Scientists watch nanoparticles grow
Analysis allows tailoring materials for switchable windows and solar cells
Left: Structure of the ammonium metatungstate dissolved in water on atomic length scale. The octahedra consisting of the tungsten ion in the centre and the six surrounding oxygen ions partly share corners and edges. Right: Structure of the nanoparticles in the ordered crystalline phase. The octahedra exclusively share corners. Credit: Dipankar Saha/Århus University
Hamburg, 27 March 2014. With DESY’s X-ray light source PETRA III, Danish scientists observed the growth of nanoparticles live. The study shows how tungsten oxide nanoparticles are forming from solution. These particles are used for example for smart windows, which become opaque at the flick of a switch, and they are also used in particular solar cells. The team around lead author Dr. Dipankar Saha from Århus University present their observations in the scientific journal "Angewandte Chemie – International Edition".
For their investigation, the scientists built a small reaction chamber, which is transparent for X-rays. “We use fine capillaries of sapphire or fused silica which are easily penetrable by X-rays,” said Professor Bo Iversen, head of the research group. In these capillaries, the scientists transformed so-called ammonium metatungstate dissolved in water into nanoparticles at high temperature and high pressure. With the brilliant PETRA III X-ray light, the chemists were able to track the growth of small tungsten trioxide particles (WO3) with a typical size of about ten nanometre from the solution in real time.
“The X-ray measurements show the building blocks of the material,” said co-author Dr. Ann-Christin Dippel from DESY, scientist at beamline P02.1, where the experiments were carried out. “With our method, we are able to observe the structure of the material at atomic length scale. What is special here is the possibility of following the dynamics of the growth process,” Dippel points out. “The different crystal structures that form in these nanoparticles are already known. But now we can track in real-time the transformation mechanism of molecules to nanocrystals. We do not only see the sequence of the process but also why specific structures form.”
On the molecular level, the basic units of many metal-oxygen compounds like oxides are octahedra, which consist of eight equal triangles. These octahedra may share corners or edges. Depending on their configuration, the resulting compounds have different characteristics. This is not only true for tungsten trioxide but is basically applicable to other materials.
The octahedra units in the solutions grow up to nanoparticles, with a ten nanometre small particle including about 25 octahedra. “We were able to determine that at first, both structure elements exist in the original material, the connection by corners and by edges,” said Saha. “In the course of the reaction, the octahedra rearrange: the longer you wait, the more the edge connection disappears and the connection by corners becomes more frequent. The nanoparticles which developed in our investigations have a predominantly ordered crystal structure.”
In the continuous industrial synthesis, this process occurs so quickly, that it mainly produces nanoparticles with mixed disordered structures. “Ordered structures are produced when nanoparticles get enough time to rearrange,” said Saha. “We can use these observations for example to make available nanoparticles with special features. This method is also applicable to other nanoparticles.”
Deutsches Elektronen-Synchrotron DESY is the leading German accelerator centre and one of the leading in the world. DESY is a member of the Helmholtz Association and receives its funding from the German Federal Ministry of Education and Research (BMBF) (90 percent) and the German federal states of Hamburg and Brandenburg (10 percent). At its locations in Hamburg and Zeuthen near Berlin, DESY develops, builds and operates large particle accelerators, and uses them to investigate the structure of matter. DESY’s combination of photon science and particle physics is unique in Europe.
“In Situ Total X-Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions”; D. Saha et al.; Angewandte Chemie - International Edition (Vol. 53, No. 14, 1 April 2014); DOI: 10.1002/anie.201311254 (online publication on 26 February 2014)
Dr. Dipankar Saha, Århus University, +45 87 15 59 05, firstname.lastname@example.org
Dr. Ann-Christin Dippel, DESY, +49 172 1695626, email@example.com
Prof. Bo B. Iversen, Århus University, +45 87 15 59 82, firstname.lastname@example.org
DESY-Pressesprecher Thomas Zoufal, +49 8998-1666, email@example.com
Real-time data oft he pair distribution function during growth of tungsten trioxide nanoparticles. The pair distribution function gives the rate of occurrence of atomic bond distances in a molecule or material. In the course of the synthesis, the bond distance at 3.3 Å disappears which represents the edge-sharing octahedra. Initially, the precursor molecule has a size of around 6 Å. Upon growth of the nanoparticles starting at ~ 5 min, structures with long-range order on the scale of nanometers evolve. Credit: Dipankar Saha/Århus University
Schematic representation of the experimental setup: The bright X-ray light from PETRA III (top left) is scattered by nano particles growing in the capillary (right) under high temperature and high pressure. The diffraction patterns (right) contain information about the nanoparticle structures and its changes in real time (lower right). Credit: Mogens Christensen/Århus University