Whereas size effects have been investigated extensively and are largely understood, it is significandy 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 c;;alled yolk-shell, or hollow character. The particles with hollow character exhibit an :m 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 wu:rtzite lattice represent deciding factors. After photoexcitation, • OOOOOOOOOO 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,.., 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 .• INTRODUGION Ceramic semiconductors continue to attract strong interest because they play a major role in many contemporary applications. For example, m;v compounds like GaN are currendy revolutionizing lighting technologies (LEDs) 1 and ll/ VI com~ounds are applied for UV protection/ in photovoltaic devices, ' 4 or as photocatalysts, for example, for the catalytic splitting of water or waste treatment. 5 -7 The performance of many of these devices depends critically on the ability of the semiconductors to generate excitonic charge carriers ( elec tron-hole pairs) when irradiated with light, as well as on the persistence of those photogenerated charge carriers (PGCCs).Thus, it is r:i prime importance to understand how the lifetime of these PGCCs is affected by the s~cific properties of the materials and how it can be controlled. The latter still remains a substantial challenge since the actual persistence of PGCCs depends strongly on numerous factors such as the coverage with stabilizers or other compounds, the density of intrinsic and extrinsic defects and, last but not least, the hierarchical construction of the material from the nanometer to the macroscopic scale. The structural motifs themselves evolve to a large extent from kinetic factors and are determined by the interplay of many parameters, which differ during the material 4593 syntheses. Unfortunately, it is very cumbersome to elucidate systematic coherences, when every material requires a different synthesis strategy. 9 -12 Nevertheless, the idea is highly tempting that the persistence of PGCCs could be controlled via the synthesis...