Patent Application
20200095686
The invention provides a method of electroless deposition of platinum group metals and their alloys and a plating bath used therein.
Platinum group metals (PGMs) consist of six elements—platinum, palladium, rhodium, iridium, ruthenium and osmium. Interest in plating of platinum group metals and PGM alloys results from a very broad range of their applications. PGM layers can provide corrosion resistance, and can be also used as decorative or tarnishing layers. Rhodium is typically plated as wear-resistant, decorative finishing layers. PGMs are also known for their catalytic activity for many chemical reactions. PGMs are used for manufacturing of electronic devices and components, as well as layers providing protection against corrosion of non-precious metals. For example, galvanized palladium, ruthenium and their alloys are used as gold substitute contacts on connectors and printed circuit boards.
Platinum layer can also replace gold-on-copper plating. In such material, the copper atoms tend to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide and/or sulphide layer. For gold-on-copper layers typically the layer of nickel has to be used to provide mechanical backing for the gold layer. However, at higher frequencies, in so modified gold-on-copper layers the skin effect is observed due to higher electrical resistance of nickel. Also soldering gold-plated parts can be problematic as gold dissolves in solder. A 2-3 μm layer of gold dissolves completely within one second during typical wave soldering conditions. Palladium and ruthenium are used as metal contacts on semiconductors, for example GaAs and InP.
Plating of PGMs is particularly suitable method for cost-effective use of PGMs since by using this approach PGMs (and their alloys) can be plated on different low cost substrates. Electroless PGMs plating is particularly suitable because in this method the layer can be plated on non-conductive supports. PGM plating on plastics can be applied in production of flexible electronics and flexible dye-sensitized solar cells [Mao V-D, et al.: Dry plasma reduction to synthesize supported platinum nanoparticles for flexible dye-sensitized solar cells. J. Mater. Chem. A 2013, 1:4436-4443].
To date, a few processes of PGM electroless deposition was described in the literature [Electroless Plating: Fundamentals and Applications; Glenn O. Mallory and Juan B. Hajdu, Ed., William Andrew, 1990]. Moreover, most of them lead to the formation of fragile and not uniform deposits with high tendency to peel. Literature survey suggests that most studies on PGM plating are focused on the development of plating bath additives with much less emphasis placed on the reducing agents. In virtually all methods of electroless deposition of PGMs one of the following reducing agents is used: hydrazine and its derivatives, borohydride or hydrogen hypophosphite. Redox potentials of these reducing agents are very low and therefore PGM plating is accompanied by hydrogen evolution, which results in unfavourable conditions for PGM deposition, such as hydrogen embrittlement, peeling, rough deposits, etc.
It is commonly known that platinum plating is a complicated and difficult process. Electrochemical methods of Pt deposition lead to formation of poorly adhesive platinum black deposits. In the classical method introduced by Keitel and Zschiegner the Pt deposition is performed form “Pt”-salts precursors. This is one of the few methods leading to relatively smooth Pt coatings [Keitel W and Zschiegner H: Electrodeposition of Platinum, Palladium and Rhodium. Trans. Electrochem. Soc. 1931, 59:273-275]. However, the plating requires high temperature operation and very low cathodic current efficiency of platinum deposition—typically lower than 10%. A solution containing sulphato-dinitritoplatinum (II) has been prepared for commercial use under the name DNS Platinum Plating Solution and patent applications covering baths of this type have been filed in a number of countries [Hopkin N and Wilson L: Bright platinum plating. Platinum Metals Review 1960, 4:56-58]. Currently used platinum plating solutions contain many additives in order to improve the quality of the deposits. However, this approach may be expensive and has its own limitations and potential problems.
Platinum metals are chemically inert, however their surfaces are extremely reactive and therefore these metals are widely used in the heterogeneous catalysis. Platinum alloyed with palladium, rhodium and other PGMs are used in various chemical industrial scale processes. Production of nitric acid and agricultural fertilizers, wherein ammonia oxidation is required, creates the highest demand for PGMs catalysts. However, important applications for platinum catalysts can also be found in manufacturing of polymer materials, for example in the process of production of silicones and vinyl acetate monomers. The PGMs are used as catalysts in reforming and isomerization reactions in the petroleum industry. Rhodium catalysts are used in the manufacturing of oxo-alcohols and acetic acid, the latter is also produced with ruthenium and iridium as catalysts. Dimensionally stable anodes, which consist of titanium coated with either ruthenium or ruthenium-iridium alloy, are used in chlorine evolution on an industrial scale. Rough PGM deposits can be applied in fuel cells, etc. For all these applications rough PGM deposits are beneficial since higher specific area results in higher catalytic activity of the layer.
However, platinum coatings obtained in brightener-free baths are usually fragile and poorly adhesive [Feltham A and Spiro M: Platinized platinum electrodes. Chem. Reviews 1971, 71:177-193]. Often the process is accompanied by hydrogen evolution and platinum is reduced in the solution. For over half a century Pt black deposition has been performed from solutions containing small amounts of lead acetate, which inhibits hydrogen evolution and improves Pt adhesion. However, in this procedure lead is incorporated into the layer and the resulting Pt deposits are contaminated with Pb, which is particularly unfavourable for catalytic applications.
Currently there is no technology for producing platinum layers on a very large specific surface area and strongly adherent to the substrates. Ultrathin layer or metal clusters can be obtained on oxides or silicon [M. B. Vukmirovic et al., ChemElectroChem 2016, 3: 1-7; Á. Muñoz-Noval, Electrochemistry Communications, 2016, 71: 9-12]. A few electroplating and electroless plating technology is available for platinum. Comparison of catalytic layers comprising platinum-ruthenium bimetallic clusters on graphite obtained by vapor deposition and electroless deposition methods were recently described in R. P. Galhenage, Phys. Chem. Chem. Phys., 2015, 17, 28354. In the most of the other known plating methods, only platinum layers are obtained (i.e. no means for plating platinum in combination with other PGMs have been disclosed). In all known methods, plating can be carried out on a specific solid support, such as copper or titanium. It is also necessary to control plating conditions, especially temperature and pH. Also, such typically used plating baths are very difficult to obtain, as they contain many additives and reducing agents, such hydrazine and its derivatives or hypophosphite. Finally, any smooth layers can be obtained only via electroplating, however with very low current efficiency.
Chemical reduction of platinum precursors is, on the other hand, widely used for production of platinum nanoparticles. Very popular reducing agents for platinum nanoparticles preparation are aliphatic alcohols and aldehydes. One of the necessary conditions for nanoparticles preparation is fast reduction step, usually performed in relatively high temperatures (typically the temperature of the reduction step is higher than 70° C.). At lower temperatures stronger reducing agents have to be used, such as hydrazine [WO2013186740].
Owing to their possible use in fuel cells, oxidation of alcohols has been extensively studied. Most extensive studies have been carried out for methanol and to the lesser extent for ethanol. Oxidation of other alcohols is occasionally reported. Polihydroxy compounds, namely sugars and sugars derivatives, have been previously used as reducing agents for plating silver and gold, e.g. glucose, ascorbic acid [Kato M et al.: Electroless Gold Plating Bath Using Ascorbic Acid as Reducing Agent-Recent Improvements. In Proceedings of AESF Technical Conf SUR/FIN'95:1995:805-813]. Aliphatic and aromatic mono-alcohols have never been previously used as reducing agents in electroless plating.
Due to a large demand for platinum and other PGM covered articles, there exists a great need for development of a method for electroless deposition of platinum and other PGMs, which would result in formation of well-adhering metal layers. Moreover, there exists a need for a PGM electroless plating method wherein the properties of the formed metal layer can be easily controlled simply by changing the process conditions. It would also be advantageous if a plating bath used in such a method of PGM electroless plating would be free of brighteners (e.g. saccharin and polyethylene glycol) and other additives and reducing agents (e.g. hydrazine, lead or hypophosphite).
The inventors of the present invention have developed a method for electroless deposition of platinum and other PGMs, as well as their alloys, in which aromatic and aliphatic alcohols are used as reducing agents. A PGM layer as used herein refers to both layers of a single metal (e.g. platinum, palladium, rhodium, iridium, ruthenium and osmium) and metal alloy layers comprising platinum group metals (e.g. platinum-rhodium, platinum-iridium, platinum-ruthenium, etc.), as well as metal alloy layers comprising platinum group metals with other metals, such as gold, nickel and copper (e.g. platinum-gold, platinum-copper, platinum-nickel, platinum-rhodium-gold, rhodium-nickel, etc.). In the most preferred aspect of the invention, an alloy layer is deposited on a substrate, wherein said alloy comprises one or more metals from a group comprising: platinum, palladium, rhodium, iridium, ruthenium, osmium, gold, nickel and copper.
In the method of the invention both primary and secondary alcohols are used and the deposition is carried out on a wide variety of substrates, including metals and alloys, polymeric materials, graphite and silicon. The process of PGM plating of the invention is carried out in the temperature range from the freezing point to the boiling point of the plating bath. Both layer thickness and surface morphology of the deposited layer can be controlled in a simple manner by adjusting the experimental conditions, in particular the reducing agent, temperature, pH and concentration of both the reducing agent and the platinum group metal precursor.
In particular, according to the invention, the method of electroless deposition of platinum group metals and their alloys, from a plating bath onto a substrate, comprises a reduction step of one or more platinum group metal precursors with a reducing agent, wherein the reducing agent is a primary or secondary monohydroxyalcohol or a mixture of primary or secondary monohydroxyalcohols. The term “monohydroxyalcohol” as used herein refers to aliphatic and aromatic alcohols comprising a single hydroxyl group. The term “primary alcohol” should be understood as an alcohol, in which the hydroxyl group is connected to a primary carbon atom—a carbon atom having only one carbon atom neighbor (i.e. an alcohol having the following group “—CH2OH”). The term “secondary alcohol” should be understood as an alcohol, in which the hydroxyl group is connected to a secondary carbon atom—a carbon atom having two carbon atom neighbors (i.e. an alcohol having the following group “—CHROH”, where “R” is a carbon-containing group (aliphatic or aromatic)). The term “a mixture of primary or secondary monohydroxyalcohols” as used herein refers to a mixture of different primary monohydroxyalcohols, a mixture of different secondary monohydroxyalcohols, as well as a mixture comprising both primary and secondary monohydroxyalcohols. The term “electroless deposition” is well-known to a person skilled in the art and refers to a process of metal deposition on a substrate by reduction of metal ions from solution without using an external source of electrons.
Preferably the monohydroxyalcohol has a general formula
wherein R1 and R2 are the same or different and each of them independently is selected from a group comprising hydrogen atom, a straight or branched C1-7-alkyl group, C3-8-aryl group, C4-8-aralkyl group and C4-8-alkaryl group. “Alkyl” means an acyclic, branched or unbranched, saturated monovalent hydrocarbyl group. Alkyl is exemplified by, but not limited to, methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl and octyl, as well as branched saturated monovalent hydrocarbyl groups. “Aryl” means a cyclic, fully unsaturated, hydrocarbyl group. Aryl is exemplified by, but not limited to, cyclopentadienyl and phenyl. “Aralkyl” means an aryl group having a pendant alkyl group. “Alkaryl” means an alkyl group having a pendant and/or terminal aryl group.
Preferably, R1 and R2 in the above formula are independently selected from a group comprising —H, —CH3, —C2H5, —C3H7, —CH(CH3)2, —C4H9, —CH2CH(CH3)2, —CH(CH3)C2H5, —C(CH3)3, −C5H11, —CH(CH3)C3H7, —CH2CH(CH3)C2H5, —C2H4CH(CH3)2, —C(CH3)2C2H5, —CH(CH3)CH(CH3)2, —CH2C(CH3)3, —CH(C2H5)2, —C6H13, —C7H15, and —CH(C2H5)(C4H9). More preferably, monohydroxyalcohol is selected from the group comprising methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methylpropan-1-ol, 1-pentanol, 3-methylbutan-1-ol, 2-methylbutan-1-ol, 2,2-dimethylpropan-1-ol, 3-pentanol, 2-pentanol, 3-methyl-2-butanol, 1-hexanol, 2-hexanol, 2-methyl-1-pentanol, 3-methyl-1-pentanol, 4-methyl-1-pentanol, 3-methyl-2-pentanol, 4-methyl-2-pentanol, 2-methyl-3-pentanol, 2,2-dimethylbutan-1-ol, 2,3-dimethylbutan-1-ol, 3,3-dimethylbutan-1-ol, 3,3-dimethylbutan-2-ol, 2-ethylbutan-1-ol, 1-heptanol, 2-heptanol, 3-heptanol, 4-heptanol, 1-octanol, 2-octanol and 2-ethylhexan-1-ol. Most preferably, monohydroxyalcohol is selected from the group comprising methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, 2-methylpropan-1-ol, 1-pentanol, 3-methylbutan-1-ol, 2-methylbutan-1-ol, 2,2-dimethylpropan-1-ol, 3-pentanol, 2-pentanol, 3-methyl-2-butanol, 1-hexanol, 1-heptanol and 1-octanol.
Certain of the above mentioned monohydroxyalcohols, exist in particular stereoisomeric forms. For example 2-butanol can be obtained as either of two stereoisomers—(R)-(−)-2-butanol and (S)-(+)-2-butanol. The present invention contemplates use of such stereoisomers as separate isomers, as well as racemic and other mixtures thereof.
The concentration of the monohydroxyalcohol in the plating bath used in the method of the invention is in the range from 0.02 to 12 M, more preferably in the range from 0.1 to 5 M. The solubility of monohydroxyalcohols poorly soluble in water can be increased by addition of 2-methylpropan-2-ol. Therefore, in one aspect of the invention the plating bath used in the method of the invention further comprises 2-methylpropan-2-ol.
According to the invention the platinum group metal precursor used in the method of the invention is preferably selected from a group comprising H2PtCl6, H6Cl2N2Pt, PtCl2, PtBr2, K2[PtCl4], Na2[PtCl4], Li2[PtCl4], H2Pt(OH)6, Pt(NO3)2, [Pt(NH3)4]Cl2, [Pt(NH3)4](HCO3)2, [Pt(NH3)4](OAc)2, (NH3)4Pt(NO3)2, (NH4)2PtBr6, K2PtCl6, PtSO4, Pt(HSO4)2, Pt(ClO4)2, K2PtI6, K2[Pt(CN)4], cis-[Pt(NH3)2Cl2], H2PdCl6, H6Cl2N2Pd, PdCl2, PdBr2, K2[PdCl4], Na2[PdCl4], Li2[PdCl4], H2Pd(OH)6, Pd(NO3)2, [Pd(NH3)4]Cl2, [Pd(NH3)4](HCO3)2, [Pd(NH3)4](OAc)2, (NH4)2PdBr6, (NH3)2PdCl6, PdSO4, Pd(HSO4)2, Pd(ClO4)2, Pd(OAc)2, RuCl2((CH3)2SO)4, RuCl3, [Ru(NH3)5(N2)]Cl2, Ru(NO3)3, RuBr3, RuF3, Ru(ClO4)3, K2RuCl6, OsI, OsI2, OsBr3, OsCl4, OsF5, OsF6, OsOF5, OsF7, IrF6, IrCl3, IrF4, IrF5, Ir(ClO4)3, K3[IrCl6], K2[IrCl6], Na3[IrCl6], Na2[IrCl6], Li3[IrCl6], Li2[IrCl6], [Ir(NH3)4Cl2]Cl, RhF3, RhF4, RhCl3, [Rh(NH3)5Cl]Cl2, RhCl[P(C6H5)3]3, K[Rh(CO)2Cl2], Na[Rh(CO)2Cl2] Li[Rh(CO)2Cl2], Rh2(SO4)3, Rh(HSO4)3 and Rh(ClO4)3, their hydrates and mixtures of these salts and/or hydrates. The term “the platinum group metal precursor” should be understood as any platinum group metal salt or complex soluble in the plating bath, which can be reduced to a metallic form, which is deposited on a substrate.
In the preferred embodiment the platinum group metal precursor concentration in the plating bath used in the method of the invention is in the range from 1 mM to 1 M, more preferably in the range from 5 mM to 100 mM, and most preferably in the range from 10 mM to 50 mM.
In the preferred embodiment, the reduction step in the method of the invention is carried out in the temperature between the freezing point to the boiling point of the plating bath. Preferably, the temperature of the reduction step in the method of the invention is between −10 to 80° C., more preferably between 0 to 40° C., and most preferably between 10 to 25° C.
In the preferred embodiment the pH of the plating bath used in the method of the invention is below 7, more preferably between 3 and 5, and most preferably about 4. The best deposits are obtained when the pH of the plating bath is about 4.
A substrate or support as used herein refers to any material onto which PGMs and their alloys are deposited. The examples of suitable substrates/supports comprise metals, such as platinum, palladium, nickel and gold, metal alloys, such as steel, iron-chromium-aluminum alloys (Kanthal®), polymer materials, such as Nafion®, polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), carbon (e.g. graphite) and silicon. Preferably, the substrate used in the method of the invention is selected from a group comprising metals, metal alloys, polymeric materials, carbon and silicon. More preferably the substrate is selected from the group comprising platinum, palladium, nickel, gold, steel, iron-chromium-aluminum alloys, Nafion®, polyethylene, polypropylene, polyethylene terephthalate, carbon, graphite and silicon.
In the preferred embodiment of the invention the surface of the substrate onto which PGM is deposited is seeded prior to the reduction step. Any standard process of surface seeding can be used according to the invention. One suitable seeding method is a zincation method, wherein zinc is deposited on the substrate surface from a solution containing zincate ions in the presence of metallic zinc. Alternatively, seeding can be performed using tin (II) ions.
After PGM is deposited on the surface it can undergo a high temperature treatment. This thermal treatment is used to convert a rough surface deposits into deposits having smooth surface. This step can be performed only for PGM layers deposited on substrates, which can withstand high temperatures (such as metals, alloys and silicon). Therefore, in the preferred embodiment the method of the invention further comprises a step of a high temperature treatment after the plating process is completed. The high temperature treatment is preferably carried out in temperature between 700-1000° C. More preferably, the substrate with the PGM deposit is flame annealed. Most preferably, the high temperature treatment is carried out in a hydrogen flame.
In order to avoid pH changes during the PGM deposition process, in the preferred embodiment the plating bath further comprises pH buffer. Preferably, the Britton-Robins buffer is used in the plating bath. Other buffers can also be used. The examples of such buffers include phosphate, acetate, citrate, and borate buffers.
Even though addition of brighteners is not required in the method of the invention, their addition is also contemplated herein. Therefore, in the preferred embodiment the plating bath used in the method of the invention further comprises brighteners, such as saccharin polyethylene glycol, paratoluene sulfonamide, benzene sulphonic acid sodium allyl sulfonate, pyridinum propyl sulfonate. Moreover, in the method of the invention further reducing agents can be used in addition to the primary or secondary monohydroxyalcohol or a mixture of primary or secondary monohydroxyalcohols. Therefore, in another aspect of the invention the plating bath used in the method of the invention further comprises hydrazine and its derivatives, borohydride or hydrogen hypophosphite as additional reducing agents.
In another aspect, the invention provides a plating bath for electroless deposition of platinum group metals and their alloys, wherein said bath is an aqueous solution comprising a primary or secondary monohydroxyalcohol or a mixture of primary or secondary monohydroxyalcohols as the reducing agent and one or more platinum group precursors. The plating bath of the invention, as well as its components are described above in reference to the method of the invention.
Finally, the invention provides a use of the plating bath as defined above in electroless deposition of platinum group metals.
Based on numerous experiments carried out by the inventors, no simple tendency in the kinetics of deposition of platinum and other PGMs, as well as in molecular weight of alcohol in the homologous series could be observed. At ambient temperature, in the series of low chain water miscible alcohols, the electroless deposition kinetics decreased in the following series of reducing agents: ethanol, methanol, 2-propanol, 1-propanol, 2-butanol. The higher PGM deposition rate (in the presence of ethanol or at higher temperatures), the higher specific surface of the deposit, which is characteristic for rough deposits. For catalytic applications, where a high specific area is beneficial, ethanol should be used as the reducing agent. Alcohols poorly soluble in water such as 1-butanol and 1-pentanol used as reducing agents also lead to the formation of very rough deposits. The most lustrous deposits were obtained for 1-propanol and 2-propanol at ambient temperature.
The observed trends for different monohydroxyalcohols used in the method of the invention can be summarized as follows:
The inventors have also observed the following mechanisms for the method of the invention:
The method of the invention has numerous advantages. First of all, well-adhering layers of PGMs and their alloys are obtained. The PGM layers deposited by the method of the invention are very stable and durable (e.g. they have remained unchanged for over twelve months after deposition; they have remained stable in the electrochemical experiments; they remain stable under mechanical stress). These layers can be compact and uniform or alternatively can have a very rough surface, depending on deposition conditions. The PGM layers are obtained on different substrates including metals (e.g. platinum, palladium, nickel and gold), metal alloys (steel, Kanthal®), polymer materials (e.g. Nafion®, PE, PP, PET), carbon (e.g. graphite) and silicon. The process can be carried out in a wide range of temperatures (from −10 to 80° C.) and, depending on reducing agent used, the temperature control is not crucial. It is, therefore, possible to carry out the plating process in room temperature. Moreover, the plating bath of the invention has a very simple composition, can be easily prepared and is very stable.
Thanks to its universal character, especially due to the fact that it can be used for plating PGMs on different supports, the method of the invention is very useful in different fields of technology, for example:
The subject of the invention is illustrated in a drawing, in which:
Platinum foil 0.7×0.7×0.01 cm in diameter was hydrogen flame annealed and quenched in deionized water. The substrate was subsequently immersed in a solution containing 2.2 M 2-propanol; 0.01 M [PtCl4]2−+Britton-Robins buffer prepared from aqueous solution containing 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH adjusted to pH=4.06 with sodium hydroxide. The plating bath was kept at room temperature for 10 hours. Metallic silver shine coating of 2 μm in thickness was thus obtained with the 98% yield. The thickness of the deposited layer and deposition yield was determined based on a weight change of the substrate.
A Kanthal® plate was immersed in a solution containing 2.2 M 2-propanol, 0.01 M [PtCl4]2−+Britton-Robins buffer prepared from aqueous solution containing 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH adjusted to pH=4.06 with sodium hydroxide. The plating bath was kept at room temperature for 12 hours. Metallic silver shine coating was obtained on Kanthal® surface.
The above method was also used for platinum deposition on polypropylene (PP), polyethylene terephthalate (PET), Nafion® and silicon. Metallic silver shine coating was obtained on silicon, PP i PET substrate, whereas grey-black coating on Nafion®. Photographs of platinum coated Nafion®, PP and PET are presented on
In a corresponding manner platinum a metallic silver shine layer was obtained on PP substrate using 2.5 M 1-propanol from the solution having pH 4 and containing 7.5 mM of [PtCl4]2−.
A gold plate 0.7×0.7×0.05 cm in diameter was seeded using a zincating method (in an aqueous solution containing 1 M of Na2[Zn(OH)4] in the presence of metallic zinc, the mass of deposited Zn was equal to 50 μg). The substrate was subsequently immersed in a solution containing 2.2 M 2-propanol; 0.01 M [PtCl4]2−+Britton-Robins buffer prepared from aqueous solution containing 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH adjusted to pH=4.06 with sodium hydroxide. The plating bath was kept at room temperature for 12 hours. Metallic silver shine coating of 2.4 μm in thickness was thus obtained with the 95% yield.
Platinum deposits on nickel, steel and graphite substrates were obtained as described in Example 3. Metallic silver shine platinum coating was obtained on all substrates.
Platinum deposits on silver, palladium, PET, PP and Nafion® substrates were obtained as described in Example 3, with that difference that instead of Na2[Zn(OH)4], tin (II) chloride (SnCl2) was used for seeding of the substrate surface. Metallic silver shine platinum coating was obtained on all substrates.
Platinum foil 0.7×0.7×0.01 cm in diameter was hydrogen flame annealed and quenched in deionized water. The substrate was subsequently immersed in a solution containing 2.85 M of ethanol; 0.01 M [PtCl4]2−+Britton-Robins buffer prepared from aqueous solution containing 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH adjusted to pH=4.06 with sodium hydroxide. The plating bath was kept at room temperature for 1 hour. An adherent grey-black coating of 0.4 μm in thickness was thus obtained with the specific roughness factor, Rf=Aspecific/Ageometric=130, where Aspecific and Ageometric is the specific and geometric surface area of the deposit, respectively. Taking into account the mass of the deposit m=1.63 mg, this correspond to 16.3 m2/g of mass specific surface area.
This type of platinum deposit is particularly suitable for use in catalytic applications.
Gold disc 0.2×0.05 cm in diameter was seeded using a zincating method (in an aqueous solution containing 1 M of Na2[Zn(OH)4] in the presence of metallic zinc, the mass of deposited Zn was equal to 20 μg). The substrate was subsequently immersed in a solution containing 4 M methanol; 0.01 M [PtCl4]2−+0.01 M [IrCl4]2−+Britton-Robins buffer prepared from aqueous solution containing 0.04 M H3BO3, 0.04 M H3PO4 and 0.04 M CH3COOH adjusted to pH=4.06 with sodium hydroxide. The plating bath was kept at room temperature for 24 hours. Metallic silver shine coating of 1 μm in thickness was obtained with the 95% yield.
The chemical composition and chemical characteristics of the deposit was investigated by X-ray photoelectron spectroscopy. This method was used to confirm the metallic characteristics of the iridium. XP spectrum in Pt 4f/Ir 4f region reveals two doublets. The doublet with components at 60.71 eV and 63.69 eV can be attributed to metallic Ir (Ir 4f7/2 and Ir 4f5/2 signals, respectively), whereas signals at 71.34 eV and 74.67 eV can be attributed to metallic Pt (Pt 4f7/2 and Pt 4f5/2 signals, respectively). Literature positions for metallic Ir and Pt: 60.81 eV and 63.76 eV for Ir 4f7/2 and Ir 4f5/2 signals, respectively and 71.09 eV and 74.42 eV for Pt 4f7/2 and Pt 4f5/2 signals, respectively (NIST X-ray Photoelectron Spectroscopy Database, Version 4.1 [National Institute of Standards and Technology, Gaithersburg, 2012); http://srdata.nist.gov/xps/.]) were marked with vertical lines. Small difference between the measured and literature peak positions are most probably caused by alloy formation [Adam Lewera et al.: Core-level Binding Energy Shifts in Pt—Ru nanoparticles: A puzzle resolved. Chem. Phys. Lett. 2007, 44: 39-43). Small doublet at ca. 76.69 eV and 80.02 eV is probably associated with presence of small amount of unreduced Pt, most probably in the form of Pt(IV) compounds. Pt to Ir ratio, determined from XPS data, is equal to 75:25 atomic percent (Pt:Ir).
A possibility of alloy formation was investigated further for different alcohols used as reducing agents and for that purpose the following alloy layers on gold substrate were prepared: platinum-rhodium, platinum-ruthenium, platinum-palladium and platinum-iridium. The process of plating was carried out in a corresponding manner as described in Example 7, however 3 mM K2PtCl6 was used as platinum precursor, in consecutive examples 5 M MeOH, 3.5 M EtOH and 2.6 M isopropanol were used as reducing agents, and 6 mM Rh2+, 6 mM Ru2+, 7 mM Pd2+, and 6 mM Ir(IV) were used as precursors of the second PGM in an alloy. The table below summarizes the conditions used for alloy plating of the substrate, as well as the properties of the obtained layer. The alloy formation and composition thereof was determined by EDS (data included in the table).
Alloy platinum-ruthenium and platinum-iridium layers were formed using procedure described in Examples 8 and 9, with a difference that a Kanthal® substrate was used as a solid support for the formed layer and different alcohols—ethanol and sec-butanol—were used as reducing agents. The table below summarizes the conditions used for alloy plating of the substrate, as well as the properties of the obtained layer. The alloy formation and composition thereof was determined by EDS (data presented in the table).
Platinum layers were obtained on PET and PP substrates, as described in Example 2. However, ethanol was used as a reducing agent instead of 2-propanol. Metallic silver shine coating of was obtained.
The influence of different reducing agent on the deposit properties was investigated. Therefore, platinum layers on platinum substrates using different reducing agents were obtained. Prior to deposition, platinum electrodes were flame annealed and quenched in deionized water. Platinum was deposited from plating bath as in Example 1 but different reducing agents were used in concentration 2 M. After 12 hours of deposition cyclic voltammograms have been recorded in 0.5 H2SO4.
The roughest deposit was obtained in methanol containing bath, which corresponds to the highest current densities at cyclic voltammograms (
Scanning electron micrographs of the surface of selected deposits are shown in
The sample obtained from a 2-propanol containing bath is much smoother in comparison to other samples. High magnification micrographs exhibit structuration material on the nanometer scale. Some tendency to fracture can be seen, however their number is much smaller. Also, their shape suggest higher plasticity of this coating. Most likely, some of fractures that can be seen are related to the stresses in the base (substrate) material. Cross-fracture of the sample exhibits no tendency to internal structuration of the coating.
Morphology of the sample obtained from 2-buthanol is shown in
Platinum coated substrates of PET, PP and Nafion® obtained in Examples 2 and 5 have undergone a mechanical stress test. All the platinum coated substrates were bent, stretched and twisted manually. After the test all substrates were examined with respect to platinum coating integrity. No changes, such as cracks or peeling, of the deposited layers were observed. The fact that the platinum layers remained intact during the mechanical stress test shows a significant integrity and a very good adherence of the deposited platinum layer to the polymer material substrate.
Electrochemical properties of PGM deposit layers on gold, Kanthal® and platinum substrates were investigated. In one of the experiments oxidation of ethanol was carried out using the deposit-substrate assemblies obtained in Examples 1-11.
Exemplary results are shown in
| Number | Date | Country | Kind |
|---|---|---|---|
| P.421812 | Jun 2017 | PL | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2018/054067 | 6/6/2018 | WO | 00 |
