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Single phasic magnetite samples co-substituted with cobalt and nickel, with the formula CoxNi(0.4−x)FeII0.6FeIII2O4 (x = 0, 0.1, 0.2, 0.3, and 0.4) were synthesized in the nanoregime via a co-precipitation technique. Being an inverse spinel, magnetite will preferentially expose the octahedral sites and make metal cations available on the surface, which will play a conducive role in both hydrogen evolution (HER) and oxygen evolution (OER) reactions. We demonstrate that the partial substitution of Fe2+ (either by Ni2+ or Co2+ ions) on the octahedral sites of the inverse spinel structure of CoxNi(0.4−x)FeII0.6FeIII2O4 significantly enhance the bifunctional electrocatalytic activity of the magnetite samples in an alkaline medium. The spinel ferrite with the formula Co0.2Ni0.2FeII0.6FeIII2O4 exhibits outstanding bifunctional electrocatalytic activity in 1 M KOH with the lowest onset overpotential (ƞOER = 190 and ƞHER = 200 mV), small overpotential at η10 (OER = 270 mV and HER = 275 mV), excellent kinetics (Tafel slopes, bOER = 44 mV dec−1 and bHER = 99 mV dec−1), and high durability (>10 h). Furthermore, Co0.2Ni0.2FeII0.6FeIII2O4 can serve as both cathode and anode for the overall water-splitting reaction, and delivered a current density of 10 mA cm−2 at a very low cell voltage of 1.72 V with excellent stability (>10 h at 10 mA cm−2). Thus, this work provides a lucid approach to engineer a highly efficient non-noble transition metal-based electrocatalyst for renewable energy applications via simple micro-structural and surface engineering.

A novel vertical non-van der Waals (non-vdW) heterostructure of graphene and hexagonal boron nitride (G/hBN) is realized and its application in direct four-electron oxygen reduction reaction (ORR) in alkaline medium is established. The G/hBN differs from previously demonstrated vdW heterostructures, where it has a chemical bridging between graphene and hBN allowing a direct charge transfer - resulting in high ORR activity. The ORR efficacy of G/hBN is compared with that of graphene-hBN vdW structure and individual layers of graphene and hBN along with that of benchmark platinum/carbon (Pt/C). The ORR activity of G/hBN is found to be on par with Pt/C in terms of current density but with much higher electrochemical stability and methanol tolerance. The onset potential of the G/hBN is found to be improved from 780 mV at a glassy carbon electrode to 930 mV and 940 mV in gold and platinum electrodes, respectively, indicating its substrate-dependent catalytic activity. This opens possibilities of new benchmark catalysts of metals capped with G/hBN atomic layers, where the underneath metal is protected while keeping the activity similar to that of pristine metal. Density functional theory-based calculations are found to be supporting the observed augmented ORR performance of G/hBN.

Due to exceptional electronic, optoelectronic and catalytic properties, MoS2 has attracted extensive research interest in various applications. In the present scenario, the exploitation of noble-metal-free catalysts for hydrogen evolution is of great interest. Herein, we report the structural, optical, magnetic and electrocatalytic properties of pure and nickel-substituted MoS2 nanostructures synthesized by the hydrothermal method. X-ray diffraction (XRD) analysis reveals that all samples exhibit the hexagonal structure of MoS2 and the formation of NiS2 at higher concentrations of nickel. Vibrating sample magnetometer (VSM) measurements of the Mo1−xNixS2 nanostructures show a hysteresis loop at room temperature with a higher saturation magnetization for 1% Ni-substituted MoS2 nanostructures, confirming the ferromagnetic behaviour of the sample. The indirect-to-direct band gap transition of few-layered nanostructures was confirmed by the optical absorption spectrum showing bands in the 600–700 nm and 350–450 nm regions. This study also highlights the excitation wavelength-dependent down- and up-conversion photoluminescence of the as-synthesized samples, providing new horizons for the design of MoS2-based optical and spintronic devices. The electrocatalytic effect of 3% Ni-substituted MoS2 nanostructures has been found to be higher than that of other deposit concentrations as it corresponds to the efficient hydrogen evolution reaction (HER).

This study reports the simple, selective and sensitive voltammetric detection of riboflavin (RF) using methylene blue (MB) incorporated sulfonic acid functionalized mesoporous silica microspheres (MSM), represented as MB-SO3H-MSM. MB-SO3H-MSM is synthesized and characterized by spectroscopic and microscopic methods. This material is coated on a glassy carbon (GC) electrode (symbolized as GC/MB-SO3H-MSM) to utilize it in electroanalytical applications. The electrochemical behavior of MB-SO3H-MSM is established using the GC/MB-SO3H-MSM electrode by cyclic voltammetry (CV) and electrochemical impedance spectroscopy techniques. The electrochemical behavior of RF at the GC/MB-SO3H-MSM electrode is also studied by CV. Compared to bare GC and SO3H-MSM coated GC, the GC/MB-SO3H-MSM electrode shows favorable electron transfer kinetics as well as an enhanced and stable electrochemical response of RF. Furthermore CV and differential pulse voltammetry (DPV) are used for the quantitative determination of RF at the GC/MB-SO3H-MSM electrode. The DPV response shows two linear calibration ranges of 10.0 nM to 15.0 μM and 15.0 to 50.0 μM. The detection limit based on the first linear calibration range is calculated as 5.0 nM with a sensitivity of 393.0 μA mM−1 cm−2. The fabricated sensing scaffold shows an excellent selectivity for RF over other soluble vitamins and interfering ions. The stability, reproducibility and determination of RF in pharmaceutical products are also demonstrated effectively.

In this paper, we report a facile route to synthesize silver nanoparticles (Ag NPs) incorporated into a copolymer of methyl methacrylate (MMA) and 2-acrylamido-2-methylpropane sulfonic acid (AMPS) (abbreviated as P(MMA-co-AMPS)) and its application to construct an electrochemical nitrite (NO2−) sensor. The copolymer nano-composite material (Ag–P(MMA-co-AMPS) is characterized by means of transmission electron microscopy, scanning electron microscopy, Fourier transform infrared spectroscopy, UV-vis spectroscopy, X-ray powder diffraction methods, thermogravimetric analysis and electrochemical impedance spectroscopy. Further, Ag–P(MMA-co-AMPS) is used to prepare an electrochemical sensing platform (ESP) on a glassy carbon electrode (GC) surface. NO2− is electrocatalytically oxidized at the ESP (GC/Ag–P(MMA-co-AMPS), which leads to a sensitive determination of NO2−. The oxidation current of NO2− is linear to its concentration in the range of 1.0–100 000.0 μM and the detection limit is found to be 0.2 μM with a sensitivity 104.6 μA mM−1 cm−2. The observed analytical parameters such as wide linear range, low detection limit, high sensitivity and short response time are comparable or superior to other previously reported NO2− sensors. Kinetic parameters are evaluated using cyclic voltammetry and chronoamperometry. Finally it is demonstrated that the proposed sensor can be used for the selective determination of NO2− present in water samples and the results are quite promising.