EP3050065A1

EP3050065A1 - Catalytic transparent electrode consisting of graphene film and application on metal nanoparticles and a method for preparation and respective usages

Info

Publication number: EP3050065A1
Application number: EP14786351.8A
Authority: EP
Inventors: Adélio Miguel MAGALHÃES MENDES; Luísa Manuela MADUREIRA ANDRADE; Rui Alberto Teixeira Cruz; F.D. De Montenegro Baptista Malheiro De Magalhães
Original assignee: Efacec - Engenharia SA
Current assignee: Efacec - Engenharia SA
Priority date: 2013-09-25
Filing date: 2014-09-24
Publication date: 2016-08-03
Also published as: WO2015044881A1

Abstract

This application describes a method for preparation and application of structured films of graphene platelets and metal particles, highly transparent and with catalytic and electrical conduction properties. In particular, the films mentioned above may be advantageously used as counter- electrodes in dye-sensitized solar cells, commonly called DSCs - "Dye-sensitized Solar Cells", as well as in other electrochemical or chemical devices, or in systems whose application also requires high electrical conductivity. Moreover, graphene is a material of great thermal, chemical and mechanical stability, and can therefore be used for applications in environments, and in more aggressive production processes.

Description

DESCRIPTION

"CATALYTIC TRANSPARENT ELECTRODE CONSISTING OF GRAPHENE FILM AND APPLICATION ON METAL NANOPARTICLES AND A METHOD FOR PREPARATION AND RESPECTIVE USAGES"

Technical Field

The present application relates to the method of preparing graphene platelets structured films applied on metal particles aiming at catalyzing chemical and electrochemical reactions. The present application also describes the application of said structured films in electrochemical systems and/or whose application also requires high electrical conductivity.

Background

Graphite may be described as the packing of graphene layers, wherein graphene may be described as carbon nanotubes longitudinally cut and flattened. Graphene is composed of a two-dimensional sheet of carbon atoms arranged in a hexagonal mesh. The great interest in graphene lies in its ultra thin geometry, as it is the thinnest material known, as well as in its high thermal conductivity, electrical conductivity and mechanical strength. The usage of graphene has gained considerable interest due to its potential to form electrical, transparent and flexible conducting films, alternative to transparent conducting oxides, usually known as TCO "Transparent Conducting Oxide". Graphene films also allow processing at high temperatures and etching processes. Moreover, when compared with other promising carbon structures, such as carbon nanotubes, graphene is a cheaper alternative and requires a less complex preparation method.

More recently, carbonaceous materials have been described as having catalytic capabilities, particularly for application in dye-sensitized cells, usually called DSCs - "Dye-sensitized Solar Cells". Dye-sensitized cells are photo-electrochemical cells able to convert solar energy into electrical energy. A typical DSC consists of three main components: one nano-structured mesoporous film of titanium dioxide (T1O2) sensitized with a dye and applied on a glass substrate coated with a transparent conducting oxide (TCO); an electrolyte based on the pair of iodide ions ( I^") /tri-iodide (I^3~); and a glass substrate coated with a transparent conducting oxide to which a catalytic material is applied which acts as counter-electrode (CE) . The CE plays a key role in the DSC since it collects the electrons coming from the external circuit, reducing the tri-iodide present in the iodide electrolyte. This ionic species is subsequently responsible for the regeneration of the dye, seeing that it gets oxidized after the injection of electrons to the conduction band. CEs typically consist of a thin platinum layer, of about 10 nm, catalytic, electrical conductor, transparent and with high corrosion stability. However, due to the shortage and price of this precious metal, it is imperative to develop new CEs using cheaper and more abundant materials and which give rise to DSCs with a similar or higher efficiency than the DSCs using platinum.

The ideal CE shall have a low electrical resistivity and a high catalytic activity in relation to the redox reaction of the couple iodide/tri-iodide, and at the same time being as transparent as possible. Previous studies were able to successfully replicate the catalytic activity shown by Pt through the use of various carbon-based materials such as carbon black, referred to as CB - "Carbon Black", activated carbon or single wall carbon nanotubes, referred to as S NT - "Single Wall Nano Tubes". These are low cost corrosion resistant materials which have good tri-iodide electroreduction activity (I^3~) . However, these CEs show very low transparency; so far it was only possible to obtain transparent CEs with some electrocatalytic activity using carbon nanotubes.

Graphite has a very low catalytic activity on the reduction of I^3~. Yet, some types of graphene have an exceptionally high surface area while exhibiting at the same time the potential to have an electrocatalytic activity comparable to platinum, especially in relation to the tri-iodide reduction in an iodide/tri-iodide redox system [1] . In addition, they have an oxidation potential similar to the Pt, thus having a great stability to electrochemical corrosion. In fact, graphene offers the most attractive combination of multiple properties: transparency, conductivity and catalytic activity. The most promising method for the preparation of graphene, in a cost and large-scale production approach, is by chemical oxidation of graphite and subsequent chemical or thermal reduction. The graphene oxide obtained through this method contains oxygen functional groups such as hydroxyl, carbonyl and epoxide groups, among others. These functional groups in carbon-based materials, together with defects on the mesh surface in the graphene sheets, are the responsible centres for the catalytic activity in relation to the reduction of the couple I³Vl^~ present in the electrolyte of a typical DSC [1] .

The graphene oxide has caused efficient CEs for DSCs. However, transparency is often compromised. Hong et al. have used graphene obtained by chemical reduction of exfoliated graphite oxide and functionalized with 1 pyrene butyrate (PET) dispersed in a composite organic matrix of PEDOT:PSS to create a very transparent CE with a relative efficiency around 30 % lower than the one presented by a DSC with a CE of Pt . The polymer used served as conductive support and was the graphene responsible for the catalysis. The polymeric materials are often used to prevent the agglomeration of reduced graphene particles, but these inhibit the property of graphene to free transfer electrons. Roy-Mayhew et al. prepared graphene CEs obtained by thermal exfoliation of graphite oxide and functionalized with oxygen groups, by using ethylcellulose (EC) as sacrificial agent. But by burning the electrodes at 350°C, and consequently by burning the polymer, a great deal of graphene was required for the film to have enough conductivity to effectively catalyze the electrolyte. Thus, the transparency of the cell was compromised even though the prepared CE presented a relative efficiency equal to that presented by a DSC with a CE of Pt . Furthermore, the graphene CEs prepared without burning or addition of conductive polymers gave very low results of electrocatalytic activity.

Graphene oxide may also be used together with conductive polymers to prepare CEs with perfectly individualized layers and functionalities. Xu et al. developed a CE by using oxidized graphene deposited with a layer by layer technique on top of conducting substrates which, in turn, contained PDDA - poly (diallyldimethylammonium) on its surface. This film was then electrochemically reduced [2] . Thereby, it was possible to obtain transparent CEs as efficient as the CEs traditionally of Pt, for electrolytes with the iodide/tri-iodide couple and with liquid solvents, and so without any ionic liquids. However, this is a highly complex procedure.

The manufacture of CEs for DSCs with graphene oxide may also be accomplished by using other kinds of structures. Guai et al. Placed platinum (Pt) particles on a conducting substrate, having then electrophoretically placed oxidized graphene over the metal particles. Besides being fairly transparent, this CE showed a performance similar to the CE of Pt . However, the produced CE uses metal particles capable of efficiently and by themselves catalyze the reaction taking place inside the DSC. It is also possible to create CEs where the metal particles are deposited on top of an oxidized graphene film which in turn is deposited on top of a conducting substrate [3].

Reasonably transparent and very efficient electrodes, using only graphene composites, or films structured with graphene have already been developed for electrolytes with another kind of ionic species responsible for the catalysis, such as cobalt and ferrocene [4] .

The synthesis of graphene may also be carried out through various techniques. One of the most used techniques is chemical vapor deposition, called CVD - "Chemical Vapor Deposition", from nickel strips. But this technique requires heating at high temperatures, above 1000°C. Recently, Tanaka et al. were able to produce graphene by using CVD with nickel strips, but at lower temperatures, around 800°C, and not requiring an atmosphere rich in unsaturated hydrocarbon gases, such as propene or propyne and flammable, such as hydrogen [5] . Choi et al managed to get graphene at even lower temperatures, around 500°C, using a nickel catalyst but with the use of a precursor, in this case oleic acid [6]. Summary

This application describes a structured film of graphene and metal to be applied over a substrate consisting of:

- a layer of conductive metal oxide over the mentioned substrate ;

metal particles substantially distributed on the surface of the conductive metal oxide layer selected from copper, nickel, iron or mixtures thereof;

-a layer of graphene platelets deposited onto those metal particles.

The intrinsic electrical conductivity of the graphene platelets increases due to the overlapping of its different layers onto the metal particles substantially distributed under the surface of metal oxides, also increasing the conductivity of the film and its catalytic properties in electrochemical reactions.

In an embodiment, the metal particles which comprise the structured film of graphene and metal are substantially distributed forming a monolayer with an average thickness between 1 nm and 1 μπι, which corresponds to a charge of the said metal between 1 x 10 —10 mol.cm2 and 1 x 101 mol.cm2, equidistantly or not distributed and existing or not contact between adjacent particles.

In another embodiment, the metal particles of the graphene and metal structured film have an equivalent diameter between 1 nm and 1 μπι.

Still in a further embodiment, the insulating substrate of the graphene and metal structured film is selected from: glass, ceramic, polymeric, composite or mixtures thereof. In an embodiment, the graphene platelets used in the film have a length between 10 nm and 100 μπι, and a thickness between 0.2 and 10 nm.

In another preferred embodiment, the graphene platelets film, shows in the film a charge of graphene between 0.00005 mg cnT² and 10 mg crrT², and a thickness between 0.2 nm and 10 um.

Still in a further embodiment, the graphene platelets of the film show an atomic ratio of carbon and oxygen between 3 - 150, preferably between 6 - 14.

In an embodiment, the graphene platelets of the film comprise oxygen functional groups just on the outskirts of these platelets.

In another embodiment, the graphene platelets of the film may show or not defects on the surface of the mentioned platelets. Those defects on the graphene surface used in the film may correspond to sp³ bond carbons or to the absence of carbon atoms.

In an embodiment, the film conductive metal oxide layer comprises fluorine doped tin oxide, fluorine doped indium oxide, or mixtures thereof, increasing the conductivity.

In another embodiment, the graphene and metal structured film shows a transmittance in the visible and near infrared spectrum between 20 % and 99 . These values are found by spectrophotometric measurements of graphene and metal structured films deposited in transparent substrates so as to reduce the interference of these substrates in the measurements. The transmittance is a quantitative measure of the transmission properties depending on a range of wavelengths (380 nm to 2500 nm) of the graphene and metal structured films.

Still in a further embodiment, the graphene and metal structured film shows a surface electrical resistivity between 0.1 Ω-sq^"1 - 10⁹ Ω-sq^"1. These values are obtained through electrical resistivity measurements of the surface of the graphene and metal structured films by using equipments which resort to the four-pin method. The surface electrical resistivity is calculated by dividing the volumetric resistivity/specific electrical resistance by the thickness of the graphene and metal structured film.

This application also describes a method for preparing the graphene and metal structured film described above comprising the following steps:

- deposition of a nanoparticle film of a metal element over a substrate;

- preparation and deposition of graphene platelets over the metal nanoparticle film;

- annealing of the graphene platelet film deposited over the substrate covered with the metal particles.

In an embodiment, the method of preparing the graphene and metal structured film still comprises a preliminary stage of coating the substrate with a film of a conductive oxide.

In another embodiment of the method of preparing the graphene and metal structured film the depositions can be made by electrophoretic deposition, cathode sputtering, vacuum deposition, printing, inkjet, spin coating, dipping, Langmuir-Blodgett deposition, layer-by-layer deposition, spraying/aerography . Still in a further embodiment, the graphene platelets of the method of preparing the graphene and metal structured film may be totally or partially reduced, partly functionalized with oxygen groups, as hydroxyl, carbonyls, carboxyls or epoxides groups, whether or not containing defects on the graphene mesh surface.

In an embodiment, the method of preparing the graphene and metal structured film may include the deposition of the graphene platelet film which includes the deposition of a graphene platelet dispersion in a solvent.

In another embodiment, a solvent of the platelet dispersion used in the method of preparing the graphene and metal structured film may not include water.

Still in a further embodiment, the solvent used in the graphene platelet dispersion may be protic or aprotic.

In an embodiment, the solvent used in the platelet dispersion may be ethanol, acetone, or their mixtures.

In another embodiment, the graphene platelet dispersion in the method of preparing the graphene and metal structured film may be prepared by sonication, preferably for a period between 30 min and 16 h.

Still in a further preferred embodiment, the annealing of the film in the method of preparing the graphene and metal structured film includes the heat treatment by annealing in a non-oxidative atmosphere or in vacuum.

In a preferred embodiment of the method of preparing the graphene and metal structured film, the non-oxidative atmosphere may comprise an inert gas, such as N₂, Ar, He, or its mixtures.

In another embodiment of the method of preparing the graphene and metal structured film, the annealing of the film may be accomplished with a heating time between 1 min - 24 h, at a level with a heating temperature ranging from 150°C - 1200°C and with a heating time during the referred level between 1 min and 24 h and with a cooling time ranging from 1 min and 24 h.

Still in a further embodiment, the method of preparing the graphene and metal structured film may still comprise, a subsequent step after annealing, for exposure of the film to ozone generated by ultraviolet radiation, preferably for a period of time between 1 min and 90 min, in an inert, reducing atmosphere or air.

This application also describes an electrode comprising at least one graphene and metal structured film as described above .

In a preferred embodiment, the electrode also comprises an insulating substrate, over which the graphene and metal structured film was deposited, and a film of a conductive oxide between that substrate and the graphene and metal structured film.

In another embodiment, the electrode shows a substrate coated with a metal conductive oxide layer, the latter being replaced by an opaque metal film, compatible in terms of corrosion with the DSC electrolyte and electrical conductor, like titanium or nickel or alloys of these materials . A DSC solar cell comprising an electrode is also disclosed in accordance with the previously described.

In a preferred embodiment, the counter-electrode of the DSC solar cell is in accordance with the previously described electrode .

A light emitting organic diode comprising the electrode with the previously described graphene and metal structured film is also disclosed.

A liquid crystal display comprising the previously described graphene and metal structured film is also disclosed.

General Description

The present invention relates to the method of preparing structured films of graphene platelets and metal particles. That structured film has high catalytic properties in electrochemical reactions, and is also highly transparent. This invention is also applicable to systems requiring high electrical conductivity.

This application describes a process for obtaining a catalytic and conductive film, which may be transparent or not, comprised by a structured set of graphene/metal (or "graphene and metal") . Preferably, the metal particles used shall not be able to efficiently catalyze electrochemical reactions taking place within an electrochemical cell. Another objective of this invention is the use of the previously described film, after being deposited in a substrate which may be conductive, in an electrochemical or electronic device. The method of preparing the graphene/metal structured film is also presented. This method comprises two stages: in the first stage there is a deposition of particles of a metal element in a substrate; in the second one graphene platelets are deposited on top of the substrate containing the metal particles, thus creating a film. The graphene platelets used may be partially or fully reduced, possibly having, in some cases, oxygen functional groups and/or superficial defects in the graphene mesh. After deposition in a substrate, the graphene/metal structured film may be reduced and/or suffer the introduction of superficial defects. This invention thus provides a method to maximize the electrocatalytic activity, conductivity and transparency of the structured film resulting from the modification of the graphene/metal structured film. The process variables are: size/length and thickness of the platelets, their state of reduction, number and type of functional groups, superficial defects and film thickness formed by them, type and amount of metal particles. In general, the thinner the graphene structured films, the greater is their transparency, the bigger the platelets and more reduced the lower levels of oxidation and sp3 bonds, the greater is their electrical conductivity; the number and type of functional groups and superficial defects of the graphene sheets are related to their electrocatalytic activity.

A preferred application of this invention is in the manufacture of the counter-electrode of dye-sensitized solar cells, using any kind of ionic species as electrolyte, in the liquid or gel/solid form - iodide/tri- iodide, cobalt, ferrocene, sulfur and similar. Over the metal oxide transparent film of the substrate, usually made of glass, a graphene/metal structured transparent film is prepared. As described above, this film is treated to have a high electrical conductivity, as well as a high electrocatalytic activity. The resulting transparency, electrical conductivity and electrocatalytic activity are comparable or superior to that exhibited by the conventional platinum counter-electrode.

Description of the Figures

For a better understanding of the technology, some figures are attached representing preferred embodiments of the present invention which, however, are not to be construed as being limiting other possible embodiments falling within the scope of protection.

Figure 1 shows a scheme of the arrangement of a dye- sensitized solar cell in accordance with the present invention. The above mentioned graphene/metal catalytic structured film is applied to the inner surface of the waterproof material (105) . The layer of the graphene/metal structured film is represented in greater detail. The items shown are not to scale. In particular, figure 1 illustrates the following elements:

(101) represents the glass plate coated with a conductive film (TCO), which supports the photoelectrode of the DSC;

(102) represents the photoelectrode or photoanode consisting of a dye-sensitized semiconductor;

(103) represents the electrolyte filling the space between photoelectrode and the counter-electrode;

(104) represents the counter-electrode, consisting of a graphene/metal structured film applied to the inner surface of the waterproof material (105);

(105) represents the waterproof material, for example a glass plate coated with a conductive film (TCO) and with the graphene/metal structured catalytic film (104), which forms the counter-electrode of the DSC;

(106) represents the metal particles applied to the inner surface of the waterproof material (105); and

(107) represents the graphene platelets, deposited over the metal particles (106) .

Figure 2 illustrates complete DSCs prepared with a Pt counter-electrode as a reference and with a counter- electrode with the graphene/metal structured film according to Example 4.

Figure 3 illustrates the DSCs current-voltage curves prepared with a Pt counter-electrode as a reference and with a counter-electrode with the graphene/metal structured film according to Example 4.

Figure 4 shows the Nyquist curves obtained at the DSCs open circuit potential prepared with a Pt counter-electrode as a reference and with a counter-electrode with the graphene/metal structured film according to Example 4; additionally, on the inside, the equivalent circuit used to model the electrochemical behaviour of the cells is shown.

Figure 5 shows the energy conversion efficiencies obtained for the DSCs prepared with a Pt counter-electrode as a reference and with a counter-electrode with the graphene/metal structured film according to Example 4 over several days.

Description of Embodiments

This application describes the manufacturing of a structured transparent film, with catalytic and/or conductive properties, comprising at least one graphene platelet monolayer organized as a mesh and deposited on top of a substrate containing metal particles at the surface. These films are used to form electrodes which can then be used in electrochemical systems or require conducting substrates .

The above mentioned graphene film is prepared from graphene platelets. The term "graphene" used from now on refers to carbon allotropes sheets/platelets, whose structure is represented by planar sheets with monoatomic thickness, composed of carbon atoms linked together in a hexagonal mesh structure. These graphene platelets may be fully or partly exfoliated, fully or partly reduced, functionali zed or not with oxygen functional groups or containing or not defects on the mesh surface of their sheets. It has been demonstrated that graphene platelets with the aforementioned changes have catalytic properties [1]. In addition, the deposition of graphene may be carried out without the mixture of binders and surfactants and without having to be functionalized, besides the preferential introduction of oxygen functional groups and defects on the mesh surface.

The method of preparing graphene/metal structured films comprises the following steps:

a) Modification of graphene platelets from graphene or graphite.

This modification stage is not critical for obtaining the graphene/metal structured film. In order to functionalize the graphene platelets surface different physical methods may be used, such as heating in oxidative or chemical atmospheres, for example the Hummer method, obtaining graphene oxide (GO - "Graphene Oxide") . These modifications are characterized by the concentration of the oxygen functional groups, which can be expressed by the atomic ratio of carbon and oxygen atoms, C/0, and hence the oxidation state of the obtained platelets. The oxidation degree interferes in the final electrical conductivity of the film. In a preferred arrangement the C/O ratio of the obtained graphene platelets must be between 3 and 150, more preferably between 5 and 50 and even more preferably between 6 and 14. Preferentially and consistently, the C/0 ratio of the graphene platelets shall be between 2 and 15. The size/length of the obtained graphene platelets may be controlled by mechanically exfoliating graphene suspensions, for example through sonication or ultrasonication . The suspensions are centrifuged in order to separate the different sizes of platelets. The degree of mechanical exfoliation also allows to control the type and amount of defects present in the surface of the graphene platelets. By varying the degree of exfoliation and/or centri fugation, one can preferably select the type and amount of defects present on the surface of the graphene platelets and the size/average length of the graphene platelets obtained. The increase of the platelet size/length makes it possible for the electrons to move through the graphene sheets with fewer interruptions. Thus, the intrinsic electrical conductivity of the platelets can be manipulated. In a preferred arrangement, the graphene platelets size/average length is controlled by the ultrasonication of a graphene suspension, for a time period between 1 min and 48 h and/or centrifugation between 100 rpm and 15 000 rpm for a time period between 1 min and 48 h. In this preferred arrangement, the size/length of a graphene platelet shall be between 10 nm and 100 μπι. b) Creation of the substrate coated with metal particles .

The substrate over which the graphene/metal structured film will be formed, shall preferably be waterproof (105), such as for example glass or a plastic material like polyethylene terephthalate (PET) , polycarbonate (PC) , polystyrene (PS), among others, rigid or flexible, electrical conductor or not. This substrate shall contain metal particles deposited on its surface (106) . These particles may consist of aluminum, gold, platinum, silver, copper, nickel, iron, cobalt, silicon, zinc, tin, palladium, molybdenum, or a combination of at least two of the aforementioned elements. The deposition of the metal particles can be achieved by electrophoretic deposition, cathode sputtering, vacuum deposition, printing, inkjet, spin coating, dipping, Langmuir-Blodgett deposition, spraying/aerography, among others. In a preferred arrangement, the substrate will be glass covered with a conductive oxide film (TCO) with nickel particles electrophoretically deposited on its surface. In a more preferred arrangement the metal particles form a film with a thickness between 0.01 nm and 10 μπι. c) Formation of the graphene film

The graphene films may be formed as a structured thin film by depositing on a substrate of graphene dispersion, followed by the evaporation of the solvent. The graphene dispersions may be prepared with different types of solvents in accordance with the hydrophobicity which is intended to be given to the platelets. Thus, if one wants platelets with a hydrophilic surface, a protic solvent shall be used, such as water, ethanol or propanol; if one wants platelets with a hydrophobic surface, an aprotic solvent shall be used, such as acetone. In both cases, a preferred procedure will consist of dispersing the graphene material in the selected solvent and submit the dispersion to sonication for a period between 30 min and 16 h. In a more preferred arrangement the solvent used shall be ethanol. The creation of graphene films may be carried out through one of the following techniques: vacuum deposition, printing, inkjet, spin coating, dipping, Langmuir-Blodgett deposition, spraying/aerography, layer-by-layer deposition, electrophoretic deposition, and similar. The films shall be applied over substrates which may be heated or not, followed by evaporation of the solvent. In a preferred procedure, the deposition is done by spraying/aerography of the graphene dispersions over a substrate with metal particles, placed against a heated plate at a temperature between 110°C and 180°C. This creates graphene/metal films. In addition, the transmission of light through the electrode may be controlled by varying the thickness of the deposited film. This thickness is controlled by varying the deposited quantity of graphene platelets per area unit. In a preferred arrangement the graphene charge shall be between 0.00005 mg cnT² and 10 mg cirT² and the thickness of the graphene film between 0.2 nm and 10 μπι. d) Creation of graphene/metal structured film

After deposition of the graphene film on top of the substrate coated with the metal particles, they are subject to a heat treatment, typically an annealing, in a non- oxidative atmosphere or in vacuum. Preferably, the deposited graphene platelets contain functional groups having oxygen and defects on their surfaces. Thus, that annealing allows the preservation of some of the oxygen functional groups while it causes a significant increase in electrical conductivity between and along the graphene sheets due to their reduction. Simultaneously, due to the action of the metal particles, there is a reduction of the electronic resistivity between the graphene plates and the substrate. This reduction leads to the appearance of the graphene/metal structured film. It is possible to vary the C/O ratio of the graphene/metal structured film, by varying the annealing time between 1 min and 24 h, at a temperature between 50°C and 1200°C, and with a cooling time between 1 min and 24 h, regardless of their initial oxidation states. Preferably the obtained final C/0 ratio of the graphene/metal structured film shall be between 3 and 150 and more preferably between 6 and 14. This range of C/0 ratio enables the platelets to have enough catalytic centres to promote an efficient catalysis and to be sufficiently conductive to allow the quick passage of electrons. During this heat treatment the supplied heat eliminates some oxygen functional groups by decomposition, causing an increase in the C/0 ratio, which leads to a realignment of the graphene platelets; consequently the carbon atoms with sp2 bonds, the ones to enable the conduction of the electrons, increase; the films become denser and their surface less irregular leading to a large increase in electrical conductivity of the graphene platelets along and among themselves. In a preferred arrangement of this heat treatment, the graphene/metal films deposited over a substrate with a conductive film shall be heated in an inert atmosphere up to 600°C and then slowly cooled, forming the graphene/metal structured films. In a more preferred arrangement the metal particles are made of nickel or copper. During the annealing stage the metal particles, such as nickel or copper, cause a further reduction of the graphene film, by nucleation, i.e., of the heteroepitaxial growth of graphene characteristic bonds on the crystalline face of the metal. This process causes a higher electronic conductivity between and along the graphene platelets and between the substrate and the graphene film. Simultaneously, the reduction brought about by this heat treatment causes a small decrease in the transparency of the structured film.

In another preferred embodiment, graphene platelets (107) fully reduced, with neither surface structural defects nor functional groups containing oxygen are deposited on top of a substrate containing metal particles (106) . This arrangement may then undergo an annealing in an inert atmosphere as described above. In a more preferred arrangement, graphene/metal films are deposited in a substrate with a conductive film (105), the metal particles being of nickel or copper.

The catalytic activity can also be increased by the introduction of defects on the graphene platelet surface in the above mentioned graphene/metal structured films. As a result, the catalytic activity increases sharply due to the increased number of active centres. Similarly, this introduction of defects enables the modification of the electrical resistivity of the graphene/metal structured films. One way to introduce these defects is by exposure of the graphene platelets of the graphene/metal structured film to ozone. This treatment doesn't cause significant changes in the transmittance of the films. In a preferred arrangement, the graphene films, having or not functional groups containing oxygen and superficial defects, shall be subject to ozone exposure, such as ozone generated by UV, for a time period between 1 min and 90 min, preferably for no more than 30 min, in an inert, reducing atmosphere, or air .

The graphene films mentioned herein refer to a single or multiple layers of graphene platelets. In all the aforementioned preferred arrangements the thickness of each graphene platelet is comprised between 0.2 nm and 10 nm. In these preferred arrangements, the graphene/metal structured film (104) shows a thickness between 0.2 nm and 10 μπι, transmittance in the visible spectrum and near infrared between 20 % e 99 % and an electrical resistivity between 0.1 Ω -sq^"1 e 10⁹ Ω -sq^"1. The optimization of the catalytic activity and the electrical conductivity of the graphene/metal structured film aimed at a given application, thus involves the proper selection of the size/length and thickness of the platelets, their reduction state through the C/O ratio, the number and type of functional groups and number and type of superficial defects of the graphene sheets; furthermore, the type and quantity of metal particles and the characteristics of the annealing stage also affect the properties of catalytic activity and conductivity.

A preferred application of this invention is the manufacture of the counter-electrode of dye-sensitized solar cells as shown in Figure 1, using as an electrolyte any ionic species such as iodide/tri-iodide, cobalt, ferrocene and related, with liquid solvents or as gel. This electrode comprises a substrate with a conductive film

(TCO) (105), on which nickel or copper particles are electrophoretically deposited. On this substrate coated with metal particles (105), a graphene oxide film is applied, preferably with an approximate charge of 0.005 mg cnT² which is heated at 550°C in a nitrogen atmosphere (N₂)

(107) . It was found that the resulting electrode has a transparency and catalytic activity similar to a conventional platinum counter-electrode.

The same electrode described above can be applied in other preferred embodiments as counter-electrode in a solid-state photovoltaic cell, in an organic light-emitting diode (OLED Organic Light-Emitting Diode) or in a liquid crystal display (LCD - Liquid Crystal Display) .

Examples For a better understanding of the invention, some examples are described below of its preferred embodiments, which, however, are not meant to limit the object of this invention .

Example 1 - Preparation of graphene oxide

50 ml of H₂SO₄ were added to 2 g of commercial graphene at room temperature; the solution was cooled to 0°C using an ice bath and then 6 g of KMn0₄ were gradually added. The mixture was heated to 35°C and stirred for 2 h. After that, 300 ml of water were added to the mixture at 0°C (ice bath) . Then H₂O₂ (30% in aqueous solution) was added until the mixture no longer produces gas. After standing for over 12 h the resulting dispersion was decanted and the solid recovered. The latter was then redispersed in distilled water and centrifuged multiple times at 4000 rpm for 15 min until reaching a pH above 4. The final dispersion was decanted and the solid recovered and lyophilized for 48 h. 0.60 g of the material was then exfoliated in 500 g of distilled water at basic pH (~9) , with the aid of NH₃ 6 M, aided by an ultrasound bath for 4 h. The dispersion of the graphene oxide was centrifuged twice in a row at 5000 rpm for 30 min in order to remove the insoluble graphite and the supernatant recovered and lyophilized for 48 h.

Example 2 - Preparation of substrates coated with metal particles of nickel

The glass plates coated with a conductive film of fluorine- doped tin (FTO - Fluorine-doped Tin Oxide) were initially washed by using an ultrasound bath with suitable detergent for 60 min and with ethanol for 45 min. These substrates were then dipped in a solution of nickel consisting of 1.14 M NiS0₄*6H₂0, 0.19 M NiCl₂*6H20 and 0.73 M H₃BO₃ (pH ~ 4.5), at a temperature of 30°C. The electrodeposition of nickel was carried out at a constant voltage of -1 V, using a reference electrode Ag/AgCl (in 4 M KC1) and a platinum mesh as a counter-electrode, for 5 seconds. These substrates were called FTO/Ni .

Example 3 - Preparation of graphene and graphene/metal (nickel) structured films

Ethanolic dispersions of graphene oxide were prepared as in Example 1 with an approximate concentration of 0.1 mg of graphene oxide per dispersion gram and each final dispersion was sonicated using an ultrasound bath, for ~1 h. These dispersions were used to produce films with an approximate charge of 0.01 mg crrT², applied on two glass plates coated with FTO/Ni measuring 2.5 cm x 2.5 cm. The application was performed by spraying the substrates placed on a heated plate at ca. 120°C. For comparison purposes dispersions were also deposited on glass plates coated only with FTO. Graphene oxide films deposited in FTO and FTO/Ni substrates were heated in a tubular oven at 550°C under nitrogen atmosphere for 15 min with a heating ramp of 5°C/min, resulting in graphene/Ni structured films and graphene films.

Transmittance measurements of the prepared films were held. The Figure 2 shows the DSCs assembled with a traditional Pt counter-electrode as a reference and with a counter- electrode with the graphene/metal structured film. Table 1 shows the transmittance values analyzed at a wavelength of 550 nm of the films that form the above mentioned counter- electrodes, and it's possible to realize that both counter- electrodes have similar transmittances . The counter- electrode with just the graphene film also shows similar transmittances . Table 1 - Transmittance values

Example 4 - Preparation of a DSC and half-cells using a graphene/metal (nickel) structured film and a graphene film as counter-electrode

A DSC consists of two glass plates coated on one side with a transparent and electrical conductive film. In one of the glass plates with FTO, and on the conductive film, a 10 μιχι layer of T1O₂ particles with ca. 20 nm diameter was applied, on which a second layer with 4 m of Ti0₂ particles with 400 nm diameter was applied. After sintering at 450°C for 30 min, the photoelectrode was dipped in a N719 dye solution (1 mM in ethanol) for over 12 h so that the latter absorbs a molecule monolayer in the whole Ti0₂ surface. The second glass plate was coated with a reaction catalyst of the I₃ ^~ a 31^" electrolyte reduction, having used the graphene/Ni structured film and the graphene film, both described in Example 3. For comparison purposes, a Pt traditional counter-electrode was prepared by printing. The photoelectrode was then sealed to the counter-electrode through a Surlyn (from DuPont), 25 μπι thick frame. The space between the two electrodes was filled with a iodide/tri-iodide liquid electrolyte of organic solvent base, EL-HPE (of the company Dyesol), comprising the following compounds and their mass fractions: organic iodine salt, 10 % - 30 %; iodine, 10 , inorganic iodide salt, 10 %, guanidinium thiocyanate 10 %, alkyl-pyridine 10 %, in acetonitrile 60 % and valeronitrile 10 - 30%. The catalytic activity of the graphene film and the graphene/Ni structured film was evaluated by the characteristic curves of the DSCs current-voltage prepared as described above. To complement the analysis, an electrochemical impedance spectroscopy was also performed.

The results show that the counter-electrode with the graphene/Ni structured film has an efficiency marginally higher than the platinum counter-electrode - Figure 3 and Table 1, and higher than the graphene film (about 15% more in terms of relative efficiency) ; the counter-electrode with the graphene/Ni structured film has an even lower resistance to charge transfer - Figure 4 and Table 2 - indicative of a greater ability to catalyze the reduction reaction that happens in its interface.

Table 2 shows the DSCs resistances prepared with a Pt counter-electrode as a reference, a counter-electrode with graphene/metal structured film and another counter- electrode just with a graphene film according to Example 4, obtained from the analysis of the corresponding Nyquist curves .

Table 2 - DSCs Resistance Values

Finally, the stability of the counter-electrode with the graphene/Ni structured film was tested and compared with the platinum traditional counter-electrode as illustrated by Figure 5, and it's possible to verify that the graphene/Ni structured film holds over 95 % of its performance after 1000 h and having a similar value to the Pt counter-electrode.

References

1. Roy-Mayhew, J.D., et al., Functionalized graphene sheets as a versatile replacement for platinum in dye- sensitized solar cells. ACS Applied Materials and Interfaces, 2012. 4(5): p. 2794-2800.

2. Xu, X., et al., Electrochemically Reduced Graphene Oxide Multilayer Films as Efficient Counter Electrode for Dye-Sensitized Solar Cells. Sci. Rep., 2013. 3 .

3. Shin, H., et al., Counter electrode for dye-sensitized solar cell in e.g. building integrated solar cell generating system,, has oxidation graphene and metal hybrid film successively laminated on metal layer, UNIST ACAD- IND RES CORP (UNIS-Non-standard) .

4. Kavan, L., et al . , Graphene nanoplatelet cathode for Co (III) / (II) mediated dye-sensitized solar cells. Acs Nano, 2011. 5(11): p. 9171-9178.

5. Tanaka, H., S. Obata, and K. Saiki, Reduction of graphene oxide at the interface between a Ni layer and a Si02 substrate. Carbon, 2013. 59(0): p. 472-478.

6. Choi, J., et al., Flexible transparent electrode useful in a display device e.g. liquid crystal display and an electronic paper like display, comprises a transparent substrate, and a transparent conductive film comprising a graphene sheet. 2009, SAMSUNG ELECTRONICS CO LTD (SMSU) UNIV SUNGKYUNKWAN FOUND CORP COLLABORATI (UYSU-Non-standard) . This embodiment, of course, is in no way limited to the embodiments described in this document and any person skilled in the art may envisage many possibilities of modifying it, sticking to the general idea, as defined in the claims.

The preferred embodiments described above may obviously be combined together. The following claims define additionally some preferred embodiments.

Claims

1. Graphene and metal structured film to cover an insulating substrate comprised of:

- a layer of conductive metal oxide on that substrate ;

- metal particles applied on the surface of the conductive metal oxide layer selected from the following list: copper, nickel, iron and their mixtures ;

- a layer of graphene platelets deposited onto those metal particles.

2. Film according to the previous claim, wherein those deposited metal particles form a monolayer with an average thickness between 1 nm and 1 μιη, to which corresponds between 1 x 1CT¹⁰ mol.cm² and 1 x 1CT¹ mol.cm² of the referred metal.

3. Film according to the previous claims, wherein those metal particles have an equivalent diameter between 1 nm and 1 μΐΐΐ.

4. Film according to the previous claims, wherein the insulating substrate is selected from the following list: glass, ceramic, polymeric, composite or mixtures thereof .

5. Film according to any of the previous claims, wherein the referred graphene platelets have a length between 10 nm and 100 μπι, and a thickness between 0.2 and 10 nm.

6. Film according to any of the previous claims, wherein the referred graphene platelet film has a graphene charge between 0.00005 mg cirT² and 10 mg cirT², and a thickness between 0.2 nm and 10 μπι.

7. Film according to any of the previous claims, wherein those graphene platelets have an atomic ratio of carbon and oxygen comprised between 3 - 150.

8. Film according to claims 1 to 6, wherein the graphene platelets comprise oxygen functional groups only in the periphery of the referred platelets.

9. Film according to any of the previous claims, wherein the referred graphene platelets may or not have defects on their surface.

10. Film according to any of the previous claims, wherein the referred graphene platelet defects correspond to carbons with sp³ bonds or absence of carbon atoms.

11. Film according to any of the previous claims, wherein the conductive metal oxide layer comprises tin oxide doped with fluorine, indium oxide doped with fluorine, or mixtures thereof.

12. Film according to any of the previous claims, wherein the graphene and metal structured film has a transmittance in the visible and near infrared spectrum between 20 % and 99 %.

13. Film according to any of the previous claims, wherein the graphene and metal structured film shows a surface electrical resistivity between 0.1 Ω-sq^"1 and 10⁹ Ω-sq^"1.

14. Method of preparing the graphene and metal structured film described in any of the previous claims, defined by the following steps:

deposition of a nanoparticle film of a metal element over a substrate;

- preparation and deposition of graphene platelets onto the metal nanoparticle films;

- annealing of the graphene platelet film deposited on the substrate covered with the metal particles.

15. Method according to the previous claim comprised of a preliminary coating stage of the substrate with a conductive oxide film.

16. Method according to any of the claims 14 to 15, wherein the referred depositions are made via electrophoretic deposition, cathode sputtering, vacuum deposition, printing, Inkjet, spin coating, dipping, Langmuir- Blodgett deposition, layer by layer deposition, spraying/aerography .

17. Method according to any of the claims 14 to 16, wherein the graphene platelets are fully or partly reduced, partially functionalized with oxygen groups, such as hydroxyl, carbonyl, carboxyl and epoxide groups, and whether or not having defects in the graphene mesh surface .

18. Method according to any of the claims 14 to 17, wherein the said deposition of the graphene platelet film includes the deposition of a graphene platelet dispersion in a solvent.

19. Method according to the previous claim, wherein the solvent of the mentioned platelet dispersion does not include water.

20. Method according to the claims 14 to 19, wherein the solvent of that graphene platelet dispersion is protic or aprotic.

21. Method according to the previous claim, wherein the solvent of the referred platelet dispersion is ethanol or acetone.

22. Method according to any of the claims 14 to 21, wherein the mentioned graphene platelet dispersion is prepared by sonication, preferably for a time period between 30 min and 16 h.

23. Method according to any of the claims 14 to 22, wherein the referred film annealing comprises the annealing heat treatment in a non-oxidative atmosphere or in vacuum.

24. Method according to the previous claim, wherein the referred non-oxidative atmosphere comprises an inert gas, such as N₂, Ar or He, or its mixtures.

25. Method according to any of the claims 14 to 24, wherein the said film annealing is carried out with a heating time between 1 min and 24 h, at a level with a heating temperature between 150°C and 1200°C and a heating time during that level between 1 min and 24 h and with a cooling time between 1 min and 24 h.

26. Method according to the claims 14 to 25, comprising a further step after annealing, of exposure of the film to ozone generated by ultraviolet radiation, preferably for a period of time between 1 min and 90 min, in an inert, reducing atmosphere or air.

27. Electrode comprising at least a metal and graphene structured film as described in the claims 1-13.

28. Electrode according to the previous claim, additionally comprising an insulating substrate, over which the graphene and metal structured film was deposited, and a film of a conductive oxide between that substrate and the graphene and metal structured film.

29. Electrode according to any of the claims 27 and 28, wherein the substrate coated with a metal conductive oxide layer is replaced by an opaque metal film, compatible in terms of corrosion with the DSC electrolyte and electrical conductor, as titanium or nickel or alloys of these materials.

30. DSC solar cell comprising an electrode according to claims 27 to 29.

31. DSC solar cell wherein the DSC counter-electrode is according to the claims 27 to 29.

32. Light emitting organic diode comprising the electrode with graphene and metal structured film described in any of the claims 27 to 29.

33. Liquid crystal display comprising the electrode with graphene and metal structured film described in any of the claims 27 to 29.