Electricity-generating solar cells are one of the greenest technologies around, but they have a problem: constant exposure to a varying environment can build up dust and grime on photovoltaic panels and reduce their efficiency. To combat this, scientists are developing superhydrophilic ‘self-cleaning’ films that attract water droplets and spread them flat as a pancake, making the surface anti-fogging and easily washable. Unfortunately, existing self-cleaning substances have light- and chemical-stability issues that limit their usability.

Fig. 1: Scanning electron microscopy image of a textured carbon film with high surface wettability.© 2010 ACS

Shuyan Gao and co-workers from Henan Normal University in China1 have now produced a very tough superhydrophilic surface — a carbon layer that can handle chemical conditions ranging from the most corrosive acid to the harshest base. The key to their approach is an etching technique that produces craggy nanoscale peaks to create a rough surface, amplifying its water-attracting abilities.

Graphitic carbon is a thermally and chemically stable material that has a natural affinity for water, making it an ideal choice for self-cleaning materials. However, its surface is normally quite flat, which is a distinct disadvantage because scientists have known since the 1930s that geometric roughness at the right length scale significantly enhances surface wetting.

Gao and her colleagues overcame this problem through the use of plasma-based etching, a method in which a high-energy excited gas containing energetic particles such as electrons, ions and free radicals is used to bombard graphitic carbon. After only five minutes of plasma treatment, a previously flat graphite layer exhibits a series of vertical carbon needles with sharp tops and thick bases, tens of nanometers high (see image). The team believes that this texture arises from a ‘gas–solid–gas’ growth mechanism involving particles on the carbon surface and in the plasma.

The researchers’ experiments revealed that the nano-patterned carbon exhibits stable superhydrophilic behavior over the entire range of pH, from acidic to basic — a remarkable range of durability that portends a bright future for this material as an anti-corrosive photovoltaic coating, according to Gao. “These surfaces can increase light adsorption by trapping light in the highly textured structures, and can help keep solar cells clean through superhydrophilic forces that allow rain or dew to wash away accumulated dirt,” she says.