Succeeded in Synthesizing a Crystal Organic-Inorganic Nano-Hybrid Film

- Breakthough Technology for Realization of Thin Film Material for Electrode Catalyst -

Keywords:
Organic molecules, non-organic molecules, rubeanic acid copper, proton conductivity, fuel-cell electrode catalyst, amorphous material, crystal organic-inorganic nano-hybrid film, nano-hybrid thin film synthesizing technology, electrode catalyst, coordination polymers, rubeanic acid copper, rubeanic acid copper thin film, crystal nano-film, crystal complex film, organic ligands, dithiooxamidato ligands, super-flat surface, bottom-up process, sapphire substrate, metal ions, surface x-ray diffraction method, atomic arrangement, high brilliance radiation, Spring-8, ligand symmetry, substrate surface smoothness, amorphous material, ion conductivity

Introduction
A nano-hybrid thin film of crystal porous coordination polymer having a laminated layer structure of organic molecules and non-organic molecules in the order of atom layer has been successfully synthesized.
It is said that the nano-hybrid thin film is a promising thin film material for the fuel-cell electrode catalyst.

Co-developed by:
* Dr. Hiroshi Kitagawa, Dr. Katsuhiko Kaneizuka (Department of Chemistry, Faculty of Sciences, Kyushu University)
* Researchers Dr. Osami Sakata and Dr. Rie Aoki (JASRI)
* Dr. Mamoru Yoshimoto (Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology)

It has been considered that a complex called "a rubeanic acid copper", listed as one of the ion conduction materials, has high proton conductivity, and will possibly serve as the fuel-cell electrode catalyst. The rubeanic acid copper is generally an amorphous material. Because of being amorphous, i.e., non-uniform structure, it is not suitable for the making devices. The lab. team synthesized a crystal organic-inorganic nano-hybrid film by a bottom-up process, which was created by the lab. team. A rubeanic acid copper and copper ions were used for synthesizing the film. The nano-hybrid film was investigated by using high brilliance synchrotron radiation (surface/interface structure analysis beam line BL13XU) of SPring-8, large radiation facility. The surface x-ray diffraction method was used for the measurement.
From the investigation results, it was confirmed that in the interlayer and intra-layer, the complexes are periodically arrayed in the order of atom layer, viz., the crystal film was formed.
The nano-hybrid thin film synthesizing technology will be applied to the making of devices such as organic electroluminescence elements and transistors, in addition to the fuel cell catalyst. The technology must have been published on "Journal of the American Chemical Society", issued on November 26, 2008.

Background
To form the fuel cell and the electrode catalyst, it is essential to develop a material having high ion conductivity.
Bear this in mind, the researchers have synthesized various coordination polymers and measured the ion conductivities of the polymers.
Through the measurement, it was found that rubeanic acid copper exhibits an extremely high ion conductivity. The researchers felt the possibility of realizing a device having high ion conductivity by sandwiching the rubeanic acid copper between the electrodes.

The amorphous material has been used for the fuel cell. A crystal material, if it could be used in place of the amorphous material, will give rise to the following advantages of decrease of the defective percentage of the resultant products and increase of ion conductivity.

A lab team has succeeded in forming a bulk crystal of the rubeanic acid copper. The structure of the crystal is unstable, however. Because of the unstable structure, its crystal has been insufficiently evaluated.

Many researchers have competitively tried to form the rubeanic acid copper thin film having a uniform structure in the inorganic chemical field in the world.
Howevr. no one has succeeded in forming the thin film of the rubeanic acid copper, so far as we know. It figures that concurrently forming of a number of crystal structures would cause low crystallinity of the formed thin film.

Experiments
Fig. 1:
To control the reactivity of copper ions with rubeanic acid as organic ligands, a try was made to laminate copper ions and rubeanic acid on the substrate interface in paired fashion, as shown in Fig. 1.
Specifically, rubeanic acid and copper were laminated on a super-flat sapphire substrate surface to form a pair of layers, and the same process was cyclically repeated to form successive paired layers, as blocks are built up (This film forming process will be referred to as a "bottom-up process".). As a result, an organic-inorganic nano-hybrid film was formed.
More specifically, a sapphire substrate having been pre-processed (modified with binder) was immersed in an aqueous solution of metal ions to fix the metal ions to the substrate. Then, the resultant was immersed in an ethanol solution of organic ligands to fix the ligands to the metal ions.
In this way, one cycle layer (rubeanic acid copper thin film having a uniform structure) was formed.
It is noted here that a thickness of the nano-thin film can be controlled by selecting the number of the film forming processes cyclically performed, and that this thin film forming process, or the bottom-up process, is very simple.
It is further noted that the bottom-up process is advantageous in that it uniformly forms the thin film over a large area, and is environment-friendly with no need of the vacuum and heat treatments. The bottom-up process comes in the category of the solution process.

Fig. 2:
Thin films of single-layer (a), bi-layers (b) and tri-layer (c) were formed.
The transmission electron spectra of those films were measured.
In each cycle layer of each layer, a fixed amount of rubeanic acid copper was fixed. This was confirmed through a measurement result that the absorbance peak increased with increase of the film thickness (see Fig. 2).

The absorption intensity is approximately proportional to the film thickness.
In Fig. 2, increased absorption intensities appear in a wavelength region from 300 to 900 nm, and from the figure it is seen that the nano-film grows as the number of cycles increases. No information about an arrangement of atoms in each cycle layer was gathered. A measurement was made to check the atomic arrangements of the films. In the measurement, the X-ray source of the laboratory was used and the diffraction method was employed. The measurement failed to present the structural information.

There would be two reasons for that the measurement failed to provide the structural information. Firstly, the thickness of the test pieces is very thin, less than 10 nm. Secondly, the diffraction intensity of the thin films is weak, unlike the semiconductor thin film of which the crystallinity is considerably high and the film forming process is matured.

To cope with this, the surface x-ray diffraction method using the high brilliance radiation in Spring-8 was used to invest the atomic arrangement of the thin film. Diffraction intensities of the x-rays diffracted in the thin films were successfully measured with well satisfaction, by the method.

Fig. 3:
Three thin films of rubeanic acids having different ligands were formed by the bottom-up method. Each thin film consists of 11 cycle layers.
The rubeanic acids were:
1) rubeanic acid (symmetric molecule, Fig. 3-1)
2) pi-extended rubeanic acid (Fig. 3-2)
3) ethanol rubeanic acid (asymmetric molecule, Fig. 3-3)

Fig. 4:
In the nano-films of the rubeanic acid and the pi-extended rubeanic acid, diffraction peaks were observed in both the out-of-plane measurement (Fig. 4-1) and the in-plane measurement (Fig. 4-2). Presence of the diffraction peaks indicates that the rubeanic acid copper nano-film has a crystal structure.

In the case of the nano-film of the ethanol rubeanic acid, diffraction peaks were observed in the out-of-plane measurement. From this, it was confirmed that the nano-film grew with increase of the number of cycles. No diffraction peak was observed in the in-plane measurement of the nano-film. This indicates that the arrangement of atoms of the rubeanic acid copper was not formed in the nano-film.

From the study, it was taught that the ligand symmetry and the smoothness of the substrate are essentially taken into accoutn when the crystal coordination polymer material is formed on the substrate.

Specifically, at least two conditions to form a crystal nano-film were derived from the study. The first condition is to use the symmetrical molecule. Three molecules, including symmetrical and asymmetrical molecules, were experimented. In the case of using the symmetrical molecule, atoms were distinctly arranged within the plane. In the asymmetrical molecule, no atoms were arranged. The second condition was to use a substrate which is flat in atomic levels. A nano-film having a 3-dimensional atom arrangement was formed only when the super-flat sapphire substrate was used.

Bottom-Up Method
A typical process having been used to crystallize the material that is amorphous in bulk state, is the heat treatment. The heat treatment is unable to crystallize such a material that is unstable, for example, decomposable by heating, however.
The bottom-up method developed this time successfully crystallized the rubeanic acid copper, which has been considered to be difficult to crystallize.

The success of the crystallization implies that a functional material, which has been considered to be impossible to crystallize, can be crystallized by the bottom-up method.
The bottom-up method as the synthesizing method comes in the category of the solution method. This method is advantageous in that it uniformly forms the thin film over a large area, and it is environment-friendly with no need of using the vacuum treatment and the heat treatment.
In this sense, the bottom-up method will be applied to electroluminescence element, transistors and the like, in addition to the fuel cells.

The source, written in Japanese, is linked at:
http://www.kyushu-u.ac.jp/pressrelease/2008/2008-11-26.pdf
http://www.spring8.or.jp/ja/current_result/press_release/2008/081126

#: For Figs. 1 to 4, reference is made to FuelCell japan