INORGANIC HOLLOW NANOCOILS AND METHOD OF MANUFACTURING THE SAME

Abstract

The present invention relates to hollow nanocoils having a three-dimensional helical structure in the form of a hollow tube and a method of manufacturing the same.

The present invention provides a method of synthesizing metal nanocoils into inorganic hollow nanocoils using the galvanic replacement reaction and an electrochemical reaction including the Kirkendall effect. The inorganic hollow nanocoil structure body of the present invention can be applied to various fields such as sensors, catalysts, batteries, or gene delivery and therapy using a large surface area.

Claims

1. An inorganic hollow nanocoil having a helical structure in the form of a hollow tube, the inorganic hollow nanocoil consists of XaZc and/or Xa′YbZc′, or XaWdYbZc, wherein in the case of XaZc and/or Xa′YbZc′, X is Cr or Mn; Y is P; Z is O; and a+a′:b:c+c′ is 1:1:4 to 1:1.4:7.5 or 1:0.1:1.5 to 1:0.3:4; and wherein in the case of XaWdYbZc, X is Mn; W is Al; Y is P; Z is O; and a:d:b:c is 1:0.7:1.4:6.5 to 1:1:1.7:9.

2. The nanocoil of claim 1, wherein the inorganic hollow nanocoil consists of Xa′YbZc′, wherein X is Cr; Y is P; Z is O; and a′:b:c′ is 1:1:4 to 1:1.4:7.5.

3. The nanocoil of claim 1, wherein the inorganic hollow nanocoil consists of XaZc and Xa′YbZc′, wherein X is Mn; Y is P; Z is O; and a:c is 1:2 to 1:4, a′:b:c′ is 1:0.15:3 to 1:0.4:5, and a+a′:b:c+c′ is 1:0.1:1.5 to 1:0.3:4.

4. The nanocoil of claim 1, wherein the inorganic hollow nanocoil consists of XaWdYbZc, wherein X is Mn; W is Al; Y is P; Z is O; and a:d:b:c is 1:0.7:1.4:6.5 to 1:1:1.7:9.

5. The nanocoil of claim 1, wherein the inorganic hollow nanocoil has an outer diameter of 50 nm to 500 nm.

6. The nanocoil of claim 1, wherein the inorganic hollow nanocoil has a shell thickness of 2 nm to 25 nm.

7. A method of manufacturing an inorganic hollow nanocoil, comprising: manufacturing a solution in which sacrificial nanocoils containing a transition metal are dispersed; and reacting by adding a metal precursor and an acidic solution to the solution in which the sacrificial nanocoils are dispersed.

8. The method of claim 7, wherein the transition metal comprises one or more selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo and Tc.

9. The method of claim 7, wherein the amount of the sacrificial nanocoils containing the transition metal is 0.005 to 0.2% (w/v) based on the total reaction solution.

10. The method of claim 7, wherein the metal precursor is a metal comprising one or more selected from the group consisting of Cr and Mn.

11. The method of claim 7, wherein the concentration of the metal precursor is 1 to 20 mM based on the total reaction solution.

12. The method of claim 7, wherein the acidic solution is a solution of hydrochloric acid (HCl), nitric acid (HNO.sub.3), sulfuric acid (H.sub.2SO.sub.4), or phosphoric acid (H.sub.3PO.sub.4).

13. The method of claim 7, wherein the acidic solution further comprises potassium phosphate (KH.sub.2PO.sub.4), dipotassium phosphate (K.sub.2HPO.sub.4), tripotassium phosphate (K.sub.3PO.sub.4), sodium dihydrogen phosphate (NaH.sub.2PO.sub.4), disodium hydrogen phosphate (Na.sub.2HPO.sub.4) or trisodium phosphate (Na.sub.3PO.sub.4).

14. The method of claim 7, wherein the reaction is carried out at 50 to 100° C.

15. The method of claim 7, wherein the reaction is carried out at a pH of 0.5 to 3.

16. The method of claim 7, wherein the reaction is carried out for 0.5 to 2 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1a is a schematic diagram of the manufacturing process of (a) hollow nanocoils and (b) a transmission electron micrograph of CrPO.sub.4 hollow nanocoils MnO.sub.x—MnP.sub.yO.sub.z hollow nanocoils, and FIG. 1B is a schematic diagram showing the formation of CrPO.sub.4 hollow nanocoils on the surface of sacrificial nanocoils, and FIG. 1c is a clearer transmission electron micrograph of CrPO.sub.4 hollow nanocoils and MnO.sub.x—MnP.sub.yO.sub.z hollow nanocoils.

[0027] FIGS. 2a to 2d are transmission electron micrograph of changes in hollow nanocoils according to reaction time (2a: sacrificial nanocoils before reaction, 2b: nanocoils in the initial stage of reaction, 2c: nanocoils in the middle stage of reaction, and 2d: hollow nanocoils after completion of reaction).

[0028] FIGS. 3a and 3b are Cr-based hollow nanocoils, and FIGS. 3c and 3d are high-resolution transmission electron micrographs and energy dispersive X-ray spectroscopy elemental analysis photographs of Mn-based hollow nanocoils.

[0029] FIG. 4a is a Cr-based hollow nanocoil, and FIG. 4b is an enlarged transmission electron micrograph and Line-EDX result graph of a Mn-based hollow nanocoil.

[0030] FIG. 5a is a transmission electron micrograph of a MnAl-based hollow nanocoil, and FIG. 5b is a high-resolution transmission electron micrograph of a MnAl-based hollow nanocoil.

[0031] FIG. 6a is an energy dispersive X-ray spectroscopy element mapping analysis photograph of a MnAl-based hollow nanocoil, and FIG. 6b is a spectrum and elemental content table of energy dispersive X-ray spectroscopy of a MnAl-based hollow nanocoil.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0032] Hereinafter, the present invention will be specifically described.

[0033] The present invention provides inorganic hollow nanocoils having a helical structure in the form of a hollow tube. The hollow nanocoil according to the present invention is completely empty inside, and has a three-dimensional cylindrical nanocoil shape having a shell of a relatively uniform thickness.

[0034] The inorganic hollow nanocoil consists of XaZc and/or Xa′YbZc′, or XaWdYbZc,

[0035] wherein in the case of XaZc and/or Xa′YbZc′, X may be Cr or Mn; Y may be P; Z may be O; and a+a′:b:c+c′ may be 1:1:4 to 1:1.4:7.5 or 1:0.1:1.5 to 1:0.3:4; and

[0036] in the case of XaWdYbZc, wherein X may be Mn; W may be Al; Y may be P; Z may be O; and a:d:b:c may be 1:0.7:1.4:6.5 to 1:1:1.7:9.

[0037] The hollow nanocoil consists of an inorganic material, and may consist of metal ions (cations) and anions. The metal ion may be Cr, Mn, etc., and the anion may be an oxide, hydroxide, phosphate, etc. In an embodiment of the present invention, the Cr-based hollow nanocoil consists of Cr, P and O, and the Mn-based hollow nanocoil consists of Mn, O, and P. The MnAl-based hollow nanocoil consists of Mn, Al, O, and P.

[0038] In addition, the hollow nanocoil manufactured in the embodiment of the present invention can control the surface roughness of the hollow nanocoil by adjusting the reduction path of the material according to a material (the kind of precursor metal used in the reaction).

[0039] In an embodiment of the present invention, the Cr-based hollow nanocoil has a relatively smooth (soft, uneven) surface in which the surface is made of a single component (CrPO.sub.4) because Cr is reduced by a single path (upper end a of FIG. 4), whereas the Mn based hollow nanocoil has a relatively rough (convoluted) surface in which Mn is reduced via multiple paths (multicomponent: MnO.sub.x—MnP.sub.yO.sub.z), the inner surface (inner shell) consists of a metal phosphate (MnP.sub.yO.sub.z) having a relatively low oxidation number, and the outer surface (outer shell) has a relatively high oxidation number on which a metal oxide (MnO.sub.x) has been positioned (lower end b in FIG. 4). The MnAl-based hollow nanocoil has a relatively smooth (smooth, non-irregular) surface because Al.sup.3+ ions present in solution penetrate the MnO.sub.x or MnP.sub.yO.sub.z structure to form a MnAl-based single-component complex.

[0040] The inorganic hollow nanocoil may consist of Xa′YbZc′, wherein X may be Cr; Y may be P; Z may be 0; and a′:b:c′ may be 1:1:4 to 1:1.4:7.5. In an embodiment of the present invention, the ratio of elements constituting the Cr-based hollow nanocoil (CrPO.sub.4) was Cr:P:O=10.85:13.38:75.77.

[0041] The inorganic hollow nanocoil may consist of XaZc and Xa′YbZc′, wherein X may be Mn; Y may be P; Z may be O, a:c may be 1:2 to 1:4, a′:b:c′ may be 1:0.15:3 to 1:0.4:5, and a+a′:b:c+c′ may be 1:0.1:1.5 to 1:0.3:4. In an embodiment of the present invention, the ratio of elements constituting the Mn-based hollow nanocoil (MnO.sub.x—MnP.sub.yO.sub.z) was inner shell Mn: P:O=19.87:5.33:74.79, outer shell Mn:O=25.52:74.48, and the total elemental ratio of the Mn-based hollow nanocoil was Mn:P:O=25.61:5.32:63.06.

[0042] The inorganic hollow nanocoil may consist of XaWdYbZc, wherein X may be Mn; W may be Al; Y may be P; Z may be 0, and a:d:b:c may be 1:0.7:1.4:6.5 to 1:1:1.7:9. In an embodiment of the present invention, the ratio of elements constituting the MnAl-based hollow nanocoil was Mn:Al:P:O=8.64:7.69:13.25:68.64. The outer diameter of the hollow nanocoil may be 50 nm to 500 nm.

[0043] The shell thickness of the hollow nanocoil may be 2 nm to 25 nm.

[0044] As used herein, the term “shell” refers to the tube wall of a hollow nanocoil in the form of a hollow tube.

[0045] The outer diameter, shell thickness, size, length, etc. of the hollow nanocoil can be adjusted to be appropriately used in the field and technology to be applied, and the size of the hollow nanocoil can be adjusted depending on the sacrificial nanocoil.

[0046] As another aspect of the present invention, the present invention provides a method of manufacturing inorganic hollow nanocoils, including:

[0047] preparing a solution in which sacrificial nanocoils containing a transition metal are dispersed; and

[0048] reacting by adding a metal precursor and an acidic solution to the solution in which the sacrificial nanocoils are dispersed.

[0049] In the present invention, the sacrificial nanocoil includes a transition metal, and the transition metal may be a metal including at least one selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo and Tc, and preferably, a metal including at least one selected from the group consisting of V, Co, Fe or Ni. Any transition metal having a relatively lower reduction potential than a metal precursor for manufacturing inorganic hollow nanocoils may be used. In the present invention, a CoFe sacrificial nanocoil having a reduction potential lower than that of Cr or Mn was used.

[0050] The sacrificial nanocoils containing the transition metal may be 0.005 to 0.2% (w/v), preferably 0.006 to 0.15% (w/v), more preferably 0.007 to 0.1% (w/v) based on the total reaction solution.

[0051] The metal precursor may be a metal including one or more selected from the group consisting of Cr and Mn. Any metal precursor capable of forming HCrO.sub.4.sup.−, Cr.sub.2O.sub.7.sup.2−, and MnO.sub.4.sup.− in an aqueous solution when the corresponding metal precursor is dissolved may be used. For example, it may be H.sub.2CrO.sub.4, CrO.sub.3, LiMnO.sub.4, NaMnO.sub.4, or KMnO.sub.4, preferably CrO.sub.3 or KMnO.sub.4, but is not limited thereto.

[0052] The metal precursor may have a concentration of 1 to 10 mM, preferably 1.5 to 8 mM, more preferably 1.7 to 7 mM, based on the total reaction solution.

[0053] The acidic solution may be a solution of hydrochloric acid (HCl), nitric acid (HNO.sub.3), sulfuric acid (H.sub.2SO.sub.4), or phosphoric acid (H.sub.3PO.sub.4).

[0054] The acidic solution may further include potassium phosphate (KH.sub.2PO.sub.4), dipotassium phosphate (K.sub.2HPO.sub.4), tripotassium phosphate (K.sub.3PO.sub.4), sodium dihydrogen phosphate (NaH.sub.2PO.sub.4), disodium hydrogen phosphate (Na.sub.2HPO.sub.4) or trisodium phosphate (Na.sub.3PO.sub.4).

[0055] In an embodiment of the present invention, phosphoric acid (H.sub.3PO.sub.4) was added as an acidic solution, and phosphoric acid provides hydrogen ions in the solution in the reaction to make the solution acidic to dissolve the CoFe sacrificial nanocoil, and served to provide phosphorylated anions participating in the binding of the final product (inorganic hollow nanocoils). Phosphorylated anions participate in the reaction, and a metal (metal 1 (in the present invention, CoFe sacrificial nanocoil) is converted to a metal phosphate (metal 2 (in the present invention, Cr, Mn or MnAl) phosphate (or oxide)). Phosphoric acid was used in the Examples because it can perform the above two roles at one time, but the reaction of synthesizing the final product may proceed even when materials capable of performing each role are simultaneously added. The reaction of the present invention may proceed even when the acidic solution, such as hydrochloric acid (HCl), nitric acid (HNO.sub.3), sulfuric acid (H.sub.2SO.sub.4), etc., and phosphorylated materials, such as potassium phosphate (KH.sub.2PO.sub.4), dipotassium phosphate (K.sub.2HPO.sub.4), tripotassium phosphate (K.sub.3PO.sub.4), sodium dihydrogen phosphate (NaH.sub.2PO.sub.4), disodium hydrogen phosphate (Na.sub.2HPO.sub.4) or trisodium phosphate (Na.sub.3PO.sub.4) are simultaneously added.

[0056] The reaction may be carried out at 50 to 100° C., preferably 60 to 90° C., more preferably 65 to 80° C.

[0057] The reaction may be carried out at a pH of 0.5 to 3, preferably a pH of 0.7 to 2.5, more preferably a pH of 0.9 to 2.

[0058] The reaction may be carried out for 0.5 to 2 hours, preferably 0.5 to 1.5 hours. The reaction time can be adjusted to prepare nanocoils with desired properties. When the reaction time becomes shorter, more characteristics of the sacrificial nanocoils can be obtained, and when the reaction time becomes longer, it can have more characteristics of the inorganic hollow nanocoils.

[0059] The reaction principle of the present invention is shown in FIG. 1, and the process in which the sacrificial nanocoils are synthesized into inorganic hollow nanocoils over time is shown in FIG. 2. The sacrificial nanocoil having a relatively low reduction potential is oxidized under acidic conditions and ionized in solution, and the generated extra electrons reduce the metal precursor ion having a relatively high reduction potential (Galvanic replacement reaction) and then are combined with phosphate anions in the solution to form an interface along the surface of the hollow nanocoils (FIG. 2b). The formed interface serves as a passage for small cations (Co.sup.2+ and Fe.sup.2+ in the present invention) of the sacrificial nanocoil, but large anions (metal precursors (HCrO.sub.4.sup.−, MnO.sub.4.sup.−) and phosphates (H.sub.2PO.sub.4.sup.−, HPO.sub.4.sup.2− and PO.sub.4.sup.3− in the present invention)) result in a difference in the ion transfer rate between the inside and outside so that a small porous pore is formed that prevents the passage of ions (Kirkendall effect). Due to this, the nanocoil gradually changes into a hollow nanocoil with an empty inside. The reaction of the hollow nanocoil has intermediate characteristics between the original nanocoil and the hollow nanocoil depending on the reaction time at the corresponding temperature. For example, in the initial stage of the reaction, it has magnetism, which is a characteristic of the CoFe sacrificial nanocoil used as an example in this experiment, but as the inside gradually becomes empty, it completely loses the magnetism and changes to a hollow nanocoil (FIG. 2).

[0060] Hereinafter, the present invention will be described in detail by the following Experimental Examples and Preparation Examples. However, the following Experimental Examples and Preparation Examples are merely illustrative of the present invention, and the content of the present invention is not limited by the following Experimental Examples and Preparation Examples. In addition, since these Experimental Examples and Preparation Examples are only for the purpose of helping the understanding of the present invention, the scope of the present invention is not limited thereto in any sense.

Experimental Example 1. Synthesis of Hollow Nanocoil

[0061] (1) Manufacture of AAO porous nanotemplate-sacrificial nanocoil (CoFe NC)

[0062] Sacrificial nanocoils were synthesized by single bath electrodeposition using an anodized aluminum oxide (AAO) porous nanotemplate having a pore size of 200 nm. First, a 300 nm-thick silver layer was deposited on one side of the AAO template and used as a working electrode, and a platinum (Pt) sheet was used as a counter electrode. Cobalt sulfate heptahydrate (CoSO.sub.4.7H.sub.2O, 0.08 M), iron sulfate heptahydrate (FeSO.sub.4.7H.sub.2O, 0.08 M), vanadium sulfate hydrate (VOSO.sub.4.xH.sub.2O, 0.06 M), L-ascorbic acid (C.sub.6H.sub.8O.sub.6, 0.06 M) were dissolved in deionized water (DI) to prepare a solution. The pH was adjusted to 2.5 by addition of nitric acid (HNO.sub.3). For electrodeposition, a constant current density of 20 mA cm.sup.−2 was applied using a source meter (Keithley 2612 B). After the electrodeposition was completed, the silver layer used as the working electrode was removed with an etchant to fabricate an AAO porous nanotemplate containing CoFe sacrificial nanocoils. The CoFe sacrificial nanocoils (CoFe NC) fabricated in the AAO porous nanotemplate had an average composition of Co50Fe50 (at %), an outer diameter of 180 nm, and an inner diameter of about 50 nm.

[0063] (1-1) Manufacture of CoFe Sacrificial Nanocoil

[0064] After the silver layer was removed with an etchant in the method of (1) above, the AAO porous nanotemplate was dissolved in a 1 to 3 M sodium hydroxide (NaOH, 3.0 M) solution at 45° C. for 15 min. Then, the dissolved (dissolved away) AAO porous nanotemplate was rinsed several times with deionized water, leaving, in the vial, only the CoFe sacrificial nanocoils (CoFe NCs) from which the AAO was completely removed.

[0065] (2) Manufacture of Inorganic Hollow Nanocoil (Crpo.sub.4 and Mno.sub.xP.sub.0.21 Hollow Nanocoil)

[0066] Synthesis Method 1: 0.83 mg of the CoFe sacrificial nanocoils (CoFe NC) manufactured in Experimental Example 1 (1-1) was dispersed (colloidal solution) in 7 mL of deionized water heated to 70° C. 2 mL of a previously prepared 10 mM aqueous CrO.sub.3 (chromium oxide) solution or aqueous KMnO.sub.4 (potassium permanganate) solution was added to the colloidal solution. At this time, the concentrations of CrO.sub.3 and KMnO.sub.4 in the colloidal solution was 2 mM. Then, 1 mL of a 5 M H.sub.3PO.sub.4 (phosphoric acid) solution was added to the 9 mL colloidal solution (the concentration of phosphoric acid in the total solution is 0.5 M) to synthesize hollow nanocoils in a water bath.

[0067] Synthesis Method 2: After heating a solution in which HCrO.sub.4.sup.− (or MnO.sub.4.sup.−) was present in a solution made by dissolving 9 mM CrO3 (or KMnO.sub.4) and 0.5 M phosphoric acid in deionized water to 70° C., 10 mL of the solution was reacted with the remaining CoFe sacrificial nanocoils after washing in Experimental Example 1 (1-1) to synthesize hollow nanocoils.

[0068] (3) Manufacture of Inorganic Hollow Nanocoil (MnAlPO.sub.4 Hollow Nanocoil)

[0069] In the method of Experimental Example 1-(1), an AAO porous nanotemplate containing a CoFe sacrificial nanocoil was placed in a vial. 7 mL of deionized water heated to 70° C. was put in the vial, and 2 mL of a previously prepared 10 mM aqueous KMnO.sub.4 (potassium permanganate) solution was put into the vial. At this time, the concentration of KMnO.sub.4 in the entire reaction solution was 2 mM. Then, 1 mL of a 5 M H.sub.3PO.sub.4 (phosphoric acid) solution was added to the 9 mL reaction solution (the concentration of phosphoric acid in the total solution is 0.5 M) to synthesize hollow nanocoils in a water bath.

[0070] Synthesis Method 2: After dissolving 9 mM of KMnO.sub.4 and 0.5 M of phosphoric acid in deionized water and heating the solution in which MnO4- exists to 70° C., 10 mL of the solution was reacted with AAO porous nanotemplate-CoFe sacrificial nanocoils prepared in Experimental Example 1 (1) to synthesize a hollow nanocoils.

[0071] For homogeneity of the reaction, all reactions were carried out in a water bath with a sonication function. After completion of the reaction, the reaction temperature was lowered by immersion in ice water for about 30 seconds, and the reaction solution was washed several times with deionized water to manufacture inorganic hollow nanocoils (HNC). Finally, the washed inorganic hollow nanocoils were dispersed in ethanol.

[0072] A schematic diagram of the corresponding process is shown in FIG. 1, and FIG. 2 shows the synthesis process of the inorganic hollow nanocoils in the following order: a) a bright field transmission electron microscope (TEM) image of a sacrificial CoFe NC template before the reaction (0 min). b) the initial stage of the interfacial formation reaction (30 min). c) the dissipation of internal material through the interface (60 min). d) the final stage of the reaction at 70° C. (90 min).

Experimental Example 2. Confirmation of Material Characteristics

[0073] (1) Method of Confirming Characteristics

[0074] The morphology and dimensions of the synthesized sacrificial nanocoil template inside the AAO template were investigated by scanning electron microscopy (SEM; SU-70, Hitachi). Transmission electron microscope (TEM) and high-resolution TEM (HRTEM; JEM-2100F, JEOL) images were acquired to analyze the shape and structure of the synthesized inorganic hollow nanocoils. High-angle annular dark-field scanning TEM (HAADF-STEM) with aberration-corrected imaging and energy dispersive X-ray spectroscopy (EDS, X-MAXn, HORIBA) was used to determine the proportions of the constituent elements of the material.

[0075] (2) Characteristics of Inorganic Hollow Nanocoils

[0076] A high-resolution transmission electron microscopy (HRTEM) image of a section of a Cr-based HNC with a smooth surface showed that the shell thickness of the hollow nanocoil was ˜5 nm and was determined to be amorphous by selective-area electron diffraction (SAED) analysis (FIG. 3a). Scanning transmission electron microscopy combined with energy dispersive X-ray spectroscopy (STEM-EDX) was carried out on the fabricated Cr-based HNC, and elemental mapping analysis revealed non-localized distributions of Cr, P and O throughout the entire nanostructure. (FIG. 3b). Mn-based HNCs were observed to have a shell thickness of ˜5 nm and were amorphous like the Cr-based HNCs. However, the surface roughness was found to be larger (FIG. 3c). Despite the presence of the same metal template to precursor ratio and the same acid concentration in the two fabricated HNCs, there is a slight difference in the amount of P present (FIG. 3b, d). The atomic percent ratio of P to Cr of Cr-based HNCs (P/Cr ratio) was 1.22, whereas the atomic percent ratio (P/Mn) of Mn-based HNCs was 0.21. Further line profile analysis showed an overall mixing of the three components (FIG. 4a, b). The two line-EDX results are consistent with the element mapping results, showing that all elements are relatively evenly distributed in the three-dimensional structure. However, as observed in the HRTEM images, the largest difference was at the surface. Elemental distribution analysis of Cr-based HNCs showed that Cr and P contents decreased simultaneously at the interface (FIG. 4a). However, in the case of Mn-based HNCs, the P content decreased more rapidly than the Mn content in the region near the interface (FIG. 4b). Therefore, it can be predicted that the material surrounding the Mn-based HNC surface is manganese oxide (MnOx). As a result of elemental distribution analysis, in the case of Cr-based HNCs, the ratio of constituent atoms was Cr:P:O=10.85:13.38:75.77. In the case of Mn-based HNCs, the ratio of constituent atoms for the inner shell was Mn:P:O=19.87:5.33:74.79, for the outer shell, Mn:O=25.52:74.48, and the overall average ratio of constituent atoms of Mn-based HNCs was Mn:P:O=25.61:5.32:63.06.

[0077] A high-resolution transmission electron microscopy (HRTEM) image of MnAl-based HNCs (MnAlPO.sub.4) (FIG. 5) showed that the hollow nanocoil had a shell thickness of 23 nm, and was determined to be amorphous like the Cr and Mn-based HNCs. The line-EDX result is consistent with the element mapping result, showing that all elements are relatively evenly distributed in the three-dimensional structure. As a result of elemental distribution analysis, the elemental ratio of the MnAl-based HNC was Mn:Al:P:O=8.64:7.69:13.25:68.64 (FIG. 6).