Comprehensive Evaluation of CuBi 2 O 4 as a Photocathode Material for Photoelectrochemical Water Splitting Sean P. Berglund,* ,† Fatwa F. Abdi, † Peter Bogdanoff, † Abdelkrim Chemseddine, † Dennis Friedrich, † and Roel van de Krol †,‡ † Institute for Solar Fuels, Helmholtz-Zentrum Berlin fü r Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109 Berlin, Germany ‡ Institut fü r Chemie, Technische Universitä t Berlin, Straße des 17. Juni 124, 10623 Berlin, Germany * S Supporting Information ABSTRACT: CuBi 2 O 4 is a multinary p-type semiconductor that has recently been identified as a promising photocathode material for photoelectrochemical (PEC) water splitting. It has an optimal bandgap energy (∼1.8 eV) and an exceptionally positive photocurrent onset potential (>1 V vs RHE), making it an ideal candidate for the top absorber in a dual absorber PEC device. However, photocathodes made from CuBi 2 O 4 have not yet demonstrated high photoconversion efficiencies, and the factors that limit the efficiency have not yet been fully identified. In this work we characterize CuBi 2 O 4 photocathodes synthesized by a straightforward drop-casting procedure and for the first time report many of the quintessential material properties that are relevant to PEC water splitting. Our results provide important insights into the limitations of CuBi 2 O 4 in regards to optical absorption, charge carrier transport, reaction kinetics, and stability. This information will be valuable in future work to optimize CuBi 2 O 4 as a PEC material. In addition, we report new benchmark photocurrent density and IPCE values for CuBi 2 O 4 photocathodes. ■ INTRODUCTION Photoelectrochemical (PEC) water splitting, the production of hydrogen and oxygen using a device submerged in aqueous electrolyte under solar irradiation, is an ideal form of renewable energy production. The hydrogen that is produced can be conveniently stored and utilized as a fuel with zero greenhouse gas emissions. In order to implement PEC water splitting on a large scale, new materials must be developed. These materials must be composed of abundant elements, remain stable under illumination in aqueous solution, and efficiently drive the water splitting half-reactions (water oxidation and proton reduction). Photoelectrochemical water splitting can be achieved using a single absorber or dual absorber (two-photon) PEC device. Dual absorber devices consist of a top absorber and bottom absorber, each facilitating one of the water splitting half- reactions. They allow for higher theoretical efficiencies and have fewer material constraints than single absorber devices. 1,2 Ultimately, the operating photocurrent density of a dual absorber device is determined by the overlap in photocurrent density of each absorber layer. 3 Therefore, it is crucial to identify new PEC materials that produce a high photocurrent density and provide a large photovoltage. One material that may be able to meet both of these requirements is CuBi 2 O 4 ,a multinary p-type metal oxide semiconductor with a bandgap energy of 1.6-1.8 eV and a photocurrent onset potential near 1 V vs RHE. 4-6 The bandgap and photocurrent onset potential of CuBi 2 O 4 make it an ideal candidate for the top absorber (larger bandgap photocathode) in dual absorber PEC device. 2,3,7 CuBi 2 O 4 was first identified as a possible photocathode material in 2007 through a combinatorial chemistry study by the group of Sayama. 4 Since then there have been relatively few reports on CuBi 2 O 4 as a PEC material, especially compared to more frequently studied metal oxides such as TiO 2 , Fe 2 O 3 , BiVO 4 , and Cu 2 O. 8 As a result the photophysical and photoelectrochemical properties of CuBi 2 O 4 are not fully understood. In this work we elucidate many of the key material properties of CuBi 2 O 4 that are relevant to PEC water splitting to broaden our understanding of its limitations in regards to optical absorption, charge carrier transport, surface reaction kinetics, and stability. In addition we have used a methodical approach to obtain record photocurrent density and IPCE values for CuBi 2 O 4 photocathodes. Our results should promote interest in CuBi 2 O 4 as a promising new photocathode material and provide direction for future work to improve the photoconversion efficiency further. ■ RESULTS Crystal Structure. The mineral name for CuBi 2 O 4 is kusachiite. It has been reported to have a tetragonal crystal structure with space group P4/ncc, a = b = 8.500-8.511 Å, c = 5.814-5.823 Å, Z = 4. 9-11 To determine the crystal structure of our CuBi 2 O 4 photocathodes we measured them by X-ray diffraction (XRD). Figure 1 shows the XRD diffractogram for a CuBi 2 O 4 photocathode synthesized by drop-casting on a fluorine-doped tin oxide (FTO) coated glass substrate followed by annealing at 450 °C. Also included are the reference patterns Received: February 29, 2016 Revised: June 6, 2016 Published: June 6, 2016 Article pubs.acs.org/cm © 2016 American Chemical Society 4231 DOI: 10.1021/acs.chemmater.6b00830 Chem. Mater. 2016, 28, 4231-4242