Room Temperature Growth of Ultrathin Au Nanowires with High Areal Density over Large Areas by <i>in Situ</i> Functionalization of Substrate

Although ultrathin Au nanowires (∼2 nm diameter) are expected to demonstrate several interesting properties, their extreme fragility has hampered their use in potential applications. One way to improve the stability is to grow them on substrates; however, there is no general method to grow these wires over large areas. The existing methods suffer from poor coverage and associated formation of larger nanoparticles on the substrate. Herein, we demonstrate a room temperature method for growth of these nanowires with high coverage over large areas by <i>in situ</i> functionalization of the substrate. Using control experiments, we demonstrate that an <i>in situ</i> functionalization of the substrate is the key step in controlling the areal density of the wires on the substrate. We show that this strategy works for a variety of substrates ranging like graphene, borosil glass, Kapton, and oxide supports. We present initial results on catalysis using the wires grown on alumina and silica beads and also extend the method to lithography-free device fabrication. This method is general and may be extended to grow ultrathin Au nanowires on a variety of substrates for other applications

Semiconductor-like Sensitivity in Metallic Ultrathin Gold Nanowire-Based Sensors

Due to the ease of modification of electronic structure upon analyte adsorption, semiconductors have been the preferred materials as chemical sensors. At reduced dimension, however, the sensitivity of semiconductor-based sensors deteriorates significantly due to passivation, and often by increased band gap caused by quantum confinement. Using first-principles density functional theory combined with Boltzmann transport calculations, we demonstrate semiconductor-like sensitivity toward chemical species in ultrathin gold nanowires (AuNWs). The sensing mechanism is governed by the modification of the electronic structure of the AuNW as well as scattering of the charge carriers by analyte adsorption. Most importantly, the sensitivity exhibits a linear relationship with the electron affinities of the respective analytes. Based on this relationship, we propose an empirical parameter, which can predict an analyte-specific sensitivity of a AuNW, rendering them as effective sensors for a wide range of chemical analytes

Synthesis of Hollow Nanotubes of Zn₂SiO₄ or SiO₂: Mechanistic Understanding and Uranium Adsorption Behavior

We report a facile synthesis of Zn₂SiO₄ nanotubes using a two-step process consisting of a wet-chemical synthesis of core–shell ZnO@SiO₂ nanorods followed by thermal annealing. While annealing in air leads to the formation of hollow Zn₂SiO₄, annealing under reducing atmosphere leads to the formation of SiO₂ nanotubes. We rationalize the formation of the silicate phase at temperatures much lower than the temperatures reported in the literature based on the porous nature of the silica shell on the ZnO nanorods. We present results from in situ transmission electron microscopy experiments to clearly show void nucleation at the interface between ZnO and the silica shell and the growth of the silicate phase by the Kirkendall effect. The porous nature of the silica shell is also responsible for the etching of the ZnO leading to the formation of silica nanotubes under reducing conditions. Both the hollow silica and silicate nanotubes exhibit good uranium sorption at different ranges of pH making them possible candidates for nuclear waste management

Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Atomic Structure of Quantum Gold Nanowires: Quantification of the Lattice Strain

Theoretical studies exist to compute the atomic arrangement in gold nanowires and the influence on their electronic behavior with decreasing diameter. Experimental studies, <i>e.g.</i>, by transmission electron microscopy, on chemically synthesized ultrafine wires are however lacking owing to the unavailability of suitable protocols for sample preparation and the stability of the wires under electron beam irradiation. In this work, we present an atomic scale structural investigation on quantum single crystalline gold nanowires of 2 nm diameter, chemically prepared on a carbon film grid. Using low dose aberration-corrected high resolution (S)TEM, we observe an inhomogeneous strain distribution in the crystal, largely concentrated at the twin boundaries and the surface along with the presence of facets and surface steps leading to a noncircular cross section of the wires. These structural aspects are critical inputs needed to determine their unique electronic character and their potential as a suitable catalyst material. Furthermore, electron-beam-induced structural changes at the atomic scale, having implications on their mechanical behavior and their suitability as interconnects, are discussed

Synergistic Effect of Mo + Cu Codoping on the Photocatalytic Behavior of Metastable TiO₂ Solid Solutions

Codoping with Cu and Mo is shown to have a synergistic effect on the photocatalytic activity of TiO₂. The enhancement in activity is observed only if the synthesis route results in TiO₂ in which (Cu, Mo) codopants are forced into the TiO₂ lattice. Using X-ray photoelectron spectroscopy, Cu and Mo are shown to be present in the +2 and +6 oxidation states, respectively. A systematic study of the ternary system shows that TiO₂ containing 6 mol % CuO and 1.5 mol % MoO₃ is the most active ternary composition. Ab initio calculations show that codoping of TiO₂ using (Mo, Cu) introduces levels above the valence band, and below the conduction band, resulting in a significant reduction in the band gap (∼0.8 eV). However, codoping also introduces deep defect states, which can have a deleterious impact on photoactivity. This helps rationalize the narrow compositional window over which the enhancement in photocatalytic activity is observed

Orientation Selection during Heterogeneous Nucleation: Implications for Heterogeneous Catalysis

Hybrids based on supported nanoparticles are used for many applications such as catalysis and sensing. It is well-known that the size and shape of the particles and their interaction with the substrate play a key role in controlling the properties of the hybrid. Here, we show that in addition to these commonly considered effects, the orientation of the particle on the substrate could play a critical role that could dominate over the other effects. For the same nominal size of the particle, changes in its orientation on the substrate lead to dramatic changes in the accessible surface area and the electronic structure, thus profoundly affecting the properties. Using analytical calculations, we show that the orientation of the crystal, heterogeneously nucleating on a substrate, is determined by the barrier for nucleation and experimentally demonstrate that it is possible to tune this orientation by tuning the interfacial energy between the nucleus and substrate. Our study provides some fundamentally new insights into the process of heterogeneous nucleation that can be exploited for practical applications

Existence of Ti²⁺ States on the Surface of Heavily Reduced SrTiO₃ Nanocubes

Existence of Ti²⁺ States on the Surface of Heavily Reduced SrTiO₃ Nanocube

Designing Diameter-Modulated Heterostructure Nanowires of PbTe/Te by Controlled Dewetting

Heterostructures consisting of semiconductors with controlled morphology and interfaces find applications in many fields. A range of axial, radial, and diameter-modulated nanostructures have been synthesized primarily using vapor phase methods. Here, we present a simple wet chemical routine to synthesize heterostructures of PbTe/Te using Te nanowires as templates. A morphology evolution study for the formation of these heterostructures has been performed. On the basis of these control experiments, a pathway for the formation of these nanostructures is proposed. Reduction of a Pb precursor to Pb on Te nanowire templates followed by interdiffusion of Pb/Te leads to the formation of a thin shell of PbTe on the Te wires. Controlled dewetting of the thin shell leads to the formation of cube-shaped PbTe that is periodically arranged on the Te wires. Using control experiments, we show that different reactions parameters like rate of addition of the reducing agent, concentration of Pb precursor and thickness of initial Te nanowire play a critical role in controlling the spacing between the PbTe cubes on the Te wires. Using simple surface energy arguments, we propose a mechanism for the formation of the hybrid. The principles presented are general and can be exploited for the synthesis of other nanoscale heterostructures

Metal−Dielectric Interface Toughening by Catalyzed Ring Opening in a Monolayer

We demonstrate a novel strategy for toughening metal−dielectric interfaces by catalyzed fissure of low-polarizability moieties in an organosilane monolayer. Photoelectron spectroscopy and ab initio calculations show that seven-fold toughening of Cu−silica interfaces is due to Cu-catalyzed disilacyclobutane ring opening and bonding. Our findings open up possibilities for directly integrating metals with molecularly derived low permittivity dielectrics for applications without using an intermediary glue layer, for example, by incorporating strained moieties into polymer precursors