Fabrication and characterization of Fe1.90Ti0.10O3 gas sensitive resistors for carbon monoxide

Sensors and Actuators B 135 (2009) 430–435

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Fabrication and characterization of Fe1.90 Ti0.10 O3 gas sensitive resistors for carbon monoxide Vandna Luthra a,1 , Keith F.E. Pratt a , Ivan P. Parkin a , David E. Williams a , R.P. Tandon b,∗ a b

Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, UK Department of Physics & Astrophysics, University of Delhi, Delhi 110007, India

a r t i c l e

i n f o

Article history: Received 29 December 2007 Received in revised form 15 September 2008 Accepted 16 September 2008 Available online 1 October 2008 Keywords: Fe2−x Tix O3 Gas sensors Resistive sensor of CO

a b s t r a c t An iron–titanium solid solution Fe1.90 Ti0.10 O3 was synthesized by a conventional ceramic route. The powder was incorporated into a gas-sensing device by screen printing a mixture of Fe1.90 Ti0.10 O3 and vehicle (Agmet ESL-400) onto an alumina tile. Heating the tile to 700 ◦ C removed the organic vehicle and left a porous, stable and uniform layer of Fe1.90 Ti0.10 O3 . This oxide layer showed a reversible n-type change in resistance on exposure to varying concentrations of carbon monoxide, for which the interfering effect of water vapour was small. Titanium was strongly surface segregated, and reduction of Fe(III) to Fe(II) near the surface was found. The behaviour is interpreted with a Mars–van Krevelen model for adsorption and reaction on a complex surface defect. The possibility to control the behaviour through control of the surface segregation is inferred. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Semi-conducting oxides have been exploited for the detection of toxic and flammable gases. The majority of these gas sensors are based on tin oxide in undoped, doped and decorated forms, for example with noble metals such as Pt, Pd, etc. Despite the poor performance of tin oxide in terms of its drift in baseline, humidity interference, poor long term stability and cross-selectivity, it is the most widely used material for commercial applications [1–3]. More recently chromium–titanium oxide solid solutions have been commercially exploited as sensor materials for detection of various gases [4]. The addition of titanium to the chromium matrix gives rise to a p-type gas sensitive resistance. The gas-sensing behaviour has in part been attributed to surface segregation of titanium [4–7]. Iron oxide has a corundum (Al2 O3 ) structure. There are a few scattered reports which discuss the potential of iron oxide as a gassensing material [8–17]. Iron oxide can exist in various forms such as ␣-Fe2 O3 , ␥-Fe2 O3 and Fe3 O4. The gas-sensing properties of the ␣ and ␥ forms are still not established and contrasts are available in literature [10,15]. Some reports attribute the gas-sensing properties of iron oxides to ␥-Fe2 O3 or Fe3 O4 forms rather than to ␣-Fe2 O3. The ␣-Fe2 O3 form has been recognized as having minimal gas-sensing

∗ Corresponding author. Tel.: +91 11 27667793; fax: +91 11 27667061. E-mail addresses: Vandna [email protected] (V. Luthra), ram [email protected] (R.P. Tandon). 1 Permanent address: Dept. of Physics, Gargi College, Siri Fort Road, New Delhi 110049, India. 0925-4005/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2008.09.023

response [10]. It has been reported that the thermal instability of the ␥-form of Fe2 O3 limits its use as a gas sensor [15]. The addition of Pd to Fe3 O4 as a decorative noble metal enhances its gas-sensing properties [10]. There are some reports on the addition of iron to titanium oxides (0.1–1.3 at.%) which show a p-type response [9]. Fe doped TiO2 fabricated by sputtering has been reported to show sensitivity to CO. This has been attributed to the presence of Fe2 O3 [8]. The improvement in gas-sensing properties of titanium dioxide when doped with nanoscaled iron has been attributed to the inhibition of grain growth due to iron segregation at the boundaries [8]. Some gas selectivity has been observed for doped iron oxides. A gold/iron oxide system has shown sensitivity to CO as a test gas [16] whereas an In2 O3 /Fe2 O3 shows no sensitivity to CO [17]. Iron–titanium oxide solid solutions have shown response to gaseous ethanol [18]. The observed variation in gas-sensing response of decorated iron oxides is still not well understood and could be attributed to the crucial roles of synthetic conditions; dopant ion/noble metal; microstructure and surface chemistry—all of which modify the gas response. In the present investigation, iron and titanium oxide solid solutions were synthesized by conventional ceramic routes. These materials were screen printed to form gas sensors. The gas sensors reported here show low humidity interference and low baseline drift. These studies also indicate that intrinsic semi-conducting oxides such as ␣-Fe2 O3 , which are not gas sensitive in their native form, can be made into effective gas sensors by formation of a solid solution with suitable metal oxides and control of surface segregation.

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2. Experimental Iron titanate (FTO) was synthesized by a conventional ceramic route. Iron oxide Fe2 O3 (Aldrich ∼99% purity) with 5 ␮m particle size and titanium dioxide (Cerac) were ball milled together for 24 h in isopropyl alcohol in the desired stoichiometry. High purity alumina balls were used as grinding medium and extra iron–titanium pick up was less than 0.01% over a period of 24 h. The resultant powder was collected by the slow evaporation of the solvent and then ground in a pestle and mortar before sintering in air at 1000 ◦ C for 4 h. The sintered powders were mixed with a commercial vehicle (Agmet ESL 400) in the form of a viscous ink suitable for screen printing onto alumina substrates. The sensor substrates used had dimensions of 3 mm × 3 mm and were composed of gold interdigitated electrodes with gap and finger widths of 200 ␮m. A platinum heater was printed on the reverse side of the sensor substrates. The substrates were supplied by City Technology Ltd. [4]. After printing, the substrates were fired in air for 45 min at 700 ◦ C with a ramp temperature of 15 ◦ C/min, followed by cooling back to room temperature. The structural characterization of the sensor material was carried out using XRD, SEM, XPS and Raman spectroscopy. X-ray diffraction was carried out using a Siemens D5000 X-ray diffractometer ( = 1.5415 Å). The Raman spectroscopy was carried out using a Renishaw Raman System 1000 instrument. The excitation source was a Renishaw He–Ne Laser operating at 632.8 nm. The sample was analysed with a 50× microscope objective. XPS was used as an alternative technique as X-ray is not adequate to detect low levels of impurity phases. XPS spectra of the sample were recorded on a VG ESCALAB 220i XL using focused (300 ␮m spot size) monochromated Al-K␣ radiation. A 4-eV flood gun was used to control sample charging and the binding energies were referred to the adventitious C 1 s peak at 284.6 eV. Spectrum quantification was performed using Shirley background and sensitivity factors obtained from Wagner [19]. XPS data has been analysed using CasaXPS Software. The sample morphology was scanned and recorded using an Hitachi s-570 SEM equipped with a LINK energy dispersive X-ray analyzer. Gas-sensing measurements were performed using an in-house constructed test rig described elsewhere [6]. The rig measures the sensor resistance using a Keithley Model 175A digital multimeter. Carbon monoxide gas (1%) was applied using computer controlled mass flow controllers (Tylan general) through three separate lines. The first carried the pre-mixed gas in air of known concentration; purchased directly from source (BOC special gases). The remaining two lines carried dry and humid air (pre-saturated to 100% relative humidity by passing through a water filled Dreschel Bottle). In this way both the gas concentration and relative humidity of the air supplied to the sensors could be controlled. The flow rate of the gas was kept at 0.5 L/min. The reverse side of the sensor had a platinum heater track that formed one arm of a Wheatstone bridge allowing the resistance and so the temperature to be both programmed and regulated. The operating temperature of the sensors as well as the gas composition and data logging are computer controlled. The measurements were carried out in 50% relative humidity (RH) (50% dry and 50% moist air). For humidity interference, three (0%, 30% and 50% RH humidity) conditions were used.

Fig. 1. X-ray powder pattern of titanium doped iron oxide synthesized by conventional ceramic route. The triangles show the positions due to iron oxide. There are no observable additional peaks or shifts due to the presence of a secondary titanium dioxide phase.

was formed for the composition Fe1.90 Ti0.10 O3 which corresponded to the hematite structure of iron oxide, Fig. 1. Well defined peaks for Fe2 O3 were seen in the diffractogram. The X-ray pattern of Fe1.90 Ti0.10 O3 synthesized in the present studies is indicative of a single-phase compound. Notably no shift in the XRD peak positions was observed as compared to pure iron oxide sintered under similar conditions. The peak assignments for pure Fe2 O3 are marked as solid points as in Fig. 1. No evidence of unreacted titanium oxide was observed by XRD. Raman spectroscopy [20] was used to analyze the Fe1.90 Ti0.10 O3 material. As both TiO2 and Fe2 O3 are good Raman scatterers the presence of unreacted starting material should be readily seen. The spectra were recorded and compared with pure iron oxide as a reference, Fig. 2. It is worth noting that no additional peaks were observed due to the presence of phase separated TiO2 . The peak positions for hematite (Fe2 O3 ) and rutile (TiO2 ) are shown in the figure. The Raman spectra indicate that a single-phase material was formed with a spectrum almost identical to pure haematite, albeit with a slight shift in peak positions. Scanning electron microscopy (Fig. 3) shows the surface morphology of the sensing layer, indicating that the Fe1.90 Ti0.10 O3 consisted of ca 500 nm particulates. Sharp, regular angular faces were seen in the SEM, indicating that the material was wellcrystallised. Energy dispersive analysis by X-rays showed that iron, titanium and oxygen were present and that the material was homogeneous and devoid of any impurity and secondary phase.

3. Results 3.1. Structural characterization A ball milled mixture of Fe2 O3 and TiO2 was heated at 1000 ◦ C for 4 h. The resultant XRD data indicated that a single-phase material

Fig. 2. Raman spectra of Fe1.9 Ti0.1 O3. For comparison the peak positions due to iron oxide are labelled by solid lines, dashed lines are used for the rutile phase of titanium dioxide.

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Fig. 3. SEM photograph of Fe1.9 Ti0.1 O3 sintered at 1000 ◦ C for 4 h.

X-ray photoelectron spectroscopy (XPS) showed that iron, titanium and oxygen were present in the sample. Fig. 4 shows the XPS spectra for Fe. The Fe 2p3/2 peaks were broad and complex due to multiple splitting and were resolved. Analysis for iron peaks clearly indicated the presence of both oxidation states for iron in the sys-

Fig. 5. The XPS spectra of Fe1.9 Ti0.1 O3 for titanium. The dotted line shows the Shirley background and Ti 2p peak is shown.

tem corresponding to a binding energy due to Fe(III) at 711.6 eV and for Fe(II) at 709.6 eV. The binding energy due to Ti 2p3/2 was observed at 458.2 eV. This is lower than for pure TiO2 at 459.1 eV (Fig. 5). XPS determined the Fe:Ti atomic ratio as 3.4:1. This indicates significant surface enrichment of Ti (expect 19:1 ratio). The O 1 s region exhibited both oxide and hydroxyl species at around 529.7 and 531.4 eV. 3.2. Gas response measurements

Fig. 4. The XPS spectra of Fe1.9 Ti0.1 O3 for Fe. The dash-dot line (a) shows the Shirley background. The dotted lines show the existence of both (b) Fe3+ and (c) Fe2+ and their resultant (d) could be well fitted to the observed data (e).

The Fe1.90 Ti0.10 O3 sample was subjected to high concentrations of CO (1000–4000 ppm) with a ramp in temperature from 30 to 600 ◦ C and then back to 30 ◦ C. After 2–3 initial burn-out ramps (to remove any remaining hydrocarbon impurities), the sample started showing a response to CO. After initial burn-out cycles the samples were exposed and shown to be sensitive to much lower concentrations of CO ∼100 ppm. Thereafter the material showed a reversible response to CO and showed good base-line stability. Intrinsic Fe2 O3 is not gas sensitive, as shown in Fig. 6. The resistance was measured at temperatures of 550, 450 and 350 ◦ C and the gas was switched on and off while the temperature was kept constant. The resistance of the sample did not show any change in the presence of a CO test gas. The activation energy has been calculated by plotting log of conductivity as a function of inverse temperature. The activation energy for the iron oxide film was found to be ∼1.1 eV which is half the band gap [21]. The Fe1.90 Ti0.10 O3 sensor was operated at temperatures of 550, 500, 450, 400 and 350 ◦ C for 10 min each. A relative humidity of 50%

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Fig. 6. The gas response of pure iron oxide at 550, 450 and 350 ◦ C in 100–4000 ppm CO. The temperatures were fixed while the gas was turned on and off. No marked change in resistance was observed on admitting pulses of CO.

(RH) was used for these measurements. Fig. 7 shows the variation of resistance in the presence of the CO test gas at temperatures of 550 and 450 ◦ C. The resistance of the sensor decreased in the presence of the gas and it also showed a reversible and speedy recovery ∼150 and 155 s as rise and fall time for 1000 ppm CO at 550 ◦ C to the baseline when the gas was turned off as shown in Fig. 7. This is noteworthy in that the sensors exhibit good reversibility. The activation energy for the conduction decreased from 1.1 eV for pure iron oxide to 0.38–0.48 eV for this composition. The response is shown as S = RAir /RGas and is plotted as a function of operating temperature for 100 and 1000 ppm of CO test gas. A maximum response was obtained at a temperature of 450 ◦ C as shown in Fig. 8. S decreased at 350 ◦ C as well as 550 ◦ C. A square root law variation showing an increase in sensitivity at low concentrations and saturation at higher concentrations has been observed. Fig. 9 shows the varia-

Fig. 7. (a) The gas response of Fe1.90 Ti0.10 O3 in the presence of 150, 300, 400, 500, 800, 1000, 2000 and 4000 ppm CO at 550 ◦ C and in the presence of 100, 200, 300, 400, 500, 800, 1000, 2000, and 4000 ppm of CO for 600 s at 450 ◦ C. The sample environment was switched alternately between clean air and test gas. The response shows a resistance decrease in the presence of the test gas.

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Fig. 8. The plot of S = RAir /RGas as a function of operating temperature for 100 and 1000 ppm of CO test gas. The plot shows maximum sensitivity at an operating temperature of 450 ◦ C.

tion of S as a function of concentration at operating temperatures of 450, 500 and 550 ◦ C. 3.3. Response transient behaviour and baseline stability Many types of metal oxide sensors exhibit a slow gas response and recovery. A commonly observed effect is that the sensor may show a fast initial response, followed by a slow upwards drift—which can be interpreted as two separate response processes with very different time constants [22]. Similarly, on returning to clean air a slow recovery to baseline may be observed. To check for this effect, data were taken by switching the samples between the air and 500 and 1000 ppm of CO for 1 h at different temperatures. The experimental details have been summarized in Fig. 10. These data show that the sensor had a fast response with no slow upward drift. On removal of the gas the sensor recovered rapidly back to the baseline. Good baseline stability has also been observed with negligible drift over a period of many hours. In order to test the humidity interference the sensors were run under dry, 30% and 50% relative humidities (Fig. 10). It is worth noting that the response was greatly influenced at 350 ◦ C by the presence of water vapour and was almost lost at 30% and 50% relative humidity.

Fig. 9. The plot of S in the presence of test gas for 100–4000 ppm CO at operating temperatures of 450, 500 and 550 ◦ C. The ratio RAir /RGas shows a value more than one in the presence of a test gas - characteristic of an n-type material.

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Fig. 10. Plot of resistance versus time for various gas concentrations and humidities; a and b show the gas response to CO in (a) 50% RH at 500 ◦ C for 500 and 1000 ppm of CO and (b) 50% RH at 450 ◦ C for 500 and 1000 ppm of CO. Parts (c), (d) and (e) show the response of the sensor to different RH conditions (0%, 30% and 50% RH) at 1000 ppm of CO at (c) 550 ◦ C, (d) 450 ◦ C and (e) 350 ◦ C.

At 450 ◦ C the response was almost independent of humidity (S = 2.03–2.18), however there was a small effect of humidity on the baseline. At 550 ◦ C the gas sensitivity was less (S = 1.32–1.40) but the humidity interference on the baseline was the least: ∼2%. The best operating temperature could be achieved by a compromise between maximising sensitivity and minimising the effects of humidity. 3.4. CO sensing mechanism The model for the charge compensating species formed on substitution of Ti4+ for Fe3+ is that Ti substitutes onto Fe sites. The addition of titanium (IV) could lead to partial reduction of Fe3+ to Fe2+ or the additional charge of the Ti(IV) could be compensated by iron vacancies. Berry et al. [23] described the Ti defects as a cluster in which four Fe3+ were replaced by three Ti4+ , two of these substituting for Fe3+ and one on an interstitial site, with two vacancies on Fe3+ sites. There was no charge change for Fe3+ at this degree of substitution though the spin density on Fe3+ was altered by the adjacent Ti4+ . The oxygen sub-lattice was unperturbed. The charges on the ions and their ionic radii play a dominant role in formulating the crystal structure and so the feasibility of such a case could be inferred by the similarity of their ionic radii as Ti4+ = 0.61 Å, Fe3+ = 0.64 Å and Fe2+ = 0.78 Å. Droubay et al. [24] suggested from computational studies a decrease in band gap for small degrees of substitution by Ti, associated with the appearance of empty Ti d states just below, but overlapping the conduction band minimum. These authors suggest that each Ti(IV)-derived donor electron may be delocalised over many Fe(III) at low degrees of substitution. They noted the appearance of Fe(II) in XPS for x > 0.3 of Fe2−x Tix O3 and suggested a gradually increasing localisation of the donor electrons with increasing degree of substitution, until each electron is effectively localised on one Fe cation, reducing it from Fe(III) to Fe(II). They also reported a mid-gap state associated with Fe(II), for x ∼ 0.3 and higher. The present investigation indicates a surface reduction of Fe(III) to Fe(II) with x = 0.1. It can be reconciled with the earlier literature by noting that, in the highly porous materials that we have

prepared, Ti is strongly surface segregated, and that the degree of substitution in the surface and near-surface region exceeds the level for which Fe(II) has previously been seen. Thus we suggest that the surface segregation of Ti results in the localisation of conduction band electrons onto Fe(II) states in the near-surface region. We have previously interpreted the gas-sensing behaviour of complex oxides with a Mars–van Krevelen type of mechanism for the surface reaction. The ideas previously proposed [5,25] suggest that the necessary elements are a surface oxygen vacancy, or a pocket where the oxygen sub-lattice has relaxed, associated with an adjacent high-valency metal ion. This cluster could adsorb oxygen to form a reactive surface oxygen species that is also an electrically active surface trap for the donor electrons provided by Ti—an Fe(II)-associated surface state in the present case. The surface oxygen vacancy or pocket could also adsorb CO and promote reaction to remove the surface trap, thus generating the signal. In this case, the Ti substituent would provide the necessary reactive metal centre, Fe the surface electron trap, and the adjacent pocket in the oxygen lattice at the surface could be provided by relaxation of the oxygen lattice away from the Fe vacancies and towards the Ti substituent. The concentration of the necessary defect clusters at the surface would be enhanced as a consequence of surface segregation, itself driven by the lattice strain and electrostatic energy associated with the defect cluster [5]. We have previously shown that the electrical responses to reactive gas and to water vapour are mediated at different surface sites [5,26]. A large response to water vapour can be interpreted as a dissociative chemisorption of water onto high-valency surface cations. Thus the control of surface composition of a complex oxide also allows the response to a reactive gas to be altered with respect to the response to water vapour [26]. In the present case, although Ti is strongly surface segregated, the dominant surface cation is Fe so the conductance response to water vapour is consequently significantly less than that for oxides such as SnO2 or TiO2 . In contrast to the behaviour of SnO2 which is inherently a gas sensor, the present investigation shows that suitable doping can induce gas-sensing properties to a material that in its undoped state shows no gas sensitivity to the test gas. Furthermore, control of the surface composition allows development of a reactive gas response without an interfering response to water vapour. 4. Conclusion Titanium substitution into Fe2 O3 using a conventional ceramic route to prepare a highly porous material develops excellent gassensing properties towards carbon monoxide, without a significant interfering response to water vapour. The behaviour is controlled by surface segregation of Ti and can accordingly to some degree be tuned. Acknowledgements One of the authors, VL is thankful to DST, India for the award of BOYSCAST Fellowship that supported this work. IPP thanks EPSRC for support in purchasing the Raman spectrometer. References [1] W. Gopel, K.D. Schierbaum, SnO2 sensors current status and future prospects, Sens. Actuators B: Chem. 26–27 (1995) 1–12. [2] K.D. Schierbaum, Engineering of oxide surfaces & metal oxides interfaces for chemical sensors: recent trends, Sens. Actuators B: Chem. 24–25 (1995) 234–297. [3] Gas Detection Mechanism in Figaro Gas Sensors, Product Catalogue 2, pp. 2–12. [4] City Technology Ltd. (www.citytech.co.uk). [5] D. Niemeyer, D.E. Williams, Peter Smith, K.F.E. Pratt, B. Slater, R.A. Catlow, A. Marshall Stoneham, Experimental and computational study of gas-sensor

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Biographies Vandna Luthra is a reader in Department of Physics, Gargi College, University of Delhi, India. She obtained PhD in physics from Department of Physics & Astrophysics and National Physical Laboratory, New Delhi on conducting polymers in 1998. She was a visiting fellow at University College London, UK. Her current field of interest are structure–property correlations of various materials such as semi-conducting oxides, polymers, ceramic–polymer composites, etc. for gas-sensing applications. Keith Pratt has a degree in chemistry from the University of Warwick and PhD in electrochemistry from the University of Southampton. He subsequently spent 10 years working on research and development of metal oxide gas sensors at University College London and at Capteur Sensors and Analysers Ltd. He is currently employed at City Technology Ltd., working in a number of fields including electrochemical gas sensors, gas sensor modeling, and exploring novel sensor technologies. Ivan P. Parkin is a professor of materials chemistry at University College London. He has research interest in solid state metal oxide gas sensors and in CVD. He has been awarded the Kroll medal in 2008 from IOM3 and the Beilby medal from the Royal Society of Chemistry in 2004. David Williams is a professor of chemistry at University of Auckland. He was previously at Inverness Medical Innovations, at University College London, and at UK Atomic Energy Authority Harwell Laboratory. He was co-founder of Capteur Sensors Ltd. and of Aeroqual Ltd. He has been studying the properties of semiconducting oxides as gas sensors for over 25 years. R.P. Tandon is a professor of physics at University of Delhi, since 1998. Previously he held position as a scientist at the National Physical Laboratory, New Delhi for nearly two decades and also served as a scientific advisor to the Government of Haryana in 1996. His research interests include glasses, crystals, ceramics and conducting polymers.