A flexible gas sensor based on single-walled carbon nanotube-Fe2O3 composite film

Accepted Manuscript Title: A flexible gas sensor based on single-walled carbon nanotube-Fe2 O3 composite film Authors: Chunfei Hua, Yuanyuan Shang, Ying Wang, Jie Xu, Yingjiu Zhang, Xinjian Li, Anyuan Cao PII: DOI: Reference:

S0169-4332(17)30328-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.01.301 APSUSC 35096

To appear in:

APSUSC

Received date: Revised date: Accepted date:

22-12-2016 18-1-2017 29-1-2017

Please cite this article as: Chunfei Hua, Yuanyuan Shang, Ying Wang, Jie Xu, Yingjiu Zhang, Xinjian Li, Anyuan Cao, A flexible gas sensor based on single-walled carbon nanotube-Fe2O3 composite film, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.01.301 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Flexible Gas Sensor Based on Single-Walled Carbon Nanotube-Fe2O3 Composite Film Chunfei Hua,1 Yuanyuan Shang,1* Ying Wang,1 Jie Xu,1 Yingjiu Zhang,1* Xinjian Li,1 Anyuan Cao2 1

School of Physical Engineering, Zhengzhou University, Zhengzhou, Henan 450052, P. R. China

2

Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China * Corresponding authors: [email protected] ;[email protected]

Graphical abstract

Stable response of SWNT-Fe2O3 composite film gas sensors under different bending angles upon exposure to 20 ppm H2S

1

"Highlights" ►In particular, a SWNT-Fe2O3 composite film obtained via a simple annealing process. ►Composite film produces a stable response to H2S and shows enhanced sensitivity to NO2 and at room temperature. ►Formation of uniform Fe2O3 nanoparticles throughout the porous film is responsible for improved performance and enabling sensing to more gases. ►Flexible sensors that can be bent to large angles repeatedly are demonstrated.

Abstract Single-walled carbon nanotubes (SWNTs) have potential for creating high performance gas sensors, but the number of gases that can be detected is still limited and the sensitivity needs further improvement. Here, large-area SWNT films directly synthesized by chemical vapor deposition are configured into gas sensors for a range of toxic gases such as NH3, NO, and NO2. In particular, a SWNT-Fe2O3 composite film obtained via a simple annealing process produces a stable response to H2S and shows enhanced sensitivity to NO2 and at room temperature, compared with pristine SWNT films. Formation of uniform Fe2O3 nanoparticles throughout the porous film is responsible for improved performance and enabling sensing to more gases, and removes conventional steps such as chemical functionalization or doping. Flexible sensors that can be bent to large angles repeatedly are also demonstrated. SWNT films containing a large amount of residual catalyst can be directly manufactured into large-area, flexible or wearable, thin film or textile-configured sensors for various toxic gases. Keywords: Single-walled carbon nanotube film, Fe2O3, H2S, Gas Sensor

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1. Introduction Single wall carbon nanotubes (SWNTs) are cylindrical tubes rolled up by honeycomb lattice carbon sheets. Upon adsorption and desorption of gas molecules on the tube surface, electron transfer between the SWNTs and foreign molecules leads to enrichment or depletion of charge carriers and consequently change in electrical conductivity. The high surface area, conductive path, porous structure, and strong interaction with adsorbed molecules make SWNTs suitable candidates for building gas sensors in versatile configurations. In the past few years, there have been extensive studies on the gas sensing behavior of SWNT-based materials and structures.[1-7] Kong et al. first demonstrated opposite resistance change when individual SWNTs were exposed to NO2 and NH3, along with several others’ work later on similar oxidizing and reducing gases.[8-13] However, for practical applications, the sensitivity toward target gases need be improved significantly, and other factors such as the fabrication cost and device reliability must be considered as well. To this end, various physical and chemical approaches have been used to improve the response and selectivity of SWNTs-based gas sensors, such as chemical group functionalization,[14] grafting of metal nanoparticles[15-19] , elemental doping[20, 21] and composites with conducting polymers[22,23]. For example, attaching poly-(m-aminobenzene sulfonic acid) could increase the resistance change of SWNTs by more than 2-fold in response to NH3.[14] Adjizian et al. concluded that nitrogen doped carbon nanotubes (CNTs) were suitable for NO2 detection while boron doped CNTs showed high sensitivity to C2H2 at ppb level concentrations, although the response remained very low.[21] Alexander Star et al. fabricated a gas sensor array by grafting CNTs with different metal nanoparticles and analyzed data by pattern-recognition analysis tools to detect and identify different toxic gases for personal safety and environmental monitoring.[16] A PANI(polyaniline)-SWNT NH3 3

gas sensor prepared by electrochemical method showed superior sensitivity, low detection limit and good repeatability, although its response and recovery time was hundreds of minutes at room temperature.[22] On the other hand, creating SWNT-metal nanoparticle hybrid structures also extends the range (or type) of gases that can be detected. For example, H2S is a well-known toxic gas with great impact on human health and environment, however, pristine SWNTs are usually insensitive to H2S. Recently, Cu nanoclusters were deposited onto SWNTs by a hydrothermal method and enhanced the sensitivity to H2S, where the resistance modulation was attributed to the catalytic effect of Cu leading to cleavage of H2 from H2S and electron transfer to SWNTs.[15] Metal-oxide semiconductors (SnO2,[24] -Fe2O3,[25,

26]

WO3,[27] In2O3[28] and ZnO[29,

30]

) have been used as gas sensors owing to their

superior sensitivity to H2S, but those materials operate at temperatures above 100

with high power

consumption. The excellent mechanical properties of CNTs also allow fabrication of flexible thin-film sensors that can operate at room temperature under ambient condition.[15,31,32] If active oxide materials can be combined with SWNTs in a convenient way, a hybrid structure with enhanced sensing capability may be constructed. Here, we show that as-synthesized SWNT films act as large-area, thin film sensors for toxic gases including NH3, NO and NO2. In particular, given the presence of a large amount of residue catalyst among SWNTs, Fe2O3 nanoparticles can be exposed by a simple annealing process. This step not only enhances sensitivity toward NO2, but also enables sensing of H2S which otherwise cannot be detected by pristine SWNTs. Reliable response for gas concentrations down to 1 ppm is observed, with complete resistance recovery after each cycle. A flexible sensor is demonstrated by anchoring the SWNT-Fe2O3 composite film onto a plastic sheet which can be bent to large angles yet 4

still produce a stable response to H2S (about 10% resistance change to10 ppm H2S). 2. Experimental 2.1 Synthesis of SWNT films SWNT films were continuously prepared by floating catalytic chemical vapor deposition method at 1160

in a horizontal quartz tube. The precursor solution consisted of xylene as carbon source,

ferrocene (0.5g/ml, higher than 0.4g/ml used before) and elemental sulfur as catalyst, which was injected into the quartz tube and then carried into the reaction zone by mixture gas (H2/Ar). As-grown SWNT films were produced and blown out of the quartz tube by the mixture gas. 2.2 Preparation of SWNT-Fe2O3 composite films An as-grown SWNT film was spread on a porcelain boat and then placed in a tube furnace which was heated up to 600

at a rate of 10 /min and held for 1h in air. For comparison, the sample

heated up to 600 oC for 1 hour in argon was also prepared under the same conditions. 2.3 Fabrication of gas sensors The as-grown SWNT or SWNT-Fe2O3 composite film (2cm×3cm) was transferred on a glass slide or plastic substrate and connected with silver wire at both ends, as two electrodes for gas sensing tests. 2.4 Characterization The morphology and structure of the as-grown SWNT films and SWNT-Fe2O3 composite films were characterized by scanning electron microscope (SEM, JEOL JSM-6700F), transmission electron microscope (TEM, JEOL JEM2100), X-ray diffraction (XRD) with Cu K radiation (=1.5406Å) and Raman spectroscopy using a 514 nm wavelength laser. Thermal gravimetric analysis (TGA) was carried out with Linseis STA PT1600 at a heating rate of 10oC/min from room 5

temperature to 1000oC in air environment. X-ray photoelectron spectroscopy (XPS) measurement was carried out on an ESCALAB250Xi apparatus at base pressure of 1 × 10−9 mbar with and X-ray source of Al Kα. 2.5 Gas sensing experiments The gas sensing properties of the samples were characterized on a CGS-1TP (Beijing Elite Technology Co., Ltd.) intelligent gas sensing analysis system at atmospheric pressure and room temperature conditions with air as the desorption gas. The relative humidity was about 45%, which was not changed during the testing process. The gas sensing device was accessed to the circuit of the gas sensing analysis system, which can apply a voltage of about 4 V on the gas sensing device, and the test chamber was closed after the resistance value of the gas sensor stable. Then the target gas with certain concentration was introduced into the test chamber, setting the adsorption time as 300 s. The chamber was then open for desorption using air as the desorption gas over a desorption time of 300s. The intelligent gas sensing analysis system also simultaneously recorded the resistance change during the testing process. The chamber was closed again to do the next test under the same testing conditions, excepted for changing the concentration of the target gas. 3. Results and discussion 3.1 Structure, morphology and chemical composition characterization The preparation process of the pristine and annealed SWNT film sensors involved the following steps (as shown in Fig. 1a-c, see Experimental for details). First, a free-standing SWNT film was directly synthesized by chemical vapor deposition (CVD) with xylene/ferrocene as the carbon precursor and catalyst, as reported by our team.[33] Second, the as-grown film was suspended on a ceramic boat and then subjected to thermal annealing at 600 6

in air for one hour to form a

SWNT-Fe2O3 composite film. Then, the composite film was transferred to a glass slide and assembled into a gas sensor by attaching sliver wires to the film sides. Here, we adopted a precursor solution with higher ferrocene concentration (0.05 g/ml), which introduced numerous iron nanoparticles into as-grown SWNTs. Scanning electron microscopy (SEM) characterization showed a porous SWNT network containing dispersed Fe nanoparticles (residual catalyst, light contrast particles in the image) (Fig. 1d). These Fe nanoparticles (5 nm in diameter) and amorphous carbon attached on the surface of the SWNT bundles as seen from transmission electron microscopy (TEM) images (Fig. 1e, f). After thermal annealing the film color was lightened and became semi-transparent (Fig. 1a, b). During this process, Fe nanoparticles on the SWNTs bundles were oxidized and agglomerated to twig-like Fe2O3 nanoparticles. These Fe2O3 nanoparticles (20-50 nm in diameter) covered well on the surface of the SWNT bundles (Fig. 1h, i). Although some SWNTs have been burned during annealing at 600

in air, the remaining SWNTs maintained the spider-web

structure which played a key role in good mechanical and electrical properties. Figure 2a showed the X-ray diffraction (XRD) patterns of the as-grown SWNT film, the annealed SWNT-Fe2O3 composite film, and pure Fe2O3 powders as reference. In addition to the characteristic graphitic (002) and (101) diffraction peaks of CNTs at 25 and 44 as seen in the pristine sample, more diffraction peaks coming from Fe2O3 emerged from the annealed SWNT sample. These peaks corresponded to reflections from (012), (104), (110), (113), (024), (116), (018), (214), (300) and (119) planes, which matched the standard -Fe2O3 sample (JCPDS card No. 33-0664) and thus suggested a well crystallized and pure hematite structure. The mass ratio of Fe2O3 nanoparticles in the composite film and the Fe nanoparticles in the as-grown SWNT film have been measured by thermal gravimetric analysis (TGA) in air. After the SWNTs were completely burned at around 1000 , the 7

remaining sample weight after combustion was measured. The mass ratio of Fe and Fe2O3 nanoparticles in the as-grown SWNT film and the annealed SWNT-Fe2O3 film was about 40% and up to 96% (Fig. 2b). Raman spectra of the SWNT and SWNT-Fe2O3 film were shown in Figure 2c. The G-band observed at ~1585 cm-1 corresponds to the vibration of sp2 hybridized graphitic carbon, while the D-band with minor intensity at 1340 cm-1 was related to the vibrations of sp3 carbon atoms in defects and disorder caused by impurities in the SWNT film.[34] The D-band was absent after heat treatment indicating that the impurities had been removed, and a higher purity SWNT film was obtained. In addition to XRD and Raman, formation of Fe2O3 in the composite film was also verified by X-ray photoelectron spectroscopy (XPS) measurements. As shown in Figure 2d, the wide XPS spectrum of the SWNT-Fe2O3 composite film indicated the presence of C, O, and Fe elements in the sample. The peaks near 711.4 eV and 724.7 eV corresponded to Fe2p3/2 and Fe2p1/2 spin orbital of the Fe2O3, respectively, and a broad satellite peak at about 719.3 eV was also observed, which was consistent with the characterization of Fe3+ (Fig. 2d Inset). 3.2 Sensor behavior of as-grown SWNT film An as-grown SWNT film was transferred onto a rigid or plastic substrate to prepare a film-shaped gas sensor in a two-probe configuration. Schematic illustration for gas sensitive test process was shown in Fig. 3a. The electrical resistance of the gas sensor would change with the type and concentration of the target gas due to interaction SWNTs and gas molecules. The response of the RS (%)  gas sensor is defined as

( Rg  Ra ) Ra

 100% , where Rg is the original device resistance in the

target gas environment and Ra is the electrical resistance of the gas sensor in air. As a reducing gas, the NH3 molecules are adsorbed on SWNTs surface and electrons are 8

transferred to SWNTs through defects and oxygen atoms present on the surface via chemical processes, leading to decrease in the hole carrier concentration and corresponding increase in the sensor resistance. Performance of the as-grown SWNT film in response to NH3 at different concentrations (10 to 200 ppm) at room temperature was shown in Fig. 3b. The response and recovery periods were set as 300 seconds, and the process was reversible as seen from the symmetrical resistance change curves. The response time t95 (period needed to reach 95% of stable output signal) was about 250 seconds and the recovery time tR (time needed to bring the resistance back to 95% of the baseline) increased with higher concentration of the target gas, which was 125 seconds for 10 ppm NH3 and 350 seconds for 200 ppm NH3.The resistance of the gas sensor could return to initial value after 300 seconds recovery in air without heating or Ultra-violet irradiation. The resistance change increased at higher NH3 concentration, which were 1.5%, 1.9%, 2.7%, 3% and 4% for 10 ppm, 20 ppm, 50 ppm, 100 ppm and 200 ppm NH3, respectively. To study the cycling behavior of NH3 gas response, cyclic test for 9 cycles under 20 ppm NH3 with the same response and recovery time was carried out (Fig. 3c). The results indicated that the SWNT film based gas sensor had excellent stability and reversibility. A continuous response-recovery curve of the as-grown SWNTs film upon exposure to NO and NO2 at different concentrations at room temperature was shown in Figure 3d. Oxidizing gases such as NO2 and NO with an unpaired electron could obtain an electron from SWNTs. This increased the number of hole carriers in the SWNTs and improved the conductivity of the SWNTs, thereby reducing the film resistance. So, the response was negative (showing decrease in resistance) and the absolute value of the response increased at higher gas concentration. From Figure 3d, the absolute values of the response to NO at concentrations of 10 ppm, 20 ppm, 50 ppm and 100 ppm, were 2.9%, 9

4.7%, 7.3% and 14.5%, respectively. And the responses to NO2 were 2.9%, 7.2%, 10.2% and 16.4% at concentrations of 10 ppm, 20 ppm, 50 ppm and 100 ppm, respectively. The curves demonstrated that the as-grown SWNT film could sense both NO and NO2 at different concentrations. But the recovery of the sensor was rather slow compared to the recovery during NH3 sensing and the resistance could not return to the original value within 300 seconds. 3.3 Sensor behavior of SWNT-Fe2O3 composite film Although pristine SWNT films showed certain response to several gases, we found that H2S could not be detected by as-grown SWNTs. There was no interaction between original SWNT films and H2S gas molecules, as studied in literature.[35] And carboxyl and hydroxyl modified SWNT sensors showed improvement in the response to H2S (1% and 8.4% resistance increase at 50 ppm H2S).[36] Here, we adopted a simple annealing step to convert the as-grown SWNT film to a SWNT-Fe2O3 composite film (Fig. 4a). Because of remaining SWNT network, the resistance of the SWNT-Fe2O3 composite film was about 200-300  which was 2.5 to 3.5 times for as-grown SWNT film. The sensing property of the latter was investigated at room temperature with response and recovery time of 300 seconds as before. Compared with the as-grown SWNT film, the composite film heat treated at 600oC for one hour in argon atmosphere exhibited a clear response to H2S, owing to the presence of Fe2O3 nanoparticles. The mechanism of the H2S detection by the SWNT-Fe2O3 composite

film

sensor

can

be

explained

2 H 2 S ( g )  3O2 (ads )  2 H 2O( g )  2 SO2 ( g )  3e 

as

following

Equation[26]

. As shown in Figure 4a, electrons released by the

reaction between H2S gas molecules and oxygen ions were adsorbed on the surface of the Fe2O3 nanoparticles and then transferred to the SWNT film. This process caused more electron-hole recombination and reduction of hole carriers in the SWNT film, which increased the resistance of the 10

gas sensor. Figure 4b demonstrated a continuous response-recovery curve of the SWNT-Fe2O3 composite film upon exposure to H2S at different concentrations with response values of 3.4%, 9.4%, 11.6%, 12.9% and 13.1% at 1 ppm, 10 ppm, 20 ppm, 50 ppm and 100 ppm, respectively. The response value increased with H2S concentration, but the starting resistances for each response-recovery cycle were different due to chemical adsorption events, resulting in incomplete recovery of the film resistance. In order to investigate the sensing performance with full recovery, we tested the device and allowed its electrical resistance return to the initial value in a sufficient recovery time (Fig. 4c). The response values for 1 ppm, 10 ppm, 20 ppm, 50 ppm and 100 ppm are 3.7%, 11.3%, 15%, 17.3% and 18.3%, respectively, higher than those obtained without full recovery. (Figure 4b). Furthermore, the response of the SWNT-Fe2O3 composite film to NO2 was also improved. Repetitive sensing behavior of the SWNT-Fe2O3 composite film to NO2 was carried out in Figure 4d. The response was stable with almost complete recovery for 5 cycles upon 1 ppm and 20 ppm NO2 with the values of 9.5% and 19%, respectively. From the response curves of the as-grown SWNT film and the SWNT-Fe2O3 composite film upon exposure to different concentration NO2 from 1 ppm to 100 ppm (Fig. 4e), we found that the response of the latter was improved greatly compared with the former. Since the composite film sensor provided two possible locations, either at the SWNT surface or at the Fe2O3 nanoparticles, for NO2 molecule adsorption, thus electron transfer may occur as following: (1) the NO2 gas molecule obtained an electron from SWNTs like in the pristine SWNT film sample, and (2) the Fe2O3 nanoparticle obtained an electron from the underlying SWNTs, and then lost his electron to the adsorbed NO2 gas molecule. Therefore, the resistance decrease of the SWNT-Fe2O3 sensor was larger than of the as-grown SWNT film, upon exposure to the same NO2 11

concentration. Previously, flexibility testing of the film-shaped gas sensor was done by bending the film substrate to certain angles.[37-39] As mentioned above, the SWNT-Fe2O3 composite film was assembled on a flexible plastic substrate. As shown in Figure 5a, the gas sensor containing a composite film anchored on a flexible plastic substrate could be manually bent to large angles (90o and 180o) and then recover to straight shape. The gas sensing behavior of the SWNT-Fe2O3 composite film was performed upon exposure to 20 ppm H2S at room temperature and atmosphere by setting a response and recovery time of 300 seconds, respectively. Such a bent-to-straightened shape change was performed repeatedly without degrading the sensor structure (Fig. 5b). In either straightened or different bending shapes, the sensor shows negligible change in response. We had deformed the sensor between the straightened and bent shapes, and repeated this shape change process for 16 times. The sensor showed mechanical robustness during such large-degree deformation, and also a stable gas sensing response. 4. Conclusions In summary, we prepared large-area SWNT films by CVD and SWNT-Fe2O3 composite films by subsequent heat treatment in air at 600 . Compared with pristine SWNTs, the SWNT-Fe2O3 composite film gas sensors showed clear sensing to H2S and improved response to NO2 at room temperature. Stable sensing behavior was observed in our gas sensors fabricated on flexible substrates which could sustain lager deformations (e.g. repeated bending). Through controlled structural design and manufacturing such as the results presented here, high-performance flexible film-shaped SWNT gas sensors could be developed with potential applications in portable, wearable environmental monitoring devices. 12

Acknowledgements The authors greatly acknowledge financial support from the National Natural Science Foundation under grants of NSFC 51502267. Startup Research Fund of Zhengzhou University (1512317001). Henan province science and technology research project (162102410069). We thank Prof. Xinchang Wang for assistance in gas sensing tests.

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Figure1 (a) Photo of as-grown SWNT films suspended on a ceramic boat, (b) Photo of as-grown SWNT films after 600

heat treatment in air for 1 hour, (c) SWNT-Fe2O3 composite film-shaped gas

sensor. (d) SEM image of as-grown SWNT film. (e) and (f) TEM images of as-grown SWNT film. (g) SEM image of SWNT-Fe2O3 composite film, (h)-(i) TEM images of SWNT-Fe2O3 film.

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Figure 2 (a) XRD pattern of as-grown SWNT and SWNT-Fe2O3 composite film. (b) TGA curves of as-grown SWNT and SWNT-Fe2O3 composite film. (c)The Raman spectra of as-grown SWNT and SWNT-Fe2O3 composite film. (d) XPS spectra of SWNT-Fe2O3 composite film.

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Figure 3 Properties of gas sensor based on as-grown SWNT film. (a) Illustration of gas sensor based on SWNT film. (b) Response and recovery curves of SWNT film sensor upon exposure to NH3 with concentrations ranging from 10-200 ppm for 5min and air for 5 min. (c) Response and recovery curves of SWNT film sensor during 9 cycles of exposure to 20 ppm NH3 for 5min and air for 5 min. (d) Response versus time for SWNT film sensor upon exposure to NO2 and NO with concentrations ranging from 10-100 ppm.

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Figure 4 Response of gas sensor based on SWNT-Fe2O3 composite film to H2S and NO2. (a) Schematic diagram of gas sensor based on SWNT-Fe2O3 composite film and the schematic representation explaining the H2S and NO2 sensing mechanism. (b) Response and recovery curves of composite film sensor upon exposure to H2S (1, 10, 20, 50, and 100 ppm) for 5min and air for 5 min. (c) Response and recovery curves of sensor exposure to H2S (1, 10, 20, 50, and 100 ppm) with completely recovery. (d) 5 cycles upon exposure to 1 ppm and 10 ppm NO2, (e) Response of gas sensor based on SWNT and SWNT-Fe2O3 composite film upon exposure to NO2 (1, 10, 20, 50, and 100 ppm).

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Figure 5 Response of SWNT-Fe2O3 composite film gas sensors under different bending angles upon exposure to 20 ppm H2S. (a) Illustration of gas sensor bended to 0o, 90o, and 180o. (b) Response of gas sensor under bending from 0o to 180o and returned to 0o again.

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