growth of monoclinic wo3 nanowire array for highly

Growth of monoclinic WO 3 nanowire array for highly sensitive NO 2 detection Baobao Cao,† Jiajun Chen,† Xiaojun Tang and Weilie Zhou * Received 23rd September 2008, Accepted 15th January 2009 First published as an Advance Article on the web 23rd February 2009 DOI: 10.1039/b816646c A monoclinic tungsten trioxide nanowire array has been grown on silicon substrates using tungsten powders as source materials by thermal evaporation under specific synthesis conditions (1000  C, 13–15 Torr, 200 sccm air flow). The morphology, chemical composition and crystal structure of the as- prepared tungsten trioxide nanowires were characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and transmission electron microscopy. The nanowires were identified as monoclinic in structure, with diameters ranging from 40 to 100 nm and lengths up to 5 mm. It was found that sufficient oxygen and air flow are the major factors to influence the nanowire array growth. The nanowire array was employed directly for gas sensor fabrication using photolithography. The gas sensing experiments revealed that the nanowire array sensors are highly sensitive to NO 2 (50 ppb), making the tungsten trioxide nanowire array a competitive candidate for highly sensitive gas sensor fabrication. Introduction In the family of transitional metal oxides, tungsten oxides have attracted great attention in the past few decades due to their unique electrochromic, optochromic and gasochromic proper- ties. Their applications have spread to various fields including flat-panel displays, smart windows, gas sensors and field emit- ters. 1–6 Since one-dimensional tungsten oxides exhibit large surface to volume ratios compared to their bulk materials, extensive efforts have been made for the synthesis of tungsten oxide nanostructures and assembled ordered superstructures for sensor applications. 7–19 Different from single nanowires, nano- wire arrays have a higher surface to volume ratio and can be potentially used to fabricate large area, highly sensitive, and more stable sensors for practical applications. However, studies on synthesis of tungsten oxide nanowire arrays are compara- tively rare due to the high melting point of tungsten metal and tungsten compounds. Several groups have performed studies on nanowire array growth. 20–23 One of the interesting works was demonstrated by Huang et al., 23 in which they successfully synthesized W 18 O 49 nanowire arrays directly on ITO (indium tin oxide) coated glass substrate using tungsten trioxide powders by a thermal evaporation method at a relatively low temperature (1100  C). Generally, among all the tungsten oxide nanostructures synthe- sized for gas sensors, W 18 O 49 are mostly obtained and studied. 24,25 However, according to the very recent study by Sun et al., 26 W 18 O 49 nanowires were completely transformed to monoclinic WO 3 at above 500  C, along with the change of morphology from nano- wires to nanoparticles. Since monoclinic WO 3 is quite stable up to 1000  C, sensors based on monoclinic WO 3 could be potentially used in severe environments. Moreover, monoclinic WO 3 contains more oxygen content than other WO 3 À x phases, which might lead to unique physical or chemical properties. In this paper, a conven- tional thermal evaporation approach is applied to synthesize a quasi-aligned, single crystal WO 3 nanowire array using tungsten powder as the source material. In addition, the gas sensors based on such WO 3 nanowire arrays demonstrate a highly sensitive capa- bility to detect NO 2 concentrations down to the 50 ppb level, demonstrating a very promising application in the field of low concentration gas detection. Experimental In a typical synthesis, 2 g tungsten powder (12 mm, 99.9% provided by Sigma–Aldrich) was loaded into the center of the tube furnace, acting as a source material. Silicon substrate (1 cm  3 cm) was cleaned by alcohol and deionized water in an ultrasonic cleaner, then positioned at the low temperature zone, 20 cm downstream of the source material. After the quartz tube was pumped down to a vacuum of 500 mTorr, the temperature of the tube was increased from room temperature to 1000  Cat a ramping rate of 50  C/min. During the whole heating process, constant air flows of 20, 100 or 200 sccm, respectively, were introduced for each experiment and pressure inside the tube was kept at 13–15 Torr in order to explore an optimum synthesis condition for nanowire array growth. After grown for 1 h, the furnace was naturally cooled to room temperature by switching off the heating power. The as-prepared products on Si substrates were characterized by Carl Zeiss 1530 VP field emission scanning electron micro- scopy (FESEM), Philips X’pert-MPD X-ray diffraction (XRD), JEOL 2010 transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS) attached on the TEM. The nanowire array gas sensor was prepared by a multiple-step photolithography process 27 and the sensor was mounted on a heating stage connected to the electrical feedthrough in Advanced Materials Research Institute, University of New Orleans, New Orleans, Louisiana, 70148, USA. E-mail: wzhou@uno.edu; Fax: +1-504- 280-318; Tel: +1-540-280-1068 † Baobao Cao and Jiajun Chen contributed equally to this work. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 2323–2327 | 2323 PAPER www.rsc.org/materials | Journal of Materials Chemistry Downloaded by Sungkyunkwan University on 13 June 2011 Published on 23 February 2009 on http://pubs.rsc.org | doi:10.1039/B816646C View Online a testing cell for sensor measurements. The gas testing experi- ments of the sensor under air-diluted NO 2 were conducted at 180  C using a Keithley 2400 source meter. Results and discussion A large area and quasi-aligned tungsten nanowire array was successfully synthesized under a pressure of 13–15 Torr with an air flow of 200 sccm at a temperature of 1000  C. Fig. 1 shows typical FESEM images of an as-synthesized nanowire array grown on Si substrate. Nanowires are distributed evenly with the length of about 3–5 mm as shown in the top view of Fig. 1(a) and cross-sectional view of Fig. 1(b), respectively. In fact, the nano- wire array grows out from a thin layer of tungsten oxide nano- particles formed prior to the nanowire growth shown in the insert of the enlarged interface in Fig. 1(b). The XRD pattern of tungsten oxide nanowires is shown in Fig. 2. The diffraction peaks can be well indexed to a monoclinic WO 3 phase with unit cell parameters as a ¼ 0.7297 nm, b ¼ 0.7539 nm, c ¼ 0.7688 nm, b ¼ 90.91  (JCPDS 43-1035). In the XRD pattern, the (002) diffraction peak is the strongest reflec- tion, indicating the (002) is the preferential growth plane of the nanowires. The strong peak marked by a star in the pattern is from the Si substrate. It is apparent that the tungsten oxide synthesized in our experiment is WO 3 phase, other than W 18 O 49 23 which was further confirmed by TEM investigation. Fig. 3(a) presents a low magnification TEM image of the nanowires with a diameter around 40–50 nm. Fig. 3(b) is the [100] high resolution electron image (HREM) of the nanowire from the rectangular area in Fig. 3(a), in which the lattice spacings are measured to be 0.385 and 0.379 nm along two orthogonal directions, corresponding to the (002) and (020) planes of monoclinic WO 3 , respectively. The selected area elec- tron diffraction pattern (SAED) proves the nanowire is single crystalline with the growth plane parallel to (002), as shown in the inset in Fig. 3(b). Furthermore, EDS analysis shows the atomic ratio of W and O are close to 3 : 1. The C and Cu peaks are from the copper grid in the EDS spectrum. From the XRD and TEM analysis, it can be further concluded that the WO 3 nanowires are single monoclinic crystalline with growth plane parallel to the (002) plane. In our experiment, the whole synthesis process follows the vapor–solid (VS) mechanism. 28 Generally, gas-phase supersatu- ration in VS growth is a most determining factor for the formation of different nanostructures, which is influenced by source materials temperature, tube pressure, gas flow rate, etc. 29 Therefore, a series of experiments were performed to investigate the optimum growth conditions. It is found that relatively low pressure (13–15 Torr), moderate temperature (1000  C), and high air flow rate (200 sccm) are able to produce a suitable gas-phase supersaturation for nanowire array growth. During the heating, the color of the source materials changed from dark gray to orange-reddish, accompanied with an obvious volume increase. Owing to the sufficient oxygen introduced from the air flow, WO 3 species were directly carried down stream, forming WO 3 nanowire arrays on the Si substrate. No catalysts were observed at the tips of the nanowires by SEM or TEM. It should be noted that a layer of WO 3 nanoparticles, acting as a seed layer, was formed prior to the nanowire array growth. A gas sensor was prepared based on this WO 3 nanowire array and the schematic diagram and corresponding SEM images of the preparation process are shown in the Fig. 4. A layer of 8 mm polymethyl methacrylate (PMMA) Resist 950 A4 (Microchem Inc.) was first coated on the nanowire array by spin coating, shown in Fig. 4(b). After the heat treatment on the PMMA resist, the nanowire array was loaded to an oxygen plasma etching machine. A 100 W input power resulted in an etch rate of 0.08 mm/min, and after 75 min etching, 2 microns of PMMA was left Fig. 1 SEM images of a WO 3 nanowire array grown on a Si substrate from a top view (a) and a cross-sectional view (b). Fig. 2 XRD spectra of the nanowire array on Si substrate. The peak marked by a star is (004) diffraction from the Si substrate. 2324 | J. Mater. Chem., 2009, 19, 2323–2327 This journal is ª The Royal Society of Chemistry 2009 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 23 February 2009 on http://pubs.rsc.org | doi:10.1039/B816646C View Online and the tips of the nanowires were exposed to the environment (Fig. 4(c)). By using sputtering and a metal mask, a 100 nm gold layer with 2 mm  2 mm area was coated onto the residue PMMA and connected to the tips of the nanowires. After that, the nanowire array was soaked into acetone, isopropyl alcohol, and deionized water, successively, to remove the PMMA. Finally, the sensor was dried by slow nitrogen flow. Silver paste was used to make connections from the sensor testing system to the bottom layer and the top gold electrode (Fig. 4(d)). By applying a voltage on the top electrode, current flowing through the nanowires could be provided and all of the nanowires were exposed to the atmosphere. The sensor was first stabilized at 180  C in pure air. IV curves were obtained when air-diluted NO 2 with different concentra- tions were introduced into the gas sensor testing cell, as shown in Fig. 5(a). All the IV curves are linear, implying ohmic contacts were formed between the deposited Au and WO 3 . The sensor can detect NO 2 with a concentration down to 50 ppb. When the sensor is exposed to 50 ppb NO 2 , the resistance of the sensor increased to about 37% from 45.5 MOhm to 62.5 MOhm. Even at a relatively low concentration, 5 ppm, the resistance change can be as high as 3400% from 45.5 MOhm to 1.6 GOhm. The sensing response curves to air-diluted NO 2 are plotted in Fig. 5(b). The sensor showed immediate response to NO 2 , the typical response times are less than 30 s; however, it took a relatively long time for the sensor recovery, generally more than 800 s, and it increased as the exposed concentration increased. The response curves for the low concentrations are also plotted in the inset of Fig. 5(b), which indicates that the response signal was much larger than the Fig. 3 (a) A typical low magnification TEM image of tungsten oxide nanowires. (b) [100] HREM image of a tungsten oxide nanowire denoted by the white rectangle in (a). The inset shows the corresponding SAED pattern. The white arrow in (a) indicates the growth direction of the nanowire. (c) EDS spectra of the as-prepared nanowires reveal that nanowires consist of W and O. The C and Cu elements are from the TEM copper grids. Fig. 4 Schematics and SEM images showing the procedures to prepare a gas sensor based on WO 3 nanowire array. All the scale bars are 4 mm. This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 2323–2327 | 2325 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 23 February 2009 on http://pubs.rsc.org | doi:10.1039/B816646C View Online background noise level, even at a concentration as low as 50 ppb and can be easily identified. Therefore, the sensor presents high sensitivity to detect NO 2 gases, which might be attributed to the large surface area of the nanowire array and intrinsic properties of the monoclinic WO 3 nanowires. WO 3 is an n-type semiconductor. Absorbing oxidizing gas molecules, such as O 2 ,NO 2 , to its surface, can induce surface states that trap electrons and result in depletion of carrier on the material surface. For nanowires, forming a carrier depletion layer will reduce the effective cross area for carrier transport and cause reduced current in our constant voltage bias measurement. Considering that the nanowires are resistors with parallel connection, overall resistance increase in our device requires the gas can cause resistance increase in most of the nanowires. Since the response speed of this sensor is very fast, gas diffusion into the nanowire array sensor must be very effective so that it can induce a resistance increase in most nanowires in a short time. Though the response trend is the same as other WO 3 based sensors, 30,31 the unique parallel connection with large surface area provides a new route to fabricate highly sensitive gas sensors. Conclusions In summary, a large area WO 3 nanowire array with diameters ranging from 40 to 100 nm and lengths up to 5 mm was synthesized on Si substrate using a convenient thermal evapo- ration method at a relatively low temperature. The nanowire grew on top of WO 3 nanoparticle seed layers, forming a quasi- aligned nanowire array. The nanowires were single crystalline with monoclinic structure. 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This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 2323–2327 | 2327 Downloaded by Sungkyunkwan University on 13 June 2011 Published on 23 February 2009 on http://pubs.rsc.org | doi:10.1039/B816646C View Online . Growth of monoclinic WO 3 nanowire array for highly sensitive NO 2 detection Baobao Cao,† Jiajun Chen,†. that the nanowire array sensors are highly sensitive to NO 2 (50 ppb), making the tungsten trioxide nanowire array a competitive candidate for highly sensitive