Method for producing hydrogenated nanodiamond particles

Abstract

Provided is a method for safely producing zeta-positive hydrogenated nanodiamond particles.　The method for producing hydrogenated nanodiamond particles according to the present disclosure is a method for obtaining hydrogenated nanodiamond particles with a zeta potential of +35 mV or more in water with a pH of 5-8 by subjecting diamond particles with a zeta potential of +30 mV or less in water with a pH of 5-8 to a hydrogenation treatment step in which the diamond particles are heated at a temperature of 300°C-850°C in a gas atmosphere with a hydrogen gas concentration of less than 1%. The hydrogen gas concentration in the gas atmosphere is preferably 0.01% or more and less than 1%, and the gas atmosphere preferably contains hydrogen gas and nitrogen gas.

Description

Method for producing hydrogenated nanodiamond particles

[0001] The present disclosure relates to a method for producing hydrogenated nanodiamond particles. This application claims priority to Japanese Patent Application No. 2024-118802 filed on July 24, 2024, and Japanese Patent Application No. 2024-159229 filed on September 13, 2024, the contents of which are incorporated herein by reference.

[0002] Nano-sized fine materials are known to possess new properties that cannot be expressed in bulk. For example, nanodiamond particles (i.e., nano-sized diamond particles) possess mechanical strength, high refractive index, thermal conductivity, insulating properties, antioxidant properties, etc. However, because nanodiamond particles have a large proportion of surface atoms, the sum of the van der Waals forces that can act between the surface atoms of adjacent particles is large, making them prone to aggregation. In addition, in the case of nanodiamond particles, the Coulomb interaction between the crystal planes of adjacent crystallites can contribute to a phenomenon known as aggregation, in which the particles aggregate very tightly. For this reason, it has been very difficult to disperse nanodiamond particles in solvents or resins in the form of primary particles. Therefore, attempts have been made to improve the dispersibility of nanodiamond particles and suppress their aggregation by modifying their surfaces.

[0003] Patent document 1 discloses that zeta positive hydrogenated nanodiamond particles have repulsive force and resist aggregation.And, it discloses that when nanodiamond particles are heated under normal pressure in a gas atmosphere with a hydrogen gas concentration of 4%, hydrogenated nanodiamond particles can be obtained, and the obtained hydrogenated nanodiamond particles are dispersed in water, and subjected to bead-assisted ultrasonic destruction treatment, the zeta potential of the hydrogenated nanodiamond particles in the dispersion liquid is more than +50mV at around pH 6, and the median diameter D50 is about 6nm.

[0004] Patent No. 6898733

[0005] However, in the method of Patent Document 1, the hydrogen gas concentration is as high as 4%, which poses a risk of explosion, and therefore poses a safety problem.

[0006] Therefore, an object of the present disclosure is to provide a method for safely producing zeta-positive hydrogenated nanodiamond particles.

[0007] The present inventors have conducted extensive research to solve the above-mentioned problems, and have found that even in a gas atmosphere with a hydrogen gas concentration of less than 1% where there is no risk of explosion, by carrying out heat treatment at high temperatures, zeta-positive hydrogenated nanodiamond particles can be obtained, just as in the case of heat treatment in a high hydrogen gas concentration atmosphere.And, surprisingly, they have found that the hydrogenated nanodiamond particles obtained by heat treatment in a gas atmosphere with a hydrogen gas concentration of less than 1% have a lower degree of aggregation than the hydrogenated nanodiamond particles obtained by heat treatment in a high hydrogen gas concentration atmosphere, and can be easily highly dispersed by simple de-dispersion treatment.The present disclosure has been completed based on these findings.

[0008] In other words, the present disclosure provides a method for producing hydrogenated nanodiamond particles, which involves subjecting diamond particles having a zeta potential of +30 mV or less in water of pH 5 to 8 to a hydrogenation treatment process in which they are heated at a temperature of 300 to 850°C in a gas atmosphere with a hydrogen gas concentration of less than 1%, to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more in water of pH 5 to 8.

[0009] The present disclosure also provides a method for producing hydrogenated nanodiamond particles, which comprises subjecting the hydrogenated nanodiamond particles produced after the hydrogenation treatment step to ultrasonic treatment or a crushing treatment using a bead mill to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more and a median diameter D50 of 20 nm or less in water of pH 5 to 8.

[0010] The present disclosure also provides a method for producing hydrogenated nanodiamond particles, wherein the hydrogen gas concentration in the hydrogenation treatment step is 0.01% or more and less than 1%.

[0011] The present disclosure also provides a method for producing hydrogenated nanodiamond particles, wherein the hydrogen partial pressure in the hydrogenation treatment step is 0.01 kPa or more and less than 1 kPa.

[0012] The present disclosure also provides a method for producing the hydrogenated nanodiamond particles, wherein the gas atmosphere in the hydrogenation process comprises hydrogen gas and nitrogen gas.

[0013] The present disclosure also provides a method for producing the hydrogenated nanodiamond particles, wherein the hydrogenation process is carried out under atmospheric pressure.

[0014] According to the method of the present disclosure, hydrogenated nanodiamond particles can be produced safely without the risk of explosion.In addition, the hydrogenated nanodiamond particles obtained by the method of the present disclosure have a lower degree of aggregation than the hydrogenated nanodiamond particles obtained by heat treatment under a high hydrogen gas concentration atmosphere, and can be easily refined by simple de-dispersion treatment such as ultrasonic treatment.The hydrogenated nanodiamond particles obtained by the method of the present disclosure have high dispersibility and have properties such as mechanical strength, high refractive index, thermal conductivity, insulation, and antioxidant properties, so they can be suitably used as an additive to impart the properties to resins, etc.

[0015] 1 shows the results of FTIR measurements of hydrogenated nanodiamond particles and raw diamond particles obtained in Examples 1 to 6 and Reference Example 1.

[0016] [Method for producing hydrogenated nanodiamond particles] The method for producing hydrogenated nanodiamond particles of the present disclosure is a method in which diamond particles (hereinafter sometimes referred to as "raw diamond particles") having a zeta potential of +30 mV or less in water of pH 5 to 8 are subjected to a hydrogenation treatment step in which they are heated at a temperature of 300 to 850°C in a gas atmosphere with a hydrogen gas concentration of less than 1%, to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more in water of pH 5 to 8. The raw diamond particles will be described in detail later.

[0017] The hydrogen gas concentration in the gas atmosphere of the hydrogenation treatment process is less than 1%. From the viewpoint of safety, the hydrogen gas concentration is preferably 0.8% or less, more preferably 0.5% or less, particularly preferably 0.3% or less, most preferably 0.2% or less, and particularly preferably 0.15% or less. Furthermore, it is preferable that the gas atmosphere contains hydrogen gas, since this allows for more excellent dispersibility of hydrogenated nanodiamond particles to be obtained. That is, the hydrogen gas concentration is preferably greater than 0%, more preferably 0.001% or more, particularly preferably 0.01% or more, and most preferably 0.05% or more. In terms of ease of control of the hydrogen gas concentration in the hydrogenation equipment, the hydrogen gas concentration is particularly preferably 0.1% or more. Here, the percentage is mole percent or volume percent.

[0018] The gas atmosphere has a hydrogen partial pressure of, for example, less than 1 kPa. From the viewpoint of safety, the hydrogen partial pressure is preferably 0.8 kPa or less, more preferably 0.5 kPa or less, particularly preferably 0.3 kPa or less, most preferably 0.2 kPa or less, and particularly preferably 0.15 kPa or less. In addition, it is preferable that the gas atmosphere contains hydrogen gas, since hydrogenated nanodiamond particles with better dispersibility can be obtained. That is, the hydrogen partial pressure is preferably greater than 0 kPa, more preferably 0.001 kPa or more, particularly preferably 0.01 kPa or more, and most preferably 0.05 kPa or more. In terms of ease of control of the hydrogen partial pressure or hydrogen gas concentration in the hydrogenation equipment, the hydrogen partial pressure is particularly preferably 0.1 kPa or more.

[0019] The gas atmosphere may contain an inert gas other than hydrogen gas. Examples of the inert gas include argon gas, nitrogen gas, helium gas, and mixtures thereof. Among these, nitrogen gas is preferred as the inert gas because it is inexpensively available.

[0020] Therefore, the gas atmosphere preferably contains hydrogen gas and nitrogen gas, and is preferably a hydrogen gas atmosphere diluted with nitrogen gas.

[0021] The inert gas concentration in the gas atmosphere is, for example, 80% or more, preferably 90% or more, particularly preferably more than 99%, most preferably 99.5% or more, and particularly preferably 99.8% or more. The upper limit of the inert gas concentration is, for example, 99.999%.

[0022] The inert gas partial pressure in the gas atmosphere is, for example, 80 kPa or more, preferably 90 kPa or more, particularly preferably more than 99 kPa, most preferably 99.5 kPa or more, and particularly preferably 99.8 kPa or more. The upper limit of the inert gas partial pressure is, for example, 99.999 kPa.

[0023] The heat treatment in the hydrotreating step is preferably carried out while continuously or intermittently supplying hydrogen gas and an inert gas (e.g., nitrogen gas) either alone or in combination into the reaction system at a supply rate of, for example, 0.01 to 5 L / min.

[0024] The temperature of the heat treatment is 300 to 850 ° C. The lower limit of the temperature is preferably 400 ° C, more preferably 450 ° C, even more preferably 500 ° C, particularly preferably 550 ° C, and most preferably 580 ° C, in order to obtain hydrogenated nanodiamond particles with a larger zeta potential and excellent dispersibility. The upper limit of the temperature is preferably 800 ° C, more preferably 750 ° C, particularly preferably 700 ° C, most preferably 650 ° C, and particularly preferably 630 ° C, in order to prevent the surface of the hydrogenated nanodiamond particles from becoming graphitized.

[0025] The heat treatment time is, for example, 1 to 24 hours, preferably 3 to 12 hours.

[0026] The hydrotreating step (or the heat treatment) can be carried out under a pressure of, for example, 0.1 to 5 atmospheres. Among these, carrying out the hydrotreating step under normal pressure (i.e., under 1 atmosphere) is preferred in that no equipment for adjusting the pressure is required and the hydrotreating can be carried out using simple equipment.

[0027] After the hydrogenation treatment step (or the heat treatment), hydrogenated nanodiamond particles with monofunctionalized surfaces are obtained.

[0028] When the hydrogenation treatment process (or the heat treatment) is carried out, for example, in a gas atmosphere in which the hydrogen gas concentration is greater than 0% and less than 1%, hydrogenated nanodiamond particles (i.e., nanodiamond particles whose surfaces are hydrogenated) are obtained.

[0029] The zeta potential of the hydrogenated nanodiamond particles obtained through the hydrogenation treatment process (or the heat treatment) in water of pH 5 to 8 is +35 mV or more, preferably +40 mV or more, more preferably +45 mV or more, and particularly preferably +46 mV or more. The upper limit of the zeta potential is, for example, +70 mV.

[0030] The raw diamond particles that are subjected to hydrogenation process (or the heat treatment) are secondary particles formed by the aggregation of primary particles, and the median diameter D50 in alkaline or acidic water is for example 50nm or more than the isoelectric point.However, when the raw diamond particles are subjected to hydrogenation process, their surface is monofunctionalized (for example, hydrogenated), so that they repel each other, and aggregation is suppressed, and they obtain high dispersibility.Therefore, when the hydrogenated nanodiamond particles that are obtained through hydrogenation process are subjected to deagglomeration treatment (for example, ultrasonic treatment or bead mill crushing treatment, etc.), they can be easily refined and highly dispersed.

[0031] The hydrogenated nanodiamond particles obtained by the method disclosed herein tend to have a lower degree of aggregation than the hydrogenated nanodiamond particles obtained by conventional heat treatment under a high hydrogen gas concentration atmosphere, and can be easily refined by simple de-agglomeration treatment such as ultrasonic treatment without performing crushing treatment using a bead mill.

[0032] The hydrogenated nanodiamond particles obtained through the hydrogenation process (or the heat treatment) are mixed with water and subjected to ultrasonic treatment (operating frequency 24KHz, 400W, output 45%, treatment time: 4 hours), and the median diameter D50 of the hydrogenated nanodiamond particles in a pH 5-8, 1 wt% aqueous dispersion is, for example, 20nm or less, preferably 15nm or less, particularly preferably 10nm or less, and most preferably 7nm or less. The lower limit of the median diameter D50 is, for example, 5nm.

[0033] In addition, the particle size D10 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by ultrasonic treatment is, for example, 15 nm or less, preferably 10 nm or less, more preferably 8 nm or less, even more preferably 7 nm or less, particularly preferably 5 nm or less. The lower limit of the particle size D10 is, for example, 3 nm.

[0034] In addition, the particle size D90 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by ultrasonic treatment is, for example, 30 nm or less, preferably 25 nm or less, more preferably 20 nm or less, more preferably 18 nm or less, particularly preferably 16 nm or less. The lower limit of the particle size D90 is, for example, 10 nm.

[0035] The hydrogenated nanodiamond particles obtained through the hydrogenation process (or the heat treatment) are mixed with water and subjected to bead mill treatment (for example, bead mill treatment using zirconia beads with a diameter of 30 μm, 2560 rpm x 3 hours), and the median diameter D50 of the hydrogenated nanodiamond particles in a pH 5-8, concentration 3 wt% aqueous dispersion is, for example, 20 nm or less, preferably 15 nm or less, particularly preferably 10 nm or less, most preferably 7 nm or less, and particularly preferably 5 nm or less. The lower limit of the median diameter D50 is, for example, 3 nm.

[0036] In addition, the particle diameter D10 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by bead mill processing is, for example, 15 nm or less, preferably 10 nm or less, more preferably 8 nm or less, even more preferably 7 nm or less, particularly preferably 5 nm or less, most preferably 4 nm or less, particularly preferably 3.5 nm or less. The lower limit of the particle diameter D10 is, for example, 1 nm.

[0037] In addition, the particle size D90 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by bead mill processing is, for example, 30nm or less, preferably 25nm or less, more preferably 20nm or less, more preferably 15nm or less, particularly preferably 10nm or less, most preferably 8nm or less, particularly preferably 7nm or less.The lower limit of the particle size D90 is, for example, 5nm.

[0038] In this specification, the particle size D10, median size D50, and particle size D90 are the 10%, 50%, and 90% cumulative particle sizes, respectively, in the volume-based particle size distribution determined by dynamic light scattering.

[0039] (Raw Diamond Particles) The zeta potential of the raw diamond particles in water of pH 5 to 8 is +30 mV or less, preferably +20 mV or less, particularly preferably +10 mV or less. The lower limit of the zeta potential is, for example, −50 mV, preferably −40 mV, more preferably −20 mV, even more preferably −15 mV, particularly preferably −10 mV, and most preferably −8 mV.

[0040] The median diameter D50 of the raw diamond particles in water that is alkaline or acidic relative to the isoelectric point is, for example, 50 nm or more, particularly 100 nm or more, and particularly 200 nm or more. There is no particular upper limit to the median diameter D50, and the raw diamond particles may be of a size that cannot be measured by dynamic light scattering (for example, 1 μm or more).

[0041] The raw diamond particles may be commercially available or may be produced by a detonation method or the like.

[0042] The detonation method includes the following steps of producing, oxidizing, and drying. If necessary, the method may include an acid treatment step after the producing step.

[0043] (Production Step) First, an explosive is placed in a pressure-resistant container for detonation and sealed, and the explosive is detonated in the container. For example, a mixture of TNT and RDX can be used as the explosive.

[0044] During detonation, the explosive used undergoes partial incomplete combustion, liberating carbon, which is used as a raw material. The pressure and energy of the shock wave generated by the explosion produces a crude diamond product (which includes diamond aggregates, soot, and metal oxides such as Fe, Co, and Ni derived from the container, etc.).

[0045] (Acid Treatment Step) The acid treatment step is a step in which a strong acid is applied to the crude diamond product in, for example, an aqueous solvent to remove metal oxides. Examples of the strong acid include hydrochloric acid, hydrofluoric acid, sulfuric acid, nitric acid, and mixtures thereof.

[0046] The acid treatment temperature is, for example, 70 to 150° C. The acid treatment time is, for example, 0.1 to 24 hours.

[0047] After the acid treatment, it is preferable to wash the solid matter with water by decantation or the like until the pH of the precipitate reaches, for example, 2 to 3.

[0048] (Oxidation Treatment Step) The oxidation treatment step is a step of removing graphite and metal oxides from the crude diamond product using an oxidizing agent. As the oxidizing agent, for example, a mixed acid (sulfuric acid / nitric acid weight ratio of, for example, 60 / 40 to 95 / 5) can be suitably used.

[0049] The amount of the oxidizing agent (particularly the mixed acid) used is, for example, 20 to 40 parts by weight per part by weight of the crude diamond product.

[0050] Furthermore, when the mixed acid is used as the oxidizing agent, a catalyst may be used together with the mixed acid. By using a catalyst, the efficiency of graphite removal can be further improved. Examples of the catalyst include copper (II) carbonate. The amount of catalyst used is, for example, about 0.01 to 10 parts by weight per 100 parts by weight of the crude diamond product.

[0051] The oxidation treatment temperature is, for example, 100 to 200° C. The oxidation treatment time is, for example, 1 to 24 hours.

[0052] (Drying step) This is a step of evaporating and drying the liquid from the diamond-containing solution obtained through the above steps. For example, a spray dryer, an evaporator, a drying oven, etc. can be used for the drying. Diamond powder is obtained through this step.

[0053] The above-described configurations and combinations thereof of the present disclosure are merely examples, and additions, omissions, substitutions, and modifications of the configurations are possible as appropriate without departing from the spirit of the present disclosure. Furthermore, each aspect disclosed in this specification can be combined with any other feature disclosed in this specification. Furthermore, the present disclosure is not limited by the embodiments.

[0054] The present disclosure will be explained in more detail below using examples, but the present disclosure is not limited to these examples.

[0055] Example 1 2.01 g of raw diamond particles (trade name "Dinobear", manufactured by Daicel Corporation, isoelectric point: around pH 7, zeta potential in pH 10 aqueous dispersion (concentration 0.1 wt%): -20 mV, average primary particle diameter: 168 nm) was placed on a quartz boat and placed in a tubular furnace. After 1.0 L / min of nitrogen gas was passed through at room temperature for 30 minutes, the gas was switched to a mixed gas of nitrogen gas (0.95 L / min) and 2% hydrogen / nitrogen gas (0.05 L / min) (hydrogen gas concentration 0.1%). Then, the temperature was increased at 10 ° C / min to 580 ° C, then increased at 1 ° C / min to 600 ° C, and maintained at 600 ° C for 5 hours. After that, the temperature was cooled to 100 ° C, the gas was stopped, and hydrogenated nanodiamond particles were obtained as the reaction product. The yield of the obtained hydrogenated nanodiamond particles was 1.80 g.

[0056] The zeta potential of the raw diamond particles in a 0.1 wt % aqueous dispersion (pH 6.46) was measured to be −6.80 mV. The pH was adjusted using dilute aqueous ammonia and dilute hydrochloric acid.

[0057] (Evaluation 1) The zeta potential of the obtained aqueous dispersion of hydrogenated nanodiamond particles (hydrogenated nanodiamond particle concentration: 0.1 wt%) was measured. The results are shown in Table 1 below.

[0058] (Evaluation 2) 0.90 g of the obtained hydrogenated nanodiamond particles was suspended in 29.2 mL of water, and 1 M hydrochloric acid was added to adjust the pH to 3.86, obtaining an aqueous dispersion of hydrogenated nanodiamond particles. Then, the aqueous dispersion of hydrogenated nanodiamond particles was added to an Imex LSG-4U disperser vessel (volume 125 mL) along with 105 g of zirconia beads with a diameter of 30 μm, and the mixture was stirred at 2560 rpm for 3 hours at a jacket temperature of 20 ° C. After allowing the zirconia beads to settle naturally, the mixture was centrifuged (20,000 g x 10 min), and the supernatant was centrifuged to obtain an aqueous dispersion of hydrogenated nanodiamond particles (approximately 15 mL). The obtained aqueous dispersion (hydrogenated nanodiamond particle concentration: 3 wt%) was subjected to dynamic light scattering to determine the volume-based particle size of the hydrogenated nanodiamond particles in the aqueous dispersion. The results are shown in Table 2 below.

[0059] In addition, the obtained aqueous dispersion was diluted with water to prepare an aqueous dispersion with a hydrogenated nanodiamond particle concentration of 0.1% by weight, and its zeta potential was measured. The results are shown in Table 2 below.

[0060] (Evaluation 3) The obtained hydrogenated nanodiamond particles were subjected to FTIR measurement (measured by the diffuse reflectance method under reduced pressure and heating at 150°C after dilution with KBr). The results are shown in Figure 1.

[0061] The raw diamond particles were also subjected to FTIR measurement, and the results are shown in Figure 1.

[0062] Examples 2 to 7, Reference Example 1 Hydrogenated nanodiamond particles were obtained in the same manner as in Example 1, except that the conditions were changed as shown in Table 1 below. In addition, evaluations 1 to 3 were performed on the obtained hydrogenated nanodiamond particles in the same manner as in Example 1. The results are shown in Tables 1 and 2 below, and in Figure 1.

[0063]

[0064]

[0065] From Tables 1 and 2 above, it can be seen that even when heat treatment is performed in a gas atmosphere with a hydrogen gas concentration of less than 1% and no risk of explosion, the surface of the nanodiamond particles can be monofunctionalized (especially hydrogenated) to the same extent or better than when heat treatment is performed in a high-concentration hydrogen gas atmosphere, and excellent dispersibility can be imparted.

[0066] Also, in FIG. －１ Observation of the peaks derived from C-H bonds present nearby reveals that the surfaces of the hydrogenated nanodiamond particles obtained under a 0.1% or 0.5% hydrogen gas atmosphere are hydrogenated to the same extent or more than those of the hydrogenated nanodiamond particles obtained under a 2% hydrogen gas atmosphere.

[0067] The hydrogenated nanodiamond particles obtained in Example 2 and Reference Example 1 were further subjected to the following evaluation 4. The results are shown in Table 3 below.

[0068] (Evaluation 4) 0.30 g of the hydrogenated nanodiamond particles obtained in Example 2 was suspended in 20 mL of water, and the pH was adjusted to 3.27 with 1 M hydrochloric acid to obtain a suspension of hydrogenated nanodiamond particles. The resulting suspension was then cooled to 20 ° C and subjected to ultrasonic irradiation (Hielscher UP400S ultrasonic homogenizer, operating frequency 24 kHz, 400 W, output 45%, 4 hours of continuous operation). A gray dispersion was then obtained. The resulting gray dispersion was allowed to stand, and impurities from the ultrasonic homogenizer horn were removed by natural sedimentation, followed by centrifugation (20,000 g x 10 min), resulting in a black-brown dispersion in the supernatant. The resulting supernatant was concentrated under reduced pressure and dried to obtain 0.23 g of black acicular powder (whiskers) (77% of the raw material). In addition, the obtained supernatant (hydrogenated nanodiamond particle concentration: 1 wt%) was subjected to dynamic light scattering to determine the volumetric particle diameter of the hydrogenated nanodiamond particle in the supernatant.Furthermore, the obtained supernatant was diluted with water to prepare an aqueous dispersion of hydrogenated nanodiamond particle concentration 0.1 wt%, and its zeta potential was measured.

[0069] In addition, in the case of the hydrogenated nanodiamond particles obtained in Reference Example 1, evaluation 4 was performed in the same manner as above, except that the pH adjustment using 1M hydrochloric acid was changed from "pH 3.27" to "pH 3.40" and the ultrasonic irradiation time was changed from "4 hours" to "5 hours."

[0070]

[0071] From Table 3 above, it can be seen that when heat treatment is performed in a gas atmosphere with a hydrogen gas concentration of more than 0% but less than 1%, the degree of aggregation is lower than when heat treatment is performed in a high-concentration hydrogen gas atmosphere, and that the particles can be easily pulverized and highly dispersed by a simple de-agglomeration treatment.

[0072] In summary, the configuration of the present disclosure and its variations are noted below. [1] A method for producing hydrogenated nanodiamond particles, which involves subjecting diamond particles having a zeta potential of +30 mV or less in water of pH 5 to 8 to a hydrogenation process in which they are heated at a temperature of 300 to 850°C in a gas atmosphere with a hydrogen gas concentration of less than 1%, to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more in water of pH 5 to 8. [2] A method for producing hydrogenated nanodiamond particles as described in [1], which involves subjecting the hydrogenated nanodiamond particles produced after the hydrogenation process to a crushing process using an ultrasonic treatment or a bead mill to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more in water of pH 5 to 8 and a median diameter D50 of 20 nm or less. [3] A method for producing hydrogenated nanodiamond particles as described in [1] or [2], in which the hydrogen gas concentration in the hydrogenation process is 0.01% or more and less than 1%. [4] A method for producing hydrogenated nanodiamond particles according to any one of [1] to [3], wherein the hydrogen gas concentration in the hydrogenation treatment step is 0.05% or more and less than 0.2%. [5] A method for producing hydrogenated nanodiamond particles according to any one of [1] to [4], wherein the hydrogen partial pressure in the hydrogenation treatment step is 0.01 kPa or more and less than 1 kPa. [6] A method for producing hydrogenated nanodiamond particles according to any one of [1] to [5], wherein the gas atmosphere in the hydrogenation treatment step contains an inert gas in addition to hydrogen gas. [7] A method for producing hydrogenated nanodiamond particles according to any one of [1] to [6], wherein the gas atmosphere in the hydrogenation treatment step contains hydrogen gas and nitrogen gas. [8] A method for producing hydrogenated nanodiamond particles according to [6], wherein the inert gas concentration in the gas atmosphere is 80% or more. [9] A method for producing hydrogenated nanodiamond particles according to [7], wherein the nitrogen gas concentration in the gas atmosphere is 80% or more.

[10] The method for producing hydrogenated nanodiamond particles according to [6] or [8], wherein the partial pressure of the inert gas in the gas atmosphere is 80 kPa or more.

[11] The method for producing hydrogenated nanodiamond particles according to [7] or [9], wherein the partial pressure of the nitrogen gas in the gas atmosphere is 80 kPa or more.

[12] A method for producing hydrogenated nanodiamond particles according to any one of [1] to

[11] , wherein the gas supply rate during the heat treatment in the hydrogenation treatment step is 0.01 to 5 L / min.

[13] A method for producing hydrogenated nanodiamond particles according to any one of [1] to

[12] , wherein the heat treatment time in the hydrogenation treatment step is 1 to 24 hours.

[14] A method for producing hydrogenated nanodiamond particles according to any one of [1] to

[13] , wherein the hydrogenation treatment step is carried out under normal pressure.

[15] A method for producing hydrogenated nanodiamond particles according to any one of [2] to

[14] , wherein the particle size D10 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by ultrasonic treatment is 15 nm or less.

[16] A method for producing hydrogenated nanodiamond particles according to any one of [2] to

[15] , wherein the particle size D90 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by ultrasonic treatment is 30 nm or less.

[17] A method for producing hydrogenated nanodiamond particles according to any one of [2] to

[14] , wherein the particle size D10 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by bead milling is 15 nm or less.

[18] A method for producing hydrogenated nanodiamond particles according to any one of [2] to

[14] and

[17] , wherein the particle size D90 of the hydrogenated nanodiamond particles in the aqueous dispersion obtained by bead milling is 30 nm or less.

[19] A method for producing hydrogenated nanodiamond particles according to any one of [1] to

[18] , wherein the median diameter D50 in alkaline or acidic water higher than the isoelectric point of the diamond particles is 50 nm or more.

Claims

1. A method for producing hydrogenated nanodiamond particles, which involves subjecting diamond particles having a zeta potential of +30 mV or less in water of pH 5 to 8 to a hydrogenation treatment process in which they are heated at a temperature of 300 to 850°C in a gas atmosphere with a hydrogen gas concentration of less than 1%, to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more in water of pH 5 to 8.

2. After the hydrogenation treatment process, the produced hydrogenated nanodiamond particles are subjected to ultrasonic treatment or crushing treatment using a bead mill to obtain hydrogenated nanodiamond particles having a zeta potential of +35 mV or more and a median diameter D50 of 20 nm or less in water of pH 5 to 8, as described in claim 1.

3. A method for producing hydrogenated nanodiamond particles as described in claim 1 or 2, wherein the hydrogen gas concentration in the hydrogenation treatment process is 0.01% or more and less than 1%.

4. A method for producing hydrogenated nanodiamond particles as described in claim 1 or 2, wherein the hydrogen partial pressure in the hydrogenation treatment process is 0.01 kPa or more and less than 1 kPa.

5. A method for producing hydrogenated nanodiamond particles as described in claim 1 or 2, wherein the gas atmosphere in the hydrogenation treatment process contains hydrogen gas and nitrogen gas.

6. A method for producing hydrogenated nanodiamond particles as described in claim 1 or 2, wherein the hydrogenation treatment process is carried out under atmospheric pressure.

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