Solar water splitting, in which water is separated into oxygen and hydrogen using sunlight, could be a clean and renewable way to produce energy, but efficient photoelectrode materials for use in this process are still few and far between. Compared with many other semiconductors, gallium phosphide could make an ideal photocathode material for solar-fuel conversion. However, the problem is that conventional GaP single-crystalline wafers do not provide enough surface area on which to place water-splitting catalysts. Single-crystal wafers are also expensive, because they are almost pure GaP.

Now, Peidong Yang and colleagues at the University of California, Berkeley, and co-workers at the Lawrence Berkeley National Laboratory may have found a way to overcome both of these problems by employing a surfactant-free solution–liquid–solid (SLS) technique to produce GaP nanowires that contain zinc dopants. The zinc helps to increase the number of charge carriers (electrons and holes) in the nanowires and thus improves the photocurrent output.

Mimicking natural photosynthesis

Although the doped nanowires contain around 3000 times less GaP than that typically found in a single crystalline wafer of the material, the photocurrent density of the nanowires is just as high. Moreover, the surfactant-free solution processing technique might easily lend itself to other III-V and II-V semiconductors and ultimately be used to make photoanodes for solar-to-fuel reactors that contain both cathodes and anodes. Such systems would mimic the processes that go on in natural photosynthesis, and so be much more efficient at converting solar energy.

GaP nanowires are usually made using high-temperature, vapour-phase techniques. Existing solution-based methods can produce colloidal nanowires but require organic surfactants and ligands to make the straight nanowire shapes required. Another disadvantage of these techniques is that the organic molecules covalently bind to the surface of the nanowires, something that hinders the transfer of photogenerated charge carriers across the semiconductor-electrolyte interface. Ideally, these surface-bound molecules need to be removed first before the wires can be used for catalyst loading in solar water splitting.

The surfactant-free SLS method developed by Yang’s team involves using the two chemicals triethylgallium and tris(trimethylsilyl)phosphine as precursors. The researchers used saturated hydrocarbons with high boiling points as solvents to synthesize straight GaP nanowires. They found squalane to be the best solvent thanks to the fact that it does not coordinate to the nanowires, because it is a viscous liquid and has a low vapour pressure. “We saw self-seeded wire growth in squalane: the precursors first thermally decomposed in situ to produce Ga nanoscale droplets that then subsequently promote the growth of the GaP nanowires via SLS,” explained Yang.

“Our experiments not only demonstrate that GaP nanowires with carefully controlled doping could make ideal electrodes for solar water splitting reactors but also enrich our understanding of nanowire photochemistry,” he told nanotechweb.org.

The work is detailed in Nano Letters.