| Introduction
Materials having dimensions about 100nm or
below commonly termed as nanomaterials. It is obvious from definition that
what makes nanomaterials are special - the size. How we determine the size
of nanomaterials - by measuring of their sizes. There are many methods how
the size of nanomaterials could be determined. With this Note we give an
introductory explanation to this problem.
Common Problems
Crystallite size of
nanomaterials commonly determined by variety of imaging techniques, such
as AFM, SEM, TEM etc. These methods are very laborious and expensive.
 For
example, if we need to determine the crystallite size distribution of
nanometals having the crystallite size let's say between 1nm to 3 nm, we
need to somehow transfer these materials into sample illumination area and
 make sure that whatever sample we see is representative sample. It may
take several days to prepare such samples, but still there are no
guarantee that this sample is representative. Let's assume that we
(me and another guy) believe that this is a representative sample. OK.
Now, take a look a AFM image shown on the left. Of course we can
measure the sizes of white spots, some of them small, some of them large.
Now you probably understand that, no way you (or me or anybody on the
earth) can tell with certainty that the spots are individual
crystals. This cannot be determined independently from fact that the spots
small or large. It is never really spelled out that we cannot assume that
"the small spots are individual crystals, but the large spots are
agglomerates of smaller ones". This is absolutely BS of course to assume
any spots on images for individual crystals. OK, even I agree that this
small spots are individual crystals (in fact I would never agree on this),
now you need to measure as many as possible crystals at least in x
and y directions then tabulate them. How many such measurements you going
to make 100, 200 or 500. Of course up to you. Then we have to make another
assumption that the areas we have not scanned are similar to those what we
see in this small image area. This whole work will take your about week of
your time and expensive equipment and still no guarantee that this was an
appropriate exercise.
Another example of
crystallite size and crystallite size distribution is taken from
respectful journal and written by well known authors. We find that this
quite common, but also highly doubtful method to determine the size of
crystals and their distribution. None of these lighter or darker looking
particles could not be taken as an individual crystal, even it well may be
so. Moreover, how to determine the particles which only partially lighter,
or laying on top of each other, simple showing not even electron
transmission properties.
What is very important to know
Particle size and
crystallite size have not the same meaning. Particles could compose (and
most often they do) from several or many small crystallites. Crystallite
size is a fundamental property of material. Important properties of
nanomaterials is dependant from the size of the crystals, but not the
size of particles.
Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM)
and other similar imaging methods cannot distinguish the size of
individual crystals unless they clearly separated from each other.
Methods based on High Resolution Electron Microscopy (HREM) or Selected
Area Electron Diffraction (SAED) are expensive, laborious and not
accurate due to only small number of particle measurements could be made
and consequently, statistically not significant
Simple laboratory diffractometer in short period of time can measure
millions of crystals and accurately determine the size distribution of
nanomaterials
Examples
Crystallite size
distribution of Anatase.
TiO2 is a
semiconductor that has been often investigated in photoelectro-chemistry
and photocatalysis. The understanding of its photophysical and
photochemical properties in aqueous media is of particularly interest
because of its extensive application in the detoxification of polluted
ecosystems and photocatalyst.
It has been known that the electronic band structure of a
semiconductor oxide is size dependent when its dimension is comparable
with the exciton (Bohr) radius (the so-called Q-size effect). The
development of the preparation and stabilization method for monodispersed
semiconductor nanoparticles in transparent colloids is very important as
offering a nice opportunity of experimental verification for theoretical
predictions. Systematic studies of the size-dependence of the
photo-properties of TiO2 sol, however, have not kept pace with
the studies in heterogeneous photocatalysis where TiO2 play a
significant role. The preparation of quantum-size TiO2
colloidal particles was reported to have a blue shift of the UV absorption
edge was observed with the decrease of the particle size. Distinct Q-size
effects even for relatively large crystallites, for instance, the band gap
increased by 0.07 eV relative to the bulk bandgap (3.0 eV) for 12-nm-sized
rutile particles and about 0.16eV relative to the bulk value of 3.2eV for
3.8-nm-sized anatase particle.
Crystallite size
distribution of Palladium nanocrystalline catalysts.
Pd, Pt, Au, Ru and many other metals play
important role in catalysis. There are direct relation between crystallite
size and catalytic activity almost any zero-valent metals.
 The
size of crystals are very sensitive to preparation methods of these
catalysts and it is well known that simple modification of support
materials, their porosity, chemistry, wet methods of applying these
metals, heating, reduction in hydrogen atmospheres etc dramatically change
the size of metal crystallites. Application of these catalysts in various
processes also have dramatic effect in the catalytic activity (or loss of
catalytic activity) of these important materials.
For example, the figure on the left shows
crystallite size distribution of Palladium crystals on titanium dioxide.
Other Application Notes are also available: |
