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Green synthesis of tungsten trioxide monohydrate nanosheets as gas sensor
Materials Chemistry and Physics 126 (2011) 717–721
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Green synthesis of tungsten trioxide monohydrate nanosheets as gas sensor Xueting Chang ∗ , Shibin Sun, Yansheng Yin Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 200135, China
a r t i c l e
i n f o
Article history: Received 5 August 2010 Received in revised form 17 October 2010 Accepted 13 December 2010 Keywords: Nanostructures Chemical synthesis Electron microscopy Semiconductivity
a b s t r a c t In this paper, orthorhombic tungsten trioxide monohydrate nanosheets in high yields were successfully generated using a very simple sonochemical method with tungsten hexachloride as the precursor and distilled water as the solvent. The tungsten trioxide monohydrate nanosheets exhibited thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. The sheet-like morphology has been well explained based on the acoustic cavitation effect as well as the crystalline structure of orthorhombic tungsten trioxide monohydrate. The tungsten trioxide monohydrate nanosheets sensor exhibited ideal room-temperature gas-sensing performances, and were found to be sensitive to various flammable organic vapors and harmful gases. The corresponding sensing mechanisms were also discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last two decades, nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical and optical properties originated from their high surface area and low dimensionality [1–3]. Among all these materials, tungsten trioxides have been shown to possess many interesting properties, making them a promising candidate for a range of applications in electrochromic display, semiconductor gas sensors and photocatalysts [4–7]. Tungsten trioxide can combine with water molecules to form crystalline phases of tungsten oxide hydrates, namely WO3 ·nH2 O (n = 1/3, 1, 2), or amorphous phases of metatungstic acids, with various molecular and crystal structures [8]. Tungsten oxide hydrates are also used for the fabrication of tungsten filaments for lamps and tungsten carbide for hard metals, or as alloying component in heavy metals [9,10]. Very recently, tungsten oxide hydrate nanowire netted-spheres were proved to possess desirable sensing characteristics towards ammonia [11]. Up till now, various nanostructured WO3 ·nH2 O (n = 1/3, 1, 2) have been successfully prepared by using wet chemical methods, i.e. hydrothermal treatment of aqueous tungstic acid sol [12,13], sol–gel synthesis [14], ion-exchange method with sodium tungstate as precursor [15], acid precipitation method [16], etc. Although wet chemical synthesis can allow for compositional tailoring and tuning of the properties of resulting nanomaterials, challenge still remains with regard to the large-scale production of the nanomaterials with enhanced properties than their commonly available bulk form, as well as their practical application.
∗ Corresponding author. E-mail address: [email protected] (X. Chang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.12.054
Sonochemistry has been proved to be an effective, environmentfriendly, and pollution-free technique in the preparation of a variety of nanomaterials with controllable morphology in relatively short reaction time, including ZnO nanosheets or nanorods [17,18], MnO2 nanowires or nanoflowers [19], Ag nanorods [20], TiO2 nanorods [21], Au nanoparticles [22], and WC/Pd nanocomposites [23]. The sonochemical synthesis is based on the process of acoustic cavitation resulting from continuous formation, growth and implosive collapse of bubbles in the liquid [24]. In this paper, based on a very simple sonochemical method, tungsten oxide monohydrate nanosheets (WONSs) in high yields were successfully generated by using WCl6 as the source material and distilled water as the solvent. The WONSs sensors exhibited ideal room-temperature gas-sensing performances, and were found to be sensitive to various flammable organic vapors and harmful gases. We expect that this study will benefit to the mass production of nanostructured tungsten oxide hydrates, and their practical use as gas sensors. 2. Experimental In the sonochemical synthesis, tungsten hexachloride (99%, WCl6 , Aladdin) was used as precursor, and distilled water as solvent. Typically, 10 g of WCl6 was dissolved in 500 ml of distilled water to make a solution in a beaker, which was then placed on a hot plate heater with temperature of 200 ◦ C. The solution was kept boiling throughout the whole reaction. Ultrasonic irradiation was accomplished using a high-intensity ultrasonic probe (P100-20 sonic processor, 20 kHz, 1000 W) that was dipped in the solution. After reaction, the solvent was evaporated completely to leave yellow powder for further examination. The morphology of the resulting products was characterized by using scanning electron microscope (SEM, Philips XL30, operated at 20 kV) and transmission electron microscope (TEM, JEOL 2000FX, 200 kV). The surface area of the as-synthesized nanosheets was calculated based on the Brunauer–Emmett–Tettler (BET) gassorption measurements, which were conducted by using Autosorb-1 sorptometer. The sensing activities towards various flammable vapors including acetone, gasoline and ethanol, and harmful gases such as ammonia and formaldehyde of
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Fig. 1. (a) Schematic illustration and (b) circuit diagram of the WONSs sensor.
sensors with WO3 ·H2 O nanosheets (WONSs sensor) were evaluated through a WS30A system (Weisheng Electronics Co., Ltd., China). The preparation process of the WONSs gas sensor was as follows. Firstly, powders of WONSs were mixed with glycerol to form a paste. The paste was then coated on a ceramic tube with a pair of Au electrodes covered by Pt wires, and then dried at 100 ◦ C for 10 min. The WONSs sensor was schematically shown in Fig. 1a. The sensitivity (S) of the WONSs sensor was defined as Rair /Rgas , where Rair and Rgas are the electrical resistances of the sensor exposed in air and in flammable vapors or harmful gases, respectively. According to the circuit diagram of the WONSs sensor, Rair and Rgas could be easily obtained from the Vout values of Rf , which were automatically recorded by the computer during the test, as shown in Fig. 1b.
3. Results and discussion Yellowish material was produced during the reaction in high yield (50 wt% of the precursor), and about 5 g product was finally collected in each run. Fig. 2a shows the morphology of the resulting
Fig. 2. (a) SEM image and (b) XRD pattern of the WONSs. Inset in (a) is a high magnification SEM image.
product produced by the simple sonochemical method. It is apparent that the resulting product was dominated by agglomerates. On close inspection (inset in Fig. 2a), the agglomerates were composed of numbers of nanosheets with thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. Actually, the morphologies of the products could be rationally controlled by simply changing the solvent, as has been reported in our previous work [26]. The XRD result indicated that the sonochemically prepared products were pure WO3 ·H2 O with orthorhombic structure (JCPDS no. 43-0679). In addition, the XRD pattern of the WO3 ·H2 O nanosheets (WONSs) exhibited sharp peaks, indicating a highly crystalline feature, as indicated in Fig. 2b. Exact information about the morphology of the WONSs can be obtained with TEM analysis. An integral square-shaped nanosheet was clearly shown in Fig. 3a, with the side length of about 250 nm. The entire nanosheet exhibits smooth surface and no boundaries have been observed. Actually, the majority of these nanosheets preferred to be agglomerated or randomly overlapped due to their high surface areas. As shown in Fig. 3b, at least four pieces of nanosheets possessing rectangular structure with side length of about 500 nm were overlapped together. The corresponding electron diffraction (ED) indicates that the surface plane of the nanosheet is parallel to the (1 1 1) plane of orthorhombic WO3 ·H2 O, agreeing well with the XRD results. Similar tungsten oxide hydrates (WO3 ·0.33H2 O) nanodiscs composed of nanosheets prepared by a hydrothermal method with tungsten acid and distilled water as raw materials has been previously reported [13]. By comparison, the sonochemical method in our work is much simpler and more controllable. In addition, the solvent used in the present work, i.e. water, is clean, nontoxic, and can be totally volatilized during the reaction, making this method environmental friendly. During the sonochemical process, the sonication treatment played an important role in the formation of the WO3 ·H2 O nanosheets. In the absence of ultrasonic treatment, only irregular, amorphous and non-uniform large particles were obtained. As proposed by Suslick, sonochemical reaction derives from the acoustic cavitation, which denotes the formation, growth, and implosive collapse of bubbles in a liquid. Bubble collapse induced by acoustic cavitation produces intense local heating, high temperature, and fast heating and cooling rates inside the collapsing bubbles or in the interfacial region of between the cavitation bubbles and the surrounding liquid [24]. This condition is available to break the chemical bonds of the water into extremely reactive H• and OH• radicals [19,25]. In addition, sonication treatment makes ionization of water molecules at the instant of the collapse of the bubbles to generated hydrated electrons through the reaction of H2 O + e− → e− (aq). Hence, the nuclei of WO3 could be generated from the reaction of W6+ + 5OH• + H• + 6e− (aq) → WO3 + 2H2 O, where W6+ may result directly from the WCl6 dissolved in the aqueous solution, or from the pyrolysis of WCl6 , or even from WO3 which is in turn formed by the fast oxidation of WCl6 . Then, driven by the ultrasonic energy, the WO3 can be easily chemically bonded with
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Fig. 3. TEM images of the WONSs. Inset in (b) is the corresponding SAED pattern.
H2 O to form the WO3 ·H2 O nuclei. The fast kinetics prohibits the growth of the WO3 ·H2 O nuclei from forming bulk shape, and in each collapsing bubble a few nucleation centers are formed whose growth is limited by the short collapse [25]. From this stage, the unique structure of orthorhombic WO3 ·H2 O dominates in determining the morphology of the final product. In general, WO3 is built by units of ReO3 -type [WO6 ] octahedra, in which six oxygen atoms surrounding the central tungsten atom connect six different octahedral by corner-sharing. Similar to that of WO3 , the WO3 ·H2 O consists of layers of edge-sharing [WO5 (OH2 )] octahedral, in which two of the oxygen atoms off the ab plane are substituted by W O bond and W–OH2 bond, respectively [13]. The WO3 ·H2 O structure is built up by stacking of the layers of edgesharing [WO5 (OH2 )] octahedral along the c direction. Within the layers (in the xy plane), all tungsten atoms are connected by covalent bonding; whilst each tungsten atom has two terminal W O and W–OH2 bonds when viewed along the z direction. The relatively weaker interaction between adjacent layers restricts their stacking along the z direction to form bulk form. In addition, the tungsten and oxygen atoms of the orthorhombic WO3 ·H2 O are closed packed in {1 1 1} planes because of their minimum surface energy. Therefore, orthorhombic WO3 ·H2 O nanosheets with {1 1 1} planes as the exposed surfaces were finally formed. Importantly, the sonication treatment during the sonochemical process can also help to disperse the resulting products effectively, and to prevent the agglomeration, a phenomenon that happens often during other soft-chemistry technique. Three different flammable organic vapors and two harmful gases were selected to evaluate the gas sensing activities of the WONSs sensor at room temperature. Fig. 4a shows the response curves to 100 ppm ethanol, acetone and 93# gasoline. Upon exposure to ethanol vapor, the output voltage increased significantly, indicating decreased electrical resistance of the sensors. The response and recovery times of the WONSs sensor to 100 ppm ethanol are about 15 and 28 s, which are short enough for practical applications [27]. The sensitivities of the WONSs sensor to different objects were calculated from the equation of S = Rair /Rgas = Vout-gas (Vc − Vout-air )/(Vc − Vout-gas ) Vout-air , where Vc , Vout-air and Vout-gas are the operating voltage, output voltage in air and output voltage in gas, respectively. Here, the WONSs sensor exhibited a high sensitivity of 15.6–100 ppm ethanol, as indicated in Fig. 4c. As far as 93# gasoline and acetone are concerned, the WONSs sensor exhibited relatively weaker responses to both organic vapors in comparison with that to ethanol, and their sensitivities decreased dramatically
to 7.3 and 5.3, respectively. However, the WONSs sensor possessed still fast response time and recovery time to acetone. The WONSs sensors are not only selective to flammable organic vapors, but also sensitive to some harmful gases. As shown in Fig. 4b, the sensitivities of the WNOSs to ammonia and formaldehyde are 13.4 and 10.6, just little lower than that to ethanol vapor. However, it should be noted that the response times of the gas sensor to both detected gases were much longer although their recovery times were short. Tungsten oxide sensors are known to behave as n-type semiconductor and their gas-sensing mechanism belongs to the surface-controlled type [28]. It has been widely accepted that the current in n-type metal-oxide-semiconductor (MOS) sensor is carried by conduction band electrons, and chemisorbed oxygen species including O2− and O− formed by atmospheric oxygen capture the electron carriers. Upon exposure to reducing chemicals, the arrested electrons are released by the reactions between the reducing gas and O2− or O− , leading to the decrease in resistance [29,30]. In the present work, the solvothermally prepared WONSs were found to be sensitive to various reducing chemicals and gases selected, and also representatively behave as n-type sensor. Based on the above discussion, the electrical responses of the WNOSs sensor are closely related to the density of the chemisorbed oxygen species, which in turn depends on the surface activities of the WNOSs. The calculated specific surface area of the present WONSs is ca. 39.6 cm3 g−1 , which is comparable to that of the Bi2 WO6 nanoplates (around 30) and higher than the reported value for the nano-sized WO3 powders (less than 25 cm3 g−1 ) [31]. The BET nitrogen sorption isotherm curve of the WONSs is shown in Fig. 4d, which exhibits a typical type IV feature. The hysteresis loop at high pressure is associated with the inter-aggregated pores between the aggregated nanosheets. The high specific surface and the existing inter-aggregated pores lead to high amount of chemisorbed oxygen species on the surfaces of the nanosheets. Upon exposing in reducing atmosphere, the surface chemisorbed oxygen such as O2− or O− , can be readily reacted with the reducing vapors or gases, releasing electrons back to the conduction band and the electrical resistance of the WNOSs sensor decreased sensitively. Additionally, the WONSs sensor exhibit high sensitivity, fast response and recovery times to ethanol vapor, ammonia and formaldehyde, but relatively weaker responses to acetone and 93# gasoline vapors, as shown in Fig. 3. The reason may be related to the different reducing powers of the detected objects, or be due to particular features associated with the solvothermally prepared WNOSs that have not yet fully addressed.
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Fig. 4. (a and b) Response curves of the WONSs sensor towards flammable organic vapors and pure gases; (c) sensitivities of the WONSs sensor towards different objects; (d) nitrogen sorption isotherm curve of the WONSs.
From what has been discussed above, it can be concluded that the WONSs sensors exhibited ideal room-temperature sensing activities towards various organic vapors and harmful gases. Here, it should be noted that a top temperature limit may exist in sensor application for the present WONSs because of its low thermal stability. According to the differential temperature analysis (DTA) and thermogravimetirc analysis (TGA), the crystal water of the WONSs would be lost at temperatures ranging from 150 ◦ C to 300 ◦ C, as shown in Fig. 5. Simultaneously, the tungsten oxide hydrate
nanosheets were transformed to tungsten trioxide plates with large size, as previously described [17]. Hence, their gas-sensing properties will be inevitably influenced. Further detailed investigations of the effects of thermal treatment on the gas-sensing properties of the WONSs are in progress. 4. Conclusions In summary, large-scale production of orthorhombic WO3 ·H2 O nanosheets was achieved using a simple sonochemical method with only water as the solvent. The WO3 ·H2 O nanosheets exhibited thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. The formation of the sheetlike morphology was attributed to a combination of the acoustic cavitation effect and the unique crystalline structure of orthorhombic WO3 ·H2 O. The WO3 ·H2 O nanosheets sensor were found to be sensitive to various flammable organic vapors and harmful gases at room temperature, and exhibited maximum sensitivity to ethanol vapor. The present sonochemistry-based method is easilyoperated, environmental friendly and capable of realizing mass production of nanomaterials with low energy consumption and short-term reaction, providing it much potential perspectives in the future nanotechnology areas. Acknowledgement
Fig. 5. Thermal analysis of the WONSs in air: (a) DTA curve and (b) TGA curve.
We wish to express our sincere appreciation to Yidong Zhang in Xuchang University for the gas-sensing measurements.
X. Chang et al. / Materials Chemistry and Physics 126 (2011) 717–721
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Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Green synthesis of tungsten trioxide monohydrate nanosheets as gas sensor Xueting Chang ∗ , Shibin Sun, Yansheng Yin Institute of Marine Materials Science and Engineering, Shanghai Maritime University, Shanghai 200135, China
a r t i c l e
i n f o
Article history: Received 5 August 2010 Received in revised form 17 October 2010 Accepted 13 December 2010 Keywords: Nanostructures Chemical synthesis Electron microscopy Semiconductivity
a b s t r a c t In this paper, orthorhombic tungsten trioxide monohydrate nanosheets in high yields were successfully generated using a very simple sonochemical method with tungsten hexachloride as the precursor and distilled water as the solvent. The tungsten trioxide monohydrate nanosheets exhibited thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. The sheet-like morphology has been well explained based on the acoustic cavitation effect as well as the crystalline structure of orthorhombic tungsten trioxide monohydrate. The tungsten trioxide monohydrate nanosheets sensor exhibited ideal room-temperature gas-sensing performances, and were found to be sensitive to various flammable organic vapors and harmful gases. The corresponding sensing mechanisms were also discussed. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In the last two decades, nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical and optical properties originated from their high surface area and low dimensionality [1–3]. Among all these materials, tungsten trioxides have been shown to possess many interesting properties, making them a promising candidate for a range of applications in electrochromic display, semiconductor gas sensors and photocatalysts [4–7]. Tungsten trioxide can combine with water molecules to form crystalline phases of tungsten oxide hydrates, namely WO3 ·nH2 O (n = 1/3, 1, 2), or amorphous phases of metatungstic acids, with various molecular and crystal structures [8]. Tungsten oxide hydrates are also used for the fabrication of tungsten filaments for lamps and tungsten carbide for hard metals, or as alloying component in heavy metals [9,10]. Very recently, tungsten oxide hydrate nanowire netted-spheres were proved to possess desirable sensing characteristics towards ammonia [11]. Up till now, various nanostructured WO3 ·nH2 O (n = 1/3, 1, 2) have been successfully prepared by using wet chemical methods, i.e. hydrothermal treatment of aqueous tungstic acid sol [12,13], sol–gel synthesis [14], ion-exchange method with sodium tungstate as precursor [15], acid precipitation method [16], etc. Although wet chemical synthesis can allow for compositional tailoring and tuning of the properties of resulting nanomaterials, challenge still remains with regard to the large-scale production of the nanomaterials with enhanced properties than their commonly available bulk form, as well as their practical application.
∗ Corresponding author. E-mail address: [email protected] (X. Chang). 0254-0584/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2010.12.054
Sonochemistry has been proved to be an effective, environmentfriendly, and pollution-free technique in the preparation of a variety of nanomaterials with controllable morphology in relatively short reaction time, including ZnO nanosheets or nanorods [17,18], MnO2 nanowires or nanoflowers [19], Ag nanorods [20], TiO2 nanorods [21], Au nanoparticles [22], and WC/Pd nanocomposites [23]. The sonochemical synthesis is based on the process of acoustic cavitation resulting from continuous formation, growth and implosive collapse of bubbles in the liquid [24]. In this paper, based on a very simple sonochemical method, tungsten oxide monohydrate nanosheets (WONSs) in high yields were successfully generated by using WCl6 as the source material and distilled water as the solvent. The WONSs sensors exhibited ideal room-temperature gas-sensing performances, and were found to be sensitive to various flammable organic vapors and harmful gases. We expect that this study will benefit to the mass production of nanostructured tungsten oxide hydrates, and their practical use as gas sensors. 2. Experimental In the sonochemical synthesis, tungsten hexachloride (99%, WCl6 , Aladdin) was used as precursor, and distilled water as solvent. Typically, 10 g of WCl6 was dissolved in 500 ml of distilled water to make a solution in a beaker, which was then placed on a hot plate heater with temperature of 200 ◦ C. The solution was kept boiling throughout the whole reaction. Ultrasonic irradiation was accomplished using a high-intensity ultrasonic probe (P100-20 sonic processor, 20 kHz, 1000 W) that was dipped in the solution. After reaction, the solvent was evaporated completely to leave yellow powder for further examination. The morphology of the resulting products was characterized by using scanning electron microscope (SEM, Philips XL30, operated at 20 kV) and transmission electron microscope (TEM, JEOL 2000FX, 200 kV). The surface area of the as-synthesized nanosheets was calculated based on the Brunauer–Emmett–Tettler (BET) gassorption measurements, which were conducted by using Autosorb-1 sorptometer. The sensing activities towards various flammable vapors including acetone, gasoline and ethanol, and harmful gases such as ammonia and formaldehyde of
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Fig. 1. (a) Schematic illustration and (b) circuit diagram of the WONSs sensor.
sensors with WO3 ·H2 O nanosheets (WONSs sensor) were evaluated through a WS30A system (Weisheng Electronics Co., Ltd., China). The preparation process of the WONSs gas sensor was as follows. Firstly, powders of WONSs were mixed with glycerol to form a paste. The paste was then coated on a ceramic tube with a pair of Au electrodes covered by Pt wires, and then dried at 100 ◦ C for 10 min. The WONSs sensor was schematically shown in Fig. 1a. The sensitivity (S) of the WONSs sensor was defined as Rair /Rgas , where Rair and Rgas are the electrical resistances of the sensor exposed in air and in flammable vapors or harmful gases, respectively. According to the circuit diagram of the WONSs sensor, Rair and Rgas could be easily obtained from the Vout values of Rf , which were automatically recorded by the computer during the test, as shown in Fig. 1b.
3. Results and discussion Yellowish material was produced during the reaction in high yield (50 wt% of the precursor), and about 5 g product was finally collected in each run. Fig. 2a shows the morphology of the resulting
Fig. 2. (a) SEM image and (b) XRD pattern of the WONSs. Inset in (a) is a high magnification SEM image.
product produced by the simple sonochemical method. It is apparent that the resulting product was dominated by agglomerates. On close inspection (inset in Fig. 2a), the agglomerates were composed of numbers of nanosheets with thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. Actually, the morphologies of the products could be rationally controlled by simply changing the solvent, as has been reported in our previous work [26]. The XRD result indicated that the sonochemically prepared products were pure WO3 ·H2 O with orthorhombic structure (JCPDS no. 43-0679). In addition, the XRD pattern of the WO3 ·H2 O nanosheets (WONSs) exhibited sharp peaks, indicating a highly crystalline feature, as indicated in Fig. 2b. Exact information about the morphology of the WONSs can be obtained with TEM analysis. An integral square-shaped nanosheet was clearly shown in Fig. 3a, with the side length of about 250 nm. The entire nanosheet exhibits smooth surface and no boundaries have been observed. Actually, the majority of these nanosheets preferred to be agglomerated or randomly overlapped due to their high surface areas. As shown in Fig. 3b, at least four pieces of nanosheets possessing rectangular structure with side length of about 500 nm were overlapped together. The corresponding electron diffraction (ED) indicates that the surface plane of the nanosheet is parallel to the (1 1 1) plane of orthorhombic WO3 ·H2 O, agreeing well with the XRD results. Similar tungsten oxide hydrates (WO3 ·0.33H2 O) nanodiscs composed of nanosheets prepared by a hydrothermal method with tungsten acid and distilled water as raw materials has been previously reported [13]. By comparison, the sonochemical method in our work is much simpler and more controllable. In addition, the solvent used in the present work, i.e. water, is clean, nontoxic, and can be totally volatilized during the reaction, making this method environmental friendly. During the sonochemical process, the sonication treatment played an important role in the formation of the WO3 ·H2 O nanosheets. In the absence of ultrasonic treatment, only irregular, amorphous and non-uniform large particles were obtained. As proposed by Suslick, sonochemical reaction derives from the acoustic cavitation, which denotes the formation, growth, and implosive collapse of bubbles in a liquid. Bubble collapse induced by acoustic cavitation produces intense local heating, high temperature, and fast heating and cooling rates inside the collapsing bubbles or in the interfacial region of between the cavitation bubbles and the surrounding liquid [24]. This condition is available to break the chemical bonds of the water into extremely reactive H• and OH• radicals [19,25]. In addition, sonication treatment makes ionization of water molecules at the instant of the collapse of the bubbles to generated hydrated electrons through the reaction of H2 O + e− → e− (aq). Hence, the nuclei of WO3 could be generated from the reaction of W6+ + 5OH• + H• + 6e− (aq) → WO3 + 2H2 O, where W6+ may result directly from the WCl6 dissolved in the aqueous solution, or from the pyrolysis of WCl6 , or even from WO3 which is in turn formed by the fast oxidation of WCl6 . Then, driven by the ultrasonic energy, the WO3 can be easily chemically bonded with
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Fig. 3. TEM images of the WONSs. Inset in (b) is the corresponding SAED pattern.
H2 O to form the WO3 ·H2 O nuclei. The fast kinetics prohibits the growth of the WO3 ·H2 O nuclei from forming bulk shape, and in each collapsing bubble a few nucleation centers are formed whose growth is limited by the short collapse [25]. From this stage, the unique structure of orthorhombic WO3 ·H2 O dominates in determining the morphology of the final product. In general, WO3 is built by units of ReO3 -type [WO6 ] octahedra, in which six oxygen atoms surrounding the central tungsten atom connect six different octahedral by corner-sharing. Similar to that of WO3 , the WO3 ·H2 O consists of layers of edge-sharing [WO5 (OH2 )] octahedral, in which two of the oxygen atoms off the ab plane are substituted by W O bond and W–OH2 bond, respectively [13]. The WO3 ·H2 O structure is built up by stacking of the layers of edgesharing [WO5 (OH2 )] octahedral along the c direction. Within the layers (in the xy plane), all tungsten atoms are connected by covalent bonding; whilst each tungsten atom has two terminal W O and W–OH2 bonds when viewed along the z direction. The relatively weaker interaction between adjacent layers restricts their stacking along the z direction to form bulk form. In addition, the tungsten and oxygen atoms of the orthorhombic WO3 ·H2 O are closed packed in {1 1 1} planes because of their minimum surface energy. Therefore, orthorhombic WO3 ·H2 O nanosheets with {1 1 1} planes as the exposed surfaces were finally formed. Importantly, the sonication treatment during the sonochemical process can also help to disperse the resulting products effectively, and to prevent the agglomeration, a phenomenon that happens often during other soft-chemistry technique. Three different flammable organic vapors and two harmful gases were selected to evaluate the gas sensing activities of the WONSs sensor at room temperature. Fig. 4a shows the response curves to 100 ppm ethanol, acetone and 93# gasoline. Upon exposure to ethanol vapor, the output voltage increased significantly, indicating decreased electrical resistance of the sensors. The response and recovery times of the WONSs sensor to 100 ppm ethanol are about 15 and 28 s, which are short enough for practical applications [27]. The sensitivities of the WONSs sensor to different objects were calculated from the equation of S = Rair /Rgas = Vout-gas (Vc − Vout-air )/(Vc − Vout-gas ) Vout-air , where Vc , Vout-air and Vout-gas are the operating voltage, output voltage in air and output voltage in gas, respectively. Here, the WONSs sensor exhibited a high sensitivity of 15.6–100 ppm ethanol, as indicated in Fig. 4c. As far as 93# gasoline and acetone are concerned, the WONSs sensor exhibited relatively weaker responses to both organic vapors in comparison with that to ethanol, and their sensitivities decreased dramatically
to 7.3 and 5.3, respectively. However, the WONSs sensor possessed still fast response time and recovery time to acetone. The WONSs sensors are not only selective to flammable organic vapors, but also sensitive to some harmful gases. As shown in Fig. 4b, the sensitivities of the WNOSs to ammonia and formaldehyde are 13.4 and 10.6, just little lower than that to ethanol vapor. However, it should be noted that the response times of the gas sensor to both detected gases were much longer although their recovery times were short. Tungsten oxide sensors are known to behave as n-type semiconductor and their gas-sensing mechanism belongs to the surface-controlled type [28]. It has been widely accepted that the current in n-type metal-oxide-semiconductor (MOS) sensor is carried by conduction band electrons, and chemisorbed oxygen species including O2− and O− formed by atmospheric oxygen capture the electron carriers. Upon exposure to reducing chemicals, the arrested electrons are released by the reactions between the reducing gas and O2− or O− , leading to the decrease in resistance [29,30]. In the present work, the solvothermally prepared WONSs were found to be sensitive to various reducing chemicals and gases selected, and also representatively behave as n-type sensor. Based on the above discussion, the electrical responses of the WNOSs sensor are closely related to the density of the chemisorbed oxygen species, which in turn depends on the surface activities of the WNOSs. The calculated specific surface area of the present WONSs is ca. 39.6 cm3 g−1 , which is comparable to that of the Bi2 WO6 nanoplates (around 30) and higher than the reported value for the nano-sized WO3 powders (less than 25 cm3 g−1 ) [31]. The BET nitrogen sorption isotherm curve of the WONSs is shown in Fig. 4d, which exhibits a typical type IV feature. The hysteresis loop at high pressure is associated with the inter-aggregated pores between the aggregated nanosheets. The high specific surface and the existing inter-aggregated pores lead to high amount of chemisorbed oxygen species on the surfaces of the nanosheets. Upon exposing in reducing atmosphere, the surface chemisorbed oxygen such as O2− or O− , can be readily reacted with the reducing vapors or gases, releasing electrons back to the conduction band and the electrical resistance of the WNOSs sensor decreased sensitively. Additionally, the WONSs sensor exhibit high sensitivity, fast response and recovery times to ethanol vapor, ammonia and formaldehyde, but relatively weaker responses to acetone and 93# gasoline vapors, as shown in Fig. 3. The reason may be related to the different reducing powers of the detected objects, or be due to particular features associated with the solvothermally prepared WNOSs that have not yet fully addressed.
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Fig. 4. (a and b) Response curves of the WONSs sensor towards flammable organic vapors and pure gases; (c) sensitivities of the WONSs sensor towards different objects; (d) nitrogen sorption isotherm curve of the WONSs.
From what has been discussed above, it can be concluded that the WONSs sensors exhibited ideal room-temperature sensing activities towards various organic vapors and harmful gases. Here, it should be noted that a top temperature limit may exist in sensor application for the present WONSs because of its low thermal stability. According to the differential temperature analysis (DTA) and thermogravimetirc analysis (TGA), the crystal water of the WONSs would be lost at temperatures ranging from 150 ◦ C to 300 ◦ C, as shown in Fig. 5. Simultaneously, the tungsten oxide hydrate
nanosheets were transformed to tungsten trioxide plates with large size, as previously described [17]. Hence, their gas-sensing properties will be inevitably influenced. Further detailed investigations of the effects of thermal treatment on the gas-sensing properties of the WONSs are in progress. 4. Conclusions In summary, large-scale production of orthorhombic WO3 ·H2 O nanosheets was achieved using a simple sonochemical method with only water as the solvent. The WO3 ·H2 O nanosheets exhibited thickness of about tens of nanometers and edge length of up to several hundreds of nanometers. The formation of the sheetlike morphology was attributed to a combination of the acoustic cavitation effect and the unique crystalline structure of orthorhombic WO3 ·H2 O. The WO3 ·H2 O nanosheets sensor were found to be sensitive to various flammable organic vapors and harmful gases at room temperature, and exhibited maximum sensitivity to ethanol vapor. The present sonochemistry-based method is easilyoperated, environmental friendly and capable of realizing mass production of nanomaterials with low energy consumption and short-term reaction, providing it much potential perspectives in the future nanotechnology areas. Acknowledgement
Fig. 5. Thermal analysis of the WONSs in air: (a) DTA curve and (b) TGA curve.
We wish to express our sincere appreciation to Yidong Zhang in Xuchang University for the gas-sensing measurements.
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