Abstract:
A method for producing a complex of metallic nanoparticles and inorganic clay and an organic promoter, wherein the organic promoter is ethanolamine, for example, monoethanolamine (MEA), diethanolamine (DEA) or triethanolamine (TEA). The metallic nanoparticles produced by this method can be stably and uniformly dispersed without adding other reducing agent or dispersant.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a method for producing a complex of metallic nanoparticles and inorganic clay and an organic promoter, and particularly to a method using ethanolamine as the organic promoter. 
         [0003]    2. Related Prior Arts 
         [0004]    It is known that silver nanoparticles (hereinafter abbreviated as AgNPs) are very effective in disinfection for six hundred kinds of bacteria. The AgNPs have higher specific areas than other metallic particles and thus more effective. The AgNPs are lower in toxicity and therefore suitable for massively-produced medicine. 
         [0005]    Currently, the AgNPs can be produced by physical processes or chemical processes. For the physical processes, expensive pieces of equipment are usually needed to achieve high vacuum or electron beams. For the chemical processes including redox reactions, reducing agents for reducing silver ions into silver atoms are needed. The reducing agents can be sodium borohydride (NaBH 4 ), formaldehyde, ethanol, or hydrazine (H 2 N—NH 2 ). 
         [0006]    To control particle sizes, stabilizers such as sodium citrate, glucose, sodium dodecyl sulfate (SDS), polyvinylpyrrolidone (PVP) and dendrimer, can be added. 
         [0007]    Particularly, concentrations of the silver ions have to be controlled in ppm ranges to avoid aggregation which will impair effects of the product. To solve this problem, continuous blending at a high temperature is required and organic solvents or surfactants are added. However, the organic solvents or surfactants may react with AgNPs to decrease surface areas thereof and result in environmental pollutions. Such processes are also difficult to operate and usually take a long time which will raise the costs. 
         [0008]    In addition, dispersants or protective agents are usually added to improve stability and avoid aggregation through electrostatic rejection or steric hindrance. The dispersants can be water soluble polymers, for example, polyvinylpyrrolidone (PVP), polyvinylalcohol (PVA), polymethylvinylether, poly(acrylic acid) (PAA), nonionic surfactants, chelating agents, etc. Inorganic silicates, for example, natural layered clay, can also be used as inorganic dispersants to attract the silver nanoparticles to form complexes. 
         [0009]    To improve the conventional processes, the present invention provides a method which is highly efficient and can be operated at lower temperatures. 
       SUMMARY OF THE INVENTION 
       [0010]    The object of the present invention is to provide a method for producing a stable and uniform complex of metallic nanoparticle and inorganic clay without a need for high temperature or complex processes. 
         [0011]    The method is to have a metal ionic compound, layered inorganic clay and an organic promoter undergo a reduction reaction at 20 to 80 degree C. for 0.5 to 24 hours. The organic promoter has a general formula: 
         [0000]      (HOCH 2 CH 2 ) 3-X N—(R) X  
 
         [0000]    wherein x=0, 1 or 2, R is H, alkyl or alkenyl of C1 to C18. The organic promoter is used to reduce the metal ions into metal atoms which then form metallic nanoparticles. The inorganic clay is preferably silicate clay, and the metallic particles are preferably gold, silver, copper or iron. 
         [0012]    The reduction reaction is preferably performed at 25 to 50 degree C. 
         [0013]    R is preferably methyl, ethyl, butyl or cyclohexanyl. Examples of the promoter include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), monopropanolamine, dipropanolamine and tripropanolamine. Methyl amine ethoxylates, butyl amine ethoxylates or fatty amine ethoxylates can be used, too. The organic promoter preferably has a concentration ranging between 5 wt % and 50 wt %. 
         [0014]    The metallic ions are preferably silver ions, and thus the complex of metallic nanoparticle and inorganic clay is the complex of AgNPs and inorganic clay. Source of the silver ions can be silver nitrate, silver sulfate or silver halide. When silver nitrate is used, the weight ratio of silver nitrate to the inorganic clay preferably ranges between 1:100 and 1:2; and the weight ratio of silver nitrate to the organic promoter preferably ranges between 1:1 and 1:3. The metallic nanoparticles produced preferably have a spherical structure. The concentration of silver nitrate preferably ranges between 0.5 wt % and 5 wt %. 
         [0015]    The inorganic clay can be montmorillonite (MMT), bentonite, laponite, sunthetical mica, kaolinite, talc, attapulgite clay, vermiculite or layered double hydroxides (LDH), preferably having an aspect ratio ranging  10  to  100 , 000 , and more preferably 100 to 1,000. The clay preferably has a structure with a ratio of Si-tetrahedron:Al-octahedron of about 2:1. The cationic exchanging equivalent (CEC) of the clay usually ranges between 0.1 and 5.0 meq/g. The concentration of the clay is preferably controlled between 0.5 wt % and 5 wt %. 
         [0016]    Ionic equivalent of the metallic particles and CEC of the clay preferably have a ratio ranging between 1 and 200. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]      FIG. 1  shows the detailed procedure of the method according to the present invention. 
           [0018]      FIG. 2  shows the photographs of the silver mirrors generated from Comparative Examples 4 to 6 in which no dispersant was added in. 
           [0019]      FIG. 3  shows the TEM images of the products of Examples 1 and 2 of the present invention. 
           [0020]      FIG. 4  shows the UV absorbance of the products of Examples 1 to 8. 
       
    
    
     ATTACHMENT 
       [0021]    Attachment 1 shows detailed procedure of Examples and Comparative Examples is described as follows and the operational conditions. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]    The detailed procedure of the method according to the present invention is shown in  FIG. 1 . In the beginning, cations between the clay layers, for example, Na + , are replaced with Ag + , and negative charges are adsorbed onto the surfaces of the clay. The organic promoter facilitates to reduce Ag + into Ag atoms, which further aggregate into silver nanoparticles (AgNPs). The clay is suitable as carriers of the AgNPs due to its strong adsorption to silver, and because AgNPs do not easily attract to each other. The clay having steric hindrance can further avoid aggregation, and thus the AgNPs can be uniformly dispersed. 
         [0023]    The main materials used in Examples and Comparative Examples of the present invention include:
   1. Monoethanolamine (MEA): an organic promoter having the following structural formula:   
 
         [0000]    
       
                 
         
             
             
         
       
       
         2. Diethanolamine (DEA): an organic promoter having the following structural formulae: 
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         3. Triethanolamine (TEA): an organic promoter having the following structural formulae: 
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         4. Ethylenediamine (EDA): replacing the above organic promoters and used in Comparative Examples, having the following structural formulae: 
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         5. Diglycolamine (DGA): replacing the above organic promoters and used in Comparative Examples, having the following structural formulae: 
       
     
         [0000]    
       
                 
         
             
             
         
       
       
         6. Laponite: layered silicate clay, having a structure with a ratio of Si-tetrahedron:Al-octahedron of about 2:1, CEC=0.69 meq/g, and acquired from Far Eastern Trading Co. 
         7. Silver nitrate: for replacing sodium ions between the clay layers and for reduction and aggregation into AgNPs; silver sulfate can be used, too. 
       
     
         [0031]    Detailed procedure of Examples and Comparative Examples is described as follows and the operational conditions are shown in Attachment 1. 
       Example 1 
       [0000]    
       
         (a) A solution of AgNO 3  (1 wt %, 3.51 g) was slowly added into a solution of Laponite (1 wt %, 30 g), Ag + /CEC=1.0, and the solution became slightly yellow after being mixed for 30 seconds. 
         (b) The slightly yellow solution was continuously mixed in a water bath (45 to 50 degree C.). 
         (c) A solution of DEA (10 wt %, 0.5 g) was slowly added into the above solution and continuously mixed for 2 to 3 hours. The solution gradually became darker and finally a dark red-brown viscous liquid containing AgNPs was obtained. The liquid was then identified by UV absorption analysis. 
       
     
       Example 2 
       [0035]    Repeat the procedure of Example 1, except that the water bath of step (b) was maintained at room temperature (about 25 to 30 degree C.) and the solution of step (c) was mixed for 5 to 6 hours. The solution containing AgNPs was also identified by UV absorption after being kept still for three days. 
       Example 3 
       [0036]    Repeat the procedure of Example 1, except that the content of DEA of step (c) was changed to 1 g, and continuously mixed for 1.5 to 2 hours. The solution containing AgNPs was also identified by UV absorption. 
       Example 4 
       [0037]    Repeat the procedure of Example 2, except that DEA of step (c) was replaced with MEA (10 wt %, 0.5 g), and continuously mixed for 5 to 6 hours. During the reaction, the solution became white and then darker. After being kept still for three days, the solution became red-brown and slightly viscous. The solution containing AgNPs was obtained and then identified by UV absorption. 
       Example 5 
       [0038]    Repeat the procedure of Example 1, except that DEA of step (c) was replaced with MEA (10 wt %, 0.5 g), and continuously mixed for 4 to 5 hours. During the reaction, the solution became darker. Finally a red-brown and slightly viscous solution containing AgNPs was obtained and identified by UV absorption. 
       Example 6 
       [0039]    Repeat the procedure of Example 1, except that DEA of step (c) was replaced with TEA (10 wt %, 1.0 g), and continuously mixed for 1.5 to 2 hours. During the reaction, the solution became darker. Finally a dark red-brown and slightly viscous solution containing AgNPs was obtained and identified by UV absorption. 
       Example 7 
       [0040]    Repeat the procedure of Example 1, except that DEA of step (c) was replaced with TEA (10 wt %, 0.5 g), and continuously mixed for 2 to 3 hours. During the reaction, the solution became darker. Finally a dark red-brown slightly viscous solution containing AgNPs was obtained and identified by UV absorption. 
       Example 8 
       [0041]    Repeat the procedure of Example 2, except that DEA of step (c) was replaced with TEA (10 wt %, 0.5 g), and continuously mixed for 5 to 6 hours. During the reaction, the solution became darker. Finally a dark red-brown and slightly viscous solution containing AgNPs was obtained and identified by UV absorption. 
       Comparative Example 1 
       [0042]    Repeat the procedure of Example 1, except that DEA of step (c) was replaced with EDA (10 wt %, 1.0 g). During the reaction, the solution became white and then yellow. After being mixed for three hours and being kept still, the solution remained yellow and precipitated and no AgNPs were generated. 
       Comparative Example 2 
       [0043]    Repeat the procedure of Example 1, except that DEA of step (c) was replaced with DGA (10 wt %, 1.0 g). During the reaction, the solution became white and then yellow. After being mixed for three hours, the solution remained yellow and no AgNPs were generated. 
       Comparative Example 3 
       [0000]    
       
         (a) A solution of AgNO 3  (1 wt %, 3.51 g) was slowly added into a solution of Laponite (1 wt %, 30 g), Ag + /CEC=1.0. After being mixed for 30 seconds, a slightly yellow mixture was generated. After being continuously mixed at room temperature for 24 hours, the mixture became slightly dark. 
         (b) The slightly yellow mixture was placed in a water bath (80 degree C.) and continuously mixed for 3 hours. A dark purple gel was obtained but no silver mirror or AgNPs occurred. 
       
     
       Comparative Example 4 
       [0000]    
       
         (a) A solution of AgNO 3  (1 wt %, 1.14 g) was added into water (9.9 g). 
         (b) A solution of DEA (10 wt %, 0.4 g) was slowly added into the solution of step (a), and then mixed by ultrasonic oscillation for 1 hour. No AgNPs were generated. 
       
     
       Comparative Example 5 
       [0048]    Repeat the procedure of Comparative Example 4, except that DEA of step (b) was replaced with TEA. No AgNPs were generated. 
       Comparative Example 6 
       [0049]    Repeat the procedure of Comparative Example 4, except that DEA of step (b) was replaced with MEA. No AgNPs were generated. 
       Analysis of Products 
     1. Silver Mirrors 
       [0050]    The photograph of  FIG. 2  showed the silver mirrors wherein the middle sample and the right sample were for Comparative Examples 4 and 5, respectively. The left sample was the slight silver mirror phenomenon of Comparative Example 6. 
       2. Particle Size Analysis 
       [0051]    The products (0.5 g for each) of Examples 1 to 8 were respectively dropped on glass substrates (1×1 cm 2 ), and then dried in an oven at 80 degree C. for 2 hours. The dried substrates were plated with carbon for particle size analysis. Table 1 showed that all of the particles were in nano scale and ranged between 5 nm and 250 nm. 
       3. TEM Analysis 
       [0052]    Silver particles obtained in Examples 1 and 2 were analyzed with the TEM (transmission electron microscopy).  FIG. 3  showed the results wherein images a and b were for Examples 1 and 2, respectively. The particles were small and dispersed uniformly on the clay. 
       4. UV Analysis 
       [0053]    In Examples 1 to 8, MEA, DEA and TEA were used as reducing agents to reduce silver ions into the AgNPs. These nanoparticles were then diluted and analyzed with UV absorption.  FIG. 4  showed peaks around 400 nm, and Table 1 also showed such results. For Comparative Examples 1 and 2, no significant peak was observed and the colors of the products were different from others. Compared with the structures of EDA and DGA, that of DEA includes N and OH respectively on the C—C chain to form a five-membered ring with silver ions through steric orientation. Such intermediate product easily combined with silver ions to form AgNPs. 
       5. Stability Analysis 
       [0054]    The product generated in the above preferred embodiments could be further heated to remove water to obtain AgNPs in the form of powders. By adding water into the powders, the original state of the product could be recovered. That is, these particles performed good stability. In addition, water and clay were used in the present invention as respectively the reacting media and the dispersant, and no organic solvents were involved. Therefore, environmental pollution could be avoided. 
         [0055]    In the present invention, small amounts of the promoters are able to cause good reductive effects, and the AgNPs can be generated without expensive equipment. Sources of the silver ions are not limited to silver nitrate. AgBrO 3 , AgBr, AgCl, AgClO 3 , etc. can be used as sources of the silver. 
         [0056]    Compared with the conventional processes, the method of the present invention can be carried out faster at moderate temperatures (25 to 50 degree C.); and the AgNPs (ranging between 5 nm and 250 nm in particle size) can be stably dispersed in a water solution containing inorganic clay. As no organic dispersant is used, the AgNPs can be easily further prepared at high concentrations (1 to 10 wt %), in the form of gel or solid powders. 
         [0000]    
       
         
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Attachment 1 
               
             
          
           
               
                   
                   
                 Reducer/ 
                 Reaction 
                 Reaction 
                   
                   
                 Particle 
               
               
                 Example/ 
                 Reducing 
                 AgNp/clay 
                 temperature 
                 time 
                 Color of 
                   
                 size 
               
               
                 Comparative Example 
                 agent 
                 (w/w/w) 
                 ( ) 
                 (hours) 
                 the solution 
                 UV (nm) 
                 (nm) 
               
               
                   
               
               
                 Example 1 
                 DEA 
                 13.5/5.9/80.6 
                 45~50 
                 2~3 
                 Dark red-brown 
                 402 
                  5~10 
               
               
                 Example 2 
                 DEA 
                 13.5/5.9/80.6 
                 25~30 
                 5~6 
                 Dark red-brown 
                 405 
                 10~20 
               
               
                 Example 3 
                 DEA 
                 23.7/5.2/71.1 
                 45~50 
                 2 
                 Dark red-brown 
                 401 
                 250 
               
               
                 Example 4 
                 MEA 
                 13.5/5.9/80.6 
                 25~30 
                 5~6 
                 Red-brown 
                 405 
                 20~25 
               
               
                 Example 5 
                 MEA 
                 13.5/5.9/80.6 
                 45~50 
                 4~5 
                 Red-brown 
                 411 
                 35~45 
               
               
                 Example 6 
                 TEA 
                 23.7/5.2/71.1 
                 45~50 
                 2 
                 Dark red-brown 
                 404 
                 30~40 
               
               
                 Example 7 
                 TEA 
                 13.5/5.9/80.6 
                 45~50 
                 2~3 
                 Dark red-brown 
                 410 
                  5~10 
               
               
                 Example 8 
                 TEA 
                 13.5/5.9/80.6 
                 25~30 
                 5~6 
                 Dark red-brown 
                 411 
                 70~80 
               
               
                 Comparative Example 1 
                 EDA 
                 23.7/5.2/71.1 
                 45~50 
                 3 
                 Yellow 
                 No peak 
                 — 
               
               
                 Comparative Example 2 
                 DGA 
                 23.7/5.2/71.1 
                 45~50 
                 2 
                 Lightly yellow 
                 No peak 
                 — 
               
               
                 Comparative Example 3 
                 none 
                   0/7/93 
                 25~30 
                 27  
                 Dark purple 
                 No peak 
                 — 
               
               
                 Comparative Example 4 
                 DEA 
                 77.8/22.2/0 
                 25~30 
                 1 
                 Silver mirror 
                 No peak 
                 —