Synthesis of a metal oxide with a room-temperature photoreversible phase transition

Journal name:
Nature Chemistry
Volume:
2,
Pages:
539–545
Year published:
DOI:
doi:10.1038/nchem.670
Received
Accepted
Published online

Abstract

Photoinduced phase-transition materials, such as chalcogenides, spin-crossover complexes, photochromic organic compounds and charge-transfer materials, are of interest because of their application to optical data storage. Here we report a photoreversible metal–semiconductor phase transition at room temperature with a unique phase of Ti3O5, λ-Ti3O5. λ-Ti3O5 nanocrystals are made by the combination of reverse-micelle and sol–gel techniques. Thermodynamic analysis suggests that the photoinduced phase transition originates from a particular state of λ-Ti3O5 trapped at a thermodynamic local energy minimum. Light irradiation causes reversible switching between this trapped state (λ-Ti3O5) and the other energy-minimum state (β-Ti3O5), both of which are persistent phases. This is the first demonstration of a photorewritable phenomenon at room temperature in a metal oxide. λ-Ti3O5 satisfies the operation conditions required for a practical optical storage system (operational temperature, writing data by short wavelength light and the appropriate threshold laser power).

At a glance

Figures

  1. Synthesis procedure for λ-Ti3O5 nanocrystals in a SiO2 matrix.
    Figure 1: Synthesis procedure for λ-Ti3O5 nanocrystals in a SiO2 matrix.

    a, A combination of reverse-micelle and sol–gel techniques is used to synthesize the λ-Ti3O5 nanocrystals in a SiO2 matrix. b, TEM image of λ-Ti3O5 nanocrystals in a SiO2 matrix. The inset is an enlarged image that shows the lattice planes.

  2. Formation and crystal structure of λ-Ti3O5.
    Figure 2: Formation and crystal structure of λ-Ti3O5.

    a, Powder XRD pattern of λ-Ti3O5 nanocrystals in SiO2. Broad deviation of the baseline caused by amorphous SiO2 is eliminated. Red dots, black lines and blue lines are the observed patterns, calculated patterns and their differences, respectively, and green bars represent the calculated positions of the Bragg reflections of λ-Ti3O5. b, Crystal structure for λ-Ti3O5 (monoclinic C2/m). Red, light red, deep red and grey balls represent Ti(1), Ti(2), Ti(3) and O atoms, respectively, and TiO6 units are drawn as polyhedra. The central square without polyhedra represents the unit cell. c, Peak position versus temperature graph of the XRD pattern in the angle range 32.0–33.0° for the flake form λ-Ti3O5. d, DSC charts of the flake form λ-Ti3O5 (red line) and conventional crystal Ti3O5 (dashed black line) with increasing temperature.

  3. Magnetic and optical properties, and electronic structures of λ-Ti3O5.
    Figure 3: Magnetic and optical properties, and electronic structures of λ-Ti3O5.

    a, χ versus T graph of the flake form λ-Ti3O5 (red line) and single-crystal β-Ti3O5 (dashed black line) under an external field of 0.5 T. b, Optical absorption spectra of the flake form λ-Ti3O5 (red line) and β-Ti3O5 (black line) in the ultraviolet–visible and infrared regions. K.M. = Kubelka–Munk transformation. c, Band structures of β-Ti3O5 and λ-Ti3O5 using VASP, showing the DOS around the Fermi level (EF) for β-Ti3O5 (i) and λ-Ti3O5 (ii). Below these are the electron-density maps around the Fermi level.

  4. Reversible photoinduced phase transition in λ-Ti3O5.
    Figure 4: Reversible photoinduced phase transition in λ-Ti3O5.

    ai, Photographs of λ-Ti3O5 at irradiations of 532 nm and 410 nm laser lights. When the flake form λ-Ti3O5 was irradiated with 532 nm nanosecond-pulsed laser light at room temperature, the irradiated area changed from black (a) to brown (b). Subsequently, on irradiating with 410 nm laser light the spots returned from brown to black (c,d). Photoinduced colour changes were observed repeatedly by alternating 532 nm and 410 nm laser-light irradiation (e–i).

  5. Phase transition between λ-Ti3O5 and β-Ti3O5 induced by one-shot laser pulses.
    Figure 5: Phase transition between λ-Ti3O5 and β-Ti3O5 induced by one-shot laser pulses.

    a, A mixed sample of λ- and β-phases (λ/β = 2/1) was irradiated with 532 nm pulsed laser light at various laser-power densities. b, The change in amount of λ-Ti3O5 from the ratio 2/1, Δ(λ-Ti3O5), versus laser-power density. Clear thresholds for laser-power densities were observed. c, Difference XRD patterns of the alternating phase transition between λ-Ti3O5 and β-Ti3O5 by irradiation of pulsed-laser (532 nm, 6 ns) shots at room temperature.

  6. Mechanism of the photoinduced phase transition in λ-Ti3O5.
    Figure 6: Mechanism of the photoinduced phase transition in λ-Ti3O5.

    Thermally populated phase versus temperature curves are based on the G versus x plots for Ti3O5 nanocrystals using the observed ΔH (= 4.8 kJ mol−1) and ΔS (= 10.4 J K−1 mol−1) values and assuming γ = 9.0 kJ mol−1. Red, pink and blue lines indicate λ-Ti3O5, α-Ti3O5 and β-Ti3O5, respectively. The photoinduced phase transition from λ-Ti3O5 to β-Ti3O5 is attributed to the phase transition from a metastable phase to a true stable phase (blue arrow pointing downwards and top inset). In the reverse photoinduced phase transition from β-Ti3O5 to λ-Ti3O5, the transition by the nanosecond (ns)-pulsed laser-light irradiation is direct (blue arrow pointing upwards and middle inset) or it thermally transits through α-Ti3O5 to give to λ-Ti3O5 (that is, β-Ti3O5 → α-Ti3O5 → λ-Ti3O5) by continuous wave (cw) laser-light irradiation (dotted blue arrow and bottom inset).

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Author information

Affiliations

  1. Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

    • Shin-ichi Ohkoshi,
    • Yoshihide Tsunobuchi,
    • Tomoyuki Matsuda,
    • Asuka Namai,
    • Fumiyoshi Hakoe &
    • Hiroko Tokoro
  2. Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

    • Kazuhito Hashimoto

Contributions

S.O. designed and coordinated this study and contributed to all measurements and calculations, and wrote the paper. Y.T. carried out synthesis, DSC and first-principle band calculation. T.M. carried out synthesis. A.N. performed XRD measurements, Rietveld analysis and ICP-MS. F.H. carried out synthesis and TEM, SEM and SQUID measurements. K.H. contributed to the discussion. H.T. carried out synthesis and thermodynamic analysis, and carried out the photoirradiation and pressure-effect experiments. All authors commented on the manuscript.

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