Abstract:
The present invention provides an approach to control the generation and grow of nanocrystal with membrane diffusion method and related apparatuses to produce inorganic oxide nanopowders and metal nanoparticles. With this method, the size and size distribution of inorganic oxide nanopowders and metal nanoparticles can be tuned. It overcomes the shortcomings possessed by the common chemical and physical method of preparing nanoparticles.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division application of, and claims the priority benefit of, U.S. application Ser. No. 11/777,090 filed on Jul. 12, 2007, which claims the priority benefit of China application No. 200610088817.4 filed on Jul 19, 2006, the contents of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to a process and an apparatus for synthesizing inorganic metal oxide nanopowders and metal nanoparticles (colloid). The nanoparticles can be used as precursors to prepare nanocatalysts and nanomaterials. 
     2. Description of Related Art 
     Nanoparticle is accumulation or cluster of atoms about 1 to 100 nm length and increasingly important material used in various areas ranging from nano-technology, non-line optics, diode laser, smart sensor, information store, gene sequencing to catalysis. During the past decades, a lot of methods have been developed for preparing nanoparticles. For example, Microwave/sonication-assisted Coprecipitation, Sol-Gel Process, Hydrothermal/Solvothermal methods, Templated Syntheses, Revise Microemulsion, Hydrolyzation, and Spray Pyrolysis have been used to synthesize metal oxide nanopowders; Vapor Deposition, Mechanical Attrition, Laser Ablation, Electrochemical Reduction, Radiolysis Reduction, Chemical reduction, and Alcohol Reduction have been employed to prepare metal nanoparticles. However, some methods mentioned above require very expensive equipments, some of them lack the ability in the precise control in the generation and growth of nanocrystals, resulting in the wide distribution of nanoparticle size. In addition, some chemical methods often involve reduction of the relevant metal salts or decomposition of organometallic precursor in the presence of a suitable surfactant that is expensive. 
     In order to control precisely the generation rate and growth of nanocrystal for preparing nanoparticles with narrow size distribution, several new apparatuses and processes have been developed recently for the synthesis of nanoparticles, especially for the synthesis of metal nanoparticle. 
     Microfluidic system has been proven to be an idea medium for nanoparticles production because both mass and thermal transfer are rapid and then the nucleation of solute molecules and growth of nanocrystal can be precisely controlled (Nature, 442, 27 Jul. 2006). Wagner used microchannel reactor to generate Au nanoparticles with the size of 11.7 nm±0.9 nm (Chemical Engineering Journal 101 (2004) 251-260). Although microfluidic method can be used to produce nanoparticles with narrow size distribution and get great attention, it is insurmountable difficult to use it to prepare metal nanoparticles in large-scale. 
     In summary, the available methods of preparing inorganic metal oxide and metal nanoparticles, especially for metal nanoparticles, are very costly and difficult to produce nanoparticles with narrow size distribution in large-scale. 
     Accordingly, there remains a great need for fabricating methods of inorganic metal oxide nanopowders and metal nanoparticles with narrow size distribution. There also remains a need for methods to control growth of inorganic metal oxide nanopowders and metal nanoparticles in the process of mass-production. 
     SUMMARY OF THE INVENTION 
     The present invention provides an approach to control the generation and growth of nanocrystal with a membrane diffusion method and related apparatuses to produce inorganic oxide nanopowders and metal nanoparticles. With this method, the size and size distribution of inorganic oxide nanopowders and metal nanoparticles can be tuned. It overcomes the shortcomings possessed by the common chemical and physical method of preparing nanoparticles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and/or other aspects and advantages of the present apparatus will become apparent and the invention will be better understood by reference to the following description of the embodiments thereof taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic view of an apparatus referring to batch reactor for preparing inorganic metal oxide nanopowders and metal nanoparticles in accordance with an embodiment of the present invention. 
         FIG. 2  is a cross-section view of the batch reactor with ceramic or polymer micro-membrane tube unit installed inside of it. 
         FIG. 3  is a schematic view of an apparatus referring to a tubal reactor for mass-preparing inorganic metal oxide nanopowders and metal nanoparticles in accordance with an embodiment of the present invention. 
         FIG. 4  is a cross-section view of the tubal reactor used for mass-preparing nanoparticles, inside of which a ceramic or polymer micro-membrane tube unit is installed. 
         FIG. 5  and  FIG. 6  are a schematic view of an apparatus referring to a tubal reactor for preparing inorganic metal oxide nanopowders and metal nanoparticles in accordance with an embodiment of the present invention. 
         FIG. 7  is a cross-section view of an apparatus referring to the tubal reactor for preparing inorganic metal oxide nanopowders and metal nanoparticles in accordance with an embodiment of the present invention. 
         FIG. 8  is a schematic view of an apparatus referring to assembled tubal reactors in parallel connection for mass production of inorganic metal oxide nanopowders and metal nanoparticles. 
         FIG. 9  is a SEM (Scanning electron microscope) image of the Ce 0.6 Zr 0.4 O 2  nanomaterial that was prepared by the method described in the invention. 
         FIG. 10  is a SEM image of the Ce 0.6 Zr 0.4 O 2  nanomaterial that was prepared by common coprecipitation method. 
         FIG. 11  is a TEM (transmission electron microscope) image of Ag nanoparticles produced according to the batch reactor used in the present invention. 
         FIG. 12  is a TEM image of Ag nanoparticles produced according to normal chemical reduction method, i.e. the Ag nanoparticles was generated by dropping the NaBH 4  solution into a mixture solution of AgNO 3  and Polyvinyl Pyrrolidone (PVP). 
         FIG. 13  is a TEM image of Au nanoparticles produced according to the batch reactor used in the present invention. 
         FIG. 14  is a TEM image of AuRh alloy nanoparticles produced according to the batch reactor used in the present invention. 
         FIG. 15  a TEM (transmission electron microscope) image of Ag nanoparticles produced according to the tubal reactor used in the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made to the drawings to describe in detail of the apparatus for producing inorganic metal oxide nanopowders and metal nanoparticles according to the present invention. 
     Referring to  FIG. 1  and  FIG. 2 , an apparatus  100  for producing inorganic oxide nanopowder and metal nanoparticles according to the embodiment of the present invention is shown. The apparatus  100  includes a stirrer  1 , a batch reactor  2 , a micro-membrane tube unit  3 , an ultrasonic generator  4 , a container  5  and a measuring pump  6 . The micro-membrane tube unit  3  is installed in the batch reactor  2 . The micro-membrane tube unit  3  is composed of a tube holder  9  made of teflon and a Polyethersulfone membrane tube that is fixed on the tube holder  9 . As an example, the inner and outer diameters as well as the length of a specific Polyethersulfone membrane tube used in one embodiment of this invention are 0.7, 1.3 and 4000 mm, respectively, with the tube wall thickness of 0.3 mm and micro-hole diameter of 0.04 μm on the tube wall. One end of the Polyethersulfone membrane tube is sealed, for example, by epoxy resin and the other is open as inlet  7  that is connected with the measuring pump  6  by a tube line. An oar  8  is installed at the end of the stirrer axis that reaches into the space surrounded by the micro-membrane tube unit  3 . As the apparatus  100  is employed to synthesize inorganic metal oxides nanopowders and metal nanoparticles, the batch reactor  2  is set up with the ultrasonic generator  4 . The measuring pump  6  is linked with container  5  with a tube line. 
     Referring to  FIG. 3  and  FIG. 4 , an apparatus  200  with a tubal reactor  2  for mass-production of inorganic oxide nanopowders and metal nanoparticles according to the embodiment of the present invention is shown. The apparatus  200  includes a stirrer  1 , a tubal reactor  2 , a micro-membrane tube unit  3  which can be made of ceramic or polymer, or other suitable materials, an ultrasonic generator  4 , a container  5  and a measuring pump  6 . The tubal reactor  2  is connected with the ultrasonic generator  4  and has one outlet  11  at one end and two inlets  10  at the other end of the tubal reactor  2 , one inlet  10  is connected to the measuring pump  6 . The micro-membrane tube unit  3  with a stirrer  1  is installed inside of the tubal reactor  2 . The micro-membrane tubes  3  are fixed, for example by epoxy resin, on two seal caps  13  that are used to seal the reactor  2 . One end of the micro-membrane tube is also sealed by the seal cap  13  and the other is open as an inlet  7  that is linked to inlet  10  of the tubal reactor  2  via a cavity  12  and to the measuring pump  6  via the inlet  10 . The measuring pump  6  is connected with container  5  by a tube line. 
     Referring to  FIG. 5  and  FIG. 6 , an apparatus  300  for producing inorganic oxide nanopowder and metal nanoparticles according to the embodiment of the present invention is shown. The apparatus  300  includes a tubal reactor  2 , a micro-membrane tube unit  3 , an ultrasonic generator  4 , two containers  5 , a measuring pump  6  and a peristaltic pump  14 . The tubal reactor  2  has two inlets  10  and one outlet  11 , one of the inlets  10  is connected with the first container  5  and the other inlet  10  is connected to the measuring pump  6  that is linked to the second container  5 . The outlet  11  is linked with a peristaltic pump  14  that is also connected with container  5 . The peristaltic pump  14 , first container  5  and tubal reactor  2  form a loop-way. The micro-membrane tubes  3  are fixed, for example by epoxy resin, on two sealed caps  13  that are used to seal the butal reactor  2 . One end of the micro-membrane tube is also sealed by the seal cap  13  and the other is open as an inlet  7  that is linked to inlet  10  of the tubal reactor  2  via a cavity  12  and to the measuring pump  6  via the inlet  10 . The measuring pump is also connected with the second container  5  by a tube line. 
     Referring to  FIG. 7 , the micro-membrane tube unit  3  comprises micro-membrane tubes that are made of ceramic or polymer materials and located at a circle line around the dummy central axis of the tubal reactor  2 . 
     Referring to  FIG. 4  and  FIG. 7 , the micro-membrane tube unit  3  comprises micro-membrane tubes that are made of ceramic or polymer materials. The micro-holes are formed on walls of the micro-membrane tubes, and the size of the micro-holes on the walls of the micro-membrane tubes is ranged from 0.03 to 0.3 μm. One example of the ceramic micro-membrane tubes is that made of α-Al 2 O 3 . The inner and outer diameters as well as the length of a specific ceramic micro-membrane tube used in one embodiment of this invention are 3, 4 and 200 mm, respectively, with the tube wall thickness of 0.5 mm and micro-hole diameter of 0.04 μm. The polymer micro-membrane tubes can be made of a polymer material selected from the group consisting of polypropylene, polyethermide, polysulfone, Polyethersulfone and polyvinylidene fluoride. 
     Referring to  FIG. 8 , as the butal reactor  2  being employed, an advantage of the invention is that the yield of inorganic oxide nanopowders and metal nanoparticles can be enhanced simply by increasing the number of butal reactors  2  without the negative effect caused by expanding the volume of reactor. 
     Referring to  FIG. 1 , a method of preparing inorganic metal oxide nanopowders and metal nanoparticles is the process described as the following:
     (I) The desired metal precursor(s) is (are) dissolved in distilled water in the presence or absence of a protective agent. The solution of the desired metal precursor(s) is transferred into the batch reactor  2 , the solution of a reductant/precipitator is transferred into the container  5 ;   (II) The solution of the precipitator or reductant stored in container  5  is injected into the lumens of micro-membrane tube unit  3  via the measuring pump  6  from the inlet  7  of the micro-membrane tube unit  3  at a desired flow rate, temperature, stirring speed and a desired supersonic frequency, and then diffuses into inside of the batch reactor  2  via the micro-holes distributed on the wall of the micro-membrane tube unit  3 . In reactor  2 , which contains a solution of a metal precursor with or without protective agent, the precipitation or reduction occurs.   (III) As the invention method is used to synthesize inorganic metal oxides, the precipitation reaction is carried out for 2-8 hours, and then the precipitate is filtered, washed with distilled water, dried in air and calcined at desired temperature for 2-8 hours, giving the product of inorganic metal oxides nanoparticles.   (IV) When the inventive method is used to prepare metal nanoparticles, the reduction reaction is not stopped until 5-20 times amount of reductant as the metal ion amount is injected into the batch reactor  2 . The resulting liquid is a colloid of metal nanoparticles with narrow metal particle size distribution.   

     Referring to  FIG. 3  and  FIG. 4 , a method of preparing inorganic metal oxide nanopowders and metal nanoparticles in mass scale is the process described as the following:
     (I) The desired metal precursor(s) is (are) dissolved in distilled water in the presence or absence of a protective agent. The solution flows via the inlet  10  that is linked with measuring pump  6  into the reactor  2  that is used for mass-preparing inorganic metal oxide nanopowers and metal nanoparticles; the solution of reductant/precipitator is transferred into the container  5 ;   (II) The solution of precipitator/reductant driven by a measuring pump  6  is injected through the inlet  9  and inlet  7  into the lumens of micro-membrane tube unit  3 , at a desired flow rate, temperature, stirring speed and a desired supersonic frequency, and then diffuses into inside of the tubal reactor  2  via the micro-holes distributed on the wall of the micro-membrane tube unit  3 . The reaction occurs immediately.   (III) As the invention method is used to synthesize inorganic metal oxides, the ration time in the tubal reactor for the reactive solution is from 2 to 8 hours, giving the products flowed out from outlet  11 . And then the product (metal oxide precursor nanoparticles) are filtered, washed with distilled water, dried in air and calcined at desired temperature for 2-8 hours, giving inorganic metal oxides nanoparticles.   (IV) When the inventive method is used to prepare metal nanoparticles, the reactive solution flows out from the outlet  11 , at which the concentration of reductant is 5-20 times as that of metal components injected into the tubal reactor  2 , giving a colloid of metal nanoparticles with narrow metal particle size distribution.   

     Referring to  FIG. 5  and  FIG. 6 , the tubal reactor  2  and other related equipments can be set up according to another way. The peristaltic pump  14 , container  5  and tubal reactor  2  form a loop. An aqueous solution of metal precursor(s) with or without protective agent is recycled through the tubal reactor  2  and container  5 , driven by a peristaltic pump  14 . 
     In an embodiment of the invention, for the batch or tubal reactor  2 , flow rate of the solution driven by measuring pump  6  is from 0.2 to 100 ml/min; the rotate speed of the stirrer is from 100 to 200 r/min; the supersonic frequency is from 60 to 120 KHz. 
     The approach of preparing inorganic metal oxide nano-powders and metal nanoparticles can be accomplished in another way: solution of precipitator or reductant is transferred into the reactor  2 . Correspondingly, the solution of metal salts is kept in container  5  and injected by measuring pump  6  into the solution of precipitator or reductant via the micro-holes distributed on the wall of the micro-membrane tube unit  3 . 
     In another embodiment of the invention, the solution of the metal precursor and protective agent is obtained by dissolving inorganic or organic metal salts of rare earth metals, alkaline-earth metals and transition group metals with protective agent in distilled water. 
     In a further embodiment the precipitation reagent are selected form the group consisting of NH 4 OH, NaOH and oxalic acid. 
     In a further embodiment the reductants are selected from the group consisting of NaBH 4 , N 2 H 4 ·H 2 O, N 2 H 4 , formaldehyde, Oxalic acid and Ascorbic acid. 
     The inorganic metal oxide nanopowders or metal nanoparticles prepared by this invention is small in size and uniform in narrow size distribution with low cost and ability in controlling the generation and growth of nanoparticles in the process of crystallization 
     EXAMPLE 1 
     In this experiment, 51.2 g of Ce(NO 3 ) 3 ·6H 2 O and 14.6 g of ZrONO 4  were dissolved in 300 ml distilled water and transferred into the batch reactor  2 . The batch reactor  2  was dipped in an ultrasonic generator  4  at frequency of 60 KHz and temperature of 60° C. The rotate speed of the stirrer  1  is 100 r/min. A desired amount of NH 4 OH solution was injected into the lumens of membrane micro-tube unit  3  at a constant rate of 0.2 ml/min by a measuring pump  6  and then diffused into the mixture solution of Ce(NO 3 ) 3 ·6H 2 O and ZrONO 4  via the micro-holes on the wall of membrane micro-tube unit  3  until the pH=10 of the solution in the batch reactor  2 . The precipitation of metal oxide precursor (hydroxid) occurred, yielding a buff color precipitate. The precipitate was filtered, washed with distilled water, and dried in air at 110° C. for 10 hours, and then calcinated at 550° C. for 4 hours, giving the products of Ce 0.6 Zr 0.4 O 2  nanoparticles with particle size of 10 nm and specific surface area of 108 m 2 /g ( FIG. 9 ). The Ce 0.6 Zr 0.4 O 2  nanoparticles prepared by the method described in the invention were smaller in size with narrow size distribution than the Ce 0.6 Zr 0.4 O 2  solid solution synthesized by common coprecipitate method ( FIG. 10 ). The oxygen storage determined by H 2 —O 2  titration of the former was larger (0.757 mmol/g) than that (0.357 mmol/g) of the later. 
     EXAMPLE 2 
     In this experiment, 0.16 g of AgNO 3  and 60 g of Polyvinyl Pyrrolidone (PVP, molecular weight is 30000) were dissolved in 300 ml distilled water and transferred into the batch reactor  2 . 0.53 g of NaBH 4  was dissolved in 30 ml distilled water and transferred into the container  5 . The batch reactor  2  was dipped in an ultrasonic bath  4  at frequency of 120 KHz and the temperature of 60° C. At same time, NaBH 4  solution was injected into the lumens of membrane micro-tube unit  3  at a constant rate of 1.2 ml/min by a measuring pump  6  and then diffused into the mixture solution of AgNO 3  and PVP via the micro-holes on the wall of membrane micro-tube unit  3 . The rotate speed of the stirrer  1  is 200 r/min. The Ag nanoparticles with size of 5-8 nm ( FIG. 11 ) were produced with uniform size distribution, which is smaller than that produced by common chemical reduction of AgNO 3  ( FIG. 12 ) 
     EXAMPLE 3 
     In this experiment, 0.836 g of HAuCl 4  and 24 g of Polyvinyl Pyrrolidone (PVP, molecular weight is 30000) were dissolved in 500 ml distilled water, and then transferred into the batch reactor  2 . 1.16 g of NaBH 4  was dissolved in 50 ml distilled water and transferred into the container  5 . The batch reactor  2  was dipped in an ultrasonic generator (100 KHz)  4  at the temperature of 50° C. At same time, NaBH 4  solution was injected into the lumens of membrane micro-tube unit  3  at a constant rate of 1 ml/min by a measuring pump  6  and then diffused into the mixture solution of AgNO 3  and PVP via the micro-holes on the wall of membrane micro-tube unit  3 , resulting the Au 3+  reduction occurred. The rotate speed of the stirrer  1  is 150 r/min In the end of this process, the color of the solution turned to be wine-reddish color, giving the Au nanoparticles with quite narrow uniform size distribution. The average size of Au nanoparticles was 3.5 nm ( FIG. 13 ). 
     EXAMPLE 4 
     In this experiment 8.8 g of NaBH 4  was dissolved in 120 ml distilled water. 3.18 g of HAuCl 4 , 3.75 g of RhCl 3  and 139 g of Polyvinyl Pyrrolidone (PVP, molecular weight is 30000) were dissolved in 1000 ml distilled water. The solutions of NaBH 4  and metal salts (HAuCl 4  and RhCl 3 ) with PVP were transferred into the container  5  and batch reactor  2 , respectively. The batch reactor  2  was dipped in an ultrasonic bath at frequency of 80 KHz and the temperature of 40° C. At same time, NaBH 4  solution was injected into the lumens of membrane micro-tube unit  3  at a constant rate of 3.5 ml/min by a measuring pump  6  and then diffused into the mixture solution of metal salts and PVP via the micro-holes on the wall of membrane micro-tube unit  3 , resulting the Au 3+  and Rh 3+  reduction occurred. The rotate speed of the stirrer  1  is 100 r/min In the end of this process, the color of the solution turned to be brown-reddish color, giving the AuRh (Au:Rh=1:1) alloy nanoparticles with quite narrow uniform size distribution. The average size of AuRh (Au:Rh=1:1) alloy nanoparticles was 2 nm ( FIG. 14 ). 
     EXAMPLE 5 
     In this experiment, 1.0 g of NaBH 4  was dissolved in 50 ml DI water (indicated as solution A). 0.2 g of AgNO 3  and 1.2 g of Polyvinyl Pyrrolidone (PVP, molecular weight is 30000) were dissolved in 200 ml distilled water (indicated as solution B). The solution A and B were transferred into the two containers  5  respectively. And then, solution B was recycled through the butal reactor  2  and the container  5 , in which the solution B was stored, at flow rate of 600 ml/min driven by the peristaltic pump  14 . The tubal reactor  2  was dipped in an ultrasonic generator  4  at frequency of 100 KHz and temperature of 40° C. At same time, solution A was inject into the lumens of membrane micro-tube unit  3  at a constant rate of 7 ml min −1  by a measuring pump  6  and diffused into solution B via the micro-holes on the wall of membrane micro-tube unit  3 , resulting the A g   +  reduction occurred. In the end of this synthesis process, the color of the solution turned to be reddish, giving the Ag nanoparticles with quite narrow uniform size distribution. The average size of Ag nanoparticles was 6.5 nm ( FIG. 15 ).