Wettability-Driven Control of ZnO-to-ZIF-8 Conversion: The Role of Surface Chemistry in Growing Continuous Metal-Organic Framework Layers.

This study focuses on the challenge of converting ZnO films into uniform zeolitic imidazolate framework-8 (ZIF-8) layers or membranes, an essential key step in shaping metal-organic frameworks (MOFs). It compares two ZnO deposition techniques: atomic layer deposition (ALD) and physical vapor deposition (PVD), examining how each method affects the surface chemistry of ZnO and its subsequent conversion into ZIF-8. The investigation includes contact angle measurements using methanol and water to assess surface wettability as well as X-ray diffraction (XRD) analysis combined with electronic microscopy to characterize the resulting ZIF-8 layers. The study indicates that ALD ZnO films are more hydrophilic, with a water contact angle of ∼75°, compared to the more hydrophobic PVD films, which exhibit a contact angle of ∼98°. XRD analysis reveals that PVD films display a pronounced (002) crystal orientation, while ALD films consist of randomly oriented nanocrystals. To optimize the conversion of ZnO to ZIF-8, the methanol-to-water ratio was adjusted, with a 3:1 mixture yielding the most uniform ZIF-8 layers. Additionally, thermal treatment of PVD films at 600 °C significantly altered their surface reactivity and conversion behavior, leading to distinct ZIF-8 morphologies. In contrast, ALD films exhibited a higher conversion efficiency, producing continuous, well-crystallized ZIF-8 layers with minimal defects. This improved performance is attributed to their superior surface wettability and reactivity. These findings underscore the critical role of ZnO surface chemistry in ZIF-8 formation and emphasize the importance of optimizing both deposition methods and conversion conditions to achieve high-quality MOF layers.