松尾 吉晃

The preparation and application of new intercalation compounds of graphite oxide are reviewed. Graphite oxide was silylated with different silylating reagents such as alkyltrichlorosilane and 3-aminopropylethoxysilanes. After they were bidentately or monodentately attached to layers of graphite oxide, Si-OH groups which are available for further functionalization were introduced. When highly silylated graphite oxide was heated under vacuum, microporous or mesoporous pillared carbons in which adjacent carbon layers are connected with siloxane or silsesquioxane pillars were obtained. Organic molecules were size-selectively intercalated into the pillared carbon obtained from graphite oxide silyated 3 times with methyltrichlorosilane. The excess hydrogen adsorption of pillared carbons at ambient temperature reached 0.6 wt% at 20 MPa. Pillared carbon thin films were prepared and they showed a selective electrical resistance change response to electron donating and smaller organic molecules.

Pillared carbons were prepared from graphite oxide silylated by a two-step method using octyltrichlorosilane and then 3-aminopropylethoxysilane. When the second step reaction was performed at 110 °C, larger amounts of silylating reagent reacted with the concurrent reduction of graphite oxide. Addition of water in the reaction system during the second step reaction accelerated the reaction at a lower temperature as the result of the hydrolysis of ethoxy groups in 3-aminopropylethoxysilane to more active silanol groups. Microporous pillared carbons with interlayer spacings of around 1.55 nm and pore sizes less than 1 nm were obtained from the pyrolysis of the resulting silylated graphite oxides. The BET surface area increased with the increase in the amount of water added during the second step reaction and reached 500 m²/g.

In order to develop metal bipolar plates for polymer electrolyte fuel cells (PEFC), various grades of stainless steels (SUS304, SUS316, and SUS430) were coated with carbonaceous thin films by plasma-assisted chemical vapor deposition method. The resulting carbon-coated stainless steels were characterized by Raman spectroscopy, interfacial contact resistance (ICR) measurement, and scanning electron microscopy (SEM). Corrosion properties were examined by dynamic polarization measurement in H₂SO₄ solution. ICR values were decreased by carbon coating due to the deposition of a conductive carbonaceous thin film. The corrosion resistance of carbon-coated SUS304 and SUS430 was superior to that of carboncoated SUS316 at ambient temperature. Based on the SEM images obtained after dynamic polarization measurements, it was found that carbonaceous thin-film layers on SUS304 and SUS430 were dense and did not peel. At 80°C (simulated operating temperature of PEFCs), corrosion resistance of carbon-coated SUS304 was superior to that of carbon-coated SUS430. The influence of the stainless steel grade on the corrosion properties provided by carbon-coating was discussed in relation to the carbon deposition mechanism.

Recent studies on the preparation and properties of carbon thin films from the dispersion of graphite oxides or organically modified graphite oxides are summarized. Silylated graphite oxide was well dispersed in an organic solvent when nhexadecylamine was intercalated into it and thin film samples were obtained from the resulting nanosheet solutions. The interlayer spacing of the silylated graphite oxide thin films reached 1.22 nm after reduction by hydrazine and their electrical nature changed from insulating to semi-conducting. Transparent and conducting carbon films which adhered well to glass substrates were obtained from the pyrolysis of silylated graphite oxide thin films. The sheet resistance of the carbon film obtained from the pyrolysis of silylated graphite oxide at 900°C reached 700 Ω/sq. When graphite oxide thin films were heated at 300°C, carbon films with a large interlayer spacing of 0.42 nm was obtained. Considerable amounts of hydrogen were electrochemically stored in this carbon film, accompanied by an increase of interlayer spacing by 0.03 nm.

The dispersion behavior of n-hexadecylamine-intercalated silylated graphite oxide in a chloroform/cyclohexane mixture was investigated. When 1.2 molecules of n-hexadecylamine per graphite oxide unit were added, the interlayer gallery of the silylated graphite oxide was saturated by n-hexadecylamine molecules, and the interlayer spacing reached 3.1 nm. The resulting intercalation compounds dispersed well in a chloroform/cyclohexane solution. After centrifugation, the intercalation compounds with lower n-hexadecylamine contents mostly precipitated, however, the dispersion of those with higher n-hexadecylamine contents were stable until the concentration reached 0.6 g of graphite oxide/L. This was ascribed to the weaker interaction between the alkyl chains bonded to the adjacent graphite oxide layers forming a bilayer of alkyl chains within a gallery.

A carbonaceous thin-film electrode was prepared by plasma-assisted chemical vapor deposition, and used as a model carbon electrode to study the interfacial reactions between a electrode and electrolyte. Lithium-ion transfer at the interface between carbonaceous thin-film electrode and electrolyte was studied by electrochemical impedance spectroscopy. In the Nyquist plots, semi-circles assigned to charge-transfer resistance were observed in high frequency region. Activation energies for lithium-ion transfer at the interface between carbonaceous thin-film electrode and electrolyte were evaluated, and the values were found to be dependent on the electrolyte solutions. The effects of ion-solvent interaction on the activation energies for the interfacial lithium-ion transfer are discussed based on theoretical calculations.

Graphite oxide was silylated by dimethyloctylchlorosilane under various conditions. Silylated graphite oxide was successfully obtained and the maximum silicon content reached 0.17 mol/graphite oxide unit when the butylamine/dimethy-loctylchlorosilane/ graphite oxide unit ratio was about 19/16/1. The above maximum silicon content was much smaller than that obtained for graphite oxide silylated by octyltrichlorosilane, probably because of the steric hindrance of methyl groups against oxygen containing groups on the surface of graphite oxide. At the same time, the smaller amount of butylamine was needed to scavenge the HC1 molecules formed as a byproduct, reflecting the smaller number of chlorine atom in the dimethy-loctylchlorosilane molecule.

Carbonaceous thin films were prepared by plasma-assisted chemical vapor deposition from acetylene, argon and nitrogen trifluoride (NF₃). Resulting carbonaceous thin films were characterized by scanning electron microscope (SEM), Raman spectroscopy, and Auger electron spectroscopy (AES). SEM observation and Raman spectra revealed that surface morphology and crystallinity of carbonaceous thin films depended on the flow rate of NF₃ to a great extent. Carbonaceous thin films composed of fine particles were obtained at high flow rate of NF₃. It was considered that active fluorine atoms derived from NF₃ accelerated the carbon growth at vapor phase and influenced the surface morphology and crystallinity. Based on the result of AES measurement, fluorine atoms were not contained in resulting thin films. It is concluded that addition of NF₃ to plasma is good method for control the surface morphology and crystallinity of carbonaceous thin films without contamination by fluorine atoms.