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Study on TMA-sensing properties of nanocrystalline titanium dioxide materials
Sensors and Actuators B 119 (2006) 370–373
Study on TMA-sensing properties of nanocrystalline titanium dioxide materials Bo Zou a , Fengqing Wu a,∗ , Cheng Chen a , Hui Ren b , Zhuyi Wang a , Lehui Zou a a
b
College of Chemistry, Jilin University, Changchun 130025, China College of Biological and Agricultural Engineering, Jilin University, Changchun 130025, China
Received 25 July 2005; received in revised form 19 December 2005; accepted 20 December 2005 Available online 25 January 2006
Abstract Nanocrystalline TiO2 materials were prepared by a sol–gel method. A series of tests on indirect-heating sensors made of these materials showed that the nanocrystalline TiO2 materials exhibited high responses to TMA with anatase having higher response than rutile. It was also found that proper doping in nanocrystalline TiO2 could increase the TMA response, reduce its resistance, and enhance the selectivity against disturbing gases. The nanocrystalline materials doped with Ag+ showed the best performance among some other metal ions. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline TiO2 ; Response; TMA
1. Introduction Examination of fish freshness is one of the most important issues in fishery business. Most sea fishes when in storage release trimethylamine (TMA), a gas with a distinct smell. Therefore, the freshness of the seafood can be examined by the content of the released TMA. However, traditional examination methods are very time-consuming and demand a lot of hard work. It has been found that some semiconductor oxides, such as SnO2 , ZnO, WO3 , In2 O3 and TiO2 are sensitive to TMA, and among them TiO2 attracts more attention because of its stability, safety, nonpoisonousness, and harmlessness [1,2]. Many research works are on rutile TiO2 with particle size of micrometer scale [3–5]. In our study, we found that anatase TiO2 showed higher sensitivity to TMA than rutile TiO2 , and the TMA sensitivity of anatase TiO2 was affected by the doping of metal ions. 2. Experimental Nanocrystalline TiO2 materials were synthesized by a stearic acid–gel method [6]. A mixture of stearic acid (need to put in producer) and Ti(OC4 H9 )4 with a mass ratio (Ti(OC4 H9 )4 :stearic acid) of 5:8 was stirred at 70 ◦ C for 2 h. The mixture was cooled ∗
Corresponding author. Tel.: +86 431 5168397 E-mail address: [email protected] (F. Wu).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.12.035
down to form gel, which was then calcined at different temperatures for 2 h to produce different types of nanocrystalline TiO2 . The indirect-heating sensor devices were fabricated by these nanocrystalline TiO2 (Fig. 1). The particle size and phase structure of the samples were then characterized by Shimadzu XD-3AX X-ray diffractometer (XRD) and Hitachi H800EM transmission electron microscope (TEM). Surface area of the samples was examined by using Quantacerome AUTOSORB1C automated chemisorption/physisorption surface area and pore size analyzer. Sensitivity to TMA of these sensors was examined by RQ-2 gas-sensitive capacity measurer. 3. Results and discussion 3.1. Characterization of nanocrystalline TiO2 Fig. 2 shows the XRD patterns of the nanocrystalline TiO2 prepared. Compared with JCPDS standard card, we found that the samples calcined at 400–450 ◦ C for 2 h were of anatase type, while those calcined above 750 ◦ C were of rutile type. Those calcined at 500–700 ◦ C for 2 h were a mixture of the two types. Fig. 3 is a TEM image of nanocrystalline TiO2 . From the image it can be seen that the resulting nanocrystalline TiO2 particles are spherical with the size around 10–20 nm and almost no conglomerates are visible. Thus, when the nanocrystalline materials were prepared by the sol–gel method, the stearic acid acted
B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373
371
Fig. 1. Structure of a gas-sensitive element: (a) before coat and (b) coated.
Fig. 4. Relationships between the response of TiO2 nanocrystals and the TMA concentration: (A) 100% rutile; (B) rutile:anatase = 2:1 (by mass); and (C): anatase 100%. Sensor response was defined as β = R0 /R. R0 and R are the resistance of sensor device in air and in TMA, respectively. Heating current was 120 mA (30 hot wire).
as a protector to prevent colloidal particles from conglomerating. As the Ti(OC4 H9 )4 molecules evenly distribute in stearic acid, the nanocrystalline particles developed after calcination are not only small in size but also distributed evenly. 3.2. Gas-sensitive character of nanocrystalline TiO2
Fig. 2. XRD patterns of nanocrystalline TiO2 calcined at different temperatures for 2 h: (a) 400 ◦ C; (b) 450 ◦ C; (c) 550 ◦ C; (d) 600 ◦ C; (e) 700 ◦ C; and (f) 750 ◦ C.
Fig. 3. A TEM image of nanocrystalline TiO2 calcined at 400 ◦ C for 2 h.
TiO2 is an n-type semiconductor [7,8]. First oxygen molecules in air are adsorbed on the surface due to chemical adsorption, electrons move along the semiconductor surface towards the adsorbed oxygen, and a layer make up of space electric charges forms on the surface of the semiconductor. This process reduces the concentration of electrons on the surface and therefore, reduces the conductivity of the semiconductor. When TMA molecules are present, the adsorbed oxygen on the TiO2 surface reacts with TMA (see the mechanism above). This leads to a decrease in the amount of adsorbed oxygen and makes more free electrons in the surface region. As a result, the conductivity of the semiconductor oxide increases. Fig. 4 shows the response to TMA of the indirect-heating sensor made of different types of nanocrystalline TiO2 . This graph reveals that the anatase-type has a higher TMA response than the rutile-type. This can be explained by different crystal structure of the two types of nanocrystalline TiO2 . It was shown in earlier researches that the amount of adsorbed O on the surface of anatase-type TiO2 is more than that on the surface of rutiletype TiO2 [11]. It is considered that there are more active centers
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B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373
Doping of nanocrystalline TiO2 with In2 O3 lowers its resistance and catalyzes the reaction of TMA with O2 because In2 O3 activates TMA and increases the reaction rate with oxygen. However, the doping with In2 O3 does not change the shape, size, or surface structure of the TiO2 particles. Therefore, the TMA sensitivity of TiO2 doped with In2 O3 is not much improved compared with that of pure TiO2 [15]. Nanocrystalline TiO2 doped with Cr2 O3 becomes a p-type semiconductor, which is not sensitive to TMA [16]. SnO2 actually lowers the sensitivity and selectivity of the material, because SnO2 tends to adsorb a variety of different gases and prevents TiO2 from adsorbing TMA. 4. Conclusion
Fig. 5. The effect of doping on the TMA response of TiO2 materials: (A) doped with 5 wt% of AgBr; (B) doped with 5 wt% of In2 O3 ; (C) doped with 5 wt% of SnO2 ; and (D) doped with 5 wt% of Cr2 O3 .
for reactions with TMA molecules on the anatase than on the rutile. And the anatase-type nanocrystalline TiO2 was formed only at a calcination temperature of 400 ◦ C for 2 h. The resulting particle size was about 19 nm, and the specific area was about 93 m2 /g. On the other hand, the rutile-type nanocrystalline TiO2 was formed at calcination temperatures above 700 ◦ C, and the resulting particle size was about 75 nm with a specific area of about 18 m2 /g. The specific area of the anatase particles is larger than that of the rutile, so that the anatase can adsorb more TMA on the surface than the rutile. It is known that doping in nanocrystalline materials can affect their surface conditions and electric properties. For example, doping a certain kind of metal ions into TiO2 can change its oxygen vacancy concentration and its electric properties such as resistance. This enhances the sensitivity of the sensor to target gases and also improves the response and recovery behavior [13,14]. Fig. 5 is the TMA response of TiO2 materials, doped with different metal ions and calcined at 500 ◦ C for 2 h. The figure shows that the sample doped with AgBr has the highest TMA response. When calcined at 500 ◦ C, the doping AgBr converts to metallic Ag, which is hard to be incorporated into the crystal lattice of TiO2 [8–10]. It is shown in literature [12] that the work function of metallic Ag is greater than that of semiconductor TiO2. When they are in contact, a Schottky barrier is created which facilitates the transfer of electrons from TiO2 to Ag. Thereby, this can increase oxygen vacancies on the oxide surface and create positive-charged centers which preferentially adsorb TMA molecules due to the presence of the non-bonded electron pair of the N atom in (CH3 )3 N. Doping with AgBr can also prevent TiO2 particle size from increasing [8–10], so that the doped sample has a larger specific area and could adsorb more target molecules, compared with the samples without doping. As a result the reaction is sped up and the TMA response is improved.
A stearic acid–gel method was very simple to synthesize nanocrystalline TiO2 materials and did not generate hard conglomerates. Anatase-type nanocrystalline TiO2 materials showed higher TMA response than rutile. The TMA response of the nanocrystalline TiO2 materials was affected by doping. Proper doping with other metal oxides could raise the TMA response. The doping also improved the response and recovery times, reduced the sensor resistance, and raised the selectivity against disturbing gases. Doping with AgBr showed the best TMA response among several other metal ions. Acknowledgements This work is supported by the National Natural Science Foundation of China (60374048). The authors are grateful to Professor Baokun Xu and Tong Zhang for help. References [1] H. Tsuji, H. Sugahara, Y. Gotoh, J. Ishikawa, Interactions with materials and atoms, Nucl. Instrum. Methods B 206 (2003) 249–253. [2] J. Hammond, B. Marquis, R. Michaels, B. Oickle, B. Segee, J. Vetelino, A. Bushway, M.E. Camire, K. Davis-Dentici, A semiconducting metaloxide array for monitoring fish freshness, Sens. Actuators B 84 (2002) 113–122. [3] N.O. Savage, S.A. Akbar, P.K. Dutta, Titanium dioxide based high temperature carbon monoxide selective sensor, Sens. Actuators B 72 (2001) 239–248. [4] Y. Yamada, Y. Seno, Y. Masuoka, T. Nakamura, K. Yamashita, NO2 sensing characteristics of Nb doped TiO2 thin films and their electronic properties, Sens. Actuators B 66 (2000) 164–166. [5] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism in Nb doped titania oxygen gas sensor, Sens. Actuators B 46 (1998) 194–201. [6] S.P. Ruan, W. Dong, F.Q. Wu, Dielectric properties of nanocrystalline BaTiO3 , Acta Phys. Chim. Sinica 19 (2003) 17–20. [7] Y. Nakato, H. Akanuma, J. Shimizu, Y. Magari, Photo-oxidation reaction of water on an n-TiO2 electrode: improvement in efficiency through formation of surface micropores by photo-etching in H2 SO4 , J. Electroanal. Chem. 396 (1995) 35–39. [8] L.Y. Su, Z.H. Lu, Photochromic and photocatalytic behaviors on immobilized TiO2 particulate films, J. Photochem. Photobiol. A 107 (1997) 245–248. [9] H.E. Chao, Y.U. Yun, H.U. Xingfang, A. Larbot, Effect of silver doping on the phase transformation and grain growth of sol–gel titania powder, J. Eur. Ceram. Soc. 23 (2003) 1457–1464.
B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373 [10] L.M. Arabatzis, T. Stergiopoulos, M.C. Bernard, D. Labou, S.G. Neophytides, P. Falaras, Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange, Appl. Catal. B: Environ. 42 (2003) 187–201. [11] S.P. Ruan, F.Q. Wu, T. Zhang, W. Gao, B.K. Xu, M.Y. Zhao, Surface state studies of TiO2 nanoparticle and photocatalyic degradation of methyl orange in aqueous TiO2 dispersions, Mater. Chem. Phys. 69 (2001) 7–9. [12] T.T.Y. Tan, C.K. Yip, D. Beydoun, R. Amal, Effects of nano-Ag particles loading on TiO2 photocatalytic reduction of selenate ions, Chem. Eng. J. 95 (2003) 179–186. [13] J. Schmal, Defect-pool model for doped a-Si:H, J. Non-Cryst. Solids 198–200 (1996) 387–390. [14] Y.Q. Wang, H.M. Cheng, L. Zhang, Y.Z. Hao, J.M. Ma, B. Xu, W.H. Li, The preparation, characterization, photoelectrochemical and photocatalytic properties of lanthanide metal-ion-doped TiO2 nanoparticles, J. Mol. Catal. A Chem. 151 (2000) 205–216. [15] J.R. Sambrano, L.A. Vasconcellos, J.B.L. Martins, M.R.C. Santos, E. Longo, A. Beltran, A theoretical analysis on electronic structure of the (1 1 0) surface of TiO2 –SnO2 mixed oxide, J. Mol. Struct. (Theochem) 629 (2003) 307–314. [16] Y.X. Li, W. Wlodarski, K. Galatsis, S.H. Moslih, J. Cole, S. Russo, N. Rockelmann, Gas sensing properties of p-type semiconducting Cr-doped TiO2 thin films, Sens. Actuators B 83 (2002) 160–163.
373
Biographies Bo Zou received the bachelor degree in chemistry at Jilin University in 2002. As PhD student, he is working at same university since 2005. His current research activities are centred on the fields of functional nanomaterials. Fenqin Wu has been graduated from the Jilin University in 1976. She is working at Jilin University since 1977. In the following year, she is mainly devoting to the research of functional nanomaterials. Cheng Chen received the bachelor degree in chemistry at Jilin University in 2002. As PhD student, he is working at same university since 2005. His current research activities are centred on the fields of functional nanomaterials. Hui Ren received the bachelor degree in College of Biological and Agricultural Engineering at Jilin University in 1984. His current research activities are centred on the fields of nanomaterials. Zhuyi Wang received the bachelor degree in chemistry at Jilin University in 2004. She is working at same university since 2005. Her current research activities are centred on the fields of functional nanomaterials. Lehui Zou has been graduated from the Jilin University in 1972. He is working at Jilin University since 1972.
Study on TMA-sensing properties of nanocrystalline titanium dioxide materials Bo Zou a , Fengqing Wu a,∗ , Cheng Chen a , Hui Ren b , Zhuyi Wang a , Lehui Zou a a
b
College of Chemistry, Jilin University, Changchun 130025, China College of Biological and Agricultural Engineering, Jilin University, Changchun 130025, China
Received 25 July 2005; received in revised form 19 December 2005; accepted 20 December 2005 Available online 25 January 2006
Abstract Nanocrystalline TiO2 materials were prepared by a sol–gel method. A series of tests on indirect-heating sensors made of these materials showed that the nanocrystalline TiO2 materials exhibited high responses to TMA with anatase having higher response than rutile. It was also found that proper doping in nanocrystalline TiO2 could increase the TMA response, reduce its resistance, and enhance the selectivity against disturbing gases. The nanocrystalline materials doped with Ag+ showed the best performance among some other metal ions. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanocrystalline TiO2 ; Response; TMA
1. Introduction Examination of fish freshness is one of the most important issues in fishery business. Most sea fishes when in storage release trimethylamine (TMA), a gas with a distinct smell. Therefore, the freshness of the seafood can be examined by the content of the released TMA. However, traditional examination methods are very time-consuming and demand a lot of hard work. It has been found that some semiconductor oxides, such as SnO2 , ZnO, WO3 , In2 O3 and TiO2 are sensitive to TMA, and among them TiO2 attracts more attention because of its stability, safety, nonpoisonousness, and harmlessness [1,2]. Many research works are on rutile TiO2 with particle size of micrometer scale [3–5]. In our study, we found that anatase TiO2 showed higher sensitivity to TMA than rutile TiO2 , and the TMA sensitivity of anatase TiO2 was affected by the doping of metal ions. 2. Experimental Nanocrystalline TiO2 materials were synthesized by a stearic acid–gel method [6]. A mixture of stearic acid (need to put in producer) and Ti(OC4 H9 )4 with a mass ratio (Ti(OC4 H9 )4 :stearic acid) of 5:8 was stirred at 70 ◦ C for 2 h. The mixture was cooled ∗
Corresponding author. Tel.: +86 431 5168397 E-mail address: [email protected] (F. Wu).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.12.035
down to form gel, which was then calcined at different temperatures for 2 h to produce different types of nanocrystalline TiO2 . The indirect-heating sensor devices were fabricated by these nanocrystalline TiO2 (Fig. 1). The particle size and phase structure of the samples were then characterized by Shimadzu XD-3AX X-ray diffractometer (XRD) and Hitachi H800EM transmission electron microscope (TEM). Surface area of the samples was examined by using Quantacerome AUTOSORB1C automated chemisorption/physisorption surface area and pore size analyzer. Sensitivity to TMA of these sensors was examined by RQ-2 gas-sensitive capacity measurer. 3. Results and discussion 3.1. Characterization of nanocrystalline TiO2 Fig. 2 shows the XRD patterns of the nanocrystalline TiO2 prepared. Compared with JCPDS standard card, we found that the samples calcined at 400–450 ◦ C for 2 h were of anatase type, while those calcined above 750 ◦ C were of rutile type. Those calcined at 500–700 ◦ C for 2 h were a mixture of the two types. Fig. 3 is a TEM image of nanocrystalline TiO2 . From the image it can be seen that the resulting nanocrystalline TiO2 particles are spherical with the size around 10–20 nm and almost no conglomerates are visible. Thus, when the nanocrystalline materials were prepared by the sol–gel method, the stearic acid acted
B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373
371
Fig. 1. Structure of a gas-sensitive element: (a) before coat and (b) coated.
Fig. 4. Relationships between the response of TiO2 nanocrystals and the TMA concentration: (A) 100% rutile; (B) rutile:anatase = 2:1 (by mass); and (C): anatase 100%. Sensor response was defined as β = R0 /R. R0 and R are the resistance of sensor device in air and in TMA, respectively. Heating current was 120 mA (30 hot wire).
as a protector to prevent colloidal particles from conglomerating. As the Ti(OC4 H9 )4 molecules evenly distribute in stearic acid, the nanocrystalline particles developed after calcination are not only small in size but also distributed evenly. 3.2. Gas-sensitive character of nanocrystalline TiO2
Fig. 2. XRD patterns of nanocrystalline TiO2 calcined at different temperatures for 2 h: (a) 400 ◦ C; (b) 450 ◦ C; (c) 550 ◦ C; (d) 600 ◦ C; (e) 700 ◦ C; and (f) 750 ◦ C.
Fig. 3. A TEM image of nanocrystalline TiO2 calcined at 400 ◦ C for 2 h.
TiO2 is an n-type semiconductor [7,8]. First oxygen molecules in air are adsorbed on the surface due to chemical adsorption, electrons move along the semiconductor surface towards the adsorbed oxygen, and a layer make up of space electric charges forms on the surface of the semiconductor. This process reduces the concentration of electrons on the surface and therefore, reduces the conductivity of the semiconductor. When TMA molecules are present, the adsorbed oxygen on the TiO2 surface reacts with TMA (see the mechanism above). This leads to a decrease in the amount of adsorbed oxygen and makes more free electrons in the surface region. As a result, the conductivity of the semiconductor oxide increases. Fig. 4 shows the response to TMA of the indirect-heating sensor made of different types of nanocrystalline TiO2 . This graph reveals that the anatase-type has a higher TMA response than the rutile-type. This can be explained by different crystal structure of the two types of nanocrystalline TiO2 . It was shown in earlier researches that the amount of adsorbed O on the surface of anatase-type TiO2 is more than that on the surface of rutiletype TiO2 [11]. It is considered that there are more active centers
372
B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373
Doping of nanocrystalline TiO2 with In2 O3 lowers its resistance and catalyzes the reaction of TMA with O2 because In2 O3 activates TMA and increases the reaction rate with oxygen. However, the doping with In2 O3 does not change the shape, size, or surface structure of the TiO2 particles. Therefore, the TMA sensitivity of TiO2 doped with In2 O3 is not much improved compared with that of pure TiO2 [15]. Nanocrystalline TiO2 doped with Cr2 O3 becomes a p-type semiconductor, which is not sensitive to TMA [16]. SnO2 actually lowers the sensitivity and selectivity of the material, because SnO2 tends to adsorb a variety of different gases and prevents TiO2 from adsorbing TMA. 4. Conclusion
Fig. 5. The effect of doping on the TMA response of TiO2 materials: (A) doped with 5 wt% of AgBr; (B) doped with 5 wt% of In2 O3 ; (C) doped with 5 wt% of SnO2 ; and (D) doped with 5 wt% of Cr2 O3 .
for reactions with TMA molecules on the anatase than on the rutile. And the anatase-type nanocrystalline TiO2 was formed only at a calcination temperature of 400 ◦ C for 2 h. The resulting particle size was about 19 nm, and the specific area was about 93 m2 /g. On the other hand, the rutile-type nanocrystalline TiO2 was formed at calcination temperatures above 700 ◦ C, and the resulting particle size was about 75 nm with a specific area of about 18 m2 /g. The specific area of the anatase particles is larger than that of the rutile, so that the anatase can adsorb more TMA on the surface than the rutile. It is known that doping in nanocrystalline materials can affect their surface conditions and electric properties. For example, doping a certain kind of metal ions into TiO2 can change its oxygen vacancy concentration and its electric properties such as resistance. This enhances the sensitivity of the sensor to target gases and also improves the response and recovery behavior [13,14]. Fig. 5 is the TMA response of TiO2 materials, doped with different metal ions and calcined at 500 ◦ C for 2 h. The figure shows that the sample doped with AgBr has the highest TMA response. When calcined at 500 ◦ C, the doping AgBr converts to metallic Ag, which is hard to be incorporated into the crystal lattice of TiO2 [8–10]. It is shown in literature [12] that the work function of metallic Ag is greater than that of semiconductor TiO2. When they are in contact, a Schottky barrier is created which facilitates the transfer of electrons from TiO2 to Ag. Thereby, this can increase oxygen vacancies on the oxide surface and create positive-charged centers which preferentially adsorb TMA molecules due to the presence of the non-bonded electron pair of the N atom in (CH3 )3 N. Doping with AgBr can also prevent TiO2 particle size from increasing [8–10], so that the doped sample has a larger specific area and could adsorb more target molecules, compared with the samples without doping. As a result the reaction is sped up and the TMA response is improved.
A stearic acid–gel method was very simple to synthesize nanocrystalline TiO2 materials and did not generate hard conglomerates. Anatase-type nanocrystalline TiO2 materials showed higher TMA response than rutile. The TMA response of the nanocrystalline TiO2 materials was affected by doping. Proper doping with other metal oxides could raise the TMA response. The doping also improved the response and recovery times, reduced the sensor resistance, and raised the selectivity against disturbing gases. Doping with AgBr showed the best TMA response among several other metal ions. Acknowledgements This work is supported by the National Natural Science Foundation of China (60374048). The authors are grateful to Professor Baokun Xu and Tong Zhang for help. References [1] H. Tsuji, H. Sugahara, Y. Gotoh, J. Ishikawa, Interactions with materials and atoms, Nucl. Instrum. Methods B 206 (2003) 249–253. [2] J. Hammond, B. Marquis, R. Michaels, B. Oickle, B. Segee, J. Vetelino, A. Bushway, M.E. Camire, K. Davis-Dentici, A semiconducting metaloxide array for monitoring fish freshness, Sens. Actuators B 84 (2002) 113–122. [3] N.O. Savage, S.A. Akbar, P.K. Dutta, Titanium dioxide based high temperature carbon monoxide selective sensor, Sens. Actuators B 72 (2001) 239–248. [4] Y. Yamada, Y. Seno, Y. Masuoka, T. Nakamura, K. Yamashita, NO2 sensing characteristics of Nb doped TiO2 thin films and their electronic properties, Sens. Actuators B 66 (2000) 164–166. [5] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism in Nb doped titania oxygen gas sensor, Sens. Actuators B 46 (1998) 194–201. [6] S.P. Ruan, W. Dong, F.Q. Wu, Dielectric properties of nanocrystalline BaTiO3 , Acta Phys. Chim. Sinica 19 (2003) 17–20. [7] Y. Nakato, H. Akanuma, J. Shimizu, Y. Magari, Photo-oxidation reaction of water on an n-TiO2 electrode: improvement in efficiency through formation of surface micropores by photo-etching in H2 SO4 , J. Electroanal. Chem. 396 (1995) 35–39. [8] L.Y. Su, Z.H. Lu, Photochromic and photocatalytic behaviors on immobilized TiO2 particulate films, J. Photochem. Photobiol. A 107 (1997) 245–248. [9] H.E. Chao, Y.U. Yun, H.U. Xingfang, A. Larbot, Effect of silver doping on the phase transformation and grain growth of sol–gel titania powder, J. Eur. Ceram. Soc. 23 (2003) 1457–1464.
B. Zou et al. / Sensors and Actuators B 119 (2006) 370–373 [10] L.M. Arabatzis, T. Stergiopoulos, M.C. Bernard, D. Labou, S.G. Neophytides, P. Falaras, Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange, Appl. Catal. B: Environ. 42 (2003) 187–201. [11] S.P. Ruan, F.Q. Wu, T. Zhang, W. Gao, B.K. Xu, M.Y. Zhao, Surface state studies of TiO2 nanoparticle and photocatalyic degradation of methyl orange in aqueous TiO2 dispersions, Mater. Chem. Phys. 69 (2001) 7–9. [12] T.T.Y. Tan, C.K. Yip, D. Beydoun, R. Amal, Effects of nano-Ag particles loading on TiO2 photocatalytic reduction of selenate ions, Chem. Eng. J. 95 (2003) 179–186. [13] J. Schmal, Defect-pool model for doped a-Si:H, J. Non-Cryst. Solids 198–200 (1996) 387–390. [14] Y.Q. Wang, H.M. Cheng, L. Zhang, Y.Z. Hao, J.M. Ma, B. Xu, W.H. Li, The preparation, characterization, photoelectrochemical and photocatalytic properties of lanthanide metal-ion-doped TiO2 nanoparticles, J. Mol. Catal. A Chem. 151 (2000) 205–216. [15] J.R. Sambrano, L.A. Vasconcellos, J.B.L. Martins, M.R.C. Santos, E. Longo, A. Beltran, A theoretical analysis on electronic structure of the (1 1 0) surface of TiO2 –SnO2 mixed oxide, J. Mol. Struct. (Theochem) 629 (2003) 307–314. [16] Y.X. Li, W. Wlodarski, K. Galatsis, S.H. Moslih, J. Cole, S. Russo, N. Rockelmann, Gas sensing properties of p-type semiconducting Cr-doped TiO2 thin films, Sens. Actuators B 83 (2002) 160–163.
373
Biographies Bo Zou received the bachelor degree in chemistry at Jilin University in 2002. As PhD student, he is working at same university since 2005. His current research activities are centred on the fields of functional nanomaterials. Fenqin Wu has been graduated from the Jilin University in 1976. She is working at Jilin University since 1977. In the following year, she is mainly devoting to the research of functional nanomaterials. Cheng Chen received the bachelor degree in chemistry at Jilin University in 2002. As PhD student, he is working at same university since 2005. His current research activities are centred on the fields of functional nanomaterials. Hui Ren received the bachelor degree in College of Biological and Agricultural Engineering at Jilin University in 1984. His current research activities are centred on the fields of nanomaterials. Zhuyi Wang received the bachelor degree in chemistry at Jilin University in 2004. She is working at same university since 2005. Her current research activities are centred on the fields of functional nanomaterials. Lehui Zou has been graduated from the Jilin University in 1972. He is working at Jilin University since 1972.