- Email: [email protected]
Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties
Accepted Manuscript Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties Tianyi Wang, Bingbing Liu, Quanjun Li, Shuangming Wang PII: DOI: Reference:
S0167-577X(18)30437-3 https://doi.org/10.1016/j.matlet.2018.03.073 MLBLUE 24034
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 November 2017 21 February 2018 13 March 2018
Please cite this article as: T. Wang, B. Liu, Q. Li, S. Wang, Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties, Materials Letters (2018), doi: https://doi.org/ 10.1016/j.matlet.2018.03.073
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 proof before it is published in its final 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.
Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties a
a*
a
Tianyi Wang , Bingbing Liu , Quanjun Li and Shuangming Wang a
b,c*
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic
of China. b
College of Electronic Science and Engineering, Jilin University, Changchun 130012, People’s Republic
of China. c
College of Physics & Materials Science, Tianjin Normal University, Tianjin 300387, People’s Republic
of China.
Abstract The designed Cr2O3-ZnO hierarchical heterostructures have been successfully synthesized by a facile two-step route. The SEM, TEM, XRD and EDS mapping characterizations demonstrate that Cr2O3 nanoparticles are well decorated on surfaces of nanosheet-assembled flowerlike ZnO, constructing three dimensional Cr2O3-ZnO hierarchical heterostructures. Such hierarchical architectures are developed and applied in formaldehyde (HCHO) gas detection. The gas sensing results indicate that Cr2O3-ZnO heterostructures based sensors exhibit superior HCHO gas sensing properties with rapid response/recovery behavior (1 s/5 s) toward 100 ppm formaldehyde (HCHO) gas, and favorable stability and repeatability with the on/off phenomenon when switching between HCHO and air at a optimal working temperature of 220 oC. The superior formaldehyde gas sensing performances are predominantly attributed to the appropriate sensitization effect induced by Cr2O3 nanoparticles and the special three dimensional hierarchical architectures. Keywords: Semiconductors; heterostructure; Formaldehyde; Sensors.
*
Corresponding author. E-mail address: [email protected]; [email protected] 1
1. Introduction The heterostructures constituted by two or more different materials, as important functional materials, have attracted widespread attention as they provide more opportunities for integrating physical and chemical characteristics of their individual counterparts [1,2]. Many methods have been invested to produced these functional materials, such as atomic layer deposition [3], chemical solution method [4,5], γ irradiation [6], chemical oxidative polymerization [7], etc. Among heterostructures, various oxide semiconductor based heterostructures have been synthesized and employed in many fields, for instance, gas sensors [8], supercapacitors [9], lithium ion batteries [10] and photocatalysis [11,12], etc. In gas sensing field, the heterojunction of the composites act as a lever in electron transfer through which electron transfer is facilitated or restrained, enormously influencing gas sensing behaviors [13]. Therefore, designing and fabricating appropriate heterostructures materials has long been regarded as an efficient and cost-effective strategy to improve the performances of gas sensors, such as n-n and p-n heterostructures [14,15]. However, in spite of the significant accomplishments achieved, constructing novel heterostructures by selecting proper oxides to enhance gas sensing properties still are confronted with great challenges. In this paper, Cr2O3 nanoparticles decorated flowerlike ZnO hierarchical heterostructures Cr2O3-ZnO are skillfully designed and successfully fabricated by a facile route. The gas sensor based on Cr2O3-ZnO hierarchical heterostructures exhibits rapid response/recovery behavior and favorable stability and repeatability for formaldehyde gas.
2. Experiments Sodium hydrate, zinc acetate, ethanol and chromic nitrate
Tianjin Guangfu
Fine Chemical Research Institute (Tianjin of China). All reagents were analytical-grade purity and were
2
used as purchased without further purification. The synthesis route of Cr2O3-ZnO heterostructures consisted of the construction of flowerlike ZnO and the decoration of Cr2O3 nanoparticles. Firstly nanosheet-assembled flowerlike ZnO hierarchical architectures were synthesized by a facile solution route. Briefly, 7.5 mmol sodium hydrate and 1.4 mmol zinc acetate was dissolved in 10 mL and 50 mL deionized water under magnetic stirring, respectively. Subsequently, sodium hydrate aqueous solution was added into zinc acetate aqueous solution drop by drop. After stirring for 5 min, 50 mL ethanol was slowly added into above mixed solution. After being stirred for another 5 min, white precipitates were collected by centrifuging, washed with deionized water and ethanol and dried in air at 60 oC for 5 h. The flowerlike ZnO products were obtained by calcining as-obtained white precipitates at 500 oC for 2 h. 0.1 g newly prepared ZnO samples were put in a porcelain boat and then 5 mL deionized water and 6 mg Cr(NO3)3 was added in turn. After ultrasonic dispersion for 30 min, the resulting suspension was dried in air at 60 oC for 20 min. Finally, Cr2O3-ZnO hierarchical heterostructures were obtained by annealing porcelain boat at 500 oC for 2 h in air. The heating rate was controlled at 2 oC/min. The morphology and crystal structure of Cr2O3-ZnO products were examined using a Hitachi SU8010, JEOL JEM-2200FS microscope and a Bruker D8A X-ray diffractometer. The detail of the gas sensor fabrication was similar to our previous report [16]. The gas sensing performances were measured by a CGS-8 intelligent gas sensing analysis system. The response was defined as S = Ra/Rg. Here, Ra and Rg was the resistance of the gas sensor in the air and target gas, respectively. The time taken by the sensor to change from Ra to Ra-90% (Ra-Rg) was defined as the response time when Cr2O3-ZnO sensor was placed into the target gas. The time taken by the sensor to change from Rg to Rg+90% (Ra-Rg) was defined as the recovery time when Cr2O3-ZnO sensor was taken out of the target gas.
3
3. Results and discussion Fig. 1a shows a typical FESEM image of as-prepared Cr2O3-ZnO hierarchical heterostructures. It can be observed that Cr2O3-ZnO samples are assembled by numerous nanosheets as primary building blocks and present three dimensional flowerlike architectures. The panoramic FESEM image in the inset of Fig. 1a indicates Cr2O3-ZnO products consist of monodisperse and uniform hierarchical architectures. The enlarged FESEM image (Fig. 1b) exhibits rough surfaces of Cr2O3-ZnO hierarchical architectures. The TEM images from Fig. 1c and 1d further demonstrate the flowerlike structure of Cr2O3-ZnO and reveal that Cr2O3 nanoparticles with diameters of tens of nanometers are well decorated on the surfaces of ZnO nanosheets without obvious aggregations. Fig. 2 shows the XRD pattern of Cr2O3-ZnO heterostructures. All diffractions peaks are indexed to the standard ZnO (JCPDS No. 79-0206) and no extra peaks from any other impurities are found, confirming the superior purity of as-prepared ZnO host. However, due to small decoration amount of Cr2O3 nanoparticles, obvious diffraction peaks corresponding to Cr2O3 are not detected. A similar phenomenon is also reported in the earlier literature [17]. To further determine the existence and specific distribution of Cr2O3, the element mapping (line scanning pattern) is conducted. As shown in Fig. 3, after Cr2O3-ZnO heterostructure is traversed by a testing line, the Cr (green line), Zn (red line) and O (yellow line) element can be clearly observed, clearly indicating the existence of Cr2O3, which matches well with the surface canning pattern (Fig. S1a-d). In addition, the BET surface area of the Cr2O3-ZnO heterostructure is 9.6 m2 g-1 (Fig. Sle). Fig.4a shows the responses of Cr2O3-ZnO heterostructures sensor to 100 ppm HCHO at various operating
temperature
(180-280
o
C).
The
Cr2O3-ZnO
heterostructure
sensor
exhibits
an
“increase-maximum-decrease” parabola tendency with the increase of working temperature. The
4
maximum response value is obtained at 220 oC which is chosen as the optimal working temperature. Furthermore, Fig. 4b presents dynamic resistance variations of Cr 2O3-ZnO heterostructures based sensors to different concentrations (5-500 ppm) of HCHO gas at 220 oC. It is apparent that the resistances change rapidly when the gas is repeatedly switched from air to HCHO, indicating a favorable reproducibility and stability of the present sensor. Even though HCHO gas is as low as 5 ppm, the heterostructure sensor still shows an obvious resistance change with a sensitivity of about 1.1. Fig. 4c shows the corresponding sensitivity-HCHO concentration curve. The sensitivities almost linearly increase with increasing HCHO concentration from 5 to 200 ppm, revealing that the heterostructure sensor is suitable for the detection of HCHO at low levels. For a gas sensor, the response and recovery time plays a vital part for the practical detection of detrimental gases, which is calculated to be about 1 s and 5 s to 100 ppm HCHO at 220 oC, respectively. In comparison with many other HCHO gas sensors, such as CuO microplates, CuO microspheres, SnO2-GO nanofibers, LaFeO3 nanospheres and SnO2 nanowires[18-21], the Cr2O3-ZnO heterostructure sensor shows rapider response and recovery kinetics, which may originate from the distinctive three dimensional open hierarchical architectures and sensitization effect of Cr2O3 nanoparticles. ZnO hierarchical architectures provide three dimensional networks for electron transport. Furthermore, as a p-type oxide semiconductor, surface modified Cr2O3 nanoparticles play a role in regulating the thickness of ZnO electron depletion layer. Thus, resistance change of the sensitive material is larger when it switches between air and formaldehyde gas.
4. Conclusions p-type Cr2O3 nanoparticles functionalized Cr2O3-ZnO hierarchical heterostructures are synthesized by a facile two-step approach and are developed for formaldehyde gas detection. The sensor based on Cr2O3-ZnO heterostructures exhibits rapid response/recovery kinetics and well stability and repeatability
5
toward HCHO gas. The facile synthesis route and superior gas sensing properties make the as-prepared composites developed for formaldehyde gas detection in practice. Considering energy band coupling effect between ZnO hierarchical nanosheets and homodisperse Cr2O3 nanoparticles, the products holds potential for use in photocatalysis applications in future.
Acknowledgements This work was funded by the Application Development Foundation (No. 135202XK1602) of Tianjin Normal University.
References [1] X. Zhou, Y. Xiao, M. Wang, P. Sun, F.M. Liu, X.S. Liang, et al. ACS Appl. Mater. Interfaces 7 (2015) 8743-8749. [2] X.L. Fu, B.W. Zhang, H.J. Liu, B.B. Zong, L.W. Huang, H. Bala, et al. Mater. Lett. 196 (2017) 149-152. [3] I. Narkeviciute, P. Chakthranont, A.J.M. Mackus, C. Hahn, B.A. Pinaud, S.F. Bent, et al. Nano Lett. 16 (2016) 7565-7572. [4] K.R. Reddy, B.C. Sin, C.H. Yoo, W.J. Park, K.S. Ryu, J.S. Lee, et al. Scripta Mater. 58 (2008) 1010-1013. [5] K.R. Reddy, K. Nakata, T. Ochiai, T. Murakami, D.A. Tryk, A. Fujishima, J. Nanosci. Nanotechno. 11 (2011) 3692-3695. [6] K.R. Reddy, K.P. Lee, A.I. Gopalan, M.S. Kim, A.M. Showkat, Y.C. Nho. J. Polym. Sci. Pol. Chem. 44 (2006) 3355-3364. [7] K.R. Reddy, K.P. Lee, A.L. Gopalan. Colloid. Surface. A 320 (2008) 49-56. [8] Y. Xiong, W.W. Xu, Z.Y. Zhu, Q.Z. Xue, W.B. Lu, D.G. Ding, et al. Sens. Actuators B 253 (2017) 523-532.
6
[9] A.K. Singh, D. Sarkar. J. Mater. Chem. A 5 (2017) 21715-21725. [10] X.L. Tong, M. Zeng, J. Li, Z.J. Liu. J. Alloy. Compd. 723 (2017) 129-138. [11] K.R. Reddy, M. Hassan, V.G. Gomes. Appl. Catal. A-Gen. 489 (2015) 1-16. [12] K.R. Reddy, V.G. Gomes, M. Hassan. Mater. Res. Express 1 (2014) 015012. [13] S. Xu, J. Gao, L.L. Wang, K. Kan, X. Xie, P.K. Shen, et al. Nanoscale 7 (2015) 14643-14651. [14] X.W. Zou, X.Y. Yan, G.M. Li, Y.M. Tian, M.G. Zhang, L.P. Liang. RSC Adv. 7 (2017) 34482-34487. [15] T.S. Wang, X.Y. Kou, L.P. Zhao, P. Sun, C. Liu, Y. Wang, et al. Sens. Actuators B 250 (2017) 692-702. [16] S.M. Wang, J. Cao, W. Cui, L.L. Fan, X.F. Li, D.J. Li. Sens. Actuators B 255 (2018) 159-165. [17] D.B. Xu, M. Chen, S.Y. Song, D.L. Jiang, W.Q. Fan, W.D. Shi, CrystEngComm 16 (2014) 1384-1388. [18] D. Meng, D.Y. Liu, G.S. Wang, X.G. Sun, Y.B. Shen, Q. Jin, et al. Vacuum 144 (2017) 272-280. [19] D. Wang, M.L. Zhang, Z.L. Chen, H.J. Li, A.Y. Chen, X.Y. Wang. Sens. Actuators B 250 (2017) 533-542. [20] H.H. Zhang, P. Song, D. Han, Q. Wang. Physica E 63 (2014) 21-26. [21] I. Castro-Hurtado, J. Herran, G.G. Mandayo, E. Castano. Thin Solid Films 520 (2012) 4792-4796.
Figure Captions Fig. 1 Low and high-magnification (a-b) FESEM and (c-d) TEM images of Cr2O3-ZnO hierarchical heterostructures. The inset is a panoramic FESEM image of Cr2O3-ZnO products. Fig. 2 The XRD pattern of as-prepared Cr2O3-ZnO hierarchical heterostructures. Fig. 3 The elemental mapping image (line scanning pattern) of Cr2O3-ZnO hierarchical heterostructures. The green, red and yellow line represents Cr, Zn and O element, respectively Fig. 4 (a) Responses versus working temperatures of Cr2O3-ZnO hierarchical heterostructures based sensors to 100 ppm HCHO at 220 oC, (b) dynamic resistance variation of Cr2O3-ZnO towards HCHO
7
gas with increasing concentration at a working temperature of 220 oC, (c) the relationships between the response values and HCHO concentrations, (d) the response transient of Cr2O3-ZnO sensor to 100 ppm HCHO at 220 oC.
Highlights ► Cr2O3-ZnO heterostructures are successfully fabricated by a facile route. ► Cr2O3 nanoparticles are well decorated on the surfaces of flower-like ZnO. ► Cr2O3-ZnO heterostructures show rapid response/recovery behavior to formaldehyde.
Fig. 1 Fig. 2
Fig. 3
Fig. 4
8
S0167-577X(18)30437-3 https://doi.org/10.1016/j.matlet.2018.03.073 MLBLUE 24034
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
7 November 2017 21 February 2018 13 March 2018
Please cite this article as: T. Wang, B. Liu, Q. Li, S. Wang, Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties, Materials Letters (2018), doi: https://doi.org/ 10.1016/j.matlet.2018.03.073
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 proof before it is published in its final 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.
Controllable construction of Cr2O3-ZnO hierarchical heterostructures and their formaldehyde gas sensing properties a
a*
a
Tianyi Wang , Bingbing Liu , Quanjun Li and Shuangming Wang a
b,c*
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, People’s Republic
of China. b
College of Electronic Science and Engineering, Jilin University, Changchun 130012, People’s Republic
of China. c
College of Physics & Materials Science, Tianjin Normal University, Tianjin 300387, People’s Republic
of China.
Abstract The designed Cr2O3-ZnO hierarchical heterostructures have been successfully synthesized by a facile two-step route. The SEM, TEM, XRD and EDS mapping characterizations demonstrate that Cr2O3 nanoparticles are well decorated on surfaces of nanosheet-assembled flowerlike ZnO, constructing three dimensional Cr2O3-ZnO hierarchical heterostructures. Such hierarchical architectures are developed and applied in formaldehyde (HCHO) gas detection. The gas sensing results indicate that Cr2O3-ZnO heterostructures based sensors exhibit superior HCHO gas sensing properties with rapid response/recovery behavior (1 s/5 s) toward 100 ppm formaldehyde (HCHO) gas, and favorable stability and repeatability with the on/off phenomenon when switching between HCHO and air at a optimal working temperature of 220 oC. The superior formaldehyde gas sensing performances are predominantly attributed to the appropriate sensitization effect induced by Cr2O3 nanoparticles and the special three dimensional hierarchical architectures. Keywords: Semiconductors; heterostructure; Formaldehyde; Sensors.
*
Corresponding author. E-mail address: [email protected]; [email protected] 1
1. Introduction The heterostructures constituted by two or more different materials, as important functional materials, have attracted widespread attention as they provide more opportunities for integrating physical and chemical characteristics of their individual counterparts [1,2]. Many methods have been invested to produced these functional materials, such as atomic layer deposition [3], chemical solution method [4,5], γ irradiation [6], chemical oxidative polymerization [7], etc. Among heterostructures, various oxide semiconductor based heterostructures have been synthesized and employed in many fields, for instance, gas sensors [8], supercapacitors [9], lithium ion batteries [10] and photocatalysis [11,12], etc. In gas sensing field, the heterojunction of the composites act as a lever in electron transfer through which electron transfer is facilitated or restrained, enormously influencing gas sensing behaviors [13]. Therefore, designing and fabricating appropriate heterostructures materials has long been regarded as an efficient and cost-effective strategy to improve the performances of gas sensors, such as n-n and p-n heterostructures [14,15]. However, in spite of the significant accomplishments achieved, constructing novel heterostructures by selecting proper oxides to enhance gas sensing properties still are confronted with great challenges. In this paper, Cr2O3 nanoparticles decorated flowerlike ZnO hierarchical heterostructures Cr2O3-ZnO are skillfully designed and successfully fabricated by a facile route. The gas sensor based on Cr2O3-ZnO hierarchical heterostructures exhibits rapid response/recovery behavior and favorable stability and repeatability for formaldehyde gas.
2. Experiments Sodium hydrate, zinc acetate, ethanol and chromic nitrate
Tianjin Guangfu
Fine Chemical Research Institute (Tianjin of China). All reagents were analytical-grade purity and were
2
used as purchased without further purification. The synthesis route of Cr2O3-ZnO heterostructures consisted of the construction of flowerlike ZnO and the decoration of Cr2O3 nanoparticles. Firstly nanosheet-assembled flowerlike ZnO hierarchical architectures were synthesized by a facile solution route. Briefly, 7.5 mmol sodium hydrate and 1.4 mmol zinc acetate was dissolved in 10 mL and 50 mL deionized water under magnetic stirring, respectively. Subsequently, sodium hydrate aqueous solution was added into zinc acetate aqueous solution drop by drop. After stirring for 5 min, 50 mL ethanol was slowly added into above mixed solution. After being stirred for another 5 min, white precipitates were collected by centrifuging, washed with deionized water and ethanol and dried in air at 60 oC for 5 h. The flowerlike ZnO products were obtained by calcining as-obtained white precipitates at 500 oC for 2 h. 0.1 g newly prepared ZnO samples were put in a porcelain boat and then 5 mL deionized water and 6 mg Cr(NO3)3 was added in turn. After ultrasonic dispersion for 30 min, the resulting suspension was dried in air at 60 oC for 20 min. Finally, Cr2O3-ZnO hierarchical heterostructures were obtained by annealing porcelain boat at 500 oC for 2 h in air. The heating rate was controlled at 2 oC/min. The morphology and crystal structure of Cr2O3-ZnO products were examined using a Hitachi SU8010, JEOL JEM-2200FS microscope and a Bruker D8A X-ray diffractometer. The detail of the gas sensor fabrication was similar to our previous report [16]. The gas sensing performances were measured by a CGS-8 intelligent gas sensing analysis system. The response was defined as S = Ra/Rg. Here, Ra and Rg was the resistance of the gas sensor in the air and target gas, respectively. The time taken by the sensor to change from Ra to Ra-90% (Ra-Rg) was defined as the response time when Cr2O3-ZnO sensor was placed into the target gas. The time taken by the sensor to change from Rg to Rg+90% (Ra-Rg) was defined as the recovery time when Cr2O3-ZnO sensor was taken out of the target gas.
3
3. Results and discussion Fig. 1a shows a typical FESEM image of as-prepared Cr2O3-ZnO hierarchical heterostructures. It can be observed that Cr2O3-ZnO samples are assembled by numerous nanosheets as primary building blocks and present three dimensional flowerlike architectures. The panoramic FESEM image in the inset of Fig. 1a indicates Cr2O3-ZnO products consist of monodisperse and uniform hierarchical architectures. The enlarged FESEM image (Fig. 1b) exhibits rough surfaces of Cr2O3-ZnO hierarchical architectures. The TEM images from Fig. 1c and 1d further demonstrate the flowerlike structure of Cr2O3-ZnO and reveal that Cr2O3 nanoparticles with diameters of tens of nanometers are well decorated on the surfaces of ZnO nanosheets without obvious aggregations. Fig. 2 shows the XRD pattern of Cr2O3-ZnO heterostructures. All diffractions peaks are indexed to the standard ZnO (JCPDS No. 79-0206) and no extra peaks from any other impurities are found, confirming the superior purity of as-prepared ZnO host. However, due to small decoration amount of Cr2O3 nanoparticles, obvious diffraction peaks corresponding to Cr2O3 are not detected. A similar phenomenon is also reported in the earlier literature [17]. To further determine the existence and specific distribution of Cr2O3, the element mapping (line scanning pattern) is conducted. As shown in Fig. 3, after Cr2O3-ZnO heterostructure is traversed by a testing line, the Cr (green line), Zn (red line) and O (yellow line) element can be clearly observed, clearly indicating the existence of Cr2O3, which matches well with the surface canning pattern (Fig. S1a-d). In addition, the BET surface area of the Cr2O3-ZnO heterostructure is 9.6 m2 g-1 (Fig. Sle). Fig.4a shows the responses of Cr2O3-ZnO heterostructures sensor to 100 ppm HCHO at various operating
temperature
(180-280
o
C).
The
Cr2O3-ZnO
heterostructure
sensor
exhibits
an
“increase-maximum-decrease” parabola tendency with the increase of working temperature. The
4
maximum response value is obtained at 220 oC which is chosen as the optimal working temperature. Furthermore, Fig. 4b presents dynamic resistance variations of Cr 2O3-ZnO heterostructures based sensors to different concentrations (5-500 ppm) of HCHO gas at 220 oC. It is apparent that the resistances change rapidly when the gas is repeatedly switched from air to HCHO, indicating a favorable reproducibility and stability of the present sensor. Even though HCHO gas is as low as 5 ppm, the heterostructure sensor still shows an obvious resistance change with a sensitivity of about 1.1. Fig. 4c shows the corresponding sensitivity-HCHO concentration curve. The sensitivities almost linearly increase with increasing HCHO concentration from 5 to 200 ppm, revealing that the heterostructure sensor is suitable for the detection of HCHO at low levels. For a gas sensor, the response and recovery time plays a vital part for the practical detection of detrimental gases, which is calculated to be about 1 s and 5 s to 100 ppm HCHO at 220 oC, respectively. In comparison with many other HCHO gas sensors, such as CuO microplates, CuO microspheres, SnO2-GO nanofibers, LaFeO3 nanospheres and SnO2 nanowires[18-21], the Cr2O3-ZnO heterostructure sensor shows rapider response and recovery kinetics, which may originate from the distinctive three dimensional open hierarchical architectures and sensitization effect of Cr2O3 nanoparticles. ZnO hierarchical architectures provide three dimensional networks for electron transport. Furthermore, as a p-type oxide semiconductor, surface modified Cr2O3 nanoparticles play a role in regulating the thickness of ZnO electron depletion layer. Thus, resistance change of the sensitive material is larger when it switches between air and formaldehyde gas.
4. Conclusions p-type Cr2O3 nanoparticles functionalized Cr2O3-ZnO hierarchical heterostructures are synthesized by a facile two-step approach and are developed for formaldehyde gas detection. The sensor based on Cr2O3-ZnO heterostructures exhibits rapid response/recovery kinetics and well stability and repeatability
5
toward HCHO gas. The facile synthesis route and superior gas sensing properties make the as-prepared composites developed for formaldehyde gas detection in practice. Considering energy band coupling effect between ZnO hierarchical nanosheets and homodisperse Cr2O3 nanoparticles, the products holds potential for use in photocatalysis applications in future.
Acknowledgements This work was funded by the Application Development Foundation (No. 135202XK1602) of Tianjin Normal University.
References [1] X. Zhou, Y. Xiao, M. Wang, P. Sun, F.M. Liu, X.S. Liang, et al. ACS Appl. Mater. Interfaces 7 (2015) 8743-8749. [2] X.L. Fu, B.W. Zhang, H.J. Liu, B.B. Zong, L.W. Huang, H. Bala, et al. Mater. Lett. 196 (2017) 149-152. [3] I. Narkeviciute, P. Chakthranont, A.J.M. Mackus, C. Hahn, B.A. Pinaud, S.F. Bent, et al. Nano Lett. 16 (2016) 7565-7572. [4] K.R. Reddy, B.C. Sin, C.H. Yoo, W.J. Park, K.S. Ryu, J.S. Lee, et al. Scripta Mater. 58 (2008) 1010-1013. [5] K.R. Reddy, K. Nakata, T. Ochiai, T. Murakami, D.A. Tryk, A. Fujishima, J. Nanosci. Nanotechno. 11 (2011) 3692-3695. [6] K.R. Reddy, K.P. Lee, A.I. Gopalan, M.S. Kim, A.M. Showkat, Y.C. Nho. J. Polym. Sci. Pol. Chem. 44 (2006) 3355-3364. [7] K.R. Reddy, K.P. Lee, A.L. Gopalan. Colloid. Surface. A 320 (2008) 49-56. [8] Y. Xiong, W.W. Xu, Z.Y. Zhu, Q.Z. Xue, W.B. Lu, D.G. Ding, et al. Sens. Actuators B 253 (2017) 523-532.
6
[9] A.K. Singh, D. Sarkar. J. Mater. Chem. A 5 (2017) 21715-21725. [10] X.L. Tong, M. Zeng, J. Li, Z.J. Liu. J. Alloy. Compd. 723 (2017) 129-138. [11] K.R. Reddy, M. Hassan, V.G. Gomes. Appl. Catal. A-Gen. 489 (2015) 1-16. [12] K.R. Reddy, V.G. Gomes, M. Hassan. Mater. Res. Express 1 (2014) 015012. [13] S. Xu, J. Gao, L.L. Wang, K. Kan, X. Xie, P.K. Shen, et al. Nanoscale 7 (2015) 14643-14651. [14] X.W. Zou, X.Y. Yan, G.M. Li, Y.M. Tian, M.G. Zhang, L.P. Liang. RSC Adv. 7 (2017) 34482-34487. [15] T.S. Wang, X.Y. Kou, L.P. Zhao, P. Sun, C. Liu, Y. Wang, et al. Sens. Actuators B 250 (2017) 692-702. [16] S.M. Wang, J. Cao, W. Cui, L.L. Fan, X.F. Li, D.J. Li. Sens. Actuators B 255 (2018) 159-165. [17] D.B. Xu, M. Chen, S.Y. Song, D.L. Jiang, W.Q. Fan, W.D. Shi, CrystEngComm 16 (2014) 1384-1388. [18] D. Meng, D.Y. Liu, G.S. Wang, X.G. Sun, Y.B. Shen, Q. Jin, et al. Vacuum 144 (2017) 272-280. [19] D. Wang, M.L. Zhang, Z.L. Chen, H.J. Li, A.Y. Chen, X.Y. Wang. Sens. Actuators B 250 (2017) 533-542. [20] H.H. Zhang, P. Song, D. Han, Q. Wang. Physica E 63 (2014) 21-26. [21] I. Castro-Hurtado, J. Herran, G.G. Mandayo, E. Castano. Thin Solid Films 520 (2012) 4792-4796.
Figure Captions Fig. 1 Low and high-magnification (a-b) FESEM and (c-d) TEM images of Cr2O3-ZnO hierarchical heterostructures. The inset is a panoramic FESEM image of Cr2O3-ZnO products. Fig. 2 The XRD pattern of as-prepared Cr2O3-ZnO hierarchical heterostructures. Fig. 3 The elemental mapping image (line scanning pattern) of Cr2O3-ZnO hierarchical heterostructures. The green, red and yellow line represents Cr, Zn and O element, respectively Fig. 4 (a) Responses versus working temperatures of Cr2O3-ZnO hierarchical heterostructures based sensors to 100 ppm HCHO at 220 oC, (b) dynamic resistance variation of Cr2O3-ZnO towards HCHO
7
gas with increasing concentration at a working temperature of 220 oC, (c) the relationships between the response values and HCHO concentrations, (d) the response transient of Cr2O3-ZnO sensor to 100 ppm HCHO at 220 oC.
Highlights ► Cr2O3-ZnO heterostructures are successfully fabricated by a facile route. ► Cr2O3 nanoparticles are well decorated on the surfaces of flower-like ZnO. ► Cr2O3-ZnO heterostructures show rapid response/recovery behavior to formaldehyde.
Fig. 1 Fig. 2
Fig. 3
Fig. 4
8