Surface spectroscopic studies on Pd-doped SnO2

Vacuum/volume 41/numbers 7-9/pages 1629 to 1632/1990

O042-207X/90S3.00 + .00 © 1990 Pergamon Press plc

Printed in Great Britain

Surface spectroscopic studies on Pd-doped SnOa J F G e i g e r , K D S c h i e r b a u m and W G 6 p e l , Institute for Physical and Theoretical Chemistry,

University of T&bingen, D- 7400 T~ibingen, FRG

Sn02 is a suitable base material for chemical sensors. Its sensitivity against reducing gases can be improved and adjusted by doping with noble metals. We studied diffusion processes and stabilities of impurities and the most important dopant Pd both, under oxidizing and reducing conditions at various temperatures by means of XPS, UPS, and ISS. Besides small amounts of Na and CL the main bulk acceptor-type impurity is Fe. It shows reversible in and out diffusion around 870 K strongly influenced by oxygen. From XPS core level shifts we deduce the low temperature metallic dopant Pd to form pd2+ O at the Sn02 surface after oxidation. Above 670K we observe diffusion of Pd 2+ into the bulk. Under reducing uhv conditions and upon H 2 exposure the reduction from surface Pd 2÷ 0 to Pd ° is driven by the thermodynamic instability of PdO.

1. Introduction Tin dioxide polycrystalline gas sensors doped with palladium are used in automatic gas alarm systems for the detection of methane, butane, hydrogen, and carbon monoxide t. Palladium dopants improve both the selectivity and sensitivity in gas sensing properties. This is attributed mainly to catalytic spillover effects influenced sensitively by chemical and electronic interactions between Pd and SnO2 2. Therefore, the aim of the present work was a surface spectroscopic characterization of Pd-induced changes of the surface electronic structure of SnO 2 and of redox reactions at the interface Pd-SnO2. The latter lead to changes in the oxidation states of Sn and Pd as well as to a subsequent segregation of Pd 2+ ions which introduce additional gap states in the n-semiconducting SnO2.

2. Experimental The XPS, UPS, and ISS experiments were performed in a combined high-pressure-uhv surface analytical system described elsewhere 3. The SnO2(110) surfaces with different bulk impurity concentrations were cleaned by sputtering (1 keV He +, 0.5/~A, 7 mm 2, 5 min), heating (T = 970 K, 20 min) and high temperature treatment with oxygen (p(O2) = 2 × 10 3 Pa, T = 970 K, 40 min) to anneal (sub)surface defects4. After this preparation procedure, palladium (Pd) was evaporated thermally onto the SnO2(lI0) surface with coverages of 0Od = 0.7 monolayers and subsequently exposed to oxygen with p(O2) = 600 Pa at 470, 670, 870, and 970 K for 6 min each. We choose 0pd <1 in order to study simultaneously reactions of this metallic overlayer with the gas phase and its interface reactions with SnO2 by our surface analytical tools.

3. Results 3.1. Ion back scattering spectroscopy (ISS). The ISS spectrum of the clean SnO2(ll0) surface (Figure l(a)) shows small amounts of contaminations of sodium (Na) and chlorine (CI) in the first atomic layer in addition to tin (Sn) and oxygen (O).

1 /

ISS

1 keV He +, 0.2 pA

Sn

o

~

(c) ¢/) c-

300

400

500

600

700

800

Kinetic E n e r g y (eV)

900

1000

,

Figure 1. ISS spectrum (He +, 1 keV, 0.2 #A) of clean SnO2 single crystals (a). For SnO2 with high Fe contamination, a reversible surface segregation of Fe could be induced by oxygen exposure (p(O 2) >/5 hPa) at high temperatures (T = 870K) (b). Segregation could subsequently be removed by uhv treatment (T = 970 K, 10 min) (c). The platinum signal results from the sample holder.

The contaminations are not observed in XPS due to the limited detection sensitivity. For single crystals with high concentrations of Fe impurities, reproducible surface segregation of Fe and back diffusion into the SnO2 bulk could be observed after annealing treatment above 870 K in oxygen (p(O2)/> 500Pa 02) and subsequent uhv treatment (970 K, 10 min) as shown in Figure l(b and c). In order to avoid significant time-dependent sputter effects during the ISS measurements, the kinetic energy range of the detected backscattered He + ions was kept within 700 <~Eki, ~< 1000 eV. For palladium doping experiments, we chose samples with low bulk impurity concentration and low oxygen pressures (p < 1 x 10 -2 Pa) to prepare stoichiometric surfaces of SnO 2. For T > 670 K and higher oxygen pressure, bulk impurities such as Na and Fe segregate at the surface as observed by ISS.

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J F Geiger et al: Surface studies on Pd-doped SnO 2

,

1

He II

XPS At

Sn3d~/

Sn3d~/~

Pd3d~2 PdO

P~dlleV

E D

"Z

E (c) ,

985

, 990

,

995

,

1000

,

1005

II

Kinetic Energy (eV)

"\ 20

15

10

5

(a)

Figure 3. AIK~-excited XPS spectrum (hv = 1486.6 eV) of Sn 3d and

0 =E F

-,--- Binding Energy E~ (eV)

Figure 2. Hell-excited UPS spectra (hv = 40.81 eV) of the clean SnO 2 surface (a) and after evaporation of 0.7 monolayers Pd (b). Spectra (c), (d), and (e) are taken after thermal treatments at 470, 670, and 870 for the same oxygen exposure (p(O2)=600Pa, 6rain) and subsequent rapid cooling down to T = 300 K. The kinetic energies are referred to the Fermi level E~.

Pd 3d core levels (a) of uncovered SnO2 (a), and of Pd-covered SnO2 (0 = 0.7 monolayers) (b). Spectra (c), (d), (e), and (f) are taken after thermal treatments at 470, 670, 870, and 970 K for the same oxygen exposure (p(O2) = 600 Pa, 6 min) and subsequent rapid cooling down to T = 300 K.

Pd 3d core levels. Changes of XPS intensities during reduction are below detection limit.

4. Discussion 3.2. Ultraviolet photoemission spectroscopy (UPS). The H e l l valence band spectrum of a clean SnO2 surface shown in Figure 2(a) indicates the mainly O 2p-derived emission with maxima at 5.2, 7.5 and I I eV. Assuming fiat band conditions for the stoichiometric surfaces, we determine the work function = 4.74 eV of SnO2 from the UPS spectrum. Changes of A/670 K). O , s "

3.3. X-ray photoemission spectroscopy (XPS). From the XPS spectra (Figure 3) obtained after oxygen exposure at different increasing temperatures, we deduce a transition from metallic Pd to oxidized Pd (PdO) at around 670 K. This is indicated by a chemical shift of the Pd 3d core levels of AE b = 1.1 eV. The Sn 3d photoelectrons show only small shifts in kinetic energies and a small broadening of the corresponding halfwidths. For temperatures above 270 K, we also observe changes in the intensities I of the Pd 3d and Sn 3d photoelectrons with quantitative results of the intensity ratios l ( P d 3d)/l(Sn 3d) given in Table 1. Under reducing conditions during H 2 exposure ( p ( H 2 ) = 1.2 x 10 -3 Pa, T = 970 K, t = 20 min) or under uhv conditions (12 min, T = 970 K), we deduce a complete transition from the oxidized to the metallic palladium from the XPS spectra of the 1630

4.1. Electronic interactions between Pd and SnO2. We will first focus on interface reactions occuring during thermal evaporation of palladium onto the SnO2. In XPS we observe small shifts of the Sn 3d core level binding energy which are not caused by a decrease of band bending. F r o m the UPS spectra we derive small positive changes--eAVs~O.2eV after Pd evaporation. We therefore conclude low concentrations of reduced Sn to be formed at the interface with binding energies attributed to oxidation states of Sn lower than 4 + . In Figure 4 the difference spectrum of curves (a) and (b) of Figure 3 for Sn 3d core levels is shown after their normalization. The presence of Sn in lower oxidation states, also confirmed by earlier studies of Huck and coworkers in their Auger electron spectroscopic (AES) investigations 5, probably results from a charge transfer from oxygen vacancies. The latter are formed thermally during dissipation of the local high heat of condensation during Pd adsorption. The corresponding interface states also affect the electrical characteristics of the Schottky-type P d - S n O 2 junction which is expected in simple theoretical models to be formed because of the larger work function of Pd if compared with SnO 2 (~s,o2 = 4.73 eV, ¢bpd = 4.82 to 5.17 eV6).

Table 1. Relative XPS peak intensities l(Pd 3d)/l(Sn 3d) of Pd-covered SnO 2 after different thermal treatments Pretreatment

Pd/Sn

clean SnO2(110) Pd-covered surface (0 = 0.7 ML) at T = 300 K after treatment in p(O2) = 600 Pa, t --- 6 min at T = after treatment in p(O2) = 600 Pa, t = 6 min at T = after treatment in p(O2) = 600 Pa, t = 6 min at T = after treatment in p(O2) = 600 Pa, t = 6 min at T = after reducing in uhv, t = 12 min at T = 970 K after reducing in p(H2) = 2 x 10 -3 Pa, t = 12 min at T = 970 K

0.000 0.122 0.0892 0.0778 0.0525 0.0478 0.0451

470 K 670 K 870 K 970 K

0.0448

J F Geiger et al. Surface studies on Pd-doped SnO 2

l

XPS

(a)

Sn3d~/2

AI K~ (b) (c)

_{5

03

E

1005

100(

Kinetic Energy (eV)

Figure 4. Two original spectra (above) and differencespectrum (below) of the normalized Sn 3d core level spectra in Figure 3(a and b). The intensities of the original curves are adjusted by a constant factor in order to obtain positive intensities of the difference spectrum at the high kinetic energy part only. The kinetic energy (a) corresponds to Sn4+, (b) to the maximum of the difference spectrum, and (c) to metallic Sn°.

For temperatures above 470 K, this Pd-SnO2 diode is thermally instable due to clustering of Pd metal overlayers and, subsequently, due to the formation of p-semiconducting PdO in the presence of oxygen.

4.2. Temperature-dependent interface reactions between O z and P d - S n O 2. From UPS and XPS measurements, different electrochemical reactions between oxygen and Pd-covered SnO 2 may be distinguished in the temperature range between 300 and 1000 K (Figure 5). At temperatures of around 300 K, we find stable Pd SnO_~ interfaces. In the Hell-excited UPS spectra, the valence band emission near Ev- is mainly determined by the metallic palladium because of the escape depth of photoelectrons in the order of about 1-2 monolayers. Even after exposure to oxygen, we find a stable homogeneous distribution of

the evaporated metallic Pd layer at the SnO 2 surface which for kinetic reasons does not react to form oxides. With increasing temperature (T ~470 K), Pd clusters are formed at the SnO2 surface. Driving force is the pronounced difference in the interface energies between the metal and the oxide and an increasing mobility of Pd with temperature. In this case, changes in the surface ratio of Pd relative to the uncovered SnO 2 surface lead to an additional valence band emission of SnO 2 observed in the Hell-excited UPS spectra (Figure 2) and to changes in the relative XPS intensities l(Pd 3d)/l(Sn 3d) in Figure 3. However, no change of the binding energies of the Pd core levels are found in XPS. At around 670 K, the thermodynamically controlled oxidation of Pd by 02 leads to PdO as it is indicated by the characteristic binding energies E b of the Pd 3d and Sn 3d core levels in XPS. As a result of the lower interface energy of PdO on SnO2, changes in the surface coverage of Pd ('spriting') occur. This was also found by Yamazoe in transmission electron microscopy (TEM) investigations of Pd-doped SnO2 polycrystalline material 7. A thermodynamically controlled oxidation of Pd by SnO2 via bulk reactions not involving oxygen vacancies in tin dioxide is impossible due to high positive values of the corresponding changes in Gibbs enthalpies8 and is therefore not observed during heating of P d - S n O , under uhv conditions. For temperatures above 870 K, we observed a pronounced Pd 2 + ion diffusion from the PdO at the surface into the bulk SnO 2 (Figure 5). This leads to thermodynamic equilibria with negligible segregation of Pd for the Pd mole-fraction in the bulk chosen in our studies. Because of larger atomic if compared with ionic radii of Pd and Pd 2+ (rpo = 137.2pm and rpd2+ = 86 pmg), we expect only Pd 2 + to penetrate into bulk SnO2. This is confirmed by the decreasing intensity of Pd 2 +derived Pd 3d emission in XPS. In UPS we observe PdOderived valence band structures with an emission onset below the Fermi edge (Figure 2(e)). The Pd 2 + bulk diffusion is observed only after formation of PdO at the surface at temperatures above 870 K (see, e.g. Table 1). In contrast to the rapid oxidation times of Pd at the surface (t ~<6 min at T = 870 K, see Figure 3(e)), the bulk diffusion of Pd 2 + is slow and not completed during thermal treatments in the order of t ~<6 min.

5. Summary and outlook Annealing temperature ISS

UPS

XPS

290 K

~

Pd°

p.O

L70 K

SnO2'~'~'

information depth [layers]

670 K

"'"

Z- i~d2. ' "

Annealing temperature 870 K

.......

--- PdO

/i/.~l//i.//i.~z//

e

e •

:

t hermodynamlcaLly controlled s t a t e e .... :

....

~. T,,

Z

Figure 5. Survey on temperature-dependent interaction mechanisms between Pd and SnO2. Further details are explained in the text.

We have investigated temperature-dependent electronic and ionic reactions between palladium overlayers and tin dioxide under oxidizing and reducing conditions. Stable Pd-SnO2 interfaces are found at low temperatures even in the presence of oxygen and are found also at higher temperatures but under reducing conditions only. The changes from metallic Pd clusters to p-type semiconducting PdO at the surface occur in oxygen under operation conditions of Pd-doped SnO2-based gas sensors, i.e., at T ~ 670 K. Under these conditions exposure of strongly reducing gases such as H2, CH4, or butane leads to changes in the opposite direction, i.e., to the formation of metallic Pd. The surface ions Pd 2 ÷ diffuse into the SnO2 bulk at temperatures above 670 K for oxygen pressures above 600 Pa. The bulk impurity ions Fe 6 + act as acceptor-type defects in SnO2 l° which segregate at the surface upon oxygen exposure at temperatures above 870 K. This effect can be reversed by pumping off 02 and cooling down. We therefore conclude oxygen to trap Fe-related defects at the surface at T = 870 K. 1631

J F Geiger et al: Surface studies on Pd-doped SnO2

Acknowledgements We acknowledge the experimental assistance of H M Schlude, U. Schmid, J. Siegle and S. Warda during their practical exercises in our group. The work was supported by the Bundesminister ffir Forschung und Technologie, Bonn ( F K Z 13 AS 00138) and Fonds der Chemischen Industrie.

References t N Yamazoe, Y Kurokawa and T Seiyama, Sensors and Actuators, 4, 283 (1983). 2 S Matsushima, Y Teraoka, N Miura and N Yamazoe, Japan J Appl Phys, 27, 1798 (1988).

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3 W G6pel, Sensors and Actuators, 16, 167 (1989). 4K D Schierbaum, H D Wiemh6fer and W G6pel, Solid St lon, 28-30, 1631 (1988). 5 R Huck, U B6ttger, D Kohl and G Heiland, Sensors and Actuators, 17, 355 (1989). 6 R C Weast, CRC Handbook of Chemistry and Physics, E-76. CRC Press, Boca Raton (1983). 7 N Yamazoe, Private communication. s I Barin and O Knacke, Thermodynamical Properties of Inorganic Substances, p 697. Springer, Berlin (1973). 9 E Fluck and K Heumann. VCH Verlagsgesellschaft, Weinheim (1985). io W G6pel, K D Schierbaum, H D Wiemh6fer and J Maier, Solid St Ion, 32/33, 440 (1989).