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Hydrogen sensing using titania nanotubes
Sensors and Actuators B 93 (2003) 338–344
Hydrogen sensing using titania nanotubes Oomman K. Varghese, Dawei Gong, Maggie Paulose, Keat G. Ong, Craig A. Grimes* Department of Electrical Engineering and Materials Research Institute, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
Abstract Titanium dioxide nanotubes, made by anodization, are highly sensitive to hydrogen; for example, cycling between nitrogen atmosphere and 1000 ppm hydrogen a variation in measured resistance of 103 is seen for 46 nm diameter nanotubes at 290 8C. The hydrogen sensors are completely reversible and have response times of approximately 150 s. Field emission scanning electron microscopy and Glancing angle Xray diffraction (GAXRD) are used to study the surface morphology and crystal structure of the nanotubes. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Titania; Nanotube; Hydrogen; Nanoporous; Gas sensor
1. Introduction We recently reported [1] the fabrication of self organized titania nanotube arrays using an anodization technique. Although the as prepared nanotubes are amorphous, they crystallize on annealing at elevated temperatures and are structurally stable to at least 600 8C. This stability of structure, which is one of the essential criteria of a gas sensing material, prompted us to study the gas sensing behavior of these nanotubes to technologically important gases, such as oxygen, carbon monoxide, ammonia, carbon dioxide and hydrogen. Titania has earned much attention for its oxygen sensing capabilities [2–7]. Furthermore with proper manipulation of the microstructure, crystalline phase and/or addition of proper impurities or surface functionalization titania can also be used as a reducing gas sensor [8–19]. The interaction of a gas with a metal oxide semiconductor is primarily a surface phenomenon, therefore nanoporous metal oxides [14,15,20,21] offer the advantage of providing large sensing surface areas. Hydrogen sensing is needed for industrial process control, combustion control, and in medical applications with hydrogen indicating certain types of bacterial infection. In this work we report on the hydrogen sensing properties of titania nanotubes made via anodization [1]. Room temperature metal oxide hydrogen sensors are generally based on Schottky barrier modulation of devices like Pd/TiO2 or * Corresponding author. E-mail address: [email protected] (C.A. Grimes).
Pt/TiO2 by hydrogen [22–24]. Elevated temperature hydrogen sensors examine the electrical resistance with hydrogen concentration; for example, Birkefeld et al. [25] observed that the resistance of anatase phase of titania varies in presence of carbon monoxide and hydrogen at temperatures above 500 8C, but on doping with 10% alumina it becomes selective to hydrogen.
2. Experimental Titania nanotubes [1] were grown from titanium foil (99.5% pure from Alfa Aesar, Ward Hill, MA, USA) of thickness 0.25 mm. The anodization was performed in an electrolyte medium of 0.5% hydrofluoric acid (J. T. BakerPhillipsburg, NJ, USA) in water, using a platinum foil cathode. Well-defined nanotube arrays were grown using anodizing potentials ranging from 12 to 20 V. Nanotube length increases with anodization time, reaching a length of 400 nm in approximately 20 min, and then remains constant. For the present study the samples were anodized for 25 min. The samples were then annealed at 500 8C in a pure oxygen ambient for 6 h, with a heating and cooling rate of 1 8C/min. A field emission scanning electron microscope (FESEM) from JEOL (model JSM6300), Peabody, MA, USA was used to study the surface morphology of the nanotubes. A glancing angle X-ray diffractometer (GAXRD) from Philips (model X’pert MRD PRO), The Netherlands was used to determine the crystalline phase. The electrode geometry of the titania nanotube sensors is shown in Fig. 1a. The sensor consists of a base titanium
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Fig. 1. Schematic representation of (a) the electrode geometry, and (b) the experimental apparatus.
metal foil with a nanotube array grown on the top. An insulating barrier layer separates the nanotubes from the conducting titanium foil. Preliminary studies using evaporated gold films as well as silver paste as interdigital electrodes showed that these materials diffuse into the titania nanotubes at elevated temperatures resulting in sensor contamination. Hence a pressure contact was used to electrically contact the nanotubes, with two spring-loaded parallel 10 mm by 2 mm platinum contact pads (100 mm thickness). A schematic diagram of the experimental set up used for the gas sensing studies is shown in Fig. 1b. The test chamber consists of a 1.3 l quartz tube, with stainless steel end caps, placed inside a furnace (Thermolyne, USA model 21100 tubular furnace). The electrical contact was formed by attaching the platinum pads to the ends of a spring-loaded ‘U’ shaped quartz rod. Gas flow through the test chamber was controlled via a computer-controlled mass flow controller (MKS instruments, Austin, TX, USA). The electrical resistance of the titania sensors were measured using a computer-controlled digital multimeter (Keithley, USA model 2000). Prior to data collection the test chamber was initially evacuated using a mechanical pump, whereupon nitrogen (99.999% pure) was passed while the sensor
under test was heated to the desired temperature. The test gases examined, oxygen, carbon dioxide, ammonia, carbon monoxide or hydrogen, were mixed in appropriate ratios with nitrogen to create the necessary test gas ambient.
3. Results and discussion The surface morphology of nanotube arrays prepared using an anodization potential of 20 V and annealed at 500 8C for 6 h in a pure oxygen ambient is shown in Fig. 2a and b. It can be seen from these images that the nanotubes are uniform over the surface. The nanotubes are approximately 400 nm in length and have a barrier layer [1] thickness of 50 nm. For the nanotubes fabricated using 20 V anodization the average pore diameter, as determined from FESEM images, is 76 nm (standard deviation 15 nm), with a wall thickness of 27 nm (standard deviation 6 nm). The sample anodized at 12 V was found to have an average pore diameter of 46 nm (standard deviation 8 nm) with a wall thickness 17 nm (standard deviation 2 nm). The porosities of the 20 and 12 V samples were calculated as 45 and 61%, respectively.
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Glancing angle X-ray diffraction patterns of a 20 V sample annealed at 500 8C for 6 h in oxygen ambient is shown in Fig. 3. It can be seen that both anatase and rutile phases of titania are present in the sample. A detailed study [26] of these structures using high resolution transmission electron microscope (HRTEM) showed that the anatase crystallites were concentrated on the walls of the nanotubes and rutile on the barrier layer. Nanotubes annealed in a pure oxygen ambient were found to be stable (intact) to temperatures of approximately 580 8C. Above this temperature protrusions were seen coming out through the nanotubes, an effect which spread with increasing temperature. These protrusions, which are due to oxidation of the titanium substrate, collapse the nanotubes. Fig. 4 shows the response of the 20 V sample as a function of ambient temperature, as it is switched from a nitrogen environment to one containing 1000 ppm hydrogen, and then back to nitrogen. The plot was made using (Rg/R0)1 versus time where R0 is the base resistance of the sensor, i.e. the sensor resistance before introducing the test gas, and Rg the measured resistance in the presence of test gas. The sensor shows increasing hydrogen sensitivity with temperature, with a three order of magnitude change in resistance at temperatures above 300 8C. At all the temperatures the original resistance it recovered without hysteresis. The sensitivity S is defined by the formula S¼
Fig. 2. The surface morphology of the titania nanotubes after annealing at 500 8C: (a) high, and (b) low magnification images of a 20 V sample, and (c) a high magnification image of a 12 V sample.
R0 Rgs Rgs
where R0 is the resistance of the sensor before passing the gas and Rgs that after passing gas and reaching the saturation value. The sensitivity of a 20 V sample with temperature, to 1000 ppm hydrogen, is shown in Fig. 5. Sensitivity is seen to increase with temperature to approximately 380 8C where the increase in sensitivity with temperature is beginning to saturate. The response time, defined as the time needed for the sensor to reach 90% of the final signal for a given concentration of gas, is plotted against temperature in Fig. 6 (the time includes that required for the gas to equilibrate inside
Fig. 3. Glancing angle X-ray diffraction pattern of a 20 V sample (glancing angle ¼ 28) annealed at 500 8C in oxygen ambient. A, R and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium substrate, respectively.
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Fig. 4. Variation of resistance Rg, normalized with respect to baseline resistance R0, of a 20 V sample with time on exposure to 1000 ppm hydrogen at different temperatures. It may be noted that the inverse of Rg/R0 was used in the plot for representing data in positive y-direction.
the measurement chamber, estimated to be 30 s). The response time reduces exponentially with temperature. To check the behavior of the sensor on repeated hydrogen exposure, the hydrogen concentration was varied in discrete steps of 100 ppm from 0 to 500 ppm while keeping the temperature constant at 290 8C; the chamber was flushed with nitrogen after each exposure to hydrogen. The response
of the 20 V prepared nanotube sensors, kept at 290 8C, is shown in Fig. 7. The behavior of the sensor is consistent, recovering its original resistance after repeated exposure to varying hydrogen gas concentrations. The sensitivity of the sensor in this concentration range is plotted in Fig. 8a; there is a linear increase in sensitivity at low concentrations. The sensitivity of the hydrogen sensor over 100 ppm to 4% (the explosive limit in the presence of oxygen) is shown in Fig. 8b. Fig. 9 shows the hydrogen sensitivities, at 290 8C, of nanotube sensors having a pore diameter of 76 nm, and a pore diameter of 46 nm. While smaller diameter nanotubes had greater sensitivity to hydrogen the samples made at lower anodizing voltages tended to be more brittle, and harder to mechanically handle without breaking. The 20 V sample was exposed to oxygen, carbon monoxide, ammonia and carbon dioxide at 290 8C. The sensor was found to have no detectable variation in resistance on exposure to carbon dioxide. The sensitivities of the titania
Fig. 5. The sensitivity temperature dependence of a 20 V sample to 1000 ppm hydrogen.
Fig. 6. Response time variation of a 20 V sample to temperature. The dots represent measured data.
Fig. 7. Resistance of a 20 V sample when exposed to different concentrations of hydrogen at 290 8C. The nitrogen–hydrogen mixture was passed for 1500 s; the chamber was then flushed with nitrogen for 3000 s before passing the nitrogen–hydrogen mixture again.
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Fig. 10. Variation of resistance of a 20 V sample when exposed to 1000 ppm carbon monoxide, 5% ammonia and 1% oxygen at 290 8C.
Fig. 8. The sensitivity variation of a 20 V sample at 290 8C for (a) low hydrogen concentrations, and (b) 0.01 to 4% hydrogen concentrations.
nanotubes to the other gases is shown in Fig. 10. The sensitivity of the nanotubes to carbon monoxide and ammonia are negligible compared to that of hydrogen. The resistance of the nanotubes increased in presence of oxygen, and did not regain their original electrical conductivity even after several hours in a nitrogen environment. Since the sensor measurements were conducted in atmospheres without oxygen, the increase in conductivity cannot be due to hydrogen removing oxygen from the lattice [27–29] or the removal of chemisorbed oxygen [30–32]. It is likely that the hydrogen molecules get dissociated at the defects on the titania surface. These molecules can diffuse into the titania lattice, and act as electron donors [25,33,34].
Fig. 9. A comparison of the variation in resistance of samples having pore diameters of 46 and 76 nm, vs. time, upon exposure to 1000 ppm of hydrogen at 290 8C.
But this process would lead to very slow response and recovery times and complete recovery would be difficult. Since the sensor completely regains its original resistance with hydrogen cycling it appears that this is not the dominant mechanism behind high hydrogen sensitivity. Hence, we believe that the major process behind the interaction between the nanotubes and hydrogen is the chemisorption of the dissociated hydrogen on the titania surface [32]. During chemisorption hydrogen acts as a surface state and a partial charge transfer takes place from hydrogen to the conduction band of titania. This creates an electron accumulation layer on the nanotube surface that enhances its electrical conductance. On removing the hydrogen ambient, electron transfer takes place back to hydrogen and it desorbs, thus restoring the original resistance of the nanotubes. Another factor that may play a role in the hydrogen sensitivity (and selectivity) is the platinum electrodes. It is possible that platinum is acting as a catalyst for interaction of hydrogen with titania. At elevated temperatures hydrogen dissociation can occur on platinum surfaces. These dissociated hydrogen atoms may spill [31,35] onto the nanotube surface where they diffuse into the nanotube surface. From the present study it was not clear how significant a role the platinum electrodes play. Anatase, the polymorph of titania has been reported to be of high sensitivity towards reducing gases like hydrogen and carbon monoxide [13,16,25]. Our nanotube samples contain anatase phase mainly on the walls and rutile in the barrier layer. As the diffusing hydrogen atoms go to the interstitial sites [25,33] and as the c/a ratio of anatase is almost four times compared to that of rutile, it appears that anatase lattice accommodates hydrogen easily and hence has a higher contribution to hydrogen sensitivity. The effect of chemisorption can be neglected in the oxygen sensing experiments. As the recovery requires several hours it appears that the nanotubes contain oxygen vacancies or titanium interstitial defects in presence of nitrogen. On exposing the sensor to oxygen ambient, the lattice reoxidizes and hence the conductivity of the sensor decreases. On removing oxygen, the reduction of the lattice
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will not immediately occur hence the sensor requires several hours to regain its original conductivity. It should be noted that the conducting titanium foil beneath the nanotubes ultimately limits the sensitivity by reducing the baseline resistance of the sensor.
4. Conclusions Titania nanotubes prepared using anodization and annealed in an oxygen atmosphere at a temperature of 500 8C were found highly sensitive to hydrogen. The nanotube sensors contain both anatase and rutile phases of titania, and showed appreciable sensitivity towards hydrogen at temperatures as low as 180 8C. The sensitivity increased drastically with temperature showing a variation of three orders in magnitude of resistance to 1000 ppm of hydrogen at 400 8C. The response time decreased with increasing temperature; at 290 8C full switching of the sensor took approximately 3 min. Results were highly reproducible with no indication of hysteresis. Our results showed these sensors are capable of monitoring hydrogen levels from 100 ppm to 4%. At 290 8C nanotubes with smaller pore diameter (46 nm) showed higher sensitivity to hydrogen compared to larger pore diameter samples (76 nm). The sensors showed high selectivity to hydrogen compared to carbon monoxide, ammonia and carbon dioxide. Although the sensor was sensitive to high concentrations of oxygen, the response time was high and the sensor did not completely regain the original condition. We believe that the hydrogen sensitivity of the nanotubes is due to hydrogen chemisorption onto the titania surface where they act as electron donors. In summary, it was demonstrated that sensors comprised of titania nanotubes prepared using anodization can successfully be used as hydrogen sensors.
Acknowledgements Partial support of this work by the National Science Foundation through grant ECS-9875104 is gratefully acknowledged. References [1] D. Gong, C.A. Grimes, O.K. Varghese, W. Hu, R.S. Singh, Z. Chen, E.C. Dickey, Titanium oxide nanotube arrays prepared by anodic oxidation, J. Mater. Res. 16 (2001) 3331. [2] T. Takeuchi, Oxygen sensors, Sens. Actuators 14 (1988) 109. [3] U. Kirner, K.D. Schierbaum, W. Gopel, Low and high temperature TiO2 oxygen sensors, Sens. Actuators B 1 (1990) 103. [4] Y. Xu, K. Yao, X. Zhou, Q. Cao, Platinum–titania oxygen sensors and their sensing mechanisms, Sens. Actuators B 13-14 (1993) 492. [5] V. Demarne, S. Balkanova, A. Grisel, D. Rosenfeld, F. Levy, Integrated gas sensor for oxygen detection, Sens. Actuators B 13-14 (1993) 497.
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Hydrogen sensing using titania nanotubes Oomman K. Varghese, Dawei Gong, Maggie Paulose, Keat G. Ong, Craig A. Grimes* Department of Electrical Engineering and Materials Research Institute, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA
Abstract Titanium dioxide nanotubes, made by anodization, are highly sensitive to hydrogen; for example, cycling between nitrogen atmosphere and 1000 ppm hydrogen a variation in measured resistance of 103 is seen for 46 nm diameter nanotubes at 290 8C. The hydrogen sensors are completely reversible and have response times of approximately 150 s. Field emission scanning electron microscopy and Glancing angle Xray diffraction (GAXRD) are used to study the surface morphology and crystal structure of the nanotubes. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Titania; Nanotube; Hydrogen; Nanoporous; Gas sensor
1. Introduction We recently reported [1] the fabrication of self organized titania nanotube arrays using an anodization technique. Although the as prepared nanotubes are amorphous, they crystallize on annealing at elevated temperatures and are structurally stable to at least 600 8C. This stability of structure, which is one of the essential criteria of a gas sensing material, prompted us to study the gas sensing behavior of these nanotubes to technologically important gases, such as oxygen, carbon monoxide, ammonia, carbon dioxide and hydrogen. Titania has earned much attention for its oxygen sensing capabilities [2–7]. Furthermore with proper manipulation of the microstructure, crystalline phase and/or addition of proper impurities or surface functionalization titania can also be used as a reducing gas sensor [8–19]. The interaction of a gas with a metal oxide semiconductor is primarily a surface phenomenon, therefore nanoporous metal oxides [14,15,20,21] offer the advantage of providing large sensing surface areas. Hydrogen sensing is needed for industrial process control, combustion control, and in medical applications with hydrogen indicating certain types of bacterial infection. In this work we report on the hydrogen sensing properties of titania nanotubes made via anodization [1]. Room temperature metal oxide hydrogen sensors are generally based on Schottky barrier modulation of devices like Pd/TiO2 or * Corresponding author. E-mail address: [email protected] (C.A. Grimes).
Pt/TiO2 by hydrogen [22–24]. Elevated temperature hydrogen sensors examine the electrical resistance with hydrogen concentration; for example, Birkefeld et al. [25] observed that the resistance of anatase phase of titania varies in presence of carbon monoxide and hydrogen at temperatures above 500 8C, but on doping with 10% alumina it becomes selective to hydrogen.
2. Experimental Titania nanotubes [1] were grown from titanium foil (99.5% pure from Alfa Aesar, Ward Hill, MA, USA) of thickness 0.25 mm. The anodization was performed in an electrolyte medium of 0.5% hydrofluoric acid (J. T. BakerPhillipsburg, NJ, USA) in water, using a platinum foil cathode. Well-defined nanotube arrays were grown using anodizing potentials ranging from 12 to 20 V. Nanotube length increases with anodization time, reaching a length of 400 nm in approximately 20 min, and then remains constant. For the present study the samples were anodized for 25 min. The samples were then annealed at 500 8C in a pure oxygen ambient for 6 h, with a heating and cooling rate of 1 8C/min. A field emission scanning electron microscope (FESEM) from JEOL (model JSM6300), Peabody, MA, USA was used to study the surface morphology of the nanotubes. A glancing angle X-ray diffractometer (GAXRD) from Philips (model X’pert MRD PRO), The Netherlands was used to determine the crystalline phase. The electrode geometry of the titania nanotube sensors is shown in Fig. 1a. The sensor consists of a base titanium
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Fig. 1. Schematic representation of (a) the electrode geometry, and (b) the experimental apparatus.
metal foil with a nanotube array grown on the top. An insulating barrier layer separates the nanotubes from the conducting titanium foil. Preliminary studies using evaporated gold films as well as silver paste as interdigital electrodes showed that these materials diffuse into the titania nanotubes at elevated temperatures resulting in sensor contamination. Hence a pressure contact was used to electrically contact the nanotubes, with two spring-loaded parallel 10 mm by 2 mm platinum contact pads (100 mm thickness). A schematic diagram of the experimental set up used for the gas sensing studies is shown in Fig. 1b. The test chamber consists of a 1.3 l quartz tube, with stainless steel end caps, placed inside a furnace (Thermolyne, USA model 21100 tubular furnace). The electrical contact was formed by attaching the platinum pads to the ends of a spring-loaded ‘U’ shaped quartz rod. Gas flow through the test chamber was controlled via a computer-controlled mass flow controller (MKS instruments, Austin, TX, USA). The electrical resistance of the titania sensors were measured using a computer-controlled digital multimeter (Keithley, USA model 2000). Prior to data collection the test chamber was initially evacuated using a mechanical pump, whereupon nitrogen (99.999% pure) was passed while the sensor
under test was heated to the desired temperature. The test gases examined, oxygen, carbon dioxide, ammonia, carbon monoxide or hydrogen, were mixed in appropriate ratios with nitrogen to create the necessary test gas ambient.
3. Results and discussion The surface morphology of nanotube arrays prepared using an anodization potential of 20 V and annealed at 500 8C for 6 h in a pure oxygen ambient is shown in Fig. 2a and b. It can be seen from these images that the nanotubes are uniform over the surface. The nanotubes are approximately 400 nm in length and have a barrier layer [1] thickness of 50 nm. For the nanotubes fabricated using 20 V anodization the average pore diameter, as determined from FESEM images, is 76 nm (standard deviation 15 nm), with a wall thickness of 27 nm (standard deviation 6 nm). The sample anodized at 12 V was found to have an average pore diameter of 46 nm (standard deviation 8 nm) with a wall thickness 17 nm (standard deviation 2 nm). The porosities of the 20 and 12 V samples were calculated as 45 and 61%, respectively.
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Glancing angle X-ray diffraction patterns of a 20 V sample annealed at 500 8C for 6 h in oxygen ambient is shown in Fig. 3. It can be seen that both anatase and rutile phases of titania are present in the sample. A detailed study [26] of these structures using high resolution transmission electron microscope (HRTEM) showed that the anatase crystallites were concentrated on the walls of the nanotubes and rutile on the barrier layer. Nanotubes annealed in a pure oxygen ambient were found to be stable (intact) to temperatures of approximately 580 8C. Above this temperature protrusions were seen coming out through the nanotubes, an effect which spread with increasing temperature. These protrusions, which are due to oxidation of the titanium substrate, collapse the nanotubes. Fig. 4 shows the response of the 20 V sample as a function of ambient temperature, as it is switched from a nitrogen environment to one containing 1000 ppm hydrogen, and then back to nitrogen. The plot was made using (Rg/R0)1 versus time where R0 is the base resistance of the sensor, i.e. the sensor resistance before introducing the test gas, and Rg the measured resistance in the presence of test gas. The sensor shows increasing hydrogen sensitivity with temperature, with a three order of magnitude change in resistance at temperatures above 300 8C. At all the temperatures the original resistance it recovered without hysteresis. The sensitivity S is defined by the formula S¼
Fig. 2. The surface morphology of the titania nanotubes after annealing at 500 8C: (a) high, and (b) low magnification images of a 20 V sample, and (c) a high magnification image of a 12 V sample.
R0 Rgs Rgs
where R0 is the resistance of the sensor before passing the gas and Rgs that after passing gas and reaching the saturation value. The sensitivity of a 20 V sample with temperature, to 1000 ppm hydrogen, is shown in Fig. 5. Sensitivity is seen to increase with temperature to approximately 380 8C where the increase in sensitivity with temperature is beginning to saturate. The response time, defined as the time needed for the sensor to reach 90% of the final signal for a given concentration of gas, is plotted against temperature in Fig. 6 (the time includes that required for the gas to equilibrate inside
Fig. 3. Glancing angle X-ray diffraction pattern of a 20 V sample (glancing angle ¼ 28) annealed at 500 8C in oxygen ambient. A, R and T represent the reflections from anatase crystallites, rutile crystallites, and the titanium substrate, respectively.
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Fig. 4. Variation of resistance Rg, normalized with respect to baseline resistance R0, of a 20 V sample with time on exposure to 1000 ppm hydrogen at different temperatures. It may be noted that the inverse of Rg/R0 was used in the plot for representing data in positive y-direction.
the measurement chamber, estimated to be 30 s). The response time reduces exponentially with temperature. To check the behavior of the sensor on repeated hydrogen exposure, the hydrogen concentration was varied in discrete steps of 100 ppm from 0 to 500 ppm while keeping the temperature constant at 290 8C; the chamber was flushed with nitrogen after each exposure to hydrogen. The response
of the 20 V prepared nanotube sensors, kept at 290 8C, is shown in Fig. 7. The behavior of the sensor is consistent, recovering its original resistance after repeated exposure to varying hydrogen gas concentrations. The sensitivity of the sensor in this concentration range is plotted in Fig. 8a; there is a linear increase in sensitivity at low concentrations. The sensitivity of the hydrogen sensor over 100 ppm to 4% (the explosive limit in the presence of oxygen) is shown in Fig. 8b. Fig. 9 shows the hydrogen sensitivities, at 290 8C, of nanotube sensors having a pore diameter of 76 nm, and a pore diameter of 46 nm. While smaller diameter nanotubes had greater sensitivity to hydrogen the samples made at lower anodizing voltages tended to be more brittle, and harder to mechanically handle without breaking. The 20 V sample was exposed to oxygen, carbon monoxide, ammonia and carbon dioxide at 290 8C. The sensor was found to have no detectable variation in resistance on exposure to carbon dioxide. The sensitivities of the titania
Fig. 5. The sensitivity temperature dependence of a 20 V sample to 1000 ppm hydrogen.
Fig. 6. Response time variation of a 20 V sample to temperature. The dots represent measured data.
Fig. 7. Resistance of a 20 V sample when exposed to different concentrations of hydrogen at 290 8C. The nitrogen–hydrogen mixture was passed for 1500 s; the chamber was then flushed with nitrogen for 3000 s before passing the nitrogen–hydrogen mixture again.
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Fig. 10. Variation of resistance of a 20 V sample when exposed to 1000 ppm carbon monoxide, 5% ammonia and 1% oxygen at 290 8C.
Fig. 8. The sensitivity variation of a 20 V sample at 290 8C for (a) low hydrogen concentrations, and (b) 0.01 to 4% hydrogen concentrations.
nanotubes to the other gases is shown in Fig. 10. The sensitivity of the nanotubes to carbon monoxide and ammonia are negligible compared to that of hydrogen. The resistance of the nanotubes increased in presence of oxygen, and did not regain their original electrical conductivity even after several hours in a nitrogen environment. Since the sensor measurements were conducted in atmospheres without oxygen, the increase in conductivity cannot be due to hydrogen removing oxygen from the lattice [27–29] or the removal of chemisorbed oxygen [30–32]. It is likely that the hydrogen molecules get dissociated at the defects on the titania surface. These molecules can diffuse into the titania lattice, and act as electron donors [25,33,34].
Fig. 9. A comparison of the variation in resistance of samples having pore diameters of 46 and 76 nm, vs. time, upon exposure to 1000 ppm of hydrogen at 290 8C.
But this process would lead to very slow response and recovery times and complete recovery would be difficult. Since the sensor completely regains its original resistance with hydrogen cycling it appears that this is not the dominant mechanism behind high hydrogen sensitivity. Hence, we believe that the major process behind the interaction between the nanotubes and hydrogen is the chemisorption of the dissociated hydrogen on the titania surface [32]. During chemisorption hydrogen acts as a surface state and a partial charge transfer takes place from hydrogen to the conduction band of titania. This creates an electron accumulation layer on the nanotube surface that enhances its electrical conductance. On removing the hydrogen ambient, electron transfer takes place back to hydrogen and it desorbs, thus restoring the original resistance of the nanotubes. Another factor that may play a role in the hydrogen sensitivity (and selectivity) is the platinum electrodes. It is possible that platinum is acting as a catalyst for interaction of hydrogen with titania. At elevated temperatures hydrogen dissociation can occur on platinum surfaces. These dissociated hydrogen atoms may spill [31,35] onto the nanotube surface where they diffuse into the nanotube surface. From the present study it was not clear how significant a role the platinum electrodes play. Anatase, the polymorph of titania has been reported to be of high sensitivity towards reducing gases like hydrogen and carbon monoxide [13,16,25]. Our nanotube samples contain anatase phase mainly on the walls and rutile in the barrier layer. As the diffusing hydrogen atoms go to the interstitial sites [25,33] and as the c/a ratio of anatase is almost four times compared to that of rutile, it appears that anatase lattice accommodates hydrogen easily and hence has a higher contribution to hydrogen sensitivity. The effect of chemisorption can be neglected in the oxygen sensing experiments. As the recovery requires several hours it appears that the nanotubes contain oxygen vacancies or titanium interstitial defects in presence of nitrogen. On exposing the sensor to oxygen ambient, the lattice reoxidizes and hence the conductivity of the sensor decreases. On removing oxygen, the reduction of the lattice
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will not immediately occur hence the sensor requires several hours to regain its original conductivity. It should be noted that the conducting titanium foil beneath the nanotubes ultimately limits the sensitivity by reducing the baseline resistance of the sensor.
4. Conclusions Titania nanotubes prepared using anodization and annealed in an oxygen atmosphere at a temperature of 500 8C were found highly sensitive to hydrogen. The nanotube sensors contain both anatase and rutile phases of titania, and showed appreciable sensitivity towards hydrogen at temperatures as low as 180 8C. The sensitivity increased drastically with temperature showing a variation of three orders in magnitude of resistance to 1000 ppm of hydrogen at 400 8C. The response time decreased with increasing temperature; at 290 8C full switching of the sensor took approximately 3 min. Results were highly reproducible with no indication of hysteresis. Our results showed these sensors are capable of monitoring hydrogen levels from 100 ppm to 4%. At 290 8C nanotubes with smaller pore diameter (46 nm) showed higher sensitivity to hydrogen compared to larger pore diameter samples (76 nm). The sensors showed high selectivity to hydrogen compared to carbon monoxide, ammonia and carbon dioxide. Although the sensor was sensitive to high concentrations of oxygen, the response time was high and the sensor did not completely regain the original condition. We believe that the hydrogen sensitivity of the nanotubes is due to hydrogen chemisorption onto the titania surface where they act as electron donors. In summary, it was demonstrated that sensors comprised of titania nanotubes prepared using anodization can successfully be used as hydrogen sensors.
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