Layered alkali iridate, layered iridic acid, and iridium oxide nanosheet

Abstract

Provided is a layered alkali iridate and a layered iridic acid to be used for producing iridium oxide nanosheets, and an iridium oxide nanosheet. A layered alkali iridate with composition of M.sub.xIrO.sub.y.nH.sub.2O (where M is a monovalent metal, x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0.5 to 1.5), wherein M.sub.xIrO.sub.y.nH.sub.2O has a layered structure. The M is potassium, and the layered alkali iridate has diffraction peaks at 2 diffraction angles of 13.0 and 26.0. A layered iridic acid with a composition of H.sub.xIrO.sub.y.nH.sub.2O (where x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0 to 1), wherein H.sub.xIrO.sub.y.nH.sub.2O has a layered structure. This layered iridic acid has diffraction peaks at 2 diffraction angles of 12.3 and 24.6. A single crystalline iridium oxide nanosheet having a thickness of 3 nm or less.

Claims

1. An alkali iridate having a composition of M.sub.xIrO.sub.y.nH.sub.2O (where M is an alkali metal, x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0.5 to 1.5), wherein M.sub.xIrO.sub.y.nH.sub.2O has a layered structure.

2. The alkali iridate according to claim 1, wherein the M is potassium and the layered alkali iridate has diffraction peaks at 2 diffraction angles of 13.0 and 26.0.

3. An iridic acid having a composition of H.sub.xIrO.sub.y.nH.sub.2O (where x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0 to 1), wherein H.sub.xIrO.sub.y.nH.sub.2O has a layered structure.

4. The iridic acid according to claim 3 having diffraction peaks at 2 diffraction angles of 12.3 and 24.6.

5. An iridium oxide nanosheet composed of single crystalline iridium oxide sheet with a nanosheet morphology having a thickness of 3 nm or less.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an electron microscope image of a layered alkali iridate.

(2) FIG. 2 is an X-ray diffraction pattern of a layered iridic acid.

(3) FIG. 3 is AFM images of an iridium oxide nanosheet thin film.

(4) FIG. 4 is an enlarged AFM image of an iridium oxide nanosheet.

(5) FIG. 5A is a TEM image, FIG. 5B is an enlarged TEM image, and FIG. 5C is an electron diffraction pattern, showing the morphology and structure of the iridium oxide nanosheet.

EMBODIMENTS OF THE INVENTION

(6) The following describes in detail a layered alkali iridate, a layered iridic acid, and an iridium oxide nanosheet according to the present invention. The present invention, however, is not limited to the following description in the range included in the technical scope thereof.

(7) [Layered Alkali Iridate and Layered Iridic Acid]

(8) A layered alkali iridate according to the present invention has a composition of M.sub.xIrO.sub.y.nH.sub.2O (where M is an alkali metal, x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0.5 to 1.5), wherein M.sub.xIrO.sub.y.nH.sub.2O has a layered structure. Examples of M include Li, Na, K, Rb, and Cs. The values of x and y can vary based on the amount of iridium oxide and alkali metal compound that is mixed during the synthetic process.

(9) The layered alkali iridate is produced by first (1) mixing iridium oxide with an alkali metal compound, (2) pelletizing the obtained mixture, (3) subjecting the formed pellets to a pre-calcination process, (4) grinding the pre-calcined pellets to a powder and making a pellet, (5) subjecting the pellet to calcination at higher temperature, and (6) final washing with water to remove water-soluble impurities. Thus, the layered alkali iridate can be obtained.

(10) The mixing step of (1) and the grinding step of (4) are preferably performed under nitrogen atmosphere. While the alkali metal compound used in the mixing step of (1) is not particularly limited, examples include potassium carbonate. While the mixture ratio of iridium oxide to the alkali metal compound is not particularly limited, the ratio may be set to, for example, iridium oxide:alkali metal compound=about 1:1 to 1:4. The pre-calcination step of (3) can be a heat treatment of several tens of minutes to several hours (one hour, as a typical example) within a range of 700 C. to 800 C. (750 C., as a typical example), for example, in an inert gas environment, and the second firing step of (5) can also be a heat treatment of several tens of minutes to several hours (1 hour, as a typical example) within a range of 700 C. to 800 C. (780 C., for example), for example, in an inert gas environment.

(11) The layered iridic acid can be obtained by treating the obtained layered alkali iridate in an acidic solution. Treatment in an acidic solution is performed by dispersing the layered alkali iridate in an acidic solution, for example 1 M HCl, for about one to four days (three days, as a typical example). The obtained layered iridic acid has a composition of H.sub.xIrO.sub.y.nH.sub.2O (where x is 0.1 to 0.5, y is 1.5 to 2.5, and n is 0 to 1), wherein H.sub.xIrO.sub.y.nH.sub.2O possesses a layered structure.

(12) [Iridium Oxide Nanosheet]

(13) An iridium oxide nanosheet according to the present invention is an iridium oxide single-crystal sheet having a thickness of 3 nm or less. This iridium oxide nanosheet is produced by first reacting an alkylammonium or an alkylamine with the layered iridate obtained above to form an alkylammonium-iridate intercalation compound with a layered structure, and subsequently mixing the alkylammonium-iridate intercalation compound with a solvent such as water to obtain an iridium oxide nanosheet colloid with a dark blue color. It should be noted that while various materials may be used as the alkylammonium or the alkylamine, tetrabutylammonium, for example, may be preferably used.

(14) Re-assembly of the iridium oxide nanosheet into a thin film can be conducted by electrostatic interaction between the obtained iridium oxide nanosheet and a counter cation. Specifically, a cleaned substrate (a Si substrate or a quartz substrate, for example) is positively charged by adsorption of a cationic polymer and then immersed in the iridium oxide nanosheet colloid. The substrate can be coated with one layer of the iridium oxide nanosheet in a single immersion step. Multilayers of iridium oxide nanosheet can be deposited by carrying out the immersion in cationic polymers solution and iridium oxide nanosheet colloid, with the amount of layers equivalent to the number of repetitions. For example, 5, 10, 10, and so on repetition affords thin films composed of 5 layers, 10 layers, 20 layers, and so on, of iridium oxide nanosheet.

(15) In the present invention, focus is placed on iridium as the central atom and the object is to obtain an iridium oxide nanosheet. The layered alkali iridate used for producing the iridium oxide nanosheet is newly synthesized, the layered iridic acid is newly synthesized from the layered alkali iridate, and the iridium oxide nanosheet is obtained from the layered iridate. Iridium oxide is used as the main component in dimensionally stable electrodes for electrolysis, ion separation, and the like, making the invented iridium oxide-based compounds applicable to these, in particular as an electrocatalyst for oxygen reduction, oxygen, chlorine and ozone evolution, and the like. Further, as iridium oxide is electrochemically stable in a wide potential range and has low solubility in acids and bases, the invented iridium oxide-based compounds can be used as a conductive metal oxide having excellent corrosion resistance as well as electronic conductivity.

EXAMPLES

(16) The present invention will now be described in detail on the basis of examples.

Example 1

(17) A layered potassium iridate and a layered iridic acid were synthesized. First, iridium oxide (IrO.sub.2) was mixed with potassium carbonate (K.sub.2CO.sub.3), in nitrogen atmosphere. The mixture ratio (IrO.sub.2:K.sub.2CO.sub.3) was set to 1:2. The mixture was pressed into pellets and pre-calcined at 750 C. for 1 hour under argon flow. The pellets were taken out and crushed into powder and again pressed into a pellet. Next, the formed pellets were calcined at 780 C. for 1 hour under argon flow. The samples exhibited a black color with a luster. The pellets were crushed again and washed with copious amount of water. Thus, a layered potassium iridate was obtained. The layered potassium iridate was reacted with 1 M HCl for three days to exchange the interlayer potassium ions with protons, resulting in a layered iridic acid. The supernatant liquid was dark brown.

(18) Scanning electron microscopy was conducted to observe the morphology of the compounds. FIG. 1 is a typical scanning electron microscope image of layered potassium iridate. The image shows a clear layered morphology. Similar layered morphology was observed for the layered iridic acid. FIG. 2 is an X-ray diffraction pattern (X-ray source: CuK) of the layered iridic acid. The layered iridic acid showed peaks at 2 values of 12.3 and 24.6, while the layered potassium iridate have peaks at 2 values of 13.0 and 26.0. It should be noted that, in both cases, peaks due to iridium oxide, the starting material, were not observed.

(19) The composition of layered iridic acid and layered potassium iridate composition was analyzed using an energy dispersive X-ray spectroscope (EDX; EX-200, manufactured by oriba, Ltd.). The atomic ratio was K/Ir/O=7.5/23/69 for the layered potassium iridate and Ir/O=28/72 for layered iridic acid. No potassium was detected in layered iridic acid.

Example 2

(20) Iridium oxide nanosheet was produced from the layered iridic acid. A 10% tetrabutylammonium hydroxide (TBAOH) solution was added to the layered iridic acid synthesized according to example 1 at the ratio of Ir:TBAOH=1:5. The dispersion was subjected to shaking for seven days at 25 C. and 160 rpm. Thus, a 0.5-g/L iridium oxide nanosheet colloid with a dark blue color was obtained.

Example 3

(21) An iridium oxide nanosheet film was obtained using the iridium oxide nanosheet colloid obtained in example 2. The nanosheet film was fabricated by layer by layer (LbL) deposition utilizing electrostatic interaction. First, a cleaned substrate (a Si substrate or a quartz substrate, for example) is positively charged by adsorption of a cationic polymer and then immersed in a 0.3-g/L iridium oxide nanosheet colloid for 20 minutes. The Si substrate was then pulled up, washed with ultrapure water, immersed in a solution of cationic polymer once again. This sequence was repeated to obtain multilayers of iridium oxide nanosheet.

(22) Atomic force microscopy (AFM) was acquired in dynamic force mode (DFM) using a scanning probe microscope (SPM; SPA400, manufactured by Seiko Instruments, Inc.). FIG. 3 shows a typical AFM images of the surface of iridium oxide nanosheet films with 1, 3, and 5 layered samples. FIG. 4 is an enlarged AFM image showing that the thickness of individual iridium oxide nanosheets are 1.65 nm0.1 nm.

(23) The transmission electron microscope (TEM) image in FIG. 5A and the enlarged TEM image in FIG. 5B of the iridium oxide nanosheet shows the two dimensional morphology. The electron diffraction pattern shown in FIG. 5C reveals that the iridium oxide nanosheet is single crystalline. These TEM images were taken by dropping the iridium oxide nanosheet colloid onto the TEM grid (carbon reinforced film, No. 1605, manufactured by JEOL Ltd.)

Example 4

(24) 20 L of the 0.3-g/L iridium oxide nanosheet colloid obtained in example 2 was dropped onto a 5-mm diameter glassy carbon rod (manufactured by Tokai Carbon Co., Ltd.) which was buff-polished with 0.05-m aluminum powder, and dried for 30 minutes at 60 C. 20 L of a 5-mass % Nafion (registered trademark of DuPont) solution was dropped on the glassy carbon, and dried for 30 minutes at 60 C. Thus, the working electrode for electrochemical studies was produced.

(25) Rotating disk electrode (RDE) measurement was performed using a standard three-electrode electrochemical cell. A Pt mesh was used as a counter electrode and a silver-silver chloride electrode was used as a reference electrode. The electrode described above was used as a working electrode. The RDE measurement was performed in 0.5 M H.sub.2SO.sub.4 electrolyte.

(26) Specific capacitance evaluation was conducted by cyclic voltammetry within a potential range of 0.2 to 1.2 V (vs. RHE), and scan rate within the range of 2 to 500 mV/s. A distinctive anodic peak was observed at 0.95 V (vs. RHE) with the corresponding cathodic peak at 0.85 V (vs. RHE).

(27) To evaluate the electrochemical stability of the iridium oxide nanosheet, the higher cut-off potential was raised from 1.2 V to 1.3 V, 1.4 V, and 1.5 V (vs. RHE), and cycled at 50 mV/s. The lower cut-off potential was set at 0.05 V (vs. RHE). In the potential range of 0.05 V to 1.5 V (vs. RHE), gas evolution associated with electrolysis did not occur, and no dissolution of iridium oxide nanosheet occurred.

Example 5

(28) The electrochemical properties in 1 M Li.sub.2SO.sub.4 were evaluated in the same manner as in example 4. In the potential range of 0.05 V to 1.5 V (vs. RHE), evolution associated with electrolysis did not occur, and no dissolution of iridium oxide nanosheet occurred.