Tag Archives: metal nanoparticles

Building metal nanoparticles: one step closer

University of Pittsburgh scientists have researched why metal nanoparticles form, a necessary first step before developing techniques for synthesizing them commercially. From a July 10, 2017 news item on ScienceDaily,

Although scientists have for decades been able to synthesize nanoparticles in the lab, the process is mostly trial and error, and how the formation actually takes place is obscure. A new study explains how metal nanoparticles form.

Caption: This is a structure of a ligand-protected Au25 nanocluster. Credit: Computer-Aided Nano and Energy Lab (C.A.N.E.LA.)

A July 10, 2017 University of Pittsburgh news release (also on EurekAlert), which originated the news item, expands on the theme (Note: A link has been removed),

“Even though there is extensive research into metal nanoparticle synthesis, there really isn’t a rational explanation why a nanoparticle is formed,” Dr. Mpourmpakis [Giannis Mpourmpakis, assistant professor of chemical and petroleum engineering] said. “We wanted to investigate not just the catalytic applications of nanoparticles, but to make a step further and understand nanoparticle stability and formation. This new thermodynamic stability theory explains why ligand-protected metal nanoclusters are stabilized at specific sizes.”

A ligand is a molecule that binds to metal atoms to form metal cores that are stabilized by a shell of ligands, and so understanding how they contribute to nanoparticle stabilization is essential to any process of nanoparticle application. Dr. Mpourmpakis explained that previous theories describing why nanoclusters stabilized at specific sizes were based on empirical electron counting rules – the number of electrons that form a closed shell electronic structure, but show limitations since there have been metal nanoclusters experimentally synthesized that do not necessarily follow these rules.

“The novelty of our contribution is that we revealed that for experimentally synthesizable nanoclusters there has to be a fine balance between the average bond strength of the nanocluster’s metal core, and the binding strength of the ligands to the metal core,” he said. “We could then relate this to the structural and compositional characteristic of the nanoclusters, like size, number of metal atoms, and number of ligands.

“Now that we have a more complete understanding of this stability, we can better tailor the nanoparticle morphologies and in turn properties, to applications from biolabeling of individual cells and targeted drug delivery to catalytic reactions, thereby creating more efficient and sustainable production processes.”

Here’s a link to and a citation for the paper,

Thermodynamic stability of ligand-protected metal nanoclusters by Michael G. Taylor & Giannis Mpourmpakis. Nature Communications 8, Article number: 15988 (2017) doi:10.1038/ncomms15988 Published online: 07 July 2017

This paper is open access.

‘Brewing up’ conductive inks for printable electronics

Scientists from Duke University aren’t exactly ‘brewing’ or ‘cooking up’ the inks but they do come close according to a Jan. 3, 2017 news item on ScienceDaily,

By suspending tiny metal nanoparticles in liquids, Duke University scientists are brewing up conductive ink-jet printer “inks” to print inexpensive, customizable circuit patterns on just about any surface.

A Jan. 3, 2017 Duke University news release (also on EurekAlert), which originated the news item, explains why this technique could lead to more accessible printed electronics,

Printed electronics, which are already being used on a wide scale in devices such as the anti-theft radio frequency identification (RFID) tags you might find on the back of new DVDs, currently have one major drawback: for the circuits to work, they first have to be heated to melt all the nanoparticles together into a single conductive wire, making it impossible to print circuits on inexpensive plastics or paper.

A new study by Duke researchers shows that tweaking the shape of the nanoparticles in the ink might just eliminate the need for heat.

By comparing the conductivity of films made from different shapes of silver nanostructures, the researchers found that electrons zip through films made of silver nanowires much easier than films made from other shapes, like nanospheres or microflakes. In fact, electrons flowed so easily through the nanowire films that they could function in printed circuits without the need to melt them all together.

“The nanowires had a 4,000 times higher conductivity than the more commonly used silver nanoparticles that you would find in printed antennas for RFID tags,” said Benjamin Wiley, assistant professor of chemistry at Duke. “So if you use nanowires, then you don’t have to heat the printed circuits up to such high temperature and you can use cheaper plastics or paper.”

“There is really nothing else I can think of besides these silver nanowires that you can just print and it’s simply conductive, without any post-processing,” Wiley added.

These types of printed electronics could have applications far beyond smart packaging; researchers envision using the technology to make solar cells, printed displays, LEDS, touchscreens, amplifiers, batteries and even some implantable bio-electronic devices. The results appeared online Dec. 16 [2016] in ACS Applied Materials and Interfaces.

Silver has become a go-to material for making printed electronics, Wiley said, and a number of studies have recently appeared measuring the conductivity of films with different shapes of silver nanostructures. However, experimental variations make direct comparisons between the shapes difficult, and few reports have linked the conductivity of the films to the total mass of silver used, an important factor when working with a costly material.

“We wanted to eliminate any extra materials from the inks and simply hone in on the amount of silver in the films and the contacts between the nanostructures as the only source of variability,” said Ian Stewart, a recent graduate student in Wiley’s lab and first author on the ACS paper.

Stewart used known recipes to cook up silver nanostructures with different shapes, including nanoparticles, microflakes, and short and long nanowires, and mixed these nanostructures with distilled water to make simple “inks.” He then invented a quick and easy way to make thin films using equipment available in just about any lab — glass slides and double-sided tape.

“We used a hole punch to cut out wells from double-sided tape and stuck these to glass slides,” Stewart said. By adding a precise volume of ink into each tape “well” and then heating the wells — either to relatively low temperature to simply evaporate the water or to higher temperatures to begin melting the structures together — he created a variety of films to test.

The team say they weren’t surprised that the long nanowire films had the highest conductivity. Electrons usually flow easily through individual nanostructures but get stuck when they have to jump from one structure to the next, Wiley explained, and long nanowires greatly reduce the number of times the electrons have to make this “jump”.

But they were surprised at just how drastic the change was. “The resistivity of the long silver nanowire films is several orders of magnitude lower than silver nanoparticles and only 10 times greater than pure silver,” Stewart said.

The team is now experimenting with using aerosol jets to print silver nanowire inks in usable circuits. Wiley says they also want to explore whether silver-coated copper nanowires, which are significantly cheaper to produce than pure silver nanowires, will give the same effect.

Here’s a link to and a citation for the paper,

Effect of Morphology on the Electrical Resistivity of Silver Nanostructure Films by Ian E. Stewart, Myung Jun Kim, and Benjamin J. Wiley. ACS Appl. Mater. Interfaces, Article ASAP
DOI: 10.1021/acsami.6b12289 Publication Date (Web): December 16, 2016

Copyright © 2016 American Chemical Society

This paper is behind a paywall but there is an image of the silver nanowires, which is not exactly compensation but is interesting,

Caption: Duke University chemists have found that silver nanowire films like these conduct electricity well enough to form functioning circuits without applying high temperatures, enabling printable electronics on heat-sensitive materials like paper or plastic.
Credit: Ian Stewart and Benjamin Wiley

Generating clean fuel with individual gold atoms

A July 22, 2016 news item on Nanowerk highlights an international collaboration focused on producing clean fuel,

A combined experimental and theoretical study comprising researchers from the Chemistry Department and LCN [London Centre for Nanotechnology], along with groups in Argentina, China, Spain and Germany, has shed new light on the behaviour of individual gold atoms supported on defective thin cerium dioxide films – an important system for catalysis and the generation of clean hydrogen for fuel.

A July ??, 2016 LCN press release, which originated the news item, expands on the theme of catalysts, the research into individual gold atoms, and how all this could result in clean fuel,

Catalysis plays a vital role in our world; an estimated 80% of all chemical and materials are made via processes which involve catalysts, which are commonly a mixture of metals and oxides. The standard motif for these heterogeneous catalysts (where the catalysts are solid and the reactants are in the gas phase) is of a high surface area oxide support that is decorated with metal nanoparticles a few nanometres in diameter. Cerium dioxide (ceria, CeO2) is a widely used support material for many important industrial processes; metal nanoparticles supported on ceria have displayed high activities for applications including car catalytic converters, alcohol synthesis, and for hydrogen production. There are two key attributes of ceria which make it an excellent active support material: its oxygen storage and release ability, and its ability to stabilise small metal particles under reaction conditions. A recent system that has been the focus of much interest has been that of gold nanoparticles and single atoms with ceria, which has demonstrated high activity towards the water-gas-shift reaction, (CO + H2O —> CO2 + H2) a key stage in the generation of clean hydrogen for use in fuel cells.

The nature of the active sites of these catalysts and the role that defects play are still relatively poorly understood; in order to study them in a systematic fashion, the researchers prepared model systems which can be characterised on the atomic scale with a scanning tunnelling microscope.

Figure: STM images of CeO2-x(111) ultrathin films before and after the deposition of Au single atoms at 300 K. The bright lattice is from the oxygen atoms at the surface – vacancies appear as dark spots

These model systems comprised well-ordered, epitaxial ceria films less than 2 nm thick, prepared on a metal single crystal, upon which single atoms and small clusters of gold were evaporated onto under ultra-high-vacuum (essential to prevent contamination of the surfaces). Oxygen vacancy defects – missing oxygen atoms in the top layer of the ceria – are relatively common at the surface and appear as dark spots in the STM images. By mapping the surface before and after the deposition of gold, it is possible to analyse the binding of the metal atoms, in particular there does not appear to be any preference for binding in the vacancy sites at 300 K.

Publishing their results in Physical Review Letters, the researchers combined these experimental results with theoretical studies of the binding energies and diffusion rates across the surface. They showed that kinetic effects governed the behaviour of the gold atoms, prohibiting the expected occupation of the thermodynamically more stable oxygen vacancy sites. They also identified electron transfer between the gold atoms and the ceria, leading to a better understanding of the diffusion phenomena that occur at this scale, and demonstrated that the effect of individual surface defects may be more minor than is normally imagined.

Here’s a link to and a citation for the paper,

Diffusion Barriers Block Defect Occupation on Reduced CeO2(111) by P.G. Lustemberg, Y. Pan, B.-J. Shaw, D. Grinter, Chi Pang, G. Thornton, Rubén Pérez, M. V. Ganduglia-Pirovano, and N. Nilius. Phys. Rev. Lett. Vol. 116, Iss. 23 — 10 June 2016 2016DOI:http://dx.doi.org/10.1103/PhysRevLett.116.236101 Published 9 June 2016

This paper is behind a paywall.

Nanowalls (like waffles) for touchscreens

ETH Zurich has announced a new technique for creating transparent electrodes in a Jan. 6, 2016 news item on ScienceDaily,

Transparent electrodes have been manufactured for use in touchscreens using a novel nanoprinting process. The new electrodes are some of the most transparent and conductive that have ever been developed.

From smartphones to the operating interfaces of ticket machines and cash dispensers, every touchscreen we use requires transparent electrodes: The devices’ glass surface is coated with a barely visible pattern made of conductive material. It is because of this that the devices recognise whether and where exactly a finger is touching the surface.

Here’s an image illustrating the new electrodes,

With a special mode of electrohydrodynamic ink-jet printing scientists can create a grid of ultra fine gold walls. (Visualisations: Ben Newton / Digit Works)

With a special mode of electrohydrodynamic ink-jet printing scientists can create a grid of ultra fine gold walls. (Visualisations: Ben Newton / Digit Works)

I think these electrodes resemble waffles,

[downloaded from https://github.com/jhermann/Stack-O-Waffles] Credit: jherman

[downloaded from https://github.com/jhermann/Stack-O-Waffles] Credit: jherman

Getting back to the electrodes themselves, a Jan. 6, 2016 ETH Zurich press release (also on EurekAlert*)by Fabio Bergamin, which originated the news item, provides more details,

Researchers under the direction of Dimos Poulikakos, Professor of Thermodynamics, have now used 3D print technology to create a new type of transparent electrode, which takes the form of a grid made of gold or silver “nanowalls” on a glass surface. The walls are so thin that they can hardly be seen with the naked eye. It is the first time that scientists have created nanowalls like these using 3D printing. The new electrodes have a higher conductivity and are more transparent than those made of indium tin oxide, the standard material used in smartphones and tablets today. This is a clear advantage: The more transparent the electrodes, the better the screen quality. And the more conductive they are, the more quickly and precisely the touchscreen will work.

Third dimension

“Indium tin oxide is used because the material has a relatively high degree of transparency and the production of thin layers has been well researched, but it is only moderately conductive,” says Patrik Rohner, a PhD student in Poulikakos’ team. In order to produce more conductive electrodes, the ETH researchers opted for gold and silver, which conduct electricity much better. But because these metals are not transparent, the scientists had to make use of the third dimension. ETH professor Poulikakos explains: “If you want to achieve both high conductivity and transparency in wires made from these metals, you have a conflict of objectives. As the cross-sectional area of gold and silver wires grows, the conductivity increases, but the grid’s transparency decreases.”

The solution was to use metal walls only 80 to 500 nanometres thick, which are almost invisible when viewed from above. Because they are two to four times taller than they are wide, the cross-sectional area, and thus the conductivity, is sufficiently high.

Ink-jet printer with tiny print head

The researchers produced these tiny metal walls using a printing process known as Nanodrip, which Poulikakos and his colleagues developed three years ago. Its basic principle is a process called electrohydrodynamic ink-jet printing. In this process scientists use inks made from metal nanoparticles in a solvent; an electrical field draws ultra-small droplets of the metallic ink out of a glass capillary. The solvent evaporates quickly, allowing a three-dimensional structure to be built up drop by drop.

What is special about the Nanodrip process is that the droplets that come out of the glass capillary are about ten times smaller than the aperture itself. This allows for much smaller structures to be printed. “Imagine a water drop hanging from a tap that is turned off. And now imagine that another tiny droplet is hanging from this drop – we are only printing the tiny droplet,” Poulikakos explains. The researchers managed to create this special form of droplet by perfectly balancing the composition of metallic ink and the electromagnetic field used.

Cost-efficient production

The next big challenge will now be to upscale the method and develop the print process further so that it can be implemented on an industrial scale. To achieve this, the scientists are working with colleagues from ETH spin-off company Scrona.

Here’s a link to and a citation for the paper,

Electrohydrodynamic NanoDrip Printing of High Aspect Ratio Metal Grid Transparent Electrodes by Julian Schneider, Patrick Rohner, Deepankur Thureja, Martin Schmid, Patrick Galliker, Dimos Poulikalos. Advanced Functional Materials DOI: 10.1002/adfm.201503705 First published: 15 December 2015

This paper is behind a paywall.

*'(also on EurekAlert)’ added on Jan. 7, 2016.