Shuttling Active Elements in and out of Ultrathin Nanowires: Toward Rational Design of Multicomponent Catalysts.

One-dimensional nanostructures, with a high ratio of surface-bulk atoms, find applications as active catalysts. Here, we report tunability in ultrathin single-crystalline AuPdPt nanowires by modifying synthesis conditions and postsynthetic treatment in a controlled ambient atmosphere. The surface microstructure modification of these nanostructures has been analyzed by integrating the results of three crucial techniques including Z-contrast HAADF-STEM imaging, X-ray photoelectron spectroscopy, and electrochemically active surface area from cyclic voltammograms.

Tuning Catalytic Activity in Ultrathin Bimetallic Nanowires via Surface Segregation: Some Insights.

The efficiency of heterogeneous catalysts critically depends on the nature of the surface. We present results on controlling the composition in ultrathin bimetallic AuPd. AuPd wires were grown using Au nanowire templates; the surface composition could be tuned by increasing the amount of Pd. Further, segregation of Pd to the surface could be induced in alloyed nanowires by annealing under a controlled CO atmosphere. Electrocatalytic activity of these bimetallic systems is assessed for the methanol oxidation reaction (MOR). While the MOR potential shows a monotonic increase with Pd content, the specific activity displays a typical volcano-type behavior. The CO-annealed nanowires show a lowering of potential owing to a higher Pd content on the surface while still maintaining the specific activity. These findings provide clear strategies to independently control the reaction potential and the activities of nanocatalysts. The experimental findings are well supported by the theoretical investigations using density functional theory (DFT) calculations.

Designing complex radial heterostructures of Te/Bi2Te3 and Te/Bi2-x Pb x Te3 nanowires: fundamental mechanistic insights into nanowire growth and evolution.

Metal telluride/Te heterostructure nanowires are important thermoelectric materials and it is important to be able to tune these materials according to the requirement of the application. In order to do so, a good understanding of the reaction mechanism and critical observation of the evolution of the nanowire heterostructure during the course of reaction is essential. Here, single crystalline, anisotropic Te core/Bi2Te3 shell nanowires have been synthesized by a facile template-based wet chemical synthesis method. The formation and evolution mechanism of the heterostructure has been elucidated by several control reactions, detailed transmission electron microscopy imaging and composition analysis using energy dispersive spectroscopy in scanning transmission electron microscopy mode of the products of the reactions. Fundamental understanding of the formation mechanism and time-dependent evolution of the core-shell structure in the nanowire have led to successful designing of higher order heterostructures involving Te/Bi2-x Pb x Te3. Through this study, interesting insights into the crystal structure evolution, crystal growth and miscibility of PbTe and Bi2Te3 into each other is obtained.

Designing Complex Radial Heterostructures of Te/Bi2Te3and Te/Bi2-xPbxTe3Nanowires: Fundamental Mechanistic Insights into Nanowire Growth and Evolution.

Metal Telluride/Te heterostructure nanowires are important thermoelectric materials and it is important to be able to tune these materials according to the requirement of the application. In order to do so, a good understanding of the reaction mechanism and critical observation of the evolution of the nanowire heterostructure during the course of reaction is essential. Here, single crystalline, anisotropic Te core/ Bi2Te3 shell nanowires have been synthesized by a facile template-based wet chemical synthesis method. The formation and evolution mechanism of the heterostructure has been elucidated by several control reactions, detailed transmission electron microscopy (TEM) and composition analysis using energy dispersive spectroscopy (EDS) in scanning transmission electron microscopy (STEM) mode of the products of the reactions. Fundamental understanding of the formation mechanism and time-dependent evolution of the core-shell structure in the nanowire have led to successful designing of higher order heterostructures involving Te/Bi2-xPbxTe3. Through this study, interesting insights into the crystal structure evolution, crystal growth and miscibility of PbTe and Bi2Te3 into each other is obtained.

Ultra-sensitive graphene-bismuth telluride nano-wire hybrids for infrared detection.

The myriad technological applications of infrared radiation sensors make the search for ultra-sensitive detectors extremely crucial. Materials such as bismuth telluride (Bi2Te3), having a small bulk band gap of 0.17 eV, are ideal infrared detectors. However, due to the high recombination rate of photo-generated charge carriers in the bulk, the electrical response under optical illumination is typically very weak in these materials. We have circumnavigated this by sensitizing graphene with Bi2Te3 nano-wires. These hybrid devices show an ultra-high sensitivity of ∼106 A W-1, under incident electromagnetic radiation from 940 nm to 1720 nm. The theoretical limit of the noise equivalent power and specific detectivity in these devices are ∼10-18 W Hz-1/2 and ∼1011 Jones respectively, which are comparable to those of some of the best known detectors.

Core-Multishell Heterostructure with Excellent Heat Dissipation for Electromagnetic Interference Shielding.

Herein, we report high electromagnetic interference (EMI) shielding effectiveness of -40 dB in the Ku-band (for a 600 µm thick film) through a unique core-shell heterostructure consisting of a ferritic core (Fe3O4) and a conducting shell (multiwalled carbon nanotubes, MWCNTs) supported onto a dielectric spacer (here SiO2). In recent times, materials with good flexibility, heat dissipation ability, and sustainability together with efficient EMI shielding at minimal thickness are highly desirable, especially if they can be easily processed into thin films. The resulting composites here shielded EM radiation mostly through absorption driven by multiple interfaces provided by the heterostructure. The shielding value obtained here is fairly superior among the different polymer nanocomposite-based EMI shielding materials. In addition to EMI shielding capability, this composite material exhibits outstanding heat dissipation ability (72 °C to room temperature in less than 90 s) as well as high heat sustainability. The composite material retained its EMI shielding property even after repeated heat cycles, thereby opening new avenues in the design of lightweight, flexible, and sustainable EMI shielding materials.

Scalable faceted voids with luminescent enhanced edges in WS2 monolayers.

A scalable approach is needed in the formation of atomically flat edges with specific terminations to enhance local properties for optoelectronic, nanophotonic and energy applications. We demonstrate point defect clustering-driven faceted void formations with luminescent enhanced edges in WS2 monolayers during large-scale CVD growth and controlled annealing. With the aid of aberration-corrected scanning transmission electron microscopy (AC-STEM) high angle annular dark field (HAADF) imaging, we probed atomic terminations of S and W to explain observed luminescence enhancement in alternate edges. Faceted void formation in monolayer WS2 was found to be sensitive to annealing temperature, time, gas environment and precursor supply. Our observations of areal coverage evolution over time revealed competition between monolayer WS2 growth and void formation at 850 °C. While the initial stage was dominated by monolayer growth, defect generation and void growth dominated at later stages and provided an optimum processing window for monolayer WS2 as well as faceted void growth. Growth of faceted voids not only followed the geometry of monolayer facets but also showed similar atomic terminations at the edges and thus enabled local manipulation of photoluminescence enhancement with an order of magnitude increase in intensity. The developed CVD processing enabled multi-fold increase in the luminescent active edge length through the formation of faceted voids within the WS2 monolayer.

Ultrathin Au-Alloy Nanowires at the Liquid-Liquid Interface.

Ultrathin bimetallic nanowires are of importance and interest for applications in electronic devices such as sensors and heterogeneous catalysts. In this work, we have designed a new, highly reproducible and generalized wet chemical method to synthesize uniform and monodispersed Au-based alloy (AuCu, AuPd, and AuPt) nanowires with tunable composition using microwave-assisted reduction at the liquid-liquid interface. These ultrathin alloy nanowires are below 4 nm in diameter and about 2 µm long. Detailed microstructural characterization shows that the wires have an face centred cubic (FCC) crystal structure, and they have low-energy twin-boundary and stacking-fault defects along the growth direction. The wires exhibit remarkable thermal and mechanical stability that is critical for important applications. The alloy wires exhibit excellent electrocatalytic activity for methanol oxidation in an alkaline medium.