Surface Converted Interconnected Tetrapodal Zinc Oxide for Selective Sensing Application

Gas sensors, a longstanding presence in the market, have yet to reach their full potential. They lack the necessary precision for specific gas measurements. For instance, while one gas may lead to cancer and another gas might be linked to tuberculosis, a conventional metal oxide gas sensor detects both indiscriminately, failing to identify the exact gas or its concentration. This leaves ordinary consumers unable to discern the particular gas or disease they may be exposed to, even after investing in a gas sensor. Thus, achieving selectivity is paramount for gas and Volatile Organic Compound (VOC) sensors. This is where porous chemical frameworks like Metal Organic Frameworks (MOFs) or zeolites enter the scene. These materials possess defined pore sizes and selective diffusivity, allowing them to filter some gases while obstructing others, contingent on the size of the gases and the pores in MOFs/zeolites. The challenge, however, lies in their predominantly insulating nature, posing an obstacle in creating chemiresistive sensors. Therefore, for effective sensor response detection, we have opted for the well-established conducting n-type semiconductor zinc oxide.<br/><br/>Zinc Oxide (ZnO) is renowned for its diverse morphology, adaptable properties, and versatile applications across various fields including sensing, battery technology, catalysis, and filtration. Among the established morphologies, the tetrapodal form (t-ZnO) synthesized through flame transport synthesis is of particular interest due to its single-crystalline structure and adjustable oxygen vacancies. A versatile method of applying different metal hydroxides/oxides to coat t-ZnO in an organized manner, creating a core@shell structure of t-ZnO@M(OH)_x (where M represents metals like Cu, Co, Al, Fe, Ni, Ti, etc.) derived from a suitable metal precursor is also studied. Subsequently, we interconnect the arms of the tetrapods using a photochemical process. This development of interconnections is particularly intriguing from a sensor application perspective, as it forms stable bridge-like structures between morphologies, even those that may not conduct electricity, such as MOFs or zeolites.<br/><br/>The surface of t-ZnO@M(OH)x can undergo conversions to adopt alternative structures based on the same metal framework. For example, a Cu(OH)₂ shell can give rise to a Cu-based MOF like HKUST-1, a Co(OH)₂ layer can yield a Co-based MOF like ZIF-67 and Al(OH)₃ can either convert to Al-based MOFs or serve as a source for zeolite. Likewise, t-ZnO can be readily surface converted to a Zn-based MOF, such as ZIF-8. This technique facilitates the solvothermal growth of MOFs based on any metal framework utilizing ZnO as the sensing material, all while preserving the morphology, thus enhancing selectivity towards various gases or VOCs. The interconnects grown can be precisely tailored with respect to the desired material growth, growth areas, thickness, and proximity of the self-organized flakes achieved. The ultimate objective is to create selective, sustainable, lightweight, and portable indoor gas sensors for VOCs.<br/><br/>In this talk, I represent how interconnected porous chemical frameworks like tetrapodal ZnO or corresponding MOFs and zeolites show promising selective filtration based on pore sizes by applying these structures to zinc oxide, a versatile semiconductor to enhance sensor selectivity, ultimately aiming to create indoor gas sensors with the capacity to detect VOCs.<br/><br/><b>Keywords: </b>Photochmical Interconnects, Volatile Organic Compounds(VOCs), Metal-Organic Framework(MOFs), selectivity, Zinc Oxide(ZnO), Metal Coating, Indoor gas sensors, Bridge-like tunable interconnects.