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Dec 6, 2007 - Chlorinated TiO2 powders of high crystallinity and good dispersion have been synthesized via one-step Ar/O2 radio frequency thermal plas...
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J. Phys. Chem. C 2007, 111, 18018-18024

Chlorinated Nanocrystalline TiO2 Powders via One-Step Ar/O2 Radio Frequency Thermal Plasma Oxidizing Mists of TiCl3 Solution: Phase Structure and Photocatalytic Performance Ji-Guang Li,*,† Masashi Ikeda,†,‡ Chengchun Tang,§ Yusuke Moriyoshi,‡ Hiromi Hamanaka,‡ and Takamasa Ishigaki† Nano Ceramics Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan, Department of Materials Science, Hosei UniVersity, Kajino 3-7-2, Koganei 184-0002, Tokyo, Japan, and Nanoscale Materials Center, National Institute for Materials Science, Namiki 1-1, Tsukuba, Ibaraki 305-0044, Japan ReceiVed: September 12, 2007; In Final Form: October 10, 2007

Chlorinated TiO2 powders of high crystallinity and good dispersion have been synthesized via one-step Ar/ O2 radio frequency thermal plasma oxidation of atomized aqueous solutions of TiCl3. The resultant powders are anatase/rutile mixtures (∼66-72 wt % of anatase), with particle sizes ranging from ∼5 nm to 5 µm. Detailed studies via X-ray diffraction and Raman spectroscopy on the nanometric (300 °C are from chemically bonded ones. It is also clear from the TDS spectra that the plasma-generated TiO2 powder has significantly higher contents of physically and chemically bonded chlorine than P25. Semiquantitative analysis of the desorption spectra in the range of 300-1000 °C for chemically bonded chlorine yielded a value on the order of 10-2 (mole Cl per mole TiO2) for the plasma-synthesized TiO2 powders, which is 1 order of magnitude higher than that (10-3) for P25. The substantially higher chlorine contents of the powders made in this work are attributable to the much higher processing temperature of the thermal plasma than flame pyrolysis, which generates highly chemically reactive species and thus enhances chlorine doping.16 TEM observations revealed that the plasma-generated TiO2 particles typically exhibit a broad size distribution. Figure 5 shows, for example, TEM morphology of the TiO2 powder synthesized with pure O2 (90 L/min) as the plasma sheath. Obviously, the majority of particles have sizes under 100 nm, but there is a small portion with particle diameters well above 1 µm. Such a wide size distribution gives rise to speculations that properties of the powders, such as phase constituent, etc., might be particle-size-dependent. To clarify this, the total powder was fractionated into two parts via free sedimentation

fraction (wt %)

rutile content (wt %)

anatase size (nm)

rutile size (nm)

34.1 100 65.9

32.1 27.9 26.6

91 65.1 42.4

96 66.1 46.8

The sample ID in this table corresponds to the labels in Figure 6.

for 24 h of its ethanol suspension (solid loading: 1 vol %) ultrasonically dispersed for 10 min, and the fractionated parts were investigated in more detail. Figure 6 compares the XRD patterns of the total powder (Figure 6b) and the fractionated parts. Apparently, the fine portion (Figure 6c) shows that the peaks are slightly broadened, whereas the coarse portion (Figure 6a) shows XRD peaks that are relatively sharp, suggesting different average crystallite sizes. Some properties of the three powders are summarized in Table 2, where it can be seen that fine particles constitute a major portion (∼66 wt %) of the whole powder and that the coarse portion has a slightly higher rutile content. Raman spectroscopy indicates that the three powders do not differ appreciably from each other in terms of oxygen stoichiometry, as they all exhibit the EAg at 149.2 cm-1, whereas the ERg is at 435.5 cm-1 (Figure 7). Morphologies of the fractionated powders are shown in Figure 8. Excellent dispersion were observed for both the coarse and the fine particles. The coarse particles are nearly perfect spheres with sizes ranging from submicrometer to ∼5 µm (Figure 8a). For the fractionated fine powder, particles of greater than 100 nm in size are hardly found and the tiniest crystallites are ∼5 nm (Figure 8b). Additionally, all the fine crystallites tend to be faceted. These two distinctly different morphologies may imply different formation pathways of the particles, that is, the large spheres (Figure 8a) might be formed via the gas-liquid-solid mechanism, whereas the fine crystallites (Figure 8b) are formed via a gas-solid route. Lattice spacing analysis via HR-TEM confirmed the existence of isolated nanocrystallites of rutile and anatase in the fine powder (Figure 8, parts c and d). The wellresolved lattice fringes suggest excellent crystallinity of the nanocrystallites, whereas the frequently observed tetragonal shapes may correspond well to the tetragonal crystal structures of anatase and rutile. For the fractionated coarse powder, the average crystallite sizes assayed from the Scherrer equation for anatase (∼91 nm) and rutile (∼96 nm) are much finer than the

18022 J. Phys. Chem. C, Vol. 111, No. 49, 2007

Figure 7. Comparison of the Raman spectra of the original and the fractionated TiO2 powders.

Li et al.

Figure 9. Typical Raman spectrum of the micrometer-sized spheres shown in Figure 8a.

Figure 8. TEM micrographs showing morphologies of the fractionated coarse (a) and fine (b) TiO2 particles. Parts c and d are HR-TEM lattice fringes of individual rutile and anatase nanocrystallites observed from the fine particles shown in part b, where A and R denote anatase and rutile, respectively.

particle diameters (Figure 8a), which may indicate that the particles are multicrystalline. Micro-Raman spectroscopy (Figure 9) of the micrometer-sized individual spheres revealed that they are composites of intergrown anatase and rutile. 3.2. Photoabsorption and Photocatalytic Evaluation. Figure 10a shows UV-vis absorption spectra of the TiO2 powders. In comparison with P25, the plasma-generated powders exhibit enhanced absorption in the UV regime and significantly redshifted absorption edges. TiO2 is known to be an indirect semiconductor,3,25 for which the relation between absorption coefficient (R) and incident photon energy (hν) may be expressed as a ) Bi(hν - Eg)2/hν, where Bi is the absorption constant for indirect transitions.3d,9a Plots of (Ahν)1/2 versus hν from the spectral data of Figure 10a are given in Figure 10b. Extrapolating the linear part of the curve for P25 gives an

Figure 10. UV-vis absorption spectra (a) and the determination of indirect interband transition energies (b) for P25 and the plasmagenerated TiO2 powders. A in the Y-axis title of part (b) represents absorbance, which is proportional to the absorption coefficient R.

indirect band gap of 2.87 eV, which is in close vicinity to the calculated value of 2.91 eV corresponding to Χ1a f G1b indirect interband transition26 and the reported value of 2.95 eV.9a The powders made with plasma show narrowed indirect band gaps of ∼2.65 eV, regardless of the O2 flow rate in the plasma sheath during powder processing. These nearly identical band gap values suggest similar contents of lattice chlorine, which is in agreement with the similar oxygen deficiency levels revealed by Raman spectroscopy (Figure 3). A careful examination of the absorption spectra in the range of 400-500 nm (Figure 10a),

Chlorinated Nanocrystalline TiO2 Powders

J. Phys. Chem. C, Vol. 111, No. 49, 2007 18023 (Figure 11b), owing to their enhanced UV absorptions (Figure 10a). The photocatalytic reactivity of the chlorinated powder is dependent on the O2 input during powder synthesis, as observed in the vis tests. Since all the powders exhibit almost identical absorptions of the UV light (Figure 10a), the decreased reactivity at a lower O2 input may thus be similarly ascribed to the enhanced surface recombination of h+/e carriers by the excessive Ti3+ ions. 4. Conclusions Ar/O2 rf thermal plasma was shown in this work to be an effective tool in synthesizing via one-step chlorinated TiO2 powders of high crystallinity, good dispersion, and excellent photocatalytic reactivity. The resultant TiO2 powders are anatase/ rutile mixtures with particle sizes distributed in the range of several nanometers to micrometers. Overall properties of the powders, in terms of phase constituent, crystallite size, chemical stoichiometry, and specific surface area, show weak dependence on the O2 input (5-90 L/min) in the plasma sheath. Fractionation studies further revealed that phase constituent and chemical stoichiometry do not appreciably influenced by the size distribution. The incorporation of chlorine into the TiO2 lattice narrows the band gap to ∼2.65 eV and enhances UV absorption and, thus, substantially improves the photocatalytic activity of the powders in the degradation of methyl orange solutions under both vis and UV illumination. References and Notes

Figure 11. Degradation kinetics of 20 µM methyl orange solutions over TiO2 photocatalysts under vis (a) and UV (b) irradiation.

within which the wavelengths (405 and 436 nm) of the vis light used for photocatalytic tests fall, shows that the absorption intensity increases steadily with decreasing O2 flow rate in the plasma sheath during powder processing. As all the powders possess almost identical band gaps; such a phenomenon is thus attributable to the different surface properties of the powders. This is plausible in view that the plasma atmosphere impacts particle surfaces more readily and more directly. At a lower O2 partial pressure (lower O2 flow rate in the plasma sheath), surfaces of the resultant TiO2 particles may have more Ti3+ ions, leading to higher absorptions of the vis light.27 Photocatalytic performances of the TiO2 powders synthesized in this work were compared with that of P25 via bleaching 20 µM methyl orange solutions under vis (Figure 11a) and UV (Figure 11b) illumination. For the vis tests, the surface area of all the tested powders has been set as that (0.5 m2) of 10 mg of P25 by varying the sample weight to exclude the effects of specific surface area. For the UV tests, the TiO2 sample has the same surface area as that of 1 mg of P25, and the intensity of the UV light used is 1 mW/cm2 on the top surface of the suspension. From Figure 11a, appreciably higher efficiencies were confirmed for all the chlorinated powders made with thermal plasma, which is attributed to the narrowed band gaps (Figure 10b) like the case of fluorinated TiO2.9 Nonetheless, the performance of the powder shows clear dependence on the O2 input during powder synthesis, exhibiting a tendency opposite to the absorption intensity of the powder in the 400-500 nm range (Figure 10a). This indicates that excessive surface Ti3+ ions, as reported previously,28 may act as surface recombination centers for h+/e and accordingly suppress the photocatalytic activity. The plasma-synthesized TiO2 powders also exhibit higher photocatalytic activities than P25 under UV irradiation

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