Patent Application
20090320894
The object of the present invention is the preparation, through a sol-gel method, of a tungsten oxide (WO3)-based colloidal paste that allows providing transparent films on conductive glasses in an easy and cost-effective manner. The conductive glasses, thus modified by the WO3 film can be used to make electrochromic devices for building glass walls, photoelectrocatalytic devices for the oxidation of organic contaminants and the parallel reduction of water to hydrogen, and for the production of transparent photoanodes that can be tandem-coupled with traditional photovoltaic or photoelectrochemical solar cells, known as Dye Sensitized Solar Cells (DSSCs), in order to obtain the reduction of water to hydrogen by means of solar energy.
Methods for WO3 deposition by means of vacuum sputtering are known and widely used in the industrial field. These methods use expensive equipment, particularly when the surfaces to be processed are large sized and require long deposition times to obtain thickness values in the range of microns.
As an alternative to the sputtering deposition method, chemical methods can be used for preparing WO3 colloidal suspensions. These methods have attracted attention because the method for the preparation and deposition of the WO3 semiconductor layer, which in principle may be carried out by means of screen printing or doctor blading, is cost-effective. The procedures known in the literature do not mention, however, examples of simple preparations of thick films. With “thick films” is meant 2˜3 micron thick films, which are required by the electrochromic devices in order to obtain a good colour contrast and by the photoelectrochemical devices to generate high photocurrents.
Augustynski's group reports in the literature the preparation of tungstic acid colloidal suspensions in water by means of sol-gel method, with procedures involving the transformation of sodium tungstate, Na2WO3, to tungstic acid, H2WO3, by using ion exchange resins. This method requires a long time, and obtaining the WO3 film from the tungstic acid colloid requires the deposition and subsequent heating of at least 6 different layers to obtain a 2 micron-thick film.
The present invention relates to a method for directly preparing colloidal WO3 suspensions which allow providing 2-3 micron thick films by depositing one or at most two layers of colloidal suspension on a conductive glass.
It has been seen that the object described above can be achieved by increasing the density while decreasing the surface tension of the WO3 colloidal suspension.
The sol-gel technique is used for forming the WO3 suspension.
This technique provides the formation of a clear and transparent WO3 colloidal solution. This colloidal solution is formed by treating a tungstate salt, preferably a tungstate of an alkali metal such as sodium tungstate (Na2WO3), in acidic medium to give a WO3 gel. This reaction is carried out in a protic solvent, preferably water. The acidizer is preferably a strong mineral acid, such as hydrochloric acid.
To obtain the colloidal solution from the WO3 gel, thus prepared, the gel is added portionwise to an acidic solution, preferably in the same solvent as used in the first step of the method, which is hold at a temperature preferably ranging between 50° C. and 100° C., more preferably between 65° C. and 75° C. In this case, the acidizer is preferably a carboxylic or polycarboxylic acid such as oxalic, malonic, succinic, glutaric acid.
A transparent and colourless WO3 colloidal solution is thus obtained.
The preparation being the object of the present invention is thus characterized by adding a thickener and a surfactant to the WO3 colloidal solution prepared above. The thickener is preferably a polyethylene glycol-based additive. The surfactant is preferably a non-ionic surfactant. These additives have the function of increasing the density and decreasing the surface tension of the WO3 aqueous colloidal solution. This allows providing a colloidal paste that can be quickly coated on a solid surface by means of screen printing or doctor blading. The thick films obtained with this method have a considerable smoothness and have optimum electrochromic, photoelectrocatalytic and photoelectrochemical characteristics.
The thickener is preferably polyethylene glycol reacted with bisphenol A diglycidyl ether, also known as Carbowax 20000. Alternatively, Mannitol, Glycerol, Ethylenglycol and 200 to about 1000 (average) MW poly PEG can be used.
The surfactant, preferably a non-ionic surfactant, in a particularly preferred embodiment of the invention, is a polyethylene glycol-based surfactant, more preferably selected from polyethylene glycol or a polyethylene glycol-ether or a polyethylene glycol-hexadecyl-ether, or a polyethylene glycol-octadecyl-ether or a polyethylene glycol-dodecyl-ether or a polyoxyethylene-stearyl-ether. In particular, the surfactant can be selected from a group of non-ionic surfactants comprising: Triton X-45, Triton X-100, Triton X-114 Triton X-165, Triton X-305, Triton X-405, Triton X 705-70 Triton CF10, Brij 30, Brij 35 P, Brij 52, Brij 56, Brij 58 P, Brij 72, Brij 76, Brij 78 P, Brij 92V, Brij 96 V.
The thickener is added in an amount ranging between 15% and 25% w/w, preferably between 18% and 23% w/w.
The surfactant is added in an amount ranging between 0.5% and 4% by weight, preferably between 1% and 3% by weight of colloidal paste.
The thickener and the surfactant allow obtaining a WO3 colloidal paste having optimum surface density and tension to obtain a thick and homogeneous film.
The deposition of the WO3 colloidal solution thus obtained on the substrate to be coated, particularly a glass plate, is preferably carried out by the “doctor blading” method (also known as “tape casting”). This method provides that the plate is coated with the colloidal solution of the invention and levelled to the desired thickness by passing a suitable blade (“doctor blade”) thereon.
The substrate thus coated is then subjected to a sintering step, normally at temperatures ranging between 500° C. and 600° C. A WO3 film is obtained, which is perfectly transparent and 2-3 micron thick. The deposition and subsequent heating of the paste can be repeated once again without the characteristics of adhesion, transparency and stability of the film being altered.
The method described in the present invention is simple, cost-effective, reproducible and can be extended to industrial outputs. The preparation of the WO3-based colloidal paste is now described by way of example.
2.5 g Na2WO3 is dissolved in 50 ml H2O mQ. 20 ml of conc. HCl is added dropwise to the solution. (about 1 drop/second). A light yellow colloidal precipitate of a gelatinous consistency is formed, which is then washed three times with H2O mQ at pH 2 to remove the NaCl resulting from the precipitation reaction and the unreacted Na2WO3, if present. The separation of the wash water from the colloid is carried out by means of 4000 rpm centrifugation for 3 minutes.
The WO3 colloidal precipitate is added to a solution consisting of 3-5 g oxalic acid in 10 ml H2O mQ that is maintained at a temperature of 90° C. Additions are carried out portionwise such that they can be completely dissolved.
After the colloid has been completely dissolved, the perfectly transparent solution is cooled at room temperature for about 10 minutes under stirring. A precipitate is formed which results from the crystallization of the excess oxalic acid that is subsequently vacuum filtered with a sintered glass filter, porosity #4. To the filtered solution is added 20% w/w Carbowax 20000, as the thickener, and about 0.015-0.030 g Triton X-100, preferably 0.0020 g, per gram of colloidal paste, as the surfactant, such as to provide the same with optimum density and surface tension for an even distribution on glass surfaces and preparation of transparent films. The films obtained by means of the doctor blading technique are finally sintered at a temperature of 550° C. for 15 minutes.
Characterization of the WO3-Based Films
From the images in
The electrochromic characteristics of a WO3 film (1.2 micron thick) that is obtained by depositing an individual layer of colloidal paste and deposited on conductive glass are illustrated in
Measurements have been carried out in the presence of a lithium perchlorate (LiClO4) solution 0.1M in methoxypropionitrile, acetonitrile or other organic solvent that is not oxidized at potentials of about +1V, and in a cell consisting of the WO3 electrode, as the working electrode, a platinum counter-electrode and a silver electrode, as the reference electrode.
−1V polarization of the WO3 electrode is shown by the appearance of a blue colour. This phenomenon is due to the injection of electrons in the WO3 conduction band. The excess electron charge is stabilized by the presence of lithium ions (Li+) capable of percolating through the WO3 nanoparticles. The blue colour disappears when WO3 is +1V polarized.
The colour process variation from transparent to blue is reversible and no alteration is seen on the film when potential differences are applied in repeated cycles between −1V and +1V.
Since the absorption spectrum of the reduced WO3 film extends to the near-infrared region, by applying these films deposited on conducting glasses building glass walls can be obtained, which in addition to changing colours, can filter the heat from solar radiation.
By irradiating WO3 film with solar light, electrons can be promoted from the valence band to the conduction band of the semiconductor. The absorption spectrum of the semiconductor, in fact, has an absorption band from 450 nm that extends to the ultraviolet region. Under UV-visible irradiation conditions, when 0.7-1 V potential difference is applied between a WO3 film on conductive glass and a platinum electrode, electrons can be promoted to the platinum electrode by maintaining a defect of electron charge, or well, on the WO3 electrode. The oxidizing power of the photogenerated wells is high, amounting to about 2.5 eV, and this allows oxidizing the water or organic species present in aqueous solution and simultaneously reducing the water at the platinum electrode with production of hydrogen.
To the luminous excitation of the semiconductor with light having wavelength less than 450 nm, there corresponds in fact the promotion of an electron to the conduction band e− (CB) and the formation of a well in the valence band h+ (VB), equation 1,
hv . . . WO3→e−(CB)+h+(VB) (1)
The well, due to its oxidizing power, is capable of oxidizing the methyl alcohol, in contact with the WO3 film, according to the equation 2,
h
+(VB)+CH3OH→.CH2OH+H+ (2)
with formation of the hydroxymethyl radical, .CH2OH and a proton, H+. The following oxidation of the hydroxymethyl radical to formaldehyde allows injecting a second electron in the conduction band of the semiconductor, equation 3
.CH2OH→HCHO+H++e−(CB) (3)
The fact that the value of the photocurrent measured in monochromatic light (IPCE) exceeds 100% testifies the system efficacy in oxidizing the methyl alcohol present in the aqueous solution. Finally, it should be noted that the oxidation of the methyl alcohol, as with other organic species, can be mediated and promoted by the formation of .OH radicals deriving from the oxidation of a water molecule on the WO3 electrode, eq 4.
H2O+h+(VB)→.OH+H+ (4)
In addition,
The characteristics of transparency of the WO3 film being the object of the present invention, together with their oxidizing capacity, allow obtaining photoelectrochemical devices for the reduction of water to hydrogen and the simultaneous oxidation of water to oxygen or the oxidation of organic species present in aqueous solution, by means of the solar energy.
That part of light which is not absorbed by the WO3 film is transmitted to the photoelectrochemical (or photovoltaic) device, which when excited produces electrons that can be transferred through an external circuit to a platinum electrode. The reduction of water to hydrogen finally takes place on this electrode.
The efficacy of these devices, which are known as the tandem cells, mainly depends on the transparency characteristics and the thickness of the WO3 film.
In the present invention, there has been developed the preparation of colloidal WO3 suspensions, which allow providing thick films through the deposition of one or at most two layers of colloidal paste by means of screen printing or doctor blading. The preparation is reproducible, easy to apply and is characterized by the use of a thickener and surfactant which have the purpose of increasing the density and decreasing the surface tension of the WO3 colloidal suspension.
The thick films obtained with this preparation have optimum characteristics for use with:
a) Electrochromic devices
b) Photoelectrocatalitic devices for the oxidation of organic substances and the production of hydrogen.
c) Tandem cells for the oxidation of organic substances and the production of hydrogen from aqueous solutions.
The preparing method described in the present invention is finally cost-effective and can be extended to industrial outputs.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/IT06/00084 | 2/17/2006 | WO | 00 | 2/6/2009 |
