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Ruijin Hu, Jing Wang, Pengpeng Chen, Yuwen Hao, Chunli Zhang, Xiaogan Li, "Preparation of Cd-Loaded In2O3 Hollow Nanofibers by Electrospinning and Improvement of Formaldehyde Sensing Performance", Journal of Nanomaterials, vol. 2014, Article ID 431956, 7 pages, 2014. https://doi.org/10.1155/2014/431956
Preparation of Cd-Loaded In2O3 Hollow Nanofibers by Electrospinning and Improvement of Formaldehyde Sensing Performance
Pure In2O3 and Cd-loaded In2O3 hollow and porous nanofibers with different Cd/In molar ratios (1/20, 1/10, 1/1) were synthesized by electrospinning method. X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), and transmission electron microscopy (TEM) were used to characterize the nanofibers. The porous nanofibers were composed of small grains. The average grain sizes and the diameters of Cd-loaded In2O3 nanofibers increased with the increasing of Cd/In molar ratios. The formaldehyde sensing properties of the sensors based on pure In2O3 and Cd-loaded In2O3 nanofibers were investigated in formaldehyde concentration range of 0.5∼100 ppm. Moreover, the selectivity of those sensors was studied by testing responses to methanol, toluene, ethanol, acetone, and ammonia. The result showed that Cd-loaded In2O3 nanofibers with Cd/In molar ratio of 1/10 possessed the highest response value and good selectivity at operating temperature 280°C. In addition, the formaldehyde sensing mechanism of the sensors based on Cd-loaded nanofibers was briefly analyzed.
Formaldehyde is a hazardous indoor pollutant among volatile organic compounds (VOCs), and prolonged exposure to formaldehyde (HCHO) can cause serious health effects such as asthma, nasopharyngeal cancer, and multiple subjective health complaints . Up to now, various materials prepared by different methods  were used to detect VOCs. Also the testing ways have been widely investigated [3–5]. Among these methods, gas sensor based on metal-oxide semiconductors (MOS) such as ZnO , SnO2 , TiO2 , and In2O3  have been attracting special interest due to their good reproducibility, compact size, ease of use, and low cost.
In2O3, an important -type semiconductor with a band gap of approximately 3.55–3.75 eV and good optical or electronic characters, has been widely used in solar cells, transparent conductors, and gas sensors. In recent years, gas sensors based on In2O3 with various morphologies have been investigated [10–12]. Among those morphologies, the controllable synthesis of one dimensional (1D) metal-oxide semiconductors nanofibers with high surface-to-volume ratio and effective electron transport have gained considerable attention in fabrication of miniature gas sensors [13–15]. In varieties of methods, electrospinning is a simple, effective, low-cost, and powerful approach to prepare 1D nanostructure nanomaterials including polymer, inorganic, and composite [16, 17].
In addition, noble metals [18–20], rare earth elements (REE) [21, 22], or metal oxide [23, 24] doping into MOS is a general method used to enhance the sensitivity, selectivity, and stability of gas sensors. And the metal additive acts as a catalyst to modify the surface reactions of MOS toward sensing gases . Xu et al. prepared Au-loaded In2O3 nanofibers-based ethanol microgas sensor and found that the 0.2 wt% Au-loaded In2O3 based sensors possessed the characterization of high response, fast response/recovery, and low power consumption . Wang et al. prepared Ag-doped In2O3 nanofibers by electrospinning and found that the response and recovery time of the sensors were about 5 and 10 s at low heating temperature of 115°C . Xu et al. investigated room H2S-sensing properties based on porous In2O3:REE (REE = Gd, Tb, Dy, Ho, Er, Tm, Yb) nanotubes and found that the response of the sensors based on In2O3 doped with Yb is around 7 times as much as pure In2O3 . There are rare reports about gas sensors based on electrospun In2O3 nanofibers doped with common metal elements such as Cd.
In this report, In2O3 nanofibers doped with low-cost metal element Cd in different Cd/In molar ratios from 1/20 to 1/1 were synthesized by electrospinning method. Then HCHO gas sensor was fabricated by coating the obtained materials on the surface of the ceramic tube attached with Au belt-shaped electrodes. Then the sensing properties of sensors based on Cd-loaded In2O3 nanofibers with different Cd/In molar ratios were tested and discussed.
2.1. Preparation of Pure In2O3 and Cd-Loaded Nanofibers
Poly(vinyl pyrrolidone) (PVP, ) was purchased from Aldrich, USA. Indium nitrate (In(NO3)34.5H2O), cadmium nitrate (Cd(NO3)24H2O), N,N Dimethylformamide (DMF), and ethanol (EtOH) (99.0%) were obtained from Sinopharm Chemical Reagent Co., Ltd., China. All of the reagents used in the experiments were analytical grade and were utilized without further purification.
Cd-loaded In2O3 nanofibers were prepared by electrospinning method. In a typical procedure, 764 mg In(NO3)34.5H2O power and 61.6 mg Cd(NO3)24H2O were dissolved in 6.0 mL DMF and 7.7 mL ethanol under vigorous stirring for 2 h. Then 382 mg PVP was added to the above solution and stirred for 6 h to transparent precursor solution for electrospinning. The obtained solution was then loaded into syringe whose internal diameter of needle is 0.6 mm. The voltage applied to the needle was 18 kV and the distance between the needle and the collector was 15 cm. The as-electrospun PVP/Cd(NO3)34H2O/In(NO3)34.5H2O composite nanofibers were annealed in tubular furnace at 500°C to obtain Cd-loaded In2O3 nanofibers with Cd/In molar ratio of 1/10. In the same way, pure In2O3 and Cd-loaded In2O3 with other Cd/In molar ratios were obtained as specific reagents proportion in Table 1.
2.2. Characterization of Cd-Loaded In2O3 Nanofibers
The structures of the nanofibers were characterized by an X-ray diffraction instrument (XRD: D/Max 2400, Rigaku, Japan) in 2θ region of 20~80° with Cu Kα1 radiation. The morphology images of nanofibers were obtained by using field emission scanning electron microscope (FE-SEM: Hitachi S-4800, Japan) and by a transmission electron microscopy (TEM) using a Philips Fei Tecnai Sprit apparatus operated at 120 kV.
2.3. Fabrication and Measurement of Gas Sensors
As-prepared Cd-loaded In2O3 nanofibers with different Cd/In molar ratios of 1/20, 1/10, and 1/1 were marked as In20Cd1, In10Cd1, and In1Cd1, respectively, as shown in Table 1. The above four kinds of obtained nanofibers were diluted with deionized water in a mortar to form a paste, respectively. Then the paste was coated onto ceramic tube with a pair of gold electrodes to form a sensing film (150~200 μm). The ceramic tube was dried at 100°C in air for 2 h and subsequently annealedat 300°C for 5 h in tubular furnace. An Ni-Cr heating wire with 30 Ω as a heater was inserted through the tube to form an inside-heated gas sensor. The mechanism of test circuit and structure of the gas sensor were shown in Figure 1(a). Figure 1(b) gives the SEM picture of the In10Cd1 showing morphology and structure of nanofibers coated on the surface of the ceramic tube.
The gas-sensing properties of gas sensor based on Cd-loaded In2O3 nanofibers were measured using a static state gas-sensing test system . The sensor was put into the test chamber for the sensing properties measurement. The environment temperature (18 ± 2°C) and relative humidity (~25% ± 5% RH) can be well controlled during the measurement. A fan was used to uniformly distribute the gas inside the chamber. The sensors were first heated by Ni-Cr heating wire for 10 min to stabilize the electrical properties of the sensor .
A given amount of target gas was injected into a test chamber (50 L in volume) by a syringe through a rubber plug. For a required concentration, the volume of the injected gas () can be calculated as: where is the concentration of target gas (ppm: parts per million), and % is volume fraction of bottled gas. The sensor was exposed to atmospheric air by opening the chamber after measurement. The gas response () was defined as a ratio of the sensor’s electrical resistance in air () to that in target gas () as: where , , , and are the voltages applied to the resistor in air and in target gas, respectively. Response time and recovery time are defined as the time spent to achieve 90% of gas response change in adsorption and desorption process, respectively.
3. Results and Discussions
3.1. Morphologies and Structures of the as-Electrospun Cd-Loaded In2O3 Nanofibers
Figure 2 shows the XRD patterns of the pure In2O3 nanofibers and Cd-loaded In2O3 nanofibers with different Cd/In molar ratios. We can see that the In2O3 nanofibers calcined at 500°C possess strong and sharp diffraction peaks identifying a high degree of crystallization. All peaks in XRD patterns perfectly match the body-centered cubic structure of In2O3 (JCPDS: 65-3170). Meanwhile, there are no obvious Cd peaks in the pattern for In20Cd1 and In10Cd1 nanofibers, which may be due to the low concentration of Cd. And we can see that three kinds of diffraction peaks of In2O3, CdO, and CdIn2O4 appeared in curve of In1Cd1 nanofibers. According to the Scherrer equation, the average grain sizes of the nanofibers were around 26 nm, 19 nm, 24 nm, and 42 nm for pure In2O3, In20Cd1, In10Cd1, and In1Cd1, respectively.
The general morphologies of the as-electrospun pure In2O3 and Cd-loaded In2O3 nanofibers with different Cd/In molar ratios (In20Cd1, In10Cd1, and In1Cd1) were studied with SEM as shown in Figures 3(a)–3(d), respectively. It can be seen from Figure 3 that the average diameter of the after-annealed In2O3 nanofibers (~156 nm) is much bigger than that of Cd-loaded In2O3 nanofibers. And the average diameter of Cd-loaded In2O3 nanofibers increased with the increase of Cd/In molar ratios. Figures 3(b), 3(c), and 3(d) show obviously that the diameters of In20Cd1, In10Cd1, and In1Cd1 were 52 nm, 76 nm, and 88 nm, respectively. In addition, the inset in the top right corner of Figures 3(a)–3(d) shows the higher resolution TEM images of the corresponding nanofibers and we found that the as-prepared nanofibers with nanohierarchical structure were indeed composed of many interconnected grains. From the SEM and TEM graphs we can see that the average grain sizes of each nanofiber accorded with the results calculated by Scherrer equation basically. Further, the center of In2O3, In20Cd1, and In10Cd1 nanofibers in TEM images seemed much brighter than the black edge of nanofibers, which indicated that the hollow and porous structure formed in the three nanofibers.
3.2. Gas Sensing Properties of Cd-Loaded In2O3 Nanofibers
Figure 4 shows the responses of the pure In2O3 nanofibers and the Cd-loaded In2O3 nanofibers to 10 ppm of HCHO at different operating temperatures. It can be seen that the responses of these sensors depends on both the operating temperature and the concentration of Cd. In the range of the operating temperature studied, with the increasing of operating temperature, the response values firstly increased and then decreased for all the sensors. Each curve presents a maximum value at an optimum operating temperature. Besides, among all the sensors based on the above four kinds of nanofibers, the sensors based on In10Cd1 nanofibers exhibit the highest response when the operating temperature was 280°C.
Figure 5 shows the responses of the sensor based on pure In2O3, In20Cd1, In10Cd1, and In1Cd1 nanofibers to different concentrations of HCHO. The concentration range is 0.5–100 ppm. The inset in Figure 5 gives the responses versus low HCHO concentration in the range of 0.5~10 ppm. It can be seen that the responses of the sensors rapidly increased with increasing HCHO concentration from 0.5 ppm to 100 ppm. Compared with sensors based on other Cd-loaded In2O3 nanofibers, the sensors based on In10Cd1 nanofibers show highest response, and there is a good linear correlation between the response of the sensors and the concentration of HCHO at high concentration range. So the specific and more HCHO-sensing properties of gas sensors based on In10Cd1 nanofibers will be discussed below.
Figure 6 shows the response and recovery transient properties of the gas sensor based on In10Cd1 nanofibers to formaldehyde in concentration range of 0.5~100 ppm at an operating temperature of 280°C and relative humidity of 25% RH. We can see from the figure that the nine-response cycles are successively recorded, and the gas sensor based on In10Cd1 nanofibers show good sensitivities to formaldehyde. As shown in the inset in Figure 6 that the response and recovery times of the sensor based on In10Cd1 nanofibers were 160 s and 70 s to 10 ppm formaldehyde, respectively.
Figure 7 shows the responses of the sensor based on In10Cd1 nanofibers to formaldehyde, ethanol, methanol, toluene, acetone, and ammonia with concentration of 10 ppm and 30 ppm, respectively. The results indicated that the sensor based on In10Cd1 nanofibers exhibited a significantly higher response to formaldehyde than to other gases at the operating temperature of 280°C.
3.3. Mechanisms of HCHO-Sensing Based on Cd-Loaded In2O3 Nanofibers
In2O3 is an -type semiconductor material and its gas sensing mechanism can be explained by the doping mechanism and external chemisorbed oxygen model. Change of material resistance is dependent on the species and the amount of oxygen chemisorbed on the surface of the sensing material. When metal ions Cd2+ was doped into In2O3 nanofibers, part of free electrons in In2O3 recombined with new generated holes as reaction (4), which is caused by that part of In3+ in In2O3, was displaced by Cd2+ as reaction (3) . Meanwhile, when the -type semiconductor oxides such as In10Cd1 are exposed in the air, the oxygen molecules will be adsorbed onto the surfaces of the materials and ionized to oxygen ions in the main form of O−  by capturing free electrons from the conduction band of the oxide as shown in Figure 8(a) and reaction (5) . The reaction (3)–(5) will cause a decrease of the carrier concentration and an increase of the resistance in air (), which will enhance the response of the material () according to the following:
(a) In air
(b) In formaldehyde
When the n-type semiconductor oxides (In10Cd1) meet reducing gas like formaldehyde, the formaldehyde gas molecules react with the adsorbed oxygen ions (O−) generated by reaction (5) and release the trapped electrons back to the conduction band of the sensitive materials as shown in Figure 8(b) and reaction (6), resulting in an increase of the carrier concentration and a decrease of the resistance () :
As we know that In2O3 nanofibers doped with certain concentration Cd may increase the selectivity and sensitivity and also reduce the response and recovery times of MOS gas sensors because the additive (Cd) changes the base oxides’ interface state, which results in a variation of the surface barrier height and finally leads to a conductance change on the base oxide . But Cd-loaded In2O3 nanofibers with other Cd/In molar ratios such as 1/20 and 1/1 did not enhance the formaldehyde gas properties as we expected. Next, the role of different concentration of Cd on improving the sensing properties will be discussed from two aspects: the surface-to-volume ratio of the sensing material and the resistance of the sensor (). Grain sizes of Cd-loaded In2O3 nanofibers increased with the increasing of Cd/In molar ratios. As we know, the surface-to-volume ratio increases with the decrease of gain size , which will cause improvement of the surface reactivity. In other words, a decrease of the gain size will enhance sensor response. As we mentioned before, with Cd2+ doping, the resistance of the sensor in air () will increase. And with the increase of the Cd-doped concentration, there will be more Cd2+ replace In3+, meaning will become bigger. Since the response of sensors is defined as: , so the increase of means the increase of response. Moreover, when Cd/In molar ratio is 1/1, new phase CdIn2O4 forms, which may cause the decrease of Cd doping concentration. The effect of those two aspects on sensing response is opposite. When the Cd/In molar ratio is 1/10, with the influence of those two aspects, the sensing response achieves the highest in this experiment.
In summary, Cd-loaded In2O3 nanofibers with different Cd/In molar ratios from 1/20 to 1/1 were prepared by an electrospinning method. The hollow and porous nanofibers are composed of nanoparticles. The average diameter and the nanoparticles size of Cd-loaded In2O3 nanofibers increase with increasing Cd/In molar ratio. Formaldehyde gas sensors were fabricated by coating the obtained materials onto a ceramic tube with a pair of gold electrodes and heaters. The sensors based on Cd-loaded In2O3 nanofibers with Cd/In molar ratio 1/10 (In10Cd1) exhibited higher response in formaldehyde concentration range of 0.5~100 ppm at operating temperature of 280°C than other materials with different Cd/In molar ratios. The response of In10Cd1 based sensors possesses good linearity with formaldehyde concentration in the range of 10~100 ppm. The response and recovery times of the sensor based on In10Cd1 nanofibers were 160 s and 70 s to 10 ppm formaldehyde, respectively. The sensor also exhibited a significantly higher response to formaldehyde than to ethanol, methanol, toluene, acetone, and ammonia. Cd-loaded In2O3 hollow and porous nanofibers with Cd/In molar ratio 1/10 (In10Cd1) have smaller grain size than In1Cd1 and larger resistance of the sensor in air than In20Cd1, which could be one reason for better sensitivity of In10Cd1 than other ratios materials.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors thank The National Natural Science Foundation of China (61176068, 61131004, and 61001054) and the 973 Projects (2011CB302105) for financial support.
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