MoO3–WO3 mixed oxide powder and thin films for gas sensing devices: A spectroscopic characterisation

Sensors and Actuators B 111–112 (2005) 28–35

MoO3–WO3 mixed oxide powder and thin films for gas sensing devices: A spectroscopic characterisation Sara Morandi a , Giovanna Ghiotti a,∗ , Anna Chiorino a , Barbara Bonelli b , Elisabetta Comini c , Giorgio Sberveglieri c a

Dipartimento di Chimica IFM, Universit`a di Torino, and INSTM Consortium, UdR Torino, Via Pietro Giuria 7, 10125 Torino, Italy b Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, C.so Duca degli Abruzzi 24, 10129 Torino, Italy c Dipartimento di Chimica e Fisica dei Materiali and INFM, Universit` a di Brescia, Via Valotti 9, 25133 Brescia, Italy Available online 15 August 2005

Abstract This work gives results about the characterisation of MoO3 –WO3 binary systems in powder or deposited in thin films on alumina and silicon substrates. Morphological and structural characterisation was carried out by XRD and SEM techniques, the powder texture by volumetric measurements. Absorbance FT-IR and diffuse reflectance UV–vis–NIR spectroscopies were employed to study the influence of different atmospheres on the electronic properties of the materials. Spectra were recorded after treatments in O2 , in vacuum, in NO2 /O2 and in CO/O2 at increasing temperature. The spectra of all the samples show, both in the vis–NIR and medium IR regions, the increase of a variety of broad absorptions after reducing atmospheres and the decrease of these absorptions after oxidising ones, so that they are all assignable to electronic transitions related to oxygen defects. Some electrical measurements on thin film supported on alumina are also presented. The electrical response toward CO at increasing temperature can be correlated with the intensity changes in the vis–NIR absorptions observed after interaction with CO/O2 at increasing temperature. © 2005 Elsevier B.V. All rights reserved. Keywords: MoO3 –WO3 ; Thin film; FT-IR; UV–vis–NIR

1. Introduction The purpose of the research on gas sensors is to obtain new materials to achieve highly sensitive and selective devices. The monitoring of CO and NO2 in an urban atmosphere is one of the applications, which require development of sensitive and selective solid-state gas sensors. MoO3 and WO3 are well known metal oxides with similar physical and chemical properties. They show n-type semi-conducting properties related to the presence of lattice defects, mainly oxygen defects, and they are extensively studied for their potential applicability in gas sensing devices. In particular, WO3 demonstrated to have good NO2 sensing capability [1,2] and MoO3 good sensitivity to CO and NO2 [3,4]. However, MoO3 exhibits a low ∗

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0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.06.037

evaporating temperature, permitting only low operating temperatures at which it shows very low conductivity changes. Metal oxide combinations can be tailored to achieve desired properties in order to modify sensor performances. Recent studies focused on MoO3 –WO3 mixed oxides report on their promising gas sensing potential [5,6]. In this work, we prepared MoO3 –WO3 mixed oxide in powder and in thin films deposited on alumina and silicon substrates and we made their characterisation with a variety of techniques: scanning electron microscopy (SEM), X-ray diffraction (XRD), absorbance/transmittance FT-IR spectroscopy, diffuse reflectance UV–vis–NIR spectroscopy and electrical measurements. In particular, the two spectroscopies were used to study the changes in the type and amount of donor levels induced by the presence of different atmospheres, comparing the results with electrical measurements on the thin film deposited on alumina.

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2. Experimental The powder of the mixed molybdenum tungsten oxide was prepared with a nominal Mo/W molar ratio equal to 80/20: (NH4 )6 Mo7 O24 ·4H2 O and (NH4 )10 W12 O41 ·7H2 O were dissolved in water at 353–363 K. Then HNO3 3 M was added in order to precipitate the mixed hydrate. The light yellow precipitate was left in the initial solution for 2 days, then washed with distilled water, dried at 333 K for 24 h and calcined for 12 h at increasing temperature up to 723 K in an oxygen flow to obtain the final mixed oxide. MoO3 –WO3 thin films were deposited by radio frequency magnetron sputtering in reactive atmosphere, starting from a target of Mo (80%, w/w) and W (20%, w/w), with the substrates held at 573 K. Films were sputtered on alumina and mono-crystalline silicon and they were obtained with a Mo/W molar ratio near to 88/12, similar to that of the powder sample. Surface area of the powder pretreated at 573 K in vacuum was determined by N2 adsorption at 77 K with the BET method using a Micromeritics ASAP 2010. XRD patterns of the powder and thin films were collected on an X’Pert Philips instrument using Cu K␣1 radiation (λ = 0.15418 nm, 40 kW, 25 mA) and the morphology of the powder and thin films was examined by a scanning electron microscope (Philips 515 SEM), operating at 30 kV. Diffuse reflectance spectra in the UV–vis–NIR region were run at room temperature on a Varian Cary 5 spectrophotometer, working in the range of wavenumbers 53,000–4000 cm−1 . Absorption/transmission FT-IR spectra were run at room temperature on a Perkin-Elmer System 2000 FT-IR spectrophotometer equipped with a Hg–Cd–Te cryodetector, working in the range of wavenumbers 7800–580 cm−1 . For UV–vis–NIR analysis, powder (about 1 g) and thin films were placed in home-made quartz cells, allowing thermal treatments in vacuum or in controlled atmospheres up to 1073 K, but the spectra registration only at room temperature (RT). For FT-IR analysis, the powder was compressed in self-supporting disk; powder disks or thin films were placed in a commercial heatable stainless steel IR cell (Aabspec) allowing in situ thermal treatments in vacuum or in controlled atmospheres up to 873 K and the spectra registration at the same temperatures. However, for sake of comparison with UV–vis–NIR experiments, the major part of FT-IR spectra were run at RT. For both UV–vis–NIR and medium FT-IR (MIR and NIR regions) analysis, the samples were first activated at 723 K in vacuum (20 min) and in dry oxygen (20 min, pO2 ≈ 60 mbar), then they were cooled down to RT and the background spectrum were run, then they underwent the following treatments: (i) evacuations at room temperature, 473, 573 and 673 K; (ii) treatments at RT, 473, 573 and 673 K, alternatively, in O2 and in a NO2 /O2 (10/1) mixture and (iii) treatments at RT, 473, 573 and 673 K in a CO/O2 (10/1) mixture. After each treatment at each temperature a spectrum was run at RT. It is important to point out that the UV–vis–NIR spectra reported in the figures are those

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of the sample after each treatment without any manipulation. Conversely, the FT-IR spectra are reported as difference spectra, where the subtrahend spectrum is always the background of the sample (outgassed and oxidised at 723 K). Electrical measurements were performed in the temperature range 423–623 K on the thin film deposited on alumina. Conductance measurements were carried out by voltamperometric technique on the as-deposited layer on alumina and on the layer annealed for 12 h at increasing temperature up to 873 K. All the measurements were executed in a sealed chamber at 293 K under controlled humidity with a constant flow rate (0.3 l/min) of synthetic air into which the desired concentration of pollutants was mixed.

3. Results and discussion 3.1. Structure and morphology The mixed oxide powder shows a BET surface area of 5 m2 /g and it does not show micro or meso-porosity. The XRD pattern of the powder does not show peaks due to pure MoO3 or pure WO3 (Fig. 1), thus suggesting the presence of a new mixed phase. Indeed, the pattern is similar to that of ␣-Mo0.5 W0.5 O3 found in PCPDFWIN database. This suggests that we obtained a solid solution (with nominal formula Mo0.8 W0.2 O3 ) with a similar structure. XRD analysis of the thin film on alumina (not reported) indicates again the formation of a mixed molybdenum-tungsten oxide. The diffraction peaks become narrower after annealing at 873 K. The peaks disappear after annealing of the film at 1073 K indicating sublimation of the oxide. In Fig. 2, SEM micrographs of the powder and thin films on alumina are reported. The SEM image of the powder (Fig. 2A) shows a sample constituted by irregular aggregates of particles with spherical shape. These aggregates appear not very homogeneous in size, their diameter ranging between 100 and 600 nm. The as-deposited thin film on alumina (Fig. 2B) appears as a compact and homogeneous base of spherical

Fig. 1. Comparison between XRD patterns of the mixed oxide powder and commercial MoO3 and WO3 oxides (Merck).

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Fig. 3. Diffuse reflectance UV–vis–NIR spectra of the mixed oxide powder (A) and thin film on alumina (B) after interaction with O2 at 723 K (1) and subsequent treatment in vacuum at: RT (2), 473 K (3), 573 K (4), 673 K (5).

Fig. 2. SEM images of the mixed oxide powder (A) and thin film on alumina (B).

particles on which irregular aggregates grow. For the asdeposited thin film on silicon SEM micrograph has been already reported by Vomiero et al. [7] and it shows a surface characterised by a continuous and compact film, which features the presence of elongated and flake-like particles. The particular shape of the elongated grains is typical of Mo oxides for which one cell parameter is so much longer than the others, resulting in anisotropic growth. 3.2. Spectroscopic characterisation For all the samples spectra run after reducing treatments (in vacuum or in CO/O2 mixture) at increasing temperature show the increase of a variety of broad absorptions both in the vis–NIR and MIR regions. In Fig. 3, UV–vis–NIR diffuse reflectance spectra of the powder and thin film on alumina treated in vacuum up to

673 K are reported. For the thin film on silicon spectra in this region are not significant because of the presence of the fundamental absorption of silicon that starts around 9000 cm−1 . For the powder sample (Fig. 3A) we can observe broad absorptions at energy below the VB–CB transition: a maximum around 9000 cm−1 , a shoulder at 16,700 cm−1 and another component (more evident at the high evacuation temperatures) around 5000 cm−1 are present and they increase in intensity on increasing outgassing temperature. For the thin film on alumina (Fig. 3B) after treatments until 473 K it is possible to observe only one broad absorption below the VB–CB transition centred at about 12,000 cm−1 , starting from 573 K on this very broad absorption two other absorptions around 11,000 and 17,000 cm−1 overlap. A component around 7000 cm−1 becomes predominant at 673 K. It is worthy to note that the integrated absorption intensities reached at 673 K for the powder are 15 times less intense then those grown at 673 K for the thin film. Probably, this effect is due to the different efficiency of the treatment on the powder and on the thin film. The analysis on the powder involves much more material with respect to the thin film and so it is more difficult to reach the equilibrium state during the treatment of the powder. A similar behaviour was obtained after interaction with the CO/O2 10/1 mixture at increasing temperature (Fig. 4). For the powder (Fig. 4A) only the absorptions around 9000 and 16,700 cm−1 appear a 20 times less intense then those

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Fig. 4. Diffuese reflectance UV–vis–NIR spectra of the mixed oxide powder (A) and thin film on alumina (B) after treatment in O2 at 723 K (1) and subsequent interaction with CO/O2 10/1 mixture at: RT (2), 473 K (3), 573 K (4), 673 K (5).

grown by treatment in vacuum at the same temperatures. So, the mixture shows a lower reducing effect than the treatments in vacuum, as expected for the presence of oxygen. Like for the treatment in vacuum, the absorptions increase in intensity on increasing outgassing temperature. The lacking of the component at 5000 cm−1 is expected, since in the evacuation experiment it appears only when the other components reach a large intensity. For the thin film on alumina (Fig. 4B), we can observe only one broad absorption at 11,000 cm−1 , about 60 times less intense than the absorptions grown in this range by treatment in vacuum and, notably, the maximum intensity is reached at 473 K. This result is very interesting being correlated with electrical measurements (see next section). In Fig. 5, the FT-IR absorption spectra (in the NIR and MIR region) of the powder and thin films treated in vacuum up to 673 K are reported. For the powder (Fig. 5A) we observe after evacuation at 473 K the formation of a very broad and weak absorption, at 573 K a predominant absorption, with maximum approximately centered between 3000 and 3500 cm−1 , overlapped with the negative band of surface hydroxyls, and at 673 K the complete absorption of the IR radiation in the examined range, making impossible to run the spectrum. For the thin films (Fig. 5B and C) we can observe a continuous absorption starting from 1000 cm−1 , growing in intensity on increasing the temperature of treatment. In the case of the mixed oxide deposited on alumina (Fig. 5B) the strong scat-

Fig. 5. Changes induced in the absorption FT-IR spectra of the mixed oxide powder (A), thin film on alumina (B) and thin film on silicon (C) after treatment in vacuum at: RT (1), 473 K (2), 573 K (3), 673 K (4). For the powder the spectrum after evacuation at 673 K is not present because the sample completely absorbs the IR radiation.

tering radiation by the substrate gives spectra not reliable at wavenumbers higher than 4500 cm−1 . However, after treatment at 673 K it is possible to distinguish a hump around 3300 cm−1 . For the sample deposited on silicon (Fig. 5C) the absence of scattering by the substrate gives reliable spectra in all the range examined, the absorption shows two hump, one around 2000 cm−1 and another one around 4500 cm−1 , suggesting the presence of two overlapped bands. For all the samples interaction with CO/O2 mixture at increasing temperature does not create appreciable absorption in the MIR region (spectra not reported). For both powder and thin films the broad absorptions observed in the examined spectral ranges after treatments in vacuum at 673 K decrease in intensity after contact with oxi-

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dising atmospheres (O2 or NO2 /O2 , 10/1 mixture) at increasing temperature, so that they are all assignable to electronic transitions due to the presence of a variety of donor levels related to oxygen loss. For both powder and thin films, at the same temperature the mixture NO2 /O2 has a higher oxidising effect than pure O2 , eroding the absorptions in a more efficient way. This behaviour is illustrated in Fig. 6 for the thin film supported on alumina, taken as an example. We note that under NO/O2 atmosphere no vibration modes assignable to NOx surface species could be observed nor for thin films, as expected for the small amount of materials present, neither for the powder. The loss of oxygen during reducing processes produces defect sites with trapped electrons, responsible for the electronic transitions observed. The lost oxygen can be either adsorbed on the surface like O2− , O− , O2 − or to be lattice oxygen at the surface or in the bulk. During reducing processes the adsorbed oxygen species are lost leaving the electrons that fill empty defect levels already present in the energy gap. The loss of oxygen in the structure creates new defect levels with trapped electrons. The electrons can be trapped in the oxygen vacancies or at lattice cationic sites. In the case of transition metal oxides with bandwidth of

Fig. 6. Diffuse reflectance UV–vis–NIR spectra (A) and absorption FT-IR spectra (B) of the mixed oxide thin film on alumina after treatment in vacuum at 723 K (1) and interaction with: O2 at RT (2), NO2 /O2 at RT (3), O2 at 473 K (4), NO2 /O2 at 473 K (5), O2 at 573 K (6), NO2 /O2 at 573 K (7), O2 at 673 K (8), NO2 /O2 at 673 K (9).

around 3 eV, as in our case, the electrons trapped at lattice cationic sites, with electron trapping being accompained by strong local lattice relaxations, are called small polarons [8]. A small polaron should show an absorption in the vis–NIR region due to a Frank–Condon exitation analogous to the intervalence transitions found in mixed-valence compounds, in other words in a reduced transition metal oxide MO3−x one can imagine an electron undergoing a transition from a M5+ cation to a neighbouring M6+ . Thus, absorptions observed in this region both for ␣-MoO3 [9–11] and for ␧-WO3 [8 and references therein] have been assigned to such polaronic transitions. However, bands in this region can be also due to: (i) M5+ d–d transitions [10] and (ii) M5+ –O2− ligand to metal charge transfers [9,10]. The existence of various oxidation states of the W atom (various possible hybridiation of W5d electrons with the O2p ones) has been also put in evidence by photoelectron spectroscopy in WO3 thin films annealed in UHV up to 573 K [2,12] and on the (0 0 1) surface of WO3 single-crystal that had been subjected to mild ion bombardement [13]. Owing to these various types of possible transitions and to the simultaneous presence of two different metallic species (Mo and W), the precise assignment of the bands observed in the range 5000–20,000 cm−1 for the mixed oxide is not easy. Concerning the MIR region, similar electronic absorptions have never been reported in literature, at our knowledge, neither for this type of mixed oxide, nor for the two pure oxides. However, for oxides like SnO2 and ZnO there are many works reporting about broad electronic absorptions in the MIR region [14–17] assigned to electronic transitions occurring from a shallow level associated to a defect site constituted by an electron trapped in an oxygen vacancy (VO + ). The assignment is supported by the fact that oxygen vacancies are the predominant point defects in these oxides and that the energies of ionisation of the bulk VO + in the single-crystals are known: 0.145 eV (1170 cm−1 ) and 0.18 eV (1450 cm−1 ) for SnO2 and ZnO, respectively [18–20]. Thus, for these oxides absorptions in the MIR due to the following photo-ionisation VO + → VO 2+ + e− (CB) are expected. Also for TiO2 , for which the predominant point defects in the bulk seems to be interstitial Ti3+ , the presence of bulk VO + has been reported with ionisation energy between 0.1 and 0.5 eV [21]. However, recent papers report that the surface oxygen vacancies are the most abundant surface donors that trap electrons about 0.75 eV below the CB [22 and references therein]. On this basis, we propose to assign the absorptions in the MIR region for Mo0.8 W0.2 O3 to transitions involving electrons trapped in the oxygen vacancies. The photo-ionisation process of an isolated VO + should originate an absorption very broad and of asymmetric shape with a steep edge at the v¯ corresponding to the ionisation energy, then a slower increase of intensity to a maximum followed by a slow decrease in intensity increasing the wavenumbers. Indeed, although the density of the final states (in the conduc-

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tion band) increases with energy, the transition probability decreases with energy so that the absorption coefficient, eventually, decreases with increasing wavenumbers [23]. If the defects are not isolated they interact giving an impurity band and we expect broadened absorption edge, in a range of wavenumbers more or less extended, depending from the concentration of defects and the strength of the interaction. For the mixed oxide powder, only a broad absorption is observed, showing an edge rather steep, extending between 1000 cm−1 (0.12 eV) and 2500 cm−1 (0.31 eV). In the case of thin films supported on silicon two broad absorption partially overlapped are present with rather steep absorption edges one in the MIR region (1000–2000 cm−1 , 0.12–0.24 eV), and the other at the boundary of the MIR and NIR region (3500–4500 cm−1 , 0.42–0.54 eV). Furthermore, the absorption increase at increasing wavenumbers in the NIR region is due to the simultaneous increase of the broad bands assigned to polaronic or d–d transitions, so that the band tails superimpose. In the case of the thin film supported on alumina, as previously underlined, the shape of the spectra is not reliable, and we can coarsely speak about a very broadened absorption edge between 1000 cm−1 (0.12 eV) and 3000 cm−1 (0.36 eV). Electrons trapped both as small polarons or in oxygen vacancies can be responsible for the semi-conducting properties of the materials examined. Indeed, small polarons can move from site to site by a thermally activated hopping process, while the electrons in oxygen vacancies can move by a thermally activated transition to the CB. In order to evaluate the response stability of the materials we maintained the powder and thin films at 723 K in oxidising atmosphere for increasing times (up to 2 h). After these treatments the powder lost progressively their capability to be defected during a reducing treatment, since after subsequent evacuation up to 673 K the absorptions related to the oxygen defects were progressively decreased in intensity. At variance, similar treatments showed a high stability of thin films, the same absorptions related to the oxygen defects with the same intensities were observed.

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3.3. Electrical measurements

of interconnection of the agglomerates (annealed at 773 K: 10−9 –10−8 S; annealed at 873 K: 10−10 –10−11 S). A mixture of carbon monoxide or nitrogen dioxide and humid air (40% relative humidity) was chosen as a test atmosphere to probe the sensing capability of the films. By defining Gf as the level of conductance when the pollutant is present and G0 as the level in air, the response R of the sensors toward CO is defined as the normalised conductance variation for an n-type semi-conductor, namely [Gf − G0 ]/G0 , while the response to NO2 is defined as the relative change of resistance. Response and recovery times are calculated as the time taken to reach 90% of [Gf − G0 ] when the gas is fed and to recover to 30% of [Gf − G0 ] when the flux of pure air is restored. The conductance of the layers rise up when CO is fed into the test chamber. This behaviour is normal for an ntype semi-conductor due to exchange of electrons between the ionosorbed species and the semi-conductor itself. Carbon monoxide reacts with oxygen species adsorbed on the semiconductor (O2− , O− , O2 − ) with a consequent increase in the conductance; as an example, for O− a possible reaction is: CO + O− → CO2 − → CO2 + e− . Fig. 7 reports the response of the mixed oxide thin film on alumina towards 10 ppm of CO as a function of temperature. The temperature of the best performance is 473 K. This result is very interesting because the temperature of the best performances corresponds to that at which the treatment with CO/O2 mixture causes the most intense absorptions in the UV–vis–NIR spectra (Fig. 4B). The response time is between 1 and 5 min at this temperature and increases with the concentration of the pollutant, while the recovery time is unaffected by CO increase; these values are limited by the dimensions of the test chamber, true response times should be even lower. Regarding NO2 interaction with metal oxides, there are proofs of reactions directly with the semi-conductor surface other than with the oxygen chemisorbed at surface. NO2 behaves like an oxidising gas and the mechanism of reaction was proposed and discussed by Tamaki et al. [24], Solymosi and Kiss [25] and Ghiotti et al. [26,27]. However, as previously noted, no surface NOx species could be detected by

Pure MoO3 is a good sensing element but its high resistance and its sublimation at low temperature prevents its use in gas sensing systems. The introduction of W in Mo oxide decreased sensor resistance facilitating the design of an electronic interface, improving S/N ratio and increasing the maximum operating temperature compatible with the film stability. Electrical measurements were performed in the temperature range 423–623 K on the thin film deposited on alumina. The conductance of the as-deposited layers is found to be about 10−8 S when the film operates in air with 40% RH and temperatures between 373 and 623 K. This is a reasonable value for gas sensing applications. As the annealing increases, the resistance of the layers increases too due to lack

Fig. 7. Response of the mixed oxide thin film on alumina towards 10 ppm of CO as a function of temperature.

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FT-IR measurements, so it is not possible to suggest the specific mechanism leading to the change in the resistance of our materials in interaction with NO2 /O2 . The electrical response to NO2 has a maximum value for 473 K (not reported), as is usual for metal oxides for this kind of gas.

4. Conclusions MoO3 –WO3 mixed oxide both in powder and thin films has shown the capability to lose oxygen leading to the formation (or repopulation) of donor sites when it undergoes reducing treatments at increasing temperature like outgassing and interaction with CO/O2 mixture. These sites can be destroyed (or empted) with an oxidising treatment like interaction with O2 or NO2 /O2 mixture at increasing temperature. UV–vis and IR spectroscopic measurements have shown that they are electrons trapped at lattice cations and at oxygen vacancies. This is consistent with the n-type semi-conducting behaviour shown by the electrical measurements here performed or reported in literature for similar materials. The changes in the amount of these defects or in their population can be related to changes in the material conductivity. In particular, we have found a good agreement between UV–vis–NIR and electrical measurements performed on the mixed oxide deposited on alumina substrate, at 473 K we observe the most intense absorptions after interaction with CO/O2 mixture and at the same temperature the highest electrical response toward CO. Concerning the stability of the material properties, spectroscopic measurements have shown a decrease in the capability of the powder to be defected after repeated oxidising treatments. At variance thin films showed a very high stability and this is a good result considering the use of thin films in gas sensor devices.

Acknowledgements This work has been partially supported by Italian MURST PRIN: “Sviluppo di materiali nanostrutturati per sensori di gas selettivi ad altissima semsibilit`a per il monitoraggio di inquinanti atmosferici” and by the European Union Projet ADVANTAGAS IST-2001-33148.

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Biographies Sara Morandi was born on April 13th, 1978 and she received her degree in chemistry at the University of Torino (Italy) in 2002. She is presently working as PhD student at the Institute of Physical Chemistry of the University of Torino. Her scientific activity concerns the spectroscopic characterisation of materials employed in chemical sensors, studying their electronic properties, and in catalysis, studying their surface site properties by means of FT-IR and UV–vis–NIR spectroscopies. At the moment, she is devoted to the characterisation of MoO3 –WO3 systems and Sn–Cr oxides in powders or deposited in thin films for sensing applications. Giovanna Ghiotti was born on September 4th, 1942 and she received her degree in chemistry from the University of Torino (Italy) in 1966. In 1967, she joined the Institute of Physical Chemistry of the University of Torino and she began her scientific activity studying the interaction of gases with dispersed solids of catalytic interest, mainly by IR and UV–vis spectroscopies. Since 1986, her research activity has also concerned the study of ZnO and SnO2-based materials for gas sensors. She has been appointed associate professor at the University of Torino in 1974. Since 2000 she has been full professor of physical chemistry at the University of Torino. Anna Chiorino was born on September 13th, 1951 and she received her degree in chemistry from the University of Torino (Italy) in 1975. In 1976, she joined the Institute of Physical Chemistry of the Univer-

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sity of Torino, where she carried out research activity in the field of spectroscopic characterisation of dispersed catalysts. Since 1985 she has been an associated professor of physical chemistry. Her current researches are devoted to both catalytic and semi-conducting material, the latter as chemical sensors for gases. Barbara Bonelli was born on May 13th, 1972 and she received her degree in industrial chemistry at the University of Torino (Italy) in 1997. In 2000, she received her PhD in chemistry at the University of Torino. In 2001, she moved to Politecnico of Torino, Department of Materials Science and Chemical Engineering, where, since 2005, she is a researcher. Her scientific interests consist mainly in the physico-chemical characterisation of oxides, zeolites, controlled mesoporosity silicas or silicoaluminas and carbon materials for applications in the fields of catalysis; adsorption; gas storage; sensors. Elisabetta Comini was born on November 21st, 1972 and she received her degree in physics at the University of Pisa (Italy) in 1996. She is presently working on chemical sensors with particular reference to deposition of thin films by PVD technique and electrical characterisation of MOS thin films. She received her PhD in material science at the University of Brescia. She is now employed as an assistant professor at the University of Brescia. Giorgio Sberveglieri was born on July 17th, 1947, and received his degree in physics at the University of Parma, where, starting in 1971, his research activities on the preparation of semi-conducting thin film solar cells was conducted. He has been appointed associate professor at the University of Brescia in 1987. In the following year, he established the Thin Film Laboratory afterwards called Gas Sensor Laboratory, which is mainly devoted to the preparation and characterisation of thin film chemical sensors. He has been the Director of the GSL since 1988. In 1994, he was appointed full professor in physics, first at the Faculty of Engineering of University of Ferrara and then in 1996, at the Faculty of Engineering of University of Brescia. He is a referee of the journals Thin Solid Films, Sensors and Actuators, Sensors and Materials, etc., and is a member of the Scientific Committee of Conferences on Sensor and Materials Science.