1670 doi:10.1017/S1431927618008838 Microsc. Microanal. 24 (Suppl 1), 2018 © Microscopy Society of America 2018 Hierarchical InGaN Nanowires for High-Efficiency Solar Water Splitting Jiseok Gim 1 , Reed Yalisove 1 , Sheng Chu 3 , Yongbum Park 4 , Srinivas Vanka 3,4 , Yichen Wang 3 , Yongjie Wang 4 , Yong-Ho Ra 3 , Hong Guo 5 , Ishiang Shih 3 , Zetian Mi 3,4 and Robert Hovden 1,2 1. Department of Materials Science & Engineering, University of Michigan, Ann Arbor, USA. 2. Applied Physics Program, University of Michigan, Ann Arbor, USA. 3. Department of Electrical & Computer Engineering, McGill University, Montreal, Canada. 4. Department of Electrical Engineering & Computer Science, University of Michigan, Ann Arbor, USA. 5. Department of Physics, McGill University, Montreal, Canada. Solar water splitting has become a promising way to alleviate supply instability of solar energy by directly storing energy in the form of hydrogen fuel. Maximizing efficiency for photoelectrochemical (PEC) water splitting requires (i) a tunable bandgap that captures the solar spectrum, (ii) an energy band edge that spans the water redox potential, and (iii) high quantum efficiency [1,2]. Although a variety of materials such as Si, Ta 3 N 5 , and BiVO 4 have been studied as photoelectrodes, most materials have not yet fulfilled these requirements. For instance, the onset potential of Si and Ta 3 N 5 is too large so charge carriers cannot be sufficiently generated under sun illumination and BiVO 4 has a wide 2.4 eV bandgap that limits the efficient utilization of the solar spectrum [1,2]. The InGaN ternary system is an optimal photoelectrode for efficient solar hydrogen production (Fig. 1a). The bandgap (E g ) of InGaN is direct and tunable from 3.4 (GaN) to 0.65 eV (InN) for indium compositions up to ~50%, allowing optimal use of the entire solar spectrum (E g  1.7 eV), which could potentially enable a solar-to-hydrogen efficiency over 25% [1,2,3]. However, creating high-performance InGaN photoelectrodes is difficult as In-rich crystals are highly strained causing phase segregation and subsequent performance degradation. Here we show low-dimensional nanostructures accommodate crystalline InGaN nanowires capable of enhanced water splitting performance (highest reported value of 10.9 mA/cm 2 at 1.23 V versus reversible hydrogen electrode (RHE)) and photoluminescence (a single emission peak at 720 nm) [2]. Using plasma- assisted molecular beam epitaxy (MBE), highly crystalline InGaN can be grown as one dimensional (1D) nanostructures. This allows high quantum efficiency with a larger surface area for PEC reaction [2,4]. Cross-sectional electron microscopy of InGaN/Si (Fig.1b, c) reveals that the crystalline InGaN nanowires have an approximate height of ~700 nm tall and diameter of ~200 nm wide which extend atop a polycrystalline growth layer on the Si substrate. The low-dimensional geometry not only allows high In concentrations, but also provides more catalytically active surface area. IrO 2 co-catalysts with a size of 1- 2 nm were uniformly loaded on the InGaN surface (Fig. 1c) to further enhance performance. The current- potential (J-V) curves of IrO 2 (Fig.1d) show that the photocurrent density of IrO 2 /InGaN reaches 10.9 mA/cm 2 at 1.23 V versus RHE due to the lower onset potential of the IrO 2 co-catalyst combined with the sufficient InGaN bandgap of ~1.7 eV. The maximum applied bias photon-to-current efficiency (ABPE) of the IrO 2 /InGaN photoanode calculated from the J−V curve is 3.6%, which is the highest among those of previously reported photoelectrodes [2]. InGaN crystals can also be grown with hierarchical order that spans the nano- to atomic- scale through 1D lithographic templating (Fig. 2a-c). Cross-sectional electron microscopy shows the periodic GaN nanowalls (width ~500 nm, height ~1 um, spacing ~400 nm) that template confined InGaN growth (Fig. 2a). InGaN grows as triangular prisms atop each nanowall (Fig. 2a) because the polar facet (001) has a faster growth rate than that of the semi-polar (101) and non-polar (100) facet sidewalls. It is also interesting that a sharply faceted single crystal InGaN nanoridge (~50 nm width) forms along the top of each nanowalls (Fig. 2c). In confined geometry heteroepitaxy, increased In incorporation can occur due to https://doi.org/10.1017/S1431927618008838 Downloaded from https://www.cambridge.org/core. IP address: 172.245.152.24, on 22 Apr 2020 at 01:53:26, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.