Review
Durability of reversible photo-induced metal-to-semiconductor
phase transitions on λ-Ti3O5
Hiroko Tokoro,a,b Fumiyoshi Hakoe,a Yoshikazu Umeta,a Toshiaki Nagata,aKenji Tanaka,a Tomomichi Nasu,a Asuka Namai,a and Shin-ichi Ohkoshi,a,c,*a

Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan, b NEXT, JSPS, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8472,
Japan, c CREST, JST, K’s Gobancho, 7 Gobancho, Chiyoda-ku, Tokyo 102-0076, JapanINTRODUCTION
Optical discs commonly used as storage media, such as DVDs and Blu-ray Discs,
employ chalcogenides as the recording material because chalcogenides exhibit
photo-induced reversible changes. Although chalcogenides are extensively attractive
materials possessing properties that are of interest in both fundamental and applied research,
they are comprised of elements that are rare and expensive. Consequently, many studies arebeing conducted to find new materials suitable for next-generation optical storage media.
As part of these efforts, in 2010, our group successfully synthesized Ti3O5
nanoparticles with a unique phase, λ-Ti3O5, which exhibits a reversible photo-induced
metal-semiconductor phase transition at room temperature.1−3 Laser-light irradiation
initiates a reversible transition between the black-colored metallic λ-Ti3O5 and thebrown-colored semiconducting β-Ti3O5 (Fig. 1). λ-Ti3O5 is also an environmentallyfriendly and sustainable material because it is comprised of only highly abundant elementswith large reserves. Due to these characteristics, λ-Ti3O5 has received increased attention
as a promising candidate for use in rewritable recording media. In order for a recording
material to be rewritable, it must be able to withstand numerous phase transition cycles. In
this work, we report our findings on the durability of λ-Ti3O5 against repeatedphotoreversible phase transitions.
Ti2
Ti1

Ti2

Ti3

Ohν

Ti1

hν ’O

Ti3

aλ-Ti3O5
Metallic conductor baβ-Ti3O5
Semiconductor

cbc

Figure 1. Reversible photo-induced metal-to-semiconductor phase transition of λ-Ti3O5
and β-Ti3O5.

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EXPERIMENTAL SECTION
Figure 2 schematically depicts our experimental setup. The flake form of λ-Ti3O5 wasformed into a pellet, and alternately irradiated by a nanosecond-pulsed laser light (532 nm,8.8 × 10−6 mJ µm−2 pulse−1) and a continuous wave (CW) semiconductor laser light (410nm, 3.9 × 10−3 mW µm−2). An optical microscope was used to record the color changes
after each irradiation as images. The pulse laser, CCD, light sensor, and shutters wereconnected to a microcomputer. The microcomputer was connected to a computer, which
controlled and automated the irradiation and image acquisition processes.
532 nm pulse laserPCPulselaser

Lens
Iris

Microcomputer

Shutter
ND Filter

CCD
Microscope

MirrorSensorShutter
410 nmcw laser
Lens
Shutter

ApertureSample

Figure 2. Schematic illustration of the experimental setup to evaluate the durability ofreversible photo-induced phase transitions.

RESULTS
Reversible durability of photo-induced phase transition on λ-Ti3O5
The color changes of the λ-Ti3O5 sample after alternately irradiating with a 532 nm
pulsed laser light and a 410 nm semiconductor laser light were examined. Before the lightirradiation, λ-Ti3O5 was black. After irradiation with a 532 nm pulsed laser light, theirradiated area became brown, but subsequent irradiation with a 410 nm laser light restored
the black color. This color change between black and brown was observed repeatedly on
the sample upon further alternately irradiating with the 532 nm pulsed laser light and the
410 nm laser light. X-ray diffraction (XRD) measurements on the brown area showed the2XRD pattern of β-Ti3O5. Therefore, the observed color change between black and brown isdue to the reversible photo-induced transition between λ-Ti3O5 and β-Ti3O5, respectively.
To quantify the color changes, RGB (Red-Green-Blue) color analysis was conductedon the obtained images, and the values of the three primary colors were obtained tocharacterize the two different phases in the photo-induced transition. Each image
consisted of 40 × 65 pixels, and each pixel can store a maximum of 255 Red (R)components. (A pixel is the smallest unit of an image on a computer that contains
information about color.) Histograms were created for each obtained image as shown inFigure 3a, where the horizontal axis represents the number of R components per pixel andthe vertical axis represents the number of pixels in the image with the corresponding
number of R components. The histograms show that the images of the brown area
(β-Ti3O5) consisted mostly of pixels with 80 to 170 R components, whereas images of theblack area (λ-Ti3O5) consisted mostly of pixels with 80 to 120 R components.

a1bafter
532 nmafter
410 nm

λ-Ti3O5
3after532 nmβ-Ti3O5
4after410 nm

λ-Ti3O5

Number of light irradiations

Number of pixels

2β-Ti3O51β-Ti3O5410 nm

2λ-Ti3O5532 nm

3β-Ti3O5410 nm

4λ-Ti3O580

Number of Red (R) components per pixel

100120140Average number of Red (R) components per pixel

Figure 3. (a) Relationship between the number of Red (R) components per pixel and thenumber of pixels containing the corresponding number of R components in images after
532 nm and 410 nm laser-light irradiation. (b) Plot of the average number of R
components per pixel.

Figure 3b illustrates the average number of R components per pixel in an image of thesample surface (55 × 90 µm2) after each iteration of alternately irradiating with a 532 nm
pulsed laser light and a 410 nm laser light. The clear alternation in the graph between two

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values of the average number of R components indicates that the reversible photo-induced
phase transition between λ-Ti3O5 and β-Ti3O5 can be quantified.
We then proceeded to measure the durability of the photo-induced phase transition on
λ-Ti3O5. We irradiated a sample up to 100 times and measured the changes in the numberof R components and Blue (B) components per pixel after exposure to each alternating light.
During the first three irradiations, some scatters were observed in the data, most likely due
to changes in the roughness of the sample surface. However, after the fourth irradiation, thephoto-induced phase transition occurred repeatedly with remarkable stability; nodegradation was detected after 100 transitions.
SUMMARY
Hereinweinvestigatedthedurabilityofreversible
photo-induced
metal-to-semiconductor phase transitions on λ-Ti3O5. RGB color analysis was used toquantify the photo-induced phase transitions from images of the sample surface taken by amicroscope. We were able to confirm that λ-Ti3O5 exhibits a reversible photo-induced
phase transition with good stability over numerous cycles (100 phase transitions). Theobserved stability is important property for applications. In the future, we plan to test morethan 1000 phase transitions, and to rigorously assess the durability of λ-Ti3O5 under various
applicative conditions.
ACKNOLEDGEMENTS
We are thankful for the Project to Create Photocatalyst Industry for Recycling-oriented
Society supported by NEDO, the CREST project of JST, Grant-in-Aid for Young Scientists(S) from JSPS, the NEXT program from JSPS, a Grant for the Global COE Program“Chemistry Innovation through Cooperation of Science and Engineering”, APSA fromMEXT, and Asahi Glass Foundation. We acknowledge the Cryogenic Research Center, The
University of Tokyo, and the Center for Nano Lithography & Analysis, The University ofTokyo, supported by MEXT Japan.
REFERENCES(1) S. Ohkoshi, Y. Tsunobuchi, T. Matsuda, K. Hashimoto, A. Namai, F. Hakoe, H. Tokoro,
Nature Chemistry, 2, 539 (2010). [This paper is highlighted in Nature Chemistry,“News & Views”, 2, 517 (2010); Nature Materials, “Commentary”, 10, 158 (2011);
Nature Japan, “Special reports”, 2010.6.25].
(2) R. Makiura, Y. Takabayashi, A. N. Fitch, H. Tokoro, S. Ohkoshi, K. Prassides,
Chemistry - An Asian Journal, 6, 1886 (2011).
(3) H. Tokoro & S. Ohkoshi, Journal of Japan Laser Processing Society, 18, 48 (2011).

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