Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials

Definitions

• the present invention relates to a process for preparing a film of a metal oxide or a metal hydroxide of an element of columns II or III of the classification, deposited on a substrate.
• Metallic oxides in thin layers are very important materials in various technological fields because of their optical, electrical and catalytic characteristics. Among their many applications, one can quote for example the use of zinc oxide for the development of conductive and transparent electrodes in solar cells.
• the thin metal oxide layers are generally obtained by vacuum deposition techniques such as sputtering, or "sputtering", chemical vapor deposition, or by deposition of successive layers by molecular beam epitaxy (E-JM ). All of these methods use expensive equipment.
• JA Switzer (cited above) and RT Coyle, et al. (US-A-, 882, 014) further describe the preparation of metal oxide and hydroxide powders as ceramic precursors. These powders are formed by precipitation in the vicinity of the cathode of an electrochemical cell, caused by the reduction of nitrate ions. These powders are then dried and sintered at high temperature to obtain the ceramic materials. Any deposits formed on the cathode are scraped off to be recovered in the form of powder. The aim is therefore to obtain powder, and neither the direct obtaining of an oxide or hydroxide film on a substrate, nor its use as such are described. Furthermore, no mention is made of an oxygen reduction reaction for the formation of an oxide or hydroxide film.
• the object of the present invention is to provide a process which does not have the drawbacks of the methods of the prior art, in order to obtain a film of a metal oxide or of a metal hydroxide on a support by electro-chemical means. , the said film having good mechanical strength and good adhesion to the support.
• the method is characterized in that oxygen is dissolved in the electrolyte and a cathode potential is imposed on the electrochemical cell lower than the oxygen reduction potential and greater than the deposit potential of the metal M in the considered electrolyte.
• the process of the present invention can be used to prepare a film of a single metal compound. It can also be used to prepare a film of a mixed compound containing at least two metallic elements. When a film of a mixed compound is prepared, at least one precursor salt of each of the desired metal species is introduced into the electrolyte and the potential imposed on the electrochemical cell is greater than the potential of the metallic deposits in the bath considered.
• the process of the present invention can be used for the preparation of a film of a compound of at least one metal M chosen from the metallic elements of columns II and III of the periodic table, and more particularly for preparing a film of a zinc, cadmium, gallium or indium compound.
• the electrochemical cell used for implementing the process of the invention comprises an electrode which functions as a cathode and which serves as a support for the film of compound of M electrodeposited, a counter-electrode and a reference electrode.
• the electrode consists of any conductive material which can be used as a cathode material.
• metallic materials such as for example iron, steels, copper or gold, conductive metallic oxides such as for example tin oxide Sn0 2 , indium ln 2 0 3 , mixed indium tin oxide (ITO) or titanium oxide Ti0 2 , or semiconductor materials such as silicon, GaAs, InP, Cu (In, Ga ) (S, Se) 2 or CdTe.
• conductive metallic oxides such as for example tin oxide Sn0 2 , indium ln 2 0 3 , mixed indium tin oxide (ITO) or titanium oxide Ti0 2
• semiconductor materials such as silicon, GaAs, InP, Cu (In, Ga ) (S, Se) 2 or CdTe.
• the counter electrode can be an unassailable electrode such as for example a platinum or gold electrode, or of a material coated with these metals. It can also be an electrode constituted by the metal M of the compound of which it is sought to form a film. In this case, the oxidation of the metal M of the counter-electrode makes it possible to keep the metal concentration M of the electrolyte constant.
• the reference electrode is chosen from the electrodes usually used as such, in particular the mercurous sulfate electrode (ESM) or the mercurous chloride electrode (ECS).
• ESM mercurous sulfate electrode
• ECS mercurous chloride electrode
• the corresponding potentials are respectively +0.65 V and +0.25 V with respect to the normal hydrogen electrode (ENH).
• the electrolyte contains at least one precursor salt of at least one metallic species M and a solvent.
• the solvent of the electrolyte is chosen from water and the nonaqueous polar solvents usually used in electrochemical cells, among which there may be mentioned alcohols, more particularly isopropanol, acetonitrile, dimethyl sulfoxide and carbonate. propylene. Water is a particularly preferred solvent.
• the precursor salt of the metallic element M can be chosen from the salts soluble in the solvent used for the electrolyte. Among these salts, mention may be made of inorganic salts such as halides, sulfates, nitrates and perchlorates, and organic salts such as acetates.
• the electrolyte may optionally contain at least one second salt, called the support salt.
• This second salt is a salt disso ⁇ ciable in the solvent used and has the main function of ensuring good electrical conductivity of the electrolyte, especially in the case where the concentration of the precursor salt of the metal M is low.
• This salt can be chosen from sodium, potassium or ammonium salts, the anion of which will not cause the precipitation of an insoluble compound with the metal cation M.
• inorganic salts such as halides, sulfates, nitrates and perchlorates, or organic salts such as acetates, lactates and formates.
• this second salt is advantageous. sow potassium chloride, preferably at a concentration of about 0.1 mole / l.
• the electrolyte can also contain, in addition to or in place of the second salt, a complexing compound with respect to the cation M, in order to adapt the conditions for the formation of the compound of M to the window permitted by the reduction of oxygen.
• a complexing compound chosen for example from oxalates, citrates, fluorides, chlorides, iodides and
• L 1 electrolysis is carried out in the presence of dis ⁇ oxygen in the electrolyte.
• the oxygen concentration is fixed between very low values, of the order of
• the oxygen can be dissolved advantageously intro ⁇ reducing in the electrolyte a gas mixture consisting of oxygen and a neutral gas.
• the neutral gas can be argon
• the oxygen concentration of the gas mixture and the gas flow rate in the electrolyte makes it possible to impose a predetermined concentration of oxygen in the electrolyte.
• the oxygen / neutral gas volume ratio is between 1 and 2.
• the potential imposed on the electrochemical cell is kept constant at a predetermined value between the potential for deposition of the metal M in the electrolyte considered and the reduction potential d 'oxygen.
• 3 ⁇ metal M in the electrolyte considered can be easily determined by a person skilled in the art by raising the intensity as a function of the potential in an electrochemical cell analogous to that in which the process of the invention is implemented, in the absence of oxygen.
• Oxygen is provided by the literature.
• the potential for depositing a zinc oxide film on a SnO2 cathode can be set between -0.75 V and -0.1 V vs ENH and for depositing a film of cadmium hydroxide on a gold cathode between -0.24 V and -0.05 V vs ENH.
• the implementation of the method according to the invention generally produces a linear growth in the thickness of the deposit as a function of time.
• the thickness of a film can therefore be predetermined by adjusting the amount of electricity used for the deposit. Thicknesses from a few nm to a few ⁇ m can be obtained.
• the particularly favorable deposition rate is between approximately 0.5 and 1 ⁇ m / h.
• the nature of the compound constituting the film deposited on the electrode of the electrochemical cell can be chosen by appropriately setting the reaction conditions.
• the process of the invention should be carried out under conditions in which the oxide is thermodynamically more stable than the hydroxide.
• favorable conditions are obtained with relatively low deposition rates and high temperatures. Therefore, for obtaining oxides from aqueous solutions, low concentrations of M (i) will be used.
• a concentration is used in Zn (II) preferably less than 10 "2 mol / 1, more particularly less than 5.10 ⁇ 3 mole / 1, a temperature at least equal to 50 ° C, and an oxygen concentration lower than the saturating concentration in the solution.
• the process of the invention should be carried out with a relatively high deposition rate and at a relatively low temperature. These conditions are met when using high M (i) concentrations.
• a concentration of Zn (II) greater than 2.10 -2 mole / 1 is used, a temperature less than 50 ° C and an oxygen concentration less than or equal at saturating concentration.
• the process of the invention leads to the deposition of layers of oxides.
• the anion A is the anion introduced into the electrolyte by the precursor salt of the metal M, or else the anion of the second dissociable salt introduced into the electrolyte to increase its conductivity.
• the anion A is chosen as a function of its propensity to form compounds defined with the metal M and with the hydroxyl ions, and as a function of the properties expected for the film deposited.
• the films obtained by the process of the invention are very adherent to the substrate, which constitutes a fundamental criterion for the applications.
• Another method for activating the substrate consists in depositing a very thin metal sublayer M, of the order of a few nanometers, by application for a very short time (for example of the order of 30 seconds) of a potential more cathodic, before applying the deposition potential of the compound of M.
• the method of the present invention makes it possible to obtain a multi-layer structure consisting of a conductive support layer and an oxide or hydroxide film M (0H) x A y , which constitutes another object of the present invention. .
• the composite structure has various applications. Multi-layer structures comprising a compact film are advantageous, in general, for applications requiring continuous layers. Such structures can be used for example as a chemical or electrochemical sensor or as a catalyst.
• the composite structures can also be used as a transparent electrode in solar cells, in flat luminescent devices, and more generally, in various optoelectronic devices.
• the support layer consists of a thin layer of a material chosen from iron, steels, copper or gold, conductive metal oxides such as for example oxide tin Sn0 2 , indium oxide ln 2 0 3 , mixed indium tin oxide (ITO) or titanium oxide Ti0 2 , semiconductor materials such as silicon, GaAs, InP, Cu (In, Ga) (S, Se) 2 or CdTe.
• the support layer consists of a thin layer of one of the preceding materials, deposited on a glass plate. Multi-layer structures comprising a film with an open structure are used for applications requiring large developed surfaces. Examples of such applications include chemical or electrochemical sensors, and catalysts. The present invention is described below in more detail by concrete examples of implementation of the process of the invention, given by way of illustration, the invention of course not being limited to these examples.
• the device used comprises an electrolysis tank, an electrode, a counter electrode and a reference electrode, all three being connected to a potentiostat.
• the electrolysis tank is provided with a stirring system and means for introducing with a predetermined flow rate an argon / oxygen gas mixture having a predetermined composition.
• the temperature is kept constant at 80 ° C. using a water bath.
• the electrode consists of a film of Sn ⁇ 2 deposited on glass.
• the counter electrode consists of a platinum plate.
• the reference electrode is a mercury sulfate electrode.
• the Sn0 2 electrode was subjected to a treatment which consists in maintaining it for 20 minutes under a potential of -1.3 V / ESM included in the field of reduction of the oxygen, in a KC1 solution (0.1 mole / 1) not containing the metallic element whose oxide is to be deposited, in the presence of dissolved oxygen at saturation.
• an electrolyte is introduced consisting of an aqueous solution of KC1 (0.1 M) and zinc chloride (5.10 ⁇ 3 M).
• the gas mixture is continued to bubbled through the electrolyte and the cell is applied to a potential of -1.3 V relative to the reference electrode (corresponding to a potential of -0 , 65 V vs ENH).
• the reaction is stopped after 1 h 30, and the film obtained has a thickness of 1 ⁇ m, determined using a mechanical profilometer. This thickness is linked to the quantity of electricity consumed during the deposit ( ⁇ 7 C for 5 cm 2 ).
• the oxide film obtained was characterized by different methods.
• X-ray analysis The X-ray diffraction diagram of the zinc oxide film obtained, preferably oriented along the ⁇ 002> axis, shows only the lines characteristic of the hexagonal phase of zinc oxide (20, 1 ° ) and the lines corresponding to the substrate.
• the infrared spectrum of the zinc oxide film obtained presents the band lying around 450-550 cm -1 , characteristic of ZnO. No characteristic band of the hydroxyl ions is visible.
• the film obtained is compact, transparent, smooth and homogeneous.
• the transmission is high, in agreement with the transparency of the film to the eye.
• an abrupt absorption front appears, which indicates the semiconductor character of the film and the presence of a forbidden band which corresponds to that of ZnO at around 3.4 eV.
• Capacitive measurements carried out in an electrolytic medium have shown that the film ZnO obtained was conductive, of type n, and that the apparent doping rate is high, of the order of 10 18 -10 19 cm -3 .
• the method of the invention was implemented under conditions similar to those of Example 1, but omitting the preliminary treatment of the SnO 2 electrode, the latter being simply degreased. Under these conditions, the oxide deposit obtained is made up of a multitude of needles with a hexagonal section, the bases being fixed to the substrate. These needles are well separated from each other and therefore constitute an open structure having a large developed surface. The height of the needles can reach several ⁇ m for a base surface of the order of ⁇ m 2 . It increases with the duration of the deposit.
• the device used is analogous to that used for the preparation of an oxide film and the operating conditions are identical, except as regards the composition of the electrolyte.
• the electrolyte is an aqueous solution of
• the film obtained has a thickness of 0.5 ⁇ m, determined using a mechanical profilometer. This thickness is related to the amount of electricity consumed during the deposit.
• the hydroxide film obtained was characterized according to different methods.
• the X-ray diffraction diagram of the hydroxide film has a preferential orientation along the line to
• the infrared spectrum of the zinc hydroxide film obtained has a dominant band located around 3500 cm --'-, characteristic of hydroxyl ions.
• the characteristic band of the Zn-0 bonds of the oxide around 500 cm ⁇ 1 is not present.
• the film obtained is covering and consists of well-defined hexagonal grains.
• the device used is analogous to that used for the preparation of a zinc oxide film and the operating conditions are identical, except as regards the following points: the potential applied to the cathode is -0.9 V / ref. (-0.3 V vs ENH); the electrolyte is an aqueous solution containing NaC10 4 (0.1 M) and CdCl 2 (5.10 -4 M), saturated with oxygen, at a temperature of 80 ° C; the reaction time is one hour.
• the film obtained has a thickness of 0.3 ⁇ m, determined under electron microscopy.
• the hydroxide film obtained was characterized according to different methods. X-ray analysis We observe the presence of the characteristic line of Cd (OH) 2 on the X-ray diffraction diagram. Electron spectroscopy analysis
• the film obtained has an open structure.
• the apparatus used is similar to that used for the preparation of a zinc oxide film and the operating conditions are identical, except as regards the following points : the potential applied to the tank is -0.15 V vs ENH.
• the electrolyte is an aqueous solution containing KC1 (0.1 mole / 1) and CdCl 2 (10 "2 mole / 1), saturated with oxygen, at a temperature of 50 ° C;
• the film obtained has a thickness of 0.4 ⁇ m, determined under electron microscopy.
• the complex hydroxide film obtained has a covering structure.
• composition Cd (OH) x Cl 1 . x was confirmed by X - ray analysis and by electron spectroscopy analysis.
• the device used is analogous to that used for the preparation of a zinc oxide film and the conditions The operating procedures are identical, except as regards the following points:
• the potential applied to the tank is -0.65 V vs ENH.
• the electrolyte is an aqueous solution at pH 3 containing potassium chloride (0.1 mole / 1), gallium sulfate (7.7xl0 ⁇ 3 mole / 1) and sodium oxalate (6xl0 ⁇ 3 mole / 1) saturated with oxygen, at a temperature of 50 ° C;
• the film obtained after one hour has a thickness of 0.5 ⁇ m, determined under electron microscopy. It is transparent and covering.
• the stoichiometric ratio Ga / 0 determined using a Ga 2 0 3 standard is 0.324.
• the gallium compound obtained therefore corresponds to gallium hydroxide Ga (OH) 3 or to hydrated gallium oxide Ga 2 0 3 .3H 2 0.

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