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TiO2-based humidity sensors elements prepared by high energy activation
Sensors and Actuators B 157 (2011) 654–661
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Structural and properties of nanocrystalline WO3 /TiO2 -based humidity sensors elements prepared by high energy activation K.O. Rocha a , S.M. Zanetti a,b,∗ a b
SENCER – Sensores Cerâmicos Ltda, Rua Santos Dumont, 800, 13566-445, São Carlos, SP, Brazil INCTMN – Instituto de Química, UNESP, Rua Francisco Degni, s/n, 14800-900–Araraquara, SP, Brazil
a r t i c l e
i n f o
Article history: Received 22 March 2011 Received in revised form 11 May 2011 Accepted 25 May 2011 Available online 1 June 2011 Keywords: WO3 /TiO2 High energy activation Humidity sensor Nanostructured oxide
a b s t r a c t Nanocrystalline WO3 /TiO2 -based powders have been prepared by the high energy activation method with WO3 concentration ranging from 1 to 10 mol%. The samples were thermal treated in a microwave oven at 600 ◦ C for 20 min and their structural and micro-structural characteristics were evaluated by X-ray diffraction, Raman spectroscopy, EXAFS measurements at the Ti K-edge, and transmission electron microscopy. Nitrogen adsorption isotherms and H2 Temperature Programmed Reduction were also carried out for physical characterization. The crystallite and particle mean sizes ranged from 30 to 40 nm and from 100 to 190 nm, respectively. Good sensor response was obtained for samples with at least 5 mol% WO3 activated for at least 80 min. Ceramics heat-treated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min, which is highly cost effective. These results indicate that WO3 /TiO2 ceramics can be used as a humidity sensor element. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Systems for air quality monitoring are of great importance for quality control of products in different industries, such as in electronic devices and precision instruments production, in textile area, in food storage, and in human comfort in domestic environments [1,2]. Another aspect of air quality is the relative humidity level which is also of critical interest for several areas, both for human comfort and for industrial processes. Therefore a large variety of oxide materials has been studied as humidity sensor elements. The requirements for practical application of humidity sensors are: good sensitivity over a wide range of humidity and temperature, short response time, good reproducibility, very small hysteresis, negligible temperature dependence, low cost, and chemical and physical stability in aggressive atmosphere [3]. Among the many different materials, titanium dioxide has been extensively studied due to its semiconductor behavior. It has received great attention for application as chemical sensors because of changes in its properties when exposed to different atmospheres. Titanium dioxide has different crystalline phases: brookite, anatase and rutile. Titanium dioxide may have its crystallization influenced by doping with transition metal oxide for instance tungsten oxide, which can alter the transformation of
anatase to rutile phase. As TiO2 does not react with WO3 forming chemical bonds, tungsten dioxide is distributed in the titanium dioxide matrix which gives peculiar properties to the compound. Also, WO3 /TiO2 oxides are important compounds for optoelectronics application especially as gas sensors since these materials are greatly sensitive to atmosphere changes [4]. WO3 /TiO2 compounds have been prepared by incipient wet impregnation of ammonium metatungstate – (NH4 )10 W12 O41 ·5H2 O [5] or NH4 OH/tungstic acid [6] aqueous solutions on the commercial TiO2 powder, by sol–gel method [2,3,7], by thermal decomposition of tungstic acid and titanium tetraisopropoxide [8], via hydrothermal template-free route [9], and by ball milling [10]. Although all these preparation methods have focused on the application of WO3 /TiO2 as catalysts; the WO3 /TiO2 system has not been exploited for humidity sensor purpose. This study aimed at obtaining WO3 /TiO2 nanopowders by high energy milling varying the WO3 content and the milling time. The powders were thermal treated either in conventional furnace or in microwave oven and their structural, microstructural, and electric properties were evaluated as a candidate for humidity sensor. 2. Experimental
∗ Corresponding author at: SENCER – Sensores Cerâmicos Ltda, Rua Santos Dumont, 800, 13566-445, São Carlos, SP, Brazil. Tel.: +55 16 3201 2090. E-mail address: [email protected] (S.M. Zanetti). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.05.048
The starting materials were the commercially available reagents: TiO2 (99%, Certronic), and H2 WO4 (99%, Aldrich). The final compounds have different WO3 contents (1 mol% – W1T, 5 mol%
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– W5T, and 10 mol% – W10T). For the mechanochemical activation, a batch of 10 g of the powder mixture with appropriate weight ratio (as required for each composition) was subjected to mechanical treatment in a vibrating mill (Fritsch, Pulverizette 6), with vial (80 cm−3 capacity) and balls made of zirconia. The ball/powder mass ratio was 6, and the milling was carried out at a rotation speed of 300 rpm during 40 min, 80 min, or 180 min. The samples were dubbed with respect to the WO3 content and the milling time, e.g. the sample with 1 mol% WO3 activated for 40 min has been named W1T40. The powders structure evolution with milling time was followed by X-ray diffraction - XRD (Philips, PW1830) using Cu K␣ radiation and the crystallite size was estimated by Scherrer’s equation using the XRD data. The particle size was obtained from the specific surface measured by N2 adsorption/desorption isotherms (Quantachrome, NOVA 1000). Their structure and microstructure were evaluated by Raman spectroscopy (RENISHAW, U-3000, with a 632.8 nm He–Ne laser coupled with an Olympus microscope) in the range of 300–1000 nm and transmission electron microscopy – TEM (Philips CM 200), respectively. The powders were also characterized by H2 Temperature Programmed Reduction (TPR-H2 ) in a Micrometricis Pulse ChemiSorb 2707 with 5%H2 /N2 and NH3 pulse. EXAFS measurements at the Ti K-edge were carried out at the D08B-XAFS2 beamline of the National Synchroton Light Laboratory (LNLS), Campinas, Brazil. This beamline is equipped with a (1 1 1) silicon channel-cut and the measurements were performed in transmission mode. Athena/Artemis software packages [11] were used to extract the EXAFS signal from the measured absorption spectra [12]. The EXAFS oscillations were fitted in R-space and k2 -weighted Fourier filtering was used. The first shell coordination scattering contribution were used for fitting EXAFS data, ˚ Kaiser–Bessel window and backscattering amplitudes R = 1.0–2.0 A, calculated by using the TiO2 -ana file. The obtained spectra of the sample were calibrated and aligned with the experimental Ti-foil EXAFS. Powders with different compositions and activated for different times were used to prepare pellets with 10 mm diameter and 2 mm thick by uniaxial pressing. The first group of pellets was thermal treated at 600 ◦ C for 2 h in a conventional furnace, at a 10 ◦ C min−1 heating rate. The second group was thermal treated at 600 ◦ C for 20 min in a domestic microwave oven (2.45 GHz, 900 W) by means of a SiC susceptor, at a 50 ◦ C min−1 heating rate. The pellet was placed between two SIC susceptor pieces (above and below the pellet) in order to ensure a uniform pellet heating. The sensor samples were prepared by using silver paste as electrodes in pellets’ both faces. The samples were submitted at different humidity levels (11–97% RH) and their electrical response was measured using a LCZ (PHILIPS, PM 6304) at room temperature and at 1 kHz frequency. The distinct humidity levels were obtained from different salts saturated solutions at room temperature.
3.1. Effect of milling time
Fig. 1. (a) XRD patterns of W5T powder: (a) activated for different times and the starting powder; (b) heat-treated at 600 ◦ C for 20 min in a microwave oven; (c) crystallite size of W5T composition calculated from XRD data using the Scherrer’s equation.
The effect of milling time for all compositions was followed by XRD. All samples showed the same crystalline phases: anatase (JCPDS 21-1272) and WO3 .H2 O (JCPDS 84-886). Fig. 1a shows the results obtained from the composition W5T (5 mol% WO3 ). It has been observed the intensity decreasing and broadening of anatase peaks as the milling time increased, showing the effect on structure distortion caused by the high energy milling. For W1T and W10T compositions the same effect was observed, except for the higher intensity of WO3 -related peaks detected for these compo-
sitions. The powders activated for different times were thermal treated at 600 ◦ C for 2 h in a microwave oven. The XRD patterns have shown only anatase and WO3 (JCPDS 72-1465) as crystalline phases, according to Fig. 1b. The crystallite size of TiO2 (anatase) for W5T composition were estimated from the XRD data and the Scherrer equation for the 25.3◦ peak is displayed in Fig. 1c. For the as-activated powder, it has been observed a linear decrease in crystallite size from 38 nm to 18 nm as
3. Results
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rutile phase were observed. This result agrees fairly well with the XRD findings. So far, from the results obtained it is possible to verify that the thermal treatment in microwave oven is highly effective even when lasting only 20 min. Hence the powders thermal treated in microwave oven had their mean particle size (MPS) obtained from the specific surface area (SSA) measurements by the BET method which are displayed in Table 1. The mean particle size was calculated through the equation: MPS =
Fig. 2. Raman spectra of WO3 /TiO2 pellets: (a) derived from powders of different compositions heat-treated at 600 ◦ C for 20 min in a microwave oven; (b) derived from 80 min-activated powder with 5 mol% WO3 heat-treated at 600 ◦ C either in conventional furnace or microwave oven.
the milling time increased. The same behavior has been observed for the heat-treated powders wherein crystallite size ranged from 39 nm to 30 nm, for powder treated in conventional furnace, and from 38 nm to 28 nm, for powder treated in microwave oven. It has also been verified that the thermal treatment at 600 ◦ C led to an increase in the mean crystallite size with respect to the as-activated powder. Moreover, the increase in crystallite size was higher for the 180 min-activated powder probably due to the high reactivity reached by high energy activation. The Raman spectra of 180 min-activated WT powders (Fig. 2a) showed a frequency shift and broadening of Raman peaks as a result of phonon confinement caused by the decrease in the crystallite dimension. It was observed that the peaks intensity decreased as the milling time increased. The intensity of the peaks related to WO3 (at 715 and 805 cm−1 ) increased as the WO3 content increased, as expected. The Raman peaks at 275 cm−1 and 430 cm−1 correspond to the deformation vibrations of O–W–O bonds and peaks in the 700–803 cm−1 range correspond to the stretching vibrations of W–O bonds [13]. The Raman peaks at 397, 518 and 640 cm−1 can be assigned as the modes of the anatase phase [14]. The thermal treatment in conventional furnace allows an increase in the intensity of Raman peaks compared to the sample treated in microwave oven, as observed for W5T pellets derived from 80 minactivated powders, as displayed in Fig. 2b. No peaks related to the
3000 (nm) SSA
(1)
where SSA is the specific surface area (m2 g−1 ) and is the theoretical density (W1T = 3.93 g/cm3 ; W5T = 4.06 g/cm3 ; W10T = 4.24 g/cm3 ). The SSA of original powders were also measured by the BET method which values were 10.0 m2 g−1 , for TiO2 , and 12.3 m2 g−1 , for H2 WO4 . For W1T composition, the mean particle size increased with the increasing of the activation time, from 27 nm to 145 nm. It is expected agglomeration of nanoparticles after activation due to the increase in surface reactivity, and probably the small WO3 content does not influence this process. For W5T composition, one can observe that the activation during 40 min promotes the highest mean particle size (187 nm) while the lowest one (108 nm) is reached after activation during 180 min. The decrease of the mean particle size with the increasing of the activation time is probably due to a more homogeneous distribution of WO3 on TiO2 matrix (conferred to the extended milling time) avoiding particles to agglomerate in spite of the reduction of the particle size. For W10T composition, it has been observed almost no influence of the milling time in the mean particle size. Probably the high WO3 content in this sample has promoted the formation of WO3 clusters avoiding a homogeneous distribution of WO3 particles in TiO2 matrix. Although its value is still high, the effect of milling time starts to be observed only for the 180 min-activated sample. Perhaps longer milling times could be more effective in reducing the particle size for these samples. Fig. 3 displays the grain morphology images of 180 minactivated powders treated at 600 ◦ C in a conventional furnace obtained by TEM. One can observe fairly agglomerated particles with dimensions ranging from 50 nm to 200 nm for all compositions, as well as a similar porous microstructure. Nevertheless, big agglomerates of smaller particles for W5T and W10T composition can be seen, probably due to the increase in WO3 content. These results agree quite well with MPS values displayed in Table 1. Fig. 4 displays the TPR-H2 profile of 80 min-activated powders treated at 600 ◦ C in a conventional furnace. Small peak of H2 consumption at 640–670 ◦ C is observed for all samples. The reduction peak at 945 ◦ C for W1T shifts to lower temperatures as WO3 content increased; an extra shoulder around 810 ◦ C can observe for W10T. According to Chen et al. [15] all peaks may be attributed to different species of WOx reduction. The shift to lower temperatures along with the shoulder observed for W10T sample may be attributed to the formation of small WO3 clusters, which contribute to activation and, consequently, to spill over of H2 to bigger clusters. All the samples were maintained at 900 ◦ C during TPR-H2 analysis until stabilization of the signal. The area under the curve indicates that all WO3 were reduced. Moreover, one may suppose that WO3 was on TiO2 surface. The acidity, calculated by NH3 pulse chemisorptions, for 80 minactivated samples treated at 600 ◦ C for 2 h in conventional furnace is displayed in Table 2. The acidity increased with the addition of WO3 to the TiO2 sample. With respect to WO3 content, a decrease of sample acidity is observed as WO3 content increased which is much lower for W10T than for W1T. In molar basis, it is possible to observe that W1T sample contains much more acid sites per mol of
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Table 1 Mean particle size of powders with different compositions activated for different times, followed by thermal treatment at 600 ◦ C for 20 min in a microwave oven. W1T
Activation time (min)
40 80 180
W5T
W10T
SSA (m2 g−1 )
MPS (nm)
SSA (m2 g−1 )
MPS (nm)
SSA (m2 g−1 )
MPS (nm)
56.4 12.1 10.5
27 125 145
7.9 11.2 13.7
187 132 108
9.9 9.7 10.9
142 147 129
Fig. 3. MET images of 180 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace.
Fig. 4. TPR-H2 of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace.
WO3 than W5T and W10T samples. Thus for those samples, only a small fraction of WO3 is available for NH3 adsorption which may be attributed to the formation of larger WO3 clusters which decreases the amount of acid sites. Similar results were also observed in [16]. According to Yang et al. [3], it is known that anatase phase is stable because of the oxygen vacancies. If a W ion substitutes a Ti ion, the valence state at Ti4+ neighborhood will be reduced and extra oxygen vacancies along with Ti3+ ions will be created, which in its turn increases the number of these defects in the anatase Table 2 Acidity calculated by NH3 pulse chemisorptions for 80 min-activated samples heattreated at 600 ◦ C for 2 h in a conventional furnace. Sample TiO2 W1T W5T W10T
Acidity (mol NH3 /gsample ) 10.1 151.3 122.8 84.3
Acidity (mol NH3 /mol WO3 ) – 3.5 0.6 0.2
phase. If the acidity of the material has increased by the addition of W, as observed, one may suppose that the Ti3± concentration has increased as well. Nevertheless, for WO3 content higher than 1%, the addition of WO3 causes a decreasing in molar basis acidity (while the total acidity remains high). In the TPR-H2 analysis, for W5T and W10T samples, the most intense peak shifts to lower temperature suggesting the formation of large WO3 particles. It is possible that these large particles block NH3 to access the Ti3± sites and, consequently, blocks also the water vapor. Fig. 5 presents the Fourier transformed of EXAFS oscillations for pure TiO2 and 80 min activated WT samples treated at 600 ◦ C for 2 h in a conventional furnace. The first peak, 1 < R < 2, corresponds to the first oxygen coordination shell around Ti (Ti-O1 at 1.937 A˚ and ˚ and the peak between 2 < R < 3 corresponds mainly Ti-O2 at 1.966 A) ˚ The decreasing of the to Ti–Ti second coordination shell (3.039 A). second peak for WT samples with respect to TiO2 may be attributed to the decreasing of particle size caused by the milling activation. Among the doped samples, the increasing of WO3 content promotes an increasing of first and second coordination shells. Table 3 presents the coordination number (CN), inter-atomic distance (r), Debye-Waller factor ( 2 ), and R factor for the first coordination shell scattering of TiO2 for 80 min activated samples. The EXAFS results indicate that the addition of 1 mol% of WO3 decreases the coordination number in the first shell in comparison to the original TiO2 powder from 3.94 to 2.42. This huge difference may be attributed to a diminishing in crystallite size caused by the
Table 3 Structural parameters obtained from EXAFS for 80 min-activated samples heattreated at 600 ◦ C/2 h in a conventional furnace. Sample
Scattering
CN
TiO2
Ti-O1 Ti-O2 Ti-O1 Ti-O2 Ti-O1 Ti-O2 Ti-O1 Ti-O2
3.94 1.97 2.42 1.21 3.36 1.68 3.58 1.80
W1T W5T W10T
˚ r (A) ± ± ± ± ± ± ± ±
0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.07
1.95 1.86 1.95 1.86 1.95 1.86 1.95 1.86
± ± ± ± ± ± ± ±
0.01 0.02 0.06 0.06 0.04 0.07 0.04 0.07
2 (A˚ 2 )
R factor
0.005
0.03
0.005
0.05
0.005
0.04
0.005
0.04
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Fig. 5. EXAFS oscillations (a, d, g, j), the Re[(q)] function data (b, e, h, k) and the magnitude of the Fourier transformed (c, f, i, l) for TiO2 (a, b, c), W1T (d, e, f), W5T (g, h, i) and W10T (j, k, l) of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace and TiO2 (reference).
activation milling, as observed in XRD patterns. One may verify a significant increasing in the coordination number as the WO3 content ranged from 1 to 5% and almost no change when WO3 increased to 10%. One can infer that WO3 clusters may have been formed, which in its turn diminish the effect of the activation milling on TiO2 particles, in contrast with what was observed for low WO3
content. In spite of the coordination number has decreased with addition of WO3 , the interatomic distance has not changed which may be attributed to the formation of TiO2 nanoparticles without any alteration of the electronic density. The alteration of electronic density could be observed through the variation of white line intensity in XANES region (Fig. 6)
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Fig. 6. XANES region spectra of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace and TiO2 (reference).
which has been focused by many authors [17–19]. Although the addition of WO3 may have promoted the increasing of Ti3+ concentration, no alteration in electronic structure have been observed by XAFS data, which could be explained by the edge chosen for data collection. The Ti K edge is associated with the electronic transition 1s1/2 → 4p1/2 , when the sublevel 4p1/2 is unoccupied. In this way, this electronic transition does not reflect any electronic alteration. The electronic alteration could be observed in Ti L3 edge [20], which is associated with the transition of 2p3/2 → 3d3/2 or 3d5/2 , when the 3d sublevel partially occupied [21]. 3.2. Electrical characterizations Capacitance measurements as a function of the relative humidity were carried out for WT ceramics. Fig. 7 displays the results obtained for samples of different compositions activated for 40 min, 80 min, or 180 min thermal treated at 600 ◦ C in a microwave oven. All samples responded as a humidity sensor but the results were analyzed with respect to the linearity in order to establish its correlation between the activation time and the WO3 content. Although the particle size have increased from 27 to 145 nm as the milling time increased from 40 to 180 min for W1T samples, it can be observed a high sensitivity for RH level below 30%, independently of the milling time. The increase in sensitivity for low humidity levels may be attributed to the greater amount of acid sites achieved in W1T (as observed in Table 2) which promote the formation of H2 O chemisorbed layer. For W5T and W10T samples, the increasing of the milling time provoked a diminishing of the particle size. Nonetheless, the increase in WO3 content causes a decreasing in the molar ratio acidity (although the total acidity remains high) and suppresses the sensibility in low humidity range. Also, as observed in TPR-H2 measurements, higher WO3 contents probably led to the formation of large WO3 particles which may block water vapor to access the Ti3± sites. The effect of particle size in the capacitance behavior as a function of relative humidity may be explained through the physical model based on adsorbed water on the surface and on capillary water condensation within the pores [22–24]. For 40 min-activated W5T and W10T samples a linear response in two segments is displayed indicating that different conduction mechanisms are involved in the phenomenon. Similar behavior is observed for 80 min-activated W10T and 180 min-activated W1T samples. Therefore at high humidity range (above 75%) an
Fig. 7. Capacitance as a function of relative humidity for WT ceramics derived from powders of different compositions activated for different times and heat-treated at 600 ◦ C for 20 min in a microwave oven.
increase in conduction is observed which may be associated to a capillary condensation process. When the capillary condensation occurs, the Warburg impedance may be originated along with the Grotthuss mechanism, and the conduction through the pores is electronic in nature. As for 80 min-activated W5T and 180 minactivated W5T and W10T samples, the electronic conduction is not observed. Fig. 8 shows the results obtained for W5T composition activated for different times. The activation for 180 min promoted a more sensitive curve yet, the response gain is not proportional to more than the double activation time. In order to compare pellets of W5T composition were thermal treated at 600 ◦ C for 2 h in a conventional furnace for electrical characterization. The results, displayed in Fig. 9, show a linear response for both thermal treatments in all RH range. The both presented no hysteresis in the 11–55% range while at RH levels above 55% a little larger hysteresis is observed for the sample heat-treated in a conventional furnace. This low hysteresis
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4. Conclusions WO3 /TiO2 nanopowders have been successfully prepared by high energy milling. XRD and Raman results for powders heattreated at 600 ◦ C indicated no other phase except WO3 and anatase. The addition of WO3 along with high energy activation promotes the reduction of particle size without any alteration of the electronic structure, as indicated by EXAFS. TPR-H2 measurements indicated that WO3 is present mainly on TiO2 surface. For the sample with 5 mol% WO3 , crystallite size and particle size, as a function of the activation time, ranged from 30 to 40 nm and 120 to 150 nm, respectively. All samples responded as a humidity sensor however, optimized preparation conditions were obtained for samples with at least 5 mol% WO3 activated for at least 80 min. Ceramics heattreated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min which is highly cost effective. These results indicate that WO3 /TiO2 ceramics can be used as a humidity sensor element. Fig. 8. Capacitance as a function of relative humidity for W5T ceramics activated for different times and heat-treated at 600 ◦ C for 20 min in a microwave oven.
Acknowledgements observed for W5T in all RH range is probably due to interparticle pores which are large sufficiently to enable the diffusion of water vapor. From these results, one can infer that the thermal treatment in microwave oven is highly indicated for preparation of porous WO3 /TiO2 ceramics.
The authors gratefully acknowledge the financial support of FAPESP through the Project JP 02/09497-0, DEQ-UFSCar for providing the apparatus for TPR-H2 analysis, LNLS/Campinas-SP for the XAFS analysis at beam line, and Dr. J. A. J. Rodrigues, coordinator of LCP/INPE/Cachoeira Paulista-SP, for NH3 pulse chemisorption analysis. References
Fig. 9. Capacitance hysteresis for 80 min-activated W5T ceramics heat-treated in: (a) a microwave oven for 20 min; (b) a conventional furnace for 2 h.
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Biographies Kleper de Oliveira Rocha He received his PhD degree in Chemical Engineering from UFSCar–São Carlos, Brazil, in 2009. Currently, he is a researcher at SENCER LTDA. His current interest is ceramic oxides applied as humidity and gaseous sensor.
Sonia Maria Zanetti She received her PhD degree in Chemistry from UFSCar–São Carlos, Brazil, in 2001. She is a visiting researcher at IQ/UNESP–Araraquara-SP and executive manager at SENCER LTDA. Her work has been mainly focused on ceramics applied as sensor
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Structural and properties of nanocrystalline WO3 /TiO2 -based humidity sensors elements prepared by high energy activation K.O. Rocha a , S.M. Zanetti a,b,∗ a b
SENCER – Sensores Cerâmicos Ltda, Rua Santos Dumont, 800, 13566-445, São Carlos, SP, Brazil INCTMN – Instituto de Química, UNESP, Rua Francisco Degni, s/n, 14800-900–Araraquara, SP, Brazil
a r t i c l e
i n f o
Article history: Received 22 March 2011 Received in revised form 11 May 2011 Accepted 25 May 2011 Available online 1 June 2011 Keywords: WO3 /TiO2 High energy activation Humidity sensor Nanostructured oxide
a b s t r a c t Nanocrystalline WO3 /TiO2 -based powders have been prepared by the high energy activation method with WO3 concentration ranging from 1 to 10 mol%. The samples were thermal treated in a microwave oven at 600 ◦ C for 20 min and their structural and micro-structural characteristics were evaluated by X-ray diffraction, Raman spectroscopy, EXAFS measurements at the Ti K-edge, and transmission electron microscopy. Nitrogen adsorption isotherms and H2 Temperature Programmed Reduction were also carried out for physical characterization. The crystallite and particle mean sizes ranged from 30 to 40 nm and from 100 to 190 nm, respectively. Good sensor response was obtained for samples with at least 5 mol% WO3 activated for at least 80 min. Ceramics heat-treated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min, which is highly cost effective. These results indicate that WO3 /TiO2 ceramics can be used as a humidity sensor element. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Systems for air quality monitoring are of great importance for quality control of products in different industries, such as in electronic devices and precision instruments production, in textile area, in food storage, and in human comfort in domestic environments [1,2]. Another aspect of air quality is the relative humidity level which is also of critical interest for several areas, both for human comfort and for industrial processes. Therefore a large variety of oxide materials has been studied as humidity sensor elements. The requirements for practical application of humidity sensors are: good sensitivity over a wide range of humidity and temperature, short response time, good reproducibility, very small hysteresis, negligible temperature dependence, low cost, and chemical and physical stability in aggressive atmosphere [3]. Among the many different materials, titanium dioxide has been extensively studied due to its semiconductor behavior. It has received great attention for application as chemical sensors because of changes in its properties when exposed to different atmospheres. Titanium dioxide has different crystalline phases: brookite, anatase and rutile. Titanium dioxide may have its crystallization influenced by doping with transition metal oxide for instance tungsten oxide, which can alter the transformation of
anatase to rutile phase. As TiO2 does not react with WO3 forming chemical bonds, tungsten dioxide is distributed in the titanium dioxide matrix which gives peculiar properties to the compound. Also, WO3 /TiO2 oxides are important compounds for optoelectronics application especially as gas sensors since these materials are greatly sensitive to atmosphere changes [4]. WO3 /TiO2 compounds have been prepared by incipient wet impregnation of ammonium metatungstate – (NH4 )10 W12 O41 ·5H2 O [5] or NH4 OH/tungstic acid [6] aqueous solutions on the commercial TiO2 powder, by sol–gel method [2,3,7], by thermal decomposition of tungstic acid and titanium tetraisopropoxide [8], via hydrothermal template-free route [9], and by ball milling [10]. Although all these preparation methods have focused on the application of WO3 /TiO2 as catalysts; the WO3 /TiO2 system has not been exploited for humidity sensor purpose. This study aimed at obtaining WO3 /TiO2 nanopowders by high energy milling varying the WO3 content and the milling time. The powders were thermal treated either in conventional furnace or in microwave oven and their structural, microstructural, and electric properties were evaluated as a candidate for humidity sensor. 2. Experimental
∗ Corresponding author at: SENCER – Sensores Cerâmicos Ltda, Rua Santos Dumont, 800, 13566-445, São Carlos, SP, Brazil. Tel.: +55 16 3201 2090. E-mail address: [email protected] (S.M. Zanetti). 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.05.048
The starting materials were the commercially available reagents: TiO2 (99%, Certronic), and H2 WO4 (99%, Aldrich). The final compounds have different WO3 contents (1 mol% – W1T, 5 mol%
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– W5T, and 10 mol% – W10T). For the mechanochemical activation, a batch of 10 g of the powder mixture with appropriate weight ratio (as required for each composition) was subjected to mechanical treatment in a vibrating mill (Fritsch, Pulverizette 6), with vial (80 cm−3 capacity) and balls made of zirconia. The ball/powder mass ratio was 6, and the milling was carried out at a rotation speed of 300 rpm during 40 min, 80 min, or 180 min. The samples were dubbed with respect to the WO3 content and the milling time, e.g. the sample with 1 mol% WO3 activated for 40 min has been named W1T40. The powders structure evolution with milling time was followed by X-ray diffraction - XRD (Philips, PW1830) using Cu K␣ radiation and the crystallite size was estimated by Scherrer’s equation using the XRD data. The particle size was obtained from the specific surface measured by N2 adsorption/desorption isotherms (Quantachrome, NOVA 1000). Their structure and microstructure were evaluated by Raman spectroscopy (RENISHAW, U-3000, with a 632.8 nm He–Ne laser coupled with an Olympus microscope) in the range of 300–1000 nm and transmission electron microscopy – TEM (Philips CM 200), respectively. The powders were also characterized by H2 Temperature Programmed Reduction (TPR-H2 ) in a Micrometricis Pulse ChemiSorb 2707 with 5%H2 /N2 and NH3 pulse. EXAFS measurements at the Ti K-edge were carried out at the D08B-XAFS2 beamline of the National Synchroton Light Laboratory (LNLS), Campinas, Brazil. This beamline is equipped with a (1 1 1) silicon channel-cut and the measurements were performed in transmission mode. Athena/Artemis software packages [11] were used to extract the EXAFS signal from the measured absorption spectra [12]. The EXAFS oscillations were fitted in R-space and k2 -weighted Fourier filtering was used. The first shell coordination scattering contribution were used for fitting EXAFS data, ˚ Kaiser–Bessel window and backscattering amplitudes R = 1.0–2.0 A, calculated by using the TiO2 -ana file. The obtained spectra of the sample were calibrated and aligned with the experimental Ti-foil EXAFS. Powders with different compositions and activated for different times were used to prepare pellets with 10 mm diameter and 2 mm thick by uniaxial pressing. The first group of pellets was thermal treated at 600 ◦ C for 2 h in a conventional furnace, at a 10 ◦ C min−1 heating rate. The second group was thermal treated at 600 ◦ C for 20 min in a domestic microwave oven (2.45 GHz, 900 W) by means of a SiC susceptor, at a 50 ◦ C min−1 heating rate. The pellet was placed between two SIC susceptor pieces (above and below the pellet) in order to ensure a uniform pellet heating. The sensor samples were prepared by using silver paste as electrodes in pellets’ both faces. The samples were submitted at different humidity levels (11–97% RH) and their electrical response was measured using a LCZ (PHILIPS, PM 6304) at room temperature and at 1 kHz frequency. The distinct humidity levels were obtained from different salts saturated solutions at room temperature.
3.1. Effect of milling time
Fig. 1. (a) XRD patterns of W5T powder: (a) activated for different times and the starting powder; (b) heat-treated at 600 ◦ C for 20 min in a microwave oven; (c) crystallite size of W5T composition calculated from XRD data using the Scherrer’s equation.
The effect of milling time for all compositions was followed by XRD. All samples showed the same crystalline phases: anatase (JCPDS 21-1272) and WO3 .H2 O (JCPDS 84-886). Fig. 1a shows the results obtained from the composition W5T (5 mol% WO3 ). It has been observed the intensity decreasing and broadening of anatase peaks as the milling time increased, showing the effect on structure distortion caused by the high energy milling. For W1T and W10T compositions the same effect was observed, except for the higher intensity of WO3 -related peaks detected for these compo-
sitions. The powders activated for different times were thermal treated at 600 ◦ C for 2 h in a microwave oven. The XRD patterns have shown only anatase and WO3 (JCPDS 72-1465) as crystalline phases, according to Fig. 1b. The crystallite size of TiO2 (anatase) for W5T composition were estimated from the XRD data and the Scherrer equation for the 25.3◦ peak is displayed in Fig. 1c. For the as-activated powder, it has been observed a linear decrease in crystallite size from 38 nm to 18 nm as
3. Results
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rutile phase were observed. This result agrees fairly well with the XRD findings. So far, from the results obtained it is possible to verify that the thermal treatment in microwave oven is highly effective even when lasting only 20 min. Hence the powders thermal treated in microwave oven had their mean particle size (MPS) obtained from the specific surface area (SSA) measurements by the BET method which are displayed in Table 1. The mean particle size was calculated through the equation: MPS =
Fig. 2. Raman spectra of WO3 /TiO2 pellets: (a) derived from powders of different compositions heat-treated at 600 ◦ C for 20 min in a microwave oven; (b) derived from 80 min-activated powder with 5 mol% WO3 heat-treated at 600 ◦ C either in conventional furnace or microwave oven.
the milling time increased. The same behavior has been observed for the heat-treated powders wherein crystallite size ranged from 39 nm to 30 nm, for powder treated in conventional furnace, and from 38 nm to 28 nm, for powder treated in microwave oven. It has also been verified that the thermal treatment at 600 ◦ C led to an increase in the mean crystallite size with respect to the as-activated powder. Moreover, the increase in crystallite size was higher for the 180 min-activated powder probably due to the high reactivity reached by high energy activation. The Raman spectra of 180 min-activated WT powders (Fig. 2a) showed a frequency shift and broadening of Raman peaks as a result of phonon confinement caused by the decrease in the crystallite dimension. It was observed that the peaks intensity decreased as the milling time increased. The intensity of the peaks related to WO3 (at 715 and 805 cm−1 ) increased as the WO3 content increased, as expected. The Raman peaks at 275 cm−1 and 430 cm−1 correspond to the deformation vibrations of O–W–O bonds and peaks in the 700–803 cm−1 range correspond to the stretching vibrations of W–O bonds [13]. The Raman peaks at 397, 518 and 640 cm−1 can be assigned as the modes of the anatase phase [14]. The thermal treatment in conventional furnace allows an increase in the intensity of Raman peaks compared to the sample treated in microwave oven, as observed for W5T pellets derived from 80 minactivated powders, as displayed in Fig. 2b. No peaks related to the
3000 (nm) SSA
(1)
where SSA is the specific surface area (m2 g−1 ) and is the theoretical density (W1T = 3.93 g/cm3 ; W5T = 4.06 g/cm3 ; W10T = 4.24 g/cm3 ). The SSA of original powders were also measured by the BET method which values were 10.0 m2 g−1 , for TiO2 , and 12.3 m2 g−1 , for H2 WO4 . For W1T composition, the mean particle size increased with the increasing of the activation time, from 27 nm to 145 nm. It is expected agglomeration of nanoparticles after activation due to the increase in surface reactivity, and probably the small WO3 content does not influence this process. For W5T composition, one can observe that the activation during 40 min promotes the highest mean particle size (187 nm) while the lowest one (108 nm) is reached after activation during 180 min. The decrease of the mean particle size with the increasing of the activation time is probably due to a more homogeneous distribution of WO3 on TiO2 matrix (conferred to the extended milling time) avoiding particles to agglomerate in spite of the reduction of the particle size. For W10T composition, it has been observed almost no influence of the milling time in the mean particle size. Probably the high WO3 content in this sample has promoted the formation of WO3 clusters avoiding a homogeneous distribution of WO3 particles in TiO2 matrix. Although its value is still high, the effect of milling time starts to be observed only for the 180 min-activated sample. Perhaps longer milling times could be more effective in reducing the particle size for these samples. Fig. 3 displays the grain morphology images of 180 minactivated powders treated at 600 ◦ C in a conventional furnace obtained by TEM. One can observe fairly agglomerated particles with dimensions ranging from 50 nm to 200 nm for all compositions, as well as a similar porous microstructure. Nevertheless, big agglomerates of smaller particles for W5T and W10T composition can be seen, probably due to the increase in WO3 content. These results agree quite well with MPS values displayed in Table 1. Fig. 4 displays the TPR-H2 profile of 80 min-activated powders treated at 600 ◦ C in a conventional furnace. Small peak of H2 consumption at 640–670 ◦ C is observed for all samples. The reduction peak at 945 ◦ C for W1T shifts to lower temperatures as WO3 content increased; an extra shoulder around 810 ◦ C can observe for W10T. According to Chen et al. [15] all peaks may be attributed to different species of WOx reduction. The shift to lower temperatures along with the shoulder observed for W10T sample may be attributed to the formation of small WO3 clusters, which contribute to activation and, consequently, to spill over of H2 to bigger clusters. All the samples were maintained at 900 ◦ C during TPR-H2 analysis until stabilization of the signal. The area under the curve indicates that all WO3 were reduced. Moreover, one may suppose that WO3 was on TiO2 surface. The acidity, calculated by NH3 pulse chemisorptions, for 80 minactivated samples treated at 600 ◦ C for 2 h in conventional furnace is displayed in Table 2. The acidity increased with the addition of WO3 to the TiO2 sample. With respect to WO3 content, a decrease of sample acidity is observed as WO3 content increased which is much lower for W10T than for W1T. In molar basis, it is possible to observe that W1T sample contains much more acid sites per mol of
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Table 1 Mean particle size of powders with different compositions activated for different times, followed by thermal treatment at 600 ◦ C for 20 min in a microwave oven. W1T
Activation time (min)
40 80 180
W5T
W10T
SSA (m2 g−1 )
MPS (nm)
SSA (m2 g−1 )
MPS (nm)
SSA (m2 g−1 )
MPS (nm)
56.4 12.1 10.5
27 125 145
7.9 11.2 13.7
187 132 108
9.9 9.7 10.9
142 147 129
Fig. 3. MET images of 180 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace.
Fig. 4. TPR-H2 of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace.
WO3 than W5T and W10T samples. Thus for those samples, only a small fraction of WO3 is available for NH3 adsorption which may be attributed to the formation of larger WO3 clusters which decreases the amount of acid sites. Similar results were also observed in [16]. According to Yang et al. [3], it is known that anatase phase is stable because of the oxygen vacancies. If a W ion substitutes a Ti ion, the valence state at Ti4+ neighborhood will be reduced and extra oxygen vacancies along with Ti3+ ions will be created, which in its turn increases the number of these defects in the anatase Table 2 Acidity calculated by NH3 pulse chemisorptions for 80 min-activated samples heattreated at 600 ◦ C for 2 h in a conventional furnace. Sample TiO2 W1T W5T W10T
Acidity (mol NH3 /gsample ) 10.1 151.3 122.8 84.3
Acidity (mol NH3 /mol WO3 ) – 3.5 0.6 0.2
phase. If the acidity of the material has increased by the addition of W, as observed, one may suppose that the Ti3± concentration has increased as well. Nevertheless, for WO3 content higher than 1%, the addition of WO3 causes a decreasing in molar basis acidity (while the total acidity remains high). In the TPR-H2 analysis, for W5T and W10T samples, the most intense peak shifts to lower temperature suggesting the formation of large WO3 particles. It is possible that these large particles block NH3 to access the Ti3± sites and, consequently, blocks also the water vapor. Fig. 5 presents the Fourier transformed of EXAFS oscillations for pure TiO2 and 80 min activated WT samples treated at 600 ◦ C for 2 h in a conventional furnace. The first peak, 1 < R < 2, corresponds to the first oxygen coordination shell around Ti (Ti-O1 at 1.937 A˚ and ˚ and the peak between 2 < R < 3 corresponds mainly Ti-O2 at 1.966 A) ˚ The decreasing of the to Ti–Ti second coordination shell (3.039 A). second peak for WT samples with respect to TiO2 may be attributed to the decreasing of particle size caused by the milling activation. Among the doped samples, the increasing of WO3 content promotes an increasing of first and second coordination shells. Table 3 presents the coordination number (CN), inter-atomic distance (r), Debye-Waller factor ( 2 ), and R factor for the first coordination shell scattering of TiO2 for 80 min activated samples. The EXAFS results indicate that the addition of 1 mol% of WO3 decreases the coordination number in the first shell in comparison to the original TiO2 powder from 3.94 to 2.42. This huge difference may be attributed to a diminishing in crystallite size caused by the
Table 3 Structural parameters obtained from EXAFS for 80 min-activated samples heattreated at 600 ◦ C/2 h in a conventional furnace. Sample
Scattering
CN
TiO2
Ti-O1 Ti-O2 Ti-O1 Ti-O2 Ti-O1 Ti-O2 Ti-O1 Ti-O2
3.94 1.97 2.42 1.21 3.36 1.68 3.58 1.80
W1T W5T W10T
˚ r (A) ± ± ± ± ± ± ± ±
0.06 0.06 0.06 0.06 0.07 0.07 0.07 0.07
1.95 1.86 1.95 1.86 1.95 1.86 1.95 1.86
± ± ± ± ± ± ± ±
0.01 0.02 0.06 0.06 0.04 0.07 0.04 0.07
2 (A˚ 2 )
R factor
0.005
0.03
0.005
0.05
0.005
0.04
0.005
0.04
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Fig. 5. EXAFS oscillations (a, d, g, j), the Re[(q)] function data (b, e, h, k) and the magnitude of the Fourier transformed (c, f, i, l) for TiO2 (a, b, c), W1T (d, e, f), W5T (g, h, i) and W10T (j, k, l) of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace and TiO2 (reference).
activation milling, as observed in XRD patterns. One may verify a significant increasing in the coordination number as the WO3 content ranged from 1 to 5% and almost no change when WO3 increased to 10%. One can infer that WO3 clusters may have been formed, which in its turn diminish the effect of the activation milling on TiO2 particles, in contrast with what was observed for low WO3
content. In spite of the coordination number has decreased with addition of WO3 , the interatomic distance has not changed which may be attributed to the formation of TiO2 nanoparticles without any alteration of the electronic density. The alteration of electronic density could be observed through the variation of white line intensity in XANES region (Fig. 6)
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Fig. 6. XANES region spectra of 80 min-activated WT powders heat-treated at 600 ◦ C for 2 h in a conventional furnace and TiO2 (reference).
which has been focused by many authors [17–19]. Although the addition of WO3 may have promoted the increasing of Ti3+ concentration, no alteration in electronic structure have been observed by XAFS data, which could be explained by the edge chosen for data collection. The Ti K edge is associated with the electronic transition 1s1/2 → 4p1/2 , when the sublevel 4p1/2 is unoccupied. In this way, this electronic transition does not reflect any electronic alteration. The electronic alteration could be observed in Ti L3 edge [20], which is associated with the transition of 2p3/2 → 3d3/2 or 3d5/2 , when the 3d sublevel partially occupied [21]. 3.2. Electrical characterizations Capacitance measurements as a function of the relative humidity were carried out for WT ceramics. Fig. 7 displays the results obtained for samples of different compositions activated for 40 min, 80 min, or 180 min thermal treated at 600 ◦ C in a microwave oven. All samples responded as a humidity sensor but the results were analyzed with respect to the linearity in order to establish its correlation between the activation time and the WO3 content. Although the particle size have increased from 27 to 145 nm as the milling time increased from 40 to 180 min for W1T samples, it can be observed a high sensitivity for RH level below 30%, independently of the milling time. The increase in sensitivity for low humidity levels may be attributed to the greater amount of acid sites achieved in W1T (as observed in Table 2) which promote the formation of H2 O chemisorbed layer. For W5T and W10T samples, the increasing of the milling time provoked a diminishing of the particle size. Nonetheless, the increase in WO3 content causes a decreasing in the molar ratio acidity (although the total acidity remains high) and suppresses the sensibility in low humidity range. Also, as observed in TPR-H2 measurements, higher WO3 contents probably led to the formation of large WO3 particles which may block water vapor to access the Ti3± sites. The effect of particle size in the capacitance behavior as a function of relative humidity may be explained through the physical model based on adsorbed water on the surface and on capillary water condensation within the pores [22–24]. For 40 min-activated W5T and W10T samples a linear response in two segments is displayed indicating that different conduction mechanisms are involved in the phenomenon. Similar behavior is observed for 80 min-activated W10T and 180 min-activated W1T samples. Therefore at high humidity range (above 75%) an
Fig. 7. Capacitance as a function of relative humidity for WT ceramics derived from powders of different compositions activated for different times and heat-treated at 600 ◦ C for 20 min in a microwave oven.
increase in conduction is observed which may be associated to a capillary condensation process. When the capillary condensation occurs, the Warburg impedance may be originated along with the Grotthuss mechanism, and the conduction through the pores is electronic in nature. As for 80 min-activated W5T and 180 minactivated W5T and W10T samples, the electronic conduction is not observed. Fig. 8 shows the results obtained for W5T composition activated for different times. The activation for 180 min promoted a more sensitive curve yet, the response gain is not proportional to more than the double activation time. In order to compare pellets of W5T composition were thermal treated at 600 ◦ C for 2 h in a conventional furnace for electrical characterization. The results, displayed in Fig. 9, show a linear response for both thermal treatments in all RH range. The both presented no hysteresis in the 11–55% range while at RH levels above 55% a little larger hysteresis is observed for the sample heat-treated in a conventional furnace. This low hysteresis
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4. Conclusions WO3 /TiO2 nanopowders have been successfully prepared by high energy milling. XRD and Raman results for powders heattreated at 600 ◦ C indicated no other phase except WO3 and anatase. The addition of WO3 along with high energy activation promotes the reduction of particle size without any alteration of the electronic structure, as indicated by EXAFS. TPR-H2 measurements indicated that WO3 is present mainly on TiO2 surface. For the sample with 5 mol% WO3 , crystallite size and particle size, as a function of the activation time, ranged from 30 to 40 nm and 120 to 150 nm, respectively. All samples responded as a humidity sensor however, optimized preparation conditions were obtained for samples with at least 5 mol% WO3 activated for at least 80 min. Ceramics heattreated in microwave oven for 20 min have shown similar sensor response as those prepared in conventional oven for 120 min which is highly cost effective. These results indicate that WO3 /TiO2 ceramics can be used as a humidity sensor element. Fig. 8. Capacitance as a function of relative humidity for W5T ceramics activated for different times and heat-treated at 600 ◦ C for 20 min in a microwave oven.
Acknowledgements observed for W5T in all RH range is probably due to interparticle pores which are large sufficiently to enable the diffusion of water vapor. From these results, one can infer that the thermal treatment in microwave oven is highly indicated for preparation of porous WO3 /TiO2 ceramics.
The authors gratefully acknowledge the financial support of FAPESP through the Project JP 02/09497-0, DEQ-UFSCar for providing the apparatus for TPR-H2 analysis, LNLS/Campinas-SP for the XAFS analysis at beam line, and Dr. J. A. J. Rodrigues, coordinator of LCP/INPE/Cachoeira Paulista-SP, for NH3 pulse chemisorption analysis. References
Fig. 9. Capacitance hysteresis for 80 min-activated W5T ceramics heat-treated in: (a) a microwave oven for 20 min; (b) a conventional furnace for 2 h.
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Biographies Kleper de Oliveira Rocha He received his PhD degree in Chemical Engineering from UFSCar–São Carlos, Brazil, in 2009. Currently, he is a researcher at SENCER LTDA. His current interest is ceramic oxides applied as humidity and gaseous sensor.
Sonia Maria Zanetti She received her PhD degree in Chemistry from UFSCar–São Carlos, Brazil, in 2001. She is a visiting researcher at IQ/UNESP–Araraquara-SP and executive manager at SENCER LTDA. Her work has been mainly focused on ceramics applied as sensor