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Hierarchical WO3·H2O porous microsphere: Hydrothermal synthesis, structure and gas-sensing performance
Materials Letters 186 (2017) 119–122
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hierarchical WO3·H2O porous microsphere: Hydrothermal synthesis, structure and gas-sensing performance
crossmark
⁎
Yangchun Yu, Wen Zeng , Zhi Zhang, Yuxin Cai, He Zhang College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Microstructure Chemical synthesis Functional
In this work, hierarchical WO3·H2O porous microspheres and conventional smooth WO3·H2O microspheres were successfully fabricated via a simple hydrothermal process. The products were characterized by X-ray diffraction and scanning electron microscope. Meanwhile, the possible evolution processes of the two products were discussed based on time-dependent experiments. Remarkably, hierarchical porous spherical architectures which possess abundant micro reaction rooms, exhibited improved gas-sensing performance compared to smooth sphere-like structures. It is believed that this unique nanostructure has great potential to enhance the gas-sensing properties of tungsten oxides and other semiconductor materials.
1. Introduction Recently, tungsten oxides, as a significant n-type semiconductor, has been extensively studied for its practical application in various areas such as gas sensors [1], electrochromic [2], photocatalysts [3], electrocatalyst [4], energy devices [5] and so forth. Among all of its applications, considerable interests has been exerted to the gas sensors based on nanostructured tungsten oxides materials, owing to its fast response, good stability, high sensitivity, and low cost [6]. To date, several noteworthy researches have already revealed that the gassensing properties of WO3·H2O materials are often affected by its microscopic morphology [7–9], which suggest that unique nanostructure possesses great potential to improve the gas sensing performance of nanomaterials. Notably, three-dimensional spherical porous structure proves to be an effective strategy to improve the gas-sensing performance of nanostructured metal oxides. In such structure, the porous architecture would provide larger specific surface areas, abundant accessible channels and prolong residence time of gas molecules, which leading to the adequate chemical reactions and high gas-sensing performance. Inspired by this novel idea, a variety of hierarchical porous microsphere of different metal oxides have been fabricated successfully, including ZnO [10], NiO [11], SnO2 [12] and so on. However, to best of our knowledge, while tungsten oxides is an important gas-sensing materials, the research about pure WO3 porous microsphere and its gas-sensing properties is very lacking, especially in template-free hydrothermal synthesis of porous tungsten oxides microspheres. Namely, preparing hierarchical WO3 microsphere with porous struc⁎
ture will be a meaningful work to explore the gas-sensing properties of tungsten oxides. Herein, in this letter, hierarchical WO3·H2O porous microspheres were prepared via a simple template-free hydrothermal process. In order to reflected the improvement in gas-sensing performance, smooth WO3·H2O nanospheres with the similar size were also synthesized and the gas sensing experiment were carried out. The test results indicate that WO3·H2O porous microspheres exhibit a superior gassensing performance than that of the smooth one, which further demonstrated the gas sensing performance of nanomaterials are closely related to their novel microstructure. Meanwhile, the possible growth process and the mechanism involved in improving gas-sensing properties were also discussed. 2. Experimental The porous WO3·H2O microspheres were synthesized by a hydrothermal process as follows. First, 0.329 g Na2WO4·2H2O was dissolved into 20 ml deionized water. Then, 25 ml of acetic acid and some drops of HCl were added into the above solution under vigorous stirring for 20 min. After that, the suspension was transferred into a 50 ml Teflonlined stainless steel autoclave and maintained at 120 °C for different time (4 h, 8 h, and 12 h). After cooling the autoclave to room temperature, the precipitates were separated by filtration and washed with deionized water and alcohol for several times, then dried in vacuum at 70 °C for 6 h. Smooth WO3·H2O nanospheres were prepared in the same procedure, while the only difference is the 25 ml acetic acid were replaced by 0.1 g trisodium citrate and 13 ml ethanol.
Corresponding author. E-mail address: [email protected] (W. Zeng).
http://dx.doi.org/10.1016/j.matlet.2016.09.106 Received 26 July 2016; Received in revised form 21 September 2016; Accepted 25 September 2016 Available online 26 September 2016 0167-577X/ © 2016 Elsevier B.V. All rights reserved.
Materials Letters 186 (2017) 119–122
Y. Yu et al.
Fig. 1. (a) XRD patterns of the products: (ⅰ) porous microspheres, (ⅱ) smooth microspheres; (b) SEM image of porous spherical architectures; (c) low – and (d) high-magnification images; (e) SEM image of smooth microspheres; (f) low – and (g) high-magnification images.
Fig. 2. SEM images of porous WO3·H2O microspheres prepared at (a) 4 h, (b) 8 h, (c) 12 h, and smooth WO3·H2O nanospheres obtained at (d) 4 h, (e) 8 h, (f) 12 h.
Fig. 3. (a) Gas response of the two samples to the 100 ppm ethanol at temperatures ranging from 100 °C to 450 °C. (b) Response-recovery curves of two sensors to ethanol gases with different concentrations at 350 °C.
characterized by a field emission scanning electron microscope, FESEM (Nova 400 Nano operate at 10.0 kV). The structure of gas sensors and gas-sensing measurements are similar to our previous
The X-ray diffraction spectra were recorded using a Rigaku D/Max1200X diffractometer with the CuKα radiation (30 kV, 100 mA) in a 2θ range from 10° to 70°. The structures and morphologies were 120
Materials Letters 186 (2017) 119–122
Y. Yu et al.
Fig. 4. Schematic illustrations of gas sensing mechanism of (a) hierarchical porous WO3·H2O spherical architectures and (b) conventional smooth WO3·H2O sphere-like structures.
concentration, the sensors both exhibit a good reversibility. Also, the gas responses of the two products keep a constant growing with the increasing ethanol concentration from 100 ppm to 400 ppm and the porous microsphere always has a better performance than the smooth one, which prove again this porous structure indeed improve the gassensing performance of WO3·H2O nanomaterials. In order to explain the improved gas-sensing properties of hierarchical WO3·H2O porous microspheres, a plausible mechanisms were proposed on the basis of classical chemical adsorption theory [13]. According to the above SEM observation, one can find that the main difference of the two products is one has porous structure while the other only has smooth surface. And we believe this is a dominate reason for the different gas sensing behaviors. As illustrated Fig. 4(a), the hierarchical porous spherical architectures possess a vast number of micro reaction rooms [14], which provide a larger effective surface area and increase the residence time of oxygen and ethanol molecules. Besides, this novel structure provides abundant open channels for gas diffusion. In stark contrast, the smooth spheres only have flat smooth surface, as displayed in Fig. 4(b), resulting in insufficient gas-sensing reaction. Consequently, hierarchical porous microsphere exhibited enhanced gas sensing behaviors.
work [9]. The gas response of the sensor in this paper was defined as Rg/Ra, where Rg and Ra are the resistances of the sensor in target gas and air, respectively. 3. Results and discussion Fig. 1(a) shows the XRD patterns of porous WO3·H2O sphere-like structure and Smooth WO3·H2O nanosphere. As one can see, the diffraction peaks are all in agreement with the standard data file (JCPDS Card no. 84-0886, a=3.471 Å, b=5.355 Å, c=2.559 Å), which indicates the samples must be WO3·H2O. No impure peaks can be found in XRD patterns, demonstrating the as-prepared products were of high purity. The micro-morphologies of the two samples were characterized by SEM observations. As illustrated in Fig. 1(b) and (e), all the products show sphere-like structures with the similar diameter ranged from 1 µm to 2 µm. Higher magnification SEM images of single hierarchical WO3·H2O porous microsphere and smooth WO3·H2O microsphere are displayed in Fig. 1(c) and (d) and Fig. 1(f) and (g), respectively. As it can be seen, porous microsphere seems to be the hierarchical structure assembled by a number of nanosheets. And these nanosheets are interweaved together, resulting the formation of numerous micro reaction rooms. The surface of the porous sphere is coarse and loose, which in contrast to the smooth surface of conventional WO3·H2O microspheres. In order to gain insight into the growth process of the two products, SEM investigations which focus on change of morphology in the two samples were conducted as a function of hydrothermal time. As shown in Fig. 2(a) and (d), the two products prepared at 4 h both failed to form regular shape while the first sample had already showed the porous structure. When the time was increased to 8 h, the spherical architectures had emerged gradually. (Fig. 2(b) and (e)) Further increasing the hydrothermal time to 12 h, as presented in Fig. 2(c) and (f), the products grew well and became uniform porous sphere-like structure and smooth nanosphere, respectively. In particular, the pores of porous nanosphere tend to become smaller and denser compared to its previous stage. Fig. 3(a) illustrates the gas response of the two samples to 100 ppm ethanol at various working temperature ranging from 100 °C to 450 °C. One observes that 350 °C is the optimum operating temperature for the two sensors under these conditions. Also, the hierarchical porous microsphere always have higher gas response than the smooth nanospheres, evidencing that WO3·H2O porous microspheres perform better in gas sensing compared to the smooth nanospheres. Fig. 3(b) shows the response and recovery characteristics for the two sensors under the different ethanol concentration at 350 °C. For each gas
4. Conclusions In summary, hierarchical WO3·H2O porous microsphere have been successfully synthesized by a one-step hydrothermal method. Gassensing measurements reveal that WO3·H2O porous spherical architectures exhibit an enhanced gas sensing behaviors to ethanol compared to the smooth WO3·H2O nanospheres. Meanwhile, we have proposed a possible mechanism to explain the gas sensing improvement of this porous microspheres. It is believed that sphere-like porous structure provide numerous micro reaction rooms and larger effective surface areas, which endow the sensors a superior gas-sensing properties, holding great promising for manufacturing high-performance gas sensors. Acknowledgements This work was supported in part by National Natural Science Foundation of China (No. 11332013) and Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006). References [1] S. Cao, C. Zhao, T. Han, L. Peng, Mater. Lett. 169 (2016) 17–20.
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Materials Letters 186 (2017) 119–122
Y. Yu et al.
[8] Y. Shen, X. Chen, W. Wang, Y. Gong, S. Chen, et al., Mater. Lett. 163 (2016) 150–153. [9] Y. Yu, W. Zeng, L. Yu, S. Wu, Mater. Lett. 180 (2016) 51–54. [10] F. Fan, P. Tang, Y. Wang, Y. Feng, A. Chen, Sens. Actuators B: Chem. 215 (2015) 231–240. [11] K. Tian, X. Wang, H. Li, et al., Sens. Actuators B: Chem. 227 (2016) 554–560. [12] B. Zhang, W. Fu, H. Li, X. Fu, Y. Wang, Hari Bala, et al., Appl. Surf. Sci. 363 (2016) 560–565. [13] D. Chen, X. Hou, T. Li, Li Yin, Sens. Actuators B: Chem. 153 (2011) 373–381. [14] Yangchun Yu, Wen Zeng, He Zhang, Mater. Lett. 171 (2016) 162–165.
[2] H. Li, J. Wang, Q. Shi, M. Zhang, C. Hou, G. Shi, et al., Appl. Surf. Sci. 380 (2016) 281–287. [3] R. Abazari, A.R. Mahjoub, L.A. Saghatfotoush, S. Aanati, Mater. Lett. 133 (2014) 208–211. [4] A. Phuruangrat, D.J. Ham, S.J. Hong, S. Thongtem, et al., J. Mater. Chem. 20 (2010) 1683–1690. [5] L. Gao, X. Wang, Z. Xie, W. Song, L. Wang, et al., J. Mater. Chem. A 1 (2013) 7167–7173. [6] F. Li, C. Li, L. Zhu, W. Guo, L. Shen, et al., Sens. Actuators B: Chem. 223 (2016) 761–767. [7] Wen Zeng, He Zhang, Zhongchang Wang, Appl. Surf. Sci. 347 (2015) 73–78.
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Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Hierarchical WO3·H2O porous microsphere: Hydrothermal synthesis, structure and gas-sensing performance
crossmark
⁎
Yangchun Yu, Wen Zeng , Zhi Zhang, Yuxin Cai, He Zhang College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Microstructure Chemical synthesis Functional
In this work, hierarchical WO3·H2O porous microspheres and conventional smooth WO3·H2O microspheres were successfully fabricated via a simple hydrothermal process. The products were characterized by X-ray diffraction and scanning electron microscope. Meanwhile, the possible evolution processes of the two products were discussed based on time-dependent experiments. Remarkably, hierarchical porous spherical architectures which possess abundant micro reaction rooms, exhibited improved gas-sensing performance compared to smooth sphere-like structures. It is believed that this unique nanostructure has great potential to enhance the gas-sensing properties of tungsten oxides and other semiconductor materials.
1. Introduction Recently, tungsten oxides, as a significant n-type semiconductor, has been extensively studied for its practical application in various areas such as gas sensors [1], electrochromic [2], photocatalysts [3], electrocatalyst [4], energy devices [5] and so forth. Among all of its applications, considerable interests has been exerted to the gas sensors based on nanostructured tungsten oxides materials, owing to its fast response, good stability, high sensitivity, and low cost [6]. To date, several noteworthy researches have already revealed that the gassensing properties of WO3·H2O materials are often affected by its microscopic morphology [7–9], which suggest that unique nanostructure possesses great potential to improve the gas sensing performance of nanomaterials. Notably, three-dimensional spherical porous structure proves to be an effective strategy to improve the gas-sensing performance of nanostructured metal oxides. In such structure, the porous architecture would provide larger specific surface areas, abundant accessible channels and prolong residence time of gas molecules, which leading to the adequate chemical reactions and high gas-sensing performance. Inspired by this novel idea, a variety of hierarchical porous microsphere of different metal oxides have been fabricated successfully, including ZnO [10], NiO [11], SnO2 [12] and so on. However, to best of our knowledge, while tungsten oxides is an important gas-sensing materials, the research about pure WO3 porous microsphere and its gas-sensing properties is very lacking, especially in template-free hydrothermal synthesis of porous tungsten oxides microspheres. Namely, preparing hierarchical WO3 microsphere with porous struc⁎
ture will be a meaningful work to explore the gas-sensing properties of tungsten oxides. Herein, in this letter, hierarchical WO3·H2O porous microspheres were prepared via a simple template-free hydrothermal process. In order to reflected the improvement in gas-sensing performance, smooth WO3·H2O nanospheres with the similar size were also synthesized and the gas sensing experiment were carried out. The test results indicate that WO3·H2O porous microspheres exhibit a superior gassensing performance than that of the smooth one, which further demonstrated the gas sensing performance of nanomaterials are closely related to their novel microstructure. Meanwhile, the possible growth process and the mechanism involved in improving gas-sensing properties were also discussed. 2. Experimental The porous WO3·H2O microspheres were synthesized by a hydrothermal process as follows. First, 0.329 g Na2WO4·2H2O was dissolved into 20 ml deionized water. Then, 25 ml of acetic acid and some drops of HCl were added into the above solution under vigorous stirring for 20 min. After that, the suspension was transferred into a 50 ml Teflonlined stainless steel autoclave and maintained at 120 °C for different time (4 h, 8 h, and 12 h). After cooling the autoclave to room temperature, the precipitates were separated by filtration and washed with deionized water and alcohol for several times, then dried in vacuum at 70 °C for 6 h. Smooth WO3·H2O nanospheres were prepared in the same procedure, while the only difference is the 25 ml acetic acid were replaced by 0.1 g trisodium citrate and 13 ml ethanol.
Corresponding author. E-mail address: [email protected] (W. Zeng).
http://dx.doi.org/10.1016/j.matlet.2016.09.106 Received 26 July 2016; Received in revised form 21 September 2016; Accepted 25 September 2016 Available online 26 September 2016 0167-577X/ © 2016 Elsevier B.V. All rights reserved.
Materials Letters 186 (2017) 119–122
Y. Yu et al.
Fig. 1. (a) XRD patterns of the products: (ⅰ) porous microspheres, (ⅱ) smooth microspheres; (b) SEM image of porous spherical architectures; (c) low – and (d) high-magnification images; (e) SEM image of smooth microspheres; (f) low – and (g) high-magnification images.
Fig. 2. SEM images of porous WO3·H2O microspheres prepared at (a) 4 h, (b) 8 h, (c) 12 h, and smooth WO3·H2O nanospheres obtained at (d) 4 h, (e) 8 h, (f) 12 h.
Fig. 3. (a) Gas response of the two samples to the 100 ppm ethanol at temperatures ranging from 100 °C to 450 °C. (b) Response-recovery curves of two sensors to ethanol gases with different concentrations at 350 °C.
characterized by a field emission scanning electron microscope, FESEM (Nova 400 Nano operate at 10.0 kV). The structure of gas sensors and gas-sensing measurements are similar to our previous
The X-ray diffraction spectra were recorded using a Rigaku D/Max1200X diffractometer with the CuKα radiation (30 kV, 100 mA) in a 2θ range from 10° to 70°. The structures and morphologies were 120
Materials Letters 186 (2017) 119–122
Y. Yu et al.
Fig. 4. Schematic illustrations of gas sensing mechanism of (a) hierarchical porous WO3·H2O spherical architectures and (b) conventional smooth WO3·H2O sphere-like structures.
concentration, the sensors both exhibit a good reversibility. Also, the gas responses of the two products keep a constant growing with the increasing ethanol concentration from 100 ppm to 400 ppm and the porous microsphere always has a better performance than the smooth one, which prove again this porous structure indeed improve the gassensing performance of WO3·H2O nanomaterials. In order to explain the improved gas-sensing properties of hierarchical WO3·H2O porous microspheres, a plausible mechanisms were proposed on the basis of classical chemical adsorption theory [13]. According to the above SEM observation, one can find that the main difference of the two products is one has porous structure while the other only has smooth surface. And we believe this is a dominate reason for the different gas sensing behaviors. As illustrated Fig. 4(a), the hierarchical porous spherical architectures possess a vast number of micro reaction rooms [14], which provide a larger effective surface area and increase the residence time of oxygen and ethanol molecules. Besides, this novel structure provides abundant open channels for gas diffusion. In stark contrast, the smooth spheres only have flat smooth surface, as displayed in Fig. 4(b), resulting in insufficient gas-sensing reaction. Consequently, hierarchical porous microsphere exhibited enhanced gas sensing behaviors.
work [9]. The gas response of the sensor in this paper was defined as Rg/Ra, where Rg and Ra are the resistances of the sensor in target gas and air, respectively. 3. Results and discussion Fig. 1(a) shows the XRD patterns of porous WO3·H2O sphere-like structure and Smooth WO3·H2O nanosphere. As one can see, the diffraction peaks are all in agreement with the standard data file (JCPDS Card no. 84-0886, a=3.471 Å, b=5.355 Å, c=2.559 Å), which indicates the samples must be WO3·H2O. No impure peaks can be found in XRD patterns, demonstrating the as-prepared products were of high purity. The micro-morphologies of the two samples were characterized by SEM observations. As illustrated in Fig. 1(b) and (e), all the products show sphere-like structures with the similar diameter ranged from 1 µm to 2 µm. Higher magnification SEM images of single hierarchical WO3·H2O porous microsphere and smooth WO3·H2O microsphere are displayed in Fig. 1(c) and (d) and Fig. 1(f) and (g), respectively. As it can be seen, porous microsphere seems to be the hierarchical structure assembled by a number of nanosheets. And these nanosheets are interweaved together, resulting the formation of numerous micro reaction rooms. The surface of the porous sphere is coarse and loose, which in contrast to the smooth surface of conventional WO3·H2O microspheres. In order to gain insight into the growth process of the two products, SEM investigations which focus on change of morphology in the two samples were conducted as a function of hydrothermal time. As shown in Fig. 2(a) and (d), the two products prepared at 4 h both failed to form regular shape while the first sample had already showed the porous structure. When the time was increased to 8 h, the spherical architectures had emerged gradually. (Fig. 2(b) and (e)) Further increasing the hydrothermal time to 12 h, as presented in Fig. 2(c) and (f), the products grew well and became uniform porous sphere-like structure and smooth nanosphere, respectively. In particular, the pores of porous nanosphere tend to become smaller and denser compared to its previous stage. Fig. 3(a) illustrates the gas response of the two samples to 100 ppm ethanol at various working temperature ranging from 100 °C to 450 °C. One observes that 350 °C is the optimum operating temperature for the two sensors under these conditions. Also, the hierarchical porous microsphere always have higher gas response than the smooth nanospheres, evidencing that WO3·H2O porous microspheres perform better in gas sensing compared to the smooth nanospheres. Fig. 3(b) shows the response and recovery characteristics for the two sensors under the different ethanol concentration at 350 °C. For each gas
4. Conclusions In summary, hierarchical WO3·H2O porous microsphere have been successfully synthesized by a one-step hydrothermal method. Gassensing measurements reveal that WO3·H2O porous spherical architectures exhibit an enhanced gas sensing behaviors to ethanol compared to the smooth WO3·H2O nanospheres. Meanwhile, we have proposed a possible mechanism to explain the gas sensing improvement of this porous microspheres. It is believed that sphere-like porous structure provide numerous micro reaction rooms and larger effective surface areas, which endow the sensors a superior gas-sensing properties, holding great promising for manufacturing high-performance gas sensors. Acknowledgements This work was supported in part by National Natural Science Foundation of China (No. 11332013) and Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006). References [1] S. Cao, C. Zhao, T. Han, L. Peng, Mater. Lett. 169 (2016) 17–20.
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Materials Letters 186 (2017) 119–122
Y. Yu et al.
[8] Y. Shen, X. Chen, W. Wang, Y. Gong, S. Chen, et al., Mater. Lett. 163 (2016) 150–153. [9] Y. Yu, W. Zeng, L. Yu, S. Wu, Mater. Lett. 180 (2016) 51–54. [10] F. Fan, P. Tang, Y. Wang, Y. Feng, A. Chen, Sens. Actuators B: Chem. 215 (2015) 231–240. [11] K. Tian, X. Wang, H. Li, et al., Sens. Actuators B: Chem. 227 (2016) 554–560. [12] B. Zhang, W. Fu, H. Li, X. Fu, Y. Wang, Hari Bala, et al., Appl. Surf. Sci. 363 (2016) 560–565. [13] D. Chen, X. Hou, T. Li, Li Yin, Sens. Actuators B: Chem. 153 (2011) 373–381. [14] Yangchun Yu, Wen Zeng, He Zhang, Mater. Lett. 171 (2016) 162–165.
[2] H. Li, J. Wang, Q. Shi, M. Zhang, C. Hou, G. Shi, et al., Appl. Surf. Sci. 380 (2016) 281–287. [3] R. Abazari, A.R. Mahjoub, L.A. Saghatfotoush, S. Aanati, Mater. Lett. 133 (2014) 208–211. [4] A. Phuruangrat, D.J. Ham, S.J. Hong, S. Thongtem, et al., J. Mater. Chem. 20 (2010) 1683–1690. [5] L. Gao, X. Wang, Z. Xie, W. Song, L. Wang, et al., J. Mater. Chem. A 1 (2013) 7167–7173. [6] F. Li, C. Li, L. Zhu, W. Guo, L. Shen, et al., Sens. Actuators B: Chem. 223 (2016) 761–767. [7] Wen Zeng, He Zhang, Zhongchang Wang, Appl. Surf. Sci. 347 (2015) 73–78.
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