COMPOSITION FOR PLATING METAL COATINGS AND METHODS OF MAKING AND USING THE SAME

Description

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of the earlier priority date of U.S. Provisional Patent Application No. 63/621,887, filed on Jan. 17, 2024, the entirety of which is incorporated herein by reference.

ACKNOWLEDGMENT OF GOVERNMENT SUPPORT

This invention was made with Government support under Contract DE-AC05-76RL01830 awarded by the U.S. Department of Energy. The Government has certain rights in the invention.

FIELD

Disclosed herein is a coating composition for depositing metal comprising a metal chloride precursor, a metal iodide precursor, or a combination thereof, along with a method for depositing metal coatings on various types of substrates using the coating composition.

BACKGROUND

Refractory metal coatings are uniquely capable of serving as chemical diffusion, corrosion, and/or erosion barriers. They have high melting temperatures and low corrosion and diffusion rates, making them ideal as barriers between systems which need to share energy but not mass. In the solar, furnace, and nuclear energy industries, the favorable thermal properties of refractories make them useful as buffers between high and low temperatures; however, refractory coatings are difficult to apply. Often, refractories are plated at high temperatures in molten salts, which can degrade the substrate onto which they are plated. Alternatively, certain metallic coatings can be electroplated using expensive ionic liquids; however, such techniques have their own set of drawbacks, such as poor/inconsistent coating coverage, brittleness, and contamination, resulting in poor-quality coatings. And, current ionic liquids cost several thousand dollars per liter, yet produce films with less than 90% surface coverage as well as cause deposition of salt species that enable interdiffusion through the deposited metal. Conventional techniques used to deposit certain metal coating, such as titanium coatings, also are known to be incompatible with certain metal precursors used in the deposition bath, such as chloride salts. There exists a need in the art for new coating compositions that can be used without relying solely on expensive ionic liquids and also a method by which durable, high-performing metal coatings can be deposited from coating compositions, particularly compositions comprising readily available and cost-effective metal precursors, such as metal chloride and/or metal iodide precursors.

SUMMARY

Disclosed herein is a coating composition, comprising: a deep eutectic solvent comprising an organic salt, a hydrogen-bond donor compound, or a combination thereof; a metal chloride precursor, a metal iodide precursor, or a combination thereof; and an alkaline salt comprising an alkali metal and a counterion selected from chloride, bromide, iodide, or fluoride.

Also disclosed is a method, comprising: (i) combining, in a cell, (a) the coating composition according to aspects of the present disclosure; and (b) a substrate; wherein the cell further comprises one or more electrodes; and (ii) exposing the cell to a voltage protocol using the one or more electrodes to thereby deposit a metal coating on the substrate from the coating composition.

The foregoing and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description, which proceeds with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.

FIG. 1 is a digital image of a steel workpiece before (left image) and after (right image) electrodeposition of a 35 m titanium coating.

FIGS. 2A and 2B show images of (i) results from using scanning electron microscopy (SEM) to show the edge of a titanium-coated steel substrate, which was obtained using a coating composition according to the present disclosure (FIG. 2A); and (ii) a cross-sectional image obtained with metallographic imaging showing a substrate plated with a titanium coating that was obtained from using a coating composition according to the present disclosure (FIG. 2B).

FIG. 3 is a graph of current (A) as a function of concentration (mg/mL) which shows results obtained from evaluating kinetic plating for an exemplary metal chloride precursor according to the present disclosure (TiCl₄) as compared with fluoride-based precursors (i.e., TiF₃ and TiF₄).

FIG. 4 is a scanning electron microscopy (SEM) image showing a titanium coating formed on a steel substrate using a coating composition according to the present disclosure.

FIG. 5 is a graph of reflectance (1-100) as a function of wavelength (nm) showing that a deposited coating made using a coating composition according to the present disclosure exhibits similar spectroscopic reflectance to reference standards (i.e., 99.9% Ti metal, Steel 316 substrate).

FIG. 6 is a graph of reflectance (1-100) as a function of wavelength (nm) showing that a deposited coating made using a coating composition according to the present disclosure exhibits similar spectroscopic reflectance to reference standards/models (i.e., 99.9% Ti, Steel 316), wherein plating was evaluated at different distances in the plated area.

FIG. 7 is a graph of integrated reflectance as a function of the radius from the center of the plating area (measured in cm), which shows that a titanium metal coating formed using a coating composition according to the present disclosure exhibits reflectance on the substrate throughout the plated area until an isolation point is reached, at which point the sample reflectance matches the steel substrate (upon which the titanium coating was deposited).

FIG. 8 is a digital image showing a substrate plated with (i) a 5 m thick titanium coating having oxide inclusions and (ii) a 5 m thick titanium metal coating.

FIGS. 9A and 9B show graphs of layer thickness (m) as a function of plating time (hours) using an exemplary coating composition comprising TiCl₄ (FIG. 9A) and comparing that with a comparison composition comprising TiF₄ (FIG. 9B).

FIGS. 10A-10C show results obtained from using compositions according to the present disclosure, wherein FIG. 10A shows digital images of titanium electrodepositions prepared with such additives; FIG. 10B shows a SEM cross section micrograph of a 1 hour-plated control at 200° C. with demarcation of the various layers including polymer, titanium, and 316 stainless-steel; and FIG. 10C shows the predicted standard reflectance spectra for polymer, titanium, and 316 stainless steel components without any brightening agents.

FIGS. 11A-11C show: darkfield images of titanium electrodeposition surfaces with leveler additives and controls (FIG. 11A); reflectance data for exemplary leveling agents (FIG. 11B); and standard deviation of the leveling agents, with the 95% confidence level shown as error bars (in a case where the error bars are smaller than the icon, the icon is to be used to evaluate the error) (FIG. 11C).

FIGS. 12A-12C show darkfield images of titanium electrodeposition surfaces with brightener additives (FIG. 12A); reflectance data for brightener additives (FIG. 12B); and integrated response of brightener additives, with the 95% confidence level shown as error bars (FIG. 12C).

FIG. 13 shows SEM images of the top of an electroplated titanium layer, wherein EDS mapping of the image shows that the titanium is situated between the steel (Fe signal) and polymer overcoat (carbon signal).

FIG. 14 shows darkfield images of titanium electrodeposition at 30° C. surfaces with polymer (left image, PVP) and zirconium salt additives (right image), both with 3-hour plating times, with island formation is evident in both cases.

FIGS. 15A-15D show results obtained from analyzing deposited coatings formed from coating compositions according to the present disclosure using cyclic voltammetry, wherein: FIG. 15A shows measured cyclic voltammogram with the regions of titanium reduction denoted by dashed lines; FIG. 15B shows square wave voltammograms taken with scan speeds from 10-40 Hz, with responses from the two observed reductive reactions indicated with arrows; FIG. 15C shows a scan speed comparison of cyclic voltammograms from 500-900 mV/s; and FIG. 15D shows a plot of the centers of the reduction responses in FIG. 15C showing a linear trend from the response occurring at about −1.64 V (line with ▪ symbols) and a non-linear influence in the response occurring at −0.8 V (line with ● symbols).

FIGS. 16A-16G show results obtained from analyzing deposited coatings made according to a method of the present disclosure utilizing different temperatures for deposition, wherein: FIG. 16A shows an optical image set showing film evolution as a function of temperature and deposition time; FIG. 16B shows a 20kX SEM image of a cross section of a steel substrate prior to deposition; FIG. 16C shows a 20kX SEM image of a cross section of the substrate after 30 seconds of deposition and at 30° C.; FIG. 16D shows a 35kX SEM image of a cross section the substrate after 5 minutes of deposition and at 90° C.; FIG. 16E provides results of an intensity analysis of the 5-minute plated cross section of FIG. 16D within the boxed region of FIG. 16D, with a 50kX SEM image background inset for layer perspective; FIG. 16F shows a 35kX SEM image of a cross section after 1 hour of deposition and at 90° C.; and FIG. 16G shows results from an XRF analysis of the 1-hour plated sample vs. a steel control.

FIGS. 17A-17E show results obtained from using potentiostatic EIS at two different temperatures for various coating compositions according to the present disclosure using three different deposition time lengths, wherein: FIG. 17A shows a comparison of the initial EIS between 30° C. (▪) and 90° C. (●) for electroplating solutions of TiBr₄ of the same concentration; FIG. 17B shows a comparison of Ohmic resistance of a Ti coating electrodeposited for 30 seconds from different coating compositions at 90° C. (namely, TiF₄ salt (●start, ▪stop), TiCl₄ salt (♦start, ▴stop), and TiBr₄ salt (▾start, ▴stop); FIG. 17C shows a comparison of Ohmic resistance of a Ti coating electrodeposited for different periods from the TiCl₄ salt at 80° C. (▴0 seconds, ♦30 seconds, ▪5 minutes, ●1 hour); FIG. 17D shows an exemplary simple EIS fitting circuit diagram; and FIG. 17E shows an exemplary EIS circuit diagram used to fit EIS post-Ti coating formation.

FIGS. 18A-18D shows results from a batch investigation of the cyclic voltammetry of the reduction to titanium (IV) chloride, wherein: FIG. 18A shows a color map denoting the magnitude of the reductive current for the response occurring at about −0.8 V vs. a standard hydrogen electrode (SHE) taken at various temperatures and scan speeds; FIG. 18B shows a color map denoting the magnitude of the reductive current for the response occurring at about −1.6 V vs. SHE taken at various temperatures and scan speeds; FIG. 18C shows a color map denoting the magnitude of the integrated charge transfer for the response occurring at about −0.8 V vs. SHE taken at various temperatures and scan speeds; and FIG. 18D shows a color map denoting the magnitude of the integrated charge transfer for the response occurring at about −1.6 V vs. SHE taken at various temperatures and scan speeds.

FIGS. 19A and 19B show a plot of the optical absorbance of 410 nm light, related to the concentration of Ti²⁺ with and without electroplating at 80° C. (FIG. 19A), and a plot of the optical absorbance of 520 nm light, related to the concentration of Ti³⁺, with and without electroplating at 80° C. (FIG. 19B).

FIGS. 20A and 20B show current plotted vs. root of the scan speed for 3 ions of titanium and at two temperatures for the reduction current at −0.8V vs. SHE (FIG. 20A) and current plotted vs. the root of the scan speed for 3 counter ions of titanium (IV) and at two temperatures for the reduction current at −1.6V vs. SHE (FIG. 20B).

DETAILED DESCRIPTION

Overview of Terms

The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, “comprising” means “including” and the singular forms “a” or “an” or “the” include plural references unless the context clearly dictates otherwise. The term “or” refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise.

Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims.

Unless otherwise indicated, all numbers expressing quantities of components, molecular weights, molarities, voltages, capacities, and so forth, as used in the specification or claims are to be understood as being modified by the term “about.” Accordingly, unless otherwise implicitly or explicitly indicated, or unless the context is properly understood by a person of ordinary skill in the art to have a more definitive construction, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods as known to those of ordinary skill in the art. When directly and explicitly distinguishing aspects of the present disclosure from discussed prior art, the stated numbers are not approximates unless the word “about” is recited.

Although there are alternatives for various components, parameters, operating conditions, etc. set forth herein, that does not mean that those alternatives are necessarily equivalent and/or perform equally well. Nor does it mean that the alternatives are listed in a preferred order unless stated otherwise. Unless otherwise stated, any of the groups defined below can be substituted or unsubstituted.

In order to facilitate review of the various aspects of the disclosure, the following explanations of specific terms are provided:

Additive Component: A component that can optionally be included in a coating composition of the present disclosure. In some aspects, additive components can be used in some aspects of the disclosure to facilitate increasing coating thickness while also decreasing deposition time. In some aspects, additive components can be added to modify brightness and/or coating surface characteristics of a deposited coating.

Aliphatic: A hydrocarbon group having at least one carbon atom to 50 carbon atoms (C_1-50), such as one to 25 carbon atoms (C_1-25), or one to ten carbon atoms (C_1-10), and which includes alkanes (or alkyl), alkenes (or alkenyl), alkynes (or alkynyl), including cyclic versions thereof, and further including straight- and branched-chain arrangements, and all stereo and position isomers as well. Aliphatic groups are distinct from aromatic groups.

Aromatic: A cyclic, conjugated group or moiety of, unless specified otherwise, from 5 to 15 ring atoms having a single ring (e.g., phenyl) or multiple condensed rings in which at least one ring is aromatic (e.g., naphthyl, indolyl, or pyrazolopyridinyl); that is, at least one ring, and optionally multiple condensed rings, have a continuous, delocalized r-electron system.

Typically, the number of out of plane r-electrons corresponds to the Hückel rule (4n+2). The point of attachment to the parent structure typically is through an aromatic portion of the condensed ring system. For example,

However, in certain examples, context or express disclosure may indicate that the point of attachment is through a non-aromatic portion of the condensed ring system. For example,

An aromatic group or moiety may comprise only carbon atoms in the ring, such as in an aryl group or moiety, or it may comprise one or more ring carbon atoms and one or more ring heteroatoms comprising a lone pair of electrons (e.g., S, O, N, P, or Si), such as in a heteroaryl group or moiety. Aromatic groups may be substituted with one or more groups other than hydrogen.

Bis-Substituted Imide: A compound comprising a di-substituted nitrogen atom that is bound to two substituents and has a negative charge. In particular aspects of the disclosure, the di-substituted nitrogen atom is bound to two heteroaliphatic groups and/or organic functional groups each of which can be the same or different.

Deep Eutectic Solvent (DES): A fluid (typically a liquid) comprising a mixture of an organic salt component and a hydrogen-bonding donor component and wherein the mixture (that is, the DES) has a melting point that is lower than the organic salt component and the hydrogen-bonding donor component. In aspects of the disclosure, the DES is distinct from an ionic liquid component used in any coating composition according to the present disclosure.

Haloaliphatic: An aliphatic group wherein one or more hydrogen atoms, such as one to 10 hydrogen atoms, independently is replaced with a halogen atom, such as fluoro, bromo, chloro, or iodo.

Heteroaliphatic: An aliphatic group comprising at least one heteroatom to 20 heteroatoms, such as one to 15 heteroatoms, or one to 5 heteroatoms, which can be selected from, but not limited to oxygen, nitrogen, sulfur, silicon, boron, selenium, phosphorous, and oxidized forms thereof within the group. Alkoxy, ether, amino, disulfide, peroxy, and thioether groups are exemplary (but non-limiting) examples of heteroaliphatic. In some aspects of the disclosure, a fluorophore can also be described herein as a heteroaliphatic group, such as when the heteroaliphatic group is a heterocyclic group.

Ionic Liquid: A salt that has a fluidic (e.g., liquid) form. In some aspects of the disclosure, the salt has a melting point lower than its surrounding ambient temperature. In aspects of the disclosure disclosed herein, ionic liquids are distinct from a DES used in any coating composition according to the present disclosure.

Metal Coating: A layer (or multiple layers) of a metal material that is deposited onto a substrate by electroplating. In particular aspects of the disclosure, the metal coating comprises a metallic form of a refractory metal. In some aspects of the disclosure, the coating can contain low levels of impurities that do not deleteriously affect the ability to form the coating or its performance, such as, sulfur (which can be present in some aspects of the disclosure at a pph level). In an independent aspect, the impurities can include Cu, Mg, Li, Ca, or the like, at trace amounts (e.g., less than 5%, such as less than 4%, or less than 3%, or less than 2%, or less than 1%).

Metal Chloride Precursor: A salt species comprising a metal component and one or more chloride counterions (e.g., a metal monochloride, metal dichloride, metal trichloride, metal tetrachloride, and the like) and that is reduced to a corresponding metal by electroplating. In some aspects of the disclosure, the metal chloride precursor comprises a refractory metal. Metal chloride precursors according to the present disclosure do not include fluoride counterions, or are other than, metal fluoride salts, including metal tetrafluoride salts, metal trifluoride salts, metal difluoride salts, or metal monofluoride salts.

Metal Iodide Precursor: A salt species comprising a metal component and one or more iodide counterions (e.g., a metal monoiodide, metal diiodide, metal triiodide, metal tetraiodide, and the like) and that is reduced to a corresponding metal by electroplating. In some aspects of the disclosure, the metal iodide precursor comprises a refractory metal. Metal iodide precursors according to the present disclosure do not include fluoride counterions, or are other than, metal fluoride salts, including metal tetrafluoride salts, metal trifluoride salts, metal difluoride salts, or metal monofluoride salts.

Organic Functional Group: A functional group that may be provided by any combination of aliphatic, heteroaliphatic, aromatic, and/or haloaliphatic groups, or that may be selected from, but not limited to, aldehyde (i.e., —C(O)H); aroxy (i.e., —O-aromatic); acyl halide (i.e., —C(O)X, wherein X is a halogen, such as Br, F, I, or Cl); halogen; nitro (i.e., —NO₂); cyano (i.e., —CN); azide (i.e., —N₃); carboxyl (i.e., —C(O)OH); carboxylate (i.e., —C(O)O— or salts thereof, wherein the negative charge of the carboxylate group may be balanced with an M⁺ counterion, wherein M⁺ may be an alkali ion, such as K⁺, Na⁺, Li⁺; an ammonium ion, such as ⁺N(R^b)₄ where R^b is H, aliphatic, heteroaliphatic, haloaliphatic, or aromatic; or an alkaline earth ion, such as [Ca²⁺]_0.5, [Mg²⁺]_0.5, or [Ba²⁺]_0.5); amide (i.e., —C(O)NR^aR^b or —NR^aC(O)R^b wherein each of R^a and R^b independently is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); ketone (i.e., —C(O)R^a, wherein R^a is selected from aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); carbonate (i.e., —OC(O)OR^a, wherein R^a is selected from aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); imine (i.e., —C(═NR^a)R^b or —N═CR^aR^b, wherein R^a and R^b independently is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); azo (i.e., —N═NR^a wherein R^a is hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); carbamate (i.e., —OC(O)NR^aR^b, wherein each of R^a and R^b independently is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); hydroxyl (i.e., —OH); thiol (i.e., —SH); sulfonyl (i.e., —SO₂R^a, wherein R^a is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); sulfonate (i.e., —SO₃—, wherein the negative charge of the sulfonate group may be balanced with an M⁺ counter ion, wherein M⁺ may be an alkali ion, such as K⁺, Na⁺, Li⁺; an ammonium ion, such as ⁺N(R^b)₄ where R^b is H, aliphatic, heteroaliphatic, haloaliphatic, or aromatic; or an alkaline earth ion, such as [Ca²⁺]_0.5, [Mg²⁺]_0.5, or [Ba²⁺]_0.5); oxime (i.e., —CR^a═NOH, wherein R^a is hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); sulfonamide (i.e., —SO₂NR^aR^b or —N(R^a)SO₂R^b, wherein each of R^a and R^b independently is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); ester (i.e., —C(O)OR^a or —OC(O)R^a, wherein R^a is selected from aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); thiocyanate (i.e., —S—CN or —N═C═S); thioketone (i.e., —C(S)R^a wherein R^a is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); thiocarboxylic acid (i.e., —C(O)SH, or —C(S)OH); thioester (i.e., —C(O)SR^a or —C(S)OR^a wherein R^a is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); dithiocarboxylic acid or ester (i.e., —C(S)SR^a wherein R^a is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); phosphonate (i.e., —P(O)(OR^a)₂, wherein each R^a independently is hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group; or wherein one or more R^a groups are not present and the phosphate group therefore has at least one negative charge, which can be balanced by a counterion, M⁺, wherein each M⁺ independently can be an alkali ion, such as K⁺, Na⁺, Li⁺; an ammonium ion, such as ⁺N(R^b)₄ where R^b is H, aliphatic, heteroaliphatic, haloaliphatic, or aromatic; or an alkaline earth ion, such as [Ca²⁺]_0.5, [Mg²⁺]_0.5, or [Ba²⁺]_0.5); phosphate (i.e., —O—P(O)(OR^a)₂, wherein each R^a independently is hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group; or wherein one or more R^a groups are not present and the phosphate group therefore has at least one negative charge, which can be balanced by a counterion, M⁺, wherein each M⁺ independently can be an alkali ion, such as K⁺, Na⁺, Li⁺; an ammonium ion, such as ⁺N(R^b)₄ where R^b is H, aliphatic, heteroaliphatic, haloaliphatic, or aromatic; or an alkaline earth ion, such as [Ca²⁺]_0.5, [Mg²⁺]_0.5, or [Ba²⁺]_0.5); silyl ether (i.e., —OSiR^aR^b, wherein each of R^a and R^b independently is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); sulfinyl (i.e., —S(O)R^a, wherein R^a is selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group); thial (i.e., —C(S)H); or combinations thereof.

Quaternary-Substituted Nitrogen: A nitrogen atom bound to four substituents and comprising a positive charge, wherein each substituent individually can be selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group; or wherein two or more substituents can, together with the nitrogen atom to which they are bound, form a saturated or unsaturated ring. In particular aspects of the disclosure, the nitrogen atom is bound to four substituents other than hydrogen.

Refractory Metal: A class of metals that exhibit resistance to heat and wear and that can have a melting point above 1,850° C., with some members of the class having a melting point above 2,200° C. Exemplary refractory metals can include, but are not limited to, titanium, vanadium, chromium, zirconium, hafnium, niobium, molybdenum, ruthenium, rhodium, rhenium, tantalum, tungsten, osmium, and iridium. In some aspects of the disclosure, technetium and rutherfordium can be considered refractory metals unless otherwise indicated.

Substrate: A physical object having a surface onto which a coating can be electroplated. Substrates can be solid and/or porous and can have any shape and can be made of any material suitable for having a metallic coating formed thereon. In some aspects of the disclosure, the substrate is a metal-based substrate or a ceramic substrate.

Tri-Substituted Sulfur: A sulfur atom bound to three substituents and comprising a positive charge, wherein each substituent individually can be selected from hydrogen, aliphatic, heteroaliphatic, haloaliphatic, aromatic, or an organic functional group; or wherein two or more substituents can, together with the sulfur atom to which they are bound, form a saturated or unsaturated ring. In particular aspects of the disclosure, the sulfur atom is bound to three substituents other than hydrogen.

INTRODUCTION

Metal coatings, and particularly refractory metal coatings, are uniquely capable of serving as chemical diffusion, corrosion, and/or erosion barriers. They have high melting temperatures and low corrosion and diffusion rates, making them ideal as barriers between systems that need to share energy but not mass. Titanium, for example, is highly prized in the aerospace and biomedical industries. In the solar, furnace, and nuclear energy industries, the favorable thermal properties of refractories make them useful as buffers between high and low temperatures. For example, nuclear reactors are an extreme environment for materials, exhibiting high temperatures and aggressive conditions. Additionally, extended service life of parts is necessary, as replacement of parts is a major undertaking. Parts used in nuclear power plants should be highly resistant to corrosion, thereby practically requiring the use of refractory metals. Titanium tubing, as an example, can last forty years or more inside nuclear power plants; however, global consumption of titanium is only increasing, as the use of titanium in the aerospace and biomedical industries is growing. While titanium is a very abundant element, the metal cannot be made through continuous processing, leading to a high price due to the laborious process required to produce the metal. This means that parts made of pure titanium can be very costly to acquire. Depending on the application, it can be significantly more cost effective to utilize titanium-plated (or other refractory metals, such as zirconium) parts, rather than the pure metals or alloys thereof. Such refractory coatings, however, are difficult to apply, particularly as thin coatings. Co-rolling or co-extrusion is considered state of the art but is limited to a select number of substrates and geometries. Refractory coatings can be plated at high temperatures in molten salts; however, this typically leads to degrading the substrate on to which they are placed. And, while molten salt electroplating has been commercialized to an extent, the substrates are even more limited than those that can be used in co-rolling processes because the molten salt electroplating process typically is performed at high temperatures (e.g., 600° C. or higher). Molten salt electroplating also has issues with uneven deposition, the formation of undesirable metal salts, and dendritic growth of the metal, all of which can lead to brittleness and coating failure.

Alternatively, the coatings can be electroplated using compositions consisting of expensive ionic liquids. This method, however, also can experience issues with coating coverage, brittleness, and contamination, resulting in poor-quality coatings for certain metals, particularly metals that are sensitive to oxidation. PVD or plasma spray can also be used to apply thin coatings of refractory metals but cannot currently produce conformal coatings. Cleanliness issues and oxidation problems both lead to highly variable coating quality. Additionally, spray coatings are constrained by line-of-sight and cannot be used for complex geometries.

Particular metal coatings that are of interest in the various industries using such coatings include refractory metal-based coatings. Conventional methods for making such coatings often rely on co-rolling and/or spray coating to obtain the coatings; however, such methods can result in scalding and, as a further drawback, poor cleaning prior to co-rolling can produce a 20% failure rate. On top of the high failure rate, the process is difficult or impossible to use on three-dimensional substrates requiring a uniform refractory metal thickness. And, conventional coatings often exhibit a matte appearance and do not exhibit a level a brightness and/or reflectance needed in certain industries, such as in industrial coatings, aerospace, medicine, and electronics.

Aspects of the present disclosure are directed to a coating composition useful for providing metal coatings at low temperatures, and a method for depositing (e.g., electroplating) such metal coatings on various substrates using the coating composition disclosed herein. The coating composition and methods of the present disclosure avoid many of the drawbacks discussed above. In some aspects of the disclosure, the method is performed at near-room temperature and uses a coating composition that can provide as much as a 200-times cost reduction in the process while also providing coatings that are smooth, uniform, and exhibit good performance. The coating composition utilizes a metal chloride precursor (or a metal iodide precursor or a combination of metal chloride and metal iodide precursors) that facilitates the ability to exclude any fluoride-containing metal precursors (and/or, in certain independent aspects of the disclosure, fluoride-containing alkaline salt components) that are used in conventional baths. While fluoride-containing components (e.g., optional metal fluoride components disclosed herein) can be used in coating compositions according to the present disclosure, they are not used as a sole metal precursor and instead are used in combination with a metal chloride or metal iodide precursor as described herein.

Additionally, the coating obtained from the coating composition can be controlled in terms of thickness and physical characteristics by modifying components of the coating composition and/or the voltage used in the method. In some aspects, the composition can further comprise additives that facilitate imparting particular properties to the resultant coating, such as tuned surface roughness/smoothness and improved appearance (e.g., brightness, reflectance, and the like). Films produced from such disclosed compositions can be used as medical implants (e.g., stents), corrosion protectors for electronics (e.g., wearable technology), and interlayers between dissimilar metals. And, in some aspects of the disclosure, the deposited coatings are conformal to the substrate, cover large surface areas of any substrate, and lack significant dendritic and/or “pinhole” features. An image of a steel substrate comprising a Ti coating formed from an exemplary coating composition and method disclosed herein is shown in FIG. 1. FIGS. 2A and 2B provide images showing the interface between the Ti coating and a steel substrate (FIG. 2A) and edges of plated substrate comprising a Ti coating (FIG. 2B).

Coating Composition and Method

Disclosed herein is a composition and method for forming coatings of metals (e.g., refractory metals, such as Zr, Ti, V, Cr, Mn, Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, or Ir) on substrates. The method of the present disclosure can be used to provide metal coatings (including thin metal coatings) for use in myriad applications, such as optical coatings (e.g., optical coatings used in solar technology), diffusion barriers, corrosion resistance, as well as in nuclear applications (e.g., diffusion barriers used for metallic fuels and oxidation-resistant outer layers for accident-tolerant fuel cladding).

In particular aspects of the disclosure, the method comprises exposing a substrate to a coating composition and applying a voltage to deposit a metal coating formed from components of the coating composition. Additional features of the method are discussed herein. In some aspects of the disclosure, the coating composition comprises, consists essentially of, or consists of (i) a deep eutectic solvent (referred to herein as “DES”); (ii) a metal chloride precursor, a metal iodide precursor, or a combination thereof; (iii) an alkaline salt; and (iv) an optional additive component, an optional metal fluoride/bromide component, or a combination thereof. In yet some additional aspects of the disclosure, the coating composition comprises, consists essentially of, or consists of (i) a DES; (ii) a metal chloride precursor, a metal iodide precursor, or a combination thereof; (iii) an ionic liquid; (iv) an alkaline salt; and (v) an optional additive component, an optional metal fluoride/bromide component, or a combination thereof. In yet some other aspects of the disclosure, the coating composition comprises, consists essentially of, or consists of (i) an ionic liquid; (ii) a metal chloride precursor, a metal iodide precursor, or a combination thereof; (iii) an alkaline salt; and (iv) an optional additive component, an optional metal fluoride/bromide component, or a combination thereof.

The DES of the coating composition can comprise a single chemical species, or a mixture of two or more chemical species. In some aspects of the disclosure, the DES comprises an organic salt, a hydrogen-bond donor compound, or a combination thereof. The organic salt can comprise a quaternary ammonium compound and a counterion. In particular aspects of the disclosure, the counterion is a halide counterion (e.g., chloride, bromide, iodide, fluoride). In representative aspects of the disclosure, the halide counterion is chloride or bromide. In yet additional aspects of the disclosure, the counterion can be an organic counterion, such as acetate, bitartrate, or the like. The quaternary ammonium compound can be selected from choline, N-ethyl-2-hydroxy-N,N-dimethylethanaminium, 2-(chlorocarbonyloxy)-N,N,N-trimethylethanaminium, N-benzyl-2-hydroxy-N,N-dimethylethanaminium, or the like. In particular aspects of the disclosure, the quaternary ammonium compound is choline and the counterion is chloride and thus the organic salt is choline chloride. In yet other aspects of the disclosure, the quaternary ammonium compound is choline and the counterion is acetate and thus the organic salt is choline acetate. In yet additional aspects of the disclosure, the quaternary ammonium compound is choline and the counterion is bromide and thus the organic salt is choline bromide. In yet additional aspects of the disclosure, the quaternary ammonium compound is choline and the counterion is bitartrate and thus the organic salt is choline bitartrate. In some aspects of the disclosure, the DES can comprise a hydrogen-bond donor compound, such as urea, acetamide, 1-methyl urea, 1,3-dimethyl urea, 1,1-dimethyl urea, thiourea, benzamide, glycerol, ethylene glycol, malonic acid, benzoic acid, adipic acid, oxalic acid, succinic acid, citric acid, acetic acid, or combinations thereof. In particular aspects of the disclosure, the hydrogen-bond donor compound is ethylene glycol, acetic acid, or urea.

In particular aspects of the disclosure, the DES comprises an organic salt and a hydrogen-bond donor compound. In representative examples of such aspects of the disclosure, the DES can comprise choline chloride and ethylene glycol (also known as ethaline when the choline chloride and ethylene glycol are present at a ratio of 1:2); choline acetate and ethylene glycol; choline bitartrate and ethylene glycol; choline bromide and ethylene glycol; choline chloride and acetic acid; choline acetate and acetic acid; choline bitartrate and acetic acid; choline bromide and acetic acid; choline chloride and urea; choline acetate and urea; choline bitartrate and urea; choline bromide and urea. The amount of the organic salt and/or hydrogen-bond donor included in the DES can be selected to provide a suitable viscosity for the method disclosed herein. In some aspects of the disclosure, the viscosity is not so viscous that atoms of components of the coating composition are immobilized when a voltage is applied. In some aspects of the disclosure, the viscosity of the coating composition is typically such that it has a “watery” consistency such that atoms are able to be mobile when a voltage is applied. In particular aspects of the disclosure, a desirable viscosity can be achieved in coating composition comprising a DES that comprises a combination of the organic salt to the hydrogen-bond donor component in a ratio ranging 1.9:1 to 2.1:1, such as 2.95:1 to 2.05:1, or 1.999:1 to 2.001:1. In particular aspects of the disclosure, the DES comprises a ratio of organic salt:hydrogen-bond donor component of 1:2.

The metal chloride precursor used in the coating composition can be selected from a metal chloride precursor capable of providing a metal coating upon exposure to a voltage protocol, such as a pulsed voltage (e.g., a pulsed DC voltage), a non-pulsed voltage (e.g., a non-pulsed DC voltage), or a combination thereof. In particular aspects of the disclosure, the metal chloride precursor is one that can provide a refractory metal coating upon exposure to pulsed voltage (e.g., a pulsed DC voltage), a non-pulsed voltage (e.g., a non-pulsed DC voltage), or a combination thereof. In some aspects of the disclosure, the metal chloride precursor can be a metal chloride salt that includes a refractory metal and that can provide a metallic form of the refractory metal upon electroplating. In some aspects of the disclosure, the refractory metal can be selected from Zr, Ti, V, Cr, Nb, Mo, Tc, Rf, Ru, Rh, Hf, Ta, W, Re, Os, or Ir. In particular aspects of the disclosure, the metal chloride precursor is a refractory metal chloride. In some aspects of the disclosure, the refractory metal chloride is a chloride salt of Zr, Ti, V, Cr, Nb, Mo, Tc, Rf, Ru, Rh, Hf, Ta, W, Re, Os, or Ir, including, but not limited to, pentachloride salts, tetrachloride salts, trichloride salts, dichloride salts, and/or monochloride salts. In representative aspects of the disclosure, the metal chloride precursor can be selected from ZrCl₄, TiCl₄, WCl₄, NbCl₄, NbCl₅, TaCl₃, TaCl₅, HfCl₄, VCl₃, VCl₄, IrCl₄, or IrCl₅. Such metal chloride precursors are either available from a commercial source or can be synthesized.

The metal iodide precursor that can be used in the coating composition (in place of, or in combination with, the metal chloride precursor) can be selected from a metal iodide precursor capable of providing a metal coating upon exposure to a voltage protocol, such as a pulsed voltage (e.g., a pulsed DC voltage), a non-pulsed voltage (e.g., a non-pulsed DC voltage), or a combination thereof. In particular aspects of the disclosure, the metal iodide precursor is one that can provide a refractory metal coating upon exposure to pulsed voltage (e.g., a pulsed DC voltage), a non-pulsed voltage (e.g., a non-pulsed DC voltage), or a combination thereof. In some aspects of the disclosure, the metal iodide precursor can be a metal iodide salt that includes a refractory metal and that can provide a metallic form of the refractory metal upon electroplating. In some aspects of the disclosure, the refractory metal can be selected from Zr, Ti, V, Cr, Nb, Mo, Tc, Rf, Ru, Rh, Hf, Ta, W, Re, Os, or Ir. In particular aspects of the disclosure, the metal iodide precursor is a refractory metal iodide. In some aspects of the disclosure, the refractory metal iodide is an iodide salt of Zr, Ti, V, Cr, Nb, Mo, Tc, Rf, Ru, Rh, Hf, Ta, W, Re, Os, or Ir, including, but not limited to, pentaiodide salts, tetraiodide salts, triiodide salts, diiodide salts, and/or monoiodide salts. In representative aspects of the disclosure, the metal iodide precursor can be selected from ZrI₄, TiI₄, WI₄, NbI₄, NbI₅, TaI₃, TaI₅, HfI₄, VI₃, VI₄, Ir4, or IrI₅. Such metal iodide precursors are either available from a commercial source or can be synthesized.

Any metal chloride precursor (or metal iodide precursor) used in the composition/method of the present disclosure is not converted to a fluoride-containing intermediate during the electrodeposition method. In a representative aspect of the disclosure, if TiCl₄ is the metal chloride precursor, it is not converted to TiF₄ or TiF₃ intermediate during the electrodeposition method. In particular aspects of the disclosure, the metal chloride precursor (or metal iodide precursor) that is used in the coating composition is present in an amount that is approximately 50% or greater than the amount of the metallic component that is to be deposited on the substrate. In some aspects of the disclosure, the metal chloride precursor (or the metal iodide precursor) that is used in the coating composition is present in an amount that provides a concentration ranging from greater than 0 nM to 10 M, such as greater than 0.01 nM to 10 M, or 0.01 M to 10 M, or 1 M to 10 M, or 5 M to 10 M. In particular aspects, the metal chloride precursor (or the metal iodide precursor) is present in an amount that provides a concentration ranging from 0.01 M to 9.964 M.

In some aspects of the disclosure, the coating composition can comprise an ionic liquid in addition to the DES. In independent aspects of the disclosure, the coating composition can comprise an ionic liquid without a DES. In such aspects of the disclosure, the ionic liquid can comprise a cationic component and an anionic component. The cationic component be a positively charged compound comprising a quaternary substituted nitrogen atom or a tri-substituted sulfur atom. The anionic component can be a negatively charged compound comprising di-substituted nitrogen atom. In particular aspects of the disclosure, the cationic component can be an ammonium salt or a sulfonium salt and the anionic component can be a bis-substituted imide compound, such as bis(trifluoromethylsulfonyl)imide. In representative aspects of the disclosure, the ionic liquid can comprise triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof. In coating compositions comprising a combination of a DES and an ionic liquid, the two can be present in amounts that provide a ratio of DES:ionic liquid ranging from 19:1 to 1:19 (DES:ionic liquid), such as 15:1 to 1:15 (DES:ionic liquid), or 10:1 to 1:10 (DES:ionic liquid), or 5:1 to 1:5 (DES:ionic liquid), or 3:1 to 1:3 (DES:ionic liquid). In particular aspects of the disclosure, the ratio can be 3:1 (DES:ionic liquid), or 2:1 (DES:ionic liquid), or 1:1 (DES:ionic liquid), or 1:2 (DES:ionic liquid), or 1:3 (DES:ionic liquid). In particular aspects of the disclosure, the ratio of DES:ionic liquid is 3:1, 1:1, or 1:3.

In some aspects of the disclosure, the coating composition can comprise an alkaline salt. In some aspects of the disclosure, the alkaline salt comprises an alkali metal and a halide counterion. Alkali metals can be selected from Group 1 of the periodic table and can include lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), and francium (Fr). In particular aspects of the disclosure, the halide counterion is a chloride ion, a bromide ion, an iodide ion, or, in certain aspects, a fluoride ion. In representative aspects of the disclosure, the alkali metal salt is LiCl, NaCl, or another charge carrier, which can, in certain aspects, be LiF. In an independent aspect of the disclosure, the halide counterion of the alkaline salt is other than a fluoride counterion. In a particular independent aspect of the disclosure, the alkaline salt is not or is other than LiF. In particular aspects of the disclosure, the alkaline salt is present in an amount that provides a concentration ranging from 0 mM to 9 mM, such as greater than 0.01 mM to 9 mM, or 1 mM to 9 mM, or 3 mM to 0.4M, or 4 mM to 9 mM. In particular aspects of the disclosure, the alkaline salt is present in an amount that provides a concentration of 4.3 mM. Interestingly, it was determined that low concentrations of a titanium chloride precursor resulted in an increased plating current, high concentrations exhibited the highest plating current, and medium concentrations resulted in the lowest plating current (see FIG. 3). FIG. 3 presents the amount of current flowing through the system versus the amount of titanium chloride added. These results are not expected based on general knowledge in the art as they are reversed from what would be expected (e.g., a medium amount of metal precursor should produce the highest current where the results in FIG. 3 show the opposite). FIG. 3 concerns a plating current and thus a more ‘negative’ value produces desirable results.

In yet some additional aspects of the disclosure, the coating composition can comprise an optional additive component. The additive component can be used to modify the viscosity of the coating composition, to adjust the time it takes to plate the coating, to modify surface smoothness, to improve the appearance of the coating, or any combination thereof. In some aspects of the disclosure, the additive component can be a urea, a pH-controlling reagent (e.g., a buffer, an acid, and/or a base), a reducing agent (e.g., lithium-based reducing agents, such as lithium aluminum hydride and the like), a brightener, a leveling agent, or combinations thereof. Without being limited to a single theory, it currently is believed that, through the addition of additives like leveling agents and/or brighteners, the electrodeposition diffusion layer can change at the molecular scale to benefit deposit formation. Additives can also create lower energy pathways for the ions to form a solid deposit or lower the transition energies if intermediate ions are formed due to a necessary reduction in oxidation states.

In some aspects, a brightener is used to improve grain orientation uniformity and reduce grain sizes below visible wavelengths, which can create a smooth surface of the coating. In some aspects, brighteners can include small molecules, which typically are molecules having fewer than 27 carbon atoms and comprising at least one carbonyl group (C═O) or sulfonyl group (S═O), a cyclic ring, for a combination thereof. In some particular aspects, the brightener can be a lithium salt (e.g., lithium iodide). In some aspects, the brightener can be selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, lithium iodide, and any combinations thereof. In some aspects, the brightener can facilitate obtaining a bright deposited metal coating that exhibits a brighter appearance than a reference metal material (e.g., a native refractory metal). In some aspects, the bright film produced using a coating composition according to the present disclosure is within 80% of the spectroscopic reflectance measured for the near infrared region of the electromagnetic spectrum (e.g., from 400 nm to 900 nm) of a reference metal material. In some such aspects, the reference metal material was titanium obtained from a commercial supplier (e.g., titanium product no. 348805 from Millipore Sigma (formerly Sigma Aldrich). The amount of the brightener used in the composition can range from 1 nM to 500 nM, such as 1 nM to 200 nM, or 1 to 100 nM, or 1 to 50 nM, or 10 nM to 50 nM. In an independent aspect of the disclosure, lithium fluoride can be used as a brightener. In such aspects of the disclosure, the lithium fluoride is not present in an amount sufficient to generate fluoride salts upon being combined with the metal chloride precursor (or the metal iodide precursor).

In some aspects, a leveling agent can be used, which can increase layer smoothness of deposited films by improving the film's ability to form in surface troughs compared to peaks. In some aspects, the leveling agent is selected from a polymeric compound or an inorganic leveling agent. In some aspects, polymer compounds useful as leveling agents can include hydrophilic polymeric molecules having molecular weights less than 10 kD. In some aspects, such polymer compounds can include polyalkylene oxides or polyalkylene amines (e.g., PEG, PPG, PEI, and the like), polyvinyl pyrrolidone, and any combinations thereof. In yet other aspects, the leveling agent can be an inorganic leveling agent, such as a chloride salt (e.g., zirconium chloride; rare earth chloride salts, such as lanthanum chloride, cerium chloride, praseodymium chloride, neodymium chloride, promethium chloride, samarium chloride, europium chloride, gadolinium chloride, terbium chloride, dysprosium chloride, holmium chloride, erbium chloride, thulium chloride, ytterbium chloride, lutetium chloride, scandium chloride, and yttrium chloride; or heavy metal chloride salts, such as hafnium chloride, molybdenum chloride, niobium chloride, tungsten chloride, and the like (wherein any such heavy metal chloride salts are used as a distinct component separate from any metal chloride/iodide precursor; and any combinations thereof). In some aspects, the leveling agent can be used in amounts ranging from 0.05 mg/mL to 0.5 mg/mL, such as 0.05 mg/mL to 0.4 mg/mL, or 0.05 mg/mL to 0.3 mg/mL, or 0.05 mg/mL to 0.2 mg/mL, or 0.05 mg/mL to 1 mg/mL. In some aspects, the leveling agent can facilitate obtaining deposited metal coatings that exhibit reduced noise of a spectroscopic reflectance signal measured for the near infrared region of the electromagnetic spectrum (e.g., from 400 nm to 900 nm) of a reference metal material. In some such aspects, the reduction in noise can be at a level of 33% or higher. In some such aspects, the reference metal material is a stainless-steel substrate.

In some aspects, the coating composition can comprise an optional metal fluoride/bromide component, which is a component that is either a metal fluoride salt or a metal bromide salt. In some such aspects, the metal fluoride salt or the metal bromide salt can comprise a metal ion that is identical to the metal ion of the metal chloride or metal iodide precursor component. In other such aspects, the metal fluoride salt of the metal bromide salt can comprise a metal ion that is different from the metal ion of the metal chloride or metal iodide precursor component. In some particular aspects, the metal ion is a refractory metal selected from Zr, Ti, V, Cr, Mn, Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, or Ir. In particular aspects, the metal fluoride/bromide component is a fluoride or bromide salt of Zr, Ti, V, Cr, Mn, Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, or Ir, including, but not limited to, pentafluoride salts, tetrafluoride salts, trifluoride salts, difluoride salts, pentabromide salts, tetrabromide salts, tribromide salts, and dibromide salts. In representative embodiments, the metal fluoride/bromide salt is selected from ZrF₄, TiF₄, WF₄, NbF₄, NbF₅, TaF₃, TaF₅, HfF₄, VF₃, VF₄, IrF₄, IrF₅, ZrBr₄, TiBr₄, WBr₄, NbBr₄, NbBr₅, TaBr₃, TaBr₅, HfBr₄, VBr₃, VBr₄, IrBr₄, or IrBr₅. In aspects of the present disclosure, the metal fluoride/bromide component is not used in the coating composition without a corresponding metal chloride or metal iodide precursor as described by the present disclosure. In particular aspects, the coating composition comprises a mixture of (i) a metal chloride or metal iodide precursor according to the present disclosure, and a metal fluoride component; or (ii) a metal chloride or metal iodide precursor according to the present disclosure, and a metal bromide component; or (iii) a metal iodide precursor and a metal fluoride component; or (iv) a metal chloride precursor and a metal fluoride component; or (v) a metal iodide precursor and a metal bromide component; or (vi) a metal chloride precursor and a metal bromide component.

Particular coating compositions can comprise choline chloride; ethylene glycol; an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some additional aspects of the disclosure, the coating composition can consist essentially of choline chloride; ethylene glycol; an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In yet additional aspects of the disclosure, the coating composition can consist of choline chloride; ethylene glycol; an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiC₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In other aspects of the disclosure, the coating composition can comprise choline chloride; ethylene glycol; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some additional aspects of the disclosure, the coating composition can consist essentially of choline chloride; ethylene glycol; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In yet additional aspects of the disclosure, the coating composition can consist of choline chloride; ethylene glycol; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In yet additional aspects, the coating composition can comprise choline chloride; ethylene glycol; LiCl; a brightener selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition can comprise choline chloride; ethylene glycol; LiCl; a leveling agent selected from a PEG polymer, a PPG polymer, a PEI polymer, polyvinyl pyrrolidone, zirconium chloride, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises choline chloride; ethylene glycol; LiCl; a brightener selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, or a combination thereof; a leveling agent selected from a PEG polymer, a PPG polymer, a PEI polymer, polyvinyl pyrrolidone, zirconium chloride, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises choline chloride; ethylene glycol; LiCl; a metal fluoride component selected from ZrF₄, TiF₄, WF₄, TaF₃, or TaF₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises choline chloride; ethylene glycol; LiCl; a metal bromide component selected from ZrBr₄, TiBr₄, WBr₄, TaBr₃, or TaBr₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises choline chloride; ethylene glycol; LiCl; a metal fluoride component selected from ZrF₄, TiF₄, WF₄, TaF₃, or TaF₅; a metal bromide component selected from ZrBr₄, TiBr₄, WBr₄, TaBr₃, or TaBr₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components.

In other aspects of the disclosure, the coating composition can comprise an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some additional aspects of the disclosure, the coating composition can consist essentially of an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiC₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In yet additional aspects of the disclosure, the coating composition can consist of an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In yet additional aspects, the coating composition can comprise an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof LiCl; a brightener selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition can comprise an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; a leveling agent selected from a PEG polymer, a PPG polymer, a PEI polymer, polyvinyl pyrrolidone, zirconium chloride, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; a brightener selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, or a combination thereof; a leveling agent selected from a PEG polymer, a PPG polymer, a PEI polymer, polyvinyl pyrrolidone, zirconium chloride, or a combination thereof; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; a metal fluoride component selected from ZrF₄, TiF₄, WF₄, TaF₃, or TaF₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; a metal bromide component selected from ZrBr₄, TiBr₄, WBr₄, TaBr₃, or TaBr₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components. In yet additional aspects, the coating composition comprises an ionic liquid selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or any combination thereof; LiCl; a metal fluoride component selected from ZrF₄, TiF₄, WF₄, TaF₃, or TaF₅; a metal bromide component selected from ZrBr₄, TiBr₄, WBr₄, TaBr₃, or TaBr₅; and (i) a metal chloride precursor selected from ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅, (ii) a metal iodide precursor selected from ZrI₄, TiI₄, WI₄, TaI₃, or TaI₅, or a combination of (i) and (ii). In some such aspects, the coating composition consists essentially of the listed components or it consists of the listed components.

In any or all of the above aspects of the disclosure, coating compositions that consist essentially of the recited components typically do not comprise components or other materials that deleteriously affect the coating formed from the coating composition, such as by (but not limited to) preventing or inhibiting the ability of the metal chloride precursor (and/or the metal iodide precursor) to be converted to the desired metal species, or preventing or inhibiting the ability of a thin coating of the metal to be formed, or preventing or inhibiting the ability to achieve a smooth coating of the metal. In some aspects of the disclosure, such components can include glycolic acid or platinum.

In some aspects of the disclosure, the coating composition can be made by combining the DES or the ionic liquid with the metal chloride precursor (and/or the metal iodide precursor) and the alkaline salt and any optional additive and/or any optional metal fluoride/bromide component. In aspects of the disclosure comprising a DES and an ionic liquid, the DES and the ionic liquid are first mixed, followed by metal chloride precursor (and/or the metal iodide precursor) and alkaline salt addition. In aspects of the disclosure comprising a DES without an ionic liquid, the metal chloride salt (and/or the metal iodide precursor) can be dissolved in the DES. The alkaline salt also can be dissolved in the DES concurrently with the metal chloride precursor (and/or the metal iodide precursor), after the metal chloride precursor (and/or the metal iodide precursor) has been dissolved, or before the metal chloride precursor (and/or the metal iodide precursor) has been dissolved. The resulting composition can be sonicated for a suitable time period such that any solids are dissolved. In some aspects of the disclosure, any water and/or air present in the coating composition can be removed by exposing the coating composition to an inert atmosphere and/or applying heat (e.g., 60-120° C.). Representative coating composition preparation methods are described in the Examples section.

The method of forming a metal coating on a substrate using the coating composition disclosed herein can comprise exposing a substrate to the coating composition and applying a suitable energy stimulus (e.g., a voltage, light, and/or thermal stimulus) to facilitate plating a metal from the metal chloride precursor (and/or the metal iodide precursor) onto the substrate.

In some aspects of the disclosure, the method comprises exposing the substrate and coating composition to a voltage. In such aspects, the voltage can be applied using either a pulsed voltage protocol, a non-pulsed voltage protocol, or a combination thereof. The voltage that is used in the method can be selected depending on the type of metal chloride precursor (and/or the metal iodide precursor) used in the coating composition and/or the desired metal to be coated on the substrate. In particular aspects of the disclosure, the voltage is a pulsed DC voltage. Solely by way of example, in aspects of the disclosure where a Ti chloride precursor (and/or the metal iodide precursor) is used in the coating composition (so as to deposit a metallic Ti coating), the applied voltage used in the method can range from −2 V to 1 V, such as −1.98 V to 1 V, −1.98 V to 0 V, or −1.98 V to −0.01 V. In particular aspects, the applied voltage can range from −2 V to −1.7 V, −1.9 V to −1.64 V, or −1.89 V to −1.64 V. In some aspects of the disclosure, the Ti metal is deposited using voltage of −1.89 V to −1.64 V. In some such aspects, the voltage can be applied using a pulsed voltage protocol.

In aspects of the disclosure utilizing a pulsed voltage protocol, the pulsed voltage protocol can comprise using one or more timed intervals where voltage is turned on and off for specified periods of time. In some exemplary aspects of the disclosure, a pulsed voltage protocol involves applying a voltage for 100 ms and then providing a 10 ms rest where no voltage is applied. These on/off voltage intervals can be repeated for a suitable amount of time to provide a desired coating thickness, which is discussed more below. In some aspects of the disclosure, the pulsed voltage protocol is carried out for one hour. Without being limited to a single theory, it currently is believed that using a pulsed voltage protocol can facilitate removing contamination from the substrate and/or coating formed thereof and can facilitate ion diffusion during the process.

In particular aspects of the disclosure, the method is carried out at temperatures ranging from 19° C. to 140° C., such as 19° C. to 130° C., 20° C. to 115° C., or 20° C. to 100° C., or 20° C. to 75° C., or 20° C. to 50° C., or 20° C. to 40° C. or 20° C. to 30° C. The disclosed coating composition and method are thus able to avoid the high temperatures typically required by conventional methods that use molten salt-based coating compositions, such as temperatures of 400° C. or higher, such as 400° C. to 600° C., and thus can preserve energy and thus result in energy/cost savings when compared to conventional methods using higher temperatures. In some aspects of the disclosure, the method is carried out at ambient temperature, such as at temperatures ranging from 19° C. to 26° C., such as 20° C. to 25° C., or 21° C. to 25° C. At least one non-limiting benefit of using ambient temperatures in some aspects of the disclosure is that the lifetime of the coating composition can be maintained for a good period of time, which can also facilitate continuous coating deposition because the coating composition can be used over several coating cycles and/or can even be reused. In some aspects of the disclosure, the lifetime of the coating composition is 6 hours or more. In some aspects of the disclosure, the method can be carried out at temperatures that are higher than ambient temperature, such as at temperatures ranging from greater than 26° C. to 130° C., such as 30° C. to 125° C., or 50° C. to 120° C., or 50° C. to 60° C.

In some aspects, the method can comprise exposing the substrate and the coating composition according to the present disclosure to a light stimulus. In such aspects, the substrate and the coating composition can be exposed to a light generating energy source to facilitate depositing a coating as described herein. In some such aspects, the light generating energy source can be any energy source capable of producing light having a wavelength that is within a range that includes the optical absorbance of the desired chemical reaction/transformation involved, wherein each range endpoint independently is value equal to the optical absorbance ±50 nm to 100 nm. Solely by way of example, the change in oxidation state from Ti³⁺ to Ti²⁺ typically requires an optical absorbance within the UV-vis region of the electromagnetic spectrum, such as 560 nm; therefore, the light produced by the energy source that is used to achieve this particular transformation can range from 460 nm to 660 nm, or 510 nm to 660 nm, or 460 nm to 610 nm, or 510 nm to 610 nm. In some aspects, the light stimulus is used in combination with other energy stimuli, such as the voltage and/or thermal stimuli discussed herein.

In some aspects of the disclosure, the method can be used to deposit metal coatings, such as refractory metal coatings, on various types of substrates. In some aspects of the disclosure, the substrate can be an inherently electrically conductive substrate or a substrate that is modified to be electrically conductive. In some aspects of the disclosure, the substrate can be a metal substrate, such as a Mo substrate, a copper (Cu) substrate, a Zr substrate, a steel substrate, a uranium (U) substrate, an aluminum (Al) substrate, a gold (Au) substrate, or substrates comprising combinations of any such materials. In particular examples, the substrate is a steel substrate or a Cu substrate. In yet additional aspects of the disclosure, the substrate can be a ceramic substrate that is functionalized with a coating that facilitates the substrate being electrically conductive, such as a metal coating. In such aspects, the metal coating formed on the ceramic substrate can be selected from metals mentioned above as the metal substrate.

The method disclosed herein can be used to deposit metal coatings on substrates, such as those described above. In some aspects of the disclosure, the metal coating can fully or partially coat the surface area of the substrate. For example, in some aspects of the disclosure, for any particular amount of surface area that is desired to be coated with a metal using the coating composition can be coated such that more than 80% to 100% of that desired surface area can be coated with the metal, such as greater than 80% to 100%, or 90% to 100%, or 95% to 100%, or 98% to 100%.

The metal coating typically is deposited as an even coating over the desired surface area of the substrate as opposed to a coating comprising patches of the metal material that is deposited randomly (or unevenly) over the desired surface area. In some aspects of the disclosure, the metal coating can be an evenly deposited (e.g., conformal) coating in the sense that there is minimal exposed surface area of the substrate after the coating has been deposited. For example, an even coating that can be formed with minimal exposed substrate surface area using coating composition and method of the present disclosure is shown by FIG. 4. As can be seen in FIG. 4, the coating composition, which comprised a DES, provided an even coating of metallic titanium on a steel substrate with minimal pinholes. In some aspects of the disclosure, the coating can be slightly porous, with particular coatings exhibiting a porosity ranging from 0.01% to 0.1%. In some aspects of the disclosure, grains having sizes ranging from 30 to 50 nm can be present.

In some aspects of the disclosure, the integrity and/or physical features of the coating provided by the coating composition and method of the present disclosure can be measured. Such measurements can involve using X-ray fluorescence (XRS) to determine atomic composition of a coating; scanning electron microscopy (SEM) to evaluate coating thickness; optical microscopy to evaluate coating uniformity; energy dispersive spectroscopy (EDS) to evaluate the chemical make-up of the coating; X-ray photoelectron spectroscopy (XPS) to evaluate surface states and chemistry; X-ray diffraction (XRD) to evaluate the crystallinity and composition of the coating; reflectance measurements to measure the reflectance of a deposited coating; and/or oxidation experiments to determine coating thickness, variation, and/or to confirm the presence of the coating. Some such techniques are discussed below.

Results from reflectance measurements of coatings made according to the present disclosure are provided by FIGS. 5-7. FIG. 5 illustrates that a deposited titanium coating formed on a steel substrate exhibits similar spectroscopic reflectance to corresponding references, including uncoated steel (Steel 316) and metallic titanium. FIG. 6 also illustrates reflectance results from analyzing a steel substrate coated with a titanium metal coating and further shows reflectance results at different regions of the substrate (measured at particular distances from the center of the coated region). Additional results are shown in FIG. 7.

Another benefit provided by the disclosed coating composition and method is the ability to obtain coatings that are substantially free of contaminants. Contaminants can include sulfur, metal oxides (e.g., lithium oxide, zirconium oxide, titanium oxide), and/or sulfides (e.g., lithium sulfides, zirconium sulfides, and titanium sulfides). The presence of such impurities can be measured using XRS and/or XRD.

In some aspects of the disclosure, oxidation experiments can be conducted to confirm that a coating has been formed on the substrate from the coating composition. In such aspects of the disclosure, a coated substrate can be exposed to an oxygenated environment and if the coating is present, it can be oxidized and can exhibit changes in visual appearance. FIG. 8 provides a photographic image of a substrate that includes a titanium coating with oxide inclusions, which shows an example of how oxidation can be visualized. An image of a titanium-coated substrate without oxide inclusions is also shown in FIG. 8.

In some aspects of the disclosure, thin coatings can be provided using coating composition and method of the present disclosure. In some aspects of the disclosure, the coating composition and method of the present disclosure can be used to provide a coating having a thickness of greater than 0 nanometers to 100 microns, such as greater than 0 nanometers to 80 microns, or 1 micron to 75 microns, or 2 microns to 65 microns, or 3 microns to 50 microns or 4 microns to 40 microns, or 5 microns to 30 microns or 6 microns to 20 microns. In some aspects of the disclosure, substrates comprising a thin coating of the metal component are desirable in certain industries and can be made using coating composition and method disclosed herein. In particular aspects of the disclosure described herein, a coating having a thickness of 10 microns or less can be obtained (e.g., 8 microns or less, 7 microns or less, or 6 microns or less). Coating thickness can be controlled by modifying the temperature and/or plating time used in the method. Results from an exemplary aspect of the disclosure wherein plating times of 1 to 5 hours were evaluated are provided by FIGS. 9A and 9B, wherein FIG. 9A shows results for a coating comprising TiCl₄ as the metal chloride precursor and FIG. 9B shows results for a comparison of a coating composition comprising TiC₄ and a comparison coating comprising TiF₄. As can be seen in FIG. 9B, the TiCl₄ precursor provides superior thickness in shorter time than what is obtained by the TiF₄ precursor. In some aspects of the disclosure, thin coats could be obtained at ambient temperature using plating time periods ranging from 2 hours to 6 hours or more.

In particular aspects of the disclosure, the coating composition and the substrate are enclosed in a container during the method. The container can be maintained under an inert atmosphere. In some aspects of the disclosure, the container can be an electrochemical cell (e.g., a cell comprising a combination of inlets and/or outlets suitable for introducing one or more electrodes into the cell, such as working, reference, and/or counter electrodes; purging and/or blanketing the contents in the cell, and/or for temperature sensing, reagent addition, and/or venting). In some aspects of the disclosure, the substrate used in the method can be pretreated by cleaning the substrate with an alcohol solution and then allowing the substrate to dry, or by using an acid etching process, or by an abrasion process. In yet some additional aspects of the disclosure, the method can further comprise post-treating the coated substrate. In some such aspects of the disclosure, the post-treatment can comprise treating the coated substrate with an alcohol solution and/or sonication and then allowing the coated substrate to dry under an inert atmosphere (e.g., N₂ or Ar). In other aspects of the disclosure, the post-treatment can comprise placing the substrate under vacuum, such as at vacuum pressures ranging from 1 mtorr to 100 mtorr.

In some aspects of the disclosure, the coating composition and method of the present disclosure provide unexpectedly superior coatings on substrates. For example, in some aspects of the disclosure, the coating composition deposits titanium from a titanium chloride precursor, which is a result contrary to conventional wisdom in the art. For example, it is generally accepted and known in the art that metallic titanium (and other refractory metal) coatings cannot be made from titanium chloride salts because it is known that Ti⁴⁺ cannot be fully reduced to Ti⁰ in the presence of chloride. In contrast to this conventional wisdom, however, the present inventors developed coating compositions that successfully provide metal coatings, such as titanium, zirconium, and other refractory metal coatings, even when using chloride-containing reagents, such as metal chloride salts. In yet additional aspects of the disclosure, the metal coating can be deposited from a metal iodide precursor at deposition rates that are much faster than deposition rates achieved with coatings using metal fluoride (or metal bromide) precursors.

Also disclosed herein is a method of converting a metal from one oxidation state to another oxidation state. Such a method comprises exposing a composition comprising a metal chloride or metal iodide precursor and a hydrogen-bond donor compound (typically a glycol, such as ethylene glycol) according to the present disclosure to an energy source to convert the metal from a first oxidation state to a second oxidation state. In some such aspects, the metal chloride or metal iodide precursor is a titanium chloride/iodide salt as described herein. The titanium chloride/iodide salt is exposed to the energy source, which is capable of generating a light and/or thermal stimulus sufficient to convert the titanium component of the titanium chloride/iodide salt from a first oxidation state (e.g., Ti⁴⁺) to a second oxidation state (e.g., Ti³⁺ and/or Ti²⁺). In some such aspects, the light stimulus can comprise exposing the metal chloride or metal iodide precursor (e.g., titanium chloride/iodide salt) to light meeting the parameters discussed herein. In some other aspects, the method can comprise using a thermal stimulus, such as elevated temperature. In some such aspects, the method can comprise exposing the metal chloride or metal iodide precursor to a temperature ranging from greater than 26° C. to 130° C., such as 30° C. to 125° C., or 50° C. to 120° C., or 50° C. to 60° C.

Overview of Several Aspects of the Disclosure

Disclosed herein is a composition, comprising: a deep eutectic solvent comprising an organic salt, a hydrogen-bond donor compound, or a combination thereof; a metal chloride precursor, a metal iodide precursor, or a combination thereof; and an alkaline salt comprising an alkali metal and a counterion selected from chloride, bromide, iodide, or fluoride.

In some aspects, the deep eutectic solvent comprises a mixture of the organic salt and the hydrogen-bond donor.

In any or all of the above aspects, the organic salt comprises a quaternary ammonium compound and a counterion selected from a halide, an acetate, or a bitartrate.

In any or all of the above aspects, the quaternary ammonium compound is selected from choline, N-ethyl-2-hydroxy-N,N-dimethylethanaminium, 2-(chlorocarbonyloxy)-N,N,N-trimethylethanaminium, N-benzyl-2-hydroxy-N,N-dimethylethanaminium.

In any or all of the above aspects, the counterion is a halide selected from a chloride or a bromide.

In any or all of the above aspects, the hydrogen-bond donor compound is selected from urea, acetamide, 1-methyl urea, 1,3-dimethyl urea, 1,1-dimethyl urea, thiourea, benzamide, glycerol, ethylene glycol, malonic acid, benzoic acid, adipic acid, oxalic acid, succinic acid, citric acid, acetic acid, or combinations thereof.

In any or all of the above aspects, the deep eutectic solvent comprises: (i) choline chloride, choline bromide, choline acetate, choline bitartrate, or a combination thereof; and (ii) urea, acetamide, 1-methyl urea, 1,3-dimethyl urea, 1,1-dimethyl urea, thiourea, benzamide, glycerol, ethylene glycol, malonic acid, benzoic acid, adipic acid, oxalic acid, succinic acid, citric acid, acetic acid, or combinations thereof.

In any or all of the above aspects, the deep eutectic solvent comprises choline chloride and ethylene glycol.

In any or all of the above aspects, the choline chloride and ethylene glycol are present in a ratio ranging from 1:3 choline chloride:ethylene glycol to 3:1 choline chloride:ethylene glycol.

In any or all of the above aspects, the coating composition further comprising an ionic liquid.

In any or all of the above aspects, the ionic liquid comprises a cationic component and an anionic component, wherein the cationic component is a positively charged compound comprising a quaternary substituted nitrogen atom or a tri-substituted sulfur atom and the anionic component is a bis-substituted imide compound.

In any or all of the above aspects, the ionic liquid is selected from triethylsulfonium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylpyridinium bis(trifluormethylsulfonyl)imide, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, diethylmethyl(2-methoxyethyl)ammonium bis(trifluoromethylsulfonyl)imide, or a combination thereof.

In any or all of the above aspects, the metal chloride precursor is a refractory metal salt compound comprising chloride and a refractory metal selected from Zr, Ti, V, Cr, Mn, Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, or Ir.

In any or all of the above aspects, the metal chloride precursor is selected from ZrCl₄, TiCl₄, WCl₄, NbCl₄, NbCl₅, TaCl₃, TaCl₅, HfCl₄, VCl₃, VCl₄, IrCl₄, or IrCl₅.

In any or all of the above aspects, the metal chloride precursor is ZrCl₄, TiCl₄, WCl₄, TaCl₃, or TaCl₅.

In any or all of the above aspects, the metal chloride precursor is TiCl₄.

In any or all of the above aspects, the metal iodide precursor is a refractory metal salt compound comprising iodide and a refractory metal selected from Zr, Ti, V, Cr, Mn, Nb, Mo, Tc, Ru, Rh, Hf, Ta, W, Re, Os, or Ir.

In any or all of the above aspects, the alkaline salt is LiCl or LiF.

In any or all of the above aspects, the composition further comprises an additive component selected from urea, a pH-controlling reagent, a reducing agent, a leveling agent, a brightener, a metal fluoride salt, a metal bromide salt, or a combination thereof.

In any or all of the above aspects, the additive component is the brightener and the brightener is selected from N-(2-ethoxyethyl)-3-morpholinopropan-1-amine, saccharin sodium salt, methyl-trioctylammonium bis(trifluoromethylsulfonyl)imide, nicotinic acid, methyl nicotinate, hydantoin, dimethyl hydantoin, lithium iodide, or any combinations thereof

In any or all of the above aspects, the additive component is the leveling agent and the leveling agent is selected from a PEG polymer, a PPG polymer, a PEI polymer, polyvinyl pyrrolidone, zirconium chloride, a rare earth chloride, a heavy metal chloride, or any combination thereof.

In any or all of the above aspects, the coating composition, comprises: a deep eutectic solvent, an ionic liquid, or a combination thereof; (i) a metal chloride precursor selected from a titanium chloride precursor, a zirconium chloride precursor, a tungsten chloride precursor, or a tantalum chloride precursor; (ii) a metal iodide precursor selected from a titanium iodide precursor, a zirconium iodide precursor, a tungsten iodide precursor, or a tantalum iodide precursor; or a combination of (i) and (ii); and an alkaline salt comprising an alkali metal and a counterion selected from chloride, bromide, iodide, or flouride.

In any or all of the above aspects, the deep eutectic solvent is a mixture of choline chloride and ethylene glycol; the metal chloride precursor is TiC₄; and the alkaline salt is LiCl.

Also disclosed is a method, comprising: (i) combining, in a cell, (a) the coating composition according to aspects of the present disclosure; and (b) a substrate; wherein the cell further comprises one or more electrodes; and (ii) exposing the cell to a voltage protocol using the one or more electrodes to thereby deposit a metal coating on the substrate from the coating composition.

In any or all of the above aspects, the substrate is a Mo substrate, a Cu substrate, a Zr substrate, a steel substrate, a U substrate, an Al substrate, or a substrate comprising any combination of Mo, Cu, Zr, U, steel, or Al.

In any or all of the above aspects, in the voltage protocol comprises applying a pulsed voltage, a non-pulsed voltage, or a combination thereof.

In any or all of the above aspects, the voltage is applied as a pulsed voltage such that voltage is applied at a timed interval comprising a 100 ms time period where voltage is applied and a 10 ms time period where no voltage is applied and wherein the timed interval is repeated at least two times.

In any or all of the above aspects, the cell is maintained at a temperature ranging from ambient temperature to 140° C.

In any or all of the above aspects, the coating composition comprises TiCl₄ as the metal precursor.

EXAMPLES

Example 1

In this example, a titanium coating was deposited on a metal substrate using a coating composition comprising 275 ml of ethaline, 300 ml TiCl₄, and 50 mg LiCl. The DES was heated at a temperature ranging from 60-120° C. under bubbling Ar (0.1 Lpm) and lip gas (1 lpm). The TiCl₄ was dissolved in the DES under sonication at 60C, 40 kHz, 40 W. The LiCl was concurrently dissolved in the DES. The coating composition was transferred under an Ar lip gas to the plating setup. A plating potential ranging from −1.64 to −1.89 V was applied and a desired wave function from DC to a pulse rest ratio of 10:1 of the second time scale was utilized to facilitate deposition of the titanium coating.

Example 2

In this example, a titanium coating was deposited on a metal substrate using a protocol as described above in Example 1, but with the parameters summarized in Table 1, below.

TABLE 1
	Depo	TiCl4		LiCl	LiF		Metal
	Time	Conc.	Temp	Conc.	Conc.	Minimum	Thickness
#	(hrs)	(mg/ml)	(C.)	(mg/ml)	(mg/ml)	Bias (V)	(um)	Result

1	3.5	17.1	90	0.29	0	−1.94	34.4	Ti metal film thickness
								at 30 um thick
2	1.5	17.1	90	0.29	0	−1.94	21.5	Ti metal film thickness
								at 15 um thick
3	3	17.1	90	0	0	−1.94	0.1	Ti metal film thickness
								at 0.1 um thick
4	0.5	17.1	120	0.29	0	−1.94	0.3 approx.	Ti Oxide produced No
								metal plating
5	3	17.1	50	0.29	0	−1.94	30 approx.	Ti metal film thickness
								at 30 um thick with
								oxide inclusions
6	3	17.1	60	0	0.29	−1.94	30 approx.	Ti metal film thickness
								at 30 um thick
7	3	17.1	90	0	0.29	−0.90	0.1	Ti metal film thickness
								at 0.01 um thick
8	3	17.1	90	0.29	0	−0.90	0.1	Ti metal film thickness
								at 0.01 um thick
9	1.5	17.1	90	0.29	0	−1.94	15 approx.	Metal Plating 15 um
								thick
10	1.5	8.6	90	0.29	0	−1.94	15.0	Metal Plating 15 um
								thick
11	1.5	2.1	90	0.29	0	−1.94	15.0	Metal Plating 15 um
								thick
12	1.5	17.1	20	0.29	0	−1.94	3.0	Oxide plating 3 um
								thick
13	1.5	17.1	50	0.29	0	−1.94	15.0	Oxide plating 15 um
								thick
14	1.5	17.1	80	0.29	0	−1.94	15.0	Metal/oxide Plating 15
								um thick
15	1.5	17.1	100	0.29	0	−1.94	13.0	Metal plating 13 um
								thick
16	1.5	17.1	120	0.29	0	−1.94	19.0	Metal plating 19 um
								thick
17	0.5	17.1	90	0.29	0	−1.94	5.1	Metal Plating 3 um thick
18	3	17.1	90	0.29	0	−1.94	31.7	Ti metal film thickness
								at 30 um thick
19	0.75	17.1	90	0.29	0	−1.94	11.6	Ti metal film thickness
								at 11 um thick

Example 3

In this example, compositions comprising different additives (namely, brighteners and leveling agent) were prepared and evaluated. The following parameters/conditions were used for this example.

The electrochemical measurements were driven and recorded by a Gamry Instruments Interface 1010E potentiostat. Electrical connections included a working sense, working electrode, counter sense and counter electrode, a reference electrode, and ground that all connected to a multipin connector that directly connects to the potentiostat. Type 316 stainless steel sheet, 0.010 in thickness, was cut to ˜¼ in wide by 6 in long strips and used as a cathode. The cathode strips were then polished with 400 grit paper and cleaned with isopropanol and wiped dry, then inserted into the central electrochemical cell port. The electrochemical cell would contain an inert atmosphere at this time to reduce any oxidation of the surface.

Plating occurred by square wave pulse plating of 0.1 s at −1.94 V and 0.05 s at −0.194 vs. Ag/AgCl₂ reference (i.e., the rest ‘0’ potential when measured against a standard hydrogen reference electrode). After the electrodeposition experiment was completed, the cathode was removed and rinsed with isopropanol and then submerged and sonicated (40 W, 40 kHz) in isopropanol for 5 minutes. After sonication, the cathode was dried under a light air stream and placed into a vacuum desiccator until reflectance measurements were taken, typically within 1 hour of electrodeposition.

A deep eutectic solvent (DES, ethaline) was created by mixing choline chloride (ThermoFisher Sci. 98+% CAS:67-48-1) and ethylene glycol (Acros Organics 99.8% CAS:107-21-1) at a 1:2 molar ratio. The two components were poured into a 500 ml round bottom, three-neck flask with a Vigreux distilling column attached, and a continuous argon gas sparge was maintained to purge and prevent the accumulation of moisture and oxygen in the DES. The argon gas line was plumbed through a desiccant cannister to remove residual moisture from the facility-supplied gas. The mixture was magnetically stirred under the controlled inert atmosphere allowing the components to mix over 72 hours prior to usage and assure the removal of any moisture. The DES was kept with a continual sparge of inert gas to ensure that it was free of water and oxygen (inline gas values of >4 ppm).

The plating solutions were prepared by weighing 10 mg of LiCl (Sigma Aldrich 99.98% CAS:7447-41-8) on an analytical balance and loading into a 20 ml vial, then 20 ml of DES was extracted from the larger flask with a gradient syringe and combined into the 20 ml vial. The selected additive was either weighed on the analytical balance, if in solid form, or pipetted using metered syringes and added to the vial. Next, 0.3 ml of TiCl₄ (Acros Organics, 99.9% CAS:7550-45-0) was filtered with a 5 μm polyvinyl difluoride (PVDF) membrane syringe filter. Then the filter was removed and TiCl₄ was inserted into the scintillation vial containing the DES inside of a fume-hood. The electrolyte was then heated to 50-60° C. while being sonicated (40 W, 40 kHz) to incorporate all the constituents. Once the electrolyte was transparent, assuring the TiCl₄ and LiCl were fully dissolved, it was poured into the electrochemical cell.

Electrodepositions were examined for reflectance with a Craic 308PV Spectrophotometer with the Craic MINERVA software application. A dark scan was completed without ambient lighting, and a reference spectroscopy was taken on a PTFE surface with a 500 W halogen lamp (FCL, J120 V-500 W) prior to sample collection of all electrodeposits. The light used produced a usable range of wavelengths from 400 to 900 nm. Polished stainless steel reflectance data was collected as a reference/control. Multiple spectra were taking and averaged for each electrodeposition with the additives. Optical images were taken of the electrodeposition surface prior to the reflectance measurements using the Craic 308PV at 20×.

Scanning electron microscopy (SEM) was performed on a JEOL IT-800 equipped with an Oxford X-Max energy dispersive spectrometer (EDS). Variable accelerating voltages ranging from 5 to 30 kV were used. Scanning transmission electron microscopy (STEM) was done on a JEOL ARM2000F spherical aberration corrected (Cs) microscope operated at 200 kV using a JEOL Centurio EDS detector with a solid collection angle of 0.9 sR.

CompleteEASE v. 3.65 from J.A. Wollman Inc. was used to predict and model the reflectance spectra. The data were produced with the vendor-supplied database. Standard reference materials (i.e., experimentally measured values) were used as inputs for the optical parameters.

The additives that were evaluated are provided in Table 2.

TABLE 2
		Purity
Name	Chemical formula	(%)	CAS #	Supplier
N-(2-ethoxyethyl)-3-	C11H24N2O2	n/a	CID 43387925	Akos GmbH
morpholinopropan-1-amine
Saccharin Sodium salt	C7H4NNaO3S × H2O	99+	82385-42-0	Acros Organics
Methyl-trioctylammonium	C27H54F6N2O4S2	>99	375395-33-8	Sigma Aldrich
bis(trifluoromethylsulfonyl)imide
Polyethylene glycol	H(OCH2CH2)nOH	n/a	25322-68-3	VWR
Polyvinylpyrrolidone	(C6H9NO)n	n/a	9003-39-8	Sigma Aldrich
Zirconium Chloride	ZrCl2	99.5	10026-11-6	Strem Chemical,
				Inc.
Lithium Iodide	LiI	99	10377-51-2	ThermoFisher
				Sci.
Nicotinic acid	C6H5NO2	>98	59-67-6	Millipore Sigma
Methyl-nicotinate	C7H7NO2	99	93-60-7	Millipore Sigma
Hydantoin	C3H4N2O2	98	461-72-3	Millipore Sigma
Dimethyl-hydantoin	C5H8N2O2	97	77-71-4	Millipore Sigma

A list of the experimental parameters for the different leveling agents that were evaluated is provided in Table 3. Experimental parameters for different brighteners that were evaluated is provided in Table 4.

TABLE 3
	Temp		TiCl4	DES	LiCl	Deposition
Additives	(° C.)	Amount	(mL)	(mL)	(mg)	Time

Polyethylene glycol,	180,	10 mg	0.3	20	10	1
8 kDa	200
Polyvinylpyrrolidone,	180,	10 mg	0.3	20	10	1
10 kDa	200
Polyvinylpyrrolidone,	30,	10 mg	0.3	20	10	3
10 kDa	200
Zirconium Chloride	30,	1 mg	0.3	20	10	3
	200

TABLE 4
	Temp		TiCl4	DES	LiCl	Deposition
Additives	(° C.)	Amount	(mL)	(mL)	(mg)	Time

N-(2-ethoxyethyl)-3-	180	0.05	ml	0.3	20	10	1
morpholinopropan-1-amine
Saccharin Sodium salt	200	50	mg	0.3	20	10	1
Methyl-trioctylammonium	180, 200	0.05	ml	0.3	20	10	1
bis(trifluoromethylsulfonyl)imide
Methyl-trioctylammonium	200	0.05	ml	0.3	20	10	1
bis(trifluoromethylsulfonyl)imide
Nicotinic acid	200	10	mg	0.3	20	10	3
Methyl nicotinate	200	10	mg	0.3	20	10	3
Hydantoin	200	10	mg	0.3	20	10	3
Dimethyl hydantoin	200	10	mg	0.3	20	10	3
Lithium lodide	200	1.46	g	0.3	20	10	3

After deposition, the deposited film was light gray in appearance and the coating covered the entire submerged area (e.g., see FIG. 10A). The titanium surfaces also appear to be un-corroded (visible and electrical resistance evaluation); many of the samples were stored in vacuum for over a month for all the samples to be completed as viewed seen in FIG. 10A. An SEM cross-section of a titanium electrodeposition in ethaline processed for 1 hour at 200° C. shows the layers of polymer, titanium and stainless steel typically seen in this process (FIG. 10B). Reference reflectance spectra has been modeled to provide insight into the spectral response of the peak polymer overcoat, titanium, and stainless steel, FIG. 10C. With initial characterization of the plate, grain structure (20-50 μm grains non-strained, FIG. 10C), and the evolution of polymers on the surface the work proceeded with additives to improve the film level and brightness (FIG. 10A).

Optical microscope images of titanium electrodeposition surfaces using leveler additives are shown in FIG. 11A. Additional optical images shown in FIG. 11A include PTFE, polished stainless steel, titanium reference, and a titanium electrodeposition without additives (control). Images were taken just prior to reflectance data collection within 24 hours of the electrodeposition. Reflectance spectra of the plates are shown in FIG. 11B. Standard deviations of the reflectance spectrum data are shown in FIG. 11C.

Optical microscope images of titanium electrodeposition surfaces using small-molecule brightener additives are shown in FIG. 12A. Reflectance spectrum of titanium electrodepositions using the brightener additives are shown in FIG. 12B. The integrated reflectance response of the brightener additives and control samples are shown in FIG. 12A.

Initially, the films of titanium are deposited at a rate of 3 microns/hr as seen by the 3 to 3.5 micron-thick film of titanium in the cross section presented in FIG. 10B and confirmed with EDS mapping shown in FIG. 13. The films are deposited at temperatures above 180 degrees Celsius, as lower temperatures have been prone to island formation (see FIG. 14). the substrate roughness appeared to be about 500 microns, with the titanium being slightly rougher at about 640 microns (as measured in the SEM cross section). On top of the titanium was a thin ˜50 nm thick polymer coat. The presence of bubble formation at the working electrode perhaps causes this increase in roughness. As such, the influence of additives is needed to increase the brightness and influence the surface of the film.

Optical modeling (FIG. 10C) predicted the reflectance spectra of each of the layers found by SEM imaging. Generally, the main distinguishing feature between titanium and the steel substrate is the increase in the reflectance from 500-650 nm that titanium has relative to the “dip” in steel's response. This aids in the identification of the titanium layer. Additionally, it was seen that the presence of the polymer overcoat spreads and narrows the response of the titanium, but does not remove it entirely. The proposed additives were evaluated for their influence on the spectroscopic reflectance of the plate (FIG. 10A).

Polymer additions to the bath showed that the films can be leveled to produce a less noisy spectroscopic response, which is used to evaluate the level of the film (see FIGS. 11A-11C). The addition of polymers can be seen to produce a film with significantly less preparation marks and, in the case of PVP and PEG, the noise in the reflectance signal is reduced by more than 33% for plating lasting longer than 3 hours, as seen in FIGS. 11A-11C. It should be noted that the addition of zirconium chloride to the bath resulted in a rougher film when compared to the control. Overall, the addition of hydrophilic polymers in 0.5 mg/ml ranges reduced the noise value by more than 33% for both PVP and PEG, in support of the original hypothesis. It should be noted by looking at FIG. 11B that most of the polymer additives produced optical responses in line with a polymer overcoat (FIG. 11B), as would be expected. It can be seen in FIG. 11B that the overall reflectance of the titanium standard is still higher than that of the plated films, leading to an investigation of the small-molecule brighteners to the bath.

The addition of small molecules to the bath, as shown in FIGS. 12A-12C, increases the brightness of the films in all but one case (saccharin). The addition of small molecules to the bath resulted in most Ti films producing an optical response in line with that of the titanium reference see in FIG. 12B. Many of the additive produced significant brightening of the deposits with methyl nicotinate, dimethyl hydantoin and lithium iodide producing films, which were within the original hypothesis of being 80% that of the reference titanium sheet (FIG. 12C). Most of the brighteners produced films with a coating more diffuse than specular reflective, except in the cases of hydantoin and nicotinic acid, which produced spectra with some specular reflectance (FIG. 12A).

The use of leveling and brightening agents typical in aqueous plating can be applied in several cases to plating in deep eutectic solvents. Lower molecular-weight hydrophilic polymers and several varieties of hydantoin and nicotinic acid produce level/bright titanium deposits with both specular and diffuse optical properties. The application of these leveling and brightening agents can produce films with appearances and surface roughnesses that may by suitable for application in medical implants, corrosion protection of electronics and aerospace components, and as interlayers between dissimilar materials.

Example 4

In this example, evaluations were conducted to determine whether aspects of the disclosed compositions and deposition methods exhibit temperature-dependent aspects that alter the film deposition kinetics upon modest heating. An ultraviolet-visible (UV-vis) spectroscopic study was used to evaluate titanium reductions, along with performing kinetics evaluations.

The chemicals used in this example are listed in Table 5; all chemicals were used without further purification.

TABLE 5
Name	CAS	Purity (%)	Supplier
Choline Chloride	67-48-1	98+	ThermoFisher Sci.
Ethylene Glycol	107-21-1	99.8	Acros Organics
Lithium Chloride	7447-41-8	99.98	Sigma Aldrich
Titanium Tetrachloride	7550-45-0	99.9	Acros Organics
Titanium Tetrabromide	7789-68-6	98	Sigma Aldrich
Titanium Tetrafluoride	7783-63-3	98	Alfa Aesar
Isopropyl Alcohol	67-63-0	99.9	Fisher Scientific

Materials and reaction parameters were used as described below.

Ethaline was synthesized from choline chloride and ethylene glycol (1:2 molar ratio) by combining them in the inert atmosphere of argon and stirring (300 RPM), and then stored under a slow constant sparge of Ar.

The general procedure for electrochemical experiments was as follows: In a 20 mL scintillation vial, 20 mL of DES was combined in the air with lithium chloride (10 mg) and a source of titanium (0.334 g TiF₄, 0.3 mL TiCl₄, 0.613 g TiBr₄). Solids were added directly into the vial, while TiCl₄ was drawn through a 5-micron polyvinyl difluoride (PVDF) membrane syringe filter into a disposable syringe and then discharged into DES solution. Obtained suspensions were put under a blanket of argon and sonicated at 60° C., resulting in a clear homogeneous solution. Next, the solution was transferred to a 5-port Gamry Inc. small Dr. Bob's Cell™ and vendor-supplied fittings. The cell was inserted into a glass bowl with metallic thermal-transfer beads and placed on a hot plate where the temperature was set from 30° C. to 90° C. during separate runs. Electrochemistry experiments were set up in the cell equipped with a DES-filled glass reference electrode, graphite counter electrode, and stainless-steel (316, McMaster Carr product: 2317K275) working electrode. The working electrode was polished with 400A sandpaper and wiped with a Kimwipe™ immediately before its insertion into the corrosion cell. During electroplating, the reaction was kept under a constant blanket of Ar.

The electrochemical measurements were performed with a Gamry Inc. Interface 1010E potentiostat. Plating occurred by square wave pulse plating of 0.1 s at −1.94 V and 0.05 s at −0.194 vs. Ag/AgCl₂ reference (i.e., the rest ‘0’ potential when measured against a standard hydrogen reference electrode). After the electrodeposition experiment was completed, the cathode was removed and rinsed with isopropanol and then submerged and sonicated (40 W, 40 kHz) in isopropanol for 5 minutes. After sonication, the cathode was dried under a light argon stream and placed into a vacuum desiccator until imaging and/or electrical resistance measurements were taken.

Optical experiments were performed in the argon-filled glove bag in a 20 mL scintillation vial equipped with a custom, 3D-printed cap. Background absorption of DES was recorded before each measurement. General procedure: 10 mg of LiCl was weighed into a 20 mL scintillation vial. Next, DES was added, and the vial was blanketed with argon. TiCl₄ was drawn through the filter and discharged into the DES solution. The obtained suspension was put under a blanket of argon and sonicated (40 kHz, 40 W) at 60° C., resulting in a clear, homogeneous solution, which was taken directly into the glove bag. Samples were heated to desired temperatures using a hot plate, and the progress of reactions was monitored using an optical exposure chamber. Data were collected using Ocean Optics OceanView software at regular intervals (1 or 60 s).

Electrochemical measurements were carried out using Gamry E1010B Potentiostat. Electrical resistance evaluation of the plated films and reference standards was accomplished with a Keysight B258A electrometer and vendor-supplied tri-ax cabling and alligator clips.

Scanning electron microscopy (SEM) was performed using a JEOL IT-800 Field Emission Scanning Electron Microscope, coupled with an Oxford 80 X-Max energy dispersive spectrometer (EDS). Variable accelerating voltages ranging from 5 to 30 kV were used. Scanning transmission electron microscopy (STEM) was done on a JEOL JEM-ARM200CF ACCELARM spherical aberration corrected (Cs) microscope operated at 200 kV using a JEOL Centurio SSD-EDS detector with a solid collection angle of 0.9 sr.

X-ray fluorescence was measured with a Bruker Scientific S2 PICOFOX total reflection X-ray fluorescence (TXRF) spectrometer. Custom acrylonitrile butadiene styrene (ABS) printed dishes were used to hold the coupons in the TXRF. All samples were measured for 1 hour with a molybdenum cathode and a nominal current of 60 microamps. All data were normalized to the intensity of the iron response to allow for comparison between samples.

Optical ultraviolet-visible (UV-vis) absorbance data were recorded with an Ocean Optics OCEAN HDX spectrometer with vendor-supplied fused silica optics and cables. All Ocean Insights products were selected for maximum light collection from 195-850 nm. The optical cell was purchased from Thor Labs Inc. (model CVH100) and connected via SubMiniature A (SMA) collimating lenses with fused silica optics (models CVH100-COL, LA4647).

Optical/SEM images were analyzed with Image J v1.53. Electrochemical data (excluding cyclic voltammetry) were analyzed with Gamry's EChem Analyst software.

Collected cyclic voltammetry (CV) data were analyzed using a custom-developed algorithm for batch processing of electrochemical data. The analysis methodology is contained in the supplement information. X-ray fluorescence (XRF) data was analyzed using Bruker's PICOFOX control/analysis software. Optical emission data was analyzed with Microsoft Excel after being recorded by Ocean Optics' OceanView control software.

Results—During electrodeposition of titanium on steel, two reduction responses were seen as evidenced by cyclic voltammetry FIG. 15A. The deposition was then analyzed by square wave voltammetry to confirm the reduction biases with minimized capacitance. The reductions appear to be centered around −0.8 V and −1.6 V (adjusted to be vs. a standard hydrogen electrode [SHE]) as shown in FIG. 15B). The initial reduction response (−0.8 V) occurred over a very broad bias range, warranting further investigation. Cyclic voltammetry of both responses was conducted from 10 to 10 k mV/s scan speeds with the cell providing ion transfer data from 10 to 1 k mV/s. The higher scan speeds had non-linearities as shown in FIG. 15C. The center of the reduction biases was plotted against the root of the scan speed to evaluate the diffusive nature of the reduction. As shown in FIG. 15D, the reduction to metal occurring at −1.6 V is linear, while the intermediary reduction response (−0.8 V) has non-linear aspects at higher scan speeds. Obtained samples were then imaged and chemically analyzed to verify that deposition was taking place.

Optically, at near-ambient temperatures, the evolution of the titanium deposition appears to happen by island growth on the seconds time scale with full film evolution proceeding after that, as shown in FIG. 16A, top row (30° C.). At higher temperatures, the island growth mechanism appears to be supplanted by a conformal layer-by-layer deposition, as shown in FIG. 16A, bottom row (95° C.). To evaluate this, the films were cross sectioned for scanning electron analysis.

FIG. 16B shows steel with caps of carbon and platinum used to illuminate the surface. After 30 seconds of electroplating the surface appears rougher, which may indicate the presence of islands of titanium (see FIG. 16C). After 5 minutes of plating, a several hundred nanometer-thick conformal film is developing on the surface, which is shown FIG. 16D. The intensity analysis of the cross section allowed for the conformation that the titanium is forming on the surface, seen in FIG. 16E. After 1 hour of deposition, the film is about 3-3.5 microns thick with small grains evident at the boundary between the steel and titanium (see FIG. 16F). Lastly, XRF was used to determine the composition of the film, with titanium appearing to be the only addition when compared to a control signal (see FIG. 16G). The micron-scale thick films shown in FIG. 16F and FIG. 16E were also tested for electrical resistance against titanium and steel reference sheets of similar roughness. The electrodeposited films had resistances in line with those of the titanium sheet standard (about 50% less resistance than the steel substrate used for plating).

To explore the different film depositions seen in FIG. 16A, potentiostatic EIS was carried out at two temperatures (30° C. and 90° C.), three bath compositions, and three different time lengths. Each of the evaluations began with a recording of the EIS of the steel electrode before applying current. At low temperatures, the EIS of the SS substrate exhibits an open-loop feature that allows for distinguishing the kinetic control region and mass transfer-controlled region (see FIG. 17A). However, at an elevated temperature of 90° C., even before the current is passed through the sample, the appearance of the inductive loop can be seen, indicating that electrode surface modification starts to take place (see FIG. 17A).

To better understand the reason for the emergence of the inductive loop (and subsequent alteration in film deposition kinetics), the influence of different anions on the mechanism of Ti electrodeposition FIG. 17B was evaluated. In each case, the inductive mechanism was shown to be temperature dependent and not ion dependent. In each case, initial EIS recorded at 30° C. showed a typical open curve with a clearly distinguishable Warburg region (circuit diagram shown in FIG. 17D), but at 90° C., even initial EIS already shows inductive behavior, as evidenced in FIG. 17C with corresponding circuit diagram shown in FIG. 17E. Further attempts at understanding the non-linearity in the reduction currents seen in FIG. 15D were then undertaken.

To better understand the reduction kinetics, a data set of over 1000 cyclic voltammetry scans was taken at varying temperatures (25° C. to 90° C.) and scan speeds (10-1000 mV/s) for the bath with saturated titanium (IV) chloride. The responses were analyzed for the magnitude of the charge transferred during reduction as well as the maximum magnitude of the reductive current, as shown in FIGS. 18A-18D.

The reductive current change of the initial reductive response is linear for slower scan speeds (5-15 mV0.5/s0.5), as seen in FIG. 18A. Also in FIG. 18A, higher scan speeds cause non-linearities to occur (diffusion is no longer the limiting kinetic step). A similar trend is seen in the reductive current of the −1.6 V response seen in FIG. 18B. This is true except for one location with higher scan speeds and temperatures from 50° C. to 60° C., where both reductive currents are lower than would be expected (see FIGS. 18A and 18B).

The total dissipated power (integrated response minus the background signal) of both reductive biases helps explain the trend seen in FIGS. 18A and 18B. The power dissipated by the initial reduction at −0.8V is relatively steady with a maximum response of around 60° C. (see FIG. 18C). Lower temperatures produce less of a response than higher ones as would be expected until an optimal plating temperature is reached. Additional data showing current plotted as a function of root of the scan speed is provided in FIGS. 20A and 20B, with FIG. 20A showing results for current at −0.8V vs. SHE and FIG. 20B showing results for current at −1.6V vs. SHE.

The reduction of Ti²⁺ to metal at −1.6 V shows a different trend, with its optimal temperature occurring around 40° C. and an unexpected dearth of signal at higher temperatures, as seen by the large blue region of FIG. 18D. This trend is unexpected, as it was hypothesized that the metal reduction would follow a similar trend of an optimal plating temperature being around 60° C., with slower pulses producing the optimal reductive environment. Since the solvent is relatively transparent across the near-ultraviolet to near-infrared wavelengths and titanium ions have different absorbances in this range, an in-situ optical absorption spectroscopy combined with the electrochemical process was undertaken to better understand the unexpected CV results of the metal reduction.

To better understand the impact of solvents on the presence and stabilization of titanium ions in the bath, the optical response of each ion was measured with and without the electrochemical pulse plating and at 60° C., as seen in FIGS. 19A and 19B. It should be noted that in the ultraviolet to near-infrared range, only Ti³⁺ and Ti²⁺ have unique absorbances that cannot be attributed to other ions. Generally, the presence of both intermediary ions is higher during the hot electroplating case as compared to the thermal process only. It should be noted that the Ti²⁺ ion is ever-increasing in concentration during the electroplating case, suggesting that there may be other production means for this intermediary ion.

In FIGS. 19A and 19B for the thermal-only cases, the presence of both intermediary ions increases in the solution with time at temperature, with Ti³⁺ being more prevalent overall at all times. It is expected, as it requires the addition of just 1 electron to Ti³⁺ vs. 2 electrons for reduction to Ti²⁺. For the pulse plating at temperature, the presence of Ti²⁺ is marginally higher and increasing on the minute time scales while the presence of Ti³⁺ remains constant.

At temperatures above 60° C., the EIS shows only layer-by-layer growth through a diffusive plane (FIGS. 17B, 17C, and 17E) rather than one reliant on island growth (FIGS. 17A and 17D). This switch is partially explained by the presence of non-linear influences in the cyclic voltammetry of the reductive response centered at about −0.8V, seen in FIG. 18A. Additionally, based on the EIS result, the initial presence of an inductive loop is evidence of the change in film growth kinetics. It has been shown that for a simple solid electrode (e.g., the metal surface before/during island formation), the impedance of the system should be able to be represented by a simple equivalent circuit with a resistor, capacitor, and Warburg impedance element (accounting for the diffusion-controlled process), as seen in FIG. 17D. This solid electrode kinetic model fits the deposition kinetics of ambient temperature electroplating. However, while using the reaction setup and heating it above 60° C., it has been found that the inductive loop should be accounted for, as well as the presence of the porous electrodes. The modeling of the inductive loop and porous electrodes in the diagram in FIG. 17E is needed as the deposition kinetics change to a layer-by-layer model with the reduction of some ions occurring in narrow pores as the films complete deposition. The use of the united transmission lines (UTLs) accounts for the rate of kinetic reaction in narrow pores of the electrode. The evident change in deposition kinetics is reflected in the optical and electrochemical behavior of the cell.

At temperatures above 60° C., there is an increase in the current which does not align with a linear trend that would be expected from a diffusion-limited process, such as is seen by the mostly linear evolution of the metal reduction response in FIGS. 15C and 15D, and FIG. 18B. The thermally-dependent non-linearities of the initial reduction response's cyclic voltammetry suggest that it is influenced by the production of ions that are not being driven by electroreduction.

UV-vis assessment during the heating of the solution (FIGS. 19A and 19B) shows that if no pulse plating is applied, the presence of both intermediate ions (Ti³⁺ and Ti²⁺) increases with time. This is suspected to be due to the polyol reduction, as the main component of the solvent is ethylene glycol. This assessment is corroborated by the batch analysis of the cyclic voltammetry, which shows non-linear aspects in current for both reduction peaks, with a particular increase evident for the primary reduction bias (−0.8V, FIGS. 18A and 18B). It should be noted that the presence of electroplating sequence increases the concentration of both intermediate ions, thus part of their kinetics is driven by electroreduction. Lastly, another aspect of the optical signature of the intermediate ions is that while the presence of Ti³⁺ remains relatively stable on the minutes time scale (FIG. 19B), the presence of Ti²⁺ increases slightly during pulse plating (FIG. 19A). Thus, with the presence of thermal and electrical reduction, there is an overabundance of Ti²⁺ in the bath and available for metal film formation.

The increasing concentration of Ti²⁺ in the bath originating from both electroreduction and solvent-based polyol reduction appears to be the mechanism by which the film growth kinetics change upon heating, as seen in FIG. 16A and FIGS. 17A and 17C. In ambient electroplating approaches, there would be a dearth of available ions for metal reduction, meaning that the electrochemical cell must provide all the power (as seen by the increased dissipated power in the lower left-hand corner of the heat map in FIG. 18D). Film growth would then naturally proceed from ideal plating locations (islands).

Taken together, these observations corroborate the hypothesis that the polyol reduction mechanism is influencing the electrodeposition kinetics at temperatures above 60° C. With an increase in temperature the concentration of the available Ti²⁺ eventually becomes more than what can diffuse to the surface (i.e., the concentration increases in the bath), thus plating occurs in a more layer-by-layer fashion, as is seen in the film depositions at higher temperatures in FIGS. 16A, 16C-F and by EIS at higher temperatures, FIGS. 17B, 17C, 17E). Thus, the polyol-induced production of intermediate ions produces a change in the film growth kinetics that is thermally dependent (as the electroless polyol reduction kinetics are). What this means from the perspective of the general state of the art is that the multi-kinetic route has an increased percentage of Ti⁴⁺ to Ti³⁺ and Ti³⁺ to Ti²⁺ production. What is not seen is an increase of metal reduction, which optically would be represented as a diminishing concentration evolution with time of the Ti²⁺ absorbance. It appears that the polyol mechanism can enhance the production of intermediate ions but cannot reduce them to elemental metal, which remains a function of the electrochemical process.

In this example, it has been shown that the electrodeposition of Ti on stainless-steel substrates is feasible using compositions according to the present disclosure. The reduction kinetics are in line with the general state of the art at ambient temperatures. Upon modest heating, the influence of the electroless polyol reduction mechanism increases the concentration of intermediate ions, altering the film growth kinetics to a more layer-by-layer method. As established with this example, temperature control can have effects on the reaction. Results also have led to a current operating theory based on an electrodeposition mechanism, as discussed above, and which is supported by multiple analytical methods, such as EIS, UV-vis, and SEM.

In view of the many possible embodiments to which the principles of the present disclosure may be applied, it should be recognized that the illustrated embodiments are only preferred examples and should not be taken as limiting the scope of the present disclosure. Rather, the scope is defined by the following claims. We therefore claim as our invention all that comes within the scope and spirit of these claims.