Amorphous Nanomaterials. Lin Guo
structure and its mechanical properties and (e‐g) amorphous alloy with micron‐scale second phase and its mechanical properties. "/>
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 amorphous structures. A large number of amorphous noble metal particles and amorphous ultrathin two-dimensional materials could be achieved through this method.
1.4 Growth Mechanisms of Amorphous Nanomaterials
1.4.1 Classical Nucleation Theory
In classical nucleation theory, the formation of solid particles in solution underwent two phases: nucleation and growth. In 1950, Lamer proposed a nucleation mechanism in the energy category based on the sulfur colloid synthesis process in solution, which is still one of the most significant self-consistent nucleation mechanisms [24].
At the nucleation stage, the transition from the liquid phase to the solid phase occurs. There are two kinds of energy changes during this transition. One is the volume free energy (ΔGV), which is defined as the decrease of the free energy of atoms during the transition from the free state in the solution to the crystal nuclei. The other is the surface free energy (ΔGγ), which demonstrates the increasing of the system’s free energy from the generated new interfaces. If the newly generated nucleus is regarded as a sphere, the former is a positive correlation function of volume (the cube of the radius r), ΔGV = 4/3πr3·ρRTln(c/c0), where c0 is the concentration of the supersaturated solution. The latter is a positive correlation function of the surface area (the square of r), ΔGγ = 4πr2·γs−l. This makes a critical radius r* in the nucleation process, which exhibit a negative correlation with the concentration of the supersaturated solution. Therefore, a higher concentration supersaturation lead to smaller nucleation radius.
Taking the formation of metal materials as an example. During the nucleation stage, the original precursors are reduced to metal atoms with external stimulation (reductive agent adding, heating, irradiating, etc.). These atoms will be the modular units for the subsequent construction of the crystal structure. With the decomposition of the precursor, the concentration of metal atoms increases. Once the atomic concentration exceeds the minimum supersaturation point, the atoms exhibit a tendency to aggregate spontaneously to reduce surface energy (surface energy is higher than volume energy before the critical radius). In somewhere of the solution, affected by thermodynamic fluctuations, initial nuclei would instantaneously aggregate by atoms and separate from solution. Once the initial clusters are formed, these seeds then accelerate their growth by adsorbing free atoms in the solution, leading to the decrease of the concentration of atoms in the solution. At this time, the decomposition of the precursor is still continuing and the free atom is continuously added. If the atomic concentration