Appl Note 1016

Copyright XRD.US

Application of Powder Diffraction for study of Nanomaterials



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


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:


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AN-1024 Powder diffraction of Semiconductors XRD.US
AN-1025 Powder diffraction of Soils XRD.US


We are currently evaluating many microspheres and nanospheres for particle and crystallite size characteristics.




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