Sequential Ligand Exchange of Coordination Polymers Hybridized

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Functional Nanostructured Materials (including low-D carbon)

Sequential ligand exchange of coordination polymers hybridized with insitu grown and aligned Au nanowires for rapid and selective gas sensing Pingping Li, Hongfeng Zhan, Suyang Tian, Jialiang Wang, Xiang Wang, Zhaohua Zhu, Jie Dai, Yihu Dai, Zhijuan Wang, Cong Zhang, Xiao Huang, and Wei Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02286 • Publication Date (Web): 19 Mar 2019 Downloaded from http://pubs.acs.org on March 24, 2019

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ACS Applied Materials & Interfaces

Sequential Ligand Exchange of Coordination Polymers Hybridized with In-Situ Grown and Aligned Au Nanowires for Rapid and Selective Gas sensing Pingping Li1, Hongfeng Zhan1, Suyang Tian2, Jialiang Wang1, Xiang Wang1, Zhaohua Zhu1, Jie Dai1, Yihu Dai2, Zhijuan Wang2, Cong Zhang1*, Xiao Huang1*, Wei Huang1,3* 1. Institute of Advanced Materials (IAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China 2. Institute of Advanced Synthesis (IAS), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, P.R. China 3. Shaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an 710072, P.R. China

*To whom correspondence should be addressed. Email: [email protected], [email protected], [email protected]. Keywords: coordination polymers, metal-organic frameworks, gold nanowires, aligned growth, ligand exchange, gas sensing

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Abstract Combining polymeric materials and conductive one-dimensional (1D) metal nanostructures is able to achieve enhanced chemical and electrical properties, but the control over their morphology and spatial arrangement remains a big challenge. Herein, by replacing benzenedicarboxylate (BDC) in ZnBDC nanoplates with oleylamine (OAM) in the presence of HAuCl4, Zn-OAM nanobelts with highly ordered laminar structure were obtained, on which ultrathin Au nanowires (Au NWs) were deposited and aligned along the long axes of the nanobelts. The resulting Zn-OAM/Au NW hybrid further underwent an OAM-to-2-methylimidazole (2-MIM) ligand exchange, resulting in the formation of porous nanobelts composed of ZIF-8 nanocrystals interwound with aligned Au NWs. Due to the synergistic effect between the polymeric and metallic structures, the Zn-OAM/Au NW hybrid nanobelts and ZIF-8/Au NW porous nanobelts demonstrated fast and selective gas sensing at ambient conditions, in sharp contrast to the non-responsive Au NWs or Zn-based polymers alone.

Keywords: coordination polymers, gold nanowires, aligned growth, ligand exchange, gas sensing, chemiresistive sensor

Introduction Coordination polymeric materials have attracted much attention due to their highly versatile chemical compositions and structures, as well as the thus endowed


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tunable functional properties.1,

2

In particular, the porous coordination polymers

(PCPs), also known as metal-organic frameworks (MOFs),3 are an emerging class of porous materials that find many applications such as in energy storage and conversion,4-7 molecular sieving and adsorption,8,

9

proton conduction,10,

11

and

sensors12-16. In addition, the porous structure and rich chemistry of MOFs make them promising candidates to form hybrids with other types of materials, such as polymer films,17-20 oxides,21-23 noble metals,24-26 carbon nanotubes27,

28

and graphene.29-31

Although numerous reports have been focused on the growth of nanoparticles on/in MOFs for enhancing their performance in catalysis,25,

32

hydrogen storage33-35 and

sensing,24, 36 only few works involve hybridization of one dimensional (1D) NWs with MOFs, such as the formation of NW@MOF core@shell structures.37, 38 Gold nanowires (Au NWs) have attracted much attention over the past years due to their appealing properties and potential applications in catalysis,39 plasmonics,40 and electronics.41, 42 Precisely controlled growth and alignment of Au NWs along a preferred direction are particularly important for study of their anisotropic properties and device fabrications. Tremendous progress has been made in the aligning, positioning, patterning or self-assembly of Au NWs via various techniques, such as the membrane-templated electrochemical deposition,43, 44 dip-pen nanolithography,45 dielectrophoresis,46 Langmuir−Blodgett method47 and microfluidic channel confined alignment.48 Unfortunately, many of these methods are complicated and time-consuming. Recently, two-dimensional (2D) materials such as graphene49 and MoS250 have been used as 2D substrate to induce the lattice-guided nucleation and



growth of Au NWs/nanorods with controlled orientation. However, long-range uni-directional surface alignment of the 1D structures has not been realized in these systems. Apart from controlling NW alignment, preventing NWs from aggregation that may cause significantly reduced surface area is also challenging. This is particularly true for ultrathin NWs which tend to bundle together due to the strong interaction between long-chain amines capped on their surfaces.51, 52 In this contribution, through post-synthetic ligand exchange of a kind of 2D PCP, i.e. Zn-1,4-benzenedicarboxylate (ZnBDC) nanoplates, the BDC ligand was exchanged with 1-amino-9-octadecene (or oleylamine, OAM) to form nanobelts of Zn-OAM complex with a well-defined laminar structure. Simultaneously, aligned ultrathin Au NWs were in-situ deposited on the nanobelts. Further exchanging OAM with 2-methylimidazole (2-MIM) led to the formation of porous networks composed of ZIF-8 nanoparticles interwound by Au NWs. These functional hybrid materials were further demonstrated as active layers of chemiresistive gas sensors that afforded selective and rapid sensing.

Results and Discussion Figure 1 schematically shows the successive ligand exchange process of Zn-based coordination polymers and in-situ growth of Au NWs. Firstly, Zn(BDC)(H2O)2 nanoplates were synthesized following a previously reported three-layer inter-diffusion method.17 As shown in the scanning electron microscopy (SEM) images and X-ray diffraction (XRD) pattern in Figure 2a and Figure S1 in the


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Supporting Information (SI), these nanoplates show lateral sizes of 2-7 µm and thicknesses in the range of 40-100 nm (Figure S1a-b in SI) with a monoclinic structure (C2/c, CCDC no. 171953) (Figure S1c in SI). Such nanoplates were then mixed with the growth solution for ultrathin Au NWs,53 containing HAuCl4, OAM and triisopropylsilane, and incubated at room temperature for 4-5 hours without disturbance. After the reaction, flexible nanobelts in the lengths of 8-15 µm and widths of 400-700 nm were observed (Figure 2b), whereas the original Zn(BDC)(H2O)2

nanoplates

disappeared.

A

close-up

transmission

electron

microscopy (TEM) image of a typical nanobelt reveals that separated Au NWs with an average diameter of about 1.8 nm were synthesized and mostly aligned along the long axis of the nanobelt (Figure 2c). A typical selected area electron diffraction (SAED) pattern of a nanobelt clearly shows two opposite diffraction arcs corresponding to an interlayer spacing of ~2.4 Å (inset of Figure 2c). This is consistent with the lattice spacing of Au (111) planes, which agrees with previously determined growth direction for ultrathin Au NWs.54 Large area energy dispersive X-ray spectroscopy (EDX) analysis of these Au NW/nanobelt hybrids indicate the existence of Zn and Au elements (Figure S2 in SI). XRD analysis on the hybrid structures further reveals a set of ordered peaks assignable to (00Ɩ) (Ɩ = 1, 2, 3, 4) reflections of a layered structure, in which the interlayer spacing is determined to be ~2.8 nm (Figure 2d). Considering the fact that most of the nanobelts were preferably lying flatly on the substrate during XRD analysis, the observed ordered XRD reflections imply the lamellar basal planes of the nanobelts. Fourier transform infrared



(FTIR) spectrum of the product confirms the presence of OAM molecules (Figure S3 in SI).55 Considering that OAM molecules in all-trans conformation have a length of about 2.1 nm,51,

56

the nanobelts could be constructed from stacking of the

OAM-Zn-OAM sandwiched layers, where adjacent OAM layers were interdigitated56 as schematically shown in Figure 1. To exclude the effect of AuCl4- ions, which can also form polymeric complex with OAM,51, 52 we exchanged BDC in Zn(BDC)(H2O)2 with OAM without presence of HAuCl4. This control experiment also resulted in the formation of Zn-OAM laminar structures based on TEM and XRD analyses (Figure 3). In order to understand the growth process of the Zn-OAM/Au NW hybrid nanobelts, we characterized the products obtained at different reaction intervals (Figure 4 and Figure S4 in SI). It is evident that the introduction of OAM to the Zn(BDC)(H2O)2 nanoplates solution caused dissolution of the nanoplates within 1 hour (Figure S4a-b in SI). In the following 1-5 hours, Zn-OAM nanobelts gradually formed from randomly shaped polymeric structures. Meanwhile, Au NWs were found to evolve from individual Au nanoparticles, to short Au NWs with big “heads”, and finally to continuous NWs (Figure 4a-c and Figure S4c-l in SI). In addition, the dark-field scanning TEM (STEM) image and the associated EDX elemental mapping of a typical Zn-OAM/Au NW nanobelt in Figure 4d clearly reveal the distribution of Au element which perfectly matches the location of the Au NWs. X-ray photoelectron spectroscopy (XPS) analysis further indicated a gradual increase in Au0 concentration and decrease in Au3+ and Au+ concentrations (Figure 4e, see the detailed analysis in


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Figure S5 and Table S1 in SI). This is consistent with previous reports that ultrathin Au NWs were typically synthesized from reduction of Au+ in the 1D Au-OAM complexes, where individual Au nanoparticles were firstly formed, then grew in length and attached with each other into longer NWs under the confinement of the 1D templates.51, 52-54 Since no Au NWs were observed in bulk solution except for those on the surfaces of the Zn-OAM nanobelts, it is likely that Au ions were attracted to Zn-OAM complex on the surfaces of the Zn-OAM nanobelts during the initial stage of their synthesis. It is worth mentioning that although Au NWs can be synthesized without the addition of Zn(BDC)(H2O)2 nanoplates, the obtained Au NWs tended to assemble into closely packed bundles (Figure S6 in SI) due to the strong interaction between the surface capped long-chain amine molecules.54 Therefore, the Zn-OAM nanobelts not only guided the aligned growth of the Au NWs, but also supported them, and prevented them from post-growth aggregation. As shown in Figure 1, the above-obtained Zn-OAM/Au NW hybrid nanobelts were further reacted with a methanolic solution of 2-MIM, in order to replace OAM in the Zn-OAM complex and form Zn-(2-MIM) (ZIF-8) at room temperature. Very interestingly, the original nanobelts became porous while their overall belt-like morphology retained (Figure 5a). A close-up TEM image of the porous network reveals interconnected ZIF-8 nanocrystals in the sizes of ~30 nm, with Au NWs interwound inbetween them (Figure 5b). The successful ligand exchange of OAM with 2-MIM to form ZIF-8 crystals was further confirmed based on their XRD pattern (Figure 5c) and FTIR spectrum (Figure S7 in SI).57



It is worth mentioning that the choice of ligands or coordination agents is crucial to enable the successful sequential ligand exchange of Zn-based coordination polymers, and thus the morphological evolution of their composites with Au NWs. It was also found in our control experiment that OAM could not replace 2-MIM in ZIF-8 crystals, and synthesis of Au NWs in presence of ZIF-8 crystals only resulted in a physical mixture of Au NW bundles and ZIF-8 crystals (Figure 5d). In order to investigate the bonding strength of the different ligand towards Zn, density functional theory (DFT) calculation was performed. According to the results, the G (Gibbs free energy at 298.15 K and 1 atm) of replacing two H2BDC in Zn(HBDC)2(H2BDC)2 with two OAM to produce an intermediate Zn(HBDC)2(HOAM)2 is -633.4 kcal/mol, which is more negative than that of forming Zn(HBDC)2(H2BDC)2 (-627.2 kcal/mol) (Table 1, Figure S8-9 and Table S2 in SI). Also, the G of four-coordinated Zn with 2-MIM is even more negative, i.e. -645.9 kcal/mol. This order of coordination strength agreed with our experimental observations. As a proof-of-concept demonstration, Au NWs hybridized with Zn-OAM nanobelts and ZIF-8 nanocrystals were both investigated for chemiresistive gas sensing. Typically, the active materials were deposited on interdigitated Au-electrodes which were then placed in a sealed chamber. The resistance of the electrode in air was recorded as R0. The response and recovery curves were measured under ambient condition by injecting a calculated amount of target gas in air to control its concentration (1-100 ppm). The chamber was pumped with air again to recover the resistance once the resistance of the electrode in the target gas reached equilibrium


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(Ra). The responses of the sensors ((Ra-R0)/R0 = R/R0) were recorded at different target gas concentrations. As shown in Figure 6a-b, the resistance of the sensor based on Zn-OAM/Au NW hybrid nanobelts decreased upon the exposure of NO2, and the response increased as the concentration of NO2 increased gradually. Importantly, a very fast response time of less than 5 s was achieved (Figure 6c), which is at least 2 orders faster than many previously reported room temperature NO2 sensors (Table S3 in SI). Based on the linear fitting of the response at a relative low concentration range of 1-10 ppm (inset of Figure 6b), a limit of detection (LOD) at a sensitivity of 0.27%, which is 3 times of the signal to noise ratio,58-60 was calculated to be 760 ppb. Compared with several other tested gases, including SO2, CO2, NH3, acetone and hexane, this sensor exhibited good selectivity towards NO2 (Figure 6c). For comparison, ZIF-8/Au NW porous hybrid was also fabricated into a NO2 sensor and showed a response time of 7 s towards 20 ppm NO2 and a calculated LOD of 190 ppb (Figure 6e-g). The fast response for both sensors based on ZIF-8/Au NWs and Zn-OAM/Au NWs can be ascribed to the presence of aligned ultrathin Au NWs that might form a conductive network in the sensing film. Note that Au NWs alone showed negligible response towards various gases (Figure S11 in SI), and therefore their main role was to provide the conductive paths. More importantly, both sensors showed opposite response (i.e. increase in resistance) towards SO2 (Figure 4d, h and Figure S12 in SI). It is well accepted that NO2 is an electron withdrawing agent, and in presence of H2O, it acted as a protonation agent.61, 62 Therefore, both sensors based on ZIF-8/Au NWs and Zn-OAM/Au NWs showed reduced resistance upon NO2



exposure. Their opposite response towards SO2 can be attributed to the fact that in presence of humid air, SO2-nH2O complex might form.63 This in turn could result in the adsorption of additional H2O molecules and cause a resistance increase of the channel material. Considering the fact that ZIF-8 crystals are more porous than Zn-OAM nanobelts and as expected the ZIF-8/Au NW porous nanobelts showed an almost 50 times larger specific surface area (1164.5 m2/g) than that of the Zn-OAM/Au NW nanobelts (23.4 m2/g) (Figure S13 in SI), more SO2-nH2O complex could be absorbed in ZIF-8/Au NWs to give a larger increase in resistance.

Conclusion In conclusion, through sequential ligand exchange of Zn-based coordination polymers, that is, from ZnBDC, to Zn-OAM and finally to ZIF-8, Au NWs were successfully in-situ grown, aligned and hybridized with the different Zn-based polymeric materials. Based on our DFT calculation results, such sequential ligand exchange processes are thermodynamically favored. It was also demonstrated that the resulting Zn-OAM/Au NW hybrid nanobelts and ZIF-8/Au NW porous nanobelts exhibited rapid and selective response towards NO2 and SO2 gases due to the coupling between the metal NWs and polymeric nanostructures. Our work provides a new way to the hybridization of metal nanostructures and coordination polymers with controlled morphology and spatial arrangement for enhanced functional properties.

MATERIALS AND METHODS Materials synthesis.


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Chemicals: All chemicals were commercially available at analytical grade and were used without further purification. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 99.0%), 1,4-benzenedicarboxylic acid (H2BDC, 98.0%), gold(III) chloride trihydrate (HAuCl4·3H2O,

≥49.0%

Au

basis),

hexane

(technical

grade,

>99.0%),

1-amino-9-octadecene (CH3(CH2)7CH=CH(CH2)8NH2, technical grade, 70%) and 2-methylimidazole (C4H6N2, 99.0%) were purchased from Sigma-Aldrich (Beijing, China). Triisopropylsilane (TIPS, >98.0%) was purchased from Tokyo Chemical Industry Co., Ltd. (Shanghai, China). N,N-dimethylformamide (DMF, 99.5%), acetonitrile (CH3CN, 99.0%), trichloromethane (CHCl3, 99.0%) and dichloromethane (CH2Cl2, 99.0%) were purchased from Shanghai Chemical Reagent Co., Ltd. (Shanghai, China). The deionized (DI) water was purified using a Milli-Q3 System (Millipore, France). Synthesis of Zn(BDC)(H2O)2: Zn(BDC)(H2O)2 nanoplates were synthesized through a diffusion-mediated method in a test tube by using Zn(CH3COO)2·2H2O and 1,4-BDC as the precursors17. Typically, 50 mg of 1,4-BDC was dissolved in a mixture of 2 mL of DMF and 1 mL of CH3CN and then poured into the bottom of the tube. Over this solution, a mixture of 1 mL of DMF and 1 mL of CH3CN was slowly and carefully added as the middle layer to prevent instant mixing of the two solutions. Finally, a solution containing 66 mg of Zn(CH3COO)2·2H2O dissolved in a mixture of 1 mL of DMF and 2 mL of CH3CN was also slowly added as the top layer. The three-layer solution in the test tube was left undisturbed at room temperature for about 24 hours, after which, precipitate at the bottom of the tube was collected, centrifuged



and washed with DMF and CHCl3 in turn. The final product was dispersed in CH2Cl2 for further investigation. Synthesis of Zn-OAM/Au NW hybrid nanobelts: In a typical synthesis, the aforementioned Zn(BDC)(H2O)2 solution was centrifuged and re-dispersed in hexane with a concentration of 30 mg/mL. To 2.5 mL of such Zn(BDC)(H2O)2 solution, 3 mg HAuCl4·3H2O mixed with 100 µL OAM was added, followed by the addition of 150 µL triisopropylsilane to accelerate the reduction reaction53. The mixed solution in a 5 mL glass bottle was left undisturbed at room temperature for 4-5 hours. The resulting solution was washed with ethanol and hexane for several times. The final product was dispersed in hexane for further investigation. Synthesis of ZIF-8/Au NW hybrid via ligand exchange: In a typical process, 2.5 mL of the obtained Zn-OAM/Au NW hybrid nanobelts in hexane (5 mg/mL) was centrifuged and re-dispersed in 1 mL methanol. Then, 5 mL of a methanol solution of 2-MIM (25 mmol/L).64 The mixture was left to react at room temperature for 24 hours without stirring. The product was collected by centrifugation, washed several times with methanol, and re-dispersed in the methanol for characterizations. Preparation of sensors and gas sensing test: Chemiresistive sensors were prepared based on the Zn-OAM/Au NW hybrid nanobelts and ZIF-8/ Au NW porous nanobelts, respectively. Typically, a drop of 100 µL solution was drop-casted onto the Au-electrode (Au IDE, with 0.1 mm spacing over a 2×1 cm area, Changchun Mega Borui Technology Co., Ltd) and the electrodes were dried at room temperature. The gas sensing test was carried out by placing the electrodes in a sealed chamber and


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monitoring the variation of the resistance as a static process.65, 66 An intelligent data acquisition system (34972A, Agilent) with a 20 channel multiplexer (34901A, Agilent) was adopted to record the changes of the device resistance. The gas response was defined as R/R0 (R = Ra-R0), where Ra and R0 are the resistance of the sample measured in the target gas and in air, respectively.

Characterizations Various samples were characterized using a scanning electron microscope (SEM, JEOL JSM-7800F, Japan) coupled with energy dispersive X-ray spectroscopy (EDX), a transmission electron microscope (TEM, JEOL 2100Plus, Japan) and a high resolution transmission electron microscope (HRTEM, JEOL 2100F, Japan). X-ray diffraction (XRD, SmartLab Rigaku) was performed with Cu Kα radiation (λ = 1.54 Å) as the X-ray source. Fourier transform infrared spectra (FTIR) were recorded on a Shimadzu IRprestige-21 FTIR spectrophotometer. X-ray photoelectron spectroscopy (XPS, PHI 5000 VersaProbe, Japan) measurements were conducted and the binding energies were corrected for specimen charging effects using the C 1 s level at 284.8 eV as the reference.

Molecular simulation. Several possible structures of four-coordinated zinc complexes were investigated by using M06-2X/6-31G(d, p)/SDD method with G09 package and some of their formation Gibbs free energies are shown in Table 1.59 In addition, different conformers of Zn(OAM)2(HOAM)2 have been optimized. The geometries are



collected and shown in Figure S9 in SI, and their energy differences are summarized in Table S2 in SI.

Acknowledgement This research was supported by the National Natural Science Foundation of China (No. 51322202) and the Young 1000 Talents Global Recruitment Program of China.

Corresponding Author ∗E-mail:

[email protected]; [email protected];

[email protected]. Conflict of Interest: The authors declare no competing financial interest.

Supporting Information The Supporting Information is available free of charge via the Internet at XXX. Additional SEM and TEM images; XPS, XRD, FTIR, BET, gas sensing data and the detailed DFT calculation (PDF).

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Figure 1. Schematic illustration of the sequential ligand exchange process of Zn-based polymers for in-situ preparation of Zn-OAM/Au NW hybrid nanobelts and ZIF-8/Au NW porous nanobelts.


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2 1/nm

Figure 2. (a) SEM image of Zn(BDC)(H2O)2 nanoplates synthesized via the three-layer interdiffusion method. (b) low-magnification SEM image and (c) zoom-in TEM image of Au NWs grown on Zn-OAM nanobelts. Inset of (c): SAED of a typical Zn-OAM/Au NW hybrid nanobelt, revealing two opposite diffraction arcs for Au(111) planes. (d) XRD pattern of as-synthesized Zn-OAM/Au NW hybrid nanobelts. Inset: the magnified region marked in the dotted rectangle, showing a peak assignable to the (111) planes of face-centered cubic (FCC) Au (JCPDS: 04-0784).



Figure 3. (a) TEM image and (b) XRD pattern of Zn-OAM synthesized by reacting Zn(BDC)(H2O)2 nanoplates with OAM without presence of HAuCl4·3H2O. Inset: proposed laminar structure model of Zn-OAM.


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Figure 4. (a-c) TEM images of products obtained at 1.5 h, 2 h and 5 h, respectively. (d) STEM image and EDX mapping of a typical Zn-OAM/Au NW nanobelt hybrid. (e) XPS Au 4f spectra of Zn-OAM/Au NW hybrid collected at different reaction intervals (upper for 5.0 h and bottom for 1.5 h).



Figure 5. (a) SEM image and (b) zoom-in TEM image of ZIF-8/Au NW porous nanobelts. (c) XRD pattern of as-synthesized ZIF-8/Au NW porous nanobelts compared with the simulated standard pattern of ZIF-8. Inset: the magnified region marked in the dotted rectangle, showing a peak assignable to the (111) planes of FCC Au. (d) SEM image of Au NWs synthesized in presence of ZIF-8 crystals.


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Table 1. Calculated Gibbs free energies for the possible reactions between Zn2+ and different ligands involved in the study. Reaction

Gibbs free energy (G)

Zn2+ + 2H2BDC + 2HBDC- → Zn(HBDC)2(H2BDC)2

- 627.2 kcal/mol

Zn2+ + 2HOAM + 2HBDC- → Zn(HBDC)2(HOAM)2

- 633.4 kcal/mol

Zn2+ + 2HMIM + 2MIM- → Zn(MIM)2(HMIM)2

- 645.9 kcal/mol



e

Air 0

10

0.3

20 30 40 Time (s)

1 ppm 5 ppm 10 ppm 20 ppm 50

-15

-2 -3 y = - 0.286x - 0.503 0

-25

f

NO2

-1

-20

60

R/R0 (%)

-4 -6

0

2

4

6

10

-1.2

Air 50

100 150 200 250 300 Time (s)

-2 -3

R/R0 (%)

R/R0 (%)

1 ppm 5 ppm 10 ppm 20 ppm

20

0.0

40 60 80 Time (s)

Recovery to 10%

-0.2 -0.3 -0.4 y = -0.046x + 0.0272 -0.5 0 2 4 6 8

-0.6

20 40 60 80 100 Concentration (ppm)

-6

NO2 Testing gas (at 50 ppm)

1.5

SO2

1.0 0.5

NH3 CO2

0.0

0

200

400 600 Time (s)

C6H14

(CH3)2CO

-1.0

Response to 90%

10

-1.0

-4

-0.5

-0.8

Concentration (ppm)

0

-0.4

C6H14

(CH3)2CO

-2

-8

h

SO2 NH3 CO2

0

-10

100

-0.2 0.0 -0.1

2

Response to 90% 0

g

0

R/R0 (%)

-0.9

-3

0 20 40 60 80 100 Concentration (ppm)

-1

-0.6

-2

-5

Concentration (ppm)

0.0 -0.3

-1

-4 8

4

Recovery to 10%

0

-10

-2

d

1

-5 R/R0 (%)

R/R0 (%)

0

c

0

R/R0 (%)

NO2

R/R0 (%)

b

2

R/R0 (%)

a

R/R0 (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

-1.5 800

NO2 Testing gas (at 50 ppm)

Figure 6. (a) Response–recovery curves of a typical chemiresistive sensor fabricated based on Zn-OAM/Au NW hybrid nanobelts in response to NO2 gas with increasing concentrations from 1 to 20 ppm. (b) Normalized resistance changes of the Zn-OAM/Au NW hybrid sensor upon exposure to 1-100 ppm NO2. Inset: linear fitting of the response versus NO2 concentration at a low concentration range. (c) Response-recovery curve of the Zn-OAM/Au NW sensor to 20 ppm NO2, from which a response time of 2 s was determined. (d) Responses of the Zn-OAM/Au NW sensor towards NO2, SO2, NH3, acetone and hexane. (e) Response–recovery curves of a typical chemiresistive sensor fabricated based on ZIF-8/Au NW porous nanobelts in response to NO2 gas with increasing concentrations from 1 to 20 ppm. (f) Normalized resistance changes of the ZIF-8/Au NW sensor upon exposure to 1-100 ppm NO2. Inset: linear fitting of the response versus NO2 concentration at a low concentration range. (g) Response-recovery curve of the ZIF-8/Au NW sensor to 20 ppm NO2, from which a response time of 7 s was determined. (h) Responses of the ZIF-8/Au NW sensor towards NO2, SO2, NH3, acetone and hexane.


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TOC


Sequential Ligand Exchange of Coordination Polymers Hybridized

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