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Magnetic and Nanostructural Properties of Cobalt–Zinc Ferrite for Environmental Sensors

  • A.-H. El Foulani
  • R. C. Pullar
  • M. Amjoud
  • K. Ouzaouit
  • A. AamoucheEmail author
Living reference work entry

Abstract

In this study, we compare nanoparticles (NPs) of Co0.5Zn0.5Fe2O4 spinel ferrite produced by a novel simple synthetic technique with those made by standard co-precipitation, sol-gel, and hydrothermal methods. The novel process is based on the addition of a very small amount of ethanol (only 2 vol% in water with a low ethanol:metals molar ratio of 0.5:1, not a co-solvent) during co-precipitation to synthesize a nanopowder, which formed single-phase magnetic spinel ferrite when heated at 700 °C. This technique produced cobalt–zinc ferrite NPs smaller than those formed by the other methods, with an average crystallite size of 17 nm calculated from X-Ray Diffraction and NPs sizes around 30 nm observed by scanning electron microscopy. A surface area of 32 m2/g, and a total pore volume of about 0.56 cm3/g, were determined by the BET isotherm. The best catalytic capabilities for converting ethanol vapor to CO, CO2, and H2O, as well as magnetic properties, were obtained for Co0.5Zn0.5Fe2O4 synthesized by the ethanol-assisted co-precipitation. The ethanol conversion rate rapidly increased above 175 °C, and the total conversion of ethanol was achieved at a relatively low temperature of 230 °C. This sample also had the largest magnetization of 58.2 A m2 kg−1 at 3 T, and a very small, near superparamagnetic, coercivity.

Keywords

Semiconductor sensors Ferrite nanoparticles Catalytic proprieties Magnetic proprieties Ethanol oxidation Scanning electron Microscopy Vibrating sample Magnetometer Thermo gravimetric analysis Ethanol-assisted co-precipitation 

Introduction

Many current concerns regarding environmental protection are focused on air quality for industry, cities, and households. Important changes and improvements need to be made to address environmental issues. Environmental pollution is due in a large part to enormous industrial growth, especially in developing countries. Strong international market demands for volatile organic compounds (VOCs) is a serious environmental problem because some of them have toxic or carcinogenic properties (Tsai 2016). They can also react with nitrogen oxides and oxygen from the air to form harmful ozone: VOC + NOx + O2 + hν → O3 + + other products (Jirátováa et al. 2016). To reduce the emission of VOC s in the air, industrialists have applied the total oxidation of VOCs to carbon dioxide and water. In the literature, researchers have a similar interest in using the catalytic process instead of thermal combustion in the oxidation of VOCs. At the industrial level, catalysts containing noble metals are currently used because they have shown high activity and stability, but their cost limits the use of this type of catalyst (Spivey 1987; Rymeša et al. 2002). In this study, a new, cheaper catalyst based on a transition metal oxide (cobalt–zinc ferrite) was developed.

Ferrite compounds can be used as catalysts as they are highly selective and stable, particularly for the N-type semiconductors (Woo et al. 2016). Ferrites are iron-oxide based materials, usually containing other transition, and they have been extensively studied for their high density (Gadkari et al. 2010), catalytic (Zhihao and Lide 1998), magnetic, and electrical applications (Dube and Darshane 1993; Wolf et al. 2001). Spinel ferrites of the formula MFe2O4, with a cubic structure, have been reported as materials sensitive to both oxidizing and reducing gas (Lalauze et al. 1992; Sberveglieri 1995; Chen et al. 2000); a wide variety of transition metal cations (M = Co, Zn, Ni, etc.) may be easily incorporated into the structure. When made as nanoparticles, they have a powerful surface activity, due to their small particle size and enormous surface area (Liu et al. 1995; Suo et al. 1997). By taking into account the fact that the catalysis phenomena take place essentially on the surface of the material (Zafeiratos et al. 2012), the surface morphology also has an essential role in the catalytic efficiency (Liu and Norskov 2001).

Cobalt ferrite (CoFe2O4) is a well-known hard magnetic material with a large magnetization of around 80 A m2 kg−1 when in bulk form, but with lower magnetization and very low coercivity when formed as nanoparticles, tending towards being superparamagnetic as they approach 20 nm in diameter with a reduced magnetization of ~50 A m2 kg−1 (Rajendran et al. 2001). These properties are highly dependent on the size, shape, and phase purity of these nanoparticles (NPs), so it is necessary to develop simple processes to control CoFe2O4 nanoparticle stoichiometry and texture. Pure ZnFe2O4 is poorly magnetic with a magnetization of 10 A m2 kg−1 or less, but the magnetic properties of the mixed cobalt–zinc spinel ferrites have been studied (Chithraa et al. 2017), and with an initial substitution of Zn for Co the magnetization increases, giving a maximum at room temperature with 25% substitution of Co by Zn. 46 nm diameter nanoparticles of Co0.5Zn0.5Fe2O4 made by sol-gel and heated to 900 °C/2 h had a magnetization of ~55 A m2 kg−1, with near-zero coercivity (Chithraa et al. 2017).

Many routes are adopted for cobalt–zinc ferrite NPs production. Chemical synthesis of nanoscale particles was studied by transition metal ion reduction (Klabunde et al. 1994), by aqueous colloidal solutions separations (Cabuil et al. 1995), in the presence of an external magnetic field (Blums 1996), in bicontinuous and lamellar solutions (Pileni et al. 1996), by template-assisted sol-gel (He 2012a), or mechanically alloyed by high-energy ball milling (Ismail et al. 2014; Zulkimi et al. 2014). Solid solutions of nonstoichiometric mixed cobalt–zinc ferrite nanoparticles were optimized using atomic absorption spectroscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy techniques (Coppola et al. 2016). For the same purpose, other authors had used X-ray diffraction and Raman scattering (Phan et al. 2017). In our previous article, we monitored the catalytic behavior using infrared spectroscopy of Co0.5Zn0.5Fe2O4 ferrite NPs synthesized using co-precipitation method (El Foulani et al. 2017). A number of synthesis techniques have been studied to determine the most reliable method to produce catalytic and magnetic spinels such as, decomposition (Ali Zamanian and Behnamghader 2016), chemical precipitation (Rajendran et al. 2001; Hedayati et al. 2016; Economos 2006), sol-gel (Lou et al. 2007; Singhal et al. 2001), self-combustion (Iftimie et al. 2005), citrate routes (Gopal Reddy et al. 2000), and co-precipitation (Hankare et al. 2009; Tao et al. 2000). The co-precipitation way is the simplest and effective for oxide multicomponent preparation (Tao et al. 2000).

In this chapter, we compare the magnetic properties and the catalytic efficiency towards ethanol of ferrite nanoparticles Co0.5Zn0.5Fe2O4 made by four different methods standard co-precipitation, an ethanol-assisted co-precipitation , hydrothermal synthesis, and sol-gel synthesis. This innovative ethanol-assisted co-precipitation is not like the ethanol–water co-solvent techniques reported in the past, as it only involves a very small amount of ethanol (2 vol% ethanol in water) with a low ethanol:metals molar ratio of 0.5:1, in similar quantities to compounds used as surfactants in nanosynthesis.

Experimental Set-Up

Sample Preparation Methods

Co0.5Zn0.5Fe2O4 nanoparticles were prepared by four different methods. All the chemical reagents FeCl2·4H2O, Co(NO3)2·6H2O and ZnCl2·2H2O were supplied by Sigma-Aldrich, of analytical grade, and used without further purification.

For co-precipitation, the precursors were mixed in the stoichiometric ratio of Co:Zn:Fe = 0.5:0.5:2 in 80 ml deionized water, with a weight ratio of metals:water = 3.3:100, and stirred until a clear solution was obtained. The NaOH precipitating agent (1 mol/l) was added dropwise to the solution, until a pH of 11 was obtained. The resulting suspension was heated to 70 °C with constant stirring at 800 rpm for 1 h. The resulting brown precipitate was washed thoroughly with deionized water, and then dried in air in an oven at 100 °C. The milled powder was calcined in air at 700 °C for 8 h.

The second method was ethanol-assisted co-precipitation, similar to the method above except for the addition to the salt solution of a small amount of ethanol, before precipitation. The ethanol was added to the salt solution before precipitation at a molar ratio of ethanol:metals of 0.5:1, and the volume ratio ethanol/water was 0.02, i.e., only 2 vol% ethanol, so it was not in sufficient quantities to be considered a co-solvent.

The third method investigated was hydrothermal synthesis , using the same reagents and precipitating agent and the same molar ratios as for the standard co-precipitation above. The mixture was dissolved in 60 ml of deionized water, put in a Teflon autoclave with a total volume 100 ml, and the autoclave was placed in an oven at 120 °C/48 h. The brown precipitates were washed, dried, milled, and calcined in air in a furnace at 700 °C/8 h.

The fourth technique studied was sol-gel synthesis , again with the same precursor metal salts in the same ratios. These three salts were separately dissolved in the minimum amount of deionized water required, and after heating at 80–90 °C, all three solutions were mixed together at room temperature. Citric acid (99% purity, Sigma-Aldrich) was added to the resulting solution with the molar ratio of metals:citric acid = 1:1, followed by ethylene glycol (99.5% purity, Sigma-Aldrich) with the molar ratio metals:EG = 1:1, to form a colloidal solution. The solution was stirred and heated at 80 °C until gel formation. After heating at 120 °C/12 h in the oven, the gel became dry in places, and after a section ignited (usually on the edge), a combustion wave spread spontaneously throughout the gel, converting it to a loose black powder containing very fine crystals. Any residual organics were removed, and the calcination is completed by heating the powder in air in a furnace at 700 °C/3 h. In all four cases, the heating and cooling rates for the calcination step was 10 °C per minute.

Characterization Methods

Thermo gravimetric analysis (TGA) were carried out on the precipitates dried in air at 100 °C using a SETARAM Labsys Evo – gas option TG-DTA up to 1000 °C at a heating rate of 10 °C/min in air. Determination of phase purity and identification of the samples were performed by X-ray diffraction (XRD) using a Bruker diffractometer, D8 ADVANCED model. The XRD patterns were taken between 10–80° with a step size 0.02° using Cu ka radiation (λ = 1.5406 Å) at room temperature. The operating parameters were a beam current of 20 mA and integration times of 20 s with an acceleration voltage of 30 kV. The crystallite sizes were calculated with the Debye-Scherrer formula using the corrected full width at half maximum (βhkl) value of the most intense (311) peak (JCPDS file numbers: 22-1086 and 22-1012). The Lattice parameters were calculated and refined using the least-squares method with the X’Pert Plus and Unitcell programs. The specific surface area of calcined powders was determined using a Quantachrome Autosorb iQ instrument with the use of nitrogen as an analytical gas. The operating conditions were bath temperature of 77.35 K, degassing temperature of 350 °C/21 h. Scanning electron microscopy (SEM) was carried out on a Hitachi S-4100 at 25 kV on samples coated with carbon. The magnetic properties were measured using a vibrating sample magnetometer (VSM, Cryogenics) at 300 K and magnetic fields up to 3 T. The sample temperature was stabilized at 300 K, and the maximum field ramp rate was 0.25 T/min. The device sensitivity is rated up to 10.6 A m2 kg−1, and the maximum recorded temperature drift was ±0.05 K, with typical values of ±0.01 K.

Catalytic Measurements

A catalytic test was performed at atmospheric pressure in a micropilot reactor tubular glass with continuous flow (Fig. 1). The mass of catalyst introduced was 200 mg. Ethanol was put in a classic saturator maintained at 20 °C and atmospheric pressure. Ethanol vapor was conveyed by air as a carrier gas with a flow rate of 20 ml/min, through a stainless steel pipe heated to 80 °C to a U-shaped reactor, which is itself contained in a tubular furnace to regulate the temperature. The catalytic tests were carried out over a temperature range of 100–350 °C, which was measured using a thermocouple projecting into the center of the chamber. The gas at the outlet of the reactor was analyzed by gas chromatography (GC) with an Agilent 7890A chromatograph, and data processing was carried out using the Cerity program.
Fig. 1

Experimental setup of the catalytic measurements coupled with a gas chromatography unit

Results and Discussion

Thermal Analysis

Typical curves TGA of the dried sample synthesized by the ethanol-assisted co-precipitation method are presented in Fig. 2. The initial mass of the sample was 31 mg. The initial weight loss of 1.08% between 40 and 100 °C was attributed to loss of physically absorbed water or ethanol due to further drying-out of the material. The greater loss of 2.56% between 100 °C and 400 °C was attributed to a combination of the loss of chemically absorbed water and OH species on the particle surface, any remaining nitrate groups from the Co(NO3)2 precursor, and the loss of any nonstoichiometric oxygen species during the calcination step and the formation of the ferrite phase, which typically occurs above 250–300 °C. The small, further loss of 0.50% between 400 °C and 900 °C could be due to the removal of any remaining persistent chloride ions, which have been seen to remain up to 800–1000 °C in ferrites made from precipitated halide salts (Pullar et al. 2001). Based on the results obtained by the thermal analysis, the optimum calcination temperature was chosen to be 700 °C for 8 h.
Fig. 2

TGA curves of ferrite Co0.5Zn0.5Fe2O4 synthesized by the ethanol-assisted co-precipitation method

Crystallographic Analysis

The XRD patterns of the four powders after calcination at 700 °C are shown in Fig. 3. All the peaks are well matched with a cubic phase spinel ferrite, as confirmed from JCPDS file numbers: 22-1086 and 22-1012. The position of the peaks and relative intensities of the XRD patterns confirm that the samples all formed single-phase spinel Co0.5Zn0.5Fe2O4 ferrite, indexed in a cubic symmetry spinel structure, and with no evidence of any secondary crystalline phases.
Fig. 3

XRD patterns of Co0.5Zn0.5Fe2O4 powders, after calcination at 700 °C, produced by four different synthetic routes: (a) co-precipitation (b) ethanol-assisted co-precipitation with, (c) hydrothermal, and d) sol-gel

The crystallite size was obtained using the Debye-Scherrer method (Lalauze et al. 1992), in which the average size of the crystallites can be estimated by applying the following equation:
$$ \mathrm{D}=\frac{k\uplambda}{\upbeta_{\mathrm{hkl}}\mathit{\cos}\uptheta} $$
where D = crystallite size, k = shape factor (~0.9), λ = X-Ray wavelength (1.5406 Å), θ = Bragg angle of the most intense (100%) XRD peak, and βhkl = corrected full width at half maximum of the peak. It should be noticed that this technique is only an approximation, becomes less reliable for smaller crystallite sizes below 100 nm, and reflects the average crystallite size, which cannot be equated to particle (or nanoparticle) size in the absence of other evidence, such as SEM or low coercivity proving them to be single nanocrystals. Nevertheless, it is a useful tool for comparing the degree of crystallinity and the likely trend in particle size between these four powders.
The dependence of the catalytic/magnetic properties of ferrites on small particle size is well established, and generally the catalytic efficiency increases simultaneously with the reduction in crystallite size, while it is more complicated for magnetic properties, which typically reach a maximum as size decreases, and then decrease with further decreases in size as the critical magnetic domain size is approached (Ali Zamanian and Behnamghader 2016; Hedayati et al. 2016; Economos 2006; Lou et al. 2007; Singhal et al. 2001; Iftimie et al. 2005; Gopal Reddy et al. 2000; Hankare et al. 2009; Tao et al. 2000). Reducing the crystallite size can also accompany an increase in the strength and hardness of a material (Tsai 2016). The calculated crystallite sizes are shown in Table 1. The largest size was found for the standard co-precipitation route at 105 nm, with the other three routes all producing much smaller sizes. The hydrothermal and sol-gel routes were broadly similar at 42 and 30 nm, respectively, while the ethanol-assisted co-precipitation route resulted in a significantly smaller size of only 17 nm. This suggests that the ethanol has a great effect during the co-precipitation process. We suggest that the added ethanol could limit nucleation, by breaking the agglomeration of the initial NPs formed, since it can replace the water molecules around the nucleus and by decreasing the critical aggregation parameter (Li et al. 2005), resulting in a finer nanomaterial, and essentially behaving similarly to a surfactant. For this this reason, such a small amount of ethanol has such a significant effect.
Table 1

Average crystallite size and lattice parameters for Co0.5Zn0.5Fe2O4 powders, calcined at 700 °C, according to the synthesis routes used

Synthetic route

Diffraction angle of (311) peak (°)

βhkl (Rad)

Average crystallite size (nm)

a Lattice parameter (Å)

Unit cell volume (Å3)

Co-precipitation

36.888

0.001395

105

8.39 ± 0.02

592.0800 ± 4.3839

Ethanol-assisted co-precipitation

37.066

0.008582

17

8.37 ± 0.01

587.5393 ± 3.6482

Hydrothermal

36.614

0.003436

42

8.44 ± 0.01

603.2331 ± 4.1161

Sol-gel

36.991

0.004814

30

8.41 ± 0.00

594.9159 ± 0.4400

Refinement of lattice parameters by the least squares method showed that all the peaks are indexed in the face-centered cubic spinel system. The crystallographic results obtained for the cobalt–zinc ferrite calcined at 700 °C for the four different synthesis routes shows that there is little difference between them, although generally the products of the two co-precipitation routes showed slightly smaller values than those from the sol-gel and hydrothermal routes, the latter giving the largest values. The values of crystalline parameters are listed in Table 1. The addition of ethanol has significantly decreased the a parameter and volume of the cell, which are given by the literature as 8.407 Å and 594.187 Å3, respectively, for Co0.5Zn0.5Fe2O4 (Nikam et al. 2015).

Specific Surface Area and Porosity

The grain size, porosity, and specific surface area (SSA) greatly control the catalytic activity of a material, because the interaction between the gas and the catalyst occurs on the surface (Punnoose et al. 2007). Hence, the use of nanomaterials with a large surface/volume ratio can significantly improve gas adsorption phenomena. Gas molecules are adsorbed more easily on porous materials (Zhu et al. 2008), as the surface is essentially made of micropores and mesopores.

Table 2 shows the evolution of the specific surface area, the average pore size and total pore volume, depending on the synthetic route. As with the XRD crystallite sizes, the standard co-precipitation and hydrothermal methods gave the samples with lower surface areas and pore volumes, while the ethanol-assisted co-precipitation and sol-gel processes lead to the formation of more porous Co0.5Zn0.5Fe2O4 powder with higher surface area. It is worth noting that the ethanol-assisted co-precipitation produced the largest SSA at 32.36 m2/g (which corresponds to the smallest particle/grain size) and highest pore volume (~0.56 cm3/g). This is considerably larger than other reported values for Co0.5Zn0.5Fe2O4 ferrites, of between 5 and 19.5 m2/g (Yasenevz et al. 2011; Wang et al. 2012). Therefore, the addition of a small amount (2 vol%) of ethanol to the water improves the specific surface area, along with the creation of a larger number of open/accessible pores in the material (~0.56 cm3/g vs. ~0.34 cm3/g without ethanol). It should be noted that the nanopores were of a similar size, ~4–5 nm, for all the four synthesis routes. It should also be noted that, as with the XRD data, the sol-gel route gave the second largest SSA and pore volume, in this case approaching the values of the ethanol-assisted co-precipitation.
Table 2

The specific surface area, average pore size, and total pore volume prepared by the synthetic routes

Synthetic route

Specific surface area (m2/g)

Average pore size (nm)

The total pore volume (cm3/g)

Co-precipitation

21.79

4.589

0.33847

Ethanol-assisted co-precipitation

32.36

5.495

0.56421

Hydrothermal

23.08

4.276

0.33400

Sol-gel

30.65

5.226

0.54215

Morphology and Microstructure

SEM images show that the morphology of samples prepared through the four different techniques varies depending on the preparation process (Fig. 4). In general, the morphology of the particles is a mixture of nanospheres and plates, with a wide distribution of the grain size in the range of 30–250 nm. Nevertheless, it can be observed that the smallest grain size is found in the samples made by ethanol-assisted co-precipitation (Fig. 4b), with most particles being well-defined and very small at around 30–50 nm, and some larger, more platy grains between 100 nm and 120 nm (see inset image, Fig. 4b). The smallest grains in the powder made from the sol-gel process are similarly sized, but this also contains larger, even more platy grains, as large as 250 nm in diameter (Fig. 4d). The standard co-precipitation and hydrothermal processes have resulted in fewer of the smallest particles, and an increased number of the larger grains, explaining their lower surface areas. However, the larger grains appear to be thicker, and less platy, than those seen in the sol-gel sample. In the absence of any TEM evidence, the SEM images suggest that the very smallest grains could be single nanocrystals or bicrystals, as they are approaching the average XRD crystallite sizes, but the majority of the grains will probably be polycrystalline nanoparticles. The variation in particle size and coalescence behavior of the agglomerated particles could be due to differing interfacial surface tension phenomena during the different synthesis conditions.
Fig. 4

SEM images of Co0.5Zn0.5Fe2O4 powders, after calcination at 700 °C, prepared with different synthesis methods: (a) standard co-precipitation, (b) ethanol-assisted co-precipitation, (c) hydrothermal, and (d) sol-gel

Magnetic Properties

Co0.5Zn0.5Fe2O4 ferrite should be a strongly ferromagnetic material, with a reported bulk magnetization of around 80 A m2 kg−1 (He 2012b), and a coercivity that can vary greatly depending if the grains are oriented or not. Co 0.5 Zn 0.5 Fe 2 O 4 ferrite nanoparticles have been studied as potential thermomagnetic materials for medical treatments via magnetic hyperthermia, and they become superparamagnetic at room temperature below around 10 nm (Arulmurugan et al. 2005), smaller than the 20 nm critical size reported for pure CoFe2O4 NPs (Rajendran et al. 2001). Coercivity (Hc) approaches zero as their size decreases below this, and Hc as low as 0.7 kA m−1 (8.8 Oe) has been reported for 8.4 nm Co0.5Zn0.5Fe2O4 nanoparticles (Arulmurugan et al. 2005). Their saturation magnetization (Ms) also decreases with size, and values of Ms = 39.6 A m2 kg−1 were reported for these 8.4 nm NPs (Jaffrezic-Renault et al. 2002), compared to 55 A m2 kg−1 for 46 nm NPs (Chithraa et al. 2017). The room temperature magnetization loops of our Co0.5Zn0.5Fe2O4 ferrites, heated to 700 °C, made by the four routes, are shown in Fig. 5.
Fig. 5

Magnetization loops of Co0.5Zn0.5Fe2O4 powders, after calcination at 700 °C, made with different synthesis methods: (1) co-precipitation, (2) ethanol-assisted co-precipitation, (3) hydrothermal, and (4) sol-gel

As expected, they are all very soft ferrites, with low Hc values. Apart from the sol-gel route, they all appear to be virtually superparamagnetic, with very little coercivity at zero field. The sol-gel sample has a slightly wider hysteresis loop, with a definite Hc of around 32 kA m−1 (400 Oe), requiring an applied field of 0.04 T to reduce the magnetization to zero and then reverse it. This may be because it contains the more plate-like particles, which may be oriented with respect to the applied magnetic field, inducing and increased coercivity, despite the small average particle size. The largest magnetization was found for the ethanol-assisted co-precipitation , which although it resulted in the smallest particles, also gave a material with a reasonably high magnetization of 58.2 A m2 kg−1 at 3 T. The magnetization of the hydrothermal sample was second highest, having an Ms (at 3 T) of 53.2 A m2 kg−1, and with values of 37.7 A m2 kg−1 and 32.3 A m2 kg−1 for the sol-gel and co-precipitated samples, respectively. Therefore, the changes in magnetization do not follow the changes in particle size/crystallite size/SSA, and other features such as particle morphology, and variation in oxidation state (e.g., presence of Fe2+ ions) play a role. Nevertheless, the sample made by our ethanol-assisted co-precipitation route has the superior magnetic properties as a strongly magnetic yet very soft/superparamagnetic ferrite, as well as having the smallest size. The very low, almost superparamagnetic coercivity values for this show that it the NPs are nearing the critical size at which this becomes superparamagnetic reported as being 20 nm for CoFe2O4 (Rajendran et al. 2001) and <45 nm for Co0.5Zn0.5Fe2O4 (Chithraa et al. 2017).

Catalytic Testing and Ethanol Conversion

The chemical composition of our material, Co0.5Zn0.5Fe2O4, is modified in the presence of an oxidizing or reducing gas (Arulmurugan et al. 2005). In the presence of ethanol vapor (a reducing gas), many reactions are possible on the surface layer, particularly as a function of temperature. For example, the following reactions have been identified (Cheong and Lee 2006):

In the presence of oxygen (an oxidizing gas):
$$ \frac{1}{2}\ {\mathrm{O}}_2\left(\mathrm{gas}\right)+1{\mathrm{e}}^{-}\to {\mathrm{O}}^{-}\left(\mathrm{adsorbed}\right) $$
In the presence of ethanol (a reducing gas):
$$ {\mathrm{CH}}_3{\mathrm{CH}}_2\mathrm{O}\mathrm{H}\ \left(\mathrm{gas}\right)+{\mathrm{O}}^{-}\left(\mathrm{adsorbed}\right)\to {\mathrm{CH}}_3\mathrm{CHO}\ \left(\mathrm{adsorbed}\right)+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}\to \, \mathrm{T}\le {230}^{{}^{\circ}}\mathrm{C} $$
$$ {\mathrm{CH}}_3\mathrm{CHO}\ \left(\mathrm{adsorbed}\right)+4\ {\mathrm{O}}^{-}\left(\mathrm{adsorbed}\right)\to \mathrm{CO}+{\mathrm{CO}}_2+2\mathrm{H}2\mathrm{O}+4{\mathrm{e}}^{-}\to \mathrm{T}>{230}^{{}^{\circ}}\mathrm{C} $$

Other reactive oxygen species such as O2− are also possible at such temperatures.

Figure 6 illustrates the results of the ethanol catalysis measurements with Co0.5Zn0.5Fe2O4 synthesized by the four different methods. The best result was obtained for the material synthesized by the ethanol-assisted co-precipitation method , in which the conversion rate rapidly increased above 175 °C, and the total conversion of ethanol was achieved at a relatively low temperature of 230 °C. This temperature is rather moderate, when compared with those of the literature (Poulopoulos 2016; Legendre and Cornet 1972; Yao 1984). The ethanol conversion of the other powders followed in the sequence of synthesis by sol-gel, hydrothermal, and standard co-precipitation routes, with the total conversion of ethanol occurring at 260 °C, 310 °C, and 340 °C, respectively. Therefore, the catalytic activity of Co0.5Zn0.5Fe2O4 spinel ferrite is related to the crystallite size and specific surface area, which determine the efficiency of these materials towards ethanol vapor in the presence of oxygen ions on their surfaces. Many studies have shown that the surface properties of the catalyst are important in studying catalysis because of the vital relationship between the catalytic activity and the catalyst surface (Wei et al. 2011).
Fig. 6

Ethanol conversion rates for Co0.5Zn0.5Fe2O4 powders, after calcination at 700 °C, synthesized with different methods: (1) co-precipitation, (2) ethanol-assisted co-precipitation, (3) hydrothermal, and (4) sol-gel

Conclusions

Environmental pollution is due in a large part to enormous industrial growth. This creates the need for serious environmental monitoring, but the high cost currently limits the control and monitoring of air quality in many countries and situations. The use of devices based on semiconductor sensors , namely from the ferrite, would be cheaper and easier than the current analytical techniques, and the detection operates via the interaction of the gas and a sensitive material, modifying physico-chemical properties (electrical conductivity, optical properties, etc.) resulting from the chemical reaction between the gas molecules and the oxygen adsorbed on the metal oxide surface. The present chapter was an opportunity to review a wide range of semiconductor-based sensing materials and how to increase the sensitivity of these sensors with the addition of catalytic metals and non-metals.

The novel process employed here involves the addition of small amount of ethanol (2:1 ratio of metals:ethanol, 2 vol% in water) to the water during the co-precipitation to synthesize a precursor nanopowder, which formed single-phase magnetic Co0.5Zn0.5Fe2O4 spinel ferrite when heated at 700 °C. The quantity of ethanol used is similar to the amounts of compounds used as surfactants in nanosynthesis. This simple technique produced cobalt–zinc ferrite nanoparticles smaller than those formed by standard co-precipitation , sol-gel , and hydrothermal methods , with an average crystallite size of 17 nm calculated from XRD, particle sizes as small as 30 nm observed by SEM, a large surface area of 32 m2/g and a high pore volume of 0.56 cm3/g evaluated from BET isotherm. We suggest that the added ethanol limits nucleation, reprecipitation, and growth on the particles surfaces during precipitation, resulting in a finer nanomaterial. These microstructural improvements promote a larger exchange surface with the gas for many applications like catalysis and gas sensing. The catalytic properties were tested with ethanol as a reducing gas (converting to CO, CO2, and H2O by reaction on the ferrite surface) using Co0.5Zn0.5Fe2O4 catalyst powders prepared through four methods. The best result was obtained for Co0.5Zn0.5Fe2O4 synthesized by the ethanol-assisted co-precipitation method , in which the conversion rate rapidly increased above 175 °C, and the total conversion of ethanol was achieved at a relatively low temperature of 230 °C. This sample also had the largest magnetization of 58.2 A m2kg−1 at 3 T, and a very small coercivity, giving an almost superparamagnetic hysteresis loop, suggesting it consisted of NPs near the critical superparamagnetic domain diameters of 20–45 nm. Therefore, this study has demonstrated the beneficial effect of a small amount of ethanol added to the water for preparing cobalt–zinc ferrite nanoparticles through the ethanol-assisted co-precipitation method, on the physicochemical features, magnetic and catalytic properties of ferrite Co0.5Zn0.5Fe2O4 powder. We also expect this material will be applicable as a gas sensor candidate for monitoring air pollution.

Cross-references

Notes

Acknowledgments

This work was supported by the Portugal-Morocco bilateral collaboration grant FCT/CNRST-2015/2016 Proc. № 441.00. This work was partly carried out at CICECO-Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT Ref. UID/CTM /50011/2013) financed by national funds through the FCT/MEC.

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Copyright information

© Springer International Publishing AG 2018

Authors and Affiliations

  • A.-H. El Foulani
    • 1
  • R. C. Pullar
    • 2
  • M. Amjoud
    • 3
  • K. Ouzaouit
    • 4
  • A. Aamouche
    • 5
    Email author
  1. 1.MSISM Research Team, Department of Physics, Polydisciplinary Faculty of SafiCadi Ayyad UniversitySafiMorocco
  2. 2.Department of Materials and Ceramic Engineering, Department of Physics, CICECO – Aveiro Institute of MaterialsUniversity of AveiroAveiroPortugal
  3. 3.Laboratory of Condensed Matter and Nanostructures, Faculty of Science and TechnologyCadi Ayyad UniversityMarrakechMorocco
  4. 4.REMINEX research center, Groupe ManagemMarrakech/MedinaMorocco
  5. 5.Applied Sciences National School (ENSA)Cadi Ayyad UniversityMarrakechMorocco

Section editors and affiliations

  • Chaudhery Mustansar Hussain
    • 1
  1. 1.Department of Chemistry and Environmental SciencesNew Jersey Institute of TechnologyNewarkUSA

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