Investigating The Oxygen Defects of Nano-Heterojunction for Toxic Gas by Using Synchrotron and Raman Spectroscopy

In this study, a multifunctional wearable sensor with high-precision of toxic gas, nitric oxide, and multiple wavelength light detection abilities was fabricated by defect-rich nano-heterojunction SnO₂-TiO₂ nanofibers. Choosing nitric oxide as the measured gas is motivated by its potential for assessing airway inflammation by measuring its concentration in exhaled breath. While Fractional Exhaled Nitric Oxide (FENO) exists for this purpose, challenging measurement techniques and methods have hindered its clinical applicability. According to the research, the nano-heterojunction device produced in this experiment exhibits a sensitivity to low concentrations of nitric oxide that is up to seven times higher. Therefore, the ultimate goal of this experiment is to utilize the prepared nano-heterojunction device for asthma sensing.<br/>The properties of dynamic internal charge transport and recombination process can be precisely controlled through the sol-gel drop casting method, which forms numerous oxygen defect structures at the nano-heterojunction gate between SnO₂ nanofibers and TiO₂ nanoparticles, allowing the tuning of the band gap within the range of 3.6 to 3.27 eV. The reason of band gap reducing might be a significant number of oxygen defect structures are formed at the nano-heterojunction gate, acting as generation-recombination centers to precisely trap or release free carriers. This process converts various photon energies (ranging from 365 to 520 nm) into different photocurrent levels.<br/>To investigate the relationship between the adsorption and desorption of oxygen defect structures, the in-situ Raman spectroscopy and electrical measurement systems can be used for toxic gases detection. Furthermore, the conductivity and sensitivity of the wearable sensor for monitoring toxic gas can be enhanced through the defect-rich toxic gas molecular adhesion layer on the surface of nanofibers. The electronic structure and functionality of each oxygen defect structures, including out-of-plane oxygen defects, bridge oxygen defects, and in-plane oxygen defects, were studied using synchrotron analysis to investigate the electron transfer between the oxygen defect energy state and the conduction bands. Based on the results, the in-plane oxygen defects will be decreased, if the nano-heterojunction increases. The electrical results indicate that defects-rich SnO₂-TiO₂ nano-heterojunction nanofiber devices could be highly sensitive to toxic gases and serve as light sensing devices in our daily lives. Moreover, they are expected to be used in asthma detection in the future.