WO2012137156A1

WO2012137156A1 - Catalytic and transparent electrode of graphene, preparation method and applications thereof

Info

Publication number: WO2012137156A1
Application number: PCT/IB2012/051672
Authority: WO
Inventors: Adélio Miguel MAGALHÃES MENDES; Rui Alberto Teixeira Cruz; David Alfredo Pacheco Tanaka
Original assignee: Efacec Engenharia E Sistemas, S.A.
Priority date: 2011-04-04
Filing date: 2012-04-04
Publication date: 2012-10-11
Also published as: PT105612A

Abstract

The present invention relates to a method of preparation and application of structured films comprised of graphene platelets (104), displaying catalytic and electrical conductivity properties while being highly transparent. The present invention also discloses techniques targeting to increase the catalytic activity of graphene films. In particular, the aforementioned films can be advantageously employed as counter-electrodes in dye-sensitized solar cells (101, 102, 103, 105), as well as other chemical or electrochemical devices. Moreover, graphene is a material exhibiting high thermal, chemical and mechanical stability and therefore can be used favourably for different applications, under aggressive environments and in aggressive production processes.

Description

D E S C R I P T I O N

"CATALYTIC AND TRANSPARENT ELECTRODE OF GRAPHENE,

PREPARATION METHOD AND APPLICATIONS"

Technical field

The present invention relates to the method of preparation of structured films of catalytic graphene platelets for chemical and electrochemical reactions. The present invention also discloses techniques for increasing the catalytic activity of a transparent graphene film. In addition the graphene film exhibits electrical conductivity properties particularly useful for electrochemical reactions, namely in dye-sensitized solar cells (DSCs).

Background Art

Graphite could be described as a stack of graphene - a flattened carbon nanotube cut along its axis made of a two- dimensional crystalline sheet of carbon atoms arranged in a honeycomb lattice. Graphene has two faces with no bulk in between and therefore reagents can attach to both faces. There is a great interest of graphene because of its ultrathin geometry (it is the thinnest known material) and properties such as high charge carrier mobility, excellent thermal conductivity and high mechanical strength. Up to now, graphene has attracted considerable attention as an alternative to transparent conducting oxides (TCOs) films, typically employed in a vastly array of photoelectronic applications. Graphene has the potential to form flexible, transparent and highly electrically conductive films. Additionally it can be used in etching and high temperature processes. Moreover, when comparing with other promising carbon structures alternatives, such as carbon nanotubes, graphene is a cheaper and practical alternative.

More recently, graphene has been described as having catalytic properties, particularly for dye-sensitized solar cells (DSCs). Dye-sensitized solar cells are photoelectrochemical cells able to convert solar energy into electric current. A DSC is generally comprised of three main components: a) a dye-adsorbed mesoporous nanocrystalline T1O₂ film deposited onto a transparent conductive oxide (TCO) coated glass substrate, b) an iodide/triiodide redox couple-based electrolyte and c) a TCO coated glass substrate covered by a catalytic material acting as a the counter-electrode (CE) . The CE plays a key role in DSC as it collects electrons from the external circuit and reduces (electrocatalyzes ) the redox species in the electrolyte that are used for regenerating the sensitizer after electron injection. Typically, CEs are usually comprised of a thin layer of platinum (Pt) because of its high catalytic activity and high corrosion stability .

However due to the cost and scarce nature of this noble metal, it is advantageous to develop new low-cost CEs using abundant materials that are simultaneously inexpensive and capable of yielding a relatively high solar conversion efficiency for DSCs, similar or possibly better than platinum - Erro! A origem da referenda nao foi encontrada ..

The ideal CE should then have low electrical resistance and high electrocatalytic activity towards the iodide/triiodide redox reaction, while being as transparent as possible. Previous studies have been able to reproduce successfully the catalytic activity exhibit by Pt through the use of different carbonaceous materials - carbon black (CB) (stand alone or with graphite powder) , activated carbon or single wall carbon nanotubes. These are low cost materials, corrosion resistant that have good electrocatalytic activity towards the reduction of triiodide (I^3~) . However, simultaneously transparent and catalytic CEs of graphene were not yet described.

Contrary to graphite that has very poor catalytic activity toward the reduction of I^3~, some forms of graphene retain the same exceptional high surface area shown by graphite while being potentially as electrocatalytic as Pt [1], particularly towards the reduction of triiodide in a iodide/triiodide redox system [1] . Additionally, as it has similar oxidation potential of Pt, exhibits great stability towards electrochemical corrosion. Consequently graphene offers the most attractive combination of multiple properties: transparency, electrical conductivity, and catalytic activity. The most promising path for the production of graphene from both scale-up and cost perspective has been regarded to be through the use of the chemical oxidation graphite method followed by either chemical of thermal reduction [2] . The graphene platelets produced through this method are a partially reduced form of graphene containing oxygen-containing functional groups (such as hydroxyls, carbonyls, epoxides and the like), moieties that along with surface lattice defects present in graphene sheets are said to be responsible (in carbonaceous electrode materials) for the catalytic activity towards the reduction of the I^3~/I^~ system, typically used as electrolyte in DSCs [1, 3-5].

However, up to the present, simultaneously transparent and catalytically efficient graphene-solely based CEs have not yet been obtained. Hong et al . [4] have used 1- pyrenebutyrate (PB^~) functionalized graphene dispersed in a organic matrix composite of PEDOT:PSS to create a very transparent CE with a relative difference in efficiency of less ca. 30 % compared with a Pt CE . The polymer was used to act as the conductive support with graphene being responsible for the catalysis. Polymer matrixes are often used to avoid agglomeration of reduced graphene powder, but they prevent the free flowing of electron-transfer, a characteristic property of graphene [6] . Roy-Mayhew et al.

[1] have prepared CEs made of graphene functionalized with oxygen-containing groups (produced from a thermal exfoliation method) with the aid of a mixture of surfactants and PEO (polyethylene oxide) . However by burning the electrodes at 350 °C, and thus burning out the polymer, a high amount of graphene was needed for the film to exhibit enough conductivity to successfully catalyse the electrolyte. Owing to that, the transparency of the cell was compromised despite achieving a relative difference in efficiency of less -10 % compared with a Pt CE . On the other hand, when graphene-based CEs are prepared without any annealing or polymer matrixes, very low electrocatalytic activities are obtained [7].

Disclosure of the Invention

The present invention relates to a method of preparation of structured films of graphene platelets for catalysing chemical and electrochemical reactions. In addition the graphene film exhibits electrical conductivity properties particularly useful for electrochemical reactions, and may also be highly transparent.

The application of graphene over a substrate is carried out using dispersions of graphene platelets. These platelets are obtained by thermal or chemical reduction of exfoliated graphite oxide flakes. The thus reduced graphene platelets can be fully or partially reduced, have oxygen-containing functional groups and possess surface lattice defects present in the graphene sheets. Upon deposition onto a substrate, the graphene film can be reduced and / or suffer the introduction of surface lattice defects.

The present invention also discloses a method to maximize the electrocatalytic activity, electrical conductivity and transparency of graphene films deposited onto substrates. Broadly, the process variables are the size and thickness of the graphene platelets, their reduction degree, number and type of oxygen-containing functional groups, number and type of surface lattice defects and the thickness of the film formed by the graphene platelets. Generally, films of structured graphene with lower thicknesses or more oxidized are more transparent; larger and more reduced (lower oxidation levels and sp3 bonds) graphene platelets are more electrical conductive; the number and type of oxygen- containing functional groups and of surface lattice defects in the graphene platelets is related with the electrocatalytic activity.

The present invention aims to obtain a catalytic film, which may or may not be transparent, comprised of single or multilayers of graphene platelets. Such film, upon deposition onto an electrically conductive support, can then be used in an electrochemical device. A preferred embodiment of the present invention is the fabrication of counter-electrodes for use in dye-sensitized solar cells, using as electrolyte the triiodide / iodide redox system. Over a transparent conductive oxide (TCO) coated substrate, typically glass, a transparent film comprised of graphene platelets is applied. Afterwards this graphene film is treated as previously described so as to have high electrical conductivity and high electrocatalytic activity. As a result, its transparency, electrical conductivity and electrocatalytic activity are comparable or superior to the conventionally used platinum counter- electrode .

The present invention discloses a method of preparing graphene films (104) to be used as transparent and catalytic electrode, comprising the following steps:

a. deposit onto a substrate (105) graphene platelets dispersed in a solvent;

b. evaporate the solvent;

c. after deposition reduce the film or previously reduce the dispersed graphene platelets, keeping the appropriate C/O ratio for ensuring an efficient catalytic activity of the film, preferably between 3 and 150.

In a preferred embodiment, the thickness of the deposited film is such to ensure its transparency, preferably between 0.2 nm and 10 pm.

In another preferred embodiment, the reduction of the film comprises a thermal treatment of the film by annealing it in under a non-oxidative environment, where the said non- oxidative environment comprises an inert gas, such as N₂, Ar or He, or is rich in unsaturated hydrocarbon, such as propene or propyne. In yet another preferred embodiment, the reduction of the film is carried out with an annealing time between 1 min and 24 h, and an annealing temperature between 50 °C and 1200 °C.

In another preferred embodiment, the preparation method of the graphene film comprises a subsequent step of exposing the film to ozone, preferably for a period of time between 1 min and 90 min.

In another preferred embodiment, the solvent of the referred graphene platelets dispersion is protic, such as ethanol, or aprotic, such as acetone and the referred graphene platelets dispersion is prepared by sonication, preferably for a period of time between 30 min and 16 h, wherein the deposited graphene platelets have a thickness between 0.2 nm and 10 nm and an average size between 10 nm and 100 pm, more preferably between 100 nm and 100 pm.

In yet another preferred embodiment, the substrate is dipped in a 50 wt . % potassium hydroxide aqueous solution, preferably for a period of time between 5 min and 60 min.

In another preferred embodiment, the deposition is carried out by vacuum deposition, spin coating, manual or screen printing, inkjet printing, spin coating, dip coating, Langmuir-Blodgett deposition, spray coating / airbrushing, chemical vapour deposition, electrophoretic deposition, electrostatic layer-by-layer self-assembly or the like.

In another preferred embodiment the method comprises a previous stage of coating the substrate (105) with a transparent conductive oxide layer.

In another preferred embodiment, the referred film of graphene platelets is deposited with a graphene load between 0.00005 mg cm^~2 and 10 mg cm^~2 and preferably comprises several depositions according to the aforementioned steps, wherein preferably the dispersed graphene platelets are fully or partially reduced, functionalized with oxygen-containing functional groups, such as hydroxyls, carbonyls, carboxylics and epoxides, and containing surface lattice defects present in the graphene sheets .

Another aspect of the present invention is a method of preparation of graphene films (104) comprising the following steps for the preparation of the referred dispersion of graphene platelets: a. chemically oxidize graphite flakes dispersed in a suspension, yielding graphite oxide;

b. chemically and / or physically exfoliate the graphite oxide, yielding platelets of graphene oxide;

c. optionally, chemically reduce a graphene oxide suspension, using a reducing agent;

d. centrifuge said suspension in order to separate the graphene oxide platelets within specific size ranges; e. disperse in a suitable solvent the thus obtained platelets .

In a preferred embodiment, the suspension from the referred exfoliation has a basic pH, preferably above 8, more preferably above 9, in particular the pH is adjusted using ammonia comprised in said suspension.

In a preferred embodiment, the reducing agent comprises hydrazine - N₂H₄, ethylene glycol, TBAB or NaBH₄, wherein the mass ratio of reducing agent to graphene oxide is between 0.05:1 and 100:1, preferably according with the desired atomic ratio C/O in the final film. In yet another preferred embodiment the referred C/O ratio should be between 3 and 150, more preferably between 5 and 50 and more preferably between 6 and 14.

Another aspect of the present invention is a graphene film (104) to be used as a catalytic and transparent electrode comprising graphene platelets having a average size between 10 nm and 100 pm, a thickness between 0.2 nm and 10 nm, and with an atomic carbon to oxygen ratio, C/O, such as to ensure an efficient catalytic activity of the film.

In another preferred embodiment, the thickness of the film is such as to ensure its transparency, preferably between 0.2 nm and 10 pm, and a graphene load between 0.00005 mg cm^~2 and 10 mg cm^~2.

In another preferred embodiment, the referred film of graphene platelets exhibits a transmittance between 40 % and 99 % over the entire visible and near infrared spectra and a sheet resistance between 1 Ω sq^-1 and 10⁹ Ω sq^_1.

Another aspect of the present invention is a graphene electrode (104, 105) that comprises one or more of the aforementioned graphene films (104).

In a preferred embodiment, the graphene electrode (104, 105) further comprises a substrate (105), over which the graphene film (104) is deposited, and a layer of a transparent conductive oxide between the aforementioned substrate (105) and the graphene film (104) .

Another aspect of the present invention is a dye-sensitized solar cell (101, 102, 103, 104, 105) comprising the graphene electrode (104, 105), previously described as the counter-electrode .

In a preferred embodiment the dye-sensitized solar cell (101, 102, 103, 104, 105) has the substrate (105) of the graphene electrode as being is one of the transparent impervious layers of the solar cell.

Brief Description of the Drawings

For easier understanding of the present invention it is attached a figure representing a preferable implementation of the invention which, however, does not intend to limit the extension of it. Additionally other figures are also attached .

Figure 1 schematically shows the dye-sensitized solar cell embodiment according to the present invention. The referred catalytic graphene film (104) is applied onto the inner surface of the impervious material (105) . The presented elements are not to scale.

In particular the aforementioned figure shows:

Where (101) represents the impervious material coated with the electrical conductive film (TCO) , that acts as support for the photoelectrode of the DSC;

Where (102) represents the photoelectrode: a nanopart iculated semiconductor sensitized with a dye photoanode ;

Where (103) represents the electrolyte that fills the space between the photoelectrode and the counter-electrode;

Where (104) represents the counter-electrode, where the aforementioned graphene film is applied onto the inner surface of the impervious material;

Where (105) represents the glass substrate coated with the electrical conductive film (TCO, not drew) and the catalytic graphene film (104), which is the counter- electrode of the DSC;

Erro ! A origem da referenda nao foi encontrada. shows the transmittance spectra of the chemically reduced graphene oxide (RGO) and the chemically and thermally reduced graphene oxide (ARGO) films, as prepared in Example 4, both deposited on top of a FTO-covered glass substrate.

Erro! A origem da referenda nao foi encontrada. shows the transmittance spectra of the graphene oxide (GO) and the thermally reduced graphene oxide (AGO) films, as prepared in Example 4, both deposited on top of a FTO-covered glass substrate .

Erro! A origem da referenda nao foi encontrada. shows the transmittance spectra of the Pt and the chemically and thermally reduced graphene oxide (ARGO) films, as prepared in Example 4, both deposited on top of a glass substrate.

Erro! A origem da referenda nao foi encontrada. shows the Nyquist diagrams of measurements taken at an applied potential of -0.4 V, of half-cells made with the Pt and the chemically and thermally reduced graphene oxide (ARGO) counter-electrodes, as prepared in Example 5; additionally inset is shown the equivalent circuit used to model the electrochemical behaviour of the half-cells.

Detailed Description of the invention

The present invention relates to a catalytic film comprising of at least a monolayer of an organized network of graphene platelets.

Platelets of fully or partially reduced graphene, functionalized with oxygen-containing functional groups (such as hydroxyls, carbonyls, and epoxides) and with surface lattice defects have been proved to have catalytic properties [1, 4] . However, it should be noticed that up until now, the state of the art does not describe a way to make such films simultaneously catalytic and transparent, without them being necessarily composite films.

The present invention discloses the preparation of simultaneously catalytic and transparent graphene films to be deposited onto substrates in order to form electrodes. These films do not need to be combined with other materials (such as electrically conductive polymers or other carbonaceous materials) in hybrid/composite structures. Additionally, the disclosing method of depositing graphene exempts the use of surfactants or binding elements (apart from the preferential introduction of oxygen-containing functional groups and surface lattice defects).

The aforementioned catalytic film is prepared from graphene platelets. The term "graphene" used hereinafter relates to sheets / platelets of carbon allotropes, whose structure is represented by one-atom-thick planar sheets of sp2-bonded carbon atoms that are densely packed in a honeycomb crystal lattice. These graphene platelets may be fully or partially reduced, functionalized with oxygen-containing functional groups and having surface lattice defects on their planar sheet .

The preparation method of graphene platelets films comprises : a) Oxidation of graphite flakes, using a chemical oxidation method, for example Hummer's method [8]) yielding platelets of graphite oxide. The amount of oxygen- containing functional groups (that can be expressed by the atomic carbon to oxygen (C/O) ratio), and consequently the oxidation state, present in the obtained graphene platelets is dependent of the time, temperature and reactants used in the reaction. b) Obtaining graphene oxide (GO) platelets through exfoliation of an aqueous suspension of graphite oxide flakes, preferably in a basic pH in order to increase the GO platelets solubility, preferably above 8, more preferably above 9, preferably adjusted with the aid of bases, more preferably ammonia, using mechanical exfoliation (sonication or ultrasonication) . The resulting suspension is centrifuged in order to separate excess material and insoluble graphite. By varying the extent of exfoliation and / or centrifugation the average size of the GO platelets can be controlled. In a preferred embodiment, the size of one platelet of graphene oxide is between 10 nm and 100 pm. c) Reduction of a GO aqueous solution, preferably in a pH basic, preferably above 8, more preferably above 9, adjusted with the aid of bases, more preferably ammonia, yielding chemically reduced graphene oxide (RGO) ; the reduction reaction is carried out by adding a reducing agent such as hydrazine (N₂H₄) , ethylene glycol, TBAB, NaBH₄ and the like, and heating the mixture preferably at a temperature between 30 °C and 90 °C for a period of time between 1 min and 48 h, in particular so that a change in the colour is observed. The C/O ratio of the graphene platelets varies with the time and temperature of the reaction and the reducing power exhibited by the used reducing agent. Owing to that, the C/O ratio varies accordingly with the massic ratio of N₂H₄ to GO (R_N2H4 /GO ) used. The R_N2H4 /GO is preferably between 0.05:1 and 100:1. In a preferred embodiment the C/O ratio in the obtained RGO platelets is between 3 and 150, more preferably between 5 and 50, more preferably between 6 and 14. Preferably and consistently, the C/O ratio of the GO platelets is between 2 and 10, more preferably between 2 and 5. The size of the obtained graphene platelets can be controlled by ultrasonication and / or centrifugation as described before. Both RGO and GO can then be used to create nanostructured films. d) Formation of the graphene films - the RGO and GO films can be formed as thin structured film by deposition of RGO and GO dispersion onto a particular substrate, followed by solvent evaporation. The RGO and GO dispersions can be prepared with a number of different solvents according to the desired surface wettability for the graphene platelets. In view of that, if platelets with a hydrophilic surface are desired, a protic solvent such as water or ethanol or propanol is used; if platelets with a hydrophobic surface are desired an aprotic solvent such as acetone is used. In both aforementioned cases the preferential procedure is to disperse the graphene material in the corresponding solvent and subject the dispersion to sonication for a period between 30 min and 16 h. In a more preferred embodiment ethanol is the used solvent.

The formation of the RGO / GO films can be carried out by one of the following techniques: vacuum deposition, spin coating, manual or screen printing, inkjet printing, spin coating, dip coating, Langmuir-Blodgett deposition, spray coating / airbrushing, chemical vapour deposition, electrophoretic deposition, electrostatic layer-by-layer self-assembly, or the like. The films should be applied onto substrates that may or may not be heated, followed by solvent evaporation. In a preferential procedure, the deposition is performed trough spray coating / airbrushing of RGO or GO dispersions onto a substrate placed on a hot plate at ca. 110 °C - 180 °C. Moreover, the transmittance of light through an electrode can be controlled by varying the thickness of the deposited film. The thickness of the film is controlled by changing the load of graphene / graphite oxide deposited per unit area of substrate. In a preferred embodiment the load of RGO / GO is between 0.00005 mg cm^~2 and 10 mg cm^~2 and the thickness of the RGO / GO film between 0.2 nm and 10 pm. For the same deposition conditions, RGO films are slightly less transparent than GO.

The substrate used herein over which the graphene film is applied, preferably an impervious material (such as glass or a plastic, such as polyethylene terephatalate (PET) , polycarbonate (PC) polysterene (PS) and the like), flexible or rigid, electrically conductive or not. The substrate may be functionalized or submitted to a wettability treatment in order to aid adhesion of graphene. In a preferred embodiment the substrate is glass covered with a fluorine- doped tin oxide (FTO) layer which, prior to the RGO / GO oxide deposition, is dipped in a 50 wt% potassium hydroxide aqueous solution for 15 minutes. This dip-coating treatment induces the attachment of functional hydroxyl groups onto the substrate that aid the adhesion of the graphene platelets .

The graphene films herein disclosed refer to single or multilayers of RGO / GO oxide platelets. In a preferred embodiment the thickness of each RGO / GO platelet is between 0.2 nm and 10 nm.

The catalytic activity of the graphene platelets is related to the size and thickness of the platelets, their reduction state and the number and type of functional groups and surface lattice defects.

The heat treatment (annealing) of films comprised of graphene (RGO or GO) platelets in a non-oxidation environment yield films with improved electrical conductivity. The aforementioned annealing also allows preserving some of the content of the oxygen-containing functional groups, while simultaneously greatly increasing the electrical conductivity of graphene sheets due to their reduction. Additionally, this thermal-induced reduction causes a slight decrease of the film's transparency.

Upon annealing, the platelets of a GO film become reduced, gaining the same characteristics as the ones defined previously for graphene films. Therefore, hereinafter they should be referred as thermally reduced graphene oxide (AGO), yielding AGO films. The annealing of RGO platelets yields chemically and thermally reduced graphene oxide platelets (ARGO) . These are capable of forming ARGO films.

It is possible to vary the C/O ratio of ARGO and AGO films by varying the annealing time between 1 min and 24 h and the annealing temperature between 50 °C and 1200 °C, independently of the oxidation levels of RGO and GO films prior to annealing. Preferably, the C/O ratio should be between 3 and 150, more preferably between 6 and 14. This C/O ratio allows the films to still have enough catalytic centres to efficiently perform catalysis and simultaneously be conductive enough to allow quick electron injection. In an example, RGO films, with an initial C/O ratio of ca. 6.5 - 7, upon annealing at 400 °C under a inert atmosphere for 30 minutes, exhibit a C/O ratio of ca. 10. In a preferred heat treatment of the embodiment, RGO and GO films are annealed under an inert environment, such as N₂, Ar or He, more preferably N₂, yielding ARGO and AGO films, respectively. During this annealing, the heat eliminates oxygen-containing functional groups by thermal decomposition, causing a small increase in C/O ratio, and a realignment of the graphene platelets; consequently the number of carbon atoms with sp2 bonds (responsible for electronic conduction) increases, films become denser and smother leading to an increase of electrical conductivity along and between the graphene platelets. In another preferred embodiment, RGO and GO films are annealed under an environment rich in an unsaturated hydrocarbon, such as propene or propyne - Chemical Vapor Deposition. This allows for a higher increase in intrinsic conductivity of the graphene platelets, as it creates cross-linking bridges between them.

The intrinsic conductivity of RGO and GO platelets can be increased increasing their size. Increasing the size of the platelets decreases the chance of disruptions in electron transport due to a poorly distributed network of platelets. In a preferred embodiment the average size of the GO (and consequently RGO) platelets is controlled by varying the extent of oxidation and exfoliation - ultrasonication - for a period of time between 1 min and 48 h and / or centrifugation between 100 rpm and 15 000 rpm for a period of time between 1 min and 48 h. In this preferred embodiment the graphene platelets average size is between 10 nm and 100 pm.

The catalytic activity can also be increased by introducing surface lattice defects. Consequently this greatly increases the catalytic activity of a graphene film, due to the increase of the active centres. A way of introducing such defects is through exposure to ozone. This procedure does not cause any significant changes in either the light transmittance nor in the sheet electrical resistance of the films. In a preferred embodiment, RGO, AGO and ARGO films are subjected to ozone exposure, such as UV-generated ozone, for a period of time between 1 min and 90 min, preferably less than 30 min, under an inert or reducing atmosphere, or air. Likewise, the surface lattice defects can also be introduced through exfoliation processes as aforementioned - ultrasonication for a period of time between 1 min and 48 h and / or centrifugation between 100 rpm and 15 000 rpm for a period of time between 1 min and 48 h.

The aforementioned techniques disclosed herein, when used individually or in a combined way, enable using only very small loads of graphene to successfully achieve a high catalytic effect, thus rendering more transparent electrodes .

The properties optimization of graphene platelets towards a given final application comprises the correct selection of the size of the platelets, the manipulation of C/O ratio through thermal or chemical treatments (e.g. using hydrazine) and the introduction of surface lattice defects (either using exfoliation processes or ozone exposure) .

Combining the different aforementioned graphene platelets

(GO, RGO, AGO and ARGO) in the formation of a film, it is possible to optimize its catalytic and electrical conductivity properties. In a proposed embodiment, the more reduced (higher C/O ratio) graphene films are deposited first, followed by deposition of the more oxidized graphene films (lower C/O ratio) on top of the first.

A preferable embodiment of the present invention is the fabrication of a counter-electrode for use in dye- sensitized solar cells employing as electrolyte the triiodide / iodide redox system. This electrode is comprised of a transparent conductive oxide (TCO) -coated substrate over which is deposited a graphene film (for example RGO) , preferably with a load of ca. 0.01 mg cm^~2 annealed at ca. 400 °C under a ₂ environment. The resulting electrode yields comparable electrocatalyt ic activity and transparency to that of a traditionally used Pt counter-electrode.

The above mentioned embodiments are combinable.

Examples

The examples given are for illustrative purposes only and are not intended to limit the scope of the invention.

Example 1 - Preparation of graphite oxide

50 ml of H₂SO₄ were added to 2 g of graphite at room temperature; the solution was cooled at 0 °C using an ice bath and then 7 g of KMn0₄ were added gradually. The mixture was then heated at 35 °C and stirred for 2 h. Afterwards 300 ml of water were added into the mixture at 0 °C (ice bath) . Then H₂O₂ (30% in aqueous solution) was added until no gas was produced. The solid was filtered, washed with 250 ml of HC1 (0.1 M) and water (500 ml) . The graphite oxide was dried under vacuum at room temperature for 24 h and then milled using a mortar.

Example 2 - Preparation of a dispersion of graphene oxide (GO)

0.60 g of graphite oxide was first exfoliated in 500g of water, for 4 hours in a basic pH (ca. 8, or preferably ca. 9), with the help of NH3 (6M), using an ultrasonic bath. Then the graphene oxide (GO) dispersion was centrifuged twice at 5000 rpm for 30 minutes to remove the insoluble graphite .

Example 3 - Preparation of a dispersion of chemically reduced graphene oxide (RGO)

200 g of the aforementioned GO dispersion (Example 2) were mixed with 2 g of hydrazine (N2H4 in water, 2M) and 10 g of NH3 (6M) and stirred for 30 minutes. Afterwards the mixture was heated at 90 °C for 3 hours and subsequently centrifuged at 5000 rpm for 30 minutes to remove aggregates and larger graphene platelets.

Example 4 - Preparation of a graphite films: GO and AGO, and RGO and ARGO

Ca. 0.25 cm³ of each of the dispersions from Examples 2 and 3, were mixed in ca. 3 g of ethanol and each of these final dispersions were then sonicated using an ultrasonic bath for ca. 3 - 4 hours. The resulting dispersions were used to produce two films - GO and RGO - so that a load of 0.01 mg CTT² would be applied to two 2.5 cm x 2.5 cm slabs of FTO- covered glass. The deposition was carried out through spray deposition onto the conductive glass substrates placed on a pre-heated hot-plate at ca. 110 °C - 170 °C. GO and RGO films as described above, were annealed in a tube furnace at 400 °C under a nitrogen atmosphere for 60 minutes with a heating rate of 5 °C/min, yielding AGO and ARGO films, respectively.

Transmittance measurements were carried out on the prepared films. Erro! A origem da referencia nao foi encontrada. and Erro! A origem da referencia nao foi encontrada. show that after the annealing treatment both the ARGO and AGO films had a slight decrease in their transparency due to thermal reduction of the graphene platelets. However, the prepared electrodes are very transparent, displaying a transmittance of more than 90 % over the entire visible and near infrared spectra .

For comparison, a conventional Pt counter-electrode was prepared by screen-printing, and a ARGO film, prepared as described above, were both deposited onto a bare glass substrate. Erro! A origem da referencia nao foi encontrada. show that the ARGO film exhibits a very similar transparency compared to that of the Pt film.

Example 5 - Preparation of a DSC and half-cells using a ARGO film as counter-electrode

The DSC comprises two glass sheets coated in one of the sides with a transparent and electrical conductive layer. In one of the glass sheets and on top of the conductive layer of FTO, a 7 μιη-thick layer of 20 nm diameter T1O₂ particles was applied, on top of which a second 5 μιη-thick layer of 400 nm diameter T1O₂ particles was later applied. The other glass sheet was coated with a catalyst for the reduction reaction of 1₃ ^" to 3I^~. In this case, and over the FTO electrical conductive layer it was applied a ARGO film as described in Example 4 - Erro! A origem da referencia nao foi encontrada.. For comparison a conventional Pt counter-electrode was also prepared by screen-printing. The photoelectrode was then sealed with the counter-electrode using a transparent frame of Surlyn (from DuPont) with a thickness of 25 μηι. The space between both electrodes was filled with an iodide / triodide based liquid electrolyte, comprised of the following compounds with the respective mass fractions: organic iodide salt 10 - 30 %, iodine 10 %, inorganic iodide salt 10 %, imidazole compound 10 %, in 3 - methoxypropinitrile, 40 - 60 %.

The catalytic activity of the graphene film was assessed by measuring the electrochemical impedance spectroscopy (EIS) of half-cells. Half-cells were constructed by sealing two identical counter-electrodes and filling the space in between with a similar electrolyte as the one used in the DSCs . Results show that the ARGO counter-electrode performs similarly to the Pt counter-electrode - Erro! A origem da referencia nao foi encontrada..

Finally, the DSC with the graphene counter-electrode yields a similar the performance to that of a DSC with a Pt counter-electrode .

References

1. Roy-Mayhew, J.D., et al . , Functionalized Graphene as a Catalytic Counter Electrode in Dye-Sensitized Solar Cells. Acs Nano, 2010. 4(10) : p. 6203-6211.

2. Li, D., et al . , Processable aqueous dispersions of graphene nanosheets. Nature Nanotechnology , 2008. 3(2) : p. 101-105.

3. Trancik, J.E., S.C. Barton, and J. Hone, Transparent and catalytic carbon nanotube films. Nano Letters, 2008. 8(4) : p. 982-987. 4. Hong, W.J., et al . , Transparent graphene/PEDOT-PSS composite films as counter electrodes of dye- sensitized solar cells. Electrochemistry Communications, 2008. 10(10): p. 1555-1558.

5. Murakami, T. and M. Gratzel, Counter electrodes for DSC: Application of functional materials as catalysts . Inorganica Chimica Acta, 2008. 361(3): p. 572-580.

6. Wang, X., L.J. Zhi, and K. Mullen, Transparent, conductive graphene electrodes for dye-sensitized solar cells. Nano Letters, 2008. 8(1): p. 323-327.

7. Wan, L., et al . , Room-temperature fabrication of graphene films on variable substrates and its use as counter electrodes for dye-sensitized solar cells. Solid State Sciences, 2011. 13(2): p. 468-475.

8. Hummers, W.S. and R.E. Offeman, Preparation of Graphitic Oxide. Journal of the American Chemical Society, 1958. 80(6): p. 1339-1339.

The following claims define additional preferred embodiments of the present invention.

Claims

1. Method of preparation a graphene film (104) for a catalytic and transparent electrode comprising the following steps:

a. deposition onto a substrate (105) graphene platelets dispersed in a solvent;

b. evaporation of the solvent;

c. either subsequent reduction of the film, or previous reduction of said dispersed graphene platelets, or both.

2. Method of preparation the graphene film (104) according to the previous claim, wherein said reduction comprises reducing until achieving an atomic carbon to oxygen ratio, C/O, between 3 and 150.

3. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the thickness of the deposited film is between 0.2 nm and 10 pm.

4. Method of preparation of a graphene film (104) according to any of the previous claims, wherein said reduction of the film comprises a thermal treatment of the film by annealing it under a non-oxidative environment .

5. Method of preparation of a graphene film (104) according to any of the previous claims, wherein said non-oxidative environment comprises an inert gas, such as N₂, Ar or He, or is rich in unsaturated hydrocarbon, such as propene or propyne.

6. Method of preparation of a graphene film (104) according to any of the previous claims, wherein said reduction of the film is carried out with an annealing time between 1 min and 24 h, and an annealing temperature between 50 °C and 1200 °C.

7. Method of preparation of a graphene film (104) according to any of the previous claims, characterized for comprising a subsequent step of exposing the film to ozone, preferably for a period of time between 1 min and 90 min

8. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the solvent of said dispersion of graphene platelets is protic, such as ethanol, or aprotic, such as acetone.

9. Method of preparation of a graphene film (104) according to the previous claims, wherein the said dispersion of graphene platelets is prepared by sonication, preferably for a period of time between 30 min and 16 h.

10. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the substrate is dipped in a 50 wt . % potassium hydroxide aqueous solution, preferably for a period of time between 5 min and 60 min.

11. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the said deposition is carried out by vacuum deposition, spin coating, manual or screen printing, inkjet printing, spin coating, dip coating, Langmuir-Blodgett deposition, spray coating, airbrushing, chemical vapour deposition, electrophoretic deposition or by electrostatic layer-by-layer self-assembly.

12. Method of preparation of a graphene film (104) according to t any of the he previous claims, wherein it comprises a previous stage of coating the substrate (105) with a transparent conductive oxide layer.

13. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the deposited graphene platelets have a thickness between 0.2 nm and 10 nm.

14. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the deposited graphene platelets have an average size between 10 nm and 100 pm.

15. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the deposited graphene platelets have an average size between 100 nm and 100 pm.

16. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the said film of graphene platelets is deposited with a graphene load between 0.00005 mg cm^~2 and 10 mg cm^-2.

17. Method of preparation of a graphene film (104) according to any of the previous claims, wherein it comprises a plurality of depositions according to the described steps.

18. Method of preparation of a graphene film (104) according to any of the previous claims, wherein the graphene platelets of the deposited suspension are fully or partially reduced, functionalized with oxygen-containing functional groups, such as hydroxyls, carbonyls, carboxylics and epoxides, and containing surface lattice defects present in the graphene sheets.

19. Method of preparation of a graphene film (104) according to any of the previous claims, wherein it comprises the following steps for the preparation of said dispersion of graphene platelets:

a. chemical oxidation of graphite flakes dispersed in a suspension, yielding graphite oxide; b. chemical and/or physical exfoliation of the graphite oxide, yielding platelets of graphene oxide ;

c. optionally, chemical reduction of a graphene oxide suspension, using a reducing agent; d. centrifugation of the said suspension in order to separate the graphene oxide platelets within specific size ranges;

e. dispersion in a suitable solvent of the said obtained platelets.

20. Method of preparation of a graphene film (104) according to the previous claim, wherein the suspension of said exfoliation has basic pH .

21. Method of preparation of a graphene film (104) according to the previous claim, wherein the suspension of said exfoliation has a pH above 8, in particular above 9.

22. Method of preparation of a graphene film (104) according to claims 19 - 21, wherein the suspension of said exfoliation comprises ammonia.

23. Method of preparation of a graphene film (104) according to claims 19 - 22, wherein said reducing agent comprises hydrazine - ₂H₄, ethylene glycol, TBAB or NaBH₄.

24. Method of preparation of a graphene film (104) according to claims 19 - 22, wherein the mass ratio of reducing agent to graphene oxide is between 0.05:1 and 100:1, according with the desired atomic ratio C/O in the final film.

25. Method of preparation of a graphene film (104) according the previous claim, wherein said C/O ratio varies between 3 and 150.

26. Method of preparation of a graphene film (104) according the previous claim, wherein said C/O ratio varies between 5 and 50.

27. Method of preparation of a graphene film (104) according to the previous claim, wherein said C/O ratio varies between 6 and 14.

28. Graphene film (104) to be used as a catalytic and transparent electrode, wherein said film comprises graphene platelets having an average size between 10 nm and 100 pm and a thickness between 0.2 nm and 10 nm, and with an atomic carbon to oxygen ratio, C/O, between 3 and 150.

29. Graphene film (104) according to the previous claim, wherein said C/O ratio varies between 5 and 50.

30. Graphene film (104) according to the previous claim, wherein said C/O ratio varies between 6 and 14.

31. Graphene film (104) according to claims 28 - 30, wherein the thickness of said film is preferably between 0.2 nm and 10 pm.

32. Graphene film (104) according to claims 28 - 31, wherein said film of graphene platelets has a graphene load between 0.00005 mg cm^~2 and 10 mg cm^-2.

33. Graphene film (104) according to claims 27 - 32, wherein said film of graphene platelets has a transmittance between 40 % and 99 % over the entire visible and near infrared spectra.

34. Graphene film (104) according to claims 27 - 33, wherein said film of graphene platelets has resistance/square between 1 Ω sq^-1 and 10⁹ Ω sq^-1.

35. Graphene electrode (104, 105) comprising one or more graphene films (104) according to claims 28 - 34.

36. Graphene electrode (104, 105) according to the previous claim, wherein it further comprises a substrate (105), over which the graphene film (104) is deposited, and a layer of a transparent conductive oxide between the aforementioned substrate (105) and the graphene film (104) .

37. Dye-sensitized solar cell (101, 102, 103, 104, 105), comprising a graphene counter-electrode (104, 105) prepared according to claims 35-66.

38. Dye-sensitized solar cell (101, 102, 103, 104, 105) according to the previous claim, wherein the substrate (105) of the graphene counter-electrode is a transparent impermeable outer layer of the solar cell.