The capture and storage of solar energy in the molecular bonds of chemical fuels provides a promising route to overcoming the current global energy dependence on fossil fuels. Solar assisted water splitting, the process usually known as artificial photosynthesis, is a favourable means of converting solar energy into transportable and end user chemical fuels. The system requires optimisation in terms of cost and performance on a scale commensurate with the global energy demand. However, most abundant semiconductors suffer from the undesirable photoelectron-hole recombination within the semiconductor due to their short diffusion path lengths and lifetimes, which results in fewer than 10% of the incident photons being used for water splitting. Hematite (α-Fe₂O₃), one of the few naturally occurring n-type metal oxides, is the most promising candidate for an efficient photoanode because of its high earth abundance (4^th most abundant element in the earth’s crust) and visible light active band gap of 2.1 eV, a scalable CMOS compatible synthesis technique, excellent photostability and favourable energy band position capable to drive off the water oxidation reactions without of any external bias. Despite having those advantages, hematite has several inherent drawbacks such as poor electron mobility (~ 10^-1 cm² V^-1 s^-1)^[1], short excited carrier lifetime (about 10 ps)^[2] and short carrier (hole) diffusion length (2−4 nm)^[3]. Hence, most of the photogenerated charge carriers (electron-hole) cannot be extracted to the semiconductor-electrolyte interface before they quickly recombine inside of the materials and dissipate as heat energy. Nanoscale design of hematite such as nanowires^[4], cauliflowers^[5] or core-shell nanostructures synthesised via solution based hydrothermal growth and expensive chemical vapour deposition or atomic layer deposition^[6] along with surface modification with oxygen evolution catalysts^[5] have led to circumvent their drawbacks and made the nanostructured hematite a promising photoanode.

In this work, electrochemical deposition of nanoporous α-Fe₂O₃ and surface modification with cobalt (Co) catalyst is described. The method involves the electrodeposition of iron (Fe) nanoporous film followed by thermal annealing in air (Fig. 1a). The kinetics of the photoelectrochemical (PEC) reactions was improved by modifying the hematite surface with oxygen evolution catalysts. The PEC activity of the optimised nanoporous α-Fe₂O₃/Co has been correlated with the morphology, thickness and Co loading under standard operational conditions (Am 1.5 G, 100 mW/cm², 1 mol/L KOH). This presentation will also discuss our current research to develop cheap and efficient photocatalysts towards fabrication of tandem PEC water splitting device.

Figure. SEM images of the electrodeposited nanoporous hematite (α-Fe₂O₃) in (a) low and (b) high magnified view.

Acknowledgements: Authors acknowledge Marie Curie Actions co-funded Irish Research Council Elevate fellowship ELEVATEPD/2014/15.

References:

[1] A. J. Bosman, H. J. Vandaal, Advances in Physics 1970, 19, 1-117.

[2] N. J. Cherepy, D. B. Liston, J. A. Lovejoy, H. M. Deng, J. Z. Zhang, Journal of Physical Chemistry B 1998, 102, 770-776.

[3] J. H. Kennedy, K. W. Frese, Journal of the Electrochemical Society 1978, 125, 709-714.

[4] L. S. Li, Y. H. Yu, F. Meng, Y. Z. Tan, R. J. Hamers, S. Jin, Nano Letters 2012, 12, 724-731.

[5] A. Kay, I. Cesar, M. Gratzel, Journal of the American Chemical Society 2006, 128, 15714-15721.

[6] Y. J. Lin, S. Zhou, S. W. Sheehan, D. W. Wang, Journal of the American Chemical Society 2011, 133, 2398-2401.