Fully Stretchable and Humidity-Resistant Quantum Dot Gas Sensors

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Fully Stretchable and Humidity-Resistant Quantum Dot Gas Sensors Zhilong Song, Zhao Huang, Jingyao Liu, Zhixiang Hu, Jianbing Zhang, Guangzu Zhang, Fei Yi, Shenglin Jiang, Jiabiao lian, Jia Yan, Jianfeng Zang, and Huan Liu ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00263 • Publication Date (Web): 08 May 2018 Downloaded from http://pubs.acs.org on May 12, 2018

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Fully Stretchable and Humidity-Resistant Quantum Dot Gas Sensors Zhilong Song1, 2, †, Zhao Huang1, 3, †, Jingyao Liu1, Zhixiang Hu1, Jianbing Zhang1, Guangzu Zhang1, Fei Yi1, Shenglin Jiang1, Jiabiao Lian2, Jia Yan2, Jianfeng Zang1, 3, *, Huan Liu1, 4, * 1

School of Optical and Electronic Information, Engineering Research Center for Functional

Ceramics, Ministry of Education, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei, 430074, P. R. China 2

Institute for Energy Research, Jiangsu University, 301 Xuefu Road, Zhenjiang, Jiangsu,

212013, P. R. China 3

Innovation Institute, Huazhong University of Science and Technology, 1037 Luoyu Road,

Wuhan, Hubei, 430074, P. R. China 4

Shenzhen Huazhong University of Science and Technology Research Institute, Shenzhen

Virtual University Park, Shenzhen 518000, P. R. China ABSTRACT: Stretchable gas sensors that accommodate the shape and motion characteristics of human body are indispensable to a wearable or attachable smart sensing system. However, these gas sensors usually have poor response and recovery kinetics when operated at room temperature, especially suffer from humidity interference and mechanical robustness issues. Here, we demonstrate the first fully stretchable gas sensors which are operated at room temperature with enhanced stability against humidity. We created a crumpled quantum dot (QD) sensing layer on elastomeric substrate with flexible graphene as electrodes. Through the control over the prestrain of the flexible substrate, we achieved a 5.8 times improvement in

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NO2 response at room temperature with desirable stretchability even under 1000 stretch/relax cycles mechanism deformation. The uniformly wavy structural configuration of the crumpled QD gas-sensing layer enabled an improvement in the anti-humidity interference. The sensor response shows a minor vibration of 15.9% at room temperature from relative humidity of 0 to 86.7% compared to that of the flat-film sensors with vibration of 84.2%. The successful assembly of QD solids into a crumpled gas-sensing layer enabled a body-attachable, mechanical robust and humidity-resistant gas sensor, opening up a new pathway to the roomtemperature operable gas sensors which may be implemented in future smart sensing systems such as stretchable electronic nose and multipurpose electronic skin. KEYWORDS: Gas sensor; Stretchable; Quantum dot; Room temperature; Humidity interference


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Human bodies are absolutely surrounded by various gases, coming from our own bodies or ambient environment, which are of vital importance to human safety and life quality. There is an increasing demand for soft gas sensors, being flexible and stretchable, that can be potentially implemented in portable, wearable and disposable electronic devices to enable awareness of healthcare and security risks anytime and anywhere1-3. The successful integration of gas-sensing materials with flexible substrates such as plastics4, polymers5 and paper6 rely on moderate processing and operating temperature conditions. For the most widely used gas-sensing materials like oxide semiconductors, the high temperatures required both for their device fabrication and sensor operation become a serious bottleneck limiting their application in soft gas sensors. For this motivation, many room-temperature gas sensing materials such as graphene7, carbon nanotubes8, organic polymer9, colloidal quantum dots10 have been explored owing to their large surface area and excellent solution processability. Room-temperature operable flexible and even stretchable gas sensors have been demonstrated. For example, NO2-sensitive single-walled carbon nanotubes deposited on paper substrates exhibited a high degree of sensitivity and flexibility, but the recovery time was longer than 7 mins11. Graphene on a polyethersulfone substrate showed a sensitive response toward NO2 at room temperature. Due to the strong chemisorption of NO2 upon the graphene surface, a heating temperature at about 165 oC was required to obtain fast and reproducible sensing performance12. Meanwhile, emerging wearable electronics requires the property of stretchability in addition to flexibility. Fundamental progress of stretchable electronic devices13, 14 have been achieved, leading to new platforms for constructing practical stretchable and body-attachable gas sensors. Recently, Lee et al. demonstrated a highly sensitive stretchable gas sensor composed of polyurethane and reduced graphene oxide on the flexible polydimethylsiloxane (PDMS) substrate, showing excellent sensitivity towards NO2 at room temperature with desirable mechanical sustainability under the static and cyclic stretching tests up to 50% of the strain15. ACS Paragon Plus Environment

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However, it should be noted that these sensors devices were not fully stretchable because that rigid electrodes such as Pt, Au were adopted, which may set a limitation for improving stability against mechanical deformation. Although fundamental progresses have been made towards the development of roomtemperature operable gas sensors employing flexible or stretchable substrates16-21, these gas sensors usually have poor response and recovery kinetics when operated at room temperature. Particularly, water molecules from ambient air prefer to be adsorbed on the surface sensing materials and thus hinder the gas-solid interaction22-24. So far, there are few studies concerned on the humidity stability issue of room-temperature operable gas sensors. In this work, we sought to realize a fully stretchable and humidity-resistant gas sensor by creating a crumpled quantum dot sensing layer onto elastomeric substrate with flexible graphene as electrodes. The excellent solution processability of colloidal quantum dots (CQDs) enables the usage of a wide variety of substrates and offers many degrees of freedom in sensor design25. Through the control over the prestrain of the stretchable substrate, we achieved a wavy structural configuration of the crumpled PbS QD films, which not only provided higher sensitivity and stretchability, but also significantly improved the antihumidity interference ability. Attractively, the device fabrication and sensor operation were both conducted at room temperature. The fully stretchable and humidity resistant gas sensors can be easily attached to the skin and show promise in wearable electronic skin for real-time healthcare and environmental safety monitoring. RESULTS AND DISCUSSION Fabrication of the Fully Stretchable Gas Sensor. In order to construct the fully stretchable gas sensors with crumpled wavy structures, we employed a facile strategy of “prestrain - film deposition - release”26, 27 as depicted in Fig. 1a. PbS CQDs were used due to their excellent gas-sensing properties and solution-processability that enable a roomtemperature films deposition on the soft substrates with good adhesion10. Graphene as ACS Paragon Plus Environment

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electrodes provide excellent stretchability and low sheet resistance to the device. Firstly, a rectangle-shaped elastomer film, VHB acrylic 4910 with thickness of 1 mm, was uniaxially stretched along one in-plane direction by the prestrain. Where L0 is the side length of the undeformed elastomer, and L is the corresponding length in the deformed elastomer. Since the VHB elastomer film is highly stretchable, the prestain = ( − )/ is set in a range from 0 to 200%. Specially, graphene papers (Fig. 1b) with thickness about 1 µm were prepared by vacuum filtration method28 (Fig. S1), and sheet resistance of the graphene papers are ~200 Ω/□. The flexible graphene paper was used as the sensor electrodes. The as-prepared graphene paper was cut into rectangles and then transferred to prestrained elastomer substrate by a dry transfer method. PbS CQDs possess extremely large surface-to-volume ratio with uniform grain size about 5 nm (Fig. 1c) capped with abundant surfactant molecules of oleic acid. Followed by layer-by-layer spin-coating of PbS CQDs and accompanied with film-level surface inorganic salts (NaNO2:NH4Cl=1:3, wt%) treatment at room temperature, flat PbS CQD film was formed. In this way, fully stretchable gas sensors were obtained when the prestrain in the VHB film was relaxed. The release step leads to the spontaneous formation of periodic, wavy structures, which can be bent, stretched, and compressed, similar to an accordion bellows. The whole crumpled film and the cross section of the as-fabricated sensors were characterized by SEM (Fig. S2). Additionally, the crumpling-unfolding process is reversible over multiple cycles under the control of VHB substrate deformation. Structural configurations that combine ultrathin, flexible geometries of such wavy shapes enable mechanical stretchability to these devices, which allows reversible deformity between wavy and planar structures. Meanwhile, our strategy enabled the delicate control covering film structures and stretchability. We take the original unreleased PbS CQD film as the frame of reference, as the uniaxially prestretched VHB substrate coated with PbS CQD film is gradually relax, the apparent length of the PbS CQD film reduces from L at the initial (flat) state to L0 at the ACS Paragon Plus Environment

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present (compressive) state. The macroscopic compressive strain in PbS CQD film along the relaxed direction is defined as26: = ( − )/

(1)

and the compressive strain in PbS CQDs film can be calculated as: = /( + 1)

(2)

where is the prestrain of the substrate. When the compressive strain in the CQDs film reaches a critical value, crumples develop with a final wavelength ()29. Therefore, we can simply regulate the compressive state ( ) of the thin films by the adjustment of the VHB prestrain (), which was favorable for the regulation of the gas-sensing performance according to the actual requirements. By the regulation of VHB prestrain (), five gas sensor devices with the compressive strain ( ) at final (totally relaxed) state of 0, 44.4%, 54.5%, 61.5% and 66.7% were obtained, respectively. Fig. 1d-h shows the SEM images of typical PbS CQD film with different wavy structures at the compressive strain of 0, 44.4%, 54.5%, 61.5% and 66.7%, respectively. We find that the PbS CQD film is flat at = 0, and the final wavelength () of the crumpled PbS CQD films are decreased with the increase of the compressive strain ( ). The calculated from SEM images is about 4.8 μm, 2.5 μm, 1.9 μm, and 1.4 μm at of 44.4%, 54.5%, 61.5% and 66.7%, respectively. The results suggested the key role of compressive strain in determining the crumpled structure of PbS CQD films. The crumpled wavy structure of the film is not only favorable for gas diffusion and adsorption, but also enable the stretchability and humidity stability which will be discussed in detail later.


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Fig. 1 a) Illustration of the fully stretchable gas sensor fabrication procedure; b) SEM image of graphene paper; c) TEM image of the PbS CQDs; d-h) SEM images of the crumpled PbS CQDs films at the uniaxial compressive strain of 0, 44.4%, 54.5%, 61.5% and 66.7%, respectively.

Stretchability of the Fully Stretchable Gas Sensor. The typical fully stretchable and body-attachable gas sensor devices based on crumpled PbS CQD film at = 120% are shown in Fig 2a, b. As shown in Fig. 2a, the as-fabricated sensor devices can attach to finger joint with “rock” and “paper” state that a tensile stain is applied along the stretchable sensors system. Meanwhile, the sensor device exhibits full stretchability under repeatable initial and tensile state condition (Fig. 2b), which exhibits the potential application in wearable gas sensing devices. Simultaneously, the sensor devices are employed for monitoring NO2 gas, one of the major hazardous gases caused by fossil fuel combustion threatening human health and environment safety. The gas sensing properties toward NO2 was detected under static gas injection conditions at room temperature in statically fixed stretching state. Fig. 2c shows real-time sensing curves toward 50 ppm NO2 based on the stretchable gas sensor at the uniaxial compressive strain of 0%, 44.4%, 54.5%, 61.5% and 66.7%, respectively. All these sensors show excellent gas-sensing performance with fast revisable response and recovery kinetics toward NO2 molecules at room temperature, significantly surpassing the existing room temperature NO2 gas sensors reported in literatures10, ACS Paragon Plus Environment

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(Table 1), which 7

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attributes to the high sensing activities of the PbS CQDs and their modest binding energy toward NO2 in reversible adsorption process. NO2 molecules could easily kicks out the originally physisorbed O2 molecules and binds to Pb2+ through O, introducing more chargetransfer-driven p-type doping and resulting in the resistance decrease shown as the gas response.10 It can be seen that compared to the compact flat films, PbS CQD-based crumpled films possessed loosely-connected channels, which maybe favorable for gas diffusion and absorption in that the utility factor of film is significantly improved with the crumple structure on the film surface34. As expected, the sensors with crumpled structures ( = 44.4%, 54.5%, 61.5% and 66.7%) exhibit superior response to that of the flat one ( = 0%) as shown in Fig.2c and Table S1, and the high sensitivity combined with fast response kinetics makes the gas sensors suitable for real-time NO2 detection. Table 1 Room-temperature NO2 sensing performance of sensor devices on various substrates Materials

Method

Substrate

Con. [ppm]

Response

T90/T10

Ref.

3D Graphene

Drop-coating

Al2O3

10

3.78

8 min/53 min

30

WS2/carbon nanofibers

Drop-coating

Al2O3

1

1.15

3.7 min/~20 min

31

NiO Nanosheets

Drop-coating

Al2O3

60

4.05

~240 s/~300 s

32

Ag/rGO

Printing

Polyimide

50

1.82

12 s/20 s

33

PbS CQDs

Spin-coating

Paper (Flexibility)

50

21.7

12 s/37 s

10

Polyurethane/rGO

Electrospinning

PDMS (Stretchability)

5

2.63

120 s/~2.5 h

15

ZnO

Sputtering

PDMS (Stretchability)

9.9

~2.2

80 s/35 min

16

MWNT/SnO2 nanowire

Punching

Ecoflex (Stretchability)

200

1.2

76.6 s/17.8 s

17

PbS CQDs

Spin-coating

VHB (Stretchability)

50

125.0

7 s/22 s

5

21.4

32 s/124 s

This work

Fig. 2d shows the real time sensing curves of the sensor toward different NO2 concentrations at uniaxial tensile strain of 0% (black) and 90% (red), respectively. Typically, the stretchable gas sensor ( = 54.5%) exhibits excellent NO2 exposure/release cycles at uniaxial tensile strain of 0% and 90%, respectively. The sensors response (Ra/Rg) increases ACS Paragon Plus Environment

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with gas concentration increases from 5 ppm to 100 ppm with a saturation tendency after 100 ppm. Exhilaratingly, the response sensitivity of the sensor at uniaxial tensile strain of 0% toward 50 ppm can reach to 108.7, with the response time (T90) and recovery time (T10) of 8 s and 48 s, respectively. In addition, the sensor devices can also work normally at the tensile strain of 90% though with degraded sensing response, which significantly benefit from the specific crumpled structure of the PbS CQD sensing film and graphene electrodes. The successful assembly of QD solids into a crumpled gas-sensing layer open a pathway to the room-temperature operable gas sensors and may find many applications in wearable gasmonitoring systems such as stretchable electronic nose and multipurpose electronic skin. To further confirm the mechanical roubstness of our stretchable sensors at large deformation, we investigated the dynamic sensing curves ( = 54.5%) toward 50 ppm NO2 at different uniaxial tensile strain from 0% to 150%. The slight change of the graphene electrodes resistance has little influence on the sensors signal under the tensile deformation (Fig. S3). Fig. 2e shows the initial resistance and response sensitivity of the sensor at the tensile deformation. We find that the resistance of the sensor increases with the increasing uniaxial tensile strain, and the response degrades slowly from the uniaxial tensile strain 0 to 90%, and then fells down at large deformation. Fortunately, the response to 50 ppm NO2 can maintain at 51.5 with fast response kinetic in 2 s at uniaxial tensile strain of 90%. We speculate it is the deformation of the PbS CQD thin film that accounts for the increasing resistance and sensitivity degradation. SEM images proved the PbS CQD film = 54.5%) at their original state is in crumples (Fig. S4a), and the wavy structure gradually disappears at uniaxial tensile strain of 90% (Fig. S4b), however, cracks are appeared at uniaxial tensile strain of 110% and 150%, respectively (Fig. S4c, d). Satisfactorily, the sensor can even work at large deformation of uniaxial tensile strain of 110% with the sensing response of 19.5 with the response time of 5 s, which is the first fully stretchable gas sensor reported that can normally work at such large deformation. ACS Paragon Plus Environment

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b)

c)

d)

e)

f)

Fig. 2 Stretchability of the fully stretchable gas sensor. a) The photograph of the gas sensor device attached on the finger joint with “paper” and “rock” state; b) the gas sensor device under initial and tensile state condition, respectively; c) Real-time sensing curves toward 50 ppm NO2 based on the stretchable gas sensor at of 0 (flat one), 44.4%, 54.5%, 61.5% and 66.7%, respectively; d) Real-time sensing curves the stretchable gas sensor ( = 54.5%) toward different concentration of NO2 at uniaxial tensile strain of 0% and 90%, respectively; e) The initial resistance and response sensitivity of the stretchable gas sensor ( = 54.5%) toward 50 ppm NO2 at different uniaxial tensile strain; f) Real-time sensing curves of the stretchable gas sensor ( = 54.5%) toward 50 ppm NO2 after different stretch times at the uniaxial tensile strain of 20%.

We further explored the fatigue properties under 200, 500 and 1000 cycles of stretching and releasing state, respectively, and the results are shown in Fig. 2f. The sensor response to 50 ppm of NO2 shows negligible changes even up to 1000 times at the repeated uniaxial tensile strain from 0% to 20% with a minor degradation of 10.7%. The response and excellent reversibility (3 cycles) of the sensor were fully retained at the repeated uniaxial tensile strain. ACS Paragon Plus Environment

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It should be mentioned that the cycling test does not mean the sensors can only undergo 1000 times fatigue deformation. The response/recovery gas-sensing property is well-maintained and the sensitivity can also reach to 6.4 toward 50 ppm NO2 at room temperature under the repeated uniaxial tensile strain even for 8000 times. The exciting finding reveals that the asfabricated stretchable gas sensors possess an excellent and stable gas-sensing behavior under reversible stretching and releasing deformation, showing great potential application in realtime wearable gas-sensing devices. Humidity Stability of the Fully Stretchable Gas Sensors. The anti-humidity interference ability is an important parameter for gas-sensing materials and in sensor design. Water vapor in the atmosphere is apt to adsorb on the surface of sensing materials and causes a deterioration of the sensor performance. Film surface wetability depends on the surface morphology and surface energy, and constructing a hydrophobic surface will prevent/alleviate the water vapor adsorption and improve humidity resistance of the gas sensors device26, 35. Here, the “prestrain - film deposition - release” strategy enables the construction of microand nanoscale hierarchical crumpled structures of the PbS CQD films on the stretchable VHB substrate. As shown in Fig. 3a, b, a water drop paced on top of the crumpled CQDs film gives a static contact angle of 91.08o and 117.05o at of 0 and 61.5%, respectively. Meanwhile, the contact angle of the crumpled CQDs film at of 44.4%, 54.5%, and 66.7% are 97.14o, 106.43o, and 136.57o, respectively (Fig. S5a-c), and the contact angle gradually increases with the increased uniaxial compressive strain (Fig. S5d). The tunable wettability of the crumpled PbS CQDs film was successfully achieved by adjusting of the VHB substrates with different levels of uniaxial prestrains, and the hydrophobicity of the films were greatly improved. We further examined the performance of humidity resistance in real-time NO2 detection, the as-fabricated five devices at different toward 50 ppm NO2 sensing at dry N2 and 55% relative humidity (RH) were carried out, respectively. Fig. 3c show the real time NO2 sensing ACS Paragon Plus Environment

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at different humidity based on the stretchable gas sensor at of 61.5%. The initial resistance of the sensor at dry N2 is 756 MΩ, and the sensor exhibits a fast response and recovery upon NO2 exposure/release at room temperature. When the H2O (g) in and the RH reach to 55%, the resistance shows a slightly vibration and increases to 865 MΩ and the real-time sensing curves upon NO2 exposure/release are well-maintained. Fortunately, the sensor resistance can also return to their original state when N2 in, which exhibits a good humidity resistance of the sensor device compared with other four devices at of 0, 44.4%, 54.5%, and 66.7% shown in Fig. S6, confirming the efficient surface structure regulation for improving the humidity resistance. The resistance (blue) and response (black) of these sensors for NO2 sensing at dry N2 and 55% were extracted from the real-time sensing curves and shown in Fig. 3d, the resistance decreased as increased from 0 to 54.5%, which might attribute to the compact contact between the nanoparticles under the compressive strain favorable for electrons transfer. However, when the compressive strain continued to increase beyond 60%, the wavy crumpled structure of the films gradually become loose and porous as shown in the SEM images (Fig. d-h) and even collapsed at large compressive strain that the electrons transfer become difficult resulting in the increased resistance. But simultaneously, the wavy and porous structure of the films were forming under the increased compressive strain, providing much more channels and active sites beneficial for gas diffusion and adsorption and resulting in higher response. The best sensing-response achieved at = 61.5% could be the result of the comprehensive effect of their crumpled porous structure and the electrons transfer ability. We can also observe that there are little changes of the resistance and sensing response (red circles) indicating that the sensor of = 61.5% possesses the best humidity resistance among these sensor devices. To quantify the humidity resistance of the gas sensing characteristics, we calculated the change ratios of the resistances in air [(R55% - Rdry)/Rdry] and the responses [(S55% - Sdry)/Sdry] (Table S1). The smaller value of the change ratios means the highly ACS Paragon Plus Environment

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improved humidity resistance. The stretchable gas sensor of = 61.5% exhibits the best humidity resistance compared with other four sensor devices, and the change ratio of the resistance and response sensitivity is the smallest with 14.9% and 5.9%, respectively. Meanwhile, the humidity resistance of the sensor of = 61.5% under different tensile strain were also conducted as shown in Fig.S7 and Table S2, the contact angle decreased as the tensile strain increased and the anti-humidity performance turned worsen, confirming the important role of the crumpled structure of the films in the anti-humidity interference ability. Besides, the sensing performance of stretchable gas sensor ( = 61.5%) at all the range relative humidity of 0, 19.9%, 37.8%, 47.6%, 58.1%, 67.4%, and 86.7% were also investigated, respectively. The sensor response toward NO2 exhibits a minor vibration of 15.9% at RH from 0 to 86.7% compared with that of the flat PbS QD-based gas sensor with vibration of 84.2% ( = 0, black line) (Fig. 3e). The real-time NO2 sensing curves at different RH are shown in Fig. 3f, which exhibits fast response/recovery kinetics with no obvious variation compared with that of the flat one in Fig. S8. We speculated that it was the synergistic effect of the wavy hierarchical structure and the hydrophobic groups of the crumpled PbS CQDs films make the stretchable gas sensors ( = 61.5%) exhibit excellent anti-humidity interference ability. First of all, the surface of the PbS CQDs were capped with hydrophobic long-chains of oleic acid and oleylamine significantly reducing the adsorption energy toward H2O molecules. In addition, the controlled crumpling of PbS CQD films leads to selforganized surface structure with controllable size ranging from nanometers to micrometres, it became more rough and difficult for the water molecules to spread out or adsorb on the PbS CQD film, and there exists the optimized electrons transfer ability and suitable crumpled structure at = 61.5%, thereby minimizing the humidity interferences and improving the response stability for target gas detection. These results clearly demonstrate that the crumpled


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PbS QD-based stretchable gas sensor ( = 61.5%) can detect NO2 with a high response, rapid response/recovery kinetics, and improved anti-humidity interference ability.

a)

c)

e)

b)

d)

f)

Fig. 3 Humidity Resistance of the Stretchable Gas Sensor. a, b) Contact angle of the crumpled PbS CQDs films at of 0 and 61.5% respectively; c) Real time sensing curves toward 50 ppm NO2 at dry N2 and 55% relative humidity based on the stretchable gas sensor ( = 61.5%). d) Gas sensing properties toward 50 ppm NO2 at dry N2 and 55% relative humidity based on the stretchable gas sensor at of 0, 28.6%, 37.5%, 44.4%, 54.5%, 61.5% and 66.7%, respectively. e) The response sensitivity toward 50 ppm NO2 at different relative humidity based on the stretchable gas sensor at of 0% and 61.5%, respectively; f) The real-time Sensing curves toward 50 ppm NO2 at different relative humidity based on the stretchable gas sensor at = 61.5%.


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a)

b)

c)

d)

Fig. 4 a, b) Real-time sensing curves and sense response toward different concentration of NO2 at room temperature, c) selectivity and d) long-term stability of the stretchable gas sensors with = 61.5% toward 50 ppm NO2.

Moreover, the champion stretchable gas sensor ( = 61.5%) is particularly attractive for the fast and sensitive response upon NO2 exposure/release cycles of different concentrations (1, 5, 10, 20, 40, 50, 60, 80, 100 and 150 ppm) at 18 oC (Fig. 4a). The sensors response increased with increasing gas concentration in the range of 1-150 ppm with a saturation tendency at higher concentrations, and the dependence of the sensor response on the NO2 gas concentration in the range of 1-20 ppm was approximately linear (Fig. 4b), and the calculated limit of detection (LOD)36 was about 13 ppb. The champion sensor shows high response sensitivity of 125.0 toward 50 ppm NO2 with response and recovery time of 7 s and 22 s, respectively, and the sensor exhibits excellent NO2-sensing selectivity against SO2, NH3, H2S, and ethanol vapor at room temperature (Fig. 4c). The response sensitivity toward 50 ppm SO2, NH3, H2S, and ethanol vapor is about 1.07, 0.72, 0.21 and 0.89, respectively. We propose that it is the modest binding energy of NO2 molecules on the PbS CQD surface which is superior to that of the other gases responsible for the activity and selectivity10. In addition, ACS Paragon Plus Environment

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the response upon 50 ppm NO2 maintains at high level without obvious degradation as long as 20 days (Fig. 4d). These results highlight that our devices possess excellent anti-humidity interference and robust response ability for practical real-time NO2 gas detection at room temperature. CONCLUSIONS We have demonstrated fully stretchable and body-attachable gas sensors with excellent gassensing performance and stretchability, especially with improved humidity resistance at room temperature. Owing to the crumpled structures of PbS CQD sensing-film and graphene electrodes, the stretchable gas sensors exhibit highly sensing response toward NO2 at room temperature with outstanding stretchability at large tensile deformation. We have also controlled the wavy structures of the crumpled QD films by the prestrain of the flexible substrate to achieve a humidity-resistant room-temperature gas sensor for accurate NO2 detection. The advances of such a device structure and geometrical design enable a fully stretchable gas sensor with high sensitivity, desirable stretchability and improved humidity resistance ability, providing potential applications in real-time health and environment monitoring with accuracy and convenience. METHODS Fully Stretchable Sensor Fabrication. A rectangle-shaped elastomer film, VHB acrylic 4910 with thickness of 1 mm, was uniaxially stretched along one in-plane direction by the prestrain and fixed it on a glass substrate. Graphene paper (Fig. S1 in Supporting Information) was cut into rectangles and then transferred to prestrained elastomer substrate along the stretch direction by a dry transfer method. The space between the two parallel graphene electrodes was 1 mm. PbS CQDs (50 mg mL-1 in octane)10 as the sensing materials were spin-coated on the prestrained VHB substrate at 2500 rpm for 30 s to form thin layers of PbS, and then treated with inorganic salts (NaNO2:NH4Cl=1:3, wt%) for 45 s and washed with methanol thrice, the procedure was repeated twice. Thereafter, a fully stretchable gas ACS Paragon Plus Environment

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sensor was obtained when the prestrain in the VHB film was relaxed. Controlling VHB prestrain ( ), five gas sensor devices with the compressive strain ( ) state of 0, 44.4%, 54.5%, 61.5% and 66.7% were obtained, respectively. Microstructure Characterization and Sensing Test. High-resolution transmission electron microscopy (HRTEM) was performed on a TEM system (JEOL, Model JEOL-2100), using an accelerating voltage of 200 kV. SEM images were obtained by a FEI Sirion 200 scanning electron microscope. Static water contact angle experiment of a pure water droplet placed on the film surface was carried out using the contact angle meter equipped with a CCD camera (Ramehart Instrument Co., USA). The gas sensors were tested by a commercial computer-connected Keithley 2450 source meter (Keithley Instrument, USA) system under static conditions at room temperature. The sensor response was defined as the ratio of Ra to

Rg, where Ra is the baseline resistance in presence of clean air and Rg the resistance of the sensor device in presence of target gas. The response time (T90) is set as the time to reach 90% of the final plateau value of response sensitivity, and the recovery time (T10) is set as time to return to 10% of the baseline response value. ASSOCIATED CONTENT Supporting Information Graphene paper preparation (Fig. S1), SEM images of the stretchable gas sensor (Fig. S2), SEM image and stretchability of the crumpled graphene film (Fig. S3), SEM images of the crumple PbS films at different uniaxial tensile strain (Fig. S4), wettability of the crumpled PbS CQDs films (Fig. S5), real time sensing curves of the stretchable gas sensors at different uniaxial compressive strain (Fig. S6), resistance and response sensitivity change ratio based on the different stretchable gas sensors at dry N2 and 55% RH condition (Table S1), contact angle of the stretchable gas sensor ( = 61.5%) at different uniaxial tensile strain (Fig. S7), resistance and response sensitivity based on the stretchable gas sensors ( = 61.5%) of


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different uniaxial tensile strain at dry N2 and 55% RH condition (Table S2), and real time sensing curves toward NO2 at different relative humidity (Fig. S8). AUTHOR INFORMATION Corresponding Author *

Jianfeng Zang: [email protected]

*

Huan Liu: [email protected]

Author Contributions H. L., J. Z., Z. S. and Z. H. designed and directed this study and analyzed the data. Z. S. and Z. H. contributed to all the experimental work. J. L., Z. H., J. Z., G. Z., F. Y. and S. J. assisted in all the experimental work. H. L., J. Z., Z. S. and Z. H. wrote the manuscript. J. L. and J. Y. contributed to the helpful discussion. †

The authors contributed equally to this work.

Notes All authors have given approval to the final version of the manuscript and declare no competing financial interest. ACKNOWLEDGMENTS Research described in this paper was supported by the National Natural Science Foundation of China (61571206 and 51572096) and National Key R&D Program of China (2016YFC0201300 and 2016YFB0402700). H. L. acknowledges the Fund for Research and Development of Science and Technology of Shenzhen (JCYJ20160414102255597). J. Z. acknowledges the National 1000 Talents Program of China. We thank the Analytical and Testing Center of HUST and the Center for Nanoscale Characterization & Devices of WNLO for the characterization support.

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Fully Stretchable and Humidity-Resistant Quantum Dot Gas Sensors

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