Preparation and characterisation of NH3 gas sensor based on PANI/Fe-doped CeO2 nanocomposite

PANI/Fe-doped CeO2nanocomposite was synthesised by the in-situ process. The produced powders were characterised by XRD, XPS, FT-IR, Raman, HRTEM and SEM-EDS tests. The sensors' function was based on PANI/Fe-doped CeO2nanocomposite with thin film deposited on top of interdigitated electrodes (IDT). NH3 detection with PANI/Fe-doped CeO2 nanocomposite sensor could be successfully performed even at room temperature (RT) and relative humidity of 45 %. Results demonstrated that PANI/Fe-doped CeO2 might be promising sensing materials for detecting the low NH3 concentration (ppm). In addition, the sensor is selective to the interfering gases, including CO, CO2 and NO2. This sensor displays acceptable repeatability and stability over time.


Introduction
The World Health Organisation (WHO) announced that 30.7 million people died in major cities from cancer chronic respiratory and cardiovascular diseases due to air pollution in the last five years.The health impacts of air pollution have received much attention in the detection of harmful pollutants in the atmospheric environment [1,2].Ammonia (NH 3 ), as a colourless gas, is harmful and foul-smelling gas, and it is one of the most abundant alkaline components in the atmosphere with a potent, pungent odour at ambient temperature [3].Fertilisers, soils, and chemical manufacturing are some additional sources of NH 3 .Developing high-performance sensors to detect NH 3 in the air rapidly and consistently appears highly necessary in this context.Besides the necessity of sensing low levels (ppm) of NH 3 , in some situations, such as the automatic management of the chemical manufacturing process, high levels (%) of NH 3 are also required [4].
In recent decades, metal oxide-based gas sensors have received much attention.The existence of active sites on the metal oxide's surface is responsible for their gas-sensing properties.Furthermore, the mechanism of gas sensors mostly depends on the size of metal oxide particles and their crystallinity [5].The intrinsic properties of metal oxide, namely low cost, thermal stability, nontoxicity, and high chemical sensitivity, are ascribed to high-density free charge carriers [6].Ceric oxide (CeO 2 ) is one of the most used oxides for developing detectors of toxic gases, such as CO, NOx, NH3, and hydrocarbons.CeO 2 -based material is an n-type semiconductor with an energy band gap of 3.19 eV [7].Applications of CeO 2 to gas sensor detection have also attracted considerable interest due to the generation of lattice defects with cubic fluorite, distinct physical and chemical features with outermost 4f shell, and high oxygen storage capacity with low cost [8].Moreover, the low redox potential between Ce +3 and Ce +4 has made this oxide an advantageous sensing material for detecting gases.Furthermore, researchers have demonstrated that CeO 2 is viable for detecting explosive, toxic, volatile organic compounds (VOCs) and hazardous gases [9][10][11].The most crucial characteristic of ceria is the capacity to store and release oxygen via facile Ce +4 /Ce +3 redox cycles [12].CeO 2 is known as an insulator; thus, due to its ionic conductivity by introducing oxygen vacancies in the lattice as charge-compensating defects, doping with different rare earth elements, alkaline cations, and/or transitional metals has been reported [13].
The introduction of noble metal nanostructures (e.g., Au, Ag, Al, Fe) has been considered an effective gas detection method [14,15].Similarly, the cubic fluorite structure of pristine CeO 2 can support the stoichiometric deviations, which, thus, proves to be advantageous when CeO 2 is doped with a small fraction of transition metal ions like Fe.Such a procedure is anticipated to enhance their properties without distorting the original structure.Among different dopants, Fe is attractive and environmentally friendly as a dopant of CeO 2 because it can improve the catalytic activity due to its redox ability since the oxygen species can alter between Fe 3+ /Fe 2+ [16,17].In addition, the amounts of Fe dopant on the humidity-sensing properties of CeO 2 NPs were validated by humidity-sensing studies [18].Essentially, the poorer electrical conductivity of metal oxides at ambient temperature limits their gas-detecting effectiveness [19].To solve this problem, superior gas sensing results can be obtained by combining CeO 2 with other materials such as ZnO, nanocrystals, graphene, WO 3 and modifying their structure [10,20].Low-dimensional nanomaterials and polyaniline (PANI) in nanocomposites produce outstanding synergies [21,22].Adsorption of gas molecules promotes de-doping of PANI particles, which alters the diameter of the space charge zone of restricted heterojunctions [23].The porous and loose nanomaterial structure could provide vast anchor sites to adhere to a PANI as a conductive substrate.

Material characterisation
Scanning electron microscope (SEM), Tescan LYRA, equipped with energy dispersive spectroscopy (EDS), Oxford Instruments, 80 mm 2 , was used for morphology determination.The EDS was utilised for chemical microanalysis of elements present and/or comprehensive analysis of element distributions within the materials.The secondary electron detector measured samples themselves while the accelerating voltage was 15 kV.Powders were placed on a double-sided adhesive tape made of carbon and covered by 2 nm of Au to ensure their excellent conductivity.Transmission electron microscopy (TEM) analysis of the synthesised nanoparticles/ nanocomposite was performed using a JEM-2200FS Jeol instrument.Fourier transform infrared spectroscopy (FTIR) measurements were performed on an iS50R FTIR spectrometer (Thermo Scientific).The measurement was performed using a DLaTGS detector and KBr beam splitter in 4000-500 cm − 1 at a resolution of 4 cm − 1 .X-ray diffraction (XRD) measurement was performed using 2nd Generation D2 Phaser X-ray diffractometer (Bruker) with Cu Kα radiation (λ = 0.15418 nm), SSD (1D mode) detector, coupled 2θ/θ scan type and continuous PSD fast scan mode.The range of measured Bragg 2θ angle was from 5 to 80 • .High-resolution X-ray photoelectron spectroscopy (XPS) measurement was performed using an ESCAProbeP Spectrometer (Omicron Nanotechnology Ltd.) with a monochromatic aluminium X-ray radiation source (1486.7 eV).Raman spectroscopy measurements were performed on a Renishaw via Raman microscope using a 532 nm laser in a backscattering geometry with a Charge Coupled Device (CCD) detector.

Synthesis of Fe-doped CeO 2 nanocomposite
The 0.01 mol of Ce(NO 3 ) 3 ⋅6H 2 O was dissolved in 100 ml of deionised (DI) water with constant stirring.NaOH (5 M) was added to the solution dropwise with constant stirring until complete precipitation.The precipitate was then stirred for 3 h, followed by hydrothermal treatment using an autoclave, maintaining the temperature of 110 • C for 24 h.The nanoparticles were washed several times with water and finally with ethanol using centrifugation.The nanoparticles were dried in an oven overnight at 80 • C followed by calcination at 500 • C for 3 h.For the synthesis of 7 mol % Fe doped CeO 2 nanocomposite, 0.282 g of Fe(NO 3 ) 3 ⋅9H 2 O was added to the aqueous solution of Ce(NO 3 ) 3 ⋅6H 2 O prior to the addition of NaOH solution and followed the similar protocol described above.

Preparation of PANI/Fe-doped CeO 2 nanocomposite layers
PANI in the form of protonated emeraldine salt was synthesised by oxidising 0.2 M aniline hydrochloride with 0.25 M ammonium persulfate at room temperature (RT), as described in the literature [24].An exothermic reaction occurred during the PANI synthesis, and the temperature of the reaction mixture was checked.The polymerisation process was completed for 15 min at 37 • C with gentle stirring.The dark green precipitate was filtered off and washed with acetone and 0.2 M hydrochloric acid several times.Afterwards, C. Esmaeili et al. the PANI was dried in a desiccator overnight.The PANI/Fe-doped CeO 2 nanocomposite was prepared by mixing 2 mg Fe-doped CeO 2 and 10 mg PANI in 1 mL xylene.
The PANI with Fe-doped CeO 2 nanocomposite suspension was ultra-sonicated for 2.0 h and then gently mixed by a vortex device for approximately 1 h.

Gas sensing measurements
All gas sensing studies were conducted in the RT gas chamber (27 • C).A Keithley 2400 source meter was used to measure the current versus time characteristics at a constant DC input voltage of 1 V.
Electrical feed through the NH 3 gas sensor was placed into an airtight testing chamber.With a specific gas concentration at RT, the resistance of the NH 3 gas sensor was continually recorded by a computer.In this work, the response of the gas sensor is defined by the ratio of (Rg − Ra)/Ra for the testing of NH 3 gas, where R a is the resistance of the sensor with synthetic air dilution, and R g is the sensor resistance in pollutant target gas.In addition, the total gas flow rate into the chamber was maintained at 200 ml per minute.A schematic diagram of the characterisation of the gas sensor is shown in Fig. 1. cubic fluorite ceria phase [26].An additional broad peak centred at 598 cm − 1 was also observed in the Fe-doped samples.This peak is attributed to the oxygen vacancy and disturbance of the local symmetry induced by dopants [27,28].This also signifies the homogeneous incorporation of Fe within the CeO 2 crystal structure.A peak centred at 520 cm − 1 is attributed to the Si substrate.Fig. 2D shows the FTIR spectra of undoped, Fe-doped CeO 2 nanocomposite and CeO 2 @ PANI composite.In the case of nanocomposite samples, the intense absorption band at 3390 and 1516-1630 cm − 1 are associated with symmetrical stretching and bonding mode of internally bonded water molecules (O-H), respectively.The O-C-O stretching band observed in 1330 cm − 1 and 1060 cm − 1 regions confirms the surface adsorbed CO 2 .The absorption band at 858 cm − 1 is produced by CeO 2, a typical peak for Ce-O stretching vibration [29].Similarly, in the case of composite samples, the peaks at 1460, 1235, and 702 cm − 1 agree with the functional groups of PANI.The peaks in the 1590 cm − 1 region are assigned to the stretching of quinonoid whereas, at 1506 cm − 1, assigned to the C-C stretching mode of the benzeneoid ring, 1300 cm − 1 is assigned to C-N stretching mode, and 1134 cm − 1 is assigned to C --N stretching of secondary aromatic amine.The N-H stretching vibration of aromatic amines is assigned at 3250 cm − 1 [30,31].No prominent peaks for CeO 2 could be observed in the composite samples.The dominance of the PANI signature peaks in the composite samples indicates the PANI matrix's encapsulation of the CeO 2 oxides during the synthesis process.Moreover, the slight shifting of peaks to lower wavenumbers compared to pure PANI [32] is presumed due to hydrogen bonding between the hydroxyl groups on the surface of the CeO 2 nanoparticles and the imine groups in the PANI molecular chain [33].

b) XPS
Wide scan XPS spectrum (Fig. 3A) exhibits evident CeO 2 features and the presence of an additional Fe 2p signal in the 705-745 eV range, indicating that the Fe particles had been successfully incorporated into the CeO 2 matrix.The fitted Ce 3d spectra of the Fe-doped CeO 2 sample (Fig. 3B) showed different peaks corresponding to different oxidation states of Ce +3 and Ce +4 .The presence of Ce +3 reveals the presence of oxygen vacancies in the samples.The presence of the Ce +3 state is due to the reduction of Ce +4 in the oxide structure.Thus, oxygen vacancies are presumed to be produced due to electron transformation between Ce +3 and Ce +4 [34].The existence of Ce +3 is a direct consequence of the presence of the Ce-O-Fe bridges on the surface.The charge compensation by Fe insertion makes part of Ce +4 transformation into Ce +3 associated with forming oxygen vacancies and lattice defects favorable for oxygen mobility [35,36].The O1s region (Fig. 3C) contained three contributions, one due to lattice oxygen with the binding energy of 537 eV, the second peak attributed to chemisorbed oxygen species on the surface (OOH), with the binding energy of 538 eV arising due to dissociative adsorption of water.In contrast, the third peak with the binding energy of 540 eV is attributed to oxygen vacancy.The  (C) and (D) are the typical high-resolution TEM (HRTEM) image and selected area electron diffraction (SAED) analysis of CeO 2 nanoparticles.The SAED analysis reveals that the prepared CeO 2 nanoparticles display typical polycrystalline rings, and the clear lattice fringes with a space of about 0.362 ± 0.003 nm can be observed corresponding to the interplanar distances of (111) of cubic fluorite CeO 2 [39].Similarly, Fig. 4 (A) shows the SEM image of the Fe-doped CeO 2 sample.The overall morphology of Fe-doped CeO 2 is similar to that of undoped CeO 2 NPs.Similar to CeO 2 nanoparticles, the HRTEM image (Fig. 4B) reveals the nanocrystalline domain in the Fe-doped CeO 2 sample.Due to the formation of mixed solutions, phase segregation of Fe and CeO 2 is not observed, which is further supported by the XRD pattern.The slight decrease in the interplanar distances (0.360 ± 0.001 nm) revealed the shrinkage of the unit cell as a result of doping metal ion (Fe) with a smaller radius into the CeO 2 lattice [38].Selected area electron diffraction patterns of both undoped (Figure S1 D) and Fe-doped CeO 2 samples (Fig. 4C) displayed similar diffraction rings demonstrating single-phase and polycrystalline nature.The inner to outer diffraction rings can be indexed to the (111), ( 220), (311), (331), (400), (511) and (531) plane of CeO 2 (JCPDS -34-0394) consistent with XRD patterns.The TEM micrograph of the Fe-doped CeO 2 sample (Fig. 4D) also shows moderate agglomeration of the NPs with quasi-spherical shapes.Elemental analysis of the Fe-doped CeO 2 sample (Fig. 3E-G) showed the presence of Ce, O and Fe, which further confirms the successful doping of Fe into the CeO 2 lattice.Fig. 4H is the SEM image of the CeOFe7@PANI composite.Agglomerated non-uniform structures of composite particles were observed.The HRTEM images of the composite (Fig. 4I-J) depict that nanoparticles (Fe doped CeO 2 ) are surrounded by the PANI matrix, forming a core-shell-like structure, which thus reveals the attachment of PANI to CeOFe7 nanoparticles.This suggests that blending the conductive nature of the PANI network with the Ceria nanoparticles leads to the interaction between the polymer particles and the nano ceria, ultimately forming a composite of CeOFe7@ PANI.Similarly, PANI, which has a large number of functional groups, provides sites for unbound CeO 2 nanoparticles through self-attachment [40].

NH3 sensing performance based on PANI/Fe-doped CeO 2 nanocomposite
The abovementioned SEM result clearly indicates that the PANI/Fe-doped CeO 2 could be described as a large specific surface of nanoparticles and multiple gaps.This result is increased gas molecule adsorption sites, and adequate time for gas molecule adsorption  and diffusion saturation may be required [41].The sensors' responses upon exposure to 25 ppm NH 3 at RT is shown in Fig. 5.A small amount of surface interacts with the air molecules in dark conditions, causing negligible response change.The response increases the chemical activity of the surface by enhancing the number of charge carriers in the conduction band as long as there is a higher number of active sites on the surface [42].Hence, it improves the adsorption capacity concentration on the surface by providing a higher number of electrons.When air is introduced to the nanoparticles, oxygen is absorbed molecularly at RT.The NH 3 gas molecules react with the excited electrons/holes and adsorbed oxygen ions and turn into the products (Fig. 5).In addition, the CeO 2 NPs alone might block the charge carriers or reduce the delocalisation length and hence increase the resistance of the nanohybrid when exposed to NH 3 gas [43].Our previous study has shown that the PANI composites' responses saturate at a higher gas concentration.It could be due to a reduced surface area with possible reaction sites on the surface of the film [24].
It could be considered that when the PANI/Fe-doped CeO 2 nanocomposite was exposed to NH 3 gas, the NH 3 molecules would diffuse into the nanocomposite surface, and PANI can cause swelling, thereby increasing the interchain distance and thus decreasing the conductivity of PANI [41].The capability of CeO 2 to retain oxygen through a unique redox reaction between Ce 3+ and Ce 4+ ions will be critical in gas detection.The doping of Fe increases oxygen vacancy and improves gas-sensing behaviour [44].The creation of oxygen vacancy not only enhances the oxygen storage capacity of the materials but also enhances the efficiency of the surface in reacting with the surrounding environment [13].
Since the Ce 3+ ions can exhibit oxygen vacancies with two units of negative charges (V O ), the content of Ce 3+ ions affect the amount of oxygen vacancies.The higher VO concentration typically causes more chemisorbed oxygen molecules on the CeO 2 surface, providing enough active sites for gas molecules and allowing more gas molecules to be adsorbed on the CeO 2 -based sensor's surface [8].As a result, oxygen vacancies improve gas responsiveness [45].The development of an oxygen defect is accompanied by the localisation of electrons left in Ce 4f states, causing the formation of two Ce 3+ ions while maintaining the cubic fluorite crystal structure (Eq (1)).
The surface of V O can be an electron donor, and more electrons would flow from the PANI-CeO 2 surface to the Fe-doped surface, directly enhancing conductivity.As for the pure CeO 2 nanoparticles, electrons would be trapped by Ce 4+ .
When the sensor is put back into the air, the electrons combine with oxygen on the surface again, and the following Eq.(2) will occur [46]: Fe-doped CeO 2 systems present a remarkable improvement in their oxygen exchange abilities compared to the pristine CeO 2 because of the Ce-Fe synergy that is achieved by combining the redox behaviour of the Ce +4 /Ce +3 and Fe +3 /Fe +2 cations [47].When transition metal ion Fe +2 /Fe +3 is doped into CeO 2 , it substitutes Ce +4 and liberates oxygen, which may take the position in interstitial lattice sites probably due to the smaller ionic state as well as the ionic size of Fe +2 (0.74A)/Fe +3 (0.78A) as compared to that of Ce +4 (0.97A) [13].
A concentration lower than 7 mol% of iron exhibited negligible response, and high concentration of 7 mol % Fe increased resistance and then higher adsorption capacity for Fe, which confirms that PANI/Fe-doped CeO 2 is more difficult to transport through media.
The resistance of the CeO 2 , PANI/Fe-doped CeO 2 composite sensor increases, whereas the resistance of the PANI sensor decreases due to changes in the depletion layer widths, as shown in Fig. 6.Hence, the as-prepared PANI with Fe-doped CeO 2 nanocomposite behaves like a p-n junction material which was evident from the positive Seebeck coefficient values, while CeO 2 and PANI are promoting n-type and p-type charge, respectively.When the sensor is switched to air, the resistance will revert to its base value with good reversibility of the sensor [48].The improvement of protonation degree and modified morphology of PANI by the addition of metal oxide nanoparticles due to the unique p-n junction between PANI and gas causes the excellent performances of sensor selectivity based on nanocomposite [24].
The sensing results towards 6.25, 12.5 and 25 ppm concentrations of NH 3 exhibited promising behaviour (Fig. 7).The increased response in the sample with PANI/Fe-doped CeO 2 nanocomposite heterojunction is attributed to the new electronic interface states.The Due to determining the sensor's stability toward 25 ppm NH 3 over a more extended period, the performance of Fe-doped CeO 2 nanocomposite after 6 months was lowered to 87.6 %.In 6 months, the response has changed slightly.
Selectivity is one of the essential parameters in the study of gas sensors.Theoretically, in the same controlled environment, a sensor that is extremely sensitive to one gas is less sensitive to other gases.Sensors, with PANI/Fe-doped CeO 2 nanocomposite as the sensing layer, were investigated for various gases, such as CO, CO 2 , and NO 2 , at ambient temperature.When the sensor is exposed to 25 ppm of various gases, including CO, CO 2 , NH 3 , and NO 2 at the ambient temperature, the PANI/Fe-doped CeO 2 nanocomposite is most sensitive to NH 3 (Fig. 9).The low selectivity of CO and CO 2 can be attributed to the crystallite's size and the surface PANI/Fe-doped CeO 2 's relative concentration.The selectivity of the PANI/Fe-doped CeO 2 composite sensor towards NH 3 over CO, CO 2 , and NO 2 is primarily due to the specific surface interactions and adsorption energies.NH3 molecules form stronger interactions with the surface functional groups of PANI/Fe-doped CeO 2 , resulting in higher adsorption energy and a more pronounced change in the sensor's electrical properties.The crystallite size and relative surface concentration of PANI/Fe-doped CeO 2 enhance this effect by providing a higher surface area and more active sites for NH 3 adsorption.
Furthermore, the surface functional groups, such as amino and hydroxyl groups, present on the composite material create selective binding sites for NH 3 through strong hydrogen bonding, which is less favorable for CO, CO 2 , and NO 2 .This selective adsorption mechanism explains why the sensor exhibits a higher response to NH 3 compared to other gases.
The gas sensing results against 25 ppm NH 3 indicated that the response of NH 3 is almost 4 times greater than that of the other gas samples.NH 3 gas sensing mechanism is measured by electrical conductivity in relation to an air atmosphere baseline.Oxygen species that are absorbed from the air onto the surface of CeO 2 materials can be ionised to absorb oxygen ions (O x − ) by absorbing the free electrons from the sensing element (Eqs (3)-( 6)) [49].The following equation can be used to explain this process: After ammonia is introduced to the gas sensor element, the ammonia gas reacts with the oxygen ions that have been adsorbed, releasing the captured electron and resulting in a lower barrier potential and thinner space charge (Eq (7)) [50].
When ammonia interacts with the oxygen adsorbed on the surface, trapped electrons are released onto the surface, increasing the surface conductivity in PANI/Fe-doped CeO 2 nanocomposite [51].This is mainly in the presence of many surface defects due to its high surface area and the higher bulk density of charge carriers, which enable the sensor to absorb more gas molecules [52].The long response time of NH 3 can be attributed to the longer migration path of gas molecules to reach the active areas within the PANI/Fe-doped CeO 2 nanocomposite s' structure [53].Table 1, summarises the comparison of the developed NH 3 performance with other materials.

Conclusion
In summary, the PANI/Fe-doped CeO 2 nanocomposite has been fabricated by in situ polymerisation of PANI in the presence of CeO 2 NPs.PANI with Fe-doped CeO 2 was characterised by various techniques and investigated for NH 3 gas detection at room temperature.The performance of the NH 3 gas sensor is considered an effect of pn-junction, the enhanced degree of protonation and its modified morphology of PANI due to the addition of Fe-doped CeO 2 nanocomposite.The selectivity of the gas sensing system was successfully tested on four different gases with responses to 6.25, 12.  NiO sphere-PANI solvothermal 43 % towars 10 ppm NH 3 [54] Camphor Sulphonic Acid (CSA)/PAni-CeO 2 facile chemical oxidative polymerisation 93 % towards 100 ppm NH 3 [55] CeO 2 nanoparticles Hydrothermal ~350 % towards 25 ppm NH 3 [56] PANI-iron oxide nanocomposite in-situ polymerisation 39 % towards 100 ppm NH 3 [57] Fuzzy nanofibrous network of PANI film in situ method ~35 % towards 50 ppm NH 3 [58] C. Esmaeili et al.

Fig. 1 .
Fig.2Ashows the XRD spectra of pure CeO 2 and Fe-doped CeO 2 nanocomposite samples.The diffraction peaks at the corresponding 2θ values well matched with the JCPDS -34-0394 for every CeO 2 sample, confirming the fluorite structured CeO 2 with Fm3m space group[25].The strong diffraction peaks indicated the good crystalline nature of the samples.No noticeable change in the diffraction patterns, i.e., any additional peaks related to Fe dopant, were detected, signifying the single phase, highly pure nature, and proper substitution of Fe ion at the Ce site in the nanocrystals.These further suggest the complete dissolution of Fe into the ceria lattice and the formation of a solid solution of Ce-O-Fe.The appearance of a broad peak centred at 19.5 2θ value that corresponds to the peak of PANI as well as the characteristics peaks of CeO 2 nanocomposite at corresponding 2θ values with diminished peak intensity (Fig.2B) representing lower crystallinity confirms the formation of PANI/Fe-doped CeO 2 nanocomposite.Raman spectra of the synthesised nanocomposite (Fig.2C) exhibit a single active mode centred at 462 cm − 1 , characteristics of the

Fig. 3 .
Fig. 3. XPS of undoped and Fe doped CeO 2 nanoparticles in the study; A) Survey spectrum of undoped and Fe doped CeO 2 nanoparticles; B), C0 and D) are the high-resolution spectra of Ce3d, O1s and Fe 2p respectively.

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Fig. 6 .
Fig. 6.(a) Schematic band structure of the PANI/CeO 2 interfaces in air and (b) in the presence of NH 3 .

Fig. 7 .
Fig. 7. Dynamic response of NH 3 gas sensor and related calibration curve.The response towards 6.25, 12.5 and 25 ppm of NH 3 gas at RT.

Fig. 8 .
Fig. 8. Repeatability of the transient response of NH 3 gas sensor to a) 6.25, b) 12.5 and c) 25 ppm NH 3 at RT.