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Facile fabrication and enhanced acetic acid sensing properties of honeycomb-like porous ZnO
Materials Letters 138 (2015) 100–103
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication and enhanced acetic acid sensing properties of honeycomb-like porous ZnO J. Lou, S.Y. Ma n, L. Cheng, H.S. Song, W.Q. Li Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2014 Accepted 24 September 2014 Available online 2 October 2014
Honeycomb-like porous ZnO architecture was fabricated by a facile solution-calcination process. The nanostructure and morphology of the porous sample were characterized by XRD, SEM and TEM techniques. This structure self-assembled by numerous irregular nanoparticles and holes, which provided more active centers and gas diffusion paths. The possible formation process was proposed as nucleation, growth and self-assembly of nanoparticles. The sensor based on this architecture shows the high response, fast response/recovery time and good selectivity toward glacial acetic acid (CH3COOH) at 370 1C. And the good reproducibility, long-term stability and liner relationship in the range of 2–1000 ppm make the sensor more practical. The results indicate that the sensor based on ZnO porous honeycomb-like shape can be used for detecting CH3COOH gas. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Crystal growth Semiconductors Sensors
1. Introduction
2. Experimental
In recent years, gas sensors based on semiconductors have been attracted extensively attention [1]. As a potential candidate for gas sensors, ZnO becomes one of the most representative materials due to direct wide band gap of 3.37 eV and large binding energy of 60 meV [2]. In particular, crystal sizes and morphology have a great important role in the performance of chemical and physical properties [3] such as gas-sensing [4], photovoltaic conversion efficiency, magnetical behavior. Therefore many researchers have explored fruitful methods to prepare ZnO nanomaterials with special shape and perfect properties [5]. The aim of the fabrication ways is low-cost, easy to handle, controllable and mass production [6]. Herein, in this paper, a facile solution-calcination synthesis approach is employed to fabrication honeycomb-like porous ZnO specimen. The nanoparticles are self-assembled with the diameter ranging from tens of nanometers to hundreds of nanometers. In addition, the release of organics after calcination leaves a numerous holes, providing more gas active center and diffusion pathways. Therefore, sensor based on this unique feature shows the high response, quick reaction time and good selectivity toward CH3COOH at the optimal working temperature of 370 1C. On the basis of the above results, this sensor based on the honeycomb-like porous shape is a promising candidate for acetic acid detection.
The typical honeycomb-like sample was obtained as follows: 0.898 g Zn (NO3)2 6H2O and 0.436 g PVP were first dissolved into a solvent of 5 ml ethanol. Then the mixed solution was stirred at 30 1C for 2 h. During this process, ammonia water (NH3 H2O) was added drop by drop for adjusting the pH value. The solution was well prepared when it was stable and clear. The final ZnO powder was collected after annealing in a furnace at 500 1C for 1.5 h. The as-calcined specimen was characterized by X-ray diffraction (XRD, D/Max-2000), scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, USA FEI TEVNAI G2 TF20). The process for the sensor fabrication was presented in our previous paper [6]. In this experiment, a test circuit voltage of 5 V was applied. The working temperature (T) was ranging from 260 1C to 440 1C through altering the heating voltage. Herein, the gas response (R) was defined as the ratio of Ra and Rg, while Ra and Rg were the resistance in air and the target gas, respectively. The response and the recovery time were defined as time required reaching 90% of the final equilibrium value [7].
n
Corresponding author. Tel.: þ 86 13919948452; fax: þ 86 9317971503. E-mail address: [email protected] (S.Y. Ma).
http://dx.doi.org/10.1016/j.matlet.2014.09.097 0167-577X/& 2014 Elsevier B.V. All rights reserved.
3. Results and discussion The XRD pattern is shown in Fig. 1, all the diffraction peak positions in the XRD spectrum can be well matched with a pure hexagonal ZnO wurtzite structure (JCPDS data card No. 36-1451).
J. Lou et al. / Materials Letters 138 (2015) 100–103
No other peak due to other phases is detected, indicating Zn(OH)2 has been completely thermal decomposed [8]. Fig. 2a and b display a low magnification and high-resolution cross-sectional SEM images of the as-synthesized product, respectively. It is clearly revealed that the sample presents honeycomblike feature, which consists of numerous nanoparticles. Spherical ZnO nanoparticles are self-assembled layer-by-layer and the holes are bound between the big particles. Careful observation in Fig. 2b can demonstrate that the particle diameters range from several
Fig. 1. XRD pattern of as-synthesis ZnO sample.
101
tens of nanometers to hundreds of nanometers. Fig. 2c and d show the typical TEM and HRTEM images where it can be seen that honeycomb-like ZnO conglomeration is composed of a variety of irregular pores, surface detects and unambiguous grain boundaries. The lattice distance is 1.91 Å which corresponds to (1 0 2) crystallographic orientation. The possible formation process of the honeycomb-like architecture is explained as nucleation, growth and self-assembly of nanoparticles. At first, Zn2 þ and OH ions are ionized in solution after the hydrolysis process. Then the process of nucleation starts between Zn2 þ and OH ions with PVP modification. With the reaction time increasing, the nucleation particles grow larger and aggregated. After calcination stage, the thermal decomposition of organics such as PVP, ethanol leaves small holes on surface of the sample [8,9]. The honeycomb-like porous architecture is obtained with nanoparticles self-assembled. Fig. 3a shows the response of the honeycomb-like ZnO sensor to 1000 ppm CH3COOH at different working temperature (T). Obviously, responses are strongly dependent on the temperatures. The response (R) is found to increase with the temperature increasing first, then obtains the maximum (R¼1051, T ¼370 1C), but minimizes with the further increases the working temperature. Therefore, based the definition of the optimal working temperature [10], the optimal working temperature is 370 1C. The sensor for practical application is required not only for high response, but also for good selectivity. The sensor is conducted to 1000 ppm various gases at 370 1C for discussing the selectivity (as shown in the inset). It is clearly shown that the response is
Fig. 2. (a) and (b) Low-magnification and high-resolution SEM images of ZnO sample, respectively. (c) and (d) TEM and HRTEM images, respectively.
102
J. Lou et al. / Materials Letters 138 (2015) 100–103
evaluated to be 1051, 320.69, 721.86, 32.93 and 20.94 for CH3COOH, ethanol, acetone, DMF and NH3 H2O, respectively. It is obvious demonstrated that the sensor is more sensitive for CH3COOH. Furthermore, Fig. 3b displays response and recovery curves of the sensor to CH3COOH as a function of gas concentration of 2 ppm, 20 ppm and 200 ppm. It is apparently seen that the response increases along with increasing concentration. The response is estimated to be 4.33, 21.45 and 207.56, respectively.
Fig. 3c shows the dynamic resistance change of sensor toward to 2 ppm, 20 ppm and 200 ppm at 370 1C in the logarithmic coordinates. The resistance of sensor is 1200.674 kΩ basically in air ambient. From the enlarged image of 20 ppm curve in Fig. 3d, when the CH3COOH gas is injected in the sensor, the resistance promptly decreases and rapidly reaches a relatively stable value. When the CH3COOH gas is switched off, the resistance quickly increases and reaches the original value (as shown in Fig. 3d).
Fig. 3. (a) The response to 1000 ppm CH3COOH at different testing temperatures; the inset is responses to different gases of 1000 ppm at 370 1C; (b) transient responserecovery curves to 2, 20 and 200 ppm CH3COOH at 370 1C, respectively; (c) resistance change of b; (d) the enlarged of the resistance change curve of the sensor to 20 ppm CH3COOH at 370 1C.
Fig. 4. (a) the response of sensor to CH3COOH concentrations in the range from 2 ppm to 6000 ppm at 370 1C; the inset shows the calibration curve in the range of 2–1000 ppm; (b) the long-term stability of the sensor to 200 ppm CH3COOH at 370 1C, the inset shows the reproducibility of the sensor to 200 ppm CH3COOH at 370 1C.
J. Lou et al. / Materials Letters 138 (2015) 100–103
The response and recovery time of 20 ppm is calculated to be about 14 s and 9 s, respectively. Further, the study on the responses to different gas concentrations is also important for the sensor design. Fig. 4a displays the relationship between the response and the CH3COOH concentrations in the range of 2–6000 ppm. The curve exhibits that the response increases rapidly when the concentration is less than 2000 ppm. But when the concentration is over 2000 ppm, the increase rate becomes slow, indicating that the sensor is more or less statured. The inset of Fig. 4a is a linear calibration curve in the range of 2–1000 ppm, which indicates sensor based on honeycomb-like ZnO can be used as promising materials for CH3COOH sensor. Certainly, the sensor for practical application requires not only for the high response to the target gas, but also good reproducibility and stability. The long-term stability is measured to 200 ppm CH3COOH at different day as shown in Fig. 4b. It is demonstrated that the responses value remains unchanged basically, which conforms the excellent long-term stability of the sensor. The inset curves make clear that the honeycomb-like ZnO based sensor has the good reproducibility. The curve in Fig. 3d can be explained by the gas-sensing mechanism. When the sensor is exposed in air, oxygen molecules are transformed to absorbed oxygen on the surface of the sensor by capturing electrons in the conduction band. Therefore the resistance increases and the depletion region becomes wider. Corresponding, the sensor is exposed in reductive gas ambient such as CH3COOH, the chemical reaction occurs between target gas and absorbed oxygen. Thus electrons will be released back to the conduction band. The resistance decreases and the depletion region becomes thinner after the reaction. The enhanced gassensing performance is attributed to the porous honeycomb-like structure. This structure provides more active center and gas diffusion pathways [8]. Thus when gas is turned on, the reaction between gas and absorbed oxygen ions is more easily to happen. Equally, when gas is turned off, gas will quickly leave the sensor through the holes. In another word, honeycomb-like porous shape enhances the gas sensing performances because of large specific surface area.
103
4. Conclusions In summary, the honeycomb-like porous ZnO nanoparticles with high efficient surface was fabricated via a very simple and low-cost solution method and subsequent calcination. The porous structure and numerous holes provide more active center, which make gas diffusion react easily. Thus, the sensor based on this structure shows the excellent performance for CH3COOH detection at the optimal operating temperature of 370 1C. The fast response, high sensitivity and good selectivity as well as the long-term stability and perfect reproducibility endows the potential significance for application in CH3COOH sensor. Moreover, the fabrication method is a candidate to obtain other materials with different shapes.
Acknowledgment This work was supported by the National Natural Science Foundations of China (Grant no. 10874140), the College Basic Scientific Research Operation Cost of Gansu province (the manufacture and characteristic research of the optical gas sensing film and the Y series superconducting materials), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Natural Science Foundational of Gansu province (Grant nos. 1308RJZA258 and 1308RJZA216).
References [1] [2] [3] [4] [5] [6] [7]
Wang SM, Wang P, Xiao CH, Li ZF, et al. Mater Lett 2014;131:358–60. Li XW, Sun P, Yang TL, Zhao J, et al. CrystEngComm 2012;15:2949–55. Guo WW, Liu TM, Zhang HJ, et al. Sens Actuators, B 2012;166–167:492–9. Ye LJ, Guan WS, Lu CY, Zhao H, Lu X. Mater Lett 2014;118:115–8. Chen XS, Liu JY, Jing XY, Wang J, Song DL, Liu LH. Mater Lett 2013;112:23–5. Luo J, Ma SY, Li FM, et al. Mater Lett 2014;121:137–40. Yu HL, Gao XM, Zhang Y, Meng FN, et al. Sens Actuators, B 2012; 171-172:679–85. [8] Li C, Yu ZS, Fang SM, Wang HX, et al. Chin Chem Lett 2008;19:599–603. [9] Cheng L, Ma SY, Li XB, Luo J, Li WQ, et al. Sens Actuators, B 2014;200:181–90. [10] Zong Y, Cao YL, Jia DZ, Bao SJ, Lu Y. Mater Lett 2010;64:243–5.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Facile fabrication and enhanced acetic acid sensing properties of honeycomb-like porous ZnO J. Lou, S.Y. Ma n, L. Cheng, H.S. Song, W.Q. Li Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 August 2014 Accepted 24 September 2014 Available online 2 October 2014
Honeycomb-like porous ZnO architecture was fabricated by a facile solution-calcination process. The nanostructure and morphology of the porous sample were characterized by XRD, SEM and TEM techniques. This structure self-assembled by numerous irregular nanoparticles and holes, which provided more active centers and gas diffusion paths. The possible formation process was proposed as nucleation, growth and self-assembly of nanoparticles. The sensor based on this architecture shows the high response, fast response/recovery time and good selectivity toward glacial acetic acid (CH3COOH) at 370 1C. And the good reproducibility, long-term stability and liner relationship in the range of 2–1000 ppm make the sensor more practical. The results indicate that the sensor based on ZnO porous honeycomb-like shape can be used for detecting CH3COOH gas. & 2014 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles Crystal growth Semiconductors Sensors
1. Introduction
2. Experimental
In recent years, gas sensors based on semiconductors have been attracted extensively attention [1]. As a potential candidate for gas sensors, ZnO becomes one of the most representative materials due to direct wide band gap of 3.37 eV and large binding energy of 60 meV [2]. In particular, crystal sizes and morphology have a great important role in the performance of chemical and physical properties [3] such as gas-sensing [4], photovoltaic conversion efficiency, magnetical behavior. Therefore many researchers have explored fruitful methods to prepare ZnO nanomaterials with special shape and perfect properties [5]. The aim of the fabrication ways is low-cost, easy to handle, controllable and mass production [6]. Herein, in this paper, a facile solution-calcination synthesis approach is employed to fabrication honeycomb-like porous ZnO specimen. The nanoparticles are self-assembled with the diameter ranging from tens of nanometers to hundreds of nanometers. In addition, the release of organics after calcination leaves a numerous holes, providing more gas active center and diffusion pathways. Therefore, sensor based on this unique feature shows the high response, quick reaction time and good selectivity toward CH3COOH at the optimal working temperature of 370 1C. On the basis of the above results, this sensor based on the honeycomb-like porous shape is a promising candidate for acetic acid detection.
The typical honeycomb-like sample was obtained as follows: 0.898 g Zn (NO3)2 6H2O and 0.436 g PVP were first dissolved into a solvent of 5 ml ethanol. Then the mixed solution was stirred at 30 1C for 2 h. During this process, ammonia water (NH3 H2O) was added drop by drop for adjusting the pH value. The solution was well prepared when it was stable and clear. The final ZnO powder was collected after annealing in a furnace at 500 1C for 1.5 h. The as-calcined specimen was characterized by X-ray diffraction (XRD, D/Max-2000), scanning electron microscopy (SEM, S-4800) and transmission electron microscopy (TEM, USA FEI TEVNAI G2 TF20). The process for the sensor fabrication was presented in our previous paper [6]. In this experiment, a test circuit voltage of 5 V was applied. The working temperature (T) was ranging from 260 1C to 440 1C through altering the heating voltage. Herein, the gas response (R) was defined as the ratio of Ra and Rg, while Ra and Rg were the resistance in air and the target gas, respectively. The response and the recovery time were defined as time required reaching 90% of the final equilibrium value [7].
n
Corresponding author. Tel.: þ 86 13919948452; fax: þ 86 9317971503. E-mail address: [email protected] (S.Y. Ma).
http://dx.doi.org/10.1016/j.matlet.2014.09.097 0167-577X/& 2014 Elsevier B.V. All rights reserved.
3. Results and discussion The XRD pattern is shown in Fig. 1, all the diffraction peak positions in the XRD spectrum can be well matched with a pure hexagonal ZnO wurtzite structure (JCPDS data card No. 36-1451).
J. Lou et al. / Materials Letters 138 (2015) 100–103
No other peak due to other phases is detected, indicating Zn(OH)2 has been completely thermal decomposed [8]. Fig. 2a and b display a low magnification and high-resolution cross-sectional SEM images of the as-synthesized product, respectively. It is clearly revealed that the sample presents honeycomblike feature, which consists of numerous nanoparticles. Spherical ZnO nanoparticles are self-assembled layer-by-layer and the holes are bound between the big particles. Careful observation in Fig. 2b can demonstrate that the particle diameters range from several
Fig. 1. XRD pattern of as-synthesis ZnO sample.
101
tens of nanometers to hundreds of nanometers. Fig. 2c and d show the typical TEM and HRTEM images where it can be seen that honeycomb-like ZnO conglomeration is composed of a variety of irregular pores, surface detects and unambiguous grain boundaries. The lattice distance is 1.91 Å which corresponds to (1 0 2) crystallographic orientation. The possible formation process of the honeycomb-like architecture is explained as nucleation, growth and self-assembly of nanoparticles. At first, Zn2 þ and OH ions are ionized in solution after the hydrolysis process. Then the process of nucleation starts between Zn2 þ and OH ions with PVP modification. With the reaction time increasing, the nucleation particles grow larger and aggregated. After calcination stage, the thermal decomposition of organics such as PVP, ethanol leaves small holes on surface of the sample [8,9]. The honeycomb-like porous architecture is obtained with nanoparticles self-assembled. Fig. 3a shows the response of the honeycomb-like ZnO sensor to 1000 ppm CH3COOH at different working temperature (T). Obviously, responses are strongly dependent on the temperatures. The response (R) is found to increase with the temperature increasing first, then obtains the maximum (R¼1051, T ¼370 1C), but minimizes with the further increases the working temperature. Therefore, based the definition of the optimal working temperature [10], the optimal working temperature is 370 1C. The sensor for practical application is required not only for high response, but also for good selectivity. The sensor is conducted to 1000 ppm various gases at 370 1C for discussing the selectivity (as shown in the inset). It is clearly shown that the response is
Fig. 2. (a) and (b) Low-magnification and high-resolution SEM images of ZnO sample, respectively. (c) and (d) TEM and HRTEM images, respectively.
102
J. Lou et al. / Materials Letters 138 (2015) 100–103
evaluated to be 1051, 320.69, 721.86, 32.93 and 20.94 for CH3COOH, ethanol, acetone, DMF and NH3 H2O, respectively. It is obvious demonstrated that the sensor is more sensitive for CH3COOH. Furthermore, Fig. 3b displays response and recovery curves of the sensor to CH3COOH as a function of gas concentration of 2 ppm, 20 ppm and 200 ppm. It is apparently seen that the response increases along with increasing concentration. The response is estimated to be 4.33, 21.45 and 207.56, respectively.
Fig. 3c shows the dynamic resistance change of sensor toward to 2 ppm, 20 ppm and 200 ppm at 370 1C in the logarithmic coordinates. The resistance of sensor is 1200.674 kΩ basically in air ambient. From the enlarged image of 20 ppm curve in Fig. 3d, when the CH3COOH gas is injected in the sensor, the resistance promptly decreases and rapidly reaches a relatively stable value. When the CH3COOH gas is switched off, the resistance quickly increases and reaches the original value (as shown in Fig. 3d).
Fig. 3. (a) The response to 1000 ppm CH3COOH at different testing temperatures; the inset is responses to different gases of 1000 ppm at 370 1C; (b) transient responserecovery curves to 2, 20 and 200 ppm CH3COOH at 370 1C, respectively; (c) resistance change of b; (d) the enlarged of the resistance change curve of the sensor to 20 ppm CH3COOH at 370 1C.
Fig. 4. (a) the response of sensor to CH3COOH concentrations in the range from 2 ppm to 6000 ppm at 370 1C; the inset shows the calibration curve in the range of 2–1000 ppm; (b) the long-term stability of the sensor to 200 ppm CH3COOH at 370 1C, the inset shows the reproducibility of the sensor to 200 ppm CH3COOH at 370 1C.
J. Lou et al. / Materials Letters 138 (2015) 100–103
The response and recovery time of 20 ppm is calculated to be about 14 s and 9 s, respectively. Further, the study on the responses to different gas concentrations is also important for the sensor design. Fig. 4a displays the relationship between the response and the CH3COOH concentrations in the range of 2–6000 ppm. The curve exhibits that the response increases rapidly when the concentration is less than 2000 ppm. But when the concentration is over 2000 ppm, the increase rate becomes slow, indicating that the sensor is more or less statured. The inset of Fig. 4a is a linear calibration curve in the range of 2–1000 ppm, which indicates sensor based on honeycomb-like ZnO can be used as promising materials for CH3COOH sensor. Certainly, the sensor for practical application requires not only for the high response to the target gas, but also good reproducibility and stability. The long-term stability is measured to 200 ppm CH3COOH at different day as shown in Fig. 4b. It is demonstrated that the responses value remains unchanged basically, which conforms the excellent long-term stability of the sensor. The inset curves make clear that the honeycomb-like ZnO based sensor has the good reproducibility. The curve in Fig. 3d can be explained by the gas-sensing mechanism. When the sensor is exposed in air, oxygen molecules are transformed to absorbed oxygen on the surface of the sensor by capturing electrons in the conduction band. Therefore the resistance increases and the depletion region becomes wider. Corresponding, the sensor is exposed in reductive gas ambient such as CH3COOH, the chemical reaction occurs between target gas and absorbed oxygen. Thus electrons will be released back to the conduction band. The resistance decreases and the depletion region becomes thinner after the reaction. The enhanced gassensing performance is attributed to the porous honeycomb-like structure. This structure provides more active center and gas diffusion pathways [8]. Thus when gas is turned on, the reaction between gas and absorbed oxygen ions is more easily to happen. Equally, when gas is turned off, gas will quickly leave the sensor through the holes. In another word, honeycomb-like porous shape enhances the gas sensing performances because of large specific surface area.
103
4. Conclusions In summary, the honeycomb-like porous ZnO nanoparticles with high efficient surface was fabricated via a very simple and low-cost solution method and subsequent calcination. The porous structure and numerous holes provide more active center, which make gas diffusion react easily. Thus, the sensor based on this structure shows the excellent performance for CH3COOH detection at the optimal operating temperature of 370 1C. The fast response, high sensitivity and good selectivity as well as the long-term stability and perfect reproducibility endows the potential significance for application in CH3COOH sensor. Moreover, the fabrication method is a candidate to obtain other materials with different shapes.
Acknowledgment This work was supported by the National Natural Science Foundations of China (Grant no. 10874140), the College Basic Scientific Research Operation Cost of Gansu province (the manufacture and characteristic research of the optical gas sensing film and the Y series superconducting materials), the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry and Natural Science Foundational of Gansu province (Grant nos. 1308RJZA258 and 1308RJZA216).
References [1] [2] [3] [4] [5] [6] [7]
Wang SM, Wang P, Xiao CH, Li ZF, et al. Mater Lett 2014;131:358–60. Li XW, Sun P, Yang TL, Zhao J, et al. CrystEngComm 2012;15:2949–55. Guo WW, Liu TM, Zhang HJ, et al. Sens Actuators, B 2012;166–167:492–9. Ye LJ, Guan WS, Lu CY, Zhao H, Lu X. Mater Lett 2014;118:115–8. Chen XS, Liu JY, Jing XY, Wang J, Song DL, Liu LH. Mater Lett 2013;112:23–5. Luo J, Ma SY, Li FM, et al. Mater Lett 2014;121:137–40. Yu HL, Gao XM, Zhang Y, Meng FN, et al. Sens Actuators, B 2012; 171-172:679–85. [8] Li C, Yu ZS, Fang SM, Wang HX, et al. Chin Chem Lett 2008;19:599–603. [9] Cheng L, Ma SY, Li XB, Luo J, Li WQ, et al. Sens Actuators, B 2014;200:181–90. [10] Zong Y, Cao YL, Jia DZ, Bao SJ, Lu Y. Mater Lett 2010;64:243–5.