Abstract:
A method for converting a metal with a relative low reduction potential into a metal with a relative high reduction potential without changing its shape is disclosed, which comprises the following steps: providing a first metal substrate and a reaction solution comprising a second metal precursor, a cation surfactant, and a weak reducing agent; and placing the first metal substrate into the reaction solution for a predetermined time to convert the first metal substrate into a second metal substrate. Herein, the reduction potential of a first metal of the first metal substrate is lower than that of a second metal of the second metal substrate, and the shapes of the first metal substrate and the second metal substrate are the same.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefits of the Taiwan Patent Application Serial Number 101139747, filed on Oct. 26, 2012, the subject matter of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for converting a metal with relative low reduction potential into another metal with relative high reducing potential and, more particularly, to a method for converting one metal with relative low reduction potential into another metal with relative high reducing potential without changing the shape of the first metal. 
         [0004]    2. Description of Related Art 
         [0005]    Metal has excellent electric conductivity, ductility and thermal conductivity, and thus is one common material used in various fields. For example, metal is greatly used in circuit designs of semiconductor fields, metal workings, battery productions, and catalyst converting in petrochemical industries. In recent years, metal nano-particles are further applied to catalysts, and biomedical materials for treatment or diagnosis as the development of the nano-technologies. However, some metal materials used in the aforementioned fields are precious metal, such as Au, Pt or Pd, and only a little amount thereof can be obtained on the surface of the earth. Hence, one important issue is to recycle and reuse these metals in wastes. 
         [0006]    Recently, several chemical reactions are known to be used in workings and syntheses of metal materials. One common chemical reaction is galvanic replacement reaction, which is a chemical reaction to convert one metal with relative low reduction potential into another metal with relative high reduction potential. 
         [0007]    However, the shapes of the original reactants cannot be kept after the conventional galvanic replacement reaction is performed. For instance, when Ag in Ag nano-particles is substituted with Au through the known galvanic replacement reaction, the products are hollow Au nano-balls, but not solid Au nano-particles. Hence, it is difficult to maintain the shapes of the original reactants in nano-scale, and further more difficult to maintain the shapes thereof in micro-scale through the known chemical reactions. 
         [0008]    Therefore, it is desirable to provide an improved method, which can convert one metal with relative low reduction potential into another metal with relative high reduction potential without changing its original shape, and be applied to the aforementioned industrial fields. 
       SUMMARY OF THE INVENTION 
       [0009]    An object of the present invention is to provide a method for converting a first metal with relative low reduction potential into a second metal with relative high reducing potential, in which the first metal is replaced by the second metal without changing the original shape of the first metal. 
         [0010]    To achieve the object, the method of the present invention comprises the following steps: providing a first metal substrate and a reaction solution comprising a second metal precursor, a cation surfactant, and a weak reducing agent; and placing the first metal substrate into the reaction solution for a predetermined time to convert the first metal substrate into a second metal substrate, wherein the reduction potential of a first metal of the first metal substrate is lower than that of a second metal of the second metal substrate and the second metal precursor, and shapes of the first metal substrate and the second metal substrate are the same. 
         [0011]    In the conventional galvanic replacement reaction, it is hard to convert a solid reactant object (i.e. the first metal substrate) into a solid product (i.e. the second metal substrate). However, according to the method of the present invention, the rate of removing the first metal and that of filling the second metal can be kept in a balance state by selecting a suitable cation surfactant and a proper weak reducing agent, and therefore the first metal of the first metal substrate can be replaced with the second metal provided by the second metal precursor without changing the original shape of the first metal substrate. Especially, the cation surfactant used in the method of the present invention is one factor for remaining the original shape of the first metal substrate after the reaction is completed. 
         [0012]    In the present invention, the phrases “without changing the (original) shape” or “identical (or the same) shape” mean that the shapes of the reactant object and the product are the same, and the sizes thereof are not different substantially. For example, when the reactant object is a solid sphere, the obtained product is also in a form of a solid sphere; and when the reactant object is a metal plate, and the obtained product is also a metal plate. More specifically, a solid reactant object made of a first metal can be converted into a solid product made of a second metal by using the method of the present invention. In the present invention, the forms of the solid reactant object or the solid product may be present in metal nano-particles, nano-wires, metal films, nano-plates, metal foil, nano-rods, nano-spheres, nano-discs, or metal bulks, but the present invention does not limit thereto. The method of the present invention can obtain a product with not only the same shape of the reactant object, but also the similar size thereof by properly adjusting the amount of the second metal precursor. 
         [0013]    In the present invention, the term “nano-plates” or “nano-discs” means that the objects have a thickness of 1-100 nm, but the diameter or the width thereof are not particularly limited and can be in micro-scale or in nano-scale. Preferably, the thickness of the nano-plates or the nano-discs is 1-50 nm. More preferably, the thickness thereof is 1-30 nm. However, the present invention is not particularly limited thereto. 
         [0014]    In addition, the term “nano-wires” or “nano-rods” means that the objects have a cross-sectional diameter of 1-100 nm, but the length thereof are not particularly limited and can be in micro-scale or in nano-scale. Preferably, the cross-sectional diameter thereof is 1-50 nm. More preferably, the cross-sectional diameter thereof is 1-30 nm. With regard to the length of the nano-rods, it can be 10-100 nm, Preferably, the length thereof is 30-100 nm. However, the present invention is not particularly limited thereto. 
         [0015]    Furthermore, the term “metal foils” or “metal films” means that the objects have a thickness of 0.1 μm-1000 μm, but the length, the width or the diameter thereof are not particularly limited. Preferably, the thickness of the metal foils or the metal films is 1 μm-500 μm. However, the present invention is not particularly limited thereto. 
         [0016]    Moreover, the term “nano-particles” or “nano-spheres” means that the objects have a diameter of 1-100 nm. Preferably, the diameter of the nano-particles or the nano-spheres is 1-50 nm. More preferably, the diameter thereof is 1-30 nm. However, the present invention is not particularly limited thereto. 
         [0017]    In the method of the present invention, the cation surfactant can be any general cation surfactant used in the art. In one aspect of the present invention, the cation surfactant is represented by the following formula (I): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    wherein each R 1 , R 2 , and R 3  independently is C 1-3  alkyl, R 4  is C 12-22  alkyl, and X −  is a halogen ion. 
         [0018]    In the aforementioned formula (I), specific examples of X −  comprise Cl − , and Br − . Preferably, X −  is Br − . Furthermore, each R 1 , R 2 , and R 3  may independently be methyl, ethyl, n-propyl, or isopropyl. Preferably, each R 1 , R 2 , and R 3  independently is methyl or ethyl. More preferably, all R 1 , R 2 , and R 3  are methyl or ethyl. Most preferably, all R 1 , R 2 , and R 3  are methyl. In addition, R 4  can be linear or branch C 12-22  alkyl. Preferably, R 4  is linear C 12-22  alkyl. More preferably, R 4  is linear C 14-20  alkyl. Most preferably, R 4  is linear C 15-18  alkyl. 
         [0019]    In the method of the present invention, a specific example of the cation surfactant shown by the formula (I) can be cetyltrimethylammonium bromide (CTAB), but the present invention is not limited thereto. Herein, the concentration of the cation surfactant used in the method of the present invention may be adjusted according to the components (for example, the second metal precursor) contained in the reaction solution or the shape of the first metal substrate, as long as the rate of removing the first metal and that of filling the second metal can be kept in a balance state 
         [0020]    In the method of the present invention, the materials of the first metal substrate, the second metal substrate and the second metal precursor are not particularly limited, as long as the reduction potential of the first metal of the first metal substrate is lower than that of the second metal of the second metal substrate and the second metal precursor. In this case, the purpose of replacing the first metal with the second metal can be achieved. For instance, the first metal of the first metal substrate can be Ag, and the second metal of the second metal substrate or the second metal precursor can be Au, Pd, or Pt. In addition, according to the materials of the first metal substrate and the second metal substrate to be obtained, suitable metal salts can be selected as the second metal precursor contained in the reaction solution. Fore examples, the second metal precursor can be metal salts of Ag, Pd or Pt. In the method of the present invention, specific examples of the second metal precursor comprise H 2 PtCl 6 , PtS 2 O 7 H 4 , HAuCl 4 , H 2 PdCl 4 , or a combination thereof, but the present invention is not limited thereto. 
         [0021]    In addition, in the method of the present invention, the weak reducing agent in the reaction solution is a reducing agent with reducing capacity lower than that of NaBH 4  or sodium citrate. Herein, a strong reducing agent cannot be selected as the reducing agent used in the reaction solution of the present invention. It is because that the reduction reaction of the second metal precursor is too fast and nano-particles of the second metal may be directly formed in the reaction solution when the reducing capacity of the reducing agent is too strong. In this case, the purpose of replacing the first metal with the second metal cannot be achieved. In the method of the present invention, a specific example of the weak reducing agent is ascorbic acid (AA), i.e. vitamin C; but the present invention is not limited thereto. In addition, a concentration of the weak reducing agent of the present invention can be adjusted according to the shape of the first substrate or each component such as the metal precursor in the reaction solution, as long as the rate of removing the first metal and that of filling the second metal can be kept in a balance state. 
         [0022]    In the method of the present invention, preferably, a molar ratio of the cation surfactant and the weak reducing agent is in a range from 1:1 to 10:1. More preferably, the molar ratio thereof is in a range from 1:1 to 9:1. Most preferably the molar ratio thereof is in a range from 2:1 to 6:1. 
         [0023]    Additionally, in the method of the present invention, preferably, a molar ratio of the second metal precursor and the weak reducing agent is in a range from 1:100 to 1:1. More preferably, the molar ratio thereof is in a range from 1:50 to 1:1. Most preferably, the molar ratio thereof is in a range from 1:10 to 1:2. 
         [0024]    Furthermore, in the method of the present invention, a second metal precursor may be further added into the reaction solution after the replacing reaction is performed for a predetermined time, in the case that the size of the first metal substrate is large. 
         [0025]    Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1A  shows TEM photos of nano-discs formed at different reaction times according to Embodiment 1 of the present invention; 
           [0027]      FIG. 1B  shows detected UV-visible spectra of nano-discs formed at different reaction times according to Embodiment 1 of the present invention; 
           [0028]      FIGS. 2A-2C  are diagrams showing element distributions of nano-discs detected by line-scanning according to Embodiment 1 of the present invention, in which the X axis thereof represents different detection points on the nano-discs; 
           [0029]      FIG. 3A  shows TEM photos of nano-discs formed in reaction solutions with different amounts of HAuCL 4  according to Embodiment 2 of the present invention; 
           [0030]      FIG. 3B  shows a detected UV-visible spectrum of nano-discs formed in reaction solutions with different amounts of HAuCL 4  according to Embodiment 2 of the present invention; 
           [0031]      FIGS. 4A-4D  show TEM photos according to Embodiment 3 of the present invention; 
           [0032]      FIGS. 5A-5C  shows TEM photos of nanoprisms formed at different reaction times according to Embodiment 4 of the present invention; 
           [0033]      FIGS. 6A-6C  are diagrams showing element distributions of nanoprisms detected by line-scanning according to Embodiment 4 of the present invention, in which the X axis thereof represents different detection points on the nano-prisms; 
           [0034]      FIG. 7  shows detected UV-visible spectra of nanoprisms formed at different reaction times according to Embodiment 4 of the present invention; 
           [0035]      FIGS. 8A-8B  are diagrams showing element distributions of metal foils detected by line-scanning according to Embodiment 5 of the present invention, in which the X axis thereof represents different detection points on the metal foils; 
           [0036]      FIGS. 9A-9B  shows SEM photos of metal foils according to Embodiment 5 of the present invention, and the magnification thereof is 500×; and 
           [0037]      FIGS. 10A-10B  shows EDX results of metal foils according to Embodiment 5 of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0038]    The present invention has been described in an illustrative manner, and it is to be understood that the terminology used is intended to be in the nature of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. Therefore, it is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. 
       Embodiment 1 
     Conversion Ag Nano-Discs into Au Nano-Discs 
       [0039]    Ag nano-discs with diameters of 51±2 nm and thickness of 19±2 nm were used in the present embodiment. 
         [0040]    For the growth of An nano-discs, a colloid solution of the Ag nano-discs was diluted into a concentration of 50 ppm. To 1 mL of the colloid solution, 600 μL of 100 mM CTAB solution, 130 μL of 100 mM AA solution and 720 μL of 5 mM HAuCl 4  solution were added therein while stirring. After around 5 min of mixing, the mixture was placed on water bath preheated at 80° C. After 16 min, the mixture was centrifuged at 8000 rpm for 10 min, suspension was discarded and the pellet was washed and purified with saturated NaCl solution to remove AgCl precipitate. Then, the precipitate was washed for twice with de-ionized water before further characterization. 
         [0041]    In the present embodiment, in order to understand the changes of the shapes and when the replacement reaction was completed, an aliquot of 300 μL was removed at 1, 5, 8, 12 and 16 min, purified by washing according to the aforementioned process, and characterized. Here, the changes of the shapes were detected by Transmission Electron Microscopy (TEM), and the results thereof are shown in  FIGS. 1A and 1B . The figures (a), (b), (c) and (d) of  FIG. 1A  respectively represent TEM photos of nano-discs at the reaction times of 0, 1, 8 and 16 min; and  FIG. 1B  shows the normalized UV-visible spectra of nano-discs obtained at the reaction times of 0, 1, 8 and 16 min. 
         [0042]    After characterization of the nano-discs with TEM, it can be found that the Ag nano-discs were converted firstly into hollow shall structure, and then into Au nano-discs, as shown in  FIG. 1A . In addition, according to the UV-visible spectra thereof, it also can be found that the absorption peak of Ag atoms were completely turned into that of Au atoms as the reaction time increased, as shown in  FIG. 1B . 
         [0043]    In addition, five different detection points on a single metal nano-disc were analyzed with High Resolution Transmission Electron Microscopy Energy-dispersive X-ray spectroscopy (HR-TEM EDX) and line scanning to understand the element changes and distributions of the metal nano-discs.  FIG. 2A  shows the result of the element distribution of the nano-disc determined by line-scanning before the replacement reaction was performed. After the replacement reaction was performed for 8 min, the element distributions thereof determined by line-scanning are shown in  FIG. 2B . After the replacement reaction was performed for 16 min, the element distributions thereof determined by line-scanning are shown in  FIG. 2C . According to the results shown in  FIGS. 2A to 2C , it can be found that Ag can be completely replaced with Au as the replacement reaction times increased. 
         [0044]    The aforementioned results confirm that the Ag nano-discs can be completely converted into Au nano-discs in the present embodiment. In addition, as shown in the TEM photos, the obtained Au nano-discs substantially have the same shapes as the original solid shapes of reactant objects (i.e. Ag nano-discs). 
       Embodiment 2 
     Optimization of Amounts of Second Metal Precursor 
       [0045]    The processes and the conditions performed in the present embodiment were the same as those performed in Embodiment 1, except that 46, 185, 555, 720 and 741 μL of 5 mM HAuCl 4  solutions were respectively used herein. The results of the present embodiment are shown in  FIGS. 3A and 3B , wherein figures (a), (b), (c), (d), (e) and (f) of  FIG. 3A  respectively show TEM photos of products obtained by using 0 (i.e. before the replacement reaction performed), 46, 185, 555, 720 and 741 μL of 5 mM HAuCl 4  solutions after the replacement reaction was completed; and  FIG. 3B  shows the normalized UV-visible spectra of products obtained by using 0 (i.e. before the replacement reaction performed), 46, 185, 555, 720 and 741 μL of 5 mM HAuCl 4  solutions after the replacement reaction was completed. 
         [0046]    As shown in  FIGS. 3A and 3B , the optimized addition amount of 5 mM HAuCl 4  solution is 720 μL. If the addition amount thereof is less than 720 μL, the replacement reaction may not be completed. If the addition amount thereof is more than 720 μL, more HAuCl 4  may be reduced, which may cause the size of the products larger than that of the reactant objects. It should be noted that the replacement reaction is considered success in the case that the size of the products is larger than that of the reactant objects. 
       Embodiment 3 
     Replacement Reaction of First Metal Substrates with Different Shapes 
       [0047]    The processes and the conditions performed in the present embodiment were the same as those performed in Embodiment 1, except that nano-particles with different shapes were used in the present embodiment. The TEM photos of the products of the present embodiment are shown in  FIGS. 4A-4D . 
         [0048]    In  FIG. 4A , figure i shows Ag nano-decahedrons; figure ii shows products obtained by performing the replacement reaction for 8 min, in which partial Ag was removed to form Au/Ag hollow decahedrons; and figure iii shows products obtained by performing the replacement reaction for 16 min, in which Ag was completely replaced with Au to form Au decahedrons. 
         [0049]    In  FIG. 4B , figure i shows Au nanorods, in which the length and the width thereof were respectively 39±3 nm and 9±1 nm; figure ii shows Au nanorods coated with Ag (Au NR@Ag), which was obtained by placing Au nanorods of figure i into silver nitrate (AgNO 3 ) solution to form Au nanorods coated with Ag shells having a thickness of 6 nm, wherein the length and the width of the Au NR@Ag were respectively 40±3 nm and 20±3 nm; figure iii shows products obtained from Au NR@Ag by performing the replacement reaction for 8 min, in which partial Ag was removed to form hollow nanorods with Au/Ag alloy shells; and figure iv shows products by performing the replacement reaction for 16 min, wherein Ag was completely replaced with Au, the hollow structures were filled with Au to form Au nanorods, and the length and the width thereof were respectively 40±3 nm and 21±3 nm. From the figures ii and iv of  FIG. 4B , these results indicate that the Ag nanorods coated with Ag, which were served as Ag substrates, can be converted into Au nanorods without changing their shapes by using the method of the present invention. 
         [0050]    In  FIG. 4C , figure i shows Ag nanoprisms with a height of 68±4 nm; figure ii shows products obtained by performing the replacement reaction for 8 min, in which partial Ag was removed to form Au/Ag hollow nanoprisms; and figure iii shows products obtained by performing the replacement reaction for 16 min, in which Ag was completely replaced with Au to form Au nanoprisms. 
         [0051]    In  FIG. 4D , figure i shows Au nanoparticles with a size of 13±2 nm; figure ii shows Au nanoparticles coated with Ag (Au NP@Ag), which was obtained by placing Au nanoparticles of figure i into silver nitrate (AgNO 3 ) solution to form Au nanoparticles coated with Ag shells having a thickness of 6 nm; figure iii shows products obtained from Au NP@Ag by performing the replacement reaction for 8 min, in which partial Ag was removed to form hollow nanoparticles with Au/Ag alloy shells; and figure iv shows products by performing the replacement reaction for 16 min, wherein Ag was completely replaced with Au, and the hollow structures were filled with Au to form Au nanoparticles. From the figures ii and iv of  FIG. 4D , these results indicate that the Ag nanoparticles coated with Ag, which were served as Ag substrates, can be converted into Au nanoparticles without changing their shapes by using the method of the present invention. 
         [0052]    The aforementioned results show that no matter what kinds of shapes (including nano-decahedrons, nanorods, nanoprisms, and nanoparticles) the Ag substrates have, all can be converted into Ag substrates through the method of the present invention. 
       Embodiment 4 
     Conversion Ag Nanoprisms into Pd Nanoprisms 
       [0053]    The processes and the conditions performed in the present embodiment were the same as those performed in Embodiment 1, except that the HAuCl 4  solutions were substituted with H 2 PdCl 4  solutions, and the first metal substrate was Ag nanoprisms. The results are shown in  FIGS. 5A-5C ,  FIGS. 6A-6C  and  FIG. 7 , wherein  FIGS. 5A-5C  respectively present TEM photos of nanoprisms at the reaction times of 0, 3 and 16 min,  FIGS. 6A-6C  respectively show element distributions of five different detection points of nanoprisms detected by line-scanning, and  FIG. 7  shows the normalized UV-visible spectra of nanoprisms obtained at the reaction times of 0, 3 and 16 min. 
         [0054]    The TEM photo and the element distribution of the nanoprisms before the replacement reaction are respectively shown in  FIGS. 5A and 6A . After the reaction was performed for a while (3 min), the TEM photo and the element distribution thereof are respectively shown in  FIGS. 5B and 6B , and these results indicate that partial Ag was replaced with Pd. After the reaction was completed (16 min), the TEM photo and the element distribution thereof are respectively shown in  FIGS. 5C and 6C . According to the results shown in  FIGS. 5A-5C  and  6 A- 6 C, it can be found that all Ag was gradually replaced with Pd as the reaction time increased. 
         [0055]    In addition, according to the UV-visible spectra shown in  FIG. 7 , it also can be found that the absorption peak of Ag atoms were gradually absent as the reaction time increased. These results indicate the reactant Ag was completely replaced with Pd, which does not have specific absorption peak. 
       Embodiment 5 
     Conversion Ag Foil into Au Foil 
       [0056]    Ag foil (99.97%) having area of 4 mm 2  and thickness of 0.005 mm was used in the present embodiment. 
         [0057]    The Ag foil (2 mm×2 mm) was placed into 3.5 ml of 200 mM CTAB solution, and then 1 ml of 200 mM AA solution was added into this mixture. In order to prevent the foil breaks into small pieces when the mixture was sonicated using ultrasonic water bath or stirred using magnetic stirrer bar on a water bath, the mixture was gently shaken using an incubator shaker in the present embodiment. However, the present embodiment only provides a preferred manner, but the present invention is not limited thereto. 
         [0058]    After the mixture was shaken for 30 min at 50° C., 600 μL of 20 mM HAuCl 4  solution was added 100 μL each in every 5 Ii during 2 days. After the reaction was completed, the product was gently washed with de-ionized water for twice before further characterization. 
         [0059]    The appearance and SEM photo (not shown in the figure) show that the Ag foil was completely converted into Au foil. In addition, five different detection points on the metal foil were analyzed with HR-TEM EDX. The results show that Ag elements can be detected at each detection points before the replacement reaction, as shown in  FIG. 8A . After the reaction was completed, no Ag elements were found but Au elements can be detected at the same detection points, as shown in  FIG. 8B . In addition,  FIGS. 9A and 9B  respectively show metal foil before and after reaction, and  FIGS. 10A and 10B  respectively show EDX analysis results of metal foil before and after reaction. According to  FIG. 8A  to  FIG. 10B , these results indicate that the Ag elements in the Ag foil can be replaced with Au elements by using the method of the present invention. 
         [0060]    According to the aforementioned results shown in Embodiments 1 to 5, these results confirm that a metal substrate can be converted into another metal substrate with relative high reduction potential through the method of the present invention. 
         [0061]    Although the present invention has been explained in relation to its preferred embodiment, it is to be understood that many other possible modifications and variations can be made without departing from the spirit and scope of the invention as hereinafter claimed.