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Chemistry Between Molecules and Particles: Nanoclusters as Advanced Functional Materials

Postdoc-stipendium i Danmark | 02/05/2016

Structures of nanoclusters containing 144, 130 and 25 gold atoms. The clusters are covered by sulfur containing ligands, hindering them to grow to bulk materials.

Research in chemical synthesis has in the last decades given chemists the tools to produce new compounds with outstanding precision, and it is now becoming possible to engineer materials on the atomic level. However, the methods usually applied to characterize the atomic arrangements in matter are not well suited to study this new class of materials. This hinders scientists in establishing the important relation between material structure and properties, needed to advance nanotechnology further. To take advantage of the potential in nanotechnology and develop new materials for e.g. energy storage, conversion and catalysis, it is therefore crucial to get a deeper understanding of nanomaterial structure. This is the heart of Kirsten Marie Ørnsbjerg Jensen’s research project. By applying newly developed methods in synchrotron X-ray scattering, she wants to solve the atomic structure of novel nanoclusters, which bridge the gap between molecules and particles. These materials have a large potential in catalysis, although their functionalities and structures are very poorly understood. Apart from studying the structure of the clusters themselves, Kirsten Marie Ørnsbjerg Jensen also wants to use X-ray scattering to investigate the chemical reactions leading to them to obtain a deeper knowledge of the intricate chemical mechanisms involved in atomic engineering.

A New Class of Materials

Over the last decades, nanotechnology has become a central theme in many different scientific disciplines such as chemistry, physics and biology.  In the quest for new and improved materials for energy production and storage, e.g. batteries, solar cells and in catalysis, chemists now know that by engineering materials on the nanoscale, new and improved properties can be obtained for a range of different applications. This development in nanotechnology has led to a whole new class of materials - nanoclusters, meaning inorganic compounds containing 10-1000 atoms. These lie between molecules and crystalline particles, and represent a completely new realm of materials with potential applications in several disciplines such as biomedicine and catalysis.1

Gold Clusters

Gold nanoparticles have long been a model system in nanotechnology. Their applications date back to ancient Mesopotamia, where they were used as dyes in pottery. As we are learning more about the cluster and their properties, new applications in e.g. catalysis, photovoltaics and biomedicince are appearing.

Magic Sized Clusters

In molecular chemistry, the chemical units have well-defined formulas, e.g. H2O (water) or CH3CH2OH (ethanol). In inorganic materials chemistry, on the other hand, chemists usually work with particles of a specific composition, e.g. NaCl (rock salt) or TiO2 (titania), but where the size of the particles can vary, which changes the specific number of atoms in each unit. In a sample of particles of e.g. TiO2 a large variety of sizes may be present, and the particles are therefore not identical in the same way molecules are. Cluster chemistry bridges the two worlds of molecular and particle chemistry. By means of newly developed methods in synthetic chemistry, clusters of e.g. gold, platinum or silver can now be synthesized with extreme precision, so that ‘magic sized’, i.e. completely identical particles with a specific number of atoms can be prepared. For example, Au102 and Au144 nanoparticles or super-molecules can now be produced routinely, and more and more ‘magic sized’ clusters in both metal and metal oxide chemistry are being synthesized. The properties of the clusters are extremely sensitive to size, and these developments thus offer the possibility to truly tailor-make materials with specific characteristics.2

Challenges in Structural Characterisation

Kirsten M. Ø. Jensen explains:

“There is still a lot of unknown land in the research of nanoclusters, and before we can fully exploit the clusters, we need further knowledge of their atomic structure. Without structural information, we have no chance of understanding neither the cluster properties nor the chemical reactions leading to them. Unfortunately, the methods we traditionally use to study atomic arrangements in either molecules or crystalline particles do not apply for this length scale – something which we in the field have coined “the nanostructure problem”. Therefore, we must look towards new techniques.”  
Development of X-ray scattering

X-rays were discovered in 1895 by W. Röntgen and in 1912, M. von Laue investigated their wave like nature. Soon after, W.L Bragg and W. H. Bragg used X-rays to examine the internal structure of crystals, and thus developed the new science of X-ray crystallography. 100 years later, we are still using the same basic principles in structural science in biology, chemistry and physics.

The solution lies in X-ray scattering.  When X-rays hit matter, they are scattered off the atoms within in the material. The atomic arrangement in the material will then give rise to interference effects in the scattered X-rays, leading to a pattern characteristic of the atomic structure. By careful data analysis, the scattering pattern can be used to extract a structural model of the atomic arrangement. This approach, using X-rays for structure solution is not new: Ever since William and Henry Bragg discovered how crystalline materials interact with X-rays in the early 19th century, X-ray diffraction has become an invaluable tool for crystal structure solution. However, Bragg diffraction, as used in the last 100 years, can only be applied for crystalline materials, where the atoms are arranged in infinite lattices and the scattering of X-rays give particular simple interference effects with clear Bragg peaks, which can routinely be analysed. This is not the case for nanoclusters and other new nanomaterials. Here, the small size of the individual particles or clusters mean that long-range order is not present, and the atomic arrangements are therefore much harder to predict using traditional methods.3

X-ray scattering pattern from gold nanoclusters. By means of advanced computational methods, the pattern can be used to solve the atomic structure of the clusters.

Instead, high energy X-ray Total Scattering can be used to overcome the challenges in nanostructure solution. Instead of only considering Bragg scattering as in traditional crystallography, the full scattering pattern is included in the analysis. By then analysing the data with advanced computational methods, where the interference effects from various atomic arrangements are considered, we can extract structural models representing the atomic arrangements in the clusters and thus understand the structure-property relations.4 The methods used for Total Scattering data analysis are still in their infancy compared to traditional crystallography, but the field is rapidly developing with new computing methods as well as new X-ray facilities appearing.




High Energy X-rays From Synchrotron Facilities for Structure Solution

Obtaining Total Scattering data suitable for nanostructure solution requires high energy X-rays, which can only be obtained at large synchrotron facilities. Synchrotrons are storage rings, where movement of electrons lead to X-ray emmitance at very high flux.  Large synchrotrons, e.g. the European Synchrotron Radiation Facility (ESRF, pictured) in Grenoble, PETRAIII in Hamburg, and in the future, at MAXIV in Lund, can furthermore produce X-rays at high energy, suitable for Total Scattering. A large part of Kirsten M. Ø. Jensen’s experimental work is therefore done at synchrotron facilities around the world, where Total Scattering data are obtained working with a team of international collaborators.

The European Synchrotron Radiation Facility in Grenoble, France. Synchrotrons are used by researcher from around the world for experiments in biology, geology, chemistry and physics as well as a range of different fields. 

Understanding Cluster Chemistry

Kirsten M. Ø. Jensen will use high energy X-ray scattering to study metal and metal oxide clusters in order to understand how the atomic arrangements in nanoclusters affect their properties. Apart from studying the structure of the magic sized clusters, she also wants to study the formation processes by performing the synthesis in situ in the X-ray beam, where Total Scattering will allow to follow the mechanisms involved in the formation of the magic sized cluster to get a much deeper understanding of the chemistry and reactivity of clusters.5,6

“The possibilities of studying chemical reactions as they happen with the X-ray beam can give much more insight into the mechanisms in cluster formation, which can make us even better chemists in the future. Even some of the most fundamental chemical reactions involved in the formation and growth of clusters and inorganic nanoparticles are still very poorly understood, and this hinders the development of new advanced materials. This project is therefore an effort to get closer to a mechanistic understanding of reactions in inorganic cluster chemistry.”

New Materials for Energy

Magic sized clusters have potential applications in a range of fields, where material tailor making is essential. For the metal clusters in the present project, Kirsten M. Ø. Jensen want to look into their catalytic activity – apart from being highly efficient catalysts in the own right, they are ideal model systems: Due to their ‘magic size’, it is possible to deduce precise size-structure-property relations, and in that way obtain knowledge that can be used in the development of tomorrow’s materials.7

“With this project, I therefore aim to obtain a much deeper understanding of some very fundamental questions in chemistry, while at the same time developing new advanced materials needed for the future society,” Kirsten M. Ø. Jensen ends.

References:

1 Jin, R. C. Quantum sized, thiolate-protected gold nanoclusters. Nanoscale 2, 343-362, (2010). 

2 Jadzinsky, P. D., Calero, G., Ackerson, C. J., Bushnell, D. A. & Kornberg, R. D. Structure of a thiol monolayer-protected gold nanoparticle at 1.1 angstrom resolution. Science 318, 430-433, (2007). 

3 Billinge, S. J. L. & Kanatzidis, M. G. Beyond crystallography: the study of disorder, nanocrystallinity and crystallographically challenged materials with pair distribution functions. Chem. Commun., 749-760, (2004). 

4 Billinge, S. J. L. & Levin, I. The problem with determining atomic structure at the nanoscale. Science 316, 561-565, (2007). 

5 Jensen, K. M. O. et al. Mechanisms for Iron Oxide Formation under Hydrothermal Conditions: An in Situ Total Scattering Study. ACS Nano 8, 10704-10714, (2014). 

6 Jensen, K. M. O. et al. Revealing the Mechanisms behind SnO2 Nanoparticle Formation and Growth during Hydrothermal Synthesis: An In Situ Total Scattering Study. J. Am. Chem. Soc. 134, 6785-6792, (2012). 

7 Li, G. & Jin, R. C. Atomically Precise Gold Nanoclusters as New Model Catalysts. Acc. Chem. Res. 46, 1749-1758, (2013). 

Reference 1, 2 and 7 are about gold nanoclusters. 

Reference 3, 4 concerns Total Scattering. 

Reference 5,6 concerns my own research in using total scattering for studying chemical reactions.