Fe₂O₃ nanowires for thermoelectric nitrogen dioxide gas sensor

A gas sensor is a device that converts the composition, concentration or other signals of a gas to be measured into easily observable signals, generally electrical signals. Different types of gas sensors are based on different principles.^1–3 For example, the most widely used conductivity-controlled semiconductor gas sensor is to convert the signals of the gases to be tested into impedances that change accordingly. The semiconductor gas sensor, thermoelectric gas sensor, based on the temperature difference converts the signals of gases into thermoelectromotive forces. In this paper, based on the thermoelectric effect, the gas sensor that is highly sensitive to NO₂ is prepared using the semiconductor Fe₂O₃ nanowires.

Transition metal oxides such as SnO₂, ZnO, TiO₂ and Fe₂O₃ were commonly used in the fabrication of solid-state devices like gas sensors.⁴ Fe₂O₃, which is the most stable among all the iron oxides, includes three forms, namely α-Fe₂O₃, β-Fe₂O₃, and γ-Fe₂O₃, is a semiconductor with a band gap of only 2.1 eV. One-dimensional Fe₂O₃ nanomaterials present good semiconductor properties and electromagnetic properties, so they find wide use in the development of nanodevices, such as lithium-ion batteries,⁵ photovoltaic devices,⁶ optoelectronic devices,⁷ and storage materials.⁸ It is also a promising material for gas sensing application. In fact, Fe₂O₃ nanowires were composed of many well-aligned nanowires, and many nanowires grain boundaries formed in it, so the sensor was expected to have short response-recovery time and high sensitivity, because grain boundaries or grain junctions are considered as the active sites and they acted positively on the performance of sensor.^5,9

In the present study, we have investigated the microstructure and morphology of Fe₂O₃ wires. These wires were produced by the oxidization process at different temperatures. Fe₂O₃ sensing element with a heater were embedded in a commercial case for the fabrication of a complete sensor. And finally, the electrical response and sensitivity of our cost effective gas sensors were measured under various concentrations of NO₂ at 120°C.

The Fe₂O₃ nanowires were synthesized in a horizontal electrical furnace, as described in detail elsewhere.¹⁰ It can be also seen in Fig. 1. First, the iron sheet with thickness 0.1 mm was ultrasonic cleaned, and then was placed on the boat. The whole was positioned in the center of the tube. Then, the furnace was heated to 600°C at a heating rate of 10°C min^-1 under Argon with a flow rate of 100 SCCM. Once the furnace temperature was up to 600°C, air was then added and maintained at this temperature for 2 h. Finally, one layer of wires products was found on the sheet after the furnaces naturally cooled down to room temperature. For the products characterization such as the morphology, and the crystallographic structure, SEM (Hitachi-S3000N) and XRD (D/MAX-gA, CuKα radiation) were used.

On the thin ceramic sheet, the heating resistor and the electrodes were painted with the conductor paste, and the back surface facing the electrode away from the heater resistor was also coated with the conductor paste so that the heat sink can be mounted. The pattern of the conductor paste on the ceramic sheet was sintered at the firing temperature. Between the two electrodes were applied sensitive films, which were connected in series to obtain a large temperature difference electromotive force. The structure of the device is shown in Fig. 2. When the power is on, the heat generated by the heater resistor raises the temperature of the ceramic, and the heat is transferred on the ceramic sheet and the heat sink. Finally, a temperature gradient is formed in a direction parallel to the ceramic sheet (shown by an arrow in the Fig. 2).

The procedure of preparing the sensitive film is as follows: firstly, grind an appropriate amount of products with an agate mortar to prepare the paste; then, apply the paste to the sensitive film shown in Fig. 2 to form a thick film by screen printing; finally, sinter the film in air at 600°C for 2 hours. The Fe₂O₃ gas sensor device based on the thermoelectric was prepared. When the device is in use, the temperature of the end of the sensitive film near the heater resistor is higher than the end away from the heater resistor, thereby generating a thermoelectromotive force. The measurement in the millivoltmeter is the accumulation of the thermoelectromotive forces generated by all sensitive films.

The polished Fe sheet was placed in a tube furnace, which was heated at 600°C under the air for 2 hours, 4 hours, 6 hours, and 8 hours, respectively. It can be seen from Fig. 3a that a few sparse Fe₂O₃ nanowires grow on the surface of the Fe sheet at the reaction time of 2 hours, and their length is about 5 μm. The base diameter of the nanowires ranges from 50 nm to 100 nm, while the tip diameter is 20 nm. When the sheet was heated for 4 hours, the Fe₂O₃ nanowires shown in Fig. 3b become denser, with the base diameter increasing to 100 nm, the tip diameter remaining 20 nm, and the length growing to nearly 10 μm. When the sheet was heated for 6 hours, the nanowires shown in Fig. 3c covered the surface of the sheet with a diameter of 150 nm and a length of more than 10 μm. After 8 hours of reaction, the surface was covered with a 20-μm-thick layer of Fe₂O₃ nanowires with a diameter of about 150 nm. The nanowires shown in Fig. 3d became denser and longer with the reaction time, while the diameter did not increase any more. As a result, the diameter of Fe₂O₃ nanowires is not necessarily related to the reaction time, which indicating that Fe₂O₃ grows in the vertical direction during the process, and the growth direction has good orientation, which can be verified by the XRD analysis.

Fig. 4 shows XRD patterns of Fe₂O₃ nanowires synthesized at different temperature. When the Fe sheet was oxidized at 300°C, only two Fe diffraction peaks were detected and the Fe₂O₃ diffraction peaks could not be detected, indicating that Fe on the surface of the sheet was not completely oxidized to Fe₂O₃. At the temperature of 400°C, weak Fe₂O₃ diffraction peaks appeared. When the temperature reached 500°C, the Fe₂O₃ diffraction peaks were enhanced, while those of Fe were weakened, which showed the presences of both Fe₂O₃ and Fe that came from the substrate or the surface of the Fe sheet that was not oxidized. When the temperature was above 600°C, the Fe₂O₃ diffraction peaks increased, while those of Fe decreased rapidly, and there were no diffraction peaks of other impurities.

By comparing the XRD patterns with the standard diffraction card JCPDS 03-0800, it can be derived that the prepared nanowire was α-Fe₂O₃, which belongs to the oblique hexagonal system with lattice constants a = b = 0.5028 nm and c = 1.726 nm. The XRD results show that the Fe diffraction peaks decrease with temperature, while the Fe₂O₃ diffraction peaks increase. When the temperature is higher than 600°C, there are almost no Fe diffraction peaks, indicating that the higher temperature leads to more thorough oxidation of Fe and higher production of Fe₂O₃.

Fig. 5 shows the measured voltage signal of the thermoelectric nitrogen dioxide sensor based on Fe₂O₃ nanowires that were synthesized at 600°C for 8 h. The output signal voltage, V_s, corresponding to the 10 ppm of NO₂ gas was 17.9 mV, when the temperature gradient was set at 120°C. The response time, T90, which is the time reaches 90% of Vs, and the recovery time, TD90, which is the time decrease down to 10% of Vs. The T90 and TD90 were 23 s and 17 s, respectively. In comparison with the results of other gas sensors for NO₂ detection used in the recent study, the thermoelectric gas sensor towards 10 ppm NO₂ were superior to them. Rakesh K.Sonker et al.,¹¹ have synthesized ZnO nanopetal using chemical route, whose response and recovery time towards 20 ppm was 1.42 min and 1.71 min, respectively. J.Y Lin et al.(2016),¹² have fabricated a novel gas sensor which based on Hall effect, whose response of 3.27 was achieved at 40 ppm of NO₂ with a response and recovery times of 36 s and 45 s, respectively. A. Mirzaei et al.(2016),¹³ reported that, a-Fe₂O₃ conductivity-controlled sensors for gas sensing applications are generally more sensitive to NO₂, their operation temperature is between 250 and 450°C, which is higher than thermoelectric nitrogen dioxide sensor. The high response and fast response-recovery of the thermoelectric nitrogen dioxide sensor could be attributed to structure of gas sensor and morphology of Fe₂O₃. Fe₂O₃ nanowires that composed of many well-aligned nanowires could give a fine pathway for electron transfer in the gas sensing process and is expected to be advantageous for gas sensing application due to availability of diffusion channel inside the nanowires. On the other hand, the nanowires provides higher surface area to volume ratio and which enormously contributes to the gas diffusion process, results in the improvement of gas sensing properties.^5,9

The voltage signal gradually increased to Vs level after T90, and the fluctuation of signal is small until the conditions such as target gas concentration or temperature gradient are changed. At the same time, the voltage signal gradually decreased to the zero level again and showed no noticeable drift after TD90.

The signal voltage, Vs of NO₂ increases linearly with temperature below 100°C and saturates above 120°C, as shown in Fig. 6. The relation between temperature gradient and the voltage signal can be explored that the voltage signal difference could be attributed to the heat generated by the oxidation reaction but not to the thermoelectric power because thermoelectric power of the Fe₂O₃ sensitive film in this temperature region is very small. Therefore, these temperature gradient dependences were mostly due to the exothermic, oxidation of the gas.¹⁴ 

The measured voltage signal, Vs, of the thermoelectric gas sensor based on Fe₂O₃, exposed to various NO₂ concentrations at temperature gradient 120°C was shown in Fig. 7. The NO₂ concentration was varied from 2 to 70 ppm, and the measured voltage was unstable when the NO₂ concentration is below 2 ppm. For the NO₂ concentration from 2 to 70 ppm, the output voltages are more stable. The sensor showed very good linearity of the Vs in lower detection, with the relationship such that 1.8 mV voltage signal roughly corresponds to 2 ppm NO₂. At higher concentration, the Vs deviates from linearity and increases more slowly. Furthermore, the response time (T90) and the recovery time (TD90) of the thermoelectric gas sensor for different concentrations of NO₂ at temperature gradient 120°C were shown in Table I. From the data shown in Table I, the T90 and TD90 for different NO₂ concentration that was varied from 2 to 70 ppm were less than 40 s, but was not monotonic increasing with increase or decrease in NO₂ concentrations. The reason is that the operating temperature are different in each active sites for surface reactions in the sensitive films.

The selectivity is a very important parameter of gas sensor, and it is defined as NO₂ against other interfering gas species. S_i=V_S(NO₂)/V_S(i), where V_S(i) is output voltage of other gas species. Theoretically, the sensors are expected to have high response to certain gas and little or no response to other gases in the same surroundings. Therefore, in order to study the selectivity of the thermoelectric sensor, the responses of thermoelectric sensor were measured for various reducing (NH₃, H₂S, C₂H₅OH) and oxidizing (Cl₂) gases, each with a various concentration from 10 to 100 ppm. All output voltages were measured at temperature gradient 120°C and the corresponding results are shown in Fig. 8.

In this part, the mechanism of the sensor is only analyzed in terms of thermoelectric effect of semiconductors and the adsorption mechanism of gases.

When there is a temperature difference between the two ends of a semiconductor, and there is a temperature gradient inside, a thermoelectromotive force is generated across the two ends, which is called the thermoelectric effect of semiconductors (also known as Seebeck effect).¹⁵ 

As a temperature gradient applied on a semiconductor, the charged carriers (electrons or holes) in the hotter region, being more energetic (and, therefore, having higher velocities) will diffuse towards region of lower temperature. The diffusion stops when the electric field generated because of movement of charges has established a strong enough field to stop further movement of charges. For an n-type semiconductor, carriers being negatively charged electrons, the colder end would become negative so that Seebeck coefficient is negative.

One-dimensional oxide nanomaterials such as ZnO and Fe₂O₃ that were used as semiconductor gas sensor materials generally operate at high temperature due to the requirement of gas chemisorption. The semiconductors are in the transition region or the high temperature intrinsic excitation region. Therefore, the Seebeck effect should be analyzed in the presence of both carriers. In addition, according to the relevant literature,^16–18 thermal radiation dominates because of the high operating temperature, so it can be considered that there are no temperature gradient on the grain junction, the temperature gradient exists only inside the grain, and the phonon enthalpy effect can be ignored.

The absolute thermoelectromotive force power of a semiconductor was defined as follows:

Where Θs(T) is the thermoelectromotive force across the ends of the thick film, dΘ_s(T) is the thermoelectromotive force across the ends of the single crystal grain, dT is the temperature difference between the two ends of the single crystal grain, and αs represents the thermoelectromotive force caused by the unit temperature difference.

According to related theories of the semiconductor thermoelectromotive force,^19,20 the thermoelectromotive force power αs of a semiconductor at a certain temperature is dependent on the concentrations of the two carriers. Based on $dΘsT=αs*dT$⁠, it can be derived that dΘ_s(T) also depends on the concentrations of the two carriers. For an oxide semiconductor thick film made of a polycrystalline material, the Seebeck effect can be regarded as the sum of the effects of many single crystals, so the thermoelectromotive force Θ_s(T) across the thick film also relies on the concentration of the two carriers.

At the operating temperature of the Fe₂O₃ nanomaterials, the gas adsorption is mainly chemisorption, that is, the adsorbed oxygen molecules capture free electrons from Fe₂O₃ and become ionized molecular or atomic oxygen species.¹³ Therefore, Fe₂O₃ shows n-type behavior.^21,22 When the sensor was exposed to air, oxygen was chemically adsorbed onto the surfaces of the nanowires to form a surface state. This process can be expressed by the following equation:^23,24

The adsorption of oxygen by the above particles causes the decreased concentration of electrons inside the microcrystal grains of the Fe₂O₃ semiconductor. If the sensor was exposed to NO₂, for one thing, NO₂ is an oxidizing gas, so electrons on the conduction band are absorbed during the process and the following reaction occurs:

For another, NO₂ also stimulates $O2ads−$ to be transformed into $Oads−$⁠, which can be expressed by the following equation:

In addition, $(NO2)ads−$ prompts the transformation of $O2ads−$ into $Oads−$⁠, which can be expressed by the following two equations:

It is the above reactions that cause a large number of free electrons to be adsorbed on the surface of the particles to form a charge layer, which reduces the electron concentration inside the particles and doesn’t change the hole concentration, thereby leading to the increases of absolute value of Seebeck coefficient and the thermoelectromotive force.

Finally, it should be noted that the Seebeck coefficient depends on the carries and that NO₂ sensing performances are closely related to the diameter of the Fe₂O₃ nanowires, associated doping and the thickness of film, which still needs more study.

NO₂ gas sensor based on thermoelectric was successfully fabricated, and it is different from the conductivity-controlled semiconductor gas sensor in structure, mechanism. The typical characteristic of this thermoelectric nitrogen dioxide gas sensor was that, the voltage signal, corresponding to the 10 ppm of NO₂ gas was 17.9 mV, the response time and the recovery time were 23 s and 17 s, respectively, when the temperature difference was set at 120°C. The lower detection limit of sensor at 120°C was lower than 2 ppm and good linearity of the signal voltage in lower detection was showed. It also had excellent selectivity, and these results indicated that the Fe₂O₃ nanowires is found to be very attractive NO₂ gas sensing material, and the Fe₂O₃ nanowires for thermoelectric nitrogen dioxide gas sensor has potential application in the field of gas sensors.

J.Y. Lin. acknowledges a fellowship from the China Scholarship Council (CSC) and thanks the support from National Display Joint Engineering Laboratory for Flat Panel Display Technology (No. KF1802). S.H.Huang thanks the support from the National Natural Science Foundation of China (No.61604041).