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In situ etching WO3 nanoplates: Hydrothermal synthesis, photoluminescence and gas sensor properties
Materials Research Bulletin 45 (2010) 1960–1963
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
In situ etching WO3 nanoplates: Hydrothermal synthesis, photoluminescence and gas sensor properties Xintai Su a,*, Yani Li a, Jikang Jian b, Jide Wang a a b
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, No. 14 Shengli Road, Urumqi, Xinjiang 830046, China College of Physics Science and Technology, Xinjiang University, Urumqi 830046, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 January 2010 Received in revised form 28 April 2010 Accepted 22 August 2010 Available online 28 September 2010
A novel hydrothermal process using p-nitrobenzoic acid as structure-directing agent has been employed to synthesize plate-shaped WO3 nanostructures containing holes. The p-nitrobenzoic acid plays a critical role in the synthesis of such novel WO3 nanoplates. The morphology, structure and optical property of the WO3 nanoplates have been characterized by transmission electron microcopy (TEM), scanning electron microcopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The lateral size of the nanoplates is 500–1000 nm, and the thickness is about 80 nm. The formation mechanism of WO3 nanoplates is discussed briefly. The gas sensitivity of WO3 nanoplates was studied to ethanol and acetone at different operation temperatures and concentrations. Furthermore, the WO3 nanoplate-based gas sensor exhibits high sensitivity for ethanol and acetone as well as quick response and recovery time at low temperature. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Crystal growth C. X-ray diffraction D. Electrical properties
1. Introduction Gas sensors based on metal-oxide semiconductors may be used in a wide variety of applications including gas monitoring and alarm applications [1,2]. Considerable research has been carried out on the development of chemical sensors based on semiconductor metal oxides such as SnO2 and ZnO [3,4]. WO3, as an n-type semiconductor, has been proven to be a highly sensitive material for the detection of both reducing and oxidizing gases [5,6]. Recently, inspired by the advantages of small size, high density of surface sites and increased surface-to-volume ratios, synthesis of these semiconductor metaloxide nanostructures and exploration of their properties are of current interest [6–8]. Many methodologies have been used to synthesize WO3 nanostructures with different morphologies. Chen et al. have prepared WO3 nanoplates through a topochemical conversion process [9], Shah has synthesized WO3 nanoplates with a freeassistant agent method [10] and Baek et al. have synthesized WO3 nanowires using thermal evaporation method [11]. Zhou et al. have synthesized WO30.33H2O nanodiscs by hydrothermal treatment of an aqueous peroxo-polytungstic acid solution [12]. Among these methods, hydrothermal method is one of the important methods to prepare different WO3 nanostructures, which requires shorter time and lower temperature. Different assisted agents such as surfactant [13], inorganic salt [14], complex
* Corresponding author. Tel.: +86 991 8581018; fax: +86 991 8582807. E-mail address: [email protected] (X. Su). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.08.011
agent [15] and some dissoluble organic acid [16] have been used to synthesize WO3 nanostructures. Recently, we found that tartaric acid or citric acid, as the dissoluble organic acid, can also influence the morphology of tungsten oxide under hydrothermal or microwave-hydrothermal conditions [17,18]. Herein, we report that an insoluble organic acid, p-nitrobenzoic acid, is used in the synthesis of WO3 nanoplates through a hydrothermal method. This novel method is based on treating freshly prepared H2WO4xH2O in the presence of p-nitrobenzoic acid under hydrothermal conditions at 120 8C for 12 h. The products are plate-shaped with a hole in the middle. The resulting nanoplates have been used as gas sensors, which exhibit good performance to ethanol and acetone when operating at low temperatures due to the large surface-to-volume ratio. Moreover, the formation mechanism for the plate-shaped WO3 nanostructures has been primarily discussed. 2. Experimental section 2.1. Sample preparation All of the chemical reagents used in the experiment were of analytical grade. A typical process for the preparation of WO3 nanoplates was as follows: 1 g of Na2WO42H2O and 0.5 g of pnitrobenzoic acid were mixed in distilled water (75 mL). 2.5 mL of 3 M hydrochloric acid aqueous solution was introduced into the aqueous solution under continuous stirring (adjust the pH value to 0.5). After 30 min of stirring, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and heated at
[(Fig._1)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
1961
120 8C for 12 h. The final products were obtained by centrifugation and washed with distilled water and alcohol for several times to remove ions possibly remaining in the final products, and finally dried at 60 8C in air for several hours. 2.2. Characterization The obtained samples were characterized by X-ray diffractometer (XRD) using a Rigaku D/max-ga X-ray diffractometer at a scanning of 28 min1 in 2u ranging from 108 to 808 with Cu Ka radiation (l = 1.54178 A˚). The transmission electronic microscopy (TEM) analysis was conducted on a model Hitachi H-600 with an accelerating voltage of 75 kV. The scanning electron microscopy (SEM) images were obtained on LEO1450VP. Photoluminescence (PL) measurements were performed on a RF-5301pc spectrofluorophotometer using a Xe lamp as the excitation light source (excitation at 250 nm) at room temperature. Gas sensing measurements were carried out on a computer-controlled WS30A system (Zhengzhou, China).
Fig. 1. Powder XRD pattern of the WO3 nanoplates.
2.3. Sensors fabrication The method and instruments of gas-sensor test were similar to the reported literature [6]. The mixture of about 36 mg of the asprepared WO3 particles and 0.1 mL of terpinol was completely ground into paste status. Then the paste was daubed onto the ceramic tube of the sensor body, which was annealed in a muffle stove at 150 8C for 1 h. The as-fabricated sensors were fixed into the gas sensing apparatus and aged at 300 8C for 24 h. The sensorsettled chamber was kept under a continuous flow of dry air for 30 min before analysis. A given amount of alcohol and acetone was injected by a micro-injector on a heating-device part of the apparatus and gasified quickly. 3. Results and discussion
shown in Fig. 2a reveals that the products are a large amount of nanoplates with uniform size. Fig. 2b shows a high magnification SEM image of the WO3 nanoplates, indicating that those nanoplates are monodisperse with lateral sizes in the range of 500–1000 nm. The thickness of the nanoplates is about 80 nm. It is interesting that many nanoplates have a hole in the middle and these nanoplates are incomplete with rough edge regions, which seems to be etched. Fig. 3a and b shows the TEM images of the WO3 nanoplates with different morphologies, which is consistent with the SEM results. Fig. 3a shows that some WO3 nanoplates have a regular square-shaped morphology as indicated by the line-frame. Fig. 3b displays an incomplete nanoplate with a hole in the middle and some defects on the edge, which may help to understand their formation mechanism.
3.1. X-ray powder diffraction analysis 3.3. Growth mechanism analysis X-ray diffraction (XRD) pattern of the as-obtained products is shown in Fig. 1. All the diffraction peaks can be indexed to the pure monoclinic phase of WO3 with lattice constants of a = 6.160 A˚, b = 4.570 A˚ and c = 5.316 A˚, which agrees well with the JCPDS card (PDF No. 54-0508). No peaks of impurities were detected from this pattern under the resolution of the diffractometer. The peaks are strong and narrow, indicating good crystallinity of the sample. 3.2. Morphology analysis Fig. 2a and b presents the SEM images of the WO3 products with different magnifications. Low-magnification SEM image
[(Fig._2)TD$IG]
In our case, the growth mechanism of the incomplete WO3 nanoplates can be explained by a two-step process, the formation of the nanoplates and the etching process. First step, H2WO4xH2O precipitation is obtained when HCl is added to Na2WO4 aqueous solution. Under the high-temperature and high-pressure hydrothermal treatment process, H2WO4xH2O precipitation is dehydrated to form WO3 nuclei in the solution. Solubility of organic compound, which is hardly dissolved in water at ambient temperature and pressure, can be enhanced under hydrothermal conditions [19]. Therefore, p-nitrobenzoic acid solid, as an insoluble compound at ambient temperature and pressure, may be partially or completely dissolved and act as capping agent under the present
Fig. 2. SEM images of the WO3 nanoplates at different magnifications. (a) Low- and (b) high-magnification SEM images.
[(Fig._3)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
1962
Fig. 3. TEM images of the WO3 nanoplates. (a) Complete nanoplates and (b) incomplete nanoplates.
[(Fig._4)TD$IG]
hydrothermal conditions. It is reported that dissoluble organic acid such as citric acid can affect the relative growth rates of different facets to form the WO3 nanoplates [18,20]. Here, the soluble pnitrobenzoic acid may have similar function of citric acid which results in the formation of the WO3 nanoplates. Second step, there exists a dynamic equilibrium between WO3 solid and soluble species of poly tungstate or metatungstate under the hydrothermal condition, as shown with WO3 $ (WOx)n. It is known that the solubility of WO3 is increased with the increase of the pH value of solution. When the reaction system cooled down, the soluble pnitrobenzoic acid will precipitate again and the pH value may increase which in turn causes the nanoplate etched partially, and the incomplete nanoplates are obtained finally. The TEM and SEM results may be explained by this speculation. However, the detailed mechanism of the formation of the incomplete WO3 nanoplates needs to be further explored. 3.4. Photoluminescence properties Fig. 4 shows the PL spectra of WO3 nanoplate samples excited by a Xe lamp with a wavelength of 250 nm. There is a sharp and strong UV emission peak centered at 360 nm (3.4 eV), which could be due to the defect states of WO3 [21,22]. Such strong emission indicates a large quantity of defects in the WO3 samples prepared here, which is consistent with their morphologies and should contribute to their good gas sensing property [22]. 3.5. Gas sensing properties To test the gas selectivity and sensitivity of the products, here, ethanol and acetone are selected and studied, respectively. Fig. 5a and [(Fig._5)TD$IG] b shows the response–recovery curves of the WO3 nanoplates
Fig. 4. PL spectra of the WO3 nanoplates.
to ethanol and acetone with different concentrations. We can see that the WO3 nanoplates have good reversibility when they are exposed to ethanol or acetone, and the measurement voltage levels up quickly after exposured to ethanol or acetone. The WO3 nanoplate sensors have good response to the alcohol and acetone gases even at low concentration of 10 ppm. The response– recovery curves of WO3 nanoplates to ethanol and acetone at different temperatures with a concentration of 1000 ppm are shown in Fig. 6a and b, which reveals that the sensitivity of the sensors is greatly enhanced with the increase of temperature. When ethanol or acetone vapor is injected into or removed from the chamber, the resistance of the sensors is quickly decreased or
Fig. 5. The response–recovery curves of the WO3 nanoplates to ethanol at 340 8C (a) and acetone at 370 8C (b) with different concentrations.
[(Fig._6)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
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Fig. 6. The response and recovery curves of the WO3 nanoplates sensor at an ethanol concentration (a) and an acetone concentration (b) of 1000 ppm.
increased. Li et al. and Chen et al. also reported the similar sensing properties [6,23]. The response time and recovery time (defined as the time required to reaching 90% of the final equilibrium value) are very short. Such a result indicates that the response speed of the fabricated sensors to ethanol and acetone is also good. 4. Conclusions In summary, an in situ etching method was used to prepare tungsten oxide nanoplates by a hydrothermal method using pnitrobenzoic acid as a structure modifier. The lateral sizes of the nanoplates range from 500 to 1000 nm and the thickness is about 80 nm. The edge and the middle of some nanoplates are defective and we propose an in situ etching mechanism for the formation of the nanostructure. The as-synthesized WO3 nanoplates exhibit UV emission at 360 nm. The WO3 nanoplates are sensitive to ethanol and acetone when operated at low temperature and concentration, thus presenting very important features for practical use. Acknowledgements We appreciate the financial supports of Key Scientific Project of Xinjiang Province (No. 200732139) and Doctoral Foundation of Xinjiang University (No. BS080115).
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
D.E. Williams, Sens. Actuators B 57 (1999) 1. K. Potje-Kamloth, Chem. Rev. 108 (2008) 367. Y.L. Wang, X.C. Jiang, Y.N. Xia, J. Am. Chem. Soc. 125 (2003) 16176. Z.P. Sun, L. Liu, L. Zhang, D.Z. Jia, Nanotechnology 17 (2006) 2266. X.S. Wang, N. Miura, N. Yamazoe, Sens. Actuators B 66 (2000) 74. X.L. Li, T.J. Lou, X.M. Sun, Y.D. Li, Inorg. Chem. 43 (2004) 5442. C. Bala´zsi, L.S. Wang, E.O. Zayim, I.M. Szila´gyi, K. Sedlackova´, J. Pfeifer, A.L. To´th, P.I. Gouma, J. Eur. Ceram. Soc. 28 (2008) 913. J. Tamaki, T. Hashishin, Y. Uno, D.V. Dao, S. Sugiyama, Sens. Actuators B 132 (2008) 234. D.L. Chen, L. Gao, A. Yasumori, K. Kuroda, Y. Sugahara, Small 4 (2008) 1813. M.A. Shah, Afr. Phys. Rev. 3 (2009) 57. Y. Baek, K. Yong, J. Phys. Chem. C 111 (2007) 1213. L. Zhou, J. Zou, M.M. Yu, P. Lu, J. Wei, Y.Q. Qian, Y.H. Wang, C.Z. Yu, Cryst. Growth Des. 8 (2008) 3993. X.C. Song, Y. Zhao, Y.F. Zheng, Mater. Lett. 60 (2006) 3405. X.C. Song, Y.F. Zheng, E. Yang, Y. Wang, Mater. Lett. 61 (2007) 3904. J.H. Ha, P. Muralidharan, D.K. Kim, J. Alloys Compd. 475 (2009) 446. Z.D. Xiao, L.D. Zhang, Z.Y. Wang, Q.F. Lu, X.K. Tian, H.B. Zeng, Mater. Lett. 61 (2007) 1718. Q.J. Sun, J.M. Luo, Z.F. Xie, J.D. Wang, X.T. Su, Mater. Lett. 62 (2008) 2992. X.T. Su, F. Xiao, Y.N. Li, J.K. Jian, Q.J. Sun, J.D. Wang, Mater. Lett. 64 (2010) 1232. E.W. Shi, C.T. Xia, B.G. Wang, W.Z. Zhong, J. Inorg. Mater. 11 (1996) 193. Y.M. Zhao, W.B. Hu, Y.D. Xia, E.F. Smith, Y.Q. Zhu, C.W. Dunnill, D.H. Gregory, J. Mater. Chem. 17 (2007) 4436. B. Yang, Y. Zhang, E. Drabarek, P.R.F. Barnes, V. Luca, Chem. Mater. 19 (2007) 5664. J.M. Wang, P.S. Lee, J. Ma, J. Cryst. Growth 311 (2009) 316. D.L. Chen, X.X. Hou, H.J. Wen, Y. Wang, H.L. Wang, X.J. Li, R. Zhang, H.X. Lu, H.L. Xu, S.K. Guan, J. Sun, L. Gao, Nanotechnology 21 (2010) 1.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
In situ etching WO3 nanoplates: Hydrothermal synthesis, photoluminescence and gas sensor properties Xintai Su a,*, Yani Li a, Jikang Jian b, Jide Wang a a b
Ministry Key Laboratory of Oil and Gas Fine Chemicals, College of Chemistry and Chemical Engineering, Xinjiang University, No. 14 Shengli Road, Urumqi, Xinjiang 830046, China College of Physics Science and Technology, Xinjiang University, Urumqi 830046, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 17 January 2010 Received in revised form 28 April 2010 Accepted 22 August 2010 Available online 28 September 2010
A novel hydrothermal process using p-nitrobenzoic acid as structure-directing agent has been employed to synthesize plate-shaped WO3 nanostructures containing holes. The p-nitrobenzoic acid plays a critical role in the synthesis of such novel WO3 nanoplates. The morphology, structure and optical property of the WO3 nanoplates have been characterized by transmission electron microcopy (TEM), scanning electron microcopy (SEM), X-ray diffraction (XRD) and photoluminescence (PL). The lateral size of the nanoplates is 500–1000 nm, and the thickness is about 80 nm. The formation mechanism of WO3 nanoplates is discussed briefly. The gas sensitivity of WO3 nanoplates was studied to ethanol and acetone at different operation temperatures and concentrations. Furthermore, the WO3 nanoplate-based gas sensor exhibits high sensitivity for ethanol and acetone as well as quick response and recovery time at low temperature. ß 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Oxides B. Crystal growth C. X-ray diffraction D. Electrical properties
1. Introduction Gas sensors based on metal-oxide semiconductors may be used in a wide variety of applications including gas monitoring and alarm applications [1,2]. Considerable research has been carried out on the development of chemical sensors based on semiconductor metal oxides such as SnO2 and ZnO [3,4]. WO3, as an n-type semiconductor, has been proven to be a highly sensitive material for the detection of both reducing and oxidizing gases [5,6]. Recently, inspired by the advantages of small size, high density of surface sites and increased surface-to-volume ratios, synthesis of these semiconductor metaloxide nanostructures and exploration of their properties are of current interest [6–8]. Many methodologies have been used to synthesize WO3 nanostructures with different morphologies. Chen et al. have prepared WO3 nanoplates through a topochemical conversion process [9], Shah has synthesized WO3 nanoplates with a freeassistant agent method [10] and Baek et al. have synthesized WO3 nanowires using thermal evaporation method [11]. Zhou et al. have synthesized WO30.33H2O nanodiscs by hydrothermal treatment of an aqueous peroxo-polytungstic acid solution [12]. Among these methods, hydrothermal method is one of the important methods to prepare different WO3 nanostructures, which requires shorter time and lower temperature. Different assisted agents such as surfactant [13], inorganic salt [14], complex
* Corresponding author. Tel.: +86 991 8581018; fax: +86 991 8582807. E-mail address: [email protected] (X. Su). 0025-5408/$ – see front matter ß 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2010.08.011
agent [15] and some dissoluble organic acid [16] have been used to synthesize WO3 nanostructures. Recently, we found that tartaric acid or citric acid, as the dissoluble organic acid, can also influence the morphology of tungsten oxide under hydrothermal or microwave-hydrothermal conditions [17,18]. Herein, we report that an insoluble organic acid, p-nitrobenzoic acid, is used in the synthesis of WO3 nanoplates through a hydrothermal method. This novel method is based on treating freshly prepared H2WO4xH2O in the presence of p-nitrobenzoic acid under hydrothermal conditions at 120 8C for 12 h. The products are plate-shaped with a hole in the middle. The resulting nanoplates have been used as gas sensors, which exhibit good performance to ethanol and acetone when operating at low temperatures due to the large surface-to-volume ratio. Moreover, the formation mechanism for the plate-shaped WO3 nanostructures has been primarily discussed. 2. Experimental section 2.1. Sample preparation All of the chemical reagents used in the experiment were of analytical grade. A typical process for the preparation of WO3 nanoplates was as follows: 1 g of Na2WO42H2O and 0.5 g of pnitrobenzoic acid were mixed in distilled water (75 mL). 2.5 mL of 3 M hydrochloric acid aqueous solution was introduced into the aqueous solution under continuous stirring (adjust the pH value to 0.5). After 30 min of stirring, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave, sealed and heated at
[(Fig._1)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
1961
120 8C for 12 h. The final products were obtained by centrifugation and washed with distilled water and alcohol for several times to remove ions possibly remaining in the final products, and finally dried at 60 8C in air for several hours. 2.2. Characterization The obtained samples were characterized by X-ray diffractometer (XRD) using a Rigaku D/max-ga X-ray diffractometer at a scanning of 28 min1 in 2u ranging from 108 to 808 with Cu Ka radiation (l = 1.54178 A˚). The transmission electronic microscopy (TEM) analysis was conducted on a model Hitachi H-600 with an accelerating voltage of 75 kV. The scanning electron microscopy (SEM) images were obtained on LEO1450VP. Photoluminescence (PL) measurements were performed on a RF-5301pc spectrofluorophotometer using a Xe lamp as the excitation light source (excitation at 250 nm) at room temperature. Gas sensing measurements were carried out on a computer-controlled WS30A system (Zhengzhou, China).
Fig. 1. Powder XRD pattern of the WO3 nanoplates.
2.3. Sensors fabrication The method and instruments of gas-sensor test were similar to the reported literature [6]. The mixture of about 36 mg of the asprepared WO3 particles and 0.1 mL of terpinol was completely ground into paste status. Then the paste was daubed onto the ceramic tube of the sensor body, which was annealed in a muffle stove at 150 8C for 1 h. The as-fabricated sensors were fixed into the gas sensing apparatus and aged at 300 8C for 24 h. The sensorsettled chamber was kept under a continuous flow of dry air for 30 min before analysis. A given amount of alcohol and acetone was injected by a micro-injector on a heating-device part of the apparatus and gasified quickly. 3. Results and discussion
shown in Fig. 2a reveals that the products are a large amount of nanoplates with uniform size. Fig. 2b shows a high magnification SEM image of the WO3 nanoplates, indicating that those nanoplates are monodisperse with lateral sizes in the range of 500–1000 nm. The thickness of the nanoplates is about 80 nm. It is interesting that many nanoplates have a hole in the middle and these nanoplates are incomplete with rough edge regions, which seems to be etched. Fig. 3a and b shows the TEM images of the WO3 nanoplates with different morphologies, which is consistent with the SEM results. Fig. 3a shows that some WO3 nanoplates have a regular square-shaped morphology as indicated by the line-frame. Fig. 3b displays an incomplete nanoplate with a hole in the middle and some defects on the edge, which may help to understand their formation mechanism.
3.1. X-ray powder diffraction analysis 3.3. Growth mechanism analysis X-ray diffraction (XRD) pattern of the as-obtained products is shown in Fig. 1. All the diffraction peaks can be indexed to the pure monoclinic phase of WO3 with lattice constants of a = 6.160 A˚, b = 4.570 A˚ and c = 5.316 A˚, which agrees well with the JCPDS card (PDF No. 54-0508). No peaks of impurities were detected from this pattern under the resolution of the diffractometer. The peaks are strong and narrow, indicating good crystallinity of the sample. 3.2. Morphology analysis Fig. 2a and b presents the SEM images of the WO3 products with different magnifications. Low-magnification SEM image
[(Fig._2)TD$IG]
In our case, the growth mechanism of the incomplete WO3 nanoplates can be explained by a two-step process, the formation of the nanoplates and the etching process. First step, H2WO4xH2O precipitation is obtained when HCl is added to Na2WO4 aqueous solution. Under the high-temperature and high-pressure hydrothermal treatment process, H2WO4xH2O precipitation is dehydrated to form WO3 nuclei in the solution. Solubility of organic compound, which is hardly dissolved in water at ambient temperature and pressure, can be enhanced under hydrothermal conditions [19]. Therefore, p-nitrobenzoic acid solid, as an insoluble compound at ambient temperature and pressure, may be partially or completely dissolved and act as capping agent under the present
Fig. 2. SEM images of the WO3 nanoplates at different magnifications. (a) Low- and (b) high-magnification SEM images.
[(Fig._3)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
1962
Fig. 3. TEM images of the WO3 nanoplates. (a) Complete nanoplates and (b) incomplete nanoplates.
[(Fig._4)TD$IG]
hydrothermal conditions. It is reported that dissoluble organic acid such as citric acid can affect the relative growth rates of different facets to form the WO3 nanoplates [18,20]. Here, the soluble pnitrobenzoic acid may have similar function of citric acid which results in the formation of the WO3 nanoplates. Second step, there exists a dynamic equilibrium between WO3 solid and soluble species of poly tungstate or metatungstate under the hydrothermal condition, as shown with WO3 $ (WOx)n. It is known that the solubility of WO3 is increased with the increase of the pH value of solution. When the reaction system cooled down, the soluble pnitrobenzoic acid will precipitate again and the pH value may increase which in turn causes the nanoplate etched partially, and the incomplete nanoplates are obtained finally. The TEM and SEM results may be explained by this speculation. However, the detailed mechanism of the formation of the incomplete WO3 nanoplates needs to be further explored. 3.4. Photoluminescence properties Fig. 4 shows the PL spectra of WO3 nanoplate samples excited by a Xe lamp with a wavelength of 250 nm. There is a sharp and strong UV emission peak centered at 360 nm (3.4 eV), which could be due to the defect states of WO3 [21,22]. Such strong emission indicates a large quantity of defects in the WO3 samples prepared here, which is consistent with their morphologies and should contribute to their good gas sensing property [22]. 3.5. Gas sensing properties To test the gas selectivity and sensitivity of the products, here, ethanol and acetone are selected and studied, respectively. Fig. 5a and [(Fig._5)TD$IG] b shows the response–recovery curves of the WO3 nanoplates
Fig. 4. PL spectra of the WO3 nanoplates.
to ethanol and acetone with different concentrations. We can see that the WO3 nanoplates have good reversibility when they are exposed to ethanol or acetone, and the measurement voltage levels up quickly after exposured to ethanol or acetone. The WO3 nanoplate sensors have good response to the alcohol and acetone gases even at low concentration of 10 ppm. The response– recovery curves of WO3 nanoplates to ethanol and acetone at different temperatures with a concentration of 1000 ppm are shown in Fig. 6a and b, which reveals that the sensitivity of the sensors is greatly enhanced with the increase of temperature. When ethanol or acetone vapor is injected into or removed from the chamber, the resistance of the sensors is quickly decreased or
Fig. 5. The response–recovery curves of the WO3 nanoplates to ethanol at 340 8C (a) and acetone at 370 8C (b) with different concentrations.
[(Fig._6)TD$IG]
X. Su et al. / Materials Research Bulletin 45 (2010) 1960–1963
1963
Fig. 6. The response and recovery curves of the WO3 nanoplates sensor at an ethanol concentration (a) and an acetone concentration (b) of 1000 ppm.
increased. Li et al. and Chen et al. also reported the similar sensing properties [6,23]. The response time and recovery time (defined as the time required to reaching 90% of the final equilibrium value) are very short. Such a result indicates that the response speed of the fabricated sensors to ethanol and acetone is also good. 4. Conclusions In summary, an in situ etching method was used to prepare tungsten oxide nanoplates by a hydrothermal method using pnitrobenzoic acid as a structure modifier. The lateral sizes of the nanoplates range from 500 to 1000 nm and the thickness is about 80 nm. The edge and the middle of some nanoplates are defective and we propose an in situ etching mechanism for the formation of the nanostructure. The as-synthesized WO3 nanoplates exhibit UV emission at 360 nm. The WO3 nanoplates are sensitive to ethanol and acetone when operated at low temperature and concentration, thus presenting very important features for practical use. Acknowledgements We appreciate the financial supports of Key Scientific Project of Xinjiang Province (No. 200732139) and Doctoral Foundation of Xinjiang University (No. BS080115).
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