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Fast-Response MoS2-Based Humidity Sensor Braced by SiO2 Microsphere Layers Ning Li , Xiang-Dong Chen, Xin-Peng Chen, Xing Ding, and Xuan Zhao Abstract — In this letter, a MoS2 -based humidity sensor braced by SiO2 microsphere layers with rapid response was developed. The results demonstrated that the humidity sensor can exhibit a subsecond response. A mechanism based on the morphology and structure of the sensing film was proposed to explain the rapid response behavior of humidity sensors. In addition, the effect of film thickness on response time was discussed. This letter demonstrated that the MoS2 based humidity sensor braced by SiO2 microspheres is suitable for the miniature drip infusion rate detection device. Index Terms — MoS2 , SiO2 microsphere, humidity sensor, fast response.
I. I NTRODUCTION
I
N GENERAL, the typical sensing materials for commercially available humidity sensors can be classified into metal oxides, ceramics, and polymers [1]–[3]. However, most commercial humidity sensors may have the disadvantage of slower response time. The conventional solid-state humidity sensors usually require a waiting period of more than 30 seconds to allow the sensor to reach a stable water adsorption state [4]. Alternatively, polymer-based humidity-sensing materials, such as polyethyleneimine (PEI), usually require a longer response time of 4 to 6 minutes [5]. Given its long response time, these traditional sensors may not meet the requirement of immediate and rapid humidity measurements, such as that in portable spirometers, apnea monitors, neonate respiration monitors, management of anesthesia patients, and atmospheric monitoring [6]–[10].The response speed of humidity sensors is heavily influenced by the thickness and microstructure of humidity-sensing films; thus, the choice of suitable sensing films is important in achieving a prompt response [11]. MoS2 , a typical example of two-dimensional (2D) layered nanomaterials, has attracted great attention due to its layered structure, in which each layer consists of molybdenum atoms sandwiched between two layers of hexagonally close-packed Manuscript received September 6, 2017; revised October 8, 2017 and November 8, 2017; accepted November 20, 2017. Date of publication November 28, 2017; date of current version December 27, 2017. This work was supported in part by the National Natural Science Foundation of China under Grant 61471305, in part by the Key Project of National Natural Science Foundation of China under Grant 61731016, and in part by the Science and Technology Support Program of Sichuan Province under Grant 2016JZ0028. The review of this letter was arranged by Editor K. Uchida. (Corresponding author: Xiang-Dong Chen.) The authors are with the School of Information Science and Technology, Southwest Jiaotong University, Chengdu 610031, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2017.2778187
sulfur atoms [12]. On the edge of the MoS2 plates, Mo-S bonding with 2-H structure exhibit good hydrophilicity [13], which gives the potential of MoS2 as a humidity sensitive material. Previous studies have demonstrated the promising sensing properties of MoS2 . Tan et al. reported the humidity sensing property of MoS2 micro@nanospheres, but its response time is 140 seconds [14]. In another study, Zhang et al. [15] fabricated a humidity sensor based on MoS2 with various layers that is controlled in different exfoliation methods, and the response time is 17 seconds. Zhang et al. [16] fabricated a humidity sensor based on MoS2 -modified SnO2 hybrid nanocomposite with extremely high sensitivity and response times of 60 seconds. Furthermore, in previous reports, the response speed of MoS2 humidity sensors is not satisfied. SiO2 microspheres, or the nanoscale spherical structures of SiO2 , show excellent thermal, chemical and biological stabilities. SiO2 microspheres are often used in infrastructures, wherein fabricated composite materials are used in many monitoring fields, including the detection of biomolecules, ions and compounds [17]–[19]. In this study, we developed fast-response humidity sensors based on MoS2 with SiO2 microspheres as bracing structures. The mechanism underlying the rapid humidity response of the sensors was discussed by analyzing the structural characteristics of sensing films and the permeation of water molecules. II. S ENSOR FABRICATION AND T ESTING All chemicals with analytical grade were purchased from xianfeng-nano company. MoS2 dispersion (1mg/ml) and SiO2 microspheres dispersion were ultrasonic treated for 30 mins before use. Interdigitated electrodes (IDEs) were prepared to fabricate these humidity sensors. Ti/Au layers with a thickness of 100 nm/300 nm were deposited on an n-type silicon wafer with a top layer of SiO2 (400 nm) using magnetron sputtering, and the electrode width and gap both was 50 μm. Before the deposition of sensing film, the IDEs were cleaned with deionized water and ethanol in an ultrasonic washing unit for 30 min and then vacuum dried for 1 h. To investigate the effects of film thickness on the humidity characteristics of sensors, MoS2 dispersion was spin-coated on the IDEs with different spin times at a speed of 500 circles/min. Basing on previous work [6], we estimated the thickness of MoS2 films to be 92, 197 and 413nm, respectively, and the samples were named as Sensors 1–3, respectively. To study the effect of SiO2 bracing layers, some IDEs were pretreated with SiO2 dispersion (1mg/ml) by dropping method.
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Fig. 1. (a) Schematic of sensor device. (b) XRD pattern of MoS2 . SEM image of (c) MoS2 , (d) SiO2 , and (e) SiO2 braced MoS2 layers.
The prepared IDEs were obtained after drying the water in the dispersion on the pretreated IDEs at 40 °C for 2 hours. MoS2 dispersion was spin-coated on the IDEs with SiO2 layers with same spin times corresponding to Sensor L1-L3, respectively, which were named as Sensor L1-L3. The structure of sensors was provided in Fig. 1 (a). To evaluate the humidity-sensing performance of the MoS2 based sensors, the sensors were placed in the chamber of temperature and humidity generator (Hong Zhan, LP-80U, China). The impedance response of all the sensors was measured using a LCR meter (Wayne Kerr, 4100, UK) connected to a PC with a local area network interface. The response and recovery times were recorded by exposing the sensor inside the homemade chamber with different levels of relative humidity (RH) for the intake of dry N2 /wet air controlled by the switch valve. The ambient humidity in the chamber was measured as ∼93% when the wet air was replenished. An oscilloscope (Tektronix, TDS 2024B, USA) with 8 ms acquisition time was used to provide detailed measurements of the response time. III. R ESULTS AND D ISCUSSION Fig. 1 (b) shows the X-ray diffractograms of MoS2 with diffraction peaks ranging from 10° to 90°. The diffraction peaks of MoS2 nanosheets are observed at 2θ of 14.4°, 33.1°, 39.7° and 58.5°, in accordance with the MoS2 nano-crystals of (002), (100), (103), and (110) planes. No peaks for other substance were observed in the spectrum, thus illustrating that MoS2 was of high-purity. Fig. 1 (c–e) shows the scanning electron microscope images of MoS2 , SiO2 and SiO2 braced MoS2 layers. MoS2 films were stacked with millions of MoS2 nanosheets with flake diameters of less than 500 nm. SiO2 bracing layers, with suboptimal continuity, were formed by one or few layers of SiO2 microspheres. As shown in Fig. 1 (e), MoS2 nanosheets were stacked over the SiO2 microspheres, which considerably formed more gaps in the MoS2 films. Fig. 2 (b) shows the impedance response of all fabricated humidity sensors. The impedance decreased with the rising relative humidity (RH) as well as the increasing thickness of MoS2 sensing films. This phenomenon showed that the
Fig. 2. (a) Response–recovery characteristic of sensors. (b) Impedance of sensors under switching RH. (c) Estimation of response and recovery times. TABLE I P ERFORMANCE C OMPARISON W ITH P REVIOUS H UMIDITY S ENSORS
sensitivity of humidity sensors was influenced by the thickness of the sensing films. A possible explanation implies that when the film thickness gradually increases, the isolated MoS2 nano-sheets can gradually form a continuous film layer, correspondingly. The sensors adsorbed water molecules more easily to form a continuous layer of such molecules. Therefore, the ionic conduction is greatly improved, decreasing the measured impedance. Moreover, SiO2 layers were intensely weakened in response to water, and Sensors L1-L3 exhibited the similar responses as with Sensors 1-3, respectively. This phenomenon occurred mainly due to the weak water absorption and discontinuity of SiO2 layers, thus providing the sensors with no contribution in humidity response. Fig. 2 (a) shows the response–recovery characteristics of all the sensors. The response time and recovery time of all the sensors are presented in Fig. 2 (c). It can be found that the response time of sensors was shortened with the decreasing thickness of sensing films. Moreover, compared with Sensors 1-3, the response and recovery times of Sensors L1-L3 were reduced. This result indicated that SiO2 bracing layers are very beneficial in improving the response speed of humidity sensors. As shown in Tab. 1, These sensors (except for Sensor 3) exhibited subsecond response, which surpassed all reported MoS2 based humidity sensors and most of GO based humidity sensors. In this study, we proposed a possible mechanism behind the fast response behavior of the MoS2 –based humidity sensors braced by SiO2 layer. The diagram of water molecules adsorption process on sensing layers was given in Fig. 3. (a). Owing
LI et al.: FAST-RESPONSE MoS2 -BASED HUMIDITY SENSOR BRACED BY SiO2 MICROSPHERE LAYERS
Fig. 3. (a) Schematic illustration for adsorption and permeation process of water molecules. (b) The adsorption process on two pieces MoS2 sheets. (c) The absorption and desorption of water molecules in convex and concave surface.
to the Mo-S bonding on the edge of the plates, along with polarity and hydrophilicity [13, 28–30], water molecules could be easily adsorbed and gathered on this position, as shown in Fig. 3 (b). This phenomenon promotes the formation of water layers, strengthens ion conduction, and sharply increases the output capacitance of sensor. Moreover, it can also accelerate the permeation of water molecules. Meanwhile, MoS2 films were supported by SiO2 microspheres layers, which can increase the number of gaps in the MoS2 film and roughen the surface. These structures can improve the speed of the water molecule adsorption process. In addition, water molecules naturally form a convex surface on MoS2 sensing films. The absorption and desorption of water molecules in convex and concave surface are presented simply in Fig. 3. (c). As is well known, a curved surface will bring added pressure. According to the Young–Laplace equation, the relationship between added pressure and the radius of curvature can be depicted as [20]: p = 2σ/r
(1)
Where σ is the surface energy and r is the radius of curvature of the surface. When the surface is concave, r is negative; when the surface is convex, r is positive. As shown in Fig. 3. (c), water molecules naturally form a convex surface on the surface of MoS2 layers braced by SiO2 microsphere. The pressure on a convex surface is higher than ambient pressure, so it is very easy for the water molecules to move away from the water surface. This feature can also accelerate the balance of dynamic adsorption and desorption of water molecules (equivalent to the response process of sensor). Due to the aforementioned reasons, the fabricated humidity sensor exhibited fast response speed. With an acceptable humidity sensitivity and subsecond response speed, more information on the performance of Sensor L1 was investigated. Fig. 4. (a) indicated that the impedance of the humidity sensor was simultaneously dependent on the frequency and RH. Fig. 4. (b) showed that the hysteresis of Sensor L1 was less than 5% RH, which corresponded to 43% RH. Fast response speed might played an important role in weakening the hysteresis. Benefit from
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Fig. 4. (a) Impedance vs. RH of Sensor L1 at various frequencies. (b) Hysteresis of Sensor L1. (c) Long-term stability of Sensor L1. (d) Reproducibility of Sensor L1.
Fig. 5. Water dripping rate monitoring with Sensor L1: (a) Experiment setup. (b) Dripping process. (c) Response of Sensor L1 towards dripping monitoring.
the intrinsically stable nature of MoS2 and SiO2 , Sensor L1 exhibited well long-term stability with 30 days experiment as shown in Fig. 4. (c). Moreover, Fig. 4. (d) showed that Sensor L1 has a relatively good reproducibility, which also profit from the rapid response. Further experiments were performed to verify the response speed of fabricated sensors. As shown in Fig. 5 (a), Sensor L1 was placed 5 mm under the dropper and 2 mm away from the centerline of the dropper. Fig. 5 (b) shows the dripping process, and the response was monitored while a drop of water passes by the sensor. The response corresponding to the drop cycles was shown in Fig. 5 (c), which indicated that the humidity sensor can sensitively catch the dripping process of water droplets. The current standard liquid transfusion control system requires optoelectronic device with light source and optical detector, which is a large and expensive device and difficult to integrate [21]. Owing to small, simple and low-cost structures, as well as the compatible integration methods with integrated circuit technology, this kind of humidity sensors is suitable to be used in miniature drip infusion rate detection device. IV. C ONCLUSION A novel humidity sensors based on MoS2 braced by a SiO2 microsphere layers was developed. Electrical characterization results showed that the sensors respond rapidly to humidity. We also demonstrated that this rapid humidity sensor can be used to monitor liquid dripping rate.
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R EFERENCES [1] S. Kozhukharov, Z. Nenova, T. Nenov, N. Nedevb, and M. Machkova, “Humidity sensing elements based on cerium doped titania-silica thin films prepared via a sol–gel method,” Sens. Actuators B, Chem., vol. 210, pp. 676–684, Apr. 2015, doi: 10.1016/j.snb.2014.12.119. [2] J. Shah, R. K. Kotnala, B. Singh, and H. Kishan, “Microstructuredependent humidity sensitivity of porous MgFe2 O4 –CeO2 ceramic,” Sens. Actuators B, Chem., vol. 128, no. 1, pp. 306–311, Dec. 2007, doi: 10.1016/j.snb.2007.06.021. [3] F.-W. Zeng, X.-X. Liu, D. Diamond, and K. T. Lau, “Humidity sensors based on polyaniline nanofibres,” Sens. Actuators B, Chem., vol. 143, pp. 530–534, Jan. 2010, doi: 10.1016/j.snb.2009.09.050. [4] Y. Li, M. Yang, N. Camaioni, and G. Casalbore-Miceli, “Humidity sensors based on polymer solid electrolytes: Investigation on the capacitive and resistive devices construction,” Sens. Actuators B, Chem., vol. 77, pp. 625–631, Jul. 2011, doi: 10.1016/S0925-4005(01)00768-7. [5] B. Chachulski, J. Gebicki, G. Jasinski, P. Jasinski, and A. Nowakowski, “Properties of a polyethyleneimine-based sensor for measuring medium and high relative humidity,” Meas. Sci. Technol., vol. 17, pp. 12–16, Nov. 2005, doi: 10.1063/1.4922049. [6] C. Laville and C. Pellet, “Comparison of three humidity sensors for a pulmonary function diagnosis microsystem,” IEEE Sensors J., vol. 2, no. 2, pp. 96–101, Apr. 2002, doi: 10.1109/JSEN.2002.1000249. [7] A. F. P. van Putten, M. J. A. M. van Putten, M. H. P. M. van Putten, and P. F. A. M. van Putten, “Multisensor microsystem for pulmonary function diagnostics,” IEEE Sensors J., vol. 2, no. 6, pp. 636–643, Dec. 2002, doi: 10.1109/JSEN.2002.807492. [8] T. Tatara and K. Tsuzaki, “An apnea monitor using a rapid-response hygrometer,” J. Clin. Monitor. Comput., vol. 13, no. 1, pp. 5–9, Jan. 1997, doi: 10.1023/A:1007380021895. [9] A. K. Kalkan, M. R. Henry, H. Li, J. D. Cuiffi, D. J. Hayes, C. Palmer, and S. J. Fonash, “Biomedical/analytical applications of deposited nanostructured Si films,” Nanotechnology, vol. 16, pp. 1383–1391, Jun. 2005, doi: 10.1088/0957-4484/16/8/068/meta. [10] P. L. Kebabian, C. E. Kolb, and A. Freedman, “Spectroscopic water vapor sensor for rapid response measurements of humidity in the troposphere,” J. Geophys. Res., Atmosp., vol. 107, pp. AAC2-1–AAC2-14, Dec. 2002, doi: 10.1029/2001JD002003. [11] J. J. Steele, M. T. Taschuk, and M. J. Brett, “Response time of nanostructured relative humidity sensors,” Sens. Actuators B, Chem., vol. 140, no. 2, pp. 610–615, Jul. 2009, doi: 10.1016/j.snb.2009.05.016. [12] B. Radisavljevic, A. Radenovic, J. Brivio, V. Giacometti, and A. Kis, “Single-layer MoS2 transistors,” Nature Nanotechnol., vol. 6, no. 3, pp. 147–150, Jan. 2011, doi: 10.1038/nnano.2010.279. [13] T. F. Jaramillo, K. P. Jørgensen, J. Bonde, J. H. Nielsen, S. Horchet, and I. Chorkendorff, “Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts,” Science, vol. 317, no. 5834, pp. 100–102, Jul. 2007, doi: 10.1126/science.1141483. [14] Y. Tan, K. Yu, T. Yang, Q. Zhang, W. Cong, H. Yin, Z. Zhang, Y. Chen, and Z. Zhu, “The combinations of hollow MoS2 micro nano-spheres: One-step synthesis, excellent photocatalytic and humidity sensing properties,” J. Mater. Chem. C., vol. 2, no. 27, pp. 5422–5430, Apr. 2014, doi: 10.1039/c4tc00423j. [15] S.-L. Zhang, H.-H. Choi, H.-Y. Yue, and W.-C. Yang, “Controlled exfoliation of molybdenum disulfide for developing thin film humidity sensor,” Current Appl. Phys., vol. 14, no. 3, pp. 264–268, Mar. 2014, doi: 10.1016/j.cap.2013.11.031. [16] D. Zhang, Y. Sun, P. Li, and Y. Zhang, “Facile fabrication of MoS2 -modified SnO2 hybrid nanocomposite for ultrasensitive humidity sensing,” ACS Appl. Mater. Interfaces, vol. 8, no. 22, pp. 14142–14149, May 2016, doi: 10.1021/acsami.6b02206.
[17] L. He, Z. Li, J. Fu, F. Wang, C. Ma, Y. Deng, Z. Shi, H. Wang, and N. He, “Preparation of SiO2 /(PMMA/Fe3 O4 ) nanoparticles using linolenic acid as crosslink agent for nucleic acid detection using chemiluminescent method,” J. Nanosci. Nanotechnol., vol. 11, no. 3, pp. 2256–2262, Mar. 2011, doi: 10.1166/jnn.2011.3149. [18] J. Shen, Y. Zhu, X. Yang, Z. Jie, and C. Li, “Multifunctional Fe3 O4 Ag/SiO2 /Au core–shell microspheres as a novel SERS-activity label via long-range plasmon coupling,” Langmuir, vol. 29, no. 2, pp. 690–695, Dec. 2012, doi: 10.1021/la304048v. [19] X.-C. Fu, X. Chen, J. Wang, J.-H. Liu, and X.-J. Huang, “Amino functionalized mesoporous silica microspheres with perpendicularly aligned mesopore channels for electrochemical detection of trace 2, 4, 6-trinitrotoluene,” Electrochim. Acta, vol. 56, no. 1, pp. 102–107, Dec. 2010, doi: 10.1016/j.electacta.2010.09.045. [20] X. Q. Fu, C. Wang, and H. C. Yu, “Fast humidity sensors based on CeO2 nanowires,” Nanotechnology, vol. 18, no. 14, Mar. 2007, Art. no. 145503, doi: stacks.iop.org/nano/18/145503. [21] N. Daimiwal, D. Ramdasi, R. Shriram, and A. Wakankar, “Wireless transfusion supervision and analysis using embedded system,” in Proc. Int. Conf. Bioinf. Biomed. Technol. (ICBBT), Jun. 2010, pp. 56–60, doi: 10.1109/ICBBT.2010.5479010. [22] H. Bi, K. Yin, X. Xie, J. Ji, S. Wan, L. Sun, M. Terrones, and M. S. Dresselhaus, “Ultrahigh humidity sensitivity of graphene oxide,” Sci. Rep., vol. 3, Sep. 2013, Art. no. 2714, doi: 10.1038/srep02714. [23] S. Borini, R. White, D. Wei, M. Astley, S. Haque, E. Spigone, N. Harris, J. Kivioja, and T. Ryhanen, “Ultrafast graphene oxide humidity sensors,” ACS Nano, vol. 7, no. 12, pp. 11166–11173, Nov. 2013, doi: 10.1021/nn404889b. [24] D. Zhang, J. Tong, B. Xia, and Q. Xue, “Ultrahigh performance humidity sensor based on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite film,” Sens. Actuators B, Chem., vol. 203, pp. 263–270, Nov. 2014, doi: 10.1016/j.snb.2014.06.116. [25] D. Zhang, J. Liu, and B. Xia, “Layer-by-layer self-assembly of zinc oxide/graphene oxide hybrid toward ultrasensitive humidity sensing,” IEEE Electron Device Lett., vol. 37, no. 7, pp. 916–919, Jul. 2016, doi: 10.1109/LED.2016.2565728. [26] D. Zhang, J. Tong, and B. Xia, “Humidity-sensing properties of chemically reduced graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly,” Sens. Actuators B, Chem., vol. 197, pp. 66–72, Jul. 2014, doi: 10.1016/j.snb.2014.02.078. [27] D. Zhang, H. Chang, P. Li, R. Liu, and Q. Xue, “Fabrication and characterization of an ultrasensitive humidity sensor based on metal oxide/graphene hybrid nanocomposite,” Sens. Actuators B, Chem., vol. 255, no. 1, pp. 233–240, Mar. 2016, doi: 10.1016/j.snb.2015.11.024. [28] J. Xie, H. Zhang, S. Li, R. Wang, X. Sun, M. Zhou, J. Zhou, X. W. D. Lou, and Y. Xie, “Defect-rich MoS2 ultrathin nanosheets with additional active edge sites for enhanced electrocatalytic hydrogen evolution,” Adv. Mater., vol. 25, no. 40, pp. 5807–5813, Aug. 2013, doi: 10.1002/adma.201302685. [29] J. Kibsgaard, Z. Chen, B. N. Reinecke, and T. F. Jaramillo, “Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis,” Nature Mater., vol. 11, no. 11, pp. 963–969, Nov. 2012, doi: 10.1038/NMAT3439. [30] C. Tsai, F. Abild-Pedersen, and J. K. Nørskov, “Tuning the MoS2 edgesite activity for hydrogen evolution via support interactions,” Nano Lett., vol. 14, no. 3, pp. 1381–1387, Feb. 2014, doi: 10.1021/nl404444k.
