Nanostructural characterization of mesoporous hematite thin film photoanode used for water splitting
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By combining high-resolution transmission electron microscopy and scanning transmission electron microscopy with analytical capability, we investigated the nanostructure of a textured hematite photoanode with columnar grains obtained by the colloidal deposition of magnetite nanocrystals. This initial report describes in detail the structure and chemistry of the a-Fe2O3/SnO2:F interface by identifying semicoherent and incoherent interfaces as well as a localized interdiffusion layer of Sn and Fe at the interface (;100 nm in length). Our study indicates that unintentional doping by tin at a high sintering temperature is not significant in enhancing hematite photoanode performance for water oxidation. The correlation of nanoscale morphology with photoelectrochemical characterization facilitated the identification of the beneficial effect of a preferential growth direction of a hematite film along the [110] axis for water-splitting efficiency. I. INTRODUCTION
Solar energy can provide sufficient power for all of our energy needs if it can be efficiently harvested. An elegant and potentially efficient route to storing solar energy is to convert light into chemical energy in the form of chemical bonds, which is a form of artificial photosynthetic process. Considering the abundance of H2O on the planet, water splitting is a natural pathway for artificial photosynthesis. Hematite (a-Fe2O3) is a candidate material to be used as a photoanode for water splitting due to intrinsic properties such as suitable band gap (2.0–2.2 eV) for visible light absorption, chemical and photoelectrochemical stability, abundance, and low cost.1–4 However, there are enormous challenges to be overcome before this material is available for technological applications. The main challenges in the implementation of this material are: (i) a large overpotential for water oxidation, (ii) a relatively low absorption coefficient requiring thick films (in the range of 400–800 nm) for efficient light absorption, (iii) a poor majority carrier conductivity (electrons), and (iv) a short diffusion length of the minority charge (hole).5–8 Actually, many of these problems contribute in a direct or indirect way to the electron–hole recombination process. In recent years, a decrease in the electron–hole recombination and an increase in the surface area through the use of a nanostructured hematite film has been the focus of researchers to obtain a high-performance photoanode.8–10 a)
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/ DOI: 10.1557/jmr.2013.249 J. Mater. Res., Vol. 29, No. 1, Jan 14, 2014
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Indeed, the electron–hole recombination can be minimized in very thin films; however, low light absorption produces low efficiency.11,12 Thus, two strategies can be used to improve the pho
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