CATALYST, ELECTRODE AND MANUFACTURING METHODS THEREOF

Abstract

The invention relates to the use of a ternary alloy having the formula Si.sub.xTi.sub.yNi.sub.z, wherein x, y and z are natural numbers, for use in electrolysis and photoelectrolysis, in particular photo-oxidation of water. One aspect of the invention relates to a method for the manufacture of an electrode, the method comprising a step of heating a carrier comprising a surface having a layer of silicon on which a layer of TiO.sub.2 is disposed, the layer of TiO.sub.2 being covered with a layer of NiO; the heating step being carried out at a temperature above 1,000 C., and preferably between 1,150 C. and 1,250 C. The invention also relates to an electrode comprising a carrier, said electrode having either: an outer surface on which particles of a ternary alloy having the formula Si.sub.xTi.sub.yNi.sub.z are positioned, wherein x, y and z are natural numbers, and wherein the particles form protrusions; or an outer surface consisting of a layer of said alloy, the layer comprising protrusions.

Claims

1. An electrode comprising a carrier the electrode having either an outer surface on which particles of a ternary alloy of formula Si.sub.xTi.sub.yNi.sub.z are positioned, wherein x, y and z are natural numbers less than or equal to 100, and wherein the particles form protrusions, either an outer surface consisting of a layer of this alloy, the layer comprising protrusions.

2. The electrode according to claim 1, wherein the electrode is a photoelectrode.

3. The electrode according to claim 1, wherein the electrode is a photocathode.

4. The electrode according to claim 1, wherein the carrier is made of light-absorbing material that comprises silicon.

5. The electrode according to claim 1, wherein the protrusions of the alloy have a size of less than 5 m.

6. A photoelectrochemical cell comprising an electrode according to claim 1.

7. (canceled)

8. A method for manufacturing an electrode based on a ternary alloy having the formula Si.sub.xTi.sub.yNi.sub.z, wherein x, y and z are natural numbers, the method comprising: a step of heating a carrier comprising a surface comprising a layer of silicon on which a layer of TiO.sub.2 is arranged, the layer of TiO.sub.2 being covered with a layer of NiO; the heating step being carried out at a temperature above 1,000 C.

9. The method according to claim 8, wherein at least one of the layers of TiO.sub.2 and NiO is applied by using the ALD technique.

10. The method according to claim 8, wherein the layer of TIO.sub.2 and/or NiO has a thickness ranging from 10 to 100 nm.

11. (canceled)

12. The electrode according to claim 1, wherein the carrier is made from photoabsorber material.

13. The electrode according to claim 2, wherein the photoelectrode is a photoanode.

14. The electrode according to claim 5, wherein the protrusions of the alloy have a size ranging from 150 nm to 1 m.

15. The method according to claim 8, wherein the temperature ranges from 1,150 C. to 1,250 C.

16. The method according to claim 10, wherein the thickness ranges from 10 to 50 nm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0049] The invention will be better understood on reading the following description given solely by way of example and with reference to the appended drawings in which:

[0050] FIG. 1 is a sectional view by Transmission Electron Microscopy (TEM) of the multilayers TiO.sub.2/NiO deposited by ALD on silicon formed in step d of example 1; (b) represents the evolution of the crystalline structure as a function of the annealing conditions under H.sub.2 by X-ray diffraction (XRD); (c) a top view of Si.sub.7Ti.sub.4Ni.sub.4 particles on Si by Scanning Electron Microscopy (SEM) of the material obtained in Example 1; (d) a silicon micropillar covered with Ni particles; (e, f, g, h) a diagram showing a preferred manufacturing mode of the Si.sub.xTi.sub.yNi.sub.7 particles by ALD and heat treatment.

[0051] FIG. 2 is a comparison of the photocurrent curves as a function of the potential for a Si electrode covered with Ni, n-Si/TiO.sub.2/Ni and Si.sub.7Ti.sub.4Ni.sub.4 (example 1 according to the invention). The position of the thermodynamic oxidation potential of the water is indicated as a dotted line.

EXAMPLE 1: MANUFACTURE OF A MATERIAL ACCORDING TO THE INVENTION

[0052] During this synthesis, the chemical products used, cited below, to clean the samples are of analytical quality, supplied by Merck.

[0053] The chosen carrier was flat n-type silicon wafers (100) doped with phosphorus (resistivity 1-10 .Math.cm) supplied by the company Sil'tronix Silicon Technologies (France).

[0054] a) Preparation of Silicon Supports

[0055] The wafers were cut into squares of 1.51.5 cm.sup.2 and were degreased in successive ultrasound baths of acetone, ethanol, isopropanol (5 min per bath). The samples are rinsed thoroughly with ultra-pure water (=18.2 M). The native oxide layer is removed by dipping in an aqueous solution of HF (10%) for 30 seconds.

[0056] b) Depositing an Amorphous TiO.sub.2 Film on Si

[0057] The TiO.sub.2 film is deposited using the ALD technique. The deposition was carried out in a commercial reactor at a temperature of 150 C. (it is generally between 70 and 250 C.) under primary vacuum (residual pressure between 10.sup.1 and 10.sup.3 Torr) under an argon carrier gas. The amount of precursor injected is determined by the opening duration of a fast membrane valve. The precursor used is tetrakis(dimethylamino)titanium (TDMAT) for Ti and ultrapure water for oxygen. The TDMAT was supplied by STREM Chemicals with a purity level of 98%. The tanks containing the Ti precursor were kept at 80 C. and the ultrapure water reservoir was left at room temperature (approximately 20 C.).

[0058] The transport of precursor is assisted by the use of a carrier gas (in this case argon) whose flow is adjusted according to the geometry of the reaction chamber and the power of the pumping unit. An exposure step is used during which the pumping system is isolated from the reactor in order to obtain a more uniform film.

[0059] The ALD cycle used in this example is therefore described as follows: [0060] Injection of Ti precursor (TDMAT) (2 s)/Exposure (7 s)/Purge (15 s) [0061] Injection of O precursor (water) (0.2 s)/Exposure (7 s)/Purge (15 s)

[0062] The cycle is repeated n times to obtain a thickness of about 40 nm.

[0063] A cross-section of the carrier thus obtained, observed by TEM, is reproduced in FIG. 1a.

[0064] c) Recrystallization of TiO.sub.2

[0065] The TiO.sub.2 film formed in step 1 is generally amorphous or very slightly crystalline. This film was therefore annealed in air at 450 C. for 2 hours in a furnace. A polycrystalline film of the anatase phase of the TiO.sub.2 is then obtained.

[0066] d) Deposition of a Layer of NiO on Si/TiO.sub.2

[0067] An NiO film was then deposited on the annealed TiO.sub.2 layer. The ALD technique described for the deposition of the layer of TiO.sub.2 was also used with the same reactor at a temperature of 250 C. The precursor used as a nickel source is Ni (EtCp).sub.2 and the ozone produced by the generator integrated into the ALD reactor constitutes the oxygen source. The reservoir containing the Ni precursor was maintained at 90 C. for Ni(EtCp).sub.2. Since this Ni precursor has a low saturated vapor pressure, it was decided to use assistance optimizing their transport from the tank to the reactor. More precisely, the carrier gas (Ar) was injected into the Ni(EtCp).sub.2 tank before opening the communication valve with the reactor. The ALD cycle consists of two injection/exposure/purge sequences, one for the Ni precursor and the other for the O precursor. The amount of precursor injected into the reactor under primary vacuum (residual pressure of between 10.sup.1 and 10.sup.3 Torr) was determined by the opening duration of a fast membrane valve. The ALD cycles are therefore described as follows: [0068] Injection of Ni(EtCp).sub.2 (2 s)/Exposure (15 s)/Purge (10 s) [0069] Injection of O.sub.3 (0.2-0.3 s)/Exposure (13 s)/Purge (10 s)

[0070] The cycle is repeated n times to obtain a thickness of about 13 nm.

[0071] e) Manufacturing a Layer of Catalvtic Material Si.sub.7Ti.sub.4Ni.sub.4 by Thermal Processing

[0072] The material consisting of the superposition of a nickel oxide film on a titanium oxide film itself placed on a doped silicon carrier was then reduced by annealing under H.sub.2 using the rapid thermal processing by infrared illumination method. The conditions of this reducing thermal processing are as follows: [0073] Temperature: 1,200 C. (temperature rise ramping 20 C./s) [0074] Annealing time: 30 s [0075] Atmosphere: Argon/H.sub.2 (ratio 1/1), pressure of 100 mbar.

[0076] This material was identified as being a ternary metal alloy Si.sub.7Ti.sub.4Ni.sub.4 (STN). The identification was performed by XRD as shown in FIG. 1b.

[0077] This figure also comprises, for comparison purposes, the obtained diagrams of materials (7) comprising layers of TiO.sub.2 and NiO superimposed on a carrier obtained according to this example except that the annealing temperature of step e) did not substantially exceed 1,000 C.

[0078] The calculations according to the density functional methods (DFT) carried out in the laboratory show that the material according to the invention is metallic. Although the literature on an Si.sub.xTi.sub.yNi.sub.z alloy is relatively limited, this is consistent with resistivity measurements performed on a film obtained by physical vapor deposition (PVD).

[0079] In addition, on the surface of the material, Si.sub.7Ti.sub.4Ni.sub.4 particles are disposed quite regularly by SEM, as shown in the top view of FIG. 1c. These particles can also be observed in FIG. 1d which shows a material according to the invention which was produced according to example 1 but from a silicon carrier configured in the form of pillars (see FIG. 1e).

[0080] These particles of sub-micrometer size increase the active area of the alloy.

EXAMPLE 2: PHOTOELECTRIC CHARACTERISTIC OF AN ELECTRODE ACCORDING TO THE INVENTION COMPRISING THE MATERIAL OF EXAMPLE 1

[0081] The photoelectrochemical characteristics of the material of Example 1 as a photoanode in the photo-oxidation of water were determined under the following conditions:

[0082] A three-electrode photoelectrochemical half-cell (photo anode, counter electrode and reference electrode) is equipped with a quartz window. This window allows the UV rays produced by a lamp emitting polychromatic light to reach the surface of the photoanode.

[0083] The photoanode consists of the carrier produced in example 1. The counter electrode is a platinum wire, the reference electrode is a Hg/HgO electrode (1 M KOH). A seal with a diameter of 6 mm ensures the sealing of the cell and allows the exposure of 0.28 cm.sup.2 of the photoanode. The rear contact between the photoanode and the circuit is ensured by a copper disc, after an InGa eutectic produced in-house is applied behind the sample. The whole is connected to a potentiostat (EG&G PAR, Model 273). The light source is a 150 W xenon lamp (Oriel, APEX, ref: 6255) calibrated using a photodiode (Newport, Cell and Meter, ref: 91150V) to obtain a power of 100 mW cm.sup.2. The electrolyte used was 1 M KOH (pH=14).

[0084] Nitrogen (nitrogen U, 99.95%, Air Liquide) is bubbled into the cell before and throughout the duration of the acquisitions in order to evacuate all the oxygen dissolved in the electrolyte.

[0085] FIG. 2 compares the photoelectrochemical performances (the photocurrents vs. the potential), via the superposition of the voltammograms obtained after several test cycles of the electrodes (these cycles consist of alternating cycling voltammetry phases with open-circuit potential measurement phases for 90 min. under illumination), of this photoanode according to the invention, an n-Si/TiO.sub.2/Ni material (obtained by reductive annealing of NiO at 900 C. for 30 seconds) and an n-Si/Ni material under the same or similar conditions of use.

[0086] The material according to the invention does not exhibit the highest current (therefore the production of O.sub.2), but this level is acceptable and can be optimized as it depends greatly on the charge and the geometry of the particles. However, the overvoltage at which the current appears is spectacular. The shifts to the negative voltages (respectively 200 and 400 mV relative to Si/Ni and Si/TiO.sub.2/Ni) are valuable. This is particularly advantageous since the absorption of the solar spectrum goes from being limited to <600 nm to a maximum located at <950 nm, or an amount of absorbed photons multiplied by 2.5.

[0087] Although the comparison is difficult to do with IrO.sub.2 (reference catalyst for the oxidation of water) since it is used in acid solution, the overvoltage obtained with the material according to the invention is comparable. In addition, the material according to the invention is functional in an alkaline medium and its cost is significantly lower. Indeed, like nickel, the alloy makes it possible to perform long (photo-)electrochemical characterizations (about ten hours under illumination) without the Si being harmed.

[0088] The invention is not limited to the embodiments presented, and other embodiments will become clearly apparent to those skilled in the art.

LIST OF REFERENCES

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