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Curly porous NiO nanosheets with enhanced gas-sensing properties
Materials Letters 190 (2017) 252–255
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Curly porous NiO nanosheets with enhanced gas-sensing properties Y. Lu a, Y.H. Ma b, S.Y. Ma a,⇑, W.X. Jin a, S.H. Yan a, X.L. Xu a, Q. Chen a a Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China b Toxicology Laboratory, Gansu Provincial Center for Disease Control and Prevention, Lanzhou 730030, China
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 28 December 2016 Accepted 5 January 2017 Available online 6 January 2017 Keywords: Porous nanosheets P-type oxide semiconductors Hydrothermal Gas-sensing Transmission electron microscopy (TEM) Scanning electron microscopy (SEM)
a b s t r a c t Curly porous NiO nanosheets were successfully fabricated by a simple hydrothermal approach. Changing the amount of ammonia, we obtained NiO nanosheets with different morphologies. Thus it could be seen the important role of ammonia in synthesizing curly porous NiO nanosheets. The structure, morphologies and gas-sensing properties of these samples were studied. Among these samples, the curly porous NiO nanosheets sensor exhibits significant improvement on ethanol gas sensing properties. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, materials with porous nanosheets structure attract many people’s eyes, owing to their potential applications in photocatalysis, magnetism, aquatic environment and many other fields [1–3]. Furthermore, it’s worth noting that this kind of porous structure has high specific surface area to the benefit of gas diffusion, and the existence of defects such as oxygen vacancies could help oxygen adsorption and further enhance the gas sensitivity and gas response speed [4,5]. As a significant p-type semiconductor material, nickel oxide (NiO) has drawn considerable research attention, various NiO porous structures such as porous nanospheres [6], porous nanotube [7], and porous nanobelts [8] have been successfully fabricated by different methods. In this paper, we report a simple and eco-friendly hydrothermal method for preparation of NiO nanosheets. The interesting thing is that NiO nanosheets with or without pores can be controlled by changing the amount of ammonia. The obtained curly porous NiO nanosheets sensor showed excellent gas-sensing properties toward ethanol. 2. Experimental All chemicals and organic solvents used were the analyticalgrade reagents and without any further purification. In a typical ⇑ Corresponding author. E-mail address: [email protected] (S.Y. Ma). http://dx.doi.org/10.1016/j.matlet.2017.01.020 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
procedure, 0.594 g of NiCl26H2O and 0.360 g of urea were first dissolved into 60 ml of glycol-water (1:2, v-v) mixture. Then ammonia (NH3H2O, 25%) was added into the above mixed solution drop by drop for adjusting the pH value under continuous magnetic stirring. Following, the resulting mixtures were transferred to a 100 ml Teflon-lined autoclave and maintained at 140 °C for 12 h then cooled down to room temperature naturally. After reaction, blue precipitated powder was collected by centrifugation and washed several times with deionized water and absolute ethanol, and then dried at 60 °C for 4 h. The final products were obtained by annealing in a furnace at 500 °C for 2 h. Three different samples (S1, S2, and S3) were obtained by adjusting the different pH value (pH = 5, pH = 8, pH = 11). The samples were examined by X-ray diffraction (XRD) (D/Max2400), field emission scanning electron microscope (FESEM) (JSM6701F), transmission electron microscopic (TEM) (USA FEI TEVNAI G2 TF20) and low temperature nitrogen adsorption technique using the JW-004 instrument. Gas-sensing performance test was operated with measurement system of WS-60A. The sensor response (S) for reducing gases was defined as Rg/Ra, where Rg or Ra was the resistance of sensor in the target gas or in air, respectively.
3. Results and discussion Fig. 1(a) shows the XRD patterns of S1, S2 and S3. Obviously, all the diffraction peaks are well indexed to JCPDS card (No. 04-0835) and no impurities peaks are observed. It indicates the high purity
Y. Lu et al. / Materials Letters 190 (2017) 252–255
of these samples. As Fig. 1(b) shows that the corresponding EDX of S2 consists of Ni, O elements and further proves the XRD results. The peaks of Cu and C are from the Cu and C grid used in the TEM measurement.
253
The effect of adjusting pH value with ammonia on the morphologies of samples was investigated in detail by FESEM and TEM. As depicted in Fig. 2(a), the structure of S1 (obtained without ammonia, the pH value is 5) consists of curled nonporous
Fig. 1. (a) XRD patterns of S1, S2 and S3. (b) EDX pattern of S2.
Fig. 2. (a) Panoramic SEM image of S1. High magnification SEM image of S1 (b), S2 (c) and S3 (d). (e) Low magnification TEM image inserted with high magnification TEM image of S2. (f) HRTEM image inserted with SAED pattern of S2.
254
Y. Lu et al. / Materials Letters 190 (2017) 252–255
nanosheets with smooth surface. A higher magnification SEM image (Fig. 2(b)) gives more information about the thickness of the NiO nanosheets (less than 10 nm). In the case of introduction of ammonia drop by drop (pH = 8), the nanosheets exhibit porous characteristics, the typical curly porous NiO nanosheets (S2) are formed as shown in Fig. 2(c). However, when the amount of ammonia increases until pH = 11 (S3), the nanosheets become fragmentary due to increased pore size, as seen from Fig. 2(d). The
Fig. 3. N2 adsorption-desorption isotherm and the corresponding BJH pore size distribution (the inset) of S2.
typical TEM image of S2 (Fig. 2(e)) are further demonstrate that the nanosheets are consist of nanoparticles, and the diameter of pores is about 5–20 nm. Ammonia maybe influent the aggregation level of nanoparticles, the porosity of materials originates from slits between nanoparticles. Without ammonia, the nanoparticles combine closely, the nanosheets are nonporous. With increasing amount of ammonia, the slits between nanoparticles increase, the nanosheets appear porosity. The HRTEM image of S2 (Fig. 2 (f)) exhibits well-defined lattice fringes, the interplanar distances of 0.241 and 0.208 nm are indexed to the (1 1 1) and (2 0 0) planes of the hexagonal phase of NiO, respectively. The inset of Fig. 2(f) shows the SAED pattern of S2 is consistent with strong ring patterns due to (1 1 1), (2 0 0), and (2 2 0) planes of hexagonal NiO. The results agree with HRTEM image and XRD pattern very well. N2 adsorption and desorption isotherms of S2 are shown in Fig. 3, which exhibit a type IV isotherm (the characteristic isotherm of mesoporous materials). The BJH pore size distribution plot can be seen in the inset of Fig. 3. The main pore size of S2 is about 5–20 nm and the result agrees with the high magnification TEM image. However, the low volume capability indicates very few mesopores, it may be due to the mesopores of nanosheets are accumulated by nanoparticles randomly, which is not a stable structure. The corresponding BET specific surface area of S2 was calculated to be 53 m2/g. As comparison, the BET specific surface area of S1 and S3 decreased to 20 m2/g and 35 m2/g, respectively. As is well-known, various gas-sensing properties are usually dependent on the working temperature. Fig. 4(a) shows 240 °C is the optimum working temperature of the ethanol sensors. At the same time, as we can see the bigger BET specific surface area of materials, the higher response, and S2 has the highest response that is 11.15. Fig. 4(b) shows the selectivity of S1 and S2, we can
Fig. 4. (a) The response of three sensors to 50 ppm ethanol at different testing temperatures from 120 to 320 °C. (b) The responses of S1 and S2 to 50 ppm different gases at 240 °C. (c) Response versus time curves of S2 to ethanol in the range from 1 to 50 ppm at 240 °C, the inset shows the relation between the response values of S2 and ethanol concentration. (d) Dynamic sensing transient of S1 and S2 to 50 ppm ethanol at 240 °C.
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Y. Lu et al. / Materials Letters 190 (2017) 252–255 Table 1 Comparison of ethanol sensor based on varied NiO nanostructures reported before and in this work. Materials
Ethanol concentration (ppm)
Working temperature (°C)
Response (Rg/Ra)
Response/recovery time (s)
Refs.
2.15 at% Al-doped NiO nanorod-flowers Flower-like NiO microspheres Fe-doped ordered mesoporous NiO NiO nanowires Curly porous NiO nanosheets
100 100 50 50 50
200 250 240 350 240
12 3.2 3.9 3.4 11.15
48/40 Non-giving 7/18 4/5 4/7
[9] [10] [11] [12] This work
see both of them had excellent selectivity on ethanol, but the response of S2 to 50 ppm ethanol is at least two times higher than the response of S1, meaning that the curly porous NiO nanosheets sensor has a more satisfactory selectivity for ethanol. Fig. 4(c) shows the response curves of S2 at different ethanol concentrations. The response increases with the increase of the ethanol concentration. The relation between the response of S2 and the ethanol concentration is close to a linear relation in the range of 1–50 ppm (the inset of Fig. 4(c)). Fig. 4(d) illustrates the response/recovery time of S2 doesn’t exceed 4/7 s, respectively. But the response/recovery time of S1 is about 18/30 s, respectively. The comparison of ethanol sensing performances between the sensor in this work and other NiO sensors reported previously is shown in Table 1. Comprehensively considering all gas-sensing properties, it is obviously that curly porous nanosheets in our work showed relatively good gas-sensing performances. Gas sensing characters of porous nanosheets structure is superior to nonporous nanosheets by comparing their sensing properties. This result is likely to attribute to porous structure, large numbers of pores on curly porous NiO nanosheets increase gas sensitive region and speed up the gas transmission, the gas is easy to diffuse into the inside surface and increase the activity site of material, which in favor of further promote the response and rapid adsorption/desorption. However pore size increases, the specific surface area of materials instead decreases thus reducing the gas-sensing response. All above studies about gas-sensing characteristics of the sensors were completed by measuring the resistance changes of the sensors. The resistance changes are due to the exchange of charges between absorbed gaseous species and the metal oxide surface. As a p-type semiconductor, when NiO sensors are in air at high temperature, oxygen molecules would adsorb onto the NiO surface and come into being ionized oxygen species (O2 , O and O2 ) by taking electrons near the surfaces of NiO. Once exposed to reducing gases, such as ethanol, the electrons are injected into the material through the oxidation reaction between the reducing gas and the
oxygen anions, which decreases the concentration of holes and increases the sensor resistance. 4. Conclusion In summary, a facile, economic and environmental friendly hydrothermal method was reported for the formation of curly porous NiO nanosheets. We found that the ammonia plays an important role in synthesis of the porous NiO nanosheets by observation on the morphology of products. Compared with the nonporous NiO nanosheets sensor, the porous NiO nanosheets sensor exhibits excellent ethanol sensing performances at 240 °C. Thus, this curly porous NiO nanosheets is more suitable for fabricating ethanol sensor. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant Nos. 10874140, 51562035 and 51462031). References [1] X.H. Li, J.Q. Wang, Y.H. Zhang, M.H. Cao, Mater. Res. Bull. 70 (2015) 728–734. [2] Q. Dong, S. Yin, C.S. Guo, X.Y. Wu, N. Kumada, T. Takei, et al., Appl. Catal. B 147 (2014) 741–747. [3] P. Mahamallik, S. Saha, A. Pal, Chem. Eng. J. 276 (2015) 155–165. [4] J.H. Lee, Sens. Actuators, B 140 (2009) 319–336. [5] Y.H. Li, W. Luo, N. Qin, J.P. Dong, J. Wei, W. Li, et al., Angew. Chem. Int. Ed. 53 (2014) 9035–9040. [6] W.L. Zhu, A. Shui, L.F. Xu, X.S. Cheng, P.A. Liu, H. Wang, Ultrason. Sonochem. 21 (2014) 1707–1713. [7] F. Cao, G.X. Pan, X.H. Xia, P.S. Tang, H.F. Chen, J. Power Sources 264 (2014) 161– 167. [8] H. Liang, L. Liu, Z.J. Yang, Y.Z. Yang, Cryst. Res. Technol. 45 (2010) 661–666. [9] C. Wang, X.B. Cui, J.Y. Liu, X. Zhou, X.Y. Cheng, P. Sun, et al., ACS Sens. 1 (2016) 131–136. [10] X. San, G. Wang, B. Liang, J. Ma, D. Meng, Y.B. Shen, J. Alloys Compd. 636 (2015) 357–362. [11] X.H. Sun, X.D. Hu, Y.C. Wang, R. Xiong, X. Li, J. Liu, et al., J. Phys. Chem. C 119 (2015) 3228–3237. [12] L.Y. Lin, T.M. Liu, B. Miao, Mater. Res. Bull. 48 (2013) 449–454.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Curly porous NiO nanosheets with enhanced gas-sensing properties Y. Lu a, Y.H. Ma b, S.Y. Ma a,⇑, W.X. Jin a, S.H. Yan a, X.L. Xu a, Q. Chen a a Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China b Toxicology Laboratory, Gansu Provincial Center for Disease Control and Prevention, Lanzhou 730030, China
a r t i c l e
i n f o
Article history: Received 2 November 2016 Received in revised form 28 December 2016 Accepted 5 January 2017 Available online 6 January 2017 Keywords: Porous nanosheets P-type oxide semiconductors Hydrothermal Gas-sensing Transmission electron microscopy (TEM) Scanning electron microscopy (SEM)
a b s t r a c t Curly porous NiO nanosheets were successfully fabricated by a simple hydrothermal approach. Changing the amount of ammonia, we obtained NiO nanosheets with different morphologies. Thus it could be seen the important role of ammonia in synthesizing curly porous NiO nanosheets. The structure, morphologies and gas-sensing properties of these samples were studied. Among these samples, the curly porous NiO nanosheets sensor exhibits significant improvement on ethanol gas sensing properties. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, materials with porous nanosheets structure attract many people’s eyes, owing to their potential applications in photocatalysis, magnetism, aquatic environment and many other fields [1–3]. Furthermore, it’s worth noting that this kind of porous structure has high specific surface area to the benefit of gas diffusion, and the existence of defects such as oxygen vacancies could help oxygen adsorption and further enhance the gas sensitivity and gas response speed [4,5]. As a significant p-type semiconductor material, nickel oxide (NiO) has drawn considerable research attention, various NiO porous structures such as porous nanospheres [6], porous nanotube [7], and porous nanobelts [8] have been successfully fabricated by different methods. In this paper, we report a simple and eco-friendly hydrothermal method for preparation of NiO nanosheets. The interesting thing is that NiO nanosheets with or without pores can be controlled by changing the amount of ammonia. The obtained curly porous NiO nanosheets sensor showed excellent gas-sensing properties toward ethanol. 2. Experimental All chemicals and organic solvents used were the analyticalgrade reagents and without any further purification. In a typical ⇑ Corresponding author. E-mail address: [email protected] (S.Y. Ma). http://dx.doi.org/10.1016/j.matlet.2017.01.020 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
procedure, 0.594 g of NiCl26H2O and 0.360 g of urea were first dissolved into 60 ml of glycol-water (1:2, v-v) mixture. Then ammonia (NH3H2O, 25%) was added into the above mixed solution drop by drop for adjusting the pH value under continuous magnetic stirring. Following, the resulting mixtures were transferred to a 100 ml Teflon-lined autoclave and maintained at 140 °C for 12 h then cooled down to room temperature naturally. After reaction, blue precipitated powder was collected by centrifugation and washed several times with deionized water and absolute ethanol, and then dried at 60 °C for 4 h. The final products were obtained by annealing in a furnace at 500 °C for 2 h. Three different samples (S1, S2, and S3) were obtained by adjusting the different pH value (pH = 5, pH = 8, pH = 11). The samples were examined by X-ray diffraction (XRD) (D/Max2400), field emission scanning electron microscope (FESEM) (JSM6701F), transmission electron microscopic (TEM) (USA FEI TEVNAI G2 TF20) and low temperature nitrogen adsorption technique using the JW-004 instrument. Gas-sensing performance test was operated with measurement system of WS-60A. The sensor response (S) for reducing gases was defined as Rg/Ra, where Rg or Ra was the resistance of sensor in the target gas or in air, respectively.
3. Results and discussion Fig. 1(a) shows the XRD patterns of S1, S2 and S3. Obviously, all the diffraction peaks are well indexed to JCPDS card (No. 04-0835) and no impurities peaks are observed. It indicates the high purity
Y. Lu et al. / Materials Letters 190 (2017) 252–255
of these samples. As Fig. 1(b) shows that the corresponding EDX of S2 consists of Ni, O elements and further proves the XRD results. The peaks of Cu and C are from the Cu and C grid used in the TEM measurement.
253
The effect of adjusting pH value with ammonia on the morphologies of samples was investigated in detail by FESEM and TEM. As depicted in Fig. 2(a), the structure of S1 (obtained without ammonia, the pH value is 5) consists of curled nonporous
Fig. 1. (a) XRD patterns of S1, S2 and S3. (b) EDX pattern of S2.
Fig. 2. (a) Panoramic SEM image of S1. High magnification SEM image of S1 (b), S2 (c) and S3 (d). (e) Low magnification TEM image inserted with high magnification TEM image of S2. (f) HRTEM image inserted with SAED pattern of S2.
254
Y. Lu et al. / Materials Letters 190 (2017) 252–255
nanosheets with smooth surface. A higher magnification SEM image (Fig. 2(b)) gives more information about the thickness of the NiO nanosheets (less than 10 nm). In the case of introduction of ammonia drop by drop (pH = 8), the nanosheets exhibit porous characteristics, the typical curly porous NiO nanosheets (S2) are formed as shown in Fig. 2(c). However, when the amount of ammonia increases until pH = 11 (S3), the nanosheets become fragmentary due to increased pore size, as seen from Fig. 2(d). The
Fig. 3. N2 adsorption-desorption isotherm and the corresponding BJH pore size distribution (the inset) of S2.
typical TEM image of S2 (Fig. 2(e)) are further demonstrate that the nanosheets are consist of nanoparticles, and the diameter of pores is about 5–20 nm. Ammonia maybe influent the aggregation level of nanoparticles, the porosity of materials originates from slits between nanoparticles. Without ammonia, the nanoparticles combine closely, the nanosheets are nonporous. With increasing amount of ammonia, the slits between nanoparticles increase, the nanosheets appear porosity. The HRTEM image of S2 (Fig. 2 (f)) exhibits well-defined lattice fringes, the interplanar distances of 0.241 and 0.208 nm are indexed to the (1 1 1) and (2 0 0) planes of the hexagonal phase of NiO, respectively. The inset of Fig. 2(f) shows the SAED pattern of S2 is consistent with strong ring patterns due to (1 1 1), (2 0 0), and (2 2 0) planes of hexagonal NiO. The results agree with HRTEM image and XRD pattern very well. N2 adsorption and desorption isotherms of S2 are shown in Fig. 3, which exhibit a type IV isotherm (the characteristic isotherm of mesoporous materials). The BJH pore size distribution plot can be seen in the inset of Fig. 3. The main pore size of S2 is about 5–20 nm and the result agrees with the high magnification TEM image. However, the low volume capability indicates very few mesopores, it may be due to the mesopores of nanosheets are accumulated by nanoparticles randomly, which is not a stable structure. The corresponding BET specific surface area of S2 was calculated to be 53 m2/g. As comparison, the BET specific surface area of S1 and S3 decreased to 20 m2/g and 35 m2/g, respectively. As is well-known, various gas-sensing properties are usually dependent on the working temperature. Fig. 4(a) shows 240 °C is the optimum working temperature of the ethanol sensors. At the same time, as we can see the bigger BET specific surface area of materials, the higher response, and S2 has the highest response that is 11.15. Fig. 4(b) shows the selectivity of S1 and S2, we can
Fig. 4. (a) The response of three sensors to 50 ppm ethanol at different testing temperatures from 120 to 320 °C. (b) The responses of S1 and S2 to 50 ppm different gases at 240 °C. (c) Response versus time curves of S2 to ethanol in the range from 1 to 50 ppm at 240 °C, the inset shows the relation between the response values of S2 and ethanol concentration. (d) Dynamic sensing transient of S1 and S2 to 50 ppm ethanol at 240 °C.
255
Y. Lu et al. / Materials Letters 190 (2017) 252–255 Table 1 Comparison of ethanol sensor based on varied NiO nanostructures reported before and in this work. Materials
Ethanol concentration (ppm)
Working temperature (°C)
Response (Rg/Ra)
Response/recovery time (s)
Refs.
2.15 at% Al-doped NiO nanorod-flowers Flower-like NiO microspheres Fe-doped ordered mesoporous NiO NiO nanowires Curly porous NiO nanosheets
100 100 50 50 50
200 250 240 350 240
12 3.2 3.9 3.4 11.15
48/40 Non-giving 7/18 4/5 4/7
[9] [10] [11] [12] This work
see both of them had excellent selectivity on ethanol, but the response of S2 to 50 ppm ethanol is at least two times higher than the response of S1, meaning that the curly porous NiO nanosheets sensor has a more satisfactory selectivity for ethanol. Fig. 4(c) shows the response curves of S2 at different ethanol concentrations. The response increases with the increase of the ethanol concentration. The relation between the response of S2 and the ethanol concentration is close to a linear relation in the range of 1–50 ppm (the inset of Fig. 4(c)). Fig. 4(d) illustrates the response/recovery time of S2 doesn’t exceed 4/7 s, respectively. But the response/recovery time of S1 is about 18/30 s, respectively. The comparison of ethanol sensing performances between the sensor in this work and other NiO sensors reported previously is shown in Table 1. Comprehensively considering all gas-sensing properties, it is obviously that curly porous nanosheets in our work showed relatively good gas-sensing performances. Gas sensing characters of porous nanosheets structure is superior to nonporous nanosheets by comparing their sensing properties. This result is likely to attribute to porous structure, large numbers of pores on curly porous NiO nanosheets increase gas sensitive region and speed up the gas transmission, the gas is easy to diffuse into the inside surface and increase the activity site of material, which in favor of further promote the response and rapid adsorption/desorption. However pore size increases, the specific surface area of materials instead decreases thus reducing the gas-sensing response. All above studies about gas-sensing characteristics of the sensors were completed by measuring the resistance changes of the sensors. The resistance changes are due to the exchange of charges between absorbed gaseous species and the metal oxide surface. As a p-type semiconductor, when NiO sensors are in air at high temperature, oxygen molecules would adsorb onto the NiO surface and come into being ionized oxygen species (O2 , O and O2 ) by taking electrons near the surfaces of NiO. Once exposed to reducing gases, such as ethanol, the electrons are injected into the material through the oxidation reaction between the reducing gas and the
oxygen anions, which decreases the concentration of holes and increases the sensor resistance. 4. Conclusion In summary, a facile, economic and environmental friendly hydrothermal method was reported for the formation of curly porous NiO nanosheets. We found that the ammonia plays an important role in synthesis of the porous NiO nanosheets by observation on the morphology of products. Compared with the nonporous NiO nanosheets sensor, the porous NiO nanosheets sensor exhibits excellent ethanol sensing performances at 240 °C. Thus, this curly porous NiO nanosheets is more suitable for fabricating ethanol sensor. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant Nos. 10874140, 51562035 and 51462031). References [1] X.H. Li, J.Q. Wang, Y.H. Zhang, M.H. Cao, Mater. Res. Bull. 70 (2015) 728–734. [2] Q. Dong, S. Yin, C.S. Guo, X.Y. Wu, N. Kumada, T. Takei, et al., Appl. Catal. B 147 (2014) 741–747. [3] P. Mahamallik, S. Saha, A. Pal, Chem. Eng. J. 276 (2015) 155–165. [4] J.H. Lee, Sens. Actuators, B 140 (2009) 319–336. [5] Y.H. Li, W. Luo, N. Qin, J.P. Dong, J. Wei, W. Li, et al., Angew. Chem. Int. Ed. 53 (2014) 9035–9040. [6] W.L. Zhu, A. Shui, L.F. Xu, X.S. Cheng, P.A. Liu, H. Wang, Ultrason. Sonochem. 21 (2014) 1707–1713. [7] F. Cao, G.X. Pan, X.H. Xia, P.S. Tang, H.F. Chen, J. Power Sources 264 (2014) 161– 167. [8] H. Liang, L. Liu, Z.J. Yang, Y.Z. Yang, Cryst. Res. Technol. 45 (2010) 661–666. [9] C. Wang, X.B. Cui, J.Y. Liu, X. Zhou, X.Y. Cheng, P. Sun, et al., ACS Sens. 1 (2016) 131–136. [10] X. San, G. Wang, B. Liang, J. Ma, D. Meng, Y.B. Shen, J. Alloys Compd. 636 (2015) 357–362. [11] X.H. Sun, X.D. Hu, Y.C. Wang, R. Xiong, X. Li, J. Liu, et al., J. Phys. Chem. C 119 (2015) 3228–3237. [12] L.Y. Lin, T.M. Liu, B. Miao, Mater. Res. Bull. 48 (2013) 449–454.