ARTICLES PUBLISHED ONLINE: 5 MAY 2013 | DOI: 10.1038/NMAT3626 H 2 evolution at Si-based metal–insulator– semiconductor photoelectrodes enhanced by inversion channel charge collection and H spillover Daniel V. Esposito 1 , Igor Levin 1 , Thomas P. Moffat 1 * and A. Alec Talin 2,3 * Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal–insulator–semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO 2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO 2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO 2 /Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting. H ydrogen production by solar-driven photoelectrochemical water splitting is an attractive means to convert intermittent solar radiation into a storable, non-polluting fuel. However, the efficiency and stability of semiconducting photoelectrodes used in PEC devices must be substantially improved to make this process economical 1–3 . One approach to achieving high solar-to-hydrogen efficiency and stability is the metal–insulator–semiconductor (MIS) photoelectrode design 4,5 . As illustrated in Fig. 1a, a typical MIS photocathode consists of metallic ‘collectors’ situated on the surface of an insulator-covered semiconductor. When the MIS photocath- ode is illuminated, minority carriers tunnel through the insulating layer to the collector, where the hydrogen evolution reaction takes place. An advantage of the MIS design is that semiconductor stability and light-harvesting efficiency are decoupled, enabling narrow bandgap semiconductors that are well-suited for absorbing sunlight to be used without being corroded by the electrolyte. A critical role is played by the thin insulating layer, typically an oxide, which must simultaneously protect the semiconductor from the corrosive electrolyte and efficiently mediate minority carrier transport across the MIS junction with minimal recombi- nation. According to the conventional view of MIS photocathode operation (Fig. 1a), photocurrent may only be produced when photogenerated electrons are created within a distance less than the sum of the depletion width (W ) and effective minority carrier diffusion length (L e ) of a collector. In a well-behaved p-type MIS junction, photogenerated minority electrons pass directly from the semiconductor conduction band edge (E C ) to the Fermi level of the metallic collector (E f,metal ), as illustrated in Fig. 1b. Both elemental and compound semiconductors have been explored for water splitting in the MIS device geometry 4–10 . Appreciable conversion efficiencies have been demonstrated for InP photoelectrodes 7,8,11 , but the low abundance of In (ref. 12) and economical scale-up of epitaxially grown III–V semiconductors are major challenges. Si is attractive for MIS photoelectrodes because 1 National Institute of Standards and Technology, Materials Measurement Laboratory, 100 Bureau Drive, Gaithersburg, Maryland 20899-1070, USA, 2 National Institute of Standards and Technology, Center for Nanoscale Science and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899-1070, USA, 3 Sandia National Laboratories, MS9161, Livermore, California 94551-0969, USA. *e-mail: thomas.moffat@nist.gov; aatalin@sandia.gov. it is highly abundant, its bandgap (E g ≈ 1.12 eV) is well-suited for absorbing sunlight, it has emerged as a low-cost material in the photovoltaic industry, and its native oxide (SiO 2 ) can serve as an insulating layer that is thermodynamically stable over a wide range of pH and potentials 13 . Recently, McIntyre et al. demonstrated a Si-based MIS-type photoanode using continuous ultrathin films of Ir and TiO 2 deposited on n-Si (ref. 6). This photoanode exhibited an appreciable inferred photovoltage of ≈550 mV and good stability under strongly oxidizing conditions. However, the conversion efficiencies of Si-based MIS photocathodes demonstrated to date 4,5 remain well below the solar-to-electricity conversion efficiencies of comparable Si photovoltaic cells. This disparity has been largely attributed to unfavorable energy band alignment in p-Si based MIS junctions and high recombination rates at the SiO 2 /Si interface 4,10 . Herein, we demonstrate two modifications to the basic MIS structure that significantly improve the conversion efficiency of Si-based MIS photocathodes. Specifically, we introduce the use of bilayer metal collectors and a high-quality tunnelling oxide layer grown by thermal annealing. Furthermore, we present evidence that hydrogen evolution rates on these MIS photocathodes are enhanced by hydrogen spillover-assisted H 2 evolution and long-distance transport of photogenerated minority carriers through inversion channels; findings with important implications for future MIS photoelectrode development. Si-based MIS photoelectrodes were fabricated using p-Si(100) wafers, on which 2 nm thick SiO 2 tunnelling layers were grown by rapid thermal oxidation (RTO) at 950 ◦ C. Previous studies have shown that similar RTO treatments can produce high-quality SiO 2 /Si interfaces with low interfacial defect densities, comparable to thermal oxidation 14 . In addition to samples with RTO-grown SiO 2 , cleaned wafers were exposed to the ambient, resulting in growth of a ≈1.4 nm native SiO 2 layer on the Si surface. Bilayer metal collectors of various diameters, thicknesses, and inter- collector spacing were deposited onto the oxide-covered p-Si(100) NATURE MATERIALS | ADVANCE ONLINE PUBLICATION | www.nature.com/naturematerials 1 © 2013 Macmillan Publishers Limited. All rights reserved.