On the Factors behind the Photocatalytic Activity of Graphene Quantum Dots for Organic Dye Degradation

Graphene quantum dots (GQD) are promising visible-light photocatalysts for organic dye degradation. Besides having improved visible-light activity compared with commercial TiO₂, GQD are versatile photocatalysts as their chemical composition and, consequently, optical properties can be tuned synthetically, with a direct impact on photoactivity. However, there is a lack of systematic comparative studies to benchmark GQD photocatalytic performance and relate it to their intrinsic properties. This is undertaken in this work for three types of GQD, which are prepared using well-established synthetic methods representative of top-down and bottom-up approaches using different precursors. Resulting GQD are similar in size but differ in chemical composition, crystallinity, bandgap (ranging from 2.63 to 3.63 eV) and visible-light absorptivity. Photoactivity measurements under comparable experimental conditions (visible-light illumination) reveal enormous activity differences for rhodamine B (RhB) degradation, with up to tenfold higher degradation yields at the same time for certain GQD types. The enormous influence of intrinsic and tunable GQD factors, like visible-light absorptivity and surface charge, on their photoactivity for the degradation of organic dyes is demonstrated, highlighting the importance of tailoring such parameters for enhanced photocatalytic performance. A plausible mechanism for GQD-catalyzed photodegradation of RhB is also proposed.     

Thermographische Indizierung einiger Reaktionen von Manganferrit mittels Exoelektronenemission

Zusammenfassung Es wurde die Temperaturabhängigkeit der Exoelektronenemission, der elektrischen Leitfähigkeit, der thermoelektrischen Kräfte und der mechanischen Au-sdehnungsfähigkeit von Manganferrit bis 700 °Cgemessen und die Curietemperatur der Proben festgestellt. Die gemessenen Kurven weisen Knicke, bzw. Extreme bei der Curietemperatur und bei zwei für Oxydations — Reduktions-Vorgänge und kristallographische Umwandlungen beim alleinigen Manganoxyd charakteristischen Temperaturen auf. Zum Vergleich wurde die Temperaturabhängigkeit der elektrischen Leitfähigkeit der in gleicher Weise wie der Ferrit wärmebearbeiteten Ausgangsstoffe der Ferritbereitung gemessen.