A high performance hydrogen sulfide gas sensor based on porous α-Fe2O3 operates at room-temperature

Applied Surface Science 351 (2015) 1025–1033

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A high performance hydrogen sulfide gas sensor based on porous ␣-Fe2 O3 operates at room-temperature Yanwu Huang a , Weimei Chen a , Shouchao Zhang a , Zhong Kuang a , Dongyi Ao a , Nooraldeen Rafat Alkurd c , Weilie Zhou c , Wei Liu a , Wenzhong Shen b , Zhijie Li a,∗ a

School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu, 610054, PR China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, 030001, PR China c Advanced Materials Research Institute, University of New Orleans, New Orleans, LA 70148, USA b

a r t i c l e

i n f o

Article history: Received 21 January 2015 Received in revised form 6 June 2015 Accepted 9 June 2015 Available online 18 June 2015 Keywords: Porous ␣-Fe2 O3 nanoparticles Hydrothermal H2 S Gas sensor

a b s t r a c t Porous ␣-Fe2 O3 nanoparticles were synthesized by simple annealing of ␤-FeOOH precursor derived from a facile hydrothermal route, the structures and morphologies of the as-prepared product were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results showed that the average crystallite size of the obtained porous ␣-Fe2 O3 was 34 nm and exits numerous irregularly distributed pores with a diameter varying from 2 nm to 10 nm on the particle surface. The gas-sensing properties of the sensor based on porous ␣-Fe2 O3 nanoparticles were investigated, and the result showed that the sensor exhibited a high performance in hydrogen sulfide (H2 S) detection at room temperature. The highest sensitivity reached 38.4 for 100 ppm H2 S, and the detection limit was as low as 50 ppb. In addition, the response of the sensor towards other gases including C2 H5 OH, CO, H2 and NH3 indicates the sensor has an excellent selectivity to detection H2 S gas. Finally, the sensing mechanism of the sensor towards H2 S was also discussed. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen sulfide (H2 S), as a commonly-used chemical gas is extensively utilized in metallurgy, medicine, pesticide and catalyst fields. However, H2 S also is an inflammable and malodorous toxic gas, which is harmful to the human nerve and respiratory system even at low concentration. As generally reported, the short term (10 min) and long term (8 h) exposure limit of H2 S has been set to 15 ppm and 10 ppm, respectively. When the concentration reaches over 15 ppm, the human olfactory nerve would be paralyzed and the ability to sense H2 S declines [1,2], which could result in lethal consequences. Hence, it is urgent to develop a high-performance H2 S gas sensor with high-efficiency, high-reliability, high sensitivity, low cost, low detection limits, and low operation temperature for environmental protection and personal safety during industrial production process. In recent years, numerous materials including metal oxide semiconductor materials, such as In2 O3 [3,4], WO3 [5,6], ZnO [7–9], SnO2 [10], and novel functional materials, such as graphene [11], carbon nanotube [12] have been exploited as H2 S gas sensor due to their

∗ Corresponding author. Tel.: +86 18086810509. E-mail addresses: [email protected], [email protected] (Z. Li). http://dx.doi.org/10.1016/j.apsusc.2015.06.053 0169-4332/© 2015 Elsevier B.V. All rights reserved.

superior sensing performance towards H2 S gas. Among them, the metal oxide semiconductor (MOS) based gas sensors have gained special focus because of their low manufacturing cost, low operation power consumption, simple design, and ease of incorporation into microelectronic devices with possible high device integration density. Of various metal oxide based H2 S gas sensors, the ␣-Fe2 O3 based sensors have been proven to be promising due to their high sensitivity, high selectivity and non-toxicity. Hematite (␣-Fe2 O3 ), which is the most stable iron oxide, possesses n-type semiconducting properties with a band gap of 2.1 eV under ambient conditions [13]. As an important and promising functional material, ␣-Fe2 O3 has been widely applied in various areas including photo-catalyst, magnetic materials, waste-water treatment, anode material of lithium batteries [14–18]. Additionally, it is considered as a good candidate in the gas sensing field. It is well known that the gas sensing properties of ␣-Fe2 O3 dependent strongly on its morphologies, such as aspect ratio, size, orientation and crystal density [19]. Thus, over the past decades, considerable efforts have been made to improve the sensor’s sensitivity and selectivity, stability and reproducibility by controllable synthesis of various ␣-Fe2 O3 nanostructures, such as nanospindles [20], nanorings [21], nanospheres [22], nanorods [23], nanowires [24] etc., Moreover, ␣-Fe2 O3 with porous structure has been demonstrated to be an favorable architecture for boosting gas sensing

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performance due to the porous structures’ significantly enhanced the surface-area-to-volume ratio and material’s active sites, which would make the gas diffusion or mass transport onto the surface of materials much more convenient and effective in comparison to the solid ones [25–27]. For example, Wang et al. synthesized porous ␣-Fe2 O3 nanorods by a facile solution approach, and then utilized the material as a gas sensor for ethanol detection [23]. Deng et al. reported a high-performance H2 S gas sensor based on porous ␣-Fe2 O3 nanospheres by microwave-assisted a hydrothermal process [28]. However, it is a disadvantage that most porous ␣-Fe2 O3 based gas sensors reported previously operate at high temperature (200–500 ◦ C), which severely restricted its practical application. Therefore, reducing working temperature or operating at roomtemperature would be of great significance. Hence, in this paper, we report a high-performance roomtemperature H2 S gas sensor based on porous ␣-Fe2 O3 , prepared by the annealing of ␤-FeOOH precursor derived from a facile hydrothermal method. The sensing characteristics including sensitivity, selectivity, stability, response time and recovery time are investigated. In addition, the H2 S sensing mechanism of porous ␣-Fe2 O3 based sensors is also discussed. 2. Experimental details 2.1. Experimental procedure All the reagents were of analytical grade and used without any further purification, purchased from Sinopharm Chemical Reagents Co., Ltd. (Shanghai, china). Besides, all the water used throughout the experiment was deionized water with a resistivity of 18.3 M cm. The preparation of the porous ␣-Fe2 O3 nanoparticles consists of two steps including the formation of ␤-FeOOH precursor prepared by hydrothermal reaction and the further annealing of the as-prepared ␤-FeOOH precursor. In a typical procedure, firstly, 1.616 g ferric nitrate (Fe(NO3 )3 ·9H2 O) and 2.4 g urea (CO(NH2 )2 ) were dissolved into 120 mL deionized water under magnetic stirring at room temperature for 15 min. Then, 0.05 g sodium dodecyl sulfonate (SDS) was added into the above homogeneous solution and continuous stirring in a water bath of 60 ◦ C for 30 min. Secondly, the resultant mixture was transformed into a 150 mL Teflon-lined stainless steel autoclave and sealed. Then, the autoclave moved into an oven and maintained at 90 ◦ C for 12 h. After the autoclave naturally cooled to ambient temperature, a yellow precipitation was collected by centrifugation and washed with deionized water and absolute ethanol, respectively. Thirdly, the clean precipitation was dried at 100 ◦ C for 2 h in air to obtain the powder of ␤-FeOOH precursor. Finally, the reddish brown ␣-Fe2 O3 powder was obtained via annealing the ␤-FeOOH precursor at 500 ◦ C for 2 h in air. 2.2. Characterization of samples The crystal structure and phase composition of samples were characterized by X-ray diffractometer (XRD, DX-2700, China) with ˚ at 2 ranging from monochromatic Cu K␣ radiation ( = 1.5418 A) 20◦ to 80◦ . The morphology of the as-prepared samples were examined by scanning electron microscopy (SEM, Hitachi S-4800, Japan) with the operating voltage of 5 kV and transmission electron microscopy (TEM, JEM-100CXII, Japan) with an accelerated voltage of 75 kV. The chemical composition of the samples was analyzed using X-ray photoelectron spectroscopy (XPS, Kratos Axis-Ultradld, Japan) with Al K␣ radiation. Fourier transform infrared (FT-IR) spectra were recorded on a FT-IR absorption spectrometer (FT-IR, Nicolet 380, USA) in the range of 400–4000 cm−1 at room temperature. The gas-sensing properties were measured using a WS-30A

Fig. 1. Schematic structure of the gas sensor.

gas sensor measurement system (Weisen Electronic Technology Co., Ltd., Zhengzhou, China). 2.3. Fabrication of gas sensors The sensors were made by coating the as-prepared gel onto the ceramic tube to form a sensing film with a thickness about 10 ␮m. The schematic diagram of the sensor was shown in the Fig. 1. It can be seen that a pair of gold electrodes connected with Pt wire were installed at each end of the ceramic tube, between the two gold electrodes was the area which used for coating sensing film. A Ni-Cr heating wire used as a heat supply source was inserted into the ceramic tube to control the operation temperature of the gas sensor. The detailed fabrication procedure of the gas sensor was similar to the literature has been reported previously [29]. 2.4. Test circuit of the ˛-Fe2 O3 based gas sensor Fig. 2 displays the schematic diagram of the measurement circuit. Where, RL is a reference resistor, RS donates a resistor of the sensor. In the test process, an appropriate working voltage (Vs = 1 V) was applied. Then, the response of the sensor could be measured by monitoring the voltage changes of RL . Thus, the gas sensing response (S) can be defined as follow. S = Ra/Rg for reducing gases, and S = Rg/Ra for oxidizing gases, where Rg and Ra are the electrical resistances of the sensor in the target gas and in the air, respectively. 3. Results and discussion 3.1. Structural and morphology characterization Fig. 3 shows the XRD patterns of the obtained precursor and the final product by annealing the precursor at 500 ◦ C for 2 h. As is shown in Fig. 3a, all of the diffraction peaks correspond well to the tetragonal ␤-FeOOH (JCPDS NO.34-1266) and no impurities peaks can be observed in the patterns, indicates the high purity of the product. Fig. 3b displays the XRD pattern of the final product. It can be seen that all the diffraction peaks are well

Fig. 2. Schematic diagram of measure circuit.

Y. Huang et al. / Applied Surface Science 351 (2015) 1025–1033

Fig. 3. XRD patterns of the samples: (a) precursor and (b) final product after calcination.

matched with the standard hexagonal structure of hematite (˛Fe2 O3 ) reflection (JCPDS NO.33-0664) with lattice parameters of ˚ c = 13.7489 A˚ and ˛ = ˇ = 90◦ ,  = 120◦ . No diffraction a = b = 5.0356 A, peaks of ␤-FeOOH can be observed, suggesting that the ␤-FeOOH has completely transformed into ␣-Fe2 O3 after annealing process. In addition, the average crystallite size of the ␤-FeOOH precursor and ␣-Fe2 O3 are calculated to be 22 nm and 34 nm respectively by Scherrer equation: D=

K Bcos 

(1)

1027

where K is scherrer constant with a value of 0.89, B is full width at half maximum (FWHM) of the diffraction peak,  is the diffraction angle,  is the wavelength of X-ray and D denotes the crystallite size. Fig. 4 shows the SEM images of the ␤-FeOOH precursor and the ␣-Fe2 O3 . Fig. 4a and b displays the images of ␤-FeOOH precursor, it can be seen clearly that the ␤-FeOOH film is dense and aggregated by numerous unevenly distributed small grains. However, in comparison to the images of ␣-Fe2 O3 film which shown in Fig. 4c and d. Obvious differences can be found between the ␤FeOOH film and ␣-Fe2 O3 film. The ␣-Fe2 O3 film is composed of a great number of uniformly dispersed particles with an average size of approximately 35 nm rather than aggregated. This difference can be attributed to the fact that the annealing process provides the sufficient energy for mobility of the particles to recrystallize. To gain further insights into the crystallographic features of the as-prepared ␣-Fe2 O3 , transmission electron microscopy (TEM), high-resolution TEM (HRTEM) and associated diffraction techniques were employed. And the images were shown in Fig. 5. Fig. 5a shows a wide-field TEM image of the as-prepared ␣-Fe2 O3 . It can be seen that the porous ␣-Fe2 O3 nanoparticles had good dispersibility and a uniform gain size of approximately 35 nm, which is good in agreement with the calculated crystallite size from XRD data. Fig. 5b shows the enlarged TEM image taken from the area marked with a red square in Fig. 5a. It can be observed that a large amount of irregularly distributed pores with a size varying from 2 nm to 10 nm generated on the surface of ␣-Fe2 O3 nanoparticles, which are pointed out by arrows in Fig. 5b and circles in Fig. 5c. The explanation for the generation of pores can be given as follow. When ␤-FeOOH transferred into ␣-Fe2 O3 by annealing, due to the

Fig. 4. The SEM image of samples: (a), (b) ␤-FeOOH precursor and (c), (d) ␣-Fe2 O3 .

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Fig. 5. (a) and (b) high-magnification TEM, (c) HRTEM image of a single ␣-Fe2 O3 particle, (d) the selected area electron diffraction pattern (SAED) of several ␣-Fe2 O3 nanoparticles. The porous structures of as-prepared ␣-Fe2 O3 are indicated by arrows in (b) and circles in (c).

thermal decomposition, the H2 O vapor and CO2 generated onto the surface of particles were evaporated, simultaneously resulting in the shrinkage of the sample, thereby, generating a series of pores on the surface of the ␣-Fe2 O3 nanoparticles [30]. As has been generally proven by the previously reported literatures, these pores located on the surface of ␣-Fe2 O3 nanoparticles will increase the active surface area, thus resulting in more active sites, which is beneficial to enhance sensor properties [31]. Fig. 5c shows the HRTEM image recorded one single ␣-Fe2 O3 nanoparticle, in which, a series of clear and continuous lattice fringes can be seen in the image even across the pores. The typical lattice fringe spacing is determined to be 0.271 nm and 0.250 nm, which corresponds to the d-spacing of the (1 0 4) and (1 1 0) planes of hexagonal ␣-Fe2 O3 , respectively. The well-crystallized nature of the ␣-Fe2 O3 nanoparticles is confirmed by the selected area electron diffraction (SAED) pattern shown in Fig. 5d. From which, the concentric diffraction rings in the pattern can be well indexed to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4), (3 0 0) lattice planes of hexagonal ␣-Fe2 O3 from inside to out.

with concentrations ranging from 50 ppb to 100 ppm at room temperature. As can be seen from the curve, the sensor’s resistance undergoes a quick decrease since the injection of H2 S gas and mostly recovered to its initial value when the test chamber was refreshed with air, which is good in agreement with the sensing mechanism of n-type semiconductor gas sensor [3]. After several cycles of gas injection and air purification alternatively, full recovery to the initial response retains the same, indicating a good reversibility of the as-prepared gas sensor.

3.2. Gas sensing properties Fig. 6 plots the typical current–voltage characteristic between two neighboring electrodes bridged by the ␣-Fe2 O3 film in the air. It can be seen that the current increased linearly with the bias voltage, which reveals the sensor possess a good ohmic contacts between the ␣-Fe2 O3 thin film and the electrodes. Fig. 7a and b shows the representative response and recovery curve of the porous ␣-Fe2 O3 based gas sensor towards H2 S gas

Fig. 6. I–V characteristic curve between two neighboring electrodes.

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Fig. 7. Response and recovery curve of the porous ␣-Fe2 O3 based gas sensor to H2 S gas with the concentration ranging from (a) 5 ppm to 100 ppm, (b) 50 ppb to 1 ppm at room temperature.

Fig. 8 shows the corresponding response versus H2 S gas with concentrations in the range of 50 ppb–100 ppm. It can be observed that the response of the sensor almost enhanced linearly with the increase of H2 S concentration, revealing the sensor presents a good linearity characteristic to the detection of H2 S with different concentration. The inset of Fig. 8 shows the response of the sensor exposure to the H2 S gas at the concentration of sub-ppm level. It demonstrates that the sensor still has a significant response to the concentration as low as 50 ppb with the response value about 1.1. Fig. 9 shows the response time and recovery time of the porous ␣-Fe2 O3 based gas sensor as a function of H2 S concentration. Here, the response time and recovery time were defined as the time to reach 90% of the maximum sensing response upon test gas injection and the time to fall to 10% of the maximum sensing response upon air purging, respectively. As can be seen from Fig. 9, the response time shows a tiny variation upon exposure to different concentrations H2 S gas. Whereas, the recovery time increased obviously with the increase of H2 S concentration from 5 to 100 ppm, which can be explained that when the sensor was exposed to the high concentration H2 S gas, in addition to the surface reaction between H2 S

molecules and the adsorbed oxygen species, the formation of iron sulfides (FeS or FeS2 ) by chemical conversion between ␣-Fe2 O3 and H2 S is another potential reaction during the response process [32]. As we all know, the operating temperature has a significant effect on the semiconductor oxide sensor’s properties [33]. Thus, the sensor’s sensing performance at high temperature was tested, as well. Fig. 10 shows the sensor’s real-time response–recovery curve at 200 ◦ C. It can be seen that the response time and recovery time all have been shortened. However, the response intensity would decrease. For example, the response at 100 ppm H2 S had a response of 6, while the response at room temperature was 38. Moreover, the detectable gas concentration increased, as well, such as, at 200 ◦ C, the lowest detectable H2 S concentration was 10 ppm. Additionally, working at a high temperature increased the device’s power consumption and the difficulty for integration. In consideration of these disadvantages, we put emphasis on studying the sensor’s sensing performance at room temperature. The reproducibility of the sensor based on porous ␣-Fe2 O3 was studied by exposing the sensor to the H2 S gas at the concentration of 1 ppm for four cycles. The response curve was shown in Fig. 11,

Fig. 8. Variation of the sensitivity as a function of H2 S concentrations (inset shows the enlarged view of the sensitivity curve at the concentration varying from 50 ppb to 1000 ppb).

Fig. 9. Response time and recovery time of the ␣-Fe2 O3 based sensors at different H2 S gas concentration.

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Fig. 10. (a) Response of ␣-Fe2 O3 based sensor as a function of H2 S concentration at an operating temperature of 200 ◦ C; (b) response and recovery time of the sensor derived from ␣-Fe2 O3 nanoparticles upon exposure to different concentration H2 S ranging from 10 to 100 ppm at an operating temperature of 200 ◦ C.

it can be seen that the response time and recovery time were basically consistent for the four consecutive tests, and the fluctuation of response value was less than 5%, demonstrating that the porous ␣-Fe2 O3 based sensor has a good reproducibility and stability. For practical application, the selectivity of the sensor was a necessary consideration. The poor selectivity would lead to a mistaken alarm, thus severely limiting its industrial potential. Therefore, the sensing response of the porous ␣-Fe2 O3 based sensor for several reducing gases (C2 H5 OH, CO, H2 , NH3 ) was measured at the same gas concentration of 100 ppm at room temperature. The result was shown in the histogram of Fig. 12. It can be seen that the sensor displayed markedly higher response to H2 S gas than those to other gases at the same test conditions. The inset of Fig. 12 showed the actual response curve with exposing the sensor in the different target gases except the H2 S gas. As is shown, the response to C2 H5 OH, CO, H2 , NH3 was just 1.48, 1.45, 1.66 and 6.08, respectively, which was far less than the response towards H2 S gas, which reached 38.4 at the concentration of 100 ppm (shown in Fig. 7a), it means that the gas response to H2 S was 16 times higher than that C2 H5 OH and CO, 14 times and 6 times higher than H2 and NH3 , respectively. This information suggested that the sensor had an excellent selective toward H2 S.

Fig. 11. Reproducibility of ␣-Fe2 O3 sensor by successive exposure to 1 ppm H2 S gas for four cycles.

3.3. Sensing mechanism It is well known that the gas sensing mechanism of gas sensors based on metal oxide semiconductor (MOS) can be ascribed to the change in the electrical parameters including current, voltage and resistance, which resulted from the chemical interaction between the test gases and the absorbed oxygen species on the sensor’s surface. Further demarcation, the mechanism can be divided into three aspects involving gas absorption, charge transfer and gas desorption. The schematic diagram of the mechanism was shown in Fig. 13. When n-type ␣-Fe2 O3 based sensors were exposed to the air, the oxygen molecules were absorbed on the surface of ␣-Fe2 O3 sensing materials, and formed negatively charged chemisorbed oxygen species by capturing electrons from conduction band and covered onto the surface [34]. The type of absorbed oxygen species was dependent on the working temperature. When the temperature was lower than 100 ◦ C, O2 − was commonly chemisorbed. However, when the temperature between 100 ◦ C and 300 ◦ C, O− was commonly chemisorbed and the O2 − disappeared rapidly. And once the temperature was higher than 300 ◦ C, the oxygen species mainly existed in the form of O2− . Thus, at room temperature,

Fig. 12. Selectivity histogram of porous ␣-Fe2 O3 gas sensor toward difference gases (C2 H5 OH, CO, H2 , NH3 and H2 S) at the same concentration of 100 ppm (inset indicates the actual measurement curve).

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Fig. 13. Schematic diagram of the reaction mechanism between H2 S gas molecule and ␣-Fe2 O3 film.

the adsorbed oxygen species is mainly O2 − [35]. Furthermore, due to the electrons of conduction band was partly captured, leading to the decrease of the electron density, which in turn produces an electron-depletion layer on the surface and results in a higher surface resistance [36]. The detail reaction process at room temperature can be described as follow: O2(g) → O2(ad)

(2)

O2(ad) + e− → O2(ad) −

(3)

where the subscripts of (g) and (ad) denote the gas molecule in the air and the oxygen ions adsorbed on the material surface, respectively. Once the sensor was placed in the presence of H2 S gas, the H2 S molecules flowed over the surface of the ␣-Fe2 O3 sensing materials and were adsorbed onto the surface, then these absorbed H2 S

Fig. 14. (1) Wide range XPS spectrum of ␣-Fe2 O3 nanoparticle: (a) before adsorption of H2 S and (b) after adsorption of H2 S gas, (2)–(4) high resolution binging energy spectra of Fe2p, O1s and S2p after adsorption H2 S gas, respectively.

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during the sensing procedure and further explained how these sulfate groups exist in this process. 4. Conclusions In summary, a novel porous ␣-Fe2 O3 nanoparticles based gas sensor was reported in this work. The fabrication process involves the formation of ␤-FeOOH as a precursor and thermal conversion ␤-FeOOH to porous ␣-Fe2 O3 by annealing treatment. As a gas sensor, it showed a high-performance in H2 S detection, the response to 100 ppm H2 S gas reached 38.4 at room temperature and the detection limit was as low as 50 ppb. Moreover, the sensor also exhibited an excellent selectivity, reproducibility and stability, which demonstrated that this porous ␣-Fe2 O3 material was very promising for the fabrication of high-performance H2 S gas sensor. Acknowledgments

Fig. 15. FT-IR spectra of ␣-Fe2 O3 : (a) before adsorption of H2 S and (b) after adsorption of H2 S.

molecules reacted with the previously absorbed oxygen species (O2 − ) and generated sulfur oxides and H2 O vapor [26], during this react, the trapped electrons were released and backed into the conduction band again, which resulting in the reduction of the electron-depletion layer. Furthermore, the concentration of the charge carriers increased, consequently, the resistance of the film decreased. The concrete surface reaction between the H2 S gas and absorbed oxygen species can be depicted as the following equation: 2H2 S(g) + 3O2(ad) − ↔ 2H2 O(g) + 2SO2(g) + 3e−

(4)

To investigate the chemical states of H2 S absorbed on the surface of ␣-Fe2 O3 during the sensing procedure, the XPS and FT-IR characterization methods were used. Fig. 14(1) shows the wide range XPS spectrum of ␣-Fe2 O3 before and after adsorption of H2 S gas. By comparison, it can be seen that the spectrum of ␣-Fe2 O3 after adsorption of H2 S appeared a significant signal of S element. Fig. 14(2) shows a high resolution spectrum of S 2p, in which, two peaks at 165.7 eV and 170.4 eV could be assigned to SO2 and the residual H2 S absorbed on the material surface, respectively [37], which indicates that the S element was introduced on the material surface after exposure to H2 S. The O1s peak shown in Fig. 14(3) was split into three asymmetrical peaks, one peak located at 529.5 eV, corresponding to lattice oxygen atoms in the ␣-Fe2 O3 , and other peaks located at 531 eV and 532.5 eV can be attributed to oxygen species (O2 − ) and the water molecule which absorbed on the surface, respectively [38]. As can be seen from Fig. 14(4), the binding energy of 710.6 eV and 723.6 eV were well matched with the binding energy values of Fe3+ in the ␣-Fe2 O3 . Fig. 15(a) and (b) displays the FT-IR spectra of ␣-Fe2 O3 before and after adsorption H2 S gas, respectively. As shown in the Fig. 15(a), three peaks were observed. The peaks at 484 cm−1 and 578 cm−1 can be attributed to the Fe–O vibration of ␣-Fe2 O3 nanoparticle. The peak at 3421 cm−1 can be assigned to O–H stretching vibration, which generally derived from the semiconductor materials weakly absorbed the water from environment. After adsorption of H2 S gas, many new peaks in the region of 700–1300 cm−1 were observed (shown in Fig. 15(b)), which can be related to sulfate groups. Namely, the peaks at 773 cm−1 and 808 cm−1 can be distinguished as S–O stretching vibration, the peaks at 995 cm−1 and 1087 cm−1 corresponding to the stretching vibration of S O bond. The existence of peaks at 1242 cm−1 is due to the asymmetrically SO2 stretching vibration. These results confirmed the formation of sulfate oxides on the surface of ␣-Fe2 O3

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