Room temperature triethylamine sensing properties of polyaniline–WO₃ nanocomposites with p–n heterojunctions

Abstract

Polyaniline (PANI)–tungsten oxide (WO₃) nanocomposites have been successfully synthesized using different weight percentages of tungsten oxide (10–50%) dispersed in a polyaniline matrix by a facile in situ chemical oxidation polymerization. The sensors based on PANI–WO₃ nanocomposites were fabricated on a substrate of polyethylene terephthalate (PET) films for detection of triethylamine (TEA) gas at room temperature. It was observed that the sensors of PANI–WO₃ nanocomposites show better sensitivity, selectivity, and reproducibility compared to pure PANI, particularly the sensor based on PANI–30% WO₃ operating at room temperature exhibits maximum response of 81 to 100 ppm TEA gas, that is 13 times higher than that of pure PANI. The sensing mechanism of the nanocomposites in the presence of TEA atmosphere was discussed in detail, and is attributed to the increase of percentage of doping protonic acid and the formation of p–n heterojunctions between p-type PANI and n-type WO₃.

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1 Introduction

Triethylamine (TEA) is a kind of volatile organic compound (VOC), which has been widely used as an organic solvent and catalyst in industrial production.^1,2 It is also released as a gas during the decay of fishes and seashells after death.^3,4 It can cause great damage to human health, causing headaches, skin burns, nausea, eye irritation and especially respiratory difficulty due to strong pungency resulting in pulmonary edema and even death. The permissible exposure limit of TEA that is recommended in the workplace is 10 ppm by the National Institute for Occupational Safety and Health (NIOSH).⁵ Although some traditional methods like gas detection tube, gas/liquid chromatography, electrochemistry analysis, and colorimetric method are powerful for TEA detection, the complex detection process and expensive equipment hinder their wide applications.^6,7 Thus accurate and fast detection of TEA, especially sensors with high sensitivity is always in great demand in biomedical, chemical and food industries.

Tungsten oxide (WO₃) is well-known as a n-type semiconductor sensing material with a band-gap of 2.6–3.0 eV,⁸ and has sensing response to some of toxic gases due to its non-stoichiometry.^9,10 But the application of pure WO₃ sensing material is limited by its high operating temperature, high power consumption, safety hazards and low selectivity. Compared with traditional inorganic semiconductor oxide materials, conducting polymers such as polyaniline (PANI), polypyrrole (PPy) and polythiopene (PTh), have attracted increasing attentions over the past decade. The most attractive character of conducting polymer gas sensors is that the response to a wide range of VOCs at ambient temperature. Among various conducting polymers, polyaniline (PANI) is of great interest because PANI shows diverse chemical structures and various response mechanisms upon exposure to different gases.^11–14 However, these conducting polymers usually show a poor thermal, mechanical, oxidative stability and low sensitivity to detect gas that limits their potential applications in future.^15–17 In order to overcome these problems, several improved methods have been studied. For example, by morphological design of PANI, doping of noble metals, and incorporating inorganic metal oxide or carbon based materials to enhance the sensitivity of PANI for specific applications in the sensors.¹⁸ Patil et al.¹⁹ prepared room-temperature ammonia sensors based on copper nanoparticle intercalated polyaniline nanocomposite thin film, and the sensor containing 0.13 at% of Cu showed the response of 1.86 to 50 ppm NH₃. Bandgar et al.²⁰ designed a polyaniline/iron-oxide (PAni/α-Fe₂O₃) nanocomposite based ammonia sensor and the response of the sensor is 1.39 when exposed to 100 ppm NH₃ gas. Mitesh et al.²¹ synthesized graphene/PANI nanocomposite with the response of 1.12 to 100 ppm toluene at 50 °C. Up to now, there are less reports relatively about TEA gas sensors operated at room temperature. In order to lower the operating temperature and improve the sensitivity of TEA gas sensors, hybridization of the polyaniline and inorganic WO₃ nanoparticles will produce new sensing materials, and their sensing performance will exceed the constituent counterparts due to the synergistic effect between the polyaniline and metal oxide.^15,22,23

In the present work, we have prepared a series of PANI–WO₃ nanocomposites and loaded on flexible polyethylene terephthalate (PET) films to fabricate the smart sensors for detection of different concentrations of TEA at room temperature (25 °C). The sensing mechanism of the material to TEA and the key factor enhanced gas sensing were discussed by XPS. The work may open the window for PANI/metal oxide composites in applications of the area of micro/nano-electronic devices.

2 Experimental details

2.1 Preparation of WO₃ nanoparticles

WO₃ nanoparticles were synthesized by a colloidal chemical method based on sodium tungstate dehydrate (Na₂WO₄·2H₂O, analytical grade). 1 g Na₂WO₄·2H₂O was dissolved into 60 mL deionized water. Then a certain concentration of aqueous solution of HCl was dropwise added to the sodium tungstate solution under stirring at room temperature till no white precipitate was further formed. Then the solution was aged for 24 h, after which a quantity of cetyltrimethyl ammonium bromide (CTAB) solution (15 mL, 0.15 mol L⁻¹) was immediately added to the solution and abundant white flocculent precipitate was formed. The white flocculent precipitate was treated by ultrasonication for 40 min and after that the precipitate was filtrated and centrifuged with deionized water and ethanol to remove Cl⁻, Br⁻ and any other possible remnants. At last the precipitate was dried at 50 °C and calcined at 600 °C for 2 h to obtain the WO₃ nanoparticles.

2.2 Synthesis of PANI–WO₃ nanocomposites

The PET (1 cm × 1 cm) films as the sensor substrates were pretreated in a 0.02 g mL⁻¹ NaOH solution at 95 °C for 90 min and dried with ethanol at 50 °C. Aniline hydrochloride, ammonium peroxydisulfate (APS), hydrochloric acid and N-methyl-2-pyrrolidone (NMP) were all of analytical grade and were used without further purification. The PANI–WO₃ nanocomposites were synthesized by a facile in situ chemical oxidation polymerization of aniline with APS as the oxidant in the presence of WO₃ nanoparticles. In a typical synthetic procedure, 1.5 mmol aniline monomer and different weight percentage of WO₃ powder (e.g. 0%, 10%, 20%, 30%, 40% and 50%) were separately dissolved in 15 mL 1 M hydrochloric acid solution. Then the solution was dealt with ultrasonication for 15 min. Another 15 mL 1 M hydrochloric acid solution containing the same mol APS which was cooled in ice cold solution in advance was immediately added to the above solution with continuous stirring at 0–5 °C for 30 min. The products were filtered and washed with deionized water, ethanol and 1 M HCl sequentially. Finally, the precipitates were dried in air at 50 °C and a series of PANI–WO₃ nanocomposites were prepared. Appropriate amount of the above obtained PANI–WO₃ powders with some amount of NMP (N-methylpyrrolidone) were mixed and grinded adequately in an agate morto to gain a uniform mixture. The obtained mixtures were smeared on the surface of PET films and denoted as PANI, PW10, PW20, PW30, PW40 and PW50 with respect to the corresponding WO₃ weight percentages of PANI. Fig. 1 shows the schematic illustration of the fabrication of the gas sensor based on PANI–WO₃ nanocomposites.

Fig. 1 Schematic diagram of the fabrication process of TEA sensor based on PANI–WO₃ nanocomposites.

2.3 Characterization of the samples

The characterization analysis of PANI–WO₃ samples were carried out using XRD, FTIR, Raman, FESEM, UV-Vis, TG and XPS techniques. For crystal structure analysis, powder X-ray diffraction (XRD) patterns of the products were recorded on a Rigaku D/MAX-2500 X-ray diffractometer at 30 kV and 100 mA with copper Kα radiation (λ = 1.54 Å). Scanning rate of 5° min⁻¹ was applied to record the patterns in range of 10–70° (2θ). Fourier transform infrared (FT-IR) spectra were recorded on a FT-IR absorption spectrometer (Nicolet 6700) at room temperature. The Raman spectra measurements were carried out on a Raman spectrometer (HR-800, λ_exc = 532 nm) at room temperature. The microscopic structure of PET samples observed by Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S-4700 instrument operated at 20.0 kV. The UV-Vis absorption spectra of the samples in methanol solvent were recorded in the range of 200–800 nm with a Shimadzu UV-2550, UV-Vis spectrophotometer (Japan). The thermogravimetric (TG) analysis was carried out in dynamic nitrogen atmosphere (75 mL min⁻¹) with a heating rate of 10 °C min⁻¹ using NETZSCH STA 449 F3 thermal analyzer. X-ray photoelectron spectra (XPS) were recorded on a VG ESCALAB-MK electron spectrometer with Al Kα as the excitation source.

2.4 Sensor fabrication and response measurement

The chemical sensors were constructed using the PET films with PANI–WO₃ nanocomposites which were welded on a special pedestal with six poles by silver pulp. The gas sensing properties of pure PANI and PANI–WO₃ nanocomposites were measured using a JF02E gas sensor test system (Guiyang Jinfeng Technology Co., Kunming, China). The constructed sensors were installed into an 18 L air chamber. TEA was used as the test gas to test the gas performance of PANI and PANI–WO₃ nanocomposites at room temperature. Test gases with different concentrations were introduced into the air chamber via a syringe. After every test, the chamber should be opened to diffuse test gas away. The resistance of the sensor in air (R_air) and in the air–test gas mixture (R_gas) was recorded, respectively. The response for reducing gas TEA is defined as the ratio of R_gas/R_air. The response time and recovery time of the sensor were measured as the time taken for the sensor output to reach 90% of its saturation after applying or switching off the gas in a step function.

3 Results and discussion

3.1 Structure, morphology and thermal stability of PANI, WO₃ and PANI–WO₃ (PW30)

Fig. 2 depicts the X-ray diffraction pattern of the pure PANI, pure WO₃ nanoparticles and PANI–WO₃ nanocomposites. Fig. 2(a) shows the pure PANI has a broad peak at 2θ = 20–30° without other obvious diffraction peaks.²⁴ Fig. 2(b) represents the X-ray diffraction pattern of WO₃, in which diffraction peaks at 2θ = 23.10°, 23.71°, 24.07°, 26.57°, 28.77°, 34.02°, 41.52°, 47.23°, 49.33° and 55.11° are assigned to the scattering from (001), (020), (200), (120), (111), (220), (221), (002), (400) and (401) planes and well match with the JCPDS card no. 20-1324.²⁵ Compared to pure WO₃, Fig. 2(c) and S1(a)† show the X-ray diffraction pattern of the PANI–WO₃ nanocomposites, which indicates that the nanocomposites appear a small broad peak at 2θ = 20–30° due to the addition of WO₃ nanoparticles during polymerization, and also indicates the structure and property of WO₃ crystalline phase have not been changed.

Fig. 2 XRD patterns of (a) PANI, (b) WO₃ and (c) PANI–WO₃ (PW30).

Fourier transform infrared spectroscopy (FT-IR) as a non-destructive detection method is used to detect the adsorbed impurities in PANI, WO₃ and PANI–WO₃ nanocomposites. Fig. 3(a) and (c) show strong bands in the region 550–1700 cm⁻¹ which are the characteristics of PANI. Fig. 3(a) shows the FT-IR spectrum of the pure PANI, which prepared by in situ chemical oxidation polymerization contains all the characteristic peaks of PANI. The spectrum shows the vibration band of the dopant anion (HCl-PANI) (1124 cm⁻¹), C–N stretching mode in Ar–NH–Ar (1295 cm⁻¹), C=C stretching mode of benzenoid rings (1475 cm⁻¹), C=C stretching mode of the quinoid rings (1558 cm⁻¹), and the C–H bending vibration out of the plane of the paradisubstituted benzene rings (797 cm⁻¹).^26,27 The IR spectra indicate that Fig. 3(a) is pure polyaniline. Fig. 3(b) shows the W–O bond stretching mode appears as a broad band around 843 cm⁻¹. As can be seen, the Fig. 3(c) spectrum exhibits the main characteristic bands of PANI. Because of the IR intensity of the inorganic WO₃ is far weaker than that of the organic PANI and the relative low amount of WO₃ in nanocomposites. The blue-shift of these bands with the addition of WO₃ indicates a synthetic effect of PANI–WO₃.²⁸

Fig. 3 FT-IR spectra of (a) PANI, (b) WO₃ and (c) PANI–WO₃ (PW30).

Raman spectroscopy was employed to study the structural characterization of the products. Fig. 4 shows a selected region of Raman spectra of the pure PANI, WO₃, as well as PANI–WO₃ (PW30). Generally, there are three regions in Raman spectrum of polyaniline that are sensitive to the level of oxidation and the protonation degree in Fig. 4(a): (1) 1167 cm⁻¹, where C–H bending vibrations of benzene or quinone type rings are most prominent; (2) 1482 cm⁻¹ with characteristic C–N, C=N and C∼N⁺ (where ∼ denotes an intermediate bond between a single and a double) stretching vibrations; (3) 1586 cm⁻¹ with the dominating C–C and C=C stretching vibration of benzene and quinone type rings, respectively.^29,30 As expected, Fig. 4(b) depicts the Raman spectrum of WO₃. The intense peaks centered at 721 and 809 cm⁻¹ correspond to the O–W–O stretching modes.

Fig. 4 Raman spectra of (a) PANI, (b) WO₃ and (c) PANI–WO₃ (PW30).

The sharp peaks at 276 and 329 cm⁻¹ are assigned to the O–W–O bending vibration.^31,32 The PANI and WO₃ peaks are still observed in Fig. 4(c), indicating the presence of PANI and WO₃ in the PANI–WO₃ nanocomposites. Compared to pure PANI, the peaks at 1190, 1496 and 1622 cm⁻¹ in PANI–WO₃ are broadened and move upward, probably due to the formation of C–O–W and N–O–W bonds between the PANI and WO₃ nanoparticles. These chemical bonds imply that WO₃ was chemically bonded to PANI via C–O–W and N–O–W bonds, rather than physically interacting with PANI and might be beneficial for effective charge transfer in the gas sensing measurement.

The morphology of PANI, WO₃, PW10 and PW30 are investigated by field emission scanning electron microscopy (FESEM) and typical results are shown in Fig. 5 and S2.† It is found that the pure PANI exhibits some nanofibers and agglomerations. Most WO₃ nanoparticles are uniform with the diameter of 200 nm approximately and a few agglomerations. Fig. 5(c), (d) and S3† are PANI–WO₃ nanocomposites. The PANI was wrapped on the WO₃ nanoparticles by an in situ chemical oxidation polymerization to obtain the composite. When WO₃ nanoparticles immersed into the aniline monomer solution, free aniline or anilinium ion in the bath may adsorb preferably at the surface of WO₃ nanoparticles for the H atoms of –NH₂– groups undergoing some degree of hydrogen bonding with surface O atoms or hydroxyls of WO₃ nanoparticles. Lots of WO₃ nanoparticles are coated on the surface of PANI and enwrapped by PANI, which is contributed to the formation of PANI–WO₃ nanocomposites.

Fig. 5 FESEM images of (a) PANI, (b) WO₃, (c) PW10 and (d) PW30.

The UV-Visible absorption spectra corresponding to pure PANI, WO₃ and PANI–WO₃ (PW30) powder samples dissolved in ethanol are shown in Fig. 6(1). The absorption peaks of PANI powder were recorded at 243 nm and 473 nm in Fig. 6(1a). The absorption band in the UV region is attributed to the chain of the aromatic nuclei and corresponds to the π–π* transitions and the absorption band appearing in the visible region, attributed to an interaction between the benzenic nuclei and the quinone di-imine structure.¹⁷ Fig. 6(1b) had shown an absorption band characteristic of WO₃ at wavelength 243 nm. Fig. 6(1c) showed the PANI–WO₃ nanocomposites have two absorption peaks at 320 nm and 471 nm. Compared with the pure PANI and WO₃, the typical peak of PANI–WO₃ at 243 nm brought about obvious red-shift, which was ascribed to the state-change of polarons from localization to delocalization, suggesting that the conformation of PANI molecular chains changed from compact to extend. Thermal gravimetric analysis curves of PANI and PANI–WO₃ are depicted in Fig. 6(2) and Fig. S1(b).† Pristine PANI exhibits a three-step mass loss, pure WO₃ almost no mass loss while PANI–WO₃ nanocomposites also show homologous three-step mass loss profiles.³³ The first mass loss occurring below 155 °C for PANI and 162 °C for PANI–WO₃ are the elimination of absorbed moisture. The second mass loss occurring in 155–476 °C for PANI and 162–492 °C for PANI–WO₃ are due to the thermal elimination of dopant species and low molecular mass polymer fragments. The third mass loss in 476–701 °C for PANI and 492–709 °C for PANI–WO₃ are attributed to the degradation of the skeletal polyaniline chain structures.²⁷ Compared the decomposition temperature in three mass loss process, it can be seen that the thermal stability of composites is enhanced.

Fig. 6 (1) UV-Vis absorption spectra of (a) PANI, (b) WO₃ and (c) PANI–WO₃ (PW30) and (2) TGA curve of (a) PANI and (b) PANI–WO₃ (PW30).

Surface sensitive X-ray photoelectron spectra (XPS) provide further evidence for the stronger interactions existing in PANI–WO₃ nanocomposites. The W 4f core-level spectra of WO₃ and PANI–WO₃ nanocomposites are shown in Fig. 7(a) and (b). Both of them can be decomposed into two components (W⁶⁺ and W⁵⁺). The peak positions corresponding to W⁶⁺ state occur at 35.1 and 37.2 eV, while for the W⁵⁺ state these occur at 34.6 and 36.4 eV.³⁴ Compared with the pure WO₃, the four peaks of PANI–WO₃ nanocomposites moved to higher binding energy levels and all of them increased 0.5–0.6 eV. The peak shift could be ascribed to the charge transfer between PANI molecular chains and WO₃ through the strong p–n heterojunction interaction. Actually, the molecular formula of the WO₃ could be written as WO_3−x, where x represents the number of oxygen vacancies. By calculating the area ratio of W⁶⁺ to W⁵⁺ from XPS spectra, the x can be confirmed 0.11 for the pure WO₃ and 0.18 for the WO₃ of PANI–WO₃ nanocomposites, which indicated the molecular formula of the pure WO₃ should be written as WO_2.89 and WO_2.82 of PANI–WO₃ nanocomposites. The improvement of the non-stoichiometric feature of PANI–WO₃ nanocomposites implies their potential gas-sensing applications.³⁵ Fig. 7(c) shows N 1s XPS peaks of the pure PANI, which are decomposed into imine nitrogen (–NH–) with a binding energy of 399.3 eV, amine nitrogen (=NH⁺=) with a binding energy of 400.2 eV and positively charged nitrogen (–NH₂⁺–) with a binding energy of 402.1 eV, respectively.³⁶ The percentage of total areas from protonated nitrogen atoms is 60.64%, and the percentage of total protonic acid areas of PANI–WO₃ is 70.51% which significantly higher than that of pure PANI. Thus the addition of WO₃ in the polymerization process improved the percentage of total protonic acid areas and may contribute to improve the gas sensing properties.

Fig. 7 W 4f XPS peaks of (a) WO₃ and (b) PANI–WO₃ (PW30) and N 1s XPS peaks of (c) PANI and (d) PANI–WO₃ (PW30).

3.2 Gas-sensing properties of PANI and PANI–WO₃ hybrids

Fig. 8 shows the responses of the sensors based on the pure PANI and PANI–WO₃ nanocomposites and transient resistances of sensors based on PW30 to TEA examined at room temperature (25 °C) and relative humidity of 40 ± 1% RH. Transient resistances of sensors based on PANI and PANI–WO₃ (PW30) were shown in Fig. 8 and S3.† The response of the PW30 to 100 ppm TEA reached 81, which is 13 times than that of pure PANI, and the latter only reached 6.3. Obviously, compared with pure PANI, the response of PANI–WO₃ sensor has been significantly improved. The response and recovery times are also important parameters for gas sensors, and these were defined as the time to reach 90% of the final equilibrium value after the detected gas was injected and removed, respectively. With the increase of TEA concentration, the response time and recovery time decrease. A comparison of the sensing performances between the sensor fabricated in this work and literature reports is summarized in Table 1. From the table, it can be observed that PANI–WO₃ nanocomposites have correspondingly high gas response and low working temperature. In conclusion, the high gas sensitivity and the fast response-recovery time at room temperature all suggest that PANI–WO₃ nanocomposites can be an ideal candidate for TEA detecting at room temperature.

Fig. 8 Responses and the transient resistances of sensors based on PANI and PANI–WO₃ (PW30) to 10–100 ppm TEA at room temperature.

Table 1 Gas responses to TEA in the present study and those reported in the literatures

Sensor material	TEA (ppm)	Operating temperature	Response	Reference
Hexagonal brick-shaped SnO2	100	160 °C	70	37
NiO/SnO2 hollow spheres	100	220 °C	267.5	38
Ag/Cr2O3 microspheres	50	92 °C	52.9	39
TiO2 nanorod arrays	100	290 °C	14.2	40
Flower-like-MoO3	100	250 °C	416	41
Polyaniline–polypyrrole coaxial nanofibers	100	Room temperature	1.5	42
PANI–WO3 nanocomposites	100	Room temperature	81	Present study

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The responses of sensors of PANI and PANI–WO₃ (PW30) to low concentration TEA were also detected at room temperature as shown in Fig. 9(1). The sensor based on PANI–WO₃ (PW30) still exhibits excellent gas sensing performance in lower TEA gas concentration with respect to pure PANI. The detection limit (D_L), defined as the lower concentration in which the response is 3 times the standard deviation of noise (instrumental detection limit).⁴³ Thus the D_L of PW30 is between 4 ppm and 2 ppm and lower than that of the pure PANI, which indicated PANI–WO₃ nanocomposites have high potential applications in detecting lower TEA level. Fig. 9(2) describes the plot of responses of PANI–WO₃ with different wt% of WO₃ to 100 ppm TEA at t room temperature (25 °C). It can be observed that the response of sensors based on PANI–WO₃ nanocomposites increased with the increase in the weight percentage of WO₃ and optimized for PANI–WO₃ nanocomposites synthesized with the addition of 30 wt% WO₃. A significant enhancement in the response of gas sensor based on PW30 has been observed. With further increase of WO₃ powder during synthesis process the sensor response decreased. The reason for the variation in the sensor response is due to varied morphology. With the addition of WO₃, the surface area will decreased owing to the agglomeration.

Fig. 9 (1) Responses of sensors of PANI and PANI–WO₃ (PW30) to 2–8 ppm TEA at room temperature and (2) plot of responses of PANI–WO₃ nanocomposites with different wt% of WO₃ to 100 ppm TEA at room temperature.

It should be noted that selectivity is an important property in a sensor system. Based on this purpose, Fig. 10 represents the sensor of PANI–WO₃ (PW30) to TEA gas for 100 ppm of 100 ppm of TEA, acetone, toluene, methanol, formaldehyde, ethanol, NH₃ and 20 ppm of TEA and NO₂ at room temperature. It can be obviously seen that the sensor based on the PANI–WO₃ nanocomposites has a larger response to TEA compared to the other gases. The results indicate that the sensor has a good selectivity to TEA at room temperature.

Fig. 10 Response of the sensor based on PANI–WO₃ (PW30) to different tested gases at room temperature.

3.3 Sensing mechanism of PANI–WO₃ nanocomposites

Here, the enhancement of sensing properties to TEA could be ascribed to the increase of the percentage of doping protonic acid, which has been confirmed by XPS analysis. The deprotonation–reprotonation process of PANI plays an important role in the enhancement of the sensing performance after exposure of the sensor to TEA. PANI is protonated by the doping protonic acid during the polymerization process. When TEA is injected into test chamber, PANI in the form of emeraldine salt was reduced to emeraldine base.⁴⁴ Therefore, the holes (polaron and bipolaron sites in PANI) density in the PANI is lowered, resulting in the increase of resistance. Thus the increase of the percentage of doping protonic acid contributes to the improvement of gas sensing properties. Meantime, the experimental results obviously suggested that the nanocomposites exhibit p-type semiconductor sensing properties due to resistance increase in TEA, which should be correlated with a p–n heterojunction formed at the interface between p-type PANI and n-type WO₃. As we all know, WO₃ shows n-type conductivity by electrons, therefore it is inferable that electron mainly flow occurs from n-WO₃ to p-PANI until establishment of equilibrium and make the Fermi level (E_F) equal in n and p regions. The mutual interaction thus produces the p–n heterojunction, as shown in Fig. 11(a).³³ Fig. 11(b) and (c) show the sensing mechanism model at room temperature for the sensor based on p-type release electrons to combine with holes in p-type PANI, leading to a decreased holes concentration. Meanwhile the electron transfer between n-WO₃ and p-PANI is hindered because of the potential barrier in the internal field. Thus, the depletion layer becomes thicker than that of it in air. This further increase the resistance of the sensor based on PANI–WO₃ nanocomposites.^38,45 In short, the response to TEA is greatly improved due to the formation of p–n heterojunction. The resistance of PANI–WO₃ nanocomposites changes so dramatically that it is easy to detect TEA with high sensitivity at room temperature.

Fig. 11 (a) The energy band structure diagram of p-type PANI-n-type WO₃ heterojunction. (b) and (c) Schematic model for the p-type PANI-n-type WO₃ heterojunction based sensors when exposed to air and TEA gas.

4 Conclusions

In summary, WO₃ nanoparticles were prepared by a facile colloidal chemical method and combined with PANI to form PANI–WO₃ nanocomposites by an in situ chemical oxidation polymerization. Then the nanocomposites loaded on the PET film to structure a smart sensor. The response of the sensor based on PANI–30%WO₃ nanocomposites to 100 ppm TEA is 81 at room temperature (25 °C), which is 13 times higher than that of pure PANI and much higher than that of reported other sensing materials to TEA at room temperature. The sensor not only exhibits high sensitivity but also has excellent selectivity to acetone, toluene, methanol, formaldehyde and ethanol, and has flexible, inexpensive and portable characters. The mechanism enhanced PANI to TEA sensing properties could be attributed to the increase of the percentage of protonic acid and the formation of p–n heterojunction between PANI and WO₃. Therefore, it is expected that the PET conducting films with PANI–WO₃ nanocomposites can be potentially used as promising gas sensors for detecting TEA at room temperature.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 21177007 and 51372013), the Fundamental Research Funds for the Central Universities (YS1406), Beijing Engineering Center for Hierarchical Catalysts and Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology.

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Footnote

† Electronic supplementary information (ESI) available: Supporting figures. See DOI: 10.1039/c5ra20843b

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