Plenary Sessions - 3&4

Self-propelled colloidal motors have shown great potential for applications in the biomedical field such as active target delivery, detoxification, minimally invasive diagnostics, and nanosurgery, owing to their tiny size, autonomous motion, and navigation capacities. To enter the clinic, biomedical colloidal motors request the biodegradability of their manufacturing materials, the biocompatibility of chemical fuels or externally-physical fields, the capability of overcoming various biological barriers (e.g. biofouling, blood flow, blood-brain barrier, cell membrane). In this talk, I will first introduce the recent advances of synthetic colloidal motors based on controlled chemical self-assembly and how such a strategy permits the realization of autonomously synthetic motors with the engineering features, such as sizes, shapes, composition, propulsion mechanism, and function. Next, I will talk how these chemically assembled motors could be designed to overcome the mentioned biological barriers. The challenges and future research priorities will be also addressed.

Just like in classical catalysis, noble metal nanoparticles are frequently applied in photocatalyst formulations. The question always arises whether the noble metal nanoparticles then act as the catalytic site, whether plasmonic effects enhance light usage, or whether the lifetime of charge carriers is extended by separation over a Schottky barrier. Possibly, more than one effect is in operation, and the mode of action likely also depends on the incident light spectrum.

In this contribution, the role of gold in photocatalysis with Au/TiO₂ is discussed down to the atomic scale. In selective alcohol oxidation reactions under UV irradiation, a predominantly catalytic role of the gold nanoparticles is detected by in situ DRIFT spectroscopy. While the alcohol can react with the photogenerated holes over the titania interface directly, the electrons can only be transferred to oxygen effectively when gold is present^[1]. Using ab initio calculations at finite temperature under consideration of the fluctuating nature of the gold nanoparticles, the primary function of the gold is discovered to be the activation of gaseous dioxygen. The oxygen molecule is split with lower activation barrier and at shorter bond lengths at the gold nanoparticle as compared to the bare titania surface^[2]. The accumulation of electrons in and near the gold nanoparticles also causes a reduction of the TiO₂ at the interface, which may serve as precursor state for rutile formation^[3].

In photocatalytic carbon dioxide reduction, the classical catalytic function of gold is also evident, because carbon monoxide, frequently observed as reaction product, is reoxidized to CO₂, so methane is the only observed product. A structure composed of gold nanoparticles embedded in a TiO₂ shell with additional gold nanoparticles on the outer surface allows significant enhancements of the methane yield; however, the outer gold nanoparticles lose their CO oxidation function, so this product is additionally obtained^[4]. Plasmonic effects, on the other hand, are not very likely in Au/TiO₂ under UV+Vis irradiation, but such effects may contribute significantly to product formation in Ag/TiO₂ systems^[5].

 References

[1] A. Lüken, M. Muhler, J. Strunk, “On the role of gold nanoparticles in the selective photooxidation of 2-propanol over Au/TiO₂”, Phys. Chem. Chem. Phys. 17 (2015) 10391.

[2] N. Siemer, A. Lüken, M. Zalibera, J. Frenzel, D. Muñoz-Santiburcio, A. Savitsky, W. Lubitz, M. Muhler,
D. Marx, J. Strunk, “Atomic scale explanation of O₂ activation at the Au-TiO₂ interface”, J. Am. Chem. Soc. 140 (2018) 18082.
[3] A. Pougin, A. Lüken, C. Klinkhammer, D. Hiltrop, M. Kauer, K. Tölle, M. Havenith, K. Morgenstern, W. Grünert, M. Muhler, J. Strunk, “Probing oxide reduction and phase transformations at the Au-TiO₂ interface by vibrational spectroscopy”, Top. Catal. 60 (2017) 1744. 

[4] A. Pougin, G. Dodekatos, M. Dilla, H. Tüysüz, J. Strunk, „Au@TiO₂ Core-Shell Composites for the Photocatalytic Reduction of CO₂“, Chem. Eur. J. 24 (2018) 12416.

[5] M. Dilla, A. Pougin, J. Strunk, “Evaluation of the plasmonic effect of Au and Ag on Ti-based photocatalysts in the reduction of CO₂ to CH₄”, J. Energy Chem. 26 (2017) 277.