Review
Photo-rewritable material based on a metal oxide
Shin-ichi Ohkoshi,*,a Yoshihide Tsunobuchi,a Kazuhito Hashimoto,b Asuka Namai,a Fumiyoshi Hakoe,a and Hiroko Tokoroa
aDepartment of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, JAPANDepartment of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, JAPANbABSTRACT
Photoinduced phase transition materials receive attention from both fundamental and technological viewpoints because theyare used in high-density optical data storage. In this paper, we report a photoreversible phase transition at room temperature
with a unique phase of Ti3O5 (hereinafter, this new phase is called O-Ti3O5). The O-Ti3O5 phase, which is generated by ananoscale size effect, is a metal conductor, and exhibits a photoinduced phase transition to E-Ti3O5, a semiconductor. The
photoreverse process from E- to O-Ti3O5 is also observed. This phenomenon originates from a particular state of O-Ti3O5,
which is trapped at thermodynamically local energy minimum. Light irradiation causes the switching between this trappedstate (O-Ti3O5) and the other energy minimum state (E-Ti3O5).
Keywords Lambda-trititanium pentaoxide, Photoinduced metal-semiconductor phase transition

INTRODUCTION
A phase-change optical disc technology was developed for practical application in 1980s, and resulted in thecommercialization of digital versatile disc (DVD) in 1990s and Blu-ray® in 2000s1),2). Currently, DVD and Blu-ray® use achalcogen alloy (e.g., GeSbTe) as an optical data storage material. Various materials, including photochromic compounds,
donor-acceptor stacked molecules, spin crossover complexes, cyano-bridged metal assemblies, and perovskite manganites,
have been studied as next-generation high-performance optical data storage3). Herein we report a unique phase of titaniumoxide, O-Ti3O5, which meets the requirements for optical data storage memory. This material shows a reversible photoinduced
metal-semiconductor phase transition between O-Ti3O5 and E-Ti3O5 at room temperature. This is the first demonstration of ametal oxide with a photoinduced phase transition at room temperature4),5).

FORMATION AND CRYSTAL STRUCTURE OF O-Ti3O5A unique phase of a titanium oxide, O-Ti3O5, was obtained as two types of morphologies through different synthetic methods:
one of them is O-Ti3O5 nanocrystal in SiO2 matrix and the other is a flake form of O-Ti3O5 assembled by nanocrystals. The
former is obtained by combining reverse-micelle and sol-gel techniques. The transmission electron microscope (TEM) imageof O-Ti3O5/SiO2 shows cubic-shape Ti3O5 nanocrystals with a size of ca. 20 nm dispersed in a SiO2 matrix (Fig. 1a). Moreover,the X-ray diffraction (XRD) pattern indicates a crystal structure of C2/m, which crystal structure does not correspond to any
reported crystal structures of Ti3O5 (D-, E-, J-, and G-phases) (Fig. 2a).
Calcinating the anatase form of the TiO2 nanoparticles (size = 7 nm) under hydrogen at 1200 °C produces the flake form
O-Ti3O5, which is black (Fig. 1b). The TEM image shows that the flake (size = 2 ± 0.5 Pm) is assembled by 25 ± 15 nm
nanocrystals. The XRD pattern of the sample at room temperature corresponds to the aforementioned O-Ti3O5 in SiO2, and theRietveld analysis shows a monoclinic structure (C2/m). Variable-temperature XRD measurements demonstrate that as thetemperature increases, the diffraction peaks of O-Ti3O5 continuously change to D-Ti3O5 peaks with a crystal structure of Cmcm.
Furthermore, heating the sample to 640 K and then cooling to 300 K causes D-Ti3O5 to return to O-Ti3O5. Differential scanningcalorimetry (DSC) measurements did not exhibit a meaningful peak, which differs vastly from the first-order phase transition
between E-Ti3O5 and D-Ti3O5 in a conventional large crystal Ti3O5. Hence, the phase transition between O-Ti3O5 and D-Ti3O5 isclassified as a second-order type phase transition.

MAGNETIC, ELECTRIC, AND OPTICAL PROPERTIES AND ELECTRONIC STRUCTURE OF O-Ti3O5Figure 3 shows the magnetic susceptibility (F) versus temperature (T) of the flake form O-Ti3O5 as well as the F versus T curveof a conventional large crystal Ti3O5. O-Ti3O5 has F values around 2×10-4 emu per Ti atom throughout the entire measuredtemperature region, suggesting that O-Ti3O5 is a Pauli paramagnet due to metallic conduction. Spin-orbital coupling on Ti3+ ion
can explain the gradual decrease below 150 K, but small amount of Curie paramagnetism (ca. 0.1%), which may be due todefects in the material, is attributed to the rapid increase below 30 K. The O-Ti3O5/SiO2 sample exhibits the same type ofF versus T curve. The electric current versus voltage curve using atomic force microscopy in the contact mode has an electricconductivity (V)value of ca. 3×101 S cm-1, indicating that O-Ti3O5 is a near metallic conductor. Additionally, both the UV-vis
and IR reflectance spectra indicate that O-Ti3O5 possesses metallic absorption over the ultraviolet and infrared wavelength
regions. In contrast, the impedance measurement and reflectance spectra suggest that E-Ti3O5 is a semiconductor with
V = 3×10-2 S cm-1 and a band gap of 0.14 eV.
The empirical relationship between the bond length and valence states gives estimated valence states for Ti(1), Ti(2), and Ti(3)1in O-Ti3O5 of +3.37, +3.20, and +3.53, respectively. These valence states are close to Ti 3 3  , indicating that O-Ti3O5 is acharge-delocalized system (Fig. 2a). This is consistent with O-Ti3O5 being a metallic conductor. The electronic structure ofO-Ti3O5 is close to that of D-Ti3O5. In contrast, the valence states of Ti(1), Ti(2), and Ti(3) in E-Ti3O5 are +3.00, +3.79, and2
1+3.32, respectively, which are close to the valence states for a charge-localized system of Ti3+ Ti 3 3   Ti 3 3  (Fig. 2b).
The band structures of E-Ti3O5 and O-Ti3O5 calculated by the first-principle calculation also supported these valence states.

REVERSIBLE PHOTOINDUCED PHASE TRANSITION
When the flake form O-Ti3O5 is irradiated with 532 nm pulsed laser light (6 ns, 3 shots, 1.5×10-5 mJ Pm-2 pulse-1) at room
temperature, the irradiated area changes from black to brown (panels a and b in Fig. 4). Successively, irradiating with 410 nm
continuous wave laser light (8×10-3 mW Pm-2) causes the irradiated spots to return to black (panel c in Fig. 4). Alternatively
irradiating with 532 nm and 410 nm lights repeatedly induces these color changes (panels d–f in Fig. 4). The XRD patternshows that the brown area is assigned to the E-phase (monoclinic structure of C2/m). Hence, the change from black to brown isdue to the transition from the O-Ti3O5 to E-Ti3O5, whereas the converse is due to the transition from the E-Ti3O5 to O-Ti3O5.
Furthermore, a similar phase transition is observed by irradiating with different pulsed-laser lights, 355 or 1064 nm.
To investigate the threshold value of laser power, a mixed sample of O-Ti3O5 and E-Ti3O5 with ratio of 2 : 1 was irradiated with
532 nm pulsed laser light (one shot) at various laser power densities. The threshold laser power of E-Ti3O5  O-Ti3O5 is
2.7×10-6 mJ Pm-2, and conversely, that of O-Ti3O5  E-Ti3O5 is 1.0×10-5 mJ Pm-2. Such a threshold is important for recording
storage media for long-lasting memory. The observed threshold values of the laser power densities used are the same order aslaser power density used on DVD.

THERMODYNAMIC INTERPRETATION OF THE PHOTOINDUCED PHASE TRANSITION
The thermodynamic analysis of O-Ti3O5 suggests that the present photoinduced metal-semiconductor phase transition is
attributed to the phase transition from O-Ti3O5, a metastable phase thermodynamically trapped at a local energy minimum state,
to E-Ti3O5, a truly stable phase by irradiation (Fig. 5). Because metallic absorption allows O-Ti3O5 to effectively absorb lightover a wide wavelength range from ultraviolet to near-IR, this metal-semiconductor phase transition can be observed byirradiating with 355, 532, and 1064 nm laser lights. The reverse photoinduced phase transition from E-Ti3O5 to O-Ti3O5 is
induced by the excitation from the valence band to the conduction band on E-Ti3O5, and then the excited state directly changes
to O-Ti3O5 in the pulsed laser light irradiation experiment or thermally transits to O-Ti3O5 through D-Ti3O5 (i.e., E-  D- O-Ti3O5) in the continuous wave laser light irradiation experiment.

CONCLUSION
We reported a photoreversible metal-semiconductor phase transition at room temperature with a new phase of Ti3O5. Thisphenomenon originates from a particular state of O-Ti3O5 trapped at thermodynamic local energy minimum. O-Ti3O5 satisfies
the operation conditions (operational temperature around room temperature, writing data by short wavelength near ultra-violet
light for high memory density, and the appropriate threshold laser power to maintain long-term memory).

ACKNOWLEDGEMENT
This work was performed under the management of the Project to Create Photocatalyst Industry for Recycling-oriented Societysupported by NEDO. We are thankful for a Grant-in-Aid for the Global COE Program, from MEXT Japan.

REFERENCES
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N. Yamada, E. Ohno, K. Nishiuchi, N. Akahira, and M. Takao, J. Appl. Phys., 69, 28492856 (1991).
M. Wuttig, and N. Yamada, Nature Mater., 6, 824832 (2007).
K. Nasu, Relaxations of Excited States and Photo-Induced Structural Phase Transitions (Springer, Berlin, 1997).
S. Ohkoshi, Y. Tsunobuchi, T. Matsuda, K. Hashimoto, A. Namai, F. Hakoe, and H. Tokoro, Nature Chem., 2, 539545 (2010).
Nature Japan, URL: http://www.natureasia.com/japan/jobs/tokushu/detail.php?id=219

(a) Reverse-micelle process

Sol-gel processCalcination

SiO2

O-Ti3O550nm

O-Ti3O5'H2(b)SiO2Calcination'H2
TiO2 antase(7 nm)

O-Ti3O5Fig. 1 (a) Schematic illustration of the synthetic procedure of O-Ti3O5 nanocrystal in SiO2 matrix using
combination method between reverse-micelle and sol-gel techniques and TEM image of O-Ti3O5nanocrystals
yin SiO2 matrix. ((b)) Schematic illustration of the synthetic
yprocedure of flake-form ofpO-Ti3O5.
(a)

(b)
Ti(3)

Ti(3)

Ti(1)
Ti(1)
Ti(2)

Ti(2)

O-Ti3O5
Charge-delocalized system
valence1Ti(1): +3.37 (+3 3 )E-Ti3O5
Charge-localized system
valenceTi(1): +3.00 (+3) S =1/2

Ti(2): +3.20 (+3 13 )Ti(2): +3.79 (+4)

Ti(3): +3.53 (+3 1 )3

Ti(3): +3.32 (+3) S =1/2

Fig. 2 Crystal structures and valence states of (a) O-Ti3O5 and (b) E-Ti3O5.

O-Ti3O5

D-Ti3O5

E-Ti3O5

Fig. 3 F versus T plots of flake-form of O-Ti3O5 (red line) and single crystal E-Ti3O5 (black dotted line)
under an external field of 0.5 T.

(a)O-Ti3O5(d) 532 nm laser irradiation(b) 532 nm laser

E-Ti3O5irradiation(e) 410 nm laser irradiation(c) 410 nm laser irradiation(f) 532 nm laser irradiation

Fig.
g 4 Reversible pphotoinduced pphase transition in O-Ti3O5.5 Photographsg p of OTi3O5 byy the irradiation
532 nm pulsed laser and 410 nm continuous wave laser lights. When flake-form of O-Ti3O5 wasirradiated with 532 nm pulsed laser light at room temperature, the irradiated area changed from black(panel a) to brown (panel b). Successively, the spots returned to black upon irradiating with 410 nm
continuous wave laser light (panel c). Photoinduced color changes were repeatedly observed byalternating 532 nm and 410 nm laser light irradiation (panels d-f).

Fig. 5 Mechanism of the photoinduced phase transition in O-Ti3O5. Schematic illustration of thepathways of the reversible photoinduced phase transition. Thermally populated phase versus temperature
curves are described based on the G versus x plots for Ti3O5 nanocrystals. Red, pink, and blue lines
indicate O-Ti3O5, D-Ti3O5, and E-Ti3O5, respectively. The photoinduced phase transition from O-Ti3O5 toE-Ti3O5 is attributed to the pphase transition from a metastable pEphase to a true stable pphase ((downwardblue arrow). In the reverse photoinduced phase transition from E-Ti3O5 to O-Ti3O5, E-Ti3O5 directlytransits to O-Ti3O5 by the nanosecond (ns)-pulsed laser light irradiation (upward blue arrow), or
thermally transits to O-Ti3O5 through D-Ti3O5 (i.e., E-  D-  O-Ti3O5) by the continuous wave (cw)laser light irradiation (dotted blue arrow).