High acetone-sensing performance of bi-phase a-/g-Fe₂O₃ submicron flowers grown using an iron plate

-p oa Dai treet Received in revised form hav per a-/g-Fe2O3 submicron flowers on a Fe foil in NH4OH. The flowers were assembled by using many controlled by the treatment time. The gas-sensing properties of the a-/g-Fe2O3 submicron flowers were The results obtained indicate that the as-synthesized material exhibits an excellent acetone-sensing characteristic and has the potential for practical applications. rs (MO Cu foil/foam and NiO nanowalls on a Ni foil) [9]. To overcome this res of a-Fe2O3, and sing applications uch as SnO2, ZnO, sors [21]. g-Fe2O3, ew materials with erefore, the com- xploit the surface as sensing proper- e ethanol-sensing 2 3 cture are at least research on this material has been conducted. In addition to the phase variation, the flower-like nano/microstructures have recently attracted great attention, because this morphology can improve the effective working/sensing areas and increase the pore size [24]. In this study, the micro flowers constructed by using a-/g-Fe2O3 nanoplates are synthesized by treating an iron plate in aqueous NH4OH near room temperature and subsequently annealing at 500
C. The effects of the treatment time on the formation of the * Corresponding author. School of Engineering Physics, Hanoi University of Sci- ence and Technology (HUST), No. 01 Dai Co Viet Street, Hanoi, Viet Nam. Fax: þ84 436231713. E-mail address: hien.vuxuan@hust.edu.vn (V.X. Hien). Contents lists availab Journal of Science: Advanc journal homepage: www.el Journal of Science: Advanced Materials and Devices 6 (2021) 27e32Peer review under responsibility of Vietnam National University, Hanoi.metal oxides grown by thermal oxidation (e.g. CuO nanowires on a thrice higher than those of g-Fe2O3 [22]. However, no furtherconductor that is naturally abundant, non-toxic and inexpensive and has interesting electrical/catalytic characteristics [1]. Many methods have been proposed to synthesize nano/microstructures of Fe2O3. Some of these methods are hydrothermal treatment, self- assembly, sputtering, chemical vapor deposition (CVD), and wet chemical processing [2e6]. Fe2O3 nanowires have been grown easily and directly on an iron plate surface by thermal oxidation or the hot-plate technique [7,8]. However, in this method, modifying the sample morphology by modulating the treatment temperature or time is nearly impossible [9,10]. This limitation is observed in all- particle-like, rod-like and plate-like nanostructu g-Fe2O3, have been investigated for gas-sen [18e20]. Most gas sensors with metal oxides, s and a-Fe2O3, are surface resistance controlled sen which has a spinel-type structure, is one of the f the bulk resistance controlled effect [22,23]. Th bination of these two phases is proposed to e versus bulk sensitivity, thereby improving the g ties of the material. Ming et al. claimed that th properties of the bi-phase a-/g-Fe O nanostruWO2, NiO, Co3O4, and Fe2O3, have attracted great attention because of their wide range of applications. Fe2O3 is an n-type semi- Nevertheless, few studies have applied the similar method to Fe. a-Fe2O3 is an excellent candidate for gas sensors [15e17]. TheKeywords: Iron oxides Metal Nanoplates Surface reaction Gas sensors 1. Introduction Many metal oxide semiconductohttps://doi.org/10.1016/j.jsamd.2020.09.011 2468-2179/© 2020 The Authors. Publishing services b (© 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. This is an open access article under the CC BY license ( Ss), such as TiO2, SnO2, issue, several studies have pre-treated the metal surface in alkaline solutions to modulate the morphology [11]. This method has been effectively used with Cu to synthesize Cu(OH)2 and CuO [12e14].Accepted 14 September 2020 Available online 17 September 2020investigated at different gas concentrations (125e1500 ppm) and operating temperatures (200e360
C).8 September 2020 nanoplates with an estimated thickness of 20e80 nm. The shapes and dimensions of the flowers can beOriginal Article High acetone-sensing performance of bi flowers grown using an iron plate Vu Xuan Hien a, *, Luong Huu Phuoc a, Cao Tien Kh Nguyen Duc Chien a a School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 b Department of Physics, Thai Nguyen University of Education, 20 Luong Ngoc Quyen S a r t i c l e i n f o Article history: Received 6 June 2020 a b s t r a c t Iron oxide nanostructures electrical, and catalytic proy Elsevier B.V. on behalf of Vietnamhase a-/g-Fe2O3 submicron b, Dang Duc Vuong a, Co Viet Street, Hanoi, Viet Nam , Thainguyen, Viet Nam e been studied extensively because of their excellent magnetic, optical, ties. This work introduces a simple process to synthesize directly bi-phase le at ScienceDirect ed Materials and Devices sevier .com/locate/ jsamdNational University, Hanoi. This is an open access article under the CC BY license micro flowers are investigated. The sample after the heat treatment was tested with acetone, ethanol, and liquified petroleum gas (LPG) at 200e360
C. 2. Experimental In this experiment, a commercial iron plate of 0.25 mm thickness and NH4OH solution of 25% concentration were used as precursors. First, the iron plate was carefully ground by a grind- stone and fine sandpaper to remove the oxide layer. Then, the iron plate was cut into 2  2 cm2 squares. The cut plate was ultra- sonically cleaned in a bath sonicator with acetone for 5 min. After being dried under a flow of N2, the plate was folded at the corners to form a table-like plate, and 25 mL of NH4OH solution in a 100- mL Duran laboratory bottle was added. The bottle was capped and placed in a large chemical bottle (total volume of 400 mL) filled with 150 mL of distilled water. After covering the cap with an aluminium foil, the large bottle was heated by a hot plate that was 3. Results and discussion The XRD patterns of the red powders treated for 24e120 h are shown in Fig. 1a. The main phase in all samples is FeOOH, and the major diffraction peaks at 2q of 27
, 29.9
, 36.2
, and 46.8
can be assigned to the (021), (110), (041) and (002) planes of FeOOH [JCPDS file No. 98-010-8876, orthorhombic structure, space group Cmcm (63)]. Two additional phases of Fe(OH)2 [JCPDS file No. 00-003- 0903, hexagonal structure, space group P-3m1 (164)] and Feþ3O(OH) [JCPDS file No. 00-017-0536, orthorhombic structure, space group Pbnm (62)] are detected with theminor peaks at 33.2
/ 36.6
and 34.7
/35.5
/67.1
, respectively. No other impurities are found. Hydroxide forms of iron can be dehydrated to form Fe2O3 above 300
C [22]. The phase transformation of the as-synthesized sample was investigated at 400
C and 500
C for 1 h (Fig. 1b). All peaks in the XRD pattern of the sample annealed at 400
C could be assigned to the cubic structures of g-Fe2O3 [JCPDS file No. 98- V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32controlled by a proportional integral derivative temperature controller (the K-type thermocouple was dipped into the distilled water). The treatment temperature was 40
C at a heating rate of 1
C/min, and different treatment times in the range of 1e5 days were carried out. After the heat treatment, the Duran bottle was treated in a bath sonicator for 5 min. A red precipitate was washed and extracted from the treated solution by distilled water and by centrifugation, respectively. The red powder was annealed in a horizontal furnace at 500
C for 1 h to produce Fe2O3 [25]. To fabricate the sensing device, the annealed powder was coated on an interdigitated electrode (gap, ~20 mm) by spin-coating (coating speed of 3000 rpm; coating time of 2 min). The electrode shape is shown in Fig. S1 (Supplemental Information). The red powder before annealing was characterized by field- emission scanning electron microscopy (FE-SEM; JEOL JSM- 7610F), and the structures were evaluated by X-ray diffraction (XRD; X'Pert-Pro) by using Cu Ka radiation (l ¼ 1.5418 Å). Gas- sensing measurement was performed in a static gas-testing sys- tem with a working chamber of 20 L. The gas-testing system is presented in Fig. S2 (Supplemental Information). The sensor response was calculated as follows: s¼Ra  Rg Rg (1) where Ra and Rg are the stable resistances of the devicewithout and with the target gas, respectively.Fig. 1. XRD patterns of the samples treated at 40
C for 24 h (a1), 48 h (a2), 72 h (a3), 96 h (a 400
C (b1) and 500
C (b2) for 1 h. 28024-7034, cubic structure, space group Fd-3m (227)]. At the annealing temperature of 500
C, the phase of a-Fe2O3 [JCPDS file No. 01-089-0597, rhombohedral structure, space group R-3c (167)] is formed along with the g-Fe2O3 background. No other phase was found in the patterns. Therefore, it is confirmed that the FeOOH phase was completely transformed to the bi-phase a-/g-Fe2O3 by annealing at 500
C in 1 h. Fig. 2 introduces the surface morphologies of all samples treated for 24e120 h after annealing at 500
C for 1 h. The sample treated for 24 h is composed of nanoplates with different dimensions and rough surfaces (Fig. 2a). When the treatment time is prolonged to 48 h, the surfaces of the nanoplates are smooth, and the plates are apparently assembled in random directions (Fig. 2b). In Fig. 2c, the sample treated for 72 h has a flower-like microstructure (with diameter of 500e2000 nm). The petals are nanoplates arranged in different orientations. The average plate thickness was estimated to be 40 nm. Many microspheres with rough surfaces are distributed in some areas of the sample. The petal/plate is thicker (~70 nm) when the treatment time is 96 h (Fig. 2d). In addition, the nano- plates are more uniform and distributed on the whole sample surface without any microspheres. When the treatment time is extended to 120 h, the sample has apparently aged, and micro- spheres are primarily observed (Fig. 2e). A graph comparing the acetone responses of the samples heated at 400
C and 500
C is shown in Fig. 3. The gas response of the bi- phase a-/g-Fe2O3 sample is nearly 9 times higher than that of the single-phase g-Fe2O3. This result agrees with that in the study of4) and 120 h (a5); XRD patterns of the sample treated at 40
C for 96 h and annealed at 6 h V.X. Hien, L.H. Phuoc, C.T. Khoa et al.Ming et al., in which the surface resistance controlled type of a- Fe2O3 has enhanced the sensing performance of the bulk resistance controlled type (g-Fe2O3) [22]. The gas-sensing behavior of the bi- phase a-/g-Fe2O3 may comprise the gas-sensing mechanism of a- Fe2O3 versus that of g-Fe2O3. As a surface resistance control sensor, the resistance of a-Fe2O3 fluctuates by the reactions of the test gas with the adsorbed oxygen ions formed by charge transfer chemi- sorption [26,27] as follows: O2ðgasÞ4O2ðadsÞ (2) Fig. 2. FE-SEM images of the samples treated at 40
C for 24 h (a), 48 h (b), 72 h (c), 9O2ðadsÞþ e4O2 ðadsÞðT < 100
CÞ (3) O2 ðadsÞþ e42OðadsÞð100
C < T < 300
CÞ (4) OðadsÞþ e4O2ðadsÞðT > 300
CÞ (5) Fig. 3. Response/recovery curves of g-Fe2O3 (sample treated at 40
C for 96 h and annealed at 400
C), and a-/g-Fe2O3 (sample treated at 40
C for 96 h and annealed at 500
C). 29where ads stands for adsorption. According to Eqs. (2)e(5), the oxygen molecules and ions take the free electrons of a-Fe2O3 to increase the surface resistance of the material. These adsorbed oxygen ions can react with reducing gases, e.g. acetone, ethanol, and LPG, to return these free electrons of a-Fe2O3 as follows: CH3COCH3ðgÞþ 8OxðadsÞ/3CO2ðgÞþ3H2OðgÞ þ 8xe (6) C2H5OHðgÞþ6OxðadsÞ/2CO2ðgÞþ3H2OðgÞ þ 6xe (7) (d), and 120 h (e). All samples were annealed at 500
C for 1 h. The scale bar is 5 mm. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32C3H8ðgÞþ10OxðadsÞ/3CO2ðgÞþ4H2OðgÞ þ 10xe (8) where x ¼ 1 or 2. Afterward, the free electrons generated by Eqs. (6)e(8) can reduce the material resistance. Maghemite g-Fe2O3 is an n-type MOS with a bandgap of 2.03 eV. When placed in a reducing gas environment, e.g. acetone, ethanol or LPG, g-Fe2O3 can be reduced to Fe3O4 [22] as follows: g Fe3þ2 O23

































! Reduction Oxidation Fe2þO2 þ Fe3þ2 O23 (9) where Fe3O4 is known as a half-metal material and acts as a conductor because of the continuous hopping of the electrons from Fe2þ to Fe3þ cations over the crystallographic B-octahedral sites of the inverse spinel structure (active by thermal energy) [28]. Therefore, the resistance of g-Fe2O3 is decreased during the expo- sure to reducing gases. According to the sensing mechanism of a- Fe2O3 versus g-Fe2O3, the sensing material is low selectivity to reducing gases. The difference in gas responses is due to several factors, such as the working temperature, morphology/structure, and the diffusion ability of the test gas [29e31]. The resistor model that explains the response enhancement of the a-/g-Fe2O3 over the g-Fe2O3 sample is shown in Fig. S3 (Sup- plemental Information). In the model, the resistance of the g-Fe2O3 sample is Rg. After reacting with the reduced gas, e.g. acetone, ethanol, or LPG, a portion of g-Fe2O3 is converted to Fe3O4 [Eq. (9)]. As a result, an electron channel is formed on the surface of g-Fe2O3 (Fig. S3b),which is equivalent to the resistorR34 (resistorof Fe3O4) in parallelwithRg. The surface of g-Fe2O3maybe converted to a-Fe2O3 because of the thermal oxidation. Therefore, the equivalent resis- tance circuit of the material possibly consists of an Ra resistor (a- Fe2O3 resistor) connected in series with Rg (Fig. S3c). After reacting with the reducing gas as presented in Eqs. (6)e(9), the surface of the material system can show two electronic conducting channels coming from the surface of the a-Fe2O3 and Fe3O4 materials. These channels are equivalent to the resistorsRa23 andR34 inparallelwith Ra-Rg (Fig. S3d). This resistor probably possesses a smaller value than that in the g-Fe2O3 sample. According to Eq. (1), the response of a-/g-Fe2O3 is evidently higher than that of g-Fe2O3. The acetone-sensing properties of all samples at operating temperatures of 200e360
C are shown in Fig. 4. The data indicate that the gas response increases with the treatment time from 24 h to 96 h (Fig. 4aed). The highest response of the samples treated for 96 h is approximately 95, and the optimal temperature is 320
C (Fig. 4d and f). The enhanced response of the samples treated for a long time may be related to the growth of the micro flowers/nanoplates. The complete formation of the nanoplates as petals may not only increase the effective working area of the sensor but also improve the pore size of the film. As mentioned above, the sample treated for 120 h has apparently aged, which may result in the decline of the sensing performance (Fig. 4e). The response versus recovery times of the sample treated for 96 h are both approximately 25 s. Fig. 5a introduces the effects of acetone/ethanol/LPG concen- tration on the sensor response of the sample treated for 96 h. The result indicates that the sample is more sensitive towards acetone than to the other gases. At a concentration of 1500 ppm, the acetone response is reasonably above 160, which is 3.2 and 10.6 times higher than the responses of ethanol and LPG, respectively. The fitting lines of these data are illustrated in Fig. 5b. The sensor Fig. 4. Acetone-sensing properties of the samples treated for 24 h (a), 48 h (b), 72 h (c), 96 h (d), and 120 h (e) at the operating temperature of 200e360
C. The concentration of acetone is 500 ppm. of t V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32Fig. 5. Influence of gas (acetone, ethanol, and LPG) concentration on the sensor response
the injected gases (b). The operating temperature is 320 C, and a is the slope of the fittin 30he sample treated for 96 h (a). A linear fit of the sensor response to the concentration of g line. response varies linearly with the concentration of the target gases. The highest coefficient of determination (R-square) reaches 99.6% for the linear function of the acetone sensing data. [3] M. Muruganandham, R. Amutha, M. Sathish, T.S. Singh, R.P.S. Suri, M. Sillanp€a€a, Facile fabrication of hierarchical a-Fe2O3 : self-assembly and its magnetic and electrochemical properties, J. Phys. Chem. C 115 (2011) 18164e18173, https://doi.org/10.1021/jp205834m. V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32Table S1 shows a comparison of the acetone-sensing perfor- mance of recent nanomaterials. However, this comparison is not entirely accurate, because the parameters of a gas sensor depend significantly on the setting of the system (e.g. static or dynamic measurement, gas flow rate, chamber volume, and whether the gas duct is exposed to the material membrane). The results indicate that the synthesized bi-phase a-/g-Fe2O3 submicron flowers possess the highest response compared with the other acetone- sensing materials. 4. Conclusion The a-/g-Fe2O3 micro flowers were well synthesized by treating iron plates in the NH4OH aqueous solution at 40
C for 72e96 h. The flower petals were obtained in form of nanoplates with thicknesses of ~40 nm and ~70 nm in the samples treated for 72 h and 96 h, respectively. The uniformity of the submicron flowers and the nanoplates were improved as the treatment time increased up to 96 h. For the 120 h treatment time, the ageing effect apparently occurred, causing the sample morphology to become less uniform and to include many microspheres. The acetone-sensing properties of the annealed samples were investigated at 200e360
C. The optimal working temperature for all samples was 320
C. The sample treated for 96 h showed the best performance among all samples, in which the sensor response to 500 ppm acetone, the response time and the recovery time were approximately 95, 25 s, and 25 s, respectively. These sensing data are higher than those of the available acetone-sensing materials. Hence, the bi-phase a-/g- Fe2O3 submicron flowers have potential for practical acetone sensor applications. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This research is funded by the Vietnam National Foundation for Science and Technology Development (NAFOSTED), under grant number 103.02-2016.20. We also thank to the NAFOSTED under grant number of 103.02-2019.25 for additional experiments and spelling correction to complete this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2020.09.011. References [1] Y. Huang, J. Yao, Y. Zheng, R. Huang, Y. Li, A simple preparation of rod-like Fe2O3 with superior lithium storage performance, Mater. Lett. 234 (2019) 105e108, https://doi.org/10.1016/j.matlet.2018.09.080. [2] S. Han, L. Hu, Z. Liang, S. Wageh, A.A. Al-Ghamdi, Y. Chen, X. Fang, One-step hydrothermal synthesis of 2D hexagonal nanoplates of a-Fe2O3/graphene composites with enhanced photocatalytic activity, Adv. Funct. Mater. 24 (2014) 5719e5727, https://doi.org/10.1002/adfm.201401279.31[4] L. Jia, K. Harbauer, P. Bogdanoff, K. Ellmer, S. Fiechter, Sputtering deposition of ultra-thin a-Fe2O3 films for solar water splitting, J. Mater. Sci. Technol. 31 (2015) 655e659, https://doi.org/10.1016/j.jmst.2014.10.007. [5] S. Park, S. Lim, H. Choi, Chemical vapor deposition of iron and iron oxide thin films from Fe(II) dihydride complexes, C