Nanotech roundup: electricity from sewage, and self-cleaning solar panels

This month’s roundup includes a device for squeezing energy from wastewater, and a coating to keep solar panels clean

Poopy Power

One resource we are never short of is wastewater. It contains plenty of organic matter that could still produce useful energy, but the challenge is to do this efficiently.

An attractive way is to enlist the help of microbes, which have already been optimised by natural selection over millions of years to extract energy from waste products. Researchers at Stanford have designed a microbial energy-harvesting device that looks like a battery, where the positive (anode) electrode consists of carbon nanomaterial coated with microorganisms and the negative (cathode) electrode contains silver oxide.

The microbes extract electrons from biomass in wastewater at the anode, and at the cathode silver oxide is converted into silver. As a result, an electrical current flows through a circuit connecting the electrodes.

Like a conventional rechargeable battery, the cathode needs to be refreshed periodically to change silver back into silver oxide. Nevertheless, even after taking this energy loss into account, a promising net energy efficiency of around 30% has been demonstrated.

Arrested Development

Scientists using fluorescent carbon nanoparticles to study the life cycle of different types of mosquito made a remarkable discovery. At low concentrations in water, the nanoparticles interfere with the growth of mosquito larvae, which die after remaining in suspended animation for four weeks.

Diseases carried by mosquitoes, such as malaria, kill around two million people worldwide every year, so a novel insecticide that curbs their breeding – particularly in stagnant pools of water – without harming the environment could save many lives.

The nanoparticles, which are thought to be environmentally benign, are made by an inexpensive process that involves burning wood wool and could be easily distributed at a wide scale.

Sun Clean

Solar cells have become of wide consumer interest, attested by Ikea’s plans to sell solar panels in the UK, in addition to their famous flat-packed furniture.

An essential part of solar cells is the anti-reflection coating, which ensures that as much sunlight as possible is captured. Nanoporous thin films have ideal optical properties for such coatings, but they are not robust in outdoor conditions and quickly become contaminated.

Now researchers at the Universities of Cambridge and Stanford have designed an antireflection coating that can keep itself clean. It is made of a porous network of silica and contains titanium dioxide nanocrystals, which are the cleaners. The nanocrystals are chemically reactive when activated by sunlight and decompose organic contaminants.

Although the nanocrystals would normally cause high reflections, they are completely hidden within the silica network so they do not interfere with the panels’ optical properties. In a first test of the technology, researchers have shown that fingerprints left on silicon coated with the new material are completely removed after two hours under a solar lamp.

The coating is scratch-resistant and could be of particular interest for solar panels covering large areas.

Light Speed

Computers could run a lot faster if they used light, in addition to electrical currents, to handle data – especially in links between chips. However, while light is perfectly suited for transmitting data fast over long distances via optical fibres, it is not easy to control and manipulate within the confines of a silicon chip. Essential but difficult to design components for this task are photodetectors that convert light signals into electrical currents.

Graphene – the wondrous ultrathin sheets made from carbon – can help out. It is easily manufactured and transferred onto silicon, absorbs light from a wide range of wavelengths and has high electrical conductivity. A problem so far has been that graphene absorbs light only weakly, but this has now been solved simultaneously by three different research groups, who have developed graphene photodetectors integrated in silicon optical waveguides.

The new graphene devices come close in performance to existing photodetectors, but offer some particular advantages: graphene’s exceptionally fast-moving electrons make high-speed signal processing possible and, in addition, the devices operate over a wide range of the electromagnetic spectrum, from visible light to telecommunications wavelengths and beyond.


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