Chapter 3 Nanomaterials Fabrication

The synthesis of nanoparticles with control over size, shape, and size distribution has been a major part of colloid chemistry 胶体化学 for decades.

Part II Principles and Methods

The ability to fabricate nanomaterials (often in the form of nanoparticles) with strictly controlled size, shape and crystalline structure, has inspired the application of nanochemistry to numerous fields, including catalysis, optics and electronics.

Under these conditions, solid matter such as metal oxides, chalcogenides 金属硫族化合物 , metals, or carbon can be obtained at the nanometric scale.

An ultra-dispersed system 超分散系统 with a high surface energy can be only kinetically 动力学 stabilized 稳定 .

Ultrafine powders 超微粉 cannot be synthesized by methods involving energies that exceed a threshold 门槛 , but rather through methods of “soft chemistry 软性化学” that maintain the forming particles in a metastable state (stable excited state).

Additives 添加剂 and/or synthesis conditions that reduce the surface energy are needed to form nanoparticles stabilized against sintering 烧结球团 , recrystallization 再结晶 , aggregation 团聚 .

Synthesis methods for nanoparticles are typically grouped into two categories

The first involves division of a massive solid into smaller portions. This “top-down” approach may involve milling or attrition 研磨 (mecanosynthesis), chemical methods for breaking specific bonds (e.g. hydrogen bonds) that hold together larger repeating elements of the bulk solid, and volatilization 挥发 of a solid by laser ablation, solar furnace, or some other method, followed by condensation of the volatilized components.

The second category of nanoparticles fabrication methods involves condensation of atoms or molecules entities in a gas phase or in solution. This is the “bottom-up” approach in which the chemistry of metal complexes in solution holds an important place. This approach is far more popular in the synthesis of nanoparticles, and many methods have been developed to obtain oxides, chalcogenides, and metals.

The liquid-phase colloidal synthetic approach is an especially powerful tool for convenient and reproducible shape-controlled synthesis of nanocrystals.

Fabrication of metal oxide nanoparticles

From molecular species to nanopaticles

Begin with individual ions or molecular complexes of metals.

metal oxides complex metal oxide nanomaterials

Metal oxides 金属氧化物One common approach is to build from the ‘bottom-up’ method, beginning with individual ions or molecular complexes 配合物 of metals.Hydroxylation 羟化 of metal cations in aqueous solution and condensation 浓缩 : Inorganic polymerization 无机纳米粒子表面引发聚合反应

Hydrolysis 水解 equilibrium 平衡

[M(H2O)n]z+ + h H2O [M(OH)h(H2O)n-h](z-h)+ + h H3O+

nanoparticleNeutralization 中和 with a base 碱

[M(H2O)n]z+ + h OH- [M(OH)h(H2O)n-h](z-h)+ + h H2O nanoparticle

The electric charge of the nanoparticle will be:

Positive complex (polycation) if h < z and is soluble

Negative complex (polyanion) if h > z and is soluble

Neutral complex if h = z and is a solid as precipitate 沉淀物

→ nanoparticle precursor 前身

Condensation 缩合反应 of aquohydroxo complexes proceed by elimination 消除 of water and formation of hydroxo bridges 桥 (olation):

P. 33 δ+ δ- δ+H2O ─ M ─ OH + HO ─ M ─ OH2 → H2O ─ M ─ O ─ M ─ OH2 + H2O

More similar reactions will make the nanoparticle grow in size!

P. 35 The precipitation of a nanoparticle involves four kinetic 动力学 steps:

1.Formation of the zero-charge precursor [M(OH)h(H2O)n-h]0 which is able to condense and form a solid phase.

2.Creation of nuclei, through condensation of zero-charge precursors.

3.Growth of the nuclei through addition of matter, until the primary 主要 particle stage is reached.

4.Aging 老化 of the reaction allows the system toward or reach stability 稳定 , usually associated with the modifications 改变 of some physical or chemical characteristics of the particles.

Figure 3.2 The four kinetic steps of the formation of nanoparticles

Control of particle size, crystalline structure 晶体结构 , and morphology 表面结构 .

There are different techniques to form the complex of zero charge and to obtain a solid. The most common method consists of adjusting the pH of the reaction.

Figure 3.4 Nanoparticle size variation against pH

P. 45 Hydrolysis 水解 of metallo-organic compounds 金属有机化合物

Metal alkoxides 金属烷氧基化合物 are precursors of hybrid 混合物 organic-inorganic materials and involved in sol-gel chemistry 溶胶 - 凝胶化学 of oxide nanomaterials 。

M(OR)z + zH2O → M(OH)z + zROH → MOz/2 + z/2 H2O + zROH

metal alkoxide metal oxide, nanoparticle

Figure 3.7 TEM (transmission electron microscope 透射电镜 ) micrographs 显微镜图片 of nanoparticles

Figure 3.9 SEM (scanning electron micrograph 扫描电镜图 ) of nanoparticles

P. 49 Non-hydrolytic 非水解 routes to oxide nanoparticles

In nonaqueous media 非水介质 in the absence of surfactant 表面活性剂 , one method is the use of metal halide 金属卤化物 complexes and alcohols.

≡ M – X + ROH → ≡ M – OH + RX metal halide alcohol metal hydroxide complex complex

≡ M – OH + ≡ M – X → ≡ M – O - M ≡ + HX nanoparticle

P. 54 From minerals 矿物 to materials

The formation of nanoparticles from inorganic metal (top-down approach) .One common example is the formation of aluminum oxide nanoparticle (Al – O – Al) from the hydrolysis of aluminum compounds.

Figure 3.15

Semiconductor Nanoparticles 半导体纳米粒子 (Quantum dots 量子点 and quantum rods 量子棒 )

The synthesis of semiconductors as nanoscale particles yields materials with properties of absorbance and fluorescence that differ considerably from those of the larger, bulk-scale material. These materials are of great interest in applications ranging from medical imaging and sensing.

Traditional semiconductors

Semiconductor is a material that has an electrical conductivity due to electron flow (as opposed to ionic conductivity) which is intermediate in magnitude between that of a conductor and an insulator. The conductivity increases with temperature and in the presence of impurities. Semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices.

In semiconductors, current is often schematized as being carried either by the flow of electrons or by the flow of positively charged “holes”. semiconductors commercially. The common semiconductor materials include silicon 硅 , germanium 锗 , gallium arsenide 砷化镓 , and silicon carbide 碳化硅 .

Two fundamental factors, both related to the size of theindividual nanocrystal, are responsible for these unique properties:The first is the large surface to volume ratio (the number of surface atoms to those in the interior increases).

The second factor is the actual size of the particle (increase of band gap energy).

The most studied nonoxide semiconductors are caddmium chalcogenides (CdE, with E=sulfide, selenide and telluride).

3 types of metallic nanoparticles

1. Precious metal 贵金属 nanoparticles, e.g. silver and gold, to produce yellow to red colored nanoparticles

2. Copper and ruthenium 钌 nanoparticles used as catalysts催化剂

3. Cobalt, iron and nickel 磁力金属 become magnetic nanoparticles can be used for information storage, and microwave composite materials

Synthesis of metallic nanoparticles by reduction 还原反应

MZ+ + reducing agent → M0 + Oxmetal salt zero valent metal

The reduction reaction involves the formation of monosized nanoparticles that is achieved by a combination of a low concentration of solute an a protective layer (polymer, surfactant or functional groups).

P.77 Carbon Based Nanomaterials

The different allotrope 同素异形体 of carbon, graphite, diamond and C60 (buckyball), which was discovered in 1985 by Curl, Kroto and Smalley who were awarded the Nobel Price in Chemistry in 1996.

Fullerenes 富勒烯

C60, C70, C74, C76, C78, etc. has to follow two principles: Euler’s theorem and the isolated pentagon rule (IPR)

Carbon fullerenes are large, closed caged carbon structures in a spherical shape. Fullerenes, discovered in 1985, are stable in gas form and exhibit many interesting properties that have not been found in other compounds before. It is a representation of a C60 Fullerene molecule. A fullerene is a spherical structure composed of both pentagonal 五角形 and hexagonal 六角形 carbon rings. Fullerenes are considered zero dimensional quantum structures which exhibit interesting quantum properties. Once fullerenes were proven to exist, research for other fullerene like structures led to the discovery of Carbon nanotubes in 1991.

TEM A fullereneMolecule diagram

Carbon nanotubes

1.Multiwalled nanotubes (MWNTs)2. Single-walled nanotubes (SWNTs)

Nanotubes are the 1 dimensional wire form of a fullerene; the diameter is typically 1 to 5 nanometers (nm), while the length can

be in the range of microns. Single Walled Nanotubes (SWNT) can be considered as a flat graphene sheet cylindrically rolled

into a tube. The tubes consist of two regions: the sidewall of the tube, and the end region of the tube.