Introduction
Metal–organic frameworks (MOFs) have been extensively studied in various fields, such as gas separation, gas storage, catalysis, energy storage system, and sensors due to their ultrahigh porosity, large surface area, and tunable structures. In particular, MOFs are highly fascinating materials as gas‐sensing layers because the signals of gas sensors rely on surface reactions of analytes. However, most MOFs have low electrical conductivity due to the lack of the orbital overlap between metal nodes and organic ligands, impeding efficient signal transduction for gas sensing. Therefore, luminescence, localized surface plasmon resonance, interferometry, and electromechanical‐based MOF sensors have been introduced to transduce gas‐sensing signals, which complicates the sensor system.
Recently, conductive MOFs (C‐MOF) with high electrical conductivity and permanent porosity have been developed, exhibiting high feasibility for applications in supercapacitors, electrocatalysts, and field‐effect transistors. In addition, Dincă et al. first reported chemiresistive sensing properties of 2D C‐MOFs. They synthesized a conductive 2D Cu3(hexaiminotriphenylene)2, and demonstrated its impedance variations upon ammonia (NH3) adsorption. In addition, they developed the chemiresistive sensor arrays consisted of Cu and Ni‐based C‐MOFs sensing layers for the detection of diverse volatile organic compounds. Although C‐MOF‐based chemiresistors have been reported in a few articles, related with the detection of hydrogen sulfide (H2S), nitrogen monoxide (NO), and NH3 gases, they still suffer from critical issues such as low sensitivity in air and poor cross‐sensitivity (selectivity), which hinders the practical application of C‐MOF‐based gas sensors. Besides, studies on C‐MOF‐based chemiresistive sensors are in the early stage, thus further in‐depth study is needed to address these limitations.
One of the noteworthy properties of MOFs is that their cavities can encapsulate noble metal nanoparticles (NPs), such as Au, Pt, and Pd. The unique porous structures of MOFs allow the encapsulation of ultra‐small and well‐dispersed metal NPs in their cavities. Since the reactivity and selectivity of catalysts highly rely on their surface properties, metal nanocatalysts embedded in highly porous MOFs are able to improve catalytic performance dramatically. For instance, Huo et al. synthesized Au or Pt NPs‐loaded zeolite imidazole frameworks (ZIF‐8) by using polyvinylpyrrolidone as a surfactant, which exhibited active and selective catalytic properties for hydrogenation. They further demonstrated a general and versatile synthesis of metal NPs‐loaded MOFs by extending the concept to various kinds of metals and MOFs. In addition, Yaghi et al. reported the site selective decoration of metal NPs in MOF and demonstrated their efficient catalytic performance in the conversion of methylcyclopentane. The functionalization of MOFs with metal NPs has been extensively studied to date. However, to the best of our knowledge, C‐MOFs decorated with metal NPs have not been reported.
Here, we propose metal NPs embedded in C‐MOFs, with integrated functionalities including moderate electrical conductivity and high porosity of C‐MOFs, and the outstanding catalytic reactivity of metal NPs. The unique porous structure of C‐MOFs inhibits the growth of metal NPs, thus ultra‐small and well‐dispersed metal NPs are embedded in the cavities of C‐MOFs. C‐MOFs provide numerous pores and high surface area, which are essential for the enhancement of the surface reaction, and their electrical conductivity realizes the direct transduction of signals from the surface reactions even at room temperature. The metal NPs in C‐MOFs not only promote the surface reactions but also tune the electrical properties of C‐MOFs, thereby improving the reactivity of C‐MOFs. As a proof of concept, we synthesized Pd or Pt NPs‐loaded 2D C‐MOFs that assembled by Cu nodes and 2,3,6,7,10,11‐hexahydroxytriphenylene (HHTP) linkers, for developing C‐MOFs‐based chemiresistors. Pd or Pt NPs (≈2 nm) functionalized Cu3(HHTP)2 (M@Cu3(HHTP)2, M = Pd or Pt) exhibited dramatically enhanced NO2 response, superior cross‐selectivity, and improved reaction kinetics (response speed). The important roles of metal NPs in C‐MOFs for the improvement of sensing characteristics are discussed based on the interpretation of the reaction kinetics and activation energy for NO2 adsorption.
Results and Discussion
Figure 1 shows the synthetic process and conceptual design of metal NPs‐loaded Cu3(HHTP)2. First, Cu3(HHTP)2 was prepared by a solvothermal reaction of copper(II) acetate and HHTP in methanol solution. Cu3(HHTP)2 is an electrically conductive 2D extended framework (Figure a). After purification, Cu3(HHTP)2 was dispersed in deionized (DI) water, and metal salts were added into the suspension. To synthesize Pd or Pt NPs, potassium tetrachloroplatinate(II) and potassium tetrachloropalladate(II) were used as precursors, respectively. The metal ions are bound to oxygen sites in Cu3(HHTP)2. Then, the reduction by sodium borohydride (NaBH4) solution results in Pd or Pt NPs embedded in Cu3(HHTP)2 (Figure b). The porous structure of Cu3(HHTP)2 that consisted of permanent pores with a diameter of 2 nm, limits the growth of Pd or Pt, thereby creating ultra‐small and well‐dispersed metal NPs throughout the structure of 2D Cu3(HHTP)2. Then, we fabricated M@Cu3(HHTP)2‐based sensors to take advantages of fascinating features of M@Cu3(HHTP)2 (Figure c).
Enlarge this image.Schematic illustration of the synthesis of M@Cu3(HHTP)2. a) Cu3(HHTP)2 prepared by solvothermal synthesis and b) M@Cu3(HHTP)2 synthesized by the infiltration of metal ions and subsequent reduction process. c) The conceptual design of the M@Cu3(HHTP)2‐based gas sensors.
The scanning electron microscopy (SEM) image exhibited the particle‐shaped pristine Cu3(HHTP)2 with an average diameter of 40 nm (Figure 2a). The morphologies of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 were similar with that of pristine Cu3(HHTP)2 (Figure b,c). The transmission electron microscopy (TEM) image of Pd@Cu3(HHTP)2 clearly showed that ultra‐small Pd NPs were well‐dispersed in Cu3(HHTP)2 (Figure d). The high‐resolution TEM (HRTEM) image of Cu3(HHTP)2 revealed the lattice distance of 2.245 Å (Figure e), which is corresponded to the crystal plane of Pd (111). In addition, the size of Pd NPs was identified to be ≈2 nm, demonstrating the confined growth of Pd NPs in the cavities of Cu3(HHTP)2. The dark‐field scanning TEM (STEM) analysis also confirmed that the nanoscale Pd was well‐dispersed in Cu3(HHTP)2 (Figure f). The energy‐dispersive X‐ray spectroscopy (EDS) elemental mapping images of Pd@Cu3(HHTP)2 showed C, Cu, O, and Pd elements (Figure S1a, Supporting Information). However, the intensity of Pd was relatively low. In the case of Pt@Cu3(HHTP)2, the samples showed a similar microstructure with that of Pd@Cu3(HHTP)2. As shown in Figure g, the tiny‐sized Cu3(HHTP)2 was decorated by Pt NPs with an average diameter of 2 nm, which were also well‐dispersed in Cu3(HHTP)2. The lattice fringe of Pt (111) plane with the spacing of 2.265 Å was clearly observed in the HRTEM image (Figure h). In addition, the STEM image of Pt@Cu3(HHTP)2 revealed that Pt NPs are well dispersed throughout the structure of the Cu3(HHTP)2 (Figure i). The presence of C, Cu, O, and Pt elements in the samples was confirmed by EDS elemental mapping (Figure S1b, Supporting Information). These results demonstrated the versatile synthesis of the metal NPs‐loaded C‐MOFs. Therefore, the proposed synthesis can be easily extended to various types of metal NPs‐loaded C‐MOFs by varying the kind of metals and C‐MOFs for the development of highly active and conductive porous materials.
Enlarge this image.SEM images of a) pristine Cu3(HHTP)2, b) Pd@Cu3(HHTP)2, and c) Pt@Cu3(HHTP)2. d) TEM image of Pd@Cu3(HHTP)2, e) HRTEM image of Pd@Cu3(HHTP)2, and f) STEM image of Pd@Cu3(HHTP)2. g) TEM image of Pt@Cu3(HHTP)2, h) HRTEM image of Pt@Cu3(HHTP)2, and i) STEM image of Pt@Cu3(HHTP)2. The scale bars of the magnified images in (e) and (h) are 1 nm.
The crystal structures of pristine Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2 were investigated by X‐ray diffraction (XRD) analysis (Figure 3a). The pristine Cu3(HHTP)2 exhibited the crystal planes of (200), (210), and (004), which were similar to observations in previous literature. After the functionalization of Cu3(HHTP)2 with metal NPs, the peaks related to Pd and Pt were clearly observed in the XRD patterns of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 (red dots for fcc Pd (JCPDS no. 46‐1043) and blue dots for fcc Pt (JCPDS no. 04‐0802) in Figure a), while the Cu3(HHTP)2 peaks were weak due to their low intensity. The chemical state of Pd and Pt in Cu3(HHTP)2 were verified by X‐ray photoelectron spectrometer (XPS) analysis. The XPS survey spectra and the high‐resolution spectra of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 revealed the presence of Cu, O, and C elements (Figures S2 and S3, Supporting Information). In the case of Pd@Cu3(HHTP)2, the Pd 3d peaks in the XPS spectrum revealed the two characteristics peaks at 335.5 eV for Pd0 3d5/2 and 336.9 eV for Pd2+ 3d5/2 with an energy gap of 5.3 eV between 3d5/2 and 3d3/2 peaks (Figure b). On the other hand, the high‐resolution spectrum of Pt@Cu3(HHTP)2 in the vicinity of Pt 4f was deconvoluted into Pt0 at 71.2 eV and Pt2+ at 72.4 eV for 4f7/2, with background peaks related to Cu 3p, which correspond to metallic Pt and PtO, respectively (Figure c). The XPS analysis of the samples confirmed that the metallic Pd and Pt NPs were embedded in the Cu3(HHTP)2 and that some of the metal NPs were partially oxidized to PdO and PtO.
Enlarge this image.a) XRD analysis of Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2. XPS spectra of b) Pd@Cu3(HHTP)2 for Pd 3d and c) Pt@Cu3(HHTP)2 for Pt 4f. d) N2 adsorption and desorption isotherms of the samples at 77 K. e) FT‐IR spectra of Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2. f) TGA of the samples under air atmosphere.
The N2 adsorption and desorption isotherms at 77 K revealed the porous structure of the samples (Figure d). The N2 uptake of pristine Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2 rapidly increased at low relative pressure (type I isotherms), revealing that the microporous structure of pristine Cu3(HHTP)2 was well preserved after the metal encapsulation. The increased N2 uptake of pristine Cu3(HHTP)2 at high relative pressure can be described by the pores between agglomerated Cu3(HHTP)2 particles, while this sharp increase did not appear after metal encapsulation. The specific surface areas calculated by Brunauer–Emmett–Teller (BET) method were 151.95 m2 g−1 for pristine Cu3(HHTP)2, 122.32 m2 g−1 for Pd@Cu3(HHTP)2, and 97.35 m2 g−1 for Pt@Cu3(HHTP)2, respectively. The decrease in the surface area was attributed to the mass contribution of nonporous metal NPs in M@Cu3(HHTP)2. To further investigate the structural stability of the samples, we carried out Fourier transform infrared (FT‐IR) analysis. The FT‐IR spectrum of pristine Cu3(HHTP)2 exhibited the major peaks at 1174 cm−1 for C–O stretching vibration and 1421 cm−1 for C–H scissoring vibration (Figure e). The Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 showed FT‐IR patterns similar to that of pristine Cu3(HHTP)2, indicating that the structure of Cu3(HHTP)2 was retained after the loading of Pd and Pt NPs. The new peaks at 1571 cm−1 for Pd@Cu3(HHTP)2 and at 1558 cm−1 for Pt@Cu3(HHTP)2 were associated with the vibration of adsorbed H2O on metal NPs, revealing the presence of metal NPs in the M@Cu3(HHTP)2. The loading amounts of Pd and Pt in Cu3(HHTP)2 were analyzed by using inductively coupled plasma optical emission spectrometry. The relative weight ratio of Pd and Pt was verified to be 3.3 wt% in Pd@Cu3(HHTP)2 and 5.6 wt% in Pt@Cu3(HHTP)2, respectively. The thermal gravimetric analysis (TGA) further confirmed the loading amounts of Pd and Pt (about 4 wt% in both samples), which remained as residues upon decomposition of organic matter at high temperature (Figure f). In addition, we observed that the thermal stability of Cu3(HHTP)2 was slightly improved after the loading of metal NPs, which perhaps resulted from the interaction of Pd and Pt with Cu3(HHTP)2.
To verify the high reactivity of metal NPs in C‐MOFs, we conducted gas‐sensing measurements against nitrogen dioxide (NO2) molecules by using Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2. NO2 is one of the toxic gases, and can be emitted from industrial sources such as vehicles and power plants, raising serious concerns on environment and human health. Thus, it is important to develop highly sensitive sensors capable of detecting NO2 in the order of sub‐part per million (ppm) levels. The sensors were fabricated by the drop‐coating of the samples on an alumina (Al2O3) substrate patterned with two parallel Au electrodes. The sensing characteristics were evaluated at room temperature in dry air (relative humidity: ≈5%). It was noted that the baseline resistance of the sensors in humid atmospheres (relative humidity: ≈95%) continuously increased to the measurement limit (100 MΩ) of our sensing system. This is because H2O molecules (hundreds to tens of thousands of ppm levels) in air can be easily adsorbed on the open Cu sites in Cu3(HHTP)2. The baseline resistances of the samples in dry air were 13 MΩ for pristine Cu3(HHTP)2, 25 MΩ for Pd@Cu3(HHTP)2, and 63 MΩ for Pt@Cu3(HHTP)2 (Figure S4, Supporting Information). The increase in baseline resistances was attributed to the creation of multiple junctions between materials of different work functions (5.99 eV for Cu3(HHTP)2, 5.12 eV for Pd, and 5.65 eV for Pt). Because the majority current carriers in Cu3(HHTP)2 are holes, the electrons transfer from Pd or Pt to Cu3(HHTP)2 increase the baseline resistance of the sensors. In addition, the refinement of XRD data revealed that the interlayer spacing (z‐axis) of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 slightly increased (3.1531 Å for Pd@Cu3(HHTP)2 and 3.1543 Å for Pt@Cu3(HHTP)2) compared with that (3.1518 Å) of pristine Cu3(HHTP)2 (Table S1, Supporting Information). The slight increase of lattice spacing to z‐axis can reduce the hopping current between C‐MOF layers. Moreover, the partially oxidized Pd and Pt (PdO and PtO as confirmed with XPS analysis) can further affect the baseline resistance by creating additional junctions. Since the work functions of PdO and PtOx are 7.90 and 5.65 eV, respectively, PtO donates electrons to Cu3(HHTP)2, whereas PdO deprives Cu3(HHTP)2 of electrons (Figure S5, Supporting Information). Therefore, from these reasons, the baseline resistance of Pt@Cu3(HHTP)2 was much higher than that of Pd@Cu3(HHTP)2.
The variations in the sensor resistances upon exposure to 5 ppm of NO2 were monitored in real time at room temperature in air (Figure S4, Supporting Information). The dynamic resistance variations were normalized to dynamic response transitions. The response is defined by the ratio of the resistance change to the baseline resistance (ΔR/R0) (Figure 4a). The sensors showed slow recovery speed, due to the difficulty in desorption of NO2 molecules at room temperature. Because NO2 is a strong electron acceptor, the resistance of the Cu3(HHTP)2, which exhibits a p‐type semiconducting behavior, decreased by the adsorption of NO2. When Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 were exposed to NO2, their resistances drastically decreased compared with that of pristine Cu3(HHTP)2. The Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 showed much improved response to 5 ppm of NO2 (−62.11% for Pd@Cu3(HHTP)2 and −57.38% for Pt@Cu3(HHTP)2) than pristine Cu3(HHTP)2 (−29.95%). In addition, Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 exhibited higher response to 1 ppm of NO2 (−13.5% for Pd@Cu3(HHTP)2 and −12.1% for Pt@Cu3(HHTP)2)) than pristine Cu3(HHTP)2 (−5.0%) (Figure b and Figure S6, Supporting Information). Response time of Cu3(HHTP)2 was also hugely improved by embedding Pd or Pt NPs in Cu3(HHTP)2 (Figure S7, Supporting Information). In particular, the response time to 1 ppm of NO2 was 13.8 min for Pd@Cu3(HHTP)2, 14 min for Pt@Cu3(HHTP)2, and 18 min for Cu3(HHTP)2. This result indicates that the M@Cu3(HHTP)2‐based sensors can detect 1 ppm of NO2 molecules within 15 min, which is the short‐term permissible exposure limit designated by the Occupational Safety and Health Administration (OSHA) in the United States. These results confirm that the sensing properties are significantly improved after the decoration Pd or Pt NPs on Cu3(HHTP)2.
Enlarge this image.Sensing characteristics of Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2 operated at room temperature in air. a) Dynamic response variations of the sensors to 5 ppm of NO2, b) response versus NO2 concentrations, c) dynamic response changes of the sensors when exposed to 5 ppm of diverse gas species (NO, NH3, H2, C7H8, C2H5OH, and CH3COCH3), d) calculated response toward various analytes, and e) response variations of the sensors during cyclic sensing tests toward 1 ppm of NO2. f) Response versus NO2 concentration for diverse state‐of‐the‐art NO2 sensors operated at room temperature in air.
Moreover, we examined the selectivity of Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2 sensing layers. The Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 exhibited high resistance changes when they were exposed to NO2, and negligible resistance changes upon exposure to nitrogen monoxide (NO), ammonia (NH3), hydrogen (H2), toluene (C6H5CH3), ethanol (C2H5OH), and acetone (CH3COCH3) (Figure c). The normalized response values of Pd@Cu3(HHTP)2 (62.11%) and Pt@Cu3(HHTP)2 (57.38%) toward NO2 molecules were tenfold higher than those (lower than 5.90% for Pd@Cu3(HHTP)2 and lower than 5.03% for Pt@Cu3(HHTP)2) toward interfering gases (Figure d), demonstrating the high NO2 selectivity of the catalyst‐loaded C‐MOFs sensors. In terms of repeatability, the resistance of pristine Cu3(HHTP)2 did not recover to the baseline value during 14 cyclic sensing tests toward 1 ppm of NO2. However, the Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 showed stable response and recovery dynamics during the cyclic tests (Figure e), demonstrating that the response and recovery kinetics of the sensors to low levels of NO2 were improved by the addition of Pd and Pt NPs. Note that the response of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 slightly decreased as the cycling number increased, which was originated from the residual adsorbed NO2 that gives rise to the change of NO2 adsorption equilibrium.
The response values (ΔR/R0) of NO2 sensors reported in recent literatures are illustrated in Figure f, and the detailed sensing properties are summarized in Table S2 in the Supporting Information. Although the response of Pd@Cu3(HHTP)2 was slightly lower than those of other materials, a significant improvement in MOF‐based sensors has been achieved in this work by combining the metal NPs with C‐MOFs. The Pd@Cu3(HHTP)2 exhibited the highest NO2 response among MOF‐based sensors reported to date, including MOFs, C‐MOFs, and MOF derivatives. The response of Pd@Cu3(HHTP)2 increased by more than 12 times with improved reaction kinetics even at room temperature, compared with the response of reported MOFs‐based sensors. These outstanding results confirmed the superiority of metal NPs‐loaded C‐MOFs, as a highly active material with electrical conductivity, for applications in NO2 sensors operated at room temperature. Although the stability of M@Cu3(HHTP)2 against humidity was not demonstrated here due to easy adsorption of H2O on Cu3(HHTP)2 at high humidity, we suppose that the introduction of additional catalysts or humidity screening membranes, as previously demonstrated, can improve the sensing properties even in humid atmospheres.
In general, the adsorption of NO2 deprived sensing materials of electrons, thereby changing the resistance of the sensors. In the case of C‐MOFs, the chemiresistive sensing behaviors originate from the charge transfer between C‐MOFs and adsorbed gas molecules. Analytes are preferentially adsorbed on open metal sites that are coordinatively unsaturated metal centers in MOFs, changing their electrical resistance. Since NO2 is an electron acceptor at room temperature, the resistance of Cu3(HHTP)2, a p‐type sensing material, decreases when C‐MOFs were exposed to NO2. The highly porous Cu3(HHTP)2 with MΩ‐level resistance and high surface area not only facilitate gas diffusion into sensing layers through a number of pores but also transduce electrical signals from the surface reaction. In addition, the metal NPs embedded in the cavities of C‐MOFs can activate the surface reaction, enabling effective modulation of resistance upon exposure to target analytes. To investigate the catalytic effect of Pd and Pt NPs on the NO2 sensing behavior of Cu3(HHTP)2, we calculated NO2 response and recovery kinetics of the sensors based on i) the mass action law of NO2 adsorption reaction on sensing materials (Cu3(HHTP)2 or M@Cu3(HHTP)2) (Reactions 1 and 2) and ii) the assumption that response is proportional to the amounts of gas adsorbed. The adsorption rate constant (kads), desorption rate constant (kdes), and equilibrium constant (K = kads/kdes) were obtained from the first reaction of the sensors to 5 ppm of NO2 by the exponential fitting of the response versus time curves (R(t)) (Figure S8, Supporting Information) as shown in the equations below [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] [Image Omitted. See PDF] where Rmax is the maximum response of the sensors, Ca is the concentration of analytes (NO2), and R0 is the response of the sensors before refreshing with air. The calculated adsorption and desorption rate constants are described in Figure 5a. The Pt@Cu3(HHTP)2 exhibited higher NO2 adsorption and desorption kinetics (5.54 × 10−2 ppm−1 s−1 for adsorption and 7.30 × 10−5 s−1 for desorption) than pristine Cu3(HHTP)2 (2.43 × 10−2 ppm−1 s−1 for adsorption and 5.03 × 10−5 s−1 for desorption), demonstrating the activation of NO2 reaction by Pt NPs. On the other hand, Pd@Cu3(HHTP)2 showed a slightly improved NO2 reaction rate kinetics (2.51 × 10−2 ppm−1 s−1 for adsorption and 5.10 × 10−5 s−1 for desorption) compared with pristine Cu3(HHTP)2. To further understand the catalytic effect of Pd and Pt NPs, we investigated the activation energy (Ea) of the sensors for NO2 adsorption and desorption by using Arrhenius equation [Image Omitted. See PDF] where A0 is a pre‐exponential factor, Ea is the activation energy for the reaction, and R is the universal gas constant. Considering that Cu3(HHTP)2 is not stable over 100 °C in air (Figure f), we conducted additional sensing measurements using 5 ppm of NO2 at 50 and 75 °C to investigate the variation of reaction rate constants at different operating temperatures. Then, we calculated the NO2 adsorption and desorption rate constants of the samples by using Equations and (Figures S9 and S10, Supporting Information). As the operating temperature increased, the adsorption and desorption rate constants also increased due to the high thermal energy of analytes (Table S3, Supporting Information). The activation energy of the samples was calculated from the slope (−Ea/R) of the natural logarithm of the reaction rate constants (kads or kdes) versus the inverse of the operating temperature (1/T) (Figure b,c). The calculated activation energy for NO2 adsorption was 511.5 cal mol−1 for pristine Cu3(HHTP)2, 1351.5 cal mol−1 for Pd@Cu3(HHTP)2, and 83.4 cal mol−1 for Pt@Cu3(HHTP)2, whereas that for NO2 desorption was 3644.8 cal mol−1 for pristine Cu3(HHTP)2, 4015.7 cal mol−1 for Pd@Cu3(HHTP)2, and 3822.7 cal mol−1 for Pt@Cu3(HHTP)2 (Figure d). The activation energy of NO2 adsorption dramatically decreased with loading of Pt NPs, indicating the chemical catalytic effect of Pt NPs that facilitate NO2 adsorption on Cu3(HHTP)2 (Figure e). The NO2 spill‐over was also observed in the system of Pt NPs with supporting materials (NO2 adsorbents). On the other hand, the decoration of Pd NPs increased the activation energy of NO2 adsorption and desorption compared with pristine Cu3(HHTP)2, revealing that the resistance change of Pd@Cu3(HHTP)2 during NO2 adsorption is not solely related to the charge transfers between NO2 and Cu3(HHTP)2. It is reported that NO2 molecules are adsorbed on Pd NPs in the form of nitrito geometry. The NO2 adsorption on Pd NPs withdrew electrons from Pd, lowering the potential barriers of Schottky junctions between Pd NPs and Cu3(HHTP)2. Therefore, the resistance of the sensors was decreased by the electronic catalytic effect of Pd NPs upon exposure to NO2. This mechanism of the electronic sensitization is also worked for the Pt catalysts on Cu3(HHTP)2 because NO2 can be adsorbed on Pt NPs. Although the minor phases of oxidized metal NPs (PdO and PtO NPs) can differently affect the sensing properties, the calculation from the sensing results included the effect of both the major phases of metallic Pd and Pt and the minor phases of PdO and PtO in Cu3(HHTP)2‐based NO2 sensors. Therefore, it is apparent that the sensing properties of Cu3(HHTP)2 were dramatically enhanced by the catalytic effect of Pd and Pt NPs in Cu3(HHTP)2.
Enlarge this image.a) Calculated adsorption rate constants (kads) and desorption rate constants (kdes) of the sensors. Arrhenius plots for b) the adsorption rate constants and c) the desorption rate constants. d) Calculated activation energy for the adsorption and desorption of NO2 on the sensors. e) Schematic illustration of sensing mechanism of the sensors.
Conclusion
In conclusion, we confirmed the facile synthesis of Pd and Pt NPs‐loaded Cu3(HHTP)2 and their potential applications in NO2 sensors. The porous structure of C‐MOFs effectively limits the growth of Pd and Pt NPs, thereby creating ultra‐small (≈2 nm) and well‐dispersed catalyst NPs in their cavities. The chemical and electronic catalytic effect of Pd and Pt NPs dramatically improved the NO2 sensing properties of Cu3(HHTP)2‐based sensors in terms of response, selectivity, and enhanced response and recovery speed. These results demonstrated the high feasibility of Pd@Cu3(HHTP)2 and Pt@Cu3(HHTP)2 as gas‐sensing layers for room temperature NO2 detection. To the best of our knowledge, this is first report on metal NPs embedded in C‐MOFs as highly active and conductive porous sensing materials. Considering that the synthetic versatility of C‐MOFs has been established, various combinations of metal NPs and C‐MOFs can be easily explored, providing new materials platform for diverse applications.
Experimental Section
Materials: 2,3,6,7,10,11‐hexahydroxytriphenylene hydrate (HHTP, C18H12O6·H2O, 95.0%) was purchased from Tokyo Chemical Industry. Copper(II) acetate monohydrate (Cu(CO2CH3)·H2O, 99.9%), methanol (CH3OH, 99.9%), ethanol (C2H5OH 99.5%), and acetone (CH3COCH3, 99.5%) were purchased from Sigma‐Aldrich. Potassium tetrachloroplatinate(II) (K2PtCl4), potassium tetrachloropalladate(II) (K2PdCl4), and sodium borohydride (NaBH4, 96%) were purchased from Aldrich. All materials were used as received.
Synthesis of Cu3(HHTP)2: Cu3(HHTP)2 was prepared by solvothermal synthesis. 0.320 g of copper(II) acetate monohydrate was dissolved in 100 mL of methanol, whereas 0.260 g of HHTP was separately dissolved in 125 mL of methanol. Two solutions were mixed in a 500 mL capped bottle, and vigorously stirred by using a magnetic bar at room temperature for 10 min. Then, the mixture was heated in a box furnace at 65 °C for 24 h, resulting in dark solutions. The solution was naturally cooled to room temperature, and the upper transparent solvent was removed. The samples were washed with fresh methanol (three times) and acetone (two times) by centrifugation, and dried at 50 °C for 12 h in air and at 100 °C for 12 h in vacuum.
Synthesis of M@Cu3(HHTP)2 (M = Pd or Pt): M@Cu3(HHTP)2 was prepared by the infiltration of metal ions followed by a reduction process. 40 mg of Cu3(HHTP)2 was homogenously dispersed in 5 mL of DI water. 5 mg of potassium tetrachloroplatinate(II) or potassium tetrachloropalladate(II) was added into the suspension, and the mixture was stirred for 30 min. To reduce metal ions in the cavities of Cu3(HHTP)2, NaBH4 solution (1 mg mL−1) was added to the mixture. After 30 min, the samples were purified by centrifugation and washing with DI water (three times) and acetone (two times). Lastly, for activation, the samples were dried at 50 °C for 6 h in air and at 100 °C for 12 h in vacuum.
Material Characterization: SEM (XL30, Philips) and TEM (Tecnai G2 F30 S‐Twin, FEI) were used to investigate the morphology and microstructure of the samples. The crystal structure was investigated by XRD (SmartLab, Rigaku) analysis with Cu Kα radiation (λ = 1.5418 Å). The composition and chemical binding states of the samples were investigated by XPS (Sigma Probe, Thermo VG Scientific). N2 adsorption/desorption isotherms at 77 K (Tristar 3020, Micromeritics) were carried out to investigate the porous structure of the samples. The surface area was calculated from the N2 isotherms of the samples by using BET method.
Gas‐Sensing Measurement: 6 mg of Cu3(HHTP)2, Pd@Cu3(HHTP)2, and Pt@Cu3(HHTP)2 were independently dispersed in 300 mL of ethanol, and sonicated for 10 min. Then, the dispersion solution of each sensing material was drop‐coated on the alumina (Al2O3) substrate (dimension: 2.5 mm × 2.5 mm and thickness: 0.2 mm). The alumina substrate was patterned with two parallel Au electrodes (width: 25 µm, distance: 70 µm) on the top side and a Pt microheater on its back side. 5 µL of the suspension was drop‐coated on the sensing substrate three times. The sensing measurements were carried out in a sealed chamber at room temperature and dry condition (relative humidity = 5%). To control the operating temperature (50 and 75 °C), a voltage was applied to the Pt microheater using a DC power supply (E3647A, Agilent). The sensors were stabilized in air for 3 h before each sensing test. Then, the sensors were exposed to target gas, and subsequently recovered by air injection. To investigate the sensing properties of the sensors, nitrogen dioxide (NO2), ammonia (NH3), hydrogen (H2), toluene (C6H5CH3), ethanol (C2H5OH), and acetone (CH3COCH3) were injected into the chamber, in turn. The dynamic resistance of the sensors was obtained in real time by using a data acquisition system (34972, Agilent). The response was calculated as the ratio of the sensor resistance before and after exposure to analytes [(ΔR/R0) × 100], (%)], where ΔR is the resistance variation of the sensors when exposed to gas and R0 is the baseline resistance of the sensors when exposed to air. The error bars were added by multiple sensing tests of three independent sensors for each sensing material.
Acknowledgements
This work was supported by the Ministry of Trade, Industry & Energy (Korea) under Industrial Technology Innovation Program (no. 10070075). This work was also supported by Wearable Platform Materials Technology Center (WMC) funded by National Research Foundation of Korea (NRF) Grant of the Korean Government (MSIT) (no. 2016R1A5A1009926). This work was supported by Nano‐Convergence Foundation(www.nanotech2020.org) funded by the Ministry of Science and ICT(MSIT, Korea) & the Ministry of Trade, Industry and Energy(MOTIE, Korea) (no. 20000230).
Conflict of Interest
The authors declare no conflict of interest.
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1 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Yuseong‐gu, Daejeon, Republic of Korea; Advanced Nanosensor Research Center, KI Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Yuseong‐gu, Daejeon, Republic of Korea
2 Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Yuseong‐gu, Daejeon, Republic of Korea