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H2 sensing performance of anodically oxidized TiO2 thin films equipped with Pd electrode
Sensors and Actuators B 121 (2007) 219–230
H2 sensing performance of anodically oxidized TiO2 thin films equipped with Pd electrode Yasuhiro Shimizu ∗ , Takeo Hyodo, Makoto Egashira Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan Available online 13 November 2006
Abstract Hydrogen sensing properties of TiO2 thin films prepared by different methods and equipped with different kinds of metal electrodes have been tested. The most developed submicron-sized pore structure was found in the TiO2 thin film prepared by anodic oxidation of a Ti metal plate in a H2 SO4 solution at 20 ◦ C. The sensor fabricated with this film and a Pd electrode showed the highest H2 response among the sensors fabricated with different TiO2 films and electrode metals at elevated temperatures. This TiO2 thin film sensor equipped with a Pd electrode can be classified into a diode-type sensor from the change in I–V curve induced by H2 , and is characterized with reversible H2 response even in N2 atmosphere. The H2 sensing mechanism is suggested to arise from dissolution of H atoms into the Pd electrode and a decrease in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film. Although the present sensor showed pretreatment-dependent H2 response properties, modification of the Pd electrode was found to be effective for reducing or improving the pretreatment-dependent H2 response properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Anodically oxidized TiO2 ; Nanoholes; Pd electrode; H2 sensor; Schottky barrier; Diodo-type sensor; Sensing mechanism
1. Introduction The use of H2 as a clean energy source has been expanding into various fields such as fuel cell vehicles, household fuel cell cogeneration systems, and so on. This results in both rapid expansion of application fields of H2 gas sensors and a rise of new research current of H2 gas sensors. Hydrogen gas can be detected by various kinds of principles, methods and materials, e.g. by utilizing semiconductor metal oxides [1,2], diodes [3,4], FETs [5,6], solid electrolytes [7,8], fiber optics [9,10], quartz crystal microbalances [11], surface acoustic wave devices [12,13], etc. The number of studies and practical applications of titanium dioxide as a sensor material was very small, especially for a semiconductor gas sensor material, due to the very low reactivity of chemisorbed oxygen on its surface in the operating temperature range adopted for conventional semiconductor gas sensors. But, recent progress in nano-processing enables us to use TiO2 nano-powder as a semiconductor gas sensor material owning
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to an increase in surface to volume ratio [14–17]. Besides these TiO2 -based semiconductor gas sensors, TiO2 -based H2 gas sensors operated by other sensing mechanisms have been studied. The number of the latter type of sensors is very limited, but typical examples are diode-type sensors studied by Tonomura et al. [18] and ourselves [19–22], and a chemisorption-type sensor studied by Varghese et al. [23,24]. The difference in sensor structure between the Tonomura group’s and ours to be described in the present paper is the kind of a TiO2 material, i.e. a single crystal is their case and anodically oxidized TiO2 thin films are our case. And, differences between the Varghese group’s and ours are the anodic oxidation conditions of TiO2 thin films and the electrode configuration, i.e. a pair of Pt electrodes at the surface of the TiO2 film in their case [23] and a sandwich configuration of Pd (top electrode)/TiO2 /Ti (served as both a bottom electrode and a substrate) in our case, as will be explained later. In the present paper, important factors affecting the H2 sensing performance of an anodically oxidized TiO2 film equipped with a Pd electrode are discussed comprehensively, by citing our published results and by adding new data on the modification of the Pd electrode. The remaining problems, which are to be solved before its practical application, are also mentioned.
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2. Experimental 2.1. Preparation and characterization of TiO2 thin films A half part of a Ti plate (5.0 mm × 10.0 mm × 0.5 mm) was inserted in an aqueous 0.5 M H2 SO4 solution, and then was anodically oxidized at a current density of 100 mA cm−2 for 30 min at 20 ◦ C, unless otherwise noted. In evaluating the effects of anodic oxidation conditions on the microstrucutre and the H2 response properties of the resulting TiO2 thin films, anodic oxidation was also conducted at 5 and 30 ◦ C. Another anodically oxidized TiO2 thin film was prepared in an aqueous 1.0 M NaOH solution under the same conditions. The two anodic TiO2 thin films are denoted as A-TiO2 and N-TiO2 , respectively. A-TiO2 thin film was also prepared by dry oxidation of a Ti plate at 600 ◦ C for 1 h in air, and is referred to as R-TiO2 . The thickness of A-TiO2 , N-TiO2 and R-TiO2 was 0.25, 0.10 and 0.15 m, respectively. The crystal phases in the TiO2 thin films were characterized with X-ray diffraction (Rigaku, RINT 2200), and the microstructure was observed with a scanning electron microscope (SEM, Hitachi, S-2250N). Morphology of the TiO2 thin films was observed by a transmission electron microscope (TEM, JEOL, JEM2010-HT). 2.2. Fabrication and measurement of H2 response of TiO2 thin film sensors Pd electrodes were sputtered (JEOL, JFC-1100 or Shimadzu, HSR-552S) for 10 min on both the TiO2 thin film and the Ti plate, and the electrical contact to lead wires was ensured by application of Pd paste and subsequent firing at 600 ◦ C for 1 h in air, unless otherwise noted. Such a sensor will be referred to as a normal sensor and is distinguished from another sensor, which was subjected to additional treatment in N2 at 600 ◦ C for 1 h before the measurement of H2 response. The latter sensor will be expressed as a N2 -treated sensor. In both cases, the electrical properties of the TiO2 thin films were measured with Pd (top) and Ti (bottom) electrodes. The electrode configuration of the sensors adopted in the present study can be seen in Fig. 1 [21]. In evaluating the effect of electrode metals on the H2 response properties, TiO2 thin film sensors equipped with sputtered Au or Pt were also fabricated, in a similar manner to that described above.
Fig. 1. Sensor structure of Pd/TiO2 thin film sensors.
A dc voltage of 0.1 V was applied to the TiO2 thin film sensors and transient changes in current flowing through the sensors upon exposure to 0.8 or 1.0% H2 balanced with dry air or dry N2 were monitored in the temperature range of 250–500 ◦ C. But, the words “dry air” or “dry N2 ” as a balance gas will be expressed as “air” or “N2 ” throughout the text for the sake of simplicity. The measurements were done under reverse {Pd(−)–TiO2 –Ti(−)} or forward {Pd(+)–TiO2 –Ti(−)} bias conditions. The response magnitude (k) was defined as a ratio (Ig /Ia or Ig /In ) of the sensor current in the sample gas (Ig ) to that in air (Ia ) or N2 (In ). Occasionally, definition of a ratio of (Ra /Rg or Rn /Rg ) of the sensor resistance in air (Ra ) or in N2 (Rn ) to that in the sample gas was also used. Responses of the A-TiO2 thin film sensor equipped with a Pd electrode (Pd/A-TiO2 ) to 2.3% H2 O water vapor, 500 ppm CO, 8000 ppm CH4 , C3 H8 and n-C4 H10 were also measured to evaluate the cross-sensitivity to these gases. 2.3. Characterization of oxidation state and morphology of a Pd electrode The change in oxidation state of the Pd electrode at the A-TiO2 sensor surface was measured with X-ray photoelectron spectroscopy (XPS, Shimadzu, ESCA-850M). The XPS measurement was conducted with three samples: a normal Pd/ATiO2 thin film sensor, a normal sensor subsequently heat treated at 250 ◦ C for 4 h in 1.0% H2 balanced with air, and a N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 . The change in morphology of the Pd electrode at the A-TiO2 sensor surface was measured with SEM and a scanning probe microscope (SPM, Shimadzu, SPM-9500PP). Morphology of four kinds of sensors, which were subjected to different heat treatments, was compared with each other: a normal Pd/A-TiO2 thin film sensor, a normal sensor subsequently heat treated at 250 ◦ C for 1 h in N2 , a N2 -treated sensor, and a N2 -treated sensor subsequently heat treated at 250 ◦ C for 1 h in air. 2.4. Effect of modification of a Pd electrode Effect of two modification methods for the Pd electrode on H2 sensing properties was studied. One is formation of Pt-Pd layered structure of a metal electrode, i.e. 5 min sputtering of Pd at a power of 500 W on an A-TiO2 thin film and additional 5 min sputtering of Pt at the same power on the Pd layer. This sensor will be expressed by a Pt-Pd/A-TiO2 thin film sensor. Another modification is coating of a normal Pd/A-TiO2 thin film sensor with a SiO2 layer, especially at the Pd surface. A SiO2 sol solution was prepared by dissolving diethoxydimethylsilane (DEMS) into a n-C4 H9 OH-based solution at a molar ratio of DEMS: n-C4 H9 OH:H2 O:HCl = 1:5:5:0.05 and refluxing the mixture at 50 ◦ C for 3 h. Then a Pd/A-TiO2 sensor was dipcoated with the SiO2 sol solution. The sensor was then subjected to drying at 80 ◦ C for 10 min in air and subsequent firing at 600 ◦ C for 10 min in air. This procedure repeated up to 5 cycles. The resulting sensor is then expressed as a (SiO2 (5)/Pd)/A-TiO2 thin film sensor.
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Fig. 3. XRD patterns of as-prepared (a) A-TiO2 and (b) R-TiO2 thin films.
The as-prepared A-TiO2 thin film was a mixture of anatase and rutile phases, while R-TiO2 was a single phase of rutile, as shown in Fig. 3(a and b), respectively [19]. The fraction of the rutile phase in A-TiO2 increased after the firing, although the data is not shown here. Fig. 4 shows the TEM photographs of the A-TiO2 fired at 600 ◦ C for 1 h in air [19]. It was obvious
Fig. 2. SEM photographs of the surface of as-prepared (a) A-TiO2 , (b) N-TiO2 and (c) R-TiO2 thin films.
3. Results and discussion 3.1. Characterization of TiO2 thin films Fig. 2 compares the surface morphology of three kinds of as-prepared TiO2 thin films [19]. Formation of submicron-sized pores in the range of 50–130 nm in diameter was observed on the surface of an as-prepared A-TiO2 thin film, as shown in Fig. 2(a). In contrast, N-TiO2 and R-TiO2 consisted of densely sintered particles and little pores were observed on their surfaces, as shown in Fig. 2(b and c). In addition, no visible change in microstructure was found in each TiO2 thin film during the firing at 600 ◦ C for 1 h in air adopted for ensuring the electrical contact as a sensor.
Fig. 4. TEM photographs of an A-TiO2 thin film fired at 600 ◦ C for 1 h in air.
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Fig. 5. Response transients of three kinds of Pd/TiO2 thin film sensors to 1.0% H2 under a reverse bias voltage of 0.1 V.
that A-TiO2 was columnar polycrystallites. Electron diffraction analysis revealed that the columnar top was rutile, but the bottom was a mixture of rutile and anatase.
3.3. Effects of anodic oxidation temperature on microstructre and H2 sensing properties of resulting A-TiO2 thin films
3.2. Effect of the kind of TiO2 thin films on H2 sensing properties
Since the highest H2 response could be achieved with the ATiO2 thin film prepared at an anodic oxidation temperature of 20 ◦ C in H2 SO4 solution, effects of anodic oxidation temperature on the microstructure and H2 sensing properties of the resulting A-TiO2 thin films were further tested. Fig. 6 shows SEM photographs of the surface of A-TiO2 thin films prepared at three different anodic oxidation temperatures [20]. Good reproducibility for the formation of submicron-size pores at an anodic oxidation temperature of 20 ◦ C could be confirmed by the comparison between Figs. 2(a) and 6(b). Although submicron-sized pores were also formed at anodic oxidation temperatures of 5 and 30 ◦ C, as shown in Fig. 6(a and c), the most developed pore structure was achieved at an anodic oxidation temperature of 20 ◦ C. All the films were again confirmed to be a mixture of anatase and rutiles phases, irrespective of the anodic oxidation temperature, although the data is not shown here. Fig. 7 shows response transients of three kinds of Pd/A-TiO2 thin film sensors prepared at different anodic oxidation temperatures to 1.0% H2 under both a reverse and a forward bias voltage of 0.1 V at 250 ◦ C [20]. Among three sensors, the Pd/A-TiO2 sensor prepared at 20 ◦ C exhibited the highest sensor current in 1.0% H2 and therefore the highest H2 response under both the bias conditions. These results suggested the importance of the existence of developed pore structure in realizing high H2 response. Therefore, A-TiO2 thin films prepared at 20 ◦ C were used for further studies aiming at improving the H2
Response transients of three kinds of Pd/TiO2 thin film sensors to 1.0% H2 balanced with air under a reverse bias voltage of 0.1 V are compared in Fig. 5 [19]. The Pd/A-TiO2 sensor exhibited very quick response with a current increase, i.e. a decrease in the sensor resistance, in H2 and rather fast recovery to an original air level after removal of H2 at 250 ◦ C, whereas the response in air was rather noisy. The magnitude of response defined as a ratio of the current in 1.0% H2 balanced with air to that in air is indicated in each figure. Both the sensor resistance and the response decreased with a rise in temperature. Similar measurement was conducted with Pd/N-TiO2 and Pd/R-TiO2 thin film sensors, and the results are also shown in Fig. 5. The sensor resistance of Pd/R-TiO2 and Pd/N-TiO2 in air was smaller than that of Pd/A-TiO2 by one or two orders of magnitude at the same temperature, and then their responses to 1.0% H2 were lower as well. Temperature dependence of the H2 response observed for these two sensors was different from that observed for the Pd/A-TiO2 thin film sensor, i.e. the maximum responses appeared at 300 ◦ C, suggesting the different H2 sensing mechanism from that of the Pd/A-TiO2 thin film sensor. Anyway, it was revealed from these results that the response properties of the sensors were markedly dependent upon the kind, i.e. preparation procedure, of the TiO2 thin films.
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depending upon the level of the sensor resistance, while the total applied voltage to the circuit, in which the sensor was connected in series with the standard resistance, remained constant to be 2.0 V. Therefore, the voltage applied actually to the sensor varied with atmosphere, and therefore the H2 sensing properties of the sensors shown in Fig. 8 cannot be compared directly with those appeared in other figures. For each sensor, figure (a) shows the response transient, (b) the sensor resistance in both air and 1.0% H2 balanced with air and (c) shows the magnitude of H2 response. When, Au was employed, the sensor showed certain H2 response, but almost one tenth of that obtained with the Pd/ATiO2 thin film sensor at the same temperature along with slower response and recovery behavior, as shown in Fig. 8(B). In the case of the Pt/A-TiO2 thin film sensor, the response magnitude to 1.0% H2 was very small, as shown in Fig. 8(C). Therefore, it is confirmed that the use of palladium is essential for achieving a high H2 response with this type of sensors. 3.5. Change in oxidation state of a Pd electrode in Pd/A-TiO2 thin film sensors
Fig. 6. SEM photographs of the surface of as-prepared A-TiO2 thin films at an anodic oxidation temperature of (a) 5 ◦ C, (b) 20 ◦ C and (c) 30 ◦ C.
sensing performance. The differences in sensor current in H2 and response time induced by the change in the direction of the dc electric field were small under an applied voltage of 0.1 V. 3.4. Effect of electrode metals on H2 sensing properties of A-TiO2 thin films Effect of the kind of metals used as a top electrode was tested. Fig. 8 compares the H2 response properties among Pd/A-TiO2 , Au/A-TiO2 and Pt/A-TiO2 thin film sensors [19]. In these measurements, the sensor resistance was calculated by referring to a voltage drop across a standard resistance connected in series with the sensor, and the value of standard resistance was changed
The change in the oxidation state of the Pd electrode of the Pd/A-TiO2 thin film sensor induced by the treatment in different atmospheres was studied by XPS. Fig. 9 shows variations in the Pd 3d photoelectron spectrum of three kinds of Pd/A-TiO2 thin film sensors with sputtering time [20]. The Pd 3d spectrum from a normal sensor shows two peaks, Pd 3d5/2 and Pd 3d3/2 , located at a binding energy of about 335.5 and 340.6 eV, respectively, after 1 min Ar sputtering, as shown in Fig. 9(a). In addition, each peak is accompanied with a small shoulder peak at a slightly higher binding energy. By referring to the data in literature [25–27], the shoulder peaks can be assigned to PdO. The intensity of these two shoulder peaks decreased obviously with sputtering time. Thus, it was confirmed that the surface of the Pd electrode was partially oxidized during the firng at 600◦ C for 1 h in air and was covered with a PdO layer. However, the Pd 3d spectrum from the normal sensor subsequently treated in 1.0% H2 balanced with air at 250 ◦ C for 4 h showed no shoulder peaks even after 1 min sputtering, as shown in Fig. 9(b), indicating the reduction of PdO to Pd in 1.0% H2 balanced with air. In addition, no PdO left behind on the surface of the Pd electrode in the case of the N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 , as shown in Fig. 9(c). These results indicate that the surface of the Pd electrode is likely oxidized in air atmosphere, but the oxide layer is likely recuded to metallic Pd under both H2 -containig air and N2 . 3.6. Other features of Pd/A-TiO2 thin film sensors Another notable feature of a Pd/A-TiO2 thin film sensor was the reversible response to H2 even in N2 atmosphere. Fig. 10 shows a response transient of the Pd/A-TiO2 thin film to 1.0% H2 balanced with N2 [19]. It is obvious that the sensor exhibits certain H2 response along with rather quick response and complete recovery to an original N2 level in a short time. These results suggest less importance of chemisorbed oxygen on both
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Fig. 7. Response transients of Pd/A-TiO2 thin film sensors prepared at different anodic oxidation temperatures to 1.0% H2 balanced with air under both a reverse and a forward bias voltage of 0.1 V at 250 ◦ C.
the surfaces of the Pd electrode and the TiO2 thin film in detecting H2 . The most important feature of a Pd/A-TiO2 thin film sensor was a change in I–V characteristics induced by H2 , as shown in Fig. 11 [19]. A nonlinear I–V curve, which is typical of a diodetype sensor, was observed in air, but it changed to an almost ohmic I–V curve in 1.0% H2 balanced with air. This result indicates the large contribution of the Schottky barrier at the interface between the Pd electrode and the A-TiO2 thin film in detecting H2 .
3.7. H2 gas sensing mechanism of Pd/A-TiO2 thin film sensors Three possible factors can be anticipated for explaining the increase in sensor current (or the decrease in sensor resistance). Factor 1 is the consumption of chemisorbed oxygen on the surface of TiO2 , which leads to electron transfer (return back of electrons) from chemisorbed oxygen to TiO2 particles. This mechanism is the most well-known basic gas-sensing mechanism in the case of semiconductor gas sensors in detecting
Fig. 8. (a) Response transient to 1.0% H2 balanced with air at 250 ◦ C and the temperature dependences of (b) resistance in air and in 1.0% H2 balanced with air and (c) response to 1.0% H2 of three A-TiO2 thin film sensors equipped with different electrodes under reverse bias conditions.
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Fig. 9. Variations in Pd 3d photoelectron spectrum of several Pd/A-TiO2 thin film sensors with sputtering time: (a) a normal sensor, (b) a normal sensor subsequently heat treated at 250 ◦ C for 4 h in 1.0% H2 balanced with air, and (c) a N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 .
reducing gases at elevated temperatures [28]. Factor 2 is the consumption of chemisorbed oxygen on the surface of the Pd electrode (or reduction of PdO on the surface of the Pd electrode), which leads to a decrease in work function of the Pd
Fig. 10. A response transient of a Pd/A-TiO2 thin film sensor to 1.0% H2 balanced with N2 at 250 ◦ C under a reverse bias voltage of 0.1 V.
Fig. 11. I–V characteristics of a Pd/A-TiO2 thin film sensor at 250 ◦ C.
electrode and in turn electron transfer from the Pd electrode to the TiO2 particles. Namely, Factor 2 is equal to the mechanism which is so-called “electronic sensitization” observed for semiconductor gas sensors catalyzed by loaded Pd [29]. However, in the case of the present Pd/A-TiO2 thin film sensor, these two factors are confirmed to be less important from the fact that the sensor shows reversible response to H2 even in N2 atmosphere. Factor 3, which is the most conceivable mechanism for the present sensor, is the decreases in work function of the Pd electrode and then in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film induced by dissolution of H atoms into the Pd electrode. The H2 sensing mechanism based on this variation in Schottky barrier height is schematically illustrated in Fig. 12 [19]. The Schottky barrier is formed reasonably at the interface between the Pd electrode and the TiO2 thin film by the electron transfer from TiO2 to Pd due to the higher value of the work function (φ) of Pd (4.95 eV) than the electron affinity (χ) of TiO2 (4.33 eV) [30], while an ohmic contact is considered to be achieved at the interface between the bottom Ti electrode (served also as a substrate) and the TiO2 thin film. Formation of chemisorbed oxygen or partial oxidation of the sputtered Pd electrode surface is likely occurred after the firing at 600 ◦ C for 1 h in air (i.e. a normal sensor) or during the measurement in the temperature range of 250–400 ◦ C in air. This results in an increase in work function of Pd (φ ) and then a higher barrier height at the interface between the Pd electrode and the TiO2 thin film in air. The existence of this barrier is responsible for a low sensor current in air under the reverse and forward bias conditions. The chemisorbed oxygen on the Pd electrode are removed with H2 , leading to a lower barrier height, although this is not the main factor for the H2 response of the present sensor. Beside this factor, H atoms dissociatively formed on the Pd electrode surface are assumed to be dissolved into Pd and migrate to the interface, resulting in further lowering of the barrier height. This is considered to be the main reason for the reversible response to H2 of the present sensor in N2 atmosphere.
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Fig. 13. Effect of 2.3% water vapor on the response behavior of a Pd/A-TiO2 thin film sensor to 1.0% H2 under a forward bias voltage of 0.1 V.
Fig. 12. H2 sensing mechanism based on variation in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film.
The lower H2 response of the Pd/N-TiO2 and Pd/R-TiO2 thin film sensors than the Pd/A-TiO2 thin film sensor can be ascribed to less formation of submicron-sized pores which allow gaseous H2 to diffuse into the interface between Pd and TiO2 particles and make more active reaction sites on the surface of TiO2 particles. The higher H2 response of the Pd/A-TiO2 sensor prepared at an anodic oxidation temperature of 20 ◦ C than those prepared at different anodic oxidation temperatures may be explained by the similar reason. The lower H2 response of the Au/A-TiO2 and the Pt/A-TiO2 sensors comes undoubtedly from the low solubility of H atoms in these metals and then the less-developed sensitive barrier height to H2 .
rent approaching a steady-state value was observed when the surrounding atmosphere was changed from dry air to 1.0% H2 balanced with wet air, as shown in Fig. 13(d). Therefore, elimination of the interference from water vapor is a big subject in achieving a high H2 response. Temperature dependence of the response of a Pd/A-TiO2 thin film sensor to 500 ppm CO balanced with air is shown in Fig. 14 [19]. The response to CO was very small over the whole temperature range of 250–400 ◦ C and the maximum response was approximately 2 at most at 400 ◦ C. Such small response may arise from consumption of chemisorbed oxygen on both the surfaces of Pd and TiO2 particles, and then may prove the validity
3.8. Response of a Pd/A-TiO2 thin film sensor to other gases Fig. 13 shows the effect of 2.3% water vapor (express as “wet air” in Fig. 13) on the H2 response behavior of a Pd/A-TiO2 thin film sensor [19]. As described above, the sensor showed a large H2 response and quick response and recovery behavior in dry air at 250 ◦ C (see Fig. 13(a)). At the same temperature, the sensor exhibited a small response of approximately 10 to 2.3% water vapor, as shown in Fig. 13(b). This may suggest the displacement of chemisorbed oxygen with hydroxyl groups, i.e. a decrease in surface coverage of chemisorbed oxygen on both the Pd electrode and the TiO2 thin film, or formation of hydrogen atoms from water vapor on the surface of the Pd electrode and then dissolution of hydrogen atoms in the Pd electrode. A more serious problem is a decrease in H2 response induced by water vapor (compare Fig. 13(c) with (a)). This decrease in response may arise from disturbance of dissociated adsorption of H2 on the surface of the Pd electrode by water vapor, though the details are not clear at present. Consequently, an abrupt increase in sensor current followed by a drastic decrease in sensor cur-
Fig. 14. Temperature dependence of response of a Pd/TiO2 sensor to 500 ppm CO balanced with air under a forward bias voltage of 0.1 V.
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Fig. 15. Response transients of a Pd/A-TiO2 thin film sensor to 8000 ppm (a) CH4 , (b) C3 H8 and (c) n-C4 H10 balanced with (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
of the H2 sensing mechanism of the Pd/A-TiO2 sensor described above. Fig. 15 shows response transients of a Pd/A-TiO2 thin film sensor to several reducing gases under both air and N2 atmosphere [21]. The sensor exhibited negligibly small responses to CH4 in both air and N2 atmosphere. However, small and stable responses to C3 H8 and n-C4 H10 were achieved in air environment. Small responses to C3 H8 and n-C4 H10 were observed even in N2 atmosphere, but the sensor showed an abrupt increase in sensor current just after the exposure to reducing gases, followed by a gradual decrease in sensor current. Therefore, hydrogen atoms may be formed on the surface of the Pd electrode, especially for butane, although the origin of a complicated response transient in N2 atmosphere is not clear at present. 3.9. Effect of pretreatment conditions on H2 response properties of a Pd/A-TiO2 thin film sensor In the above sections, gas responses to H2 and other gases were measured with a normal Pd/A-TiO2 thin film sensor, i.e. the sensor after the firing at 600 ◦ C for 1 h in air. Here, effect of additional heat treatment in N2 at 600 ◦ C for 1 h on the H2 response properties was tested in both air and N2 atmospheres. Namely, the H2 response properties of a N2 -treated Pd/A-TiO2 thin film sensor were measured in air and N2 atmospheres. Fig. 16 shows response transients of a normal and a N2 -treated Pd/A-TiO2 thin film sensor to 0.8% H2 in both air and N2 at 250 ◦ C [22]. For easy comparison, a normal and a N2 -treated sensor are expressed as Ta and TN , respectively, and measurements in air and in N2 atmosphere are indicated as Ma and MN , respectively, in Fig. 16. H2 response properties of the normal sensor in both air and N2 atmospheres have already described (see response transients under the conditions “Ta –Ma ” and “Ta –MN ”). However, the N2 treated sensor showed almost no response in air (see response transient under the condition “TN –Ma ”), while a reversible H2 response, but a slightly decreased response, compared with that
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Fig. 16. Response transients of (a) a normal and (b) a N2 -treated Pd/A-TiO2 thin film sensor to 0.8% H2 measured in (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
in air, was observed in N2 (see response transient under the condition “TN –MN ”). Therefore, this diode-type Pd/A-TiO2 sensor was revealed to show the pretreatment-dependent H2 response. Such behavior is of course to be improved before being considered for practical application. To get some information to account for such pretreatmentdependent H2 response, changes in morphology of Pd induced by heat treatment were studied by SEM and SPM observation. Fig. 17 shows SEM and SPM images of the surfaces of Pd/ATiO2 thin films after different thermal treatments. In the case of a normal sensor, it is obvious that the electrode consists of very fine Pd particles (see small white dots in SEM image (a-2)). The size of Pd particles remained almost unchanged even after the normal sensor was placed under the measurement conditions in N2 atmosphere (see SEM image (b-2)). In contrast, aggregation of Pd particles is clearly observed in the case of a N2 -treated sensor (see large white particles in SEM image (c-2)). Then, such aggregation was kept under the measurement conditions in air (see SEM image (d-2)). These results suggest that such morphological change is responsible for no response to H2 in air at 250 ◦ C, due to decreases in both the activity of the Pd surface and the interface area between the Pd electrode and the A-TiO2 thin film. Less interference from such morphological changes of Pd particles in the case of H2 response measurement in N2 atmosphere may arise from a small amount of chemisorbed oxygen on the surface of large Pd particles, i.e. the chemisorbed oxygen under the condition of air atmosphere may highlight the deterioration of the H2 sensing ability induced by the morphological changes of Pd particles. 3.10. Effects of Pd electorde modification on H2 response properties of a Pd/A-TiO2 thin film sensor To realize pretreatment-independent H2 response, two modification methods for a Pd electrode were tested aiming at limiting
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Fig. 17. SEM and SPM images of the surfaces of Pd/A-TiO2 thin films after different thermal treatments: (A) a normal sensor, (B) a normal sensor treated at 250 ◦ C for 1 h in N2 (measurement conditions in N2 ), (C) a N2 -treated sensor, and (D) a N2 -treated sensor treated in 250 ◦ C for 1 h in air (measurement conditions in air).
or reducing the morphological changes of Pd particles. The first one was the adoption of a Pt-Pd layered structure instead of the Pd electrode, and the second one is the formation of a SiO2 layer over the Pd electrode. As described in Fig. 16, a N2 -treated Pd/A-TiO2 thin film sensor showed no H2 response in air, i.e. no H2 response under the TN –Ma condition in the case of an unmodified Pd electrode. But, the Pt-Pd/A-TiO2 thin film sensor showed certain H2 response under the TN –Ma condition, while its response was lower than that observed under the Ta –Ma condition, as shown in Fig. 18. In contrast, the (SiO2 (5)/Pd)/A-TiO2 thin film sensor showed almost the same H2 sensing properties, i.e. response magnitude and response and recovery speed, under both Ta –Ma and TN –Ma conditions. Therefore, it was confirmed that the SiO2 coating reduced the changes in morphology of Pd particles and then achieved almost pretreatment-independent H2 response. Another merit of the SiO2 coating was improvement of the magnitude of H2 response under all pretreatment and measurement conditions. This may arise from the limitation of O2 gas diffusion on the surfaces of both Pd and TiO2 particles by the SiO2 coating layer. Anyway, it is concluded that the H2 response
Fig. 18. Response transients of (a) normal and (b) N2 -treated Pd/A-TiO2 , PtPd/A-TiO2 and (SiO2 (5)/Pd)/A-TiO2 thin film sensors to 0.8% H2 measured in (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
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properties of a diode-type Pd/A-TiO2 thin film sensor could be improved by the modification of the Pd electrode. 4. Conclusions The diode-type TiO2 thin film sensor sandwiched with a sputtered Pd layer as a top electrode and a Ti plate as a bottom electrode showed reversible H2 response in both air and N2 atmospheres at elevated temperatures. Introduction of a welldeveloped submicron-sized pore structure into a TiO2 thin film and the choice of a Pd electrode were essential for achieving the high H2 response properties. The reversible H2 response in N2 atmosphere was suggested to come from its unique sensing mechanism different from that for the conventional semiconductor gas sensors: dissolution of H atoms into the Pd electrode and a decrease in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film. The H2 response decreased in the presence of water vapor, therefore elimination of the interference from water vapor is a subject to be solved in future study. But, the sensor showed almost no response to CO, while it exhibited certain response to butane. Although the present sensor showed pretreatment-dependent H2 response properties, the coating of the Pd electrode with an SiO2 layer was found to be useful for reducing or improving the pretreatment-dependent H2 response properties, probably by limitation or reduction of morphological changes of Pd particles during preheating under different conditions. Acknowledgement The present work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 18360333) from Japan Society for the Promotion of Science. References [1] A. Setkus, C. Baratto, E. Comini, G. Faglia, A. Galdikas, Z. Kancleris, G. Sberveglieri, D. Senuliene, Influence of metallic impurities on response kinetics in metal oxide thin film gas sensors, Sens. Actuators B 103 (2004) 448–456. [2] J. Wollenstein, J.A. Plaza, C. Cane, Y. Min, H. Bottner, H.L. Tuller, A novel single chip thin film metal oxide array, Sens. Actuators B 93 (2004) 350–355. [3] B.S. Kang, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, AlGaN/GaNbased metal-oxide-semiconductor diode-based hydrogen gas sensor, Appl. Phys. Lett. 84 (2004) 1123–1125. [4] W.-C. Liu, K.-W. Lin, H.-I. Chen, C.-K. Wang, C.-C. Cheng, S.-Y. Cheng, C.-T. Lu, A new Pt/oxide/In0.49 Ga0.51 P MOS Schottky diode hydrogen sensor, IEEE Electron. Device Lett. 23 (2002) 640–642. [5] I. Lundstr¨om, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogensensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [6] T. Tsukada, T. Kiwa, T. Yamaguchi, S. Migitaka, Y. Goto, K. Yokosawa, A study of fast response characteristics for hydrogen sensing with platinum FET sensor, Sens. Actuators B 114 (2006) 158–163. [7] G. Velayutham, C. Ramesh, N. Murugesan, V. Manivannan, K.S. Dhathathreyan, G. Periaswami, Nafion based amperometric hydrogen sensor, Ionics 10 (2004) 63–67. [8] B.K. Narayanan, S.A. Akbar, P.K. Dutta, A phosphate-based proton conducting solid electrolyte hydrocarbon gas sensor, Sens. Actuators B 87 (2002) 480–486.
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Biographies Yasuhiro Shimizu received his B Eng degree in applied chemistry in 1980 and Dr Eng degree in 1987 from Kyushu University. He has been a professor at Nagasaki University since 2005. His current research concentrates on design of intelligent sensors by controlling gas diffusivity and reactivity, development of new sensor materials and application of microwave-induced plasma. Takeo Hyodo received his B Eng degree in applied chemistry and M Eng degree in materials science and technology in 1992 and 1994, respectively, and Dr Eng
degree in 1997 from Kyushu University. He has been a research associate at Nagasaki University since 1997. His current interests are the development of electrochemical devices such as chemical sensors and lithium batteries, and mesoporous and macroporous materials. Makoto Egashira received his B Eng degree and M Eng degree in applied chemistry in 1966 and 1968, respectively, and Dr Eng degree in 1974 from Kyushu University. He has been a professor at Nagasaki University since 1985. His current interests include the development of new chemical sensors and surface modification of ceramics, preparation of hollow ceramic microspheres and porous films and application of microwave-induced plasma.
H2 sensing performance of anodically oxidized TiO2 thin films equipped with Pd electrode Yasuhiro Shimizu ∗ , Takeo Hyodo, Makoto Egashira Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan Available online 13 November 2006
Abstract Hydrogen sensing properties of TiO2 thin films prepared by different methods and equipped with different kinds of metal electrodes have been tested. The most developed submicron-sized pore structure was found in the TiO2 thin film prepared by anodic oxidation of a Ti metal plate in a H2 SO4 solution at 20 ◦ C. The sensor fabricated with this film and a Pd electrode showed the highest H2 response among the sensors fabricated with different TiO2 films and electrode metals at elevated temperatures. This TiO2 thin film sensor equipped with a Pd electrode can be classified into a diode-type sensor from the change in I–V curve induced by H2 , and is characterized with reversible H2 response even in N2 atmosphere. The H2 sensing mechanism is suggested to arise from dissolution of H atoms into the Pd electrode and a decrease in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film. Although the present sensor showed pretreatment-dependent H2 response properties, modification of the Pd electrode was found to be effective for reducing or improving the pretreatment-dependent H2 response properties. © 2006 Elsevier B.V. All rights reserved. Keywords: Anodically oxidized TiO2 ; Nanoholes; Pd electrode; H2 sensor; Schottky barrier; Diodo-type sensor; Sensing mechanism
1. Introduction The use of H2 as a clean energy source has been expanding into various fields such as fuel cell vehicles, household fuel cell cogeneration systems, and so on. This results in both rapid expansion of application fields of H2 gas sensors and a rise of new research current of H2 gas sensors. Hydrogen gas can be detected by various kinds of principles, methods and materials, e.g. by utilizing semiconductor metal oxides [1,2], diodes [3,4], FETs [5,6], solid electrolytes [7,8], fiber optics [9,10], quartz crystal microbalances [11], surface acoustic wave devices [12,13], etc. The number of studies and practical applications of titanium dioxide as a sensor material was very small, especially for a semiconductor gas sensor material, due to the very low reactivity of chemisorbed oxygen on its surface in the operating temperature range adopted for conventional semiconductor gas sensors. But, recent progress in nano-processing enables us to use TiO2 nano-powder as a semiconductor gas sensor material owning
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to an increase in surface to volume ratio [14–17]. Besides these TiO2 -based semiconductor gas sensors, TiO2 -based H2 gas sensors operated by other sensing mechanisms have been studied. The number of the latter type of sensors is very limited, but typical examples are diode-type sensors studied by Tonomura et al. [18] and ourselves [19–22], and a chemisorption-type sensor studied by Varghese et al. [23,24]. The difference in sensor structure between the Tonomura group’s and ours to be described in the present paper is the kind of a TiO2 material, i.e. a single crystal is their case and anodically oxidized TiO2 thin films are our case. And, differences between the Varghese group’s and ours are the anodic oxidation conditions of TiO2 thin films and the electrode configuration, i.e. a pair of Pt electrodes at the surface of the TiO2 film in their case [23] and a sandwich configuration of Pd (top electrode)/TiO2 /Ti (served as both a bottom electrode and a substrate) in our case, as will be explained later. In the present paper, important factors affecting the H2 sensing performance of an anodically oxidized TiO2 film equipped with a Pd electrode are discussed comprehensively, by citing our published results and by adding new data on the modification of the Pd electrode. The remaining problems, which are to be solved before its practical application, are also mentioned.
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2. Experimental 2.1. Preparation and characterization of TiO2 thin films A half part of a Ti plate (5.0 mm × 10.0 mm × 0.5 mm) was inserted in an aqueous 0.5 M H2 SO4 solution, and then was anodically oxidized at a current density of 100 mA cm−2 for 30 min at 20 ◦ C, unless otherwise noted. In evaluating the effects of anodic oxidation conditions on the microstrucutre and the H2 response properties of the resulting TiO2 thin films, anodic oxidation was also conducted at 5 and 30 ◦ C. Another anodically oxidized TiO2 thin film was prepared in an aqueous 1.0 M NaOH solution under the same conditions. The two anodic TiO2 thin films are denoted as A-TiO2 and N-TiO2 , respectively. A-TiO2 thin film was also prepared by dry oxidation of a Ti plate at 600 ◦ C for 1 h in air, and is referred to as R-TiO2 . The thickness of A-TiO2 , N-TiO2 and R-TiO2 was 0.25, 0.10 and 0.15 m, respectively. The crystal phases in the TiO2 thin films were characterized with X-ray diffraction (Rigaku, RINT 2200), and the microstructure was observed with a scanning electron microscope (SEM, Hitachi, S-2250N). Morphology of the TiO2 thin films was observed by a transmission electron microscope (TEM, JEOL, JEM2010-HT). 2.2. Fabrication and measurement of H2 response of TiO2 thin film sensors Pd electrodes were sputtered (JEOL, JFC-1100 or Shimadzu, HSR-552S) for 10 min on both the TiO2 thin film and the Ti plate, and the electrical contact to lead wires was ensured by application of Pd paste and subsequent firing at 600 ◦ C for 1 h in air, unless otherwise noted. Such a sensor will be referred to as a normal sensor and is distinguished from another sensor, which was subjected to additional treatment in N2 at 600 ◦ C for 1 h before the measurement of H2 response. The latter sensor will be expressed as a N2 -treated sensor. In both cases, the electrical properties of the TiO2 thin films were measured with Pd (top) and Ti (bottom) electrodes. The electrode configuration of the sensors adopted in the present study can be seen in Fig. 1 [21]. In evaluating the effect of electrode metals on the H2 response properties, TiO2 thin film sensors equipped with sputtered Au or Pt were also fabricated, in a similar manner to that described above.
Fig. 1. Sensor structure of Pd/TiO2 thin film sensors.
A dc voltage of 0.1 V was applied to the TiO2 thin film sensors and transient changes in current flowing through the sensors upon exposure to 0.8 or 1.0% H2 balanced with dry air or dry N2 were monitored in the temperature range of 250–500 ◦ C. But, the words “dry air” or “dry N2 ” as a balance gas will be expressed as “air” or “N2 ” throughout the text for the sake of simplicity. The measurements were done under reverse {Pd(−)–TiO2 –Ti(−)} or forward {Pd(+)–TiO2 –Ti(−)} bias conditions. The response magnitude (k) was defined as a ratio (Ig /Ia or Ig /In ) of the sensor current in the sample gas (Ig ) to that in air (Ia ) or N2 (In ). Occasionally, definition of a ratio of (Ra /Rg or Rn /Rg ) of the sensor resistance in air (Ra ) or in N2 (Rn ) to that in the sample gas was also used. Responses of the A-TiO2 thin film sensor equipped with a Pd electrode (Pd/A-TiO2 ) to 2.3% H2 O water vapor, 500 ppm CO, 8000 ppm CH4 , C3 H8 and n-C4 H10 were also measured to evaluate the cross-sensitivity to these gases. 2.3. Characterization of oxidation state and morphology of a Pd electrode The change in oxidation state of the Pd electrode at the A-TiO2 sensor surface was measured with X-ray photoelectron spectroscopy (XPS, Shimadzu, ESCA-850M). The XPS measurement was conducted with three samples: a normal Pd/ATiO2 thin film sensor, a normal sensor subsequently heat treated at 250 ◦ C for 4 h in 1.0% H2 balanced with air, and a N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 . The change in morphology of the Pd electrode at the A-TiO2 sensor surface was measured with SEM and a scanning probe microscope (SPM, Shimadzu, SPM-9500PP). Morphology of four kinds of sensors, which were subjected to different heat treatments, was compared with each other: a normal Pd/A-TiO2 thin film sensor, a normal sensor subsequently heat treated at 250 ◦ C for 1 h in N2 , a N2 -treated sensor, and a N2 -treated sensor subsequently heat treated at 250 ◦ C for 1 h in air. 2.4. Effect of modification of a Pd electrode Effect of two modification methods for the Pd electrode on H2 sensing properties was studied. One is formation of Pt-Pd layered structure of a metal electrode, i.e. 5 min sputtering of Pd at a power of 500 W on an A-TiO2 thin film and additional 5 min sputtering of Pt at the same power on the Pd layer. This sensor will be expressed by a Pt-Pd/A-TiO2 thin film sensor. Another modification is coating of a normal Pd/A-TiO2 thin film sensor with a SiO2 layer, especially at the Pd surface. A SiO2 sol solution was prepared by dissolving diethoxydimethylsilane (DEMS) into a n-C4 H9 OH-based solution at a molar ratio of DEMS: n-C4 H9 OH:H2 O:HCl = 1:5:5:0.05 and refluxing the mixture at 50 ◦ C for 3 h. Then a Pd/A-TiO2 sensor was dipcoated with the SiO2 sol solution. The sensor was then subjected to drying at 80 ◦ C for 10 min in air and subsequent firing at 600 ◦ C for 10 min in air. This procedure repeated up to 5 cycles. The resulting sensor is then expressed as a (SiO2 (5)/Pd)/A-TiO2 thin film sensor.
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Fig. 3. XRD patterns of as-prepared (a) A-TiO2 and (b) R-TiO2 thin films.
The as-prepared A-TiO2 thin film was a mixture of anatase and rutile phases, while R-TiO2 was a single phase of rutile, as shown in Fig. 3(a and b), respectively [19]. The fraction of the rutile phase in A-TiO2 increased after the firing, although the data is not shown here. Fig. 4 shows the TEM photographs of the A-TiO2 fired at 600 ◦ C for 1 h in air [19]. It was obvious
Fig. 2. SEM photographs of the surface of as-prepared (a) A-TiO2 , (b) N-TiO2 and (c) R-TiO2 thin films.
3. Results and discussion 3.1. Characterization of TiO2 thin films Fig. 2 compares the surface morphology of three kinds of as-prepared TiO2 thin films [19]. Formation of submicron-sized pores in the range of 50–130 nm in diameter was observed on the surface of an as-prepared A-TiO2 thin film, as shown in Fig. 2(a). In contrast, N-TiO2 and R-TiO2 consisted of densely sintered particles and little pores were observed on their surfaces, as shown in Fig. 2(b and c). In addition, no visible change in microstructure was found in each TiO2 thin film during the firing at 600 ◦ C for 1 h in air adopted for ensuring the electrical contact as a sensor.
Fig. 4. TEM photographs of an A-TiO2 thin film fired at 600 ◦ C for 1 h in air.
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Fig. 5. Response transients of three kinds of Pd/TiO2 thin film sensors to 1.0% H2 under a reverse bias voltage of 0.1 V.
that A-TiO2 was columnar polycrystallites. Electron diffraction analysis revealed that the columnar top was rutile, but the bottom was a mixture of rutile and anatase.
3.3. Effects of anodic oxidation temperature on microstructre and H2 sensing properties of resulting A-TiO2 thin films
3.2. Effect of the kind of TiO2 thin films on H2 sensing properties
Since the highest H2 response could be achieved with the ATiO2 thin film prepared at an anodic oxidation temperature of 20 ◦ C in H2 SO4 solution, effects of anodic oxidation temperature on the microstructure and H2 sensing properties of the resulting A-TiO2 thin films were further tested. Fig. 6 shows SEM photographs of the surface of A-TiO2 thin films prepared at three different anodic oxidation temperatures [20]. Good reproducibility for the formation of submicron-size pores at an anodic oxidation temperature of 20 ◦ C could be confirmed by the comparison between Figs. 2(a) and 6(b). Although submicron-sized pores were also formed at anodic oxidation temperatures of 5 and 30 ◦ C, as shown in Fig. 6(a and c), the most developed pore structure was achieved at an anodic oxidation temperature of 20 ◦ C. All the films were again confirmed to be a mixture of anatase and rutiles phases, irrespective of the anodic oxidation temperature, although the data is not shown here. Fig. 7 shows response transients of three kinds of Pd/A-TiO2 thin film sensors prepared at different anodic oxidation temperatures to 1.0% H2 under both a reverse and a forward bias voltage of 0.1 V at 250 ◦ C [20]. Among three sensors, the Pd/A-TiO2 sensor prepared at 20 ◦ C exhibited the highest sensor current in 1.0% H2 and therefore the highest H2 response under both the bias conditions. These results suggested the importance of the existence of developed pore structure in realizing high H2 response. Therefore, A-TiO2 thin films prepared at 20 ◦ C were used for further studies aiming at improving the H2
Response transients of three kinds of Pd/TiO2 thin film sensors to 1.0% H2 balanced with air under a reverse bias voltage of 0.1 V are compared in Fig. 5 [19]. The Pd/A-TiO2 sensor exhibited very quick response with a current increase, i.e. a decrease in the sensor resistance, in H2 and rather fast recovery to an original air level after removal of H2 at 250 ◦ C, whereas the response in air was rather noisy. The magnitude of response defined as a ratio of the current in 1.0% H2 balanced with air to that in air is indicated in each figure. Both the sensor resistance and the response decreased with a rise in temperature. Similar measurement was conducted with Pd/N-TiO2 and Pd/R-TiO2 thin film sensors, and the results are also shown in Fig. 5. The sensor resistance of Pd/R-TiO2 and Pd/N-TiO2 in air was smaller than that of Pd/A-TiO2 by one or two orders of magnitude at the same temperature, and then their responses to 1.0% H2 were lower as well. Temperature dependence of the H2 response observed for these two sensors was different from that observed for the Pd/A-TiO2 thin film sensor, i.e. the maximum responses appeared at 300 ◦ C, suggesting the different H2 sensing mechanism from that of the Pd/A-TiO2 thin film sensor. Anyway, it was revealed from these results that the response properties of the sensors were markedly dependent upon the kind, i.e. preparation procedure, of the TiO2 thin films.
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depending upon the level of the sensor resistance, while the total applied voltage to the circuit, in which the sensor was connected in series with the standard resistance, remained constant to be 2.0 V. Therefore, the voltage applied actually to the sensor varied with atmosphere, and therefore the H2 sensing properties of the sensors shown in Fig. 8 cannot be compared directly with those appeared in other figures. For each sensor, figure (a) shows the response transient, (b) the sensor resistance in both air and 1.0% H2 balanced with air and (c) shows the magnitude of H2 response. When, Au was employed, the sensor showed certain H2 response, but almost one tenth of that obtained with the Pd/ATiO2 thin film sensor at the same temperature along with slower response and recovery behavior, as shown in Fig. 8(B). In the case of the Pt/A-TiO2 thin film sensor, the response magnitude to 1.0% H2 was very small, as shown in Fig. 8(C). Therefore, it is confirmed that the use of palladium is essential for achieving a high H2 response with this type of sensors. 3.5. Change in oxidation state of a Pd electrode in Pd/A-TiO2 thin film sensors
Fig. 6. SEM photographs of the surface of as-prepared A-TiO2 thin films at an anodic oxidation temperature of (a) 5 ◦ C, (b) 20 ◦ C and (c) 30 ◦ C.
sensing performance. The differences in sensor current in H2 and response time induced by the change in the direction of the dc electric field were small under an applied voltage of 0.1 V. 3.4. Effect of electrode metals on H2 sensing properties of A-TiO2 thin films Effect of the kind of metals used as a top electrode was tested. Fig. 8 compares the H2 response properties among Pd/A-TiO2 , Au/A-TiO2 and Pt/A-TiO2 thin film sensors [19]. In these measurements, the sensor resistance was calculated by referring to a voltage drop across a standard resistance connected in series with the sensor, and the value of standard resistance was changed
The change in the oxidation state of the Pd electrode of the Pd/A-TiO2 thin film sensor induced by the treatment in different atmospheres was studied by XPS. Fig. 9 shows variations in the Pd 3d photoelectron spectrum of three kinds of Pd/A-TiO2 thin film sensors with sputtering time [20]. The Pd 3d spectrum from a normal sensor shows two peaks, Pd 3d5/2 and Pd 3d3/2 , located at a binding energy of about 335.5 and 340.6 eV, respectively, after 1 min Ar sputtering, as shown in Fig. 9(a). In addition, each peak is accompanied with a small shoulder peak at a slightly higher binding energy. By referring to the data in literature [25–27], the shoulder peaks can be assigned to PdO. The intensity of these two shoulder peaks decreased obviously with sputtering time. Thus, it was confirmed that the surface of the Pd electrode was partially oxidized during the firng at 600◦ C for 1 h in air and was covered with a PdO layer. However, the Pd 3d spectrum from the normal sensor subsequently treated in 1.0% H2 balanced with air at 250 ◦ C for 4 h showed no shoulder peaks even after 1 min sputtering, as shown in Fig. 9(b), indicating the reduction of PdO to Pd in 1.0% H2 balanced with air. In addition, no PdO left behind on the surface of the Pd electrode in the case of the N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 , as shown in Fig. 9(c). These results indicate that the surface of the Pd electrode is likely oxidized in air atmosphere, but the oxide layer is likely recuded to metallic Pd under both H2 -containig air and N2 . 3.6. Other features of Pd/A-TiO2 thin film sensors Another notable feature of a Pd/A-TiO2 thin film sensor was the reversible response to H2 even in N2 atmosphere. Fig. 10 shows a response transient of the Pd/A-TiO2 thin film to 1.0% H2 balanced with N2 [19]. It is obvious that the sensor exhibits certain H2 response along with rather quick response and complete recovery to an original N2 level in a short time. These results suggest less importance of chemisorbed oxygen on both
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Fig. 7. Response transients of Pd/A-TiO2 thin film sensors prepared at different anodic oxidation temperatures to 1.0% H2 balanced with air under both a reverse and a forward bias voltage of 0.1 V at 250 ◦ C.
the surfaces of the Pd electrode and the TiO2 thin film in detecting H2 . The most important feature of a Pd/A-TiO2 thin film sensor was a change in I–V characteristics induced by H2 , as shown in Fig. 11 [19]. A nonlinear I–V curve, which is typical of a diodetype sensor, was observed in air, but it changed to an almost ohmic I–V curve in 1.0% H2 balanced with air. This result indicates the large contribution of the Schottky barrier at the interface between the Pd electrode and the A-TiO2 thin film in detecting H2 .
3.7. H2 gas sensing mechanism of Pd/A-TiO2 thin film sensors Three possible factors can be anticipated for explaining the increase in sensor current (or the decrease in sensor resistance). Factor 1 is the consumption of chemisorbed oxygen on the surface of TiO2 , which leads to electron transfer (return back of electrons) from chemisorbed oxygen to TiO2 particles. This mechanism is the most well-known basic gas-sensing mechanism in the case of semiconductor gas sensors in detecting
Fig. 8. (a) Response transient to 1.0% H2 balanced with air at 250 ◦ C and the temperature dependences of (b) resistance in air and in 1.0% H2 balanced with air and (c) response to 1.0% H2 of three A-TiO2 thin film sensors equipped with different electrodes under reverse bias conditions.
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Fig. 9. Variations in Pd 3d photoelectron spectrum of several Pd/A-TiO2 thin film sensors with sputtering time: (a) a normal sensor, (b) a normal sensor subsequently heat treated at 250 ◦ C for 4 h in 1.0% H2 balanced with air, and (c) a N2 -treated sensor subsequently heat treated at 250 ◦ C for 4 h in 0.8% H2 balanced with N2 .
reducing gases at elevated temperatures [28]. Factor 2 is the consumption of chemisorbed oxygen on the surface of the Pd electrode (or reduction of PdO on the surface of the Pd electrode), which leads to a decrease in work function of the Pd
Fig. 10. A response transient of a Pd/A-TiO2 thin film sensor to 1.0% H2 balanced with N2 at 250 ◦ C under a reverse bias voltage of 0.1 V.
Fig. 11. I–V characteristics of a Pd/A-TiO2 thin film sensor at 250 ◦ C.
electrode and in turn electron transfer from the Pd electrode to the TiO2 particles. Namely, Factor 2 is equal to the mechanism which is so-called “electronic sensitization” observed for semiconductor gas sensors catalyzed by loaded Pd [29]. However, in the case of the present Pd/A-TiO2 thin film sensor, these two factors are confirmed to be less important from the fact that the sensor shows reversible response to H2 even in N2 atmosphere. Factor 3, which is the most conceivable mechanism for the present sensor, is the decreases in work function of the Pd electrode and then in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film induced by dissolution of H atoms into the Pd electrode. The H2 sensing mechanism based on this variation in Schottky barrier height is schematically illustrated in Fig. 12 [19]. The Schottky barrier is formed reasonably at the interface between the Pd electrode and the TiO2 thin film by the electron transfer from TiO2 to Pd due to the higher value of the work function (φ) of Pd (4.95 eV) than the electron affinity (χ) of TiO2 (4.33 eV) [30], while an ohmic contact is considered to be achieved at the interface between the bottom Ti electrode (served also as a substrate) and the TiO2 thin film. Formation of chemisorbed oxygen or partial oxidation of the sputtered Pd electrode surface is likely occurred after the firing at 600 ◦ C for 1 h in air (i.e. a normal sensor) or during the measurement in the temperature range of 250–400 ◦ C in air. This results in an increase in work function of Pd (φ ) and then a higher barrier height at the interface between the Pd electrode and the TiO2 thin film in air. The existence of this barrier is responsible for a low sensor current in air under the reverse and forward bias conditions. The chemisorbed oxygen on the Pd electrode are removed with H2 , leading to a lower barrier height, although this is not the main factor for the H2 response of the present sensor. Beside this factor, H atoms dissociatively formed on the Pd electrode surface are assumed to be dissolved into Pd and migrate to the interface, resulting in further lowering of the barrier height. This is considered to be the main reason for the reversible response to H2 of the present sensor in N2 atmosphere.
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Fig. 13. Effect of 2.3% water vapor on the response behavior of a Pd/A-TiO2 thin film sensor to 1.0% H2 under a forward bias voltage of 0.1 V.
Fig. 12. H2 sensing mechanism based on variation in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film.
The lower H2 response of the Pd/N-TiO2 and Pd/R-TiO2 thin film sensors than the Pd/A-TiO2 thin film sensor can be ascribed to less formation of submicron-sized pores which allow gaseous H2 to diffuse into the interface between Pd and TiO2 particles and make more active reaction sites on the surface of TiO2 particles. The higher H2 response of the Pd/A-TiO2 sensor prepared at an anodic oxidation temperature of 20 ◦ C than those prepared at different anodic oxidation temperatures may be explained by the similar reason. The lower H2 response of the Au/A-TiO2 and the Pt/A-TiO2 sensors comes undoubtedly from the low solubility of H atoms in these metals and then the less-developed sensitive barrier height to H2 .
rent approaching a steady-state value was observed when the surrounding atmosphere was changed from dry air to 1.0% H2 balanced with wet air, as shown in Fig. 13(d). Therefore, elimination of the interference from water vapor is a big subject in achieving a high H2 response. Temperature dependence of the response of a Pd/A-TiO2 thin film sensor to 500 ppm CO balanced with air is shown in Fig. 14 [19]. The response to CO was very small over the whole temperature range of 250–400 ◦ C and the maximum response was approximately 2 at most at 400 ◦ C. Such small response may arise from consumption of chemisorbed oxygen on both the surfaces of Pd and TiO2 particles, and then may prove the validity
3.8. Response of a Pd/A-TiO2 thin film sensor to other gases Fig. 13 shows the effect of 2.3% water vapor (express as “wet air” in Fig. 13) on the H2 response behavior of a Pd/A-TiO2 thin film sensor [19]. As described above, the sensor showed a large H2 response and quick response and recovery behavior in dry air at 250 ◦ C (see Fig. 13(a)). At the same temperature, the sensor exhibited a small response of approximately 10 to 2.3% water vapor, as shown in Fig. 13(b). This may suggest the displacement of chemisorbed oxygen with hydroxyl groups, i.e. a decrease in surface coverage of chemisorbed oxygen on both the Pd electrode and the TiO2 thin film, or formation of hydrogen atoms from water vapor on the surface of the Pd electrode and then dissolution of hydrogen atoms in the Pd electrode. A more serious problem is a decrease in H2 response induced by water vapor (compare Fig. 13(c) with (a)). This decrease in response may arise from disturbance of dissociated adsorption of H2 on the surface of the Pd electrode by water vapor, though the details are not clear at present. Consequently, an abrupt increase in sensor current followed by a drastic decrease in sensor cur-
Fig. 14. Temperature dependence of response of a Pd/TiO2 sensor to 500 ppm CO balanced with air under a forward bias voltage of 0.1 V.
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Fig. 15. Response transients of a Pd/A-TiO2 thin film sensor to 8000 ppm (a) CH4 , (b) C3 H8 and (c) n-C4 H10 balanced with (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
of the H2 sensing mechanism of the Pd/A-TiO2 sensor described above. Fig. 15 shows response transients of a Pd/A-TiO2 thin film sensor to several reducing gases under both air and N2 atmosphere [21]. The sensor exhibited negligibly small responses to CH4 in both air and N2 atmosphere. However, small and stable responses to C3 H8 and n-C4 H10 were achieved in air environment. Small responses to C3 H8 and n-C4 H10 were observed even in N2 atmosphere, but the sensor showed an abrupt increase in sensor current just after the exposure to reducing gases, followed by a gradual decrease in sensor current. Therefore, hydrogen atoms may be formed on the surface of the Pd electrode, especially for butane, although the origin of a complicated response transient in N2 atmosphere is not clear at present. 3.9. Effect of pretreatment conditions on H2 response properties of a Pd/A-TiO2 thin film sensor In the above sections, gas responses to H2 and other gases were measured with a normal Pd/A-TiO2 thin film sensor, i.e. the sensor after the firing at 600 ◦ C for 1 h in air. Here, effect of additional heat treatment in N2 at 600 ◦ C for 1 h on the H2 response properties was tested in both air and N2 atmospheres. Namely, the H2 response properties of a N2 -treated Pd/A-TiO2 thin film sensor were measured in air and N2 atmospheres. Fig. 16 shows response transients of a normal and a N2 -treated Pd/A-TiO2 thin film sensor to 0.8% H2 in both air and N2 at 250 ◦ C [22]. For easy comparison, a normal and a N2 -treated sensor are expressed as Ta and TN , respectively, and measurements in air and in N2 atmosphere are indicated as Ma and MN , respectively, in Fig. 16. H2 response properties of the normal sensor in both air and N2 atmospheres have already described (see response transients under the conditions “Ta –Ma ” and “Ta –MN ”). However, the N2 treated sensor showed almost no response in air (see response transient under the condition “TN –Ma ”), while a reversible H2 response, but a slightly decreased response, compared with that
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Fig. 16. Response transients of (a) a normal and (b) a N2 -treated Pd/A-TiO2 thin film sensor to 0.8% H2 measured in (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
in air, was observed in N2 (see response transient under the condition “TN –MN ”). Therefore, this diode-type Pd/A-TiO2 sensor was revealed to show the pretreatment-dependent H2 response. Such behavior is of course to be improved before being considered for practical application. To get some information to account for such pretreatmentdependent H2 response, changes in morphology of Pd induced by heat treatment were studied by SEM and SPM observation. Fig. 17 shows SEM and SPM images of the surfaces of Pd/ATiO2 thin films after different thermal treatments. In the case of a normal sensor, it is obvious that the electrode consists of very fine Pd particles (see small white dots in SEM image (a-2)). The size of Pd particles remained almost unchanged even after the normal sensor was placed under the measurement conditions in N2 atmosphere (see SEM image (b-2)). In contrast, aggregation of Pd particles is clearly observed in the case of a N2 -treated sensor (see large white particles in SEM image (c-2)). Then, such aggregation was kept under the measurement conditions in air (see SEM image (d-2)). These results suggest that such morphological change is responsible for no response to H2 in air at 250 ◦ C, due to decreases in both the activity of the Pd surface and the interface area between the Pd electrode and the A-TiO2 thin film. Less interference from such morphological changes of Pd particles in the case of H2 response measurement in N2 atmosphere may arise from a small amount of chemisorbed oxygen on the surface of large Pd particles, i.e. the chemisorbed oxygen under the condition of air atmosphere may highlight the deterioration of the H2 sensing ability induced by the morphological changes of Pd particles. 3.10. Effects of Pd electorde modification on H2 response properties of a Pd/A-TiO2 thin film sensor To realize pretreatment-independent H2 response, two modification methods for a Pd electrode were tested aiming at limiting
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Fig. 17. SEM and SPM images of the surfaces of Pd/A-TiO2 thin films after different thermal treatments: (A) a normal sensor, (B) a normal sensor treated at 250 ◦ C for 1 h in N2 (measurement conditions in N2 ), (C) a N2 -treated sensor, and (D) a N2 -treated sensor treated in 250 ◦ C for 1 h in air (measurement conditions in air).
or reducing the morphological changes of Pd particles. The first one was the adoption of a Pt-Pd layered structure instead of the Pd electrode, and the second one is the formation of a SiO2 layer over the Pd electrode. As described in Fig. 16, a N2 -treated Pd/A-TiO2 thin film sensor showed no H2 response in air, i.e. no H2 response under the TN –Ma condition in the case of an unmodified Pd electrode. But, the Pt-Pd/A-TiO2 thin film sensor showed certain H2 response under the TN –Ma condition, while its response was lower than that observed under the Ta –Ma condition, as shown in Fig. 18. In contrast, the (SiO2 (5)/Pd)/A-TiO2 thin film sensor showed almost the same H2 sensing properties, i.e. response magnitude and response and recovery speed, under both Ta –Ma and TN –Ma conditions. Therefore, it was confirmed that the SiO2 coating reduced the changes in morphology of Pd particles and then achieved almost pretreatment-independent H2 response. Another merit of the SiO2 coating was improvement of the magnitude of H2 response under all pretreatment and measurement conditions. This may arise from the limitation of O2 gas diffusion on the surfaces of both Pd and TiO2 particles by the SiO2 coating layer. Anyway, it is concluded that the H2 response
Fig. 18. Response transients of (a) normal and (b) N2 -treated Pd/A-TiO2 , PtPd/A-TiO2 and (SiO2 (5)/Pd)/A-TiO2 thin film sensors to 0.8% H2 measured in (A) air and (B) N2 at 250 ◦ C under a forward bias voltage of 0.1 V.
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properties of a diode-type Pd/A-TiO2 thin film sensor could be improved by the modification of the Pd electrode. 4. Conclusions The diode-type TiO2 thin film sensor sandwiched with a sputtered Pd layer as a top electrode and a Ti plate as a bottom electrode showed reversible H2 response in both air and N2 atmospheres at elevated temperatures. Introduction of a welldeveloped submicron-sized pore structure into a TiO2 thin film and the choice of a Pd electrode were essential for achieving the high H2 response properties. The reversible H2 response in N2 atmosphere was suggested to come from its unique sensing mechanism different from that for the conventional semiconductor gas sensors: dissolution of H atoms into the Pd electrode and a decrease in Schottky barrier height at the interface between the Pd electrode and the TiO2 thin film. The H2 response decreased in the presence of water vapor, therefore elimination of the interference from water vapor is a subject to be solved in future study. But, the sensor showed almost no response to CO, while it exhibited certain response to butane. Although the present sensor showed pretreatment-dependent H2 response properties, the coating of the Pd electrode with an SiO2 layer was found to be useful for reducing or improving the pretreatment-dependent H2 response properties, probably by limitation or reduction of morphological changes of Pd particles during preheating under different conditions. Acknowledgement The present work was partly supported by a Grant-in-Aid for Scientific Research (B) (No. 18360333) from Japan Society for the Promotion of Science. References [1] A. Setkus, C. Baratto, E. Comini, G. Faglia, A. Galdikas, Z. Kancleris, G. Sberveglieri, D. Senuliene, Influence of metallic impurities on response kinetics in metal oxide thin film gas sensors, Sens. Actuators B 103 (2004) 448–456. [2] J. Wollenstein, J.A. Plaza, C. Cane, Y. Min, H. Bottner, H.L. Tuller, A novel single chip thin film metal oxide array, Sens. Actuators B 93 (2004) 350–355. [3] B.S. Kang, F. Ren, B.P. Gila, C.R. Abernathy, S.J. Pearton, AlGaN/GaNbased metal-oxide-semiconductor diode-based hydrogen gas sensor, Appl. Phys. Lett. 84 (2004) 1123–1125. [4] W.-C. Liu, K.-W. Lin, H.-I. Chen, C.-K. Wang, C.-C. Cheng, S.-Y. Cheng, C.-T. Lu, A new Pt/oxide/In0.49 Ga0.51 P MOS Schottky diode hydrogen sensor, IEEE Electron. Device Lett. 23 (2002) 640–642. [5] I. Lundstr¨om, S. Shivaraman, C. Svensson, L. Lundkvist, A hydrogensensitive MOS field-effect transistor, Appl. Phys. Lett. 26 (1975) 55–57. [6] T. Tsukada, T. Kiwa, T. Yamaguchi, S. Migitaka, Y. Goto, K. Yokosawa, A study of fast response characteristics for hydrogen sensing with platinum FET sensor, Sens. Actuators B 114 (2006) 158–163. [7] G. Velayutham, C. Ramesh, N. Murugesan, V. Manivannan, K.S. Dhathathreyan, G. Periaswami, Nafion based amperometric hydrogen sensor, Ionics 10 (2004) 63–67. [8] B.K. Narayanan, S.A. Akbar, P.K. Dutta, A phosphate-based proton conducting solid electrolyte hydrocarbon gas sensor, Sens. Actuators B 87 (2002) 480–486.
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Biographies Yasuhiro Shimizu received his B Eng degree in applied chemistry in 1980 and Dr Eng degree in 1987 from Kyushu University. He has been a professor at Nagasaki University since 2005. His current research concentrates on design of intelligent sensors by controlling gas diffusivity and reactivity, development of new sensor materials and application of microwave-induced plasma. Takeo Hyodo received his B Eng degree in applied chemistry and M Eng degree in materials science and technology in 1992 and 1994, respectively, and Dr Eng
degree in 1997 from Kyushu University. He has been a research associate at Nagasaki University since 1997. His current interests are the development of electrochemical devices such as chemical sensors and lithium batteries, and mesoporous and macroporous materials. Makoto Egashira received his B Eng degree and M Eng degree in applied chemistry in 1966 and 1968, respectively, and Dr Eng degree in 1974 from Kyushu University. He has been a professor at Nagasaki University since 1985. His current interests include the development of new chemical sensors and surface modification of ceramics, preparation of hollow ceramic microspheres and porous films and application of microwave-induced plasma.