Page 28 - Book Hosokawa Nanoparticle Technology Handbook

CHAPTER 1 BASIC PROPERTIES AND MEASURING METHODS OF NANOPARTICLES

1.1 Size effect and properties of The diameter of the smallest hydrogen atom is
0.074nm, and that of the relatively large lead atom
nanoparticles
(atomic number is 82) is 0.35nm. From these sizes, it
is estimated that the particle with a size of 2nm con-
1.1.1 Definition of nanoparticles sists of only several tens to thousands atoms. When
the particle is constructed by larger molecules, the
The nanoparticles are ultrafine particles in the size number decreases furthermore. Table 1.1.1 shows the
of nanometer order. “Nano” is a prefix denoting the relationship between the particle size and the fraction
minus 9th power of ten, namely one billionth. Here it of number of atoms located at the particle surface
means nanometer (nm) applied for the length. One with an assumption of atomic distance of 0.2nm [2].
nm is extremely small length corresponding to one It is indicated that the fraction of surface atoms of a
billionth of 1m, one millionth of 1mm, or one thou- 20 m cubic particle is only 0.006%, but it increases
sandth of 1 m. to 0.6% for a 200nm particle and then it is estimated
The definition of nanoparticles differs depending almost half of the atoms are situated at the surface of
upon the materials, fields and applications concerned. a 2nm particle.
In the narrower sense, they are regarded as the parti-
cles smaller than 10–20nm, where the physical prop- (2) Increase of surface area
erties of solid materials themselves would drastically On the other hand, as the micronization of solid
change. On the other hand, the particles in the three particles, the specific surface area increases generally
digit range of nanometer from 1nm to 1 m could be in reversal proportion to the particle size. In the
called as nanoparticles. In many cases, the particles above-mentioned case, when the particle of 1cm is
from 1 to 100nm are generally called as nanoparti- micronized to 1 m and 10nm, the specific surface area
cles, but here they will be regarded as the particles becomes ten thousand times and million times, respec-
smaller than those called conventionally “submicron tively. As the increase in the specific surface area
particles”, and concretely less than the wavelength of directly influences such properties like the solution and
visible light (its lower limit is about 400nm) as a reaction rates of the particles, it is one of major reasons
measure, which need to be treated differently from the for the unique properties of the nanoparticles different
submicron particles. Figs 1.1.1 and 1.1.2 show exam- from the bulk material together with the change in the
ples of nanoparticles and the related phenomena and surface properties of the particles itself.
an example of electron microscope picture of gold
colloid nanoparticles made by liquid phase chemical
method [1], respectively. 1.1.3 Evaluation of size of nanoparticles
In order to elucidate the change in properties and
1.1.2 Features of nanoparticles characteristics of nanoparticles with the particle size,
it is essential first of all to measure the size of the
(1) Activation of particle surface nanoparticles accurately. The most basic method to
All the solid particles consist of the atoms or the mol- measure the size of nanoparticles is the size analysis
ecules. As they are micronized, they tend to be from the picture image using the transmission elec-
affected by the behavior of atoms or the molecules tron microscope, which could also give the particle
themselves and to show different properties from size distribution. For this analysis, preparation of
those of the bulk solid of the same material. It is the well-dispersed particles on the sample mount is
attributable to the change of the bonding state of the the key issue. The grain size of the particles can be
atoms or the molecules constructing the particles. For obtained from peak width at half height in the X-ray
example, as shown in Fig.1.1.3, if a cube with a side diffraction analysis and it is regarded as an average
length of 1cm is divided into a cube of 1 m, the par- primary particle size of particles.
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ticle number increases to 10 and being divided into Meanwhile, the laser diffraction and scattering
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the one of 10nm, then it amounts to 10 , where the method, which is popular for the size analysis of
fraction of the atoms or the molecules located at the micron-sized particles, would hardly measure the par-
surface on the particles plays a great role, since they ticle size of individual nanoparticles but that of the
are more active than those inside the solid particles agglomerated particles. The photon correlation
because of the free hand, which leads to easy bonding method often used for the particle analysis in the
with the contacting materials and causes various nanosized range might not give accurate results in
changes in particle properties. many cases, when the particle size distribution is wide.

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