Читать книгу Amorphous Nanomaterials - Lin Guo - Страница 20

The polymerized amorphous structure represented by amorphous sulfur and polymers, and the multielement metallic amorphous structure represented by metallic glass together constitute the traditional amorphous materials. They demonstrate the standard structure characteristics of amorphous represented by glass, which is the existence of glass transition temperature. The former amorphous structure is formed due to the complexity of the basic chain structure, the flexibility and the homogeneous site would confuse the connection of monomer. The construction of latter amorphous structure is due to the hindrance of multiple components during quenching, which prevent the atoms to move to the regular position in crystal.

In addition to these two strategies, by manipulating the synthesis step and introducing additional factors to disrupt the regular arrangement, it is possible to obtain amorphous nanomaterials whose compositions are basically the same as their crystals, but whose structure is chaotic. These materials also have a random atomic arrangement. Here we simply summarize some methods and will describe them in chapter 4 to chapter 8.

1 For most metal, the amorphous structure of a single elemental metal cannot be easily obtained by quenching. By means of chemical synthesis in solution, the strong reducing agent NaBH4/KBH4 could be used to rapidly reduce the transition metal from the solvent. This process is similar to the fast quenching of metallic glass in which metal atoms are frozen at the disordered arrangement in solution. Apart from it, the residual small atoms B hinders the nucleation and regular arrangement of metal atoms, producing amorphous M–B nanomaterials.

2 For Si and Ge, which are the most important materials in the semiconductor field, their industrial amorphous materials cannot be obtained by the quenching method. That was because of the distinct difference between the six-coordinated structure in the liquid phase and the four-coordinated structure in the solid phase. Therefore, the preparation of amorphous silicon relies on gas-phase synthesis methods. However, the directly obtained amorphous silicon from gaseous Si exhibit a high concentration of dangling bonds. The product with abundant defects shows no practical industrial value. Thus, amorphous Si always replace to amorphous silicon–hydrogen alloy obtained from the decomposition of the precursor silane (SiH4) by the vapor deposition method or the glow discharge method. Hydrogen atoms can saturate the dangling bonds, thereby reducing the concentration of the paramagnetic center by four to five orders of magnitude. In addition to the advantage of simple and convenient, the amorphous silicon can be easily adjusted to a p-type or n-type semiconductor mixing a small amount of borane (BH3) or phosphane (PH5).

3 Amorphous nanomateriasl could also be achieved in solution by adjusting the decomposition process of the precursor to obtain a partially decomposed product in which the obtained amorphous structure is stabilized by the remaining atoms/coordination groups. For example, as the most polar solvent, water molecules can easily coordinate with cations to form ionic hydrates, which is widely used in the formation of amorphous oxides, hydroxides, et al. For example, in the preparation of titanium dioxide in solution, the amorphous hydrated oxidation state is usually obtained first. In the field of biomineralization, amorphous calcium carbonate is the most important intermediate state in the mineralization process. Its formation also depends on the water carried out from the solution. Its composition is mostly understood as hydrated calcium carbonate. The addition of Mg ions, polyacids, amino acids, etc., which mimic the biological environment, can effectively improve the stability of its amorphous structure.

4 In addition to the introduction of additional components, taking away the structural atoms to construct an unsaturated environment can also destroy the regular atomic arrangement of the original structure to obtain an amorphous structure. For example, the classic semiconductor material titanium dioxide can change to black/blue titanium dioxide with a large number of oxygen atoms missing under a strong reducing atmosphere. Due to the abundant oxygen defects, its surface usually exhibits an amorphous structure. This is a universal method for many transition metal oxides like ZnO, CeO2, SnO2.

5 For ultrasmall/ultrathin nanomaterials, the position of the atoms on the surface could be affected by surface ligand. The design of surface ligands could reduce the order of atoms to construct amorphous structures. A large number of amorphous noble metal particles and amorphous ultrathin two-dimensional materials could be achieved through this method.