Thermal enhancement of water affinity on the surface of undoped hematite photoelectrodes Waldemir M. Carvalho-Jr, Flavio L. Souza n Centro de Ciências Naturais e Humanas, Laboratory of Alternative Energy and Nanomaterial-LEAN, Universidade Federal do ABC, 09090-400 Santo André, SP, Brazil article info Article history: Received 20 April 2015 Received in revised form 17 September 2015 Accepted 18 September 2015 Available online 23 October 2015 Keywords: Hematite thin film Nanostructure Hydrothermal-microwave process Photoelectrochemical performance Wettability abstract Hematite is one of the most promising semiconductors used to convert solar irradiation directly into chemical energy (molecular hydrogen and oxygen) via water splitting. Although several methods have been developed in the last decades, the repertoire of a method to fabricate hematite electrodes with a large covered area, relative low cost, industrial viability and remarkable properties still remains a chal- lenge. This paper describes a microwave-assisted hydrothermal process to produce hematite films. The photoelectrodes prepared consisted of rods of hematite on the FTO substrate (FTO-fluorine doped tin oxide) forming mesoporous nanostructured. According to the X-ray diffraction data, thin films thermally treated at high temperature grew preferentially to the [110] direction, i.e., growth perpendicular to the substrate. Moreover, high temperature of thermal treatment influenced positively on the stability, cur- rent density and charge transfer efficiency of photoelectrodes. The impedance spectroscopy (EIS) data aided us to understand the best photoelectrochemical performance exhibited by hematite electrode synthesized during 2 h with additional thermal treatment at 750 °C. The high temperature treatments seem to promote higher N D (charge density) and V fb values estimated from EIS data, which are related to an efficient photogenerated charge separation (e À Àh þ ). Additionally, our results suggested that the hematite surface roughness and wettability should be considered as important parameters for producing active electrodes for solar water oxidation. A quasi-super hydrophilic surface observed for electrode sintered at high temperature and illustrated by contact angle images could be favoring solid/liquid interface to increase the surface efficiency for chemical reaction. Finally, this work gives a new insight into the role of high temperatures in the sintering process for the activation of pure hematite photo- electrode surface. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Iron oxide (hematite) has been attracting the scientific com- munity interest for decades due to its potential to carry out the water oxidation under solar irradiation in a photoelectrochemical cell (PEC) configuration [1–3]. Hematite is an abundant material, has an excellent chemical stability in an aqueous environment and favorable optical band gap that allows it to absorb a wide solar irradiation spectrum. Despite its attractive properties, the poor optoelectronic properties necessitate application of large external potential to split water with solar irradiation assistance, limiting the high performance of this material [4–7]. Since the nanoscience has opened new perspectives [8] and several synthetic routes have been developed to build efficient nanostructured materials for PEC applications many advances were reported [9–14]. Despite recent advances, more fundamental research to understand the hema- tite's properties and limitations is needed in order to make real progress in making PEC economically viable [5,15–19]. Indeed, it is a consensus in the scientific community that the state-of-art in this field is to find ways to selectively change or overcome one limitation without adversely altering the other properties [20–23]. Therefore, the experimental effort is being focused on developing strategies to reach the perfect harmony between nanostructured morphologies and active surfaces for chemical reactions as pre- dicted by theoretical calculations [4,24]. Bard and Hardee through chemical vapor deposition (CVD) reported the first performance of hematite films in a PEC [25,26]. Nowadays, aqueous chemical routes under hydrothermal conditions have been considered a very promising and versatile method to synthesize films with innumerous morphologies at relative low cost and easily scalable [27–30]. In addition to the development of synthetic routes, investigations have been carried out in order to activate the sur- face of electrodes prepared with a wide variety of morphologies. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/solmat Solar Energy Materials & Solar Cells http://dx.doi.org/10.1016/j.solmat.2015.09.028 0927-0248/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ55 11 4996 8353; fax: þ55 11 4996 0090. E-mail addresses: fleandro.ufabc@gmail.com, flavio.souza@ufabc.edu.br (F.L. Souza). Solar Energy Materials & Solar Cells 144 (2016) 395–404