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Tin oxide nanobelts electrical and sensing properties
Sensors and Actuators B 111–112 (2005) 2–6
Tin oxide nanobelts electrical and sensing properties E. Comini a,∗ , G. Faglia a , G. Sberveglieri a , D. Calestani b , L. Zanotti b , M. Zha b a b
Sensor Lab., INFM, Universita di Brescia, Via Valotti 9, 25133 Brescia, Italy IMEM-CNR, Parco Area delle Scienze 37/A, 43010 Fontanini PARMA, Italy Available online 8 August 2005
Abstract Semiconducting oxides nanobelts of tin oxide have been obtained by vapour phase deposition, using SnO as source material. In this work, we present the results obtained using tin oxide nanobelts as conductometric gas sensors. Electrical characterization showed that the nanobelts were sensitive to oxygen and environmental polluting species, like CO and NO2 as well as ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance or resistance due to the introduction of the gas, is 200% for 30 ppm of CO at 350 ◦ C, 900% for 200 ppb NO2 at 300 ◦ C, and 2500% for 10 ppm of ethanol at 350 ◦ C. It has been studied the variation of the response as a function of the density of the nanobelts. The results demonstrate the potential of fabricating nanosize sensors using the integrity of a single nanobelt with sensitivity at the level of a few ppb and the necessity to control nanobelts density to optimize the sensing performances. © 2005 Elsevier B.V. All rights reserved. Keywords: Semiconductor oxide; Nonobelts; Electrical and sensing properties
1. Introduction In the past few years, progress has been achieved in the synthesis, structural characterization and physical investigation of nanostructure properties. Nanoscale science and technology are experiencing a rapid development and they are likely to have a profound impact on every field of research in the first decades of the 21st century. Due to their peculiar characteristics and size effects, these materials often exhibit novel physical properties that are different from those of the bulk, and are of great interest both for fundamental study and for potential nanodevice applications. Carrier transport takes place in a regime in which the Boltzmann equation is invalid. Mesoscopic devices are structures large compared to the atomic scale but small compared to the macroscopic scale where Boltzmann equation holds. Carriers do not exhibit fully wave-like behaviour but are sufficiently de-localized to exhibit also some particle-like properties. The finite size of the metal oxide particles confines the electrons wave functions, leading to quantized energy levels and a complete ∗ Corresponding author at: INFM, Dipartimento di Chimica e Fisica, Via Valotti 9, 25133 Brescia, Italy. Tel.: +39 030 3715706; fax: +39 030 2091271. E-mail address: [email protected] (E. Comini).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.031
modification of the transport and optical properties of the material. The hugely enhanced surface/volume ratio augments the role of surface states in the sensor response. Sensing mechanisms controlled at the nano scale level will therefore bring many benefits to the three “S” of sensor technology (sensitivity, selectivity and stability). Interest in nanowires of semiconducting oxides has exponentially grown in the last years, due to their attracting potential applications in electronic, optical, and sensor field. Above all, the possibility to control the dimensionality of these materials is crucial to build devices with specific physical properties. Among these studies, several investigations have been devoted to tin oxide (SnO2 ) nanowires/nanobelts and different approaches to their synthesis have been developed [1–4]. At the present, vapour deposition processes seem to be the most promising ones, due to their simplicity and low costs. Some works [4,5] drew the attention on the VLS (vapour–liquid–solid) mechanism, through which thermal evaporation techniques can favour a fast growth below 1000 ◦ C (crystal growth by thermal evaporation of SnO2 powders usually occurs at temperatures higher than 1300 ◦ C [1]). A specific study on SnO2 nanobelts has been conducted to obtain large scale growths on different substrates (alumina, SiO2 , Si, . . .) [6].
E. Comini et al. / Sensors and Actuators B 111–112 (2005) 2–6
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2. Experimental procedure Source material and substrates have been placed at the center of a quartz tube, in which is possible to have vacuum and flow different gasses. The quartz tube is kept inside a tubular furnace that can reach at least 1000 ◦ C. Commercial (99.9% pure) SnO powder has been selected as source material and it was preferred to elemental Sn in order to have a better control of the evaporation process. Infact, the dissociation 2SnO(s) → Sn(l) + SnO2 (s)
(1)
rapidly develops only when the temperature reaches the working range (800–1000 ◦ C). Further to that, the mixture of SnO2 and liquid Sn does not flow inside the boat, as pure liquid Sn would do, thus covering the substrates. The reactor is heated until 850–900 ◦ C and, at this temperature, Sn vapours coming from reaction (1) are carried by an Ar flux in the substrates region, where they condense in a liquid phase as micron-size droplets. An oxygen flow is then introduced to promote the crystallization of the SnO2 nanobelts. The obtained samples have been studied by a Thermo ARL X’tra X-ray diffractometer (Cu K␣, grazing angle configuration) to confirm the structural nature of wires. A more detailed insight as to structure and morphology has been gained by scanning electron microscope and transmission electron microscope investigations. For electrical characterisation, nanobelts were deposited onto alumina 3 mm × 3 mm blank substrates. Different deposition densities have been obtained through the control of the growing parameters (temperature, gradients, distance from source material, . . .). Substrates with nanobelts were then equipped with a platinum meander on the backside, acting as a heater and a temperature sensor. Platinum interdigitated contacts were sputtered onto the nanobelts layer. The flow-through technique was used to test the gassensing properties of the thin films. A constant flux of synthetic air of 0.3 l/min was the gas carrier, into which the desired concentration of pollutants—dispersed in synthetic air—was mixed. All the measurements were executed in a temperature-stabilised sealed chamber at 20 ◦ C under controlled humidity. Electrical characterisation was carried out by volt-amperometric technique; the sensor was biased by 1 V and film resistance was measured by a picoammeter.
3. Results SEM images (Fig. 1) revealed that nanobelts grows in a homogeneous entanglement on a large area (3–5 cm2 ). The shape of the nanocrystals is mainly “belt-like”, with a few tens of nanometers rectangular cross section. Width to thick-
Fig. 1. SEM image of the nanobelts deposited on alumina substrates.
ness ratio is generally about 5:1, while the nanowires length can reach several hundreds of micrometers. Although, they usually appear bended, TEM investigations revealed that they are single-crystals. Generally no extended defect is present along the wire. Few straight thin wires with a rounded cross section have also been observed. X-ray diffraction has been performed in grazing angle configuration to reduce drastically the contribution of the substrate in the diffraction pattern. All measurement have confirmed that the structure corresponds to rutile-like tetragonal SnO2 . Only peaks of this crystal structure are present in the pattern (with the exception of those from the substrate), meaning that neither crystalline spurious phases nor unreacted Sn can be found in the growth product. The first electrical measurements were addressed to study the variations of the electrical conductance of the nanobelts as a function of the operating temperature, tin oxide as all the metal oxide semiconductor is sensitive to the oxygen in the atmosphere and changes its conductivity due to this interaction. The sensing properties of a chemoresistive metal-oxide semiconductor (MOX) rely just on surface reactions between the material and the gases in the atmosphere. The existing theory on gas-sensing via MOX accounts for physisorption and chemisorption of gases occurring at the surface of the material. The resulting charge exchange between adsorbed molecules and the material modifies the energy barrier, eVs , for grain-to-grain current percolation and, in turn, the electrical conductance of the layers [7,8]. Accordingly, the conductance of the layer can be expressed by: −eVs G = G0 exp , (2) kT where G0 is a proportionality constant [8]. Operation of the films in ambient air involves coverage by oxygen as different species (O2 , O2 − and O− ) depending on temperature. The dependence of resistance on temperature for the semiconducting thin films is a competition between adsorption of oxygen species and capture of electrons [8].
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Fig. 2. Conductance of tin oxide nanobelts with different densities exposed to a humid synthetic air flux of 300 l/min, RH 50% at 20 ◦ C.
The interaction of an oxidizing (reducing) gas occurs with either the film surface or the oxygen coverage. It results in absorption (desorption) of electrons at surface, which leads to a positive (negative) conductance variation for an n-type MOX. Fig. 2 shows the conductance of nanobelts with different densities as a function of the operating temperature in an atmosphere of synthetic humid air with a constant flux of 0.3 l/min. Samples SnO2 A, SnO2 B, SnO2 C have a decreasing density of nanobelts and show a consequent decrease in the conductance. Sample B and C have a minimum in the conductance at 300 ◦ C, that can probably be ascribed to the maximum oxygen coverage, while the minimum for sample A is at higher temperature. After the first electrical characterization with an humid air flux, the sensing behaviour of the tin oxide nanobelts obtained has been tested toward different gases like CO, NO2 and ethanol. Following the literature, the response of a sensor is defined as the normalised variation of resistance for a ntype semiconductor and oxidising gases (S = R/R0 ) while for reducing gases it is defined as the normalised variation of conductance for a n-type semiconductor (S = G/G0 ).
4. CO sensing In the particular case of CO the working mechanism is well established. As CO is fed into the test chamber, the conductance of the layers rise up, as is normal for an ntype semiconductor due to exchange of electrons between the ionosorbed species and the semiconductor itself. CO reacts with oxygen species adsorbed on the semiconductor (O2− , O− , O2 − ) with a consequent increase in the conductance; as an example, for O− : CO + O− → CO2 − → CO2 + e− Fig. 3 shows the response towards 30 ppm CO for the three different density of tin oxide nanobelts as a function of operating temperature. There is a maximum of the response at
Fig. 3. Response towards 30 ppm CO as a function of the operating temperature with 50% RH at 20 ◦ C.
temperature corresponding to the maximum oxygen coverage for the layers (i.e. 300 for SnO2 B and C and 350 ◦ C for the other layer). This behaviour is consistent to the proposed reaction mechanism that is dependent on the oxygen coverage. The response is influenced by the density of the nanobelts as can be inferred by Fig. 3. The value of the response is similar or even higher compared to the one found in literature for sputtered tin oxide for example [9].
5. NO2 sensing In the case of NO2 , there are proofs of reactions directly with the semiconductor surface other than with the oxygen chemisorbed at surface. Tamaki et al. [10–12] studied nitrogen dioxide absorption on tin oxide surfaces by temperature programmed desorption measurements and found that the adsorbates originating from NO2 are essentially the same as those for NO, since NO2 molecule dissociate easily over the tin oxide surface. Fig. 4 shows the variation of the response toward 200 ppb of nitrogen dioxide as a function of the operating temperature; the conductance of the layer decreases rapidly and, when the air flux is restored, it returns to the stable value, as is normal for a n-type semiconductor. The maximum of the response is obtained for relatively low temperatures, due to the particular reaction mechanism of NO2 with MOX, not related to oxygen chemisorbed species. Also, in the case of nitrogen dioxide the influence of the nanobelts density is remarkable. The value of the response is even higher compared to the one found in literature for tin oxide prepared by pulsed laser PLD tin oxide [15].
6. Ethanol sensing It has recently been proposed a scheme for interpretation of the transformation of ethanol to aldehyde [14] through
E. Comini et al. / Sensors and Actuators B 111–112 (2005) 2–6
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Fig. 4. Response towards 200 ppb of nitrogen dioxide as a function of the operating temperature with 50% RH at 20 ◦ C.
Fig. 5. Response towards 10 ppm of ethanol as a function of the operating temperature with 50% RH at 20 ◦ C.
catalytic interaction with a MOS surface. The dissociation of ethanol produces an ionic bond between the oxygen in the molecule and unsaturated metal site, while the hydrogen atom is bound to a nearby oxygen anion:
ethanol as demanded to a material for gas sensing. The maximum of the response is between 300 and 350 ◦ C for all the layers and there is a difference in the response between the different layers, while the sensing performances are good compared to literature data [15].
CH3 CH2 OH + O(s) + M(s) → CH3 CH2 O M(ads) + O H(ads) → CH3 CHO + H M(ads) + O H(ads)
7. Conclusions (3)
were (s) and (ads) indicate a surface sites or an adsorbed species, respectively. The residual hydrogen from ethanol adsorption generally desorbs as H2 O or as H2 . H2 O can originate from the recombination of M H and OH adsorbed from the environment. This product then desorbs, leaving an oxygen vacancy and a partially reduced metal, which in presence of gas phase O2 is re-oxidized. Ethanol exhibits a reducing behaviour; hence for a n-type semiconductor the conductance increases with the gas introduction. Based on this scheme, one would conclude that the gas-sensing qualities of titania towards ethanol should not only rely on the acknowledged model of interaction with the oxygen coverage, but rather one should consider direct interaction with the semiconductor surface sites. We experimentally investigated at concentration of ethanol below 200 ppm: this range is wide enough to extrapolate general information on the processes involved and to envisage a possibility for technological applications. As an example, a breath analyzer should be designed to detect at least 200 ppm of ethanol, which corresponds approximately to 0.6 g of alcohol per litre in the human blood [15]. Fig. 5 reports the response of the nanobelts towards concentration as low as 10 ppm of ethanol as a function of the operating temperatures at 50% RH. Feeding of ethanol resulted in an increase of electric conductance, a firm sign of the reducing behaviour of ethanol as proposed through the model. All the layers performed reversible response toward
We proposed the use of nanostructures with reduced dimensionality of SnO2 in the form of nanobelts for gas sensing and we proved their capability in the revealing of gases like CO, NO2 , and ethanol. This topology experimentally showed good electrical responses to gases, comparable to the 3D counterpart. The sensor response, defined as the relative variation in conductance due to the introduction of the gas, is high compared to literature data: 200% for 30 ppm of CO at 350 ◦ C, 900% for 200 ppb NO2 at 300 ◦ C, and 2500% for 10 ppm of ethanol at 350 ◦ C. In this communication, the authors report also the influence of the density of these structures on the electrical and sensing properties of the nanobelts, this is a crucial thing if these nanostructures have to be implemented in a reliable gas sensing device. It has been proved that the conductance of the layers increase with the density of the nanobelts. While the gas sensing properties of the layers depend on the surface reaction that lead to a change in the conductance. It has been shown for example that the CO and NO2 responses are strongly affected by changes in the density of nanobelts with respect to ethanol sensing.
Acknowledgements This work has been founded by the European Strep project “Nano-structured solid-state gas sensors with superior performance” (NANOS4) no.: 001528.
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References [1] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947. [2] A. Kolmakov, Y. Zhang, G. Cheng, M Moskovits, Adv. Mater. 15 (2003) 997. [3] J. Hu, Y. Bando, Q. Liu, D. Golberg, Adv. Funct. Mater. 13 (2003) 493. [4] Y. Chen, X. Cui, K. Zhang, D. Pan, S. Zhang, B. Wang, J.G. How, Chem. Phys. Lett. 369 (2003) 16. [5] H.Y. Dang, J. Wang, S.S. Fan, Nanotechnology 14 (2003) 738. [6] L. Zanotti, et al., J. Crystal Growth 275 (2005) e2083–e2087. [7] N. Yamazoe, N. Miura, in: S. Yamauchi, Kodansha (Eds.), Some Basic Aspects of Semiconductor Gas Sensors in Chemical Sensor Technology, vol. 4, Elsevier, Amsterdam, 1992, p. 19. [8] S.R. Morrison, The Chemical Physics of Surfaces, Plenum Press, NewYork, 1977, p. 25 (Chapter 2). [9] Wollenstein, et al., Sens. Actuators B 70 (2000) 196–202. [10] J. Tamaki, N. Nagaishi, Y. Teraoka, N. Miura, Y. Yamazoe, Surf. Sci. 221 (1989) 183. [11] F. Solymosi, J. Kiss, J. Catal. 41 (1976) 202. [12] G. Ghiotti, A. Chiorino, W.X. Pan, L. Marchese, Sens. Actuators B 7 (1992) 691. [14] H. Idriss, E.G. Seebauer, J. Mol. Catal. A: Chem. 152 (2000) 201. [15] T. Brousse, D.M. Schleich, Sprayed and thermally evaporated SnO2 thin film for ethanol sensors, Sens. Actuators B 31 (1996) 77–79.
Biographies Elisabetta Comini was born on 21st November 1972 and she received her degree in physics at the University of Pisa in 1996. She is presently working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterisation of MOS thin films. She received her Ph.D. in material science at the University of Brescia. She is now a researcher at the University of Brescia. Guido Faglia was born in 1965 and has received an M.S. degree from the Polytechnic of Milan in 1991 with a thesis on gas sensors. In 1992, he has been appointed as a researcher by the Thin Film Lab at the University of Brescia. He is involved in the study of the interactions between gases and semiconductor surfaces and in gas sensors electrical characterization. In 1996, he has received the Ph.D. degree by discussing a thesis on semiconductor gas sensors. In 2000, he has been appointed as associate professor in experimental physics at University of Brescia. During his career, Guido Faglia has published more than 50 articles on International Journals with referee Giorgio Sberveglieri was born on 17 July 1947, and received his degree in physics from the University Parma, where, starting in 1971, his research
activities on the preparation of semiconducting thin film solar cells was conducted. He has been appointed associate professor at the University of Brescia in 1987. In the following year, he established the Thin Film Laboratory afterwards called Gas Sensor Laboratory, which is mainly devoted to the preparation and characterisation of thin film chemical sensors. He has been the director of the GSL since 1988. In 1994, he was appointed full professor in physics, first at the Faculty of Engineering of University of Ferrara and then in 1996, at the Faculty of Engineering of University of Brescia. He is a referee of the journals Thin Solid Films, Sensors and Actuators, Sensors and Materials, etc., and is a member of the Scientific Committee of Conferences on Sensor and Materials Science. During his 25 years of scientific activities, Giorgio Sberveglieri has published more than 140 papers on international reviews; he has presented more than 50 oral communications to international congresses and numerous oral communications to national congresses. Davide Calestani was born on 20th October 1978 and he received his degree in material science at the University of Parma in 2002. As Ph.D. student, he is working at IMEM in Technology Department since 2003, with particular reference to the development of oxide nanowires growth techniques. Ming Zheng Zha was born on 15th February 1942, has worked as associated Professor at the University of Bengjin till 1984, when he got a researcher position at Zurich ETH. Here Professor Zha has been involved in special project of NASA/ESA based on study of crystallization phenomena in microgravity conditions. This activity has been taken up at MASPEC-CNR (presently IMEM) Institute, where specific crystal growth process have been studied and performed.Recently he is involved in research oriented to the development of oxide nanowires for gas sensor application. He is author of about 90 papers in the field of material science and technology, with many innovative contribute in the field of crystal preparation from the vapour phase. Lucio Zanotti was born on 15th July 1944. He has been graduated from the University of Bologna in 1969 with a specialization in chemistry. He is one of the founders of MASPEC Institute of the Italian National Council of Research and has been working there since 1970. Since 1983 he is head of the Technology Department at MASPEC. His research has been carried out mainly in the field of semiconductor compounds for use in microand opto-electronics, infrared detectors and photovoltaic cells and in the field of inorganic\organic materials for non-linear optical application. His activity has been centered around: inorganic synthesis, purification procedures, impurity and compositional-inhomogeneity analysis, crystal growth of binary, ternary and multinary compounds from the melt, vapour and solution, defect (chemical etching) analysis, electrical and optical characterization. He is director of MASPEC (presently IMEM Institute) since 1987, is author of well over 150 scientific papers, reviews, patents in the field of material science and technology.
Tin oxide nanobelts electrical and sensing properties E. Comini a,∗ , G. Faglia a , G. Sberveglieri a , D. Calestani b , L. Zanotti b , M. Zha b a b
Sensor Lab., INFM, Universita di Brescia, Via Valotti 9, 25133 Brescia, Italy IMEM-CNR, Parco Area delle Scienze 37/A, 43010 Fontanini PARMA, Italy Available online 8 August 2005
Abstract Semiconducting oxides nanobelts of tin oxide have been obtained by vapour phase deposition, using SnO as source material. In this work, we present the results obtained using tin oxide nanobelts as conductometric gas sensors. Electrical characterization showed that the nanobelts were sensitive to oxygen and environmental polluting species, like CO and NO2 as well as ethanol for breath analyzers and food control applications. The sensor response, defined as the relative variation in conductance or resistance due to the introduction of the gas, is 200% for 30 ppm of CO at 350 ◦ C, 900% for 200 ppb NO2 at 300 ◦ C, and 2500% for 10 ppm of ethanol at 350 ◦ C. It has been studied the variation of the response as a function of the density of the nanobelts. The results demonstrate the potential of fabricating nanosize sensors using the integrity of a single nanobelt with sensitivity at the level of a few ppb and the necessity to control nanobelts density to optimize the sensing performances. © 2005 Elsevier B.V. All rights reserved. Keywords: Semiconductor oxide; Nonobelts; Electrical and sensing properties
1. Introduction In the past few years, progress has been achieved in the synthesis, structural characterization and physical investigation of nanostructure properties. Nanoscale science and technology are experiencing a rapid development and they are likely to have a profound impact on every field of research in the first decades of the 21st century. Due to their peculiar characteristics and size effects, these materials often exhibit novel physical properties that are different from those of the bulk, and are of great interest both for fundamental study and for potential nanodevice applications. Carrier transport takes place in a regime in which the Boltzmann equation is invalid. Mesoscopic devices are structures large compared to the atomic scale but small compared to the macroscopic scale where Boltzmann equation holds. Carriers do not exhibit fully wave-like behaviour but are sufficiently de-localized to exhibit also some particle-like properties. The finite size of the metal oxide particles confines the electrons wave functions, leading to quantized energy levels and a complete ∗ Corresponding author at: INFM, Dipartimento di Chimica e Fisica, Via Valotti 9, 25133 Brescia, Italy. Tel.: +39 030 3715706; fax: +39 030 2091271. E-mail address: [email protected] (E. Comini).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.031
modification of the transport and optical properties of the material. The hugely enhanced surface/volume ratio augments the role of surface states in the sensor response. Sensing mechanisms controlled at the nano scale level will therefore bring many benefits to the three “S” of sensor technology (sensitivity, selectivity and stability). Interest in nanowires of semiconducting oxides has exponentially grown in the last years, due to their attracting potential applications in electronic, optical, and sensor field. Above all, the possibility to control the dimensionality of these materials is crucial to build devices with specific physical properties. Among these studies, several investigations have been devoted to tin oxide (SnO2 ) nanowires/nanobelts and different approaches to their synthesis have been developed [1–4]. At the present, vapour deposition processes seem to be the most promising ones, due to their simplicity and low costs. Some works [4,5] drew the attention on the VLS (vapour–liquid–solid) mechanism, through which thermal evaporation techniques can favour a fast growth below 1000 ◦ C (crystal growth by thermal evaporation of SnO2 powders usually occurs at temperatures higher than 1300 ◦ C [1]). A specific study on SnO2 nanobelts has been conducted to obtain large scale growths on different substrates (alumina, SiO2 , Si, . . .) [6].
E. Comini et al. / Sensors and Actuators B 111–112 (2005) 2–6
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2. Experimental procedure Source material and substrates have been placed at the center of a quartz tube, in which is possible to have vacuum and flow different gasses. The quartz tube is kept inside a tubular furnace that can reach at least 1000 ◦ C. Commercial (99.9% pure) SnO powder has been selected as source material and it was preferred to elemental Sn in order to have a better control of the evaporation process. Infact, the dissociation 2SnO(s) → Sn(l) + SnO2 (s)
(1)
rapidly develops only when the temperature reaches the working range (800–1000 ◦ C). Further to that, the mixture of SnO2 and liquid Sn does not flow inside the boat, as pure liquid Sn would do, thus covering the substrates. The reactor is heated until 850–900 ◦ C and, at this temperature, Sn vapours coming from reaction (1) are carried by an Ar flux in the substrates region, where they condense in a liquid phase as micron-size droplets. An oxygen flow is then introduced to promote the crystallization of the SnO2 nanobelts. The obtained samples have been studied by a Thermo ARL X’tra X-ray diffractometer (Cu K␣, grazing angle configuration) to confirm the structural nature of wires. A more detailed insight as to structure and morphology has been gained by scanning electron microscope and transmission electron microscope investigations. For electrical characterisation, nanobelts were deposited onto alumina 3 mm × 3 mm blank substrates. Different deposition densities have been obtained through the control of the growing parameters (temperature, gradients, distance from source material, . . .). Substrates with nanobelts were then equipped with a platinum meander on the backside, acting as a heater and a temperature sensor. Platinum interdigitated contacts were sputtered onto the nanobelts layer. The flow-through technique was used to test the gassensing properties of the thin films. A constant flux of synthetic air of 0.3 l/min was the gas carrier, into which the desired concentration of pollutants—dispersed in synthetic air—was mixed. All the measurements were executed in a temperature-stabilised sealed chamber at 20 ◦ C under controlled humidity. Electrical characterisation was carried out by volt-amperometric technique; the sensor was biased by 1 V and film resistance was measured by a picoammeter.
3. Results SEM images (Fig. 1) revealed that nanobelts grows in a homogeneous entanglement on a large area (3–5 cm2 ). The shape of the nanocrystals is mainly “belt-like”, with a few tens of nanometers rectangular cross section. Width to thick-
Fig. 1. SEM image of the nanobelts deposited on alumina substrates.
ness ratio is generally about 5:1, while the nanowires length can reach several hundreds of micrometers. Although, they usually appear bended, TEM investigations revealed that they are single-crystals. Generally no extended defect is present along the wire. Few straight thin wires with a rounded cross section have also been observed. X-ray diffraction has been performed in grazing angle configuration to reduce drastically the contribution of the substrate in the diffraction pattern. All measurement have confirmed that the structure corresponds to rutile-like tetragonal SnO2 . Only peaks of this crystal structure are present in the pattern (with the exception of those from the substrate), meaning that neither crystalline spurious phases nor unreacted Sn can be found in the growth product. The first electrical measurements were addressed to study the variations of the electrical conductance of the nanobelts as a function of the operating temperature, tin oxide as all the metal oxide semiconductor is sensitive to the oxygen in the atmosphere and changes its conductivity due to this interaction. The sensing properties of a chemoresistive metal-oxide semiconductor (MOX) rely just on surface reactions between the material and the gases in the atmosphere. The existing theory on gas-sensing via MOX accounts for physisorption and chemisorption of gases occurring at the surface of the material. The resulting charge exchange between adsorbed molecules and the material modifies the energy barrier, eVs , for grain-to-grain current percolation and, in turn, the electrical conductance of the layers [7,8]. Accordingly, the conductance of the layer can be expressed by: −eVs G = G0 exp , (2) kT where G0 is a proportionality constant [8]. Operation of the films in ambient air involves coverage by oxygen as different species (O2 , O2 − and O− ) depending on temperature. The dependence of resistance on temperature for the semiconducting thin films is a competition between adsorption of oxygen species and capture of electrons [8].
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E. Comini et al. / Sensors and Actuators B 111–112 (2005) 2–6
Fig. 2. Conductance of tin oxide nanobelts with different densities exposed to a humid synthetic air flux of 300 l/min, RH 50% at 20 ◦ C.
The interaction of an oxidizing (reducing) gas occurs with either the film surface or the oxygen coverage. It results in absorption (desorption) of electrons at surface, which leads to a positive (negative) conductance variation for an n-type MOX. Fig. 2 shows the conductance of nanobelts with different densities as a function of the operating temperature in an atmosphere of synthetic humid air with a constant flux of 0.3 l/min. Samples SnO2 A, SnO2 B, SnO2 C have a decreasing density of nanobelts and show a consequent decrease in the conductance. Sample B and C have a minimum in the conductance at 300 ◦ C, that can probably be ascribed to the maximum oxygen coverage, while the minimum for sample A is at higher temperature. After the first electrical characterization with an humid air flux, the sensing behaviour of the tin oxide nanobelts obtained has been tested toward different gases like CO, NO2 and ethanol. Following the literature, the response of a sensor is defined as the normalised variation of resistance for a ntype semiconductor and oxidising gases (S = R/R0 ) while for reducing gases it is defined as the normalised variation of conductance for a n-type semiconductor (S = G/G0 ).
4. CO sensing In the particular case of CO the working mechanism is well established. As CO is fed into the test chamber, the conductance of the layers rise up, as is normal for an ntype semiconductor due to exchange of electrons between the ionosorbed species and the semiconductor itself. CO reacts with oxygen species adsorbed on the semiconductor (O2− , O− , O2 − ) with a consequent increase in the conductance; as an example, for O− : CO + O− → CO2 − → CO2 + e− Fig. 3 shows the response towards 30 ppm CO for the three different density of tin oxide nanobelts as a function of operating temperature. There is a maximum of the response at
Fig. 3. Response towards 30 ppm CO as a function of the operating temperature with 50% RH at 20 ◦ C.
temperature corresponding to the maximum oxygen coverage for the layers (i.e. 300 for SnO2 B and C and 350 ◦ C for the other layer). This behaviour is consistent to the proposed reaction mechanism that is dependent on the oxygen coverage. The response is influenced by the density of the nanobelts as can be inferred by Fig. 3. The value of the response is similar or even higher compared to the one found in literature for sputtered tin oxide for example [9].
5. NO2 sensing In the case of NO2 , there are proofs of reactions directly with the semiconductor surface other than with the oxygen chemisorbed at surface. Tamaki et al. [10–12] studied nitrogen dioxide absorption on tin oxide surfaces by temperature programmed desorption measurements and found that the adsorbates originating from NO2 are essentially the same as those for NO, since NO2 molecule dissociate easily over the tin oxide surface. Fig. 4 shows the variation of the response toward 200 ppb of nitrogen dioxide as a function of the operating temperature; the conductance of the layer decreases rapidly and, when the air flux is restored, it returns to the stable value, as is normal for a n-type semiconductor. The maximum of the response is obtained for relatively low temperatures, due to the particular reaction mechanism of NO2 with MOX, not related to oxygen chemisorbed species. Also, in the case of nitrogen dioxide the influence of the nanobelts density is remarkable. The value of the response is even higher compared to the one found in literature for tin oxide prepared by pulsed laser PLD tin oxide [15].
6. Ethanol sensing It has recently been proposed a scheme for interpretation of the transformation of ethanol to aldehyde [14] through
E. Comini et al. / Sensors and Actuators B 111–112 (2005) 2–6
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Fig. 4. Response towards 200 ppb of nitrogen dioxide as a function of the operating temperature with 50% RH at 20 ◦ C.
Fig. 5. Response towards 10 ppm of ethanol as a function of the operating temperature with 50% RH at 20 ◦ C.
catalytic interaction with a MOS surface. The dissociation of ethanol produces an ionic bond between the oxygen in the molecule and unsaturated metal site, while the hydrogen atom is bound to a nearby oxygen anion:
ethanol as demanded to a material for gas sensing. The maximum of the response is between 300 and 350 ◦ C for all the layers and there is a difference in the response between the different layers, while the sensing performances are good compared to literature data [15].
CH3 CH2 OH + O(s) + M(s) → CH3 CH2 O M(ads) + O H(ads) → CH3 CHO + H M(ads) + O H(ads)
7. Conclusions (3)
were (s) and (ads) indicate a surface sites or an adsorbed species, respectively. The residual hydrogen from ethanol adsorption generally desorbs as H2 O or as H2 . H2 O can originate from the recombination of M H and OH adsorbed from the environment. This product then desorbs, leaving an oxygen vacancy and a partially reduced metal, which in presence of gas phase O2 is re-oxidized. Ethanol exhibits a reducing behaviour; hence for a n-type semiconductor the conductance increases with the gas introduction. Based on this scheme, one would conclude that the gas-sensing qualities of titania towards ethanol should not only rely on the acknowledged model of interaction with the oxygen coverage, but rather one should consider direct interaction with the semiconductor surface sites. We experimentally investigated at concentration of ethanol below 200 ppm: this range is wide enough to extrapolate general information on the processes involved and to envisage a possibility for technological applications. As an example, a breath analyzer should be designed to detect at least 200 ppm of ethanol, which corresponds approximately to 0.6 g of alcohol per litre in the human blood [15]. Fig. 5 reports the response of the nanobelts towards concentration as low as 10 ppm of ethanol as a function of the operating temperatures at 50% RH. Feeding of ethanol resulted in an increase of electric conductance, a firm sign of the reducing behaviour of ethanol as proposed through the model. All the layers performed reversible response toward
We proposed the use of nanostructures with reduced dimensionality of SnO2 in the form of nanobelts for gas sensing and we proved their capability in the revealing of gases like CO, NO2 , and ethanol. This topology experimentally showed good electrical responses to gases, comparable to the 3D counterpart. The sensor response, defined as the relative variation in conductance due to the introduction of the gas, is high compared to literature data: 200% for 30 ppm of CO at 350 ◦ C, 900% for 200 ppb NO2 at 300 ◦ C, and 2500% for 10 ppm of ethanol at 350 ◦ C. In this communication, the authors report also the influence of the density of these structures on the electrical and sensing properties of the nanobelts, this is a crucial thing if these nanostructures have to be implemented in a reliable gas sensing device. It has been proved that the conductance of the layers increase with the density of the nanobelts. While the gas sensing properties of the layers depend on the surface reaction that lead to a change in the conductance. It has been shown for example that the CO and NO2 responses are strongly affected by changes in the density of nanobelts with respect to ethanol sensing.
Acknowledgements This work has been founded by the European Strep project “Nano-structured solid-state gas sensors with superior performance” (NANOS4) no.: 001528.
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Biographies Elisabetta Comini was born on 21st November 1972 and she received her degree in physics at the University of Pisa in 1996. She is presently working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterisation of MOS thin films. She received her Ph.D. in material science at the University of Brescia. She is now a researcher at the University of Brescia. Guido Faglia was born in 1965 and has received an M.S. degree from the Polytechnic of Milan in 1991 with a thesis on gas sensors. In 1992, he has been appointed as a researcher by the Thin Film Lab at the University of Brescia. He is involved in the study of the interactions between gases and semiconductor surfaces and in gas sensors electrical characterization. In 1996, he has received the Ph.D. degree by discussing a thesis on semiconductor gas sensors. In 2000, he has been appointed as associate professor in experimental physics at University of Brescia. During his career, Guido Faglia has published more than 50 articles on International Journals with referee Giorgio Sberveglieri was born on 17 July 1947, and received his degree in physics from the University Parma, where, starting in 1971, his research
activities on the preparation of semiconducting thin film solar cells was conducted. He has been appointed associate professor at the University of Brescia in 1987. In the following year, he established the Thin Film Laboratory afterwards called Gas Sensor Laboratory, which is mainly devoted to the preparation and characterisation of thin film chemical sensors. He has been the director of the GSL since 1988. In 1994, he was appointed full professor in physics, first at the Faculty of Engineering of University of Ferrara and then in 1996, at the Faculty of Engineering of University of Brescia. He is a referee of the journals Thin Solid Films, Sensors and Actuators, Sensors and Materials, etc., and is a member of the Scientific Committee of Conferences on Sensor and Materials Science. During his 25 years of scientific activities, Giorgio Sberveglieri has published more than 140 papers on international reviews; he has presented more than 50 oral communications to international congresses and numerous oral communications to national congresses. Davide Calestani was born on 20th October 1978 and he received his degree in material science at the University of Parma in 2002. As Ph.D. student, he is working at IMEM in Technology Department since 2003, with particular reference to the development of oxide nanowires growth techniques. Ming Zheng Zha was born on 15th February 1942, has worked as associated Professor at the University of Bengjin till 1984, when he got a researcher position at Zurich ETH. Here Professor Zha has been involved in special project of NASA/ESA based on study of crystallization phenomena in microgravity conditions. This activity has been taken up at MASPEC-CNR (presently IMEM) Institute, where specific crystal growth process have been studied and performed.Recently he is involved in research oriented to the development of oxide nanowires for gas sensor application. He is author of about 90 papers in the field of material science and technology, with many innovative contribute in the field of crystal preparation from the vapour phase. Lucio Zanotti was born on 15th July 1944. He has been graduated from the University of Bologna in 1969 with a specialization in chemistry. He is one of the founders of MASPEC Institute of the Italian National Council of Research and has been working there since 1970. Since 1983 he is head of the Technology Department at MASPEC. His research has been carried out mainly in the field of semiconductor compounds for use in microand opto-electronics, infrared detectors and photovoltaic cells and in the field of inorganic\organic materials for non-linear optical application. His activity has been centered around: inorganic synthesis, purification procedures, impurity and compositional-inhomogeneity analysis, crystal growth of binary, ternary and multinary compounds from the melt, vapour and solution, defect (chemical etching) analysis, electrical and optical characterization. He is director of MASPEC (presently IMEM Institute) since 1987, is author of well over 150 scientific papers, reviews, patents in the field of material science and technology.