Plasma-induced oxidation of Ag(110)

L276

Surface Science 155 (1985) L276-L280 North-Holland, Amsterdana

SURFACE SCIENCE LETTERS PLASMA-INDUCED OXIDATION OF Ag(ll0) M. BOWKER New Science Group, ICI PLC, P.O. Box 11, The Heath, Runcorn WA7 4QE, UK Received 3 December 1984; accepted for publication 7 February 1985

The exposure of a clean Ag(ll0) surface to a glow discharge oxygen plasma has been found to induce considerable surface oxidation which otherwise cannot be achieved under the same pressure/temperature conditions. As a result of such treatment several states of oxygen appear to be adsorbed/absorbed onto the crystal, which have been characterised primarily- by thermal desorption. Interestingly, the main desorption occurs at 570 K indicating that desorption from bulk oxide occurs at the same temperature as the surface chemisorbed layer. This result explains previous observations of exchange of surface oxygen with subsurface oxygen in desorption experiments.

The adsorption of oxygen on silver has been studied by numerous workers and a multiplicity of adsorption states has been identified [1-7]. Thus two molecular states of different stability have been found, one desorbs at 200 K [1-4], the other at - 350 K [5,6]; atomic states have been seen desorbing at 600 K [1,2,7], even a doublet has been identified in this temperature range [4,6]. Finally a state described as subsurface oxygen has recently been identified by thermal desorption from clean silver wire [6]. It is important to note that Backx et al. [2] have investigated this system and claim to have seen exchange between subsurface, predosed a60 and surface chemisorbed aSo occurring in the "normal" desorption peak from Ag(ll0) which is observed at 600 K, even though no "subsurface" oxygen state was directly identified by desorption. In the present work new light is shed on this adsorption/absorption system by the use of an unusual dosing method, that is, treatment of a Ag(ll0) surface with the highly excited molecular oxygen species, ionic species, and dissociated species, produced in a DC glow discharge plasma of oxygen [8,9]. The techniques used to study this system were XPS (Vacuum Science Workshops model HA50, with 50 mm diameter hemispherical electron analyser), Auger electron spectroscopy and mass spectrometric thermal desorption. The sample was heated by radiation from a tungsten filament close to the crystal. Typical conditions for a DC glow discharge to be obtained were 0.1 Torr oxygen pressure, under which conditions a plasma of 1.0 mA is produced at a voltage of 400 V. The plasma is produced between the negatively charged sample and 0039-6028/85/$03.30 © Elsevier Science Publishers B.V. (North.Holland Physics Publishing Division)

M. Bowker / Plasma-inducedoxidation of Ag(l lO)

L277

M5Na,sN4.5

/1 I ~J

o,ls,

C.ao---A

/

"~ Plasma J~ ~ea~/xl~~

t=:

--v

f ~. -~% ~~_

Binding Energy(eV) I

360

i

i

I

t

I

I

I

I

i

I

i

I

r

i

I

355 350 Kinetic Energy (eV)

Fig. 1. XPS and XAES spectra for plasma oxidation of Ag(ll0) using 25 eV pass energy in the analyser. The plasma treatment was carried out at 0.1 Torr with 0.36 W discharge power for 60 s.

earthed parts of the sample holder. Observation of the glow so produced indicates a homogeneous light intensity in the region of the sample surface, the glow being extinguished - 2 cm away from it. The result of such a plasma treatment for 1 rain is shown in fig. 1 which presents both O(ls) XPS and Ag M4,sN4,sN4,5 XAES spectra. It can be seen that there is an intense O(ls) signal (O(ls) : Ag(3ds/2) area ratio of 0.12), but more importantly significant shifting of the silver Auger peaks has occurred (by 1.4 eV for M4N4,sN4,5 and 1.0 eV for MsN4,sN4,5) indicative of the formation of silver oxide and similar to changes observed for oxidation of Cd [10], and In [11]. The main O(ls) peak is at 529.6 eV binding energy: this is measured relative to the C(ls) line - taken as a reference and assumed to be 285.0 eV, from which the Ag(3d5/2) peak for the clean metal is at 368.2 eV, in agreement with others [12]. This peak is usually associated with a smaller one (seen as a shoulder at - 531.2 eV) whose relative intensity is dependent upon the exact plasma conditions used. In contrast to other reports [10,13] the Ag(3d5/2) peak was found to shift only slightly (by - 0 . 2 eV) to a lower binding energy upon oxidation, a value which is within the individual reproducibility value of + 0.2 eV. Fig. 2 shows the thermal desorption spectrum obtained after such a dose of oxygen onto the surface. The main desorbate was oxygen, although much smaller amounts of CO 2 were also evolved. This is probably produced from "clean-off" reactions of the plasma with other areas of the support assembly. The small amount of CO 2 is probably adsorbed/absorbed in the form of silver

L278

M. Bowker / Plasma-induced oxidation of Ag(l lO) 0.1 torr 0.36 Watts 60 secondstreatment, 300 K

Mass Spectrometer Signal (m/e 32, arb. unit.•

J I

I 500

I

I I I 600 700 Temperature(K)

I

I 800

Fig. 2. Thermal desorption spectrum after plasma oxidation of the Ag(110) sample under the conditions specified. The heating rate was 3.5 K s -1 and the amount of oxygen desorbed corresponds with about 230 layers of oxide, assuming the stoichiometry AgzO.

carbonate and desorbs in the same temperature region seen for surface carbonates on silver [7], that is, 450 K. The oxygen desorption spectrum is particularly fascinating since the majority of the desorption appears at 570 K, very close to the desorption temperature from the chemisorbed atomic state [1,2,7]; indeed analysis of the leading edge of the desorption gives a desorption activation energy of 170 (+ 15) kJ mol, with a pre-exponential of 1015(± 0.7) s-1 in good agreement with previous findings [7]. This is very significant since in their surface studies Backx et al. have reported that if they pretreat a Ag(ll0) surface in 1602, then heat to 720 K to desorb the majority of the oxygen, and then redose with 1802 at 300 K, subsequent desorption yields mixed isotopic products in the desorption peak which they see at 580 K [2]. Exactly why all the 160 has not been lost by heating to 720 K is not clear, presumably some 160 is dissolved in the lattice, may be still re-emerging at 720 K, and could be quenched near the surface on cooling. Nevertheless the results here show that mixed isotopic desorption should be expected from a subsurface oxide layer having a different label from the surface layer since desorption can occur from both in the same temperature regime. Indeed the present results appear to show that "surface" oxygen is really

M. Bowker / Plasma-induced oxidation of Ag(110)

L279

very little different from bulk oxide and therefore the chemisorbed atomic layer itself should be considered to be oxidic in nature. The desorption peak at 570 K is actually composed of two peaks very close in desorption temperature, and again their relative population is dependent upon the exact dosing conditions. There are also peaks present in the desorption at 700-750 K and it is likely that these are due to oxygen atoms dissolved in the substrate since there is relatively little O(ls) signal left after heating to 610 K and the Auger peaks have reverted to their clean Ag metal appearance, even though this desorption represents about 80 monolayers equivalent of absorbed oxygen (assuming monolayer stoichiometry of Ag20 ). This desorption state appears at a similar desorption temperature to the so-called y-state reported by Haul et al. for oxygen desorption from silver [6], and also considered by them to be subsurface. The state of oxide produced is most likely to be Ag20, although this shows little distinction from AgO in electron spectra (the Auger peak shape and position, in particular, are very similar [13]). However since little Ag(3ds/2) shifting has been observed it is unlikely that the oxidation is in the form of AgO since the peak should shift - 0 . 8 eV to lower binding energy [9,13], a value well outside the reproducibihty limits in this work and those in the references cited. The O(ls) spectrum shows a single major peak, unlike the spectra shown in the work of Schon [13] which shows a doublet a 529.0 and 530.4 eV for bulk Ag20 powder. It may be that the shoulder observed in the present work at - 531.0 eV is a small amount of the higher binding energy state seen by Schon, since it is lost last by thermal processing as also seen by him. Schon interprets the two states as lattice-oxygen (low BE state) and adsorbed oxygen (high BE state), but the latter choice seoms extremely unlikely since chemisorbed oxygen is lost [1] at the same temperature as the bulk decomposition (shown above). Furthermore, the relative intensity of the two peaks in Schon's work is - 1, whereas the bulk lattice oxygen should dominate the spectrum. It is unlikely that this peak is due to carbonate or hydroxyl impurities since they are less thermally stable than the oxide. The exact nature of this state, then, is not clear, but could possibly be due to a defect form of oxygen within the Ag20 lattice. In conclusion, then, silver can be readily oxidised in a DC plasma of molecular oxygen to produce predominantly an Ag 20 layer at the surface. This layer then decomposes thermally at - 570 K, an identical temperature seen for desorption of chemisorbed oxygen from Ag(110) [1,2]. Thus, it would appear that the chemisorbed layer is, in fact, very little different from silver oxide - a fact which is of some significance for the industrial catalytic process of ethylene epoxidation [14], where "surface" and "subsurface" states are thought to be important for the selective production of ethylene oxide [15].

L280

M. Bowker / Plasma-induced oxidation of Ag(l l O)

References [1] M.A. Barteau and R.J. Madix, Surface Sci. 97 (.1980) 101. [2] C. Backx, C. de Groot and P. Biloen, Surface Sci. 104 (1981) 300. [3] C.-T. Au, S. Singh-Boparai and M.W. Roberts, J. Chem. Soc., Faraday Trans. I, 79 (1981) 1779. [4] C.J. Campbell and M.T. Paffett, Surface Sci. 143 (1984) 517. [5] R.M. Lambert and R; Grant, Surface Sci. 146 (1984) 256. [6] R. Haul, G. Neubauer, D. Fischer, D. Hoge and U. Zeeck, in: Proc. 8th Intern. Congr. on Catalysis, Berlin, 1984, Vol. 3, p. 265. [7] M. Bowker, M.A. Barteau and R.J. Madix, Surface Sci. 92 (1980) 528. [8] J.R. Hollahan and A.T. Bell, Eds., Techniques and Applications of Plasma Chemistry (Wiley, New York, 1974) pp. 26-56. [9] P. Koeian, Phys. Letters 73A (1979) 17. [10] S.W. Garenstroom and N. Winograd, J. Chem. Phys. 67 (1977) 3500. [11] D. Briggs and M.P. Scab, Eds., Practical Surface Analysis (Wiley, Chichester, 1983) p. 103. [12] D. Briggs and M.P. Seah, Eds., Practical Surface Analysis (Wiley, Chichester, 1983) p. 503. [13] G. Schon, Acta Chem. Scand. 27 (1973) 24. [14] X. Verykios, F. Stein and R. Coughlin, Catalysis Rev. Sci. Eng. 22 (1980) 197. [15] C. Baekx, J. Moolhuysen, P. Geenen and R. van Santen, J. Catalysis 72 (1981) 364.