Preparation of Cd-Loaded In 2 O 3 Hollow Nanofibers by Electrospinning and Improvement of Formaldehyde Sensing Performance

Pure In 2 O 3 and Cd-loaded In 2 O 3 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 In 2 O 3 nanofibers increased with the increasing of Cd/In molar ratios. The formaldehyde sensing properties of the sensors based on pure In 2 O 3 and Cd-loaded In 2 O 3 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 In 2 O 3 nanofibers with Cd/In molar ratio of 1/10 possessed the highest response value and good selectivity at operating temperature 280C. In addition, the formaldehyde sensing mechanism of the sensors based on Cd-loaded nanofibers was briefly analyzed.


Introduction
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 [1].Up to now, various materials prepared by different methods [2] were used to detect VOCs.Also the testing ways have been widely investigated [3][4][5].Among these methods, gas sensor based on metal-oxide semiconductors (MOS) such as ZnO [6], SnO 2 [7], TiO 2 [8], and In 2 O 3 [9] have been attracting special interest due to their good reproducibility, compact size, ease of use, and low cost.
In 2 O 3 , an important -type semiconductor with a band gap of approximately 3.55-3.75eV 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 In 2 O 3 with various morphologies have been investigated [10][11][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][14][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][19][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 [25].Xu   Cd-loaded In 2 O 3 nanofibers were prepared by electrospinning method.In a typical procedure, 764 mg In(NO 3 ) 3 ⋅ 4.5H 2 O power and 61.6 mg Cd(NO 3 ) 2 ⋅4H 2 O 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 aselectrospun PVP/Cd(NO 3 ) 3 ⋅4H 2 O/In(NO 3 ) 3 ⋅4.5H 2 O composite nanofibers were annealed in tubular furnace at 500 ∘ C to obtain Cd-loaded In 2 O 3 nanofibers with Cd/In molar ratio of 1/10.In the same way, pure In 2 O 3 and Cd-loaded In 2 O 3 with other Cd/In molar ratios were obtained as specific reagents proportion in Table 1.

Characterization of Cd-Loaded
In 2 O 3 Nanofibers.The structures of the nanofibers were characterized by an X-ray diffraction instrument (XRD: D/Max 2400, Rigaku, Japan) in 2 region of 20∼80 ∘ with Cu K1 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.

Fabrication and Measurement of Gas Sensors.
As-prepared Cd-loaded In 2 O 3 nanofibers with different Cd/In molar ratios of 1/20, 1/10, and 1/1 were marked as In 20 Cd 1 , In 10 Cd 1, and In 1 Cd 1 , 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 m).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).The gas-sensing properties of gas sensor based on Cdloaded In 2 O 3 nanofibers were measured using a static state gas-sensing test system [28].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 [29].
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 V% 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   =   (10 −  air )/ air ,   =   (10 −  gas )/ gas ,  air , and  gas are the voltages applied to the resistor   in air and in target gas, respectively.Response time  1 and recovery time  2 are defined as the time spent to achieve 90% of gas response change in adsorption and desorption process, respectively.

Gas Sensing Properties of Cd-Loaded In 2 O 3 Nanofibers.
Figure 4 shows the responses of the pure In 2 O 3 nanofibers and the Cd-loaded In 2 O 3 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 In 10 Cd 1 nanofibers exhibit the highest response when the operating temperature was 280 ∘ C. nanofibers, the sensors based on In 10 Cd 1 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 In 10 Cd 1 nanofibers will be discussed below.recovery times of the sensor based on In 10 Cd 1 nanofibers were 160 s and 70 s to 10 ppm formaldehyde, respectively.Figure 7 shows the responses of the sensor based on In 10 Cd 1 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 In 10 Cd 1 nanofibers exhibited a significantly higher response to formaldehyde than to other gases at the operating temperature of 280 ∘ C.

Mechanisms of HCHO-Sensing Based on Cd-Loaded
In 2 O 3 Nanofibers.In 2 O 3 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 Cd 2+ was doped into In 2 O 3 nanofibers, part of free electrons in In 2 O 3 recombined with new generated holes as reaction (4), which is caused by that part of In 3+ in In 2 O 3, was displaced by Cd 2+ as reaction (3) [30].Meanwhile, when the -type semiconductor oxides such as In 10 Cd 1 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 − [31] by capturing free electrons from the conduction band of the oxide as shown in Figure 8(a) and reaction (5) [32].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:  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 (  ) [33]: As we know that In 2 O 3 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 [34].But Cd-loaded In 2 O 3 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 In 2 O 3 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 [35], 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 Cd 2+ doping, the resistance of the sensor in air (  ) will increase.And with the increase of the Cd-doped concentration, there will be more Cd 2+ replace In 3+ , 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 CdIn 2 O 4 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.

Conclusion
In summary, Cd-loaded In 2 O 3 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 In 2 O 3 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 In 2 O 3 nanofibers with Cd/In molar ratio 1/10 (In 10 Cd 1 ) 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 In 10 Cd 1 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 In 10 Cd 1 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 In 2 O 3 hollow and porous nanofibers with Cd/In molar ratio 1/10 (In 10 Cd 1 ) have smaller grain size than In 1 Cd 1 and larger resistance of the sensor in air than In 20 Cd 1 , which could be one reason for better sensitivity of In 10 Cd 1 than other ratios materials.
et al. prepared Au-loaded In 2 O 3 nanofibers-based ethanol microgas sensor and found that the 0.2 wt% Au-loaded In 2 O 3 based sensors possessed the characterization of high response, fast response/recovery, and low power consumption [26].Wang et al. prepared Agdoped In 2 O 3 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 [21].Xu et al. investigated room H 2 S-sensing properties based on porous In 2 O 3 :REE (REE = Gd, Tb, Dy, Ho, Er, Tm, Yb) nanotubes

Figure 1 (
b) gives the SEM picture of the In 10 Cd 1 showing morphology and structure of nanofibers coated on the surface of the ceramic tube.

Figure 1 :Figure 2 :
Figure 1: (a) The mechanism of test circuit and structure of the gas sensor and (b) SEM images of In 10 Cd 1 nanofibers coated on the surface of the ceramic tube.
Cd-Loaded In 2 O 3 Nanofibers.Figure 2 shows the XRD patterns of the pure In 2 O 3 nanofibers and Cd-loaded In 2 O 3 nanofibers with different Cd/In molar ratios.We can see that the In 2 O 3 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 In 2 O 3 (JCPDS: 65-3170).Meanwhile, there are no obvious Cd peaks in the pattern for In 20 Cd 1 and In 10 Cd 1 nanofibers, which may be due to the low concentration of Cd.And we can see that three kinds of diffraction peaks of In 2 O 3 , CdO, and CdIn 2 O 4 appeared in curve of In 1 Cd 1 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 In 2 O 3 , In 20 Cd 1 , In 10 Cd 1 , and In 1 Cd 1 , respectively.The general morphologies of the as-electrospun pure In 2 O 3 and Cd-loaded In 2 O 3 nanofibers with different Cd/In molar ratios (In 20 Cd 1 , In 10 Cd 1, and In 1 Cd 1 ) 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 afterannealed In 2 O 3 nanofibers (∼156 nm) is much bigger than that of Cd-loaded In 2 O 3 nanofibers.And the average diameter of Cd-loaded In 2 O 3 nanofibers increased with the increase of Cd/In molar ratios.Figures 3(b), 3(c), and 3(d) show obviously that the diameters of In 20 Cd 1 , In 10 Cd 1, and In 1 Cd 1 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 In 2 O 3 , In 20 Cd 1, and In 10 Cd 1 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.

Figure 5 Figure 3 :Figure 4 :
Figure 3: SEM images of (a) pure In 2 O 3 nanofibers, (b) In 20 Cd 1 nanofibers, (c) In 10 Cd 1 nanofibers, and (d) In 1 Cd 1 nanofibers; The inset in each graph is high magnification TEM image of each nanofiber.

Figure 6 Figure 5 :Figure 6 :
Figure 5: Response versus HCHO concentration for sensors based on pure In 2 O 3 and Cd-loaded In 2 O 3 nanofibers at each optimum operating temperature.The inset gives the responses versus low HCHO concentration.

Figure 7 :
Figure 7: Responses of the sensor based on In 10 Cd 1 nanofibers to 10 and 30 ppm different gases.

Figure 8 :
Figure 8: The adsorption model of Cd-loaded In 2 O 3 nanofibers in air and in formaldehyde gases, respectively.

Table 1 :
[27]rimental parameters and characterized results.theresponse of the sensors based on In 2 O 3 doped with Yb is around 7 times as much as pure In 2 O 3[27].There are rare reports about gas sensors based on electrospun In 2 O 3 nanofibers doped with common metal elements such as Cd.In this report, In 2 O 3 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 beltshaped electrodes.Then the sensing properties of sensors based on Cd-loaded In 2 O 3 nanofibers with different Cd/In molar ratios were tested and discussed.