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Electrical characterization of nanocrystalline titania—I: (Impedance spectroscopy studies between 300 K and 473 K)
I~AIi'IllU[II~S
ELSEVIER
A
Materials Science and Engineering A204 (1995) 258 266
Electrical characterization of nanocrystalline titania I: (Impedance spectroscopy studies between 300 K and 473 K) S i d d h a r t h a B h o w m i k , a Kristen P. C o n s t a n t a,* J o h n C. Parker, b M. Ali b ~Department of Materials Science and Engineering, Iowa State University, Ames, IA 5001 l, USA bNanophase Technologies, Darien, IL 60561. USA
Abstract
The electrical properties of nanocrystalline titania with gold electrodes were characterized using impedance spectroscopy in the frequency range 10 -2 to 106 Hz and temperature range 300 K to 500 K. An attempt has been made to correlate the microstructural properties of these specimens to the electrical response of the material over the different ranges of temperatures. The results indicate that the conductivity of nanocrystalline titania is dependent on the porosity, grain size, and grain boundaries. The impedance and polarization behavior of the samples, especially at the electrodes, is also affected by the humidity. The electrical behavior of the nanocrystalline material makes it a candidate for use as a low temperature gas sensor. Keywords: Electrical characterisation; Nanocrystalline titania; Impedance spectroscopy
1. Introduction Nanocrystalline materials have in recent years been of interest because of the possibility to generate properties that may differ significantly from the microcrystalline material. These differences may arise from the smaller particle size, increased grain boundaries, a larger fraction of atoms residing at these interfaces, and differences in the porosity of the nanocrystalline materials. Differences may also originate due to the high purity of the nanocrystalline material compared with the microcrystalline form. The defect structure of microcrystalline titanium oxide has been studied in detail [1 3] in attempts to describe the conductivity mechanism. These studies included measurements of the d.c. conductivity at high temperatures and the possibility of using titania as an oxygen gas sensor. An attempt was made by Azad et al. [4] to characterize microcrystalline titania using impedance spectroscopy at room temperature. Impedance spectroscopy (IS) has been used to study the electrical properties of ceramics since Baurle's [5] work with zirconia, where he showed that the effects of * Corresponding author.
electrode, grain interiors and grain boundaries could be resolved in the admittance plane or the impedance plane. IS has since proven to be an effective method in evaluating the microstructural effects on conductivity when combined with other methods of characterization. Characterization of microstructural effects on conductivity is essential in the development of materials for use in electronic components. In the case of ceramic materials, impedance, Z, usually arises due to capacitive reactance, Xc, and resistance, R. For a capacitor, the capacitive reactance, X,, is the opposition to current: X c = l/~oC,
(l)
where v ) = v/2n, v is the frequency, and C the capacitance. Z is a complex quantity and can be described [6] in terms of various related functions such as admittance, Y = 1/Z,
(2)
and complex dielectric permittivity, ¢ = Y/jcoC= ¢' - j¢",
(3)
where j = x / ( - 1). In capacitors [7] having dielectric losses, the loss tangent, 0921-5093/95/$09.50 © 1995
Elsevier Science S.A. All rights reserved SSD1 0921-5093(95)09971-9
259
S. B h o ~ m i k et al. / Materials Science and Eng#Teering A 2 0 4 (1995) 258 266
Table 1 The density and cell constant, Ko, of nanocrystalline and microcrystalline TiO, Sample
Nanocrystalline Nanocrystalline Nanocrystalline Nanocrystalline Nanocrystalline Microcrystalline
TiOx TiO~ TiO 2 TiO~ TiO 2 TiO~
Calcination temperature (°C)
Soak time (h)
Bulk density (gcm 3)
True density (pycnometer) (g cm ~)
Ko ~' (cm ~)
Structure ~
NA (Green) 400°C 500°C 650°C 800°C 800°C
NA u 4 4 4 4 4
~2.2 ~2.3 ~2.3 ~2.8 ~ 3.6 ~3.4
44.1 ~4.1 ~4.1
NA 0.215 0.185 0.165 0.340 0.227
Dominantly rutile: some anatase NA NA NA NA Ruffle ~ 80% Anatase ~ 20%
~4.1
~' K o - d / A , where d - thickness of pellet and A = area of applied electrode.
b NA = not available/applicable. c Structures obtained from Ref. [12] indicate the dominance of rutile in all these cases.
tan(5 = e"/e',
(4)
is used as an estimate of the power dissipation. For capacitors used for charge storage the value of tang is usually low. Sensors can be classified [8] according to the basis of the generation of the charge carrier and its transport. Accordingly, the sensing mechanism may be due to bulk effects, i.e. the physical properties of the grain itself, grain boundary effects, as in the case of PTC thermistors, or through surface absorption, as in the case of humidity sensors. In characterizing the electrical properties of microcrystalline titania, the emphasis has been in understanding the bulk or grain interior conductivity. However, in the case of nanocrystalline titania, the effect of grain interior effects is unlikely to be the sole effective mechanism. Porosity and grain boundaries are expected to significantly affect the properties. The purpose of this paper is to discuss the experimental results in terms of porosity, grain size and electrodes and the possible use of nanocrystalline titania as an effective low temperature chemical sensor.
2. Experimental procedure Nanocrystalline TiO2 was prepared through inert gas phase condensation [9,10]. In this method atomic clusters of TiO 2 were first formed by the evaporation of Ti in a pure He gas. The Ti vapor was then condensed in a liquid N2 cold finger and oxidized with the rapid introduction of oxygen into the chamber to form TiO2. The loose powders were compacted uniaxialty, with pressures ranging from 15,000 to 20,000 psi. The pressure used is an important factor as the pressure is responsible for the breakdown of the particle agglomerates. Test samples were usually ~ 1 cm in diameter and 0.1 cm in thickness in the green state. The pellets formed were dry pressed with a light coating of stearic acid on the punch to facilitate mold release. To avoid
lamination cracks, care was taken in expelling the pellets. The green nanocrystalline samples were calcined in air at temperatures between 400°C and 800°C in a tube furnace, with a soak time of 4 h (see Table 1). Calcining at 400°C improved the strength of the samples for handling purposes. Higher temperature calcining of the nanocrystalline TiOz enabled examination of the grain growth and porosity on the electrical measurements. Bulk density measurements and 'true density' were measured using pycnometery to monitor changes in porosity. The evolution of the pore structure was examined as a function of temperature and calcination time. Microstructural studies were done in parallel with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to see the corresponding increase in grain sizes. Since the impedance measurements done to date have been at temperatures much below the calcining temperatures, grain growth was not expected to occur during impedance measurements. Microcrystalline TiO2 (Aldrich Chemicals, 99.999%) was pressed between pressures ranging from 14,000 to 20,000 psi and the pellets formed were calcined in air at 800°C for 4 h (soak time). X-ray diffraction (XRD) measurements were done to indicate the phases present at this stage. Electrodes were applied to all the calcined specimens, which were sputter-coated (Polaron SC 502, Sputter Coater) with high purity gold. Sputter-coating ensures the formation of porous electrodes; additionally, all the parameters were kept constant to form electrodes of equal thickness. The application and monitoring of the electrodes are important factors since they play an important role in impedance measurements. Impedance measurements were performed using a Solarton 1260 Impedance Gain-Phase Analyzer (Schlumberger Technologies, Billerica, MA). The measurements were done in an apparatus designed to take into account the characteristic impedance of the instru-
260
S. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258 266 400000
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1000000
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Fig. 2. Cole-Cole plot of nanocrystalline TiO2 calcined at 400°C (673 K), showing the three major effects of (i) the electrode, (ii) the grain boundaries and (iii) the grain interior.
S. Bhowmik et al. I Materials Science and Engineering A204 (1995) 258-266 20
type-K thermocouple. A constant amplitude sinusoidal voltage was applied across the sample and the phase shift and amplitude of the current measured at each frequency. The frequency was automatically scanned through the range 10 -2 Hz to 10 MHz, and data were collected and analyzed [11].
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+ 3O3
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3. Results and discussion
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Fig. 1 shows the impedance as a function of frequency for nanocrystalline TiO 2 calcined at 400°C (673 K) at some representative temperatures. The impedance, Z, of the sample initially decreases with increasing temperatures but then the trend reverses around 378 K and is seen to increase. The Cole-Cole [12] plot of the impedance data for this sample, Fig. 2, shows that there are three distinct polarizations or arcs. When a system is subjected to an applied potential difference there is a polarization effect at each interface [6]. When the voltage or electric field is reversed, the rate at which the polarized region changes is dependent on the characteristic of that interface. This is slow for chemical reactions at the TiO2/electrode/air, triple phase contacts, leading to polarizations only at low frequencies. This polarization is at higher frequencies for grain boundaries and even greater for grain interiors [5,13]. Thus, from the Cole-Cole plots we can suggest the following effects: (i) the effect of electrodes
-
8
261
16
20
10 6 f ? e m )
Fig. 3. Cole Cole plot of nanocrystalline TiO2 calcined at 800°C (1073 K). ment and also to reduce noise, especially at higher frequencies. Four terminal measurements were used to eliminate contact resistance and lead effects. Rigid cables (or leads) from the sample were of short length and consisted of platinum as the inner conductor and stainless steel as the outer conductor. The sample was then heated within the conductivity apparatus, in air, and the temperature of the sample was monitored using a
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262
S. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258 266 '
]
:
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10 6
10 7
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10 8
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Fig. 5. (a) C o l e - C o l e plot of a microcrystalline TiO 2 sample calcined at 800°C. (b) Impedance vs. frequency for a microcrystalline TiO2 sample calcined at 800°C.
(space charge polarizations) at low frequency; (ii) the grain boundary effect at intermediate frequencies; and (iii) the bulk effect at the highest frequency range. Within the bulk effect we actually see the distortion o f the semicircle, which is the effect of t w o arcs with different relaxation times. This may be due to m o r e than one phase (the anatase and the rutile) being present in the material. For samples calcined at 500°C
(773 K) there is no appreciable change in the magnitudes of the impedance or the trends. The Cole-Cole plot also shows a similar pattern for this sample. Fig. 3 shows the Cole-Cole plots of a nanocrystalline sample calcined at 800°C (1073 K) in the temperature range 303 453 K. As in Fig. 2, there are three distinct semicircles for all ranges of temperature. However, there is less distortion of the grain interior arc.
S. Bhowmik et al./ Materials Science and Engineering A204 (1995) 258 266 j
,
•
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a
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a
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10,0 pore radius
(angstroms)
Fig. 6. Pore size distribution in nanocrystalline samples calcined at various temperatures.
Interestingly, there are no changes in the patterns or magnitudes of the arcs through the entire range of temperatures. Obviously such constancy is also reflected in the impedance vs. frequency response . The overall change from the previous behavior of Figs. 1 and 2 is actually noticed at samples calcined above 650°C (923 K). The other important behavior is seen in the flattening of the arcs from the electrode and grain boundary effects or polarizations. The overall difference in the
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~!~
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impedance between samples sintered at 400°C, 500°C, and 800°C is observed in Fig. 4. It is seen that the impedance of the samples calcined at lower temperatures is less than those at higher temperatures by more than an order of magnitude. Fig. 5(a) shows the Cole-Cole plot of a microcrystalline sample calcined at 800°C. The overall impedance of the microcrystalline titania pellet is significantly higher than that of the nanocrystalline material at any particular temperature. In this case the Cole-Cole plots still suggest three major effects. However, the effect of electrodes and that of grain boundaries are comparatively less than that of the bulk effect. The arc due to the bulk (grain interior) effect seems to be highly skewed in shape, which is indicative of the existence of another phase. Fig. 5(b)shows the impedance vs. frequency response for the same sample at different temperatures. Table 1 shows the bulk density, 'true density', and the physical properties of some of the representative samples. For the nanocrystalline samples significant densification (bulk density) occurs from 400°C to 800°C. This densification is consistent with the decreasing pore structure of the samples with increasing calcination temperatures (Fig. 6). Bulk and 'true density' measurements of samples in the green state and samples calcined at 400°C also indicate no significant change in the porosity of these materials. TEM studies also indicate no significant grain growth of the nanocrystalline
' ~, (b)
264
S. Bhowmik et al. Materials Science and Engineering A204 (1995) 258-266
(c)
(d)
(e)
Fig. 7. (a) SEM micrograph of a micro-TiO2 sample calcined at 800°C. (b) TEM micrograph of a nano-TiO 2 sample calcined at 400°C. (c) TEM micrograph of a nano-TiO2 sample calcined at 500°C. (d) TEM micrograph of a nano-TiO2 sample calcined at 600°C. (e) TEM micrograph of a nano-TiO 2 sample calcined at 8000C.
material below 500°C, as was also reported by Hahn et al. [14]. On the basis of these studies, our present hypothesis is that increased porosity and grain boundaries of the nanocrystalline TiO2 are the most important factors in showing the increased sensitivity of the response. This is also observed by comparing the impedance responses at
different temperatures (Figs. 1 and 5(b)). However, we see from the Cole-Cole plots, Figs. 2 and 5(a), that for both the nano and microcrystalline TiO2 the resistivity is dominated by the grain interior (bulk). It also appears from Figs. 2 and 3 that the increased grain boundaries in nanocrystalline TiO2 have the net effect of increasing impedance. The effects of densification by
S. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258-266
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Frequency
---.-Y" . . . . .
I llllulJ
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10 6
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Fig. 8. Dielectric losses as a function of frequencies of nano-TiO2 samples calcined at various temperatures. sintering may reduce the magnitude of the resistance across the grain boundaries and cause a simultaneous decrease in the surface area associated with interface capacitance, and these elements are obviously in parallel. The conductivity of the grain boundaries is complex and is a function of any other phase present, porosity and grain size. At this point we do not have enough information to entail the detailed behavior of the grain boundaries. The initial decrease in the overall impedance in Fig. 1 (or the increase in conductivity) and the behavior at the electrode interface is attributed to moisture within the apparatus and sample. Initially, with increasing temperature, the mobility of the ( O H ) - ions, which are initially formed by the dissociation of the water molecule into H + and ( O H ) - , increases. The introduction of moisture at the surface probably occurs through simple adsorption and might even be due to the 'donor-like' behavior of adsorbed water in semiconductors [15]. Subsequently, with increasing temperature, drying of the sample and its surrounding occurs; with escape of total moisture present, there is a decrease in conductivity or an increase in impedance. Although the mobility of the ( O H ) - ions at higher temperatures may increase, there is an overall decrease in the amount of moisture and (OH) ions available at higher temperatures. Therefore an increase in impedance occurs at temperatures around 378 K. The samples with high porosity have an enhanced response as they have a larger exposed surface. The samples calcined at higher tempera-
tures ( > 650°C) do not have high porosity and have decreased grain boundaries, and subsequently the effect of moisture is not exhibited. Figs. 7(a)-(e) show the SEM and TEM micrographs of microcrystalline TiO2 and nanocrystalline TiO 2 samples calcined at different temperatures. From these photo-micrographs it is observed that the grain growth does occur after calcining at 650°C. The bulk conductivity of microcrystalline TiO2 (rutile) has been proposed as due to the presence of titanium (both Ti 3 + and Ti 4+) interstitials and oxygen vacancies [1]. Considering the simple semiconductor band model, the conductivity is expected to increase with increasing temperature since there is an increase in the number of charge carriers. However, complex defect reactions in this material are not well understood. These defect reactions are also not expected to dominate at lower temperatures in the case of microcrystalline TiO2. An analysis of XRD spectra confirm the presence of two phases in both nanocrystalline and microcrystalline TiO 2. Microcrystalline TiO2 specimens sintered at 800°C reveal the dominant rutile phase ( ~ 80%) and the rest as anatase. XRD studies of nanocrystalline TiO 2 samples show a similar dominant rutile phase and some anatase [16]. The presence of the two phases is consistent with the distorted arcs in Figs. 2 and 5(a), representing the bulk impedance. It has also been reported by Eastman [16] that the conversion of anatase to rutile in nanocrystalline TiO2 is completed after
266
s. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258 266
annealing at 1073 K. This is also consistent with our previous observation for the bulk effect (arc) in Fig. 3, although there still exists some indication of another phase. On the basis of the above discussions, we can see the presence of a resistive (R~) and a capacitive element (C~) at the electrode interface. The resistance may be associated with a chemical reaction at the electrode, TiO2 and air interface. There is also resistive (R0 and capacitive (C~) behavior (in parallel) at the grain boundaries. Finally, we have the bulk or the grain resistance (Rg) and capacitance (Cg), which may be the combination of more than one phase present. The combination of all these elements can be represented as an equivalent circuit, for nanocrystalline TiO2. The magnitudes of the individual elements associated with each of the effects could be estimated from extrapolation of the data in Fig. 2.
4. Conclusions Nanocrystalline titania calcined at lower temperatures with a detailed pore structure exhibits enhanced sensitivity to changes in ambient atmosphere, especially in the case of humidity, which is reflected by changes in impedance (or conductivity). It is evident from the above studies that impedance spectroscopy is an effective method to observe the said behavior. It is also evident that further studies are required to determine the effects of different kinds of electrode material, different partial pressures.of oxygen, controlled humidity conditions, and ~different (both lower and higher temperature ranges) before any definite conclusion can be drawn. Our IS studies, have also shown that the loss tangent, tan(d), Eq. (4), for nanocrystalline samples declines with increased sintering temperature (Fig. 8) in the frequency range 1 Hz to 10 6 Hz and, for samples calcined at high temperatures, this loss remains constant in the temperature range studied. This is one of
the important features of low lossy capacitors. Further studies of this aspect should also be considered. An attempt will be made to further discuss these topics in the next paper of this series.
Acknowledgments The authors gratefully acknowledge funding from Nanophase Technologies and Iowa State University. They also wish to thank Dr. Hitendra Patel, Joseph Kinc and Jim Hudgens for helpful suggestions.
References [1] R.N. Bluementhal, J. Baukus and W.M. Hirthe, J. Electrochem. Soc.: Solid State Science, 114 (1967) 172. [2] D.S. Tannhauser, Experimental Evidence j?om Conductivity Measurements/br Interstitial Titanium in Reduced 7702, Solid State Communications, Vol. l, Pergamon Press, US, 1963, p. 223. [3] A.L. Micheli, Am. Ceram. Soc. Bull., 63 (5) (1984) 694.
[4] A.M. Azad, L.B. Younkman, S.A. Akbar and M.A. Alim, J. Am. Ceram. Soc., 77 (2) (1994) 481. [5] J.E. Bauerle~ J. Phys. Chem. Solids, 30 (1969) 2657. [6] J. Ross Macdonald, lmpedanee Spectroscopy Emphasizing Solid Materials and Systems, Wiley, New York, 1987, pp. 3 20. [7] B. Tareev, Physics of Dielectric Materials, Mir, Moscow, 1979, pp. 141 173. [8] L. Ketron, Ceram. Soc. Bull., 68 (4)(1989). [9] R.W. Siegel, S. Ramaswamy, H. Hahn, L. Zongquan, L. Ting and R. Gronsky, J. Mater. Res., 3 (1988) 1367. [10] J.C. Parker and R.W. Siegel, J. Mater. Res., 5(6)(1990) 1246. [11] H.K. Patel, Ph.D. Thesis, Iowa State University, 1993. [12] K.S. Cole and R.H. Cole, J. Chem. Phys., 9 (1941) 341. [13] E.J.L. Schouler, N. Mesbahi and G. Vitter, Solid State lonics, 9&lO (1983) 989. [14] H. Hahn, J. Logas and R.S. Averback, J. Mater. Res., 5 (3) (1990) 609. [15] M.J. Madou and S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989, pp. 506 513. [16] J.A. Eastman, J. Appl. Phys., 75 (2) (1994).
ELSEVIER
A
Materials Science and Engineering A204 (1995) 258 266
Electrical characterization of nanocrystalline titania I: (Impedance spectroscopy studies between 300 K and 473 K) S i d d h a r t h a B h o w m i k , a Kristen P. C o n s t a n t a,* J o h n C. Parker, b M. Ali b ~Department of Materials Science and Engineering, Iowa State University, Ames, IA 5001 l, USA bNanophase Technologies, Darien, IL 60561. USA
Abstract
The electrical properties of nanocrystalline titania with gold electrodes were characterized using impedance spectroscopy in the frequency range 10 -2 to 106 Hz and temperature range 300 K to 500 K. An attempt has been made to correlate the microstructural properties of these specimens to the electrical response of the material over the different ranges of temperatures. The results indicate that the conductivity of nanocrystalline titania is dependent on the porosity, grain size, and grain boundaries. The impedance and polarization behavior of the samples, especially at the electrodes, is also affected by the humidity. The electrical behavior of the nanocrystalline material makes it a candidate for use as a low temperature gas sensor. Keywords: Electrical characterisation; Nanocrystalline titania; Impedance spectroscopy
1. Introduction Nanocrystalline materials have in recent years been of interest because of the possibility to generate properties that may differ significantly from the microcrystalline material. These differences may arise from the smaller particle size, increased grain boundaries, a larger fraction of atoms residing at these interfaces, and differences in the porosity of the nanocrystalline materials. Differences may also originate due to the high purity of the nanocrystalline material compared with the microcrystalline form. The defect structure of microcrystalline titanium oxide has been studied in detail [1 3] in attempts to describe the conductivity mechanism. These studies included measurements of the d.c. conductivity at high temperatures and the possibility of using titania as an oxygen gas sensor. An attempt was made by Azad et al. [4] to characterize microcrystalline titania using impedance spectroscopy at room temperature. Impedance spectroscopy (IS) has been used to study the electrical properties of ceramics since Baurle's [5] work with zirconia, where he showed that the effects of * Corresponding author.
electrode, grain interiors and grain boundaries could be resolved in the admittance plane or the impedance plane. IS has since proven to be an effective method in evaluating the microstructural effects on conductivity when combined with other methods of characterization. Characterization of microstructural effects on conductivity is essential in the development of materials for use in electronic components. In the case of ceramic materials, impedance, Z, usually arises due to capacitive reactance, Xc, and resistance, R. For a capacitor, the capacitive reactance, X,, is the opposition to current: X c = l/~oC,
(l)
where v ) = v/2n, v is the frequency, and C the capacitance. Z is a complex quantity and can be described [6] in terms of various related functions such as admittance, Y = 1/Z,
(2)
and complex dielectric permittivity, ¢ = Y/jcoC= ¢' - j¢",
(3)
where j = x / ( - 1). In capacitors [7] having dielectric losses, the loss tangent, 0921-5093/95/$09.50 © 1995
Elsevier Science S.A. All rights reserved SSD1 0921-5093(95)09971-9
259
S. B h o ~ m i k et al. / Materials Science and Eng#Teering A 2 0 4 (1995) 258 266
Table 1 The density and cell constant, Ko, of nanocrystalline and microcrystalline TiO, Sample
Nanocrystalline Nanocrystalline Nanocrystalline Nanocrystalline Nanocrystalline Microcrystalline
TiOx TiO~ TiO 2 TiO~ TiO 2 TiO~
Calcination temperature (°C)
Soak time (h)
Bulk density (gcm 3)
True density (pycnometer) (g cm ~)
Ko ~' (cm ~)
Structure ~
NA (Green) 400°C 500°C 650°C 800°C 800°C
NA u 4 4 4 4 4
~2.2 ~2.3 ~2.3 ~2.8 ~ 3.6 ~3.4
44.1 ~4.1 ~4.1
NA 0.215 0.185 0.165 0.340 0.227
Dominantly rutile: some anatase NA NA NA NA Ruffle ~ 80% Anatase ~ 20%
~4.1
~' K o - d / A , where d - thickness of pellet and A = area of applied electrode.
b NA = not available/applicable. c Structures obtained from Ref. [12] indicate the dominance of rutile in all these cases.
tan(5 = e"/e',
(4)
is used as an estimate of the power dissipation. For capacitors used for charge storage the value of tang is usually low. Sensors can be classified [8] according to the basis of the generation of the charge carrier and its transport. Accordingly, the sensing mechanism may be due to bulk effects, i.e. the physical properties of the grain itself, grain boundary effects, as in the case of PTC thermistors, or through surface absorption, as in the case of humidity sensors. In characterizing the electrical properties of microcrystalline titania, the emphasis has been in understanding the bulk or grain interior conductivity. However, in the case of nanocrystalline titania, the effect of grain interior effects is unlikely to be the sole effective mechanism. Porosity and grain boundaries are expected to significantly affect the properties. The purpose of this paper is to discuss the experimental results in terms of porosity, grain size and electrodes and the possible use of nanocrystalline titania as an effective low temperature chemical sensor.
2. Experimental procedure Nanocrystalline TiO2 was prepared through inert gas phase condensation [9,10]. In this method atomic clusters of TiO 2 were first formed by the evaporation of Ti in a pure He gas. The Ti vapor was then condensed in a liquid N2 cold finger and oxidized with the rapid introduction of oxygen into the chamber to form TiO2. The loose powders were compacted uniaxialty, with pressures ranging from 15,000 to 20,000 psi. The pressure used is an important factor as the pressure is responsible for the breakdown of the particle agglomerates. Test samples were usually ~ 1 cm in diameter and 0.1 cm in thickness in the green state. The pellets formed were dry pressed with a light coating of stearic acid on the punch to facilitate mold release. To avoid
lamination cracks, care was taken in expelling the pellets. The green nanocrystalline samples were calcined in air at temperatures between 400°C and 800°C in a tube furnace, with a soak time of 4 h (see Table 1). Calcining at 400°C improved the strength of the samples for handling purposes. Higher temperature calcining of the nanocrystalline TiOz enabled examination of the grain growth and porosity on the electrical measurements. Bulk density measurements and 'true density' were measured using pycnometery to monitor changes in porosity. The evolution of the pore structure was examined as a function of temperature and calcination time. Microstructural studies were done in parallel with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to see the corresponding increase in grain sizes. Since the impedance measurements done to date have been at temperatures much below the calcining temperatures, grain growth was not expected to occur during impedance measurements. Microcrystalline TiO2 (Aldrich Chemicals, 99.999%) was pressed between pressures ranging from 14,000 to 20,000 psi and the pellets formed were calcined in air at 800°C for 4 h (soak time). X-ray diffraction (XRD) measurements were done to indicate the phases present at this stage. Electrodes were applied to all the calcined specimens, which were sputter-coated (Polaron SC 502, Sputter Coater) with high purity gold. Sputter-coating ensures the formation of porous electrodes; additionally, all the parameters were kept constant to form electrodes of equal thickness. The application and monitoring of the electrodes are important factors since they play an important role in impedance measurements. Impedance measurements were performed using a Solarton 1260 Impedance Gain-Phase Analyzer (Schlumberger Technologies, Billerica, MA). The measurements were done in an apparatus designed to take into account the characteristic impedance of the instru-
260
S. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258 266 400000
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S. Bhowmik et al. I Materials Science and Engineering A204 (1995) 258-266 20
type-K thermocouple. A constant amplitude sinusoidal voltage was applied across the sample and the phase shift and amplitude of the current measured at each frequency. The frequency was automatically scanned through the range 10 -2 Hz to 10 MHz, and data were collected and analyzed [11].
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(space charge polarizations) at low frequency; (ii) the grain boundary effect at intermediate frequencies; and (iii) the bulk effect at the highest frequency range. Within the bulk effect we actually see the distortion o f the semicircle, which is the effect of t w o arcs with different relaxation times. This may be due to m o r e than one phase (the anatase and the rutile) being present in the material. For samples calcined at 500°C
(773 K) there is no appreciable change in the magnitudes of the impedance or the trends. The Cole-Cole plot also shows a similar pattern for this sample. Fig. 3 shows the Cole-Cole plots of a nanocrystalline sample calcined at 800°C (1073 K) in the temperature range 303 453 K. As in Fig. 2, there are three distinct semicircles for all ranges of temperature. However, there is less distortion of the grain interior arc.
S. Bhowmik et al./ Materials Science and Engineering A204 (1995) 258 266 j
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Interestingly, there are no changes in the patterns or magnitudes of the arcs through the entire range of temperatures. Obviously such constancy is also reflected in the impedance vs. frequency response . The overall change from the previous behavior of Figs. 1 and 2 is actually noticed at samples calcined above 650°C (923 K). The other important behavior is seen in the flattening of the arcs from the electrode and grain boundary effects or polarizations. The overall difference in the
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impedance between samples sintered at 400°C, 500°C, and 800°C is observed in Fig. 4. It is seen that the impedance of the samples calcined at lower temperatures is less than those at higher temperatures by more than an order of magnitude. Fig. 5(a) shows the Cole-Cole plot of a microcrystalline sample calcined at 800°C. The overall impedance of the microcrystalline titania pellet is significantly higher than that of the nanocrystalline material at any particular temperature. In this case the Cole-Cole plots still suggest three major effects. However, the effect of electrodes and that of grain boundaries are comparatively less than that of the bulk effect. The arc due to the bulk (grain interior) effect seems to be highly skewed in shape, which is indicative of the existence of another phase. Fig. 5(b)shows the impedance vs. frequency response for the same sample at different temperatures. Table 1 shows the bulk density, 'true density', and the physical properties of some of the representative samples. For the nanocrystalline samples significant densification (bulk density) occurs from 400°C to 800°C. This densification is consistent with the decreasing pore structure of the samples with increasing calcination temperatures (Fig. 6). Bulk and 'true density' measurements of samples in the green state and samples calcined at 400°C also indicate no significant change in the porosity of these materials. TEM studies also indicate no significant grain growth of the nanocrystalline
' ~, (b)
264
S. Bhowmik et al. Materials Science and Engineering A204 (1995) 258-266
(c)
(d)
(e)
Fig. 7. (a) SEM micrograph of a micro-TiO2 sample calcined at 800°C. (b) TEM micrograph of a nano-TiO 2 sample calcined at 400°C. (c) TEM micrograph of a nano-TiO2 sample calcined at 500°C. (d) TEM micrograph of a nano-TiO2 sample calcined at 600°C. (e) TEM micrograph of a nano-TiO 2 sample calcined at 8000C.
material below 500°C, as was also reported by Hahn et al. [14]. On the basis of these studies, our present hypothesis is that increased porosity and grain boundaries of the nanocrystalline TiO2 are the most important factors in showing the increased sensitivity of the response. This is also observed by comparing the impedance responses at
different temperatures (Figs. 1 and 5(b)). However, we see from the Cole-Cole plots, Figs. 2 and 5(a), that for both the nano and microcrystalline TiO2 the resistivity is dominated by the grain interior (bulk). It also appears from Figs. 2 and 3 that the increased grain boundaries in nanocrystalline TiO2 have the net effect of increasing impedance. The effects of densification by
S. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258-266
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Fig. 8. Dielectric losses as a function of frequencies of nano-TiO2 samples calcined at various temperatures. sintering may reduce the magnitude of the resistance across the grain boundaries and cause a simultaneous decrease in the surface area associated with interface capacitance, and these elements are obviously in parallel. The conductivity of the grain boundaries is complex and is a function of any other phase present, porosity and grain size. At this point we do not have enough information to entail the detailed behavior of the grain boundaries. The initial decrease in the overall impedance in Fig. 1 (or the increase in conductivity) and the behavior at the electrode interface is attributed to moisture within the apparatus and sample. Initially, with increasing temperature, the mobility of the ( O H ) - ions, which are initially formed by the dissociation of the water molecule into H + and ( O H ) - , increases. The introduction of moisture at the surface probably occurs through simple adsorption and might even be due to the 'donor-like' behavior of adsorbed water in semiconductors [15]. Subsequently, with increasing temperature, drying of the sample and its surrounding occurs; with escape of total moisture present, there is a decrease in conductivity or an increase in impedance. Although the mobility of the ( O H ) - ions at higher temperatures may increase, there is an overall decrease in the amount of moisture and (OH) ions available at higher temperatures. Therefore an increase in impedance occurs at temperatures around 378 K. The samples with high porosity have an enhanced response as they have a larger exposed surface. The samples calcined at higher tempera-
tures ( > 650°C) do not have high porosity and have decreased grain boundaries, and subsequently the effect of moisture is not exhibited. Figs. 7(a)-(e) show the SEM and TEM micrographs of microcrystalline TiO2 and nanocrystalline TiO 2 samples calcined at different temperatures. From these photo-micrographs it is observed that the grain growth does occur after calcining at 650°C. The bulk conductivity of microcrystalline TiO2 (rutile) has been proposed as due to the presence of titanium (both Ti 3 + and Ti 4+) interstitials and oxygen vacancies [1]. Considering the simple semiconductor band model, the conductivity is expected to increase with increasing temperature since there is an increase in the number of charge carriers. However, complex defect reactions in this material are not well understood. These defect reactions are also not expected to dominate at lower temperatures in the case of microcrystalline TiO2. An analysis of XRD spectra confirm the presence of two phases in both nanocrystalline and microcrystalline TiO 2. Microcrystalline TiO2 specimens sintered at 800°C reveal the dominant rutile phase ( ~ 80%) and the rest as anatase. XRD studies of nanocrystalline TiO 2 samples show a similar dominant rutile phase and some anatase [16]. The presence of the two phases is consistent with the distorted arcs in Figs. 2 and 5(a), representing the bulk impedance. It has also been reported by Eastman [16] that the conversion of anatase to rutile in nanocrystalline TiO2 is completed after
266
s. Bhowmik et al. / Materials Science and Engineering A204 (1995) 258 266
annealing at 1073 K. This is also consistent with our previous observation for the bulk effect (arc) in Fig. 3, although there still exists some indication of another phase. On the basis of the above discussions, we can see the presence of a resistive (R~) and a capacitive element (C~) at the electrode interface. The resistance may be associated with a chemical reaction at the electrode, TiO2 and air interface. There is also resistive (R0 and capacitive (C~) behavior (in parallel) at the grain boundaries. Finally, we have the bulk or the grain resistance (Rg) and capacitance (Cg), which may be the combination of more than one phase present. The combination of all these elements can be represented as an equivalent circuit, for nanocrystalline TiO2. The magnitudes of the individual elements associated with each of the effects could be estimated from extrapolation of the data in Fig. 2.
4. Conclusions Nanocrystalline titania calcined at lower temperatures with a detailed pore structure exhibits enhanced sensitivity to changes in ambient atmosphere, especially in the case of humidity, which is reflected by changes in impedance (or conductivity). It is evident from the above studies that impedance spectroscopy is an effective method to observe the said behavior. It is also evident that further studies are required to determine the effects of different kinds of electrode material, different partial pressures.of oxygen, controlled humidity conditions, and ~different (both lower and higher temperature ranges) before any definite conclusion can be drawn. Our IS studies, have also shown that the loss tangent, tan(d), Eq. (4), for nanocrystalline samples declines with increased sintering temperature (Fig. 8) in the frequency range 1 Hz to 10 6 Hz and, for samples calcined at high temperatures, this loss remains constant in the temperature range studied. This is one of
the important features of low lossy capacitors. Further studies of this aspect should also be considered. An attempt will be made to further discuss these topics in the next paper of this series.
Acknowledgments The authors gratefully acknowledge funding from Nanophase Technologies and Iowa State University. They also wish to thank Dr. Hitendra Patel, Joseph Kinc and Jim Hudgens for helpful suggestions.
References [1] R.N. Bluementhal, J. Baukus and W.M. Hirthe, J. Electrochem. Soc.: Solid State Science, 114 (1967) 172. [2] D.S. Tannhauser, Experimental Evidence j?om Conductivity Measurements/br Interstitial Titanium in Reduced 7702, Solid State Communications, Vol. l, Pergamon Press, US, 1963, p. 223. [3] A.L. Micheli, Am. Ceram. Soc. Bull., 63 (5) (1984) 694.
[4] A.M. Azad, L.B. Younkman, S.A. Akbar and M.A. Alim, J. Am. Ceram. Soc., 77 (2) (1994) 481. [5] J.E. Bauerle~ J. Phys. Chem. Solids, 30 (1969) 2657. [6] J. Ross Macdonald, lmpedanee Spectroscopy Emphasizing Solid Materials and Systems, Wiley, New York, 1987, pp. 3 20. [7] B. Tareev, Physics of Dielectric Materials, Mir, Moscow, 1979, pp. 141 173. [8] L. Ketron, Ceram. Soc. Bull., 68 (4)(1989). [9] R.W. Siegel, S. Ramaswamy, H. Hahn, L. Zongquan, L. Ting and R. Gronsky, J. Mater. Res., 3 (1988) 1367. [10] J.C. Parker and R.W. Siegel, J. Mater. Res., 5(6)(1990) 1246. [11] H.K. Patel, Ph.D. Thesis, Iowa State University, 1993. [12] K.S. Cole and R.H. Cole, J. Chem. Phys., 9 (1941) 341. [13] E.J.L. Schouler, N. Mesbahi and G. Vitter, Solid State lonics, 9&lO (1983) 989. [14] H. Hahn, J. Logas and R.S. Averback, J. Mater. Res., 5 (3) (1990) 609. [15] M.J. Madou and S.R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, New York, 1989, pp. 506 513. [16] J.A. Eastman, J. Appl. Phys., 75 (2) (1994).