1. Introduction
A porous microstructure has made different ceramic oxides good candidates for humidity and gas sensing [
1]. Many oxides have been investigated and applied for gas sensing. They have included metal oxides such as α-Fe
2O
3 [
2], SnO
2 [
3,
4,
5] and complex oxides such as spinel ferrites [
6]. Tin oxide (SnO
2) is a wide band semiconductor (~3.6 eV) widely applied in gas sensing as a morphologically and chemically stable and low cost material that is highly sensitive to different gases but suffers from a lack of selectivity [
6]. Research still continues in the search for a more effective gas sensing material that will have high sensitivity, a fast response and recovery time and good selectivity to detect low concentrations of gases [
7]. Cubic spinel ferrites are a class of oxide semiconducting materials especially interesting for gas sensing applications as a change in the chemical composition and structure reflected in the cation distribution on the tetrahedral or octahedral sites of the cubic spinel structure has a profound influence on gas sensing properties such as sensitivity, selectivity, response and recovery and long term stability [
7]. Thus many mixed and cation substituted spinel ferrites have been investigated [
8,
9,
10,
11,
12].
Spinel ferrites have been investigated as resistive-type humidity sensors due to active sites for water vapor dissociation [
7]. They have a high resistance that decreases with increase in humidity and a chemically stable structure [
13]. Their response to change in humidity is closely related to the surface morphology, so the aim is to achieve a higher specific surface area or improved porous structure containing pores of different shape, size and connectivity. Tin oxide nanoparticles synthesized by the microwave irradiation method have shown promising properties for application in humidity sensing [
4]. The humidity sensing mechanism of oxide ceramics can be described as adsorption of water molecules on the ceramic sample surface [
14]. First chemisorption occurs when the first layer of water is chemisorbed on the active ceramic oxide surface forming a monolayer of immobile hydroxyl groups [
7,
13]. This is followed by physisorption of water when double hydrogen bonds form that cannot move freely. High electrostatic fields in the chemisorbed layer cause easy dissociation of physisorbed water to form hydronium ions (H
3O
+). Physisorption of water molecules onto available oxygen sites by single hydrogen bonding continues with the formation of further physisorbed layers. Protonic conduction starts from the free second physisorbed layer by proton transfer from one water molecule to another enabled by single hydrogen bonding. This conduction mechanism is known as the Grotthus charge mechanism [
15]. At high humidity levels electrolytic conduction takes place together with protonic conduction. Water molecules can also adsorb into small pores leading to capillary condensation.
Another way of improving properties of spinel ferrites as gas sensing materials is doping with metal ions or metal oxides. Their aim is to increase the spinel ferrite porosity. Some examples include doping MgFe
2O
4 with Pr [
16] or Mo and Sn [
9], Y [
17] or adding CeO
2 [
13] or WO
3 [
18]. Relatively recently much work has been devoted to the idea of combining different metal oxide materials together to form heterostructures [
19]. The simplest are a mixture of two (or more) constituent materials without a specific distribution forming mixed compounds.
In this work we have investigated how addition of SnO2 nanopowder to MgO and α-Fe2O3 nanopowders used to obtain MgFe2O4 by solid state sintering, influences the resulting composition, microstructure and complex impedance in different humidity conditions (relative humidity 30–90%) in view of applying this compound in humidity sensing.
2. Materials and Methods
Nanopowders of MgO (Alfa Aesar, <100 nm), SnO2 (Sigma Aldrich, <100 nm) and α-Fe2O3 (Alfa Aesar, 20–60 nm) were homogenized for 15–20 min in the appropriate weight ratio to obtain (1 − x)·MgFe2O4 + x·SnO2 (x = 0–0.5) in an agate mortar with pestle.
Green samples (0.25 g powder mixture, diameter 8 mm) were obtained by uniaxial double-sided pressing (3 t/cm2). The average green sample thickness decreased with increase in SnO2 content and was 2 mm for green samples composed of MgO and Fe2O3 nanopowders and 1.95, 1.88, 1.85, 1.80 and 1.74 mm for samples with 10, 20, 30, 40 and 50 wt.% SnO2, respectively. Sintering of green samples was performed in air in a chamber furnace at 1000 and 1100 °C (heating time 4 h, holding time 4 h). Samples were denoted as M100-M105 (sintering temperature 1000 °C, hence M10x, where x denotes the initial SnO2 content as 0 for no SnO2, 0.1 is 10 wt.% SnO2, 0.2 is 20, 0.3 is 30, 0.4 is 40 and 0.5 is 50 wt.% SnO2) and M110–M115 (sintering temperature 1100 °C, hence M11x with the last number denoting the tin oxide content in the same way as previously described for the samples sintered at 1000 °C). Sample density was determined from weight and volume (dimension) measurements.
X-ray diffraction (XRD) patterns were recorded on a Rigaku RINT2000 diffractometer CuKα = 1.54178 Å. Scanning electron microscopy (SEM) analysis of freshly fractured samples was performed in a TESCAN Electron Microscope VEGA TS 5130MM device in scanning electron (SE) and back scattering electron (BSE) modes. Electron dispersive X-ray spectroscopy (EDS) analysis was performed on an INCA Penta FETX3 energy dispersive system.
Silver electrodes were deposited on the disc samples on both sides for electrical measurements in capacitor form electrically equivalent to a capacitance Cp in parallel with resistance Rp. Measurements of impedance (R and X) in the frequency range 42 Hz–1 MHz were conducted in a JEIO TECH TH-KE-025 temperature and humidity climatic chamber on a HIOKI 3532-50 LCR HiTESTER device (TEquipment, Long Branch, NJ, USA) at 25 °C (room temperature) in the relative humidity (RH) range 30–90% and applied voltage 5 V.
4. Discussion
Bulk MgFe
2O
4-Fe
2O
3-SnO
2 compounds were obtained from starting nanopowders by sintering at 1000 and 1100 °C with varying SnO
2 content (0–50 wt.%), denoted as M100–105 and M110–M115, respectively. Both sintering temperatures have been previously applied to obtain magnesium ferrite [
1,
9], so we aimed to determine which one would potentially yield better samples for humidity sensing. Reliable gas sensors need to fulfill the requirement of high sensitivity, fast response and good selectivity [
2]. Oxide semiconducting materials have been both investigated and widely applied as gas sensing materials in commercial and industrial gas sensors. Advantages of oxide gas sensors also include its low cost, easy fabrication and these devices are simple to use. Their disadvantages include poor selectivity, low sensitivity and high power consumption due to the required often high operating temperature. This is the reason why heterostructures have been widely investigated [
19]. Compounds (the simplest heterostructure) composed of two or more known good gas sensing materials could potentially address some of these issues.
Obtaining phase pure magnesium ferrite powder is difficult and often hematite and even magnesium oxide remain depending on the calcination temperature, though only hematite was noted at higher calcination temperatures, such as 900 and 1100 °C [
23]. Rezlescu et al [
9] noted that Sn
4+ could also enter the MgFe
2O
4 lattice to a certain degree. Tin oxide has also been doped with Mg where up to 10 wt% Mg was incorporated into the SnO
2 lattice, changing the peak intensity [
24]. Hematite has been doped with Sn and up to 12 at% was incorporated into the hematite lattice resulting in a lattice distortion with cell volume increase [
25]. As we started from MgO, α-Fe
2O
3 and SnO
2 nanopowders that were mixed thoroughly and then sintered all these variations are possible and probable. Analysis of the measured XRD showed that we did not note any hematite impurity peaks in the samples with no added SnO
2 and obtained phase pure MgFe
2O
4 at both applied sintering temperatures (samples M100 and M110, sintered at 1000 and 1100 °C, shown in
Figure 1) However, with SnO
2 addition hematite peaks can also be noted besides SnO
2 peaks, as shown in
Figure 1 for samples with the smallest amount of added SnO
2 (10 wt.%, M101 and M111, sintered at 1000 and 1100 °C, respectively). The SnO
2 peak intensity increased as expected with the increase in SnO
2 content and we showed an example for sample M102 (20 wt.% SnO
2). Thus, even though we obtained phase pure MgFe
2O
4 from starting nanopowders (MgO and Fe
2O
3), addition of SnO
2 to the starting mixture resulted in the appearance of remaining hematite, besides SnO
2. A more in depth analysis of this compound on powder samples could determine whether Mg has been incorporated in the SnO
2 lattice, Sn in the hematite lattice and MgFe
2O
4, as determined in literature [
23,
24,
25].
Our compound samples had a porous structure and the morphology changed with increase in SnO
2 content and the sintering temperature (
Figure 3,
Figure 4 and
Figure 5). The gas sensing mechanism of oxide semiconducting materials is linked to the sample surface, that when exposed to gases reacts with the gas molecules, making the microstructure of the gas sensing material of great significance. The SEM image of the pure MgFe
2O
4 sample (M100,
Figure 3a) showed a porous microstructure with a relatively homogenous grain size. Similar morphologies have been obtained before for magnesium ferrite [
1,
16]. At 1000 °C addition of SnO
2 resulted in an inhomogeneous morphology composed of parts with larger and parts with smaller particles, as shown in
Figure 3b–f for samples with different SnO
2 content (M101–M105). Rezlescu et al [
9] noted that incorporation of Sn into magnesium ferrite resulted in a substantial decrease in grain size and significant reduction of electrical resistivity with increase in relative humidity. In this work the solid state synthesis procedure we applied using starting nanopowders resulted in a mixture of larger hematite and magnesium ferrite grains mixed with smaller tin oxide ones, confirmed by analysis of BSE images and EDS analysis of characteristic grains and particles noted (
Figure 5). Smaller amounts of SnO
2 improved the morphology for gas sensing by improving sensitivity, though the largest decrease of impedance with increase in RH was obtained for sample M105, with the highest amount of SnO
2 with a morphology consisting of a mixture of larger magnesium ferrite and hematite particles mixed with many small tin-oxide particles.
In the case of humidity sensing the humidity sensing mechanism is based on chemisorption (low relative humidity) and physisorption of water vapor on the surface of the sensing material [
1,
11,
13]. At higher relative humidity with the formation of several physisorbed layers hopping of H
+ via water molecules occurs by the Grotthus mechanism [
1,
14,
15]. At 1000 °C all samples showed a noticeable decrease in impedance with increase in relative humidity (
Figure 5 and
Figure 6a) but differing due to the differences in microstructure. Thus, the decrease in impedance with increase in relative humidity was almost linear for samples M102, M103 and M104, as shown in the inset on
Figure 7a, with the impedance decreasing 5–9 times. The decrease of impedance for pure MgFe
2O
4, became more rapid with increasing RH. Addition of SnO
2 reduced the impedance at lower RH, for sample M101 for which we determined the highest sensitivity. Even though the decrease in impedance for low RH was relatively small for M105 with the highest amount of tin oxide, it became much more rapid for RH 40–70% and also 80–90%, resulting in the overall decrease of impedance with RH of ~26 times. Changes in the slope of resistivity-humidity curves have been observed before when MgFe
2O
4 was doped with Sn and Mo [
9]. This shows that further fine tuning of the microstructure by changing synthesis parameters, composition and synthesis method could lead to even better humidity sensing properties. The higher sintering temperature (1100 °C) resulted in grain growth. The sample density increased and though still porous, as the sintering process advanced leading to lower complex impedance and smaller change with humidity. The influence of humidity on complex impedance was higher at lower frequencies, as noted before for magnesium ferrite and other oxides [
1,
14,
21]. The highest sensitivity was obtained for the sample with a small amount of SnO
2 sintered at 1000 °C showing that this compound has potential for application in humidity sensing. This was confirmed by low hysteresis values obtained for this compound that is one of the requirements of a good sensing material [
13].
The influence of change in relative humidity can be analyzed by following the change in the imaginary part of complex impedance in the analyzed frequency range. We noted a shift in the relaxation peak to higher frequency with increase in relative humidity, as shown in
Figure 8b, for sample M105. This has been noted before for MgFe
2O
4 [
1] and can be attributed to increasing relaxation time due to increase in ionic diffusion [
26].
The determined Cole-Cole plots of complex impedance resembled diagrams obtained before for other oxide semiconducting materials [
14,
21]. As the relative humidity decreases the arc resembles more of a semicircle and shrinks. This is due to the humidity sensing mechanisms of these materials, as explained in the introduction and in more detail in References [
14,
21]. We can note that depending on the material composition and microstructure the semicircular arc or semicircle change and shrink slightly less or more rapidly. As observed before, the semicircles and semicircular arcs were depressed, indicating non-ideal Debye behavior as described in detail in Reference [
22]. In this case a CPE element is used to replace capacitance and model this behavior. Analysis of complex impedance data using an equivalent circuit showed dominant influence of grain boundaries at low relative humidity, while the influence of the grain (bulk) component could be analyzed at higher relative humidity.