Counting electrons on supported nanoparticles
Whether it is in catalytic processes in the chemical industry, environmental catalysis, new types of solar cells or new electronic components, nanoparticles are everywhere in modern production and environmental technologies, where their unique properties ensure efficiency and save resources. The special properties of nanoparticles often arise from a chemical interaction with the support material that they are placed on. Such interactions often change the electronic structure of the nanoparticle because electrical charge is exchanged between the particle and the support. Working groups led by FAU and the University of Barcelona have now succeeded in counting the number of elementary charges that are lost by a platinum nanoparticle when it is placed onto a typical oxide support. Their work brings the possibility of developing tailor-made nanoparticles a step closer.
One of the main questions that nanoscience researchers have been discussing for some time now is how nanoparticles interact with the support that they are placed on. It is now clear that various physical and chemical factors such as the electronic structure, the nanostructure and – crucially – their interaction with the support control the properties of nanoparticles. Although this interaction – specifically the transfer of electrical charge – has already been observed to a great extent, previous studies have not investigated how much charge is transferred and whether there is a relationship between the transfer and the size of the nanoparticle.
In order to measure the electrical charge that is exchanged the international team of researchers from Germany, Spain, Italy and the Czech Republic led by Prof. Dr. Jörg Libuda, Professor of Physical Chemistry, and Prof. Dr. Konstantin Neyman, University of Barcelona, prepared an extremely clean and atomically well-defined oxide surface, onto which they placed platinum nanoparticles. Using a highly sensitive detection method at the Materials Science beamline at Elettra the researchers were able to quantify the effect for the first time. Looking at particles with various numbers of atoms, from several to many hundred, they counted the number of electrons transferred and showed that the effect is most pronounced for small nanoparticles with around 50 atoms. The magnitude of the effect is surprisingly large: approximately every tenth metal atom loses an electron when the particle is in contact with the oxide. The researchers were also able to use theoretical methods to show how the effect can be controlled, allowing the chemical properties to be adapted to better suit their intended application. This would allow valuable raw materials and energy to be used more efficiently in catalytic processes in the chemical industry, for example.
In Figure 1, the valence band spectra obtained under the conditions of resonant photoemission enhancement in Ce4+ and Ce3+ ions provided a basis for quantification of the charge transfer from Pt nanoparticles to CeO2 support. Analysis of these spectra yielded the total number of transferred electrons per surface area. Combining this information with structural data from STM, the researchers were able to link the number of transferred electrons with the size of Pt particle. In Figure 2, the number of electrons transferred per Pt particle, the partial charge per Pt atom, and the total amount of transferred electrons per surface area are plotted as functions of the Pt particle size.
Figure 1. Valence band spectra obtained under resonant enhancement conditions for Ce4+ (a) and Ce3+ (b) ions. The amplitudes of the resonances D(Ce4+) and D(Ce3+) are determined with respect to the valence band obtained under off-resonant conditions (hv=115 eV). D(Ce4+) and D(Ce3+) (c) and RER (d) as a function of the Pt deposition time.
Figure 2. The number of electrons transferred per Pt particle to the ceria support (green squares), the partial charge per Pt atom (yellow circles), and the total amount of transferred electrons per surface area (red squares) as functions of the Pt particle size.
The project was funded in part by the EU and by FAU’s Cluster of Excellence ‘Engineering of Advanced Materials’ (EAM). The researchers at EAM aim to bring together basic research in the natural sciences and applied research in engineering to investigate and develop new hierarchically structured materials with specific electronic, optical, catalytic and mechanical properties.
The authors also acknowledge the CERIC-ERIC Consortium for access to experimental facilities.
This research was conducted by the following research team:
Yaroslava Lykhach1, Sergey M. Kozlov2, Tomáš Skála3, Andrii Tovt3, Vitalii Stetsovych3, Nataliya Tsud3, Filip Dvorák3, Viktor Johánek3, Armin Neitzel1, JosefMyslivecek3, Stefano Fabris4, Vladimír Matolín3, Konstantin M. Neyman2,5 and Jörg Libuda1,61 Lehrstuhl für Physikalische Chemie II, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
2 Departament de Química Física and Institut de Quimica Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Barcelona, Spain.
3 Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Science, Czech Republic.
4 CNR-IOM DEMOCRITOS, Istituto Officina dei Materiali, Consiglio Nazionale delle Ricerche and SISSA, Trieste, Italy.
5 Institucio Catalana de Recerca i Estudis Avançats (ICREA), 08010 Barcelona, Spain.
6 Erlangen Catalysis Resource Center, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany
Contact person:
Y. Lykhach, S. M. Kozlov, T. Skála, A. Tovt, V. Stetsovych, N. Tsud, F. Dvorák, V. Johánek, A. Neitzel, J. Myslivecek, S. Fabris, V. Matolín, K. M. Neymanand and J. Libuda “Counting electrons on supported nanoparticles” Nature Materials, (2015), DOI: 10.1038/NMAT4500
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