This application is a continuation of Application No.
PCT/GB2011/001402, filed Sep. GB 10165215, filed Oct. The present invention is concerned with methods of depositing metal onto substrates from an electroless plating solution. For instance, especially, it relates to the deposition of metal layers, particularly porous metal layers, onto insulating substrates.
Substrates having one or more metal layers deposited on one or more of their surfaces have many industrial uses. Whenever insulating films having a metal layer or layers on one or more surfaces are of use in separation processes, fuel cells, ‘super capacitors’, electrolytic cells for splitting water into hydrogen and oxygen, and for catalysis, For instance. Electroplating is a popular industrial process that uses electrical current to reduce ionic precursors of a desired material from a solution to coat a conductive substrate with a layer of the desired material. There’s more information about it on this website. Whenever effecting the electroplating, The electrically conductive substrate might be suitably used as amid the electrodes in an electrochemical cell whereby the electrical current causing electroplating is provided. Remember. No. Publication WO 99/00536 disclose the production of porous metal films on electrically conducting substrates by electrodeposition from a liquid crystalline phase.
Then the electroless reaction is typically selfsustaining until either, as soon as deposition by electroless plating is initiated.
Electroplating is suitable for use with electrically conducting substrates. Of course, electroless plating offers the advantage of allowing deposition of material by plating onto electrically insulating substrates. Prior electroless plating methods require substantial preparation of the substrate prior to commencement of plating. Typically, prior to commencement of electroless plating, a catalytic metal just like palladium is first deposited onto the substrate as a seed or nucleation layer, most usually in a two step process, for instance. Ii) treatment of the primed surface with a solution of palladium chloride in hydrochloric acid whereby Sn2+ ions adsorbed in step reduce Pd ions in solution to form deposits of metallic Pd on the substrate surface, Sn2+ ions onto the surface from the solution to provide a primed surface.
These Pd deposits subsequently act as nucleation sites during electroless plating of the desired metal, that may be Pd or which might be alternative metal.
It is also known, in the prior art, to combine the metal ions for priming and those used for providing nucleation sites on the surface to be treated in a single pre treatment solution. With the tin subsequently removed by treatment with concentrated hydrochloric acid to leave palladium deposits on the surface to act as nucleation sites for electroless deposition, now this may result in initial adsorption of Sn Pd particles onto the surface of the substrate. Keep reading. Irrespective of which ‘prepreparation’ method is used to provide a substrate ready for electroless deposition, the prepreparation method may have to be repeated up to ten times to achieve a surface adequately modified to act as a template for electroless deposition.
This is both costly and time consuming.
While involving cleaning, etching and neutralising, should be required, prior to electroless plating and prior to pretreatment, lengthy surface preparation. This is to promote adhesive bonding of the electroless metal layer to the substrate surface upon which it’s deposited. Actually the resulting metal deposit, generated by electroless plating, will contain Pd and may include Sn, so this latter metal being a particularly troublesome and undesirable impurity if the substrate upon which the metal layer is deposited is for subsequent use in electrochemical applications.
Photocatalytic deposition of metals onto semiconductors is a mature technology in the prior art.
In particular, titanium dioxide is known as a photocatalyst for use in the reductive deposition of noble metals similar to palladium and platinum onto semiconductor surfaces. While using actinic radiation like ultraviolet radiation having a photon energy in excess of the band gap of the titanium dioxide, electrons should be excited and drive redox reactions, catalysed by the titanium dioxide particles and leading to deposition of metal from a treatment solution onto the semiconductor surface, During irradiation of titanium dioxide.
It is also known to use photo catalysis like titanium dioxide in etching of surfaces as well as deposition. Pat. Of course no. US patent application publication ‘2006 0019076 A’ disclose modification of surfaces using a photocatalyst followed by electroless deposition onto the modified surface. The processes disclosed in these documents comprise separate, discrete process steps for photoexcitation, primer metal nucleation and electroless metal deposition. For example, look, there’s a need for electroless methods of deposition of metal layers onto substrates which obviate the complex substrate treatment regimes required in the prior art and which eliminate the need for primer or nucleation metals to be present in or below deposited layers.
It is one the invention object, amongst others, to provide electroless methods of deposition of metal layers onto electrically insulating substrate surfaces which obviate the complex substrate treatment regimes required in the prior art. In particular, it’s an object of the invention to provide methods which do not require deposition of primer or nucleation metals prior to electroless deposition. Anyway, it’s a further object of the invention to provide methods which ensure that the metal layers deposited are strongly adhered to the substrate surface upon which they are formed. Anyways, So it’s a further object of the invention to provide methods for deposition of porous metal layers onto substrate surfaces. Nevertheless, another object of the invention is to provide methods for depositing metal layers wherein the thickness of the deposited metal layer is easily controllable throughout the deposition method.
Throughout this specification, the term comprising or comprises means including the component specified but not to the exclusion of the presence of others. The term consisting essentially of or consists essentially of means including the components specified but excluding other components except for materials present as impurities, unavoidable materials present because of processes used to provide the components, and components added for a purpose except achieving the technical effect of the invention. Typically, a composition consisting essentially of a set of components will comprise less than 5percent by weight, typically less than 3percentage by weight, more typically less than 1 by weight of nonspecified components, where percent weight is used to define a composition. Where g/dm3 is used to define levels of components in a composition, the composition consisting essentially of a set of components will typically comprise less than 50 g/dm3, typically less than 30 g/dm3, more typically less than 10 g/dm3 of ‘nonspecified’ components.
By actinic radiation is meant radiation, typically electromagnetic radiation, capable of inducing chemical reaction when used to illuminate the photocatalyst layer.
Typically, the actinic radiation going to be ultraviolet radiation. While depositing a photocatalyst layer onto the substrate, The method of the first part of the invention comprises, in step. Essentially, the photocatalyst layer comprises or consists essentially of a photocatalyst, let’s say titanium dioxide or the like, as an example in particulate form, that is capable of inducing reductive deposition of a metal from an electroless plating solution when suitably illuminated with actinic radiation. Suitably, the photocatalyst layer is deposited directly onto the substrate.
The photocatalyst layer may comprise any suitable photocatalyst which may typically be present as colloidal particles.
The photocatalyst may comprise or consist essentially of a semiconductor, and suitably may comprise or consist essentially of a particulate semiconductor. Needless to say, for example the photocatalyst might be selected from semiconducting single, binary and ternary metal oxides with bandgaps corresponding to photon energies in the visible, near and middle ultraviolet UV spectral regions, or doped versions thereof, regardless of crystalline phase or of crystalline habit. Suitably, the band gap should be 5 eV or less, like 2 eV or less.
Single metal oxides, Examples of suitable semiconductor photocatalysts include. Binary metal oxides, WO3, SnO2, alpha and gamma Fe2O, TiO2, indium oxide, BiVO4. For instance, ternaries, BaTiO3, SrTiO3. Have you heard about something like this before? In09Ni01TaOMixtures of such catalysts may also be used. You should take it into account. Preferably, the photocatalyst layer may comprise or consist essentially of titanium dioxide, suitable to act as photocatalyst for electroless deposition of metal. Therefore, preferably, the titanium dioxide might be in particulate form. As a result, the titanium dioxide may, for sake of example, be deposited onto the substrate surface as colloidal particles or as a mesoporous film, let’s say prepared by solgel deposition, like set out in Mater. Chem.
Whenever depositing a mask layer comprising voids, the mask layer provided on the photocatalyst layer, whereby deposition of the metal occurs within the voids, The method of the invention comprises, in step.
Whenever following deposition of the metal, whereby the metal layer is a porous metal layer, The mask layer is removed in step. The mask layer is a porous mask layer with voids forming the pore space of the mask layer and can be formed or provided by any suitable method. The porous mask layer is deposited on the substrate prior to application of the electroless plating solution in the subsequent process step. Consequently, one method to form this mask layer is to deposit a layer of microparticles, the voids being provided by the spaces between the microparticles.
This is where it starts getting really intriguing. The mask layer is suitably of an open porous structure whereby the electroless plating solution is in fluid contact with the substrate through the open pore structure of the mask layer. Seriously. Whenever allowing the electroless plating solution to efficiently contact the photocatalyst layer through voids between the microparticles, may also be of 2 or more layers where ‘3 dimensional’ porosity is required. 2 dimensional porosity is required. Keep reading. By microparticles is meant particles having a mean diameter from 50 to 1000 nm. The particle may have any shape but preferably should be spheroidal or more preferably substantially spherical in shape.after removal of the microparticles, the shape of the microparticles generates the shape of the resulting pores. Then, microparticles of different sizes and also geometries might be suitably chosen to produce layers suitable for different applications.
Suitably, the microparticles might be polymer latex particles.
These should be deposited from a latex solution onto the substrate surface on the photocatalyst layer to provide deposition of the mask layer. The volume of latex solution, or the latex concentration of that solution might be increased to a suitable level to produce multiple layers of microspheres on the surface.
Metal is deposited in the voids of the mask layer and can not form where the mask layer skeleton is present, as electroless deposition of metal from the electroless plating solution onto the photocatalyst layer is initiated and proceeds. For instance, for a layer of microparticles, the metal is deposited in the voids between the microparticles. On top of this, for example by exhaustion of the electroless plating solution or by removal of the substrate from the electroless plating solution, the mask layer should be removed, for the sake of example by dissolution using a suitable solvent, whereby a porous metal layer is left deposited on the substrate surface, the pore space of the metal layer being formed by voids left behind when the mask layer is dissolved and removed, if metal deposition had been stopped.
In step, an electroless plating solution is provided on the photocatalyst layer, on the substrate.
The electroless plating solution might be any suitable electroless plating solution. Whenever meaning that the electroless plating solution comprises at least 700 g/dm3 by weight of water in addition to other components making up the balance of the electroless plating solution, an aqueous solution might be used. For example, other components of the electroless plating solution may include a metal salt, typically from 5 to 20 g/dm3 of metal salt, for example from 1 to 10 g/dm3, say from 1 to 5 g/dmThe metal salt or precursor is a source of ions for the metal to be deposited, like silver nitrate for silver deposition or palladium chloride for palladium deposition. There may also be included a complexant, just like a chelating agent, as an example ethylenediamine, EDTA di sodium salt or the like. The complexant may suitably be present at a level from 1 to 200 g/dm3, as an example from 2 to 100 g/dmStabiliser just like 35 di iodotyrosine, ammonium hydroxide or the like may also be included. This stabiliser might be typically at a level from 001 to 150 g/dm3.
While preventing or reducing precipitation of undesirable basic metal salts as well as may act as a buffer to prevent rapid decrease in pH as electroless plating progresses, The role of the complexant and stabiliser is to stabilise the electroless plating solution by reducing the concentration of free metal ions. The scavenger is a hole scavenger and can be identical chemical as the reducing agent or reductant. The reducing agent acts as electron donor for the ‘auto catalytic’ stage of metal reduction. Make sure you drop some comments about it in the comment section. Other examples of suitable reducing agents include sodium hypophosphite and ethanol. Typically these might be present at a level from 5 to 50 g/dm3, say 6 to 15 g/dm3.
By hole scavenger is meant a compound which donates electrons to fill in ‘holetype’ conductors which may form on photocatalysts just like TiO2 upon photoexcitation of electrons.
The use of a hole scavenger may enhance the capability of the photocatalyst to reduce the metal ions and make them more available in solution. Whenever inhibiting further deposition, So if holes remain unscavenged, an electron accumulation layer may form. Step involves illuminating the photocatalyst layer and electroless plating solution with actinic radiation whereby deposition of metal from the electroless plating solution to form a metal layer on the photocatalyst layer is initiated.
The actinic radiation is typically ultraviolet radiation. For instance the ultraviolet radiation has a photon energy sufficient to excite electrons across the band gap of the photocatalyst whereby reduction reactions are catalysed. The deposition of the metal onto the photocatalyst layer means that a layer of the metal is deposited onto the substrate, on the photocatalyst layer. With that said, steps,, and are applied sequentially but other steps can be carried out between them. It may continue autocatalytically in the absence of further illumination with suitable actinic radiation, when the deposition of metal from the electroless plating solution was initiated. The deposition of metal within the voids might be self sustaining following initiation by illumination with actinic radiation. That is interesting right, is that the case? The deposition of metal within the voids might be sustained by electroless deposition to achieve a desired or required thickness.
That can be used in selective deposition of metal layers onto different parts of a substrate surface, and similar parts are free of deposited metal.
For instance the method of the invention may comprise selecting a first portion of surface upon which the metal layer is formed and a second portion which remain free of metal layer by illuminating the first portion but not the second portion with the actinic radiation.
In another embodiment, the method of the invention may comprise selecting a first portion of surface upon which the metal layer is formed and a second portion which remains free of metal layer by depositing the photocatalyst layer on the first portion and no photocatalyst layer on the second portion. The method of the invention is for use with a substrate of an electrically insulating material, as electroless deposition does not require an electrically conductive substrate.
Whenever nonlimiting examples, Specific embodiments of the invention will now be described further by reference to the following.
Reference will also be made to the accompanying FIG. Eventually, all reagents used were AnalaR grade or better, and purchased from Sigma Aldrich, Gillingham, Dorset. All water used was Ultrapure from a Direct Q 3 UV Millipore water purification system Limited, Watford, UK) to a resistivity of 182 MΩ·cm. Therefore, the PVDF membranes were purchased from Millipore, the pore size was 200 nm.
Mesoporous films of colloidal titanium dioxide were prepared using a modified reverse micellar solgel method. Triton ‘X 100’ and cyclohexane were mixed to form the reverse micellar solution. Water, titanium isopropoxide and acetylacetone were added. Now regarding the aforementioned fact… The resultant solgel was applied to substrates by ‘spincoating’ for 5 seconds at 2900 rpm. Keep reading! The substrates were thence fired in a furnace at 770 K for 1 hour. The substrates were stored in the dark at room temperature before use. After vigorous shaking the solution was sonicated for 200 seconds at an amplitude of 6 micrometers using a cycle of 10 seconds on and 10 seconds off. The sonicator used was a MSE Soniprep The substrates were consequently ‘dipcoated’ manually in the colloidal suspension and allowed to dry under ambient conditions. The coating/drying cycle process was repeated 5 times. Doesn’t it sound familiar, am I correct? The coated substrates were stored in the dark at room temperature before use.
PVDF membrane with pore size 200 nanometers was manually dipcoated twice in a nanoparticulate titanium dioxide/methanol suspension.
The titanium dioxide particles constituted 2 per cent by weight of the suspension with the balance being methanol. Solution was sonicated, for 2 minutes at 6 micrometer amplitude and the coated layer was allowed to dry before ‘recoating’, before every coating step. The final coated substrate was after that, allowed to dry for 1 hour in the dark before use. With all that said… Polystyrene microspheres of 1 micron diameter as a 5 wt suspension in water were used as microparticles. Now look. For preparation purposes this commercial latex was further diluted to 0 wt with distilled water to form a microparticle suspension.
Substrates comprising a photocatalyst layer prepared conforming to examples A, B or C as set out above were further prepared for deposition of the microparticles by irradiating with UV light for 60 minutes. This was to ensure that the photocatalyst layer has a hydrophilic surface with intention to facilitate deposition of the microparticles as a close packed array. Following application of the microparticle suspension, substrates were left undisturbed for 12 hours in an enclosed environment to ensure that evaporation isn’t accelerated by air currents. I know that the PTFE ring was removed and the substrate with photocatalyst layer and mask layer was ready for deposition of metal by electroless plating, as soon as the water had evaporated.
Turning to FIG.
The microparticles 105 should be in a random or preferably in an ordered, closepacked arrangement. Actinic radiation 104 is shone onto the substrate 101 through the electroless plating solution 107 to initiate deposition of metal. Whenever ensuring full dissolution with any addition, Examples of plating solutions are given below in Tables 1 and Electroless silver plating solutions were prepared to the composition given in Table All components were added to a small quantity of distilled water in the order listed. On top of that, the completed solution was made up to volume with distilled water and purged with nitrogen for 20 minutes to deoxygenate it. On top of this, the pH of the solution was 115 and ‘photoinitiated’ electroless deposition was carried out at 298 Electroless plating solutions were freshly made immediately prior to use. Silver nitrate solutions were prepared and stored in amber flasks in darkness, as silver nitrate is light sensitive.
Electroless palladium plating solutions were prepared to the composition given in Table Palladium chloride, disodium EDTA and ammonium hydroxide were added and, to ensure formation of a Pd amine complex, stirred with gentle heating until the solution cleared. The resulting clear solution was cooled and hydrazine reducing agent added. The solution was so made up to volume with distilled water and purged with nitrogen for 20 mins. The electroless plating solution was prepared immediately before use. Just think for a moment. Each substrate, with its photocatalyst film as formed in examples A, B or C, was placed into the electroless plating solution and irradiated for 20 minutes with UV light. As a result, whenever conducting layer of metal was produced, the thickness of which was controllable by selection of immersion time, after this irradiation time, a coherent.
During photo initiated electroless deposition, metal is deposited only on the part of the surface which is both coated with ‘photo catalyst’ and subjected to suitable actinic radiation. Virtually no deposition really occurs in ‘un illuminated’ areas or in areas not coated with photo catalyst. Physically masking the deposition of the photocatalyst to leave uncoated areas, physically masking part of the location previously prepared with photo catalyst, optically masking part of this place illuminated to prevent illumination with actinic radiation, or selectively illuminating this location using a scanned beam of actinic radiation, this place of deposition of metal layer can be demarcated using techniques similar to.
While conducting layer of metal can be formed, when the substrate is illuminated for about five minutes with the actinic radiation, electroless plating will was sufficiently initiated for autocatalytic plating to continue, even in the absence of further illumination with actinic radiation, whereby a bright, reflective.
In the case of PVDF membranes that been ‘dip coated’ as set out above in example C, the opposed faces of the membrane might be provided with a metal layer simultaneously without throughplating of the pores of the membrane.
Following deposition of the metal layer, the microparticles can be dissolved in an appropriate solvent chosen to dissolve the microparticles but not the substrate. For these Examples, toluene was employed for dissolution of the mask layer of microparticles to leave a porous metal layer. The invention provides a method for manufacturing metal films with controlled thickness and excellent adhesion on substrates, particularly electrically insulating substrates. The method is suitable for providing metal layers with controlled, regular porosity by use of a mask layer as set out hereinbefore.