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Microwave-assisted solvothermal synthesis of shape-controlled CoFe2O4 nanoparticles for acetone sensor
Journal of Alloys and Compounds 788 (2019) 1103e1112
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Microwave-assisted solvothermal synthesis of shape-controlled CoFe2O4 nanoparticles for acetone sensor Hai-Jun Zhang a, b, Li-Zhu Liu a, *, Xiao-Rui Zhang a, Shuang Zhang b, Fan-Na Meng b a b
Harbin University of Science and Technology, Harbin, 150080, China Heilongjiang University of Science and Technology, Harbin, 150027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2018 Received in revised form 28 February 2019 Accepted 1 March 2019 Available online 2 March 2019
Acetone sensor is very important to human health. However high sensitivity to trace acetone and low working temperature are highly required. In this work, we report a facile method for the preparation of CoFe2O4 nanoparticles with a small size of 10 nm. The CoFe2O4 exhibited high sensitivity to acetone. Furthermore, CoFe2O4 nanoparticles fabricated through microwave-assisted solvothermal method have higher sensitive property than that synthesized by solvothermal method. Such outstanding acetone gas sensing property might be attributed to its grain size, surface adsorbed oxygen, specific surface area, structure characteristic of spinel ferrites and the possible sensing mechanism is also discussed. © 2019 Published by Elsevier B.V.
Keywords: CoFe2O4 nanoparticles Microwave solvothermal method Acetone sensing characteristics
1. Introduction Zinc oxide, tin oxide and iron oxide are the three major series of practical gas sensitive substrate material, but the gas-sensing properties of single oxide are not very ideal, such as sensitivity, selectivity, stability, etc [1e7]. Spinel AB2O4 composite oxide is a kind of important inorganic non-metallic materials and it has good gas sensitive properties [8e14]. Exploiting high-performance gas sensors based on spinel ferrite materials has been attracting researchers' attentions attribute to chemical composition and structure features of spinel compounds. The divalent transition metal ion which incorporated into the lattice of the parent structure, is oxidized into trivalent metal ions due to hopping of electrons [15e20]. Cobalt ferrite is a particularly interesting semiconductor material in spinel ferrite materials. Cobalt ferrite embodies the characteristics of n-type or p-type semiconductor which is determined by the predominant conduction mechanism, due to the migration of electrons from Fe2þ to Fe3þ or the migration of holes from Co2þ to Co3þ [21]. Bhosale and Chu research teams [22,23] both claimed that CoFe2O4 prepared under different conditions exhibited different conducting behaviors. As a typical semiconductor metal oxide, cobalt ferrite is widely used in gas sensor. Some researchers
* Corresponding author. E-mail address: [email protected] (L.-Z. Liu). https://doi.org/10.1016/j.jallcom.2019.03.009 0925-8388/© 2019 Published by Elsevier B.V.
have previously reported the application of CoFe2O4 as gas sensors. Bodade et al. reported Ni and Sn doped CoFe2O4 prepared by twostep solegel method, was highly selective to H2S with good response at 200 C [24]. Bagade et al. prepared thin films of cobalt ferrite using simple chemical spray pyrolysis technique on the quartz substrates and found that the gas sensor response can reach 95% for 80 ppm NO2 at the operating temperature of 150 C [25,26]. Kumar synthesized Mn substituted CoFe2O4 nanoparticles by two methods of the auto-combustion and evaporation method. The response of the auto-combustion sample showed higher response of 0.19e1000 ppm of LPG gas at an optimum operating temperature of 250 C than that of the evaporation sample [27]. Lin's group observed that the response value of the sensor based on CoFe2O4@SiO2@In2O3 nanocomposite microspheres can detect 10 ppm acetone at 260 C [28]. Khandekar et al. prepared Ce doped CoFe2O4 nanoparticles, which has selectivity towards acetone gas and the response value is 1.38e100 ppm acetone gas at the optimized operating temperature of 225 C [29]. Moreover, Cobalt ferrite exhibits high sensitivity and selectivity to NH3 at 450 C as sensing electrode [30]. Otherwise, few publications reported the application of CoFe2O4 as gas sensor of acetone. Herein, we first reported a modified solvothermal method to prepare CoFe2O4 nanoparticles, which can remarkably decrease the reaction time and effectively control the particle size through the combination of solvothermal and microwave processes. In order to demonstrate the potential applications of gas sensing, the assynthesized samples were used to fabricate gas sensor. As
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expected, the measurement results showed that it exhibited good sensing properties to acetone gas at low temperature and the influencing factors on the sensors performance were investigated in details. 2. Experiment section CoFe2O4 nanoparticles were synthesized via a modified solvothermal method. In brief, 1.4 g of iron acetylacetonate (Fe (C5H47O2)3$H2O, >95%) and 0.7 g of cobalt acetylacetonate (Co(C5H7O2)2$H2O, >95%) were dissolved in a beaker, which contained 60 ml ethanol. Then the mixed solution was continuous stirred for 20.0 min at room temperature and put into autoclaves with a capacity of 70 mL vessels, which was maintained at the microwave-solvothermal condition of 800 W power, 150 C for 3 h. After heating, the resulting products were collected by centrifugation and then washed several times with deionized water and ethanol. Finally, powder slurry was freeze dried overnight at 35 C and collected. In order to understand the effect of microwave on the preparation of CoFe2O4 nanoparticles, we prepared CoFe2O4 nanoparticles only by normal solvothermal method as mentioned in the literature [31]. The samples obtained were denoted as SM and S respectively. The phase of the as-obtained product was identified using Xeray powder diffraction (XRD, D/max 2550 V, Cu Ka radiation with the wavelength l ¼ 0.1546 nm) at 40 kV and 150 mA in a scanning range of 20e80 (2q). The morphology and size of the prepared samples were investigated by transmission electron microscopy (TEM, JEOL 2010) with a Gatan Ultrascan 100 TEM camera for selective area electron diffraction patterns (SAED) and highresolution TEM (HRTEM) images. The elemental composition of the material was examined by X-Ray photoelectron spectrometer (XPS, MULTILAB-2000 Base system with XR4 Twin Anode Mg/Al (300/400 W) X-Ray spectrometer). Surface properties of the nanoparticles were studied by the BrunauereEmmetteTeller (BET) method via nitrogen adsorption and desorption measurements (3H-2000PS2). Pore diameter and the pore size distributions were calculated by the BarreteJoynereHalenda (BJH) method from the desorption isotherm. Gas sensors were based on two samples: CoFe2O4 nanoparticles with different particle sizes prepared by solvothermal and microwave-assisted solvothermal method. The fabrication and measuring principle of the gas sensor were similar with those described in our previous reports [32,33]. Briefly, the powder prepared was mixed with ethanol in a weight ratio of 3:1 and ground in an agate mortar for 20 min to form a paste. The paste of the thick sensing film (with a thickness of about 0.3 mm) was evenly brushed on a ceramic tube with a particularly small bristle brush. Nickel chrome resistor with 27e30 U in the ceramic tube was used to control the working temperature of the sensor by varying the heating voltage during detection. The film was heat-treated for 24 h at an aging temperature of 300 C after coating to keep the sensor stability. Fig. 1(a) displayed the sketch of the gas-sensor structure. The actual graph and the measurement principle of gas sensor were also showed in Fig. 1(b). RL denotes as a constant load resistor connected in series with the gas sensor, RS denotes the resistance of the sensing material which could be adjusted in different gas. For the equal current in a series circuit, the resistance of RS can be calculated by measuring the voltage of RL. The Gas-sensing properties of sensor were tested by WS-30A gas sensing measurement system (Wei Sheng Electronics Science and Technology Co. Ltd., Henan Province, China) under a static process with a test chamber of 18 L and 30 testing channels. The sensor response (S) was defined as: S¼ Ra/Rg, where Ra and Rg are the sensor resistance values of the sensor measured in air and the target gas, respectively. The
Fig. 1. (a) Sketch of the gas-sensor structure (b) actual graph of gas sensor and the measurement principle of gas sensors.
response time is defined as the time taken by the sensor to achieve 90% of its maximum sensing response upon test gas injection, whereas the recovery time is the time taken by the sensor to reach 10% of its initial resistance after the removal of the gas.
3. Results and discussion 3.1. Structure and characterization of CoFe2O4 nanoparticles The crystal structures and purity of all the products were
Fig. 2. XRD patter of CoFe2O4 nanoparticles.
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characterized by powder X-ray diffraction. Fig. 2 shows the XRD pattern of CoFe2O4 nanoparticles. The diffraction peaks at 2q values of (220), (311), (400), (422), (511), (440) and (533) match with JCPDS 22-1086, which confirms the formation of CoFe2O4. No other peaks from impurities are detected, which indicates CoFe2O4 nanoparticles have high crystalline purity. At the same time, we can observe that the diffraction peak of sample SM is wider than that of sample S, suggesting that the particle size of sample SM is smaller than that of S. The average crystalline size of CoFe2O4 nanoparticles can be calculated by Scherrer's equation [34].
d¼
0:9l b cos q
(1)
Where l is the wavelength of X-ray, b is the full width at half maximum which was determined from the experimental integral peak width of the (311) peak, q is diffraction angle, d is the crystallite size. The average crystallite size of S and SM were calculated as 23.5 and 9.8 nm, respectively. The morphology and sizes of the obtained samples were further investigated by transmission electron microscopy (TEM). Fig. 3 shows TEM images with different magnification. From TEM images with the low magnification (Fig. 3(a), (c)), it can be found that the as-synthesized nanoparticles were spherical shape. TEM image with high magnification (Fig. 3(b)) can reveal that there is a little agglomeration phenomenon in sample S and the size is uneven. The inset of Fig. 3(b) is the particle size distribution of sample S, which reveals that the average diameter of S is 23.5 nm. Furthermore, more details of sample SM can be observed from Fig. 3(d), the assynthesized nanoparticles are very uniform. Their average size is
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9.8 nm, which is in accordance with the XRD measurement. By freeze-drying, there is no obvious agglomeration of samples in TEM images of Fig. 3. Lots of small pores may be presented in CoFe2O4 nanoparticles for loose contact of nanoparticles. In order to further obtain the information about the as-prepared samples, the BET surface area and pore-size distribution are investigated based on nitrogen adsorption-desorption isotherms as shown in Fig. 4. The sample SM (Fig. 4(b)) which shows a BET surface area of 90.11 m2g1, is greatly larger than sample S (53.24 m2g1, Fig. 4(a)). The cumulative pore volumes were calculated via the desorption branch of the nitrogen isotherms in terms of BJH method, exhibiting cumulative pore volumes of 0.383 and 0.382 cm3g1, respectively. It could be found that freeze drying method makes the particles equally distributed and effectively prevents agglomeration [35]. The insets in Fig. 4 exhibit the corresponding pore size distribution of the nanoparticles. The pore size of nanoparticles mainly distributes in about 10 and 18 nm, which also can be speculated that there are differences in particle size and they are consistent with the TEM images. Further details of the structure sample SM are shown in Fig. 5. The d-values obtained from the analysis of the lattice fringes is ~0.214 nm which corresponds to the (400) reflections of CoFe2O4 (JCPDF 22-1086) by HRTEM image Fig. 5(a). There are the equally and clear lattice planes in HRTEM image, which shows that CoFe2O4 nanoparticles prepared has a high degree of crystallinity. The diffraction rings show in SAED image of Fig. 5(b), are assigned to (311), (400), (511), (440) of CoFe2O4, suggesting the polycrystalline characteristics of the nanoparticles. The Energy Dispersion X-ray analysis spectrometry (EDS) spectrum is shown in Fig. 5(c). It is confirmed that the prepared SM shows the existence of iron, oxygen
Fig. 3. Structural characterization of CoFe2O4 nanoparticles (a) (c) low magnification TEM images of S and SM (b) (d) high magnification TEM images and distribution of particle size (inset) of S and SM.
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Fig. 4. Nitrogen adsorption-desorption isotherms of CoFe2O4 nanoparticles (a) The sample S (b) The sample SM. Insets showing the BJH pore-size distribution.
Fig. 5. CoFe2O4 nanoparticles SM (a) HRTEM image (b) SAED pattern (c) Energy dispersive X-ray spectrum.
and cobalt. The Ka lines of iron and oxygen in the energy dispersion energy spectrum are observed at 0.53 and 6.4 eV respectively. Besides, cobalt Ka line is observed at 6.9 eV. A typical nanoparticle of the cobalt-ferrite analyzed and the Co/Fe atomic ratio of asproduced nanoparticles is near 1/2, thus the initial formula should be CoFe2O4. The chemical constituents and elemental valence states of samples were analyzed by XPS in Fig. 6. The survey of XPS spectrum clearly indicated the existence of elemental Co, Fe, and O, which was consistent with the XRD and EDS results. All the results before analysis were calibrated with the binding energy of the C 1s peak at 284.6 eV. The spectrum for Fe 2p (Fig. 6 (a)) contained two strong peaks centered at 712.7 and 725.5 eV is assigned to Fe 2p3/2 and Fe 2p1/2 and the satellite peaks at 718.6 eV, which is consistent with
Fe 2p binding energy for cobalt ferrite nanoparticles. In addition to the binding energy at 711.2 and 725 eV which can confirm the existence of Fe3þ species in CoFe2O4. In detail, the Fe 2p3/2 binding energy at 713.4 eV is due to the contributions from Fe3þ ions in tetrahedral sites. The Fe 2p3/2 binding energy at 711.2 eV and Fe 2p1/2 binding energy at 725 eV are due to the contributions from Fe3þ ions in octahedral sites. It should be noted that Fe3þ will also be reduced to Fe2þ with the presence of oxygen vacancies. It has been reported that conductivity in CoFe2O4 will occur due to electron hopping between Fe ions existing in different valence states (Fe2þ and Fe3þ) on equivalent lattice sites [36,37]. As shown in the Co 2p spectrum (Fig. 6(b)), two peaks located at 781.1 and 786.9 eV correspond to Co 2p3/2, while another two peaks at 797.1 and 803.6 eV can be attributed to Co 2p 1/2. The Co 2p spectrum
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Fig. 6. High resolution XPS spectrum of SM (a) Fe 2p (b) Co 2p; High resolution XPS O 1s spectrum (c) S (d) SM.
indicates Co exist in 2 þ oxidation state because the low spin Co3þ cations can only give rise to much weaker satellite features than high spin Co2þ cations with unpaired valence 3d electron orbitals [38]. According to the XPS data, the atomic ratio of Fe to Co is 2.1:1, indicating the excess of iron on the material surface. Fig. 6(c and d) show that the spectrum diagram of the O 1s to study the oxidation state of oxygen and oxygen vacancies of CoFe2O4 nanoparticles, which fitted using Gaussian functions and divided into two peaks of 530.6 eV and 532.2 eV. The peak marked O1 is attributed to oxygen vacancies and surface hydroxyl group. The peak O2 is assigned to the lattice oxygen [39]. In addition, the fraction of oxygen vacancies (0.35) in SM was higher than that in S (0.22), which was obtained from the relative integrated peak area. We all know that oxygen vacancies play an important role in the gas-sensing performances of sensor [8]. As we know, the decrease of particle diameter, high BET surface and a lot of oxygen vacancies could help to improve electron transfer and enhance gas sensitivity, which can be useful to application such as gas sensor and will be verified in the following gas sensitive test.
temperatures ranging from 150 to 320 C. The optimum operating temperatures of two sensors were 220 C. Fig. 8 shows the dynamic response characteristics of the samples S and SM when they are exposed to various concentrations of acetone gas ranging from 50 to 500 ppm at the optimum operating temperatures, which demonstrate an evident tendency that the responses increase with the tested gas concentrations. The responses of SM and S sensors to acetone with concentration of 100 ppm reached the maximum value of about 17.3 and 5.5, respectively. Furthermore, the concentration of acetone increases
3.2. Gas sensing properties of CoFe2O4 nanoparticles The gas sensors of sample S and sample SM were fabricated and the gas sensing performances were assessed by their resistance changes upon exposure to target gases with a controlled gas concentration. It is well known that the most important properties of a gas sensor are sensitivity, operating temperature, reproducibility, selectivity and long-term stability [40]. The working temperature is a key parameter among these factors, which determines the quality of the gas sensor [31]. Fig. 7 shows that the sensor responses of CoFe2O4 nanostructures to 100 ppm acetone gas at the
Fig. 7. The sensor response of CoFe2O4 nanoparticles to 100 ppm acetone gas at different working temperatures.
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Fig. 8. (a) The sensor response of sample S and SM to different concentrations acetone at 220 C (b) The response of the two sensors to different acetone concentrations.
and the response value of the sensing material increases accordingly, while the sensors do not saturate less than 500 ppm, as shown in Fig. 8 (a). The SM sensor displays much higher response values to different acetone vapor concentration than that of S sensor, which consists well with the results of BET analysis. The sensor response of sample S is very low and is only 3.7e50 ppm acetone gas in Fig. 8(b), which is less than half of sample SM. Obviously, it is confirmed that the acetone sensor based on SM shows higher gas response to acetone gas, which sensor response value can reach 11.8e50 ppm acetone gas. So, we take the SM sensor as the main research object to analyze its gas sensitivity characteristics. In order to find the lower detection limit and make better use of it, we have tested at a low concentration acetone gas to the SM sensor in Fig. 9. It is worth to note that the response to 5 ppm acetone vapor achieves 3.2, showing practical application potential for low concentration acetone vapor detection. As shown in Fig. 10, all response and recovery time of samples at the optimum working temperature are less than 30 s to either high or low concentration acetone gas. For example, the response and recovery time of SM sensor decreased to 7 and 5 s to 10 ppm acetone gas at a working temperature of 220 C, as shown in Fig. 10(c). Selectivity is the ability that a gas sensor to distinguishes with different kinds of gases, which is also important for evaluating the gas sensing properties of sensor. Fig. 11 depicts the histogram for the sensors which are made of two samples at optimum operating temperature of 220 C for different target gases with a
concentration of 100 ppm. The response of two sensors to acetone gas are the highest comparing with gases such as H2S, H2, ammonia and ethanol, indicating both good selectivity to acetone gas. It is obvious that sample SM has better selectivity to acetone gas than sample S. It can be found that SM has remarkably smaller response values to H2S, H2, ammonia and ethanol, Sacetone/SH2S, Sacetone/SH2, Sacetone/Sammonia and Sacetone/Sethanol were 12.3, 15.7, 15.6 and 3.8, respectively. Therefore, the sample SM has good selectivity to acetone gas. Fig. 12 depicts the long-term stability results of as-prepared SM sensor measured for 100 ppm acetone under their optimum working temperature every several days. The response of SM based sensor changes over time but nearly fluctuated around the initial value and the maximal deviations of the response to acetone are less than 5.3% within two months, which did not decrease obviously. The results show that the sensor responses can be duplicated for a long time. The reproducibility of SM based sensor was further investigated by successively exposing the sensor to 100 ppm acetone gas for 4 cycles at 220 C, and the corresponding curve of the responseerecovery characteristic is shown in Fig. 12 (inset). In every test cycle, we can observe that the recovery time is a few second and the sensing material came back to the initial state, indicating that the sensor based SM possessed good reproducibility. Hence, the relevant acetone sensing performances of gas sensors were illustrated in Table 1 to compare the sensing performances of CoFe2O4 with other works. It is confirmed that the acetone sensor based on CoFe2O4 nanoparticles prepared by this improved method, shows higher gas response and lower working temperature than most of those reported in the literatures [41e48]. The response value of PANI/a-Fe2O3 nanocomposite in the 48th reference is very close to 17.3 in this work to 100 ppm acetone. Although its response temperature (180 C) is lower than 220 C, its two-step preparation method is more complicated. 3.3. Gas sensing mechanism
Fig. 9. The sensor response of sample SM to low concentration acetone at 220 C.
As a typical n-type semiconductor, the gas sensing mechanism of CoFe2O4 nanoparticles is based on the changes of the resistance before and after being exposed to the test gas, the gas sensing mechanism is schematic illustrated in Fig. 13. Oxygen species are chemisorbed onto CoFe2O4 nanoparticles and ionized by capturing free electrons from conduction band in air (Fig. 13(a)). In a low temperature (<100 C), oxygen molecules adsorbs on the surface of the sensor and forms absorbed oxygen. Then, absorbed oxygen is ionized by the electrons in the sensing materials conduction band to form O 2 (ads), as shown in equations (2) and (3). With the temperature exceeds 100 C and at the range 2 of 100e300 C, O 2 (ads) continue be ionized to form Oads , which is
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Fig. 10. The response and recovery curves of samples to different concentration acetone gas at 220 C (a) S (b) SM (c) SM.
O2 ðadsÞ þ e /O 2 ðadsÞ
(3)
O 2 ðadsÞ þ e /2O ðadsÞ
(4)
Furthermore, the grain size also plays an important role in the gas sensing response enhancement, based on the grain-control model [5]. The electrons almost completely depleted to participate in gas sensing reaction for the small size, which leads to CoFe2O4 nanoparticles highly sensitive to low concentration acetone gas. As shown in Fig. 13(b), when the sensor is exposed to acetone gas, the chemisorbed oxygen ions will react with acetone to form CO2 and H2O, which can be described as equation (5). CH3COCH3 þ 8O (ads) / 3CO2 þ 3H2O þ 8ee
Fig. 11. The response values of samples to 100 ppm different gases at 220 C.
Fig. 12. The long-term stability of sample SM to 100 ppm acetone gas for 60 days, the inset is four test cycles of the sensor resistance to 100 ppm acetone gas at the working temperature of 220 C.
shown in equation (4) [49]. This produces a resistance layer from an electron depleted space charge on the n-type nanoparticle surface and higher resistance. O2(gas) / O2 (ads)
(2)
(5)
The reactions will cause a decrease of the resistance since the trapped electrons can be released back to the conduction band of CoFe2O4, resulting in a decrease of the thickness in the depletion region and an increase in the carrier concentration. Therefore, the adsorption-desorption based kinetics in a chemical sensing process is actuated here by the increased surface area and small size of the sample SM, which can provide more active sites for sensing reactions as compared to the sample S. Structure characteristic of spinel ferrites is also one important affecting factor to improve sensitivity. For CoFe2O4, Co2þ ions greatly prefer the octahedral sites while the Fe3þ ions prefer both octahedral and tetrahedral sites. The oxidationereduction reaction of Fe2þ/Fe3þ and Co2þ/Co3þ on the surface of n-type spinel ferrite surface is reversible [50]. A greater Fe2þ concentration increases e conductivity. After chemisorption of oxygen on the n-type spinel ferrite, Fe2þ is oxidized to Fe3þ. When oxygen reacts with gas, e is released back to the material and Fe3þ is reduced to Fe2þ, thus the amount of Fe3þ and Fe2þ pairs and over-all electrical conductivity are increased. A greater amount of Fe2þ increases the oxygen chemisorption capacity, which has been demonstrated in the literature based on the preference energies for divalent and trivalent ions in the spinel structure [51]. However, the concentration of Fe2þ plays an important role in gas sensitivity. Sutka et al. reported that the response of the sensor is reduced since the Fe2þ concentration is too high [52]. Oxygen vacancy is considered as an important reagent of many adsorbents. Therefore, many surface reactions are affected by this point defect. Oxygen vacancy is not only the direct adsorption site, but also the electron donor site. Every oxygen deficient site can offer two electrons and oxygen molecules on the sensing material surface take electrons and become oxygen ions, which can lead to the electrical conductance decrease and thus the resistance of the sensor increase [53,54]. There are a lot of oxygen vacancies on the surface of sample SM and the ratio of oxygen vacancy reaches 35% of the total oxygen, which is calculated by XPS. Oxygen vacancies play an important role in the gas-sensing performances of sensor and can drastically alter the
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Table 1 Comparison of the acetone sensing performance of the current work with other reported literatures. Materials
Acetone
temperature
Gas Response
Reference
SnO2 nanoparticles EuFeO3 nanoparticles Porous -Fe2O3 nanotubes Fe2O3 nanoparticles Fiber-like Co3O4 Hierarchical In2O3 a-Fe2O3 nanoparticles PANI/a-Fe2O3 nanocomposite CoFe2O4 nanoparticles (S) CoFe2O4 nanoparticles (SM)
50 ppm 100 100 100 100 50 100 100 100 100
310 295 240 340 160 260 250 180 220 220
11.4 6 11 8.8 4 8.1 9.5 16 5.5 17.3
[41] [42] [43] [44] [45] [46] [47] [48] This work This work
Fig. 13. A schematic energy band diagram (band bending) of CoFe2O4 nanoparticles (a) before and (b) after chemisorptions of charged species (acetone gas). EC, EV, and EF represent the conduction band, valence band, and the Fermi level energy, respectively.
surface electronic properties of materials. Therefore, the as prepared CoFe2O4 nanoparticles by this method with considerable oxygen deficient sites are promising for the gas sensing application. Based on the discussion above, both SM and S have good selectivities to acetone. However, SM has higher response value than S. The reasons can be explained by the smaller size, larger content of oxygen vacancy.
nanoscale. Furthermore, the CoFe2O4 nanoparticles exhibits the best gas sensing properties to acetone vapor at the low working temperature of 220 C. The CoFe2O4 nanoparticles can detect acetone gas down to 5 ppm and the sensor response can be up to 17.3e100 ppm acetone gas. From the observation of sensing response, we conclude that CoFe2O4 nanoparticles can be efficiently used as a chemical sensor for the sensing of low concentration acetone vapor at a low temperature.
4. Conclusions Acknowledgments In general, an uncomplicated synthesis route was developed for the preparation of uniform CoFe2O4 nanoparticles, which are homodispersed and less than 10 nm. The strategy presented here could be expended as a general method to synthesize other transition bimetallic oxide nanoparticles by rational designation in
We thank the National Science and Technology Support Program of China (Grant No. 2013BAE04B02) and also Heilongjiang University of science and technology college students' scientific research project.
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Alloys Compd. 657 (2016) 414e421. [26] A.A. Bagade, V.V. Ganbavle, S.V. Mohite, T.D. Dongale, B.B. Sinha, K.Y. Rajpure, Assessment of structural, morphological, magnetic and gas sensing properties of CoFe2O4 thin films, J. Colloid Interface Sci. 497 (2017) 181e192. [27] E.R. Kumar, A.S. Kamzin, K. Janani, Effect of annealing on particle size, microstructure and gas sensing properties of Mn substituted CoFe2O4 nanoparticles, J. Magn. Magn. Mater. 417 (2016) 122e129. [28] G. Lin, H. Wang, X.F. Li, X.Y. Lai, Y.Z. Zou, X.F. Zhou, D. Liu, J.W. Wan, H. Xin, Chestnut-like CoFe2O4@SiO2@In2O3 nanocomposite microspheres with enhanced acetone sensing property, Sens. Actuators, B 255 (2018) 3364e3373.
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Jiang, Highly sensitive acetone gas sensor based on ultrafine a-Fe2O3 nanoparticles, Sens. Actuators, B 238 (2017) 923e927. [45] Y.L. Lin, H.M. Ji, Z.R. Shen, Q.Q. Jia, D.H. Wang, Enhanced acetone sensing properties of Co3O4 nanosheets with highly exposed (111) planes, J. Mater. Sci. Mater. Electron. 27 (2016) 2086e2095. [46] S.M. Wang, J. Cao, W. Cui, X.F. Li, D.J. Li, Facile synthesis and high acetone gas sensing performances of popcorn-like In2O3 hierarchical nanostructures, Mater. Lett. 186 (2016) 256e258. [47] J.T. Li, L.W. Wang, Z. Liu, Y.H. Wang, S.L. Wang, Au-modified a-Fe2O3 columnar superstructures assembled with nanoplates and their highly improved acetone sensing properties, J. Alloys Compd. 728 (2017) 944e951. [48] L.W. Wang, H. Fu, H.J. Liu, K.F. Yu, Y.H. Wang, J.M. Ma, In-situ packaging ultrauniform 3D hematite nanotubes by polyaniline and their improved gas sensing properties, Mater. Res. Bull. 107 (2018) 46e53. [49] W. Zhang, Y.B. Shen, J. Zhang, H.S. Bi, S.K. Zhao, P.F. Zhou, C. Han, D.Z. Wei, N. Cheng, Low-temperature H2S sensing performance of Cu-doped ZnFe2O4 nanoparticles with spinel structure, Appl. Surf. Sci. 470 (2019) 581e590. [50] A. Sutka, G. Mezinskis, M. Zamovskis, D. Jakovlevs, I. Pavlovska, Monophasic ZnFe2O4 synthesis from a xerogel layer by auto combustion, Ceram. Int. 39 (2013) 8499e8502. [51] D.S. Mathew, R.S. Juang, An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions, Chem. Eng. J. 129 (2007) 51e65. [52] A. Sutka, G. Mezinskis, A. Lusis, D. Jakovlevs, Influence of iron nonstoichiometry on spinel zinc ferrite gas sensing properties, Sens. Actuators, B 171e172 (2012) 204e209. [53] J.J. Liu, G. Chen, Y.G. Yu, Y.L. Wu, M.J. Zhou, H.Q. Zhang, Chade Lv, Y. Zheng, F. He, Controllable synthesis of In2O3 octodecahedra exposing {110} facets with enhanced gas sensing performance, RSC Adv. 55 (2015) 44306e44312. [54] T. Jinkawa, G. Sakai, J. Tamaki, N. Miura, N. Yamazoe, Relationship between ethanol gas sensitivity and surface catalytic property of tin oxide sensors modified with acidic or basic oxides, J. Mol. Catal. A Chem. 155 (2000) 193e200.
H. J. Zhang is currently studying for Ph.D. degree at Harbin University of Science and Technology. He has been an associate professor at Heilongjiang University of Science and Technology since 2012. Her main research interest is in the development of nanostructured materials for their applications.
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L. Z. Liu has been a professor at Harbin University of Science and Technology since 2004. He received his Ph.D. degree in 2006 from Harbin University of Science and Technology. His research interests focus on the nanostructured materials for the nanodevice applications.
X. R. Zhang has been a lecturer at Harbin University of Science and Technology since 2016. He received her Ph.D. degree in 2015 from Harbin University of Science and Technology. His research interests focus on the nanostructured materials for the nanodevice applications.
S. Zhang is currently studying for bachelor's degree at Heilongjiang University of Science and Technology. His main research interest is in the development of nanostructured materials for their applications.
F. N. Meng has been an associate professor at Heilongjiang University of Science and Technology since 2015. She received her Ph.D. degree in 2015 from Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microwave-assisted solvothermal synthesis of shape-controlled CoFe2O4 nanoparticles for acetone sensor Hai-Jun Zhang a, b, Li-Zhu Liu a, *, Xiao-Rui Zhang a, Shuang Zhang b, Fan-Na Meng b a b
Harbin University of Science and Technology, Harbin, 150080, China Heilongjiang University of Science and Technology, Harbin, 150027, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 December 2018 Received in revised form 28 February 2019 Accepted 1 March 2019 Available online 2 March 2019
Acetone sensor is very important to human health. However high sensitivity to trace acetone and low working temperature are highly required. In this work, we report a facile method for the preparation of CoFe2O4 nanoparticles with a small size of 10 nm. The CoFe2O4 exhibited high sensitivity to acetone. Furthermore, CoFe2O4 nanoparticles fabricated through microwave-assisted solvothermal method have higher sensitive property than that synthesized by solvothermal method. Such outstanding acetone gas sensing property might be attributed to its grain size, surface adsorbed oxygen, specific surface area, structure characteristic of spinel ferrites and the possible sensing mechanism is also discussed. © 2019 Published by Elsevier B.V.
Keywords: CoFe2O4 nanoparticles Microwave solvothermal method Acetone sensing characteristics
1. Introduction Zinc oxide, tin oxide and iron oxide are the three major series of practical gas sensitive substrate material, but the gas-sensing properties of single oxide are not very ideal, such as sensitivity, selectivity, stability, etc [1e7]. Spinel AB2O4 composite oxide is a kind of important inorganic non-metallic materials and it has good gas sensitive properties [8e14]. Exploiting high-performance gas sensors based on spinel ferrite materials has been attracting researchers' attentions attribute to chemical composition and structure features of spinel compounds. The divalent transition metal ion which incorporated into the lattice of the parent structure, is oxidized into trivalent metal ions due to hopping of electrons [15e20]. Cobalt ferrite is a particularly interesting semiconductor material in spinel ferrite materials. Cobalt ferrite embodies the characteristics of n-type or p-type semiconductor which is determined by the predominant conduction mechanism, due to the migration of electrons from Fe2þ to Fe3þ or the migration of holes from Co2þ to Co3þ [21]. Bhosale and Chu research teams [22,23] both claimed that CoFe2O4 prepared under different conditions exhibited different conducting behaviors. As a typical semiconductor metal oxide, cobalt ferrite is widely used in gas sensor. Some researchers
* Corresponding author. E-mail address: [email protected] (L.-Z. Liu). https://doi.org/10.1016/j.jallcom.2019.03.009 0925-8388/© 2019 Published by Elsevier B.V.
have previously reported the application of CoFe2O4 as gas sensors. Bodade et al. reported Ni and Sn doped CoFe2O4 prepared by twostep solegel method, was highly selective to H2S with good response at 200 C [24]. Bagade et al. prepared thin films of cobalt ferrite using simple chemical spray pyrolysis technique on the quartz substrates and found that the gas sensor response can reach 95% for 80 ppm NO2 at the operating temperature of 150 C [25,26]. Kumar synthesized Mn substituted CoFe2O4 nanoparticles by two methods of the auto-combustion and evaporation method. The response of the auto-combustion sample showed higher response of 0.19e1000 ppm of LPG gas at an optimum operating temperature of 250 C than that of the evaporation sample [27]. Lin's group observed that the response value of the sensor based on CoFe2O4@SiO2@In2O3 nanocomposite microspheres can detect 10 ppm acetone at 260 C [28]. Khandekar et al. prepared Ce doped CoFe2O4 nanoparticles, which has selectivity towards acetone gas and the response value is 1.38e100 ppm acetone gas at the optimized operating temperature of 225 C [29]. Moreover, Cobalt ferrite exhibits high sensitivity and selectivity to NH3 at 450 C as sensing electrode [30]. Otherwise, few publications reported the application of CoFe2O4 as gas sensor of acetone. Herein, we first reported a modified solvothermal method to prepare CoFe2O4 nanoparticles, which can remarkably decrease the reaction time and effectively control the particle size through the combination of solvothermal and microwave processes. In order to demonstrate the potential applications of gas sensing, the assynthesized samples were used to fabricate gas sensor. As
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expected, the measurement results showed that it exhibited good sensing properties to acetone gas at low temperature and the influencing factors on the sensors performance were investigated in details. 2. Experiment section CoFe2O4 nanoparticles were synthesized via a modified solvothermal method. In brief, 1.4 g of iron acetylacetonate (Fe (C5H47O2)3$H2O, >95%) and 0.7 g of cobalt acetylacetonate (Co(C5H7O2)2$H2O, >95%) were dissolved in a beaker, which contained 60 ml ethanol. Then the mixed solution was continuous stirred for 20.0 min at room temperature and put into autoclaves with a capacity of 70 mL vessels, which was maintained at the microwave-solvothermal condition of 800 W power, 150 C for 3 h. After heating, the resulting products were collected by centrifugation and then washed several times with deionized water and ethanol. Finally, powder slurry was freeze dried overnight at 35 C and collected. In order to understand the effect of microwave on the preparation of CoFe2O4 nanoparticles, we prepared CoFe2O4 nanoparticles only by normal solvothermal method as mentioned in the literature [31]. The samples obtained were denoted as SM and S respectively. The phase of the as-obtained product was identified using Xeray powder diffraction (XRD, D/max 2550 V, Cu Ka radiation with the wavelength l ¼ 0.1546 nm) at 40 kV and 150 mA in a scanning range of 20e80 (2q). The morphology and size of the prepared samples were investigated by transmission electron microscopy (TEM, JEOL 2010) with a Gatan Ultrascan 100 TEM camera for selective area electron diffraction patterns (SAED) and highresolution TEM (HRTEM) images. The elemental composition of the material was examined by X-Ray photoelectron spectrometer (XPS, MULTILAB-2000 Base system with XR4 Twin Anode Mg/Al (300/400 W) X-Ray spectrometer). Surface properties of the nanoparticles were studied by the BrunauereEmmetteTeller (BET) method via nitrogen adsorption and desorption measurements (3H-2000PS2). Pore diameter and the pore size distributions were calculated by the BarreteJoynereHalenda (BJH) method from the desorption isotherm. Gas sensors were based on two samples: CoFe2O4 nanoparticles with different particle sizes prepared by solvothermal and microwave-assisted solvothermal method. The fabrication and measuring principle of the gas sensor were similar with those described in our previous reports [32,33]. Briefly, the powder prepared was mixed with ethanol in a weight ratio of 3:1 and ground in an agate mortar for 20 min to form a paste. The paste of the thick sensing film (with a thickness of about 0.3 mm) was evenly brushed on a ceramic tube with a particularly small bristle brush. Nickel chrome resistor with 27e30 U in the ceramic tube was used to control the working temperature of the sensor by varying the heating voltage during detection. The film was heat-treated for 24 h at an aging temperature of 300 C after coating to keep the sensor stability. Fig. 1(a) displayed the sketch of the gas-sensor structure. The actual graph and the measurement principle of gas sensor were also showed in Fig. 1(b). RL denotes as a constant load resistor connected in series with the gas sensor, RS denotes the resistance of the sensing material which could be adjusted in different gas. For the equal current in a series circuit, the resistance of RS can be calculated by measuring the voltage of RL. The Gas-sensing properties of sensor were tested by WS-30A gas sensing measurement system (Wei Sheng Electronics Science and Technology Co. Ltd., Henan Province, China) under a static process with a test chamber of 18 L and 30 testing channels. The sensor response (S) was defined as: S¼ Ra/Rg, where Ra and Rg are the sensor resistance values of the sensor measured in air and the target gas, respectively. The
Fig. 1. (a) Sketch of the gas-sensor structure (b) actual graph of gas sensor and the measurement principle of gas sensors.
response time is defined as the time taken by the sensor to achieve 90% of its maximum sensing response upon test gas injection, whereas the recovery time is the time taken by the sensor to reach 10% of its initial resistance after the removal of the gas.
3. Results and discussion 3.1. Structure and characterization of CoFe2O4 nanoparticles The crystal structures and purity of all the products were
Fig. 2. XRD patter of CoFe2O4 nanoparticles.
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characterized by powder X-ray diffraction. Fig. 2 shows the XRD pattern of CoFe2O4 nanoparticles. The diffraction peaks at 2q values of (220), (311), (400), (422), (511), (440) and (533) match with JCPDS 22-1086, which confirms the formation of CoFe2O4. No other peaks from impurities are detected, which indicates CoFe2O4 nanoparticles have high crystalline purity. At the same time, we can observe that the diffraction peak of sample SM is wider than that of sample S, suggesting that the particle size of sample SM is smaller than that of S. The average crystalline size of CoFe2O4 nanoparticles can be calculated by Scherrer's equation [34].
d¼
0:9l b cos q
(1)
Where l is the wavelength of X-ray, b is the full width at half maximum which was determined from the experimental integral peak width of the (311) peak, q is diffraction angle, d is the crystallite size. The average crystallite size of S and SM were calculated as 23.5 and 9.8 nm, respectively. The morphology and sizes of the obtained samples were further investigated by transmission electron microscopy (TEM). Fig. 3 shows TEM images with different magnification. From TEM images with the low magnification (Fig. 3(a), (c)), it can be found that the as-synthesized nanoparticles were spherical shape. TEM image with high magnification (Fig. 3(b)) can reveal that there is a little agglomeration phenomenon in sample S and the size is uneven. The inset of Fig. 3(b) is the particle size distribution of sample S, which reveals that the average diameter of S is 23.5 nm. Furthermore, more details of sample SM can be observed from Fig. 3(d), the assynthesized nanoparticles are very uniform. Their average size is
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9.8 nm, which is in accordance with the XRD measurement. By freeze-drying, there is no obvious agglomeration of samples in TEM images of Fig. 3. Lots of small pores may be presented in CoFe2O4 nanoparticles for loose contact of nanoparticles. In order to further obtain the information about the as-prepared samples, the BET surface area and pore-size distribution are investigated based on nitrogen adsorption-desorption isotherms as shown in Fig. 4. The sample SM (Fig. 4(b)) which shows a BET surface area of 90.11 m2g1, is greatly larger than sample S (53.24 m2g1, Fig. 4(a)). The cumulative pore volumes were calculated via the desorption branch of the nitrogen isotherms in terms of BJH method, exhibiting cumulative pore volumes of 0.383 and 0.382 cm3g1, respectively. It could be found that freeze drying method makes the particles equally distributed and effectively prevents agglomeration [35]. The insets in Fig. 4 exhibit the corresponding pore size distribution of the nanoparticles. The pore size of nanoparticles mainly distributes in about 10 and 18 nm, which also can be speculated that there are differences in particle size and they are consistent with the TEM images. Further details of the structure sample SM are shown in Fig. 5. The d-values obtained from the analysis of the lattice fringes is ~0.214 nm which corresponds to the (400) reflections of CoFe2O4 (JCPDF 22-1086) by HRTEM image Fig. 5(a). There are the equally and clear lattice planes in HRTEM image, which shows that CoFe2O4 nanoparticles prepared has a high degree of crystallinity. The diffraction rings show in SAED image of Fig. 5(b), are assigned to (311), (400), (511), (440) of CoFe2O4, suggesting the polycrystalline characteristics of the nanoparticles. The Energy Dispersion X-ray analysis spectrometry (EDS) spectrum is shown in Fig. 5(c). It is confirmed that the prepared SM shows the existence of iron, oxygen
Fig. 3. Structural characterization of CoFe2O4 nanoparticles (a) (c) low magnification TEM images of S and SM (b) (d) high magnification TEM images and distribution of particle size (inset) of S and SM.
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Fig. 4. Nitrogen adsorption-desorption isotherms of CoFe2O4 nanoparticles (a) The sample S (b) The sample SM. Insets showing the BJH pore-size distribution.
Fig. 5. CoFe2O4 nanoparticles SM (a) HRTEM image (b) SAED pattern (c) Energy dispersive X-ray spectrum.
and cobalt. The Ka lines of iron and oxygen in the energy dispersion energy spectrum are observed at 0.53 and 6.4 eV respectively. Besides, cobalt Ka line is observed at 6.9 eV. A typical nanoparticle of the cobalt-ferrite analyzed and the Co/Fe atomic ratio of asproduced nanoparticles is near 1/2, thus the initial formula should be CoFe2O4. The chemical constituents and elemental valence states of samples were analyzed by XPS in Fig. 6. The survey of XPS spectrum clearly indicated the existence of elemental Co, Fe, and O, which was consistent with the XRD and EDS results. All the results before analysis were calibrated with the binding energy of the C 1s peak at 284.6 eV. The spectrum for Fe 2p (Fig. 6 (a)) contained two strong peaks centered at 712.7 and 725.5 eV is assigned to Fe 2p3/2 and Fe 2p1/2 and the satellite peaks at 718.6 eV, which is consistent with
Fe 2p binding energy for cobalt ferrite nanoparticles. In addition to the binding energy at 711.2 and 725 eV which can confirm the existence of Fe3þ species in CoFe2O4. In detail, the Fe 2p3/2 binding energy at 713.4 eV is due to the contributions from Fe3þ ions in tetrahedral sites. The Fe 2p3/2 binding energy at 711.2 eV and Fe 2p1/2 binding energy at 725 eV are due to the contributions from Fe3þ ions in octahedral sites. It should be noted that Fe3þ will also be reduced to Fe2þ with the presence of oxygen vacancies. It has been reported that conductivity in CoFe2O4 will occur due to electron hopping between Fe ions existing in different valence states (Fe2þ and Fe3þ) on equivalent lattice sites [36,37]. As shown in the Co 2p spectrum (Fig. 6(b)), two peaks located at 781.1 and 786.9 eV correspond to Co 2p3/2, while another two peaks at 797.1 and 803.6 eV can be attributed to Co 2p 1/2. The Co 2p spectrum
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Fig. 6. High resolution XPS spectrum of SM (a) Fe 2p (b) Co 2p; High resolution XPS O 1s spectrum (c) S (d) SM.
indicates Co exist in 2 þ oxidation state because the low spin Co3þ cations can only give rise to much weaker satellite features than high spin Co2þ cations with unpaired valence 3d electron orbitals [38]. According to the XPS data, the atomic ratio of Fe to Co is 2.1:1, indicating the excess of iron on the material surface. Fig. 6(c and d) show that the spectrum diagram of the O 1s to study the oxidation state of oxygen and oxygen vacancies of CoFe2O4 nanoparticles, which fitted using Gaussian functions and divided into two peaks of 530.6 eV and 532.2 eV. The peak marked O1 is attributed to oxygen vacancies and surface hydroxyl group. The peak O2 is assigned to the lattice oxygen [39]. In addition, the fraction of oxygen vacancies (0.35) in SM was higher than that in S (0.22), which was obtained from the relative integrated peak area. We all know that oxygen vacancies play an important role in the gas-sensing performances of sensor [8]. As we know, the decrease of particle diameter, high BET surface and a lot of oxygen vacancies could help to improve electron transfer and enhance gas sensitivity, which can be useful to application such as gas sensor and will be verified in the following gas sensitive test.
temperatures ranging from 150 to 320 C. The optimum operating temperatures of two sensors were 220 C. Fig. 8 shows the dynamic response characteristics of the samples S and SM when they are exposed to various concentrations of acetone gas ranging from 50 to 500 ppm at the optimum operating temperatures, which demonstrate an evident tendency that the responses increase with the tested gas concentrations. The responses of SM and S sensors to acetone with concentration of 100 ppm reached the maximum value of about 17.3 and 5.5, respectively. Furthermore, the concentration of acetone increases
3.2. Gas sensing properties of CoFe2O4 nanoparticles The gas sensors of sample S and sample SM were fabricated and the gas sensing performances were assessed by their resistance changes upon exposure to target gases with a controlled gas concentration. It is well known that the most important properties of a gas sensor are sensitivity, operating temperature, reproducibility, selectivity and long-term stability [40]. The working temperature is a key parameter among these factors, which determines the quality of the gas sensor [31]. Fig. 7 shows that the sensor responses of CoFe2O4 nanostructures to 100 ppm acetone gas at the
Fig. 7. The sensor response of CoFe2O4 nanoparticles to 100 ppm acetone gas at different working temperatures.
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Fig. 8. (a) The sensor response of sample S and SM to different concentrations acetone at 220 C (b) The response of the two sensors to different acetone concentrations.
and the response value of the sensing material increases accordingly, while the sensors do not saturate less than 500 ppm, as shown in Fig. 8 (a). The SM sensor displays much higher response values to different acetone vapor concentration than that of S sensor, which consists well with the results of BET analysis. The sensor response of sample S is very low and is only 3.7e50 ppm acetone gas in Fig. 8(b), which is less than half of sample SM. Obviously, it is confirmed that the acetone sensor based on SM shows higher gas response to acetone gas, which sensor response value can reach 11.8e50 ppm acetone gas. So, we take the SM sensor as the main research object to analyze its gas sensitivity characteristics. In order to find the lower detection limit and make better use of it, we have tested at a low concentration acetone gas to the SM sensor in Fig. 9. It is worth to note that the response to 5 ppm acetone vapor achieves 3.2, showing practical application potential for low concentration acetone vapor detection. As shown in Fig. 10, all response and recovery time of samples at the optimum working temperature are less than 30 s to either high or low concentration acetone gas. For example, the response and recovery time of SM sensor decreased to 7 and 5 s to 10 ppm acetone gas at a working temperature of 220 C, as shown in Fig. 10(c). Selectivity is the ability that a gas sensor to distinguishes with different kinds of gases, which is also important for evaluating the gas sensing properties of sensor. Fig. 11 depicts the histogram for the sensors which are made of two samples at optimum operating temperature of 220 C for different target gases with a
concentration of 100 ppm. The response of two sensors to acetone gas are the highest comparing with gases such as H2S, H2, ammonia and ethanol, indicating both good selectivity to acetone gas. It is obvious that sample SM has better selectivity to acetone gas than sample S. It can be found that SM has remarkably smaller response values to H2S, H2, ammonia and ethanol, Sacetone/SH2S, Sacetone/SH2, Sacetone/Sammonia and Sacetone/Sethanol were 12.3, 15.7, 15.6 and 3.8, respectively. Therefore, the sample SM has good selectivity to acetone gas. Fig. 12 depicts the long-term stability results of as-prepared SM sensor measured for 100 ppm acetone under their optimum working temperature every several days. The response of SM based sensor changes over time but nearly fluctuated around the initial value and the maximal deviations of the response to acetone are less than 5.3% within two months, which did not decrease obviously. The results show that the sensor responses can be duplicated for a long time. The reproducibility of SM based sensor was further investigated by successively exposing the sensor to 100 ppm acetone gas for 4 cycles at 220 C, and the corresponding curve of the responseerecovery characteristic is shown in Fig. 12 (inset). In every test cycle, we can observe that the recovery time is a few second and the sensing material came back to the initial state, indicating that the sensor based SM possessed good reproducibility. Hence, the relevant acetone sensing performances of gas sensors were illustrated in Table 1 to compare the sensing performances of CoFe2O4 with other works. It is confirmed that the acetone sensor based on CoFe2O4 nanoparticles prepared by this improved method, shows higher gas response and lower working temperature than most of those reported in the literatures [41e48]. The response value of PANI/a-Fe2O3 nanocomposite in the 48th reference is very close to 17.3 in this work to 100 ppm acetone. Although its response temperature (180 C) is lower than 220 C, its two-step preparation method is more complicated. 3.3. Gas sensing mechanism
Fig. 9. The sensor response of sample SM to low concentration acetone at 220 C.
As a typical n-type semiconductor, the gas sensing mechanism of CoFe2O4 nanoparticles is based on the changes of the resistance before and after being exposed to the test gas, the gas sensing mechanism is schematic illustrated in Fig. 13. Oxygen species are chemisorbed onto CoFe2O4 nanoparticles and ionized by capturing free electrons from conduction band in air (Fig. 13(a)). In a low temperature (<100 C), oxygen molecules adsorbs on the surface of the sensor and forms absorbed oxygen. Then, absorbed oxygen is ionized by the electrons in the sensing materials conduction band to form O 2 (ads), as shown in equations (2) and (3). With the temperature exceeds 100 C and at the range 2 of 100e300 C, O 2 (ads) continue be ionized to form Oads , which is
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Fig. 10. The response and recovery curves of samples to different concentration acetone gas at 220 C (a) S (b) SM (c) SM.
O2 ðadsÞ þ e /O 2 ðadsÞ
(3)
O 2 ðadsÞ þ e /2O ðadsÞ
(4)
Furthermore, the grain size also plays an important role in the gas sensing response enhancement, based on the grain-control model [5]. The electrons almost completely depleted to participate in gas sensing reaction for the small size, which leads to CoFe2O4 nanoparticles highly sensitive to low concentration acetone gas. As shown in Fig. 13(b), when the sensor is exposed to acetone gas, the chemisorbed oxygen ions will react with acetone to form CO2 and H2O, which can be described as equation (5). CH3COCH3 þ 8O (ads) / 3CO2 þ 3H2O þ 8ee
Fig. 11. The response values of samples to 100 ppm different gases at 220 C.
Fig. 12. The long-term stability of sample SM to 100 ppm acetone gas for 60 days, the inset is four test cycles of the sensor resistance to 100 ppm acetone gas at the working temperature of 220 C.
shown in equation (4) [49]. This produces a resistance layer from an electron depleted space charge on the n-type nanoparticle surface and higher resistance. O2(gas) / O2 (ads)
(2)
(5)
The reactions will cause a decrease of the resistance since the trapped electrons can be released back to the conduction band of CoFe2O4, resulting in a decrease of the thickness in the depletion region and an increase in the carrier concentration. Therefore, the adsorption-desorption based kinetics in a chemical sensing process is actuated here by the increased surface area and small size of the sample SM, which can provide more active sites for sensing reactions as compared to the sample S. Structure characteristic of spinel ferrites is also one important affecting factor to improve sensitivity. For CoFe2O4, Co2þ ions greatly prefer the octahedral sites while the Fe3þ ions prefer both octahedral and tetrahedral sites. The oxidationereduction reaction of Fe2þ/Fe3þ and Co2þ/Co3þ on the surface of n-type spinel ferrite surface is reversible [50]. A greater Fe2þ concentration increases e conductivity. After chemisorption of oxygen on the n-type spinel ferrite, Fe2þ is oxidized to Fe3þ. When oxygen reacts with gas, e is released back to the material and Fe3þ is reduced to Fe2þ, thus the amount of Fe3þ and Fe2þ pairs and over-all electrical conductivity are increased. A greater amount of Fe2þ increases the oxygen chemisorption capacity, which has been demonstrated in the literature based on the preference energies for divalent and trivalent ions in the spinel structure [51]. However, the concentration of Fe2þ plays an important role in gas sensitivity. Sutka et al. reported that the response of the sensor is reduced since the Fe2þ concentration is too high [52]. Oxygen vacancy is considered as an important reagent of many adsorbents. Therefore, many surface reactions are affected by this point defect. Oxygen vacancy is not only the direct adsorption site, but also the electron donor site. Every oxygen deficient site can offer two electrons and oxygen molecules on the sensing material surface take electrons and become oxygen ions, which can lead to the electrical conductance decrease and thus the resistance of the sensor increase [53,54]. There are a lot of oxygen vacancies on the surface of sample SM and the ratio of oxygen vacancy reaches 35% of the total oxygen, which is calculated by XPS. Oxygen vacancies play an important role in the gas-sensing performances of sensor and can drastically alter the
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Table 1 Comparison of the acetone sensing performance of the current work with other reported literatures. Materials
Acetone
temperature
Gas Response
Reference
SnO2 nanoparticles EuFeO3 nanoparticles Porous -Fe2O3 nanotubes Fe2O3 nanoparticles Fiber-like Co3O4 Hierarchical In2O3 a-Fe2O3 nanoparticles PANI/a-Fe2O3 nanocomposite CoFe2O4 nanoparticles (S) CoFe2O4 nanoparticles (SM)
50 ppm 100 100 100 100 50 100 100 100 100
310 295 240 340 160 260 250 180 220 220
11.4 6 11 8.8 4 8.1 9.5 16 5.5 17.3
[41] [42] [43] [44] [45] [46] [47] [48] This work This work
Fig. 13. A schematic energy band diagram (band bending) of CoFe2O4 nanoparticles (a) before and (b) after chemisorptions of charged species (acetone gas). EC, EV, and EF represent the conduction band, valence band, and the Fermi level energy, respectively.
surface electronic properties of materials. Therefore, the as prepared CoFe2O4 nanoparticles by this method with considerable oxygen deficient sites are promising for the gas sensing application. Based on the discussion above, both SM and S have good selectivities to acetone. However, SM has higher response value than S. The reasons can be explained by the smaller size, larger content of oxygen vacancy.
nanoscale. Furthermore, the CoFe2O4 nanoparticles exhibits the best gas sensing properties to acetone vapor at the low working temperature of 220 C. The CoFe2O4 nanoparticles can detect acetone gas down to 5 ppm and the sensor response can be up to 17.3e100 ppm acetone gas. From the observation of sensing response, we conclude that CoFe2O4 nanoparticles can be efficiently used as a chemical sensor for the sensing of low concentration acetone vapor at a low temperature.
4. Conclusions Acknowledgments In general, an uncomplicated synthesis route was developed for the preparation of uniform CoFe2O4 nanoparticles, which are homodispersed and less than 10 nm. The strategy presented here could be expended as a general method to synthesize other transition bimetallic oxide nanoparticles by rational designation in
We thank the National Science and Technology Support Program of China (Grant No. 2013BAE04B02) and also Heilongjiang University of science and technology college students' scientific research project.
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H. J. Zhang is currently studying for Ph.D. degree at Harbin University of Science and Technology. He has been an associate professor at Heilongjiang University of Science and Technology since 2012. Her main research interest is in the development of nanostructured materials for their applications.
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L. Z. Liu has been a professor at Harbin University of Science and Technology since 2004. He received his Ph.D. degree in 2006 from Harbin University of Science and Technology. His research interests focus on the nanostructured materials for the nanodevice applications.
X. R. Zhang has been a lecturer at Harbin University of Science and Technology since 2016. He received her Ph.D. degree in 2015 from Harbin University of Science and Technology. His research interests focus on the nanostructured materials for the nanodevice applications.
S. Zhang is currently studying for bachelor's degree at Heilongjiang University of Science and Technology. His main research interest is in the development of nanostructured materials for their applications.
F. N. Meng has been an associate professor at Heilongjiang University of Science and Technology since 2015. She received her Ph.D. degree in 2015 from Harbin Engineering University. Her main research interest is in the development of nanostructured materials for their applications.