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Article

Novel Mixed-Phase α/γ-Fe2O3 Micro-Flower Assembled with Nanosheets for Enhancing Acetone Detection

by
Ruonan Tian
1,
Huai Tan
1,
Gang Chen
1,
Hongtao Guan
1,
Chengjun Dong
1,2,* and
Zongyou Yin
2,*
1
School of Materials and Energy, Yunnan University, Kunming 650091, China
2
Research School of Chemistry, Australian National University, Canberra, ACT 2601, Australia
*
Authors to whom correspondence should be addressed.
Crystals 2023, 13(5), 810; https://doi.org/10.3390/cryst13050810
Submission received: 29 March 2023 / Revised: 6 May 2023 / Accepted: 8 May 2023 / Published: 13 May 2023
(This article belongs to the Special Issue Advances in Low-Dimensional Materials for Electronics and Sensing Applications)
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Abstract

:
Although individual γ-Fe2O3 and α-Fe2O3 have been widely fabricated for gas sensors, their mixed phase of α/γ-Fe2O3 might deliver excellent sensing properties. In this study, a facile solvothermal method was used to fabricate Fe-alkoxide. After thermal treatment, it was converted into γ-Fe2O3, α-Fe2O3 and their mixed-phase α/γ-Fe2O3 with a nanosheets-assembled flower-like structure. We studied the influence of calcination temperature on the phase and sensing properties on acetone detection. The α/γ-Fe2O3 which annealed at 400 °C included 18% α-Fe2O3 and it exhibited excellent sensing performance towards acetone compared to that of γ-Fe2O3 and α-Fe2O3. It showed a response of 353 to acetone with a concentration of 200 ppm, and a low limit of detection of 0.5 ppm at 160 °C. In addition, the change in responses with acetone concentration from 50 to 200 ppm shows a good linear relationship. Moreover, this material has good reproducibility and selectivity as well as a fast response time of 22 s and recovery time of 14 s to 200 ppm. Therefore, our mixed phase of α/γ-Fe2O3 possesses great prospects for acetone detection.

1. Introduction

As a typical volatile organic compound (VOC), acetone is a generally used chemical reagent in the industry of paints, plastics, coating, laboratories, pharmaceuticals, and so on. However, acetone easily evaporates into the atmosphere, which risks human beings to undergo irreversible damage to physical health even at ppb-level concentrations [1]. In addition, the acetone concentration in respiratory gases of diabetic persons (over 1.8 ppm) is higher than those healthy individuals in the range of 0.3–0.9 ppm [2,3]. The effective acetone detection is used for diabetes diagnoses with the advantages of being non-invasive and painless. Hence, it is very necessary to construct a dependable gas sensor to quickly detect acetone with high performance and low cost. Significantly, chemoresistive gas sensors constructed from metal oxide semiconductors (MOSs) are greatly favorable [4]. Iron oxide (Fe2O3), as a kind of n-type MOS, is quite promising in the gas-sensing field [5]. So far, many approaches have been proposed to boost the sensing properties of Fe2O3, such as the regulation of morphologies [6,7], design of heterojunctions [8,9,10], doping of elements [11,12], functionalization of noble nanoparticles [13,14] and exposure of high-energy crystal facets [15,16,17]. However, another convenient method of adjustment of the crystalline phase is worth a try. For instance, Zhang’s group discussed the relationship of the In2O3 crystalline phase and sensing properties with ozone, in which they found that the cubic bixbyite-type exhibited an extraordinary ozone-sensing performance [18]. It was reported that SnO2 with a microstructure of nanorods and tetragonal and orthorhombic phases showed better sensitivity to detect isopropanol gas [19]. Wand and co-authors converted a metal organic framework into α-Fe2O3, γ-Fe2O3 and their mixed phase for n-butanol detection, in which the α-Fe2O3-based sensor exhibited the best sensing performance [20]. Therefore, it is extremely desirable to explore the effects of the Fe2O3 phase with a unique microstructure for acetone detection.
Usually, Iron oxide (Fe2O3) shows four crystal phases which are α-Fe2O3, ε-Fe2O3, β-Fe2O3 and γ-Fe2O3. Among them, α-Fe2O3 with various microstructures has been extensively studied for different applications [21]: gas sensors in particular. Very recently, studies showed that γ-Fe2O3 is a potential candidate for gas sensors as well. However, α-Fe2O3 [22,23] and γ-Fe2O3 [24,25] have been studied separately for the application of gas sensors, which motivates us to synthesize Fe2O3 with different crystalline phases by a special precursor for acetone detection. The precursor will impact on the formation of a certain phase of Fe2O3. Apart from the general precursors of MOF [26] and FeOOH [27], the iron alkoxides coordinated with metal ions based on hydroxyl groups of alcohols are a good alternative to fabricate Fe2O3 through thermal treatment [28]. In this study, an Fe-alkoxide precursor was first synthesized under assistance of ethylene glycol using a solvothermal method. Then, the Fe-alkoxide was converted into Fe2O3 and the influences of annealing temperature on phases were studied. The gas-sensing investigation demonstrated that the mixed phase of α/γ-Fe2O3 exhibited better sensing performance than that of single α-Fe2O3 and γ-Fe2O3 for acetone detection. Moreover, the potential sensing mechanism is discussed in detail with respect to the unique microstructure and the mixed phases of α/γ-Fe2O3.

2. Experimental Section

2.1. Material and Chemicals

Ferric chloride (FeCl3·6H2O), hexadecyl trimethyl ammonium bromide (CTAB), urea and ethylene glycol (EG) were purchased from Shanghai Aladdin Bio-Chem Technology Co., Ltd. (Shanghai, China), and used without further purification.

2.2. Synthesis of Hierarchical Fe2O3 Micro-Flower

Firstly, the Fe-alkoxides were synthesized by a one-step solvothermal route. Typically, 2 mmol FeCl3·6H2O and 3 g urea were dissolved into 100 mL of EG solution. Then, 1 g CTAB was added to obtain a homogeneous solution with stirring for 0.5 h. The solution was transferred to a 200 mL conical flask and refluxed in an oil bath at 180 °C for 30 min with vigorous stirring. When it cooled down naturally, green precipitates were rinsed with ethanol for three times, and dried in an oven at 60 °C for 12 h. The precursor was annealed at 350 °C, 400 °C and 450 °C for 2 h in air to prepare Fe2O3. The as-prepared samples after annealing were red in color, which are named as α-Fe2O3-350, α/γ-Fe2O3-400 and γ-Fe2O3-450, respectively.

2.3. Materials Characterization

X-ray diffraction (XRD) measurements were taken on Rigaku TTRIII with a Cu-Kα radiation source. The scanning speed was set to 0.1°/min with a step of 0.02° from a 2θ range of 10–90°. Field emission scanning electron microscope (FESEM) images were observed on a FEI QUANTA 200 microscope (USA) instrument at an accelerating voltage of 10 kV. Transmission electron microscopy (TEM) analysis was performed on a JEM-2100 (JEOL Ltd., Tokyo, Japan) microscope. The thermogravimetric analysis (TGA) was performed for Fe-alkoxides from room temperature to 800 °C in air with a heating rate of 10 °C/min using an American TA SDT-2960 thermal analyzer. Nitrogen adsorption–desorption isotherms were measured in a Micrometrictics ASAP 2010 automated adsorption analyzer. The materials were pre-outgassed in a vacuum at 300 °C for 2 h before nitrogen adsorption at 77.3 K. We used the Barrett–Joyner–Halenda (BJH) method to calculate the distribution of pore size. The chemical compositions of the sample were characterized by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific Corporation, Waltham, MA, USA). All spectra were calibrated with C 1s peak at 285.0 eV.

2.4. Fabrication and Testing of Gas Sensor

Following our previous procedures [29,30], the gas sensors were fabricated. At the very beginning, a homogeneous paste was formed by mixing 0.1 gas-synthesized powder with 0.2 mL deionized water. Then, the paste was coated on a 4 mm alumina ceramic tube. The thickness of the sensing layer was around 2 μm. Two Au electrodes were deposited at each end of an alumina ceramic tube, which was connected by Pt wires. Next, the resulting sensors were sintered at 350 °C for 2 h to facilitate good contact between the Au electrode and sensing material. Subsequently, a Ni–Cr coil heater was inserted into the alumina tube to provide the operating temperature through the heating current. Figure 1b shows the basic testing principle with the heating voltage (Vh), load resistance (RL), export voltage (Vout) and working voltage (VC). Finally, a base was used to connect the alumina tube and Ni–Cr heater to form a complete sensor unit, as shown in Figure 1c. A WS-30 gas-sensing measurement system provided by Weisheng Instruments Co. (Zhengzhou, China) (Figure 1a) with a test chamber of 18 L was used to measure and record sensing properties. According to the following static liquid gas distribution method, the amount of organic liquid was calculated and injected into the test chamber. After being evaporated, the vapor was mixed with air to form VOC gas with a desirable concentration.
C = (22.4 × ρ × d × V1)/(M × V2)
where C (ppm), ρ (g/mL) and d represent the gas concentration, density of the liquid, and purity of the liquid. In addition, V1 (L), V2 (L) and M correspond to the volume of the chamber and molecular weight of the liquid, respectively. We used the change in the ratio of real electrical resistance of the sensor upon air (Ra) and gas (Rg) to estimate the gas response (β = Ra/Rg). The response and recovery time are defined as the time to achieve 90% of total resistance changes by switching the sensor in target gas and air, respectively. During the test of sensing performance, the relative humidity of the atmosphere was about 30%.

3. Results and Discussion

3.1. Morphology and Structure

The precursors of Fe-alkoxides were synthesized via a solvothermal method and then annealed in air at different temperatures including 350 °C, 400 °C and 450 °C to form pure-phase α-Fe2O3 and γ-Fe2O3, and the mixed-phase α/γ-Fe2O3. The synthesis procedure, as illustrated in Figure 2, involves a solvent of EG and CTAB as a surfactant. Initially, the EG molecules reduced Fe3+ to form Fe2+ ions. In addition, EG molecules coordinate with Fe2+ ions to form an iron alkoxide complex. Urea was further added to provide OH− ions through hydrolysis, which neutralize the by-product of H+ to favor the coordination reaction [28,31]. Under the assistance of CTAB, the nucleation and growth of Fe-alkoxides begin to form nanosheets. With the prolongation in reaction time, these nanosheets could further self-assemble into three-dimensional flower-like microspheres to minimize the free energy of nanostructures. The XRD result of the Fe-alkoxides in Figure 3a shows a strong diffraction peak around 11° along with a weak peak at 18°, which matches well with the reported results [32,33]. Subsequently, we investigated the effects of calcination temperature on the crystal phase and morphology of products. The Fe-alkoxides were further thermally transformed into pure α-Fe2O3, γ-Fe2O3 and especially their mixed phase of α/γ-Fe2O3 for high-sensing performance towards acetone.
X-ray diffraction (XRD) analysis was performed to investigate the crystallinity after calcination. As displayed in Figure 3b, the phase of products is strongly dependent on the annealing temperature. Obviously, the sample calcined at 350 °C matches perfectly with the standard pattern of a maghemite (γ-Fe2O3) crystalline structure (JCPDS: 39-1346). This phase possesses a rhombic-centered corundum-type hexagonal structure which is associated with Fe(III) ions for two-thirds of the octahedral positions and a tight oxygen lattice [34]. At the higher temperature of 450 °C, all detectable peaks are indexed to the hematite (α-Fe2O3) by comparing with JCPDS: 33-0664. The γ-Fe2O3 shows an anti-spinel-type cubic crystal structure, in which the Fe(III) occupy tetrahedral and octahedral positions [34]. Interestingly, the mixed phase of α/γ-Fe2O3-400 is obtained when the annealing temperature is 400 °C. Using MDI Jade refinement analysis, the proportions of γ-Fe2O3 and α-Fe2O3 in the α/γ-Fe2O3-400 are estimated to be 82% and 18%, indicating the partial conversion of γ-Fe2O3 to α-Fe2O3 with increase in temperature. Moreover, the diffraction peaks of α-Fe2O3 show smaller half-widths and strong diffraction intensities, indicating that the higher calcination temperature promotes the crystallinity of the products.
The morphology of precursor and samples calcined at 400 °C (α/γ-Fe2O3-400) was observed by FESEM. As displayed in Figure 4a,c, the Fe-alkoxides are flower-like architectures in shape with smooth surfaces, which are assembled from a bunch of nanosheets. After being annealed at 400 °C in air, the overall flower-like structures of the precursor are preserved, as shown in Figure 4b,d. However, the rough nanosheet surface is observed due to the decomposition of organic parts in Fe-alkoxides. To characterize the microstructure of α/γ-Fe2O3 in detail, TEM analyses were carried out. As shown in Figure 5, typical α/γ-Fe2O3 flower-like structures are 1–2 μm in size and the nanosheets are approximately 40 nm in thickness. Meanwhile, HRTEM images of the α/γ-Fe2O3-400 (Figure 5d,e) show a lattice spacing of 0.295 nm, 0.482 nm and 0.592 nm, corresponding to the (220), (111), and (110) planes of γ-Fe2O3, respectively. Furthermore, the clear lattice spacing of 0.270 nm and 0.252 nm are attributed to the α-Fe2O3 in the (104) and (110) planes. Importantly, the interface derived from the dual phase of α-Fe2O3 and γ-Fe2O3 can be clearly seen in Figure 5d,e, which further proves the coexistence of α-Fe2O3 and γ-Fe2O3. Furthermore, the selected area electron diffraction (SAED) in Figure 5f illustrates the polycrystalline nature of the mixed phase of α/γ-Fe2O3. The diffraction pattern consists of polycrystalline diffraction rings, which matches well with the (220), (111), (311) and (520) crystal planes of γ-Fe2O3, and the (042), (113) and (220) crystal planes of α-Fe2O3, respectively. The compositional analysis of α/γ-Fe2O3-400 was investigated by EDS testing (Figure 5g–i), which more visually reveals the highly homogeneous distribution of Fe and O within the material.
In order to understand the conversion of Fe-alkoxides to Fe2O3, we studied the thermal decomposition procedure of the precursor by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). In Figure 6a, a clear exothermic peak at 308 °C on the DSC curve is observed. Apart from the removal of residual water at low temperature, a big weight loss of 42.47% from 240 °C to 400 °C occurs, corresponding to the decomposition of Fe-alkoxides and the formation of Fe2O3. Furthermore, the N2 adsorption–desorption isotherm was recorded to evaluate the specific surface area and pore structure of the flower-like Fe2O3 by taking α/γ-Fe2O3-400 as an example. As shown in Figure 6b, the isotherm of this sample can be classified as a type IV, with a small hysteresis loop with relative pressure ranging from 0.3–1.0. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, this hysteresis loop belongs to the type H4 return line, indicating the presence of mesopores. Brunauer–Emmett–Teller (BET) gas adsorption measurements exhibit that the specific surface area and pore volume of the α/γ-Fe2O3 are 67.324 m2/g and 0.238 cc/g. The average Barrett–Joyner–Halenda (BJH) pore diameter verifies that the pores are mostly distributed around 3.406 nm. The loose interspaces in the flower-like structure with large BET surface area and suitable pore size distribution are beneficial to provide more active reaction sites and effective channels for acetone detection [35,36].
X-ray photoelectron spectroscopy (XPS) was further used to study the chemical compositions and valance states of the elements in the mixed-phase α/γ-Fe2O3. In the Fe 2p spectrum (Figure 6d), the peak at binding energy of 710.45 eV corresponds to Fe 2p3/2. On the other hand, the peak at 723.96 eV is assigned to Fe 2p1/2. These two major peaks are accompanied by satellite peaks centered at 718.67 eV and 732.45 eV, demonstrating the formation of the Fe2O3 [11,13]. After deconvolution of the O 1s spectrum (Figure 6c), two peaks are observed at binding energies of 529.76 eV and 530.97 eV, which are associated with the lattice oxygen atom (Olattice) and the adsorbed oxygen (Ox−), respectively. In particular, the adsorbed oxygen occupies 38.7% according to the area shared by the two peaks, which is active to react with acetone molecules to contribute a high response.

3.2. Gas-Sensing Properties

For metal oxide semiconductors, sufficient thermal energy is generally needed to favor the surface chemisorption and reaction of gas molecules [37,38], which is an important parameter of a gas sensor. The responses of the sensors to 200 ppm acetone were measured at a series of temperatures from 120 °C to 220 °C, as shown in Figure 7a. At low working temperatures, the responses are very weak due to insufficient thermal energy. With the rising of the working temperature, the responses increase gradually and then reach a maximum of 163, 353, and 74 for the sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 at 160 °C. However, the decrease in sensor response is observed with further increments of the working temperature because of the accelerated gas desorption from the surface of sensing materials. Therefore, the temperature of 160 °C is selected as the best working temperature in the following tests.
Figure 7b,c are dynamic sensing responses to different concentrations from 0.5 to 200 ppm at 160 °C. Apparently, the responses of the sensors increase abruptly when they are exposed to acetone gas. After exposure in fresh air again, they recover to the initial value. The changes in responses to acetone concentration are plotted in Figure 7d. The mixed-phase α/γ-Fe2O3-400 sensor performs better than the pure γ-Fe2O3-350 and α-Fe2O3-450 at each concentration. For the α/γ-Fe2O3-400 sensor, a linear relationship between responses and tested acetone concentrations from 50 to 200 ppm is fitted (inset in Figure 7d) with R2 = 0.9992. In addition, γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 sensors show response values of 1.7, 2.25, and 1.2 to a low limit of detection (LOD) of 0.5 ppm acetone, demonstrating the quantitative detection for trace acetone, as shown in Figure 7b. The response and recovery times of α/γ-Fe2O3-400 are 22/13 s and 27/37 s to 200 ppm and 10 ppm acetone (Figure 7e,f). Furthermore, gas sensors must possess good repeatability and long-term stability and excellent selectivity towards practical gas detection. Figure 8a presents the reproducibility of gas sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 to 200 ppm acetone during five consecutive tests, which maintain the initial responses. For instance, the sensor based on α/γ-Fe2O3-400 shows a slight fluctuation in the range of 366–374, indicating an excellent reproducibility. Figure 8b shows long-term stability to 200 ppm acetone in 30 days, in which the response values remain relatively stable with standard deviations (S) of 5.89, 4.90 and 5.96 for γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 to 200 ppm acetone. The selectivity of our sensors has also been evaluated, as shown in Figure 8c. Some typical VOC gases including acetone, isopropanol, ethanol, methanol, toluene, benzene, formaldehyde and ammonia were tested at 160 °C with a concentration of 200 ppm. It is obvious that all three different sensors show a pronounced response to acetone over other gases, which is determined by the different stability and activity of the gases. Especially, the response of the α/γ-Fe2O3-400 gas sensor is highest, which is probably correlated with the mixed phase. Table 1 lists the acetone-sensing properties of α/γ-Fe2O3-400 prepared in this work as well as other recent reports on α-Fe2O3 and γ-Fe2O3. Numerous reports show a higher optimum working temperature and lower response. Indeed, our mixed phase of α/γ-Fe2O3 exhibits a comparable sensing performance to acetone detection.
As reported, the gas-sensing mechanism for metal oxide-based sensors usually arises from the surface chemical reaction of target gas molecules, leading to the change in resistance [43,44]. In our work, when n-type Fe2O3 materials are exposed in air, the absorbed oxygen molecules on the surface will capture electrons from the conduction band of Fe2O3 to generate active oxygen ions of O2−, O− and O2− at a certain temperature (as shown in Figure 9a and Equations (2)–(5)) [45,46]. The absorbed oxygen has been confirmed by the above XPS measurements.
O2 (gas)→O2 (ads)
O2 (ads) + e−→O2− (ads)
O2− (ads) + e−→2O− (ads)
2O− (ads) + e−→O2− (ads)
For an n-type semiconductor, the loss of electrons produces a high resistance by forming an electron depletion layer. When the gas sensor is exposed to acetone atmosphere, the reaction between these oxygen species and acetone molecules leads to the release of the captured electrons. As a result, the increase in electrons in the conduction band of Fe2O3 leads to a decrease in the electron depletion layer. The supposed chemical reaction is expressed as follows [47,48]:
CH3COCH3 + 8O−→3CO2 + 3H2O + 8e−
This process will give rise to a big change in resistance and thus endows a high response. For instance, the real resistance of the α/γ-Fe2O3-400 gas sensor changed from 0.89 MΩ in air to 0.0685 MΩ and 0.0025 MΩ in 10 ppm and 200 ppm acetone, as shown in Figure 9b,c. The excellent gas-sensing performance of mixed-phase α/γ-Fe2O3 for acetone detection is ascribed to the intrinsic nature of Fe2O3 unique nanosheets’ assembled hierarchical structure, and the formation of a dual-phase interface. Firstly, the different valences of iron ions in Fe2+ and Fe3+ could favor the transfer of electrons [24]. The merits of a large surface area with a large number of active sites is beneficial for the accommodation of more oxygen molecules and acetone gas molecules, which promotes the reaction to produce a large change in resistance. In addition, the interconnection of nanosheets could form abundant inner space to increase the pathway for gas diffusion. Last but not least, due to the difference in the crystal structure of γ-Fe2O3 and α-Fe2O3, the rich defects may be introduced by mixing them to form a dual-phase structure [49]. Furthermore, the boundary between γ-Fe2O3 and α-Fe2O3 creates a large amount of phase heterojunction interface, which will be beneficial for improving the diffusion of charge carriers. Therefore, the mixed-phase α/γ-Fe2O3 exhibits a higher gas-sensing performance over the counterparts of single-phase of γ-Fe2O3 and α-Fe2O3 for acetone detection.

4. Conclusions

In this work, we reported the synthesis of Fe-alkoxides via a facile solvothermal method, which further converts into single γ-Fe2O3 and α-Fe2O3 as well as the mixed-phase α/γ-Fe2O3 depending on the calcination temperature. The hierarchical structure is assembled by a bunch of nanosheets, which provides a large surface area and number of channels for gas molecules reaction and diffusion. Compared with the single phases of γ-Fe2O3 and α-Fe2O3, the mixed-phase α/γ-Fe2O3 sensor shows a high response of 353 to 200 ppm acetone at 160 °C with a fast response and recovery time of 22/14 s, good selectivity and long-term stability. Importantly, a linear relationship between responses and the acetone concentration from 50 to 200 ppm is fitted for the α/γ-Fe2O3 sensor, making it more practical for application. The present work offers a new vision of the crystalline-phase regulation of Fe2O3 from Fe-alkoxides to improve the sensing properties for effective acetone detection. However, it is necessary to note that more work should be done to extend the current findings for real diagnostic use in the future.

Author Contributions

Conceptualization, C.D.; methodology, R.T.; formal analysis, H.T.; funding acquisition, G.C. and H.G.; writing—original draft preparation, R.T.; writing—review and editing, C.D. and Z.Y. All authors have read and agreed to the published version of the manuscript.

Funding

Financial support from the National Natural Science Foundation of China (22165032), and the Application Basic Research Fund of Yunnan Province (2019FB129, and 202301AT070207) and the Major Science and Technology Project of Precious Metal Materials Genetic Engineering in Yunnan Province (2019ZE001-6) is greatly appreciated. The authors acknowledge the Advanced Analysis and Measurement Center of Yunnan University for the sample testing service.

Data Availability Statement

The data that support the findings of this study are available within this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 13 00810 g001 550
Figure 1. (a) The WS−30 gas−sensing measurement system, (b) basic test circuit, (c) photo of a gas sensor, and (d) schematic illustration of a gas sensor.
Figure 1. (a) The WS−30 gas−sensing measurement system, (b) basic test circuit, (c) photo of a gas sensor, and (d) schematic illustration of a gas sensor.
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Crystals 13 00810 g002 550
Figure 2. Schematic diagram of the synthesis procedure of hierarchical Fe2O3 micro-flower.
Figure 2. Schematic diagram of the synthesis procedure of hierarchical Fe2O3 micro-flower.
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Figure 3. XRD characterizations of (a) the Fe-alkoxides, (b) γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450.
Figure 3. XRD characterizations of (a) the Fe-alkoxides, (b) γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450.
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Figure 4. FESEM images of (a,c) Fe-alkoxides, and (b,d) the mixed phase of α/γ-Fe2O3-400.
Figure 4. FESEM images of (a,c) Fe-alkoxides, and (b,d) the mixed phase of α/γ-Fe2O3-400.
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Figure 5. (a–c) TEM, (d,e) HRTEM, (f) SAED images, and EDS maps of (h) Fe, and (i) O for selected area of (g), and (j) the EDS spectra of α/γ-Fe2O3-400.
Figure 5. (a–c) TEM, (d,e) HRTEM, (f) SAED images, and EDS maps of (h) Fe, and (i) O for selected area of (g), and (j) the EDS spectra of α/γ-Fe2O3-400.
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Figure 6. (a) TGA and DSC curves of precursors, and (b) nitrogen adsorption–desorption isotherms of mixed-phase α/γ-Fe2O3-400, and high-resolution XPS spectra of α/γ-Fe2O3-400, (c) O 1s and (d) Fe 2p.
Figure 6. (a) TGA and DSC curves of precursors, and (b) nitrogen adsorption–desorption isotherms of mixed-phase α/γ-Fe2O3-400, and high-resolution XPS spectra of α/γ-Fe2O3-400, (c) O 1s and (d) Fe 2p.
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Figure 7. (a) Temperature-dependent responses of the gas sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 to 200 ppm, (b,c) response–recovery curves of the gas sensors to various acetone concentrations from 0.5 ppm to 200 ppm, (d) responses as a function of acetone concentration (inset is the linear relationship between the responses and concentration of the α/γ-Fe2O3-400), response and recovery time of α/γ-Fe2O3-400 to (e) 200 ppm acetone and (f) 10 ppm acetone. All sensing properties were tested at optimal operating temperature of 160 °C.
Figure 7. (a) Temperature-dependent responses of the gas sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 to 200 ppm, (b,c) response–recovery curves of the gas sensors to various acetone concentrations from 0.5 ppm to 200 ppm, (d) responses as a function of acetone concentration (inset is the linear relationship between the responses and concentration of the α/γ-Fe2O3-400), response and recovery time of α/γ-Fe2O3-400 to (e) 200 ppm acetone and (f) 10 ppm acetone. All sensing properties were tested at optimal operating temperature of 160 °C.
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Figure 8. (a) Repeatable response–recovery curves of gas sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 over five cycles of exposure to 200 ppm acetone, (b) Long-term stability of gas sensors to 200 ppm acetone at 160 °C (S represents standard deviation), and (c) a comparison of the response for gas sensors towards 200 ppm different gases.
Figure 8. (a) Repeatable response–recovery curves of gas sensors based on γ-Fe2O3-350, α/γ-Fe2O3-400 and α-Fe2O3-450 over five cycles of exposure to 200 ppm acetone, (b) Long-term stability of gas sensors to 200 ppm acetone at 160 °C (S represents standard deviation), and (c) a comparison of the response for gas sensors towards 200 ppm different gases.
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Figure 9. (a) Schematic illustration of the acetone-sensing mechanism, and the real resistance changes in α/γ-Fe2O3-400 gas sensor to acetone with a concentration of (b) 10 ppm and (c) 200 ppm at 160 °C.
Figure 9. (a) Schematic illustration of the acetone-sensing mechanism, and the real resistance changes in α/γ-Fe2O3-400 gas sensor to acetone with a concentration of (b) 10 ppm and (c) 200 ppm at 160 °C.
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Table
Table 1. Comparison of reported gas sensor based on α-Fe2O3 and γ-Fe2O3 to acetone.
Table 1. Comparison of reported gas sensor based on α-Fe2O3 and γ-Fe2O3 to acetone.
MaterialsT
(°C)		C
(ppm)		Response (β)tres/trec (s)LODRef.
Porous α-Fe2O330010026.36/<10010[39]
Nanorods α-Fe2O328010032.50.4/2.410[40]
Nanoparticles α-Fe2O33401009.1--5[22]
Nanospheres α-Fe2O3170100162/500.1[41]
Microrods γ-Fe2O3220100125.50.9/1510[24]
Nanocubes γ-Fe2O3350100161--10[42]
Flower-like α/γ-Fe2O316020035322/140.5 ppmThis work
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Tian, R.; Tan, H.; Chen, G.; Guan, H.; Dong, C.; Yin, Z. Novel Mixed-Phase α/γ-Fe2O3 Micro-Flower Assembled with Nanosheets for Enhancing Acetone Detection. Crystals 2023, 13, 810. https://doi.org/10.3390/cryst13050810

AMA Style

Tian R, Tan H, Chen G, Guan H, Dong C, Yin Z. Novel Mixed-Phase α/γ-Fe2O3 Micro-Flower Assembled with Nanosheets for Enhancing Acetone Detection. Crystals. 2023; 13(5):810. https://doi.org/10.3390/cryst13050810

Chicago/Turabian Style

Tian, Ruonan, Huai Tan, Gang Chen, Hongtao Guan, Chengjun Dong, and Zongyou Yin. 2023. "Novel Mixed-Phase α/γ-Fe2O3 Micro-Flower Assembled with Nanosheets for Enhancing Acetone Detection" Crystals 13, no. 5: 810. https://doi.org/10.3390/cryst13050810

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