Sensors and Actuators B 161 (2012) 447–452
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
NO2 -sensing properties of porous WO3 gas sensor based on anodized sputtered tungsten thin ﬁlm Jing Zeng, Ming Hu ∗ , Weidan Wang, Huiqing Chen, Yuxiang Qin School of Electronics and Information Engineering, Tianjin University, Tianjin 300072, PR China
a r t i c l e
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Article history: Received 31 May 2011 Received in revised form 9 October 2011 Accepted 23 October 2011 Available online 29 October 2011 Keywords: Porous WO3 Tungsten thin ﬁlm Anodization Gas sensor Nitrogen dioxide
a b s t r a c t In this paper, a novel porous WO3 sensor was prepared by anodic oxidation of DC magnetron sputtered metallic tungsten (W) ﬁlm deposited on alumina substrate. The structural and morphological properties of these ﬁlms are investigated using ﬁeld emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). Coral-like porous crystalline WO3 ﬁlm with a grain size of about 9.3 nm was obtained after annealing of the anodized W ﬁlm. The porous WO3 sensor achieved its maximum response value to NO2 at a low operating temperature of 150 ◦ C. In comparison to sputtered WO3 sensor, the porous WO3 sensor showed markedly higher responses, much better response–recovery characteristics and lower optimal operating temperature to different concentration of NO2 gas due to its high speciﬁc surface area and small grain size. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As the air pollution becoming more and more serious, requirements to gas sensitive devices increase. In these years, there has been increasing interest to detect nitrogen oxide gases since they are main source of acid rain and photochemical smog . The air quality standard for NO2 , suggest by Italian legislation for ambient air (“long-term exposure”), is 100 ppb (attention level) . American standards, concentration of NO and NO2 should not exceed 3 and 25 ppm respectively while in Japan, these fall in ppb range . Thus, there is a strong demand for cheap, reliable, sensitive gas sensors targeting NOX . Metal oxide gas sensors are promising for use in detection of low concentrations of NOX due to their low cost, compatibility with microfabrication technologies, high sensitivity and availability of a large variety of metal oxides with different gas sensing characteristics . Transition metal oxides, such as TiO2 , SnO2 , ln2 O3 , WO3 , ZnO, Nb2 O5 or MoO3 , and several combinations of these have been widely investigated so far [5–8]. Among them, tungsten trioxide (WO3 ), which shows promising properties especially if possesses a large surface area , is considered as one of the most interesting materials in the ﬁeld of gas sensors based on metals oxides semiconductors . An evident positive correlation relationship was found between the surface areas of gas sensors and their sensor response . Accordingly, the performance of WO3 gas sensor can
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be signiﬁcantly enhanced by increasing their surface areas, which provides more surface adsorption sites for the reaction of NO2 gas. At the same time, the deposition process affects the sensor performance largely since it affects the morphology and structure of the sensing material (and thus, its properties) [3,12]. Porous WO3 materials, which provide high surface areas have been prepared by sol–gel process , anodization of tungsten foils [14,15] or sputtered tungsten thin ﬁlms [16,17]. Porous WO3 materials obtained by anodization of tungsten thin ﬁlms deposited on conductive substrates are not suitable for the fabrication of resistive-type gas sensor for their substrate conductivity. In this article, anodization of sputtered tungsten thin ﬁlm deposited on insulating substrate was studied for the application of gas sensor. Metallic tungsten ﬁlms were deposited on alumina substrates, which were attached with a pair of interdigitated Pt electrodes, through a DC magnetron sputtering process. Porous WO3 ﬁlms were then prepared by anodic oxidation of the W ﬁlms. We studied the gas sensing properties of porous WO3 towards NO2 gas ranging from 100 ppb to 5 ppm at operating temperature of 100–200 ◦ C. The selectivity to NO2 at operating temperature of 100–250 ◦ C was investigated as well. 2. Experiment The alumina substrates were ultrasonically cleaned ﬁrst with deionized water, acetone, ethanol consecutively for 15 min each, in order to remove grease and other possible pollutants from the substrate surface, and dried at room temperature in ambient air. After cleaned, a pair of interdigitated platinum (Pt) electrodes with
J. Zeng et al. / Sensors and Actuators B 161 (2012) 447–452
Table 1 The sputtering conditions for W and WO3 thin ﬁlms. Sputtering condition
Target composition Base pressure Sputtering pressure Sputtering gas Sputtering power Sputtering time Substrate temperature
99.995% W metal 4 × 10−4 Pa 6.0 Pa Pure argon 100 W 30 min Room temperature
99.995% W metal 4 × 10−4 Pa 1.0 Pa 60% Ar:40% O2 100 W 60 min Room temperature
a thickness of 100 nm was deposited on the alumina substrate using RF magnetron sputtering method by using an interdigitated shadow mask. A DPS-III ultra-high vacuum facing target magnetron sputtering system was used for the deposition of ﬁlms. The W ﬁlms were deposited by means of a DC facing target magnetron sputtering using a circular tungsten target of 2 in. in diameter with a purity of 99.995% supplied by GRIKIN Advanced Materials Co. The sputtering conditions for W thin ﬁlms were shown in Table 1. Anodization of the sputtered W ﬁlm was carried out in the aqueous electrolyte consist of 0.2 wt.% NaF and 0.3% HF by a conventional anode (W ﬁlm)–cathode (platinum plate) system, where voltage was held at 40 V for 60 s. Upon the completion of anodization, samples were rinsed in deionized water, ethanol, and allowed to dry naturally. The schematic diagram of anodization device is shown in Fig. 1. For comparison, a compact WO3 ﬁlm was deposited on an alumina substrate with another sputtering condition. The sputtering conditions for W and WO3 thin ﬁlms fabrication were summarized in Table 1. Transparent WO3 ﬁlms were obtained by annealing the asanodized sample and just sputtered WO3 sample by a standard laboratory furnace at 450 ◦ C for 4 h in air with a slow temperature ramping rate of 2.5 ◦ C min−1 in order to avoid the occurrence of cracks in the ﬁlms. The morphology, crystal structures, and chemical composition of the ﬁlms were characterized using a ﬁeld emission scanning electron microscope (FESEM, FEI Nanosem 430, and Hitachi S-4800), a X-ray diffractometer (XRD, RIGAKU D/MAX 2500 V/PC, Cu K␣ radiation). The gas sensing characteristics were measured in a static gas sensing testing system consisting of a polymethyl methacrylate (PMMA) test chamber, a ﬂat heating plate, a professional digital multimeter and an automatic data acquisition system . The
Fig. 1. Schematic diagram of anodization device.
schematic diagram of the gas sensing test system is shown in Fig. 2(b). The samples could be ﬁxed on the heating plate by a pair of spring-loaded Au-coated copper probes, which connected the Pt electrodes with the digital multimeter. The pure target gas was introduced into the test chamber by static volumetric method. Certain amount of pure target gas was injected into the chamber directly by a micro-injector to get the desired concentration, and the sensor was recovered by opening the movable side cover of the test chamber. The mini-fan ﬁxed at the bottom of the chamber was kept blowing during the testing time to promote the gas diffusion. The digital multimeter will measure the resistance of the sensor and send the data to the computer. The acquired resistance data were stored in a PC for further analysis. The sampling interval was set to 1 s. The operating temperature can be controlled by adjusting the temperature controller of heat plate. The sensor response (S) to NO2 was deﬁned as S = (Rgas − Rair )/Rair , where Rgas and Rair denote the resistance of the sensor under exposure to the measuring gas and clean air, respectively. The response time is deﬁned as the time for 90% of the total resistance change. Conversely, the recovery time is the time for 90% recovery of the resistance change.
3. Results and discussion 3.1. Microstructure characterizations To investigate the effect of anodization on the morphology of the ﬁlm, the top view SEM images of different ﬁlms were taken (Fig. 3). It can be seen from Fig. 3(a), the sputtered W layer exhibits a compact stratiﬁed structure. The size of W particles is about several microns. After anodization, many cracks appear on the surface of W ﬁlm, which makes it porous (inset in Fig. 3(b)). Compared with the porous morphology before annealing, it can be seen that the thermal annealing results in the cracks becoming wider and deeper. As shown in Fig. 3(b), aggregates in a porous coral-like structure can be observed and the voids within the structure provide direct conduits for gas molecules to ﬂow in from the environment, which means larger speciﬁc surface area and more adsorption sites for gas molecules. The unannealed sputtered WO3 ﬁlm consists of many small grains in narrow size distribution, which exhibits a compact ﬂat surface with no crystalline structure observed (inset in Fig. 3(c)). After annealing, clear grain boundaries can be seen on the ﬁlm (Fig. 3(c)), which indicates that the ﬁlm is crystallized. As we know, the performance of gas sensor can be signiﬁcantly enhanced by increasing their surface area . It is easy to conclude that the anodic porous WO3 has a more potential structure for gas sensor than sputtered WO3 ﬁlm. XRD analysis was used to investigate the difference of the crystalline structures of the porous WO3 and sputtered WO3 . Fig. 4 displays the X-ray diffraction (XRD) patterns obtained for the porous WO3 and sputtered WO3 ﬁlms after annealing. As shown in Fig. 4(a), the main diffraction peaks of the porous WO3 ﬁlm can be well indexed to a monoclinic phase [space group P21 /n (14)] of ˚ b = 7.53 A, ˚ c = 7.68 A, ˚ and WO3 with lattice parameters of a = 7.30 A, ˇ = 90.9◦ (JCPDS No. 72-1465), whereas those of the sputtered WO3 ﬁlm can be well indexed to a monoclinic phase [space group P21 /n ˚ b = 7.539 A, ˚ (14)] of WO3 with lattice parameters of a = 7.297 A, ˚ and ˇ = 90.91◦ (JCPDS No. 43-1035). The inset on the left c = 7.688 A, of Fig. 4 is a magniﬁed image of Fig. 4 in the range of 22.0–25.0◦ , which provides further details of XRD patterns. As shown in Fig. 4, the strongest diffraction peak appear at 2 = 23.14◦ corresponding to (0 0 2) plane exhibits a ﬁne preferential growth of the anodic porous WO3 in the [0 0 2] direction, whereas the peak of the (2 0 0) plane at 2 = 25.58◦ indicated that the growth of the sputtered WO3 ﬁlm was preferentially along the [2 0 0] direction. Therefore,
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Fig. 2. Schematic diagrams of (a) the sensor, (b) the gas sensing test system.
both the porous WO3 and the sputtered WO3 ﬁlm were monoclinic phase, but had different preferential orientation of crystal faces. The average particle size was estimated by Scherrer’s formula using FWHM of main prominent peaks, the calculated grain sizes of porous WO3 and sputtered WO3 ﬁlm are approximately 9.3 nm, and 70.8 nm, respectively. It is well known that the sensor sensitivity will be improved by decreasing the particle size [19,20]. It can be inferred that, compared with the sputtered WO3 ﬁlm, the porous WO3 may have a higher sensor response for its smaller particle size. 3.2. NO2 -sensing properties In order to determine the optimal operating temperature, the gas sensing characteristics of the porous WO3 and sputtered WO3 ﬁlms towards 1 ppm NO2 were measured at different operating temperatures ranging from 100 to 250 ◦ C. Fig. 5 shows the measured sensor responses of the porous WO3 and sputtered WO3 ﬁlm
as a function of operating temperature. To ensure the reliability of the testing data, each sensor sample was tested three times at every operating temperature. The three continual tests carried out at the same temperature showed no difference in sensor responses, every data displayed in Fig. 5 is the average value of the three data obtained from the tests. The measured resistances increased upon exposure to NO2 gas, which is consistent with typical tendency of WO3 towards oxidizing gases. The gas sensing mechanism will be discussed below. It is well known that the sensor response of metal oxide gas sensor is much dependent on the operating temperatures as illustrated in Fig. 5. Both of the sensors have optimal operating temperatures at which the maximum response values were achieved. The porous WO3 achieved its maximum response value of 41.2 at 150 ◦ C, and 7.2 for the sputtered WO3 ﬁlm at 200 ◦ C. Sensor responses for both sensors were found to increase with increase in temperature up to their optimal operating temperatures and decrease continuously thereafter. The gas responses of porous
Fig. 3. SEM images of (a) W ﬁlm, (b) anodic tungsten ﬁlm before and after annealing, (c) sputtered WO3 ﬁlm before and after annealing.
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Fig. 4. XRD patterns of the (a) porous WO3 and (b) sputtered WO3 after annealing.
Fig. 6 illustrates the relation between operating temperature and the response and recovery times of the two sensors to 1 ppm NO2 . The porous WO3 ﬁlm shows faster response–recovery than the sputtered WO3 ﬁlm at various operating temperatures. Both response and recovery times for the sensors were improved with increasing in temperature. While it is noteworthy that, the resistances of both samples failed to recover effectually after a long recovery time (>30 min) at low temperatures (<100 ◦ C). Fig. 7 shows the dynamic response of both sensors towards NO2 gas in varying concentration. The measurements are carried out at their respective optimal operating temperature. Notably, the resistances of both sensors increased marginally after NO2 removal, indicating that small amounts of NO2 molecules failed to desorb
Fig. 5. Relationship between operating temperature and the sensor responses for the porous WO3 and sputtered WO3 ﬁlm to 1 ppm NO2 .
WO3 were markedly higher than those of the sputtered WO3 ﬁlm, especially in the operating temperature range of 100–175 ◦ C. Meanwhile, when the temperature exceeds 200 ◦ C, the NO2 response of WO3 ﬁlm is higher than that of porous WO3 ﬁlm.
Fig. 6. Response time and recovery time of the porous WO3 and sputtered WO3 to 1 ppm NO2 as a function of operating temperature.
Fig. 7. Dynamic response of (a) porous WO3 at 150 ◦ C, (b) sputtered WO3 at 200 ◦ C to various concentration of NO2 .
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Fig. 8. Relationship between the responses and NO2 concentration for sensors at respective optimal operating temperature.
from the surface, which might have little inﬂuence on the gas sensing properties of the sensors. The resistances increments of porous WO3 ﬁlm were much smaller than those of the sputtered WO3 ﬁlm, indicating a better reversibility. Fig. 8 illustrates the sensor responses of both sensors to various concentration of NO2 at their respective optimal operating temperature. The responses of both sensors increase with increasing of the NO2 gas concentration in the range of 0.1–5 ppm. It can be seen that the porous WO3 ﬁlm show markedly higher responses to NO2 . As the detected NO2 concentration reaches 1 ppm or above, the porous WO3 sensor exhibits several times higher than the sputtered WO3 ﬁlm with lower operating temperature. The reasons for the better gas sensing properties were also taken into consideration. It is widely known that WO3 is n-type semiconductor whose electron concentration is determined mainly by the concentration of stoichiometric defects such as oxygen vacancy like other metal oxide semiconductors . When exposed to air, the surface of WO3 grains adsorbs molecules or atoms (O2 or O) from air, which extracts electrons from the conduction band of WO3 . As a result, an electron-depleted space–charge layer is formed under the surface. When oxidizing gases such as NO2 present in air, the oxidizing gas molecules may be directly adsorbed onto the surface by trapping electrons from the conduction band. It is also possible that the gas molecules interact with the chemisorbed oxygen on the surface. Anyway, the presence of oxidizing gases in air will lead to the increase of thickness of the electron-depleted layer and height of the Schottky barrier. As a result, the resistance of WO3 semiconductor material increases. Researchers have demonstrated that parameters such as ﬁlm thickness, grain size, agglomeration, porosity, faceting, grain network, surface geometry, and ﬁlm texture have a great effect on the gas-sensing properties of metal oxide conductometric-type sensors . According to the SEM images of both sensors (Fig. 3(b and c)), porous microstructure was formed after anodization, which showed much larger speciﬁc surface area than sputtered WO3 ﬁlm. The formation of porous microstructure also provided more adsorption sites for gas molecules. In other words, more gas molecules will be adsorbed upon exposure to the same concentration of NO2 , which results in much larger change in resistance. As been reported, the grain size has a signiﬁcant impact on the sensitivity of metal-oxide gas sensors. Xu et al. reported that [19,20], when the average grain size D > 20 nm, the sensor response is nearly independent of the grain size, but below about 20 nm it increases with decreasing grain size, where below about 10 nm this increase is remarkable. According to the results of XRD analysis, the grain size of porous WO3 is about an order of magnitude smaller than that
Fig. 9. Sensor responses of porous WO3 to 1 ppm NO2 , 100 ppm NH3 and 500 ppm C2 H5 OH at the operating temperatures ranging from 100 to 250 ◦ C.
of sputtered WO3 ﬁlm. Therefore, smaller grain size also contributes to the increase of sensor response. The response time consists of two parts: time for gas molecules to diffuse to the surface and time for the gas molecules to be absorbed on the surface. Since the mini-fan ﬁxed at the bottom of the chamber was kept blowing during the testing and the voids within the structure acted as diffusion channels for gas molecules, the time for gas molecules to diffuse to the surface can be ignored. As increasing the surface by means of making the ﬁlms porous, more NO2 molecules may be adsorbed on the surface, which results in a sensor response increase. However, the density of gas molecules absorbed on the surface decreased, which means less gas molecules were absorbed per unit area. As a result, the relationship between the surface areas and sensor responses is not always linear. It is obvious that the time taken for lower density of gas molecules to be absorbed on the surface will be less. For all these reasons, porous sensor shows faster response than just sputtered sensor. 3.3. Selectivity It has been demonstrated that tungsten oxide sensors can response to various gases, such as NO2 [3,9,11,13,18], NH3 , ethanol (C2 H5 OH) , O3 , CO , H2 , NO  and H2 S , and so on. It seems to be necessary to make research on the selectivity of porous WO3 . Fig. 9 shows the response values of the porous WO3 to 1 ppm NO2 , 100 ppm NH3 and 500 ppm ethanol at the operating temperatures ranging from 100 to 250 ◦ C. The sensor response (S) to reducing NH3 and ethanol was deﬁned as S = (Rair − Rgas )/Rgas . It can be seen from the ﬁgure that the sensor responses to 100 ppm NH3 and 500 ppm C2 H5 OH were improved with increasing in temperature, indicating that the optimal operating temperatures of porous WO3 to NH3 and C2 H5 OH were not lower than 250 ◦ C. It is obvious that the sensor responses to 1 ppm NO2 were markedly higher than those of 100 ppm NH3 and 500 ppm C2 H5 OH at every operating temperature. It is easy to conclude that the porous WO3 exhibits highly selective to NO2 gas. 4. Conclusions In this work, a novel porous WO3 gas sensor was fabricated by applying electrochemical anodization treatment to metallic tungsten ﬁlm deposited on alumina substrates, which had been attached
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with a pair of interdigitated Pt electrodes. SEM showed a porous coral-like structure was formed after annealing. The results of XRD conﬁrmed the formation of crystalline WO3 with a grain size of about 9 nm. Compared with sputtered WO3 sensor, the porous WO3 sensor exhibited markedly higher response value, lower optimal operating temperature and much better response–recovery characteristics to different concentration of NO2 gas due to the high speciﬁc surface area and the small grain size. The porous WO3 shows faster response than just sputtered WO3 sensor since less gas molecules were absorbed on the surface per unit area. These results indicate the necessity of further research on the porous WO3 ﬁlm gas sensor. Acknowledgments Project supported by the National Natural Science Foundation of China (grant nos. 60771019 and 60801018), Tianjin Key Research Program of Application Foundation and Advanced Technology, China (grant no. 11JCZDJC15300), Tianjin Natural Science Foundation, China (grant no. 09JCYBJC01100), and the New Teacher Foundation of Ministry of Education, China (grant no. 200800561109) References  G. Eranna, B.C. Joshi, D.P. Runthala, R.P. Gupta, Oxide materials for development of integrated gas sensors – a comprehensive review, Crit. Rev. Solid State 29 (2004) 111–188.  C. Baratto, G. Faglia, E. Comini, G. Sberveglieri, A. Taroni, V. La Ferrara, L. Quercia, G. Di Francia, A novel porous silicon sensor for detection of sub-ppm NO2 concentrations, Sens. Actuators B: Chem. 77 (2001) 62–66.  E.K. Heidari, C. Zamani, E. Marzbanrad, B. Raissi, S. Nazarpour, WO3 -based NO2 sensors fabricated through low frequency AC electrophoretic deposition, Sens. Actuators. B: Chem. 146 (2010) 165–170.  S. Kannan, L. Rieth, F. Solzbacher, NOx sensitivity of In2 O3 thin ﬁlm layers with and without promoter layers at high temperatures, Sens. Actuators B: Chem. 149 (2010) 8–19.  C.Y. Lin, Y.Y. Fang, C.W. Lin, J.J. Tunney, K.C. Ho, Fabrication of NOx gas sensors using In2 O3 –ZnO composite ﬁlms, Sens. Actuators B: Chem. 146 (2010) 28–34.  T. Waltz, B. Becker, T. Wagner, T. Sauerwald, C.D. Kohl, M. Tiemann, Ordered nanoporous SnO2 gas sensors with high thermal stability, Sens. Actuators B: Chem. 150 (2010) 788–793.  H.G. Moon, H.W. Jang, J.S. Kim, H.H. Park, S.J. Yoon, A route to high sensitivity and rapid response Nb2 O5 -based gas sensors: TiO2 doping, surface embossing, and voltage optimization, Sens. Actuators B: Chem. 153 (2011) 37–43.  M.R. Mohammadi, D.J. Fray, Semiconductor TiO2 –Ga2 O3 thin ﬁlm gas sensors derived from particulate sol–gel route, Acta Mater. 55 (2007) 4455–4466.  Y.G. Choi, G. Sakai, K. Shimanoe, N. Yamazoe, Wet process-based fabrication of WO3 thin ﬁlm for NO2 detection, Sens. Actuators B: Chem. 101 (2004) 107–111.  A. Labidi, C. Jacolin, M. Bendahan, A. Abdelghani, J. Guerin, K. Aguir, M. Maaref, Impedance spectroscopy on WO3 gas sensor, Sens. Actuators B: Chem. 106 (2005) 713–718.  T. Yamazaki, C.J. Jin, Y. Shirai, T. Yoshizawa, T. Kikuta, N. Nakatani, H. Takeda, Dependence of NO2 gas sensitivity of WO3 sputtered ﬁlms on ﬁlm density, Thin Solid Films 474 (2005) 255–260.  G. Korotcenkov, The role of morphology and crystallographic structure of metal oxides in response of conductometric-type gas sensors, Mater. Sci. Eng. R 61 (2008) 1–39.  L.G. Teoh, Y.M. Hon, J. Shieh, W.H. Lai, M.H. Hon, Sensitivity properties of a novel NO2 gas sensor based on mesoporous WO3 thin ﬁlm, Sens. Actuators B: Chem. 96 (2003) 219–225.
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Biographies Jing Zeng received his bachelor degree in Electronic Science and Technology from Tianjin University in 2009. He is now a graduate student at Tianjin University. His current research is focused on the metal oxide based gas-sensing materials and gas-sensing mechanism. Ming Hu received a M.S. in microelectronics and solid-state electronics from Tianjin University in 1991. She is now a full professor in department of electronics science and technology in Tianjin University. Her research interests include MEMS, gas sensor, and sensitive materials and functional thin ﬁlm devices. Weidan Wang received her bachelor degree in Electronic Science and Technology from North University of China in 2008. She is now a graduate student at Tianjin University. Her current research is focused on the tungsten oxide based gas sensor and material adsorption properties simulation. Huiqing Chen received his bachelor degree in Applied Physics from Hebei Normal University in 2009. He is now a graduate student at Tianjin University. His current research is focused on the porous silicon based gas-sensing materials and gas-sensing mechanism. Yuxiang Qin received a Ph.D. in microelectronics and solid-state electronics from Tianjin University in 2007. She is currently an associate professor in department of electronics science and technology in Tianjin University. Her research interest is in the areas of oxide semiconductor gas sensor, ﬁeld emission materials and devices.