Sensors and Actuators B 117 (2006) 128–134 Gas sensor response of pure and activated WO 3 nanoparticle films made by advanced reactive gas deposition L.F. Reyes a,∗,1 , A. Hoel a , S. Saukko b , P. Heszler a,2 , V. Lantto b , C.G. Granqvist a a Department of Engineering Sciences, The ˚ Angstr¨om Laboratory, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden b Microelectronics and Materials Physics Laboratories, University of Oulu, Linnanmaa, FIN-90570 Oulu, Finland Received 6 July 2005; received in revised form 2 November 2005; accepted 7 November 2005 Available online 7 December 2005 Abstract Pure and activated (doped) nanocrystalline WO 3 films, produced by advanced reactive gas deposition, were investigated for gas sensing appli- cations. Activation took place by co-evaporation of Al or Au with tungsten oxide as the particles were produced. Structural characterization of the films was performed by electron microscopy and X-ray diffractometry. Sensitivity, response time, and recovery time of the sensors were systematically investigated as a function of annealing and operating temperature, using H 2 S, CO, and NO 2 as test gases. The sensitivity was found to lie below and around the ppm level for H 2 S and NO 2 , respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Microstructure; Electrical properties; WO 3 ; Au; Sensor 1. Introduction Nanocrystalline materials are in the focus of contemporary materials research as a consequence of the superior properties that are achievable when materials are built up from structural units with sizes on the nanometer scale [1]. Materials of this type can be produced by a variety of techniques including gas deposition [2], sputtering [3], sol–gel technology [4], microbi- ological preparation [5], etc. Gas-phase synthesis stands out as being of particular interest [6]. Materials consisting of crystalline nanoparticles with a porous structure can have extremely large surface areas and be well suited for applications in semiconductor-based gas sensors. For example, it has been recently shown that nanocrystalline WO 3 films exhibit very interesting sensor properties and are candidates for detecting toxic gases such as H 2 S [7],NO 2 [8], and ozone [9]. The sensor properties can be boosted by activa- tion with metals such as Au, Pd, and Pt [10]. The purpose of ∗ Corresponding author. Tel.: +51 1 2922528. E-mail address: lfreyesh@yahoo.com (L.F. Reyes). 1 Permanent address: Facultad de Ciencias, Universidad Nacional de Inge- nier ´ ıa, Av. T ´ upac Amaro 210, CP 31-139 Lima, Peru. 2 Also at Research Group on Laser Physics of Hungarian Academy of Sci- ences, P.O. Box 406, H-6701 Szeged, Hungary. activation is to improve the sensitivity as well as the selectiv- ity of the sensor films for the gas to be detected. We note that nanocrystalline WO 3 is well known also as an electrochromic material capable of sustaining reversible and persistent changes of its optical properties [11,12]. Analogously with the case for sensors, the electrochromic properties can be radically changed by inclusion of Au nanoparticles [13]. Below we present results from a study on gas sensing using nanocrystalline films of WO 3 in pure (intrinsic) form and after activation with Au or Al. The films were prepared by reactive gas deposition. Materials characterization was accomplished by X- ray diffraction (XRD) and scanning electron microscopy (SEM). The performance of sensor devices, based on the WO 3 films, was investigated by a two-point measuring system capable of in situ conductivity determinations as a function of sensor temperature and gas concentration. Test gases of H 2 S, NO 2 , and CO were used at different concentrations on the ppm level. 2. Experimental details A reactive gas deposition unit (Ultra Fine Particle equip- ment, ULVAC Ltd., Japan) was used for producing nanocrys- talline WO 3 -based films. A detailed description of this unit was given elsewhere [14]. Briefly, the system consists of an evap- oration chamber, a deposition chamber, a transfer pipe, and an 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.11.008 L.F. Reyes et al. / Sensors and Actuators B 117 (2006) 128–134 129 evacuation pipe. The chambers were evacuated to a base pressure of ∼3 × 10 −2 mbar prior to the experiments. For producing WO 3 nanoparticles, a tungsten pellet was placed inside an induction coil in the evaporation chamber and was heated up to ∼1200 ◦ C. Reactive evaporation was performed using synthetic air (80% N 2 and 20% O 2 ) introduced with a flow of 10 l/min at the bottom of the induction coil so that an operating pressure of ∼20 mbar was maintained. The surface of the tungsten pellet was oxidized, and the chosen temperature allowed vaporization of the tung- sten oxide layer. The gas flow moved the tungsten oxide vapor upward, leading to a cooling of the vapor as it was removed from the heating zone and thereby yielding first molecular oxide clusters and then WO 3 nanoparticles. One part of the gas flow in the nanoparticle formation zone was diverted by the transfer pipe to the deposition chamber. This selection, and the fact that the gas flow is highly laminar, results in a narrow size distribu- tion of the particles [15,16]. For producing the activated WO 3 nanoparticle films, additional Au or Al pellets were placed in the heating zone, thus resulting in co-evaporation or sublimation of the metallic dopant. Doping was manifest as a color induced in the film, which made the visual impression different from that of the unactivated WO 3 nanoparticle films. Both unactivated and activated WO 3 films were deposited on alumina substrates with previously prepared gold electrodes, separated by 0.3 mm, on the front side and a platinum wire heater on the back side. Sam- ples were heat treated at temperatures between 100 and 600 ◦ C for 1 h to examine the annealing effect on the sensitivity of the WO 3 nanoparticle films. Thickness measurements were made by a Tencor Alpha-Step 200 mechanical stylus instrument with a maximal resolution of 0.5 nm. An average thickness of 11 ␮m was found for the as- prepared films. The crystal structure of the deposited materials was examined by X-ray diffraction, using a Siemens D5000 diffractrometer (40 kV, 40 mA) with monochromatized Cu K␣ radiation having a wavelength of 1.5406 ˚ A. The mean grain size was determined using Scherrer’s equation [17]. Microstructures of the nanocrystalline WO 3 films were investigated by scanning electron microscopy using a LEO 1550 Gemini instrument with an in-lens detector. The conductance of the samples was measured by a two- point set-up in an evacuated system comprising a gas source, a gas blender (Signal series 850), and a test chamber. Synthetic air was used as a carrier gas at a constant flow rate 1 l/min. Diverse parameters – such as sensor temperature, gas concen- tration, exposition time to different gases, etc. – were computer controlled and monitored in real time. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows X-ray diffraction patterns of as-deposited and annealed (sintered) nanocrystalline WO 3 films. Reflection peaks due to tetragonal (t) and monoclinic (m) phases of WO 3 and of the substrate (Al 2 O 3 ) can be seen. Clearly the WO 3 films con- sist of a mixture of tetragonal and monoclinic phases. It should be noted, though, that films produced at low heating powers Fig. 1. X-ray diffractograms of nanocrystalline WO 3 films on alumina sub- strates in as-deposited state and after sintering at a temperature T between 100 and 600 ◦ C. Peaks denoted by m, t, and (*)Al 2 O 3 are characteristic for the mon- oclinic and tetragonal phases of WO 3 and for the alumina substrate, respectively. only gave evidence for the tetragonal phase [7,18]. The tetrag- onal structure corresponds to a high-temperature phase of WO 3 that is stable above 740 ◦ C, while a monoclinic phase is stable at lower temperatures. It is evident that the high temperature associated with the W oxide evaporation – being ∼1200 ◦ C during film fabrication – can lead to a tetragonal phase which remains metastable during cooling in the gas stream. No sig- nificant changes in the diffractograms could be observed for sintering temperatures up to 300 ◦ C. However, the tetragonal phase started to transform to monoclinic above 300 ◦ C, and only the monoclinic phase appeared to be present at a sintering tem- perature of 600 ◦ C. The crystallite grain size was ∼10 nm for the as-deposited films, and no grain growth could be seen up to 300 ◦ C. However, grain growth could be observed above this temperature, and the crystallite size was ∼40 nm at 600 ◦ C. 3.2. SEM analysis Scanning electron microscopy data on as-deposited nanocrystalline WO 3 films are shown in Fig. 2. A minimum feature size of ∼12 nm can be seen, which is in agreement with XRD measurements analyzed by using Scherrer’s equa- tion. Aggregates in a porous network-like structure could be observed; this is an important characteristic for gas sensors [19]. Fig. 2. Scanning electron micrograph (inset with higher magnification) of an as-deposited nanocrystalline WO 3 film. 130 L.F. Reyes et al. / Sensors and Actuators B 117 (2006) 128–134 Transmission electron microscopy data on samples produced at slightly lower temperatures were reported elsewhere [18]. 4. Sensitivity measurements The sensitivity of the active layer upon gas exposure was defined as the ratio G gas /G air , where G gas and G air denote the conductance of the sensor exposed to a test gas (H 2 S, NO 2 , or CO) and in pure synthetic air, respectively. First the pure nanocrystalline WO 3 films, sintered at temperatures between 100 and 600 ◦ C, were tested upon exposure to 10 ppm of H 2 S at room temperature. Fig. 3 shows that the sample annealed at 300 ◦ C displayed the highest sensitivity. Fig. 4(a) depicts room-temperature sensitivity upon 10 min exposures to several concentrations of H 2 S gas of a sample sin- tered at 300 ◦ C. This exposure produced a drastic increase of the sensitivity; specifically an increment by more than three orders of magnitude can be observed at 35 ppm of H 2 S. One can also note that the increase in sensitivity does not reach satura- tion for the applied concentration range, between 5 and 35 ppm, but the sensitivity increases linearly with the H 2 S concentration as apparent from Fig. 4b. The slope of the sensitivity curve is somewhat below 100 per ppm, indicating sensitivity below the ppm level for H 2 S at room temperature. However, the change of the sensitivity as a function of time was very slow at room temperature; the response time was several minutes, and the recovery time amounted to some hours after evacuation of the gas. However, high-temperature operation at 600 K significantly improved the response and recovery times of the sensors (see Fig. 5a), but there is a concomitant decrease of the sensitivity by more than two orders of magnitude compared to the case of room-temperature operation (see Fig. 5b). The sensitivity always reached its initial value after the H 2 S gas had been evacuated, which shows that the adsorption and desorption processes were reversible under cycles of gas exposure and ensuing evacuation. A slightly non-linear behavior of the sensitivity versus H 2 S con- centration was apparent at 600 K (see Fig. 5b). Fig. 3. Sensitivity as a function of annealing temperature for a pure nanocrys- talline WO 3 sensor exposed to 10 ppm of H 2 S at room-temperature operation. The line is a guide for the eye. Fig. 4. (a) Sensitivity as a function of time for a pure nanocrystalline WO 3 sensor (sintered at 300 ◦ C) exposed to several H 2 S concentrations at room- temperature operation. H 2 S injection and evacuation took place after 5 and 15 min, respectively. (b) Sensitivity S (maximum values) as a function of H 2 S concentration C for the experiment described in (a). The line corresponds to the stated equation. A comparison of the room-temperature sensitivity for as- deposited unactivated and activated films is presented in Fig. 6. Clearly the WO 3 nanoparticle film activated by Al exhibited a sensitivity increase by more than one order of magnitude, while activation with Au increased the sensitivity approximately by a factor of two. Neither the concentration nor phase of the dopant was investigated in this study, but it is likely that the Al co-evaporation resulted in an inclusion of aluminum oxide. Sensitivity measurements are in progress to determine optimal concentrations of the activation material. Slow response and long recovery time prevailed for room- temperature operation of activated films (see Fig. 6), just as for unactivated films (see Fig. 4a). However, these characteristics could be significantly improved by having the sensor oper- ate at higher temperature (see Fig. 7a and b). Fig. 7a shows that the processes of adsorption and desorption were reversible for several concentrations of H 2 S gas at 600 K. We note that these processes were irreversible for continued H 2 S exposures in 10 min-intervals for the 300–450 K temperature range. It is also observed that Au is a better dopant for high-temperature oper- ation than Al, contrary to the situation for room-temperature sensing (see Fig. 6). A slight non-linearity can be seen in the sensitivity versus concentration curves (see Fig. 7b). L.F. Reyes et al. / Sensors and Actuators B 117 (2006) 128–134 131 Fig. 5. (a) Sensitivity as a function of time for a pure nanocrystalline WO 3 film (sintered at 300 ◦ C) exposed to several H 2 S concentrations at 600 K operation temperature. The solid lines represent the H 2 S concentration (right-hand scale), while the dotted curves denote measured data (left-hand scale). (b) Sensitivity S as a function of H 2 S concentration C for the experiment described in part (a). The curve corresponds to the stated equation. Fig. 8 depicts the conductance response of an Au-activated sensor film under exposure to CO and NO 2 gases at an operating temperature of 700 K. For a reducing gas such as CO, the conductance was increased, while a conductance Fig. 6. Sensitivity as a function of time for pure and activated as-deposited nanocrystalline WO 3 sensors upon exposure to 10 ppm H 2 S at room-temperature operation. H 2 S injection and evacuation took place after 5 and 15 min, respec- tively. Fig. 7. (a) Sensitivity as a function of time for pure and activated as-deposited nanocrystalline WO 3 sensors exposed to several H 2 S concentrations at 600 K operation temperature. Solid bars represent H 2 S concentrations (right-hand scale), while curves represent measured data (left-hand scale). (b) Sensitivity as a function of H 2 S concentration for the experiments described in (a). decrease could be observed for an oxidizing gas such as NO 2 . In order to determine the optimal operating temperature, the sensitivity of an Al-activated sensor film was tested at different operation temperatures in H 2 S, CO, and NO 2 . Results are shown in Fig. 9 for gas concentrations being 10 ppm H 2 S, 100 ppm CO, and 5 ppm NO 2 . The sensor exhibited different optimum operating temperatures, specifically being 400, 525, and 700 K for H 2 S, NO 2 , and CO, respectively. Importantly, the sensor did Fig. 8. Conductance response of an as-deposited Au-activated WO 3 sensor upon CO (50 and 100 ppm) and NO 2 (5 and 10 ppm) exposures at 700 K operating temperature. 132 L.F. Reyes et al. / Sensors and Actuators B 117 (2006) 128–134 Fig. 9. Sensitivity as a function of operating temperature for as-deposited Al- activated WO 3 films exposed to 10 ppm H 2 S (right-hand scale), 5 ppm NO 2 (left-hand scale), and 100 ppm CO (left-hand scale). The lines are guide for the eye. not show any overlap in its maximum gas-specific sensitivities, which implies that chemical selectivity can be obtained when the sensor is operated at 400, 525, and 700 K. At optimal operating temperatures, the sensor is ∼20 and ∼2000 times more sensitive to H 2 S than to NO 2 and CO, respectively, normalized to the concentration. 5. Discussion Semiconductor gas sensors are mostly used at in air at atmo- spheric pressure, so that the surface of the sensor is continuously exposed to oxygen with a partial pressure of ∼0.2 atm. Stud- ies on the operating mechanism of semiconductor gas sensors indicated that most target gases are detected via the influence they exert on the adsorbed oxygen [20]. In particular, several investigations showed that the key reaction for detecting reduc- ing gases involves oxygen ions on the surface of the sensor [19]. We first consider processes that are – or at least may be – responsible for conductivity changes in sensor films. At low temperatures, it has been suggested [20] that resistance changes due to H 2 S adsorption occur as a consequence of the reactions O 2 (g) + e − ↔ O 2 − (ads) (1) 2H 2 S(g) + 3O 2 − (ads) ↔ 2H 2 O + 2SO 2 + 3e − (2) where (g) and (ads) denote gas phase and adsorbed species, respectively. Reaction (1) takes place prior to sensing and creates a thin electron-depleted layer at the surfaces of the WO 3 grains. As H 2 S is adsorbed, electrons are released into the conduction band according to reaction (2), resulting in conductivity and thus sensitivity increase. For high-temperature operation, the reactions O 2 − (ads) + e − ↔ 2O − (ads) (3) H 2 S(ads) + O − (ads) ↔ H 2 O + S + e − (4) are relevant [20]. Reaction (3) assumes that O 2 − has already been formed on the surface by reaction (1). Another mechanism, that can play a role in the gas sensing, is the formation of additional surface oxygen vacancies, created by the interaction of H 2 S with lattice oxygen according to [21] 3WO 3 + 7H 2 S → 3WS 2 + SO 2 + 7H 2 O (5) This reaction takes place on the surface and involves a reduction of W 6+ to W 4+ . Oxygen leaves the surface thereby releasing electrons into the grains so that the conductivity of the film is increased. However, re-oxidation of the vacancies by O 2 results in a competition with the formation of the oxygen vacancies by H 2 S. Samples sintered at 300 ◦ C exhibited the largest sensitivity. This may be associated with structural modifications, shown by the XRD analysis which indicated that a transformation from a tetragonal to a monoclinic phase commenced at ∼300 ◦ C. It therefore appears that a certain ratio of tetragonal to monoclinic phase is required for optimal sensitivity. The high sensitivity at room temperature is remarkable and in accordance with earlier results of ours [22]. It may be explained as a consequence of several different effects such as (i) the occur- rence of a tetragonal phase of WO 3 in the film [22]; (ii) a high surface area of the nanocrystalline films; and (iii) a porous struc- ture with percolating networks that can be extremely sensitive to minor changes in the conductivity of the individual nanopar- ticles ensuing from H 2 S adsorption. The room-temperature sensitivity could be increased by active materials, especially by Al as found from Fig. 6. Although we could not identify the pertinent phase of Al, it is very likely that it was in oxide form in the films since the Al evaporation took place in synthetic air. It is known that Al 2 O 3 can be active for oxygen adsorption in an air ambient [23], and it follows that it can exert a catalytic role for reaction (1) in the WO 3 layers. Thisleads to a lower intrinsic conductivity for the Al-doped films than for the non-activated ones. According to reaction (2),H 2 S exposure increases the conductivity of the sensor. In addition, active Al- oxide centers can be neutralized by water produced by reaction (2), thus resulting in further conductivity increase. Considera- tions ofthis kind may explain theenhanced sensing capability for the Al-doped films at room temperature. For high-temperature sensing, the significant water desorption makes the neutraliza- tion of the active Al oxide centers less effective, thereby yielding less sensitivity than at low-temperature operation. For room-temperature sensing, the long response times can be explained by the slow reaction rate of H 2 S with adsorbed oxygen ions (reaction (2)), and the long recovery time is due to incomplete desorption of the adsorbed reaction products and H 2 S after finishing the H 2 S exposure. By increasing the temper- ature, the reactions described by (1) and (2) as well as desorption of the products are faster, according to an Arrhenius-type behav- ior, which explains the decrease of the response time as well as the recovery time (see Fig. 7a). However, both pure and activated WO 3 nanoparticle films exhibit low sensitivity for high-temperature operation, as found from Fig. 7b. In addition, Au-activated films were more sensitive than Al-activated ones – in contrast to the behavior at room temperature (see Fig. 6)– indicating that the catalytic effects are temperature sensitive. L.F. Reyes et al. / Sensors and Actuators B 117 (2006) 128–134 133 Several reasons can be given for the sensitivity loss at high temperatures. The rates of the reactions (3) and (4) become sig- nificant at high temperatures and as a consequence – although the net result of these reactions is neutral in terms of the number of electrons – the involved electrons tend to block reaction (2) so that fewer electrons are injected into the conduction band, thus leading to a loss of sensitivity. Formation of hydrogen tungsten bronze upon exposure to H 2 S may also contribute to the sensitiv- ity loss at high temperatures. Similar effects have been reported in electrochromic tungsten oxide [11,12]. It is also worth noting that transformation to WS 2 occur for WO 3 powder reacting with H 2 S at atmospheric pressure and elevated temperature according to reaction (5) [24]. This implies that the amount of surface- adsorbed oxygen available for reaction’s (2) and (4) decreases, i.e., a thin layer containing W S bonds works as an inhibitor, which would result in lesssensitivity. The decrease of the amount of the tetragonal phase as the operating temperature was elevated may also contribute to the sensitivity loss [22]. Furthermore, the desorption rate for adsorbed O 2 − ions is accelerated at high tem- peratures, thus leading to less sensitivity toward H 2 S according to reaction (2). The WO 3 nanoparticle sensor films are also sensitive to CO and NO 2 at 700 K, as seen from Fig. 9. The conductivity increased for CO exposure while it decreased for NO 2 expo- sure. Reactions similar to those described by (1)–(4) above can be used to account for the conductivity increase due to CO. As the molecule is adsorbed, it is oxidized to CO 2 by the help of an adsorbed 1 2 O 2 − , thereby leaving one electron to the conduc- tion band. Adsorption of NO 2 on WO 3 nanoparticles results in a decrease of the conductivity, which may be explained by the reactions [25] NO 2 (g) + e − ↔ NO 2 − (ads) (6) NO 2 (g) + e − ↔ NO(g) + O − (ads) (7) Both of these reactions require electrons from the conduction band of WO 3 , which then leads to a decrease of the conductivity. It is very likely that the reactions indicated for the various test gases have different rates depending on the operating tempera- ture. This may lead to different optimal operating temperatures and, indeed, these temperatures were found to be 400 K for H 2 S, 525 K for NO 2 , and700 K for CO, as seen from Fig. 9. It is impor- tant to observe that this effect, with no noticeable cross-over, can be exploited for chemical selectivity. 6. Conclusions Pure and activated nanocrystalline WO 3 films were pro- duced by advanced reactive gas deposition on alumina substrates prepared for gas sensor application. Activation took place by co-evaporation of Al and Au with the tungsten source. As- deposited films exhibited ∼10 nm average crystal size, high electrical resistivity, and tetragonal structure. Agglomerates of WO 3 nanoparticles with a porous structure – hence being suit- able for gas sensing – could be observed by electron microscopy. The tetragonal phase changed gradually to a monoclinic phase as the sintering temperature was elevated from 100 to 600 ◦ C. Importantly, the WO 3 -nanoparticle-based sensors were sensitive to H 2 S even at room temperature. However, room-temperature operation resulted in a slow response (some minutes) and a long recovery time (several hours). Both of these times could be sig- nificantly decreased by operating at elevated temperature. Activation by Al or Au increased the sensitivity of the WO 3 nanoparticle-based devices by about an order of magnitude. It was also found that the activity of the dopants is temperature dependent. 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Sensors and Actuators B 117 (2006) 128–134 Gas sensor response of pure and activated WO 3 nanoparticle films made by advanced reactive gas deposition L.F system capable of in situ conductivity determinations as a function of sensor temperature and gas concentration. Test gases of H 2 S, NO 2 , and CO were used