Patent Application
20140291591
This application claims priority to and the benefit of Korean Patent Application No. 10-2013-0033835 filed in the Korean Intellectual Property Office on Mar. 28, 2013, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
A nanocomposite structure, an electrode including the nanocomposite structure, a manufacturing method of the electrode, and an electrochemical device including the electrode are provided.
2. Description of the Related Art
Titanium dioxide receives more attention as a material for a photoanode of a dye-sensitized solar cell as compared with zinc oxide (ZnO) and tin dioxide (SnO2) since it is combinable with various dye-sensitizers and has relatively fewer surface states.
Recently, one-dimensional structured materials, such as nanofibers, nanotubes, and nanowires, have been applied for a photoanode of a dye-sensitized solar cell. Particularly, vertically grown titanium dioxide nanotubes have shown excellent electron mobility and ion mobility to improve the photocurrent and open circuit voltage, and thus, have been widely studied.
Most titanium dioxide nanotubes are possible to be synthesized by an electrochemical oxidation method using a titanium foil. This method can easily control the morphology of nanotubes having excellent crystallinity and self-aligning property.
However, the thus manufactured titanium dioxide nanotubes may have a decreased internal surface area due to the formation of a soft wall thereof, which restricts the efficiency in dye adsorption and the current density of a dye-sensitized solar cell.
In order to solve the problem such as a decrease in internal surface area, there have been attempts to form nanoparticles on the internal surface or modify the internal surface. The thus formed nanotubes have advantages and disadvantages according to the morphology and crystal structure thereof.
For example, when nanoparticles are formed on internal surfaces of nanotubes through a titanium tetrachloride (TiCl4) treatment, the open circuit voltage may be reduced due to an increase in recombination at an interface between the nanoparticles and an electrolyte. Moreover, when nanoparticles are formed through modification of internal surfaces of nanotubes, charge transfer characteristics may be deteriorated.
Therefore, research on the improvement of surface characteristics of nanotubes, without the deterioration of charge transfer characteristics, is needed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.
An embodiment provides a nanocomposite structure, including: TiO2 nanotube,; and nanoparticles uniformly formed on wall surfaces of the TiO2 nanotubes.
The nanoparticles may include a metal or a metal oxide.
The nanoparticles may include titanium (Ti), titanium oxide, silver (Ag), silver oxide, gold (Au), gold oxide, platinum (Pt), platinum oxide, or a combination thereof.
The average outer diameter of the TiO2 nanotubes may be from 10 to 1000 nm.
The average wall thickness of the TiO2 nanotubes may be 0.1 to 100 nm.
An embodiment provides an electrode including the nanocomposite structure.
An embodiment provides an electrochemical device including the electrode.
An embodiment provides a manufacturing method of an electrode, the method including: preparing amorphous TiO2 nanotubes by anodizing a Ti substrate; crystallizing the amorphous TiO2 nanotubes through a heat treatment; bonding the crystallized TiO2 nanotubes to a substrate; and forming a coating layer on wall surface of the crystallized TiO2 nanotubes through atomic layer deposition (ALD), the coating layer containing metal or metal oxide; and subjecting the coated nanotubes to a water treatment.
The crystallizing of the amorphous TiO2 nanotubes through heat treatment may be performed at a temperature of from 200 to 800° C.
The substrate may be a transparent conductive oxide (TCO) substrate, a silicon substrate, a plastic substrate, a glass substrate, a metal substrate, a quartz substrate, a metal oxide substrate, a metal nitride substrate, or a combination thereof.
The atomic layer deposition (ALD) may be remote plasma atomic layer deposition (RPALD).
The atomic layer deposition (ALD) may be performed by using a metal precursor or a metal oxide precursor; and plasma.
The atomic layer deposition (ALD) may be remote plasma atomic layer deposition (RPALD) using titanium tetraisopropoxide [Ti(OC(CH3)2)4] as a metal precursor and O2 plasma as a reactant.
The atomic layer deposition (ALD) may be performed at a temperature of 70 to 150° C.
In the forming of the coating layer, the metal may be titanium (Ti), silver (Ag), gold (Au), platinum (Pt), or a combination thereof, and the metal oxide is titanium oxide, silver oxide, gold oxide, platinum oxide, or a combination thereof.
The thickness of the coating layer may be from 1 to 50 nm.
The coating layer may be converted into nanoparticles through the water treatment of the nanotubes with the coating layer formed thereon.
The water treatment may be performed for from 10 to 100 hours.
Specific descriptions of other embodiments are included in the following detailed description.
According to an embodiment, the nanocomposite structure can induce efficient dye adsorption and charge transfer, and the electrode including the nanocomposite structure and the electrochemical device including the electrode can have improved photo conversion efficiency.
a) is an HR-SEM image of a TiO2 nanotube array after ALD coating before water treatment in Example 1;
b) is an HR-SEM image of the ALD-coated TiO2 nanotube array after water treatment for 24 hours in Example 1;
c) is a high resolution-transmission electron microscopy (HR-TEM) image of a TiO2 nanotube array after ALD coating before water treatment in Example 1;
d) is an enlarged image of a circle part of
e) is an HR-TEM image of a nanotube after water treatment for 48 hours in Example 2;
Hereinafter, embodiments will be described in detail. However, these embodiments are merely exemplified, and the scope of protection is not limited thereto but defined by the appended claims.
In an embodiment, a nanocomposite structure including TiO2 nanotubes and nanoparticles uniformly formed on wall surface of the TiO2 nanotubes is provided.
The nanocomposite structure may have a form in which several TiO2 nanotubes are densely formed in parallel with each other and several nanoparticles are uniformly formed on wall surface of the TiO2 nanotubes.
The nanocomposite structure can induce efficient dye adsorption and charge transfer, and the electrode including the nanocomposite structure and the electrochemical device including the electrode can having improved photo conversion efficiency.
The nanoparticles may include a metal or a metal oxide. The nanoparticles may include titanium (Ti) or titanium oxide. In addition, the nanoparticles may optionally further include silver (Ag), silver oxide, gold (Au), gold oxide, platinum (Pt), platinum oxide, or a combination thereof. For example, the nanoparticles may be TiO2 nanoparticles.
The average outer diameter of the TiO2 nanotubes may be from 10 to 1000 nm. The average outer diameter thereof may be from 10 to 800 nm, 10 to 600 nm, 10 to 400 nm, 10 to 200 nm, 10 to 150 nm, 50 to 150 nm, or 90 to 110 nm. When the average outer diameter falls within the above range, the nanocomposite structure can exhibit excellent performance when being applied to an electrode of an electrochemical device.
The average wall thickness of the TiO2 nanotubes may be 0.1 to 100 nm. The average wall thickness thereof may be 0.1 to 80 nm, 0.1 to 60 nm, 0.1 to 40 nm, 0.1 to 20 nm, 0.1 to 10 nm, 1 to 10 nm, or 5 to 7 nm. When the average wall thickness falls within the above range, the nanocomposite structure can exhibit excellent performance when being applied to an electrode of an electrochemical device.
The existing method of forming nanoparticles inside the nanotubes by subjecting titanium dioxide nanotubes to water treatment has disadvantages in that the nanotubes become thinner due to the water treatment, thus original shapes thereof may be lost and the thicknesses thereof is difficult to control. However, according to the nanocomposite structure of an embodiment, the thicknesses of the nanotubes can be maintained within the above range and the thicknesses thereof can be controlled.
According to an embodiment, an electrode including the nanocomposite structure is provided.
The electrode has a form in which several TiO2 nanotubes are densely formed in parallel with each other on a substrate, and several nanoparticles are uniformly formed on wall surfaces of the TiO2 nanotubes.
According to an embodiment, a manufacturing method of the electrode is provided.
The manufacturing method of the electrode includes: preparing amorphous TiO2 nanotubes by anodizing a Ti substrate; crystallizing the amorphous TiO2 nanotubes through heat treatment; bonding the crystallized TiO2 nanotubes on a substrate; forming a coating layer on wall surface of the crystallized TiO2 nanotubes through atomic layer deposition (ALD), the coating layer containing a metal or a metal oxide; and subjecting the nanotubes with the coating layer formed thereon to water treatment.
The thus manufactured electrode can induce efficient dye adsorption and charge transfer, and the electrochemical device including the same can realize excellent photo conversion efficiency.
The Ti substrate may be a Ti foil.
In the preparing of the amorphous TiO2 nanotubes, the anodizing may be performed by a general anodizing method that can be used in the art. The anodizing may be performed by potentiostatic anodization using a cell having two electrodes.
The anodizing may be performed by using an electrolyte containing NH4F and ethylene glycol.
For example, in the preparing of the amorphous TiO2 nanotubes, the Ti substrate is washed, anodized at 10 to 100 V for 1 to 3 hours, subjected to ultrasonic treatment while being immersed in an oxidizing solution of H2O2, and then washed.
In addition, the anodizing may be performed twice or more.
The anodizing may be performed by a double anodization method to suppress the formation of nanograss. For example, the Ti substrate is anodized at 10 to 100 V for 1 to 3 hours, subjected to ultrasonic treatment in an oxidizing solution of H2O2 or the like, anodized while being immersed in an electrolytic solution, subjected to ultrasonic treatment in a solution of isopropanol or the like, and then dried.
The preparing of the amorphous TiO2 nanotubes may be performed at room temperature.
The crystallizing of the amorphous TiO2 nanotubes through heat treatment may be performed at a temperature of from 200 to 800° C. The crystallizing may be performed at from 300 to 700° C., 400 to 600° C., or 400 to 500° C. When the heat treatment is performed within the above temperature range, the amorphous TiO2 can be sufficiently crystallized.
The heat treatment may be performed while the temperature is slowly raised.
The heat treatment may be performed for 1 to 5 hours.
The nanotubes crystallized through the heat treatment may be anodized again.
The bonding of the TiO2 nanotubes to the substrate may be performed by separating the crystallized TiO2 nanotubes, transferring the separated TiO2 nanotubes to the substrate, and bonding the TiO2 nanotubes to the substrate, in that order.
The separating of the crystallized TiO2 nanotubes may be performed by, for example, dissolving the crystallized TiO2 nanotubes in an oxidizing solution of H2O2 or the like.
The bonding of the TiO2 nanotubes to the substrate may be performed by, for example, dripping a titanium alkyl oxide solution on the substrate and then fixing the TiO2 nanotubes to the substrate, followed by heating at 100 to 600° C.
The substrate may be a transparent conductive oxide (TCO) substrate, a silicon substrate, a plastic substrate, a glass substrate, a metal substrate, a quartz substrate, a metal oxide substrate, a metal nitride substrate, or a combination thereof.
The ALD may be one that can be generally employed in the art.
The ALD may be performed at least once, and may be performed once or more but 70 times or less.
The ALD may be, for example, remote plasma ALD (RPALD).
The ALD may be performed by using a metal precursor or a metal oxide precursor, and plasma. The metal may be titanium (Ti), silver (Ag), gold (Au), platinum (Pt), or a combination thereof, and the metal oxide may be titanium oxide, silver oxide, gold oxide, platinum oxide, or a combination thereof.
Examples of the metal precursor or metal oxide precursor may include alkyl oxides of the above metals.
An example of the plasma may be O2 plasma.
The ALD may be performed at a temperature of from 70 to 150° C.
The coating layer formed by the ALD may have a thickness of 1 to 50 nm. The thickness thereof may be 1 to 40 nm, 1 to 30 nm, or 1 to 20 nm. The coating layer may have a layered type. According to the ALD, a coating layer with a very uniform thickness may be formed on the wall surfaces of the nanotubes.
Here, in the subjecting of the nanotubes with the coating layer to water treatment may be performed by immersing the nanotubes with the coating layer in water.
The water treatment may be performed for 10 to 100 hours. The water treatment may be performed for 10 to 50 h or 20 to 50 hours.
Through the subjecting of the nanotubes with the coating layer to water treatment, the coating layer may be converted into nanoparticles.
The existing method of forming nanoparticles inside nanotubes by subjecting titanium dioxide nanotubes to water treatment has disadvantages in that the nanotubes becomes thinner due to the water treatment, thus original shapes thereof may be lost and the thickness thereof is difficult to control.
However, according to the manufacturing method of an electrode of an embodiment, since the water treatment is performed after the coating layer is formed on the surfaces of the nanotubes by an ALD, the basic frame of the nanotubes is maintained, so that the thickness of the nanotubes can be maintained at a predetermined level and the thickness of the nanotubes can be controlled. Accordingly, a nanocomposite structure having a form in which nanoparticles are formed on wall surface of nanotubes and a shape of the nanotubes is maintained intact can be manufactured. Also, an electrode including the nanocomposite structure and a substrate can be provided.
Further, nanoparticles are difficult to form on surface of the nanotubes according to the existing method, in which nanoparticles are directly injected, but the nanoparticles can be very uniformly introduced to the wall surfaces of the nanotubes according to an embodiment.
According to an embodiment, an electrode manufactured by the above method is provided.
In the electrode, TiO2 nanotubes are densely packed and vertically parallel with each other on a base, and nanoparticles are uniformly formed on wall surfaces of the nanotubes.
According to an embodiment, an electrochemical device including the electrode is provided.
The electrochemical device includes a display device, an energy device, and the like. Examples of the electrochemical device include a light emitting diode (LED), an organic light emitting diode (OLED), an optical sensor, a transistor, a solar cell, a fuel cell, a photocatalytic device, a photo detection device, a photoelectrochemical water splitting device, and the like. For example, the electrochemical device may be a dye-synthesized solar cell.
An embodiment has been made in an effort to provide a nanocomposite structure, an electrode, and a manufacturing method of the electrode, having advantages of inducing efficient dye adsorption and charge transfer.
An embodiment has been also made in an effort to provide an electrochemical device having advantages of having excellent photo conversion efficiency.
Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are merely for illustrating the present invention, but the present invention is not limited thereto.
Preparing nanotubes by anodizing Ti substrate
A highly arranged TiO2 nanotube array was prepared by potentiostatic anodization using a cell having two electrodes. Ti foil (0.127 mm, purity 99.7%, Aldrich) was used as an electrode and Pt gauze was used as counter electrode. Prior to anodization, the Ti foil was washed by ultrasonic treatment in an acetone, isopropanol, or ethanol solvent. Then, the Ti foil was washed with deionized (DI) water and then dried with nitrogen steam. The two electrodes were disposed at an interval of 1.5 cm, and 0.5 wt % of NH4F (Aldrich, purity: 99.8%) and ethylene glycol (Aldrich, purity: 99.9%) were used as an electrolyte.
A double anodization method was employed to prevent the formation of nanograss. First, the Ti foil was anodized at 50 V for 2 hours, subjected to ultrasonic treatment while being immersed in a 30% solution of H2O2 (Aldrich), washed with DI water, and then dried. Then, the Ti foil was anodized at 50 V for 3 hours, while being immersed in the electrolyte, and subjected to ultrasonic treatment for 15 min while being immersed in isopropanol, followed by drying in air. The two anodizing processes were performed at room temperature while stifling was continuously and slowly conducted.
Crystallizing Nanotubes through Heat Treatment
In order to crystallize the amorphous TiO2 nanotubes array formed on the Ti foil, heat treatment was performed thereon at 450° C. for 3 hours while the temperature was raised at a rate of 2° C./min. The heat-treated Ti foil and amorphous TiO2 nanotube array were again anodized at 50 V for 30 min, and then immersed in a 30% solution of H2O2 (Aldrich).
Bonding Nanotubes to Substrate
The amorphous TiO2 layer formed through anodization after heat treatment was dissolved in H2O2 to separate a crystalline white TiO2 nanotube array (TNTA) film therefrom. The TNTA film was transferred to a Petri dish containing isopropyl alcohol. This crystalline TNTA film was transferred to a transparent conductive oxide substrate (TCO substrate). In order to strongly fix the TNTA film to a fluorine-doped tin oxide (FTO) film, some droplets of a solution in which titanium butoxide was added to isopropanol were dripped thereon. Last, the TNTA film on the FTO film was heated on a heating substrate at 200° C. for 30 min, and then heated in air at 450° C. for 30 min.
The thus prepared crystalline anatase TiO2 nanotube array was analyzed through X-ray spectroscopy (XRD), and the resultant graph is shown in
Coating TiO2 Layer by Atomic Layer Deposition
A thin amorphous TiO2 layer of ˜15 nm was coated on the TNTA film on the FTO film by using atomic layer deposition (ALD). Here, the deposition was conducted at 100° C. by remote plasma atomic layer deposition (RPALD) using titanium tetraisopropoxide [Ti(OC(CH3)2)4] as a Ti precursor and O2 plasma as a reactant.
The state in which the TiO2 layer is coated on the wall surface of the TiO2 nanotube array by the ALD as described above was observed by high-resolution scanning electron spectroscopy (HR-SEM), and the results thereof are shown in
Water Treatment
After the TiO2 layer was coated on the wall surface of the TiO2 nanotube array by the ALD as described above, the resultant structure was immersed in water (watery NH4F solution) for 24 hours.
The nanotube array after the above water treatment was photographed by HR-SEM, and the results are shown in
Manufacture of Dye-Sensitized Solar Cell
The thus manufactured TNTA electrode was immersed in a solution, in which 0.3 mM N719 dye (cis-bis(isothiocyanato)bis(2,20-bipyridyl-4,40-dicarboxylato)-ruthenium(II)bis-tetrabutylammonium, Solaronix) was added into a 1:1 (volume ratio) mixed solvent of acetonitrile (Sigma-Aldrich) and t-butanol (Sigma-Aldrich), at room temperature for 18 hours. Then, washing with acetonitrile and drying with nitrogen gas were conducted.
The Pt counter electrode was manufactured by thermal decomposition of a solution in which 0.01 M H2PtCl6 (Sigma-Aldrich) was added to isopropyl alcohol (Sigma-Aldrich), and then applied to the FTO film, followed by sintering at 450° C. for 30 min.
Two holes were formed in the Pt counter electrode for electrolyte injection. Spacers were disposed between the TNTA photoelectrode and the Pt counter electrode while Surlyn (25 μm, Solaronix) was used for the spacers. Last, an electrolyte was injected between the spacers through one of the two holes of the Pt counter electrodes, and then the holes were sealed with the Surlyn film and cover glass.
An electrolyte in which 0.5 M 1-methyl-3-propyl imidazoliumiodide (MPII, Sigma-Aldrich) and 0.05 M iodine (Sigma-Aldrich) were added to a solution of 3-methoxypropionitrile (Aldrich) was used for the electrolyte in the example. LiI (Sigma-Aldrich), 4-tert-butylpyridine (Aldrich), and guanidinium thiocyanate (Sigma) were used as an electrolyte additive.
A dye-sensitized solar cell was manufactured by the above method.
An electrode and a solar cell including the same were manufactured by the same method as in Example 1, except that the coated nanotubes were immersed in water for 48 hours in the step of water treatment.
The nanotubes subjected to water treatment for 48 hours in Example 2 above were photographed by HR-SEM, and the results are shown in
An electrode and a solar cell including the same were manufactured by the same method as in Example 1, except that a TiO2 nanotube array subjected to neither ALD nor water treatment was used as an electrode.
a) is an HR-SEM image of a TiO2 nanotube array after ALD coating but before water treatment. It can be seen from
c) is a high resolution-transmission electron microscopy (HR-TEM) image of a TiO2 nanotube array after ALD coating but before water treatment, and
b) is an HR-SEM image of the ALD-coated TiO2 nanotube array after water treatment for 24 hours. It can be confirmed from
e) is an HR-TEM image of a nanotube after water treatment for 48 hours in Example 2, and
In order to evaluate performances of solar cells of Example 1, Example 2, and Comparative Example 1, J-V measurement was employed. Evaluation was conducted with a Newport, USA solar simulator (1000 W Xe source) and Keithley 2400 source meter (device area: 0.25 cm2) under 1 sun illumination (AM 1.5 G, 100 mWcm−2) conditions. The intensity of incident sunlight illumination was fixed to a 1 sun condition by using the NREL-certified Si standard cell equipped with the KG-5 filter.
It can be seen from
Charge transfer characteristics of the solar cells manufactured in Example 1 and Comparative Example 1 were evaluated by electrochemical impedance spectroscopy (EIS). The results are tabulated in Table 2.
In Table 2, Rs represents series resistance, Rpt represents charge transfer resistance at an interface of the electrode and electrolyte, Rt represents transport resistance, Rrec represents charge transfer resistance at the interface of TiO2 nanotubes and the electrolyte, and ηcol represents column efficiency.
It can be seen from Table 2 that electron conduction characteristics were excellent in Example 1 as compared with Comparative Example 1.
While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2013-0033835 | Mar 2013 | KR | national |
