Sol–gel synthesis of nanosized λ-Ti3O5 crystals

In this study, we show a synthesis of λ-Ti3O5 nanocrystals dispersed in silica by sol–gel method. The X-ray diffraction measurements, Rietveld analyses, and transmission electron microscope images of the obtained samples showed that tuning the sintering temperature in the synthesis process can control the size of the λ-Ti3O5 nanocrystals, i.e., 8±2 nm (1123°C; sample 1), 9±3 nm (1133°C; 2), 9±2 nm (1143°C; 3), 10±3 nm (1153°C; 4), 11±4 nm (1163°C; 5), 13±4 nm (1173°C; 6), 25±12 nm (1200°C; 7), and 36±15 nm (1250°C; 8), whereas adjusting the hydrogen flow rate can tune the oxidation-reduction state of the sample without apparent change in the crystal size. At the lowest sintering temperature of 1123°C, the smallest X-Ti3O5 nanocrystals of 8 nm in size were produced.

In our efforts to find a suitable next-generation optical storage medium, we synthesized Ti 3 O 5 nanoparticles with a novel phase, -Ti 3 O 5 , in 2010 [20]. Irradiating -Ti 3 O 5 with laser light at room temperature causes a reversible photo-induced metal-to-semiconductor phase transition between black metallic -Ti 3 O 5 and brown semiconducting -Ti 3 O 5 (figure 1). -Ti 3 O 5 is an environmentally friendly and sustainable material that consists of highly abundant elements [21,22]. Consequently, -Ti 3 O 5 is a promising candidate for use in rewritable recording optical media.
The morphology of -Ti 3 O 5 depends on its synthesis [20]. A combination of reverse-micelle and sol-gel techniques provides -Ti 3 O 5 nanocrystals dispersed in a SiO 2 matrix, where the nanocrystal size is 21±11 nm. In contrast, a sintering method, such as sintering titanium dioxide (TiO 2 ) nanoparticles, provides the flake form -Ti 3 O 5 , where the -Ti 3 O 5 size is several micrometers assembled from 25±15 nm nanocrystals. The possibility to reducing the size of -Ti 3 O 5 is important from the viewpoints of basic and applied science. In this work, we demonstrate that tuning the sintering temperature can control the size of the -Ti 3 O 5 nanocrystals prepared via sol-gel synthesis, and report -Ti 3 O 5 nanocrystals below ten nanometers size.

Characterization
Transmission electron microscope (TEM) measurements were acquired with JEOL JEM-2000EXII. The X-ray diffraction (XRD) measurements were performed using Rigaku Ultima IV with Cu Kradiation ( =1.5418 Å). Rietveld analyses were performed using the Rigaku PDXL program.      [20]. For samples 1-3, the XRD patterns indicate that the -Ti 3 O 5 fraction is 100%, whereas those for samples 4-8 contain minor peaks, which are assigned to a few percent of Ti 2 O 3 impurity. Figure 4 plots the crystal size versus sintering temperature. Crystals of samples 1-8 have average sizes between 8 and 36 nm, i.e., 8±2 nm, 9±3 nm, 9±2 nm, 10±3 nm, 11±4 nm, 13±4 nm, 25±12 nm, and 36±15 nm for samples 1, 2, 3, 4, 5, 6, 7, and 8     TEM images and size distribution. Rietveld analyses indicate that the main phase of sample 9 is anatase TiO 2 , while samples 10-12 are mainly comprised of -Ti 3 O 5 (table 2). Samples 11 and 12 contain a minor impurity phase of Ti 2 O 3 , and the Ti 2 O 3 fraction increases as the hydrogen flow rate increases. Samples 9-12 have an average size of ten or more nanometers, i.e., varying the hydrogen flow rate has less effect on the nanocrystal size, compared to varying the sintering temperature.

Temperature dependence of crystal structure
Variable-temperature XRD measurements were conducted for the -Ti 3 O 5 nanocrystals. Figure 6 plots the peak position versus temperature for the XRD patterns of sample 10 measured in the range of 31.4-      [20].

Mechanism
Next, we considered the reason why -Ti 3 O 5 forms as nanocrystals by this sol-gel synthesis. In this method, the precursor Ti(OH) 4 particles are uniformly dispersed in the aqueous solution. The hydrolysis of TEOS into SiO 2 occurs slowly and homogeneously in the sol-gel process, resulting in a homogeneous SiO 2 shell over the precursor particle surfaces. Due to the SiO 2 matrix, the sintering process suppresses crystalline growth. Consequently, a combination of the sol-gel method and sintering process is suitable to form -Ti 3 O 5 in the size of ca. ten nanometers. Furthermore, the dependence of the crystal size on temperature is due to the viscosity of the SiO 2 matrix; the viscosity of SiO 2 increases as the temperature decreases, which results in smaller nanocrystals at lower temperature.

Conclusion
Synthesizing small-sized nanocrystals of a photo-induced phase transition material is a promising method to develop high-density recording media. Herein we demonstrate that tuning the sintering temperature can control the size of the -Ti 3 O 5 nanocrystals. A sintering temperature of 1123°C gives the smallest -Ti 3 O 5 nanocrystals, 8 nm. Adjusting the hydrogen flow rate can tune the oxidationreduction state of the sample without apparent change in the crystal size. The memory density in the recording media is estimated to be 1 terabit inch -2 with nanocrystal size of 21 nm, which is our previously reported crystal size [20], while the density is expected to achieve 10 terabit inch -2 with 8 nm nanocrystals. Although we prepared the smallest sized -Ti 3 O 5 nanocrystals in this work, a study to synthesize even smaller nanocrystals is currently underway.