BACKGROUND OF THE INVENTION Field of the Invention [0001] The present invention relates to a method of preparing iron oxide nanoparticles, and more particularly, to a method of preparing iron oxide nanoparticles, iron oxide nanoparticles prepared thereby, and an anode material including the iron oxide nanoparticles. Description of the Related Art [0002] Recently, as issues, such as the depletion of fossil fuels and environmental destruction, have emerged, many researchers have devoted to develop alternative energies that may replace fossil fuels. As a kind of such alternative energies, a significant amount of research into secondary batteries that are applicable to various fields has been conducted. [0003] The application fields of secondary batteries have been further expanded to batteries for vehicles and batteries for power storage as well as typical portable systems. [0004] A secondary battery may be composed of a cathode, an anode, an electrolyte solution, and a separator, and among these, components that mostly affect the battery may be the cathode and the anode in which electrochemical reactions actually occur. [0005] A graphite material has mainly been used as the anode due to its competitiveness in terms of stability, lifetime, and price. However, as high-capacity batteries, which may be used in electric vehicles and batteries for power storage, have been increasingly required, research into developing a new anode material having high theoretical capacity has emerged. [0006] In line with such requirements, metal oxides have recently received attention as a high-capacity anode active material, and particularly, iron (III) oxide (Fe2O3) 3 among these oxides has received attention as an anode material due to its high theoretical capacity (1007 mAh/g). [0007] However, since capacity and cycle efficiency may decrease during a charge/discharge process when a particle diameter of the metal oxide is large (>1 μm), research into preparing nanometer-sized iron oxide particles by various chemical syntheses has continued. [0008] As a synthesis method of the iron oxide nanoparticles, a method has been known, in which a ferrous hydroxide (Fe(OH)2) aqueous solution, which is prepared by adding sodium hydroxide to a ferric salt aqueous solution, is irradiated with ultrasonic waves. [0009] However, with respect to a typical synthesis method of iron oxide, difficult synthesis conditions (inert atmosphere) must be maintained and an expensive high purity raw material must be used. In addition, since the particles obtained by the irradiation of ultrasonic waves may not be uniform and may have poor crystallinity, the application range thereof may be narrow. Also, since it takes a long time to irradiate with ultrasonic waves, mass production may be impossible. [0010] Furthermore, since iron (III) oxide nanoparticles prepared by a typical method may be difficult to be prepared as nanoparticles having a fine and uniform particle size, cracks may occur in an electrode due to volume expansion and shrinkage of the electrode during a charge/discharge process. Thus, capacity loss is high and cycle efficiency rapidly decreases. [0011] Therefore, development of a method of preparing iron oxide nanoparticles is urgent, in which limitations of a typical preparation method of iron oxide nanoparticles, such as complex process and long preparation time, may be addressed, particle diameter and shape of powder may be uniformly controlled, and simultaneously, capacity loss 4 may be low even during numerous charge/discharge processes. SUMMARY OF THE INVENTION [0012] An aspect of the present invention provides a method of preparing iron oxide nanoparticles, in which mass production may be possible by a simple process condition and capacity loss may be low even during numerous charge/discharge processes. [0013] Another aspect of the present invention provides iron oxide nanoparticles prepared by the above method and an anode material including the iron oxide nanoparticles. [0014] Another aspect of the present invention provides an anode for an electrochemical device that is formed of the above anode material and an electrochemical device including the anode. [0015] According to an aspect of the present invention, there is provided a method of preparing iron (III) oxide nanoparticles including: preparing a ferric chloride (FeCl3) aqueous solution; preparing a sodium hydroxide (NaOH) aqueous solution; mixing both aqueous solutions; adding a sodium sulfate (Na2SO4) aqueous solution while stirring a mixed aqueous solution prepared by the mixing; and reacting the mixed aqueous solution having the sodium sulfate aqueous solution added thereto in an electric convection oven. [0016] According to a method of the present invention, iron oxide nanoparticles may be prepared in which particle diameter and particle distribution of the powder may be controlled by a simple preparation process and capacity loss may be low even during numerous charge/discharge processes. Also, an anode having excellent capacity and stability, and an electrochemical device including the anode may be prepared by using the iron oxide nanoparticles. 5 BRIEF DESCRIPTION OF THE DRAWINGS [0017] FIG. 1 is a scanning electron microscope (SEM) image of iron oxide nanoparticles prepared by a method of the present invention; [0018] FIG. 2 is a scanning electron microscope (SEM) image of iron oxide nanoparticles prepared by a method of the present invention; and [0019] FIG. 3 is a graph comparing the results of the measurement of capacities of secondary batteries prepared in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0020] Hereinafter, the present invention will be described in detail. [0021] The present invention provides a method of preparing iron (III) oxide nanoparticles including: preparing a ferric chloride (FeCl3) aqueous solution; preparing a sodium hydroxide (NaOH) aqueous solution; mixing both aqueous solutions; adding a sodium sulfate (Na2SO4) aqueous solution while stirring a mixed aqueous solution prepared by the mixing; and reacting the mixed aqueous solution having the sodium sulfate aqueous solution added thereto in an electric convection oven. [0022] Specifically, in the method of the present invention, a concentration of the FeCl3 aqueous solution may be in a range of 2 M to 3 M and a concentration of the NaOH aqueous solution may be in a range of 5 M to 6 M. Also, a concentration of the Na2SO4 aqueous solution may be in a range of 0.5 M to 1 M, and for example, may be 0.6 M. [0023] In this case, when the concentration of the FeCl3 aqueous solution is 2 M or less or 3 M or more, metal oxide having a particle diameter of 1 μm or more may be synthesized. Also, when the concentration of the NaOH aqueous solution is 6 M or 6 more, a pH level of the synthesized aqueous solution may be increased and thus, metal oxide may not be synthesized. Furthermore, when the concentration of the Na2SO4 aqueous solution is 1 M or more, metal oxide having a uniform shape may not be prepared. In this case, a sodium phosphate (Na2PO4) aqueous solution may be used instead of the Na2SO4 aqueous solution. [0024] In the method of the present invention, the ferric chloride aqueous solution, the sodium hydroxide aqueous solution, and the sodium sulfate aqueous solution may be mixed in a volume ratio of 10:9:x (where x satisfies 0