Letter pubs.acs.org/JPCL
Stable Solar-Driven Water Oxidation to O2(g) by Ni-Oxide-Coated Silicon Photoanodes Ke Sun,†,‡ Matthew T. McDowell,†,‡ Adam C. Nielander,† Shu Hu,†,‡ Matthew R. Shaner,†,‡ Fan Yang,†,‡ Bruce S. Brunschwig,§ and Nathan S. Lewis*,†,‡,§,∥ †
Division of Chemistry and Chemical Engineering, ‡Joint Center for Artificial Photosynthesis, §Beckman Institute and Molecular Materials Research Center, and ∥Kavli Nanoscience Institute, California Institute of Technology, Pasadena, California 91125, United States S Supporting Information *
ABSTRACT: Semiconductors with small band gaps (60 °C24) are ∼100 and 2000 times slower than the etch rates for the Si (111) and Si (100) facets, respectively. The observed behavior is therefore consistent with expectations for the formation of a passivating oxide on Si sites exposed by defects/pin-holes in the prepared NiOx films, which in turn suppresses further etching of the Si and results in a sustained high branching ratio for oxygen evolution relative to further oxidation of the photoanode. Assuming that the stability of the photoanode under steady-state conditions is related to the amount of charge passed and accounting for the ∼20% capacity factor of sunlight,25 the 1200 h of continuous operation contained the same amount of charge as would be passed during >8 months of outdoor operation, which therefore plausibly represents a lower limit on the actual temporal stability of the NiOx-coated Si photoanodes in an operational device. Accelerated long-term stability tests would be required to precisely identify the degradation/corrosion mechanisms in more detail.
presumably be minimized further with improved control over processing conditions as well as over exposure to particulates. Figure 4a shows a cross-sectional scanning-electron microscope (SEM) image of a Si substrate coated by a NiOx film. The columnar appearance of the film suggests that the film is polycrystalline. Figure 4b shows an atomic-force microscope (AFM) image of a sputtered NiOx film with a surface roughness of 4 nm and a grain size of about 18 nm. Figure 4c shows the Xray diffraction (XRD) patterns obtained for sputtered NiOx films. The XRD patterns for the films agreed well with those for cubic NiO (NaCl crystal structure, JCPDS file 4-835). The preferred orientation of the grains was in the ⟨111⟩ direction. Figure 4d shows the transmittance spectra for a 75 nm NiOx film and for an ultrathin Ni metal (∼2 nm) film, both coated on a quartz substrate. The 75 nm thick NiOx sample showed a comparable transmittance to that of an ultrathin (∼2 nm) Ni metal film that had been simultaneously optimized for catalytic activity, conductivity, and minimization of optical obscuration (Figure S2). Tauc plots indicated a direct transition at 3.74 eV (Figure 4d inset) in the sputter-deposited NiOx film. Using ellipsometry, the refractive index (n) of the NiOx films was determined to be ∼2.23 at a wavelength (λ) of 550 nm. The decrease in transmittance at wavelengths greater than 400 nm was thus not primarily due to absorption of light by NiOx, but was due to the enhancement in reflection resulting from the larger difference in the refractive indices of air (n = 1) and NiOx (n ∼ 2.23) relative to the difference in the refractive indices of air and quartz (n ∼ 1.46). Although the electrochromic behavior of Ni oxide due to the adsorption of ions under anodic bias can reduce the transmittance of crystalline NiOx films,16 the observed optical absorption of crystalline NiOx films prepared using hightemperature deposition (Figure S3) was negligible compared to Ni(OH)2 or Ni(OOH) alone.17 Using a high estimate (3 × 104 cm−1) for the absorption coefficients of Ni(OH)2 and NiOOH based on the range of reported values,18 an increase after 1000 h in the thickness of the catalytically active Ni layer from 7.6 to 10.2 nm would be expected to result in
