Contents lists available at ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod Controlling phase fraction and crystal orientation via thermal oxidation of iron foils for enhanced photoelectrochemical performance Rambabu Yalavarthi, Alberto Naldoni*, Radek Zbořil, Štěpán Kment* Regional Centre of Advanced Technologies and Materials, Faculty of Science, Palacký University Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic ARTICLE INFO Keywords: Fe 2 O 3 nanoflakes Mixed-phase Charge recombination Intensity modulated photocurrent spectroscopy (IMPS) Impedance spectroscopy (EIS) PEC water splitting ABSTRACT It has been known that the intrinsic properties of a semiconducting photoanodes significantly influence the overall photoelectrochemical (PEC) performance. Here, we report on the fabrication of layered structure of mixed-phase FeO (wustite), Fe 3 O 4 (magnetite), and α-Fe 2 O 3 (hematite) iron oxide nanoflake/nanowire morphologies through the thermal oxidation of pristine Fe foils, and the role of metastable FeO phase on the PEC performance discussed. X-ray diffraction and Raman spectroscopic measurements revealed the variation in phase fraction of wustite, magnetite, and hematite with respect to oxidation temperature. The PEC measurements indicate a dependence of onset potential and photocurrent density on phase proportion. The sample, which contains metastable wustite phase FeO, along with Fe 3 O 4 and α-Fe 2 O 3, shows a lower onset and higher photo- current density, followed by the sample that contains a nearly equal ratio of magnetite to hematite phase (∼ 42:58) than that of relatively higher magnetite phase content samples. It is attributed to the improvement in the intrinsic transport of photogenerated charge carriers from hematite via the magnetite and wustite phases to the back contact of the photoanode. It consequently led to a decrease in bulk charge recombination across the interfaces of multiple phases. We carried out electrochemical impedance (EIS) and light intensity-modulated photocurrent measurements (IMPS) to elucidate the mechanism behind the charge separation across the multiple phases. 1. Introduction Photoelectrochemical (PEC) water splitting is regarded as one of the direct approaches to produce hydrogen from water using semi- conductor upon light illumination [1–3]. Realization of low cost and highly abundant metal oxide semiconductors for PEC water splitting has drawn much attention in recent years [4,5]. The hematite (α- Fe 2 O 3 ), an n-type semiconductor, has been widely studied material for photoelectrochemical water splitting due to its suitable bandgap (∼ 2.1 eV) [6–11]. Besides, it is a low-cost, abundant, and stable photo- electrode in wide pH electrolytes. However, short hole diffusion length (2-4 nm) compared to light penetration depth (∼1/α = 118 nm at λ = 550 nm) can cause rapid recombination of photogenerated elec- tron-hole pairs; thus the overall performance of hematite is limited in comparison to the theoretical efficiency ∼15% [9,7–11]. Another main drawback of α-Fe 2 O 3 is a poor oxygen evolution reaction kinetics (OER) and a high density of surface states acting as recombination centers [12,13]. In order to address the shortcomings associated with hematite photoelectrodes, several approaches including elemental doping to improve the intrinsic conductivity, co-catalyst decoration for enhancing the surface redox reactions, band engineering, and nanostructuring to improve the charge carriers dynamics have been proposed [14–18]. Morphological engineering through the fabrication of nanorods, nano- tubes, nanowires, and nanoflake structures is an effective strategy to improve the surface area, light absorption, and thus minimizing the electron-hole recombination [7,14–20]. Apart from that, modification of the hematite surface with TiO 2 nanoparticles and making hetero- junction nanocomposites have also been explored to improve the charge separation rate across the electrode/electrolyte interface [21–23]. Notably, in order to achieve a balance between the extended photon penetration depth and short hole diffusion lengths, the fabrication of ultrathin films (< 50 nm) has been explored in the recent past [10,24–26]. The thermal annealing of pristine Fe foils is an easy and direct approach for the fabrication of iron oxide nanoflake and nano- wire-like structures [26–28]. However, thermal oxidation at a higher temperature can lead to the formation of different polymorphic content films of iron oxide. The three main polymorphs of iron oxide are α- Fe 2 O 3 (hematite), Fe 3 O 4 (magnetite), and metastable FeO (wustite). The Fe 3 O 4 is a less photoactive phase than hematite; however, its https://doi.org/10.1016/j.cattod.2020.01.044 Received 1 October 2019; Received in revised form 16 January 2020; Accepted 29 January 2020 ⁎ Corresponding authors. E-mail addresses: alberto.naldoni@upol.cz (A. Naldoni), stepan.kment@upol.cz (Š. Kment). Catalysis Today xxx (xxxx) xxx–xxx 0920-5861/ © 2020 Published by Elsevier B.V. Please cite this article as: Rambabu Yalavarthi, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.01.044