Search

Water splitting electrolysis is a promising sustainable methodology to produce clean and green hydrogen fuel, which is considered as a prominent replacement for conventional fossil fuels. Developing highly active, stable and low-cost electrocatalysts plays a critical role for achieving efficient production of hydrogen by electrolysis for practical applications. Herein, we report the synthesis of Co3O4 spinel oxides by simple calcination of commercially available cobalt salts at different temperatures (400, 600 and 800 °C) and the disordered structure induced highly enhanced hydrogen evolution reaction (HER) in an alkaline medium (1.0 M KOH). Powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) indicated the formation of a normal spinel Co3O4 structure at 400 (Co3O4-400) and 600 °C (Co3O4-600) calcination, whereas disordered Co3O4 at 800 °C (Co3O4-800). HR-TEM analysis revealed the formation of spherical crystalline particles in Co3O4-400 and featureless aggregated crystalline particles at higher temperatures. The d-spacing analysis using PXRD and the lattice fringe pattern showed a decrease of the spacing with increasing calcination temperature, which suggested higher strain for Co3O4-800. The electrocatalytic HER activity of Co3O4-400 and Co3O4-600 in an alkaline medium showed moderate activity. Co3O4-400 and Co3O4-600 required an overpotential of 293 and 240 mV, respectively, to achieve a geometric current density of 10 mA cm−2. In contrast, disordered Co3O4-800 required a very low overpotential (93 mV) for producing the geometric current density of 10 mA cm−2. Electrochemical impedance studies showed faster kinetics and charge transfer for disordered spinel Co3O4-800 and supported the enhanced HER activity. The electrocatalytic stability studies for Co3O4-800 were tested up to 48 h at 100 mA cm−2 current density and confirmed the excellent stability. Thus, a low-cost earth abundant HER electrocatalyst was fabricated by a simple method. [ABSTRACT FROM AUTHOR]

Cobalt hydroxide Co(OH)2 is considered to be a potential material for electrocatalyst, especially for oxygen evolution reaction (OER), owing to the earth's abundance, environmentally benign nature, and redox-active properties of cobalt. Herein, we report the fabrication of pristine Co(OH)2 and AgNP-integrated Co(OH)2 (Ag–Co(OH)2) by simple wet chemical methods and explore the electrocatalytic OER activity in the alkaline medium. High-resolution transmission electro-microscopic (HR-TEM) analysis revealed featureless nanostructures for Co(OH)2 with coexisting amorphous and crystalline phases, as well as the inclusion of crystalline AgNPs in Ag–Co(OH)2. X-ray photoelectron spectroscopic (XPS) analysis confirmed the inclusion of metallic AgNPs and the presence of Co at a mixed oxidation state. Electrocatalytic OER studies indicated that pristine Co(OH)2 required the overpotential of 299 mV to achieve a geometric current density of 10 mA cm−2. The integration of AgNPs in Co(OH)2 (Ag–Co(OH)2) showed a gradual improvement in the OER activity. The optimized sample, Ag–Co(OH)2-5, required the overpotential of 253 mV to produce 10 mA cm−2 current density. Tafel slope analysis revealed a lower value upon AgNP integration and electrochemical impedance showed lower charge transfer resistance. The lower Tafel value and charge transfer resistance of Ag–Co(OH)2-5 indicated good chemical coupling and faster reaction kinetics at the electrode surface. AgNPs incorporation with Co(OH)2 also showed enhanced turn-over frequency, electrochemical active surface area, and double-layer capacitance. The fabricated hybrid Ag–Co(OH)2-5 catalyst also exhibited good stability over 60 h. Thus, the electrocatalytic activity of low-cost Co(OH)2 was improved by fabricating coexisting amorphous and crystalline phases and integrating noble silver nanoparticles. [ABSTRACT FROM AUTHOR]

The coordination environment and networks of metal–organic frameworks (MOFs) play a significant role in their functional properties, especially catalysis. In recent years, the use of MOFs for electrocatalytic water-splitting reactions to produce green hydrogen fuel has gained increasing attention. Herein, we report the preparation of Co-benzene dicarboxylic acid-based MOFs (Co-MOF-1 and Co-MOF-2) with different coordination environments and correlated their structure-controlled electrocatalytic OER activity in an alkaline medium. Both Co-MOF-1 and Co-MOF-2 showed 3D network structures with different coordination environments around the Co metal centre. In Co-MOF-1, the Co metal centre is coordinated with DMF and pyridine along with a benzene dicarboxylic acid (BDC) ligand, whereas the Co metal centre is coordinated only with BDC ligands in Co-MOF-2. The Co metal centres in Co-MOF-1 contain labile solvent coordination but have completely saturated coordination with the BDC ligand alone in Co-MOF-2. Interestingly, Co-MOF-1 showed relatively strong OER activity and required the overpotential (η) of 294 mV to produce 10 mA cm−2 current density. However, Co-MOF-2 required the overpotential of 343 mV to achieve the 10 mA cm−2 current density. Further Co-MOF-1 showed comparatively low Tafel slope and charge transfer resistance and high electrochemical surface area than Co-MOF-2 and supported the enhanced OER activity. XPS analysis of Co-MOF-1 before and after catalysis suggested the conversion of Co-MOF-1 to CoOOH during the electrocatalysis that boosted the OER activity. The presence of a labile coordination site in Co-MOF-1 might have facilitated the interaction of hydroxyl electrolyte with the metal centre and active site generation by forming CoOOH easily which enhanced the OER activity. Co-MOF-1 also showed excellent stability for over 48 h. Hence, the present work provides structural insight for designing new MOFs for electrocatalytic water-splitting reactions. [ABSTRACT FROM AUTHOR]

Developing an efficient transition metal-based non-noble electrocatalyst is important for the industrial water splitting process and producing green sustainable hydrogen (H2) fuel. Particularly, the direct fabrication of electrocatalysts on an electrode surface can exhibit strong electronic coupling that results in increased catalytic activity. Herein, copper (Cu) and cuprous oxide (Cu2O) nanoparticles were directly deposited on a nickel foam (NF) substrate and the controlled calcination of the Cu produced mixed copper–copper oxides (CuO, Cu2O–CuO and Cu–Cu2O–CuO) that exhibited excellent composition-controlled bifunctional electrocatalytic (oxygen evolution reaction (OER) and hydrogen evolution reaction (HER)) activity in an alkaline medium with good stability. Powder X-ray diffraction (PXRD) and X-ray photoelectron spectroscopic (XPS) studies confirmed the formation of pure and mixed copper–copper oxide nanomaterials on NF. Mixed copper oxide (Cu2O–CuO@NF) and pure CuO@NF exhibited excellent electrocatalytic OER and HER activity, respectively, in alkaline media. To the best of our knowledge, Cu2O–CuO@NF required an overpotential of 262 mV to achieve the lowest geometric current density (10 mA cm−2) among the pure copper-based electrocatalysts reported. Cu2O–CuO@NF also produced a high current density (130.9 mA cm−2) at a relatively low applied potential (1.56 V) compared to other catalysts. The low Tafel slope (45.8 mV dec−1) and charge-transfer resistance indicated that good electronic coupling and fast charge transport led to the higher activity. In the HER, pure CuO@NF required a much lower overpotential of 165 mV to produce geometric current density. It is worth noting that CuO@NF showed strong HER activity in the alkaline medium. The Tafel slope of 121.81 mV dec−1 for CuO@NF suggests that the desorption process could be the rate-determining step in the HER process. Cu2O–CuO@NF and CuO@NF exhibited good stability in the alkaline medium. The highly enhanced current density and good stability of Cu2O–CuO@NF in 6.0 M KOH at 60 °C highlights its potential for practical applications. Overall, an earth-abundant copper-based electrocatalyst with excellent OER and HER activity in alkaline media has been developed by directly fabricating on a conducting substrate and controlling the copper oxides composition. [ABSTRACT FROM AUTHOR]

Smart fluorescent materials exhibit a controlled fluorescence response to different external stimuli such as pressure, heat, light, pH, etc. Molecular structure and supramolecular assembly via weak intermolecular interactions strongly influence the fluorescence efficiency and colour. In particular, non-planar molecular structures play a significant role in developing solid state fluorescent materials. Triphenylamine (TPA), a typical non-planar propeller molecule with interesting optoelectronic properties, provides an excellent opportunity for developing a variety of molecular fluorescent materials by taking advantage of synthetic feasibility. For example, a donor–acceptor (D–A) aggregation induced emissive (AIE) fluorophore can be developed by integrating an acceptor group into the phenyl unit of a TPA donor. In this review, we focus exclusively on TPA-based smart fluorescent materials that exhibit distinct and reversible fluorescence switching towards different stimuli. Molecular engineering of the TPA fluorophore resulted in the development of different types of stimuli-responsive materials and the conformational flexibility of non-planar phenyl groups often produced polymorphism induced fluorescence tuning. A detailed discussion of the structure–property relationships of TPA derivatives could provide insights for developing new TPA-based smart fluorescent materials with versatile properties. [ABSTRACT FROM AUTHOR]

Terpyridine (TP) integrated with tetraphenylethylene (TPE-TP), pyrene (Py-TP) and triphenylamine (TPA-TP) exhibited structure controlled molecular packing and distinct fluorescence in the solid state. TPE-TP exhibited infinite aromatic π⋯π stacking between terpyridine units, Py-TP produced an isolated pyrene dimer, and TPA-TP displayed helical assembly via weak intermolecular H-bonding in the crystal lattice. The different molecular assembly of TPE-TP, TPA-TP and Py-TP produced a locally excited (LE) state, charge transfer (CT) state and excimeric emission at 460, 520 and 568 nm, respectively, in the solid state. Interestingly, the different molecular packing of TPE-TP, Py-TP and TPA-TP produced varied mechanofluorochromism. TPA-TP exhibited on–off reversible fluorescence switching upon mechanical crushing and heating, whereas TPE-TP and Py-TP displayed switching between two fluorescence states but in opposite directions. Crushing caused red shift of emission in TPE-TP and blue shift in Py-TP. Both TPA-TP and Py-TP showed strong fluorescence in solution and crystalline states, but TPE-TP showed typical aggregation induced emission (AIE). Further, Py-TP and TPE-TP showed selective fluorescence sensing of picric acid in solution and solid matrix, respectively. TPA-TP exhibited sensing of nitroaromatics both in solution as well as in the solid state. Solid state fluorescence of TPA-TP and TPE-TP was further utilized for fabricating PVA polymer composite thin films and demonstrated reusable fluorescence sensing of nitroaromatics in aqueous medium. The PVA–fluorophore composite film exhibited clear fluorescence quenching upon immersing the thin film into 10−7 M picric acid. Thus, the present study demonstrates the role of molecular structure in developing stimuli-responsive fluorescence materials and reusable sensors. [ABSTRACT FROM AUTHOR]

Biochar (BC), prepared from the carbonization of neem wood, was modified with nickel ferrite (NiFe2O4) nanorod/poly(3,4-ethylenedioxythiophene) (PEDOT) composite and directly employed as a free-standing and binder-free anode in microbial fuel cells (MFCs). The morphological and X-ray diffraction analyses demonstrate that the cubic spinel-structured NiFe2O4 nanorods are homogeneously distributed over the sheet-like amorphous PEDOT. The bioelectrochemical activity of exoelectrogens is improved with PEDOT/NiFe2O4/BC as endorsed by the superior oxidation current in the voltammograms. Furthermore, the MFC with PEDOT/NiFe2O4/BC demonstrates a peak power of 1200 mW m−2 because of the unique catalytic nanoarchitectures, compact biofilm formation, continuous electron conduction paths, and elevated structural and chemical stabilities of PEDOT/NiFe2O4. The waste matter-derived electrode constituent, binder-free electrode modification, mediator-less electron transference, and considerable power generation associated with a proficient lifespan performance of PEDOT/NiFe2O4 open up a versatile avenue towards the development of cost-efficient, high-performance, and durable MFCs. [ABSTRACT FROM AUTHOR]

Metal organic framework (MOF) based electrocatalysts have received increased attention in recent years because of their tailorable structure, functionality and porosity. Herein, cobalt-terephthalic acid (CoTPA) based microcrystalline MOFs particles are synthesized in the presence of four different organic amines (DABCO (D), butylamine (B), hexylamine (H) and triethylamine (T)) and the electrocatalytic oxygen evolution reaction was explored. Powder X-ray diffraction (PXRD) analysis confirmed the formation of crystalline CoTPA MOFs with a similar structure but with different faceted orientation. Further, morphological studies revealed the formation of CoTPA MOFs microparticles with different morphologies depending on the amine utilized in the reaction. BET analysis confirmed the porosity with a different surface area and pore diameter. Thermogravimetric analysis indicated the water coordination in all four MOFs. X-ray photoelectron spectroscopy (XPS) analysis indicated the coexistence of Co2+ and Co3+ in the MOFs. All four MOFs exhibited strong OER activity in alkaline medium. CoTPA-D exhibited relatively higher activity and required an overpotential of 273 mV to produce the benchmark current density of 10 mA cm−2 compared to CoTPA-B (281 mV), CoTPA-H (287 mV) and CoTPA-T (278 mV). The low Tafel slope and charge transfer resistance indicated the faster reaction kinetics whereas the higher electrochemical surface area and double layer capacitance (Cdl) suggested the availability of more active sites in CoTPA-D MOFs. The post catalytic analysis suggested the conversion of CoMOFs into CoOOH during electrocatalysis, which resulted in the enhanced OER reaction. Thus, the present work reports the facile preparation of CoMOFs with varied porosity and functionality that resulted in different electrocatalytic OER activity. [ABSTRACT FROM AUTHOR]

Molecular structural tailoring is one of the versatile approaches for developing functional organic fluorescent molecules with desired properties for practical applications. In this manuscript, we report the synthesis of triphenylamine based boron complexes (TPA-BF2) with unusual thermofluorochromism and alkyl chain length controlled molecular fluorescent liquids/solids. TPA-BF2 complexes with ethyl, propyl, butyl and pentyl groups produced fluorescent solids whereas with hexyl, heptyl and octyl groups yielded fluorescent liquids. The longer-alkyl chain compounds significantly improved the fluorescence efficiency (quantum yield (Φf = 12.6%)) by inhibiting close arrangement of fluorophores. TPA-BF2-2 and TPA-BF2-3 showed very weak fluorescence (Φf = 1.9%). On the other hand, all complexes exhibited strong solvent polarity dependent tunable fluorescence in the solution state. Importantly, the ability to form multiple excited states (locally excited (LE), charge transfer (CT) and twisted intramolecular charge transfer (TICT) states) resulted in unusual fluorescence tuning and reversible thermofluorochromism. The conversion of CT emission at RT to LE state emission at 77 K (under liquid nitrogen) of TPA-BF2 complexes in CHCl3/toluene resulted in strong blue shifting, while warming towards RT, the TPA-BF2 CHCl3 solution showed unusual fluorescence tuning. The fluorescence λmax of TPA-BF2-2 was red shifted from 476 to 600 nm and then blue shifted to RT fluorescence (550 nm). Interestingly, TPA-BF2 complexes in solution and in the PMMA matrix showed clear phosphorescence at 77 K. The unusual fluorescence tuning while warming from 77 K is attributed to the phosphorescence emission coupled with fluorescence at room temperature. Notably, the fluorescence intensity at RT was relatively stronger compared to red shifted fluorescence intensity while warming. TPA-BF2 complexes in toluene showed strong linear enhancement of fluorescence intensity (1.9 to 2.5 times) with a blue shifted peak position upon increasing the temperature from 0 to 100 °C. The alkyl chain length exhibited a strong influence on the rate and extent of fluorescence tuning with temperature. Thus, alkyl group variations in simple Schiff base–boron complexes produced unusual thermofluorochromism and molecular fluorescent liquids. [ABSTRACT FROM AUTHOR]

Interlinked aggregation induced emissive (AIE) fluorophores (triphenylamine (TPA) donor (D) and indanedione acceptor (A)) via non-conjugated spacers (1–3) showed concentration-dependent tunable fluorescence from blue to deep red/NIR in solution and deep red emission in the solid-state. 1–3 exhibited weak broad solid-state fluorescence covering the blue to deep red region (433 to 706 nm, ΦF = 2.3 to 5.3%). It is noted that the non-interlinked TPA-indanedione fluorophore was non-emissive in solution but showed strong solid-state fluorescence at 611 nm (ΦF = 17.7%). In contrast, 1–3 showed strong fluorescence in the solution state (ΦF = 0.001 to 0.09 compared to fluorescein). Importantly, 1–3 exhibited concentration-dependent tunable fluorescence from blue (457 nm) to deep red/NIR (720 nm) in CHCl3 due to varied aggregation of fluorophores. Dynamic light scattering (DLS) studies confirmed the molecular aggregation even at very low concentrations (10−7 M). The increasing concentration lowered the solubility and produced bigger aggregates with wide size distribution that caused tunable fluorescence. The deep red fluorescence of 1 was utilized for live-cell imaging applications using E. coli cells. Fluorescence microscopic images revealed clear red fluorescent E. coli cells with a rod morphology. Furthermore, the non-toxic nature of 1 was confirmed by in vitro as well as in vivo studies. Thus, deep red/NIR emissive fluorophores were achieved by interlinking two AIEgens using a non-conjugated spacer and utilized for live cell imaging. [ABSTRACT FROM AUTHOR]

pH-responsive fluorescence molecules are interesting candidates for achieving tunable as well as white light emission from a single molecule. Herein, we have synthesized pyridine-integrated carbazole and triphenylamine donor–acceptor isomers, (Z)-3-(4-(9H-carbazol-9-yl)phenyl)-2-(pyridinyl)acrylonitrile (1–3) and (Z)-3-(4-(diphenylamino)phenyl)-2-(pyridinyl)acrylonitrile (4–6), and studied the effects of the structure and solvent on acid-responsive fluorescence modulation in the solution state. Both carbazole (1–3) and triphenylamine (4–6) isomers produced twisted intramolecular charge transfer (TICT) states and showed tunable fluorescence depending on the solvent polarity. Highly blue shifted fluorescence was observed in non-polar toluene, while polar DMF showed large red-shifted fluorescence. Further, 1–3 isomers exhibited relatively strong fluorescence compared to 4–6. Interestingly, 1–6 exhibited organic acid-dependent (trifluoroacetic acid (TFA) and p-toluene sulphonic acid (PTSA)) and solvent-dependent fluorescence tuning. Carbazole isomers (1–3) showed wide fluorescence colour tuning from blue to red as well as white light emission in toluene, whereas green to red transition was observed in CHCl3. Addition of TFA, a stronger acid, to 1 and 3 afforded blue, green and red emission along with white emission in toluene, whereas weak PTSA produced blue and red emission along with white emission. In contrast, addition of TFA or PTSA to the triphenylamine isomers (4–6) exhibited complete quenching of fluorescence in all solvents. The meta-isomers did not show significant fluorescence tuning with acids. The wide fluorescence tuning of 1 in toluene with TFA has also been successfully transformed into a PMMA polymer matrix, which also exhibited clear fluorescence tuning from blue to red. Density functional theoretical (DFT) studies indicated high molecular twisting in the excited state and clear separation of charge transfer from the donor carbazole to the acceptor unit compared to the propeller-shaped triphenylamine isomer; this may contribute to the wide fluorescence tuning in the carbazole isomers. The present studies could be useful for choosing appropriate molecular structures and solvent systems for achieving wide fluorescence tuning and white light emission from a single donor–acceptor molecule. [ABSTRACT FROM AUTHOR]

The current study demonstrates a facile one-pot pyrolysis route to develop photoluminescent graphitic carbon nitride nanosheets (g-C3N4) for the fluorescence detection of hydrazine. The morphological images show that MnO2 nanotubes are decorated over the surface of g-C3N4 nanosheets, and the addition of hydrazine completely topples the MnO2 nanotubes; this uncovers the g-C3N4 nanosheets. The turn-off fluorescence was achieved with the decoration of MnO2 nanostructures on the g-C3N4-nanosheets via the fluorescence resonance energy transfer mechanism. Morphological influences of MnO2 nanostructures on the dynamic quenching of the fluorescence behavior of g-C3N4 nanosheets were analyzed. The fluorescence energy transfer between g-C3N4 and MnO2 was restricted with the addition of hydrazine; this led to the recovery of the fluorescence behavior of g-C3N4. The proposed sensor exhibits high sensitivity, the wide linear range of 2–55 μM, and the low detection limit of 0.12 μM. The developed nanocomposite exhibits excellent selectivity towards hydrazine over common metal ions and molecules with analogous structures. Furthermore, the prepared nanocomposite exhibits good recovery in environmental water samples; this unveils its practical competence in real samples. Conspicuously, this study provides an excellent platform for the development of a competent fluorescent probe and dynamic opportunities for the effectual monitoring of hydrazine in environmental water samples. [ABSTRACT FROM AUTHOR]

Correction for 'Cobalt metal–organic framework microcrystalline particles with strong electrocatalytic activity: amine controlled morphology and OER activity' by Gunasekaran Arunkumar et al., New J. Chem., 2023, https://doi.org/10.1039/d3nj01457f. [ABSTRACT FROM AUTHOR]