Bimetallic Nanoparticles-Stabilized Conductive Metal-Organic Frameworks for Superior Chemiresistors

Abstract

Introduction Recently, conductive metal-organic frameworks (cMOFs), having electrical conductivity with ultra-high specific surface area, have attracted much attention in diverse applications. In particular, cMOFs are promising materials for gas sensors, because their high surface area facilitates surface reactions of gas molecules, which can be transduced by electrical signals. However, cMOFs-based gas sensors are still suffering from some challenges, such as low response and poor detection limits [1]. On the other hand, bimetallic nanoparticles (BNPs) are known to have superior activities for surface reactions than single-elemental counterparts. In this regard, we envisioned that ultra-small BNPs can be encapsulated in the cavities of cMOFs and these BNPs-decorated cMOFs (BNP@cMOFs) can exhibit exceptionally high surface reactivity with efficient electrical responses. Here, for the first time, we report the combination of cMOFs with ultra-small BNPs in order to translate bimetallic synergies for surface reactions into cMOFs-based chemiresistors. As a proof of our concept, we fabricated 2 nm-sized Pt–Ru BNPs encapsulated in cMOFs (PtRu@cMOFs). PtRu@cMOFs-based sensors show hugely improved NO2 sensing performances at room temperature in air, in terms of response, cross-selectivity, and detection limits. Method Cu3(HHTP)2, a typical cMOF, is prepared by hydrothermal method using copper(II) acetate and HHTP. The Cu3(HHTP)2 has numerous rigid pores with a diameter of 2 nm, which act as an encapsulation site for BNPs. Two kinds of metal precursors, RuCl3 and K2PtCl4, are added in the suspension of Cu3(HHTP)2 in deionized water. Then, the suspension is stirred for 30 min to bind heterogeneous metal ions with the cavities of Cu3(HHTP)2. The metal ions are electrostatically bound to oxygen groups in HHTP of Cu3(HHTP)2. Subsequently, the metal precursors are reduced with sodium borohydride (NaBH4), resulting in ultra-small (~2 nm) PtRu BNPs encapsulated in the cavities of Cu3(HHTP)2. The prepared PtRu@cMOFs are used as a chemiresistor for NO2 detection, in order to utilize bimetallic synergies in Pt–Ru nanocatalysts for chemiresistive sensing. Results and Conclusions We investigated the morphology of PtRu NPs in PtRu@cMOFs by using transmission electron microscopy (TEM) analysis. TEM image showed that ~2 nm PtRu NPs were well dispersed in the cMOF-matrix. X-ray photoelectron spectroscopy (XPS) analysis was conducted to confirm the formation of Pt–Ru BNPs. In the region of Pt and Ru spectra, PtRu@cMOFs display shifts of Pt0 4f and Ru0 3p peaks compared to monometallic counterparts. As reported in previous literature, these peak shifts indicate the interaction between atomic Pt and Ru in bimetallic Pt–Ru, demonstrating the formation of Pt–Ru BNPs in cMOFs. To investigate bimetallic synergy of Pt–Ru BNPs in cMOFs-based chemiresistive sensing, the resistance changes (ΔR/R0) of PtRu@cMOFs in response to exposures of NO2 2 ppm are compared with those of pristine cMOFs, Ru@cMOFs, and Pt@cMOFs. Importantly, PtRu@cMOFs showed hugely improved NO2 responses compared to pristine cMOFs, Pt@cMOFs, and Ru@cMOFs, demonstrating the superior catalytic effect of Pt–Ru BNPs on NO2 sensing. Responses of PtRu@cMOFs-based sensors were measured in 0.2–3 ppm of NO2. The sensors showed high responses of 46.8% to 1 ppm, which is the regulatory permissible NO2 exposure limit designated by the occupational safety and health administration (OSHA) in U.S.A. Moreover, PtRu@cMOFs exhibited 8-fold higher responses to NO2 than other analytes, demonstrating the ultrahigh cross-selectivity. Furthermore, PtRu@cMOFs showed superior sensing response compared to the state-of-art NO2 sensors in air at room temperature. In conclusion, we have demonstrated the synthesis of BNPs embedded in cMOFs, which have superior NO2 sensing performances. The cavities of cMOFs do not only act as a binding site of metal precursors but also suppress the growth of BNPs during the chemical reduction, generating ultra-small (~ 2nm) PtRu BNPs in cMOFs. PtRu@cMOFs exhibit greater NO2 responses compared to their monometallic counterparts. These outstanding sensing performances are attributed to the bimetallic synergies of Pt and Ru in Pt–Ru BNPs with highly porous and conductive matrix. Our approaches provide general and facile methods to create various beneficial BNPs in versatile cMOFs [2].