Stabilization of a Highly Concentrated Colloidal Suspension of Pristine Metallic Nanoparticles

A colloidal suspension containing a high concentration of metallic nanoparticles (NPs) finds potential applications in flexible electronic printing, nanofluids, healthcare, antifouling coating, and so on. Here, we demonstrate a generic, easily scalable, simple, and contamination-free cryogenic temperature grinding method, which can effectively be used to prepare pristine NPs that can be stabilized in polar liquids in high concentrations. These surfactant-free pristine NPs have been found to remain dispersed in different polar liquids (CH3OH, C2H5OH, glycol, etc.) for weeks. The long-term stability of the NPs in these liquids has been investigated using zeta potential, in situ Fourier transform infrared spectroscopy, indicating electrostatic stabilization for ultrapure, surfactant-free NPs. Furthermore, stabilization of the NPs has been probed with detailed calculations using the Derjaguin Landau Verwey Overbeek theory as well as atomistic molecular dynamics simulation (MD). Experimental measurements along with theoretical calculations categorically indicate that the electrostatic energy is helping these NPs to be stabilized in a polar liquid

Understanding the evolution of catalytically active multi-metal sites in a bifunctional high-entropy alloy electrocatalyst for zinc–air battery application

Zinc–air batteries are known for high theoretical energy density and environmental friendliness. The successful commercial utilization of rechargeable zinc–air batteries is limited by unstable electrochemical interfaces and sluggish kinetics with poor round-trip efficiency. In this study, we report a nanocrystalline high entropy alloy (HEA) comprising Cu–Co–Mn–Ni–Fe (CCMNF) prepared by casting-cum-cryomilling method. This multi-component HEA embodies multiple catalytically active sites with diverse functionalities, thus enhancing the electrochemical redox reactions, e.g., oxygen reduction (ORR) and oxygen evolution reaction (OER). The bifunctional electrocatalytic performance of this HEA is comparable to that of standard catalysts, RuO2 and Pt/C, as evidenced by low overpotential requirements towards OER and ORR. The HEA was tested for use in the air electrode catalyst in the zinc–air battery, where it performed stable oxygen electrocatalysis that was durable over 1045 charging–discharging cycles for ∼90 hours of continuous operation. The microstructural analysis of HEA at different time scales (0, 24, 87 h) during the zinc–air battery operation suggested a dynamic participation of multiple metal active sites on the catalyst surface. Detailed studies revealed that despite leaching in harsh alkaline operation conditions, the synergistic electronic interactions between the component metal sites sustained good electrocatalytic performance and promoted oxygen electrocatalysis through the modification of electronic and chemical properties

High-Entropy Alloys as Catalysts for the CO2 and CO Reduction Reactions: Experimental Realization

Conversion of carbon dioxide into selective hydrocarbon using a stable catalyst remains a holy grail in the catalysis community. The high overpotential, stability, and selectivity in the use of a single-metal-based catalyst still remain a challenge. In current work, instead of using pure noble metals (Ag, Au, and Pt) as the catalyst, a nanocrystalline high-entropy alloy (HEA: AuAgPtPdCu) has been used for the conversion of CO2 into gaseous hydrocarbons. Utilizing an approach of multimetallic HEA, a faradic efficiency of about 100% toward gaseous products is obtained at a low applied potential (−0.3 V vs reversible hydrogen electrode). The reason behind the catalytic activity and selectivity of the high-entropy alloy (HEA) toward CO2 electroreduction was established through first-principles-based density functional theory (DFT) by comparing it with the pristine Cu(111) surface. This is attributed to the reversal in adsorption trends for two out of the total eight intermediates—*OCH3 and *O on Cu(111) and HEA surfaces

Low-density Three-dimensional Foam Using Self-reinforced Hybrid Two-dimensional Atomic Layers

Low-density nanostructured foams are often limited in applications due to their low mechanical and thermal stabilities. Here we report an approach of building the structural units of three-dimensional (3D) foams using hybrid two-dimensional (2D) atomic layers made of stacked graphene oxide layers reinforced with conformal hexagonal boron nitride (h-BN) platelets. The ultra-low density (1/400 times density of graphite) 3D porous structures are scalably synthesized using solution processing method. A layered 3D foam structure forms due to presence of h-BN and significant improvements in the mechanical properties are observed for the hybrid foam structures, over a range of temperatures, compared with pristine graphene oxide or reduced graphene oxide foams. It is found that domains of h-BN layers on the graphene oxide framework help to reinforce the 2D structural units, providing the observed improvement in mechanical integrity of the 3D foam structure. © 2014 Macmillan Publishers Limited.5Reinfried, M., Hybrid foams-A new approach for multifunctional applications (2011) Adv. Eng. Mater., 13, pp. 1031-1036Studart, A.R., Gonzenbach, U.T., Tervoort, E., Gauckler, L.J., Processing routes to macroporous ceramics: A review (2006) Journal of the American Ceramic Society, 89 (6), pp. 1771-1789. , DOI 10.1111/j.1551-2916.2006.01044.xReddy, E.S., Schmitz, G.J., Superconducting foams (2002) Supercond. Sci. Technol., 15, pp. L21-L24Banhart, J., Metal foams: Production and stability (2006) Advanced Engineering Materials, 8 (9), pp. 781-794. , DOI 10.1002/adem.200600071Banhart, J., Manufacture, characterisation and application of cellular metals and metal foams (2001) Progress in Materials Science, 46 (6), pp. 559-632. , DOI 10.1016/S0079-6425(00)00002-5, PII S0079642500000025Svagan, A.J., Samir, M.A.S.A., Berglund, L.A., Biomimetic foams of high mechanical performance based on nanostructured cell walls reinforced by native cellulose nanofibrils (2008) Adv. Mater., 20, pp. 1263-1269Cao, X., Preparation of novel 3D graphene networks for supercapacitor applications (2011) Small, 7, pp. 3163-3168Cao, A., Dickrell, P.L., Sawyer, W.G., Ghasemi-Nejhad, M.N., Ajayan, P.M., Materials science: Super-compressible foamlike carbon nanotube films (2005) Science, 310 (5752), pp. 1307-1310. , DOI 10.1126/science.1118957Butler, S.Z., Progress, challenges, and opportunities in two-dimensional materials beyond graphene (2013) ACS Nano, 7, pp. 2898-2926Zhou, W., Wang, Z.L., (2011) Three-Dimensional Nanoarchitectures: Designing Next-Generation Devices, , SpringerNovoselov, K.S., Jiang, D., Schedin, F., Booth, T.J., Khotkevich, V.V., Morozov, S.V., Geim, A.K., Two-dimensional atomic crystals (2005) Proceedings of the National Academy of Sciences of the United States of America, 102 (30), pp. 10451-10453. , DOI 10.1073/pnas.0502848102Keller, S.W., Kim, H.-N., Mallouk, T.E., Layer-by-layer assembly of intercalation compounds and heterostructures on surfaces: Toward molecular "beaker" epitaxy (1994) Journal of the American Chemical Society, 116 (19), pp. 8817-8818Fendler, J.H., Self-assembled nanostructured materials (1996) Chemistry of Materials, 8 (8), pp. 1616-1624Jones, M.R., Mirkin, C.A., Materials science: Self-assembly gets new direction (2012) Nature, 491, pp. 42-43Zhang, X., Mechanically strong and highly conductive graphene aerogel and its use as electrodes for electrochemical power sources (2011) J. Mater. Chem., 21, pp. 6494-6497Xu, Y., Sheng, K., Li, C., Shi, G., Self-assembled graphene hydrogel via a onestep hydrothermal process (2010) ACS Nano, 4, pp. 4324-4330Sudeep, P.M., Covalently interconnected three-dimensional graphene oxide solids (2013) ACS Nano, 7, pp. 7034-7040Yan, Z., Three-dimensional metal-graphene-nanotube multifunctional hybrid materials (2013) ACS Nano, 7, pp. 58-64Lu, X., Macroporous foam of reduced graphene oxides prepared by lyophilization (2012) Mater. Res. Bull., 47, pp. 4335-4339Novoselov, K.S., Geim, A.K., Morozov, S.V., Jiang, D., Zhang, Y., Dubonos, S.V., Grigorieva, I.V., Firsov, A.A., Electric field in atomically thin carbon films (2004) Science, 306 (5696), pp. 666-669. , DOI 10.1126/science.1102896Geim, A.K., Novoselov, K.S., The rise of graphene (2007) Nature Materials, 6 (3), pp. 183-191. , DOI 10.1038/nmat1849, PII NMAT1849Allen, M.J., Tung, V.C., Kaner, R.B., Honeycomb carbon: A review of graphene (2010) Chem. Rev., 110, pp. 132-145Stankovich, S., Dikin, D.A., Dommett, G.H.B., Kohlhaas, K.M., Zimney, E.J., Stach, E.A., Piner, R.D., Ruoff, R.S., Graphene-based composite materials (2006) Nature, 442 (7100), pp. 282-286. , DOI 10.1038/nature04969, PII NATURE04969Han, Z., Ammonia solution strengthened three-dimensional macro-porous graphene aerogel (2013) Nanoscale, 5, pp. 5462-5467Gao, G., Artificially stacked atomic layers: Toward new van der waals solids (2012) Nano Lett., 12, pp. 3518-3525Britnell, L., Field-effect tunneling transistor based on vertical graphene heterostructures (2012) Science, 335, pp. 947-950Rafiee, M.A., Hexagonal boron nitride and graphite oxide reinforced multifunctional porous cement composites (2013) Adv. Funct. Mater., 23, pp. 5624-5630Liu, Z., In-plane heterostructures of graphene and hexagonal boron nitride with controlled domain sizes (2013) Nat. Nanotechnol., 8, pp. 119-124Peng, Q., Zamiri, A.R., Ji, W., De, S., Elastic properties of hybrid graphene/boron nitride monolayer (2012) Acta Mech., 223, pp. 2591-2596Pacile, D., Meyer, J.C., Girit, C.O., Zettl, A., The two-dimensional phase of boron nitride: Few-atomic-layer sheets and suspended membranes (2008) Appl. Phys. Lett., 92, pp. 133107-1331073Pakdel, A., Zhi, C., Bando, Y., Nakayama, T., Golberg, D., Boron nitride nanosheet coatings with controllable water repellency (2011) ACS Nano, 5, pp. 6507-6515Kimura, Y., Wakabayashi, T., Okada, K., Wada, T., Nishikawa, H., Boron nitride as a lubricant additive (1999) Wear, 232 (2), pp. 199-206. , DOI 10.1016/S0043-1648(99)00146-5, PII S0043164899001465Taha-Tijerina, J., Electrically insulating thermal nano-oils using 2D fillers (2012) ACS Nano, 6, pp. 1214-1220Song, L., Large scale growth and characterization of atomic hexagonal boron nitride layers (2010) Nano Lett., 10, pp. 3209-3215Nag, A., Graphene analogues of BN: Novel synthesis and properties (2010) ACS Nano, 4, pp. 1539-1544Neto, A.H.C., Novoselov, K., Two-dimensional crystals: Beyond graphene (2011) Mater. Exp., 1, pp. 10-17Van Duin, A.C.T., Dasgupta, S., Lorant, F., Goddard, W.A., ReaxFF: A reactive force field for hydrocarbons (2001) J. Phys. Chem. A, 105, pp. 9396-9409Paci, J.T., Belytschko, T., Schatz, G.C., Computational studies of the structure, behavior upon heating, and mechanical properties of graphite oxide (2007) J. Phys. Chem. C, 111, pp. 18099-18111Grantab, R., Shenoy, V.B., Ruoff, R.S., Anomalous strength characteristics of tilt grain boundaries in graphene (2010) Science, 330, pp. 946-948Perim, E., Autreto, P.A.S., Paupitz, R., Galvao, D.S., Dynamical aspects of the unzipping of multiwalled boron nitride nanotubes (2013) Phys. Chem. Chem. Phys., 15, p. 19147Marcano, D.C., Improved synthesis of graphene oxide (2010) ACS Nano, 4, pp. 4806-4814Coleman, J.N., Two-dimensional nanosheets produced by liquid exfoliation of layered materials (2011) Science, 331, pp. 568-571Mortier, W.J., Ghosh, S.K., Shankar, S., Electronegativity-equalization method for the calculation of atomic charges in molecules (1986) J. Am. Chem. Soc., 108, pp. 4315-4320Plimpton, S., Fast parallel algorithms for short-range molecular dynamics (1995) J. Comput. Phys., 117, pp. 1-1

Multi-component (Ag–Au–Cu–Pd–Pt) alloy nanoparticle-decorated p-type 2D-molybdenum disulfide (MoS2) for enhanced hydrogen sensing

Molybdenum disulfide (MoS2) has emerged as a promising material for the development of efficient sensors. Here, we have exfoliated and decorated MoS2 flakes with the novel, single-phase multi-component silver–gold–copper–palladium–platinum (Ag–Au–Cu–Pd–Pt) alloy nanoparticles, popularly named High Entropy Alloy (HEA) nanoparticles, using facile and scalable low-temperature grinding, followed by the sonochemical method. It was found that the decoration of HEA nanoparticles imparts the surface-enhanced Raman scattering effect and reduction in the work function of the material from 4.9 to 4.75 eV as measured by UV photoelectron spectroscopy. This change in the work function resulted in a Schottky barrier between the gold contact and HEA decorated MoS2 flakes as a result of drastic changes in the surface chemical non-stoichiometry. The response to hydrogen gas was studied at temperatures in the range of 30 to 100 °C, and it showed an unusual p-type nature due to surface-adsorbed oxygen species. The nanoscale junction formed between HEA and MoS2 showed a ten-time increase in the response towards hydrogen gas at 80 °C. The experimental observations have been explained with DFT simulation showing more favourable hydrogen adsorption on HEA-decorated MoS2 resulting in an enhanced response

A Perspective on the Catalysis Using the High Entropy Alloys

The near equimolar and non-equimolar high entropy alloys (HEAs) having five or more major components along with their mingled sites over the surface have made them unique materials for various catalytic reactions involving renewable energies. HEAs provide a platform to tune the surface microstructure and chemistry by selecting and controlling the elements, opening up vistas to design new materials for catalysis. The present perspective aims to provide the correlation between HEAs' structure and catalytic performance in various applications with views on challenges and unique opportunities. The scientific and technological curiosity needs to dig deep into the multicomponent phase space to discover various new materials with unique catalytic properties

Formic acid and methanol electro-oxidation and counter hydrogen production using nano high entropy catalyst

Renewable harvesting of clean energy using the benefits of multi-metallic catalytic materials of high entropy alloy (HEA, equimolar CueAgeAuePtePd) from formic acid with minimum energy input has been achieved in the present investigation. The synergistic effect of pristine elements in the multimetallic HEA drives the electro-oxidation reaction towards non-carbonaceous pathway. The atomistic based simulations based on DFT rationalize the distinct lowering of the d-band center for the individual atoms in the HEA as compared to the pristine counterparts. Further this catalytic activity of the HEA has also been extended to methanol electro-oxidation to show the unique capability of the novel catalyst. The nanostructured HEA, prepared using a combination of casting and cryomilling techniques can further be utilized as the fuel cell anode in the direct formic acid/methanol fuel cells (DFFE)

Nanodiamond-Based Thermal Fluids

Dispersions of nanodiamond (average size ∼6 nm) within dielectric insulator mineral oil are reported for their enhanced thermal conductivity properties and potential applications in thermal management. Dynamic and kinematic viscositiesvery important parameters in thermal management by nanofluidsare investigated. The dependence of the dynamic viscosity is well-described by the theoretical predictions of Einstein’s model. The temperature dependence of the dynamic viscosity obeys an Arrhenius-like behavior, where the activation energy and the pre-exponential factor have an exponential dependence on the filler fraction of nanodiamonds. An enhancement in thermal conductivity up to 70% is reported for nanodiamond based thermal fluids. Additional electron microscopy, Raman spectroscopy and X-ray diffraction analysis support the experimental data and their interpretation

Methanol Electrolysis for Hydrogen Production Using Polymer Electrolyte Membrane: A Mini-Review

Hydrogen (H2) has attained significant benefits as an energy carrier due to its gross calorific value (GCV) and inherently clean operation. Thus, hydrogen as a fuel can lead to global sustainability. Conventional H2 production is predominantly through fossil fuels, and electrolysis is now identified to be most promising for H2 generation. This review describes the recent state of the art and challenges on ultra-pure H2 production through methanol electrolysis that incorporate polymer electrolyte membrane (PEM). It also discusses about the methanol electrochemical reforming catalysts as well as the impact of this process via PEM. The efficiency of H2 production depends on the different components of the PEM fuel cells, which are bipolar plates, current collector, and membrane electrode assembly. The efficiency also changes with the nature and type of the fuel, fuel/oxygen ratio, pressure, temperature, humidity, cell potential, and interfacial electronic level interaction between the redox levels of electrolyte and band gap edges of the semiconductor membranes. Diverse operating conditions such as concentration of methanol, cell temperature, catalyst loading, membrane thickness, and cell voltage that affect the performance are critically addressed. Comparison of various methanol electrolyzer systems are performed to validate the significance of methanol economy to match the future sustainable energy demands.Scopu

Low-density three-dimensional foam using self-reinforced hybrid two-dimensional atomic layers

Low-density nanostructured foams are often limited in applications due to their low mechanical and thermal stabilities. Here we report an approach of building the structural units of three-dimensional (3D) foams using hybrid two-dimensional (2D) atomic layers made of stacked graphene oxide layers reinforced with conformal hexagonal boron nitride (h-BN) platelets. The ultra-low density (1/400 times density of graphite) 3D porous structures are scalably synthesized using solution processing method. A layered 3D foam structure forms due to presence of h-BN and significant improvements in the mechanical properties are observed for the hybrid foam structures, over a range of temperatures, compared with pristine graphene oxide or reduced graphene oxide foams. It is found that domains of h-BN layers on the graphene oxide framework help to reinforce the 2D structural units, providing the observed improvement in mechanical integrity of the 3D foam structur