Patent Publication Number: US-2013243974-A1

Title: Method of preparing nickel-coated nanocarbon

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Korean Patent Application No. KR 10-2012-0026650, filed on Mar. 13, 2012, which is hereby incorporated by reference in its entirety into this application. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a method of preparing a nickel-coated Nanocarbon, and, more particularly, to a method of preparing a nickel-coated nanocarbon whose shape is controlled by controlling process variables in electroless plating. 
     2. Description of the Related Art 
     Nanocarbons have high strength, a high elastic modulus, excellent thermal and electrical conductivity and excellent mechanical and physical properties. Therefore, recently, there have been attempts to put nanocarbons to practical use in various fields by coating nanocarbons with metal materials to prepare nanocarbon-metal composites. For example, since nickel-coated nanocarbons exhibit excellent performance as electromagnetic-wave shielding materials or far-field electromagnetic-wave absorbing materials, attempts to coat nanocarbons with nickel have been made. 
     Recently, electroless plating has been frequently used as a method of coating nanocarbons with nickel. 
     For example, Korean Unexamined Patent Application Publication No. 2006-0073019 discloses a method of preparing a carbon nanotube-metal composite by coating carbon nanotubes with a metal using electroless plating. 
     Meanwhile, carbon nanotubes, a typical example of nanocarbons, have a nanosized cylindrical graphite sheet and an sp2 bond structure. Nanocarbons exhibit conductive characteristics or semiconductive characteristics depending on the rolling angle and structure of the graphite sheet. Carbon nanotubes may be classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), multi-walled carbon nanotubes (MWCNTs) and rope carbon nanotubes according to the number of bonds constituting a wall. 
     Among these carbon nanotubes, single-walled carbon nanotubes (SWCNTs) exhibit various electrical, chemical, physical and optical characteristics because they have both metallic characteristics and semiconductive characteristics. Generally, at the time of synthesizing single-walled carbon nanotubes (SWCNTs), metallic single-walled carbon nanotubes (SWCNTs) are necessarily mixed with semiconductive single-walled carbon nanotubes (SWCNTs). 
     Metallic nanocarbons, such as CNF (carbon nanofiber), MWCNT, TWCNT, DWCNT, metallic SWCNT and the like, are different from semiconductive nanocarbons, such as semiconductive SWCNT, SWCNT bundles and the like, in electrical characteristics. In electroless plating, a chemical reaction occurs on the surface of a subject to be plated. Therefore, at the time of coating nanocarbons with a metal using electroless plating, differentiated processes are required depending on electrical properties. However, conventional technologies do not provide a solution for the problem. 
     SUMMARY OF THE INVENTION 
     Accordingly, the present invention has been made to solve the above-mentioned problems, and the present invention intends to provide a coating method that is differentiated according to the electrical properties of a nanocarbon in the process of coating a nanocarbon with nickel using electroless plating. 
     More specifically, the present invention intends to provide a method of coating a metallic nanocarbon, such as a CNF, a MWCNT, a TWCNT, a DWCNT or a metallic SWCNT, with nickel using electroless plating. 
     Further, the present invention intends to provide a method of coating a semiconductive nanocarbon, such as a semiconductive SWCNT or a SWCNT bundle, with nickel using electroless plating. 
     Further, the present invention intends to provide a method of coating a nanocarbon with nickel using electroless plating to prepare a shape-controlled nickel-coated nanocarbon. 
     In order to accomplish the above objects, the present invention provides a method of preparing a nickel-coated nanocarbon using electroless plating, including the steps of: washing a nanocarbon with a solvent or thermally-oxidizing the nanocarbon to remove impurities from the nanocarbon; immersing the washed or thermally-oxidized nanocarbon into a Pd-containing solution to form an activated Pd seed on a surface of the nanocarbon; treating the nanocarbon having the Pd seed with a strong acid; immersing the strong acid-treated nanocarbon into an electroless nickel plating solution to form a nickel plated layer on a surface of the nanocarbon; and heat-treating the nanocarbon having the nickel plated layer at a high temperature to crystallize the nanocarbon. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a conceptional view of the present invention; 
         FIG. 2  shows the results of thermogravimetric analysis (TGA) of carbon nanofibers (CNFs) before and after heat treatment; 
         FIG. 3  shows scanning electron microscope (SEM) images of amorphous and crystalline carbon nanofibers of Examples 1 to 3 according to fibrous, scalelike and spherical nickel-plating shapes; 
         FIG. 4  shows transmission electron microscope (TEM) images of amorphous and crystalline carbon nano-fibers of Examples 1 to 3 according to fibrous, scalelike and spherical nickel-plating shapes; 
         FIG. 5  shows the results of thermogravimetric analysis (TGA) of amorphous carbon nanofibers (CNFs) of Examples 1 to 3 according to fibrous, scalelike and spherical nickel-plating shapes; and 
         FIG. 6  shows the results of thermogravimetric analysts (TGA) of crystalline carbon nanofibers (CNFs) of Examples 1 to 3, which were prepared by performing high-temperature heat treatment at 400° C. (celsius degree) for 3 hours under an air atmosphere, according to fibrous, scalelike and spherical nickel-plating shapes. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, preferred embodiment of the present invention will be described in detail. 
     The present invention provides a method of preparing a nickel-coated nanocarbon using electroless plating, including the steps of: 1) washing a nanocarbon with a solvent or thermally-oxidizing the nanocarbon to remove impurities from the nanocarbon; 2) immersing the washed or thermally-oxidized nanocarbon into a Pd-containing solution to form an activated Pd seed on a surface of the nanocarbon; 3) treating the nanocarbon having the Pd seed with a strong acid; 4) immersing the strong acid-treated nanocarbon into an electroless nickel plating solution to form a nickel plated layer on a surface of the nanocarbon; and 5) heat-treating the nanocarbon having the nickel plated layer at a high temperature to crystallize the nanocarbon. 
     In the present invention, nanocarbons are classified into metallic nanocarbons, such as CNFs, MWCNTs, TWCNTs, DWCNTs, metallic SWCNTs and the like, and semiconductive nanocarbons, such as semiconductive SWCNTs, semiconductive SWCNT bundles and the like. 
     In the present invention, nickel coating may include Ni—P coating and Ni—B coating according to the kind of reductant used in electroless plating. That is, Ni—P coating is formed at the time of electroless-plating nickel using a P-type reductant, and Ni—B coating is formed at the time of electroless-plating nickel using a B-type reductant. 
     In the method of preparing a nickel-coated nanocarbon using electroless plating according to the present invention, in the step 1), for the purpose of purity improvement, the nanocarbon is washed with ultrasonic waves in an organic solvent or an aqueous acid solution. For example, the nanocarbon is immersed into an organic solvent such as an alcohol or an aqueous acid solution and is then treated with ultrasonic waves, thus removing impurities such as amorphous carbon and the like. 
     Meanwhile, in the step 1), the nanocarbon is thermally oxidized at 400˜600° C. (celsius degree) for 30 minutes˜5 hours under an air atmosphere. Referring to  FIG. 2 , it can be ascertained that when a carbon nanofiber (CNF) was thermally oxidized at 400˜600° C. (celsius degree) for 3 hours under an air atmosphere, the purity of CNF was increased from 87 wt % to 99 wt %. Therefore, the process of washing CNF with a solvent such as an alcohol may be replaced by thermal oxidization treatment. The thermal oxidization treatment is advantageous in terms of economy and environment compared to a washing process. 
     In the step 2), the washed or thermally-oxidized nanocarbon is immersed into a Pd-containing solution to reduce palladium (Pd) ions on the surface of the nanocarbon, thereby forming an activated palladium (Pd) seed on the surface of the nanocarbon. 
     Electroless plating is performed only on the activated surface of the nanocarbon, and the degree of activation of the surface of the nanocarbon influences the adhesivity of an electroless plated layer. 
     Therefore, in the step 2), the washed or thermally-oxidized nanocarbon is immersed into a Pd-containing solution to reduce palladium (Pd) ions on the surface of the nanocarbon, so that an activated palladium (Pd) seed is formed on the surface of the nanocarbon, thereby activating the surface of the nanocarbon. 
     When the nanocarbon is a semiconductive SWCNT or a SWCNT bundle, the method may further include the step of immersing a semiconductive nanocarbon into a tin (Sn)-containing solution to adsorb tin ions (Sn 2+ ) on the surface of the semiconductive nanocarbon and then washing the tin ion-adsorbed semiconductive nanocarbon with water, that is, the step of sensitizing the semiconductive nanocarbon. 
     When the nanocarbon is a CNF, a MWCNT, a TWCNT, a DWCNT or a metallic SWCNT, the step of sensitizing the semiconductive nanocarbon is not required. However, when the nanocarbon is a semiconductive SWCNT or a SWCNT bundle, the step of sensitizing the semiconductive nanocarbon is performed before the step of activating the nanocarbon. 
     In the step 3) which is an acceleration procedure, when the nanocarbon is a metallic nanocarbon (a CNF, a MWCNT a TWCNT, a DWCNT or a metallic SWCNT), the nanocarbon having the Pd seed is treated with a strong acid to deposit purified palladium (Pd). 
     Further, in the step 3), when the nanocarbon is a semiconductive nanocarbon (a semiconductive SWCNT or a SWCNT bundle), a tin (Sn) component remaining on the surface of the nanocarbon after sensitization treatment and activation treatment is removed to deposit purified palladium (Pd). That is, when the semiconductive nanocarbon is sensitized and activated, a reaction of Sn 2+ +Pd 2+ =Sn 4+ +Pd 0  is caused, so that a PD seed is formed and Sn 4+  remains. The remaining Sn 4+  is removed by treating it with a strong acid. 
     In the step 4), the strong acid-treated nanocarbon is immersed into an electroless nickel plating solution to form a nickel plated layer on the surface of the nanocarbon. 
     Even though a Pd catalyst is activated on the surface of the nanocarbon, it must be maintained at a predetermined temperature or more in order to continuously perform an antocatalytic plating reaction, and moreover, a plating reaction rate increases as the temperature increases. The nickel plating solution may be classified into a normal temperature type of nickel plating solution (a reaction is conducted at 40° C. (celsius degree) or lower) and a high temperature type of nickel plating solution (a reaction is conducted at 100° C. (celsius degree) or lower). 
     Further, the plating rate can be controlled by controlling pH. That is, the plating rate increases when pH is higher than 4.8 
     Since plating thickness increases in proportion to plating time, the plating rate is controlled according to the target plating thickness. 
     In the present invention, the step 4) may be performed at  20-˜40 ° C. (celsius degree) for 5˜20 minutes when the electroless nickel plating solution is a normal temperature type of nickel plating solution, and may be performed at 70˜100° C. (celsius degree) for 1˜10 minutes when the electroless nickel plating solution is a high temperature type of nickel plating solution. 
     Further, in the step 4), the pH may be maintained at 4˜6. When the pH is maintained at 4˜6, the electroless nickel plating solution can be more stably maintained, the plating rate becomes high, and the plating efficiency becomes high. 
     In the method of preparing a nickel-coated nanocarbon using electroless plating according to the present invention, the loadage, shape, density and particle size of a metal cast be controlled by controlling the concentration, deposition time, reaction temperature and pH of a plating solution. 
     Plating solutions are classified into a high phosphorus concentration plating solution (phosphorus content: 10˜13%), a middle phosphorus concentration plating solution (phosphorus content: 7˜9%) and a low phosphorus concentration plating solution (phosphorus content: 1˜5%) according to the content of phosphorus. As the content of phosphorus increases, a plating rate decreases, corrosion resistance increases, and heat resistance decreases. 
     According to the present invention, the loadage, shape, density and particle size of Ni—P, Ni—B or Ni can be controlled by controlling process variables such as electroless plating solution concentration, deposition time, traction temperature, pH and the like. 
     In particular, various shapes of Ni—P coating layers or Ni—B coating layers, such as a fibrous Ni—F or Ni—B coating layer, a scalelike Ni—P or Ni—B coating layer, a spherical Ni—P or Ni—B coating layer and the like, can be formed on the surface of the nanocarbon by controlling process variables. 
     As shown in  FIG. 1 , a fibrous coating layer can be formed when a reaction rate is slow under the conditions of a large amount of Pd ions, a low temperature and low pH (reference: 4.8). 
     Further, a scalelike coating layer can be formed when a reaction is rapidly conducted under the conditions of a large amount of Pd ions, a high temperature and high pH (reference: 4.8). 
     Further, a spherical coating layer can be formed under the conditions of a small amount of Pd ions, a high temperature and high pH (reference: 4.8). That is, when the concentration of Pd serving as a seed of nickel plating becomes low, and temperature and pH becomes high, a reaction is rapidly conducted, with the result that nickel ions are collected only on the circumference of Pd, thereby forming a spherical coating layer. 
     Specifically, the step 4) may be conducted under the conditions of a Pd concentration of 0.4˜1 g/L, a nickel plating solution concentration of 5˜10 g/L, a deposition time of 10˜15 minutes, a reaction temperature of 70˜80° C. (celsius degree) and a pH of 4˜5, thus forming a fibrous nickel plated layer. 
     Further, the step 4) may be conducted under the conditions of a Pd concentration of 0.4˜1 g/L, a nickel plating solution concentration of 5˜10 g/L, a deposition time of 5˜10 minutes, a reaction temperature of 80˜100° C. and a pH of 5˜6, thus forming a scalelike nickel plated layer. 
     Further, the step 4) may be conducted under the conditions of a Pd concentration of 0.125˜0.2 g/L, a nickel plating solution concentration of 5˜10 g/L, a deposition time of 5˜10 minutes, a reaction temperature of 80˜100° C. (celsius degree) and a pH of 5˜6, thus forming a spherical nickel plated layer. 
     The nanocarbon provided with a fibrous coating layer is advantageous in that it has high strength, the nanocarbon provided with a scalelike coating layer is advantageous in that it can effectively block magnetic waves and easily adsorb hydrogen because its surface area is large, and the nanocarbon provided with a spherical coating layer is advantageous in that it can be easily used as a catalyst support of a fuel cell. 
     The electroless nickel plating solution includes a main component and a subsidiary component. 
     The main component may be a reductant for reducing a nickel salt or nickel ions into nickel by donating electrons thereto. As the nickel salt, nickel chloride, nickel sulfate, nickel sulfamate or the like may be used. As the reductant, hypophosphite, hydrogenated borate, dimethylamineborane, hydrazine or the like may be used. 
     As the subsidiary component, a complexing agent, a buffering agent, a pH adjuster, a promoter, a stabilizer, an improver or the like may be used. The subsidiary component is added in order to increase the life cycle of a plating solution and improve the efficiency of a reductant. 
     The complexing agent serves to help the stabilization of metal ions by forming metal ions into metal complex ions and thus adjusting the total amount of metal ions participating in a reduction reaction or by retarding the precipitation of meal ions into a metal salt. The kind of the complexing agent is not particularly limited. As the complexing agent, an organic acid such as sodium acetate, ethylene glycol or the like, or salts thereof may be used. 
     The buffering agent is used to decrease the width in change of pH at the time of electroless plating, and the kind thereof is not particularly limited. 
     The pH adjuster is used to prevent the pH change influencing the rate and efficiency of electroless plating and the state of a plating film, and the kind thereof is not particularly limited. As the pH adjuster, ammonium hydroxide, an inorganic acid, an organic acid, caustic soda or the like may be used. 
     The promoter serves to improve the efficiency of metal deposition by accelerating the plating rate, and the kind thereof is not particularly limited. As the promoter, sulfide, fluoride or the like may be used. 
     The stabilizer serves to prevent a reduction reaction from occurring on the site other than the surface to be plated and to prevent a plating bath composition from being naturally decomposed, and the kind thereof is not particularly limited. As the stabilizer, lead chloride, lead sulfide, lead nitride or the like may be used. 
     The improver serves to improve the gloss of a plating film. 
     in the step 5), the nanocarbon provided with the nickel plated layer is heat-treated at a high temperature of 300˜700° C. (celsius degree) for 3 hours under an inert gas atmosphere (Ar, N 2 , He or the like), a vacuum atmosphere (10 −3  tor) or an air atmosphere. 
     The nickel plated layer formed on the nanocarbon, obtained in the step 4), may be an amorphous nickel plated layer. This amorphous nickel plated layer can be converted into a crystalline nickel plated layer by thermally oxidizing the amorphous nickel plated layer. For example, when a nanocarbon is coated with Ni—P or Ni—B by electroless plating, an amorphous Ni—P or Ni—B coating layer formed by immersing the nanocarbon into an electroless nickel plating solution may be converted into a crystalline Ni—P or Ni—B coating layer by heat treatment. 
     Consequently, the method of preparing a nickel-coated nanocarbon using electroless plating according to the present invention is characterized in that pre-treatment, activation treatment, acceleration treatment and then plating treatment are sequentially performed when a nanocarbon is a CNF, a MWCNT, a TWCNT, a DWCNT or a metallic SWCNT, and in that pre-treatment, sensitization treatment, activation treatment, acceleration treatment and then plating treatment are sequentially performed when a nanocarbon is a semiconductive SWCNT or a SWCNT bundle. 
     The method of preparing a nickel-coated nanocarbon using electroless plating according to the present invention is effective in improving the result and reliability of a plating process because electroless plating is performed according to differentiated processes in consideration of electrical properties of nanocarbons. 
     Hereinafter, the present invention will be described in more detail with reference to the following Examples. However, these Examples are set forth to illustrate the present invention, and the scope of the present invention is not limited thereto. The following Examples may be modified and changed within the scope of the present invention by those skilled in the art. 
     EXAMPLES 1 to 3 
     Nickel Coating of CNF 
     CNF (VGCF®-H, manufactured by Showa Denko Corporation) was immersed into an ethanol solution, was treated with ultrasonic waves for 30 minutes, was immersed into a solution including PdCl 2 , HCl and H 2 O, and was then further treated with ultrasonic waves for 10 minutes. Thereafter, the CNF was immersed into a concentrated sulfuric acid solution, was treated with ultrasonic waves for 3 minutes, was immersed into a nickel plating solution including SX-A, SX-M and H 2 O, and was then stirred at 90° C., (celsius degree) for 10 minutes at a rotation speed of 200 rpm to obtain Ni—P-coated CNF. 
     Here, SX-A is a nickel plating solution including nickel in a concentration of 2.138 M, and SX-M is a reducing solution including sodium hypophosphite in a concentration of 2.36 M. 
     The Ni—P-coated CNF was heat-treated at a temperature of 300 to 700° C. (celsius degree) for 3 hours under an air atmosphere. 
     
       
         
           
               
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Pd 
                 Ni 
                   
                 Reaction 
                   
                   
               
               
                   
                 concen- 
                 plating 
                   
                 temperature 
               
               
                   
                 tration 
                 solution 
                 Deposition 
                 (celcius 
               
               
                   
                 (g/L) 
                 (g/L) 
                 time (min) 
                 degree) 
                 pH 
                 Remark 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Ex. 1 
                 0.8 
                 7 
                 12 
                 75 
                 4.2 
                 fibrous 
               
               
                 Ex. 2 
                 0.8 
                 7 
                 7 
                 90 
                 5.5 
                 scalelike 
               
               
                 Ex. 3 
                 0.2 
                 7 
                 7 
                 90 
                 5.5 
                 spherical 
               
               
                   
               
            
           
         
       
     
     Preparation of Crystalline Nickel-Coated CNF 
     The Ni—P-coated CNF obtained from Examples 1 to 3 was heat-treated at a high temperature of 400° C. (celsius degree) for 3 hours under an air atmosphere to prepare crystalline Ni—P-coated CNF. 
     EXAMPLE 4 
     Nickel Coating of SWCNT Bundle 
     A semiconductive SWCNT bundle was immersed into an ethanol solution, was treated with ultrasonic waves for 30 minutes (washing process), was immersed into a solution including PdCl 2 , HCl and H 2 O, and was then further treated with ultrasonic waves for 10 minutes. Thereafter, the semiconductive SWCNT bundle was immersed into a solution including 0.1 mol SnCl 2  and 0.1 mol HCl for several seconds to sensitize the semiconductive SWCNT bundle, and then the sensitized semiconductive SWCNT bundle was immersed into a concentrated sulfuric acid solution, was treated with ultrasonic waves for 3 minutes, was immersed into a nickel plating solution including SX-A, SX-M and H 2 O, and was then stirred at 90° C. (celsius degree) for 10 minutes at a rotation speed of 200 rpm to obtain a nickel-coated SWCNT bundle. 
     Test Example: Evaluation of Characteristics of Nickel-Coated CNFs 
     The SEM analysis and TEM analysis of the amorphous and crystalline Ni—P-coated CNFs obtained from Example 1 to 3 were carried out.  FIG. 3  shows SEM images of the amorphous and crystalline carbon nanofibers of Examples 1 to 3. Amorphous fibrous, scalelike and spherical nickel plated layers formed on the surface of carbon nanofibers are shown in the upper portion of  FIG. 3 . Further, crystalline fibrous, scalelike and spherical nickel plated layers formed on the surface of carbon nanofibers are shown in the lower portion of  FIG. 3 . From the SEM images of  FIG. 3 , it can be ascertained that the surface shape of the amorphous nickel plated layer was changed by high-temperature heat treatment. 
       FIG. 4  shows TEM images of amorphous and crystalline carbon nanofibers of Examples 1 to 3. From the TEM images of  FIG. 4 , the thickness and shapes of the N—P-plated layers laminated on carbon nanofiber can be ascertained by the sectional area thereof. 
     Further, the TGA analysis of the amorphous and crystalline fibrous, scalelike and spherical Ni—P-coated CNFs obtained from Example 1 to 3 were carried out. 
       FIG. 5  shows the results of TGA analysis of the thermal characteristics of the fibrous, scalelike and spherical nickel plated layers of amorphous carbon nanofibers (CNFs). From the results shown in  FIG. 5 , it can be ascertained that the thermal characteristics thereof are changed depending on the shape of the nickel plated layer and that the carbon nanofiber is stable as the distribution of the nickel plated layer becomes wider. 
       FIG. 6  shows the results of TGA analysis of the thermal characteristics of the fibrosis, scalelike and spherical nickel plated layers of the crystalline carbon nanofibers (CNFs), which were prepared by heat-treating the amorphous carbon nanofibers (CNFs) at a high temperature of 400° C. (celsius degree). From the results shown in  FIG. 6 , it can be ascertained that the nickel plate layer becomes thermally stable as it is crystallised. 
     As described above, according to the present invention, variously-shaped metal-coated nanocarbons can be simply and easily prepared in large quantities using electroless plating in consideration of electrical properties of nanocarbons. 
     Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.