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A novel cactus-like WO3-SnO2 nanocomposite and its acetone gas sensing properties
Materials Letters 231 (2018) 5–7
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A novel cactus-like WO3-SnO2 nanocomposite and its acetone gas sensing properties Ling Zhu a, Wen Zeng a,⇑, Yanqiong Li b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing 400030, China
a r t i c l e
i n f o
Article history: Received 31 March 2018 Received in revised form 3 August 2018 Accepted 4 August 2018 Available online 4 August 2018 Keywords: Semiconductors Hydrothermal Sensors Heterojunction Functional
a b s t r a c t Herein, a novel hierarchical cactus-like WO3-SnO2 nanocomposite was synthesized by a one-step hydrothermal route. It was a surprise to find that such nanostructure consisting of diminutive SnO2 nanospheres decorated bulky WO3 nanosphere exhibited a high gas response and fast response-recovery speed towards acetone, mainly attributed to their synergetic effect as well as the heterojunction at the interface between the bulky WO3 nanosphere and diminutive SnO2 nanosphere, resulting in adequate surface adsorption and efficient electron transport. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors demonstrate great potential in the detection of various gases due to their fascinating physical and chemical features, and have been widely applied in the field of gas-sensing materials [1–6]. However, the further enhancement of gas-sensing properties for a pristine oxide nanomaterials is largely limited [7]. Specifically, the preparation of binary metal-oxide nanocomposites is one of the desirable approaches to resolve this difficulty via combining their respective catalytic activity, gas adsorption ability and promoted electron transfer at the heterointerface [8,9]. WO3 and SnO2, as two of the prominent n-type oxide semiconductors, have extensive uses in gas sensors [10,11], photocatalysts [12,13], lithium-ion batteries [14,15], and so forth. The incorporation of such two oxides has also been proved to be able to substantially strengthen the capabilities of gas sensor. For instance, Li et al. [16] developed a sensor based on WO3-SnO2 hollow nanospheres which displayed excellent high-temperature humidity sensing properties in comparison to the pristine WO3 nanoparticles and SnO2 nanoparticles. Yin et al. [17] synthesized WO3-SnO2 nanosheet composites through a two-step hydrothermal method, and the composites with coverage-controlled SnO2 nanoparticles showed the improved sensor response and selectivity to acetone. ⇑ Corresponding author. E-mail address: [email protected] (W. Zeng). https://doi.org/10.1016/j.matlet.2018.08.007 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Hence, design of a distinctive nanostructure for WO3-SnO2 hybrid nanocomposite is expected to attain superb gas-sensing performances. However, one-step synthesis of WO3-SnO2 composite with unique structure, especially many SnO2 spheres adhering to the surface of WO3 spheres, is hardly controlled and is still a huge challenge. In this work, a novel WO3-SnO2 composite nanostructure composed of bulky WO3 nanosphere functionalized by diminutive SnO2 nanospheres was designed and fabricated via one-step hydrothermal method. Such WO3-SnO2 heterostructure exhibited desirable gas-sensing properties to acetone with large gas response and fast response-recovery speed. The synergetic effect arising from WO3 and SnO2 and the variation in the heterojunction barrier accounted for the origin of the desirable properties. It is expected that our work could offer a reference for the rational design of other binary metal-oxide nanocomposites.
2. Experiments Synthesis: The cactus-like WO3-SnO2 nanocomposite was prepared by a hydrothermal method. Initially, 1 mmol Na2WO42H2O was dissolved in 40 mL of deionized water under magnetic stirring to form a homogeneous solution. Subsequently, 0.5 mmol Na2SnO33H2O and 5 g glucose were added into the above mixture. After vigorous stirring for 10 min, the desired mixture was transferred into a 50 mL Teflon-lined stainless autoclave and kept at 180 °C for 20 h. After naturally cooled to room temperature, the
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L. Zhu et al. / Materials Letters 231 (2018) 5–7
obtained solid products were washed with deionized water and ethanol in sequence several times and collected by centrifugation. Finally, the as-prepared products were dried in air at 60 °C overnight for further characterization. Characterization: The composition and crystalline structures of the sample were confirmed by X-ray diffraction (XRD) on a Rigaku TTRIII X-ray diffractometer using a Cu-Ka radiation source. The morphologies were observed by field emission scanning microscopy (FESEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, JEOL JEM-2100). Fabrication of gas sensor: The as-prepared powders were mixed with terpineol and then coated onto an alumina ceramic tube which was previously equipped with a pair of gold wires. The operating temperature of the sensor could be precisely controlled by adjusting the current flow across the Ni-Cr alloy coil which was inserted into the tube. Finally, the fabricated sensor was further aged at 200 °C for 24 h to enhance the stability and repeatability. Measurement of gas-sensing properties: The gas-sensing behavior towards acetone was tested on a static system controlled by a central computer. To reach the required gas concentration, a given amount of target gas was injected into the test chamber with a needle valve. The gas response in this work was defined as the ratio S = Ra/Rg, from which Ra and Rg represented the resistances of the sensor in air and target gas, respectively. 3. Results and discussion The crystal structure and purity of the sample were first identified by X-ray diffraction as depicted in Fig. 1. All the diffraction peaks in the XRD pattern can be well accorded with the standard monoclinic WO3 (JCPDS No. 32-1324) and tetragonal rutile SnO2 (JCPDS No. 41-1445). No other diffraction peaks belonging to any impurities was detected, thus suggesting the high purity of the as-prepared sample. The typical morphology of the obtained WO3-SnO2 nanocomposite was analyzed through SEM. As can be seen from Fig. 2a and b, the detailed morphology of the sample presents an apparent similarity to a cactus. The morphology is mainly composed of the bulky nanosphere with a diameter of 500 nm decorated with numerous diminutive nanospheres ranging from 20 to 50 nm in diameter on the surface. More specifically, an individual sphere was further investigated by TEM, as shown in Fig. 2c. It is further identified that a great deal of diminutive nanospheres are firmly adhered upon the bulky nanosphere. Furthermore, the interplanar spacing of 0.335 nm and 0.358 nm observed from the HRTEM images (Fig. 2d and e) corresponds to the (1 1 0) plane of tetragonal SnO2 and (2 0 0) plane of monoclinic WO3, respectively. It is well known that the response of a gas sensor based on metal oxide semiconductors is closely dependent on operating temperature. Fig. 3a displays the gas response of the sensor
Fig. 1. XRD spectra of the as-prepared WO3-SnO2 composite.
Fig. 2. (a and b) SEM images with different magnification of the obtained composite. (Inset: the natural cactus.) (c) TEM image of the sample. HRTEM images of individual (d) diminutive nanosphere and (e) bulky nanosphere.
exposed to 600 ppm of acetone under different temperatures ranging from 160 to 400 °C. The result indicates that the gas response to acetone continuously increases, up to 360 °C, and then gradually decreases with the further increase of the temperature. It is evident that the fabricated sensor possesses high acetone response, and the maximum response to 600 ppm acetone is 26 at 360 °C. Such change in gas response is caused by the quantity of adsorption and desorption of target gas. The amount of gas adsorption will reduce when the operating temperature surpasses the optimum value (360 °C), and the desorption-dominated case leads to a decrease in gas response. Therefore, all further gas sensing measurements for acetone were recorded at the optimum temperature of 360 °C. Fig. 3b shows the gas response of the sensor as a function of acetone concentrations at 360 °C, from which one can see that the gas response increases almost linearly with the acetone concentration in the investigated range. The gas responses are 12.1, 13.8, 16.5, 19.8, 23.5, 26, 29.5 and 32 for 100, 200, 300, 400, 500, 600, 700 and 800 ppm acetone gas, respectively. Additionally, the response and recovery curve of the sensor at 360 °C to 600 ppm acetone was also tested, as illustrated in Fig. 3c. It is clearly seen that the sensor voltage presents a rapid increase when the acetone is in, but suddenly goes back to its initial sate when the target gas is out. Obviously, the WO3-SnO2 composite based gas sensor exhibits the fast response and recovery speed towards acetone. Moreover, Fig. 3d depicts the gas response and recovery curve exposed to 600 ppm acetone after 4 cycles of gas in and out at 360 °C, indicating an excellent stability of the sensor. As both n-type oxides, the basic mechanism for acetone gas sensing can be elucidated by the resistance change due to the adsorption and desorption of oxygen [18]. Briefly, when the sensor is exposed to air, the oxygen molecules get adsorbed onto the surface and consume free electrons from the conduction band thorough oxygen ionization process, and then transform to oxygen ions. The adsorbed oxygen leads to the formation of thick electron depletion layer and an increase in the sensor resistance. However, once introducing acetone gas, the acetone molecules are oxidized by the adsorbed oxygen ions. Meanwhile, the trapped electrons would be released back to the surface of the composite, narrowing the depletion layer and thus decreasing the sensor resistance. In our study, the following two points should be considered to explain the satisfactory performance of the as-synthesized nanocomposite. On the one hand, from the SEM images, it could be readily observed that the surface of bulky WO3 matrix is not completely covered by the diminutive SnO2 nanospheres (Fig. 2a
L. Zhu et al. / Materials Letters 231 (2018) 5–7
7
Fig. 3. (a) Gas response of the WO3-SnO2 composite exposed to 600 ppm acetone at different temperatures. (b) Gas response of the WO3-SnO2 composite towards different gas concentrations at 360 °C. (c) Response-recovery curve of the WO3-SnO2 composite to 600 ppm acetone at 360 °C. (d) Response-recovery curve of the composite after 4 cycles to 600 ppm acetone at 360 °C.
and b). Therefore, both of them are highly accessible for the gas adsorption and could offer more reactive sites. Their synergetic effects contribute to the response enhancement of the gas sensor. On the other hand, the presence of heterojunction at the interface between the WO3 and SnO2 is considerably favorable for the improvement in sensing properties. The electron transfer from WO3 to SnO2 occurs owing to higher Femi level of WO3 comparing to that of SnO2, resulting in the formation of a new potential barrier at the heterojunction between the WO3 and SnO2. The directional flow of electrons further promotes the formation of depletion layer and thereby increases the sensor resistance [19]. As a result, the synergetic effect and the formation of heterojunction cause the excellent sensing properties. 4. Conclusions In summary, a novel cactus-like WO3-SnO2 composite, where bulky WO3 nanosphere is functionalized by diminutive SnO2 nanospheres, has been successfully prepared by the one-step hydrothermal process. The sensor was exposed to acetone for various operating temperatures and gas concentrations. The sensing measurement exhibited that the cactus-like WO3-SnO2 composite possesses a high gas response and fast response-recovery speed towards acetone. The striking synergetic effect and the heterojunction played an important role in the gas-sensing reaction.
Acknowledgment This work was supported by Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
A.K. Nayak, R. Ghosh, S. Santra, et al., Nanoscale 7 (2015) 12460–12473. L. Zhu, W. Zeng, H. Ye, et al., Mater. Res. Bull. 100 (2018) 259–264. L. Zhu, Y. Li, W. Zeng, Appl. Surf. Sci. 427 (2018) 281–287. H. Long, W. Zeng, H. Wang, et al., Adv. Sci. 5 (2018) 1700634. Y. Zhang, W. Zeng, Y. Li, J. Alloy. Compd. 749 (2018) 355–362. Y. Zhang, W. Zeng, H. Ye, et al., Appl. Surf. Sci. 442 (2018) 507–516. K. Xu, N. Li, D. Zeng, et al., ACS Appl. Mater. Inter. 7 (2015) 11359–11368. L. Zhu, W. Zeng, Sens. Actuators A: Phys. 267 (2017) 242–261. H. Li, W. Xie, B. Liu, et al., Sens. Actuators B: Chem. 240 (2017) 349–357. L. Han, J. Chen, Y. Zhang, et al., Mater. Lett. 210 (2018) 8–11. S. Wang, W. Yu, C. Cheng, et al., Mater. Res. Bull. 89 (2017) 267–272. S. Yao, F. Qu, G. Wang, et al., J. Alloy. Compd. 724 (2017) 695–702. D. Zhao, X. Wu, Mater. Lett. 210 (2018) 354–357. P. Li, X. Li, Z. Zhao, et al., Electrochim. Acta 192 (2016) 148–157. D. Narsimulu, S. Vinoth, E.S. Srinadhu, et al., Ceram. Int. 44 (2018) 201– 207. H. Li, B. Liu, D. Cai, et al., J. Mater. Chem. A 2 (2014) 6854–6862. M. Yin, Y. Yao, H. Fan, et al., J. Alloy. Compd. 736 (2018) 322–331. Z.-X. Cai, H.-Y. Li, J.-C. Ding, et al., Sens. Actuators B: Chem. 246 (2017) 225– 234. V.K. Tomer, K. Singh, H. Kaur, et al., Sens. Actuators B: Chem. 253 (2017) 703– 713.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
A novel cactus-like WO3-SnO2 nanocomposite and its acetone gas sensing properties Ling Zhu a, Wen Zeng a,⇑, Yanqiong Li b a b
College of Materials Science and Engineering, Chongqing University, Chongqing 400030, China School of Electronic and Electrical Engineering, Chongqing University of Arts and Sciences, Chongqing 400030, China
a r t i c l e
i n f o
Article history: Received 31 March 2018 Received in revised form 3 August 2018 Accepted 4 August 2018 Available online 4 August 2018 Keywords: Semiconductors Hydrothermal Sensors Heterojunction Functional
a b s t r a c t Herein, a novel hierarchical cactus-like WO3-SnO2 nanocomposite was synthesized by a one-step hydrothermal route. It was a surprise to find that such nanostructure consisting of diminutive SnO2 nanospheres decorated bulky WO3 nanosphere exhibited a high gas response and fast response-recovery speed towards acetone, mainly attributed to their synergetic effect as well as the heterojunction at the interface between the bulky WO3 nanosphere and diminutive SnO2 nanosphere, resulting in adequate surface adsorption and efficient electron transport. Ó 2018 Elsevier B.V. All rights reserved.
1. Introduction Metal oxide semiconductors demonstrate great potential in the detection of various gases due to their fascinating physical and chemical features, and have been widely applied in the field of gas-sensing materials [1–6]. However, the further enhancement of gas-sensing properties for a pristine oxide nanomaterials is largely limited [7]. Specifically, the preparation of binary metal-oxide nanocomposites is one of the desirable approaches to resolve this difficulty via combining their respective catalytic activity, gas adsorption ability and promoted electron transfer at the heterointerface [8,9]. WO3 and SnO2, as two of the prominent n-type oxide semiconductors, have extensive uses in gas sensors [10,11], photocatalysts [12,13], lithium-ion batteries [14,15], and so forth. The incorporation of such two oxides has also been proved to be able to substantially strengthen the capabilities of gas sensor. For instance, Li et al. [16] developed a sensor based on WO3-SnO2 hollow nanospheres which displayed excellent high-temperature humidity sensing properties in comparison to the pristine WO3 nanoparticles and SnO2 nanoparticles. Yin et al. [17] synthesized WO3-SnO2 nanosheet composites through a two-step hydrothermal method, and the composites with coverage-controlled SnO2 nanoparticles showed the improved sensor response and selectivity to acetone. ⇑ Corresponding author. E-mail address: [email protected] (W. Zeng). https://doi.org/10.1016/j.matlet.2018.08.007 0167-577X/Ó 2018 Elsevier B.V. All rights reserved.
Hence, design of a distinctive nanostructure for WO3-SnO2 hybrid nanocomposite is expected to attain superb gas-sensing performances. However, one-step synthesis of WO3-SnO2 composite with unique structure, especially many SnO2 spheres adhering to the surface of WO3 spheres, is hardly controlled and is still a huge challenge. In this work, a novel WO3-SnO2 composite nanostructure composed of bulky WO3 nanosphere functionalized by diminutive SnO2 nanospheres was designed and fabricated via one-step hydrothermal method. Such WO3-SnO2 heterostructure exhibited desirable gas-sensing properties to acetone with large gas response and fast response-recovery speed. The synergetic effect arising from WO3 and SnO2 and the variation in the heterojunction barrier accounted for the origin of the desirable properties. It is expected that our work could offer a reference for the rational design of other binary metal-oxide nanocomposites.
2. Experiments Synthesis: The cactus-like WO3-SnO2 nanocomposite was prepared by a hydrothermal method. Initially, 1 mmol Na2WO42H2O was dissolved in 40 mL of deionized water under magnetic stirring to form a homogeneous solution. Subsequently, 0.5 mmol Na2SnO33H2O and 5 g glucose were added into the above mixture. After vigorous stirring for 10 min, the desired mixture was transferred into a 50 mL Teflon-lined stainless autoclave and kept at 180 °C for 20 h. After naturally cooled to room temperature, the
6
L. Zhu et al. / Materials Letters 231 (2018) 5–7
obtained solid products were washed with deionized water and ethanol in sequence several times and collected by centrifugation. Finally, the as-prepared products were dried in air at 60 °C overnight for further characterization. Characterization: The composition and crystalline structures of the sample were confirmed by X-ray diffraction (XRD) on a Rigaku TTRIII X-ray diffractometer using a Cu-Ka radiation source. The morphologies were observed by field emission scanning microscopy (FESEM, JEOL JSM-7500F) and transmission electron microscopy (TEM, JEOL JEM-2100). Fabrication of gas sensor: The as-prepared powders were mixed with terpineol and then coated onto an alumina ceramic tube which was previously equipped with a pair of gold wires. The operating temperature of the sensor could be precisely controlled by adjusting the current flow across the Ni-Cr alloy coil which was inserted into the tube. Finally, the fabricated sensor was further aged at 200 °C for 24 h to enhance the stability and repeatability. Measurement of gas-sensing properties: The gas-sensing behavior towards acetone was tested on a static system controlled by a central computer. To reach the required gas concentration, a given amount of target gas was injected into the test chamber with a needle valve. The gas response in this work was defined as the ratio S = Ra/Rg, from which Ra and Rg represented the resistances of the sensor in air and target gas, respectively. 3. Results and discussion The crystal structure and purity of the sample were first identified by X-ray diffraction as depicted in Fig. 1. All the diffraction peaks in the XRD pattern can be well accorded with the standard monoclinic WO3 (JCPDS No. 32-1324) and tetragonal rutile SnO2 (JCPDS No. 41-1445). No other diffraction peaks belonging to any impurities was detected, thus suggesting the high purity of the as-prepared sample. The typical morphology of the obtained WO3-SnO2 nanocomposite was analyzed through SEM. As can be seen from Fig. 2a and b, the detailed morphology of the sample presents an apparent similarity to a cactus. The morphology is mainly composed of the bulky nanosphere with a diameter of 500 nm decorated with numerous diminutive nanospheres ranging from 20 to 50 nm in diameter on the surface. More specifically, an individual sphere was further investigated by TEM, as shown in Fig. 2c. It is further identified that a great deal of diminutive nanospheres are firmly adhered upon the bulky nanosphere. Furthermore, the interplanar spacing of 0.335 nm and 0.358 nm observed from the HRTEM images (Fig. 2d and e) corresponds to the (1 1 0) plane of tetragonal SnO2 and (2 0 0) plane of monoclinic WO3, respectively. It is well known that the response of a gas sensor based on metal oxide semiconductors is closely dependent on operating temperature. Fig. 3a displays the gas response of the sensor
Fig. 1. XRD spectra of the as-prepared WO3-SnO2 composite.
Fig. 2. (a and b) SEM images with different magnification of the obtained composite. (Inset: the natural cactus.) (c) TEM image of the sample. HRTEM images of individual (d) diminutive nanosphere and (e) bulky nanosphere.
exposed to 600 ppm of acetone under different temperatures ranging from 160 to 400 °C. The result indicates that the gas response to acetone continuously increases, up to 360 °C, and then gradually decreases with the further increase of the temperature. It is evident that the fabricated sensor possesses high acetone response, and the maximum response to 600 ppm acetone is 26 at 360 °C. Such change in gas response is caused by the quantity of adsorption and desorption of target gas. The amount of gas adsorption will reduce when the operating temperature surpasses the optimum value (360 °C), and the desorption-dominated case leads to a decrease in gas response. Therefore, all further gas sensing measurements for acetone were recorded at the optimum temperature of 360 °C. Fig. 3b shows the gas response of the sensor as a function of acetone concentrations at 360 °C, from which one can see that the gas response increases almost linearly with the acetone concentration in the investigated range. The gas responses are 12.1, 13.8, 16.5, 19.8, 23.5, 26, 29.5 and 32 for 100, 200, 300, 400, 500, 600, 700 and 800 ppm acetone gas, respectively. Additionally, the response and recovery curve of the sensor at 360 °C to 600 ppm acetone was also tested, as illustrated in Fig. 3c. It is clearly seen that the sensor voltage presents a rapid increase when the acetone is in, but suddenly goes back to its initial sate when the target gas is out. Obviously, the WO3-SnO2 composite based gas sensor exhibits the fast response and recovery speed towards acetone. Moreover, Fig. 3d depicts the gas response and recovery curve exposed to 600 ppm acetone after 4 cycles of gas in and out at 360 °C, indicating an excellent stability of the sensor. As both n-type oxides, the basic mechanism for acetone gas sensing can be elucidated by the resistance change due to the adsorption and desorption of oxygen [18]. Briefly, when the sensor is exposed to air, the oxygen molecules get adsorbed onto the surface and consume free electrons from the conduction band thorough oxygen ionization process, and then transform to oxygen ions. The adsorbed oxygen leads to the formation of thick electron depletion layer and an increase in the sensor resistance. However, once introducing acetone gas, the acetone molecules are oxidized by the adsorbed oxygen ions. Meanwhile, the trapped electrons would be released back to the surface of the composite, narrowing the depletion layer and thus decreasing the sensor resistance. In our study, the following two points should be considered to explain the satisfactory performance of the as-synthesized nanocomposite. On the one hand, from the SEM images, it could be readily observed that the surface of bulky WO3 matrix is not completely covered by the diminutive SnO2 nanospheres (Fig. 2a
L. Zhu et al. / Materials Letters 231 (2018) 5–7
7
Fig. 3. (a) Gas response of the WO3-SnO2 composite exposed to 600 ppm acetone at different temperatures. (b) Gas response of the WO3-SnO2 composite towards different gas concentrations at 360 °C. (c) Response-recovery curve of the WO3-SnO2 composite to 600 ppm acetone at 360 °C. (d) Response-recovery curve of the composite after 4 cycles to 600 ppm acetone at 360 °C.
and b). Therefore, both of them are highly accessible for the gas adsorption and could offer more reactive sites. Their synergetic effects contribute to the response enhancement of the gas sensor. On the other hand, the presence of heterojunction at the interface between the WO3 and SnO2 is considerably favorable for the improvement in sensing properties. The electron transfer from WO3 to SnO2 occurs owing to higher Femi level of WO3 comparing to that of SnO2, resulting in the formation of a new potential barrier at the heterojunction between the WO3 and SnO2. The directional flow of electrons further promotes the formation of depletion layer and thereby increases the sensor resistance [19]. As a result, the synergetic effect and the formation of heterojunction cause the excellent sensing properties. 4. Conclusions In summary, a novel cactus-like WO3-SnO2 composite, where bulky WO3 nanosphere is functionalized by diminutive SnO2 nanospheres, has been successfully prepared by the one-step hydrothermal process. The sensor was exposed to acetone for various operating temperatures and gas concentrations. The sensing measurement exhibited that the cactus-like WO3-SnO2 composite possesses a high gas response and fast response-recovery speed towards acetone. The striking synergetic effect and the heterojunction played an important role in the gas-sensing reaction.
Acknowledgment This work was supported by Chongqing Research Program of Basic Research and Frontier Technology (No. cstc2016jcyjA0006). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
A.K. Nayak, R. Ghosh, S. Santra, et al., Nanoscale 7 (2015) 12460–12473. L. Zhu, W. Zeng, H. Ye, et al., Mater. Res. Bull. 100 (2018) 259–264. L. Zhu, Y. Li, W. Zeng, Appl. Surf. Sci. 427 (2018) 281–287. H. Long, W. Zeng, H. Wang, et al., Adv. Sci. 5 (2018) 1700634. Y. Zhang, W. Zeng, Y. Li, J. Alloy. Compd. 749 (2018) 355–362. Y. Zhang, W. Zeng, H. Ye, et al., Appl. Surf. Sci. 442 (2018) 507–516. K. Xu, N. Li, D. Zeng, et al., ACS Appl. Mater. Inter. 7 (2015) 11359–11368. L. Zhu, W. Zeng, Sens. Actuators A: Phys. 267 (2017) 242–261. H. Li, W. Xie, B. Liu, et al., Sens. Actuators B: Chem. 240 (2017) 349–357. L. Han, J. Chen, Y. Zhang, et al., Mater. Lett. 210 (2018) 8–11. S. Wang, W. Yu, C. Cheng, et al., Mater. Res. Bull. 89 (2017) 267–272. S. Yao, F. Qu, G. Wang, et al., J. Alloy. Compd. 724 (2017) 695–702. D. Zhao, X. Wu, Mater. Lett. 210 (2018) 354–357. P. Li, X. Li, Z. Zhao, et al., Electrochim. Acta 192 (2016) 148–157. D. Narsimulu, S. Vinoth, E.S. Srinadhu, et al., Ceram. Int. 44 (2018) 201– 207. H. Li, B. Liu, D. Cai, et al., J. Mater. Chem. A 2 (2014) 6854–6862. M. Yin, Y. Yao, H. Fan, et al., J. Alloy. Compd. 736 (2018) 322–331. Z.-X. Cai, H.-Y. Li, J.-C. Ding, et al., Sens. Actuators B: Chem. 246 (2017) 225– 234. V.K. Tomer, K. Singh, H. Kaur, et al., Sens. Actuators B: Chem. 253 (2017) 703– 713.