A Highly Robust and Conducting Ultramicroporous 3D Fe(II)-Based Metal-Organic Framework for Efficient Energy Storage.

Exploitation of metal-organic framework (MOF) materials as active electrodes for energy storage or conversion is reasonably challenging owing to their poor robustness against various acidic/basic conditions and conventionally low electric conductivity. Keeping this in perspective, herein, a 3D ultramicroporous triazolate Fe-MOF (abbreviated as Fe-MET) is judiciously employed using cheap and commercially available starting materials. Fe-MET possesses ultra-stability against various chemical environments (pH-1 to pH-14 with varied organic solvents) and is highly electrically conductive (σ = 0.19 S m-1) in one fell swoop. By taking advantage of the properties mentioned above, Fe-MET electrodes give prominence to electrochemical capacitor (EC) performance by delivering an astounding gravimetric (304 F g-1) and areal (181 mF cm-2) capacitance at 0.5 A g-1 current density with exceptionally high cycling stability. Implementation of Fe-MET as an exclusive (by not using any conductive additives) EC electrode in solid-state energy storage devices outperforms most of the reported MOF-based EC materials and even surpasses certain porous carbon and graphene materials, showcasing superior capabilities and great promise compared to various other alternatives as energy storage materials.

Electro-Oxidation Reaction of Methanol over La2-xSrxNi1-y(Mn/Fe/Co)yO4+δ Ruddlesden-Popper Oxides.

Solution combustion-synthesized Ruddlesden-Popper oxides La1.4Sr0.6Ni0.9(Mn/Fe/Co)0.1O4+δ were explored for the methanol electro-oxidation reaction. With optimal doping of Sr2+ in the A site and Co2+ in the B site, Ni3+ with t2g6 dx2-y21 configuration in La1.4Sr0.6Ni0.9Co0.1O4+δ exhibited a tetragonal distortion with compression in axial bonds and elongation in equatorial bonds. This structural modification fostered an augmented overlap of dz2 orbitals with axial O 2p orbitals, leading to a heightened density of states at the Fermi level. Consequently, this facilitated not only elevated electrical conductivity but also a noteworthy reduction in the charge transfer resistance. These effects collectively contributed to the exceptional methanol oxidation activity of La1.4Sr0.6Ni0.9Co0.1O4+δ, as evidenced by an impressive current density of 21.4 mA cm-2 and retention of 95% of initial current density even after 10 h of prolonged reaction. The presence of Ni3+ further played a pivotal role in the creation of NiOOH, a crucial intermediate species, facilitated by the presence of surface oxygen vacancies. These factors synergistically enabled efficient methanol oxidation. In summary, our present study not only yields substantial insights but also paves the way for a novel avenue to fine-tune the activity of Ruddlesden-Popper oxides for the successful electro-oxidation of methanol.

Co-containing metal-organic framework for high-performance asymmetric supercapacitors with functionalized reduced graphene oxide.

Nowadays, supercapacitors are the most coveted eco-friendly and sustainable next-generation energy storage devices. In this regard, developing supercapacitors with high energy density and power density has always been a challenge for researchers. Herein, we have exploited an electroactive Co-containing metal-organic framework (Co-MOF) using cheap and commercially available starting materials under refluxing conditions and explored its energy storage properties in three- and two-electrode methods. The Co-MOF exhibited a specific capacitance of 425 F g-1 at 2 A g-1, maintaining a capacitance of ∼78% over 2200 successive charge-discharge cycles in a three-electrode system. The two-electrode asymmetric supercapacitor (ASC) using Co-MOF as the working electrode and as-synthesized p-phenylenediamine (PPD)-functionalized reduced graphene oxide (PPD-rGO) as the counter electrode divulged a specific capacitance of 72.5 F g-1 at 2 A g-1 current density with ∼70% capacitive retention after 2200 successive charge-discharge cycles over a broad potential window of 0-1.6 V. Moreover, the ASC demonstrated a maximum power density of 11.9 kW kg-1 at 10 A g-1 and a maximum energy density of 25.8 W h kg-1 at 2 A g-1 current density. Owing to the stable electrochemical redox (Co2+/Co3+)-mediated pseudocapacitive behavior of the Co-MOF and the high surface area and electrical conductivity of in situ generated PPD-intercalated rGO, the fabricated ASC unveiled high-performance supercapacitive behaviors. To investigate the practical applicability of this material, solid-state (ASC) devices were fabricated by employing the Co-MOF as the positive electrode and PPD-rGO as the negative electrode in a KOH-based gel electrolyte, which could power a commercially available light-emitting diode bulb (∼1.8 V) for several seconds. Therefore, the elucidated high electrochemical energy storage performance of the prepared Co-MOF makes it a very promising electrode material for supercapacitors.

Side-Chain Modification in Conjugated Polymer Frameworks for the Electrocatalytic Oxygen Evolution Reaction.

Conjugated polymer frameworks (CPFs) have recently sparked tremendous research interest due to their broad potentials in various frontline application areas such as photocatalysis, sensing, gas storage, energy storage, etc. These framework materials, without sidechains or functional groups on their backbone, are generally insoluble in common organic solvents and less solution processable for further device applications. There are few reports on metal-free electrocatalysis, especially oxygen evolution reaction (OER) using CPF. Herein, we have developed two triazine-based donor-acceptor conjugated polymer frameworks by coupling a 3-substituted thiophene (donor) unit with a triazine ring (acceptor) through a phenyl ring spacer. Two different sidechains, alkyl and oligoethylene glycol, were rationally introduced into the 3-position of thiophene in the polymer framework to investigate the effect of side-chain functionality on the electrocatalytic property. Both the CPFs demonstrated superior electrocatalytic OER activity and long-term durability. The electrocatalytic performance of CPF2, which achieved a current density of 10 mA/cm2 at an overpotential (η) of 328 mV, is much superior to CPF1, which reached the same current density at an overpotential of 488 mV. The porous and interconnected nanostructure of the conjugated organic building blocks, which allowed for fast charge and mass transport processes, could be attributed to the higher electrocatalytic activity of both CPFs. However, the superior activity of CPF2 compared to CPF1 may be due to the presence of a more polar oxygen-containing ethylene glycol side chain, which enhances the surface hydrophilicity, promotes better ion/charge and mass transfer, and increases the accessibility of the active sites toward adsorption through lower π-π stacking compared to hexyl side chain present in CPF1. The DFT study also supports the plausible better performance toward OER for CPF2. This study confirms the promising potentiality of metal-free CPF electrocatalysts for OER and further sidechain modification to improve their electrocatalytic property.

A terpyridine based hydrogel system for reversible transmissive-to-dark electrochromism and bright-to-quenched electrofluorochromism.

A carboxylic acid-containing terpyridine-based hydrogelator (TPPCA) is synthesized to afford a self-assembly induced TPPCA hydrogel, which was used as an all-in-one electrochrome in electrochromic devices (ECDs) to demonstrate reversible transparent-to-black electrochromism with fast darkening and bleaching time of 8.3 s and 9.5 s, respectively, high photopic coloration efficiency of 65.8 cm2 C-1 and high optical memory. The ECD also revealed bluish-white to quenched emission simultaneously under the -3.5 V to 0 V voltage range.

Electrochromic and photochromic behaviour in a single metal-organic framework containing a redox-active linker.

A Zr-based metal organic framework with naphthalene diimide teracarboxylate linkers is reported for its dual electrochromic and photochromic behavior. MOF crystals display reversible yellow to green photochromism upon exposure to visible light and colourless to dark-brown reversible electrochromism on applying a potential of 0 to -2.5 V. The MOF thin film shows good colouration efficiency at 550 nm, which is the highest sensitivity of the human eye.

Transmissive to blackish-green NIR electrochromism in a Co(II)-based interfacial co-ordination thin film.

Herein, a Co(ii)-based metallo-polymer (Co-tpy-L) with a three armed non-conjugated terpyridine ligand is synthesized using solvent-solvent interfacial polymerization. The thin film exhibits durable transmissive-to-blackish green electrochromism, selectively covering both the visible and NIR regions with a moderate voltage range of -1.4 to 0 V.

Metal-Immobilized Micellar Aggregates of a Block Copolymer from a Mixed Solvent for a SERS-Active Sensing Substrate and Versatile Dip Catalysis.

Here, we have reported micellar aggregations of an amphiphilic block copolymer in mixed solvent and their subsequent use as a template for the fabrication of a very dense, tunable metal nanoparticle-decorated surface for SERS and flexible dip catalysis applications. A silver nanoparticle-immobilized layer on silicon substrates shows excellent SERS (surface-enhanced Raman scattering)-based sensing performance for model analyte rhodamine B up to 10-6 M concentration with a well-defined calibration curve. Furthermore, a facile approach to the preparation of metal NP-immobilized BCP membranes as efficient dip catalyst for two model reactions (the reduction of nitrophenol and the Suzuki-Miyaura reaction of iodobenzene or 2,7-diiodofluorene with phenyl boronic acid) is also demonstrated. The Ag NP-decorated film exhibits high efficiency and extensive reusability in a prototype reaction such as the reduction of nitrophenol by sodium borohydride with a very high turnover number, >126 (for a single use), whereas the Pd NP-immobilized film also has a high, ∼100%, reaction yield and extensive reusability and applicable for different aromatic systems. This work provides a new platform for the design and synthesis of a functionalizable, flexible, and highly mechanically stable dip catalyst which is highly demanded in the catalytic production of value-added chemicals and environmental applications such as wastewater treatment.

Targeted Bioimaging of Cancer Cells Using Free Folic Acid-Sensitive Molybdenum Disulfide Quantum Dots through Fluorescence "Turn-Off".

In the present study, a proficient way for targeted bioimaging of folate receptor (FR)-positive cancer cells using free folic acid (FA)- and MoS2 QD-based nanoprobes is discussed along with its advantages over the preparation of orthodox direct FA-nanoprobe bioconjugates for the imaging. The water-soluble MoS2 QDs of size 4-5 nm with cysteine functionalization are synthesized by a simplistic bottom-up hydrothermal method. The as-prepared MoS2 QDs exhibit the blue emission with the highest emission intensity at 444 nm upon excitation of 370 nm. The MoS2 QDs are too sensitive toward FA to produce an effective and stable nanofiber structure through supramolecular interaction, which demonstrates ∼97% quenching of fluorescence. Moreover, the high selectivity and sensitivity of MoS2 QDs toward FA make the MoS2 QD-based nanoprobe an appropriate candidate for FA-targeted "turn-off" imaging probes for in vivo study of FA-pretreated FR-overexpressed cancer cells. It is obvious from the confocal microscopy images that the FA-pretreated B16F10 cancer cells show higher population of dimmed fluorescence compared to untreated cancer cells and HEK-293 normal cells. The flow cytometry study quantitatively reveals the significant difference of the geometric mean of fluorescence between FA-pretreated and untreated B16F10 cancer cells. Hence, these MoS2 QD-based nanoprobes can be applied as potential nanoprobes for the prediagnosis of cancer through targeted bioimaging.

Interfacial Coordination Nanosheet Based on Nonconjugated Three-Arm Terpyridine: A Highly Color-Efficient Electrochromic Material to Converge Fast Switching with Long Optical Memory.

An electrochromic (EC) hyperbranched coordination nanosheet (CONASH) comprising a three-arm terpyridine (3tpy)-based ligand and Fe(II) ion has been synthesized by interfacial complexation at the liquid-liquid interface. The film can be easily deposited on the desired substrate such as indium tin oxide (ITO) glass. Characterization of CONASH deposited on ITO by microscopic methods reveals the homogeneous nanosheet film with an ∼350 nm thickness after 48 h of reaction. The fabricated solid-state EC device (ECD) undergoes a reversible redox reaction (Fe2+ → Fe3+) in the potential range of +3 to -2 V in ECDs accompanied with a distinct color change from intense pink to colorless for several switching cycles with a coloration time of 1.15 s and a bleaching time of 2.49 s along with a high coloration efficiency of 470.16 cm2 C-1. Besides, the nonconjugated 3tpy ligand restricts the easy electron redox conduction inside the EC film to enhance the EC memory in open-circuit condition as it shows 50% retention of its colorless state until 25 min. The long EC memory compared to other metallo-supramolecular polymers having a conjugated ligand suggests the potentiality of the 3tpy-Fe CONASH film to be used as a power-efficient EC material for modern display device applications.

Structure-Sensitive Electrocatalytic Reduction of CO2 to Methanol over Carbon-Supported Intermetallic PtZn Nano-Alloys.

The electrochemical reduction of CO2 (CO2RR) to produce valuable synthetic fuel like CH3OH not only mitigates the accumulated greenhouse gas from the environment but is also a promising direction toward attenuating our continuous reliance on fossil fuels. However, CO2RR to yield CH3OH suffers because of large overpotential, competitive H2 evolution reaction (HER), and poor product selectivity. In this regard, intermetallic alloy catalysts open up a wide possibility of fine-tuning the electronic property and attain appropriate structures that facilitate selective CO2RR. Here, we report for the first time the CO2RR over carbon-supported PtZn nano-alloys and probed the crucial role of structures and interfaces as active sites. PtZn/C, Pt3Zn/C, and PtxZn/C (1 < x < 3) synthesized from the metal-organic framework material were characterized structurally and morphologically. The catalysts demonstrated structure dependency toward CH3OH selectivity, as the mixed-phase PtxZn/C outperformed the phase-pure PtZn/C and Pt3Zn/C. The structure-dependent reaction mechanism and the kinetics were elucidated over the synthesized catalysts with the help of detail experiments and associated density functional theory calculations. Results showed that in spite of low electrochemically active surface area, PtxZn could not only have facilitated the single electron transfer to adsorbed CO2 but also showed better binding of the intermediate CO2•- over its surface. Moreover, the lower bond energy between the mixed-phase surface and -OCH3 compared to the phase-pure catalysts has enabled higher CH3OH selectivity over PtxZn. This work opens a wide possibility of studying the role of interfaces between phase-pure nano-alloys toward CO2RR.

Probing the photo- and electro-catalytic degradation mechanism of methylene blue dye over ZIF-derived ZnO.

Due to the severe water pollution from effluent dyes, the need of the hour is to find a suitable dye degradation technology, and appropriate catalyst materials. Semiconducting ZnO was produced by pyrolysis of ZIF-8 template. The materials were well characterized with in situ and ex situ XRD and TGA, FE-SEM, HRTEM, UV-DRS, PL, and FRET. The results showed that upon calcination the body centered cubic ZIF-8 produces hexagonal primitive ZnO while retaining the truncated cubic shaped particles. The materials were screened for photo- and electro-catalytic oxidation of methylene blue. In both the different degradation technologies, ZnO synthesized from ZIF-8 outperformed the ZIF-8. The FRET dynamics showed significant spectral overlap of ZnO emission and the methylene blue absorption. It was found to be responsible for the better photocatalytic efficacy of ZnO samples than ZIF-8. The proposed reaction mechanism showed that the surface-bound reactive oxygen species produced either by light exposure or due to applied bias is key to dye degradation. The cytotoxicity of the untreated and ZnO and ZIF-8 treated dye over melanoma cells was evaluated, and it was found that the cytotoxicity of the degraded dye from ZIF-derived ZnO was less compared to that of ZIF-8 treated one.

Determination of band edges and their influences on photocatalytic reduction of nitrobenzene by bulk and exfoliated g-C3N4.

Thermally and chemically exfoliated metal-free semiconducting g-C3N4 are synthesized from bulk g-C3N4. Thorough characterization of the synthesized materials is performed with the help of XRD, FTIR, FE-SEM, PL, surface area analysis and DRS to probe differences in structural, morphological and optical properties between thermally and chemically exfoliated g-C3N4. The synthesized materials are exposed to light for photocatalytic reduction of nitrobenzene. The complete reduction reaction mechanism and product selectivity over the synthesized catalysts are studied in this report. The rate of reduction of nitrobenzene is found to be higher with thermally exfoliated g-C3N4, and the selectivity of aniline is found to be higher in the case of chemical exfoliated g-C3N4. The differences in the reactivity are explained in terms of structure, surface morphologies and band edge positions.

Electrochromic Os(II)-Based Metallo-Supramolecular Polymers.

This work presents the preparation of a series of novel Os(II)-based metallo-supramolecular polymers (polyOss: linear polyOsL1100% and hyperbranched polyOsL1x% L2y% ) that show a broad absorption spanning 312 to 677 nm and a low Os(II)/(III) redox potential of 0.94 V. The electrochromic properties of a polyOs film cast on an ITO substrate is investigated. The change in transmittance (ΔT) of polyOsL1100% is 49.9%, and the switching times for coloration (t c ) and bleaching (t b ) are 0.70 and 0.82 s, respectively. The introduction of a 10% branching structure (polyOsL190% L210% ) further enhanced the electrochromic performance with ΔT = 59.4%, t c  = 0.41 s, and t b  = 0.54 s. The coloration efficiency (η) increased from 396.1 to 467.5 cm2  C-1 upon branching. A solid-state electrochromic device with polyOsL1100% is successfully fabricated to use the polymer for potential applications.

Enhanced Photoinduced Electrocatalytic Oxidation of Methanol Using Pt Nanoparticle-Decorated TiO2-Polyaniline Ternary Nanofibers.

Herein, perylene-3,4,9,10-tetracarboxylic acid-doped polyaniline (PTP) nanofibers with/without photoreactive anatase TiO2 (TiO2-PTP and PTP, respectively) have been successively synthesized and subsequently decorated by Pt nanoparticles (Pt NPs) to prepare Pt-PTP and Pt-TiO2-PTP composites. High-resolution transmission electron microscopy confirms the presence of ∼3 nm spherical-shaped Pt NPs on both the composites along with TiO2 on Pt-TiO2-PTP. Pt loading on the composites is deliberately kept similar to compare the methanol electro-oxidation in the two composites. The Pt nanocomposites along with the precursor polyanilines are characterized by optical characterization, X-ray diffraction study, X-ray fluorescence spectroscopy, and Raman spectroscopy. The ternary composite-modified (Pt-TiO2-PTP) electrode demonstrates high electrocatalytic performance for methanol oxidation reaction in acid medium than Pt-PTP and Pt-TiO2. The higher electrochemical surface area (1.7 times), high forward/backward current ratio, and the higher CO tolerance ability for Pt-TiO2-PTP make it a superior catalyst for methanol oxidation reaction in the electrochemical process than Pt-PTP. Moreover, the catalytic activity of Pt-TiO2-PTP is further enhanced significantly with light irradiation. The cooperative effects of photo- and electrocatalysis on methanol oxidation reaction in Pt-TiO2-PTP enhance the methanol oxidation catalytic activity approximately 1.3 times higher in light illumination than in dark. Therefore, the present work will be proficient to get a light-assisted sustainable approach for developing the methanol oxidation reaction activity of Pt NP-containing catalysts in direct methanol fuel cells.

Modulation of a coordination structure in a europium(iii)-based metallo-supramolecular polymer for high proton conduction.

Developing high proton conducting solid materials is significant in the field of fuel cells. A europium(iii)-based metallo-supramolecular polymer with uncoordinated carboxylic acids (PolyEu-H) was successfully synthesized by modifying the synthesis conditions. The proton conductivity was enhanced with increasing the relative humidity (RH) from 30 to 95% RH. PolyEu-H showed about 104 times higher proton conductivity than the polymer with coordinated carboxylic acids (PolyEu) and about 400 times higher than the polymer without carboxylic acids (PolyEu-2). The proton conductivity of PolyEu-H reached 4.45 × 10-2 S cm-1 at 95% RH and 25 °C and 5.6 × 10-2 S cm-1 at 75 °C. The activation energy, E a was ultralow (0.04 eV), which indicates proton conduction based on the Grotthuss mechanism. The results indicate that efficient proton conduction occurs through proton channels formed by moisture in PolyEu-H.

One-Dimensional Anhydrous Proton Conducting Channel Formation at High Temperature in a Pt(II)-Based Metallo-Supramolecular Polymer and Imidazole System.

One dimensional (1D) Pt(II)-based metallo-supramolecular polymer with carboxylic acids (polyPtC) was synthesized using a new asymmetrical ditopic ligand with a pyridine moiety bearing two carboxylic acids. The carboxylic acids in the polymer successfully served as apohosts for imidazole loaded in the polymer interlayer scaffold to generate highly ordered 1D imidazole channels through the metallo-supramolecular polymer chains. The 1D structure of imidazole loaded polymer (polyPtC-Im) was analyzed in detail by thermogravimetric analysis, powder X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and ultraviolet-visible and photoluminescence spectroscopic measurements. PolyPtC-Im exhibited proton conductivity of 1.5 × 10-5 S cm-1 at 120 °C under completely anhydrous conditions, which is 6 orders of magnitude higher than that of the pristine metallo-supramolecular polymer.

Selective DNA Recognition and Cytotoxicity of Water-Soluble Helical Metallosupramolecular Polymers.

Water-soluble helical Fe(II)-based metallosupramolecular polymers ((P)- and (M)-polyFe) were synthesized by 1:1 complexation of Fe(II) ions and bis(terpyridine)s bearing a (R)- and (S)-BINOL spacer, respectively. The binding affinity to calf thymus DNA (ct-DNA) was investigated by titration measurements. (P)-PolyFe with the same helicity as B-DNA showed 40-fold higher binding activity (Kb = 13.08 × 107 M-1) to ct-DNA than (M)-polyFe. The differences in binding affinity were supported by electrochemical impedance spectroscopy analysis. The charge-transfer resistance (Rct) of (P)-polyFe increased from 2.5 to 3.9 kΩ upon DNA binding, while that of (M)-polyFe was nearly unchanged. These results indicate that ionically strong binding of (P)-polyFe to DNA chains decreased the mobility of ions in the conjugate. Unique rod-like images were obtained by atomic force microscopy measurement of the DNA conjugate with (P)-polyFe, likely because of the rigid binding between DNA chains and the polymer. Differences in polymer chirality lead to significantly different cytotoxicity levels in A549 cells. (P)-PolyFe showed higher binding affinity to B-DNA and much higher cytotoxicity than (M)-polyFe. The helicity in metallosupramolecular polymer chains was important not only for chiral recognition of DNA but also for coordination to a biological target in the cellular environment.

Proton Conductive Nanosheets Formed by Alignment of Metallo-Supramolecular Polymers.

Linear Fe(II)-based metallo-supramolecular polymer chains were precisely aligned by the simple replacement of the counteranion with an N,N'-bis(4-benzosulfonic acid)perylene-3,4,9,10-tetracarboxylbisimide (PSA) dianion, which linked the polymer chains strongly. A parallel alignment of the polymer chains promoted by the PSA dianions yielded nanosheets formation. The nanosheets' structure was analyzed with FESEM, HRTEM, UV-vis, and XRD in detail. The nanosheets showed more than 5 times higher proton conductivity than the original polymer due to the smooth ionic conduction through the aligned polymer chains. The complex impedance plot with two semicircles also suggested the presence of grain boundaries in the polymer nanosheets.