Graphene/Nickel Oxide-Based Nanocomposite of Polyaniline with Special Reference to Ammonia Sensing.

Polyaniline@graphene/nickel oxide (Pani@GN/NiO), polyaniline/graphene (Pani/GN), and polyaniline/nickel oxide (Pani/NiO) nanocomposites and polyaniline (Pani) were successfully synthesized and tested for ammonia sensing. Pani@GN/NiO, Pani/NiO, Pani/GN, and Pani were characterized using X-ray diffraction, UV-vis spectroscopy, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The as-prepared materials were studied for comparative dc electrical conductivity and the change in their electrical conductivity on exposure to ammonia vapors followed by ambient air at room temperature. It was observed that the incorporation of GN/NiO in Pani showed about 99 times greater amplitude of conductivity change than pure Pani on exposure to ammonia vapors followed by ambient air. The fast response and excellent recovery time could probably be ascribed to the relatively high surface area of the Pani@GN/NiO nanocomposite, proper sensing channels, and efficaciously available active sites. Pani@GN/NiO was observed to show better selectivity toward ammonia because of the comparatively high basic nature of ammonia than other volatile organic compounds tested. The sensing mechanism was explained on the basis of the simple acid-base chemistry of Pani.

Introduction

A variety of materials such as carbon nanomaterials, inorganic semiconductors, and conjugated polymers have been explored to fabricate gas/vapor sensors.[1−7] Conducting polymer-based sensors have high sensitivity because of the large surface area, fast response, lower power consumption, small size, and lightweight. Among them, polyaniline (Pani) is considered to be the most promising and widely applied sensing material because of its low cost, easy preparation, high environmental stability, tuneable electrical properties, and unique functions by controlled charge-transfer processes.[8−10] However, its low sensitivity and inadequate thermal stability still restrict its application in operable sensors. In order to improve the sensing performance of Pani, many efforts have been made by loading the noble metals, semiconductors, and other components to develop composite materials.[11−13] Recently, nickel oxide (NiO) nanoparticles have drawn considerable attention because of their low price, acceptable sensing properties, and good environmental stability. Researchers developed gas sensors based on NiO films which have demonstrated significant sensing properties.[14] Because of its high resistivity, there is still drawback in using NiO in composite materials for practical sensing applications. To overcome this problem, many carbonaceous materials with high electrical conductivity have been introduced to prepare carbon/NiO nanocomposites.[15]

A number of composites of Pani have been reported with metal oxides, metal nanoparticles, graphene (GN), and carbon nanotubes widely used in gas sensing applications.[16]

Also, many other nanocomposites have also been reported as ammonia sensors.[17−21] We have decorated NiO nanoparticles on GN sheets, which increases the resultant surface area. The material obtained GN/NiO nanocomposite is also conductive with enhanced surface area, which may be a suitable material for making conducting nanocomposites with Pani for the purpose of gas/vapor sensing studies.

In this work, the polyaniline@graphene/nickel oxide (Pani@GN/NiO) nanocomposite was selected and studied as an ammonia sensor by examining the dynamic response of electrical conductivity using a simple four-in-line-probe dc electrical conductivity measurement setup.

Results and Discussion

X-ray Diffraction Studies

X-ray diffraction (XRD) spectra of Pani, polyaniline/nickel oxide (Pani/NiO), polyaniline/graphene (Pani/GN), and Pani@GN/NiO nanocomposites are shown in Figure . Figure a exhibits the XRD spectrum of Pani, which has two characteristic broad peaks at 2θ = 20.31° and 25.23°. The XRD spectrum of Pani/GN has broad peaks similar to those for Pani slightly shifted to 2θ = 20.45° and 25.27°, which is attributed to the presence of GN in the Pani/GN nanocomposite, as shown in Figure b. In Figure c, there are five extra sharp peaks along with characteristic broad peaks of Pani (at 2θ = 20.25° and 25.34°), which are observed at 2θ = 37.42°, 43.35°, 62.86°, 75.26°, and 79.42°, respectively, attributed to the presence of NiO nanoparticles in the Pani/NiO nanocomposite. The XRD spectrum of the Pani@GN/NiO nanocomposite is similar to that of the Pani/NiO nanocomposite, as shown in Figure d. The peaks due to Pani at 2θ = 20.05° and 25.69°, respectively, shifted from 2θ = 20.31° and 25.23°, which may be attributed to the interaction of Pani with NiO embedded on GN sheets. The five intense peaks observed in the Pani@GN/NiO nanocomposite at 2θ = 37.13°, 43.48°, 63.12°, 75.39°, and 79.56° may be ascribed to the (111), (200), (322), (311), and (222) planes, respectively, due to the reflection of NiO deposited on GN sheets with a slight shift compared to those in the Pani/NiO nanocomposite, indicating the successful preparation of the Pani@GN/NiO nanocomposite.[22,23]

Figure 1

XRD patterns of (a) Pani, (b) Pani/GN, (c) Pani/NiO, and (d) Pani@GN/NiO.

Scanning Electron Microscopy Studies

Figure presents the scanning electron microscopy (SEM) images of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites. The SEM images of the Pani@GN/NiO nanocomposite demonstrated the sheet-type morphology due to the presence of GN/NiO, as shown in Figure a,b. However, the NiO nanoparticles deposited on GN sheets are not visible here, which indicates that the NiO nanoparticles deposited on GN sheets have a nucleating effect on the polymerization of aniline and are completely covered by the Pani shell. Figure c,d presents the SEM images of Pani, which seems to possess a nanotubelike morphology. Figure e presents the SEM image of the Pani/NiO nanocomposite, in which the polymer matrix is well deposited on NiO nanoparticles and the Pani is agglomerated by several nanoparticles. In Figure f, it may clearly be seen that the Pani is well deposited on GN, giving some sheetlike morphology in the Pani/GN nanocomposite.

Figure 2

SEM micrographs of (a,b) Pani@GN/NiO nanocomposite, (c,d) Pani, (e) Pani/NiO, and (f) Pani/GN.

Transmission Electron Microscopy Studies

The morphologies of Pani@GN/NiO, Pani/NiO, Pani/GN, and Pani were further analyzed by transmission electron microscopy (TEM), as shown in Figure . In this synthesis, we deposited NiO nanoparticles on GN sheet and then aniline monomers were polymerized on the surface of GN/NiO. Figure a shows that the NiO nanoparticles were deposited on GN sheets upon which Pani nanotubes are deposited, signifying the successful incorporation of NiO nanoparticles onto GN sheets for a nanocomposite with Pani nanotubes. A lattice spacing of 0.818 nm can be attributed to the lattice spacing of the (200) plane of the NiO nanoparticles in the Pani@GN/NiO nanocomposite (see the magnified image of Figure a). Figure b shows that the NiO particles are uniformly distributed in the outer shell of the polymer matrix. In Figure c, it can be observed that the short tubes of Pani are precisely deposited on the GN sheets. We also synthesized Pani using the same procedure in the absence of the GN and NiO; short tubes of Pani were obtained, as shown in Figure d. In the light of the above observations of TEM analysis, it can be inferred that the NiO nanoparticles were precisely deposited on GN sheets and covered by Pani nanotubes. Furthermore, to evaluate the Brunauer–Emmett–Teller (BET) surface area, the N2 adsorption–desorption isotherms of the Pani@GN/NiO nanocomposite were measured. The BET surface area of the Pani@GN/NiO nanocomposite was found to be 275 m2 g–1 (Figure S1).

Figure 3

TEM micrographs of (a) Pani@GN/NiO nanocomposite, (b) Pani/NiO nanocomposite, (c) Pani/GN nanocomposite, and (d) Pani.

Optical Studies

The UV–vis absorption spectra of Pani@GN/NiO, Pani/NiO, Pani/GN, and Pani are given in Figure . In Figure a, the band observed at 327 nm for Pani may be attributed to the π–π* electronic transition of benzenoid rings.[24] In the case of the Pani/GN nanocomposite, the band red-shifted to 332 nm from 327 nm may be attributed to the interaction of π-electrons of benzenoid rings with π-bonds of GN, as shown in Figure b.[25] In Figure c, the Pani/NiO nanocomposite illustrates the two bands at 281 and 323 nm, which may be related to the electronic transition from the valence band to the conduction band in the NiO and due to the π–π* transition of the benzenoid rings, respectively.[26,27] In the case of Pani/NiO, a blue shift is observed in contrast to that in Pani, which may be attributed to the decrease in the conjugation of Pani by loading of NiO in Pani, reducing the path of polarons in Pani. From Figure d, it is illustrated that the Pani@GN/NiO nanocomposite reflected the two maxima at 276 nm, blue-shifted in comparison with Pani/NiO, and the other at 375 nm, red-shifted in comparison with Pani from 327 nm, which may be attributed to the increase in the extent of conjugation of Pani by forming an efficient network by electronic transition from NiO to GN.[28] The significant red shift in the Pani@GN/NiO nanocomposite supports the enhanced dc electrical conductivity because of the ease in the movement of polarons by extended conjugation in Pani@GN/NiO.

Figure 4

UV–vis spectra of (a) Pani, (b) Pani/GN nanocomposite, (c) Pani/NiO nanocomposite, and (d) Pani@GN/NiO nanocomposite.

Raman Spectroscopy

Figure shows the Raman spectra of Pani@GN/NiO, Pani/NiO, Pani/GN, and Pani. In Figure a, the different modes of vibration are observed. The sharp peak at 1410 cm–1 may be associated with the presence of C–N+ polarons.[29] The peaks observed at 1330 and 1570 cm–1 may be due to the presence of GN sheet bands (D and G), and the bands at 1370 and 1592 cm–1 may be due to two-magnon (2M) and two-phonon (2P) bands of NiO. In Figure b, the Raman spectrum of Pani/NiO displays two vibrational bands at 1363 and 1574 cm–1, which may be due to the presence of NiO nanoparticles that correspond to the 2M and 2P models.[30−32]Figure c displays the Raman spectrum of the Pani/GN nanocomposite, showing the two bands at 1345(D) cm–1 due to disordered structures in the GN layers and 1576(G) cm–1 due to sp2 electronic configuration of graphitic carbon phase in the Pani/GN nanocomposite.[33] In the Raman spectra of Pani, the bands at 1350 and 1565 cm–1 may be attributed to C–N+ and C=C modes of vibration, respectively, as shown in Figure d. Significant shifts of Raman bands of Pani@GN/NiO are observed in comparison with the Pani/GN and Pani/NiO nanocomposites. The D and G bands of GN in the Pani/GN nanocomposite are blue-shifted from 1345 and 1576 to 1330 and 1570 cm–1, respectively, in the Pani@GN/NiO nanocomposite, whereas the 2M and 2P bands of NiO in Pani/NiO are red-shifted from 1363 and 1574 to 1370 and 1592 cm–1, respectively. The substantial band shifts of GN and nickel oxide constituent in the Pani/GN and Pani/NiO nanocomposites indicate a strong interaction between NiO and GN sheets, favoring the charge transfer from NiO to GN sheets[28] and forming more holes in Pani. Thus, an increment in the electrical conductivity of Pani@GN/NiO also strongly supports the interaction and charge transportation between NiO and GN.

Figure 5

Raman spectra of (a) Pani@GN/NiO nanocomposite, (b) Pani/NiO nanocomposite, (c) Pani/GN nanocomposite, and (d) Pani.

Electrical Conductivity

The initial dc electrical conductivities of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites were measured by a standard four-in-line-probe method, as shown in Figure a. The electrical conductivity of Pani was observed to be 1.832 S/cm. The electrical conductivity of the Pani/NiO nanocomposite prepared by the method mentioned above was observed to be 0.853 S/cm, which is slightly less than that of Pani. It may be proposed that the movement of polarons of Pani get retarded because of the Coulombic interaction between the polarons of Pani and lone pairs of oxygen of NiO, as shown in Figure b. Although, the d-orbitals of Ni may increase the number of polarons in the Pani chains by accommodating electrons from Pani, the Coulombic interaction seems to be a dominating factor. Shambharkar and Umare[23] also reported that the electrical conductivity of the Pani/NiO nanocomposite decreases with a low NiO content in the nanocomposite.

Figure 6

(a) Initial dc electrical conductivities of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites and the mechanism of conductivity behavior of (b) Pani/NiO, (c) Pani/GN, and (d) Pani@GN/NiO.

The initial electrical conductivity at room temperature of the as-prepared Pani/GN nanocomposite was observed to be 9.2 S/cm, which was much higher than that of Pani (1.2 S/cm). It may be proposed that the charge carriers hop from Pani to GN, where they gain high mobility along the π-conjugated system of GN, as shown in Figure c. The overall mobility of charge carriers thus enhanced because of unobstructed movement along the GN nanosheets, leading to an increase in the electrical conductivity of the Pani/GN nanocomposite. Ansari et al.[34] also reported that a sulfonated-Pani/GN nanocomposite exhibited a high electrical conductivity because of π–π interaction between sulfonated-Pani and GN.

In the case of Pani@GN/NiO, the electrical conductivity was observed to be 11.34 S/cm, which was the highest among all samples. It seems that the NiO acts as a bridge between Pani and GN, which helps in the movement of charge carriers from Pani to GN and thus increases electrical conductivity because of a greater mobility of charge carriers along the π-conjugated system of GN nanosheets, as shown in Figure d. Thus, NiO bridges additionally facilitate the hopping movement of charge carriers between Pani and GN in the Pani@GN/NiO nanocomposites.

Stability under Isothermal Ageing Conditions

The isothermal ageing stabilities of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites were calculated, as shown in Figure , by using the following equation:[13]where σ is the electrical conductivity at time t and σ0 is the electrical conductivity at time zero at experimental temperature. The as-prepared materials were examined for stability in terms of dc electrical conductivity retention with respect to time at different temperatures. The electrical conductivity of each of the samples was measured at the temperatures 50, 70, 90, 110, and 130 °C versus time at an interval of 5 up to 20 min. From Figure a,b, it can be observed that Pani and Pani/NiO are fairly stable up to 90 °C, whereas the Pani/GN and Pani@GN/NiO nanocomposites are stable up to 110 °C in terms dc electrical conductivity, as shown in Figure c,d. It may be inferred that the relative electrical conductivity of Pani@GN/NiO and Pani/GN nanocomposites showed greater stability than the Pani and Pani/NiO nanocomposite under isothermal ageing conditions.

Figure 7

Relative electrical conductivity of (a) Pani, (b) Pani/NiO, (c) Pani/GN, and (d) Pani@GN/NiO under isothermal ageing conditions.

Stability under Cyclic Ageing Conditions

The cyclic ageing stabilities of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites were studied, as shown in Figure .[13] The relative electrical conductivity (σr) was calculated using the following equation:where σ is the electrical conductivity (S/cm) at temperature T (°C) and σ50 is the electrical conductivity (S/cm) at 50 °C, that is, at the beginning of each cycle.

Figure 8

Relative electrical conductivity of (a) Pani, (b) Pani/NiO, (c) Pani/GN, and (d) Pani@GN/NiO under cyclic ageing conditions.

The electrical conductivity was recorded for successive cycles and observed to be increased gradually for each of the cycle with a regular trend in all of these cases, which may be due to the increment of the number of charge carriers as polarons or bipolarons at elevated temperatures. Figure a shows the relative electrical conductivity of Pani following the same trend for each of the cycle with gain in the dc electrical conductivity as the temperature increases. Also, the relative electrical conductivities of Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites increased as the temperature increases from 50 to 130 °C, as shown in Figure b–d. Among all of these nanocomposites, the Pani@GN/NiO nanocomposite showed the lowest gain in conductivity with less deviation. Thus, it may be inferred that the Pani@GN/NiO nanocomposite is a more stable semiconductor than Pani and among all of these nanocomposites under cyclic ageing conditions. The difference observed may be attributed to the removal of moisture, excess HCl, or low-molecular-weight oligomers of aniline.[35]

Sensing

The dc electrical conductivity responses of the sensors based on Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO were measured on exposure to ammonia (NH3) at room temperature (∼25 °C). A four-in-line-probe PID-200 (Scientific Equipment, Roorkee, India) dc electrical conductivity measuring instrument was used to study ammonia sensing properties. The pellet fabricated was in contact with the four probes and placed in a closed chamber of ammonia vapors (Figure ).

Figure 9

Instrumental setup of the ammonia sensor and the schematic presentation of the four-in-line probe.

The electrical conductivity immediately decreases on exposure to ammonia for 60 s and rapidly reverts back on exposure to ambient air in next 60 s in all of the as-prepared materials, as shown in Figure a. However, the greatest change in electrical conductivity was observed in the case of the Pani@GN/NiO nanocomposite. Therefore, the Pani@GN/NiO nanocomposite was also tested for different concentrations of ammonia, as shown in Figure b. The highest change in electrical conductivity was observed for 1703 ppm of ammonia. As the concentration of ammonia increases from 170 to 1703 ppm, the more polarons of Pani get neutralized by the lone pair of electrons of ammonia molecules, leading to a decrease of electrical conductivity. The electrical conductivity change of the Pani@GN/NiO nanocomposite could be observed on exposure to ammonia as low as 170 ppm concentration.

Figure 10

(a) Effect on the dc electrical conductivity of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO nanocomposites on exposure to (1703 ppm) ammonia vapors followed by exposure to ambient air with respect to time, (b) effect on the dc electrical conductivity of the Pani@GN/NiO nanocomposite on exposure to ammonia vapors at different concentrations, and (c) steady-state response of dc electrical conductivity of the Pani@GN/NiO nanocomposite on exposure to 1703 ppm ammonia followed by exposure to ambient air with respect to time.

The steady-state response of Pani@GN/NiO was determined by first keeping the sample in 1703 ppm of ammonia environment for 180 s followed by 120 s in air for a total duration of 300 s (Figure c). It can be observed that the electrical conductivity immediately decreases on exposure to ammonia for about 65 s and attains saturation for further exposures until exposed to ambient air. The conductivity reverted in ambient air after 180 s of exposure to ammonia and became saturated at about 240 s.

Reversibility

The sensitivity and reversibility are the two important parameters for a gas sensor. Sensitivity may be defined as the time taken to reach the final value after exposure to vapors and to reach the initial value on exposure to ambient air, whereas the reversibility may be considered as the cycling between the analyte and ambient air without any loss in its sensing ability. The reversibility of Pani and Pani@GN/NiO nanocomposite was measured in terms of the dc electrical conductivity. The reversibility of both the samples was determined by first keeping the sample in 1703 ppm ammonia vapors for 30 s followed by 30 s in air for a total duration of 150 s. Figure a represents the reversibility response of Pani in terms of the dc electrical conductivity with a variation range from 1.849 to 1.812 S/cm in ambient air and on exposure to 1703 ppm of ammonia vapors, respectively. The change in conductivity observed was 0.037 S/cm, whereas in the case of the Pani@GN/NiO nanocomposite, the reversibility was observed with an excellent variation range compared to Pani, which is from 11.146 to 7.489 S/cm in ambient air and on exposure to 1703 ppm of ammonia vapors, respectively, as shown in Figure b. In the case of Pani@GN/NiO, the observed decrease in conductivity by 3.657 S/cm is indicative of much higher efficiency than that of Pani in terms of reversibility. There was around about 99 times greater variation observed in the conductivity of the Pani@GN/NiO nanocomposite than that of Pani.

Figure 11

Variation in the electrical conductivity of (a) Pani and (b) Pani@GN/NiO on alternate exposure to (1703 ppm) ammonia vapors and ambient air.

Sensing under Dry and Wet Atmospheres

The NH3 response properties under completely dry and wet atmospheres were also studied. The effect of relative humidity (RH) on ammonia sensing properties was studied by mixing of humidity and NH3 vapors. It was seen that the dc electrical conductivity of the Pani@GN/NiO nanocomposite sharply decreases and reaches its saturation level within 50 s when exposed to 1700 ppm of dry ammonia prepared by heating of ammonium chloride and slaked lime mixture. Also, the sample Pani@GN/NiO was tested for pure water vapors, and it was observed that the decrease in electrical conductivity was not rapid as that it was in the case of completely dry ammonia, as shown in Figure . To confirm the effect of water vapors on the ammonia sensing properties of the Pani@GN/NiO nanocomposite, the sample was also exposed to an NH3 atmosphere at 1703 ppm at different humidity levels viz. 20, 30, 40, and 60% RH. The electrical conductivity of the sample continuously decreases with time at different RHs. The variations of humidity levels for the same concentration keep the conductivity stable. The conductivity variations of Pani@GN/NiO are not much effected at any RH, and the four curves were almost similar, from which we may conclude that humidity has only small effect on NH3 sensing properties.

Figure 12

dc electrical conductivity response as a function of time of Pani@GN/NiO nanocomposite exposed to 1700 ppm dry ammonia, 20% (1703 ppm ammonia), 30% (1703 ppm ammonia), 40% (1703 ppm ammonia), 60% (1703 ppm ammonia) RH, and pure water vapors.

Selectivity

For a worthy and applicative gas sensor, high selectivity is also an important requirement. The conductivity responses of the Pani@GN/NiO nanocomposite to ammonia and volatile organic compounds (VOCs) viz. isopropanol, methanol, ethanol, acetone, acetaldehyde, and formaldehyde at room temperature (25 °C) are shown in Figure . It was observed that the conductivity response of the Pani@GN/NiO nanocomposite to 1703 ppm ammonia was 9.8, 6.2, 6.6, 7.9, 6.5, and 6.1 times higher than those for isopropanol, methanol, ethanol, acetone, acetaldehyde, and formaldehyde, respectively. Such a selectivity results from the difference of their sensing mechanisms, that is, the neutralization of polarons of Pani that plays an important role in changing electrical conductivity on exposure to VOCs. The greater the availability of lone pairs in a vapor/gas, the greater the observed electrical conductivity change. Ammonia has more electron-donating nature and thus neutralizes more number of polarons of Pani. On the other hand, in VOCs, there are interactions of lone pairs of electrons of oxygen of greater −I effect than that of nitrogen of Pani. Therefore, the lower electron-donating tendency of VOCs leads to a lower conductivity change. This also infers the higher selectivity of the Pani@GN/NiO nanocomposite toward NH3.

Figure 13

Selectivity of the Pani@GN/NiO nanocomposite to 1703 ppm of ammonia and different VOCs (1 M) in water.

The efficient gas sensing properties of Pani@GN/NiO may be attributed to the large surface area of Pani which anchored more adsorption and desorption of gaseous molecules as well as high polarons mobility due to the presence of GN, essential requirement for rapid response of any electrical conductivity-based sensor.[13] It is expected that the NiO embedded on GN sheets interacts with lone pairs of nitrogen atoms of Pani, thus increasing the number of polarons in Pani and creating a more number of active sites at a large surface area for the interaction of lone pairs of ammonia molecules to polarons of Pani. However, the detailed understanding on the role of NiO and GN sheets in the sensing mechanism of the Pani@GN/NiO nanocomposite requires further investigations on this aspect.

Proposed Mechanism for Sensing

Sensing by the Pani@GN/NiO nanocomposite is based on the decrease in electrical conductivity on exposure to analyte vapors and to revert on exposure to ambient air. Therefore, the different aspects of emergence of high electrical conductivity in the nanocomposite have been discussed above in order to understand the sensing behaviour of the Pani@GN/NiO nanocomposite. The sensing mechanism of the Pani@GN/NiO nanocomposite was explained through dc electrical conductivity response by simple adsorption and desorption mechanism of ammonia vapors at room temperature, as shown in Scheme . In the Pani@GN/NiO nanocomposite, NiO nanoparticles embedded on GN sheets interact with lone pairs of nitrogen atoms of Pani; thus, the numbers of polarons and bipolarons increase in Pani chains. In the presence of ammonia vapors, the lone pairs of ammonia molecules bind with polarons of the Pani@GN/NiO nanocomposite, which impedes the mobility of polarons, leading to a decrease in electrical conductivity. When Pani@GN/NiO is exposed to ambient air, the ammonia molecules are desorbed from the surface and thus the electrical conductivity reverts to its initial value. Thus, the simple adsorption and desorption of ammonia vapors on the Pani@GN/NiO nanocomposite govern the mobility of polarons.

Scheme 1

Proposed Mechanism of Interaction of Ammonia with the Pani@GN/NiO Nanocomposite

However, the humidity present in atmosphere may also interfere in ammonia adsorption on the Pani@GN/NiO nanocomposite, but the conductivity changes of Pani@GN/NiO are not much affected at any RH so that humidity has only small effect on NH3 sensing properties. Therefore, in competition between ammonia and water molecules, it may be said that the change in conductivity of Pani@GN/NiO may be mainly due to ammonia vapors because ammonia has more available electrons than water.

The selective response of the Pani@GN/NiO nanocomposite toward ammonia may be due to the basic nature of ammonia, which readily interacts with polarons of Pani, whereas all other VOCs tested are much less basic than ammonia because they contain an oxygen atom which is more electronegative than nitrogen, so the electronic interaction with Pani in these compounds is comparatively poorer.

Conclusions

Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO were successfully prepared and characterized by XRD, SEM, TEM, UV–vis, and XRD analysis. The Pani@GN/NiO nanocomposite showed the higher thermal stability in terms of dc electrical conductivity retention under isothermal and cyclic ageing conditions than pristine Pani. The results highlighted that the Pani@GN/NiO nanocomposite was found to be an excellent material for ammonia sensing with excellent selectivity against VOCs and rapid response. There was about 99 times greater variation in the electrical conductivity of Pani@GN/NiO nanocomposite than that of Pani during sensing. The enhancement of sensing properties in the Pani@GN/NiO nanocomposite may be attributed to the synergistic effect between NiO, GN, and Pani. Therefore, the sensor based on the nanocomposite of GN/NiO with Pani may be useful and efficient while the material is promising for ammonia vapor sensing.

Experimental Section

Materials

Aniline (99%, E. Merck, India) and commercially available natural graphite powder (Sigma-Aldrich, USA) were used to prepare graphene oxide (GO). Sodium dodecylsulfate, ammonium persulfate (APS), sulfuric acid (H2SO4), hydrated nickel nitrate (Ni(NO3)26H2O), sodium hydroxide (NaOH), and hydrazine hydrate (85%) were obtained from the local suppliers and used as received.

Preparation of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO

Pani nanofibers were prepared through the interfacial polymerization reaction in which aniline monomers (10 mmol) were dissolved in hexane (10 mL) under stirring for 30 min. APS (30 mmol) was gently added to 10 mL of 1 M sulfuric acid solution under constant stirring. The solution of APS was added dropwise into aniline solution, and the mixture was allowed to react for 1 h. The final greenish reaction product was then filtered, washed with double distilled water, and then dried in an air oven at 70 °C for 10 h.

In a typical synthesis procedure of the Pani/NiO nanocomposite, 2 mL of aniline was dissolved in the solution of 2 mL of 1 M H2SO4 in 90 mL of distilled water. A solution of 0.5 g of nickel oxide (NiO) was prepared in accordance with the existing literature[36] and was then added to aniline solution. APS (5 g) dissolved in 10 mL of 1 M H2SO4 was added to the aniline solution, and the reaction mixture was stirred for 24 h at room temperature. The final reaction product was then filtered and washed thoroughly with distilled water and acetone until the filtrate became colorless. The resulting product was dried at 70 °C for 10 h.

To prepare the Pani/GN nanocomposite, GO (20 mg) was prepared in accordance with the existing literature[37] and was then dissolved in 10 mL of 1 M H2SO4 with constant stirring for 1 h. Subsequently, aniline solution (10 mmol) prepared in 1 M H2SO4 was mixed to the GO solution and stirred for another 1 h. The APS (30 mmol) solution prepared in 1 M H2SO4 was then added to the above solution. The final reaction mixture was kept under constant stirring, followed by a conventional microwave treatment at 300 W for 30 min. The resulting suspension was separated by centrifugation and washed with double distilled water and ethanol. The final product was dried at 70 °C under vacuum for 12 h.

In the synthesis of GN/NiO, the as-prepared GO (25 mg) was ultrasonically dispersed in 10 mL of double distilled water for 1 h and 0.5 g of Ni(NO3)2·6H2O in 25 mL of distilled water was added into the above dispersion. After constant stirring for 6 h, 10 mL of 1 M NaOH aqueous solution was added into the above reaction mixture drop by drop. The pH value was adjusted to ∼10 by 10% aqueous ammonia. After that, 10 mL of hydrazine hydrate (85%) was added to it. The reaction mixture was stirred for several minutes before being treated with microwave at 300 W for 30 min. The product was separated by centrifuging and washed with double distilled water followed by ethanol. The product was dried in a vacuum oven at 50 °C for 24 h. The dried sample was heated in a N2 atmosphere at 300 °C for 3 h to obtain GN/NiO. The Pani@GN/NiO nanocomposite was prepared by mixing GN/NiO to aniline monomer in a weight ratio of 5:1. GN/NiO was dispersed in ethanol and water (1:1 weight ratio) by stirring for 60 min and then added to the aniline solution prepared in 1 M H2SO4. An aqueous solution of APS (10 mL) prepared in 1 M H2SO4 was added dropwise to the mixture of aniline and GN/NiO at 5–10 °C and kept under constant stirring for 24 h. The resultant product was washed several times by vacuum filtration using ethanol and double distilled water. The final product was dried at 50 °C for 12 h.

Characterization

The morphology, structure, and chemical composition of Pani, Pani/NiO, Pani/GN, and Pani@GN/NiO were investigated by a variety of methods. XRD pattern were recorded by a Bruker D8 diffractometer with Cu Kα radiation at 1.5418 Å. SEM studies were carried out by JEOL, JSM, 6510-LV (Japan). TEM studies were carried out by using JEM 2100, JEOL (Japan). The Raman spectra were taken using a Varian FT-Raman spectrometer. UV–vis spectra were recorded at room temperature using a Shimadzu UV–vis spectrophotometer (model 1601). dc electrical conductivity measurements were performed by using a four-in-line probe electrical conductivity retention measuring instrument with a PID controlled oven (Scientific Equipment, Roorkee, India). The thermal stability of all as-prepared materials in terms of dc electrical conductivity under isothermal and cyclic ageing conditions was also studied. The equation used in the calculation of dc electrical conductivity waswhere I, V, W, and S are the current (A), voltage (V), thickness of the pellet (cm), and probe spacing (cm), respectively, and σ is the conductivity (S/cm).[11,38] In thee testing of isothermal stability, the pellets were heated at 50, 70, 90, 110, and 130 °C in an air oven, and the dc electrical conductivity was measured at particular temperatures at an interval of 5 min in the accelerated ageing experiments. In the testing of the stability under cyclic ageing conditions, dc conductivity measurements were taken five times within the temperature range of 50–130 °C.