Hierarchical and Hollow Fe2O3 Nanoboxes Derived from Metal

Subscriber access provided by Binghamton University | Libraries

Article 2

3

Hierarchical and hollow FeO nano-boxes derived from metal-organic frameworks with excellent sensitivity to HS 2

Kuan Tian, Xiao-Xue Wang, Zhu-Ying Yu, Huayao Li, and Xin Guo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b07069 • Publication Date (Web): 03 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Hierarchical and hollow Fe2O3 nano-boxes derived from metalorganic frameworks with excellent sensitivity to H2S Kuan Tian1,3†, Xiao-Xue Wang1†, Zhu-Ying Yu1, Hua-Yao Li2 and Xin Guo1* 1. Laboratory of Solid State Ionics, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China 2. Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea 3. Department of Material and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450001, China Abstract Hierarchical and hollow porous Fe2O3 nano-boxes (with an average edge length of ~500 nm) were derived from metal-organic frameworks (MOFs) and the gas sensing characteristics were investigated. Sensors based on Fe2O3 nano-boxes exhibited a response (resistance ratio) of 1.23 to 0.25 parts per million (ppm) hydrogen sulfide (H2S) at 200 °C, the response/recovery speed is fast and the selectivity to H2S is excellent. Remarkably, the sensor showed fully reversible response to 5 ppm H2S at 50 °C, demonstrating its promise for operating at near room temperature, which is favourable for medical diagnosis and indoor/outdoor environment monitoring. The excellent performance of the Fe2O3 nano-boxes can be ascribed to the unique morphology with high specific surface area (SSA) and porous nanostructure.

Keywords: Fe2O3; gas sensor; H2S; hierarchical porous nanostructure; nano-box ________________________ †

The authors contributed equally to this work. * Author to whom correspondence should be addressed. Tel/Fax: +86-27-87559804; E-mail: [email protected] 1

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

1. INTRODUCTION Hydrogen sulfide (H2S), often results from the microbial breakdown of plants and animals, is a colorless, corrosive, flammable, and very poisonous gas with the distinctive stink smell of rotten eggs. Although the odor is pungent enough at very low concentration, it quickly paralyzes the olfactory system, leading to the unconsciousness of its presence. H2S is a broad-spectrum poison, i.e. it attacks not only the nervous system but also several other systems in the body; it can bind with iron in the mitochondrial cytochrome enzymes and preventing cellular respiration. According to the American Conference of Governmental Industrial Hygienist (ACGIH) and the Occupational Safety and Health Administration (US), the threshold limit values (TLV) for H2S is only 10 ppm and the permissible exposure is 10 ppm for 8 h. Moreover, the recommended acceptable ambient levels of H2S by the Scientific Advisory Board on Toxic Air Pollutants (USA) are in the range of 20−100 ppb

1, 2

.

H2S is also a trace biomarker in early diagnosis of lung disease (the third endogenous gas transmitter in the pathophysiological process)

3-5

. Accordingly, the selective

detection of H2S at ppm and sub-ppm levels is of great significance to environmental monitoring, disease diagnosis and food safety. Metal oxide semiconductors are very promising sensing materials and many of them are applied in gas sensors to detect trace concentrations of hazardous gases. Till date, materials such as ZnO 6-11, SnO2 12-14, Fe2O3 15-22, WO3 23, 24, CuO

25-27

, MoO3 25-

27

, and BaTiO3 29, 30 have been investigated for the detection of ppm and sub-ppm

levels of H2S. Among them, Fe2O3 exhibits several unique advantages such as good 2


Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


sensing performance, easy synthesis, low cost, non-toxic and abundant in the earth’s crust. In addition, Fe2O3 is the best desulfurizer among various metal oxides that are used in the Hot Gas Desulfurization (HGD) in the purification process of coal gas. Many efforts have been applied to improve the selectivity and sensitivity of H2S gas sensors based on Fe2O3, including loading with noble metals, doping with catalytic oxides, and composing p-n junctions. However, as shown in Table 1, the long recovery time and high concentration of the detection limit are still two cruxes for H2S sensors based on Fe2O3. Moreover, most metal oxide semiconductors require heating to 200−400 °C to promote the sensing reaction, and usually sensors based on semiconductors cannot thoroughly recover at low temperatures. High operating temperatures can lead to high power consumption, which is inimical to portable applications. To date, hierarchical and hollow nanostructures have received great attention owing to their high SSA and well-aligned porous morphology. They can provide effective gas diffusion paths without sacrificing the high surface area as a result of their less agglomerated configurations31. Li et al.

32

prepared the acetone and ethanol

sensor based on hierarchical SnO2/α-Fe2O3 bilayer hollow spheres, and the sensor showed a high sensitivity and ultra-fast response/recovery speed (3-4 s/6-9 s). Zhang et al.

33

synthesized hierarchical Au-loaded In2O3 porous nanocubes by a two-step

approach. The gas sensor based on these porous nanocubes showed excellent sensing performance to formaldehyde, and its response/recovery time (3/8 s) were exceedingly short. Tan et al.34 prepared hollowed-out hierarchical α-Fe2O3 nanorods. 3



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The gas sensor based on the α-Fe2O3 hierarchical nanorods exhibited high sensitivity and fast response/recovery speed to acetone (0.4/2.4 s) and ethanol (0.8/3.2 s). Sui et al. 35 synthesized monodisperse, hierarchical α-MoO3 hollow spheres using a facile template-free solvothermal method. The 2.04 wt.% Au-loaded α-MoO3 sensor exhibited high response to benzene, toluene and xylene and the response time were sharply shortened. Our previous works also showed that the hierarchical and hollow porous WO3 and NiO microspheres exhibited good gas-sensing performances towards NO and NO2 36, 37. Therefore, it is worthwhile developing hierarchical and hollow Fe2O3 to simultaneously enhance the response and response/recovery speed to H2S. Metal-organic frameworks (MOFs) perfectly assemble metal ions and organic ligands in the crystal lattice and provide a porous structure with high surface area. Owing to the high porosity, high internal SSA, and exceptional thermal stability and chemical stability, MOFs have received great interest and been applied in various fields, such as gas adsorption and separation, energy storage and catalysis38-45. Therefore, MOFs are ideal self-sacrificial templates for fabricating porous metal oxides. Prussian blue (PB), a well-known MOFs for a wide range of applications 46-52, was applied as the templates in this work to fabricate the hierarchical and hollow porous Fe2O3 nano-boxes. In this work, hierarchical and hollow porous Fe2O3 nano-boxes (with edge length of about 500 nm) were prepared by wet chemical method derived from MOFs, and their ability to detect H2S was investigated. The sensor based on the Fe2O3 nanoboxes with a high SSA (113 m2/g) exhibited high response, excellent selectivity and 4


Page 4 of 37

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fast response/recovery speed to ppb-level concentration of H2S at 200 °C, and showed fully reversible sensing properties at 50 °C. The improved H2S gas sensing properties can be ascribed to the nanostructure of the Fe2O3 nano-boxes with high SSA and wellaligned porous morphology.

2. EXPERIMENTAL 2.1 Synthesis of Prussian blue cubes In a typical procedure (shown in Figure 1), the Prussian blue cubes were prepared as follows: 15.2 g polyvinypyrrolidone (PVP, K30, Mw ≈ 40,000) and 0.44 g K4Fe(CN)6•3H2O were added into 200 mL 0.1 M HCl solution. After vigorous stirring for 30 min, a clear solution was formed and kept at 80 °C for 24 h. The product was then collected by centrifugation and washing several times with deionized water and alcohol. Finally, the blue product was dispersed into 50 mL alcohol for further use. 2.2 Synthesis of Fe(OH)3 nano-boxes The hollow Fe(OH)3 nano-boxes were prepared as follows: 17 mL of 2 M NaOH aqueous solution was added into 10 mL of the Prussian blue cube suspension. After shaking for about 5 min, the precipitation was collected after 5 rinse-centrifugation cycles. The hollow Fe(OH)3 nano-boxes were calcined at 400°C in air for 3 h to obtain hollow Fe2O3 nano-boxes. The heating rate was 1 °C /min. 2.3 Fabrication and characterization of gas sensors based on Fe2O3 nano-boxes Some Fe(OH)3 nano-boxes were dispersed into ethanol with ultrasonic treatment 5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

for 30 min to obtain a homogeneous suspension. The substrate was a piece of FTO glass (Nippon Sheet Glass, Japan), on which a gap of about 60 µm was cut by laser. 20 µL of the Fe(OH)3 suspension was added dropwise on the FTO substrate (see Figure 1). To obtain Fe2O3 nano-boxes, the Fe(OH)3 nano-boxes on the FTO glass substrate was calcined at 400 °C in air for 3 h at a heating rate of 1 °C/min. In order to improve the sensor stability, an aging treatment (300 °C for 48 h in air) was introduced before the gas-sensing measurement. The sensor resistance was measured by the two-probe method, and the data were collected automatically every second using an Agilent B2901A Source Measurement Unit (SMU). A flow system consisting of two mass flow controllers (MFC) was used to introduce gases with specified concentrations of H2S in air into the gas chamber at a flow rate of 500 Standard Centi-Cubic per Minute (SCCM). The H2S concentration was monitored by an Agilent 7890A Gas Chromatography System. To accurately monitor the sensor temperature, a thermocouple was placed near to the sample. 2.4 Characterization Scanning electron microscopy (SEM) images were taken by Sirion 200 equipped with EDS elemental composition analyzer at an acceleration voltage of 20 kV. Transmission electron microscopic (TEM) investigations were performed on JEOL JEM-2100F (FEI, Holland). The X-ray diffraction (XRD) was recorded by Philips diffractometer/PW3050 X equipped with a source of Cu-Kα1 radiation. The surface and pore characterization of the Fe2O3 nano-boxes were characterized by N2 adsorption/desorption measurements at −196 °C using an ASAP 2020 surface area 6


Page 6 of 37

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and porosity analyzer (Micromeritics Instrument Corporation, USA). Before measurement, the Fe2O3 nano-boxes were degassed at 160 °C using the VacPrep 061 degasser (Micromeritics Instrument Corporation, USA) to remove moisture and adsorbed contaminants. The specific surface area of the Fe2O3 nano-boxes was calculated from the N2 adsorption isotherm using the Brunauer-Emmett-Teller (BET) equation in the relative pressure range of 0.1 to 0.3. Porosity distribution was calculated by the Barrett-Joyner-Halenda (BJH) model from the N2 adsorption in slit pores. XPS investigations were carried out with an SECA Lab2200i-XL spectrometer by using an unmonochromated Al Kα (1486.6 eV) X-ray source. The surface analysis using the diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) were conducted by a VERTEX 70-FTIR spectrometer equipped with a SMART collector and an MCT detector cooled by water. Raman spectra were obtained using a LabRAM HR800 spectrometer operated under He-Cd laser excitation.

3. RESULTS AND DISCUSSION The crystal structures of the as-prepared cubes and nano-boxes were determined by X-ray diffraction. Figure 2 depicts the XRD patterns of the Prussian blue (PB) cubes, Fe(OH)3 and Fe2O3 nano-boxes. The PB cubes can be identified as facecentered-cubic (FCC) phase (JCPDS NO.73-0687), and no sign for any second phase can be observed. The XRD pattern of the Fe(OH)3 nano-boxes displays a phase of Fe(OH)3 (JCPDS NO.22-0346). After annealing at 400 °C for 3h, Fe(OH)3 transformed into Fe2O3. The XRD pattern of the Fe2O3 nano-boxes (Figure 2C) 7



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

reveals a hexagonal structure (JCPDS NO.33-0664). The SEM and TEM images (Figure 3A to C) of the Prussian blue confirm the formation of uniform nanocubes with an average edge length of 500 to 800 nm. These cubes were employed as the template to synthesize the hierarchical and hollow porous Fe2O3 nano-boxes. Figure 1 shows the schematic diagram for the synthetic process of the Fe2O3 nano-boxes. In the first step, an ion exchange described by Equation (1) occurs:

12OH () + Fe Fe(CN) () → 3Fe(CN)

() + 4Fe(OH)()

(1)

In the alkaline solution, the chemical reaction that Fe3+ reacts with OH- to form Fe(OH)3 firstly occurs at the interface between PB and alkaline, forming a thin layer on the surface of PB. Then OH- permeates into PB to react with more Fe(CN)64-, finally Fe(OH)3 nano-boxes are obtained. Figure 3D shows the SEM image of the as-prepared Fe(OH)3 nano-boxes; the Fe(OH)3 nano-boxes display a uniform cubic morphology with an average edge length of 500 to 800 nm. Some nanosheets can be observed on the surfaces of the Fe(OH)3 nano-boxes. TEM investigation further reveals the subtle structure of the Fe(OH)3 nano-boxes. As demonstrated in Figure 3E and F, the nano-boxes are hollow and porous with thin shells (about 45 nm thick), and the shells are constructed by nanosheets with an average thickness of 3 to 5 nm. The SEM and TEM images of the Fe2O3 nano-boxes are displayed in Figure 3G, H and I; it can be seen that the hierarchical hollow structure, and the nanosheet subunits in the surface layer are all maintained. But some differences can be observed 8


Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


between the Fe(OH)3 and Fe2O3 nano-boxes; there are more pores in the shells of the Fe2O3 nano-boxes, which are formed by the vapor evaporation and recrystallization during the annealing at 400 °C. The nitrogen adsorption/desorption isotherms and the pore-size distribution curves of the Fe(OH)3 and Fe2O3 nano-boxes are shown in Figure 4. The two isotherms are very similar, exhibiting a distinct hysteresis loop in the P/P0 range of 0.5 to 1, which is typical for the presence of mesopores. According to the BJH poresize distribution (Figure 4B), the two samples exhibit porous structures with mainly micropores (˂ 2 nm) and mesopores (2-50 nm). A small quantity of macropores (50100 nm) can also be detected. The SSA of the Fe(OH)3 and Fe2O3 nano-boxes are 166.6 m2/g and 113 m2/g, respectively. The Fe(OH)3 nano-boxes have an average pore diameter about 19.1 nm and high pore volume about 0.8 cm3/g. In addition to the slightly smaller specific surface area, the Fe2O3 nano-boxes have a slightly larger average pore diameter (22.2 nm) and a slightly smaller pore volume (0.63 cm3/g). As can be seen in Figure 3H, the micro-, meso- and macropores are located in the shells, which are favorable for gas permeating and diffusing into the nano-boxes. XPS was applied to further investigate the chemical statuses of the elements in the Fe2O3 nano-boxes. Figure 5A shows the high-resolution XPS spectrum of Fe 2p. It reveals the doublet Fe 2p3/2 and 2p1/2 with binding energies of 710.9 and 724.8 eV, respectively. Both peaks are accompanied by satellite structures with higher binding energy (approximately 8 eV), which is characteristic of the Fe3+ species in Fe2O3 53-55. The high-resolution XPS spectrum of O 1s is displayed in Figure 5B. After fitting 9



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

with Gaussian-Lorentzian functions, three peaks are found in the spectrum; two of them are O 1s peaks at 529.8 and 531.2 eV, corresponding to O 1s in Fe2O3 and absorbed oxygen species56, respectively. Moreover, the peak at 532.8 eV is identified as H2O absorbed in the Fe2O3 nano-boxes. All the above features confirm that the sample is Fe2O3. A low operating temperature ensures low energy consumption and high operation safety. Thus, the sensor based on the Fe2O3 nano-boxes was tested at low working temperatures (below 250 °C). Figure 6 exhibits the response-recovery curves of the sensor to 5 ppm H2S gas at 250, 200 and even 50 °C; the sensor exhibits good repeatability among individual alternating cycles. In this work, the response (S) is defined as Ra/Rg, where Rg and Ra refer to the sensor resistances in H2S gas and air, respectively. The response/recovery times (tresp/treco) are defined as the time required for reaching 90% variation of the sensor resistance upon exposure to the analyte gas or air. From Figure 6A, the response to 5 ppm H2S at 250 °C can be determined to be about 5.36 and the response/recovery time be 31/187 s, respectively. At 200 °C, the sensor’s response decreases to 2.74, and the response speed becomes slower. Noticeably, the sensor based on the Fe2O3 nano-boxes still manifests a reversible response of 2.58 to 5 ppm H2S even at a low temperature of 50 °C (Figure 6C). The sensing properties of the sensor based on the Fe2O3 nano-boxes are compiled in Table 1. Compared with various H2S sensors based on Fe2O3 listed in Table 1, it is easy to see that the H2S gas sensor based on the Fe2O3 nano-boxes exhibits fast response/recovery speed (especially the recovery speed is very fast) and high response 10


Page 10 of 37

Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to low concentration H2S. It has to be pointed out that some data in Table 1 (marked by “*”) were obtained from the static test, which is different from our dynamic one. More importantly, the sensor can detect ppb-level H2S with good response (Figure 7A, B and C). The response to 250 ppb H2S is about 1.23 at 200 °C, and tresp increases (145 s) while treco has almost no change (134 s). Despite of the low concentration of H2S, the overall performance of the sensor based on the Fe2O3 nanoboxes is still quite good. The responses of the sensor based on the Fe2O3 nano-boxes to NH3, CO and NO were also measured at 200 °C, and the results are given in Figure 7D. The sensor exhibits the best sensing performance toward 1 ppm H2S, while the response to NH3 is less than 1.2, and the response to CO and NO is even less than 1.03 (Rg /Ra). Therefore, the sensor shows good selectivity. To test the sensor’s stability and repeatability, the sensing properties of the sensor based on the Fe2O3 nano-boxes to alternating cycles of 5 ppm H2S at 200 °C were investigated after one month and three months. As shown in Figure 8, the response of the sensor slightly decreased from 2.74 to 2.3 after one month, but the response remained almost unchanged (S = 2.47) after three months. The long-term stability of the sensor based on the Fe2O3 nano-boxes shows a very promising prospect for practical applications. The excellent gas-sensing performance to H2S can be ascribed to the hierarchical and hollow porous nanostructure. Figure 9 schematically shows the gas sensing mechanism. When exposed to air, an electron depletion layer (EDL) with high resistance is formed due to negatively charged chemisorbed oxygen species (O2-, O-, 11



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

and O2-) on the surface of Fe2O3. The chemisorption of oxygen species is described by55-57 : () → ()

(2)

() + → ()

(3)

When the sensor is exposed to H2S gas, the ionized oxygen anions oxidize H2S gas, and electrons are released and transfer to the Fe2O3, decreasing the resistance of Fe2O3; the oxidation reaction is

2! "() + 3() ↔ 2! () + 2"() + 3

(4)

Therefore, when exposed to air, Fe2O3 becomes resistive; when exposed to H2S, Fe2O3 turns into conductive. The porous and hollow nanostructure of the Fe2O3 nanoboxes provides pores for effective gas diffusion and decreases the gas diffusion distance 31. Therefore, the Fe2O3 nano-boxes can quickly convert into the conductive state when exposed to H2S. Similarly, the special nanostructure can also make the Fe2O3 nano-boxes quickly turn into the resistive state when exposed to air. As a result, a high response and fast response/recovery are achieved. To further understand the gas sensing mechanism, the surface reactions of the Fe2O3 nano-boxes were investigated using DRIFT (Figure 10). To make the interaction between H2S and Fe2O3 more pronounced, 50 ppm H2S was used for the DRIFT measurements. As shown in Figure 10A, the bands around 570, 1116 and 1260 cm-1 correspond to the Fe-O bond, physisorbed SO2 with symmetric and asymmetric stretching modes58,59, respectively. The presence of SO2 confirms the sensing mechanism described by Equation (4). The DRIFT spectra of the Fe2O3 nano-boxes 12


Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


exposed to H2S at different temperatures (from 50 to 250 °C) are shown in Figure 10B; the bands around 1116 and 1260 cm-1 related to the physisorbed SO2 are clearly visible even at 50 °C, which explains the sensor performance at 50 °C (Figure 6C). Fe2O3 is a desulfurizer for hot gas desulfurization60. The works on the H2S adsorption behavior on the Fe2O3 surface demonstrate that the conversion to FeS needs high energy, therefore, a high temperature (about 700 °C) is necessary. However, at the initial stage, there are two more chemisorption states for the H2S adsorbed on the surface of Fe2O3, and the chemisorption states are not stable. In our DRIFT results, no Fe-S characteristic peak is observed, therefore, FeS is not formed. In other words, the H2S adsorption remains in the relatively unstable initial state, thus the gas sensors based on Fe2O3 nano-boxes can recover quickly and completely.

4. Conclusions Fe2O3 nano-boxes with a hierarchical and hollow porous nanostructure were prepared by a simple method from MOFs. The Fe2O3 nano-boxes have a unique complex nanostructure with a high specific surface area (113 m2/g), which is very advantageous for gas sensing. Compared with previous works on H2S gas sensors based on Fe2O3, the sensor based on the Fe2O3 nano-boxes showed an excellent sensitivity to ppb-level H2S at low operating temperature (at 200 °C, S = 1.23, tresp = 145 s, treco = 134 s), and showed fully reversible response at near room temperature (50 °C). Moreover, the selectivity, reproducibility and stability of this sensor were outstanding, demonstrating that the hierarchical porous Fe2O3 nano-boxes are very 13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

promising for high-performance H2S gas sensors.

Acknowledgements

The Analytical and Testing Center of Huazhong University of Science and Technology is acknowledged for the TEM, SEM and XPS investigations.

Supporting Information

Low-magnification SEM images of Fe(OH)3 and Fe2O3 nano-boxes.

14


Page 14 of 37

Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References [1] Wetchakun, K.; Samerjai, T.; Tamaekong, N.; Liewhiran, C.; Siriwong, C.; Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Semiconducting Metal Oxides as Sensors for Environmentally Hazardous Gases. Sens. Actuators B: Chem. 2011, 160, 580-591. [2] Kaur, M.; Jain, N.; Sharma, K.; Bhattacharya, S.; Roy, M.; Tyagi, A.K.; Gupta, S.K.; Yakhmi, J.V. Room-Temperature H2S Gas Sensing at ppb level by Single Crystal In2O3 Whiskers. Sens. Actuators B: Chem. 2008, 133, 456-461. [3] Beauchamp, R.O.; Bus, J.S.; Popp, J.A.; Boreiko, C.J.; Andjelkovich, D.A.; Leber, P. A Critical Review of the Literature on Hydrogen Sulfide Toxicity. Crit. Rev. Toxicol. 1984, 13, 25-97. [4] Van den Broek, A.M.W.T.; Feenstra, L.; De Baat, C. A Review of the Current Literature on Aetiology and Measurement Methods of Halitosis. J. Dent. 2007, 35, 627-635. [5] Bain, M.A.; Faull, R.; Fornasini, G.; Milne, R.W.; Evans, A.M. Accumulation of Trimethylamine and Trimethylamine-N-oxide in End-Stage Renal Disease Patients Undergoing Haemodialysis. Nephrol. Dial. Transpl. 2006, 21, 1300. [6] Hosseini, Z.S.; Zad, A.I.; Mortezaali, A. Room Temperature H2S Gas Sensor Based on Rather Aligned ZnO Nanorods with Flower-Like Structures. Sens. Actuators B: Chem. 2015, 207, 865-871. [7] Deng, J.; Fu, Q.; Luo, W.; Tong, X.; Xiong, J.; Hu, Y.; Zheng, Z. Enhanced H2S Gas Sensing Properties of Undoped ZnO Nanocrystalline Films From QDs by LowTemperature Processing. Sens. Actuators B: Chem. 2016, 224, 153-158. [8] Diao, K.; Zhou, M.; Zhang, J.; Tang, Y.; Wang, S.; Cui, X. High Response to H2S Gas with Facile Synthesized Hierarchical ZnO Microstructures. Sens. Actuators B: Chem. 2015, 219, 30-37. [9] Kim, S.; Na, C.W.; Hwang, I.; Lee, J. One-pot Hydrothermal Synthesis of CuO– ZnO Composite Hollow Spheres for Selective H2S Detection. Sens. Actuators B: Chem. 2012, 168, 83-89. 15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[10] Mortezaali, A.; Moradi, R. The Correlation Between the Substrate Temperature and Morphological ZnO Nanostructures for H2S Gas Sensors. Sens. Actuators A: Phys. 2014, 206, 30-34. [11] Y. Chen, P. Xu, T. Xu, D. Zheng and X. Li, ZnO-nanowire size effect induced ultra-high sensing response to ppb-level H2S, Sens. Actuators B: Chem., 240 (2017) 264-272. [12] Nakla, W.; Wisitsora-at, A.; Tuantranont, A.; Singjai, P.; Phanichphant, S.; Liewhiran, C. H2S Sensor Based on SnO2 Nanostructured Film Prepared by High Current Heating. Sens. Actuators B: Chem. 2014, 203, 565-578. [13] Hwang, I.; Choi, J.; Kim, S.; Dong, K.; Kwon, J.; Ju, B.; Lee, J. Enhanced H2S Sensing Characteristics of SnO2 Nanowires Functionalized with CuO. Sens. Actuators B: Chem. 2009, 142, 105-110. [14] Katti, V.R.; Kaur, M.; Muthe, K.P.; Dua, A.K.; Gadkari, S.C.; Gupta, S.K.; Sahni, V.C. H2S Sensors Based on Chemically Treated SnO2: Pd Thin Films. Proc. Spie. 2003, 5062, 93-99. [15] Deng, J.; Ma, J.; Mei, L.; Tang, Y.; Chen, Y.; Lv, T.; Xu, Z.; Wang, T. Porous αFe2O3 Nanosphere-Based H2S Sensor with Fast Response, High Selectivity and Enhanced Sensitivity. J. Mater. Chem. A. 2013, 1,12400-12403. [16] Balouria, V.; Kumar, A.; Samanta, S.; Singh, A.; Debnath, A.K.; Mahajan, A.; Bedi, R.K.; Aswal, D.K.; Gupta, S.K. Nano-Crystalline Fe2O3 Thin Films for ppm level Detection of H2S. Sens. Actuators B: Chem. 2013, 181, 471-478. [17] Wang, Y.; Kong, F.; Zhu, B.; Wang, S.; Wu, S.; Huang, W. Synthesis and Characterization of Pd-doped α-Fe2O3 H2S Sensor with Low Power Consumption. Mat. Sci. Eng. B. 2007, 140, 98-102. [18] Wang, Y.; Wang, S.; Zhao, Y.; Zhu, B.; Kong, F.; Wang, D.; Wu, S.; Huang, W.; Zhang, S. H2S Sensing Characteristics of Pt-doped α-Fe2O3 Thick Film Sensors. Sens. Actuators B: Chem. 2007, 125, 79-84.

16


Page 16 of 37

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[19] Balouria, V.; Ramgir, N.S.; Singh, A.; Debnath, A.K.; Mahajan, A.; Bedi, R.K.; Aswal, D.K.; Gupta, S.K. Enhanced H2S Sensing Characteristics of Au Modified Fe2O3 Thin Films. Sens. Actuators B: Chem. 2015, 219, 125-132. [20] Wang, Y.; Wang, Y.; Cao, J.; Kong, F.; Xia, H.; Zhang, J.; Zhu, B.; Wang, S.; Wu, S. Low-Temperature H2S Sensors Based on Ag-doped α-Fe2O3 Nanoparticles. Sens. Actuators B: Chem. 2008, 131, 183-189. [21] Sun, G.; Kheel, H.; Lee, J.K.; Choi, S.; Lee, S.; Lee, C. H2S Gas Sensing Properties of Fe2O3 Nanoparticle-Decorated NiO Nanoplate Sensors. Surf. Coat. Tech. 2016, 307, 1088-1095. [22] Kheel, H.; Sun, G.; Lee, J.K.; Lee, S.; Dwivedi, R.P.; Lee, C. Enhanced H2S Sensing Performance of TiO2-Decorated α-Fe2O3 Nanorod Sensors. Ceram. Int. 2016, 42, 18597-18604. [23] Kruefu, V.; Wisitsoraat, A.; Tuantranont, A.; Phanichphant, S. Ultra-sensitive H2S Sensors Based on Hydrothermal/Impregnation-made Ru-Functionalized WO3 Nanorods. Sens. Actuators B: Chem. 2015, 215, 630-636. [24] Tao, W.; Tsai, C. H2S Sensing Properties of Noble Metal Doped WO3 Thin Film Sensor Fabricated by Micromachining. Sens. Actuators B: Chem. 2002, 81, 237-247. [25] Kim, H.; Jin, C.; Park, S.; Kim, S.; Lee, C. H2S Gas Sensing Properties of Bare and Pd-Functionalized CuO Nanorods. Sens. Actuators B: Chem. 2012, 161, 594-599. [26] Liang, X.; Kim, T.; Yoon, J.; Kwak, C.; Lee, J. Ultrasensitive and Ultraselective Detection of H2S Using Electrospun CuO-loaded In2O3 Nanofiber Sensors Assisted by Pulse Heating. Sens. Actuators B: Chem. 2015, 209, 934-942. [27] Wang, Y.; Qu, F.; Liu, J.; Wang, Y.; Zhou, J.; Ruan, S. Enhanced H2S Sensing Characteristics of CuO-NiO Core-Shell Microspheres Sensors. Sens. Actuators B: Chem. 2015, 209, 515-523. [28] Li, H.; Huang, L.; Wang, X.; Lee, C.; Yoon, J.; Zhou, J.; Guo, X.; Lee, J. Molybdenum Trioxide Nanopaper as a Dual Gas Sensor for Detecting Trimethylamine and Hydrogen Sulfide. RSC Adv. 2017, 7, 3680-3685.

17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

[29] Jain, G.H.; Patil, L.A.; Wagh, M.S.; Patil, D.R.; Patil, S.A.; Amalnerkar, D.P. Surface Modified BaTiO3 Thick Film Resistors as H2S Gas Sensors. Sens. Actuators B: Chem. 2006, 117, 159-165. [30] Huang, H.; Li, H.; Wang, X.; Guo, X. Detecting Low Concentration of H2S Gas by BaTiO3 Nanoparticle-Based Sensors, Sens. Actuators B: Chem. 2017, 238, 16-23. [31] Lee, J. Gas Sensors Using Hierarchical and Hollow Oxide Nanostructures: Overview. Sens. Actuators B: Chem. 2009, 140, 319-336. [32] Li, H.; Xie, W.; Liu, B.; Wang, Y.; Xiao, S.; Duan, X.; Li, Q.; Wang, T. Ultra-fast and Highly-Sensitive Gas Sensing Arising From Thin SnO2 Inner Wall Supported Hierarchical Bilayer Oxide Hollow Spheres. Sens. Actuators B: Chem. 2017, 240, 349-357. [33] Zhang, S.; Song, P.; Li, J.; Zhang, J.; Yang, Z.; Wang, Q. Facile Approach to Prepare Hierarchical Au-loaded In2O3 Porous Nanocubes and Their Enhanced Sensing Performance Towards Formaldehyde. Sens. Actuators B: Chem. 2017, 241, 11301138. [34] Tan, J.; Huang, X. Ultra-thin Nanosheets-Assembled Hollowed-Out Hierarchical α-Fe2O3 Nanorods: Synthesis via an Interface Reaction Route and Its Superior Gas Sensing Properties. Sens. Actuators B: Chem. 2016, 237, 159-166. [35] Sui, L.; Zhang, X.; Cheng, X.; Wang, P.; Xu, Y.; Gao, S.; Zhao, H.; Huo, L. AuLoaded Hierarchical MoO3 Hollow Spheres with Enhanced Gas-Sensing Performance for the Detection of BTX (Benzene, Toluene, And Xylene) And the Sensing Mechanism. ACS Appl. Mater. Inter. 2017, 9, 1661-1670. [36] Tian, K.; Wang, X.; Li, H.; Nadimicherla, R.; Guo, X. Lotus Pollen Derived 3Dimensional Hierarchically Porous NiO Microspheres for NO2 Gas Sensing. Sens. Actuators B: Chem. 2016, 227, 554-560. [37] Wang, X.; Tian, K.; Li, H.; Cai, Z.; Guo, X. Bio-templated Fabrication of Hierarchically Porous WO3 Microspheres From Lotus Pollens for NO Gas Sensing at Low Temperatures. RSC Adv. 2015, 5, 29428-29432.

18


Page 18 of 37

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[38] Fang, G.; Zhou, J.; Liang, C.; Pan, A.; Zhang, C.; Tang, Y.; Tan, X.; Liu, J.; Liang, S. MOFs Nanosheets Derived Porous Metal Oxide-Coated Three-Dimensional Substrates for Lithium-ion Battery Applications. Nano Energy. 2016, 26, 57-65. [39] Li, J.; Kuppler, R.J. Zhou, H. Selective Gas Adsorption and Separation in MetalOrganic Frameworks. Chem. Soc. Rev. 2009, 38, 1477-1504. [40] Rowsell, J.L.C.; Yaghi, O.M. Strategies for Hydrogen Storage in Metal–Organic Frameworks. Angew. Chem. Int. Edit. 2005, 44, 4670-4679 [41] Zhou, H.; Long, J.R.; Yaghi, O.M. Introduction to Metal–Organic Frameworks. Chem. Rev. 2012, 112, 673-674. [42] Zhang, C.; Ai, L.; Jiang, J. Solvothermal Synthesis of MIL-53(Fe) Hybrid Magnetic Composites for Photoelectrochemical Water Oxidation and Organic Pollutant Photodegradation Under Visible Light. J. Mater. Chem. A. 2015, 3, 30743081. [43] Jiang, J.; Huang, L.; Liu, X.; Ai, L. Bioinspired Cobalt–Citrate Metal–Organic Framework as an Efficient Electrocatalyst for Water Oxidation. ACS Appl. Mater. Inter. 2017, 9, 7193-7201. [44] Ai, L.; Zhang, C.; Li, L.; Jiang, J. Iron Terephthalate Metal–Organic Framework: Revealing the Effective Activation of Hydrogen Peroxide for the Degradation of Organic Dye Under Visible Light Irradiation. Appl Catal B-Environ. 2014, 148–149, 191-200. [45] Ai, L.; Tian, T.; Jiang, J.; Ultrathin Graphene Layers Encapsulating Nickel Nanoparticles

Derived

Metal–Organic

Frameworks

for

Highly

Efficient

Electrocatalytic Hydrogen and Oxygen Evolution Reactions. ACS Sustain. Chem. Eng. 2017, 5, 4771-4777. [46] Li, Z.; Li, B.; Yin, L.; Qi, Y. Prussion Blue-Supported Annealing Chemical Reaction Route Synthesized Double-Shelled Fe2O3/Co3O4 Hollow Microcubes as Anode Materials for Lithium-Ion Battery. ACS Appl. Mater. Inter. 2014, 6, 80988107.

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

[47] Hu, M.; Belik, A.A.; Imura, M.; Mibu, K.; Tsujimoto, Y.; Yamauchi, Y. Synthesis of Superparamagnetic Nanoporous Iron Oxide Particles with Hollow Interiors by Using Prussian Blue Coordination Polymers. Chem. Mater. 2012, 24, 2698-2707. [48] Zhang, L.; Wu H.B.; Lou, X.W.D.; Metal–Organic-Frameworks-Derived General Formation of Hollow Structures with High Complexity. J. Am. Chem. Soc. 2013, 135, 10664-10672. [49] Cai, D.; Liu, B.; Wang, D.; Wang, L.; Liu, Y.; Qu, B.; Duan, X.; Li, Q.; Wang, T. Rational Synthesis of Metal-Organic Framework Composites, Hollow Structures and Their Derived Porous Mixed Metal Oxide Hollow Structures. J. Mater. Chem. A. 2016, 4, 183-192. [50] Hu, M.; Belik, A.A.; Imura, M.; Mibu, K.; Tsujimoto, Y.; Yamauchi, Y. Synthesis of Superparamagnetic Nanoporous Iron Oxide Particles with Hollow Interiors by Using Prussian Blue Coordination Polymers. Chem. Mater. 2012, 24, 2698-2707. [51] Yan, L.; Liu, Y.; Zha, K.; Li, H.; Shi, L.; Zhang, D. Scale–Activity Relationship of MnOx-FeOy Nanocage Catalysts Derived from Prussian Blue Analogues for LowTemperature NO Reduction: Experimental and DFT Studies. ACS Appl. Mater. Inter. 2017, 9, 2581-2593. [52] Mo, Q.; Wei, J.; Jiang, K.; Zhuang, Z.; Yu, Y. Hollow α-Fe2O3 Nanoboxes Derived from Metal–Organic Frameworks and Their Superior Ability for Fast Extraction and Magnetic Separation of Trace Pb2+. ACS Sustain. Chem. Eng. 2017, 5, 1476-1484. [53] Yamashita, T.; Hayes, P. Analysis of XPS Spectra of Fe2+ and Fe3+ Ions in Oxide Materials. Appl. Surf. Sci. 2008, 8, 2441-2449. [54] Cui, H.; Liu, Y.; Ren, W. Structure Switch Between α-Fe2O3, γ-Fe2O3 and Fe3O4 During the Large Scale and Low Temperature Sol–gel Synthesis of Nearly Monodispersed Iron Oxide Nanoparticles. Adv. Powder Technol. 2013, 254, 93-97. [55]

Mullet,

M.;

Khare,

V.;

Ruby,

C.

XPS

Study

of

Fe(II)/Fe(III)

(oxy)hydroxycarbonate Green Rust Compounds. Surf. Interface. Anal. 2008, 40, 323328. 20


Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[56] Wang, Y.; Wang, S.; Zhao, Y.; Zhu, B.; Kong, F.; Wang, D.; Wu, S.; Huang, W.; Zhang, S. H2S Sensing Characteristics of Pt-doped α-Fe2O3 Thick Film Sensors. Sens. Actuators B: Chem. 2007, 125, 79-84. [57] Kim, H.; Lee, J. Highly Sensitive and Selective Gas Sensors Using p-type Oxide Semiconductors: Overview. Sens. Actuators B: Chem. 2014, 192, 607-627. [58] Zhao, Y.; Hu, G.; Removal of SO2 by a Mixture of Caprolactam Tetrabutyl Ammonium Bromide Ionic Liquid and Sodium Humate Solution. RSC Adv, 2013, 3, 2234-2240. [59] Zhang, H.; Liu.Y. L. Fabrication of Magnetic and Photocatalytic Polyamide Fabric Coated with Fe2O3 Particles. Fiber Polym. 2015, 16, 378-387. [60] Song, J.; Niu, X.; Ling L. Wang, B.; A Density Functional Theory Study on the Interaction Mechanism Between H2S and the α-Fe2O3(0001) Surface. Fuel Process Technol. 2013, 115, 26-33. [61] Li, Z.; Lin, Z.; Wang, N.; Huang, Y.; Wang, J.; Liu, W.; Fu Y.; Wang, Z. Facile Synthesis of α-Fe2O3 Micro-ellipsoids by Surfactant-Free Hydrothermal Method for sub-ppm level H2S Detection. Mater. Design. 2016, 110, 532-539. [62] Huang, Y.; Chen, W.; Zhang, S.; Kuang, Z.; Ao, D.; Alkurd, N.R.; Zhou, W.; Liu, W.; Shen W.; Li, Z. A High Performance Hydrogen Sulfide Gas Sensor Based on Porous α-Fe2O3 Operates at Room-Temperature. Appl. Surf. Sci. 2015, 351, 10251033. [63] Li, Z.; Huang, Y.; Zhang, S.; Chen, W.; Kuang, Z.; Ao, D.; Liu W.; Fu, Y. A Fast Response & Recovery H2S Gas Sensor Based on α-Fe2O3 Nanoparticles with ppb level Detection Limit. J. Hazard. Mater. 2015, 300, 167-174.

21



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table Caption: Table 1 Comparison of the sensing properties of various H2S sensors based on Fe2O3

22


Page 22 of 37

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure Caption: Figure 1. Schematic illustration of the preparation process of hierarchical hollow Fe2O3 nano-boxes from Prussian blue cubes and the fabrication of gas sensors. Figure 2. XRD patterns of (A) Prussian blue cubes, (B) Fe(OH)3 nano-boxes and (C) Fe2O3 nano-boxes. Figure 3. SEM and TEM images of samples: (A) low-magnification SEM image of Prussian blue cubes, (B) high-magnification SEM image of Prussian blue cubes, (C) TEM image of Prussian blue cubes, (D) SEM image of Fe(OH)3 nano-boxes derived from the Prussian blue cubes, (E) low-magnification TEM image of Fe(OH)3 nanoboxes, (F) high-magnification TEM image of Fe(OH)3 nano-boxes, (G) SEM image of Fe2O3 nano-box derived from Fe(OH)3, (H) low-magnification TEM image of Fe2O3 nano-box, (I) high-magnification TEM image of Fe2O3 nano-box. Figure 4. (A) Nitrogen adsorption-desorption isotherm, and (B) pore-size distribution derived from nitrogen isotherms by the BJH model. Figure 5. XPS spectra of Fe2O3 nano-boxes: (A) Fe 2p spectrum, (B) C 1s spectrum. Figure 6. Response-recovery curves of Fe2O3 nano-boxes to 5 ppm H2S at (A) 250 °C, (B) 200 °C, and (C) 50 °C. Figure 7. (A) Response-recovery curve of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, (B) response of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, (C) response and recovery time of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, and (D) responses of 23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the Fe2O3 nano-boxes to 1 ppm H2S, 50 ppm NH3, 2000 ppm CO and 200 ppm NO at 200 °C. Figure 8. Stability and repeatability of the sensor based on Fe2O3 nano-boxes to 5 ppm H2S at 200 °C. Figure 9. Schematic diagram showing the sensing mechanism. Figure 10. DRIFT spectra of Fe2O3 nano-boxes: (A) exposed to 50 ppm H2S or air at 250 °C, and (B) exposed to 50 ppm H2S at different temperatures (50, 100, 150, 200, and 250 °C).

24


Page 24 of 37

Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1 Comparison of the sensing properties of various H2S sensors based on Fe2O3 Material

H2S (ppm)

T (°C)

Response

tResp/tReco (s)

Ref.

200

300

7.4

～150/150

22

200

300

26.55

~11/18

21

Ag: Fe2O3*

100

160

220

42/26

20

Au: Fe2O3

10

250

5.38

99/1620

19

Pt: Fe2O3*

10

160

147.5

/

18

Pd: Fe2O3*

10

160

46.6

/

17

Nanocrystalline Fe2O3 thin films

1

250

~0.5

64/390

16

Porous α-Fe2O3 nanospheres*

1

350

3.4

/

15

α-Fe2O3 microellipsoids*

0.5

350

1.45

80/7

61

Porous α-Fe2O3 nanoparticles*

0.05

RT

1.08

~250/~200

62

0.05

300

1.25

/

63

Fe2O3 nano-boxes

5

50

2.58

806/1100

This work

Fe2O3 nano-boxes

5

200

2.74

75/132

This work

Fe2O3 nano-boxes

5

250

5.36

31/187

This work

Fe2O3 nano-boxes

0.25

200

1.23

145/134

This work

TiO2-decorated α-Fe2O3 nanorods* Fe2O3 nanoparticle decorated NiO nanoplates*

α-Fe2O3 nanoparticles*

*Static testing was used in these works, consequently the response is higher.

25



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. Schematic illustration of the preparation process of hierarchical hollow Fe2O3 nano-boxes from Prussian blue cubes and the fabrication of gas sensors.

26


Page 26 of 37

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. XRD patterns of (A) Prussian blue cubes, (B) Fe(OH)3 nano-boxes and (C) Fe2O3 nano-boxes.

27



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. SEM and TEM images of samples: (A) low-magnification SEM image of Prussian blue cubes, (B) high-magnification SEM image of Prussian blue cubes, (C) TEM image of Prussian blue cubes, (D) SEM image of Fe(OH)3 nano-boxes derived from the Prussian blue cubes, (E) low-magnification TEM image of Fe(OH)3 nanoboxes, (F) high-magnification TEM image of Fe(OH)3 nano-boxes, (G) SEM image of Fe2O3 nano-box derived from Fe(OH)3, (H) low-magnification TEM image of Fe2O3 nano-box, (I) high-magnification TEM image of Fe2O3 nano-box.

28


Page 28 of 37

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (A) Nitrogen adsorption-desorption isotherm, and (B) pore-size distribution derived from nitrogen isotherms by the BJH model.

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. XPS spectra of Fe2O3 nano-boxes: (A) Fe 2p spectrum, (B) C 1s spectrum.

30


Page 30 of 37

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Response-recovery curves of Fe2O3 nano-boxes to 5 ppm H2S at (A) 250 °C, (B) 200 °C, and (C) 50 °C.

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. (A) Response-recovery curve of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, (B) response of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, (C) response and recovery time of Fe2O3 nano-boxes to different concentrations (0.25, 0.5, 1, 2, 3, 4 and 5 ppm) of H2S at 200 °C, and (D) responses of the Fe2O3 nano-boxes to 1 ppm H2S, 50 ppm NH3, 2000 ppm CO and 200 ppm NO at 200 °C.

32


Page 32 of 37

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Stability and repeatability of the sensor based on Fe2O3 nano-boxes to 5 ppm H2S at 200 °C.

33



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9. Schematic diagram showing the sensing mechanism.

34


Page 34 of 37

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 10. DRIFT spectra of Fe2O3 nano-boxes: (A) exposed to 50 ppm H2S or air at 250 °C, and (B) exposed to 50 ppm H2S at different temperatures (50, 100, 150, 200, and 250 °C).

35



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

36


Page 36 of 37

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphic abstract

Hierarchical and hollow Fe2O3 nano-boxes derived from metal-organic frameworks (MOFs) demonstrate an excellent sensitivity to H2S.

37


Hierarchical and Hollow Fe2O3 Nanoboxes Derived from Metal

Recommend Documents