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Quartz resonator with thin TiO2 for NH3 detection
ARTICLE IN PRESS
Vacuum 76 (2004) 203–206 www.elsevier.com/locate/vacuum
Quartz resonator with thin TiO2 for NH3 detection V. Georgievaa,, P. Stevchevb, P. Vitanovb, L. Spassova a Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
b
Abstract The sorption properties of TiO2 thin films are investigated, in order to detect ammonium vapor existence. For the purpose mass-sensitive quartz resonators covered by TiO2 film are used. TiO2 thin films are prepared using a new method called liquid phase deposition (LPD). By this method metal oxide layers are deposited on surfaces dipped in solutions. The layers are obtained as a result of a balance reaction, which happens between the metal fluorocomplex and metal oxide in aqueous solution. By this process substances with complex morphology are easily coated using no special equipment. The main advantage of this method is its low synthesis temperature–up to 200 1C. The sorption sensitiveness of the created structure is registered by the Quartz Crystal Microbalance (QCM) method. An interval of ammonium concentration (from 100 to 10,000 ppm) is investigated. A dependence is obtained between relative change of resonator frequency (Df/f) and ammonium solution concentration. The obtained initial results show that this sort of systems could be used for creating sensor elements aiming the detection of ammonium contamination in the air. r 2004 Elsevier Ltd. All rights reserved. Keywords: TiO2; Quartz resonator; Quartz crystal microbalance
1. Introduction Recently, the different modifications of TiO2 as active elements of various types of gas sensors are investigated. The studies in this direction are based on TiO2 surface chemical activity and sorption properties. Corresponding author. Fax: +359-2-975-3632
E-mail address: [email protected] (V. Georgieva).
In this paper, [1] the possibility of nanocrystalline TiO2 film sensitivity towards ethanol and methanol to be improved by Nb and Pt doping are investigated. The films were prepared using the sol–gel process by the spin coating technique. The sensing characteristics indicated that doped TiO2 films exhibit enhanced sensitivity compared with undoped ones. Thin films of TiO2 power dispersed in poly (venylidenfluoride) are used, too, as sensors for
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.014
ARTICLE IN PRESS 204
V. Georgieva et al. / Vacuum 76 (2004) 203–206
benzene, ethanol and methanol vapors at room temperature. In this way, concentration over the range 150–350 ppm are determined [2]. Two different sensing materials, TiO2 and WO3, are investigated as a TiO2–WO3 system for NO2 determination. The sorption and sensitivity properties of complex films with nanometer grains are improved compared to those of pure TiO2 and WO3 films [3]. The gas sensing properties of platinum-dispersed TiO2 thin films are studied, too. These thin films show high selectivity to trimethylamine and ammonium [4]. The aim of this paper is to present the primary investigation on the sorption properties of thin TiO2 films prepared by the liquid-phase deposition process to ammonium vapors. The sorption process is esimated by the method of quartz crystal microbalance (QCM).
result the working solution is prepared: 0.1 M (NH4)2TiF6 and 0.2 M H3BO3. The quartz resonators are immersed vertically in the working solution for 30, 60 and 90 min, respectively. The solution temperature is kept 5071 1C. Its increase results in a substantial acceleration of the LPD process. At room temperature the LPD TiO2 layer formation will take more than 16 h [8]. The TiO2 samples are annealed at 2001C in N2 atmosphere for 1 h. The dielectric coatings thus formed are transparent and exhibit high adhesion to the substrate. The thickness of the reference samples is limited by the process duration and is measured by Taylor Hobson Talystep. The obtained values are 40, 110 and 160 nm for duration of LPD process 30, 60 and 90 min, respectively.
2. Experimental
3. Results and discussion
Recently, a new wet process for preparation of metal oxide thin films has been developed and it is called LPD process [5]. In this process, metal oxide thin films can be deposited on immersed substrates by using a chemical equilibrium reaction between the metal fluorocomplex ions and metal oxide in aqueous solution. For the hydrolysis of [TiF6]2 ion in aqueous solution the following equilibrium scheme has been proposed by Schmitt et al. [6]:
The studied sensor elements are made by TiO2 LPD deposition method on quartz resonators. The latter have been realized on AT-cut polished quartz plates of 8 mm diameter and with electrodes of 4 mm diameter. The Ag metal electrodes are 1200 A˚ thick and deposited by vacuum evaporation. The resonator frequency is 14 MHz.The experimental equipment is described in detail in Ref. [9]. To determine the sorption abilities of the formed structures a method is used based on the dependence between the additional mass of the sorbed NH3 vapors, charging the sensitive resonator, and the shift of resonant frequency. The method is known as the quartz crystal microbalance (QCM). The dependence between the additional mass and the frequency shift in case of AT-cut of quartz is given by equation (1) of Ref. [10].
½TiF6 2 þ nH2 O ¼ TiF6n ðOHÞ2 þ nHF: The equilibrium can be changed by adding H3BO3 as F scavenger into the solution. The boric acid (H3BO3) reacts with F ion and forms more a stable complex as H3 BO3 þ 4HF ¼ HBF4 þ 3H2 O: The addition of H3BO3 leads to the consumption of non-coordinated F ions and accelerates the hydrolysis reaction [7]. Two solutions are the starting materials for the LPD process–a 0.2 M aqueous solution of (NH4)2TiF6 with pH=5.6 and a 0.4 M aqueous solution of H3BO3. Those are mixed immediately before the experiment in 1:1 proportion and as a
Df ¼ 2:26 106 f 2p Dm=S L ; where fp is the resonant frequency in MHz, Df is resonant frequency change in Hz, Dm is the additional sorbed mass in grams, SL is the covered area in cm2.
ARTICLE IN PRESS V. Georgieva et al. / Vacuum 76 (2004) 203–206
The sorption sensitivity of such TiO2 films to NH3 solutions of different concentration is evaluated as difference in frequency measured at saturation above H2O vapors and the one at saturation above the corresponding NH3 concentration. On the basis of the dependence obtained it can be presumed that there are three types of sorption mechanisms. The first is simply a surface one, characterized by the highest velocity; the second has limited surface and bulk sorption and the third includes diffusion to the film depth, which has the lowest velocity.
13993540 13993520
Frequency, Hz
Each of the sensor elements (bulk quartz resonators with a thin film of TiO2) has been kept in NH3 aqueous solutions of various concentrations. To avoid temperature influence on resonant frequency stability and NH3 solution the system is tempered at 26 7 0.51C. The resonant frequency shift is measured as a result of NH3 sorption. Thus, the possibility of different concentrations that can be registered is evaluated. Each sensor element is initially saturated to water vapor in order to avoid their influence on NH3 sorption. The typical time-frequency dependence of a quartz resonator with a 110 nm thick TiO2 film is shown in Fig. 1. Frequency changes with a velocity of 5 102 Hz/s and saturation, i.e. a dynamic equilibrium between sorbed and desorbed water molecules, is reached for a time of the order of 300 s. After the sensor element has been saturated to water vapors, it is subsequently put and kept above aqueous solution of various NH3 concentrations. The typical time-frequency characteristics of a quartz resonator with a TiO2 film are shown in Fig. 2. The TiO2 film thickness is 110 nm, the resonator is kept in NH3 vapor ambient of 5000 ppm solution concentration. The course of the curve illustrates three different sorption regions, which determine three different velocities in frequency decrease as: 8 102, 1.5 102 and 0.36 102 Hz/s.
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13993500 13993480 13993460 13993440 0
1000
2000
3000
4000
5000
6000
Time, s Fig. 2. Time–frequency characteristics of quartz resonator with thin TiO2 film over 5000 ppm ammonia solution. The thickness of the TiO2 film is 110 nm.
24
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20
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16
∆f/f.10
Frequency, Hz
13993955
13993950
13993945
12
1
8 4
13993940 0
100
200
300
400
500
600
Time, s Fig. 1. Time–frequency characteristics of quartz resonator with thin TiO2 film in water vapor during saturation. The thickness of the TiO2 film is 110 nm.
100
1000
Ammonia concentration, ppm
10000
Fig. 3. Relative change of resonator frequency versus ammonia concentration. 1—quartz resonator with 45 nm thin TiO2 film, 2—quartz resonator with 110 nm thin TiO2 film.
ARTICLE IN PRESS 206
V. Georgieva et al. / Vacuum 76 (2004) 203–206
Fig. 3 illustrates the changes in the relative frequency of the explored structures as a function of NH3 concentration variations in the interval of 100–10,000 ppm. The quartz resonators covered with thin TiO2 film thickness of 40, 110 and 160 nm are examined. Curve 1 shows the changes of Df/f for resonator structures with TiO2 thickness of 40 nm, curve 2 concern the one with 110 nm thick TiO2 film. Obviously, with ammonium concentration grown Df/f expresses a tendency for increasing over the whole investigated interval, i.e., the quantity of NH3 sorbed grows. The dependence of Df/f on the sensitive TiO2 film thickness is weekly expressed at low concentrations (100 ppm) and it is rather stronger for the higher ones. While for TiO2 thickness of 40 nm Df =f ¼ 2:8 106 ; for 110 nm it is 4.2 106, for NH3 concentration of 100 ppm. For higher concentrations (5000 ppm) the values for Df/f are 7 106 and 20 106, respectively. For 160 nm thick TiO2 films single measurements for Df/f show a rapid growth to 15 106 at NH3 concentration of 100 ppm. The growth of Df/f with film thickness at one and the same NH3 concentration confirms our supposition for the existence of a third sorption mechanism—the diffusion one. The treated sensor elements restore their initial resonant frequency values a few minutes after the measurements stop and no additional thermal treatment is necessary. In this case a physical sorption can be presumed.
3. Conclusions The results obtained in this work show that the structure quartz resonator covered with thin LPD deposited TiO2 film sorbs NH3 vapor. It makes the sensor element interesting for further studies. The main advantage of the studied system is the low temperature of TiO2 synthesis and stabilization by the LPD method (200 1C). This makes them compatible with the quartz resonator technology and, hence, the use of the QSM method for pollutant registration in the ambient.
References [1] Comini E, Faglia G, Sberveglieri G, Li Y, Wlodarski W, Ghantasala M. Sensors Activators 2000;64:169–74. [2] Mabrook M, Hawkins P. Sensors Activators 2001;73:197–202. [3] Lee D, Han S, Lee S, Huh J, Lee D. Sensors Activators 2000;65:331–5. [4] Hayakawa I, Iwamoto Y, Kikuto K, Hirano S. Sensors Activators 2000;62:55–60. [5] Hishinuma A, Goda T, Kitaoka M, Hayashi S, Kawahara H. Appl Surf Sci 1991;49:405–9. [6] Schmitt R, Glove E, Brown R. J Am Chem Soc 1960;82:5292–4. [7] Wamser C. J Am Chem Soc 1951;73:409–55. [8] Richardson T, Rubin M. Electrochem acta 2001;46:2119–23. [9] Lazarova V, Spassov L, Georgiev V, Andreev S, Manolov E, Popova L. Vacuum 1996;47:1423–5. [10] Malov VV. Piezoresonansnie datchiki. Moskow: Energoatomizdat; 1989 p. 68 (in Russian).
Vacuum 76 (2004) 203–206 www.elsevier.com/locate/vacuum
Quartz resonator with thin TiO2 for NH3 detection V. Georgievaa,, P. Stevchevb, P. Vitanovb, L. Spassova a Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria Central Laboratory of Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria
b
Abstract The sorption properties of TiO2 thin films are investigated, in order to detect ammonium vapor existence. For the purpose mass-sensitive quartz resonators covered by TiO2 film are used. TiO2 thin films are prepared using a new method called liquid phase deposition (LPD). By this method metal oxide layers are deposited on surfaces dipped in solutions. The layers are obtained as a result of a balance reaction, which happens between the metal fluorocomplex and metal oxide in aqueous solution. By this process substances with complex morphology are easily coated using no special equipment. The main advantage of this method is its low synthesis temperature–up to 200 1C. The sorption sensitiveness of the created structure is registered by the Quartz Crystal Microbalance (QCM) method. An interval of ammonium concentration (from 100 to 10,000 ppm) is investigated. A dependence is obtained between relative change of resonator frequency (Df/f) and ammonium solution concentration. The obtained initial results show that this sort of systems could be used for creating sensor elements aiming the detection of ammonium contamination in the air. r 2004 Elsevier Ltd. All rights reserved. Keywords: TiO2; Quartz resonator; Quartz crystal microbalance
1. Introduction Recently, the different modifications of TiO2 as active elements of various types of gas sensors are investigated. The studies in this direction are based on TiO2 surface chemical activity and sorption properties. Corresponding author. Fax: +359-2-975-3632
E-mail address: [email protected] (V. Georgieva).
In this paper, [1] the possibility of nanocrystalline TiO2 film sensitivity towards ethanol and methanol to be improved by Nb and Pt doping are investigated. The films were prepared using the sol–gel process by the spin coating technique. The sensing characteristics indicated that doped TiO2 films exhibit enhanced sensitivity compared with undoped ones. Thin films of TiO2 power dispersed in poly (venylidenfluoride) are used, too, as sensors for
0042-207X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2004.07.014
ARTICLE IN PRESS 204
V. Georgieva et al. / Vacuum 76 (2004) 203–206
benzene, ethanol and methanol vapors at room temperature. In this way, concentration over the range 150–350 ppm are determined [2]. Two different sensing materials, TiO2 and WO3, are investigated as a TiO2–WO3 system for NO2 determination. The sorption and sensitivity properties of complex films with nanometer grains are improved compared to those of pure TiO2 and WO3 films [3]. The gas sensing properties of platinum-dispersed TiO2 thin films are studied, too. These thin films show high selectivity to trimethylamine and ammonium [4]. The aim of this paper is to present the primary investigation on the sorption properties of thin TiO2 films prepared by the liquid-phase deposition process to ammonium vapors. The sorption process is esimated by the method of quartz crystal microbalance (QCM).
result the working solution is prepared: 0.1 M (NH4)2TiF6 and 0.2 M H3BO3. The quartz resonators are immersed vertically in the working solution for 30, 60 and 90 min, respectively. The solution temperature is kept 5071 1C. Its increase results in a substantial acceleration of the LPD process. At room temperature the LPD TiO2 layer formation will take more than 16 h [8]. The TiO2 samples are annealed at 2001C in N2 atmosphere for 1 h. The dielectric coatings thus formed are transparent and exhibit high adhesion to the substrate. The thickness of the reference samples is limited by the process duration and is measured by Taylor Hobson Talystep. The obtained values are 40, 110 and 160 nm for duration of LPD process 30, 60 and 90 min, respectively.
2. Experimental
3. Results and discussion
Recently, a new wet process for preparation of metal oxide thin films has been developed and it is called LPD process [5]. In this process, metal oxide thin films can be deposited on immersed substrates by using a chemical equilibrium reaction between the metal fluorocomplex ions and metal oxide in aqueous solution. For the hydrolysis of [TiF6]2 ion in aqueous solution the following equilibrium scheme has been proposed by Schmitt et al. [6]:
The studied sensor elements are made by TiO2 LPD deposition method on quartz resonators. The latter have been realized on AT-cut polished quartz plates of 8 mm diameter and with electrodes of 4 mm diameter. The Ag metal electrodes are 1200 A˚ thick and deposited by vacuum evaporation. The resonator frequency is 14 MHz.The experimental equipment is described in detail in Ref. [9]. To determine the sorption abilities of the formed structures a method is used based on the dependence between the additional mass of the sorbed NH3 vapors, charging the sensitive resonator, and the shift of resonant frequency. The method is known as the quartz crystal microbalance (QCM). The dependence between the additional mass and the frequency shift in case of AT-cut of quartz is given by equation (1) of Ref. [10].
½TiF6 2 þ nH2 O ¼ TiF6n ðOHÞ2 þ nHF: The equilibrium can be changed by adding H3BO3 as F scavenger into the solution. The boric acid (H3BO3) reacts with F ion and forms more a stable complex as H3 BO3 þ 4HF ¼ HBF4 þ 3H2 O: The addition of H3BO3 leads to the consumption of non-coordinated F ions and accelerates the hydrolysis reaction [7]. Two solutions are the starting materials for the LPD process–a 0.2 M aqueous solution of (NH4)2TiF6 with pH=5.6 and a 0.4 M aqueous solution of H3BO3. Those are mixed immediately before the experiment in 1:1 proportion and as a
Df ¼ 2:26 106 f 2p Dm=S L ; where fp is the resonant frequency in MHz, Df is resonant frequency change in Hz, Dm is the additional sorbed mass in grams, SL is the covered area in cm2.
ARTICLE IN PRESS V. Georgieva et al. / Vacuum 76 (2004) 203–206
The sorption sensitivity of such TiO2 films to NH3 solutions of different concentration is evaluated as difference in frequency measured at saturation above H2O vapors and the one at saturation above the corresponding NH3 concentration. On the basis of the dependence obtained it can be presumed that there are three types of sorption mechanisms. The first is simply a surface one, characterized by the highest velocity; the second has limited surface and bulk sorption and the third includes diffusion to the film depth, which has the lowest velocity.
13993540 13993520
Frequency, Hz
Each of the sensor elements (bulk quartz resonators with a thin film of TiO2) has been kept in NH3 aqueous solutions of various concentrations. To avoid temperature influence on resonant frequency stability and NH3 solution the system is tempered at 26 7 0.51C. The resonant frequency shift is measured as a result of NH3 sorption. Thus, the possibility of different concentrations that can be registered is evaluated. Each sensor element is initially saturated to water vapor in order to avoid their influence on NH3 sorption. The typical time-frequency dependence of a quartz resonator with a 110 nm thick TiO2 film is shown in Fig. 1. Frequency changes with a velocity of 5 102 Hz/s and saturation, i.e. a dynamic equilibrium between sorbed and desorbed water molecules, is reached for a time of the order of 300 s. After the sensor element has been saturated to water vapors, it is subsequently put and kept above aqueous solution of various NH3 concentrations. The typical time-frequency characteristics of a quartz resonator with a TiO2 film are shown in Fig. 2. The TiO2 film thickness is 110 nm, the resonator is kept in NH3 vapor ambient of 5000 ppm solution concentration. The course of the curve illustrates three different sorption regions, which determine three different velocities in frequency decrease as: 8 102, 1.5 102 and 0.36 102 Hz/s.
205
13993500 13993480 13993460 13993440 0
1000
2000
3000
4000
5000
6000
Time, s Fig. 2. Time–frequency characteristics of quartz resonator with thin TiO2 film over 5000 ppm ammonia solution. The thickness of the TiO2 film is 110 nm.
24
13993960
2
20
-6
16
∆f/f.10
Frequency, Hz
13993955
13993950
13993945
12
1
8 4
13993940 0
100
200
300
400
500
600
Time, s Fig. 1. Time–frequency characteristics of quartz resonator with thin TiO2 film in water vapor during saturation. The thickness of the TiO2 film is 110 nm.
100
1000
Ammonia concentration, ppm
10000
Fig. 3. Relative change of resonator frequency versus ammonia concentration. 1—quartz resonator with 45 nm thin TiO2 film, 2—quartz resonator with 110 nm thin TiO2 film.
ARTICLE IN PRESS 206
V. Georgieva et al. / Vacuum 76 (2004) 203–206
Fig. 3 illustrates the changes in the relative frequency of the explored structures as a function of NH3 concentration variations in the interval of 100–10,000 ppm. The quartz resonators covered with thin TiO2 film thickness of 40, 110 and 160 nm are examined. Curve 1 shows the changes of Df/f for resonator structures with TiO2 thickness of 40 nm, curve 2 concern the one with 110 nm thick TiO2 film. Obviously, with ammonium concentration grown Df/f expresses a tendency for increasing over the whole investigated interval, i.e., the quantity of NH3 sorbed grows. The dependence of Df/f on the sensitive TiO2 film thickness is weekly expressed at low concentrations (100 ppm) and it is rather stronger for the higher ones. While for TiO2 thickness of 40 nm Df =f ¼ 2:8 106 ; for 110 nm it is 4.2 106, for NH3 concentration of 100 ppm. For higher concentrations (5000 ppm) the values for Df/f are 7 106 and 20 106, respectively. For 160 nm thick TiO2 films single measurements for Df/f show a rapid growth to 15 106 at NH3 concentration of 100 ppm. The growth of Df/f with film thickness at one and the same NH3 concentration confirms our supposition for the existence of a third sorption mechanism—the diffusion one. The treated sensor elements restore their initial resonant frequency values a few minutes after the measurements stop and no additional thermal treatment is necessary. In this case a physical sorption can be presumed.
3. Conclusions The results obtained in this work show that the structure quartz resonator covered with thin LPD deposited TiO2 film sorbs NH3 vapor. It makes the sensor element interesting for further studies. The main advantage of the studied system is the low temperature of TiO2 synthesis and stabilization by the LPD method (200 1C). This makes them compatible with the quartz resonator technology and, hence, the use of the QSM method for pollutant registration in the ambient.
References [1] Comini E, Faglia G, Sberveglieri G, Li Y, Wlodarski W, Ghantasala M. Sensors Activators 2000;64:169–74. [2] Mabrook M, Hawkins P. Sensors Activators 2001;73:197–202. [3] Lee D, Han S, Lee S, Huh J, Lee D. Sensors Activators 2000;65:331–5. [4] Hayakawa I, Iwamoto Y, Kikuto K, Hirano S. Sensors Activators 2000;62:55–60. [5] Hishinuma A, Goda T, Kitaoka M, Hayashi S, Kawahara H. Appl Surf Sci 1991;49:405–9. [6] Schmitt R, Glove E, Brown R. J Am Chem Soc 1960;82:5292–4. [7] Wamser C. J Am Chem Soc 1951;73:409–55. [8] Richardson T, Rubin M. Electrochem acta 2001;46:2119–23. [9] Lazarova V, Spassov L, Georgiev V, Andreev S, Manolov E, Popova L. Vacuum 1996;47:1423–5. [10] Malov VV. Piezoresonansnie datchiki. Moskow: Energoatomizdat; 1989 p. 68 (in Russian).