TiO₂ is one of the most widely used ceramic oxides in various applications, such as in pigments, cosmetics, fillers, photocatalysts, and dye sensitized solar-cells [1]. TiO₂ has interesting properties such as brightness, high reflective index, and strong UV light absorbing capabilities. The particle size, particle size distribution, and particle morphology of TiO₂is an important factor which affects properties of the materials in application and can be varied by different preparing methods [2]. Recently, TiO₂ quantum dots have attracted much attention as TiO₂ anode materials in lithium ion batteries due to high activity which stems from large surface area [3-5].

Various preparation methods, such as the sol-gel method [6], hydrothermal method [7], and solvothermal method [8], are known in synthesizing nano-sized TiO₂. In this study, we synthesized TiO₂ quantum dots by hydrothermal method after precipitating TiO₂ by dropping Ti(SO₄)₂ solution into NaOH solution. Prepared TiO₂ quantum dots were characterized using UV/Vis/NIR Spectrophotometer, X-ray diffractometer, and transmission electron microscopy (TEM).

TiO₂ quantum dots were prepared as shown in Fig. 1.

Briefly, Ti(SO₄)₂ solution was prepared by diluting Ti(SO₄)₂ solution (Kanto Chemical, reagent grade) with distilled water. NaOH solution with OH^- concentration of 6 times of Ti⁴⁺ ion concentration of Ti(SO₄)₂ solution was prepared by dissolving NaOH (Sigma Aldrich, 98% purity) in distilled water. Ti(SO₄)₂ solution was dropped into NaOH solution while stirring the solution. Prepared pH of NaOH solution was 13.7. And with the assumption of precipitates were Ti(OH)4, final pH of the mixed precipitated solution was 12.8. After precipitation, the resulting slurry was stored undisturbed until precipitates separated from solution of remaining ions of Na⁺ and SO₄^2-. Then, the supernatant above the precipitates was removed. This procedure was repeated several times to completely remove Na⁺ and SO₄^2- ions. The resulting washed precipitated precursor slurry solution was transferred to the hydrothermal apparatus built in the laboratory. The solution was hydrothermalized at temperatures 100℃, 130℃, and 160℃ for 24 hours. Hydrothermally synthesized TiO₂ quantum dots were characterized using UV/Vis/NIR Spectrophotometer (Hitachi, U-4100), X-ray diffractometer (Bruker, D8 Advance), and transmission electron microscopy (Jeol, JEM2010).

Figure 2 shows the XRD spectra of the TiO₂ quantum dots prepared by hydrothermal method at temperatures 100℃, 130℃, and 160℃. The phase of the TiO₂ quantum dots was shown to be anatase (JCPDS #89-4921). Crystalline TiO₂ quantum dots can be synthesized even at temperatures as low as 100℃. As the hydrothermal temperature increases, crystallinity increases steadily. The average crystalline size of the TiO₂ powder were obtained (Table 1) from the half-width of the full maximum (HWFM) of the most intense peak of (101) plane using the Scherrer equation given by Eq. (1):

where D is the average crystalline size , λ is the wavelength of the CuKα radiation, θ is the Bragg angle of (101) plane of TiO₂, and β is the corrected line broadening of the sample.

Figure 3 shows the TEM image of the TiO₂ quantum dots prepared at hydrothermal temperature of 160℃ for 24 hours. The particles were observed to be very small size, around 15nm, which is fairly consistent with calculated average crystalline size from XRD data. Nearly identical crystalline size between TEM and XRD are considered due to the almost uniform size of the TiO₂ quantum dots prepared by hydrothermal method. Notably, the hydrothermally prepared TiO₂ quantum dots showed no agglomeration of the TiO₂ powders. Due to the non-aggregated nature of the TiO₂ powders, the TiO₂ slurries of quantum dots did not fall out of suspension for extended periods of time (over one month of observation).

Figure 4 shows the absorption spectra of TiO₂ quantum dots prepared at temperatures 100℃, 130℃, and 160℃.

Inflection points of absorption spectra are strong absorption edges which can be obtained by differentiating the absorption spectra.

Strong absorption edges obtained from Fig. 5 are tabulated in Table 1. The band gaps of the TiO₂ quantum dots were obtained using the Kebelka-Munk function. Figure 6 shows the plot of (αhv)^1/2 versus hv for indirect transition (a) and (αhv)² versus hv for direct transition (b) taking several linear points around inflection points for temperatures of 100℃, 130℃, and 160℃. Band gaps was obtained by extrapolating the photon energy to α=0 [9,10]. Calculated band gaps of hydrothermally prepared TiO₂ quantum dots for indirect transition and direct transition are tabulated in Table 1. Calculated band gaps of indirect transition are 2.88 eV, 3.03 eV, and 2.73 eV and direct transition are 3.38 eV, 3.47 eV, and 3.25 eV at hydrothermal temperature of 100℃, 130℃, and 160℃, respectively. Anatase TiO₂ has indirect type transitions and band gap is around 3.2 eV [10]. For smaller particles, direct transition has been also reported to be possible [9]. At this point, the TiO₂ quantum dots prepared in this study are not certain to have a direct or an indirect transition band gap.

Although the crystal size and band gaps did not show any temperature dependence (Table 1), the crystallinity increases as the hydrothermal temperature increases. Heat treatment in the liquid state seems to be different with that of solid state heat treatment for particle size variation and needs to be studied further.

Anatase phase TiO₂ quantum dots with nearly uniform size 15nm and complete crystallinity were prepared by a hydrothermal method. The TiO₂ quantum dots had no agglomeration and completely spherical shapes. The TiO₂ quantum dots can be synthesized at temperatures as low as 100℃.