TiO2

Sensors and Actuators

87

B, 4 (1991) 87-94

Schottky-harrier and Conductivity Gas Sensors Based upon Pd/SnO, and Pt/TiO* K. D. SCHIERBAUM, Institute

U. K. KIRNER,

of Physical and Theoretical

J. F. GEIGER

Chemistry,

University

and W. GdPEL of Tiibingen, Morgenstelle

Abstract Electronic conductivity, mixed conductivity and Schottky-barrier sensors based upon the oxides SnOz and TiOz which are modified or contacted by the metals Pd and Pt can be used to detect gases like CO, CH4, H2 and 0,. The response signals of these different types of sensors result from changes in surface and bulk conductivities or in Schottkybarrier heights, which are measured using different geometric arrangements of metal (Pd or Pt) contacts. The atomistic understanding of sensing mechanisms and corresponding sensor structures is deduced from comparative spectroscopic and electrical measurements. Our studies show that reliable sensor properties can only be achieved either by forming stable metal/oxide interfaces or by adjusting a stable dopant distribution.

1. Introduction Different charge-transfer reactions of surfaces, interfaces, grain boundaries and bulk oxide material often occur simultaneously and determine the properties of chemical sensors which change their conductivity with the partial pressure of the monitored particles [ 1,2]. We now report first results of a systematic approach to separate, understand and utilize these different reactions by controlling the atomic composition of the gas/metal/bulk oxide three-phase boundary. Different metal/ oxide/metal sandwich systems were chosen to optimize the following sensor principles (see Fig. 1). (1) Electronic conductivity (type 1) sensors consist of undoped and Pd-doped SnO, thin 0925-4005/91/$3.50

8, D- 7400 Tiibingen (F.R.G.)

layers evaporated onto Pt interdigital structures [3]. Two-point d.c. and a.c. conductivities were measured parallel to the surface as a function of the temperature and of the partial pressures of oxidizing and reducing gases such as 02, N02, CO, CH4 and H2. (2) Mixed conductivity (type 2) sensors consist of undoped Ti02 thin layers and twoand four-point arrangements of Pt contacts to determine d.c. and a.c. conductivities as a function of O2 partial pressure at high temperatures [4]. (3) Schottky-barrier (type 3) sensors consist of Schottky-type front contacts (Pt/TiO, or Pd/SnO,) formed by low-temperature evaporation of Pt or Pd onto single-crystalline TiOz (rutile phase) [4] and SnOz. Ohmic Zr back contacts were used. The non-ohmic current-voltage characteristics were measured for different partial pressures of oxidizing and reducing gases such as 02, NO2 and CO. The influence of the electronic and geometric structures of the different interfaces on their electrical sensor properties was studied. Drastic changes of the sensor properties are observed during even small variations of the structure of metal/oxide interfaces. Conditions were determined for the preparation and operation of stable sensors. Schottky

barrier or ohmic contact

-

Fl2

type type

3 sensors 2 senscrs

IIV)[DCorACI

ohmic contacts w IWI

\

type 1 and

2 sensors

Fig. 1. Schematic experimental set-up for the different types of sensors investigated in this study. For details, see text. 0

Elsevier Sequoia/Printed in The Netherlands

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2. Experimental All prototype devices prepared in this study operate with metal contacts in different geometrical arrangements to monitor d.c. and a.c. conductivities of electronic, ionic and/or mixed conduction contributions at different temperatures for different oxygen partial pressures and concentrations of reducing gases. These measurements were done in synthetic air under flow conditions using different test chambers (heated quartz tubes and polyethylene chambers). For experimental details of the electrical measurements, see [3-S] and of the surface spectroscopies, see [6].

3. Results and Discussion 3.1. Results from Surface Spectroscopies Thermodynamically and kinetically controlled reactions at metal/oxide interfaces and at gas/metal/bulk oxide three-phase boundaries determine the stability of metallic surface dopants and electrical contacts. They were studied first with different surface spectroscopic techniques as a function of temperature under oxidizing and reducing conditions (i.e., for different oxygen partial pressures poZ). To illustrate typical interface phenomena at binary oxides, results on Pd overlayers (which can act as metallic surface dopants) and SnO, bulk substrates obtained from Xray and ultraviolet photoemission spectroscopy (XPS and UPS) and ion backscattering spectroscopy (ISS) experiments are discussed here; they are summarized in Fig. 2 [7]. Evaporation of Pd in the submonolayer range leads to strongly reduced intensities of Sn- and O-related substrate signals in the surface-sensitive ISS and HeII-excited UPS spectra. Clustering of palladium atoms occurs at low temperatures. This leads to only small changes in the XPS relative core level intensities of Pd and Sn and makes it possible to cluster sizes (typically 0.5 < determine r’ < 1.7 nm in diameter) [ 81. Oxidation of Pd in

the presence of oxygen at T 2 470 K is identified by the chemical shift of Pd core levels from metallic (PdO) to oxidic (Pd’+) binding energies. It is a prerequisite for segregation of the small Pd2+ ions (if compared to the large Pd atoms) into Sn02 substrates at higher temperatures. This effect can be monitored by a decreased XPS core level intensity of Pd or by depth profiling in secondary ion mass spectrometry (SIMS) and secondary neutral mass spectrometry (SNMS) [ 81. Stabilities and reactions of interfaces also determine the preparation conditions for stable metallic contacts. As an example, Schottky diodes are obtained after low-temperature evaporation of platinum onto single-crystalline rutile. In contrast, ohmic contacts are formed after the same preparation using Ti02 Magneli phases instead of rutile [9]. In Auger electron spectroscopy (AES) depth profiles, they show comparatively broad interfaces and large in-diffused regions. The same is also observed for Pt on Ti02 rutile phases after high-temperature and high oxygen-pressure treatments. From XPS measurements (Fig. 3) it can be deduced that PtO, and PtO are

rmatcn Ith

WSI

Annealing

temperature

Annealing

temperature

WOK

thetmod"namlcally

‘Pd”

interaction Fig. 2. Survey on temperature-dependent mechanisms between Pd overlayers and SnO, substrates (as determined from XPS, HeI-, and HeII-excited UPS, and ISS by taking into account their specific information depths): Pd” overlayers with coverages 0 >>1 may be used as electrical contacts for type 3 sensors based upon Pd/SnO, provided that the operation temperature is below 400 K. Type 1 or 2 sensors are based on Pd sub-surface or bulk doping after thermal treatment of Pd/SnO, overlayer systems. Further details are explained in the text.

89

MgK,

Pti4f)

A

T = 470 K in air 60% r.h.1

Partial pressure p (ppmld0

-

7%

7'0

Eb[eV,E,=OI

Fig. 3. Identification of metal overlayers and dopants: XPS (MgK,) spectra of Pt 4f core levels of platinum deposited on TiO, ( 110) before (a) and after (b) heating in ultra-high vacuum, as well as after annealing in oxygen (T= 1070 K, po2 = lo2 Pa) for 10 min (c). Curve (c) can be fitted by different Pt 4f contributions (dashed lines) from Pt, PtO and PtO, (d) as determined independently from reference measurements of Pt and Pt02. The binding energy of the Pt 4f core level in PtO is assumed to be E,(PtO) = &(PtO,) - E,(Pt)]/2. Binding energies Eb are related to the valence band edge E.,.

formed under these experimental conditions in a kinetically controlled oxidation of platinum at the three-phase boundary. This leads to an in-diffusion of intermediate Pt*+ and Pt4+ ions into Ti02, thereby donating electrons to the conduction band of Ti02. Hence, the Schottky diode is destroyed and ohmic contacts are formed with characteristic I-V curves described in detail in Section 3.4. 3.2. Electronic

Surface (Type 1 Sensors)

Conductivity

Sensors

For Pd/SnO, junctions, PdO is formed at comparatively low temperatures (T x 470 K). As described above in Section 3.1, bulk doping of Sn02 subsurface regions occurs under these conditions. In addition, the diffusion-

Fig. 4. Type 1 sensor: typical changes of the sheet conductivity Aa, of electronic conductivity sensors based upon thin-film SnO, and Pd-doped SnO, structures for different temperatures and for different gases.

controlled Pd*+ profile (determined in our SIMS and SNMS measurements [8]) may even be changed during exposure to reducing gases at the elevated temperatures (370 < T < 770 K) usually applied for the operation of the devices. To characterize differences in the sensor properties of the different Pd/SnO, interfaces indicated in Fig. 2, we studied electronic conductivitites of pure Sn02 thin films evaporated on interdigital structures [3] after their subsequent Pd surface doping by evaporation of Pd and after establishing their bulk doping by thermally induced diffusion. The latter leads to acceptors in Sn02. Typical conductivity changes after exposure to CO and H2 are shown in Fig. 4. Temperature-dependent charge-transfer chemisorption effects and/or catalytic reactions involving intrinsic defects (substrate-derived oxygen vacancies) of undoped SnO, can be identified. Drastic differences in the sensor chraracteristics are obtained for both H2 and CO detection between pure and Pd-doped SnO, due to extrinsic (dopant-derived Pd atoms) species at and below the surface. As an example, conductivity

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Fig. 5. Band scheme of pure (1) and acceptor-doped (2a) GO, at T = 1600 K as derived from high-temperature conductivity measurements [ 1I] where EF is the Fermi energy and EC and Ev are conduction and valence band edges. The density n(E) of occupied electronic states is denoted by n(E) = D(E) -f(E) where D(E) is the density of all states and f(E) is the Fermi function. Ionization energies EDCAjiof intrinsic donor-type defects (oxygen vacancies VO) and acceptor-type extrinsic (impurity) are derived from conductivity measurements at low temperatures (2b and 2~). For details of the defect calculations, see [12].

changes, AoO, are larger for H2 if compared with CO at T = 470 K (Fig. 4). This ratio of CO and H2 effects changes with temperature. 3.3. Mixed Conductivity Sensors (Type 2 Sensors) Ohmic Pt/TiOz junctions can be obtained after annealing Pt/TiO, layer systems in oxygen They can be used for high-temperature bulk conductivity sensors to measure the oxygen partial pressure in the range lo- ‘* < po2 ,< lo5 Pa [4]. Characteristic slopes 8[log(a))/8[log(p,,)] of - l/6 and - l/4 are found in regimes where the concentration [V,] of oxygen vacancies is determined by poz and where [VO] is controlled by extrinsic acceptor-type defects such as Al or Fe impurities [lo]. The condition of a complete ionization of these defects can be verified from an analysis of the conductivities at high temperatures. As a typical example, characteristic band schemes for pure ( 1) and acceptor-doped Sn02 (2) at T = 1600 K are shown in Fig. 5 [ 131 with the Fermi energy EF near the midgap.

Although the mean conductivities are predominantly determined by electrons, the thermodynamically controlled response and decay times are given by the mixed conduction of both the ionic defects Vo and electrons through the oxide bulk, because intrinsic defect concentrations change with the partial pressure in the gas phase. 3.4. Current - Voltage Curves of Pt/Ti02 and Pd/Sn02, Junctions. The electrical characteristics of non-ohmic Pt/TiO, and Pd/SnOz contacts are sensitively controlled by the geometric and electronic structure of the metal/oxide interface. They can be investigated by different electrical techniques, such as current-voltage (I- V) measurements curves, capacitance-voltage and impedances [ 141. Typical results of Z-V curves for different O2 and CO partial pressures at constant temperatures are shown in Fig. 6 for chemically abrupt Pt/TiO, interfaces. The voltage-dependent conductance of the Pt/TiO,/Zr device (given by the slope aZ/a V)

91

voltage V [VI

-

Fig. 6. Type 2 and 3 sensors: current-voltage curves of Pt/TiOz junctions showing Schottky behaviour (type 3 sensors) with ohmic Zr back contacts for different oxygen partial pressures po2at T = 500 K (solid curves) and for pure carbon monoxide (dashed curve). Ohmic behaviour (type 2 sensors) was found after annealing at T = 1070 K and po, = 2 x lo4 Pa (dotted curve).

is determined by the Pt/TiO* interface and the TiO, (sub-) surface with characteristic regimes of the I-I/ curves. A pinning occurs of the Fermi energy EF relative to the conduction band EC in an accumulation layer of TiOz at the Zr contact over the entire voltage range and thereby leads to ohmic properties of the back contact. 3.4.1. Reverse voltage regime As indicated in Fig. 7 (dot-dashed line EF and solid lines EC and E,), a negative voltage equal to the contact potential difference V,,, applied to the Pt contact corresponds to a constant Fermi energy EF, i.e., to the thermodynamic equilibrium under which no electrical current flows through the system. The higher work function OPt = 5.6 eV [ 151, if compared with 4.6 < @Tio2d 5.5 eV (this value depends on the degree of chemical reduction [ 16]), leads to a band bending eAF/, at the TiOz surface and hence to a Schottky-barrier height @se = eA vs + (EC - EF)~ = % - xTi02 (1) between Pt and Ti02. Here, XTiozdenotes the

Fig. 7. Type 3 sensor: band scheme of the Pt/TiO,/Zr structure with depletion in the TiOz (reverse voltage region V = AE, 6 0) at the Pt/TiO, interface and weak accumulation at the ohmic Zr/TiO, contact. Evac,i denotes the vacuum energy, EF the Fermi energy, QptcTioz) the work functions of the platinum (titanium dioxide), E, and EC the valence and conduction band edges of semiconducting TiOz, respectively, and xTioz the electron affinity at the Pt/TiO, interface. For EF = const, the contact potential difference V,--, is given by (apt - @=,)/e with e as elementary charge. Compensation of the band bending eAV, leads to flat-band conditions (dotted lines) with a decreased Fermi energy E& of the Pt. In the forward voltage regime (dashed lines and E’k) the Schottky barrier mSs decreases and the voltage decay in Ti02 is significant.

electron affinity and (EC - EF),, the bulk position of the conduction band edge EC relative to the Fermi energy EF which is deduced from the bulk electron density &,. For Ti02 samples used in this study, 1OL6? & 2 10” cm-3 are calculated from temperaturedependent conductivities ob = ep&, with e as the elementary charge and ~1= 1.0 x 106(T/ K) -2.5 cm2( V s) -’ as the temperature-dependent mobility [ 171. At T = 500 K, 0:28 6 (EC - EF)b 6 0.58 eV was determined from (EC - EF)b = kT h(N&.$,) with NC as bulk density of conduction band states [ 181. As shown schematically in Fig. 7 by dotted lines, flat-band conditions and hence a resistance R = o; ‘j” of Ti02 (f denotes a geometric factor determined by the coritact arrangement and sample geometry) are adjusted to compensate for the work function difference @, - @Tio2z 0.9 eV (of reduced

92

TiOz single crystals with Q,x 4.6 eV). A constant Fermi energy across the TiO, can be assumed due to the fact that the resistance R of the TiOz is far below the total resistance of the Pt/TiO* structure. 3.4.2. Forward voltage regime In the forward voltage regime, i.e., with a positive voltage applied to the platinum, an accumulation layer is formed in TiOz (Fig. 7, dashed lines). For weak accumulation, i.e., for V<5V, from the straight lines In I =f(V) values [aV/a(ln I)] [e/W] % 1.5 for pure CO and x8 for pure O2 are obtained, which are higher than is expected for the ideal Schottky barrier, i.e., [aV/a(ln I)] [e/W] = 1 [14] where e is the elementary charge and k is the Boltzmann factor. This deviation cannot be explained [ 141 from a(lnI) -=kT f3V

e

aA@ l+w+e

kTa(lnA**) av

(2) > ( where AQ, is the effective change of the barrier height (due to the combined effect of image force lowering and the electric field) and A** is the Richardson coefficient. For the highly dielectric TiO, material (E x 100 [ 181) variations A@ = (e3E/s.s,) ‘/* % 15 meV [ 141 are negligible (e.g., for an applied voltage of 1 V) in spite of high electric fields E in the depletion regions (E = 1.6 x lo5 V/ cm for 1 V). The thermionic emission current is therefore most probably enhanced by physical processes such as tunnelling or recombination currents. For V >, 5 V, deviations from the straight lines In I =f(V) are observed, which can be attributed to an ohmic voltage decay within the Ti02 (compare Fig. 7, dashed lines). Schottky-type junctions are also formed between Pd overlayers and single crystalline SnO,. They show values [aV/a(ln I)] [e/kT] z 3.3 in the forward voltage region, which are lower as compared with Pt/Ti02. In the reverse voltage region, the conductivities are relatively high. This can be explained by donor states formed during evaporation of Pd onto SnO,. From XPS results, they can

be formally attributed to Sn in lower oxidation states [7]. In contrast to Pt/Ti02, however, the temperature stability in the presence of oxygen is low for Pd/SnO,. 3.5. Schottky-barrier Sensors (Type 3 Sensors) At low temperatures the effect of in-diffusion of metal ions into the oxide is avoided and a Schottky-barrier characteristic can be observed. The effective barrier heights @sB vary with occupation of the extrinsic surface states which can be formed, e.g., by chemisorption of oxidizing and reducing gases such as 02, NO2 and CO at Ti02. The formation of intrinsic point defects, e.g, oxygen vacancies, at the surface can be neglected in the presence of O2 at T < 500 K. For both possible operation modes of Schottky-type sensors, i.e., for constant voltage (chosen in this study) and for constant current, the partial pressure-dependent variations of the interface resistance result from processes at the three-phase Pt/TiO,/gas boundary whereas the two-phase boundaries, i.e., the covered Pt/TiO, and the Zr/Ti02 interfaces, are not affected. 3.5.1. O2 adsorption At low temperatures, oxygen is chemisorbed as 0, or O- (the latter may be formed by dissociative adsorption at platinum and a subsequent spill-over effect to Ti02). As indicated in Fig. 8, acceptor-type chemisorption leads to trapped electrons in surface states and hence to an additional band-bending at the Ti02 surface which affects the I- V curves in the forward-voltage regime (see also Fig. 6). Here, the effective surface state energy of 0, states (relative to the Fermi energy under flat-band conditions) E:F = 700 meV [ 191, is assumed to be the same as the value for uncovered clean Ti02 surfaces. 3.5.2. CO adsorption and CO/O,/H,O coaa5orption Interaction with CO leads to a donor-type chemisorption with an effective surface state

93

E

three-phase boundary

Fig. 8. Type 3 sensor: band scheme of Pt/TiOz interfaces with acceptor-type chemisorption of 0, and donor-type chemisorption of CO for a Schottky-type sensor operated in the forward-voltage region. Different occupation- of surface states 0; and CO+ leads to depletion (I), flat bands (2) and accumulation (3) with different band bending eAV, at the surface and different Schottky-barrier heights 4sn. Further explanations are given in the text.

partial charge 6 of CO-related surface states CO:,+ with decreasing band bending [21]. For partial pressures pco < po,/2, the trapping of electrons in oxygen-related acceptortype species exceeds the donor properties of CO, i.e., the interaction with CO leads to an increase of the Schottky barrier and hence to a decrease in the current through the Pt/ TiOJZr device upon CO exposure. This can be explained by an increased number of electrons trapped in acceptor-type chemisorption complexes CO’- formed at low CO coverages during the catalytic oxidation of CO under steady-state conditions. This reaction leads to thermodynamically favoured formation of CO* molecules which desorb into the gas phase. The existence of acceptor-type chemisorption complexes CO:- (‘carbonatelike’) can be ruled out from our COz/02 coadsorption experiments. Under these conditions complicated time-dependent relaxation phenomena are observed. 4. Summary and Outlook

0

60

120

la0 t

240

[minl

300

360

-

Fig. 9. Type 3 sensor: typical response curves (relative current Z/Z, vs. time t) monitored at Vf = 3 V of Pt/TiO, Schottky-type sensors upon exposure of CO in humid synthetic air (PHI0 = 50% r.h.) (a) and for different at a constant total pressure CO/O* mixtures ptot = IO5Pa (b).

energy E:F = - 133 meV [20] (compare the argument above) and hence to a decrease of the interface resistance. This is also observed for CO/O* coadsorption for partial pressures pco >poz/2, which is indicated in Fig. 9. Saturation effects in the I/lo =f(pco) characteristics may result from changes in the effective

From our spectroscopic and electrical characterization of metal/oxide interfaces, implications can be deduced for designing chemical sensors SnOl - and TiO,-based which monitor electronic conductivities, mixed conductivities with the latter and voltage-dependent conductivities determined by Schottky-barrier heights. Reliable sensors based on these principles require stable dopant distributions in the subsurface regions. To improve our Schottky-barrier sensors, we are currently developing thin-film structures with ohmic Pt back contacts (prepared at high temperatures) and with microstructured Pt Schottky contacts with large three-phase boundaries subsequently prepared at low temperatures. References 1 M. J. Madou and S. R. Morrison, Chemical Sensing with Solid State Devices, Academic Press, San Diego, CA, 1989, p. 419 and p. 479.

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2 W. GBpel, Chemisorption

and charge transfer at semiconductor surfaces: implications for designing gas sensors, Prog. Surf Sci., 20 (1985) 9-255. 3 K. D. Schierbaum, S. Vaihinger, W. Gopel, H. H. van den Vlekkert, B. Kloeck and N. F. de Rooij, Prototype structure for systematic investigations of thin-film gas sensors, Sensors and Actuators B, I

(1990) 171-175. 4 U. Kirner, B. Leibold,

D. Fischer, K. D. Schierbaum, N. Nicoloso, W. F. Chu, W. Weppner and W. Gopel, Low and high temperature TiO, oxygen sensors, Sensors and Actuators B, I (1990) 103- 107. 5 K. D. Schierbaum, U. Weimar and W. Giipel, Multicomponent gas analysis: an analytical chemistry approach applied to modified SnO, sensors, Sensors and Actuators B, 2 (1990) 71-78. 6 W. Giipel, Solid state chemical sensors: atomistic models and research trends, Sensors and Actuators, I6 (1989) 167-193. 7 J. F. Geiger, K. D. Schierbaum

Surface spectroscopic

and W. Giipel, studies on Pd-doped SnO,,

11th Int. Vacuum Congr. (IVC,ll)and 7th Int. Conf Solid Surfaces (ICSS.7), K&r, F.R.G., 1989, and Vacuum, to be published.

8 J. F. Geiger, P. Beckmann, K. D. Schierbaum and W. Giipel, Interaction of Pd-overlayers with SnO,: comparative XPS, SIMS, and SNMS studies, 6. Arbeitstagung Angewandte OberJIiichenanalytik, Kaiserslautern, F.R.G., 1990, and Fresenius’ Z. Anal. Chem., to be published.

9 U. K. Kirner, K. D. Schierbaum and W. Gopel, Interface-reactions of Pt/TiO,: comparative electrical, XPS, and AES-depth profile investigations. 6, Arbeitstagung Angewandte OberJtichenanalytik, Kaiserslautern, F.R.G., 1990 and Fresenius’ Z. Anal. Chem., to be published. 10 N. Nicoloso, TiO, and Bi,O, thin film sensors, Ber. Bunsenges. Phys. Chem., 94 (1990) 731-736. Alter-

native defect models for TiO, are described in J. Penneweiss and B. Hoffmann, Electrical conductivity of aluminium-doped TiO, ceramic after quenching, Mater. Lett., 5 (1987) 121-125.

11 C. G. Fonstad and R. H. Rediker, Electrical properties of high-quality stannic oxide crystals, J. Appl. Phys., 42 (1971) 2911-2918; S. Samson and C. G. Fonstad, Defect structure and electronic donor levels in stannic oxide crystals, J. Appl. Phys., 44 (1973) 4618-4621. 12 W. GBpel and U. Lampe, Influence of defects on the electronic structure of zinc oxide surfaces, Phys. Rev. B, 22 (1980) 6447-6462.

13 K. D. Schierbaum, Elektrische und spektroskopische Untersuchungen an Dtinnschicht-SnO,-Gassensoren, Thesis, Tiibingen, F.R.G., 1987, p. 27. 14 L. J. Brillson, The structure and properties of metal-semiconductor interfaces, Surf. Sci. Reps., 2 (1982) 123-326.

15 D. E. Eastman, Photoelectric work functions of transition, rare-earth and noble metals, Phys. Rev. B, 2 (1970) l-2. 16 Y. W. Chung, W. J. Lo and G. A. Somorjai, Low energy electron diffraction and electron spectroscopy studies of the clean (110) and (100) titanium dioxide (rutile) crystal surfaces, Surf. Sci., 64 (1977) 588-602. 17 V. N. Bogomolov and V. P. Zhuse, Anisotropy of the Hall effect in partially reduced single-crystal rutile (TiO,), Fiz. Tverd. Tela. (Leningrad), 10 (1962) 100 [Sov. Phys. Solid State, 5 (1963) 24041. 18 G. Rocker and W. Giipel, Titanium overlayers on TiO, (1 lo), Surf. Sci., 181 (1987) 530-558. 19 H. W. Gundlach and K. E. Heusler, Kinetics of the incorporation of oxygen into non-stoichiometric titanium dioxide films, Z. Phys. Chem. N. F., 119 (1980) 213. 20 W. Gopel, G. Rocker and R. Feierabend,

Intrinsic defects of TiO*( 110): interaction with chemisorbed O,, H,, CO, and CO,, Phys. Rev. B, 28 (1983) 3427-3438. 21 P. Esser, R. Feierabend and W. Giipel, Comparative study on the reactivity of polycrystalline and single crystal ZnO surfaces: catalytic oxidation of CO, Ber. Bunsenges. Phys. Chem., 85 (1981) 447455.