Photothermal responsive ultrathin Cu-TCPP nanosheets/sulfonated polystyrene nanocomposite photo-switch proton conducting membranes
Graphical abstract
Smart proton conductive membrane: ultrathin Cu-TCPP nanosheets decorated polystyrene membrane shows nice light switchable proton conductivity and durability.
Introduction
Solid proton conducting/exchange membranes based on advanced nanostructured have gotten immense attention in the horizon of emerging materials due to their widespread potential applications [[1], [2], [3]]. These membranes are practically used in electrochemical devices like fuel cells, batteries, electronics, supercapacitors, sensors etc. [4]. A state-of-art proton conducting membranes ought to exhibit conduction of high magnitude of protons with low electronic conductivity [3]. Furthermore, such membranes also be highly durable i.e., they have good chemical, electrochemical, thermal and hydrolytic stability. Mechanical strength (hydrated and dry states) and stability in operating environment ensures the operation time of membranes [5]. Moreover, such membranes not only transport high quantity of water, but also maintain uniform water content and prevent the localized drying. And the production cost may also be reasonable [6].
Proton conducting membranes based on organic polymers are usually fabricated from the polymeric chains containing ion conducting or exchange groups providing channels for proton transport. The polymeric chains (mostly hydrophobic) provide mechanical stability, while the ion exchange groups (hydrophilic domain) serve as proton conductors [6]. As the content of ion conducting groups increases, the protonic conductivity enhances due to generation of much protonic flow channels. But on the same time, the membrane loses its mechanical stability and, as a result, the overall efficiency of proton conductivity of membrane becomes declined [7]. In order to overcome the related problems, different strategies like surface modification, polymers blending, nanocomposite formation etc. are applied to improve the performance of proton conducting membranes. Among different techniques, nanocomposite formation is the best way to engender the desired characteristics [7,8]. But this method strongly depends on type and content (weight %) of nanofillers incorporation [9]. Homogenous dispersion and utilization of high surface area of nanofillers enhances the interactions with the polymer matrix resulting mechanically, thermally and electrically stable membranes with improved protonic conductivity [10]. Using MOFs as a nanofiller while nanocomposite membranes fabrication is a novel strategy and has renowned outcomes. The MOFs nanofillers not only enhances the protonic conductivity of a polymeric membrane, but also enhances the membrane durability [4]. MOFs with highly ordered crystalline structure, well designed tailorable pores, high surface area [[11], [12], [13], [14], [15]] can generate good interfacial interactions if they have good compatibility with the polymer [10]. Various MOFs functionalized hybrid polymer proton conductive membranes have been reported with improved proton conductivity and suppressed methanol permeability as well as enhanced mechanical stability [[16], [17], [18], [19]].
Among different types of MOFs, photothermal responsive MOFs and MOFs based nanomaterials are novel emergent area which significantly converts light into heat. The localized light-to-heat conversion upon irradiation in MOFs has many advantages like solvent removal to unlock the potential porosity of MOFs without damaging the crystalline integrity [20], chemo-photothermal therapies [[21], [22], [23]], sterilization [24] etc. The phenomenon of photo-thermal effect of MOFs also acts as photo-switch protons conduction, which has not been explored extensively. This photo-switch proton conductivity provides a platform for smart protonic solids having potential technological applications in remote-controllable chemical sensors, proton-conducting field-effect transistors and switchable sensors [25,26]. The photo-switch proton conductivity is not limited to the smart protonic solids, but also has a key role in bio-ionic functions [27] like light-driven proton and chloride pumping rhodopsin, acting as active ion transport by using energy (light) in the biological system [28,29]. For artificial ATP synthesis, a photoinduced reversible proton gradient in a silica matrix-based gel was employed along the usage of proton gradients for biomimetic power generation with solid-liquid interfaces [27]. Amongst MOFs based nanomaterials, MOFs nanosheets with photothermal potential have gotten much attention in the recent research due to their special inherit properties like ultrathin and uniform thickness, large accessible surface area, numerous available active sites [11,23], hence quite distinctive for nanocomposite formation with polymers [10,30].
Sulfonated polystyrene (SPS), which is one of the utmost significant and low cast thermoplastic polymer can serve as proton conductive material and can be synthesized by simple and low temperature sulfonation of polystyrene. The resulting SPS can conduct a high magnitude of protons by retaining more water and creating excellent hydrodynamic environment. But higher degree of sulfonation not only increases swelling behavior of SPS membrane in water and declining its proton conductive efficiency, but also disprove its mechanical properties [31]. In order to overcome these serious issues SPS nanocomposite formation is the best strategy to incite the desired properties. Several nanomaterials like particles, fibers, spheres etc. are incorporated into SPS matrix either by solution casting or melt-compounding technique to improve SPS properties, ultimately enhances its efficiency [32]. Porphyrins, light harvesting materials, absorb incident light and channel the photon energy to reaction centers where light-induced charge separation takes place like in chlorophyll while photosynthesis. So, porphyrin are considered as light harvesters for semiconductors and photocatalysis due to good chromophore activities over the solar spectrum and electron donating properties (π-electron systems) and offer a wide variety of photochemical and redox properties, which can be easily tuned by peripheral substituent modifications or metal complexation [33]. Based on porphyrin complexation with copper, ultrathin and highly ordered Cu-TCPP MOFs nanosheets have good protonic conductivity due to dangling coordination water and COOH groups (acting as Lewis acid), porous structure of pore size of 1 nm [34] along water adsorption in meso and macro pores [35,36] and optimum negative zeta potential revealing its stability [37]. But due to Cu-TCPP nanosheets intrinsic fragile nature, its assembling into a free standing and uniform nanofilm is hardly achieved employing traditional techniques [36]. Furthermore, high Cu-TCPP content causes accumulation (stacking) while composite formation which blocks the active exposed sites and becomes less proton conductive [38].
Herein, we report the synthesis of ultrathin and highly ordered porous Cu-TCPP nanosheets as a novel photothermal responsive nanofiller for preparation of SPS-Cu-TCPP photo-switch proton conductive nanocomposite membranes for the first time. The Cu-TCPP nanosheets were well dispersed in the SPS matrix with the incorporating capacity up to 10 wt %. The resulting nanocomposite membranes conduct good protonic conductivity, 196% amplified than the pristine SPS polymer and give remarkable response to light due to the nice photothermal response of Cu-TCPP nanosheets. The visible light ON declines the protonic conductivity of nanocomposite membranes by more than sixteen times and the parental protonic conductivity can be regained after switching the light OFF with good precision. Furthermore, the MOFs nanosheets not only enhance the water content, hindered the swelling ratio, but also enhances the membrane durability in terms of thermal and mechanical stability.
Section snippets
Chemicals
Polystyrene (M.W. 100,000) was bought from Alfa Aesar. Cupper II Nitrate trihydrate [Cu(NO3)2·3H2O] was bought from Sigma-Aldrich. Polyvinyl pyrrolidone (PVP M.W. 58K), Trifluroaceic acid (TFA), tetrakis(4-carboxyphenyl)porphyrin (TCPP) were bought from Aladdin. Dimethyl formamide (DMF analytical grade), ethanol (analytical grade) and dimethyl acetamide (DMAc analytical grade) were bought from Merck. All reagents and solvents were used as received.
Synthesis of 2D Cu-TCPP nanosheets
The Cu-TCPP nanosheets were synthesized via
Results and discussion
Fig. 1 (a, b) show that randomly oriented Cu-TCPP nanosheets were formed with smooth surface and uniform morphology in uniform thickness of 2 ± 1 nm, width of 9897 nm and length of 1.03 μm, revealing the nanosheets ultrathin nature. The XRD diffractogram of Cu-TCPP nanosheets Fig. 1 (c) describing the tetragonal structure of nanosheets with broad peak corresponding to the (004) plane is observed, indicates the layered crystal structure [39]. The FT-IR of Cu-TCPP nanosheets Fig. 1 (d) gives two
Conclusions
In summary, a photo-responsive, swelling resistant and good photo-switch proton conductive membrane was prepared by incorporating ultrathin Cu-TCPP MOFs nanosheets into SPS matrix. The Cu-TCPP nanosheets were homogenously dispersed and generated a good interfacial interaction with SPS resulting dense nanocomposite membranes. The obtained membrane illustrated good protonic conductivity of 1.24 × 10−4 S. cm−1, 196% higher than sulfonated blank polymer along excellent light ON/OFF ratio. Based on
CRediT authorship contribution statement
Shabab Hussain: Writing - original draft, Writing - review & editing, Formal analysis, Methodology. Zheng Deng: Methodology, Investigation. Amin Khan: Investigation. Peipei Li: Investigation, Methodology. Zhuoyi Li: Investigation. Zhou Fang: Investigation. Xinyi Wan: Investigation. Xinsheng Peng: Conceptualization, Supervision, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by Major R & D plan of Zhejiang Natural Science Foundation (LD18E020001), the Key program of National Natural Science and Foundation (51632008), and the National Natural Science Foundation of China of China (21875212) and National Key Research and Development Program (2016YF-A0200204).
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