Soumen Dutta

Silver Nanoparticle Decorated Reduced Graphene Oxide (rGO) Nanosheet: A Platform for SERS Based Low-Level Detection of Uranyl Ion

Herein, a simple wet-chemical pathway has been demonstrated for the synthesis of silver nanoparticle conjugated reduced graphene oxide nanosheets where dimethylformamide (DMF) is judiciously employed as an efficient reducing agent. Altogether, DMF reduces both silver nitrate (AgNO3) and graphene oxide (GO) in the reaction mixture. Additionally, the presence of polyvinylpyrolidone (PVP) assists the nanophasic growth and homogeneous distribution of the plasmonic nanoparticle Ag(0). Reduction of graphene oxide and the presence of aggregated Ag NPs on reduced graphene oxide (rGO) nanosheets are confirmed from various spectroscopic techniques. Finally, the composite material has been exploited as an intriguing platform for surface enhanced Raman scattering (SERS) based selective detection of uranyl (UO2(2+)) ion. The limit of detection has been achieved to be as low as 10 nM. Here the normal Raman spectral (NRS) band of uranyl acetate (UAc) at 838 cm(-1) shifts to 714 and 730 cm(-1) as SERS bands for pH 5.0 and 12.0, respectively. This distinguished Raman shift of the symmetric stretching mode for UO2(2+) ion is indicative of pronounced charge transfer (CT) effect. This CT effect even supports the higher sensitivity of the protocol toward UO2(2+) over other tested oxo-ions. It is anticipated that rGO nanosheets furnish a convenient compartment to favor the interaction between Ag NPs and UO2(2+) ion through proximity induced adsorption even at low concentration.

Redox-Switchable Copper(I) Metallogel: A Metal–Organic Material for Selective and Naked-Eye Sensing of Picric Acid

Thiourea (TU), a commercially available laboratory chemical, has been discovered to introduce metallogelation when reacted with copper(II) chloride in aqueous medium. The chemistry involves the reduction of Cu(II) to Cu(I) with concomitant oxidation of thiourea to dithiobisformamidinium dichloride. The gel formation is triggered through metal-ligand complexation, i.e., Cu(I)-TU coordination and extensive hydrogen bonding interactions involving thiourea, the disulfide product, water, and chloride ions. Entangled network morphology of the gel selectively develops in water, maybe for its superior hydrogen-bonding ability, as accounted from Kamlet-Taft solvent parameters. Complete and systematic chemical analyses demonstrate the importance of both Cu(I) and chloride ions as the key ingredients in the metal-organic coordination gel framework. The gel is highly fluorescent. Again, exclusive presence of Cu(I) metal centers in the gel structure makes the gel redox-responsive and therefore it shows reversible gel-sol phase transition. However, the reversibility does not cause any morphological change in the gel phase. The gel practically exhibits its multiresponsive nature and therefore the influences of different probable interfering parameters (pH, selective metal ions and anions, selective complexing agents, etc.) have been studied mechanistically and the results might be promising for different applications. Finally, the gel material shows a highly selective visual response to a commonly used nitroexplosive, picric acid among a set of 19 congeners and the preferred selectivity has been mechanistically interpreted with density functional theory-based calculations.

Mesoporous Gold and Palladium Nanoleaves from Liquid−Liquid Interface: Enhanced Catalytic Activity of the Palladium Analogue toward Hydrazine-Assisted Room-Temperature 4‑Nitrophenol Reduction

The importance of an interfacial reaction to obtain mesoporous leafy nanostructures of gold and palladium has been reported. A new synthetic strategy involving 1,4-dihydropyridine ester (DHPE) as a potential reducing agent performs exceptionally well for the desired morphologies of both the noble metals at room temperature. The DHPE in turn transforms into its oxidized aromatic form. The as-synthesized gold leaves exhibit high surface-enhanced Raman scattering activity with rhodamine 6G (R6G) due to their hyperbranched structure. It is worthwhile that as-synthesized porous architectures of palladium support the room-temperature hydrogenation of 4-nitrophenol (4-NP) by hydrazine hydrate (N2H4·H2O), reported for the first time. Furthermore, MPL exhibits exceptionally good catalytic activity toward electrooxidation of formic acid. Therefore, an aromaticity driven synthetic technique achieves a rationale to design leafy nanostructures of noble metals from the liquid-liquid interface for multifaceted applications.

Au@Pd core–shell nanoparticles-decorated reduced graphene oxide: a highly sensitive and selective platform for electrochemical detection of hydrazine

The tailored fabrication of noble metal-based bimetallic nanoparticles-decorated reduced graphene oxide (rGO) is highly demanding for its use as a clean and recyclable substrate for electrochemical performances. Herein, we have successfully prepared Aucore@Pdshell with an average size of ∼11.5 nm on an rGO support (denoted as GAP) through a surfactant-free one-step synthetic protocol. The use of 2-propanol as a solvent as well as a reducing agent for the precursors demonstrates that the designed process is economically preferable for the scalable synthesis of the desired GAP material. Comparative electrocatalytic efficiency in terms of hydrazine (N2H4) oxidation reveals the optimized noble-metal loading on rGO nanosheets and also the composition to capitalize maximum advantage from the as-synthesized GAP material. The most effective electrocatalyst, GAP3 with optimized constituents, was then applied as a substrate to electrochemically determine N2H4 down to a trace concentration level with high selectivity and sensitivity. The limit of detection (LOD) from GAP3 is calculated to be 0.08 μM with a linear range of 2–40 μM from the chronoamperometric (CA) result at −0.15 V vs. SCE. The novelty of N2H4 detection by the designed substrate becomes evident while all of the related reports are reviewed. This electrochemical sensor was successfully applied further to detect trace-level N2H4 in various real water samples, indicating its promising practical application.

Hierarchical Au–CuO nanocomposite from redox transformation reaction for surface enhanced Raman scattering and clock reaction

Electrochemical processing has already manifested its prominence for obtaining well structured composite materials. The method is highly precise and is reduction potential driven. This methodology has resulted in a hierarchical Au–CuO nanocomposite from a chosen redox transformation reaction between the newly synthesized spherical and roughened Cu2O nanoparticles and HAuCl4. Thus the attractive shape as well as reducing capability of Cu2O made it promising as a starting material. The proposed redox transformation reaction does not need any additional reducing or stabilizing agents. The reduction potential value of CuO/Cu2O (+0.66 V vs. SHE) supports the quantitative reduction of Au(III) ions because of its higher reduction potential (+1.69 V vs. SHE) i.e., for the AuCl4−/Au half cell. The prescribed conditions, spontaneous redox reaction and the morphology of Cu2O nanoparticles help to produce the unique Au–CuO nanoflowers. The formation of Au–CuO nanocomposites from Cu2O nanospheres is characterised by several physical techniques such as XRD, XPS and FTIR. Finally the flower like Au–CuO nanocomposite shows higher SERS activity with 4-aminothiophenol (4-ATP) as a probe molecule than what is evident from the individual components (Au or Cu2O or CuO). Additionally the derived Au–CuO nanocomposite has also been found to be an effective catalyst for the clock reaction employing methylene blue and ascorbic acid in solution.

Fabrication of dog-bone shaped Au NRcore–Pt/Pdshell trimetallic nanoparticle-decorated reduced graphene oxide nanosheets for excellent electrocatalysis

Herein, a wet-chemical synthetic approach has been adopted, taking advantage of electrostatic interactions between positively charged [CTA]+ capped Au NRs and negative functionalities of graphene oxide (GO), to fabricate dog-bone shaped Au NRcore–Pt/Pdshell decorated reduced graphene oxide (rGO) nanosheets (GMTs) for efficient ethanol electrooxidation reaction (EOR). This new strategy is based on an exceptionally efficient immobilization of trimetallic nanoparticles onto rGO sheets. It is important to note that the sequential addition of Pt and Pd precursor salts into the reaction mixture evolves Pd as the surface layer in the trimetallic assembly (described as GMT-1), whereas reversal of the sequence of addition results in spongy Pt NPs at the outer surface (designated as GMT-2), and thus two different types of Aucore–Pt/Pdsandwich–Pd/Ptshell nanoparticle-decorated rGO nanocomposites have been fabricated. The well recognized electrocatalytic performance of Pt is improved towards EOR by controlled assembling of Au and Pd in the rGO support in both the GMTs, exceeding the performance of the corresponding bimetallic composites. The presence of Pd and rGO decreases the CO poisoning of Pt and rGO supports restrict the aggregation of the nanoparticles during CV cycles. Thus the durability of the GMT catalysts has been improved in contrast to other related materials. Interestingly, the exposed spongy Pt NPs, present at the exterior of the trimetallic particles in GMT-2, help to exhibit better EOR activity (∼1.22 times) but lower durability (∼10%) in comparison to its analogue, GMT-1. The observed electrocatalytic efficiencies of GMTs are superior to commercial Pt/C (more than 6 times better mass utilization and ∼40% higher durability) and most of the recently reported related materials, which suggest cost-effective and efficient practical fuel cell application of GMTs.

Benzoin derived reduced graphene oxide (rGO) and its nanocomposite: application in dye removal and peroxidase-like activity

Benzoin, a common reagent, has been used as a reducing agent for large scale production of reduced graphene oxide nanosheets and also for a gold-reduced graphene oxide nanocomposite (Au–rGO) by a co-reduction method. The pH of the reaction medium plays a crucial role in controlling the reaction kinetics. Systematic microscopic characterization reveals the sheet like nanostructure of reduced graphene oxide (rGO). A two-electron transfer mechanism has been proposed to support the reduction phenomenon. The reduced graphene oxide (rGO) has been successfully exploited for the removal of different dye molecules from aqueous solution and the recyclability has been tested. Here the adsorption capacity of the adsorbent is calculated to be 338.65 mg g−1 for methylene blue. Finally peroxidase like activity of Au–rGO has been identified for pyrogallol oxidation using tert-butyl hydroperoxide.

A facile synthesis of 1D nano structured selenium and Au decorated nano selenium: catalysts for the clock reaction

Elemental selenium nanowires (Se NWs) are obtained from the easily available, less-toxic, low-cost reagents sodium selenite and ethylenediamine (en) through a modified hydrothermal reaction (MHT). The reaction supports the habitual growth of selenium nanowires and the amine molecules act as a reducing agent. Then, an interesting galvanic exchange reaction deposits Au nanoparticles on the as-synthesized Se evolving an Au decorated composite material. But a similar galvanic exchange produces Ag2Se because of the lower diffusion activation energy of Ag. Selenium nanowires, even as metalloids, have been found to catalyze the reduction of methylene blue (MB) using NaBH4 as a reducing agent. The blue colour of MB vanishes due to the formation of leucomethylene blue (LMB). On shaking the reaction mixture, the blue colour reappears by aerial oxidation. This oscillation of reversible colour change is taken as a clock reaction. The reversible reaction is monitored by a UV-visible spectrophotometer with respect to the absorbance of MB at λmax = 663 nm. Common reducing agents such as hydrazine hydrate, ammonium thiocyanate and glucose do not show this stunning phenomenon. Sodium borohydride fails to assist the reduction process in the presence of bulk selenium. The reduction process proceeds through two steps. Initially, the rate is slower due to an ‘induction period’ and then it becomes facile. A higher catalyst dose reduces the time lag, i.e., the induction period. The Se NWs performed well for about 30–50 cycles for a clock reaction set up. Finally, the composite materials have been found to exhibit a faster clock reaction cycle in comparison to neat Se NWs.

Facile Synthesis of Unique Hexagonal Nanoplates of Zn/Co Hydroxy Sulfate for Efficient Electrocatalytic Oxygen Evolution Reaction

Cost-effective, highly active water oxidation catalysts are increasingly being demanded in the field of energy conversion and storage. Herein, a simple modified hydrothermally (MHT) synthesized zinc and cobalt based hydroxyl double salt, that is, Zn4-xCoxSO4(OH)6·0.5H2O (ZCS), has been exfoliated for the first time as an efficient electrocatalyst for oxygen evolution reaction (OER) in alkaline medium. Morphology investigation suggests the evolution of unique hexagonal nanoplates of ZCS material. As OER catalyst, it requires only 370 and 450 mV overpotential to achieve 10 and 100 mA cm-2 current density, respectively. More importantly, performance at the overpotential over 400 mV and durability of the designed material have been found to be superior to those of commercial RuO2 catalyst. In the designed ZCS material trace amounts of cobalt species lead to higher mass activity of 146 A g-1, compared to that of the RuO2 catalyst (83 A g-1) at the same overpotential of 370 mV. The outstanding activity and stability of the cost-effective material emerges from the promotional effect of Zn ions, which are present as the principal constituent in the electrocatalyst, and they also protect the cobalt ions in the matrix during its long-term electrochemical test. It is important to note that an appropriate ratio of zinc and cobalt ions synergistically helps to create an economically viable and environmentally suitable electrocatalyst in comparison to other related transition metal based materials.

Suitable Morphology Makes CoSn(OH)6 Nanostructure a Superior Electrochemical Pseudocapacitor

Morphology of a material with different facet, edge, kink, etc., generally influences the rate of a catalytic reaction.1,2 Herein, we account for the importance of altered morphology of a nanomaterial for a supercapacitor device and employed CoSn(OH)6 as an electrode material. Suitable fabrication of a stable aqueous asymmetric supercapacitor (AAS) using metal hydroxide as positive electrode can be beneficial if the high energy density is derived without sacrificing the power density. Here we have synthesized an uncommon hierarchical mesoporous nanostructured (HNS) CoSn(OH)6 to fabricate a pseudocapacitor. In this endeavor, NH3 is found to be a well-suited hydrolyzing agent for the synthesis.3 Serendipitously, HNS was transformed into favored cubic nanostructure (CNS) in NaOH solution. In solution, NaOH acts as a structure directing as well as an etching agent. Both the samples (HNS & CNS) were used as pseudocapacitor electrodes in KOH electrolyte independently, which is reported for the first time. The HNS exhibits very high specific capacitance value (2545 F/g at 2.5 A/g specific current) with better cyclic durability over CNS sample (851 F/g at 2.5 A/g specific current). To examine the real cell application, we used HNS sample as the positive electrode material with the activated carbon (AC) as the negative electrode material for the development of an aqueous asymmetric supercapacitor (AAS). The as-fabricated AAS exhibited very high specific capacitance value of 713 F/g at a specific current of 1.5 A/g and retained 92% specific capacitance value even after 10 000 charge-discharge cycles. A maximum energy density of 63.5 Wh kg(-1) and a maximum power density of 5277 W kg(-1) were ascertained from the as-fabricated AAS, HNS CoSn(OH)6//AC.