Anisotropic Growth of Silver Nanocubes: The Role of Bromide Adsorption and Hydrophilic Polymers

The application of capping agents to modulate the colloidal synthesis of metal nanocrystals offers an effective avenue for shape control, but the roles of such agents are not yet completely understood. This study uses seed-mediated growth, single-crystal electrochemistry, and surface-enhanced Raman spectroscopy (SERS) to illuminate the roles of polyvinyl­pyrrolidone (PVP) and bromide (Br–) in the anisotropic growth of Ag nanocubes. Synthetic results show that Ag nanocubes only form in the presence of both PVP and sufficiently high concentrations of Br–. Truncated octahedra form in the presence of PVP, and truncated cubes form in the presence of Br– alone. Electrochemical measurements indicate that elevated concentrations of Br– consistently passivate Ag(100) facets more than Ag(111) facets regardless of the presence of PVP. The critical condition for the growth of cubes, however, materializes only when both PVP and Br– are present, which sufficiently suppresses atomic deposition to Ag(100) relative to Ag(111). SERS measurements show that high concentrations of Br– displace PVP from Ag(100) and Ag(111), but electrochemical measurements suggest PVP acts as a strong passivator under such conditions. We propose that the chemisorption of Br– beneath a physisorbed layer of PVP creates a unique condition for the growth of Ag nanocubes. Furthermore, our investigation points to a generalizable adsorption structure for the synthesis of {100}-faceted Ag nanocrystals, where Br– adsorption under other hydrophilic polymers can similarly guide the formation of Ag nanocubes. This study enhances our understanding of the synergistic roles of Br– and hydrophilic polymers in the controlled morphogenesis of Ag nanocrystals

Effect of Morphology on the Electrical Resistivity of Silver Nanostructure Films

The relatively high temperatures (>200 °C) required to sinter silver nanoparticle inks have limited the development of printed electronic devices on low-cost, heat-sensitive paper and plastic substrates. This article explores the change in morphology and resistivity that occurs upon heating thick films of silver nanowires (of two different lengths; Ag NWs), nanoparticles (Ag NPs), and microflakes (Ag MFs) at temperatures between 70 and 400 °C. After heating at 70 °C, films of long Ag NWs exhibited a resistivity of 1.8 × 10^–5 Ω cm, 4000 times more conductive than films made from Ag NPs. This result indicates the resistivity of thick films of silver nanostructures is dominated by the contact resistance between particles before sintering. After sintering at 300 °C, the resistivity of short Ag NWs, long Ag NWs, and Ag NPs converge to a value of (2–3) × 10^–5 Ω cm, while films of Ag MFs remain ∼10× less conductive (4.06 × 10^–4 Ω cm). Thus, films of long Ag NW films heated at 70 °C are more conductive than Ag NP films sintered at 300 °C. Adding 10 wt % nanowires to a film of nanoparticles results in a 400-fold improvement in resistivity

Synthesis of Oxidation-Resistant Cupronickel Nanowires for Transparent Conducting Nanowire Networks

Nanowires of copper can be coated from liquids to create flexible, transparent conducting films that can potentially replace the dominant transparent conductor, indium tin oxide, in displays, solar cells, organic light-emitting diodes, and electrochromic windows. One issue with these nanowire films is that copper is prone to oxidation. It was hypothesized that the resistance to oxidation could be improved by coating copper nanowires with nickel. This work demonstrates a method for synthesizing copper nanowires with nickel shells as well as the properties of cupronickel nanowires in transparent conducting films. Time- and temperature-dependent sheet resistance measurements indicate that the sheet resistance of copper and silver nanowire films will double after 3 and 36 months at room temperature, respectively. In contrast, the sheet resistance of cupronickel nanowires containing 20 mol % nickel will double in about 400 years. Coating copper nanowires to a ratio of 2:1 Cu:Ni gave them a neutral gray color, making them more suitable for use in displays and electrochromic windows. These properties, and the fact that copper and nickel are 1000 times more abundant than indium or silver, make cupronickel nanowires a promising alternative for the sustainable, efficient production of transparent conductors

Stretchable Conductive Composites from Cu–Ag Nanowire Felt

Materials that retain a high conductivity under strain are essential for wearable electronics. This article describes a conductive, stretchable composite consisting of a Cu–Ag core–shell nanowire felt infiltrated with a silicone elastomer. This composite exhibits a retention of conductivity under strain that is superior to any composite with a conductivity greater than 1000 S cm^–1. This work also shows how the mechanical properties, conductivity, and deformation mechanism of the composite changes as a function of the stiffness of the silicone matrix. The retention of conductivity under strain was found to decrease as the Young’s modulus of the matrix increased. This was attributed to void formation as a result of debonding between the nanowire felt and the elastomer. The nanowire composite was also patterned to create serpentine circuits with a stretchability of 300%

Real-Time Visualization of Diffusion-Controlled Nanowire Growth in Solution

This Letter shows that copper nanowires grow through the diffusion-controlled reduction of dihydroxycopper­(I), Cu­(OH)₂^–. A combination of potentiostatic coulometry, UV–visible spectroscopy, and thermodynamic calculations was used to determine the species adding to growing Cu nanowires is Cu­(OH)₂^–. Cyclic voltammetry was then used to measure the diffusion coefficient of Cu­(OH)₂^– in the reaction solution. Given the diameter of a Cu nanowire and the diffusion coefficient of Cu­(OH)₂^–, we calculated the dependence of the diffusion-limited growth rate on the concentration of copper ions to be 26 nm s^–1 mM^–1. Independent measurements of the nanowire growth rate with dark-field optical microscopy yielded 24 nm s^–1 mM^–1 for the growth rate dependence on the concentration of copper. Dependence of the nanowire growth rate on temperature yielded a low activation energy of 11.5 kJ mol^–1, consistent with diffusion-limited growth

Photocatalytic Growth of Copper Nanowires from Cu₂O Seeds

This article describes the photocatalytic growth of copper nanowires from Cu₂O octahedra. When exposed to visible light with an energy greater than the band gap of Cu₂O, electrons excited from the valence band to the conduction band within Cu₂O octahedra reduce Cu­(OH)₂^– onto the octahedra to form copper nanowires. This phenomenon was used to turn nanowire growth on and off with visible light, as well as pattern the growth of nanowires on a substrate

Reversible Sliding in Networks of Nanowires

This work demonstrates that metal nanowires in a percolating network can reversibly slide across one another. Reversible sliding allows networks of metal nanowires to maintain electrical contact while being stretched to strains greater than the fracture strain for individual nanowires. This phenomenon was demonstrated by using networks of nanowires as compliant electrodes for a dielectric elastomer actuator. Reversible nanowire sliding enabled actuation to a maximum area strain of 200% and repetitive cycling of the actuator to an area strain of 25% over 150 times. During actuation, the transmittance of the network increased 4.5 times, from 13% to 58%. Compared to carbon-based compliant electrodes, networks of metal nanowires can actuate across a broader range of optical transmittance. The widely tunable transmittance of nanowire-based actuators allows for their use as a light valve

Synthesis and Purification of Silver Nanowires To Make Conducting Films with a Transmittance of 99%

Metal nanowire (NW) networks have the highest performance of any solution-coatable alternative to ITO, but there is as yet no published process for producing NW films with optoelectronic performance that exceeds that of ITO. Here, we demonstrate a process for the synthesis and purification of Ag NWs that, when coated from an ink to create a transparent conducting film, exhibit properties that exceed that of ITO. The diameter, and thus optoelectronic performance, of Ag NWs produced by a polyol synthesis can be controlled by adjusting the concentration of bromide. Ag NWs with diameters of 20 nm and aspect ratios up to 2000 were obtained by adding 2.2 mM NaBr to a Ag NW synthesis, but these NWs were contaminated by nanoparticles. Selective precipitation was used to purify the NWs, resulting in a transmittance improvement as large as 4%. At 130.0 Ω sq^–1, the transmittance of the purified Ag NW film was 99.1%

Synthesis of Cu–Ag, Cu–Au, and Cu–Pt Core–Shell Nanowires and Their Use in Transparent Conducting Films

This article describes a room-temperature solution-phase process for the synthesis of copper–silver (Cu–Ag), copper–gold (Cu–Au), and copper–platinum (Cu–Pt) core–shell nanowires (NWs) in which ascorbic acid removes the passivating copper oxide coating from the Cu NWs and reduces noble metal ions onto the Cu NWs while preventing galvanic replacement. Cu–Ag NWs are conductive as printed, and the resulting NW films exhibit optoelectronic properties equivalent to films of Ag NWs with a similar aspect ratio. Unlike Cu NWs, Cu–Ag NWs were resistant to oxidation in dry air at 160 °C and under humid conditions (85% RH) at 85 °C for 24 h