strength of 650 Nm g−1, which reached the highest record for the strength of metal materials.
At the same time, the introduction of micro/nanoscale heterogeneous structures or the second phase in bulk amorphous materials could significantly improve the toughness of amorphous materials. In 2007, according to Poisson’s ratio criterion, Wang Weihua et al. [18] adjusted the composition of the Zr–Cu–Ni–Al metallic alloy and prepared an amorphous alloy system with a multilevel microscale heterogeneous structure, which showed high strength (1.7 GPa) and very large compressive plasticity (strain > 150%). These amorphous alloys can even be bent to 90° at room temperature (Figure 1.6a–d). In 2008, WL Johnson et al. [19] improved the composition of the amorphous alloy and controlled the content of each component to synthesize the Zr–Ti–Nb–Cu–Be metallic alloy with a micron-scale precipitated second phase. For the first time, the fracture deformation has been increased to more than 10%, and up to 14%. At the same time, the fracture toughness of the amorphous alloy reached 170 MPa m0.5, indicating an excellent toughness (Figure 1.6e–g).
In addition to the mechanical properties, another key point for amorphous materials attracting widespread attention is the application in catalysis. Amorphous materials exhibit high activity surface with unsaturated coordination sites and unique local environment with uniform chemical states and atomic structures. Whether in theoretical research as a model catalyst or performance exploration as a practical catalyst, the study of amorphous materials is significant. For example, in 1925, Constable systematically discussed the difference between amorphous materials and crystalline materials during the process of catalytic decomposition by calculating the active center and pointed out that amorphous materials may have higher performance [20]. In 1980, at the Seventh International Conference on Catalysis, Gerard V. Smith of the University of Southern Illinois at Carbondale [21] firstly demonstrated amorphous alloys as a new system for catalytic reactions. In 1983, Brower et al. reported in Nature that Pd-based metallic glass has a higher selectivity than crystalline Pd in hydrogenation reactions [22], which gradually opened the prelude to the research of amorphous alloys in the field of catalysis.
Figure 1.6 Amorphous metal with micro/nanomicrostructure. (a–d) Amorphous alloy with micron-scale heterogeneous structure and its mechanical properties and (e–g) amorphous alloy with micron-scale second phase and its mechanical properties. Source: Panels (a–d) reproduced with permission from Liu et al. [18]. Copyright 2007, AAAS. Panels (e–g) reproduced with permission from Hofmann et al. [19]. Copyright 2008, Nature Publishing Group.
However, because of the low specific surface area, amorphous alloys always failed to fully realize the true activity. For example, in the degradation of direct blue, the degradation ability of iron-based amorphous alloy (46 h) is not significantly improved, compared to pure iron (>50 h). However, after grinding the amorphous alloy to the micron level, it can completely degrade the same concentration of dye in a short period of 1 h [23], showing the important application prospects of micro–nanoamorphous materials in the field of catalysis.
1.3.5 Modern Amorphous Materials 3-Nontraditional Amorphous Nanomaterials
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
