Ammonia gas sensors based on In2O3/PANI hetero-nanofibers operating at room temperature

Indium nitrate/polyvinyl pyrrolidone (In(NO3)3/PVP) composite nanofibers were synthesized via electrospinning, and then hollow structure indium oxide (In2O3) nanofibers were obtained through calcination with PVP as template material. In situ polymerization was used to prepare indium oxide/polyaniline (In2O3/PANI) composite nanofibers with different mass ratios of In2O3 to aniline. The structure and morphology of In(NO3)3/PVP, In2O3/PANI composite nanofibers and pure PANI were investigated by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), transmission electron microscopy (TEM) and current–voltage (I–V) measurements. The gas sensing properties of these materials towards NH3 vapor (100 to 1000 ppm) were measured at room temperature. The results revealed that the gas sensing abilities of In2O3/PANI composite nanofibers were better than pure PANI. In addition, the mass ratio of In2O3 to aniline and the p–n heterostructure between In2O3 and PANI influences the sensing performance of the In2O3/PANI composite nanofibers. In this paper, In2O3/PANI composite nanofibers with a mass ratio of 1:2 exhibited the highest response values, excellent selectivity, good repeatability and reversibility.


Introduction
With the development of modern industry, environmental pollution in the form of air pollution, water pollution and soil pollution has become ever more serious [1]. With regard to this, considerable attention has been paid to air pollution. Ammonia (NH 3 ), as a highly toxic gas, can be emitted by natural and industrial sources and threaten human health [2][3][4]. NH 3 at concentrations of 50 ppm may irritate the human respiratory system, skin and eyes [4]. Higher concentrations of NH 3 will cause blindness, seizures, lung disease and even death [5][6][7]. So, there is an urgent need to develop a kind of gas sensor with high sensitivity and selectivity to detect NH 3 at room temperature.
Metal oxide semiconductors can be applied as sensing materials for monitoring NH 3 . Ammonia sensors based on In 2 O 3 [8], TiO 2 [9], SnO 2 [10], ZnO [11] and WO 3 [12] have been reported. Indium oxide (In 2 O 3 ) is an n-type semiconductor with a band gap of approximately 3.55-3.75 eV, which has been widely used due to its excellent electrical and optical properties. In 2 O 3 also exhibits sensitivity to various vapors and gases, such as NO 2 [13], CO [14], H 2 [15], acetone [16] and formaldehyde [17]. However, for most metal oxides, there is the drawback of a required high operation temperature, about 300 °C, which will increase the energy consumption [18]. Compared with metal oxides, sensors based on conducting polymers show low power consumption and can be operated at room temperature. In addition, they exhibit a large specific area, small size and low weight, and they are easy to integrate with existing electronics [19,20]. Because of the environmental stability, easy synthesis and reversible doping behavior, polyaniline (PANI), as one of the most commonly used conducting polymers has received considerable attention. However, the sensitivity of PANI remains to be improved [21,22]. To conquer the limitations mentioned above, the combination of metal oxide and conducting polymers have been developed as an effective way to achieve enhanced performance [21, [23][24][25][26][27].
In this paper, In 2 O 3 /PANI composite nanofibers were prepared by the combination of electrospinning technique, calcination method and in situ polymerization. This study presents the improved response capabilities of gas sensors based on In 2 O 3 / PANI composite nanofibers, which were synthesized with different ratios between In 2 O 3 and aniline during the in situ polymerization. All sensors were tested at room temperature in a concentration range of NH 3 from 100 to 1000 ppm.

Preparation of hollow In 2 O 3 nanofibers
The In(NO 3 ) 3 /PVP composite nanofibers were fabricated via single-nozzle electrospinning. In(NO 3 ) 3 ·4.5H 2 O (1.059 g) and PVP (3.529 g) were added into 10 mL ethyl alcohol and 10 mL DMF. The mixture was stirred at 65 °C until all the solutes were fully dissolved. The precursor solution was poured into the syringe for electrospinning. The parameters of the electrospinning were: a needle-to-collector distance of 16 cm, a voltage of 16 kV, and a feed rate of 0.5 mL/h. Then the In 2 O 3 nanofibers were synthesized by annealing the precursor composite nanofibers at 800 °C for 3 h after heating from room temperature at a rate of 0.5 °C/min.

Preparation of In 2 O 3 /PANI composite nanofibers
Firstly, 0.1 g In 2 O 3 nanofibers, which had been ground in an agate mortar, were added into 200 mL 1.2 mol/L HCl solution with ultrasonication treatment. Then a certain amount of aniline in HCl solution was added to the above suspension. After that, 30 mL 1.2 mol/L HCl solution containing APS was slowly dripped into the suspension to initiate the polymerization. The mass ratios of In 2 O 3 nanofibers to aniline were 1:1, 1:2 and 1:4. The molar ratio of aniline to APS was 1:1. The in situ polymerization of aniline was carried out in an ice bath at 0-5 °C. The reaction lasted for 5 h. The suspension was taken out and left for 30 min, and then washed with deionized water and centrifuged for 5 min. At last, the composite nanofibers were filtered and dried in vacuum at 50 °C for 48 h. The schematic of the preparation of In 2 O 3 /PANI composite nanofibers is illustrated in Figure 1.

Fabrication of In 2 O 3 /PANI gas sensors
The ground In 2 O 3 /PANI nanofibers and pure PANI were mixed with m-cresol to form pastes, in which the weight ratio of In 2 O 3 /PANI or PANI to m-cresol was 1:10. Each paste was coated onto interdigital electrodes to construct a sensing film and dried at 55 °C for 2 h in air. Four thin film sensors with different mass ratios of In 2 O 3 to aniline (0, 1:1, 1:2 and 1:4) were prepared. Correspondingly, the four sensors were denoted as PANI sensor, In 2 O 3 /PANI-1 nanofibers sensor, In 2 O 3 /PANI-2 nanofibers sensor and In 2 O 3 /PANI-3 nanofibers sensor.

Structural characterization and gas sensing test
The crystal structure of In 2 O 3 nanofibers was characterized by X-ray diffraction (XRD; D8, Bruker AXS, Germany) in a 2θ region of 3-90° with Cu Kα radiation. The morphologies and structures of the In 2 O 3 nanofibers, PANI and PANI/In 2 O 3 composite nanofibers were examined by field emission scanning electron microscopy (FESEM, S-4800 and SU-1510, Hitachi, Tokyo, Japan), transmission electron microscopy (TEM; JEM-2100HR, JEOL), and Fourier transform infrared (FTIR) spectroscopy in the range of 4000-400 cm −1 with a 4 cm −1 spectral resolution (NEXUS 470 spectrometer, Nicolet, Madison, WI, USA). I-V measurements were carried out on a CHI 660E electrochemical workstation (CH Instruments, Shanghai, China) with a three-electrode system.
The gas sensing performance of the prepared In 2 O 3 /PANI nanofibers was measured using a custom-built static state gas sensing test system at room temperature (25 ± 1 °C) with a relative humidity of 60 ± 1%. During the gas measurement, the aqueous ammonia was injected into the test chamber using a syringe through a rubber plug. The volume of ammonia injected into chamber were 1.3468 μL, 4.0404 μL, 6.734 μL, 10.7744 μL and 13.468 μL resulting in ammonia vapor with concentrations of 100, 300, 500, 800 and 1000 ppm, respectively. The gas response value (S) is defined as a ratio of (R i − R 0 )/ R 0 , in which R i and R 0 are the resistance of the sensor in testing gas and air, respectively. Each result was the average value of five tests.

Materials characterization
The XRD patterns of the nanofibers obtained by annealing In(NO 3 ) 3 /PVP composite nanofibers at 800 °C are shown in Figure 2a. It can be seen that the crystal phase of the material was In 2 O 3 , and the diffraction peak of 30.56° was indexed to the (222) crystal plane of the cubic structure of In 2 O 3 . This result confirmed that the final product of calcination was In 2 O 3 .
The chemical structure of the precursor nanofibers, In 2 O 3 nanofibers and In 2 O 3 /PANI nanofibers were analyzed by FTIR. As shown in Figure 2b, the FTIR spectrum of In(NO 3 ) 3 /PVP composite nanofibers exhibits a broad characteristic band around 3382 cm −1 , which is related to the O-H stretching vibration. It could be the result of absorbing moisture from air. The -CH 2 stretching vibration and bending vibration of PVP are attributed to the peaks of around 2951 cm −1 and 1423 cm −1 . The peaks at 1647 cm −1 and 1292 cm −1 were assigned to the C=O stretching vibration and C≡N antisymmetrical stretching vibration, respectively, in the ring skeleton of PVP. But the characteristic peaks of In(NO 3 ) 3 could not be found in the FTIR spectra. In the spectrum of In 2 O 3 nanofibers (Figure 2c), the characteristic peaks of PVP have almost vanished. Instead, peaks around 600 cm −1 , 567 cm −1 and 538 cm −1 appeared, which are associated with the cubic bixbyite-type structure of In 2 O 3 . The results indicates that PVP was resolved and In(NO 3 ) 3 was converted into In 2 O 3 during annealing.   Figure 3a, it can be seen that the surface of In(NO 3 ) 3 /PVP composite nanofibers was relatively smooth and no beads and droplets appeared. The diameter distribution of In(NO 3 ) 3 /PVP nanofibers were mostly in the range of 700-1000 nm. The In 2 O 3 nanofibers were relatively uniform with diameters of 150-220 nm. These results show that the In 2 O 3 nanofibers were much rougher and smaller than the precursor nanofibers. In addition, it shows that some nanofibers adhered together in Figure 3b, which is not the case in Figure 3a. It is possible that the solvent was not completely volatilized from the precursor nanofibers membrane. The residual solvent may have re-dissolved the precursor nanofibers and then the dissolved nanofibers were connected along each other during the calcinations. As the calcination temperature increased, the residual solvent started to volatilize and the In 2 O 3 nanofibers were gradually formed, but the bonded nanofibers would not segregate during this process, leading to some nanofibers connected along each other.
The cross-sectional image and the TEM image show the detailed structure of In 2 O 3 nanofibers. In Figure 3c and 3d, it is confirmed that the In 2 O 3 nanofibers consited of small grains and the hollow structure can be clearly observed. The hollow structure of In 2 O 3 nanofibers was synthesized by a templateassisted method. PVP as the supporting material of precursor nanofibers was decomposed during the annealing process, and the In(NO 3 ) 3 was transformed into crystalline In 2 O 3 . During calcination, PVP decomposed and gases diffused from the interior to the exterior of the composite nanofibers, leading to crystalline In 2 O 3 grains constantly moving and tending to be regularly arrayed [16]. As a result, hollow structure of In 2 O 3 nanofibers was formed during the annealing process.
Current-voltage (I-V) measurements of PANI, In 2 O 3 /PANI-1, In 2 O 3 /PANI-2 and In 2 O 3 /PANI-3 nanofibers were carried out at room temperature. As shown in Figure 4, the I-V characteristics of all In 2 O 3 /PANI nanofibers clearly exhibit a nonlinear be-havior and it can be observed the rectifying behavior in Figure 4, which might result from the p-n junction between the p-type PANI and n-type In 2 O 3 [28,29]. It can be observed that the current of In 2 O 3 /PANI showed exponential rise at low voltage and then almost linear rise at high voltages. But for pure PANI, the current showed nearly linear behavior in the forward region, which is attributed to raidply forming polarons and bipolarons in PANI. As some researches mentioned [28][29][30], the ohmic behavior in this case was related to the formation of an ohmic contact between PANI and In 2 O 3 . Compared with pure PANI, the In 2 O 3 /PANI nanofibers reach a higher current due to the smaller width of the depletion layer between PANI and In 2 O 3 . Moreover, the addition of PANI reduced the width of the depletion layer at the interface and was helpful to form a typical ohmic system [31]. Thus, it can be confirmed that a p-n junction between PANI and In 2 O 3 had been formed.

Gas sensing properties
To study the ammonia sensing behavior of the sensors with different ratios of In 2 O 3 to aniline, the dynamic response of the sensors based on pure PANI and In 2 O 3 /PANI nanofibers towards different NH 3 concentrations ranging from 100 to 1000 ppm at room temperature were investigated. As exhibited in Figure 5, it can be seen that the trends of the response and recovery were consistent among the pure PANI and the three In 2 O 3 /PANI nanofibers sensors. The In 2 O 3 /PANI-1 and In 2 O 3 / PANI-2 nanofibers always show a higher response value than pure PANI at the same concentration of NH 3 .
The response values of pure PANI and these three In 2 O 3 /PANI nanofibers sensors to different concentrations of NH 3 are displayed in Figure 6. It can be found that the response values increased with the growth of gas concentration. The responses of pure PANI to 100 ppm, 300 ppm, 500 ppm, 800 ppm, 1000 ppm NH 3   that at low concentrations of NH 3 , the responses of these sensors were similar. But with increasing NH 3 concentrations, it was very clear that the responses of In 2 O 3 /PANI gas sensors were much higher than pure PANI. The response of In 2 O 3 / PANI-2 exhibited the highest value. The response of In 2 O 3 / PANI-2 to 1000 ppm was about twice as large as that of In 2 O 3 / PANI-1. When the weight ratio of In 2 O 3 to aniline was raised to 1:4 (In 2 O 3 /PANI-3), the responses to NH 3 decreased.
These results revealed that the mass ratio of In 2 O 3 to aniline had an obvious influence on the NH 3 sensing performance of the composite nanofibers. Comparing the sensitivity of three In 2 O 3 /PANI nanofibers sensors, it can be found that the In 2 O 3 / PANI-2 nanofibers sensor delivers the best performance. Therefore, In 2 O 3 /PANI-2 was selected to further investigate the sensing properties.
As mentioned in the Introduction section, 50 ppm NH 3 will cause harm to human health. Accordingly, the response of the In 2 O 3 /PANI-2 sensor to 50 ppm, 30 ppm and 10 ppm were investigated. As shown in Figure 7, the response of the In 2 O 3 / PANI-2 sensor to low concentration (10-50 ppm) NH 3 was 0.12, 0.48 and 0.94, respectively. Thus it can be seen the In 2 O 3 / PANI-2 sensor had good response performance towards low concentrations of NH 3 .
The cross-response test was used to evaluate the selectivity of In 2 O 3 /PANI-2 nanofibers sensor. Figure 8 shows the dynamic response of In 2 O 3 /PANI-2 nanofibers sensor to methanol, ethanol, acetone and ammonia at a concentration of 1000 ppm. It is obvious that the In 2 O 3 /PANI-2 nanofibers sensor was almost insensitive to methanol, ethanol and acetone vapors. According to the test results, it can be concluded that In 2 O 3 /PANI-2 nanofibers sensor exhibited unique selectivity to ammonia. A possible mechanism for the selectivity to NH 3 is the chemisorption of NH 3 on PANI in In 2 O 3 /PANI-2 forming ammonium [32]. Besides, the different gases show different electron affinity values [33], and the varying sensitivity of In 2 O 3 /PANI-2 nanofibers to different gases may be explained by this. In 2 O 3 /PANI-2 nanofibers sensor was exposed to 1000 ppm ammonia for five times to investigate the repeatability and reversibility. As shown in Figure 9, the recovery of the In 2 O 3 / PANI-2 nanofibers sensor could not fully return to the initial state, and there was a baseline drift of 4% after the first exposure to NH 3 . This bias was smaller than the results in other reports [34][35][36]. On the other hand, the response of this sensor slightly decreased with the increasing number of tests. The final response reached 47.42, which was about 89% of the first test. Hence, the In 2 O 3 /PANI-2 nanofibers sensor showed good repeatability and reversibility.

Gas sensing mechanism
It is well known that the chemical sensors are composed of two parts, an active part and a transduction part, whose function is sensitive to gas analytes and produces a signal that is related to the concentration [19]. In this study, PANI acted as an active element which can react with NH 3 resulting in the transformation of PANI from emeraldine salt to emeraldine base by dedoping. The reaction between PANI and NH 3 can be described as follows: The absorption of NH 3 caused the deprotonation of the N-H + site of the emeraldine salt, leading to a significantly increased  resistance [37,38]. When this reaction reached equilibrium in NH 3 atmosphere, the resistance of the PANI-based sensor maintained a constant value. When the sensor was exposed to air, NH 3 is volatilized and the resistance of the PANI composite nanofibers is reduced. Therefore, due to the unique mechanism, PANI (emeraldine salt) based sensors exhibited a great selectivity to NH 3 .
As the I-V characteristics show, it is confirmed that p-n heterojunctions had been formed between PANI and In 2 O 3 nano-  fibers, in which PANI is a p-type semiconductor and In 2 O 3 nanofibers presents as an n-type semiconductor [21, 27,34]. The changes of the depletion layer of the p-n heterojuction are shown in Figure 10. The width of the depletion section is related to the doping concentration [39]. With low concentration doping, it needs a sufficiently thick depletion layer to provide impurity atoms to build an internal field. Accordingly, on exposure to NH 3 , the protons from PANI are transferred to the NH 3 molecules, which results in a widening of the depletion layer in the PANI section [40]. Simultaneously, the variation of the PANI region width would have effects on the width of the In 2 O 3 region and on the p-n junction. The electrons of In 2 O 3 and holes of PANI move in opposite directions until the new Fermi level (E F-NH3 ) reaches equilibrium. In this process, the electron transfer between the n-type In 2 O 3 and p-type PANI is obstructed due to the potential barrier. Thus the depletion layer between PANI and In 2 O 3 becomes wider and the resistance of the material increases [27,41,42]. According to the Figure 10: Schematic of p-n junction of In 2 O 3 /PANI nanofibers and its potential energy barrier change when exposed to NH 3 . response definition (S = (R i − R 0 )/R 0 ), the increase in resistance attributed to the p-n junction increases the sensitivity of composite nanofibers sensors. In addition, the sensitivity of the composite gas sensor materials is also connected to the mass ratio of In 2 O 3 and aniline.
Because of the smaller amount of polyaniline, which acts as active material in this composite system, the In 2 O 3 /PANI-1 gas sensor showed a lower sensitivity. However, for the In 2 O 3 / PANI-3 gas sensor, the external surface of In 2 O 3 nanofibers was coated with excess polyaniline, yielding a gas sensing perfomance similar to that of pure PANI. Even the response of In 2 O 3 /PANI-3 to 100 ppm and 500 ppm NH 3 was less than that of pure PANI. The p-n junction of In 2 O 3 and PANI in In 2 O 3 / PANI-3 does not work efficiently. In general, the characteristic of the gas sensitive material response was discrete instead of ideally linear [34,36,43]. For In 2 O 3 /PANI-3 and pure PANI, the characteristic responses were similar. Therefore, it was possible that the response of In 2 O 3 /PANI-3 were less than that of pure PANI, especially towards lower concentrations of NH 3 . In this study, when the mass ratio of In 2 O 3 to aniline was 1:2, the gas sensor material exhibited optimum performance in detecting NH 3 .

Conclusion
In 2 O 3 /PANI nanofibers with reliable sensing properties towards NH 3 were synthesized by electrospinning, calcination and in situ polymerization. The gas sensors based on In 2 O 3 /PANI nanofibers exhibited a higher sensitivity than pure PANI. The In 2 O 3 /PANI-2 nanofiber sensor exhibited the best sensitivity to NH 3 vapor at room temperature, and this sensor was further investigated for its selectivity by interfering with methanol, ethanol and acetone vapors. The results indicated that the In 2 O 3 / PANI-2 nanofiber sensor had excellent selectivity, good repeatability and reversibility. The enhancement of gas sensing performance of In 2 O 3 /PANI nanofiber sensor may be attributed to formation of a p-n junction between In 2 O 3 and PANI, which existence is confirmed by the I-V characteristics.