Self-Modification of Defective TiO2 under Controlled H2/Ar Gas Environment and Dynamics of Photoinduced Surface Oxygen Vacancies.

In recent years, defective TiO2 has caught considerable research attention because of its potential to overcome the limits of low visible light absorption and fast charge recombination present in pristine TiO2 photocatalysts. Among the different synthesis conditions for defective TiO2, ambient pressure hydrogenation with the addition of Ar as inert gas for safety purposes has been established as an easy method to realize the process. Whether the Ar gas might still influence the resulting photocatalytic properties and defective surface layer remains an open question. Here, we reveal that the gas flow ratio between H2 and Ar has a crucial impact on the defective structure as well as the photocatalyic activity of TiO2. In particular, transmission electron microscopy (TEM) in combination with electron energy loss spectroscopy (EELS) revealed a larger width of the defective surface layer when using a H2/Ar (50 %-50 %) gas mixture over pure H2. A possible reason could be the increase in dynamic viscosity of the gas mixture when Ar is added. Additionally, photoinduced enhanced Raman spectroscopy (PIERS) is implemented as a complementary approach to investigate the dynamics of the defective structures under ambient conditions which cannot be effortlessly realized by vacuum techniques like TEM.

A novel method for the synthesis of core-shell nanoparticles for functional applications based on long-term confinement in a radio frequency plasma.

A novel combined setup of a Haberland type gas aggregation source and a secondary radio frequency discharge is used to generate, confine, and coat nanoparticles over much longer time scales than traditional in-flight treatment. The process is precisely monitored using localized surface plasmon resonance and Fourier-transform infrared spectroscopy as in situ diagnostics. They indicate that both untreated and treated particles can be confined for extended time periods (at least one hour) with minimal losses. During the entire confinement time, the particle sizes do not show considerable alterations, enabling multiple well-defined modifications of the seed nanoparticles in this synthesis approach. The approach is demonstrated by generating Ag@SiO2 nanoparticles with a well-defined surface coating. The in situ diagnostics provide insights into the growth kinetics of the applied coating and are linked to the coating properties by using ex situ transmission electron microscopy and energy dispersive X-ray spectroscopy. Surface coating is shown to occur in two phases: first, singular seeds appear on the particle surface which then grow to cover the entire particle surface over 3 to 5 minutes. Afterwards, deposition occurs via surface growth which coincides with lower deposition rates. Our setup offers full control for various treatment options, which is demonstrated by coating the nanoparticles with a SiO2 layer followed by the etching of the part of the applied coating using hydrogen. Thus, complex multi-step nanofabrication, e.g., using different monomers, as well as very large coating thicknesses is possible.