Method for producing silicon nanowires

Abstract

A method for producing a material based on silicon nanowires is provided. The method includes the steps of: i) bringing into contact, in an inert atmosphere, a sacrificial support based on a halogenide, a carbonate, a sulfate or a nitrate of an alkali metal, an alkaline earth metal or a transition metal having metal nanoparticles, with the pyrolysis vapours of a silicon source having a silane compound, by which silicon nanowires are deposited on the sacrificial support; and optionally ii) eliminating the sacrificial support and recovering the silicon nanowires produced in step ii).

Claims

1. A process for the preparation of a material based on silicon nanowires, comprising the stages of: i) bringing a sacrificial support based on an alkali metal or alkaline earth metal halide, carbonate, sulfate or nitrate, comprising metal nanoparticles, into contact, under an inert atmosphere, with the pyrolysis vapors from a silicon source comprising a silane compound, whereby silicon nanowires deposited on the sacrificial support are obtained, the sacrificial support being obtained by mixing alkali metal or alkaline earth metal halide, carbonate, sulfate, or nitrate particles in an anhydrous nonpolar solvent, with the metal nanoparticles; and optionally ii) removing the sacrificial support and recovering the silicon nanowires obtained in stage i).

2. The process as claimed in claim 1, in which the sacrificial support is based on sodium chloride.

3. The process as claimed in claim 1, in which stage ii) comprises a stage of washing with water the silicon nanowires deposited on the sacrificial support.

4. The process as claimed in claim 1, in which stage i) is carried out in the presence of a doping agent.

5. The process as claimed in claim 4, in which the doping agent is chosen from organophosphines, organoarsines, organoboranes and aromatic amines.

6. The process as claimed in claim 1, in which the sacrificial support is a powder of an alkali metal or alkaline earth metal halide, carbonate, sulfate or nitrate, the diameter of the particles which is between 10 nm and 50 m.

7. The process as claimed in claim 1, in which the silicon nanowires resulting from stages i) or ii) are deposited on a support which is a conductor or semiconductor.

Description

DETAILED DESCRIPTION

EXAMPLES

Example 1

Synthesis of a Batch of Undoped Silicon Nanowires

(1) 1/ The gold nanoparticles are synthesized according to the process of the state of the art described in the paper by Brust et al..sup.[14]. Their diameter is from 1 to 2 nm and their surface is covered with dodecanethiol. They are dispersed in toluene at a concentration of 50 mg/ml in order to constitute the gold nanoparticles mother solution.

(2) 2/ The sacrificial salt support is prepared by grinding anhydrous sodium chloride. 200 g of anhydrous sodium chloride are placed in a grinding cylinder comprising zirconia beads. The cylinder is closed and rotated for 24 h. The salt powder obtained has a mean size of 10 m. It is stored with exclusion of the air.

(3) 3/ 40 l of gold nanoparticles mother solution are mixed with 1 g of sacrificial salt support in 20 ml of dry hexane. The hexane is evaporated under a stream of inert gas (argon or nitrogen) in 3 h. The dry solid is transferred into a mortar and finely ground with a pestle under an inert atmosphere. The powder is deposited in an alumina crucible.

(4) 4/ The reactor is a tube with an external diameter of 16 mm made of pyrex with a thickness of 1 mm, comprising two constrictions at 3 cm and 6 cm from the bottom approximately. In the bottom of the reactor (on the left in FIG. 1), 184 mg of diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2, i.e. 1 mmol, are deposited and then, on the top of the reactor (on the right in FIG. 1), the crucible containing the sacrificial support impregnated with gold nanoparticles is deposited. The reactor is subsequently placed on a vacuum line and sealed with a blow torch at 10 cm from the bottom approximately.

(5) 5/ The reactor is placed in an oven at 450 C. for 1 h and then it is removed from the oven and left to cool to ambient temperature over 30 minutes. The reactor is broken open under ambient conditions.

(6) 6/ The sacrificial salt support covered with silicon nanowires is transferred from the crucible into a 40 ml plastic centrifuge tube with 5 ml of chloroform. The suspension in chloroform is washed three times with 30 ml of water under ultrasound with an ultrasonic bath.

(7) 7/ 20 ml of ethanol are added to the suspension of silicon nanowires in chloroform. The mixture is passed through the ultrasonic bath and then centrifuged at 8000 rpm for 5 minutes, and the solvent is removed and replaced with 20 ml of ethanol and 5 ml of chloroform. This washing operation is repeated 3 times.

(8) Finally, the solid obtained consists of 5 to 10 mg of silicon nanowires ready for use.

Example 2

Synthesis of Doped Nanowires

(9) Stages 3/ to 7/ described above are repeated with the sole modification being that, in stage 4/, diphenylphosphine P(C.sub.6H.sub.5).sub.2H is introduced as a mixture in the diphenylsilane in a proportion of 0.1 to 3% by weight.

Example 3

Preparation of the Electrodes for Lithium Batteries

(10) 54 mg of undoped silicon nanowires are ground in a mortar with 7 mg of carbon black and 7 mg of carboxymethylcellulose in water (1 ml).

(11) The paste obtained is deposited by coating on a metal film at 0.8 mg/cm.sup.2 and dried at 60 C. under vacuum for 6 h.

(12) The electrode is fitted as a cathode in a lithium battery, with a Viledon separator impregnated with an electrolyte consisting of a solution of LiPF.sub.6 in a 1/1 mixture by weight of ethylene carbonate and diethyl carbonate, against a lithium-metal anode. The combination is sealed in a button cell.

(13) The lithium battery is tested in discharge/charge cycles over 70 cycles at a rate of C/20 in the first cycle and C/5 in the following cycles. By definition, a charge rate of C/20 (C/5 respectively) indicates that the battery is completely charged in 1/20 hour ( hour respectively). The charge and discharge capacity, with respect to the weight of silicon nanowires/carbon black/carboxymethylcellulose composite, is shown in FIG. 2 as a function of the number of cycles. The figure shows that, despite the initial fall in capacity due to the formation of a passivation layer on the silicon nanowires, the capacity of the battery stabilizes from the 20.sup.th cycle and subsequently remains constant over at least 50 cycles. This high stability demonstrates the excellent resistance of the silicon nanowires to the mechanical stresses imposed by the lithiation/delithiation cycling, in comparison with silicon nanopowders, which induce, in the lithium battery, a continuous fall in the capacity.sup.[3].

Example 4

Preparation of the Electrodes for a Supercapacitor

(14) 1 mg of doped silicon nanowires are suspended in 200 l of chloroform. A 1 cm.sup.2 piece of ultradoped silicon wafer is stripped by dipping in a 10% by weight aqueous hydrofluoric acid solution. The suspension of silicon nanowires of the invention is deposited on this substrate. The deposit is dried in ambient air. Two identical electrodes are prepared for the manufacture of a supercapacitor. The two electrodes according to the invention are assembled as a sandwich one face to the other, separated by a Whatman filter paper separator impregnated with electrolyte consisting of the ionic liquid 1-methyl-1-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide. The supercapacitor is assembled in an inert atmosphere.

(15) The capacitive behavior of the supercapacitor is tested by cyclic voltammetry. The voltammogram presented in FIG. 3 shows a virtually ideal capacitive behavior within a wide voltage window of 3 V.

(16) The stability to cycling is measured by cycling the supercapacitor over a window of 2.5 V over 30 000 cycles, under a current density of 0.050 mA/cm.sup.2. The initial capacitance of the supercapacitor is 17 F/cm.sup.2. The change in the capacitance as a function of the number of cycles is given in FIG. 4. The capacitance of supercapacitor is remarkably stable with a fall of barely 7% in the first 3000 cycles and then an unvarying capacitance over time.

Example 5

Synthesis of a Batch of Undoped Silicon Nanowires on Calcium Carbon Nanoparticles

(17) 1/ The gold nanoparticles are synthesized according to the process of the state of the art described in the paper by Brust et al..sup.[14]. The diameter is from 1 to 2 nm and their surface is covered with dodecanethiol. They are dispersed in toluene at a concentration of 50 mg/ml in order to constitute the gold nanoparticles mother solution.

(18) 2/ The sacrificial support of calcium carbonate nanoparticles is a commercial product which has an appearance of a white powder with the mean nanoparticle size of 67 nm. The product is stored with exclusion of moisture.

(19) 3/ 20 l of gold nanoparticles mother solution are mixed with 200 mg of sacrificial support of calcium carbonate nanoparticles in 20 ml of dry hexane. After stirring for 30 min, the hexane is evaporated using a rotary evaporator. The dry solid is transferred into an alumina crucible. All the handling operations of this part are carried out in the open air.

(20) Stages 4/ and 5/ described in example 1 are repeated with the sole modification being that, in stage 4/, 368 mg of diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2, i.e. 2 mmol, are used.

(21) 6/ The sacrificial support of calcium carbonate nanoparticles is transferred from the crucible into a 40 ml plastic centrifuge tube with 5 ml of chloroform. The suspension is washed three times with 20 ml of the aqueous hydrochloric acid HCl solution (1 M) and then twice with 20 ml of water under ultrasound in the ultrasonic bath.

(22) Stage 7/ is repeated.

(23) Finally, the solid obtained consists of 17 to 20 mg of silicon nanowires which are ready for use.

Example 6

Synthesis of a Batch of Doped Silicon Nanowires on Calcium Carbonate Nanoparticles

(24) Stages 4/ to 7/ described in example 5 are repeated with the sole modification being that, in stage 4/, diphenylphosphine P(C.sub.6H.sub.5).sub.2H is introduced as a mixture in the diphenylsilane Si(C.sub.6H.sub.5).sub.2H.sub.2 in a proportion of 0.1 to 3% by weight.

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