Structure–Property–Function Relationships in Nanoscale Oxide Sensors: A Case Study Based on Zinc Oxide**

Transition metal oxides exhibit an amazing spectrum of properties and applications. ZnO is an excellent example as it possesses interesting piezoelectric and electromechanical coupling properties, and it has been used in UV light-emitting diodes, lasers, photovoltaic solar cells, UV-photodetectors, varistors, and even heterogeneous catalysis. For the current manuscript, the combination of semiconducting properties and catalytic surface activity is most relevant: the electrical resistance of ZnO changes depending on the amount of surfacebound oxygen atoms. Interestingly, ZnO was one of the first materials to be explored for chemical sensing. However, to date most of the attention in the field of metal-oxide sensors was devoted to SnO2, [18,19] and not ZnO sensors, presumably because the latter devices suffered from insufficient longterm stability. It is already well established for metal-oxide sensors that the sensitivity depends on various factors that can be summarized as textural parameters: low-area intergrain and interagglomerate contacts, lower film thickness, and high porosities of the sensor matrix are beneficial for performance. Consequently, a considerable amount of effort has been made to achieve a highly sensitive gas sensor by engineering the above-mentioned texture parameters. However, the intrinsic properties of semiconducting oxides are determined by more than morphological factors. Therefore, efforts to understand the contribution of nontexture factors (local or nonlocal deviations in chemical composition as well as microstructural defects) regarding the performance of chemical sensors are of enormous importance.