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Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection
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Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection Min Li a , Dongxiang Zhou a , Jun Zhao a , Zhiping Zheng a , Jungang He a , Long Hu b , Zhe Xia b , Jiang Tang b , Huan Liu a,∗ a b
School of Optical and Electronic Information, Huazhong University of Sciences and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China Wuhan National Laboratory for Optoelectronics, Huazhong University of Sciences and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China
a r t i c l e
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Article history: Available online xxx Keywords: Gas sensor Hydrogen sulfide Colloidal quantum dots Lead sulfide
a b s t r a c t Colloidal quantum dot (CQD) is emerging as new substitution gas sensing materials due to the excellent accessibility of gas molecules to CQD surfaces realized via surface ligand removal. Here we demonstrated highly sensitive and selective H2 S gas sensors based on PbS CQD solids. The sensor resistance decreases upon H2 S gas exposure and the response is defined as the ratio of the sensor resistance in clean air to that in H2 S gas. As the operating temperature increased within the range 50–135 ◦ C, the sensor response increased while the response and the recovery time decreased. The sensor was fully recoverable toward 50 ppm of H2 S at 108 ◦ C and the highest response was 2389 at 135 ◦ C with the response and recovery time being 54 s and 237 s, respectively. The dependence of sensor response on the H2 S gas concentration in the range of 10–50 ppm is linear, suggesting a theoretical detection limit of 17 ppb toward H2 S at 135 ◦ C. Meanwhile, the sensor showed superb response selectivity toward H2 S against SO2 , NO2 and NH3 . We propose that the PbS CQDs film where the surface states determine the conduction type via remote doping may undergo a p-to-n transition due to H2 S exposure at elevated temperatures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide is an irritant and extremely toxic gas even at concentrations as low as hundreds of parts per million [1–3]. Gas sensors capable of sensitively and selectively detecting low concentrations of H2 S are important to human safety. Conventionally semiconductor gas sensors are widely employed for H2 S detection but suffering from high operating temperature typically above 150 ◦ C [4]. For instance, the ZnO nanostructure sensor was operated at 300 ◦ C and the response toward 20 ppm of H2 S gas was 5 with 35 s and 390 s for response and recovery time, respectively [5]. When operated at low temperatures, the semiconductor gas sensors usually show weak and sluggish response and recovery. The Pt-loaded WO3 thin films had highest response of 3512 to 5 ppm H2 S at 100 ◦ C, but it took 36.5 min to recover to 90% of the initial resistance [6]. The SnO2 sensor promoted by In2 O3 loading had highest response of 1921 toward 100 ppm of H2 S at 40 ◦ C, but only at higher temperatures was the sensor recoverable and the recovery time was about 5 min upon gas release [7]. Overall, fast,
sensitive and fully reversible H2 S gas sensor that could be operated at low temperatures remains a very important yet challenging task. As a new class of nanostructured semiconductors, colloidal quantum dots (CQD) combine low-cost solution processibility with precise control over particle size and have been shown to be promising candidates for applications in high performance solar cells [8], photodetectors [9] and light emitters [10]. Their extremely small size (a few nanometers in diameter), large and sensitive surface also suggest their great promise for the construction of fast and sensitive gas sensors. We previously introduced PbS CQD solids into the field of gas sensing. The PbS CQDs gas sensor had rapid and recoverable response toward NO2 at room temperature [11]. Here we exploited the PbS CQDs as H2 S gas-sensing materials. Through operating temperature control, we obtained a fast, sensitive, selective, and fully recoverable H2 S gas sensor. The interaction of H2 S with PbS and its surface oxygen is discussed and the underlying mechanism is proposed to be an enhanced electron transport induced by H2 S exposure. 2. Experimental 2.1. Lead sulfide CQDs synthesis
∗ Corresponding author. Tel.: +86 27 87558482; fax: +86 27 87558482. E-mail address: [email protected] (H. Liu).
PbS CQDs were synthesized following the published procedures through the reaction of Pb-oleate and bis(trimethylsilyl) sulfide
http://dx.doi.org/10.1016/j.snb.2014.07.058 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058
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(TMS) [11]. Briefly, PbO (1.8 g, 8.0 mmol), oleic acid (19.2 mol, 6 ml) and 1-octadecene (20 ml) were mixed in a three-neck flask and heated to 90 ◦ C under vacuum for 8 h. The flask temperature was increased to 120 ◦ C and then 280 l TMS in 10 ml ODE was rapidly injected. After the injection, the heating mantle was turned off and the flask was then transferred into a cold water bath to let the flask temperature decrease to 36 ◦ C. The product was rinsed with acetone and finally dispersed in octane at a concentration of 50 mg/ml. 2.2. Sensor fabrication The sensor device was fabricated via layer-by-layer (LBL) spincoating of the CQDs onto alumina substrates. Commercial Ag paste was screen-printed onto the substrates and sintered at 600 ◦ C to form interdigital sensor electrode. For each layer, three drops of PbS CQDs solution were dropped onto the substrates and spun at 2500 RPM for 15 s. NaNO2 treatment followed by absolute methanol washing and drying was carried out after the deposition of each PbS layer to remove the oleic acid ligands surrounding the PbS CQDs from synthesis: (1) drop 1 ml of diluted NaNO2 in methanol (10 mg/ml) onto the film, wait for 45 s and spin at 2500 RPM for 10 s, repeat the NaNO2 treatment twice; (2) wash the film by methanol flush and then spun dry, repeat the methanol washing for 3 times. For the sensor device fabrication, two LBL cycles were carried out inside a fumehood in air ambient at room temperature. 2.3. Characterization The gas-sensing response curves of the sensor samples were measured using a gas sensing characterization station (QMCS-I, HUST, China). The details of the station configuration and gassensing test steps were described in our previous publications [12]. The sensor samples were mounted over the test-board in a chamber of 0.7 L volume with controlled temperature for DC electrical measurement. The accuracy of the temperature control is ±2 ◦ C. The static method was employed and the gas concentration was determined by the volume ratio. The gas-sensing tests throughout the work were performed under atmospheric pressure with a relative humidity of (45 ± 1)% at 25 ◦ C. The gas-sensing response is defined as the ratio of the sensor resistance in clean air to that in the target gas. The response time (T90) and the recovery time (T10) are specified in terms of the time for the sensor to reach 90% of its final response upon target gas exposure and the time interval over which the sensor response reduces to 10% of its maximal value after gas release, respectively.
Fig. 1. Response curves toward 50 ppm of H2 S of the sensor at different temperatures.
was better compared to 50 ◦ C. Fully recovery was observed only at higher temperature, e.g. 108 ◦ C. Fig. 2 clearly shows that the sensor became more sensitive as the operating temperature increased from 50 to 135 ◦ C. The monotonically increased response with increasing temperature suggests a thermally activated interaction between the PbS CQDs sensor with H2 S. In addition, both the response time and recovery time decreased with increasing temperature, which were ascribed to the accelerated reaction rate and the desorption rate, respectively. At 135 ◦ C, the gas sensor showed highest response of 2389 to 50 ppm of H2 S, with shortest response and recovery time being 54 and 237 s, respectively. With the goal of low-
3. Results and discussion The as-synthesized PbS CQDs (about 4 nm in diameter) were capped by oleate ligands used during synthesis to avoid agglomeration, which actually protect the CQDs from interaction with gas molecules and hinder the carrier transfer due to the long chains (18 carbons). We have found that the NaNO2 treatment could effectively remove the oleate ligands, consequently producing excellent accessibility of the incoming gas molecules to CQD surfaces and enhancing the interdot carrier transport, thereby enables the PbS CQDs to be gas-sensitive [11]. The NaNO2 -treated PbS CQDs sensor has been found to be very sensitive to NO2 at room temperature, but for H2 S gas detection, it is not the case. We thereby explored the effect of operating temperature on the sensor response upon H2 S exposure/release cycles (Fig. 1). At elevated temperatures, the sensor resistance significantly decreased upon 50 ppm-H2 S exposure; when H2 S gas was removed, the resistance increased and tended to recover its initial value in clean air. However, the sensor was not fully recoverable when operated at 80 ◦ C although the recovery
Fig. 2. (a) Dependence of the sensor response on the operating temperature. (b) Dependence of the response time and recovery time on the operating temperature.
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058
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Fig. 4. Sensor response toward various gases at 135 ◦ C.
RMSnoise = DL = 3
Fig. 3. (a) Response curves of the PbS CQDs sensor upon seven cycles of gas exposure/release at 135 ◦ C. (b) The dependence of the response on gas concentration. As shown in the inset, the sensor response depended linearly on NO2 concentration in the range of 10–50 ppm.
temperature gas sensing in mind, we did not operate the sensor at higher temperatures although we may expect higher sensitivity with more rapid response and recovery at temperatures above 135 ◦ C. We studied the dynamic response of the sensor upon multiple H2 S exposure/release cycles, successively corresponding to H2 S concentrations of 10, 20, 30, 40, 50, 70 and 100 ppm, respectively. As shown in Fig. 3a, the device had excellent reversibility and little baseline drift. The resistance returned completely to its initial value once the H2 S gas was pumped out. We show the dependence of the response upon H2 S gas concentration in Fig. 3b. The sensor response increased with gas concentration in the range of 10–100 ppm with a saturation tendency at higher concentrations. The inset indicated a linear dependence of response upon gas concentration in the range below 50 ppm, suggesting advantages in low-concentration H2 S detection. We estimated the theoretical detection limit according to the least-squares method of fitting in the linear regime. The slope is 26.8 ppm−1 with a fitting quality R2 = 0.972. The sensor noise was calculated using the variation in the relative sensor response in the baseline using the root-mean-square deviation (RMSD) [13]. We replotted 200 data points of Fig. 3a at the baseline of the sensor before the H2 S exposure. The points were averaged and a standard deviation (S) was gathered as 2.13. Therefore, the sensor noise is 0.15 according to the Eq. (1) and the theoretical detection limit (for signal-to-noise ratio of 3) is approximately 17 ppb according to the Eq. (2).
S2 = 0.15 N
RMSnoise = 17 ppb Slope
(1) (2)
The PbS CQDs sensor was not only fast and sensitive, but very selective toward H2 S. Fig. 4 shows the response values of the sensor upon H2 S, SO2 , NO2 and NH3 (50 ppm) at 135 ◦ C for comparison. The response curves toward those gases except H2 S are shown in the inset for details. As can be clearly seen, the sensor showed remarkably high response with little cross-sensitivity against SO2 , NO2 and NH3 at 135 ◦ C, suggestive of a desirable selectivity which is critical to practical use. The simplicity of CQD solids make it very attractive and promising for gas detection. It has been understood that although PbS CQDs are inherently n-type because of lead excess [14], the PbS CQDs films fabricated in ambient air are prone to take on the p-type character when subjected to even a low level of air exposure due to oxygeninduced p-type doping [15,16]. In this case, the response direction (resistance decrease) of PbS CQDs sensor toward H2 S seems to be inverted compared to n-type SnO2 films which also show resistance decrease upon H2 S that commonly acts as a reducing agent [4,17]. We hence propose that for the PbS CQDs film where the surface states dictate charge transfer and determine the conduction via remote doping [18–20], they might undergo a p-to-n transition as a result of H2 S exposure at elevated temperatures. That is to say, for the PbS quantum dots that are inherently surface dominated [21], oxygen-related surfaces states from ambient air act as acceptors and inject extra holes so that the hole conduction becomes dominant, thus forming a partially compensated p-type PbS CQDs film. Upon H2 S exposure, the surface oxygen of PbS CQDs may be consumed via the chemical reaction with the adsorbed H2 S at elevated temperature. This oxygen desorption process upon H2 S exposure may lead to a p-to-n transition due to a clean surface allowing for the intrinsic n-type character of the Pb-rich PbS CQDs. More intriguingly, H2 S may bound to the PbS CQD surface and introduce additional energy levels near the conduction band of PbS via remote doping, forming donor-like surface states in PbS CQDs and contributing to the p-to-n transition as well. As a result, the electron transport throughout the sensor was enhanced and became dominant due to H2 S exposure, so the resistance decreases shown as the response. When H2 S is released, the PbS CQDs film would take on the p-type character again due to oxygen-induced p-type doping. This explains the switch-like sensor response upon H2 S gas exposure/release cycles. Based on the experimental data, the p-to-n transition of PbS CQDs film switched by H2 S is thermally activated. A comprehensive investigation clearly describing the
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058
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underlying sensing mechanism is underway and may provide effective strategies to further improve PbS CQDs sensor performance for H2 S detection. 4. Conclusions We have demonstrated rapid-response and recoverable H2 S gas sensors based on PbS CQD solids. Thanks to the solutionprocessibility of CQDs, the sensor fabrication is simply employing spin-coating at room temperature in ambient air. The removal of surface ligands surrounding PbS CQDs realized by NaNO2 treatment ensures excellent accessibility by gas molecules to CQD surfaces. The sensor was highly sensitive (down to 17 ppb) and selective (little cross-sensitivity to SO2 , NO2 and NH3 ) to H2 S gas at elevated temperatures. The response was 2389 toward 50 ppm of H2 S at 135 ◦ C, with the response and recovery time being 54 s and 237 s, respectively. We propose that at elevated temperatures the PbS CQDs film undergoes a p-to-n transition induced by H2 S which may consume the surface adsorbed oxygen and/or induce additional energy levels near the conduction band. As a result, the electron conduction in the sensor was promoted and the resistance decreased. Acknowledgments This work was financially supported by National Natural Science Foundation of China (61006012 and 61274055). J. Tang and H. Liu acknowledge the “National 1000 Young Talents” project and the Program for New Century Excellent Talents in University (NCET-120216), respectively. The authors thank the Analytical and Testing Center of HUST and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for facility access. References [1] S. Pandey, K. Kim, K. Tang, A review of sensor-based methods for monitoring hydrogen sulfide, TrAC – Trends Anal. Chem. 32 (2012) 87–99. [2] J. Sarfraz, P. Ihalainen, A. Määttänen, T. Gulin, J. Koskela, C. Wilén, A. Kilpelä, J. Peltonen, A printed H2 S sensor with electro-optical response, Sens. Actuators B 191 (2014) 821–827. [3] W. Mickelson, A. Sussman, A. Zettl, Low-powder, fast, selective nanoparticlebased hydrogen sulfide gas sensor, Appl. Phys. Lett. 100 (2012) 173110. [4] H. Liu, S. Gong, Y. Hu, J. Liu, D. Zhou, Properties and mechanism study of SnO2 nanocrystals for H2 S thick-film sensors, Sens. Actuators B 140 (2009) 190–195. [5] A. Mortezaali, R. Moradi, The correlation between the substrate temperature and morphological ZnO nanostructures for H2 S gas sensors, Sens. Actuators A 206 (2014) 30–34. [6] Y. Shen, B. Zhang, X. Cao, D. Wei, J. Ma, L. Jia, S. Gao, B. Cui, Y. Jin, Microstructure and enhanced H2 S sensing properties of Pt-loaded WO3 thin films, Sens. Actuators B 193 (2014) 273–279. [7] H. Liu, S. Wu, S. Gong, J. Zhao, J. Liu, D. Zhou, Nanocrystalline In2 O3 –SnO2 thick films for low-temperature hydrogen sulfide detection, Ceram. Int. 37 (2011) 1889–1894. [8] J. Tang, K. Kemp, S. Hoogland, K. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. Chou, A. Fischer, A. Amassian, J. Asbury, E. Sargent, Colloidalquantum-dot photovoltaics using atomic-ligand passivation, Nat. Mater. 10 (2011) 765–771. [9] E. Sargent, Photodetectors: a sensitive pair, Nat. Nanotechnol. 7 (2012) 349–350. ´ Emergence of colloidal [10] Y. Shirasaki, G. Supran, M. Bawendi, V. Bulovic, quantum-dot light-emitting technologies, Nat. Photonics 7 (2013) 13–23. [11] H. Liu, M. Li, O. Voznyy, L. Hu, Q. Fu, D. Zhou, Z. Xia, E. Sargent, J. Tang, Physically flexible, rapid-response gas sensor based on colloidal quantum dot solids, Adv. Mater. 26 (2014) 2718–2724. [12] H. Liu, J. Wan, Q. Fu, M. Li, W. Luo, Z. Zheng, H. Cao, Y. Hu, D. Zhou, Tin oxide films for nitrogen dioxide gas detection at low temperatures, Sens. Actuators B 177 (2013) 460–466.
[13] V. Dua, S.P. Surwade, S. Ammu, S.R. Agnihotra, S. Jain, K.E. Roberts, S. Park, R.S. Ruoff, S.K. Manohar, Angew. Chem. Int. Ed. 49 (2010) 2154–2157. [14] D. Zhitomirsky, M. Furukawa, J. Tang, P. Stadler, S. Hoogland, O. Voznyy, H. Liu, E. Sargent, N-type colloidal quantum dot solids for photovoltaics, Adv. Mater. 24 (2012) 6181–6185. [15] E. Klem, H. Shukla, S. Hinds, D. MacNeil, L. Levina, E. Sargent, Impact of dithiol treatment and air annealing on the conductivity, mobility, and hole density in PbS colloidal quantum dot solids, Appl. Phys. Lett. 92 (2008) 212105. [16] G. Konstantatos, L. Levina, A. Fischer, E. Sargent, Engineering the temporal response of photoconductive photodetectors via selective introduction of surface trap states, Nano Lett. 8 (2008) 1446–1450. [17] N. Barsan, Transduction in semiconducting metal oxide based gas sensors – implications of the conduction mechanism, Procedia Eng. 25 (2011) 100–103. [18] D. Norris, A. Efros, S. Erwin, Doped nanocrystals, Science 319 (2008) 1776–1779. [19] M. Shim, P. Guyot-Sionnest, N-type colloidal semiconductor nanocrystals, Nature 407 (2000) 981–983. [20] S. Geyer, P. Allen, L. Chang, C. Wong, T. Osedach, N. Zhao, V. Bulovic, M. Bawendi, Control of the carrier type in InAs nanocrystal films by predeposition incorporation of Cd, ACS Nano 4 (2010) 7373–7378. [21] J. Luther, J. Pietryga, Stoichiometry control in quantum dots: a viable analog to impurity doping of bulk materials, ACS Nano 7 (2013) 1845–1849.
Biographies Min Li received her Bachelor’s degree in Department of Electronic Science and Technology from Huazhong University of Science and Technology, China in 2011. She is currently a PhD degree candidate in School of Optical and Electronic Information at Huazhong University of Science and Technology, China. Her research focuses on colloidal quantum dots and thin film gas sensor. Dongxiang Zhou has been a professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 1991. He received Master’s degree from Huazhong University of Science and Technology in 1980. He has continuously conducted systematic research into theories and application technologies of semiconductor ceramics physics, functional materials design and virtual instruments. He has authored more than 300 refereed journals and proceedings, and holds more than 20 patents. Jun Zhao acquired his PhD degree in micro- and solid-state electronics from Huazhong University of Science and Technology in 2007. He is now working as a Lecturer and his research interests include electronic circuit and material physics and chemistry. Zhiping Zheng has been an associate professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 2008. She received her Master’s degree and PhD degree in Micro- and Solid-state Electronics from Huazhong University of Science and Technology in 2002 and 2005, respectively. Her research interests include functional ceramics and sensors. Jungang He is currently a PhD degree candidate in School of Optical and Electronic Information at Huazhong University of Science and Technology. His research focuses on colloidal quantum dot photodetectors. Long Hu is currently a PhD degree candidate in Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. His research focuses on thin film photovoltaics. Zhe Xia received her Bachelor’s degree in Huazhong University of Science and Technology, China in 2012. She is currently a PhD degree candidate in Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. Her research focuses on thin film photovoltaics. Jiang Tang is a full professor at Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. He obtained his Bachelor’s degree from University of Science and Technology of China, and his PhD from University of Toronto under the supervision of Prof. Edward H. Sargent. After graduation, he moved to IBM T. J. Watson research center working on copper zinc tin sulfide solar cells as a postdoctoral researcher. Based on his intensive research experience in PbS colloidal quantum dot (CQDs) solar cells and photodetectors, he started working on PbS CQDs gas sensors at WNLO, as well as thin film photovoltaics. Huan Liu has been an associate professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 2011. She received her Master’s degree and PhD degree in micro- and solid-state electronics from Huazhong University of Science and Technology in 2004 and 2008 respectively. From 2009 to 2011, she had been a post-doctoral fellow in the Department of Electrical and Computer Engineering at University of Toronto, Canada. Her research interests include nanostructured functional materials, gas sensors and photovoltaics.
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058
ARTICLE IN PRESS
SNB-17212; No. of Pages 4
Sensors and Actuators B xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection Min Li a , Dongxiang Zhou a , Jun Zhao a , Zhiping Zheng a , Jungang He a , Long Hu b , Zhe Xia b , Jiang Tang b , Huan Liu a,∗ a b
School of Optical and Electronic Information, Huazhong University of Sciences and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China Wuhan National Laboratory for Optoelectronics, Huazhong University of Sciences and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China
a r t i c l e
i n f o
Article history: Available online xxx Keywords: Gas sensor Hydrogen sulfide Colloidal quantum dots Lead sulfide
a b s t r a c t Colloidal quantum dot (CQD) is emerging as new substitution gas sensing materials due to the excellent accessibility of gas molecules to CQD surfaces realized via surface ligand removal. Here we demonstrated highly sensitive and selective H2 S gas sensors based on PbS CQD solids. The sensor resistance decreases upon H2 S gas exposure and the response is defined as the ratio of the sensor resistance in clean air to that in H2 S gas. As the operating temperature increased within the range 50–135 ◦ C, the sensor response increased while the response and the recovery time decreased. The sensor was fully recoverable toward 50 ppm of H2 S at 108 ◦ C and the highest response was 2389 at 135 ◦ C with the response and recovery time being 54 s and 237 s, respectively. The dependence of sensor response on the H2 S gas concentration in the range of 10–50 ppm is linear, suggesting a theoretical detection limit of 17 ppb toward H2 S at 135 ◦ C. Meanwhile, the sensor showed superb response selectivity toward H2 S against SO2 , NO2 and NH3 . We propose that the PbS CQDs film where the surface states determine the conduction type via remote doping may undergo a p-to-n transition due to H2 S exposure at elevated temperatures. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Hydrogen sulfide is an irritant and extremely toxic gas even at concentrations as low as hundreds of parts per million [1–3]. Gas sensors capable of sensitively and selectively detecting low concentrations of H2 S are important to human safety. Conventionally semiconductor gas sensors are widely employed for H2 S detection but suffering from high operating temperature typically above 150 ◦ C [4]. For instance, the ZnO nanostructure sensor was operated at 300 ◦ C and the response toward 20 ppm of H2 S gas was 5 with 35 s and 390 s for response and recovery time, respectively [5]. When operated at low temperatures, the semiconductor gas sensors usually show weak and sluggish response and recovery. The Pt-loaded WO3 thin films had highest response of 3512 to 5 ppm H2 S at 100 ◦ C, but it took 36.5 min to recover to 90% of the initial resistance [6]. The SnO2 sensor promoted by In2 O3 loading had highest response of 1921 toward 100 ppm of H2 S at 40 ◦ C, but only at higher temperatures was the sensor recoverable and the recovery time was about 5 min upon gas release [7]. Overall, fast,
sensitive and fully reversible H2 S gas sensor that could be operated at low temperatures remains a very important yet challenging task. As a new class of nanostructured semiconductors, colloidal quantum dots (CQD) combine low-cost solution processibility with precise control over particle size and have been shown to be promising candidates for applications in high performance solar cells [8], photodetectors [9] and light emitters [10]. Their extremely small size (a few nanometers in diameter), large and sensitive surface also suggest their great promise for the construction of fast and sensitive gas sensors. We previously introduced PbS CQD solids into the field of gas sensing. The PbS CQDs gas sensor had rapid and recoverable response toward NO2 at room temperature [11]. Here we exploited the PbS CQDs as H2 S gas-sensing materials. Through operating temperature control, we obtained a fast, sensitive, selective, and fully recoverable H2 S gas sensor. The interaction of H2 S with PbS and its surface oxygen is discussed and the underlying mechanism is proposed to be an enhanced electron transport induced by H2 S exposure. 2. Experimental 2.1. Lead sulfide CQDs synthesis
∗ Corresponding author. Tel.: +86 27 87558482; fax: +86 27 87558482. E-mail address: [email protected] (H. Liu).
PbS CQDs were synthesized following the published procedures through the reaction of Pb-oleate and bis(trimethylsilyl) sulfide
http://dx.doi.org/10.1016/j.snb.2014.07.058 0925-4005/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058
G Model SNB-17212; No. of Pages 4
ARTICLE IN PRESS M. Li et al. / Sensors and Actuators B xxx (2014) xxx–xxx
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(TMS) [11]. Briefly, PbO (1.8 g, 8.0 mmol), oleic acid (19.2 mol, 6 ml) and 1-octadecene (20 ml) were mixed in a three-neck flask and heated to 90 ◦ C under vacuum for 8 h. The flask temperature was increased to 120 ◦ C and then 280 l TMS in 10 ml ODE was rapidly injected. After the injection, the heating mantle was turned off and the flask was then transferred into a cold water bath to let the flask temperature decrease to 36 ◦ C. The product was rinsed with acetone and finally dispersed in octane at a concentration of 50 mg/ml. 2.2. Sensor fabrication The sensor device was fabricated via layer-by-layer (LBL) spincoating of the CQDs onto alumina substrates. Commercial Ag paste was screen-printed onto the substrates and sintered at 600 ◦ C to form interdigital sensor electrode. For each layer, three drops of PbS CQDs solution were dropped onto the substrates and spun at 2500 RPM for 15 s. NaNO2 treatment followed by absolute methanol washing and drying was carried out after the deposition of each PbS layer to remove the oleic acid ligands surrounding the PbS CQDs from synthesis: (1) drop 1 ml of diluted NaNO2 in methanol (10 mg/ml) onto the film, wait for 45 s and spin at 2500 RPM for 10 s, repeat the NaNO2 treatment twice; (2) wash the film by methanol flush and then spun dry, repeat the methanol washing for 3 times. For the sensor device fabrication, two LBL cycles were carried out inside a fumehood in air ambient at room temperature. 2.3. Characterization The gas-sensing response curves of the sensor samples were measured using a gas sensing characterization station (QMCS-I, HUST, China). The details of the station configuration and gassensing test steps were described in our previous publications [12]. The sensor samples were mounted over the test-board in a chamber of 0.7 L volume with controlled temperature for DC electrical measurement. The accuracy of the temperature control is ±2 ◦ C. The static method was employed and the gas concentration was determined by the volume ratio. The gas-sensing tests throughout the work were performed under atmospheric pressure with a relative humidity of (45 ± 1)% at 25 ◦ C. The gas-sensing response is defined as the ratio of the sensor resistance in clean air to that in the target gas. The response time (T90) and the recovery time (T10) are specified in terms of the time for the sensor to reach 90% of its final response upon target gas exposure and the time interval over which the sensor response reduces to 10% of its maximal value after gas release, respectively.
Fig. 1. Response curves toward 50 ppm of H2 S of the sensor at different temperatures.
was better compared to 50 ◦ C. Fully recovery was observed only at higher temperature, e.g. 108 ◦ C. Fig. 2 clearly shows that the sensor became more sensitive as the operating temperature increased from 50 to 135 ◦ C. The monotonically increased response with increasing temperature suggests a thermally activated interaction between the PbS CQDs sensor with H2 S. In addition, both the response time and recovery time decreased with increasing temperature, which were ascribed to the accelerated reaction rate and the desorption rate, respectively. At 135 ◦ C, the gas sensor showed highest response of 2389 to 50 ppm of H2 S, with shortest response and recovery time being 54 and 237 s, respectively. With the goal of low-
3. Results and discussion The as-synthesized PbS CQDs (about 4 nm in diameter) were capped by oleate ligands used during synthesis to avoid agglomeration, which actually protect the CQDs from interaction with gas molecules and hinder the carrier transfer due to the long chains (18 carbons). We have found that the NaNO2 treatment could effectively remove the oleate ligands, consequently producing excellent accessibility of the incoming gas molecules to CQD surfaces and enhancing the interdot carrier transport, thereby enables the PbS CQDs to be gas-sensitive [11]. The NaNO2 -treated PbS CQDs sensor has been found to be very sensitive to NO2 at room temperature, but for H2 S gas detection, it is not the case. We thereby explored the effect of operating temperature on the sensor response upon H2 S exposure/release cycles (Fig. 1). At elevated temperatures, the sensor resistance significantly decreased upon 50 ppm-H2 S exposure; when H2 S gas was removed, the resistance increased and tended to recover its initial value in clean air. However, the sensor was not fully recoverable when operated at 80 ◦ C although the recovery
Fig. 2. (a) Dependence of the sensor response on the operating temperature. (b) Dependence of the response time and recovery time on the operating temperature.
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Fig. 4. Sensor response toward various gases at 135 ◦ C.
RMSnoise = DL = 3
Fig. 3. (a) Response curves of the PbS CQDs sensor upon seven cycles of gas exposure/release at 135 ◦ C. (b) The dependence of the response on gas concentration. As shown in the inset, the sensor response depended linearly on NO2 concentration in the range of 10–50 ppm.
temperature gas sensing in mind, we did not operate the sensor at higher temperatures although we may expect higher sensitivity with more rapid response and recovery at temperatures above 135 ◦ C. We studied the dynamic response of the sensor upon multiple H2 S exposure/release cycles, successively corresponding to H2 S concentrations of 10, 20, 30, 40, 50, 70 and 100 ppm, respectively. As shown in Fig. 3a, the device had excellent reversibility and little baseline drift. The resistance returned completely to its initial value once the H2 S gas was pumped out. We show the dependence of the response upon H2 S gas concentration in Fig. 3b. The sensor response increased with gas concentration in the range of 10–100 ppm with a saturation tendency at higher concentrations. The inset indicated a linear dependence of response upon gas concentration in the range below 50 ppm, suggesting advantages in low-concentration H2 S detection. We estimated the theoretical detection limit according to the least-squares method of fitting in the linear regime. The slope is 26.8 ppm−1 with a fitting quality R2 = 0.972. The sensor noise was calculated using the variation in the relative sensor response in the baseline using the root-mean-square deviation (RMSD) [13]. We replotted 200 data points of Fig. 3a at the baseline of the sensor before the H2 S exposure. The points were averaged and a standard deviation (S) was gathered as 2.13. Therefore, the sensor noise is 0.15 according to the Eq. (1) and the theoretical detection limit (for signal-to-noise ratio of 3) is approximately 17 ppb according to the Eq. (2).
S2 = 0.15 N
RMSnoise = 17 ppb Slope
(1) (2)
The PbS CQDs sensor was not only fast and sensitive, but very selective toward H2 S. Fig. 4 shows the response values of the sensor upon H2 S, SO2 , NO2 and NH3 (50 ppm) at 135 ◦ C for comparison. The response curves toward those gases except H2 S are shown in the inset for details. As can be clearly seen, the sensor showed remarkably high response with little cross-sensitivity against SO2 , NO2 and NH3 at 135 ◦ C, suggestive of a desirable selectivity which is critical to practical use. The simplicity of CQD solids make it very attractive and promising for gas detection. It has been understood that although PbS CQDs are inherently n-type because of lead excess [14], the PbS CQDs films fabricated in ambient air are prone to take on the p-type character when subjected to even a low level of air exposure due to oxygeninduced p-type doping [15,16]. In this case, the response direction (resistance decrease) of PbS CQDs sensor toward H2 S seems to be inverted compared to n-type SnO2 films which also show resistance decrease upon H2 S that commonly acts as a reducing agent [4,17]. We hence propose that for the PbS CQDs film where the surface states dictate charge transfer and determine the conduction via remote doping [18–20], they might undergo a p-to-n transition as a result of H2 S exposure at elevated temperatures. That is to say, for the PbS quantum dots that are inherently surface dominated [21], oxygen-related surfaces states from ambient air act as acceptors and inject extra holes so that the hole conduction becomes dominant, thus forming a partially compensated p-type PbS CQDs film. Upon H2 S exposure, the surface oxygen of PbS CQDs may be consumed via the chemical reaction with the adsorbed H2 S at elevated temperature. This oxygen desorption process upon H2 S exposure may lead to a p-to-n transition due to a clean surface allowing for the intrinsic n-type character of the Pb-rich PbS CQDs. More intriguingly, H2 S may bound to the PbS CQD surface and introduce additional energy levels near the conduction band of PbS via remote doping, forming donor-like surface states in PbS CQDs and contributing to the p-to-n transition as well. As a result, the electron transport throughout the sensor was enhanced and became dominant due to H2 S exposure, so the resistance decreases shown as the response. When H2 S is released, the PbS CQDs film would take on the p-type character again due to oxygen-induced p-type doping. This explains the switch-like sensor response upon H2 S gas exposure/release cycles. Based on the experimental data, the p-to-n transition of PbS CQDs film switched by H2 S is thermally activated. A comprehensive investigation clearly describing the
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underlying sensing mechanism is underway and may provide effective strategies to further improve PbS CQDs sensor performance for H2 S detection. 4. Conclusions We have demonstrated rapid-response and recoverable H2 S gas sensors based on PbS CQD solids. Thanks to the solutionprocessibility of CQDs, the sensor fabrication is simply employing spin-coating at room temperature in ambient air. The removal of surface ligands surrounding PbS CQDs realized by NaNO2 treatment ensures excellent accessibility by gas molecules to CQD surfaces. The sensor was highly sensitive (down to 17 ppb) and selective (little cross-sensitivity to SO2 , NO2 and NH3 ) to H2 S gas at elevated temperatures. The response was 2389 toward 50 ppm of H2 S at 135 ◦ C, with the response and recovery time being 54 s and 237 s, respectively. We propose that at elevated temperatures the PbS CQDs film undergoes a p-to-n transition induced by H2 S which may consume the surface adsorbed oxygen and/or induce additional energy levels near the conduction band. As a result, the electron conduction in the sensor was promoted and the resistance decreased. Acknowledgments This work was financially supported by National Natural Science Foundation of China (61006012 and 61274055). J. Tang and H. Liu acknowledge the “National 1000 Young Talents” project and the Program for New Century Excellent Talents in University (NCET-120216), respectively. The authors thank the Analytical and Testing Center of HUST and the Center of Micro-Fabrication and Characterization (CMFC) of WNLO for facility access. References [1] S. Pandey, K. Kim, K. Tang, A review of sensor-based methods for monitoring hydrogen sulfide, TrAC – Trends Anal. Chem. 32 (2012) 87–99. [2] J. Sarfraz, P. Ihalainen, A. Määttänen, T. Gulin, J. Koskela, C. Wilén, A. Kilpelä, J. Peltonen, A printed H2 S sensor with electro-optical response, Sens. Actuators B 191 (2014) 821–827. [3] W. Mickelson, A. Sussman, A. Zettl, Low-powder, fast, selective nanoparticlebased hydrogen sulfide gas sensor, Appl. Phys. Lett. 100 (2012) 173110. [4] H. Liu, S. Gong, Y. Hu, J. Liu, D. Zhou, Properties and mechanism study of SnO2 nanocrystals for H2 S thick-film sensors, Sens. Actuators B 140 (2009) 190–195. [5] A. Mortezaali, R. Moradi, The correlation between the substrate temperature and morphological ZnO nanostructures for H2 S gas sensors, Sens. Actuators A 206 (2014) 30–34. [6] Y. Shen, B. Zhang, X. Cao, D. Wei, J. Ma, L. Jia, S. Gao, B. Cui, Y. Jin, Microstructure and enhanced H2 S sensing properties of Pt-loaded WO3 thin films, Sens. Actuators B 193 (2014) 273–279. [7] H. Liu, S. Wu, S. Gong, J. Zhao, J. Liu, D. Zhou, Nanocrystalline In2 O3 –SnO2 thick films for low-temperature hydrogen sulfide detection, Ceram. Int. 37 (2011) 1889–1894. [8] J. Tang, K. Kemp, S. Hoogland, K. Jeong, H. Liu, L. Levina, M. Furukawa, X. Wang, R. Debnath, D. Cha, K. Chou, A. Fischer, A. Amassian, J. Asbury, E. Sargent, Colloidalquantum-dot photovoltaics using atomic-ligand passivation, Nat. Mater. 10 (2011) 765–771. [9] E. Sargent, Photodetectors: a sensitive pair, Nat. Nanotechnol. 7 (2012) 349–350. ´ Emergence of colloidal [10] Y. Shirasaki, G. Supran, M. Bawendi, V. Bulovic, quantum-dot light-emitting technologies, Nat. Photonics 7 (2013) 13–23. [11] H. Liu, M. Li, O. Voznyy, L. Hu, Q. Fu, D. Zhou, Z. Xia, E. Sargent, J. Tang, Physically flexible, rapid-response gas sensor based on colloidal quantum dot solids, Adv. Mater. 26 (2014) 2718–2724. [12] H. Liu, J. Wan, Q. Fu, M. Li, W. Luo, Z. Zheng, H. Cao, Y. Hu, D. Zhou, Tin oxide films for nitrogen dioxide gas detection at low temperatures, Sens. Actuators B 177 (2013) 460–466.
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Biographies Min Li received her Bachelor’s degree in Department of Electronic Science and Technology from Huazhong University of Science and Technology, China in 2011. She is currently a PhD degree candidate in School of Optical and Electronic Information at Huazhong University of Science and Technology, China. Her research focuses on colloidal quantum dots and thin film gas sensor. Dongxiang Zhou has been a professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 1991. He received Master’s degree from Huazhong University of Science and Technology in 1980. He has continuously conducted systematic research into theories and application technologies of semiconductor ceramics physics, functional materials design and virtual instruments. He has authored more than 300 refereed journals and proceedings, and holds more than 20 patents. Jun Zhao acquired his PhD degree in micro- and solid-state electronics from Huazhong University of Science and Technology in 2007. He is now working as a Lecturer and his research interests include electronic circuit and material physics and chemistry. Zhiping Zheng has been an associate professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 2008. She received her Master’s degree and PhD degree in Micro- and Solid-state Electronics from Huazhong University of Science and Technology in 2002 and 2005, respectively. Her research interests include functional ceramics and sensors. Jungang He is currently a PhD degree candidate in School of Optical and Electronic Information at Huazhong University of Science and Technology. His research focuses on colloidal quantum dot photodetectors. Long Hu is currently a PhD degree candidate in Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. His research focuses on thin film photovoltaics. Zhe Xia received her Bachelor’s degree in Huazhong University of Science and Technology, China in 2012. She is currently a PhD degree candidate in Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. Her research focuses on thin film photovoltaics. Jiang Tang is a full professor at Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology. He obtained his Bachelor’s degree from University of Science and Technology of China, and his PhD from University of Toronto under the supervision of Prof. Edward H. Sargent. After graduation, he moved to IBM T. J. Watson research center working on copper zinc tin sulfide solar cells as a postdoctoral researcher. Based on his intensive research experience in PbS colloidal quantum dot (CQDs) solar cells and photodetectors, he started working on PbS CQDs gas sensors at WNLO, as well as thin film photovoltaics. Huan Liu has been an associate professor of School of Optical and Electronic Information at Huazhong University of Science and Technology, China since 2011. She received her Master’s degree and PhD degree in micro- and solid-state electronics from Huazhong University of Science and Technology in 2004 and 2008 respectively. From 2009 to 2011, she had been a post-doctoral fellow in the Department of Electrical and Computer Engineering at University of Toronto, Canada. Her research interests include nanostructured functional materials, gas sensors and photovoltaics.
Please cite this article in press as: M. Li, et al., Resistive gas sensors based on colloidal quantum dot (CQD) solids for hydrogen sulfide detection, Sens. Actuators B: Chem. (2014), http://dx.doi.org/10.1016/j.snb.2014.07.058