Bimetallic Two-Dimensional Metal-Organic Frameworks for the Chemiresistive Detection of Carbon Monoxide

Abstract

This paper describes the demonstration of a series of heterobimetallic, isoreticular 2D conductive metal-organic frameworks (MOFs) with metallophthalocyanine (MPc, M=Co and Ni) units interconnected by Cu nodes towards low-power chemiresistive sensing of ppm levels of carbon monoxide (CO). Devices achieve a sub-part-per-million (ppm) limit of detection (LOD) of 0.53 ppm toward CO at a low driving voltage of 0.1 V. MPc-based Cu-linked MOFs can continuously detect CO at 50 ppm, the permissible exposure limit required by the Occupational Safety and Health Administration (OSHA), for multiple exposures, and realize CO detection in air and in humid environment. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), density functional theory (DFT) calculations, and comparison experiments suggest the contribution of Cu nodes to CO binding and the essential role of MPc units in tuning and amplifying the sensing response.

Keywords: carbon monoxide; chemiresistors; conductive; metallophthalocyanines; two-dimensional.

Figure 1.

(a) Experimental pXRD diffraction patterns of CoPc-O₈-Cu (blue) and NiPc-O₈-Cu (green) MOFs and simulated pXRD of MPc-O₈-Cu MOFs with eclipsed (orange) and staggered (grey) stacking. (b) Structure models of MPc-O₈-Cu MOFs with eclipsed (left) and staggered (right) stacking. SEM of images of (c) CoPc-O₈-Cu and (d) NiPc-O₈-Cu. Insets, TEM images.

Figure 2.

Saturation sensing traces of (a) CoPc-O₈-Cu and (b) NiPc-O₈-Cu MOF after 30 min exposure to 80, 40, 20, and 10 ppm of CO. (c) Responses (−ΔG/G₀) of CoPc-O₈-Cu MOF and NiPc-O₈-Cu after 30 min exposure versus concentration of CO. Error bars represent standard deviation from the average response based on at least three devices.

Figure 3.

(a) Sensing traces of 7 sequential exposure-recovery cycles to 50 ppm CO using CoPc-O₈-Cu (blue) and NiPc-O₈-Cu (green). Each cycle comprised a 5 min exposure and 10 min recovery. (b) Response of CoPc-O₈-Cu (blue) and NiPc-O₈-Cu (green) to 80 ppm CO in N₂, air, and humid N₂ with 5000 ppm of H₂O (18% relative humidity, RH). (c) Sensing traces of CoPc-O₈-Cu to consecutive exposure-recovery cycles of 80, 40, 20, and 10 ppm of CO in the air with 5000 ppm of H₂O. For each cycle, the exposure and recovery time were 5 and 10 min, respectively. For each concentration, three exposure-recovery cycles were performed. (d) Response-concentration relationship of CoPc-O₈-Cu under consecutive CO exposures in humidified air (5000 ppm H₂O, 18% relative humidity, RH). (e) Response of the CoPc-O₈-Cu (blue) and NiPc-O₈-Cu (green) to 80 ppm of NO, NO₂, and CO₂. (f) PCA for NiPc-O₈-Cu (green) and CoPc-O₈-Cu (blue) showing capability for differentiating 80 ppm of NO₂, NO, and CO.

Figure 4.

(a) DRIFTS spectra of CoPc-O₈-Cu and (b) NiPc-O₈-Cu after exposure to 1% CO (10000 ppm) of for 6 min. The spectra are presented as double beam experiments with pristine MPc-O₈-Cu MOFs used as the reference. (c) The optimized structures of CoPc-O₈-Cu, CO@Co/CoPc-O₈-Cu, CO@Cu/CoPc-O₈-Cu. (d) The optimized structures of NiPc-O₈-Cu, CO@Ni/NiPc-O₈-Cu, and CO@Cu/NiPc-O₈-Cu. The calculated values of the Mulliken charge are labeled with blue. The CO•••M lengths are labeled with black. (e) Bindning free energies of CO at different sites of the MPc-O₈-Cu MOFs. (f) Comparison of the sensing response of MPc-O₈-Cu and M₃(HHTP)₂ MOFs to 80 ppm of CO in N₂.