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Synthesis and Gas Sensing Properties of Au@In2O3 Core-shell Nanoparticles
Available online at www.sciencedirect.com
ScienceDirect Procedia Engineering 168 (2016) 227 – 230
30th Eurosensors Conference, EUROSENSORS 2016
Synthesis and gas sensing properties of Au@In2O3 core-shell nanoparticles Yeon-Tae Yua,*, Sanjit Manohar Majhia, Ho-Geun Songb a
Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, South Korea b Ogam Technology Co., Jeonju 54882, South Korea
Abstract The interest in hybrid nanoparticles (NPs) arises from their combined and often synergetic properties exceeding the functionality of the individual components. Herein, a general, environmentally friendly, and facile hydrothermal approach to synthesize the metal-semiconductor, Au@In2O3 core-shell heteronanostructures is described. These Au@In2O3 core-shell NPs are used as chemiresistive gas sensors for the detection of gases such H2, CO, NO2, VOCs in air. Sensor device based on Au@In2O3 coreshell NPs showed greater response than that of In2O3 NPs sensor device. The enhanced activity can be attributed to the catalytic effect of Au, and synergistic interactions between the Au and In2O3 NPs formed into the core-shell heteronanostructures in such a way that favors the efficient electron transfer at the interface © Published by Elsevier Ltd. This ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Gas sensor; Au@In2O3; core-shell nanoparticles
Motivation and results The semiconducting metal oxide gas sensors have attracted much attention in the field of gas sensing under atmospheric conditions due to their low cost and flexibility in production, simplicity of their use, large number of detectable gases. Recently, great efforts have been centered on the design of the semiconductor oxides with excellent sensing performance. Owing to their exceptional catalytic activities, noble metals such as Pt, Pd, Ag, and Au are often introduced into metal oxide semiconductors as sensitizers or promoters to improve the sensor performance under certain conditions.1 However, the noble metals, especially Au nanoparticles, will suffer from undesired aggregation arising from its low melting point and increased mobility at high operating temperature, which results in a loss of catalytic activity. Thus, to overcome such issues, noble metal particles were encapsulated into a metal oxide semiconductor shell. By encapsulating noble metal nanoparticles in a protective shell, the hybrid
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.168
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Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
core-shell structure could effectively increase the stability of catalyst against undesirable aggregation during practical operation. Metal@metal oxide core–shells NPs have attracted great interest in gas sensing applications because the heteronanostructures can leads to synergetic effects that enhance gas sensing properties.2,3 Indium oxide (In2O3), as a typical n-type metal oxide semiconductor with a direct band gap of 3.55–3.75 eV, could potentially be used to detecting number of gases because of its high electrical conductivity, abundant defects. In2O3 with high specific surface areas and hierarchical structures have been shown to be promising for sensing applications. If noble metal nanoparticles can be encapsulated into In2O3 nanostructures enhanced activity and sensitivity can be expected because they can improve the adsorption of gas molecules and accelerate the electron exchange between the sensor materials and target gas. In a typical synthesis of Au@In2O3 core–shell nanostructures, at first 5 ml of citrate-stabilized Au NPs prepared in the above manner are kept under stirring for 5 min. The well-dissolved solution containing 2.4 mM of Na3cit and 2 mM of InCl3 was added under stirring. The solution was stirred at room temperature for 20 min and then 2 mM of urea was added and kept on stirring for another 20 min. After being stirred for 20 min, the light reddish purple colored solution was transformed into a 40 ml Teflon-lined stainless steel autoclave and was heated at 140 °C for 24 hours. Then the autoclave was cooled to room temperature naturally. The obtained purple colored precipitate was washed and centrifuged with deionized water at 9000 rpm/10 min and the purification and centrifugation were repeated five times. The obtained sample was dried at 60 °C overnight so as to obtain the Au@InOOH core–shell NPs. Finally, the Au@In2O3 core–shell NPs were obtained by calcining the Au@InOOH core–shell NPs synthesized in the above manner at 350 °C for 2 hours. The heating rate was controlled at 2 °C min−1. The core–shell nanostructures produced from Au nanospheres are displayed in Fig. 1. The average diameter of the well dispersed Au nanocrystal seeds (Fig. 1a) was about 15 nm. After being coated with an InOOH shell (Fig. 1b and c), the total size increased up to 90 nm. The InOOH shell is in close contact with the Au core and appears rough on the outer surface. It is probably due to the aggregated InOOH nanoparticles. After calcination, the InOOH shell was transformed into In2O3 and the corresponding Au@In2O3 core–shell NPs are shown in Fig 1(d–f). The shapes of the core–shell nanostructures resemble those of their corresponding cores except that the overall sizes become larger.
Figure 1: TEM images of (a) Au NPs, (b,c) Au@InOOH and (d-f) Au@In2O3 core-shell NPs.
Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
Figure 2: Dependence of the sensor response of Au@In2O3 core-shell and In2O3 NPs at different temperatures to 100 ppm H2 gas.
To test the sensing response (Rs=Rair/Rgas) of Au@ In2O3 core-shell nanoparticles for various gases, we investigated our sensors sensitivity for H2, CO, NO2, C2H5OH, and CH3CHO at the operating temperature of 300oC to 100 ppm levels. The response was estimated by measuring the electrical resistance in the presence of air (Ra) and in gas (Rg).2,3 Fig. 2 shows the response to a fixed H2 of 100 ppm as a function of working temperature on both samples. Obviously, the response of the tested sensors varied with operating temperature, both the responses continuously increased and reached maximum at 300 °C, and after that the response of both sensors decreased. At this temperature, the Au@In2O3 core–shell and In2O3 nanostructures showed a higher response of 34.4 and 9.3, respectively. As shown in Figure 3, the response towards CO, NO2, C2H5OH, and CH3CHO is lower than that of hydrogen, underscoring a remarkably high selectivity toward H2. It might be due to the diameters of all gas molecules are much larger than those of H2, therefore it is difficult for the larger molecules to penetrate into the inner layers of Au@In2O3 core-shell NPs film and react with the chemisorbed oxygen. The sensing performance of our sensor to all the target gases could be attributed to the redox reaction with the surface absorbed oxygen species. Here we also observed that the sensitivity/selectivity of Au@In2O3 core-shell NPs to H2 gas was greatly increased by combining In2O3 with Au into a core-shell heterostructured form. In this core-shell heterostructured form, Au acts as electronic sensitizer which causes a dominant mechanism to increase the selectivity as well as sensitivity. Electronic sensitization is due to a changing Schottky barrier at the interface between Au and In2O3 gas sensing material as a consequence of the interaction of the Au with the gas phase. In this study, we have synthesized Au@In2O3 core-shell hybrid heteronanostructures by microwave hydrothermal method and the gas sensing property of such Au@In2O3 core-shell nanostructures over In2O3 was investigated. The diameter of Au core was around 10 nm and whereas the In2O3 shell thickness was found to be about 80 nm. We significantly enhanced the gas-sensing properties of n-type In2O3 by creating core-shell hybrid heteronanostructures with Au metal NPs and the enhancement was observed in the response to other interfering gases such as CO, NO2, C2H5OH, CH3CHO. Au@In2O3 core-shell NPs showed a greater sensitivity and selectivity towards H2 gas with highest response of 34.38 at operating temperature of 300oC to 100 ppm gas level and whereas In2O3 NPs showed a response of 9.26 only. This remarkable enhanced hydrogen gas sensing performance of Au@In2O3 core-shell NPs over bare In2O3 NPs was due to the electronic sensitization effects induced by changing Schottky barrier at the interface between the Au NPs and the In2O3 shell, and also due to the chemical sensitization of the catalytic metal Au NPs, i.e., they activate the target gas by dissociation and subsequent spillover of dissociation fragments onto the
229
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Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
gas sensing material. A greater understanding of the role of metal NPs in metal oxide based sensing devices will help in optimizing the sensor devices and will results in developing more sensitive and selective gas sensors.
40
Au@In2O3 In2O3
35
Response / Rs
30 25 20 15 10 5 0
H2
C2H5OH CH3CHO NO2 CO Gases to 100 ppm level
Figure 3: Response of Au@In2O3 core-shell and bare In2O3 NPs based sensor devices to different gases to a testing temperature of 300oC at 100 ppm gas levels.
Acknowledgements This paper was supported by 1) BK21 plus program from the Ministry of Education and Human-Resource Development, 2) National Research Foundation grant funded by the Korea government (MSIP) (BRL No. 2015042417, 2016R1A2B4014090), and 3) Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2016 (Grant No. C0396231).
References [1] R. Costi, A. E. Saunders, U. Banin (2010): Colloidal Hybrid Nanostructures: A New Type of Functional Materials, Angew. Chem. Int. Ed. 49, 4878-4897. [2] T. Yanagimoto, Y. Yu, K. Kaneko (2012): Microstructure and CO Gas Sensing Property of Au/SnO2 Core–Shell Structure Nanoparticles Synthesized by Precipitation Method and Microwave-assisted Hydrothermal Synthesis Method, Sens. Actuators B 166-167, 31-35. [3] P. Rai, R. Khan, S. Raj, S. Majhi, K. Park, Y. Yu, I. Lee, P. Sekhar (2014): Au@Cu2O core–shell nanoparticles as chemiresistors for gas sensor applications: effect of potential barrier modulation on the sensing performance. Nanoscale, 6, 581-588.
ScienceDirect Procedia Engineering 168 (2016) 227 – 230
30th Eurosensors Conference, EUROSENSORS 2016
Synthesis and gas sensing properties of Au@In2O3 core-shell nanoparticles Yeon-Tae Yua,*, Sanjit Manohar Majhia, Ho-Geun Songb a
Division of Advanced Materials Engineering, Chonbuk National University, Jeonju 54896, South Korea b Ogam Technology Co., Jeonju 54882, South Korea
Abstract The interest in hybrid nanoparticles (NPs) arises from their combined and often synergetic properties exceeding the functionality of the individual components. Herein, a general, environmentally friendly, and facile hydrothermal approach to synthesize the metal-semiconductor, Au@In2O3 core-shell heteronanostructures is described. These Au@In2O3 core-shell NPs are used as chemiresistive gas sensors for the detection of gases such H2, CO, NO2, VOCs in air. Sensor device based on Au@In2O3 coreshell NPs showed greater response than that of In2O3 NPs sensor device. The enhanced activity can be attributed to the catalytic effect of Au, and synergistic interactions between the Au and In2O3 NPs formed into the core-shell heteronanostructures in such a way that favors the efficient electron transfer at the interface © Published by Elsevier Ltd. This ©2016 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference. Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference Keywords: Gas sensor; Au@In2O3; core-shell nanoparticles
Motivation and results The semiconducting metal oxide gas sensors have attracted much attention in the field of gas sensing under atmospheric conditions due to their low cost and flexibility in production, simplicity of their use, large number of detectable gases. Recently, great efforts have been centered on the design of the semiconductor oxides with excellent sensing performance. Owing to their exceptional catalytic activities, noble metals such as Pt, Pd, Ag, and Au are often introduced into metal oxide semiconductors as sensitizers or promoters to improve the sensor performance under certain conditions.1 However, the noble metals, especially Au nanoparticles, will suffer from undesired aggregation arising from its low melting point and increased mobility at high operating temperature, which results in a loss of catalytic activity. Thus, to overcome such issues, noble metal particles were encapsulated into a metal oxide semiconductor shell. By encapsulating noble metal nanoparticles in a protective shell, the hybrid
1877-7058 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of the 30th Eurosensors Conference
doi:10.1016/j.proeng.2016.11.168
228
Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
core-shell structure could effectively increase the stability of catalyst against undesirable aggregation during practical operation. Metal@metal oxide core–shells NPs have attracted great interest in gas sensing applications because the heteronanostructures can leads to synergetic effects that enhance gas sensing properties.2,3 Indium oxide (In2O3), as a typical n-type metal oxide semiconductor with a direct band gap of 3.55–3.75 eV, could potentially be used to detecting number of gases because of its high electrical conductivity, abundant defects. In2O3 with high specific surface areas and hierarchical structures have been shown to be promising for sensing applications. If noble metal nanoparticles can be encapsulated into In2O3 nanostructures enhanced activity and sensitivity can be expected because they can improve the adsorption of gas molecules and accelerate the electron exchange between the sensor materials and target gas. In a typical synthesis of Au@In2O3 core–shell nanostructures, at first 5 ml of citrate-stabilized Au NPs prepared in the above manner are kept under stirring for 5 min. The well-dissolved solution containing 2.4 mM of Na3cit and 2 mM of InCl3 was added under stirring. The solution was stirred at room temperature for 20 min and then 2 mM of urea was added and kept on stirring for another 20 min. After being stirred for 20 min, the light reddish purple colored solution was transformed into a 40 ml Teflon-lined stainless steel autoclave and was heated at 140 °C for 24 hours. Then the autoclave was cooled to room temperature naturally. The obtained purple colored precipitate was washed and centrifuged with deionized water at 9000 rpm/10 min and the purification and centrifugation were repeated five times. The obtained sample was dried at 60 °C overnight so as to obtain the Au@InOOH core–shell NPs. Finally, the Au@In2O3 core–shell NPs were obtained by calcining the Au@InOOH core–shell NPs synthesized in the above manner at 350 °C for 2 hours. The heating rate was controlled at 2 °C min−1. The core–shell nanostructures produced from Au nanospheres are displayed in Fig. 1. The average diameter of the well dispersed Au nanocrystal seeds (Fig. 1a) was about 15 nm. After being coated with an InOOH shell (Fig. 1b and c), the total size increased up to 90 nm. The InOOH shell is in close contact with the Au core and appears rough on the outer surface. It is probably due to the aggregated InOOH nanoparticles. After calcination, the InOOH shell was transformed into In2O3 and the corresponding Au@In2O3 core–shell NPs are shown in Fig 1(d–f). The shapes of the core–shell nanostructures resemble those of their corresponding cores except that the overall sizes become larger.
Figure 1: TEM images of (a) Au NPs, (b,c) Au@InOOH and (d-f) Au@In2O3 core-shell NPs.
Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
Figure 2: Dependence of the sensor response of Au@In2O3 core-shell and In2O3 NPs at different temperatures to 100 ppm H2 gas.
To test the sensing response (Rs=Rair/Rgas) of Au@ In2O3 core-shell nanoparticles for various gases, we investigated our sensors sensitivity for H2, CO, NO2, C2H5OH, and CH3CHO at the operating temperature of 300oC to 100 ppm levels. The response was estimated by measuring the electrical resistance in the presence of air (Ra) and in gas (Rg).2,3 Fig. 2 shows the response to a fixed H2 of 100 ppm as a function of working temperature on both samples. Obviously, the response of the tested sensors varied with operating temperature, both the responses continuously increased and reached maximum at 300 °C, and after that the response of both sensors decreased. At this temperature, the Au@In2O3 core–shell and In2O3 nanostructures showed a higher response of 34.4 and 9.3, respectively. As shown in Figure 3, the response towards CO, NO2, C2H5OH, and CH3CHO is lower than that of hydrogen, underscoring a remarkably high selectivity toward H2. It might be due to the diameters of all gas molecules are much larger than those of H2, therefore it is difficult for the larger molecules to penetrate into the inner layers of Au@In2O3 core-shell NPs film and react with the chemisorbed oxygen. The sensing performance of our sensor to all the target gases could be attributed to the redox reaction with the surface absorbed oxygen species. Here we also observed that the sensitivity/selectivity of Au@In2O3 core-shell NPs to H2 gas was greatly increased by combining In2O3 with Au into a core-shell heterostructured form. In this core-shell heterostructured form, Au acts as electronic sensitizer which causes a dominant mechanism to increase the selectivity as well as sensitivity. Electronic sensitization is due to a changing Schottky barrier at the interface between Au and In2O3 gas sensing material as a consequence of the interaction of the Au with the gas phase. In this study, we have synthesized Au@In2O3 core-shell hybrid heteronanostructures by microwave hydrothermal method and the gas sensing property of such Au@In2O3 core-shell nanostructures over In2O3 was investigated. The diameter of Au core was around 10 nm and whereas the In2O3 shell thickness was found to be about 80 nm. We significantly enhanced the gas-sensing properties of n-type In2O3 by creating core-shell hybrid heteronanostructures with Au metal NPs and the enhancement was observed in the response to other interfering gases such as CO, NO2, C2H5OH, CH3CHO. Au@In2O3 core-shell NPs showed a greater sensitivity and selectivity towards H2 gas with highest response of 34.38 at operating temperature of 300oC to 100 ppm gas level and whereas In2O3 NPs showed a response of 9.26 only. This remarkable enhanced hydrogen gas sensing performance of Au@In2O3 core-shell NPs over bare In2O3 NPs was due to the electronic sensitization effects induced by changing Schottky barrier at the interface between the Au NPs and the In2O3 shell, and also due to the chemical sensitization of the catalytic metal Au NPs, i.e., they activate the target gas by dissociation and subsequent spillover of dissociation fragments onto the
229
230
Yeon-Tae Yu et al. / Procedia Engineering 168 (2016) 227 – 230
gas sensing material. A greater understanding of the role of metal NPs in metal oxide based sensing devices will help in optimizing the sensor devices and will results in developing more sensitive and selective gas sensors.
40
Au@In2O3 In2O3
35
Response / Rs
30 25 20 15 10 5 0
H2
C2H5OH CH3CHO NO2 CO Gases to 100 ppm level
Figure 3: Response of Au@In2O3 core-shell and bare In2O3 NPs based sensor devices to different gases to a testing temperature of 300oC at 100 ppm gas levels.
Acknowledgements This paper was supported by 1) BK21 plus program from the Ministry of Education and Human-Resource Development, 2) National Research Foundation grant funded by the Korea government (MSIP) (BRL No. 2015042417, 2016R1A2B4014090), and 3) Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2016 (Grant No. C0396231).
References [1] R. Costi, A. E. Saunders, U. Banin (2010): Colloidal Hybrid Nanostructures: A New Type of Functional Materials, Angew. Chem. Int. Ed. 49, 4878-4897. [2] T. Yanagimoto, Y. Yu, K. Kaneko (2012): Microstructure and CO Gas Sensing Property of Au/SnO2 Core–Shell Structure Nanoparticles Synthesized by Precipitation Method and Microwave-assisted Hydrothermal Synthesis Method, Sens. Actuators B 166-167, 31-35. [3] P. Rai, R. Khan, S. Raj, S. Majhi, K. Park, Y. Yu, I. Lee, P. Sekhar (2014): Au@Cu2O core–shell nanoparticles as chemiresistors for gas sensor applications: effect of potential barrier modulation on the sensing performance. Nanoscale, 6, 581-588.