Underpinning the conductivity mechanism in wide bandgap metal organic framework through chemical sensing

Metal organic frameworks are an emerging class of materials with a broad range of electronic properties with diverse applications such as sensors, catalysts, and permeable membranes. Here, three isostructural wide bandgap (WB) zeolitic imidazole frameworks (ZIFs) are synthesized having metal ion sites occupied by Zn (ZIF-8, Eg = 5.3 eV), Co (ZIF-67, Eg = 4.3 eV), and both Zn and Co (50%–50% mixture). The conductivity mechanism in these WB ZIFs involves the Mott variable range hopping of charge carriers from one metal site to the other. The hopping probability in the mixture is governed by the constitute having a lesser activation energy. Thus, it reveals that the incorporation of a different metal ion (Co in place of Zn) in the lattice forms a parallel low resistance path through hopping at Co sites and hence reduces the sensor response as well as selectivity toward ammonia. This parallel resistance path of the Co channel does not get affected by ammonia since it is found that ammonia has high affinity toward Zn ions and not toward Co ions. Thus, the incorporation of new metal ions hinders the hopping charge transport mechanisms in ZIFs.