Rigorous substrate cleaning process for reproducible thin film hematite (α-Fe 2 O 3 ) photoanodes

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Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel

Ki Ro Yoon and Il-Doo Kim Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea

Avner Rothschilda)

Department of Materials Science and Engineering, Technion – Israel Institute of Technology, Haifa 32000, Israel (Received 6 August 2015; accepted 16 September 2015)

Hematite (a-Fe2O3) photoanodes are widely studied as candidates for water splitting photoelectrochemical (PEC) cells. To speed up the development of high efficiency hematite photoanodes, systematic investigations of the effect of material properties such as dopants and microstructure on PEC properties that determine the photoanode performance are crucial. Toward this end, this work presents a route for reproducible fabrication of thin film hematite photoanodes with reproducible microstructure and PEC properties. Hematite thin (50 nm) films are deposited by pulsed laser deposition from a Ti-doped (1 cation%) Fe2O3 target onto cleaned transparent conducting substrates (fluorinated tin oxide, FTO, coated glass substrates). Special attention is paid to rigorous cleaning of the substrates prior to the hematite deposition, which is found to be crucial for achieving highly reproducible results. Specimens prepared by this route display homogenous conformal coating with very little spread in PEC properties between different specimens, meeting the necessary prerequisite for systematic investigation of hematite photoanodes. I. INTRODUCTION

The need for cost effective and sustainable technologies for storing intermittent solar power spurs a growing interest in artificial photosynthesis and solar fuels.1,2 Photoelectrochemical (PEC) cells combined in tandem with photovoltaic cells offer a viable solution to this need by splitting water, using solar power, to hydrogen, and oxygen.3,4 The hydrogen can be stored and converted to electricity on-demand (e.g., by fuel cells), or be used as renewable feedstock for sustainable synthesis of liquid fuels (e.g., by hydrogenation of CO2).5,6 One of the greatest challenges in this route is the development of stable, efficient, and inexpensive photoanodes for water photo-oxidation. Hematite (a-Fe2O3) is a leading photoanode candidate7–11 due to its abundance, stability in alkaline aqueous solutions, high catalytic activity for water oxidation12 and band gap energy of 2.1 eV which is nearly optimal for tandem cells in combination with lower band gap semiconductors such as Si.13 Unfortunately, hematite also displays deleterious charge transport properties14 that give rise to photocurrent and photovoltage losses due to

Contributing Editor: Artur Braun a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.300 J. Mater. Res., Vol. 31, No. 11, Jun 14, 2016

charge carrier recombination, which complicates the development of high efficiency photoanodes. Various strategies have been scrutinized to red