Mechanistic Insights into the Synthesis of Nickel-Graphene Nanostructures for Gas Sensors.

Toxic gases are used in different types of industries and thus, present a potential health hazard. Therefore, highly sensitive gas sensing materials are essential for the safety of those operating in their environments. A process involving electrospinning polymer solutions impregnated with transition metal ions are developed to yield nanofibers that are annealed to form graphitic carbon / nickel nanoparticle-based fibers for gas sensing applications. The performance of these gas sensors is strongly related to the ability to control the material parameters of the active material. As the formation of these nanostructures, which nucleate within solid carbon scaffolds, have not been investigated, the growth mechanisms are look to understand in order to exert control over the resulting material. Evaluation of these growth mechanisms are conducted through a combination of thermogravimetric analysis with mass spectrometry (TGA-MS), x-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) and reveal nucleation of nickel at the onset of the polymer scaffold decomposition with subsequent growth processes, including surface diffusion, aggregation, coalescence and evaporation condensation, that are activated at different temperatures. Gas sensing experiments conducted on analyte gases demonstrate good sensitivity and response times, and significant potential for use in other energy and environmental applications.

Bio-Inspired Functional Materials for Environmental Applications.

With the global population expected to reach 9.7 billion by 2050, there is an urgent need for advanced materials that can address existing and developing environmental issues. Many current synthesis processes are environmentally unfriendly and often lack control over size, shape, and phase of resulting materials. Based on knowledge from biological synthesis and assembly processes, as well as their resulting functions (e.g., photosynthesis, self-healing, anti-fouling, etc.), researchers are now beginning to leverage these biological blueprints to advance bio-inspired pathways for functional materials for water treatment, air purification and sensing. The result has been the development of novel materials that demonstrate enhanced performance and address sustainability. Here, an overview of the progress and potential of bio-inspired methods toward functional materials for environmental applications is provided. The challenges and opportunities for this rapidly expanding field and aim to provide a valuable resource for researchers and engineers interested in developing sustainable and efficient processes and technologies is discussed.

Rational Design of Highly Efficient Perovskite Hydroxide for Electrocatalytic Water Oxidation.

The production of hydrogen from ecofriendly renewable technologies like water electrolysis and fuel cells involves oxygen evolution reaction (OER), which plays a major role, but the slow kinetics of OER is a bottleneck of commercialization of such technologies. Herein, we have reported the formation of an efficient OER catalyst from SnCo(OH)6 (SCH) by leaching of Sn atoms during electrochemical OER studies. According to density functional theory calculations, adsorption of OH* species on Sn atoms is energetically more favorable than that of Co atoms, and as a result, highly active CoOOH is generated by leaching of Sn atoms from surface layers. We observed enhanced OER performance with superior mass activity by blending SCH with activated charcoal, which displays a low overpotential of 293 mV and higher mass activity than that of pristine SCH. More importantly, it outperforms Co(OH)2 and RuO2 having the same carbon composition because of the formation of thermodynamically stable and amorphous CoOOH on the surface of single-crystalline SCH and strong tethering ability of activated charcoal.

Cobalt tungsten oxide hydroxide hydrate (CTOHH) on DNA scaffold: an excellent bi-functional catalyst for oxygen evolution reaction (OER) and aromatic alcohol oxidation.

A material with interdisciplinary properties is of wide interest for use in environmental applications. Currently, hydrogen generation by electrolysis and formation of carbonyl derivatives from alcohols are two different fields that focus on energy and environmental applications. In this work, a new material, Cobalt Tungsten Oxide Hydroxide Hydrate (CTOHH) on deoxyribonucleic acid (DNA) scaffold having chain-like morphology has been prepared for the first time by a facile microwave heating method. The same CTOHH was also prepared without the DNA scaffold and resulted in irregular aggregated molecular structures. Further, both CTOHH-DNA and CTOHH were converted into CoWO4-DNA and CoWO4, respectively by annealing them at a temperature of 600 °C. All the four catalysts were used for electrocatalytic oxygen evolution reaction (OER) and for oxidation of aromatic alcohols. In OER, CTOHH-DNA delivered fruitful results compared to all other electrocatalysts. For attaining a current density of 10 mA cm-2, it just required an overpotential of 355 mV with a Tafel slope value of 47.5 mV dec-1. Similarly, all four catalysts were also analyzed for selective and controlled oxidation of aromatic alcohols to their respective aldehydes and ketones using molecular oxygen as a green oxidant where CTOHH-DNA showed better results. Chemo-selectivity has been observed for CTOHH-DNA in the co-presence of hydroxyl and cyano functional groups. The durability of CTOHH-DNA was analyzed and it showed excellent catalytic activity retention up to five cycles.

Advanced Cu3Sn and Selenized Cu3Sn@Cu Foam as Electrocatalysts for Water Oxidation under Alkaline and Near-Neutral Conditions.

Water electrolysis is a field growing rapidly to replace the limited fossil fuels for harvesting energy in future. In searching of non-noble and advanced electrocatalysts for the oxygen evolution reaction (OER), here we highlight a new and advanced catalyst, selenized Cu3Sn@Cu foam, with overwhelming activity for OER under both alkaline (1 M KOH) and near-neutral (1 M NaHCO3) conditions. The catalysts were prepared by a double hydrothermal treatment where Cu3Sn is first formed which further underwent for second hydrothermal condition for selenization. For comparison, Cu7Se4@Cu foam was prepared by a hydrothermal treatment under the same protocol. The as-formed Cu3Sn@Cu foam, selenized Cu3Sn@Cu foam, and Cu7Se4@Cu foam were utilized as electrocatalysts and showed their potentiality in terms of activity and stability. In 1 M KOH, for attaining the benchmarking current density of 50 mA cm-2, selenized Cu3Sn@Cu foam required a low overpotential of 384 mV and increased charge transfer kinetics with a lower Tafel slope value of 177 mV/dec comparing Cu3Sn@Cu foam, Cu7Se4@Cu foam, and pristine Cu foam. Furthermore, potentiostatic analysis (PSTAT) was carried out for 40 h for selenized Cu3Sn@Cu foam and with minimum degradation in activity assured the long-term application for hydrogen generation. Similarly, under neutral condition selenized Cu3Sn@Cu foam also delivered better activity trend at higher overpotentials in comparison with others. Therefore, the assistance of both Sn and Se in Cu foam ensured better activity and stability in comparison with only selenized Cu foam. With these possible outcomes, it can also be combined with other active, non-noble elements for enriched hydrogen generation in future.

Nanosheets of Nickel Iron Hydroxy Carbonate Hydrate with Pronounced OER Activity under Alkaline and Near-Neutral Conditions.

Evaluation of unique catalysts of the iron group metals with activity in the OER region similar to that of scarce metals is of great importance to achieve sustainable production of H2 on a large scale. Herein, we report the unique nanosheets of nickel iron hydroxy carbonate hydrate (NiFeHCH) which were prepared through a wet-chemical route within 1 h of reaction time, acting as an efficient electrocatalyst for the oxygen evolution reaction (OER) in both alkaline and near-neutral media. The NiFeHCH was prepared with different concentrations of Fe in different ratios: 1:0.2, 1:0.4, 1:0.6, 1:0.8, and 1:1. Among them, nanosheets of NiFeHCH (1:0.2) were found to have superior OER activity, which required an overpotential of 250 mV to reach 20 mA cm-2 with a very low Tafel slope value of 39 mV/decade in 1 M KOH. Nanosheets with other ratios also had comparable OER activity with overpotentials ranging from 256 to 290 mV with Tafel slope values from 39 to 49 mV/decade. Nanosheets of NiFeHCH electrocatalysts screened for the OER in 1 M NaHCO3 (pH ∼8.5) required overpotentials for all of the ratios ranging from 389 to 507 mV at 10 mA cm-2 and Tafel slope values from 155 to 205 mV/decade, of which nanosheets of NiFeHCH (1:0.4) showed better activity by requiring an overpotential of 389 mV at 10 mA cm-2 and Tafel slope value of 155 mV/decade. With these fruitful advantages, these prepared nanosheets of NiFeHCH can be a better alternative to scarce metals for industrial water electrolysis.

Shape-selective synthesis of Sn(MoO4)2 nanomaterials for catalysis and supercapacitor applications.

Size and shape-selective Sn(MoO4)2 nanomaterials have been synthesized for the first time using a simple hydrothermal route by the reaction of Sn(ii) chloride salt with sodium molybdate in CTAB micellar media under stirring at 60 °C temperature for about three hours. Needle-like and flake-like Sn(MoO4)2 nanomaterials were synthesized by optimizing the CTAB to metal salt molar ratio and by controlling other reaction parameters. The eventual diameter and length of the nanoneedles are ∼100 ± 10 nm and ∼850 ± 100 nm respectively. The average diameter of the flakes is ∼250 ± 50 nm. The synthesized Sn(MoO4)2 nanomaterials can be used in two potential applications, namely, catalytic reduction of nitroarenes and as an anodic material in electrochemical supercapacitors. From the catalysis study, it was observed that the Sn(MoO4)2 nanomaterials could act as a potential catalyst for the successful photochemical reduction of nitroarenes into their respective aminoarenes within a short reaction time. From the supercapacitor study, it was observed that the Sn(MoO4)2 nanomaterials of different shapes show different specific capacitance (Cs) values and the highest Cs value was observed for Sn(MoO4)2 nanomaterials having a flake-like morphology. The highest Cs value observed was 109 F g(-1) at a scan rate of 5 mV s(-1) for the flake-like Sn(MoO4)2 nanomaterials. The capacitor shows an excellent long cycle life along with 70% retention of the Cs value, even after 4000 consecutive cycles at a current density of 8 mA cm(-2). Other than the applications in catalysis and supercapacitors, the synthesized nanomaterials can find further applications in photoluminescence, sensor and other energy-related devices.

Bio-molecule assisted aggregation of ZnWO4 nanoparticles (NPs) into chain-like assemblies: material for high performance supercapacitor and as catalyst for benzyl alcohol oxidation.

ZnWO4 nanoparticles (NPs) that are assembled and aggregated together as chain-like morphology have been synthesized via the reaction of Zn(II) salt solution with sodium tungstate in the presence of the DNA scaffold under 5 min of microwave heating. The reaction parameters have been tuned to control the size of the individual particles and diameter of the chains. The significance of different reaction parameters and specific growth mechanism for the formation of particles is elaborated. The DNA-ZnWO4 nanoassemblies have been used in two potential applications for the first time, namely, supercapacitor and catalysis studies. Supercapacitor study revealed that DNA-ZnWO4 nanoassemblies exhibited good electrochemical properties having high specific capacitance value ∼72 F/g at 5 mV s(-1), and electrodes possessed a good cyclic stability with more than 1000 consecutive times of cycling. Catalysis studies have been done for benzyl alcohol oxidation, and it was observed that DNA-ZnWO4 nanoassemblies having smaller diameter gives better catalytic efficiency compared to other morphology. This is further authenticated from their BET surface area analysis. In the future, the self-assembled DNA-ZnWO4 nanoassemblies could be a promising candidate for the synthesis of other mixed metal oxides and should be applicable in various emerging fields like Li ion batteries or photocatalysis, or as luminescent materials.

DNA-encapsulated chain and wire-like ß-MnO2 organosol for oxidative polymerization of pyrrole to polypyrrole.

A DNA-encapsulated chain and wire-like ß-MnO2 organosols have been synthesized utilizing a two-phase water-toluene extraction procedure at room temperature (RT). The ß-MnO2 organosol was prepared by transferring KMnO4 and DNA from aqueous solution separately to an organic solvent (toluene) using a phase transfer catalyst, mixing both organic solutions together, and subsequent reduction with NaBH4. The eventual diameters of the MnO2 particles in chain-like and wire-like morphologies were ∼1-2 nm and ∼1.8 ± 0.2 nm, respectively, whereas the nominal length of the DNA-MnO2 chains was ∼2-3 µm. Different morphologies of the MnO2 organosol were synthesized by simply tuning the DNA to KMnO4 molar ratio. The synthesized particles were successfully re-dispersed in different organic solvents for application in various organic reactions. The potential of the DNA-MnO2 organosol as a catalyst has been tested in the organic catalytic reaction for the oxidative polymerization of pyrrole to polypyrrole, using the DNA-MnO2 organosol as a potential catalyst. The synthesis process was simple, reproducible and robust. In future, the present process might be utilized for the formation of other nanomaterials in organic solvents, with specific morphologies and uses in a variety of catalytic reactions and energy storage applications.

Enhanced catalytic and supercapacitor activities of DNA encapsulated ß-MnO2 nanomaterials.

A new approach is developed for the aqueous phase formation of flake-like and wire-like ß-MnO2 nanomaterials on a DNA scaffold at room temperature (RT) within a shorter time scale. The ß-MnO2 nanomaterials having a band gap energy ∼3.54 eV are synthesized by the reaction of Mn(II) salt with NaOH in the presence of DNA under continuous stirring. The eventual diameter of the MnO2 particles in the wire-like and flake-like morphology and their nominal length can be tuned by changing the DNA to Mn(ii) salt molar ratio and by controlling other reaction parameters. The synthesized ß-MnO2 nanomaterials exhibit pronounced catalytic activity in organic catalysis reaction for the spontaneous polymerization of aniline hydrochloride to emeraldine salt (polyaniline) at RT and act as a suitable electrode material in electrochemical supercapacitor applications. From the electrochemical experiment, it was observed that the ß-MnO2 nanomaterials showed different specific capacitance (Cs) values for the flake-like and wire-like structures. The Cs value of 112 F g(-1) at 5 mV s(-1) was observed for the flake-like structure, which is higher compared to that of the wire-like structure. The flake-like MnO2 nanostructure exhibited an excellent long-term stability, retaining 81% of initial capacitance even after 4000 cycles, whereas for the wire-like MnO2 nanostructure, capacitance decreased and the retention value was only 70% over 4000 cycles. In the future, the present approach can be extended for the formation of other oxide-based materials using DNA as a promising scaffold for different applications such as homogeneous and heterogeneous organic catalysis reactions, Li-ion battery materials or for the fabrication of other high performance energy storage devices.

Enhanced catalytic and SERS activities of CTAB stabilized interconnected osmium nanoclusters.

A new route for the formation of osmium nanoparticles (NPs) having different morphologies like aggregated clusters, chain-like networks, and small spheres are reported. The synthesis was done by utilizing a simple wet-chemical method at room temperature (RT) by the reaction of OsO4, cetyl trimethyl ammonium bromide (CTAB), 2,7-dihydroxynaphthalene (2,7-DHN) and NaOH under 30 min of reaction. The diameter of the individual particles in all the morphologies was ∼1-3 nm. The synthesized materials have been tested for catalysis and SERS studies. The catalysis study was examined taking different organic nitro compounds and the catalysis rate was found superior as compared to other reports. The surface enhanced Raman scattering (SERS) study was done taking Rose Bengal (R Be) as a probe molecule and the observed enhancement factor (EF) value was found superior or comparable to most of other noble metal NPs. The overall synthesis process was simple, less time consuming and cost-effective. The enhanced catalytic and SERS activities of the Os NCs might open up a new avenue for the application in other organic catalysis reactions, SERS based detection of environmentally important trace bio-molecules and sensors.

DNA mediated wire-like clusters of self-assembled TiO2 nanomaterials: supercapacitor and dye sensitized solar cell applications.

A new route for the formation of wire-like clusters of TiO2 nanomaterials self-assembled in DNA scaffold within an hour of reaction time is reported. TiO2 nanomaterials are synthesized by the reaction of titanium-isopropoxide with ethanol and water in the presence of DNA under continuous stirring and heating at 60 °C. The individual size of the TiO2 NPs self-assembled in DNA and the diameter of the wires can be tuned by controlling the DNA to Ti-salt molar ratios and other reaction parameters. The eventual diameter of the individual particles varies between 15 ± 5 nm ranges, whereas the length of the nanowires varies in the 2-3 µm range. The synthesized wire-like DNA-TiO2 nanomaterials are excellent materials for electrochemical supercapacitor and DSSC applications. From the electrochemical supercapacitor experiment, it was found that the TiO2 nanomaterials showed different specific capacitance (Cs) values for the various nanowires, and the order of Cs values are as follows: wire-like clusters (small size) > wire-like clusters (large size). The highest Cs of 2.69 F g(-1) was observed for TiO2 having wire-like structure with small sizes. The study of the long term cycling stability of wire-like clusters (small size) electrode were shown to be stable, retaining ca. 80% of the initial specific capacitance, even after 5000 cycles. The potentiality of the DNA-TiO2 nanomaterials was also tested in photo-voltaic applications and the observed efficiency was found higher in the case of wire-like TiO2 nanostructures with larger sizes compared to smaller sizes. In future, the described method can be extended for the synthesis of other oxide based materials on DNA scaffold and can be further used in other applications like sensors, Li-ion battery materials or treatment for environmental waste water.