Shengrong Ye

Metal Nanowire Networks: The Next Generation of Transparent Conductors

There is an ongoing drive to replace the most common transparent conductor, indium tin oxide (ITO), with a material that gives comparable performance, but can be coated from solution at speeds orders of magnitude faster than the sputtering processes used to deposit ITO. Metal nanowires are currently the only alternative to ITO that meets these requirements. This Progress Report summarizes recent advances toward understanding the relationship between the structure of metal nanowires, the electrical and optical properties of metal nanowires, and the properties of a network of metal nanowires. Using the structure-property relationship of metal nanowire networks as a roadmap, this Progress Report describes different synthetic strategies to produce metal nanowires with the desired properties. Practical aspects of processing metal nanowires into high-performance transparent conducting films are discussed, as well as the use of nanowire films in a variety of applications.

Copper as a Robust and Transparent Electrocatalyst for Water Oxidation

Copper metal is in theory a viable oxidative electrocatalyst based on surface oxidation to Cu(III) and/or Cu(IV) , but its use in water oxidation has been impeded by anodic corrosion. The in situ formation of an efficient interfacial oxygen-evolving Cu catalyst from Cu(II) in concentrated carbonate solutions is presented. The catalyst necessitates use of dissolved Cu(II) and accesses the higher oxidation states prior to decompostion to form an active surface film, which is limited by solution conditions. This observation and restriction led to the exploration of ways to use surface-protected Cu metal as a robust electrocatalyst for water oxidation. Formation of a compact film of CuO on Cu surface prevents anodic corrosion and results in sustained catalytic water oxidation. The Cu/CuO surface stabilization was also applied to Cu nanowire films, which are transparent and flexible electrocatalysts for water oxidation and are an attractive alternative to ITO-supported catalysts for photoelectrochemical applications.

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%.

Solution-processed copper–nickel nanowire anodes for organic solar cells

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%.

Copper Nanowire Networks with Transparent Oxide Shells That Prevent Oxidation without Reducing Transmittance

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.

Optically Transparent Water Oxidation Catalysts Based on Copper Nanowires

Let the light shine through: A transparent film of copper nanowires was transformed into an electrocatalyst for water oxidation by electrodepostion of Ni or Co onto the surface of the nanowires. These core-shell nanowire networks exhibit electrocatalytic performance equivalent to metal oxide films of similar composition, but are several times more transparent.

Optically transparent hydrogen evolution catalysts made from networks of copper–platinum core–shell nanowires

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).

The Role of Cuprous Oxide Seeds in the One-Pot and Seeded Syntheses of Copper Nanowires

This paper demonstrates that Cu2O nanoparticles form in the early stages of a solution-phase synthesis of copper nanowires, and aggregate to form the seeds from which copper nanowires grow. Removal of ethylenediamine from the synthesis leads to the rapid formation of Cu2O octahedra. These octahedra are introduced as seeds in the same copper nanowire synthesis to improve the yield of copper nanowires from 12% to >55%, and to enable independent control over the length of the nanowires. Transparent conducting films are made from nanowires with different lengths to examine the effect of nanowire aspect ratio on the film performance.

How Copper Nanowires Grow and How To Control Their Properties

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.

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.