Nanometer, definition

According to their diameter, pores are conventionally classified as macropores (J>50nm), mesopores (2< J<50nm) and micropores (J<2nm). For nanometer-sized pores the term nanopores has been also used for some time (Handbook of Porous Solids, F. Schiith, K. Sing, J. Weitkamp (eds.), Wiley-VCH, Berlin, 2002) but the definition of nanopores is not fully established. In this chapter the term nanopore will be used for pores with 1 < J < 10 nm. 

Chemists have been working for a long time with particles having sizes of nanometers. The novelty of recent developments concerns the ability to make nanostructured substances with uniform particle sizes and in regular arrays. In this way it becomes feasible to produce materials that have definite and reproducible properties that depend on the particle size. The development began with the discovery of carbon nanotubes by Ijima in 1991 (Fig. 11.15, p. 116). 

Pores are found in many solids and the term porosity is often used quite arbitrarily to describe many different properties of such materials. Occasionally, it is used to indicate the mere presence of pores in a material, sometimes as a measure for the size of the pores, and often as a measure for the amount of pores present in a material. The latter is closest to its physical definition. The porosity of a material is defined as the ratio between the pore volume of a particle and its total volume (pore volume + volume of solid) [1]. A certain porosity is a common feature of most heterogeneous catalysts. The pores are either formed by voids between small aggregated particles (textural porosity) or they are intrinsic structural features of the materials (structural porosity). According to the IUPAC notation, porous materials are classified with respect to their sizes into three groups microporous, mesoporous, and macroporous materials [2], Microporous materials have pores with diameters < 2 nm, mesoporous materials have pore diameters between 2 and 50 nm, and macroporous materials have pore diameters > 50 nm. Nowadays, some authors use the term nanoporosity which, however, has no clear definition but is typically used in combination with nanotechnology and nanochemistry for materials with pore sizes in the nanometer range, i.e., 0.1 to 100 nm. Nanoporous could thus mean everything from microporous to macroporous. 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

We do have to be careful in the way we apply the definition of a phase to the n-butylammonium vermiculite system. According to Gibbs [13], a phase is any homogeneous and physically distinct part of a system that is separated from other parts of the system by definite boundary surfaces. Because the gel can be lifted out of the supernatant fluid on a spatula, it clearly justifies description as a phase in the latter sense, but it is inhomogeneous on the nanometer-to-micron (colloidal) length scale. It can only be defined as homogeneous on the macroscopic length scale. The same considerations apply to the tactoid phase. 

For the purposes of this article, zeolites are defined as crystalline nanoporous substances whose frameworks are inorganic oxides. This is a narrower definition than the most general, which includes organic and noncrystalline materials, and which remains open with regard to the framework anion it may perhaps be sulfide or nitride. "Nanoporous" indicates that the dimensions of the pores are of the order of 1 nanometer (1 nm =10 A), so it is exactly the right word to use in discussing zeolites, whose pores range in size from ca 0.3 to 3 nm. 

What makes a nanoparticle a nanoparticle Definitions of the size ranges for molecules, nanoparticles, and macroscopic solids must be compound specific. However, a useful upper limit for nanoparticles is the size at which one of its properties deviates from the value for the equivalent bulk material by an amount that is significantly larger than the error of the method used to make the measurement (a few percent). In practice, some characteristic will probably be different enough to warrant description as a hanoparticle if it is less than a few tens of nanometers in diameter, and perhaps less than a fraction of a micron in diameter. 

Naturally occurring nanomaterials exist in a variety of complex forms. In this chapter a short set of definitions will be stated for clarity. Nanocrystals are single crystals with sizes from a few nm up to about 100 nm. They may be aggregated into larger units with a wide spectrum of microstructures. Nanoparticles are units of minerals, mineraloids or solids smaller in size than 100 nm, and composed of aggregated nanocrystals, nanoclusters or other molecular units, and combinations of these. Nanoclusters are individual molecular units that have well-defined structure, but too small to be true crystals. Al and ZnsSs solution complexes are types of nanoclusters with sizes from sub nanometer to a few nm. Nanoporous materials are substances with pores or voids of nanoscale dimensions. These materials can be single crystals, such as zeolites or... 

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