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Surface composition of alkali-dope TiO2 films for sensors investigated by XPS
B
CHlS4lCAL
Sensors and Actuators B 24-2.5 (1995) 886-888
Surface composition of alkali-doped Ti02 films for sensors investigated by XPS Roberto
Zanoni a, Guido Righini b, Angelo Montenero ‘, Guglielmina Gnappi d, Andrea Bearzotti e, Giampiero Montesperelli ‘, Enrico Traversa f aDipiimento
di Chimica, Universild!di Roma ‘La Sapienza~ P.k. A. More 5, 00185 Rome, Italy
b Servizio ESCA, National Research Council of Italy (CNR), Area della Ricerca di Roma, CP 10, 00016 Monterotondo Stazione, Rome, Ita& ’Diprtimenfo di Chimica Gene&e e Inorganica, Chimica Analitica, Chimico Firica, Universitd di Parma, viale delle Science, 43100 Parma, Italy ’ CO.R.I.vE, viak delle Science, 43100 Parma, Italy
’Diprtimento
’Istitito di Elettronica deilo Stat0 Solid0 (IESS), CNR, via Cineto Romano 42, 00156 Rome, It& di Science e Tecnologie Chimiche, Universitd di Roma ‘To? Verpata: via della Ricerca Scientifica, 00133 Rome, Ita&
Abstract X-ray photoelectron spectroscopy (XPS) results are presented on a series of alkali-doped and undoped TiO, films prepared by the sol-gel technique. TiOz films are deposited onto AIrOr and quartz substrates, using titanium isopropylalkoxide as the precursor, by dip-coating from the appropriate solutions. After dipping, the films are dried at 100 “C for one day and then heated to 300 “C for 30 min. The surface composition of the films is discussed, and the effects of Li, Na and K doping are compared. The effect of doping on the humidity-sensitive electrical properties of the films is evaluated by measuring the current flowing at various relative-humidity values upon application of a constant d.c. voltage. Keywordr.
Humidity sensors; Titania; X-ray photoelectron spectroscopy
ions on the electrical response of prototype devices with TiO, films as the active element.
1. Introduction TiO, shows interesting properties as a functional material, and has been used for the detection of various gases [l-4] and of humidity [5,6]. The current trend in sensor fabrication is towards integration and miniaturization, giving rise to interest in the production of ceramic thin films. The preparation of very uniform and highly pure TiO, films for humidity sensors has recently been achieved via the sol-gel process [7,8]. The addition of alkali atoms has been reported to enhance the humidity-sensitive electrical properties of sintered TiO, [9,10]. The addition of alkali also improved the performance of sol-gel processed TiOz thin films
WI* In this investigation we used XPS to probe the surface composition of alkali-doped and undoped TiO, films prepared by the sol-gel technique on different substrates. Our investigation of Na addition has been reported in a previous paper [12], and here we have completed our study of the addition of Li and K. We have studied the effect of doping with different alkali 09254005/9S/$O9.50 B 1995 Elsevier SSDI 0925-4005[94)01487-3
Science
S.A. All rights reserved
2. Experimental Titania films were prepared using titanium isopropylalkoxide (TIPT) as the precursor compound in the sol-gel process. Alkali-doped TiO, films were prepared by adding an appropriate volume of LiNO,, CH,COONa or CH,COOK to the TIPT mixed solution, to a final content of alkali equal to 6 at.%. Na-free quartz slides or alumina wafers were used as the substrates for the deposition, which was performed by dip-coating. The films were then dried at 100 “C for one day and successively heated in an oven for 1 h at 300 “C. XPS spectra were run on a Vacuum Generators BSCALAB spectrometer. The binding energy (BE) scale was calibrated by taking the Au 4f,, peak at 84.0 eV. Correction of the energy shifts due to static charging was accomplished by referencing to the C Is line from the residual pump-line oil contamination, taken at 285.0 eV.
R. Zanoni et al. I Sensors and Achdon
For electrical measurements, prototype devices were prepared by depositing the ceramic films on alumina substrates with comb-type Au electrodes, submitted to the same treatments as the films used for XPS analysis. The electrical response of the sensing elements was analysed by measuring the dependence of resistivity on relative humidity (rh). The current upon application of a constant d.c. bias was recorded whilst increasing the rh gradually from 2 to 90%, at a constant rate of 1% rh min-‘.
3. Results and discussion XPS results are reported in Table 1. The results for the TiO, films on AIZO, and quartz substrates showed some minor but significant differences. In both samples, a symmetrical Ti 2~~~ peak was obtained. The Ti 2p,, BEs of the centre of the band corresponded to those of the Ti4’ oxidation state. The shape of the Ti 2p peaks excludes the presence of traceable amounts of Ti2+ or Ti’, which are separated by more than -3 eV from the Ti4+ peak [13], while some TT’ oxides/ hydroxides could be present, on the basis of their limited relative shift from TiOz [14]. The corresponding 0 1s bands can be fitted with (a minimum of’)two components. Following the relevant literature 1151, the lower BE component is assigned to 02- in Ti4’ oxidic species, while the higher BE component falls within the typical BE range reported for various oxygenated species. 0 1s BE for Tiz03, Ti hydroxides or metal alkoxides can contribute to this signal due to the temperature of the heat treatment. The O/Ti atomic ratios, based on the 0 1s component at the lower BE, is lower than expected for a perfectly stoichiometric TiO, surface. Table 1 XPS binding energies and elemental atomic ratios for reported samples (M=Li, Na, K). RAR values are the relative atomic ratios among 0 1s peak components, from curve fitting Sample
Ti 2p
01s
RAR
Ofli
Undoped film on quartz
458.9
530.4 531.7
73 27
1.77
6% Li-doped film on quartz
459.0
530.4 531.1
71 29
1.90
1.6 (1.7) 8
6% Na-doped film on quartz
458.8
530.3 531.7
75 25
2.04
0.10 (0.04) a
6% K-doped film on quartz
458.8
530.3 531.6
79 21
1.78
0.08 (0.07) a
Undopcd film on alumina
458.9
530.4 531.8
61 33
1.83
6% K-doped film on alumina
458.8
530.3 531.7
64 36
1.80
M/C
0.12 (0.09) a
‘After 4 min AI+ ion etching conducted at 2 kV, 200 PA, lo-’ mbar.
B 24-25 (1995) 886-888
887
In the case of films on alumina, the substrate peaks were detectable, in contrast with the films on quartz. A previous morphological characterization of the 300 “C undoped films, conducted by SEM, showed that while the films deposited onto quartz were compact and continuous, the films deposited on alumina showed some cavities. This was because the surface of the alumina substrates that we used was rough, and during dipping the film was not able to cover completely the roughness of the substrate. As a consequence of the presence of cavities in the alumina-supported films, the relative percentages of the two peak components under the 0 Is signal are moderately but reproducibly different in the two types of films. An analysis of Ti 2p and Ti Auger spectra, coupled with the relative quantitative results, showed clearly that the 300 “C film surfaces were defective. Surface defects in TiO, are mainly due to the bond breaking of bridging 0 atoms, resulting in Ti3* species and 0 vacancies [16]. We have observed experimental evidence for both effects. The shake-up satellites accompanying Ti 2p ionization peaks have not been considered in the O/Ti calculation. The O/I? ratios are, as a consequence, systematically overestimated. Therefore, if we exclude a systematic error in the sensitivity factors employed, the quantitative results also favour O-defective surfaces. Minor differences between single samples are within *lo%, the error typically associated with the calculation method of XPS relative quantitative measurements we used. There were only minor differences between doped and undoped sets of films, in so far as the Ti and 0 ionization peaks are concerned. Therefore, the above discussion of these two peaks also fits for doped films. In the series of doped samples, a large surface enrichment was found in all the investigated samples. The effect is greater for Li, and follows the relative differences in ionic mobility for the alkali elements: Li> Na > K. In the case of K-doped samples, where a comparison can be drawn between the two substrates, the surface enrichment was slightly higher for the film supported on alumina. Li-doped samples presented the most homogeneous surface composition in the series, as evidenced by Ar+ ion etching. Experimental relative Ar+ sputtering yields for Li, Na, and K are not available. For the heavier alkali metals, the yields are close. In this case, we cannot exclude differential sputtering effects. Fig. 1 shows the rh dependence of the current for the undoped and 6 at.% Na- and K-doped TiO, films annealed at 300 “C!. The behaviour was different depending on the doping element. The results of the films doped with 6 at.% Li are not reported in Fig. 1 because they showed a very high resistivity, higher than 1Ol4R, which only changed very slightly with increasing rh. The intrinsic resistance of the films doped with Na
R. Zanoni et al. I Sensors and Actuators B 24-25 (1995) 886-888
888
Acknowledgements This work was supported by the National Research Council of Italy (CNR), under the auspices of the Targeted Project ‘Special Materials for Advanced Technologies’.
7 z $
-6atl Na -...- 6 at% K 1
0
, 20
I
I
40 60 rh (%j
80
I loo
Fig. 1. The relative humidity (rh) dependence of the current of undoped and 6 at.% alkali (Na, Q-doped TiOz films annealed at 300 “C, measured with 10 V d.c., at room temperature.
was smaller than that of the films doped with K, and the intrinsic resistance of the undoped films was intermediate. However, the films doped with K showed the largest variation of resistivity over the entire rh range tested, which was of about three orders of magnitude. The variation of the logarithm of the resistivity with rh was linear up to about 80% rh. Above this rh value, a larger increase in current was observed. It is possible that the overall current sensitivity over the entire rh range of the doped TiO, films is related to surface enrichment: the larger the surface enrichment of alkali ions, the worse the humidity-sensing properties of the films. The effect of Li addition at 6 at.% was surprisingly negative, while Na and K addition at the same concentration had a positive, yet limited, effect. Electrical measurements showed, however, that the humidity-sensitive electrical response of doped TiO, films was dependent on the dopant concentration [ll]. Further XPS measurements are in progress to clarify the influence of the alkali concentration on the sensing properties of the TiO, films.
4. Conclusions
The surface composition of sol-gel processed undoped and alkali-doped TiO, films was investigated in detail by XPS. The surface of TiO, films was found to be sub-stoichiometric. The presence of surface defects was clearly shown and their nature discussed. TiO, films on quartz are continuous, while cavities are present on films formed on Al,O,. A comparison between differently doped samples showed different surface enrichment, which is definitely larger for the case of Li, and follows the different ionic mobility values of the alkali ions. The enrichment for Li ions is deeper than for Na and K.
References 111 S.R. Morrison, Semiconductor gassensors, Serrrors andActuators, 2 (1982) 329-341.
121 W. Gopel, U. Kimer, H.D. Wiemhijfer and G. Rocker, Surface and bulk properties of TiOa in relation to sensor applications, Solid State lonics, 28-30 (1988) 1423-1430. r31 KD. Schierbaum, U.K. Kimer, J.F. Geiger and W. Gopel, Schottky-barrier and conductivity gas sensors based upon Pd/ SnOl and Pt,‘Ti02, Senrors and Actunfors B, 4 (1991) 87-94. [41 R.M. Geatches, A.V. Chadwick and J.D. Wright, Single-crystal metal oxide gas sensors, Sensors and Actuators L?, 4 (1991) 467472. PI Y.C. Yeh, T.Y. Tseng and D.A. Chang, Electrical properties of porous titania ceramic humidity sensors, I. Am. &ram. Sot., 72 (1989) 1472-1475. 161 K. Katayama, K. Hasegawa, T. Takahashi, T. Akiba and H. Yanagida, Humidity-sensitivity of NbaOs-doped TiO* ceramics, Sensors and Actuators A, 24 (1990) 55-60. [71 N. Yoshimura, S. Sate, M. ltoi and H. Taguchi, Electrical and humidity sensitive characteristics of TiG, and TiOrSnOz films prepared by sol-gel method, Sozai Buweigaku Zasshi, 3 (1990) PI
41-56. G. Gusmano, G. Montesperelli, P. Nunziante, E. Traversa, A.
Montenero, M. Braghini, G. Mattogno and A. Bearzotti, Humidity-sensitive properties of titania films prepared using the sol-gel process, J. C&am. Sot. Jprz., 101 (1993) 1095-1100. PI K. Katayama, K. Hasegawa, T. Noda, T. Akiba and H. Yanagida, Effect of alkaline oxide addition on the humidity-sensitivity of Nb,O,-doped TiO,, Sensors and Actuators B, 2 (1990) 143149. PJI Y.C. Yeh, T.Y. Tseng and D.A. Chang, Electrical properties of TiOrKzT&,0t3 porous ceramic humidity sensor,J. Am. Ceram. Sm., 73 (1990) 1992-1998. WI G. Montespcrelli, A. Pumo, E. Traversa, G. Gusmano, A. Bearzotti, A. Montenero and G. Gnappi, Sol-gel processed TiOa-based thin films as innovative humidity sensors, Sensors and Actuators B, 24-25 (1995) 705-709. WI R. Zanoni, G. Righini, A. Montenero, G. Gnappi, G. Montesperelli, E. Travena and G. Gusmano, XPS analysis of sol-gel processed doped and. undoped TiOz films for sensors, Su$ ~nferjfuceA&., 22 (1994) 376. [I31 W. Gopal, J.A. Anderson, D. Frankel, M. Jaehnig, K Phillips, J.A. Schafer and G. Rocker, Surface defects of TiO,(llO): a combined XPS, XAES and ELS study, Surjirce Sci., 139 (1984) 333-346. [I41 G. Lewis, X-ray photoelectron study of surface layers on orthopaedic alloys, I. Ti-6Al-4V (ASTM F-136), J. Vat. Sci. Tecknol., All (1993) 325-335. WI C.D. Wagner, in D. Briggs and M.P. Seah (eds.), Practical Surface Anal@, Vol. 1, Wiley, Chichester, 2nd edn., 1990, Appendix 5. PI J.-M. Pan, B.L. Maschhoff, U. Diebold and T.E. Madey, Interaction of water, oxygen, and hydmgen withTiOz(l 10) surfaces having different defect densities, J. Vat. Sci. TecknoL, AI0 (1932) 2470-2476.
CHlS4lCAL
Sensors and Actuators B 24-2.5 (1995) 886-888
Surface composition of alkali-doped Ti02 films for sensors investigated by XPS Roberto
Zanoni a, Guido Righini b, Angelo Montenero ‘, Guglielmina Gnappi d, Andrea Bearzotti e, Giampiero Montesperelli ‘, Enrico Traversa f aDipiimento
di Chimica, Universild!di Roma ‘La Sapienza~ P.k. A. More 5, 00185 Rome, Italy
b Servizio ESCA, National Research Council of Italy (CNR), Area della Ricerca di Roma, CP 10, 00016 Monterotondo Stazione, Rome, Ita& ’Diprtimenfo di Chimica Gene&e e Inorganica, Chimica Analitica, Chimico Firica, Universitd di Parma, viale delle Science, 43100 Parma, Italy ’ CO.R.I.vE, viak delle Science, 43100 Parma, Italy
’Diprtimento
’Istitito di Elettronica deilo Stat0 Solid0 (IESS), CNR, via Cineto Romano 42, 00156 Rome, It& di Science e Tecnologie Chimiche, Universitd di Roma ‘To? Verpata: via della Ricerca Scientifica, 00133 Rome, Ita&
Abstract X-ray photoelectron spectroscopy (XPS) results are presented on a series of alkali-doped and undoped TiO, films prepared by the sol-gel technique. TiOz films are deposited onto AIrOr and quartz substrates, using titanium isopropylalkoxide as the precursor, by dip-coating from the appropriate solutions. After dipping, the films are dried at 100 “C for one day and then heated to 300 “C for 30 min. The surface composition of the films is discussed, and the effects of Li, Na and K doping are compared. The effect of doping on the humidity-sensitive electrical properties of the films is evaluated by measuring the current flowing at various relative-humidity values upon application of a constant d.c. voltage. Keywordr.
Humidity sensors; Titania; X-ray photoelectron spectroscopy
ions on the electrical response of prototype devices with TiO, films as the active element.
1. Introduction TiO, shows interesting properties as a functional material, and has been used for the detection of various gases [l-4] and of humidity [5,6]. The current trend in sensor fabrication is towards integration and miniaturization, giving rise to interest in the production of ceramic thin films. The preparation of very uniform and highly pure TiO, films for humidity sensors has recently been achieved via the sol-gel process [7,8]. The addition of alkali atoms has been reported to enhance the humidity-sensitive electrical properties of sintered TiO, [9,10]. The addition of alkali also improved the performance of sol-gel processed TiOz thin films
WI* In this investigation we used XPS to probe the surface composition of alkali-doped and undoped TiO, films prepared by the sol-gel technique on different substrates. Our investigation of Na addition has been reported in a previous paper [12], and here we have completed our study of the addition of Li and K. We have studied the effect of doping with different alkali 09254005/9S/$O9.50 B 1995 Elsevier SSDI 0925-4005[94)01487-3
Science
S.A. All rights reserved
2. Experimental Titania films were prepared using titanium isopropylalkoxide (TIPT) as the precursor compound in the sol-gel process. Alkali-doped TiO, films were prepared by adding an appropriate volume of LiNO,, CH,COONa or CH,COOK to the TIPT mixed solution, to a final content of alkali equal to 6 at.%. Na-free quartz slides or alumina wafers were used as the substrates for the deposition, which was performed by dip-coating. The films were then dried at 100 “C for one day and successively heated in an oven for 1 h at 300 “C. XPS spectra were run on a Vacuum Generators BSCALAB spectrometer. The binding energy (BE) scale was calibrated by taking the Au 4f,, peak at 84.0 eV. Correction of the energy shifts due to static charging was accomplished by referencing to the C Is line from the residual pump-line oil contamination, taken at 285.0 eV.
R. Zanoni et al. I Sensors and Achdon
For electrical measurements, prototype devices were prepared by depositing the ceramic films on alumina substrates with comb-type Au electrodes, submitted to the same treatments as the films used for XPS analysis. The electrical response of the sensing elements was analysed by measuring the dependence of resistivity on relative humidity (rh). The current upon application of a constant d.c. bias was recorded whilst increasing the rh gradually from 2 to 90%, at a constant rate of 1% rh min-‘.
3. Results and discussion XPS results are reported in Table 1. The results for the TiO, films on AIZO, and quartz substrates showed some minor but significant differences. In both samples, a symmetrical Ti 2~~~ peak was obtained. The Ti 2p,, BEs of the centre of the band corresponded to those of the Ti4’ oxidation state. The shape of the Ti 2p peaks excludes the presence of traceable amounts of Ti2+ or Ti’, which are separated by more than -3 eV from the Ti4+ peak [13], while some TT’ oxides/ hydroxides could be present, on the basis of their limited relative shift from TiOz [14]. The corresponding 0 1s bands can be fitted with (a minimum of’)two components. Following the relevant literature 1151, the lower BE component is assigned to 02- in Ti4’ oxidic species, while the higher BE component falls within the typical BE range reported for various oxygenated species. 0 1s BE for Tiz03, Ti hydroxides or metal alkoxides can contribute to this signal due to the temperature of the heat treatment. The O/Ti atomic ratios, based on the 0 1s component at the lower BE, is lower than expected for a perfectly stoichiometric TiO, surface. Table 1 XPS binding energies and elemental atomic ratios for reported samples (M=Li, Na, K). RAR values are the relative atomic ratios among 0 1s peak components, from curve fitting Sample
Ti 2p
01s
RAR
Ofli
Undoped film on quartz
458.9
530.4 531.7
73 27
1.77
6% Li-doped film on quartz
459.0
530.4 531.1
71 29
1.90
1.6 (1.7) 8
6% Na-doped film on quartz
458.8
530.3 531.7
75 25
2.04
0.10 (0.04) a
6% K-doped film on quartz
458.8
530.3 531.6
79 21
1.78
0.08 (0.07) a
Undopcd film on alumina
458.9
530.4 531.8
61 33
1.83
6% K-doped film on alumina
458.8
530.3 531.7
64 36
1.80
M/C
0.12 (0.09) a
‘After 4 min AI+ ion etching conducted at 2 kV, 200 PA, lo-’ mbar.
B 24-25 (1995) 886-888
887
In the case of films on alumina, the substrate peaks were detectable, in contrast with the films on quartz. A previous morphological characterization of the 300 “C undoped films, conducted by SEM, showed that while the films deposited onto quartz were compact and continuous, the films deposited on alumina showed some cavities. This was because the surface of the alumina substrates that we used was rough, and during dipping the film was not able to cover completely the roughness of the substrate. As a consequence of the presence of cavities in the alumina-supported films, the relative percentages of the two peak components under the 0 Is signal are moderately but reproducibly different in the two types of films. An analysis of Ti 2p and Ti Auger spectra, coupled with the relative quantitative results, showed clearly that the 300 “C film surfaces were defective. Surface defects in TiO, are mainly due to the bond breaking of bridging 0 atoms, resulting in Ti3* species and 0 vacancies [16]. We have observed experimental evidence for both effects. The shake-up satellites accompanying Ti 2p ionization peaks have not been considered in the O/Ti calculation. The O/I? ratios are, as a consequence, systematically overestimated. Therefore, if we exclude a systematic error in the sensitivity factors employed, the quantitative results also favour O-defective surfaces. Minor differences between single samples are within *lo%, the error typically associated with the calculation method of XPS relative quantitative measurements we used. There were only minor differences between doped and undoped sets of films, in so far as the Ti and 0 ionization peaks are concerned. Therefore, the above discussion of these two peaks also fits for doped films. In the series of doped samples, a large surface enrichment was found in all the investigated samples. The effect is greater for Li, and follows the relative differences in ionic mobility for the alkali elements: Li> Na > K. In the case of K-doped samples, where a comparison can be drawn between the two substrates, the surface enrichment was slightly higher for the film supported on alumina. Li-doped samples presented the most homogeneous surface composition in the series, as evidenced by Ar+ ion etching. Experimental relative Ar+ sputtering yields for Li, Na, and K are not available. For the heavier alkali metals, the yields are close. In this case, we cannot exclude differential sputtering effects. Fig. 1 shows the rh dependence of the current for the undoped and 6 at.% Na- and K-doped TiO, films annealed at 300 “C!. The behaviour was different depending on the doping element. The results of the films doped with 6 at.% Li are not reported in Fig. 1 because they showed a very high resistivity, higher than 1Ol4R, which only changed very slightly with increasing rh. The intrinsic resistance of the films doped with Na
R. Zanoni et al. I Sensors and Actuators B 24-25 (1995) 886-888
888
Acknowledgements This work was supported by the National Research Council of Italy (CNR), under the auspices of the Targeted Project ‘Special Materials for Advanced Technologies’.
7 z $
-6atl Na -...- 6 at% K 1
0
, 20
I
I
40 60 rh (%j
80
I loo
Fig. 1. The relative humidity (rh) dependence of the current of undoped and 6 at.% alkali (Na, Q-doped TiOz films annealed at 300 “C, measured with 10 V d.c., at room temperature.
was smaller than that of the films doped with K, and the intrinsic resistance of the undoped films was intermediate. However, the films doped with K showed the largest variation of resistivity over the entire rh range tested, which was of about three orders of magnitude. The variation of the logarithm of the resistivity with rh was linear up to about 80% rh. Above this rh value, a larger increase in current was observed. It is possible that the overall current sensitivity over the entire rh range of the doped TiO, films is related to surface enrichment: the larger the surface enrichment of alkali ions, the worse the humidity-sensing properties of the films. The effect of Li addition at 6 at.% was surprisingly negative, while Na and K addition at the same concentration had a positive, yet limited, effect. Electrical measurements showed, however, that the humidity-sensitive electrical response of doped TiO, films was dependent on the dopant concentration [ll]. Further XPS measurements are in progress to clarify the influence of the alkali concentration on the sensing properties of the TiO, films.
4. Conclusions
The surface composition of sol-gel processed undoped and alkali-doped TiO, films was investigated in detail by XPS. The surface of TiO, films was found to be sub-stoichiometric. The presence of surface defects was clearly shown and their nature discussed. TiO, films on quartz are continuous, while cavities are present on films formed on Al,O,. A comparison between differently doped samples showed different surface enrichment, which is definitely larger for the case of Li, and follows the different ionic mobility values of the alkali ions. The enrichment for Li ions is deeper than for Na and K.
References 111 S.R. Morrison, Semiconductor gassensors, Serrrors andActuators, 2 (1982) 329-341.
121 W. Gopel, U. Kimer, H.D. Wiemhijfer and G. Rocker, Surface and bulk properties of TiOa in relation to sensor applications, Solid State lonics, 28-30 (1988) 1423-1430. r31 KD. Schierbaum, U.K. Kimer, J.F. Geiger and W. Gopel, Schottky-barrier and conductivity gas sensors based upon Pd/ SnOl and Pt,‘Ti02, Senrors and Actunfors B, 4 (1991) 87-94. [41 R.M. Geatches, A.V. Chadwick and J.D. Wright, Single-crystal metal oxide gas sensors, Sensors and Actuators L?, 4 (1991) 467472. PI Y.C. Yeh, T.Y. Tseng and D.A. Chang, Electrical properties of porous titania ceramic humidity sensors, I. Am. &ram. Sot., 72 (1989) 1472-1475. 161 K. Katayama, K. Hasegawa, T. Takahashi, T. Akiba and H. Yanagida, Humidity-sensitivity of NbaOs-doped TiO* ceramics, Sensors and Actuators A, 24 (1990) 55-60. [71 N. Yoshimura, S. Sate, M. ltoi and H. Taguchi, Electrical and humidity sensitive characteristics of TiG, and TiOrSnOz films prepared by sol-gel method, Sozai Buweigaku Zasshi, 3 (1990) PI
41-56. G. Gusmano, G. Montesperelli, P. Nunziante, E. Traversa, A.
Montenero, M. Braghini, G. Mattogno and A. Bearzotti, Humidity-sensitive properties of titania films prepared using the sol-gel process, J. C&am. Sot. Jprz., 101 (1993) 1095-1100. PI K. Katayama, K. Hasegawa, T. Noda, T. Akiba and H. Yanagida, Effect of alkaline oxide addition on the humidity-sensitivity of Nb,O,-doped TiO,, Sensors and Actuators B, 2 (1990) 143149. PJI Y.C. Yeh, T.Y. Tseng and D.A. Chang, Electrical properties of TiOrKzT&,0t3 porous ceramic humidity sensor,J. Am. Ceram. Sm., 73 (1990) 1992-1998. WI G. Montespcrelli, A. Pumo, E. Traversa, G. Gusmano, A. Bearzotti, A. Montenero and G. Gnappi, Sol-gel processed TiOa-based thin films as innovative humidity sensors, Sensors and Actuators B, 24-25 (1995) 705-709. WI R. Zanoni, G. Righini, A. Montenero, G. Gnappi, G. Montesperelli, E. Travena and G. Gusmano, XPS analysis of sol-gel processed doped and. undoped TiOz films for sensors, Su$ ~nferjfuceA&., 22 (1994) 376. [I31 W. Gopal, J.A. Anderson, D. Frankel, M. Jaehnig, K Phillips, J.A. Schafer and G. Rocker, Surface defects of TiO,(llO): a combined XPS, XAES and ELS study, Surjirce Sci., 139 (1984) 333-346. [I41 G. Lewis, X-ray photoelectron study of surface layers on orthopaedic alloys, I. Ti-6Al-4V (ASTM F-136), J. Vat. Sci. Tecknol., All (1993) 325-335. WI C.D. Wagner, in D. Briggs and M.P. Seah (eds.), Practical Surface Anal@, Vol. 1, Wiley, Chichester, 2nd edn., 1990, Appendix 5. PI J.-M. Pan, B.L. Maschhoff, U. Diebold and T.E. Madey, Interaction of water, oxygen, and hydmgen withTiOz(l 10) surfaces having different defect densities, J. Vat. Sci. TecknoL, AI0 (1932) 2470-2476.