Vertically aligned hematite nanopillars for PEC water splitting

Poster, Vladimiro Dal Santo, 054
Publication date: 31st March 2013

Photoelectrochemical (PEC) water splitting offers anelegantand clean direct route toward the storage of solar energy on a global scale. Solar energy can be captured and stored in the form of chemical bonds to yields solar fuels such as hydrogen. In thepastdecades, metal oxide semiconductors have been extensively studied as photoelectrodes for water splitting.¹ Among many different materials, hematite (alpha-Fe₂O₃) has emerged as a strong candidate photoanode because it is stable in water, is prepared from aboundant and low cost elements, and has a favorable band gap (E_g=2.1 eV) that enable it to strongly absorb visible light.² However, PEC activity of a-Fe₂O₃ is severly limited by its low light penetration depht, very short excited lifetimes (10^-12s), and a short hole diffusion lenght (2-4 nm). To address these limitations and improve solar conversion efficiency, enormous efforts have been focused on the synthesis of hematite nanostructured films, modification of electronic structure via elemental doping (e.g., Si, Sn, reduction), passivation under- or overlayer, and coupling with electrocatalysts for the oxygen evolution reaction (i.e., IrO₂).^3,4

Here, we report the plasma enhanced chemical vapour deposition (PE-CVD) of nanostructured alpha-Fe₂O₃ films on FTO. Nanostructured alpha-Fe₂O₃ films were produced by decomposition of an organometallic precursor (ferrocene), promoted by an Ar-oxygen (10-25% by vol.) cold RF plasma. The alpha-Fe₂O₃ films are formed by vertically aligned nanopillars with features size of about 25x80 nm assembled in submicrometers platelet morphology (Figure 1).

Film thickness (and then light absorption) can be tuned by changing the amount of precursor, that in turn, influences the features size of nanostructures. An advantage of our deposition technique is that we obtain crystalline alpha-Fe₂O₃ films without the need of further thermal treatment. Interestingly, increasing the amount of precursor we observe an increased preferential growth of the a-Fe₂O₃ along the [110] crystallographic plane, perpendicular to the FTO substrates. This preferential growth closely resembles that of nanowires and it is of special interests since alpha-Fe₂O₃ presents a four-fold enhancement of conductivity along this direction than ortogonal to it. This property makes our novel a-Fe₂O₃ nanopillars an interesting candidate morphology for efficient photoanodes. At the moment, we are testing the alpha-Fe₂O₃ nanopillars films in the PEC water splitting and studying the effects of doping and co-catalysts addition on their photoactivity.

Figure 1. SEM micrograph (left) and AFM morphology image (right) of hematite nanopillars organized in submicrometric plateletes.
[1] Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, J. Photoelectrochemical cells for solar hydrogen production: current state of promising photoelectrodes, methods to improve their properties, and outlook. Energy Environ. Sci., 2013, 6, 347-370. [2] Sivula, K. ; Florian Le Formal, F.; Grätzel M. Solar Water Splitting: Progress Using Hematite (α-Fe2O3) Photoelectrodes. ChemSusChem, 2001, 4, 432–449. [3] Ling, Y.; Wang, G.; Wheeler, D. A.; Zhang, J. Z.; Li, Y. Sn-Doped Hematite Nanostructures for Photoelectrochemical Water Splitting. Nano Lett. 2011, 11, 2119–2125. [4] Tilley, S. D.; Cornuz, M.; Sivula, K.; Grätzel, M. Light-Induced Water Splitting with Hematite: Improved Nanostructure and Iridium Oxide Catalysis. Angew. Chem. Int Ed. 2010, 49, 6405–6408.

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