Sulfate-functionalized hafnium-organic frameworks as a highly effective chemiresistive sensor for low-temperature detection of hazardous NH₃ gas

This study proposes a solvothermal method for synthesizing sulfate-functionalized hafnium-organic frameworks (Hf-BTC-SO₄) for application in low-temperature NH₃ gas sensors. Prior to the gas-sensing studies, solvothermal-processed Hf-BTC-SO₄ is characterized using various techniques to obtain structural, elemental, morphological, and thermal stability information. Results of structural and thermal-stability analysis demonstrate that Hf-BTC-SO₄ exhibits good crystallinity and high thermal stability with the functionalization of SO₄ in the Hf-framework. Microstructural analysis reveals that nanoparticles aggregated to form compact clusters of Hf-BTC-SO₄. In addition, Hf-BTC-SO₄ has an ultra-high specific surface area of 1100 m²g⁻¹ (with a pore size of 15 Å), suitable for gas detection owing to enhanced surface reactions. Gas-sensing studies confirm that the fabricated Hf-BTC-SO₄ sensor exhibits selective detection of NH₃ gas at a lower working temperature of 100 ºC. Notably, the Hf-BTC-SO₄ sensor detected up to 1 ppm of NH₃ (response = 1.41), with excellent response reversibility. The functionalized sulfate bonds and Hf-clusters within the framework form strong bonds with NH₃, enhancing their interaction with the metal-organic frameworks. This study can motivate future research on the synthesis of functional organic frameworks for applications in low-temperature NH₃ detection devices.