Template- and etching-free fabrication of two-dimensional hollow bimetallic metal-organic framework hexagonal nanoplates for ammonia sensing

This work reports the template- and etching-free fabrication of hollow bimetallic nickel-cobalt benzenetricarboxylic acid (Ni-Co BTC) hexagonal nanoplates by a polyvinylpyrrolidone (PVP)-assisted approach. The incorporation of PVP can reduce the stacking of these nanoplates along the vertical direction and generate depletion forces between them to reduce aggregation. When employed for the quartz crystal microbalance (QCM) sensing of ammonia (NH3), the hollow Ni-Co BTC hexagonal nanoplates exhibit 1.6, 3.8, and 7.5 times higher sensitivity to 69.5 ppm of NH3 than non-hollow Ni-Co BTC nanoplates, Ni-BTC nanobelts, and Co-BTC microrods, respectively, and a low limit of detection (LOD) of 1.53 ppm. Additionally, they show good selectivity to NH3 in the presence of other interfering compounds and excellent stability with only a very small decrease of 2.86 % in sensitivity after 6 months. The NH3 adsorption on the hollow Ni-Co BTC hexagonal nanoplates follows a pseudo first-order kinetic model with the adsorption rate being 6.1 and 7.1 times faster than Ni-BTC nanobelts and Co-BTC microrods, respectively. The good sensing performance of the hollow Ni-Co BTC hexagonal nanoplates to NH3 is attributed to the existence of carboxyl and hydroxyl groups which can provide energetic sites for the chemisorption of NH3 molecules and the increased adsorption sites provided by the hollow two-dimensional structure and the bimetallic composition of this MOF.