Sensors and Actuators B 127 (2007) 137–142 Hybrid metal oxide and multiwall carbon nanotube films for low temperature gas sensing E.H. Espinosa a , R. Ionescu a , B. Chambon b , G. Bedis b , E. Sotter a , C. Bittencourt c , A. Felten c , J J. Pireaux c , X. Correig a , E. Llobet a,∗ a MINOS, Universitat Rovira i Virgili, ETSE-DEEEA, Av. Pa¨ısos Catalans 26, E-43007 Tarragona, Spain b Institut Universitaire de Technologie GEII Bordeaux 1, 15 rue Naudet, 33175 Gradignan Cedex, France c LISE, Falcult ´es Universitaires Notre Dame de la Paix, 61 rue de Bruxelles, B-5000 Namur, Belgium Available online 8 August 2007 Abstract Active layers for detecting NO 2 have been obtained by adding a low amount of oxygen-functionalized MWCNTs to three different types of metal oxides (SnO 2 ,WO 3 or TiO 2 ). We show that the responsiveness towards NO 2 of micro-sensors based on some of the resulting metal oxide/MWCNT hybrid films is considerably improved. In particular, the sensors based on hybrid SnO 2 /MWCNTs films present excellent sensitivity towards NO 2 when operated at room temperature. Additionally, the sensors show a total recovery of their baseline resistance and reasonable response and recovery times. A mechanism of response based on the development of two depletion layers, one at the surface of metal oxide grains and another at the interface of the n-metal oxide/p-MWCNT hetero-structure, is postulated as responsible for the improvement observed. Our results suggest that there should be an optimum amount of carbon nanotubes to be added to each specific metal oxide in order to enhance sensitivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas sensors; Hybrid metal oxides/MWCNTs; Low temperature operation 1. Introduction Metal oxides are well-known materials suitable for detecting a wide spectrum of gases with enough sensitivity. These mate- rials are typically operated at temperatures that range between 200 and 800 ◦ C [1–5]. When the detection of toxic species is devised, metal oxide sensors work usually as surface conductiv- ity devices. Ambient oxygen is adsorbed at oxygen vacancies and abstracts electrons via the conduction band. When the equilibrium concentration of oxygen adsorbates at the surface of metal oxides is altered by the presence of either reducing of oxidizing species, the material’s conductivity dramatically changes. When metal oxide sensors are operated at high tem- peratures, changes in the microstructure of the gas sensitive film are likely to occur (i.e., structural changes or coalescence [6]). At higher temperatures, the mobility of oxygen vacancies becomes appreciable and the mechanism of conduction becomes mixed ionic–electronic. The diffusion of oxygen vacancies is known ∗ Corresponding author. Tel.: +34 977558502; fax: +34 977559605. E-mail address: eduard.llobet@urv.cat (E. Llobet). to be a mechanism responsible for long-term drift in metal oxide gas sensors [7,8]. Therefore, a strategy to avoid long-term changes in their response could consist in operating the sen- sors at temperatures low enough so that appreciable structural variation never occurs, provided that gas reactions occur at a reasonable rate. Additionally, the development of sensors that work at room temperature would help in developing new applications where power supply is an issue (e.g. micro-labs with gas sensing capa- bilities). Recently, hybrid films based on tin or tungsten oxide and carbon nanotubes have been introduced as new gas sensitive materials with improved sensitivity [9–12]. These works indi- cate that the detection at ambient temperature of toxic gases such as nitrogen dioxide, carbon monoxide and ammonia or ethanol vapors can be highlyimproved by dispersingan adequate quantity of carbon nanotubes into a metal oxide matrix. In this paper, we study and compare the performance in gas sensing ofhybrid materials consisting of O 2 -functionalized mul- tiwall carbon nanotubes (MWCNTs) dispersed in a metal oxide matrix. Three different metal oxides namely SnO 2 ,WO 3 and TiO 2 were considered. 0925-4005/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2007.07.108 138 E.H. Espinosa et al. / Sensors and Actuators B 127 (2007) 137–142 2. Experimental 2.1. Carbon nanotube functionalization The MWCNTs used were obtained from Nanocyl, S.A. [13]. They were grown by chemical vapor deposition (CVD), had purity higher than 95%, were up to 50 ␮m in length and their outer and inner diameters ranged from 15 to 3 nm and 7 to 2 nm, respectively. A uniform functionalization with oxygen was applied to the as-provided carbon nanotubes in order to improve their disper- sion and surface reactivity. For this activation step the MWCNTs were placed inside a glass vessel and a magnet, externally con- trolled from the plasma chamber, was used to stir the nanotube powder during the plasma treatment.Inductively coupled plasma at a RF frequency of 13.56 MHz was used during the process [14]. Once the MWCNT powder was placed inside the plasma glow discharge, the treatment was performed at a pressure of 0.1 Torr, using two different powers 30 and 100 W, while the processing time was adjusted to 10 min. A controlled flow of oxygen was introduced inside the chamber. 2.2. Active layers The sensing layers were prepared using commercially avail- able SnO 2 and WO 3 nanopowders (Sigma–Aldrich). On the other hand, TiO 2 was prepared from alkoxide precursors via a sol–gel route in dry nitrogen atmosphere. The precursors used were titanium isopropoxide(IV) Ti[OCH(CH 3 ) 2 ] 3 with 99.99% purity [15]. Hybrid materials were obtained by adding 0.7 mg of oxygen-functionalized MWCNTs to 350 mg of metal oxide (which gave a proportion of 1/500 wt.%). An adequate mix- ture of the two components was obtained by dissolving them in glycerol (employed as organic vehicle) and stirring the resulted solution in an ultrasonic bath at 75 ◦ C for 2 h. The amount of nanotubes to be added to the metal oxide matrix was based on a previous study [9]. The pastes thus obtained were subsequently dropped onto the electrode area of micro-hotplate transducers [16] (fabricated at the Centre Nacional de Microelectr ` onica, Barcelona, Spain) using a microinjector (JBE1113 Dispenser, I&J FISNAR Inc., USA). The as-deposited sensing films were dried at 170 ◦ C for 1 h in order to burn out the organic vehicle and after that annealed in situ at 400 ◦ C for 2 h in ambient atmosphere. During the anneal- ing process, the temperature was raised from ambient to 400 ◦ C using a ramp of 1 h. In total, six different types of active layers were produced: • Sample A: SnO 2 /MWCNTs type 1 • Sample B: SnO 2 /MWCNTs type 2 • Sample C: WO 3 /MWCNTs type 1 • Sample D: WO 3 /MWCNTs type 2 • Sample E: TiO 2 /MWCNTs type 1 • Sample F: TiO 2 /MWCNTs type 2 where MWCNTs type 1 and type 2 were functionalized in an oxygen plasma of 30 and 100 W, respectively. 2.3. Material characterization The morphology of the hybrid films deposited onto the micro-hotplate substrates was investigated by scanning elec- tron microscopy.SEM measurements were performed using Joel JSM 6400 equipment, with a resolution of 0.3 nm. The magnifi- cation during thisstudy was set to values varying between30,000 and 150,000. A 30 kV accelerating voltage was employed. The system allows for sample rotation (360 ◦ ) and sample inclination (90 ◦ ). 2.4. Gas sensing measurements The gas sensing properties of the different sensing films were tested in the presence of NO 2 and CO both at ambient tem- perature and 150 ◦ C. Three concentrations of each gas under study were measured: 100, 200 and 500 ppb for NO 2 ; and 1, 10 and 50 ppm for CO. The sensors were placed inside an airtight 5.3 dm 3 measurement chamber and the desired concentrations of the gases were introduced by the direct injection method. 3. Results and discussion 3.1. Morphological characterization SEM analyses were recorded in order to observe the mor- phology of the active films deposited. Fig. 1 shows SEM images recorded on the different hybrid films studied. These micro- graphs show the presence of metal oxide grains. The surfaces of the different films prepared had the appearance shown in Fig. 1, where carbon nanotubes are not revealed. However, regardless the metal oxide employed, carbon nanotubes could be observed in a few regions of the films’ surface (see Fig. 2). Therefore, car- bon nanotubes are still present in the films after the deposition process and annealing at 400 ◦ C in air. It is due to the low propor- tion of carbon nanotubes dispersed in the active layers (1/500 in wt.%) that most of the nanotubes are actually embedded within the metal oxide matrix. 3.2. Gas response analysis The measurement process was as follows: data acquisition started 5 min before injecting into the measurement chamber, by means of a gas chromatography syringe, the required vol- ume corresponding to the lowest concentration of the gas to be measured. Fifteen minutes later (meanwhile the steady state of the sensor response had been reached), a new amount of the same gas was injected, so the next concentration of this gas was reached. The same procedure was repeated until all desired con- centrations of the different gases under study were measured. After a series of continuous injection measurements was com- pleted, air was used to flush the measurement chamber for 1 h, while the sensors were heated at 150 ◦ C to speed up gas des- orption. Finally, the airflow was interrupted and all sensors fully recovered their baseline resistance in less than 2 h. For assessing the gas sensing properties of the different sen- sors fabricated, sensor responsiveness (S) was defined as the E.H. Espinosa et al. / Sensors and Actuators B 127 (2007) 137–142 139 Fig. 1. SEM images ofthe different hybrid films fabricated: (a)SnO 2 /MWCNTs; (b) WO 3 /MWCNTs; (c) TiO 2 /MWCNTs. Fig. 2. SEM image of a particular region of a TiO 2 /MWCNT hybrid film, where carbon nanotubes are found laying inside the metal oxide matrix. Similar images could be recorded for SnO 2 and WO 3 -based films. ratio between the changes experienced by the sensor resistance after its exposure to a test gas and the sensor baseline resistance in air: S = R air − R gas R air (1) where R air is the sensor resistance in air and R gas is the sensor resistance in the presence of the pollutant after reaching a steady state. Responsiveness results are summarized in Fig. 3. In order to further evaluate the performance in gas sensing for the different hybrid materials investigated, the sensitivity defined as the ratio between the changes in responsiveness over the change in gas concentration was calculated as follows: Sen| C = S| C 1 − S| C 2 C 1 − C 2 (2) whereS| C i is the sensor responsiveness after the injection of a concentration C i of a toxic gas (as defined in Eq. (1)), C 1 and C 2 correspond to 500 and 100 ppb (case of NO 2 ) or 50 and 1 ppm (case of CO). Table 1 shows the sensitivities of the different sensors studied when operated at room temperature (25 ◦ C) and at 150 ◦ C. Sensors based on SnO 2 and carbon nanotubes (samples A and B) were the most sensitive to NO 2 among the different materials studied. They were able to detect as low as 100 ppb Table 1 Sensitivities of the different sensors studied as calculated from Eq. (2) Sample type NO 2 ,25 ◦ CNO 2 , 150 ◦ C CO, 25 ◦ C CO, 150 ◦ C A 0.39 0.40 0.01 0.10 B 0.51 0.15 0.01 0.09 C 0.00 0.02 0.00 0.12 D 0.02 0.05 0.00 0.14 E 0.00 0.00 0.00 0.00 F 0.00 0.00 0.00 0.00 C 1 is 500 ppb (NO 2 ) or 50 ppm (CO); C 2 is 100 ppb (NO 2 ) or 1 ppm (CO). 140 E.H. Espinosa et al. / Sensors and Actuators B 127 (2007) 137–142 Fig. 3. Sensor responsiveness: (a) NO 2 detection at room temperature; (b) NO 2 detection at 150 ◦ C; (c) CO detection at 150 ◦ C. of this pollutant gas with responsiveness near 120 even when operated at room temperature. A similar responsiveness value was obtained when the sensors were operated at 150 ◦ C. Actu- ally, Table 1 shows that sensitivity is higher when these sensors are operated at room temperature. For these sensors, the power used during the functionalization of MWCNTs does not seem to play a determinant role in their responsiveness to NO 2 . This is not the case for sensors based on the other two metal oxides considered, since those employing MWCNTs functionalized at 100 W showed higher responses. A typical response of a SnO 2 and MWCNT sensor to successive concentrations of NO 2 can be observed in Fig. 4. Sensors based on WO 3 and carbon nanotubes (samples C and D) showed some responsiveness towards NO 2 . Operated at room temperature, their responsiveness reached a value of 0.8 after the injection of 100 ppb of NO 2 , and this value rose to nearly 10 in the presence of 500 ppb of NO 2 . These val- ues, although significantly lower than the ones for tin oxide and carbon nanotube-based sensors, compare favorably to those previously published for WO 3 and MWCNT hybrid sen- sors [9].In[9] a responsiveness of 0.02 towards 500 ppb of NO 2 was reported for 1/100 wt.% MWCNT/WO 3 sensors and 1/1000 wt.% MWCNT/WO 3 sensors were irresponsive to NO 2 . Therefore, sensor responsiveness heavily depends onthe amount of MWCNTs dispersed on the metal oxide matrix. Sensors based on TiO 2 and carbon nanotubes (samples E and F) showed the lowest responsiveness to NO 2 . Responsive- ness did not exceed 0.1 after being exposed to the highest NO 2 concentration tested (i.e., 500 ppb). When CO was measured, similar results were obtained when either SnO 2 or WO 3 was employed as metal oxide in the hybrid films (see Fig. 3). Responsiveness peaked at 6.8 for MWCNT/WO 3 operated at 150 ◦ C in the presence of CO 50 ppm. This compares favorably with the previously reported responsiveness towards 50 ppm of CO (i.e., 0.4) for MWCNT/WO 3 hybrid films [9]. Table 1 shows that the sensi- tivity towards CO of tungsten oxide hybrid films is higher than the one of tinoxide hybrid films. All sensors were irresponsive to CO when operated at room temperature and TiO 2 -based sensors were irresponsive to CO even at 150 ◦ C. Fig. 4. Response of a sensor type B operated at room temperature after exposure to increasing concentrations of NO 2 and recovery in air. E.H. Espinosa et al. / Sensors and Actuators B 127 (2007) 137–142 141 One important aspect of operating the sensors at 150 ◦ C was their faster recovery of the baseline after gas expo- sure. For instance, the recovery time of SnO 2 /MWCNTs- based sensors was reduced from 45 min when operated at room temperature down to 20 min, while in the case of WO 3 /MWCNTs sensors,recovery time was reducedfrom 120 to 45 min. 3.3. Discussion In a previous paper [9] we investigated the responsiveness of pure WO 3 sensors operated at room temperature. These sensors were found to be insensitive to NO 2 , CO and NH 3 at low temper- atures of operation. Additionally, Sberveglieri and co-workers [17] reported that, thick film TiO 2 -based sensors obtained via a sol–gel route (equivalent to the one employed in this paper) were irresponsive to gases when operated at temperatures below 350 ◦ C. Finally, it is well known that pure SnO 2 -based sensors are almost insensitive to gases when operated at room tempera- ture [10,18]. On the other hand, sensors based on pure MWCNTs (these nanotubes had been functionalized in an oxygen plasma) were found to be sensitive to NO 2 when operated at room temperature [19]. However, the responsiveness reported for these pure MWCNTs sensors to NO 2 was far lower than the one reported here for SnO 2 /MWCNT and WO 3 /MWCNT sensors. On the basis of the images of the hybrid films recorded by SEM analysis shown in Fig. 2, it can be derived that MWCNTs are embedded within the metal oxide matrix. Since SnO 2 or WO 3 films behave as n-type semiconductors and oxygen-functionalized MWCNTs films behave as p-type semiconductors [19], the hetero-structure n-SnO 2 /p-MWCNTs (n-WO 3 /p-MWCNTs) are formed at the interface between tin oxide (tungsten oxide) and carbon nanotubes. The exis- tence of such a hetero-structure has been suggested earlier for SnO 2 /MWCNT films [10,11]. In hybrid films, two differ- ent depletion layers (and associated potential barriers) co-exist [10,11]. The first type of depletion layers is located at the surface of the grains of the metal oxide film and the second type at the interface between MWCNTs and metal oxide films. Considering the sensitivity of hybrid films (see Table 1), it can be derived that the adsorption of NO 2 at the metal oxide modifies the depletion layer at the surface of its grains and also at the n-metal oxide/p-MWCNTs heterostructures. This combined effect may explain the improvement in responsive- ness shown by the tin or tungsten oxide-based hybrid sensors. Our previous results [9] and the results presented here support the fact that there should be an optimum amount of carbon nanotubes to be added to the metal oxide matrix in order to enhance the responsiveness of hybrid sensors operated at room temperature. The lack of responsiveness observed for TiO 2 -based hybrid sensors can be associated to the non-appropriateness of this metal oxide for detecting NO 2 and/or to the fact that the amount of nanotubes added to the TiO 2 matrix is far from optimal. 4. Conclusions In this paper we have shown that the addition of a small quantity of O 2 -functionalized MWCNTs to metal oxides can significantly improve the detection capability of metal oxide- based sensors at low operating temperatures. In particular, micro-sensors basedon SnO 2 /MWCNTs hybrid films operatedat room temperature showed the higher sensitivity towards NO 2 in the ppb range, among the different materials studied. The response mechanism is fully reversible, since the sensors can recover their baseline resistance after each exposure to pollutant gases. Our resultssuggest that there is an optimum amount of carbon nanotubes to be added to each particular metal oxide in order to enhance responsiveness. SEM analysis showed that the nanotubes survived theprocess of deposition and subsequent annealing at 400 ◦ C in air and that most of the nanotubes lay embedded within the matrix of metal oxide. Based on these results, the modulation of the width of two depletion layers existing at the surface of metal oxide grains and at the interface of metal oxide grains and MWCNTs, respec- tively is postulated as the mechanism that could explain the enhanced performance of hybrid metal oxide/MWCNT sensors in comparison with pure metal oxide or pure MWCNT sensors. Acknowledgements This work was funded in part by the Spanish Commis- sion for Science and Technology (CICYT) under grant no. TIC2003-06301. E.H. Espinosa gratefully acknowledges a PhD studentship from Rovira i Virgili University. R. Ionescu holds a ‘Juan de la Cierva’ research fellowship funded by the Span- ish Ministry for Science and Education. Parts of this work are directly connected to the Belgian Program on Interuniversity Attraction Poles (PAI5/1/1) on Quantum Size Effects in Nanos- tructured Materials, sponsored by the Communaut ´ e Franc¸aise de Belgique. References [1] I. Simon, N. Barsan, M. Bauer, U. 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