Porous materials are a huge family of inorganic materials, possessing open pore frameworks and large surface area (including the inner and outer surfaces). Based on IUPAC, porous materials according to the pore diameter can be classified into three categories: those with pore diameters less than 2 nm are microporous; pore sizes between 2 and 50 nm are mesoporous; and pore diameters greater than 50 nm are called macroporous materials. "Nano" is a concept with the size from 1 to 100 nm. Therefore all the above three kinds of porous materials can be designated as porous nanomaterials.
Fig 1 Representative electron microscopy images of porous Cu2O nanospheres.
Porous nanomaterials have large surface areas due to appreciable internal and external surface areas. The presence of pores in nanostructured materials greatly promotes their physical and chemical properties. Nowadays, the well understanding, designed porous materials have significantly advanced science and technology, and have been playing increasingly important roles in modern society.
Fig 2 Structure of Zeolites
Microporous materials: Microporous materials are often used in laboratory environments to facilitate contaminant-free exchange of gases. Mold spores, bacteria, and other airborne contaminants will become trapped, while allowing gases to pass through the material. Examples of microporous materials include zeolites (Fig 2) and metal-organic frameworks. Zeolites can be used for ion exchange and adsorption processes. The presence of well-defined porosity affords a more homogeneous environment for matrix-drug complexes and hence a better control of the drug release rate.
Mesoporous materials: Mesoporous materials are of great interests because of their high surface areas and large pore sizes, making them good candidates for hosting molecules of different sizes, shapes, and functionalities. Recently, mesoporous nanomaterials have received much attention in the fields of adsorption, separation, sensing and biomedicine. For example, a great amount of research has focused on the synthesis and electrochemical lithium storage behavior of various nanostructured MoS2 materials, due to its high theoretical capacity and excellent cycling performance.
Macroporous materials: Electrodes with macroporous structure could offer many opportunities for improved performance in catalysis and energy storage. For example, the 3D Ni foam is a low cost and conductive metal with a high surface area, which is ideal for use as a template to host catalysts for increasing the number of reaction sites.