Scientists watch nanoparticles grow: Analysis allows tailoring materials for switchable windows, solar cells

Mar 27, 2014
Left: This is the structure of the ammonium metatungstate dissolved in water on atomic length scale. The octahedra consisting of the tungsten ion in the center and the six surrounding oxygen ions partly share corners and edges. Right: This is the structure of the nanoparticles in the ordered crystalline phase. The octahedra exclusively share corners. Credit: Dipankar Saha/Århus University

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 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 nanometres 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.

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

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."

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

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."

Explore further: Researchers X-ray living cancer cells: Nanodiffraction opens up new insights into the physics of life

More information: "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)

add to favorites email to friend print save as pdf

Related Stories

A layered nanostructure held together by DNA

Mar 20, 2014

(Phys.org) —Dreaming up nanostructures that have desirable optical, electronic, or magnetic properties is one thing. Figuring out how to make them is another. A new strategy uses the binding properties ...

Lungs may suffer when certain elements go nano

Jan 28, 2014

(Phys.org) —Nanoparticles are used in all kinds of applications—electronics, medicine, cosmetics, even environmental clean-ups. More than 2,800 commercially available applications are now based on nanoparticles, ...

Nanoparticle networks' design enhanced by theory

Feb 27, 2014

For close to two decades, Cornell scientists have developed processes for using polymers to self-assemble inorganic nanoparticles into porous structures that could revolutionize electronics, energy and more.

Recommended for you

A new way to make microstructured surfaces

Jul 30, 2014

A team of researchers has created a new way of manufacturing microstructured surfaces that have novel three-dimensional textures. These surfaces, made by self-assembly of carbon nanotubes, could exhibit a ...

Tough foam from tiny sheets

Jul 29, 2014

Tough, ultralight foam of atom-thick sheets can be made to any size and shape through a chemical process invented at Rice University.

User comments : 0