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Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe2O3 microparticles
Author’s Accepted Manuscript Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe 2O3 microparticles W.X. Jin, S.Y. Ma, Z.Z. Tie, X.L. Xu, X.H. Jiang, W.Q. Li, T.T. Wang, Y. Lu, S.H. Yan www.elsevier.com
PII: DOI: Reference:
S0167-577X(15)30389-X http://dx.doi.org/10.1016/j.matlet.2015.08.030 MLBLUE19391
To appear in: Materials Letters Received date: 1 July 2015 Revised date: 30 July 2015 Accepted date: 5 August 2015 Cite this article as: W.X. Jin, S.Y. Ma, Z.Z. Tie, X.L. Xu, X.H. Jiang, W.Q. Li, T.T. Wang, Y. Lu and S.H. Yan, Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe 2O3 microparticles, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe2O3 microparticles W.X. Jin a, S.Y. Ma a, , Z.Z. Tie b, X.L. Xu a, X.H. Jiang a, W.Q. Li a, T.T. Wang a, Y. Lu a, S.H. Yan a a
Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province,
College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China b
College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract: Hamburger-like alpha-iron oxide (α-Fe2O3) microparticles were fabricated by a simple and low-cost hydrothermal route at 140oC for 16 h and subsequent by annealing process. The microparticles showed monodisperse porous structures with the length and the width about 2.7 and 2.2 μm, respectively. The sensor based on this unique structure displayed excellent gas-sensing properties to acetone at the optimal working temperature of 270oC, which with the response value about 236 and the response-recovery time around 10 s for 500 ppm acetone. It indicated that this structure had high sensitivity, fast response-recovery time and good selectivity to acetone. Thus, the peculiar structure of the sample endows acetone detection with widely application prospects. Keywords: Sensors; Single crystal α-Fe2O3; Microstructure; Semiconductors; Monodisperse 1.
Introduction As we all know, it is harmful to human’s safety and health inhaling too much acetone due to its
easy volatilization and toxic. Hence it is necessary to detect and monitor acetone concentration in the workplace. As a typical n-type transition metal oxide semiconductor, α-Fe2O3 with a band gap of 2.1 eV has been extensively investigated in magnetic materials [1], gas sensors [2] and lithium rechargeable batteries [3]. Especially, in the field of gas sensor, α-Fe2O3 plays an important role for the detection of toxic, harmful, flammable and explosive gases due to its advantages such as small dimensions, low cost, low power consumption and high compatibility with microelectronic processing [4].
Corresponding author: S.Y. Ma
Tel.: +86 13893422608
E-mail address: [email protected] (S.Y. Ma)
1
Fax: +86 9317971503
A variety of methods for synthesizing the α-Fe2O3 nanostructures have been reported [5-7]. Among them, the hydrothermal method is a facile, common and effective way to fabricate the three-dimensional (3D) structures. In this paper, the monodisperse porous hamburger-like α-Fe2O3 microparticles are successfully synthesized via a simple hydrothermal process. The sensor based on this structure exhibits the excellent performance toward acetone at 270oC. 2.
Experimental The typical hamburger-like sample was obtained as follows: 2.0 g FeCl3·6H2O and 0.2 g sodium
citrate (Na3C6H5O7·2H2O) were dissolved into 30 mL of ethanol-water (1:2, v-v) mixture with vigorous stirring for 30 min at 25oC, then a homogeneous brownish red solution was obtained. After that, it was transferred into a 50 ml Teflon-lined autoclave and maintained at 140°C for 16 h in a thermostat, and then cooled down to room temperature naturally. Following, the precipitation was collected and washed with de-ionized water and absolute ethanol for several times by centrifugation, respectively, and then dried at 60°C for 4 h. Finally, the sample was obtained after annealing at 550°C for 3 h. The morphology, structure and gas-sensing performance of the as-prepared sample were characterized via scanning electron microscopy (SEM, S-4800), transmission electron microscopy (TEM, USA FEI Tecnai G2 TF20), X-ray diffraction (XRD, D/Max-2400) and WS-60A gas-sensing measurement device (Weisheng Electronics Science and Technology Co. Ltd. China). The response of the sensor defined as S = Ra/Rg, where Ra and Rg represented the initial resistance of sensor in air and the target gas, respectively. The response time could be defined as the time needed to reach 90% of its saturated pulse height, while the recovery time was the time needed for the pulse to reach 10% from its base line [8]. Results and discussion 500
Fe
(104)
(a)
(b)
(110)
400
(116)
(214) (300)
100
O
(018)
(113)
200
(024)
300 (102)
Intensity (a.u.)
3.
Fe Cu
Cu Fe Cu
0 20
30
40
50
60
70
2 Theta (deg.)
0
10
20
30
Energy (keV)
Fig.1. (a) XRD and (b) EDS patterns of hamburger-like α-Fe2O3 microparticles. 2
40
Fig. 1(a) shows the typical XRD pattern of the as-synthesized α-Fe2O3 microparticles. All the diffraction peaks are well indexed to the rhombohedral phase of α-Fe2O3 (JCPDS Card No. 33-0664) with ao = 5.035 Å, co = 13.74 Å. No diffraction peaks of any other impurities are observed, which indicates the high purity of our product. The corresponding EDS is shown in Fig. 1(b), which displays the sample consists of Fe, O and Cu elements. The Cu signal from the spectrum is attributed to the copper grid used in the TEM measurement. Fig. 2 shows the SEM and TEM images of as-prepared α-Fe2O3 product. As shown in Fig. 2(a), the sample is composed of uniform hamburger-like microparticles and these microparticles evenly tiled into a monolayer instead of piling up together, demonstrating the high dispersity and uniformity. Fig. 2(b) displays the magnified SEM image of Fig. 2(a), which can be observed obviously that the length and the width of the hamburger-like microparticle is about 2.7 and 2.2 μm, respectively. It also can be found that the surface of hamburger-like microparticle is rough and porous by careful observation, this unique structure with large specific surface area, low density and well dispersing property, enable the monodisperse porous materials to have potential applications in gas sensors [9].
(a)
(b)
1 μm
(c)
200 nm
(d)
0.27 nm α-Fe2O3 (104)
1 μm
3
Fig.2. (a) SEM, (b) magnified SEM, (c) TEM and (d) HRTEM images of hamburger-like α-Fe2O3 microparticles. The TEM and HRTEM images of hamburger-like microparticles are shown in Fig. 2(c) and (d) combine with the selected area electron diffraction (SAED) analysis technique. It can be observed from Fig. 2(c) that the size and shape of the products are in good accordance with the SEM observations. The lattice fringes are clear visible with a spacing of 0.27 nm as shown in Fig. 2(d), which corresponding to the spacing of the (104) planes of α-Fe2O3. The corresponding SAED pattern (inset of Fig. 2(d)) clearly suggests a single-crystal nature of α-Fe2O3 microparticles.
(a)
Acetone at 270oC
1200
(b)
1000
160
Response
Response
200
1400
500 ppm ammonia 500 ppm ethanol 500 ppm acetone 500 ppm acetic acid
240
120 80
250
800
200
600
150
400
100 50
40
200 0
0
0
0
150
200
250
300
350
400
0
2000
Oprating temperature (oC) 300
300
400
6000
500
8000
260 500 ppm acetone at 270oC
(d)
250
200
240
Response
Response
200
Concentration (ppm)
o 500 ppmacetone at 270 C
(c)
250
100
4000
Gas out ~8s
150 100
Gas in ~ 10 s
50
230 220 210
0 0
30
60
90 120 150 180 210 240 270 300
Time (s)
200 0
10
20
30
40
50
60
Time (day)
Fig.3. (a) The response of the sensor based on the monodisperse porous hamburger-like α-Fe2O3 microparticles to 500 ppm ammonia, ethanol, acetone and acetic acid at different temperatures, (b) responses of the sensor versus acetone concentration at 270 oC, the inset shows the linear relationship dependence on response in the range of 25–500 ppm, (c) dynamic sensing transient of the sensor toward 500 ppm acetone, and (d) the long-term stability of the sensor toward 500 ppm acetone at 270oC. It is well known that the response of gas sensor is greatly influenced by the operating temperature. In order to find out the optimum operating temperature, the response of the sensor based on the 4
hamburger-like α-Fe2O3 to 500 ppm different gases are examined at the temperature ranging from 160 to 400oC, as shown in Fig. 3(a). It can be observed that the response value of the sensor to 500 ppm ammonia, ethanol, acetone and acetic acid is 21.2, 126.5, 236 and 103.3 at the corresponding optimum operating temperature (340, 300, 270 and 300oC), respectively. Moreover, it also can be seen that the response of the sensor to acetone is about 15.3 times higher than that of ammonia. The result indicates that the sensor has a higher response and a lower optimum operating temperature to acetone, it means that the sensor with a good selective ability to acetone and is more appropriate for using as acetone detection compared with other gases. Fig. 3(b) shows the response of sensor to acetone from 25 to 8000 ppm at 270°C. Between 25 and 500 ppm, the response reaches almost a linear relationship (inset of Fig. 3(b)). Above 4000 ppm, the response increases very slowly with the concentration further increasing, this may be due to the sensor tends to be saturated. Fig. 3(c) gives the response-recovery characteristic curve of the sensor to 500 ppm acetone. The response and recovery time is around 10 and 8 s, respectively. Compare with some previous reports on the sensing properties [10, 11], the sensor based on the monodisperse porous hamburger-like α-Fe2O3 microparticles exhibits a significantly improve on sensing properties. As shown in Fig. 3(d), the sensor exhibits nearly constant response to acetone in 60 days, demonstrating the as-prepared sensor has a good long-time stability.
In air ambient O2
O-ads + 2e
-
In acetone ambient Ra O
C3H6O
-
CO2 + 3H2O + 8e+ O-ads
O2-
Hamburger-like
Rg CO2
α-Fe2O3 microparticles
O2-
e-
Section
H2O
Electron depleted layer
Conducting channel
+ 2e-
+ O-ads
of
hamburger-like α-Fe2O3 microparticles
O2
O-ads
Ra
C3H6O
CO2 + 3H2O + 8e-
Rg
Fig.4. The schematic illustration of the gas-sensing mechanism of hamburger-like α-Fe2O3 5
microparticles. Gas-sensing mechanism of semiconductor oxide gas sensor is that the change of resistance which is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing materials [12]. As shown in Fig. 4, when the sensor is exposed to air atmosphere, O2 is absorbed onto the surface of α-Fe2O3 and form O2−(ads), O−(ads) and O2−(ads) by capturing free electrons from the conduction band of the sensing material. The decrease of the electron concentration in the conduction band results an increase in the resistance. Furthermore, when the sensor is exposed to reductive gas such as acetone atmosphere at a moderate temperature, the gas will react with the chemisorbed oxygen species and release electrons back to the conduction band, leading to a decrease in the resistance. The enhanced gas-sensing performance of α-Fe2O3 microparticles in this study is attributed to the unique 3D monodisperse porous hamburger-like microstructure. On the one hand, the 3D hamburger-like porous structure can help accelerate gas diffusion, offering a completely activated surface due to their high surface permeability and low density, which can vastly enable the increase of gas-sensing response. On the other hand, the structure feature of monodispersity is also effectively influences the sensing property owing to the more exposed surfaces of hamburger-like α-Fe2O3 microparticles adsorb the more oxygen molecules from the ambient gas components, which can capture more electrons from the conduction band and form more oxygen species. Perhaps this is the reason why monodisperse porous hamburger-like α-Fe2O3 microparticles enhanced gas-sensing properties observably. 4.
Conclusions In summary, we have synthesized the monodisperse porous hamburger-like α-Fe2O3 microparticles
by a simple hydrothermal route combine with subsequent annealing treatment. The sensor exhibits excellent acetone sensing properties at 270oC due to the 3D monodisperse porous structure of hamburger-like α-Fe2O3, demonstrating the highly promising applications of this unique structure. In addition, the gas-sensing mechanism of the sensor is also proposed. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140), the State Education Ministry and Natural Science Foundational of Gansu province (Grant No. 1308RJZA258 and 1308RJZA216), and the basic scientific research business expenses of Gansu province. 6
References [1] Sun LN, Cao MH, Hu CW. Solid State Sci. 2010; 12: 2020–2023. [2] Yan W, Fan HQ, Zhai YC, Yang C, Ren PR, Huang LM, et al. Sens. Actuators B: Chem. 2011; 160: 1372–1379. [3] Liang HF, Chen W, Yao YW, Wang ZC, Yang Y. Ceram Int. 2014; 40: 10283–10290. [4] Zhou X, Liu JY, Wang C, Sun P, Hu XL, Li XW, et al. Sens. Actuators B: Chem. 2015; 206: 577–583. [5] Cui HT, Liu Y, Ren WZ. Adv. Powder Technol. 2013; 24: 93–97. [6] Wang Y, Cao JL, Yu MG, Sun G, Wang XD, Zhang ZY, et al. Mater. Lett. 2013; 100: 102–105. [7] Nang LV, Kim ET. Mater. Lett. 2013; 92: 437–439. [8] Jin WX, Ma SY, Tie ZZ, Wei JJ, Luo J, Jiang XH, et al. Sens. Actuators B: Chem. 2015; 213: 171–180. [9] Zhang FH, Yang HQ, Xie XL, Li L, Zhang LH, Yu J, et al. Sens. Actuators B: Chem. 2009; 141: 381–389. [10] Su C, Liu CB, Liu L, Ni MC, Li HY, Bo XQ, et al. Appl. Surf. Sci. 2014; 314: 931–935. [11] Sun P, Wang WN, Liu YP, Sun YF, Ma J, Lu GY. Sens. Actuators B: Chem. 2012; 173: 52–57. [12] Yamazoe N, Sakai G, Shimanoe K. Catal. Surv. Asia. 2003; 7: 63–75. Highlights:
The hamburger-like α-Fe2O3 microparticles are synthesized by a low-cost and simple hydrothermal approach.
Our sample has highly selectivity and sensitivity to 500 ppm acetone at 270°C.
The gas-sensing mechanism of hamburger-like α-Fe2O3 microparticles is proposed.
7
PII: DOI: Reference:
S0167-577X(15)30389-X http://dx.doi.org/10.1016/j.matlet.2015.08.030 MLBLUE19391
To appear in: Materials Letters Received date: 1 July 2015 Revised date: 30 July 2015 Accepted date: 5 August 2015 Cite this article as: W.X. Jin, S.Y. Ma, Z.Z. Tie, X.L. Xu, X.H. Jiang, W.Q. Li, T.T. Wang, Y. Lu and S.H. Yan, Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe 2O3 microparticles, Materials Letters, http://dx.doi.org/10.1016/j.matlet.2015.08.030 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Enhanced acetone sensing performance of monodisperse porous hamburger-like α-Fe2O3 microparticles W.X. Jin a, S.Y. Ma a, , Z.Z. Tie b, X.L. Xu a, X.H. Jiang a, W.Q. Li a, T.T. Wang a, Y. Lu a, S.H. Yan a a
Key Laboratory of Atomic and Molecular Physics & Functional Materials of Gansu Province,
College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China b
College of Chemistry & Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
Abstract: Hamburger-like alpha-iron oxide (α-Fe2O3) microparticles were fabricated by a simple and low-cost hydrothermal route at 140oC for 16 h and subsequent by annealing process. The microparticles showed monodisperse porous structures with the length and the width about 2.7 and 2.2 μm, respectively. The sensor based on this unique structure displayed excellent gas-sensing properties to acetone at the optimal working temperature of 270oC, which with the response value about 236 and the response-recovery time around 10 s for 500 ppm acetone. It indicated that this structure had high sensitivity, fast response-recovery time and good selectivity to acetone. Thus, the peculiar structure of the sample endows acetone detection with widely application prospects. Keywords: Sensors; Single crystal α-Fe2O3; Microstructure; Semiconductors; Monodisperse 1.
Introduction As we all know, it is harmful to human’s safety and health inhaling too much acetone due to its
easy volatilization and toxic. Hence it is necessary to detect and monitor acetone concentration in the workplace. As a typical n-type transition metal oxide semiconductor, α-Fe2O3 with a band gap of 2.1 eV has been extensively investigated in magnetic materials [1], gas sensors [2] and lithium rechargeable batteries [3]. Especially, in the field of gas sensor, α-Fe2O3 plays an important role for the detection of toxic, harmful, flammable and explosive gases due to its advantages such as small dimensions, low cost, low power consumption and high compatibility with microelectronic processing [4].
Corresponding author: S.Y. Ma
Tel.: +86 13893422608
E-mail address: [email protected] (S.Y. Ma)
1
Fax: +86 9317971503
A variety of methods for synthesizing the α-Fe2O3 nanostructures have been reported [5-7]. Among them, the hydrothermal method is a facile, common and effective way to fabricate the three-dimensional (3D) structures. In this paper, the monodisperse porous hamburger-like α-Fe2O3 microparticles are successfully synthesized via a simple hydrothermal process. The sensor based on this structure exhibits the excellent performance toward acetone at 270oC. 2.
Experimental The typical hamburger-like sample was obtained as follows: 2.0 g FeCl3·6H2O and 0.2 g sodium
citrate (Na3C6H5O7·2H2O) were dissolved into 30 mL of ethanol-water (1:2, v-v) mixture with vigorous stirring for 30 min at 25oC, then a homogeneous brownish red solution was obtained. After that, it was transferred into a 50 ml Teflon-lined autoclave and maintained at 140°C for 16 h in a thermostat, and then cooled down to room temperature naturally. Following, the precipitation was collected and washed with de-ionized water and absolute ethanol for several times by centrifugation, respectively, and then dried at 60°C for 4 h. Finally, the sample was obtained after annealing at 550°C for 3 h. The morphology, structure and gas-sensing performance of the as-prepared sample were characterized via scanning electron microscopy (SEM, S-4800), transmission electron microscopy (TEM, USA FEI Tecnai G2 TF20), X-ray diffraction (XRD, D/Max-2400) and WS-60A gas-sensing measurement device (Weisheng Electronics Science and Technology Co. Ltd. China). The response of the sensor defined as S = Ra/Rg, where Ra and Rg represented the initial resistance of sensor in air and the target gas, respectively. The response time could be defined as the time needed to reach 90% of its saturated pulse height, while the recovery time was the time needed for the pulse to reach 10% from its base line [8]. Results and discussion 500
Fe
(104)
(a)
(b)
(110)
400
(116)
(214) (300)
100
O
(018)
(113)
200
(024)
300 (102)
Intensity (a.u.)
3.
Fe Cu
Cu Fe Cu
0 20
30
40
50
60
70
2 Theta (deg.)
0
10
20
30
Energy (keV)
Fig.1. (a) XRD and (b) EDS patterns of hamburger-like α-Fe2O3 microparticles. 2
40
Fig. 1(a) shows the typical XRD pattern of the as-synthesized α-Fe2O3 microparticles. All the diffraction peaks are well indexed to the rhombohedral phase of α-Fe2O3 (JCPDS Card No. 33-0664) with ao = 5.035 Å, co = 13.74 Å. No diffraction peaks of any other impurities are observed, which indicates the high purity of our product. The corresponding EDS is shown in Fig. 1(b), which displays the sample consists of Fe, O and Cu elements. The Cu signal from the spectrum is attributed to the copper grid used in the TEM measurement. Fig. 2 shows the SEM and TEM images of as-prepared α-Fe2O3 product. As shown in Fig. 2(a), the sample is composed of uniform hamburger-like microparticles and these microparticles evenly tiled into a monolayer instead of piling up together, demonstrating the high dispersity and uniformity. Fig. 2(b) displays the magnified SEM image of Fig. 2(a), which can be observed obviously that the length and the width of the hamburger-like microparticle is about 2.7 and 2.2 μm, respectively. It also can be found that the surface of hamburger-like microparticle is rough and porous by careful observation, this unique structure with large specific surface area, low density and well dispersing property, enable the monodisperse porous materials to have potential applications in gas sensors [9].
(a)
(b)
1 μm
(c)
200 nm
(d)
0.27 nm α-Fe2O3 (104)
1 μm
3
Fig.2. (a) SEM, (b) magnified SEM, (c) TEM and (d) HRTEM images of hamburger-like α-Fe2O3 microparticles. The TEM and HRTEM images of hamburger-like microparticles are shown in Fig. 2(c) and (d) combine with the selected area electron diffraction (SAED) analysis technique. It can be observed from Fig. 2(c) that the size and shape of the products are in good accordance with the SEM observations. The lattice fringes are clear visible with a spacing of 0.27 nm as shown in Fig. 2(d), which corresponding to the spacing of the (104) planes of α-Fe2O3. The corresponding SAED pattern (inset of Fig. 2(d)) clearly suggests a single-crystal nature of α-Fe2O3 microparticles.
(a)
Acetone at 270oC
1200
(b)
1000
160
Response
Response
200
1400
500 ppm ammonia 500 ppm ethanol 500 ppm acetone 500 ppm acetic acid
240
120 80
250
800
200
600
150
400
100 50
40
200 0
0
0
0
150
200
250
300
350
400
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2000
Oprating temperature (oC) 300
300
400
6000
500
8000
260 500 ppm acetone at 270oC
(d)
250
200
240
Response
Response
200
Concentration (ppm)
o 500 ppmacetone at 270 C
(c)
250
100
4000
Gas out ~8s
150 100
Gas in ~ 10 s
50
230 220 210
0 0
30
60
90 120 150 180 210 240 270 300
Time (s)
200 0
10
20
30
40
50
60
Time (day)
Fig.3. (a) The response of the sensor based on the monodisperse porous hamburger-like α-Fe2O3 microparticles to 500 ppm ammonia, ethanol, acetone and acetic acid at different temperatures, (b) responses of the sensor versus acetone concentration at 270 oC, the inset shows the linear relationship dependence on response in the range of 25–500 ppm, (c) dynamic sensing transient of the sensor toward 500 ppm acetone, and (d) the long-term stability of the sensor toward 500 ppm acetone at 270oC. It is well known that the response of gas sensor is greatly influenced by the operating temperature. In order to find out the optimum operating temperature, the response of the sensor based on the 4
hamburger-like α-Fe2O3 to 500 ppm different gases are examined at the temperature ranging from 160 to 400oC, as shown in Fig. 3(a). It can be observed that the response value of the sensor to 500 ppm ammonia, ethanol, acetone and acetic acid is 21.2, 126.5, 236 and 103.3 at the corresponding optimum operating temperature (340, 300, 270 and 300oC), respectively. Moreover, it also can be seen that the response of the sensor to acetone is about 15.3 times higher than that of ammonia. The result indicates that the sensor has a higher response and a lower optimum operating temperature to acetone, it means that the sensor with a good selective ability to acetone and is more appropriate for using as acetone detection compared with other gases. Fig. 3(b) shows the response of sensor to acetone from 25 to 8000 ppm at 270°C. Between 25 and 500 ppm, the response reaches almost a linear relationship (inset of Fig. 3(b)). Above 4000 ppm, the response increases very slowly with the concentration further increasing, this may be due to the sensor tends to be saturated. Fig. 3(c) gives the response-recovery characteristic curve of the sensor to 500 ppm acetone. The response and recovery time is around 10 and 8 s, respectively. Compare with some previous reports on the sensing properties [10, 11], the sensor based on the monodisperse porous hamburger-like α-Fe2O3 microparticles exhibits a significantly improve on sensing properties. As shown in Fig. 3(d), the sensor exhibits nearly constant response to acetone in 60 days, demonstrating the as-prepared sensor has a good long-time stability.
In air ambient O2
O-ads + 2e
-
In acetone ambient Ra O
C3H6O
-
CO2 + 3H2O + 8e+ O-ads
O2-
Hamburger-like
Rg CO2
α-Fe2O3 microparticles
O2-
e-
Section
H2O
Electron depleted layer
Conducting channel
+ 2e-
+ O-ads
of
hamburger-like α-Fe2O3 microparticles
O2
O-ads
Ra
C3H6O
CO2 + 3H2O + 8e-
Rg
Fig.4. The schematic illustration of the gas-sensing mechanism of hamburger-like α-Fe2O3 5
microparticles. Gas-sensing mechanism of semiconductor oxide gas sensor is that the change of resistance which is mainly caused by the adsorption and desorption of gas molecules on the surface of the sensing materials [12]. As shown in Fig. 4, when the sensor is exposed to air atmosphere, O2 is absorbed onto the surface of α-Fe2O3 and form O2−(ads), O−(ads) and O2−(ads) by capturing free electrons from the conduction band of the sensing material. The decrease of the electron concentration in the conduction band results an increase in the resistance. Furthermore, when the sensor is exposed to reductive gas such as acetone atmosphere at a moderate temperature, the gas will react with the chemisorbed oxygen species and release electrons back to the conduction band, leading to a decrease in the resistance. The enhanced gas-sensing performance of α-Fe2O3 microparticles in this study is attributed to the unique 3D monodisperse porous hamburger-like microstructure. On the one hand, the 3D hamburger-like porous structure can help accelerate gas diffusion, offering a completely activated surface due to their high surface permeability and low density, which can vastly enable the increase of gas-sensing response. On the other hand, the structure feature of monodispersity is also effectively influences the sensing property owing to the more exposed surfaces of hamburger-like α-Fe2O3 microparticles adsorb the more oxygen molecules from the ambient gas components, which can capture more electrons from the conduction band and form more oxygen species. Perhaps this is the reason why monodisperse porous hamburger-like α-Fe2O3 microparticles enhanced gas-sensing properties observably. 4.
Conclusions In summary, we have synthesized the monodisperse porous hamburger-like α-Fe2O3 microparticles
by a simple hydrothermal route combine with subsequent annealing treatment. The sensor exhibits excellent acetone sensing properties at 270oC due to the 3D monodisperse porous structure of hamburger-like α-Fe2O3, demonstrating the highly promising applications of this unique structure. In addition, the gas-sensing mechanism of the sensor is also proposed. Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 10874140), the State Education Ministry and Natural Science Foundational of Gansu province (Grant No. 1308RJZA258 and 1308RJZA216), and the basic scientific research business expenses of Gansu province. 6
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The hamburger-like α-Fe2O3 microparticles are synthesized by a low-cost and simple hydrothermal approach.
Our sample has highly selectivity and sensitivity to 500 ppm acetone at 270°C.
The gas-sensing mechanism of hamburger-like α-Fe2O3 microparticles is proposed.
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