Î±-Fe2O3 Porous Nano

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Self-sacrificial template driven LaFeO3/#-Fe2O3 porous nano-octahedrons for acetone sensing Nan Zhang, Shengping Ruan, Yanyang Yin, Feng Li, Shanpeng Wen, and Yu Chen ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00932 • Publication Date (Web): 10 Aug 2018 Downloaded from http://pubs.acs.org on August 13, 2018

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ACS Applied Nano Materials

Self-sacrificial Template Driven LaFeO3/α-Fe2O3 Porous Nano-octahedrons for Acetone Sensing

Nan Zhanga, Shengping Ruana, *, Yanyang Yina, Feng Lia, Shanpeng Wen c,*, Yu Chen a,b,*

a State Key Laboratory on Integrated Optoelectronics, Jilin University, Changchun 130012, P. R. China. b Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, PR China c College of Electronic Science & Engineering, Jilin University, Changchun 130012, P. R. China.

*

Corresponding author. Tel.: +86 431 85168242; fax: +86 431 85168242. E-mail address: [email protected] (S. Ruan), [email protected] (Y. Chen), [email protected] (S Wen).

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Abstract Semiconducting metal oxides (SMOs) with a porous structure have been paid close attention due to large specific surface area. Three-dimensional LaFeO3/α-Fe2O3 nano-octahedrons were synthesized by a facile one-step solvothermal method, and a metal-organic framework was used as a self-sacrificing template. The method included the synthesis of MIL-53(Fe)/Fe-La hydroxide precursor and its conversion into LaFeO3/α-Fe2O3 porous nanostructure by thermal annealing in air. A variety of techniques were employed to study the structures and morphology of the nano-octahedrons, including XRD, FESEM, TEM, XPS, ICP-OES and BET analysis. The gas sensing performances of LaFeO3/α-Fe2O3 nano-octahedrons to acetone were studied. By constructing a hetero-structure, the gas sensing performances of LaFeO3/α-Fe2O3 nano-octahedrons were significantly improved comparing with that of pure α-Fe2O3. In addition, the sensor was noted to have superior reversibility and selectivity. Such good performance may be due to porous nanostructure and p-n heterojunctions between two types of oxides. Key words: LaFeO3/α-Fe2O3 composites, metal-organic framework, gas sensor, heterojunction, porosity


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1. Introduction Driven by growing concerns about environmental protection and physical health in modern society, effective detection towards various gases has become of vital importance. Acetone, a kind of toxic and colorless saturated ketone with special pungent smell, has been used as an important organic raw material widely. Short-term exposure to acetone will inhibit and anesthetize the central nervous system, and long-term inhaled acetone may cause irreversible damage to liver, kidney and pancreas.1 In addition, the concentration of breathed out acetone in patients with diabetes were obtained to be above 1.8 ppm and that from a healthy person should be below 0.9 ppm.2 Thus, the sensors with a low detection limitation and rapid response and recovery speed to acetone are crucial to enable successful diagnosis of diabetes. Up to now, a rich variety of SMOs with micro and nanostructure were successfully found to improve gas sensing properties. Among them, porous micro-/nano-materials, a unique category of functional materials, play an important role because of their large volume of gas adsorption.3 Porous materials that can be divided into three major branches, including microporous, mesoporous and large pore materials, could be synthesized using various templated strategy.4-5 Among these methods, the approach using self-sacrificial template based on pyrolysis of solid precursors has been regarded as the most potential one in the synthesis of porous micro-/nanostructured materials.6 In order to obtain porous micro-/nanostructures, the self-sacrificial templates are removable well through pyrolysis process as well as maintain the



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primary shape. Metal-organic

framework

(MOF)

was

a

burgeoning

representation

of

self-sacrificing templates, and the morphology of MOFs are well adjusted by controlling the number of metal ions and various organic ligands. 7-10 Among a wide variety of known MOFs, the MIL (materials of institute Lavoisier frameworks) family, first synthesized by Professor Fe´rey of the University of Versailles in France, has been particularly paid close attention by researchers due to their enhanced stability, enormous porosity, and very large pores.11-13 For example, MIL-53 (Fe) with formula FeIII(OH)[(O2C–C6H4–CO2)]· H2O, having a rhombo-dodecahedron shape, are one of the most attractive MOF materials because of their facile synthesis methods and potential applications in gas adsorption, catalysis and energy storage.14-17 In particular, MOFs-templated SMOs could be applied in chemical sensors owing to high specific surface area and high gas accessible structures.18-19 Hematite (α-Fe2O3), as an important n-type semiconductor, possess a narrow band gap (~2.1 eV), which exhibits exciting application in gas detection.20-21 Meanwhile, the multifarious morphologies has also been reported, such as nanoparticles, nanowires, nanotubes, nanosheets and hierarchical nanostructures.20,22-23 Besides, it is regarded as one of the most important considerations that gas sensing performance can also be modified significantly by heterojunction.24-25 Previous studies on nano-/micro-structured SMO sensors paid mostly attention to single metal oxide (SnO2, Co3O4, Cr2O3 etc.), which exhibited relatively low selectivity for gases of similar properties. Polynary compounds have been a viable alternative to avoid such


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problem. Lanthanum ferrite (LaFeO3), one of the perovskite (ABO3) materials, have been regarded as brilliant candidates for lots of technical applications that range from gas sensor and catalysts to solar cells due to their catalysis properties, chemical and magnetic stability.26-30 Herein, we synthesized MOF-templated LaFeO3/α-Fe2O3 nano-octahedron by a one-step solvothermal method, which included synthesizing a nanostructured MIL-53(Fe)/La-Fe hydroxide and subsequent transformation to LaFeO3/α-Fe2O3 nanostructure through pyrolysis of the as-prepared precursor in air. To enhance the performance, different contents of LaFeO3 nanoparticles were grown on the surface of the α-Fe2O3 nano-octahedron, forming p-n heterostructures. The LaFeO3-decorated α-Fe2O3 showed higher sensitivity, lower optimum operation temperature, and rapid response/recovery speed. As a result, the excellent gas-sensing performance was highly associated with the p-n heterojunction structure. 2. Experiment section 2.1 Chemical reagent. All the starting materials were analytical grade (AR grade) and unpurified. Fe(NO3)3·9H2O (Ferric nitrate nonahydrate) and La(NO3)3·6H2O (lanthanum nitrate hexahydrate) were supplied by Xilong chemical reagent Co. Ltd. H2BDC (p-Phthalic acid) was purchased from inopharm Chemical Reagent Co. Ltd. HCON(CH3)3 (DMF) was obtained from Beijing Chemicals Works. 2.2 Preparation of pure and LaFeO3-decorated α-Fe2O3 nano-octahedrons. 2.1.1 Synthesis of MIL-53. Typically, Fe(NO3)3·9H2O (0.675g) and H2BDC (0.206 g) were dissolved in DMF (15 ml) and agitated magnetically to obtain



well-proportioned and stable solutions. The prepared homogenous solution was then moved into a 20 mL Teflon-lined stainless-steel autoclave and maintained at 110 °C for 20 h. The red precipitate was picked up through centrifugation, washed with absolute ethanol, and dried at 80 °C for 24 hours. 2.2.2 Synthesis of α-Fe2O3 nano-octahedrons. Nano-octahedron was obtained by annealing the as-prepared MIL-53 powder directly at 400 °C for 2 hours. 2.2.3 Synthesis of LaFeO3-decoratedα-Fe2O3 nano-octahedrons. 0.04, 0.08 and 0.16g La(NO3)3·6H2O was dissolved in 25 ml ethanol and stirred magnetically for 10 minutes, respectively. Then, 0.04 g of the as-prepared MIL-53 was dispersed in the above three solutions under ultrasonication for 5 minutes and stirred for 30 minutes. The as-synthesized precursor, combined by MIL-53/La-Fe double hydroxides, was collected by centrifugation and dried at 60 °C for 12 hours. Finally, the LaFeO3-decorated α-Fe2O3 nano-octahedrons were obtained by annealing the precursor at 400 ºC for 2 hours. The four samples obtained by the above method are named S1-S4, respectively, for convenience. 2.3 Characterization. The crystal texture for the S1-S4 samples were analyzed using X-ray Diffraction (XRD) patterns acquired with an X-ray Diffractometer (Shimadzu XRD-6000, Cu Kα radiation). A Scanning Electron Microscopy (SEM, XL30ESEM FEG) and Transmission Electron Microscope (TEM, JEOL JEM-3010) were used to respectively characterize the microstructures of the materials and morphologies. The chemical state and element composition are then characterized by X-ray photoelectron spectroscopy (XPS, MultiLab 2000, ThermoScientic, USA). ICP


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analysis were examined using the Ultima2 Inductively that was Coupled Plasma OES Spectrometer. BET gas-sorption measurements and BJH methods were performed to investigate the pore size distribution and the specific surface areas. 2.4 Fabrication and measurement of gas sensors. The fabrication was similar with the previous work.31 The α-Fe2O3 or LaFeO3/α-Fe2O3 composite powder was dispersed in deionized water to form a slurry, and then each powder was coated on the surface of an alumina tube with two Au electrodes and Pt lead wires to form a 300 µm thickness of sensing film and a Ni-Cr alloy heater was inserted into the ceramic tube to control the operating temperature. CGS-8, provided Beijing Elite Tech Co., Ltd., China, is used to measure the large resistance, and the maximum measurable value is 500 M. The response value (S) was defined as Ra/Rg, for a n-type device, where Ra and Rg is the resistance of the devices in air and in the presence of target gases, respectively. The response-recovery time was taken by the sensor changing 90 % of the total resistance change. 3. Results and discussion 3.1 Structural and Morphological Characteristics. The synthesis strategy of Fe2O3/LaFeO3 nano-octahedrons is schematically illustrated in Scheme 1. Firstly, MIL-53(Fe) was formed through the reaction of Fe3+ and H2BDC in DMF solution under high temperature and pressure conditions. Then, the as-synthesized MIL-53 particles were added into the ethanol solution of La(NO3)3 under stirring for 30 minutes to prepare the MIL-53/La-Fe hydroxides precursor. Finally, these precursors were transformed into Fe2O3/LaFeO3 nano-octahedrons under calcination at 400 °C



for 2 h. The morphologies of MIL-53 and MOF-templated α-Fe2O3 nano-octahedrons were observed by FESEM. From Figure 1a, we could see MIL-53 samples showed morphologies of octahedrons with a uniform grain diameter of ∼500 nm and good dispersivity. After conversing to α-Fe2O3 nano-octahedrons through thermal annealing process, α-Fe2O3 still retained an octahedral shape with a smooth surface as shown in Figure 1b. In addition, after annealing at 400 ºC for 2 h, the particle size of the nnano-octahedrons were decreased from 500 nm to 300 nm due to the disappearance of the metal organic skeleton. Further observing a single nano-octahedron in Figure 1c, it can be found that the nano-octahedrons were assembled by many 0-D particles (∼20 nm). When S1 sample was characterized by TEM (Figure 1d), the products had a porous octahedral structure, which correspond with the FESEM observation. The SAED pattern of α-Fe2O3 was shown in the inset of Figure 1d, where diffraction rings were distinctly observable, showing the Fe2O3 nano-octahedrons are polycrystalline. An HRTEM image (Figure 1e) further revealed that the crystal texture of α-Fe2O3 with fringe spacings of 0.25 nm and 0.272 nm were corresponding to the (110) and (104) planes, respectively. Figure 1f-h is the scanning TEM (STEM) image of S1 sample, where the spatial distribution of Fe and O in the porous composite structure were clearly detected. As shown in Figure 2a, we could find that the structure of MIL-53/La-Fe hydroxides precursor was maintained after the process of growing La-Fe hydroxides. The thermal annealing process will lead to the conversion from the MIL-53


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(Fe)/La-Fe

hydroxide

precursor

to

Fe2O3/LaFeO3.

The

morphology

of

LaFeO3/α-Fe2O3 (S3) was shown in Figure 2b. Comparing with the morphology of S1 sample, the diameter of these particles has increased from 300 nm to 350 nm owing to the growth of the hetero-layer of LaFeO3. Figure 2c exhibits the high-magnification SEM image of S3, where the surface of particles could be clearly observed. Further, the grain size distributions were measured to estimate the size of nano-octahedrons statistically, as shown in Figure S-1. The results revealed that the average grain sizes of Fe2O3 and LaFeO3-decorated Fe2O3 nano-octahedrons were found to be 0.33 and 0.37 µm, respectively. The morphology of S3 sample was further explored by TEM and HRTEM. As shown in Figure 2d, S3 sample has an average diameter of about 350 nm in accord with the SEM image. Highly homogeneous nanoparticles are dispersed around the nano-octahedron with rich pores and the thickness of heterogeneous layer is about 50 nm. In addition, primary particle sizes of 4 samples were estimated from TEM images, as shown in Figure S-2. The particle sizes of 4 samples was calculated to be 19.69, 16.80, 18.98 and 23.51 nm, respectively. Figure 2e showed a high-resolution TEM (HRTEM) image of LaFeO3/α-Fe2O3 composites. The measured interplanar distance of 0.25 nm and 0.27 nm was in good agreement with the (110) and (121) lattice planes of α-Fe2O3 and LaFeO3, respectively. It was preliminarily determined that hetero-structure was produced in sample S3. The EDS elemental mapping images (Figure 2g-i) showed that most of LaFeO3 nanoparticles are distributed on the surface of Fe2O3 porous nano-octahedron without noticeable aggregation on the whole structure.



X-ray diffraction (XRD) was employed to measure the crystal structures and purity. The XRD patterns of all samples were shown in Figure S-4. For α-Fe2O3 sample (S1), all of the diffraction peaks could be seen from the curves, and correspond with JCPDS Card No. 33-0664, which had lattice parameters of a = 5.0397 Å and c = 13.7676 Å, and no other crystal impurities were obvious. For the LaFeO3/α-Fe2O3 composites (S2, S3, S4), the diffraction peaks of hematite structured α-Fe2O3 can be found, and the two peaks of perovskite structure were seen from the curves, corresponding to the (121) and (240) planes of the LaFeO3 phase (JCPDS Card No. 75-0541). It shows a good agreement with the HRTEM result. In addition, when the peak intensity of LaFeO3 was compared with that of α-Fe2O3, we can easily find that the peaks of LaFeO3 were weaker, which may be caused by a small content of LaFeO3. The average sizes of Fe2O3 in S1-S4 samples calculated from the Debye−Scherrer equation based on the (104) peaks were 20.9, 19.7, 21.2 and 19.9 nm, respectively, which were in general agreement with the values estimated by Figure S-2. 3.2 Chemical composition and oxidation state. In order to get further information about the chemical composition and oxidation state, the XPS of S1-S4 samples have been provided in Figure 3. Table 1 listed the energy binding of La, Fe and O elements and the component of all oxygen species. The high resolution scan of La 3d for S2, S3 and S4 was shown in Figure 3(b). The peaks at ∼839.9 and ∼853.1 eV were indexed to La−O bonds of 3d5/2 and 3d3/2. As shown in Figure 3c, two major peaks at binding energies of ∼711.9 eV and ∼725.6 eV with a shakeup satellite at ∼719.1 eV, corresponding to the Fe 2p3/2 and Fe 2p1/2 pin-orbit peaks of Fe2O3. The capacity of


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adsorbing oxygen has a great influence for the resistance-type sensing materials. Therefore, the O 1s XPS spectrum of S1-S4 samples are shown in Figure 3d. The curves could be fitted into two peaks, which indicated that the samples had two kinds of considerable oxygen species, including lattice oxygen (Olattice) and surface adsorbed oxygen (Oads). Obviously, the relative intensity of adsorbed oxygen on the surface of S3 sample (Figure 3d) is higher than three other samples, which would lead to increase more surface adsorbed oxygen species and thus bring about a larger change in sensor resistance. X-ray photoelectron spectroscopy (XPS) analysis showed that the values of La/Fe were 4.09%, 9.68%, and 18.51%. In addition, ICP-OES analysis was employed in order to more accurately determine the composition of pure Fe2O3 and three different compositions of LaFeO3-decorated Fe2O3 nano-octahedrons, as shown in Table S-1. The La/Fe were calculated to be 3.31%, 7.26%, and 14.58%, which basically coincided with the result of XPS. 3.3

Pore-Size

Distribution

and Surface

Area

measurement.

Usually

MOF-templated materials have a large specific surface area, which will be helpful for improving response of sensitive devices.32 The specific surface area and pore size distribution of S1-S4 samples were determined by Nitrogen isotherm desorption (Figure 4). The N2-BET surface area of pure Fe2O3 and three different compositions of LaFeO3-decorated Fe2O3 nano-octahedrons was calculated to be 35.81, 52.33, 62.23 and 63.55 m2g−1, respectively (as shown in Figure 4a,c). The specific surface area of self-sacrificial template driven LaFeO3/α-Fe2O3 porous nano-octahedrons were higher than that of the pure Fe2O3 nano-octahedrons and previously reported



α-Fe2O3 hierarchical microcubes.33 S1-S4 samples all showed a shape between a type II and a type IV isotherm with an H3-type hysteresis loop for 0.7-0.9 S1 sample, 0.6–1.0 for S2 sample, 0.4–1.0 for S3 sample, and 0.6–1.0 for S4 sample, which indicated the existence of a mesoporous structure.34 A shift of the hysteresis loop toward lower relative pressure indicated that the average pore size of the LaFeO3-decorated Fe2O3 nano-octahedrons was larger than that of pure Fe2O3. The pore size distribution of S1-S4 samples calculated using the BJH method was shown in the inset of Figure 4a, b, c and d. And it indicated a uniform pore size of S1-S4 samples centered at 17.46, 27.50, 24.38 nm 17.64, respectively. 3.4 Gas sensing properties. In order to evaluate the potential practicality and reliability of the gas sensors, the fundamental properties of gas sensors have been investigated. The responses of the sensors based on S1-S4 samples to 100 ppm acetone at various operating temperatures (160-280 °C) were shown in Figure 5a. As reported by the sensing trend of all fabricated sensors in curves, the response increased as the temperature raised until a point is reached where a decrease in response sets in as the temperature is further increased.35 As a result, the optimum operating temperature of the sensors based on LaFeO3/α-Fe2O3 composites were shown to be 230 ºC, and the response value increased to 21.0 toward 100 ppm acetone, which was almost 3 times higher than that of pure one. The relatively low response at low temperatures was mainly due to the lack of sufficient thermal energy for the target gas molecules to react with adsorbed oxygen species. The rise of response can be explained by two factors with the increase of the operating


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temperature. On one hand, enough energy was acquired by acetone molecules to overcome the energy barrier, resulting in reacting with the surface oxygen species. On the other hand, due to the sufficient thermal energy, more electrons could jump from the valence band, which would lead to form more surface oxygen ions (O-). As the working temperature went up further, high temperature would reduce the gas adsorption capacity, leading to the decrease of response.19 Selectivity is also one of the significant factors to find out the performance of SMO-based gas sensors. For example, acetone, ethanol, toluene and xylene etc. possess similar chemical and physical properties so that selective detection may be more difficult.36 The responses of S3 and S1 samples to 100 ppm common pollutants, including acetone, ethanol, xylene, formaldehyde, ammonium, hydrogen and carbon monoxide, was shown in Figure 5b. The response values of the sensor based on S1 sample to seven different kinds of target gases illustrated less differences that did not acquire adequate selectivity. On the contrary, the sensor based on S3 sample showed the higher selectivity, because the response was more than 3 times higher than that towards other interference gases. Therefore, selectivity coefficient (K) was introduced to analyze selectivity of the sensor quantitatively, which was calculated using following relation: K=RCH COCH /RX 3

3

where RCH3COCH3 and RX are the responses of the sensor in presence of CH3COCH3 and other target gases (X). The ‘K’ value for ethanol, xylene, formaldehyde, ammonia, hydrogen and carbon monoxide are 3.01, 6, 6.28, 18.26, 17.35 and 17.8, respectively.



Higher ‘K’ value suggested that the sensor has a superior selectivity to discriminate the target gas among the mixtures of gas. As shown in Figure 3d, XPS spectrum analysis showed that −OH group located in 531±0.2 eV could form hydrogen bonds with the lone pair electrons on the carbonyl group of acetone, so that the electron adsorption of acetone was enhanced.34,37 According to the previous report, LaFeO3 modified on the surface of samples can raise the active region, resulting in enhanced ability to adsorb acetone.29,38

However, further increase of LaFeO3 modification did

not lead to higher ‘K’ value, which may be due to the decrease of the active area by excessive LaFeO3 nanoparticles. Thus the sensor based on S3 samples showed the best selectivity to acetone. Figure 6a reveals the gas responses of S1 and S3 samples with the concentrations of acetone ranging from 1 to 200 ppm at 230 °C. The S1 and S3 sensors displayed a rapid growing trend at the ranging from 1 to 50 ppm. However, the S1 and S3 sensors showed a slower increase in response and the curve would tend to be stable under high concentrations (50−200 ppm). The sensor based on S3 sample were approximately 1.75, 2.5, 3.5, 5.2, 8, 15.2, 20 and 23 to 1, 2, 5, 10, 20, 50, 100 and 200 ppm of acetone, respectively, while the responses of S1 sample were only 1.5, 1.9, 2.4, 3.2, 4.75, 6.3, 8.3 and 9.45, respectively. This phenomenon can be explained that a dynamic equilibrium between oxygen species adsorbed on the surface of the material and oxygen species consumed by reacting with acetone species tends to be achieved. Meanwhile, on the basis of the criterion defined by IUPAC, acetone detection limit of the sensor based on S3 sample was measured to be 1 ppm.39


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Figure 6(b) and (c) showed the response and recovery time based on S1 and S3 samples at the ranging from 1 to 200 ppm acetone. The gas sensing performances of all samples to acetone were listed in Table 2. The resistance of LaFeO3/Fe2O3 composites (S3 sample) was observably higher (264 MΩ) than that of Fe2O3 octahedron (S1 sample) (23 MΩ). The results of HRTEM and XRD showed that LaFeO3 on the surface of materials could increase depletion widths, leading to an increase in resistance. The response and recovery times based on S3 sample to 100 ppm acetone were about 1 s and 3 s, while response and recovery times based on S1 sample were 3 s and 5 s, respectively. The S3 sensor exhibited fast response and recovery process compared with the S1 sensors, which was attributed to the introduction of LaFeO3 nanoparticles. Figure 7(a) showed the cycling performances based on S3 sample to 100 ppm acetone at the optimum operating temperature. The response remained nearly constant after 50 cycles. Further, in Figure 7(b), the response of S3 sample was nearly constant after the dynamic 5-cycle response measurements, which verified the excellent response and recovery characteristics. Moreover, a long-term stability based on S3 sample was also measured within 30 days as shown in Figure 7(c), and the results indicated that the response of S3 sample exhibited a decrease of about 4%, which indicates a good long-term stability. Table 3 summarized the acetone sensing performances reported in the references.40-46 Until now, a large amount of SMO have been utilized to detect acetone, including Au/Fe2O3 thick films, Cu-doped WO3 hollow fibers, Pt-SnO2 nanofiber,



WO3 nanoplates, BaO·6Fe2O3 particles, Fe2O3/SnO2 composites, Ce-doped SnO2 hollow spheres. Our LaFeO3/α-Fe2O3 sensor has a high response of approximately 21 (1.75) to 100 (1) ppm acetone at 230 °C. Thus, it was obvious that the LaFeO3/α-Fe2O3 porous nano-octahedrons (S3 sensor) showed relatively lower operating temperature, faster response and recovery speed. 3.5 Gas sensing mechanism The sensing response of SMOs based on the variation of resistance is attributed to the chemical adsorption and desorption process of oxygen molecules and reaction of target gas with adsorbed oxygen species (O2−, O− and O2−).47 The O 1s peak located at 532.2 eV in Figure 3(d) confirmed the existence of oxygen species.34 As an n-type semiconducting transition-metal oxide, gas sensor based on α-Fe2O3 nano-material can absorb oxygen onto the surface, and then these oxygen molecules are ionized into oxygen species by electrons of the conduction band,48 as shown in Scheme 2a and b. In this process, oxygen molecules performed the role of electron acceptor to decrease the electronic concentration, which increased in resistance of the sensors. Besides, the depletion layer on the surface of the materials was brought about by adsorbed oxygen, which caused the downward bending of the energy band, as shown in Scheme 2c. When test gas was detected by the sensors, acetone molecules would react with oxygen ions (O-) on the surface, leading to release free electrons back to the conduction band of α-Fe2O3.49 This surface reaction leads to the transfer of electron toward sensing materials, so that depletion layer on the surface of a sensitive material became narrow, as shown in Scheme 2e. Thus, the resistance of the sensitive


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material would become low when the sensor was exposed to reduced gas (acetone). The possible process is shown in the Formula (1) and (2): 50 CH3COCH3 (g) → CH3COCH3 (ads)

(1)

CH3COCH3 (ads) + 4O− → 3CO2 (g) +3H2O + 4e−

(2)

The enhanced gas sensing properties of LaFeO3/α-Fe2O3 nano-octahedron are due to the specific surface structure (Figure 1d) and p–n hetero-structure (Figure 2e). First, the nano-octahedron assembled by many zero-dimensional particles is more conducive to produce a porous structure using a self-sacrifice template, leading to large specific surface area of the material (62.23 m2 g−1), which provides more active sites to adsorb oxygen and acetone molecules. Second, the hetero-structures between p- and n-type metal oxides play an important role in the enhanced response of composites. When more oxygen species adsorb on the surface of materials react with acetone molecules, electrons are released back to sensing material, which will lead to increase in response eventually. In our case, the Fermi level of α-Fe2O3 is higher than that of LaFeO3. As shown in Scheme 2d, when two different semiconducting metal oxides contact each other, electrons will transfer from α-Fe2O3 to LaFeO3 while holes will flow from LaFeO3 to α-Fe2O3 until their Fermi energy levels become equal, which would lead to the band bending and the formation of depletion layer. The electrons is depleted on side of α-Fe2O3, while holes is depleted on the other side of LaFeO3. G. Lu and his cooperators indicated that formation of the depletion layer could increase the amount of absorbed oxygen species, leading to an enhanced response.51 As shown in Scheme 2f, once the composites is exposed to reducing gas of



acetone, more oxygen ions react with acetone molecules and release electrons. Finally, the electrons release back to conduction band in n-type Fe2O3 and a smaller resistance (Rg) is obtained, leading to enhance the response (Ra/Rg). In addition, the normal ambient resistance (Ra) of hetero-structure will be usually higher than that without the heterojunction, because the widening of the depletion region decrease the width of the charge conduction channel.52 Thus, according to Ra/Rg, the response of composites towards acetone is significantly enhanced. As a result, the excellent gas-sensing performance was highly associated with the p-n heterojunction structure. 4. Conclusions In the current study, the novel porous composites composed of octahedron-like α-Fe2O3 and LaFeO3 nanoparticle were successfully synthesized through a facile one-step solvothermal method, and a metal-organic framework was used as a self-sacrificing template. Furthermore, the LaFeO3/Fe2O3 nano-octahedrons were used as the gas sensing material. The LaFeO3/Fe2O3 composites possess a regular octahedron-like morphology, larger surface area, and porous structure, which could provide a high degree of gas adsorption and improve diffusion rate of gas molecules. The result demonstrated that LaFeO3/α-Fe2O3 composites exhibited high sensitivity with the response of 21 to 100 ppm acetone, good selectivity, rapid response and recovery time, and cyclic stability at 230 °C. These findings showed that LaFeO3 and α-Fe2O3 (p-n heterojunction) nanomaterial is a very bright applicant to improve the performance of gas sensor. Associated content


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Supporting Information The histograms of the sizes of pure Fe2O3 and LaFeO3-decorated Fe2O3 nano-octahedrons from SEM images, the respective insets are the histograms of primary particle sizes of 4 samples from TEM images, XRD patterns of the MIL-53 and MIL-53(Fe)/Fe-La precursors and S1−S4 samples, ICP analysis about the composition of pure Fe2O3 and three different compositions of LaFeO3-decorated Fe2O3 nano-octahedrons, and dynamical response transients of the LaFeO3/α-Fe2O3 composites (S2, S3) to different concentrations of acetone. Author information Corresponding Authors *

E-mail: [email protected]. Tel.: +86-431-85168242. fax: +86-431-85168270.

*

E-mail: [email protected].

ORCID Shengping Ruan: 0000-0001-6923-9405 Shanpeng Wen: 0000-0001-5114-6307 Notes There is no economic benefit among the authors Acknowledgements Our study was supported by (1) National Natural Science Foundation of China (Grant No. 11574110), (2) Project of Science and Technology Plan of Jilin Province (201602041013GX,

20180414020GH),

and (3) Project of Jilin Provincial

Development and Reform Commission (Grant No.2018C040-2).



References (1) Navale, S. T.; Yang, Z. B.; Liu, C.; Cao, P. J.; Patil, V. B.; Ramgir, N. S.; Mane, R. S.; Stadler, F. J., Enhanced Acetone Sensing Properties of Titanium Dioxide Nanoparticles with a Sub-ppm Detection Limit. Sens. Actuat. B: Chem. 2018, 255, 1701-1710. (2) Jaisutti, R.; Lee, M.; Kim, J.; Choi, S.; Ha, T. J.; Kim, J.; Kim, H.; Park, S. K.; Kim, Y. H., Ultrasensitive Room-Temperature Operable Gas Sensors Using p-Type Na:ZnO Nanoflowers for Diabetes Detection. ACS Appl. Mater. Inter. 2017, 9, 8796-8804. (3) Gu, C.; Cui, Y.; Wang, L.; Sheng, E.; Shim, J.-J.; Huang, J., Synthesis of the Porous Nio/Sno 2 Microspheres And Microcubes and their Enhanced Formaldehyde Gas Sensing Performance. Sens. Actuat. B: Chem. 2017, 241, 298-307. (4) Bai, S.; Guo, J.; Shu, X.; Xiang, X.; Luo, R.; Li, D.; Chen, A.; Liu, C. C., Surface functionalization of Co3O4 hollow spheres with ZnO nanoparticles for modulating sensing properties of formaldehyde. Sens. And Actuat. B: Chem. 2017, 245, 359-368. (5) Wang, D.; Wang, Q.; Wang, T., Controlled Synthesis of Mesoporous Hematite Nanostructures and their Application as Electrochemical Capacitor Electrodes. Nanotechnology 2011, 22, 13560135616. (6) Tian, D.; Zhou, X. L.; Zhang, Y. H.; Zhou, Z.; Bu, X. H., MOF-Derived Porous Co3O4 Hollow Tetrahedra with Excellent Performance as Anode Materials for Lithium-Ion Batteries. Inorg. Chem. 2015, 54, 8159-8161. (7) Wu, P.; Liu, Y.; Li, Y.; Jiang, M.; Li, X.-l.; Shi, Y.; Wang, J., A Cadmium(II)-Based Metal–Organic Framework for Selective Trace Detection of Nitroaniline Isomers and Photocatalytic Degradation of Methylene Blue in Neutral Aqueous Solution. J. Mater. Chem. A 2016, 4, 16349-16355. (8) Li, Z. X.; Zhang, X.; Liu, Y. C.; Zou, K. Y.; Yue, M. L., Controlling the BET Surface Area of Porous Carbon by Using the Cd/C Ratio of a Cd-MOF Precursor and Enhancing the Capacitance by Activation with KOH. Chem.-Eur. J. 2016, 22, 17734-17747. (9) Li, Z. X.; Zou, K. Y.; Zhang, X.; Han, T.; Yang, Y., Hierarchically Flower-like N-Doped Porous Carbon Materials Derived from an Explosive 3-Fold Interpenetrating Diamondoid Copper Metal-Organic Framework for a Supercapacitor. Inorg. Chem. 2016, 55, 6552-62. (10) Chen, L.; Luque, R.; Li, Y., Controllable Design of Tunable Nanostructures inside Metal-Organic Frameworks. Chem. Soc. Rev. 2017, 46, 4614-4630. (11) Gordon, J.; Kazemian, H.; Rohani, S., MIL-53(Fe), MIL-101, and SBA-15 Porous Materials: Potential Platforms for Drug Delivery. Mat. Sci. Eng. C-Mater. 2015, 47, 172-9. (12) Sun, M.; Sun, M.; Yang, H.; Song, W.; Nie, Y.; Sun, S., Porous Fe2O3 Nanotubes as Advanced Anode for High Performance Lithium Ion Batteries. Ceram. Int. 2017, 43, 363-367. (13) Chen, Y.; Wang, Y.; Yang, H.; Gan, H.; Cai, X.; Guo, X.; Xu, B.; Lü, M.; Yuan,


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Figure captions Figure 1. (a) Typical FESEM images of MIL-53 nano-octahedrons. (b, c) High-magnification SEM images of α-Fe2O3 nano-octahedrons. (d) TEM image of α-Fe2O3 nano-octahedrons. (e) HRTEM image of α-Fe2O3 nano-octahedron. (f-h) Scanning TEM (STEM) image and corresponding elemental mapping images. Figure 2. (a) Typical FESEM images of MIL-53/La-Fe hydroxides precursor and (b, c) LaFeO3/α-Fe2O3 composites. (d) Typical TEM images and (e) HRTEM image of LaFeO3/α-Fe2O3 composites. (f−i) STEM image and corresponding elemental mapping images. Figure 3. XPS of the LaFeO3/α-Fe2O3 composites (S1-S4): (a) global survey spectrum, (b) La 3d, (c) Fe 2p, and (d) O 1s. Figure 4. Nitrogen adsorption−desorption isotherms of (a) S1 sample, (b) S2 sample. (c) S3 sample and (d) S4 sample. Figure 5. (a) Responses of α-Fe2O3 and LaFeO3/α-Fe2O3 composites at operating temperature to 100 ppm of acetone. (b) Selectivity of α-Fe2O3 and LaFeO3/α-Fe2O3 composites to 100 ppm of various gases. Figure 6. (a) Responses of the pure α-Fe2O3 (S1) and LaFeO3/α-Fe2O3 composites (S3) to different concentrations of acetone, respectively. (b) Dynamical response transients of pure α-Fe2O3 (S1) and (c) LaFeO3/α-Fe2O3 composites (S3) at 230 °C to acetone concentration. Figure 7. (a) Response of the LaFeO3/α-Fe2O3 sensor to 100 ppm acetone at 230 °C versus cycle number. (b) The dynamic 5-cycles response measurements to 100 ppm


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actone for LaFeO3/α-Fe2O3 at 230 ºC. (c) Long-term stability of the LaFeO3/α-Fe2O3 sensor to 100 ppm acetone at 230 °C. Scheme 1. Schematic of the process to fabricateα-Fe2O3 nano-octahedronswith porous structure, and LaFeO3/α-Fe2O3 composites. Scheme 2. Schematic illustration of the proposed gas sensing mechanism: (a-b) Diagrams of gas adsorption on hollow structure LaFeO3/α-Fe2O3 composite. (c-f) The energy band structure of pure α-Fe2O3 and LaFeO3/α-Fe2O3 hetero-structures.



Page 26 of 37

Table captions

Sample

Fe 2p3/2

Fe 2p1/2

La 3d5/2

La 3d3/2

OC

Peak (eV)

Peak (eV)

Peak (eV)

Peak (eV)

Peak (eV)

Relative Percentage (%)

Peak (eV)

Relative Percentage (%)

—

—

530.1

73.47

531.45

26.53

OL

S1

711.9 725.6

S2

712.0 725.7 839.9 853.1

530.3

79.32

531.7

20.68

S3

712.1 725.9 840.7 853.9 530.55

71.28

532.2

28.72

S4

711.9 725.6 840.3 853.6 530.25

81.05

531.7

18.95

Table 1. XPS analysis results of S1-S4 samples.


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material

Ra (MΩ)

Res (Ra/Rg)

Tres

Treco

temp (ºC)

S1 S2 S3 S4

23 187 264 437

8.3 9.6 20 3.6

1 4 3 3

3 4 5 2

230 230 230 230

Table 2. Comparison of Acetone Sensing Performances of S1−S4 Sensors.



materials

structure

Page 28 of 37

Particle

gas

working

size

concentration

temp. (ºC)

Ra/Rg

Tres

Treco

(s)

v(s)

ref

Au-Fe2O3

thick films

190-200

1-100

270

45

0.5

20

[40]

Cu-WO3

hollow fibers

300-500

20

300

6.5

15

40

[41]

Pt-SnO2

fibers

230

0.12-3

200-400

2

11

6

[42]

WO3

nanoplates

200-300

1000

300

3.5

∼8

∼13

[43]

BaO·6Fe2O3

particles

70

20

325

6

26

80

[44]

Fe2O3/SnO2

composites

300-500

100

250

10

∼3

∼90

[45]

Ce-SnO2

hollow spheres

10-15

100

250

12

∼18

∼7

[46]

Pure Fe2O3

nano-octahedrons

300

100

230

7.5

3

5

Fe2O3/LaFeO3

nano-octahedrons

350

100

230

21

1

3

Our work:

Table 3. Summary on the Gas Sensing Properties of SMO for Acetone. Table of Content


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Scheme 1. Schematic of the process to fabricateα-Fe2O3 nano-octahedronswith porous structure, and LaFeO3/α-Fe2O3 composites. 153x85mm (300 x 300 DPI)



Figure 1. (a) Typical FESEM images of MIL-53 nano-octahedrons. (b, c) High-magnification SEM images of αFe2O3 nano-octahedrons. (d) TEM image of α-Fe2O3 nano-octahedrons. (e) HRTEM image of α-Fe2O3 nanooctahedron. (f-h) Scanning TEM (STEM) image and corresponding elemental mapping images. 205x188mm (300 x 300 DPI)


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Figure 2. (a) Typical FESEM images of MIL-53/La-Fe hydroxides precursor and (b, c) LaFeO3/α-Fe2O3 composites. (d) Typical TEM images and (e) HRTEM image of LaFeO3/α-Fe2O3 composites. (f−i) STEM image and corresponding elemental mapping images. 174x135mm (300 x 300 DPI)



Figure 3. XPS of the LaFeO3/α-Fe2O3 composites (S1-S4): (a) global survey spectrum, (b) La 3d, (c) Fe 2p, and (d) O 1s. 194x168mm (300 x 300 DPI)


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Figure 4. Nitrogen adsorption−desorption isotherms of (a) S1 sample, (b) S2 sample. (c) S3 sample and (d) S4 sample. 163x121mm (300 x 300 DPI)



Figure 5. (a) Responses of α-Fe2O3 and LaFeO3/α-Fe2O3 composites at operating temperature to 100 ppm of acetone. (b) Selectivity of α-Fe2O3 and LaFeO3/α-Fe2O3 composites to 100 ppm of various gases. 318x450mm (300 x 300 DPI)


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Figure 6. (a) Responses of the pure α-Fe2O3 (S1) and LaFeO3/α-Fe2O3 composites (S3) to different concentrations of acetone, respectively. (b) Dynamical response transients of pure α-Fe2O3 (S1) and (c) LaFeO3/α-Fe2O3 composites (S3) at 230 °C to acetone concentration. 145x101mm (300 x 300 DPI)



Figure 7. (a) Response of the LaFeO3/α-Fe2O3 sensor to 100 ppm acetone at 230 °C versus cycle number. (b) The dynamic 5-cycles response measurements to 100 ppm actone for LaFeO3/α-Fe2O3 at 230 ºC. (c) Long-term stability of the LaFeO3/α-Fe2O3 sensor to 100 ppm acetone at 230 °C. 124x47mm (300 x 300 DPI)


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Scheme 2. Schematic illustration of the proposed gas sensing mechanism: (a-b) Diagrams of gas adsorption on hollow structure LaFeO3/α-Fe2O3 composite. (c-f) The energy band structure of pure α-Fe2O3 and LaFeO3/α-Fe2O3 hetero-structures. 190x107mm (300 x 300 DPI)


Î±-Fe2O3 Porous Nano

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