Transcript Chemistry: The Study of Change

Part II Principles and Methods
Chapter 3 Nanomaterials Fabrication
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.
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.
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+
nanoparticle
Neutralization 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 (topdown 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 the
individual 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 + Ox
metal 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 fullerene
Molecule
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.