Unraveling the Morphology-Function Correlation of Mesoporous ZnO Films upon Water Exposure

Ubiquitous moisture in synthetic conditions and ambient environments can strongly influence the conductivity of ZnO semiconductors via the chemisorption and physisorption of water molecules on the ZnO surface. Such an intrinsically water-sensitive nature will become more evident in mesoporous ZnO films where a large surface area and active sites are created simultaneously. However, fundamental insights underlying water-mediated ZnO surface chemistry and electrical conductivity and the factors affecting them remain ambiguous due to the complexity of ZnO surfaces and the difficulties of in situ characterizations at multi-dimensions. Here, self-assembling diblock copolymers are exploited as structure-directing agents to achieve mesoporous ZnO thin films with highly tailorable structural characteristics ranging from nanomorphologies, over crystalline levels, to defect contents. As verified by theoretical calculations, the presence of oxygen vacancy will facilitate favorable water adsorption and subsequent dissociation on the polar ZnO surfaces. Upon humidity exposure with progressively increased levels, mesoporous ZnO films are revealed to follow an almost positive relationship between adsorption and electrical conductivity but show superior morphological stability. This work not only elucidates the water-governed ZnO surface chemistry but may also promote a comprehensive understanding of the morphology-function relationship on ZnO-based electronics. Mesoporous ZnO films demonstrate a close correlation between water adsorption and electrical properties but high morphological stability upon water exposure. The highly tunable structure features from the nanoscale and crystal level to defect levels will pave the way for further development of ZnO-based electronics.image