Sensors and Actuators B 137 (2009) 27–31 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb Room temperature ammonia sensing properties of W 18 O 49 nanowires Y.M. Zhao, Y.Q. Zhu ∗ Nanotubes Laboratory, Division of Materials, Mechanics and Structures, Faculty of Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, UK article info Article history: Received 10 July 2008 Received in revised form 22 December 2008 Accepted 3 January 2009 Available online 19 January 2009 Keywords: Gas sensor Tungsten oxide nanowire Ammonia abstract This paper describes the room temperature ammonia sensing properties of ultra-thin W 18 O 49 nanowire (diameter less than 5nm) bundles prepared by a solvothermal technique. The W 18 O 49 nanowires of high surface areas exhibit a concentration-dependent response when exposed to ammonia. An abnormal behavior of resistance increases first and followed by a decrease was observed when ammonia concentra- tion is above 1 ppm. The response of W 18 O 49 nanowires to ammonia transited from n-type to p-type when the concentration decreased to sub-ppm level. The W 18 O 49 nanowires are highly sensitive to sub-ppm and ppb level ammonia at room temperature, which is attributed to the small diameters, high surface areas and non-stoichiometric crystal structure. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Metal semiconductor oxides, such as SnO 2 ,TiO 2 and WO 3 , etc., have been widely explored for application in detecting toxic gases [1–4]. The principle for the applications of these materials in gas sensing is based on the fact that the electrical resistance of the materials will be changed when they are exposed to the gases. Typically, semiconducting oxide type gas sensors are clas- sified to p-type (resistance increases, e.g. CuO and Cr 2 O 3 ) and n-type (resistance decreases, e.g. SnO 2 and ZnO) when exposed to reducing gas (e.g.NH 3 and CO) [4,5]. Reducing the crystal size and increasing the specific surface area of the sensing materi- als are an effective way to enhance the sensing signals, since the reduction/oxidation reactions are mainly affected by the sur- faces of the oxide [6–9]. Gas sensors based on single nanotube or nanowire have been reported to exhibit high sensitivity, fast response time, and operable ability at room temperature that are unattainable for conventional materials [10–13]. At present, inves- tigations on single nanowire sensor are rather rare because of the challenge in manipulation, fabrication, reproducibility and low mechanical stability of a single nanowire. Most of the studies are based on films sensors comprising of nanowires. Room temper- ature detections of low concentration level (sub-ppm) ammonia are of great importance in clinical diagnosis and food safety. We here report the ammonia detection properties of ultra-thin tung- sten oxide nanowires with a diameter less than 5nm at room temperature. ∗ Corresponding author. E-mail address: yanqiu.zhu@nottingham.ac.uk (Y.Q. Zhu). 2. Experimental Tungsten oxide nanowires were prepared by a solvothermal technique. 100 mg of WCl 6 (Sigma–Aldrich, 99.99%) was slowly dis- solved in 50 ml cyclohexanol to obtain a uniform solution. The solution was transferred to a 125 ml Teflon-lined stainless steel pressure vessel for heating at 200 ◦ C for 5 h. After reaction, a blue precipitate was centrifuged and washed with deionized water and acetone for several times. The resulting powder was pure tung- sten oxide nanowires (WONWs), which was subject to structural and morphological characterization by using scanning electron microscopy, transmission electron microscopy and powder X-rays diffraction utilizing a Cu K␣ radiation source having a wavelength of 0.154 nm. An image of the sample assembly for the gas sensing mea- surement is shown in Fig. 1. In order to assembly the tungsten oxide nanowires gas sensors, two gold electrodes with a gap of about 80 m were first fabricated by cold sputtering of a thin layer of gold onto a masked quartz plate. Then, a layer of tungsten oxide nanowire film was casted onto the substrate spinning across the two gold electrodes using a casting suspension of WONWs in ethanol. The sensor sample was heated at 200 ◦ C for 2 h to evap- orate the organic species and to improve the contact between the nanowires and the gold electrodes. After the heat treatment, two conducting wires were attached to the two gold electrodes by sil- ver paste or by conductive tape. The electrical resistance of the nanowire samples bridging the gap between the two gold elec- trodes at room temperature will be measured as a response to the gas. The sensor sample was placed in a quartz test chamber (about 50 ml). A continuous flow of mixed example gas passes through the test chamber. Gases were applied through two separate lines. 0925-4005/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2009.01.004 28 Y.M. Zhao, Y.Q. Zhu / Sensors and Actuators B 137 (2009) 27–31 Fig. 1. The sample assembly (the two gold electrodes with a gap of about 80 m deposited on glass by cold sputtering, top; with W 18 O 49 nanowires deposition bridg- ing the gold gap, and silver paste electrode connections, bottom). The first one passes dry air (compressed air, zero grade, BOC gases) which is used to dilute the target toxic gas. The other one car- ried calibration gas of known concentration (Ammonia, 1000 ppm balanced with air or 5ppm balanced with nitrogen, BOC gases). The desired gas concentration was obtained by varying the flow rates of the target gases while the flow rate of dry compressed was fixed. The large flow rate dry compressed air, air usually at least ten times faster than that of the target gas, was introduced into the test chamber continuously throughout the whole measurement. The electrical current of the sensor sample was monitored when a fixed potential was applied between the two gold electrodes by a computer controlled CHI 650 potentiostat. 3. Results and discussion The SEM images of the WONWs are shown in Fig. 2a, exhibiting the highly pure and uniform nanowires with a high aspect ratio. Fur- ther characterization by TEM (Fig. 2b) reveals that the nanowires shown in the SEM image (Fig. 2a) are actually WONW bundles consisting of ultra-thin nanowires with diameters of 2–5 nm, with orientation parallel to each other. The electron diffraction (ED) pat- tern collected from an isolated bundle displays elongated spots in a direction vertical to the nanowire growth direction attributed to the alignment of the ultra-thin nanowires comprising the bun- dle, as shown in the inset of Fig. 1b. The ED pattern suggests that the nanowires grow along [0 1 0]. Fig. 3 shows the XRD profile of the WONWs, and all of the peaks can be indexed to the mon- oclinic W 18 O 49 (JCPDF No.71-2450). The specific surface area of the WONW samples based on Brunauer–Emmett–Teller (BET) gas- sorption measurements have been calculated to b e 151 m 2 /g, which is higher than that of the mesoporous WO 3 thin films (143 m 2 /g) [14], about one hundred times of the best for commercial WO 3 particles (1.7 m 2 /g) [15], and five times of the reported value for nano-sized monoclinic WO 3 powders (less than 25 m 2 /g) [15]. Pho- toluminescence characterization of the tungsten oxide nanowires by the solvothermal treatment of tungsten chloride in cyclohex- anol, as we published previously [16], showed a ultraviolet (UV) Fig. 2. (a) SEM and (b) TEM images of the WONWs synthesized by solvothermal process; the inset is the ED pattern. Fig. 3. XRD pattern of WONWs. Y.M. Zhao, Y.Q. Zhu / Sensors and Actuators B 137 (2009) 27–31 29 Fig. 4. Electrical response of the WONWs film to 200 ppm NH 3 . emission peak at 369 nm (3.36 eV) assigned to band–band transi- tion and a strong blue emission peak at 423 nm (2.94 eV) attributed to large amounts of oxygen vacancies that are often implicit in the preparation of oxide semiconductors. The room temperature electrical response of the nanowire sen- sor to a pulse of 200 ppm (balance air) NH 3 is shown in Fig. 4. The current drops sharply when NH 3 was introduced into the cham- ber, and it is then increased slowly to reach a stable state. When the NH 3 flow was cut off, the electrical current recovered and increased quickly. Heat treatment at elevate temperatures usually is used to stabilize the current. However, heat treatment at 400 ◦ C and above will significantly change the morphology and reduce the surface areas of the ultra-thin WONWs, as we have reported elsewhere [17]. Therefore, in this work the sensor sample was annealed at 200 ◦ C for 2 h. The baseline drift of the currents occurred during the mea- surement due to the low annealing temperature and low operation temperature [15,18,19]. It is well-known that for a typical n-type metal oxide semiconducting material, the electrical resistance normally decreases when exposed to a reducing gas, such as ammo- nia [5]. However, the electrical response of the present WONW sensor exhibits something unusual, as shown in Fig. 4, and a resis- tance increase was observed. Non-stoichiometric tungsten oxide nanowires has been reported to experience a conductivity-type change from p-type at room temperature to n-type at temperature above 150 ◦ C when exposed to 100 ppm ammonia [6]. Abnormal behavior of an abrupt decrease followed by a slow increase of the electrical resistance of a nanocrystalline WO 3 sensor has also been recorded [20]. Fig. 5 shows the electrical response of the WONWs to 45 and 400 ppm NH 3 . When exposed to 45 ppm NH 3 , a similar behavior of sharp decrease and then slow increase in the electrical current was observed; when a pulse of 400 ppm NH 3 was introduced, the current decreased sharply again and followed by a fast increase. The stabilized current is higher than that in the synthetic dry air. The dynamic electrical response of the WONWs sensor to different concentrations of NH 3 ranging from 0.1 ppm to 10 ppm was shown in Fig. 6. In the range of 0.1–1 ppm NH 3 , the sensor exhibits a good p-type response (the current decreases fast and then maintains at a stable value) and the signal intensity increases with increasing gas concentration, as shown in Fig. 6. Above 5 ppm, the abnor- mal behavior again appears (the current decreases suddenly and then followed by an increase). The responses of the WONWs to ammonia are dependent on the concentration of the ammonia and experience a conductivity-type change with the increasing of the ammonia concentration. The electrical resistance changes of the Fig. 5. Electrical response of the WONWs to 45 ppm and 400 ppm NH 3 . W 18 O 49 nanowires when exposed to ammonia at room tempera- ture is believed to be caused by the variation of the surface acceptor states density related to the chemisorbed oxygen. The surfaces of the ultra-thin diameter (less than 5 nm) of the W 18 O 49 nanowires are more active than the conventional bulk fully oxidized tung- sten oxide materials. The large amounts of oxygen vacancies in the reduced tungsten oxide W 18 O 49 can serve as adsorption site. The small diameter and large amounts of oxygen vacancies in the WONWswill facilitatethe chemisorptionof oxygen at alow temper- ature. William has predicted that if the grain size of the materials is smaller thanthe depletion layer thickness (Debye-length), the grain could be considered as completely depleted so that the conductiv- ity would b ecome surface-trap limited [5]. Taking into account of the equilibrium between gaseous oxygen and surface oxygen ions, the Debye-length can be defined as d = 2εε 0 KT e 2 N D , (1) where ε is the material’s relative dielectric constant and ε 0 is dielectric constant of vacuum, K is the Boltzmann constant, T is the absolute temperature, e is the electron charge, and N D is bulk donor density. The conductivity of such materials can be expressed in terms of thesurface acceptor states density formulated Fig. 6. Electrical response of WONWs to NH 3 of different concentrations. 30 Y.M. Zhao, Y.Q. Zhu / Sensors and Actuators B 137 (2009) 27–31 Fig. 7. Electrical responses of WONWs to (a) 100 ppb and (b) 10 ppb NH 3 ; (c) and (d) after the subtraction of the baseline in (a) and (b). as chemisorbed oxygen species: e = e K I N D N A + p K II N A K I N D (2) where K I = n(1 −f A )/f A and K II = pn, where e and p are electron and hole mobility respectively, p and n are concentration of hole and electron respectively, f A is fraction of acceptors with trapped electrons. The interaction of the surface with the gas presented in air will cause the surface acceptor state density change. From the above equation it can be seen that the conductivity is not lin- early dependent on the surface acceptor state. The conductivity will decrease pass through a minimum and then increase again with the increases of N A . The minimum occurs when N A is satisfying the following condition: N A = N D e K I p K II (3) If the bulk donor density N D is too large or too small the achiev- able surface state density N A could not reach the minimum and the material will exhibit a pure n-type orp-type response to the gas pre- sented. However, if the N D is in a proper range that N A could reach the minimum with variation of gas concentrations and a change of the sign in conductivity variation will be observed. The abnor- mal behavior of the response of the WONWs to ammonia can be explained if the diameter of the WONWs (less than 5 nm) is less than the Debye-length at room temperature when the bulk donor density of the WONWs is in a proper range. By adopting the typical values of √ ε = 2.29 and N D = 4–6 ×10 15 cm 3 for typically sputtered tungsten oxide thin films [21,22], the Debye-length at room tem- perature is calculated to be about 45–50 nm. We cannot use this value directly as the Debye-length for the WONWs since the donor density of the WONWs is affected by the oxygen vacancies in the nanowires. However, for a reference, the ultra-small diameter of the WONWs and the conductive sign change observed in the experi- mental suggest that the diameter of the nanowires may be smaller than the Debye-length of the WONWs at room temperature. Nevertheless, the electrical response of the WONWs in Fig. 6 is indicative that the WONWs sensor is particularly suitable for NH 3 detection at sub-ppm level. In fact, low concentration ammo- nia detection is actually desired for the development of highly sensitive sensor. Electrical responses of the sensor to low concen- tration of NH 3 down to 10 ppb were studied. Fig. 7a and b shows the electrical response to 100 ppb and 10 ppb NH 3 . Good signals were recorded at low concentration of 10 ppb NH 3 , even with the general baseline drift at room temperature. Fig. 7c and d displays the electrical response of the WONW sensor to 100 ppb and 10 ppb NH 3 , after the subtraction of the baseline . It is clear that stable and repeatable signals were obtained for both low concentrations of 100 ppb and 10ppb NH 3. These results show that this type of WONWs sensor is indeed capable of NH 3 detection at ppb level, which is still a big challenge for conventional sensing materials [23–25]. A simple comparison with other forms of tungsten oxide in NH 3 detection reveals the advantages of the present WONW sen- sors. It has been reported that pure WO 3 do not respond to 100 ppm ammonia at room temperature [26].W 18 O 49 nanowires have been found to be responsive to 10 ppm ammonia at room temperature Y.M. Zhao, Y.Q. Zhu / Sensors and Actuators B 137 (2009) 27–31 31 [6]. Fully oxidized WO 3 only have several active sites, however the large amounts of oxygen vacancies in the reduced tungsten oxide W 18 O 49 can serve as adsorption sites of gas molecular [27]. The current high specific surface areas of the oxide nanowires may be due in part to a combination of the small diameters of individual nanowires comprising the bundles and the unique packing char- acteristic of the bundles themselves. Barret–Joyner–Halenda (BJH) analysis for the pore size distribution reveal an apex centred at about 2.2 nm. The pores are considered to arise from two possible mechanisms: (1) direct formation within nanowires during the low temperature solvothermal preparation technique and (2) indirect formation via the inter-nanowire spaces within a bundle. The high sensitivity of the W 18 O 49 nanowires sensor should be attributed to the small diameter (less than 5 nm), the high surface area and the non-stoichiometric crystal structure. 4. Conclusion We have investigated the ammonia sensing properties of W 18 O 49 nanowires prepared by a low temperature solvother- mal technique. The electrical resistance of W 18 O 49 nanowires is increased when exposed to sub-ppm level ammonia, exhibiting a p-type behavior. The electrical resistance of W 18 O 49 nanowires increases first and then decreases when exposed to ammonia with a concentration more than 1 ppm. The abnormal behavior of the W 18 O 49 nanowires to ammonia is related with the ultra-thin diam- eter (less than 5 nm). The high sensitivity of the W 18 O 49 nanowires in the detection of sub-ppm to ppb level ammonia at room tem- perature is attributed to the small diameter, high surface area and non-stoichiometric crystal structure. References [1] N. Barsan, U. Weimar, Understanding the fundamental principles of metal oxide based gas sensors; the example of CO sensing with SnO 2 sensors in the presence of humidity, J. Phys.: Condens. Mater. 15 (2003) R813–R839. 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She received her first degree and master degree in Materials Science and Engineering in University of Science and Technology (Beijing, China) in 1999 and 2002, respectively. Her research interests focus on the preparation, characterization and application of novel inorganic one-dimensional nanomaterials. Yanqiu Zhu obtained his BSc and MSc degree at Harbin Institute of Technology (Harbin, China) in 1989 and 1992, and his PhD degree from Tsinghua University (Beijing China) in 1996. His research covers carbon nanotubes and a variety of inorganic nanomaterials. He is a Reader in Nanomaterials at the University of Not- tingham (UK) with a research interest focusing on the synthesis and application of nanomaterials. . www.elsevier.com/locate/snb Room temperature ammonia sensing properties of W 18 O 49 nanowires Y.M. Zhao, Y.Q. Zhu ∗ Nanotubes Laboratory, Division of Materials, Mechanics. the ammonia sensing properties of W 18 O 49 nanowires prepared by a low temperature solvother- mal technique. The electrical resistance of W 18 O 49 nanowires