Next Article in Journal
Application of Starch Based Coatings as a Sustainable Solution to Preserve and Decipher the Charred Documents
Previous Article in Journal
Preparation and Properties of Textured Ni–W Coatings Electrodeposited on the Steel Surface from a Pyrophosphate Bath
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Coatings
Volume 13
Issue 9
10.3390/coatings13091520
coatings-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

Thickness and Humidity on Proton Conductivity in MOF-508 Thin Film by Twin-Zinc-Source Method

by
Kun Zhang
1,
Chunxia Wang
2,
Feng Yang
2,
Jing Li
2,
Shuguang Yan
3 and
Yue Qi
2,*
1
Sichuan Puhua Chinese Traditional Medicine Technology Co., Ltd., Chengdu 611135, China
2
College of Chemistry, Sichuan University, Chengdu 610064, China
3
Analytical &Testing Center, Sichuan University, Chengdu 610064, China
*
Author to whom correspondence should be addressed.
Coatings 2023, 13(9), 1520; https://doi.org/10.3390/coatings13091520
Submission received: 30 July 2023 / Revised: 20 August 2023 / Accepted: 24 August 2023 / Published: 30 August 2023
Browse Figures
Review Reports Versions Notes

Abstract

:
To achieve structurally stable and high proton conductive materials, preferably under ambient humidity and pressure, the well-controlled thickness and conductivity of the MOF thin films represent an effective approach. Electrodes are the most important part of fuel cells; proton conducting materials are often used for electrodes, but today high proton conducting materials are expensive and use harsh conditions. Therefore, the goal of researchers is the pursuit of stable structure high proton conductive materials. We prepared well controlled thickness and conductive MOF-508a thin films on a Zn substrate by the “twin zinc source” method, which is very rare in conventional proton conductive materials. The results show that when the thickness of the MOF-508a/Zn thin film was at its minimum (16 µm), the resistivity and proton conductivity reached 2.5 × 103 Ω cm and 4 × 10−4 S cm−1, respectively. The MOF-508b/Zn thin film can absorb water molecules in a high humidity atmosphere and the conductivity decreases significantly with increasing humidity. When the film was put into the atmosphere with a relative humidity of 85%, the resistivity reached 200 Ω cm significantly. This work provides a simple, low cost, and environmentally friendly strategy for fabricating high proton conducting MOF films by exploring the “twin-zinc-source” method, which is critically important for PEMFC. It is believed that higher conductivity MOF films can be obtained with further modifications, indicating the potential of such films as humidity detectors.

1. Introduction

Proton conducting materials are frequently used in the construction of electrodes [1]. While electrodes are the most important part of a fuel cell, conventional high proton conductive materials are expensive and subject to harsh conditions for their use [2]. Therefore, the goal of researchers is the establishment of materials with stable structures and high proton conducting properties, and to find new proton-conducting materials to replace existing ones [3].
In recent decades, coordination polymers (CPs) or metal–organic frameworks (MOFs) have received extensive attention in research because they are tunable solids modulated by a limitless number of combinations of metal ions and organic ligands and have unique properties such as high porosity with a large surface area [4,5]. In addition, the proton conductivity of MOFs has received increasing attention from researchers [6,7] due to the presence of metal centers, organic linkers, and pore spaces. There have been several achievements in MOF materials with high proton conductivity by systematic design modifications, such as the introduction of NH4+, H3O+, and HSO4− into the pores of the framework [8]. However, there have been few studies on the physical properties of MOF thin films [9,10,11], such as adjusting the film morphology and the fabrication of dense MOF films [12,13,14]. Therefore, the simple preparation of stable and high proton conductive MOF thin films remains a great challenge [15].
At present, two fundamental issues need to be addressed before MOF films can be used as electrolytes in fuel cells, namely how to easily prepare stable films and how to obtain high proton conductivity. The dense thin films are very important to the proton exchange mechanism in fuel cells [16]; the means of preparing dense and electrical conductivity-controlled thin films are studied in this paper [17,18]. MOF-508a (Zn(BDC)(4,4′-bipy)0.5DMF(H2O)0.5 with a 3D structure containing a 1D channel (valid pore diameter of 4 × 4 Å) was selected [19]; from this we prepared dense, micron-sized thin films on a Zn substrate via the “twin-zinc-source” method [20,21]. Solid metal foils were chosen for the twin metal source method to force the reaction to occur close to the substrate surface, allowing dense MOF films to grow compactly on the substrate. To ensure close contact between MOFs and these substrates, direct growth of MOFs on the substrates as crystalline thin films seems to be the way to go for achieving good proton conductivity and mechanical stability. Additionally, the thickness of the film also changed with the reaction time, which provides another effective way to control thickness. This convenient method of introducing the twin-zinc-source into MOFs provides useful inspiration for us to find out the practical application of sch MOFs in fuel cells [22].

2. Materials and Methods

2.1. Materials

All the materials were used as received unless specified otherwise. Terephthalic acid (H2BDC, terephthalic acid, 95%, Alfa Aesar, Haverhill, MA, USA), anhydrous ethanol (EtOH, AR), and N,N-dimethylformamid (DMF, 99.8%, AR) were commercially available and used without further purification; the water was twin distilled and deionized.

2.2. Preparation of Zinc Substrate

First, the zinc substrate was cut into a rectangle of 1.0 × 1.2 cm and ultrasonically washed with acetone for 10 min and then deionized water 3 times for 10 min each time. Finally, it was dried in a vacuum oven at room temperature for 12 h, then put aside until use.

2.3. Preparation of MOF-508a Thin Film by “Twin-Zinc-Source” Method

Zn(NO3)2·6H2O powder 2.365 mmol, H2BDC powder 2.365 mmol, and 4′-bipy powder 1.18 mmol was dissolved in DMF and ethanol (1:1) mixed solution of 200 mL, put the solution into the three flasks, put vertically the clean zinc into three flasks, reflux condensation under 90 °C oil bath reaction 24 h, taking out after cooling and cleaning, respectively, with DMF and n-hexane each 3 times, drying in the air, removing the guest method in a vacuum drying oven at 120 °C, 24 h.

2.4. Preparation of MOF-508a Thin Film by the Solvent Thermal Method

Zn(NO3)2·6H2O powder 0.118 mmol, H2BDC powder 0.118 mmol, and 4,4′-bipy powder 0.059 mmol was dissolved in DMF and ethanol (1:1) mixed solution of 10 mL, then placed the solution into a Teflon container and vertically inserted the clean zinc into the container for 24 h at 90 °C, taking out after cooling and cleaning, respectively, with DMF and n-hexane each 3 times, drying in the air, removing the guest method in a vacuum drying oven at 120 °C, 24 h.

2.5. Characterization

X-ray diffraction (XRD) data were taken with a Bruker D8 ADVANCE powder X-ray diffractometer with Cu Kα. Scanning electron micrographs (SEM) were constructed with data taken using a JSM-6330F microscope (JEOL, Akishima shi, Japan). Thermogravimetric analysis (TGA) was performed on a Netzsch TG-209 F1 Thermogravimetric Analyzer (Netzsch, Ulm, Germany) at a heating rate of 10 °C min−1 under a nitrogen gas flow of 20 mL·min−1. The relative humidity was controlled by sealing the MOF-508a microfilm in a quartz cell that contained salt solutions at various saturations at room temperature (25 °C kept in the cell for 1 to 3 days, Table S1) [23,24]. N2 adsorption isotherms were recorded on a BEL-max gas adsorption analyzer.

2.6. The Method of the Conductivity

The conductivity of the sample was derived from the impedance value using the following equation.
σ = L Z · A
where σ is the conductivity (S cm−1), L is the measured thickness of the sample (cm), A is the electrode area (cm−2), and Z is the impedance (Ω). The electrical properties were measured using a solarton 1260 Impedance/Gain-Phase analyzer. The AC impedance was measured with a solarton 1260 Impedance/Gain-Phase analyzer in the frequency range of 0.1–10 MHz and an input voltage amplitude of 300 mV. The conductivity of the microfilm was calculated via the method described in Ref [25]. Two platinum wires were attached to the same side of the MOF-508a thin film with silver paste (purchased from Alfa Aesar). The distance between the two electrodes was 6.5 mm (L). The thickness of the films were about 62 µm, 37 µm, 22 µm, 18 µm, and 16 µm, respectively. The width of the film was 1 cm. The area of the film (A) was the film’s thickness times its width.

3. Result and Discussion

3.1. PXRD Pattern of the MOF-508a Thin Film

Figure 1 depicts a MOF-508a thin film prepared via the atmospheric “twin-zinc-source” method using powder, and shows this material corresponding with all characteristic peaks, indicating that the preparation of the MOF-508a thin film via this method was successful. However, the characteristic peaks of MOF-508a prepared via the solvent thermal method using XRD (Figure S1) are not shown, indicating that the preparation of zinc foil using this method was not successful. Further, this shows that reaction times, pressure, and temperature during the MOF-508a thin film preparation were important, and so the most simple and feasible method is the “twin-zinc-source” method.

3.2. The Effect of Zn2+ Concentration on the Thickness of MOF-508a Film

The conductivity of the MOF-508a film of a given thickness was studied by changing the Zn2+ concentration and reaction time. First, the concentration of Zn2+ was changed from 0 to 12 mmol L−1; the PXRD pattern of this sample is shown in Figure 2. Compared with the PXRD pattern yielded by the single-crystal MOF-508a, when the concentration of Zn2+ decreased to 0.75 mmol L−1, the target product was not produced on the Zn substrate. However, when the concentration of Zn2+ was 0, the target product again almost did not form. This indicates that this reactant must be added to Zn2+ and that with the lowest concentration of Zn2+ (1 mmol L−1), the corresponding target product could be obtained on the Zn substrate. Therefore, this method is called the twin-zinc-source method.

3.3. The SEM Images of the MOF-508a/Zn Thin Films Obtained under Different Zn2+ Concentrations

The morphology and thickness properties of the MOF-508a/Zn thin films obtained using different concentrations of Zn2+ were assessed using a scanning electron microscope (SEM). In Figure 3, the SEM images show the surface (a–f) and cross-sectional microstructure (g–l) of the MOF-508a/Zn thin films in which the concentrations of Zn2+ were about 12 mmol L−1, 6 mmol L−1, 3 mmol L−1, 1.5 mmol L−1, 1 mmol L−1, and 0 mmol L−1. The film surface was homogeneous and free of cracks. Figure 3a–f shows that a high degree of intergrowth within the MOF-508a film was obtained. The size and density of the MOF-508a crystals growing on the Zn substrate were not decreased with the reduction in the concentration of Zn2+, that is, the size of the crystals remained consistent. Figure 3g–k illustrates that the film layer is well bonded with the support, and the thickness of the film was controllable. The thickness values of the thin films were about 62 µm, 37 µm, 22 µm, 18 µm, 16 µm, and 0 µm, which, obtained by the section—the thickness of the MOF-508a thin film—decreased with a reduction in the concentration of Zn2+. A photo of the MOF-508a/Zn thin film is shown in Figure S3.

3.4. The Growth Mechanism of MOF-508 Crystals on the Zinc Foil

The growth mechanism of MOF-508 crystals on the zinc foil is more complicated [26]. The only clear fact is that the substrate is also the zinc source that led to the crystals growing bigger. To the best of our knowledge, the closest work on growth mechanism is by Yang et al. [27] on the growth of a semiconductor with metal substrates; they reported a tip-growth mechanism in which the growth is on the NWs tip and the feeding stocks came from the roots. The crystals with a rough surface produced by an instable metal ion source at the liquid-solid interface was a possible feature [28]. However, in our case, when the concentrations of Zn2+ were zero, only a few MOF-508 crystals formed on the Zn surface, but not enough to provide a continuous film (Figure 3f). This suggests that the zinc substrate provides a zinc source by self-sacrifice and the zinc substrate was one of the reactants, but the nucleation density at the support surface was not enough to provide a continuous film. When the Zn2+ is added, the dense controllable film is formed increasingly, meaning that the reaction has two metal sources, the first from the substrates and the second from additional Zn2+. The “twin-zinc-source” method was easier for an intergrown, dense, and controllable thin film.

3.5. The Linear Relationship of the Zn2+ Concentration and the Thickness of MOF-508a/Zn Thin Film

The data on the different concentrations of Zn2+ indicate a good linear relationship between this property and the thickness of the MOF-508a/Zn thin films, as shown in Figure 4. The linear equation obtained is as follows: thickness = 11.10 + 4.20CZn2+ (R = 0.99909, R2 = 0.99756). Therefore, the thickness of the MOF-508a thin film could be controlled in this reaction system simply by adjusting the concentration of Zn2+ as needed. In order to verify the results presented in this study, the concentration of Zn2+ was selected as 2 mmol L−1. The SEM image displays that the thickness of the thin film was about 20 µm (Figure S4), which verified the integrity of the mathematical modeling.

3.6. The Effects of Different Reaction Times on the MOF-508/Zn Thin Film

In order to explore the effects of different reaction times, we maintained the concentration of Zn2+ at 12 mmol L−1 and reduced the reaction time to 12 h and 6 h. Figure 5 shows the PXRD patterns of the MOF-508a films synthesized over different reaction times. Compared with the simulated PXRD pattern derived from the single-crystal MOF-508a data, it is possible to prepare an MOF-508a/Zn thin film by shortening the reaction time to 6 h. However, with the reduced time, the intensity of the diffraction peak is also reduced, which may indicate that the thickness of the MOF-508a/Zn thin film decreases.
Similarly, Figure 6 shows the SEM images of the surfaces (a,c) and profiles (b,d) of the MOF-508a/Zn thin film samples produced via reaction times of 6 h and 12 h. As the figure shows, the thickness of the MOF-508a/Zn thin film changed when the reaction time was reduced. With a shorter reaction time, the thickness of the thin film decreased from 36 µm (12 h) to 21 µm (6 h), which is consistent with the predictions given by the PXRD results in Figure 5. The Zn2+ may not have only come from the substrate, but could also have come from the solution in the reaction system. Thus, this provides another effective way to control the thickness of an MOF-508/Zn thin film.

3.7. The Effect of the Thickness on the Resistivity of the MOF-508a/Zn Thin Film

Alternating current (ac) impedance measurements were carried out using a two-probe method with Pt-pressed electrodes. The data are shown in Figure 7.
Different concentrations of Zn2+ affected the thickness of the films and the thickness of the film also affected its resistivity. The proton conductivity decreases in general under higher film thickness conditions. The thickness of the film was measured for the samples kept under different concentrations of Zn2+, i.e., 12 mmol L−1, 6 mmol L−1, 3 mmol L−1, 1.5 mmol L−1, and 1 mmol L−1 for 24 h. Table 1 shows the resistivity of the MOF-508a/Zn thin film. With the decreased concentration of Zn2+, the resistivity value decreased obviously from the maximum value of 62 µm (12 mmol L−1) to 2.5 × 103 Ω cm at 16 µm (1 mmol L−1) for the microfilm, and the proton conductivity increased to 4 × 10−4 S cm−1. This result proved that it is effective to increase the proton conductivity of MOF-508a/Zn by reducing the thickness of the microfilm.
To the best our knowledge, the highest σ value of the carboxylate-based (Zn)-related material was 1D MOF, in which all of the water molecules were aligned by H-bonds and hence yielded a conductivity of 1.0 × 10−5 S cm−1 (25 °C and 100% RH), as reported by D. Sara vanabharathi [29]. This work reported a product showing a high proton conductivity of ~10−4 S cm−1, which is one order of magnitude higher than similar work. This study opens a new window for the study of the solid state proton conduction of MOF film.

3.8. Arrhenius-Type Plot of MOF-508a Sample at Various Temperatures

The temperature dependence of ion conductivity performed from 60 °C to 150 °C under ambient conditions is shown in Figure 6. Activation energies were 1.04 eV and 0.77 eV for 60–90 °C and 100–150 °C, respectively, obtained from the equation below.
σ T = σ 0 exp ( − E a k B T )
where σ is the ionic conductivity, σ0 is the pre-exponential factor, kB is the Boltzmann constant, and T is the temperature.
The temperature dependence of ion conductivity is performed from 60 °C to 150 °C under ambient conditions, shown in Figure 8. The σ value increased gradually with the increase in temperature owing to the increase in thermal movement of the molecules. [30] The abrupt decrease in conductivity at 100 °C was due to the evaporation of guest water. After that, the contribution from the less volatile high boiling point guest molecules (DMF) in the cavity provide multiple proton delocalization pathways for efficient proton transport [31]. Therefore, the guest DMF molecules in this pristine MOF-508a also play a vital role in proton conduction in addition to coordinated water [32]. The synthesized MOFs have good proton conductivity over a very wide range of 60 °C to 150 °C. The activation energies were 1.04 eV and 0.77 eV for 60–90 °C and 100–150 °C, respectively, indicating that MOF-508a thin films belong to semiconductor conductive materials [33,34].

3.9. The Effect of H2O Molecules on the Resistivity of the MOF-508b/Zn Thin Film

MOF-508b/Zn thin film was fabricated from MOF-508a/Zn thin film after removing the guest water molecules (Figure S2). Complex impedance plots (CIPs) are often used to investigate the conduction mechanism of the impedance humidity sensors at different RHs [35]. Figure 9 shows the complex impedance plots of the MOF-508b/Zn film humidity sensor measured at the different RHs, in which the Z′ and Z″ are the real part and imaginary part of the complex impedance, respectively. At 63% and 75% RHs, the MOF-508b/Zn film has no proton conductivity in the absence of a continuous hydrogen bonding network. At 85% RH the CIP presents as a semicircle, and then the arc length of the semicircle becomes shorter with the increase in RH [36]. In this case (RH > 85%), there are a lot of water molecules adsorbed on the surface of the MOF-508b/Zn film; the water molecules will form hydrogen bonds leading to extra proton (H+) hopping. The impedance of the sensor is mainly controlled by ion conduction (Grotthuss chain reaction: H2O + H3O+ ↔ H3O+ + H2O) [37]. In general, the resistivity reached 200 Ω cm(RH > 85%), which was beyond the capacity of the coordination polymer, as shown in previous reports [38]. This phenomenon indicates that the conductivity of MOF-508b/Zn film decreases significantly with increasing humidity. From this, we were able to judge the content of the adsorbed guest water molecules from the electrical resistance of the film, indicating the potential of such films as humidity detectors [39].
However, according to the PXRD data (Figure S6), when the relative humidity was 85%, the molecular structure began to collapse (Figure S7). This phenomenon is not unique to MOFs [40]. Therefore, this process was extremely useful to obtaining a stable proton conductive MOF thin film. The N2 sorption isotherms of the MOF-508b are shown in Figure S5.

3.10. The Mechanism of Proton Conductivity

The mechanism for the proton conductivity of MOF-508 crystals on the zinc foil is more complicated. As discussed at the introduction part of the manuscript, the proton conduction of MOF is closely related to its structure; a hydrogen-bonding network was formed in the 1D channels with interaction with guest molecules, thus forming an efficient conducting pathway. Consequently, Zn(BDC)(4,4′-bipy)0.5DMF(H2O)0.5 (MOF-508a) contained a 1D channel, with a pore aperture size of 4 × 4 Å, which were filled with water and DMF molecules to form hydrogen-bonding networks, while the MOF-508b lacks the 1D channel due to the removal of the guest molecules of MOF-508a (Scheme 1). Presumably, this continuous hydrogen-bonding network in the as-prepared intergrowth crystals film significantly increased the proton conductivity of the MOF-508a, while an MOF-508b sample with no proton conductivity could be yielded by not forming the continuous hydrogen-bonding network. The second Zinc foil was employed not only to provide metal sources for the preparation of the film, but also to enable the root of the crystals to attach close enough to form dense films.

4. Conclusions

In this paper, we have mainly addressed the preparation of MOF-508a/Zn thin films via the atmospheric “twin-zinc-source” method. The controllability of this method was shown to be greater as the thickness of the MOF-508a/Zn film was controlled by adjusting the reaction concentration and time. The approach was facile and environmentally friendly. In contrast to conventional methods which typically require multiple steps in situ substrate growth can be accomplished with ambient pressure capable of producing dense films. Zinc foils were employed not only to provide a metal source for the preparation of the film, but also to enable the attachment of the root of the crystal close enough to form a dense film.
We also found that the resistivity of the MOF-508a/Zn film can be changed by varying the thickness of the film, and that a film with no proton conductivity can be yielded by removing the guest molecules. This finding is relatively rare in conventional proton conducting materials and may be related to the presence of a 1D channel in MOF-508a. While the thickness of the MOF-508a/Zn thin film was at its minimum (16 µm), the resistivity and proton conductivity reached 2.5 × 103 Ω cm and 4 × 10−4 S cm−1, respectively, which is one order of magnitude higher than similar work.
The MOF-508b/Zn film can absorb water molecules at a high atmospheric humidity and the conductivity decreases significantly with increasing humidity. When the film was put into the atmosphere with a relative humidity of 85%, the resistivity reached 200 Ω cm. But according to the PXRD data, when the relative humidity is 85%, the molecular structure starts to collapse. Such a simple approach affords the development of films of MOFs with high proton conduction properties under stable conditions, which is critically important for PEMFC. We believe that this approach could be extended to other MOFs or modified to obtain films with higher proton conductivity. From this, we were able to judge the content of the adsorbed guest water molecules from the resistance of the film, indicating the potential of this film as a humidity detector.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/coatings13091520/s1, Figure S1. PXRD pattern of (a) the standard powder, (b) the thin film incorporated on the Zn foil via the solvent thermal method. Figure S2. PXRD pattern of (a) the standard powder of MOF-508b, (b) the MOF-508b/Zn thin film with the additional molecules removed at 120 °C. Figure S3. The photo of the MOF-508a/Zn thin film with different concentrations of Zn2+ and different reaction times. Figure S4. The thickness of the thin film when the concentration of Zn2+ was 2 mmol L−1. Figure S5. N2 sorption isotherms of the MOF-508b at 77 K. Figure S6. The PXRD pattern of MOF-508b/Zn thin film kept at different relative humidity values (63%, 85%, 75%, and 98%) for 1–3 days (* is Zn). Figure S7. Thermogravimetric analysis curve of the as-made MOF-508b/Zn thin film kept at different relative humidity values, i.e., 63%, 75%, 85%, and 98% RH, and at room temperature for 1 to 3 days. The ramp rate was 10 °C min−1. Figure S8. (A) Representative diagrams for the conductivity recovery studies, with the conductivity of MOF-508b/Zn thin film after five runs of recycling experiments. (B) PXRD of the MOF-508b/Zn thin film after each step of relative humidity atmosphere of 63%. Table S1. The specific saturated salt solutions and corresponding relative humidity (RH)% values. Table S2. The proton conductivities of carboxylate-based MOF. Refs. [23,41,42] are cited in Supplementary Materials.

Author Contributions

K.Z.: Conceptualization, formal analysis, investigation, methodology, writing—original draft, C.W.: Formal analysis, investigation, F.Y.: Formal analysis, investigation, J.L.: Data curation, S.Y.: Manuscript modification, writing—review and editing, supervision, Y.Q.: Resources, project administration, funding acquisition, supervision. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the Sichuan Science and Technology Program (2021YFG0229).

Data Availability Statement

Y.Q. designed the work. K.Z. performed the experiment and data analysis.

Conflicts of Interest

The authors declare no conflict of interest.

Sample Availability

Samples of the compounds Kun Zhang are available from the authors.

References

  1. Ye, Y.; Gong, L.; Xiang, S.; Zhang, Z.; Chen, B. Metal–Organic Frameworks as a Versatile Platform for Proton Conductors. Adv. Mater. 2020, 32, e1907090. [Google Scholar] [CrossRef] [PubMed]
  2. Mendes, R.F.; Barbosa, P.; Domingues, E.M.; Silva, P.; Figueiredo, F.; Paz, F.A.A. Enhanced proton conductivity in a layered coordination polymer. Chem. Sci. 2020, 11, 6305–6311. [Google Scholar] [CrossRef] [PubMed]
  3. Aluru, N.R.; Aydin, F.; Bazant, M.Z.; Blankschtein, D.; Brozena, A.H.; de Souza, J.P.; Elimelech, M.; Faucher, S.; Fourkas, J.T.; Koman, V.B.; et al. Fluids and Electrolytes under Confinement in Single-Digit Nanopores. Chem. Rev. 2023, 123, 2737–2831. [Google Scholar] [CrossRef] [PubMed]
  4. Liu, C.; Gu, Y.; Liu, C.; Liu, S.; Li, X.; Ma, J.; Ding, M. Missing-Linker 2D Conductive Metal Organic Frameworks for Rapid Gas Detection. ACS Sens. 2021, 6, 429–438. [Google Scholar] [CrossRef]
  5. Dutta, S.; More, Y.D.; Fajal, S.; Mandal, W.; Dam, G.K.; Ghosh, S.K. Ionic metal–organic frameworks (iMOFs): Progress and prospects as ionic functional materials. Chem. Commun. 2022, 58, 13676–13698. [Google Scholar] [CrossRef]
  6. Que, Z.; Ye, Y.; Yang, Y.; Xiang, F.; Chen, S.; Huang, J.; Li, Y.; Liu, C.; Xiang, S.; Zhang, Z. Solvent-Assisted Modification to Enhance Proton Conductivity and Water Stability in Metal Phosphonates. Inorg. Chem. 2020, 59, 3518–3522. [Google Scholar] [CrossRef]
  7. Beaubras, F.; Rue, J.-M.; Perez, O.; Veillon, F.; Caignaert, V.; Jean-François, L.; Cardin, J.; Rogez, G.; Jestin, C.; Couthon, H.; et al. M(H2O)(PO3C10H6OH)·(H2O)0.5 (M = Co, Mn, Zn, Cu): A new series of layered metallophosphonate compounds obtained from 6-hydroxy-2-naphthylphosphonic acid. Dalton Trans. 2020, 49, 3877–3891. [Google Scholar] [CrossRef]
  8. Lim, D.W.; Kitagawa, H. Rational strategies for proton-conductive metal-organic frameworks. Chem. Soc. Rev. 2021, 50, 6349–6368. [Google Scholar] [CrossRef]
  9. Monjezi, B.H.; Okur, S.; Limbach, R.; Chandresh, A.; Sen, K.; Hashem, T.; Schwotzer, M.; Wondraczek, L.; Wöll, C.; Knebel, A. Fast Dynamic Synthesis of MIL-68(In) Thin Films in High Optical Quality for Optical Cavity Sensing. ACS Nano 2023, 17, 6121–6130. [Google Scholar] [CrossRef]
  10. Okada, K.; Mashita, R.; Fukatsu, A.; Takahashi, M. Polarization-dependent plasmonic heating in epitaxially grown multilayered metal–organic framework thin films embedded with Ag nanoparticles. Nanoscale Adv. 2023, 5, 1795–1801. [Google Scholar] [CrossRef]
  11. Berry, J.F.; Lu, C.C. Metal-Metal Bonds: From Fundamentals to Applications. Inorg. Chem. 2017, 56, 7577–7581. [Google Scholar] [CrossRef] [PubMed]
  12. Hwang, J.; Ejsmont, A.; Freund, R.; Goscianska, J.; Schmidt, B.V.K.J.; Wuttke, S. Controlling the morphology of metal–organic frameworks and porous carbon materials: Metal oxides as primary architecture-directing agents. Chem. Soc. Rev. 2020, 49, 3348–3422. [Google Scholar] [CrossRef] [PubMed]
  13. Dadashi, J.; Khaleghian, M.; Hanifehpour, Y.; Mirtamizdoust, B.; Woo Joo, S. Lead(II)-Azido Metal-Organic Coordination Polymers: Synthesis, Structure and Application in PbO Nanomaterials Preparation. Nanomaterials 2022, 12, 2257. [Google Scholar] [CrossRef] [PubMed]
  14. Wang, H.; Lustig, W.P.; Li, J. Sensing and capture of toxic and hazardous gases and vapors by metal-organic frameworks. Chem. Soc. Rev. 2018, 47, 4729–4756. [Google Scholar] [CrossRef]
  15. Han, B.-X.; Jiang, Y.-F.; Sun, X.-R.; Li, Z.-F.; Li, G. Proton conductive N-heterocyclic metal–organic frameworks. Coord. Chem. Rev. 2021, 432, 213754. [Google Scholar] [CrossRef]
  16. Siman, P.; Trickett, C.A.; Furukawa, H.; Yaghi, O.M. l-Aspartate links for stable sodium metal–organic frameworks. Chem. Commun. 2015, 51, 17463–17466. [Google Scholar] [CrossRef]
  17. Hwang, K.; Ahn, J.; Cho, I.; Kang, K.; Kim, K.; Choi, J.; Polychronopoulou, K.; Park, I. Microporous Elastomer Filter Coated with Metal Organic Frameworks for Improved Selectivity and Stability of Metal Oxide Gas Sensors. ACS App. Mater. Inter. 2020, 12, 13338–13347. [Google Scholar] [CrossRef]
  18. Zhang, Y.; Zhang, W.; Bian, Y.; Liu, Y.; Zhang, X.; Chen, M.; Hu, B.; Jin, Q. Tuning luminescence of the fluorescent molecule 2-(2-hydroxyphenyl)-1H-benzimidazole via zeolitic imidazolate framework-8. RSC Adv. 2022, 12, 9342–9350. [Google Scholar] [CrossRef]
  19. Chen, B.; Liang, C.; Yang, J.; Contreras, D.S.; Clancy, Y.L.; Lobkovsky, E.B.; Yaghi, O.M.; Dai, S. A microporous metal-organic framework for gas-chromatographic separation of alkanes. Angew. Chem. Int. Ed. Engl. 2006, 45, 1390–1393. [Google Scholar] [CrossRef]
  20. Hailing, G.G.Z.; Ian, J.H.; Qiu, S. “Twin Copper Source” Growth of Metal-Organic Framework Membrane: Cu3(BTC)2 with High Permeability and Selectivity for Recycling H2. J. Am. Chem. Soc. 2009, 131, 1646. [Google Scholar]
  21. Sarango-Ramírez, M.K.; Lim, D.-W.; Kolokolov, D.I.; Khudozhitkov, A.E.; Stepanov, A.G.; Kitagawa, H. Superprotonic Conductivity in Metal–Organic Framework via Solvent-Free Coordinative Urea Insertion. J. Am. Chem. Soc. 2020, 142, 6861–6865. [Google Scholar] [CrossRef] [PubMed]
  22. Kolokolov, D.I.; Lim, D.; Kitagawa, H. Characterization of Proton Dynamics for the Understanding of Conduction Mechanism in Proton Conductive Metal-Organic Frameworks. Chem. Rec. 2020, 20, 1297–1313. [Google Scholar] [CrossRef]
  23. Carotenuto, A.; Dell’Isola, M. An experimental verification of saturated salt solution-based humidity fixed points. Int. J. Thermophys. 1996, 17, 1423–1439. [Google Scholar] [CrossRef]
  24. Arman Kuzubasoglu, B. Recent Studies on the Humidity Sensor: A Mini Review. ACS App. Elec. Mater. 2022, 4, 4797–4807. [Google Scholar] [CrossRef]
  25. Fontanella, J.J.W.M.; Wainright, J.S.; Savinell, R.F.; Litt, M. High pressure electrical conductivity studies of acid doped polybenzimidazole. Electrochim Acta 1998, 43, 1289–1294. [Google Scholar] [CrossRef]
  26. Biemmi, C.S.E.; Bein, T. Oriented Growth of the Metal Organic Framework Cu3(BTC)2(H2O)3·xH2O Tunable with Functionalized Self-Assembled Monolayers. J. Am. Chem. Soc. 2007, 129, 8054–8055. [Google Scholar] [CrossRef]
  27. Zhang, W.; Yang, S. In Situ Fabrication of Inorganic Nanowire Arrays Grown from and Aligned on Metal Substrates. Acc. Chem. Res. 2009, 42, 1617–1627. [Google Scholar] [CrossRef]
  28. Schmidt, V.; Wittemann, J.V.; Senz, S.; Gösele, U. Silicon Nanowires: A Review on Aspects of their Growth and their Electrical Properties. Adv. Mater. 2009, 21, 2681–2702. [Google Scholar] [CrossRef]
  29. Saravanabharathi, D.; Obulichetty, M.; Rameshkumar, S.; Kumaravel, M. New 2-methyl benzimidazole based zinc carboxylates: Supramolecular structures, biomimetic proton conductivities and luminescent properties. Inorganica Chim. Acta 2015, 437, 167–176. [Google Scholar] [CrossRef]
  30. Colomban, P. Proton Conductors; Cambridge University Press: Cambridge, UK, 2010. [Google Scholar]
  31. Ponomareva, V.G.; Kovalenko, K.A.; Chupakhin, A.P.; Dybtsev, D.N.; Shutova, E.S.; Fedin, V.P. Imparting High Proton Conductivity to a Metal–Organic Framework Material by Controlled Acid Impregnation. J. Am. Chem. Soc. 2012, 134, 15640–15643. [Google Scholar] [CrossRef]
  32. Peterson, G.W.W.; Balboa, A.; Mahle, J.; Sewell, T.; Karwacki, C.J. Ammonia Vapor Removal by Cu3(BTC)2 and Its Characterization by MAS NMR. J. Phys. Chem. C 2009, 113, 13906–13917. [Google Scholar] [CrossRef]
  33. Morikawa, S.; Yamada, T.; Kitagawa, H. Crystal Structure and Proton Conductivity of a One-dimensional Coordination Polymer, {Mn(DHBQ)(H2O)2}. Chem. Let. 2009, 38, 654–655. [Google Scholar] [CrossRef]
  34. Prestipino, C.R.L.; Vitillo, J.G.; Bonino, F.; Damin, A.; Lamberti, C.; Zecchina, A.; Solari, P.L.; Kongshaug, K.O.; Bordiga, S. Local Structure of Framework Cu(II) in HKUST-1 Metallorganic Framework: Spectroscopic Characterization upon Activation and Interaction with Adsorbates. Chem. Mater. 2006, 18, 1337–1346. [Google Scholar] [CrossRef]
  35. Yu, S.; Zhang, H.; Chen, C.; Lin, C. Investigation of humidity sensor based on Au modified ZnO nanosheets via hydrothermal method and first principle. Sens. Actuators B Chem. 2019, 287, 526–534. [Google Scholar] [CrossRef]
  36. Duan, Z.; Xu, M.; Li, T.; Zhang, Y.; Zou, H. Super-fast response humidity sensor based on La0.7Sr0.3MnO3 nanocrystals prepared by PVP-assisted sol-gel method. Sens. Actuators B Chem. 2018, 258, 527–534. [Google Scholar] [CrossRef]
  37. Zhao, H.; Zhang, T.; Qi, R.; Dai, J.; Liu, S.; Fei, T.; Lu, G. Humidity sensor based on solution processible microporous silica nanoparticles. Sens. Actuators B Chem. 2018, 266, 131–138. [Google Scholar] [CrossRef]
  38. Cao, J.; Ma, W.; Lyu, K.; Zhuang, L.; Cong, H.; Deng, H. Twist and sliding dynamics between interpenetrated frames in Ti-MOF revealing high proton conductivity. Chem. Sci. 2020, 11, 3978–3985. [Google Scholar] [CrossRef] [PubMed]
  39. Duan, Z.; Zhao, Q.; Wang, S.; Huang, Q.; Yuan, Z.; Zhang, Y.; Jiang, Y.; Tai, H. Halloysite nanotubes: Natural, environmental-friendly and low-cost nanomaterials for high-performance humidity sensor. Sens. Actuators B Chem. 2020, 317, 128204. [Google Scholar] [CrossRef]
  40. Yang, L.; Qian, S.; Wang, X.; Cui, X.; Chen, B.; Xing, H. Energy-efficient separation alternatives: Metal-organic frameworks and membranes for hydrocarbon separation. Chem. Soc. Rev. 2020, 49, 5359–5406. [Google Scholar] [CrossRef]
  41. Li, X.M.; Dong, L.Z.; Liu, J.; Ji, W.X.; Li, S.L.; Lan, Y.Q. Intermediate-Temperature Anhydrous High Proton Conductivity Triggered by Dynamic Mo-lecular Migration in Trinuclear Cluster Lattice. Chem 2020, 6, 2272–2282. [Google Scholar] [CrossRef]
  42. Marsh, K.N. (Ed.) Recommended Reference Material for the Realization of Physicochemical Properties; Blackwell Scientific Publications: Oxford, UK, 1987; pp. 157–162. [Google Scholar]
Coatings 13 01520 g001
Figure 1. PXRD pattern of MOF-508a from (a) a standard powder and (b) a thin film incorporated in Zn foil via the “twin-zinc-source” method.
Figure 1. PXRD pattern of MOF-508a from (a) a standard powder and (b) a thin film incorporated in Zn foil via the “twin-zinc-source” method.
Coatings 13 01520 g001
Coatings 13 01520 g002
Figure 2. PXRD pattern of MOF-508a/Zn thin film prepared by changing the concentration of Zn2+.
Figure 2. PXRD pattern of MOF-508a/Zn thin film prepared by changing the concentration of Zn2+.
Coatings 13 01520 g002
Coatings 13 01520 g003
Figure 3. SEM images of the surface (a–f) and profile (g–l) of the MOF-508a/Zn thin films (the concentrations of Zn2+ are 12 mmol L−1, 6 mmol L−1, 3 mmol L−1, 1.5 mmol L−1, 1 mmol L−1, and 0 mmol L−1, respectively).
Figure 3. SEM images of the surface (a–f) and profile (g–l) of the MOF-508a/Zn thin films (the concentrations of Zn2+ are 12 mmol L−1, 6 mmol L−1, 3 mmol L−1, 1.5 mmol L−1, 1 mmol L−1, and 0 mmol L−1, respectively).
Coatings 13 01520 g003
Coatings 13 01520 g004
Figure 4. The linear relationship between the concentration of Zn2+ and the thickness of MOF-508a/Zn thin film.
Figure 4. The linear relationship between the concentration of Zn2+ and the thickness of MOF-508a/Zn thin film.
Coatings 13 01520 g004
Coatings 13 01520 g005
Figure 5. The XRD pattern of the MOF-508a/Zn thin film synthesized by the twin-zinc-source method over different reaction times (* is Zn).
Figure 5. The XRD pattern of the MOF-508a/Zn thin film synthesized by the twin-zinc-source method over different reaction times (* is Zn).
Coatings 13 01520 g005
Coatings 13 01520 g006
Figure 6. The SEM images of MOF-508/Zn thin films produced over different reaction times. (a,b) show the surface and profile images of the MOF-508/Zn thin films produced over reaction times of 6 h. (c,d) show the surface and profile images of the MOF-508/Zn thin films produced over reaction times of 12 h.
Figure 6. The SEM images of MOF-508/Zn thin films produced over different reaction times. (a,b) show the surface and profile images of the MOF-508/Zn thin films produced over reaction times of 6 h. (c,d) show the surface and profile images of the MOF-508/Zn thin films produced over reaction times of 12 h.
Coatings 13 01520 g006
Coatings 13 01520 g007
Figure 7. Resistance graphs of the samples with different concentrations of Zn2+.
Figure 7. Resistance graphs of the samples with different concentrations of Zn2+.
Coatings 13 01520 g007
Coatings 13 01520 g008
Figure 8. The temperature dependence of the conductivity of the MOF-508a crystals intergrowth film on zinc foil. Least-squares fitting is shown as a dotted line.
Figure 8. The temperature dependence of the conductivity of the MOF-508a crystals intergrowth film on zinc foil. Least-squares fitting is shown as a dotted line.
Coatings 13 01520 g008
Coatings 13 01520 g009
Figure 9. The effect of H2O molecules on the resistivity of the MOF-508b/Zn thin film held at different humidity for 1–3 days.
Figure 9. The effect of H2O molecules on the resistivity of the MOF-508b/Zn thin film held at different humidity for 1–3 days.
Coatings 13 01520 g009
Coatings 13 01520 sch001
Scheme 1. Illustration of space-filling representations of the structures of (a) the open phase Zn(BDC)(4,4′-bipy)0.5DMF(H2O)0.5 (MOF-508a) contained a 1D channel, with a pore aperture size of 4 × 4 Å, which were filled with water and DMF molecules to form hydrogen-bonding networks, and (b) the dense phase Zn(BDC)(4,4′-Bipy)0.5 (MOF-508b) lacking in a 1D channel due to the removal of the guest molecules of MOF-508a. The two interpenetrating frameworks are shown in yellow and pink [24].
Scheme 1. Illustration of space-filling representations of the structures of (a) the open phase Zn(BDC)(4,4′-bipy)0.5DMF(H2O)0.5 (MOF-508a) contained a 1D channel, with a pore aperture size of 4 × 4 Å, which were filled with water and DMF molecules to form hydrogen-bonding networks, and (b) the dense phase Zn(BDC)(4,4′-Bipy)0.5 (MOF-508b) lacking in a 1D channel due to the removal of the guest molecules of MOF-508a. The two interpenetrating frameworks are shown in yellow and pink [24].
Coatings 13 01520 sch001
Table 1. The effect of the thickness on the resistivity of the MOF-508a/Zn thin film.
Table 1. The effect of the thickness on the resistivity of the MOF-508a/Zn thin film.
Thickness of Film/μmResistivity × 104 Ω cm
62--
3727.88
226.21
180.40
160.25
Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Share and Cite

MDPI and ACS Style

Zhang, K.; Wang, C.; Yang, F.; Li, J.; Yan, S.; Qi, Y. Thickness and Humidity on Proton Conductivity in MOF-508 Thin Film by Twin-Zinc-Source Method. Coatings 2023, 13, 1520. https://doi.org/10.3390/coatings13091520

AMA Style

Zhang K, Wang C, Yang F, Li J, Yan S, Qi Y. Thickness and Humidity on Proton Conductivity in MOF-508 Thin Film by Twin-Zinc-Source Method. Coatings. 2023; 13(9):1520. https://doi.org/10.3390/coatings13091520

Chicago/Turabian Style

Zhang, Kun, Chunxia Wang, Feng Yang, Jing Li, Shuguang Yan, and Yue Qi. 2023. "Thickness and Humidity on Proton Conductivity in MOF-508 Thin Film by Twin-Zinc-Source Method" Coatings 13, no. 9: 1520. https://doi.org/10.3390/coatings13091520

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop