WO2006113719A2

WO2006113719A2 - Methods of preparing nanoparticles by reductive sonochemical synthesis

Info

Publication number: WO2006113719A2
Application number: PCT/US2006/014571
Authority: WO
Inventors: Zhengrong Li; Dominick J. Casadonte, Jr.
Original assignee: Texas Tech University
Priority date: 2005-04-19
Filing date: 2006-04-18
Publication date: 2006-10-26
Also published as: WO2006113719A3

Abstract

A method of synthesizing a binary alloy nanoparticle represented by the general formula AxBy comprising contacting in solution under ultrasonic irradiation a source of component A, a source of component B, and a reducing agent wherein A and B are present in amounts consistent with the final stoichiometry of the desired alloy. A method of producing a semiconductor nanoparticle comprising ultrasonically irradiating a reaction mixture comprising a solvent and reactants to form a first reagent in solution, introducing of a second reagent to the reaction mixture, further ultrasonically irradiating said reaction mixture to produce a desired semiconductor nanoparticle, and recovering the semiconductor nanoparticle. A III-V semiconductor nanoparticle comprising less than about a 1:1 mole ratio of capping agent to semiconductor nanoparticle.

Description

METHODS OF PREPARING NANOPARTICLES BY REDUCTIVE SONOCHEMICAL SYNTHESIS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority under 35 U.S.C. 119(e) to provisional U.S. Patent Application Serial No. 67/672,958 filed April 19, 2005 and entitled "Facile Sonochemical Synthesis of Nanostructured InP and GaP," which is hereby incorporated herein by reference in its entirety for all purposes. STATEMENT REGARDING FEDERALLY SPONSORED

RESEARCH OR DEVELOPMENT

[0002] This invention is from work sponsored by the Texas Advanced Technology

Program under Grant No. 003644-00778-2001, Project Title "Enhanced Degradation of

Environmental Contaminants Using Pulsed and Heterodyne Sonochemistry" and the Donors of the Petroleum Research Fund administered by the American Chemical Society under Grant No.

37884-AC4, Project Title "Enhancing Sonochemical Efficiency Using Power-Modulated

Pulsed and Heterodyne Ultrasound."

BACKGROUND OF THE INVENTION

Field of the Invention [0003] This disclosure relates to methods of synthesizing nanoparticles. More specifically, this disclosure relates to methods of sonochemical synthesis of semiconductor nanoparticles.

Background of the Invention

[0004] A semiconductor is a material with an electrical conductivity that is intermediate between that of an insulator and a conductor. A semiconductor behaves as an insulator at very low temperature and may have an appreciable electrical conductivey at room temperature. Some examples of commonly used semiconducting elements are silicon, germanium and compounds of these metals with arsenic and phosphorus. Semiconductor compounds formed through the use of an element from Group ITI of the Periodic Table such as gallium and an element from Group V of the periodic table such as phosphorus are referred to as ItI-V semiconductors.

[0005] Semiconductor nanocrystals (NC) have shown novel physical properties that are remarkably different from those of the bulk solid. HI-V semiconductors have received increasing attention due to their usefulness in high-speed digital circuits, microwave devices and optoelectronics. In the past decade, many studies concerning the synthesis of III-V NCs have been reported, including the use of colloid chemistry involving toxic precursors, ion implantation, molecular beam epitaxy, metal-organic chemical vapor deposition and metal- organic vapor phase epitaxy. These processes may have significant drawbacks including the use of high reaction temperature, the presence of complex and unwanted side reactions and, the formation of toxic precursors. Thus, a need exists for an improved methodology for the synthesis of semiconductor nanoparticles.

BRIEF SUMMARY OF SOME OF THE PREFERRED EMBODIMENTS [0006] Disclosed herein is a method of synthesizing a binary alloy nanoparticle represented by the general formula A_xB_y comprising contacting in solution under ultrasonic irradiation a source of component A, a source of component B, and a reducing agent wherein A and B are present in amounts consistent with the final stoichiometry of the desired alloy. [0007] Also disclosed herein is a method of producing a semiconductor nanoparticle comprising ultrasonically irradiating a reaction mixture comprising a solvent and reactants to form a first reagent in solution, introducing of a second reagent to the reaction mixture, further ultrasonically irradiating said reaction mixture to produce a desired semiconductor nanoparticle, and recovering the semiconductor nanoparticle.

[0008] Further disclosed herein is a III-V semiconductor nanoparticle comprising less than about a 1 :1 mole ratio of capping agent to semiconductor nanoparticle. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] For a detailed description of the preferred embodiments of the invention, reference will now be made to the accompanying drawings in which:

[0010] Figures Ia-Ib are scanning electron micrographs of InP.

[0011] Figures 2a-2b are transmission electron micrographs of InP and GaP. [0012] Figures 3a-3b are x-ray diffraction patterns of GaP. [0013] Figures 4a-4b are x-ray diffraction patterns of InP.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [0014] Disclosed herein are methods of synthesizing nanoparticles. hi an embodiment, the nanoparticles are binary alloys, alternatively semiconductor nanoparticles, alternatively ITl-V semiconductor nanoparticles. Said methodology may comprise the use of reductive sonochemical methods to generate reactive species in situ which result in the formation of a desired product. The methodologies disclosed herein for synthesis of nanoparticles may provide significant advantages over existing methodologies such as reduced reaction temperatures and the reduction or elimination of unwanted side reactions. [0015] In an embodiment, a method for the production of a binary alloy nanoparticle generally represented by the formula A_xB_y comprises contacting in a solvent under ultrasonic irradiation a source of component A (e.g., a compound comprising A), a source of component B (e.g., a compound comprising B)₅ and a reducing agent, wherein A and B are present in amounts consistent with the final stoichiometry of the desired alloy. In such embodiments, the reducing agent may function to donate electrons to component A, component B, or both rendering them able to complex with the other component and form a binary alloy. In an embodiment, the reducing agent may also be a source of component A, component B, or both. The reducing agents useful in this methodology are known to one of ordinary skill in the art and may be chosen to meet any number of reaction criteria such as for example compatibility with the other components of the mixture and reduction potential. Without limitation, examples of reducing agents suitable for use in this disclosure include sodium metal, sodium phosphide, lithium phosphide, sodium napthalide potassium metal, lithium metal, lithium phosphide, lithium napthalide, or combinations thereof.

[0016] In an embodiment, the binary alloy A_xB_y may be produced by contacting in solution under ultrasonic irradiation a source of component A and a source of component B at a frequency of from about 20 kHz to about 100 MHz, alternatively from about 20 kHz to about 1 MHz, alternatively from about 20 kHz to about 500 kHz, at a temperature of from about 0 ⁰C to about 20 ⁰C. Ultrasonic irradiation also termed herein sonication may be broadly classified as irradiating the sample with sound having a frequency greater than about 20 kHz and equal to or less than about 100 MHz. Ultrasonic irradiation of a solvent results in acoustic cavitation, which is defined herein as the formation, growth and implosive collapse of bubbles in a liquid. This can produce local conditions inside the bubbles (cavitation sites) of about 5000 K and 1000 atmospheres for nanosecond lifetimes. Without wishing to be limited by theory, it is believed that the interaction of these cavitation sites with the reagents in solution is what drives the reactions to produce the binary alloys. [0017] A binary alloy formed by the methodology disclosed herein may have a particle size of from about 2 nm to about 20 nm, alternatively from about 4 nm to about 6 nm. In an embodiment, the binary alloys produced by this methodology may be nanoparticles that display bulk alloy properties. In many instances the desirable physical and mechanical properties observed with use of the bulk alloy are not maintained in smaller particles of that same alloy. For example, the magnetic properties of 4d-metal clusters may differ significantly from the magnetic properties of the bulk alloy. Without wishing to be limited by theory, this difference may originate due to a narrowing of electronic states and the high ratio of surface to volume atoms in the nanoparticles compared to the bulk alloy that give rise to enhanced magnetic orbital moments. Without limitation examples of binary alloys that may be produced by this methodology include PtRu and PtSn alloys

[0018] In the production of the binary alloy nanoparticles, the individual components of the final alloy may be introduced to the reaction mixture in amounts consistent with the final stoichiometry of the desired product. For example, for the production of a NiAl alloy the reaction mixture may contain an equimolar concentration of Ni and Al allowing for the formation of NiAl, whereas for the formation Of ZrPt₃, the Pt may be present in the reaction at three times the molar concentration of Zr in order to allow for the formation of ZrPt₃. Calculations of the amount of each component necessary for the formation of the desired binary alloy are known to one of ordinary skill in the art. [0019] hi an embodiment, a methodology for the synthesis of a semiconductor nanoparticle (SNP) comprises the formation of precursor material by ultrasonic irradiation of a reaction solution to provide a first reagent (R₁) which may then be further sonicated in the presence of a second reagent (R₂) to produce a desired SNP. In an embodiment, the SNP is a ITI-V SNP comprising GaP, GaAs, InP or hiAs, alternatively the III- V SNP comprises GaP or InP. [0020] hi an embodiment, the SNP is a IEt-V SNP and R₁ may be a source of a Group V element, a source of a Group m element, a reducing agent or a combination thereof. In an embodiment, R₁ is a reducing agent. Reducing agents are well known in the art and may be chosen by one of ordinary skill in the art to meet the desired needs of the reaction. Examples of reducing agents include without limitation sodium phosphide, sodium napthalide, lithium napthalide, sodium metal, or combinations thereof. In an alternative embodiment, R₁ is a source of a Group V element, alternatively a source of a phosphide. Without limitation examples of such R₁S include AlP, ZnP, KP, NaP, or combinations thereof. In an alternative embodiment, R₁ is a source of a Group V element and a reducing agent alternatively, R₁ is NaP. In an embodiment where R₁ is both a reducing agent and a source of a Group V element (e.g. NaP), R₁ may be formed in situ through the ultrasonic irradiation in solution of materials such as for example in the case of NaP, sodium metal and phosphorus as is known to one of ordinary skill in the art. [0021] In an embodiment, a methodology for the production of a III-V semiconductor nanoparticle may further comprise contacting a second reagent (R₂) with Rj to generate the desired product. In an embodiment, R₂ is any compound containing a Group HI element that is chemically compatible with the other components of the reaction mixture. Alternatively, R₂ is a salt of a Group HI element, alternatively R₂ is a halide salt of a Group III element, alternatively R₂ is a chloride salt of the Group El element. Examples of suitable R₂ include without limitation GaCl₃, InCl₃, or combinations thereof.

[0022] Contacting of R₂ with R₁ may occur through any means known to one of ordinary skill in the art while the sample is under ultrasonic irradiation. For example, R₂ may be introduced directly to the reaction solution containing R₁ that was formed in situ as described above, hi an embodiment, R₂ is the chloride salt of In or Ga and is directly added to the reaction solution containing R₁.

[0023] In an embodiment, the reaction solution for production of a III-V SNP may comprise a suitable solvent. A suitable solvent is any solvent chemically compatible with the other components of the reaction mixture. Alternatively, the solvent is an etherate with a boiling point of greater than about 60 ⁰C, alternatively greater than about 80 ⁰C. Examples of suitable solvents include without limitation tetrahydrofuran (THF), diethyl ether, p-dioxane, polyethylene glycol, or combinations thereof. [0024] Ultrasonic irradiation to produce either R₁ or to facilitate the reaction OfR₁ and R₂ may be carried out in a temperature range of from about -10 ⁰C to about 45 ⁰C, alternatively from about 0 ⁰C to about 25 ⁰C, alternatively at about 25 ⁰C using a sonicator irradiating at a frequency of from about 20 kHz to about 500 kHz, alternatively upward from about 20 kHz. [0025] The resultant IH-V SNPs formed by the disclosed methodologies may be particles having a size of from about 4 nm to about 50 nm, alternatively from about 6 nm to about 30 nm, alternatively from about 6 nm to about 10 nm. When formed as disclosed the III-V SNPs have an amorphous structure, a nanocrystalline structure or a mixture of both. The methodology for production of a III-V SNP may further comprise annealing of the ffl-V SNP to induce crystallization. Such annealing may be carried out in the temperature range of from about 150 ⁰C to about 700 ⁰C, alternatively from about 200 ⁰C to about 650 ⁰C, alternatively from about 200⁰C to about 600⁰C.

[0026] In an embodiment, a III-V SNP formed by the disclosed methodologies and comprising indium may be annealed at a temperature of from about 150 ⁰C to about 450 ⁰C, alternatively of from about 200 ⁰C to about 300 ⁰C, alternatively of from about 200 ⁰C to about 250 ⁰C for from about 1 to about 12 hrs, alternatively for from about 2 to about 6 hrs, alternatively for from about 2 hrs. In an embodiment, a III-V SNP formed by the disclosed methodologies and comprising gallium may be annealed at a temperature of from about 200 ⁰C to about 750 ⁰C, alternatively of from about 300 ⁰C to about 650 ⁰C, alternatively of from about 350⁰C to about 600 ⁰C for from about 2 to about 48 hrs, alternatively for from about 6 to about 36 hrs, alternatively for from about 24 hrs.

[0027] In an embodiment, the reductive sonochemical synthetic methodology disclosed herein provides a rapid and efficient method for the room temperature synthesis of binary alloy nanoparticles and more specifically, SNP displaying bulk alloy properties. Furthermore, the synthetic methodology disclosed herein produces SNPs in the absence of capping agents. Capping agents are compounds commonly used in the synthesis of SNPs to control particle size and prevent the conglomeration of particles. Examples of capping agents include without limitation tetraoctylammonium bromide (TOAB) and trioctylphosphine (TOPO). The presence of such capping agents may adversely affect the conductivity of the nanoparticle resulting in particles lacking the desired semiconducting ability. In an embodiment, the synthetic methodology disclosed herein produces pure SNPs comprising less than about a 1:1 mole ratio of capping agent to SNP. The purity of the SNP product may be assessed by any means known to one of ordinary skill in the art. Alternatively, the purity of the SNP may be assessed by X- ray diffraction (XRD). [0028] The disclosed methodologies may provide the desired binary alloys in yields of greater than about 50 %, alternatively greater than about 80 %, alternatively greater than about 90 %, with significant improvements over existing methodologies such as minimizing the number of undesirable side reactions and a low reaction temperature. Without wishing to be limited by theory, the methodology for the production of ITI-V SNPs disclosed herein employs the use of ultrasonic irradiation to form cavitation sites, which contain the dissolved phosphorous atoms and subsequently bombard the surface of the sodium metal at high velocity. The collapse of the bubbles not only bring phosphorous atom directly into contact with sodium metal, but also may provide enough energy to yield sodium phosphide. After having been added to the reaction under ultrasonic irradiation, the dissolved metal chloride molecules (i.e InCl₃ or GaCl₃) may also be present in or at the surface of the cavitation sites, and, through either asymmetric collapse or by direct in situ atomization, collided with sodium phosphide to initiate reaction and the eventual formation of InP or GaP. As the result, utilizing ultrasonic irradiation may shorten the reaction period to from about 2 hours to about 24 hours. EXAMPLES EXAMPLE l

[0029] The reductive sonochemical synthesis of GaP and InP were carried out. The reactions were carried out under an atmosphere of dry nitrogen using Schlenk techniques. In a typical reaction, sodium metal (0.40 g, 17.4 mmol) and yellow phosphorous (0.31 g, 10 mmol) were added to a sonication cell containing 15 mL of tetrahydrofuran (THF) freshly distilled from sodium. The mixture was then subjected to high-intensity ultrasound under dried nitrogen at a 5 ⁰C cell temperature for 2 hours using a Sonics & Materials 500 W Vibra-Cell sonicator operating at 20 kHz with an acoustic power density of 17 W/cm² as measured calorimetrically. An InCl₃/THF solution (5.0 mmol, with the same mole amount of GaCl₃ in a diethyl ether solution being used in the GaP synthesis) was then added. Sonication continued for an additional three hours. The resulting solid was first washed with ethanol and then by deionized water three times. The solid was collected, air-dried overnight, and dried under dynamic vacuum at room temperature for 24 hours. The yield based on metal chlorides was 85%. For annealing, the samples were heated under nitrogen for 6 hours at 200-250⁰C for InP, and at 600 ⁰C for GaP.

[0030] This synthesis was a one-pot process. In contrast to the thermal method which requires days of reflux, sodium phosphide was synthesized within 2 hours using ultrasound irradiation, with the black phosphide then being reacted in situ by direct addition of InCl₃ or GaCl₃ to form InP or GaP nanoparticles.

EXAMPLE 2

[0031] The morphology and structure of the products from Example 1 were further characterized. Scanning electron micrographs of as-prepared InP and an InP standard (purchased from Aldrich Chemical Co.) are shown in Figure Ia and Ib, respectively. Unlike a plate-like morphology from the conventional InP, the sonochemical InP exists as a porous agglomeration of clusters of particles with an average size of 50 nm, which are aggregates of smaller particles. Energy dispersive X-ray (EDX) analysis performed on these particles shows an In/P atomic ratio close to 1. The transmission electron microscopic (TEM) images of as- prepared InP and GaP are shown in Figures 2a and 2b respectively. Both images reveal that the agglomerates of larger micron-sized particles consist of nanoparticles of about 6-10 nm in size. [0032] The X-ray diffraction (XRD) patterns of GaP before and after annealing, Figures 3a and 3b and InP nanoparticles before and after annealing Figures 4a and 4b were recorded. The absence of peaks and a broad hump around 17° Wangle indicate the amorphous nature of the as- prepared GaP nanoparticles (Figure 3a). After annealing at 600 ⁰C, the pattern in Figure 3b clearly shows the (111), (200), (220), and (311) reflections of cubic GaP. Figure 4a shows that elemental indium is formed concomitantly with as-prepared InP, where very weak (111), (220) and (311) reflection peaks of cubic InP are observed. After annealing the sample at 200 ⁰C, substantial InP nanocrystals were formed, indicated by the intensity loss of indium metal as well as the more indexed and intensive presence of peaks corresponding to cubic InP, shown in Figure 4b.

EXAMPLE 3 [0033] The synthesis of PtRu nanoparticles was carried out under an atmosphere of dry nitrogen using Schlenk techniques. In a typical reaction, a dried capped flask containing 15 mL THF distilled from sodium was added to lithium metal (0.05 g, 7.2 mmol) and naphthalene (0.36 g, 2.8 mmol). The mixture was irradiated with a high-intensity ultrasonic probe (Sonics and Materials 500W Vibra Cell™ operating at 20 kHz, with an acoustic power density of 17 W /cm² as measured calorimetrically ) for 2 hours under a stream of N₂ gas at 5 ⁰C cell temperature. Then stoichiometric quantities of RuCl₃ • xH₂O and PtBr₄ THF solution (total metal ions: 0.5 mmol) were added slowly during sonication. The sonication was then continued for no more than 90 minutes. The solid was centrifuged and washed with THF five times, de-ionized water/THF mixture three times, and THF three times under air. The black metal powder was collected by cenπϊfugation, air-dried overnight, and dried under dynamic vacuum for 24 hours.

[00341 The protocol is a one-pot process. The reaction was easily carried out at low temperature using ultrasound with yields greater than 90 %. Lithium and naphthalene, the electron carrier, were in large excess to ensure that the both hydrates and metal ions could be reduced completely. The reaction mixture remained dark green after the addition of metal halides. The sonication time of 90 minutes after the addition of the metal salts was chosen in an attempt to reduce agglomeration due to interparticle collisions which occur among the metal particles at high velocity accelerated by cavitation and Shockwaves. [0035] The invention having been generally described, the following examples are given as particular embodiments of the invention and to demonstrate the practice and advantages thereof. It is to be understood that the examples are given by way of illustration and are not intended to limit the specification of the claims in any manner. [0036] -While preferred embodiments of the invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit and teachings of the invention. The embodiments described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the invention disclosed herein are possible and are within the scope of the invention. Where numerical ranges or limitations are expressly stated, such express ranges or limitations should be understood to include iterative ranges or limitations of like magnitude falling within the expressly stated ranges or limitations (e.g., from about 1 to about 10 includes, 2, 3, 4, etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). Use of the term "optionally" with respect to any element of a claim is intended to mean that the subject element is required, or alternatively, is not required. Both alternatives are intended to be within the scope of the claim. Use of broader terms such as comprises, includes, having, etc. should be understood to provide support for narrower terms such as consisting of, consisting essentially of, comprised substantially of, etc. [0037] Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present invention. Thus, the claims are a further description and are an addition to the preferred embodiments of the present invention. The discussion of a reference herein is not an admission that it is prior art to the present invention, especially any reference that may have a publication date after the priority date of this application. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference, to the extent that they provide exemplary, procedural or other details supplementary to those set forth herein.

Claims

CLAIMSWhat is claimed is:

1. A method of synthesizing a binary alloy nanoparticle represented by the general formula A_xB_y comprising contacting in solution under ultrasonic irradiation a source of component A, a source of component B, and a reducing agent wherein A and B are present in amounts consistent with the final stoichiometry of the desired alloy.

2. The method of claim 1 wherein the ultrasonic irradiation is carried out a frequency of from about 20 kHz to 100 MHz.

3. The method of claim 1 wherein the reducing agent reduces component A, component B or both.

4. The method of claim 1 wherein the reaction is carried out in a temperature range of from about -10 ⁰C to about 45 ⁰C.

5. The method of claim 1 wherein the product is a binary alloy nanoparticle with a size of from about 2 nm to about 20 nm.

6. The method of claim 1 wherein the binary alloy nanoparticle is a semiconductor nanoparticle.

7. The method of claim 1 wherein the yield of binary alloy nanoparticle is greater than about 50%.

8. A method of producing a semiconductor nanoparticle comprising: (a) ultrasonically irradiating a reaction mixture comprising a solvent and reactants to form a first reagent in solution;

(b) introducing of a second reagent to the reaction mixture;

(c) further ultrasonically irradiating said reaction mixture to produce a desired semiconductor nanoparticle; and (d) recovering the semiconductor nanoparticle.

9. The method of claim 8 wherein the semiconductor nanoparticle is a HI-V semiconductor nanoparticle.

10. The method of claim 8 wherein the first reagent is a source of a Group V element, a source of a Group III element, a reducing agent or combinations thereof.

11. The method of claim 8 wherein the first reagent is NaP.

12. The method of claim 8 wherein the second reagent is a salt of a Group IH element, a halide salt of a Group HI element, a chloride salt of the Group III element or combinations thereof.

13. The method of claim 8 wherein the second reagent is GaCl₃, InCB or combinations thereof.

14. The method of claim 8 wherein the ultrasonic irradiation is carried out a frequency of from about 20 kHz to about 100 MHz.

15. The method of claim 1 wherein the solvent is an etherate.

16. The method of claim 15 wherein the etherate has a boiling point of greater than about 60⁰C.

17. The method of claim 15 wherein the etherate has a boiling point of greater than about 80⁰C.

18. The method of claim 8 wherein the semiconductor nanoparticle has a particle size of from about 2 nm to about 20 nm.

19. The method of claim 8 further comprising annealing the semiconductor nanoparticle at a temperature of from about 150⁰C to about 750⁰C.

20. A πi-V semiconductor nanoparticle comprising less than about a 1:1 mole ratio of capping agent to semiconductor nanoparticle.

21. The nanoparticle of claim 20 wherein the capping agent is trioctylphosphine oxide.

22. The nanoparticle of claim 20 wherein the particle size is from about 2 nm to about 6 nm.