Catalytic Metal Nanoparticles Embedded in Conductive Metal–Organic Frameworks for Chemiresistors: Highly Active and Conductive Porous Materials

Abstract

Conductive porous materials having a high surface reactivity offer great promise for a broad range of applications. However, a general and scalable synthesis of such materials remains challenging. In this work, the facile synthesis of catalytic metal nanoparticles (NPs) embedded in 2D metal–organic frameworks (MOFs) is reported as highly active and conductive porous materials. After the assembly of 2D conductive MOFs (C‐MOFs), i.e., Cu3(hexahydroxytriphenylene)2 [Cu3(HHTP)2], Pd or Pt NPs are functionalized within the cavities of C‐MOFs by infiltration of metal ions and subsequent reduction. The unique structure of Cu3(HHTP)2 with a cavity size of 2 nm confines the bulk growth of metal NPs, resulting in ultra‐small (≈2 nm) and well‐dispersed metal NPs loaded in 2D C‐MOFs. The Pd or Pt NPs‐loaded Cu3(HHTP)2 exhibits remarkably improved NO2 sensing performance at room temperature due to the high reactivity of catalytic metal NPs and the high porosity of C‐MOFs. The catalytic effect of Pd and Pt NPs on NO2 sensing of Cu3(HHTP)2, in terms of reaction rate kinetics and activation energy, is demonstrated.

Large-area synthesis of nanoscopic catalyst-decorated conductive MOF film using microfluidic-based solution shearing

“…These outstanding sensing performances outperform the current state-of-the-art 2D material-based NO2 chemiresistors operated at room temperature in air (Fig. 5 and Supplementary Table 8) 15,41,42,43,44,45,46,47,48,49,50 . To the best of our knowledge, the NO2 responses of our sensors are the highest among MOF-based NO2 sensors, and even superior to other 2D materials, including graphene, transition metal dichalcogenides (TMDs), and their composites.…”

“…Comparing the HR-TEM images of Pt@Cu3(HHTP)2 made using conventional bulk synthesis technique 15 to that of MiCS (Supplementary Fig. 7) showed that the size variance of the Pt particles was much higher for the former.…”

“…These properties broadened the applicability of metal organic frameworks (MOFs) (which have traditionally been electrically insulating) to novel applications such as transistors 6 , electrodes 7, 8, 9, 10 , and resistive chemical sensors 11,12,13,14 .The key features of MOFs are their high porosity and regularly arranged pores. These pores can be utilized to immobilize nanoscale catalyst such as Au, Pd, and Pt, and since the reactivity of the catalysts improves with increasing surface area, well-dispersed nanoscale catalysts can drastically enhance the overall catalytic performance of MOFs 15,16,17,18,19 .Recently, the immobilization of nanocatalyst has been applied to C-MOFs, and was demonstrated of enhancing the performance of Li-S batteries 20 and gas sensors 15 .The C-MOF applications mentioned above require the formation of high quality thinfilms with controlled film thickness down to nanometer scale (less than 100 nm), smooth and uniform surface, and densely-packed thin-film MOF particles to ensure fast transport of charge carriers across the thin-film 21 . Furthermore, immobilization of nanocatalyst into the pores is needed to optimize the catalytic performance of C-MOF thin-films.…”

“…The key features of MOFs are their high porosity and regularly arranged pores. These pores can be utilized to immobilize nanoscale catalyst such as Au, Pd, and Pt, and since the reactivity of the catalysts improves with increasing surface area, well-dispersed nanoscale catalysts can drastically enhance the overall catalytic performance of MOFs 15,16,17,18,19 .…”

“…The Pt@Cu3(HHTP)2 thin-film shows higher NO2 adsorption kinetics (kads = 2.34 × 10 −3 ppm −1 s −1 ) than the Cu3(HHTP)2 powder (0.877 × 10 −3 ppm −1 s −1 ) and the Cu3(HHTP)2 thin-film (1.51 × 10 −3 ppm −1 s −1 ), demonstrating that NO2 reactions on Cu3(HHTP)2 are promoted by two factors: (1) the thinfilm structure of Cu3(HHTP)2 and (2) the catalytic effect of ultra-small Pt NPs. The thin-film structure induces high gas accessibility into sensing layers 14 , and ultra-small Pt NPs (~2 nm) cause NO2 spillover onto Cu3(HHTP)2 15 . Therefore, NO2 molecules are easily accessible to Cu3(HHTP)2 layers and their reactions are activated by nanoscopic Pt catalysts, leading to the exceptionally high sensing performances.…”