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Hollow cauliflower-like WO3 nanostructures: Hydrothermal synthesis and their CO sensing properties
Materials Letters 186 (2017) 259–262
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
crossmark
Hollow cauliflower-like WO3 nanostructures: Hydrothermal synthesis and their CO sensing properties ⁎
⁎
Shaohong Weia,b, , Lixiao Hana,b, Mingyue Wanga,b, Huihui Zhanga,b, Weimin Dua,b, , Meihua Zhouc a b c
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000, China Henan Province Key Laboratory of New Opto-Electronic Functional Materials, Anyang 455000, China College of Environmental Science and Engineering, Donghua University, Shanghai 200051, China
A R T I C L E I N F O
A BS T RAC T
Keywords: WO3 Cauliflower-like Crystal growth CO Sensors
Hollow cauliflower-like WO3 nanostructures have been successfully synthesized via a facile hydrothermal process. The 3D novel structure is assembled by lots of non-uniform hexagonal WO3 nanorods, which shows the porosity surface and the hollow interiors. According to the time-dependent morphology evolution process, a possible formation mechanism was proposed. Gas sensing investigations reveal that the sample sensor exhibits excellent CO sensing properties at 270 °C, which may substantially benefit from the (001) reactive surfaces of hexagonal phase and the hollow porous structure.
1. Introduction Carbonmonoxide (CO), which is mainly released from industrial process and transportation activities, is considered as one of the most important air pollutants. Detection of CO is of great importance in the control of both indoor and outdoor air quality [1]. In the past few years, gas sensors have received additional consideration driven by their practical applications in flammable or toxic gases detection [2,3]. Recent researches indicate that hierarchical architectures, especially with hollow interiors, often exhibit superior performance for gas sensor because of their high surface area and less agglomerated configurations with outstanding porous structure [4–6]. Therefore, researching simple synthetic methods to fabricate hollow and hierarchical structure will be a meaningful and interesting work for gas sensor. Tungsten trioxide (WO3), as a wide band gap n-type semiconductor, is a promising gas sensing material. owing to its highly sensitive and simple preparation [1,7]. Recently, the most metastable hexagonal phase WO3 (h-WO3) have attracted peoples attention not only because of its unique hexagonal and trigonal tunnels, but also because its highly reactive surfaces, which makes it a very prominent material for gas sensors [8,9]. So far, researches have been done to achieve the hexagonal WO3, especially the surfacecontrolled synthesis of WO3, which remains a challenge. In the present study, we have controllably prepared the hollow hierarchical cauliflower-like (CFL) WO3 assembled by lots of nonuniform hexagonal WO3 nanorods via the simple hydrothermal pro⁎
cess. Furthermore, the sensing investigations reveal that the CFL-WO3 sensor exhibits excellent CO sensing properties at 270 °C which may substantially benefit from the (001) reactive surfaces of hexagonal phase and the hollow porous structure. It is hoped that this study will provide a new insight into the facile synthesis and potential applications of the novel hollow CFL-WO3 hierarchical structures. 2. Experimental All the chemicals were of analytic purity and used directly without further purification. In a typical synthesis process, 6.1 mmol sodium tungstate (Na2WO4·2H2O) was dissolved in 35 ml deionized water and stirred for 30 min. Then 6 mol/L HCl solution was dropped into the upper solution with stirring until the pH value reached 1.0 and the light yellow precipitation was produced. Subsequently, 2.5 mmol ammonium tartrate (C4H12N2O6) was added to above precursor solution. The mixture was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated in oven at 200 °C for 18 h. After being collected by centrifugation, washed with deionized water and absolute ethanol, and dried in air at 80 °C for 10 h, the yellow-colored powders were turned into the CFL-WO3 product. Microstructures of the product were characterized by X-ray diffraction (XRD, Rigaku Ultima III) with a Cu Kα (λ=1.5418 Å), field emission scanning electron microscopy (FE-SEM, JEOL JSM-6701F) and transmission electron microscopy (TEM, JEOL JEM-2100). Gassensing property measurements were performed through a static test
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Du).
http://dx.doi.org/10.1016/j.matlet.2016.10.016 Received 30 July 2016; Received in revised form 29 September 2016; Accepted 5 October 2016 Available online 06 October 2016 0167-577X/ © 2016 Published by Elsevier B.V.
Materials Letters 186 (2017) 259–262
S. Wei et al.
270 °C and the responses are 6.7, 12.9, 16.6, 22.7, 32.6 and 41.9 for 10, 20, 50, 100, 200 and 300 ppm CO, respectively (Fig. 3b). The response and recovery times are in the range of 9–15 s and 5–9 s, respectively. Subsequently, the responses of the sensor to different concentration of gases were tested at 270 °C (Fig. 3c). It is clear that our sensor shows high selectivity to CO in the presence of interfering gases such as HCHO, CH4, H2 and LPG, which may benefit from the surface reaction between WO3 and CO. Further discussion will be performed in following. In addition, the stability of the sample sensor toward 300 ppm CO was evaluated over a period of 60 days (Fig. 3d). The response values vary slightly from 41.6 to 40.9 and to 41.3 after 20 days, 40 days and 60 days respectively, confirming the good stability of the sensor. Table S1 summarizes the CO sensing performances of our sample and those of WO3 base sensors reported in Refs. [10–14]. Compared with Refs. [10–13], our sensor exhibits obvious advantages. Significantly, with the modification of Pt, the Pt-WO3 sensor in Ref. [14] displays better sensing properties than ours. The pity is the response-recovery times prolong a little. Based on an overall consideration, our CFL-WO3 sensor in this work has more advantages and will be a potential advanced sensing material for CO detection. The excellent CO sensing performance of our sensor can be benefited from the highly reactive (001) surfaces of h-WO3. Nagarajan and Zhao et al. have carried out the theory research through a density functional theory and reported that h-WO3 (001) surface has prominent reactivity to CO gas for the large adsorption energy and reaction enthalpy of the surface reaction between CO and WO3 [15,16]. The reaction is shown as follows: WnO3 n+CO WnO3 n-1+VO·+e-+CO2. From the thermodynamic point, the reaction enthalpy can be calculated through the formula: ΔH=E (WnO3 n-1)+E (CO2)–E (WnO3 n)–E (CO) and the result is −2.42 eV which is quite exothermic [16,17]. That means CO will be oxidized into CO2 directly followed by forming oxygen vacancy and negative charge carrier on the sensor surface, which will directly increase the sensor conductivity and effectively improve the CO sensing response of our h-WO3 with the novel structure. In addition, the hierarchical and hollow nanostructures can provide more convenient pathway via the porous architectures without sacrificing the large surface area, which will allow the rapid gas diffusion and easy conversion of the entire hollow structures into conducting phase through the permeable shells and then improve the gas sensing properties [5].
system (WS-30A, Winsen Electronics Co. Ltd., Zhengzhou, China). The response (R) was defined as Ra/Rg, where Ra and Rg were the resistance of the sensor in air and test gas, respectively. The response and recovery time is defined as the time for the sensor to reach 90% of its maximum response and fall to 10% of its maximum response, respectively. The humidity was 30% RH. 3. Results and discussion XRD pattern of the as-synthesized product is shown in Fig. 1a. All the diffraction peaks can be well indexed to pure hexagonal WO3 with the lattice parameters of a=b=7.298 Å, c=3.899 Å and a space group of P6/mmm (JCPDS NO. 33–1387). The tungsten trioxide hexagonal structure bases on the tungstenoxygen framework built up of layers (Fig. 1b), which contains the distorted corner sharing of WO6 octahedral (Fig. 1c). The layers are stacking along the [001] axis, resulting in the formation of narrow triangular and large hexagonal tunnels as shown in Fig. 1b. Then, hWO3 with the special open structure should exhibit the enhanced adsorption-desorption capacity and fast electron transport property, which are valuable for gas sensing applications. The overview SEM image of the sample is displayed in Fig. 2a, indicating the product is a CFL hierarchical structure with the flower diameters of 450–600 nm. The higher magnification SEM image in Fig. 2b suggests that the 3D CFL WO3 is assembled by lots of nonuniform nanorods with the sizes in the range of 30–95 nm. More importantly, various sizes of nanorods gathered with each other in the spherical way, resulting in the porosity surface and the hollow interiors as shown in Fig. 2b. The typical cauliflowers TEM image is given in Fig. 2c which clearly presents that the loose-packed nanorods are assembled into the hollow and permeable interior nanostructures. The HRTEM image of the individual WO3 nanorod (Fig. 2d) displays the lattice spaces of 0.634 and 0.365 nm which is corresponding to (010) and (110) facets respectively (JCPDS 33–1387). The angle of 60° between [100] (or [010]) and [110] is consistent with the hexagonal WO3. Meanwhile, the lattice space of 0.391 nm is corresponding well to the lattice space of (001) planes. From these results, we concluded that our WO3 sample exhibits hexagonal prism crystal and leads to the dominant (001) and (1−10) facets, as illustrated in Fig. 2e. In order to investigate the growth mechanism of such novel CFLWO3, time-dependent experiments were operated. The resultant products were analyzed by SEM (Fig. S1 a-d) and the schematic illustration is shown in Fig. S1e. In addition, the possible growth mechanism is explained in the SI. CO sensing properties of the sensor using CFL-WO3 have been investigated and the results are displayed in Fig. 3. We first measured the gas response of our sample at various operating temperature under 200 ppm CO. As shown in Fig. 3a, it is obvious that 270 °C is the optimal temperature. Furthermore, the dynamic responses of the obtained product to different concentration of CO were studied at
4. Conclusion In this letter, the unique hollow hierarchical CFL-WO3 assembled by hexagonal WO3 nanorods has been successfully synthesized via a simple hydrothermal method. Further gas sensing investigations reveal that the sample sensor exhibits high response, rapid response-recovery characteristics and good selectivity to CO at 270 °C, demonstrating the highly promising application for CO detection. Hexagonal tunnel
Triangular tunnel
Fig. 1. (a) XRD patterns (b) crystal structure (c) WO6 octahedra of h-WO3.
260
Materials Letters 186 (2017) 259–262
S. Wei et al.
100 nm
500 nm
120
( 10
0)
(001)
(1-10)
[110]
[001]
Fig. 2. (a, b)SEM, (c,d)TEM of CFL-WO3, and (e) illustration of the single crystal h-WO3 nanorod..
Fig. 3. (a) Dependence of CO response on operating temperature, (b) Response of the sample to different concentration of CO, (c) selectivity and (d) the stability of CFL-WO3 sensor at 270 °C.
261
Materials Letters 186 (2017) 259–262
S. Wei et al. [4] [5] [6] [7]
Acknowledgement This work was supported by National Natural Science Foundation of China (U1404203), Program for Innovative Research Team of Science and Technology in the University of Henan Province (16IRTSTHN003).
[8] [9] [10]
Appendix A. Supplementary material
[11]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2016.10.016.
[12] [13]
References
[14] [15] [16]
[1] V. Nagarajan, R. Chandiramouli, Superlattices Microstruct. 78 (2015) 22–39. [2] H. Tian, H. Fan, M. Li, L. Ma, ACS Sens. 1 (2016) 243–250. [3] W. Yan, H. Fan, Y. Zhai, C. Yang, P. Ren, L. Huang, Sens. Actuators B 160 (2011) 1372–1379.
[17]
262
Y. Liu, G. Li, R. Mi, C. Deng, P. Gao, Sens. Actuators B 191 (2014) 537–544. S. Zhang, P. Song, H. Yan, Q. Wang, Sens. Actuators B 231 (2016) 245–255. J. Lee, Sens. Actuators B 140 (2009) 319–336. X. Chu, T. Hu, F. Gao, Y. Dong, W. Sun, L. Bai, Mater. Sci. Eng. B 193 (2015) 97–104. B. Miao, W. Zeng, S. Hussain, Q. Mei, S. Xu, H. Zhang, et al., Mater. Lett. 147 (2015) 12–15. W. Zeng, B. Miao, T. Li, H. Zhang, S. Hussain, Y. Lia, et al., Thin Solid Films 584 (2015) 294–299. S. Park, H. Kim, C. Jin, S. Choi, S. Kim, C. Lee, Thermochim. Acta 542 (2012) 69–73. G. Buono-Core, A. Klahn, G. Cabello, E. Muñoz, M. Bustamante, C. Castillo, et al., Polyhedron 41 (2012) 134–139. A. Azad, M. Hammoud, Sens. Actuators B 119 (2006) 384–391. M. Ahsan, T. Tesfamichael, M. Ionescu, J. Bell, N. Mott, Sens. Actuators B 162 (2012) 14–21. T. Lei, Q. Deng, S. Zhang, S. Cai, C. Xie, Sens. Actuators B 232 (2016) 506–513. V. Nagarajan, R. Chandiramouli, Superlattice Microstruct. 78 (2015) 22–39. L. Zhao, F. Tian, X. Wang, W. Zhao, A. Fu, Y. Shen, et al., Comp. Mater. Sci. 79 (2013) 691–697. V. Oison, L. Saadi, C. Lambert-Mauriat, R. Hayn, Sens. Actuators B 160 (2011) 505–510.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
crossmark
Hollow cauliflower-like WO3 nanostructures: Hydrothermal synthesis and their CO sensing properties ⁎
⁎
Shaohong Weia,b, , Lixiao Hana,b, Mingyue Wanga,b, Huihui Zhanga,b, Weimin Dua,b, , Meihua Zhouc a b c
College of Chemistry and Chemical Engineering, Anyang Normal University, Anyang 455000, China Henan Province Key Laboratory of New Opto-Electronic Functional Materials, Anyang 455000, China College of Environmental Science and Engineering, Donghua University, Shanghai 200051, China
A R T I C L E I N F O
A BS T RAC T
Keywords: WO3 Cauliflower-like Crystal growth CO Sensors
Hollow cauliflower-like WO3 nanostructures have been successfully synthesized via a facile hydrothermal process. The 3D novel structure is assembled by lots of non-uniform hexagonal WO3 nanorods, which shows the porosity surface and the hollow interiors. According to the time-dependent morphology evolution process, a possible formation mechanism was proposed. Gas sensing investigations reveal that the sample sensor exhibits excellent CO sensing properties at 270 °C, which may substantially benefit from the (001) reactive surfaces of hexagonal phase and the hollow porous structure.
1. Introduction Carbonmonoxide (CO), which is mainly released from industrial process and transportation activities, is considered as one of the most important air pollutants. Detection of CO is of great importance in the control of both indoor and outdoor air quality [1]. In the past few years, gas sensors have received additional consideration driven by their practical applications in flammable or toxic gases detection [2,3]. Recent researches indicate that hierarchical architectures, especially with hollow interiors, often exhibit superior performance for gas sensor because of their high surface area and less agglomerated configurations with outstanding porous structure [4–6]. Therefore, researching simple synthetic methods to fabricate hollow and hierarchical structure will be a meaningful and interesting work for gas sensor. Tungsten trioxide (WO3), as a wide band gap n-type semiconductor, is a promising gas sensing material. owing to its highly sensitive and simple preparation [1,7]. Recently, the most metastable hexagonal phase WO3 (h-WO3) have attracted peoples attention not only because of its unique hexagonal and trigonal tunnels, but also because its highly reactive surfaces, which makes it a very prominent material for gas sensors [8,9]. So far, researches have been done to achieve the hexagonal WO3, especially the surfacecontrolled synthesis of WO3, which remains a challenge. In the present study, we have controllably prepared the hollow hierarchical cauliflower-like (CFL) WO3 assembled by lots of nonuniform hexagonal WO3 nanorods via the simple hydrothermal pro⁎
cess. Furthermore, the sensing investigations reveal that the CFL-WO3 sensor exhibits excellent CO sensing properties at 270 °C which may substantially benefit from the (001) reactive surfaces of hexagonal phase and the hollow porous structure. It is hoped that this study will provide a new insight into the facile synthesis and potential applications of the novel hollow CFL-WO3 hierarchical structures. 2. Experimental All the chemicals were of analytic purity and used directly without further purification. In a typical synthesis process, 6.1 mmol sodium tungstate (Na2WO4·2H2O) was dissolved in 35 ml deionized water and stirred for 30 min. Then 6 mol/L HCl solution was dropped into the upper solution with stirring until the pH value reached 1.0 and the light yellow precipitation was produced. Subsequently, 2.5 mmol ammonium tartrate (C4H12N2O6) was added to above precursor solution. The mixture was transferred into a 50 ml Teflon-lined stainless steel autoclave and heated in oven at 200 °C for 18 h. After being collected by centrifugation, washed with deionized water and absolute ethanol, and dried in air at 80 °C for 10 h, the yellow-colored powders were turned into the CFL-WO3 product. Microstructures of the product were characterized by X-ray diffraction (XRD, Rigaku Ultima III) with a Cu Kα (λ=1.5418 Å), field emission scanning electron microscopy (FE-SEM, JEOL JSM-6701F) and transmission electron microscopy (TEM, JEOL JEM-2100). Gassensing property measurements were performed through a static test
Corresponding author. E-mail addresses: [email protected], [email protected] (W. Du).
http://dx.doi.org/10.1016/j.matlet.2016.10.016 Received 30 July 2016; Received in revised form 29 September 2016; Accepted 5 October 2016 Available online 06 October 2016 0167-577X/ © 2016 Published by Elsevier B.V.
Materials Letters 186 (2017) 259–262
S. Wei et al.
270 °C and the responses are 6.7, 12.9, 16.6, 22.7, 32.6 and 41.9 for 10, 20, 50, 100, 200 and 300 ppm CO, respectively (Fig. 3b). The response and recovery times are in the range of 9–15 s and 5–9 s, respectively. Subsequently, the responses of the sensor to different concentration of gases were tested at 270 °C (Fig. 3c). It is clear that our sensor shows high selectivity to CO in the presence of interfering gases such as HCHO, CH4, H2 and LPG, which may benefit from the surface reaction between WO3 and CO. Further discussion will be performed in following. In addition, the stability of the sample sensor toward 300 ppm CO was evaluated over a period of 60 days (Fig. 3d). The response values vary slightly from 41.6 to 40.9 and to 41.3 after 20 days, 40 days and 60 days respectively, confirming the good stability of the sensor. Table S1 summarizes the CO sensing performances of our sample and those of WO3 base sensors reported in Refs. [10–14]. Compared with Refs. [10–13], our sensor exhibits obvious advantages. Significantly, with the modification of Pt, the Pt-WO3 sensor in Ref. [14] displays better sensing properties than ours. The pity is the response-recovery times prolong a little. Based on an overall consideration, our CFL-WO3 sensor in this work has more advantages and will be a potential advanced sensing material for CO detection. The excellent CO sensing performance of our sensor can be benefited from the highly reactive (001) surfaces of h-WO3. Nagarajan and Zhao et al. have carried out the theory research through a density functional theory and reported that h-WO3 (001) surface has prominent reactivity to CO gas for the large adsorption energy and reaction enthalpy of the surface reaction between CO and WO3 [15,16]. The reaction is shown as follows: WnO3 n+CO WnO3 n-1+VO·+e-+CO2. From the thermodynamic point, the reaction enthalpy can be calculated through the formula: ΔH=E (WnO3 n-1)+E (CO2)–E (WnO3 n)–E (CO) and the result is −2.42 eV which is quite exothermic [16,17]. That means CO will be oxidized into CO2 directly followed by forming oxygen vacancy and negative charge carrier on the sensor surface, which will directly increase the sensor conductivity and effectively improve the CO sensing response of our h-WO3 with the novel structure. In addition, the hierarchical and hollow nanostructures can provide more convenient pathway via the porous architectures without sacrificing the large surface area, which will allow the rapid gas diffusion and easy conversion of the entire hollow structures into conducting phase through the permeable shells and then improve the gas sensing properties [5].
system (WS-30A, Winsen Electronics Co. Ltd., Zhengzhou, China). The response (R) was defined as Ra/Rg, where Ra and Rg were the resistance of the sensor in air and test gas, respectively. The response and recovery time is defined as the time for the sensor to reach 90% of its maximum response and fall to 10% of its maximum response, respectively. The humidity was 30% RH. 3. Results and discussion XRD pattern of the as-synthesized product is shown in Fig. 1a. All the diffraction peaks can be well indexed to pure hexagonal WO3 with the lattice parameters of a=b=7.298 Å, c=3.899 Å and a space group of P6/mmm (JCPDS NO. 33–1387). The tungsten trioxide hexagonal structure bases on the tungstenoxygen framework built up of layers (Fig. 1b), which contains the distorted corner sharing of WO6 octahedral (Fig. 1c). The layers are stacking along the [001] axis, resulting in the formation of narrow triangular and large hexagonal tunnels as shown in Fig. 1b. Then, hWO3 with the special open structure should exhibit the enhanced adsorption-desorption capacity and fast electron transport property, which are valuable for gas sensing applications. The overview SEM image of the sample is displayed in Fig. 2a, indicating the product is a CFL hierarchical structure with the flower diameters of 450–600 nm. The higher magnification SEM image in Fig. 2b suggests that the 3D CFL WO3 is assembled by lots of nonuniform nanorods with the sizes in the range of 30–95 nm. More importantly, various sizes of nanorods gathered with each other in the spherical way, resulting in the porosity surface and the hollow interiors as shown in Fig. 2b. The typical cauliflowers TEM image is given in Fig. 2c which clearly presents that the loose-packed nanorods are assembled into the hollow and permeable interior nanostructures. The HRTEM image of the individual WO3 nanorod (Fig. 2d) displays the lattice spaces of 0.634 and 0.365 nm which is corresponding to (010) and (110) facets respectively (JCPDS 33–1387). The angle of 60° between [100] (or [010]) and [110] is consistent with the hexagonal WO3. Meanwhile, the lattice space of 0.391 nm is corresponding well to the lattice space of (001) planes. From these results, we concluded that our WO3 sample exhibits hexagonal prism crystal and leads to the dominant (001) and (1−10) facets, as illustrated in Fig. 2e. In order to investigate the growth mechanism of such novel CFLWO3, time-dependent experiments were operated. The resultant products were analyzed by SEM (Fig. S1 a-d) and the schematic illustration is shown in Fig. S1e. In addition, the possible growth mechanism is explained in the SI. CO sensing properties of the sensor using CFL-WO3 have been investigated and the results are displayed in Fig. 3. We first measured the gas response of our sample at various operating temperature under 200 ppm CO. As shown in Fig. 3a, it is obvious that 270 °C is the optimal temperature. Furthermore, the dynamic responses of the obtained product to different concentration of CO were studied at
4. Conclusion In this letter, the unique hollow hierarchical CFL-WO3 assembled by hexagonal WO3 nanorods has been successfully synthesized via a simple hydrothermal method. Further gas sensing investigations reveal that the sample sensor exhibits high response, rapid response-recovery characteristics and good selectivity to CO at 270 °C, demonstrating the highly promising application for CO detection. Hexagonal tunnel
Triangular tunnel
Fig. 1. (a) XRD patterns (b) crystal structure (c) WO6 octahedra of h-WO3.
260
Materials Letters 186 (2017) 259–262
S. Wei et al.
100 nm
500 nm
120
( 10
0)
(001)
(1-10)
[110]
[001]
Fig. 2. (a, b)SEM, (c,d)TEM of CFL-WO3, and (e) illustration of the single crystal h-WO3 nanorod..
Fig. 3. (a) Dependence of CO response on operating temperature, (b) Response of the sample to different concentration of CO, (c) selectivity and (d) the stability of CFL-WO3 sensor at 270 °C.
261
Materials Letters 186 (2017) 259–262
S. Wei et al. [4] [5] [6] [7]
Acknowledgement This work was supported by National Natural Science Foundation of China (U1404203), Program for Innovative Research Team of Science and Technology in the University of Henan Province (16IRTSTHN003).
[8] [9] [10]
Appendix A. Supplementary material
[11]
Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2016.10.016.
[12] [13]
References
[14] [15] [16]
[1] V. Nagarajan, R. Chandiramouli, Superlattices Microstruct. 78 (2015) 22–39. [2] H. Tian, H. Fan, M. Li, L. Ma, ACS Sens. 1 (2016) 243–250. [3] W. Yan, H. Fan, Y. Zhai, C. Yang, P. Ren, L. Huang, Sens. Actuators B 160 (2011) 1372–1379.
[17]
262
Y. Liu, G. Li, R. Mi, C. Deng, P. Gao, Sens. Actuators B 191 (2014) 537–544. S. Zhang, P. Song, H. Yan, Q. Wang, Sens. Actuators B 231 (2016) 245–255. J. Lee, Sens. Actuators B 140 (2009) 319–336. X. Chu, T. Hu, F. Gao, Y. Dong, W. Sun, L. Bai, Mater. Sci. Eng. B 193 (2015) 97–104. B. Miao, W. Zeng, S. Hussain, Q. Mei, S. Xu, H. Zhang, et al., Mater. Lett. 147 (2015) 12–15. W. Zeng, B. Miao, T. Li, H. Zhang, S. Hussain, Y. Lia, et al., Thin Solid Films 584 (2015) 294–299. S. Park, H. Kim, C. Jin, S. Choi, S. Kim, C. Lee, Thermochim. Acta 542 (2012) 69–73. G. Buono-Core, A. Klahn, G. Cabello, E. Muñoz, M. Bustamante, C. Castillo, et al., Polyhedron 41 (2012) 134–139. A. Azad, M. Hammoud, Sens. Actuators B 119 (2006) 384–391. M. Ahsan, T. Tesfamichael, M. Ionescu, J. Bell, N. Mott, Sens. Actuators B 162 (2012) 14–21. T. Lei, Q. Deng, S. Zhang, S. Cai, C. Xie, Sens. Actuators B 232 (2016) 506–513. V. Nagarajan, R. Chandiramouli, Superlattice Microstruct. 78 (2015) 22–39. L. Zhao, F. Tian, X. Wang, W. Zhao, A. Fu, Y. Shen, et al., Comp. Mater. Sci. 79 (2013) 691–697. V. Oison, L. Saadi, C. Lambert-Mauriat, R. Hayn, Sens. Actuators B 160 (2011) 505–510.