Research ArticleINORGANIC CHEMISTRY

Radially oriented mesoporous TiO2 microspheres with single-crystal–like anatase walls for high-efficiency optoelectronic devices

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Science Advances  08 May 2015:
Vol. 1, no. 4, e1500166
DOI: 10.1126/sciadv.1500166
  • Fig. 1 Schematic representation of the formation process through evaporation-driven oriented assembly.

    Step 1: Formation of the PEO-PPO-PEO/titania oligomer composite spherical micelles with PPO segments as a core and titania-associated PEO segments as a shell with the initial preferential evaporation of THF solvent at 40°C for 6 hours. Step 2: Aggregation of the composite spherical micelles into big spheres on the interface of the poor solvent water-rich phase, which is driven by the increasing concentration of the spherical micelles and the requirement of minimization of interface energy. Step 3: The second-step evaporation of THF and residual solvents hydrolyzed from titanium tetrabutoxide (TBOT) precursor (further treated at 80°C for 8 hours) could further drive the composite spherical micelles to fuse into cylinders, leading to continuous 3D radially-oriented growth of cylindrical micelles and TiO2 nanoentities. Step 4: The 3D open radially oriented mesoporous TiO2 microspheres with single-crystal–like anatase walls that dominate (101) facets are obtained by removing the triblock copolymer templates after calcination in air at 400°C for 2 hours.

  • Fig. 2 Microstructure characterization of the radially oriented mesoporous TiO2 microspheres.

    (A) In situ synchrotron radiation 1D SAXS patterns of the mesoporous TiO2 microsphere products harvested at different intervals of reaction time. Insets: Corresponding schematic representation of the four samples. a.u., arbitrary units. (B to E) 2D SAXS images of the four samples. (F) SEM image of the mesoporous TiO2 microspheres. Inset: SEM image of a single mesoporous TiO2 microsphere. (G) SEM image of a single ultramicrotomed, radially-oriented mesoporous TiO2 microsphere with a large number of interchannel pores (~5 to 15 nm in diameter, marked by red circles). Inset: Corresponding schematic representation of the structure models for the radially oriented channels with interchannel pores. (H and I) TEM images of a single ultramicrotomed, mesoporous TiO2 microsphere.

  • Fig. 3 Single-crystal pore wall characterizations.

    (A) HRTEM images taken from the area of the cylindrical mesopore bundles of an ultramicrotomed, mesoporous TiO2 microsphere with [010] incidence, perpendicular to the mesopore channels. (B) A fast Fourier transform–filtered TEM image recorded from the dotted square area in (A). (C) The SAED pattern taken from the cylindrical pore bundles region with [010] incidence. (D) The WAXRD pattern of the mesoporous TiO2 microspheres, compared to the standard anatase (space group I41/amd, JCPDS card no. 21-1272). (E) Nitrogen adsorption-desorption isotherms; inset: pore size distributions of the mesoporous TiO2 microspheres with two sets of pores. The primary pore size is centered at 5.7 nm, and the secondary pore size at 10 to 30 nm. (F and G) XPS core-level spectra of Ti2p and O1s, respectively, for the mesoporous TiO2 microspheres.

  • Fig. 4 Photovoltaic device characterization.

    (A) Diffuse reflectance spectra of TiO2 films with a thickness of about 12 μm. Inset: Photograph of the 3D open, radially-oriented mesoporous TiO2 microsphere-based film. (B) J-V curves of DSSCs fabricated from the three TiO2 samples with N719 dye under AM 1.5G simulated sunlight with a power density of 100 mW cm−2. (C) A cross-sectional SEM image of a DSSC composed of 3D open mesoporous TiO2 microspheres. Inset: Photograph of the sliced films for SEM. (D) IPCE spectra of the DSSCs based on the radially oriented mesoporous TiO2 spheres with a uniform size of 800 nm, conventional mesoporous TiO2 bulk, and commercial Degussa TiO2 P25. The pink marked shadow region shows that the photo-response region is extended over 800 nm for the single-crystal–like mesoporous TiO2 microspheres. (E) Electron transport time and electron lifetime. (F) Electron diffusion coefficient (Dn) for the mesoporous TiO2 microspheres.

  • Table 1 Photovoltaic parameters of DSSCs based on the photoanodes of radially oriented mesoporous TiO2 microspheres, bulk mesoporous TiO2, and commercial P25 after TiCl4 treatment [measured under AM 1.5 sunlight illumination (100 mW cm−2)].

    The active area of the devices with a metal mask was about 0.16 cm2. FF, fill factor.

    SamplesVoc
    (mV)
    Jsc
    (mA cm−2)
    FF
    (%)
    η
    (%)
    Adsorbed dye
    (×10−7 mol cm−2)*
    Mesoporous TiO2
    microspheres
    75122.970.612.12.13
    Bulk mesoporous TiO275915.863.17.61.45
    P2573412.375.06.81.12

    *Dye-adsorbed films with an area of ~10 cm2 were used for estimating the adsorbed dye concentration. The commercial N719 dye was first desorbed into a 0.1 M NaOH solution in water and ethanol [1:1 (v/v)], and the desorbed N719 dye concentration was then measured by using an ultraviolet-visible spectrophotometer.

    Supplementary Materials

    • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/4/e1500166/DC1

      Materials and Methods

      Fig. S1. Photographs of the mixed solution for the preparation of the mesoporous TiO2 microspheres during the evaporation of the solvent THF.

      Fig. S2. TEM images with different magnifications of the sample taken from the preparation solution after the solvent evaporation at 40°C for 6 hours.

      Fig. S3. STEM image and EDX elemental maps of the sample taken from the preparation solution after THF evaporation at 40°C for 6 hours.

      Fig. S4. WAXRD patterns of the precipitates obtained at different solvent evaporation times without calcination.

      Fig. S5. The proposed mechanism for oriented growth of the anatase single-crystal–like pore walls.

      Fig. S6. HAADF-STEM images of a single ultramicrotomed 3D open radially oriented mesoporous TiO2 microspheres.

      Fig. S7. SEM images of the products obtained after the second-step evaporation at a high temperature of 120°C.

      Fig. S8. TEM images of an ultramicrotomed radially oriented mesoporous TiO2 microsphere with different integrity.

      Fig. S9. SAED and HRTEM images taken from the cylindrical pore bundle regions with different orientation.

      Fig. S10. HRTEM and SEAD images of the mesoporous TiO2 spheres taken from the area of the cylindrical pore bundles.

      Fig. S11. Detail photovoltaic parameters of 15 individual mesoporous TiO2 microsphere-based cells.

      Fig. S12. Independent certificate by the National Center of Supervision and Inspection on Solar Photovoltaic Products Quality.

      Fig. S13. Cross-sectional and top-view SEM images of the photoanode film based on the uniform mesoporous TiO2 microspheres.

      Fig. S14. Photocatalytic decomposition of methylene blue dye over the mesoporous TiO2 microspheres.

      Fig. S15. Cryo-SEM images of the samples harvested at different intervals of solvent evaporation time.

      Fig. S16. STEM image and EDX elemental maps of a single ultramicrotomed mesoporous TiO2 microsphere.

      Fig. S17. SEM images of the products obtained under similar conditions in the absence of Pluronic F127.

      Fig. S18. Field-emission SEM (FESEM) and TEM images of the ordered mesoporous TiO2 bulks with randomly oriented cylindrical pores.

      Fig. S19. HRTEM images, nitrogen adsorption-desorption isotherms, pore size distributions, and SAXS integral curves of the randomly oriented mesoporous TiO2 bulk samples.

    • Supplementary Materials

      This PDF file includes:

      • Materials and Methods
      • Fig. S1. Photographs of the mixed solution for the preparation of the mesoporous TiO2 microspheres during the evaporation of the solvent THF.
      • Fig. S2. TEM images with different magnifications of the sample taken from the preparation solution after the solvent evaporation at 40°C for 6 hours.
      • Fig. S3. STEM image and EDX elemental maps of the sample taken from the preparation solution after THF evaporation at 40°C for 6 hours.
      • Fig. S4. WAXRD patterns of the precipitates obtained at different solvent evaporation times without calcination.
      • Fig. S5. The proposed mechanism for oriented growth of the anatase single-crystal–like pore walls.
      • Fig. S6. HAADF-STEM images of a single ultramicrotomed 3D open radially oriented mesoporous TiO2 microspheres.
      • Fig. S7. SEM images of the products obtained after the second-step evaporation at a high temperature of 120°C.
      • Fig. S8. TEM images of an ultramicrotomed radially oriented mesoporous TiO2 microsphere with different integrity.
      • Fig. S9. SAED and HRTEM images taken from the cylindrical pore bundle regions with different orientation.
      • Fig. S10. HRTEM and SEAD images of the mesoporous TiO2 spheres taken from the area of the cylindrical pore bundles.
      • Fig. S11. Detail photovoltaic parameters of 15 individual mesoporous TiO2 microsphere-based cells.
      • Fig. S12. Independent certificate by the National Center of Supervision and Inspection on Solar Photovoltaic Products Quality.
      • Fig. S13. Cross-sectional and top-view SEM images of the photoanode film based on the uniform mesoporous TiO2 microspheres.
      • Fig. S14. Photocatalytic decomposition of methylene blue dye over the mesoporous TiO2 microspheres.
      • Fig. S15. Cryo-SEM images of the samples harvested at different intervals of solvent evaporation time.
      • Fig. S16. STEM image and EDX elemental maps of a single ultramicrotomed mesoporous TiO2 microsphere.
      • Fig. S17. SEM images of the products obtained under similar conditions in the absence of Pluronic F127.
      • Fig. S18. Field-emission SEM (FESEM) and TEM images of the ordered mesoporous TiO2 bulks with randomly oriented cylindrical pores.
      • Fig. S19. HRTEM images, nitrogen adsorption-desorption isotherms, pore size distributions, and SAXS integral curves of the randomly oriented mesoporous TiO2 bulk samples.

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