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Functional Inorganic Materials and Devices
VOC gas sensing properties of bimodal porous #Fe2O3 with ultrahigh sensitivity and fast response. Wangchang Geng, Shaobing Ge, Xiaowei He, Shan Zhang, Junwei Gu, Xiaoyong Lai, Hong Wang, and Qiuyu Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02435 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 7, 2018
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ACS Applied Materials & Interfaces
VOC gas sensing properties of bimodal porous α-Fe2O3 with ultrahigh sensitivity and fast response. Wangchang Geng *†, Shaobing Ge †, Xiaowei He †, Shan Zhang †, Junwei Gu †,Xiaoyong Lai*‡ , Hong Wang §, Qiuyu Zhang † †
MOE Key Laboratory of Material Physics and Chemistry under Extraordinary Conditions，School of Science, Northwestern Polytechnical University, Xi’an 710072, P.R. China. ‡
Key Laboratory of Energy Resource and Chemical Engineering, State Key Laboratory Cultivation Base of Natural Gas Conversion, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan 750021, P.R. China. §
Sichuan Univ Sci & Engn, Dept Mat Sci & Engn, Key Lab Mat Corros & Protect Sichuan Prov, Zigong 643000, Peoples R China. KEYWORDS: Gas sensor, VOCs, bimodal pore size, α-Fe2O3, Nanocasting, Mass transportation
ABSTRACT: Porous solid with multimodal pore size distribution provides plenty of advantages including large specific surface area and superior mass transportation to achieve high gas-sensing performances. In this study, α-Fe2O3 nanoparticles with bimodal porous structures were prepared successfully through a nanocasting pathway, adopting the bicontinuous 3D cubic symmetry mesoporous silica KIT-6 as the hard template. Its structure and morphology were characterized by X-ray Diffraction, Nitrogen adsorptiondesorption, Transmission electron microscope and so on. Furthermore, the gas sensor fabricated from this material exhibited excellent gas-sensing performance to several Volatile Organic Compounds (acetone, ethyl acetate, isopropyl alcohol, N-butanol, ethanol and methanol), such as ultrahigh sensitivity, rapid response speed (less than 10s) and recovery time, good reproducibility as well as stability. These would be associated with the desirable pore structure of the material, facilitating the molecules diffusion toward the entire sensing surface, and providing more active sensing sites for analytical gas.
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molecules.7 Consequently, the composition and structure (crystal phase, morphology, grain size, texture) of sensitivematerials play crucial roles in their sensing performances. 8-13 Stable alpha-iron oxide (α-Fe2O3), an n-type semiconductor (Eg=2.1 eV) with good sensitivity, environmentally friendliness, high corrosion resistance and easy fabrication properties, has been extensively applied in many fields including gas sensors.14-19 Up to now, various morphologies and architectures such as nanorod,20 nanotube,21 network,22 flower,23 urchin,24 nanoparallelepiped,25 spindle,26 27 28 29 polyhedron, yolk-shell, hollow nanoboxe and special exposed crystal facet30 of α-Fe2O3 have been investigated for detecting of toxic and flammable gases. On the other hand, hierachically porous structure containing multimodal pore sizes can usually maintain a large surface-tovolume ratio, facilitate pleasurable gas diffusion toward the entire sensing surface and achieve fast response and high sensitivity for analytical molecules.31-34 Liu 35 et al validated that the H2S sensing response of hierarchical porous ZnO synthesized through a hydrothermal bioinspired approach, was 5.1 times higher than non-templated ZnO due to inheritance of
Introduction
A contaminant group of highly reactive, toxic, carcinogenic and mutagenic Volatile Organic Compounds (VOC) have been widely emitted into environment media due to their highly volatile characteristics in the massive anthropogenic activities, such as the production of chemicals, pharmaceuticals and petroleum.1-3 It is reported that higher incidence of respiratory, leukemia and liver/lung cancers has been taking place in the urban areas between developing and developed countries.4 Accordingly, serious online monitoring and controlling the quality of waste-gas as well as air indoors are extremely indispensable to guarantee human health and sustainability of our society. In recent decades, metal oxide semiconductor material-based gas sensors have attracted considerable attention in this case, due to their high sensitivity, low detection limits, large number of detectable gases, simple fabrication, stability, robustness and low-cost.5,6 Theoretically, gas sensing mechanism of this type of sensor is based on a conspicuous conductivity change of oxide semiconductor, deriving from the adsorption of oxygen and the chemical reaction between oxygen species and target gas
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wood’s hierarchical porous structure. Mao 36 et al revealed the hierarchically bimodal mesoprous hematite microsphere which was synthesized by combining surfactant-inorganic aggregates and heat-treatment processes had enhanced sensing properties for HCHO and also affirmed that the structure is of importance. It is well known that the nanocasting method is usually adopted to prepare porous materials because of its valid control to pore diameter range and structure,37,38 especially, in fabricating mesoporous metal oxides with crystalline wall.37 Some hierarchically porous metal oxides such as NiO, 39 αMnO2,40 β-MnO241 have been synthesized successfully depending on the growing property of metal oxide in the bicontinuous 3D cubic channel of the hard template mesoprous silica KIT-6. However, to the best of our knowledge, there are few reports on the VOC gas-sensing properties of hierarchically porous α-Fe2O3 prepared by this approach. Therefore, in this work, we employed the mesoporous KIT-6 as a hard template to obtain α-Fe2O3 nanoparticles with bimodal pore sizes, whose gas sensing performance for a class of Volatile Organic Compounds were studied in detail. Results demonstrated that this α-Fe2O3 material held excellent sensing properties for many VOC vapors and exhibited a potential application in detection of these gases.
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humidity was 30～42%. The response of sensor was defined as the ratio of Ra/Rg, where Rg was the resistance of sensor in target vapor, Ra was the resistance of sensor in air. High relative humidity (RH) atmosphere was obtained through adding water vapor into the glass chamber. RH value was checked by a temperature＆humidity meter (HygroPalm HP22-A, Rotronic Instruments). 2.3. Characterization. Nitrogen physicsorption measurement were obtained from TriStar Ⅱ 3020 instrument (MICRO Corporation, America) at -196oC, where the sample was degassed for 12 h at 80oC. Specific surface area was computed with Brunauer-Emmet-Teller (BET) analysis. Pore size distribution was evaluated from desorption branch of isotherm through the Barrett-Joyner-Halenda (BJH) method. Xray diffraction (XRD) patterns were recorded at an X’Pert Pro MPD X-ray diffraction instrument (PANalytical Co., Holland, 40 kV and 35 mA). Morphology and pore structure of the sample was observed in Transmission Electron Microscope (TEM, JEOL JEM-3010).
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Results and discussion
Figure 1a displays that the template KIT-6 holds a IV-type isotherm with H1 type hysteresis loop according to the IUPAC definition, indicating this material has a typically mesoporous structure. 44 Corresponding pore-size distribution curve (Figure 1b) suggests that the pore diameter of mainly mesoporous channel in template KIT-6 is around 8.1 nm. And Figure 1c shows there are some small mesopores in this sample. Actually, 3D cubic structure of KIT-6 is composed of two sets of independently mesoporous channel, where they are interconnected by micropores and small mesopores. 45,46 With the increasing hydrothermal temperature in its preparation (40~130oC), the size of channels would be larger and interconnected pores would become more developed. 46,47 Therefore, it can be confirmed that these weakly small mesopores are assigned to some relatively undeveloped connected-pores in the material. Moreover, high BET surface area (715.2 cm2·g-1) and large pore volume (0.997 cm3·g-1) also demonstrate this material is well suitable as a template.
Experimental section
2.1. Synthesis. The mesoprous silica KIT-6 was synthesized at 100oC via hydrothermal method according to the reported procedure.42,43 Bimodal porous α-Fe2O3 material (α-Fe2O3-HT) was prepared by adopting the KIT-6 as hard template. Typically, 1 g of calcined KIT-6 was vacuumed for 2 h to remove gas in the channel of template. Subsequently, 3.4 mL of ethanol solution of Fe(NO3)3 (1 M) was added into the template and kept for 0.5 h to ensure the precursor enter into the channel completely. Afterwards, the mixture was dried at 80 oC for 3h, heated to 500oC by 1oC/min and calcined at this temperature for 3 h to decompose the precursor Fe(NO3)3 . In order to achieve higher loading, the vacuum, filling and decomposition procedure was repeated once, except the volume of precursor solution decreased to 3.2 mL. Then, the template was removed by 2 M NaOH aqueous solution. Finally, sample was centrifuged, washed with water to neutral and dried at 80oC in air overnight. Moreover, α-Fe2O3-Cal was prepared by directly decomposing Fe(NO3)3·6H2O at 500oC for 3 h. 2.2. Preparation and measurement of gas sensor. The typical fabrication procedure of gas-sensor based on the αFe2O3 was as follows: a Ni-Cr alloy wire and a ceramic tube (1.4 mm in diameter, 4 mm in length) mounted with a couple of Au electrodes as well as four Pt wires, were welded onto a sixbracket pedestal. The Ni-Cr alloy wire through the tube was served as heater by tuning heating voltage. Then, the paste from mixing 0.01 g sample with 0.02 mL deionized water was coated onto the ceramic tube. At last, the sensor was aged at 300oC for 24 h to improve the thermal stability and repeatability. The image of gas sensor was shown in Figure S1a. Gas sensing test were carried out on a commercial Gas-sensing measurement system (WS-30A, Weisheng Electronics Co., Ltd., Henan, China). Working principle of the WS-30A system was briefly described in Figure S1b. Target gas was introduced into testing chamber via a microsyringe at room temperature. In this testing process, air was used as dilution and reference gas, relative
From the wide angle XRD patterns of α-Fe2O3-HT, αFe2O3-Cal depicted in Figure 2a, it can be found that their clearly visible diffraction peaks can be well indexed to standard hexagonal α-Fe2O3 (JCPDS No. 33-0664), where the strong and sharp diffraction peaks at 33.15o, 35.61o correspond to the (104) and (110) plane of α-Fe2O3 crystal phase, respectively. However, based on the N2 physicsorption measurement, the isotherms (Figure 2b) of α-Fe2O3 materials prepared through diverse methods are extremely different. For α-Fe2O3-Cal obtained through direct calcination procedure, a type II isotherm from the non-pore or macro-pore is observed.44 Linearly nitrogen adsorption takes place in a long range of relative pressure (
