User talk:Zaenal abidin

Nano-ball Allophane and Nano-tube Imogolite
The discovery of fullerene nano-ball and carbon nano-tube has accelerated rapid development in the field of nanoscale materials. Spherical and tubular nanoparticles of several inorganic materials have already been synthesized such as boron nitride nano-tube, MoS2, silica and alumina. Imogolite and allophane are nano-sized aluminum silicates with tubular and hollow spherical morphology that exist in various soil environments. Imogolite and allophane have been already found in the soil environment long before fullere nano-ball and carbon nano-tube was discovered. However, development in research of imogolite and allophane is still stagnant compared to other nano-materials. One of the main reasons is they cannot be synthesized except in the dilute concentration in the parent solution resulting in low yield of product. Imogolite and allophane are frequently found together as main clay components in soils derived from pyrocalstic materials such as volcanic ash and pumice. They have occasionally been found to occur in soils developed on other parent materials such as basalt and tills derived from various types of rock, and in association with basaltic saprolite. In particular, imogolite and allophane have also been recognized as major components of the B horizon clays of many pozdolized soils worldwide. The two clays form in similar natural and experimental conditions and have similar local chemical structure, but have different morphology. Imogolite was first identified as a fibrous acid-dispersible clay component of weathered pumice in Japan. Imogolite has nano-tubular structure having inside diameter of 1.0 nm and outside diameter of 2.0 nm with a well-defined fibrous electron-diffraction pattern indicating that the tubes are uni-dimensional crystals. These tubes may be several hundred-nano meters in length. Combining this information with chemical and IR evidence for the presence of silicon only as orthosilicate, the structure model of imogolite was proposed. The imogolite structure has a composition of Al2(OH)3SiO3OH with a Si/Al ratio of 0.5. The basic structure of imogolite is built up largely of a gibbsite sheet, with orthosilicic acid is coordinated from inside via oxygen of the three Al atoms. Allophane is fundamentally composed of a gibbsite sheet with a SiO4 tetrahedra attached to it similar to that of the imogolite structural unit except for some defects or pores. It has hollow spherical morphology with the SiO4 tetrahedra located at the inner surface of the sphere. It was difficult to determine the chemical structure of allophane by X-ray diffraction analysis because of the short-range ordered nature of its structure. Some scientists considered allophane to be an amorphous aluminosilicate mineral. However, upon separation of the pure mineral, and with the aid of a high-resolution electron microscope, the mineral has been found to be hollow spherical particles composed of polyhedrons with a diameter ranging between 3.5 and 5.0 nm. The wall of the sphere has a thickness between 0.7 and 1.0 nm and has some holes or ‘defects’. The chemical compositions of these hollow spherules vary in Si/Al atomic ratio from 0.5 to 1.0. Studies on allophanes by using 29Si-NMR showed that orthosilicates are a main component of allophanes with low Si/Al atomic ratio. On the other hand, allophane with high Si/Al atomic ratio showed that allophane contains Si atoms in the orthosilicate structure as well as several different states of polymerized silicates with various SiOSi angles and SiO bond strengths. 27Al-NMR also showed that six-coordinated Al is dominant in the allophane. These studies concluded that the chemical structure of the wall of allophane is same as that of imogolite.