EP3874075B1

EP3874075B1 - Method for aluminum electroless deposition

Info

Publication number: EP3874075B1
Application number: EP19880829.7A
Authority: EP
Inventors: Mohammed Taha ELSHARKAWI; Amal ESAWI
Original assignee: American University in Cairo
Current assignee: American University in Cairo
Priority date: 2018-10-30
Filing date: 2019-10-30
Publication date: 2026-02-25
Anticipated expiration: 2039-10-30
Also published as: EP3874075A2; WO2020089690A3; US20220002877A1; WO2020089690A2; EP3874075A4; US20240158919A1

Description

TECHNICAL FIELD

The present invention relates to a method for electroless deposition of aluminum on a substrate which employs materials which are commercially available at a cost suitable for large scale production quantities.

BACKGROUND

Aluminum coatings are applied to many substrates to impart strength, abrasion resistance corrosion resistance, barrier properties, thermal conductivity and electrical conductivity. The substrate may include, but is not limited to glass, metal, metal oxide, ceramic, organic materials, or polymer and may have any geometry varying from a simple geometry such as a flat sheet to a more complex geometry.
Conventional methods for applying aluminum coatings or thin films include methods such as aluminum cladding wherein aluminum and another metal are bonded together by application of pressure at a suitable elevated pressure, thermal or slurry spray methods wherein molten or semi-molten aluminum or an aluminum containing slurry is sprayed onto a substrate, physical or chemical vapor deposition wherein thin aluminum film is produced through condensation of a vaporized form of aluminum under high vacuum or electrolytic deposition (also referred to as electrolytic plating) wherein electrical current is used to reduce dissolved aluminum cations to form an aluminum film on an electrode. Each of these methods incurs energy, equipment and environmental requirements which may be problematic and is limited to a substrate of simple geometry. For example, aluminum cladding is generally limited to flat sheets and difficult to apply to complex geometries. Thermal or slurry spraying also has limitation to the geometry of the substrate and requires line of sight for the coating to be deposited. In addition, the spraying methods tend to have non-uniformity issue and the product may contain contaminants from the process and therefore limiting the application of this method. Physical or chemical vapor deposition, on the other hand, requires expensive and specialized equipment, is conducted at a high temperature, and is only applicable to selected substrates with simple geometries. Aluminum electrodeposition in an aqueous medium is difficult because the standard reduction potential of aluminum is -1.66E (V) and therefore, water is electrolyzed in favor of electrodeposition of aluminum.
Electrolytic deposition of aluminum may be accomplished in a solvent system. However, in addition to the problem associated with the use of highly flammable solvents, this method is only applicable to the deposition of pure aluminum. When using ionic liquid based aluminum alloy deposition, due to the low solution conductivity of aluminum or aluminum alloys, it is often difficult to plate alloy onto complex geometries, and also difficult to produce a coating of uniform thickness. In addition, this process requires very high power and is only applicable to conductive substrates. In the case of non-conductive substrates the surface has to be modified with a conductive or catalyst layer prior to the electrolytic deposition.
In view of the issues described regarding conventional aluminum application methods, electroless deposition of aluminum has become of increasing interest.
Shitanda et al. (Electrochimica Acta, 54 (2009) 5889-5893) describes a method for electroless aluminum deposition on glass wherein the glass surface is first treated with a catalytic coating in a two stage process. The surface is initially treated with a tin chloride solution and then treated with a palladium chloride solution to prepare a Pd catalyst surface on the glass substrate. Next the part to be plated is dipped in 1-ethyl 3-methylImidazolium chloride (EMIC) - aluminum chloride Room Temperature Ionic liquid (RTIL) that has diisobutyl aluminum hydride in toluene as a liquid reducing agent. The main source of aluminum according to this method is attributed to reduction of Al₂Cl₇ ^- ions according to the equation:

4Al₂Cl₇ ^- + 3e^- → 7AlCl₄ ^- + Al
The present inventors have studied this electroless plating process and determined that this method has two significant negative attributes which inhibit its use as an industrial scale process. The cost of the ionic liquid EMIC is extremely high and this material is not readily available as a commercial material. Additionally the method employs a two-step catalyzation process that must be optimized according to the type of surface to be plated.
Thus, there is a need for a method to apply aluminum coatings to various and multiple substrates that is economical on an industrial scale and is adaptable to a wide range of substrates and substrates of complex geometries and varying size including nanostructures such as nanofibers, nanoparticles, nanotubes, nano-rods and quantum dots.
CN 101210319 A describes a chemical aluminum plating solution and a chemical aluminum plating method. The chemical aluminum plating solution is composed of an aluminum-containing room-temperature molten salt and a reducer, wherein 0.05 to 2.5 mol reducer is contained in every liter of the aluminum-containing room-temperature molten salt. The method comprises the following steps of: soaking a workpiece to be plated in SnCl₂ solution for sensitizing treatment, cleaning with water, and soaking in a PdCl₂ solution for activation; or soaking a mixture solution containing SnCl₂ and PdCl₂ for activation; and chemically plating aluminum using the chemical aluminum plating solution in a dry inert atmosphere.

SUMMARY OF THE INVENTION

These and other objects are provided by the present invention, the first embodiment of which includes a method for electroless deposition of aluminum or an aluminum alloy on a substrate surface, comprising:

activating the surface of the substrate to be coated by applying a coating of a catalyst metal;
preparing a mixture of urea (NH₂CONH₂) and anhydrous aluminum chloride (AlCl₃) wherein a molar ratio of AlCl₃: NH₂CONH₂ is greater than 1:1 to obtain a Lewis acid room temperature ionic liquid (RTIL);
dissolving a hydride reducing agent in an aprotic anhydrous solvent to obtain a hydride solution;
optionally, when an Al alloy coating is to be deposited, adding an anhydrous alloy metal salt as a solution to the RTIL;
mixing the hydride solution and the AlCl₃: NH₂CONH₂ RTIL to obtain an electroless Al solution;
exposing the activated surface of the substrate to the electroless Al solution; and
removing the electroless Al solution from the substrate surface;
wherein upon exposure of the activated substrate surface to the electroless Al solution, an Al or Al alloy coating is obtained on the activated substrate surface,
wherein the hydride reducing agent is selected from the group consisting of lithium hydride, lithium aluminum hydride, diisobutyl aluminum hydride and combinations thereof.

In one aspect of the first embodiment the AlCl₃: NH₂CONH₂ molar ratio of the AlCl₃: NH₂CONH₂ RTIL is from greater than 1:1 to 2:1 and in a special aspect the AlCl₃: NH₂CONH₂ molar ratio of the AlCl₃: NH₂CONH₂ RTIL is 2:1.
In one aspect of the first embodiment the hydride reducing agent is lithium aluminum hydride.
In one aspect of the first embodiment, the aprotic anhydrous solvent is selected from the group consisting of tetrahydrofuran, diethyl ether, dibutyl ether, dioxane, toluene and hexane.
In one aspect of the first embodiment when an Al alloy coating is deposited, the alloy metal salt is dissolved in an aprotic solvent; and added to the RTIL prior to the addition of the hydride reducing agent; wherein the metal salt is selected from the group consisting of a halide salt of zinc, chromium, iron, nickel, tin, lead, copper, silver, gold and combinations thereof.
In one aspect of the first embodiment the catalyst metal is selected from the group consisting of iron, palladium, silver, gold, platinum and combinations thereof and in a special aspect the catalyst metal in Pd.
In another aspect of the first embodiment the activation of the surface of the substrate to be coated comprises: treating the surface with a colloidal solution of palladium-tin (Pd-Sn) nanoparticles in the presence of HCl and water to cover the surface of the substrate with a layer of adsorbed catalytic Pd-Sn nanoparticles comprising stannous hydroxide covered on their surface; cleaning the substrate surface from the residues of the colloidal solution; and placing the substrate in an acidic accelerator solution wherein the excess stannous hydroxide layer is removed from the surface of the substrate for an increased catalytic activity.
In one aspect of the first embodiment the substrate surface is non-reactive to Al deposition and/or is non-conductive.
In an aspect of the first embodiment the substrate is a nanostructure and in a special aspect the nanostructure is selected from the group consisting of a nanofiber, a nanoparticle, a nanotube, a nano-rod and a quantum dot.
In one special aspect of the first embodiment the substrate is composed of a polymer selected from the group consisting of ABS, PL A, Nylon, Teflon and PMMA and the AlCl₃: NH₂CONH₂ molar ratio is from 1.3:1 to 1.5:1.
In another special aspect of the first embodiment, the substrate is a metal coated polymer and the AlCl₃: NH₂CONH₂ molar ratio is 2:1.
In another special aspect of the first embodiment the substrate is selected from the group of fibers consisting of a glass fiber, an aramid fiber and a carbon fiber.
In a further special aspect of the first embodiment the substrate is selected from the group of yarns consisting of a glass fiber yarn, a Kevlar fiber yarn and a carbon fiber yarn.
In a further special aspect of the first embodiment the substrate is selected from the group consisting of a fullerene, a Bucky paper and a Bucky sheet.
In a further special aspect of the first embodiment the substrate is selected from the group of 2-D materials consisting of graphene, molybdenum disulfide (MOS₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), and zinc oxide(ZnO).
In a further special aspect of the first embodiment the substrate is selected from the group consisting of graphene powder and graphene nanoparticles.
In a further special aspect of the first embodiment the substrate is selected from the group consisting of a ZnO microtube and a ZnO nanowire.
In an additional aspect of the first embodiment the substrate is selected from the group consisting of steel, a steel alloy, glass and a ceramic.
In a second embodiment the present invention includes a method for coating a substrate with an anodized aluminum oxide layer, comprising:

activating the surface of the substrate to be coated by applying a coating of a catalyst metal;
preparing a mixture of urea (NH₂CONH₂) and anhydrous aluminum chloride (AlCl₃) wherein a molar ratio of AlCl₃: NH₂CONH₂ is greater than 1:1 to obtain a Lewis acid room temperature ionic liquid (RTIL);
dissolving a hydride reducing agent in an aprotic anhydrous solvent to obtain a hydride solution;
mixing the hydride solution and the AlCl₃: NH₂CONH₂ RTIL to obtain an electroless Al solution;
exposing the activated surface of the substrate to the electroless Al solution; and
removing the electroless Al solution from the substrate surface to obtain an electroless aluminum plated substrate;
submerging the electroless aluminum plated substrate in an electrolytic solution;
applying an anode current to the aluminum coat to form an aluminum oxide coat comprising a barrier layer; and
treating the aluminum oxide layer to form pores in the aluminum oxide structure.

The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

Fig. 1 is a schematic representation of the coating stages of the method according to one embodiment of the invention.
Fig. 2 is a schematic representation of the experimental procedure of the example.
Fig. 3 shows a chemical equation description of the Al deposition according to an embodiment of the invention.
Fig. 4A shows a TEM image of CNTs coated with Al as prepared in the example.
Fig. 4B shows a SEM image of CNTs coated with Al as prepared in the example.
Fig. 5A shows a TEM image of the surface morphology of CNTs coated with Al as prepared in the example.
Fig. 5B shows a SEM of the surface morphology of CNTs coated with Al as prepared in the example.
Fig. 6 shows an EDX spectrum of CNTs coated with Al as prepared in the example.
Fig. 7 shows and XRD diffraction pattern of CNTs coated with Al as prepared in the example.
Fig. 8 shows a Raman analysis of CNTs coated with Al as prepared in the example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description the words "a" and "an" and the like carry the meaning of "one or more." The phrases "selected from the group consisting of," "chosen from," and the like include mixtures of the specified materials. Terms such as "contain(s)" and the like are open terms meaning 'including at least' unless otherwise specifically noted. Where a numerical limit or range is stated, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.
In view of the above described disadvantages of conventionally known methods of Al electroless deposition, the inventors conducted research to find a low cost and readily available RTIL that would resolve the previously mentioned problems. Such an alternative ionic liquid would significantly lower the manufacturing cost of high surface area substrates such as Al-CNT powders and make it feasible to explore the wide variety of options in using such a composite with different Al-CNT percentages in industrial metallic parts. As described above the inventors recognized that Al₂Cl₇ ^- ions are the main ions that promote the electrodeposition of aluminum in any ionic liquid. Therefore, a search was conducted for a cost effective ionic liquid which supports the formation of Al₂Cl₇ ^- ions. It was discovered that an ionic liquid formed from AlCl₃ and Urea results in the formation of Al₂Cl₇ ^- ions when used with 2:1 molar ratio or greater of AlCl₃ and Urea respectively. This ionic compound was originally created as an electrolyte for batteries due to its high conductivity (Angell et al. Proc. Natl. Acad. Sci. 2017, 114 (5), 834-839). However, when used at higher molar ratios, aluminum undesirably deposited on battery electrodes which was problematic for battery function and performance,
The inventors have discovered that when AlCl₃-Urea at a molar ratio greater than 1:1 is employed as a RTIL in combination with specific hydride reducing agents in an anhydrous aprotic solvent an efficient and economical method for aluminum electroless plating was obtainable. The newly developed RTIL was found to generate the desired Al₂Cl₇ ^- ions when used at a molar ratio of AlCl₃ to urea greater than 1:1. Although not wishing to be bound by theory the inventors believe the Al plating process may be explained by the chemical processes shown in Fig. 3. Interestingly and unexpectedly, as shown in Fig. 3, Al deposition occurs not only by reduction of the Al₂Cl₇ ^- ion but also via reduction of an ion derivative of the AlCl_3' NH₂CONH₂ RTIL shown as [AlCl_2·(urea)_n]⁺ and by catalyzed decomposition of the LiAlH₄.
Due to the low cost of urea, the process could be scaled up to industrial levels in a cost effective manner without affecting the quality of aluminum coatings. In addition, a one-step colloidal palladium surface catalyzation process was adopted to render the surfaces to be electroless plated catalytic prior to the electroless plating process. The colloidal palladium resulted in a major cut in the optimization time for the catalyzation step of the entire electroless plating process compared to the conventional two-step system. Further the method may be universally applied to a wide variety of substrates of different chemical composition, size and geometries.
Thus in a first general embodiment of the present disclosure a method for electroless deposition of aluminum or an aluminum alloy on a substrate surface is provided. The method comprises:

activating the surface of the substrate to be coated by applying a coating of a catalyst metal;
preparing a mixture of urea (NH₂CONH₂) and anhydrous aluminum chloride (AlCl₃) wherein a molar ratio of AlCl₃: NH₂CONH₂ is greater than 1:1 to obtain a Lewis acid room temperature ionic liquid (RTIL);
dissolving a hydride reducing agent in an aprotic anhydrous solvent to obtain a hydride solution;
optionally, when an Al alloy coating is to be deposited, adding an anhydrous alloy metal salt as a solution to the RTIL;
mixing the hydride solution and the AlCl₃: NH₂CONH₂ RTIL to obtain an electroless Al solution;
exposing the activated surface of the substrate to the electroless Al solution; and
removing the electroless Al solution from the substrate surface;
wherein upon exposure of the activated substrate surface to the electroless Al solution, an Al or Al alloy coating is obtained on the activated substrate surface,
wherein hydride reducing agent is selected from the group consisting of lithium hydride, lithium aluminum hydride, diisobutylaluminum hydride and combinations thereof.

The AlCl₃: NH₂CONH₂ ratio may vary from greater than 1:1 to 2:1. As the ratio increases toward 2:1 the Lewis acidity of the RTIL increases and the ratio may be adjusted within the described limits to provide a Lewis acidity compatible with the substrate to be plated.
In an aspect of the present method the AlCl₃: NH₂CONH₂ molar ratio may be 2:1.
In one preferred aspect the hydride reducing agent may be lithium aluminum hydride (LiAlH₄).
The hydride reducing agent may be dissolved in an aprotic anhydrous solvent to be added to the AlCl₃: NH₂CONH₂ RTIL. The aprotic anhydrous solvent may be one or more of tetrahydrofuran (THF), diethyl ether, dibutyl ether, dioxane, toluene and hexane. In some RTIL compositions where viscosity is high, the electroless mixture may be further diluted with one or more of these aprotic anhydrous solvents to lower the viscosity.
An aluminum alloy plating may be obtained by dissolving an anhydrous alloy metal salt in the AlCl₃: NH₂CONH₂ RTIL prior to addition of the solution of the hydride reducing agent solution. The alloy metal salt may be first dissolved in one or more of the listed aprotic anhydrous solvents and the obtained solution added to the AlCl₃: NH₂CONH₂ RTIL. Although any solvent soluble salt may be useful, halide salts (F, Cl, Br and I) may be preferred and chloride salts may be most preferred. The alloy element may be any metal which alloys with aluminum and preferably may be one or more selected from zinc, chromium, iron, nickel, tin, lead, copper, silver and gold.
When the substrate is a metal activation of the surface may not be necessary. However, when the substrate surface is not reactive to electroless Al deposition and/or not conductive, the surface may be activated with application of a metal catalyst. The catalyst metal may be selected from the group consisting of iron, palladium, silver, gold, platinum and combinations thereof. In one preferred aspect the catalyst may be palladium.
In one special aspect of the first embodiment the activation of the surface of the substrate to be coated comprises: treating the surface with a colloidal solution of palladium-tin (Pd-Sn) nanoparticles in the presence of HCl and water to cover the surface of the substrate with a layer of adsorbed catalytic Pd-Sn nanoparticles comprising stannous hydroxide covered on their surface; cleaning the substrate surface from the residues of the colloidal solution; and placing the substrate in an acidic accelerator solution wherein the excess stannous hydroxide layer is removed from the surface of the substrate for an increased catalytic activity. A schematic drawing of the application of the Pd-Sn nanoparticles and subsequent plating of aluminum is shown in Fig. 1. This method is based upon the description of Cohen et al. (The Chemistry of Palladium-Tin Colloid Sensitizing Processes, J. Colloid Interface Sci. 1976, 55 (1), 156-162).
As indicated in Fig. 1 the substrate surface need not be functionalized to promote application of the Pd-Sn nanoparticles. According to the method a layer of colloidal Pd-Sn nanoparticles is coated to the substrate surface (Carbon nanotubes in Fig. 1). The initial coating contains an excess amount of stannous hydroxide which is removed by treatment with an aqueous acid solution known as accelerator solution. The acid may be any inorganic acid capable of dissolving and removing stannous hydroxide and may be selected from H₂SO₄, HCl, HBF₄ and NH₄BF₄.
As indicated above the substrate may be a metal wherein application of a catalyst layer is not necessary or a non-reactive surface where catalytic activation is necessary.
Examples of substrates where catalytic activation is necessary include non-metal nanostructures including nanofibers, nanoparticles, nanotubes, nano-rods and quantum dots. The Al coating of carbon nanotubes, including and multi-wall carbon nanotubes (MWCNT) is described in the Example and supporting analytical information shown in Figs. 4-8 as described in the Example. The method may also be applied to single walled carbon nanotubes (SWCNT).
The method for electroless deposition of Al or an Al alloy may be employed to coat polymers including ABS, PLA, polyamides, polyimides such as Kapton films, Teflon, fluorinated sulfones, polyethylene oxide and PMMA. Polyelectrolytes such as poly(ethylene dioxythiophene):polystyrene sulfonate (PEDOT:PSS) may also be coated. Due to the potential for polymer degradation in a strong Lewis environment, the AlCl₃: NH₂CONH₂ molar ratio employed in the coating of these materials may be from 1.3:1 to 1.5:1.
However, if the polymer is already metal coated, thus protected from the AlCl₃ Lewis acidity, the AlCl₃: NH₂CONH₂ molar ratio employed in the coating may be 2:1.
Other substrates that may be coated or plated according to the present invention include fibers, such as glass fibers, aramid fibers and carbon fibers; yarns, such as glass fiber yarns, Kevlar fiber yarns and carbon fiber yarns; allotropes of carbon such as fullerenes, Bucky paper and Bucky sheets; graphene powder; graphene nanoparticles, NMC532/graphite; hollow carbon nanospheres; Li₂FeSiO₄/C nanospheres. polystyrene nanospheres; ZnO microtubes; ZnO nanowires; silver nanowires and 2-D materials such as graphene, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), zinc phthalocyanine (ZnPc) and zinc oxide( ZnO).
Other substrates which may be Al or Al alloy coated according to the present invention include all grades of steel and steel alloys, elemental and precious metals, metal alloys, glass including sulfide based glasses and ceramic materials including Li conducting lanthanum zirconate ceramic structures (LLZO). Included in this group of substrates may be copper, silver, gold, aluminum, zinc, nickel, platinum, iron, carbon steel, stainless steel, lead, bronze, brass, boron, gallium, indium, and lithium. Further possible substrates may include amorphous, polycrystalline, and single crystalline silicon, amorphous and polycrystalline silicon germanium (SiGe), silicon dioxide, silicon doped with any of antimony, phosphorous, arsenic, boron, gallium and indium as well as very large scale integration (VLSI) semi conducting surfaces, complimentary metal-oxide-semiconductors (CMOS), P-type semi conductor, N-type semi conductor, PN junctions, PNP junctions and NPN junctions.
Additional substrates suitable for Al or Al coating according to the present invention may include microelectromechanical systems (MEMS), solar cells, and transparent electrodes for solar cells.
Metal salts may also be substrates which may be plated or coated according to the present invention. Examples of such metal salts may include LiS, MoO₃, MnO₂, LiNi_0.5Mn_1.5O₄, indium tin oxide and MnCO₃.
Advantageously, the method for electroless deposition of aluminum or an aluminum alloy according to the present invention provides a low cost approach for aluminum or Al alloy electroless plating which is virtually universally applicable to a wide range of substrate materials. The materials employed are inexpensive and readily available in comparison to materials employed in previously described methods. The AlCl₃: NH₂CONH₂ RTIL has a wide electrochemical window and may be used to plate on non-conductive and non-reactive surfaces. Further, the method may be applied to coat or plate substrates of complex 3 dimensional structure.
A further embodiment of the present invention includes a method to coat a substrate with an anodized aluminum oxide layer wherein an Al coated substrate obtained according to the first embodiment and the various aspects thereof may be submerged in an electrolytic solution and an anode current applied to the Al coating to obtain an aluminum oxide coating having an outer barrier layer. Anodization of aluminum is conventionally known and may be conducted in an electrolyte such as chromic acid, sulfuric acid, oxalic acid or phosphoric acid. According to known theory, during the anodization a thin aluminum oxide film is formed on the aluminum coating. As the electric current flows at the aluminum - electrolyte border there grows a thin dense electrolyte film as a barrier layer which forms due to the migration of aluminum ions towards oxygen ions. The thickness of this barrier layer may be from 0.01 - 0.1 nm and may not change throughout the process as it dissolves at the outer side exposed to the electrolyte.
The electrochemical field localizes on inhomogeneities of the surface of formed aluminum oxide and the oxide dissolves under the influence of the inhomogeneity of the field thus leading to the growth of pores. The alumina layer may then be dissolved, leaving a regular array of porous aluminum and when anodization is repeated a layer of porous aluminum oxide is obtained.
In an extended application of this process organic or inorganic pigments may be inserted within the aluminum oxide pores to give the aluminum oxide an aesthetic look. The colored substrate may then be inserted in boiling water to seal the pores by forming a transparent outer aluminum hydroxide Al(OH)₃ layer via a method known in industry as "hydration pore closure".
The combination of the electroless deposition of aluminum according to the present invention and such methods to form an anodized aluminum oxide layer and porous alumina layer allows for application of these processes to a wide range of substrates as previously described and provides products and decorative structures not previously readily available.
The above description is presented to enable a person skilled in the art to make and use the embodiments and aspects of the disclosure, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the scope of the appended claims. Thus, this disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the disclosure may not show every benefit of the disclosure, considered broadly.

EXAMPLE

Multi-wall carbon nanotubes were obtained from Thomas Swan Corporation (average diameter of 10-15 nm) were used in the present study. The colloidal palladium-tin solution and the accelerator acids were obtained from Macdermid Enthony USA. Aluminum chloride anhydrous was obtained from Alfa-Aeser. Urea (99.9%) was obtained from Lobachemie India.
The growth of aluminum on CNTs has taken place in 3 different steps. The first step is the catalytic activation of CNTs by palladium nanoparticles. Then, excess stannous hydroxide was removed from the surface via a group of accelerating acids.
CNTs were catalyzed using a colloidal Pd-Sn solution based on the description of Cohen et al. (The Chemistry of Palladium-Tin Colloid Sensitizing Processes, J. Colloid Interface Sci. 1976, 55 (1), 156-162). The colloidal solution was prepared from 62.5 ml of commercial colloidal Pd-Sn concentrate, 50 ml of HCl (37%), and 137.5 ml of DI water. CNTs of 0.1 g were immersed in the prepared solution and left for 1-minute sonication agitation and extra 3 to 4 minutes of stir agitation. The CNTs were then filtered using a 0.22 µm PTFE filter membrane on a microfiltration kit. The collected CNTs were then dispersed in what is known industrially as the accelerator solution. The acceleration step is composed of a group of acids beneficial for the removal of excess stannous hydroxide from the surface of palladium nanoparticles coated on CNTs. The concentration of the activator solution was 50 g/L. The CNTs were refiltered and collected using teasers.
For aluminum electroless deposition, the entire experiment was carried out in a glove box filled with dry argon gas at ambient conditions. To prepare the aluminum electroless deposition electrolyte, 50 grams of 2:1 molar ratio of anhydrous aluminum chloride were used to form an electrolyte that is rich with Al2Cl7- ions.
The aluminum chloride urea reaction is an exothermic reaction and excess heat may result in the decomposition of the entire electrolyte. Failure in controlling the exothermic heat of the reaction leads to a great failure in the electroless deposition. For this reason, strict procedures were carried out to prevent the thermal decomposition of the electrolyte by preparing the volume needed on 4 separate parts to reduce the heat created as a result of the exothermic reaction. The previous step was not sufficient in preventing the decomposition. Therefore, the volumetric flask was cooled with a sealed rubber ice bucket that preserved the dry environment of the chamber.
An ideal electrolyte has a pale yellow color. If light brown color is observed, this will be a sign of the electrolyte decomposition.
After preparing the ionic liquid, Lithium Aluminum Hydride (LiAlH₄) (LAH) was dissolved in Toluene, hexane, or diethyl ether and used as a reducing agent. 1.5, 1.9, 2.5, and 5 grams of LAH were tested. The activated CNTs were immersed in the electroless solution using sonication for 5 minutes and magnetic stirring for 10 minutes. The ionic liquid containing CNTs was viscous and could not be filtered without dilution using an organic solvent. This dilatant solvent had to be the same solvent used in diluting the LAH.
After dilution, the CNTs were filtered and washed thoroughly with hexane.
A schematic representation of the coating stages is shown in Figure 1 and a schematic representation of the experimental procedures is shown in Figure 2.
The Al coating was confirmed using SEM and TEM imaging. Chemical analysis was performed using EDX. Crystal structure of aluminum was confirmed using XRD. Raman analysis was carried out to confirm the existence of CNTs that are coated with aluminum.
The aluminum coated MWCNTs were characterized by scanning electron microscopy (SEM) analysis using (LEO SUPRA 55VP FEG, Zeiss, equipped with Oxford EDS detector), transmission electron microscopy (TEM) using (JEM-2100 LaB6, JEOL, operating at 200 kV and equipped with Gatan SC200B CCD camera), energy dispersive X-ray (EDX) attached to the SEM, X-ray diffraction (XRD) using (Cu Ka, Panalytical Xpert Pro diffractometer).
It was confirmed by TEM imaging shown in Figure 4-A and SEM imaging shown in Figure 4-B that there is a considerable increase in the diameter of CNTs indicating that aluminum was coated successfully on the CNT's surface.
It was observed that the aluminum coated on CNTs is nanostructured as shown in Figure 5-A. When the aluminum coated CNTs were crushed with a mortar and pestle, it was possible to break some of the aluminum coated parts on the CNTs. Therefore, the difference of the coated and uncoated part of a CNT that has two branches could be shown in Figure 5-B. The aluminum coat is much thicker than the CNT itself which would be preferable when used in a composite material.
Chemical analysis of the sample was conducted to confirm the existence of aluminum on the CNTs surface. The chemical analysis was conducted using an Energy Dispersive Xray (EDX). The EDX spectrum shown in Figure 6 indicates the presence of aluminum, carbon and some aluminum oxides in the tested sample as listed in Table 1. Table 1

ELEMENT WEIGHT %

Aluminum 84.67

Oxygen 13.67 (due to Al activity)

Carbon 1.67

TOTAL 100
Aluminum in nature cannot be formed in an amorphous form. So, to confirm the FCC crystal structure of aluminum an XRD analysis was performed. Figure 7 shows the XRD diffraction pattern of aluminum coated CNTs which confirmed the existence the aluminum coat in a crystalline form. As the diffraction depends on how heavy the atom is, it is difficult to observe the CNTs peak at 26° because of their low percentage in the sample as well as the large difference of atomic weight between aluminum and carbon.
The presence of intact CNT inside the aluminum coat was confirmed via Raman analysis shown in Figure 8 which indicated the position of the Dband and the G-band of CNTs.

Claims

A method for electroless deposition of aluminum or an aluminum alloy on a substrate surface, comprising:
activating the surface of the substrate to be coated by applying a coating of a catalyst metal;

preparing a mixture of urea (NH₂CONH₂) and anhydrous aluminum chloride (AlCl₃) wherein a molar ratio of AlCl₃: NH₂CONH₂ is greater than 1:1 to obtain a Lewis acid room temperature ionic liquid (RTIL);

dissolving a hydride reducing agent in an aprotic anhydrous solvent to obtain a hydride solution;

optionally, when an Al alloy coating is to be deposited, adding an anhydrous alloy metal salt as a solution to the RTIL;

mixing the hydride solution and the AlCl₃: NH₂CONH₂ RTIL to obtain an electroless Al solution;

exposing the activated surface of the substrate to the electroless Al solution; and

removing the electroless Al solution from the substrate surface;

wherein upon exposure of the activated substrate surface to the electroless Al solution, an Al or Al alloy coating is obtained on the activated substrate surface,

wherein the hydride reducing agent is selected from the group consisting of lithium hydride, lithium aluminum hydride, diisobutylaluminum hydride and combinations thereof.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the AlCl₃: NH₂CONH₂ molar ratio is from greater than 1.1 to 2:1, preferably 2:1.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the hydride reducing agent is lithium aluminum hydride (LiAlH₄).
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the aprotic anhydrous solvent is selected from the group consisting of tetrahydrofuran (THF), diethyl ether, dibutyl ether, dioxane, toluene and hexane.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein an anhydrous alloy metal salt is added to the RTIL by a method comprising dissolving the alloy metal salt in an aprotic solvent; and
adding the solution to the RTIL;

wherein the metal salt is selected from the group consisting of a halide salt of zinc, chromium, iron, nickel, tin, lead, copper, silver, gold and combinations thereof.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the catalyst metal is selected from the group consisting of iron, palladium, silver, gold, platinum and combinations thereof, preferably palladium.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the activation of the surface of the substrate to be coated comprises:
treating the surface with a colloidal solution of palladium-tin (Pd-Sn) nanoparticles in the presence of HCl and water to cover the surface of the substrate with a layer of adsorbed catalytic Pd-Sn nanoparticles comprising stannous hydroxide covered on their surface;

cleaning the substrate surface from the residues of the colloidal solution; and

placing the substrate in an acidic accelerator solution wherein the excess stannous hydroxide layer is removed from the surface of the substrate for an increased catalytic activity.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 7, wherein the substrate surface is non-conductive.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 7, wherein the substrate is selected from the group consisting of a nanofiber, a nanoparticle, a nanotube, a nano-rod and a quantum dot.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 9, wherein the substrate is a carbon nanotube (CNT), preferably a multi-wall carbon nanotube (MWCNT) or a single-wall carbon nanotube (SWCNT).
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the substrate is composed of a polymer selected from the group consisting of ABS, PLA, Nylon, Teflon and PMMA and the AlCl₃: NH₂CONH₂ molar ratio is from 1.3:1 to 1.5:1, or
wherein the substrate is a metal coated polymer and the AlCl₃: NH₂CONH₂ molar ratio is 2:1.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the substrate is selected from the group of fibers consisting of a glass fiber, an aramid fiber and a carbon fiber, or
wherein the substrate is selected from the group of yarns consisting of a glass fiber yarn, a Kevlar fiber yarn and a carbon fiber yarn.
The method for electroless deposition of aluminum or an aluminum alloy according to claim 1, wherein the substrate is selected from the group consisting of a fullerene, a Bucky paper and a Bucky sheet, or
wherein the substrate is selected from the group of 2-D materials consisting of graphene, molybdenum disulfide (MoS₂), tungsten disulfide (WS₂), tungsten diselenide (WSe₂), and zinc oxide( ZnO), or

wherein the substrate is selected from the group consisting of graphene powder and graphene nanoparticles, or

wherein the substrate is selected from the group consisting of a ZnO microtube and a ZnO nanowire, or

wherein the substrate is selected from the group consisting of steel, a steel alloy, glass and a ceramic.
A method for coating a substrate with an anodized aluminum oxide layer, comprising:
the method for electroless deposition of aluminum or an aluminum alloy on a substrate surface according to claim 1, followed by

submerging the electroless aluminum plated substrate in an electrolytic solution;

applying an anode current to the aluminum coat to form an aluminum oxide coat comprising a barrier layer; and

treating the aluminum oxide layer to form pores in the aluminum oxide structure.