This article demonstrates a two-step method to 3D print double network hydrogels at room temperature with a low-cost ($300) 3D printer. A first network precursor solution was made 3D printable via extrusion from a nozzle by adding a layered silicate to make it shear-thinning. After printing and UV-curing, objects were soaked in a second network precursor solution and UV-cured again to create interpenetrating networks of poly(2-acrylamido-2-methylpropanesulfonate) and polyacrylamide. By varying the ratio of polyacrylamide to cross-linker, the trade-off between stiffness and maximum elongation of the gel can be tuned to yield a compression strength and elastic modulus of 61.9 and 0.44 MPa, respectively, values that are greater than those reported for bovine cartilage. The maximum compressive (93.5 MPa) and tensile (1.4 MPa) strengths of the gel are twice that of previous 3D printed gels, and the gel does not deform after it is soaked in water. By 3D printing a synthetic meniscus from an X-ray computed tomography image of an anatomical model, we demonstrate the potential to customize hydrogel implants based on 3D images of a patient’s anatomy.Keywords: 3D printing; double network hydrogel; tissue engineering; tough hydrogel;

This article describes a fully printed memory in which a composite of Cu–SiO2 nanowires dispersed in ethylcellulose acts as a resistive switch between printed Cu and Au electrodes. A 16-cell crossbar array of these memristors was printed with an aerosol jet. The memristors exhibited moderate operating voltages (∼3 V), no degradation over 104 switching cycles, write speeds of 3 μs, and extrapolated retention times of 10 years. The low operating voltage enabled the programming of a fully printed 4-bit memristor array with an Arduino. The excellent performance of these fully printed memristors could help enable the creation of fully printed RFID tags and sensors with integrated data storage.

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.Keywords: electrical resistivity; silver microflakes; silver nanoparticles; silver nanowires; sintering;

Scalable, solution-phase nanostructure synthesis has the promise to produce a wide variety of nanomaterials with novel properties at a cost that is low enough for these materials to be used to solve problems. For example, solution-synthesized metal nanowires are now being used to make low cost, flexible transparent electrodes in touch screens, organic light-emitting diodes (OLEDs), and solar cells. There has been a tremendous increase in the number of solution-phase syntheses that enable control over the assembly of atoms into nanowires in the last 15 years, but proposed mechanisms for nanowire formation are usually qualitative, and for many syntheses there is little consensus as to how nanowires form. It is often not clear what species is adding to a nanowire growing in solution or what mechanistic step limits its rate of growth. A deeper understanding of nanowire growth is important for efficiently directing the development of nanowire synthesis toward producing a wide variety of nanostructure morphologies for structure–property studies or producing precisely defined nanostructures for a specific application.This Account reviews our progress over the last five years toward understanding how copper nanowires form in solution, how to direct their growth into nanowires with dimensions ideally suited for use in transparent conducting films, and how to use copper nanowires as a template to grow core–shell nanowires. The key advance enabling a better understanding of copper nanowire growth is the first real-time visualization of nanowire growth in solution, enabling the acquisition of nanowire growth kinetics. By measuring the growth rate of individual nanowires as a function of concentration of the reactants and temperature, we show that a growing copper nanowire can be thought of as a microelectrode that is charged with electrons by hydrazine and grows through the diffusion-limited addition of Cu(OH)2–. This deeper mechanistic understanding, coupled to an understanding of the structure–property relationship of nanowires in transparent conducting films, enabled the production of copper nanowires that can be coated from solution to make films with properties that rival the dominant transparent conductor, indium tin oxide. Finally, we show how copper nanowires can be coated with Zn, Sn, In, Ni, Co, Ag, Au, and Pt to protect them from oxidation or enable their use as transparent electrocatalysts.

The synthesis of metal nanostructures usually requires a capping agent that is generally thought to cause anisotropic growth by blocking the addition of atoms to specific crystal facets. This work uses a series of electrochemical measurements with a quartz crystal microbalance and single-crystal electrodes to elucidate the facet-selective chemistry occurring in the synthesis of Cu nanowires. Contrary to prevailing hypotheses, ethylenediamine, a so-called capping agent in the synthesis of Cu nanowires, causes anisotropic growth by increasing the rate of atomic addition to (111) facets at the end of a growing nanowire relative to (100) facets on the sides of a nanowire. Ethylenediamine increases the reduction rate of Cu(OH)2– on a Cu(111) surface relative to Cu(100) by selectively inhibiting the formation of Cu oxide on Cu(111). This work demonstrates how studying facet-selective electrochemistry can improve the understanding of the processes by which atoms assemble to form anisotropic metal nanostructures.

Printable electronics has the potential to drastically reduce the environmental and economic costs associated with the production of electronic devices, as well as enable rapid prototyping of circuits and their printing on demand, similar to what 3D printing has done for structural objects. A major barrier to the realization of printable computers that can run programs is the lack of a solution-coatable non-volatile memory with performance metrics comparable to silicon-based devices. Here we demonstrate a non-volatile memory based on Cu–SiO2 core–shell nanowires that can be printed from solution and exhibits on-off ratios of 106, switching speeds of 50 ns, a low operating voltage of 2 V, and operates for at least 104 cycles without failure. Each of these metrics is similar to or better than Flash memory (the write speed is 20 times faster than Flash). Memory architectures based on the individual memory cells demonstrated here could enable the printing of the more complex, embedded computing devices that are expected to make up an internet of things.

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%.

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.

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.

Since the discovery of the role of oxidative etching in shape-controlled metal nanostructure synthesis in 2004, it has become a versatile tool to precisely manipulate the nucleation and growth of metal nanocrystals at the atomic level. Subsequent research has shown that oxidative etching can be used to reshape nanocrystals via atomic addition and subtraction. This research has attracted extensive attention from the community because of its promising practical applications and theoretical value, and as a result, tremendous efforts from numerous research groups have been made to expand and apply this method to their own research. In this review, we first outline the merits of oxidative etching for the controlled synthesis of metal nanocrystals. We then summarize recent progress in the use of oxidative etching to control the morphology of a nanostructure during and after its synthesis, and analyze its specific functions in controlling a variety of nanocrystal parameters. Applications enabled by oxidative etching are also briefly presented to show its practical impact. Finally, we discuss the challenges and opportunities for further development of oxidative etching in nanocrystals synthesis.

This article reports the fabrication of copper–platinum core–shell nanowires by electroplating platinum onto copper nanowires, and the first demonstration of their use as a transparent, conducting electrocatalyst for the hydrogen evolution reaction (HER). Cu–Pt core–shell nanowire networks exhibit mass activities up to 8 times higher than carbon-supported Pt nanoparticles for the HER. Electroplating minimizes galvanic replacement, allowing the copper nanowires to retain their conductivity, and eliminating the need for a conductive substrate or overcoat. Cu–Pt core–shell nanowire networks can thus replace more expensive transparent electrodes made from indium tin oxide (ITO) in photoelectrolysis cells and dye sensitized solar cells. Unlike ITO, Cu–Pt core–shell nanowire films retain their conductivity after bending, retain their transmittance during electrochemical reduction, and have consistently high transmittance (>80%) across a wide optical window (300–1800 nm).

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.

This work describes a process to make anodes for organic solar cells from copper–nickel nanowires with solution-phase processing. Copper nanowire films were coated from solution onto glass and made conductive by dipping them in acetic acid. Acetic acid removes the passivating oxide from the surface of copper nanowires, thereby reducing the contact resistance between nanowires to nearly the same extent as hydrogen annealing. Films of copper nanowires were made as oxidation resistant as silver nanowires under dry and humid conditions by dipping them in an electroless nickel plating solution. Organic solar cells utilizing these completely solution-processed copper–nickel nanowire films exhibited efficiencies of 4.9%.

This communication presents a way to produce copper nanowires with aspect ratios as high as 5700 in 30 min, and describes the growth processes responsible for their formation. These nanowires were used to make transparent conducting films with a transmittance >95% at a sheet resistance <100 Ω sq−1.

Transparent conducting films of solution-synthesized copper nanowires are an attractive alternative to indium tin oxide due to the relative abundance of Cu and the low cost of solution-phase nanowire coating processes. However, there has to date been no way to protect Cu nanowires with a solution-phase process that does not adversely affect the optoelectric performance of Cu nanowire films. This article reports that the electrodeposition of zinc, tin, or indium shells onto Cu nanowires, followed by oxidation of these shells, enables the protection of Cu nanowire films against oxidation without decreasing film performance.Keywords: copper nanowires; core−shell structure; metal oxide; oxidation-resistant; transparent conductor;

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.

This article describes the solution-phase synthesis of 4 nm gold nanoparticles with 0.7 atom-thick, 1.9 atom-thick, and 3.8 atom-thick layers of Pd on their surfaces. These well-defined core–shell nanoparticles were deposited on a silica support, calcined, and reduced at 300 °C to create alloyed nanoparticles containing 10.9, 20.2, and 28.5% Pd (w/w). Monometallic Pd nanoparticles sintered during calcination at 300 °C, but no sintering was observed for AuPd nanoparticles. Diffuse reflectance infrared Fourier transform (DRIFT) spectra of adsorbed CO suggests that Au donates d electron density to Pd in the core–shell and alloy structures and confirms the presence of Au and Pd atoms on the surface of the nanoparticles after calcination and reduction. The properties of the AuPd alloy catalysts were tested in the vapor-phase conversion of α-limonene to p-cymene. AuPd nanoparticles containing 20% or more Pd per particle produced p-cymene yields greater than 80%, equivalent to conventional Pd catalysts prepared by incipient wetness and ion exchange methods. Very low yields of p-cymene were obtained from dehydrogenation of p-menthane under equivalent conditions, suggesting that the production of p-cymene from α-limonene proceeds through terpinene intermediates.

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.

This article describes how the dimensions of nanowires affect the transmittance and sheet resistance of a random nanowire network. Silver nanowires with independently controlled lengths and diameters were synthesized with a gram-scale polyol synthesis by controlling the reaction temperature and time. Characterization of films composed of nanowires of different lengths but the same diameter enabled the quantification of the effect of length on the conductance and transmittance of silver nanowire films. Finite-difference time-domain calculations were used to determine the effect of nanowire diameter, overlap, and hole size on the transmittance of a nanowire network. For individual nanowires with diameters greater than 50 nm, increasing diameter increases the electrical conductance to optical extinction ratio, but the opposite is true for nanowires with diameters less than this size. Calculations and experimental data show that for a random network of nanowires, decreasing nanowire diameter increases the number density of nanowires at a given transmittance, leading to improved connectivity and conductivity at high transmittance (>90%). This information will facilitate the design of transparent, conducting nanowire films for flexible displays, organic light emitting diodes and thin-film solar cells.

The conventional anode for organic photovoltaics (OPVs), indium tin oxide (ITO), is expensive and brittle, and thus is not suitable for use in roll-to-roll manufacturing of OPVs. In this study, fully solution-processed polymer bulk heterojunction (BHJ) solar cells with anodes made from silver nanowires (Ag NWs) have been successfully fabricated with a configuration of Ag NWs/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer:phenyl-C61-butyric acid methyl ester (PCBM)/Ca/Al. Efficiencies of 2.8 and 2.5% are obtained for devices with Ag NW network on glass and on poly(ethylene terephthalate) (PET), respectively. The efficiency of the devices is limited by the low work function of the Ag NWs/PEDOT:PSS film and the non-ideal ohmic contact between the Ag NW anode and the active layer. Compared with devices based on the ITO anode, the open-circuit voltage (Voc) of solar cells based on the Ag NW anode is lower by ∼0.3 V. More importantly, highly flexible BHJ solar cells have been firstly fabricated on Ag NWs/PET anode with recoverable efficiency of 2.5% under large deformation up to 120°. This study indicates that, with improved engineering of the nanowires/polymer interface, Ag NW electrodes can serve as a low-cost, flexible alternative to ITO, and thereby improve the economic viability and mechanical stability of OPVs.Keywords: flexible solar cell; organic photovoltaics; silver nanowires; solution processing; transparent electrode;

Reduction of Pd ions by hydroquinone in the presence of gold nanoparticles and polyvinylpyrrolidone resulted in the formation of nanoflowers with a Au core and Pd petals. Addition of HCl to the synthesis halted the reduction by hydroquinone and enabled the acquisition of snapshots of the nanoflowers at different stages of growth. TEM images of the reaction after 10 s show that the nanoflower morphology resulted from the homogeneous nucleation of Pd clusters in solution and their subsequent attachment to gold seeds coated with a thin (0.8 ± 0.1 nm) shell of Pd. UV–visible spectra also indicate Pd clusters formed in the early stages of the reaction and disappeared as the nanoflowers grew. The speed at which this reaction can be halted is useful not only for producing a variety of bimetallic nanostructures with precisely controlled dimensions and morphologies but also for understanding the growth mechanism of these structures. The ability of the AuPd core–shell structure to catalyze the Suzuki coupling reaction of iodobenzene to phenylboronic acid was probed and compared against the activity of Pd nanocubes and thin-shelled AuPd core–shell nanoparticles. The results of this study suggest that Suzuki coupling was not affected by the surface structure or subsurface composition of the nanoparticles, but instead was primarily catalyzed by molecular Pd species that leached from the nanostructures.Keywords: gold; nanoclusters; nanoflowers; nanostructures; palladium

In this work, silver (Ag) nanocubes with different sizes were rapidly synthesized with a modified HCl-based polyol approach by employing carbamide (CO(NH2)2) as the additive or promoter, which could shorten the reaction time from about 25 h to less than 4 h and the method could be confirmed as facile and robust. In the reaction system, the NH3 molecules play the role of aggregating Ag+ and [Ag(NH3)2]+ could gradually release Ag+, which in turn results in formation of a more homogeneous product in a short time (50 min–4 h). Some factors affecting the synthesis including the concentration, reaction time and agitator speed have also been investigated, which could be adjusted to control the size, morphology, purity and uniformity of the Ag nanocubes. A mechanism for the rapid synthesis of the Ag nanocubes was proposed. To overcome the lower repeatability of reported methods, we have supplied a robust method to synthesize Ag nanocubes and this procedure may provide a useful guide for the future synthesis of Ag or other metal nanoparticles. The transmittance properties of the different Ag nanocubes have also been detected, which demonstrated that the transmittance of the Au film coupled with the Ag nanocubes is very sensitive to not only the size of the Ag nanocubes but also the thickness of the polyelectrolyte molecular spacer layers.

This communication presents a way to produce copper nanowires with aspect ratios as high as 5700 in 30 min, and describes the growth processes responsible for their formation. These nanowires were used to make transparent conducting films with a transmittance >95% at a sheet resistance <100 Ω sq−1.

Since the discovery of the role of oxidative etching in shape-controlled metal nanostructure synthesis in 2004, it has become a versatile tool to precisely manipulate the nucleation and growth of metal nanocrystals at the atomic level. Subsequent research has shown that oxidative etching can be used to reshape nanocrystals via atomic addition and subtraction. This research has attracted extensive attention from the community because of its promising practical applications and theoretical value, and as a result, tremendous efforts from numerous research groups have been made to expand and apply this method to their own research. In this review, we first outline the merits of oxidative etching for the controlled synthesis of metal nanocrystals. We then summarize recent progress in the use of oxidative etching to control the morphology of a nanostructure during and after its synthesis, and analyze its specific functions in controlling a variety of nanocrystal parameters. Applications enabled by oxidative etching are also briefly presented to show its practical impact. Finally, we discuss the challenges and opportunities for further development of oxidative etching in nanocrystals synthesis.