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TiO2 nanostructured thin films for control of drunken driving
Sensors and Actuators B 83 (2002) 230±237
Sol±gel TiO2 and W/TiO2 nanostructured thin ®lms for control of drunken driving C. Garzellaa,*, E. Cominia, E. Bontempib, L.E. Deperob, C. Frigeric, G. Sberveglieria a
I.N.F.M., Department of Chemistry and Physics for Engineering and Materials, University of Brescia, via Valotti 9, 25133 Brescia, Italy b I.N.S.T.M., Structural Chemistry Laboratory, University of Brescia, via Branze 38, 25133 Brescia, Italy c C.N.R.-MASPEC Institute, Parco Area delle Scienze 37/A, 43100 Parma, Italy
Abstract TiO2 and W/TiO2 thin ®lms have been prepared by a chemically modi®ed sol±gel technique. Oxide±polymer thin ®lms were deposited by dip-coating. Annealing at 500 8C results in nanosized structurally stable oxides ®lms. The dopant concentration led to nominal W:Ti ratio of 1:33, 5:33, 10:33. The morphological and structural characteristics of thin ®lms were studied through microraman, glancing incidence X-ray diffraction (GIXRD), SEM and TEM. A microstructural comparison between pure and doped TiO2 layers is reported. Ethanol sensing properties are tested. TiO2 and W/TiO2 structural features and sensing characteristics are compared. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol±gel; TiO2; Ethanol sensor
1. Introduction Ethanol sensors ®nd application in various areas, such as the control of drunken driving and the monitoring of fermentation and other processes in chemical industries. Deposition of oxide thin ®lm by sol±gel method has recently attracted considerable attention because it is an economical and energy saving method to deposit ®lms of precisely controlled microstructure [1]. Since the sensor community is looking for low cost sensors, our efforts were aimed in that direction and we used a novel sol±gel technique to deposit TiO2 thin ®lm for ethanol sensing. In a previous work we reported the ®rst application of the chemically modi®ed sol±gel technique in the gas sensor ®eld [2]. This technique results an effective method for performing nanosized structurally stable titanium dioxide thin ®lms. The role of polymer as ``steric stabilizer'' during the ®lm structural evolution has been described. TiO2 thin ®lms presented interesting alcohol sensing performances. In this work we undertake an experimental study to investigate the doping effects on structural features and sensing response of TiO2. It has been observed that doping with suitable cation provides the shortest route by altering electronic and catalytic properties for gas interaction at the interface [3]. Pentavalent or trivalent TiO2 dopants as Cr3 * Corresponding author. Tel.: 39-30-3715749; fax: 39-30-2091271. E-mail address: [email protected] (C. Garzella).
and Nb5 have been studied by several authors [4±6]. The improvement in the oxygen sensing properties of these materials has been explained on the basis of oxide electronic surface structure and structural properties. More recently, it was observed that TiO2 oxygen sensitivity could be enhanced with increasing Ta concentration [7]. W±Ti±O thin ®lm oxides were deposited by Depero et al. [8] by rf sputtering. They noted that the addition of Ti into the W±O oxide inhibited the formation of coarse-grained structure and led to a formation of a high surface to volume layer. An enhanced gas sensitivity was observed. We have deposited tungsten doped TiO2 thin ®lms using the chemically modi®ed sol±gel technique. To our knowledge this dopant has never been studied for sol±gel titanium dioxide in the gas sensor ®eld. The relationships between the morphology, crystalline structure and chemical composition of these ®lms and the gas sensing properties will be discussed. 2. Fabrication 2.1. Synthesis of TiO2 and W/TiO2 solutions TiO2 sol preparation was made using the chemically modi®ed sol±gel procedure proposed by Nagpal et al. [9] which implies hydrolysis and condensation of tetraethylortotitanate (TEOT) in the presence of hydroxypropylcellulose
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 1 0 4 6 - 2
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(HPC) dissolved in ethanol. Concentration of TEOT and water were 0.02 and 2.3 M, respectively, the addition of HPC led to a ®nal TiO2:HPC ratio of 48:52 wt.%. Under these conditions a full conversion of TEOT is obtained. Equal volumes of TEOT in anhydrous ethanol were mixed with solutions of HPC in ethanol and water. The TEOT solution was prepared by dripping TEOT in ethanol under nitrogen. The mixture of the two solutions was stirred for 24 h at ambient temperature and concentrated at 70 8C. For doping tungsten(V) ethoxide was used in three different concentrations that led to a ®nal nominal atomic ratio W:Ti as 1:33, 5:33 and 10:33. The dopant was added to solution of TEOT in ethanol under nitrogen, and then hydrolysed at the same time of TEOT. 2.2. Film formation For ®lm formation the alumina support (3 mm 3 mm) with platinum interdigitated system and already bonded gold wires were dipped and immersed into the TiO2±HPC or W/TiO2±HPC solutions. Silicon substrates were used in order to prepare samples for TEM analyses. The ®lms were then dried at 160 8C for 1 h, heated (30 8C/h) up to 500 8C an held at this temperature for 5 h in air. The ®lm deposition and annealing were repeated four times. The organic matrix was completely removed after annealing as just checked and reported in the previous work [2]: ®lms of pure oxide were obtained.
technique, applying a constant potential of 1 V to the sensing layer and registering with a picoammeter the conductance variation as a function of time. Either the gas ¯ow or the electrical parameters were controlled by a PC, which also registers the conductance change of the sensors. 3. Results 3.1. Microstructural characterization SEM micrographs of pure and W doped TiO2 ®lms showed that the ®lms are uniform and nanostructured. As just observed for pure TiO2 [2], the microstructure of W doped TiO2 ®lms was stable after the annealing treatment. TEM micrographs of pure and W doped TiO2 ®lms were observed in order to compare the particles size of the ®lms (Figs. 1 and 2). Bare TiO2 had particles of 3±30 nm. Doped TiO2 has more de®ned and homogeneous structure with particles size of 5±20 nm. The maximum particles size are therefore lower in the latter: the doping agent allows to change the oxide particles growth. This fact was just observed by Depero et al. [8] for TiO2 thin ®lms obtained by rf sputtering. Fig. 3 shows the GIXRD spectra of pure TiO2 and W/TiO2 sample with a nominal W:Ti atomic ratio of 5:33, collected with an incidence angle ®xed at 0.58. The GIXRD spectra showed for all the samples the TiO2 anatase peaks. No other
2.3. Film characterization The microstructural characterization of pure and doped TiO2 thin ®lms was performed by SEM, TEM, microraman and glancing incidence X-ray diffraction (GIXRD). SEM observations were carried out with a Cambridge 360 microscope with beam energies in the 10±25 keV range. TEM analyzer was a JEOL 2000FX and operated at 2000 keV. The GIXRD data were collected by a Bruker ``D8 Advance'' diffractometer equipped with a GoÈbel mirror. The angular accuracy was 0.0018 and the angular resolution was better than 0.018. Microraman spectra were collected by a Dilor Labram spectrograph. The spectra were measured at room temperature. The gas sensing response of these materials were tested toward two different kinds of alcohol (methanol and ethanol) and to some possible interfering gases as nitrogen dioxide and carbon monoxide. The technique used was the ¯owtrough technique that was reported by Sberveglieri et al. [10] elsewhere. A constant ¯ux of synthetic air of 0.3 l/min was used as gas carrier and the desired concentrations of pollutants were obtained mixing the certi®ed gas with the synthetic air. All the measures were made under controlled relative humidity (RH 30 or 50%) and the temperature of the chamber containing the sensors was set at 20 8C. The changes in the electrical properties due to variations of the atmosphere were registered with the Volt Amperometric
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Fig. 1. TEM micrograph of TiO2 thin film (bar 20 nm).
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phases were detected, suggesting that the W atoms were substitutional to Ti in the anatase structure. The average crystallite sizes were calculated by means of the P. Topas program [11]. The results are in accord to TEM observations: pure TiO2 has grains size of about 30 nm and doped TiO2 has grains size lower than 20 nm. Since the size of coherently diffracting domains is always lower than 100 nm, all the ®lms result to be nanostructured. Fig. 4 shows the microraman spectrum of pure TiO2 thin ®lm. The pattern presents three intense bands at 400, 515 and 640 cm 1. These bands are characteristic of TiO2 anatase phase. All the microraman spectra of the samples, with different W content, present similar patterns to that obtained for pure TiO2 thin ®lm. 3.2. Electrical characterization Gas test measurements were made under costant temperature conditions for ethanol and methanol varing the gases concentration in the range 100±600 ppm at 30 and 50% of RH The working temperature was varied from 300 to 500 8C. Possible interfering gases as nitrogen dioxide and carbon monoxide were tested to complete these gas tests.
Fig. 2. TEM micrographs of W/TiO2 thin film (W:Ti ratio of 1:33) (bar 20 nm).
3.2.1. Gas response to ethanol Fig. 5 shows the dynamic variation of the current ¯owing through the W/TiO2 ®lm with a nominal atomic W:Ti ratio
Fig. 3. GIXRD spectra of pure TiO2 (bottom curve) and W/TiO2 sample with a nominal W:Ti atomic ratio of 5:33 (upper curve), collected with an incidence angle fixed at 0.58. Vertical bars indicate the Al2O3 substrate contribution and stars indicate film contribution.
C. Garzella et al. / Sensors and Actuators B 83 (2002) 230±237
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Fig. 4. Microraman spectrum of pure TiO2 thin film.
of 5:33 when increasing concentration of ethanol were introduced in the test chamber at 300 8C, RH 30%. The relative variation of the conductance is very high in these tests, 2100% for 600 ppm of ethanol and already for 100 ppm is 400%. This concentration is well below the limit imposed for breath analyzer (200 ppm) [12]. The measurements highlight that these thin ®lms are stable and that the recovery of the signal is complete when the air ¯ux is restored after the gas test.
The dynamic of these sensors is very fast as it can be noticed. The response and recovery times are de®ned as the times the conductance takes to reach 90% of (Rf R0 ) when the gas is introduced and to recover 30% of (Rf R0 ) when the ¯ux of air is restored. The response and the recovery times are about 1 min both for these sensors and for the ones doped at different concentration. Similar times values are obtained at 400 8C: as just observed for pure TiO2 ®lms the working temperature does not in¯uence the response time [2].
Fig. 5. Dynamic variation of the current flowing through the W/TiO2 (W:Ti ratio of 5:33) when increasing concentration of ethanol was introduced in the test chamber at 300 8C at 30% RH
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C. Garzella et al. / Sensors and Actuators B 83 (2002) 230±237
Fig. 6. Responses of two different W/TiO2 films (a and b) with a nominal atomic W:Ti ratio of 5:33 through increasing concentration of EtOH at 300 and 400 8C at 30% RH
Fig. 7. Response of three different W/TiO2 films (W:Ti ratio of 1:33, 5:33, 10:33) as function of EtOH concentration at the working temperature of 300 8C, RH 30%.
3.2.2. The response as a function of temperature The responses of two different W/TiO2 ®lms (a and b) with a nominal atomic W:Ti ratio of 5:33 are reported in Fig. 6 as function of ethanol concentration for two different temperatures (300 and 400 8C, RH 30%). We noted that the response got a rise increasing the temperature up to 400 8C and then decreased. This fact was observed for all the W/ TiO2 thin ®lms, whereas for the bare TiO2 the highest response was obtained at 500 8C [2]. 3.2.3. Comparison between different doped W/TiO2 films W/TiO2 ®lms doped with increasing concentration of W are then compared. Fig. 7 reports the responses obtained for three different ®lms (W:Ti atomic ratios: 1:33, 5:33, 10:33) through increasing concentrations of ethanol at 300 8C and
30% RH. We noted a considerable enhancement in the response performances as W concentration increased. The response obtained for the ®lm with the W:Ti atomic ratio of 10:33 through 600 ppm of EtOH at 300 8C was as high as 18,000%. Table 1 A and B coefficients for three different W/TiO2 sensors (W:Ti 1:33, 5:33, 10:33), resulted from the fits with a power low (DG/G A[gas]B, where concentration of the gas is expressed in ppm) W:Ti
A
B
1:33 5:33 10:33
0.028 0.009 0.067
0.95 1.28 1.25
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Fig. 8. Dynamic variation of the current flowing through the W/TiO2 thin films (W:Ti ratio of 1:33) when 100 ppm of ethanol are introduced in the test chamber at two different values of RH at 400 8C.
Fig. 9. Comparison of responses of W/TiO2 thin films through two different alcohols: (a) methanol (300 8C, 30% RH); (b) ethanol (300 8C, 30% RH).
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3.2.4. Modelling Consideration on sensitivity of these thin ®lms towards ethanol can be made by plotting the response, de®ned as relative change in the conductance, as a function of the concentration, as in Fig. 7. The dependence of the response towards the concentration of the gas follows the well-known power behavior DG/G A[gas]B for these W/TiO2 thin ®lms. Three different ®ts were made with this kind of power expression, and the ®tting parameters obtained for the different doped W/TiO2 thin ®lms of Fig. 7 are reported in Table 1. The slope of the lines in the logarithmic scale is similar for the three ®lms. 3.2.5. Detection limit Potentially these sensors could detect a concentration of ethanol as low as 20 ppm with a relative variation of 30%. We did not investigate this point yet because it is not interesting for breath analysers. 3.2.6. Cross sensitivity The in¯uence on the response amplitude of RH of the environment atmosphere has been tested for the target gas. Fig. 8 plots the different responses of two W/TiO2 ®lms (W:Ti 1:33) to 100 ppm of ethanol at 400 8C at two different values of RH 30 and 50%. The response varies about 14% in the two ambient, which is <30 times the variation due to the presence of 100 ppm of ethanol (400%). Methanol was tested besides to evaluate the possible interfering effect of this common alcohol with ethanol. The response comparison between the two gases at 300 8C is reported in Fig. 9 for different doped thin ®lms. We noted that the response to MeOH is even quite an half less than the response to EtOH. This ``discrimination'' between two alcohols is more pronounced in the W/TiO2 ®lms than in the previous tested TiO2 ®lms [2]. In order to complete these gas measurements we tested possible interfering gases as NO2 and CO in concentrations of 1±3 and 15±30 ppm, respectively. The responses to these gases, in high concentration with respect to the alarm level for indoor and outdoor conditions, were more than one order of magnitude lower than the responses to the alcohols. 4. Conclusions TiO2 and W/TiO2 thin ®lms with increasing W content were made via the chemically modi®ed sol±gel technique by dip-coating onto alumina substrates. All the layers prepared resulted nanostructured, but we observed that the doping determined smaller oxide grain sizes. Different doped W/ TiO2 thin ®lms presented the same anatase phase as bare TiO2, and no other phases are detected by GIXRD and microraman analyses. This allows us to suppose that W is substitutional to Ti in the TiO2 structure. TiO2 and W/TiO2 thin ®lms presented interesting ethanol sensing performances. We found a high response in the
presence of concentrations of ethanol well below the limit imposed for breath analyzers (100 ppm versus 200 ppm) [12]. The best responses were obtained for W/TiO2 sensors at the temperature of 400 8C, whereas for bare TiO2 resulted 500 8C [2]. That indicates that doping with W allows to operate with a lower sensor working temperature. Otherwise a considerable enhancement in alcohol sensing was noted increasing the sensors W content. These behaviors may be ascribed to the microstructural change of the oxide. Indeed the smaller crystalline sizes obtained with doping implies a higher surface to volume ratio. These W/TiO2 thin ®lms showed very high ethanol response compared with those already presented for TiO2 [13] and SnO2 sensors [14]. Furthermore the responses toward possible interfering gases were more than one order of magnitude lower than the responses to the alcohols. We performed a low cost ethanol-sensing device that could be employed for control of drunken driving. References [1] Y. Takahashi, Y. Wada, Dip coating of Sb-doped SnO2 films by ethanolamine-alkoxide method, J. Electrochem. Soc. 137 (1990) 267±272. [2] C. Garzella, E. Comini, E. Tempesti, C. Frigeri, G. Sberveglieri, TiO2 thin film by a novel sol±gel processing for gas sensor applications, Sens. Actuators B 68 (2000) 189. [3] A. Akami, T. Matsuura, S. Miyata, K. Furusaki, Y. Wantanabe, Effect of precious metal catalyst in TiO thick film HEGO sensor with multilayer alumina substrates, Soc. Automot. Eng. 870 (290) (1988) 1.1183±1.1192. [4] K. Zakrzeka, M. Radecka, M. Rekas, Effect of Nb, Cr, Sn additions on gas sensing properties of TiO2 thin films, Thin Solid Films 310 (1997) 161±166. [5] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism of highly sensitive and fast response Cr doped TiO2 oxygen gas sensor, Sens. Actuators B 45 (1997) 209±215. [6] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism in Nb doped titania oxygen gas sensor, Sens. Actuators B 46 (1998) 194± 201. [7] K. Kajihara, T. Yao, Oxigen detection in sol±gel derived titania thin films doped with tantalum, Phys. Chem. Chem. Phys. 1 (1999) 1979± 1983. [8] L.E. Depero, M. Ferroni, V. Guidi, G. Marca, G. Martinelli, P. Nelli, L. Sangaletti, G. Sberveglieri, Preparation and microstructural characterization of nanosized thin film of WO3±TiO2 as novel material with high sensitivity towards NO2, Sens. Actuators B 35/36 (1996) 381±383. [9] V.J. Nagpal, R.M. Davis, S.B. Desu, Novel thin films of titanium dioxide particles synthesised by a sol±gel process, J. Mater. Res. 10 (12) (1995) 3068±3078. [10] G. Sberveglieri, L.E. Depero, S. Groppelli, P. Nelli, WO3 sputtered thin films for NOx monitoring, Sens. Actuators B 26/27 (1995) 89±92. [11] P. Topas, Copyright Bruker AXS Version 1.0.1, 1999. [12] T. Brousse, D.M. Schleich, Sprayed and thermally evaporated SnO2 thin film for ethanol sensors, Sens. Actuators B 31 (1996) 77±79. [13] G. Sberveglieri, M.Z. Atashbar, Y. Li, W. Wlodarski, E. Comini, G. Faglia, M.K. Ghantasala, Nanocristalline TiO2 thin films prepared by the sol±gel process for alcohol sensing, in: Proceedings of the 10th International Conference on Solid-State Sensors and Actuators (Transducers '99), Senday, Japan, June 7±10 1999, pp. 1690±1693.
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Biographies Carlotta Garzella was born on 14 June, 1972. She received graduate degree in chemistry at University of Milan in 1997. She is now concluding the PhD in materials science at the University of Brescia, where she is working on chemical sensors with particular interest to the sol±gel technology for oxide thin film deposition and to the chemico-physical characterization of the oxide surfaces. Elisabetta Comini was born in 1972. She received graduate degree in physics at University of Pisa in 1996. She is presently, working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterization of MOS thin films. She received a PhD in material science at the University of Brescia. Elza Bontempi holds a degree in engineering in 1996. After a period study at the CNRS of Grenoble, where she worked on X-ray magnetic resonant reflectivity in the Raoux group. She received a PhD in materials for engineering application at the Brescia University with a dissertation on X-ray reflectivity and glancing incidence X-ray diffraction for thin film characterization. At the moment she has a permanent position at the Brescia University. Laura E. Depero has a background in solid state physics and physical chemistry. She received her degree in solid state physics (full marks) at the University of Milano in 1984. From January 1985 to April 1987 she had a permanent position at the Electron Microscopy Laboratory of the Quality Control Department of the GTE Telecomunication Company (Cassina de Pecchi, Milan). From April 1987 to August 1988 she was Visiting Scientist at the IBM Almaden (California) Research Laboratories in the group led
237
by Dr. William Parrish. From September 1988 to 1989 she obtained a fellowship at the Mechanical Department of the University of Brescia, where she performed XRD studies on the stress analysis of metallic samples. Actually she is Full Professor in Chemistry in Brescia. Giorgio Sberveglieri was born on 17 July 1947 and received his degree in physics from the University Parma, where starting in 1971 his research activities on the preparation of semiconducting thin film solar cells. He has been appointed Associate Professor at the University of Brescia in 1987. In the following year he established the Thin Film Laboratory, afterwards called Gas Sensor Lab, mainly devoted to the preparation and characterization of thin film chemical sensors, he is the Director of the GSL, since 1988. In 1994 he has been appointed Professor in Physics, formerly at the Faculty of Engineering of University of Ferrara and then in 1996 at the Faculty of Engineering of University of Brescia. He is referee of the journals Thin Solid Films, Sensors and Actuators, Sensors and Materials and other journals and member of scientific Committee of conferences on Sensor and Materials Science. During his 25 years of scientific activities, Giorgio Sberveglieri published more than 140 papers on International Reviews, he presented more than 50 oral communications to international congresses and numerous oral communications to national congresses. Cesare Frigeri got his PhD degree in solid state physics at the University of Bologna. For 8 years he was a research scientist in Agip Nucleare where he performed researches in the field of sintering of ceramic materials and relationship between mechanical properties and crystalline textures in metal alloys. In 1982, he joined CNR-MASPEC Institute in Parma where he is senior scientist. Since then his activity has dealt with the study of extended defects by electron microscopy, their interaction with point defects and their correlation with growth parameters in both bulk and epitaxial III±V compound semiconductors, as well as with the development of SEM and TEM methods for defect analysis in such compounds. He is the author of 85 publications in international journals and proceedings with international refereeing committee. He has given six invited talks in international conferences and is member of the scientific or steering committee of two international conferences.
Sol±gel TiO2 and W/TiO2 nanostructured thin ®lms for control of drunken driving C. Garzellaa,*, E. Cominia, E. Bontempib, L.E. Deperob, C. Frigeric, G. Sberveglieria a
I.N.F.M., Department of Chemistry and Physics for Engineering and Materials, University of Brescia, via Valotti 9, 25133 Brescia, Italy b I.N.S.T.M., Structural Chemistry Laboratory, University of Brescia, via Branze 38, 25133 Brescia, Italy c C.N.R.-MASPEC Institute, Parco Area delle Scienze 37/A, 43100 Parma, Italy
Abstract TiO2 and W/TiO2 thin ®lms have been prepared by a chemically modi®ed sol±gel technique. Oxide±polymer thin ®lms were deposited by dip-coating. Annealing at 500 8C results in nanosized structurally stable oxides ®lms. The dopant concentration led to nominal W:Ti ratio of 1:33, 5:33, 10:33. The morphological and structural characteristics of thin ®lms were studied through microraman, glancing incidence X-ray diffraction (GIXRD), SEM and TEM. A microstructural comparison between pure and doped TiO2 layers is reported. Ethanol sensing properties are tested. TiO2 and W/TiO2 structural features and sensing characteristics are compared. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Sol±gel; TiO2; Ethanol sensor
1. Introduction Ethanol sensors ®nd application in various areas, such as the control of drunken driving and the monitoring of fermentation and other processes in chemical industries. Deposition of oxide thin ®lm by sol±gel method has recently attracted considerable attention because it is an economical and energy saving method to deposit ®lms of precisely controlled microstructure [1]. Since the sensor community is looking for low cost sensors, our efforts were aimed in that direction and we used a novel sol±gel technique to deposit TiO2 thin ®lm for ethanol sensing. In a previous work we reported the ®rst application of the chemically modi®ed sol±gel technique in the gas sensor ®eld [2]. This technique results an effective method for performing nanosized structurally stable titanium dioxide thin ®lms. The role of polymer as ``steric stabilizer'' during the ®lm structural evolution has been described. TiO2 thin ®lms presented interesting alcohol sensing performances. In this work we undertake an experimental study to investigate the doping effects on structural features and sensing response of TiO2. It has been observed that doping with suitable cation provides the shortest route by altering electronic and catalytic properties for gas interaction at the interface [3]. Pentavalent or trivalent TiO2 dopants as Cr3 * Corresponding author. Tel.: 39-30-3715749; fax: 39-30-2091271. E-mail address: [email protected] (C. Garzella).
and Nb5 have been studied by several authors [4±6]. The improvement in the oxygen sensing properties of these materials has been explained on the basis of oxide electronic surface structure and structural properties. More recently, it was observed that TiO2 oxygen sensitivity could be enhanced with increasing Ta concentration [7]. W±Ti±O thin ®lm oxides were deposited by Depero et al. [8] by rf sputtering. They noted that the addition of Ti into the W±O oxide inhibited the formation of coarse-grained structure and led to a formation of a high surface to volume layer. An enhanced gas sensitivity was observed. We have deposited tungsten doped TiO2 thin ®lms using the chemically modi®ed sol±gel technique. To our knowledge this dopant has never been studied for sol±gel titanium dioxide in the gas sensor ®eld. The relationships between the morphology, crystalline structure and chemical composition of these ®lms and the gas sensing properties will be discussed. 2. Fabrication 2.1. Synthesis of TiO2 and W/TiO2 solutions TiO2 sol preparation was made using the chemically modi®ed sol±gel procedure proposed by Nagpal et al. [9] which implies hydrolysis and condensation of tetraethylortotitanate (TEOT) in the presence of hydroxypropylcellulose
0925-4005/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 4 0 0 5 ( 0 1 ) 0 1 0 4 6 - 2
C. Garzella et al. / Sensors and Actuators B 83 (2002) 230±237
(HPC) dissolved in ethanol. Concentration of TEOT and water were 0.02 and 2.3 M, respectively, the addition of HPC led to a ®nal TiO2:HPC ratio of 48:52 wt.%. Under these conditions a full conversion of TEOT is obtained. Equal volumes of TEOT in anhydrous ethanol were mixed with solutions of HPC in ethanol and water. The TEOT solution was prepared by dripping TEOT in ethanol under nitrogen. The mixture of the two solutions was stirred for 24 h at ambient temperature and concentrated at 70 8C. For doping tungsten(V) ethoxide was used in three different concentrations that led to a ®nal nominal atomic ratio W:Ti as 1:33, 5:33 and 10:33. The dopant was added to solution of TEOT in ethanol under nitrogen, and then hydrolysed at the same time of TEOT. 2.2. Film formation For ®lm formation the alumina support (3 mm 3 mm) with platinum interdigitated system and already bonded gold wires were dipped and immersed into the TiO2±HPC or W/TiO2±HPC solutions. Silicon substrates were used in order to prepare samples for TEM analyses. The ®lms were then dried at 160 8C for 1 h, heated (30 8C/h) up to 500 8C an held at this temperature for 5 h in air. The ®lm deposition and annealing were repeated four times. The organic matrix was completely removed after annealing as just checked and reported in the previous work [2]: ®lms of pure oxide were obtained.
technique, applying a constant potential of 1 V to the sensing layer and registering with a picoammeter the conductance variation as a function of time. Either the gas ¯ow or the electrical parameters were controlled by a PC, which also registers the conductance change of the sensors. 3. Results 3.1. Microstructural characterization SEM micrographs of pure and W doped TiO2 ®lms showed that the ®lms are uniform and nanostructured. As just observed for pure TiO2 [2], the microstructure of W doped TiO2 ®lms was stable after the annealing treatment. TEM micrographs of pure and W doped TiO2 ®lms were observed in order to compare the particles size of the ®lms (Figs. 1 and 2). Bare TiO2 had particles of 3±30 nm. Doped TiO2 has more de®ned and homogeneous structure with particles size of 5±20 nm. The maximum particles size are therefore lower in the latter: the doping agent allows to change the oxide particles growth. This fact was just observed by Depero et al. [8] for TiO2 thin ®lms obtained by rf sputtering. Fig. 3 shows the GIXRD spectra of pure TiO2 and W/TiO2 sample with a nominal W:Ti atomic ratio of 5:33, collected with an incidence angle ®xed at 0.58. The GIXRD spectra showed for all the samples the TiO2 anatase peaks. No other
2.3. Film characterization The microstructural characterization of pure and doped TiO2 thin ®lms was performed by SEM, TEM, microraman and glancing incidence X-ray diffraction (GIXRD). SEM observations were carried out with a Cambridge 360 microscope with beam energies in the 10±25 keV range. TEM analyzer was a JEOL 2000FX and operated at 2000 keV. The GIXRD data were collected by a Bruker ``D8 Advance'' diffractometer equipped with a GoÈbel mirror. The angular accuracy was 0.0018 and the angular resolution was better than 0.018. Microraman spectra were collected by a Dilor Labram spectrograph. The spectra were measured at room temperature. The gas sensing response of these materials were tested toward two different kinds of alcohol (methanol and ethanol) and to some possible interfering gases as nitrogen dioxide and carbon monoxide. The technique used was the ¯owtrough technique that was reported by Sberveglieri et al. [10] elsewhere. A constant ¯ux of synthetic air of 0.3 l/min was used as gas carrier and the desired concentrations of pollutants were obtained mixing the certi®ed gas with the synthetic air. All the measures were made under controlled relative humidity (RH 30 or 50%) and the temperature of the chamber containing the sensors was set at 20 8C. The changes in the electrical properties due to variations of the atmosphere were registered with the Volt Amperometric
231
Fig. 1. TEM micrograph of TiO2 thin film (bar 20 nm).
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phases were detected, suggesting that the W atoms were substitutional to Ti in the anatase structure. The average crystallite sizes were calculated by means of the P. Topas program [11]. The results are in accord to TEM observations: pure TiO2 has grains size of about 30 nm and doped TiO2 has grains size lower than 20 nm. Since the size of coherently diffracting domains is always lower than 100 nm, all the ®lms result to be nanostructured. Fig. 4 shows the microraman spectrum of pure TiO2 thin ®lm. The pattern presents three intense bands at 400, 515 and 640 cm 1. These bands are characteristic of TiO2 anatase phase. All the microraman spectra of the samples, with different W content, present similar patterns to that obtained for pure TiO2 thin ®lm. 3.2. Electrical characterization Gas test measurements were made under costant temperature conditions for ethanol and methanol varing the gases concentration in the range 100±600 ppm at 30 and 50% of RH The working temperature was varied from 300 to 500 8C. Possible interfering gases as nitrogen dioxide and carbon monoxide were tested to complete these gas tests.
Fig. 2. TEM micrographs of W/TiO2 thin film (W:Ti ratio of 1:33) (bar 20 nm).
3.2.1. Gas response to ethanol Fig. 5 shows the dynamic variation of the current ¯owing through the W/TiO2 ®lm with a nominal atomic W:Ti ratio
Fig. 3. GIXRD spectra of pure TiO2 (bottom curve) and W/TiO2 sample with a nominal W:Ti atomic ratio of 5:33 (upper curve), collected with an incidence angle fixed at 0.58. Vertical bars indicate the Al2O3 substrate contribution and stars indicate film contribution.
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Fig. 4. Microraman spectrum of pure TiO2 thin film.
of 5:33 when increasing concentration of ethanol were introduced in the test chamber at 300 8C, RH 30%. The relative variation of the conductance is very high in these tests, 2100% for 600 ppm of ethanol and already for 100 ppm is 400%. This concentration is well below the limit imposed for breath analyzer (200 ppm) [12]. The measurements highlight that these thin ®lms are stable and that the recovery of the signal is complete when the air ¯ux is restored after the gas test.
The dynamic of these sensors is very fast as it can be noticed. The response and recovery times are de®ned as the times the conductance takes to reach 90% of (Rf R0 ) when the gas is introduced and to recover 30% of (Rf R0 ) when the ¯ux of air is restored. The response and the recovery times are about 1 min both for these sensors and for the ones doped at different concentration. Similar times values are obtained at 400 8C: as just observed for pure TiO2 ®lms the working temperature does not in¯uence the response time [2].
Fig. 5. Dynamic variation of the current flowing through the W/TiO2 (W:Ti ratio of 5:33) when increasing concentration of ethanol was introduced in the test chamber at 300 8C at 30% RH
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Fig. 6. Responses of two different W/TiO2 films (a and b) with a nominal atomic W:Ti ratio of 5:33 through increasing concentration of EtOH at 300 and 400 8C at 30% RH
Fig. 7. Response of three different W/TiO2 films (W:Ti ratio of 1:33, 5:33, 10:33) as function of EtOH concentration at the working temperature of 300 8C, RH 30%.
3.2.2. The response as a function of temperature The responses of two different W/TiO2 ®lms (a and b) with a nominal atomic W:Ti ratio of 5:33 are reported in Fig. 6 as function of ethanol concentration for two different temperatures (300 and 400 8C, RH 30%). We noted that the response got a rise increasing the temperature up to 400 8C and then decreased. This fact was observed for all the W/ TiO2 thin ®lms, whereas for the bare TiO2 the highest response was obtained at 500 8C [2]. 3.2.3. Comparison between different doped W/TiO2 films W/TiO2 ®lms doped with increasing concentration of W are then compared. Fig. 7 reports the responses obtained for three different ®lms (W:Ti atomic ratios: 1:33, 5:33, 10:33) through increasing concentrations of ethanol at 300 8C and
30% RH. We noted a considerable enhancement in the response performances as W concentration increased. The response obtained for the ®lm with the W:Ti atomic ratio of 10:33 through 600 ppm of EtOH at 300 8C was as high as 18,000%. Table 1 A and B coefficients for three different W/TiO2 sensors (W:Ti 1:33, 5:33, 10:33), resulted from the fits with a power low (DG/G A[gas]B, where concentration of the gas is expressed in ppm) W:Ti
A
B
1:33 5:33 10:33
0.028 0.009 0.067
0.95 1.28 1.25
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Fig. 8. Dynamic variation of the current flowing through the W/TiO2 thin films (W:Ti ratio of 1:33) when 100 ppm of ethanol are introduced in the test chamber at two different values of RH at 400 8C.
Fig. 9. Comparison of responses of W/TiO2 thin films through two different alcohols: (a) methanol (300 8C, 30% RH); (b) ethanol (300 8C, 30% RH).
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3.2.4. Modelling Consideration on sensitivity of these thin ®lms towards ethanol can be made by plotting the response, de®ned as relative change in the conductance, as a function of the concentration, as in Fig. 7. The dependence of the response towards the concentration of the gas follows the well-known power behavior DG/G A[gas]B for these W/TiO2 thin ®lms. Three different ®ts were made with this kind of power expression, and the ®tting parameters obtained for the different doped W/TiO2 thin ®lms of Fig. 7 are reported in Table 1. The slope of the lines in the logarithmic scale is similar for the three ®lms. 3.2.5. Detection limit Potentially these sensors could detect a concentration of ethanol as low as 20 ppm with a relative variation of 30%. We did not investigate this point yet because it is not interesting for breath analysers. 3.2.6. Cross sensitivity The in¯uence on the response amplitude of RH of the environment atmosphere has been tested for the target gas. Fig. 8 plots the different responses of two W/TiO2 ®lms (W:Ti 1:33) to 100 ppm of ethanol at 400 8C at two different values of RH 30 and 50%. The response varies about 14% in the two ambient, which is <30 times the variation due to the presence of 100 ppm of ethanol (400%). Methanol was tested besides to evaluate the possible interfering effect of this common alcohol with ethanol. The response comparison between the two gases at 300 8C is reported in Fig. 9 for different doped thin ®lms. We noted that the response to MeOH is even quite an half less than the response to EtOH. This ``discrimination'' between two alcohols is more pronounced in the W/TiO2 ®lms than in the previous tested TiO2 ®lms [2]. In order to complete these gas measurements we tested possible interfering gases as NO2 and CO in concentrations of 1±3 and 15±30 ppm, respectively. The responses to these gases, in high concentration with respect to the alarm level for indoor and outdoor conditions, were more than one order of magnitude lower than the responses to the alcohols. 4. Conclusions TiO2 and W/TiO2 thin ®lms with increasing W content were made via the chemically modi®ed sol±gel technique by dip-coating onto alumina substrates. All the layers prepared resulted nanostructured, but we observed that the doping determined smaller oxide grain sizes. Different doped W/ TiO2 thin ®lms presented the same anatase phase as bare TiO2, and no other phases are detected by GIXRD and microraman analyses. This allows us to suppose that W is substitutional to Ti in the TiO2 structure. TiO2 and W/TiO2 thin ®lms presented interesting ethanol sensing performances. We found a high response in the
presence of concentrations of ethanol well below the limit imposed for breath analyzers (100 ppm versus 200 ppm) [12]. The best responses were obtained for W/TiO2 sensors at the temperature of 400 8C, whereas for bare TiO2 resulted 500 8C [2]. That indicates that doping with W allows to operate with a lower sensor working temperature. Otherwise a considerable enhancement in alcohol sensing was noted increasing the sensors W content. These behaviors may be ascribed to the microstructural change of the oxide. Indeed the smaller crystalline sizes obtained with doping implies a higher surface to volume ratio. These W/TiO2 thin ®lms showed very high ethanol response compared with those already presented for TiO2 [13] and SnO2 sensors [14]. Furthermore the responses toward possible interfering gases were more than one order of magnitude lower than the responses to the alcohols. We performed a low cost ethanol-sensing device that could be employed for control of drunken driving. References [1] Y. Takahashi, Y. Wada, Dip coating of Sb-doped SnO2 films by ethanolamine-alkoxide method, J. Electrochem. Soc. 137 (1990) 267±272. [2] C. Garzella, E. Comini, E. Tempesti, C. Frigeri, G. Sberveglieri, TiO2 thin film by a novel sol±gel processing for gas sensor applications, Sens. Actuators B 68 (2000) 189. [3] A. Akami, T. Matsuura, S. Miyata, K. Furusaki, Y. Wantanabe, Effect of precious metal catalyst in TiO thick film HEGO sensor with multilayer alumina substrates, Soc. Automot. Eng. 870 (290) (1988) 1.1183±1.1192. [4] K. Zakrzeka, M. Radecka, M. Rekas, Effect of Nb, Cr, Sn additions on gas sensing properties of TiO2 thin films, Thin Solid Films 310 (1997) 161±166. [5] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism of highly sensitive and fast response Cr doped TiO2 oxygen gas sensor, Sens. Actuators B 45 (1997) 209±215. [6] R.K. Sharma, M.C. Bhatnagar, G.L. Sharma, Mechanism in Nb doped titania oxygen gas sensor, Sens. Actuators B 46 (1998) 194± 201. [7] K. Kajihara, T. Yao, Oxigen detection in sol±gel derived titania thin films doped with tantalum, Phys. Chem. Chem. Phys. 1 (1999) 1979± 1983. [8] L.E. Depero, M. Ferroni, V. Guidi, G. Marca, G. Martinelli, P. Nelli, L. Sangaletti, G. Sberveglieri, Preparation and microstructural characterization of nanosized thin film of WO3±TiO2 as novel material with high sensitivity towards NO2, Sens. Actuators B 35/36 (1996) 381±383. [9] V.J. Nagpal, R.M. Davis, S.B. Desu, Novel thin films of titanium dioxide particles synthesised by a sol±gel process, J. Mater. Res. 10 (12) (1995) 3068±3078. [10] G. Sberveglieri, L.E. Depero, S. Groppelli, P. Nelli, WO3 sputtered thin films for NOx monitoring, Sens. Actuators B 26/27 (1995) 89±92. [11] P. Topas, Copyright Bruker AXS Version 1.0.1, 1999. [12] T. Brousse, D.M. Schleich, Sprayed and thermally evaporated SnO2 thin film for ethanol sensors, Sens. Actuators B 31 (1996) 77±79. [13] G. Sberveglieri, M.Z. Atashbar, Y. Li, W. Wlodarski, E. Comini, G. Faglia, M.K. Ghantasala, Nanocristalline TiO2 thin films prepared by the sol±gel process for alcohol sensing, in: Proceedings of the 10th International Conference on Solid-State Sensors and Actuators (Transducers '99), Senday, Japan, June 7±10 1999, pp. 1690±1693.
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Biographies Carlotta Garzella was born on 14 June, 1972. She received graduate degree in chemistry at University of Milan in 1997. She is now concluding the PhD in materials science at the University of Brescia, where she is working on chemical sensors with particular interest to the sol±gel technology for oxide thin film deposition and to the chemico-physical characterization of the oxide surfaces. Elisabetta Comini was born in 1972. She received graduate degree in physics at University of Pisa in 1996. She is presently, working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterization of MOS thin films. She received a PhD in material science at the University of Brescia. Elza Bontempi holds a degree in engineering in 1996. After a period study at the CNRS of Grenoble, where she worked on X-ray magnetic resonant reflectivity in the Raoux group. She received a PhD in materials for engineering application at the Brescia University with a dissertation on X-ray reflectivity and glancing incidence X-ray diffraction for thin film characterization. At the moment she has a permanent position at the Brescia University. Laura E. Depero has a background in solid state physics and physical chemistry. She received her degree in solid state physics (full marks) at the University of Milano in 1984. From January 1985 to April 1987 she had a permanent position at the Electron Microscopy Laboratory of the Quality Control Department of the GTE Telecomunication Company (Cassina de Pecchi, Milan). From April 1987 to August 1988 she was Visiting Scientist at the IBM Almaden (California) Research Laboratories in the group led
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by Dr. William Parrish. From September 1988 to 1989 she obtained a fellowship at the Mechanical Department of the University of Brescia, where she performed XRD studies on the stress analysis of metallic samples. Actually she is Full Professor in Chemistry in Brescia. Giorgio Sberveglieri was born on 17 July 1947 and received his degree in physics from the University Parma, where starting in 1971 his research activities on the preparation of semiconducting thin film solar cells. He has been appointed Associate Professor at the University of Brescia in 1987. In the following year he established the Thin Film Laboratory, afterwards called Gas Sensor Lab, mainly devoted to the preparation and characterization of thin film chemical sensors, he is the Director of the GSL, since 1988. In 1994 he has been appointed Professor in Physics, formerly at the Faculty of Engineering of University of Ferrara and then in 1996 at the Faculty of Engineering of University of Brescia. He is referee of the journals Thin Solid Films, Sensors and Actuators, Sensors and Materials and other journals and member of scientific Committee of conferences on Sensor and Materials Science. During his 25 years of scientific activities, Giorgio Sberveglieri published more than 140 papers on International Reviews, he presented more than 50 oral communications to international congresses and numerous oral communications to national congresses. Cesare Frigeri got his PhD degree in solid state physics at the University of Bologna. For 8 years he was a research scientist in Agip Nucleare where he performed researches in the field of sintering of ceramic materials and relationship between mechanical properties and crystalline textures in metal alloys. In 1982, he joined CNR-MASPEC Institute in Parma where he is senior scientist. Since then his activity has dealt with the study of extended defects by electron microscopy, their interaction with point defects and their correlation with growth parameters in both bulk and epitaxial III±V compound semiconductors, as well as with the development of SEM and TEM methods for defect analysis in such compounds. He is the author of 85 publications in international journals and proceedings with international refereeing committee. He has given six invited talks in international conferences and is member of the scientific or steering committee of two international conferences.