- Email: [email protected]
Au-modified α-Fe2O3 columnar superstructures assembled with nanoplates and their highly improved acetone sensing properties
Accepted Manuscript Au-modified α-Fe2O3 columnar superstructures assembled with nanoplates and their highly improved acetone sensing properties Jintao Li, Liwei Wang, Zhuo Liu, Yinghui Wang, Shengli Wang PII:
S0925-8388(17)33068-2
DOI:
10.1016/j.jallcom.2017.09.039
Reference:
JALCOM 43095
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2017 Revised Date:
2 September 2017
Accepted Date: 4 September 2017
Please cite this article as: J. Li, L. Wang, Z. Liu, Y. Wang, S. Wang, Au-modified α-Fe2O3 columnar superstructures assembled with nanoplates and their highly improved acetone sensing properties, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.039. 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.
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Au-modified α-Fe2O3 columnar superstructures assembled with
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nanoplates and their highly improved acetone sensing properties
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Jintao Li a, Liwei Wang b,*, Zhuo Liu a, Yinghui Wang b,*, Shengli Wang a
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a
Experimental Practising & Teaching center, Hebei GEO University, Shijiazhuang, 050031, China b
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School of Marine Sciences, Guangxi University, Nanning, 530004, China
ABSTRACT
A highly improved acetone sensing hybrid material of Au nanoparticles (Au
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NPs)-modified α-Fe2O3 columnar superstructures (Au/α-Fe2O3 CSs) was successfully
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fabricated by two-stage solution processes. Firstly, a simple glycerin-assisted
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hydrothermal method was used to assemble single crystalline hematite α-Fe2O3
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nanoplates into three dimensional (3D) CSs. Afterward, the as-prepared α-Fe2O3 CSs
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were further employed as supports for loading Au NPs via precipitating HAuCl4
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aqueous solution with ammonia. The obtained samples were analyzed by means of
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SEM, TEM, XRD and EDX. Both pristine and Au-functionalized α-Fe2O3 CSs were
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practically applied as gas sensors. The results indicated that the hybrid sensor
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exhibited enhanced responses and selectivity to acetone than the pristine one at the
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optimal working temperature of as low as 150 oC. Meanwhile, the detection limit
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could extend down to ppb-level. Such excellent sensing performances are better than
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those previously reported sensors based on iron oxide nanocomposites, indicating its
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original sensor application in detecting acetone. The strong spillover effect of the Au
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*Corresponding author. Tel: +86 771 3227746; Fax: +86 771 3227522. Email addresses: [email protected]; [email protected] 1
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NPs and the electronic interaction between Au NPs and α-Fe2O3 CSs support are
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believed to contribute to the improved sensor performances.
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Keywords: Au; α-Fe2O3; Columnar superstructures; Acetone; Sensor
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Graphical abstract
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1. Introduction As one of the highly volatile organic compounds, acetone is often used as organic
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solvent (plastic, fiber and spray-paint) and easily-made drug chemicals. However, its
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easy volatilization and toxic nature make it harmful to people at high concentration in
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air [1]. Some investigations have indicated that chronic exposure may cause
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inflammation and damage to the liver, kidney or nerve. In addition, acetone is also
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one of the important biomarkers for diabetes mellitus [2]. According to medical
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reports, higher acetone concentration (usually in the range of 1.7 to 3.7 ppm) could be
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detected in breath gas from diabetic patients, while the breath gas of healthy human
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typically contains less than 0.9 ppm [3-6]. Detection of diabetes is usually carried out
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by invasive method, such as blood test, which is painful, and may lead to cross
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infection of blood-transmitted diseases. Therefore, a noninvasive method is in a
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pressing need by the patients and physicians. Besides, the traditional analysis
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technologies for determining acetone for diabetes diagnosis are based on table-top
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equipments, such as gas chromatography-mass spectrometry and high performance
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liquid chromatography [7, 8]. The above-mentioned detection methods often suffer
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from such disadvantages like sophisticated procedures, bulky equipment and low
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detection limit, which hinder the progress of clinical or at-home applications.
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Currently, various types of gas sensors based on different sensing principles have been
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fabricated due to their simplicity, accuracy and convenience. Recent studies on gas
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sensing have focused on enhancements of sensor characteristics targeting higher
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environment pollution monitoring, disease diagnosis, security check, and industrial
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processing control [9-14]. Sensors may be one of the most promising methods to
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measure low concentration of acetone in human expiration or indoor environment
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[15-17].
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Among the vast majority of sensing materials, α-Fe2O3, an n-type semiconductor
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with a narrow band-gap (Eg) of 2.2 eV, has been recognized as a promising
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multifunctional metal oxide for catalyst [18, 19], gas sensors [20-22] and batteries [23,
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24] due to its nontoxic, stable and economical properties. However, pure iron oxide
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gas sensors usually suffer from limitations of showing either low responses or high
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operation temperatures (>250 oC) [25-27]. Therefore, a number of α-Fe2O3-based
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nanocomposites have been investigated to improve the performances of pristine
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α-Fe2O3. The assembly of noble metals (Au, Ag, Pt, Pd, etc.) onto α-Fe2O3 has been
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widely applied in gas sensors [28-31], since noble metals can act as promoters to
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enhance the physical and chemical properties of the functional materials due to their
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outstanding electronic and catalytic properties. Moreover, the synergic electronic
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interaction between noble metals and the α-Fe2O3 support can also enhance the
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surface depletion layers to promote the sensing performance [30].
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It is widely known that the shape or morphology of metal oxide has a great
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influence on their properties and many efforts have been made to prepare α-Fe2O3
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nanostructures in improved (textural) forms. For example, various α-Fe2O3
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ACCEPTED MANUSCRIPT nanostructures including nanospindles [29], nanorods [30, 31], nanowires [32],
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nanotubes [33] and complex hierarchical structures constructed with nanoscale
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building blocks [34-36] have been reported, with beneficial attributes such as high
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surface areas and porosities. In particularly, the 3D hierarchical architectures which
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are assembled by 1D or 2D nanoscale building blocks are currently the subject of
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intensive research because of their unique properties and potential applications
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[34-36]. Herein, we focus on fabricating the 3D hierarchical architectures with highly
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designed structural characteristics which can be potential for gaining high
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performance acetone sensors. In this work, we present a facile two-stage solution
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process to successfully fabricate the Au-modified 3D α-Fe2O3 CSs. Au NPs with small
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sizes are anchored onto the surface of α-Fe2O3 CSs via precipitating HAuCl4 aqueous
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solution with ammonia. The as-fabricated Au/α-Fe2O3 CSs were used to fabricate gas
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sensor devices and results revealed that the sensor exhibited a significant
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enhancement in the specific response to acetone at the optimal operating temperature
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of as low as 150 oC. To explain the enhanced gas sensing properties of the Au/α-Fe2O3
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CSs sensor, the gas sensing mechanism have also been discussed.
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2. Experimental
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2.1 Synthesis of α-Fe2O3 CSs
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All the reagents such as glycerin, FeCl3·6H2O, anhydrous ethanol and ammonia
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were of analytical grade and used without further purification. Distilled water was
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used throughout the experiments.
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ACCEPTED MANUSCRIPT 3D α-Fe2O3 CSs were synthesized by a glycerin assisted hydrothermal reaction [37].
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In a typical synthesis, 0.55 g of FeCl3·6H2O was dissolved into 78 mL of deionized
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water and stirred for 10 min to form a clear solution. Then, 2 mL of glycerin was
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added to the above solution. After magnetic stirring for 30 min, the mixture was
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transferred to a 100 mL Teflon-lined stainless steel autoclave and maintained at 140
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C for 24 h. After the autoclave was cooled down to room temperature naturally, red
precipitation was collected by centrifugation and washed with deionized water and
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anhydrous ethanol for several times, and finally dried at 60 oC for 12 h to get the
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support. For comparison, the preparation of the α-Fe2O3 nanoparticles was achieved
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by the similar protocol in the absence of glycerin (Fig. S1, S2 and S3, ESI†).
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2.2 Synthesis of Au/α-Fe2O3 CSs
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Au/α-Fe2O3 CSs were synthesized by adding 0.10 g of the above α-Fe2O3 CSs into
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20 mL of deionized water under stirring and then ultrasound treated for 20 min. Then
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1.00 mL of 0.01 mol/L HAuCl4 aqueous solution was introduced into the system,
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followed by adding the diluted ammonia solution until the pH was adjusted to 9. After
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stirring for 0.5 h, the precipitate was collected by centrifugation, and washed
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alternately with deionized water and ethanol until the pH was down to 7, and then
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dried at 80 oC overnight.
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2.3 Characterization
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XRD analysis was performed on a Bruker D8 diffractometer with Cu Kα radiation
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(λ = 0.1542 nm) in the range of 20-70° (2θ) to examine the crystal phase and purity of
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microscope. TEM and HRTEM images with EDX were obtained by Tecnai G2 F20 to
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study the morphology and chemical composition of the materials.
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2.4 Gas sensing tests
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The fabrication and testing processes of gas sensors were described detailedly in
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our previous work [38]. Typically, the aqueous slurry of the materials was directly
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coated on the outer surface of an alumina tube, which was equipped with two gold
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electrodes to acquire the electrical resistance and a Ni-Cr alloy coil through the tube
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as a heater to provide required operating temperatures by tuning the heating voltage
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(Vh). The working voltage for gas sensing test was 5 V, and a load resistance (RL) was
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connected in series to the sensor. Before test, the sensors were aged at 200 oC for one
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week in order to make them stable. The gas sensing experiments were performed in an
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airtight housing fabricated static system equipped with a multimeter/DC power supply,
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and a certain concentration of acetone was injected into the gas-chamber with sample
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in the system. The acetone concentration in the chamber can be calculated based on
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the injected amount. During the measurements, clean dry air was used as the dilluted
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gas. All the sensing tests were conducted at room temperature (25 oC) with ca. 40%
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relative humidity. The sensor response S (S = Ra/Rg) was defined as the ratio of sensor
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resistance in fresh air (Ra) to that in test gases (Rg).
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3. Result and discussion
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3.1 Characterization
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ACCEPTED MANUSCRIPT The size and morphology of the α-Fe2O3 CSs were characterized by SEM (Fig. 1).
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From the low magnification SEM image (Fig. 1 (a)), it could be clearly observed that
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the obtained α-Fe2O3 support presented the uniform 3D columnar superstructures
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(CSs) patterns with different lengths (about 0.5 – 1.0 µm), which were formed by the
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arrays of many uniform α-Fe2O3 nanoplates. The magnified SEM image (Fig. 1 (b))
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showed that the diameters of the nanoplates were in the range of about 250 – 300 nm,
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while the thickness was about 45 nm. As shown in Fig. 1(c) and its inset, the α-Fe2O3
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CSs were assembled with α-Fe2O3 nanoplates through the face-to-face mode. The
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clear resolved lattice fringes were calculated to be around 0.27 and 0.22 nm, which
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can be indexed to the (104) and (113) plane of the α-Fe2O3 phase. Fig. 1 (d) displayed
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the XRD patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs. Compared with the data in
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JCPDS No. 86-2368, it could be seen clearly that all diffraction peaks indicated by
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Miller indices in the patterns should be indexed to hexagonal α-Fe2O3 [34]. From the
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XRD pattern of Au/α-Fe2O3 CSs, no diffraction peaks from Au were observed
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probably due to its small content in the α-Fe2O3 CSs, where Au nanoparticles should
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be highly dispersed with a small size, or the absence of large crystalline Au clusters
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which affected its peak appearance. So the existence of Au will be approved latter by
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the following TEM and EDX results.
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The detailed TEM and HRTEM observations in Fig. 2 exhibited deeper insights
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into the composite structures. In the TEM images of Fig. 2 (a) and (b), a high density
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of Au nanoparticles with small sizes could be observed clearly and uniformly
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dots contrasted against the support. Fig. 2 (c) showed the HRTEM image of Au
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nanoparticles. The interplanar spacing of 0.238 nm can be indexed to the (111) plane
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of Au nanoparticles. The Au nanoparticles distributed as narrow sizes and almost all
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Au particle diameters were smaller than 12 nm, with an average diameter of ca. 7.5
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nm (Fig. 2 (d)). Furthermore, EDX analysis was carried out in Fig. 2 (e) to confirm
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the surface chemical compositions of the hybrid and the result displayed the existence
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of Au, Fe and O elements, confirming the successful assembly of Au on α-Fe2O3 CSs.
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In addition, the EDX result revealed that the Au loading content in the hybrid sample
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is 2.32 wt.%, which was more or less equal with the theoritical value of 2 wt.%. The
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small difference was probably due to the inevitable loss of α-Fe2O3 support in
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preparation.
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Based on the above results, the possible formation processes for Au/α-Fe2O3 CSs
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was proposed and depicted in Fig. 3, which could be divided into two steps. Step 1
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represents the formation of 3D α-Fe2O3 CSs synthesized by a glycerin assisted
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hydrothermal method. In this process, firstly, Fe3+ ions in the aqueous solution
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coordinated with glycerin molecules to form Fe(III)-glycerin complexes, which
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aggregated to form a quasi-emulsion system [39, 40]. Then the Fe3+ ions hydrolyzed
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into Fe(OH)3 minicrystals, which were followed with the decomposition into large
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number of α-Fe2O3 nuclei under hydrothermal treatment. Afterwards, the neighboring
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α-Fe2O3 nanocrystal began to orientedly aggregate and further grew into the plate-like
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into various 3D column-like structures through the continuous self-assembly growth.
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In step 2, the as-prepared α-Fe2O3 CSs were pre-dispersed in water by ultrasonic
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treatment to enhance their hydrophilicity. Then the HAuCl4 solution was added to the
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above solution, during which AuCl4- species were adsorbed onto the α-Fe2O3 surface
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by strong interparticle forces. As ammonia was added dropwise, the OH− groups also
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increased, which promoted the hydrolysis of AuCl4- to form various gold complexes
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([AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]− and [Au(OH)4]−) [41, 42]. Au(OH)3
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precipitates were formed at the pH of about 9, and then decomposed into pink
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thermally-unstable Au2O3. Finally, Au NPs were generated after drying at 80 oC to
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obtain the final Au/α-Fe2O3 CSs products.
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3.2 Gas sensing performance
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The sensing performances of our Au/α-Fe2O3 CSs have been systematically
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investigated and compared to that of the pure hematite support, and acetone vapor was
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chosen as the main probe gas due to its important detection significance. To optimize
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the operating temperature to achieve the best sensing response, 50 ppm acetone was
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used as the standard, and the sample response was evaluated from 50 to 300 °C. As
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shown in Fig. 4 (a), the operating temperature had a great influence on the acetone
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vapor responses and both of the sensors exhibited an “increase–maximum–decrease”
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tendency. The responses of α-Fe2O3 and Au/α-Fe2O3 sensors steadily increased with
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the increase of operating temperature from 50 to 150 °C, and then reached the
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ACCEPTED MANUSCRIPT maximum value of 18 and 31, respectively. However, the response reduced when
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further increasing the operating temperature above 150 °C, which might be ascribed
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to the dynamic balance between the adsorption and desorption of the acetone
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molecules, then the response of the gas sensor achieved the maximum. Hence, 150 °C
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was chosen as the optimum operating temperatures for α-Fe2O3 and Au/α-Fe2O3 CSs
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to carry out the further investigations. Fig. 4 (b) showed the response of the sensors
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versus different concentrations of acetone from 0.8 ppm to 100 ppm at 150 °C. It was
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worth noting that both of the profiles exhibited different linear responses versus
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acetone concentration in the testing range. Moreover, the response of the sensor based
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on Au/α-Fe2O3 CSs enhanced quite significantly to acetone than that of the pure one,
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and the response amplitude improved as the acetone concentration increased. For
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example, the response of the Au/α-Fe2O3 was 42 for the 100 ppm acetone, which was
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about 2 times of the pristine α-Fe2O3 CSs at 150 °C.
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To investigate the sensing ability of Au/α-Fe2O3 based sensor, different
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concentrations of acetone in the sequence of 800 ppb, 1, 10, 20, 30, 40, 50, 60, 70, 80,
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90 and 100 ppm were tested at 150 oC, and the dynamic response-recovery curves
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were presented in Fig. 5. It could be seen from Fig. 5 (a) that the response amplitudes
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of both sensors were significantly enhanced towards the increasing gas concentrations,
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and the characteristics of response and recovery were almost reproducible. Meanwhile,
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the output signal undergoes a drastic and then gradual upward trend when injecting
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the higher and higher concentrations of acetone, and the speeds of returning back to
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acetone concentration was as low as 800 ppb, the gas response for Au/α-Fe2O3 CSs
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still could reach 2.5, which indicated that this kind of sensor could detect lower
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acetone concentration down to ppb-level and might be used to measure acetone which
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breathed out from the diabetic patients (usually in the range of 1.7 ppm to 3.7 ppm)
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[3-6].
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Response and recovery times are two important factors of gas sensors. The
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response and recovery time is defined as the time required for a change in output
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voltage to reach 90% of the equilibrium value after injecting or removing the acetone
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gas. When exposed to 50 ppm acetone, the response and recovery times were about 17
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and 12 s for the Au/α-Fe2O3, and 10 and 17 s for α-Fe2O3 CSs (Fig. 5 (b)),
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respectively, indicating such columnar superstructures, especially after the
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modification of noble metal Au NPs, can meet the practical demands of fast detection.
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For practical applications, the gas sensors are required not only to possess high
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response but also excellent selectivity to the targeted gas. Thus, the responses of
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Au/α-Fe2O3 CSs to 50 ppm various pollutant gases (acetone, ethanol, hydrogen
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sulfide, methanol, n-butylamine, toluene and heptane) were also measured at the
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optimal working temperature of 150 oC to further examine its selectivity. As shown in
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Fig. 6, the corresponding response values were 31, 5, 4, 3, 3, 1.6 and 1.5, respectively.
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Just as expected, the compositive sensor exhibited obviously highest response to
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acetone, then ethanol and hydrogen sulfide were succedent. It meant that the highest
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response (31) to acetone was about 8 times of that for hydrogen sulfide (4), and 10
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times of that for methanol and n-butylamine (3), while the sensor presented nearly no
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responses to toluene and heptane. In addition, long term stability, also called reproducibility, is another important
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index to evaluate the practical application of gas sensors. To investigate the long term
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stability of Au/α-Fe2O3 CSs sensor, we performed five response-recovery
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characteristic cycles to 50 ppm acetone at 150 °C after three months (Fig. 7). It was
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obvious that the response–recovery curves were similar for five continuous cycles with
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nearly no changes in response, response time and recovery time, indicating its good
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reproducibility property.
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Table 1 compared the sensing performances of several α-Fe2O3-based sensors to
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100 ppm acetone between our work and previously reports. According to the results,
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our gas sensor based on Au/α-Fe2O3 CSs exhibited relatively higher acetone response,
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lower working temperature and shorter response/recovery times than those reported in
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the literatures [27, 29, 31, 43-47].
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3.3 Sensing mechanisms
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As is well known, the sensing mechanism of the n-type semiconductor gas sensors
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such as α-Fe2O3 is related to the surface-adsorbed oxygen species (O2-, O- and O2-),
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which can produce a depletion layer on the surface of α-Fe2O3 thus increase the
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resistance. When the sensor was exposed to reductive gases, for instance, acetone, the
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gases would react with the adsorbed oxygen species, which resulted in the release of
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[20-22]. Besides, the prepared α-Fe2O3 CSs in this work presented unique 3D
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nanostructures, and compared with the traditional bulk materials, such composite
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could bring about quick adsorption and desorption of O2 to facilitate the acetone
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consuming at the surface of the sensing layer, thus could improve the sensing
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properties [20]. In addtion, the α-Fe2O3 CSs were composed of α-Fe2O3 nanoplates
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with face-to-face stacking, where the presence of α-Fe2O3/α-Fe2O3 homojunction
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could be used for additional active sites, leading to enhancement of sensing
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performances [48], just as illustrated in Fig. 8.
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After the α-Fe2O3 CSs were decorated by Au NPs, the sensor response was
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significantly increased. After the surface of the α-Fe2O3 CSs were modified by the
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chemical and electrical effect of Au NPs, the catalytic activity of Au NPs would cause
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more oxygen to be absorbed on the surface of α-Fe2O3 CSs and dissociated into large
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quantity of adsorbed oxygen species, resulting in a greater and faster degree of
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electron depletion. As shown in the right part of Fig. 8, the depletion layer at the
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Au/α-Fe2O3 interface is wider than that at the surface of the pristine α-Fe2O3 CSs.
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When the Au/α-Fe2O3 composite was exposed to the acetone gas, the sensing reaction
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between adsorbed oxygen species and the tested gases would lead to a larger
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resistance change and a higher sensor response [27, 28]. Besides, there exists a
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catalytic synergy effect between the Au NPs and α-Fe2O3 supporter. In this effect, the
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Au NPs played the role of active sites for gas sensing reactions between
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surface-adsorbed oxygen ion species and the reductive gases, as well as an excellent
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medium to supply nanochannels for electron transfer to enhance the sensing
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performances [31, 43].
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4. Conclusions
A novel sensing hybrid-material of Au-modified α-Fe2O3 CSs was successfully
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synthesized by two-stage solution processes. SEM and TEM results revealed that the
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obtained α-Fe2O3 CSs were composed of uniform columnar 3D superstructures, which
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were formed by the arrays of many building blocks, namely α-Fe2O3 nanoplates.
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Furthermore, the small Au NPs (avg. 7.5 nm) were uniformly dispersed onto the
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α-Fe2O3 CSs, which was confirmed by high-resolution of TEM and EDX.
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Comparisons of the gas sensing performances between pure α-Fe2O3 CSs and the
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as-fabricated Au/α-Fe2O3 hybrid revealed that Au/α-Fe2O3 based sensor exhibited
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remarkably improved response, good selectivity and low detection limit down to
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ppb-level to acetone at 150 oC, which was significantly more effective to acetone than
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previously reported sensors based on α-Fe2O3 nanocomposites. The enhanced
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gas-sensing behavior should be attributed to the unique 3D column-like morphology
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of α-Fe2O3 CSs, the catalytic effect of Au NPs, and the synergetic effect induced by
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the strong interfacial interaction between Au NPs and α-Fe2O3 support. The
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as-prepared Au/α-Fe2O3 CSs composites can be used as a potentially promising
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candidate for acetone detection, which may lead for the non-invasive testing of
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diabetics.
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Acknowledgements
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This work was funded by the National Experimental Teaching Demonstration
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Center of Geology of Hebei GEO University, the Doctoral Scientific Research
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Foundation of Hebei GEO University (BQ 201501), the Natural Science Foundation of
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Guangxi Province, China (NO. 2016GXNSFBA380232), and the National Natural
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Science Foundation of China (NOs. 41473118, 41673105).
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found in ESI†.
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References
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[1] T. Godish, Indoor Air Pollution Control. Chelsea, MI: Lewis Publishers, 1991.
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[2] P. Mayes, R. Murray, D. Granner, V. Rodwell, Harper’s Biochemistry;
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McGraw-Hill Companies Inc.: New York, NY, USA, (2000) 130-136.
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[3] C.N. Tassopoulos, D. Barnett, T.R. Fraser, Breath-acetone and blood-sugar
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measurements in diabetes, Lancet 293 (1969) 1282-1286.
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[4] K.M. Veloso, S.S. Likhodi, S.C. Cunnane, Breath acetone is a reliable indicator of
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ketosis in adults consuming ketogenic meals, Am. J. Clin. Nutr. 76 (2002) 65-70.
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ACCEPTED MANUSCRIPT [5] C.H. Deng, J. Zhang, X.F. Yu, W. Zhang, X.M. Zhang, Determination of acetone
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Table 1. Comparison of the responses for various α-Fe2O3-based sensors toward 100
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ppm acetone Operating Temperature Sensor Materials
Response/recovery Response (S)
References time (s)
α-Fe2O3 nanospindles
300
3.0
Au/α-Fe2O3 nanospindles
300
7.2
α-Fe2O3 porous nanorods
270
20.9
Au/α-Fe2O3 porous nanorods
270
Pt/α-Fe2O3 porous nanorods
300
α-Fe2O3 porous nanorods
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(°C)
[29]
5/10
[29]
0.5/10
[27]
0.5/20
[27]
10.0
0.5/0.5
[27]
300
10.0
30/31
[31]
Au/α-Fe2O3 porous nanorods
270
20.0
25/28
[31]
Pt/α-Fe2O3 porous nanoparticles
260
44.0
10/14
[43]
350
7.0
4/15
[44]
260
22.5
<3/<3
[45]
Nanoscale α-Fe2O3 nanoparticles
240
15.7
0.8/27
[46]
α-Fe2O3 nanoparticles
240
7
1.7/76
[46]
Mesoporous α-Fe2O3 nanostructures
150
28.0
<3/<3
[47]
α-Fe2O3 nanoparticles
250
9.5
28/7
This work
α-Fe2O3 CSs
150
23.5
13/19
This work
Au/α-Fe2O3 CSs
150
42.0
13/16
This work
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Fig. 1. (a, b) SEM images and (c) TEM image of the pure α-Fe2O3 CSs, and (d) XRD
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patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs.
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Fig. 2. (a, b) TEM and (c) HRTEM images of the Au/α-Fe2O3 CSs composites, (d) the
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corresponding Au diameter distribution histogram from (b), and (e) EDX of the
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compositive product shown in (a).
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Fig. 3. Schematic illustration of the synthetic process of the Au/α-Fe2O3 CSs.
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Fig. 4. Sensor responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus (a) different
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operating temperatures with a fixed acetone concentration of 50 ppm, and (b) different
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acetone gas concentrations at the optimal operating temperatures of 150°C.
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Fig. 5. (a) Responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus different concentrations
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of acetone at 150 °C, and (b) dynamic response-recovery sensing curves of α-Fe2O3
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and Au/α-Fe2O3 CSs at the acetone concentrations of 50 ppm.
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Fig. 6. Selectivity test of the Au/α-Fe2O3 sensor to 10 ppm different tested gases at
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150 °C.
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Fig. 7. The long term stability of Au/α-Fe2O3 CSs sensor to 50 ppm acetone after
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three months at 150 °C.
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Fig. 8. Schematic illustrations of the proposed sensing mechanism of the α-Fe2O3 and
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Au/α-Fe2O3 CSs.
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Fig. 1. (a, b) SEM images and (c) TEM image of the pure α-Fe2O3 CSs, and (d) XRD
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patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs.
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corresponding Au diameter distribution histogram from (b), and (e) EDX of the
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compositive product shown in (a).
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Fig. 4. Sensor responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus (a) different
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operating temperatures with a fixed acetone concentration of 50 ppm, and (b) different
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acetone gas concentrations at the optimal operating temperatures of 150°C.
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Fig. 5. (a) Responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus different concentrations
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of acetone at 150 °C, (b) Dynamic response-recovery sensing curves of α-Fe2O3 and
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Au/α-Fe2O3 CSs at the acetone concentrations of 50 ppm.
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Highlights: 3D Au/α-Fe2O3 columnar superstructures were synthesized by solution processes. Au/α-Fe2O3 CSs showed enhanced sensing properties to acetone vapor.
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The mechanism was discussed to help explain the improved sensor performance.
S0925-8388(17)33068-2
DOI:
10.1016/j.jallcom.2017.09.039
Reference:
JALCOM 43095
To appear in:
Journal of Alloys and Compounds
Received Date: 13 May 2017 Revised Date:
2 September 2017
Accepted Date: 4 September 2017
Please cite this article as: J. Li, L. Wang, Z. Liu, Y. Wang, S. Wang, Au-modified α-Fe2O3 columnar superstructures assembled with nanoplates and their highly improved acetone sensing properties, Journal of Alloys and Compounds (2017), doi: 10.1016/j.jallcom.2017.09.039. 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.
ACCEPTED MANUSCRIPT 1
Au-modified α-Fe2O3 columnar superstructures assembled with
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nanoplates and their highly improved acetone sensing properties
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Jintao Li a, Liwei Wang b,*, Zhuo Liu a, Yinghui Wang b,*, Shengli Wang a
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a
Experimental Practising & Teaching center, Hebei GEO University, Shijiazhuang, 050031, China b
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School of Marine Sciences, Guangxi University, Nanning, 530004, China
ABSTRACT
A highly improved acetone sensing hybrid material of Au nanoparticles (Au
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NPs)-modified α-Fe2O3 columnar superstructures (Au/α-Fe2O3 CSs) was successfully
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fabricated by two-stage solution processes. Firstly, a simple glycerin-assisted
10
hydrothermal method was used to assemble single crystalline hematite α-Fe2O3
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nanoplates into three dimensional (3D) CSs. Afterward, the as-prepared α-Fe2O3 CSs
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were further employed as supports for loading Au NPs via precipitating HAuCl4
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aqueous solution with ammonia. The obtained samples were analyzed by means of
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SEM, TEM, XRD and EDX. Both pristine and Au-functionalized α-Fe2O3 CSs were
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practically applied as gas sensors. The results indicated that the hybrid sensor
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exhibited enhanced responses and selectivity to acetone than the pristine one at the
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optimal working temperature of as low as 150 oC. Meanwhile, the detection limit
18
could extend down to ppb-level. Such excellent sensing performances are better than
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those previously reported sensors based on iron oxide nanocomposites, indicating its
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original sensor application in detecting acetone. The strong spillover effect of the Au
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*Corresponding author. Tel: +86 771 3227746; Fax: +86 771 3227522. Email addresses: [email protected]; [email protected] 1
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NPs and the electronic interaction between Au NPs and α-Fe2O3 CSs support are
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believed to contribute to the improved sensor performances.
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Keywords: Au; α-Fe2O3; Columnar superstructures; Acetone; Sensor
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Graphical abstract
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1. Introduction As one of the highly volatile organic compounds, acetone is often used as organic
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solvent (plastic, fiber and spray-paint) and easily-made drug chemicals. However, its
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easy volatilization and toxic nature make it harmful to people at high concentration in
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air [1]. Some investigations have indicated that chronic exposure may cause
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inflammation and damage to the liver, kidney or nerve. In addition, acetone is also
64
one of the important biomarkers for diabetes mellitus [2]. According to medical
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reports, higher acetone concentration (usually in the range of 1.7 to 3.7 ppm) could be
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detected in breath gas from diabetic patients, while the breath gas of healthy human
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typically contains less than 0.9 ppm [3-6]. Detection of diabetes is usually carried out
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by invasive method, such as blood test, which is painful, and may lead to cross
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infection of blood-transmitted diseases. Therefore, a noninvasive method is in a
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pressing need by the patients and physicians. Besides, the traditional analysis
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technologies for determining acetone for diabetes diagnosis are based on table-top
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equipments, such as gas chromatography-mass spectrometry and high performance
73
liquid chromatography [7, 8]. The above-mentioned detection methods often suffer
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from such disadvantages like sophisticated procedures, bulky equipment and low
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detection limit, which hinder the progress of clinical or at-home applications.
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Currently, various types of gas sensors based on different sensing principles have been
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fabricated due to their simplicity, accuracy and convenience. Recent studies on gas
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sensing have focused on enhancements of sensor characteristics targeting higher
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ACCEPTED MANUSCRIPT sensitivity and lower detection limit for applications in different fields, such as
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environment pollution monitoring, disease diagnosis, security check, and industrial
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processing control [9-14]. Sensors may be one of the most promising methods to
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measure low concentration of acetone in human expiration or indoor environment
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[15-17].
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Among the vast majority of sensing materials, α-Fe2O3, an n-type semiconductor
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with a narrow band-gap (Eg) of 2.2 eV, has been recognized as a promising
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multifunctional metal oxide for catalyst [18, 19], gas sensors [20-22] and batteries [23,
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24] due to its nontoxic, stable and economical properties. However, pure iron oxide
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gas sensors usually suffer from limitations of showing either low responses or high
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operation temperatures (>250 oC) [25-27]. Therefore, a number of α-Fe2O3-based
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nanocomposites have been investigated to improve the performances of pristine
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α-Fe2O3. The assembly of noble metals (Au, Ag, Pt, Pd, etc.) onto α-Fe2O3 has been
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widely applied in gas sensors [28-31], since noble metals can act as promoters to
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enhance the physical and chemical properties of the functional materials due to their
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outstanding electronic and catalytic properties. Moreover, the synergic electronic
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interaction between noble metals and the α-Fe2O3 support can also enhance the
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surface depletion layers to promote the sensing performance [30].
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It is widely known that the shape or morphology of metal oxide has a great
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influence on their properties and many efforts have been made to prepare α-Fe2O3
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nanostructures in improved (textural) forms. For example, various α-Fe2O3
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nanotubes [33] and complex hierarchical structures constructed with nanoscale
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building blocks [34-36] have been reported, with beneficial attributes such as high
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surface areas and porosities. In particularly, the 3D hierarchical architectures which
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are assembled by 1D or 2D nanoscale building blocks are currently the subject of
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intensive research because of their unique properties and potential applications
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[34-36]. Herein, we focus on fabricating the 3D hierarchical architectures with highly
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designed structural characteristics which can be potential for gaining high
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performance acetone sensors. In this work, we present a facile two-stage solution
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process to successfully fabricate the Au-modified 3D α-Fe2O3 CSs. Au NPs with small
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sizes are anchored onto the surface of α-Fe2O3 CSs via precipitating HAuCl4 aqueous
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solution with ammonia. The as-fabricated Au/α-Fe2O3 CSs were used to fabricate gas
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sensor devices and results revealed that the sensor exhibited a significant
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enhancement in the specific response to acetone at the optimal operating temperature
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of as low as 150 oC. To explain the enhanced gas sensing properties of the Au/α-Fe2O3
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CSs sensor, the gas sensing mechanism have also been discussed.
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2. Experimental
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2.1 Synthesis of α-Fe2O3 CSs
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All the reagents such as glycerin, FeCl3·6H2O, anhydrous ethanol and ammonia
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were of analytical grade and used without further purification. Distilled water was
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used throughout the experiments.
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In a typical synthesis, 0.55 g of FeCl3·6H2O was dissolved into 78 mL of deionized
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water and stirred for 10 min to form a clear solution. Then, 2 mL of glycerin was
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added to the above solution. After magnetic stirring for 30 min, the mixture was
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transferred to a 100 mL Teflon-lined stainless steel autoclave and maintained at 140
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C for 24 h. After the autoclave was cooled down to room temperature naturally, red
precipitation was collected by centrifugation and washed with deionized water and
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anhydrous ethanol for several times, and finally dried at 60 oC for 12 h to get the
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support. For comparison, the preparation of the α-Fe2O3 nanoparticles was achieved
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by the similar protocol in the absence of glycerin (Fig. S1, S2 and S3, ESI†).
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2.2 Synthesis of Au/α-Fe2O3 CSs
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Au/α-Fe2O3 CSs were synthesized by adding 0.10 g of the above α-Fe2O3 CSs into
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20 mL of deionized water under stirring and then ultrasound treated for 20 min. Then
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1.00 mL of 0.01 mol/L HAuCl4 aqueous solution was introduced into the system,
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followed by adding the diluted ammonia solution until the pH was adjusted to 9. After
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stirring for 0.5 h, the precipitate was collected by centrifugation, and washed
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alternately with deionized water and ethanol until the pH was down to 7, and then
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dried at 80 oC overnight.
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2.3 Characterization
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XRD analysis was performed on a Bruker D8 diffractometer with Cu Kα radiation
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(λ = 0.1542 nm) in the range of 20-70° (2θ) to examine the crystal phase and purity of
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microscope. TEM and HRTEM images with EDX were obtained by Tecnai G2 F20 to
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study the morphology and chemical composition of the materials.
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2.4 Gas sensing tests
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The fabrication and testing processes of gas sensors were described detailedly in
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our previous work [38]. Typically, the aqueous slurry of the materials was directly
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coated on the outer surface of an alumina tube, which was equipped with two gold
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electrodes to acquire the electrical resistance and a Ni-Cr alloy coil through the tube
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as a heater to provide required operating temperatures by tuning the heating voltage
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(Vh). The working voltage for gas sensing test was 5 V, and a load resistance (RL) was
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connected in series to the sensor. Before test, the sensors were aged at 200 oC for one
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week in order to make them stable. The gas sensing experiments were performed in an
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airtight housing fabricated static system equipped with a multimeter/DC power supply,
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and a certain concentration of acetone was injected into the gas-chamber with sample
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in the system. The acetone concentration in the chamber can be calculated based on
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the injected amount. During the measurements, clean dry air was used as the dilluted
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gas. All the sensing tests were conducted at room temperature (25 oC) with ca. 40%
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relative humidity. The sensor response S (S = Ra/Rg) was defined as the ratio of sensor
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resistance in fresh air (Ra) to that in test gases (Rg).
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3. Result and discussion
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3.1 Characterization
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ACCEPTED MANUSCRIPT The size and morphology of the α-Fe2O3 CSs were characterized by SEM (Fig. 1).
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From the low magnification SEM image (Fig. 1 (a)), it could be clearly observed that
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the obtained α-Fe2O3 support presented the uniform 3D columnar superstructures
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(CSs) patterns with different lengths (about 0.5 – 1.0 µm), which were formed by the
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arrays of many uniform α-Fe2O3 nanoplates. The magnified SEM image (Fig. 1 (b))
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showed that the diameters of the nanoplates were in the range of about 250 – 300 nm,
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while the thickness was about 45 nm. As shown in Fig. 1(c) and its inset, the α-Fe2O3
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CSs were assembled with α-Fe2O3 nanoplates through the face-to-face mode. The
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clear resolved lattice fringes were calculated to be around 0.27 and 0.22 nm, which
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can be indexed to the (104) and (113) plane of the α-Fe2O3 phase. Fig. 1 (d) displayed
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the XRD patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs. Compared with the data in
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JCPDS No. 86-2368, it could be seen clearly that all diffraction peaks indicated by
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Miller indices in the patterns should be indexed to hexagonal α-Fe2O3 [34]. From the
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XRD pattern of Au/α-Fe2O3 CSs, no diffraction peaks from Au were observed
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probably due to its small content in the α-Fe2O3 CSs, where Au nanoparticles should
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be highly dispersed with a small size, or the absence of large crystalline Au clusters
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which affected its peak appearance. So the existence of Au will be approved latter by
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the following TEM and EDX results.
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The detailed TEM and HRTEM observations in Fig. 2 exhibited deeper insights
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into the composite structures. In the TEM images of Fig. 2 (a) and (b), a high density
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of Au nanoparticles with small sizes could be observed clearly and uniformly
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ACCEPTED MANUSCRIPT anchored on the surface of α-Fe2O3 CSs, because they presented as the small black
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dots contrasted against the support. Fig. 2 (c) showed the HRTEM image of Au
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nanoparticles. The interplanar spacing of 0.238 nm can be indexed to the (111) plane
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of Au nanoparticles. The Au nanoparticles distributed as narrow sizes and almost all
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Au particle diameters were smaller than 12 nm, with an average diameter of ca. 7.5
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nm (Fig. 2 (d)). Furthermore, EDX analysis was carried out in Fig. 2 (e) to confirm
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the surface chemical compositions of the hybrid and the result displayed the existence
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of Au, Fe and O elements, confirming the successful assembly of Au on α-Fe2O3 CSs.
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In addition, the EDX result revealed that the Au loading content in the hybrid sample
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is 2.32 wt.%, which was more or less equal with the theoritical value of 2 wt.%. The
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small difference was probably due to the inevitable loss of α-Fe2O3 support in
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preparation.
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Based on the above results, the possible formation processes for Au/α-Fe2O3 CSs
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was proposed and depicted in Fig. 3, which could be divided into two steps. Step 1
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represents the formation of 3D α-Fe2O3 CSs synthesized by a glycerin assisted
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hydrothermal method. In this process, firstly, Fe3+ ions in the aqueous solution
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coordinated with glycerin molecules to form Fe(III)-glycerin complexes, which
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aggregated to form a quasi-emulsion system [39, 40]. Then the Fe3+ ions hydrolyzed
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into Fe(OH)3 minicrystals, which were followed with the decomposition into large
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number of α-Fe2O3 nuclei under hydrothermal treatment. Afterwards, the neighboring
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α-Fe2O3 nanocrystal began to orientedly aggregate and further grew into the plate-like
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ACCEPTED MANUSCRIPT nanostructures [37]. With the reaction time increased, the nanoplates were assembled
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into various 3D column-like structures through the continuous self-assembly growth.
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In step 2, the as-prepared α-Fe2O3 CSs were pre-dispersed in water by ultrasonic
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treatment to enhance their hydrophilicity. Then the HAuCl4 solution was added to the
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above solution, during which AuCl4- species were adsorbed onto the α-Fe2O3 surface
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by strong interparticle forces. As ammonia was added dropwise, the OH− groups also
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increased, which promoted the hydrolysis of AuCl4- to form various gold complexes
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([AuCl3(OH)]−, [AuCl2(OH)2]−, [AuCl(OH)3]− and [Au(OH)4]−) [41, 42]. Au(OH)3
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precipitates were formed at the pH of about 9, and then decomposed into pink
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thermally-unstable Au2O3. Finally, Au NPs were generated after drying at 80 oC to
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obtain the final Au/α-Fe2O3 CSs products.
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3.2 Gas sensing performance
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The sensing performances of our Au/α-Fe2O3 CSs have been systematically
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investigated and compared to that of the pure hematite support, and acetone vapor was
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chosen as the main probe gas due to its important detection significance. To optimize
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the operating temperature to achieve the best sensing response, 50 ppm acetone was
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used as the standard, and the sample response was evaluated from 50 to 300 °C. As
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shown in Fig. 4 (a), the operating temperature had a great influence on the acetone
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vapor responses and both of the sensors exhibited an “increase–maximum–decrease”
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tendency. The responses of α-Fe2O3 and Au/α-Fe2O3 sensors steadily increased with
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the increase of operating temperature from 50 to 150 °C, and then reached the
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ACCEPTED MANUSCRIPT maximum value of 18 and 31, respectively. However, the response reduced when
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further increasing the operating temperature above 150 °C, which might be ascribed
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to the dynamic balance between the adsorption and desorption of the acetone
229
molecules, then the response of the gas sensor achieved the maximum. Hence, 150 °C
230
was chosen as the optimum operating temperatures for α-Fe2O3 and Au/α-Fe2O3 CSs
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to carry out the further investigations. Fig. 4 (b) showed the response of the sensors
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versus different concentrations of acetone from 0.8 ppm to 100 ppm at 150 °C. It was
233
worth noting that both of the profiles exhibited different linear responses versus
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acetone concentration in the testing range. Moreover, the response of the sensor based
235
on Au/α-Fe2O3 CSs enhanced quite significantly to acetone than that of the pure one,
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and the response amplitude improved as the acetone concentration increased. For
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example, the response of the Au/α-Fe2O3 was 42 for the 100 ppm acetone, which was
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about 2 times of the pristine α-Fe2O3 CSs at 150 °C.
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To investigate the sensing ability of Au/α-Fe2O3 based sensor, different
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concentrations of acetone in the sequence of 800 ppb, 1, 10, 20, 30, 40, 50, 60, 70, 80,
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90 and 100 ppm were tested at 150 oC, and the dynamic response-recovery curves
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were presented in Fig. 5. It could be seen from Fig. 5 (a) that the response amplitudes
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of both sensors were significantly enhanced towards the increasing gas concentrations,
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and the characteristics of response and recovery were almost reproducible. Meanwhile,
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the output signal undergoes a drastic and then gradual upward trend when injecting
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the higher and higher concentrations of acetone, and the speeds of returning back to
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ACCEPTED MANUSCRIPT their initial values were also fast after the gases were out. Furthermore, when the
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acetone concentration was as low as 800 ppb, the gas response for Au/α-Fe2O3 CSs
249
still could reach 2.5, which indicated that this kind of sensor could detect lower
250
acetone concentration down to ppb-level and might be used to measure acetone which
251
breathed out from the diabetic patients (usually in the range of 1.7 ppm to 3.7 ppm)
252
[3-6].
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Response and recovery times are two important factors of gas sensors. The
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response and recovery time is defined as the time required for a change in output
255
voltage to reach 90% of the equilibrium value after injecting or removing the acetone
256
gas. When exposed to 50 ppm acetone, the response and recovery times were about 17
257
and 12 s for the Au/α-Fe2O3, and 10 and 17 s for α-Fe2O3 CSs (Fig. 5 (b)),
258
respectively, indicating such columnar superstructures, especially after the
259
modification of noble metal Au NPs, can meet the practical demands of fast detection.
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For practical applications, the gas sensors are required not only to possess high
261
response but also excellent selectivity to the targeted gas. Thus, the responses of
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Au/α-Fe2O3 CSs to 50 ppm various pollutant gases (acetone, ethanol, hydrogen
263
sulfide, methanol, n-butylamine, toluene and heptane) were also measured at the
264
optimal working temperature of 150 oC to further examine its selectivity. As shown in
265
Fig. 6, the corresponding response values were 31, 5, 4, 3, 3, 1.6 and 1.5, respectively.
266
Just as expected, the compositive sensor exhibited obviously highest response to
267
acetone, then ethanol and hydrogen sulfide were succedent. It meant that the highest
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response (31) to acetone was about 8 times of that for hydrogen sulfide (4), and 10
269
times of that for methanol and n-butylamine (3), while the sensor presented nearly no
270
responses to toluene and heptane. In addition, long term stability, also called reproducibility, is another important
272
index to evaluate the practical application of gas sensors. To investigate the long term
273
stability of Au/α-Fe2O3 CSs sensor, we performed five response-recovery
274
characteristic cycles to 50 ppm acetone at 150 °C after three months (Fig. 7). It was
275
obvious that the response–recovery curves were similar for five continuous cycles with
276
nearly no changes in response, response time and recovery time, indicating its good
277
reproducibility property.
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Table 1 compared the sensing performances of several α-Fe2O3-based sensors to
279
100 ppm acetone between our work and previously reports. According to the results,
280
our gas sensor based on Au/α-Fe2O3 CSs exhibited relatively higher acetone response,
281
lower working temperature and shorter response/recovery times than those reported in
282
the literatures [27, 29, 31, 43-47].
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3.3 Sensing mechanisms
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As is well known, the sensing mechanism of the n-type semiconductor gas sensors
285
such as α-Fe2O3 is related to the surface-adsorbed oxygen species (O2-, O- and O2-),
286
which can produce a depletion layer on the surface of α-Fe2O3 thus increase the
287
resistance. When the sensor was exposed to reductive gases, for instance, acetone, the
288
gases would react with the adsorbed oxygen species, which resulted in the release of
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ACCEPTED MANUSCRIPT free electrons to the conduction band, thus leading to a decrease in the resistances
290
[20-22]. Besides, the prepared α-Fe2O3 CSs in this work presented unique 3D
291
nanostructures, and compared with the traditional bulk materials, such composite
292
could bring about quick adsorption and desorption of O2 to facilitate the acetone
293
consuming at the surface of the sensing layer, thus could improve the sensing
294
properties [20]. In addtion, the α-Fe2O3 CSs were composed of α-Fe2O3 nanoplates
295
with face-to-face stacking, where the presence of α-Fe2O3/α-Fe2O3 homojunction
296
could be used for additional active sites, leading to enhancement of sensing
297
performances [48], just as illustrated in Fig. 8.
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After the α-Fe2O3 CSs were decorated by Au NPs, the sensor response was
299
significantly increased. After the surface of the α-Fe2O3 CSs were modified by the
300
chemical and electrical effect of Au NPs, the catalytic activity of Au NPs would cause
301
more oxygen to be absorbed on the surface of α-Fe2O3 CSs and dissociated into large
302
quantity of adsorbed oxygen species, resulting in a greater and faster degree of
303
electron depletion. As shown in the right part of Fig. 8, the depletion layer at the
304
Au/α-Fe2O3 interface is wider than that at the surface of the pristine α-Fe2O3 CSs.
305
When the Au/α-Fe2O3 composite was exposed to the acetone gas, the sensing reaction
306
between adsorbed oxygen species and the tested gases would lead to a larger
307
resistance change and a higher sensor response [27, 28]. Besides, there exists a
308
catalytic synergy effect between the Au NPs and α-Fe2O3 supporter. In this effect, the
309
Au NPs played the role of active sites for gas sensing reactions between
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surface-adsorbed oxygen ion species and the reductive gases, as well as an excellent
311
medium to supply nanochannels for electron transfer to enhance the sensing
312
performances [31, 43].
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4. Conclusions
A novel sensing hybrid-material of Au-modified α-Fe2O3 CSs was successfully
316
synthesized by two-stage solution processes. SEM and TEM results revealed that the
317
obtained α-Fe2O3 CSs were composed of uniform columnar 3D superstructures, which
318
were formed by the arrays of many building blocks, namely α-Fe2O3 nanoplates.
319
Furthermore, the small Au NPs (avg. 7.5 nm) were uniformly dispersed onto the
320
α-Fe2O3 CSs, which was confirmed by high-resolution of TEM and EDX.
321
Comparisons of the gas sensing performances between pure α-Fe2O3 CSs and the
322
as-fabricated Au/α-Fe2O3 hybrid revealed that Au/α-Fe2O3 based sensor exhibited
323
remarkably improved response, good selectivity and low detection limit down to
324
ppb-level to acetone at 150 oC, which was significantly more effective to acetone than
325
previously reported sensors based on α-Fe2O3 nanocomposites. The enhanced
326
gas-sensing behavior should be attributed to the unique 3D column-like morphology
327
of α-Fe2O3 CSs, the catalytic effect of Au NPs, and the synergetic effect induced by
328
the strong interfacial interaction between Au NPs and α-Fe2O3 support. The
329
as-prepared Au/α-Fe2O3 CSs composites can be used as a potentially promising
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candidate for acetone detection, which may lead for the non-invasive testing of
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diabetics.
332
Acknowledgements
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This work was funded by the National Experimental Teaching Demonstration
335
Center of Geology of Hebei GEO University, the Doctoral Scientific Research
336
Foundation of Hebei GEO University (BQ 201501), the Natural Science Foundation of
337
Guangxi Province, China (NO. 2016GXNSFBA380232), and the National Natural
338
Science Foundation of China (NOs. 41473118, 41673105).
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Appendix A. Supplementary data
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Supplementary data associated with this article can be found in ESI†.
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Table 1. Comparison of the responses for various α-Fe2O3-based sensors toward 100
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ppm acetone Operating Temperature Sensor Materials
Response/recovery Response (S)
References time (s)
α-Fe2O3 nanospindles
300
3.0
Au/α-Fe2O3 nanospindles
300
7.2
α-Fe2O3 porous nanorods
270
20.9
Au/α-Fe2O3 porous nanorods
270
Pt/α-Fe2O3 porous nanorods
300
α-Fe2O3 porous nanorods
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(°C)
[29]
5/10
[29]
0.5/10
[27]
0.5/20
[27]
10.0
0.5/0.5
[27]
300
10.0
30/31
[31]
Au/α-Fe2O3 porous nanorods
270
20.0
25/28
[31]
Pt/α-Fe2O3 porous nanoparticles
260
44.0
10/14
[43]
350
7.0
4/15
[44]
260
22.5
<3/<3
[45]
Nanoscale α-Fe2O3 nanoparticles
240
15.7
0.8/27
[46]
α-Fe2O3 nanoparticles
240
7
1.7/76
[46]
Mesoporous α-Fe2O3 nanostructures
150
28.0
<3/<3
[47]
α-Fe2O3 nanoparticles
250
9.5
28/7
This work
α-Fe2O3 CSs
150
23.5
13/19
This work
Au/α-Fe2O3 CSs
150
42.0
13/16
This work
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ACCEPTED MANUSCRIPT Figure Captions
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Fig. 1. (a, b) SEM images and (c) TEM image of the pure α-Fe2O3 CSs, and (d) XRD
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patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs.
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Fig. 2. (a, b) TEM and (c) HRTEM images of the Au/α-Fe2O3 CSs composites, (d) the
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corresponding Au diameter distribution histogram from (b), and (e) EDX of the
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compositive product shown in (a).
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Fig. 3. Schematic illustration of the synthetic process of the Au/α-Fe2O3 CSs.
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Fig. 4. Sensor responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus (a) different
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operating temperatures with a fixed acetone concentration of 50 ppm, and (b) different
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acetone gas concentrations at the optimal operating temperatures of 150°C.
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Fig. 5. (a) Responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus different concentrations
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of acetone at 150 °C, and (b) dynamic response-recovery sensing curves of α-Fe2O3
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and Au/α-Fe2O3 CSs at the acetone concentrations of 50 ppm.
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Fig. 6. Selectivity test of the Au/α-Fe2O3 sensor to 10 ppm different tested gases at
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150 °C.
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Fig. 7. The long term stability of Au/α-Fe2O3 CSs sensor to 50 ppm acetone after
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three months at 150 °C.
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Fig. 8. Schematic illustrations of the proposed sensing mechanism of the α-Fe2O3 and
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Au/α-Fe2O3 CSs.
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Fig. 1. (a, b) SEM images and (c) TEM image of the pure α-Fe2O3 CSs, and (d) XRD
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patterns of α-Fe2O3 CSs and Au/α-Fe2O3 CSs.
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Fig. 2. (a, b) TEM and (c) HRTEM images of the Au/α-Fe2O3 CSs composites, (d) the
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corresponding Au diameter distribution histogram from (b), and (e) EDX of the
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compositive product shown in (a).
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Fig. 3. Schematic illustration of the synthetic process of the Au/α-Fe2O3 CSs.
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Fig. 4. Sensor responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus (a) different
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operating temperatures with a fixed acetone concentration of 50 ppm, and (b) different
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acetone gas concentrations at the optimal operating temperatures of 150°C.
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Fig. 5. (a) Responses of α-Fe2O3 and Au/α-Fe2O3 CSs versus different concentrations
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of acetone at 150 °C, (b) Dynamic response-recovery sensing curves of α-Fe2O3 and
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Au/α-Fe2O3 CSs at the acetone concentrations of 50 ppm.
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Fig. 6. Selectivity test of the Au/α-Fe2O3 sensor to 10 ppm different tested gases at
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Fig. 7. The long term stability of Au/α-Fe2O3 CSs sensor to 50 ppm acetone after
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CO2 CO2
e-
e-
e-
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e-
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Fig. 8. Schematic illustrations of the proposed sensing mechanism of the α-Fe2O3 and
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Au/α-Fe2O3 CSs.
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Highlights: 3D Au/α-Fe2O3 columnar superstructures were synthesized by solution processes. Au/α-Fe2O3 CSs showed enhanced sensing properties to acetone vapor.
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Au/α-Fe2O3 sensor could detect low concentration of acetone down to ppb-level.
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The mechanism was discussed to help explain the improved sensor performance.