Haonan Zhanga,
Yazi Luoa,
Ming Zhuoa,
Ting Yangc,
Jiaojiao Liangc,
Ming Zhangc,
Jianmin Mac,
Huigao Duanc and
Qiuhong Li*ab
aCollege of Electrical and Information Engineering, Hunan University, China. E-mail: liqiuhong@xmu.edu.cn; Fax: +86 731 88664019; Tel: +86 731 88664019
bPen-Tung Sah Institute of Micro-Nano Science and Technology of Xiamen University, Xiamen, 361005, China. Tel: +86-592-2187198
cKey Laboratory for Micro-Nano Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha 410082, P. R. China
First published on 12th January 2016
V2O5-decorated α-Fe2O3 composite nanorods were synthesized successfully by electrospinning and an environmentally-friendly soak-calcination method. The composite showed high selectivity and stability to diethylamine gas as well as ultra fast response times within 2 s to 100 ppm diethylamine gas.
α-Fe2O3 (hematite), a classical n-type semiconductor (Eg = 2.1 eV), which is a stable, nontoxic, low-cost iron oxide under ambient conditions, is of great scientific and technological importance.23 V2O5 is also an n-type semiconducting oxide whose resistance decreases when the V5+ species are reduced to V4+ during the interaction with reducing gases.24 These transition metal oxides have been extensively investigated because of the unique electrical and catalytic properties. Sensitive materials incorporated with V2O5 can significantly enhance the sensitivity.25,26 Diethylamine is colorless, strong alkali and corrosive liquid with volatile and flammable nature. Diethylamine gas is used in the manufacture of pharmaceuticals, pesticides, dyes, rubber vulcanization accelerator, textile auxiliaries and metal preservatives, emulsifying agent, inhibitor, etc. Diethylamine gas has strong excitant, which can stimulate eye, trachea, lung, skin, and excretory system. Meanwhile there is no report on diethylamine gas sensing as far as we know. However it is very meaningful for the detection of diethylamine gas in chemical plant, pharmacy workshop, dye workshop, etc. Inhalation of vapor or mist of diethylamine, which is strong alkali, excitant, corrosive and flammable with ammonia smell, can cause laryngeal edema, bronchitis, chemical pneumonia, and pulmonary edema. High concentrations of diethylamine may lead to death. In this work, we present an attempt to enhance the response of semiconductor oxide gas sensor to diethylamine gas by synthesizing V2O5-decorated α-Fe2O3 composite with novel morphology and structure. Materials with nanoscale and porous structure as gas sensors are thought to not only provide a large number of channels for gas diffusion but also possess significantly large surface areas for gas-sensing reactions.27,28 Moreover, the addition of V2O5 is supposed to improve the gas sensing performance of the composite. The synthesis process and experiment of raw materials are introduced in detail in the ESI.†
X-ray diffraction (XRD) analysis was used to determine the crystal structure of the samples before and after soak-calcination. As shown in Fig. 1, all the reflection peaks of the sample before soak-calcination (the read line) can be indexed as standard α-Fe2O3 sample (Joint Committee on Powder Diffraction Standards (JCPDS) card no. 33-0664), which indicates that the sample synthesised by ES is α-Fe2O3. Moreover, the reflection peaks of the sample after soak-calcination (the black line) can be indexed as standard α-Fe2O3 sample (JCPDS card no. 33-0664) and standard V2O5 sample (JCPDS card no. 89-2482) demonstrating that through the soak-calcination strategy method, V2O5 nanoparticles were successfully synthesized in the composite.
The morphologies of the as-prepared α-Fe2O3 nanorods and V2O5-decorated α-Fe2O3 nanorods were investigated based on the field-emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM) and element EDS mapping images shown in Fig. 2. Fig. 2a and b give a representative SEM image of the as-synthesized α-Fe2O3 nanorods, showing that the surface is smooth and uniform with diameters about 100 nm. Fig. 2c gives typical high magnification SEM images of the V2O5-decorated α-Fe2O3 nanorods showing that the surface is rougher than α-Fe2O3 nanorods (Fig. 2a) and are uniform with the diameters about 100 nm. The EDS spectrum of V2O5-decorated α-Fe2O3 nanorods from SEM certificated the presence and proportion of Fe, O and V (Fig. S1†). TEM, high-resolution TEM (HRTEM) and the corresponding EDS elemental mapping images provide further insight into the microstructure and morphology of the product. Fig. 2d shows a typical TEM image of α-Fe2O3 nanorods. Structural information was further characterized by HRTEM in Fig. 2e. Lattice spacings of 0.2271 nm and 0.2710 nm for the nanorods correspond to (113) and (024) plane of rhombohedral α-Fe2O3 (Fig. 1). TEM image (Fig. 2f and g) shows that V2O5 nanoparticles were modified on the surface of α-Fe2O3 nanorods. HRTEM (Fig. 2h) of the nanoparticle showed that lattice spacing of 0.2503 nm correspond to the (005) plane of rutile V2O5. Moreover, Fig. 2i shows element mapping of Fe, V and O in one single V2O5-decorated α-Fe2O3 nanorod. The results demonstrate that Fe, V and O are distributed throughout nanorods which confirm V2O5 nanoparticles were modified on the surface of α-Fe2O3 nanorods. In other words, through the soak-calcination strategy, V2O5 nanoparticles were successfully decorated on the α-Fe2O3 nanorods.
The operating temperature is closely linked with the response for semiconductor oxide sensors. As shown in Fig. 3a, the responses of three sensors to 100 ppm diethylamine were measured under different operating temperatures from 150 °C to 350 °C. Evidently, the sensitivity of the V2O5-decorated α-Fe2O3 nanorods increases with working temperature and reaches a maximum value of 8.9 at 350 °C. Because V2O5-decorated α-Fe2O3 nanorods were calcined at 350 °C for 2 h in air, the working temperature in this work was limited to 350 °C. Similar tendencies are observed for the sensors based on α-Fe2O3 nanorods and c-V2O5, respectively. The maximum sensitivities of pure α-Fe2O3 nanorods is 2.97 at 250 °C and that of c-V2O5 is 1.63 at 200 °C, both of which are much smaller than that of V2O5-decorated α-Fe2O3 nanorods. According to the oxygen adsorption theory, when the operating temperature is low, physical adsorption is happened in general, when operating temperature is high there comes the chemical adsorption. Optimum operating temperature is related to morphology, synthetic process, environment conditions, etc. Although the optimum operating temperature was increased after combination of V2O5, sensitivity of the V2O5-decorated α-Fe2O3 composite is obviously higher than those of the pristine α-Fe2O3 nanorods and c-V2O5. Fig. 3b shows the sensitivity–concentration curves of the V2O5-decorated α-Fe2O3 nanorods, α-Fe2O3 nanorods and c-V2O5. α-Fe2O3 nanorods and c-V2O5 show sensitivities of approximately 1–3.2 and 1.1–1.9, to 5–300 ppm diethylamine, respectively. In contrast, V2O5-decorated α-Fe2O3 nanorods show a sensitivity of about 1.8–9.5 to 5–300 ppm diethylamine. The sensitivity of the V2O5-decorated α-Fe2O3 nanorods is significantly increased with increasing diethylamine concentration, while the increase in the sensitivities of α-Fe2O3 nanorods and c-V2O5 is very little. These results indicate that for detecting diethylamine, the sensitivity of the V2O5-decorated α-Fe2O3 nanorods is obviously higher than those of the pristine α-Fe2O3 nanorods and c-V2O5. Fig. 3c–e display the response of sensors to 100 ppm diethylamine at 350 °C. The response time is defined as the time taken for the sensor to achieve 90% of total resistance, and the recovery time is the time for the resistance to recover to 90% of the initial level after removal of diethylamine vapor. The response curve shows a drastic decline once the sensor is exposed to target gases and achieves a near steady state, then rises to its initial value in air. It can be observed that the three sensors display fast response/recovery time: 2 s/40 s (V2O5-decorated α-Fe2O3 nanorods), 14 s/1 s (α-Fe2O3 nanorods), and 9.5 s/40 s (c-V2O5) to 100 ppm diethylamine, respectively.
The responses of the three sensors towards a variety of flammable, toxic gases including diethylamine, formaldehyde, n-hexane, trichloromethane, glycol, ammonium hydroxide, ethylenediamine, CH4, CO and SO2 are studied to evaluate the selectivity of the sensors. As shown in Fig. 4a, the responses of the V2O5-decorated α-Fe2O3 nanorods sensor to the ten gases are all improved compared with those of the α-Fe2O3 nanorods and c-V2O5. In addition, the sensor based on V2O5-decorated α-Fe2O3 nanorods shows superior sensitivity to diethylamine compared with other gases; the response is 3–8.1 times higher than those for the other tested gases, which indicates an excellent selectivity to diethylamine. Fig. 4b shows the responses of V2O5-decorated α-Fe2O3 nanorods, α-Fe2O3 nanorods and c-V2O5 sensors to 30 ppm diethylamine at 350 °C. In our practical gas sensor test, the resistance of α-Fe2O3 sensor in the air is not stable but regular. So we choose and calculate the average resistance of the sensor to calculate the response of it. As the resistance baseline of V2O5 is very stable, when the V2O5 was decorated on the surface of α-Fe2O3 nanorods, the sensor of V2O5-decorated α-Fe2O3 composite become stable because of the synergistic effect. The sensor of V2O5-decorated α-Fe2O3 nanorods demonstrates remarkable repeatability with outstanding stability compared with α-Fe2O3 nanorods sensor.
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Fig. 4 (a) Selectivity tests to 100 ppm of different gases at 350 °C. (b) The response of V2O5-decorated α-Fe2O3 nanorods, α-Fe2O3 nanorods and c-V2O5 sensors to 30 ppm diethylamine at 350 °C. |
We confirmed this structure feature by N2 adsorption–desorption measurements. As expected, the V2O5-decorated α-Fe2O3 composite shows a Brunauer–Emmett–Teller (BET) specific surface area of 30.5 m2 g−1 with pore volume of 0.19 cm3 g−1, higher than α-Fe2O3 nanorods with specific surface area of 24.6 m2 g−1 and pore volume of 0.077 cm3 g−1 which is showed in Fig. S3.† The larger surface area could facilitate the absorption of gas molecule. Fig. S4† shows the pore size distribution of V2O5-decorated α-Fe2O3 composite and α-Fe2O3 nanorods obtained from the desorption branch of the isotherm. With regard to α-Fe2O3 nanorods, there are numerous micropores around 1.6 nm and 1.8 nm, and the mesopores between 2 and 5 nm. After soak-calcination method, there are more micropores existed around 0.9 nm and mesopores between 2 and 10 nm. The high specific surface area and optimized pore structure are beneficial for absorption of gas molecule.
Based on the above results, the reason was discussed for the improvement in gas-sensing properties of α-Fe2O3 nanorods decorated with the V2O5 nanoparticles. It is well known that both Fe2O3 and V2O5 are n-type semiconducting oxides and their sensing mechanism is the surface-controlled type.29,30 Their gas sensitivity depends on grain size, porosity, active surface state, oxygen adsorption quantity, active energy of adsorption of the test gas on the surface, lattice defect and so on.31 According to the above FESEM and TEM analysis, it is observed that after being sintered, α-Fe2O3 nanorods with V2O5 nanoparticles form some pores in different sizes. Those pores with small diameters become closed gas pores in the sintering process, or move to the grain boundaries and gradually disappear, while those pores with larger diameters will shrink slightly, but will not disappear. The porous structure on the surface of the element not only provides enough space to reduce space hindrance induced by the gas adsorption, but also increases its inner surface, thus the gas can be adsorbed on the inner surface of the element through van der Waals force. This could help to improve the gas adsorption and sensitivity. The amount of V2O5 in the α-Fe2O3 nanorods was calculated which is about 17% according to presence of Fe and V atom in the EDS of SEM (Fig. S1†). As we know the presence of atom in EDS of SEM is inaccuracy with certain reference. The modification of metal oxides by additives or dopants is well-known for improving the performances of resulting chemoresistive gas sensors.32,33 Additives may induce electronic sensitization, based on the modification of the electronic properties of the host oxide,34,35 or spillover.36–39 Nanomaterials incorporated with V2O5 can significantly enhance the sensing performances in this paper and other literatures.40,41 The sensing results showed that by introducing V in the α-Fe2O3 structure it was possible to obtain larger responses, shorter response time and much stable at appropriated temperatures, which is just the aim of surface modifications. In Fe2O3–V2O5, more effective diethylamine oxidation took place, while in pure α-Fe2O3 a reaction with lower activation energy was operative. In addition, the one-dimensional nanorods form a network in the V2O5-decorated α-Fe2O3 nanocomposite, which are favorable for conducting the charges induced when being exposed to the samples. As far as we know, there is no diethylamine gas sensing literature, the mechanism of why sensing selectivity enhanced by doping V2O5 may be as follows. Hence, it is reasonable to search for hints in catalysis field about how to improve chemoresistive sensors. A well-known catalyst system is the titania-supported vanadium pentoxide42,43 which is similar to our V2O5-decorated α-Fe2O3 nanorods. Diethylamine gas is one of the volatile organic compounds (VOCs). Particularly, it is known as a promoter of oxidation reactions of many organic compounds,44,45 which makes it an ideal candidate for detection of volatile organic compounds.46 As aforementioned, V2O5 nanoparticles significantly enhance the gas-sensing properties of α-Fe2O3 nanorods. More details of the enhancing effect of the V2O5 nanoparticles on the sensing properties of the Fe2O3 nanorods need further investigation.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23032b |
This journal is © The Royal Society of Chemistry 2016 |