In this article, we demonstrate the fabrication of tailored multicomponent cryoaerogels from colloidal nanoparticles via the cryogelation method. With this method, it is possible to control the amount of components very precisely. Furthermore, the microscopic distribution of the different nanoparticle components in the resulting monolithic structure is shown to be adjustable by simply mixing calculated amounts of colloidal nanoparticle solutions with a suitable surface charge. We focus on titania cryoaerogels due to their potential for optical applications and investigate the properties of synthesized titania-gold cryoaerogels in dependency of the composition. In addition, titania-platinum cryoaerogels were tested for photocatalytic applications such as hydrogen evolution and showed a significant increase in performance and stability compared to their respective colloidal solutions. While showing comparable results for hydrogen evolution with aerogels as reported in literature, the fabrication is much faster and less complex and therefore might enable future industrial application.

Halide perovskites are a promising optical gain medium with high tunability and simple solution synthesis. In this study, two gain regimes, namely, amplified spontaneous emission and random lasing, are demonstrated in the same MAPbBr3 halide perovskite single crystal. For this, photoluminescence is measured at a temperature of 4 K with pulsed femtosecond pumping by UV light with an 80 MHz repetition rate. Random lasing is observed in areas of the sample where a random resonator is formed due to cracks and crystal imperfections. In more homogeneous regions of the sample, the dominant regime is amplified spontaneous emission. These two regimes are reliably distinguished by the line width, the mode structure, the growth of the intensity after the threshold, and the degree of polarization of the radiation. The spectral localization of the stimulated emission well below the bound exciton resonance raises a question concerning the origin of the emission in halide perovskite lasers.

For the understanding of the activity of TiO2 photocatalysts, knowledge of the activities of different crystal facets is necessary. This information can be achieved by the investigation of well-defined single-crystalline TiO2 surfaces. In this study, the photocatalytic activity of different anatase, brookite, and rutile single-crystal wafers with only one exposed surface has been investigated via the oxidation of methanol and the hydroxylation of terephthalic acid, respectively. X-ray diffraction, atomic force microscopy, and scanning electron microscopy measurements have shown that all surfaces are clearly defined and possess a smooth surface, which allows a reliable comparison of the photocatalytic activities. The investigated anatase surfaces show higher activity than the rutile surfaces, while the brookite surface is interestingly the least active one. To the best of our knowledge, there are no other reports based on the investigation and comparison of well-defined TiO2 anatase (100), anatase (001), and brookite (100) single-crystalline surfaces concerning their photocatalytic activity. Furthermore, the influence of the coordination of the superficial titanium and the oxygen ions on the photocatalytic activity is discussed.

In our work, we employed Cs3Bi2I9 as a visible-light-active photocatalyst, synthesized with a low-temperature solvothermal method. The morphological and structural properties of the as-prepared perovskite were investigated, and the results were compared to previous studies to confirm its nature and the quality of the synthesis procedure. Transient absorption spectroscopy was applied in order to investigate the generation and lifetime of photogenerated charge carriers, revealing their formation after visible light excitation. The potential photocatalytic activity of the as-prepared metal halide perovskite was applied for the removal of Rhodamine B in aqueous solution, demonstrating an excellent activity of 93% after 180 min under visible-light irradiation. The current research aims to provide insights into the design of a new visible-light-active photocatalyst, Cs3Bi2I9, selected for its high application value in the field of advanced materials for light harvesting.

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