 3+ +Ce 4+ ) = 0.06.

[0142] Similarly, 7nm CeO 2+δ From the experimental results, the Curie constant C = 0.005 is obtained. From this C, CeO 2+δ Ce 3+ The proportion R is given by R = Ce 3+ / (Ce 3+ +Ce 4+ ) = 0.006, and 1 nm CeO 2+δ This is approximately 10% of the previous figure.

[0143] Although not shown in the diagram, 1 nm CeO 2+δ The mass magnetization of Mg at 300K is approximately 47 × 10⁻⁶ -3 The values ​​are emu / g, and the volume magnetization Ms is approximately 340 Am -1 This was an order of magnitude larger than that of conventional magnetic nanoparticles.

[0144] <Contribution of Oxidation Structure to Magnetism> The inventors performed density functional theory (DFT) calculations using the VASP (Vienna Ab-initio Simulation Package) package. They applied the Hubbard U correction for 4f orbitals to the PBE (Perdew-Burke-Ernzerho) functional and set its on-site potential to 5 eV.

[0145] Analysis of the Density of States (DOS) shows that Ce has high symmetry. 13 O 32 A clear spin splitting was observed due to the arrangement of surface oxygen atoms in the cluster (Figure 8(a)). This spin splitting resulted in a magnetic moment m = 12μ B μ occurred. B This is a Bohr magneton.

[0146] On the other hand, if we remove six oxygen atoms from this cluster, we get a stoichiometrically appropriate Ce 13 O 26 When we created clusters and performed calculations, no magnetic properties were observed (Figure 8(b)).

[0147] These results suggest that the surface oxidation structure of the nanoclusters is responsible for the emergence of magnetism.

Claims

1. Metal oxide magnetic nanoparticles with an oxygen-rich or deficient oxygen content, wherein at least one of the mean primary particle diameter and crystallite diameter is 3 nm or less.

2. The nanoparticle according to claim 1, wherein the metal constituting the metal oxide magnetic nanoparticle is one or more selected from alkaline earth metals, transition metals, post-transition metals, metalloids, lanthanides, and actinides.

3. The nanoparticle according to claim 1, wherein the surface is modified with organic molecules.

4. The nanoparticles according to claim 1, which are undoped.

5. The nanoparticle according to claim 1, wherein the type of metal, the presence or absence of doping, and the dopant element are the same, and the mass magnetization and volume magnetization are five times or more compared to a metal oxide nanoparticle with a diameter of 7 nm.

6. A magnetic resonance imaging contrast agent containing nanoparticles according to any one of claims 1 to 5.

7. A drug delivery system containing nanoparticles according to any one of claims 1 to 5.

8. A biosensor containing nanoparticles according to any one of claims 1 to 5.

9. A magnetic adsorbent containing nanoparticles according to any one of claims 1 to 5.

10. A recording medium containing nanoparticles according to any one of claims 1 to 5.

11. A spintronics device containing nanoparticles according to any one of claims 1 to 5.

12. A negative electrode material for a secondary battery containing nanoparticles according to any one of claims 1 to 5.

13. The process includes a mixing step of mixing a supercritical, subcritical, or gas-phase aqueous material with a metal complex solution. Using Kolmogorov's theory, the mixing rate k ｍｉｘ Seeking, The aforementioned mixing speed k ｍｉｘ and apparent reaction rate k ａｐｐ Using this, the true reaction rate k is 1 / k = 1 / k ａｐｐ -1 / k ｍｉｘ From, Damkeller number Da = k / k ｍｉｘ By setting it to <<1, the mixing time can be controlled within a range of 0.015 seconds to 1 second. A method for producing metal oxide magnetic nanoparticles, wherein, in addition to controlling the mixing time, crystal growth due to collision and coalescence is controlled by the residence time, thereby controlling at least one of the average primary particle diameter and crystallite diameter of the nanoparticles to a range of 3 nm or less.

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