Metal chalcogenide thin film electrode, method for the production thereof and use

Abstract

The invention relates to a method for producing a metal chalcogenide thin film electrode, comprising the steps: (a) contacting a metal or metal oxide with an elementary halogen in a non-aqueous solvent, producing a metal halide compound in the solution, (b) applying a negative electric voltage to an electrically conducting or semiconducting substrate which is in contact with the solution from step (a), and (c) during and/or after step (b) contacting the substrate with an elementary chalcogen forming a metal chalcogenide layer on the substrate. The invention also relates to a metal chalcogenide thin film electrode which can be produced by the method and its use as an anode for releasing oxygen during (photo)electrochemical water splitting.

Claims

1. A method for producing a metal chalcogenide thin film electrode, comprising the steps: (a) contacting a metal or metal oxide with an elementary halogen in a solution comprising a non-aqueous solvent, producing a metal halide compound in the solution, (b) applying a negative electric voltage to an electrically conducting or semiconducting substrate which is in contact with the solution from step (a), and (c) during and/or after step (b), contacting the substrate with an elementary chalcogen, forming a metal chalcogenide layer on the substrate, wherein contacting the substrate with the elementary chalcogen is performed by contacting the substrate with a chalcogen-containing gaseous atmosphere.

2. The method according to claim 1, wherein the metal contained in the metal or metal oxide of step (a) is able to form a metal halide compound in which the metal is present in the oxidation state +2 or higher.

3. The method according to claim 1, wherein in step (b) the metal contained in the metal or metal oxide of step (a) is deposited onto the substrate by reduction, while the substrate functions as an electron transmitter during the reduction due to the negative voltage applied to the substrate.

4. The method according to claim 1, wherein the metal contained in the metal or metal oxide of step (a) comprises at least one transition metal.

5. The method according to claim 1, wherein the metal or metal oxide applied in step (a) is a solid metal body.

6. The method according to claim 1, wherein the elementary chalcogen is elementary oxygen, elementary sulphur or elementary selenium.

7. The method according to claim 1, wherein the substrate comprises an n-semiconductor material.

8. The method according to claim 1, wherein the elementary halogen is iodine (I.sub.2) or bromine (Br.sub.2).

9. The method according to claim 1, wherein the non-aqueous solvent is an organic solvent.

10. The method according to claim 1, wherein a proportion of water in the non-aqueous solvent is at most 0.2 wt. %.

11. The method according to claim 1, comprising the step: (d) thermal after treatment of the substrate comprising the metal chalcogenide layer.

12. The method according to claim 4, wherein the transition metal is selected from the group consisting of: iron, cobalt, nickel, a mixture or alloy thereof.

13. The method according to claim 5, wherein the solid metal body is an industrial metal or scrap metal.

14. The method according to claim 7, wherein the n-semiconductor material is selected from n-doped silicon and fluorine-doped tin oxide (FTO).

15. The method according to claim 9, wherein the organic solvent comprises a carbonyl group or cyanide group.

16. The method according to claim 1, wherein the non-aqueous solvent is one of acetone and acetonitrile.

Description

(1) The invention is explained in the following by way of example embodiments with reference to the associated drawings. In the latter:

(2) FIG. 1 shows the morphology and electrochemical properties of metal oxide FTO thin film electrodes according to the invention after the cathode deposition of Ni (a), Co (b), Fe (c) and Cu (d);

(3) FIG. 2 shows results of the chemical analysis of an Ni.sub.xO.sub.y layer deposited onto Si (111); a) XPS before and after anode use, b) EDX;

(4) FIG. 3 shows results of the chemical analysis of a Co.sub.xO.sub.y layer deposited onto Si (111); a) XPS before and after anode use, b) EDX;

(5) FIG. 4 shows results of the chemical analysis of a Fe.sub.xO.sub.y-layer deposited onto Si (111); a) XPS before and after anode use, b) EDX;

(6) FIG. 5 shows results of the chemical analysis of a Cu(0)-layer deposited onto Si (111); a) XPS before and after anode use, b) EDX;

(7) FIG. 6 shows studies on an Ni.sub.xO.sub.y/Si(100) thin film electrode; a) current voltage behaviour during photoelectrochemical oxygen development with continual illumination (continuous curve) and periodic illumination (broken curve); b) SEM image in oblique view; c) SEM image in plan view; d) TEM image in plan view;

(8) FIG. 7 SEM images of an Ni.sub.xO.sub.y/Si(100) thin film electrode; produced a) in a water-free solution according to the invention and b) in a solvent mixture with 25 vol. % H.sub.2O;

(9) FIG. 8 shows an analysis of a Fe/Si/Ni/Cr/Co/Mn oxide layer deposited on FTO; a) SEM images before and after a thermal aftertreatment; b) EDX analysis;

(10) FIG. 9 shows an electrochemical investigation of a tempered Fe/Si/Ni/Cr/Co/Mn oxide/FTO electrode with regard to the electrochemical oxygen development; on the left) dark reaction with an external voltage of 0-1.4 V; on the right) intermittent illumination at a constant applied potential of 0.65 V);

(11) The method according to the invention is explained in more detail in the following. In this case for a clearer overview iodine is used as the halogen, oxygen is used as the chalcogen and a solid metal body is used as the provider for the metal of the metal chalcogenide layer, but the invention is not restricted to this.

(12) In an optional step 1 a metal is cleaned of any possible surface dirt and/or oxide or hydroxide layers on the surface. The cleaning step can be performed mechanically for example by using abrasive materials such as sandpaper or the like. Alternatively, the cleaning can be performed by chemical treatment, such as for example oxide-dissolving reactions. Preferably, a solid metal body is used as the metal, which can come in particular from industrial metal or scrap metal. Solid bodies of any geometric form can be used, for example in the form of metal sheets, powders or the like. In chemical terms preferably metals are used which comprise iron, cobalt and/or nickel or are made of the latter.

(13) In step 2 the metal halide is formed, here metal iodide. For this purpose the metal possibly cleaned in step 1 is placed into a non-aqueous solvent with a water content of at most 0.2 wt. %. Preferably, acetone or acetonitrile is used as the solvent. A halogen, here crystallised iodine, is added to the solvent in a mass ratio of solvent:iodine of at least 1:1 or a greater amount of iodine. Preferably, ultrasound is applied to the mixture to achieve a better mixing result. The reaction is carried out for a period of at least 5 minutes, preferably at ambient temperature. It is assumed that in this way the iodine, with a partial detachment of metal from the solid body to the corresponding metal iodide in the solution reacts according to the following equation for example:
M(0)+I.sub.2.fwdarw.MI.sub.2.

(14) Electrochemical processing takes place in the next step 3. For this two electrically conductive or semiconductive electrodes are moved into contact with the metal iodide-containing solution from step 2, once the remaining solid metal body has been removed from the latter. One of said electrodes is used as a substrate for the thin film electrode to be produced, whereas the other one represents the counter electrode for electrochemical processing. The substrate electrode is made for example from a metal or a metal alloy, FTO, n-doped silicon or carbon. The counter electrode can be made in principle from the same material or from a different material than the substrate electrode. Voltage is applied to the electrodes, wherein the substrate electrode is allocated a negative voltage, i.e. is connected as a cathode. The applied voltage is 2 Volt, in particular it is within a range of 5 to 10 Volt, wherein the () sign refers to the substrate electrode on which the metal oxide film is to be deposited. The electrochemical deposition is preferably performed at ambient temperature. The length of the reaction corresponds to the thickness of the metal oxide layer to be deposited and depends on the applied voltage.

(15) Without wishing to commit to a specific theory it is assumed that the metal iodide in the solution is coordinated by the organic solvent, in particular its carbonyl or cyanide groups with the formation of metal organic complexes. Said complexes exhibit high reactivity compared to free oxygen, which is already present in traces in the solvent or in the environmental air. The oxygen is reduced at the cathode (i.e. negatively) polarised substrate electrode with the capture of electrons to O.sup.2 anions which react with the metal halide to form the corresponding metal oxide. This leads to a direct deposition of the metal oxide on the substrate (see following reaction equations). Possibly a reaction with oxygen takes place and the associated metal oxide deposition only upon later contact with the air oxygen, after the still wet, negatively polarised electrode has been removed from the solution.
O.sub.2+e.sup..fwdarw.O.sup.2
O.sup.2+xMI.fwdarw.M.sub.xO.sub.y

(16) Although the chemical processes have already been described in detail the organic solvent appears to participate in the reactions such that in the produced metal oxide layer significant proportions of carbon and/or carbon-containing compounds are also deposited.

(17) Afterwards the substrate electrode with the layer deposited thereon is removed from the solvent/iodine-bath. This is preferably performed under dry nitrogen, to enable the evaporation of possibly formed hydrogen iodide with the exclusion of humidity.

(18) In an optional subsequent step 4 a chemical or electrochemical aftertreatment of the deposited layers takes place with the aim of increasing the stability of the metal oxide layer. In particular, the aim of the aftertreatment is to increase the oxidation state of the metal, that is to oxidise the latter further. For this purpose the electrode can be introduced for example into an aqueous hydroxide-containing electrolyte solution and processed electrochemically.

(19) In a further optional step 5 a thermal aftertreatment of the electrode takes place for increasing the crystallinity of the deposited metal chalcogenide layer. For this the electrode is tempered at temperatures in a range of 150 to 800 C. for a duration of 1 minute to 10 hours.

(20) The metal chalcogenide thin film electrode obtained by the method according to the invention are characterised by having a particularly impervious and stable metal chalcogenide layers, which also contain carbon.

EXAMPLES

(21) Measurement Techniques

(22) XPS.

(23) X-ray photoelectron spectroscopy, (XPS), was performed for the chemical analysis of the samples. The allocation of the core contour lines to specific oxidation states was performed using published data (Chastain & King (Ed.), Handbook of X-Ray Photoelectron Spectroscopy, Physical Electronics, Minnesota, USA, 1995).

(24) EDX.

(25) Energy dispersive X-ray analysis (EDX) was performed for chemical element analysis both integrally, i.e. averaging over the whole sample surface, and also locally, i.e. with later resolution (smallest resolution limit about 100 nm) on the scanning electron microscope. The excitation energies were selected so that the expected element-specific K or L lines of the elements can be detected, i.e. between 3 keV and 10 keV. The allocation of the measured X-ray lines was automated by means of database values by the control software (NSS 2.2, Thermo Fisher Scientific, USA).

(26) Electrochemical Characterisation

(27) The electrodes produced in the examples were tested in an electrochemical standard cell with respect to their suitability for generating oxygen in 0.1 mol/l NaOH (pH 13). For this the samples were measured either in a three electrode configuration with a Pt counter electrode and an Ag/AgCl reference electrode or in a two electrode configuration with the short-circuiting of the Pt counter and Ag/AgCl reference electrode. The potential was controlled respectively by a potentiostat (VSP, BioLogic, France).

(28) For photochemical studies lighting was provided using a W-I source of white light (MI-150, Dolan-Jenner, the Netherlands) by fibre optics.

Example 1: Production of an NixOy/FTO Electrode

(29) As the solid metal body 2 cm.sup.2 of an extremely pure Ni-metal film (Goodfellow Corp. USA, purity >99.95 wt. %) was placed in a mixture of acetone (15 ml, w (H.sub.2O)<0.2%) and iodine crystal powder (80 mg). This mixture was mixed for 5 min in an ultrasound bath at 37 kHz. During this treatment the temperature increased from ambient temperature to about 35 C. Afterwards the metal film was removed from the solution.

(30) FTO films (Solaronix, Switzerland, sheet resistance 7 cm.sup.2, 31.5 cm) were precleaned with acetone. An FTO sample was placed as a cathode (substrate) and a second as a counter electrode (anode) at a distance of 5-10 mm from another in the acetone solution. A potential of 10 V was applied between the substrate and counter electrode for 5 min.

(31) Afterwards the substrate electrode was removed from the solution and dried.

Example 2: Production of a CoxOy/FTO Electrode

(32) The method was performed as in example 1, except that as the solid metal body 2 cm.sup.2 of an extremely pure Co metal film was used (Goodfellow Corp. USA, purity >99.95 wt. %).

Example 3: Production of a FexOy/FTO Electrode

(33) The method was performed as in example 1, except that as the solid metal body 2 cm.sup.2 of an extremely pure Fe metal film was used (Goodfellow Corp. USA, purity >99.95 wt. %).

Example 4: Production of a Cu/FTO Electrode

(34) The method was performed as in example 1, except that as the solid metal body 2 cm.sup.2 of an extremely pure Cu metal film was used (Goodfellow Corp. USA, purity >99.95 wt. %).

Examples 5-8: Production of Different MxOy/Si(100) Electrodes

(35) The method was performed as in examples 1-4, except that 2 cm.sup.2 of an n-type Si(100) wafer (ABC Company, Germany; doping N.sub.D610.sup.15) was used as the substrate electrode (cathode) respectively. For the pre-treatment the Si(100) wafer was precleaned with ethanol and water and then chemically etched in a solvent mixture of hydrofluoric acid (50%) and ethanol (HF:C.sub.2H.sub.5OH=3:1) for 30 s and 10 s, then rinsed with water and dried with N.sub.2. FTO was used as the counter electrode as in examples 1-3 FTO.

Examples 9-12: Production of Different MxOy/Si(111) Electrodes

(36) The method was performed as in examples 1-4, except that 2 cm.sup.2 of an n-type Si(111) wafer (ABC Company, Germany; doping N.sub.D610.sup.15) was used as the substrate electrode (cathode) with. For the pre-treatment the Si(111) wafer was precleaned with ethanol and water and then chemically etched firstly with NH.sub.4F (100 s) and then with hydrofluoric acid (50%, 10 min) and then dried with N.sub.2. FTO was used as a counter electrode as in examples 1-3.

Example 13: Production of a Mixed Oxide/FTO Electrode

(37) The method was performed as in example 1, but instead as the solid metal body 2 cm.sup.2 of a steel alloy of the metals Fe/Si/Ni/Cr/Co/Mn was used. Furthermore, unlike example 1 the sample was tempered after drying at 300 C. for 10 min.

Example 14: Production of a CoO/ZnO/CoZnO/Si Electrode

(38) The method was performed as in example 1, but instead chemically synthesized CoZnO nanoparticles were used and n-doped Si(100) was used as the substrate.

(39) The substrates and metals used in the examples 1 to 14 are listed in table 1.

(40) TABLE-US-00001 TABLE 1 Metal/metal Example Substrate oxide Chalcogen 1 FTO Ni 2 FTO Co 3 FTO Fe 4 FTO Cu 5 Si(100) Ni 6 Si(100) Co 7 Si(100) Fe 8 Si(100) Cu 9 Si(111) Ni 10 Si(111) Co 11 Si(111) Fe 12 Si(111) Cu 13 FTO Steel (FeSiNiCrCoMn) 14 Si(100) CoZnO nanoparticles

Comparison Example 1: Production of an NixOy/Si(100) Electrode in the Presence of Water

(41) The method of production was as described in example 5 with the use of an Ni metal film and crystalline iodine, but instead of the acetone a solvent mixture of acetone and 25 volume % H.sub.2O was used.

Comparison Example 2: Production of a FexOy/Si(100) Electrode in the Presence of Water

(42) The method was performed as described in example 7 except that instead of acetone a solvent mixture of acetone and 25 volume % H.sub.2O was used.

(43) Characterisation of the Produced Metal Oxide Thin Film Electrodes

(44) FIG. 1 a) to d) shows the current densities as a function of the applied potential of the thin film electrodes obtained in examples 1 to 4 during the development of oxygen in a 0.1 M NaOH solution, which were measured in the dark between 0 and 2 volt against Ag/AgCl in a three electrode configuration. By comparison the behaviour of pure FTO is shown as a dashed-line in the respective graphs. The nickel, cobalt and iron oxides show clear activity with respect to the oxygen development (FIGS. 1 a) to c)). The overpotentials determined by the curves range between 340 and 420 mV, wherein the cobalt and nickel oxide electrodes have the highest activity and the iron oxide electrodes the lowest activity. The copper electrode is only active however at high external potentials (FIG. 1 d)). The oxygen development was confirmed respectively by means of differential electrochemical mass spectroscopy (DEMS).

(45) SEM images of the respective surfaces are also shown in FIGS. 1 a) to d). They confirm a surface morphology with a roughness in the nanometre range.

(46) FIGS. 2 to 5 shows the results of the chemical analyses of the deposited metal oxide layers on Si(111) according to the examples 9 to 12. In this case a) shows respectively the results of the XPS analysis of the metal oxide layer after its deposition (upper graph respectively) and after the use of the electrode for electrochemical acid development (lower graph respectively). b) of FIGS. 2 to 5 shows the results of the EDX measurements of the metal oxide layers after their production (before electrochemical use).

(47) The allocation of individual XPS signals to specific oxidation states was made more difficult by electrostatically charging the fresh oxide layers after their production (see FIGS. 2 a) to 5 a) above respectively). By comparison after the use of the electrodes for electrochemical oxygen development an allocation to different oxidation states of the metals was possible (see respectively FIGS. 2 a) to 5 a) below). For example the results in FIG. 2 a) show the simultaneous presence of Ni.sub.2O.sub.3 and NiO within the information depth of the method of about 2 to 3 nm.

(48) The EDX results show for nickel, cobalt and iron the presence of bonded oxygen in the form of the corresponding oxides (see FIGS. 2 to 4 b)). In the case of copper in the deposited layer only a comparatively small amount of oxygen could be found (FIG. 5 b)), which in accordance with the XPS analysis indicates the predominant deposition of metallic copper Cu(0). In line expectations this is associated with the low activity with regard to the oxygen development (cf. FIG. 1 d)). It is assumed that the insufficient deposition of copper oxide is associated with the property of copper of forming only monovalent iodide CuI during production. During a single electron transfer from the cathode the formation of metal copper appears to be preferable to the complexing in the organic solvent and subsequent oxidation.

(49) Interestingly carbon could be found in all of the thin films produced (FIG. 2 b) to 5 b)), which presumably originates from the used solvent (acetone).

(50) Based on the example of the Ni.sub.xO.sub.y/Si(100) thin film electrode produced according to example 5 the photoelectrocatalytic behaviour is shown in FIG. 6 a). In this case the continuous curve shows the current-voltage behaviour with continuous lighting and the interrupted curve shows the current-voltage behaviour with periodic lighting. The measurements were made respectively in a three electrode configuration. It can be seen that the Ni.sub.xO.sub.y/Si(100) thin film electrode according to the invention has good photoactivity. However in the dark it is inactive itself with an applied voltage of 2 Volt, which is caused by the drop in current density after switching off the light.

(51) FIGS. 6 b), c) and d) show SEM or TEM images of the Ni.sub.xO.sub.y/Si(100) thin film electrodes from example 5. It can be seen that the nickel oxide layer has a roughness in the nanometre range and a layer thickness of about 100 to 150 nm.

(52) FIG. 7 shows for comparison with the Ni.sub.xO.sub.y/Si(100) thin film electrode produced by according to the water-free method according to the invention of example 5 (FIG. 7a) the product according to comparison example 1, in which the solution contained 25 vol. % (FIG. 7b). It can be seen that the presence of water largely prevents the formation of a layer. Only individual Ni, O and C-containing islands can be observed (confirmed by EDX-analysis).

(53) A similar result was obtained in the comparison example 2 with iron. Here too the inhibiting effect of water on the film formation was clearly evident. The resulting flat islands made EDX analysis possible which could here also show Fe, O and C in the deposited islands (results not shown).

(54) FIG. 8 shows the results of the analysis of the metal oxide layer according to example 13 (before and after the thermal aftertreatment), in which a steel alloy was used as a starting material for the metal, which in addition to the main components iron and chromium contained additives of nickel, cobalt, manganese and silicon. In the EDX analysis shown in FIG. 8 b) it should be noted that the power range is shown from 0 to 5 keV at the top and from 5 to 10 keV at the bottom. The steel staring material (curve I) is shown, the metal oxide thin layer after its deposition (curve II) and after its tempering at 300 C. for 10 minutes (curve III). The EDX analysis shows that all of the elements contained in the steel are deposited onto the FTO substrate as oxides.

(55) FIG. 9 shows the electrochemical characterisation of the same sample as in FIG. 8 as an anode for oxygen development. In this case the left side shows the current-voltage behaviour with an applied external voltage of between 0 and 1.4 V without the action of light. The electrode shows rode an excess voltage relative to the oxygen development of about 300 mV, whereby its electrocatalytic activity is confirmed. The measurement with an applied constant potential of 0.65 Volt with intermittent illumination is shown on the right hand side of FIG. 9. This measurement confirms the photoactivity of the electrode.

(56) The thin film produced according to example 14 with the use of CoZnO nanoparticles as a starting material for the thin film to be deposited was also analysed (results not shown). The analysis shows two different layers deposited on top of one another. A thin film of the oxides CoO and Zno could be identified immediately on the n-Si(100)-substrate, which also contained amounts of carbon. On said amorphous CoO/ZnO/C layer a second layer could be identified which consisted of deposited CoZnO particles according to the starting material. It is assumed that the two thin films of this heterostructure were produced by different reaction routes. In this case the metal oxide particles in contact with the iodine-containing solution were partly dissolved, and the metal components enter into solution as metal iodides. The following electrochemical treatment produces (in a fast reaction phase) the amorphous, carbon-containing boundary layer on the substrate. However, the remaining undissolved oxide particles (in a slower reaction phase) are deposited onto said boundary layer by electrophoretic transport.

(57) Example 14 shows that chemically synthesised metal oxides produced as powder can be deposited by the method onto the substrate so that an (amorphous) protective layer is formed on the substrate. This enables the corrosion-free operation of the thus forming substrate/oxide heterostructure. This is an advantage particularly for light-based water splitting by using sensitive semiconductor substrates.

(58) As a by-product of the iron oxide layer formation in example 3 in the aforementioned cathodic conditions there was still a dispersion of black particles in solution. Said particles exhibited paramagnetic properties, i.e. they were attracted by the magnetic field of a permanent magnet, without being permanently magnetisedas with ferromagnetism. For the particles in the solution this means that they disperse again and the external magnetic field is removed. The method can thus also be used to produce nanoparticles of iron or other metal materials.