A vailable online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 183–189 Low-temperature H 2 S sensors based on Ag-doped ␣-Fe 2 O 3 nanoparticles Yan Wang, Yanmei Wang, Jianliang Cao, Fanhong Kong, Huijuan Xia, Jun Zhang, Baolin Zhu, Shurong Wang, Shihua Wu ∗ College of Chemistry, Nankai University, Tianjin 300071, China Received 29 July 2007; received in revised form 2 November 2007; accepted 2 November 2007 Available online 20 February 2008 Abstract Ag-doped ␣-Fe 2 O 3 nanoparticles were synthesized by a chemical coprecipitation method and characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller specific surface area analysis (BET) techniques. Obtained results indicated that spherical Ag grains with size of about 5 nm are highly dispersed on the surface of ␣-Fe 2 O 3 nanoparticles. The surface area of the Ag/Fe 2 O 3 nanoparticles is several times as large as that of pure ␣-Fe 2 O 3 . The H 2 S sensing properties of these Ag/Fe 2 O 3 sensors were systematically investigated. In comparison with pure ␣-Fe 2 O 3 , all of the Ag-doped sensors showed better sensing performance in respect of response, selectivity and optimum operating temperature. The effects of Ag content, calcination and operation temperature on the sensing characteristics of the Ag/␣- Fe 2 O 3 sensors were also investigated. The sensor containing 2 wt% Ag and calcined at 400 ◦ C exhibited the maximum response to H 2 Sat160 ◦ C. A possible mechanism for the influence of Ag on the H 2 S-sensing properties of Ag/␣-Fe 2 O 3 sensors was proposed. © 2007 Published by Elsevier B.V. Keywords: Gas sensor; Ag-doped ␣-Fe 2 O 3 ;H 2 S; Low operating temperature 1. Introduction In recent years, the concern over environmental protection and increasing demands to monitor hazardous gases in industry and home has attracted extensive interests in developing gas sen- sors for various polluting and toxic gases. Due to the advantages of small size, low cost, simple operation and good reversibil- ity, the semiconductor sensors have become the most promising devices among the solid-state chemical sensors. Hence, the metal oxide gas-sensing materials are widely investigated. Many semiconductor oxides such as SnO 2 , ZnO, Fe 2 O 3 ,In 2 O 3 ,WO 3 , and CuO, have been explored to detect the polluting, toxic and inflammable gases, such as CO, CO 2 ,NO X ,H 2 S, and ethanol [1–7]. Hematite (␣-Fe 2 O 3 ), the most stable iron oxide, is of sci- entific and technological importance as catalysts, pigments, ion exchangers, magnetic materials andlithium-ionbatteries [8–11]. ∗ Corresponding author. Tel.: +86 22 23505896; fax: +86 22 23502458. E-mail address: wushh@nankai.edu.cn (S. Wu). Recently, ␣-Fe 2 O 3 has been proved to be an important solid- state gas sensor. However, the low sensitivity, poor selectivity and high operating temperature discourage its extensive appli- cation. In order to fit the increasing demands of sensors in more complicated systems and strict conditions, many attempts have been made to improve the sensing properties of ␣-Fe 2 O 3 . Based on the sensitization effects of noble metals on metal oxides through chemical and/or electronic interactions, modi- fying the base materials with noble metals is an efficient way to the base materials to promote their response towards various gases [12–16]. Up to now, ␣-Fe 2 O 3 -based sensors have been investigated for the detection of some organic gases such as ethanol, ace- tone, methanol, and LPG [17–21]. However, there are few reports on the study of the gas sensing properties of ␣-Fe 2 O 3 to H 2 S. In our previous works, the effects of Au, Pd and Pt on the sensing properties of ␣-Fe 2 O 3 sensors to H 2 S have been investigated [22–24]. The gas sensing properties of ␣-Fe 2 O 3 were markedly promoted by doping with such noble metals. However, the high cost discourages their extensive applica- tion. Hence, it is important to search an alternative component 0925-4005/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.snb.2007.11.002 184 Y. Wang et al. / Sensors and Actuators B 131 (2008) 183–189 to replace Au, Pd and Pt. Silver is probably a suitable one. In this paper, we reported for the first time the preparation of Ag/␣-Fe 2 O 3 sensors by a coprecipitation method. The structural properties of the prepared Ag/␣-Fe 2 O 3 nanoparticles were char- acterized by means of XRD, TG-DTA, TEM, HRTEM, BET and XPS. The effects of Ag content, calcination and operating tem- perature on the gas sensing properties to H 2 S were investigated in detail. The sensing mechanism of the Ag/␣-Fe 2 O 3 sensors to H 2 S was also discussed. 2. Experimental 2.1. Preparation of Ag/α-Fe 2 O 3 All the reagents are ofanalytical grade andused as purchased. Ag/␣-Fe 2 O 3 nanoparticles were prepared by a coprecipita- tion method. A small quantity of polyglycol was added to an aqueous solution of AgNO 3 (0.25, 0.5, 1.0, 1.5, 2.0, 3.0 and 5.0 wt%) and Fe(NO 3 ) 3 ·9H 2 O. The aqueous mixture was then added dropwise to an aqueous solution of Na 2 CO 3 under vig- orous stirring at 80 ◦ C. The pH of the solution was adjusted by a diluted Na 2 CO 3 aqueous solution in the reaction process. After stirring for 1 h, a solid precipitate Ag/FeOOH was formed. After digesting the precipitate overnight at room temperature, it was washed with deionized water, dried at 80 ◦ C and calcined at 400 ◦ C for 1 h. Finally, a series of Ag-doped ␣-Fe 2 O 3 powders with 0.25, 0.5, 1.0, 1.5, 2.0, 3.0 and 5.0 wt% Ag were obtained. 2.2. Characterization of Ag/α-Fe 2 O 3 Thermal analyses of the Ag/␣-Fe 2 O 3 powders were carried out on a ZRY-2P thermal analyzer. Ten milligrams of samples were heated from room temperature to 600 ◦ C in air at a heating rate of 20 ◦ C min −1 . X-ray diffraction (XRD) analyses were per- formed on a D/MAX-RAX diffractometer with Cu K␣ radiation (λ = 0.15418 nm) operating at 40 kV and 100 mA. Diffraction peaks of crystalline phases were compared with those of stan- dard compounds reported in the JCPDS Data File. Transmission electron microscopy (TEM) was carried out on a Philips-T20ST electron microscope, operating at 200 kV. X-ray photoelec- tron spectroscopy (XPS) measurements were performed with a Perkin-Elmer PHI 5600 spectrophotometer with Mg K␣ radi- ation. The operating conditions were kept constant at 187.85 eV and 250.0 W. In order to subtract the surface charging effect, the C 1s peak has been fixed at a binding energy of 284.6 eV. The specific surface areas (S BET ) of the samples were calculated following the multi-point Brunauer–Emmett–Teller (BET) pro- cedure on a Quantachrome NOVA 2000e sorption analyzer at liquid nitrogen temperature. 2.3. Fabrication and analysis of gas sensors The gas sensing behavior was investigated by using a com- mercial gas sensing measurement system of HW-30A from Henan Hanwei Electronical Technology Co., Ltd. An alumina substrate tube of 4 mm in length was used for the heater and sensing base. A small Ni–Cr alloy coil was placed through the tube to supply the operating temperatures from 100 to 500 ◦ C. Two platinum wires attaching to each gold electrode were used as electrical contacts. The schematic diagrams of the typical gas sensor and the measuring principle were shown in our previ- ous publications [23,24]. The Ag/␣-Fe 2 O 3 powders were mixed with terpineol to form apaste. The pastewas then coated onto the outside surface of an alumina tube. In order to improve their sta- bility and repeatability, the gas sensors were sintered at 300 ◦ C for 10 days in air. The sensing properties of the sensors were tested in a chamber with a volume of 0.015 m 3 . The test gases were injected into the closed chamber by a microinjector. The sensitivity of the gas sensors was measured under a steady-state condition. The gas response S is defined as the ratioR a /R g , where R a and R g are the resistances measured in air and in a test gas, respectively. 3. Results and discussion 3.1. Results of characterization Fig. 1 shows the TG-DTA curves of as-prepared Ag/FeOOH powders. A dramatic weight loss occurs around 100 ◦ C, accom- panied with an endothermic peak on the DTA curve, which may be due to the dehydration process of physically adsorbed water and the release of attached nitrates in the products. Inthe range of 200–400 ◦ C, another weight loss is observed, corresponding to an obvious exothermic peak around 300 ◦ C on the DTA curve. It can be attributed to the reaction 2FeOOH → Fe 2 O 3 +H 2 O. There is no obvious weight loss peak above 400 ◦ C on the TG curve, indicating that the hydroxide products have already been well crystallized. According to the TG-DTA results, we can safely conclude that perfect Ag/Fe 2 O 3 crystals can be obtained after calcined at 400 ◦ C under air atmosphere. The typical XRD patterns of Ag/Fe 2 O 3 nanoparticles cal- cined at 400 ◦ C with different Ag content are shown in Fig. 2. The diffraction patterns of the samples match perfectly with the stan- dard ␣-Fe 2 O 3 reflections (JCPDS No. 33-664). The sharp peaks indicate thatthe crystals of␣-Fe 2 O 3 are perfect,which is ingood agreement with the TG-DTA analysis results. The average size Fig. 1. TG-DTA curves of as-prepared Ag/FeOOH. Y. Wang et al. / Sensors and Actuators B 131 (2008) 183–189 185 Fig. 2. XRD patterns of Ag/␣-Fe 2 O 3 with different Ag content calcined at 400 ◦ C. of ␣-Fe 2 O 3 particles, calculated by the Deby–Scherrer formula, is 20 nm. When Ag content is low, no obvious Ag peaks pre- sented, which may be due to the high dispersion of Ag particles on thesurface of ␣-Fe 2 O 3 . Theparticles of Ag are too small to be identified by the conventional X-ray diffraction method. When Ag content increased to 5 wt%, the diffraction peaks attributed to Ag crystal phases appeared at 38.1 ◦ and 44.3 ◦ , which indi- cates that Ag particles become bigger with the increase of Ag content. TEM and HRTEM images of 2 wt% Ag/␣-Fe 2 O 3 calcined at 400 ◦ C are shown in Fig. 3. It can be seen that the morphol- ogy of ␣-Fe 2 O 3 particles is almost spherical. The diameter of the particle is about 30–50 nm, which is larger than the crys- talline size obtained by XRD. That indicates the presence of polycrystallites because of the high temperature treatment. As is shown in Fig. 3b, there are a few small spherical grains on the ␣-Fe 2 O 3 surface with the size about 5 nm, which may be the Ag nanoparticles. X-ray photoelectron spectroscopy was performed to illu- minate the surface composition of the studied Ag/␣-Fe 2 O 3 nanoparticles. The spectra of Fe 2p and Ag 3d of 2 wt% Ag/␣- Fe 2 O 3 calcined at 400 ◦ C are shown in Fig. 4.InFig. 4a, the Table 1 The specific surface areas (S BET )of␣-Fe 2 O 3 and Ag/␣-Fe 2 O 3 Sample S BET (m 2 /g) Undoped 18.31 Ag/Fe 2 O 3 with different Ag content (wt%) 0.25% 44.72 0.5% 57.21 1.0% 61.19 1.5% 62.62 2.0% 69.60 3.0% 57.75 5.0% 40.62 bending energies ofFe 2p 3/2 and Fe 2p 1/2 are 710.7 and 724.3 eV, respectively, which is in well agreement with the literature val- ues of Fe 3+ in ␣-Fe 2 O 3 [25].InFig. 4b, the peaks of Ag 3d 5/2 and Ag 3d 3/2 are centered at 368.0 and 374.0 eV, respectively, which indicates that the state of Ag in the sample is metallic [26]. Surface elemental analysis reveals that the atomic ratio of Ag/Fe is 1/30. This value is higher than the theoretical one,which indicates that the Ag dopant is well dispersed on the surface of ␣-Fe 2 O 3 . The specific surface areas (S BET ) of the undoped and Ag- doped ␣-Fe 2 O 3 with different Ag content were measured by N 2 -sorption analysis. Asshown in Table 1, theundoped ␣-Fe 2 O 3 has a small surface area (18.31 m 2 /g). After Ag is loaded, the sur- face area of ␣-Fe 2 O 3 can be promoted remarkably, and reaches the maximum (69.6 m 2 /g) when the content of Ag is 2 wt%. When the Ag content is larger than 2 wt%, the surface areas become to decrease unexpectedly. This should be attributed to the aggregation of Ag particles, resulting in the decrease of the surface area of ␣-Fe 2 O 3 . Generally, a large BET surface area will lead to improvement of the sensing properties of the sensor. 3.2. Gas sensing properties It is well known that the gas sensitivity is greatly influenced by the operating temperature and the amounts of additives. In order to determine the optimum operating temperature and addi- tive amount, the responses of Ag/␣-Fe 2 O 3 sensors calcined at 400 ◦ C with different Ag content to 100 ppm H 2 S gas were mea- Fig. 3. TEM (a) and HRTEM (b) images of 2 wt% Ag/␣-Fe 2 O 3 . 186 Y. Wang et al. / Sensors and Actuators B 131 (2008) 183–189 Fig. 4. XPS of 2 wt% Ag/␣-Fe 2 O 3 (a) Fe 2p (b) Ag 3d. sured at different operating temperatures. The response of an undoped ␣-Fe 2 O 3 sensor to H 2 S was also measured for compar- ison. The results are shown in Fig. 5. It can be seen obviously from Fig. 5 that the undoped sensor has a poor response to H 2 S, while the doped sensors with different amounts of Ag all exhibit much higher responses than the undoped one. Among all the Ag-doped ␣-Fe 2 O 3 sensors, the one with 2 wt% Ag shows the largest response to H 2 S. Whilst, the gas sensing property change trend is similar to the specific surface area change trend of the samples. Additionally, the responses of the sensors to H 2 S are also affected by the operating temperature. Each curve reveals a maximum response at an optimum operating temper- ature. The undoped sensor has the maximum gas response at 200 ◦ C, whereas all the doped sensors have the maximum gas response at about 160 ◦ C. The operating temperature of all the Ag-doped sensors is lower than that of the reported H 2 S sen- sors [27–30]. The lower operating temperature could lead to lower energy consumption, which is one of current pursuits in solid-state gas sensors. Based on the above results, we can see that the Ag/␣-Fe 2 O 3 sensors exhibit much better response and lower operating temperature than pure ␣-Fe 2 O 3 , and the opti- mum performance is obtained at 160 ◦ C for the sensor of 2 wt% Fig. 5. Responses of undoped ␣-Fe 2 O 3 and Ag/␣-Fe 2 O 3 with different Ag content to 100 ppm H 2 S. Ag/␣-Fe 2 O 3 . Therefore,all further experiments were carried out using this particular composition (2 wt%) and operating temper- ature (160 ◦ C) to explore the effect of other factors on the sensing performance of Ag/␣-Fe 2 O 3 to H 2 S. Compared with our previ- ously reported Au, Pt and Pd-doped ␣-Fe 2 O 3 sensors [22–24], the gas sensitivity of our present prepared Ag/␣-Fe 2 O 3 sensor is relatively lower than that of Pt-doped one, but a little higher than that of Au- and Pd-doped ␣-Fe 2 O 3 . Since Ag is cheaper than Au, Pt and Pd, the present Ag/␣-Fe 2 O 3 sensor system is worthy to further investigation. As generally mentioned earlier, the gas-sensing mechanism of ␣-Fe 2 O 3 -based sensors belongs to the surface-controlled type, which is based on the change in conductance of the semiconductor. The oxygen adsorbed on the surface directly influences the conductance of the ␣-Fe 2 O 3 -based sensors. The amount of oxygen adsorbed on sensor surface depends on the operating temperature, particle size, and specific surface area of the sensor [31]. The state of oxygen on the surface of Ag/␣- Fe 2 O 3 sensor undergoes the following reaction [15], O 2 (gas) → O 2 (ads) (1) O 2 (ads) + e − → O 2 − (ads) (2) O 2 − (ads) + e − → 2O − (ads) (3) O − (ads) + e − → O 2− (ads) (4) The oxygen species capture electrons from the material, which results in the concentration changes of holes or electrons in the Ag/␣-Fe 2 O 3 semiconductor. When the sensor is exposed to H 2 S, the reductive gas reacts with the oxygen adsorbed on the sensor surface. Then the electrons are released back into the semiconductor, resulting in the change in electrical conductance of the Ag/␣-Fe 2 O 3 sensor. It can be expressed in the following reaction, H 2 S + 3O 2− → H 2 O + SO 2 + 6e − (5) For the Ag/␣-Fe 2 O 3 sensors, the low response at low operating temperature can be attributed to the low thermal energy of the gas molecules, which is not enough to react with the surface adsorbed oxygen species. As a result, the reaction rate between them is essentially low [31,32] and low response is observed. Y. Wang et al. / Sensors and Actuators B 131 (2008) 183–189 187 On the other hand, the reduction in response after the optimum operating temperature may be due to the difficulty in exother- mic gas adsorption at higher temperature [33]. Therefore, the maximum response can just be observed at the right operating temperature. The enhanced response of Ag/␣-Fe 2 O 3 sensors can be attributed to two factors. First, the higher specific surface areas of the Ag-doped sensors can lead to an increase in active sur- face for gas sensing. Secondly, the Ag dopants, as a catalyst, enhance the adsorption of gas molecules and accelerate the elec- tron exchange between the sensor and the test gas [34]. The two factors together contribute to the improvement of gas sensing properties of the Ag-doped ␣-Fe 2 O 3 sensors. Furthermore, the maximum response is observed for 2 wt% Ag/␣-Fe 2 O 3 , perhaps due to the largest amount of active reaction sites formed by the random dispersion of Ag on the Fe 2 O 3 surface in this compo- sition, as revealed by above BET analysis. The abrupt decrease of response for the sensors doped with more than 2 wt% Ag is probably due to the reduction of active sites correlated with the agglomeration of Ag grains [35]. In this paper, the effect of calcination temperature on the response of the as-prepared Ag/␣-Fe 2 O 3 sensors to H 2 S is also investigated. Fig. 6 shows the variation of the responses of a series of 2 wt% Ag/␣-Fe 2 O 3 sensors calcined at different tem- peratures to 100 ppm H 2 S. It can be seen that the calcination temperature has an obvious influence on the sensor response to H 2 S gas. The obtained results show that the response of the sensor to H 2 S is enhanced with the increase of calcination temperature from 200 to 400 ◦ C, and the Ag/␣-Fe 2 O 3 sensor calcined at 400 ◦ C has the highest response, being in good agree- ment with the TG-DTA results that 400 ◦ C is the temperature at which the perfect ␣-Fe 2 O 3 nanocrystallines can be obtained. But, the sample is amorphous FeOOH when calcined below 400 ◦ C, and the gas sensing property of amorphous FeOOH is far worse than that of the crystalline ␣-Fe 2 O 3 . The responses of the sensor decrease gradually when the calcination tempera- ture is higher than 400 ◦ C, which may be due to an increase in Fig. 6. Responses to 100 ppm H 2 S of 2 wt% Ag-doped ␣-Fe 2 O 3 calcined at different temperatures. Fig. 7. Responses of 2 wt% Ag/␣-Fe 2 O 3 to various gases at different operating temperatures. the particle size. As a result, the 400 ◦ C-calcined sensor which possesses a high surface area exhibits the highest response to H 2 S. Selectivity is another important parameter of a gas sensor. The sensor must have rather high selectivity for its application. Because ␣-Fe 2 O 3 -based sensors also respond to other gases, such as ethanol and acetone, the responses of our 2 wt% Ag/␣- Fe 2 O 3 sensor to other seven gases of 1000 ppm at different operating temperatures are also examined and the results are shown in Fig. 7. From Fig. 7, it can be seen clearly that the sensor exhibits the largest response to 100 ppm H 2 S, moderate responses to ethanol and acetone, and negligible responses to n-hexane, NH 3 ,H 2 and CO even at such a high concentration (1000 ppm). On the other hand, the optimum operating temper- ature to ethanol and acetone is 200 ◦ C, which is higher than that to H 2 S. The selectivity to H 2 S is good enough to detect H 2 S, especially when the operating temperature is in the range of 120–160 ◦ C. According to the experiment results, it is no prob- lem to detect low concentration H 2 S at relatively low operating temperature. Furthermore, if there is no H 2 S in the atmosphere, the sensor can be used to monitor ethanol or acetone vapor at 200 ◦ C. Response and recovery times are the basic parameters of gas sensors, which are defined as the time required to reach 90% of the final resistance. Rapid response and recovery to a target gas are demanded for practical application. Fig. 8 shows the typi- cal response-recovery characteristics of the 2 wt% Ag/␣-Fe 2 O 3 sensor to different H 2 S concentrations (from 50 to 500 ppm). Corresponding response values of the sensor to these H 2 S con- centrations are shown in the inset picture. As can be seen from Fig. 8, the response of the 2 wt% Ag/␣-Fe 2 O 3 sensor to H 2 S sharply increases with an increase in gas concentration. The response and recovery times to 50, 100, 200 and 500 ppm H 2 S are 68 and 35 s, 42 and 26 s,31 and 25 s, 25 and 21 s, respectively. These times are short enough for practical application. The reversibility of the Ag/␣-Fe 2 O 3 sensors was also inves- tigated. The sensors exhibited excellent responses to H 2 Seven 188 Y. Wang et al. / Sensors and Actuators B 131 (2008) 183–189 Fig. 8. Response-recovery characteristics of 2 wt% Ag-doped ␣-Fe 2 O 3 to H 2 S of different concentrations. after several months. This indicates that the sensors possess excellent stability and reversibility. The Ag/␣-Fe 2 O 3 sensors should have a promising application to detect H 2 S and is worthy of further investigation. 4. Conclusions In summary, the Ag/␣-Fe 2 O 3 nanoparticles were successfully synthesized by a convenient chemical coprecipitation method, and Ag/Fe 2 O 3 sensors were made and tested. The XPS analysis indicated that Ag existed in metallic form and highly dispersed on the surface of sensors. The specific surface area (S BET )of ␣-Fe 2 O 3 nanopowders was remarkably promoted by the Ag- doping. Compared with pure ␣-Fe 2 O 3 , the Ag-doped sensors showed much higher response, better selectivity and ratherlower optimum operating temperature to H 2 S. Especially, the opti- mum performance was obtained at an operating temperature of 160 ◦ C for the 2 wt% Ag/␣-Fe 2 O 3 sensor calcined at 400 ◦ C. The sensor could be used to detect H 2 S at the temperature range of 120–160 ◦ C and to detect ethanol and acetone at 200 ◦ C. The doped sensor also presented long-term stability and rela- tively short response/recovery times. 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Actuators B 84 (2002) 258–264. [34] T. Maosong, G.R. Dai, D.S. Gao, Surface modification of oxide thin- film and its gas-sensing properties, Appl. Surf. Sci. 171 (2001) 226– 230. [35] G. Xu, Y.W.Zhang, X. Sun, C.L. Xu,C.H. Yan, Synthesis, structure, texture, and CO sensing behavior of nanocrystalline tin oxide doped with scandia, J. Phys. Chem. B 109 (2005) 3269–3278. Biographies Yan Wang received her M.S. degree in chemistry from Nankai University in 2006. Now she is a Ph.D. candidate in the College of Chemistry, Nankai Univer- sity in China. Her research focuses on the synthesis, characterization of metal oxide nanomaterials and their gas-sensing properties. Yanmei Wang received her B.S. degree in chemistry from Shenyang Institute of Gold Technology in 1995. Now she works at Nankai University and is also a M.S. candidate. Her research is focused on the development and application of catalysis and gas-sensitive materials. Jianliang Cao received his M.S. degree in chemistry from Nankai University in 2006. Now he is a Ph.D. candidate in the College of Chemistry, Nankai University. His research interests include the synthesis of nanomaterials and their application in catalysis. Fanhong Kong received her B.S. degree in chemistry from Qufu Normal Uni- versity in 2005. Currently, she is a master student in the College of Chemistry in Nankai University. Her research is focused on the development and application of gas-sensitive materials. Huijuan Xia received her B.S. degree in chemistry from Liaocheng Normal University in 2006. Now she is a master student in the College of Chemistry, Nankai University. Her interest is devoted to the preparation and application of gas-sensitive materials. Jun Zhang received his B.S. degree in chemistry from Qufu Normal University in 2006. Now he is a master student in the College of Chemistry in Nankai University. His research interest focuses on nanomaterials gas sensors. Baolin Zhu received her Ph.D. degree in chemistry from Nankai University in 2006. Now she is a faculty in the College of Chemistry, Nankai University. Her research is focused on the preparation of nanomaterials. Shurong Wang received her Ph.D. degree in chemistry from Nankai University in 2007. Now she is a faculty in the College of Chemistry, Nankai University. Her research covers nanomaterials, catalysis and gas sensors. Shihua Wu received his degree in chemistry from Nankai University in 1970. At present, he is a professor in the College of Chemistry, Nankai University, where he has been working for many years in the field of prepa- ration, characterization and catalytic and gas-sensing properties of metal oxides nanomaterials. . A vailable online at www.sciencedirect.com Sensors and Actuators B 131 (2008) 183–189 Low-temperature H 2 S sensors based on Ag-doped ␣-Fe 2 O 3 nanoparticles Yan. investigation. As generally mentioned earlier, the gas-sensing mechanism of ␣-Fe 2 O 3 -based sensors belongs to the surface-controlled type, which is based on