Properties of nanoparticles

Nanoparticles are of great scientific interest as they are, in effect, a bridge between bulk materials and atomic or molecular structures. Materials of dimensions close to the atom (≤ 100 nm) show very different properties and great potentiality. These new properties are derived from the very small size of the nanoparticles, very close to the size of the atoms (1-100 nm) and causing substantial changes in the material.

A bulk material should have constant physical properties regardless of its size, but at the nano-scale size-dependent properties are often observed. Thus, the properties of materials change as their size approaches the nanoscale and as the percentage of the surface in relation to the percentage of the volume of a material becomes significant.

Because almost all the atoms in a nanoparticle are on the surface, these particles have chemical and physical properties that differ from those of individual molecules or larger aggregates of atoms or molecules.

The causes of these behavioral differences in their properties are mainly two:

1. The large increase in the surface area of the nanoparticle.

2. The quantum confinement of the electrons inside the nanoparticle (new quantum effect).

Nanomaterials have a much greater surface area to volume ratio than their conventional forms, which can lead to greater chemical reactivity and affect their strength. Also at the nanoscale, quantum effects can become much more important in determining the materials properties and characteristics, leading to novel optical, electrical and magnetic behaviors.

The extraordinary surface area of the nanoparticles causes a redistribution of the atoms, increasing the fraction of atoms that are in its surface. The number of atoms in the surface grows in a parabolic form, as the nanoparticle is smaller. When the particle size is 3 nm there is about 50% of atoms on the surface and when a nanoparticle size is 1nm there is more than 90% of its atoms located on its surface. This is the main reason for the highest catalytic activity of nanoparticles in chemical and biochemical reactions.

The most popular term in the nano world is quantum confinement effect which is essentially due to changes in the atomic structure as a result of direct influence of ultra-small length scale on the energy band structure. The length scale corresponds to the regime of quantum confinement ranges from 1 to 25 nm. In which the spatial extent of the electronic wave function is comparable with the particle size. As a result of these “geometrical” constraints, electrons “feel” the presence of the particle boundaries and respond to changes in particle size by adjusting their energy. This phenomenon is known as the quantum-size effect.

Nanoparticles have unique properties compared with their bulk counterparts. Many of these properties, including physical, chemical, optical, electrical, and magnetic, can be controlled by relatively simple tuning of their sizes, shapes, compositions, protecting organic ligands, and interparticle distance.

ü Physical properties

The melting point generally decreases as a result of its large specific surface area and the greater number of atoms on the surface. This affects the thermodynamic behavior of the volume of the nanoparticle. The atoms of the surface need less energy to move because there are fewer atoms inside the nanoparticle and they need less energy to overcome the intermolecular forces of attraction. Changes in melting point occur because nanoscale materials have a much larger surface to volume ratio than bulk materials, drastically altering their thermodynamic and thermal properties. Changing the metal melting temperature, depends on the particle size. With decreasing the size the melting temperature may be lowered by several hundreds of degrees, as for gold, for example, a compact metal melts at a temperature of 1340 K, and the nanoparticles of 2 nm melt at 340 K.

Other physical properties of nanoparticles caused by quantum effects are: high electrical and thermal conductivity of some nanoparticles, such as silver; Low electrical and thermal conductivity of other nanoparticles such as gold.

Conversion of some metals into semiconductors (ZnO, Si ...); Some semiconductors become insulators.

Absorption of solar radiation is much higher in materials composed of nanoparticles than it is in thin films of continuous sheets of material.

The changes in physical properties are not always desirable. Ferromagnetic materials smaller than 10 nm can switch their magnetization direction using room temperature thermal energy, thus making them unsuitable for memory storage.

ü Chemical properties

One of the important factors for the chemical applications of nanomaterials is the increment of their surface area which increases the chemical activity of the material. Due to their enhanced chemical activity, nanostructured materials can be used as catalysts to react with such noxious and toxic gases as carbon monoxide and nitrogen oxide in automobile catalytic converters and power generation equipment to prevent environmental pollution arising from burning gasoline and coal.

Nano metallic or ionic particles exhibit very important chemical properties: self-assembly and the exceptional properties as catalysts.

Its high chemical reactivity is due to its high specific surface area and the important number of atoms at the surface that give rise to a high surface energy of the nanoparticles.

ü Mechanical properties

The mechanical properties of nanoparticles and nanostructured materials change with size; At Nano-metric scale, the atomic structure of the nanocrystals is modified, becoming more resistant and acquiring mechanical properties superior to those of macro materials.

For a given nanomaterial, its hardness and strength are increased, generally following a growth inversely proportional to its diameter.

An important property of some nanoparticles is their greater capacity of tensile deformation, before breaking, without reducing their resistance, being able to reach the maximum theoretical resistance of the material. This behavior is due to the fact that the nanocrystal structure is practically devoid of defects.

The elongation or tensile deformation of some nanoparticles may be in the range of 100% - 1000%, prior to rupture, without fracture.

ü Optical properties (quantum effects)

The optical properties of the nanoparticles of some metals such as gold, silver and semiconductors are produced by the interaction between their plasmon surface and the incident electromagnetic wave (natural light, laser), producing a quantum effect as a consequence of the change of the electronic structure induced by the size and shape of the nanoparticle.

The color that acquires a nanoparticle on which a white ray of light (in which all the wavelengths in the visible spectrum 400-700 nm, with a similar intensity are present) is white color; If the nanoparticle absorbs some wavelength (color), it will change the color of the nanoparticle perceived by the naked eye and the color of the transmitted light.[T1] The absorbed wavelength is that whose energy causes the plasmon on the surface of the nanoparticles to vibrate at the same wavelength as the absorbed wave. The wavelength of the electron pool of the surface (plasmon) can be varied with the size and shape of the nanoparticles (surface plasmon resonance).

The red color of the stained-glass windows of cathedrals occurs when the contained gold nanoparticles have the appropriate size and shape (10-20 nm), to absorb the photons corresponding to the wavelengths of the received light, which correspond with the colors blue and green.

Conclusions

A nanomaterial differs from a conventional polycrystalline material not only because of the size of its structures, but also in the way we can use it. The electronic, optical, magnetic, chemical, and mechanical properties are substantially affected by the scale of a material’s features.

Fundamental material properties that were long considered constants (melting temperature, electrical conductivity, ductility, etc.) are suddenly subject to manipulation. We can tailor these properties by engineering the dimensions of a material’s features. QUIERO PONER QUE LAS NANOPARTÍCULAS SE PUEDEN MANIPULAR CONOCIENDO SUS PROPIEDADES Y DISEÑAR NUEVOS MATERIALES.

if the dimensions of a material are reduced, its properties are modified and accordingly materials with properties on demand can be designed.

Control questions

1. What does Nanoscience mean?

2. Give the definition of Nanochemistry.

3. How nanomaterials can be classified?

4. Explain which are the reasons for the differences in nanomaterials properties and bulk properties.

5. Mention some characteristics of nanomaterials.

[T1]No entiendo!!