Rechargeable zinc-air batteries (ZABs) with high-performance and stability is desirable for encouraging the transition of the technology from academia to industries. However, achieving this balance remains a formidable challenge, primarily due to the requirement of robust, earth-abundant reversible oxygen electrocatalyst. The present study introduces a simple strategy to synthesize Co-Nx rich nanoalloy with N-doped porous carbon tubes (NiCo@NPCTs). The optimized catalyst is bestowed with high electrochemical surface area, and three dimensional (3D) interwoven N-doped PCTs. Moreover, the presence of dual redox-active sites synergistically promotes rapid mass/charge transfer for oxygen electrocatalysis. These features offer excellent reversibility for oxygen electrocatalysis with a reversible oxygen potential gap (ΔE) of 0.74 V. The NiCo@NPCTs is utilized as an air-electrode for designing ZABs and using the same electrode-material asymmetric supercapacitor device (ASC) is fabricated. The assembled ZAB delivers an impressive peak power density of 298 mW cm−2 and specific capacity of 731mAh g−1 at 50 mA cm−2, along with high rate-capability, durable round-trip voltaic-efficiency. The as-fabricated ASC also shows exciting performance with negligible fading in capacitance and columbic efficiency after 10,000 continuous charge–discharge cycles at a 10 A/g current density. In addition, ZAB-ASC integrated device is assembled, showing real-time application. Thus, the synthesized electrode-material holds great promise for electrocatalysis and also for diverse energy storage applications.

Integration of different active sites by heterostructure engineering is pivotal to optimize the intrinsic activities of an oxygen electrocatalyst and much needed to enhance the performance of rechargeable Zn–air batteries (ZABs). Herein, a biphasic nanoarchitecture encased in in situ grown N-doped graphitic carbon (MnO/Co-NGC) with heterointerfacial sites are constructed. The density functional theory model reveals formation of lattice oxygen bridged heterostructure with pyridinic nitrogen atoms anchored Co species, which facilitate adsorption of oxygen intermediates. Consequently, the well-designed catalyst with accessible active sites, abundant oxygen vacant sites, and heterointerfacial coupling effects, simultaneously accelerate the electron/mass transfer and thus promotes the trifunctional electrocatalysis. The assembled aqueous ZAB delivers maximum power density of ≈268 mW cm−2 and a specific capacity of 797.8 mAh gzn −1 along with excellent rechargeability and extremely small voltage gap decay rate of 0.0007 V h−1. Further, the fabricated quasisolid-state ZAB owns a remarkable power density of 163 mW cm−2 and long cycle life, outperforming the benchmark air-electrode and many recent reports, underlining its robustness and suitability for practical utilization in diverse portable applications. transfer rates while simultaneously reducing the size and investment costs of industrial devices. In this comprehensive review

The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are considered to be the most important processes in metal–air batteries and regenerative fuel cell devices. Metal–organic polymers are attracting interest as promising precursors of advanced metal/carbon electrocatalysts because of their hierarchical porous structure along with the integrated metal–carbon framework. We developed carbon-coated CNTs with Ni/Fe and Cu/Fe as active sites. Experimental observations from X-ray photoelectron spectroscopy and X-ray absorption analysis suggest that C@CNT[Ni] outperforms C@CNT[Cu] in the ORR and OER, which is further supported by density functional theory calculations. C@CNT[Ni] exhibits a higher onset potential (0.99 V vs RHE) and a smaller Tafel slope (40.2 mV decade–1) compared to those of C@CNT/[Cu] in an alkaline electrolyte (0.94 V vs RHE and 46.5 mV decade–1, respectively). Such circumstances are attributed to the alloying effect between Ni and Fe in C@CNT[Ni], in contrast to the existing copper iron oxide phase in C@CNT/[Cu]. It is noteworthy that C@CNT[Ni] also displayed an improved OER, demanding its bifunctional property. As a proof of concept, C@CNT[Ni] was utilized in zinc–air batteries, which shows a high energy efficiency of ∼60%, a small charge–discharge voltage gap of 0.78 V, and excellent cycling performance (∼120 h) at 5 mA cm–2 and 25 °C. This protocol expands the utility of novel metal–organic hyper-cross-linked polymer-derived bimetallic electrocatalysts for clean energy research.

Electrochemical water-splitting in alkaline medium has gained massive attention for generating large-scale renewable hydrogen. Yet, it encounters challenges such as poor stability and high voltage at higher current density, particularly for insufficient electron transport kinetics. Therefore, the practical application of water electrolysis, a resilient electrocatalyst with superior efficiency and maximum metal utilization, is essential. In this research, a rational design of quadruple-phase interface-derived noble-metal-unbounded hierarchical 3D-interconnected multi-layered heterostructure CoCu-LDH@FeNi2S4–FeNiS2@CoNi2S4/NF as a self-sacrificed highly efficient electrode for affordable green H2 production through electrochemical water splitting in alkaline electrolyte is discussed. The resultant bifunctional electrode was synthesized through a controllable two-step hydrothermal approach. The hybrid electrocatalyst exhibited outstanding electrocatalytic performance towards oxygen evolution reaction (η10 ∼240 mV) and hydrogen evolution reaction (η10 ∼87 mV) to achieve a current density of 10 mA cm−2 with long-term stability. Impressively, the alkaline water electrolyzer delivered a cell voltage of 1.56 V@10 mA cm−2 and remarkable stability due to the significant synergistic interfacial effect among the different phases, high electrical conductivity, rich exposed active sites with optimized free energy of chemisorbed reaction intermediates, high intrinsic activity, and numerous open channels for ion diffusion with mass transport at the interface. This research strategy provided insight into designing non-noble transition metal-based electrocatalysts by engineering interfacial active sites toward industrial-scale green hydrogen production.

Effective integration of multiple active moieties and strategic engineering of coordinated interfacial junctions are crucial for optimizing the reaction kinetics and intrinsic activities of heterogeneous electrocatalysts. Herein, a simple integrated heterostructure of biphasic Co0.7Fe0.3/Fe3C embedded on in situ grown N-doped carbon sheets is constructed. Rationally designed CoFe/Fe3C-T2 owns more accessible active sites and interfacial junction effects, cooperatively boosting the electron and mass transfer, needed for multifunctional electrocatalysis. Leveraging the synergistic effect of dual active sites, CoFe/Fe3C-T2 demonstrates outstanding oxygen electrocatalytic activity in alkaline medium with an ultra-low potential gap of 0.58 V, surpassing the recently available state-of-the-art catalysts. Moreover, CoFe/Fe3C-T2 air-electrode achieves a high peak power density of 249 mW cm−2, a large specific capacity of 808 mAh g−1 and excellent cycling stability for aqueous Zn-air batteries. Remarkably, the solid-state flexible ZAB also exhibits satisfactory performance, showcasing an open-circuit voltage of 1.43 V and a peak power density of 66 mW cm−2. These outstanding results push this catalyst to the top of the list of non-noble metal-based electrode materials. This work offers a viable method for using the active-site-uniting strategy to create double-active-site catalysts, which may find real-time applications in energy conversion/storage devices.

Herein, a self-supported, robust, and noble-metal-free 3D hierarchical interface-rich Fe-doped Co-LDH@MoS2-Ni3S2/NF heterostructure electrocatalyst has been prepared through a controllable two-step hydrothermal process. The resultant electrode shows low overpotential of ~95 mV for hydrogen evolution reaction (HER), ~220 mV for the oxygen evolution reaction (OER), and the two-electrode system requires only a cell voltage of ~1.54 V at 10 mA cm−2 current density, respectively. Extensive ab initio calculations were carried out to find out the overpotential for HER, orbital interaction through the determination of electron density of states and quantification of charge transfer by Bader charge analysis. The computed overpotential matched closely with the experimental data. The superior HER performance of the tri-layer is enhanced due to the charge transfer (1.7444 e) to Fe-doped Co-LDH from Ni3S2-MoS2 hybrid. This research strategy paves an effective pathway for affordable green H2 production and future efficient non-precious bifunctional electrocatalyst design for overall water electrolysis.

The development of efficient and reliable trifunctional electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) is a crucial but extremely challenging endeavor. In this study, a simple temperature assisted carbohydrate combustion approach was utilized to fabricate 3D-hierarchical fluffy Co3O4/IrO2 hybrid nanostructure. To investigate the catalytic behavior, the catalyst was synthesized at different calcination temperatures, among which the catalyst synthesized at 550 °C exhibits superior trifunctional electrocatalytic activity. The as-synthesized hierarchical 3D-Co3O4/IrO2 fluffy structure possesses impressively high ORR performance (Eonset = 1.02 V, E1/2= 0.84 V) in 0.1 M KOH as well as displays decent HER and OER performance requiring overpotential of 143 mV and 352 mV respectively, to achieve the benchmark current density (j = 10 mA cm–2) in 1 M KOH. Moreover, the catalyst shows very small bifunctional index value (ΔE = 0.74 V), depicting excellent oxygen electrocatalysis. This multifunctional activity is attributed to the synergistic coupling between spinel/oxide structure and distinctive fluffy nanostructure, which offers a sea of active sites and passageways for the electrocatalytic process.

Rechargeable aqueous Zn-ion batteries (ZiBs) have attracted extensive attention in the field of large-scale electrical grid energy storage owing to their high level of safety, high volumetric energy density, and low cost. The usage of water as solvent facilitates the intrinsic properties. However, the battery performance is impeded by the narrow electrochemical stability window of the aqueous electrolyte, sluggish Zn2+-ion kinetics association with free water molecules, and undesired side reaction with cathode and anode materials. The viability of practical ZiBs largely depends on suitable electrolyte formulation. This review aims to provide a comprehensive understanding on the electrolyte formulation in association with basic characteristics, electrode/electrolyte interface mechanics, and their optimization strategies. The choice of suitable electrolyte salt, solvent, and other parameters related to the electrolyte along with their electrochemical performances are discussed. Finally, advanced strategies to modulate the suitable electrolyte and future perspectives are discussed to mitigate the issues for advanced ZiBs development.

Development of cost-effective and environmental friendly energy storage devices (ESDs) has attracted widespread attention in recent scenario of energy research. Recently, the environmentally viable “water-in-salt” (WiS) electrolytes has received significant interest for the development of advanced high performance ESDs. The WiS electrolyte exhibits wide electrochemical stability window (ESW), high-safety, non-flammability and superior electrochemical performance compared to the conventional “salt-in-water” electrolytes. This review aims to provide a comprehensive discussion on WiS electrolyte based on theoretical, electrochemical and physicochemical characteristics. A strategic way for the usage of WiS electrolyte in rechargeable metal-ion batteries and supercapacitors with potentially improved electrochemical performance has been reviewed systematically. This review also discussed the unique advantages of WiS electrolytes as well as the future scope and challenges.

Metal-air batteries, especially, rechargeable zinc–air batteries (ZABs) have recently rejuvenated extensive research attention as a promising sustainable energy technology, owing to its environment friendliness, low manufacturing cost, and high theoretical specific energy density. However, the real-time application of ZABs is yet to be achieved and it is mainly hindered due to the sluggish kinetics of oxygen-involved reactions. The two fundamental electrode reactions, specifically, oxygen reduction reaction and oxygen evolution reaction reinforce discharging and charging processes of ZABs. Therefore, uninterrupted research endeavours in developing novel design strategies is the crucial step to realize effective bifunctional electrocatalysts for oxygen electrocatalysis, which will rapid-up further progress of ZABs for commercialization. Recently, single atom catalysts (SACs) supported on various carbon scaffolds with maximized atom-utilization efficiency, unique metal coordination environments, abundant anchoring sites with exceptional tunability, ordered porosity, and selectivity have emerged as potential candidates for motivating oxygen electrocatalysis. Significant advances have been accomplished in designing SACs with commendable oxygen electrocatalytic activity, still great challenges need to be surmounted in order to make them viable for electrochemical energy conversion and storage devices. In this review, we have provided an overview of synthesis strategies, oxygen electrocatalytic performances, and identification of carbon-supported SACs. We have critically examined the role of coordination environment, engineering of isolated reactive centers, tailoring of the metal active centers, and modulation of the configuration of carbon substrates on the oxygen electrocatalytic activity. Finally, we concluded by highlighting the existing challenges and future research directions for further innovation of carbon-supported SACs for ZAB applications.

Electrochemical energy storage devices with stable performance, high power output, and energy density are urgently needed to meet the global energy demand. Among the different electrochemical energy storage devices, batteries have become the most promising energy technologies and ranked as a highly investigated research subject. Recently, metal–air batteries especially Zn–air batteries (ZABs) have attracted enormous scientific interest in the electrochemical community due to their ease of operation, sustainability, environmental friendliness, and high efficiency. The oxygen electrocatalytic reactions [oxygen reduction reaction (ORR) and oxygen evolution reaction (OER)] are the two fundamental reactions for the development of ZABs. Noble metal-based electrocatalysts are widely considered as the benchmark for oxygen electrocatalysis, but their practical application in rechargeable ZAB is hindered due to several shortcomings. Thus, to replace noble metal-based catalysts, a wide range of transition-metal-based materials and heteroatom-doped metal-free carbon materials has been extensively investigated as oxygen electrocatalysts for ZABs. Recently, metal–organic frameworks (MOFs) with unique structural flexibility and uniformly dispersed active sites have become attractive precursors for the synthesis of a large variety of advanced functional materials. Herein, we summarize the recent progress of MOF-derived oxygen electrocatalysts (MOF-derived carbon nanomaterials, MOF-derived alloys/nanoparticles, and MOF-derived single-atom electrocatalysts) for ZABs. Specifically, we highlight MOF-derived single-atom electrocatalysts owing to the wide exploration of these emerging materials in electrocatalysis. The influence of the active sites, structural/compositional design, and porosity of MOF-derived advanced materials on the oxygen electrocatalytic performances is also discussed. Finally, the existing challenges and prospects of MOF-derived electrocatalysts in ZABs are briefly highlighted.

Industrial alkaline water electrolysis is facing major difficulties in developing inexpensive, abundant and active bifunctional non-noble metal electrocatalysts. Herein, a heterogeneous three-dimensional (3D) FeNi2S4@Mo-doped Ni3S2/NF heterostructure electrocatalyst is synthesized by a controlled two-step hydrothermal method. Based on the unique structural advantages, the number of redox active sites and the conductivity of the heterostructure electrode are improved remarkably resulting the shortening of ion diffusion path, effective penetration of the electrolyte and decrease in equivalent series resistance. The electrode achieves 10 mA cm−2 current density in hydrogen evolution reaction and oxygen evolution reaction at ∼121 and 150 mV overpotential, respectively in alkaline solution. The bifunctional electrochemical performance of the heterostructure is confirmed from the low cell voltage of 1.501 V and higher stability in overall water electrolysis. This work provides a useful strategy to tune the electrocatalytic performance by doping engineering and hetero-interface formation that offers an efficient self-supported 3D heterostructure electrode material for overall water electrolysis.

Aqueous rechargeable Zn-ion batteries (ZiBs) are receiving increasing attention worldwide owing to their inherent safety and cost effectiveness. However, ZiBs are still struggling with rapid performance degradation caused by poor Zn anode reversibility and cathode dissolution into aqueous electrolytes. Inspired by the knowledge of industrial-scale Zn electroplating, a cyanide functional group containing a succinonitrile (SN) neutral ligand-based hydrated eutectic electrolyte was used to mitigate these issues. The ligand-oriented SN partially replaced free water molecules from the Zn2+ ion primary solvation sheath, resulting in delayed oxidation and a smaller Zn2+ ion desolvation energy barrier which promoted uniform Zn nucleation. Moreover, the MoO3@Mn3O4 cathode and Zn anode-based ZiB in an eutectic hydrated electrolyte with a 10:10 molar ratio of ZnCl2 and SN exhibited an ∼2.3 V working potential window which delivered a maximum of ∼476 mAh g–1 specific capacity and ∼232.2 Wh kg–1 energy density at a 0.2 A g–1 current density. The fabricated device exhibited ∼79.56% specific capacity retention after 5000 cycles at a 10 A g–1 current density. The coinsertion/extraction of H+ and Zn2+ ions and the Zn deposition/dissolution mechanism of the optimized hydrated eutectic electrolyte-based ZiBs are investigated by ex situ physicochemical and electrochemical studies. Overall, this work provides a new path on exploring green electrolytes and layered-structure materials for the development of high-performance ZiBs.

Conventional rechargeable aqueous Zn-ion batteries (ZiBs) store charges through the ion insertion/extraction process involving the cationic redox conversion mechanism. However, the ZiB based on the cationic redox conversion mechanism provides limited specific capacity and energy density, which can be enhanced by triggering the oxygen redox chemistry (ORC). Therefore, the ORC has received significant attention from the perspective of innovative material design and electrolyte formulation. Only a few reports are available on the ORC process in ZiBs, but the in-depth understanding of the process is still unclear. A layered-type V2P2O9 cathode material is proposed for enhancing the ZiB performance, which exhibited the ORC process at a high working potential window along with cationic redox conversion in the 1 M Zn(OTf)2 + 21 M LiTFSI “water-in-salt” (WiS) electrolyte. The ORC process not only enhanced the capacity performance but also improved the electrochemical stability window up to ∼3.0 V. The ORC and cationic redox conversion processes were investigated through several ex situ and in situ techniques by 18O isotope labeling. The in situ Raman spectra by 18O isotope labeling and ex situ XPS spectra confirmed the ORC pathway demonstrating the 2O2– ↔ O2n– redox conversion. This finding opens up an opportunity to design a suitable cathode material and electrolyte formulation for high-performance ZiBs.

Noble metal-based materials are enormously used as a cathode material for electrocatalytic oxygen reduction reaction (ORR), which plays an important role in determining the performance of energy conversion and storage devices such as fuel cells, metal-air battery, and so on. The practicability of these energy devices is mainly related to the cost of the cathodic ORR electrocatalyst. Hence, a cost-effective and environmentally benign approach is highly demanding to design the electrocatalyst for ORR and replacing noble metal-based electrocatalyst. In this regard, biomass-derived hierarchically porous carbon-based materials have become attractive options compared to metal-based electrocatalysts due to their several advantages such as abundance in nature, economic viability, characteristic sustainability, environmental friendliness, and excellent physicochemical properties. Moreover, harsh chemicals are not being involved during their synthesis, and they intrinsically possess a variety of heteroatoms (N, P, S, etc.), which are key for augmenting the electrocatalytic activity. In the present review article, the recent progress on biomass-derived cathode electrocatalysts has been summarized for ORR including a brief account of bioresource selection, synthesis methods, and processing criteria that greatly influences the electrocatalytic activity.

Designing effective multifunctional electrocatalysts with robust activity and durability is crucial for developing different electrochemical energy storage/conversion devices. In this study, we established a general approach to synthesize a multilayer N-doped graphitic carbon nanotube (CNT)-encapsulated NiCo alloy, which exhibits robust trifunctional electrocatalytic activity toward three fundamental electrochemical reactions: oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). Remarkably, the synthesized NiCo alloy with melamine as the N-dopant (NiCo-M) exhibits admirable trifunctional activity with a low overpotential of 109 mV for HER and 295 mV for OER to achieve the benchmark current density of 10 mA cm–2 as well as superior ORR performance (onset: 0.96 V; E1/2: 0.856 V) in alkaline medium. More importantly, the NiCo-M electrocatalyst possesses excellent oxygen electrocatalytic activity with a small potential difference (ΔE) of 0.669 V, which is very close to that of the state-of-the-art Pt/C–RuO2 (0.651 V) couple. Such overwhelming performances stem from the presence of sufficient active sites and the formation of a unique N-doped graphitic CNT-like structure, which encourages electronic structure modulation as well as synergism for improved trifunctional electrocatalytic activities. Moreover, the catalyst shows outstanding electrochemical stability with a negligible decay of overpotential or E1/2 values after exhaustive CV cycles for HER, OER, and ORR. This is attributed to the encapsulation of the NiCo alloy into a N-doped multilayer graphitic carbon shell. The present study will offer a general approach to rationally synthesize robust trifunctional electrocatalysts required for fabricating energy-efficient electrochemical devices.

Development of additive-free electrode material on conductive substrates is a cost-effective strategy and is essential for the fabrication of high-performance supercapacitor. Herein, the effect of transition metal ions on the preparation of bimetallic (Ni & Mn)/reduced graphene oxide (RGO) composites through multi-step electrodeposition technique was extensively analysed. Two different composites; nickel manganese oxide/RGO (NiMn-G/NF) and manganese nickel oxyhydroxide/RGO (MnNi-G/NF) were synthesized through step-wise electrodeposition technique. The NiMn-G/NF and MnNi-G/NF were prepared through interchanging the source of host transition metal ions (Ni and Mn). Various physicochemical characterization techniques confirmed that the crystal structure as well as the morphology and electrochemical performance of the composites were largely dependent on the host metal ions. The formation of surface functional groups may play a key role in the electrochemical performance of supercapacitors. The MnNi-G/NF electrode showed high specific capacitance of ~1525 F g−1 at 1.5 A g−1 current density compared to NiMn-G/NF (~1312.5 F g−1) and other synthesized electrode materials. An asymmetric supercapacitor (ASC) device was fabricated using MnNi-G/NF as positive and thermally-RGO as negative electrode materials. The fabricated device exhibited highest specific capacitance of ~173.3 F g−1 at 2 A g−1 current density and maximum energy density of ~54.1 Wh kg−1 and power density of 9 kW kg−1.

Detection of toxic hexavalent chromium in soil, groundwater, industrial effluent, etc., is of significant interest. We demonstrate the electrochemical detection of Cr(VI) using a surface-engineered mesoporous carbon-based material. The mesoporous carbon-based material is obtained by the controlled pyrolysis of a homogenous mixture of Fe and Co complexes of hydrolyzed collagen. The as-synthesized material is subjected to surface engineering by acid treatment. The surface-engineered mesoporous carbon-based material has a surface area as large as 443.28 m2 g−1 with interconnected mesopores. Detection of Cr(VI) was achieved at parts per billion level by electrochemical reduction using the surface-engineered carbon-based electrode at the potential of 0.65 V (Ag/AgCl). The modified electrode has excellent sensitivity (7.75 ± 0.03 × 10−4 µA/ppb) and selectivity, low detection limit (8 ppb), and wide linear range (40–800 ppb). The coexisting other metal ions do not interfere with the amperometric measurement of Cr(VI). The electrode is highly stable and it retains >65% of the initial current during the long-term durability test for an hour. The highly porous nature of the material favors facile mass transport and facilitate electron transfer kinetics. The trace amount of CoFe alloy present in the carbon-based material catalyses the reduction of Cr(VI) at a favourable potential.

The electrochemical reduction of oxygen to water and the evolution of oxygen from water are two important electrode reactions extensively studied for the development of electrochemical energy conversion and storage technologies based on oxygen electrocatalysis. The development of an inexpensive, highly active, and durable nonprecious-metal-based oxygen electrocatalyst is indispensable for emerging energy technologies, including anion exchange membrane fuel cells, metal-air batteries (MABs), water electrolyzers, etc. The activity of an oxygen electrocatalyst largely decides the overall energy storage performance of these devices. Although the catalytic activities of Pt and Ru/Ir-based catalysts toward an oxygen reduction reaction (ORR) and an oxygen evolution reaction (OER) are known, the high cost and lack of durability limit their extensive use for practical applications. This review article highlights the oxygen electrocatalytic activity of the emerging non-Pt and non-Ru/Ir oxygen electrocatalysts including transition-metal-based random alloys, intermetallics, metal-coordinated nitrogen-doped carbon (M–N–C), and transition metal phosphides, nitrides, etc., for the development of an air-breathing electrode for aqueous primary and secondary zinc-air batteries (ZABs). Rational surface and chemical engineering of these electrocatalysts is required to achieve the desired oxygen electrocatalytic activity. The surface engineering increases the number of active sites, whereas the chemical engineering enhances the intrinsic activity of the catalyst. The encapsulation or integration of the active catalyst with undoped or heteroatom-doped carbon nanostructures affords an enhanced durability to the active catalyst. In many cases, the synergistic effect between the heteroatom-doped carbon matrix and the active catalyst plays an important role in controlling the catalytic activity. The ORR activity of these catalysts is evaluated in terms of onset potential, number of electrons transferred, limiting current density, and durability. The bifunctional oxygen electrocatalytic activity and ZAB performance, on the other hand, are measured in terms of potential gap between the ORR and OER, ΔE = Ej10OER – E1/2ORR, specific capacity, peak power density, open circuit voltage, voltaic efficiency, and charge–discharge cycling stability. The nonprecious metal electrocatalyst-based ZABs are very promising and they deliver high power density, specific capacity, and round-trip efficiency. The active site for oxygen electrocatalysis and challenges associated with carbon support is briefly addressed. Despite the considerable progress made with the emerging electrocatalysts in recent years, several issues are yet to be addressed to achieve the commercial potential of rechargeable ZAB for practical applications.

The synthesis of nonprecious electrocatalysts for oxygen electrocatalysis is of considerable interest for the development of electrochemical energy devices. Herein, we demonstrate a facile approach for the synthesis of bamboo-like nitrogen-doped carbon nanotube-encapsulated Co0.25Ni0.75 alloy electrocatalyst (Co0.25Ni0.75@NCNT) and its bifunctional oxygen electrocatalytic performance toward oxygen reduction and oxygen evolution reactions. The Co0.25Ni0.75 alloy wrapped with NCNT is obtained by a one-step carbothermal reduction approach using dicyandiamide and NiCo-MOF precursors. Dicyandiamide acts as a nitrogen source, and the in situ generated Co0.25Ni0.75 alloy nanoparticles catalyze the growth of bamboo-like NCNTs. The hollow NiCo-MOF plays a sacrificial role in providing a suitable environment for the controlled growth of Co0.25Ni0.75 alloy and NCNT. Co0.25Ni0.75@NCNT efficiently catalyzes both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) at a favorable overpotential. It shows a low potential gap (ΔE) of ∼0.8 V between the two reactions, and it qualifies for the development of air cathode in metal–air batteries. The enhanced bifunctional activity and excellent durability stem from the chemical composition and the synergistic effect between Co0.25Ni0.75 alloy and encapsulating NCNT. The original phase and morphology of the catalyst is preserved after an extensive durability test. Aqueous rechargeable Zn–air battery (ZAB) is fabricated using a Co0.25Ni0.75@NCNT-based air cathode. The battery has high open-circuit voltage (1.53 V) and a maximum peak power density of 167 mW cm–2 with only 1.6% loss in the voltaic efficiency after 36 h charge–discharge cycles. As a proof-of-concept demonstration, the as-fabricated ZAB is successfully used for the electrochemical water splitting in alkaline solution.

Carbon dots (CDs) are a new class in carbonaceous family and have instigated remarkable research interests over the past decade both from fundamental and technological point of view due to its astonishing photoluminescence (PL) property, though the underlying PL mechanisms for CD are strongly disputed. Herein, we have synthesized nitrogen functionalized CDs (N-CDs) utilizing a facile and one-step hydrothermal approach. The synthesized CDs exhibit excellent solubility in a series of organic solvents and we have extensively investigated the CD-solvent interactions to understand the solvatochromic behavior of CDs which have hardly been studied. Our CDs show excitation wavelength and solvent dependent PL across nearly the entire visible spectrum without compromising the PL quantum yield (CD shows high quantum yield for both blue and red region in some selected solvents). The origin and spectral shift of PL property in different solvents are also thoroughly studied. These observations suggest that the hydrogen bonding between N-CDs and protic solvents is the primary driving force in controlling the PL in the system whereas both dipolar interaction and hydrogen bond acceptance basicity (β) of aprotic.

The production of hydrogen and oxygen via water electrolysis has become a sustainable and encouraging pathway for the establishment of new energy sources. Herein, we report the successful growth of hierarchical NiCo2O4-carbon dots (CDs) nanoneedle arrays supported on nickel foam through a simple and environmentally benign hydrothermal self-assembly technique. The designed material acts as a binder free electrode and shows bifunctional electrocatalytic activity for both hydrogen evolution reaction (HER) as well as oxygen evolution reaction (OER) in alkaline medium. An electrocatalyst sample with an optimal loading of CDs (25 mg) requires a low overpotential of 146 mV to achieve a current density of 10 mA/cm2 for the HER in an alkaline medium, whereas it requires an overpotential of 390 mV to achieve a current density of 50 mA/cm2 for the OER in the same alkaline medium. The excellent electrocatalytic activities of the sample with loading of CD can be ascribed due to the presence of large number of exposed active sites offered by CD/NiCo2O4 and the enhanced electron transfer processes occurring as a result of hierarchical structure composed of three-dimensional nickel foam and the NiCo2O4−CDs nanoneedle arrays. Thus, the synthesis method introduced in this present work is a facile and cost-effective approach for the construction of bifunctional electrocatalysts with high reactivity and excellent durability.

Electrochemical water splitting is known as a potential approach for sustainable energy conversion; it produces H2 fuel by utilizing transition metal-based catalysts. We report a facile synthesis of FeCo2O4@carbon dots (CDs) nanoflowers supported on nickel foam through a hydrothermal technique in the absence of organic solvents and an inert environment. The synthesized material with a judicious choice of CDs shows superior performance in hydrogen and oxygen evolution reactions (HER and OER) compared to the FeCo2O4 electrode alone in alkaline media. For HER, the overpotential of 205 mV was able to produce current densities of up to 10 mA cm−2, whereas an overpotential of 393 mV was needed to obtain a current density of up to 50 mA cm−2 for OER. The synergistic effect between CDs and FeCo2O4 accounts for the excellent electrocatalytic activity, since CDs offer exposed active sites and subsequently promote the electrochemical reaction by enhancing the electron transfer processes. Hence, this procedure offers an effective approach for constructing metal oxide-integrated CDs as a catalytic support system to improve the performance of electrochemical water splitting.

Heavy metal pollution is a potential threat because it exerts severe harmful effects on the environment and human health. Hence, the rational design and fabrication of fluorescent probes for the simple, selective, and sensitive detection of heavy metal ions are of great significance. In this article, we have reported an environmentally benign, green and cost-effective approach for the synthesis of red luminescent gold nanoclusters (AuNCs) using wheat flour as the stabilizing and capping agent. The resultant AuNCs have been characterized by several spectroscopic and microscopic techniques. We have achieved a high quantum yield (9.02%) for the red fluorescent AuNCs, with a maximum emission wavelength of B640 nm under 370 nm excitation. We have successfully applied the synthesized AuNCs for the nanomolar detection of Hg2+ in an aqueous medium via selective fluorescence quenching of the AuNCs in the presence of several other metal ions. We have attained a limit of detection (LOD) as low as 7 nM for Hg2+ and the selectivity of detection is attributed to the specific interaction between the Hg2+ ions and the Au+ ions present in the AuNCs.

Exploration of proficient electrocatalyst from earth-abundant nonprecious metals in lieu of noble metal-based catalysts to obtain clean hydrogen energy through large-scale electrochemical water splitting is still an ongoing challenge. Herein, iron-doped nickel cobalt phosphide nanoplate arrays grown on a carbon cloth (NiCoFexP/CC) are fabricated using a simple hydrothermal route, followed by phosphorization. The electrochemical analysis demonstrates that the NiCoFexP/CC electrode possesses high electrocatalytic activity for water splitting in alkaline medium. Benefits from the synergistic effect between the metal centers, two-dimensional porous nanoplates, and unique three-dimensional electrode configuration of NiCoFexP/CC provide small overpotentials of 39 at 10 mA cm–2 and 275 mV at 50 mA cm–2 to drive the hydrogen evolution reaction and oxygen evolution reaction, respectively. Furthermore, the assembled two-electrode (NiCoFexP/CC∥NiCoFexP/CC) alkaline water electrolyzer can achieve 10 mA cm–2 current density at 1.51 V. Remarkably, it can maintain stable electrolysis over 150 h. The excellent activity and stability of this catalyst is proved to be a economical substitute of commercial noble metal-based catalysts in technologies relevant to renewable energy.

Luminescent nanomaterials are encouraging scaffolds for diverse applications such as chemical sensors and biosensors, imaging, drug delivery, diagnostics, catalysis, energy, photonics, medicine, and so on. Carbon dots (CDs) are a new class of luminescent carbonaceous nanomaterial that have appeared recently and reaped tremendous scientific interest. Herein, we have exploited a simple approach to prepare tuneable and highly fluorescent CDs via surface functionalization. The successful synthesis of CDs is manifested from several investigations like high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The CDs exhibit excellent water solubility and with increasing nitrogen content fluorescence quantum yield increases whereas cell toxicity decreases. The CD synthesized at high temperature (180 °C) shows very high quantum yield (more than 56%). The tuneable optical properties of CDs are systematically studied using UV-vis and fluorescence spectroscopy. The cell viability evaluation and in vitro imaging study reveals that the synthesized CDs can be employed as a potential fluorescent probe for bio-imaging without further modification.

Developing binder-free, low-cost, and efficient electrocatalysts for water splitting is very important to meet the ever-increasing global energy demands. We have judiciously designed a polyaniline (PANI)-mediated protocol for the synthesis of nickel–cobalt nitride (NCN) heterostructures on carbon cloth (CC) to be applied as catalysts for full electrochemical splitting of water. Controlled pyrolyzation of the nickel–cobalt precursor on PANI-coated CC generates assembled grass-like nanostructures of cobalt nitride (CoN) and nickel nitride (Ni3N) along with beneficial, conductive nitrogen-doped carbon layers on CC for improved electrochemical activity. The generation of numerous catalytically active centers with expeditious charge and mass transportation due to the incorporated nickel and high mechanical stability owing to the self-supporting nature of the designed material result in excellent and stable electrocatalytic performance. The designed NCN/CC electrode requires low overpotentials (η10) of 247 and 68 mV to attain a current density of 10 mA cm−2 during oxygen evolution and hydrogen evolution reactions, respectively, with appreciable stability (>90% retention of the initial current density) over a 24 h long electrochemical test in 1.0 M KOH. Finally, the NCN/CC electrocatalyst is utilized to demonstrate full alkaline water splitting at a cell voltage of 1.56 V to deliver the current density of 10 mA cm−2 with tremendous stability over 240 h. Moreover, NCN/CC could afford a stable current density of 10 mA cm−2 towards full water splitting at 1.59 V cell voltage in acidic electrolyte with 100 h long-term stability. These results suggest its prospects as a substitute for expensive noble-metal-based water splitting electrocatalysts in practical applications.

Layered materials provide good electrochemical performance, but insufficient rate capability, which is the main issue in energy storage. Herein, we propose a facile synthesis of Co2(CO3)(OH)2 nanoflakes and polyhedron flowers supported on Ni foam, a novel binder-free electrode. Power law revealed that Co2(CO3)(OH)2 stores charge by a battery-type mechanism at the peak potential. The nanoflakes store more internal surface charge than the polyhedron-flower, which was confirmed via Trasatti plot. Benefiting from amorphous nanostructure, unique morphology and high surface area, the nanoflakes shows good performance. The areal capacitance (2111 mF cm−2), rate capability (80%), and energy density (0.152 mWh cm−2) are comparable to recent reports. The results suggest that the amorphous Co2(CO3)(OH)2 nanoflakes are a suitable cathode candidate for the supercapattery. The assembled supercapattery (ASC) provides high specific capacitance (91 F g−1), high energy density (26.22 Wh kg−1 at power density 828 W kg−1), and long cycle life (specific capacitance retention of 85% over 4000 cycles). The ASC device shows good potential in the field of energy storage devices.

One-dimensional (1D) nanostructure exhibits excellent electrochemical performance because of their unique physico-chemical properties like fast electron transfer, good rate capability, and cyclic stability. In the present study, Co3(PO4)2 1D nanograsses are grown on Ni foam using a simple and eco-friendly hydrothermal technique with different reaction times. The open space with uniform nanograsses displays a high areal capacitance, rate capability, energy density, and cyclic stability due to the nanostructure enhancing fast ion and material interactions. Ex-situ microscope images confirm the dependence of structural stability on the reaction time, and the nanograsses promoted ion interaction through material. Further, the reproducibility of the electrochemical performance confirms the binder-free Co3(PO4)2 1D nanograsses to be a suitable high-performance cathode material for application to hybrid supercapacitor. Finally, the assembled hybrid supercapacitor exhibits a high energy density (26.66 Wh kg−1 at 750 W kg−1) and longer lifetimes (80% retained capacitance after 6000 cycles). Our results suggests that the Co3(PO4)2 1D nanograss design have a great promise for application to hybrid supercapacitor.

Nucleic acid based fabrication of nanomaterials has fascinated scientists since the past two decades and exciting challenges have been surmounted. Recently, nucleic acid is successfully combined with other nanometre-scale entities, sometimes by modifying with chemical functional groups, to obtain a wide range of nanomaterials which in certain cases have been characterized with atomic level precision. These nanomaterials are highly focused due to their new physico-chemical properties, which confer several advantages in multi-disciplinary field of research leading to advanced technologies. This review highlights the systematic advances in the synthesis, properties (optical and electronic) and versatile applications of nucleic acid based nano-biomaterials produced from polymer and metal or semiconductor nanoparticles.

Recent advances in the development of two-dimensional transition-metal chalcogenides (2D TMCs) have opened up new avenues for supercapacitor applications. However, they still suffer from limited specific capacitance and poor rate capability due to their poor interfacial properties and simple geometry. Here, we propose a facile strategy for the synthesis of yolk–shell NiGa2S4 microspheres comprising crumpled nanosheets supported on nickel foam. The robust structure not only highly facilitates the electron and charge transportation but also efficiently alleviates the volume expansion during redox reactions, contributing to excellent electrochemical behaviors in terms of specific capacitance and rate capability. Significantly, an asymmetric supercapacitor based on the prepared NiGa2S4 as the positive electrode and N,S-codoped graphene/Fe2O3 (N,S-G/Fe2O3) as the negative electrode delivers a high energy density of 43.6 W h kg−1 at a power density of 961 W kg−1 and retains an energy density of 22.2 W h kg−1 even at 15[thin space (1/6-em)]974 W kg−1. These impressive results may provide a new perspective to develop high energy and power density storage systems for practical applications.

NiO has been intensively studied as a promising electrode material for supercapacitors because of its high theoretical specific capacitance, well-defined redox behavior, and good chemical compatibility with nickel foam. However, it still suffers from inferior rate capability and cycling stability because of the simple component and random structural integration. Herein, we report a tunable sulfuration process of NiO nanosheets constructed on porous nickel foam for supercapacitor applications. The resulting NiO/Ni3S2 with distinct structural features exhibits an ultra-high specific capacitance of 2153 F g−1 at a current density of 1 A g−1, and the capacitance is retained at 1169 F g−1 even at a current density as high as 30 A g−1. An asymmetric supercapacitor device fabricated with NiO/Ni3S2 as the positive electrode and activated carbon as the negative electrode delivers high energy and power densities (52.9 W h kg−1 at 1.6 kW kg−1; 26.3 W h kg−1 at 6.4 kW kg−1), and good cycling stability (a capacitance retention of 92.9% over 5000 cycles).

Transition-metal-based heteronanoparticles are attracting extensive attention in electrode material design for supercapacitors owing to their large surface-to-volume ratios and inherent synergies of individual components; however, they still suffer from limited interior capacity and cycling stability due to simple geometric configurations, low electrochemical activity of the surface, and poor structural integrity. Developing an elaborate architecture that endows a larger surface area, high conductivity, and mechanically robust structure is a pressing need to tackle the existing challenges of electrode materials. This work presents a supercapacitor electrode consisting of honeycomb-like biphasic Ni5P4–Ni2P (NixPy) nanosheets, which are interleaved by large quantities of nanoparticles. The optimized NixPy delivers an ultrahigh specific capacity of 1272 C g–1 at a current density of 2 A g–1, high rate capability, and stability. An asymmetric supercapacitor employing as-synthesized NixPy as the positive electrode and activated carbon as the negative electrode exhibits significantly high power and energy densities (67.2 W h kg–1 at 0.75 kW kg–1; 20.4 W h kg–1 at 15 kW kg–1). These results demonstrate that the novel nanostructured NixPy can be potentially applied in high-performance supercapacitors.

Herein, we have reported a facile method to synthesize surface passivated carbon dots (CDs) with high quantum yield. The structure and optical properties of the CDs are successfully investigated using high-resolution transmission electron microscopy; dynamic light scattering; and UV–vis, fluorescence, and Fourier transform infrared spectroscopy (FTIR). The synthesized CDs with excellent water solubility have been utilized to prepare nanostructured polyaniline (PANI) where CDs act both as a dopant as well as nucleating agent. The synthesis of PANI is achieved through a simple and one pot chemical oxidative polymerization. The UV–vis and FTIR data clearly indicates the formation of highly doped PANI (emeraldine salt form), which will really be helpful for fabricating photovoltaic device. We have examined the current–voltage characteristics of CD-PANI samples both in dark and illuminated state, which evidently designates the enhancement of current in the illuminated state. Hence, we have employed the CD-PANI samples for photovoltaic study, and the DSSCs are studied under illumination of 100 mW/cm2. Among the CD-PANI samples, CDPA10 shows a maximum power conversion efficiency of 3.65%.

Over the couple of decades scientific community has witnessed the robust utilization of DNA as soft template for the directed growth of conducting polymers and the so formed DNA-conducting polymer hybrid finds wide application in various biomedical fields. Herein, we have employed DNA templated enzymatic polymerization technique to prepare water soluble, biocompatible, fluorescent DNA-polymer hybrid having boronic acid moiety utilizing the berberine intercalated fluorescent DNA. To achieve the DNA templated polymerization of 3-aminophenylboronic acid (APBA), catalytic property of the enzyme, horseradish peroxidase (HRP) is exploited in presence of hydrogen peroxide (H2O2). Fluorescence property is induced within this DNA-polymer conjugate, using the excellent intercalating propensity of berberine (B) towards base pairs of DNA. This synthesized fluorescent DNA- berberine-polymer conjugate (DBPAB) has effectively been employed as a cell imaging probe, owing to the presence of boronic acid groups having cell adhesive property. In addition, cellular uptakes as well as cell viability (evaluated using MTT assay) of DBPAB have also been investigated.

Selective drug release becomes an indispensable criterion for anticancer drug delivery to minimize its hazardous side effects. Here utilizing poly(N-isopropyl acryl amide) anchored graphene oxide (GPNM) having π-electron rich graphitic basal plane, is employed for carrying drug molecule through non-covalent interactions due to its sufficient stability in cell medium and good dispersibility under physiological condition. The intrinsic fluorescence property of the anticancer drug, berberine becomes quenched on loading with GPNM and after it’s selective release to nucleic acids the quenched fluorescence of loaded berberine turns on. But other biomolecules like protein (HSA), sugar etc. are unable to release the drug from the GPNM surface and ds-DNA is more effective than RNA for its release. The intercalative interaction of berberine into the base pairs of nucleic acids has been attributed to the cause of specificity. UV-vis and circular dichroism spectroscopy support the above intercalative mechanism of specific drug delivery.

Carbon dots (CDs) are a new representative in carbonaceous family and have initiated remarkable research interests over the past one decade in a large variety of fields. Herein, we have utilized a facile, one-step carbonization method to prepare fluorescent carbon dots using poly(vinyl alcohol) (PVA) both as a carbon source and as a surface passivating agent. The as prepared CDs emit bright blue fluorescence under ultraviolet illumination. The structure and optical properties of the CDs are thoroughly investigated by several methods such as high-resolution transmission electron microscopy; dynamic light scattering; UV–vis, fluorescence and Fourier transform infrared spectroscopy. The CDs exhibit excellent water solubility and demonstrate average hydrodynamic diameter of 11.3 nm, holding great promise for biological applications. The biocompatibility evaluation and in vitro imaging study reveals that the synthesized CDs can be used as effective fluorescent probes in bio-imaging without noticeable cytotoxicity. In addition, a unique sensor for the detection of vitamin B2 in aqueous solution is proposed on the basis of spontaneous fluorescence resonance energy transfer from CD to vitamin B2. These findings therefore suggest that the CDs can find potential applications in cellular imaging along with sensing of vitamin B2.

Functionalized graphene-based drug delivery vehicles have conquered a significant position because functionalization improves its biocompatibility and stability in cell medium, leaving sufficient graphitic basal plane for drug loading through π–π stacking. In this study, poly(N-isopropylacrylamide) (PNIPAM) is covalently grafted from the surface of graphene oxide (GO) via a facile, eco-friendly and an easy procedure of free radical polymerization (FRP) using ammonium persulfate initiator. Various spectroscopic and microscopic studies confirm the successful grafting of PNIPAM from GO surface. PNIPAM-grafted GO (GPNM) exhibits enhanced thermal stability, improved dispersibility both in aqueous and cell medium, and better biocompatibility and cell viability compared to GO. Interestingly, GPNM displays an exciting fluorescence property in aqueous medium, which is a hike of intensity at 36 °C due to the lower critical solution temperature (LCST) of PNIPAM chains (32 °C). Moreover both hydrophilic (doxorubicin (DOX)) and hydrophobic (indomethacin (IMC)) drugs loaded on the surface of GPNM hybrid exhibits its efficacy as an efficient carrier for both types of drugs. Cellular uptakes of free DOX and DOX-loaded GPNM (GPNM-DOX) are evidenced both from optical and fluorescence imaging of live cells, and the efficiency of drug is significantly improved in the loaded system. The release of DOX from GPNM-DOX was achieved at pH 4, relevant to the environment of cancer cells. The pH-triggered release of hydrophobic drug was also studied using UV–vis spectroscopy via alginate encapsulation, showing a great enhancement at pH = 7.4. The IMC is also found to be released by human serum albumin using dialysis technique. The GPNM nanomaterial shows the property of simultaneous loading of DOX and IMC as well as pH-triggered simultaneous release of both of the drugs.

A facile approach of nanojacketing DNA in intact conformation is evolved by the in situ polymerization of o-methoxyaniline (OMA) at 30 °C using HAuCl4 as an oxidant and DNA as a soft template. It concomitantly produces poly(o-methoxyaniline) (POMA) and a Au nanojacket encapsulating the double stranded DNA (ds-DNA). The POMA chains remain adhered to the Au nanojacket, facilitating the dissolution of nanojacketed DNA (DNA–Au–POMA) in organic solvent without affecting its conformation. Digestion of the nanojacketed system with saturated iodine solution dejackets the ds-DNA with retention of its conformation, leaving the POMA nanotube. The nanojacketing and dejacketing phenomena are established by transmission electron microscopy (TEM), UV–vis spectroscopy, and CD spectroscopy, and the nanostructure is further characterized by FTIR, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The impedance study of the DNA–Au–POMA sample suggests the Cole–Cole plots at both the impedance and modulus planes and the values of capacitance and electron-transfer resistance of the material (Ret) are calculated to be 13.74 pF and 388 kΩ, respectively. The presence of a single Debye peak in both the impedance and modulus vs frequency plots suggests an isotropic nature of the system, and the frequency dependent ac-conductivity suggests the presence of short-range translational and reorientational (localized) hopping of charge carriers at lower and higher frequency region.

High-performance nanocomposites of NaCMC with GO are produced by solution casting. FESEM images reveal a good homogeneous dispersion of GO in the NaCMC matrix. The composite formation is facilitated by H-bonding interaction between GO and NaCMC. Tg of the composites increases with increasing GO concentration. The storage modulus (G′) exhibits a maximum 174% increase over NaCMC at 1 wt% GO. The mechanical properties of the composites exhibit highest increase of tensile stress and Young's modulus of 188 ± 4% and 154 ± 11%, respectively, for 1 wt% GO. Analysis of Young's modulus (Ey) data using the Halpin-Tsai equation suggests that the Ey data are close to the unidirectional orientation at >0.5 wt% GO, indicating more efficient load transfer at these compositions.

We have prepared sulfonated graphene (SG) by diazonium coupling technique and it has been characterized by UV–vis absorption spectroscopy, Raman spectroscopy, electron microscopy, energy-dispersive spectroscopy (EDS), EDS elemental mapping, X-ray photoelectron spectroscopy (XPS), and FTIR spectroscopy. The photoluminescence (PL) property of SG at different pH (pH 4, 7, and 9.2) has been investigated and SG shows highest PL-intensity and quantum yield at pH 4 compared to those at higher pH and that of GO at pH 4. Due to the strong overlap between the emission spectrum of SG and absorption spectrum of riboflavin (RF, vitamin B2) at pH 4, it has been tactfully used as donor for the fluorescence resonance energy transfer (FRET) process. However, graphene oxide (GO) does not exhibit any FRET with RF at an identical condition due to its much lower quantum yield. We have demonstrated a selective detection of vitamin B2 in presence of nucleic acid (DNA, RNA), protein (BSA), amino acid (Lysine) and other water-soluble vitamins (Becosules, Zevit capsules) based on the spontaneous FRET from PL-active SG (donor) to RF (acceptor). The calibration curve indicates excellent affirmation to detect vitamin B2 using FRET and it is superior to the ordinary fluorescence method of detecting RF in presence of different biomolecules.

Poly(vinylidene fluoride) (PVDF)-graft-poly(dimethyl amino ethyl methacrylate) (PDMAEMA) (PD copolymer) is produced via atom transfer radical polymerization from PVDF solution in N-methyl-2-pyrrolidone. PD copolymer is doped with 1% and 5% (w/w) Li+ ion to produce PDLi1 and PDLi5 samples, respectively. In PD copolymer, the crystalline structure of PVDF changes from α polymorph to a mixture of α and β polymorph, and it transforms completely to piezoelectric β polymorph on doping with 1% (w/w) Li+ ion. The impedance behavior of PVDF changes on grafting, and that of the PD graft copolymer also changes with increasing Li+ ion dopant concentration. In the Nyquist plots, PVDF exhibits a straight line character, and a curvature has appeared in the PD graft copolymer; on doping the latter with Li+ ion (1% w/w), the curvature increases and a semicircle is completed on 5% Li+ doping. Fitting the data from the Z-view program, the Ohmic resistance of PDLi1 is found to be 78 MΩ having capacitance with constant phase element (CPE) = 1.38 nF while for the PDLi5 sample the resistance decreases to16.1 MΩ with a small increase in CPE to 1.46 nF. The modulus plane plots for PDLi1 and PDLi5 samples also exhibit only one peak supporting the presence of only one equivalent resistance–capacitance circuit with constant phase element in both PDLi1 and PDLi5 samples. Both the impedance and modulus vs frequency plots of PDLi1 and PDLi5 samples exhibit a single Debye peak suggesting isotropic nature of the samples. For PVDF and PDMAEMA, ac-conductivity increases linearly with angular frequency, but in the case of PDLi1 and PDLi5 samples, it remains at first invariant in the frequency range 1–102 Hz, and above 102 Hz, an increase in conductivity with frequency occurs obeying the double power law. In the temperature variation of conductivity, PVDF exhibits its typical insulating nature, and in the PD graft copolymer, the conductivity decreases with increase of temperature (metallic-like behavior) due to gradual breaking of supramolecular interaction. The temperature variation of ac-conductivity of the Li+-doped PD graft copolymer suggests that both the ionic and supramolecular contributions of conductivity operate; the former increases and the latter decreases with rise in temperature showing a maximum. The temperature-dependent FTIR spectra of PDLi1 and PDLi5 samples support the gradual breaking of supramolecular interactions with increase of temperature.

We have developed a new highly fluorescent graphene oxide (GO)/poly(vinyl alcohol) (PVA) hybrid (GO-PVA) in an acidic medium (pH 4). Fourier transform infrared (FTIR) spectra indicate the formation of hydrogen bonds between the hydroxy group of PVA and the hydroxy groups of GO. The hybrid is highly fluorescent, because of passivation by hydrogen bonding, as evident from Raman spectra. The quantum yields of GO-PVA hybrids are higher than that of GO. The fluorescent microscopic images of the hybrids exhibit a fibrillar morphology, and all of them emit highly intense green light. Field-emission scanning electron microscopy (FESEM) micrographs also show a fibrillar morphology, which is produced due to the supramolecular organization of GO-PVA complex. The highly fluorescent GO-PVA1 hybrid has been used as a fascinating tool for selective sensing of Au3+ ions in aqueous media with a detectable limit of ∼275 ppb. The sensitivity of the Au3+ ion (300 μM) in the presence of 600 μM concentrations of each ion (Cu2+, Ag+, Mg2+, Ca2+, Zn2+, K+, Pb2+, Co2+, Ni2+, Pd2+, Fe2+, Fe3+, and Cr3+), taken together, is unique, exhibiting a quenching efficiency of 76%. The quenching efficiency in the presence of a biologically analogous mixture (d-glucose, d-lysine, BSA, Na+, K+, Ca2+, Mg2+, Zn2+) (600 μM each) is 73%, which suggests that the GO-PVA1 hybrid is an efficient sensor of Au3+ ions. The average lifetime of GO at pH 4 increases in the GO-PVA1 hybrid, indicating the formation of a more stable excited state but the increase in lifetime value after addition of Au3+ salt solution to the hybrid solution indicates dynamic quenching. The selectivity of sensing of Au3+ is attributed to its reduction potential being higher than that of other metal ions and XPS data of GO-PVA1 hybrid with 300 μM Au3+ substantiate the reduction of Au3+ to Au0, because of the transfer of excitons from the hybrid facilitating the selective photoluminescence (PL) quenching.

Graphene oxide (GO) in acidic media (pH = 4) emits blue light but in neutral and alkaline media (pH = 7 and 9.2) the emission is negligible. On addition of 0.85, 1.7 and 3.4% (w/v) methyl cellulose (MC) to GO solution (0.005% w/v) the emission intensity increases dramatically at every pH but with an increase in pH the PL (photoluminescence) intensity decreases for every composition of the hybrid solution. The average lifetime of GO at pH = 4 increases on addition of MC. Fluorescent microscopic images of GO–MC hybrids for different MC content indicate that the morphology of the hybrids at pH 4 is ribbon type but at pH 7 and 9.2 no characteristic morphology is produced. The decrease of glass transition temperature by 9 °C of the GMC0.85 system (produced from drying GO-MC hybrid solution containing 0.85% MC solution) from that of pure MC suggests the presence of supramolecular interaction in the system. There is a drastic decrease in PL intensity on addition of nitroaromatics to the GMC0.85 system and it is very large (91%) for the addition of picric acid. Thus, the hybrid system acts as a good sensor for the detection of nitro aromatics by instantaneous photoluminescence quenching with a detectable limit of 2 ppm.