Single Crystalline Hematite Films for Solar Water Splitting: Ti-Doping and Thickness Effects Maxime Rioult, He ́ le ̀ ne Magnan,* Dana Stanescu, and Antoine Barbier CEA-Saclay, DSM/IRAMIS/SPEC, F-91191 Gif-sur-Yvette Cedex, France ABSTRACT: Undoped and Ti-doped (2 at. %) epitaxial hematite thin films, in the thickness range 5-50 nm, were grown by atomic oxygen assisted molecular beam epitaxy (AO-MBE) on Pt(111) substrates in the framework of hydrogen harvesting from sunlight-induced water splitting. Such single crystalline samples are suitable model systems to study thickness and doping effects on the photoelectrochemical properties; we demonstrate that they also allow disentangling intrinsic transport properties from mingled overall properties due to the usually unknown contributions from morphology or crystalline structure defects. From their photoelectrochemical characteristics (I(V) curves, incident photon to current efficiency measurements, and electrochemical impedance spectroscopy), we evidence the existence of an optimum layer thickness, which is higher for Ti-doped samples (30 nm) as compared to undoped ones (20 nm). Our results suggest that this effect is due to an increase of the carrier concentration combined with higher carriers’ diffusion lengths in the doped samples stressing intrinsic modifications of the hematite layer upon titanium doping that cannot be accounted for by simple structural or electronic structure changes. I. INTRODUCTION Solving the concomitant worldwide increasing energy con- sumption problem and the need for greenhouse gas reduction to avoid or limit climate changes imposes a higher fraction of renewable energies in the total energy production. Sustainable growth cannot anymore be dissociated from fostering novel renewable energies that become more and more a first-class issue. Within this framework, increasing attention is paid to the sunlight-assisted water splitting as a clean method of hydrogen production. As a matter of fact, hydrogen is an energy carrier of choice which does not lead to any greenhouse gas production. Although the idea of producing hydrogen using water splitting assisted by solar light is very seductive, it remains unfortunately also very tricky, and many materials science issues have to be solved. During the process, electron-hole pairs are generated in a semiconductor electrode, upon solar light absorption, and are subsequently used to promote the oxido-reduction reactions of water leading to oxygen production at the photoanode and hydrogen production at the photocathode. 1,2 Since the pioneering discovery of water-photoassisted electrolysis using semiconducting TiO 2 in 1972 by Fujushima and Honda, 1 several materials were investigated as photo- anodes 3 where water oxidation occurs (2OH - +2h + → (1/ 2)O 2 +H 2 O). Hematite, i.e., the α-Fe 2 O 3 iron oxide, is one of the most promising materials regarding its characteristics. It is an n-type semiconductor with a quasi-ideal band gap (∼2.2 eV) for solar water splitting applications. Indeed this material is able to absorb ca. 40% of the solar light spectrum, and its theoretical solar-to-hydrogen conversion yield reaches 13%. 3 It is abundant on earth and very stable in aqueous environments, which makes it a serious candidate in the framework of green energy production. 4 Moreover, the valence band edge of hematite is located below the H 2 O/O 2 redox potential which favors the water oxidation reaction. 5 Unfortunately, hematite has not only advantages since it has been demonstrated to have weak transport properties 6,7 (low conductivity and low carrier lifetime) and poor surface kinetics. 8 Also, the conduction band edge of α-Fe 2 O 3 is not well positioned with respect to the potential of the water reduction reaction (2H 2 O+2e + → H 2 + 2OH - ), thus an external bias is necessary to promote water splitting. 9 Various strategies were proposed to overcome hematite drawbacks 3,4,10 including (i) doping with different elements such as Ti, Si, or Sn 7,11-21 which can improve electrical properties and also modify the electronic structure; (ii) nanostructuration (e.g., nanowires, mesoporous materials, ...) to compensate small carrier diffusion lengths; 5,22,23 (iii) more complex semiconductor heterostructures 24,25 to optimize light absorption and photogenerated carrier separation; and finally (iv) surface engineering with overlayers or catalysts. 8,26-28 The case of hematite doped with Ti is a seductive idea to improve hematite properties and has been the subject of numerous studies in recent years. 7,11-18 However, the improve- ment of the electrical conductivity induced by Ti doping has not been discussed in detail with respect to possible changes of the crystalline structure. As a matter of fact, previous studies concerning polycrystalline films or nanostructures include a high density of grain boundaries which may dominate the electric conduction; they can either limit conduction (if the conduction occurs perpendicularly to them) or increase Received: January 10, 2014 Revised: January 20, 2014 Published: January 23, 2014 Article pubs.acs.org/JPCC © 2014 American Chemical Society 3007 dx.doi.org/10.1021/jp500290j | J. Phys. Chem. C 2014, 118, 3007-3014