Capturing hot electrons created under chemical reactions

Hot electron generation on the surface of nanocatalysts during chemical reactions, which is direct evidence of the electronic effect in heterogeneous catalysis, was successfully observed using the catalytic nanodiodes composed of a metal–semiconductor Schottky nanodiode.

Article  |  Fall 2015

Catalysis is a core task for developing next-generation energy technology as well as in the traditional chemical industry because catalysts are used in more than 85% of practical chemical processes. In recent year, “nanocatalyts”, combined with nanotechnology, have been intensively studied to improve catalytic activity and selectivity. One of the key issues in heterogeneous catalysis is the role of hot electrons on metal catalysts deposited on an oxide support.

In particular, with the rising development of nanocatalysts for industrial applications, there is great need for a fundamental understanding of hot electron dynamics on metal nanocatalysts. Elucidation of the electronic effect on catalytic activity requires direct detection of hot electrons on metal nanoparticles; however, this is challenging because of the quick thermalization of hot electrons via electron–electron scattering, electron–phonon coupling, and the electrical disconnection of the nanoparticles.

Prof. Jeong Young Park’s group in the Graduate School of EEWS (Energy, Environment, Water, and Sustainability) at the Korea Advanced Institute of Science and Technology (KAIST) has succeeded in the direct detection of hot electrons induced by exothermic hydrogen oxidation on Pt nanoparticles using an Au/TiO2 Schottky nanodiode, which provides further understanding of the catalytic properties of nanoparticle catalysts. To design a well-defined model catalyst system enabling current measurement under dynamic reactions, catalytically active Pt nanoparticles of two different sizes (1.7 nm and 4.5 nm) are deposited as two-dimensional arrays on the chemically inert Au surface of Schottky nanodiodes via the Langmuir–Blodgett technique. Using this nanoparticle–nanodiode hybrid system, the current measurements are carried out at different temperatures and chemical reaction conditions to elucidate the relationship between hot electron flows and reaction rates on the Pt nanoparticles.

Interestingly, the chemicurrent signal (i.e., the flow of hot electrons excited on a Pt nanoparticle during hydrogen oxidation) is obviously proportional to the reaction rates measured by gas chromatography. The team also clearly showed that smaller Pt nanoparticles lead to higher chemicurrent yields, which is associated with the shorter travel length for the hot electrons, and consequently, results in much higher catalytic activity. Additionally, the research group verified the impact of capping layers on nanoparticles on the charge carrier transfer by comparing the activation energy values obtained from both chemicurrent and reaction rate measurements and from quantitatively estimating the additional potential barrier formed by the capping layer.

The study of hot electron dynamics under catalytic surface reactions could lead to a better understanding of the electronic effect in various catalyst systems and provide a chance to develop advanced heterogeneous catalysts with remarkably enhanced catalytic activity and selectivity.

The present work was published in Angewandte Chemie International Edition (2015, 54, 2340-2344) and was selected as a frontispiece on the issue due to the importance of the work in this rapidly evolving field of high interest.

Additional links for more information:
http://onlinelibrary.wiley.com/doi/10.1002/anie.201410951/abstract
http://scale.kaist.ac.kr
http://www.brainmedia.co.kr/brainWorldMedia/ContentView.aspx?contIdx=15368