TiO2 system

vacuum/volume 35/number Printed in Great Britain

Reactivity E RomBn,

3lpages

0042-207X/85$3.00 + .OO Pergamon Press Ltd

of the H,02/Ti02

M Sgnchez-Avedillo

Fisica de Materiales

125 to 126/l 985

del CENFA

system

and J L de Segovia, ‘L Torres Quevedo’

Laboratorio de Fisica de Superficies, CSIC Serrano 144, 28006 Madrid, Spain

lnstituto

de

received 20 August 1984

The reactivity of the H202/Ti02 system for samples treated at atmospheric pressure is studied by means of Auger Electron Spectroscopy. The most significant changes on the TiOp ‘final’ state are: (i) The l(O)fl(Ti) peak-topeak ratio is diminished by 15%. (ii) The LMV transition suffers an energy shift of +2 eV, whereas the peak-topeak width diminishes 1.5 eV. (iii) Its structure changes with respect to that of the TiOp ‘initial’ state although it is not identified with any one of the known stoichiometric oxides. (iv) When the samples that represent the ‘final’ state are heated up to 650 K, the AES spectrum matches quite well that of the TiO2 ‘initial’ state.

Whereas the interaction of TiO, with 0, or other molecules containing oxygen (e.g. H,O) has been thoroughly studied by means of different techniques of secondary electron spectroscopies, there is no reference on the changes which can be induced on the various spectra upon treatment of such a component with such a high oxygen containment and reactivity, as H,O,. Works by Boonstra and Mutsaers’ have demonstrated that this molecule is chemisorbed on TiO,. In this paper some preliminary results found for TiO, samples treated with H,O, at atmospheric pressure and using AES are presented. The analysis has been performed with a CMA with an axial electron gun (2500 eV and 2 PA current emission), placed in a uhv system. This system reached an ultimate base pressure of4. lo- ’ Pa after being exposed at atmospheric pressure and pumped down for 24 h without any bakeout. TiO, samples were prepared as pressed powder in an inert atmosphere (Ar) in stainless steel cylinders, 6 mm in diameter and 3 mm in thickness. These samples were placed in the uhv system and their AES spectra correspond with those of the so-called TiO, ‘initial’ state. Another group of samples prepared in the same way was treated with H,O, and introduced later into the uhv system. The spectra recorded correspond with the so-called TiOz ‘final’ state. In both cases, AES spectra were recorded at a pressure of 5.10- ’ Pa. The AES spectra in dN(E)/dE corresponding to various transitions, LMM, LMV and LVV for Ti and KLL for 0, are presented in Figure 1 for the different samples studied: (a) Ti(poli); (b) TiO, (anatase) ‘initial’ state; (c) TiO, reached after treatment with H,O, ‘final’ state; (d) this latter after heating at 650 K. Spectra for Ti (curve (a)) and TiO, (curve (b)) are included only for reference. Both of them present the characteristic structure well described in the literature. In the spectrum corresponding to TiO, (curve (b)) ‘initial’ state, the LMV transition does not show the characteristic double peak which is probably due to a low resolution in the spectrum. However, there is a remarkable asymmetry and a bump which is present in the low energy zone.

significant changes are observed in the spectrum corresponding to the ‘final’ state sample treated with H,Oz, (curve (c)): (i) the structure of the LMV transition becomes more symmetric and the bump in the low energy zone disappears; (ii) this transition has an energy shift; (iii) the peak-to-peak width diminishes; (iv) the I(O)(KLL)/I(Ti)(LMM) ratio also diminishes. The energies corresponding to LMM and LMV Ti transitions and the O(KLL) transition are presented in Table 1 as calculated in the present work. Whereas there is no shift for the LMM transition, both in the ‘initial’ and in the ‘final’ state with respect to those corresponding to metallic Ti, the LMV transition presents an energy shift of - 1 eV for the TiO, ‘initial’ state. These results agree with observations by Solomon and Baut?. However, this transition has a shift of N + 2 eV with respect to metallic Ti and of 3 eV with respect to TiO, for samples corresponding to the ‘final state, namely after being treated with H,O,. The values for the peak-to-peak width in the samples studied are shown in Table 2 along with some of the values found by different authors. It can be noted that the treatment with H,O, causes a decrease of 1.5 eV in the peak-to-peak width. Finally, the Ti(LMV)/Ti(LMM) and O(KLL)/Ti(LMM) peak-to-peak ratios are shown in Table 3. Whereas the former remains constant at 1.86 for all the samples studied, the latter decreases from 2.14 in TiO, to 1.82 in samples corresponding to the ‘final’ state. When these are heated up to 650 K, the ratio moves towards its original value, although not completely, reaching a value of 2.04 instead of the initial value of 2.14. From these preliminary experiments it is not possible to explain either the mechanism and reactions that take place between the initial and the final state, or the electronic structure which characterizes the final state. Other techniques, specially UPS, would be necessary to establish it. However, some hypotheses which are based on these results can be considered. First, the reversibility of the reaction can be pointed out: the ‘final’ state when it is thermally treated regains the structure of the TiO, ‘initial’ state except for cases where the O/Ti ratio does not Some

125

E. RomBn,

M SBnchez-Avedillo

Reactivity of the H202/Ti02 system

and J L de Segovia:

Table 1. Energy in eV of the AES Ti(LMM), Ti(LMV) and O(KLL) of

the different samples

I

400

I ’ 500

I

600

(eV)

Ti

TiO 2

Ti02/H,0,

Transition

380 415.7

380.0 414.7 511.5

380 417.6 511.4

LMM LMV KLL(0)

Table 2. Peak to peak width of the LMV transition for Ti, TiO, and TiO,/H,O,

(c) Ti 021 H2 0,

Reference

Ti

TiO,

TiO,/H,O,

2 4 5 This work

4.1 3.8 5.5 3.3

11.0 9.7 11.7 10.0

~ 8.5

Table 3. Peak to peak ratios of TiO, and TiO,/H,O,

I

600

400

500

600

(eV)

(e’.‘)

Figure 1. AES spectra of the different samples: (a) Ti (polycrystalline);

(b) TiO, ‘initial’ state; (c) TiO,/H,O, 650 K.

‘final’ state; (d) after heating at

Sample

Ti(380)/Ti(418)

0(511),QI(380)

TiO, TiO,/H,O, After heating at 650 K

1.87 1.86 1.86

2.14 1.82 2.04

LMV transition which is very sensitive to such changes, as shown, among others, by Nishigaki3. The ‘final’ state is characterized by its uniformity (see curve (c)) and the decrease of the peak-to-peak width. This structure does not correspond with known stoichiometric oxides, but it is clear that it is not that of initial TiO,.

Acknowledgement

reach the initial value of 2.14. This fact might be explained on the basis of the TiO, initial sample containing some additional 0, which is not part of the oxide. This additional O,, along with some oxygen coming from TiO, might have been reduced by H,O,. When the sample is heated in vacuum, oxygen diffusing from the bulk rebuilds the oxygen on the surface and this regains the structure corresponding to TiO, (see curve (d)). Finally, it is worth pointing out the change in the electronic structure of the valence band. This feature is clearly seen in the

126

We gratefully manuscript.

thank

J. P. Adrados

for his comments

on the

References

’ A H Boonstra and C A H A Mutsaers, J Phys Chem, 79, 1940 (1975). 2 J S Solomon and W L Baun, Surface Sci, 51, 228 (1975). 3 S Nishigaki, Surface Sci, 125, 762 (1983). 4 M L Knotek and J E Houston, Phys Req B25, 3563 (1982). ’ S Thomas, Surface Sci, 55, 754 (1976).