Solar water splitting, in which water is separated into oxygen and hydrogen using sunlight, could be a clean and renewable way to produce energy. However, scientists are still looking for efficient photoelectrode materials to use in this process.

Peidong Yang and colleagues first made single-crystalline InGaN nanowires with high In concentrations in 2007 and found that these nanowires had bandgaps in the 1 eV to 3.3 eV range depending on the In concentration. The present study shows that these materials can be used as efficient photoanodes for solar water splitting thanks to their small bandgap that be tuned to cover a broad part of the solar spectrum for optimal sunlight absorption. The nanostructures also have a high surface area so the electrodes can be "decorated" with a large number of catalysts to run the water oxidation reaction.

Measuring photocurrents

The researchers grew the Si/InGaN structures by coating n-doped Si wire arrays with n-type InGaN nanowires and annealing the ensemble at high temperatures. Working electrodes were then made from the wire arrays and illuminated by a 300 W xenon lamp fitted with a diffuser that produces uniform light intensity and another filter to simulate sunlight. Photocurrents were then measured through the electrodes.

The measurements revealed that the photocurrent density through Si/InGaN nanowire arrays increases by five times compared to the photocurrent density through InGaN nanowire arrays grown on planar Si.

Stability not a problem

One of the main worries with metal nitride semiconductors is how chemically stable they are under prolonged photooxidation. Yang's team tested the stability of the electrodes by illuminating the InGaN nanowires with intense light (350 mW/cm2) for 15 hours in an acidic electrolyte that had a pH of three. "To our delight, the photocurrent density remained stable and did not change after illumination," says Yang. "Also, we did not observe any noticeable decomposition of the InGaN nanowires – as observed with transmission electron microscopy – after illumination."

High-resolution TEM images of the nanowires that had been illuminated for 15 hours also clearly showed that the crystalline structure of the wires remained intact without any surface oxidation, he added.

"Although these preliminary results are promising, there are still many issues out there for us such as low overall extracted photocurrent in the anode and short carrier (electron and hole) lifetimes," Yang told "We now plan to optimise the nanostructures to solve these problems."

The present work is described in Nano Letters.