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

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This work introduces a simple process to synthesize directly bi-phase a-/g-Fe2O3 submicron flowers on a Fe foil in NH4OH. The flowers were assembled by using many nanoplates with an estimated thickness of 20e80 nm. The shapes and dimensions of the flowers can be controlled by the treatment time.

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

Journal of Science: Advanced Materials and Devices 6 (2021) 27e32 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article High acetone-sensing performance of bi-phase a-/g-Fe2O3 submicron ﬂowers grown using an iron plate Vu Xuan Hien a, *, Luong Huu Phuoc a, Cao Tien Khoa b, Dang Duc Vuong a, Nguyen Duc Chien a a School of Engineering Physics, Hanoi University of Science and Technology (HUST), 01 Dai Co Viet Street, Hanoi, Viet Nam b Department of Physics, Thai Nguyen University of Education, 20 Luong Ngoc Quyen Street, Thainguyen, Viet Nam a r t i c l e i n f o a b s t r a c t Article history: Iron oxide nanostructures have been studied extensively because of their excellent magnetic, optical, Received 6 June 2020 electrical, and catalytic properties. This work introduces a simple process to synthesize directly bi-phase Received in revised form a-/g-Fe2O3 submicron ﬂowers on a Fe foil in NH4OH. The ﬂowers were assembled by using many 8 September 2020 nanoplates with an estimated thickness of 20e80 nm. The shapes and dimensions of the ﬂowers can be Accepted 14 September 2020 controlled by the treatment time. The gas-sensing properties of the a-/g-Fe2O3 submicron ﬂowers were Available online 17 September 2020 investigated at different gas concentrations (125e1500 ppm) and operating temperatures (200e360 C). The results obtained indicate that the as-synthesized material exhibits an excellent acetone-sensing Keywords: Iron oxides characteristic and has the potential for practical applications. Metal © 2020 The Authors. Publishing services by Elsevier B.V. on behalf of Vietnam National University, Hanoi. Nanoplates This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Surface reaction Gas sensors 1. Introduction issue, several studies have pre-treated the metal surface in alkaline solutions to modulate the morphology [11]. This method has been Many metal oxide semiconductors (MOSs), such as TiO2, SnO2, effectively used with Cu to synthesize Cu(OH)2 and CuO [12e14]. WO2, NiO, Co3O4, and Fe2O3, have attracted great attention because Nevertheless, few studies have applied the similar method to Fe. of their wide range of applications. Fe2O3 is an n-type semi- a-Fe2O3 is an excellent candidate for gas sensors [15e17]. The conductor that is naturally abundant, non-toxic and inexpensive particle-like, rod-like and plate-like nanostructures of a-Fe2O3, and and has interesting electrical/catalytic characteristics [1]. Many g-Fe2O3, have been investigated for gas-sensing applications methods have been proposed to synthesize nano/microstructures [18e20]. Most gas sensors with metal oxides, such as SnO2, ZnO, of Fe2O3. Some of these methods are hydrothermal treatment, self- and a-Fe2O3, are surface resistance controlled sensors [21]. g-Fe2O3, assembly, sputtering, chemical vapor deposition (CVD), and wet which has a spinel-type structure, is one of the few materials with chemical processing [2e6]. Fe2O3 nanowires have been grown the bulk resistance controlled effect [22,23]. Therefore, the com- easily and directly on an iron plate surface by thermal oxidation or bination of these two phases is proposed to exploit the surface the hot-plate technique [7,8]. However, in this method, modifying versus bulk sensitivity, thereby improving the gas sensing proper- the sample morphology by modulating the treatment temperature ties of the material. Ming et al. claimed that the ethanol-sensing or time is nearly impossible [9,10]. This limitation is observed in all- properties of the bi-phase a-/g-Fe2O3 nanostructure are at least metal oxides grown by thermal oxidation (e.g. CuO nanowires on a thrice higher than those of g-Fe2O3 [22]. However, no further Cu foil/foam and NiO nanowalls on a Ni foil) [9]. To overcome this 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 * Corresponding author. School of Engineering Physics, Hanoi University of Sci- this study, the micro ﬂowers constructed by using a-/g-Fe2O3 ence and Technology (HUST), No. 01 Dai Co Viet Street, Hanoi, Viet Nam. Fax: þ84 436231713. nanoplates are synthesized by treating an iron plate in aqueous E-mail address: hien.vuxuan@hust.edu.vn (V.X. Hien). NH4OH near room temperature and subsequently annealing at Peer review under responsibility of Vietnam National University, Hanoi. 500 C. The effects of the treatment time on the formation of the https://doi.org/10.1016/j.jsamd.2020.09.011 2468-2179/© 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 (http://creativecommons.org/licenses/by/4.0/).
V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32 micro ﬂowers are investigated. The sample after the heat treatment 3. Results and discussion was tested with acetone, ethanol, and liquiﬁed petroleum gas (LPG) at 200e360 C. 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 2. Experimental 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 In this experiment, a commercial iron plate of 0.25 mm ﬁle No. 98-010-8876, orthorhombic structure, space group Cmcm thickness and NH4OH solution of 25% concentration were used as (63)]. Two additional phases of Fe(OH)2 [JCPDS ﬁle No. 00-003- precursors. First, the iron plate was carefully ground by a grind- 0903, hexagonal structure, space group P-3m1 (164)] and stone and ﬁne sandpaper to remove the oxide layer. Then, the iron Feþ3O(OH) [JCPDS ﬁle No. 00-017-0536, orthorhombic structure, plate was cut into 2 2 cm2 squares. The cut plate was ultra- space group Pbnm (62)] are detected with the minor peaks at 33.2 / sonically cleaned in a bath sonicator with acetone for 5 min. After 36.6 and 34.7 /35.5 /67.1, respectively. No other impurities are being dried under a ﬂow of N2, the plate was folded at the corners found. to form a table-like plate, and 25 mL of NH4OH solution in a 100- Hydroxide forms of iron can be dehydrated to form Fe2O3 mL Duran laboratory bottle was added. The bottle was capped and above 300 C [22]. The phase transformation of the as-synthesized placed in a large chemical bottle (total volume of 400 mL) ﬁlled sample was investigated at 400 C and 500 C for 1 h (Fig. 1b). All with 150 mL of distilled water. After covering the cap with an peaks in the XRD pattern of the sample annealed at 400 C could aluminium foil, the large bottle was heated by a hot plate that was be assigned to the cubic structures of g-Fe2O3 [JCPDS ﬁle No. 98- controlled by a proportional integral derivative temperature 024-7034, cubic structure, space group Fd-3m (227)]. At the controller (the K-type thermocouple was dipped into the distilled annealing temperature of 500 C, the phase of a-Fe2O3 [JCPDS ﬁle water). The treatment temperature was 40 C at a heating rate of No. 01-089-0597, rhombohedral structure, space group R-3c (167)] 1 C/min, and different treatment times in the range of 1e5 days is formed along with the g-Fe2O3 background. No other phase was were carried out. After the heat treatment, the Duran bottle was found in the patterns. Therefore, it is conﬁrmed that the FeOOH treated in a bath sonicator for 5 min. A red precipitate was washed phase was completely transformed to the bi-phase a-/g-Fe2O3 by and extracted from the treated solution by distilled water and by annealing at 500 C in 1 h. centrifugation, respectively. The red powder was annealed in a Fig. 2 introduces the surface morphologies of all samples treated horizontal furnace at 500 C for 1 h to produce Fe2O3 [25]. To for 24e120 h after annealing at 500 C for 1 h. The sample treated fabricate the sensing device, the annealed powder was coated on for 24 h is composed of nanoplates with different dimensions and an interdigitated electrode (gap, ~20 mm) by spin-coating (coating rough surfaces (Fig. 2a). When the treatment time is prolonged to speed of 3000 rpm; coating time of 2 min). The electrode shape is 48 h, the surfaces of the nanoplates are smooth, and the plates are shown in Fig. S1 (Supplemental Information). apparently assembled in random directions (Fig. 2b). In Fig. 2c, the The red powder before annealing was characterized by ﬁeld- sample treated for 72 h has a ﬂower-like microstructure (with emission scanning electron microscopy (FE-SEM; JEOL JSM- diameter of 500e2000 nm). The petals are nanoplates arranged in 7610F), and the structures were evaluated by X-ray diffraction different orientations. The average plate thickness was estimated to (XRD; X'Pert-Pro) by using Cu Ka radiation (l ¼ 1.5418 Å). Gas- be 40 nm. Many microspheres with rough surfaces are distributed sensing measurement was performed in a static gas-testing sys- in some areas of the sample. The petal/plate is thicker (~70 nm) tem with a working chamber of 20 L. The gas-testing system is when the treatment time is 96 h (Fig. 2d). In addition, the nano- presented in Fig. S2 (Supplemental Information). The sensor plates are more uniform and distributed on the whole sample response was calculated as follows: surface without any microspheres. When the treatment time is extended to 120 h, the sample has apparently aged, and micro- Ra Rg spheres are primarily observed (Fig. 2e). s¼ (1) A graph comparing the acetone responses of the samples heated Rg at 400 C and 500 C is shown in Fig. 3. The gas response of the bi- where Ra and Rg are the stable resistances of the device without and phase a-/g-Fe2O3 sample is nearly 9 times higher than that of the with the target gas, respectively. single-phase g-Fe2O3. This result agrees with that in the study of Fig. 1. XRD patterns of the samples treated at 40 C for 24 h (a1), 48 h (a2), 72 h (a3), 96 h (a4) and 120 h (a5); XRD patterns of the sample treated at 40 C for 96 h and annealed at 400 C (b1) and 500 C (b2) for 1 h. 28
V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32 Fig. 2. FE-SEM images of the samples treated at 40 C for 24 h (a), 48 h (b), 72 h (c), 96 h (d), and 120 h (e). All samples were annealed at 500 C for 1 h. The scale bar is 5 mm. Ming et al., in which the surface resistance controlled type of a- where ads stands for adsorption. According to Eqs. (2)e(5), the Fe2O3 has enhanced the sensing performance of the bulk resistance oxygen molecules and ions take the free electrons of a-Fe2O3 to controlled type (g-Fe2O3) [22]. The gas-sensing behavior of the bi- increase the surface resistance of the material. These adsorbed phase a-/g-Fe2O3 may comprise the gas-sensing mechanism of a- oxygen ions can react with reducing gases, e.g. acetone, ethanol, Fe2O3 versus that of g-Fe2O3. As a surface resistance control sensor, and LPG, to return these free electrons of a-Fe2O3 as follows: the resistance of a-Fe2O3 ﬂuctuates by the reactions of the test gas with the adsorbed oxygen ions formed by charge transfer chemi- CH3 COCH3 ðgÞ þ 8Ox ðadsÞ / 3CO2 ðgÞ þ 3H2 OðgÞ þ 8xe (6) sorption [26,27] as follows: C2 H5 OHðgÞ þ 6Ox ðadsÞ / 2CO2 ðgÞ þ 3H2 OðgÞ þ 6xe (7) O2 ðgasÞ4O2 ðadsÞ (2) C3 H8 ðgÞ þ 10Ox ðadsÞ / 3CO2 ðgÞ þ 4H2 OðgÞ þ 10xe (8) O2 ðadsÞ þ e 4O 2 ðadsÞðT < 100 CÞ (3) 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 O 2 ðadsÞ þ e 42O ðadsÞð100 C < T < 300 CÞ (4) 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 O ðadsÞ þ e 4O2 ðadsÞðT > 300 CÞ (5) be reduced to Fe3O4 [22] as follows: Reduction 2 ! g Fe3þ 2 O3 Fe2þ O2 þ Fe3þ 2 2 O3 (9) Oxidation 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, Fig. 3. Response/recovery curves of g-Fe2O3 (sample treated at 40 C for 96 h and ethanol, or LPG, a portion of g-Fe2O3 is converted to Fe3O4 [Eq. (9)]. annealed at 400 C), and a-/g-Fe2O3 (sample treated at 40 C for 96 h and annealed at As a result, an electron channel is formed on the surface of g-Fe2O3 500 C). (Fig. S3b), which is equivalent to the resistor R34 (resistor of Fe3O4) in 29
V.X. Hien, L.H. Phuoc, C.T. Khoa et al. Journal of Science: Advanced Materials and Devices 6 (2021) 27e32 parallel with Rg. The surface of g-Fe2O3 may be converted to a-Fe2O3 320 C (Fig. 4d and f). The enhanced response of the samples because of the thermal oxidation. Therefore, the equivalent resis- treated for a long time may be related to the growth of the micro tance circuit of the material possibly consists of an Ra resistor (a- ﬂowers/nanoplates. The complete formation of the nanoplates as Fe2O3 resistor) connected in series with Rg (Fig. S3c). After reacting petals may not only increase the effective working area of the with the reducing gas as presented in Eqs. (6)e(9), the surface of the sensor but also improve the pore size of the ﬁlm. As mentioned material system can show two electronic conducting channels above, the sample treated for 120 h has apparently aged, which coming from the surface of the a-Fe2O3 and Fe3O4 materials. These may result in the decline of the sensing performance (Fig. 4e). The channels are equivalent to the resistors Ra23 and R34 in parallel with response versus recovery times of the sample treated for 96 h are Ra-Rg (Fig. S3d). This resistor probably possesses a smaller value both approximately 25 s. than that in the g-Fe2O3 sample. According to Eq. (1), the response of Fig. 5a introduces the effects of acetone/ethanol/LPG concen- a-/g-Fe2O3 is evidently higher than that of g-Fe2O3. tration on the sensor response of the sample treated for 96 h. The The acetone-sensing properties of all samples at operating result indicates that the sample is more sensitive towards acetone temperatures of 200e360 C are shown in Fig. 4. The data indicate than to the other gases. At a concentration of 1500 ppm, the that the gas response increases with the treatment time from 24 h acetone response is reasonably above 160, which is 3.2 and 10.6 to 96 h (Fig. 4aed). The highest response of the samples treated times higher than the responses of ethanol and LPG, respectively. for 96 h is approximately 95, and the optimal temperature is 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. Fig. 5. Inﬂuence of gas (acetone, ethanol, and LPG) concentration on the sensor response of the sample treated for 96 h (a). A linear ﬁt of the sensor response to the concentration of the injected gases (b). The operating temperature is 320 C, and a is the slope of the ﬁtting line. 30
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