SnO2/SnS2 nanotubes for flexible room-temperature NH3 gas sensors

College of Physics and Mathema Magneto-Photoelectrical Composite and Int Technology Beijing, Beijing 100083, China. Institute & Hospital of Hepatobiliary Surger Surgery of Chinese PLA, Chinese PLA Medic Beijing 100853, China. E-mail: jiangk301@ State Key Laboratory for Superlattice Semiconductors, Chinese Academy of Sci gzshen@semi.ac.cn † Electronic supplementary informa 10.1039/c7ra10537a Cite this: RSC Adv., 2017, 7, 52503


Introduction
Ammonia (NH 3 ), a kind of colorless gas, has been widely used in various elds, including compound fertilizers, biofuels, textiles and paper products.However, this highly toxic gas is not only harmful to public health, but also has a negative effect on the surrounding environment.][9][10] Unfortunately, the exible ammonia sensors reported until now have displayed either relatively low sensing performance or have poor long-term environmental stability mainly caused by the sensing material characteristics.][13] Tin oxide (SnO 2 ), an n-type semiconductor, has been widely applied in gas sensors and exhibits a sensing response to various gases, including CO, 14 CH 4 , 15 NO 2 , 16 and NH 3 , 17 because of its low cost and high electrical conductivity.2][23][24] Combined sensing materials with a particular interface between both crystalline materials can display outstanding sensing performance due to their specic synergistic effect.For example, SnO 2 /SnS 2 heterojunction based sensors exhibited enhanced sensitivity and selectivity to different concentrations of NO 2 at the testing temperature of 80 C. 25 Zhang's group reported an interesting NH 3 sensor based on hybrid Co 3 O 4 /SnO 2 core-shell nanospheres, which displayed a fast response time and good reproducibility to ammonia gas. 26Kim et al. synthesized composite SnO 2 -ZnO nanobers from an electrospinning method and found a superior sensing performance towards H 2 gas. 27erein, by using electrospinning followed by in situ hydrothermal sulfuration routes in the presence of CH 3 CSCH 2 , we prepared tubular SnO 2 /SnS 2 composites and fabricated highperformance exible ammonia gas sensors.As-fabricated NH 3 sensors exhibited better sensing performance than pure SnO 2 nanotubes in terms of sensitivity, response time, and cycle stability.In addition, the exible sensors also showed reliable exibility and mechanical stability, making them ideal candidates for practical sensor applications.

Synthesis of SnO 2 nanotubes and SnO 2 /SnS 2 nanotubes
Tubular SnO 2 precursors were rst prepared from the facile electrospinning process.Typically, SnCl 4 $5H 2 O (3 g) and PVP (2.8 g) were dissolved in an ethanol/DMF mixture solution (26 g) with a weight ratio of 1 : 1 under magnetic stirring.Following vigorous stirring for 12 h at room temperature, the solution was electrospun from a stainless steel needle on an aluminium foil collector placed at a distance of 22 cm with an applied voltage of 20 kV and a constant ow rate of 0.3 ml h À1 .The as-spun SnO 2 precursors were then calcinated at 600 C for 3 h with a heating rate of 0.5 C min À1 , resulting in the formation of pure SnO 2 nanotubes. 28Aer that, the as-synthesized SnO 2 nanotubes were added into 20 ml of 10 vol% acetic acid solution, and 12 mmol of thioacetamide was dissolved in 20 ml of distilled water.Aer mixing both solutions and stirring continuously for 30 min at room temperature, the mixture was transferred into a 50 ml Teon-lined autoclave and heated at 150 C for 3 h.The obtained yellow products were washed with deionized water and ethanol and dried in a vacuum drying oven for 6 h at 100 C.

Characterization and gas sensing measurement
The surface morphology and microstructure of the obtained products were characterized by eld emission scanning electron microscopy (FESEM, SUPRA 55) and transmission electron microscopy (TEM, JEM 2200FS).The crystallographic structures of the products were determined by X-ray diffraction (XRD, DMAX-RB) and electron energy-loss spectroscopy (EELS, SUPRA 55).The chemical composition of the products was analyzed by X-ray photoelectron spectroscopy (XPS Thermo escalab 250XI).The Brunauer-Emmett-Teller (BET) specic surface area of the products was examined by measuring the N 2 adsorptiondesorption isotherm (QS-18, 0.01 M).
Flexible gas sensors were fabricated with the photolithographic process using PET as a substrate.In a typical process, we ground the appropriate SnO 2 /SnS 2 samples with a small amount of ethanol solution to form a paste.Then we spin coated the paste on the PET substrate with an interdigital electrode.The gas-sensing properties of the exible sensors were measured by a CGS-1 TP intelligent gas sensitivity analysis system.The gas-sensing sensitivity was assessed through the response value of the electric resistance, which was dened as S¼ I g /I a , where I a and I g were the sensor current in dry air and in the target gas, respectively.

Results and discussion
The crystal structure and phase of the obtained samples from the in situ hydrothermal sulfuration process with a CH 3 CSNH 2 concentration of 12 mmol at 150 C for 3 h were rst characterized by XRD, as shown in Fig. 1.Clearly, all the diffraction peaks can be indexed to rutile SnO 2 (JCPDS card No. 41-1445) and hexagonal phase SnS 2 (JCPDS card No. 83-1705), indicating the formation of composite SnO 2 /SnS 2 samples.The XRD pattern of the calcinated SnO 2 precursor is also shown in Fig. 1, revealing the formation of a pure SnO 2 sample aer calcination.
To obtain information about the morphology and microstructures of the samples, we studied the samples with SEM and TEM, respectively.Fig. 2a shows the SEM image of the precursor SnO 2 samples before in situ hydrothermal sulfuration.SnO 2 nanotubes with uniform diameters of about 300 nm were prepared on a large scale via the electrospinning/calcination process.When the precursor SnO 2 nanotubes were sulfurated in the presence of CH 3 CSNH 2 at 150 C for 3 h, the resulting SnO 2 /SnS 2 composites retain the nanotube shape of the precursor, as can be seen in the SEM images shown in Fig. 2b  and c, conrming the successful synthesis of composite SnO 2 / SnS 2 nanotubes.X-ray photoelectron spectroscopy (XPS) was employed to analyse the surface chemical compositions and the valence states of the SnO 2 /SnS 2 nanotubes.As shown in Fig. 3a, the obtained full spectrum suggested the existence of several elements, including C, Sn, O, and S. Fig. 3b shows the XPS spectra of Sn 3d and two signals at $487 eV and $497 eV can be attributed to Sn 4+ 3d 3/2 and Sn 4+ 3d 5/2 , respectively.The peaks in Fig. 3c centered at $158 eV and $166 eV can be attributed to the S 2p.The peak of O 1s (Fig. 3d) can be attributed to the O 2À of SnO 2 /SnS 2 composites that appeared at a binding energy of $531 eV.These results further indicated that the as-synthesized samples were composite SnO 2 /SnS 2 nanotubes.
To study the effects of experimental parameters on the nal samples, we performed control experiments at different   We also studied the effect of reaction time on the nal samples by increasing the reaction time from 3 h to 5 h, 7 h, and 9 h and the corresponding SEM images are shown in Fig. 4d-f, respectively.Nanotubes are obtained aer 5 h and the XRD data (Fig. S3a †) conrms that they are still composite SnO 2 /SnS 2 nanotubes.Aer reacting for 7 h, XRD revealed that the sample is composed of both SnO 2 and SnS 2 (Fig. S3b †).However, nanoowers were found coexisting with nanotubes (Fig. 4e).When the reaction time was further prolonged to 9 h, only nanoowers were obtained, as shown in Fig. 4f, and XRD data (Fig. S3c †) revealed they are pure SnS 2 .
Flexible chemical sensors based on nanomaterials with high sensitivity, stability and workability under ambient conditions are of great interest for wearable sensing applications.Aer obtaining the samples, we then fabricated exible gas sensors on PET substrates with interdigitated electrodes to detect ammonia gas using the conventional photolithographic technique.Fig. 5a shows the NH 3 gas sensing response of the gas sensors built on the samples obtained with different CH 3 CSNH 2 concentrations of 8 mmol, 10 mmol, 12 mmol, and 14 mmol, to 100 ppm NH 3 gas at room temperature.From the data, it was found that the sensitivity rst increased and then dropped for the samples treated with increased CH 3 CSNH 2 concentrations.The highest sensitivity is about 2.48 for the sample treated with 12 mmol of CH 3 CSNH 2 .The sensitivity of the samples treated with 12 mmol of CH 3 CSNH 2 at different reaction times was then investigated and the corresponding results are shown in Fig. 5b.This revealed that the sample aer reacting for 3 h showed the best sensitivity with a value of 2.48.All these results suggested that composite SnO 2 /SnS 2 nanotubes aer sulfuration for 3 h in 12 mmol of CH 3 CSNH 2 exhibited the highest sensitivity to NH 3 gas at room temperature, which were therefore chosen as the target material to investigate the gas sensing properties in the following work.
First, we compared the room-temperature NH 3 sensing performance of our SnO 2 /SnS 2 nanotubes with that of pure SnO 2 nanotubes.Fig. 6a and b show the sensing performance of the SnO 2 /SnS 2 nanotubes and the pure SnO 2 nanotubes to NH 3 gas with concentrations of 10-500 ppm at room temperature, respectively.It is clear that both samples showed an obvious response to NH 3 gas.With increased concentration of NH 3 gas, the sensitivity also increased gradually.However, the sensitivity of the SnO 2 /SnS 2 nanotubes is much higher than that of pure SnO 2 nanotubes.For example, when exposed to NH 3 gas with a concentration of 100 ppm, the sensitivity for the SnO 2 /SnS 2 nanotubes is 2.48, while it is 1.25 for pure SnO 2 nanotubes, indicating a great NH 3 gas sensing enhancement.The fast response-recovery time is an important assessment standard for evaluating a good gas sensor.Fig. 6c illustrates the real-time dynamic response curve of the SnO 2 /SnS 2 nanotube sensor to 100 ppm NH 3 at room temperature.The response curve indicated that the sensor has a relatively rapid response to NH 3 gas.0][31] The repeatability of the SnO 2 /SnS 2 nanotube sensor toward 100 ppm NH 3   gas is shown in Fig. 6d.The test was performed under the same conditions for four exposure/recovery cycles.No obvious changes in the response amplitude of response and recovery time were observed, revealing the outstanding stability of our exible composite sensor.
The sensing selectivity of the fabricated exible SnO 2 /SnS 2 nanotube based sensors was investigated by exposing the devices to different volatile organic gases at the same concentration of 500 ppm, including acetone, ethanol, ammonia, toluene, and chloroform.As shown in Fig. 7a, the SnO 2 /SnS 2 nanotube based sensors show much higher response to NH 3 than to other gases at room temperature, indicating that the assynthesized SnO 2 /SnS 2 nanotubes can be chosen to be the sensing material for NH 3 sensors.Fig. 7b further displays the response of the SnO 2 /SnS 2 nanotube based sensors to NH 3 at various concentrations, in which the sensors exhibited a very broad gas sensing range toward NH 3 from 1 to 1000 ppm.The sensing saturation concentration is found to be $500 ppm for the SnO 2 /SnS 2 nanotube sensors.In addition, the minimum detection limit for the SnO 2 /SnS 2 nanotubes to NH 3 is about 1 ppm and the response has a linear relationship with the NH 3 concentration in the low concentration region, as can be seen in the Fig. 7b inset.
Generally, the mechanical exibility of exible gas sensors is very important for potential applications in wearable electronics.Fig. 7c shows the long-term mechanical stability of our exible sensors evaluated by the current ratio aer bending for different times with the initial current.When the nanotube sensor was bent 500, 1000, 1500, 2000, 2500 and 3000 times, the calculated current ratio obviously remained almost constant, conrming the reliable and robust exibility of the device.The inset in Fig. 7c is a photograph of the fabricated ammonia sensor, also demonstrating its good exibility.Fig. 7d further shows the response of our gas sensor to 100 ppm NH 3 under different bending angles of 0, 30, 60, 90, 120 and 150 at room temperature.With an increase in bending angle, the gas sensors still retained a high sensing performance to ammonia at room temperature, indicating the good exibility and mechanical stability of the device.Aer the sensing test, the samples were characterized by SEM again.As shown in Fig. S4 (ESI), † the samples still retain a tubular morphology, demonstrating that the microstructure of the samples is not affected by the sensing event.
Table 1 depicts a brief comparison of the performance of our ammonia gas sensor with that of other sensors.Apparently, compared to other room-temperature NH 3 sensors, including SnO 2 /SnS 2, 38 In 2 O 3 /PANI, 39 CeO 2 /Pani, 40 and Pd/SnO 2 /RGO 41 based devices, our SnO 2 /SnS 2 nanotube sensors show faster response and recovery times to ammonia.Moreover, this SnO 2 / SnS 2 based sensor also exhibits a lower operating time than other oxide based sensors, 26,35,36 demonstrating that this tubular composite based sensor with high sensing performance could be applied to the potential monitoring of ammonia at room temperature.
All the above-mentioned testing results demonstrated that our tubular SnO 2 /SnS 2 composites exhibited excellent sensing performance to ammonia gas at room temperature, including high sensitivity, good selectivity, and outstanding repeatability and stability.Fig. 8 shows the proposed mechanism for the sensing behaviour of SnO 2 /SnS 2 nanotube based sensors in air and in ammonia gas at room temperature.From the energy band structure diagram in Fig. 8 le, a heterojunction is formed at the boundaries of SnO 2 and SnS 2 crystallites in the   33 The depletion layer is widened, leading to an increase in the measured resistance of the sensor.When the sensor is exposed to ammonia gas, NH 3 molecules react with O À on the surface of SnO 2 and SnS 2 as below: The depletion layer is narrowed, leading to a decrease in the measured resistance of the sensor, as shown in Fig. 8 right.In this case, the response of SnO 2 /SnS 2 nanotube based sensors to ammonia is much higher than that of both pure SnO 2 and SnS 2 based sensors at room temperature, which could be attributed to the following two factors.Firstly, the hollow structure SnO 2 / SnS 2 composites with a high aspect ratio provide more active sites for ammonia to adsorb on the surface. 34Secondly, the synergistic effect of both SnO 2 and SnS 2 particles is helpful to the reversible adsorption of more NH 3 gases for an enhanced sensing response.Thus, the sensing response was effectively increased in SnO 2 /SnS 2 nanotubes.

Conclusions
In summary, tubular SnO 2 /SnS 2 composites composed of highly aggregated nanoparticles have been synthesized on the backbones of pristine SnO 2 nanotubes from an in situ hydrothermal sulfuration process in the presence of CH 3 CSNH 2 .Flexible gas sensors based on the SnO 2 /SnS 2 nanotubes were fabricated and exhibited good sensing performance to ammonia at room temperature, including fast sensing response/recovery time, good selectivity and mechanical stability.Besides, the results showed that the composite SnO 2 /SnS 2 nanotube sensors exhibited an enhanced sensing performance towards ammonia gas at room temperature when compared to pristine SnO 2 or SnS 2 , mainly due to the hollow structure and synergistic effect of both grains.Our experimental results highlight that these tubular composites are promising candidates for building ammonia gas sensors at room temperature.

CH 3
CSNH 2 concentrations and different reaction times, respectively.Fig. 4a-c show the SEM images of the synthesized samples with CH 3 CSNH 2 concentrations of 8 mmol, 10 mmol, and 14 mmol at 150 C for 3 h, respectively.From these images, we can see that, similar to the above sample with a CH 3 CSNH 2 concentration of 12 mmol, nanotubes were still formed with CH 3 CSNH 2 concentrations of 8 mmol and 10 mmol except that the sample obtained at a lower CH 3 CSNH 2 concentration was composed of nanoparticles with lower densities.While for the sample at 14 mmol, almost no hollow nanotubes were observed and the sample consisted of nanobers instead.Although samples with different morphologies were obtained under different CH 3 CSNH 2 concentrations, XRD patterns conrmed that all of them are still composite SnO 2 /SnS 2 nanostructures (Fig. S2, ESI †).

Fig. 2
Fig. 2 SEM images of the as-prepared (a) SnO 2 nanotubes and (b, c, g) SnO 2 /SnS 2 nanotubes.(d and e) TEM images and (f) HRTEM image of the SnO 2 /SnS 2 nanotubes.(h-j) Elemental mapping images of each element corresponding to Sn, O, and S, respectively.

Fig. 4
Fig. 4 SEM images of as-synthesized products from the in situ hydrothermal sulfuration process with different CH 3 CSNH 2 concentrations of (a) 8 mmol, (b) 10 mmol, and (c) 14 mmol with the same reaction time; with different reaction times of (d) 5 h, (e) 7 h, and (f) 9 h with the same CH 3 CSNH 2 concentration.

Fig. 6
Fig. 6 Response curves of (a) SnO 2 NTs and (b) SnO 2 /SnS 2 NTs flexible sensors toward different concentrations of ammonia at room temperature.(c) The dynamic response-recovery and (d) cyclic response curves of the SnO 2 /SnS 2 sensor toward 100 ppm of ammonia at room temperature.

Fig. 7
Fig. 7 (a) Response of the flexible SnO 2 /SnS 2 nanotubes based sensors to various gases (500 ppm) at room temperature.(b) Gas response of the sensors to NH 3 with different concentrations at room temperature.(Inset: response of the sensor to 1-50 ppm NH 3 ).Mechanical stability of the flexible SnO 2 /SnS 2 nanotube based sensors with (c) various bending times and (d) various bending angles at room temperature.

Table 1
Comparison of the sensing performance of various NH 3 gas sensors Electrons can transfer from the conduction band of SnS 2 to that of SnO 2 due to the work function of SnS 2 being higher than that of SnO 2 , 32 resulting in the formation of a thin electron depletion layer on the side of SnS 2 and an accumulation layer on the side of SnO 2 .When exposed to air, oxygen molecules physically adsorbed onto the surface of SnO 2 and SnS 2 form O À by capturing electrons from the conduction bands of SnO 2 and SnS 2 .