Field of the invention
[0001] This invention generally relates to titanium dioxide nanostructures. More particularly,
the invention relates to anodic growth of titanium dioxide nanostructures.
Background of the invention
[0002] Titanium dioxide (TiO
2, TiO2 or titania) is a non-toxic, highly photoactive, mechanically stabile, and cheap
substance, which also has a favourable overlap with the UV portion of the solar spectrum.
Titanium dioxide can be used for self-cleaning windows, photovoltaic cells, antibacterial
agents, antifogging agents, hydrogen generation, for reducing pollution etc.
[0003] Titanium dioxide nanostructures can be used for sensors, dye sensitised solar cells,
and photoelectrocatalysis which can be used for self-cleaning surfaces and hydrogen
generation.
[0004] Titanium dioxide nanostructures can be prepared on a substrate, and the method for
manufacturing titanium dioxide nanostructures may typically comprise anodization and
a thermal process. A substrate is in the remainder of the application defined as a
physical object, subject, component, surface, or material with a physical or spatial
extent. The word substrate is not used in its chemical sense.
[0005] A method of growing titanium dioxide nanostructures on substrates can be by means
of an electrochemical process as described in
J.M. Macak et al. Current Opinion in Solid State and Materials Science 11 (2007) 3-18 and in
C. A. Grimes, J. Mater. Chem., 2007, 17, 1451-1457. A substrate covered by a titanium (Ti) layer, i.e. a metal layer, is submerged in
an electrolyte solution and works as the anode, i.e. the electrode where oxidation
occurs. At the counter electrode, the cathode, reduction occurs. A voltage difference
is applied to the circuit, and oxidation and anodizing of the titanium metal occurs
and converts the titanium into titanium dioxide. However, the titanium metal layer
in contact with the electrolyte in air may be rapidly etched away, whereby the electrical
contact with the submerged portion of the titanium metal layer undergoing anodization
is cut off, before all the titanium metal is converted into titanium dioxide nanostructures.
[0006] Transparent titanium dioxide nanotube arrays on non-conducting supports can be grown
using a "bilayer technique" where two layers of titanium are used, a top layer and
a bottom layer. The top layer of titanium is converted to a top layer of titanium
dioxide nanotubes by anodizing, thus the top layer of titanium dioxide nanotubes is
thereby grown on the bottom layer of titanium. The bottom layer of titanium is thus
not converted, and this bottom layer of titanium must subsequently be oxidized thermally
to give a transparent film. The bottom layer of titanium metal is necessary to obtain
a complete conversion of the top layer.
[0007] WO2008/127508 discloses a method of preparing titania nanotubes involving anodization of titanium
in the presence of chloride ions and at low pH (1-7) in the absence of fluoride. Inclusion
of organic acids in the electrolyte solution leads to the incorporation into the nanotubes
of up to 50 atom percent of carbon. In a two-stage method, a titanium anode is pre-patterned
using a fluoride ion containing electrolyte and subsequently anodized in a chloride
ion containing electrolyte to provide more evenly distributed nanotube arrays.
[0011] Thus, growing titanium dioxide nanostructures on semi-conductor substrates, such
as silicon, is well-known from the documents described above.
[0012] However, it remains a problem to provide an easier method for growing titanium dioxide
nanostructures on non-conducting substrates.
Summary
[0013] Disclosed is a method of producing nanostructures of titanium dioxide (TiO2) by anodisation
of titanium (Ti) in an electrochemical cell, comprising the steps of:
- immersing a non-conducting substrate coated with a layer of titanium, defined as the
anode, in an electrolyte solution in the electrochemical cell;
- immersing a conducting substrate, defined as the cathode, in the electrolyte solution
in the electrochemical cell; whereby the anode and the cathode are in electrical contact
through the electrolyte solution;
- providing a voltage difference between the anode and the cathode; and
- providing an electrical contact to the layer of titanium on the anode, where the electrical
contact is made in the electrolyte solution.
[0014] In some embodiments the method further comprises controlling the oxygen concentration
in the electrolyte solution.
[0015] In some embodiments the electrical contact to the layer of titanium is provided at
a point on the anode corresponding to a substantially lowest oxygen concentration
in the electrolyte solution.
[0016] This may be the most optimal oxygen concentration for avoiding loss of electrical
contact.
[0017] In some embodiments the anode is defined as having a distal end and a proximal end,
and where the distal end is at the bottom of the electrochemical cell, and where the
proximal end is at the air interface of the electrochemical cell, and wherein the
electrical contact is made to the distal end of the anode.
[0018] In some embodiments the electrical contact to the layer of titanium is provided at
a point on the anode which is at the largest possible distance from the surface of
the electrolyte solution.
[0019] In some embodiments the electrical contact is provided by means of a cable immersed
in the electrolyte solution, where the cable is defined as having a distal end and
a proximal end, where the distal end contacts the titanium layer of the anode, and
where the cable comprises a conducting core for providing the electrical contact to
the titanium layer on anode.
[0020] It is understood that there may be one or more cables for providing the electrical
contact.
[0021] Furthermore, the proximal end of the cable may be connected to the electrical circuit
of the electrochemical cell, whereby the entire assembly of the non-conducting substrate
and the conducting cable constitute the anode
[0022] In some embodiments the conducting core is made from a self-passivating material,
which suppresses oxygen evolution.
[0023] In some embodiments the conducting core of the cable is covered by an electrical
insulator material, where the electrical insulator material is removed at the distal
end of the cable for exposing the conducting core for providing the electrical contact
to the titanium layer on anode
[0024] In some embodiments the length of the insulator material removed at the distal end
of the cable is in a range from about 0.5 mm to about 2 mm.
[0025] In some embodiments the non-conducting anode substrate is selected from the group
consisting of:
- glass;
- polymer;
- ceramics.
[0026] In some embodiments the conducting cathode substrate is selected from the group consisting
of:
- carbon (C);
- titanium (Ti);
- nickel (Ni);
- a steel alloy such as mild steel or stainless steel;
- platinum (Pt);
- iron (Fe);
- cobalt (Co);
- palladium (Pd);
- gold (Au).
[0027] Alternatively, the conducting cathode substrate can be made of any other conducting
material.
[0028] In some embodiments the conducting cable core is made of a material selected from
the group consisting of:
- titanium (Ti);
- aluminium (Al);
- tungsten (W);
- silicon (Si);
- zirconium (Zr);
- niobium (Nb);
- tantalum (Ta);
- hafnium (Hf).
[0029] Alternatively, the conducting cable core can be made of any other self-passivating
material. Self-passivating materials work because they tend to suppress the oxygen
evolution by forming a passivation layer. Consequently, oxygen evolving materials
such as gold (Au) or platinum (Pt) can not be used as material for the cable core,
since oxygen is developed and causes oxidation around the contact point.
[0030] In some embodiments the temperature in the electrolyte solution is in a range from
about 5°C to about 60°C.
[0031] In some embodiments the voltage difference between the anode and cathode is in a
range from about 5 V to about 40 V.
[0032] In some embodiments the produced titanium dioxide is amorphous assynthesized and
in the anatase form after temperature treatment, where both are active forms.
[0033] Consequently, it is an advantage that electrical contact by means of a contact point
is made in the electrolyte solution, since the nanostructures grow towards this contact
point, and hence electrical contact to the virgin titanium metal is maintained until
all titanium metal is converted into titanium dioxide. This provides that a substantially
complete conversion of titanium to titanium dioxide is obtained. In prior art approaches,
the electrical contact is made above or near the interface between the electrolyte
solution and the air, in which case the anodization proceeds fastest near the contact
point causing loss of the electrical connection to the remaining titanium metal which
is then left unconverted.
[0034] It is an advantage to grow the anodic film of titanium nanostructures on non-conducting
substrates by controlling the oxygen concentration gradient, oxygen partial pressure
or oxygen content over the growing nanostructures in such a way that the nanostructures
anodizes faster further away from the contact point than closer to the contact point.
This enables complete anodization of the entire surface without premature loss of
contact. Thus the oxygen gradient controls the formation of nanostructures on the
non-conducting substrate.
[0035] The electrical contact can be below the surface of the electrolyte solution near
the part of the titanium film that is immersed deepest into the bath. Hereby, the
electrical contact point can be located furthest from the highest oxygen concentration
in the electrolyte solution.
[0036] Thus, a suitable oxygen (02) gradient may be achieved in practice by having a vertical
geometry of the anode, and where the electrical contact is established at the bottom
of the electrochemical cell, since the oxygen concentration will be lowest at the
bottom of the electrochemical cell; and at the surface of the electrolyte solution
the oxygen concentration will be highest.
[0037] It is an advantage that the oxygen concentration determines where the process proceeds
fastest, so by controlling that the oxygen concentration is highest furthest away
from the electrical contact, the process will start furthest away from the electrical
contact, and thus the electrical connection is not cut-off before the entire titanium
film is converted and oxidized to titanium dioxide.
[0038] It is an advantage that the oxygen concentration can be controlled for example by
controlling the atmosphere of the electrolyte solution; or by providing an oxygen-free
atmosphere in the electrolyte solution and then provide oxygen from an air tube or
develop oxygen from an oxygen-developing auxiliary electrode, such as a gold wire,
by means of electrolysis and placing the air tube or the oxygen-developing auxiliary
electrode, e.g. a gold wire, furthest away from the electrical contact.
[0039] It is an advantage that the nanostructures can be grown in any direction, such as
from the top to the bottom or from the bottom to the top in a vertical arrangement,
or in a horizontal arrangement from one side to another or vice versa. The nanostructures
can for example be grown from the bottom to the top, if the atmosphere around the
arrangement is oxygen-free, and an oxygen-developing auxiliary electrode, such as
a gold wire, for producing oxygen or an air tube is provided at the bottom of the
anode and the electrical contact is provided at the top. An air tube or gas reservoir
can provide an oxygen gradient, whereby the oxygen concentration is controlled. Thus,
the oxygen supply in an oxygen-free atmosphere can be from a point source such as
an auxiliary electrode, e.g. a gold wire, or an air tube, or it can be from a number
of sources or from an entire surface.
[0040] It is an advantage that the growth direction of the nanostructures can be manipulated
in any direction by controlling the oxygen gradient in such a manner that the anodization
rate is enhanced by high oxygen concentration. In this manner, the anodization can
be made to start in the distant point from the contact point avoiding loosing electrical
contact during the anodization.
[0041] It is an advantage that titanium dioxide nanostructures such as nanotubes organise
in vertically-oriented arrays, which provides an easy path for electron transport.
The high surface area of titanium dioxide nanotubes facilitates faster hole transport
for oxygen evolution. The internal reflections within the titanium dioxide nanotubes
enhance light absorption, and titanium dioxide nanotubes can become super hydrophilic.
[0042] It is an advantage that the titanium dioxide film on the substrate is transparent,
because then the film can be used for windows etc. without having to perform further
process steps.
[0043] It is an advantage that the method is faster and with less complicated steps, e.g.
no annealing may be necessary, compared to the prior art bilayer method where a thermal
oxidation step is necessary.
[0044] It is an advantage that according to the present method, the titanium dioxide nanostructures
is directly deposited on the substrate, whereas in the bilayer method, the titanium
dioxide nanotube array is created on an other titanium layer of the bilayer and not
directly on the substrate.
[0045] It is an advantage that the present method potentially can provide more homogenous
titanium dioxide films, since there is a complete conversion of titanium into titanium
dioxide directly on the substrate.
[0046] It is an advantage that if using a high voltage for the potential difference, no
further thermal treatment for crystallisation of the titanium dioxide nanostructures
may be provided.
[0047] It is an advantage that the present method is straightforward, cheap, and reproducible
and suitable for large-scale production.
[0048] It is an advantage that titanium dioxide nanostructures can be grown to be directly
deposited on non-conducting substrates instead of on conducting or semi-conductor
substrates, since thereby substrates such as polymers, plastics, glass, ceramics,
paper etc. can be used as substrate.
[0049] It is an advantage that when using e.g. glass as the substrate for the titanium dioxide
nanostructures, self-cleaning and anti-fogging windows can be produced. These windows
can be used as windows in buildings and may thus require less maintenance or no maintenance,
less or no cleaning, and this saves money, and time, and is an environment-friendly
product.
[0050] Furthermore, it is an advantage that the titanium dioxide nanostructures can be grown
on materials for making them anti-bacterial. This could be on glass, plastics, food,
cosmetics, fibers, papers, packing, as coatings etc. for use in e.g. hospitals and
on medical devices.
[0051] On an electrically conducting substrate electron transfer occurs, whereby electrical
contact is created. However, when not using a conducting or semiconducting substrate,
as in the present method, a contact point should be created for making an electrical
circuit so that electron transfer can occur. The contact point may be necessary when
using a non-conducting substrate even though there is a conducting layer of titanium
metal present, because it may be necessary to maintain a constant electrical contact
to the titanium layer. This contact point is achieved by means of the exposed and
bare laid tip of the cable touching the non-conducting substrate.
[0052] A conductor is a material which contains movable electric charges. A flow of charges
is electric current. Non-conducting materials or insulators lack mobile charges, and
so resist the flow of electric current. Electrical resistance is a measure of the
degree to which a material opposes an electric current through it. Since all conductors
have some resistance, and all insulators will carry some current, there is no theoretical
dividing line between conductors and insulators. However, there is a large gap between
the conductance of materials that will carry a useful current at working voltages
and those that will carry a negligible current for the purpose in hand, so the categories
of insulator and conductor do have practical utility. Thus when the expressions conductor
or conducting material are used in this application, what is meant is a material that
will carry a useful current at working voltages, and when the expressions non-conductor,
non-conducting material or insulator are used in this application, what is meant is
a material that will only carry a negligible current at working voltages. Thus in
practice a non-conducting material does not carry any current.
[0053] Electrical resistivity (also known as specific electrical resistance) is a measure
of how strongly a material opposes the flow of electrical current. A low resistivity
indicates a material that readily allows the movement of electrical charges. The SI
unit of electrical resistivity is the ohm meter (Ω m). According to the present method,
the cathode may comprise the conducting material carbon, and carbon has a resistivity
of 3.5×10
-5 Ω m at 20 °C. The anode may comprise the non-conducting material glass, and glass
has a resistivity of 10
10 to 10
14 Ω m at 20 °C. As opposed to this, prior art discloses anodes comprising the material
silicon, which has resistivity of 6.40×10
2 Ω m at 20 °C, and is defined as a semi-conductor.
[0054] According to the present method, prior to the formation of the titanium dioxide nanostructures,
the anode comprises the non-conducting substrate and the metallic layer of titanium.
Titanium has fairly low electrical conductivity with an electrical resistivity of
0.420 µΩ·m at 20 °C. Thus the anode comprises a poorly conducting substance besides
the non-conducting substrate. After the formation of the titanium dioxide nanostructures
the anode comprises the non-conducting substrate and the layer of titanium dioxide
nanostructures. Titanium dioxide has an electrical resistivity of 10
12 Ω·m at 25°C.
[0055] According to the present method, e.g. the anode electrode and the cathode electrode
are immersed in the electrolyte solution in the electrochemical cell, and by immersion
is meant "at least partial immersion", i.e. "at least partly immersing". Thus, the
electrodes may also be fully immersed.
[0056] According to the present method, the conducting core of the cable may be covered
by an electrical insulator material, and by covering is meant "at least partial covering",
i.e. "at least partially covered".
[0057] Nanostructures are defined as structures of the size of nanometers and thus with
dimensions on nanometer-scale, and examples of nanostructures are arrays of nanotubes,
films such as thin films, etc. Titanium dioxide nanostructures are defined as nanostructures
comprising titanium dioxide. A nanotube is defined as a nanometer-scale tube-like
structure. Nanostructures may be defined as having a size of 100 nanometers or smaller,
however nanostructures may also be defined as having a size of more than 100 nanometers,
such as for example less than 1000 nanometers.
[0058] The present invention relates to different aspects including the nanostructures of
titanium dioxide described above and in the following, and corresponding methods,
processes, products, apparatus, devices, uses and/or product means, each yielding
one or more of the benefits and advantages described in connection with the first
mentioned aspect, and each having one or more embodiments corresponding to the embodiments
described in connection with the first mentioned aspect and/or disclosed in the appended
claims.
[0059] In particular, disclosed herein is an apparatus for producing nanostructures of titanium
dioxide (TiO2) by anodisation of titanium (Ti), comprising:
- an electrochemical cell comprising an electrolyte solution; an anode comprising a
non-conducting substrate coated with a layer of titanium; and a cathode comprising
a conducting substrate;
- means for providing a voltage difference between the anode and the cathode;
- means for providing an electrical contact to the layer of titanium on the anode made
below the surface of the electrolyte solution.
[0060] In particular, disclosed herein is a kit for preparing nanostructures of titanium
dioxide by anodisation of titanium comprising:
- an electrochemical cell;
- an electrolyte solution;
- an anode comprising a non-conducting substrate coated with a layer of titanium;
- a cathode comprising a conducting substrate;
- a cable comprising a conducting core for enabling an electrical contact to the titanium
layer on the anode.
[0061] In particular, disclosed herein is a non-conducting substrate covered with nanostructures
of titanium dioxide.
[0062] In particular, disclosed herein is a non-conducting substrate covered with nanostructures
in the form of nanotube arrays of titanium dioxide, where a nanotube has a tube length
of about 400 nm, an inner tube diameter of about 50 nm, and a thickness of the tube
wall of about 10 nm.
[0063] Alternatively, the size and dimensions of the nanotubes can be varied.
[0064] In particular, disclosed herein is a use of a non-conducting substrate covered with
nanostructures of titanium dioxide for photo-oxidation processes, such as self-cleaning
surfaces.
Brief description of the drawings
[0065] The above and/or additional objects, features and advantages of the present invention,
will be further elucidated by the following illustrative and nonlimiting detailed
description of embodiments of the present invention, with reference to the appended
drawings, wherein:
Fig. 1 shows an example of the experimental setup used for growing titanium dioxide
nanostructures in the form of nanotube arrays.
Fig. 2 shows the region of the contact point before and after formation of nanostructures.
Fig. 3 shows the process of nanostructure formation.
Fig. 4 shows recorded images of titanium dioxide nanostructures in the form of nanotube
arrays.
Fig. 5 shows a flowchart of an example of the process for producing the titanium dioxide
nanostructures according to the present method.
Fig. 6 shows prior art.
Fig. 7 shows an example of an experimental set-up.
Fig. 8 shows an example of an experimental set-up.
Fig. 9 shows an example of an experimental set-up.
Fig. 10 shows an example of an experimental set-up.
Detailed description
[0066] In the following description, reference is made to the accompanying figures, which
show by way of illustration how the invention may be practiced.
[0067] Figure 1 shows an example of the experimental setup used for growing titanium dioxide
nanostructures in the form of nanotube arrays. In fig. 1a) the nanotube arrays are
grown in an electrochemical cell 101 comprising a non-conducting substrate 102 with
a layer of titanium (Ti) 103. The non-conducting substrate and the titanium layer
constitute the anode 104 of the electrochemical cell. The cathode 105 of the electrochemical
cell comprises a conducting material. The anode 104 and the cathode 105 are immersed
in and thereby in contact with an electrolyte solution 106 (indicated by waves in
the solution). The interface 107 between the electrolyte solution 106 and the air
is indicated by a dotted line. A cable 108 is also immersed in the electrolyte solution
106. The cable 108 comprises a self-passivating conducting core 109 covered by an
electrical insulator material 110. The conducting core may be made of metal or other
conducting, self-passivating materials. The electrical insulator material 110 is removed
from the conducting core 109 at one end 111 of the cable 108, and this end 111 is
in contact with the titanium layer 103. The length of the insulator material removed
at the end of the cable may be in a range from about 0.5 mm to about 2 mm. The cable
108 is shown to be arranged between the anode 104 and the wall 113 of the electrochemical
cell, the cable 108 then passes under the anode 104, and the cable 108 is then bended
upwards and inwards towards the titanium layer 103 in order to make contact with the
titanium layer 103. See details in fig. 2 and 3. The exposed conducting core 109 at
the end 111 of the cable 108 touching the titanium layer 103 constitutes an electrical
contact point 112.
[0068] A voltage (V) 114 is applied to the anode, oxidation at the anode 104 electrode occurs,
and anodizing of the titanium metal 103 occurs and converts the titanium 103 into
titanium dioxide. Anodizing is an electrolytic process, where the thickness of the
natural oxide layer on the anode electrode in an electrical circuit is increased.
Thus the metal layer of titanium 103 is gradually converted to titanium dioxide nanotube
arrays. The arrays of nanotubes grow towards the contact point 112, and hence electrical
contact to the virgin titanium metal 103 is maintained until all titanium metal 103
is converted into titanium dioxide. This provides that a substantially complete conversion
of titanium 103 to titanium dioxide is obtained. In prior art approaches, the electrical
contact is made above or near the interface between the electrolyte solution and the
air, in which case the anodization proceeds fastest near the contact point causing
loss of the electrical connection to the remaining titanium metal which is then left
unconverted.
[0069] Thus, the growth of titanium nanostructures is controlled by the oxygen concentration,
oxygen content, oxygen partial pressure or oxygen gradient in the electrolyte. So
when the electrical contact is made on a part of the anode which is arranged at the
bottom of the electrochemical cell, the oxidation proceeds on the anode from the surface
of the electrolyte and downwards on the anode, because the oxygen content is highest
at the surface of the electrolyte.
[0070] The growth of titanium nanostructures may start and proceed fastest at the point
on the anode, where the oxygen content in the electrolyte solution is highest, so
if an air bobble, which contains 21 % oxygen is placed for instance halfway down the
anode, then the formation of nanostructures happens at that point on the anode first.
[0071] For example, a cable having a core of gold for making the electrical contact would
not work in the experimental setup shown in fig. 1, because gold will develop oxygen
at the high pressure potential at the bottom of the electrochemical cell, and the
nanostructures will then form around the contact point due to the produced oxygen
from gold, and when nanostructures are formed at the contact point by converting the
metallic titanium layer, the electrical contact is cut off, because the metallic titanium
disappear.
[0072] The non-conducting substrate of the anode may comprise glass, polymer, plastics,
ceramics etc.
[0073] The conducting cathode substrate may comprise carbon (C) or other conducting materials,
such as for example platinum (Pt), gold (Au), iron (Fe), cobalt (Co), palladium (Pd),
titanium (Ti), nickel (Ni), steel alloys such as mild steel, stainless steel etc.
[0074] It may be an advantage to have some amount of platinum coated on the cathode substrate
to facilitate continuous removal of oxygen (02) via reaction with the hydrogen (H2),
which is formed due to the anodization at the anode, and hereby the chemical reaction
02 + H2 -> H2O can take place.
[0075] The electrolyte solution may comprise Ammonium flouride (NH4F or NH
4F), water (H2O or H
2O), and Ethylene glycol (CH
2OHCH
2OH). Alternatively, the electrolyte solution may comprise other substances. However,
fluoride (F) may be an essential substance in the electrolyte solution.
[0076] The temperature of the electrolyte solution may be in a range from about 5°C to about
60°C.
[0077] The voltage difference between the anode and cathode may be in a range from about
5 V to about 40 V. The magnitude of the voltage difference may determine the size
of the formed titanium dioxide nanotubes.
[0078] The distance between the anode and the cathode in the electrochemical cell may be
35 mm. Alternatively, the distance the may be shorter or larger than this.
[0079] The produced titanium dioxide may be in the amorphous, rutile or anatase form.
[0080] The conducting core 109 of the cable 108 may be made of the metal titanium (Ti).
Alternatively and/or additionally, other self-passivating, conducting materials, such
as metals, may be used for the core material, e.g. aluminium (Al), tungsten (W), silicon
(Si), zirconium (Zr), niobium (Nb), tantalum (Ta) and Hafnium (Hf) etc.
[0081] The electrical insulating material 110 of the cable 108 may be Teflon (brand name).
The name Teflon is a collective terms used for the chemical names poly(tetrafluoroethylene)
or poly(tetrafluoroethene) (PTFE), as well as PFA (perfluoroalkoxy resin) and FEP
(fluorinated ethylene-propylene). Alternatively and/or additionally, other insulating
materials may be used for the coating of the conducting core, e.g. rubber-like polymers,
composite polymers, plastics, silicon rubber, glass, porcelain etc.
[0082] The arrangement of the cable in relation to the anode can be different from what
is shown in fig. 1. Alternatively, the cable can be arranged to contact the titanium
layer from below instead of from the side as shown in fig. 1. The cable can also be
arranged between the anode and cathode, and the cable can then be bended inwards towards
the titanium layer. Other arrangements for the internal positions of the different
parts relative to each other are possible.
[0083] The titanium layer 103 of the anode 103 is deposited on the non-conducting substrate
prior to the method of growing titanium dioxide nanotube arrays from metallic titanium.
The titanium may be deposited on the non-conducting substrate by means of physical
vapor deposition, such as sputtering or evaporation or chemical methods such as chemical
vapor deposition, etc..
[0084] After the titanium dioxide nanotube arrays are formed, they can be annealed for increasing
the activity.
[0085] Fig. 1b) shows an example of the experimental setup used for growing titanium dioxide
nanostructures in the form of nanotube arrays. All the reference numbers refer to
the same as in fig. 1a). However, in fig. 1b) the anode 104 and the cathode 105 are
completely immersed in the electrolyte solution 106 (indicated by waves in the solution).
The interface 107 between the electrolyte solution 106 and the air is indicated by
a dotted line.
[0086] Fig. 2 shows the region of the contact point before and after formation of nanostructures
in the form of nanotube arrays. Fig. 2a) shows the region of the contact point before
the formation of nanotube arrays. The contact point 212 is between the titanium layer
203 on the non-conducting substrate 202 and the exposed conducting core 209 of the
cable 208. Thus, the cable core is covered by an electrical insulating material 210,
which is removed at the end 211.
[0087] Fig. 2b) shows the region of the contact point after the formation of nanotube arrays.
The contact point 212 is between the titanium remaining metal layer 203 (see fig.
2a) on the non-conducting substrate 202 and the exposed conducting core 209 of the
cable 208. The titanium diode nanotube arrays 215 have formed above the contact point
212. Below the contact point 212 titanium nanotube arrays 215 have also formed.
[0088] If using the setup as described above, and examining the substrate afterwards, there
is titanium dioxide on the substrate and no titanium left on the substrate, except
for the small spot where the contact point was made. At the spot of the contact point,
no conversion from titanium to titanium dioxide has occurred, and consequently the
spot is dark and not transparent as the rest of the substrate.
[0089] When using the substrate covered with titanium dioxide nanostructures in the form
of nanotube arrays for e.g. a window glass, the region of the substrate, where the
dark spot from the contact point is present, may be cut away, so that the entire window
glass mounted in a window frame is transparent.
[0090] If a window glass, e.g. having a size suitable for being mounted in a typical building,
is produced, then there may be provided more than one contact point when producing
the substrate with the titanium dioxide nanostructures in the form of nanotube arrays,
for ensuring that the entire substrate can be covered with titanium dioxide nanotube
arrays. As mentioned above, the region of the substrate, where the dark, non-transparent
spots from the contact points are present, can be cut off, so that the entire substrate
is transparent, when mounted as e.g. a window glass in a building façade.
[0091] Fig. 2c) shows the region of the contact point before the formation of nanotube arrays,
when the cable core is not covered by an electrical insulating material. The contact
point 212 is between the titanium layer 203 on the non-conducting substrate 202 and
the conducting core 209 of the cable 208. Thus in this figure, the cable core 209
is exposed along its entire length, and it is thus not covered by an electrical insulating
material, being removed at the end.
[0092] Fig. 3 shows the process of formation of nanostructures in the form of nanotube array.
Fig. 3a) shows the direction of the titanium dioxide nanotube array formation is from
the top and down (TiO
2 NT forming in the figure). The conducting core 309 of the cable 308 is connected
to the titanium layer 303 on the non-conducting substrate 302 by means of the contact
point 312. The titanium dioxide nanotube arrays 315 are initially formed at the top
end, the nanotube arrays 315 then gradually form downwards and finally all titanium
is converted to titanium dioxide nanotube arrays, see fig. 3b). The arrow with Δt
indicates that the titanium dioxide nanotube arrays are formed over a period of time.
[0093] By examining the anode with the growing titanium dioxide nanotube arrays during the
formation, it was verified that the direction of growth is as described above, since
the top part of the anode had titanium dioxide nanotube arrays while the bottom part
of the anode had not. The examination was by means of visual inspection and scanning
electron microscopy (SEM).
[0094] The growth of the nanostructures is regulated by the oxygen concentration gradient.
However, the direction of growth of the nanostructures can alternatively and/or additionally
be due to electrical field effects, chemical transport phenomena, and/or other physical
or chemical effects.
[0095] Fig. 4 shows recorded images of titanium dioxide nanotubes arrays.
[0096] The images are made by means of scanning electron microscopy (SEM). Fig. 4a) shows
titanium dioxide nanotubes arrays seen in a top-view. From above the nanotubes are
round and hollow. Fig. 4b) shows the titanium dioxide nanotube arrays seen from a
side-view. From the side the nanotubes are thin and elongated.
[0097] A nanotube may have a tube length of about 400 nm, an inner tube diameter of about
50 nm, and a thickness of the tube wall of about 10 nm. However, other dimensions
of a nanotube are also possible.
[0098] Fig. 5 shows a flowchart of an example of the process for producing the titanium
dioxide nanostructures in the form of nanotube arrays according to the present method.
[0099] In step 501, a layer of metallic titanium is deposited on a non-conducting substrate.
The titanium may be deposited by means of physical vapor deposition, such as sputtering
or evaporation or chemical methods such as chemical vapor deposition.
[0100] In step 502, the non-conducting substrate with the titanium layer is immersed in
an electrolyte solution. The non-conducting substrate with the titanium layer may
be attached at its top end to a stand by means of a clamp. The non-conducting substrate
with the titanium layer works as the anode in the electrochemical cell.
[0101] In step 503, a cable with a self-passivating conducting core and e.g. an insulating
cover is also attached at its top end to a stand by means of a clamp. If the core
is covered by insulating material, the insulating material at the bottom end of the
cable which is immersed in the electrolyte, is removed to expose the conducting core
for providing an electrical contact point to the titanium layer on the non-conducting
substrate.
[0102] In step 504, a conducting substrate is also immersed in the electrolyte solution.
The conducting substrate may be attached at its top end to a stand by means of a clamp.
The conducting electrode works as the cathode in the electrochemical cell.
[0103] In step 505, a voltage difference is applied between the anode and the cathode, and
oxidation and anodizing of the anode begins, whereby the titanium on the non-conducting
substrate begins to convert to titanium dioxide nanotube arrays.
[0104] In step 506, a time period passes, while the anodizing of the titanium to titanium
dioxide nanotube arrays is in progress, and eventually, the conversion from titanium
to titanium dioxide nanotube arrays is complete, and the non-conducting substrate
is covered with the to titanium dioxide nanotube arrays.
[0105] Fig. 6 shows an example of prior art.
[0106] Fig. 6 shows that in the prior art process using a non-conducting support, the electrical
contact point to the titanium is made at the top of the anode, that is above or near
the interface between the electrolyte solution and the air, and in this case the anodization
proceeds fastest near the contact point causing loss of the electrical connection
to the remaining titanium metal which is then left unconverted.
[0107] In a different prior art process (not shown), transparent titanium dioxide nanotube
arrays can be grown on non-conducting supports using a "bilayer-technique" where two
layers of titanium are used, a top layer and a bottom layer. The top-layer of titanium
is converted to a top-layer of titanium dioxide nanotubes by anodizing, thus the top
layer of titanium dioxide nanotubes is thereby grown on the bottom layer of titanium.
The bottom layer of titanium is thus not converted, and this bottom layer of titanium
must subsequently be oxidized thermally to give a transparent film.
Experimental examples
[0108] In the following, some experimental examples according to the present invention are
given. The invention is not restricted to them, but may also be embodied in other
ways within the scope of the subject matter.
Example A
[0109] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0110] The conducting cable core for providing the electrical contact point is made of titanium.
[0111] See also fig. 1-3.
Example B
[0112] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0113] The conducting cable core for providing the electrical contact point is made of aluminium.
[0114] Regarding the conducting cable core, it should be made of a self-passivating material,
which titanium (Ti) and aluminium (Al) as used above are, but materials such as tungsten
(W), silicon (Si), zirconium (Zr), niobium (Nb), tantalum (Ta) and hafnium (Hf) can
also be used since these are also self-passivating. These materials work because they
tend to suppress the oxygen evolution by forming a passivation layer.
[0115] Consequently, oxygen evolving materials such as gold (Au) or platinum (Pt) can not
be used as material for the cable core, since oxygen bobbles up and causes oxidation
around the contact point.
Example C
[0116] The electrolyte solution comprises: 0.2 M Ammonium fluoride (NH4F, 0.3%) and 0.1
M H
3PO
4 aqueous electrolyte. The temperature of the electrolyte solution is: 298 K, and the
voltage difference between the anode and the cathode is: 10 V.
Example D
[0118] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0119] Oxygen in the electrolyte solution is removed by sonication and N
2 (or Ar) purging for a couple of hours for providing an oxygen-free atmosphere in
the electrolyte solution.
[0120] A gold (Au) wire 701 is placed near the bottom of the anode 702. A voltage V1 of
5 V is applied to the Au wire 701 until oxygen covers the Au wire 701. The wire for
voltage bias V2 is connected at the top of the anode 702. The growth direction is
controlled by the oxygen release from the gold wire, and the nanostructures grow from
below where the oxygen release occurs and up while having the wire connected at the
top.
[0121] Thus the growth of nanostructures is controlled by providing an oxygen-free atmosphere
of the electrolyte solution, and then developing oxygen from a gold wire by means
of electrolysis and placing the oxygen-developing gold wire furthest away from the
electrical contact.
Example E
[0123] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0124] Oxygen in the electrolyte solution is removed by sonication and N
2 (or Ar) purging for a couple of hours for providing an oxygen-free atmosphere in
the electrolyte solution.
[0125] A small air tube 801 for providing oxygen is placed under the anode 802. The wire
for voltage bias V2 is connected at the top of the anode 802. By using the N2 atmosphere
and the tube with air at the bottom of the anode, the nanostructures grow from below
and up while having the wire connected at the top.
[0126] Thus the growth of nanostructures is controlled by providing an oxygen-free atmosphere
of the electrolyte solution, and then providing oxygen from an air tube and placing
the air tube furthest away from the electrical contact.
Example F
[0128] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0129] Oxygen in the electrolyte solution is removed by sonication and N
2 (or Ar) purging for a couple of hours for providing an oxygen-free atmosphere in
the electrolyte solution.
[0130] The anode 902 is placed horizontally in the bottom of the container with electrolyte
solution, and a Ti wire 903 for voltage bias V1 is connected at the end of the anode
902. A gold (Au) wire 901 is placed at the other end of the anode 902. A voltage V2
of 5 V is applied to the Au wire 901 until oxygen covers the Au wire 901. The growth
direction of the nanostructures is controlled by the oxygen release from the gold
wire, and the nanostructures grow from the end of the anode where the oxygen release
occurs and to the other end of the anode where the wire is connected.
[0131] Thus the growth of nanostructures is controlled by providing an oxygen-free atmosphere
of the electrolyte solution, and then developing oxygen from a gold wire by means
of electrolysis and placing the oxygen-developing gold wire furthest away from the
electrical contact.
Example G
[0133] The electrolyte solution comprises: Ammonium fluoride (NH4F, 0.3%) water (H2O, 2%),
and Ethylene glycol (CH2OHCH2OH, 97.7%). The temperature of the electrolyte solution
is: 298 K, and the voltage difference between the anode and the cathode is: 10 V.
[0134] Oxygen in the electrolyte solution is removed by sonication and N
2 (or Ar) purging for a couple of hours for providing an oxygen-free atmosphere in
the electrolyte solution.
[0135] The anode 1002 is placed horizontally in the bottom of the container with the electrolyte
solution and a Ti wire 1003 for voltage bias V is connected at the end of the anode
1002. A small air tube 1001 for providing oxygen is placed at the other end of the
anode 1002. By using the N2 atmosphere and the tube with air at one end of the anode,
the nanostructures grow from this end of the anode and to the other end of the anode
where the wire is connected. Thus the growth of nanostructures is controlled by providing
an oxygen-free atmosphere of the electrolyte solution, and then providing oxygen from
an air tube and placing the air tube furthest away from the electrical contact.
[0136] Many variations of the electrolyte solution, temperature and voltage difference are
known to work.
[0137] Although some embodiments have been described and shown in detail, the invention
is not restricted to them, but may also be embodied in other ways within the scope
of the subject matter defined in the following claims. In particular, it is to be
understood that other embodiments may be utilised and structural and functional modifications
may be made without departing from the scope of the present invention.
[0138] In device claims enumerating several means, several of these means can be embodied
by one and the same item of hardware. The mere fact that certain measures are recited
in mutually different dependent claims or described in different embodiments does
not indicate that a combination of these measures cannot be used to advantage.
[0139] It should be emphasized that the term "comprises/comprising" when used in this specification
is taken to specify the presence of stated features, integers, steps or components
but does not preclude the presence or addition of one or more other features, integers,
steps, components or groups thereof.