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Gas sensing properties of W6 +–TiO2 thin film prepared by R.F. magnetron sputtering based on PN junction substrate
Surface & Coatings Technology 228 (2013) S77–S80
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Gas sensing properties of W 6 +–TiO2 thin film prepared by R.F. magnetron sputtering based on PN junction substrate Huijun Zhang a, Tingyu Han a, Jingliang Cheng a, Tooru Harigai b, Akimitsu Hatta b, Dianzhong Wen a,⁎ a b
School of Electronics Engineering, Heilongjiang University, Harbin, 150080, PR China Dept. of Electronic and Photonic Systems Engineering, Kochi University of Technology, Kochi, 782-8502, Japan
a r t i c l e
i n f o
Available online 19 August 2012 Keywords: R.F. magnetron sputter O2 sensors PN junction Sensing response
a b s t r a c t The techniques outlined here offer new means for preparing novel metal-oxide gas sensor with structural substrate which is compatible with IC technology. N-type W6+–TiO2 gas-sensing layers have been deposited on PN junction substrate by R.F. magnetron sputtering. Structural characterization of TiO2 thin films has been analyzed by XRD and AFM in order to correlate physical properties with gas sensing performance. XRD and AFM indicate that the TiO2 keeps its anatase phase and its crystal lattice becomes larger with annealing temperature. The particle size at 400, 500 and 600 °C is 40, 60, and 100 nm, respectively. The sensors using W-doped TiO2 films showed promising gas sensing characteristics, such as low operation temperature (200 °C) and sufficient gas response, the sensitivity is 524%, respond and recover times are 11.2 s and 11.7 s, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting TiO2 films have been widely used for a plenty of applications such as dye-sensitized solar cells [1], photocatalysts [2], anti-reflection coatings [3], lithium batteries [4], self-cleaning coatings [5], and so on. In addition, TiO2 is considered to be a key material for reliable gas sensors with long life time owing to its superior chemical stability at the elevated temperatures [6,7]. Several efforts have been done to enhance the selectivity of these materials towards a specific gas. TiO2 is a high resistive n-type semiconductor, the conductivity of titanium dioxide can be improved through the introduction of proper dopants or by producing mechanical mixtures with different oxides. Particular doping metals offer the possibility to tune the selectivity of sensor devices: Pt and W are employed for oxygen detection [8,9], Nb for ethanol sensor [10], and Cu for CO sensing [11,12]. During the past few years, oxygen sensors based on the n type titanium dioxide (TiO2) films with the thickness of microns or sub-microns have been studied extensively [9–13]. Among these reported devices, the sensing principle is always based on the significant and abrupt change in resistance [8–13], and these devices are fabricated on Ti metal plate or foil. Disadvantage of the Ti metal-based devices is the difficulty in size-shrinking, due to the millimeter-scaled metal substrate, the oxide/metal interface is vulnerable to stress. Cracks are easily formed, causing mechanical failure. On considering these problems, fabrication of a nanometer-thick TiO2 film on a stiff and durable substrate with good adhesion, which is resistant to thermal and/or mechanical stress, is very important for robust oxygen-detection devices operating at special temperatures. In ⁎ Corresponding author. Tel.: +86 451 86608413; fax: +86 451 86609142. E-mail address: [email protected] (D. Wen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.023
addition, fabrication of TiO2 film oxygen sensor is of great interest to improve gas sensing performance by structured substrate [14]. In this work, we report the fabrication and gas sensing properties of a TiO2 films gas sensor based on PN junction substrate. W-inserted TiO2 have been deposited by R.F. magnetron sputtering, using Ti targets with a different number of W inserts. The annealing treatments and the electrical conductivity of thin films have been investigated. 2. Experimental MEMS (silicon-based Micro-electromechanical Systems) have adopted to manufacture c-type silicon cup on n-type b 100> singlecrystal silicon in order to cut down applied power. On the top of the silicon cup, PN junction substrate is accomplished by micro-electronic technology. Sputtering Pt film on surface of sensor structure, under the protection of the mask of photoresist, the Pt thin film is patterned to 10 μm wide resistance strip by HF(15%)/HNO3(15%)/H2O2(2%) H2O(68%) solutions at 293 K in order to achieve Pt electrodes on PN junction substrate for measuring electrode and on SiO2 film for heating electrode. The thickness of Pt electrodes was approximately 80 nm. The structure of sensor is shown in Fig. 1. The W-doped TiO2 thin films were deposited on PN junction substrates by R.F. (13.56 MHz) magnetron co-sputtering system in a high vacuum reactor. The background pressure was 2 × 10−5 Pa. The targets were metallic titanium disks with different numbers of W inserts of 60 mm in diameter. High purity oxygen was used as reactive gas and argon was used as sputtering gas (O2:Ar= 2:1). The substrate-to-target distance was determined to be 100 mm. The R.F. power of the Ti was 125 W. The deposition pressure was fixed at 0.5 Pa. The thickness of as-deposited films was 200 nm approximately, and the W content of TiO2 film is 3.4%. Before each
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H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
W-TiO2 film Pt measuring electrodes +
P Si Depletion region +
N Si Silicon cup Pt heating electrodes SiO2 Fig. 1. Structure graph of TiO2 sensor on PN junction substrate.
run, targets have been pre-sputtered in a pure argon atmosphere for 15 min to clean the surfaces of the targets. After the deposition, the as-deposited films were heated from 300 to 600 °C in a furnace at a rate of 3 °C/min to a desired temperature up to 1200 °C and maintained at that temperature for 1 h, then allowed to cool naturally, in order to avoid additional stresses or cracks in the thin layer. The crystal phase of the prepared thin films is determined by X-ray diffraction (XRD), and atomic force microscopy (AFM) is used to examine the morphology of the thin films. 3. Results and discussion 3.1. Structural properties Fig. 2 shows the XRD patterns of the W-doped TiO2 thin films. The XRD results indicate that the TiO2 thin film calcined at 300 °C is amorphous, and anatase peaks appeared at 400 °C. W–TiO2 thin films calcined at temperatures between 400 and 500 °C are the anatase phase and stimulate preferential orientation of TiO2 anatase in (101) and (004) peaks. The TiO2 thin film calcined at 600 °C is a mixture of anatase and rutile phases. Usually, the anatase phase of TiO2 thin films is formed at ~300 °C calcination temperature and is transformed into the rutile phase between 650 and 750 °C. In our experiment, W–TiO2 films are weak anatase structure at 400 °C and a mixture of anatase and rutile phases at 600 °C. These results suggest that the doped-W
Fig. 2. XRD patterns of W6+–TiO2 calcined at various temperatures.
particles in the TiO2 thin films hinder to form the anatase phase. These results suggest that the doped-W particles in the TiO2 thin films hinder to form the anatase phase, and facilitate the phase transformation from anatase to rutile structure; this is due to lower surface energy which the formed W atoms are substituted for Ti atoms of the intrinsic TiO2 at the anatase temperature. AFM micrographs of the TiO2 thin films are shown in Fig. 3. The surface morphology of the calcined thin films is quite different, indicating that calcination strongly affects the surface morphology. The surface morphology of W–TiO2 film annealed at 300 °C is quite similar to the as-deposited thin film. Even though some grains are about 30 nm in diameter, the film is the amorphous structure according to the XRD pattern. Some grain structures of about 40–50 nm in diameter are found at 400 °C. Grain structure of film annealed at 500 °C is almost the same diameter, and is about 60 nm, these grains can be confirmed as an anatase structure by XRD. The film surface annealed at 600 °C is very similar to that of the W–TiO2 film annealed at 500 °C, but the diameter of crystal grains is slightly larger (about 100 nm) and the rutile structure is found by the XRD pattern (Fig. 2). 3.2. Sensitivity properties The sensing properties of TiO2 films on PN junction substrate were studied by using a home-made gas sensor unit [15]. Through the external connections (as shown in Fig. 4), the reverse biased junction I–V characteristics were achieved by 10 V negative bias voltage. The electrical currents ISurface (from Pt electrode on P + region to P + Si region, next to TiO2 film, then to N + Si region) which changed with detected gas (ISO with detected O2 gas, ISN without detected O2 gas) and IR which was reverse biased PN junction current (constant) were measured. Using the following relation, the gas response was calculated. Sð% Þ ¼
ðI SO −I SN Þ
. ðI SN þIR Þ
Δ
100% ¼ I
. ICons tant
100%
or Sð% Þ ¼
RM
. RI
100%:
During measuring, the working temperature of sensor is limited at 200 °C. Fig. 5 shows the sensitivity versus the concentration of O2 for the TiO2 sensor at different calcined temperatures. A linear relationship between the sensitivity and concentration was obtained. The relative change of resistances can be demonstrated by comparing Fig. 5. RM and RI are the resistances upon gas exposure and the initial resistance, respectively. The sensitivity (S = RM/RI) of the TiO2 film is gradually higher with raising calcined temperatures. This result indicates that the sensitivity strongly depends on the crystalline structures and amorphous structures, and that the crystal boundary is strongly effective in blocking transportation of charge carriers inside the TiO2 film and converting carriers between O2 and TiO2 film, thus the sensitivity of the sensors is cut down. Crystalline structures of films are more perfect and crystal boundary is reduced with raising calcined temperatures (according to XRD pattern and AFM). But the resistance of the sensor calcined at 600 °C is higher than at 500 °C. This is due to the diameter of crystal grains which is slightly larger (about 100 nm, as shown in AFM), causing a nonuniform thickness of the film, a sharply high resistance ratio of relatively thin region on the film, and a higher measuring resistance. The response increases from 122% to 524% for O2 with increasing calcined temperatures, and the highest sensitivity is observed at 500 °C. Fig. 6 shows the typical response curves of TiO2 gas sensors based on a plain TiO2 film to 0–2% of O2 gases at 500 °C calcined temperatures. With on and off of O2 gases, sensors quickly respond and
H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
S79
Fig. 3. Surface morphologies by AFM of W-doped TiO2 films calcined at: (a) 300 °C, (b) 400 °C, (c) 500 °C, and (d) 600 °C.
recover, the response times and recovery times are measured to be 11.2 s and 11.7 s, respectively. 4. Conclusions W-doped TiO2 thin film calcined at 300 °C is amorphous, the anatase phase between 400 and 500 °C, and a mixture of anatase and rutile phases at 600 °C. The doped W particles in the TiO2 thin films hinder to form the anatase phase, and facilitate the phase transformation from anatase to rutile structure. Grain diameters are about 40–50 nm
Fig. 4. A schematic representation of W6+–TiO2 sensing layer: reverse biased junction.
at 400 °C and are about 60 nm at 500 °C, and can be confirmed as an anatase structure by XRD. But the diameter of crystal grains is slightly larger (about 100 nm) and the rutile structure is found by the XRD pattern at 600 °C. Gas sensor with structural PN junction substrate which is compatible with IC technology has been achieved. A linear relationship between the sensitivity and concentration was obtained.
Fig. 5. The sensitivity versus the concentration of O2 for the TiO2 sensor at different calcined temperatures.
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H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
Acknowledgment The project is supported by Youth-Innovation Fund of Heilongjiang Province (QC07C13) and Key Lab of Electronics Engineering of the College of Heilongjiang Province (DEED20100001).
References
Fig. 6. The response curves of TiO2 gas sensors to 0–2% of O2 gases calcined at a temperature of 600 °C.
The sensitivity of the TiO2 film is gradually higher with raising calcined temperatures. The response increases from 122% to 524% for O2 with increasing calcined temperatures, and the highest sensitivity is observed at 500 °C, and the response and recovery times of oxygen sensor are measured to be 11.2 s and 11.7 s, respectively.
[1] B. O'Regan, M. Grätzel, Nature 353 (1991) 737. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [3] J.Q. Xi, M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.Y. Lin, W. Liu, J.A. Smart, Nat. Photon. 1 (2007) 176. [4] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) 140. [5] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [6] H. Tang, K. Prasad, R. Sanjines, F. Levy, Sens. Actuators B: Chem. 26 (1995) 71. [7] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111. [8] Y. Xu, K. Yao, X. Zhou, Q. Cao, Sens. Actuators B: Chem. 13–14 (1993) 492. [9] A. Ruiz, J. Arbiol, A. Cirera, A. Cornet, J.R. Morante, Mater. Sci. Eng. C 19 (2002) 105. [10] G. Sberveglieri, E. Comini, G. Faglia, M.Z. Atashbar, W. Wlodarski, Sens. Actuators B: Chem. 66 (2000) 139. [11] P.K. Dutta, A. Ginwalla, B. Hogg, B.R. Patton, B. Chwieroth, Z. Liang, P. Gouma, M. Mills, S. Akbar, J. Phys. Chem. B 103 (1999) 4412. [12] N.O. Savage, S.A. Akbar, P.K. Dutta, Sens. Actuators B: Chem. 72 (2001) 239. [13] H. Kim, W. Moon, Y. Jun, S. Hong, Sens. Actuators B: Chem. 120 (2006) 63. [14] Lu. Chi, Zhi Chen, Sens. Actuators B: Chem. 140 (2009) 109. [15] S.S. Joshi, C.D. Lokhande, S.H. Han, Sens. Actuators B: Chem. 123 (2007) 240.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Gas sensing properties of W 6 +–TiO2 thin film prepared by R.F. magnetron sputtering based on PN junction substrate Huijun Zhang a, Tingyu Han a, Jingliang Cheng a, Tooru Harigai b, Akimitsu Hatta b, Dianzhong Wen a,⁎ a b
School of Electronics Engineering, Heilongjiang University, Harbin, 150080, PR China Dept. of Electronic and Photonic Systems Engineering, Kochi University of Technology, Kochi, 782-8502, Japan
a r t i c l e
i n f o
Available online 19 August 2012 Keywords: R.F. magnetron sputter O2 sensors PN junction Sensing response
a b s t r a c t The techniques outlined here offer new means for preparing novel metal-oxide gas sensor with structural substrate which is compatible with IC technology. N-type W6+–TiO2 gas-sensing layers have been deposited on PN junction substrate by R.F. magnetron sputtering. Structural characterization of TiO2 thin films has been analyzed by XRD and AFM in order to correlate physical properties with gas sensing performance. XRD and AFM indicate that the TiO2 keeps its anatase phase and its crystal lattice becomes larger with annealing temperature. The particle size at 400, 500 and 600 °C is 40, 60, and 100 nm, respectively. The sensors using W-doped TiO2 films showed promising gas sensing characteristics, such as low operation temperature (200 °C) and sufficient gas response, the sensitivity is 524%, respond and recover times are 11.2 s and 11.7 s, respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Semiconducting TiO2 films have been widely used for a plenty of applications such as dye-sensitized solar cells [1], photocatalysts [2], anti-reflection coatings [3], lithium batteries [4], self-cleaning coatings [5], and so on. In addition, TiO2 is considered to be a key material for reliable gas sensors with long life time owing to its superior chemical stability at the elevated temperatures [6,7]. Several efforts have been done to enhance the selectivity of these materials towards a specific gas. TiO2 is a high resistive n-type semiconductor, the conductivity of titanium dioxide can be improved through the introduction of proper dopants or by producing mechanical mixtures with different oxides. Particular doping metals offer the possibility to tune the selectivity of sensor devices: Pt and W are employed for oxygen detection [8,9], Nb for ethanol sensor [10], and Cu for CO sensing [11,12]. During the past few years, oxygen sensors based on the n type titanium dioxide (TiO2) films with the thickness of microns or sub-microns have been studied extensively [9–13]. Among these reported devices, the sensing principle is always based on the significant and abrupt change in resistance [8–13], and these devices are fabricated on Ti metal plate or foil. Disadvantage of the Ti metal-based devices is the difficulty in size-shrinking, due to the millimeter-scaled metal substrate, the oxide/metal interface is vulnerable to stress. Cracks are easily formed, causing mechanical failure. On considering these problems, fabrication of a nanometer-thick TiO2 film on a stiff and durable substrate with good adhesion, which is resistant to thermal and/or mechanical stress, is very important for robust oxygen-detection devices operating at special temperatures. In ⁎ Corresponding author. Tel.: +86 451 86608413; fax: +86 451 86609142. E-mail address: [email protected] (D. Wen). 0257-8972/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2012.08.023
addition, fabrication of TiO2 film oxygen sensor is of great interest to improve gas sensing performance by structured substrate [14]. In this work, we report the fabrication and gas sensing properties of a TiO2 films gas sensor based on PN junction substrate. W-inserted TiO2 have been deposited by R.F. magnetron sputtering, using Ti targets with a different number of W inserts. The annealing treatments and the electrical conductivity of thin films have been investigated. 2. Experimental MEMS (silicon-based Micro-electromechanical Systems) have adopted to manufacture c-type silicon cup on n-type b 100> singlecrystal silicon in order to cut down applied power. On the top of the silicon cup, PN junction substrate is accomplished by micro-electronic technology. Sputtering Pt film on surface of sensor structure, under the protection of the mask of photoresist, the Pt thin film is patterned to 10 μm wide resistance strip by HF(15%)/HNO3(15%)/H2O2(2%) H2O(68%) solutions at 293 K in order to achieve Pt electrodes on PN junction substrate for measuring electrode and on SiO2 film for heating electrode. The thickness of Pt electrodes was approximately 80 nm. The structure of sensor is shown in Fig. 1. The W-doped TiO2 thin films were deposited on PN junction substrates by R.F. (13.56 MHz) magnetron co-sputtering system in a high vacuum reactor. The background pressure was 2 × 10−5 Pa. The targets were metallic titanium disks with different numbers of W inserts of 60 mm in diameter. High purity oxygen was used as reactive gas and argon was used as sputtering gas (O2:Ar= 2:1). The substrate-to-target distance was determined to be 100 mm. The R.F. power of the Ti was 125 W. The deposition pressure was fixed at 0.5 Pa. The thickness of as-deposited films was 200 nm approximately, and the W content of TiO2 film is 3.4%. Before each
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H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
W-TiO2 film Pt measuring electrodes +
P Si Depletion region +
N Si Silicon cup Pt heating electrodes SiO2 Fig. 1. Structure graph of TiO2 sensor on PN junction substrate.
run, targets have been pre-sputtered in a pure argon atmosphere for 15 min to clean the surfaces of the targets. After the deposition, the as-deposited films were heated from 300 to 600 °C in a furnace at a rate of 3 °C/min to a desired temperature up to 1200 °C and maintained at that temperature for 1 h, then allowed to cool naturally, in order to avoid additional stresses or cracks in the thin layer. The crystal phase of the prepared thin films is determined by X-ray diffraction (XRD), and atomic force microscopy (AFM) is used to examine the morphology of the thin films. 3. Results and discussion 3.1. Structural properties Fig. 2 shows the XRD patterns of the W-doped TiO2 thin films. The XRD results indicate that the TiO2 thin film calcined at 300 °C is amorphous, and anatase peaks appeared at 400 °C. W–TiO2 thin films calcined at temperatures between 400 and 500 °C are the anatase phase and stimulate preferential orientation of TiO2 anatase in (101) and (004) peaks. The TiO2 thin film calcined at 600 °C is a mixture of anatase and rutile phases. Usually, the anatase phase of TiO2 thin films is formed at ~300 °C calcination temperature and is transformed into the rutile phase between 650 and 750 °C. In our experiment, W–TiO2 films are weak anatase structure at 400 °C and a mixture of anatase and rutile phases at 600 °C. These results suggest that the doped-W
Fig. 2. XRD patterns of W6+–TiO2 calcined at various temperatures.
particles in the TiO2 thin films hinder to form the anatase phase. These results suggest that the doped-W particles in the TiO2 thin films hinder to form the anatase phase, and facilitate the phase transformation from anatase to rutile structure; this is due to lower surface energy which the formed W atoms are substituted for Ti atoms of the intrinsic TiO2 at the anatase temperature. AFM micrographs of the TiO2 thin films are shown in Fig. 3. The surface morphology of the calcined thin films is quite different, indicating that calcination strongly affects the surface morphology. The surface morphology of W–TiO2 film annealed at 300 °C is quite similar to the as-deposited thin film. Even though some grains are about 30 nm in diameter, the film is the amorphous structure according to the XRD pattern. Some grain structures of about 40–50 nm in diameter are found at 400 °C. Grain structure of film annealed at 500 °C is almost the same diameter, and is about 60 nm, these grains can be confirmed as an anatase structure by XRD. The film surface annealed at 600 °C is very similar to that of the W–TiO2 film annealed at 500 °C, but the diameter of crystal grains is slightly larger (about 100 nm) and the rutile structure is found by the XRD pattern (Fig. 2). 3.2. Sensitivity properties The sensing properties of TiO2 films on PN junction substrate were studied by using a home-made gas sensor unit [15]. Through the external connections (as shown in Fig. 4), the reverse biased junction I–V characteristics were achieved by 10 V negative bias voltage. The electrical currents ISurface (from Pt electrode on P + region to P + Si region, next to TiO2 film, then to N + Si region) which changed with detected gas (ISO with detected O2 gas, ISN without detected O2 gas) and IR which was reverse biased PN junction current (constant) were measured. Using the following relation, the gas response was calculated. Sð% Þ ¼
ðI SO −I SN Þ
. ðI SN þIR Þ
Δ
100% ¼ I
. ICons tant
100%
or Sð% Þ ¼
RM
. RI
100%:
During measuring, the working temperature of sensor is limited at 200 °C. Fig. 5 shows the sensitivity versus the concentration of O2 for the TiO2 sensor at different calcined temperatures. A linear relationship between the sensitivity and concentration was obtained. The relative change of resistances can be demonstrated by comparing Fig. 5. RM and RI are the resistances upon gas exposure and the initial resistance, respectively. The sensitivity (S = RM/RI) of the TiO2 film is gradually higher with raising calcined temperatures. This result indicates that the sensitivity strongly depends on the crystalline structures and amorphous structures, and that the crystal boundary is strongly effective in blocking transportation of charge carriers inside the TiO2 film and converting carriers between O2 and TiO2 film, thus the sensitivity of the sensors is cut down. Crystalline structures of films are more perfect and crystal boundary is reduced with raising calcined temperatures (according to XRD pattern and AFM). But the resistance of the sensor calcined at 600 °C is higher than at 500 °C. This is due to the diameter of crystal grains which is slightly larger (about 100 nm, as shown in AFM), causing a nonuniform thickness of the film, a sharply high resistance ratio of relatively thin region on the film, and a higher measuring resistance. The response increases from 122% to 524% for O2 with increasing calcined temperatures, and the highest sensitivity is observed at 500 °C. Fig. 6 shows the typical response curves of TiO2 gas sensors based on a plain TiO2 film to 0–2% of O2 gases at 500 °C calcined temperatures. With on and off of O2 gases, sensors quickly respond and
H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
S79
Fig. 3. Surface morphologies by AFM of W-doped TiO2 films calcined at: (a) 300 °C, (b) 400 °C, (c) 500 °C, and (d) 600 °C.
recover, the response times and recovery times are measured to be 11.2 s and 11.7 s, respectively. 4. Conclusions W-doped TiO2 thin film calcined at 300 °C is amorphous, the anatase phase between 400 and 500 °C, and a mixture of anatase and rutile phases at 600 °C. The doped W particles in the TiO2 thin films hinder to form the anatase phase, and facilitate the phase transformation from anatase to rutile structure. Grain diameters are about 40–50 nm
Fig. 4. A schematic representation of W6+–TiO2 sensing layer: reverse biased junction.
at 400 °C and are about 60 nm at 500 °C, and can be confirmed as an anatase structure by XRD. But the diameter of crystal grains is slightly larger (about 100 nm) and the rutile structure is found by the XRD pattern at 600 °C. Gas sensor with structural PN junction substrate which is compatible with IC technology has been achieved. A linear relationship between the sensitivity and concentration was obtained.
Fig. 5. The sensitivity versus the concentration of O2 for the TiO2 sensor at different calcined temperatures.
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H. Zhang et al. / Surface & Coatings Technology 228 (2013) S77–S80
Acknowledgment The project is supported by Youth-Innovation Fund of Heilongjiang Province (QC07C13) and Key Lab of Electronics Engineering of the College of Heilongjiang Province (DEED20100001).
References
Fig. 6. The response curves of TiO2 gas sensors to 0–2% of O2 gases calcined at a temperature of 600 °C.
The sensitivity of the TiO2 film is gradually higher with raising calcined temperatures. The response increases from 122% to 524% for O2 with increasing calcined temperatures, and the highest sensitivity is observed at 500 °C, and the response and recovery times of oxygen sensor are measured to be 11.2 s and 11.7 s, respectively.
[1] B. O'Regan, M. Grätzel, Nature 353 (1991) 737. [2] R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. Taga, Science 293 (2001) 269. [3] J.Q. Xi, M.F. Schubert, J.K. Kim, E.F. Schubert, M. Chen, S.Y. Lin, W. Liu, J.A. Smart, Nat. Photon. 1 (2007) 176. [4] S.Y. Huang, L. Kavan, I. Exnar, M. Gratzel, J. Electrochem. Soc. 142 (1995) 140. [5] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [6] H. Tang, K. Prasad, R. Sanjines, F. Levy, Sens. Actuators B: Chem. 26 (1995) 71. [7] G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Crit. Rev. Solid State Mater. Sci. 29 (2004) 111. [8] Y. Xu, K. Yao, X. Zhou, Q. Cao, Sens. Actuators B: Chem. 13–14 (1993) 492. [9] A. Ruiz, J. Arbiol, A. Cirera, A. Cornet, J.R. Morante, Mater. Sci. Eng. C 19 (2002) 105. [10] G. Sberveglieri, E. Comini, G. Faglia, M.Z. Atashbar, W. Wlodarski, Sens. Actuators B: Chem. 66 (2000) 139. [11] P.K. Dutta, A. Ginwalla, B. Hogg, B.R. Patton, B. Chwieroth, Z. Liang, P. Gouma, M. Mills, S. Akbar, J. Phys. Chem. B 103 (1999) 4412. [12] N.O. Savage, S.A. Akbar, P.K. Dutta, Sens. Actuators B: Chem. 72 (2001) 239. [13] H. Kim, W. Moon, Y. Jun, S. Hong, Sens. Actuators B: Chem. 120 (2006) 63. [14] Lu. Chi, Zhi Chen, Sens. Actuators B: Chem. 140 (2009) 109. [15] S.S. Joshi, C.D. Lokhande, S.H. Han, Sens. Actuators B: Chem. 123 (2007) 240.