High selectivity Fe3O4 nanoparticle to volatile organic compound (VOC) for MEMS gas sensors

In the current study, XRD analysis shows the polycrystalline form an inverse spinel Fe3O4 structure. Fe3O4 film is prepared by dip coating method on MEMS gas sensors to test the sensitivity on volatile organic compound (VOC) gas. VOC is being tested at 92 mW (∼300 °C) power consumption with different VOC gas concentrations and also tested with different gases like NO2, SO2, NH3 and CO gas. The results showed that the Fe3O4 gas sensor has better selectivity and high response with VOC 1.2 ppm concentration. Structural morphology is seen and reaction mechanism when VOC gas reacts with Fe3O4 material is also being discussed.


Introduction
There is an industrial impact all over the world which concerns about the quality of air in our environment. Air quality monitoring includes indoor, outdoor and local air monitoring. It all depends on the types of pollutants. Greenhouse gases are the primary concern for polluting the air but volatile organic compounds (VOC) gases like alkanes, alkenes, ketones and aldehydes causes health defects in living beings. VOC gases like methane, benzene and toluene causes breathing problems anemia and narcosis in nervous system [1,2]. Emission of VOC gases occurs mostly by production industries, combustion, transport, agriculture, solvents and landfill [3]. So, there is a high demand for VOC gas detection in homeland security, medical and space exploration applications [4]. Volatile compounds are also used as solvents in paints, adhesives and cleaning liquids which are used in daily life and they easily evaporate causing respiratory problems [5]. In case of food market, quality and odor of the food have became primary issue due to off-odors and pungent smells. Gas sensor arrays have been suggested to detect the VOC gases that release from the plants and crops. This helps in packaging foods and off-odors detection [6].
For gas sensing applications, Metal oxide semiconductors (MOS) are most widely studied and used materials [7]. The sensing mechanism works based on the change in resistance when the gas reacts with the material. In case of VOC gas detection, there has been many research studies and used different kinds of metal oxide semiconductors like ZnO [8][9][10], SnO 2 [11], Co 3 O 4 [12], Graphene composites [13] and Fe 3 O 4 [14,15]. In the current study, Fe 3 O 4 is considered for the detection of VOC gas. Fe 3 O 4 has one of the most unique magnetic properties such as the solubility and simplicity in synthesis in acidic media when compared to other metal oxide nano materials like SnO 2 , TiO 2 . So, Fe 3 O 4 is one of the most researched material for gas sensing [16]. Fe 3 O 4 material is also used for detecting different gases like ethanol at room temperature. Some of the other applications of Fe 3 O 4 material are magnetic storage devices, biomedical filed and separation processes.
In the current study, Fe 3 O 4 has been prepared in various thickness by using dip coating method and different concentrations of VOC gas is being tested. The performance of Fe 3 O 4 gas sensor is being evaluated by monitoring the sensitivity of VOC gas and comparing with some other gas sensitivity like NH 3 , SO 3 , NO 2 and CO gases. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Figure 1 shows a schematic diagram of the general gas sensor MEMS structure. The structure includes a suspension, a dielectric layer, a micro heater and a sensing material. This study uses bilayer materials for the micro heater, the interdigital transducer (IDT) electrode (Ti and Au) and the sensing film (Fe 3 O 4 ). Iron oxide (Fe 3 O 4 ) purchased from LIWEI Nano Tech Co., Ltd at Taiwan with a particle size of about 20 nm (±15%Φ).

Experimental procedure
A p-type 400 μm thick Si wafer was used as the substrate. A 0.6 μm thick SiO 2 and 0.3 μm thick SiNx isolation layer was formed using plasma-enhanced chemical vapor deposition (PECVD). A 400 nm thick Ti layer and a 50 nm thick Au layer were then deposited on the defined pattern. The Ti and Pt layers were deposited using electron gun evaporation. A positive-type photoresist (PR) was spin-coated onto the isolation layer and then standard photolithography was used to define the bottom sensing layer area (400 μm×400 μm). The SiO 2 isolation layer was wet-etched. Finally, the structures on the reverse side were formed using SF 6

Experimental results and discussion
The XRD pattern shows the polycrystalline form of an inverse spinel Fe 3 O 4 structure (JCPDS No. 65-3107) in figure 2. No diffraction peaks for notable impurities (e.g., hematite) are found. From the values of 2θ and β of the (311) peak, the grain size (D) can be estimated by Scherrer's formula D=0.89λ/βcosθ [17], where λ is the x-ray wavelength equal to 0.1542 nm, θ is the (311) peak angle, and β is half the peak width. The average grain size of nanoparticle was about 21.5 nm.
SEM image of Fe 3 O 4 surface structure is shown in figure 3. As the SEM image showed different sizes of Fe 3 O 4 nanoparticles and that led to assume the size of nanoparticles can be in the rage of microns to nano. Different size particles are combined together to form a bulky structures after baking at 70°C. The temperature is set to medium high which is 70°C, where it helps to remove the moisture content. Since the process dispensing coating is used, there are many pores on the surface area which helps the adsorption process when the gas is tested for sensing properties. In the figure, particles size ranges from few microns to hundreds of nanomaters because of the calcination process, clusters will start to form between the particles which results in different particle sizes. Some researchers have suggested that particle agglomeration happens because the particle has relatively large surface area which corresponds to high surface area energy which results in mutual influence between surface areas.
The gas sensor is baked at 70°C and placed inside a glass chamber for detecting the gas sensing characteristics. The temperature should be set to the working temperature and the resistance will be stabilized after some time and then the VOC gas is sent into the chamber for gas sensing. Every gas sensor has different working temperature and it is advised that the temperature is being set to required value to get better sensing results. Figure 4 shows three samples of the response graph of Fe 3 O 4 nanomaterial working at different operating temperatures are considered and VOC gas concentration is taken to be 2.4 ppm. The sensing characteristics of gas sensor with different power consumption is discussed.
The response can be expressed for a particular VOC gas, is calculated using the relation, where Rg is the resistance of Fe 3 O 4 film in presence of VOC gas, Ra is the resistance in presence of the atmosphere. Until 92 mW (∼300°C), higher the power consumption, gas sensitivity is higher, but the sensitivity drops tremendously after 90 mW. Therefore, the operating temperature should be the peak value at 90 mW where the gas sensitivity is about 60%. The reason for sensitivity fall might be due to the resistance value  fluctuations of semiconductor sensor which is caused by the oxidation reaction between the surface oxygen atoms adsorption and VOC gas. At very high temperatures, the kinetic energy of the gas reduces the adsorption capacity of semiconductor sensor which causes the reduction in gas sensing capability. The oxygen atoms in Fe 3 O 4 are of two kinds which are adsorbed oxygen and lattice oxygen. When VOC gas reacts with Fe 3 O 4 material, CO 2 is produced where the oxygen atoms are adsorbed continuously, and lattice oxygen atoms are released continuously. For this phenomenon to happen, the working temperatures should be very high. According to the experimental result, the temperature is found to be 300°C which is not high value. So, this phenomenon does not happen in this procedure since the working temperature is not high. Different gas concentration curves of gas sensor at same temperature are discussed in figure 5. The working temperature of VOC gas sensor is set to be 300°C and the gas concentrations tested are 0.6 ppm, 1.2 ppm, 1.8 ppm, 2.4 ppm. In figure 5. Resistance change in the VOC gas sensor is detected for various gas concentrations. The resistance changes significantly with increase in the gas concentrations. In this study, various VOC gas concentrations are introduced into the chamber from 0.6 ppm to 2.4 ppm and the recorded responses suggest that higher the gas concentration, higher the gas response to the sensor. When the gas is removed, the sensor showed good recovery response and resistance is back to the original position which shows the stability of the gas sensor. The response of an oxide semiconductor is commonly expressed as R=A[C] n +B, where A and B are constants, n is an exponent, and [C] is the target gas concentration. Data fitting provided the following equations for Fe 3 O 4 film: R=21.1[C]+6.6. We found that when the VOC gas concentration exceeds 5 ppm, the response   figure 6. The maximum response is recorded is 30% for VOC gas at 1.2 ppm concentration. The other gases like CO, SO 2, NO 2, NH 3 are also tested with Fe 3 O 4 sensor at a similar concentration of 1.2 ppm and all the four gases has less than 2% gas response. The increasing order of gas sensitivity response is VOC>NH 3 >CO>SO 2 >NO 2 .
The reaction mechanism characteristics for Fe 3 O 4 gas sensor is being shown in figure 7. In the n-type semiconductor metal oxides, the surface of the film is accumulated with the oxygen molecules when the film is exposed to air. The adsorbed oxygen molecules extracts the free electrons in the conduction band and forms oxygen ion species like O 2 − , O − and O 2− . So, electrons from the conduction band are consumed by the oxygen ions and forms a depletion layer over the surface of Fe 3 O 4 . This tends to decrease the concentration of charge carriers in the sensing layers and this results in the increased resistance and decrease in conductivity of sensor [14]. When the Fe 3 O 4 gas sensor is exposed to VOC gas the reaction takes place on the surface of the material as shown in figure 7.  When the sensor is exposed to the VOC gas, the following reaction is expected to happen [18]. The VOC gas molecules interact with the adsorbed oxygen species and release the trapped electrons which leads to decrease the depletion layer. And this reduction in the depletion layer of an n-type Fe 3 O 4 causes the resistance to decrease in the presence of VOC gas [15]. However in the presence of air or humidity, the depletion layer is enlarged due to the oxygen ions accumulation and the resistance is increased [19].

Conclusion
In this study, the variables like power consumption and gas concentration are used to study the sensitivity and material properties. At 92 mW (∼300°C) power consumption, the sensitivity increased linearly with increase in the VOC gas concentration. Gas sensitivity is tested with different gases. The gas of CO, SO 2 , NO 2 and NH 3 are tested at a similar concentration of 1.2 ppm and all the four gases has less than 2% gas response. However, the maximum response is recorded is 30% for VOC gas at 1.2 ppm concentration for Fe 3 O 4 MEMS gas sensor.