- Email: [email protected]
The preparation and characterization of TiO2 ultrafine particles
Materials Science and Engineering B56 (1999) 211 – 214 www.elsevier.com/locate/mseb
The preparation and characterization of TiO2 ultrafine particles Zili Xu a,*, Jing Shang a, Chunming Liu a, Chunli Kang a, Haichen Guo b, Yaoguo Du a a b
Department of En6ironmental Science, Jilin Uni6ersity, Changchun 130023, People’s Republic of China Analysis Test and Experiment Center, Jilin Uni6ersity, Changchun 130023, People’s Republic of China Received 29 October 1998; received in revised form 8 March 1999; accepted 19 March 1999
Abstract Ultrafine particles (UFP) were prepared by means of the colloidal chemical method. The structure and properties of the as-prepared TiO2 UFP, having been submitted to different heat-treatment tests, were studied using TEM, XRD, IR, SPS, Raman, XPS and UV–vis absorption spectrum. The results of the XPS, SPS and UV – vis absorption spectrum indicated that on the surface of TiO2 UFP, there were surface active species such as Ti3 + and hydroxyl which can improve the photocatalytic activity of TiO2 UFP. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Colloidal chemical method; TiO2; Ultrafine particle; Characterization
1. Introduction Semiconductor TiO2 ultrafine particle (UFP) as a new material has found widespread application [1–3]. Recently the sol-gel method has become an efficient method for preparing ultrafine particles [4,5]. However, metal alkoxide is required in the method and it is expensive. Knowing the structure and properties of UFP will contribute to its application. We prepared TiO2 UFP using the colloidal chemical method and determined their characteristics such as particle size structure and surface state. Those can give useful information about the photocatalytic activity of TiO2 UFP.
which gave rise to an organosol. After separating the solvent from the organosol by diminished distillation and heat treatment processes, ultrafine particles of titanium dioxide were prepared. The most favorable experimental conditions for the preparation of TiO2 UFP were studied. The concentrations of Ti(SO4)2 solution and NaOH solution were over a range of 0.1–0.25 and 1.0–2.5 mol l − 1, respectively. The pH value was over the range 0.3–0.7. When the mole ratio of hydrosol to DBS was 6.25, the transparent organosol was obtained. The calcining temperature was an important factor for grain size and properties of the final product. In this paper the particles were calcined at 320, 480 or 550°C for 2 h, respectively.
2. The preparation of TiO2 ultrafine particles 3. The characterization of TiO2 UFP Ti(SO4)2 solution reacted with NaOH solution to produce Ti(OH)4 precipitate, which was transformed into a transparent hydrosol of titanium dioxide by means of washing, centrifuging and peptizing processes. The hydrosol was then changed into hydrophobic colloid particles with an anionic surfactant (DBS). The hydrophobic particles were transferred into chloroform
* Corresponding author.
3.1. The particle size and composition of TiO2 UFP The particle size and morphology were measured with a Hitachi H-8100 transmission electron microscope (TEM). The TEM photomicrographs showed that the TiO2 UFP were small grains with a narrow size range. The averaged particle sizes of TiO2 UFP calcined at 320, 480 and 550°C were 3.8, 10 and 20 nm, respectively. The averaged particle size increased as the calcining temperature increased.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 0 8 4 - 7
212
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214 Table 1 The unit cell parameters of TiO2 particlesa
The averaged particle sizes and compositions of TiO2 UFP and a commercial TiO2 sample (600 nm) were 24 determined by means of a D/MAX-rA powder diffractometer with nickel-filtered Cu Ka source. The averaged particle size D of the ultrafine particles can also be estimated from the width of the lines in the X-ray diffraction spectrum with the aid of the Scherer equation: D = Kl/b cos u where l is the wavelength of the X-ray used, b is the width of the line at the half-maximum intensity, and K is a constant [6]. The estimated values D of TiO2 UFP calcined at 320, 480 and 550°C were 4.2, 10.6 and 19.4 nm, respectively. This means that the particle size D is in good agreement with the values of the particle size D measured by electron microscope. Fig. 1 shows the X-ray diffraction patterns of TiO2 UFP calcined at 320, 480 and 550 for 2 hand and a commercial TiO2 sample. From Fig. 1, it can be seen that the TiO2 UFP heated at 320°C only had a tetragonal anatase structure (the values of 2u were 25.360° (100%), 48.140° (35%), 37.876° (20%), 53.993° (20%), 55.220° (20%), and 62.791° (14%)) and had a widened spectral line because of the crystal imperfection and incomplete defect layer owing to the small grain size. Other TiO2 particles were mainly tetragonal anatase structures having little rutile structure content (the values of 2u were 27.483° (100%), and 36.070° (50%)). The results of unit cell parameter determination of TiO2 particles are shown in Table 1. It should be noted that TiO2 UFP has a larger unit cell volume and widened diffraction spectrum than the commercial TiO2 sample. The ultrafine particle is so-called mesomorphous, and the grain size of UFP only has an order of magnitude difference from that of the atom or molecule. Because of its very small grain size,
the UFP cannot be seen as an ideal crystal which has innumerable crystal faces and the atom malposition is obvious. The second strain root-mean-square values (ss) of TiO2 UFP is greater than that of the commercial TiO2 sample, indicating that UFP is easily of unit cell distortion.
Fig. 1. XRD patterns for TiO2 UFP calcined at 320 (a), 480 (b) and 550°C (c) and a commercial TiO2 sample (d).
Fig. 2. SPS response of a commercial TiO2 sample(a) and TiO2 UFP calcined at 320 (b), 480 (c) and 550°C (d).
Sample
V
HKL
IW
SS
Commercial TiO2
136.444
101 200 105 211
0.3449 0.3528 0.3580 0.3412
2.7644 2.3610 0.5007 0.4709
TiO2 UFP about 10 nm
136.8398
101 200 105 211
0.9859 0.9944 1.4351 1.4639
6.5856 6.0910 7.2815
a V, unit cell volume; HKL, crystal face exponent; IW, integral width; SS, second strain root-mean-square value.
3.2. The quantum size effect The spectra such as IR (acquired from a Nicolet Impact 410 infrared spectrophotometer), Raman spectrum (measured with a Bruker RFS-100 FT Raman spectrophotometer), surface photovoltage spectroscopy (SPS, as shown in Fig. 2) and UV–vis absorption spectrum of TiO2 UFP showed a large deviation from those of the commercial TiO2 sample. It is commonly observed that with decreasing size, the optical edge shifts to the blue which is often attributed to the quantum size effect [7,8].
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214
213
3.3. The characterization of Ti 3 + on the TiO2 UFP surface Fig. 2 shows the surface photovoltage spectroscopy of a commercial TiO2 sample (600 nm) and TiO2 UFP calcined at 320 (3.8 nm), 480 (10 nm) and 550°C (20 nm). The surface photovoltage spectrometer was built by the Department of Chemistry in our University [9]. It can be seen from Fig. 2 that the surface photovoltage signal decreased as the ultrafine particle size decreased. It was found that the ultrafine particles are small in diameter and the light-induced electron and hole are very easily trapped. Therefore, the results are different from those of the usual powder photocatalysis, i.e. the weaker the surface photovoltage signal, the higher the photocatalytic activity [10]. From Fig. 2 we can see that for all the TiO2 particles there is a broad band absorption from 300 to 400 nm which is due to the transition of the O2 − antibonding orbital to the lowest empty orbital of Ti4 + [11,12]. For TiO2 UFP, the signal of 400 – 600 nm was responsible for the surface and ultrafine characteristics. The signal of TiO2 UFP heated at 480 or 550°C is very weak in the region, while for TiO2 UFP heated at 320°C the signal is strong which can be associated with the presence of Ti3 + . Similarly, the UV – vis absorption spectrum of TiO2 UFP heated at 320°C showed that there was also a peak at about 430 nm which was attributed to Ti3 + . The presence of surface Ti3 + species causes distinct differences in the nature of the chemical bonding between the adsorbed molecule and the substrate surface [13].
co-workers [17,18] showed that the surface photoactivity for a TiO2 powder is proportional to the number of hydroxyl groups on the surface. As shown in Fig. 3, the ratio of the chemisorbed oxygen to crystal lattice oxygen of TiO2 UFP heated at 320°C was greater (26.09%) than that of the chemisorbed oxygen to crystal lattice oxygen of TiO2 UFP heated at 480°C (19.86%), i.e. the surface hydroxyl disappeared as the temperature increased. Thus we can speculate that the photocatalytic activity of TiO2 UFP heated at 320°C is larger than that of TiO2 UFP heated at 480°C.
3.4. The characterization of surface hydroxy
3.5. The characterization of photocatalytic acti6ity
The oxygen on the surfaces of the TiO2 UFP was examined by means of a X-ray photoelectron spectrometer. The X-ray source emitted Al Ka radiation (1436.8 eV). For all the spectra obtained the pressure was maintained at the 6.3×10 − 5 Pa. Binding energies were calibrated with respect to the signal for adventitious carbon (binding energy=284.6 eV) The X-ray photoelectron spectra (XPS) for O1s of TiO2 UFP heated at 320 and 480°C are shown in Fig. 3. The O1s XPS peak at 530 eV is asymmetric (the right side is wider than that of the left side), indicating that at least two oxygen species were present in the near surface region. The peak at about 530 eV is due to the crystal lattice oxygen, while the peak at about 532 eV is due to chemisorbed oxygen [14]. The comparison of oxygen binding energies of metal oxide and metal hydroxide shows that the latter is higher than the former. For example, the binding energies of NiO and Ni(OH)2 are 529.8 and 531.5 eV, respectively [15] and those of Al2O3 and Al(OH)3 are 531.6 and 533.2 eV, respectively [16]. So we speculate that the chemisorbed oxygen was the contribution of surface hydroxyl. Boonstra and
We applied the three kinds of TiO2 UFP and commercial TiO2 sample to the photocatalytic reaction system of heptane (or SO2) (4000 ppm), oxygen (20%) and ultrapure nitrogen (99.9999%). The band gap radiation was provided by a 400 W high pressure mercury lamp. Fig. 4 shows the photocatalytic activity of TiO2 UFP calcined at 320, 480 and 550°C and commercial TiO2 sample in the system of heptane or SO2. As shown in Fig. 4, it can be seen that increasing the treatment temperature of TiO2 UFP from 320 to 550°C, the degradation process of the reactant (heptane or SO2) is retarded, indicating that it is related to the surface properties of the TiO2 particle of the process. When the particle diameter decreases the chance of recombination for photoinduced electron–hole pairs decreases because of their faster arrival at the reaction site on the surface [19], so the photocatalytic activity is increased. The surface hydroxyl groups can act as the centers of photocatalytic reactions [20]. The hole (h + ) attacks the surface hydroxyl and yields a surface-bound OH radical. The Ti3 + species are responsible for oxygen photoad-
Fig. 3. O1s XPS of TiO2 UFP calcined at 320 (a) and 480°C (b)
214
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214
Acknowledgements This work was supported by the National Nature Science Foundation of China.
References
Fig. 4. The relationship between concentration of reactant with different TiO2 particles and irradiation time. (a) The heptane system and (b) the SO2 system. − sorption to form Oads [21] which is, together with the OH radical, essential to photocatalytic oxidation of heptane (or SO2).
4. Conclusions The TiO2 UFP prepared by the colloid chemical method has a tetragonal anatase structure. As the calcining temperature decreases, the TiO2 UFP has a decreased particle size and increased contents of surface Ti3 + and hydroxyl active species which are considered to be essential to the photocatalytic activity of the TiO2 UFP. .
[1] R.W. Siegel, S. Ramasamy, H. Hahn, J. Mater. Res. 3 (1988) 1367. [2] S. Lalshmi, R. Renganathan, S. Fujta, J. Photochem. Photobiol. A: Chem. 89 (1995) 163. [3] M.R. Hoffmann, S.T. Martin, W. Chio, P.W. Bahnemann, Chem. Rev. 95 (1995) 69. [4] E. Haro-Poniatowski, R. Rodriguez-Talavera, J. Mater. Res. 9 (1994) 2102. [5] D.C. Hague, M.Y. Mayo, J. Am. Ceram. Soc. 77 (1994) 365. [6] H. Ogawa, A. Abe, J. Electrochem. Soc. 128 (1981) 685. [7] L.E. Brus, J. Chem. Phy. 80 (1984) 4403. [8] L.E. Brus, J. Phys. Chem. 90 (1986) 2555. [9] D.J. Wang, L.Z. Xiao, W. Liu, Chin. Chem. Bull. 10 (1989) 32. [10] C.M. Zhang, W. Liu, Photogr. Sci. Photochem. 9 (1991) 225. [11] N. Daude, C. Gout, C. Jouanin, Phys. Rev. B15 (1977) 3229. [12] K. Vos, H.K. Krusemeyer, Solid. State. Commun. 15 (1975) 949. [13] W.J. Lo, Y.W. Chung, G.A. Somorjai, Surf. Sci. 71 (1978) 199. [14] M. Koudelka, A. Monnier, J. Sanchez, J. Augustynski, J. Mol. Catal. 25 (1984) 295. [15] K.S. Kim, W.E. Baitinger, J.W. Amy, N. Winogrod, J. Electron. Spectrosc. 5 (1974) 351. [16] J.C. Fuggle, L.M. Watson, D.J. Fabian, S. Affrossman, J. Phys F. Metal. Phys. 5 (1975) 375. [17] A.H. Boonstra, C.A.H.A. Mutsaers, J. Phys. Chem. 79 (1975) 1694. [18] E. Pelizzetti, C. Minero, Electrochim. Acta 38 (1993) 47. [19] Y. Nosaka, K. Yamaguchi, H. Miyama, Chem. Lett. 4 (1988) 605. [20] R.I. Bickley, F.S. Stone, J. Catal. 31 (1973) 389. [21] G. Munuera, V. Rires-Arnau, A. Saucedo, J. Chem. Soc. Faraday Trans. I. 75 (1979) 736.
.
The preparation and characterization of TiO2 ultrafine particles Zili Xu a,*, Jing Shang a, Chunming Liu a, Chunli Kang a, Haichen Guo b, Yaoguo Du a a b
Department of En6ironmental Science, Jilin Uni6ersity, Changchun 130023, People’s Republic of China Analysis Test and Experiment Center, Jilin Uni6ersity, Changchun 130023, People’s Republic of China Received 29 October 1998; received in revised form 8 March 1999; accepted 19 March 1999
Abstract Ultrafine particles (UFP) were prepared by means of the colloidal chemical method. The structure and properties of the as-prepared TiO2 UFP, having been submitted to different heat-treatment tests, were studied using TEM, XRD, IR, SPS, Raman, XPS and UV–vis absorption spectrum. The results of the XPS, SPS and UV – vis absorption spectrum indicated that on the surface of TiO2 UFP, there were surface active species such as Ti3 + and hydroxyl which can improve the photocatalytic activity of TiO2 UFP. © 1999 Elsevier Science S.A. All rights reserved. Keywords: Colloidal chemical method; TiO2; Ultrafine particle; Characterization
1. Introduction Semiconductor TiO2 ultrafine particle (UFP) as a new material has found widespread application [1–3]. Recently the sol-gel method has become an efficient method for preparing ultrafine particles [4,5]. However, metal alkoxide is required in the method and it is expensive. Knowing the structure and properties of UFP will contribute to its application. We prepared TiO2 UFP using the colloidal chemical method and determined their characteristics such as particle size structure and surface state. Those can give useful information about the photocatalytic activity of TiO2 UFP.
which gave rise to an organosol. After separating the solvent from the organosol by diminished distillation and heat treatment processes, ultrafine particles of titanium dioxide were prepared. The most favorable experimental conditions for the preparation of TiO2 UFP were studied. The concentrations of Ti(SO4)2 solution and NaOH solution were over a range of 0.1–0.25 and 1.0–2.5 mol l − 1, respectively. The pH value was over the range 0.3–0.7. When the mole ratio of hydrosol to DBS was 6.25, the transparent organosol was obtained. The calcining temperature was an important factor for grain size and properties of the final product. In this paper the particles were calcined at 320, 480 or 550°C for 2 h, respectively.
2. The preparation of TiO2 ultrafine particles 3. The characterization of TiO2 UFP Ti(SO4)2 solution reacted with NaOH solution to produce Ti(OH)4 precipitate, which was transformed into a transparent hydrosol of titanium dioxide by means of washing, centrifuging and peptizing processes. The hydrosol was then changed into hydrophobic colloid particles with an anionic surfactant (DBS). The hydrophobic particles were transferred into chloroform
* Corresponding author.
3.1. The particle size and composition of TiO2 UFP The particle size and morphology were measured with a Hitachi H-8100 transmission electron microscope (TEM). The TEM photomicrographs showed that the TiO2 UFP were small grains with a narrow size range. The averaged particle sizes of TiO2 UFP calcined at 320, 480 and 550°C were 3.8, 10 and 20 nm, respectively. The averaged particle size increased as the calcining temperature increased.
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 9 ) 0 0 0 8 4 - 7
212
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214 Table 1 The unit cell parameters of TiO2 particlesa
The averaged particle sizes and compositions of TiO2 UFP and a commercial TiO2 sample (600 nm) were 24 determined by means of a D/MAX-rA powder diffractometer with nickel-filtered Cu Ka source. The averaged particle size D of the ultrafine particles can also be estimated from the width of the lines in the X-ray diffraction spectrum with the aid of the Scherer equation: D = Kl/b cos u where l is the wavelength of the X-ray used, b is the width of the line at the half-maximum intensity, and K is a constant [6]. The estimated values D of TiO2 UFP calcined at 320, 480 and 550°C were 4.2, 10.6 and 19.4 nm, respectively. This means that the particle size D is in good agreement with the values of the particle size D measured by electron microscope. Fig. 1 shows the X-ray diffraction patterns of TiO2 UFP calcined at 320, 480 and 550 for 2 hand and a commercial TiO2 sample. From Fig. 1, it can be seen that the TiO2 UFP heated at 320°C only had a tetragonal anatase structure (the values of 2u were 25.360° (100%), 48.140° (35%), 37.876° (20%), 53.993° (20%), 55.220° (20%), and 62.791° (14%)) and had a widened spectral line because of the crystal imperfection and incomplete defect layer owing to the small grain size. Other TiO2 particles were mainly tetragonal anatase structures having little rutile structure content (the values of 2u were 27.483° (100%), and 36.070° (50%)). The results of unit cell parameter determination of TiO2 particles are shown in Table 1. It should be noted that TiO2 UFP has a larger unit cell volume and widened diffraction spectrum than the commercial TiO2 sample. The ultrafine particle is so-called mesomorphous, and the grain size of UFP only has an order of magnitude difference from that of the atom or molecule. Because of its very small grain size,
the UFP cannot be seen as an ideal crystal which has innumerable crystal faces and the atom malposition is obvious. The second strain root-mean-square values (ss) of TiO2 UFP is greater than that of the commercial TiO2 sample, indicating that UFP is easily of unit cell distortion.
Fig. 1. XRD patterns for TiO2 UFP calcined at 320 (a), 480 (b) and 550°C (c) and a commercial TiO2 sample (d).
Fig. 2. SPS response of a commercial TiO2 sample(a) and TiO2 UFP calcined at 320 (b), 480 (c) and 550°C (d).
Sample
V
HKL
IW
SS
Commercial TiO2
136.444
101 200 105 211
0.3449 0.3528 0.3580 0.3412
2.7644 2.3610 0.5007 0.4709
TiO2 UFP about 10 nm
136.8398
101 200 105 211
0.9859 0.9944 1.4351 1.4639
6.5856 6.0910 7.2815
a V, unit cell volume; HKL, crystal face exponent; IW, integral width; SS, second strain root-mean-square value.
3.2. The quantum size effect The spectra such as IR (acquired from a Nicolet Impact 410 infrared spectrophotometer), Raman spectrum (measured with a Bruker RFS-100 FT Raman spectrophotometer), surface photovoltage spectroscopy (SPS, as shown in Fig. 2) and UV–vis absorption spectrum of TiO2 UFP showed a large deviation from those of the commercial TiO2 sample. It is commonly observed that with decreasing size, the optical edge shifts to the blue which is often attributed to the quantum size effect [7,8].
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214
213
3.3. The characterization of Ti 3 + on the TiO2 UFP surface Fig. 2 shows the surface photovoltage spectroscopy of a commercial TiO2 sample (600 nm) and TiO2 UFP calcined at 320 (3.8 nm), 480 (10 nm) and 550°C (20 nm). The surface photovoltage spectrometer was built by the Department of Chemistry in our University [9]. It can be seen from Fig. 2 that the surface photovoltage signal decreased as the ultrafine particle size decreased. It was found that the ultrafine particles are small in diameter and the light-induced electron and hole are very easily trapped. Therefore, the results are different from those of the usual powder photocatalysis, i.e. the weaker the surface photovoltage signal, the higher the photocatalytic activity [10]. From Fig. 2 we can see that for all the TiO2 particles there is a broad band absorption from 300 to 400 nm which is due to the transition of the O2 − antibonding orbital to the lowest empty orbital of Ti4 + [11,12]. For TiO2 UFP, the signal of 400 – 600 nm was responsible for the surface and ultrafine characteristics. The signal of TiO2 UFP heated at 480 or 550°C is very weak in the region, while for TiO2 UFP heated at 320°C the signal is strong which can be associated with the presence of Ti3 + . Similarly, the UV – vis absorption spectrum of TiO2 UFP heated at 320°C showed that there was also a peak at about 430 nm which was attributed to Ti3 + . The presence of surface Ti3 + species causes distinct differences in the nature of the chemical bonding between the adsorbed molecule and the substrate surface [13].
co-workers [17,18] showed that the surface photoactivity for a TiO2 powder is proportional to the number of hydroxyl groups on the surface. As shown in Fig. 3, the ratio of the chemisorbed oxygen to crystal lattice oxygen of TiO2 UFP heated at 320°C was greater (26.09%) than that of the chemisorbed oxygen to crystal lattice oxygen of TiO2 UFP heated at 480°C (19.86%), i.e. the surface hydroxyl disappeared as the temperature increased. Thus we can speculate that the photocatalytic activity of TiO2 UFP heated at 320°C is larger than that of TiO2 UFP heated at 480°C.
3.4. The characterization of surface hydroxy
3.5. The characterization of photocatalytic acti6ity
The oxygen on the surfaces of the TiO2 UFP was examined by means of a X-ray photoelectron spectrometer. The X-ray source emitted Al Ka radiation (1436.8 eV). For all the spectra obtained the pressure was maintained at the 6.3×10 − 5 Pa. Binding energies were calibrated with respect to the signal for adventitious carbon (binding energy=284.6 eV) The X-ray photoelectron spectra (XPS) for O1s of TiO2 UFP heated at 320 and 480°C are shown in Fig. 3. The O1s XPS peak at 530 eV is asymmetric (the right side is wider than that of the left side), indicating that at least two oxygen species were present in the near surface region. The peak at about 530 eV is due to the crystal lattice oxygen, while the peak at about 532 eV is due to chemisorbed oxygen [14]. The comparison of oxygen binding energies of metal oxide and metal hydroxide shows that the latter is higher than the former. For example, the binding energies of NiO and Ni(OH)2 are 529.8 and 531.5 eV, respectively [15] and those of Al2O3 and Al(OH)3 are 531.6 and 533.2 eV, respectively [16]. So we speculate that the chemisorbed oxygen was the contribution of surface hydroxyl. Boonstra and
We applied the three kinds of TiO2 UFP and commercial TiO2 sample to the photocatalytic reaction system of heptane (or SO2) (4000 ppm), oxygen (20%) and ultrapure nitrogen (99.9999%). The band gap radiation was provided by a 400 W high pressure mercury lamp. Fig. 4 shows the photocatalytic activity of TiO2 UFP calcined at 320, 480 and 550°C and commercial TiO2 sample in the system of heptane or SO2. As shown in Fig. 4, it can be seen that increasing the treatment temperature of TiO2 UFP from 320 to 550°C, the degradation process of the reactant (heptane or SO2) is retarded, indicating that it is related to the surface properties of the TiO2 particle of the process. When the particle diameter decreases the chance of recombination for photoinduced electron–hole pairs decreases because of their faster arrival at the reaction site on the surface [19], so the photocatalytic activity is increased. The surface hydroxyl groups can act as the centers of photocatalytic reactions [20]. The hole (h + ) attacks the surface hydroxyl and yields a surface-bound OH radical. The Ti3 + species are responsible for oxygen photoad-
Fig. 3. O1s XPS of TiO2 UFP calcined at 320 (a) and 480°C (b)
214
Z. Xu et al. / Materials Science and Engineering B63 (1999) 211–214
Acknowledgements This work was supported by the National Nature Science Foundation of China.
References
Fig. 4. The relationship between concentration of reactant with different TiO2 particles and irradiation time. (a) The heptane system and (b) the SO2 system. − sorption to form Oads [21] which is, together with the OH radical, essential to photocatalytic oxidation of heptane (or SO2).
4. Conclusions The TiO2 UFP prepared by the colloid chemical method has a tetragonal anatase structure. As the calcining temperature decreases, the TiO2 UFP has a decreased particle size and increased contents of surface Ti3 + and hydroxyl active species which are considered to be essential to the photocatalytic activity of the TiO2 UFP. .
[1] R.W. Siegel, S. Ramasamy, H. Hahn, J. Mater. Res. 3 (1988) 1367. [2] S. Lalshmi, R. Renganathan, S. Fujta, J. Photochem. Photobiol. A: Chem. 89 (1995) 163. [3] M.R. Hoffmann, S.T. Martin, W. Chio, P.W. Bahnemann, Chem. Rev. 95 (1995) 69. [4] E. Haro-Poniatowski, R. Rodriguez-Talavera, J. Mater. Res. 9 (1994) 2102. [5] D.C. Hague, M.Y. Mayo, J. Am. Ceram. Soc. 77 (1994) 365. [6] H. Ogawa, A. Abe, J. Electrochem. Soc. 128 (1981) 685. [7] L.E. Brus, J. Chem. Phy. 80 (1984) 4403. [8] L.E. Brus, J. Phys. Chem. 90 (1986) 2555. [9] D.J. Wang, L.Z. Xiao, W. Liu, Chin. Chem. Bull. 10 (1989) 32. [10] C.M. Zhang, W. Liu, Photogr. Sci. Photochem. 9 (1991) 225. [11] N. Daude, C. Gout, C. Jouanin, Phys. Rev. B15 (1977) 3229. [12] K. Vos, H.K. Krusemeyer, Solid. State. Commun. 15 (1975) 949. [13] W.J. Lo, Y.W. Chung, G.A. Somorjai, Surf. Sci. 71 (1978) 199. [14] M. Koudelka, A. Monnier, J. Sanchez, J. Augustynski, J. Mol. Catal. 25 (1984) 295. [15] K.S. Kim, W.E. Baitinger, J.W. Amy, N. Winogrod, J. Electron. Spectrosc. 5 (1974) 351. [16] J.C. Fuggle, L.M. Watson, D.J. Fabian, S. Affrossman, J. Phys F. Metal. Phys. 5 (1975) 375. [17] A.H. Boonstra, C.A.H.A. Mutsaers, J. Phys. Chem. 79 (1975) 1694. [18] E. Pelizzetti, C. Minero, Electrochim. Acta 38 (1993) 47. [19] Y. Nosaka, K. Yamaguchi, H. Miyama, Chem. Lett. 4 (1988) 605. [20] R.I. Bickley, F.S. Stone, J. Catal. 31 (1973) 389. [21] G. Munuera, V. Rires-Arnau, A. Saucedo, J. Chem. Soc. Faraday Trans. I. 75 (1979) 736.
.