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Efficient solar-driven water splitting by nanocone BiVO4-perovskite tandem cells

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Science Advances  17 Jun 2016:
Vol. 2, no. 6, e1501764
DOI: 10.1126/sciadv.1501764
  • Fig. 1 Schematic illustration of the optical absorption mechanism and electron transport of nanoporous BiVO4 on the flat substrate and the conductive nanocone substrate.

    The total thickness (h) of the BiVO4 film is limited by its short carrier lifetime and poor electron transport properties. A large proportion of photoexcited electrons recombine in the flat photoelectrode film before reaching the electrode. However, the h of the BiVO4 film can be increased by introducing the nanocone arrays as a result of the shortened charge transport path that enables efficient charge collection around the conductive nanocones. In addition, its light absorption capability can be further enhanced by multiple light scattering in the unique structure. The half-pitch d/2 (~150 to 200 nm) is an optimized carrier diffusion length for our nanoporous Mo:BiVO4.

  • Fig. 2 Schematic illustration of the fabrication process of the conductive nanocone substrate and electron microscope images of Mo:BiVO4 on the nanocone substrate.

    (A) First, the glass/Si substrate was deposited with one close-packed monolayer of SiO2 to produce a mask for etching and then the shrinking stage was used to adjust the diameter and spacing of the SiO2 nanoparticles through a selective and isotropic RIE process. Second, a Si nanocone array structure was generated via Cl2-based RIE of the Si substrate. The subsequent third process is to oxidize Si nanocone arrays at high temperature in air. The fourth step is to prepare conductive nanocone substrates by coating one layer of Pt and another functional layer of SnO2. The final step is to deposit the nanoporous BiVO4 photoactive layer through a sol-gel process. (B) Scanning electron microscope (SEM) images (60° tilting) of the final SiOx/Pt/SnO2 nanocone arrays. (C) Cross-sectional SEM images of Mo:BiVO4 on the SiOx/Pt/SnO2 nanocone substrate. Some exposed nanocones were also marked in the figure. Scale bars, 500 nm.

  • Fig. 3 Optical absorption measurement and simulation.

    (A) UV-VIS optical absorption (solid line) of Mo:BiVO4 on the nanocone substrate and the FTO-coated glass and their corresponding simulated air mass (AM) 1.5-G spectrum–integrated absorption (dashed line). (B) Comparison of simulated cross-sectional |E| distribution of the EM wave at 500 nm in Mo:BiVO4 on the nanocone substrate and the FTO-coated glass. The red hot area, which represents high generation, indicates effective light trapping by nanocone arrays. A 200-nm-thick BiVO4 on FTO-coated glass and a 700-nm-thick BiVO4 on the SnO2/Pt (50 nm/80 nm) nanocone substrate were used to perform the simulations, respectively. a.u., arbitrary units.

  • Fig. 4 PEC test.

    (A) J-V curves of the Mo:BiVO4 on the FTO-coated glass and the nanocone substrate tested in a 0.5 M KH2PO4 buffer solution (pH 7) and at a scan rate of 20 mV s−1. The corresponding dark currents are also shown. (B) J-V curves of the nanocone/Mo:BiVO4 film measured in phosphate buffer solution containing 0.5 M Na2SO3 and the nanocone/Mo:BiVO4/Fe(Ni)OOH film measured in phosphate buffer solution. The J-V curve of the nanocone/Mo:BiVO4 is also shown. (C) IPCE spectra of the nanocone/Mo:BiVO4 film tested in 0.5 M Na2SO3 and the nanocone/Mo:BiVO4/Fe(Ni)OOH film tested in phosphate buffer solution with bias at 1.23 V versus RHE. (D) Comparison of the angular independence of photocurrent between the flat substrate and the nanocone substrate, showing only a slight decrease in photocurrent for the latter going from 0° to 60° irradiation.

  • Fig. 5 PEC-PSC tandem device.

    (A) The configuration of the PEC-PSC tandem device. (B) J-V curves of the PSC (>515 nm) and the nanocone/Mo:BiVO4 photoanode (<515 nm) measured at 1 sun. (C) JOP versus time profiles measured for the PEC-PSC tandem over 10 hours.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/6/e1501764/DC1

    fig. S1. SEM images of the Si nanocone arrays with the tips of SiO2, Si nanocone arrays, and Mo:BiVO4 on SiOX/Pt/SnO2 nanocone arrays.

    fig. S2. SEM images with different magnifications of the conductive nanocone substrate.

    fig. S3. Band energy diagram of the designed BiVO4-based photoanode.

    fig. S4. SEM images of nanoporous Mo:BiVO4 on the Pt-coated glass and the FTO-coated glass substrates.

    fig. S5. XRD pattern of nanoporous Mo:BiVO4.

    fig. S6. PEC water-splitting performance of BiVO4 on the FTO-coated glass with different Mo-doping concentrations tested in pH 7 phosphate buffer solution.

    fig. S7. High-resolution Bi, V, and Mo XPS spectra of 3% Mo:BiVO4.

    fig. S8. Band gap of 3% Mo-doped BiVO4.

    fig. S9. The ABPE obtained from J-V curves in Fig. 4B according to the equation η = I (1.23 − Vapp)/Plight.

    fig. S10. A typical TEM image of Fe(Ni)OOH nanoparticles on Mo:BiVO4 and its corresponding EDX spectrum.

    fig. S11. Photocurrent density versus time responses of the optimized nanocone/Mo:BiVO4/Fe(Ni)OOH photoanode over 5 hours at 1.23 V versus RHE.

    fig. S12. Sulfite oxidation measurements of Mo:BiVO4 on the conductive flat substrate and the nanocone substrate with the same film thickness of ~700 nm (see Fig. 2 and fig. S3) performed in pH 7 phosphate buffer solution containing 0.5 M Na2SO3.

    fig. S13. J-V characteristics of the optimized nanocone Mo:BiVO4/Fe(Ni)OOH photoanode measured using a two-electrode (a working electrode and a Pt counter electrode) method in pH 7 phosphate buffer solution.

    fig. S14. The distribution of ABPE by measurements of batches of photoanodes performed in a two-electrode system.

    fig. S15. J-V curve of a PSC measured under 1-sun irradiation.

    fig. S16. The morphology of the Mo:BiVO4/Fe(Ni)OOH after 10 hours of PEC test.

    fig. S17. H2 and O2 production from the tandem device and the theoretical gas production rate of the tandem device.

  • Supplementary Materials

    This PDF file includes:

    • fig. S1. SEM images of the Si nanocone arrays with the tips of SiO2, Si nanocone arrays, and Mo:BiVO4 on SiOX/Pt/SnO2 nanocone arrays.
    • fig. S2. SEM images with different magnifications of the conductive nanocone substrate.
    • fig. S3. Band energy diagram of the designed BiVO4-based photoanode.
    • fig. S4. SEM images of nanoporous Mo:BiVO4 on the Pt-coated glass and the FTO-coated glass substrates.
    • fig. S5. XRD pattern of nanoporous Mo:BiVO4.
    • fig. S6. PEC water-splitting performance of BiVO4 on the FTO-coated glass with different Mo-doping concentrations tested in pH 7 phosphate buffer solution.
    • fig. S7. High-resolution Bi, V, and Mo XPS spectra of 3% Mo:BiVO4.
    • fig. S8. Band gap of 3% Mo-doped BiVO4.
    • fig. S9. The ABPE obtained from J-V curves in Fig. 4B according to the equation η = I (1.23 − Vapp)/Plight.
    • fig. S10. A typical TEM image of Fe(Ni)OOH nanoparticles on Mo:BiVO4 and its corresponding EDX spectrum.
    • fig. S11. Photocurrent density versus time responses of the optimized nanocone/Mo:BiVO4/Fe(Ni)OOH photoanode over 5 hours at 1.23 V versus RHE.
    • fig. S12. Sulfite oxidation measurements of Mo:BiVO4 on the conductive flat substrate and the nanocone substrate with the same film thickness of ~700 nm (see Fig. 2 and fig. S3) performed in pH 7 phosphate buffer solution containing 0.5 M Na2SO3.
    • fig. S13. J-V characteristics of the optimized nanocone Mo:BiVO4/Fe(Ni)OOH photoanode measured using a two-electrode (a working electrode and a Pt counter electrode) method in pH 7 phosphate buffer solution.
    • fig. S14. The distribution of ABPE by measurements of batches of photoanodes performed in a two-electrode system.
    • fig. S15. J-V curve of a PSC measured under 1-sun irradiation.
    • fig. S16. The morphology of the Mo:BiVO4/Fe(Ni)OOH after 10 hours of PEC test.
    • fig. S17. H2 and O2 production from the tandem device and the theoretical gas production rate of the tandem device.

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