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Nano Energy

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Self-powered ammonia nanosensor based on the integration of the gassensor and triboelectric nanogenerator

Siwen Cuia,1, Youbin Zhenga,b,1, Tingting Zhanga, Daoai Wanga,b,⁎, Feng Zhoua, Weimin Liua

a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, ChinabQingdao Center of Resource Chemistry and New Materials, Qingdao 266100, China

A R T I C L E I N F O

Keywords:Triboelectric nanogeneratorPolyaniline nanofibersSelf-poweredAmmonia nanosensor

A B S T R A C T

A new self-powered ammonia (NH3) nanosensor with flexibility, portability, good selectivity and sensitivity hasbeen developed from conducting polyaniline nanofibers (PANI NFs) based triboelectric nanogenerator (TENG).The power supply and gas sensor have been successfully integrated into one device. The PANI NFs with NH3

sensing property work both as a frictional layer and an electrode in the TENG. The TENG shows high outputperformance with the maximum short current circuit of 45.70 μA and output voltage of 1186 V in air, while itsoutput voltage is obviously reduced in varying degrees after being exposed to NH3 with different concentrations,resulting from the change of electroconductivity of PANI, which is the design principle of the NH3 sensor.Meanwhile, this NH3 nanosensor exhibits good selectivity and sensitivity with the limit detection of 500 ppm atroom temperature. This work proposes a new thought to design the self-powered NH3 nanosensor, which has thewidespread application prospect to harvest ambient energy for detecting toxic NH3 without any external powersources.

1. Introduction

Today atmospheric pollution problem is becoming more and moreserious and has already threatened human survival and development[1,2]. Ammonia (NH3), one of chief culprits for air pollution, has hightoxicity and strong corrosiveness, resulting in severe damage to humanhealth and even death at high concentrations when it is released in theindustrial production and daily life [3]. Thus, it is very essential todevelop a portable and highly sensitive NH3 sensor for the real-timedetection of NH3 concentration in industrial and living environments.

Until now, the most commonly used NH3 sensing materials havebeen mainly focused on semiconductors (WO3 [4], ZnO [5], In2O3 [6],SnO2 [7], etc.) and conducting polymers including polypyrrole [8],polyaniline (PANI) [9,10], and so on. And the conventional gas sensingmethod is to measure the electrical variation of the NH3 sensing ma-terials. Different from the inorganic semiconductors based NH3 sensorsthat have to operate at high temperature to ensure the sensitivity andselectivity, the conducting polymers as the NH3 sensing materials canwork at room temperature with high sensitivity [11,12]. Among theseconducting polymers, PANI has attracted more attention because of itslow cost, high stability, ease of synthesis, mechanical flexibility andtunable electrical property, which can be reversibly controlled by

protonation/deprotonation process in a single acid or base reaction[13,14]. Once exposed to NH3 environment, the deprotonation of PANIhappens with the conducting emeraldine salt transforming to the non-conducting emeraldine base form. In this process, the change of theelectroconductivity of PANI can be easily measured, which makes itpossible to be used as a remarkable NH3 sensing material [15]. More-over, to speed up the sensor response rate, nanostructured PANI withhigh surface-to-volume ratio has been extensively studied, which wouldenhance the interaction between sensing material and NH3 for highsensitivity and fast adsorption/desorption kinetics of the gas on thesensing materials to obtain a rapid response and recovery [16]. Al-though many studies have been carried out to enhance the sensingperformance in the field of NH3 sensor, some urgent problems likefabrication complexity and high energy consumption are still unsolved[17]. Moreover, the disadvantage without continual working functionfor the commercial NH3 sensors has to be overcome. Therefore, a smartself-powered NH3 sensor with simple preparation, portability andcontinuous working ability is highly expected.

Very recently, triboelectric nanogenerator (TENG), which is workedby the physical contact of two materials with opposite triboelectricpolarity, has attracted much research interest for converting all kinds ofmechanical energies into electricity since it was first invented by Wang

https://doi.org/10.1016/j.nanoen.2018.04.033Received 4 February 2018; Received in revised form 30 March 2018; Accepted 10 April 2018

⁎ Corresponding author at: State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China.

1 The authors equally contributed to this work.E-mail address: wangda@licp.cas.cn (D. Wang).

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Available online 12 April 20182211-2855/ © 2018 Elsevier Ltd. All rights reserved.

T

and coworkers in 2012 [18]. Because of the advantages of low cost,simple fabrication, sustainability and high efficiency, TENG has beenwidely used as a self-powered sensors for effectively detecting the ex-ternal changes using the current and voltage output signals, respec-tively, with applications as pressure sensor [19], motion detection [20],vibration monitor [21], chemical and environmental detection [22,23],and gas sensors [24–27]. Such a sensor can be operated without apower to drive it, which is completely different from the conventionalsensors. However, for most of the gas sensors based on TENGs, TENGsas the power supply and gas sensor measurement are separated. TENGsand the gas sensors have not been integrated into one device, which goagainst the requirements of the portability and convenience in practicaluse.

In this work, conducting PANI based TENG as a self-powered gasnanosensor has been designed for detecting NH3 at room temperature.PANI nanofibers (PANI NFs) as a common NH3 sensing material wereprepared by a dilute chemical polymerization method [28], and thenassembled with a polyvinylidene fluoride (PVDF) triboelectrode toconstruct TENG. Two forms of PANI, conducting emeraldine salt (C-PANI) and non-conducting emeraldine base (N-PANI), were used toform TENGs, respectively. When they act both as frictional layer andelectrode, the output performance of these two TENGs is considerablydifferent due to the dissimilar electroconductivity of PANI. By using thisproperty, we designed the self-powered NH3 nanosensor. The NH3

sensing property of the PANI based TENG can be attributed to thetransformation of C-PANI to N-PANI through the absorption of NH3 onPANI NFs film, resulting in the change of the output performance of theTENG. Moreover, this type of NH3 nanosensor could be operated byharvesting the mechanical energy in the ambient environment to rea-lize self-powered device, solving the energy supply problem in practicalapplications.

2. Experimental

2.1. Preparation of PANI NFs

PANI NFs were grown on Kapton substrate by a simple dilute che-mical polymerization method [28]. In a typical procedure, 10mmolaniline monomer was dissolved into 1 L of 1M HClO4 aqueous solutionby stirring at 0 °C. Kapton film (thickness of 25 µm) with one sidecovered with the commonplace adhesive tape was placed in the abovesolution. And then 15mmol ammonium persulfate (APS) was addedinto the reaction solution to initiate the polymerization reaction. Afterchemical polymerization for 24 h at 0 °C, the Kapton film was taken outof the solution, and washed with deionized water and dried at roomtemperature. Only one side of the Kapton film was covered with PANINFs when the adhesive tape was peeled off. The prepared PANI NFs onKapton substrate were reversibly deprotonated and protonated bysoaking in 0.1M NH3·H2O and 1M HCl to obtain N-PANI and C-PANI,respectively.

2.2. Fabrication of PANI NFs based TENG

Two kinds of TENG structures were designed with PANI and PVDFtriboelectrodes as the friction pairs. The first was labelled as C-PANI-TENG or N-PANI-TENG in which PANI as both frictional layer andelectrode with a Cu wire connected directly to PANI surface, as shownin Fig. 2a. The second was labelled as C-PANI/TENG or N-PANI/TENGin which PANI only worked as frictional layer and a piece of Cu tapepasted on the other side of the Kapton substrate as electrode, as shownin Fig. 4a. The aim of the two compared designs is also to study thereason for the different output performance of the C-PANI-TENG and N-PANI-TENG. The other triboelectrode was prepared by spin-coatingPVDF on Kapton film with Cu tape as back electrode. Typically, 3.75 g

Fig. 1. (a) Schematic diagram of PANI NFs preparation process on Kapton substrate by a diluted polymerization method. (b–d) FESEM images of PANI NFs. (e)Photographs of protonation and deprotonation process of N-PANI and C-PANI with different colors and transmittances by soaking in HCl and NH3·H2O solutions.

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PVDF power was poured into a solution of 12.75 g acetone and 8.5 g N,N-dimethylacetamide (DMAC) under stirring at 60 °C for 30min to forma transparent viscous solution. And then the obtained PVDF solutionwas spin-coated on a piece of Kapton film (thickness of 25 µm) with therotation speed of 3000 rpm for 30 s. The back side of the Kapton filmwas attached with a piece of Cu tape and a Cu wire was fixed onto theCu tape by using conductive silver epoxy. Subsequently, the PANIsurface and the PVDF triboelectrode were placed face to face, leaving agap between them to form an arched structure. The total effectivecontact area of the TENGs is 25 cm2. The TENGs were both operatedunder the vertically contact/separation modes.

2.3. Characterization

The morphology of the PANI NFs was characterized by a fieldemission scanning electron microscopy (FESEM, JSM-6701F, JEOL Inc.,Japan). The conductivity of two forms of PANI was measured by a four-point probe technique (RTS-8, Four Probes Tech., Guangzhou, China).To measure the output performance of the TENGs, a commercial linearmechanical motor with the operating frequency of 3 Hz was used todrive the TENGs. The short circuit current was measured using a SR570low-noise current amplifier (Stanford Research System) and the outputvoltage was measured by a NI-PCI6259 (National Instruments) with aload resistor of 100MΩ.

2.4. Self-powered NH3 nanosensor

C-PANI-TENG was worked as the self-powered NH3 nanosensor dueto its output performance highly related to NH3 concentration. Thecharacterization of the NH3 nanosensor was carried out in a sealed gaschamber at room temperature. The TENG was driven by the linearmechanical motor at the frequency of 3 Hz and the output voltagesignal was collected by a NI-PCI6259 with a load resistor of 100MΩ.When the output voltage reached to a stable state, NH3 gas with in-creasing concentrations was injected into the gas chamber. After ex-posure to a certain concentration of NH3, the voltage was recorded bythe measurement system in real time. Gas response was defined as theratio of (Vair−Vgas)/Vair, in which Vair and Vgas represent the outputvoltage in air and in NH3 atmosphere, respectively. To analyze the re-sponse time and recovery time of the C-PANI-TENG based NH3 nano-sensor, the real-time continuous responding/ recovering process against1000 ppm NH3 concentration was conducted. To test the gas sensingselectivity, the NH3 nanosensor was also tested with 10,000 ppmethanol, acetone, and isopropanol and methylbenzene under the samecondition.

3. Results and discussion

Polyaniline with nanofiber structure as the triboelectrification ma-terial and NH3 sensing material was synthesized on the Kapton surfaceby a simple dilute chemical polymerization method. The schematic

Fig. 2. Schematic illustrate (a) and photograph (b) of the designed PANI-based TENG with an arch-shaped structure. (c) The detailed working mechanism of PANINFs based TENG based on the coupling effect of the triboelectrification and electrostatic induction.

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diagram of preparation process of PANI NFs is shown in Fig. 1a. Firstly,when very dilute aniline monomer is used, most active nucleation sitesare generated more preferentially on the solid Kapton substrate than inthe bulk solution, which would minimize the interfacial energy barrierfor the further growth of PANI on the Kapton surface. Subsequently,PANI would be also formed in the bulk solution with the consumptionof some reactive aniline cation radicals and oligomeric intermediates,resulting in the inhibition of PANI growth on the substrate. As a result,PANI NFs can be grown vertically from the initial nucleation sites onKapton substrate [28]. The morphology of the prepared PANI NFs ischaracterized using FESEM and the relevant images are presented inFig. 1b–d. It can be seen that the conical nanofibers were uniformlyformed on the surface of Kapton substrate with the top diameter ofabout 70 nm and bottom diameter of about 110 nm. In addition, fromthe cross-section FESEM image of PANI NFs (Fig. 1d), the averagelength of the vertically aligned nanofibers is about 420 nm. Generally,the morphology of the triboelectrode surface has a great effect on theoutput performance of TENG. As the triboelectric material, the nanos-tructured PANI NFs is effective for enhancing the friction contact area

and the triboelectrification property. On the other hand, the preparedPANI NFs with large surface area are also conducive to fast the diffusionof NH3 molecules into the PANI nanofiber surface, resulting in moresensitive and shorter response time as NH3 nanosensor.

PANI is a unique polymer with various redox states. The electricaland optical properties of PANI would be changed dramatically in theprocess of the reversible protonation and deprotonation process byacid/base [15]. As shown in Fig. 1e, N-PANI could be protonated byHCl acid to obtain the C-PANI. Conversely, C-PANI can also be depro-tonated by NH3·H2O to come back to the N-PANI. These two formswould be reversibly transformed by the treatment of acid/base. In ad-dition, different colors and transmittances were presented for these twoforms as shown in Fig. 1e. The C-PANI with high transparency showsemerald while the N-PANI with poor transparency is deep hunter green.According to the electrical conductivity measured by a four-point probetechnique, the resistivity of C-PANI is determined as 0.232Ωm, in-dicating its good conductivity to transfer charges. But the N-PANI ex-hibits very high resistivity of larger than 103 Ωm, which is beyond thelargest measurement range of the four-point probe instrument. Besides,

Fig. 3. Output performance of C-PANI based TENG (C-PANI-TENG) and N-PANI based TENG (N-PANI-TENG) in which PANI as both frictional layer and conductingelectrode. Short circuit current (a, b), charge density (c, d) and output voltage (e, f) of the C-PANI-TENG and N-PANI-TENG, respectively.

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the electroconductivity of PANI changes dramatically during the re-versible change from C-PANI to N-PANI or from N-PANI to C-PANI,which makes PANI as a highly sensitive sensor material.

To design the PANI NFs triboelectrode based TENG, the arch-shapedstructure, as one of the most classical designs, is chosen as shown inFig. 2a due to its simple fabrication, universal feasibility and highperformance [29,30]. Fig. 2a and b show the structural diagram andphotograph of the TENG which utilizes the contact electrification be-tween two triboelectrodes with an arch-shaped gap between PANI andPVDF friction layers. In this type of PANI-based TENG, the PANI NFsgrown on Kapton substrate act as both frictional layer and conductingelectrode. Another triboelectrode is PVDF spin-coated on anotherKapton substrate with Cu tape as conducting electrode on the backside.The working mechanism of the TENG is illustrated by the couplingeffect of triboelectrification and electrostatic induction as shown inFig. 2c (i-v). At the initial state, there is no charges transfer, and noelectrons flow in the external circuit before the contact of the two tri-boelectrodes (Fig. 2c, i). When the two triboelectrodes are pressed byan external force to make them fully contact, the electrons would betransferred from the materials with positive polarity to the one withnegative polarity. Generally, the polymers (PANI) containing nitrogenelement usually develop the positive charge while the fluoropolymers(PVDF) have the negative charge [31]. Therefore, during the frictionprocess the electrons would be injected from PANI into PVDF, leavingpositive charges on the PANI and negative charges on the PVDF(Fig. 2c, ii). Once released, the two triboelectrodes will separate apartdue to the elasticity of the arch-shaped Kapton films, with a potentialdifference established between the two friction electrodes which woulddrive the electrical current to flow from the PANI electrode to the Cuelectrode through external circuit because of the higher potential ofPANI electrode (Fig. 2c, iii). When the separation between the twotriboelectrodes reaches to its limit, the accumulated charges on the twotriboelectrodes will realize an electrical equilibrium and there is nocurrent flow in the external circuit (Fig. 2c, iv). Subsequently, once thetwo triboelectrodes are pressed again, the reversed potential differencewill be built by electrostatic induction, resulting in the electrical cur-rent flowing in a reverse direction from the Cu electrode to PANIelectrode (Fig. 2c, v). Then, a new electrical equilibrium is achievedwhen the two triboelectrodes are completely contacted again (Fig. 2c,ii). Therefore, during the process of the TENG being pressed and re-leased periodically, alternative current and voltage pulse signals wouldbe observed.

To study the influence of the redox state of PANI on the TENGoutput performance, C-PANI NFs triboelectrode and N-PANI NFs

triboelectrode have been used to assemble TENGs, respectively, whichare marked as C-PANI-TENG and N-PANI-TENG. The assembled TENGswith the effective friction contact area of 5 cm×5 cm were operated bya linear motor at a frequency of 3 Hz to simulate the mechanical energyin ambient. The short circuit current, charge density and output voltageof these TENGs are shown in Fig. 3(a, c, e) for C-PANI-TENG andFig. 3(b, d, f) for N-PANI-TENG. It can be seen that C-PANI-TENG ex-hibits much higher output performance than that of N-PANI-TENG. Thepeak value of short circuit current and output voltage of C-PANI-TENGreach to 45.70 μA and 1186 V, respectively, which are hundred timesmore than that of N-PANI-TENG (0.26 μA and 6.70 V). The surfacecharge density is also one of the most important parameters to evaluatethe performance of a TENG because a material figure of merit is thesquare of the charge density [32]. The triboelectric surface chargedensity produced by C-PANI-TENG is up to 87.1 μC/cm2, while thecharge density of N-PANI-TENG is only 4.27 μC/cm2. Therefore, C-PANI-TENG creates much more charges than N-PANI-TENG. In thisstructured TENG, since PANI plays the dual roles of frictional layer andconducting electrode, the huge difference of the output performancebetween C-PANI-TENG and N-PANI-TENG is possibly resulted from twoaspects: first, the ability to produce triboelectric charges during thecontact and separation process; second, the ability to transport elec-trons. In order to explore the real reason for the difference in theiroutput performance, a new structured TENG was designed as shown inFig. 4a. In this device, PANI only acts as frictional layer while Cu tape asthe conducting electrode to transport electrons. Similarly, the C-PANINFs friction electrode and N-PANI NFs friction electrode were used toassemble the TENGs, respectively, namely C-PANI/TENG and N-PANI/TENG, and the other friction electrode is still the PVDF based triboe-lectrode. Obviously, it can been seen in Fig. 4b and c that the peakvalues of short circuit current and output voltage of C-PANI/TENG andN-PANI/TENG are nearly the same. Based on this, we conclude that theC-PANI and N-PANI have no difference in creating triboelectric charges.So the ability to transport electrons become the main reason for thedifferent output performance of C-PANI-TENG and N-PANI-TENGowing to the greatly different electroconductivity of C-PANI and N-PANI.

As a new energy collection and conversion device, TENG with highoutput performance can be used as electricity provider to drive portableelectronics. However, since TENG is operated by repetitively pressingand releasing, the short circuit current signal shows periodic changeand exhibits alternating current behavior. To transform the alternatingcurrent to pulse output in the same direction, a full-wave rectifyingbridge is applied. The rectified short circuit current of C-PANI-TENG is

Fig. 4. Schematic illustrate (a) of the designed PANI based TENG with Cu tape as conducting electrode on the backside. Short circuit current (b) and output voltage(c) of C-PANI/TENG and N-PANI/TENG, respectively.

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Fig. 5. (a) The rectified output current of the C-PANI-TENG. (b) Photograph of 1240 LEDs which are lighted by the C-PANI-TENG. (c) A stability test of C-PANI-TENGunder continuous working for 15,000 cycles.

Fig. 6. (a, b) The output voltage of C-PANI-TENG based nanosensor at room temperaturein air and NH3 with various concentrations(from 500 ppm to 10,000 ppm) (c) The re-lationship between the response of the C-PANI-TENG ((Vair−Vgas)/Vair) and the concentrationof NH3. Inset is the response curve with NH3

concentration ranging from 500 ppm to10,000 ppm. (d) Photographs show that 1240LEDs connected to the same TENG are poweredin air and NH3 atmosphere with concentrationof 3000 ppm, respectively.

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shown in Fig. 5a. It can be seen that the current is in one direction,which can light up 1240 commercial red LEDs as shown in Fig. 5b.Moreover, in order to test the stability of C-PANI-TENG, the TENG wascontinuously worked for 15,000 cycles at the frequency of 3 Hz. Asshown in Fig. 5c, the short circuit current has no decay after 15,000cycles, indicating that C-PANI-TENG has good stability for practicalapplication.

Remarkably, PANI has been the most common NH3 sensing materialby measuring its resistance change in NH3 atmosphere. Given that theresistance change of C-PANI may have an influence on the outputperformance of C-PANI-TENG, we designed a novel self-powered NH3

nanosensor based on PANI-TENG which can harvest mechanical en-ergies from the ambient environment to generate electricity. The sen-sitivity has been an important parameter to assess the performance ofsensor. Generally, the lower detection limit reflects the better perfor-mance of sensor. The NH3 sensing selectivity and response of C-PANI-TENG were measured by exposing the device to NH3 with differentconcentrations. Fig. 6a shows the output voltage of C-PANI-TENG uponexposure to air and NH3 with the concentrations ranging from 500 ppmto 10,000 ppm under the same operating frequency of 3 Hz at roomtemperature. The results show that the output voltage of the TENGshows the highest value in air condition and it has obvious decreasewith increasing of the concentration of NH3, resulting from the trans-formation of C-PANI to N-PANI during the absorption of NH3 moleculeson the surface of C-PANI. The decrease of output voltage here is coin-cidence with the results in Fig. 3e and f. It can be seen that the detectionlimit is 500 ppm. Upon exposure to 0, 500, 750, 1000, 1500, 2000,3000, 4000, 6000, 8000, 10,000 ppm NH3, the output voltage of C-PANI-TENG is 1155.67, 1118.33, 1075.67, 985.67, 769.00, 611.33,458.00, 369.67, 306.33, 259.33, and 220.29 V, respectively, as shownin Fig. 6b. And the output voltage value of TENG after decreasing re-mains at a relatively stable value for thousands of working cycles withconstant NH3 concentration, as shown in Fig. S1. Fig. 6c displays thecorresponding response curve ((Vair−Vgas)/Vair) with a testing rangefrom 500 ppm to 10000 ppm. The response value increases with theincrease of NH3 concentration. Especially, as shown in the inset ofFig. 6c, when the NH3 concentration is lower than 3000 ppm, the sen-sing response greatly enhances with the concentration of NH3, showinga linear relationship. However, at a higher concentration range above3000 ppm, the increase of response is no longer linear, resulting fromthat the gas adsorption sites reduce and a saturation is graduallyformed. In Fig. 6d, 1240 red LEDs are connected to the rectified TENGin air and NH3 atmosphere with concentration of 3000 ppm, respec-tively. The TENG in NH3 atmosphere could not provide enough powerto light up 1240 red LEDs (Video S1).

Supplementary material related to this article can be found online athttp://dx.doi.org/10.1016/j.nanoen.2018.04.033.

To obtain response time and recovery time of the C-PANI-TENGbased NH3 nanosensor device, a real-time continuous responding-

recovering process was taken at room temperature against 1000 ppmNH3 as shown in Fig. 7a. The response time and recovery time aredefined as the time to reach 90% of the final equilibrium value afterinjecting and removing the detected gas, respectively [33]. When theworking atmosphere of the device changes from air to NH3, the outputof the TENG reduces quickly to a relatively stable value within 40 s(response time). After that, pure air is rapidly delivered to remove NH3.The output voltage recovers to the original value within 225 s (recoverytime). The longer recovery time than response time may contribute tothe slower desorption rate than absorption rate of NH3 molecules ontoPANI film. In addition, the selectivity is a significant parameter of gassensor in practical application for the accurate detection of the targetgas. Fig. 7b shows the responses of the NH3 nanosensor upon exposureto 10,000 ppm NH3, ethanol, isopropanol, acetone, and methylbenzene.It can be observed that the C-PANI-TENG based nanosensor has poorresponse to these volatile gases except for NH3, indicating a good se-lectivity for NH3 detection.

The detailed working mechanism of the self-powered NH3 nano-sensor based on C-PANI-TENG is shown in Fig. 8. Firstly, the highspecific surface area of PANI NFs is conducive to the adsorption/des-orption of more NH3 molecules and the enhancement of the sensingproperty. Secondly, PANI NFs has NH3 gas sensing response due to itsunique electroconductivity change during the transformation processfrom C-PANI to N-PANI in NH3 atmosphere. Once it is exposed to NH3

atmosphere, NH3 molecules are easily absorbed onto the C-PANI NFssurface to provide electrons to C-PANI and capture the protons fromN+−H site of C-PANI. In this process, C-PANI form is transformed to N-PANI form, resulting in the decrease of its electroconductivity and theincrease of its resistance. When N-PANI is exposed to air again afterremoving NH3 atmosphere, NH3 would be desorbed and N-PANI form istransformed to C-PANI form again. The whole NH3 adsorption/deso-rption process is reversible so that the electroconductivity of PANIcould be also reversibly changed between C-PANI and N-PANI. Thirdly,because C-PANI works as both frictional layer and electrode of C-PANI-TENG, the electroconductivity change of C-PANI has a significant im-pact on the electron transport and the output performance of the TENG.When C-PANI-TENG is operated in NH3 atmosphere, the electro-conductivity of C-PANI would be reduced and then the output voltagesignal shows obvious drop. Different NH3 concentrations lead to thedecrease of electroconductivity in different degrees. As a result, C-PANI-TENG could be applied as NH3 sensor to detect different NH3

concentrations by monitoring the change of its output voltage.

4. Conclusions

In summary, C-PANI NFs based TENG as a new self-powered NH3

nanosensor was designed through implanting the NH3 sensing PANINFs to TENG. The high specific surface area of PANI NFs is convenientfor the absorption/desorption process of NH3 molecules. The C-PANI-

Fig. 7. (a) Real-time continuous responding/recovering process of the C-PANI-TENG against 1000 ppm NH3. (b) Gas-sensing selectivity of C-PANI-TENG based NH3

nanosensor upon exposure to 10,000 ppm NH3, ethanol, isopropanol, acetone, and methylbenzene.

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TENG as a self-powered NH3 nanosensor not only shows the responsesignals in NH3 detection, but also provides the power for the sensormeasurement process by converting the mechanical energy in ambientinto electric energy without any external power sources. This new NH3

nanosensor exhibits good performance for NH3 detection at roomtemperature. Because the electroconductivity of PANI would bechanged a lot in NH3 atmosphere, the output voltage signal of the C-PANI-TENG is greatly dependent on the NH3 concentrations. The de-tection limit of NH3 concentration is about 500 ppm, showing its goodsensitivity. Moreover, the NH3 nanosensor has poor response to someother volatile gases, indicating its outstanding selectivity for NH3.Therefore, a new self-powered NH3 nanosensor based on C-PANI-TENGwith high sensitivity and selectivity in our work creates a new thoughtto develop the novel NH3 sensor with low-cost fabrication, flexibility,portability and self-powered property.

Acknowledgements

This work was supported by National Natural Science Foundation ofChina (No.51722510, 21603242, 21573259), the Outstanding YouthFund of Gansu Province (1606RJDA31), Qingdao Science andTechnology Plan Application Foundation Research Project (17-1-1–70-JCH) and the “Hundred Talents Program” of Chinese Academy ofSciences (D. Wang).

Appendix A. Supporting information

Supplementary data associated with this article can be found in theonline version at http://dx.doi.org/10.1016/j.nanoen.2018.04.033.

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Siwen Cui received her B.S. (2012) in Chemistry fromHenan University and Ph.D. (2017) in State Key Laboratoryof Solid Lubrication, Lanzhou Institute of Chemical Physics(LICP), Chinese Academy of Sciences (CAS). She mainlyfocuses on the research of fabrication of triboelectric na-nogenerators and self-powered devices.

Fig. 8. Illustration of NH3-sensing mechanism in the C-PANI-TENG system.

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Youbin Zheng received his B.S. (2010) in Electronic Deviceand Materials Engineering and Ph.D. (2015) in MaterialPhysics and Chemistry from Lanzhou University. He iscurrently a research assistant at LICP, CAS. His researchmainly focuses on designing and fabrication of triboelectricgenerators that harvest and convert ambient friction energyinto electricity.

Tingting Zhang received her B.S. (2012) in Chemistry fromNorthwest Normal University and Ph.D. (2017) in State KeyLaboratory of Solid Lubrication, LICP, CAS. She mainly fo-cuses on the research of nanomaterials and nanoenergyconversion technology.

Daoai Wang received his Ph.D. in Lanzhou Institute ofChemical Physics, CAS. From 2009–2010, he worked as aPostdoc in Max Planck Institute of Microstructure Physics.From 2010–2013, he worked as a JSPS researcher in theUniversity of Tokyo. And he is now a professor in LICPsupported by ‘‘Top Hundred Talents’’ Program of ChineseAcademy of Sciences. His research interests include na-noenergy technology, photoelectrochemistry, and marinetribology.

Feng Zhou got his Ph.D. in 2004 in Lanzhou Institute ofChemical Physics. He spent three years (2005–2008) in thedepartment of Chemistry, University of Cambridge as apostdoctoral research associate. And he is currently a pro-fessor and deputy director in State Key Lab of SolidLubrication, LICP, CAS. His research interests are themicro/nanostructured surfaces for lubrication, drag/noisereduction and anti-biofouling applications, high perfor-mance lubricants. Details can be found at: http://www.licp.cas.cn/zfz/.

Weimin Liu got his Ph.D. in Lanzhou Institute of ChemicalPhysics in 1990 and spent a year (1993–1994) in depart-ment of Chemistry, Penn State University, USA. Currently,he is a full Professor and the director in the State KeyLaboratory of Solid Lubrication. He was elected asAcademicians of CAS in 2013. His research interests covertribology of polymer composites, organic and nanoparti-cular lubricating oil additives, ultra-thin lubricating films,and particularly in space tribology.

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