IEEE SENSORS JOURNAL, VOL. 7, NO. 12, DECEMBER 2007
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Dissociative Gas Sensing at Metal Oxide Surfaces Andreas Helwig, Gerhard Müller, Martin Eickhoff, and Giorgio Sberveglieri
Abstract—The low- and high-temperature gas sensing behavior of hydrogenated diamond (HD) and metal oxide (MOx) materials is compared and contrasted. We present evidence that at room temperature and above both kinds of materials are coated with a thin surface electrolyte layer in which gas molecules can be adsorbed and in which adsorbed gases may undergo electrolytic dissociation. We show that both kinds of materials respond in a very similar way when exposed to acid and base vapors and that no gas response is observed otherwise. Heating beyond 200 C removes the surface electrolyte layer from both kinds of materials. Whereas at MOx surfaces, the established combustive gas sensing effect sets in, the surface conductivity and the gas sensitivity of HD samples is lost due to the disappearance of the surface transfer doping effect. Index Terms—Diamond, electrolytic dissociation, gas sensing mechanism, metal oxide, pH sensitivity, surface combustion, surface transfer doping.
I. INTRODUCTION
W
E HAVE SHOWN in a recent publication that hydrogenated diamond (HD) samples selectively respond to NO and NH gases at room temperature [1]. We have provided further evidence that this kind of gas sensitivity is due to electrolytic dissociation of NO and NH molecules in a thin adsorbed liquid electrolyte layer. This conclusion is consistent with several experimental observations, which have been made in investigations aimed at elucidating the phenomenon of surface transfer doping at hydrogen-terminated diamond (HD) surfaces [2]–[6]. Diamond with a bandgap of 5.5 eV, in principle, is an insulator. In case diamond surfaces are hydrogen-terminated, however, a thin, highly conductive, near-surface p-type layer is observed. This conductive layer is assumed to originate from the fact that the top of the HD valence band is well aligned with the H O/H O redox level in the adsorbed surface electrolyte [6]. The ensuing conducting channel in the subsurface diamond region has attracted considerable fundamental interest and it has also paved the way to interesting device applications of HD samples such as field effect transistors [7] or pH-sensors [8]. Manuscript received May 3, 2007; revised July 11, 2007; accepted September 24, 2007. The work at the Walter Schottky Institut was supported in part by the Deutsche Forschungsgemeinschaft DFG (Ei 182/1-3). The work at EADS Innovation Works was supported in part by the European Union under Project NANOS4, FP6-2002-NMP-1, No. 001528. The associate editor coordinating the review of this paper and approving it for publication was Prof. Istvan Barsony. A. Helwig and G. Müller are with the EADS Innovation Works Germany, EADS Deutschland GmbH, D-81663 München, Germany (e-mail:
[email protected]). M. Eickhoff is with the Walter Schottky Institut, Technische Universität München, D-85748 Garching, München, Germany. G. Sberveglieri is with the Sensor Laboratory, CNR-INFM, University of Brescia, Brescia, I-25133 Italy. Digital Object Identifier 10.1109/JSEN.2007.909428
The presence of adsorbed water layers, however, is by no means limited to diamond. Water adsorption rather is a very general phenomenon that occurs on virtually all solid surfaces [9]. For this reason, we have investigated the gas sensing properties of metal oxide (MOx) and HD surfaces under experimental conditions, which make a surface electrolyte layer likely to exist. In normal operation, MOx gas sensors are heated to temperatures in the order of 400 C, i.e., into a range of temperatures at which adsorbed water layers are highly unlikely to exist. In this high-temperature range, electrically detectable surface combustion is promoted [10], [11], which involves surface oxygen ions and adsorbed analyte gas species. In recent years, however, there have also been reports on room temperature gas sensing effects on MOx surfaces that are enhanced by UV light and that seems to exhibit some selectivity towards NO [12]–[15]. In more recent reports, room-temperature gas sensitivity to short-chain alcohols and to acetone was also demonstrated [16], [17]. In these latter investigations, however, very few clues were given to the actual mechanisms that underlie this low-temperature gas sensing effect. In the present communication, we show that the room-temperature gas sensitivity in both MOx and HD materials is photoactivated by UV light and strongly biased towards NO and NH , i.e., towards analyte gases that are capable of electrolytic dissociation in the adsorbed surface electrolyte. We further show that once the surface electrolyte has been evaporated from the MOx surfaces, MOx gas sensors return to their normal and widely known broadband combustive gas sensing behavior. II. EXPERIMENTAL The HD samples investigated were nominally undoped (100)oriented type Ib diamond substrates. In a first processing step, the diamond samples were cleaned in CrO /H SO for 1 h at 180 C to remove nondiamond components. Afterwards, the diamond surfaces were oxygen-terminated by a 15 min dip in a 30% H O -solution. The functional hydrogen termination of the diamond surfaces was accomplished by exposing the samples to a hot-wire generated flow of atomic hydrogen in a high vacuum chamber for 30 min at a substrate temperature of 540 C [8]. Sensing resistors were delineated by standard photolithography and oxygen-plasma treatments were used to form insulating surface areas around the sensing resistors. Finally, Ti/Au (20 nm/200 nm) metal pads were deposited on the remaining patches of H-terminated diamond to form ohmic contacts. After a set of initial measurements, the level of the H-termination at the diamond surfaces was reduced in several steps by ozone (O ) treatments in an arc-discharge ionization system (Sander Lab-Ozonizer). After each ozonization step, gas sensing tests were performed to assess the impact of the oxygen-terminated surface sites on the gas sensing behavior of the HD samples.
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Fig. 1. Response of a HD sensor to various gases at room temperature. A large increase in the gas response is observed upon a partial exchange of H-termination sites against O-termination ones.
The metal oxide sensors used during the present investigations consisted of granular layers of SnO with a thin Au catalyst layer on top [18], [19]. At the beginning of the preparation process, a thin layer of Sn (300 nm) was sputtered with the substrate being kept at 350 C, i.e., well above the melting point of Sn (232 C) to form an array of Sn droplets. Subsequently, the droplets were annealed and oxidized at 600 C for several hours in ambient air to form a polycrystalline layer of SnO . Afterwards, a thin layer of Au (5–10 nm) was sputtered onto the SnO grains at a substrate temperature of 300 C. After a final annealing step at 600 C in air, the sensing layers consisted of small grains of SnO with interspersed clusters of Au. In order to allow heating of the SnO sensor layer, Pt heater meanders were sputter-deposited on the rear side of the ceramic substrates. All gas response measurements were carried out in a custombuilt gas mixing facility at EADS. This test rig allows preparing controlled mixtures of up to six gases or vapors, diluted in a background of synthetic air or nitrogen. In our experiments, all analyte gases investigated (H , various hydrocarbons, CO, NH , various nitrous oxides, and O ) were diluted in synthetic air (80% N /20% O ). III. RESULTS AND DISCUSSION Comparing and contrasting the gas sensing behavior of HD and MOx surfaces, we first draw attention to the room-temperature gas response of HD samples. Fig. 1 shows that HD samples selectively respond to NO and NH vapors at roomtemperature, however, with long response and particularly long recovery time constants. In comparison, the response to other gases, which are all well detected at heated MOx surfaces, is very small. An item which has already been discussed in our previous report [1] is that the gas response of HD samples is significantly enhanced when a certain number of hydrogen-terminated sites are replaced by oxygen-terminated ones. It has been argued that this latter effect is due to a catalytic effect of OH-termination sites which are amphoteric in the sense that they can easily convert into O or OH termination ones and that they, thus, facilitate the transfer of electronic charges between surface electrolyte and the HD bulk [8]. Concerning the HD gas sensitivity,
Fig. 2. Effect of UV light exposure on the NH response of a HD sample.
Fig. 3. Result of a contact angle experiment as performed on a nanogranular SnO film deposited on an oxidized silicon wafer.
we further note that this room-temperature gas response is significantly enhanced when the HD samples are illuminated with UV light during the gas exposure. This effect is shown in Fig. 2, which compares the response to an NH exposure pulse both under dark and UV-light conditions. A substantial reduction of both the response and recovery time constants is observed. Aiming at a comparison of the room-temperature gas sensing effects at HD and MOX surfaces, we reflect on the case that the HD gas sensitivity is believed to be supported by an adsorbed liquid electrolyte layer. To this end, we consider a contact angle measurement, which had been performed on a MOx sensing layer. This result is presented in Fig. 3 and shows that a MOx surface is highly attractive for water adsorption, as the water droplet on a SnO surface rapidly spreads out across the oxide surface forming a thin sheet of adsorbed water with a concomitantly small contact angle. As the magnitude of the contact angle can be related to the adsorption energy of the water molecules on a solid surface [20], it is evident that MOx surfaces are very likely to adsorb a liquid surface electrolyte layer. With this result in mind we compare room-temperature gas sensitivity tests on HD and MOx samples. This comparison is made in Fig. 4. From this comparison, it is evident that both kinds of samples respond relatively well to NO and NH but are irresponsive to all other kinds of gases which are well detected on MOx surfaces. Having attributed the HD gas sensing effect to electrolytic dissociation of NO and NH molecules in the surface electrolyte layer and to the ensuing changes in the pH value in this electrolyte, a similar explanation is suggested in the MOx case as
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Fig. 4. Result of room-temperature gas sensing tests on a p-type HD and an n-type MOx sample. Fig. 6. Response of a MOX sensor towards reactive gases at different sensor operation temperatures. In the temperature range from RT to 180 C the cross sensitivity pattern suggests a dissociative gas sensing effect as in HD. After 180 C the well-established combustive desorption of the humidity at T gas sensing effect takes over.
>
Fig. 5. Effect of UV illumination on the NO gas response of a SnO layer.
well. In particular, it is known that MOx-surfaces exhibit a high pH-sensitivity ( mV/pH) that has been explained in terms of the site-dissociation model [21]. The similarity of both cases can be further corroborated by considering the effect of UV light exposure on the room-temperature MOx gas sensing effect. This is done in Fig. 5, which shows the enhancing and accelerating effect of UV light exposure on the NO gas response on a MOx surface. As a further test to this proposition, we have performed gas sensing tests on the same MOx sample at successively higher sensor operation temperatures. In Fig. 6, these results are displayed as a long and repeated sequence of the same gas exposure tests as in Fig. 4, however, at successively higher sensor operation temperatures. This latter figure clearly shows that the dissociative gas sensing pattern is observed at room-temperature and above, persisting up to almost 200 C. Thereafter, a gradual transition to the normal combustive gas sensing mechanism can be observed. This latter effect reveals itself by a large and reversible response to species such as H , ethene, and CO, which are combustible but do not undergo electrolytic dissociation upon absorption in water. Comparative experiments on HD samples cannot be performed, as these latter samples loose their surface conductivity
Fig. 7. Change of the HD baseline resistance and of the NH gas sensing effect in response to repeated water desorption steps at 350 C. Arrows indicate the NH -induced resistance changes after the different desorption steps.
and thus their gas response as the surface electrolyte layer is desorbed in the vicinity of 200 C. This latter effect has already been described in our previous report [1]. For the sake of completeness of the present discussion, we repeat this critical result here in Fig. 7. In concluding our discussion, we should like to obtain estimates for the detection limits that are associated with this novel type of dissociative gas response. To this end, we draw attention to Fig. 8, which summarizes NH and NO calibration curves for an ozonized HD sample. In this plot, the NO gas sensi, where R stands tivity was calculated from for the sensor resistance under clean-air conditions (baseline for the sensor resistance under the influresistance) and ence of NO . In the case of NH , we used to account for the fact that opposite resistance changes are encountered in the case of NO and NH exposures and to arrive consistently positive values of in both cases. Considering the for calibration curves in Fig. 8 and a practical value of
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Fig. 8. Response of a 380 min ozonized HD sample towards different NO and NH concentrations at room temperature.
the detection threshold, minimum detectable concentrations of about 1 ppm NH and 0.1 ppm NO are obtained. IV. CONCLUSION In conclusion, we have shown that the room-temperature gas response of HD and MOx surfaces exhibits striking similarities such as UV light enhancement and selective response to NO and NH molecules. The wide body of data on the HD transfer doping effect suggests that these similarities are due to the presence of a very thin adsorbed water layer at both kinds of surfaces. From the application point-of-view, it is important to note that the dissociative room-temperature effect exhibits a significantly different cross sensitivity behavior than the established surface combustion effect that is operative on heated MOx surfaces.
[11] S. Ahlers, G. Müller, and T. Doll, “Factors influencing the gas sensitivity of metal oxide materials,” in Encyclopedia of Sensors, ISBN: 1-58883-059-4, C. A. Grimes, E. C. Dickey, and M. V. Pisko, Eds., 2007, vol. 3, pp. 413–447. [12] E. Comini, G. Faglia, and G. Sberveglieri, “UV light activation of tin oxide thin films for NO sensing at low temperatures,” Sens. Actuators B, vol. 78, p. 73, 2001. [13] E. Comini, A. Cristalli, G. Faglia, and G. Sberveglieri, “Light enhanced gas sensing properties of indium oxide and tin dioxide sensors,” Sens. Actuators B, vol. 65, p. 260, 2000. [14] K. Anothainart, M. Burgmair, A. Karthigeyan, M. Zimmer, and I. Eisele, “Light enhanced NO gas sensing with tin oxide at room temperature: Conductance and work function measurements,” Sens. Actuators B, vol. 93, p. 580, 2003. [15] T.-Y. Yang, H.-M. Lin, B.-Y. Wei, C.-Y. Wu, and C.-K. Lin, “UV enhancement of the gas sensing properties of nano-TiO ,” Rev. Adv. Mater. Sci., vol. 4, p. 48, 2003. [16] Z. Jie, H. Li-Hua, G. Shan, Z. Hui, and Z. Jing-Gui, “Alcohols and acetone sensing properties of SnO thin films deposited by dip coating,” Sens. Actuators B, vol. 115, p. 460, 2006. [17] H. C. Wang, Y. Li, and M. J. Yang, “Fast response thin film SnO gas sensors operating at room temperature,” Sens. Actuators B, vol. 119, p. 380, 2006. [18] G. Sberveglieri, G. Faglia, S. Groppelli, and P. Nelli, “RGTO: A new technique for preparing SnO sputtered thin-film as gas sensors,” in Proc. Solid-State Sensors and Actuators, 1991. Digest of Technical Papers, Transducers’91, 1991, p. 165. [19] W. Hellmich, Ch. Bosch-v. Braunmühl, G. Müller, G. Sberveglieri, M. Berti, and C. Perego, “The kinetics of formation of gas-sensitive RGTO-SnO films,” Thin Solid Films, vol. 263, p. 231, 1995. [20] A. W. Adamson and A. P. Gast, Physical Chemistry of Surfaces, 6th ed. New York: Wiley, 1997, ISBN 0-417-14873-3. [21] D. E. Yates, S. Levine, and T. W. Healy, “Site-binding model of the electrical double layer at the oxide/water interface,” J. Chem. Soc. Farady. Trans., vol. 170, p. 1807, 1974.
Andreas Helwig was born in 1975. After graduating in precision engineering and microengineering in 2001, he received the M.E. degree in the field of nanotechnology from the Munich University of Applied Sciences, Munich, Germany, in 2003. He is currently working towards the Ph.D. degree at the Department of Sensors, Electronics and Systems Integration, Innovation Works Germany, München, Germany. Currently, he is working amongst other things in the field of smart sensors for maintenance, safety, and
REFERENCES [1] A. Helwig, G. Müller, O. Weidemann, A. Härtl, J. A. Garrido, and M. Eickhoff, “Gas sensing interactions at hydrogenated diamond surfaces,” IEEE Sensors J., vol. 7, no. 9, pp. 1349–1353, Sep. 2007. [2] M. I. Landstrass and K. V. Ravi, “Hydrogen passivation of electrically active defects in diamond,” Appl. Phys. Lett., vol. 55, p. 1391, 1989. [3] G. S. Gi, T. Mizumasa, Y. Akiba, Y. Hirose, T. Kurosu, and M. Iida, “Formation mechanism of p-type surface conductive layer on deposited diamond films,” Jpn. J. Appl. Phys., vol. 34, p. 5550, 1995. [4] G. S. Gi, K. Tashiro, S. Tanaka, T. Fujisawa, H. Kimura, T. Kurosu, and M. Iida, “Hall effect measurements of surface conductive layer on undoped diamond films in NO and NH atmospheres,” Jpn. J. Appl. Phys., vol. 38, p. 3492, 1999. [5] T. Maki, S. Shikama, M. Komori, Y. Sakaguchi, K. Sakuta, and T. Kobayashi, “Hydrogenating effect of single-crystal diamond surface,” Jpn. J. Appl. Phys., vol. 31, p. 1446, 1992. [6] F. Maier, M. Riedel, B. Mantel, J. Ristein, and L. Ley, “Origin of surface conductivity in diamond,” Phys. Rev. Lett., vol. 85, p. 3472, 2000. [7] A. Hokazono and H. Kawarada, “Enhancement/depletion surface channel field effect transistors of diamond and their logic circuits,” Jpn. J. Appl. Phys., vol. 36, p. 7133, 1997. [8] J. A. Garrido, A. Härtl, S. Kuch, and M. Stutzmann, “pH sensors based on hydrogenated diamond surfaces,” Appl. Phys. Lett., vol. 86, no. 073504, 2005. [9] B. Ostrick, J. Mühlsteff, M. Fleischer, H. Meixner, T. Doll, and C.-D. Kohl, “Adsorbed water as key to room temperature gas-sensitive reactions in work function type sensors: The carbonate-carbon dioxide system,” Sens. Actuators B, vol. 57, p. 115, 1999. [10] D. E. Williams, “Conduction and gas response of semiconductor gas sensors,” in Solid State Gas Sensors. Bristol: Adam Hilger, 1987, pp. 71–123.
emission control.
Gerhard Müller graduated in physics from the University of Heidelberg, Heidelberg, Germany, in 1974. He received the Ph.D. degree from the University of Heidelberg in 1976. Subsequently, he was employed at the MaxPlanck-Institute for Nuclear Physics in Heidelberg, where he performed work on ion implantation and nuclear solid-state physics. In 1979, he changed to the University of Dundee, U.K., where he started research on hydrogenated amorphous silicon. In 1981, he moved to Messerschmitt-Boelkow-Blohm GmbH (MBB), where he performed development work on thin-film solar- cell modules. Since 1986, he has been active in the field of silicon micromachining and sensors working in leading positions for a number of employers: MBB/DASA (1986–1993): building up clean room laboratories for silicon micromachining and thin-film technologies; DaimlerChrysler Central Research (1994–2000): sensors for automotive safety and exhaust gas monitoring; EADS Corporate Research Centre, Germany (2000 onwards): sensors for aircraft safety, security, maintenance and diagnosis. He is currently managing the Biological and Chemical Sensors Group, EADS Innovation Works ( 15 employees). He is author and coauthor of about 250 articles in scientific journals and conference proceedings. Since 2000, he has been a Lecturer with the Munich University of Applied Sciences.
HELWIG et al.: DISSOCIATIVE GAS SENSING AT METAL OXIDE SURFACES
Martin Eickhoff received the Ph.D. degree in experimental physics from the Technische Universität München, München, Germany, in 2000. He has worked on SiC material and device development with DaimlerChrysler Research and Technology, Munich, from 1995 to 2000. After that he worked for Infineon Technologies AG, Munich, on the development of surface micromachined integrated sensors. Since 2001, he has been a Work Group Leader of the Sensors and Materials at the Walter Schottky Institut, TU München, focusing on growth and characterization of wide bandgap semiconductors, surface functionalization and on the application in biochemical sensors.
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Giorgio Sberveglieri was born on July 17, 1947. He received the degree in physics (cum laude) from the University of Parma, Parma, Italy. He started his research activities on the preparation of semiconducting thin-film solar cells at the University of Parma in 1971. He is now the Director of the CNR-INFM Sensor Laboratory (http://sensor.ing. unibs.it) at Brescia University, where there are more than 20 researchers. In 1988, he established the Gas Sensor Laboratory, mainly devoted to the preparation and characterization of thin-film chemical sensors based on nanostructured metal oxide semiconductors and, since the mid 1990s, to the area of electronic noses. In 1994, he was appointed Full Professor of Physics. He is referee of many international journals. During 30 years of scientific activity he published more than 250 papers in international journals; he presented more than 250 oral communications to international congresses (12 plenary talks and 45 invited talks). He also is an Evaluator of the European Union, in the area of Nanoscience and Nanomaterials, and the Coordinator of the EU Project NANOS4 (Nano-structured solid-state gas sensors with superior performance) and several Italian projects on gas sensors. Prof. Sberveglieri is an Associate Editor of the IEEE SENSOR JOURNAL and has acted as Chairman in several Conferences on Materials Science and on Sensors. He has been the General Chairman of IMCS11th (11th International Meeting on Chemical Sensors) and is the Chair of the Steering Committee of the IMCS Series Conference.
