Ultramicroscopy 23 (1987) 299-304 North-Holland, Amsterdam

299

THE ELECTRON-BEAM-INDUCED REDUCTION OF TRANSITION METAL OXIDE SURFACES TO METALLIC LOWER OXIDES

David J. SMITH and M.R. McCARTNEY

Center for Sofid State Science and Department of Pt(vsics, Arizona State University. Tempe, Arizona 85287, USA

and

L.A. BURSILL

School of Pt(vsics, Unieersitv of Melbourne, Parkt,ille, Victoria 3052, Australia

Received 21 January 1987; received at editorial office 15 June 1987

The clean surfaces of sevcral maximally valent transition metal oxides, namely ".riO 2, Nb20.~, V:O 5 and WO 3, have been reduced to the corresponding binary oxide during extended observation inside a 400 keV high-resolution electron microscope. Selected-area electron diffraction patterns and optical transforms direct from the high-resolution lattice images were used to identify the oxide, while electron energy loss spectra from a sample of VzO5 eliminated the possibility that the isostructural carbide had been formed. "lhese reduced binary oxides have a defective rocksalt structure, and they art, metallic in nature. which may thereby stifle further electron-stimulated desorption of oxygen.

1. Introduction

The high current densit2 ' of the electron beam which is used to irradiate inorganic materials dur- ing high-resolution observations can lead to con- siderable modification of the surface and bulk of the materials. Within the bulk, any susceptibility to damage is reduced because of an "encapsula- tion" effect due to surrounding material [1], par- ticularly at incident beam energies below the threshold for knock-on atomic displacement. Closer to the specimen surface, however, there is a greater likelihood of structural change eventually occurring because constituent atoms of the material may be permanently lost to the nearby v~r,,,,m ,~J' tho microscope. Our tn..art;o,,l~r ;.'~xar~ct"

here is in the surface reduction of transition metal oxides (TMO) which is induced by an incident electron beam during extended periods of observa- tions [2]. By using a combination of surface profile imaging, selected-area electron diffraction and optical diffractograms, we have established, for a number of maximally valent TMO (namely TiO.

V205, Nb205, and WO~), that the surfaces of these materials are reduced only as far as the corre- sponding binary oxide. Further reduction, which would otherwise lead to the accumulation of metal on the oxide surface, appears to be stifled by the metallic nature of these reduced oxides. In this short paper, we concentrate primarily on a posi- tive identification of the surface oxide layer. A comprehensive report, which includes numerous micrographs and structural models, will appear elsewhere [3].

2. Experimental details and .esuits

• ,,,. ,_,A,u,.. , . ,~, .~ml~ u ~ u in th i s " - ~ ' ~ u u ) w e r e ifi

the form of high-purity powders. Samples ,,,,'ere prepared for electron microscopy in the normal fashion by crushing the powder under ethanol or isopropanol with an agate mortar and pestle, and allowing a drop of the suspension to dry on a holey carbon support film. Most observations were made with a JEM-4000EX hioh-resolution elec-

0304-3991/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

300 D.J. Smith et al. / Oxide surface reduction to metallic lower oxides

tron microscope (HREM), operated primarily at 400 keV. Typical magnifications were 600,000 and 800,000, with current densities at the specimen in the range 10-50 A/cm 2 for irradiation of the sample and 5-10 A/cm 2 for recording micro- graphs. A Philips 400T electron microscope with Gatan 607 electron energy ~9ss spectrometer was used at 100 keV to record spectra from vanadium oxide crystals (originally V205) after various peri- ods of irradiation.

Figs. l a - l c show a series of selected-area elec- tron diffraction (SAED) patterns taken at 400 keV from a specimen of vanadium oxide. The first pattern (la) was recorded as soon as the crystal had been tilted into a zone axis projection which, in this case, corresponded to the [001] projection of V205. The crystal was irradiated for a period of 25 min at an average current density of 10 A / c m 2, and patterns lb and lc were then recorded. Fig. lb was recorded from a comparatively large re- gion (1/~m) of the original crystal, most of which had only received a small fraction of the previous irradiation, whereas fig. lc came from a smaller, strongly irradiated, region ( - 0 . 2 /tm) along the crystal edge. l~,ote the extra array of spots visible in fig. lb wh:Lch is absent in fig. la and which appears to ha ve a well-defined orientational rela- tionship with the V205 spots. Fig. 2 shows a high-resolutioa micrograph subsequently recorded from the edge region: a disorganized, but basically orthogonal, lattice is clearly ,~isible. Further crystals of V20 ~ in [101] and [011] projections

were also irradiated. Extra sets of spots were again observed in these cases, parallel to one major axis, and the spacings only corresponded to those visi- ble in figs. lb and lc. In particular, no spots were ever observed which corresponded to larger lattice spacings.

The dimensions of the SAED pattern from the o

original V205 crystal (orthorhombic, a = 11.51 A, b = 3.559 A, c - 4 . 3 7 1 A) provided a reference calibration for accurate measurement of the extra spots present in figs. lb and lc. These spots were measured to have lattice spacings of 2.04 + 0.02 ~, and 1.42 + 0.02 .A. Reference to the ASTM card index files reveals a considerable number of lower valence vanadium oxides, such as V405, V6013 , V8015, and VO2, which have comparatively large unit cell dimensions and poredominant lattice spac- ings in the range 3.3-3.6 A. Only vanadium metal (bcc; a0 = 3.027 ,~,) and the vanadum oxide ,, V n ,, "-'0.9 (a0 = 4.12,4,) (which has a basic rocksalt structure but with cation and /or anion vacancies depending on the exact stoichiometry [4]) would seem to be serious candidates for the modified surface layer, and a - 4 % distortion would be required in the case of vanadium metal. In later discussion we present further evidence and rea- sons for proposing that the surface layers of the V205 have been changed to "VO0.9".

Crystals of rutile, TiO2, were irradiated for extended periods, and modifications of the surfaces again took place. Fig. 3a shows an optical diffrac- togram taken from the edge of a high-resolution

Fig. 1. Selected-area electron diffraction patterns from crystal of vanadium oxide (a) V205, [001] projection; (b) same as (a) after 25 min, 10 A/cm2; (c) edge region of (b).

D.J. Smith et al. / Oxide surface reduction to metallic lower oxides 301

Fig. 2. Surface region of vanadium oxide crystal after period of electron irradiation (25 min, 10 A/cm 2).

image of a rutile crystal, in a [100] projection, after it had been irradiated for about 60 min at a current density of 2 0 A / c m 2. The characteristic geometry of the rutile cross-lattice is easily identi- fied and, by reference to the known rutile lattice spacings, the extra spots visible in the O D M are found to correspond to spacings of 3.76 _+ 0.02 and 2.78 _ 0.02 ,~. These values are within 1% of the listed values for (011 } and {220} reflections in TiO [5]. It is significant, in view of our results for vanadium oxide, that TiO is reported to have a structure similar to that of NaCl but with ordered arrays of vacant lattice sites [5]. It is also interest- ing that there was always a well defined angular relationship between the directions of the major futile lattice and those of the surface monoxide phase [3]. As represented by the schematic of fig. 3a drawn in fig. 3b, there were two possible orien- tations of the surface oxide for [100]-oriented crystals of rutile.

g lt. ~ l t %,, t t ~ l I ~ , , U 0 L UU ~,' l 1 ¢,~ ~,D U t=,,, I~,, 1 1 ~,,~ ~O' 1 1 1 [ J I t,,, l, k,,,'klt I,...P 1 It 1 l K ;

effects of beam reduction on WO 3 surfaces [3], and fig. 4 shows how the surface of a [ll0]-ori- ented crystal of WO 3 has developed a tetragonal array of fringes. With the resolution available from the JEM-4000EX, it was possible, with care, to distinguish clearly between the three {100}-type projections of the pseudo-cubic WO 3 lattice, and

b *011 ®022 O

O

"011

* "020 o @

o

• T i 0

• . TiO 2 Fig. 3. (a) Optical diffractogram from the imag..~: ~,i" ~ crxstal of rutile after it had been irradiated at 4..00 keV ({~0 rain, 20 A/cm:) , (b) schematic of (a) i,aentifying the tutile spots and

the extra spots due to the surface monoxide phase.

accurate estimates could be made of the lattice spacings of the reduced surfacc layer. Such accu- racy was presumably not possible in ti~e 200 keV study of WO 3 by Petford et al. [6], who apparently could not distinguish between the three projec- tions of WO~ because of limited microscope r. so- ~uuon. in ~g. 4, mr example, two sets of fringes have spacings of 2.38 + 0.03 A and the other set measures 2.06 +_ 0.02 ,~, which is consistent with a [110]-type projection of a cubic material (a,~ = 4.21

Electron beam irradiation of Nb:O 5 crystals led to surface fringes which could again be identified v,:ith the monoxide phase by reference to the char-

302 D.J. Smith et aL / Oxide su~flace reduction to metallic lower oxides

Fig. 4. Re#on of WO3 in a [110] projection following electron irradiation at 400 keV. Note the modified surface lattice with

spacings of 2.38 and 2.06 ,~,.

acteristic 3.82 A, lattice spacings of Nb205. The observed 2.09 Ai spacings of the modified surface region correspond to the {200}-lattice spacings of

NbO (a = 4.210 A,) but they do not match closely with any of the spacings of Nb(a 0 = 3.307 A). It is interesting that a similar spacing had been previ- ously observed in our earlier studies involving the "metallization" of Ti 2NbmO29 surfaces under in- tense electron irradiation [7], though positive iden- tification of the surface phase was not made at that time.

We had previously reported [7] the observation of a square surface lattice on WO 3 with fringe spacings of 2.06 4-0.03 A, which we had tenta- tively assigned as being {211} fringes of the fl- phase of tungsten/tungsten oxide [8]. We now believe, on the basis of our subsequent observa- tions of TiO2, V205 and Nb205, that we are devel- oping a novel tungsten oxide, with a defective rocksalt structure and with nominal composition WO, as a result of the electron irradiation [3]. This conclusion is not in conflict with earlier Auger studies of oxide layers on transition metal surfaces

200 K

U

8

i t

I , , ,,,, ,,

, , , , , , , , , ,, , , , ,

6154

. . . . . : . . . . . . . . . . . .

o o ' ' ' do ' do ' 1oo

] 36O 740 g30

[nergy Loss ( eV )

Fig. 5. EELS spectrum from a sample of V205 after irradiation at 100 keV. Note the presence of a low-loss peak at 10 eV/see inset). which is not due to vanadium metal but could be due to oxygen [13]. and the position of the second low-loss peak at -- 24 eV

indicating tha' vanadium is present in an ionized state.

D.J. Smith et al. / Oxide surface reduction to metallic lower oxides 303

which showed that the electron-stimulated-desorp- tion (ESD) processes had depleted the surfaces of oxygen but had not caused its complete removal [9,10]. Moreover, it is significant that TiO, NbO and "~O0.9" are metallic oxides [4,5]. It seems possible that the ready availability of conduction- band electrons would quickly de-excite the triter- atomic Auger transition processes which might otherwise cause further oxygen desorption to oc- cur [11].

Several of the transition metal binary oxides, in particular those of V, Nb ?rid Ti, are isostructural with the corresponding carbide. Given that surface contamination by carbonaceous material is some- times known to occur during obsen'ation within the electron microscope, it was considered im- portant to establish whether the modified surface layer contained any significant amounts of carbon. A sample of V205 was accordingly placed in the Philips 400T and a suitable V205 crystal was ori- ented into the [001] projection. Electron irradia- tion quickly led to a fading, and eventual loss, of the V205 spots in the SAED pattern, with the concurrent development of a square lattice like that visible in fig. l c. EELS spectra were then recorded from the modified surface legion and an example is shown in fig. 5. Careful analysis of the spectra established the absence of a carbon peak at 283 eV and the presence of a peak at 532 eV which is most likely due to the oxygen K-edge. Unfortunately, however, the near-edge structure of the L2, 3 peak of vanadium commencing at 513 eV is too close to the oxygen K-edge for proper background removal. An extra low-loss peak is clearly visible at 10 eV which is not present in spectra from vanadium metal and could be due to oxygen [12]. Moreover, the peak in the low-loss region at about 24 eV is shifted to higher energy by about 3 eV relative to that of V metal, suggest- ing that the majority of the V atoms arc present in

, " i t I _ ! an ionized state, tne EELS results thus snow that effectively no carbon is prese~,t and also suggest that some oxygen still remains after the surface modification has occurred.

3. Conclusion

InterLse electron irradiation of several maxi- mally valent transition metal oxides causes their reduction by an electron-stimulated-desorption process to the corresponding binary oxides. These oxides have a defective rocksalt structure, and they are metallic in nature: their ready supply of conduction electrons serves to stifle any further desorption of oxygen.

Acknowledgemer.ts

This work was undertaken in the National Facility for High-Resolution Electron Microscopy, within the Center for Solid State Science at Arizona State University, established with support from NSF Grant DMR-8306501. Financial sup- port for this research from the Office for Naval Research MAPS program is gratefully acknowl- edged. We are grateful to B. Miner for her assis- tance in producing fig. 5, and to Dr. M. Skiff for useful discussions.

References

[l! S. Salih and V.E. Cosslett, Phil. Mag. 30 (1974) 225. [2] See, for example, D. Lichtman, Ultramicroscopy 23 (19,~7)

291. [3] D.J. Smith. L.A. Bursill and M.R. McCaI"-.,ex, Surface

Eci., submitted. [4] P. Kofstad, Nonstoichiometry, Diffusion and Electrical

Conductivity in Binary Metal Oxides (Wiley-lnterscience. New York, 1972) p. 175. Ref. [4], p. 138. A.K. Petford, L.D. Marks and M. O'Keeffe, Surface Sci. 172 (1986) 496. D.J. Smith and L.A. Bursill, Ultraplicroscopy 17 11985)

387. H. Hartmann, F. Ebert and O. Bretschneider, Z. Anorg, Chem. 129 (1931) 116. T.T. Lin and D. Lichlman, J. Appl. Phys. 50 11979) 1298. T.T. Lin and D. Lichtman, J. Vacuum Sci. Technol. 15 (1q78) !680, M.L. Knotek and P,J. Feibelman. Surface Sci. 90 ~107~

78. C.C. Ahn and O.L. Krivanek, EELS Atlas IHREM Fatal- ity, Arizona State University, Tempe, AZ, 19831.

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