Journal of Sol-Gel Science and Technology 22, 167–179, 2001c© 2001 Kluwer Academic Publishers. Manufactured in The Netherlands.

Sol-Gel Processed TiO2-Based Nano-Sized Powders for Use in Thick-FilmGas Sensors for Atmospheric Pollutant Monitoring

ENRICO TRAVERSA, MARIA LUISA DI VONA AND SILVIA LICOCCIAConsorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali (INSTM) & Dipartimento

di Scienze e Tecnologie Chimiche, Universita di Roma “Tor Vergata”, Via della Ricerca Scientifica,00133 Roma, Italy

MICHELE SACERDOTIIstituto di Mineralogia, Universita di Ferrara, Corso Ercole I d’Este 4, 44100 Ferrara, Italy

MARIA CRISTINA CAROTTA, LUIGI CREMA AND GIULIANO MARTINELLIINFM—Dipartimento di Fisica, Universita di Ferrara, Via Paradiso 12, 44100 Ferrara, Italy

Received July 26, 2000; Accepted March 20, 2001

Abstract. Sol-gel routes were used to prepare pure and 5 at% and 10 at% Ta- or Nb-dope TiO2 nano-sized powders.The thermal decomposition behaviour of the precursors was studied using simultaneous thermogravimetric anddifferential thermal analysis (TG/DTA). X-ray diffraction (XRD) analysis showed that the powders heated to 400◦Cwere crystalline in the anatase TiO2 structure. The pure TiO2 powder heated to 850◦C showed the rutile structure.The addition of Ta and Nb inhibited the anatase-to-rutile phase transformation up to 950–1050◦C. Ta was solublein the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films werefabricated with these powders by screen printing technology and then fired for 1 h at different temperatures inthe 650–1050◦C range. Scanning electron microscopy (SEM) observations showed that the anatase-to-rutile phasetransformation induces a grain growth of about one order of magnitude for pure TiO2. The addition of Ta and Nbis effective to keep the TiO2 grain size at a nanometric level even at 950◦C, though grain growth was observedwith increasing temperature. The gas-sensitive electrical response of the thick films were tested in laboratory, inenvironments with CO in dry and wet air. Conductance measurements showed a good gas response only for thenanostructured titania-based films. For field tests, the prototype sensors were placed beside a conventional stationfor atmospheric pollutant monitoring. The electrical response of the thick films was compared with the results ofthe analytical instruments. The same trend was observed for both systems, demonstrating the use of gas sensors forthis aim.

Keywords: gas sensors, titania, nano-sized powders, thick films, atmospheric pollutant monitoring

1. Introduction

Atmospheric pollutant monitoring is one of the mostimportant applications that need the development ofreliable and cheap solid-state gas sensors [1]. Environ-mental problems, such as global warming and emis-sion of pollutants, are amongst the main concerns of

our age. Very small concentrations of these gases arealready able to dramatically deteriorate air quality, andthus atmospheric pollutant monitoring methods shouldbe able to evaluate very low gas concentration levels.The methods currently accepted by international envi-ronmental standards are based on spectroscopic analy-sis techniques [2], which require the use of expensive

168 Traversa et al.

equipment that limits the diffusion of air quality con-trol stations. Methods based on solid-state gas sensorswould be dramatically cheaper than analytical equip-ment, and their use would lead to the possibility of awider distribution of environmental monitoring loca-tions than exist at present, resulting in an improvedpicture of air quality [3].

As first discovered in the early 60s, semiconductingoxides are able to sense gases upon changes of theirconductance [4, 5]. The most widely accepted detectionmechanism is related to the presence of surface oxygenadsorbates, which build up a space-charge layer at themetal oxide surface. Reaction of the adsorbates withreducing gases induces an increase in the oxide con-ductance in the case of n-type semiconducting oxides.This sensing mechanism is well described in [6]. There-fore, one of the main factors enhancing the detectionproperties of semiconducting oxides used for gas sens-ing is their grain size reduction [7]. Sharp increases insensitivity are expected when the grain size becomessmaller than the space-charge depth [6, 8].

However, sensors based on SnO2 are commerciallyavailable but used only for indoor applications. Thereis the need to improve their sensitivity, selectivity, andstability to perform outdoor air quality monitoring [9].Given the gas sensing mechanism of semiconductingoxides, the use of nano-sized powders is proposed toimprove the performance of gas sensors in order toallow their use in atmospheric pollutant monitoring.

Controlled, ultrafine and homogeneously-sized ce-ramic powders can be prepared using chemical meth-ods [10]. Both chemical and physical methods havebeen proposed for the production of nano-sized ce-ramic powders [11, 12]. In particular, sol-gel chemistryroutes have been widely applied for the preparation ofnano-sized materials [13] because of their versatility,simplicity, relatively low cost, and ease of homoge-neous doping [14].

TiO2 is an n-type semiconducting oxide, well-known for photoelectrical, photocatalytic, and op-tical applications, which has been used as oxygen[15, 16] and polluting gas sensor (in the anatase phase)[17]. The main advantage in using TiO2 as reduc-ing gas sensor is that its response is much less af-fected by humidity than the response of SnO2, whichmakes TiO2 suitable for environmental monitoring[18, 19].

Sol-gel techniques have been reported to synthesizenano-sized powders of TiO2 using alkoxides as precur-sors [20]; extremely high water/alkoxide ratios during

the hydrolysis of titanium isopropoxide result in theformation of 5-nm anatase crystallites [20]. The useof nano-sized semiconducting oxide powders has beenfound to be very promising for the fabrication of thick-film gas sensors able to detect polluting gases duringenvironmental monitoring field tests [21–24].

This paper reports the sol-gel synthesis of pure andTa- and Nb-doped TiO2 nano-sized powders togetherwith their structural and microstructural characteriza-tion. The sol-gel method was particularly suitable forhomogeneous doping, and to control the phase andgrain size evolution. These powders were used to pre-pare thick-film gas sensors by screen-printing tech-nology. Screen-printing technology is one of the mostpromising technologies to produce low-cost commer-cial semiconductor chemical sensors with good repro-ducibility [25, 26]. Electrical responses to pollutinggases were measured in laboratory tests, in dry and wetenvironments. Electrical responses were also measuredin the field and the results obtained were compared withthe concentration of some major polluting gases (CO,NO, NO2 and O3), measured by the internationally-standard analytical techniques.

2. Experimental Procedure

2.1. Powder Synthesis

Pure titania powders were prepared according to aprocedure described in the literature [20]: titaniumtetraisopropoxide, Ti(OiPr)4 (8 ml, 27 mmole), dis-solved in abs. ethanol (92 ml) under N2, was addeddrop by drop to 250 ml of a solution of ethanol/water1:1 under rapid stirring. The suspension obtained waskept stirring for 10 min., then filtered to obtain a whiteprecipitate, which was dried in air (100–110◦C) for15 hours.

We have slightly modifed the literature procedure toobtain the doped powders.

Powders of TiO2 doped with 5 at% Ta were pre-pared by dissolving Ti(OiPr)4 (8 ml, 27 mmole)and tantalum pentaethoxide, Ta(OEt)5 (0.35 ml,1.35 mmole) in abs. ethanol (92 ml) under N2, andthen following the same procedure above described.0.7 ml, 2.7 mmole of Ta(OEt)5 were used to pre-pare 10 at% Ta-doped TiO2 powders. Powders of TiO2

doped with 5 at% Nb were prepared by dissolv-ing Ti(OiPr)4 (8 ml, 27 mmole) and niobium pen-taethoxide, Nb(OEt)5 (0.34 ml, 1.35 mmole) in abs.

Sol-Gel Processed TiO2-Based Nano-Sized Powders 169

ethanol (92 ml) under N2, and then following the sameprocedure described above. 0.68 ml, 2.7 mmole ofNb(OEt)5 were used to prepare 10 at% Ta-doped TiO2

powders.

2.2. Powder Characterization

Simultaneous thermogravimetry and differential ther-mal analysis (TG/DTA), with a heating rate of10◦C/min in flowing air, was used to study the ther-mal decomposition behaviour of these precursors.

The oxide powders were obtained by firing the pre-cursors at 400◦C for 2 hours. Other samples wereheated to selected temperatures (650, 850, 950, and1050◦C) to check the crystalline structure and result-ing phases using X-ray diffraction (XRD) analysis.The XRD measurements were performed using a ver-tical diffractometer (Philips) with a Bragg-Brentanogeometry, provided with a graphite monochromatoralong the diffracted beam and equipped with an x-raysource operating at 1000 W (Cu Kα radiation withλ = 0.154 nm). The diffraction patterns were collectedin the range 5–140◦ (2θ ) with steps of 0.02◦ and 10 s ofdwell time. The XRD data were elaborated using theRietveld analysis program DBWS-9411 [27].

The microstructure of the powders was observed us-ing scanning electron microscopy (SEM, model Stere-oscan 360, Leica Cambridge) and transmission electronmicroscopy (TEM, model H-800, Hitachi, at an accel-erating voltage of 200 kV). Selected area diffractionpattern (SADP) measurements were also performedduring TEM analysis.

2.3. Thick-Film Sensor Fabrication

Thick-film prototype gas sensors were fabricated byscreen-printing technology [22] using the nano-sizedtitania-based powders prepared with the sol-gel proce-dure described above. The thick films were preparedfrom pastes obtained by adding to each oxide powderan organic vehicle, and a small percentage of a glass fritfor improving the adhesion to the substrates. The pasteswere printed on alumina substrates pre-cut in squaresof 2 by 2 mm2 area, each one provided with a heaterelement, a Pt-100 resistor for the control of the sensoroperating temperature, and comb-type gold electrodesfor electrical measurements. The films were fired at se-lected temperatures (650, 850, 950, and 1050◦C) for

1 h. The thickness of the deposited layers was in the15–30 µm range. The morphology of the films wasanalysed by SEM.

2.4. Electrical Measurements: Laboratoryand Field Tests

The conductance measurements were performed in asealed test chamber (400 cm3) with different gas in dryor wet (at 40% of relative humidity, RH) air, at a flowrate of 0.5 l/min. The measurements were performedusing 100 ppm of CO, varying the operating tempera-ture in the 350–450◦C range.

To evaluate the activation energy for the testedsamples, the electrical d.c. conductivity was mea-sured in dry and wet air by varying the temperature.Temperature-stimulated conductance measurements,which consist in measuring conductance as a functionof time after a fast temperature variation, were also per-formed to determine the barrier energy as a function ofthe temperature, with a procedure described elsewhere[28].

The field tests were performed by measuring thevariation of conductance of 10 at% Ta- and Nb-dopedTiO2 850◦C-heated thick-film prototype sensors ex-posed to real atmosphere. An array containing alsosensors based on other semiconducting oxides [22] wasplaced in a small box that contains also the electroniccircuitry related to each sensor, an electronic-controlunit provided with serial connection, and a portablecomputer to record the sensor electrical signals. Detailsare reported elsewhere [24]. Signals were obtained bymaintaining the sensors biased at a constant voltage of5 V and measuring voltage drops on calibrated resis-tors. The relative humidity (RH) and the temperature(T ) in the chamber allocating the sensors were simul-taneously monitored [21].

The sensor array was positioned near a conventionalatmospheric pollutant monitoring station located in thecity centre of Ferrara. The analytical monitoring stationis managed by the Regional Agency for EnvironmentPrevention (ARPA)-section of Ferrara. The electricalresponse was measured at 400◦C for the TiO2-basedsensors. The sensors’ response was compared with theconcentrations of the major pollutants (CO, NO, NO2,and O3) in the atmosphere, as measured with the ana-lytical equipment. According to the Italian regulations,these data are given as average concentrations in ahour.

170 Traversa et al.

3. Results and Discussion

3.1. Powder Analysis

The TG curves for all the precursors showed that theweight loss ended at about 400◦C. The TG/DTA curvesfor the pure and 5 at% Ta- and Nb-doped TiO2 werevery similar. As an example, Fig. 1 shows the TG-DTAcurves for the precursor of 5 at% Ta-doped TiO2. Theweight loss measured from the TG curves was verylimited, between 12.5 and 13.5 wt% for all the threesamples. The DTA curves showed two main exother-mic effects. The first peak, with its maximum at about260◦C, was weak and broad, and it was attributed to thedecomposition of the organic groups. The second peakat 400◦C was very sharp and narrow, and was attributedto a crystallization process [20].

The TG/DTA curves for the 10 at% Ta and Nb dopedTiO2 were somewhat different from the curves of theother samples. Figure 2 shows the TG/DTA curves forthe precursor of 10 at% Ta-doped TiO2. The weight lossmeasured from the TG curves was slightly larger, being14.5 wt% for the Ta-doped sample and 17.3 wt% for theNb-doped sample. The DTA curves show an endother-mic peak at about 120◦C, due to the loss of physisorbedwater, which can explain the slightly larger weight loss.Their DTA curves still showed the weak and broadexothermic effect with its maximum at about 260◦C.The second exothermic peak was however much lessintense and split in two broader peaks with their max-

Figure 1. TG and DTA curves of the 5 at% Ta-doped TiO2 precursor.

ima at 400 and 425◦C for the Ta-doped sample, andat 400 and 440◦C for the Nb-doped sample, showingthat the crystallization processes occurred in a differentway for the doped samples.

XRD analysis showed that the precipitates dried at100◦C were amorphous. Table 1 summarizes the XRDresults, showing the percentage of the phases observedon each sample for the heated precursors. The presenceof crystalline TiO2 in the anatase phase (JCPDS FileNo. 21-1272) was observed for all the samples heatedto 400◦C. The pure TiO2 powder heated to 650◦C stillshowed the presence of the anatase phase. Heating upto 850◦C induced the phase transition from anatase torutile (JCPDS File No. 21-1276). No peaks for thisphase transition were observed in the DTA curves.

Figure 3 shows the TEM micrograph with the corre-sponding SADP of the precursor of pure TiO2 heatedto 400◦C. The powder was made of agglomeratednanoparticles with average size of 10 nm. From XRDanalysis, the crystallite size measured for the same sam-ple using Scherrer’s equation was 11 nm, demonstrat-ing that each grain corresponds in average to a singlecrystallite. SADP measurements showed the presenceof polycrystalline TiO2 in the anatase structure. Similarresults were obtained for the doped powders heated to400◦C.

XRD analysis showed that the presence of Ta and Nbdramatically affected the phase transition from anataseto rutile observed for the pure titania. The Ta-dopedpowders showed the presence of only the anatase phase

Sol-Gel Processed TiO2-Based Nano-Sized Powders 171

Figure 2. TG and DTA curves of the 10 at% Ta-doped TiO2 precursor.

up to a heating temperature of 850◦C. At 950◦C, theanatase phase was accompanied by small amounts ofrutile. At 1050◦C, the only crystalline phase observedwas TiO2 in the rutile structure. Crystalline phases con-taining Ta were not observed. The crystallite size of the

Table 1. Percentage of the crystal phases present in the TiO2-based powder samples fromXRD results (elaboration with Rietveld analysis program DBWS 94411 [27]).

Sample Heating T (◦C) Anatase (TiO2) Rutile (TiO2) Nb2TiO7

Pure TiO2 400 100

650 100

850 1 99

5 at% Nb-doped TiO2 400 100

650 100

850 95 5

1050 100

10 at% Nb-doped TiO2 400 100

650 100

850 88 8 4

950 27 66 6

1050 93 7

10 at% Ta-doped TiO2 400 100

650 100

850 100

950 95 5

1050 100

anatase phase increased from 10 nm at 400◦C to 15 nmat 650◦C, and 25 nm at 850◦C.

The 10 at% Nb-doped powder showed the presenceof only the anatase phase up to 650◦C, while the sam-ple heated to 850◦C contained mainly anatase, but also

172 Traversa et al.

Figure 3. TEM micrograph with SADP of the pure TiO2 precursor decomposed at 400◦C.

rutile and Nb2TiO7 (JCPDS File No. 39-1407). Thecrystallite size of the anatase phase were 10 nm at400◦C, 12 nm at 650◦C, and 30 nm at 850◦C. At 950◦C,the rutile phase was predominant with respect to theanatase phase, and at 1050◦C the transformation ofanatase into rutile was completed. At both tempera-tures, the peaks of Nb2TiO7 were also observed. The5 at% Nb-doped powder heated to 850◦C showed thepresence of anatase and a small amount of rutile, andat 1050◦C the presence of only the rutile structure.

3.2. Thick-Film Morphology

SEM observations of the thick films showed that thephase transformation from anatase to rutile was ac-companied by a significant grain growth, particularlyin the case of pure TiO2. The pure TiO2 film fired at650◦C was highly porous and made of homogeneousspherical particles with grain size in the range 40–60nm [18]. The grain size of pure titania film fired at850◦C was in the 200–600 nm range, showing a graingrowth of about one order of magnitude, with a reduc-tion in the total porosity [22]. The grain size furtherincreased for the pure TiO2 film fired at 1050◦C, reach-

ing the 500–800 nm range, with formation of sinteringnecks.

Figure 4 shows the SEM micrographs of the 10 at%Nb-doped (Fig. 4(a)) and of the 10 at% Ta-doped (Fig.4(b)) TiO2 films heated to 850◦C. The morphology ofthe doped TiO2 films heated to 650◦C and up to 850◦Cwas similar to that of the pure titania films fired at650◦C, with particles having grain size in the 40–70 nmrange. Only in the case of the 10 at% Nb-doped filmfired at 850◦C the grain size was slightly larger.

Figure 5 shows the SEM micrographs of the 10 at%Nb-doped (Fig. 5(a)) and of the 10 at% Ta-doped(Fig. 5b) TiO2 films heated to 950◦C. The grain size forthe Nb-doped film increased up to 70–100 nm, due tothe formation of rutile phase. The increase in the grainsize was limited to the 50–80 nm range in the case of theTa-doped film, which still mainly consisted of anatase.

As one can observe in Fig. 6, which shows the SEMmicrographs of the 10 at% Nb-doped (Fig. 6(a)) and ofthe 10 at% Ta-doped (Fig. 6(b)) TiO2 films heated to1050◦C, the increase in the grain size was more signifi-cant, and it was larger for the Nb-doped film. The grainsize was in the 150–300 nm range for the Nb-dopedTiO2 film, and 100–250 nm for the Ta-doped TiO2

film heated to 1050◦C. The sintering of the particles

Sol-Gel Processed TiO2-Based Nano-Sized Powders 173

Figure 4. SEM micrographs of the 10 at% Nb-doped TiO2 film (a) and of the 10 at% Ta-doped TiO2 film (b) heated to 850◦C.

proceeded for both the films, with a stronger effect forthe Nb-doped film.

Similar results were obtained for the TiO2 filmsdoped at the 5 at% level. The doping at both the concen-trations was effective in hindering the grain growth andthe anatase-to-rutile phase transformation. The effectof Ta was observed up to 950◦C, while Nb additionwas up to 850◦C. The higher valence of Ta and Nb thatreduces the oxygen vacancy concentration in titaniamight be the cause of the inhibition of the anatase-to-rutile phase transformation [29].

3.3. Electrical Analysis in Laboratory

Figure 7 shows the temperature-dependence of the con-ductance in dry air for the pure and 10 at% Ta- and Nb-doped TiO2 thick films heated to 650◦C. The morphol-ogy of all the films fired at this temperature was verysimilar. Also the thickness of all the films was about15 µm (as measured from cross sections by SEM). Forall the films, two different regions of conduction can beobserved; at temperatures lower than 380◦C, the con-ductance increased at slower rate, while conductance

174 Traversa et al.

Figure 5. SEM micrographs of the 10 at% Nb-doped TiO2 film (a) and of the 10 at% Ta-doped TiO2 film (b) heated to 950◦C.

increased sharply above this temperature. The conduc-tance for the various samples followed the order:10 at%Nb-doped TiO2 > 10 at% Ta-doped TiO2 > pureTiO2. Therefore, the Nb doping was more effectivethan the Ta doping to decrease the resistance of thesamples.

Figure 8 shows the Arrhenius plots of the conduc-tance in wet (40% RH) air for the pure and 10 at% Ta-and Nb-doped TiO2 thick films heated to 650◦C. Theconductance above 180◦C was very similar to the be-haviour observed in dry air, confirming that RH affectsonly slightly the electrical response of the TiO2-based

films [18]. At temperature lower than 180◦C, only theconductance of the pure TiO2 film was dramaticallyaffected by RH; an increase in conductance can be as-cribed to the presence of adsorbed water molecules onthe surface of pure titania.

The conductance G of a semiconducting oxide at agiven temperature T can be expressed by [30]:

G = G0 exp(−eVs/kT) (1)

where eVs is the surface potential barrier energy be-tween particles, k is the Boltzmann’s constant, and G0

Sol-Gel Processed TiO2-Based Nano-Sized Powders 175

Figure 6. SEM micrographs of the 10 at% Nb-doped TiO2 film (a) and of the 10 at% Ta-doped TiO2 film (b) heated to 1050◦C.

is a factor which includes the bulk intragranular con-ductance. On the basis of Eq. (1), it is not possibleto directly obtain the values of the barrier energy eVs

from the Arrhenius plots because both constant andlinearly changing eVs will give straight lines in theArrhenius plot formalism [31]. However, the barrierenergy value at a given temperature can be directly de-termined by temperature-stimulated conductance mea-surements [32]. Given that G0 can be considered asa constant, independent of the temperature [31], weused the temperature-stimulated conductance measure-ments to determine the G0 value for each sample tested

[28]. The starting temperature for these measurementswas 370◦C.

Given the measured G0 values, the barrier energyvalues were calculated from Eq. (1) using the measuredG values (from the Arrhenius plots). Figure 9 shows thetemperature dependence of the barrier energy in dry andin wet air of the pure and 10 at% Ta- and Nb-doped TiO2

thick films heated to 650◦C. In dry air, the measuredG0 values were 2.2 �−1 for pure TiO2, 21.5 �−1 forTa-doped TiO2, and 248.6 �−1 for Nb-doped TiO2. Thebarrier energy values in the whole temperature rangewere in the order: 10 at% Nb-doped TiO2 > 10 at%

176 Traversa et al.

Figure 7. Temperature dependence of the conductance in dry airof the pure and 10 at% Ta- and Nb-doped TiO2 thick films heated to650◦C.

Figure 8. Temperature dependence of the conductance in wet(40% RH) air of the pure and 10 at% Ta- and Nb-doped TiO2 thickfilms heated to 650◦C.

Ta-doped TiO2 > pure TiO2. The fact that the largestbarrier energy was observed for the film having thelargest conductance indicates that the barrier energy ismainly due to surface species adsorbed on the oxidesurface and not to bulk phenomena. The increase in

Figure 9. Temperature dependence of the barrier energy in dry andin wet (40% RH) air of the pure and 10 at% Ta- and Nb-doped TiO2

thick films heated to 650◦C.

conductance has thus to be attributed to the increasednumber of electrons in the conduction band releasedby the donor-like Nb and Ta atoms.

The barrier energy increased with the temperatureup to a maximum at 380◦C. It has been reported thatthe barrier energy for SnO2 thick films has a mini-mum at around 180◦C and then increased up to a max-imum at 380◦C [21]. The trend of the barrier energyfor tin dioxide has been explained by the beginningof the dissociation of surface O2

− ions into O− ions atthe temperature at which the minimum was observed[31]. Oxygen desorption starts above the temperature atwhich the maximum was observed. The trend observedfor the TiO2-based films can be thus explained by sur-face adsorption of only O− ions. Given that TiO2 is an-type semiconducting oxide, the presence of O− ionscauses an increase in the barrier energy, while oxygendesorption causes a decrease in the barrier energy.

In wet air, the measured G0 values were 0.02 �−1

for pure TiO2, 1.9 �−1 for Ta-doped TiO2, and 4.9 �−1

for Nb-doped TiO2. In wet air, the barrier energy valueswere much smaller than the values measured in dry airfor all the films (Fig. 9). The trend for the doped-filmswas very similar to the trend observed in dry air, thoughthe barrier energy became larger for the Ta-doped filmsthan for the Nb-doped films. For pure TiO2, the trendof the barrier energy was different at low temperatures,as expected from the Arrhenius plot. This behaviour

Sol-Gel Processed TiO2-Based Nano-Sized Powders 177

might be due to the competition between adsorption ofOH− ions with adsorption of O− ions. The oxide resis-tivity decreases when water molecules are chemisorbedat relatively high temperatures (200–500◦C) on an-type semiconducting oxide, indicating that electronsare apparently transferred from water molecules to theoxide [33]. This results in a further decrease in the bar-rier energy.

The TiO2-based films were tested in laboratory asgas sensors. In previous papers [18, 22], we have re-ported that the titania-based sensors were able to de-tect polluting gases only if their particle dimensionswere smaller than approximately 100 nm. For pure ti-tania samples, the response to 100 ppm of CO wasgood when they were fired at 650◦C, but the filmswere insensitive to CO when their firing temperaturewas 850◦C, because of grain growth occurrence ac-companied by anatase-to-rutile phase transformation[18]. Previous work demonstrated also that the CO re-sponse was larger for the 10 at% Ta-doped film fired at850◦C [19], in the 350–400◦C temperature range [18].This is in agreement with the barrier energy measure-ments; the best gas response has to be expected in cor-respondence of the barrier-energy maximum (380◦Cin this case), because at this temperature the surfacecoverage by ionosorbed oxygen reaches its maximumvalue. The operating temperature for TiO2-based filmshas thus to be at least 400◦C, also to increase the ox-ide conductivity [22]. This temperature would be toohigh to safely operate the pure titania films that aresensitive when fired to 650◦C. The nanostructure forNb- and Ta-doped films is maintained at higher firingtemperature, making them more suitable for practicalapplications.

Figure 10 shows the electrical response (evaluatedas the ratio between the conductance in the tested gas,Ggas, and the conductance in air, G i) to CO (100 ppm)in wet air (40% RH) of the 10 at% Ta- and 10 at%Nb-doped TiO2 thick films fired at 650, 850, and 950◦C,measured at 380◦C. The differences in the films’ mi-crostructure affected their CO response, even thoughthe best gas response was observed for the films firedat 850◦C, as previously reported [18, 19]. It must benoted that the doped samples fired at 650◦C and 850◦Chave the same particle size and similar morphologies,even though the crystallite size is slightly larger forthe 850◦C-heated films. However, the presence of dif-ferent surface species also affects the gas response,as observed for other oxides [34]. Therefore, also inthis case the improved CO response can be ascribed

Figure 10. Electrical response to CO (100 ppm) in wet air(40% RH) of the 10 at% Ta- and 10 at% Nb-doped TiO2 thick filmsfired at 650◦, 850◦, and 950◦C, measured at 380◦C.

to a different surface coverage induced by the thermaltreatments at higher temperature. The samples fired at950◦C showed smaller responses and this is certainlydue to the increased grain size, particularly in the caseof Nb-doped films; in fact, conductance values mea-sured for both samples fired at 950◦C were one orderof magnitude larger than those measured for the 850◦Cfilms, due to the formation of sintered necks and theresulting decrease in specific surface area.

For the films fired at the same temperature, the CO re-sponse of Ta-doped films was always larger than the re-sponse of the corresponding Nb-doped films, althoughtheir microstructure was very similar (in particular atthe lower firing temperatures). Again, the different COresponse might be ascribed to a different surface cov-erage and further studies are needed to fully clarify thispoint.

3.4. Atmospheric Pollutant Monitoring Field Tests

The field tests were performed on an array of sensorsinstalled in March, 2000. The responses of the sensorswere considered fifteen days later to allow the sensorstabilisation. Figure 11 shows a typical electrical re-sponse of the 10 at% Ta- and Nb-doped TiO2 thick filmsfired at 850◦C during field tests. These measurementsrefers to a continuous monitoring (the signals wererecorded every 60 seconds) from the 29th of Marchto the 1st of April 2000. In general, the activity of thesensors was larger during rush hours, and two peaks of

178 Traversa et al.

Figure 11. Time dependence of the conductance of 10 at% Ta- andNb-doped TiO2 thick films fired at 850◦C during field tests on March29–April 1, 2000.

pollution can be observed for each day in the morningand in the afternoon. This indicates that pollutant con-centrations are mainly due to automotive emissions. Toconfirm this, the peak referring to the second afternoonin the Fig. 11 is much smaller, due to the fact that inthis day the access to cars in downtown Ferrara wascontrolled and limited.

The sensor signal intensity followed well the pollu-tants’ concentration, as measured with the analyticalequipment. These results are surprisingly interesting,because although the concentrations of toxic gases inthe atmosphere are very low, the sensors are still able tomonitor the concentration changes. To find a quantita-tive correlation between the response of the sensors andthe concentrations measured by the analytical equip-ment, we tried to correlate the electrical response ofthe titania-based sensors with the output of the analyt-ical equipment. A good correlation was found only forthe CO concentration. Figure 12 shows, as an exam-ple, the correlation between the response of the 10 at%Nb-doped TiO2 thick film fired at 850◦C (given as av-erage values in a hour) and the CO concentration formeasurements performed in different days than for thetests reported in Fig. 11. The fitting of the data is verygood. These calibration curves were used to evaluatequantitatively the CO concentrations and the obtainedvalues were compared with the CO concentration val-ues measured with the IR analyser in the same period oftime as in Fig. 11. Figure 13 shows that the correspon-dence with the conventional measurements is excellent.Although the large errors involved in the calibrationprocedure, which inevitably neglected the influence onthe sensor response of other reducing gases that might

Figure 12. Correlation between the response of the 10 at% Nb-doped TiO2 sensor during field tests on April 8–14, 2000, and theCO concentration measured with the conventional environmentalmonitoring station.

Figure 13. Comparison between the CO concentration values(ppm) measured with the conventional environmental monitoringstation and evaluated from the response of the 10 at% Ta- and Nb-doped TiO2 sensors using calibration curves (Fig. 12), during fieldtests on March 29–April 1, 2000.

be present in the atmosphere but not recorded by theanalytical equipment, the sensors were able to reliablydetect CO concentrations above 0.5 ppm, as shown inFig. 13. We reported elsewhere [24] that the total errorcan be very small for this type of sensors.

4. Conclusions

Nano-sized TiO2-based powders were prepared us-ing a sol-gel procedure. The synthesis method al-lows controllling particle nanostructure and homoge-neously doping with Ta and Nb. The sol-gel powders

Sol-Gel Processed TiO2-Based Nano-Sized Powders 179

have suitable characteristics for use in screen-printingtechnology. Thick-film prototype sensors were thusfabricated using screen-printing technology. Deposi-tion and firing of thick films were optimised to ob-tain TiO2-based films preserving the size of grains atthe nanometer scale. The addition of Ta and Nb is ef-fective in controlling grain growth at relatively highertemperatures. Good performance as CO sensors wasrecorded when the TiO2-based gas-sensing materialsare nanostructured. Given that titania-based sensorsneeds to work at operating temperatures larger than400◦C, the sensors based on TiO2 films doped with10 at% Ta and fired at 850◦C offer the best combinationof characteristics for CO control in air quality moni-toring. They show enhanced thermal stability keepingthe grain size at the nano-sized level, and thus a largeCO response, and their response is almost unaffectedby humidity. Field tests demonstrated that the nanos-tructured TiO2-based thick-film sensors were able todetect CO with the same performance of the conven-tional analytical techniques. The calibration performeddirectly in the field allowed the quantitative evaluationof CO concentrations. Therefore, the use of TiO2-basedsensors can be considered for application in innovativeand low-cost atmospheric pollutant monitoring.

5. Acknowledgments

This work was supported by the National ResearchCouncil of Italy (CNR), under the auspices of the Tar-geted Project “MADESS II”. The technical assistanceof Ms. C. D’Ottavi (University of Roma “Tor Vergata”)is gratefully acknowledged.

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