Construction of a Cu-Based Metal-Organic Framework by Employing a Mixed-Ligand Strategy and Its Facile Conversion into Nanofibrous CuO for Electrochemical Energy Storage Applications.

Recently, metal-organic frameworks (MOFs) have been widely employed as a sacrificial template for the construction of nanostructured materials for a range of applications including energy storage. Herein, we report a facile mixed-ligand strategy for the synthesis of a Cu-MOF, [Cu3(Azopy)3(BTTC)3(H2O)3·2H2O]n (where BTTC = 1,2,4,5-benzenetetracarboxylic acid and Azopy = 4,4'-azopyridine), via a slow-diffusion method at room temperature. X-ray analysis authenticates the two-dimensional (2D)-layered framework of Cu-MOF. Topologically, this 2D-layered structure is assigned as a 4-connected unimodal net with sql topology. Further, nanostructured CuO is obtained via a simple precipitation method by employing Cu-MOF as a precursor. After analysis of their physicochemical properties through various techniques, both materials are used as surface modifiers of glassy carbon electrodes for a comparative electrochemical study. The results reveal a superior charge storage performance of CuO (244.2 F g-1 at a current density of 0.8 A g-1) with a high rate capability compared to Cu-MOF. This observation paves the pathway for the strategic design of high-performing supercapacitor electrode materials.

Nitrogen-Doped Mixed-Phase Cobalt Nanocatalyst Derived from a Trinuclear Mixed-Valence Cobalt(III)/Cobalt(II) Complex for High-Performance Oxygen Evolution Reaction.

Because of a continuous increase in energy demands and environmental concerns, a focus has been on the design and construction of a highly efficient, low-cost, environmentally friendly, and noble-metal free electrocatalyst for energy technology. Herein we report facile synthesis of the mixed-valence trinuclear cobalt complex 1 by the reaction of 2-amino-1-phenylethanol and CoCl2·6H2O in methanol as the solvent at room temperature. Further, 1 was reduced by using aqueous N2H4 as a simple reducing agent, followed by calcination at 300 °C for 3 h, yielding a nitrogen-doped mixed phase cobalt [ß-Co(OH)2 and CoO] nanocatalyst (N@MPCoNC). Both 1 and N@MPCoNC were characterized by various physicochemical techniques. Moreover, 1 was authenticated by single-crystal X-ray diffraction studies. The hybrid N@MPCoNC reveals a unique electronic and morphological structure, offering a low overpotential of 390 mV for a stable current density of 10 mA cm-2 with high durability. This N@MPCoNC showed excellent electrocatalytic as well as photocatalytic activity for oxygen evolution reaction compared to 1.

Mixed-Ligand-Architected 2D Co(II)-MOF Expressing a Novel Topology for an Efficient Photoanode for Water Oxidation Using Visible Light.

The structural diversity of Co(II) metal centers is known to influence their physicochemical properties. A novel two-dimensional (2D) Co(II)-MOF {[Co5(HL)4(dpp)2(H2O)2(µ-OH)2]·21H2O} n has been designed and synthesized by adopting a mixed-ligand strategy, using 1,3-di(4-pyridyl)propane (dpp) colinker with a flexible spacer H3L (H3L: 5-(2 carboxybenzyloxy)isophthalic acid). Co(II)-MOF features a 2D network, which is further interpenetrated among the equivalent sets and therefore results in a 3D supramolecular network. Topologically, the entire network can be viewed as a (3,4,8)-connected three-nodal net with the extended point symbol of {4.5.7}4{412.52.710.94}{52.8.92.10}2, duly assigned to the novel topological type smm2. The functional utility of Co(II)-MOF is demonstrated by employing it toward oxygen evolution reaction (OER) in a photoelectrochemical cell, exhibiting appreciable photocurrents of up to 5.89 mA/cm2 when used as an anode in a photoelectrochemical cell, while also displaying encouraging electrocatalytic currents of 9.32 mA/cm2 (at 2.01 V vs RHE) for the OER. Moreover, detailed electrochemical impedance spectroscopy studies confirm enhanced charge-transfer kinetics and improved conductivity under illumination with minimal effect of interfacial phenomena. This work provides a reference for the expanding field of research into applications of MOF materials and establishes MOF materials as favorable candidates for sustainable and efficient design of electrodes for water splitting.

Visible light driven water splitting through an innovative Cu-treated-δ-MnO2 nanostructure: probing enhanced activity and mechanistic insights.

In this study, we have fabricated nanostructured thin films of δ-MnO2 on FTO glass substrates by a facile, room-temperature and low cost chemical bath deposition method. A copper treatment procedure in the synthesis steps results in a film of Cu-δ-MnO2, which displays significant photoactivity when used as a photocathode for hydrogen evolution reaction, with a photocurrent of 3.59 mA cm-2 (at 0 V vs. RHE) in a mild acidic solution. Furthermore, the electrodes also display significant electrocatalytic activity towards water oxidation reaching up to 10 mA cm-2 (at only 1.67 V vs. RHE). The Cu-δ-MnO2 film has been thoroughly characterized via various physicochemical, optical and electrochemical techniques, and an attempt has been made to explain the conductivity mechanism. It is suggested that Cu treatment enhances the photoactivity of δ-MnO2 films through a series of surface dominated processes, which facilitate reduced recombination and enhanced hole consumption at the interface of the electrode and electrolyte. These results establish birnessite-based manganese dioxides as suitable candidates for electrodes in water splitting cells and pave the way for atomic-level engineering of earth abundant materials to reach the ultimate goal of low-cost, sustainable generation of hydrogen.

Emerging Robust Heterostructure of MoS2-rGO for High-Performance Supercapacitors.

The intermittent nature of renewable energy resources has led to a continuous mismatch between energy demand and supply. A possible solution to overcome this persistent problem is to design appropriate energy-storage materials. Supercapacitors based on different nanoelectrode materials have emerged as one of the promising storage devices. In this work, we investigate the supercapacitor properties of a molybdenum disulfide-reduced graphene oxide (rGO) heterostructure-based binder-free electrode, which delivered a high specific capacitance (387.6 F g-1 at 1.2 A g-1) and impressive cycling stability (virtually no loss up to 1000 cycles). In addition, the possible role of rGO in the composite toward synergistically enhanced supercapacitance has been highlighted. Moreover, an attempt has been made to correlate the electrochemical impedance spectroscopy studies with the voltammetric analyses. The performance exceeds that of the reported state-of-the-art structures.

Investigating the Role of Substrate Tin Diffusion on Hematite Based Photoelectrochemical Water Splitting System.

Hematite (α-Fe2O3) nanostructures have been extensively studied as photo-anodes for the conversion of sunlight into chemical fuels by water splitting. A number of factors limit the photo-activity of pristine hematite nanostructures, including poor electrical conductivity and long penetration depth of light. Previous studies have shown that use of tin (Sn) as an n-type dopant can substantially enhance the photoactivity of hematite photoanodes by modifying their morphological, optical and electrical properties. This article presents impedance spectroscopic investigation of interplay between Sn-doping and the photoanode performance for photoelectrochemical water splitting using hematite nanostructure. Mott-Schottky measurements show that the Sn dopant serves as electron donor and increases the donor density of Sn-doped α-Fe2O3 nanostructured layer to 2.39 × 1019 cm-3. Photoelectrochemical impedance spectroscopy shows efficient photogenerated charge transfer from hematite to electrolyte in Sn-doped α-Fe2O3 nanostructure. The Sn-doped α-Fe2O3 nanostructure exhibit a photocurrent density of 1.2 mA/cm2 at 1.4 V versus RHE electrode.

Small biomolecule sensors based on an innovative MoS2-rGO heterostructure modified electrode platform: a binder-free approach.

The requirement of sensitive diagnostic chips for small biomolecules has triggered the urgent development of versatile nanomaterial based platforms. Therefore, numerous materials have been designed with fascinating properties. Herein, we report a facile one-pot synthesis of MoS2-rGO nanoflowers grown by the hydrothermal method and their applicability in the simultaneous sensing of AA, DA and UA. The structure and morphology of nanoflowers have been probed by various physico-chemical techniques such as XRD, SEM/TEM, AFM, Raman and XPS. Furthermore, these nanoflowers were used to construct a glassy carbon based working electrode (MoS2-rGO/GCE), by a facile drop-casting method in the absence of any commercial binder. The electrochemical investigations revealed high separating potency of the MoS2-rGO/GCE towards AA, DA and UA with distinguishable oxidation potentials (AA-DA = 204 mV and DA-UA = 122 mV) and a notable detection limit and reasonable sensitivity for each of these biomolecules. The charge transfer resistance and capacitive components obtained by electrochemical impedance spectroscopy (EIS) were found to be in agreement with the voltammetric observations. The observed synergy between MoS2 and rGO opens up new possibilities to consider the MoS2-rGO nanostructures as the cutting edge material for electrochemical sensor development.

A new multitalented azine ligand: elastic bending, single-crystal-to-single-crystal transformation and a fluorescence turn-on Al(iii) sensor.

We report a rare combination of two unique properties of an azine based ligand (H3L): in a solid-state crystalline material it shows highly flexible and elastic behavior which on triggering with light results in slight deviation with phase transformation at the Single-Crystal-to-Single-Crystal (SCSC) level. Furthermore, in the solution state it acts as a highly selective, sensitive and reversible Al3+ sensor with a detection limit of 42 nM.

Visible-Light-Induced Water Splitting Based on a Novel α-Fe2O3/CdS Heterostructure.

In this work, CdS nanoparticles were grown on top of a hematite (α-Fe2O3) film as photoanodes for the photoelectrochemical water splitting. Such type of composition was chosen to enhance the electrical conductivity and photoactivity of traditionally used bare hematite nanostructures. The fabricated thin film was probed by various physicochemical, electrochemical, and optical techniques, revealing high crystallinity of the prepared nanocomposite and the presence of two distinct phases with different band gaps. Furthermore, photoassisted water splitting tests exhibit a noteworthy photocurrent of 0.6 mA/cm2 and a relatively low onset potential of 0.4 V (vs reversible hydrogen electrode) for the composite electrode. The high photocurrent generation ability was attributed to the synergistic interplay between conduction and valence band (VB) levels of CdS and α-Fe2O3, which was further interpreted by J-V curves. Finally, electrochemical impedance spectroscopy investigation of the obtained films suggests that the photogenerated holes could be transferred from the VB of α-Fe2O3 to the electrolyte more efficiently in the hybrid nanostructure.

Non-enzymatic amperometric sensing of glucose by employing sucrose templated microspheres of copper oxide (CuO).

We report a facile hydrothermal synthesis of copper oxide microspheres (CMS) for the enzymeless amperometric detection of glucose in an alkaline medium. The crystallinity, morphology and size were examined by powder X-ray diffraction (PXRD), scanning and transmission electron microscopy (SEM/TEM) and dynamic light scattering (DLS) techniques, respectively. The fabricated CMS were grafted onto the working area of a carbon screen printed electrode (CSPE) and covered with a thin Nafion layer (Nafion/CMS/CSPE), forming a modified carbon screen printed electrode (MCSPE) which acts as a working electrode. Further, the electrochemical behavior of MCSPE was investigated under optimized conditions through cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV) and chronoamperometry (CA) techniques. The CV results showed a drastic enhancement of the current response in the presence of glucose. The amperometry results reveal the catalytic ability of CMS for glucose oxidation with a notable limit of detection (LOD) of 20.6 µM in a wide linear range of 2-9 mM with a high sensitivity of 26.59 µA mM(-1) cm(-2). Moreover, the anti-interference test confirmed the selectivity of the fabricated sensor towards glucose in the presence of interfering agents such as uric acid (UA), ascorbic acid (AA) and dopamine (DA).