Journal of Nanomaterials
Volume 2018 (2018), Article ID 9236450, 11 pages
https://doi.org/10.1155/2018/9236450

Research Article

Fast-LPG Sensors at Room Temperature by α-Fe₂O₃/CNT Nanocomposite Thin Films

B. Chaitongrat and S. Chaisitsak

Department of Electronics, Faculty of Engineering, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Correspondence should be addressed to S. Chaisitsak; sutichai.ch@kmitl.ac.th

Received 24 November 2017; Accepted 11 January 2018; Published 14 February 2018

Academic Editor: Chengyuan Wang

Copyright © 2018 B. Chaitongrat and S. Chaisitsak. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

We present performance of a room temperature LPG sensor based on α-Fe₂O₃/CNT (carbon nanotube) nanocomposite films. The nanocomposite film was fabricated via the metallic Fe catalyst particle on CNTs in which both the catalyst particles and the CNT were simultaneously synthesized by chemical vapor deposition (CVD) synthesis and were subsequently annealed in air to create α-Fe₂O₃. These methods are simple, inexpensive, and suitable for large-scale production. The structure, surface morphologies, and LPG response of nanocomposite films were investigated. Raman spectroscopy and XPS analysis showed the formation of α-Fe₂O₃ on small CNTs (SWNTs). Morphological analysis using FE-SEM and AFM revealed the formation of the porous surface along with roughness surface. Additionally, the sensing performance of α-Fe₂O₃/CNTs showed that it could detect LPG concentration at lower value than 25% of LEL with response/recovery time of less than 30 seconds at room temperature. These results suggest that the α-Fe₂O₃/CNTs films are challenging materials for monitoring LPG operating at room temperature.

1. Introduction

Liquefied petroleum gas (LPG) is a complex mixture of hydrocarbon compounds, which mostly consist of propane (C₃H₈) and butane (C₄H₁₀). LPG is widely used as a combustion apparatus in a heater, cooking equipment, and automotive vehicles. Besides, LPG is of most harmful gases due to its flammable, explosive nature, which presents many hazards to the human being as well as environment. To avoid the damage caused by leaks and gas explosions, there is prevailing need to detect LPG leakage at the lower explosive limit (~2.0 vol.% of LPG) [1]. In last few decades, metal oxide semiconducting (MOS) materials have been extensively used as an LPG sensor [2, 3]. However, the optimal sensing properties of them operated along with high temperature (300–500°C). In such case, a heater needs to be installed for the sensors to function, causing increased power consumption, complexity, and investment budgets. To overcome these limitations, the fabrication of a LPG sensor operable at room temperature has gained essence importance [4, 5].

Many reports are available on the development of the LPG sensors that operate at room temperature, the reduction of the MOS size [6, 7], and the mixing of MOS with metal nanoparticles [8] including the fabrication of heterojunctions [4, 9–11], which is one of the most strategies used. Due to the mixing of MOS and other materials, it formed a variety of the unique properties for sensing material, such as a change in conductance, improved surface catalytic property, increasing surface reaction sites, and producing a high porosity [12]. In addition, a formed contact potential at the interface between MOS and other nanostructured materials has also enhanced the sensing performance of a gas sensor [13, 14]. Therefore, more studies in fabricating devices based on heterojunctions are necessary, especially in nanostructure systems.

Iron oxide (Fe₂O₃) is n-type semiconductor, of which hematite (α-Fe₂O₃) is the most stable of Fe₂O₃. It has been widely studied for the various applications, including magnetic devices, pigments, catalysts, sensors, and medical fields [15]. In gas sensor applications, Fe₂O₃ has been continuously researched because it can provide high electron mobility, high chemical/thermal stability, multiple functions, and low cost [16, 17]. Recently, the nanostructures of Fe₂O₃ materials have attracted much attention as an effective material for various gases because of their high surface activities, high surface-to-volume ratios, and high carrier mobility [18]. However, their electrical conductivity is limited, because it requires high temperatures to work [19]. Meanwhile, CNT can be considered as highly conductive material at room temperature. Additionally, CNTs have a hollow structure, nanosized morphology (diameter 1–10 nm), and high surface area (500–1500 m²/g) [20–24]. Therefore, CNTs have been used for mixtures enhanced in the conductivity of Fe₂O₃ at room temperature [19, 25]. Moreover, Fe₂O₃ and CNTs nanocomposites have been fabricated through asynchronous methods, and then the mixture of Fe₂O₃ and CNT has been deposited on the substrate, usually using the usually spinning and screen-printing technique [25, 26]. However, these techniques are the major performance-limiting factors in the sensor because of its inhomogeneous film thickness. Therefore, the sensor fabrication process also provides the enhancement in sensing performance, which can control the porosity and the roughness, resulting in the improvement of sensing properties of the sensor. Recently, a new strategy to synthesize metal nanoparticle/CNT hybrid materials has been reported. Both of the Fe catalyst particles and the CNT are simultaneously synthesized. Then, the metallic Fe was transformed into maghemite (ɤ-Fe₂O₃) nanoparticles via a potential‐cycling method [27].

Here, we report an effective approach to fabricate the α-Fe₂O₃/CNT nanocomposite films, in which the CNT was uniformly coated with α-Fe₂O₃ particles (α-Fe₂O₃/CNTs). According to our previous work [28], we found that the Fe catalyst particles adhere to as-grown CNTs. In this work, those particles were annealed in air to create α-Fe₂O₃ into CNTs films. In addition, we also demonstrate the application of the α-Fe₂O₃/CNTs thin films as a LPG sensor. Obtained results indicated that the response and recovery time of sensor have been significantly improved for detecting LPG at room temperature (28°C). Moreover, the sensing mechanisms of α-Fe₂O₃/CNTs-based LPG sensor were also discussed.

2. Experimental Section

2.1. Fabrication of α-Fe₂O₃/CNT Thin Films

An α-Fe₂O₃/CNTs films-based sensor was fabricated through the following processes. Firstly, as-grown CNTs were synthesized through the vertical floating catalyst chemical vapor deposition method (FC-CVD) using pyrolysing solutions of ethanol-ferrocene. The details of synthesized as-grown CNTs have been previously reported [28]. Briefly, a precursor was prepared by dissolving ferrocene powder (Fe (C₅H₅)₂; Sigma-Aldrich) in ethanol solution (C₂H₅OH; Carlo Erba) at a 0.25 wt.% ferrocene/ethanol ratio. The mist of precursor solution was carried into the 960°C reactor via argon (Ar) gas with a flow rate of 1000 sccm. The synthesized CNTs were collected onto a membrane filter directly from the FC-CVD reactor at the bottom of the reactor under room temperature (hereinafter called “as-grown CNTs”). The density of as-grown CNTs was controlled by adjusting deposition time (study; 15, 30, 45, 60, 90, and 120 min). Secondly, as-grown CNT films were then transferred from the filter to a slide glass or silicon (Si) substrate by pressing and then dissolving the filter in acetone. Finally, the as-grown CNTs on substrate were put into a laboratory oven and annealed in air atmosphere at 350°C for 8 h to create α-Fe₂O₃ (hereinafter “α-Fe₂O₃/CNTs”). In order to investigate the effect of stand-alone CNTs on the gas sensing properties, α-Fe₂O₃/CNTs films were purified by immersing in 3 M hydrochloric acid (HCl) to remove α-Fe₂O₃, resulting in a purified CNTs film (hereinafter called “purified CNTs”).

2.2. Characterizations

The characteristics of CNTs were analyzed by Raman spectroscopy (Renishaw inVia Reflex) with an Ar laser (514.5 nm) as the excitation source. The surface composition changes of as-grown and α-Fe₂O₃/CNTs were measured by X-ray photoelectron spectroscopy (XPS; PHI 5700), which were carried out on an AXIS Ultra DLD-X-ray photoelectron spectrometer using a monochromatic Al Kα source (1486.6 eV). In order to analyze the chemical functional group of the samples, the Fe 2p, O 1s, and C 1s core level were deconvoluted using the Shirley background function and Vigt fit. The surface morphology of films was performed by field-emission scanning electron microscopy (FE-SEM: JEOL, Hitachi s4700), atomic force microscopy (AFM: Park Systems XE100), and transmission electron microscopy (TEM: JEOL, JSM-2010). The sensor was placed into the chamber; N₂ (or zero air) and LPG were then injected via a mass flow controller unit (MFC). The gas sensing characteristics of the sensor were examined by monitoring changes in resistance with a constant voltage of 5 V, using Keithley source meter (Model 2004). Data acquisitions (DAQ), storage, and plotting in real time were realized using a personal computer with LabView™ software via a GPIB interface control.

3. Results and Discussion

3.1. Raman Spectroscopy Analyses

Figure 1(a) presents the Raman spectra of the as-grown CNT and α-Fe₂O₃/CNT nanocomposite films in the low-frequency (~100–500 cm⁻¹) region. The as-grown CNT films show several characteristic peaks at 162 and 267 cm⁻¹. These peaks are the Raman spectra in radial breathing mode (RBM: ~100–350 cm⁻¹) of the small-diameter CNTs (SWNTs: single-walled CNTs), which corresponded to the diameters () of the CNTs films varying in the range of 0.9–1.5 nm [29]. It is clearly seen from this figure that CNTs with 1.5 mm diameter remain after the anneal, while the small tube (0.9 nm) is easily burned at 350°C due to curvature strain [30]. In addition, Raman spectra of α-Fe₂O₃/CNT nanocomposite films also reveal additional peaks at 227, 244, 298, and 414 cm⁻¹ but these peaks are small signals (see inset of Figure 1(a)). These peaks remained after removing CNTs by annealing at 550°C, which can clearly observe the main peak at 298 cm⁻¹, 414 cm⁻¹, and 1320 cm⁻¹ and minor peaks at 227 cm⁻¹ and 244 cm⁻¹. These peaks are identified to the hematite (α-Fe₂O₃) nanostructure [31, 32].

(a)

(b)

Figure 1: The Raman spectrum of the as-grown CNT and α-Fe₂O₃/CNT nanocomposite films measured in (a) low-frequency and (b) high-frequency regions. The inset in (a) shows a comparison between Raman spectra of as-grown CNTs, α-Fe₂O₃/CNTs, and α-Fe₂O₃ reference.

Figure 1(b) presents the Raman spectra of the as-grown CNT and α-Fe₂O₃/CNT nanocomposite films in the high-frequency (~1200–1800 cm⁻¹). In the high-frequency regions, two apparent peaks at 1570 cm⁻¹ and 1590 cm⁻¹ are observed and correspond to the D and G band of CNTs [29]. The intensity ratio of G and D mode (/): a higher ratio indicates a lower defect, therefore, better structural quality [29]. / of as-grown CNT and α-Fe₂O₃/CNTs are 11.9 and 12.3, respectively. It can be seen that / of α-Fe₂O₃/CNT films had higher than that of as-grown CNT films. / results indicated that the purity of CNTs increased and the structure of CNTs was not damaged even when annealed in air.

3.2. XPS Analyses

The XPS technique is used to analyze chemical composition on the top surface of the materials (depth < 10 nm). Compositional elements can be identified by the peak position in terms of binding energy and the peak intensity (peak area) can be related to the amount of elements in the material.

Figure 2(a) demonstrates that the XPS survey spectrum of both as-grown CNTs and α-Fe₂O₃/CNT films was carbon (C 1s; ~284 eV), oxygen (O 1s; ~530 eV), silicon (Si 2p; ~100 eV), silver (Ag 3d; ~370 eV), and iron (Fe 2p; ~700 eV) [33–35]. In α-Fe₂O₃/CNT thin films, peak at 100 eV is observed and corresponds to silicon of silicon substrate; this is because of the ultra-thin films of the Fe₂O₃/CNTs. In addition, the atomic percent (At%) table (see the inset table) demonstrated that the O : Fe atomic ratio on the α-Fe₂O₃/CNT films was close to 2 : 3, which is probably Fe₂O₃ [25].

(a)

(b)

(c)

(d)

Figure 2: (a) XPS survey scan of as-grown CNTs and α-Fe₂O₃/CNT films. (b) XPS deconvolution of (b) Fe 2p, (c) O 1s, and (d) C 1s. The inset in (a) shows the atomic percent (At%) and weight percent (Wt%) of as-grown CNTs and α-Fe₂O₃/CNT films.

The phase analyses in α-Fe₂O₃ have been achieved via the Fe 2p deconvolution, as shown in Figure 2(b). The peaks in the Fe 2p spectrum were considered as species of Fe in films, which is referenced in the previous reports of Fe₂O₃ [36–38]. Fe 2p1/2 and Fe 2p3/2 are doublet spin orbit component of Fe 2p. According to Figure 2(b), the as-grown CNT films were composed mostly of Fe metal, followed by Fe₃C (Fe-C), FeO, Fe₃O₄, and Fe₂O₃, respectively. After annealing, the dominant component in α-Fe₂O₃/CNT films was Fe₂O₃. In addition, two distinct peaks are observed at binding energies of 711.4 (Fe 2p3/2) and 724.4 (Fe 2p1/2) eV with a broad shake-up-satellite peaking at 719.3 eV. These peaks suggested that the phase formation of α-Fe₂O₃ was as good as those reported in the previous α-Fe₂O₃ studies [18, 36]. The occurrence of α-Fe₂O₃ is similar to that obtained by our Raman results. These results indicated that the most of Fe elements in α-Fe₂O₃/CNT films were α-Fe₂O₃, followed by the Fe₃O₄, Fe (OH)x, and Fe-Si, respectively. The increase in intensity of the O 1s deconvoluted peak at 530.4 eV indicated that the surface of α-Fe₂O₃ had mostly adsorbed oxygen ions [39], as shown in Figure 2(c). The peaks at 532.4, 533.4, and 534.2 eV corresponded to carboxylate/carbonyl (O–C=O/C=O), epoxy/hydroxyl (C–O–C/C–OH), and silicon oxide (Si–O) respectively. It should be noted that the α-Fe₂O₃/CNTs showed the occurrence of a new peak at 531.7 eV (in O 1s deconvolution; Figure 2(c)) and that Fe–C (in Fe 2p deconvolution; Figure 2(b)) disappeared. This may be that bonding of metallic Fe and CNTs (Fe–C) was changed to α-Fe₂O₃ and CNTs (Fe–O–C), of which Fe atom and CNTs were contacted via carbonyl groups (–O–C). The peak at 531.7 eV, therefore, possibly is assigned to the binding of α-Fe₂O₃ and oxygen functional group on CNTs through Fe–O–C bonds.

Furthermore, more structure and characterization of the CNTs were carried out by the C 1s deconvolution [39], as shown in Figure 2(d). Both of as-grown CNT and α-Fe₂O₃/CNT films presented the main peak at 284.9 eV, which can be assigned the C 1s (Sp²) binding energy of CNTs. The peaks in the shoulder of the main peak at 285.1–285.4, 286.5, and 288.9–289.8 eV could be assigned to Sp³/C–OH/C–O–C, C–O, and π–/O–C=O, respectively. Sp³ was attributed to amorphous carbon/defects in the nanotube structure, which, at this defect, could be grafted to the –OH and –C–O [40, 41]. It is widely recognized that the mixing of oxide NPs by the formation of nucleus Fe⁺ on CNTs induces damage in CNTs [25]. However, our method did not further significantly affect the CNTs due to the Fe spontaneously grow and uniformly assemble on the entire surface of each nanotube by CVD. In addition, the sp²/sp³ ratio of the α-Fe₂O₃/CNT films was higher than that of the as-grown CNT films. This is consistent with / ratios in the Raman results, implying that the structure of CNTs was not damaged even when annealed in air.

3.3. FE-SEM/AFM Analyses

From the obtained results from XPS and Raman spectroscopy, it is confirmed that the as-grown CNTs were decorated with metallic Fe. However, these metallic Fe could be easily transformed to α-Fe₂O₃ by air annealing. In this section, we investigated that the variation of deposition time effected on the morphology of α-Fe₂O₃/CNTs.

Figures 3(a)–3(c) present the SEM images of as-grown CNT films on silicon substrates, which were prepared at different deposition times of 15, 60, and 90 min, respectively. As-grown CNT films consist of CNTs and metallic Fe particles. The decrease in the distribution of CNTs and Fe particles suggest that the density of CNTs and Fe particles increases with increasing the deposition time. In addition, the hydroxide (OH–) part was observed onto CNTs and Fe particles in XPS result. These components may be attributed to the remained filters. The CNTs were the loosely packed and were consisted of the randomly oriented entangled CNT bundles, which were decorated with Fe NPs (size averaged 7.33 nm in diameter; Supplement Figure S1).

Figure 3: Morphology of (a–c) as-grown CNT films and (d–f) α-Fe₂O₃/CNT composite films on glass substrate.

Figures 3(d), 3(e), and 3(f) present the SEM images of α-Fe₂O₃/CNT films prepared with different deposition time of 15, 60, and 90 min, respectively. The hydroxide part was removed during air annealing leading to the formation of α-Fe₂O₃ nanoparticles. It is seen that the CNTs network with interconnected α-Fe₂O₃ nanoparticles is formed in Figures 3(d) and 3(e). In addition, the small agglomeration of the α-Fe₂O₃ particles is also observed at the interconnections of network structure along leading to the formation of the high porous surface. The α-Fe₂O₃/CNT structure with high porous is beneficial towards LPG sensing application because of the gas molecules enable to enter quickly through the porous structure.

Figures 4(a), 4(b), and 4(c) present that the AFM images of α-Fe₂O₃/CNT films. Root mean square (RMS) roughness of α-Fe₂O₃/CNT films is ~42, ~120, and ~2 nm for α-Fe₂O₃/CNT films prepared under deposition time of 15, 60, and 90 min, respectively. In our previous report [47], the sensor response of LPG sensor increases as a result of the high RMS value, because of the increase in the number of the active adsorption sites for oxygen or hydrocarbon molecules on the sensor surfaces.

(a)α-Fe₂O₃/CNTs prepared under 15 min

(b)α-Fe₂O₃/CNTs prepared under 60 min

(c)α-Fe₂O₃/CNTs prepared under 90 min

Figure 4: Morphology of surface measured using AFM on samples of the α-Fe₂O₃/CNT films with different deposition times. The 3D images recorded at 5 μm × 5 μm planar in contact mode.

According to SEM and AFM observations, it is seen that the porous structure and the surface roughness of α-Fe₂O₃/CNT films were controlled by deposition time along with spatial distribution of CNTs and α-Fe₂O₃ particles. SEM and AFM images (Figures 3(f) and 4(a)) of such α-Fe₂O₃/CNT films prepared under deposition time of 90 min clearly indicate that their surface had a bulk structure and low roughness. For a spatial distribution of Fe₂O₃/CNT films under prepared 90 min, small particles were closed together and easily aggregated into a flat surface with reduced surface area of α-Fe₂O₃/CNT films. Obtained results indicated that choosing a suitable deposition time could generate α-Fe₂O₃/CNT films along with a highly porous structure and high roughness.

3.4. Sensing Properties of LPG Sensors

The sensor response to LPG was defined as , is , where and denote the resistance of sensor when exposed to LPG gas and that when exposed to baseline gas, respectively [48]. Response/recovery time [49] is an important sensing performance indicator for a sensor to detect a flammable gas. A sensor’s response time () is defined as 90% change in resistance from its baseline value to the maximum value in the presence of LPG. Recovery time () is defined as the time required for recovering of the original resistance. In the examination of the sensor, carbon conductive electrodes were added onto the top of sensing films.

Figures 5(a), 5(b), and 5(c) display the change in the resistance of as-grown CNTs, α-Fe₂O₃/CNTs, and purified CNTs sensors in the presence of LPG at various concentrations. The baseline resistances in N₂ for as-grown CNTs, α-Fe₂O₃/CNTs, and purified CNTs were approximately 2.5 kΩ, 1.9 MΩ, and 360 kΩ, respectively. Figure 5(a) presents that the p-type LPG sensing behavior is observed for as-grown CNTs, which resulted from the CNTs sensing while the Fe catalyst was encapsulated with carbon [27]. Meanwhile, α-Fe₂O₃/CNT films display an n-type sensing behavior due to the formations of α-Fe₂O₃ that act as LPG sensing. It is well known that the sensing behavior of gas sensor has been related to the properties of sensing materials such as p-type material, which had hole majority carriers. The amount of hole will be decreased dramatically when reducing gas (LPG) exposure, due to the fact that an electron of reducing gas was injected to p-type material resulting in increased resistance of the sensor [50]. This could explain the sensing behavior of as-grown CNTs and α-Fe₂O₃/CNTs composite based on LPG sensor. Figure 5(c) clearly depicts that the sensing behavior of CNTs without α-Fe₂O₃ is of p-type.

(a)

(b)

(c)

(d)

Figure 5: Dynamic responses of (a) as-grown CNTs; (b) α-Fe₂O₃/CNTs; and (c) purified CNTs to LPG in air atmosphere. (d) demonstrates sensor responses by sensor type to varying LPG concentrations of 1–5 vol.%.

Figure 5(d) presents the sensor response, and α-Fe₂O₃/CNT films showed the maximum in response along with fast response/recovery time, while as-grown CNT and purified CNT films had a low response. This was due to the fact that high bonding energy among LPG atoms allows limited electron transfer from LPG molecules to the CNTs [51]. However, the purified CNTs had a quick response/recovery time because of the highly purified CNTs than that of CNT films. These results indicated that the α-Fe₂O₃/CNT films had an excellent responsive to detect the LPG.

Moreover, we observed that the time response of α-Fe₂O₃/CNTs was faster than that of as-grown CNTs. This observation can be explained by the different sensing mechanism. For as-grown CNT films, the response is attributed to the adsorption between the LPG and the surface of the carbon nanotube. On the other hand, the fast response of α-Fe₂O₃/CNT films might be due to the sensing process of the interface between the CNTs and the α-Fe₂O₃. According to this mechanism, the quick response of Fe₂O₃/CNT-based sensors could be attributed the Schottky barrier formed at α-Fe₂O₃ and CNTs junction [52]. Moreover, the annealing in air could improve the response time of α-Fe₂O₃/CNT films [53].

In next section, we investigated the effect of the deposition time on LPG sensing, as showed in Figure 6. “S1” and “S2” are defined as the sensor response of n- and p-type, respectively. The α-Fe₂O₃/CNT films were prepared under varying deposition time of 15, 30, 45, 60, and 90 min. For the sensing behavior of α-Fe₂O₃/CNT films, we found that the α-Fe₂O₃/CNT films present the both of n- and p-type behavior. However, notable behaviors of the sensor were attributed to the deposition time. α-Fe₂O₃/CNTs prepared for 15 min showed the p-type sensing behavior while α-Fe₂O₃/CNTs prepared under deposition time of 30, 45, 60, and 90 min presented n-type sensing behavior. Note that the resistance of the α-Fe₂O₃/CNT composites prepared under the deposition time of 120 min was also investigated, because of its extremely high resistance at room temperatures, which is out of the range of our instrument. Moreover, SEM and AFM images of α-Fe₂O₃/CNT films indicated that the choosing a suitable deposition time could be generated the α-Fe₂O₃/CNT films along with a highly porous structure and high roughness. This could explain the α-Fe₂O₃/CNT films prepared under deposition time of 45 min, which is the maximum response of ~6% to 5 vol.% of LPG.

Figure 6: Sensing performances of α-Fe₂O₃/CNT films prepared under varying the deposition time of 15, 30, 45, 60, and 90 min.

To examine the sensor’s multiple-cycle sensing performance under air environment, the α-Fe₂O₃/CNT films were measured under at various concentrations of 0.1, 0.4, and 0.7 vol.% of LPG diluted in zero air, as shown in Figure 7. The electrical resistance of sensors decreased upon LPG exposure and increased after replacing LPG with zero air. The sensor response, response, and recovery times of α-Fe₂O₃/CNTs on Figure 7 are summarized in Figure 8. It is observed that the sensor response and response/recovery times were stable and nearly equal at each concentration, indicating a good reproducibility of the sensing performance. Moreover, the α-Fe₂O₃/CNT films could detect LPG at concentration levels of less than 0.5 vol.% of LPG, which corresponds to 25% LEL of LPG. Note that LEL (lower explosive limit) is defined as the minimum level of concentration of LPG contained in the air sufficient enough to propagate a flame when exposed to a source of ignition.

Figure 7: Dynamic responses of α-Fe₂O₃/CNT composite films to LPG concentrations varying from 0.1 to 0.7 vol.% in a different mix environment of air.

Figure 8: Summarizing the sensing properties of α-Fe₂O₃/CNT composite films to LPG concentrations varying from 0.1 to 0.7 vol.% in a different mix environment of air.

3.5. LPG Sensing Mechanism

The sensing mechanism of the reducing gas (electron donors) like LPG results from the chemical reaction (between LPG molecules and the surface of materials) and relates to change in electrical properties of the samples, as discussed in many reports [48, 54]. For example, LPG sensing of the n-Fe₂O₃ semiconductor has been shown to be n-type sensing behaviors due to reducing in resistance when exposed to LPG [18]. In air, oxygen molecules in the ambient air absorb continuously on the empty absorption sites Fe₂O₃, which can be described by the following equations:

According to (1)–(3), the oxygen extracts electrons from the conduction band of Fe₂O₃ to form (at room temperature), leading to an increase in resistance [25]. When Fe₂O₃ is exposed to LPG (), on the film surface interacts with the LPG molecules and produces gas intermediates (CnH2n : O), water (H₂O) vapor, and electron. An electron in (3) led to a decrease in resistance of Fe₂O₃ materials. In contrast, p-CNTs presented the resistance reduced upon exposure to LPG, showing p-type sensing behaviors, as illustrated in sensor response of as-grown CNTs (see in Figure 5).

Moreover, all α-Fe₂O₃/CNT films exhibited excessive recovery (point “i” in Figure 6), which is similar to that reported by Dai et al. in a gas sensor made of monolayer α-Fe₂O₃ [55]. They explained the excessive recovery due to the one-electron response, one initial electron recovery, and two-electron eventual recovery. In our work, the one-electron response has received from the LPG molecules; the two-electron recovery might be received from the oxygen/substance intermediates gas () which may be due to the high active site on α-Fe₂O₃.

The advantageous properties of the CNTs-based gas sensor are high sensitivity and fast response; however, the CNTs-based LPG sensor is slow to respond and slow to recover. This was due to the fact that high bonding energy among LPG atoms allows limited electron transfer from LPG molecules to the CNTs [51]. In our experiment, the α-Fe₂O₃/CNTs-based LPG sensor’s improved response was achieved by converting Fe, which served as catalyst for the growth of CNTs, to α-Fe₂O₃, causing an increase in LPG active sites in the CNTs. In addition, the porous morphology and the new characteristics of the nanocomposite materials such as n-Fe₂O₃/p-CNT nanocomposites make it possible to improve LPG sensing performance. Further investigations are required to clarify this kind of sensing mechanism, which will be undertaken in our future work. Moreover, our literature review shows that various materials have been used in LPG sensor applications intended for room temperature use, as shown in Table 1. Our α-Fe₂O₃/CNT films-based LPG sensors exhibited a fast response/recovery time, indicating the promise of possible applications in LPG leak detection at room temperature.

Table 1: Nanocomposite materials-based LPG sensors operating at room temperature.

Years [ref.]
Materials
LPG (vol.%)
(%)
/@90%

2011 [1]
Ferric oxide
2.0
5100.0
~120 s/~150 s

2013 [4]
Ppy/TiO₂
0.10
55.0
~112 s/~130 s

2013 [11]
PTh/SnO₂ hybrid
0.5–2.5
~9.5–56.2
196–94 s/182–466 s

2014 [42]
CuO-SnO₂
0.5
~4200.0
~180 s/~200 s

2014 [10]
PANI/ɤ-Fe₂O₃
0.02
~140
~60 s/N/A

2014 [43]
ZnFe₂O₄
5.0
~1600.0
~120 s/~150 s

2014 [44]
Cu₂ZnSnS₄
0.12
~20.0
~70 s/~40 s

2016 [45]
Cu₂ZnSnS₄
0.1
~60.0
~40 s/~40 s

2017 [46]
BaTiO₃
0.5
50
~30 s/~60 s

This study
As-grown CNTs
5.0
~0.25
~290 s/N/A

Purified CNTs
5.0
~0.25
~20 s/45 s

α-Fe₂O₃/CNTs
0.1–5.0
~1.0–6.0
~25–13 s/~30–80 s

4. Conclusions

A simple fabrication of liquid petroleum gas sensor based on α-Fe₂O₃/CNT nanocomposite films is reported. α-Fe₂O₃/CNT films were successfully synthesized through the vertical floating catalyst chemical vapor deposition method (FC-CVD) using ferrocene-ethanol mist. To fabricate a sensor, the metallic Fe particles on CNTs were changed into α-Fe₂O₃ by annealing in air at 350°C. Raman spectroscopy, XPS analysis, SEM, and AFM images reveal the Fe₂O₃/CNT structure with porous and roughness structure. The sensing performance of the sensor was tested at room temperature. The sensing performance of α-Fe₂O₃/CNTs showed that it could detect LPG concentration at lower value than 25% of LEL with response/recovery time of less than 30 seconds at room temperature. These results suggest that the α-Fe₂O₃/CNTs films are a challenging material for monitoring LPG operating at room temperature.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors are grateful to the Western Digital (Thailand) for the Raman measurements. The work has been supported, in part, by the King Mongkut’s Institute of Technology Ladkrabang (KMITL) Research Fund. B. Chaitongrat gratefully acknowledges the financial support for this work provided by the Higher Education Research Promotion and National Research University Project of Thailand, Office of the Higher Education Commission.

Supplementary Materials

Figure S1: (a) TEM images which represent iron nanoparticles (Fe NPs) at surface of the growth of CNTs; (b) particle-size distribution of the Fe NPs performed by measuring 45 individual NPs on the TEM images. Figure   S1(a) shows the TEM image of the as-grown CNT films, which consisted of the randomly oriented entangled CNTs. The small-diameter CNTs were coated with Fe NPs, and some of these CNTs were coated with Fe NP clusters. These NPs were metallic iron (using XPS). An estimation of the distribution of Fe NPs’ sizes performed by measuring each NP obtained from TEM images yielded an average particle size of ~7.33 nm, as shown in Figure  1S(b). (Supplementary Materials)

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