Journal of Science: Advanced Materials and Devices (2016) 45e50 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original article Synthesis and gas-sensing characteristics of a-Fe2O3 hollow balls Chu Manh Hung, Nguyen Duc Hoa**, Nguyen Van Duy, Nguyen Van Toan, Dang Thi Thanh Le, Nguyen Van Hieu* International Training Institute for Materials Science, Hanoi University of Science and Technology, No Dai Co Viet Street, Hanoi, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Received March 2016 Accepted 21 March 2016 Available online 11 April 2016 The synthesis of porous metal-oxide semiconductors for gas-sensing application is attracting increased interest In this study, a-Fe2O3 hollow balls were synthesized using an inexpensive, scalable, and template-free hydrothermal method The gas-sensing characteristics of the semiconductors were systematically investigated Material characterization by XRD, SEM, HRTEM, and EDS reveals that singlephase a-Fe2O3 hollow balls with an average diameter of 1.5 mm were obtained The hollow balls were formed by self assembly of a-Fe2O3 nanoparticles with an average diameter of 100 nm The hollow structure and nanopores between the nanoparticles resulted in the significantly high response of the aFe2O3 hollow balls to ethanol at working temperatures ranging from 250 C to 450 C The sensor also showed good selectivity over other gases, such as CO and NH3 promising significant application © 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: a-Fe2O3 hollow balls Hydrothermal Gas sensors Introduction Chemical and gas sensors are attracting increased interest worldwide because of the growing demand for monitoring gaseous molecules in various applications [1,2] Various wide-bandgap metal oxide semiconductors, such as SnO2, TiO2, ZnO, In2O3, Fe2O3, and WO3, have been synthesized for gas-sensing applications [3] The synthesis of earth-abundant metal oxides, such as Fe2O3, with a three-dimensional configuration and a porous structure for advanced applications has been the topic of interest in recent years [4e6] a-Fe2O3 is a nontoxic, stable, and earthabundant transition metal oxide [7,8] This compound has been used as a sensing material for the detection of various gases [9], such as CO [10], xylene [11], and acetone [12], among others [13] A hollow spherical structure has been reported to show significantly faster response and recovery times, as well as higher response to analytic gases, compared with other structures Thus, recent studies have focused on the synthesis of this material for sensing * Corresponding author International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road, Hanoi, Viet Nam Tel.: ỵ84 38680787; fax: ỵ84 38692963 ** Corresponding author International Training Institute for Materials Science (ITIMS), Hanoi University of Science and Technology (HUST), No.1, Dai Co Viet Road, Hanoi, Viet Nam Tel.: ỵ84 38680787; fax: ỵ84 38692963 E-mail addresses: ndhoa@itims.edu.vn (N.D Hoa), hieu@itims.edu.vn (N Van Hieu) Peer review under responsibility of Vietnam National University, Hanoi applications [14,15] Hollow balls are typically fabricated by a template-assisted method, in which the scarified template is prepared first, then the desired materials are coated, and finally the template is removed [16] For instance, hollow sphere Fe2O3 composed of ultrathin nanosheets were prepared by templateassisted method, in which monodispersed Cu2O spheres were used as scarified template to synthesize FeOOH, which was subsequently converted into a-Fe2O3 nanospheres [17] Wang et al [15] prepared Fe2O3 hollow spheres using ZnS-cyclohexylamine as a template-assisted agent However, the use of template in the synthesis of hollow balls has some limitations, such as multiple-step processes and contamination by foreign elements [17] In this study, we synthesized a-Fe2O3 hollow balls using a facile, inexpensive, and scalable hydrothermal method using glucose and ferric chloride hexahydrate as precursors for gas-sensing applications The a-Fe2O3 hollow balls were formed by the aggregation of single-crystal a-Fe2O3 nanoparticles with an average diameter of 100 nm The interspace between aggregated nanoparticles facilitates the entry of the gas molecules into the hollow balls and adsorption on the total surface of the a-Fe2O3 nanoparticles, thus enhancing the sensing performance Experimental Large-scale a-Fe2O3 hollow balls were synthesized using a facile and template-free hydrothermal method with glucose and ferric chloride as precursors In a typical synthesis, 2.7 g of ferric chloride hexahydrate (99%, SigmaeAldrich) and 3.7 g of glucose (99.5%, http://dx.doi.org/10.1016/j.jsamd.2016.03.003 2468-2179/© 2016 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/) 46 C.M Hung et al / Journal of Science: Advanced Materials and Devices (2016) 45e50 SigmaeAldrich) were dissolved in 50 ml of deionized water at room temperature to obtain a clear solution Then, ammonium hydroxide (25%) was added dropwise to adjust the pH level to pH to obtain a milky solution The milky solution was then poured into a Teflonlined autoclave for hydrothermal treatment at 180 C for 24 h before cooling to room temperature naturally The precipitates were washed several times with deionized water and ethanol and then collected by centrifugation at 4000 rpm Finally, the collected products were air dried at 60 C for 24 h and then calcined at 600 C for h prior to use in sensor fabrication and characterizations The morphology and crystal structure of the synthesized materials were characterized using field-emission scanning electron microscopy (FESEM, JSM, 7600F), transmission electron microscopy (TEM, Tecnai, G20, 200 kV, FEI), and X-ray powder diffraction (XRD, Bruker D8 Advance) [18] The gas sensor was fabricated by dispersing the obtained powders in dimethyl formamide solution and then coating the mixture onto a pair of comb-type Pt electrode deposited on thermally oxidized silicon substrate The gas-sensing characteristics were measured by a flow-through technique with a standard flow rate of 400 sccm for both dry air and balanced gas using a homemade sensing system Details of the sensing system can be found in our recent publication [19] Gas-sensing characteristics were tested using ethanol, CO, and NH3 at temperatures ranging from 250 C to 450 C The sensor response S was defined as S ¼ Rair/Rgas for reducing gases, where Rgas and Rair are the sensor resistances in the presence of test gas and dry air, respectively Results and discussion 3.1 Material characterization The morphology of the synthesized materials was characterized by FESEM [Fig 1] The as-hydrothermal products have a spherical shape with an average diameter of approximately 1.5 mm [Fig 1(A) and (B)] The glucose may have decomposed and grown into carbon spheres under the hydrothermal treatment [20] The ferric particles were then aggregated on the surface of carbon spheres to form the coreeshell materials [21] The carbon cores were burned out after calcination at 600 C, forming the a-Fe2O3 hollow spheres [Fig 1(C)e(F)] The a-Fe2O3 hollow balls were formed from the aggregated nanoparticles with an average diameter of 100 nm The shell of the hollow balls is not a dense material, but porous as a result of the nanoparticle aggregation The shell thickness of the hollow sphere from the broken area is estimated to be approximately one layer of nanoparticles [Fig 1(E)] TEM images and elemental analytical results by EDS of the aFe2O3 hollow balls are shown in Fig The hollow structure of the a-Fe2O3 balls is clearly shown in Fig 2(A), in which the central part is brighter than the surrounding region The HRTEM image of the sample demonstrates the high crystallinity of the a-Fe2O3 phase where the gap between two adjunction fringes is approximately 0.25 nm, corresponding to the interspace of (110) planes [22] The inter-grain boundary between nanoparticles can also be seen in the HRTEM image Selective area electron diffraction of the selected Fig Scanning electron micrographs of (A, B) as-fabricated and (CeF) calcined a-Fe2O3 hollow balls C.M Hung et al / Journal of Science: Advanced Materials and Devices (2016) 45e50 47 Fig (A, B) Transmission electron micrographs and (C) EDS results of a-Fe2O3 hallow balls; inset of (B) is the corresponding FFT area marked by white square in the HRTEM image exhibits diffraction spots revealing the single crystallinity of a-Fe2O3 The EDS analytical results of the sample shown in Fig 2(C) demonstrate the peaks of C, O, Fe, and Cu Elements C and Cu came from the carbon-coated Cu grid used for TEM characterization, whereas O and Fe were from the sample The ratio [O]/[Fe] ¼ 1.64 is higher than the composition of stoichiometric Fe2O3, possibly because of contamination of some OH groups on the surface of sample Crystal structures of the as-hydrothermal and calcined materials characterized by XRD are shown in Fig 3(A) and (B), respectively The XRD patterns of the as-hydrothermal product shown in Fig 3(A) illustrate unresolved peaks The metastable phase, such as Fe(OH)2 or Fe(OOH), may have been formed after the hydrothermal process [6] The metastable phase was then converted to Fe2O3 by thermal oxidation at high temperature The XRD pattern of the calcined sample [Fig 3(B)] demonstrates that the materials have a rhombohedral crystal structure, with the main peaks indexed to the standard profile of a-Fe2O3 phase (JCPDS No 86e0550) [22] No detectable peaks of FeOOH or Fe3O4 impurities and other phases were observed, indicating the formation of single-phase a-Fe2O3 No template was used in the fabrication of hollow balls, thus the products were not contaminated by any foreign element [15] 3.2 Gas-sensing characteristics Gas-sensing characteristics of the synthesized a-Fe2O3 hollow balls were tested using ethanol at different temperatures ranging from 250 C to 450 C [Fig 4] Fig 4(A) shows that the initial resistance of the a-Fe2O3 hollow ball sensor measured in air at 250 C, 300 C, 350 C, 400 C, and 450 C were approximately 85, 58, 43, 31, and 18 k U, respectively The decrease in the initial resistance of a-Fe2O3 sensor with increasing operating temperature reveals the semiconducting nature of metal oxide, that is, the thermal energy excites electrons from valence band to conduction Fig X-ray diffraction patterns of the (A) as-hydrothermal and (B) calcined a-Fe2O3 hollow balls band to contribute to the conductivity of the material [23] The aFe2O3 hollow balls showed n-type semiconducting characteristics at all measured temperatures The sensor resistance decreased significantly upon exposure to reducing gases (ethanol, NH3, and 48 C.M Hung et al / Journal of Science: Advanced Materials and Devices (2016) 45e50 Fig Ethanol-sensing characteristics of a-Fe2O3 hollow balls: (A) transient resistance versus time of sensor upon exposure to different concentrations of ethanol at various temperatures; (B) sensor response as functions of ethanol concentrations, (C) response and recovery times; (D) short-term stability of sensors Fig (A) CO and NH3 (B) sensing characteristics of a-Fe2O3 hollow balls CO) [6] The semiconducting characteristics of metal oxide are determined by the deficiency or excess of material composition The excess or deficiency of oxygen in the crystal structure of aFe2O3 generally leads to p-type or n-type semiconducting characteristics [24,25] Long et al [26] demonstrated that the polyhedral a-Fe2O3 particles showed p-type gas-sensing characteristics, in which the sensor resistance increased upon exposure to reducing gases, such as H2, CO, and C2H5OH They reported that the p-type characteristic of materials was due to the incorporation of Na into a-Fe2O3 oxide Heat treatment temperature significantly influences the electrical properties of a-Fe2O3, such that high-temperature treatment can result in p-type characteristics [25] In this study, the synthesized a-Fe2O3 hollow balls were heat-treated at a relatively low temperature of approximately 600 C for h, so the balls exhibited n-type characteristics This result is consistent with other reports, where the n-type nature of metal oxide semiconductor was attributed to the presence of oxygen vacancies [12,27] The effect of temperature heat treatment on the ethanol-sensing characteristics of a-Fe2O3 hollow balls was determined by annealing the sample at 800 C for h However, high-temperature heat treatment led to the distortion of sensor response [Fig S1, Supplementary] Sensor response as a function of ethanol concentration measured at different temperatures is shown in Fig 4B The sensor response increases with increasing working temperature from 250 C and reaches a maximum value at 400 C Further increase in the working temperature results in a slight decrease in the sensor response At 400 C, the sensor response also increases from 1.77 to 4.29 with increasing ethanol concentration from 50 ppm to 500 ppm Fast response and recovery times of the sensor are also important in real-time measurements of the device [18] The 90% response and recovery times of the sensor at different C.M Hung et al / Journal of Science: Advanced Materials and Devices (2016) 45e50 temperatures were calculated [Fig 4(C)] The response and recovery times to 50 ppm ethanol were approximately 16/30, 7/20, 4/15, and 3/12 s at working temperatures of 250 C, 300 C, 350 C, 400 C, and 450 C, respectively The response and recovery times decrease with increasing working temperature because of the acceleration of thermal energy for the adsorption and desorption processes [18] The fast response recovery times of less than is sufficient for practical application [27] The transient stability of the fabricated sensor was also tested at 450 C for several cycles switching from air to analytic gas and back to air [Fig 4(D)] A slight deviation from the baseline resistance is observed after several cycles possibly due to the poor adhesion of sensing layer and substrate The experiment was repeated for a day, and negligible distortion in response was found, indicating sufficient stability Selectivity of the sensor to CO and NH3 was also tested at different temperatures [Fig 5(A) and (B), respectively] The sensor resistance decreased upon exposure to CO and NH3 gases The response and recovery characteristics to CO gas improve with increase in temperature At 450 C, the sensor response to 25, 50, and 100 ppm CO is very low, that is, approximately 1.22, 1.30, and 1.33, respectively The sensor also shows weak response to NH3 (50÷500 ppm) [Fig 5(B)] At 450 C, the sensor responses to 50, 100, 250, and 500 ppm NH3 are 1.04, 1.11, 1.18, and 1.27, respectively The response of a-Fe2O3 hollow balls to ethanol (500 ppm) is 3.38 times higher than that to NH3 (500 ppm), at low working temperature, suggesting the possibility of using this material for sensing ethanol 49 between nanoparticles acted as diffusion path for analytic gas molecules to enter deeply into the balls to be adsorbed on the total surface of sensing materials, thereby enhancing sensing performance [31] Conclusion The synthesis of a-Fe2O3 hollow balls by a facile hydrothermal method for gas-sensing application is introduced The a-Fe2O3 hollow balls were formed by the aggregation of highly crystalline aFe2O3 nanoparticles The average diameters of a-Fe2O3 nanoparticles and hollow balls were 100 nm and 1.5 mm, respectively The interspace between nanoparticles and hollow structure of the materials facilitate the fast diffusion of analytic gas molecules into the sensing layer and adsorption on the total surface of sensing materials These characteristics ensured the high sensitivity of materials Thus, the a-Fe2O3 hollow balls were found to be sufficient for ethanol sensor application Acknowledgment The present study was funded by the Vietnam Ministry of Education and Training under Code No KB2015e01e100 Appendix A Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jsamd.2016.03.003 3.3 Gas-sensing mechanism The gas-sensing mechanism of the fabricated sensor can be explained by the spaceecharge layer mode [28] The gas-sensing characteristics were measured under a continuous flow of dry air Thus, the oxygen molecules in air can capture the free electron from a-Fe2O3 crystals to form the electron-depletion region The oxygen molecules adsorb on the surface of the sensing layer in the form of À 2À OÀ , O and O , as follows [29] O2 gasị ỵ e 4O adsị (1) O gasị ỵ e 42O adsị (2) O adsị ỵ e 4O2 (3) The analytic molecules interact with the pre-adsorbed oxygen upon exposure to ethanol gas, according to the following equations: À C2 H5 OH þ 3OÀ 42CO2 þ 3H2 O þ 3e (4) C2 H5 OH ỵ 6O 42CO2 ỵ 3H2 O ỵ 6e (5) C2 H5 OH ỵ 6O2 42CO2 ỵ 3H2 O ỵ 12e (6) The interactions between analytic ethanol molecules and preadsorbed oxygen release electrons back to the crystals and reduce the spaceecharge layer, resulting in decreased sensor resistance The porosity of the sensing layer is also very important in controlling the sensitivity of the device because it decides the diffusion rate of analytic gas molecules into the sensing layer The diffusion constant (DK) can be calculated based on the Knudsen diffusion model as DK ¼ 4r/3(2RT/pM)1/2, where r is the pore size, R is the universal gas constant, T is the temperature, and M is the molecular weight of the diffusing gas [30] In this study, the shell of the hollow balls was 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O? ? , O and O , as follows [29 ] O2 gasị ỵ e 4O adsị (1) O gasị ỵ e 4 2O adsị (2) O adsị