11.3.1 Co-precipitation

Co-precipitation is used to prepare multicomponent and/or doped oxides via precursor formation, a pure or a variously complex multicomponent intermediate precipitate. The process depends on the ions in the starting materials, usually inorganic salts, precipitated by the addition of stoichiometric amounts of bases or acids. The reaction medium is usually water. The precursor is irreversibly converted into the oxide by calcination, typically performed between 400 and 900 °C. The time and temperature of the calcination belong to the chemical nature of the precursor itself and are evaluated via thermal analysis. The morphology and the specific surface area of the final oxide result as a trade-off between those two parameters. Co-precipitation allows an intimate mixing of all the desired ions in the oxide, maintaining chemical homogeneity on the molecular scale (Segal, 1997). Doping is a chance to enhance or inhibit the chemical and/or physical behavior of nanopowders whether the behavior is intentional or not. In the reaction, close attention to experimental conditions (pH, temperature, concentration) is required. Chemical homogeneity can be enhanced by sonochemical treatment with the application of ultrasound waves, ranging between 20 kHz and 10 MHz, on the reacting mixture. The resulting precursor is a function of the frequency imparted (Buzby et al., 2007). Moreover, SEM analysis assisted by Energy Dispersive Spectrometry (EDS) can be useful to control the elemental composition of small areas of certain samples.

A crucial point of co-precipitation is the kinetically driven control of the particle size and its distribution. In fact, very often the precipitation runs uncontrolled due to the difficulty in matching the suitable experimental conditions. After calcination, the nanopowder undergoes morphological and structural characterization, usually through transmission or scanning electron microscopy (TEM, SEM), specific surface area measurement by BET (Brauner, Emmett, Teller) method and finally by x-ray diffraction (XRD) analysis.

11.3.2 Sol-gel synthesis method

The sol-gel route consists in the preparation of pure or multicomponent metal oxides by the hydrolysis of inorganic salts in aqueous media or metal-organic compounds in non-aqueous solutions (see for example: Segal, 1997; Buzby et al., 2007; Brinker and Scherer, 1990). A sol-gel reactive solution is a colloidal suspension (the ‘sol’), which leads to a solid and wet 3D-skeleton (the wet gel) via sol-gel transition (gelation, the ‘gel’). When the solvent is removed under atmospheric pressure, the precursor of the final oxide (the xerogel) is yielded, which is typically an hydroxide, irreversibly converted into the final oxide by calcination at temperatures between 400 and 900 °C.

During the ‘sol’ formation, the starting material undergoes hydrolysis and then condensation, or cross-condensation in the case of a route for multi-component materials. The essence of sol-gel chemistry is quite simple (Brinker and Scherer, 1990; Gopalakrishnan and Mani, 2009), but considerable efforts are still required in tuning the experimental conditions for tailoring the properties of the powders. The precise choice of both pH and concentration of the reacting mixture play a key role in driving the rate of hydrolysis and, in turn, the morphology, the specific surface area and the presence of by-products. Figure 11.3(a) shows the general scheme of the sol-gel route via inorganic salts.

For the synthesis of multi-component oxides, the route proceeds essentially by the same steps, just considering the co-presence of different cations M₁ ≠ M₂ starting from step (1).

Metal-organics, hereinafter labeled as M(OR)_n consist of a metal or a metalloid, M, directly linked with the oxygen, and R is usually an alkyl group. Sol-gel processing using metal-organics results in a better-controlled hydrolysis than processing using inorganic salts. Metal-organics are susceptible to atmospheric moisture: the M-O bonds readily react with nucleophiles so they have to be handled under inert conditions and, to avoid by-products, they could be purified before their use. Although a wide set of metal-organics is commercially available, their delicate reactivity and high cost suggest the use of an inorganic salt as starting material, or to synthesize the molecule in-situ and use it in a one-pot reaction. Figure 11.3(b) shows the general scheme of sol-gel route via metal-organic compounds.

11.3.3 Hydrothermal synthesis

The hydrothermal (HY) processing is ‘any heterogeneous chemical reaction in the presence of a solvent, whether aqueous or non-aqueous, above room temperature and at pressure greater than 1 atm in a closed system’ (Byrappa and Yoshimura, 2001a). The use of different solvents, or a mixture of them, helps to push down the operating pressure and favors both the solubility of the starting materials and the dispersion of the growing particles within the solvent. The reagents used in the HY processing are both inorganic and metal-organic. The precursor of the desired oxide can be synthesized via wet chemistry or via a HY route and then submitted to HY conversion into the oxide.

The equipment used for the processing is an autoclave or a ‘bomb’ (Byrappa and Yoshimura, 2001b). The technique is being extensively employed to prepare both pure single crystals and polycrystalline materials (Somiya and Roy, 2000). An important application is the synthesis of zeolites, crystalline alumino-silicates used in combination with chemical sensors to enhance their selectivity (Hugon et al., 2000; Sasaki et al., 2002). The powders prepared via HY hinder particle agglomeration yielding fine grain size down to the nanometric domain, and a narrower grain size distribution in comparison with those obtained via wet chemistry routes.

Together with SnO₂, ZnO is one of the first nanomaterials studied for thick-film gas sensing devices. Because of its poor selectivity and high working temperature (400–500 °C), over the years it has become gradually less popular, favoring the study of more promising materials. With emerging powders processing applied in the field of nanoceramics, such as the HY technique, ZnO has been reconsidered with the aim of improving its performances by tailoring, for example, its morphology (Baruwati et al., 2006; Asamoto and Yahiro, 2009).

11.3.4 Rheology, pastes and screen-printing technology

The study of compressing and deforming strengths on a system is the base of rheology, which is crucial to selecting the methodology and the equipment for mass processing the powder. Thick-films are fabricated from printable pastes of functional materials in the form of powder, inorganic additives and organic binders. The structure of a paste easily changes with time, temperature and shear stress. Its physical characteristics depend on all components added to the paste and their interactions (Reed, 1995a). Moreover, the pastes for screen-printing must be thixotropic: during the printing processing, the viscosity of the paste must decrease under an applied shear stress and increase when the stress is removed (Reed, 1995b).

The viscosity of the paste depends on the loading in powder, resins and solvents percentage. Resins dissolved in the solvents yield the organic vehicle, whose viscosity affects that of the paste. The most common formulation for screen-printable paste is listed in Table 11.9; specific paste compositions are highly proprietary.

In screen-printing technology a patterned woven mesh screen defines the pattern to be printed onto a substrate by a squeegee, a polymeric blade whose stiffness is chosen according to the process to be carried out. The screen is a set of regularly spaced woven mesh anchored on a rigid frame, under calibrated-tension conditions.

The caliber and type of the cables depend on the detail required for the pattern, the rheology of the paste, shape and roughness of the substrate and the thickness of the final layer. To print a layer with a specific pattern, the paste must flow only through the empty areas of the screen; each printed block will merge into a compact film.

The variables influencing the printing processes are listed in Table 11.10. For both the best pattern resolution and the best printing quality, a trade-off between all the tabulated parameters is a must.

After printing, drying allows the removal of the volatile components from the paste (100–200 °C) (Reed, 1995c).

Shrinkage occurs when the liquid between the solid particles is removed and inter-particle separation decreases; this effect can be reduced when the grain size distribution of the powder is spread over a wide dimensional range. Drying is the step prior to firing, the thermal process that imparts and consolidates the shape of the printed layer, and also develops its microstructure and properties. Unfired ceramics are weak and can sustain only weak stresses without deformation or local fracture. Firing is performed in furnaces under ambient atmosphere between 600 and 1000 °C. The oxygen partial pressure drives the content of metals or metalloids with the proper oxidation state, in the functional material; rarely, some layers need to be treated in a different atmosphere to tailor the stoichiometry of the material itself.

Firing proceeds in three steps: (1) organics burnout and gaseous by-products elimination; (2) sintering; and (3) cooling (Reed, 1995d).

The glass frit, in the paste formulation, is essential to improve the adhesion of the printed film onto the substrate. During sintering it achieves its softening point, turning from a solid to a viscous glassy matrix wetting the grains uniformly. The matrix draws the particles together enabling their sliding and rearrangement into a denser configuration. Rarely more than 1% w/w of glass frit, with respect to the weight of the powder, is required to coat entirely the grains at the softening point. When the frit is inhomogeneously distributed within the paste, different rates of densification and grain growth may cause inhomogeneous microstructure and a low average density of the resulting film. In Fig. 11.6 the flow chart of the total process is shown.

11.4.1 SnO₂-based materials

Among the above-mentioned materials, a case in point is tin dioxide, the most widely used material for gas sensing, by which in 1970 Figaro Engineering Inc. fabricated and commercialized the first metal oxide gas sensor device (Taguchi, 1970). SnO₂ exhibits n-type semiconductor behavior due to lattice defects, in particular oxygen vacancies acting as electronic donor levels. Indeed, Samson and Fonstad (1973) experimentally determined the presence of two donor levels, Ed₁ = 0.034 and Ed₂ = 0.145 eV below the bottom of the conduction band.

Figure 11.8 shows the Arrhenius plot obtained by changing the temperature between 360 and 800 K at the heating rate of 3 K/min (A) and the height of the surface barrier potential versus temperature (B) of a SnO₂ thick-film. To determine the energy barrier (the difference in energy between the conduction band bottom at the surface and that in the bulk) temperature-stimulated conductance measurements were performed with a procedure described elsewhere (Lantto et al., 1988; Carotta et al., 1991).

Different conductivity regions were observed, demonstrating the occurrence of different surface adsorption phenomena, in particular the O₂⁻ – O⁻ transformation, as experimentally proven by Chang through the correlation between electrical conductivity and EPR measurements (Chang, 1980). The typical behaviour of the SnO₂ energy barrier versus temperature exhibits a minimum and a maximum corresponding to the increase of the depletion layer depth with temperature attributed to a negative surface charge accumulation, typically O⁻ ions. The ability of tin dioxide to sense either reducing agents at high temperature or oxidizing ones, like NO₂, at low temperature could be explained according to these characteristics. The interaction between the analyte and the surface of the semiconductor causes changes to the potential barrier height at the inter-granular contact and therefore to electrical conductance variations.

All the electrical measurements here shown have been obtained in a sealed test chamber using the flow-through technique maintaining a constant flow of 0.5 l/min in dry or wet air or in mixtures of air and test gases.

As the first material extensively investigated in gas sensing, many studies have been performed to improve the sensing and selectivity properties of SnO₂, above all by adding small amounts of noble metals on the surface (Yamazoe et al., 1983; Schierbaum et al., 1991; Yamazoe, 1991; Fliegel et al., 1994; Xu et al., 1996; Cabot et al., 2000, 2002). On this subject, in Batzill and Diebold (2005) the role of additives in gas-sensing materials has been thoroughly reviewed. In Chiorino et al. (1997), the role of Pd has been examined on two materials, a commercial SnO₂ powder from CERAC and a SnO₂ powder prepared in the laboratory by a precipitation route, differing in grain size and the shape of grains. It turned out that the role of the nanostructure associated with a high dispersion of small additive particles resulted in a higher gas response and reduction of the operating temperature. Figure 11.9(a) shows a result of TEM investigations performed on powders calcined at 550 °C and on films loaded with 0.4 at.% of Pd and heated up to 850 °C (Fig. 11.9(b)). The round-shaped particles exhibit an average diameter of 20 nm, a little wider (< 30 nm) in the films.

Moreover, in Fig. 11.10, the SEM micrographs performed on Pd-loaded films show the crack-free film surface (Fig. 11.10(a)) and the optimum degree of particle sintering control (Fig. 11.10(b)).

From the electrical point of view, Fig. 11.11 illustrates the different sensing effect of Pd or Au (0.4 at.%) addition on the detection of carbon monoxide and methane.

Fourier-transform-infrared-spectroscopy (FT-IR) measurements enabled us to clarify the role of the oxygen vacancies for the laboratory-prepared material. Indeed, in the presence of reducing gases, a growth of a very intense and broad absorption peak due to electronic transitions involving oxygen vacancies appeared, which is not present in the commercial powder. On the contrary, for the latter material, only an increase of the scattering of the radiation by free electrons could be found, thus highlighting the difference between a nanostructured material and a coarser one and the importance of a high stoichiometric defectiveness of the sensing material associated with the nanostructure.

Other selectivity properties can be obtained by adding different ions to SnO₂ crystal lattice, such as Mo, W, V, La, etc. The addition of Mo and W generally resulted in a decrease of the response to reducing gases, like CO, and in an increase versus oxidizing gases, such as NO₂, while for V and La addition, a good sensitivity to SO₂ and CO₂, respectively, is reported (Chiorino et al., 1999, 2001; Das et al., 2008; Marsal et al., 2003).

As stated above, the synthesis method involves different electronic properties, while the decrease in grain dimension down to the depletion width provides an enhanced gas response (Malagù et al., 2004a, 2004b; Xu et al., 1991; Vuong et al., 2005; Zhang and Liu, 2000). Actually, in tin oxide thick-film sensors it is unusual to observe the grain size dependence of the response. Indeed, the firing at high temperature causes a grain coalescence of up to 20–30 nm. Because the depletion layer depth in tin oxide is less than 20 nm, in order to observe this phenomenon the sensing layers must be deposited starting from very small powders (< 10 nm) and heated at a moderate temperature (≤ 650 °C) (Malagù et al., 2004c). Under these conditions, we have experimentally verified that the response to carbon monoxide doubles when the grain size is about 10 nm with respect to that of a grain size of about 30 nm.

In spite of the many improvements obtained through material modifications (such as the synthesis of nanostructured powders and the addition of ions or catalysts), a significant cross-sensitivity caused by the presence of a small amount of water vapor greatly hindered the exploitation of tin oxide-based gas sensor devices. Several authors visited the subject following different approaches: first of all by studying the mechanism of interaction between the semiconductor surface, detecting gas and water vapor and afterwards trying to solve the drawback through material modification, filters or processing the sensor signal by mathematical algorithms. Some papers on this subject are listed: Yamazoe et al., 1979; McAleer et al., 1988; Gercher and Cox, 1995; Ghiotti et al., 1997; Bârsan and Weimar, 2003; Harbeck et al., 2003; Marsal et al., 2003; Ankara et al., 2004; Ivanov et al., 2004; Hossein-Babaei and Ghafarinia, 2010. In particular, in Carotta et al. (2007b), a compensation algorithm has been obtained through experimental tests and positively applied to the monitoring of atmospheric pollutants. The resulting equation, which represents a surface, contains three terms: two of them represent the response to a single gas and the third one is an interaction term between the water vapor and the testing gas.

11.4.2 TiO₂-based materials

Among the large variety of TiO₂ applications, the photo-assisted degradation of organic molecules is certainly the most exploited. So, the major knowledge of titania as a functional material comes from catalysis and photocatalysis (Fujishima and Honda, 1972; Matsuda and Kato, 1983; Linsebigler et al., 1995; Paz, 2010; Diebold, 2003). In the gas sensing field, titania was first investigated for its application in automotive exhaust sensors, i.e. in the measurement of the oxygen partial pressure in the exhaust gas, and hence for the control of the air/fuel ratio (Tien et al., 1975; Takami, 1988; Zhu et al., 1996; Sharma et al., 1996). Since the 1990s, titania has been studied to detect atmospheric pollutants at moderate temperature (300–600 °C) (Tang et al., 1995). Actually, titania is suitable for sensing polluting gases in air, allowing good and stable responses only if the grain dimension of the poly-crystalline material is kept at nanometric level. Indeed, in contrast to SnO₂, titanium dioxide, when heated, undergoes a phase transition from anatase into rutile, simultaneously resulting in very large grain coalescence (Hague and Mayo, 1993; Guidi et al., 2003). The anatase phase can be stabilized through the addition of ions with valence 5⁺ (Guidi et al., 2003; Akhtar et al., 1992; Martinelli et al., 1999; Carotta et al., 1999; Traversa et al., 2001; Ruiz et al., 2004a; Sacerdoti et al., 2004). In particular, Ta (10 at. %) allowed to hinder the transition to rutile up to 950 °C (Traversa et al., 2001). A different method of stabilizing titania in the anatase phase consists of synthesizing the oxide by using the sol-gel route (SG) followed by an appropriate treatment HY (Ruiz et al., 2004b; Carotta et al., 2007a). In contrast to the traditional SG synthesis, the HY process allows for the maintenance of the anatase phase up to 850 °C without any introduction of foreign elements (Carotta et al., 2007a). In Fig. 11.12, a SEM image of a HY–titania thick-film fired at 850 °C shows nanosized particles with an anatase structure as confirmed by XRD analysis.

In spite of the excellent results in terms of anatase phase stabilization with the consequent maintenance of the grain dimension at nanometric size, the TiO₂-based gas sensors differ in gas-sensing behavior from SnO₂-based ones, the latter being much more reactive versus reducing gases. A theoretical model has been developed to justify this behavior. The idea was to generalize the Schottky relation (Morrison, 1982) to the case of spherical symmetry and to the cases where the depletion approximation fails. The result is a strong grain-size dependence of the TiO₂ gas-sensing properties caused by the variation of the charged surface states, N_s, with grain radius. SnO₂, instead, exhibits a modest dependence of the gas response on the particle size (as confirmed by experimental evidence) due to an almost constancy of N_s (see section 11.2).

TiO₂ strongly differs from SnO₂ in that it has a much smaller electrical conductivity and the shape of the Arrhenius plot is different (see Fig. 11.13(a)). Indeed, the sigma shape caused in SnO₂ by the occurrence of surface reactions (in particular the O₂⁻–O⁻ transformation in the 200–400 °C range) is not visible in TiO₂ because in this temperature range the bulk donor states are not yet ionized. In fact, the n-type behavior, in TiO₂, as with SnO₂, depends on donor-like stoichiometric defects, generally ascribed to oxygen vacancies, but with energetic positions in TiO₂ that are much deeper inside the band gap with respect to SnO₂ (Samson and Fonstad, 1973; Munnix and Schmeits, 1985). Actually some authors assign donor levels 0.8 eV deep, at Ti^3 + ions rather than at oxygen vacancies (Diebold, 2003; Göpel et al., 1988). Moreover, titania films feature heights of energy barriers versus temperature that are much higher than SnO₂, where almost three activation energies are exhibited, differently from TiO₂, in which only two regions of barrier are present (see Fig. 11.13(b)).

With respect to the structural properties, SnO₂ and TiO₂ exhibit the same rutile crystalline structure, the space group P4₂/mnm easily giving rise to a solid solution with chemico-physical properties varying with the Ti–Sn molar ratio. In fact, the SnO₂-TiO₂ system has been widely investigated; in some of the papers mentioned here, the characteristics of the immiscibility gap can be found (Park et al., 1975; Yuan and Virkar, 1988; Radecka et al., 1998; Dusastre and Williams, 1999; Radecka et al., 2001). Actually, the SnO₂-TiO₂ mixed oxide is a very interesting material for thick-film gas sensors and its synthesis and investigation (as Ti_xSn_1-xO₂, 0 < x < 1 with x = 0, 0.1, 0.2, 0.3, 0.5, 0.7, 0.9, 1) allowed us to discover a completely different phenomenon occurring during the sensing process in SnO₂-like materials compared with what happens in TiO₂-like ones. Indeed, through FT-IR analysis under a CO atmosphere, for the materials with x ≤ 0.2, as already observed in pure SnO₂ (Chiorino et al., 1997), a very broad absorption in the medium IR (MIR) region, related to the photoionization of mono-ionized oxygen vacancies VO^., occurred, while samples with 0.3 ≤ x ≤ 1 exhibited a completely different phenomenon consisting in an erosion of the absorption edge related to the skeletal M–O vibration modes (Carotta et al., 2009b). This difference of behavior can contribute, together with the definition of the phenomenon of the unpinning of the Fermi level (Malagù et al., 2004b; Carotta et al., 2007a) to understanding the electrical differences between SnO₂ and TiO₂.

In summary, with regard to the electrical properties of the various solid solutions obtained by varying the Ti molar ratio, all examined samples with 0.1 ≤ x ≤ 0.9 exhibit good sensing performance, even if they exhibit a crystalline rutile structure. However, it must be noted that these materials do not undergo a phase transformation, exhibiting a rutile phase as synthesized. For this reason, the grains undergo coalescence only moderately when heated at high temperature. In Fig. 11.14(a), the crystallite dimension of the powders calcined at 650 °C, estimated through XRD analysis using Scherrer’s equation, is reported, whereas that of pure titania is not reported because at 650 °C the crystalline phase is anatase instead of rutile. In Fig. 11.14(b), the response to 50 ppm of carbon monoxide (working temperature: 550 °C) of the thick-films fired at 650 °C, as a function of the crystallite size of the films, is shown. We note that the crystallite size is strongly dependent on composition, exhibiting the lowest size for 0.3 ≤ x ≤ 0.5, while the gas response increases with decreasing grain size rather than with the Ti molar ratio, as might have been supposed (Carotta et al., 2008, 2009a, 2009b).

From the electrical point of view, the materials with x ≤ 0.2 exhibit conductivity much higher than the compositions with x > 0.2; the two classes of materials also show different shapes of conductance curve versus temperature (Carotta et al., 2009b). Concerning surface barrier height trend versus temperature, the sample with x = 0.1 behaves similarly to SnO₂ with an increased barrier height corresponding to the decreased conductivity. The samples with 0.3 ≤ x ≤ 0.9 behave like titania (temperature dependence), but the height of the energy barrier was determined by the size of the nanoparticles. Indeed, the grain radius of the samples with x = 0.3 and x = 0.5 are smaller than Λ (the depletion layer width) and the band bending cannot be fully developed (Carotta et al., 2009a, 2009b). In summary, among the various synthesized Ti_xSn_1 − xO₂ solid solutions, the stoichiometry with x = 0.2 exhibits a borderline behavior between that SnO₂-like and TiO₂-like oxides, for all examined properties (see Figs. 11.15(a), 11.15(b)).

11.4.3 WO₃-based materials

Tungsten trioxide has been investigated mainly because of its wide number of crystalline forms, in contrast to its simple stoichiometry (Boulova and Lucazeau, 2002; Vogt et al., 1999; Howard et al., 2002). Since then it has been appreciated in many applications and mainly in electrochromic devices (Jiao et al., 2010; Kharade et al., 2010), photo-catalysis under UV and visible light (Bamwenda and Arakawa, 2001; Kim et al., 2010) and, especially from the 1990s, in gas sensing (Akiyama et al., 1991; Tamaki et al., 1994; Tomchenko et al., 1999; Lee et al., 1999; Solis et al., 2001; Guidi et al., 2004; Kanda and Maekawa, 2005; Reyes et al., 2006; Meng et al., 2009; Szilágyi et al., 2010; Senguttuvana et al., 2010). Regarding this application, WO₃ appears mainly investigated in NO and NO₂ sensing. Also H₂S and ammonia have been considered, but very few papers assess the capacity of WO₃ to detect reducing gases, like VOC or hydrocarbons. According to the majority of the authors, the WO₃ thick-films prepared by our group exhibit a strong attitude to oxidizing gases (Guidi et al., 2004; Blo et al., 2004).

Like SnO₂, WO₃ behaves as a n-type semiconductor because of latttice defects, in particular, oxygen vacancies acting as electronic donor levels. However, it shows a band gap ranging from 2.5 to 2.8 eV (Erbs et al., 1984) and contemporaneously a conductance value, at low temperature, almost three orders of magnitude higher than SnO₂ (Fig. 11.16(a)). Both materials exhibit three regions of conductance and therefore the same trend versus temperature of the surface energy barrier, but with a very low value in the case of WO₃ (Fig. 11.16(b)). This fact could explain the ability of WO₃ to detect only oxidizing gases.

Also the preparation of mixed oxides of tungsten and tin tends to exhibit the same behaviour in relation to oxidizing gases. Indeed, as shown in Chiorino et al. (2001), adding 1 or 5% of W in SnO₂ progressively modifies the sensing properties of the sensors from the detection of reducing to the detection of oxidizing gases. Furthermore, the solid solution of (Sn,W)O₃ (Sn: 10 at.%) did not increase the ability of the mixed oxide with respect to pure WO₃ to detect oxidizing gases (Fig. 11.17).

Under thermal treatment, WO₃ suffers from exaggerated grain coalescence assisted by phase transition as highlighted in Guidi et al. (2004) and shown in Fig. 11.18, in which a pure WO₃ film (a) is compared with a film of (Sn,W)O₃ (Sn: 10 at.%) (b), both fired at 750 °C. Moreover, it has been proved that NO₂ response depends on particle size (Guidi et al., 2004; Meng et al., 2009), but the phenomenon becomes significant only if the grain size is very small (< 30 nm) as reported in Tamaki et al. (1994). Unfortunately, to obtain a good electrical stability of the sensors, it is essential to operate them at a sufficiently high temperature. For this reason, specifically for WO₃, particles smaller than 30 nm are not consistent with the manufacturing process. On the other hand, responses to other oxidizing gases, like ozone, are quite independent of particle size, therefore, for WO₃, it does not appear clear if the gas response might even depend on gas molecule size and gas diffusion effect.

11.4.4 ZnO-based materials

ZnO has been a pioneering material in gas sensing (Seiyama et al., 1962). Later it was abandoned, but in the past two decades it has again met the interest of researchers as witnessed by an exponential growth of publications. Zinc oxide is a direct wide band-gap (3.37 eV) semiconductor and exhibits high exciton binding energy (60 meV), which causes an efficient UV emission (~ 380 nm), making it suitable for UV- and blue-optoelectronic applications. Furthermore, a wide range of other technological applications such as photocatalysis, piezoelectricity, optoelectronics, photovoltaic conversion and gas sensing make ZnO a very attractive material (Özgür et al., 2005; Klingshirn, 2007; Ruske et al., 2007; Dodd et al., 2009; Martins et al., 2004). With the advent of modern nanotechnologies, a lot of metal oxide semiconductors have been synthesized in a large variety of nanostructures such as nanoparticles, nanowires, nanobelts, nanorods or nanotetrapods. Among the various oxides, ZnO is particularly suitable to be synthesized in such nano-forms (Schmidt-Mende et al., 2007; Wang, 2004). In this context, recent works in gas sensing claim the superior performance of two-dimensional nano-morphologies with respect to traditional round-shaped three-dimensional nanoparticles (Schmidt-Mende et al., 2007; Wang, 2004; Lu et al., 2006). In Carotta et al. (2009c), a comparison between round-shaped and two-dimensional nanoparticles in the form of tetrapods (TP) has been performed. It turned out that, despite the enormous difference in morphology, both types of sensors behaved similarly with all tested gases, mainly exhibiting very good responses towards oxidizing gases, in particular with ozone. TP films best responded to ozone with faster response time, while sol-gel thick-films best responded to NO₂ with a different temperature of maximum response compared to ozone.

ZnO has been successfully employed also to detect other gases, such as H₂, O₂, H₂S, LPG, HCHO and NH₃ (Patil et al., 2010; Baruwati et al., 2006; Trivikrama Rao and Tarakarama Rao, 1999; Sarala et al., 2006; Xiangfeng et al., 2005). In addition to thermally activated gas sensors, ZnO is particularly suitable to be assisted by UV irradiation. In this operational mode, the sensors work at room temperature overcoming the main technical limitation of the high operating temperature, essential in chemo-resistive gas sensors. Actually, the photoactivation has been investigated for other metal oxide semiconductors, such as, for example, SnO₂ or TiO₂ (Prades et al., 2009; Yan et al., 2010; Yang et al., 2003). However, ZnO is the most investigated material with UV light instead of thermal energy as the excitation source (de Lacy Costello et al., 2008; Gui et al., 2008). Our group successfully applied the gas detection assisted by an UV light-emitting diode (LED) to ZnO sensors for ozone detection (Carotta et al., 2011); for this application (see also the next section), the most appropriate morphology has been obtained by firing the sensing film at 650 °C (see Fig. 11.19).

11.5.1 Environmental monitoring

The worldwide emission of pollutant gases and fine particulate matter caused by vehicular traffic, industrial activities, domestic heating and biomass burning is steadily increasing. In particular, emerging industrial activity in developing countries will further add to the global pollution during the next decades. Increasing concerns for the proven impact of atmospheric pollution on the health of citizens strengthens the necessity of emission control and mitigation, which have to be verified through steady public and private air quality monitoring activities.

Recent developments in the European legislation have enabled proposals for the use of more flexible devices, such as reliable solid-state gas sensors, in addition to cumbersome analytical devices (Commission Directive 2008/50/EC). Indeed, real-time in-situ air quality data acquisition is necessary for assessing pollution, but also as an input for pollutant diffusion models and for the validation and correlation with remote sensing measurements. Nowadays, monitoring is usually performed in few fixed stations in urban areas, and with mobile stations (where the conventional equipment is installed on vehicles) for temporary measurement campaigns or occasional environmental impact estimations. The equipment used in these stations is often complex, expensive and difficult to deploy, requiring controlled environmental conditions and frequent maintenance and calibration. These analyzers are mainly rack-mounted devices with several requirements like external gas cylinders, consumables and periodic fetching and processing of the acquired data by trained personnel.

In this frame, the technology of thick-film gas sensors is an optimal candidate to be implemented in portable devices with low requirements in terms of power, consumables, maintenance and installation costs. Besides an evident cost reduction, this implementation will enable on-board correlation and processing of the acquired data, which can be autonomously distributed in a user-friendly format via the integrated wireless network interfaces and/or radio-modems (Qi and Shimamoto, 2012).

The advanced thick-film gas sensors are prepared as described in section 11.3 with optimal levels of reliability, stability, reproducibility and response speed for the different target gases, such as CO, NOx and O₃. These sensors are used to monitor the air quality with sensitivity, selectivity and reliability comparable to that of conventional analytical devices. In Fig. 11.20 both the sensor components of the system and the assembled monitoring unit are reported.

Figure 11.21 shows a comparison between dynamic measurements of (a) CO, (b) NOx and (c) O₃ performed through the described system with those of conventional spectrophotometers, while in Fig. 11.22 the correlation between the results of the two instrumentation is reported for the case of ozone.

The sensors shown in Fig. 11.20 can also be employed inside a gas chromatograph or in an array controlled by suitable electronics; therefore, they fit perfectly a scenario in which integrating measurements of outdoor pollutants are performed, from both fixed and mobile web-enabled stations. In this context, an integrated system realized in collaboration with CNR-IMM is sketched in Fig. 11.23.

The integrated innovative system performs detection and quantification of atmospheric pollutants, in particular carbon monoxide, nitrogen oxides, ozone and aromatic volatiles. Details about monitoring gaseous pollutants in the atmosphere using semiconductor sensors as detectors can be found, e.g. in Viricelle et al., 2006; Zampolli et al., 2009; Carotta et al., 2006. The employment of this system on city buses together with geostatistic modelling results in the reconstruction of spatial distribution of pollutants in the area of interest. The system was tested in the year 2010 in the frame of GEOBUS, an Emilia Romagna PRRIITT Programme, a result of which was a picture of the distribution map of the carbon monoxide in a certain area of Bologna.

11.5.2 Photoactivation and photocatalysis

The wide-ranging features of MOX sensors also promoted research into photonic applications, such as light-emitting or laser diodes, transparent conducting oxides (TCO), hybrid solar cells, and material photo-activation and photocatalysis. Moreover, the recent emphasis given to the research on nanostructures and the advent of modern nanotechnologies has opened the way to further improvement in this direction.

In particular, in the case of nanostructured metal oxides used as gas sensors, many studies on exposure to light have been performed (Rao and Rao, 1999; Izu et al., 2004). The basic idea arises from the fact that some metal oxides, known for their good sensing capabilities, also show photocatalytic properties, the photo-generated carriers promoting chemical reactions with the adsorbed species on the semiconductor surface.

The mechanism mainly involved in the described application, is the following: given that the absorption of a photon with enough energy promotes an electron in the conduction band and it generates a hole in the valence band, two specific mechanisms can contribute to the phenomenon: (1) the photo-generated holes recombine with electrons trapped on the surface causing desorption of the adsorbed oxygen ions. This results in a conductivity increase, due to the increased free electron density; (2) since the sensing layer is nanocrystalline, it is reasonable that many of the electron–hole pairs are generated within the depletion region causing a decrease in the energy barrier height.

An experimental result in this frame consisted in the observation that, under UV irradiation, the conductivity of ZnO increased by two orders of magnitude in ZnO. An important application, as reported in Chapter 4, is a ZnO ozone sensor exhibiting excellent performance in sensitivity, in response and recovery times also at room temperature (see Fig. 11.24). This result finds its application both in particularly harsh environments, e.g. in factories and plants where explosives may be present, and in situations where a very low power supply is required.

In the same context, a novel employment of nanostructured metal oxides has been tested by our group consisting in monitoring the photoactivity of functionalized tiles for atmospheric pollution abatement, as can be observed in Fig. 11.25, in which a test of abatement of 2 ppm of NO is monitored through two sensors, one based on ZnO and the second one based on (Sn,W)O₃ (Sn: 10 at.%). The opposite response of the two sensors is caused by the contemporaneous presence of NO and NO₂ in the atmosphere induced by operating functionalized tiles.

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