.

Ageing iron nanoparticles feel the strainRusting occurs much more rapidly on the

nanoscale compared to the macroscale,

scientists have discovered. This is caused

by strain present in nanoparticles due to

their small size.

Iron and iron oxide nanoparticles are

being utilized in an ever increasing number

of applications; from targeted drug delivery,

to cancer treatment, to catalysis in water

treatment, to use in clean fuel technologies.

But little is known about how they age,

something Andrew Pratt from the Univer-

sity of York, UK, and his team were keen to

address.

‘Understanding the reactivity and ageing

of nanomaterials is of vital importance for

their use in environmental applications or

inside the body,’ Pratt told Materials Today.

‘For example, Fe and Fe oxide nanoparticles

show great promise in removing toxic

metals and chemicals from contaminated

water and soil systems. However, if [over

time] a particle reacts and degrades too

much, its performance will be compro-

mised and it may even rerelease a captured

contaminant back into the environment.’

For the study, published in Nature Materi-

als [Pratt, et al., Nat. Mater. (2013),

doi:10.1038/nmat3785], the team tracked

how natural oxidation progresses in cubic

iron nanoparticles. This was done over sev-

eral years using state-of-the-art aberration-

corrected transmission electron micro-

scopes. After several months, ‘we noticed

some interesting effects that could only

arise because of the nanoscale size of the

particles and their cubic geometry,’ Pratt

says. The centers of each of the cube’s faces

were oxidizing faster than the regions

nearer the corners. The nanoparticles were

also observed to be rusting several magni-

tudes faster that bulk iron crystals. ‘We

postulated that this was due to strain

introduced because of confinement at the

nanoscale.’

To prove this theory, the scientists

mapped the distribution of strain on the

individual iron oxide molecules within the

particles. ‘We found the strain is greatest in

the center of the nanoparticle’s faces and. .

that the atoms in the oxide are further away

from each other than would normally be

found in the bulk material,’ he says. ‘The

atoms in the oxide shell are being pulled

apart because of the need for the oxide

domains on the six faces of the cubic core

to stay connected. This opens up the atomic

lattice which promotes the inward move-

ment of oxygen ions and outward move-

ment of iron ions.’ This acts to speed up the

oxidation process, meaning the nanoparti-

cles were fully oxidized in just two years.

Currently, the team is studying silver and

copper nanoparticles to see if they behave

the same. ‘We are also looking at ways

to engineer nanoparticle properties to

improve performance or to mitigate some

of their more damaging effects,’ Pratt adds.

Nina Notman

Lithium–air batteries go viralPower density could be boosted consider-

ably if air is added to the recipe for recharge-

able lithium batteries make long-distance

electric vehicles a much more viable propo-

sition. But first, materials scientists need to

develop better materials for such lithium–

air batteries ones that can undergo many

more charge–discharge cycles than current

experimental systems. Now, researchers at

Massachusetts Institute of Technology have

used genetically modified viruses to make

nanowire electrodes.

Writing in Nature Communications,

Dahyun Oh and colleagues explain how

they use the virus M13 to sequester manga-

nese from solution to sculpt wires just

80 nm in diameter for use in a lithium–air

electrode. The viral approach makes a rough

spiky wiry surface which greatly increases

the surface area of the wire relative to other

manufacturing processes provide more sur-

face for a given volume to charge and dis-

charge [Oh, et al., Nat. Commun. 4 (2013)

2756]. The growth process is not unlike the

manner in which an abalone assimilates

calcium ions from seawater to grow its

shell.

Aside from giving rise to a high surface to

volume ratio, the viral approach to making

nanowires avoids the energy-intensive,

high temperature approaches of conven-

tional electronics manufacturing as well

as precluding the need for toxic solvents,

working as it does in room temperature

water. Moreover, rather generating isolated

wires, the viral method produces three-

dimensional structure of cross-linked wires,

which make for a more stable electrode.

In order to activate their electrodes, the

team dopes the nanowires with palladium

to boost conductivity and to facilitate the

necessary catalytic processes that must

occur during charge and discharge. The

amount of noble metal dopant required is

much lower than was reported by other

groups for their electrode materials, again

by virtue of the biological underpinning of

the fabrication.

The team suggests that a lithium–air

battery using their electrode materials

might have a density more than double

that of the best conventional lithium ion

batteries. However, the team admits that

more work is now required to make this

viral approach a commercial viable

method for electrode manufacture. In par-

ticular, they have demonstrated proof of

principle with the material for charge–dis-

charge cycles but a commercial battery

needs to be able to operate over thousands

of cycles.

The team also points out that while

viruses have been used in the laboratory,

it would be most likely that a more conven-

tional fabrication process that emulated the

viral approach would be developed in a

commercial manufacturing system, the

viruses are simply the pathfinders.

David Bradley

Materials Today � Volume 17, Number 1 � January/February 2014 NEWS

A cuboid iron nanoparticle after six monthsexposure to the environment. Credit: Amish Shah

and Roland Kroger.

9