Hierarchical Assembly of Î±-Fe2O3 Nanorods on Multiwall Carbon

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Hierarchical assembly of #-FeO nanorods on multiwall carbon nanotubes as a high-performance sensing material for gas sensors Mingjun Dai, Liupeng Zhao, Hongyu Gao, Peng Sun, Fengmin Liu, Sean Zhang, Kengo Shimanoe, Noboru Yamazoe, and Geyu Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00805 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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ACS Applied Materials & Interfaces

Hierarchical Assembly of α-Fe2O3 Nanorods on Multiwall Carbon Nanotubes as a High-Performance Sensing Material for Gas Sensors Mingjun Daia, Liupeng Zhaoa, Hongyu Gaoa, Peng Suna*, Fengmin Liua， Sean Zhangc, Kengo Shimanoeb, Noboru Yamazoeb, Geyu Lua* a. College of Electron Science and Engineering, JiLin University, 2699 Qianjin Street, Changchun 130012, China

b. Department of Energy and Material Sciences, Faculty of Engineering Sciences, Kyushu University, Kasuga-shi, Fukuoka 816-8580, Japan

c. State Key Laboratory of Supramolecular Structure and Materials College of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, China

KEYWORDS: gas sensors, CNTs, α-Fe2O3, p-n heterojunction

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Abstract

This paper presents a facile hydrolysis reaction and annealing for preparing a novel hierarchical nanoheterostructure via assembly of α-Fe2O3 nanorods onto multiwall carbon nanotubes (MWCNTs) backbones. The as-synthesized nanocomposites were characterized using XRD (X-ray diffraction), FESEM (Field emission scanning electron microscopy), TEM (Transmission electron microscopy), XPS (X-ray photoelectron spectroscopy) and BET (Surface Area and Porosity System). The observations showed uniform α-Fe2O3 nanorods approximately 100 - 200 nm in length and 50 - 100 nm in diameter that were hierarchically assembled onto the surface of the MWCNTs. The formation of the heterostructure was investigated by observing the evolution of the microstructure of the products at different reaction times. The X-ray photoelectron spectra (XPS) showed that the ability of the absorbing oxygen was enhanced by the formation of the heterostructure composites. Moreover, as a proof-of-concept presentation, the novel CNTs@α-Fe2O3 hierarchical heterostructure acted as a gas sensitive material. Significantly, the composites exhibited excellent sensing properties for acetone with high sensitivity, exceptional selectivity and good reproducibility. The response of the CNTs@α-Fe2O3 sensor to 100 ppm acetones at 225 °C was nearly 35, which was superior to the single α-Fe2O3 nanorods with a response of 16, and the detection limit of the sensor was 500 ppb.

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The enhanced properties were mainly attributed to the unique structure and p-n heterojunction between the CNTs and the α-Fe2O3 nanorods.

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1. INTRODUCTION Increased attention to environmental pollution and industrial emissions monitoring has led to extensive research on gas sensors that can detect toxic, inflammable, and explosive gases in real time. Oxide semiconductor gas sensors have been widely investigated1-3 because of their excellent sensing performances and simple fabrication. Over the past few decades, although many satisfactory results have been

obtained for the gas-sensitive performances of oxide semiconductors,4-8 the development of highly selective and controllably sensitized devices is still a future challenge for oxides. The sensing mechanism for sensors using semiconductor oxides is primarily the adsorption and reactions of oxygen and target gases causing a conspicuous change in the electrical conductivity.9−12 Thus, the morphology, microstructure, and composition of the sensors play important roles in their gas-sensitive properties. For this reason, the design and preparation of new sensing materials with unique morphologies and architectures is of interest to researchers. Recent research has shown that the composites of different oxide semiconductors with heterostructures that are composed of diverse components have enhanced sensing performances in comparison to single oxides.13−16 Moreover, as a functional metal oxide, n-type alpha-iron oxide (α-Fe2O3) has attracted enormous attention. Because of its excellent sensitivity, low fabrication cost, good permanence, and environmental friendliness, it has been extensively used in various fields, including gas sensors.17 Various materials related to α-Fe2O3 were prepared for use as gas sensors, such as porous α-Fe2O3 microspheres,17 Zn-doped 4


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α-Fe2O3

microcubes,18

and

heterogeneous

SnO2/α-Fe2O3.19

Sun17

prepared

monodisperse α-Fe2O3 discoid crystals using a hydrothermal method, and the crystals exhibited an excellent sensing performance for acetone. Afterwards, Wang19 prepared a SnO2/α-Fe2O3 heterostructure using an electrospun and hydrothermal method, and the heterostructure exhibited enhanced sensing properties compared to the single oxide. In that paper, it was concluded that a heterogeneous structure could improve the response to a test gas by increasing the adsorption of the oxygen species, which has been confirmed by other investigators.20-22 In addition, the discovery of carbon nanotubes (CNTs) has attracted attention in various fields because of their unique physical, chemical, electrical, and mechanical properties.23-25 The electrical properties of CNTs have been shown to alter and exhibit a p-type property when various molecules adsorb onto their surface.26 This behavior is the basis for the fabrication of gas sensors based on CNTs as a sensing material.27 Nevertheless, gas sensors using bare CNTs have some limitations, i.e., insensitivity to certain gases with a low adsorption energy or affinity, lack of selectivity, and long recovery time. With more researches available, people have discovered that surface defects and residual contaminants significantly impact the sensing performances of CNTs. Therefore, the functionalization of CNTs with different materials has been recognized as an effective strategy to change their chemical characteristics and enhance their gas sensing properties.28 Although noble metal modified CNTs display excellent sensing properties, these hybrid materials lead to a high cost and limited detection range because of the selective catalysis of the noble metal nanoparticles. 5



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Moreover, improving gas sensor performance can be achieved by decorating CNTs with semiconductor oxide nanocrystals because the CNT-supported oxides are believed to have collaborative or synergetic actions between the semiconductor oxides and the carbon nanotubes. Currently, significant progress has been made in the preparation of various CNT-based binary hybrids obtained from different synthetic routes.29 For instance, Chen et al. demonstrated that the sensing platform using hybrid nanostructures composed of discrete SnO2 nanocrystals dispersed on the surface of multiwall CNTs exhibited a high response to low concentrations of NO2, CO, and H2 at room temperature.30 Asad et al. reported a wireless H2S gas sensor based on CuO-single wall CNT hybrid nanomaterials that could detect H2S at a concentration as low as 100 ppb at room temperature.31 CNTs exhibit p-type properties when used as gas sensors. If the heterojunctions of the CNTs-α-Fe2O3 nanocrystals are synthesized, enhanced gas performance will be obtained. In this paper, we present a strategy for synthesizing hierarchical CNTs@α-Fe2O3 heterojunction composites composed of α-Fe2O3 nanorods uniformly assembled on the surface of CNT backbones (Figure 1). The combination of the extraordinary characteristics of the CNTs and the high-performance sensing material α-Fe2O3 allowed for the detection of acetone at a concentration as low as 500 ppb. The hybrid α-Fe2O3-CNT nanostructure was superior to either of its separate units. The growth of the nanocrystals onto the CNTs resulted in more active sites to adsorb gaseous molecules, which resulted in a higher sensitivity compared to the bare CNTs and the α-Fe2O3 nanostructure. Moreover, the formation of the heterostructure and the 6


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synergetic effect between the carbon nanotubes and the α-Fe2O3 nanorods were also sources of the enhancement in the sensing properties. 2. EXPERIMENT SECTION 2.1 Materials Synthesis CNTs with a range of 110 - 150 nm in diameter and 5 - 9 µm in length were provided by Sigma-Aldrich, America. In an archetypal procedure, 0.2 g of the CNTs were refluxed in 40 mL of an acid mixture (HNO3 (68 wt%):H2SO4 (72 wt%) = 1:3, v/v) for 5 h at 80 °C. After that, the products were rinsed with ethanol and DI (deionized) water alternately until the filtrate reached a neutral pH. Then, the products were collected via filtration and dried at 80°C. For the growth of the FeOOH nanospindles on the CNT backbones, 5 mg of the CNTs was dispersed in 10 mL of an aqueous solution of FeCl3 (0.1 M) via ultrasonication for 60 min in a capped bottle. The mixed black sol was kept in an oil bath with stirring at 80 °C for 5 h. The resultant black powder was separated using suction filtration with a microfiltration membrane (10 µm bore diameter), washed with ethanol and DI water alternately several times and subsequently dried at 60 °C overnight. Last, certain amounts of the CNTs@FeOOH hierarchical structures were annealed at 450 °C in air for 8 h with a slow ramp rate of 1°C·min-1 to form α-Fe2O3 nanorods on the CNT backbones. 2.2 Materials Characterization The crystal structures of the as synthesized products were characterized using X-ray diffraction (XRD) on a Rigaku TTRIII X-ray diffractometer operating at 40 kV and •

200 mA using Cu Kα radiation ( λ = 1.5418 Α ). The morphologies and particle 7



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sizes of the products were directly observed using a JEOL JSM-7500F field emission scanning electron microscope (FESEM) operating at 15 kV. Transmission electron microscope (TEM) and high-resolution TEM image patterns were measured using an H-7650 instrument (Hitachi, Ltd) operating at 120 kV. A STA 449F3 (NETZSCH) simultaneous TG analyzer under air from 30 to 1000 ℃ at a heating rate of 10 ℃•min-1 was used for the thermogravimetric (TG) and differential scanning calorimetric (DSC) analyses. An INVIA Micro-Raman spectrometer (RENISHAW) was used for the Raman spectroscopy analyses. A PREVAC X-ray photoelectron spectroscopy (XPS) system was used to analyze the composition of the products. A Micromeritics Gemini VII apparatus (Surface Area and Porosity System) using the Brunauer-Emmett-Teller (BET) equation based on the nitrogen adsorption isotherms was used to estimate the specific surface areas of the as-synthesized samples. 2.3 Fabrication and Measurement of the Gas Sensor For the gas sensors, the obtained products were coated on the outer surface of a commercially produced ceramic tube to form a thick sensing film. The sensor structure is illustrated in Figure S1 (a)†. The ceramic tube had a 0.8 mm internal diameter, a 1.2 mm external diameter, and a 4 mm length, and each end of the tube had a couple of Au electrodes. Simultaneously, two Pt wires were attached to each electrode. Some as-synthesized powders were diffused in deionized water and ground to form viscous slurries that were evenly coated on the ceramic tube to form a thick film. After drying in air for 30 min, the sensors were annealed at 400 °C for 2 h. Then, we placed a Ni–Cr alloy coil as a heater into the alumina tube to provide the 8


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operating temperature for the sensor. A static system was utilized to investigate the gas sensing properties of the device. The sensors were placed in a sealed chamber (volume of 50 L) filled with pristine air, and a given quantity of the target gases was injected into the chamber using a microsyringe to measure the sensing properties. The detailed testing procedure is provided in the support information. In the testing process, more than two sensors based on each material were prepared and measured to cross-validate their gas-sensing performances to reduce the error caused by the different devices. When the test gases were VOC (volatile organic compounds), we utilized the static liquid gas distribution method to obtain the required concentration of the target gas, which was calculated using the following formula:

c =

22.4 × ρ × δ × V1 M × V2

where c (ppm) is the required concentration of the test gas, ρ (g/mL) is the density of the corresponding liquid, δ is the desired gas volume fraction, V1 (µL) is the volume of the liquid, M (g/mol) is the molecular weight of the liquid, and V 2 (L) is the volume of the chamber. When testing reducing (VOC) gases, the gas sensitivity was defined as Rg/Ra (p-type) or Ra/Rg (n-type), where Ra was the electrical resistance of the gas sensor in the air, and Rg was the resistance in the test gases. The response time and recovery time were defined as the time taken by the sensor to reach 90% of the total resistance change in the process of adsorption and desorption, respectively. 3. RESULTS AND DISCUSSION 3.1 Structural and Morphological Characteristics 9



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The crystallographic structure and phase purity of the precursor CNTs@FeOOH sample were investigated using XRD, as shown in Figure 2a. All the diffraction peaks belonged to the tetragonal β-FeOOH (JCPDS no. 3-440), excluding the peak at ca. 26.30 next to the peak of β-FeOOH at ca. 26.70, which was assigned to the (002) planes of the CNTs. The weak peak of the CNTs in the pattern was attributed to its low percentage and masking by the outer β-FeOOH. A panoramic view of the SEM images shows that the product is composed of meandering and closely intertwined CNTs evenly adorned on the whole surface with nanospindles approximately 400 nm in diameter and tens of micrometers in length, as shown in Figure 2b-d. TEM observation further confirmed the hierarchical CNTs@β-FeOOH structures, which were characterized by the radiate assembly of high density nanospindles with lengths of approximately ~180 nm and diameters of hundreds of nanometers on the long, curved CNT backbones (Figure 2e-f). The SAED (selected area electron diffraction) analysis (Figure 2g) was superimposed with a pair of short diffraction arcs from the CNTs and the diffraction rings from the polycrystalline β-FeOOH nanospindles. Figure 2f presents an HRTEM (High-resolution TEM) characterization of the β-FeOOH nanospindles, which reveals that the nanospindles are polycrystalline. The lattice fringes are visible, and the fringe spacing are 0.23 nm, 0.25 nm, and 0.33 nm, which correspond with the interplanar spacing of the (140), (210), and (001) planes of the tetragonal β-FeOOH, respectively. We suggest that the formation of the structure occurs because the Fe3+ ions are preferentially adsorbed onto the polar oxygen-containing groups on the surface of the CNTs due to electrostatic adsorption. 10


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Subsequently, the hydrolysis of Fe3+ might result in the heterogeneous nucleation of FeOOH nanocrystals and further growth on the CNT backbones. During the process, the size of the FeOOH nanocrystals coated on the CNT backbones will increase with the reaction time, as shown in Figure S2†. The hierarchical property of the CNTs@β-FeOOH nanostructures allows the compositions of their individual constituents to be altered without compromising the whole construction. After annealing in air, the β-FeOOH phase transformed into hexagonal α-Fe2O3 on the CNT backbones, and the CNTs@α-Fe2O3 hierarchical structure was obtained, as illustrated by the XRD examination (Figure 3a) and SEM analysis (Figure 3b-d). The diffraction peaks of the composite were assigned to the hexagonal α-Fe2O3 (hematite, JCPDS no. 33-664), except for the peaks from the CNT (002) planes at ca. 26.30. Figure 3b shows that the single α-Fe2O3 nanorods with good uniformity had an average size of ~230 nm in length and ~80 nm in diameter. In the composite, the α-Fe2O3 nanoparticles coated the CNT surface to form the CNTs@α-Fe2O3 composite (with a size of approximately 450 nm in diameter and tens of micrometers in length) and loosely piled together, as shown in Figure 3c-d. The loose stacks allow for faster gas diffusion and create a larger specific surface area. The TEM examination (Figure 3e) revealed that the thermal transformation from β-FeOOH to α-Fe2O3 nanorods was a topotactic conversion process because of their similar structure. The α-Fe2O3 nanorods coated on the CNTs were 100 - 250 nm in length. The SAED pattern (inset of Figure 3e) had diffraction rings from the polycrystalline α-Fe2O3 nanorods, which indicated the transformation from the 11



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assembly of FeOOH nanospindles to α-Fe2O3 nanorods on the CNT backbones. The HRTEM characterization further indicated the transformation, as shown in Figure 3f. The lattice fringes are visible, and the fringe spacing are 0.22 nm and 0.27 nm, which agree with the interplanar spacing of the (113) and (104) crystal faces of hexagonal α-Fe2O3, respectively. The TEM characterization of the α-Fe2O3 is provided in the supporting information. As shown in Figure S3, the TEM images show that after annealing in air at 450 ºC, the α-Fe2O3 had good crystallinity and was a polycrystal. The lattice fringes are obvious with fringe spacing of 0.37 nm and 0.25 nm, which correspond to the interplanar spacing of the (012) and (110) planes of hexagonal α-Fe2O3, respectively. Because the CNTs were closely surrounded by α-Fe2O3, they were hard to directly observe in the composite using SEM or TEM. To confirm the presence of the CNTs, thermogravimetric analysis of the composites and acid-treated CNTs was performed. As shown in Figure S5†, the acid-treated CNTs decomposed after 530 ℃, so a lower temperature of 450 ℃ was chosen to anneal the precursor CNTs@FeOOH. The mass of the CNTs@α-Fe2O3 composites decreased by ~18% above 530 ℃, which was the weight percentage of the CNTs present in the composite. On the other hand, Raman spectroscopy excited with the 514.5 nm laser line has been obtained. All Raman experiments were carried out in ambient Ar at 4 mW laser power, 514 nm laser source, 30 sec acquisition time and the average of 20 sample spots obtained with identical conditions. The Raman spectra (Figure 4) revealed the significant structural alterations happening in the chemical process from the original 12


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CNTs to the acid-treated CNTs and the CNTs@α-Fe2O3 composite. The general characteristics of the carbon materials in the Raman spectra are the D band (a peak at ~1350 cm-1) and G band (a peak at ~1580 cm-1).32 The disordered or amorphous carbon in the CNT sample contributes to the D band. The disorder is primarily due to the nanosized or finite graphitic planes and other carbon forms, e.g., defects on the CNTs - walls, vacancies, heteroatoms and kinks.32 The in-plane tangential stretching of the C-C bonds (C sp2 atoms) in the graphene sheets causes the G band. As shown in Figure 4, there are two primary bands in the range of 1200 - 1800 cm-1 in the Raman spectra of the three tested samples, and they are the G band and D band. In the process of the mixed acid treatment of the CNTs, some hexagonal ring structures of the carbon atoms are destroyed due to the violent oxidation by the strong oxidizing acid, and the hybridization form transforms from sp2 to sp3, which generates a variety of new chemical bonds and dangling bonds, introduces many defects and produces a strong D band. The ratio of the D and G bands, ID/IG, is usually a main parameter in evaluating the defect and disorder degree of carbon materials, i.e., the higher the ratio of ID/IG, the higher the degree of disorder in the carbon materials.33-34 The ratio of ID/IG for the CNTs@α-Fe2O3 composite was estimated to be ~1.05, which is higher than that of the CNTs (~0.12) or the acid-treated CNTs (~0.56). In contrast, the increased ID/IG ratio of the composite and acid-treated CNTs in comparison to the initial CNTs demonstrates an increase in the degree of defects, which results in a stronger D band signal. 13



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XPS (X-ray photoelectron spectra) measurements were taken to determine the electronic-states and surface chemical composition of the elements in the CNTs, single α-Fe2O3 nanorods and CNTs@α-Fe2O3 composite. As shown in Figure S4†, for the Fe 2p spectrum, the peaks of the binding energies for Fe 2p3/2 at ~710.7 eV and ~711.9 eV match the A site (tetrahedral site) and B site (octahedral site), respectively. In addition, other peaks at ~719.7 eV and ~725.0 eV correspond to the Fe 2p1/2 and shakeup structures, respectively.35 As for the surface resistance-type sensing material, the oxygen absorption capacity is important for the gas-sensing performance of sensors. Therefore, the O 1s spectra of the single α-Fe2O3 and CNTs@α-Fe2O3 composites displayed in Figure 5b-c, respectively, with curves that can be decomposed into three fitted peaks, indicate that there are significant differences between the oxygen states on the surface of the samples. The OL component with an energy peak at ~530.1 eV was attributed to the lattice oxygen species, and the middle of the three peaks with an energy of ~530.8 eV was the OV component in the oxygen vacancy regions in the Fe2O3 of the samples. The OC component with the energy peak at ~532.3 eV was identified as the chemisorbed and dissociated oxygen species.36 Figure 5d is the O 1s XPS spectra of the CNTs, and it revealed the presence of some hydroxyl and carboxylic functions on the CNT surface at 533.7 eV and 532.7 eV, respectively. Furthermore, the peak at ~531.6 eV for O 1s was identified as chemisorbed and dissociated oxygen (OC).

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We estimated the percentage of different

oxygen species in different samples using the intensity of the relevant peaks in the O 1s XPS peak. Their center position and the relative percentage of each peak are listed 14


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in Table 1. For the different samples, the relative percentages of the OC component were approximately 8.6% (single α-Fe2O3 nanorods) and 25.7% (CNTs@α-Fe2O3 composites), which showed that the CNTs@α-Fe2O3 composites had a stronger oxygen adsorbing ability. This was attributed to the transfer of OC from the CNTs to the α-Fe2O3 in the CNTs@α-Fe2O3 composites. Thus, the excellent chemisorption of oxygen greatly contributes to the high-performance gas sensing of the target gases. To confirm the conductivity type of the CNTs in this work, the Hot-Probe38 method was performed using a hot side temperature from 200 ℃ to 400 ℃. If the carriers are holes, the hot side is a lack of holes, and the cold side has an excess of holes. Therefore, an electric field was created that was directed from the cold side to the hot side with a higher potential on the hot side. The absolute value of the EMF increased with the increase in the temperature difference, and the results agreed with those of p-type semiconductors. As shown in Figure S7†, when the cold sheet metal was connected to the positive electrode of the multimeter and the hot side was connected to the negative electrode, the measured EMF was positive. When the hot side temperature changed from 200 ℃ to 400 ℃, the measured EMF changed from 6.2 mV to 19.3 mV. This result proves that the CNTs in this work are p-type. 3.2 Gas Sensing Properties The operating temperature is a significant parameter for a metal oxide semiconductor gas sensor because it dramatically influences the response to a gas. Figure 6a shows the correlation between the operating temperatures and the responses to 100 ppm acetones for the sensors based on the CNTs@α-Fe2O3 structures, single 15



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α-Fe2O3 nanorods, and single CNTs. The response of the CNTs@α-Fe2O3 sensor to 100 ppm acetones increased sharply with the temperature until it reached a maximum at 200 °C, and the response decreased with a further increase in temperature. Consequently, the response of the CNTs@α-Fe2O3 sensor to 100 ppm acetones reached its maximum of 38.7 at 200 °C. However, we used 225 °C as the optimal operating temperature because the sensor had a high response (34.6) with a faster response and recovery time at that temperature. As shown in Figure S9†, the response time and recovery time decreased with the increasing operating temperature, and the values were 10 s and 210 s at 200 °C, respectively, and 2 s and 35 s at 225 °C, respectively. A similar behavior was observed for the sensor based on the single α-Fe2O3 nanorods with a much lower response value. The response of the α-Fe2O3 sensor to 100 ppm acetones reached a maximum of 16.3 at 200 °C and 15.2 at 225 °C. The response of the sensor based on the CNTs@α-Fe2O3 composite structure to 100 ppm acetones was more than 2 times higher than that of the single α-Fe2O3 sensor, which clearly indicated that the sensitivity to acetone was enhanced by fabricating the CNTs@α-Fe2O3 heterojunction. The sensing transients of the three types of sensors to 100 ppm acetones at 225 °C are displayed in Figure S8†. The CNT gas sensors had a very low response, and the change in the sensor resistance from air to acetone gas was extremely small, less than 1% of the Ra (Resistance of the CNT gas sensors in air). When exposed to acetone gas, the resistance of the CNT sensors increased, which confirms that the CNTs in this work are p-type.

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Figure 6b shows the responses of three different types of sensors to 100 ppm of various VOC gases, including methanol, formaldehyde, ethanol, acetone, toluene and benzene. The CNTs@α-Fe2O3 sensor showed a higher response to the testing gases than the α-Fe2O3 and CNTs sensors. Furthermore, the CNTs@α-Fe2O3 sensor had the strongest response to acetone, which showed an outstanding selectivity for acetone. Figure 7a displays the gas responses for the CNTs@α-Fe2O3 and α-Fe2O3 sensors as the acetone concentration varied at 225 °C. The results show that the responses increased with the acetone concentration from 10 to 100 ppm. The CNTs@α-Fe2O3 sensor had a superior gas response compared to the α-Fe2O3 sensor. Moreover, the CNTs@α-Fe2O3 sensor had a lower detection limit (1.6 to 500 ppb of acetone) than the α-Fe2O3 sensor (2.5 to 10 ppm acetone), as shown in Figure 7b. The inset of Figure 7a shows a simple linear relationship between the response of the CNTs@α-Fe2O3 sensor to the acetone concentration from 10 ppm to 100 ppm. The composite structures of the CNTs@α-Fe2O3 composite could effectively improve the response to the acetone gas in comparison to the single α-Fe2O3. The two types of sensors displayed excellent response and recovery properties for different concentrations of acetone. Figure S10a† and Figure S10b† show the responses for the sensors based on CNTs@α-Fe2O3 and α-Fe2O3 to 100 ppm acetones at 225 °C, respectively. The response and recovery time of the sensor based on the as-synthesized CNTs@α-Fe2O3 composite were 2 s and 45 s, respectively. Meanwhile, the sensor using the single α-Fe2O3 had similar response and recovery times of 2 s and 30 s, respectively. As exhibited in the inset of Figure S10†, the three 17



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reversible cycles of the response curve showed reproducible and stable properties. Table 2 shows the sensing performance comparisons between the CNTs@α-Fe2O3 composites and acetone gas sensors based on Fe2O3. Table 2 shows that the devices based on the CNTs@α-Fe2O3 composites exhibited a higher acetone response along with faster response and recovery times than other devices reported in the literature. The long-term stability of a gas sensor is significant for practical applications. For the CNTs@α-Fe2O3 sensor, the gas response to 100 ppm acetones at 225 °C was nearly constant after near 200 unceasing loop tests, as shown in Figure 8. In addition, continuous testing for dozens of days showed that the CNTs@α-Fe2O3 sensor was stable in air, which further demonstrated the long-term stability of the gas sensors using CNTs@α-Fe2O3 composites. 3.3 Gas Sensing Mechanism For gas sensors based on metal oxide semiconductors, the most widely accepted sensing mechanism is based on the change in the resistance of a material due to the adsorption and desorption of oxygen molecules on the surface of the sensing materials.39-42 For an n-type semiconductor such as α-Fe2O3, the oxygen adsorption occurs on the surface of the material when it is exposed to air. In this process, the oxygen molecules capture the electrons of the conduction band and are simultaneously ionized to oxygen ions (O2−(ads), O−(ads), or O2−(ads)). Therefore, the electron concentration in the conduction band decreases, and the resistance of the sensor increases. Upon exposure to the reducing gas (like acetone), the absorbed oxygen reacts with the organic molecules on the surface of the sensing material to 18


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simultaneously form CO2 and H2O and release the captured electrons into the conduction band, which results in a reduction in the sensor resistance. In this work, the enhanced gas sensing performance of the CNTs@α-Fe2O3 composites can be attributed to the following aspects. First, the formation of a p-n junction between the CNTs and α-Fe2O3 plays a leading role in the enhanced performance of the composites. The calculated band gap and work function of the p-type CNTs reported in previous literature are approximately 5.6 eV and 0.5 eV, respectively.43 Otherwise, the electron affinity and band gap of the n-type α-Fe2O3 are 4.78 eV and 2.2 eV, respectively. Hence, after the α-Fe2O3 was coated on the CNTs, p-n heterojunctions formed at the interface between the CNTs and α-Fe2O3, which led to band bending in the depletion layers. The qualitative band diagram of the CNTs@α-Fe2O3 shows that the electrons can be moved from the conduction band of α-Fe2O3 to the CNTs44-46 and that the holes will move in the opposite direction of the electrons due to the relative position of the conduction band in the CNTs and α-Fe2O3. The hole depletion layer is produced in this process, as shown in Figure 9b, and enhances the oxygen adsorption, especially the chemisorbed oxygen. In addition, the XPS analysis (in Figure 5 and Table 1) shows a large increase in the amount of chemisorbed oxygen. Eventually, the electron concentration in the α-Fe2O3 decreases, which leads to an increase in the resistance of the CNTs@α-Fe2O3 composite. Figure S11† shows that the resistance of the CNTs@α-Fe2O3 composites is higher than the single α-Fe2O3 in air at the same temperature. When exposed to acetone gas at an appropriate temperature, the surface reaction between the target gas and the adsorbed 19



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oxygen species occurs and releases electrons back to the composites, which causes a sharp reduction in the electrical resistance of the composites. Thus, the enhanced response will be obtained. Furthermore, for gas sensors, increasing the active surface area of the sensing material is an effective method to improve the performance. As shown in Figure S12†, the CNTs@α-Fe2O3 composite has a larger surface area (22.6 m2/g vs. 12.6 m2/g for pure α-Fe2O3), and the larger surface area provides more active sites for the adsorbing gas molecules and promotes electron diffusion to the CNTs, which contributes to the enhanced response of the sensors. 4. CONCLUSIONS In summary, a novel CNTs@α-Fe2O3 hierarchical heterostructure was prepared by assembling α-Fe2O3 nanorods on CNT backbones. The characterization results demonstrated that the α-Fe2O3 nanorods were successfully coated on the surface of the CNTs. The as-obtained CNTs@α-Fe2O3 composite was used as the sensing material for the gas sensor. The enhanced sensing properties, including high response, good reproducibility and excellent selectivity to acetone, were achieved via the construction of the heterostructure. The improvement in the sensor may be ascribed to variations in the heterojunction barrier in the diverse gas atmosphere, the stronger ability for chemisorbing oxygen and the larger specific surface area of the CNTs@α-Fe2O3 hierarchical nanostructure composite.

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FIGURE CAPTIONS:

Figure 1. Schematic illustration of the formation of α-Fe2O3 nanorods coated on the CNT backbone: (I) heterogeneous growth of β-FeOOH nanospindles on CNTs via force hydrolysis of Fe3+ ions; (II) thermal transformation of β-FeOOH nanospindles into α-Fe2O3 nanorods on CNTs via annealing CNTs@FeOOH structures in air.

21



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Figure 2. (a) XRD patterns; (b-c) SEM images of the precursor CNTs@β-FeOOH; (d-e) TEM images of the precursor; (f) HRTEM image; (g) corresponding SAED pattern inset of (e).

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Figure 3. (a) XRD patterns of the CNTs@α-Fe2O3 nanostructures and α-Fe2O3; (b) SEM image of a single α-Fe2O3 nanorod; (c-d) SEM images of the CNTs@α-Fe2O3 composites; (e) TEM image of the CNTs@α-Fe2O3 composites and corresponding SAED pattern inset on (e); (f) HRTEM image of the CNTs@α-Fe2O3 composites.

23



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Figure 4. Raman spectra of CNTs, acid-treated CNTs, and the CNTs@α-Fe2O3 composite.

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Figure 5. XPS analyses of (a) Fe 2p and (b) O 1s for the CNTs@α-Fe2O3 composites; (c) O 1s spectra of single Fe2O3 and (d) O 1s spectra of the acid-treated CNTs.

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Figure 6. (a) Response of the CNTs@α-Fe2O3 composites, single α-Fe2O3 and CNTs to 100 ppm of acetone vs. the operating temperature. (b) Comparison of the response of the CNTs@α-Fe2O3 composites, single α-Fe2O3 and CNTs sensors to 100 ppm of various gases at 225 °C. For the CNTs@α-Fe2O3 composites and single α-Fe2O3 sensors, the response is Ra/Rg, and for the CNTs sensor, the response is Rg/Ra. The error bars represent the standard errors of the average values for three independent measurements based on three devices of the same material.

26


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Figure 7. (a) Response of the sensors based on the CNTs@α-Fe2O3 composites and single α-Fe2O3 at 225 °C vs. the acetone concentration. (b-c) Dynamic response of the sensors based on the CNTs@α-Fe2O3 composites and single α-Fe2O3 at 225 °C to different concentrations of acetone. The error bars represent the standard errors of the average values of three independent measurements based on three devices of the same material.

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Figure 8. Responses of the sensors based on the CNTs@α-Fe2O3 composites to 100 ppm of acetone as a function of the number of (a) cyclic tests and (b) test days at 225 °C. The error bars represent the standard errors of the average values of three independent measurements based on three devices of the same material.

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Figure 9. (a) Qualitative band diagrams of α-Fe2O3 and CNTs and (b) the energy band structure of the CNTs@α-Fe2O3 heterostructures in air.

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TABLE CAPTIONS:

Table 1. Fitting Results of the O 1s XPS Spectra of Two Samples

Sample

Oxygen species

Binding energy (eV)

Relative percentage (%)

Pure Fe2O3

OL (Fe-O)

530.1

72.1%

OV(vacancy)

530.8

19.3%

OC (chemisorbed)

532.3

8.6%

530.1

43.0%

OV(vacancy)

530.6

31.3%

OC (chemisorbed)

532.1

25.7%

CNTs@α-Fe2O3 OL (Fe-O) composites

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Table 2. Comparison of Acetone Sensing Performance for Gas Sensors Based on Fe2O3

Sensing material

Synthesis approach

Operation Acetone Respons Response References temperatur concentratio e time (s) e (℃) n (ppm)

Porous α-Fe2O3hydrothermal 350 hollow sphere

100

6

9

17

Zn-doped α-Fe2O3 microcubes

100

22

3

18

Co3O4/a-Fe2O3 single nozzle240 electrospinni CSNF ng

50

11.7

2

21

SnO2/α-Fe2O3 single nozzle340 heterogeneous electrospinni ng, hydrothermal

100

30

15

19

SnO2 nanowireshydrothermal 290

50

8

7

40

CNTs@α-Fe2O water bath 3 composites

100

38.7

2

this work

hydrothermal 240

225

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Page 32 of 42

ASSOCIATED CONTENT

Supporting Information. Figure S1. Schematic diagrams of (a) the sensor, (b) the test circuit and (c) the test equipment. Figure S2. SEM images of the precursor CNTs@β-FeOOH with different growing times, (a) 1 h, (b) 3 h, (c) 5 h, (d) 8 h. Figure S3. (a) SEM image of α-Fe2O3 nanorods, (b) HRSEM image of α-Fe2O3 nanorods, (c) TEM image of α-Fe2O3 nanorods, (d) HRTEM image of α-Fe2O3 nanorods. Figure S4. XPS analyses of Fe 2p of (a) α-Fe2O3 nanorods and (b) CNTs@α-Fe2O3 composites. Figure S5. Thermogravimetric analysis of CNTs@α-Fe2O3 composites, α-Fe2O3 nanorods and CNTs. The test was conducted in air at a heating rate of 10 ℃•min-1. Figure S6. (a) SEM image of a section of one of the fabricated sensors, (b) SEM image of a section of another fabricated sensor. Figure

S7.

(a) Schematic

diagram

of the

Hot-Probe

method.

(b) The

thermoelectromotive force (thermal EMF) measured with a hot probe at 200 ℃ to 400 ℃. Each point in the figure represents the average of three experimental values obtained from different points of the sample. Figure S8. The sensing transients of the three sensors to 100 ppm acetones at 225 ℃: (a) and (b) Dynamic resistance and response-recovery curves of the sensors based on single α-Fe2O3, respectively; (c) and (d) Dynamic resistance and response-recovery curves of the sensors based on the CNTs@α-Fe2O3 composites, respectively; (e) and (f) Dynamic resistance and response-recovery curves of the sensors based on CNTs, respectively. Figure S9. 32


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Correlations among the gas response, τres (response time) and τrec (recovery time) to 100 ppm acetones at the operating temperature for the sensor using the as-obtained CNTs@α-Fe2O3 composites. Figure S10. The dynamic response-recovery curves of the sensor based on (a) α-Fe2O3 nanorods and (b) CNTs@α-Fe2O3 composites to 100 ppm acetones at 225 ℃. Figure S11. The resistance of the sensors based on α-Fe2O3 nanorods and CNTs@α-Fe2O3 composites in air at different temperatures. Figure S12. (a) N2 adsorption-desorption isotherm and the corresponding pore size distribution curve inset of (a) α-Fe2O3 nanorods and (b) CNTs@α-Fe2O3 composite, respectively. (c) Textural parameters of the two as-obtained materials.

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Page 34 of 42

AUTHOR INFORMATION

Corresponding author

*E-mail:

[email protected].

Telephone:

+86-431-85167808.

Fax:

+86-431-85167808.

*E-mail: [email protected].

Author Contributions

The manuscript was written with contributions from all authors. All authors have approved the final version of the manuscript.

Notes

The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENTS

This work is supported by the National Key Research and Development Program (No. 2016YFC0207300); the National Nature Science Foundation of China (Nos. 61503148, 61520106003, 61327804, 61374218); the Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT13018); the National High-Tech Research and Development Program of China (863 Program, Nos. 2013AA030902 and 2014AA06A505); the Science and Technology Development Program of Jilin Province (No. 20170520162JH); the China Postdoctoral Science Foundation funded project No. 2015M580247. 34


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Hierarchical Assembly of Î±-Fe2O3 Nanorods on Multiwall Carbon

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