Synthesis and No_x Gas Sensing Properties of In_1.82 ni_0.18o₃ Electrospun Nanofibers

Abstract

The detection and control of nitrogen oxides (NOX) in exhaust gases emitted by combustion engines has been an important subject in the last decades. Regulations of vehicle emissions focus on the minimization of NOX in automotive exhaust gases, particularly in lean combustion exhaust gases. Fast response times and high sensitivity of NOX sensor in lean combustion environments are necessary to meet those regulations. In this paper a new sensing material (In_1.82Ni_0.18O₃ nanofibers) was synthesized via a simple and effective electrospinning method. The morphology and crystal structure of the as-prepared samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectra (XPS). Potentiometric-type NOX sensor based on yttria-stabilized zirconia (YSZ) with In_1.82Ni_0.18O₃ nanofibers sensing electrode was prepared and its gas sensing properties were also tested. The results show that large-scale In_1.82Ni_0.18O₃ nanofibers with diameters ranging from 40 to 80 nm and lengths of several tens of micrometers were successfully synthesized by this technique. A loose reticular porous non-woven lap structure was formed by many fibers. The results of sensing tests show that the sensitivity ∆EMF of sensor prepared can reach 85 mV for 500 ppm NO, and the ∆EMF is stable. Moreover, the sensor also exhibited fast response time and good selectivity.

F2012-D02-016

1 Introduction

Lean-burn gasoline and direct-injection diesel engines offer the possibility of significant improvements in automotive fuel efficiency. Their developments, however, also induce high NOX emissions [1, 2]. NOX gas has caused serious damage to human health and the surrounding environment. In order to reduce the emission of nitrogen oxide and implementation of stringent regulations of NOX emission require the development of new technologies for NOX gas sensor which can work steady in cruel combustion gas environment. Several publications have described a method of NOX sensing based on the electrochemical oxygen pump cell using yttria-stabilized zirconia (YSZ). According to the introduction of Patent EP1942338A1, the NOX sensor consists of two internal cavities and three oxygen pumping cells [3]. Its measuring concept consists of lowing an oxygen concentration of a measuring gas to a predetermined level in the first internal cavity, in which NOX does not decompose, and further lowing the oxygen concentration of the measuring gas to a predetermined level in the second internal cavity. The second cavity also contains a NOX detection cell (the third oxygen pumping cells) with a rhodium catalytic electrode which has NOX reduction catalytic activity. Therefore, NOX decomposes on the measuring electrode and the oxygen generated is detected as an oxygen pumping current which is in proportion to NOX concentrations.

Patent 97117135.1[4] has also reported a new method by measuring the electromotive force (EMF) between measuring electrode and reference electrode. This new method works as potentiometric sensor. It should be noted that, regardless of the pump current or electromotive force (EMF), the catalytic electrode materials was very critical, because it direct effects the sensitivity and response time of the NOX sensor.

At present, the catalytic electrode usually using noble metals, such as Pt, Rh et al., as Patent US2010/0243447 A1, EP 2107366A2, US 2008/0156644 A1 reported[5–7]. However, the noble metal will lose its catalytic activity after undergoing long-term high temperature aging and exposure to toxic gases (such as SO₂ and Pb). The failure of noble metal seriously affects the life and sensitivity of gas sensor.

Therefore, it is urgent to develop non noble metal catalyst electrode materials. In₂O₃ with wide band-gap, good catalysis and high electric conductance has aroused significant interests in recent years. It has been proven to be an excellent sensing material for detection of many toxic and combustible gases after doped with metal ions in its crystal lattice. Because the dopant metal ions could be introduced into the structure of host material to change its lattice parameters, leading to a larger lattice distortion. The larger lattice distortion is beneficial for interaction between gas and material surface.

In addition, as a result of indium oxide gas-sensitive mechanism is based on the tested gas adsorption and surface reaction, therefore, higher specific surface area of gas sensitive material is beneficial for gas sensing performance improvement. Recently, interest in one-dimensional (1D) has been greatly stimulated because of its increased surface-to-volume ratio and high density of surface sites. Considerable efforts have been made to fabricate 1D sensing nanomaterials via thermal oxidation, thermal evaporation, self-catalytic growth, molten salt synthesis, and electrospinning [8]. Electrosping, as a simple and versatile method, has gained great interest because it can produce 1D nanofibers with high long-diameter ratio, which makes the electron transport more effective and improves the performance of gas sensors [9].

In this paper, a new type gas sensitive material (In_1.82Ni_0.18O₃ nanofiber material) was prepared by a relatively simple electrospinning technique at the first time. Potentiometric-type NOx sensor based on yttria-stabilized zirconia (YSZ) with In_1.82Ni_0.18O₃ nanofibers sensing electrode was prepared and its gas sensing properties were also tested.

2 Experimental

To prepare In_1.82Ni_0.18O₃ solid solution nanofibers, 0.38 g In(NO₃)₃·4.5H₂O and 0.025 g Ni(CH₃COO)₂·4H₂O powders were added to 8.8 g mixed solvent contained DMF/EtOH with the weight ratio of 1:1 and stirred for 2 h. Then 0.8 g PVP was added to the above solution with stirring for 6 h. The obtained solution was then loaded into a plastic syringe and connected to a high-voltage power supply. 20 kV was provided between the cathode (a flat aluminum foil) and the anode (syringe) at a distance of 25 cm. In order to remove PVP completely, the composite nanofibers were calcined in air at 600 °C for 4 h. Then, 10 at. % In_1.82Ni_0.18O₃ nanofibers were obtained.

Details of the design and fabrication of the potentiometric gas sensors are given in [10]. Figure 1 shows a schematic image of the as-fabricated sensor. Figure 2 shows an image of a real sensor.

Fig. 1

Schematic image of the sensor

Fig. 2

An image of a real sensor

Gas sensing properties were measured using a dynamic test system. The sensors were tested in a Micro reactor which was connected to several gas reservoirs. Gas mixtures were regulated by mass flow controllers and computer control. Available gases were NO, NO₂, CO₂, CO. A temperature sensor was installed near the gas sensor. Temperature in the gas chamber was stabilized at 600  C. The sensitivity of the sensor is defined as the ∆EMF. The time taken by the sensor to achieve 90 % of the total EMF change was defined as the response time or the recovery time.

The samples were characterized by X-ray diffractometer (XRD) (Shimadzu XD-3AX), field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700F at 3 kV), transmission electron microscopy (TEM) (HITACHI H-8100 using an acceleration voltage of 200 keV), high–resolution transmission electron microscopy (HRTEM JEM–3010), X-ray photoelectron spectra (XPS) (ESCALAB Mark II spectrometer with Al Kα radiation).

3 Results and Discussion

3.1 Materials Characterizatics

In order to confirm that the Ni ions were incorporated into the lattice structure, XRD was performed. The crystalline structures of In₂O₃ nanofibers and In_1.82Ni_0.18O₃ solid solution nanofibers were characterized by X-ray diffraction (XRD) patterns, as shown in Fig. 3. It can be seen that a slight shift of XRD peak to higher angle for the In_1.82Ni_0.18O₃ samples (Fig. 3b) compared with that of pure In₂O₃ (Fig. 3a). For each samples, all the observed diffraction peaks can be indexed to cubic indium oxide (JCPDS file NO. 06-0416), and no additional peaks for other phases have been found. The lattice parameters of In_1.82Ni_0.18O₃ samples (a = 10.091 Å) is slightly less than that of pure In₂O₃ (a = 10.118 Å), which is consistent with the formation of a substitution solid solution (In_1.82Ni_0.18O₃) [11]. It should be noted that although the ionic radius of Ni²⁺ (r = 0.78 Å) is bigger than that of In³⁺ (r = 0.72 Å), the lattice parameters decrease after substitution. This phenomenon can be explained by considering the change of Ni ions state as follow.

Fig. 3

XRD patterns of a pure In₂O₃ and b In_1.82Ni_0.18O₃ nanofibers

In order to maintain the charge neutrality, an electron exchange process takes place in In_1.82Ni_0.18O₃ solid solution structure. It leads to a partial transition of Ni²⁺ ions into Ni³⁺ which has smaller ionic radius (0.56 Å).

$$ Ni^{2 + } + In^{3 + } \leftrightarrow Ni^{3 + } + In^{2 + } $$

(1)

The presence of Ni³⁺ was confirmed by the next XPS.

Determination of the state of the Ni ions was carried out by measuring Ni 2p3/2 binging energy (BE) with XPS and shown in Fig. 4. The BE 855.6 eV is assigned to Ni³⁺ [12] which comes from the process of an electron exchange (formula 1), and in an octahedral oxygen neighbourhood in the In₂O₃ crystal lattice [12]. The appearance of Ni³⁺ ions is further confirmed the formation of In_1.82Ni_0.18O₃ substitution solid solution. The BE 854.1 eV is assigned to Ni²⁺ which also in the In₂O₃ crystal lattice, and created more vacant oxygen which leading more oxygen species absorbed on the surface of In_1.82Ni_0.18O₃ solid solution nanofibers.

Fig. 4

XPS spectrum of Ni 2p3/2 peak of In_1.82Ni_0.18O₃ nanofibers

The general morphologies of the In_1.82Ni_0.18O₃ nanofibers were studied with field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), and high–resolution transmission electron microscopy (HRTEM). Large-scale nanofibers with diameters ranging from 40 to 80 nm and lengths of several tens of micrometers can be found in the FE-SEM images (Fig. 5) at different magnifications. A loose reticular porous non-woven lap structure was formed by many fibers and the average diameter of these nanofibers is about 50 nm. From the TEM image (Fig. 6a), it can be seen that each nanofiber consists of many ultrafine particles with an average diameter of 20 nm. Lattice images are clearly observed from the HRTEM image (Fig. 6b), indicating the In_1.82Ni_0.18O₃ nanofibers are highly crystalline. The interplaner spacing of 0.41 nm are corresponds to the (211) planes of cubic In₂O₃.

Fig. 5

FE-SEM images of In_1.82Ni_0.18O₃ solid solution nanofibers at different magnifications

Fig. 6

a TEM and b HRTEM images of In_1.82Ni_0.18O₃ solid solution nanofibers

4 Sensing Characteristics

Figure 7 shows the correlation between the sensitivity ∆EMF and the NO concentration for the sensor using In_1.82Ni_0.18O₃ nanofibers-measuring electrode. It is seen that the sensitivity ∆EMF can reach 85 mV for 500 ppm NO. The sensitivity of the sensor as a function of stepwise increasing the NO concentration from 0 to 500 ppm was shown in Fig. 8. At an NO concentration of 40 ppm, the response time was very fast (about 1 s). After purging of NH₃ from the gas phase, the sensitivity was quickly recovered to the initial level. In addition, at each NO concentration, a stable ∆EMF value was observed. The quick response and recovery characteristics of our sample are based on its structures. The large surface of the nanofibers makes the absorption of target gas molecules on the surface of the sensor easily. Simultaneously, the high long-diameter ratio of the nanofibers makes the electron transport more effective.

Fig. 7

Dependence of ∆EMF on the NO concentrations for the sensor

Fig. 8

The response and recovery characteristics of the sensor at different NO concentrations

To further understand the practicability of our fibers, the sensor was exposed to various 400 ppm gases at 600 °C. Most of the tested gas mixtures were similar to typical exhaust gases from lean burn engines. The selectivity shown in Fig. 9 indicates that the In_1.82Ni_0.18O₃ nanofibers are less sensitive to NO₂, totally insensitive to CO₂, and negative sensitivity to CO. Thus the obtained nanofibers exhibit prominent and good selectivity.

Fig. 9

The selectivity of the sensor at different gases

5 Conclusion

In summary, large-scale In_1.82Ni_0.18O₃ nanofibers with diameters ranging from 40 to 80 nm and lengths of several tens of micrometers were successfully synthesized through an electrospinning method. A loose reticular porous non-woven lap structure was formed by many fibers. Potentiometric-type NOx sensor based on yttria-stabilized zirconia (YSZ) with In_1.82Ni_0.18O₃ nanofibers sensing electrode was prepared. The results of sensing tests show that the sensor exhibited high and stable sensitivity ∆EMF, fast response time and good selectivity. The results demonstrate that In_1.82Ni_0.18O₃ nanofibers have excellent potential applications for fabrication high performance NO_X sensors.