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Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
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Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies Yanbai Shen a,*, Wei Wang a, Anfeng Fan a, Dezhou Wei a, Wengang Liu a, Cong Han a, Yansong Shen b, Dan Meng c, Xiaoguang San c a
College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia c College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China b
article info
abstract
Article history:
SnO2 nanomaterials with different morphologies, such as nanofilms, nanorods, and
Received 3 June 2015
nanowires, were fabricated by sputtering and thermal evaporation methods. Their
Received in revised form
hydrogen sensing properties were then investigated. The structural characterizations
20 September 2015
showed that the SnO2 in these nanomaterials was tetragonal. The surface-to-volume ratio
Accepted 22 September 2015
of the nanofilms, nanorods, and nanowires increased, leading to an increase in the
Available online xxx
effective surface area. Gas sensors based on these SnO2 nanomaterials showed a reversible response to hydrogen at various concentrations. The response order of the nanofilms,
Keywords:
nanorods and nanowires was enhanced while the peak operating temperature was
SnO2
decreased from 250 to 150 C, and the response or recovery time became shorter. The re-
Nanomaterial
sults indicated that the sensor response effectively increased as the effective surface area
Morphology
of the SnO2 nanomaterials increased, demonstrating that gas-sensing properties could be
Hydrogen
significantly improved by changing the nanomaterial morphology.
Gas sensor
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen (H2) has promise as a clean energy carrier that can be generated from renewable energy sources, helping solve some critical problems such as the depletion of fossil fuel resources, pollution, and climate change due to greenhouse gas emissions [1]. Currently, hydrogen is widely used in ammonia synthesis, oil refining, fuel cells, and rocket engines [2]. However, hydrogen production, storage, and transport can be hazardous because hydrogen is flammable or explosive if not handled properly [3]. Therefore, hydrogen sensors that are
capable of leak detection, especially at the ppm level, are important. To date, many studies have been conducted to develop hydrogen sensors based on metal oxide semiconductors, such as SnO2, WO3, ZnO, NiO, and Nb2O5, all of which were prepared by various techniques, including sputtering, thermal evaporation, chemical vapor deposition, hydrothermal synthesis, and solegel [4e8]. Among these metal oxide semiconductors, SnO2, with a wide band gap of 3.6 eV at 300 K, has proved to be a promising candidate for gas sensors [9,10], photocatalysts [11,12], dye-sensitized solar cells [13,14], and lithium-ion batteries [15,16]. Considerable efforts have been
* Corresponding author. Tel./fax: þ86 24 83692711. E-mail address: [email protected] (Y. Shen). http://dx.doi.org/10.1016/j.ijhydene.2015.09.077 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
made to prepare SnO2 nanomaterials to improve their sensitivity, selectivity and stability because gas-sensing properties are strongly dependent on its morphology or shape. SnO2 nanomaterials with different morphologies and structures, such as nanowires [17], nanorods [18], nanoparticles [19], nanobelts [20], nanotubes [21], and nanoribbons [22], have been successfully synthesized to investigate their sensing properties. Although it has been stressed that the morphology and structure of SnO2 nanomaterials significantly affect their gas sensing properties [9,10], the major structural influence parameter is not yet clear and still needs to be investigated for the further development of high-performance gas sensing devices. Additionally, most gas sensors based on metal oxide semiconductors have shown a maximum response at operating temperatures above 200 C, resulting in high power consumption and complexities in integration. These results may reduce the sensor life and limit the applications of these gas sensors [23]. Therefore, there is still a need to develop gas sensors that have high sensing performance at low operating temperatures. In this study, SnO2 nanomaterials with different nanofilm, nanorod, and nanowire morphologies were fabricated by sputtering and thermal evaporation methods. The microstructural characterizations of these SnO2 nanomaterials were investigated by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and the BrunauereEmmetteTeller (BET) method. The hydrogen sensing properties of gas sensors based on these SnO2 nanomaterials were investigated with respect to their response, operating temperature, reversibility, and hydrogen concentration.
Experimental Preparation of SnO2 nanomaterials In this study, SnO2 nanofilms were prepared by the sputtering method, and SnO2 nanorods and nanowires were synthesized by the thermal evaporation method. In the case of the SnO2 nanofilms, the films were deposited on oxidized Si substrates at a discharge gas pressure of 24 Pa and a substrate temperature of room temperature (25 C) by reactive dc magnetron sputtering. A circular tin target that was 100 mm in diameter with a purity of 99.99% was used. The discharge gas was an argoneoxygen mixture with a ratio of Ar:O2 ¼ 2:3. The discharge current was fixed at 80 mA, and the discharge voltage showed a value of 270 V. Before deposition, the target was pre-sputtered for 5 min with a shutter closed. After the deposition, the films were annealed to stabilize the film structure at 450 C for 1 h in air with a heating and cooling rate of 2 C min1. In the case of the SnO2 nanorods, tin grains with a high purity of 99.99% were placed at the bottom of an alumina boat. The oxidized Si substrates were placed on the top of the tin grains with a vertical distance of 5 mm. The boat was then positioned in the central part of a 45 cm-long horizontal quartz tube in a tubular electric furnace. Argon gas mixed with 5 vol.% oxygen was introduced into the quartz
tube at a flow rate of 50 mL min1 at ambient pressure. Then, the furnace was heated to 900 C at a heating rate of 10 C min1 and maintained at this temperature for 1 h. After the furnace was naturally cooled to room temperature, one layer of the rod-shaped products was obtained on the substrates. In the case of the SnO2 nanowires, the preparation procedure was similar to that of the SnO2 nanorods except for two points: 1) tin grains were placed on the surface of the oxidized Si substrates in the alumina boat; 2) only argon gas was introduced into the quartz tube.
Microstructure characterization of SnO2 nanomaterials The crystallographic structure of the SnO2 nanomaterials was investigated by XRD using an X-ray diffractometer (PANA) in the 2q alytical X'Pert Pro) with Cu Ka1 radiation (l ¼ 1.5406 range of 20e60 . The operating voltage and current were 40 kV and 40 mA, respectively. The morphology of the SnO2 nanomaterials was observed by a field emission scanning electron microscope (ZEISS Ultra Plus) with an operating voltage of 20 kV. The effective surface areas were estimated by N2 gas isotherms at a relative pressure (P/P0) ranging from 0.05 to 0.3 using the BET method described in Ref. [24] on a Quantachrome AUTOSORB-1-C facility after the samples were vacuumdried at 150 C for 1 h.
Hydrogen sensing measurement of SnO2 nanomaterials Gas sensors were fabricated by depositing SnO2 nanomaterials onto the oxidized Si substrates with a pair of interdigitated Pt electrodes. The SnO2 nanofilms were directly deposited on the sensor substrates, and the SnO2 nanorod or nanowire gas sensors were fabricated by pouring a few drops of SnO2 nanorod- or nanowire-suspended ethanol onto the sensor substrates. Subsequently, the sensor samples with a thickness of approximately 300 nm were aged at 400 C for 1 h in air to stabilize the sensing layer. It was confirmed from FESEM images that the morphologies and structures of these SnO2 nanomaterials were not changed on the SiO2 and Pt surfaces of the sensor substrates. Hydrogen sensing measurements were conducted in a sealed quartz tube furnace as reported elsewhere [24]. The operating temperature was controlled by the electric furnace and varied from room temperature of 25 Ce300 C. Hydrogen was introduced into the quartz tube to a predefined concentration of 100e1000 ppm by mixing dry synthetic air and hydrogen. The mass flow controllers adjusted the gas flow; the total flow rate was kept constant at 200 mL min1. The resistance of the sensors was determined by measuring the electric current, which flowed when a voltage of 10 V was applied between interdigitated Pt electrodes. In this measurement, a computerized Agilent 34972A multimeter was used. The sensor response was defined as (Ra Rg)/Rg, where Ra and Rg were the electrical resistances before and after introducing hydrogen, respectively.
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
Results and discussion Microstructure characterization Fig. 1 shows the XRD patterns of the SnO2 nanofilms, nanorods, and nanowires. All of the diffraction peaks of these SnO2 nanomaterials are identified as tetragonal in the SnO2 structure, as indicated in JCPDS card No. 41e1445. SnO2 nanofilms and nanorods tend to be (110) plane oriented, while SnO2 nanowires tend to be (101) plane oriented. The full width at half maximums (FWHM) of the (101) peak are 1.3 , 0.4 , and 0.2 for SnO2 nanofilms, nanowires, and nanorods, respectively. The decrease in the FWHM indicates a crystallinity improvement in the nanofilms, nanowires, and nanorods. More importantly, no other crystal forms were found, except for a Si peak that came from the Si substrate used for the preparation of the SnO2 nanorods, revealing that single-phase SnO2 nanomaterials were obtained. FESEM images of the SnO2 nanofilms, nanorods, and nanowires are illustrated in Fig. 2. Fig. 2(a) and (b) show the surface and cross-sectional FESEM images of the SnO2 nanofilms deposited at 24 Pa and room temperature. The film is composed of columnar nanograins separated by voids, and the grain size is approximately 40 nm in diameter and 300 nm in thickness. These columnar nanograins are loosely packed, indicating a high film porosity, which is consistent with the microstructure model of sputtered films presented by Thornton [25,26]. The low- and high-magnification FESEM images of the SnO2 nanorods are illustrated in Fig. 2(c) and (d). It is shown that the nanorods with high mass production are uniformly distributed on the substrate (Fig. 2(c)). SnO2 nanorods with a smooth surface are 30 nm in diameter and several tens to several hundreds of nanometers in length (Fig. 2(d)). Fig. 2(e) and (f) present the low- and high-magnification FESEM images of the SnO2 nanowires. It is noted that SnO2 nanowires of 30e200 nm in diameter and several tens of micrometers in length are observed, indicating a high surface-to-volume ratio. Based on the above FESEM images, it is concluded that
Fig. 1 e XRD patterns of the SnO2 nanomaterials. (a) SnO2 nanofilms. (b) SnO2 nanorods. (c) SnO2 nanowires.
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the surface-to-volume ratio increases in the order of nanofilms, nanorods, and nanowires. The effective surface areas of three SnO2 nanomaterials were measured, and the corresponding result is illustrated in Fig. 3. It is indicated that the effective surface areas are 12.6 m2 g1 for the SnO2 nanofilms, 21.7 m2 g1 for the SnO2 nanorods, and 34.5 m2 g1 for the SnO2 nanowires. In the film deposited at a high pressure of 24 Pa, the nanograins surrounded by voids appear to be loosely packed. Thus, the effective surface area is attributed to the top surface area and the sidewall area of the naked nanograins, which results in a relatively low effective surface area compared with the SnO2 nanorods and nanowires. The effective surface area of the SnO2 nanowires is the highest among the three SnO2 nanomaterials, mainly resulting from its highest surface-tovolume ratio. It is expected that the nanomaterial with the highest effective surface area will show a large response to the detected gas, as identified in the following experiments.
Gas sensing properties The changes in the resistance of the gas sensors based on three SnO2 nanomaterials upon exposure to 1000 ppm H2 at an operating temperature of 150 C are shown in Fig. 4. For each sensor sample, the resistance decreases upon exposure to H2 and then recovers to its initial value after H2 removal, indicating a good reversibility of these gas sensors. Additionally, some important results can be obtained. First, the resistance in air Ra decreases in the order of nanofilms, nanorods, and nanowires; these results demonstrate that the nanomaterial with the larger effective surface area shows a lower Ra. More importantly, a low sensor resistance is very promising to fabricate the sensor device used for a practical measurement system. Second, the change in the resistance and thus the response increases in the order of nanofilms, nanorods, and nanowires. Third, the response and recovery times become shorter in the order of nanofilms, nanorods, and nanowires, demonstrating that the nanomaterial with a larger effective surface area demonstrates a quicker reaction between H2 and the nanomaterials. The responses of the SnO2 nanomaterials upon exposure to 1000 ppm H2 as a function of the operating temperature are shown in Fig. 5. The figure shows that each sensor has a peak operating temperature at which the response shows the maximum value. The peak responses are 2.3 for the SnO2 nanofilms at 250 C, 2.8 for the SnO2 nanorods at 200 C, and 5.5 for the SnO2 nanowires at 150 C. Based on the results obtained in Fig. 3, it is concluded that as the effective surface area of three SnO2 nanomaterials increases, the peak operating temperature gradually decreases. This decrease in peak operating temperature is accompanied by an abrupt increase in the peak response. This result is very useful and promising, and it demonstrates the potential to develop SnO2 nanomaterial-based gas sensors with low power consumption. It should be noted that two sensor samples were fabricated for each type of gas sensor. The responses of the two sensor samples agreed well with each other, indicating a good reproducibility. Generally, the dependence of the conductivity of an n-type semiconductor on operating temperature can be expressed by
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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Fig. 2 e (a) Surface and (b) cross-sectional FESEM images of SnO2 nanofilms. (c) Low- and (d) high-magnification FESEM images of SnO2 nanorods. (e) Low- and (f) high-magnification FESEM images of SnO2 nanowires. the following relationship: s ¼ soexp(Ea/(kBT)) [27e29], where s is the electrical conductivity; Ea is the activation energy; kB is the Boltzmann constant; and T is the operating temperature in Kelvin. By plotting a logarithm of the electrical conductivity before the introduction of H2 versus 1/T, the activation energies calculated from the slope of the Arrhenius plots for the
Fig. 3 e Effective surface areas of SnO2 nanomaterials.
three SnO2 nanomaterials can be obtained in the operating temperature range of 25e150 C, as presented in Fig. 6. The determined activation energies are 0.47 eV for the SnO2 nanofilms, 0.31 eV for the SnO2 nanorods, and 0.08 eV for the
Fig. 4 e Changes in the resistance of gas sensors based on SnO2 nanomaterials upon exposure to 1000 ppm H2 at an operating temperature of 150 C.
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e7
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Fig. 5 e Responses of SnO2 nanomaterials upon exposure to 1000 ppm H2 as a function of the operating temperature.
SnO2 nanowires. The smaller the activation energy, the easier it is to accomplish the sensing reaction. The activation energy decreases in the order of nanofilms, nanorods, and nanowires, which could be indicative of superiority in the high response and quick response/recovery time for SnO2 nanowires. Additionally, it reveals that the morphology and structure of semiconductor materials play key roles in determining the gas sensing properties of a semiconductor. Fig. 7 demonstrates the typical dynamic responses of SnO2 nanomaterial gas sensors upon exposure to H2 with various concentrations at each peak operating temperature. The resistance decreases rapidly upon exposure to H2 and recovers to its initial value after removing H2, indicating a good reversibility of these sensors. It should be noted that the change in the resistance increases with increasing H2 concentration in the range of 100e1000 ppm for each sensor, demonstrating that the response greatly enhances at a high H2 Fig. 7 e Typical dynamic responses of SnO2 nanomaterial gas sensors upon exposure to H2 with various concentrations at each peak operating temperature. (a) SnO2 nanofilms at 250 C. (b) SnO2 nanorods at 200 C. (c) SnO2 nanowires at 150 C.
Fig. 6 e Arrhenius plots representing the electrical conductivities of SnO2 nanomaterials before H2 introduction versus the reciprocal of the operating temperature (1/T).
concentration. At a fixed H2 concentration, the change in the resistance increases in the order of nanofilms, nanorods, and nanowires. Particularly, the response and recovery times gradually become short in the order of nanofilms, nanorods, and nanowires. Such quick response and recovery speeds are attributed to the large surface-to-volume ratio and great surface activities for the SnO2 nanowires compared with the nanofilms and nanorods, which results in the rapid and effective diffusion of the gaseous species into nanowires, thereby favoring fast adsorption and desorption kinetics [30,31].
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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Fig. 7 shows that these sensors generally have two response/recovery steps, especially the SnO2 nanofilms. When H2 is introduced, a rapid and then slow resistance decrease occurs; during the recovery period, the resistance increases quickly first, followed by a slow increase. This response/recovery of the sensor based on the semiconductor is a complicated physicalechemistry process, including surface adsorption/desorption, oxygen inter-crystallite diffusion, gas diffusion inside porous structure, and bulk oxygen diffusion [24,32]. The surface adsorption/desorption is the first step of gas response/recovery, which causes a quick surface potential change of the sensors. Subsequently, the diffusion process occurs. It takes a relatively longer time to make gas species disperse or be removed from the sensors, depending on the surface-to-volume ratio of the nanomaterials and the operating temperature. According to the results obtained in Fig. 7, the relationships between the response and the H2 concentration for the three SnO2 nanomaterials at each peak operating temperature can be obtained. As shown in Fig. 8, there is a relatively linear relationship between the response and H2 concentration for each sensor. Thus, it is possible to determine a wide range of H2 concentrations from the response signal. At a fixed H2 concentration, it can clearly be observed that the response increases in the order of SnO2 nanofilms, nanorods, and nanowires. The response of the SnO2 nanofilms upon exposure to H2 is relatively low, and the response is greatly enhanced for SnO2 nanorods and nanowires. The highest responses are obtained for the SnO2 nanowire based sensor, and the responses upon exposure to 100, 200, 400, 800, and 1000 ppm H2 are 0.9, 1.7, 3.0, 4.2, and 5.5, respectively.
Conclusions The influence of the morphology of SnO2 nanomaterials including nanofilms, nanorods, and nanowires, on H2 sensing properties were systematically investigated in this study.
Fig. 8 e Relationships between the response and H2 concentration for SnO2 nanofilms at 250 C, SnO2 nanorods at 200 C, and SnO2 nanowires at 150 C.
Microstructure characterizations demonstrated that these tetragonal SnO2 nanomaterials showed an increasing surfaceto-volume ratio and effective surface area in the order of nanofilms, nanorods, and nanowires. The sensors based on these SnO2 nanomaterials obtained the highest response to H2 at 250 C for nanofilms, 200 C for nanorods, and 150 C for nanowires. The response to H2 was reversible and increased linearly with the increasing H2 concentration. The activation energy decreased in the order of nanofilms, nanorods, and nanowires, thus gradually enhancing the gas sensing properties. In summary, the morphology and structure of semiconductor materials play a key role in determining its gassensing properties.
Acknowledgments This project was supported by the National Natural Science Foundation of China (51422402, 61403263), the Fundamental Research Funds for the Central Universities (N140105002, N130301003), the Program for Liaoning Excellent Talents in University (LJQ2013025), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20130042120033), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-3).
references
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ScienceDirect journal homepage: www.elsevier.com/locate/he
Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies Yanbai Shen a,*, Wei Wang a, Anfeng Fan a, Dezhou Wei a, Wengang Liu a, Cong Han a, Yansong Shen b, Dan Meng c, Xiaoguang San c a
College of Resources and Civil Engineering, Northeastern University, Shenyang 110819, China Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia c College of Chemical Engineering, Shenyang University of Chemical Technology, Shenyang 110142, China b
article info
abstract
Article history:
SnO2 nanomaterials with different morphologies, such as nanofilms, nanorods, and
Received 3 June 2015
nanowires, were fabricated by sputtering and thermal evaporation methods. Their
Received in revised form
hydrogen sensing properties were then investigated. The structural characterizations
20 September 2015
showed that the SnO2 in these nanomaterials was tetragonal. The surface-to-volume ratio
Accepted 22 September 2015
of the nanofilms, nanorods, and nanowires increased, leading to an increase in the
Available online xxx
effective surface area. Gas sensors based on these SnO2 nanomaterials showed a reversible response to hydrogen at various concentrations. The response order of the nanofilms,
Keywords:
nanorods and nanowires was enhanced while the peak operating temperature was
SnO2
decreased from 250 to 150 C, and the response or recovery time became shorter. The re-
Nanomaterial
sults indicated that the sensor response effectively increased as the effective surface area
Morphology
of the SnO2 nanomaterials increased, demonstrating that gas-sensing properties could be
Hydrogen
significantly improved by changing the nanomaterial morphology.
Gas sensor
Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen (H2) has promise as a clean energy carrier that can be generated from renewable energy sources, helping solve some critical problems such as the depletion of fossil fuel resources, pollution, and climate change due to greenhouse gas emissions [1]. Currently, hydrogen is widely used in ammonia synthesis, oil refining, fuel cells, and rocket engines [2]. However, hydrogen production, storage, and transport can be hazardous because hydrogen is flammable or explosive if not handled properly [3]. Therefore, hydrogen sensors that are
capable of leak detection, especially at the ppm level, are important. To date, many studies have been conducted to develop hydrogen sensors based on metal oxide semiconductors, such as SnO2, WO3, ZnO, NiO, and Nb2O5, all of which were prepared by various techniques, including sputtering, thermal evaporation, chemical vapor deposition, hydrothermal synthesis, and solegel [4e8]. Among these metal oxide semiconductors, SnO2, with a wide band gap of 3.6 eV at 300 K, has proved to be a promising candidate for gas sensors [9,10], photocatalysts [11,12], dye-sensitized solar cells [13,14], and lithium-ion batteries [15,16]. Considerable efforts have been
* Corresponding author. Tel./fax: þ86 24 83692711. E-mail address: [email protected] (Y. Shen). http://dx.doi.org/10.1016/j.ijhydene.2015.09.077 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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made to prepare SnO2 nanomaterials to improve their sensitivity, selectivity and stability because gas-sensing properties are strongly dependent on its morphology or shape. SnO2 nanomaterials with different morphologies and structures, such as nanowires [17], nanorods [18], nanoparticles [19], nanobelts [20], nanotubes [21], and nanoribbons [22], have been successfully synthesized to investigate their sensing properties. Although it has been stressed that the morphology and structure of SnO2 nanomaterials significantly affect their gas sensing properties [9,10], the major structural influence parameter is not yet clear and still needs to be investigated for the further development of high-performance gas sensing devices. Additionally, most gas sensors based on metal oxide semiconductors have shown a maximum response at operating temperatures above 200 C, resulting in high power consumption and complexities in integration. These results may reduce the sensor life and limit the applications of these gas sensors [23]. Therefore, there is still a need to develop gas sensors that have high sensing performance at low operating temperatures. In this study, SnO2 nanomaterials with different nanofilm, nanorod, and nanowire morphologies were fabricated by sputtering and thermal evaporation methods. The microstructural characterizations of these SnO2 nanomaterials were investigated by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and the BrunauereEmmetteTeller (BET) method. The hydrogen sensing properties of gas sensors based on these SnO2 nanomaterials were investigated with respect to their response, operating temperature, reversibility, and hydrogen concentration.
Experimental Preparation of SnO2 nanomaterials In this study, SnO2 nanofilms were prepared by the sputtering method, and SnO2 nanorods and nanowires were synthesized by the thermal evaporation method. In the case of the SnO2 nanofilms, the films were deposited on oxidized Si substrates at a discharge gas pressure of 24 Pa and a substrate temperature of room temperature (25 C) by reactive dc magnetron sputtering. A circular tin target that was 100 mm in diameter with a purity of 99.99% was used. The discharge gas was an argoneoxygen mixture with a ratio of Ar:O2 ¼ 2:3. The discharge current was fixed at 80 mA, and the discharge voltage showed a value of 270 V. Before deposition, the target was pre-sputtered for 5 min with a shutter closed. After the deposition, the films were annealed to stabilize the film structure at 450 C for 1 h in air with a heating and cooling rate of 2 C min1. In the case of the SnO2 nanorods, tin grains with a high purity of 99.99% were placed at the bottom of an alumina boat. The oxidized Si substrates were placed on the top of the tin grains with a vertical distance of 5 mm. The boat was then positioned in the central part of a 45 cm-long horizontal quartz tube in a tubular electric furnace. Argon gas mixed with 5 vol.% oxygen was introduced into the quartz
tube at a flow rate of 50 mL min1 at ambient pressure. Then, the furnace was heated to 900 C at a heating rate of 10 C min1 and maintained at this temperature for 1 h. After the furnace was naturally cooled to room temperature, one layer of the rod-shaped products was obtained on the substrates. In the case of the SnO2 nanowires, the preparation procedure was similar to that of the SnO2 nanorods except for two points: 1) tin grains were placed on the surface of the oxidized Si substrates in the alumina boat; 2) only argon gas was introduced into the quartz tube.
Microstructure characterization of SnO2 nanomaterials The crystallographic structure of the SnO2 nanomaterials was investigated by XRD using an X-ray diffractometer (PANA) in the 2q alytical X'Pert Pro) with Cu Ka1 radiation (l ¼ 1.5406 range of 20e60 . The operating voltage and current were 40 kV and 40 mA, respectively. The morphology of the SnO2 nanomaterials was observed by a field emission scanning electron microscope (ZEISS Ultra Plus) with an operating voltage of 20 kV. The effective surface areas were estimated by N2 gas isotherms at a relative pressure (P/P0) ranging from 0.05 to 0.3 using the BET method described in Ref. [24] on a Quantachrome AUTOSORB-1-C facility after the samples were vacuumdried at 150 C for 1 h.
Hydrogen sensing measurement of SnO2 nanomaterials Gas sensors were fabricated by depositing SnO2 nanomaterials onto the oxidized Si substrates with a pair of interdigitated Pt electrodes. The SnO2 nanofilms were directly deposited on the sensor substrates, and the SnO2 nanorod or nanowire gas sensors were fabricated by pouring a few drops of SnO2 nanorod- or nanowire-suspended ethanol onto the sensor substrates. Subsequently, the sensor samples with a thickness of approximately 300 nm were aged at 400 C for 1 h in air to stabilize the sensing layer. It was confirmed from FESEM images that the morphologies and structures of these SnO2 nanomaterials were not changed on the SiO2 and Pt surfaces of the sensor substrates. Hydrogen sensing measurements were conducted in a sealed quartz tube furnace as reported elsewhere [24]. The operating temperature was controlled by the electric furnace and varied from room temperature of 25 Ce300 C. Hydrogen was introduced into the quartz tube to a predefined concentration of 100e1000 ppm by mixing dry synthetic air and hydrogen. The mass flow controllers adjusted the gas flow; the total flow rate was kept constant at 200 mL min1. The resistance of the sensors was determined by measuring the electric current, which flowed when a voltage of 10 V was applied between interdigitated Pt electrodes. In this measurement, a computerized Agilent 34972A multimeter was used. The sensor response was defined as (Ra Rg)/Rg, where Ra and Rg were the electrical resistances before and after introducing hydrogen, respectively.
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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Results and discussion Microstructure characterization Fig. 1 shows the XRD patterns of the SnO2 nanofilms, nanorods, and nanowires. All of the diffraction peaks of these SnO2 nanomaterials are identified as tetragonal in the SnO2 structure, as indicated in JCPDS card No. 41e1445. SnO2 nanofilms and nanorods tend to be (110) plane oriented, while SnO2 nanowires tend to be (101) plane oriented. The full width at half maximums (FWHM) of the (101) peak are 1.3 , 0.4 , and 0.2 for SnO2 nanofilms, nanowires, and nanorods, respectively. The decrease in the FWHM indicates a crystallinity improvement in the nanofilms, nanowires, and nanorods. More importantly, no other crystal forms were found, except for a Si peak that came from the Si substrate used for the preparation of the SnO2 nanorods, revealing that single-phase SnO2 nanomaterials were obtained. FESEM images of the SnO2 nanofilms, nanorods, and nanowires are illustrated in Fig. 2. Fig. 2(a) and (b) show the surface and cross-sectional FESEM images of the SnO2 nanofilms deposited at 24 Pa and room temperature. The film is composed of columnar nanograins separated by voids, and the grain size is approximately 40 nm in diameter and 300 nm in thickness. These columnar nanograins are loosely packed, indicating a high film porosity, which is consistent with the microstructure model of sputtered films presented by Thornton [25,26]. The low- and high-magnification FESEM images of the SnO2 nanorods are illustrated in Fig. 2(c) and (d). It is shown that the nanorods with high mass production are uniformly distributed on the substrate (Fig. 2(c)). SnO2 nanorods with a smooth surface are 30 nm in diameter and several tens to several hundreds of nanometers in length (Fig. 2(d)). Fig. 2(e) and (f) present the low- and high-magnification FESEM images of the SnO2 nanowires. It is noted that SnO2 nanowires of 30e200 nm in diameter and several tens of micrometers in length are observed, indicating a high surface-to-volume ratio. Based on the above FESEM images, it is concluded that
Fig. 1 e XRD patterns of the SnO2 nanomaterials. (a) SnO2 nanofilms. (b) SnO2 nanorods. (c) SnO2 nanowires.
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the surface-to-volume ratio increases in the order of nanofilms, nanorods, and nanowires. The effective surface areas of three SnO2 nanomaterials were measured, and the corresponding result is illustrated in Fig. 3. It is indicated that the effective surface areas are 12.6 m2 g1 for the SnO2 nanofilms, 21.7 m2 g1 for the SnO2 nanorods, and 34.5 m2 g1 for the SnO2 nanowires. In the film deposited at a high pressure of 24 Pa, the nanograins surrounded by voids appear to be loosely packed. Thus, the effective surface area is attributed to the top surface area and the sidewall area of the naked nanograins, which results in a relatively low effective surface area compared with the SnO2 nanorods and nanowires. The effective surface area of the SnO2 nanowires is the highest among the three SnO2 nanomaterials, mainly resulting from its highest surface-tovolume ratio. It is expected that the nanomaterial with the highest effective surface area will show a large response to the detected gas, as identified in the following experiments.
Gas sensing properties The changes in the resistance of the gas sensors based on three SnO2 nanomaterials upon exposure to 1000 ppm H2 at an operating temperature of 150 C are shown in Fig. 4. For each sensor sample, the resistance decreases upon exposure to H2 and then recovers to its initial value after H2 removal, indicating a good reversibility of these gas sensors. Additionally, some important results can be obtained. First, the resistance in air Ra decreases in the order of nanofilms, nanorods, and nanowires; these results demonstrate that the nanomaterial with the larger effective surface area shows a lower Ra. More importantly, a low sensor resistance is very promising to fabricate the sensor device used for a practical measurement system. Second, the change in the resistance and thus the response increases in the order of nanofilms, nanorods, and nanowires. Third, the response and recovery times become shorter in the order of nanofilms, nanorods, and nanowires, demonstrating that the nanomaterial with a larger effective surface area demonstrates a quicker reaction between H2 and the nanomaterials. The responses of the SnO2 nanomaterials upon exposure to 1000 ppm H2 as a function of the operating temperature are shown in Fig. 5. The figure shows that each sensor has a peak operating temperature at which the response shows the maximum value. The peak responses are 2.3 for the SnO2 nanofilms at 250 C, 2.8 for the SnO2 nanorods at 200 C, and 5.5 for the SnO2 nanowires at 150 C. Based on the results obtained in Fig. 3, it is concluded that as the effective surface area of three SnO2 nanomaterials increases, the peak operating temperature gradually decreases. This decrease in peak operating temperature is accompanied by an abrupt increase in the peak response. This result is very useful and promising, and it demonstrates the potential to develop SnO2 nanomaterial-based gas sensors with low power consumption. It should be noted that two sensor samples were fabricated for each type of gas sensor. The responses of the two sensor samples agreed well with each other, indicating a good reproducibility. Generally, the dependence of the conductivity of an n-type semiconductor on operating temperature can be expressed by
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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Fig. 2 e (a) Surface and (b) cross-sectional FESEM images of SnO2 nanofilms. (c) Low- and (d) high-magnification FESEM images of SnO2 nanorods. (e) Low- and (f) high-magnification FESEM images of SnO2 nanowires. the following relationship: s ¼ soexp(Ea/(kBT)) [27e29], where s is the electrical conductivity; Ea is the activation energy; kB is the Boltzmann constant; and T is the operating temperature in Kelvin. By plotting a logarithm of the electrical conductivity before the introduction of H2 versus 1/T, the activation energies calculated from the slope of the Arrhenius plots for the
Fig. 3 e Effective surface areas of SnO2 nanomaterials.
three SnO2 nanomaterials can be obtained in the operating temperature range of 25e150 C, as presented in Fig. 6. The determined activation energies are 0.47 eV for the SnO2 nanofilms, 0.31 eV for the SnO2 nanorods, and 0.08 eV for the
Fig. 4 e Changes in the resistance of gas sensors based on SnO2 nanomaterials upon exposure to 1000 ppm H2 at an operating temperature of 150 C.
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Fig. 5 e Responses of SnO2 nanomaterials upon exposure to 1000 ppm H2 as a function of the operating temperature.
SnO2 nanowires. The smaller the activation energy, the easier it is to accomplish the sensing reaction. The activation energy decreases in the order of nanofilms, nanorods, and nanowires, which could be indicative of superiority in the high response and quick response/recovery time for SnO2 nanowires. Additionally, it reveals that the morphology and structure of semiconductor materials play key roles in determining the gas sensing properties of a semiconductor. Fig. 7 demonstrates the typical dynamic responses of SnO2 nanomaterial gas sensors upon exposure to H2 with various concentrations at each peak operating temperature. The resistance decreases rapidly upon exposure to H2 and recovers to its initial value after removing H2, indicating a good reversibility of these sensors. It should be noted that the change in the resistance increases with increasing H2 concentration in the range of 100e1000 ppm for each sensor, demonstrating that the response greatly enhances at a high H2 Fig. 7 e Typical dynamic responses of SnO2 nanomaterial gas sensors upon exposure to H2 with various concentrations at each peak operating temperature. (a) SnO2 nanofilms at 250 C. (b) SnO2 nanorods at 200 C. (c) SnO2 nanowires at 150 C.
Fig. 6 e Arrhenius plots representing the electrical conductivities of SnO2 nanomaterials before H2 introduction versus the reciprocal of the operating temperature (1/T).
concentration. At a fixed H2 concentration, the change in the resistance increases in the order of nanofilms, nanorods, and nanowires. Particularly, the response and recovery times gradually become short in the order of nanofilms, nanorods, and nanowires. Such quick response and recovery speeds are attributed to the large surface-to-volume ratio and great surface activities for the SnO2 nanowires compared with the nanofilms and nanorods, which results in the rapid and effective diffusion of the gaseous species into nanowires, thereby favoring fast adsorption and desorption kinetics [30,31].
Please cite this article in press as: Shen Y, et al., Highly sensitive hydrogen sensors based on SnO2 nanomaterials with different morphologies, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/j.ijhydene.2015.09.077
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Fig. 7 shows that these sensors generally have two response/recovery steps, especially the SnO2 nanofilms. When H2 is introduced, a rapid and then slow resistance decrease occurs; during the recovery period, the resistance increases quickly first, followed by a slow increase. This response/recovery of the sensor based on the semiconductor is a complicated physicalechemistry process, including surface adsorption/desorption, oxygen inter-crystallite diffusion, gas diffusion inside porous structure, and bulk oxygen diffusion [24,32]. The surface adsorption/desorption is the first step of gas response/recovery, which causes a quick surface potential change of the sensors. Subsequently, the diffusion process occurs. It takes a relatively longer time to make gas species disperse or be removed from the sensors, depending on the surface-to-volume ratio of the nanomaterials and the operating temperature. According to the results obtained in Fig. 7, the relationships between the response and the H2 concentration for the three SnO2 nanomaterials at each peak operating temperature can be obtained. As shown in Fig. 8, there is a relatively linear relationship between the response and H2 concentration for each sensor. Thus, it is possible to determine a wide range of H2 concentrations from the response signal. At a fixed H2 concentration, it can clearly be observed that the response increases in the order of SnO2 nanofilms, nanorods, and nanowires. The response of the SnO2 nanofilms upon exposure to H2 is relatively low, and the response is greatly enhanced for SnO2 nanorods and nanowires. The highest responses are obtained for the SnO2 nanowire based sensor, and the responses upon exposure to 100, 200, 400, 800, and 1000 ppm H2 are 0.9, 1.7, 3.0, 4.2, and 5.5, respectively.
Conclusions The influence of the morphology of SnO2 nanomaterials including nanofilms, nanorods, and nanowires, on H2 sensing properties were systematically investigated in this study.
Fig. 8 e Relationships between the response and H2 concentration for SnO2 nanofilms at 250 C, SnO2 nanorods at 200 C, and SnO2 nanowires at 150 C.
Microstructure characterizations demonstrated that these tetragonal SnO2 nanomaterials showed an increasing surfaceto-volume ratio and effective surface area in the order of nanofilms, nanorods, and nanowires. The sensors based on these SnO2 nanomaterials obtained the highest response to H2 at 250 C for nanofilms, 200 C for nanorods, and 150 C for nanowires. The response to H2 was reversible and increased linearly with the increasing H2 concentration. The activation energy decreased in the order of nanofilms, nanorods, and nanowires, thus gradually enhancing the gas sensing properties. In summary, the morphology and structure of semiconductor materials play a key role in determining its gassensing properties.
Acknowledgments This project was supported by the National Natural Science Foundation of China (51422402, 61403263), the Fundamental Research Funds for the Central Universities (N140105002, N130301003), the Program for Liaoning Excellent Talents in University (LJQ2013025), the Specialized Research Fund for the Doctoral Program of Higher Education of China (20130042120033), and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry (47-3).
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