A nanonet lattice comprising tiny titanium disilicide wires coated with iron oxide can be used as a catalyst for the hydrolysis of water to produce clean hydrogen for powering fuel cells, according to US researchers [Wang et al., J Am Chem Soc (2011) 133, 2398; doi:10.1021/ja110741z].
Chemist Dunwei Wang and colleagues at Boston College in Massachusetts explain that semiconductors are promising materials in the field of high-efficiency solar water splitting. Titanium dioxide and iron oxide have been investigated for this application. However, while the band-gap of iron oxide is close to optimum, efforts are stymied by the very short charge-diffusion distance, which makes collection of photoinduced charges difficult. Wang's team believe they have a solution to this problem in the form of nanonets, which they first developed in 2008.
A nanoscale network of the readily available semiconductor titanium disilicide provides an increased surface area and improved conductivity and a substrate on to which iron oxide in the form of haematite can be deposited. On this nanonet, haematite is free to absorb light efficiently without the need for oxygen-releasing catalytic additives. The nanonet serves not only as a structural support but also as an efficient charge collector, allowing for maximum photon-to-charge conversion, Wang explained. Quantum efficiency is 46 % for an incident wavelength of 400 nm. Photocurrents of 1.6 and 2.7 mA/cm2 were possible at voltages of 1.23 and 1.53 V, respectively.
"Recent research has shown that the use of a catalyst can boost the performance of haematite," Wang says. "What we have shown is the potential performance of haematite at its fundamental level, without a catalyst." He adds that, "The result highlights the importance of charge transport in semiconductor-based water splitting, particularly for materials whose performance is limited by poor charge diffusion."
The team concedes that there is still much room for improvement; quantum efficiencies of less than 50 % are much lower than the 80+ % possible with tungsten oxide or titanium dioxide. The team is currently investigating the various factors that are affecting the overall efficiency of their system. They point out, however, that the very low cost and accessibility of haematite compared to tungsten or titanium oxides does represent a significant incentive for optimizing their system, especially given the high current density possible thanks to haematite's near-perfect band-gap.
David Bradley