Magnesium Ferrite Sensor for H2S Detection

The composite effects of p-type MgO and n-type MgFe2O4 on H2S detection were investigated. The mixture state of these oxides was fixed under coprecipitation conditions: mixture ratio of iron source to magnesium source, and pH control of precursor solution by dropping of an alkaline solution. The composite mixture of MgO, Fe2O3, and MgFe2O4 was obtained by adjusting the mixture ratio of Fe to Mg. The resistance in air monotonically increased with increasing MgO content in the matrix of MgFe2O4. In contrast, the resistance in air decreased with increasing Fe2O3 content. The sensing properties of these composites to 3 ppm H2S were evaluated as a sensor response (S), defined as the ratio of the resistance in air (Ra) to that in the gas mixture of H2S and air (Rg). The sensor response depended on the composite state of MgFe2O4 and MgO because the amount of H2S adsorbed on the composite was increased by the formation of the p–n junction. In contrast, the sensor response was not improved in the presence of excess MgO in the matrix of MgFe2O4, suggesting that the excess MgO serves as an insulator in the electron transfer between the semiconductor composite MgO–MgFe2O4 and an interdigitated Pt microelectrode.


Introduction
Semiconductor metal oxide (SMO) gas sensors have become one of the major tools in gas detection. Among SMOs, n-type SnO 2 , (1)(2)(3)(4) ZnO, (5)(6)(7)(8) TiO 2 , (9)(10)(11) and WO 3 (12,13) are widely used as gas sensors. The sensing properties of a solid solution sensor with a spinel structure, i.e., MFe 2 O 4 (M = divalent metal), have been evaluated using CO, CO 2 , CH 4 , C 2 H 5 OH, H 2 S, C 2 H 5 COOH, O 2 , H 2 , Cl 2 , NH 3 , CH 3 COOH, gasoline, acetylene, and liquid petroleum gas (LPG). (14) In the case of M = Ni, nickel ferrite was used for the first time as a sensor to detect a low concentration of chlorine gas. (15) In this study, the detection limit was 2 ppm. Besides, nanocrystalline NiFeMnO 4 thick films indicated high selectivity at 350 °C to 1000 ppm H 2 S, i.e., 4-fold higher than that to LPG, CO 2 , C 2 H 5 OH, NH 3 , and Cl 2 . (16) This selectivity was based on the fact that H 2 S is oxidized by the adsorbed oxygen (O − ) generated at more than 350 °C. The ferrite compounds MFe 2 O 4 (M = Cu, Zn, Cd, and Mg) have been developed for the detection of acetylene, CO, LPG, H 2 , and C 2 H 5 OH in the concentration range of 1000 to 6000 ppm. (17) The hole concentration was decreased by the reaction of alcohol with ZnO/ZnFe 2 O 4 film such as methanol, ethanol, and propanol. The doubly charged oxygen vacancy formed during the reaction was eliminated by the hole of the ZnO/ZnFe 2 O 4 film. (18) Ferrite compounds have high corrosion resistance (19) and oxidation resistance, (20) which are important features of a gas sensor. Thus, ferrite could be an attractive candidate gas sensor with low cost and high stability. The MgFe 2 O 4 pellet sensor showed that the sensor responses (ratio of resistance in air to resistance in gas) to petrol gas and hydrogen sulfur were 3.0 and 1.2 at 250 °C, respectively. (21) The response of the ferrite gas sensor should be improved for domestic and industrial purposes.
In general, p-n junction effects are adopted to enhance the sensor response of an oxide semiconductor. (22,23) The formation of a p-n junction contributes to the enlargement of the depletion layer on the surface of the oxide semiconductor, leading to the increase in the amount of adsorbed oxygen (O − ). As a result, the sensor responses to oxidizing or reducing gases increase with the interaction of the adsorbed oxygen with the gases.
Magnesium ferrite is composed of p-type MgO and nonconductor Fe 2 O 3 . The solid solution of both oxides behaves as an n-type semiconductor. In this study, we focused on the complex effects of p-type MgO and n-type MgFe 2 O 4 on H 2 S detection. The coprecipitation method was adopted to obtain magnesium ferrite, on which both cations of Mg and Fe were simultaneously deposited as oxides by adjusting the pH of a highly alkaline solution. The effects of the difference in mixture ratio between Fe and Mg on the product phases and composition were investigated. A water suspension of magnesium ferrite was dropped on the interdigitated Pt electrodes to be a sensor element. The sensing properties of a magnesium ferrite gas sensor to 3 ppm H 2 S were discussed on the basis of the correlation between the existence ratio of MgFe 2 O 4 to MgO and the resistance in air.

Preparation of ferrite
As a typical precipitant, 96 g of NaOH was dissolved in 400 mL of H 2 O to obtain 6 M NaOH aqueous solution. In this study, iron nitrate and magnesium acetate were used as starting substances on the basis of a report (24) indicating that magnesium ferrite could be obtained at low temperatures using an iron nitrate-barium acetate system rather than an iron nitrate-barium nitrate system. The starting materials, i.e., Fe(NO 3 )·9H 2 O and Mg(CH 3 COO) 2 ·4H 2 O, were stirred for 1 h in 200 mL of H 2 O to obtain a predetermined ratio of Fe to Mg. Continuously, the 6 M NaOH aq. solution was added dropwise in the solution mixture containing Fe and Mg until a pH of 11 was reached. Herein, note that the excessive dropping of the NaOH aq. solution contributed to the increase in sodium ion concentration, leading to sodium contamination. The coprecipitate obtained after stirring and aging for 1 d was washed in water to remove impurity ions. The precipitate was calcined at 800 °C for 3 h to promote crystallization.

Preparation of ferrite suspension
0.5 g of coprecipitate obtained in Sect. 2.1 was dispersed in 20 mL of deionized water. This suspension and 35 g of zirconia balls of 2 mm diameter were placed in a 50 mL ball milling vial, and this vial was rotated for 20 h to obtain a suspension with fine particles.

Dropping ferrite suspension on interdigitated Pt electrodes
Pt was adopted as the component of interdigitated electrodes to avoid peeling metal electrodes off the SiO 2 /Si substrate at a high temperature of more than 600 °C. The interdigitated Pt electrodes were fabricated on the SiO 2 /Si substrate by ultraviolet photolithography, as seen in Fig. 1(a). 0.2 µL of suspension prepared in Sect. 2.2.1 was dropped five times on the electrodes and dried gradually at 35 °C for half a day to obtain a sensing film. The sensing film was calcined at 800 °C for 3 h to obtain a ferrite microsensor.

Identification of product phase
The coprecipitate obtained in Sect. 2.1 dried at 100 °C for 1 d was hand-grounded for 10 min to avoid the aggregation of ferrite grains. The grounded particles were calcined at 800 °C for 3 h and became fine via hand grinding for powder X-ray diffraction. The product phases were identified using an X-ray diffractometer (XRD, Ultima IV, Rigaku, Japan), equipped with a Ni-filtered Cu Kα radiation source (λ = 0.154178 nm).

Sensing properties of ferrite microsensor to 3 ppm H 2 S
The ferrite microsensor was introduced into a flow apparatus equipped with an electric furnace. The sensor resistance (R) of the microsensor was connected to a direct current circuit with power source (E) and standard resistance (r), as shown in Fig. 1(b). The output voltage (V) was corrected at both edges of the standard resistance (r) using a digital multimeter. The sensor resistance (R) was calculated using the equation R = (E/V − 1)r. The electrical resistance of the microsensor was measured in air (R a ) and in H 2 S-containing air (R g ) at 350 °C. The sensor response was defined as R g /R a . The H 2 S concentration was precisely fixed to 3 ppm using a mass flow controller (MFC).

Effects of Fe and Mg contents on product phase
The XRD patterns of the coprecipitate calcined at 800 °C for 3 h are shown in Fig. 2    Relative peak intensity (%)

Deposition state of ferrite film on microsensor
The surface morphologies of the sensing film obtained at 100 mol% Fe (Fe:Mg = 10:0) were inhomogeneous, and the film was partially peeled off the interdigitated Pt electrodes. In the cases of 90 mol% Fe (Fe:Mg = 9:1) and 80 mol% Fe (Fe:Mg = 8:2), the sensing films had irregular concave and convex surfaces without cracks. The sensing film obtained at 33.3 mol% Fe (Fe:Mg = 1:2) had many large cracks on its surface. The sensing film with peeled parts and large cracks is the cause of the irregularity of the sensing properties to gases. From the above surface morphologies of the sensing films, the film with peeled parts and large cracks was not used for gas detection, whereas the sensing films with smooth surfaces and small cracks could be used for gas detection, as seen in

Sensing properties of ferrite microsensor to 3 ppm H 2 S
The complex effects of MgFe 2 O 4 and MgO are shown in Fig. 5. The R a of the MgFe 2 O 4 and MgO composite monotonically increased with increasing Mg content. The p-n junction between p-type MgO and n-type MgFe 2 O 4 contributed to the enlargement of the depletion layer caused by increasing amount of oxygen adsorbate (O − ). In general, the sensor with a higher R a indicates a higher sensor response without an insulator. The sensor response of the sensing film with 50 mol% Fe (Fe:Mg = 5:5) is maximum, but those with 40 mol% Fe (Fe:Mg = 4:6) and 30 mol% Fe (Fe:Mg = 3:7) tend to be lower, as shown in Fig. 6. On the other hand, the sensing film composed of MgO did not   Figure 7 shows representative response-recovery characteristics of the magnesium ferrite microsensor obtained in Fe:Mg = 5:5. , which was ranging from 20 to 60 nm. From the above results, the roles of three oxides on the sensor response of the magnesium ferrite microsensor could be proposed as shown in Fig. 9. On the basis of the discussion on Fig. 3, it is considered that

Conclusions
The optimum processes in the fabrication of a ferrite microsensor for H 2 S detection are concluded as follows: A complex oxide of MgFe 2 O 4 and MgO was coprecipitated at the mixing ratios of Fe:Mg = 5:5 and 6:4. The coprecipitate after calcination at 800 °C for 3 h was handgrounded for 10 min. 0.2 µL of the water suspension was added dropwise five times on the interdigitated Pt electrodes and dried gradually at 35 °C for half a day to avoid peeling and cracks on the surface.
The higher sensor response to H 2 S was realized as follows: the sensing films with 40 and 50 mol% Mg (Fe:Mg = 6:4, 5:5) contributed to the formation of the p-n junction between the p-type MgO and the n-type MgFe 2 O 4 ; the optimum detection temperature for 3 ppm H 2 S was shifted to a lower value of 200 °C by the p-n junction effect.