EP1920082A2

EP1920082A2 - Substrate with spatially selective metal coating method for production and use thereof

Info

Publication number: EP1920082A2
Application number: EP06775802A
Authority: EP
Inventors: Wolfgang Pompe; Michael Mertig; Alexander Kirchner; Nina Schreiber; Anja BLÜHER; Steffen Roos; Daniela Keck; Beate Katzschner
Original assignee: Hofinger Juergen
Current assignee: Namos GmbH
Priority date: 2005-07-29
Filing date: 2006-07-29
Publication date: 2008-05-14
Also published as: KR20080041673A; US20090124488A1; WO2007012333A3; BRPI0614239A2; CN101273156B; DE112006002640A5; WO2007012333A2; CA2620514A1; CN101273156A; JP2009502456A

Abstract

The invention relates to a substrate with spatially selective metal coating and method for production thereof wherein the regions of metal coating on the substrate can be influenced. The invention further relates to the use of such substrates for catalysts, solid body electrolyte sensors or optical, transparent conducting layers. Said substrate with spatially selective metal coating the surfaces of which partly comprise biological templates with a metallic coating may be obtained by carrying out the metallic coating after deposition of the biological templates on the substrate.

Description

Substrate with spatially selective metal coating, process for its preparation and its use

The invention relates to substrates with spatially selective metal coating, to processes for their production, wherein the locations of the metal coating on the substrate can be influenced. Furthermore, the invention relates to the use of such substrates for catalysts, solid-state electrolyte sensors or optically transparent conductive layers.

Catalysts are substances which reduce the activation energy to the end of a particular reaction, thereby increasing the reaction rate without being consumed in the reaction. As catalysts colloidal metals are known, which are prepared by reduction of metal salts or metal complexes.

On the one hand, the size, the type and distribution of the metallically active clusters have an essential influence on the activity of noble metal catalysts, but on the other hand their accessibility within the support structures.

From Sleytr. et al. WO 89/09406 has patented a process for the immobilization or deposition of molecules or substances on a carrier. The support consists of at least one layer of identical protein-containing molecules, which are arranged in the form of a crystal lattice with a lattice constant of 1 to 50 nm.

WO 97/48837 describes metallic nanostructures based on self-assembled, geometrically highly ordered proteins and a process for their preparation. The assembled proteins are activated with a metal salt or metal complex and can then be electrolessly metallized in a metallization under conditions compatible with proteins.

Also by Sleytr et al. For example, AT 410 805 B describes a method for depositing S-layer proteins, in which the S-layer proteins have a net electrical charge, and establishing an electrical potential difference between the solution and the carrier surface by adjusting the electrical potential of the carrier surface under which effect the S-layer proteins from the solution accumulate on the carrier surface. For the selective coating of surfaces with precious metals, publications are known which relate to applications in the field of microelectronics. Thus, DE 692 31 893 T2 describes a process for electroless metallization in which a selective deposition of metals takes place by pretreating the substrate with chemical groups.

By contrast, DE 199 52 018 C1 describes a process in which substrates decorated in the nanometer range are produced. The method is based on the positioning of polymeric core-shell systems in wells of a photoresist layer structured by lithographic techniques.

All of the techniques described in the literature achieve selective deposition of metals onto surfaces either serially through a writing or positioning operation using a positionable device or via masking techniques. Serial processes are very slow, especially in the production of small structures and therefore too expensive for many applications. In masking methods, prefabricated patterns, e.g. using lithographic masks or stamping techniques transferred to the surface and thus used multiple times. However, both the serial and cover methods require surface accessibility for the patterning process.

DE 199 30 893 B4 discloses the use of highly ordered proteins which are occupied by clustered clusters of a catalytically active metal as carrier-fixed catalyst for chemical hydrogenations, in which the clusters occupied proteins remain unchanged. The highly ordered proteins serve as carriers on which metallic clusters are deposited in more or less regular form, that is, a structuring of the clusters is achieved in the best case by the regular structure of the self-assembled proteins. Use of the proteins for the selective deposition of the metallic clusters on the underlying substrate by incomplete coverage and thus the prevention of metal deposition in the undesired locations is not disclosed.

DE 102 28 056 A1 includes a method for creating nucleation centers for the selective heterogeneous growth of metal clusters on DNA molecules. The DNA molecules are metallized in an aqueous solution in the presence of metal salts and reducing agents. The nucleation centers act as a particularly good template, so that with a suitable process control the homogeneous nucleation of metal clusters in the solution can be prevented. There are no other ones Support materials in the solution, which may also come as nucleation in question. In particular, the DNA molecules are not deposited on carrier surfaces prior to metallization. The selectivity of the deposition thus relates to the suppression of homogeneous nucleation as well as the possibility of partial metallization of the DNA molecules by influencing the base sequences of the DNA.

New applications of catalytic processes, e.g. in fuel cell technology and ever greater challenges in the effectiveness of catalytic processes have led to the development of new catalyst support. These have a more or less controlled internal microstructure and thus bring the gases and liquids to be catalyzed into intensive contact with the catalytically active centers of the catalyst. However, by far not all metal clusters deposited on the carrier are equally active. Neither are all deposited clusters readily accessible for the gases or liquids to be catalyzed. Due to the high price and the expected shortage of precious metal resources, a better use of the precious metals in catalysts would therefore be desirable.

The object of the invention is therefore to provide substrates with a spatially selective metal coating and method for their production in which the locations of the metal coating on the substrate can be influenced.

According to the invention the object is achieved by a substrate with spatially selective metal coating, the surface of which partially has biological template with a metallic coating and which is obtainable in that the metallic coating takes place only after deposition of the biological template on the substrate.

The metal coating is according to the invention on the biological template.

In an advantageous embodiment of the invention, the biological template surface layer proteins (S-layer).

The metallic coating may consist of metal clusters and / or at least one metal layer. Metal clusters and metal layers can be made of different materials Consist of metals. As metals are preferably noble metals, such as. Ex. Pt, Pd used.

The substrate preferably consists of Al ₂ O ₃ , silicon, carbon, or a solid state electrolyte.

According to the invention, the object is achieved by a method for producing a substrate with a spatially selective metal coating, in which biological templates are deposited on the substrates and then metallized under compatible conditions for the biological template or activated in the biological template in metal salt solution, then on the substrates deposited and then metallized under conditions compatible with the biological template.

According to the invention, the metal coating is not carried out directly on the substrates, but on biological templates with which the substrates are previously coated. Due to their selectable size and chemical or physical properties, the biological templates allow control of the deposition site. According to one embodiment of the invention, the biological template can be activated prior to deposition on the substrate surface in metal salt solution. As a result, the effectiveness of the nucleation centers of the biotemplate is increased even before the substrate is coated, and the metallization process on the substrate can be accelerated. The activation is achieved by mixing a suspension of Biotemplate with a Metallaltzlösung over several hours.

Self-organizing biological templates, especially surface layer proteins (S-layer), are preferred as biological templates.

Many bacteria form periodic protein membranes in their cell walls. In them nanopores with species-dependent crystal symmetry are arranged with great regularity. The spacing of adjacent equal morphological units is 5 to 30 nanometers, depending on the species. Since the structural units are composed of identical proteins or glycoproteins, they have a precise spatial modulation of the physico-chemical surface properties. This makes it an ideal object for the construction of artificial supramolecular structures. On them can be arranged regularly nanometer-sized metal cluster arrays are generated. The ability to self-assemble the monomers makes it possible to reconstitute the two-dimensional protein arrangements at the water-air interface on solid surfaces as large-area protein membranes. This makes it possible to deposit metallic nanostructures in a defined manner on the surfaces of catalyst supports or sensors with the aid of the S-layer.

Precious metals are preferably deposited as metals. As a method of metal deposition of the metallic clusters on a biological template, electroless metallization is preferred. This metal complexes are bound to a surface and reduced by a subsequent process to metals and formed metal cluster.

According to the invention, first the biological template on the substrate, for. For example, one suitable for catalysts, deposited. The biological templates then act as nuclei for preferential deposition of noble metal clusters on their surface, since the deposition of metal on the template is energetically favored over direct deposition on the substrate. Selective deposition of the membrane at the sites preferred for catalysis can thus lead, with suitable process control, to an exclusive deposition of catalytically active noble metal clusters in the optimum form for the desired catalytic reaction on the substrate.

In a further embodiment, metal complexes are already bound to the membrane-like structures in a metal salt solution. After controlled deposition to the desired locations on the substrate, the metal complexes are reduced by suitable processes to metallic clusters.

When depositing biological templates on substrate surfaces provided with meso- or nanopores, the deposition can be controlled on the basis of the size and structure of the biological template, so that accessible or effective centers are created in the subsequent metal coating for the catalysis. The diffusion of noble metal complexes and the deposition of noble metal clusters in greater depths of the porous substrate is not advantageous because of the low accessibility for the gases or liquids to be catalyzed. The selective metal deposition on the biological template prevents the formation of ineffective metal clusters and thus the uncontrolled loss of expensive precious metal resources. In a further preferred embodiment, the biological template has a nanostructure with respect to its property as a nucleating agent and with respect to its geometric shape, which supports a homogeneous and dense arrangement in a narrow size distribution in the process of depositing the metallic clusters.

According to the invention, biological templates are used to cover the surface. In contrast to the previously known structuring methods, other techniques can be used for selective deposition:

• Due to the influence of the adsorption of biotemplates in solution on surfaces, a selective coverage by locally different flow conditions can take place. Thus, when flowing through complex carrier structures with inner surfaces, a selective coating of areas covered by the flow can take place to a different extent. With a correspondingly low concentration of the biomolecules in solution, complete coverage of the surfaces with biotemplates can be achieved by increasing the throughflow velocities and / or by providing a longer flow through the strongly flown areas. In the weakly traversed areas, however, much less biomolecules are offered from the solution at the same time, so that deposition takes place to a much lesser extent. In the case of customary dip coatings, an opposite effect is observed, since the coating solution keeps particularly well during drying, especially in areas with low flow.

• Depending on the size of the biotemplate or its aggregates, the penetration into pores of the surface to be coated may be hindered. The biotemplates are then selectively deposited only on the surface or in pores above a certain size. Biomolecules have a defined structure and are therefore present in an equally defined size. In addition, the size of the biomolecules can be controlled by the formation of aggregates. In this case, it is possible to control the number of biomolecules involved, in order in turn to produce a defined size.

• Specific binding mechanisms of biological templates can be used to achieve a variation of chemical and / or physical properties on material surfaces for a selective deposition. The direct deposition of metallic clusters, however, is much more unspecific. • The deposition of biological templates can be controlled by electric fields. This effect can also be used selectively for a selective coverage of the surface with biotemplates. In contrast to the metallic clusters, biomolecules can use different surface charges to achieve preferential deposition on areas of the substrate surface which have opposite charge and thus cause electrostatic attraction. Likewise, a charge on the substrate surface with the same sign thus prevent deposition. For example, a varying surface charge can be achieved very easily by means of a geometric structuring of a charged surface. The charges then concentrate on local corners and edges.

Usually, in the chemical coating of surfaces with metals via reduction, the formation of clusters takes place both in the solution (homogeneous nucleation) and on the substrate to be coated. It is known that with the help of a suitable pretreatment of surfaces and appropriate process control, the homogeneous nucleation can be largely suppressed. The formation of the metallic clusters then takes place exclusively on the surface and leads to their more or less uniform coverage. However, not only homogeneous nucleation in the solution can be prevented by the selective coverage of the surface with biological templates according to the invention, but also the occupancy of adjacent, biotemplate-free areas with metallic clusters. Only then is the selective coating of the surface with biotemplates transferred into a selective coating with metallic clusters or layers.

This effect does not occur in previous substrates for catalysts and was therefore not expected.

An essential feature of the invention is the avoidance of the deposition of metals in places where this does not require the application or the application is detrimental. Examples of this are the noble metal catalysis, in which the deposition of precious metals, which do not participate in the catalytic reaction, represent a significant cost factor as well as sensor surfaces, where the sensory effect is created only by a structuring of the layer. Both metal clusters and / or metal layers can be deposited on the biological templates. Metal clusters and metal coatings can be made of different metals. Preference is given to precious metals, such as. Eg platinum, palladium.

The deposition of metallic clusters is always the first step in coating. A continued cluster deposition first leads to the mutual contact of an increasing number of clusters, so that finally closed layers are formed. Once a continuous conductivity is achieved, the process can also be continued with electrochemical coating techniques. If the clusters deposited in the first step consist of sufficiently noble metals, the further coating can also be mixed with other metals such as e.g. Nickel, cobalt or copper can be continued. For this purpose, methods of electroless metallization according to the prior art are used.

Substrates of Al ₂ O ₃ , silicon, carbon, a solid electrolyte or a transparent, electrically conductive layer are used as substrates for the process.

The invention also includes the use of the substrates according to the invention for catalysts, solid-state electrolyte sensors or optically transparent, electrically conductive layers.

Heterogeneous catalysts consist of a carrier through which the gases or liquids to be catalyzed flow. The carrier consists of a catalytically active material or is coated in the case of noble metal catalysts with particles of the catalytically active noble metal. In contrast to a closed metallic coating, the deposition of fine clusters, typically in the range of 1 to 50 nm, offers the advantage of a larger surface area with the same noble metal volume used. In order to increase the surface further, it is customary to carry out the deposition of the metallic clusters on an intermediate carrier, which usually also takes the form of particles and is deposited on the actual carrier as a coating. This intermediate carrier has a large inner surface (eg gamma alumina or activated carbon). Thus, much more precious metal particles can be deposited thereon than on the actual carrier surface, so that the catalytic activity is increased. However, the penetration of the metal salt solution into the pore structure of the intermediate carrier is relatively uncontrolled. A significant proportion of the total porosity of these materials, however, accounts for very small pores. Due to the high flow resistance is thus a contact of the difficult to catalyze gases or liquids in the application or even impossible. In this case, the use of biological template according to the invention makes it possible to select the precipitation sites on the basis of the template size. The subsequent deposition of the metallic clusters on the biological structure thus prevents the uncontrolled loss of expensive precious metal resources with constant catalytic activity.

The coating according to the invention makes it possible to provide substrate surfaces with a high proportion of three-phase boundary surfaces (metal coating / substrate gas phase / liquid phase)). Such substrates are suitable for solid-state electrolyte sensors.

The substrates according to the invention are also suitable for optically transparent, electrically conductive layers, for. Eg displays. On optically transparent conductive substrates to biological templates are deposited, which are then metallized. In the construction of displays, layers are required that can dissipate electrical charges. Naturally, however, these layers must at the same time have a high optical transparency in order not to impair their optical function. Likewise, there are many applications for coating non-conductive substrates in which the reduction of the electrostatic charge is advantageous. At the same time, however, the appearance should not be changed.

With reference to accompanying drawings embodiments of the invention will be explained in more detail. Show

1 REM image of a substrate according to embodiment 1

Fig. 2 DSC diagram

Fig. 3 DSC diagram

Embodiment 1

Targeted occupation of surfaces with metal clusters by controlled coating with biological templates (S-layer patches of Bacillus sphaericus NCTC 9602).

The preparation of the S-layer is based on the publication by Engelhard H .; Saxton, W .; Baumeister, W., "Three-dimensional structure of tetragonal surface layer of Sporosarcina urea, J. Bacteriol. 168 (1), 309, 1986. The standard buffer for storage at 4 ° C of the isolated and purified S-layer consists of a 50 mM TRIS / HCl solution, with the addition of 3 mM NaN ₃ and ImM MgCl ₂ .

The S-layer solution for all further experimental work has a standard concentration of 10 mg / ml.

A 3 mM K ₂ PtCl ₄ solution prepared at least 24 h beforehand is mixed with 13 μl of the protein solution in accordance with the calculations for occupying the protein with metal clusters. The interaction between S-layer solution and metal complex solution takes place in a time of 24 h and with light termination. After this incubation period, the number of metal complexes required for clustering is already attached to the template. After addition of the Al ₂ O ₃ particles as a substrate and a again 24 h lasting adsorption time, in which the activated biomolecules attach to the substrate, the substrate material is removed from the solution and subjected to several washing steps. The subsequent addition of hydrazine as a reducing agent to the coated substrate reduces the bound metal salt complexes to noble metal clusters.

The thus produced, inter alia catalytically active materials are applied for characterization and investigation on conductive films and examined in a scanning electron microscope. Fig. 1 shows an electron micrograph of a sample thus prepared. Clearly recognizable is the exclusive deposition of the metal clusters on the areas with biological material. The example thus demonstrates the possibility of selective deposition of metal clusters on substrates. With a further chemical metal coating according to the prior art, the existing clusters can be converted into closed metallic layers. A surface produced in this way then has the property of electrical conductivity with a simultaneously high proportion of three-phase boundary surfaces (metal coating-substrate-gas phase or metal coating-substrate-liquid phase). Substrates prepared in this way can be used as a solid-state electrolyte sensor with particularly high sensitivity. Embodiment 2

Targeted coating of surfaces with metal clusters by controlled coating with biological templates as in Example 1, but by prior recrystallization of protein monomers on the respective substrates.

The standard S-layer solution was lyophilized and then suspended in a 0.8 M TRIS-buffered guanidine hydrochloride solution so that the final concentration of the protein solution is 10 mg / ml. After an interaction time of 30 minutes between the reagents, the solution is transferred to a prepared dialysis tube (VISKING type 27/32) or a dialysis chamber and dialyzed against water and then the standard buffer without MgCl ₂ . The existing after this step in the dialysis tube solution is transferred to a suitable reaction vessel and at 4 ⁰ C, 20,000 g min centrifuged for 10 degrees. The resulting after this step pellet is discarded, the supernatant monomer solution used for the following works (the monomer solution is known to last about 5 days, after which self-assembly products are already formed).

The freshly prepared monomer solution is recrystallized directly on a Si substrate with the addition of MgCl ₂ (final concentration 1 mM). At 30 ° C and a very high humidity, the protein monomers recrystallise within 24 h on the Si substrate in a monolayer. After several washing steps, the Si substrate thus functionalized is brought into contact with a metal complex solution in order subsequently to be coated with metallic clusters as in Working Example 1.

The advantage of recrystallization of protein monomers directly on the Si substrate over the deposition of S-layer patches is the formation of a monolayer of protein and the associated lower use of biological material. The proportion of the surface covered with biotemplates can be influenced by external parameters (eg temperature, pH value of the solution.) The substrate produced in this way is suitable as in Example 1 as a three-phase interface of a solid electrolyte sensor. Embodiment 3

Targeted coating of ablation catalytic surfaces with noble metal clusters by controlled coating with biological templates (S-layer patches of Bacillus sphaericus NCTC 9602), bypassing the use of chlorides and hydrazine.

The preparation of the S-layer is based on the publication by Engehard H .; Saxton, W; Baumeister, W., "Tree-dimensional structure of tetragonal surface layer of Sporosarcina urea", J. Bacteriol. 168 (1), 309, 1986. The standard buffer for storage (4 ⁰ C) of the isolated and purified S-layer is from a 50 mM Tris / HCl solution, with the addition of 3 mM NaN ₃ and ImM MgCl ₂ .

The S-Layer solution for all further experimental work has a standard concentration of 10 mg / l.

Alumina particles (100 mg each) are mixed with 825 μl of the activated S-layer solution and allowed to interact for 24 hours. Thereafter, twice with dest. H ₂ O rinsed.

10.83 ml of Pt (NO ₃ ) ₂ solution are then added to the particles coated with S-layer, mixed, and incubated for 72 h at room temperature with exclusion of light. During this time, the binding of the Pt complexes to the S-layer proteins, which is necessary for cluster formation, takes place.

The supernatant is discarded and the particles are washed twice with dist. H ₂ O rinsed.

The following reduction to metallic platinum is induced by administration of 2.4 ml of NaBH ₄ to the alumina particles. Gas evolution can be considered as an indicator for the completion of the reduction. This should be completed after 30 - 60 min.

The supernatant is again discarded. This is followed by two rinsing steps with 10 ml dist. H ₂ O and drying of the preparations at 40 ⁰ C.

For the visual characterization of Pt cluster deposition, scanning electron micrographs are suitable. To assess the catalytic activity, a reference preparation is made, which is carried out according to the same procedure but without the biological template. 2 shows the results of a DSC Measurement (differential thermal analysis) for the assessment of catalytic activity. The catalyst using the biological template shows a comparable catalytic effect (light-off temperature only 10 ⁰ C above the reference catalyst). A determination of the platinum contained, however, shows a significant saving (reduction of platinum content from 1.1% to 0.24%). A repetition of the experiment with a modified concentration of the platinum solution using the biotemplate shows that both the catalytic activity and the amount of platinum contained in the catalyst is independent of the platinum concentration used in the process. This clearly indicates that the amount of platinum deposited is only determined by and controlled by the biotemplate (Figure 3).

Claims

claims:

1. Substrate with spatially selective metal coating, the surface of which partially has biological template with a metallic coating, obtainable in that the metallic coating takes place only after deposition of the biological template on the substrate.

2. Substrate according to claim 1, characterized in that the biological template surface layer proteins (S-layer).

3. Substrate according to claim 1 or 2, characterized in that the metallic coating consists of metal clusters and / or at least one metal layer.

4. Substrate according to one of claims 1 to 3, characterized in that the metallic coating consists of precious metals.

5. Substrate according to claim 3 or 4, characterized in that metal clusters and metal layer consist of different metals.

6. Substrate according to one of claims 1 to 5, characterized in that the substrate consists of Al ₂ O ₃ , silicon, carbon, or a solid electrolyte.

7. A method for spatially selective deposition of metallic clusters on substrates, characterized in that biological template deposited on the substrates and then metallized under conditions compatible for the biological template or that biological template activated in metal salt solution, then deposited on the substrates and then under metallized for the biological template compatible conditions

8. The method according to claim 6, characterized in that metallic clusters and / or metal layers are deposited.

9. The method according to claim 7 or 8, characterized in that the metal coating takes place electrolessly in at least one metal salt solution.

10. The method according to any one of claims 7 to 9, characterized in that the deposition of the biological template is used by changing the concentration or the flow rate of the biological template used for the deposition, containing solution.

11. The method according to any one of claims 7 to 9, characterized in that the size of the biological template and their binding mechanisms are used for targeted deposition.

12. The method according to any one of claims 7 to 9, characterized in that the deposition of the biological template is controlled by applying electric fields.

13. The method according to any one of claims 7 to 12, characterized in that the biological template are recrystallized as monomers on the substrate.

14. The method according to any one of claims 7 to 9, characterized in that used as a biological template surface layer proteins (S-layer) and coated with precious metals.

15. Use of substrates according to one of claims 1 to 6 for catalysts, solid-state electrolyte sensors or optically, transparent conductive layers.

16. Catalyst comprising at least one substrate according to one of claims 1 to 6.

17. Solid-state electrolyte sensor, comprising at least one substrate according to one of claims 1 to 6.