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Highly sensitive hydrogen sulphide sensors operable at room temperature
Sensors and Actuators B 115 (2006) 270–275
Highly sensitive hydrogen sulphide sensors operable at room temperature Shashwati Sen a , Vinit Bhandarkar a , K.P. Muthe a , M. Roy b , S.K. Deshpande c , R.C. Aiyer d , S.K. Gupta a,∗ , J.V. Yakhmi a , V.C. Sahni a a
Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India b Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India c UGC-DAE Consortium for Scientific Research, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India d Department of Physics, University of Pune, Pune 411007, India Received 15 April 2005; received in revised form 8 September 2005; accepted 15 September 2005 Available online 20 October 2005
Abstract Tellurium thin films have been investigated for use as hydrogen sulphide gas sensors. The films were prepared by thermal evaporation on alumina substrates at a temperature of 373 K and were found to be sensitive towards 0.1 ppm of H2 S at room temperature. The response was reproducible and the films were stable for operation over 3 months. Detection mechanism of the sensors was investigated and it was found that hydrogen sulphide reduced the amount of adsorbed oxygen on the Te film surface leading to increase in resistance. © 2005 Elsevier B.V. All rights reserved. Keywords: Sensors; Semiconductor; Thin Films; Grain boundaries
1. Introduction Hydrogen sulphide (H2 S) is a toxic, corrosive and inflammable gas, is produced in sewage, coal mines, oil and natural gas industries, etc., and is utilized in many chemical industries. It has an occupational exposure limit of 10 ppm for 8 h exposure. Even at low concentration it produces severe effect on the nervous system. Therefore, there is a need for low cost H2 S sensors operable in sub-ppm range. Various solid-state hydrogen sulphide sensors based on semiconductor metal oxides, such as SnO2 , ZrO2 , CeO2 , WO3 , have been reported in literatures [1–5]. Most of them can detect H2 S at concentrations higher than ∼10 ppm. Further, these sensors require high temperatures (∼523 K) for operation that increase power requirement and reduce their long-term stability. A few studies have been reported on sensors usable at low concentrations as well as those operable at room temperature. Solis et al. [6] have reported nano-crystalline WO3 sensors operable at room temperature and suitable for detection of 1 ppm H2 S. However, these sensors need short heating pulses of 523 K for stable operation. Fang et al. [5] have reported room temperature ∗
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operating CeO2 –SnO2 sensors, prepared by a sol–gel method, suitable for detection of 5 ppm H2 S. Similarly, Smith et al. [7] have reported noble metal doped WO3 sensors operable at 473 K and suitable for H2 S detection in a ppb range. However, sensors that can detect low concentration of H2 S as well as operate at room temperature have not been reported. We report here, first H2 S sensors suitable for a very low concentration range of 0.1–1 ppm and also operable at room temperature. In a few studies reported earlier, tellurium thin films have been found to be sensitive to some of reducing/oxidizing gases as NO2 , CO and NH3 [8,9]. However, interaction of gases with these films has not been studied except in case of NH3 . In the present study, Te films were deposited on single crystal and polycrystalline alumina substrates. Interaction of H2 S with the films has also been investigated. 2. Experimental Tellurium films of ∼200 nm thickness were deposited on polycrystalline and single crystal Al2 O3 substrates by thermal evaporation of (∼99.99%) pure tellurium in a tantalum boat. Depositions were carried out at a substrate temperature of 373 K and a background pressure of 10−6 mbar. Electrical contacts on the films were made by evaporating gold pads
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Fig. 1. Schematic diagram of: (a) a tellurium film deposited on an alumina substrate attached to a nichrome heater and Pt-100 temperature sensor with adhesive alumina paste, and (b) a sensor evaluation system having a stainless steel chamber with coupling for sensor housing and rubber gasket for insertion of gas using a syringe.
and attaching silver wires to them with indium solder. Several sensor films were prepared for different studies and to check reproducibility of response. For measurement of response, a miniature heater of nichrome wire wound on a alumina piece (5 mm × 5 mm × 1 mm) and a Pt-100 temperature sensor (2 mm × 2 mm) were attached to the film with alumina paste, as shown in Fig. 1(a). The heater and temperature sensor were connected to a temperature controller circuit that enabled control of sensor temperature to a desired value. To expose the Te film to H2 S, the sensor assembly was loaded in a stainless steel housing having a volume of 250 cm3 . A measured quantity of gas having 1000 ppm concentration (of H2 S) was introduced in the housing with a micro-syringe so as to yield a desired concentration. The resistance of the film was measured as a function of time with a multimeter (Keithley 2700 multimeter/data acquisition system). After a steady state was achieved, recovery of sensors was studied by opening the housing to the atmosphere. The mechanism of gas detection was investigated by using Raman, X-ray photoelectron (XPS) and impedance spectroscopy techniques. The spectra were recorded before and after exposure to the gas. The films were exposed to a high concentration (1000 ppm) of gas for ∼10 min and the Raman spectra were recorded immediately after exposure. For XPS study, the films were loaded immediately after exposure and measurements were
carried out in the shortest time. Impedance spectroscopy measurements were carried out with a sensor loaded in a test chamber (see Fig. 1) and the sensor was maintained under a fixed concentration of H2 S during the measurement. Raman spectra of the films were recorded in a back scattering geometry with a spectral resolution of 2 cm−1 . The 514.5 nm line of an Ar+ laser was used for excitation. The Raman scattered light was analyzed by using an optically aligned triple monochromator Raman spectrometer (Dilor-XY). XPS spectra was recorded using a Mg K␣ (1253.6 eV) source in an XPS system (M/s RIBER) comprising of a twin anode X-ray source (Model CX700) and a MAC-2 electron analyzer. The binding energy scale was calibrated to the Au 4f7/2 line of 84 eV. Impedance measurements on Te films were carried out in a frequency range of 100 Hz to 40 MHz using an impedance analyzer (Hewlett Packard model 4192-A). 3. Results and discussion The resistance of the Te films was seen to increase reversibly on exposure to H2 S. To determine the optimum operating temperature, sensitivity was measured at different temperatures (between 300 K and 373 K) for a fixed concentration of H2 S (5 ppm). The measurements were carried out on the films deposited on single crystal (sapphire) as well as polycrystalline
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Fig. 2. Response (S) of typical Te films deposited on: (a) single crystal alumina (sapphire), and (b) polycrystalline alumina measured after exposure to 5 ppm of H2 S at different temperatures.
alumina. Temperatures lower than 300 K were not used as room temperature varied from 293 K to 298 K and the temperature controller could only control temperatures higher than room temperature. We may add that room temperature in this paper refers to 300 K. Typical results are shown in Fig. 2. Here, the response is defined as S(%) = (Rg − Ra )(100/Ra ), where Rg and Ra are resistances in gas and air, respectively. It can be seen that films on both substrates show the maximum response at room temperature and the response decreases with an increase in operating temperature. At room temperature, the films deposited on a polycrystalline alumina substrate exhibited a much higher response to H2 S than the films deposited on sapphire. However, at temperatures above 323 K, the response of the films deposited on both substrates was found to be similar to each other. In order to investigate the origin of the smaller response on the single crystal substrate, scanning electron micrographs were obtained for the films deposited on both substrates and the typical results are shown in Fig. 3. The grain size for the film on the polycrystalline substrate is seen to be much smaller than that on the single crystal substrate. Further, grains on sapphire substrate show coalescence, which reduces the surface area of the film. As discussed later, the sensitivity of film to H2 S arises due to interaction of the gas with adsorbed oxygen in the grain boundary region. Thus, the increased response with the polycrystalline substrate is attributed to the enhanced grain boundary region. In view of better sensitivity, further studies were carried out with the films on polycrystalline substrates. The larger response of the films at 300 K may partly be caused by an increase in carrier concentration with temperature. The sensitivity towards a gas arises due to a change in its carrier concentration upon interaction with the gas [9]. With an increase in temperature, the carrier concentration of the Te film increases, and therefore, the relative change in carrier concentration decreases on exposure to a gas. Thus the response of the film decreases. Further experiments to understand the effect of temperature are being carried out. For investigation of the reproducibility of the gas response, 15 different depositions were carried out under similar conditions with two films in each deposition. Response of these films
Fig. 3. SEM micrographs of Te films deposited on: (A) single crystal and (B) polycrystalline alumina substrates.
to 1 ppm of H2 S varied from 80% to 120%, indicating that the Te sensors can be fabricated in a reproducible manner. Response of the films deposited on polycrystalline alumina was also measured as a function of H2 S concentration and the results obtained at room temperature (300 K) on a typical Te film are shown in Fig. 4. The films are seen to be sensitive to very low H2 S concentration of 0.1 ppm. A log–log plot of the response versus concentration (C) is also shown in the inset of Fig. 4. The linear behavior shows a power law dependence of the response on the concentration, i.e. S = ACα , as also reported in earlier studies [10,11]. From Fig. 4, the value of α was found to be 0.495. This is in agreement with predictions (based on the surface reaction mechanism) of a rational value (1 or 1/2) for the power law exponent [11]. To determine response and recovery times, a film was exposed to different concentrations of H2 S and its resistance was recorded as a function of time. The results are shown in Fig. 5. The response time has been defined as the time taken to attain 90% of final value, and the recovery time as the time taken to regain 10% of the base value. The response time was seen to decrease with an increase in gas concentration and had a value
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Fig. 6. Raman Spectra of: (a) as-prepared and (b) H2 S-exposed Te films.
Fig. 4. Response of a typical film at room temperature as a function of H2 S concentration in 0.1–20 ppm range. Inset shows the log–log plot of response vs. concentration.
of ∼5 min for 1 ppm of H2 S gas. On the other hand, the recovery time was found to increase with an increase in gas concentration and was ∼20 min for 1 ppm H2 S. The other important characteristic of a sensor is its selectivity. To check the sensitivity of these Te films towards other gases, we have exposed a film to 10 ppm of different oxidizing and reducing gases. The response on exposure to 10 ppm of CO, NH3 , H2 S and NO was found to be 3%, 40%, 200% and −67% (negative sign indicates reduction in resistance), respectively. Thus we see that the Te films have much larger response towards H2 S gas in comparison to other gases. To commercially implement any material for gas sensing purpose it is desirable to understand the detection mechanism.
Fig. 5. Response and recovery transients of a typical film after exposure to 0.4 ppm, 1 ppm and 4 ppm H2 S gas.
We have investigated the mechanism of H2 S gas detection by using Raman, X-ray photoelectron (XPS) and impedance spectroscopy (IS) techniques. The Raman spectra of the films before and immediately after exposure to a high concentration (1000 ppm) of H2 S are shown in Fig. 6. The as-prepared films showed peaks corresponding to TeO2 [12] at 680 cm−1 and 811 cm−1 in addition to those of Te. On exposure to H2 S, TeO2 was seen to be reduced to Te. Comparing the intensities of the peaks corresponding to Te and TeO2 , it is seen that the fraction of TeO2 in the films is very small. The results of XPS measurements are shown in Fig. 7. Te 3d5/2 spectra of the unexposed samples (Fig. 7a) showed the presence of Te metal (572.2 eV) and TeO2 (575.6 eV) [13] with intensity of the TeO2 peak being much larger. As XPS has a
Fig. 7. XPS spectra in S 2p, O 1s and Te 3d5/2 regions for: (a) unexposed and (b) H2 S exposed film. Curve (c) gives XPS in the Te 3d5/2 region for film recovered after exposure to H2 S.
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Fig. 8. Impedance spectra of a Tellurium film before (open squares) and after (solid triangles) exposure to 1 ppm H2 S. Curves (a) and (b) were obtained by fitting of the experimental data to the equivalent circuit given in the inset.
˚ comparison with the small depth resolution of typically 30 A, Raman result, where the intensity of Te is much larger, indicates that TeO2 is formed on the surface only. On exposure to 1000 ppm H2 S (Fig. 7b), TeO2 was found to be reduced to Te in agreement with the Raman results. O 1s spectra before exposure could be de-convoluted into two peaks attributed to TeO2 (529 eV) and physisorbed oxygen (530.5 eV). After exposure to H2 S, the peak corresponding to TeO2 was not observed, but a new peak was seen at higher binding energy (532 eV), which could arise due to formation of OH upon interaction with H2 S. The S 2p signal was also observed in a freshly exposed film. It may be noted that in case of interaction with NH3 no peak pertaining to N was observed in the XPS [9]. Fig. 7 also shows the Te 3d5/2 XPS spectra of a film after recovery from exposure to H2 S gas (curve c). The spectra is seen to be similar to curve a obtained before exposure. This confirms that the change in resistivity on exposure to H2 S is due to reduction of TeO2 to Te metal. The recovery of the sensor to its original state is also indicated by the recovery in response shown in Fig. 5. Impedance spectroscopy helps to distinguish between the significant contributions from the grain boundary, intra-grain region, contacts or surface region to the film resistivity and sensitivity [14]. Impedance spectra before and after exposure to H2 S are shown in Fig. 8 in a form of complex impedance plot [14]. This spectra was analyzed by using the circuit given in the inset of Fig. 8. The optimum values of resistance and capacitance in the equivalent circuit are also given in the inset. Frequency-independent resistance R0 is attributed to the bulk and surface contributions [15]. Resistance Ri and capacitance Ci are attributed to the intra-grain region as the value of capacitance is typical of the intragrain region, while resistance Rg and capacitance Cg are attributed to grain boundary region. On exposure to 1 ppm H2 S, both Ri and Rg were seen to increase, though the increase in Rg was found to be much larger. It is seen that upon interaction with H2 S, adsorbed oxygen on the surface of the Tellurium films is removed and the increase in resistance on
Fig. 9. Time dependence of: (A) base resistance and (B) response of two Te films for 0.4 ppm and 2 ppm of H2 S, respectively. Data has been measured over a period of 3 months.
exposure is mainly contributed by the intragrain region of the films. Based on these results, we propose the following detection mechanism. Te is known to be a p-type lone pair (electrons that are not shared between atoms are localized as lone-pair electrons on one or other atom(s) within a molecule or crystal [16]) semiconductor, where the upper part of the valence band is formed by lone-pair electrons [17,18]. Oxygen atoms adsorbed on the surface act as acceptor sites and traps these electrons, leading to an increase in hole concentration and conductivity in the intragrain region. On removal of adsorbed oxygen, the hole density decreases thereby increasing the resistivity of the film. This also explains the reduced response of the films on single crystal sapphire substrates, where due to the large grain size the amount of adsorbed oxygen is reduced on the surfaces of grain boundaries. These films were evaluated for their long-term stability by recording their base resistance, as well as the gas response for a period of over 3 months. Two films (1 and 2) were used for this study and these were repeatedly exposed to 0.4 ppm and 2 ppm of gas, respectively. Fig. 9(A) and (B) show the base resistance and the response of these two films for a period of 3 months.
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It can be seen that the base resistance as well as the sensitivity remains stable within a range of ±10%. Thus these films are suitable for gas sensor applications. 4. Conclusions Vacuum-deposited Te thin films were sensitive to very low concentrations (0.1–1 ppm) of H2 S gas. The films deposited on polycrystalline alumina showed larger response, compared to those on single crystal sapphire substrates. Response of the films towards other oxidizing and reducing gases was lower than that to H2 S. XPS, Raman and impedance spectroscopy studies showed that upon interaction with H2 S, adsorbed oxygen on the surface of Te grains was removed which lead to changes in resistivity of the film. The films were found to show stable response for a period of more than 3 months. References [1] J. Watson, The tin oxide gas sensor and its application, Sens. Actuators 5 (1984) 29–42. [2] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Sensing behavior of CuO-loaded SnO2 element for H2 S detection, Chem. Lett. 4 (1991) 575–578. [3] E.P.S. Barrett, G.C. Georgiades, P.A. Sermon, The mechanism of operation of WO3 -based H2 S sensors, Sens. Actuators B 1 (1990) 116–120. [4] N. Miura, Y. Yan, G. Lu, N. Yamazoe, Sensing characteristics and mechanism of hydrogen sulphide sensor using stabilized zirconia and oxide sensing electrode, Sens. Actuators B 34 (1996) 367–372. [5] G. Fang, Z. Liu, C. Liu, K. Yao, Room temperature H2 S sensing properties and mechanism of CeO2 –SnO2 -sol–gel thin films, Sens. Actuators B 66 (2000) 46–48. [6] J.L. Solis, S. Saukko, L.B. Kish, C.G. Granqvist, V. Lantto, Nanocrystalline tungsten oxide thick-films with high sensitivity to H2 S at room temperature, Sens. Actuators B 77 (2001) 316–321. [7] D.J. Smith, J.F. Vetelino, R.S. Falconer, E.L. Wittman, Stability, sensitivity and selectivity of tungsten trioxide films for sensing applications, Sens. Actuators B 13/14 (1993) 264–268. [8] D. Tsiulyanu, S. Marian, H.D. Liess, Sensing properties of tellurium based thin films to propylamine and carbon oxide, Sens. Actuators B 85 (2002) 232–238. [9] S. Sen, K.P. Muthe, N. Joshi, S.C. Gadkari, S.K. Gupta, Jagannath, M. Roy, S.K. Deshpande, J.V. Yakhmi, Room temperature operating ammonia sensor based on tellurium thin films, Sens. Actuators B 98 (2004) 154–159. [10] N. Barsan, A. Tomescu, Calibration procedure for SnO2 -based gas sensors, Thin Solid Films 259 (1995) 91–95. [11] D.E. Williams, K.F.E. Pratt, Microstructure effects on the response of gas-sensitive resistors based on semiconducting oxides, Sens. Actuators B 70 (2000) 214–221. [12] A.P. Mirgorodsky, T. Merle-Mejean, J.-C. Champarnaud, P. Thomas, B. Frit, Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new ␥- and ␦-phases, J. Phys. Chem. Solids 61 (2000) 501–509. [13] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, J.E. Millenberg, Handbook of the X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Draivie, MN, 1979. [14] J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987.
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[15] U. Weimar, W. Gopel, A.C. measurements on tin oxide sensors to improve selectivities and sensitivities, Sens. Actuators B 26/27 (1995) 13–18. [16] F.A. Cotton, G. Wilkinson, P.L. Gaus, Basic Inorganic Chemistry, John Wiely & Sons, 1995, p. 74. [17] A.K. Ray, R. Swan, C.A. Hogarth, Conduction mechanisms in amorphous tellurium films, J. Non-Cryst. Solids 168 (1994) 150– 156. [18] J.P. Hernandez, Charge transfer states in a chain molecule with a dangling bond, J. Phys: Condens. Matter 9 (1997) L285–L289.
Biographies Dr. Shashwati Sen joined BARC in 1996. She obtained PhD degree from University of Mumbai for her work on “Dissipation mechanisms in high temperature superconductors”. Currently she is working on gas sensors based on elemental and metal oxide semiconductor thin films. V.B. Bhandarkar has received his MSc degree in physics and is pursuing his PhD at the Bhabha Atomic Research Centre, Mumbai. He is involved in the design of XPS beamline for synchrotron radiation source. He has studied H2 S and NO2 gas-sensing properties of tellurium thin films and NH3 sensors based on tungsten oxide thin films. K.P. Muthe has been working in the field of thin film growth and characterization for the last 17 years. He has studied the growth behavior of HTSC films using MBE. His current interests include development of toxic gas sensors and synthesis of advanced radiation sensors for Personnel Dosimetry. M. Roy joined Novel Materials & Structural Chemistry Division, BARC, in 1999 after completing his MSc (chemistry) from the Burdwan University. His research interests include thin films of diamond and diamond-like materials and their spectroscopic characterization. Dr. S.K. Deshpande obtained PhD in physics from University of Pune in 1994 and worked at the Institute for Plasma Research, Gandhinagar, on Tokamak plasma spectroscopic diagnostics. Later, he was a lecturer at the Department of Physics, University of Pune. Currently, he is working as scientist at the Mumbai Centre of UGC-DAE Consortium for Scientific Research. He is involved in research and developmental work on neutron and X-ray diffraction, and dielectric studies. Dr. S.K. Gupta joined BARC in 1975 and is presently Head of Thin Films Devices Section in TPPED. Over the years, he has worked on space quality silicon solar cells, high-temperature superconductor thin films and single crystals, gas sensors and thermoelectric materials. Dr. J.V. Yakhmi, Head, Technical Physics and Prototype Engineering Division, has worked in BARC for the past 37 years on diverse areas of research in materials science, such as, high Tc superconductors, magnetic alloys, molecular materials, etc. Dr V.C. Sahni joined Bhabha Atomic Research Centre in the year 1965 and has made significant contributions in the areas of lattice dynamics of complex crystals, Raman spectroscopy, measurement of electron momentum densities, quasi crystals, high-temperature superconductors, solid-state gas sensors and mass spectrometers. Presently, he is Director of Centre for Advanced Technology, Indore, and Director, Physics Group at BARC, Mumbai. Prof. R.C. Aiyer is working as physics professor at University of Pune, Maharashtra, India. She has carried out several investigations on preparation of semiconductor oxide gas sensors, optical fiber-based sensors and nanomaterials.
Highly sensitive hydrogen sulphide sensors operable at room temperature Shashwati Sen a , Vinit Bhandarkar a , K.P. Muthe a , M. Roy b , S.K. Deshpande c , R.C. Aiyer d , S.K. Gupta a,∗ , J.V. Yakhmi a , V.C. Sahni a a
Technical Physics and Prototype Engineering Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India b Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India c UGC-DAE Consortium for Scientific Research, Mumbai Centre, Bhabha Atomic Research Centre, Mumbai 400085, India d Department of Physics, University of Pune, Pune 411007, India Received 15 April 2005; received in revised form 8 September 2005; accepted 15 September 2005 Available online 20 October 2005
Abstract Tellurium thin films have been investigated for use as hydrogen sulphide gas sensors. The films were prepared by thermal evaporation on alumina substrates at a temperature of 373 K and were found to be sensitive towards 0.1 ppm of H2 S at room temperature. The response was reproducible and the films were stable for operation over 3 months. Detection mechanism of the sensors was investigated and it was found that hydrogen sulphide reduced the amount of adsorbed oxygen on the Te film surface leading to increase in resistance. © 2005 Elsevier B.V. All rights reserved. Keywords: Sensors; Semiconductor; Thin Films; Grain boundaries
1. Introduction Hydrogen sulphide (H2 S) is a toxic, corrosive and inflammable gas, is produced in sewage, coal mines, oil and natural gas industries, etc., and is utilized in many chemical industries. It has an occupational exposure limit of 10 ppm for 8 h exposure. Even at low concentration it produces severe effect on the nervous system. Therefore, there is a need for low cost H2 S sensors operable in sub-ppm range. Various solid-state hydrogen sulphide sensors based on semiconductor metal oxides, such as SnO2 , ZrO2 , CeO2 , WO3 , have been reported in literatures [1–5]. Most of them can detect H2 S at concentrations higher than ∼10 ppm. Further, these sensors require high temperatures (∼523 K) for operation that increase power requirement and reduce their long-term stability. A few studies have been reported on sensors usable at low concentrations as well as those operable at room temperature. Solis et al. [6] have reported nano-crystalline WO3 sensors operable at room temperature and suitable for detection of 1 ppm H2 S. However, these sensors need short heating pulses of 523 K for stable operation. Fang et al. [5] have reported room temperature ∗
Corresponding author. Tel.: +91 22 25593863; fax: +91 22 25505296. E-mail address: [email protected] (S.K. Gupta).
0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.09.013
operating CeO2 –SnO2 sensors, prepared by a sol–gel method, suitable for detection of 5 ppm H2 S. Similarly, Smith et al. [7] have reported noble metal doped WO3 sensors operable at 473 K and suitable for H2 S detection in a ppb range. However, sensors that can detect low concentration of H2 S as well as operate at room temperature have not been reported. We report here, first H2 S sensors suitable for a very low concentration range of 0.1–1 ppm and also operable at room temperature. In a few studies reported earlier, tellurium thin films have been found to be sensitive to some of reducing/oxidizing gases as NO2 , CO and NH3 [8,9]. However, interaction of gases with these films has not been studied except in case of NH3 . In the present study, Te films were deposited on single crystal and polycrystalline alumina substrates. Interaction of H2 S with the films has also been investigated. 2. Experimental Tellurium films of ∼200 nm thickness were deposited on polycrystalline and single crystal Al2 O3 substrates by thermal evaporation of (∼99.99%) pure tellurium in a tantalum boat. Depositions were carried out at a substrate temperature of 373 K and a background pressure of 10−6 mbar. Electrical contacts on the films were made by evaporating gold pads
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Fig. 1. Schematic diagram of: (a) a tellurium film deposited on an alumina substrate attached to a nichrome heater and Pt-100 temperature sensor with adhesive alumina paste, and (b) a sensor evaluation system having a stainless steel chamber with coupling for sensor housing and rubber gasket for insertion of gas using a syringe.
and attaching silver wires to them with indium solder. Several sensor films were prepared for different studies and to check reproducibility of response. For measurement of response, a miniature heater of nichrome wire wound on a alumina piece (5 mm × 5 mm × 1 mm) and a Pt-100 temperature sensor (2 mm × 2 mm) were attached to the film with alumina paste, as shown in Fig. 1(a). The heater and temperature sensor were connected to a temperature controller circuit that enabled control of sensor temperature to a desired value. To expose the Te film to H2 S, the sensor assembly was loaded in a stainless steel housing having a volume of 250 cm3 . A measured quantity of gas having 1000 ppm concentration (of H2 S) was introduced in the housing with a micro-syringe so as to yield a desired concentration. The resistance of the film was measured as a function of time with a multimeter (Keithley 2700 multimeter/data acquisition system). After a steady state was achieved, recovery of sensors was studied by opening the housing to the atmosphere. The mechanism of gas detection was investigated by using Raman, X-ray photoelectron (XPS) and impedance spectroscopy techniques. The spectra were recorded before and after exposure to the gas. The films were exposed to a high concentration (1000 ppm) of gas for ∼10 min and the Raman spectra were recorded immediately after exposure. For XPS study, the films were loaded immediately after exposure and measurements were
carried out in the shortest time. Impedance spectroscopy measurements were carried out with a sensor loaded in a test chamber (see Fig. 1) and the sensor was maintained under a fixed concentration of H2 S during the measurement. Raman spectra of the films were recorded in a back scattering geometry with a spectral resolution of 2 cm−1 . The 514.5 nm line of an Ar+ laser was used for excitation. The Raman scattered light was analyzed by using an optically aligned triple monochromator Raman spectrometer (Dilor-XY). XPS spectra was recorded using a Mg K␣ (1253.6 eV) source in an XPS system (M/s RIBER) comprising of a twin anode X-ray source (Model CX700) and a MAC-2 electron analyzer. The binding energy scale was calibrated to the Au 4f7/2 line of 84 eV. Impedance measurements on Te films were carried out in a frequency range of 100 Hz to 40 MHz using an impedance analyzer (Hewlett Packard model 4192-A). 3. Results and discussion The resistance of the Te films was seen to increase reversibly on exposure to H2 S. To determine the optimum operating temperature, sensitivity was measured at different temperatures (between 300 K and 373 K) for a fixed concentration of H2 S (5 ppm). The measurements were carried out on the films deposited on single crystal (sapphire) as well as polycrystalline
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Fig. 2. Response (S) of typical Te films deposited on: (a) single crystal alumina (sapphire), and (b) polycrystalline alumina measured after exposure to 5 ppm of H2 S at different temperatures.
alumina. Temperatures lower than 300 K were not used as room temperature varied from 293 K to 298 K and the temperature controller could only control temperatures higher than room temperature. We may add that room temperature in this paper refers to 300 K. Typical results are shown in Fig. 2. Here, the response is defined as S(%) = (Rg − Ra )(100/Ra ), where Rg and Ra are resistances in gas and air, respectively. It can be seen that films on both substrates show the maximum response at room temperature and the response decreases with an increase in operating temperature. At room temperature, the films deposited on a polycrystalline alumina substrate exhibited a much higher response to H2 S than the films deposited on sapphire. However, at temperatures above 323 K, the response of the films deposited on both substrates was found to be similar to each other. In order to investigate the origin of the smaller response on the single crystal substrate, scanning electron micrographs were obtained for the films deposited on both substrates and the typical results are shown in Fig. 3. The grain size for the film on the polycrystalline substrate is seen to be much smaller than that on the single crystal substrate. Further, grains on sapphire substrate show coalescence, which reduces the surface area of the film. As discussed later, the sensitivity of film to H2 S arises due to interaction of the gas with adsorbed oxygen in the grain boundary region. Thus, the increased response with the polycrystalline substrate is attributed to the enhanced grain boundary region. In view of better sensitivity, further studies were carried out with the films on polycrystalline substrates. The larger response of the films at 300 K may partly be caused by an increase in carrier concentration with temperature. The sensitivity towards a gas arises due to a change in its carrier concentration upon interaction with the gas [9]. With an increase in temperature, the carrier concentration of the Te film increases, and therefore, the relative change in carrier concentration decreases on exposure to a gas. Thus the response of the film decreases. Further experiments to understand the effect of temperature are being carried out. For investigation of the reproducibility of the gas response, 15 different depositions were carried out under similar conditions with two films in each deposition. Response of these films
Fig. 3. SEM micrographs of Te films deposited on: (A) single crystal and (B) polycrystalline alumina substrates.
to 1 ppm of H2 S varied from 80% to 120%, indicating that the Te sensors can be fabricated in a reproducible manner. Response of the films deposited on polycrystalline alumina was also measured as a function of H2 S concentration and the results obtained at room temperature (300 K) on a typical Te film are shown in Fig. 4. The films are seen to be sensitive to very low H2 S concentration of 0.1 ppm. A log–log plot of the response versus concentration (C) is also shown in the inset of Fig. 4. The linear behavior shows a power law dependence of the response on the concentration, i.e. S = ACα , as also reported in earlier studies [10,11]. From Fig. 4, the value of α was found to be 0.495. This is in agreement with predictions (based on the surface reaction mechanism) of a rational value (1 or 1/2) for the power law exponent [11]. To determine response and recovery times, a film was exposed to different concentrations of H2 S and its resistance was recorded as a function of time. The results are shown in Fig. 5. The response time has been defined as the time taken to attain 90% of final value, and the recovery time as the time taken to regain 10% of the base value. The response time was seen to decrease with an increase in gas concentration and had a value
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Fig. 6. Raman Spectra of: (a) as-prepared and (b) H2 S-exposed Te films.
Fig. 4. Response of a typical film at room temperature as a function of H2 S concentration in 0.1–20 ppm range. Inset shows the log–log plot of response vs. concentration.
of ∼5 min for 1 ppm of H2 S gas. On the other hand, the recovery time was found to increase with an increase in gas concentration and was ∼20 min for 1 ppm H2 S. The other important characteristic of a sensor is its selectivity. To check the sensitivity of these Te films towards other gases, we have exposed a film to 10 ppm of different oxidizing and reducing gases. The response on exposure to 10 ppm of CO, NH3 , H2 S and NO was found to be 3%, 40%, 200% and −67% (negative sign indicates reduction in resistance), respectively. Thus we see that the Te films have much larger response towards H2 S gas in comparison to other gases. To commercially implement any material for gas sensing purpose it is desirable to understand the detection mechanism.
Fig. 5. Response and recovery transients of a typical film after exposure to 0.4 ppm, 1 ppm and 4 ppm H2 S gas.
We have investigated the mechanism of H2 S gas detection by using Raman, X-ray photoelectron (XPS) and impedance spectroscopy (IS) techniques. The Raman spectra of the films before and immediately after exposure to a high concentration (1000 ppm) of H2 S are shown in Fig. 6. The as-prepared films showed peaks corresponding to TeO2 [12] at 680 cm−1 and 811 cm−1 in addition to those of Te. On exposure to H2 S, TeO2 was seen to be reduced to Te. Comparing the intensities of the peaks corresponding to Te and TeO2 , it is seen that the fraction of TeO2 in the films is very small. The results of XPS measurements are shown in Fig. 7. Te 3d5/2 spectra of the unexposed samples (Fig. 7a) showed the presence of Te metal (572.2 eV) and TeO2 (575.6 eV) [13] with intensity of the TeO2 peak being much larger. As XPS has a
Fig. 7. XPS spectra in S 2p, O 1s and Te 3d5/2 regions for: (a) unexposed and (b) H2 S exposed film. Curve (c) gives XPS in the Te 3d5/2 region for film recovered after exposure to H2 S.
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Fig. 8. Impedance spectra of a Tellurium film before (open squares) and after (solid triangles) exposure to 1 ppm H2 S. Curves (a) and (b) were obtained by fitting of the experimental data to the equivalent circuit given in the inset.
˚ comparison with the small depth resolution of typically 30 A, Raman result, where the intensity of Te is much larger, indicates that TeO2 is formed on the surface only. On exposure to 1000 ppm H2 S (Fig. 7b), TeO2 was found to be reduced to Te in agreement with the Raman results. O 1s spectra before exposure could be de-convoluted into two peaks attributed to TeO2 (529 eV) and physisorbed oxygen (530.5 eV). After exposure to H2 S, the peak corresponding to TeO2 was not observed, but a new peak was seen at higher binding energy (532 eV), which could arise due to formation of OH upon interaction with H2 S. The S 2p signal was also observed in a freshly exposed film. It may be noted that in case of interaction with NH3 no peak pertaining to N was observed in the XPS [9]. Fig. 7 also shows the Te 3d5/2 XPS spectra of a film after recovery from exposure to H2 S gas (curve c). The spectra is seen to be similar to curve a obtained before exposure. This confirms that the change in resistivity on exposure to H2 S is due to reduction of TeO2 to Te metal. The recovery of the sensor to its original state is also indicated by the recovery in response shown in Fig. 5. Impedance spectroscopy helps to distinguish between the significant contributions from the grain boundary, intra-grain region, contacts or surface region to the film resistivity and sensitivity [14]. Impedance spectra before and after exposure to H2 S are shown in Fig. 8 in a form of complex impedance plot [14]. This spectra was analyzed by using the circuit given in the inset of Fig. 8. The optimum values of resistance and capacitance in the equivalent circuit are also given in the inset. Frequency-independent resistance R0 is attributed to the bulk and surface contributions [15]. Resistance Ri and capacitance Ci are attributed to the intra-grain region as the value of capacitance is typical of the intragrain region, while resistance Rg and capacitance Cg are attributed to grain boundary region. On exposure to 1 ppm H2 S, both Ri and Rg were seen to increase, though the increase in Rg was found to be much larger. It is seen that upon interaction with H2 S, adsorbed oxygen on the surface of the Tellurium films is removed and the increase in resistance on
Fig. 9. Time dependence of: (A) base resistance and (B) response of two Te films for 0.4 ppm and 2 ppm of H2 S, respectively. Data has been measured over a period of 3 months.
exposure is mainly contributed by the intragrain region of the films. Based on these results, we propose the following detection mechanism. Te is known to be a p-type lone pair (electrons that are not shared between atoms are localized as lone-pair electrons on one or other atom(s) within a molecule or crystal [16]) semiconductor, where the upper part of the valence band is formed by lone-pair electrons [17,18]. Oxygen atoms adsorbed on the surface act as acceptor sites and traps these electrons, leading to an increase in hole concentration and conductivity in the intragrain region. On removal of adsorbed oxygen, the hole density decreases thereby increasing the resistivity of the film. This also explains the reduced response of the films on single crystal sapphire substrates, where due to the large grain size the amount of adsorbed oxygen is reduced on the surfaces of grain boundaries. These films were evaluated for their long-term stability by recording their base resistance, as well as the gas response for a period of over 3 months. Two films (1 and 2) were used for this study and these were repeatedly exposed to 0.4 ppm and 2 ppm of gas, respectively. Fig. 9(A) and (B) show the base resistance and the response of these two films for a period of 3 months.
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It can be seen that the base resistance as well as the sensitivity remains stable within a range of ±10%. Thus these films are suitable for gas sensor applications. 4. Conclusions Vacuum-deposited Te thin films were sensitive to very low concentrations (0.1–1 ppm) of H2 S gas. The films deposited on polycrystalline alumina showed larger response, compared to those on single crystal sapphire substrates. Response of the films towards other oxidizing and reducing gases was lower than that to H2 S. XPS, Raman and impedance spectroscopy studies showed that upon interaction with H2 S, adsorbed oxygen on the surface of Te grains was removed which lead to changes in resistivity of the film. The films were found to show stable response for a period of more than 3 months. References [1] J. Watson, The tin oxide gas sensor and its application, Sens. Actuators 5 (1984) 29–42. [2] T. Maekawa, J. Tamaki, N. Miura, N. Yamazoe, Sensing behavior of CuO-loaded SnO2 element for H2 S detection, Chem. Lett. 4 (1991) 575–578. [3] E.P.S. Barrett, G.C. Georgiades, P.A. Sermon, The mechanism of operation of WO3 -based H2 S sensors, Sens. Actuators B 1 (1990) 116–120. [4] N. Miura, Y. Yan, G. Lu, N. Yamazoe, Sensing characteristics and mechanism of hydrogen sulphide sensor using stabilized zirconia and oxide sensing electrode, Sens. Actuators B 34 (1996) 367–372. [5] G. Fang, Z. Liu, C. Liu, K. Yao, Room temperature H2 S sensing properties and mechanism of CeO2 –SnO2 -sol–gel thin films, Sens. Actuators B 66 (2000) 46–48. [6] J.L. Solis, S. Saukko, L.B. Kish, C.G. Granqvist, V. Lantto, Nanocrystalline tungsten oxide thick-films with high sensitivity to H2 S at room temperature, Sens. Actuators B 77 (2001) 316–321. [7] D.J. Smith, J.F. Vetelino, R.S. Falconer, E.L. Wittman, Stability, sensitivity and selectivity of tungsten trioxide films for sensing applications, Sens. Actuators B 13/14 (1993) 264–268. [8] D. Tsiulyanu, S. Marian, H.D. Liess, Sensing properties of tellurium based thin films to propylamine and carbon oxide, Sens. Actuators B 85 (2002) 232–238. [9] S. Sen, K.P. Muthe, N. Joshi, S.C. Gadkari, S.K. Gupta, Jagannath, M. Roy, S.K. Deshpande, J.V. Yakhmi, Room temperature operating ammonia sensor based on tellurium thin films, Sens. Actuators B 98 (2004) 154–159. [10] N. Barsan, A. Tomescu, Calibration procedure for SnO2 -based gas sensors, Thin Solid Films 259 (1995) 91–95. [11] D.E. Williams, K.F.E. Pratt, Microstructure effects on the response of gas-sensitive resistors based on semiconducting oxides, Sens. Actuators B 70 (2000) 214–221. [12] A.P. Mirgorodsky, T. Merle-Mejean, J.-C. Champarnaud, P. Thomas, B. Frit, Dynamics and structure of TeO2 polymorphs: model treatment of paratellurite and tellurite; Raman scattering evidence for new ␥- and ␦-phases, J. Phys. Chem. Solids 61 (2000) 501–509. [13] C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, J.E. Millenberg, Handbook of the X-ray Photoelectron Spectroscopy, Perkin-Elmer, Eden Draivie, MN, 1979. [14] J.R. Macdonald, Impedance Spectroscopy, Wiley, New York, 1987.
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[15] U. Weimar, W. Gopel, A.C. measurements on tin oxide sensors to improve selectivities and sensitivities, Sens. Actuators B 26/27 (1995) 13–18. [16] F.A. Cotton, G. Wilkinson, P.L. Gaus, Basic Inorganic Chemistry, John Wiely & Sons, 1995, p. 74. [17] A.K. Ray, R. Swan, C.A. Hogarth, Conduction mechanisms in amorphous tellurium films, J. Non-Cryst. Solids 168 (1994) 150– 156. [18] J.P. Hernandez, Charge transfer states in a chain molecule with a dangling bond, J. Phys: Condens. Matter 9 (1997) L285–L289.
Biographies Dr. Shashwati Sen joined BARC in 1996. She obtained PhD degree from University of Mumbai for her work on “Dissipation mechanisms in high temperature superconductors”. Currently she is working on gas sensors based on elemental and metal oxide semiconductor thin films. V.B. Bhandarkar has received his MSc degree in physics and is pursuing his PhD at the Bhabha Atomic Research Centre, Mumbai. He is involved in the design of XPS beamline for synchrotron radiation source. He has studied H2 S and NO2 gas-sensing properties of tellurium thin films and NH3 sensors based on tungsten oxide thin films. K.P. Muthe has been working in the field of thin film growth and characterization for the last 17 years. He has studied the growth behavior of HTSC films using MBE. His current interests include development of toxic gas sensors and synthesis of advanced radiation sensors for Personnel Dosimetry. M. Roy joined Novel Materials & Structural Chemistry Division, BARC, in 1999 after completing his MSc (chemistry) from the Burdwan University. His research interests include thin films of diamond and diamond-like materials and their spectroscopic characterization. Dr. S.K. Deshpande obtained PhD in physics from University of Pune in 1994 and worked at the Institute for Plasma Research, Gandhinagar, on Tokamak plasma spectroscopic diagnostics. Later, he was a lecturer at the Department of Physics, University of Pune. Currently, he is working as scientist at the Mumbai Centre of UGC-DAE Consortium for Scientific Research. He is involved in research and developmental work on neutron and X-ray diffraction, and dielectric studies. Dr. S.K. Gupta joined BARC in 1975 and is presently Head of Thin Films Devices Section in TPPED. Over the years, he has worked on space quality silicon solar cells, high-temperature superconductor thin films and single crystals, gas sensors and thermoelectric materials. Dr. J.V. Yakhmi, Head, Technical Physics and Prototype Engineering Division, has worked in BARC for the past 37 years on diverse areas of research in materials science, such as, high Tc superconductors, magnetic alloys, molecular materials, etc. Dr V.C. Sahni joined Bhabha Atomic Research Centre in the year 1965 and has made significant contributions in the areas of lattice dynamics of complex crystals, Raman spectroscopy, measurement of electron momentum densities, quasi crystals, high-temperature superconductors, solid-state gas sensors and mass spectrometers. Presently, he is Director of Centre for Advanced Technology, Indore, and Director, Physics Group at BARC, Mumbai. Prof. R.C. Aiyer is working as physics professor at University of Pune, Maharashtra, India. She has carried out several investigations on preparation of semiconductor oxide gas sensors, optical fiber-based sensors and nanomaterials.