U.S. patent application number 16/328570 was filed with the patent office on 2021-03-04 for capacitors, electrodes, reduced graphene oxide and methods and apparatuses of manufacture.
This patent application is currently assigned to Swinburne University of Technology. The applicant listed for this patent is Swinburne University of Technology. Invention is credited to Baohua Jia, Han Lin.
Application Number | 20210065996 16/328570 |
Document ID | / |
Family ID | 1000005252929 |
Filed Date | 2021-03-04 |
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United States Patent
Application |
20210065996 |
Kind Code |
A1 |
Lin; Han ; et al. |
March 4, 2021 |
CAPACITORS, ELECTRODES, REDUCED GRAPHENE OXIDE AND METHODS AND
APPARATUSES OF MANUFACTURE
Abstract
A method, including irradiating graphene oxide (GO) with a beam
of light or radiation to form reduced graphene oxide (RGO) in a
three-dimensional (3D) pattern, wherein the RGO is porous RGO with
pores having sizes tuned by controlling the beam of light or
radiation.
Inventors: |
Lin; Han; (Hawthorn, AU)
; Jia; Baohua; (Hawthorn, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Swinburne University of Technology |
Hawthorn |
|
AU |
|
|
Assignee: |
Swinburne University of
Technology
Hawthorn
AU
|
Family ID: |
1000005252929 |
Appl. No.: |
16/328570 |
Filed: |
August 29, 2017 |
PCT Filed: |
August 29, 2017 |
PCT NO: |
PCT/AU2017/050916 |
371 Date: |
February 26, 2019 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01G 11/32 20130101;
H01G 11/24 20130101; C01B 32/192 20170801; C01B 32/198 20170801;
H01G 11/86 20130101; B05D 3/06 20130101; H01G 11/28 20130101; C01B
2204/22 20130101 |
International
Class: |
H01G 11/86 20060101
H01G011/86; C01B 32/198 20060101 C01B032/198; C01B 32/192 20060101
C01B032/192; B05D 3/06 20060101 B05D003/06; H01G 11/28 20060101
H01G011/28; H01G 11/32 20060101 H01G011/32; H01G 11/24 20060101
H01G011/24 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 30, 2016 |
AU |
2016903449 |
Claims
1.-56. (canceled)
57. A method of forming cross-linked reduced graphene oxide
including: irradiating graphene oxide (GO) in a graphene oxide
solution containing cross-linker, with a beam of light or radiation
wherein the irradiation cross-links and reduces the GO.
58. The method according to claim 1, further including: focusing
the beam of light or radiation to a point on or approximate to the
surface of the GO solution.
59. The method according to claim 1, further including: adding
cross-linkers to the GO solution.
60. The method according to claim 1, further including: oxidising
graphite to form oxidised graphite; and exfoliating the oxidised
graphite in a solvent to form the GO solution.
61. The method according to claim 1, further including: submerging
a substrate in the GO solution to receive the formed RGO.
62. The method according to claim 1, further including: providing a
substrate in the graphene oxide solution to receive the reduced
graphene oxide; and moving the substrate down relative to the
surface of the graphene oxide solution to fabricate a 3D pattern of
reduced graphene oxide.
63. The method according to claim 1, including moving the GO
relative to the beam in a pattern with an anode and a cathode that
are intertwined.
64. The method according to claim 1, wherein the beam of light or
radiation includes a continuous-wave (CW) laser beam or a pulsed
laser beam.
65. The method according to claim 1, wherein the beam of light or
radiation includes a femtosecond laser.
66. The method according to claim 1, further comprising forming the
cross-linked reduced graphene oxide into an electrode.
Description
RELATED APPLICATIONS
[0001] The originally filed specification of the following related
patent application is hereby incorporated by reference herein in
its entirety: Australian provisional patent application 2016903449
(filed 30 Aug. 2016).
TECHNICAL FIELD
[0002] The present invention generally relates to reduced graphene
oxide for electrodes for capacitors and supercapacitors, and to
methods and apparatuses for making the capacitors, the
supercapacitors, and the electrodes.
BACKGROUND
[0003] Supercapacitors (also known as "ultracapacitors" or
"electric double-layer capacitors") are electrochemical capacitors
with capacitance values much higher than other capacitors. Due to
their high energy densities, supercapacitors are widely used for
energy storage and energy supply.
[0004] A typical supercapacitor comprises two electrodes separated
by an ion-permeable membrane ("separator"), and a pair of current
collectors respectively connected to the electrodes.
[0005] For some applications, the electrodes do not have large
enough surface areas to achieve a sufficiently high
capacitance.
[0006] It is desired to address or ameliorate one or more
disadvantages or limitations associated with the prior art, or to
at least provide a useful alternative.
SUMMARY
[0007] In accordance with a first aspect of the present invention,
there is provided a method, including:
[0008] irradiating graphene oxide (GO) with a beam of light or
radiation to form reduced graphene oxide (RGO) in a
three-dimensional (3D) pattern, wherein the RGO is porous RGO with
pores having sizes tuned by controlling the beam of light or
radiation.
[0009] In accordance with the present invention, there is further
provided an electrode, including reduced graphene oxide (RGO) in a
three-dimensional (3D) pattern, wherein the 3D pattern includes a
3D pattern in which anode and cathode are intertwined.
[0010] In accordance with the present invention, there is further
provided an apparatus for making reduced graphene oxide (RGO),
including:
[0011] a container for containing graphene oxide (GO) solution;
[0012] a substrate for receiving the formed RGO;
[0013] an irradiating device for irradiating a beam of light or
radiation, which simultaneously crosslinks and reduces the GO,
thereby forming RGO.
[0014] In accordance with the present invention, there is further
provided a method, including:
[0015] irradiating a solution, including graphene oxide (GO) and
cross-linkers, by a beam of light or radiation to crosslink and
reduce the GO simultaneously, thereby forming reduced graphene
oxide (RGO).
[0016] In accordance with the present invention, there is further
provided a method, including:
[0017] irradiating graphene oxide (GO) with a beam of light or
radiation to form reduced graphene oxide (RGO), wherein the GO
includes a plurality of layers of porous GO film.
[0018] In accordance with the present invention, there is further
provided a method, including:
[0019] irradiating graphene oxide (GO) with a beam of light or
radiation to form reduced graphene oxide (RGO), wherein the GO
includes a GO solution.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Some embodiments of the present invention are hereinafter
further described, by way of example only, with reference to the
accompanying drawings, in which:
[0021] FIG. 1(a) is a cross-sectional diagram of a capacitor
without porous electrodes;
[0022] FIG. 1(b) is a cross-sectional diagram of a supercapacitor
including porous electrodes with pores having large sizes;
[0023] FIG. 1(c) is a cross-sectional diagram of a supercapacitor
including porous electrodes with nanopores;
[0024] FIG. 1(d) is a graph of an exemplary relationship of
specific surface area of a porous material as a function of pore
size;
[0025] FIG. 2(a) is a schematic diagram of an apparatus for
photo-reduction of graphene oxide film attached to a 3D
surface;
[0026] FIG. 2(b) is a schematic diagram of generation of conductive
porous reduced graphene oxide (RGO);
[0027] FIG. 3 is a schematic diagram of an apparatus for
simultaneously cross-linking and reducing graphene oxide from
graphene oxide solution;
[0028] FIG. 4(a) is a graph of an exemplary relationship between
the pore size and laser power;
[0029] FIG. 4(b) is a graph of an exemplary relationship between
the pore size and scanning speed;
[0030] FIG. 4(c) is a graph of an exemplary relationship between
resistivity and conductivity, and the laser power;
[0031] FIG. 5(a) and FIG. 5(b) are schematic diagrams of two types
of a sandwich structure of a RGO supercapacitor;
[0032] FIG. 6(a) and FIG. 6(b) are schematic diagrams of two types
of RGO supercapacitor with interdigital structures;
[0033] FIG. 7(a) and FIG. 7(b) are schematic diagrams of two types
of RGO supercapacitor with a 3D structure: (a) a 3D multilayer
structure, and (b) a 3D intertwined structure;
[0034] FIGS. 8(a) to 8(c) are schematic diagrams showing an
increase in lateral flux of a capacitor using fractal
electrodes;
[0035] FIG. 9 is a flow chart of a described method for forming
RGO;
[0036] FIG. 10(a) to FIG. 10(c) are graphs of XPS spectra of the
graphene oxide before reduction (a), and photo reduced once (b) and
twice (c);
[0037] FIG. 11(a) to FIG. 11(c) are graphs of Raman spectra of a
graphene oxide film prepared using a filtration technique (a), and
photo reduced once (b) and twice (c);
[0038] FIG. 12(a) and FIG. 12(b) are graphs of a Raman spectra of
graphene oxide film prepared using a self-assembly method (a), and
reduced by a femtosecond laser (b);
[0039] FIG. 13(a) to FIG. 13(e) are graphs of XPS spectra of a
drop-casted GO film reduced by the femtosecond laser, with
different pulse widths;
[0040] FIG. 14(a) is a graph of corresponding Raman spectra of the
drop-casted film reduced by the femtosecond laser with the
different pulse widths;
[0041] FIG. 14(b) is a graph of a I.sub.D:I.sub.G ratio showing
defect density, and an I.sub.2D:I.sub.G ratio showing formation of
sp.sup.2 graphene domains of the drop-casted film reduced by
femtosecond laser;
[0042] FIG. 15 is a schematic diagram of an exemplary fabrication
process of RGO interdigital supercapacitors including: (a) a free
standing graphene oxide film; (b) the graphene oxide film attached
to a flexible substrate; (c) gold current collectors deposited on
the graphene oxide film; and (d) graphene oxide supercapacitors
fabricated by laser patterning;
[0043] FIG. 16(a) is an optical photo of the fabricated
supercapacitors, and FIGS. 16(b)-(d) are scanning electron
microscopic images of one of the supercapacitors with different
magnification rates;
[0044] FIG. 17(a) is a 3D schematic of the RGO supercapacitor;
[0045] FIG. 17(b) is a schematic diagram showing different
parameters in the RGO supercapacitors;
[0046] FIGS. 17(c)-(f) are graphs of cyclic voltammetry curves of
the graphene oxide supercapacitors with electrode widths of 50
.mu.m, 100 .mu.m, 150 .mu.m, 200 .mu.m;
[0047] FIGS. 17(g)-(j) are graphs of measured specific capacitances
corresponding to FIGS. 15(g)-(j);
[0048] FIG. 18(a) is a graph of specific capacitances of
supercapacitors with different electrode widths at different
voltage scan rate;
[0049] FIG. 18(b) is a graph of the energy density of the
supercapacitors with different electrode width at different voltage
scan rate;
[0050] FIG. 19 is a graph of the linewidth of the generated RGO
structure for different laser powers;
[0051] FIG. 20(a) is a graph showing a design of a fractal
supercapacitor with Hilbert fractal pattern;
[0052] FIG. 20(b) is an optical photo of the fabricated fractal
supercapacitor with the design of FIG. 20(a); and
[0053] FIG. 21 are graphs showing two designs of fractal
supercapacitor and their measured performances respectively.
DETAILED DESCRIPTION
Overview
[0054] A conventional capacitor, e.g., as shown in FIG. 1(a),
includes two conventional electrodes, a separator between the two
electrodes, and a pair of current collectors (one for each
electrode). The conventional electrodes do not have pores, and
charge is stored on facing surfaces of the conventional electrodes.
The current collectors are electrically connected to the electrodes
to conduct charge from and to the electrodes.
[0055] A supercapacitor, e.g., as shown in FIG. 1(b) and FIG. 1(c),
may include porous electrodes, and charge can attach to porous
surfaces of the porous electrodes, i.e., in the pores as well as on
facing surfaces of the porous electrodes.
[0056] The supercapacitor theoretically has a capacitance C
proportional to a specific surface area A of the electrodes,
i.e.,
C.varies.A. (1)
[0057] The specific surface area A is defined as the total surface
area of a material per unit of mass, or solid or bulk volume.
[0058] Therefore, the capacitance C can be increased by enlarging
the specific surface area A.
[0059] The specific surface area of a porous material theoretically
increases significantly, as pore size decreases. In most
circumstances, the specific surface area is inversely proportional
to the pore size, e.g., as shown in FIG. 1(d) for a theoretical
pore size defined as a radius of the pores.
[0060] Electrodes for supercapacitors may be made from activated
carbon, which typically has a complex porous structure that
provides a high surface area. However, measured capacitances of
supercapacitors with activated carbon electrodes are generally much
lower than calculated "theoretical" capacitances, e.g., due to some
pores in the activated carbon being too small for electrolyte ions
to diffuse into, and because forming a double-layer structure with
a narrow distance between the two layers is difficult.
[0061] Graphene is an allotrope of carbon. Graphene includes at
least one two-dimensional sheet composed of a monolayer of
sp.sup.2-bonded carbon atoms arranged in a hexagonal honeycomb
structure. Graphene has a very stable structure, high conductivity,
high toughness, high strength, and a large specific surface area,
which can be desirable properties for the electrode material in
supercapacitors.
[0062] However, making electrodes directly from graphene has
challenges or limitations. Despite the large surface areas,
supercapacitors formed using a monolayer of graphene may have
limited volume capacitance. Although stacks of graphene layers may
achieve high volume capacitance, the surface may be poorly
accessible to ions due to the small spaces between layers.
Furthermore, conventional methods of producing graphene often
consume a large amount of energy and involve high costs, thus do
not suit mass production.
[0063] Graphene oxide ("GO") is an oxidized form of graphene, in
which the oxygen containing groups are attached to the graphene
basal plane. Graphene oxide can be chemically reduced to convert
the graphene oxide to reduced graphene oxide ("RGO"): RGO is a
material with higher electrical conductivity than GO.
[0064] Described herein is a method for preparing (i.e., making or
manufacturing) a reduced graphene oxide structure for porous
electrodes for a supercapacitor.
[0065] The described method may allow generating one or more pores
of a selected size (e.g., having diameters between 1 and 1000 nm,
known as "nanopores") between the graphene oxide layers, and allow
mass production (i.e., production in large quantities) of the
reduced graphene oxide structure and supercapacitor with electrodes
having the RGO structure. The described method may also allow
simplified (e.g., one-step) fabrication of a supercapacitor having
RGO electrodes, with selected properties such as geometric design
and/or device footprint (i.e., the amount of space the electrode or
the supercapacitor occupies), and allow direct integration of the
supercapacitor with other electric devices. Using the described
method, supercapacitors with RGO electrodes of selectable
two-dimensional (2D) and three-dimensional (3D) structures may be
fabricated in a simple, efficient and low-cost manner.
[0066] The described method for preparing a reduced graphene oxide
structure for porous electrodes for a supercapacitor includes:
irradiating GO with a beam of light or radiation to form RGO.
Porous Graphene Oxide (GO) Film
[0067] In some embodiments, the GO being irradiated with a beam of
light or radiation includes one or a plurality of layers of porous
GO film.
[0068] The porous graphene oxide film employed in the method of the
embodiments described herein comprises a multilayer array
comprising graphene oxide sheets.
[0069] As used herein, the term "multilayer array" generally refers
to an arrangement comprising a plurality of planar graphene-based
sheets that are stacked on one another in an overlapping manner to
resemble a layered structure. Planar sheets in the multilayer array
may partially overlap or completely overlap one another. The
multi-layer array is generally a three-dimensional arrangement.
[0070] The expression "graphene-based" may be used herein as a
convenient reference to material comprising graphene, including
graphene oxide and reduced graphene oxide.
[0071] Planar sheets in a multilayer may be composed of graphene
oxide (e.g. in the case of a graphene oxide film). Alternatively,
the sheets may be composed of reduced graphene oxide or mixtures of
graphene oxide and reduced graphene oxide (e.g. in the case of a
reduced graphene oxide film).
[0072] The porous graphene oxide film used herein comprises
graphene oxide sheets, wherein at least some of the graphene oxide
sheets comprise one or more pores. In some embodiments, a portion
of the graphene oxide sheets in the multilayer array comprise at
least one pore, while a further portion of the graphene oxide
sheets do not comprise a pore. In other embodiments, each graphene
oxide sheet in the graphene oxide film comprises at least one pore.
A skilled person would appreciate that an individual graphene oxide
sheet in the graphene oxide film can comprise a plurality of
pores.
[0073] Pores in a graphene oxide sheet are carbon atoms vacancies
in the plane of the sheet, which disrupt the regular hexagonal
carbon lattice of the sheet. Such pores may be distributed randomly
or with high regularity in a graphene oxide sheet. Depending on
their diameters, the pores may be classified as micropores
(diameters below 2 nm), mesopores (diameters in the range of from
about 2 nm to about 50 nm) or macropores (diameters above 50
nm).
[0074] Graphene oxide sheets in the porous graphene oxide film are
also separated or spaced apart from one another in the multilayer
structure. Accordingly, an interlayer space exists between the
graphene oxide sheets. The extent (i.e. distance) by which graphene
oxide sheets are separated from one another when in the graphene
oxide film may be referred to herein as the separation distance or
interlayer spacing between the sheets.
[0075] The porous graphene oxide film employed in the process of
the embodiments described herein comprises at least one oxygen
containing functional group. In some embodiments, the graphene
oxide film may comprise a plurality of oxygen containing functional
groups. Such oxygen containing functional groups are generally
present in at least one graphene oxide sheet that forms part of the
porous graphene oxide film.
[0076] As used herein, the term "oxygen containing functional
group" is generally a reference to functional groups such as
epoxides, hydroxyls, ketones, ketone pairs, phenols, carboxyls,
cyclic ethers and the like, that are covalently bound to a carbon
atom of a graphene oxide sheet. Such oxygen containing functional
groups may be a result of oxidation reactions.
[0077] In one set of embodiments, the porous graphene oxide film
comprises an oxygen containing functional group situated in at
least one selected from (i) a pore of a graphene oxide sheet and
(ii) between two or more graphene oxide sheets.
[0078] In one set of embodiments, the porous graphene oxide film
comprises an oxygen containing functional group situated both in a
pore of a graphene oxide sheet and in between two or more graphene
oxide sheets.
[0079] Oxygen containing functional groups situated in a pore of a
graphene oxide sheet may be positioned at the edge of the pore.
Pores in a graphene oxide sheet may comprise at least one oxygen
containing functional group and may comprise a plurality of oxygen
containing functional groups. When an individual graphene oxide
sheet comprises a plurality of pores, each pore may comprise at
least one oxygen containing functional group.
[0080] Oxygen containing functional groups that are situated in
between two or more graphene oxide sheets may be covalently bound
to a surface of a graphene oxide sheet and extend from the basal
plane of the graphene oxide sheet into the interlayer space that
exists between overlapping sheets. In this manner, overlapping
graphene oxide sheets can be spaced apart or separated from one
another by the oxygen containing functional groups. The porous
graphene oxide film comprises at least one oxygen containing
functional group, and may comprise a plurality of oxygen containing
functional groups, situated in between two or more graphene oxide
sheets.
[0081] In one set of embodiments, porous graphene oxide films
useful for the embodiments described herein have a high oxidation
degree. A porous graphene oxide film having a high oxidation degree
may comprise an amount of oxygen containing functional groups to
provide an oxygen content of at least about 15%, preferably at
least about 20%, more preferably at least about 25%, in the
graphene oxide film.
[0082] The oxygen content of the porous graphene oxide film may be
determined by suitable techniques. For example, oxygen content and
hence oxidation degree may be determined by X-ray photoelectron
spectroscopy (XPS), which measures the type and percentage of each
type of chemical element present in a material. In one form,
graphene oxide sheets forming the graphene oxide film have a carbon
to oxygen (C:O) ratio as determined by XPS in a range of from about
2:1 to about 4:1, preferably from about 2.5:1 to 3:1.
[0083] Porous graphene oxide films with a high oxidation degree may
have a large number of pores in the graphene oxide sheets as well
as a large interlayer spacing between the sheets. For instance, a
porous graphene oxide film with a high degree of oxidation may have
graphene oxide sheets that are separated by a distance of up to 8
.ANG..
[0084] Further, porous graphene oxide films with a high oxidation
degree may have a high electric resistivity. In some embodiments,
the graphene oxide film without reduction may be used as the
separator of a supercapacitor, which requires high electric
resistivity to prevent self-discharge. Thus, the use of a graphene
oxide film with a high oxidation degree (e.g., about 28%) may be
advantageous.
[0085] The porous graphene oxide film employed in the process of
the embodiments described herein may be obtained from commercial
sources. Alternatively, the porous graphene oxide film may be
synthesised from graphite, for example, by generating GO film from
GO solution.
Graphite Oxide (GO) Solution
[0086] The GO solution used for forming the GO film may be prepared
by: [0087] oxidising graphite to form oxidised graphite; [0088]
exfoliating the oxidised graphite in a solvent to form a graphene
oxide solution.
[0089] An exemplary process of preparing the graphene oxide
solution is described below.
Oxidisation of Graphite
[0090] In some embodiments, purified natural graphite powder (e.g.,
natural graphite powder of ultrahigh purity) may be used to for
oxidised graphite.
[0091] Graphite may be oxidised using conventional methods to
produce graphite oxide. In some embodiments, oxidising methods such
as Hummers method (Journal of the American Chemical Society, 1958,
80(6), 1339) or modified Hummers method (ACS nano, 2010, 4(8),
4806) may be employed.
Exfoliation of Graphite Oxide
[0092] The graphite oxide that is produced from the oxidation of
graphite comprises a plurality of planar graphene oxide sheets,
with each of the graphene oxide sheets comprising at least one
oxygen containing functional group.
[0093] The graphite oxide is exfoliated to produce sheets of
graphene oxide. The exfoliation of the graphite oxide may be
performed using exfoliation techniques and conditions known in the
art.
[0094] In some embodiments, the graphite oxide can be suspended in
a solvent and exfoliated in the solvent under conditions sufficient
to cause separation of the graphene oxide sheets, resulting in the
formation of a graphene oxide solution. The graphene oxide solution
comprises separated sheets of graphene oxide suspended in the
solvent. The separated graphene oxide sheets may be in monolayer or
few-layer form.
[0095] The graphite oxide may be suspended in any suitable solvent.
In one set of embodiments, the graphite oxide is suspended in an
aqueous solvent. In one embodiment the aqueous solvent is
substantially free of organic solvent. In one preference, the
aqueous solvent is water. The use of an aqueous solvent allows the
graphene oxide film to be prepared in an environmentally friendly
manner.
[0096] The exfoliation of graphite oxide in a solution can be
performed using a suitable exfoliation technique.
[0097] In one set of embodiments, a graphite oxide in a solution
may be subjected to mechanical exfoliation to produce graphene
oxide sheets, which are then dispersed in the solvent. Mechanical
exfoliation may be achieved using sonication.
[0098] A person skilled in the art would appreciate that sonication
involves the application of sound energy to agitate the graphite
oxide and ultimately result in disruption of the graphene oxide
lattice layers in the graphite material. Disruption of the lattice
layers leads to separation of the layers of graphene oxide sheets.
Sonication means and conditions known to be useful for exfoliating
graphite oxide may be used. Sonication may be performed with a
sonifier or sonication bath.
[0099] In some embodiments, graphite oxide may be sonicated at a
frequency in a range of from about 20 kHz to about 400 kHz,
preferably at a frequency of about 20 kHz.
[0100] In one set of embodiments, graphite oxide is ultrasonicated
to produce graphene oxide sheets.
[0101] Sonication may be carried out for a time period ranging from
seconds to hours. Time periods may vary depending on for example,
the quantity of graphite oxide to be exfoliated and the frequency
of sonication. In one set of embodiments, the graphite oxide may be
sonicated for a time period in a range of from about 5 minutes to
several hours, preferably from about 20 minutes to about 1 hour,
more preferably for about 30 minutes.
[0102] After exfoliation of the graphite oxide in solution, a
graphene oxide solution is then formed. The graphene oxide solution
may comprise graphene oxide in monolayer and/or in few-layer form.
Few-layer form may comprise from 2 to 10 graphene-based sheets.
[0103] At least some of the graphene oxide in the graphene oxide
solution comprise at least one pore. In some embodiments, at least
some of the graphene oxide in the solution comprise a plurality of
pores. The pores may arise as a result of defects that are
introduced in the sheets of graphene oxide.
[0104] The graphene oxide solution can be used to form a porous
graphene oxide film. The graphene oxide film may be prepared using
conventional film formation techniques that would be known to a
skilled person.
Formation of the Porous GO Film
[0105] The graphene oxide film may be formed by film formation
techniques that would be known to a skilled person.
[0106] In one set of embodiments, formation of a porous GO film
involves applying a graphene oxide solution to a substrate to form
a coating and removing the solvent from the coating to leave a
porous graphene oxide film on the substrate. If desired, the
resulting graphene oxide film may be removed from the substrate.
For example, the film may be peeled off the substrate.
[0107] In some embodiments, the porous graphene oxide film may be
prepared by at least one film forming technique selected from
filtration, spin coating, spray coating and drop casting.
Filtration
[0108] In one set of embodiments, a graphene oxide solution is
subjected to a filtration process to form a porous graphene oxide
film. An example of a filtration process is described in Dikin, D.
A. et al, Nature 448, 457-460 (2007). The graphene oxide solution
may be passed through a filter substrate in order to form a porous
graphene oxide film. The porous graphene oxide in the solution is
thereby retained on the filter substrate while the solvent passes
through. Filtration of the graphene oxide solution may be aided by
a vacuum filtration apparatus. The overall dimensions of the porous
graphene oxide film may be influenced by the filtration setup,
including the size of the filter substrate, while the thickness of
the graphene oxide film may be controlled by adjusting the amount
of graphene oxide in the solution and the time of filtration. A
free-standing (i.e. unsupported) porous graphene oxide film may be
produced by removing the as prepared film from the filter
substrate.
Spray Coating
[0109] In one set of embodiments, formation of the porous graphene
oxide film may involve a spray coating process. An example of a
spray coating process is described in Moon, In Kyu, et al,
Scientific Reports 3 (2013). In some embodiments, a graphene oxide
solution is sprayed onto a substrate to form a porous graphene
oxide film. The graphene oxide solution may be sprayed onto the
substrate using a suitable spray device, such as a spray gun. The
sprayed graphene oxide solution thereby coats the surface of the
substrate. In carrying out the process, the substrate may be heated
to allow the solvent from the sprayed graphene oxide solution to be
rapidly removed by evaporation after the coating is applied. When
the solvent is an aqueous solvent (for example, water), the
substrate may be heated at a temperature of up to about 80.degree.
C. The thickness of the porous graphene oxide film may be
controlled by the concentration of graphene oxide in the solution
and/or the amount of graphene oxide solution applied to the
substrate. Application of the graphene oxide solution can be
controlled by adjusting the flow rate of the solution and/or the
spray time. The flow rate of the graphene oxide solution spray can
be controlled by the nozzle size of the spray device and the
pressure at which the spray of graphene oxide solution is
applied.
Spin Coating
[0110] In one set of embodiments, formation of the porous graphene
oxide film may involve a spin coating process. Spin coating may be
used to deposit uniform thin graphene oxide film onto flat
substrates. An example of spin coating process is described in Guo,
Yunlong, et al, ACS nano 4.10 (2010): 5749-5754. In some
embodiments, a graphene oxide solution may initially be applied to
a stationary or spinning substrate that is rotating at a low speed.
The substrate is subsequently rotated at high speed in order to
spread the graphene oxide solution on the substrate by centrifugal
force. Rotation is continued while the fluid spins off the edges of
the substrate, until the desired film thickness is achieved. The
thickness of porous graphene oxide film may be controlled by the
spinning speed, which can vary from 400 to 6000 revolutions per
minute (rpm).
Drop Casting
[0111] In one set of embodiments, formation of the porous graphene
oxide film may involve a drop casting process. An example of a drop
casting process is described in El-Kady, Maher F., et al, Science
335.6074 (2012): 1326-1330. In such embodiments, a graphene oxide
solution may be dropped onto a substrate to form a coating on the
substrate. The coating is then dried under ambient atmospheric
conditions to remove the solvent from the coating and form a
graphene oxide film. To accelerate the drying process, a flow of
air may be passed over the coating. The size of the substrate
and/or the size of the drops may determine the size of the porous
graphene oxide film. The thickness of the graphene oxide film may
be determined by the concentration of graphene oxide in the
solution.
Spacers
[0112] In one form of the embodiments, the porous graphene oxide
film may further comprise one or more spacers. When present, the
spacers are generally situated in between two or more graphene
oxide sheets of the graphene oxide film.
[0113] Spacers may be derived from one or more suitable spacer
compounds. For example, spacers may be polymeric spacers, which are
derived from one or more polymeric compounds. When present, spacers
may act in conjunction with oxygen containing functional groups to
control the interlayer spacing between graphene oxide sheets in the
graphene oxide film. For instance, spacers may help to enlarge the
interlayer spacing between graphene oxide sheets, such that the
separation distance between the sheets is greater than that
observed without the spacer.
[0114] Depending on the nature of the spacer, the mechanical
properties of the porous graphene oxide film and consequently, the
porous reduced graphene oxide film, may be altered by the presence
of the spacer.
[0115] In some embodiments, spacers can act to crosslink graphene
oxide sheets, such that a porous crosslinked graphene oxide film is
then produced. In this manner, at least two graphene oxide sheets
in the multilayer array may be covalently bonded together via
crosslinks provided by the spacer.
[0116] In some embodiments, a porous crosslinked graphene oxide
film comprises at least one graphene oxide sheet that is
crosslinked to a graphene oxide sheet overlapping it via a spacer.
Preferably, the spacer is bonded to each of the graphene oxide
sheets and extends between the graphene oxide sheets.
[0117] In embodiments, crosslinking occurs prior to the reduction
process so as to produce a porous crosslinked graphene oxide film.
A crosslinked graphene oxide film may help to ensure that the
resulting porous reduced graphene oxide film formed after the
reduction process maintains its physical integrity and does not
degrade or dissolve when in use.
[0118] Spacer compounds useful for producing a porous crosslinked
graphene oxide film may be of any suitable molecular weight or
size. The size of the spacer compound may influence the interlayer
spacing and hence separation distance between graphene oxide
sheets, with larger (i.e. higher molecular weight) spacers giving
rise to larger separation distances.
[0119] Crosslinking of the porous graphene oxide film may proceed
via covalent or non-covalent bonding interactions, or mixtures
thereof.
[0120] A porous covalently crosslinked graphene oxide film may be
produced when a spacer compound contains functional groups that are
capable of covalently reacting with oxygen containing functional
groups (such as epoxy or carboxyl functional groups) present on a
surface of a graphene oxide sheet, resulting in covalent attachment
of the spacer to the graphene oxide sheet.
[0121] A spacer compound may have any suitable functional group. In
some embodiments, the spacer compound may comprise a functional
group selected from the group consisting of hydroxy, amino, amido
and thiol, and mixtures thereof. A spacer compound may be
multifunctional and may comprise two or more of these functional
groups.
[0122] Metal nanoparticles that are capable of covalently bonding
with oxygen containing functional groups of a graphene oxide sheet
may also be used as spacer compounds.
[0123] In one set of embodiments, the spacer compound may be a
polyol. Accordingly, the graphene oxide film may comprise a spacer
derived from at least one polyol compound.
[0124] Polyol compounds are multifunctional and comprise two or
more hydroxy functional groups. The hydroxy functional groups are
generally terminal functional groups. Polyol compounds suitable as
spacers for the graphene oxide film may comprise two, three, four
or more hydroxy functional groups.
[0125] When a polyol compound is used as a spacer compound,
covalent attachment of the polyol to a graphene oxide sheet may be
via functional groups, such as ester (--C(O)O), ether (--O--) or
anhydride (--(O)COC(O)--) groups, formed between the polyol and the
graphene oxide sheet.
[0126] When covalent reactions occur between a polyol compound
comprising at least two terminal hydroxyl functional groups and two
separate overlapping graphene oxide sheets, a crosslink can be
formed between the graphene oxide sheets. For instance, a first
terminal hydroxyl group on the polyol compound may covalently react
with an oxygen containing functional group on a first graphene
oxide sheet while a second terminal hydroxyl group on the polyol
compound covalently reacts with an oxygen containing functional
group on a second graphene oxide sheet. The polyol therefore
extends between the first and second graphene oxide sheets and acts
as a crosslinker between the graphene oxide sheets.
[0127] In some embodiments, the porous graphene oxide film may
comprise a spacer derived from a polyol compound selected from the
group consisting of ethylene glycol (EG), 1,2-propylene glycol
(PG), butylene glycol (BG), 1,6-hexylene glycol (HG), neopentyl
glycol (NPG), glycerol (GL), pentaerythritol (PER), and mixtures
thereof. Thus the porous graphene oxide film may comprise one or
more spacers derived from at least one or a mixture of the
aforementioned polyols.
[0128] A porous non-covalently crosslinked graphene oxide film may
be produced when a spacer compound is capable of interacting with a
graphene oxide sheet via non-covalent bonding interactions.
Examples of a non-covalent bonding interaction include ionic,
hydrogen bonding and Van der Waals interactions. The spacer is
therefore bound to the graphene oxide sheets via non-covalent bonds
and crosslinks overlapping graphene oxide sheets via the
non-covalent bonds, producing a porous non-covalently crosslinked
graphene oxide film.
[0129] In one set of embodiments, the porous graphene oxide film is
crosslinked via ionic or electrostatic interactions. In such
embodiments, the porous graphene oxide film may comprise a spacer
derived from an ionisable spacer compound.
[0130] An ionisable spacer compound is a compound that is capable
of carrying a net charge at a selected pH. Ionisable spacer
compounds may comprise functional groups such as carboxylic acid,
carboxylic acid ester, amino, amido, nitro, phospho, sulpho, thiol,
and the like.
[0131] In some embodiments, the ionisable spacer compound may be
selected from the group consisting of pyrenebutanoic acid
succidymidyl ester, 1,5-diaminonaphthalene (DAN) and 1- nitropyrene
(NP), polydimethylsiloxane (PDMS) and DNA.
[0132] The crosslinking of a porous graphene oxide film may be
achieved using a range of techniques. In one set of embodiments,
crosslinking may be achieved by adding a suitable spacer compound
to a graphene oxide solution. A porous graphene oxide film is then
prepared from the solution. The spacer compound interacts with
graphene oxide sheets present in the graphene oxide solution by
covalent or non-covalent bonding interactions and becomes arranged
in between sheets of graphene oxide during formation of the
graphene oxide film. The resulting porous graphene film is then
crosslinked by the spacer. The crosslinked graphene oxide film may
then be subsequently subjected to a reduction process, as described
below.
[0133] Crosslinking of the porous graphene oxide film may also
proceed under suitable conditions. In one set of embodiments,
crosslinking is facilitated by the application of heat.
Self-Assembly
[0134] In some embodiments, a porous crosslinked graphene oxide
film may be prepared by layer-by-layer (LbL) self-assembly of
alternating layers of a spacer material and suitable graphene oxide
sheets. Thus in one set of embodiments, formation of a porous
crosslinked graphene oxide film involves subjecting the graphene
oxide solution to a self-assembly process. A self-assembly process
may enable a porous graphene oxide film of controlled thickness to
be prepared.
[0135] A self-assembly process for the formation of a porous
graphene oxide film may comprise the following steps: [0136] (1)
providing a negatively charged surface; [0137] (2) depositing a
layer of positively charged material onto the negatively charged
surface to form a positively charged surface; and [0138] (3)
depositing a layer of negatively charged graphene oxide sheets onto
the positively charged surface.
[0139] The negatively charged graphene oxide layer may provide a
negatively charged surface on to which another layer of positively
charged material may be subsequently deposited. The alternating
layers of positive charged and negative charged material are bound
to each other via electrostatic interactions.
[0140] A porous graphene oxide film formed by layer-by-layer
assembly may be supported by a suitable substrate. The substrate
may provide an initial charged surface on which a layer of graphene
oxide or polymer may be deposited when the layer-by-layer assembly
process is commenced. The resulting porous graphene oxide film is
therefore bound to the underlying substrate via electrostatic
interactions. Any suitable substrate may be used. In one set of
embodiments, the substrate is a glass substrate.
[0141] The deposition of a layer of a positively charged material
may be achieved through the use of a solution comprising an
appropriately charged compound or molecule. For example, a
substrate having a negatively charged surface may be immersed in a
solution comprising a positively charged material, such as a
positively charged compound or a positively charged polymer. This
leads to deposition of a layer of the positively charged material
onto the negatively charged surface and the formation of a
positively charged surface.
[0142] Following deposition of the positively charged material, a
layer of negatively charged graphene oxide is then deposited onto
the positively charged surface. Deposition of the negatively
charged graphene oxide layer may be achieved through the use of a
graphene oxide solution as described herein. For example, a
substrate having a positively charged polymer-modified surface may
be immersed in a graphene oxide solution comprising sheets of a
negatively charged graphene oxide. This leads to deposition of a
layer of graphene oxide onto the positively charged surface and the
formation of a negatively charged graphene oxide surface.
[0143] The alternating deposition of layers of positively charged
material and negatively charged graphene oxide may be repeated a
number of times to assemble a porous graphene oxide film having
layers of material interspersed in between layers of graphene oxide
sheets. Each layer of material may act as a spacer to separate and
space apart the layers of graphene oxide. The number deposition
steps determine the thickness of the graphene oxide film.
[0144] Pores are introduced to the self-assembled graphene oxide
film through pores present in the graphene oxide material forming
the graphene oxide layer that is part of the film structure.
[0145] In between each deposition step, any unattached material
(e.g. unattached polymer or unattached graphene oxide) may be
removed by washing the substrate-supported sample.
[0146] Once the desired number of layers has been achieved, the
porous graphene oxide film may then be dried. Sample drying may be
carried out with compressed air or a flow of nitrogen gas.
[0147] In one set of embodiments, positively charged material
useful for the production of a porous graphene oxide film by
self-assembly comprises a functional group that is capable of
carrying a net positive charge at a selected pH. In one embodiment,
the positively charged material may comprise a nitrogen-containing
functional group that is ionised at a selected pH to form a
cationic group carrying a positive charge. Nitrogen-containing
functional groups present in the positively charged material may be
primary, secondary or tertiary amino groups, amido groups, imino
groups and the like. In some embodiments, the positively charged
material may be positively charged polymer such as polyethylenimine
(PEI), polydiallyldimethylammonium chloride (PDDA),
poly[2-(N,N-dimethylamino)ethyl methacrylate] (PDMAEMA) and
chitosan, or a positively charged compound such as chlorophyll. In
one preference, the positively charged material is a positively
charged polymer.
Formation of RGO
[0148] The method of the embodiments includes irradiating the GO
film or GO solution with a beam of light or radiation to form RGO.
The irradiation process for reducing graphene oxide may also be
referred to below as "photo reduction" or "3D laser printing".
[0149] The reduction process can reduce one or more oxygen
containing functional groups present in one or more GO sheets
within the porous GO film. In some embodiments, the reduction
process reduces at least one oxygen containing functional group in
a plurality of GO sheets.
[0150] During the reduction process of the embodiments, an oxygen
containing functional group situated (i) in a pore of a graphene
oxide sheet and/or (ii) in between two or more graphene oxide
sheets is reduced.
[0151] The reduction process may therefore reduce an oxygen
containing functional group situated either in a pore of a graphene
oxide sheet or in between graphene oxide sheets, and in some
embodiments, the irradiation reduces at least a portion of the
oxygen containing functional groups between the graphene oxide
sheets.
[0152] Alternatively, the reduction process reduces oxygen
containing functional groups present both in a pore of a graphene
oxide sheet and in between graphene oxide sheets.
[0153] The reduction of an oxygen containing functional group
removes that functional group from a graphene oxide sheet and
results in the formation of a reduced graphene oxide sheet.
[0154] Following the reduction process, a porous reduced graphene
oxide film is produced. The porous reduced graphene oxide film
comprises at least one reduced graphene oxide sheet and may
comprise a plurality of reduced graphene oxide sheets. A reduced
graphene oxide sheet is formed when at least one oxygen containing
functional group in a graphene oxide sheet is reduced and
removed.
[0155] It would be appreciated by a skilled person that it is not
essential to the embodiments that all the graphene oxide sheets in
the porous graphene oxide film be reduced. However, the process of
the embodiments provides that at least one of the graphene oxide
sheets in the film is reduced.
[0156] In some embodiments, a portion of the graphene oxide sheets
in the porous graphene oxide film are reduced. In such embodiments,
the resultant film comprises a mixture of graphene oxide sheets and
reduced graphene oxide sheets. The resultant film may therefore be
a partially reduced graphene oxide film. However, such partially
reduced films are still regarded as reduced graphene oxide films in
accordance with the embodiments.
[0157] In some embodiments, each of the graphene oxide sheets in
the graphene oxide film is reduced.
[0158] Reduced graphene oxide sheets present in a porous reduced
graphene oxide film are also part of a multilayer array.
[0159] A skilled person would understand that the reduction process
conditions can be adjusted so as to vary the amount of oxygen
containing functional groups that are reduced and thus alter the
degree or extent of reduction. As explained further below, changes
in the degree of reduction can enable the properties (which may
include one or more of the following: pore/defect size, layer
spacing, electrical conductivity, hydrophilicity, surface charge
properties, surface roughness, or mechanical properties) of the
porous reduced graphene oxide film to be adjusted.
[0160] In some embodiments, the process of the embodiments may
selectively reduce an oxygen containing functional group that is
located in a pore or in an interlayer space of the porous graphene
oxide film. The selectivity may be possible as the type of oxygen
containing functional groups present in a pore may be different
from that in an interlayer space. For instance, a porous graphene
oxide film as described herein may comprise hydroxyl and epoxy
functional groups attached to the basal plane of a graphene oxide
sheet, which extend into the interlayer space in between graphene
oxide sheets. Meanwhile, carbonyl and carboxyl functional groups
may be attached to a defect edge of a graphene oxide sheet and thus
such functional groups may be present in a pore of the graphene
oxide sheet.
[0161] Reduction processes described herein may be capable of
distinguishing between different types of oxygen containing
functional groups and thus the process of the embodiments may be
able to selectively reduce different oxygen containing functional
groups that are positioned at different locations in a graphene
oxide film.
[0162] The reduction of an oxygen containing functional group in
accordance with the process of the embodiments results in the
removal of that oxygen containing functional group and sp.sup.3
carbon atoms from the graphene oxide sheet and the formation of
more hydrophobic graphene domains.
[0163] The reduction of an oxygen containing functional group that
is situated in a pore of a graphene oxide sheet results in a change
in the size of the pore. This change in pore size occurs due to the
removal of the oxygen containing functional group. In one set of
embodiments, the pore size (determined as pore diameter) of a
reduced graphene oxide sheet is increased in comparison to the
original pore size in the corresponding graphene oxide sheet.
[0164] The reduction of an oxygen containing functional group that
is situated in between graphene oxide sheets produces in a change
in the separation distance or interlayer spacing between the
sheets, as the oxygen containing functional group is removed from
the basal plane of a graphene oxide sheet. The reduced graphene
oxide sheet that is formed after the reduction step is therefore
separated from a graphene-based sheet that overlaps it by a
different distance, compared to the original corresponding graphene
oxide sheets in the graphene oxide film prior to the reduction
process. In one set of embodiments, the separation distance between
a reduced graphene oxide sheet and a sheet overlapping is
reduced.
[0165] Any change in the pore size and/or the sheet separation
distance is determined by comparison to a corresponding graphene
oxide sheet that is present in the porous graphene oxide film prior
to the reduction process. A "corresponding" graphene oxide sheet
relates to a selected reduced graphene oxide sheet in that it is
positioned at the same location as that reduced graphene oxide
sheet within the multilayer array. Thus the corresponding graphene
oxide sheet is the original, oxidised graphene sheet prior to it
being subjected to the reduction process.
[0166] Accordingly, a selected pore in a reduced graphene oxide
sheet will be compared to a corresponding pore in the graphene
oxide sheet prior to the reduction process being performed, and any
change in pore size (diameter) following reduction will be
ascertained relative to the size of the original pore in the
graphene oxide sheet.
[0167] Similarly, the separation distance between selected
graphene-based sheets in a porous reduced graphene oxide film will
be compared to the separation distance between corresponding
graphene oxide sheets prior to the reduction process being
performed, and any change in the separation distance between
selected sheets following reduction will be ascertained relative to
the original separation distance between equivalent sheets in the
graphene oxide film.
[0168] In one set of embodiments, the porous reduced graphene oxide
film comprises a plurality of reduced graphene oxide sheets, where
the separation distance or interlayer spacing between the reduced
graphene oxide sheets is decreased, relative to that of the
corresponding graphene oxide sheets in the graphene oxide film
prior to the reduction process.
[0169] Changes in the pore size and/or the separation distance
produced as a result of the reduction process allow the properties
(the properties may include one or more of the following:
pore/defect size, layer spacing, electrical conductivity,
hydrophilicity, surface charge properties, surface roughness, or
mechanical properties) of the porous reduced graphene oxide film to
be adjusted. In some embodiments, the reduction process may
selectively reduce oxygen containing functional groups situated in
one or more pores and/or in between two or more sheets of graphene
oxide to enable the pore size and/or interlayer spacing to be
controlled.
[0170] As previously mentioned, the GO film or GO solution is
irradiated with a beam of light or radiation to form the RGO. The
irradiation may induce a thermal (i.e. photo-thermal) or chemical
(i.e. photo-chemical) effect, which reduces at least one oxygen
containing functional group present in the porous graphene oxide
film.
[0171] Photo-thermal reduction may involve the use of the light or
radiation to irradiate the porous graphene oxide film and generate
localized heat in the film. The heat that is generated following
irradiation depends on the source of the light or radiation and
thermal properties of the graphene oxide film. Parameters such as
the wavelength and/or intensity of the source and the time (i.e.
duration) of irradiation can influence the pore size and/or the
interlayer spacing of the porous reduced graphene oxide film due to
the amount of thermal energy (or heat) that is generated. In one
embodiment, photo-thermal reduction is performed in a substantially
oxygen-free environment, such in a vacuum or in an inert atmosphere
such as a nitrogen or argon atmosphere. In photo-thermal reduction,
the light or radiation can include different forms of
electromagnetic radiation, including optical radiation.
[0172] Photo-thermal reduction may be performed using light or
radiation of any suitable wavelength. Suitable wavelengths may vary
from the UV range (approximately 10 nm) up to the infrared range
(approximately 100 .mu.m).
[0173] In some embodiments, suitable wavelengths may be from
approximately 248 nm up to 10.6 .mu.m from a CO.sub.2 laser.
[0174] Photo-thermal reduction may be performed using any suitable
type of light or radiation source. A suitable light or radiation
source preferably has sufficient power to generate a minimum amount
of heat. In some embodiments, a suitable light or radiation source
has sufficient power to heat the porous graphene oxide film to a
temperature of at least about 200.degree. C. during the reduction
process. Some examples of light sources that may be used facilitate
photo-thermal reduction include but are not limited to UV lamps,
focused sunlight and flash lights.
[0175] Photo-thermal reduction of the porous graphene oxide film
can involve irradiation of the graphene oxide film with a beam of
light or radiation with sufficient irradiance to generate the
minimum amount of heat. A suitable spot size can be selected based
on the radiant power of the source of the light or radiation--i.e.,
the provided light or radiation source--to provide sufficient
radiant flux (power) at the surface per unit area, i.e., sufficient
"irradiance", measured in Watts per square metre (W/m2). Thus, the
higher the source power, the larger the treated surface area can
be. For a femtosecond laser, the selected average power can be in
the range of 1 to 1000 micro-Watts (.mu.W) For a continuous-wave
(CW) laser, the selected average power can be in the range of from
10 to several hundred milli-Watts (mW), preferably in the range of
from 10 to 100 milli-Watts (mW). For a UV lamp or other light
source, the selected power output can be in the range of 100 to
1000 Watts, such as for example, a power output of about 100 W. The
source can include pulsed sources (including pulsed lasers, and
camera flashes) and CW sources (including sunlight, UV lamps, and
laser diodes).
[0176] In some embodiments, photo-thermal reduction may
advantageously permit the controlled removal of oxygen functional
groups by adjusting the power of the source of light or radiation.
Different powers can be used to generate different temperatures. In
turn, as different oxygen containing functional groups may have
different binding energies, different oxygen containing functional
groups may dissociate at different temperatures, allowing
particular oxygen containing functional groups to be selectively
removed.
[0177] Photo-chemical reduction uses a shaped pulse of light or
radiation to control chemical reactions that occur during the
reduction of the porous graphene oxide film. Thus light or
radiation may facilitate the chemical reduction of one or more
oxygen containing functional groups in the graphene oxide film. In
some embodiments, it may be possible to control the pore size
and/or the interlayer spacing of a porous reduced graphene oxide
film by selectively removing certain oxygen containing functional
groups that are situated in the pores of graphene oxide sheets
and/or in between graphene oxide sheets. The selective removal of
an oxygen containing functional group may be facilitated by the use
of a shaped pulse of light or radiation.
[0178] A shaped pulse, such as a shaped light pulse, may be
provided by a suitable source of light or radiation. In some
embodiments, a shaped pulse may be provided from a femtosecond
laser. Any suitable femtosecond laser can be used. Furthermore, any
suitable spot size can be used. The spot size depends on the laser
power and the average power of the laser depends on the repetition
rate of the laser pulses (for 1 kHz it requires several tens of
.mu.W and for 80 MHz it requires several mW).
[0179] In some embodiments, the selective reduction of oxygen
containing functional groups may be manipulated by changing the
pulse shape. In such embodiments, the pulse shape can be
iteratively updated by a feedback loop taking input from an in-situ
monitoring method, including published techniques to characterise
graphene oxide and reduced graphene oxide using Raman spectroscopy
or Fourier transform infrared (FTIR) spectroscopy, which is known
to a skilled person in the art.
[0180] When spacers or crosslinks are present in the porous
graphene oxide film, the interlayer spacing of the resultant porous
reduced graphene oxide film may be influenced by both the size of
the spacers and whether the spacers are removed by the reduction
process. For example, organic or polymer-like spacers may be
removed by the irradiation, while the nanoparticles or quantum dots
spacers may not not be removed.
[0181] Irradiation with the beam of light or radiation may provide
the ability to accurately control the reduction process and hence
the pore size and interlayer spacing in the porous reduced graphene
oxide film. For instance, reduction of an oxygen containing
functional group in a pore of a graphene oxide sheet and/or in
between two or more graphene oxide sheets may be selectively
controlled by adjusting the radiation power. In some embodiments,
the higher the power of the beam, the higher the proportion of
oxygen containing functional groups in the graphene oxide film that
are reduced.
[0182] The ability to control the reduction process through
irradiation of the porous graphene oxide film using a controlled
beam enables porous reduced graphene oxide films having different
pore sizes and/or interlayer spacing in different regions of the
film to be produced. Accordingly, it is possible to tune the
properties (for example, one or more of the following: pore/defect
size, layer spacing, electrical conductivity, hydrophilicity,
surface charge properties, surface roughness, or other mechanical
properties) of the porous reduced graphene oxide film to suit a
desired application by controlling the power of the beam to adjust
the pore size and/or interlayer spacing with high precision in the
sub-nanometer region.
[0183] Reducing the oxygen containing functional groups in the
porous graphene oxide film by irradiation allow the type and
coverage of oxygen containing functional groups in the film to be
manipulated by adjusting irradiation parameters (including
wavelength, power, and exposure time) of the light source.
Consequently, the surface properties of the porous graphene oxide
films can be selectively controlled to suit different
applications.
[0184] For example, when light or radiation of short wavelength is
used for irradiation, the power can be reduced due to higher photon
energy. Furthermore, for a given wavelength and power of light or
radiation, an increase in exposure time can increase the number of
oxygen containing functional groups being removed and thus increase
the extent by which the graphene oxide film is reduced.
Additionally, for a shaped light pulse, the repetition rate, pulse
width and pulse shape may also influence the extent of
reduction.
[0185] For a given source of radiation, the operating power range
can be ascertained by scanning the power. The lower power threshold
(i.e. the reduction threshold) of a beam can be determined by
observing a transmission change under an optical microscope. The
upper power threshold (i.e. the ablation/burning threshold) of a
beam can be ascertained by visually observing when ablation of the
GO film occurs, e.g., by using a microscope. The upper and lower
thresholds can dictate the operating range of power in which
irradiation can be performed. Selective oxygen containing
functional group removal can be achieved by controlling the power
of the beam within the operating range. For example, changing the
pulse widths of the laser can lead to change of the peak power of
the femtosecond laser while maintaining the same average power, and
the ratio of C--O and C.dbd.O bonds can be changed in accordance to
the change of the pulse widths, as described in further detail
below.
[0186] The porous graphene oxide film is irradiated at least once,
and may be irradiated multiple times, in order to reduce the oxygen
containing functional groups in the film. Multiple irradiations may
result in the removal of an increasing number of oxygen containing
functional groups in the film.
[0187] The irradiation affords the ability to locally reduce oxygen
containing functional groups in at least one selected area of the
porous graphene oxide film. Thus, it may be possible to form a
patterned film comprising selected areas of graphene oxide and
reduced graphene oxide for specific applications.
[0188] For instance, patterning with an irradiation process can be
achieved by laser patterning or photo-illumination, which can be
facilitated with a mask. The mask can cover a defined area of a
porous graphene oxide film and may help to direct or control how
light or radiation reaches that area of the film. This in turn may
help to control how oxygen containing functional groups are reduced
in that particular area of the film. In this manner, a porous
reduced graphene oxide film having different localised areas with
different degrees of reduction can be formed. Such a patterned
reduced graphene oxide film may be advantageous as it can enable
the fabrication of filters having multiple integrated regions with
different properties (e.g., pore/defect size, layer spacing,
electrical conductivity, hydrophilicity, surface charge properties,
surface roughness, or mechanical properties) in the different
regions.
[0189] The beam may also be able to selectively remove carbon atoms
from a graphene-based sheet by selectively breaking carbon-carbon
bonds and removing sp.sup.3 carbon atoms in the graphene basal
plane of the sheet. In this manner, additional pores may be
generated in the porous reduced graphene oxide film.
[0190] Moreover, the reduction process can be combined with a
graphene oxide film synthesis technique that controls the
properties of the interlayer space, providing a larger tuning range
for the interlayer spacing. Thus the interlayer spacing may be
tuned within a distance ranging from tens of nanometres down to
sub-nanometres. Accordingly, the process of the embodiments may
provide porous reduced graphene oxide films having a range of
versatile properties.
Irradiation Using Graphene Oxide (GO) Film
[0191] In some embodiments, the method of the embodiments described
herein includes irradiation of a GO film with the beam of light or
radiation.
[0192] FIG. 2 illustrates an exemplary process of irradiation of
the GO film.
[0193] As shown in FIG. 2(a), the GO film may be attached to a
three-dimensional (3D) surface. An emitting device is used as the
radiation source, which can be a laser emitting device or laser.
For example, the emitting device may be a laser, including a laser
diode or a femtosecond laser source. The emitting device may be
mounted on a movement control device to form a movable emitting
system (which may be referred to as a "laser 3D printer"), in which
the 2D and/or 3D position of the emitting device and the beam are
controllable and adjustable.
[0194] As shown in FIG. 2(a), a focussing element, which can be an
objective lens, may also be used to focus the laser beam on the 3D
surface the GO film is attached to. The focussing element may be
connected to the emitting device and/or the movement control
device, so as it may move together with the emitting device. The
objective lens may be conventional objective lens that can be used
to focus a beam of light or radiation. The focussing element may
also be part of a commercially available emitting device package,
e.g., a laser or a light that has a focusing lens. In some other
embodiments, the emitted laser may be directly used for the
reduction, without using the focussing element, which can be used
for large area reduction.
[0195] The beam of light or radiation may be movable relative to
the GO film during the irradiation, thereby allowing reduction of
the GO film according to a selected pattern. The selected pattern
may be any suitable 2D or 3D pattern. The movement of the beam may
be controlled manually. Alternatively, the movement of the beam may
be controlled automatically, e.g., by running a pre-programmed
controlling program based on the selected pattern.
[0196] The emitting device may include a 3D surface detecting unit
that automatically detects the 3D surface, which may allow the GO
film to be attached to an arbitrary surface, e.g., surface with an
arbitrary 2D/3D shape or structure.
[0197] FIG. 2(b) is a schematic diagram showing the reduction of
the GO film. As shown in FIG. 2(b), the oxygen functional groups,
including oxygen and hydrogen atoms, locate between the GO sheets.
Under the irradiation, the oxygen functional groups are removed to
form reduced graphene oxide (RGO), conductivity of the graphene
layers, and generate pores in the GO film.
Irradiation Using Graphene Oxide (GO) Solution
[0198] In some embodiments, the method includes irradiation of a GO
solution with the beam of light or radiation.
[0199] The reduction process by irradiation may be combined with a
GO film synthesis technique that controls the properties of the
interlayer space, providing a larger tuning range for the
interlayer spacing. Thus the interlayer spacing may be tuned within
a distance ranging from tens of nanometres down to sub-nanometres.
Accordingly, the process of the embodiments may provide reduced
graphene oxide (RGO) films having a range of versatile
properties.
[0200] The GO solution used in the irradiation process may be
prepared using known methods, e.g., oxidisation and subsequent
exfoliation, as described above.
[0201] Further, for GO solution with cross-linkers, cross-linking
may be achieved by the irradiation. Thus, it is possible to
simultaneously cross-link and reduce the GO, by using GO solution
with cross-linkers, or by adding cross-linkers to GO solution prior
to the irradiation.
[0202] FIG. 3 shows an exemplary process of simultaneously
photo-crosslinking and reducing the GO from its solution.
[0203] As shown in FIG. 3, the GO solution with cross-linkers is
stored in a container. The substrate to which the structure can
attach is submerged in the GO solution. An emitting device is used
as the radiation source. For example, the emitting device may be a
laser diode or a femtosecond laser source. The emitting device may
be mounted on a movement control device to form a movable emitting
system (which may be referred to as a "laser 3D printer"), in which
the 2D and/or 3D position of the emitting device is controllable
and adjustable.
[0204] A beam is emitted by the emitting device, and focused by a
focussing element to a point on or close to the surface of the GO
solution. The cross-linking and photo reduction occurs at the
centre of the irradiated spot on the surface of the GO solution.
The focussing element may be connected to the emitting device
and/or the movement control device, so as it may move together with
the emitting device. The objective lens may be conventional
objective lens that can be used to focus a beam of light or
radiation. The objective lens may also be part of a commercially
available emitting device package, e.g., with a laser emitting
device.
[0205] The beam of radiation may be movable relative to the
substrate during the irradiation, thereby allowing fabrication of
the RGO according to a selected pattern. The selected pattern may
be any suitable 2D or 3D pattern, thereby allowing fabrication of a
RGO of a desired structure. The movement of the beam may be
controlled manually. Alternatively, the movement of the beam may be
controlled automatically, e.g., by running a pre-programmed
controlling program based on the pattern.
[0206] After the cross-linking and reduction process, the
fabricated sample may be washing by water, so that the GO flakes
that are cross-linked and reduced may remain on the sample, while
the ones not cross-linked may be washed away.
[0207] In this way, it is possible to combine the film synthesis,
reduction and device fabrication into a single step, which may
boost the efficiency of the fabrication process of the RGO
structure.
[0208] Further, as the cross-linking and reduction occurs around
the surface of the GO solution, the RGO structure may be fabricated
in a layer-by-layer manner, by moving the substrate down (e.g.,
towards the bottom of the container). The linewidth of the
generated structure may be controlled by selecting the numerical
aperture of the focusing element, and/or controlling the laser
power, as shown in FIG. 19.
Controlling the Conductivity and Pore Size
[0209] The conductivity and the pore size of the RGO material may
be controlled by selecting or controlling the irradiation
parameters.
[0210] By the irradiation, the oxygen functional groups may be
removed and hydrophobic graphene domains may be formed. In this
process, gases, such as CO, CO.sub.2 and H.sub.2O vapour, may be
generated due to the removal of oxygen functional groups and the
water between the plurality of layers of GO sheets. During the
irradiation, the gases may be heated up in a high speed, which may
result in expansion of the volume of the gases, thus generating
pores between layers.
[0211] FIG. 4(a) shows an exemplary relationship between the pore
sizes and the laser power. FIG. 4(b) shows an exemplary
relationship between the pore sizes and the scanning speed. FIG.
4(c) shows an exemplary relationship between the resistivity and
the laser power.
[0212] As shown in FIG. 4(a)-FIG. 4(c), it is possible to
continuously tune, with high precision, the pore size and the
conductivity of the RGO structure, in selected sub-micron region
via the control of laser power and scanning speed.
Supercapacitor with RGO Electrodes
[0213] The RGO structure generated according to the above-described
method may be used for a range of applications, including making
electrodes of a capacitor.
[0214] The RGO structure generated according to the above-described
method may be used for making electrodes of a supercapacitor.
[0215] The supercapacitor including electrodes made of the RGO
structures prepared using the above-described method (which may be
referred to below as "the RGO supercapacitor") may have any one of
the following structures: a sandwich structure, an interdigital
structure, and a 3D structure.
[0216] Alternatively, the RGO supercapacitor may have any suitable
structure other than the sandwich structure, the interdigital
structure, and the 3D structure.
Supercapacitor with a Sandwich Structure
[0217] In some embodiments, the RGO supercapacitor may have a
sandwich structure.
[0218] FIG. 5 illustrates two types of sandwich structure of the
RGO supercapacitor. Each of the sandwich structures shown in FIG.
5(a) and FIG. 5(b) includes two electrodes, a separator sandwiched
between the two electrodes, and a pair of current collectors
connected to the electrodes.
[0219] In the RGO supercapacitor shown in FIG. 5(a), the RGO
electrodes with pores are sandwiched between two metal current
collectors, which are separated by a separator (e.g., a dielectric
separator). The RGO electrodes may be fabricated using the
irradiation process as described above, and may include nanopores
generated during the photo-reduction process.
[0220] The RGO electrodes may be nanostructured, as shown in FIG.
5(b) (which may be referred to as "nanostructured sandwich
design"), by fabrication using the simultaneous cross-linking and
photo-reduction process. In the nanostructured electrodes, the pore
size and layer spacing may be selectively controlled by controlling
the cross-linking and photo-reduction process.
[0221] The process of making RGO supercapacitors with sandwich
structures may include the following steps: [0222] (1) fabricating
the RGO structure, which will be used as the electrodes of the
supercapacitor; and [0223] (2) assembling the electrodes with metal
current collectors and separators.
[0224] The process of making RGO supercapacitors may further
include any additional steps of making capacitors known to a
skilled person. For example, the electrodes, the separator and the
collectors may be attached layer-by-layer (as shown in FIG. 5(a)),
which may then be filled in a plastic package. Next, the
electrolyte is added into the package. Finally, the package is
sealed, for example by using a vacuum sealer. Therefore, the
electrodes, the separator and the collectors are held in place by
the sealed package, and the pressure from the sealed package may
facilitate the attachment of the elements.
[0225] The separator and the collectors may be made in any
conventional methods known to a skilled person. The collectors may
be metals, for example, any one or more of the following: Al, Pt,
Au, Ag, Cu, or steel. The separator may be made using materials
including nonwoven fibers (e.g., cotton, nylon, polyesters, and
glass), and polymer films (e.g., polyethylene, polypropylene, poly
such as tetrafluoroethylene, and polyvinyl chloride). In some
embodiments, the separator may be made of the graphene oxide film
made according to the methods described above.
[0226] In some other embodiments, the RGO supercapacitor may have
any suitable sandwich structure other than the ones shown in FIG.
5.
[0227] The sandwich supercapacitor may be made in any suitable
shape and size, for example, in a cuboid shape with the height of
80 .mu.m, the width of 1 cm, and the length of 1 cm.
Supercapacitor with an Interdigital Structure
[0228] In some embodiments, the RGO supercapacitor may have an
interdigital structure.
[0229] FIG. 6 shows two types of RGO supercapacitor with
interdigital structure, and both may be fabricated by the
photo-reduction process described above, using different design
patterns.
[0230] In an interdigital supercapacitor design, the anode and
cathode intersect in one plane, which is parallel to the plane of
graphene oxide layers. Therefore, the ions travel within the plane.
In this way, the mean ionic path is shortened.
[0231] In the interdigital structures shown in FIG. 6, the graphene
oxide film without reduction is used as the separator of a
supercapacitor. The supercapacitor further includes a pair of
current collectors connected to the electrodes. In this way, the
interdigital design does not require adding a separator. As a
result, the volume ratio of the electrode material may be higher
than the sandwich design, which may improve the energy density and
power density of the supercapacitor.
[0232] In addition, as the interdigital supercapacitor design
requires only a layer of graphene oxide material (the sandwich
requires two layers of reduced graphene oxide, and a layer of
separator), the thickness of the supercapacitor may be reduced.
Moreover, it may be easy to stack the interdigital supercapacitors
along the direction normal to the plane to further use the 3D
space. Last but not least, the interdigital structure can be
on-chip integrated with other electronic devices.
[0233] Compared to the sandwich structures, the interdigital
structures may provide shorter mean ionic paths, and may be easier
to be integrated with on-chip devices. Further, the interdigital
structures may allow more efficient use of the 3D volume of the
device, i.e., storing more energy within a limited volume.
[0234] Compared to the interdigital structure shown in FIG. 6(a),
the interdigital structure shown in FIG. 6(b) (which may be
referred to as a "fractal interdigital design") allows more
efficient use of the area of the device, thus may enhance the
overall energy density of the supercapacitor, i.e., storing more
energy within a limited volume.
[0235] As shown in FIG. 6(b), a fractal interdigital design bears a
pattern that each part of which has the same or similar geometrical
character as the whole.
[0236] Electrodes having a fractal interdigital supercapacitor
design may further increase the capacitance of the interdigital
supercapacitor by adopting patterns in which the fractal curves
enclose a modest area with a long boundary, as the increase in the
capacitance is in proportion to the increase in the boundary length
due to lateral fringing. In the meantime, the fractal pattern is
able to fill more electrode material within the same area. In this
way, the volume ratio the electrode material is increased, which is
beneficial for storing more charges to enhance the energy density.
Further, the fractal pattern minimize the distance between
electrodes, which shortens the mean ionic path and enhance the
power density.
[0237] The process of making RGO supercapacitors with interdigital
structures may include the following steps: [0238] (1) attaching or
depositing the metal current collectors onto the graphene oxide
film; and [0239] (2) forming the RGO electrodes by photo reduction
process.
[0240] As previously described, for RGO supercapacitors with
interdigital structures, no separator is needed. In assembly, the
collectors may be attached to the patterned electrodes. The
electrodes with the collectors may then be put in a plastic
package, into which electrolyte can be filled. After adding
electrolyte, the package may be sealed by a vacuum sealer.
[0241] The current collectors may be made of metals, for example,
any one or more of the following: Al, Pt, Au, Ag, Cu, or steel. The
electrodes and the collectors may be connected by using any
suitable means, for example by using conductive tape/glue or
applying pressure using clip.
[0242] For making RGO supercapacitors with interdigital structures,
these two steps may be performed in any selected order, according
to the device design and applications.
[0243] Further, due to the capability of drawing arbitrary patterns
using the 3D laser printing technique, when making supercapacitors
with the interdigital structure as shown in FIG. 6(b), it is
possible to increase the capacitance of the supercapacitor by
adopting patterns in which the fractal curves enclose a modest area
with a long boundary, as the increase in the capacitance is in
proportion to the increase in the boundary length due to lateral
fringing, as shown in FIGS. 8(b) and 8(c) The meaning of "lateral
fringing" is known to a skilled person in the art, as described for
example in Samavati, H., et al. (1998). "Fractal capacitors." IEEE
Journal of solid-state circuits 33(12): 2035-2041
[0244] In some other embodiments, the RGO supercapacitor may have
any suitable interdigital structure other than the ones shown in
FIG. 6.
[0245] The RGO supercapacitors with an interdigital structure may
have any suitable shape and size, for example, a cuboid shape with
the height of 25 .mu.m, the width of 5 mm, and the length of 1.5
cm.
Supercapacitor with a 3D Structure
[0246] In some embodiments, the RGO supercapacitor may have a 3D
structure.
[0247] Compared to the sandwich structure and the interdigital
structure, the 3D structure may allow more efficient use of the 3D
volume of the device, i.e., storing more energy within a limited
volume.
[0248] The 3D structure may include one of: a 3D multilayer
structure, and a 3D intertwined structure. FIG. 7(a) shows an
example of a 3D multilayer structure. FIG. 7(b) shows an example of
a 3D intertwined structure.
[0249] In some embodiments, the 3D multilayer structure as shown in
FIG. 7(a) may be made using multiple layers of graphene oxide film
which are separated by insulating dielectric material that is
transparent, e.g. polymer such as the same polymer for
self-assembly as previously mentioned, or photo-polymer, which can
be polymerized upon light irradiation. By this arrangement, the
irradiation in the photo-reduction process may simultaneous reduce
and fabricate the multilayer graphene oxide structure, which may
allow the supercapacitor to be fabricated in one go.
[0250] The 3D intertwined structure shown in FIG. 7(b) may be made
by 3D fabrication using the method described above, e.g., the
simultaneously cross-linking and photo-reduction process.
[0251] As shown in FIG. 7(b), the two electrodes are intertwined
three-dimensionally with each other, the thickness of the solid
lines and the distance between the two electrodes are kept
constant. The ions are attached to the surface of the two
electrodes. Electrolyte in a gel form is injected between the
electrodes, which is able to provide positive and negative ions and
act as the separator. The overall surface area for ion attachment
is controlled by the thickness of the solid lines and the distance
between the two electrodes. The thinner the thickness the larger
the surface area and the smaller the distance the larger the
surface area.
[0252] 3D fabrication of the 3D intertwined structure may allow
minimizing of the mean ionic path by intertwining the
nanostructured anode and cathode. In this way, the ions may be
stored in the nanopores between electrodes. As a result, the ions
may only have to travel nanometre distances during the charge and
discharge processes.
[0253] In the 3D multilayer structure shown in FIG. 7(a), the
graphene oxide film without reduction is used as the separator of
the supercapacitor. In the 3D intertwined structures shown in FIG.
7(b), the separator of the supercapacitor may be electrolyte in gel
form.
[0254] The supercapacitors in both FIG. 7(a) and FIG. 7(b) further
include a pair of current collectors connected to the
electrodes.
[0255] In some other embodiments, the RGO supercapacitor may have
any suitable 3D structure other than the ones shown in FIG. 7.
[0256] The RGO supercapacitors with a 3D multilayer structure may
have any suitable shape and size, for example, a cuboid shape with
the height of 5 .mu.m, the width of 5 mm, and the length of 1.5
cm.
[0257] The RGO supercapacitors with a 3D intertwined structure may
have any suitable shape and size, for example, a cubic shape with
the length being any value between 100 .mu.m and 1 mm.
[0258] The current collectors for supercapacitors having the 3D
multilayer structure or the 3D intertwined structure may be made of
metals, for example, any one or more of the following: Al, Pt, Au,
Ag, Cu, or steel.
[0259] For the 3D multilayer structure, the electrodes and the
collectors may be connected by: first etching through the side the
electrodes by high power laser, and then depositing the collectors,
which connects to the electrodes in each layer.
[0260] For the 3D intertwined structure, the electrodes and the
collectors may be connected by: connecting the collectors to the
two sides (anode and cathode, left and right in the FIG. 7(b)) of
the whole structure of the electrodes.
Supercapacitor with a Fractal Pattern
[0261] Fractals are infinitely complex patterns that are
self-similar across different scales. They are created by repeating
a simple process over and over in an ongoing feedback loop. The
number of loops decides the scale of the largest pattern and
smallest pattern.
[0262] In some other embodiments, the electrodes of the RGO
supercapacitor may have a fractal pattern. FIG. 6(b) shows one
example of the fractal pattern. The fractal pattern may have other
suitable shapes different from the one shown in FIG. 6(b).
[0263] Fractals are infinitely complex patterns that are
self-similar across different scales. The fractal pattern may be a
2D fractal pattern, or a 3D fractal pattern.
[0264] As shown in FIG. 8, the fractal pattern may enhance lateral
flux of the capacitor, thus increasing the total amount of
capacitance. Further, the fractal design may increase the
capacitance per unit area as the distance between the electrodes
scales down. In this way, supercapacitors with electrodes in
fractal patterns may exploit both lateral and vertical electric
fields to increase the capacitance per unit area. Further, fractal
structures may maximize periphery, which increases field usage, and
may minimize internal resistance while maximizing surface-to-volume
ratios.
[0265] As previously described, the capacitance of the
supercapacitor can be increased by increasing the overall surface
area and the lateral fringing. The increase is proportional to the
length of the boundary of the electrodes. As fractal curves enclose
a modest area with a very long boundary, having fractal patterns
may allow the supercapacitor to provide increased capacitance.
[0266] FIG. 6(b) shows a supercapacitor with a 2D fractal
interdigital structure.
[0267] Further, the supercapacitor with electrodes including a
fractal pattern is not limited to interdigital structures. Rather,
the fractal structure may be applied to supercapacitors of other
types, such as the supercapacitor with the sandwich structures, or
the supercapacitor with the 3D structures.
Exemplary Processing Method
[0268] As shown in FIG. 9, a method 900 of forming RGO according to
some embodiments commences at step 902.
[0269] At step 904, graphite is oxidised to generate graphite
oxide. The generated graphite oxide is then exfoliated at step 906
to form GO solution. At step 908, spacer compound is added to the
GO solution for crosslinking of GO.
[0270] The GO solution formed at step 908 may then be used to form
a GO film at step 910. At step 912, the GO film is irradiated with
a beam of light or radiation to form a RGO structure that will be
used as electrode(s) in a RGO supercapacitor.
[0271] Alternatively, GO solution formed at step 908 may be
irradiated with a beam of light or radiation at step 914, to
simultaneously cross-link and reduce the GO, thereby forming a RGO
structure that will be used as electrode(s) in a RGO
supercapacitor.
[0272] At step 916, the formed RGO structure is assembled with
metal current collectors, and electrolyte is added to form a RGO
supercapacitor.
Example Applications
[0273] The reduced graphene oxide (RGO) structure, the RGO
electrodes or the RGO supercapacitor made according to the methods
as described above, may provide a number of advantages or technical
effects. The energy density may be similar to that of lithium
batteries. The graphene oxide solution may be synthesised directly
from bulk graphite material with oxidants, and the graphene oxide
films may be manufactured with economical synthesis techniques such
as vacuum filtration, self-assembly, spray coating and drop
casting. Only nanometre to microns thickness is needed without
reinforcement using other materials, so only a small amount of GO
material may be necessary for manufacturing a large number of
supercapacitors. The reduction of the graphene oxide material may
be performed using inexpensive laser diodes. The thin film
structure may be flexible to attach to any structures and surfaces.
The film synthesis techniques may allow the attachment of graphene
oxide films to any 3D structures or surfaces, thus saving space for
storing graphene oxide supercapacitors. With the laser 3D printing
reduction techniques, it is possible to achieve film coating and
fabrication of supercapacitors in one step, without further
transferring processes. This may allow easy integration of the
graphene oxide supercapacitor with other electronic devices, e.g.,
solar panels. The thin film structure may be stitched to cloths,
bags or shoes for powering personal electronic devices. The RGO
supercapacitors may be integrated with helmets, e.g., bike helmets,
to power built-in head lights (e.g., white light LEDs with high
brightness). By using the high resolution laser 3D printer, the
size of the RGO electrodes may be reduced down to nanometre scales,
which allows the fabrication of supercapacitors with footprints of
several microns that can be easily integrated with microelectronic
chips. The flexibility of the laser 3D printer fabrication system
may allow design and fabrication of RGO supercapacitors with
selected parameters, such as selected footprint, capacitance,
voltage and/or current. Further, it is possible to vary the
geometric shape from device to device by simply changing the design
pattern. Using the simultaneous cross-linking and reduction of
graphene oxide, it is possible to further save raw materials, as
there is no need to further make a separator, and only the material
for making the electrodes are required. This may further reduce the
weight of the supercapacitors. The ultrahigh power density may
provide high current for electronic devices, while charging the RGO
supercapacitors may be completed within a very short time period.
The RGO supercapacitors may be thermally stable and chemically
inert, which allows application in demanding environments. The RGO
films may have high tolerance to high temperatures, oxidants,
strong acidic/alkaline regents, or organic solvents. The RGO films
may have a high mechanical strength. With high mechanical strength,
thermal and chemical stability, the lifetime of the RGO
supercapacitor can be significantly longer than existing
supercapacitors.
[0274] The RGO structure, the RGO electrodes and the RGO
supercapacitors may be prepared in an environmentally friendly
manner, employing environmentally friendly solvents. Furthermore,
the RGO films may be non-toxic and compatible with biological
samples.
[0275] The methods for preparing the RGO structure for making the
electrodes, as described above, may provide a number of advantages
or technical effects. As the 3D laser printing technique is able to
fabricate 3D nanostructures layer-by-layer, it may be possible to
design novel sandwich supercapacitors with nanostructured
electrodes with precisely-controlled pore sizes. Due to the
capability of drawing arbitrary patterns using the 3D laser
printing technique, when making supercapacitors with fractal
interdigital structures, it may be possible to utilise the lateral
fringing to further increase the capacitance of the supercapacitor.
By controlling the focal depth of the irradiation beam, it may be
possible to simultaneously reduce multilayers of graphene oxide
film to make 3D supercapacitors. The flexibility of the 3D laser
printing allows fabrication of intertwined 3D supercapacitors, in
which the surface area is maximized and the mean ionic path is
well-defined and minimized. Thus it may be possible to achieve high
energy density and power density. The high spatial resolution and
precision of 3D laser printing, down to nanometer scale, may allow
the fabrication of supercapacitors with overall size in micron
scale that can be integrated with on-chip electronic circuits. The
3D laser printing technique may allow photo reduction of the
graphene oxide film attached to an arbitrary 3D surface, thus it
may be possible to spray-coat the graphene oxide film on the
surface of any object, then one-step fabricate the supercapacitors
without requiring any film transferring process.
[0276] The supercapacitors made using the method as described above
may be used for suitable applications, which may include one or
more of the following: a solar battery (e.g., by integrating the
supercapacitor with a solar panel); a power supply for unmanned
aerial vehicle (UAV); a power supply for electrical bikes or
vehicles; a night-vision-goggles power source; a power supply for
military radio; a power supply for military GPS devices; a power
supply for solar powered road illumination; a power supply for
solar powered irrigation system; a power supply for mobile houses;
in biomedical applications, e.g., power supply for bio-implants; a
power supply for consumer electronics, e.g., cell phone batteries;
a power supply for light-rails and trams; a smart and microgrid; a
biosensor; a chargeable coat for powering personal devices; a
chargeable bag for powering personal devices; a chargeable bike
helmet with built-in head lights; and a power supply for green
houses or other horticulture-related applications.
[0277] The supercapacitors made using the method as described above
may be characterized by known electrochemical techniques, e.g., any
one or more of the following techniques: cyclic voltammetry, cyclic
charge discharge, leakage current measurement, self-discharge
measurement, and electrochemical impedance spectroscopy.
[0278] The embodiments will now be described with reference to the
following examples. However, it is to be understood that the
examples are provided by way of illustration of the embodiments and
that they are in no way limiting to the scope of the invention.
EXAMPLES
[0279] Described below are exemplary experiments involved a process
of making RGO structures and RGO supercapacitors, and the
corresponding experimental results.
Preparation of Graphene Oxide Solution
[0280] The natural graphite powder (SP-1, Bay Carbon) (20 g) was
put into an 80.degree. C. solution of concentrated H.sub.2SO.sub.4
(30 mL), K.sub.2S.sub.2O.sub.8 (10 g), and P.sub.2O.sub.5 (10 g).
The resultant dark blue mixture was thermally isolated and allowed
to cool to room temperature over a period of 6 hours. The mixture
was then carefully diluted with distilled water, filtered, and
washed on the filter until the rinse water pH became neutral. The
product was dried in air at ambient temperature overnight. This
peroxidised graphite was then subjected to oxidation by Hummers'
method. The oxidised graphite powder (20 g) was put into cold
(0.degree. C.) concentrated H.sub.2SO.sub.4 (460 mL). KMnO.sub.4
(60 g) was added gradually with stirring and cooling, so that the
temperature of the mixture was not allowed to reach 20.degree. C.
The mixture was then stirred at 35.degree. C. for 2 hours, and
distilled water (920 mL) was added. In 15 min, the reaction was
terminated by the addition of a large amount of distilled water
(2.8 L) and 30% H.sub.2O.sub.2 solution (50 mL), after which the
colour of the mixture changed to bright yellow. The mixture was
filtered and washed with 1:10 HCl solution (5 L) in order to remove
metal ions. The graphite oxide product was suspended in distilled
water to give a viscous, brown, 2% dispersion, which was subjected
to dialysis to completely remove metal ions and acids.
As-synthesized graphite oxide was suspended in water to give a
brown dispersion, which was subjected to dialysis to completely
remove residual salts and acids. Ultrapure Milli-Q water was used
in all experiments. As-purified graphite oxide suspensions were
then dispersed in water to create a 0.05 wt % dispersion.
Exfoliation of graphite oxide to GO was achieved by ultrasonication
of the dispersion using a Brandson Digital Sonifier (S450D, 500 W,
30% amplitude) for 30 min. The obtained brown dispersion was then
subjected to 30 min of centrifugation at 3,000 r.p.m. to remove any
unexfoliated graphite oxide (usually present in a very small
amount) using an Eppendorf 5702 centrifuge with a rotor radius of
14 cm.
Preparation of Porous Graphene Oxide Films
[0281] The graphene oxide solution was used to prepare porous
graphene oxide (GO) films via three different film synthesis
techniques. The prepared GO films were then subjected to a
reduction process by irradiation with a laser diode or femtosecond
laser to produce a reduced graphene oxide (RGO) film.
Example 1: Formation of Porous GO Film Formed by Filtration and
Reduction of the GO Film by Laser Diode
[0282] The graphene oxide solution prepared above (the total weight
of graphene oxide used is 1 mg) was used to made graphene oxide
film by using filtration method (Sigma-Aldrich.RTM. vacuum
filtration assembly, for 47 mm filter) through an Anodisc membrane
filter (47 mm in diameter, 0.2 mm pore size; Whatman). A fully
dried porous GO film was achieved in approximately 5 hours at
ambient conditions.
[0283] A laser diode (650 nm, 200 mW) mounted on a homemade 3D
printer frame (Prusa i3) was used to prepare a reduced graphene
oxide (RGO) film. The prepared graphene oxide film was reduced by
using the laser diode working at 30 mW power focused by a
10.times., 0.25 NA objective with a scanning speed of 2 mm/s. The
pattern was designed using Inscape or Coreldraw, then converted to
Python codes by a homemade program.
[0284] Irradiation by laser diode produced a porous reduced
graphene oxide (RGO) film. If desired, multiple writing processes
were performed to further reduce the graphene oxide film.
Example 2: Formation of Porous GO Film Formed by Filtration and
Reduction of the GO Film by Femtosecond Laser
[0285] Following the procedure described in Example 1, a porous GO
film was formed by filtration.
[0286] A femtosecond laser (Coherent Libra, 800 nm, 10 kHz
repetition rate, 3 W output power) working at 10 .mu.W power
focused by a high numerical aperture objective (100.times.0.85 NA)
was used to prepare a reduced graphene oxide (RGO) film. The
prepared graphene oxide film was mounted on a 3D nanoscanning stage
(Physik Instrumente P-517) and scanned at 10 .mu.m/s. The scanning
stage was driven by a homemade Labview program. The pattern was
designed as bitmap and converted to a txt file by a homemade Matlab
program.
[0287] Irradiation by femtosecond laser produced a porous reduced
graphene oxide (RGO) film.
Example 3: Formation of Porous GO Film Formed by Self-Assembly and
Reduction of the GO Film by Laser Diode
[0288] A glass slide substrate was sonicated in acetone, methanol
and Milli-Q water for 5 minutes to fully clean the surface. The
following steps were then performed: (1) the substrate was
submerged in a 2% poly(diallyldimethylammonium chloride) (PDDA)
water solution for 1 minute and then taken out; (2) the
PDDA-modified substrates was cleaned by soaking in Milli-Q water to
remove excess PDDA at the surface and completely dried by
compressed air, (3) the dried substrate was submerged in 5 mg/ml
graphene oxide solution for 1 minute and then take out, (4) the
graphene-oxide modified substrate was soaked in Milli-Q water and
dried by compressed air. Steps (1) to (4) were repeated for N times
to get N self-assembled layers. In this way, a self-assembled
porous graphene oxide film was made.
[0289] The self-assembled GO film was reduced by using the laser
diode according to the procedure described in Example 1 to form a
porous reduced graphene oxide (RGO) film.
Example 4: Formation of Porous GO Film Formed by Self-Assembly and
Reduction of the GO Film by Femtosecond Laser
[0290] Following the procedure described in Example 3, a porous GO
film was formed by self-assembly.
[0291] The GO film was then subjected to a reduction by femtosecond
laser following the procedure described in Example 2 to form a
porous reduced graphene oxide (RGO) film.
Example 5: Formation of Porous GO Film Formed by Drop Casting and
Reduction of the GO Film by Laser Diode
[0292] A glass slide substrate was sonicated in acetone, methanol
and Milli-Q water for 5 minutes to fully clean the surface. A 5
mg/ml graphene oxide solution was dropped onto the surface of the
substrate to cover the whole surface. The resulting sample was
dried in fume hood for 8 hours in room temperature to produce a
porous graphene oxide (GO) film.
[0293] The prepared GO film was reduced by using the laser diode
according to the procedure described in Example 1 to form a porous
reduced graphene oxide (RGO) film.
Example 6: Formation of Porous GO Film Formed by Drop Casting and
Reduction of the GO Film by Femtosecond Laser
[0294] Following the procedure described in Example 5, a porous GO
film was formed by drop casting.
[0295] The GO film was then subjected to a reduction by femtosecond
laser following the procedure described in Example 2 to form a
porous reduced graphene oxide (RGO) film.
Results
[0296] Porous reduced graphene oxide films prepared in the above
examples were analysed by Raman spectroscopy and X-ray
photo-electron spectroscopy (XPS). Some results are discussed
below.
Laser Diode Reduction of Porous GO Films Prepared by Filtration
Method (Raman and XPS)
[0297] The X-ray photo-electron spectroscopic (XPS) results of the
porous GO film produced in accordance with Example 1 and reduced by
laser diode (wavelength=785 nm, power=18 mW) is shown in FIG. 10.
As one can see in FIG. 10, the strength of the C--O bond peak is
significantly reduced by the resulted C:O ratio and the percentage
of the C--C bonds (including sp.sup.2 and sp.sup.3 bonding). After
irradiation by writing the GO film with laser diode twice, the
reduction results slightly improved.
[0298] The Raman spectrum of the GO film produced by filtration
technique is shown in FIG. 11(a). The spectra of a porous reduced
graphene oxide (RGO) film produced by irradiation with laser diode
(LD) either once and twice is shown in FIGS. 11(b) and 11(c),
respectively. The LD reduction significantly decreased the
I.sub.D/I.sub.G ratio, which corresponds to lower defect density.
After second reduction, the I.sub.D/I.sub.G ratio is increased
slightly.
Femtosecond Laser Reduction of Porous GO Films Prepared by the
Self-Assembly Method (Raman Spectra)
[0299] The Raman spectrum of the porous GO film produced in
accordance with Example 4 is shown in FIG. 12(a). The GO film
reduced by femtosecond laser (wavelength=800 nm, repetition
numerical simulation rate=10 kHz, pulse width=85 fs) is shown in
FIG. 12(b).
[0300] As seen in FIG. 12(b), the I.sub.D/I.sub.G ratio increased
slightly after laser reduction, however, one can see significantly
increase of the I.sub.2D/I.sub.G ratio which confirms the formation
of sp.sup.2 graphene domains.
Femtosecond Laser Reduction of Porous GO Films Prepared by the
Drop-Casting Method (Raman and XPS)
[0301] The X-ray photo-electron spectroscopic (XPS) results of the
drop-casted film produced in accordance with Example 6 and reduced
by femtosecond laser (wavelength=800 nm, repetition rate=10 kHz)
with different pulse width is shown in FIG. 13. As one can see in
FIG. 13, the resultant C:O ratio and the percentage of the C--C
bonds (including sp.sup.2 and sp.sup.3 bonding) are affected by the
pulse width. The ratio of C--O and C.dbd.O bonds are different when
the pulse widths are tuned (larger pulse width corresponds to a
lower peak power, given the same average power of the femtosecond
laser). The C--O bond corresponds to the C--O--C (epoxy) and C--OH
(hydroxyl) functional groups and the C.dbd.O bond corresponds to
>C.dbd.O carbonyl and --COOH carboxyl functional groups. This
shows that selective reduction of different oxygen functional
groups has been achieved.
[0302] The corresponding Raman spectra are shown in FIG. 14(a). The
I.sub.D:I.sub.G ratio showing the defect density and the
I.sub.2D:I.sub.G ratio showing the formation of sp.sup.2 graphene
domain are shown in FIG. 14(b).
Example 7: Fabrication and Characterization of Interdigital
Supercapacitors
[0303] A fabrication process of an interdigital graphene oxide
supercapacitor is shown in FIG. 15. The fabrication process
includes 4 steps: (a) synthesising a graphene oxide film via
filtration method and peeling the graphene oxide film off the
filter; (b) attaching the graphene oxide film to a flexible
substrate; (c) depositing gold current collectors on the graphene
oxide film; and (d) fabricating reduced graphene oxide
supercapacitors by photo reduction.
[0304] As shown in FIG. 16(a), four supercapacitors with different
widths of electrode patterns were made. The scanning electron
microscopic images of these supercapacitors are shown in FIGS.
16(b)-(d). In FIG. 16(b), the bright areas show the gold current
collectors which have high conductivity; the grey areas and the
dark areas show the reduced graphene oxide and the graphene oxide
respectively, the reduced graphene oxide having higher conductivity
than the graphene oxide. As shown in FIG. 16(c), the surface of the
reduced graphene oxide is higher than the graphene oxide, due to
the micro pores generated during the photo-reduction process. The
details of the micro pores are shown in FIG. 16(d).
[0305] The performances of the fabricated supercapacitors were
measured using an electrochemical stat (Metro Autolab N series
potentiostat/galvanostat instrument). The electrolyte used was 1
mol/L H.sub.2SO.sub.4, and the voltage window was 0 to 1 V. The
design pattern of the tested supercapacitors is shown in FIG.
17(a), and the definitions of the parameters are shown in FIG.
17(b), where L represents the length of the reduced graphene oxide
unit in the electrodes, w represents the width of the reduced
graphene oxide unit in the electrodes, and s represents the
interspace between the reduced graphene oxide units in the
electrodes. In the experiment, the values of L and s were fixed,
while w had varied values in different supercapacitors, in order to
test the relationship between the value of w and the performance of
the supercapacitors. The value of w was selected to increase from
50 .mu.m to 200 .mu.m, with the step being 50 .mu.m. The resulting
cyclic voltammetry curves with different voltage scan rates are
shown in FIGS. 17(c)-(f). The corresponding measured specific
capacitances are shown in FIGS. 17(g)-(j).
[0306] FIG. 18 shows a comparison of the performance of the
supercapacitors with different width w. As shown in FIG. 18, the
specific capacitance changes as the width w changes, which leads to
the change of the energy density of the supercapacitor.
[0307] FIG. 19 shows a comparison of the linewidth of the generated
RGO structure with different laser power. As shown in FIG. 19, as
the laser power changes (increases) the linewidth of the generated
RGO structure changes (increases nonlinearly). The numerical
aperture of the focusing lens used in this example is 1.4.
Example 8: Fabrication and Characterization of Fractal
Supercapacitors
[0308] A design diagram of a fractal supercapacitor with Hilbert
fractal pattern is shown in FIG. 20(a), in which the gap is 300
.mu.m, and the area is 5.times.5 mm.sup.2. FIG. 20(b) shows a
fabricated fractal supercapacitor according to the design of FIG.
20(a).
[0309] FIG. 21 illustrates two fractal supercapacitors with Hilbert
fractal pattern of third and fourth iteration, respectively, the
pitch interval being 100 .mu.m. FIG. 21 also shows measured
performances of the two fractal supercapacitors respectively,
including: the resulting cyclic voltammetry curves with different
voltage scan rates; and the corresponding measured specific
capacitances.
Interpretation and Definition
[0310] As used herein, the singular forms "a," "an," and "the"
designate both the singular and the plural, unless expressly stated
to designate the singular only.
[0311] The term "about" and the use of ranges in general, whether
or not qualified by the term about, means that the number
comprehended is not limited to the exact number set forth herein,
and is intended to refer to ranges substantially within the quoted
range while not departing from the scope of the invention. As used
herein, "about" will be understood by persons of ordinary skill in
the art and will vary to some extent on the context in which it is
used. If there are uses of the term which are not clear to persons
of ordinary skill in the art given the context in which it is used,
"about" will mean up to plus or minus 10% of the particular
term.
[0312] Percentages (%) referred to herein are based on weight
percent (w/w or w/v) unless otherwise indicated.
[0313] The reference in this specification to any prior publication
(or information derived from it), or to any matter which is known,
is not, and should not be taken as an acknowledgment or admission
or any form of suggestion that that prior publication (or
information derived from it) or known matter forms part of the
common general knowledge in the field of endeavour to which this
specification relates.
[0314] Throughout this specification and the claims which follow,
unless the context requires otherwise, the word "comprise", and
variations such as "comprises" and "comprising", will be understood
to imply the inclusion of a stated integer or step or group of
integers or steps but not the exclusion of any other integer or
step or group of integers or steps.
[0315] Many modifications will be apparent to those skilled in the
art without departing from the scope of the present invention.
* * * * *