graphene 'moth eyes' to power future smart technologies

• New ultra-thin, patterned graphene sheets will be essential in designing future technologies such as 'smart wallpaper' and internet-of-things applications
• Advanced Technology Institute uses moth-inspired ultrathin graphene sheets to capture light for use in energy production and to power smart sensors
• Graphene is traditionally an excellent electronic material, but is inefficient for optical applications, absorbs only 2.3% of the light incident on it. A new technique enhances light absorption by 90%.

New research published today in Science Advances has shown how graphene can be manipulated to create the most light-absorbent material for its weight, to date. This nanometre-thin material will enable future applications such as 'smart wallpaper' that could generate electricity from waste light or heat, and power a host of applications within the growing 'internet of things'.

Using a technique known as nanotexturing, which involves growing graphene around a textured metallic surface, researchers from the University of Surrey's Advanced Technology Institute took inspiration from nature to create ultra-thin graphene sheets designed to more effectively capture light. Just one atom thick, graphene is very strong but traditionally inefficient at light absorption. To combat this, the team used the nano-patterning to localise light into the narrow spaces between the textured surface, enhancing the amount of light absorbed by the material by about 90%.

"Nature has evolved simple yet powerful adaptations, from which we have taken inspiration in order to answer challenges of future technologies," explained Professor Ravi Silva, Head of the Advanced Technology Institute.

"Moths' eyes have microscopic patterning that allows them to see in the dimmest conditions. These work by channelling light towards the middle of the eye, with the added benefit of eliminating reflections, which would otherwise alert predators of their location. We have used the same technique to make an amazingly thin, efficient, light-absorbent material by patterning graphene in a similar fashion."

Graphene has already been noted for its remarkable electrical conductivity and mechanical strength. Professor Ravi's team understood that for graphene's potential to be realised as material for future applications, it should also harness light and heat effectively.

Professor Silva commented: "Solar cells coated with this material would be able to harvest very dim light. Installed indoors, as part of future 'smart wallpaper' or 'smart windows', this material could generate electricity from waste light or heat, powering a numerous array of smart applications. New types of sensors and energy harvesters connected through the Internet of Things would also benefit from this type of coating."

Dr José Anguita of the University of Surrey and lead author of the paper commented: "As a result of its thinness, graphene is only able to absorb a small percentage of the light that falls on it. For this reason, it is not suitable for the kinds of optoelectronic technologies our 'smart' future will demand."

"Nanotexturing graphene has the effect of channelling the light into the narrow spaces between nanostructures, thereby enhancing the amount of light absorbed by the material. It is now possible to observe strong light absorption from even nanometre-thin films. Typically a graphene sheet would have 2-3% light absorption. Using this method, our ultrathin coating of nanotextured few-layer graphene absorbs 95% of incident light across a broad spectrum, from the UV to the infrared."

water-splitting

Splitting water is a two-step process, and in a new study, researchers have performed one of these steps (reduction) with

 100% efficiency. The results shatter the previous record of 60% for hydrogen production with visible light, and 

emphasize that future research should focus on the other step (oxidation) in order to realize practical overall water 

splitting. The main application of splitting water into its components of oxygen and hydrogen is that the hydrogen can 

then be used to deliver energy to fuel cells for powering vehicles and electronic devices.

work shows that it is possible to obtain a perfect 100% photon-to-hydrogen production efficiency, under visible 

light illumination, for the photocatalytic water splitting reduction half-reaction. These results shatter the previous 

benchmarks for all systems, and leave little to no room for improvement for this particular half-reaction. With a stable 

system and a turnover frequency of 360,000 moles of hydrogen per hour per mole of catalyst, the potential here is real."

When an H2O molecule splits apart, the three atoms don't simply separate from each other. The full reaction requires 

two H2O molecules to begin with, and then proceeds by two separate half-reactions. In the oxidation half-reaction, four

 individual hydrogen atoms are produced along with an O2 molecule (which is discarded). In the reduction half-reaction, 

the four hydrogen atoms are paired up into two H2 molecules by adding electrons, which produces the useful form of 

hydrogen: H2 gas.

The 100% efficiency refers to the photon-to-hydrogen conversion efficiency, and it means that virtually all of the photons 

that reach the photocatalyst generate an electron, and every two electrons produce one H2 molecule. At 100% yield, the 

half-reaction produces about 100 H2 molecules per second (or one every 10 milliseconds) on each nanorod, and a 

typical sample contains about 600 trillion nanorods.

One of the keys to achieving the perfect efficiency was identifying the bottleneck of the process, which was the need to 

quickly separate the electrons and holes (the vacant places in the semiconductor left after the electrons leave), and 

remove the holes from the photocatalyst. To improve the charge separation, the researchers redesigned the nanorods to

 have just one platinum catalyst instead of two. The researchers found that the efficiency increased from 58.5% with two 

platinum catalysts to 100% with only one.

Going forward, the researchers plan to further improve the system. The current demonstration requires a very high pH,

 but such strong basic conditions are not always ideal in practice. Another concern is that the cadmium sulfide (CdS) 

used in the nanorod becomes corroded under prolonged light exposure in pure water. The researchers are already 

addressing these challenges with the goal to realize practical solar-to-fuel technology in the future.

published by:  Philip Kalisman, Yifat Nakibli, and Lilac Amrivan

for paper published click here