WO2014040128A1

WO2014040128A1 - Method for manufacture and assembly of modified graphene nano-sheets into thin film electrodes

Info

Publication number: WO2014040128A1
Application number: PCT/AU2013/001036
Authority: WO
Inventors: Linda ZOU; Baoping JIA
Original assignee: Murdoch University
Current assignee: Murdoch University
Priority date: 2012-09-11
Filing date: 2013-09-11
Publication date: 2014-03-20
Anticipated expiration: 2015-03-11

Description

Method for manufacture and assembly of modified graphene nano-sheets into thin film electrodes

Field of Invention

Method for manufacture and assembly of modified graphene nano-sheets thin film electrodes Background.

The unique two dimensional (2D) planar structure of graphene nano-sheets combined with outstanding electrical conductivity, high optical transmittance, exceptional mechanical flexibility and durability, makes Graphene nano-sheets an ideal candidate for a wide range of potential applications including flat display, optoelectronics, sensors, thin film electrodes for energy conversion and storage system, and electrodes for capacitive deionization water treatment.

For most potential applications, the ability to arrange and control the structures of graphene nano-sheets based thin films is essential and will fundamentally affect their ultimate performances. Technically, epitaxial growth and chemical vapor deposition (CVD) could directly deposit mono or few-layered graphene nano-sheet films on certain substrates, but their high demand on equipment and the rigid reaction conditions as well as the following transferring related issues severely limit the chance of scalable production by these methods.

Comparatively, bottom-up assembly of graphene nano-sheets building blocks presents to be much more convenient and cost-effective due to that they are highly flexible and are compatible with large-scale, heterogeneous production processes. Conventional solution- based routes such as vacuum filtration, dip coating, spin coating and electrostatic sequential adsorption have been used to assemble the graphene nano-sheets into 2D and 3D structures, but, in most cases, these method are lack of control on the thickness and in-plane structure of the assembled films.

Langmuir-Blodgett assembly, on the other hand, presents a possible approach for arranging 2D nano-sheets into high-quality film architecture via controlled compressing the air/liquid interface and promoting their lateral packing. One of the prerequisites of Langmuir-Blodgett assembly is the well dispersed building blocks in a volatile solvent. But in the case of graphene nano-sheets, the strong interlayer restacking tendency makes their assembly a by this technique impractical.

There are several reports on the Langmuir-Blodgett assembly of graphene oxide nano-sheets instead of graphene nano-sheets however graphene oxide does not have all of the desirable properties of graphene. Also graphene tends to be hydrophobic and so the techniques are not without its problems. Hydrophobic graphene tends to aggregate together and therefore establishing an evenly well dispersed coating of graphene is difficult.

The more hydrophilic graphene oxide can be dispersed more easily in water than graphene and form a uniform film at the liquid/air interface. After the coating, the film assembly could be further transformed to graphene nano-sheet film by post-reduction, i.e. using heat treatment. It has been found, these ultrathin nano-sheets films are easily crumpled and deformed into fragments during high temperature post-treatment and therefore the technique is not suitable to produce a graphene nano-sheet having the advantageous physical properties described.

Summary of Invention

A method for fabrication and assembly of electrodes having at least one layer of modified graphene nano-sheet including depositing modified Graphene nano-sheets dispersion onto a subphase, forming a stable floating mono-layer, compressing the stable floating monolayer to the required packing density and depositing the floating stable monolayer onto a substrate.

Preferably the modified graphene nano-sheet is modified by the addition of sulphonate groups.

An electrode including at least one layer of modified graphene nano-sheets wherein at a current density of 20 A/g, the layer exhibits a specific capacitance of at least 198 F/g

In a preferred embodiment an inducer is used in combination with the subphase preferably this inducer is an alcohol and more preferable ethanol or methanol. It is preferred that a number of monolayers are deposited onto the substrate it achieve a substantially uniform and complete coverage of the substrate. In a preferred embodiment it was found that about 10 monolayers deposited on the substrate provide a uniform and substantially continuous coverage of the substrate.

It has been found that when a sufficiently uniform and complete coverage of the substrate is achieved that the electrical properties of the deposited monolayers at a current density of 20A/g exhibit a specific capacitance of at least 198F/g.

In an application of the method an electrode can be prepared by the process comprising depositing modified graphene nano-sheets dispersion onto a subphase, forming a stable floating monolayer, compressing the stable floating monolayer to the required packing density and depositing the floating stable monolayer onto a substrate including at least one layer of modified graphene nano-sheets.

The preferred graphene nano-sheet is a partially sulphonated graphene nano- sheet

An electrode at a current density of 20 A/g, the electrode exhibits a specific capacitance of at least 198 F/g.

Brief Description of the Drawings

Figure 1. Schematic illustration of the Langmuir-Blodgett assembly of graphene nano-sheet monolayer film

Figure 2. Isothermal surface pressure-area (π-Α) plot of sequential compression/expansion cycles of graphene nano-sheet monolayer in Langmuir-Blodgett assembly.

Figure 3. SEM images of graphene nano-sheet films collected at different surface pressures. (a,b) 1 mN/m; (c,d) 8 mN/m; (e,f) 14 mN/m; (g,h) 18 mN/m

Figure. 4 (a) CV curves of 10 layered graphene nano-sheet Langmuir-Blodgett film and (b) corresponding specific capacitance at different scan rate from 0.5 to 10 rnWs; (c) Galvanostatic charge/discharge tests at different current density; (d) Variation of the specific capacitance of graphene nano-sheet Langmuir-Blodgett film as function of cycle number measured at 5 mV/s.

Detailed Description

It will now be convenient to describe the present invention with respect to the accompanying drawings. Modifications and variations to the methods described in the subsequent description may be apparent to the skilled reader of this disclosure. Such modifications and variations are deemed within the scope of the present invention.

In the present invention the above identified problem has been addresses by direct assembly of ordered graphene nano-sheet films, instead of graphene oxide, via Langmuir-Blodgett method. By the direct arranging of already reduced graphene nano-sheet, hash post-reduction thermal treatment is not required and thus assembled graphene nano-sheet film with uniform 2D structures, and highly desirable physical properties can be maintained.

Referring to Figure 1 the overall Langmuir-Blodgett assembly procedure is illustrated A Langmuir-Blodgett trough is a laboratory apparatus (2) that is used to compress monolayers (25) of molecules on the surface of a given subphase (12) and in this application to deposit single or multiple monolayers on a solid substrate (30).

The apparatus (2) consists of a trough (10) to hold a subphase (12) such as water. Within the trough (10) there is contained a number of barriers (20) these barriers are movable to allow for the compaction of the stable floating monolayer (25) at the subphase (12) air interface to achieve a desired packing density.

As with all surface chemistry work cleanliness and purity of components is required. Even small contaminations can have substantial effects on results. Preparation of the apparatus (2) requires that the trough (10) and barriers (20) be thoroughly cleaned by a solvent such as ethanol to remove any residual organics. The liquid subphase (12) is added to a height such that the meniscus just touches the barriers (20).

The Langmuir-Blodgett technique can provide precision control of the film thickness, and is possible to make multilayer structures with varying layer composition. An advantage of Langmuir-Blodgett assembly is that thin films can be deposited on almost at any kind of solid substrate.

In one embodiment of the present invention a homogeneous dispersion of graphene nano- sheets was synthesized through a multistep process of adding functional groups and reduction of graphene oxide. The functional groups in a preferred embodiment are sulphonate but could be any one or a combination of functional groups that have the effect of altering the hydrophilicity of the resultant modified graphene.

In order to directly assemble the graphene nano sheets, by using Langmuir-Blodgett method, it is required to obtain well dispersed building blocks of graphene in a dispensing solution (5) Due to the improved wettability of the modified graphene nano-sheets and hydrophilicity, the modified graphene nano-sheets can be dispersed effectively in an aqueous solution. However such dispersion will not form a stable floating mono layer (25) at the liquid/air interface on the subphase (12) in the apparatus (5).

An inducer is employed to aid in the formation of a stable floating mono-layer (25) at the liquid/air interface on the subphase (12). The chemical used as inducer should be a volatile solvent and good for dispersing the targeted materials. The inducer interacts with the modified graphene nano-sheets and reduces the surface charge, which in turn changes the graphene nano-sheets contact angle to 90 degree and accelerate the graphene nano-sheets accumulation at the liquid/air interface.

Without wishing to be limited to any specific theory it is believed that a possible mechanism of the inducer is that the surface potential of nano-sheets can be reduced by adding external inducer so that the contact angle is approaching 90°. 90° is the most favorable condition for their liquid/air interfacial entrapment.

In the present invention it was found that the modified graphene, can form stable dispersion at the liquid/air interface of the subphase (12) when applied from a dispersion (5) having a water/alcohol solvent. An alcohol such as methanol or ethanol may be employed as an inducer. In the present invention the inducer employed was preferably ethanol. The dispersion (5) was carefully spread onto the subphase (12) in this case water surface with a speed of between 100-300 μΐ/min. After stabilizing for about 20 rnin, a stable floating monolayer (25) is achieved. In the present example modified graphene nano-sheets were compressed by reducing the distance between barriers (20) at a speed of between 2-15 mm/min. Surface pressure was monitored by a tensiometer attached to a Wilhelmy plate (not shown). Then, the resultant compacted stable floating monolayer (27) was transferred onto substrate (30) by vertically dipping the substrate into the trough (10) and lifting up at a speed of between 0.5 and 5 mm/min. in a preferred embodiment the speed of 1 mm/min may be used.

As water was used as sub-phase (12) the substrate surface (30) needs to be hydrophilic for good deposition of compacted stable monolayer (27) onto the substrate (30). The Substrate (30) may consist of materials such as glass and quartz slide, activated carbon or metal alloy or a number of other materials that have the ability to be effectively cleaned to remove contaminants and have sufficient hydrophilic surface for the modified graphene nano-sheet to deposit onto. In the present invention the slide was preferably quartz. The slides were firstly cleaned with ethanol and acetone respectively, and then treated with Piranha solution (H₂S0₄:H2C>2=4:1) for about 30 min to remove all traced of organic contaminants.

For multilayered film coating, the subsequent layers may be coated onto the previously applied compacted stable monolayer (27) in the present invention the compacted stable mono-layer (27) modified graphene nano-sheet film, provides a sufficient hydrophilic surface for efficient subsequent deposition. In a preferred embodiment the compacted stable monolayer (27) is a sulphonated graphene nano-sheet film

This direct fabrication and assembly technique has the advantage of producing a graphene electrode without the need to undergo post assembly thermal treatment The post assembly thermal treatment has been found to have a detrimental effect on the properties of the resultant graphene nano-sheet.

The morphology and structure of the resultant Langmuir-Blodgett film was characterised by Scanning electron microscopy (SEM, Philips XL30) imaging. The cyclic voltammetry (CV) and galvanostatic charge/discharge analysis were carried out. Figure 2 showed the isothermal surface pressure-area (π-Α) plot at 25 °C for the formed graphene nano-sheet film. It was found that the increase of surface pressure was divided into three different stages. In stage I, until the surface area was reduced to 280 cm², no obvious increase in surface pressure was observed with the increased compression before the first turn point.

In stage II, a gradual and steady increase in surface pressure (up to ~16 mN/m) was recorded with continuing increase of compression.

In stage ΠΤ, the increase rate of surface pressure was slowed down after the second turn point. Each stage reflected different type of interaction between the graphene nano-sheets during the compression of Langmuir-Blodgett film. In addition, it was also noticed that the π-Α plot of sequential expansion was almost the same as that of compression, which suggested the monolayer film possessed excellent stability and reversibility during the compression/expansion cycles. The slight shift toward the lower area direction indicated the loss of a small amount of graphene nano-sheet during the compression, which could be assigned to the overlapping, winkle and restacking of the closely packed Graphene nano- sheets.

In Figure 3 SEM images of the film at different stages of the plot were shown. In stage I of the assembly, the Graphene nano-sheets were loosely scattered. It was also found that the Graphene nano-sheets were not uniformly dispersed, some nano-sheets formed clusters-like patterns (Figure 3a, b), which represented a compromised result of restored hydrophobility and modified hydrophilicity of sulphonated graphene nano-sheet.

The overall coverage of the substrate by the graphene nano-sheet film is about 18%. As the nano-sheets were further compressed in the stage Π, these nano-sheet clusters move closer to each other and the increasing inter-sheet repulsion results in the higher surface pressure. SEM images (Figure 3c, d) indicated that the size of each cluster become larger due to the merging of smaller clusters and the coverage of the substrate was increased to about 31%. With compression approaching the second turn point, closely packed graphene nano-sheet monolayer graphene nano-sheet film was formed, and the total coverage by nano-sheets reached 60% at surface pressure of 14 mN/m (Figure 3e, f).

When the pressure was further increased to 18 mN/m in stage III, the densely packed film with obvious folding, overlapping and wrinkles at inter-sheets area were observed and the overall coverage reached 89% (Figure 3g, h).

Figure 4 (a) showed the CV curves of graphene nano-sheet film with 10 times of depositions (lxl cm²) under surface pressure of 18 mN/m. The symmetrical curve with respect to the X- axis indicated that the capacitive process was a reversible and stable process and the quasi- rectangular shape implied graphene oxide electrochemical double-layer capacitance behavior. The calculated specific capacitance from the first cycle reached as high as 287 F/g at a scan rate of 0.5 mV/s, which almost doubled the value of corresponding powder graphene nano- sheet sample. With the increase of scan rate up to 10 mV/s, the specific capacitances were gradually reduced to 198 F/g (Figure 4b).

In Figure 4c corresponding galvanostatic charge/discharge curves are shown. It can be seen that all curves were highly linear and symmetrical at various current densities from 1 to 20 A/g, which represented typical characteristic of standard capacitor. The cyclic stability of the graphene nano-sheet film was also evaluated by repeating the CV test at a scan rate of 5 mV/s for 50 cycles. The specific capacitance as a function of cycle number is presented in Figure. 4d. After 50 cycles, the capacitance decreases only 0.93%, exhibiting excellent cycle stability and a very high degree of reversibility over the entire cycle numbers.

Assembled sulphonated graphene nano-sheets have been fabricated into mono-layered film by Langmuir-Blodgett deposition without any surfactant or stabilizing agent involved.

It has been found that the film packing density and in-plane structure can be tailored to produce different topographical features, namely from loosely scattered, closely-packed to densely over-packed, by adjusting the surface pressure. While folding, overlapping and wrinkles could be generated at the inter-sheet edges as well as interior areas of graphene nano-sheet under high surface pressure. A Multi-layered film can be readily fabricated by repeating the Langmuir-Blodgett deposition.

The advantage of this novel and inventive technique is that the harsh post-reduction by thermal treatment is not required and thus the assembled graphene nano-sheet film with uniform 2D structures can be well preserved.

The specific capacitance of graphene nano-sheet film with 10 layers of coating reached as high as 287 F/g at a scan rate of 0.5 mV/s, which was nearly doubled that for corresponding powder graphene nano-sheet sample. Even at high current density of 20 A/g, the film still exhibited a specific capacitance of 198 F/g. In addition, the graphene nano-sheet film showed excellent cyclic stability in long-term charge/discharge test.

After 50 cycles, the capacitance decreased only 0.93%, exhibiting a very high degree of reversibility over the entire cycle numbers. The outstanding electrochemical properties of resultant multi-layered graphene nano-sheet film showed high potential in applications including as electrodes for super capacitor and for capacitive deionization of brackish water.

Claims

1. A method for fabrication and assembly of electrodes having at least one layer of modified graphene nano-sheet including depositing modified graphene nano- sheets dispersion onto a subphase, forming a stable floating mono-layer, compressing the stable floating monolayer to the required packing density and depositing the floating stable monolayer onto a substrate.

2. The method of claim 1 wherein the modified graphene nano-sheet is a partially sulphonated graphene nano- sheet.

3. The method of claim 1 where an inducer is used in combination with the

subphase

4. The method of claim 3 where the inducer is an alcohol.

5. The method of claim 1 where in a plurality of monolayers are deposited onto the substrate.

6. The method of claim 1 wherein about 10 monolayers are deposited on the

substrate.

7. The method of claims 1 where in the deposited monolayers at a current density of 20A/g exhibits a specific capacitance of at least 198F/g.

8. The method of claims 5 where in the deposited monolayers at a current density of 20A/g exhibit a specific capacitance of at least 198F/g.

9. An electrode prepared by the process comprising depositing modified graphene nano-sheets dispersion onto a subphase, forming a stable floating mono-layer, compressing the stable floating monolayer to the required packing density and depositing the floating stable monolayer onto a substrate including at least one layer of modified graphene nano-sheets.

10. The electrode of claim 9 wherein the modified graphene nano-sheet is a partially sulphonated graphene nano- sheet

11. An electrode of claim 9 wherein at a current density of 20 A/g, the electrode exhibits a specific capacitance of at least 198 F/g.

12. The method of producing an electrode substantially as herein described with reference to the accompanying specification and figures.