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titanium dioxide nanohybrid film
Accepted Manuscript Title: Ultrahigh-Performance Impedance Humidity Sensor Based on Layer-by-layer Self-Assembled Tin Disulfide/Titanium Dioxide Nanohybrid Film Authors: Dongzhi Zhang, Xiaoqi Zong, Zhenling Wu, Yong Zhang PII: DOI: Reference:
S0925-4005(18)30485-4 https://doi.org/10.1016/j.snb.2018.03.007 SNB 24292
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
22-11-2017 9-2-2018 1-3-2018
Please cite this article as: Dongzhi Zhang, Xiaoqi Zong, Zhenling Wu, Yong Zhang, Ultrahigh-Performance Impedance Humidity Sensor Based on Layer-by-layer SelfAssembled Tin Disulfide/Titanium Dioxide Nanohybrid Film, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.03.007 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.
Ultrahigh-Performance Impedance Humidity Sensor Based on Layer-by-layer Self-Assembled Tin Disulfide/Titanium Dioxide Nanohybrid Film
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Dongzhi Zhang*, Xiaoqi Zong, Zhenling Wu, Yong Zhang
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College of Information and Control Engineering, China University of Petroleum (East
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China), Qingdao 266580, China
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*Corresponding authors: Dongzhi Zhang
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E-mail address: [email protected]
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Tel: +86-532-86982928
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Fax: +86-532-86983326
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Highlights SnS2/TiO2
film–based
impedance
humidity
sensor
was
fabricated
via
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layer-by-layer self-assembly.
Ultrahigh sensitivity and response of SnS2/TiO2 film sensor toward humidity was
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demonstrated.
The sensing mechanism for the SnS2/TiO2 film sensor toward humidity was discussed.
Abstract An ultrahigh-performance impedance-type humidity sensor based on tin disulfide Page 1 of 42
(SnS2)/titanium dioxide (TiO2) nanocomposite was demonstrated via layer-by-layer self-assembly technique in this work. The nanostructures, morphologies, composition properties of the SnS2/TiO2 nanocomposite were fully examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS),
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transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The humidity sensing characteristics of the SnS2/TiO2 film sensor were
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investigated at room temperature. The results show that the SnS2/TiO2 film sensor has an impedance response up to 200050 and a sensitivity of 442000 Ω/%RH, which is
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much better than that of the existing humidity sensors. Excellent stability and
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repeatability is also exhibited for the SnS2/TiO2 film sensor. Moreover, the underlying
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sensing mechanism for the SnS2/TiO2 sensor toward humidity was explored with the
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complex impedance spectroscopy and the Bode diagram. The SnS2/TiO2
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nanocomposite provides a promising building block for ultrahigh-sensitive humidity
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sensing and detection of human respiratory.
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Key words: Tin disulfide, layer-by-layer assembly, complex impedance spectroscopy, humidity sensing
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1. Introduction
In recent years, humidity sensors have been used in many applications such as commercial and defense aircrafts, textile industry, environmental monitoring, industrial and biomedical processing [1-3]. Until now, various transduction techniques
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such as capacitance [3], resistance [4], optical fiber [5], field effect transistor (FET) [6], surface acoustic wave (SAW) [7], and quartz crystal microbalance (QCM) [8] have been widely used to fabricate humidity sensors. So far, the humidity sensors are mainly based on ceramics, nanomaterials, organic polymers and organic-inorganic
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composites [9]. A large number of nanomaterials, such as metal oxides, carbon nanotubes and graphene, have been extensively used to fabricate humidity sensing
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devices [10-12]. As is well known, graphene has been widely used by many
researchers in various applications such as photocatalysis [13], electronics [14],
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supercapacitors [15], and sensors [16]. However, its semi-metallic properties have
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hindered its use in nanoelectronics and optoelectronic devices applications [3]. More
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recently, graphene-like 2D nanomaterials such as MoS2 [4, 17, 18], WS2 [19, 20],
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MoSe2 [21], WSe2 [22], SnSe2 [23], MoTe2 [24], WTe2 [25], black phosphorus [26]
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and TiS3 [27] have been extensively investigated in many applications such as photodetector, sensors, transistors and catalysis.
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In the family of transition-metal dichalcogenides (TMDCs), tin disulfide (SnS2)
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has attracted more and more attention as an emerging and promising nanomaterial in recent years [28]. SnS2 is a naturally rich material that exists in CdI2 type structure consisting of a Sn atom layer sandwiched between two layers of hexagonal enclosed S
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atoms. The adjacent layers of sulfur atoms are connected by weak van der Waals
force. Currently, the SnS2 is widely studied due to its superb properties, non-toxicity and high stability, and various applications such as lithium ion batteries [29], glucose sensing [30], gas sensing [31-35], have been reported. Xu et al. fabricated a
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SnO2-SnS2 composite for ammonia gas detection by the oxidation of SnS2 at 300°C and exhibited high response to NH3 at room temperature [33]. Ou et al. demonstrated a NO2 gas sensor based on SnS2 flakes and showed high sensitivity, superior selectivity and excellent reversibility for NO2 sensing at low operating temperatures
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[34]. Bharatula et al. fabricated layered SnS2 nanoflakes by hydrothermal synthesis method, and demonstrated high sensitivity towards humidity and selectivity towards
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alcohol gas at room temperature [35].
Humidity sensors based on metal oxides obtain special interest due to their
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nano-sized grains, nanoporous structures, and high surface contact of water molecules
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[3]. Moreover, doped metal oxides exhibit different types of morphologies, various
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properties and have various applications toward humidity sensing. Misra and Pandey
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et al. investigated humidity sensing properties of Ag-loaded WO3, SnO2-doped ZnO,
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Al2O3-doped ZnO, Cu-doped ZnO nanomaterials prepared by solid-state reaction method [36-39]. It was noticed that the doped metal oxides annealed at high
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temperature showed higher sensitivity, lower hysteresis, better reproducibility and
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faster response and recovery time compared to the sensing element of undoped metal oxide. Titanium dioxide (TiO2) has been attracted great attention recently as a promising material for fabricating sensors for the detection of CO [40], H2S [41],
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alcohol [42], NOx [43], and humidity [44]. However, there still remain some problems including long recovery time, high operating temperature (∼300°C) and low sensitivity at low temperature. Currently, some approaches such as inkjet-printing [45], dip coating [3], spin coating [46], self-assembly [11] and solid-state reaction methods
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[36-39] are widely used to fabricate humidity sensors. Among them, the layer-by-layer (LbL) self-assembly technique is based on sequential electrostatically adsorptions of ionized polyelectrolytes and oppositely charged materials in aqueous solutions. The LbL self-assembly technique has many advantages over other
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alternative methods, such as simplicity, substrate-independence, low-cost, low temperature deposition, controllable thickness from nanometers to micrometers, and
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no complex or costly equipment required [47]. It is a bottom-up approach to fabricate uniform nanocomposite film as humidity sensing material. Su et al. fabricated an
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impedance-type humidity sensor via layer-by-layer self-assembly of
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poly(2-acrylamido-2-methylpropane sulfonate) (PAMPS) and its salt complex on a
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flexible polyethylene terephthalate (PET) substrate, and enhanced sensitivity, good
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linearity, small hysteresis, short response and recovery times and long-term
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stability are demonstrated [48]. In our previous work, we presented a high-performance humidity sensor based on layer-by-layer self-assembly of
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chemically reduced graphene oxide/polymer nanocomposite film on flexible
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polyimide (PI) substrate [11]. In this work, we demonstrated an original and facile fabrication of tin
disulfide/titanium dioxide nanocomposite (SnS2/TiO2) nanocomposite on flexible
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substrate towards humidity sensing via layer-by-layer self-assembly technique. The composition and nanostructure of the as prepared SnS2/TiO2 nanocomposite were characterized by XRD, SEM, XPS and TEM. The humidity-sensing properties of the sensor were investigated in the range of 11%RH to 97%RH. The SnS2/TiO2 sensor
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exhibited much higher response than the existing humidity sensors. Fast response/recovery time, excellent repeatability and good reversibility towards humidity are demonstrated. At last, the possible sensing mechanism of SnS2/TiO2 film sensor toward humidity was revealed using the complex impedance spectroscopy and
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the Bode diagram. The SnS2/TiO2 film sensor has many technological advancements in terms of flexibility, small hysteresis, long-term stability, ease of integration, and
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respiratory monitoring capability, indicating great potential applications in various fields.
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2. Experimental
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2.1 Materials
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Thiourea (NH2CSNH2, ≥99%), titanium sulfate (Ti(SO4)2, ≥96%) and urea
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(CO(NH2)2, ≥99%) were obtained from Sinopharm Chemical Reagent Co. Ltd
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(Shanghai, China). Tin chloride pentahydrate (SnCl4·5H2O, ≥99%) was offered by Shanghai Hansi Chemical Industry Co. Ltd (Shanghai, China). Polyelectrolytes
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including 1.5 wt% poly(diallyldimethylammonium chloride) (PDDA) and 0.3 wt%
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poly(sodium 4-styrenesulfonate) (PSS) were obtained from Sigma-Aldrich Inc. In this work, SnS2 nanosheets and TiO2 nanospheres were synthesized via a
facile hydrothermal method [49]. Precursors, thiourea (1.218 g) and SnCl4·5H2O
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(1.402 g) were dissolved into 50 mL deionized (DI) water in a beaker. After magnetic stirring for 0.5 h, the resulting solution was transferred into a 70 mL Teflon-lined autoclave and hydrothermally treated at 180ºC for 24 h. After cooling naturally to room temperature, the SnS2 nanosheets were collected by centrifuging and washing
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with ethanol and DI water several times. The synthesis of TiO2 nanosphere was similar to that of SnS2 nanosheet. 6 g of urea and 12 g of Ti(SO4)2 were dissolved into 100 mL of DI water. Subsequently, magnetic stirred for 1.5 h and ultrasonication treated for 0.5 h to form a homogeneous
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suspension. At last, the suspension was transferred into a Teflon-lined autoclave and heated at 180°C for 3 h. Finally, the TiO2 solution was obtained after centrifuging and
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washing several times [50]. 2.2 Sensor fabrication
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The sensor device was fabricated on a polyimide substrate with a pair of
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interdigitated electrodes (IDEs), which was described in our previous work [51]. The
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sensing film of the sensor was fabricated by LbL self-assembly method, as shown in
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Figure 1(a). Two bilayers of PDDA/PSS were sequentially deposited on the substrate
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as precursor layer, followed by alternative immersing the device into TiO2 and SnS2 solutions for five cycles. The immersing time used was 10 min for PDDA and PSS
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mono-layer, and 15 min for SnS2 and TiO2 monolayer. Intermediate rinsing with DI
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water and drying with nitrogen stream were required after each monolayer assembly to reinforce the interconnection between layers. Finally, the device was dried in an oven at 50oC for 8 h. The optical image of the sensor prototype on a flexible PI
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substrate was shown in Figure 1(b). For making a comparison, the pure TiO2 and pure SnS2 film sensors were prepared by drop-casting method. 2.3 Experimental setup The humidity sensing measurement was performed at an ambient temperature of
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20°C. Various RH levels were yielded via several saturated salt solutions, as reported in our previous work [52]. The schematic of humidity-sensing experimental setup was illustrated in Figure 1(c). Saturated solutions of LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl and K2SO4 in a closed vessel were used to yield
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approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97%RH levels, respectively. The sensor response as a function of RH was achieved by exposing the
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sensor inside the closed vessels with different RH levels. A computer-coupled
TH2828 precision LCR meter was applied to record the impedance and complex
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impedance spectroscopy of the SnS2/TiO2 film sensor during humidity sensing. The
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time taken to reach 90% of the initial total capacitance variation is defined as
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response/recovery time during the humidification and desiccation processes [37]. The
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normalized response R and sensitivity S are used as figures of merit to evaluate the
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performance of the SnS2/TiO2 film sensor, which are defined by R=Z11/Zx and S=ΔZ/ΔRH, respectively, where Z11 and Zx are the impedance of the sensor at 11%RH
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and x%RH, respectively, ΔZ is the change in impedance, and ΔRH is the RH change.
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The hysteresis H is used to evaluate the drawback of the SnS2/TiO2 film sensor, which are defined by H=(ZD-ZI)/S (%RH), where ZD and ZI are the impedance of the sensor at same RH in the decreasing humidity and increasing humidity process, respectively.
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3. Results and discussion 3.1 Characterization results The surface morphologies of the SnS2, TiO2, and SnS2/TiO2 nanocomposite were examined using a field emission scanning electron microscope (FESEM, Hitachi
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S-4800, Japan). Figure 2(a) and 2(b) indicate that SnS2 has a regular hexagonal nanosheet shape. Figure 2(c) shows the hydrothermally synthesized TiO2 possesses a nanosphere shape. Figure 2(d) illustrates SnS2 nanosheet and TiO2 nanosphere have a good contact in the LbL self-assembled SnS2/TiO2 nanocomposite.
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The nanostructure and crystallinity of the as-prepared samples were observed via a transmission electron microscope (TEM, JEOL JEM2100). Figure 3(a) and 3(b)
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show that SnS2/TiO2 nanocomposite consists of TiO2 nanospheres decorated on the
SnS2 hexagonal nanosheets with good distribution. The side length about 350 nm for
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hexagonal SnS2 nanosheet can be clearly observed. Figure 3(c) and 3(d) show the
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HRTEM lattice fringe images of SnS2/TiO2 nanocomposite. As observed, an
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interplanar distance of 0.352 nm is attributed to the (101) plane for the TiO2. The
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lattice fringes spacing of 0.316 nm and 0.27 nm for SnS2 nanocrystal, which is
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attributed to the (100) and (011) planes of the SnS2 nanosheets, respectively. The XRD analysis of SnS2, TiO2 and SnS2/TiO2 samples was performed by an
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X-ray diffractometer (Rigaku D/Max 2500PC) using Cu Kα radiation (λ=1.5418Å).
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The diffraction peaks were scanned in the range of 20°-80°, and the XRD results are illustrated in Figure 4. The XRD spectrum of the hexagonal SnS2 nanosheet exhibits a crystalline phase without impurity peaks. The main diffraction peaks of SnS2
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nanosheets at 15.04, 28.27, 32.17, 41.95, 50.05, 52.45, and 60.62o are ascribed to the (001), (100), (101), (102), (110), (111), and (201) planes, respectively, which are in accordance with the hexagonal 2H SnS2 structure (ICDD 23-0677) [34]. The diffraction peaks of TiO2 sample are observed at 2θ of 25.3°, 37.9°, 48.0°, 53.99°,
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54.98° and 62.2°, which correspond to the (101), (004), (200), (105), (211) and (204) planes of the anatase TiO2, respectively (JCPDS Card no. 21-1272) [45]. The result indicates the successfully synthesis of TiO2 nanocrystals. The XRD pattern of SnS2/TiO2 nanocomposite demonstrates the main characteristic peaks of the both SnS2
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and TiO2, confirming the existence of SnS2 and TiO2. The elements composition of the SnS2/TiO2 nanocomposite was identified by
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Hitachi S-4800 equipped with an energy dispersive spectroscopy (EDS). As shown in Figure 5(a), only Sn, S, Ti and O elements in the SnS2/TiO2 nanocomposite are
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detected, and no other impurity element has been observed. Furthermore, the
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elemental chemical states of the SnS2/TiO2 nanocomposite was also inspected by
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X-ray photoelectron spectroscopy (XPS, Thermo Scientific instrument), and the XPS
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spectra for survey of SnS2/TiO2 nanocomposite is shown in Figure 5(b). The Sn-to-S
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ratio of 1:2.235 was obtained through the XPS measurement, which is slightly substoichiometric, indicating sulfur vacancies in the SnS2 nanosheets. No other
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impurity elements were observed, which is consistent with the XPS analysis. The
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spectra of S 2p shown in Figure 5(c) exhibits two peaks at 161.1 eV and 162.4 eV, corresponding to S 2p3/2 and S 2p1/2, respectively. There are no extra peaks are found in the region between 168 eV and 170 eV, indicating that the sulfur atoms in SnS2
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remain unoxidized without no oxide phase formed [34]. Figure 5(d) shows that Sn atom has two binding energy peaks at 486.1 eV (3d5/2) and 494.4 eV (3d3/2), which confirm the presence of Sn4+. No extra peaks are observed at ~485 eV and ~493 eV, indicating that there is no Sn2+ in the sample [34]. The O 1s peaks shown in Figure
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5(e) reveal three peaks at 529.73, 532.03, and 533.53 eV, attributed to the lattice oxygen ions of the anatase TiO2, surface hydroxyl groups and adsorbed water molecules, respectively. The XPS spectra of Ti 2p in Figure 5(f) illustrates doublet peaks at 458.6 eV and 464.3 eV with a spin-orbit splitting of 5.7 eV, which was
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ascribed to the Ti 2p3/2 and Ti 2p1/2 spin-orbit components of Ti4+-O surface species unique to TiO2. These binding energies peaks are in consistent with the values
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reported for pure TiO2 and prove the exclusive presence of Ti4+ [53, 54]. 3.2 Humidity-sensing performances
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The measured impedance of the SnS2/TiO2 film sensor upon exposure to various
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RH levels was shown in Figure 6(a). The applied operation frequencies were chosen
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at 100 Hz, 1k Hz and 10k Hz, respectively. We can observe that the impedance
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decreases with the increasing RH at fixed operation frequency. This is due to the
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adsorption of water molecules promoted the ionic conductivity and enhanced the dielectric constant of the sensing film. Among the three different operation
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frequencies, the sensor shows the largest impedance variation upon exposure to
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11%-97%RH at 100 Hz. Figure 6(b) shows the sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100, 1000 and 10000 Hz, and the inset indicates the magnified curve for 10k Hz. In the measuring range of 11-97%RH, the sensor yields
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the sensitivity of 442000 Ω/%RH, 65461 Ω/%RH and 7111 Ω/%RH at 100 Hz, 1k Hz and 10k Hz, respectively. Obviously, the sensitivity at 100 Hz is higher than that at the two other frequencies. The sensor sensitivity decreases with the increasing of operating frequency, and becomes flatly with the increasing RH. This is due to the
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polarization of the adsorbed water molecules is hard to catch up the electrical field direction changes at higher frequencies [10]. Therefore, 100 Hz is selected as the operating frequencies in the subsequent experiments. In order to highlight the remarkable properties of SnS2/TiO2 film sensor, the
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comparison with pure SnS2 and TiO2 film sensor is further investigated. Figure 6(c) shows the response of the three sensors versus different RH at 100 Hz. The responses
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for the SnS2/TiO2 film, pure SnS2 and TiO2 film sensors measured at 97%RH are
200050, 55882 and 737, respectively, indicating the SnS2/TiO2 film sensor exhibits
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the most striking impedance-RH characteristics. Table 1 shows the humidity sensing
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performance of the SnS2/TiO2 film sensor compared with the previous works [45, 46,
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48, 55-61]. The comparison is made with the existing humidity sensors made from
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metal oxide, conductive polymer, and 2D materials (i.e., graphene, MoS2, black
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phosphorus) via various methods. We can observe from the comparison in Table 1, our SnS2/TiO2 film sensor has unprecedented sensitivity, which is 3.5 times higher
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that of the best one among conventional sensors ever reported.
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Figure 7(a) shows the time-dependent impedance measurement of the SnS2/TiO2 film sensor toward various RH levels at 100 Hz. The sensor was switched between the tested RH and 11%RH for humidity sensing and recovery, respectively. The time
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interval for response/recovery duration was 150 s. The sensor impedance decreases about 200000-fold of magnitude from 38.01 MΩ to 190 Ω toward RH varies from 11% to 97%RH. Figure 7(b) shows the impedance value of the SnS2/TiO2 film sensor as a function of RH in the range of 11%-97%. The logarithm of sensor impedance
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(lgY) presents a good linearity with relative humidity X, which can be fitted as lgY=8.60-0.069X. Figure 7(c) shows the typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. Response time and recovery time of less than 58 s are observed, respectively, which is much better
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than that of conventional humidity sensors [39, 62-64]. Figure 7(d) illustrates the repeatability performance of the SnS2/TiO2 film sensor exposed to 52%, 67%, and
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75%RH from 11%RH over five cycles. A good repeatability can be observed and the stable-state impedance were 117769 Ω (52%RH), 6848 Ω (67%RH), and 909 Ω
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(75%RH), respectively.
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Figure 8(a) shows the impedance of the SnS2/TiO2 film sensor for
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humidity-increasing and humidity-decreasing measurement. The switching RH test is
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performed by exposing the sensor to 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%,
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97%RH, and then conversely from high RH to low RH. The hysteresis of the SnS2/TiO2 film sensor is shown in Figure 8(b). The maximum hysteresis is 1.13%RH,
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indicating a highly reversible property [38, 65]. Figure 8(c) shows the long-term
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stability of the SnS2/TiO2 film sensor tested at 33%, 62% and 85%RH, respectively. The impedance of the sensor did not change significantly over three months, indicating a good stability. Figure 8(d) demonstrated the sensor in the application of
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human respiration monitoring. The impedance response of the sensor was recorded during the breathing for an adult, and 12 repetitive cycles in 50 s were clearly observed. The impedance response of the sensor exhibited sharp rise during exhaling and dropped while inhaling corresponding to the breathing cycles, indicating the
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sensor is capable of capturing the humidity change during human breath. 3.3 Humidity-sensing mechanism The SnS2/TiO2 nanocomposite humidity sensor exhibited excellent sensing properties in terms of high response, low hysteresis, fast response and recovery times,
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and good reproducibility. The nanostructure of SnS2/TiO2 nanohybrid contributes to the enhanced humidity sensing properties. As well is known, TiO2 nanospheres have
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nanosize grains, high physicochemical properties, broad bandgap of about 3.2 eV, and large excition binding energy [65, 66]. The incorporation of TiO2 into SnS2
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nanosheets brings more active sites (i.e., oxygen vacancies, defects) and nanopores,
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which can provide high surface exposure for adsorption of water molecules. SnS2
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nanosheets possess natural band gap, low resistivity and high carrier mobility, serving
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as direct conduction paths for the electrons transfer in the TiO2 nanospheres, which
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play a dominant role in improving the electrical properties of TiO2. The synergistic effect of the binary nanomaterials endows the SnS2/TiO2 nanocomposite sensor has
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better properties compared to the pure SnS2 and TiO2 sensor. Moreover, SnS2
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nanosheets have much hydrophilic functional groups attached on its surface capable of capturing much more water molecules [67]. This may be attributed to the fact that
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the pure SnS2 sensor exhibits higher humidity sensing ability than that of pure TiO2. The humidity sensing mechanism of the sensor is ascribed to the mutual
interdependence between the SnS2/TiO2 sensing film and water adsorption/desorption. The adsorption of water molecules on the SnS2/TiO2 nanocomposite is shown in Figure 9. At low humidity, only a few water molecules are chemically adsorbed on the
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surface of SnS2 and TiO2, and the coverage of water on the film surface is discontinuous. As the humidity increases, much more water molecules are physically adsorbed to the SnS2 nanosheets and TiO2 nanospheres through the hydrogen bonding. At high humidity, the adsorbed water molecules form a serial water layer on the film
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surface, in which the hydronium (H3O+) is generated and the proton hopping transport occurs according to the Grotthuss chain reaction [68].
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In order to further discover the potential sensing mechanism of the SnS2/TiO2
film, the complex impedance spectroscopy (CIS) and bode diagram were investigated.
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Figure 10 shows the CIS measurements and equivalent circuit models for the
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SnS2/TiO2 sensor at different RH levels. The real and imaginary parts of CIS curves
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are magnified synchronously to compile them in one Nyquist plot. The operation
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frequency was scanned from 100 Hz to 1M Hz. At low humidity level (11%RH), the
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CIS curve behaves like a straight line, which can be modeled as a constant phase element (CPE), as shown in Figure 10(b). The CPE ZCPE is defined as ZCPE =1/A(j2πf)n,
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in which A is a real parameter and 0≤n≤1. The corresponding Bode diagram is shown
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in Figure 11(a). The logarithmic of impedance magnitude has a linear relationship with the logarithmic frequency, and the impedance angle is near to -90o. At 23%RH level, the CIS curve starts to bend downwardly. This indicates that
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the film resistance decreases and the capacitance increases due to the adsorption of water molecules. The Bode diagram shown in Figure 11(b) confirms its impedance magnitude and the phase angle are decreased with the variation of frequency. The CIS curve appears semicircular-shape when the humidity reaches 33%RH, 43%RH and
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52%RH. The corresponding equivalent circuit for the sensor can be described as a CPE and a resistor connected in parallel (Figure 10(c)), indicating a relaxation effect owing to the polarization from intrinsic impedance of SnS2/TiO2 nanocomposite. Figure 11(c) shows the Bode diagram for the sensor at 52%RH, which is a typical
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behavior for a RC parallel circuit. The phase angle increases and spans a wide range of 0-90o with increasing frequency.
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At high RH (67%, 75%, 85%, 97%RH), the CIS curve shows a line at the tail of
semicircle at low frequency region, semicircle gradually disappears at high frequency
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region. This is ascribed to the Warburg impedance (Zw) caused by the diffusion
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process of ions or charge carriers at the sensing film/electrode interfaces, and the main
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charge carrier is the hydronium (H3O+) [69, 70]. The equivalent circuit for the sensor
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at high RH is described as shown in Figure 10(d). Figure 11(d) shows the Bode
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diagram for the sensor at 97%RH. The impedance magnitude becomes much smaller, and the phase angle is near to 0 at low frequency region. This is due to the serial water
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layer results in the proton transfer, which makes a great contribution to the sharp
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decrease of film impedance. 4. Conclusion
In summary, a novel humidity sensor based on SnS2/TiO2 nanocomposites was
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successfully fabricated via layer-by-layer self-assembly method. The nanostructures, morphology and composition of SnS2/TiO2 samples were characterized by XRD, SEM, TEM and EDS. The humidity sensing characteristics of SnS2/TiO2 sensors were measured at room temperature over a wide RH range. The result shows that the
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impedance response of SnS2/TiO2 nanocomposite film sensor is much higher than capacitance and resistance response of SnS2/TiO2 nanocomposites. Comparing with pure SnS2 and pure TiO2, the SnS2/TiO2 nanocomposite film sensor possesses not only high response, but also excellent repeatability. Besides, it has great potential for
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application in the detection of human respiratory. The humidity sensing mechanism was investigated with the complex impedance spectroscopy and the Bode diagram.
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This work reveals the possibility of using SnS2/TiO2 nanocomposite for constructing low-cost humidity sensor.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China
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(51777215), the Fundamental Research Funds for the Central Universities of China
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(18CX07010A), the Science and Technology Development Plan Project of Qingdao
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(16-6-2-53-nsh), and the Open Fund of National Engineering Laboratory for Mobile
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Source Emission Control Technology (NELMS2017B03).
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References [1] C. Giaconia, A. Orioli, A.D. Gangi, Air quality and relative humidity in commercial aircrafts: an experimental investigation on short-haul domestic flights, Build. Environ. 67 (2013) 69-81.
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[2] G.H. Zhou, J.H. Byun, Y. Oh, B.M. Jung, H.J. Cha, D.G. Seong, M.K. Um, S. Hyun, T.W. Chou, Highly sensitive wearable textile-based humidity sensor made
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of high-strength, single-walled carbon nanotube/poly(vinyl alcohol) filaments, ACS Appl. Mater. Inter. 9 (2017) 4788-4797.
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[3] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2
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Interfaces 8 (2016) 14142-14149.
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hybrid nanocomposite for ultrasensitive humidity sensing, ACS Appl. Mater.
M
[4] M. Bhattacharjee, H.B. Nemade, D. Bandyopadhyay, Nano-enabled paper
ED
humidity sensor for mobile based point-of-care lung function monitoring, Biosen. Bioelectron. 94 (2017) 544-551.
PT
[5] L. Xia, L.C. Li, W. Li, T. Kou, D.M. Liu, Novel optical fiber humidity sensor
CC E
based on a no-core fiber structure, Sens. Actuators A 190 (2013) 1-5. [6] C. Sun, K.R.G. Karthik, S.S. Pramana, L.H. Wong, J. Zhang, Y.Z. Huang, C.H.
A
Sow, N. Mathews, S.G. Mhaisalkar, The role of tin oxide surface defects in determining nanonet FET response to humidity and photoexcitation, J. Mater. Chem. C 2 (2014) 940-945.
[7] Y.L. Tang, Z.J. Li, J.Y. Ma, L. Wang, J. Yang, B. Du, Q.K. Yu, X.T. Zu, Highly sensitive surface acoustic wave (SAW) humidity sensors based on sol-gel SiO2
Page 18 of 42
films: investigations on the sensing property and mechanism, Sens. Actuators B 215 (2015) 283-291. [8] Y. Yao, H. Zhang, J. Sun, W. Y. Ma, L. Li, W. Z. Li, J. Du, Novel QCM humidity sensors using stacked black phosphorus nanosheets as sensing film, Sens.
IP T
Actuators B 244 (2017) 259-264. [9] J. Dai, T. Zhang, H. Zhao, T. Fei, Preparation of organic-inorganic hybrid
SC R
polymers and their humidity sensing properties, Sens. Actuators B 242 (2017) 1108-1114.
U
[10] D. Zhang, J. Tong, B. Xia, Q. Xue, Ultrahigh performance humidity sensor based
N
on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite
A
film, Sens. Actuators B 203 (2014) 263-270.
M
[11] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced
ED
graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly, Sens. Actuators B 197 (2014) 66-72.
PT
[12] Y. Li, T.T. Wu, M.J. Yang, Humidity sensors based on the composite of
CC E
multi-walled carbon nanotubes and cross linked polyelectrolyte with good sensitivity and capability of detecting low humidity, Sens. Actuators B 203 (2014) 63-70.
A
[13] J. Di, J.X. Xia, H.M. Li, Z. Liu, Freestanding atomically-thin two-dimensional materials beyond graphene meeting photocatalysis: opportunities and challenges, Nano Energy 35 (2017) 79-91. [14] M. Shekhirev, H. Vo, M.M. Pour, A. Sinitskii, Interfacial self-assembly of
Page 19 of 42
atomically precise graphene nanoribbons into uniform thin films for electronics applications, ACS Appl. Mater. Interfaces 9 (2017) 693-700. [15] M.S. Lee, H.J. Choi, J.B. Baek, D.W. Chang, Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors, Appl.
IP T
Energy 195 (2017) 1071-1078. [16] T.R. Zhan, Z.W. Tan, X. Tian, W.G. Hou, Ionic liquid functionalized graphene
SC R
oxide-Au nanoparticles assembly for fabrication of electrochemical 2, 4-dichlorophenol sensor, Sens. Actuators B 246 (2017) 638-646.
U
[17] D.J. Late, Y.K. Huang, B. Liu, J. Acharya, S.N. Shirodkar, J. Luo, A. Yan, D.
N
Charles, U.V. Waghmare, V.P. Dravid, C.N.R. Rao, Sensing behavior of atomically
A
thin-layered MoS2 transistors, ACS Nano 7 (2013) 4879-4891.
M
[18] R.S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A.C. Ferrari, P. Avouris, M.
1416-1421.
ED
Steiner, Electroluminescence in single layer MoS2, Nano Lett 13(2013)
PT
[19] D. Ovchinnikov, A. Allain, Y.S. Huang, D. Dumcenco, A. Kis, Electrical
CC E
transport properties of single-layer WS2, ACS Nano 8 (2014) 8174-8181. [20] A.S. Pawbake, R. Waykar, D.J. Late, S.R. Jadkar, Highly transparent wafer-scale
A
synthesis of crystalline WS2 nanoparticle thin film for photodetector and humidity-sensing applications, ACS Appl. Mater. Interfaces 8 (2016) 3359-3365.
[21] Y. Chang, W. Zhang, Y. Zhu, Y. Han, J. Pu, J. Chang, W. Hsu, J. Huang, C. Hsu, M. Chiu, T. Takenobu, H. Li, C. Wu, W. Chang, A.T.S. Wee, L. Li, Monolayer MoSe2 grown by chemical vapor deposition for fast photodetection, ACS Nano 8
Page 20 of 42
(2014) 8582-8590. [22] J.K. Huang, J. Pu, C.L. Hsu, M.H. Chiu, Z.Y. Juang, Y.H. Chang, W.H. Chang, Y. Iwasa, T. Takenobu, L.J. Li, Large-area synthesis of highly crystalline WSe2 monolayers and device applications, ACS Nano 8 (2013) 923-930.
IP T
[23] X. Zhou, L. Gan, W. Tian, Q. Zhang, S. Jin, H. Li, Y. Bando, D. Golberg, T. Zhai,
photodetectors, Adv. Mater. 27 (2015) 8035-8041.
SC R
Ultrathin SnSe2 flakes grown by chemical vapor deposition for high-performance
[24] L. Zhou, K. Xu, A. Zubair, A.D. Liao, W. Fang, F. Ouyang Y. Lee, K. Ueno, R.
U
Saito, T. Palacios, J. Kong, M.S. Dresselhaus, Large-area synthesis of high-quality
N
uniform few-layer MoTe2, J. Am. Chem. Soc. 137 (2015) 11892-11895.
A
[25] M.N. Ali, J. Xiong, S. Flynn, J. Tao, Q.D. Gibson, L.M. Schoop, T. Liang, N.
M
Haldolaarachchige, M. Hirschberger, N.P. Ong, R.J. Cava, Large, non-saturating
ED
magnetoresistance in WTe2, Nature 514 (2014) 205-208. [26] L. Li, Y. Yu, G.J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X.H. Chen, Y. Zhang, Black
PT
phosphorus field-effect transistors, Nat. Nanotechnol. 9 (2014) 372-377.
CC E
[27] J.O. Island, M. Buscema, M. Barawi, J.M. Clamagirand, J.R. Ares, C. S´anchez, I.J. Ferrer, G.A. Steele, H.S.J. Zant, A. Castellanos-Gomez, Ultrahigh
A
photoresponse of few-Layer TiS3 nanoribbon transistors, Adv. Opt. Mater. 2 (2014) 641-645.
[28] Y. Huang, E. Sutter, J.T. Sadowski, M. Cotlet, O.L. Monti, D.A. Racke, M.R. Neupane, D. Wickramaratne, R.K. Lake, B.A. Parkinson, Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and
Page 21 of 42
device characteristics, ACS Nano 8 (2014) 10743-10755. [29] X.Y. Li, M.C. Si, F.H. Jian, G.K. Xin, M.P. Li, Preparation of hierarchical SnS2/SnO2 anode with enhanced electrochemical performances for lithium-ion battery, Electrochim. Acta 238 (2017) 168-177.
IP T
[30] K.T. Lee, Y.C. Liang, H.H. Lin, C.H. Li, S.Y. Lu, Exfoliated SnS2 nanoplates for enhancing direct electrochemical glucose sensing, Electrochim. Acta 219 (2016)
SC R
241-250.
[31] R. Li, K. Jiang, S. Chen, Z. Lou, T. Huang, D. Chen, G. Shen, SnO2/SnS2
U
nanotubes for flexible room temperature NH3 gas sensors, RSC Adv. 7(2017)
N
52503-52509.
A
[32] D. Gu, X. Li, Y. Zhao, J. Wang, Enhanced NO2 sensing of SnO2/SnS2
M
heterojunction based sensor, Sens. Actuators B 244 (2017) 67-76.
ED
[33] K. Xu, N. Li, D. Zeng, S. Tian, S. Zhang, D. Hu, C. Xie, Interface bonds determined gas-sensing of SnO2-SnS2 hybrids to ammonia at room temperature,
PT
ACS Appl. Mater. Interfaces 7 (2015) 11359-11368.
CC E
[34] J. Ou, W. Ge, B. Carey, T. Daeneke, A. Rotbart, W. Shan, Y. Wang, Z. Fu, A.F. Chrimes, W. Wlodarski, S.P. Russo, Y.K. Li, Physisorption-based charge transfer
A
in two-dimensional SnS2 for selective and reversible NO2 gas sensing, ACS Nano 9 (2015) 10313-10323.
[35] L.D. Bharatula, M.B. Erande, I.S. Mulla, C.S. Rout, D.J. Late, SnS2 nanoflakes for efficient humidity and alcohol sensing at room temperature, RSC Adv. 2016, 6, 105421-105427.
Page 22 of 42
[36] S.K. Misra, N.K. Pandey, Analysis on activation energy and humidity sensing application of nanostructured SnO2-doped ZnO material, Sens. Actuators A 249 (2016) 8-14. [37] S.K. Misra, N.K. Pandey, Study of activation energy and humidity sensing
IP T
application of nanostructured Cu-doped ZnO thin films, J. Mater. Res. 31 (2016) 3214-3222.
SC R
[38] S.K. Misra, N.K. Pandey, V. Shakya, A. Roy, Application of undoped and Al2O3-doped ZnO nanomaterials as solid-state humidity sensor and its
U
characterization studies, IEEE Sens. J. 15 (2015) 3582-3589.
N
[39] N.K. Pandey, K. Tiwari, A. Roy, A. Mishra, A. Govindan, Ag-loaded WO3
M
Ceram. Technol. 10 (2013) 150-159.
A
ceramic nanomaterials: characterization and moisture sensing studies, Int. J. Appl.
ED
[40] M. Duta, L. Predoana, J.M. Calderon-Moreno, S. Preda, M. Anastasescu, A. Marin, I. Dascalu, P. Chesler, C. Hornoiu, M. Zaharescu, P. Osiceanu, M. Gartner,
PT
Nb-doped TiO2 sol-gel films for CO sensing applications, Mater. Sci. Semicond.
CC E
Process. 42 (2016) 397-404. [41] C. Liu, R. Zhang, S. Wei, Selective removal of H2S from biogas using a referable Hybrid TiO2/Zeolite composite, Fuel 157 (2015) 183-190.
A
[42] L. Zhou, M. Chen, Y. Wang, Y. Su, X. Yang, Au/Mesoporous-TiO2 as catalyst for the oxidation of alcohols to carboxylic acids with molecular oxygen in water, Appl. Catal. A 475 (2014) 347-354. [43] L. Sivachandiran, F. Thevenet, P. Gravejat, A. Rousseau, Investigation of NO and
Page 23 of 42
NO2 adsorption mechanisms on TiO2 at room temperature, Appl. Catal B-Environ. 142 (2013) 196-204. [44] W.D. Lin, C.T. Liao, T.C. Chang, S.H. Chen, R.J. Wu, Humidity sensing properties of novel graphene/TiO2 composites by sol-gel process, Sens. Actuators
IP T
B 209 (2015) 555-561. [45] T. Yang, Y.Z. Yu, L.S. Zhu, X. Wu, X.H. Wang, J. Zhang, Fabrication of silver
SC R
interdigitated electrodes on polyimide films via surface modification and
ion-exchange technique and its flexible humidity sensor application, Sens.
U
Actuators B 208 (2015) 327-333.
N
[46] C.Y. Zhu, F. Xu, L. Zhang, L.M. Li, J. Chen, S.H. Xu, G.G. Huang, W.H. Chen,
A
L.T. Sun, Ultrafast preparation of black phosphorus quantum dots for efficient
M
humidity sensing, Chem. Eur. J. 22 (2016) 7357-7362.
ED
[47]P.-G. Su, H.-C. Shieh, Flexible NO2 sensors fabricated by layer-by-layer covalent anchoring and in situ reduction of graphene oxide, Sens. Actuators B 190 (2014)
PT
865-872.
CC E
[48] P.-G. Su, W.C. Li, J.Y. Tseng, C.J. Ho, Fully transparent and flexible humidity sensors fabricated by layer-by-layer self-assembly of thin film of
A
poly(2-acrylamido-2-methylpropane sulfonate) and its salt complex. Sens. Actuators B 153 (2011) 29-36.
[49] L. Zhuo, Y. Wu, L. Wang, Y. Yu, X. Zhang, F. Zhao, One step hydrothermal synthesis of SnS2/graphene composites as anode material for highly efficient rechargeable lithium ion batteries, RSC Adv. 2 (2012) 5084-5087.
Page 24 of 42
[50] N. Jiang, N. Lu, K.F. Shang, J. Li, Y. Wu, Effects of electrode geometry on the performance of dielectric barrier/packed-bed discharge plasmas in benzene degradation, J. Hazard. Mater. 262 (2013) 387-393. [51] D. Zhang, H. Chang, P. Li, R. Liu, Q. Xue, Fabrication and characterization of an
nanocomposite, Sens. Actuators B 225 (2016) 233-240.
IP T
ultrasensitive humidity sensor based on metal oxide/graphene hybrid
SC R
[52] D. Zhang, N. Yin, B. Xia, Y. Sun, Y. Liao, Z. He, S. Hao, Humidity-sensing
properties of hierarchical ZnO/MWCNTs/ZnO nanocomposite film sensor based
U
on electrostatic layer-by-layer self-assembly, J. Mater. Sci.: Mater. Electron. 27
N
(2016) 2481-2487.
A
[53] J.S. Cai, J.Y. Huang, M.Z. Ge, J. Iocozzia, Z.Q. Lin, K.Q. Zhang, Y.K. Lai,
M
Immobilization of Pt nanoparticles via rapid and reusable electropolymerization of
ED
dopamine on TiO2 nanotube arrays for reversible SERS substrates and nonenzymatic glucose sensors, Small 13 (2017) 1604240
PT
[54] C. Konstantinos, Christoforidis, S. Armelle, K. Valérie, K. Nicolas, Single-step
CC E
synthesis of SnS2 nanosheet-decorated TiO2 anatase nanofibers as efficient photocatalysts for the degradation of gas phase diethylsulfide, ACS Appl. Mater. Interfaces 7 (2015) 19324-19334.
A
[55] Y. Zhou, J.D. Grunwaldt, F. Krumeich, K.B. Zheng, G.R. Chen, J. Stotzel, R. Frahm, G.R. Patzke, Hydrothermal synthesis of Bi6S2O15 nanowires: structural, in situ EXAFS, and humidity-sensing studies, Small 6 (2010) 1173-1179. [56] M. Parthibavarman, V. Hariharan; C. Sekar. High-sensitivity humidity sensor
Page 25 of 42
based on SnO2 nanoparticles synthesized by microwave irradiation method, Mater. Sci. Eng. 31 (2011) 840-844. [57] P. Pascariu, A. Airinei, N. Olaru, I. Petrila, V. Nica, L. Sacarescu, F. Tudorache, Microstructure, electrical and humidity sensor properties of electrospun NiO-SnO2
IP T
nanofibers, Sens. Actuators B 222 (2016) 1024-1031. [58] M. Zhuo, Y. Chen, J. Suna, H. Zhang, D. Guo, H. Zhang, Q. Li, T. Wang, Q. Wan,
SC R
Humidity sensing properties of a single Sb doped SnO2 nanowire field effect transistor, Sens. Actuators B 186 (2013) 78-83.
U
[59] D. Toloman, A. Popa, M. Stan, C. Socaci, A.Biris, G. Katona, F. Tudorache, I.
N
Petrila, F. Iacomi, Reduced graphene oxide decorated with Fe doped SnO2
A
nanoparticles for humidity sensor, Appl. Surf. Sci. 402 (2017) 410-417.
M
[60] H.C. Bi, K.B. Yin, X. Xie, J. Ji, S. Wan, L.T. Sun, M. Terrones, M.S. Dresselhaus,
ED
Ultrahigh humidity sensitivity of graphene oxide, Sci. Rep. 3 (2013) 2714. [61] Y. Tan, K. Yu, T. Yang, Q. Zhang, W. Cong, H. Yin, Z. Zhang, Y. Chen, Z. Zhu,
PT
The combinations of hollow MoS2 micro@nano-spheres: one-Step synthesis,
CC E
excellent photocatalytic and humidity sensing properties, J. Mater. Chem. C 2 (2014) 5422-5430.
A
[62] X.J. Chen, J. Zhang, Z.L. Wang, Q. Yan, S.C. Hui, Humidity sensing behavior of silicon nanowires with hexamethyldisilazane modification, Sens. Actuators B 156 (2011) 631-636. [63] Y. Wang, S. Park, J.T.W. Yeow, A. Langner, F.A Müller, Capacitive humidity sensor based on ordered macroporous silicon with thin film surface coating, Sens.
Page 26 of 42
Actuators B 149 (2010) 136-142. [64] Y. Kim, B. Jung, H. Lee, H. Kim, K. Lee, H. Park, Capacitive humidity sensor design based on anodic aluminum oxide, Sens. Actuators B 141 (2009) 441-446. [65] M.Q. Liu, C. Wang, N.Y. Kim, High-sensitivity and low-hysteresis porous
IP T
MIM-type capacitive humidity sensor using functional polymer mixed with TiO2 microparticles, Sensors 17 (2017) 284-295.
SC R
[66]D. Zhang, J. Liu, C. Jiang, P. Li, Y. Sun, High-performance sulfur dioxide sensing properties of layer-by-layer self-assembled titania-modified graphene hybrid
U
nanocomposite, Sens. Actuators B 245 (2017) 560-567.
N
[67]Y. Liu, P. Geng, J. Wang, Z. Yang, H. Lu, J. Hai, Z. Lu, D. Fan, M. Li, In-situ
A
ion-exchange synthesis Ag2S modified SnS2 nanosheets toward highly
M
photocurrent response and photocatalytic activity, J. Colloid Interface Sci. 512
ED
(2018) 784-791.
[68] N. Agmon, The grotthuss mechanism, Chem. Phys. Lett. 244 (1995) 456-462.
PT
[69] D. Zhang, J. Liu, B. Xia, Layer-by-layer self-assembly of zinc oxide/graphene
CC E
oxide hybrid toward ultrasensitive humidity sensing, IEEE Electron Dev. Lett. 37 (2016) 916-919.
A
[70] K. Jiang, T. Fei, T. Zhang, Humidity sensor using a li-loaded microporous organicpolymer assembled by 1,3,5-trihydroxybenzene and terephthalic aldehyde, RSC Adv. 4 (2014) 28451-28455.
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Biographies
Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D.
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degree from South China University of Technology in 2011. He is currently an associate professor at China University of Petroleum (East China), Qingdao, China.
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His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics.
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Xiaoqi Zong received her B.S. degree from China University of Petroleum (East
N
China) in 2016. Currently, she is graduate student at China University of Petroleum
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(East China), Qingdao, China. Her fields of interests include nanomaterials-based
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humidity and gas sensors, precision measurement technology.
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Zhenling Wu received his B.S. degree from Qingdao University in 2016. Currently, he is graduate student at China University of Petroleum (East China), Qingdao, China.
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His fields of interests include nanomaterials based gas sensors, precision
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measurement technology.
Yong Zhang received his Ph. D degree in ocean information detection and treatment from Ocean University of China in 2008. Currently, he is an associate professor at
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China University of Petroleum (East China), Qingdao, China. His main research interests are precision measurement technology and instruments.
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Figure captions
Figure 1. (a) Schematic of layer-by-layer fabrication of SnS2/TiO2 film, (b) optical image of the sensor prototype on a flexible PI substrate, and (c) schematic of
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humidity-sensing experimental setup. Figure 2. SEM images of (a-b) SnS2 nanosheets, (c) TiO2 nanospheres, and (d)
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SnS2/TiO2 nanocomposite.
Figure 3. (a-b) TEM images and (c-d) HRTEM lattice fringe images of SnS2/TiO2
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nanocomposite.
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Figure 4. XRD patterns of TiO2, SnS2 and SnS2/TiO2 samples.
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Figure 5. (a) EDS spectrum of SnS2/TiO2 nanocomposite. (b) XPS spectra for survey
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SnS2/TiO2 nanocomposite.
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of SnS2/TiO2 nanocomposite. XPS spectra of (c) S, (d) Sn, (e) O, and (f) Ti of
Figure 6. (a) Impedance of the SnS2/TiO2 film sensor as a function of RH at 100 Hz,
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1k Hz, 10k Hz. (b) Sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100
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Hz, 1k Hz, 10k Hz. (c) Response of the SnS2/TiO2 film sensor versus different RH at 100 Hz, compared with pure SnS2 and TiO2 film sensor. Figure 7. (a) Impedance of the SnS2/TiO2 film sensor upon exposure to various RH
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levels at 100 Hz. (b) Impedance of the SnS2/TiO2 film sensor as a function of RH. (c) Typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. (d) Repeatability of the SnS2/TiO2 film sensor exposed to 52%, 67%, and 75%RH from 11%RH.
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Figure 8. (a) Impedance of the SnS2/TiO2 film sensor for humidity-increasing and humidity-decreasing measurement. (b) Hysteresis of the SnS2/TiO2 film sensor. (c) Long-term stability of the SnS2/TiO2 film sensor. (d) Response of the SnS2/TiO2 film sensor to person’s breath.
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Figure 9. Schematic of humidity sensing for the SnS2/TiO2 film sensor. Figure 10. (a) Complex impedance plots at the range of 11%-97%RH and equivalent
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circuits of the SnS2/TiO2 film sensor at (b) low, (c) medium, and (d) high RH.
Figure 11. Bode diagram of the SnS2/TiO2 film sensor toward (a) 11%RH, (b)
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23%RH, (c) 52%RH, and (d) 97%RH.
Figure 1. (a) Schematic of layer-by-layer fabrication of SnS2/TiO2 film, (b) optical
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image of the sensor prototype on a flexible PI substrate, and (c) schematic of
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humidity-sensing experimental setup.
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SnS2/TiO2 nanocomposite.
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Figure 2. SEM images of (a-b) SnS2 nanosheets, (c) TiO2 nanospheres, and (d)
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Figure 3. (a-b) TEM images and (c-d) HRTEM lattice fringe images of SnS2/TiO2
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nanocomposite.
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Figure 4. XRD patterns of TiO2, SnS2 and SnS2/TiO2 samples.
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Figure 5. (a) EDS spectrum of SnS2/TiO2 nanocomposite. (b) XPS spectra for survey
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of SnS2/TiO2 nanocomposite. XPS spectra of (c) S, (d) Sn, (e) O, and (f) Ti of SnS2/TiO2 nanocomposite.
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Figure 6. (a) Impedance of the SnS2/TiO2 film sensor as a function of RH at 100 Hz, 1k Hz, 10k Hz. (b) Sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100 Hz, 1k Hz, 10k Hz. (c) Response of the SnS2/TiO2 film sensor versus different RH at 100 Hz, compared with pure SnS2 and TiO2 film sensor. Page 36 of 42
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Figure 7. (a) Impedance of the SnS2/TiO2 film sensor upon exposure to various RH
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levels at 100 Hz. (b) Impedance of the SnS2/TiO2 film sensor as a function of RH. (c)
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Typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. (d) Repeatability of the SnS2/TiO2 film sensor exposed to
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52%, 67%, and 75%RH from 11%RH.
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Figure 8. (a) Impedance of the SnS2/TiO2 film sensor for humidity-increasing and humidity-decreasing measurement. (b) Hysteresis of the SnS2/TiO2 film sensor. (c)
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Long-term stability of the SnS2/TiO2 film sensor. (d) Response of the SnS2/TiO2 film
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sensor to person’s breath.
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Figure 9. Schematic of humidity sensing for the SnS2/TiO2 film sensor.
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Figure 10. (a) Complex impedance plots at the range of 11%-97%RH and equivalent circuits of the SnS2/TiO2 film sensor at (b) low, (c) medium, and (d) high RH.
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23%RH, (c) 52%RH, and (d) 97%RH.
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Figure 11. Bode diagram of the SnS2/TiO2 film sensor toward (a) 11%RH, (b)
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Table 1. Humidity sensing properties of the SnS2/TiO2 film sensor compared with previous work. Fabrication method
Meas. range
Sensitivity
Ref.
Bi6S2O15
Suspension dripping
11-95%RH
118928.5 Ω/%RH
[55]
SnO2
Microwave irradiation
5-95%RH
149.89 Ω/%RH
[56]
NiO/SnO2
Electrospinning technique
0-100%RH
3254.22 Ω/%RH
[57]
Sb-SnO2
Vapor liquid solid method
22-44%RH
100454 Ω/%RH
[58]
RGO/Fe-SnO2
Electrostatic interaction
0-100%RH
7.56 Ω/%RH
[59]
Black phosphorus
Spin-coating
10-90%RH
Polyimide
Inkjet-printing
16-90%RH
Graphene oxide (GO)
Solution dripping
15-95%RH
MoS2
Hydrothermal
GO/polyelectrolyte
LbL self-assembly
SnS2/TiO2
LbL self-assembly
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Sensing material
[46]
24.5 pF/%RH
[45]
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124975 Ω/%RH
[60]
17.2−89.5%RH
81.9 pF/%RH
[61]
11−97%RH
1552.3 pF/%RH
[48]
11-97%RH
442000 Ω/%RH
This work
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46.253 pF/%RH
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S0925-4005(18)30485-4 https://doi.org/10.1016/j.snb.2018.03.007 SNB 24292
To appear in:
Sensors and Actuators B
Received date: Revised date: Accepted date:
22-11-2017 9-2-2018 1-3-2018
Please cite this article as: Dongzhi Zhang, Xiaoqi Zong, Zhenling Wu, Yong Zhang, Ultrahigh-Performance Impedance Humidity Sensor Based on Layer-by-layer SelfAssembled Tin Disulfide/Titanium Dioxide Nanohybrid Film, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2018.03.007 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.
Ultrahigh-Performance Impedance Humidity Sensor Based on Layer-by-layer Self-Assembled Tin Disulfide/Titanium Dioxide Nanohybrid Film
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Dongzhi Zhang*, Xiaoqi Zong, Zhenling Wu, Yong Zhang
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College of Information and Control Engineering, China University of Petroleum (East
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China), Qingdao 266580, China
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*Corresponding authors: Dongzhi Zhang
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E-mail address: [email protected]
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Tel: +86-532-86982928
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Fax: +86-532-86983326
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Highlights SnS2/TiO2
film–based
impedance
humidity
sensor
was
fabricated
via
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layer-by-layer self-assembly.
Ultrahigh sensitivity and response of SnS2/TiO2 film sensor toward humidity was
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demonstrated.
The sensing mechanism for the SnS2/TiO2 film sensor toward humidity was discussed.
Abstract An ultrahigh-performance impedance-type humidity sensor based on tin disulfide Page 1 of 42
(SnS2)/titanium dioxide (TiO2) nanocomposite was demonstrated via layer-by-layer self-assembly technique in this work. The nanostructures, morphologies, composition properties of the SnS2/TiO2 nanocomposite were fully examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS),
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transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The humidity sensing characteristics of the SnS2/TiO2 film sensor were
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investigated at room temperature. The results show that the SnS2/TiO2 film sensor has an impedance response up to 200050 and a sensitivity of 442000 Ω/%RH, which is
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much better than that of the existing humidity sensors. Excellent stability and
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repeatability is also exhibited for the SnS2/TiO2 film sensor. Moreover, the underlying
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sensing mechanism for the SnS2/TiO2 sensor toward humidity was explored with the
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complex impedance spectroscopy and the Bode diagram. The SnS2/TiO2
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nanocomposite provides a promising building block for ultrahigh-sensitive humidity
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sensing and detection of human respiratory.
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Key words: Tin disulfide, layer-by-layer assembly, complex impedance spectroscopy, humidity sensing
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1. Introduction
In recent years, humidity sensors have been used in many applications such as commercial and defense aircrafts, textile industry, environmental monitoring, industrial and biomedical processing [1-3]. Until now, various transduction techniques
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such as capacitance [3], resistance [4], optical fiber [5], field effect transistor (FET) [6], surface acoustic wave (SAW) [7], and quartz crystal microbalance (QCM) [8] have been widely used to fabricate humidity sensors. So far, the humidity sensors are mainly based on ceramics, nanomaterials, organic polymers and organic-inorganic
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composites [9]. A large number of nanomaterials, such as metal oxides, carbon nanotubes and graphene, have been extensively used to fabricate humidity sensing
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devices [10-12]. As is well known, graphene has been widely used by many
researchers in various applications such as photocatalysis [13], electronics [14],
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supercapacitors [15], and sensors [16]. However, its semi-metallic properties have
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hindered its use in nanoelectronics and optoelectronic devices applications [3]. More
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recently, graphene-like 2D nanomaterials such as MoS2 [4, 17, 18], WS2 [19, 20],
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MoSe2 [21], WSe2 [22], SnSe2 [23], MoTe2 [24], WTe2 [25], black phosphorus [26]
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and TiS3 [27] have been extensively investigated in many applications such as photodetector, sensors, transistors and catalysis.
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In the family of transition-metal dichalcogenides (TMDCs), tin disulfide (SnS2)
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has attracted more and more attention as an emerging and promising nanomaterial in recent years [28]. SnS2 is a naturally rich material that exists in CdI2 type structure consisting of a Sn atom layer sandwiched between two layers of hexagonal enclosed S
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atoms. The adjacent layers of sulfur atoms are connected by weak van der Waals
force. Currently, the SnS2 is widely studied due to its superb properties, non-toxicity and high stability, and various applications such as lithium ion batteries [29], glucose sensing [30], gas sensing [31-35], have been reported. Xu et al. fabricated a
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SnO2-SnS2 composite for ammonia gas detection by the oxidation of SnS2 at 300°C and exhibited high response to NH3 at room temperature [33]. Ou et al. demonstrated a NO2 gas sensor based on SnS2 flakes and showed high sensitivity, superior selectivity and excellent reversibility for NO2 sensing at low operating temperatures
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[34]. Bharatula et al. fabricated layered SnS2 nanoflakes by hydrothermal synthesis method, and demonstrated high sensitivity towards humidity and selectivity towards
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alcohol gas at room temperature [35].
Humidity sensors based on metal oxides obtain special interest due to their
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nano-sized grains, nanoporous structures, and high surface contact of water molecules
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[3]. Moreover, doped metal oxides exhibit different types of morphologies, various
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properties and have various applications toward humidity sensing. Misra and Pandey
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et al. investigated humidity sensing properties of Ag-loaded WO3, SnO2-doped ZnO,
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Al2O3-doped ZnO, Cu-doped ZnO nanomaterials prepared by solid-state reaction method [36-39]. It was noticed that the doped metal oxides annealed at high
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temperature showed higher sensitivity, lower hysteresis, better reproducibility and
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faster response and recovery time compared to the sensing element of undoped metal oxide. Titanium dioxide (TiO2) has been attracted great attention recently as a promising material for fabricating sensors for the detection of CO [40], H2S [41],
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alcohol [42], NOx [43], and humidity [44]. However, there still remain some problems including long recovery time, high operating temperature (∼300°C) and low sensitivity at low temperature. Currently, some approaches such as inkjet-printing [45], dip coating [3], spin coating [46], self-assembly [11] and solid-state reaction methods
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[36-39] are widely used to fabricate humidity sensors. Among them, the layer-by-layer (LbL) self-assembly technique is based on sequential electrostatically adsorptions of ionized polyelectrolytes and oppositely charged materials in aqueous solutions. The LbL self-assembly technique has many advantages over other
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alternative methods, such as simplicity, substrate-independence, low-cost, low temperature deposition, controllable thickness from nanometers to micrometers, and
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no complex or costly equipment required [47]. It is a bottom-up approach to fabricate uniform nanocomposite film as humidity sensing material. Su et al. fabricated an
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impedance-type humidity sensor via layer-by-layer self-assembly of
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poly(2-acrylamido-2-methylpropane sulfonate) (PAMPS) and its salt complex on a
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flexible polyethylene terephthalate (PET) substrate, and enhanced sensitivity, good
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linearity, small hysteresis, short response and recovery times and long-term
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stability are demonstrated [48]. In our previous work, we presented a high-performance humidity sensor based on layer-by-layer self-assembly of
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chemically reduced graphene oxide/polymer nanocomposite film on flexible
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polyimide (PI) substrate [11]. In this work, we demonstrated an original and facile fabrication of tin
disulfide/titanium dioxide nanocomposite (SnS2/TiO2) nanocomposite on flexible
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substrate towards humidity sensing via layer-by-layer self-assembly technique. The composition and nanostructure of the as prepared SnS2/TiO2 nanocomposite were characterized by XRD, SEM, XPS and TEM. The humidity-sensing properties of the sensor were investigated in the range of 11%RH to 97%RH. The SnS2/TiO2 sensor
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exhibited much higher response than the existing humidity sensors. Fast response/recovery time, excellent repeatability and good reversibility towards humidity are demonstrated. At last, the possible sensing mechanism of SnS2/TiO2 film sensor toward humidity was revealed using the complex impedance spectroscopy and
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the Bode diagram. The SnS2/TiO2 film sensor has many technological advancements in terms of flexibility, small hysteresis, long-term stability, ease of integration, and
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respiratory monitoring capability, indicating great potential applications in various fields.
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2. Experimental
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2.1 Materials
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Thiourea (NH2CSNH2, ≥99%), titanium sulfate (Ti(SO4)2, ≥96%) and urea
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(CO(NH2)2, ≥99%) were obtained from Sinopharm Chemical Reagent Co. Ltd
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(Shanghai, China). Tin chloride pentahydrate (SnCl4·5H2O, ≥99%) was offered by Shanghai Hansi Chemical Industry Co. Ltd (Shanghai, China). Polyelectrolytes
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including 1.5 wt% poly(diallyldimethylammonium chloride) (PDDA) and 0.3 wt%
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poly(sodium 4-styrenesulfonate) (PSS) were obtained from Sigma-Aldrich Inc. In this work, SnS2 nanosheets and TiO2 nanospheres were synthesized via a
facile hydrothermal method [49]. Precursors, thiourea (1.218 g) and SnCl4·5H2O
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(1.402 g) were dissolved into 50 mL deionized (DI) water in a beaker. After magnetic stirring for 0.5 h, the resulting solution was transferred into a 70 mL Teflon-lined autoclave and hydrothermally treated at 180ºC for 24 h. After cooling naturally to room temperature, the SnS2 nanosheets were collected by centrifuging and washing
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with ethanol and DI water several times. The synthesis of TiO2 nanosphere was similar to that of SnS2 nanosheet. 6 g of urea and 12 g of Ti(SO4)2 were dissolved into 100 mL of DI water. Subsequently, magnetic stirred for 1.5 h and ultrasonication treated for 0.5 h to form a homogeneous
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suspension. At last, the suspension was transferred into a Teflon-lined autoclave and heated at 180°C for 3 h. Finally, the TiO2 solution was obtained after centrifuging and
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washing several times [50]. 2.2 Sensor fabrication
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The sensor device was fabricated on a polyimide substrate with a pair of
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interdigitated electrodes (IDEs), which was described in our previous work [51]. The
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sensing film of the sensor was fabricated by LbL self-assembly method, as shown in
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Figure 1(a). Two bilayers of PDDA/PSS were sequentially deposited on the substrate
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as precursor layer, followed by alternative immersing the device into TiO2 and SnS2 solutions for five cycles. The immersing time used was 10 min for PDDA and PSS
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mono-layer, and 15 min for SnS2 and TiO2 monolayer. Intermediate rinsing with DI
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water and drying with nitrogen stream were required after each monolayer assembly to reinforce the interconnection between layers. Finally, the device was dried in an oven at 50oC for 8 h. The optical image of the sensor prototype on a flexible PI
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substrate was shown in Figure 1(b). For making a comparison, the pure TiO2 and pure SnS2 film sensors were prepared by drop-casting method. 2.3 Experimental setup The humidity sensing measurement was performed at an ambient temperature of
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20°C. Various RH levels were yielded via several saturated salt solutions, as reported in our previous work [52]. The schematic of humidity-sensing experimental setup was illustrated in Figure 1(c). Saturated solutions of LiCl, CH3COOK, MgCl2, K2CO3, Mg(NO3)2, CuCl2, NaCl, KCl and K2SO4 in a closed vessel were used to yield
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approximately 11%, 23%, 33%, 43%, 52%, 67%, 75%, 85% and 97%RH levels, respectively. The sensor response as a function of RH was achieved by exposing the
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sensor inside the closed vessels with different RH levels. A computer-coupled
TH2828 precision LCR meter was applied to record the impedance and complex
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impedance spectroscopy of the SnS2/TiO2 film sensor during humidity sensing. The
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time taken to reach 90% of the initial total capacitance variation is defined as
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response/recovery time during the humidification and desiccation processes [37]. The
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normalized response R and sensitivity S are used as figures of merit to evaluate the
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performance of the SnS2/TiO2 film sensor, which are defined by R=Z11/Zx and S=ΔZ/ΔRH, respectively, where Z11 and Zx are the impedance of the sensor at 11%RH
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and x%RH, respectively, ΔZ is the change in impedance, and ΔRH is the RH change.
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The hysteresis H is used to evaluate the drawback of the SnS2/TiO2 film sensor, which are defined by H=(ZD-ZI)/S (%RH), where ZD and ZI are the impedance of the sensor at same RH in the decreasing humidity and increasing humidity process, respectively.
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3. Results and discussion 3.1 Characterization results The surface morphologies of the SnS2, TiO2, and SnS2/TiO2 nanocomposite were examined using a field emission scanning electron microscope (FESEM, Hitachi
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S-4800, Japan). Figure 2(a) and 2(b) indicate that SnS2 has a regular hexagonal nanosheet shape. Figure 2(c) shows the hydrothermally synthesized TiO2 possesses a nanosphere shape. Figure 2(d) illustrates SnS2 nanosheet and TiO2 nanosphere have a good contact in the LbL self-assembled SnS2/TiO2 nanocomposite.
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The nanostructure and crystallinity of the as-prepared samples were observed via a transmission electron microscope (TEM, JEOL JEM2100). Figure 3(a) and 3(b)
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show that SnS2/TiO2 nanocomposite consists of TiO2 nanospheres decorated on the
SnS2 hexagonal nanosheets with good distribution. The side length about 350 nm for
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hexagonal SnS2 nanosheet can be clearly observed. Figure 3(c) and 3(d) show the
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HRTEM lattice fringe images of SnS2/TiO2 nanocomposite. As observed, an
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interplanar distance of 0.352 nm is attributed to the (101) plane for the TiO2. The
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lattice fringes spacing of 0.316 nm and 0.27 nm for SnS2 nanocrystal, which is
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attributed to the (100) and (011) planes of the SnS2 nanosheets, respectively. The XRD analysis of SnS2, TiO2 and SnS2/TiO2 samples was performed by an
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X-ray diffractometer (Rigaku D/Max 2500PC) using Cu Kα radiation (λ=1.5418Å).
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The diffraction peaks were scanned in the range of 20°-80°, and the XRD results are illustrated in Figure 4. The XRD spectrum of the hexagonal SnS2 nanosheet exhibits a crystalline phase without impurity peaks. The main diffraction peaks of SnS2
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nanosheets at 15.04, 28.27, 32.17, 41.95, 50.05, 52.45, and 60.62o are ascribed to the (001), (100), (101), (102), (110), (111), and (201) planes, respectively, which are in accordance with the hexagonal 2H SnS2 structure (ICDD 23-0677) [34]. The diffraction peaks of TiO2 sample are observed at 2θ of 25.3°, 37.9°, 48.0°, 53.99°,
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54.98° and 62.2°, which correspond to the (101), (004), (200), (105), (211) and (204) planes of the anatase TiO2, respectively (JCPDS Card no. 21-1272) [45]. The result indicates the successfully synthesis of TiO2 nanocrystals. The XRD pattern of SnS2/TiO2 nanocomposite demonstrates the main characteristic peaks of the both SnS2
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and TiO2, confirming the existence of SnS2 and TiO2. The elements composition of the SnS2/TiO2 nanocomposite was identified by
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Hitachi S-4800 equipped with an energy dispersive spectroscopy (EDS). As shown in Figure 5(a), only Sn, S, Ti and O elements in the SnS2/TiO2 nanocomposite are
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detected, and no other impurity element has been observed. Furthermore, the
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elemental chemical states of the SnS2/TiO2 nanocomposite was also inspected by
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X-ray photoelectron spectroscopy (XPS, Thermo Scientific instrument), and the XPS
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spectra for survey of SnS2/TiO2 nanocomposite is shown in Figure 5(b). The Sn-to-S
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ratio of 1:2.235 was obtained through the XPS measurement, which is slightly substoichiometric, indicating sulfur vacancies in the SnS2 nanosheets. No other
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impurity elements were observed, which is consistent with the XPS analysis. The
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spectra of S 2p shown in Figure 5(c) exhibits two peaks at 161.1 eV and 162.4 eV, corresponding to S 2p3/2 and S 2p1/2, respectively. There are no extra peaks are found in the region between 168 eV and 170 eV, indicating that the sulfur atoms in SnS2
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remain unoxidized without no oxide phase formed [34]. Figure 5(d) shows that Sn atom has two binding energy peaks at 486.1 eV (3d5/2) and 494.4 eV (3d3/2), which confirm the presence of Sn4+. No extra peaks are observed at ~485 eV and ~493 eV, indicating that there is no Sn2+ in the sample [34]. The O 1s peaks shown in Figure
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5(e) reveal three peaks at 529.73, 532.03, and 533.53 eV, attributed to the lattice oxygen ions of the anatase TiO2, surface hydroxyl groups and adsorbed water molecules, respectively. The XPS spectra of Ti 2p in Figure 5(f) illustrates doublet peaks at 458.6 eV and 464.3 eV with a spin-orbit splitting of 5.7 eV, which was
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ascribed to the Ti 2p3/2 and Ti 2p1/2 spin-orbit components of Ti4+-O surface species unique to TiO2. These binding energies peaks are in consistent with the values
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reported for pure TiO2 and prove the exclusive presence of Ti4+ [53, 54]. 3.2 Humidity-sensing performances
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The measured impedance of the SnS2/TiO2 film sensor upon exposure to various
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RH levels was shown in Figure 6(a). The applied operation frequencies were chosen
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at 100 Hz, 1k Hz and 10k Hz, respectively. We can observe that the impedance
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decreases with the increasing RH at fixed operation frequency. This is due to the
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adsorption of water molecules promoted the ionic conductivity and enhanced the dielectric constant of the sensing film. Among the three different operation
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frequencies, the sensor shows the largest impedance variation upon exposure to
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11%-97%RH at 100 Hz. Figure 6(b) shows the sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100, 1000 and 10000 Hz, and the inset indicates the magnified curve for 10k Hz. In the measuring range of 11-97%RH, the sensor yields
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the sensitivity of 442000 Ω/%RH, 65461 Ω/%RH and 7111 Ω/%RH at 100 Hz, 1k Hz and 10k Hz, respectively. Obviously, the sensitivity at 100 Hz is higher than that at the two other frequencies. The sensor sensitivity decreases with the increasing of operating frequency, and becomes flatly with the increasing RH. This is due to the
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polarization of the adsorbed water molecules is hard to catch up the electrical field direction changes at higher frequencies [10]. Therefore, 100 Hz is selected as the operating frequencies in the subsequent experiments. In order to highlight the remarkable properties of SnS2/TiO2 film sensor, the
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comparison with pure SnS2 and TiO2 film sensor is further investigated. Figure 6(c) shows the response of the three sensors versus different RH at 100 Hz. The responses
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for the SnS2/TiO2 film, pure SnS2 and TiO2 film sensors measured at 97%RH are
200050, 55882 and 737, respectively, indicating the SnS2/TiO2 film sensor exhibits
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the most striking impedance-RH characteristics. Table 1 shows the humidity sensing
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performance of the SnS2/TiO2 film sensor compared with the previous works [45, 46,
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48, 55-61]. The comparison is made with the existing humidity sensors made from
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metal oxide, conductive polymer, and 2D materials (i.e., graphene, MoS2, black
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phosphorus) via various methods. We can observe from the comparison in Table 1, our SnS2/TiO2 film sensor has unprecedented sensitivity, which is 3.5 times higher
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that of the best one among conventional sensors ever reported.
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Figure 7(a) shows the time-dependent impedance measurement of the SnS2/TiO2 film sensor toward various RH levels at 100 Hz. The sensor was switched between the tested RH and 11%RH for humidity sensing and recovery, respectively. The time
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interval for response/recovery duration was 150 s. The sensor impedance decreases about 200000-fold of magnitude from 38.01 MΩ to 190 Ω toward RH varies from 11% to 97%RH. Figure 7(b) shows the impedance value of the SnS2/TiO2 film sensor as a function of RH in the range of 11%-97%. The logarithm of sensor impedance
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(lgY) presents a good linearity with relative humidity X, which can be fitted as lgY=8.60-0.069X. Figure 7(c) shows the typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. Response time and recovery time of less than 58 s are observed, respectively, which is much better
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than that of conventional humidity sensors [39, 62-64]. Figure 7(d) illustrates the repeatability performance of the SnS2/TiO2 film sensor exposed to 52%, 67%, and
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75%RH from 11%RH over five cycles. A good repeatability can be observed and the stable-state impedance were 117769 Ω (52%RH), 6848 Ω (67%RH), and 909 Ω
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(75%RH), respectively.
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Figure 8(a) shows the impedance of the SnS2/TiO2 film sensor for
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humidity-increasing and humidity-decreasing measurement. The switching RH test is
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performed by exposing the sensor to 11% and 23%, 33%, 43%, 52%, 67%, 75%, 85%,
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97%RH, and then conversely from high RH to low RH. The hysteresis of the SnS2/TiO2 film sensor is shown in Figure 8(b). The maximum hysteresis is 1.13%RH,
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indicating a highly reversible property [38, 65]. Figure 8(c) shows the long-term
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stability of the SnS2/TiO2 film sensor tested at 33%, 62% and 85%RH, respectively. The impedance of the sensor did not change significantly over three months, indicating a good stability. Figure 8(d) demonstrated the sensor in the application of
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human respiration monitoring. The impedance response of the sensor was recorded during the breathing for an adult, and 12 repetitive cycles in 50 s were clearly observed. The impedance response of the sensor exhibited sharp rise during exhaling and dropped while inhaling corresponding to the breathing cycles, indicating the
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sensor is capable of capturing the humidity change during human breath. 3.3 Humidity-sensing mechanism The SnS2/TiO2 nanocomposite humidity sensor exhibited excellent sensing properties in terms of high response, low hysteresis, fast response and recovery times,
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and good reproducibility. The nanostructure of SnS2/TiO2 nanohybrid contributes to the enhanced humidity sensing properties. As well is known, TiO2 nanospheres have
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nanosize grains, high physicochemical properties, broad bandgap of about 3.2 eV, and large excition binding energy [65, 66]. The incorporation of TiO2 into SnS2
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nanosheets brings more active sites (i.e., oxygen vacancies, defects) and nanopores,
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which can provide high surface exposure for adsorption of water molecules. SnS2
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nanosheets possess natural band gap, low resistivity and high carrier mobility, serving
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as direct conduction paths for the electrons transfer in the TiO2 nanospheres, which
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play a dominant role in improving the electrical properties of TiO2. The synergistic effect of the binary nanomaterials endows the SnS2/TiO2 nanocomposite sensor has
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better properties compared to the pure SnS2 and TiO2 sensor. Moreover, SnS2
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nanosheets have much hydrophilic functional groups attached on its surface capable of capturing much more water molecules [67]. This may be attributed to the fact that
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the pure SnS2 sensor exhibits higher humidity sensing ability than that of pure TiO2. The humidity sensing mechanism of the sensor is ascribed to the mutual
interdependence between the SnS2/TiO2 sensing film and water adsorption/desorption. The adsorption of water molecules on the SnS2/TiO2 nanocomposite is shown in Figure 9. At low humidity, only a few water molecules are chemically adsorbed on the
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surface of SnS2 and TiO2, and the coverage of water on the film surface is discontinuous. As the humidity increases, much more water molecules are physically adsorbed to the SnS2 nanosheets and TiO2 nanospheres through the hydrogen bonding. At high humidity, the adsorbed water molecules form a serial water layer on the film
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surface, in which the hydronium (H3O+) is generated and the proton hopping transport occurs according to the Grotthuss chain reaction [68].
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In order to further discover the potential sensing mechanism of the SnS2/TiO2
film, the complex impedance spectroscopy (CIS) and bode diagram were investigated.
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Figure 10 shows the CIS measurements and equivalent circuit models for the
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SnS2/TiO2 sensor at different RH levels. The real and imaginary parts of CIS curves
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are magnified synchronously to compile them in one Nyquist plot. The operation
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frequency was scanned from 100 Hz to 1M Hz. At low humidity level (11%RH), the
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CIS curve behaves like a straight line, which can be modeled as a constant phase element (CPE), as shown in Figure 10(b). The CPE ZCPE is defined as ZCPE =1/A(j2πf)n,
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in which A is a real parameter and 0≤n≤1. The corresponding Bode diagram is shown
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in Figure 11(a). The logarithmic of impedance magnitude has a linear relationship with the logarithmic frequency, and the impedance angle is near to -90o. At 23%RH level, the CIS curve starts to bend downwardly. This indicates that
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the film resistance decreases and the capacitance increases due to the adsorption of water molecules. The Bode diagram shown in Figure 11(b) confirms its impedance magnitude and the phase angle are decreased with the variation of frequency. The CIS curve appears semicircular-shape when the humidity reaches 33%RH, 43%RH and
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52%RH. The corresponding equivalent circuit for the sensor can be described as a CPE and a resistor connected in parallel (Figure 10(c)), indicating a relaxation effect owing to the polarization from intrinsic impedance of SnS2/TiO2 nanocomposite. Figure 11(c) shows the Bode diagram for the sensor at 52%RH, which is a typical
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behavior for a RC parallel circuit. The phase angle increases and spans a wide range of 0-90o with increasing frequency.
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At high RH (67%, 75%, 85%, 97%RH), the CIS curve shows a line at the tail of
semicircle at low frequency region, semicircle gradually disappears at high frequency
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region. This is ascribed to the Warburg impedance (Zw) caused by the diffusion
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process of ions or charge carriers at the sensing film/electrode interfaces, and the main
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charge carrier is the hydronium (H3O+) [69, 70]. The equivalent circuit for the sensor
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at high RH is described as shown in Figure 10(d). Figure 11(d) shows the Bode
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diagram for the sensor at 97%RH. The impedance magnitude becomes much smaller, and the phase angle is near to 0 at low frequency region. This is due to the serial water
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layer results in the proton transfer, which makes a great contribution to the sharp
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decrease of film impedance. 4. Conclusion
In summary, a novel humidity sensor based on SnS2/TiO2 nanocomposites was
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successfully fabricated via layer-by-layer self-assembly method. The nanostructures, morphology and composition of SnS2/TiO2 samples were characterized by XRD, SEM, TEM and EDS. The humidity sensing characteristics of SnS2/TiO2 sensors were measured at room temperature over a wide RH range. The result shows that the
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impedance response of SnS2/TiO2 nanocomposite film sensor is much higher than capacitance and resistance response of SnS2/TiO2 nanocomposites. Comparing with pure SnS2 and pure TiO2, the SnS2/TiO2 nanocomposite film sensor possesses not only high response, but also excellent repeatability. Besides, it has great potential for
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application in the detection of human respiratory. The humidity sensing mechanism was investigated with the complex impedance spectroscopy and the Bode diagram.
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This work reveals the possibility of using SnS2/TiO2 nanocomposite for constructing low-cost humidity sensor.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China
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(51777215), the Fundamental Research Funds for the Central Universities of China
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(18CX07010A), the Science and Technology Development Plan Project of Qingdao
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(16-6-2-53-nsh), and the Open Fund of National Engineering Laboratory for Mobile
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Source Emission Control Technology (NELMS2017B03).
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References [1] C. Giaconia, A. Orioli, A.D. Gangi, Air quality and relative humidity in commercial aircrafts: an experimental investigation on short-haul domestic flights, Build. Environ. 67 (2013) 69-81.
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[2] G.H. Zhou, J.H. Byun, Y. Oh, B.M. Jung, H.J. Cha, D.G. Seong, M.K. Um, S. Hyun, T.W. Chou, Highly sensitive wearable textile-based humidity sensor made
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of high-strength, single-walled carbon nanotube/poly(vinyl alcohol) filaments, ACS Appl. Mater. Inter. 9 (2017) 4788-4797.
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[3] D. Zhang, Y. Sun, P. Li, Y. Zhang, Facile fabrication of MoS2-modified SnO2
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Interfaces 8 (2016) 14142-14149.
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hybrid nanocomposite for ultrasensitive humidity sensing, ACS Appl. Mater.
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[4] M. Bhattacharjee, H.B. Nemade, D. Bandyopadhyay, Nano-enabled paper
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humidity sensor for mobile based point-of-care lung function monitoring, Biosen. Bioelectron. 94 (2017) 544-551.
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[5] L. Xia, L.C. Li, W. Li, T. Kou, D.M. Liu, Novel optical fiber humidity sensor
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based on a no-core fiber structure, Sens. Actuators A 190 (2013) 1-5. [6] C. Sun, K.R.G. Karthik, S.S. Pramana, L.H. Wong, J. Zhang, Y.Z. Huang, C.H.
A
Sow, N. Mathews, S.G. Mhaisalkar, The role of tin oxide surface defects in determining nanonet FET response to humidity and photoexcitation, J. Mater. Chem. C 2 (2014) 940-945.
[7] Y.L. Tang, Z.J. Li, J.Y. Ma, L. Wang, J. Yang, B. Du, Q.K. Yu, X.T. Zu, Highly sensitive surface acoustic wave (SAW) humidity sensors based on sol-gel SiO2
Page 18 of 42
films: investigations on the sensing property and mechanism, Sens. Actuators B 215 (2015) 283-291. [8] Y. Yao, H. Zhang, J. Sun, W. Y. Ma, L. Li, W. Z. Li, J. Du, Novel QCM humidity sensors using stacked black phosphorus nanosheets as sensing film, Sens.
IP T
Actuators B 244 (2017) 259-264. [9] J. Dai, T. Zhang, H. Zhao, T. Fei, Preparation of organic-inorganic hybrid
SC R
polymers and their humidity sensing properties, Sens. Actuators B 242 (2017) 1108-1114.
U
[10] D. Zhang, J. Tong, B. Xia, Q. Xue, Ultrahigh performance humidity sensor based
N
on layer-by-layer self-assembly of graphene oxide/polyelectrolyte nanocomposite
A
film, Sens. Actuators B 203 (2014) 263-270.
M
[11] D. Zhang, J. Tong, B. Xia, Humidity-sensing properties of chemically reduced
ED
graphene oxide/polymer nanocomposite film sensor based on layer-by-layer nano self-assembly, Sens. Actuators B 197 (2014) 66-72.
PT
[12] Y. Li, T.T. Wu, M.J. Yang, Humidity sensors based on the composite of
CC E
multi-walled carbon nanotubes and cross linked polyelectrolyte with good sensitivity and capability of detecting low humidity, Sens. Actuators B 203 (2014) 63-70.
A
[13] J. Di, J.X. Xia, H.M. Li, Z. Liu, Freestanding atomically-thin two-dimensional materials beyond graphene meeting photocatalysis: opportunities and challenges, Nano Energy 35 (2017) 79-91. [14] M. Shekhirev, H. Vo, M.M. Pour, A. Sinitskii, Interfacial self-assembly of
Page 19 of 42
atomically precise graphene nanoribbons into uniform thin films for electronics applications, ACS Appl. Mater. Interfaces 9 (2017) 693-700. [15] M.S. Lee, H.J. Choi, J.B. Baek, D.W. Chang, Simple solution-based synthesis of pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors, Appl.
IP T
Energy 195 (2017) 1071-1078. [16] T.R. Zhan, Z.W. Tan, X. Tian, W.G. Hou, Ionic liquid functionalized graphene
SC R
oxide-Au nanoparticles assembly for fabrication of electrochemical 2, 4-dichlorophenol sensor, Sens. Actuators B 246 (2017) 638-646.
U
[17] D.J. Late, Y.K. Huang, B. Liu, J. Acharya, S.N. Shirodkar, J. Luo, A. Yan, D.
N
Charles, U.V. Waghmare, V.P. Dravid, C.N.R. Rao, Sensing behavior of atomically
A
thin-layered MoS2 transistors, ACS Nano 7 (2013) 4879-4891.
M
[18] R.S. Sundaram, M. Engel, A. Lombardo, R. Krupke, A.C. Ferrari, P. Avouris, M.
1416-1421.
ED
Steiner, Electroluminescence in single layer MoS2, Nano Lett 13(2013)
PT
[19] D. Ovchinnikov, A. Allain, Y.S. Huang, D. Dumcenco, A. Kis, Electrical
CC E
transport properties of single-layer WS2, ACS Nano 8 (2014) 8174-8181. [20] A.S. Pawbake, R. Waykar, D.J. Late, S.R. Jadkar, Highly transparent wafer-scale
A
synthesis of crystalline WS2 nanoparticle thin film for photodetector and humidity-sensing applications, ACS Appl. Mater. Interfaces 8 (2016) 3359-3365.
[21] Y. Chang, W. Zhang, Y. Zhu, Y. Han, J. Pu, J. Chang, W. Hsu, J. Huang, C. Hsu, M. Chiu, T. Takenobu, H. Li, C. Wu, W. Chang, A.T.S. Wee, L. Li, Monolayer MoSe2 grown by chemical vapor deposition for fast photodetection, ACS Nano 8
Page 20 of 42
(2014) 8582-8590. [22] J.K. Huang, J. Pu, C.L. Hsu, M.H. Chiu, Z.Y. Juang, Y.H. Chang, W.H. Chang, Y. Iwasa, T. Takenobu, L.J. Li, Large-area synthesis of highly crystalline WSe2 monolayers and device applications, ACS Nano 8 (2013) 923-930.
IP T
[23] X. Zhou, L. Gan, W. Tian, Q. Zhang, S. Jin, H. Li, Y. Bando, D. Golberg, T. Zhai,
photodetectors, Adv. Mater. 27 (2015) 8035-8041.
SC R
Ultrathin SnSe2 flakes grown by chemical vapor deposition for high-performance
[24] L. Zhou, K. Xu, A. Zubair, A.D. Liao, W. Fang, F. Ouyang Y. Lee, K. Ueno, R.
U
Saito, T. Palacios, J. Kong, M.S. Dresselhaus, Large-area synthesis of high-quality
N
uniform few-layer MoTe2, J. Am. Chem. Soc. 137 (2015) 11892-11895.
A
[25] M.N. Ali, J. Xiong, S. Flynn, J. Tao, Q.D. Gibson, L.M. Schoop, T. Liang, N.
M
Haldolaarachchige, M. Hirschberger, N.P. Ong, R.J. Cava, Large, non-saturating
ED
magnetoresistance in WTe2, Nature 514 (2014) 205-208. [26] L. Li, Y. Yu, G.J. Ye, Q. Ge, X. Ou, H. Wu, D. Feng, X.H. Chen, Y. Zhang, Black
PT
phosphorus field-effect transistors, Nat. Nanotechnol. 9 (2014) 372-377.
CC E
[27] J.O. Island, M. Buscema, M. Barawi, J.M. Clamagirand, J.R. Ares, C. S´anchez, I.J. Ferrer, G.A. Steele, H.S.J. Zant, A. Castellanos-Gomez, Ultrahigh
A
photoresponse of few-Layer TiS3 nanoribbon transistors, Adv. Opt. Mater. 2 (2014) 641-645.
[28] Y. Huang, E. Sutter, J.T. Sadowski, M. Cotlet, O.L. Monti, D.A. Racke, M.R. Neupane, D. Wickramaratne, R.K. Lake, B.A. Parkinson, Tin disulfide-an emerging layered metal dichalcogenide semiconductor: materials properties and
Page 21 of 42
device characteristics, ACS Nano 8 (2014) 10743-10755. [29] X.Y. Li, M.C. Si, F.H. Jian, G.K. Xin, M.P. Li, Preparation of hierarchical SnS2/SnO2 anode with enhanced electrochemical performances for lithium-ion battery, Electrochim. Acta 238 (2017) 168-177.
IP T
[30] K.T. Lee, Y.C. Liang, H.H. Lin, C.H. Li, S.Y. Lu, Exfoliated SnS2 nanoplates for enhancing direct electrochemical glucose sensing, Electrochim. Acta 219 (2016)
SC R
241-250.
[31] R. Li, K. Jiang, S. Chen, Z. Lou, T. Huang, D. Chen, G. Shen, SnO2/SnS2
U
nanotubes for flexible room temperature NH3 gas sensors, RSC Adv. 7(2017)
N
52503-52509.
A
[32] D. Gu, X. Li, Y. Zhao, J. Wang, Enhanced NO2 sensing of SnO2/SnS2
M
heterojunction based sensor, Sens. Actuators B 244 (2017) 67-76.
ED
[33] K. Xu, N. Li, D. Zeng, S. Tian, S. Zhang, D. Hu, C. Xie, Interface bonds determined gas-sensing of SnO2-SnS2 hybrids to ammonia at room temperature,
PT
ACS Appl. Mater. Interfaces 7 (2015) 11359-11368.
CC E
[34] J. Ou, W. Ge, B. Carey, T. Daeneke, A. Rotbart, W. Shan, Y. Wang, Z. Fu, A.F. Chrimes, W. Wlodarski, S.P. Russo, Y.K. Li, Physisorption-based charge transfer
A
in two-dimensional SnS2 for selective and reversible NO2 gas sensing, ACS Nano 9 (2015) 10313-10323.
[35] L.D. Bharatula, M.B. Erande, I.S. Mulla, C.S. Rout, D.J. Late, SnS2 nanoflakes for efficient humidity and alcohol sensing at room temperature, RSC Adv. 2016, 6, 105421-105427.
Page 22 of 42
[36] S.K. Misra, N.K. Pandey, Analysis on activation energy and humidity sensing application of nanostructured SnO2-doped ZnO material, Sens. Actuators A 249 (2016) 8-14. [37] S.K. Misra, N.K. Pandey, Study of activation energy and humidity sensing
IP T
application of nanostructured Cu-doped ZnO thin films, J. Mater. Res. 31 (2016) 3214-3222.
SC R
[38] S.K. Misra, N.K. Pandey, V. Shakya, A. Roy, Application of undoped and Al2O3-doped ZnO nanomaterials as solid-state humidity sensor and its
U
characterization studies, IEEE Sens. J. 15 (2015) 3582-3589.
N
[39] N.K. Pandey, K. Tiwari, A. Roy, A. Mishra, A. Govindan, Ag-loaded WO3
M
Ceram. Technol. 10 (2013) 150-159.
A
ceramic nanomaterials: characterization and moisture sensing studies, Int. J. Appl.
ED
[40] M. Duta, L. Predoana, J.M. Calderon-Moreno, S. Preda, M. Anastasescu, A. Marin, I. Dascalu, P. Chesler, C. Hornoiu, M. Zaharescu, P. Osiceanu, M. Gartner,
PT
Nb-doped TiO2 sol-gel films for CO sensing applications, Mater. Sci. Semicond.
CC E
Process. 42 (2016) 397-404. [41] C. Liu, R. Zhang, S. Wei, Selective removal of H2S from biogas using a referable Hybrid TiO2/Zeolite composite, Fuel 157 (2015) 183-190.
A
[42] L. Zhou, M. Chen, Y. Wang, Y. Su, X. Yang, Au/Mesoporous-TiO2 as catalyst for the oxidation of alcohols to carboxylic acids with molecular oxygen in water, Appl. Catal. A 475 (2014) 347-354. [43] L. Sivachandiran, F. Thevenet, P. Gravejat, A. Rousseau, Investigation of NO and
Page 23 of 42
NO2 adsorption mechanisms on TiO2 at room temperature, Appl. Catal B-Environ. 142 (2013) 196-204. [44] W.D. Lin, C.T. Liao, T.C. Chang, S.H. Chen, R.J. Wu, Humidity sensing properties of novel graphene/TiO2 composites by sol-gel process, Sens. Actuators
IP T
B 209 (2015) 555-561. [45] T. Yang, Y.Z. Yu, L.S. Zhu, X. Wu, X.H. Wang, J. Zhang, Fabrication of silver
SC R
interdigitated electrodes on polyimide films via surface modification and
ion-exchange technique and its flexible humidity sensor application, Sens.
U
Actuators B 208 (2015) 327-333.
N
[46] C.Y. Zhu, F. Xu, L. Zhang, L.M. Li, J. Chen, S.H. Xu, G.G. Huang, W.H. Chen,
A
L.T. Sun, Ultrafast preparation of black phosphorus quantum dots for efficient
M
humidity sensing, Chem. Eur. J. 22 (2016) 7357-7362.
ED
[47]P.-G. Su, H.-C. Shieh, Flexible NO2 sensors fabricated by layer-by-layer covalent anchoring and in situ reduction of graphene oxide, Sens. Actuators B 190 (2014)
PT
865-872.
CC E
[48] P.-G. Su, W.C. Li, J.Y. Tseng, C.J. Ho, Fully transparent and flexible humidity sensors fabricated by layer-by-layer self-assembly of thin film of
A
poly(2-acrylamido-2-methylpropane sulfonate) and its salt complex. Sens. Actuators B 153 (2011) 29-36.
[49] L. Zhuo, Y. Wu, L. Wang, Y. Yu, X. Zhang, F. Zhao, One step hydrothermal synthesis of SnS2/graphene composites as anode material for highly efficient rechargeable lithium ion batteries, RSC Adv. 2 (2012) 5084-5087.
Page 24 of 42
[50] N. Jiang, N. Lu, K.F. Shang, J. Li, Y. Wu, Effects of electrode geometry on the performance of dielectric barrier/packed-bed discharge plasmas in benzene degradation, J. Hazard. Mater. 262 (2013) 387-393. [51] D. Zhang, H. Chang, P. Li, R. Liu, Q. Xue, Fabrication and characterization of an
nanocomposite, Sens. Actuators B 225 (2016) 233-240.
IP T
ultrasensitive humidity sensor based on metal oxide/graphene hybrid
SC R
[52] D. Zhang, N. Yin, B. Xia, Y. Sun, Y. Liao, Z. He, S. Hao, Humidity-sensing
properties of hierarchical ZnO/MWCNTs/ZnO nanocomposite film sensor based
U
on electrostatic layer-by-layer self-assembly, J. Mater. Sci.: Mater. Electron. 27
N
(2016) 2481-2487.
A
[53] J.S. Cai, J.Y. Huang, M.Z. Ge, J. Iocozzia, Z.Q. Lin, K.Q. Zhang, Y.K. Lai,
M
Immobilization of Pt nanoparticles via rapid and reusable electropolymerization of
ED
dopamine on TiO2 nanotube arrays for reversible SERS substrates and nonenzymatic glucose sensors, Small 13 (2017) 1604240
PT
[54] C. Konstantinos, Christoforidis, S. Armelle, K. Valérie, K. Nicolas, Single-step
CC E
synthesis of SnS2 nanosheet-decorated TiO2 anatase nanofibers as efficient photocatalysts for the degradation of gas phase diethylsulfide, ACS Appl. Mater. Interfaces 7 (2015) 19324-19334.
A
[55] Y. Zhou, J.D. Grunwaldt, F. Krumeich, K.B. Zheng, G.R. Chen, J. Stotzel, R. Frahm, G.R. Patzke, Hydrothermal synthesis of Bi6S2O15 nanowires: structural, in situ EXAFS, and humidity-sensing studies, Small 6 (2010) 1173-1179. [56] M. Parthibavarman, V. Hariharan; C. Sekar. High-sensitivity humidity sensor
Page 25 of 42
based on SnO2 nanoparticles synthesized by microwave irradiation method, Mater. Sci. Eng. 31 (2011) 840-844. [57] P. Pascariu, A. Airinei, N. Olaru, I. Petrila, V. Nica, L. Sacarescu, F. Tudorache, Microstructure, electrical and humidity sensor properties of electrospun NiO-SnO2
IP T
nanofibers, Sens. Actuators B 222 (2016) 1024-1031. [58] M. Zhuo, Y. Chen, J. Suna, H. Zhang, D. Guo, H. Zhang, Q. Li, T. Wang, Q. Wan,
SC R
Humidity sensing properties of a single Sb doped SnO2 nanowire field effect transistor, Sens. Actuators B 186 (2013) 78-83.
U
[59] D. Toloman, A. Popa, M. Stan, C. Socaci, A.Biris, G. Katona, F. Tudorache, I.
N
Petrila, F. Iacomi, Reduced graphene oxide decorated with Fe doped SnO2
A
nanoparticles for humidity sensor, Appl. Surf. Sci. 402 (2017) 410-417.
M
[60] H.C. Bi, K.B. Yin, X. Xie, J. Ji, S. Wan, L.T. Sun, M. Terrones, M.S. Dresselhaus,
ED
Ultrahigh humidity sensitivity of graphene oxide, Sci. Rep. 3 (2013) 2714. [61] Y. Tan, K. Yu, T. Yang, Q. Zhang, W. Cong, H. Yin, Z. Zhang, Y. Chen, Z. Zhu,
PT
The combinations of hollow MoS2 micro@nano-spheres: one-Step synthesis,
CC E
excellent photocatalytic and humidity sensing properties, J. Mater. Chem. C 2 (2014) 5422-5430.
A
[62] X.J. Chen, J. Zhang, Z.L. Wang, Q. Yan, S.C. Hui, Humidity sensing behavior of silicon nanowires with hexamethyldisilazane modification, Sens. Actuators B 156 (2011) 631-636. [63] Y. Wang, S. Park, J.T.W. Yeow, A. Langner, F.A Müller, Capacitive humidity sensor based on ordered macroporous silicon with thin film surface coating, Sens.
Page 26 of 42
Actuators B 149 (2010) 136-142. [64] Y. Kim, B. Jung, H. Lee, H. Kim, K. Lee, H. Park, Capacitive humidity sensor design based on anodic aluminum oxide, Sens. Actuators B 141 (2009) 441-446. [65] M.Q. Liu, C. Wang, N.Y. Kim, High-sensitivity and low-hysteresis porous
IP T
MIM-type capacitive humidity sensor using functional polymer mixed with TiO2 microparticles, Sensors 17 (2017) 284-295.
SC R
[66]D. Zhang, J. Liu, C. Jiang, P. Li, Y. Sun, High-performance sulfur dioxide sensing properties of layer-by-layer self-assembled titania-modified graphene hybrid
U
nanocomposite, Sens. Actuators B 245 (2017) 560-567.
N
[67]Y. Liu, P. Geng, J. Wang, Z. Yang, H. Lu, J. Hai, Z. Lu, D. Fan, M. Li, In-situ
A
ion-exchange synthesis Ag2S modified SnS2 nanosheets toward highly
M
photocurrent response and photocatalytic activity, J. Colloid Interface Sci. 512
ED
(2018) 784-791.
[68] N. Agmon, The grotthuss mechanism, Chem. Phys. Lett. 244 (1995) 456-462.
PT
[69] D. Zhang, J. Liu, B. Xia, Layer-by-layer self-assembly of zinc oxide/graphene
CC E
oxide hybrid toward ultrasensitive humidity sensing, IEEE Electron Dev. Lett. 37 (2016) 916-919.
A
[70] K. Jiang, T. Fei, T. Zhang, Humidity sensor using a li-loaded microporous organicpolymer assembled by 1,3,5-trihydroxybenzene and terephthalic aldehyde, RSC Adv. 4 (2014) 28451-28455.
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Biographies
Dongzhi Zhang received his B.S. degree from Shandong University of Technology in 2004, M.S. degree from China University of Petroleum in 2007, and obtained Ph.D.
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degree from South China University of Technology in 2011. He is currently an associate professor at China University of Petroleum (East China), Qingdao, China.
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His fields of interests are gas and humidity sensing materials, nanotechnology, and polymer electronics.
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Xiaoqi Zong received her B.S. degree from China University of Petroleum (East
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China) in 2016. Currently, she is graduate student at China University of Petroleum
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(East China), Qingdao, China. Her fields of interests include nanomaterials-based
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humidity and gas sensors, precision measurement technology.
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Zhenling Wu received his B.S. degree from Qingdao University in 2016. Currently, he is graduate student at China University of Petroleum (East China), Qingdao, China.
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His fields of interests include nanomaterials based gas sensors, precision
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measurement technology.
Yong Zhang received his Ph. D degree in ocean information detection and treatment from Ocean University of China in 2008. Currently, he is an associate professor at
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China University of Petroleum (East China), Qingdao, China. His main research interests are precision measurement technology and instruments.
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Figure captions
Figure 1. (a) Schematic of layer-by-layer fabrication of SnS2/TiO2 film, (b) optical image of the sensor prototype on a flexible PI substrate, and (c) schematic of
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humidity-sensing experimental setup. Figure 2. SEM images of (a-b) SnS2 nanosheets, (c) TiO2 nanospheres, and (d)
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SnS2/TiO2 nanocomposite.
Figure 3. (a-b) TEM images and (c-d) HRTEM lattice fringe images of SnS2/TiO2
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nanocomposite.
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Figure 4. XRD patterns of TiO2, SnS2 and SnS2/TiO2 samples.
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Figure 5. (a) EDS spectrum of SnS2/TiO2 nanocomposite. (b) XPS spectra for survey
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SnS2/TiO2 nanocomposite.
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of SnS2/TiO2 nanocomposite. XPS spectra of (c) S, (d) Sn, (e) O, and (f) Ti of
Figure 6. (a) Impedance of the SnS2/TiO2 film sensor as a function of RH at 100 Hz,
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1k Hz, 10k Hz. (b) Sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100
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Hz, 1k Hz, 10k Hz. (c) Response of the SnS2/TiO2 film sensor versus different RH at 100 Hz, compared with pure SnS2 and TiO2 film sensor. Figure 7. (a) Impedance of the SnS2/TiO2 film sensor upon exposure to various RH
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levels at 100 Hz. (b) Impedance of the SnS2/TiO2 film sensor as a function of RH. (c) Typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. (d) Repeatability of the SnS2/TiO2 film sensor exposed to 52%, 67%, and 75%RH from 11%RH.
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Figure 8. (a) Impedance of the SnS2/TiO2 film sensor for humidity-increasing and humidity-decreasing measurement. (b) Hysteresis of the SnS2/TiO2 film sensor. (c) Long-term stability of the SnS2/TiO2 film sensor. (d) Response of the SnS2/TiO2 film sensor to person’s breath.
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Figure 9. Schematic of humidity sensing for the SnS2/TiO2 film sensor. Figure 10. (a) Complex impedance plots at the range of 11%-97%RH and equivalent
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circuits of the SnS2/TiO2 film sensor at (b) low, (c) medium, and (d) high RH.
Figure 11. Bode diagram of the SnS2/TiO2 film sensor toward (a) 11%RH, (b)
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23%RH, (c) 52%RH, and (d) 97%RH.
Figure 1. (a) Schematic of layer-by-layer fabrication of SnS2/TiO2 film, (b) optical
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image of the sensor prototype on a flexible PI substrate, and (c) schematic of
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humidity-sensing experimental setup.
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SnS2/TiO2 nanocomposite.
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Figure 2. SEM images of (a-b) SnS2 nanosheets, (c) TiO2 nanospheres, and (d)
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Figure 3. (a-b) TEM images and (c-d) HRTEM lattice fringe images of SnS2/TiO2
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nanocomposite.
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Figure 4. XRD patterns of TiO2, SnS2 and SnS2/TiO2 samples.
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Figure 5. (a) EDS spectrum of SnS2/TiO2 nanocomposite. (b) XPS spectra for survey
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of SnS2/TiO2 nanocomposite. XPS spectra of (c) S, (d) Sn, (e) O, and (f) Ti of SnS2/TiO2 nanocomposite.
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Figure 6. (a) Impedance of the SnS2/TiO2 film sensor as a function of RH at 100 Hz, 1k Hz, 10k Hz. (b) Sensitivity of the SnS2/TiO2 film sensor as a function of RH at 100 Hz, 1k Hz, 10k Hz. (c) Response of the SnS2/TiO2 film sensor versus different RH at 100 Hz, compared with pure SnS2 and TiO2 film sensor. Page 36 of 42
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Figure 7. (a) Impedance of the SnS2/TiO2 film sensor upon exposure to various RH
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levels at 100 Hz. (b) Impedance of the SnS2/TiO2 film sensor as a function of RH. (c)
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Typical response and recovery curves of the SnS2/TiO2 film sensor to a RH pulse from 11%RH to other RH levels. (d) Repeatability of the SnS2/TiO2 film sensor exposed to
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52%, 67%, and 75%RH from 11%RH.
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Figure 8. (a) Impedance of the SnS2/TiO2 film sensor for humidity-increasing and humidity-decreasing measurement. (b) Hysteresis of the SnS2/TiO2 film sensor. (c)
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Long-term stability of the SnS2/TiO2 film sensor. (d) Response of the SnS2/TiO2 film
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sensor to person’s breath.
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Figure 9. Schematic of humidity sensing for the SnS2/TiO2 film sensor.
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Figure 10. (a) Complex impedance plots at the range of 11%-97%RH and equivalent circuits of the SnS2/TiO2 film sensor at (b) low, (c) medium, and (d) high RH.
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23%RH, (c) 52%RH, and (d) 97%RH.
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Figure 11. Bode diagram of the SnS2/TiO2 film sensor toward (a) 11%RH, (b)
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Table 1. Humidity sensing properties of the SnS2/TiO2 film sensor compared with previous work. Fabrication method
Meas. range
Sensitivity
Ref.
Bi6S2O15
Suspension dripping
11-95%RH
118928.5 Ω/%RH
[55]
SnO2
Microwave irradiation
5-95%RH
149.89 Ω/%RH
[56]
NiO/SnO2
Electrospinning technique
0-100%RH
3254.22 Ω/%RH
[57]
Sb-SnO2
Vapor liquid solid method
22-44%RH
100454 Ω/%RH
[58]
RGO/Fe-SnO2
Electrostatic interaction
0-100%RH
7.56 Ω/%RH
[59]
Black phosphorus
Spin-coating
10-90%RH
Polyimide
Inkjet-printing
16-90%RH
Graphene oxide (GO)
Solution dripping
15-95%RH
MoS2
Hydrothermal
GO/polyelectrolyte
LbL self-assembly
SnS2/TiO2
LbL self-assembly
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Sensing material
[46]
24.5 pF/%RH
[45]
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124975 Ω/%RH
[60]
17.2−89.5%RH
81.9 pF/%RH
[61]
11−97%RH
1552.3 pF/%RH
[48]
11-97%RH
442000 Ω/%RH
This work
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46.253 pF/%RH
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