Abstract:
A method for generating oxidic nanoparticles from a material forming oxide particles, comprising the steps of: preparation of an aqueous solution containing ions of the material forming the oxide particles, film evaporation of the solution at a temperature above 200° C., and skimming off the nanoparticles floating on the surface of the aqueous solution generated in the vicinity of the vapor film on film evaporation. A device for performing the method is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application represents a National Stage application of PCT/DE2007/001444 entitled “Method for Generating Oxidic Nanoparticles from a Material Forming Oxide Particles” filed Aug. 14, 2007, pending. 
     BACKGROUND OF THE INVENTION 
     The invention relates to the production of oxidic nanoparticles from a material that forms oxide articles, in particular metal-oxide particles, and the formation of a film of such particles on any substrate. 
     Nanostructures that are themselves nanoparticles or are formed by a uniform distribution and/or regular arrangement of nanoparticles on a substrate, are presently at the centre of research. They represent a class of materials that exhibit novel electrical, optical, magnetic, and thermodynamic properties among others due to quantum effects. In addition to the primarily academic issues, the problem of reproducible mass production with means as simple as possible arises in the context, so that the field of commercial utilization of nanostructures can be achieved in a short period of time. 
     Regarding the general state of the art concerning the manufacture of nanostructures, the following printed publications are referred to: D. Rovillain et al.: “ Film boiling chemical vapor infiltration—An experimental study on carbon/carbon composite materials ”, Carbon 39, pp. 1355-1365 (2001); B. J. Urban and C T. Avedisian, W. Tsang: “ The Film Boiling Reactor: A New Environment for Chemical Processing ”, AIChE J. 52(7), pp. 2582-2595 (2006); DE 10 2005 060 407 B3; DE 103 92 447 T5; Dongsheng Wen and Yulong Ding: “ Experimental investigation into the pool boiling heat transfer of aqueous based y - alumina nanofluids ”, Journal of Nanoparticle Research 7, pp. 265-274 (2005) and S. M. You, J. H. Kim, K. H. Kim: “ Effect of nano - particles on critical heat flux of water in pool boiling heat transfer ”, Appl. Phys. Lett. 83(16), pp. 2274-2276 (2003). 
     In particular the production of a uniformly distributed film of nanoparticles on the surface of a substrate has until today been a difficult process that is usually associated with several steps and high costs, in particular if the nanoparticles are furthermore to be arranged in a specific manner. Typical methods for the production of such structures are Vapor Liquid Solid (VLS) or MOCV methods. Although these methods can be employed relatively universally, however both the control of the atmosphere (UHV) and also the necessity for high temperatures (600-1000° C.) require expensive equipment and make the synthesis time-consuming. Pre-structured substrates as for example MEMS cannot be exposed to such high temperatures just like that. 
     To reduce the outlay, the person skilled in the art has knowledge also of wet chemical manufacturing methods from an aqueous solution that lead to the desired results at temperatures below 100° C. and at atmospheric pressure (for example see Law et al., “ Nanowire dye - sensitized solar cells , Nature Materials, 4, 455-459, 2005). However, the wet chemical methods have other disadvantages in addition to proceeding very slowly (process times of several hours up to days). For example, no epitaxial growth on silicon is possible (see J. Phys. Chem. B 2001, 105, 3350-3352). Also in some cases solvents are used that could lead to disposal problems. 
     Today the interest is very high regarding the manufacture of zinc oxide (ZnO) nanostructures such as nanorods and nanotubes. This is the case in particular on account of the fact that ZnO as a semiconductor can form a great variation in terms of nanostructures. On top of this, versatile applications such as opto-electronic components, lasers, field emission and gas sensor materials are envisaged (for the manufacture and application of nanotubes and nanorods see also Advanced Materials 2005, 17, 2477). So that ZnO structures can be produced epitaxially, either special substrates such as gallium nitride (GaN) are used or silicon substrates are coated with a so-called “seeding layer” that usually consists of a ZnO thin film heated to 400° C. 
     The patent application DE 10 2005 060 407 A1 reveals a direct, non-epitaxial production of ZnO structures distributed over the surface of substrates. In the process, metal salts, in particular zinc acetate, are dissolved in water and made to drip on a previously heated substrate (for example a hot plate). The drops evaporate, floating on the steam cushion (Leidenfrost effect), and distributed nanostructures remain on the wetted substrate surface, in particular for example small tufts of ZnO wires. 
     The Leidenfrost effect seems essential for the formation of the nanoparticles from the metal ions that have previously been dissolved in water. For this effect, a very fast reaction kinetics in non-equilibrium at the phase boundary fluid/steam at the bottom side of the drop is assumed to be the cause. The precise nature of the processes there has so far not been explained. 
     Apart from further advantages for example in terms of the directional arrangement of nanowires, the method described in DE 10 2005 060 407 A1, however, also exhibits the disadvantage that it is not suited just like that for coating any substrates. Problems seem to occur with curved, locally recessed or above all angled-off substrate surfaces if the vaporizing drops are to float across. Furthermore, the substrate must be able to withstand temperatures above 200° C. for longer periods of time—it is not to oxidize in the process, which limits the choice of material among others in the field of plastics. 
     It has, however, been pointed out in a publication that appeared recently (Urban and Avedisian, “ The Film Boiling Reactor: A New Environment for Chemical Processing ”, AIChE Journal, 52, 7, pp. 2582-2595 (2006)), that the process of film boiling, that resembles the Leidenfrost effect, makes effective chemical conversions possible. Specifically a so-called FIBOR (“film boiling reactor”) for producing hydrogen gas from methanol is described there. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to specify a method for the fast production of nanoparticles that at the same time permits a very simple application of films of such particles on largely any substrates. 
     According to the invention, the intention is to produce the nanoparticles by chemical conversion of ions dissolved in water, in particular metal ions, at the phase boundary water/steam between the water volume and a surface (for example hot plate) that supplies heat to the water volume. To this extent, the concept of particle production in DE 10 2005 060 407 A1 is used. The realization of this configuration, however, does not take place here by depositing drops of the solution, but by film boiling in a possibly continuously operating reactor. The acronym FIBOR is used for this reactor in the following text. 
     The invention further provides that at least the upper side of the water volume is freely accessible. It has indeed been shown that the nanoparticles formed according to the invention are subject to lift after their production, that preferably takes place at the bottom side of the water volume, and they finally float in large numbers on the water surface. If largely any substrate is immersed into the water volume from the surface provided with the floating particles, they immediately bind to the substrate, so that the substrate is covered with a relatively firmly adhering film of nanoparticles after being pulled out. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the following text, the invention is explained in more detail. The following drawings also serve this purpose, in which: 
         FIGS. 1A-1C  outline the different boiling behavior of water for different values of the temperature difference ΔT=Twall−T—water between a wall that contacts the water and gives off heat energy and the water volume itself; 
         FIG. 2  shows the known “boiling curve” for water, in which the amount of heat, transmitted into the water volume, is plotted versus ΔT; and 
         FIGS. 3A and 3B  show a silicon substrate coated with ZnO particles according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     A FIBOR implements in a manner known per se the continuous film boiling of a liquid. It is known in particular for water that heating the water volume starting from its boundary surfaces proceeds differently as a function of the temperature difference ΔT between the water volume and the heating surface. 
     For ΔT&lt;5 K, the heat transport takes place mainly convectively, steam bubbles forming sporadically now and then on the heated wall, possibly detaching themselves and rising in the water volume and being dissolved again in the process (compare  FIG. 1A ). If ΔT is increased further to values up to 30 K, increased bubble formation takes place little by little, these being formed over a large area, they detach themselves in a rapid sequence and rise to the surface of the water (compare  FIG. 1B ). It can be gathered from  FIG. 2 , that the heat amount dissipated into the water volume increases particularly strongly from the start of the increased bubble formation, until ΔT=30 K is reached. 
     Conventional household stoves are indeed for this reason designed such that the cooktops reach absolute temperatures around 130° C. 
       FIG. 2  likewise shows the finding that the heat transfer into the water volume decreases again for still higher values of ΔT and reaches a minimum at approximately ΔT=200 K. The reason is to be found in the increased evaporation in the immediate vicinity of the heated wall. Thus the steam can no longer be absorbed fast enough by the water volume and carried away, so that an insulating steam cushion forms between the water and the wall across ever larger surfaces. Herein lies the analogy to the known Leidenfrost effect. The minimum heat transfer obviously occurs when the entire heating surface is covered by a steam film, thus largely blocking the convective heat transport. This state marks the onset of film boiling (compare  FIG. 1C ). A FIBOR has the task of initiating the film boiling and of maintaining it through suitable closed-loop control—for example of the heating temperature. 
     The precise implementation of the FIBOR lies within the framework of the expert action. However, it should be stressed that a FIBOR that is advantageously designed for the purpose of the invention should allow free access to the surface of the water volume to carry out the inventive immersion coating of substrates with nanoparticles. In this respect a FIBOR, that is designed as an open trough with a heating device that acts from the bottom side, is particularly suitable for realizing the invention. 
     The following general advantages of the invention should be stressed:
     1. The inventive method does not require any defined atmospheric conditions, and in particular no excess pressure is required. It can be carried out in every laboratory at indoor air.   2. Only energy (heat) and an aqueous solution of metal salts, that can be prepared fast and at short notice, have to be supplied to the FIBOR. Catalysts are not necessarily requisite, and in principle no dangerous by-products are produced.   3. The production of the nanoparticles from the aqueous solution, the floating of the nanoparticles to the water surface and coating a substrate by immersion in the solution only require a total of a few minutes. The method is thus many times faster than other wet-chemical methods where particle growth takes place.   

     The most simple implementation of a suitable FIBOR that can be realized at any time is a simple beaker on a hot plate. Without further measures, such a “beaker reactor” will not be able to sustain a continuous production of nanoparticles. As mentioned above, it is assumed, that these additional measures can be realized by a person skilled in the art. 
       FIGS. 3A and 3B  show electron microscope pictures of a film, manufactured according to the invention, from zinc oxide nanoparticles on a silicon substrate in two amplifications. To prepare the film, at first a laboratory hot plate was preheated to 250-270° C. A 100 ml beaker with an aqueous zinc acetate solution was placed on the preheated plate. Measurements confirm that the temperature of the glass underside corresponded to that of the hot plate after 1-2 minutes. 
     After approximately 10-15 minutes, a film that floated on the water surface developed, and a silicon substrate was dipped slowly and steadily into the solution over the time interval of approximately one minute and pulled out again. During the dipping movement, the film material that previously floated then adhered to the substrate. 
     The pictures in  FIGS. 3A and 3B  prove that they were relatively uniformly distributed nanoparticles of approximately identical size. Their composition was confirmed by means of energy-dispersive x-ray analysis (EDX) and x-ray diffractometry (x-ray diffraction, XRD) as being ZnO particles. 
     If the dipping process is repeated with a second silicon substrate after a few minutes, a film of nanoparticles is formed whose particle size distribution is scattered further than that of the film on the first substrate, the reason for this being on the one hand the agglomeration of the nanoparticles already present and on the other hand the addition of further ions to these particles from the solution, that is, the particle growth proceeds over time. This fact has to be taken into account when implementing the method in a continuously operating FIBOR. 
     Since the substrate is coated with the particle film by dipping in water at atmospheric pressure, in practice the temperature to which the substrate is exposed is limited upwards to approximately 100° C. For this reason, both pre-structured and also many organic substrates can be provided for coating, only with the proviso that the particles produced adhere to them. Of particular interest in this context are substrates of commercially common, chemically inert polymers such as polymethyl methacrylate (PMMA) or any other fluoropolymer (for example Teflon®). In this case, the adhesion of the nanoparticles may well be promoted by a controlled, temperature-dependent softening of the polymers. 
     Here, the example zinc oxide in analogy to DE 10 2005 060 407 A1 is not to be understood in a limiting sense. In this area, research is still at the beginning, but it is clear even today that other particles, too, in particular metal-oxide particles, may be produced in the same way. 
     Titanium oxide is of particular commercial interest (for example as UV absorber). 
     It is possible here that the oxygen for the formation of the oxide particles is being provided by thermal dissociation of water molecules. It can, however, also be imagined (and it has not been clarified so far), that the oxygen dissolved in the solution plays a role for the reaction. If this should be confirmed, monitoring and possibly the closed-loop control of the concentration of the dissolved oxygen would have to be envisaged when setting up a continuous FIBOR. This can be implemented by a simple gas pump that for example draws in the indoor air and conducts it via a tube into the reactor vessel as far as possible into the proximity of the reactive phase limit water/steam. As an alternative, pure oxygen from cylinders can also be introduced. 
     It shall finally be pointed out that film boiling is per se a fixed technical term that is associated with the minimum of heat transfer into the fluid as a starting point. In the case of the present invention, the term film boiling shall also refer to the case of a steam film that is possibly not (yet) continuous (conventionally termed as “transition boiling”), to the extent that it is already able to produce nanoparticles. In view of DE 10 2005 060 407 A1, which is incorporated herein by reference, it is to be assumed that the nanoparticles can be produced already starting at a wall temperature above 200° C., where the classical film boiling does not necessarily have to have started. In view of this, film boiling in the inventive sense is to be understood in a broader meaning.