Electrochemical Deposition of N-Heterocyclic Carbenes

FIELD

The field of the invention is deposition methods for coating metallic surfaces. Specifically, deposition of coatings of N-heterocyclic carbenes (NHC).

BACKGROUND

Carbon-based ligands known as N-heterocyclic carbenes (NHCs) have played a role in the field of transition metal complexes (Herrmann, Angew. Chem. Int. Ed. 41, 1290 (2002), Peris, et al. Coord. Chem. Rev. 248, 2239 (2004)). These ligands are part of catalysts such as the Grubbs second generation metathesis catalyst (Thomas, et al. Organometallics 30, 6713 (2011)), and NHC-based cross-coupling catalysts (Kantchev, et al. Angew. Chem. Int. Ed. 46, 2768 (2007)). Unlike most carbenes, which are reactive with limited stability, NHCs typically have one or two heteroatoms adjacent to a carbene carbon (Igau, et al. J. Am. Chem. Soc. 110, 6463 (1988), Arduengo, et al. J. Am. Chem. Soc. 113, 361 (1991)). These heteroatoms increase NHCs' stability such that they can usually be prepared on a gram scale (Niehues, et al. Organometallics 21, 2905 (2002)), crystallized (Arduengo, Harlow, Kline, J. Am. Chem. Soc. 113, 361 (1991)), distilled (Niehues, et al. Organometallics 21, 2905 (2002)), and stored for longer periods of time.

An Au-NHC bond is estimated to be on an order of 90 KJ/mol stronger than a corresponding Au-phosphine bond, and twice as strong as metal sulfide bonds in molecular complexes (Pyykkö, et al. Chem. Asian J. 1, 623 (2006)). As such, NHCs have potential to be valuable ligands for protecting and functionalizing gold and other metal surfaces. Application of these carbenes in materials science, and other fields outside of homogeneous catalysis, has been limited (Mercs, et al. Chem. Soc. Rev. 39, 1903 (2010)).

SUMMARY

An aspect of the invention provides a method of forming a coating on a substrate that comprises at least two metal surfaces, comprising applying a positive potential to a first metal surface to form a working electrode, applying a negative potential to a second metal surface or a conductive surface to form a counter electrode, disposing the working electrode and counter electrode in a solution comprising free carbene, a salt of carbene, or a combination, and producing a coated metal surface on the substrate, wherein the positive potential and negative potential are referenced to a reference electrode of substantially equivalent potential to a Ag/AgCl electrode.

In one embodiment, the conductive surface comprises carbon, graphite, silicon, germanium, gallium arsenide, or a combination thereof. In one embodiment, the second metal surface comprises a metal that is the same as the metal of the working electrode. In one embodiment, the first metal surface comprises gold. In one embodiment, the first metal surface is substantially unprotected and is not oxidized. In one embodiment, substantially unprotected is substantially unoxidized. In one embodiment, the salt of carbene is a N-heterocyclic carbene benzimidazolium hydrogen carbonate salt. In one embodiment, the N-heterocyclic carbene benzimidazolium hydrogen carbonate salt is 5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate. In one embodiment, the positive potential is in a range of 0.1 to 2.0 V. In one embodiment, the working electrode comprises gold, copper, iron, or a combination. In one embodiment, the positive potential is in a range of 0.8 to 1.0 V. In one embodiment, the coating is an electron passivation layer. In one embodiment, the coating prevents or reduces corrosion.

In one embodiment, the coated first metal surface has reduced reactivity relative to its uncoated form. Examples of reduced reactivity include reduced reactivity for oxidation, reduction, corrosion, dissolution, hydrogenation, halogenation, adsorption, addition reactions, thiolation, addition reactions with proteins, and surface fouling. In one embodiment, the disposing in the solution occurs for about 1 min to about 5 h. In one embodiment, disposing occurs for about 1 min to about 120 min. In one embodiment, the method further comprises a reference electrode, a salt bridge, or both. In one embodiment, the metal of the first and second metal surfaces are selected from the group consisting of scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, palladium, silver, cadmium, lanthanum, hafnium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, actinium, rutherfordium, dubnium, seaborgium, bohrium, hassium, meitnerium, darmstadtium, roentgenium, copernicium, beryllium, magnesium, calcium, strontium, barium, radium, lithium, sodium, potassium, rubidium, cesium, francium, scandium, lanthanum, cerium, praseodymium, neodymium, samarium, yttrium, dysprosium, holmium, erbium, thulium, ytterbium, lutetium, and combinations of metals such as an alloy, brass, steel, bronze, aluminum alloys, tin, and monel. In one embodiment, the N-heterocyclic carbene benzimidazole is 1,3-diisopropyl-5-(dodecyloxy)-1H-benzo[d]imidazole-2-ylidene (NHC-2).

An aspect of the invention provides a method of forming a carbene coating on a metal surface, comprising applying a positive potential to a metal surface, applying a negative potential to a conductive surface, disposing the metal surface and the conductive surface in (i) a solution comprising free carbene or a salt of carbene; or (ii) an inert atmosphere housed in a reaction chamber and contacting the metal surface with gaseous free carbene, and producing a carbene coated metal surface.

In one embodiment, the applying a positive potential to a metal surface is applying a positive potential to a substantially unprotected metal surface. In one embodiment, substantially unprotected is substantially unoxidized. In one embodiment, the carbene salt is a N-heterocyclic carbene benzimidazolium hydrogen carbonate salt. In one embodiment, the N-heterocyclic carbene benzimidazolium hydrogen carbonate salt is 5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate. In one embodiment, the positive potential is in a range of 0.1 to 2.0 V. In one embodiment, the metal is copper or iron. In one embodiment, the positive potential is in a range of 0.8 to 1.0 V. In one embodiment, the metal is gold. In one embodiment, the carbene coating is an electron passivation layer. In one embodiment, the disposing of the metal surface occurs for about 1 min to about 5 h. In one embodiment, the disposing of the metal surface occurs for about 1 min to about 120 min. In one embodiment, the method further comprises a reference electrode. In one embodiment, the method further comprises a salt bridge connecting the solution comprising free carbene or a salt of carbene, to the aqueous solution.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, wherein:

FIG. 1A shows a schematic representation of a 3-electrode, two half-cell experimental set up.

FIG. 1B shows structural formulae for: 5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate (NHC-2·HCO₃); 1,3-diisopropyl-5-(dodecyloxy)-1H-benzo[d]imidazole-2-ylidene (NHC-2); and 1,3-diisopropylbenzimidazolium N-heterocyclic carbene

FIG. 2A shows an example cyclic voltammogram of a gold electrode which undergoes surface modification with NHC-2·HCO₃for 30 min; solid line (—) is 0 min time point on bare gold, dash line ( custom-character ) is 1 min time point, dotted line () is 5 min time point, dash dot () line is 10 min time point, and dash dot dot () line is 30 min time point.

FIG. 2B shows a cyclic voltammogram of a surface stability study of the electrode of FIG. 2A (subsequent to the surface modification); surface stability was investigated by measuring 100 CV cycles, where a solid line (—) is cycle 2, dash line ( custom-character ) is cycle 10, dotted line () is cycle 20, dash dot () is cycle 40, dash dot dot () line is cycle 60, short dashes () is cycle 80, and short dots () is cycle 100.

FIG. 2C shows two plots of absolute peak current values from oxidation (▪) and reduction (●) peaks; on the left side with an x-axis of time, data is plotted of an electrode modified with NHC-2·HCO₃up to the 30 min time point; on the right side with an x-axis of CV cycle, data is plotted up to 100 CV cycles and this shows stability of the modified electrode surface.

FIG. 3A shows two plots of oxidative applied potentials against time and against CV cycle where error bars represent standard deviation from three experiments run with three different electrodes; ♦ solution phase deposition (control, no applied potential), ▪ 0.2 V applied potential, ● 0.4 V applied potential, ▴ 0.8 V applied potential, and ▾ 1.0 V applied potential.

FIG. 3B shows two plots for reductive applied potentials against time and against CV cycle where error bars represent standard deviation from three experiments run with three different electrodes; ♦ solution phase deposition (control, no applied potential), *−0.2 V applied potential, custom-character −0.4 V applied potential, −0.8 V applied potential, and *−1.0 V applied potential.

FIG. 4A shows cyclic voltammogram overlay that started with a bare gold electrode (solid line) and changes as the gold electrode surface was modified with NHC-2·HCO₃via solution phase deposition with no applied potential up to 30 min time point.

FIG. 4B shows cyclic voltammogram overlay for a stability study performed from 2 to 100 CV cycles on the modified gold electrode of FIG. 4A.

FIG. 4C shows cyclic voltammogram overlay that started with a bare gold electrode (solid line) and changes as the gold electrode surface was modified with NHC-2·HCO₃via solution phase deposition with an applied potential of −0.2 V for up to 30 min.

FIG. 4D shows cyclic voltammogram overlay for a stability study performed from 2 to 100 CV cycles on the modified gold electrode of FIG. 4C.

FIG. 4E shows cyclic voltammogram overlay that started with a bare gold electrode (solid line) and changes as the gold electrode surface was modified with NHC-2·HCO₃via solution phase deposition with an applied potential of 1.0 V for up to the 30 min.

FIG. 4F shows cyclic voltammogram overlay for a stability study performed from 2 to 100 CV cycles on the modified gold electrode of FIG. 4E.

FIG. 5 shows two plot of absolute peak current values versus time and versus CV cycle for oxidation and reduction peaks of optimized conditions for solution phase deposition of NHC-2·HCO₃at the following conditions: ♦ (control, no applied potential), ▪ −0.2 V applied potential, and ▾ 1.0 V applied potential.

FIG. 6A shows two plots of absolute peak current values versus time, and versus CV cycle, ● represents peak current for oxidation CV peaks, and ▪ represents peak current for reduction CV peaks for an electrode modified with NHC-2·HCO₃via 24 h solution phase modification with no applied potential, where the error bars represent standard deviation from three experiments run with three different electrodes.

FIG. 6B shows two plots of absolute peak current values versus time, and versus CV cycle, ● represents peak current for oxidation CV peaks, and ▪ represents peak current for reduction CV peaks for an electrode modified with NHC-2·HCO₃via solution phase modification with no applied potential up to 120 min time point.

FIG. 6C shows two plots of absolute peak current values versus time, and versus CV cycle, ● represents peak current for oxidation CV peaks, and ▪ represents peak current for reduction CV peaks for an electrode modified with NHC-2·HCO₃via solution phase modification via −0.2 V applied potential modification up to 120 min time point.

FIG. 6D shows two plots of absolute peak current values versus time, and versus CV cycle, ● represents peak current for oxidation CV peaks, and ▪ represents peak current for reduction CV peaks for an electrode modified with NHC-2·HCO₃via solution phase modification via 1.0 V applied potential modification up to 120 min time point.

FIG. 7 shows a plot of absolute peak current values versus time for a negative control experiment where a gold electrode was exposed to plain methanol (in the absence of NHC-2·HCO₃) under the following conditions: ♦ control, no applied potential solution phase, ▪ −0.2 V, and ▾ 1.0 V applied potential conditions; where error bars represent standard deviation from three experiments run with three different electrodes.

FIG. 8A shows N 1s XPS spectra of Au on Si chip modified with NHC-2·HCO₃under solution phase conditions with no applied potential for 5 min time point.

FIG. 8B shows 1s Is XPS spectra of Au on Si chip modified with NHC-2·HCO₃under −0.2 V applied potential conditions for 5 min time point.

FIG. 8C shows N 1s XPS spectra of Au on Si chip modified with NHC-2·HCO₃under 1.0 V applied potential conditions for 5 min time point.

FIG. 8D shows N 1s XPS spectra of Au on Si chip modified with NHC-2·HCO₃under solution phase conditions (i.e., no applied potential) for 120 min time point.

FIG. 8E shows N 1s XPS spectra of Au on Si chip modified with NHC-2·HCO₃under −0.2 V applied potential conditions for 120 min time point.

FIG. 8F shows N 1s XPS spectra of Au on Si chip modified with NHC-2·HCO₃under 1.0 V applied potential conditions for 120 min time point.

FIG. 9 shows a plot of adsorption energy versus potential, which is simulated results for adsorption energy of 1,3-diisopropylbenzimidazolium N-heterocyclic carbene bound to gold surface at reductive and oxidative potentials.

FIG. 10 shows an experimental overview of free-carbene (NHC-2) experiments, where ▪ represent solution phase experiments with no applied potential and ● represent solution phase experiments with +1 V applied potential; error bars are standard deviation from three different trials.

FIG. 11A shows an example cyclic voltammogram from studies of solution phase surface modification with no applied potential using free-carbene (NHC-2).

FIG. 11B shows an example cyclic voltammogram from a surface stability of the modified electrode of FIG. 11A.

FIG. 11C shows an example cyclic voltammogram from studies of solution phase surface modification with +1 V applied potential using free-carbene (NHC-2).

FIG. 11D shows an example cyclic voltammogram from a surface stability of the modified electrode of FIG. 11C.

DETAILED DESCRIPTION OF EMBODIMENTS

A deposition method is described for electrochemically modifying metal surfaces by applying a carbene coating (i.e., monolayer or overlays). As used herein, “NHC” or “carbene” refers to an N-heterocyclic carbene. An embodiment of this method includes applying a positive potential to a metal surface to form a working electrode, and disposing it and a counter electrode in a media that includes free carbene, a salt of carbene, or a combination. The positive potential was referenced relative to a Ag/AgCl reference electrode. The media can be a liquid with free carbene, carbonic salt, or a combination in solution. In one embodiment, the media is an inert gaseous atmosphere that includes free carbene. Following contact of the charged metal surface (i.e., working electrode) with the NHC media, the metallic surface will have a coating of NHC bound to metal.

In the liquid environment, a solution is used that includes free carbene, carbene salt, or a combination. In one embodiment, the solution includes an NHC salt, meaning that a solid salt that is stable in ambient conditions was added to a solvent (e.g., methanol). The carbenic salt is made up of an anion that is a negatively charged carbenic moiety, and a cationic counterion. In one embodiment, the counterion is a hydrogen carbonate moiety (e.g., HCO₃⁻). Deprotonation of carbenic moiety (i.e., base attacking H) results in formation of a neutral free carbene, which reacts with surfaces.

In the gaseous embodiment, a reaction chamber is used to house an inert atmosphere (e.g., N₂, Ar). A positive potential is applied to a metal surface on a substrate to form a working electrode, and disposed in the inert atmosphere. The positive potential was referenced to a Ag/AgCl reference electrode. Free carbene in gaseous form is generated in the reaction chamber. Carbene deposits on the metal surface.

As used herein, the term ‘substrate” refers to an item on which a coating is applied, which may be made of one or more materials that include for example, metal, polymer, semi-conductor material, plastic, glass, electrodes, sensors, electrical circuits, etc.

Any metal that is conductive and that can have a positive potential applied to it is suitable for this method. Examples of suitable metals include: transition metals, alkaline earth metals, alkali metals, rare earth elements, and also include combinations of metals such as alloys.

Transition metals include: scandium; titanium; vanadium; chromium; manganese; iron; cobalt; nickel; copper; zinc; yttrium; zirconium; niobium; molybdenum; technetium; ruthenium; rhodium; palladium; silver; cadmium; lanthanum; hafnium; tantalum; tungsten; rhenium; osmium; iridium; platinum; gold; mercury; actinium; rutherfordium; dubnium; scaborgium; bohrium; hassium; meitnerium; darmstadtium; roentgenium; and copernicium.

Alkaline earth metals include beryllium; magnesium; calcium; strontium; barium; and radium. Alkali metals include lithium; sodium; potassium; rubidium; cesium; and francium.

Rare earth elements include: scandium; lanthanum; cerium; prascodymium; neodymium; samarium; yttrium; dysprosium; holmium; erbium; thulium; ytterbium; and lutetium.

Alloys include brass, steel, bronze, aluminum alloys, tin, and monel.

In one embodiment, non-limiting examples of metals that are appropriate for this electrochemical method include gold, copper, and iron. In one embodiment, the metal surface is a substantially unprotected metal surface. As used herein “substantially unprotected” means substantially no metal oxide coating, or no protective coating, so the metal atoms are available for reaction. In one embodiment, the metal comprises metal oxide (e.g., cupric oxide).

Once such an NHC coating has been deposited on the metal surface, the coating formed an electron passivation layer that inhibited the metal from participating in reactions. Such inhibited reactions may include corrosion, oxidation, etc. This inhibition was evidenced by suppression of the reduction and oxidation peak current values in characterization tests described herein.

Such characterization tests included overlayed cyclic voltammograms shown in FIGS. 2A, 4A, 4C, 4E and 11C where surface passivation obstructed electron transfer between the gold surface and a ferri/ferro cyanide reaction from an electrolyte solution.

An electrochemical cell is a device that is capable of generating electrical energy from chemical reactions occurring in it, or it is capable of using electrical energy supplied to it to facilitate chemical reactions in it. As described in the Working Examples, many experiments with electrochemical cells were conducted to investigate a wide range of electrochemical voltages (e.g., from 31 1 V to 1 V). It was determined by experimental and theoretical evidence that applying a positive potential in the range of 0.01 V to 2 V formed an NHC coating. In one embodiment, a positive potential in the range of 0.8 to 1.0 V was used. The positive potential was referenced to a Ag/AgCl reference electrode.

In experiments using a 3-electrode, two half-cell electrochemical cell (see FIG. 1A), gold surfaces, which were present as a Working Electrode (WE), were coated with N-heterocyclic carbene (NHC) molecules (see structural formulae of representative examples in FIG. 1B). The presence of the coating caused a modification of the metallic surfaces' stability properties. This modification was quantified in surface stability studies of the coated WE. As used herein, the term “surface stability” refers to reactivity level. For example, increased surface stability may refer to decreased reactivity to oxygen, or decreased susceptibility to electron transfer processes. Data showing modification of the surface stability of the coated metallic surfaces relative to controls are presented herein (sec FIGS. 2B, 4B, 4D, 4F and 11D).

As described in the Working Examples, in characterization tests an electrochemical cell was used that included two half cells that were connected to one another via a salt bridge. When modifying the WE with NHC molecules, a first half-cell contained a methanolic solution wherein 10 mM of a hydrogen bicarbonate benzimidazolium salt was dissolved, and when electrochemically evaluating the WE after surface modification, the other half-cell contained an aqueous solution with 5 mM Fe(CN)₆^3−/4− electrolyte and 1 M NaCIO₄supporting electrolyte.

Electrode modification was performed as described in the Working Examples. Instability in the modified surface was demonstrated by the increase in absolute peak current at the reduction and oxidation peak as the number of cycles increased, illustrated in FIG. 2B, 4B and 4D. Where, instability of the modified surface demonstrates an inefficient reaction of the NHC to the metal surface. A side-by-side comparison was performed of suppression of absolute peak current due to surface modification over time, and instability of the modified surface via an increase in the absolute peak current with continued cycling. This side-by-side comparison was used to determine the effectiveness of this electrochemical deposition methodology. That is, a solution phase deposition with no applied potential (control) was compared to a solution phase deposition through the use of an applied potential (see FIGS. 4F and 6D).

Referring to FIGS. 3A and 3B, a side-by-side comparison is provided of surface modification and modified surface stability for all the potentials evaluated. Each potential was repeated in triplicate using three different electrodes. Of the oxidative potentials evaluated (see FIG. 3A), an applied potential of 0.2 and 0.4 V demonstrated suppressed surface modification. After the 30 min time point, the absolute oxidation peak current values were 31.9±4.1 and 28.4±3.0 μA, respectively, and the absolute reduction peak current values were -34.6±5.2 and −20.9 +0.9 μA, respectively. The control experiment had an oxidation and reduction peak current of 19.6±3.9 and −32.4±6.9 μA, respectively. Since the oxidation peak current values for the surfaces modified with an applied potential of 0.2 and 0.4 V demonstrated a larger potential than the control, it appeared that the surface modification was not as effective as the control. When observing the reduction peak current there was a slight enhancement of surface coverage for the 0.2 V applied potential condition. The 0.4 V applied potential condition fell within error of the control. However, when evaluating the stability of the modified surface, significant current increase occurred for electrodes modified using a potential of 0.4 V. After 100 Cyclic Voltammetry (CV) cycles, the oxidation peak current was measured at 50.7±4.3 μA. This result was 10 μA smaller than the zero time point. The reduction peak current measurement was −46.0±5.3 μA, which is 15 μA smaller than the zero time point. The control had a measured oxidation and reduction peak current after 100 CV cycles of 23.2±4.0, and 30.5±7.0 μA, respectively. This result was 24 and 28 μA smaller than the zero time point.

When 0.8 and 1.0 V of applied potential was used for surface modification, significant current suppression was observed. An applied potential of 1.0 V demonstrated fast onset of surface modification with oxidation and reduction peak currents of 20.5±1.5 μA and −25.7+2.3 μA, respectively, after the 1 min time point. For comparison, the oxidation and reduction peak currents for the control experiment after the 1 min time point were 39.0±2.9 μA and −46.2±4.9 μA, respectively. These results are current measurements that are 53 and 56% larger than those of an electrode modified using an applied potential of 1.0 V. The electrodes modified with an applied potential of 0.8 V had current measurements within error of the control experiment after the 30 min time point. The oxidation and reduction peak currents were 21.1±2.4 μA and -25.7 #1.8 μA, respectively. The stability of surfaces modified with 0.8 V applied potential were less than those of the solution phase control experiment where the change in current from cycle 2 to cycle 100 for the oxidation and reduction peaks were found to be 41 and 27%, respectively. For these conditions, the surface passivation was quite fast and there was a stronger reduction in peak current at 1 min., which is good relative to the reduction in peak current of 30 min for the control at zero potential.

The control experiment demonstrated a change in current from cycle 2 to cycle 100 for the oxidation and reduction peaks of 26 and 11%, respectively. However, an enhanced surface stability in comparison to the control was observed for the electrode modified using an applied potential of 1.0 V. This result was obtained with a change in peak current from cycle 2 to 100 for the oxidation and reduction peaks of 9 and -19%, respectively. The negative value for the percentage change between cycle 2 and cycle 100 for electrodes modified with 1.0 V applied potential were thought to be due to the current value decreasing as the electrode was subjected to 100 cycles of CV. The decrease in the current value may come from a reorganization of the adsorbed species on the surface such that there was more uniform coverage. Uniformity would minimize defects which would enhance the signal of the Fe(CN)₆^3−/4− redox probe. Accordingly, due to the enhanced surface modification and superior surface stability observed for electrodes modified with an applied potential of 1.0 V, it was chosen as the desirable potential for modification under oxidative conditions.

When evaluating the reductive potentials (FIG. 3B), electrodes which were modified with an applied potential of −0.8 and −0.4 V demonstrated minimal surface modification where the measured current of the oxidation and reduction peak at the 30 min time point for each of the conditions were 52.5±0.5, 42.9±0.8, −49.1±0.2 and −40.7±0.3 A, respectively. The current suppression observed at these conditions is 32.9, 23.3, 16.7, and 8.3 μA larger than the control experiment for the oxidation and reduction peaks of the electrodes modified under an applied potential of −0.8 and 0.4 V, respectively.

When considering the electrodes which were modified using an applied potential of −1.0 and −0.2 V, the oxidation and reduction peak currents were found to be 28.0±8.7, 26.1±2.0, −49.1±0.2, and −27.7±2.3 μA, respectively. Where the reduction peak current measurements for the electrodes modified under both conditions are within error of the control experiment and the oxidation peak currents are 6.5 and 8.4 μA larger than the control measurement for the electrodes modified under an applied potential of −0.2 and −1.0 V, respectively. When probing the stability of the surfaces which were modified under an applied potential of −0.2 V a 36 and 14% change in current was observed for the oxidation and reduction peaks from cycle 2 to cycle 100 which is similar to the change observed for the control experiment. The change in current observed upon stability investigation for the electrodes modified with an applied potential of −1.0 V was 26 and 16% from cycle 2 to cycle 100 of the oxidation and reduction peaks, respectively which is also aligns with the stability observed for the control experiment. However, an applied potential of −0.2 V demonstrated a faster onset of surface modification in comparison to electrodes modified under an applied potential of −1.0 V, thus an applied potential of −0.2 V was chosen as the desired potential for experiments at reductive potentials. As a note, when observing the reduction peak all the reductive potentials evaluated a faster onset of surface modification was demonstrated when compared to the control.

Optimal reductive and oxidation conditions of −0.2 and 1.0 V applied potential were investigated further. It was noted that the modified surfaces demonstrated more stability under reductive conditions.

The side-by-side comparison of the optimized conditions in comparison with the control is shown in FIG. 4A. When considering −0.2 V of applied potential to the control of solution phase deposition, the onset of surface modification is expedited for the reduction half cycle of the CV voltammogram. Conversely, surface modification under oxidative conditions at 1.0 V applied potential demonstrates a faster onset of surface modification for both oxidative and reductive potentials. When visually evaluating the CV voltammograms (see FIG. 4A-4F) for both the surface modification and modified surface stability for the electrodes corresponding to the control, −0.2 and 1.0 V applied potential, it was evident the electrode modified at a potential of 1.0 V demonstrates the fastest onset of surface modification in addition to the most current suppression from a bare gold surface, which was also supported by the peak current values. Additionally, it was also visually evident that the electrode modified at a potential of 1.0 V had the largest surface stability with minimal change in peak current and rather more of a shift in the position of the curve. Within all the CV voltammograms for both the surface modification and the redox stability of the resultant modified surface, there was a visual difference between the oxidation and reduction peak, where the current suppression of the reduction peak as the surface was being modified is less than the oxidation peak and the increase of the current of the reduction peak when performing 100 CV cycles to test the stability of the modified surface.

Moving forward with the optimal conditions, a longer time frame was evaluated FIG. 6A-6D. Non-electrochemical, solution phase modification for a 24 h time frame is a well-known method to generate NHC self-assembled monolayers on surfaces (Crudden, et al., (2016) Nature Comm., 7, 12654). Using the 24 h surface modification as the control, experiments were carried out evaluating solution phase modification and modification at −0.2 and 1.0 V applied potentials up to the 120 minute time point. After the 120 minute time point of solution phase modification, the peak suppression was comparable to that seen of the 24 h solution phase deposition with the oxidation and reduction peak currents are 14.14±7.5, 14.6, −21.7±5.4, and −25.7 μA, respectively for the solution phase modification for 120 mins and 24 h solution phase deposition. Likewise, the stability of the modified surface was comparable to that of the 24 h solution phase deposition where the oxidation and reduction peak currents were measured to be 9.7±1.1, 10.1, −15.5±0.6, and −19.1 μA, respectively for the 100^thcycle of the 24 h solution phase and 120 minute solution phase modification.

When applying a reductive potential of −0.2 V, the difference in surface modification based on the suppression of the absolute peak current was negligible between 30 mins and 120 mins where the currents were measured to be 29.3 and 25.4 μA, respectively for the oxidation peak and −34.8 and −29.0 μA, respectively for the reduction peak. Upon evaluating the stability of the modified surface with 100 CV cycles, the modified surface after 30 mins demonstrates comparable stability as the surface modified for 120 mins where the oxidation and reduction peak current measurements are 25, 24.6±2.4, −22.8, and −23.1±3 μA, respectively for the 100^thcycle of the electrodes modified under an applied potential of −0.2 V for 120 and 30 minutes, respectively. This suggests when modifying a surface using an applied potential of −0.2 V, the modification of the electrode surface is self limiting at the 30 min time point and no additional modification is possible, yet the stability of the modified surface is maintained.

Lastly, the surface modification was studied when applying an oxidative potential of 1.0 V. Not only did the electrode modified under an applied potential of 1.0 V demonstrate greatly expedited onset of surface modification and enhanced surface modification, there was also negligible change in surface stability from CV cycle 2 to 100. Where after the 1 min time point the oxidation and reduction peaks had current values of 24.0 and −34.4 μA, respectively which is a current value 61 and 50% smaller than the 0 time point. Additionally, when probing the surface stability the oxidation and reduction peak current changes from 1.3 μA to 0.9 μA and −1.2 μA to −0.9 μA, respectively from cycle 2 to cycle 100. Similarly to the experiments run up to the 30 min time point, there is a reduction in peak current with continued CV cycling which may again be attributed to molecular reorganization on the surface. As the 120 min time point modification experiment was not completed in triplicate, it is not consistent with the previous results for surface modification up to the 30 min time point for the applied oxidative potential of 1.0 V. However, there is consistency in the rapid onset of surface modification which demonstrates enhanced modification when compared to the control as well as good stability and decrease in current measurement over 100 redox cycles by CV.

A negative control experiment was performed where solution phase modification and modification under an applied potential of −0.2 and 1.0 V was completed in methanol with no imidazolium salt added. As seen in FIG. 7, after modifying the surface up to the 120 min time point under all conditions no surface modification was observed when the electrodes were probed using electrochemical methods. Here the change in peak current from the zero time point to the 120 minute time point for the solution phase, −0.2 V applied potential and 1.0 V applied potential is 7.3, 4, and 0.2 μA for the oxidation peak current, respectively and 8.8, 2.7, and 0.2 μA for the reduction peak current, respectively. This suggested that the solvent used for dissolving the imidazolium salt had no effect on the system both in a solution phase deposition experiment in addition to applying either a reductive or oxidative potential. Thus the surface modification observed in the presence of the imidazolium salt must come exclusively from the organic molecule.

To investigate the system further, x-ray photoelectron spectroscopy (XPS) measurements were performed. The method for surface modification which was applied to the disk electrodes was used on a PVD gold silica wafer. As can be seen in FIG. 8A-8C, a N1s peak after the 5 min time point for the solution and negative potential (−0.2 V) have similar intensities, whereas the N1s peak for the positive potential (1.0 V) was much sharper. The average % N/Au signal for the solution, negative, and positive conditions was 1.64, 0.79, and 1.68%, respectively. Thus, after the 5 min time point of surface modification with the imidazolium precursor, the modification with an oxidative potential demonstrated the similar coverage as that of solution phase modification and the reductive potential has the least amount of coverage. The enhanced surface coverage for modification under oxidative potentials became more evident at the 120 min time point. When looking at the N1s signal, again the modification at a negative potential and under solution phase conditions demonstrated similar broad signals and the modification under a positive potential has a much sharper and obvious signal. Where the average % N/Au signal for solution, negative, and positive were 3.86, 7.07, and 33.56%, respectively. Here, the N1s signal for the surface modified under oxidative conditions was 10-fold larger than the control solution phase modification. Additionally, under reductive conditions there was also a small enhancement in surface modification when compared to the solution phase control. From this XPS study there was strong evidence to support both the enhanced surface coverage and faster onset of surface modification for the surfaces modified by oxidative potentials seen previously in the electrochemical experiments.

Gold surface modification with NHC molecules through the use of electrochemical conditions was investigated where a wide range of oxidative and reductive potentials were investigated. Additionally, a range in modification time from 1 to 120 min time point was also probed. The optimal reductive potential to apply was found to be −0.2 V and the optimal oxidative potential to apply was found to be 1.0 V. From the experiments where −0.2 V of potential was applied to facilitate surface modification, it was found that there is no significant change to surface coverage or modified surface stability between the 30 and 120 min time point. Additionally, after the 30 min time point the surface modification was demonstrated to be comparable with the solution phase control experiment.

In all instances where the 1.0 V applied potential condition was evaluated, a faster on set of surface modification was observed, in addition to an enhanced surface coverage when compared to the solution phase control. Additionally, when evaluating the stability of the modified surface, the peak current measurements demonstrated a slight suppression in value suggesting even further surface coverage. The redox cycling when evaluating the stability of the modified surface may be facilitating the re-arrangement of the NHC molecules adsorbed onto the surface, increasing the density of the coverage and reducing any pinholes.

These findings were supported by some XPS measurements which were performed (sec FIGS. 8A-8F). After the 120 minute time point, the surfaces which were subjected to the 1.0 V applied potential condition demonstrated a 10 fold increase in the % Au/N signal when compared to the control (see FIGS. 8D and 8F). There was an enhancement of the % Au/N signal in comparison to the control in surface coverage for the surface modified with the applied potential of −0.2 V at the 120 minute time point (see FIGS. 8D and 8E).

A negative control experiment was performed up to the 120 minute time point, and found that methanol solvent was not contributing to the surface modification under any condition. Simulations were performed which demonstrated an enhanced adsorption energy for the NHC—Au bond under oxidative conditions (see FIG. 9).

It was determined that modifying a gold surface with NHC molecules under oxidative conditions by applying a potential of 1.0 V demonstrated faster and better surface coverage than the solution phase control, which was done at zero potential. Additionally, applying a reductive potential of −0.2 V produced surface coverage results comparable with the solution phase control.

Reporting of N-heterocyclic carbene modification in 2014 (see Crudden et al., Nature Communications 7 (2016), 12654) employed a method of surface functionalization using a free-carbene, that had been generated in a glove box under an inert atmosphere. This method for modifying surfaces with NHCs is still widely employed. This method was employed to demonstrate its applicability for electrochemical methods in which two conditions were chosen. As a control, solution-phase deposition was performed with no potential applied. Surface modification was performed for a total time of 46 mins. The 1.0 V applied potential experiments utilized the methodology of Example 1A wherein surface modification was sequentially carried out up to the 30 min timepoint, totaling 46 mins of surface modification.

FIG. 10 illustrates a summary of results for both deposition with and without 1.0 V applied potential experiments. The left side of FIG. 10 demonstrates peak current reduction from surface modification up to 46 mins total and the left hand side demonstrates redox stability of the modified surface over 100 CV cycles. As there was no change in peak current for the solution phase with no applied potential experiment from the zero time point to the 46 min time point. This result suggests that surface modification in methanolic solution with NHC-2 was not possible after 46 mins without applied potential of 1.0 V. Conversely, there was an absolute peak current change of 28.3 μA and 34.0 μA for the reduction and oxidation peak currents respectively, from the zero time point to the 30 min time point when an applied potential of 1.0 V was used the 51.7% and 61.9% reduction in absolute peak current at the reduction and oxidation peaks, respectively, is strong evidence for the passivation of the gold electrode with NHC-2. Additionally, since there was no surface modification under solution phase conditions, there was also no changes in the peak currents when the electrode was evaluated for redox stability. Conversely, when evaluating redox stability of the electrodes which were modified using 1.0 V applied potential, an absolute peak current change of 3.1 μA and 6.0 μA was observed at the reduction and oxidation peak respectively from CV cycle 2 to 100. The change in peak current for both the reduction and oxidation peaks is within error of the peak current measured for the 2nd CV cycle suggesting zero removal of the modified surface at the gold electrode.

From the solution phase and 1.0 V applied potential experiments, evidence is shown to suggest the efficient and strong modification of NHC-2 when an applied potential of 1.0 V was used, further supporting the claim that applying a potential to the surface can expedite and enhance the stability of NHCs modified on gold surfaces. Referring to FIG. 1A, a schematic is shown for an embodiment of the method having a carbene precursor (i.e., imidazolium salt). The same layout of set-ups in FIG. 1 was used for electrochemical deposition of NHC, and for evaluation of the NHC coating once formed. with half cell 1 (C1) solution being different containing a 10 mM solution of NHC-2·HCO₃in methanol.

Referring to FIG. 1B, structural formulae are shown of 5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate (NHC-2·HCO3); 1,3-diisopropyl-5-(dodecyloxy)-1H-benzo[d]imidazole-2-ylidene (NHC-2); And 1,3-diisopropylbenzimidazolium N-heterocyclic carbene on a metal surface

Referring to FIGS. 2A-2C, CV spectra and summary plots are shown that illustrate a data analysis method and range of electrochemical conditions that were studied. Surface modification with NHC led to limiting electron transfer and decreasing electrochemical current peaks. A formed NHC layer was tested against electrochemical recycling experiment where 100 sequential CVs were performed to see if any instability was detected. Instability of NHC layers led to degradation, which created defects (i.e., gaps) in the NHC layer. Such defects led to increased current during a redox instability test. Maximum currents from surface modification and redox instability tests were plotted for comparison.

Referring to FIGS. 3A and 3B, results are illustrated from a wide range of electrochemical conditions. The deposition was consistently carried out for 30 minutes. The results show a range of −0.2 V and 1 V to be the most promising in formation of NHC layers which were stable after formation.

Referring to FIGS. 4A-4F, shows cyclic voltammograms showing an overlay of surface stability. Electrochemical deposition by −0.2 V and 1 V was isolated out for comparison together with solution deposition.

Referring to FIG. 5, a plot is shown of absolute peak current values versus time and versus

CV cycle for oxidation and reduction peaks of optimized conditions of solution phase.

Referring to FIGS. 6A-6D, these plots summarize results from further investigation on −0.2 V and 1 V with extended deposition time. The deposition time was extended from 30 minutes to 120 minutes. This experiment further shows electrochemical deposition by 1 V was effective in formation of an electron blocking layer.

Referring to FIG. 7, plots illustrate results from control experiments. When NHC molecules were not used, none of the solution deposition or electrochemical deposition conditions could form electron blocking layers.

Referring to FIGS. 8A-8F, spectra are shown for X-ray photo electron spectroscopy showing consistent results for electrochemical analysis. By monitoring chemical composition on electrode surfaces, including nitrogen to gold signal ratio, it was shown that electrochemical deposition by 1 V leads to a rapid formation of NHC layer.

Referring to FIGS. 8A-8F XPS spectra are shown that confirm the presence of the carbene coating after treatment under the stated conditions.

Referring to FIG. 9, a plot is shown of adsorption energy versus potential, which is simulated results for adsorption energy at reductive and oxidative potentials of 1,3-diisopropylbenzimidazolium N-heterocyclic carbene (see FIG. 1C) on a gold surface. These results are from a modelling study that was performed using Density Functional Theory (DFT). The result suggested a strong absorption energy from using 1 V and produced a strong NHC—Au bond, with fast formation and good stability.

Referring to FIG. 10, a plot is shown of free-carbene (NHC-2) solution-phase experiments with an without applied potential.

Referring to FIG. 11A-11D, CVs are shown for modification and stability studies with solution-phase free-carbene (NHC-2), that were deposited without applied potential or were electrodeposited with applied potential.

The following working examples further illustrate the invention and are not intended to be limiting in any respect.

EXAMPLE 1A
Electrochemical Deposition of NHC Coatings Using Solution NHC Solution

5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate was synthesized as reported by Crudden et. al (Nature Communications 7 (2016), 12654). A 10 mM methanolic benzimidazolium salt solution was prepared by dissolving 5-(dodecyloxy)-1,3-diisopropyl-1H-benzo[d]imidazole-3-ium hydrogen carbonate in methanol.

Working Electrode

A Working Electrode was prepared from a gold disc that was partially embedded in Kel-F polymer (2 mm diameter, available from CH Instruments, Austin, TX, USA). The Working Electrode (WE) was connected to a potentiostat. The potentiostat, which is an electronic instrument that controls voltage difference between the Working Electrode and the Reference Electrode, provided sufficiently high current capacity to accommodate necessary current levels during the coating process. The WE was charged with a selected positive potential that is specified in the figures (see the electrochemical cell schematic shown in FIG. 1A). A Counter Electrode (CE) (e.g., platinum wire) was present to balance the charges.

Coating Process

As shown in FIG. 1A, an electrochemical cell was set up with the WE as described above. A counter electrode (CE) included a conductive surface (e.g., platinum wire) (CH Instruments, Austin, TX). A salt bridge included a 3% Agar solution (Fisher Scientific, Ottawa ON) in 1 M KNO₃solution (VWR Chemicals, Radnor, PA).

The WE and CE were immersed in the methanolic benzimidazolium salt solution at room temperature, the salt bridge connected this solution to another container which included a Ag/AgCl reference electrode (RE) in a 3 mol/L KCI solution.

NHC coating on a metal surface was deposited by applying a potential to the WE while it was immersed in the methanolic solution, using a technique of chronoamperometry. The electrode modification (i.e., NHC coating) at each potential was performed for incrementally longer time points The potentials of −1.0, −0.8, −0.4, 0.2, 0.4, 0.8, and 1.0 V were applied to the WE using the technique of chronoamperometry up to the 30 minute time point and up to the 120 min time point for −0.2 and 1.0 V applied potential. The WE was immersed for a selected period of time. When the selected amount of time had elapsed, the WE was removed from the methanolic benzimidazolium salt solution and disconnected from the potentiostat. A carbene coating was present on the parts of the gold disc that had been in contact with the NHC solution. The coating was not visible to the eye.

EXAMPLE 1B.
Electrochemical Deposition of NHC Coatings Using Free Carbene

5-(Dodecyloxy)-1,3-diisopropyl-1H-benzo][d]imidazole-3-ium (NHC-2) was prepared in a glovebox as reported by Crudden et. al (Nature Communications 7 (2016), 12654. When employing a solution phase methodology, a clean gold disk electrode was immersed in 2 mL 10 mM methanolic solution of NHC-2 under a continuous stream of argon gas for 46 mins. When employing electrochemical methods, a clean gold disk electrode was immersed in 2 mL of a 10 mM methanolic solution of NHC-2 under a continuous stream of argon gas sequentially for 1, 5, 10, and 30 mins each (total time 46 mins) with an applied potential of 1 V chronoamperometry technique, which applied a fixed potential for a given time. Between each modification, the electrode was moved to a 5 mM K₄Fc(CN)₆/5 mM K₃Fe(CN)₆redox couple and 1 M NaCIO₄supporting electrolyte solution under ambient conditions and evaluated using CV. For both the solution phase with and without applied potential methodologies, the electrodes were subjected to 100 CV cycles after the final time point had been achieved to evaluate the stability of the modified electrode surface.

EXAMPLE 2.
Characterization of NHC Coatings Using Electrochemical Cells

An electrochemical cell (see FIG. 1A) was used to characterize NHC coatings that were deposited on WEs under different conditions of time and potential. A control experiment involved use of a WE that had undergone solution phase deposition at zero potential The cell included two half cells (C1 and C2) that were connected to one another via a salt bridge. C1 contained a working electrode (WE) and a counter electrode (CE). The WE included a gold disk (CH Instruments, Austin, TX) that had been coated with NHC coating as described in Example 1. The counter electrode (CE) included a conductive surface (e.g., platinum wire) (CH Instruments, Austin, TX). The salt bridge included a 3% Agar solution (Fisher Scientific, Ottawa ON) in 1 M KNO₃solution (VWR Chemicals, Radnor, PA). C2 included a Ag/AgCl reference electrode (RE) in a 3 mol/L KCI solution.

A wide range of reductive and oxidative potentials were evaluated (as illustrated in FIGS. 3A and 3B).

Characterization of the coating of Example 1, such as surface stability tests, were conducted. An electrochemical cell was set up containing an aqueous solution of Fe(CN)₆^3−/4− electrolyte and 1 M NaCIO4 supporting electrolyte. A WE and CE were immersed in the aqueous solution for this electrochemical cell. This cell was used to evaluate the coating which was deposited as described in Example 1. This cell was used to probe the redox activity of the Fe(CN)₆^3−/4− and to determine if there was a coating on the metal surface by evaluating the measured current values from a cyclic voltammetry (CV) experiment.

Evaluating the modified surface by electrochemical means was performed using cyclic voltammetry (CV). A CV was acquired for each electrode after each deposition time point. Surface modification was indicated by suppression in the absolute peak current for both the reduction and oxidation peaks over time as illustrated in FIGS. 2A, 4C, 4E, 11A, and 11C). After the final modification time point, the stability of the modified surface was evaluated by CV where 100 cycles were performed (see FIGS. 2B, 4B, 4D, 4F, 11B, 11D).

The characterization tests determined that samples obtained through electrochemical deposition were superior to samples obtained through solution deposition at zero applied potential (control). In the case of the electrochemically-prepared NHC-coated metal that was prepared using a positive potential on the metal surface, the surface stability was superior relative to NHC-coated metal prepared using non-electrochemical methods.

Example 3
DFT Modelling Study

A modeling study was performed using Density Functional Theory (DFT) with PBEsol (Perdew, et al., Phys. Rev. Lett. (1996) 77, 3865; and Perdew, et al., J. Chem. Phys. (1999) 110, 6158) which is an exchange-correlation functional. A simple NHC was chosen for this modeling study, specifically 1,3-diisopropylbenzimidazolium N-heterocyclic carbene (see structural formulae in FIG. 1B). Projector augmented wave potentials for the core electrons, and a planewave basis set expanded to kinetic energy cutoffs of 45 Ry and 450 Ry for the electronic wavefunctions and densities, respectively, were also used. A 3×3×1 k-point grid was used for an adsorbed system and Au slab and a 1×1×1 k-point grid was used for isolated NHC. These methods reproduced a lattice constant of Au to 0.009 Å. Voltage induced by applying an electric field normal to surface that shifted Fermi level by the desired voltage.

As can be seen in FIG. 9, the adsorption energy of the bound NHC increased as the potential became more positive. The stronger adsorption energy was indicative of a stronger NHC—Au bond, suggesting that applying a potential of 1.0 V will provide favorable conditions for the NHC to form a strong bond with the gold surface.

Equivalents

It will be understood by those skilled in the art that this description is made with reference to certain embodiments and that it is possible to make other embodiments employing the principles of the invention which fall within its spirit and scope.

Electrochemical Deposition of N-Heterocyclic Carbenes

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