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In situ diffuse reflectance infrared Fourier transform spectroscopy study of formaldehyde adsorption and reactions on nano γ-Fe2O3 films
Applied Surface Science 270 (2013) 405–410
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In situ diffuse reflectance infrared Fourier transform spectroscopy study of formaldehyde adsorption and reactions on nano ␥-Fe2 O3 films Kaijin Huang a,b,c,∗ , Lingcong Kong a , Fangli Yuan b , Changsheng Xie a a b c
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Science, Beijing 100190, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 17 June 2012 Received in revised form 21 December 2012 Accepted 7 January 2013 Available online 18 January 2013 Keywords: Gas sensors Nano ␥-Fe2 O3 films Formaldehyde DRIFTS Gas sensing mechanism
a b s t r a c t The nano ␥-Fe2 O3 films gas sensor was fabricated by the screen printing technology. The phase structures and morphologies of nano ␥-Fe2 O3 films were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively. The gas sensitivity of the films to 100 ppm formaldehyde was investigated. The surface adsorption and reaction process between nano ␥-Fe2 O3 films and formaldehyde was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method at different temperatures. DRIFTS results showed that dioxymethylene, formate ions, polyoxymethylene and molecularly formaldehyde surface species were detected when the nano ␥-Fe2 O3 films exposed to 100 ppm formaldehyde at different temperatures. A possible mechanism of the reaction process was discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Formaldehyde (CH2 O) is one of important chemicals and is widely used in construction, wood processing, furniture, textiles, carpeting industries. However, it is highly toxic and volatile. It has been classified as a human carcinogen that causes nasopharyngeal cancer and probably leukemia [1]. So it is very important to monitor and control its exposure in both industrial and residential environments. There are many papers covering the monitor and detection of formaldehyde. At present, the major methods are spectrophotometric analysis [2], electrochemical analysis [3] and chromatography techniques [4]. In spite of their advantages, these methods share a disadvantage that the monitor and detection are based on field sampling and laboratory analysis resulting in tedious operation and time-consuming, which cannot meet the demands of number of measuring points at the test site. However, gas sensors for formaldehyde have developed quickly because they are easy to operate, rapid to feedback and convenient to carry. To date, microfabricated formaldehyde gas sensors including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, nanowire-based sensors and polymer-based sensors for low-temperature, low-power operation have been reviewed by Flueckiger et al. [5]. Among these gas
∗ Corresponding author. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (K. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.038
sensors, ␥-Fe2 O3 -based gas sensor has been widely investigated by many researchers due to its high sensitivity, low cost, quick response and low power consumption [6]. To the best of our knowledge, most ␥-Fe2 O3 gas sensing mechanisms are focused on the resistance change of the sensing materials caused by bulk reactions and variations within the materials [7,8]. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique, which follows the physicochemical processes taking place in an active sensing element in real time and under operating conditions, is a very relevant tool for the characterization of the surface reactivity of gas sensing materials [9]. This technique has been applied to gas adsorption and oxidation on metallic oxide catalysts, such as formaldehyde, formic acid and methy formiate over TiO2 [10], ␥-Al2 O3 [11] and gaseous isopropanol over BiVO4 [12], on metal doping oxide catalysts, such as CO adsorption and oxidation on Au/Fe2 O3 catalysts [13], Cu/SiO2 and Co/SiO2 catalysts [14] and Pt/SiO2 [15], water-gas shift reaction on chromium-free iron oxide catalysts [16], formaldehyde catalytic oxidation over Pt/TiO2 [17] and Cu/Al2 O3 [18], NO reduction by NH3 over Fe–Ce–Mn/ZSM-5 catalysts [19], methanol oxidation by lattice oxygen over Cu/ZnO catalyst [20] and N2 O decomposition on Rh/CeO2 and Rh/␥-Al2 O3 catalysts [21], and on composite metallic oxide catalysts, such as CO adsorption and oxidation on CuCo/SiO2 catalysts [14], CO2 adsorption and desorption on MgO/Al2 O3 [22], toluene oxidation over V2 O5 /TiO2 [23] and CuO/CeO2 catalysts doped with alkaline earth metal oxides [24]. Besides, DRIFTS has also been applied to analyze different
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gas adsorption for CO selective oxidation on Cu–Zr–Ce–O catalysts [25], surface reactivity to NO2 by zinc nanoparticle aggregates and zinc hollow nanofibers [26] and the surface characterization of modified silica nanoparticles [27]. Few papers are focused on the underlying relationships between surface reactions and gas sensing properties on the ␥-Fe2 O3 films by DRIFTS. On the other hand, gas–solid adsorption is one of the most common interface adsorption phenomenon, therefore, it is of important significance to study the law and influencing factors of gas–solid interface adsorption for both industrial production and scientific research. The DRIFTS technique, taken into account, plays a significant role to study the gas–solid interface adsorption. Hence, in this paper, the adsorption and reactions of formaldehyde on nano ␥Fe2 O3 films were studied by DRIFTS to understand the ␥-Fe2 O3 gas sensing mechanism. 2. Experimental 2.1. Fabrication of -Fe2 O3 gas sensor All the reagents were of analytical grade and used as purchased. The ␥-Fe2 O3 nano-powder with 20–30 nm was mixed with organic slurry at a mass ratio of 10:9 to form a paste, which was milled at a speed of 250 rpm for 1 h. Then the milled paste was coated onto the surface of an Al2 O3 substrate on which the Au electrodes and RuO2 heaters had been pre-deposited by the screen printing technology. The area of each gas sensing film is about 1 × 1 mm2 . Heat-treatments of the Al2 O3 substrate coated with the ␥-Fe2 O3 gas sensor films were carried out in the air in a muffle furnace at 400◦ C for 2 h, with a heating rate of 5◦ C/min. Then the gas sensor array was soldered to form gas sensing devices using gold wires and a welding machine. In the end, the gas sensing device was aged at 320◦ C for 72 h in the air to improve its stability and repeatability. 2.2. Characterization of nano -Fe2 O3 films and measurement of gas sensing properties The phase structures of the nano ␥-Fe2 O3 films before and after sintered were determined by XRD with an X’ Pert Pro model X-ray diffractometer using Cu K␣ radiation ( = 0.1542 nm). The morphologies of the nano ␥-Fe2 O3 films were analyzed by fieldemission scanning electron microscopy (FESEM). The gas sensing device was tested in a glass chamber with a volume of 30 L. Gas sensitivity of the gas sensor was measured under a steady-state condition. The sensitivity to formaldehyde is defined as the ratio S = Ra /Rg , where Ra and Rg are the electrical resistance measured in the air and in formaldehyde, respectively.
to the predetermined temperature under flowing formaldehyde gas. A background spectrum was collected before each test. The spectra are smoothed and measured in the Kubelka–Munk mode because the peak intensities in these units are proportional to the concentration of the adsorbed species. 3. Results and discussion 3.1. Structural characterization The XRD patterns of nano ␥-Fe2 O3 films are shown in Fig. 1. The XRD patterns indicate that the phase of the films consists of cubic ␥-Fe2 O3 phase. No characteristic peaks of ␣-Fe2 O3 are detected. The result shows that the phase transition of ␥-Fe2 O3 → ␣-Fe2 O3 did not take place after sintering at 400 ◦ C for 2 h. FESEM images of the pure nano ␥-Fe2 O3 films are shown in Fig. 2. The films are characterized by a relatively rough surface with the presence of some pores and cracks, as presented in Fig. 2(a), due to the decomposition of organic slurry during heat-treatment of the films. In the meantime, Fig. 2(b) shows that the films consist of original spherical ␥-Fe2 O3 nano-particles with 20–30 nm. Small nano-particles are good for gas sensing properties by increasing the surface area of the sensing layer and favoring the interaction with the targeted gases. The evenly distributed pores in the films are likely to improve the surface area, which can greatly facilitate gas diffusion and mass transport in sensor materials. This plays a significant role in the surface reactions resulting in an increase in the gas sensitivity of the films. 3.2. Gas sensing properties The gas sensitivity is greatly influenced by the operating temperature. In order to research the optimum operating temperature, response of the nano ␥-Fe2 O3 gas sensor to 100 ppm formaldehyde gas at different operating temperatures was examined. Fig. 3 shows that the operating temperature has a great influence on the response. The response increases with operating temperature, reaches its maximum of 5.5 at 320 ◦ C, and begins to reduce above the temperature span. The sensor samples were fabricated for each type of the sensors, and the gas sensing properties of these samples were found to be similar, indicating a good reproducibility. The changes in response with operating temperature can be attributed to the fact that the adsorption type of oxygen molecules is chemisorption at higher temperature, and physisorption at lower
2.3. In situ DRIFTS study Infrared spectra were collected using a Bruker VERTEX 70 FTIR Spectrometer equipped with a MCT detector cooled by liquid N2 with KBr windows and a diffuse reflectance DRIFT accessory. The spectra were collected in the range of 4000–400 cm−1 at a resolution of 4 cm−1 and an accumulating of 128 scans. A vacuum chamber was connected to gas pipelines for materials treatment, whose temperature was controlled by a thermostat ranging from room temperature to 600 ◦ C. The Al2 O3 substrate printed with nano ␥-Fe2 O3 films was cut into 2.8 mm × 2.8 mm square. In this experiment, the cut sample was placed in the reactive cell. Pre-treatment process at 200 ◦ C and 5.0 × 10−3 Pa was performed for 15 min in order to make the sample reproducible and the spectra of KBr were recorded for background removal. Then a flow (30 ml/min) of 100 ppm formaldehyde was introduced into the cell at different working temperatures until the spectra stabilized. The reference spectra were taken after the sample was pretreated and heated
Fig. 1. XRD patterns of nano ␥-Fe2 O3 films sintered at 400 ◦ C for 2 h.
K. Huang et al. / Applied Surface Science 270 (2013) 405–410
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temperature, as reported by Xie et al. [28]. The stronger adsorption attraction of the chemisorptions could result in the higher concentration of adsorbed oxygen, trap more electrons from the conductance band and cause the larger change of the resistance. Hence, the response enhanced with the increasing temperature. But the reduction in response above 320 ◦ C is due to the fact that the adsorption reaction is proverbially exothermic. At higher temperature, the basic reaction (1/2O2 (g) + e− → O− (ads)) will proceed to left, which leads to the decrease of the trapped electrons from the conductance band. Thus the response decreases above 320 ◦ C. ␥-Fe2 O3 is a typical n-typed semiconductor, whose gas sensing mechanism belongs to the bulk-controlled type. The change of resistance is dependent on the species and chemisorbed oxygen on the surface. The oxygen absorbed on the surface of a ␥-Fe2 O3 sensor causes the electron depletion, consequently the resistance of the sensor increases. The process can be expressed in the following equations [29]: O2 (g) → O2 (ads) −
(1) −
O2 (ads) + e → O2 (ads) −
−
(2)
O2 (ads) + e− → 2O (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
wherein “g” and “ads” refer to gas and adsorbate, respectively. After CH2 O was introduced, it would be oxidized by these chemisorbed oxygen species (O2 − , O− , O2− ) on the surface of the test sensor. During the reaction, the electrons went back into the semiconductor, resulting in a decrease in resistance of the sensor. When the sensor was exposed in air again, the gases are desorbed as H2 O and CO2 . This reaction process may be expressed in the following equation. HCHO + 2O2− (ads) → CO2 + H2 O + 4e−
Fig. 2. FSEM images of nano ␥-Fe2 O3 films after sintering at 400 ◦ C for 2 h: (a) low magnification image; (b) high magnification image.
(5)
In addition, it is also well known that when ␥-Fe2 O3 exposed to reducing ambinent, it may convert to Fe3 O4 by a reversible redox reaction, because both the ␥-Fe2 O3 and Fe3 O4 have the same spinel crystal structure. However, the electrical resistivity of Fe3 O4 is lower than that of ␥-Fe2 O3 . Therefore, phase transition between ␥Fe2 O3 and Fe3 O4 is accompanied by a large change in the electrical resistance in the gas atmospheres. When the ␥-Fe2 O3 films exposure to formaldehyde gas, one part of formaldehyde gas molecules will react with Fe3+ in ␥-Fe2 O3 lattice, part of Fe3+ in the octahedral was reduced to Fe2+ , forming low resistance Fe3 O4 phase. Half of the Fe3+ ions are distributed at tetrahedral sites, and the other half of the Fe3+ ions together with the Fe2+ ions are distributed at octahedral sites. Since the emergence of sub-lattice location of iron ions with different valences, so the Fe2+ – Fe3+ – Fe2+ – Fe3+ . . . between the atomic electrons can hop from one defect atoms to another, so that the body resistance of ␥-Fe2 O3 films drops. The Fermi level of Fe3 O4 is a little higher than that of ␥-Fe2 O3 , electrons will migrate from the Fermi level of Fe3 O4 to that of ␥-Fe2 O3 until their Fermi levels equalize [30]. In the equilibrium state, an electron depletion layer might be generated between Fe3 O4 and ␥-Fe2 O3 , which plays an important role in the electron transfer of surface reactions. 3.3. DRIFTS study of formaldehyde adsorption on the gas sensor
Fig. 3. Response of nano ␥-Fe2 O3 to 100 ppm formaldehyde at different temperatures.
Figs. 4–7 show the dynamic changes in the DRIFTS spectra of the nano-␥-Fe2 O3 films in a flow of 100 ppm formaldehyde at 250 ◦ C, 300 ◦ C, 320 ◦ C and 350 ◦ C, respectively. The IR assignments of formaldehyde adsorption on ␥-Fe2 O3 films at different temperatures are shown in Table 1. As shown in Fig. 4, after 30 min of formaldehyde exposure at 250 ◦ C, the bands at 3743, 3626, 2931, 2863, 1413 and 1264 cm−1 were observed in the spectra, but relatively weak. With increasing absorption time, the bands intensities increased and reached a
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Fig. 4. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 250 ◦ C.
Fig. 6. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 320 ◦ C.
Fig. 5. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 300 ◦ C.
Fig. 7. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 350 ◦ C.
steady level after 45 min. The bands at 3743 cm−1 , which is accompanied by weak shoulder at 3626 cm−1 , was due to the (OH) mode of different types of isolated hydroxyl groups [10,11,31]. As we know, if a crystal of ␥-Fe2 O3 is formed, the network of Fe3+ and O2− comes to an abrupt end at the gas/solid interface, which leads to Fe3+ and O2− species at the surface coordinatively unsaturated. Therefore, ␥-Fe2 O3 can provide a spot of hydroxyls sites,
because the surface of ␥-Fe2 O3 is hydroxylated when exposure to formaldehyde gas containing water vapor. Dissociation of water forms two distinctive hydroxyl groups: one OH− group bridges two surface vicinal Fe3+ and the other forms a terminal Fe3+ OH− group, which is the reason of the existence of two (OH) mode of different types of isolated hydroxyl groups in the DRIFTS spectra. The band at 2863 cm−1 were assigned to the C H stretching
Table 1 IR assignments of formaldehyde adsorption on nano ␥-Fe2 O3 films at different temperatures. Surface species
Wavenumber (cm−1 ) at 250 ◦ C
Wavenumber (cm−1 ) at 300 ◦ C
Wavenumber (cm−1 ) at 320 ◦ C
Wavenumber (cm−1 ) at 350 ◦ C
References
(OH) (CH) di-oxymethylene (C O) CO2 ı(OH) Poly-oxymethylene as (COO) ıas(CH) s (COO) Formaldehyde ıs(CH)
3743/3626 2863 2931
3744/3633/3475 2859 2935
3736/3447
3736/3613
2935
2937
2349/2306 1642
2349/2306
2349/2312
1150 1594 1487 1354 1413/1255
1168 1593 1495 1358 1409/1258
[10,11,31] [11,31,32] [11] [33] [10,20] [11,33] [11,31,33] [11,32] [10] [11,31–33] [10,33] [33]
1413/1264
1241/1010 1433
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vibration ((CH)) [31,32] and the bands at 1413 and 1264 cm−1 to molecularly adsorbed formaldehyde [10,33] on the nano ␥-Fe2 O3 films. The band at 2931 cm−1 , which was the most prominent in the spectra, was assigned to the characteristic peak of dioxymethylene [11]. We can infer that CH2 O was molecularly adsorbed as a whole at first. Then the adsorbed formaldehyde gas was oxided to form dioxymethylene by chemisorbed oxygen species. At the same time, Fe3+ ions were deoxidated to Fe2+ . This reaction process may be expressed in the following Eqs. (6)–(8). HCHO(g) → HCHO(ads) 2−
HCHO(ads) + O
(6) −
(ads) → H2 COO (ads) + e−
Fe3+ + e− ↔ Fe2+
(7) (8)
As shown in Fig. 5, the DRIFTS spectra on nano ␥-Fe2 O3 films at 300 ◦ C, the new bands at 2306, 1642 and 1433 cm−1 appeared, except the bands at 3744, 3633 and 3475 assigned to the OH stretching vibration [10,11,31], the band at 2859 cm−1 to the C H stretching vibration [11,31,32] and the band at 2935 cm−1 to the characteristic peak of dioxymethylene [11], which was formed from the adsorbed formaldehyde gas by chemisorbed oxygen species, as shown in Table 1. The band at 1642 cm−1 was assigned to the OH bending vibration (ı(OH)) [11,33]. The low intensity band at 1433 cm−1 observed can be associated with the deformation vibration ıs(CH) in the formate species [33]. The bands at 1241 and 1010 cm−1 were also assigned to molecularly adsorbed formaldehyde on the ␥-Fe2 O3 films [10,33], indicating that the formaldehyde gas was adsorbed on the surface at first. As shown in Table 1, the new weak bands between 2400 and 2300 cm−1 indicated the appearance of carbon dioxide [10,20]. We can infer that the formed intermediate product dioxymethylene was oxided to form formate ions with the breakage of C H band. At last, CO2 and H2 O were formed after the oxidation of formate ions. The same products are produced on TiO2 [10], on ␥-Al2 O3 [11], on Pt/TiO2 [17] and over Cu/Al2 O3 [18]. The possible way for the adsorbed carbon dioxide can be formed by the following equations: H2 COO− (ads) + O2− (ads) → HCOO2− (ads) + OH− (ads) 2HCOO2− (ads) + O2− (ads) → 2CO2 + H2 O + 6e−
(9) (10)
Fig. 6 shows the DRIFTS spectra on nano ␥-Fe2 O3 films at 320 ◦ C, in which the bands at 3736, 3447, 3067, 2935, 2349, 2305, 1594, 1487, 1413, 1354, 1255 and 1150 cm−1 were observed. The intensities of most of the adsorbed species change obviously. The two (OH) [10,11,31] modes of different types of isolated hydroxyl groups (3736 and 3447 cm−1 ), as above, were due to the dissociation of water to form two distinctive hydroxyl groups: one OH− group bridges two surface vicinal Fe3+ and the other forms a terminal Fe3+ OH− group. The band at 2935 cm−1 was assigned to the characteristic peak of dioxymethylene [11]. The sharp absorption bands at 1594 cm−1 , 1487 cm−1 and 1354 cm−1 observed at higher temperature can be associated with COO asymmetric stretching (as (COO)) [11,32], CH bending (ıas(CH)) [10] and COO symmetric stretching(s (COO)) [11,31–33] vibrations in the formate structure. In this case, the spectra in the lower frequency region, showing very intense absorptions at 1150 cm−1 , indicate that formaldehyde is polymerized to linear polyoxymethylene [11,31], which was formed in the following process: nHCHO → [CH2 − O]n The bands at 1413 and 1255 cm−1
(11)
were assigned to molecularly adsorbed formaldehyde on the ␥-Fe2 O3 film [10,33]. The peaks that were attributed to carbon dioxide appeared obviously. Therefore, most of the species were found at this temperature, which indicated that the reactions went actively with temperature increasing as Eqs. (6)–(10) went.
409
After the ␥-Fe2 O3 sensor was heated to 350 ◦ C, as shown in Fig. 7, the adsorption species were similar to Fig. 6, but the bands intensities appeared weakly. Maybe the intermediate reactions were so fast that what we could get were the final products H2 O and CO2 . That was the reason why the band of carbon dioxide (2312 cm−1 ) [10,20] increased, suggesting that it contained active oxygen for the reaction. The infrared bands intensities at 3736 and 3613 cm−1 assignable to (OH) frequencies [10,11,31] declined while the others increased with progression of the reaction, indicating that the hydrogen of the surface isolated OH groups may be related to the lone pair electrons of the oxygen of formaldehyde to form hydrogen bonds and formaldehyde can be adsorbed, with consumption of the surface hydroxyls. This is in accord with the bands at 1409 and 1258 cm−1 assigned to molecularly adsorbed formaldehyde [10,33]. The band at 2937 cm−1 that increased during the reaction corresponded to the characteristic peak of dioxymethylene [11]. By increasing the adsorbed temperature of formaldehyde, the characteristic peak of polyoxymethylene [11,31,33] of the band at 1168 cm−1 became prominent showing that polymerization occurred. The absorption bands at 1593 cm−1 , 1354 cm−1 and 1495 cm−1 were assigned to COO asymmetric stretching (as (COO)) [11,32], COO symmetric stretching (s (COO)) [11] and CH bending (ıas(CH)) [10] vibrations in the formate structure. Analysis of the infrared spectra showed that formate was the most important product of the reaction on the surface of nano ␥-Fe2 O3 films. According to the above results, the activities of all the adsorbed species on nano ␥-Fe2 O3 films changed with the temperature obviously. When the temperature was raised from 250 ◦ C to 320 ◦ C, the activities of the main species reached the maximum. Nevertheless, when the temperature was raised up to 350 ◦ C, there were very weak adsorptions on the surface of nano ␥-Fe2 O3 films. We speculated that when the temperature increased, the activities of the electrons enhanced, and as a result, more electrons could jump over the potential barrier of the crystal to join the adsorption of formaldehyde. When the temperature increased continuously, the process of thermal decomposition of the main adsorbed species occurred on the surface (Eqs. (7), (9) and (10)). Formate species and polyoxymethylene were the dominated species at high temperature 320 ◦ C and 350 ◦ C. We speculated on its fast decomposed from further reaction on the nano ␥-Fe2 O3 films surfaces at high temperatures. In summary, the possible way of the reaction and decomposition process of formaldehyde on the surface of nano ␥-Fe2 O3 films can be discussed by the following equations: HCHO(g) → HCHO(ads) 2−
HCHO(ads) + O
(6) −
−
(ads) → H2 COO (ads) + e
(7)
H2 COO− (ads) + O2− (ads) → HCOO2− (ads) + OH− (ads) 2−
2HCOO
2−
(ads) + O
−
(ads) → 2CO2 + H2 O + 6e
(9) (10)
Electrons are released in the above reactions. As a result, the resistance of gas sensing materials decreases and the detection of formaldehyde gas is realized. 4. Conclusions The nano ␥-Fe2 O3 films gas sensor was fabricated by the screen printing technology. The results of gas sensing properties indicate that the nano ␥-Fe2 O3 films gas sensor could effectively monitor formaldehyde gas. The mechanism of formaldehyde sensing was proposed according to the results of DRIFTS experiments, which show that dioxymethylene, formate ions, polyoxymethylene and molecularly formaldehyde surface species are formed during the interaction of formaldehyde with nano ␥-Fe2 O3 films at different temperatures. The possible mechanism of the reaction process
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discussed as above may be helpful to the development of gas sensor for formaldehyde. Acknowledgments This work was supported by the Opening Project (MPCS-2011D-13) of State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Science, and the Opening Project (2012-KF-4) of State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology). The authors are also grateful to Analytical and Testing Center of Huazhong University of Science and Technology. References [1] X.J. Tang, Y. Bai, A. Duong, Formaldehyde in China: production, consumption, exposure levels, and health effects, Environment International 35 (2009) 1210–1224. [2] Q. Li, P. Sritharathikhum, M. Oshima, S. Motomizu, Development of novel detection reagent for simple and sensitive determination of trace amounts of formaldehyde and its application to flow injection spectrophotometric analysis, Analytica Chimica Acta 612 (2008) 165–172. [3] B.P. Andreini, R. Baroni, E. Galimberti, G. Sesana, Aldehydes in the atmospheric environment: evaluation of human exposure in the north-west area of Milan, Microchemical Journal 67 (2000) 11–19. [4] S.S.H. Ho, J.Z. Yu, Feasibility of collection and analysis of airborne carbonyls by on-sorbent derivatization and thermal desorption, Analytical Chemistry 74 (2002) 1232–1240. [5] J. Flueckiger, F.K. Ko, K.C. Cheung, Microfabricated formaldehyde gas sensors, Sensors 9 (2009) 9196–9215. [6] K. Taeyang, S. Andrew, G. Bing, Effect of synthesis condition and annealing on the sensitivity and stability of gas sensors made of Zn-doped ␥-Fe2 O3 particles, Korean Journal of Chemical Engineering 27 (2010) 1003–1009. [7] Z.H. Jing, S.H. Wu, Synthesis, characterization and gas sensing properties of undoped and Co-doped ␥-Fe2 O3 -based gas sensors, Materials Letters 60 (2006) 952–956. [8] Z.H. Jing, Synthsis, characterization and gas sensing properties of undoped and Zn-doped ␥-Fe2 O3 -based gas sensors, Materials Science and Engineering A 441 (2006) 176–180. [9] G. Alexander, R. Ralf, In situ and operando spectroscopy for assessing mechanism of gas sensing, Angewandte Chemie International Edition 46 (2007) 3826–3848. [10] S. Sun, J.J. Ding, J. Bao, C. Gao, Z.M. Qi, C.X. Li, Photocatalytic oxidation of gaseous formaldehyde on TiO2 : an in situ DRIFTS study, Catalysis Letters 137 (2010) 239–246. [11] B.Y. Xu, J. Shang, T. Zhu, X.Y. Tang, Heterogeneous reaction of formaldehyde on the surface of ␥-Al2 O3 , Atmospheric Environment 45 (2011) 3569–3575. [12] C.M. Huang, G.T. Pan, P.Y. Peng, In situ DRIFT study of photocatalytic degradation of gaseous isopropanol over BiVO4 under indoor illumination, Journal of Molecular Catalysis A: Chemical 327 (2010) 38–44. [13] S. Goran, S. Neven, W.J.C. Menno, L. Karaly, A comparative study of CO adsorption and oxidation on Au/Fe2 O3 catalysts by FT-IR and in situ DRIFTS spectroscopies, Journal of Molecular Catalysis A: Chemical 252 (2006) 163–170.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
In situ diffuse reflectance infrared Fourier transform spectroscopy study of formaldehyde adsorption and reactions on nano ␥-Fe2 O3 films Kaijin Huang a,b,c,∗ , Lingcong Kong a , Fangli Yuan b , Changsheng Xie a a b c
State Key Laboratory of Materials Processing and Die & Mould Technology, Huazhong University of Science and Technology, Wuhan 430074, PR China State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Science, Beijing 100190, PR China State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, PR China
a r t i c l e
i n f o
Article history: Received 17 June 2012 Received in revised form 21 December 2012 Accepted 7 January 2013 Available online 18 January 2013 Keywords: Gas sensors Nano ␥-Fe2 O3 films Formaldehyde DRIFTS Gas sensing mechanism
a b s t r a c t The nano ␥-Fe2 O3 films gas sensor was fabricated by the screen printing technology. The phase structures and morphologies of nano ␥-Fe2 O3 films were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FESEM), respectively. The gas sensitivity of the films to 100 ppm formaldehyde was investigated. The surface adsorption and reaction process between nano ␥-Fe2 O3 films and formaldehyde was studied by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) method at different temperatures. DRIFTS results showed that dioxymethylene, formate ions, polyoxymethylene and molecularly formaldehyde surface species were detected when the nano ␥-Fe2 O3 films exposed to 100 ppm formaldehyde at different temperatures. A possible mechanism of the reaction process was discussed. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Formaldehyde (CH2 O) is one of important chemicals and is widely used in construction, wood processing, furniture, textiles, carpeting industries. However, it is highly toxic and volatile. It has been classified as a human carcinogen that causes nasopharyngeal cancer and probably leukemia [1]. So it is very important to monitor and control its exposure in both industrial and residential environments. There are many papers covering the monitor and detection of formaldehyde. At present, the major methods are spectrophotometric analysis [2], electrochemical analysis [3] and chromatography techniques [4]. In spite of their advantages, these methods share a disadvantage that the monitor and detection are based on field sampling and laboratory analysis resulting in tedious operation and time-consuming, which cannot meet the demands of number of measuring points at the test site. However, gas sensors for formaldehyde have developed quickly because they are easy to operate, rapid to feedback and convenient to carry. To date, microfabricated formaldehyde gas sensors including silicon microhotplates for metal oxide-based detection, enzyme-based electrochemical sensors, nanowire-based sensors and polymer-based sensors for low-temperature, low-power operation have been reviewed by Flueckiger et al. [5]. Among these gas
∗ Corresponding author. Tel.: +86 27 87556544; fax: +86 27 87543776. E-mail address: [email protected] (K. Huang). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.01.038
sensors, ␥-Fe2 O3 -based gas sensor has been widely investigated by many researchers due to its high sensitivity, low cost, quick response and low power consumption [6]. To the best of our knowledge, most ␥-Fe2 O3 gas sensing mechanisms are focused on the resistance change of the sensing materials caused by bulk reactions and variations within the materials [7,8]. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique, which follows the physicochemical processes taking place in an active sensing element in real time and under operating conditions, is a very relevant tool for the characterization of the surface reactivity of gas sensing materials [9]. This technique has been applied to gas adsorption and oxidation on metallic oxide catalysts, such as formaldehyde, formic acid and methy formiate over TiO2 [10], ␥-Al2 O3 [11] and gaseous isopropanol over BiVO4 [12], on metal doping oxide catalysts, such as CO adsorption and oxidation on Au/Fe2 O3 catalysts [13], Cu/SiO2 and Co/SiO2 catalysts [14] and Pt/SiO2 [15], water-gas shift reaction on chromium-free iron oxide catalysts [16], formaldehyde catalytic oxidation over Pt/TiO2 [17] and Cu/Al2 O3 [18], NO reduction by NH3 over Fe–Ce–Mn/ZSM-5 catalysts [19], methanol oxidation by lattice oxygen over Cu/ZnO catalyst [20] and N2 O decomposition on Rh/CeO2 and Rh/␥-Al2 O3 catalysts [21], and on composite metallic oxide catalysts, such as CO adsorption and oxidation on CuCo/SiO2 catalysts [14], CO2 adsorption and desorption on MgO/Al2 O3 [22], toluene oxidation over V2 O5 /TiO2 [23] and CuO/CeO2 catalysts doped with alkaline earth metal oxides [24]. Besides, DRIFTS has also been applied to analyze different
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gas adsorption for CO selective oxidation on Cu–Zr–Ce–O catalysts [25], surface reactivity to NO2 by zinc nanoparticle aggregates and zinc hollow nanofibers [26] and the surface characterization of modified silica nanoparticles [27]. Few papers are focused on the underlying relationships between surface reactions and gas sensing properties on the ␥-Fe2 O3 films by DRIFTS. On the other hand, gas–solid adsorption is one of the most common interface adsorption phenomenon, therefore, it is of important significance to study the law and influencing factors of gas–solid interface adsorption for both industrial production and scientific research. The DRIFTS technique, taken into account, plays a significant role to study the gas–solid interface adsorption. Hence, in this paper, the adsorption and reactions of formaldehyde on nano ␥Fe2 O3 films were studied by DRIFTS to understand the ␥-Fe2 O3 gas sensing mechanism. 2. Experimental 2.1. Fabrication of -Fe2 O3 gas sensor All the reagents were of analytical grade and used as purchased. The ␥-Fe2 O3 nano-powder with 20–30 nm was mixed with organic slurry at a mass ratio of 10:9 to form a paste, which was milled at a speed of 250 rpm for 1 h. Then the milled paste was coated onto the surface of an Al2 O3 substrate on which the Au electrodes and RuO2 heaters had been pre-deposited by the screen printing technology. The area of each gas sensing film is about 1 × 1 mm2 . Heat-treatments of the Al2 O3 substrate coated with the ␥-Fe2 O3 gas sensor films were carried out in the air in a muffle furnace at 400◦ C for 2 h, with a heating rate of 5◦ C/min. Then the gas sensor array was soldered to form gas sensing devices using gold wires and a welding machine. In the end, the gas sensing device was aged at 320◦ C for 72 h in the air to improve its stability and repeatability. 2.2. Characterization of nano -Fe2 O3 films and measurement of gas sensing properties The phase structures of the nano ␥-Fe2 O3 films before and after sintered were determined by XRD with an X’ Pert Pro model X-ray diffractometer using Cu K␣ radiation ( = 0.1542 nm). The morphologies of the nano ␥-Fe2 O3 films were analyzed by fieldemission scanning electron microscopy (FESEM). The gas sensing device was tested in a glass chamber with a volume of 30 L. Gas sensitivity of the gas sensor was measured under a steady-state condition. The sensitivity to formaldehyde is defined as the ratio S = Ra /Rg , where Ra and Rg are the electrical resistance measured in the air and in formaldehyde, respectively.
to the predetermined temperature under flowing formaldehyde gas. A background spectrum was collected before each test. The spectra are smoothed and measured in the Kubelka–Munk mode because the peak intensities in these units are proportional to the concentration of the adsorbed species. 3. Results and discussion 3.1. Structural characterization The XRD patterns of nano ␥-Fe2 O3 films are shown in Fig. 1. The XRD patterns indicate that the phase of the films consists of cubic ␥-Fe2 O3 phase. No characteristic peaks of ␣-Fe2 O3 are detected. The result shows that the phase transition of ␥-Fe2 O3 → ␣-Fe2 O3 did not take place after sintering at 400 ◦ C for 2 h. FESEM images of the pure nano ␥-Fe2 O3 films are shown in Fig. 2. The films are characterized by a relatively rough surface with the presence of some pores and cracks, as presented in Fig. 2(a), due to the decomposition of organic slurry during heat-treatment of the films. In the meantime, Fig. 2(b) shows that the films consist of original spherical ␥-Fe2 O3 nano-particles with 20–30 nm. Small nano-particles are good for gas sensing properties by increasing the surface area of the sensing layer and favoring the interaction with the targeted gases. The evenly distributed pores in the films are likely to improve the surface area, which can greatly facilitate gas diffusion and mass transport in sensor materials. This plays a significant role in the surface reactions resulting in an increase in the gas sensitivity of the films. 3.2. Gas sensing properties The gas sensitivity is greatly influenced by the operating temperature. In order to research the optimum operating temperature, response of the nano ␥-Fe2 O3 gas sensor to 100 ppm formaldehyde gas at different operating temperatures was examined. Fig. 3 shows that the operating temperature has a great influence on the response. The response increases with operating temperature, reaches its maximum of 5.5 at 320 ◦ C, and begins to reduce above the temperature span. The sensor samples were fabricated for each type of the sensors, and the gas sensing properties of these samples were found to be similar, indicating a good reproducibility. The changes in response with operating temperature can be attributed to the fact that the adsorption type of oxygen molecules is chemisorption at higher temperature, and physisorption at lower
2.3. In situ DRIFTS study Infrared spectra were collected using a Bruker VERTEX 70 FTIR Spectrometer equipped with a MCT detector cooled by liquid N2 with KBr windows and a diffuse reflectance DRIFT accessory. The spectra were collected in the range of 4000–400 cm−1 at a resolution of 4 cm−1 and an accumulating of 128 scans. A vacuum chamber was connected to gas pipelines for materials treatment, whose temperature was controlled by a thermostat ranging from room temperature to 600 ◦ C. The Al2 O3 substrate printed with nano ␥-Fe2 O3 films was cut into 2.8 mm × 2.8 mm square. In this experiment, the cut sample was placed in the reactive cell. Pre-treatment process at 200 ◦ C and 5.0 × 10−3 Pa was performed for 15 min in order to make the sample reproducible and the spectra of KBr were recorded for background removal. Then a flow (30 ml/min) of 100 ppm formaldehyde was introduced into the cell at different working temperatures until the spectra stabilized. The reference spectra were taken after the sample was pretreated and heated
Fig. 1. XRD patterns of nano ␥-Fe2 O3 films sintered at 400 ◦ C for 2 h.
K. Huang et al. / Applied Surface Science 270 (2013) 405–410
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temperature, as reported by Xie et al. [28]. The stronger adsorption attraction of the chemisorptions could result in the higher concentration of adsorbed oxygen, trap more electrons from the conductance band and cause the larger change of the resistance. Hence, the response enhanced with the increasing temperature. But the reduction in response above 320 ◦ C is due to the fact that the adsorption reaction is proverbially exothermic. At higher temperature, the basic reaction (1/2O2 (g) + e− → O− (ads)) will proceed to left, which leads to the decrease of the trapped electrons from the conductance band. Thus the response decreases above 320 ◦ C. ␥-Fe2 O3 is a typical n-typed semiconductor, whose gas sensing mechanism belongs to the bulk-controlled type. The change of resistance is dependent on the species and chemisorbed oxygen on the surface. The oxygen absorbed on the surface of a ␥-Fe2 O3 sensor causes the electron depletion, consequently the resistance of the sensor increases. The process can be expressed in the following equations [29]: O2 (g) → O2 (ads) −
(1) −
O2 (ads) + e → O2 (ads) −
−
(2)
O2 (ads) + e− → 2O (ads)
(3)
O− (ads) + e− → O2− (ads)
(4)
wherein “g” and “ads” refer to gas and adsorbate, respectively. After CH2 O was introduced, it would be oxidized by these chemisorbed oxygen species (O2 − , O− , O2− ) on the surface of the test sensor. During the reaction, the electrons went back into the semiconductor, resulting in a decrease in resistance of the sensor. When the sensor was exposed in air again, the gases are desorbed as H2 O and CO2 . This reaction process may be expressed in the following equation. HCHO + 2O2− (ads) → CO2 + H2 O + 4e−
Fig. 2. FSEM images of nano ␥-Fe2 O3 films after sintering at 400 ◦ C for 2 h: (a) low magnification image; (b) high magnification image.
(5)
In addition, it is also well known that when ␥-Fe2 O3 exposed to reducing ambinent, it may convert to Fe3 O4 by a reversible redox reaction, because both the ␥-Fe2 O3 and Fe3 O4 have the same spinel crystal structure. However, the electrical resistivity of Fe3 O4 is lower than that of ␥-Fe2 O3 . Therefore, phase transition between ␥Fe2 O3 and Fe3 O4 is accompanied by a large change in the electrical resistance in the gas atmospheres. When the ␥-Fe2 O3 films exposure to formaldehyde gas, one part of formaldehyde gas molecules will react with Fe3+ in ␥-Fe2 O3 lattice, part of Fe3+ in the octahedral was reduced to Fe2+ , forming low resistance Fe3 O4 phase. Half of the Fe3+ ions are distributed at tetrahedral sites, and the other half of the Fe3+ ions together with the Fe2+ ions are distributed at octahedral sites. Since the emergence of sub-lattice location of iron ions with different valences, so the Fe2+ – Fe3+ – Fe2+ – Fe3+ . . . between the atomic electrons can hop from one defect atoms to another, so that the body resistance of ␥-Fe2 O3 films drops. The Fermi level of Fe3 O4 is a little higher than that of ␥-Fe2 O3 , electrons will migrate from the Fermi level of Fe3 O4 to that of ␥-Fe2 O3 until their Fermi levels equalize [30]. In the equilibrium state, an electron depletion layer might be generated between Fe3 O4 and ␥-Fe2 O3 , which plays an important role in the electron transfer of surface reactions. 3.3. DRIFTS study of formaldehyde adsorption on the gas sensor
Fig. 3. Response of nano ␥-Fe2 O3 to 100 ppm formaldehyde at different temperatures.
Figs. 4–7 show the dynamic changes in the DRIFTS spectra of the nano-␥-Fe2 O3 films in a flow of 100 ppm formaldehyde at 250 ◦ C, 300 ◦ C, 320 ◦ C and 350 ◦ C, respectively. The IR assignments of formaldehyde adsorption on ␥-Fe2 O3 films at different temperatures are shown in Table 1. As shown in Fig. 4, after 30 min of formaldehyde exposure at 250 ◦ C, the bands at 3743, 3626, 2931, 2863, 1413 and 1264 cm−1 were observed in the spectra, but relatively weak. With increasing absorption time, the bands intensities increased and reached a
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Fig. 4. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 250 ◦ C.
Fig. 6. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 320 ◦ C.
Fig. 5. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 300 ◦ C.
Fig. 7. In situ DRIFTS spectra of 100 ppm formaldehyde adsorption on nano ␥-Fe2 O3 film at 350 ◦ C.
steady level after 45 min. The bands at 3743 cm−1 , which is accompanied by weak shoulder at 3626 cm−1 , was due to the (OH) mode of different types of isolated hydroxyl groups [10,11,31]. As we know, if a crystal of ␥-Fe2 O3 is formed, the network of Fe3+ and O2− comes to an abrupt end at the gas/solid interface, which leads to Fe3+ and O2− species at the surface coordinatively unsaturated. Therefore, ␥-Fe2 O3 can provide a spot of hydroxyls sites,
because the surface of ␥-Fe2 O3 is hydroxylated when exposure to formaldehyde gas containing water vapor. Dissociation of water forms two distinctive hydroxyl groups: one OH− group bridges two surface vicinal Fe3+ and the other forms a terminal Fe3+ OH− group, which is the reason of the existence of two (OH) mode of different types of isolated hydroxyl groups in the DRIFTS spectra. The band at 2863 cm−1 were assigned to the C H stretching
Table 1 IR assignments of formaldehyde adsorption on nano ␥-Fe2 O3 films at different temperatures. Surface species
Wavenumber (cm−1 ) at 250 ◦ C
Wavenumber (cm−1 ) at 300 ◦ C
Wavenumber (cm−1 ) at 320 ◦ C
Wavenumber (cm−1 ) at 350 ◦ C
References
(OH) (CH) di-oxymethylene (C O) CO2 ı(OH) Poly-oxymethylene as (COO) ıas(CH) s (COO) Formaldehyde ıs(CH)
3743/3626 2863 2931
3744/3633/3475 2859 2935
3736/3447
3736/3613
2935
2937
2349/2306 1642
2349/2306
2349/2312
1150 1594 1487 1354 1413/1255
1168 1593 1495 1358 1409/1258
[10,11,31] [11,31,32] [11] [33] [10,20] [11,33] [11,31,33] [11,32] [10] [11,31–33] [10,33] [33]
1413/1264
1241/1010 1433
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vibration ((CH)) [31,32] and the bands at 1413 and 1264 cm−1 to molecularly adsorbed formaldehyde [10,33] on the nano ␥-Fe2 O3 films. The band at 2931 cm−1 , which was the most prominent in the spectra, was assigned to the characteristic peak of dioxymethylene [11]. We can infer that CH2 O was molecularly adsorbed as a whole at first. Then the adsorbed formaldehyde gas was oxided to form dioxymethylene by chemisorbed oxygen species. At the same time, Fe3+ ions were deoxidated to Fe2+ . This reaction process may be expressed in the following Eqs. (6)–(8). HCHO(g) → HCHO(ads) 2−
HCHO(ads) + O
(6) −
(ads) → H2 COO (ads) + e−
Fe3+ + e− ↔ Fe2+
(7) (8)
As shown in Fig. 5, the DRIFTS spectra on nano ␥-Fe2 O3 films at 300 ◦ C, the new bands at 2306, 1642 and 1433 cm−1 appeared, except the bands at 3744, 3633 and 3475 assigned to the OH stretching vibration [10,11,31], the band at 2859 cm−1 to the C H stretching vibration [11,31,32] and the band at 2935 cm−1 to the characteristic peak of dioxymethylene [11], which was formed from the adsorbed formaldehyde gas by chemisorbed oxygen species, as shown in Table 1. The band at 1642 cm−1 was assigned to the OH bending vibration (ı(OH)) [11,33]. The low intensity band at 1433 cm−1 observed can be associated with the deformation vibration ıs(CH) in the formate species [33]. The bands at 1241 and 1010 cm−1 were also assigned to molecularly adsorbed formaldehyde on the ␥-Fe2 O3 films [10,33], indicating that the formaldehyde gas was adsorbed on the surface at first. As shown in Table 1, the new weak bands between 2400 and 2300 cm−1 indicated the appearance of carbon dioxide [10,20]. We can infer that the formed intermediate product dioxymethylene was oxided to form formate ions with the breakage of C H band. At last, CO2 and H2 O were formed after the oxidation of formate ions. The same products are produced on TiO2 [10], on ␥-Al2 O3 [11], on Pt/TiO2 [17] and over Cu/Al2 O3 [18]. The possible way for the adsorbed carbon dioxide can be formed by the following equations: H2 COO− (ads) + O2− (ads) → HCOO2− (ads) + OH− (ads) 2HCOO2− (ads) + O2− (ads) → 2CO2 + H2 O + 6e−
(9) (10)
Fig. 6 shows the DRIFTS spectra on nano ␥-Fe2 O3 films at 320 ◦ C, in which the bands at 3736, 3447, 3067, 2935, 2349, 2305, 1594, 1487, 1413, 1354, 1255 and 1150 cm−1 were observed. The intensities of most of the adsorbed species change obviously. The two (OH) [10,11,31] modes of different types of isolated hydroxyl groups (3736 and 3447 cm−1 ), as above, were due to the dissociation of water to form two distinctive hydroxyl groups: one OH− group bridges two surface vicinal Fe3+ and the other forms a terminal Fe3+ OH− group. The band at 2935 cm−1 was assigned to the characteristic peak of dioxymethylene [11]. The sharp absorption bands at 1594 cm−1 , 1487 cm−1 and 1354 cm−1 observed at higher temperature can be associated with COO asymmetric stretching (as (COO)) [11,32], CH bending (ıas(CH)) [10] and COO symmetric stretching(s (COO)) [11,31–33] vibrations in the formate structure. In this case, the spectra in the lower frequency region, showing very intense absorptions at 1150 cm−1 , indicate that formaldehyde is polymerized to linear polyoxymethylene [11,31], which was formed in the following process: nHCHO → [CH2 − O]n The bands at 1413 and 1255 cm−1
(11)
were assigned to molecularly adsorbed formaldehyde on the ␥-Fe2 O3 film [10,33]. The peaks that were attributed to carbon dioxide appeared obviously. Therefore, most of the species were found at this temperature, which indicated that the reactions went actively with temperature increasing as Eqs. (6)–(10) went.
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After the ␥-Fe2 O3 sensor was heated to 350 ◦ C, as shown in Fig. 7, the adsorption species were similar to Fig. 6, but the bands intensities appeared weakly. Maybe the intermediate reactions were so fast that what we could get were the final products H2 O and CO2 . That was the reason why the band of carbon dioxide (2312 cm−1 ) [10,20] increased, suggesting that it contained active oxygen for the reaction. The infrared bands intensities at 3736 and 3613 cm−1 assignable to (OH) frequencies [10,11,31] declined while the others increased with progression of the reaction, indicating that the hydrogen of the surface isolated OH groups may be related to the lone pair electrons of the oxygen of formaldehyde to form hydrogen bonds and formaldehyde can be adsorbed, with consumption of the surface hydroxyls. This is in accord with the bands at 1409 and 1258 cm−1 assigned to molecularly adsorbed formaldehyde [10,33]. The band at 2937 cm−1 that increased during the reaction corresponded to the characteristic peak of dioxymethylene [11]. By increasing the adsorbed temperature of formaldehyde, the characteristic peak of polyoxymethylene [11,31,33] of the band at 1168 cm−1 became prominent showing that polymerization occurred. The absorption bands at 1593 cm−1 , 1354 cm−1 and 1495 cm−1 were assigned to COO asymmetric stretching (as (COO)) [11,32], COO symmetric stretching (s (COO)) [11] and CH bending (ıas(CH)) [10] vibrations in the formate structure. Analysis of the infrared spectra showed that formate was the most important product of the reaction on the surface of nano ␥-Fe2 O3 films. According to the above results, the activities of all the adsorbed species on nano ␥-Fe2 O3 films changed with the temperature obviously. When the temperature was raised from 250 ◦ C to 320 ◦ C, the activities of the main species reached the maximum. Nevertheless, when the temperature was raised up to 350 ◦ C, there were very weak adsorptions on the surface of nano ␥-Fe2 O3 films. We speculated that when the temperature increased, the activities of the electrons enhanced, and as a result, more electrons could jump over the potential barrier of the crystal to join the adsorption of formaldehyde. When the temperature increased continuously, the process of thermal decomposition of the main adsorbed species occurred on the surface (Eqs. (7), (9) and (10)). Formate species and polyoxymethylene were the dominated species at high temperature 320 ◦ C and 350 ◦ C. We speculated on its fast decomposed from further reaction on the nano ␥-Fe2 O3 films surfaces at high temperatures. In summary, the possible way of the reaction and decomposition process of formaldehyde on the surface of nano ␥-Fe2 O3 films can be discussed by the following equations: HCHO(g) → HCHO(ads) 2−
HCHO(ads) + O
(6) −
−
(ads) → H2 COO (ads) + e
(7)
H2 COO− (ads) + O2− (ads) → HCOO2− (ads) + OH− (ads) 2−
2HCOO
2−
(ads) + O
−
(ads) → 2CO2 + H2 O + 6e
(9) (10)
Electrons are released in the above reactions. As a result, the resistance of gas sensing materials decreases and the detection of formaldehyde gas is realized. 4. Conclusions The nano ␥-Fe2 O3 films gas sensor was fabricated by the screen printing technology. The results of gas sensing properties indicate that the nano ␥-Fe2 O3 films gas sensor could effectively monitor formaldehyde gas. The mechanism of formaldehyde sensing was proposed according to the results of DRIFTS experiments, which show that dioxymethylene, formate ions, polyoxymethylene and molecularly formaldehyde surface species are formed during the interaction of formaldehyde with nano ␥-Fe2 O3 films at different temperatures. The possible mechanism of the reaction process
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