FIELD

The present disclosure generally relates to a method for the synthesis and production of indium nanoparticles. More particularly, disclosed is a method of making indium nanoparticles by reduction of an indium fatty acid carboxylate in the presence of a surfactant in a nonpolar solvent.

BACKGROUND

Indium is a useful electrocatalyst and electrode material for performing certain electrochemical reactions. Because electrochemical reactions generally occur on the electrode surface, a high surface area of electrode material, or material coated on an electrode is desirable. One way to achieve high surface area is to use a coating of small particles such as nanoparticles.

Currently, however, there are relatively few methods for the production of sub- 100 nm indium nanoparticles (NPs) because it is challenging to identify combinations of precursor (i.e. indium salt, etc.), solvent, and surfactant (i.e. growth-limiting agent) that are mutually soluble while still affording the generation of < 100 nm diameter indium NPs.

SUMMARY

In summary, the invention is a method of preparing indium nanoparticles by mixing an indium fatty acid carboxylate salt (In(OFA) ₃ ) with a surfactant; heating the mixture of the indium fatty acid carboxylate and surfactant; and adding a reducing agent. The fatty acid is a C12-C24 saturated or unsaturated fatty acid. Non-limiting examples of fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, lignoceric acid, myristoleic acid, palmitoleic acid, sapienic acid, oleic acid, elaidic acid, vaccenic acid, linoleic acid, linoelaidic acid, -linolenic acid, arachidonic acid, eicosapentaenoic acid, erucic acid, and docosahexaenoic acid. In embodiments, the fatty acid is oleic acid. The reducing agent may be, for example, sodium borohydride, lithium borohydride, lithium aluminum hydride, lithium hydride, sodium hydride, hydrogen gas, and sodium metal. Sodium borohydride is an exemplary reducing agent. Suitable surfactants include alkyl thiols, phosphines, phosphine oxides, pyrrolidone, silanes, and amines. Alkyl thiols, for example dodecanethiol are exemplary. The mixture of the indium fatty acid salt and surfactant may be heated to a temperature from about 75°C to about 200°C, for example about 175°C. The indium fatty acid salt and the surfactant may be combined in a solvent. Non- limiting examples of solvents include decane, dodecane, tetradecane, hexadecane, octadecene, octadecane, trioctylphosphine oxide, mesitylene, and trichlorobenzene. The fatty acid carboxylate may be prepared by reacting an indium salt with a fatty acid to prepare the indium fatty acid carboxylate. The indium salt amy be, for example, an indium halide, indium nitrate, indium sulfate, indium carbonate, indium formate, indium acetate, and indium propionate.

Indium acetate is a typical starting material. The method of preparing the indium fatty acid carboxylate may include removing an acid by-product formed from the reaction of the indium salt with the fatty acid.

In an exemplary embodiment, the invention is a method of preparing indium

nanoparticles by reacting an indium salt with a fatty acid (HOFA) to prepare an indium fatty acid carboxylate (In(OFA) ₃ ); mixing the indium fatty acid carboxylate with a surfactant; heating the mixture of the indium fatty acid carboxylate and surfactant; and adding a reducing agent. In other exemplary embodiments, the invention is a method of preparing indium nanoparticles by reacting indium acetate with oleic acid in a nonpolar solvent to prepare indium oleate; removing the acetic acid by product; mixing the indium oleate with an alkylthiol surfactant in a solvent; heating the mixture of the indium oleate and surfactant; and adding a reducing agent; and then isolating or separating the indium nanoparticles from the solvent. Indium nanoparticles produced according to the invention may have a particle size that is less than 100 nm. In embodiments, a significant portion of the nanoparticles may have an average particle size of greater than about 5 nm or greater than about 10 nm. In embodiments, particle sizes may be in the range of from about 10 nm to about 100 nm; about 20 nm to about 100 nm; or about 25 nm to about 100 nm. In exemplary embodiments, 10% or more of the nanoparticles are larger than 10 nm, 10% or more of the nanoparticles are larger than 20 nm, or 10% or more of the nanoparticles are larger than 25 nm

Further objectives and advantages, as well as the structure and function of preferred embodiments will become apparent from a consideration of the description, drawings, and examples.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not necessarily restrictive of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain the principles of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the invention will be apparent from the following, more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings.

FIG. 1 is a transmission electron microscopy image of indium nanoparticles produced according to an embodiment of the invention; and

FIG. 2 is an exemplary transmission electron microscopy image of indium nanoparticles produced according to alternative methods of higher surfactant concentration or lower temperature.

DETAILED DESCRIPTION

Reference will now be made in detail to the subject matter disclosed, which is illustrated in the accompanying drawings. Embodiments of the invention are discussed in detail below. In describing embodiments, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected. While specific exemplary embodiments are discussed, it should be understood that this is done for illustration purposes only. A person skilled in the relevant art will recognize that other components and configurations may be used without parting from the spirit and scope of the invention. All references cited herein are incorporated by reference as if each had been individually incorporated. The present description is exemplary and explanatory only and not restrictive nor limiting of the present disclosure. The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate subject matter of the disclosure. Together, the descriptions and the drawings serve to explain but not limit the principles of the disclosure.

The present disclosure is directed to a method for the synthesis and production of indium nanoparticles (NPs) from an indium fatty acid carboxylate that is soluble in a nonpolar solvent. A variety of surfactants or ligands that are highly effective at limiting NP growth to less than about 100 nm may thus be utilized. The terms surfactant and ligand are used interchangeably herein. In using a combination of a nonpolar solvent and a surfactant, the nonpolar portion of the surfactant face outward and lend to the nanocrystal the property of solubility in a non-polar solvent and the polar head group faces the indium core. This is contrary to to the association generally used and observed in the art in which reactions are conducted in polar solvents and leads to advantages described elsewhere herein. Without being bound by theory, it is believed that by utilizing an indium fatty acid carboxylate precursor, and/or a nonpolar solvent with the surfactant, the reaction kinetics of NP nucleation and growth are slowed down allowing the use of higher temperatures, as well as providing sufficient time for the size of nanoparticles to be easily controlled by control of reaction time, as well as temperature and the nature and concentration of surfactant. Alternative methods that utilize a bare indium anion are characterized by rapid (virtually instantaneous) nucleation and growth kinetics and cannot realistically be temporally controlled.

Suitable indium fatty carboxylates may be prepared from an indium salt and a fatty acid. In exemplary embodiments, the reaction is driven toward completion by removal of the conjugate acid formed during the reaction. After removal of the conjugate acid, the indium fatty acid carboxylate may be reduced in situ without isolation.

This method permits the synthesis of indium nanoparticles at elevated temperatures, including temperatures of greater than 100 °C, thereby facilitating rapid nucleation of extremely small indium nanocrystals at high concentration and high-yield of sub 100-nm NPs. Other synthetic routes typically utilize reaction temperatures of < 100 °C due to the tendency of NPs to grow too rapidly and reach > 100 nm dimensions if synthesized at higher temperatures.

According to the present disclosure, indium nanoparticles are synthesized from an indium fatty acid carboxylate. As used herein, an indium fatty acid carboxylate is a salt that includes an indium cation and a carboxylate anion from a fatty acid. For ease of description, the fatty acid is designated as HOFA, the carboxylate anion derived from a fatty acids as OFA, and the indium carboxylate as In(OFA) ₃ . An example of a fatty acid, HOFA, is stearic acid

(HOOC(CH ₂ )CH ₃ ) in which the corresponding anion, OFA, is stearate ( ^" OOC(CH ₂ )CH ₃ ), and the indium fatty acid carboxylate, (In(OFA) ₃ ), is indium stearate (In(OOC(CH ₂ )CH ₃ ) ₃ ). Exemplary of fatty acids that may be used with the invention include saturated and unsaturated fatty acids having 12 or more carbons, including Ci ₂ -C ₂ saturated and unsaturated fatty acids. Examples of suitable saturated fatty acids include lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, and lignoceric acid. Examples of suitable unsaturated fatty acids include myristoleic acid, palmitoleic acid, sapienic acid, oleic acid, elaidic acid, vaccenic acid, linoleic acid, linoelaidic acid, -linolenic acid, arachidonic acid, eicosapentaenoic acid, erucic acid, and docosahexaenoic acid. The use of a fatty acid carboxylate provides an indium salt that is soluble in nonpolar solvents. Exemplary fatty acids are oleic acid (forming the oleate salt) and stearic acid (forming the stearate salt).

The general reaction of an embodiment of the invention is illustrated in Scheme 1. An indium fatty acid carboxylate is combined with a surfactant, typically in a non-polar solvent. The reaction mixture is heated and a reducing agent added. The resulting indium nanoparticles may be isolated by precipitation, for example by the addition of a polar solvent, filtration, and centrifugation, or by other physical separation method known in the art. Indium nanoparticles obtained by the method of the invention have a particle size that is less than 100 nm. In embodiments, a significant portion of the nanoparticles may have a particle size greater than about 5 nm or greater than about 10 nm. In embodiments, particle sizes may be in the range of from about 10 nm to about 100 nm. In some embodiments, particle sizes may be in the range of from about 20 nm to about 100 nm. In other embodiments, particle sizes may be in the range of from about 25 nm to about 100 nm. In exemplary embodiments, 10% or more of the nanoparticles are larger than 10 nm. In exemplary embodiments, 10% or more of the nanoparticles are larger than 20 nm. In other exemplary embodiments, 10% or more of the nanoparticles are larger than 25 nm. In other exemplary embodiments, the indium nanoparticles prepared may have a very narrow size distribution, for example, +/- 5 nm. Thus, embodiments may have average sizes distributions of, for example, 25 nm, with a size distribution of 25 nm +/- 5 nm; 50 nm, with a size distribution of 50 nm +/- 5 nm; 75 nm, with a size distribution of 75 nm +/- 5 nm; or 100, with a size distribution of 100 nm +/- 5 nm.

Scheme 1

Surfactant

In(OFA) ₃ + Reducing Agent * ^In ° W

Solvent

Heat

Varying reaction conditions may be utilized to control the particle size of nanoparticles resulting from the invention. For example, the use of higher concentrations of surfactant or lower temperature tends to produce smaller particles. Conversely, lower concentrations of surfactant or higher temperature tends to produce larger particles. For example, in comparing the nanoparticles shown in the transmission electron microscopy images of Figure 1 and 2, the smaller particles observed in Figure 2 are the result of using increased surfactant concentration and/or lower temperature as compared with the methods used to prepare the nanoparticles of Figure 1.

In alternate embodiments, methods employing high speed mixing and reagent addition techniques in the reactions forming the nanoparticles may be used to enhance and control the nanoparticle size and distribution. Also anticipated are other methods, such as employing solution viscosity agents in the reaction step, which may also enhance and control the reactions to modify the nanoparticle size and distribution.

Prior art methods have been used to synthesize indium nanoparticles of small size in polar solvents, for example having average diameters of less than 10 nm or less than 5 nm. Larger nanoparticles may offer some advantages, however. For example, larger particles may permit better macroscopic flow of an electrolyte when assembled into a macroscopic structure by virtue of the larger inter-particle pore size. This improved flow may be useful when the nanoparticles are used in the formation of an electrode for use in electrochemical reactions in solution. Larger particles may also be expected to retain their nanoscale geometry at higher temperature based on reduced surface energy. Larger particles may also be more resistant to chemical attack/corrosion based on reduced surface energy.

Nanoparticles produced in nonpolar solvents may have additional advantages. These advantages may be enhanced by the presence of surfactant remaining on the surface. For example, most prior art process use polar synthesis solvents and result in nanoparticles in which the corresponding polar head groups face outward. Such structures are useful for applying to (or decorating) only hydrophilic substrates. By using non-polar ligands and a non-polar solvent, the nanoparticles may be dispersed in a range of solvents not previously reported. This may be useful when applying the nanoparticles onto hydrophobic substrates such as plastics or carbon.

The invention includes the step of mixing the indium fatty acid carboxylate with a surfactant. Suitable surfactants include, for example, alkyl thiols, trialkylphosphines or trialkylphosphine oxides, pyrrolidones, silanes, and amines. Although amine surfactants have commonly been used to produce very small (< 10 nm or <5 nm) indium nanoparticles, the use of amines in nonpolar solvents may provide a different size range and distribution. Non-limiting examples of surfactants include Cs-Cjs alkyl thiols, trioctyl phosphine (TOP), trioctyl phosphine oxide (TOPO), poly vinyl pyrrolidone (PVP) and silanes having amino-, carboxylic acid- and poly(ethylene glycol) terminal groups together with nonpolar (typically alkyl chain) nonpolar groups. A particular surfactant is dodecane thiol. The amount or concentration of surfactant required to make nanoparticles of a desired size may be readily determined by routine repeated experiments. For example, when dodecane thiol or other thiols are used, the concentration may range from about lOmM to about lOOmM.

The choice of alkyl thiol surfactant affects the size of indium nanoparticles. The size of the NPs may be controlled by varying the concentration and identity of the alkyl thiol surfactant. Alkylated thiols would not be expected to be a viable surfactant for other synthetic routes which utilize either polar solvents or reactive solvents such as amines.

In exemplary embodiments, the indium fatty acid carboxylate and surfactant are combined in a non-polar solvent. Non-limiting examples of solvents include saturated and unsaturated aliphatic hydrocarbons and substituted or unsubstituted aromatic hydrocarbons. Non limiting examples include decane, dodecane, tetradecane, hexadecane, octadecene, mesitylene, and trichlorobenzene. The identity of the solvent is not particularly important so long as it is non-polar; nonreactive with the indium fatty acid carboxylate, surfactant, and reducing agent; stable at the temperatures under which reaction occurs; and sufficiently high boiling to be useful and the desired reaction termperature. The temperature at which the reaction of the present is carried out is generally higher than temperatures used for preparation of indium nanoparticles in prior art methods. Accordingly, the solvent selected should have a boiling point at or above the reaction temperature. By utilizing such solvents, it is possible to reach high temperatures and employ aggressive reducing agents without concern for reaction of the solvent with other species. This is not the case for other commonly used solvents such as amines, phosphine oxides, glycols and the like. Persons skilled in the art will recognize suitable non-reactive nonpolar solvents with boiling points appropriate for the desired reaction in addition to those specifically described herein.

Reaction temperatures may vary and routine experimentation may be used to determine the temperature needed to obtain nanoparticles of a desired size. For example, the temperature range may be from about 75°C to about 200°C, for example about 150°C to about 200°C, or at about 175°C. By using indium fatty acid carboxylate as the indium source, the reaction kinetics of NP nucleation and growth are slowed down allowing the use of higher temperatures. Indium oleate is known to decompose at high temperature to form indium oxide, so the temperature should be selected such that no decomposition of the indium fatty acid carboxylate occurs. The slower reaction kinetics may play a part in the lack of decomposition to indium oxide. The slower kinetics also allows sufficient time for the size of nanoparticles to be easily controlled by control of reaction time. Alternative methods in the prior art that utilize a "naked" indium cation are characterized by rapid (virtually instantaneous) nucleation and growth kinetics and cannot be controlled by changes in reaction time. In addition to reaction time, the size of the nanoparticles may be controlled in part by the reaction temperature. Generally, the higher the temperature, the larger the particles that are formed.

Using high-boiling point non-polar solvents allows for degassing at high temperature and effectively remove most oxygen and water, which may greatly reducing the tendency of the indium to oxidize. It is also possible that the presence of the oleate anion reduces the tendency of the indium to oxidize when compared to bare indium anions (such as those present during direct conversion of InCl ₃ ) or when the heating is conducted in a nonpolar solvent. Higher temperature also allows initiation of nucleation and growth of the nanoparticles even with a high concentration of ligands present. The high concentration of ligands will lead to good passivation and increases stability of the nanoparticles at lower temperatures. The high boiling point of the solvent also helps produce a dry reaction mixture as describe above.

After combining the indium fatty acid carboxylate and surfactant and heating to the reaction temperature, a reducing agent is added. The reducing agent is generally added relatively slowly or dropwise, and may be present in an inert solvent such as, for example, tetraglyme. Any suitable reducing agent may be used including, but not limited to, sodium borohydride, lithium borohydride, lithium aluminum hydride, lithium hydride, sodium hydride, hydrogen gas, sodium metal or others. Exemplary embodiments of the invention us a hydride reducing agent such as sodium borohydride, lithium borohydride, lithium aluminum hydride, lithium hydride, sodium hydride. Sodium borohydride is an exemplary reducing agent.

Indium fatty acid carboxylates may be prepared by the reaction of a suitable indium salt with a fatty acid as shown in Scheme 2. Although the indium salt starting material is shown as InX ₃ , it will be appreciated that the formula of the indium salt may be different depending on the oxidation sate of the indium and the charge on the anion. For example if indium (III), the most common oxidation state, is used and the counter-ion X has other than a charge other than -1, the formula of the salt will be different. For example, indium (III) sulfate has the formula In ₂ (S0 ₄ )3. Suitable indium salts that may be used in the reaction of Scheme 2 for the preparation of indium fatty acid carboxylates include the halide (bromide, chloride, fluoride and iodide), nitrate, sulfate carbonate, formate, acetate, and propionate, although others are considered within the scope of the invention. The most desirable starting salts are those which, after reaction with the fatty acid, form a conjugate acid HX that may be readily removed, for example by distillation, in order to drive the reaction to completion. Indium acetate is a specific example of an indium salt. Suitable fatty acids are described above.

Scheme 2

InX ₃ + HOFA In(OFA) ₃ + HX

The molar ratio of fatty acid to indium (HOFA:In) must be three (3) or greater, and an excess of fatty acid is typically used, for example a HOFA:In ratio of four (4). In most instances, the indium salt and fatty acid are combined in a non-reactive nonpolar solvent. In exemplary embodiments, the solvent has a boiling point sufficiently high that the conjugate acid HX may be removed by heating under reduced pressure. For example, if indium acetate is the indium salt and oleic acid the fatty acid, octadecane, octadecene and tetradecane and the like may be utilized as solvents. This allows for heating to temperatures of 200°C during the reaction and drying, and for removal of residual acetic acid at temperatures of about 100°C and reduced pressure. Depending upon the counterion, fatty acid, and temperature necessary for reaction, persons skilled in the art will recognize solvents that are suitable for the reaction.

In exemplary embodiments, the indium fatty acid carboxylate is not isolated prior to addition of surfactant and reduction. Surfactant may be added directly to the solution of indium fatty acid carboxylate after removal of HX. The conversion to indium nanoparticles then proceeds as described above.

In an exemplary embodiment of the invention, an indium salt, for example indium acetate, and a fatty acid such as oleic acid, are dissolved in a solvent. The solvent may be octadecene. The solution is dried, purged with an inert gas (e.g. Ar or N ₂ ) and heated. Vacuum drying may be accomplished at about 1 10°C and the final temperature may be about 200°C. During heating, the reaction mixture tends to become clear as the insoluble indium salt is converted to the soluble indium fatty acid carboxylate, e.g. indium oleate. The reaction mixture may then be cooled, for example to about 110°C and vacuum dried to remove any residual acid, such as acetic acid. This solution may be used without further isolation of the indium fatty acid carboxylate or the indium fatty acid carboxylate may be isolated and dissolved in another solvent for conversion to the nanoparticles. A suitable surfactant, for example dodecanethiol may then be added to the solution of indium fatty acid carboxylate. This solution may then again be vacuum dried (110°C, for example) and then heated to the final reaction temperature, for example about 175°C. A reducing agent such as sodium borohydride is then slowly added. Prior to addition, the reducing agent may be dissolved in a suitable nonreactive solvent, e.g. tetraglyme, to allow for dropwise addition. After reaction is complete and the solution cooled, a polar solvent such as methanol may be added to precipitate the indium nanoparticles which are then isolated by centrifugation or another suitable method. The In nanoparticles thus obtained range in size from about 10 nm to about 100 nm.

In another embodiment, the addition of other salts of other metals with the indium salts may be employed, such that an alloy or co-metallic nanoparticle may be formed as the nanoparticle catalyst product. Metals may include transition metals, such as Cu, Fe, Co, Ni, V, Cr, as well as other metals such as Sn, Pt, Ru, Pd, Sb, Cd, Pb, and Bi and their mixtures and combinations with indium as examples. These nanoparticles may form substantially useful and improved catalysts in electrochemical processes such as in the electrochemical reduction of carbon dioxide to formate as well well as other single and multicarbon chemical products. In addition, another embodiment is the use of this process in the preparation of nanoparticles of these listed metals alone, or in combination, without indium in the nanoparticle composition.

It is believed that the present disclosure and many of its attendant advantages will be understood by the foregoing description, and it will be apparent that various changes may be made in the form, construction and arrangement of the components without departing from the disclosed subject matter or without sacrificing all of its material advantages. The form described is merely explanatory, and it is the intention of the following claims to encompass and include such changes.

The embodiments illustrated and discussed in this specification are intended only to teach those skilled in the art to make and use the invention. Nothing in this specification should be considered as limiting the scope of the present invention. All examples presented are representative and non-limiting. The above-described embodiments of the invention may be modified or varied, without departing from the invention, as appreciated by those skilled in the art in light of the above teachings. The accompanying methods present elements of the various steps in a sample order, and are not necessarily meant to be limited to the specific order or hierarchy presented. It is therefore to be understood that, within the scope of the claims and their equivalents, the invention may be practiced otherwise than as specifically described.

EXAMPLES

Example 1 - Preparation of indium (III) oleate

Example la

To a solution of octadecane (24 ml) and oleic acid (6 ml) in a 100 ml flask was added In (III) acetate (0.876 g). The mixture was warmed to 100 °C for 30 minutes. At the end of this time the reaction vessel was filled with argon then subjected to vacuum. The argon flushing process was repeated two more times and the mixture was placed under an argon atmosphere. The reaction mixture was heated to 200 °C for 1 hour, cooled to 120 °C for 30 minutes, and finally allowed to cool to room temperature. After the reaction mixture reached room temperature, the solution of indium (III) oleate in octadecane was collected and stored.

Example lb

Indium acetate (0.35 g) and oleic acid (1.4 g) were combined in octadecene (16 ml). This solution was vacuum dried at 110 °C and then purged with argon and heated to 200 °C. The solution was held at this temperature for one hour, during which time the solution turned from cloudy to clear, indicating the formation of indium(III) oleate. The solution was then cooled to 1 10 °C and vacuum dried to remove any residual acetic acid, purged with argon and cooled to room temperature.

Example lc

To a solution of hexadecane (15.5 ml) and stearic acid (2.276 g, 8 mmol) in a 50 ml flask was added In (III) acetate (0.525 g). The mixture was warmed to 1 10 °C for 60 minutes. At the end of this time the reaction vessel was filled with argon then subjected to vacuum. The argon flushing process was repeated two more times and the mixture was placed under an argon atmosphere. The reaction mixture was heated to 200 °C for 1 hour, cooled to room temperature. After the reaction mixture reached room temperature, the solution of indium (III) oleate in octadecane was 8500 rpm for 15 minutes. The product was collected and stored. Example 2 - Preparation of indium nanoparticles

Example 2a

To trioctylphosphine oxide (5 g) was added a solution of indium (III) oleate (from Example la) in octadecane (5 ml). The mixture was stirred and dodecane thiol (0.625 ml) was added. The mixture was warmed to 100 °C for one hour. At the end of this time the reaction vessel was filled with argon and then subjected to vacuum. The argon flushing process was repeated a total of five times. The mixture was heated to a temperature of 175 °C under an argon atmosphere. At this time sodium borohydride solution (1 ml of 2M solution) was slowly added. The reaction mixture turned from grey to black over a period of 20 seconds and the temperature increased to 183 °C. The mixture was cooled to 175 °C over a period of five minutes and then cooled to room temperature. The reaction mixture was centrifuged and washed with acetone. The resulting product was suspended in hexane. FIG. 1 is a transmission electron microscopy (TEM) image of indium nanoparticles obtained.

Example 2b

To the indium oleate solution from Example lb, dodecanethiol (0.75 ml ) was added. The solution was again dried at 110 °C and purged with argon before heating to 175 °C. Once at this temperature, sodium borohydride in a tetraglyme solution resulting in the formation indium NPs ranging in size from < 10 nm to >100 ^' nm. Methanol was then added to precipitate the indium NPs which were isolated by centrifugation.

Example 2c

To trioctylphosphine oxide (6 g) was added a solution of indium (III) oleate in octadecane (6 ml, 0.6 mmol). The mixture was warmed to 110 °C for one hour. At the end of this time the reaction vessel was filled with argon and then subjected to vacuum. The argon flushing process was repeated a total of five times. The mixture was cooled to 73 °C under an argon atmosphere. At this time sodium borohydride solution (0.5 ml of 3M solution) was slowly added. The reaction mixture was cooled to room temperature. The reaction mixture was washed with ethanol, centrifuged at 9,000 rpm for 15 minutes, and suspended in hexane.

Example 2d

To trioctylphosphine oxide (6 g) was added a solution of indium (III) oleate in octadecane (6 ml, 0.6 mmol). The mixture was stirred and dodecane thiol (0.5 ml) was added. The mixture was warmed to 110 °C for one hour. At the end of this time the reaction vessel was filled with argon and then subjected to vacuum. The argon flushing process was repeated a total of five times. The mixture was heated to a temperature of 200 °C under an argon atmosphere and then cooled to 175 °C. At this time sodium borohydride solution (0.6 ml of 3M solution) was slowly added. The reaction mixture turned from grey to black after 0.1 ml of the NaBH4 solution was added. The mixture was cooled to 150 °C and the remaining NaBH4 solution was added. After cooling to room temperature the product was precipitated with acetone, centrifuged for 3 minutes at 9,000 rpm, and then dispersed in toluene.

Example 2e

Dodecane thiol (1ml) was added a solution of indium (III) oleate in octadecane (6 ml). The mixture was stirred at 1 10 °C for one hour. At the end of this time the reaction vessel was filled with argon and then subjected to vacuum. The argon flushing process was repeated a total of three times. The mixture was heated to a temperature of 200 °C under an argon atmosphere. At this time sodium borohydride solution (1 ml of 2M solution) was slowly added. The mixture was cooled to room temperature. The mixture was washed with isopropyl alcohol and centrifuged for 15 minutes at 8,000 rpm. The pellet was sonicated in chloroform and then centrifuged for 15 minutes at 4,000 rpm. The pellet was dispersed in toluene, sonicated, and then centrifuged for 20 minutes at 5,500 rpm. The clear supernatant was removed and the pellet was dispersed in chloroform, sonicated, and centrifuged for 10 minutes at 5,000 rpm.