Sensors and Actuators B 222 (2016) 95–105
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Review
Spinel ferrite oxide semiconductor gas sensors a,∗ ˇ ¯ Andris Sutka , Karlis A. Gross a,b a b
Faculty of Material Science and Applied Chemistry, Riga Technical University, Paula Valdena 3/7, LV-1048, Latvia Department of Materials Engineering, Monash University, VIC 3168, Australia
a r t i c l e
i n f o
Article history: Received 25 February 2015 Received in revised form 5 August 2015 Accepted 7 August 2015 Available online 17 August 2015 Keywords: Ferrite Gas sensor Nanomaterials
a b s t r a c t The demand for portable gas sensors is increasing following the progress in the electronics industry; there is an equal need to increase the quality of gas sensors. Spinel ferrites have been used as electronic materials for more than 50 years and offer a suitable ceramic base for the gas sensor market. They are simple, low cost, and compared to other gas sensors have structural and compositional versatility. This review highlights the recent developments and shows the potential of the spinel ferrites on gas sensor technology. Sensing mechanisms for a range of gasses and humidity are explained for n-type, p-type, mixed and substituted spinel ferrite gas sensors. The change in conduction mechanism is discussed outlining electronic and chemical sensitization that both increase the conductivity. Some cation substitutions are shown to change the oxidation state, thereby increasing sensitivity, but noble metals are shown to chemically sensitize spinel ferrites. This review surveys synthesis methods for producing spinel ferrites and discusses future prospects for further improvements. © 2015 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
4. 5.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Cation site occupation in the spinel ferrite structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Gas sensing mechanism in spinel ferrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.1. N-type spinel ferrite gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98 3.2. P-type spinel ferrite gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.3. Mixed and substituted spinel ferrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 3.4. Spinel ferrite humidity sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99 Spinel ferrite gas sensor synthesis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Latest developments and future prospects of spinel ferrite gas sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102 Biography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
1. Introduction Gas detection is attracting an increasingly important role for a range of different needs; environmental protection, safety (detecting explosive gases), fermentation control in the food industry, diagnostics and patient monitoring, as well as household use to detect combustible gases [1–3]. The global gas sensor market in 2012 was about 1.7 billion USD and is expected to grow for the next six years at a compound annual growth
∗ Corresponding author. ˇ E-mail address:
[email protected] (A. Sutka). http://dx.doi.org/10.1016/j.snb.2015.08.027 0925-4005/© 2015 Elsevier B.V. All rights reserved.
rate of 5.1% [4]. This repositions spinel ferrites as alternative materials for the gas sensor market. Initial consideration will be placed on different sensors before focusing on ferrites as gas sensors, their crystal structure and synthesis methods. Gas sensor detection is based on variability of electrical, acoustic, optical, mass or calorimetric properties of the material. Detection based on variability of electrical properties is simple, fast and less expensive and as a result is attracting the most interest [3]. There is a growing need to incorporate sensors into smart devices for remote sensing, and so this portability and compatibility with operating systems is supporting faster development of sensors based on electrical detection.
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96 Table 1 Electrical gas sensors employing different materials. Material Metal oxide semiconductors Carbonaceous allotropes Carbon polymer composites Conducting polymers
High sensitivity √ √
Fast response √ √
√
√
Gas detection involving changes of electrical properties is predominantly based on changes in electrical resistance. The most common gas sensors are based on single crystalline or polycrystalline metal oxide semiconductors [5], carbons [6,7], carbon polymer composites [8,9] and conducting polymers [10]. Comparison of different gas sensing materials shows that metal oxides are the most promising both for manufacture and gas sensing characteristics (Table 1). Metal oxide semiconductor gas sensor materials have been intensively investigated since Seiyama et al. first proposed gas sensing properties of semiconducting metal oxide thin films back in 1962 [11]. Metal oxides hold promise for long-term detection at low concentrations without the need of complex measurement techniques (Table 1). An alternative characteristic of metal oxide semiconductor gas sensors is the reversible interaction of the gas with the pre-adsorbed ambient oxygen, thus significantly changing the electrical resistance [12]. For n-type semiconductors, the resistance increases or decreases after exposure to oxidizing and reducing gases, respectively; for p-type oxide semiconductor, an opposite change in the electrical resistance is expected [13]. Hundreds of metal oxide materials are deposited in the form of thick or thin films, as active layers in gas-sensing devices. They can be divided into three categories: pre-transition metal oxides, transition metal oxides and post-transition metal oxides. The pretransition metal oxides, such as Al2 O3 , contain elements with only one preferred oxidation state and exhibit an exceedingly high resistance. They can neither be reduced nor oxidized, the large band gap does not alter the electrical conductivity required for gas sensors [14]. Transition-metal oxides, such as TiO2 , WO3 , NiO and Fe2 O3 are based on multivalent cations that can be oxidized and reduced by a change in the oxidation state and thus are very sensitive. Unfortunately, the instability of spinels with a stoichiometric composition prevents wider use in semiconductor gas sensors [14]. A higher gas response stability occurs in post-transition metal oxides. Post-transition metals with a filled d10 shell form oxides such as ZnO, In2 O3 and SnO2 which exhibit a high gas response. These d10 oxides are the most stable and most sensitive gas sensor materials, since they can be reduced by altering the d10 cation electron configuration. Free charge carriers can be formed by creating oxygen vacancies or aliovalent substitution of host cations. Cations may function as donors, acceptors, or interstitials. An oxygen vacancy effectively introduces an overall 2+ charge, thus the site will be missing two electrons compared to O2− but cation interstitial will carry positive charges due to additional positive charges in interstitial sites. Extra positive charge is also introduced by donors (cations with higher oxidation state than host cations). The overall charge neutrality of the whole lattice would then cause compensation of these defects by means of free electrons, which however are less strongly bound at a lattice site, and therefore will contribute to gas sensing process. Substitution of host cations with acceptors (cations with lower oxidation state than host cations) will introduce additional negative charges that are compensated by holes and thus materials has p-type conductivity. Despite the large variety of available oxide based gas sensors, researchers continue to search for more effective gas sensor materials with high sensitivity, fast response time, and selectivity to detect gases at lower concentrations or to eliminate cross sensitivity. In the last decades, some complex oxides attracted interest
Long term stability √
Simplicity √
High selectivity √ √
Low cost √ √ √
√
Fig. 1. Number of papers published per year on spinel ferrite gas sensor materials (articles published in Scopus database on November, 2014).
with the possibility to optimize physical and chemical properties of gas sensor. Spinel ferrites with a AB2 O4 formula unit are very promising complex oxides for gas sensing applications. Spinel ferrites are better known as magnetic materials used in high frequency applications as micro-electronic/magnetic devices [15]. The most interesting point for spinel ferrites in gas detection is the chemical composition and structure; two different cation sites are occupied by either transition or post-transition cations. Cations differing in chemical nature and charge state are arranged in two types of polyhedra (at a different bond energy) situated between the cations and the surrounding lattice oxygen [16]. Combining various transition and post-transition metal cations in spinel ferrites bring new possibilities for gas sensor material design to improve many characteristics such as sensitivity and selectivity, response and long term stability [17]. During the last decade, various spinel ferrite gas sensors have been published in an increasing intensity (articles published in Scopus database on November, 2014) (Fig. 1). Spinel ferrites have shown sensitivity to a wide range of gases [18–60]. Various spinel ferrite compounds tested to different gasses are gathered together in Table 2. The most readily detected is H2 O, followed by VOCs, then CO, NH3 , elemental gases and H2 S. Very few spinel compositions can sense NO2 , and only the MgFe2 O4 ferrite can sense SOx . Gas sensing properties of other spinel oxide compounds such as stannates [61–64], gallates [65–67], aluminates [68,69], cobaltites [70–72], chromates [72–75] and manganites [72,74,76] have also been studied. Table 3 depicts various spinel compounds used as gas sensors. These oxides have shown good thermal stability and gas response to VOC’s or nitrogen oxides and could be promising for automobile emission control applications [72]. However spinel ferrites are the most widely investigated for gas sensor applications. This review is focused on spinel ferrite gas sensor materials, their physical–chemical properties and modification possibilities through cation substitution, stoichiometry control or optimization of synthesis conditions. 2. Cation site occupation in the spinel ferrite structure The spinel structure is a cubic crystal system constructed of 32 closely packed oxygen atoms with 64 tetrahedral sites and 32
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97
Table 2 Spinel ferrite compounds tested as gas sensors. Type of spinel ferrite
Structure type
Simple
ZnFe2 O4 NiFe2 O4 CdFe2 O4 MgFe2 O4 CuFe2 O4 CoFe2 O4
Normal Inverse Normal Inverse Inverse Inverse
Solid solutions
NiZnFe2 O4 MgZnFe2 O4 NiCoFe2 O4 ZnMnFe2 O4 CuMnFe2 O4 MnNiFe2 O4 CuZnFe2 O4 MgCuFe2 O4 NiCuZnFe2 O4
Mixed Mixed Inverse Mixed Inverse Mixed Mixed Inverse Mixed
Gases
Ref.
Elemental gases √ √ √ √ √ √
NOx √ √
SOx
√
√
√
√
VOC’s √ √ √ √ √ √
CO √ √ √ √ √ √
√ √ √ √ √ √
√ √ √ √
NH3 √
H2 S √ √
H2 O √ √
√ √ √
√ √ √
√ √ √
√
√
√
√ √
√ √ √ √ √
[18–26] [26–32] [33] [17,26,34–40] [26,41,42] [26,42,43] [44] [45–47] [48–51] [52,53] [54] [55] [56–58] [59] [60]
Table 3 Spinel oxide gas sensor materials. Spinel
General formula
Ferrite Stannate Gallate Aluminate Cobaltite Chromate Manganite
(M2+ )Fe2 O4 (M2+ )2 SnO4 (M2+ )Ga2 O4 (M2+ )Al2 O4 (M2+ )Co2 O4 (M2+ )Cr2 O4 (M2+ )Mn2 O4
Divalent cations (M2+ ) Zn √ √ √ √ √ √ √
Cd √
Cu √
√
√ √ √ √ √
√ √ √
Table 4 The preferred spinel ferrite structure and its dependence on the type of cation. Spinel structure
Normal Mixed Inverse
Cation
2+
Zn Cd2+ Mn2+ Fe2+ Mg2+ Ni2+ Co2+ Cu2+
Co-ordination Tetrahedral √ √ √
Ref.
Octahedral
√ √ √ √ √ √
octahedral sites. In spinel ferrite compounds, electrical neutrality in (Me2+ )[Fe3+ ]2 O4 is maintained by Me2+ and Fe3+ in 8 tetrahedral and 16 octahedral sites, respectively [77]. Three spinel ferrite structures are possible depending on the Me2+ and Fe3+ cation balance between the tetrahedral and octahedral sites. In the normal spinel structure, Me2+ is located in tetrahedral sites while Fe3+ is found in octahedral sites. In the inverse spinel structure, Fe3+ cations are equally distributed between tetrahedral and octahedral sites, while Me2+ cations are located in octahedral sites. In the mixed spinel structure both Me2+ and Fe3+ randomly occupy both tetrahedral and octahedral sites. Table 4 shows occupation of different cations in the tetrahedral or octahedral co-ordinated sites. The inverse spinel structure occurs more frequently than the normal or mixed spinel structure. Conventionally, zinc ferrite (ZnFe2 O4 ) is a spinel ferrite with a normal spinel structure, where divalent Zn2+ cations occupy tetrahedral sites. However, NiFe2 O4 has an inverse spinel structure with divalent Ni2+ cations in octahedral sites of the spinel structure. A solid solution of zinc and nickel ferrites (Ni1−x Znx Fe2 O4 ) exhibits a mixed spinel ferrite structure. Preference for site occupation depends on (i) electrostatic contribution
Co √
Ni √
Mn √ √
√ √ √
√ √
√
[18–60] [61–64] [65–67] [68,69] [70–72] [72–75] [72,74,76]
to the lattice energy, (ii) cation radii, (iii) cation charge and (iv) crystal-field effects. The reader is directed to the work of Sickafus et al. [77] for more details. The cation distribution in spinels has been determined by a variety of spectroscopic, diffraction and magnetism based characterization methods. These include infrared absorption [78], Raman spectroscopy [79], nuclear magnetic resonance (NMR) [80], electron spin/paramagnetic resonance [81], neutron diffraction [82], Mössbauer spectroscopy [83] and X-ray photoelectron spectroscopy (XPS) [84]. XPS is the most widely used to study antisite defects (structural inversion) in ZnFe2 O4 , where some Zn2+ cations occupy octahedral sites instead of tetrahedral sites. For zinc ferrite, Zn2+ in the octahedral site results in an adjoining peak on the Zn 2p peak in the XPS spectra (Fig. 2). Properties of spinel ferrites depend on the nature of divalent cation in the spinel structure (besides Fe3+ ). The size range of cation radii in the spinel structure is 40–90 pm [85]. Owing to the difference in geometry and ion bonding energy between the cations and the surrounding oxygen ions (in octahedral and tetrahedral polyhedron) the physicochemical characteristics of spinel ferrites can be tuned by the composition, charge state, and cation arrangement [86]. For example, changes to cation positioning in the ZnFe2 O4 spinel structure from a normal (Zn in tetrahedral site) to a mixed structure (Zn in both tetrahedral and octahedral sites) can significantly change the properties of ZnFe2 O4 . The ferrimagnetic order in mixed spinel ZnFe2 O4 unlike the antiferromagnetic order in normal spinel exhibits non-linear magnetization with increased magnetic field [87–89]. This establishes conditions for improved gas sensing properties of mixed spinel ZnFe2 O4 but the instability of Zn2+ cations in the octahedral site at temperature higher than 300 ◦ C prevents exploitation [84]. The operating temperature of ZnFe2 O4 gas sensors is between 200 ◦ C and 400 ◦ C [90].
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gas sensing oxide particle. The layer thickness is further modified by the charge carrier concentration. The higher charge carrier concentrations form thinner space charge layer due to accessibility of charge carriers to the surface [92]. For oxygen chemisorption by spinel ferrites, divalent transition metal cations are in octahedral sites where they can be oxidized. For example, when a divalent cation in an octahedral site is oxidized to Me3+ , then a change in electrical conductivity will follow. This will be the case for Fe2+ in ZnFe2 O4 and NiFe2 O4 Fe2+ or Ni2+ in NiFe2 O4 [51,93]. In accordance with the mechanism proposed by Verwey, charge conduction occurs by hopping type conductivity between transition metal cations from the same element with different oxidation state located in octahedral sites [94]. Electron hopping establishes the basis for conductivity
Fig. 2. XPS shows antisite defects by zinc cation in octahedral site. Shoulder peak in Zn2p spectra, shown with an arrow, arises from octahedral zinc. An increase in annealing temperature decreases the antisite defects and lowers the concentration of zinc in octahedral sites, as shown a decrease in the shoulder height. Reproduced with permission from Ref. [84].
Me3+ + e− ↔ Me2+
n-type
Me2+ + h+ ↔ Me3+
p-type
More Me2+ and Me3+ pairs of the same element increase the cation conductivity. During oxygen chemisorption, divalent cations are oxidized to trivalent ones. Trivalent cations are reduced back to divalent ones after reaction with test gas and thus increasing or decreasing conductivity in n-type or p-type spinel ferrites, respectively. Similar to other oxide semiconductor chemical gas sensors, resistivity decreases in n-type spinel ferrites but increases in p-type spinel ferrites after reaction with a reducing analyte. Changing the transition metal Me2+ cation concentration in octahedral sites not only changes the amount of chemisorbed oxygen, but also the thickness of the space charge layer. The increase in the Me2+ concentration decreases the thickness of the space charge layer since Me2+ is more accessible for oxidation at the surface [93]. 3.1. N-type spinel ferrite gas sensors
Fig. 3. Gas sensing mechanism of chemical semiconductor oxide gas sensors.
3. Gas sensing mechanism in spinel ferrites Spinel ferrite gas sensors operate according to the wellknown semiconductor gas sensor mechanism [91]. Oxygen species chemisorb onto metal oxide particles thereby trapping electrons (Fig. 3). This produces a resistance layer from an electron depleted space charge on the n-type particle surface or conducting layer from accumulated holes on the p-type particle. When gas molecules react with chemisorbed oxygen, electrons are released from oxygen back to oxide according to the following reaction: CO + O− ads → CO2 + e− . The release of electrons changes conductivity in the space charge layer. For n-type oxides, the conductivity increases, from a greater charge carrier (electron) concentration, but for p-type oxide conductivity decreases since electrons recombine with holes. The change of conductivity in the space charge layer changes the overall electrical resistance of the oxide. A thicker space charge layer increases the sensitivity. Consequently, the smaller particle size is beneficial for a higher gas response, because the volume of space charge layer is comparable to volume of
N-type spinel ferrite gas sensors are slightly reduced from their stoichiometric composition and before oxygen chemisorption contain Fe2+ and Fe3+ pairs in octahedral sites. An exception is magnetite (Fe3 O4 ) that in its stoichiometric form contains equal amounts of Fe2+ and Fe3+ in the octahedral site and exhibits n-type conductivity [95]. Regardless, it can’t be used as chemical gas sensor due to a lack of stability on heating. Magnetite nanoparticles at 200 ◦ C irreversibly transform to maghemite ␥-Fe2 O3 and then further to the hematite (a high-temperature polymorph) [96–98]. Charge carrier transport in n-type spinel ferrites is provided by hopping type conductivity of e− between iron cations located in octahedral sites. A greater Fe2+ concentration increases e− conductivity (Fe3+ + e− ↔ Fe2+ ). After chemisorption of oxygen on the n-type spinel ferrite, Fe2+ is oxidized to Fe3+ . When oxygen reacts with gas, e− are released back to the material and Fe3+ is reduced to Fe2+ , thus increasing the amount of Fe3+ and Fe2+ pairs and overall electrical conductivity. The oxidation–reduction reaction on the surface of n-type spinel ferrite surface is reversible as demonstrated by others [90]. Most popular examples of n-type spinel ferrite gas sensors are ZnFe2 O4 , CdFe2 O4 and MgFe2 O4 . The Fe2+ concentration in ZnFe2 O4 can be set either at the synthesis stage or the annealing stage. Ferrous iron can be maintained by synthesis of iron-excess compounds or annealing in a reducing atmosphere [99]. In iron-excess zinc ferrites Fe2+ incorporates in octahedral site, but Fe3+ is moved to tetrahedral sites and substitutes zinc cations providing formula:
Zn2+ Fe3+ x 1−x
Fe3+ O4 Fe2+ x 2−x
Increasing the iron content also increases the amount of Fe2+ [100]. A greater amount of Fe2+ increases the oxygen chemisorption capacity, however, if the Fe2+ concentration is too high then the
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Fig. 4. Schematic of (a) isolated and (b) embedded tubes based sensing elements. Reproduced with permission from Ref. [102].
Debye width is narrower, and the gas response of the sensor is reduced [93]. Annealing in a reducing atmosphere forms oxygen vacancies which are compensated by reducing Fe3+ to Fe2+ in accordance with equation: • 1 ZnFe2 O4 → O2 + V2O + 2FeFe , 2 •
where V2O is oxygen vacancy with effective charge +2 and +2FeFe is Fe3+ substitution with Fe2+ thus compensating positive charge from oxygen vacancies. In spinel ferrites such as ZnFe2 O4 or CdFe2 O4 annealing at about 1000 ◦ C preferentially vaporizes Zn2+ or Cd2+ , leading to an excessiron ferrite [101]. Evaporation of zinc alters the stoichiometry (Zn1−x Fe2+x O4 ) allowing the newly formed Fe2+ to chemisorb oxygen. However, non-stoichiometry of zinc or cadmium ferrites by heating at high temperatures is unlikely, because exposure to high temperature reduces the specific surface area and consequently, the ability to react with the test gas. The conductivity of n-type spinel ferrites such as Mg0.5 Zn0.5 Fe2 O4 can be tailored by surface engineering [102]. Hollow Mg0.5 Zn0.5 Fe2 O4 tubes made by alumina template assisted wet chemical synthesis may be embedded in alumina (Fig. 4). Isolated tubes exhibit p-type sensing characteristics, but when embedded in alumina template, an n-type behaviour is displayed. An inversion of the main charge (from n- to p-type) is associated with the higher oxygen chemisorption on Mg0.5 Zn0.5 Fe2 O4 nano-tubes than on embedded tubes. Incorporation of the oxygen into Mg0.5 Zn0.5 Fe2 O4 creates defects; negatively charged oxygen interstitials or cation vacancies. To maintain charge neutrality, these defects are compensated by holes thereby imparting p-type conductivity. 3.2. P-type spinel ferrite gas sensors P-type spinel ferrites are usually inverse spinel structure ferrites with transition Me2+ cations in octahedral alongside besides Fe3+ . A common p-type spinel ferrite is NiFe2 O4 . The p-type conductivity arises from hole (h+) hopping between Ni2+ and Ni3+ in octahedral sites (Ni2+ + h+ ↔ Ni3+ ). The Ni3+ in NiFe2 O4 is from the cation vacancy. Cation vacancies in nickel ferrite form due to the tendency of nickel to attract excess oxygen during synthesis. To maintain electrical neutrality in the lattice Ni2+ oxidizes to Ni3+ [103]. Divalent nickel cation oxidation to Ni3+ in lattice can be described as follows in Kröger–Vink notation: • 1 O + 2NixNi → OxO + VNi + 2NiNi 2 2(g)
where “x” represents zero charge, “• ” represents a positive charge (hole) and “ ” represents a negative charge (electron). Higher Ni3+ concentration in the bulk lattice of nickel ferrite lowers the sensitivity. For compositions with high intrinsic Ni3+ concentrations, only a small amount of additional Ni3+ can be formed by chemisorption of oxygen adsorbates because, in the spinel structure, the
99
Ni3+ does not occupy a higher oxidation state. In contrast, for nickel ferrite with lower intrinsic Ni3+ concentration and a higher concentration of Ni2+ , the higher oxygen chemisorption occurs. Chemisorbed oxygen reacts with the test gas, to alter the resistance. Higher chemisorbed oxygen concentrations will, therefore, lead to an improved gas response. This assumption is illustrated schematically in Fig. 5. To improve the gas response, a higher Ni2+ surface concentration in NiFe2 O4 is needed [51]. It is also possible to synthesize an n-type nickel ferrite by changing composition. As little as five at% of Ni2+ replacement of Fe2+ produces n-type conductivity after sol–gel combustion [104]. With a greater amount of Fe2+ over the Ni3+ , electron transport occurs by electron hopping between Fe3+ + e− ↔ Fe2+ . 3.3. Mixed and substituted spinel ferrites Substituted spinel ferrites incorporate more than two different cations. Ni-Zn ferrites are a typical example. The substitution of Ni2+ by Zn2+ in NiFe2 O4 strongly affects its electronic structure. Zinc ions preferentially occupy tetrahedral sites, thus displacing Fe3+ ions from tetrahedral sites to octahedral site resulting in a mixed spinel with uniformly distributed divalent and trivalent ions in tetrahedral and octahedral sites [105]. Resistivity of sol–gel autocombustion derived Ni–Zn ferrite nanoparticles increases with more Zn2+ and type of conductivity changes from p-type to n-type. The dominating charge carriers become electrons. Ni-Zn ferrites, with x − 0.5 and x = 0.7 in Ni1−x Znx Fe2 O4 , do not show a uniform response and operate according to different mechanisms. After exposure to a fixed acetone concentration at 225 ◦ C, the resistance increases reaching a peak after 50 s followed by a slow decay (Fig. 6). Moreover, compositions at x = 0.5 and x = 0.7 show an n-type behaviour below 225 ◦ C and a p-type behaviour above 225 ◦ C. The change in conductance arises from change of major charge carriers from electrons to holes. Upon heating Ni-Zn ferrites, oxidation from Fe2+ to Fe3+ occurs during oxygen chemisorption. Higher temperatures increase oxygen chemisorption leading to Ni2+ oxidation to Ni3+ and the conductivity mechanism changing to a p-type with holes responsible for conduction. At 225 ◦ C, after reaction with the test gas, trapped electrons are returned to the semiconductor, reducing Ni3+ to Ni2+ to increase the resistivity and after Ni3+ expended reduction of Fe3+ to Fe2+ occur decreasing resistivity [106]. Substituted spinel ferrites will exhibit a change in gas response, but the direction of change is not always predictable. Solid solutions of Ni-Zn in ferrites lower the gas response compared to NiFe2 O4 and ZnFe2 O4 . Replacing Co2+ for Ni2+ in NiFe2 O4 also lowers the gas response [51]. The Co2+ in octahedral sites increases resistivity by interrupting the hole transfer between Ni2+ and Ni3+ and hindering transfer of chemical signal to electric one. Some other substitutions are more favourable. Replacing Fe3+ with Ce2+ increases the gas response in CoFe2 O4 [43]. Vanadium (V5+ ) can be incorporated into substitutional and interstitial positions and can further increase the sensitivity to VOCs. Vanadium addition to ZnFe2 O4 decreases the gas response in ethanol and acetone, but increases the response to benzene, especially at higher temperatures, due to its catalytic oxidation [22]. Vanadium incorporation also enhances temperature stability. 3.4. Spinel ferrite humidity sensors Spinel ferrites also display active sites for water vapour dissociation and so can be used for sensing water. Spinel ferrite humidity sensors display good sensitivity, reversibility and a long lifetime. The humidity sensing mechanism of spinel ferrites is the same as that of other ceramic-based humidity sensors [38]. Firstly,
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Fig. 5. Schematic representation of gas sensing mechanism for nickel ferrite with different Ni3+ content: Sample No 1 contains Ni3+ and has lower oxygen chemisorption ability and thus lower change of electrical resistance R and gas response.
chemisorbed water displaces oxygen and forms a monolayer of OH groups [53]. Secondly, water molecules are physisorbed and then dissociate due to the high electrical fields in the chemisorbed layers. The conduction known as the Grothus chain reaction is generated by H+ and H3 O+ movement [39]. At elevated humidity levels, the electrolytic conductivity becomes dominant [107]. Often, the response behaviour of ceramic humidity sensors is related to morphology. A higher specific surface area favours the water adsorption. Introducing pores of different sizes, shape and connectivity enhances water physisorption. Water molecules adsorbing into small pores (3–50 nm) will condense by capillary action and lower the electrical resistivity due to electrolytic conduction [108]. Research on spinel ferrite humidity sensors has mostly focused on microstructural issues – specific surface area, grain size and porosity, which has been regulated by physical or chemical modification. A reduction in humidity sensitivity occurs when a Mn-Zn ferrite is annealed in vacuum, that could be attributed to densification [53,109]. Mn2+ and Ni2+ ions were used to modify the microstructure of MgFe2 O4 [37]. Mn2+ addition increases the specific surface area by five times and increases the response to
humidity over a wide humidity range. Ga3+ , Y3+ and La3+ ions substitution for iron in Mg0.5 Cu0.5 Fe2 O4 ferrite modifies porosity, surface area, grain size and electric resistivity [59]. The best humidity detection has been found with Mg0.5 Cu0.5 Fe1.8 Ga0.2 O4 attributed to higher porosity, larger specific surface area and smaller grain sizes. Similar results were observed when Mg was substituted by Sn in MgFe2 O4 [110]. The response to humidity can be improved by substituting a foreign element such as Pr in MgFe2 O4 . The electropositive nature of praseodymium increases the reaction with the highly electronegative OH− [38]. The humidity response of spinel ferrite MgFe2 O4 can be improved by including other oxides such as CeO2 [39]. Cerium oxide uptakes oxygen from MgFe2 O4 creating more surface defect lattice sites or oxygen vacancies, resulting in a greater number of efficient water adsorption sites. Spinel ferrites are promising materials for humidity sensors, but there is insufficient information on various spinel ferrite compounds and the influence of point defects on response to humidity. A direct comparison of humidity detection for p-type or n-type ferrites is missing.
Fig. 6. The response characteristics for Ni0.3 Zn0.7 Fe2 O4 gas sensor material. Reproduced with permission from Ref. [106].
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Fig. 7. SEM image of sol–gel auto combustion derived products (left) and a schematic drawing of sol–gel auto combustion reaction products (right) – primary grains assembles into larger entities. Two different kinds of pores are formed – the pores between agglomerates and pores between adjacent grains. The pores between agglomerates are larger than the pores between adjacent grains.
4. Spinel ferrite gas sensor synthesis methods Wet synthesis is the preferred synthesis route in producing nanopowders for gas sensors. Sol–gel combustion, co-precipitation and hydrothermal synthesis are the most popular methods, representing 70% of all cases. Properties of spinel ferrite gas sensors are very sensitive to synthesis conditions and are, therefore, dependent on stoichiometry, inversion and point defect concentration. Sol–gel auto-combustion creates porous agglomerated nanoparticle aggregates after thermally induced self-sustaining anionic redox reactions between metal nitrates and reducing agents (Fig. 7). Organic reducing agents (glycine, urea or citric acid) act as complexing agents and effectively chelate metal ions in xerogels by evaporation of nitrate/complexant water solutions. Synthesis involves dissolving metal nitrates and organic reducing agents in water, stirring and heating until water evaporates and then forming dry xerogel. Further heating above 200 ◦ C causes exothermic combustion, a rapid evolution of heat (up to 1000 ◦ C) and a large mass loss (c.a. 90%). The high processing temperature creates high crystallinity ferrites, and the gas release creates open pore structures with a high surface area (Fig. 7). These open pore networks enhance gas diffusion. Co-precipitation is low-temperature synthesis method for precipitating nanoparticles from metal salt aqueous solutions (chlorides, nitrates or sulphates) in basic conditions. The particle size is less than 10 nm at temperatures lower than 100 ◦ C. The size, aspect ratio, phase purity, inversion, stoichiometry and point defect concentration of the spinel ferrite nanoparticles is adjusted by precursors, molarity, temperature, pH and time [111]. However, once the co-precipitation conditions are fixed, the properties of the spinel ferrite nanoparticles are nearly fully reproducible. Due to the low synthesis temperature, co-precipitation synthesis frequently produces low crystallinity ferrites with additional (oxy)-hydrate and amorphous impurity phases [112]; further annealing at temperatures above 500 ◦ C is necessary to increase phase purity and crystallinity. Additional annealing reduces porosity and specific surface area [96]. Hydrothermal synthesis uses the same reactants as coprecipitation at elevated temperatures and pressures to produce high crystallinity crystals [113–115]. Particle shape and size are controlled with capping agents or surfactants such as polyethyleneglycol [116] and cetyltrimethylammonium bromide (CTAB) [117]. CTAB-assisted hydrothermal synthesis has been used to obtain single-crystalline CoFe2 O4 high aspect ratio nanorods. Cobalt ferrite high aspect ratio nanorods have formed due to the
adsorption of CTAB surfactant, which affected the growth rate and orientation of crystals. Overall, the size, shape, phase purity and structural peculiarities of hydrothermally synthesized spinel ferrite nanoparticles depends on the type and concentration of precursors used and most important the pressure into the autoclave [119]. Varying hydrothermal synthesis conditions (pH value, temperature and reaction time) leads to shape anisotropy and a different type of conductivity, possibly attributed to the formation of iron-excess non-stoichiometric compounds [31]. The hydrothermal technique has the lowest yield, but the highest ability to control particle size (Table 5). Spinel ferrite nanoparticles play an important part in the preparation of ferrite based gas sensor elements. The gas sensor elements usually consist of a ceramic alumina tube assembled with platinum wire electrodes for electrical contacts, a heater fixed inside the tube and a chromel–alumel thermocouple for temperature control. Ferrite nanoparticles are deposited on the sensor elements in layers by dip-coating, spin-coating, screen-printing or drop deposition from suspensions (containing viscous alcohols such as terpineol or glycerol). Gas sensor materials must be well integrated with prototype systems to validate their industrial relevance before downscaling to smaller sensors. In producing the gas sensors, it is necessary to deposit films on small 100 m2 heated sensing areas (Fig. 8(a)). Micro-drop deposition can deliver nanoparticle suspension or sol containing precursors [118]. Micro-drop deposition is a simple two step process. A micro-capillary delivers sol or suspension to the microsensor platform (Fig. 8b), removes excess sol or suspension (Fig. 8c), leaving a uniformly dispersed deposit (about 100 nm thick and about 500 m wide) (Fig. 8d). This process leaves a thin circular layer, which is later thermally treated if necessary. 5. Latest developments and future prospects of spinel ferrite gas sensors Gas response of metal oxide semiconductor gas sensors improves by the addition of noble metal nanoparticles [93,119]. Sensitization of spinel ferrites by noble metals is achieved electronically (Ag or Pd) or chemically (Pt or Au) [52,30,120,121]. Electronic sensitizers deplete the electron space charge layer on the surface and also increase the gas response volume. Chemical sensitizers facilitate catalytic oxidation of the test gas on the surface without changing the resistance of the sensors [91]. Noble metals increase gas response and selectivity and decrease the operating temperature. Best results were achieved by Liu et al.
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Table 5 Synthesis methods used for the synthesis of spinel ferrite gas sensor materials. Method
Reaction temp. (◦ C)
Reaction period
Sol–gel auto combustion Co-precipitation
550–1100
Hours
Hydro-(solvo-) thermal
105–270
a
20–90
Particle size (nm) >25
Minutes
