Nanoarchitecture Effects on Persistent Room Temperature Photoconductivity and Thermal Conductivity in Ceramic Semiconductors: Mesoporous, Yolk-Shell, and Hollow ZnO Spheres

Whereas size effects have been investigated extensively and are largely understood, it is significantly more challenging to elucidate how functional properties of semiconductors can be altered and ultimately be improved by a hierarchical nanoarchitecture. For semiconductor applications, such as in photovoltaics or photocatalysis, it is of great importance to learn how to avoid the recombination of photogenerated charge carriers and how to enhance their lifetime. A gas-phase synthesis method is explored, which enables the generation of spherical zinc oxide nanostructures with compact, mesoporous, a special type of core shell, so-called yolk shell, or hollow character. The particles with hollow character exhibit an extraordinarily long persistence of photogenerated charge carriers. It is demonstrated that the presence of the ZnO shell and its special orientation with respect to the polar character of the wurtzite lattice represent deciding factors. After photoexcitation, electrons and holes migrate to opposite sides of the interfaces, where they are stabilized. Moreover, photoluminescence thermometry was used to determine the thermal conductivity of the samples, which is lowered by a factor of similar to 100 compared with bulk ZnO. The thermal conductivity of this type of nanostructure is found to be only 10 times larger than that of air, and this points toward potential applications as thermoelectrics.