Highly responsive and selective NO₂ gas sensors based on titanium metal organic framework (Ti-MOF) with pyromellitic acid

Metal-organic framework (MOF) is a class of materials with high specific surface area, and is synthesized using metal ions and organic ligands. Due to its ability to reduce metal ions, the MOF materials are beneficial to react with gas molecules and are suitable as sensing materials for gas sensors. This study adopted the one-step hydrothermal method to synthesize terephthalic acid (TPA) and pyromellitic acid (PMA), and combine them with titanium metal ions to produce the Ti-MOF(TPA) and Ti-MOF(PMA) powders, which can be used as the sensing materials of the gas sensors. Surface morphology, crystallinity, chemical bond, molecular vibration, and specific surface area of the synthesized Ti-MOF(TPA) and Ti-MOF(PMA) powders were determined, respectively, using field-emission scanning electron microscope, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, and surface area and porosimetry analyzer. The dynamic response of the Ti-MOF(PMA) gas sensor was higher than the Ti-MOF(TPA) sensor. Moreover, the responses of the Ti-MOF(PMA) gas sensors were 6.4, 16.1, 35.6, and 48.5, respectively, with the applied NO₂ concentrations of 50, 100, 150, and 200 ppm at room temperature. The Ti-MOF(PMA) gas sensors demonstrated excellent reproducibility and selectivity to NO₂ compared with acetone, methanol, ethanol, ammonia, and carbon dioxide at 100 ppm concentration.