Method for preparing silicon and/or germanium nanowires

Abstract

The invention relates to a method for preparing a material made of silicon and/or germanium nanowires, comprising the steps of: i) placing a source of silicon and/or a source of germanium in contact with a catalyst comprising a binary metal sulfide or a multinary metal sulfide, said metal(s) being selected from among Sn, In, Bi, Sb, Ga, Ti, Cu, and Zn, by means of which silicon and/or germanium nanowires are obtained, ii) optionally recovering the silicon and/or germanium nanowires obtained in step (i); the catalyst and, optionally, the source of silicon and/or the source of germanium being heated before, during and/or after being placed in contact under temperature and pressure conditions that allow the growth of the silicon and/or germanium nanowires.

Claims

1. Method for preparing a material made of silicon and/or germanium nanowires, that comprises the steps of: i) placing a silicon source and/or a germanium source in contact with a catalyst comprising a binary metal sulfide or a multinary metal sulfide, said metal(s) being selected from Sn, In, Bi, Sb, and Ga, by means of which the silicon and/or germanium nanowires are obtained, ii) optionally recovering the silicon and/or germanium nanowires obtained in step (i); the catalyst, and optionally the silicon source and/or the germanium source, being heated before, during, and/or after the placing in contact under temperature and pressure conditions allowing the growth of silicon and/or germanium nanowires, the heating being done at a temperature of between 200° C. and 500° C.

2. Method according to claim 1, wherein the metal is tin.

3. Method according to claim 1, wherein the silicon source is selected from silanes and organosilanes.

4. Method according to claim 1, wherein the germanium source is selected from germanes and organogermanes.

5. Method according to claim 1, wherein the catalyst is in the form of nanoparticles.

6. Method according to claim 1, wherein said catalyst is present on a substrate.

7. Method according to claim 1, wherein the catalyst is present on a substrate, said catalyst being in the form of one or more thin layers on said substrate.

8. Method according to claim 1, wherein the catalyst is present on a substrate, said catalyst being in the form of nanoparticles on said substrate.

9. Method according to claim 1, wherein the silicon source is selected from SiH.sub.4 and SiCl.sub.4.

10. Method according to claim 1, wherein the germanium source is selected from GeH.sub.4 and GeCl.sub.4.

Description

FIGURES

(1) FIG. 1 is an image of the nanowires obtained in example 1, taken by scanning electron microscopy.

(2) FIG. 2 is a thermogravimetric analysis (TGA) measurement under argon of SnS nanoparticles which could be used in the context of the present invention.

(3) The curve (1) shows the loss of mass during heating a sample of SnS nanoparticles which could be used in the context of the present invention: the sample contains 60% organic stabilisers in order to keep the nanoparticles in suspension (suspension stable over several years). These organic stabilisers are thermally decomposed at 220 and 350° C., leaving the inorganic core, which does not show any decomposition until 1000° C.

(4) The curve (2) shows the loss of mass during heating of a sample of the same SnS nanoparticles where the majority of organic stabilisers have been removed by washing (20 mg of nanoparticles are washed in 100 ml of methanol with 1.4% acetone added, separated by centrifugation at 20000 g for 10 minutes, and then dried at 50° C. under argon for 10 minutes). The loss of mass due to the decomposition of the organic stabilisers is still visible on the curve (2) for 20% of the total mass, and then the inorganic core does not undergo decomposition before 700° C.

(5) FIG. 3 illustrates the analysis by X-ray diffraction of silicon nanowires obtained in the context of the present invention.

EXAMPLES

Example 1: Synthesis of a Batch of Silicon Nanowires

(6) 1/ SnS nanoparticles 5.5 nm in diameter are prepared from two solutions A and B, prepared as follows. Solution A containing 3 ml of trioctylphosphine (6.7 mmol), 380 mg of tin chloride SnCl.sub.2 (2 mmol) and 5 ml of octadecene (15.6 mmol) is placed under argon and heated to 60° C., and then 100° C. for 5 minutes. The temperature is increased to 150° C. and 1.5 ml of oleic acid is added. Solution B containing 3 ml of trioctylphosphine (6.7 mmol), 150 mg of thioacetamide (1 mmol) and 5 ml of oleylamine (15.2 mmol) is placed under argon and heated to 60° C., and then to 100° C. for 5 minutes. Solution B is added into solution A quickly with a syringe. The reaction is stopped at the end of 2 minutes by cooling in an ice bath.

(7) 2/ After adding 20 ml of methanol, the solution is centrifuged (9000 rpm, 2 minutes). The lower part is re-dispersed in 6 ml of dichloromethane. The solution is centrifuged (3000 rpm, 2 minutes) to remove the lower part. The supernatant is taken up in 24 ml of chloroform and 2 ml of oleylamine. 30 ml of methanol is added to precipitate the nanocrystals by centrifugation (9000 rpm, 2 minutes). The lower part is re-dispersed in 6 ml of chloroform and centrifuged (3000 rpm, 2 minutes), to remove the lower part. The supernatant is a stable solution of SnS nanocrystals, 5.5 nm in diameter.

(8) It must be noted that calibrated nanoparticles of a size selected from within a relevant diameter range for the growth of nanowires (in particular, from 7 to 23 nm) can be obtained using protocol A defined in Reiss et al..sup.17

(9) 3/ 0.45 mg of SnS nanocrystals in suspension in 450 μl of chloroform is placed in contact with 90 mg of calcium carbonate microparticles in 5 ml of cyclohexane. The cyclohexane is evaporated under argon flow, under heating to 60° C. The calcium carbonate particles covered with SnS nanocrystals are mixed with 285 μL of diphenylsilane (1.5 mmol). It all is placed in a 10 ml glass tube, vacuum-sealed. The tube is heated to 350° C. for one hour, then cooled.

(10) 4/ The solid is washed with 15 ml of 2M hydrochloric acid and 10 ml of chloroform. The silicon nanowires are recovered at the interface between chloroform and aqueous phase after (8000 rpm, 10 minutes). A second identical washing is carried out. The nanowires are then washed with 25 ml of ethanol and centrifuged (8000 rpm, 10 minutes). The lower part taken up in the chloroform contains silicon nanowires, 20 nm in diameter, ready to be used.

Example 2: Synthesis of Doped Nanowires

(11) Steps 1/ to 4/ defined above are repeated, with the only modification in step 3/, of diphenylphosphine P(C.sub.6H.sub.5).sub.2H being introduced in a mixture in the diphenylsilane in a proportion of 0.1 to 3% by mass.

Example 3: Preparation of Electrodes for Lithium Batteries

(12) 54 mg of silicon nanowires obtained in example 1 are ground in a mortar with 7 mg of black carbon and 7 mg of carboxy-methylcellulose in water (1 ml).

(13) The paste obtained is deposited by coating on an 0.8 mg/cm.sup.2 metal film and dried for 6 hours at 60° C., under vacuum.

(14) The electrode is mounted as a cathode in a lithium battery, with a Villedon separator impregnated with an electrolyte consisting of an LiPF.sub.6 solution in a 1/1 by mass mixture of ethylene carbonate and diethyl carbonate, against a metal lithium anode. It all is sealed in a button battery.

(15) The lithium battery is tested in discharging/charging cycles, over 70 cycles, at a speed of C/20 for the first cycle and C/5 for the following cycles. By definition, a charging speed of C/20 (C/5 respectively) indicates that the battery is fully charged in 1/20 hour (1/5 hour respectively).

Example 4: Preparation of Electrodes for a Supercondenser

(16) 1 mg of doped silicon nanowires obtained in example 2 are suspended in 200 μL of chloroform. A 1 cm.sup.2 piece of ultra-doped silicon wafer is stripped by soaking in an aqueous solution of hydrofluoric acid, 10% by mass. The silicon nanowire suspension of the invention is deposited on this substrate. The deposition is dried in ambient air. Two identical electrodes are prepared for producing a supercondenser. The two electrodes according to the invention are assembled into a sandwich opposite each other, separated by a Whatman filter paper separator impregnated with electrolyte consisting of 1-methyl-1-propylpyrolidinium bis-trifluoromethylsulfonide imide ionic water. The assembly of the supercondenser is done in an inert atmosphere.

(17) The capacitive performance of the supercondenser is tested by cyclic voltametrics.

Example 5: Study of the Stability of Metal Sulfides, According to the Temperature, Under Atmospheric Pressure

(18) At ambient pressure, metal sulfides are known to be stable at a temperature up to 700-900° C. generally, and are decomposed beyond that. This is highlighted by the thermogravimetric analysis (TGA) measurement under argon of the SnS nanoparticles used in the context of the present invention and presented in FIG. 2.

(19) Thus the tin sulfide SnS nanoparticles are not decomposed, under the conditions of obtaining the curves (1) and (2) in FIG. 2, into tin Sn nanoparticles, at a temperature of between 380 and 450° C., as particularly used in the present invention.

Example 6: Analysis of Silicon Nanowire Rays by X-Ray Diffraction

(20) The use of SnS nanoparticles as silicon nanowire growth catalysts makes it possible to obtain nanowires having a high proportion of amorphous silicon, as demonstrated by the analysis by X-ray diffraction presented in FIG. 3. In FIG. 3, the vertical lines indicate the positions of the diffraction peaks that are typical of crystalline silicon. The diffractogram shows the peaks at the expected positions (29°, 47°, 56°, 76°, 88°, 104°), showing the presence of crystalline silicon in the form of crystals of a nanometric size inside the nanowires. In addition to these indexed peaks, the diffractogram shows a large peak centred at 82° and spreading from 65° to 105°. Other less intense wide peaks are visible under the indexed diffraction peaks towards 25° and 51°. These wide peaks are due to an amorphous form of silicon present in the nanowires. This amorphous form represents a large quantity of material with respect to the crystallised part, as demonstrated by the high intensity of the amorphous silicon signal with respect to the crystalline silicon signal on the diffractogram. The high proportion of silicon in amorphous form in the nanowires is not observed for the silicon nanowires obtained on a gold catalyst.

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