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The crystallographic shear planes of the non-stoichiometric molybdenum oxides as revealed by RHEED: Investigation from the Mo18O52(100) surface
Surface Science 164 (1985) 305-319 North-Holland, Amsterdam
305
T H E C R Y S T A L L O G R A P H I C S H E A R P L A N E S OF T H E NON-STOICHIOMETRIC MOLYBDENUM OXIDES AS REVEALED BY RHEED: I N V E S T I G A T I O N F R O M T H E MolsOsz(100) S U R F A C E O. B E R T R A N D , N. F L O Q U E T and D. J A C Q U O T Laboratoire de recherches sur la Rbactivitb des Solides, LA 23 CNRS, Facultb des Sciences Mirande, BP 138, F-21004 Do'on Cedex, France
Received 14 May 1985; accepted for publication 6 August 1985 Large M018052 crystals are obtained by an appropriate crystallization method. The examination of their well developed surfaces by the use of reflection high energy electron diffraction (RHEED) proves that these are (100) surfaces stepped along [010] directions. It is concluded that this oxide surface conformation can be connected to the particular MolsO52 structure which is built up of MoO3 slabs of finite width mutually joined by crystallographic shear planes (CS planes). Transmission electron microscopy (TEM) analysis from MOlsO52 crystal flakes confirms that these are single crystals without disorder in the periodicity of the CS planes.
1. Introduction The crystal structures of the higher m o l y b d e n u m oxides MoO~ (2.75 ~< x < 3) were proved by m a n y authors to be shear structures consisting of largely u n c h a n g e d slabs of a base structure ( M o O 3 or R e O 3 ) j o i n e d along crystallographic shear planes [1-3]. N u m e r o u s studies have been made to propose models for the mechanism of p r o d u c t i o n of these ordered and disordered shear structures, in the belief that similar principles occurring in the R e O 3 and TiO2 based oxides would also occur in the non-stoichiometric MoO3 oxides [4,5]. Most of these investigations were performed with microcrystallites. In order to identify their shear structure, selected area diffraction in a transmission electron microscope has been used, occasionally c o m b i n e d with lattice fringe imaging. N o work reported the structure of these oxide surfaces. Nevertheless, there has been a considerable interest in the study of defect structures of surfaces, because of its impact on the electronic, chemical and mechanical properties of materials. For this purpose, attempts were made to crystallize well developed Mo~8052 single crystals. These crystals offer nearly perfect specimens for R H E E D analysis. The results of this study are reported here. 2. MotsOs2 crystallography: idealized structure The crystal structure of Mo18052 has been determined and refined by Kihlborg [6]. The M018052 structure belongs to the triclinic system. The unit 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
306
O, Bertrand et al. / Mot,s052(lO0 ) surface
a3=_3.96'IA6/~ ~ (b)
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Fig. l. (a) MoO 3 structure formed by two levels of MoO 6 octahedra layers linked by Van der Waals interactions, (b) [01013, (c) [00113, (d) [100]~ projections of MoO 3 (a = 3.96 A, b = 13.86 ,~, and c = 3.69 A) [7].
cell c o n t e n t h a s t w o f o r m u l a u n i t s a n d its d i m e n s i o n s are: a=8.145A, c~ = 1 0 2 ° 6 7 ,
b=ll.89A, /~ = 6 7 ° 8 2 ,
c=21.23A, y = 109°97.
T h e a t o m i c a r r a n g e m e n t in t h e u n i t cell e x h i b i t s a m a r k e d s u b s t r u c t u r e o f t h e M o O ; - t y p e . A s r e p r e s e n t e d i n fig. 1, t h e o r t h o r h o m b i c M o O 3 s t r u c t u r e c a n b e d e s c r i b e d as a l a y e r s t r u c t u r e w i t h l a y e r s o f M o O 6 o c t a h e d r a a t t w o levels
O. Bertrand et al. / MolnOs:(lO0) surface
307
Fig. 2. Splitted representation of the idealized Mo18052 structure: in Mo18052 , the MOO3(010) layers can be regarded as being cut into strips. The width of the strips is determined by the number 18 of octahedra within the zig-zag MOO3[001] row that correspond to the 6 octahedra in the perpendicular MOO3[100] row (6).
parallel to MOO3(010 ). The octahedra shared edges to give zigzag rows in the MOO3[001 ] direction. Each octahedron has one unshared corner corresponding to an oxygen a t o m b o u n d to only one metal atom. Successive layers are stacked so that these free apices in one layer point between the apices of the neighbouring layers. The idealized Mo18052 structure can be regarded as being built up of M o O 3 slabs of finite width joined by crystallographic shear planes (CS planes) as illustrated by the split representation of the structure in fig. 2. These shear planes are parallel to MoO3(351 ) when referred to the basic M o O 3 unit cell and b e c o m e the basal Mot8052(001 ) planes in the M018052 lattice. The width of each slab is regular with 18 octahedra along the zig-zag MOO3[001 ] rows and 6 octahedra in the perpendicular MOO3[100 ] rows. Three projections of the M018052 structure are shown in fig. 3. The MOO3(010 ) layers of each slabs are mutually connected by edge sharing between the outer octahedra of neighbouring layers. So a M018052(100 ) layer is derived from a MOO3(010 ) layer by a shear m e c h a n i s m described by the displacement vector R = ½a 3 + vb 3 where a 3 and b 3 are the M o O 3 lattice vectors. This shear m e c h a n i s m occurs along regularly spaced, stepped lines parallel to M003[103 ] and ordered in the CS M003(351 ) planes. Since the shear direction R is inclined to the MOO3(010 ) layers, the resulting M018052(100 ) layers are regularly stepped, thus forming a step lattice [6]. Fig. 4 illustrated in a stereographic projection the orientation relationships between the two M0~8052 and M o O 3 lattices as deduced from the idealized Mo~sOs2 structure. In this stereographic projection and in the following the
308
O. Bertrand et al. / M o l s O s 2 ( l O 0 ) surface
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Fig. 3. Idealized structure of Mo18052; arrangement of the M o O 6 octahedra according to the main cross sections of MOO3: (a) MOO3(010); (b) M003(001); (c) MOO3(100). Each slab of MoO 3 is displaced relative to the next by the shear operation R = [2a3 + ~vb3] in the crystallographic M003(351 ) plane. The orientation relationships between the main axes of the MolsO52 structure and the parent M o O 3 structure are given.
O. Bertrand et al. / Mo18052(100) surface
309
Fig. 4. Stereographic projection of real crystallographic axes of Mo18052 and MoO 3 lattices giving the orientation relationships between both structures in agreement with Mo180521010]II MOO3[103] and Mo18052[100] IIMoO3[1121.
subscripts 3 and 18 will be used for referring to the lattices of MoO 3 and M018052 , respectively.
3. MotsOsz crystal preparation Mo18052 crystals were prepared by mixing appropriate amounts of MoO 3 oxide and Mo metal according to the stoichiometric composition MOO2.891. The MoO 3 and Mo powders used were commercial specimens: MoO 3 was pro analysis grade from Merck and Mo was 4N powder from Metals Research. The mixture, well ground in an agate mortar, was spread into a silica tube with iodine. The silica tube was then sealed under dynamic vacuum and the specimen heated at uniform temperature 960 K for two to three days. At the end of the heating treatment the specimen was quenched in air.
4. Results
4.1. Morphological analysis Phases formed are identified by X-ray powder diffractometry to be essentially Mo18052 [6] including very small amounts of MoO 3 [7] and M08023 [8]. The MoxsO52 crystals grew as thin bluish-black plates. They are pinacoids with dimensions of 5 × 2 x 0.1 mm, often twinned as it is shown on the SEM micrograph in fig. 5a. The well developed faces have irregularly spaced steps (0.1 to 0.25/~m). From an analysis with an optical goniometer, the habit of these crystals was
310
O. Bertrand et al. / Mo~,~Os2(lO0) surface
(100)
b
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Fig. 5. (a) S E M micrograph of the stepped surface of a twinned Mo18052 crystal (b) Mo18052 crystal faces indexing as deduced from an analysis by an optical goniometer.
indexed as reported in fig. 5" the well developed, stepped faces are oriented (100)l ~, whereas the ledges of the steps are either (102)1~ or (10])l~. The narrow side faces are (001)l~ and (010)18.
4.2. T E M analysis Transmission electron microscopy was carried out with a J E O L 100C electron microscope operating at 100 kV and fitted with a goniometer stage. Electron microscope specimens were prepared by crushing MolsO52 crystals in an agate mortar and dispersing the resultant fragments in ethanol. A drop of suspension was allowed to dry on a net like carbon film resting on a conventional copper support grid.
O. Bertrand et aL / MoH¢05:(I O0) surface
311
Fig. 6. (a)-(e) Electron diffraction patterns of a Mo1~O52 crystal flake. (f) Electron diffraction pattern of a M%O3~_2 crystal (with 14 ~ n ~<21) recorded by Bur:sill[9].
M o s t crystal flakes p r e s e n t o n the grid give d i f f r a c t i o n p a t t e r n s as shown in fig. 6. A t a b o u t zero tilt (0 < + 5 ° ) , the p a t t e r n t y p e of fig. 6b is o b t a i n e d mostly. S t r o n g spots are observed, c o r r e s p o n d i n g n e a r l y to the MOO3[010 ] zone axis
312
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O. Bertrand et al. / MozsOse(lO0) surface
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ture. But it proves that all observed crystal flakes are MotsO52 single crystals. We have never observed a mixture of different phases as M017049, M018052, MotgOs 5 as it has been previously reported by other authors [9,10].
4.3. R H E E D analysis Surface analysis was carried out in a standard UHV chamber with R H E E D and C M A - A E S equipment. The R H E E D gun was operated at 40 kV and the electron beam made a glancing angle with the specimen between 5 ° and 1 ° The specimen manipulator allowed R H E E D observations along a 120 ° azimuth of the crystal. The crystals were mounted on the specimen holder with silver paste, then placed in the UHV chamber without previous treatment. The only impurity detected by AES was carbon usually found on metal oxides having been in contact with the ambient atmosphere. No attempt to remove this carbon contamination was made, because of the possible structural change which could occur through oxygen treatments, even at moderate temperatures (about 770 K). Diffraction patterns were obtained from the well developed face of MotsOs2 single crystals. Fig. 10 shows these patterns recorded at several azimuthal angles. Several types of information about the
O. Bertrand et al. / Mo1~052(100 ) surface
315
Fig. 10. RHEED patterns from a stepped M018052(100) surface along several azimuths: (a) [05111s, (b) [010118,(c) [041118, (d) [021118, (e) [011118,(f) [001118 (Vp = 40 kV).
surface structure can be extracted from these R H E E D patterns: [11]. (1) The short streaks determine the surface symmetry. It is characteristic of the unit cell in the MoasOs2(100) surface: the measured parameters are in agreement with the (100) unit cell of the MotsO52 bulk lattice. Figs. l l a and l l b illustrate this surface cell in real and reciprocal space, respectively. It can be noticed that, because of the MoxsOs2 triclinic structure, there is no connection
316
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O. Bertrand et al. / MolsOse(lO0 ) surface
317
between the reciprocal surface cell and a singular reciprocal bulk cell. Consequently, the measured spacings between the streaks do not agree with a dhk t interplanar spacing of the real bulk lattice. In this case, the reciprocal surface cell is close to the [611h8 zone axis patterns (about 7°). (2) Continuous sharp streaks connect the zero order and next Laue zones. They are very much curved in the azimuths close to the [010118 azimuth. Besides, in this particular [010118 azimuth, they are disappearing while the background intensity increases. These features form regularly spaced sheets (reciprocal planes of diffuse intensity) with a periodicity which matches the parameter of the Mo18052[010 ] row. The representation of these sheets is illustrated in fig. l l b . Such sheets are indicative of one-dimensional disorder generally due to atomic steps or to a particular type of boundary [12,13] along the Mo18052[010] direction. Considering the idealized structure of Mol8052 described in section 2, the Mo18052(100 ) layers were stepped along these Mo18052[010 ] rows, due to the CS planes developed within the Mo18052 structure. Therefore an explanation of the one-dimensional disorder evident on the crystal surface is provided, assuming, in relation to its bulk crystal structure, that the Mo~8052(100) surface is, consequently, a stepped surface along the Mo~8052[010 ] direction. (3) Along specific azimuths, there is an increase in the intensity of some extended streaks and sharp symmetric Kikuchi patterns. These superimposed features are derived from the Mo~8052 substructure, i.e. from the MoO 3 lattice [14]. They allow identification of the orientation relationships between Mo18052 surface structure and its MoO 3 substructure. The MoO 3 surface symmetry matches nearly the symmetry characteristic of the known unit cell of a MOO3(010 ) surface [15]. Its orientation with respect to the Mo~8052(100) surface symmetry agrees with the orientation relationship between an idealized MOa8052(100 ) layer and its parent MOO3(010), which is given by the relation: Mo18052 (100)[0x0j at a few degrees off MoO3 (010)[1031.
KS.lanesboundary MolBOs2(lO0) __sur
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Fig. 12. Schematic picture of the Mo18052(100) surface assuming monoatomic steps in the M018052[010] direction. This surface is built up of MoO3(010 ) terraces, MOO3(100) ledges and MOO3(001 ) kinks. These two latter are the boundaries of the crystallographic shear M003(351 ) planes.
318
O. Bertrand et al. / MozsO~z(lO0 ) surface
From these results, it can be assumed that the Mo1~O52(100 ) surface has the same atomic arrangement as an idealized Mo1~O52(100) layer and can be described as a particular stepped surface composed of MOO3(010 ) terraces, MOO3(100 ) ledges and MOO3(001 ) kinks, which are the boundaries of the CS MoO3(351 ) planes. Fig. 12 illustrates this surface structure. Such an assumption concerning the atomic arrangement of the MolsO52 (] 00) surface can explain the lack of typical features in the R H E E D patterns from a stepped surface [16,17]. So, in relation to surface MOO3(001 ) kinks, when the beam was incident along the terraces, the produced pattern of fig. 10b did not give clear evidence concerning the width of the terraces and the angle between the surface and its terraces. 5. Summa~ and conclusions The crystal structure of Mo~8052 was previously proved to be a shear structure consisting of largely unchanged slabs of MoO 3 structure joined along crystallographic shear planes (CS planes) [6]. These CS planes are regularly spaced in the ordered structure of a M018052 single crystal. Preserving, yet, the average M01~O52 composition (MOO2 asg), disorder can appear in the CS planes periodicity: in that case the oxide phase is described as a mixture of different phases such as M017049, M018052, M019055, M020058 [9,10]. With our crystallization method, large M018052 crystals are obtained. TEM analysis proves that these crystals have the known MolsOsz structure without disorder in the periodicity of the CS planes. But from an intensity analysis of diffraction spots, the orientation relationships between the M0~8052 structure and its MoO 3 substructure is ambiguously determined. From R H E E D investigation, it can be concluded that the well developed surface of these M%8052 single crystals is a MolsOs2(100) surface stepped along the Mo~sO52[010] directions (that gives an explanation to the macroscopic steps observed on the surface). This surface conformation is connected to the particular Mo~aO~2 structure described by MoO 3 base structure: the MOO3(010 ) terraces are the surfaces of MoO 3 slabs and the MOO3(100) ledges and MoO_~(001) kinks are the boundaries of the CS planes. From this investigation of M018052 surfaces by R H E E D it can be presumed that the surfaces of other stable phases of molybdenum oxide such as M040~, MosO23, M09026 are one-dimensional disorder surfaces due to the CS planes boundaries ordered in their bulk structure. Some recent R H E E D analyses from M0401~ and M0~O23 surfaces confirm the expected disorder on these particular oxide surfaces. Such superficial structures should induce on non-stoichiometric oxides of MoO_~ a hyperreactivity of their surfaces when interacting with gas. Work in this direction will be pursued to try to elucidate the structural changes taking place during oxidation or catalytic reactions with these molybdenum oxide surfaces.
O. Bertrand et al. / Mol~052(100) surface
319
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [151 [16] [17]
A. Magneli, Acta Cryst. 6 (1953) 495. L. Kihlborg, Arkiv Kemi 21 (44) (1963) 471. L.A. Bursill, Proc. Roy. Soc. (London) A311 (1969) 267. B.G. Hyde, A.N. Bagshaw, S. Anderson and M. O'Keefe, Ann. Rev. Mater. Sci. 4 (1974) 43. B.G. Hyde and L.A. Bursill, in: The Chemistry of Extended Defects in Non-Metallic Oxides, Eds. L. Eyring and M. O'Keefe (North-Holland, Amsterdam, 1970) p. 377. L. Kihlborg, Arkiv Kemi 21 (1963) 443. L. Kihlborg, Arkiv Kemi 21 (1963) 357. L. Kihlborg, Arkiv Kemi 21 (1963) 461. L.A. Bursill, Acta Cryst. A28 (1972) 187. G. Liljestrand, in: Reactivity of Solids, Eds, J.W. Wood, O. Lindquist, C. Helgesson and N.G. Vannerberg (Plenum, New York, 1977) p. 385. E. Bauer, in: Techniques of Metals Research, Vol. II, Part 2 (Interscience, New York, 1969) p. 502. P. Delescluse and A. Masson, Surface Sci. 100 (1980) 423. J. Dobson, J.M. Neave and B,A. Joyce, Surface Sci. 119 (1982) I_339. K. Hayakawa and N. Aizawa, Japan. J. Appl. Phys. 21 (1982) 1105. L.C. Dufour, O. Bertrand and N. Floquet, Surface Sci. 147 (1984) 396. M. Henzler, in: Electron Spectroscopy for Surface Analysis, Topics in Current Physics, Vol. 4, Ed. H. Ibach (Springer, Berlin, 1977) p. 117. R.H. Milne, Surface Sci. 122 (1982) 474.
305
T H E C R Y S T A L L O G R A P H I C S H E A R P L A N E S OF T H E NON-STOICHIOMETRIC MOLYBDENUM OXIDES AS REVEALED BY RHEED: I N V E S T I G A T I O N F R O M T H E MolsOsz(100) S U R F A C E O. B E R T R A N D , N. F L O Q U E T and D. J A C Q U O T Laboratoire de recherches sur la Rbactivitb des Solides, LA 23 CNRS, Facultb des Sciences Mirande, BP 138, F-21004 Do'on Cedex, France
Received 14 May 1985; accepted for publication 6 August 1985 Large M018052 crystals are obtained by an appropriate crystallization method. The examination of their well developed surfaces by the use of reflection high energy electron diffraction (RHEED) proves that these are (100) surfaces stepped along [010] directions. It is concluded that this oxide surface conformation can be connected to the particular MolsO52 structure which is built up of MoO3 slabs of finite width mutually joined by crystallographic shear planes (CS planes). Transmission electron microscopy (TEM) analysis from MOlsO52 crystal flakes confirms that these are single crystals without disorder in the periodicity of the CS planes.
1. Introduction The crystal structures of the higher m o l y b d e n u m oxides MoO~ (2.75 ~< x < 3) were proved by m a n y authors to be shear structures consisting of largely u n c h a n g e d slabs of a base structure ( M o O 3 or R e O 3 ) j o i n e d along crystallographic shear planes [1-3]. N u m e r o u s studies have been made to propose models for the mechanism of p r o d u c t i o n of these ordered and disordered shear structures, in the belief that similar principles occurring in the R e O 3 and TiO2 based oxides would also occur in the non-stoichiometric MoO3 oxides [4,5]. Most of these investigations were performed with microcrystallites. In order to identify their shear structure, selected area diffraction in a transmission electron microscope has been used, occasionally c o m b i n e d with lattice fringe imaging. N o work reported the structure of these oxide surfaces. Nevertheless, there has been a considerable interest in the study of defect structures of surfaces, because of its impact on the electronic, chemical and mechanical properties of materials. For this purpose, attempts were made to crystallize well developed Mo~8052 single crystals. These crystals offer nearly perfect specimens for R H E E D analysis. The results of this study are reported here. 2. MotsOs2 crystallography: idealized structure The crystal structure of Mo18052 has been determined and refined by Kihlborg [6]. The M018052 structure belongs to the triclinic system. The unit 0 0 3 9 - 6 0 2 8 / 8 5 / $ 0 3 . 3 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics Publishing Division)
306
O, Bertrand et al. / Mot,s052(lO0 ) surface
a3=_3.96'IA6/~ ~ (b)
(o)
S
0
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(d)
Fig. l. (a) MoO 3 structure formed by two levels of MoO 6 octahedra layers linked by Van der Waals interactions, (b) [01013, (c) [00113, (d) [100]~ projections of MoO 3 (a = 3.96 A, b = 13.86 ,~, and c = 3.69 A) [7].
cell c o n t e n t h a s t w o f o r m u l a u n i t s a n d its d i m e n s i o n s are: a=8.145A, c~ = 1 0 2 ° 6 7 ,
b=ll.89A, /~ = 6 7 ° 8 2 ,
c=21.23A, y = 109°97.
T h e a t o m i c a r r a n g e m e n t in t h e u n i t cell e x h i b i t s a m a r k e d s u b s t r u c t u r e o f t h e M o O ; - t y p e . A s r e p r e s e n t e d i n fig. 1, t h e o r t h o r h o m b i c M o O 3 s t r u c t u r e c a n b e d e s c r i b e d as a l a y e r s t r u c t u r e w i t h l a y e r s o f M o O 6 o c t a h e d r a a t t w o levels
O. Bertrand et al. / MolnOs:(lO0) surface
307
Fig. 2. Splitted representation of the idealized Mo18052 structure: in Mo18052 , the MOO3(010) layers can be regarded as being cut into strips. The width of the strips is determined by the number 18 of octahedra within the zig-zag MOO3[001] row that correspond to the 6 octahedra in the perpendicular MOO3[100] row (6).
parallel to MOO3(010 ). The octahedra shared edges to give zigzag rows in the MOO3[001 ] direction. Each octahedron has one unshared corner corresponding to an oxygen a t o m b o u n d to only one metal atom. Successive layers are stacked so that these free apices in one layer point between the apices of the neighbouring layers. The idealized Mo18052 structure can be regarded as being built up of M o O 3 slabs of finite width joined by crystallographic shear planes (CS planes) as illustrated by the split representation of the structure in fig. 2. These shear planes are parallel to MoO3(351 ) when referred to the basic M o O 3 unit cell and b e c o m e the basal Mot8052(001 ) planes in the M018052 lattice. The width of each slab is regular with 18 octahedra along the zig-zag MOO3[001 ] rows and 6 octahedra in the perpendicular MOO3[100 ] rows. Three projections of the M018052 structure are shown in fig. 3. The MOO3(010 ) layers of each slabs are mutually connected by edge sharing between the outer octahedra of neighbouring layers. So a M018052(100 ) layer is derived from a MOO3(010 ) layer by a shear m e c h a n i s m described by the displacement vector R = ½a 3 + vb 3 where a 3 and b 3 are the M o O 3 lattice vectors. This shear m e c h a n i s m occurs along regularly spaced, stepped lines parallel to M003[103 ] and ordered in the CS M003(351 ) planes. Since the shear direction R is inclined to the MOO3(010 ) layers, the resulting M018052(100 ) layers are regularly stepped, thus forming a step lattice [6]. Fig. 4 illustrated in a stereographic projection the orientation relationships between the two M0~8052 and M o O 3 lattices as deduced from the idealized Mo~sOs2 structure. In this stereographic projection and in the following the
308
O. Bertrand et al. / M o l s O s 2 ( l O 0 ) surface
513
_
section of (351) 3 ~lane
~-~-~~~~-+ [°1°]a
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[015]3 = [210]18
I~12 ~12]3 = [100]18
Fig. 3. Idealized structure of Mo18052; arrangement of the M o O 6 octahedra according to the main cross sections of MOO3: (a) MOO3(010); (b) M003(001); (c) MOO3(100). Each slab of MoO 3 is displaced relative to the next by the shear operation R = [2a3 + ~vb3] in the crystallographic M003(351 ) plane. The orientation relationships between the main axes of the MolsO52 structure and the parent M o O 3 structure are given.
O. Bertrand et al. / Mo18052(100) surface
309
Fig. 4. Stereographic projection of real crystallographic axes of Mo18052 and MoO 3 lattices giving the orientation relationships between both structures in agreement with Mo180521010]II MOO3[103] and Mo18052[100] IIMoO3[1121.
subscripts 3 and 18 will be used for referring to the lattices of MoO 3 and M018052 , respectively.
3. MotsOsz crystal preparation Mo18052 crystals were prepared by mixing appropriate amounts of MoO 3 oxide and Mo metal according to the stoichiometric composition MOO2.891. The MoO 3 and Mo powders used were commercial specimens: MoO 3 was pro analysis grade from Merck and Mo was 4N powder from Metals Research. The mixture, well ground in an agate mortar, was spread into a silica tube with iodine. The silica tube was then sealed under dynamic vacuum and the specimen heated at uniform temperature 960 K for two to three days. At the end of the heating treatment the specimen was quenched in air.
4. Results
4.1. Morphological analysis Phases formed are identified by X-ray powder diffractometry to be essentially Mo18052 [6] including very small amounts of MoO 3 [7] and M08023 [8]. The MoxsO52 crystals grew as thin bluish-black plates. They are pinacoids with dimensions of 5 × 2 x 0.1 mm, often twinned as it is shown on the SEM micrograph in fig. 5a. The well developed faces have irregularly spaced steps (0.1 to 0.25/~m). From an analysis with an optical goniometer, the habit of these crystals was
310
O. Bertrand et al. / Mo~,~Os2(lO0) surface
(100)
b
, [.ooll
Fig. 5. (a) S E M micrograph of the stepped surface of a twinned Mo18052 crystal (b) Mo18052 crystal faces indexing as deduced from an analysis by an optical goniometer.
indexed as reported in fig. 5" the well developed, stepped faces are oriented (100)l ~, whereas the ledges of the steps are either (102)1~ or (10])l~. The narrow side faces are (001)l~ and (010)18.
4.2. T E M analysis Transmission electron microscopy was carried out with a J E O L 100C electron microscope operating at 100 kV and fitted with a goniometer stage. Electron microscope specimens were prepared by crushing MolsO52 crystals in an agate mortar and dispersing the resultant fragments in ethanol. A drop of suspension was allowed to dry on a net like carbon film resting on a conventional copper support grid.
O. Bertrand et aL / MoH¢05:(I O0) surface
311
Fig. 6. (a)-(e) Electron diffraction patterns of a Mo1~O52 crystal flake. (f) Electron diffraction pattern of a M%O3~_2 crystal (with 14 ~ n ~<21) recorded by Bur:sill[9].
M o s t crystal flakes p r e s e n t o n the grid give d i f f r a c t i o n p a t t e r n s as shown in fig. 6. A t a b o u t zero tilt (0 < + 5 ° ) , the p a t t e r n t y p e of fig. 6b is o b t a i n e d mostly. S t r o n g spots are observed, c o r r e s p o n d i n g n e a r l y to the MOO3[010 ] zone axis
312
O. Bertrand et al.
/ Mo18052(100) surface
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Fig 7. The diffraction patterns (a)-(e) correspond to the zone axis [lll]l 8, [611118,[210118,[310115 and [100], respectively.The sharp spots are indexed with respect to the Mo18052 lattice [61 and the strong spots, with respect to the MoO3 sublattice
O. Bertrand et al. / MozsOse(lO0) surface
313
o01"
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Fig. 8. Stereographicprojections: (a) cross sections in reciprocalspace, (b) zone axes in real space, corresponding to the diffraction patterns indexed in fig. 7. pattern. Only short rows of Mo18052 lattice spots appear, indicating the MolaO52 lattice rows are inclined to this zero tilt plane. In order to obtain MoasO52 zone axis patterns, careful rotation-tilt had to be performed. These recorded patterns consist of single crystal spots; no extra spots appear as in the pattern of fig. 6f obtained by Bursill from MolsO52 crystals with disordered structure [9]. The spot indexing is unambiguous in the Mo18052 reciprocal lattice corresponding to the Mo~8052 cell parameters given in section 2. This indexing is listed in fig. 7 and each indexed zone axis is indicated in the stereographic projection given in fig. 8. The MoO 3 substructure of Mo18052 appears in these patterns in the higher intensity of particular diffraction spots. Consequently, these strong spots patterns do not correspond exactly to the MoO 3 cell parameters. Due to the orthorhombic structure of MoO 3, where the two main [10013 and [00113 axes have close parameter values (but do not correspond in the structure to the same atomic ordering - see fig. 1), the strong spots were indexed ambiguously. In this way fig. 9 proves that the indexing of the strong spots in the main Mo~8052[100 ] zone axis pattern is a double one; it is impossible to choose between two non-equivalent indexings: the MOO3121:[] zone or the MoO31112 ] zone. However, only the first one is in agreement with the orientation relationship between Mo18052 and MoO 3 structures as previously given in fig. 4. Therefore only detailed measurements of interplanar spacings and angles from whole patterns allow to assign the right orientation relationships between the Mo18052 and MoO 3 lattices, that is to say: MoO3 (35i)llO31 IIM°lsOsz (001)tin01. This TEM analysis gives evidence for the problem to identify the only orientation relationship between the Mo~8Os2 structure and its MoO 3 substruc-
314
O. Bertrand et aL / Mot ecOse(lO0) surface
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ture. But it proves that all observed crystal flakes are MotsO52 single crystals. We have never observed a mixture of different phases as M017049, M018052, MotgOs 5 as it has been previously reported by other authors [9,10].
4.3. R H E E D analysis Surface analysis was carried out in a standard UHV chamber with R H E E D and C M A - A E S equipment. The R H E E D gun was operated at 40 kV and the electron beam made a glancing angle with the specimen between 5 ° and 1 ° The specimen manipulator allowed R H E E D observations along a 120 ° azimuth of the crystal. The crystals were mounted on the specimen holder with silver paste, then placed in the UHV chamber without previous treatment. The only impurity detected by AES was carbon usually found on metal oxides having been in contact with the ambient atmosphere. No attempt to remove this carbon contamination was made, because of the possible structural change which could occur through oxygen treatments, even at moderate temperatures (about 770 K). Diffraction patterns were obtained from the well developed face of MotsOs2 single crystals. Fig. 10 shows these patterns recorded at several azimuthal angles. Several types of information about the
O. Bertrand et al. / Mo1~052(100 ) surface
315
Fig. 10. RHEED patterns from a stepped M018052(100) surface along several azimuths: (a) [05111s, (b) [010118,(c) [041118, (d) [021118, (e) [011118,(f) [001118 (Vp = 40 kV).
surface structure can be extracted from these R H E E D patterns: [11]. (1) The short streaks determine the surface symmetry. It is characteristic of the unit cell in the MoasOs2(100) surface: the measured parameters are in agreement with the (100) unit cell of the MotsO52 bulk lattice. Figs. l l a and l l b illustrate this surface cell in real and reciprocal space, respectively. It can be noticed that, because of the MoxsOs2 triclinic structure, there is no connection
316
O. Bertrand et aL /MolaQs2(lO0 ) surface
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Fig. 1 l. Mo18057(100 ) surface cell illustrated in (a) real space with b~8 = 11.89 A, c18 = 21.33 A, a ~ 1 0 2 . 6 7 ° ; (b) reciprocal space with b~8=l/(b18 sin e 0 - 1 / 1 1 . 6 0 A, c ' ~ 8 = l / ( c l g s i n a ) = 1/20.71 A. The orientation relationships between the Mo18057 surface cell ( ) and the MoO 3 subcell ( - - - - - ) are given. ( . . . . . . ) azimuths of recorded R H E E D patterns; ( , ) reciprocal planes of diffuse intensity.
O. Bertrand et al. / MolsOse(lO0 ) surface
317
between the reciprocal surface cell and a singular reciprocal bulk cell. Consequently, the measured spacings between the streaks do not agree with a dhk t interplanar spacing of the real bulk lattice. In this case, the reciprocal surface cell is close to the [611h8 zone axis patterns (about 7°). (2) Continuous sharp streaks connect the zero order and next Laue zones. They are very much curved in the azimuths close to the [010118 azimuth. Besides, in this particular [010118 azimuth, they are disappearing while the background intensity increases. These features form regularly spaced sheets (reciprocal planes of diffuse intensity) with a periodicity which matches the parameter of the Mo18052[010 ] row. The representation of these sheets is illustrated in fig. l l b . Such sheets are indicative of one-dimensional disorder generally due to atomic steps or to a particular type of boundary [12,13] along the Mo18052[010] direction. Considering the idealized structure of Mol8052 described in section 2, the Mo18052(100 ) layers were stepped along these Mo18052[010 ] rows, due to the CS planes developed within the Mo18052 structure. Therefore an explanation of the one-dimensional disorder evident on the crystal surface is provided, assuming, in relation to its bulk crystal structure, that the Mo~8052(100) surface is, consequently, a stepped surface along the Mo~8052[010 ] direction. (3) Along specific azimuths, there is an increase in the intensity of some extended streaks and sharp symmetric Kikuchi patterns. These superimposed features are derived from the Mo~8052 substructure, i.e. from the MoO 3 lattice [14]. They allow identification of the orientation relationships between Mo18052 surface structure and its MoO 3 substructure. The MoO 3 surface symmetry matches nearly the symmetry characteristic of the known unit cell of a MOO3(010 ) surface [15]. Its orientation with respect to the Mo~8052(100) surface symmetry agrees with the orientation relationship between an idealized MOa8052(100 ) layer and its parent MOO3(010), which is given by the relation: Mo18052 (100)[0x0j at a few degrees off MoO3 (010)[1031.
KS.lanesboundary MolBOs2(lO0) __sur
aC
oO
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Fig. 12. Schematic picture of the Mo18052(100) surface assuming monoatomic steps in the M018052[010] direction. This surface is built up of MoO3(010 ) terraces, MOO3(100) ledges and MOO3(001 ) kinks. These two latter are the boundaries of the crystallographic shear M003(351 ) planes.
318
O. Bertrand et al. / MozsO~z(lO0 ) surface
From these results, it can be assumed that the Mo1~O52(100 ) surface has the same atomic arrangement as an idealized Mo1~O52(100) layer and can be described as a particular stepped surface composed of MOO3(010 ) terraces, MOO3(100 ) ledges and MOO3(001 ) kinks, which are the boundaries of the CS MoO3(351 ) planes. Fig. 12 illustrates this surface structure. Such an assumption concerning the atomic arrangement of the MolsO52 (] 00) surface can explain the lack of typical features in the R H E E D patterns from a stepped surface [16,17]. So, in relation to surface MOO3(001 ) kinks, when the beam was incident along the terraces, the produced pattern of fig. 10b did not give clear evidence concerning the width of the terraces and the angle between the surface and its terraces. 5. Summa~ and conclusions The crystal structure of Mo~8052 was previously proved to be a shear structure consisting of largely unchanged slabs of MoO 3 structure joined along crystallographic shear planes (CS planes) [6]. These CS planes are regularly spaced in the ordered structure of a M018052 single crystal. Preserving, yet, the average M01~O52 composition (MOO2 asg), disorder can appear in the CS planes periodicity: in that case the oxide phase is described as a mixture of different phases such as M017049, M018052, M019055, M020058 [9,10]. With our crystallization method, large M018052 crystals are obtained. TEM analysis proves that these crystals have the known MolsOsz structure without disorder in the periodicity of the CS planes. But from an intensity analysis of diffraction spots, the orientation relationships between the M0~8052 structure and its MoO 3 substructure is ambiguously determined. From R H E E D investigation, it can be concluded that the well developed surface of these M%8052 single crystals is a MolsOs2(100) surface stepped along the Mo~sO52[010] directions (that gives an explanation to the macroscopic steps observed on the surface). This surface conformation is connected to the particular Mo~aO~2 structure described by MoO 3 base structure: the MOO3(010 ) terraces are the surfaces of MoO 3 slabs and the MOO3(100) ledges and MoO_~(001) kinks are the boundaries of the CS planes. From this investigation of M018052 surfaces by R H E E D it can be presumed that the surfaces of other stable phases of molybdenum oxide such as M040~, MosO23, M09026 are one-dimensional disorder surfaces due to the CS planes boundaries ordered in their bulk structure. Some recent R H E E D analyses from M0401~ and M0~O23 surfaces confirm the expected disorder on these particular oxide surfaces. Such superficial structures should induce on non-stoichiometric oxides of MoO_~ a hyperreactivity of their surfaces when interacting with gas. Work in this direction will be pursued to try to elucidate the structural changes taking place during oxidation or catalytic reactions with these molybdenum oxide surfaces.
O. Bertrand et al. / Mol~052(100) surface
319
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [151 [16] [17]
A. Magneli, Acta Cryst. 6 (1953) 495. L. Kihlborg, Arkiv Kemi 21 (44) (1963) 471. L.A. Bursill, Proc. Roy. Soc. (London) A311 (1969) 267. B.G. Hyde, A.N. Bagshaw, S. Anderson and M. O'Keefe, Ann. Rev. Mater. Sci. 4 (1974) 43. B.G. Hyde and L.A. Bursill, in: The Chemistry of Extended Defects in Non-Metallic Oxides, Eds. L. Eyring and M. O'Keefe (North-Holland, Amsterdam, 1970) p. 377. L. Kihlborg, Arkiv Kemi 21 (1963) 443. L. Kihlborg, Arkiv Kemi 21 (1963) 357. L. Kihlborg, Arkiv Kemi 21 (1963) 461. L.A. Bursill, Acta Cryst. A28 (1972) 187. G. Liljestrand, in: Reactivity of Solids, Eds, J.W. Wood, O. Lindquist, C. Helgesson and N.G. Vannerberg (Plenum, New York, 1977) p. 385. E. Bauer, in: Techniques of Metals Research, Vol. II, Part 2 (Interscience, New York, 1969) p. 502. P. Delescluse and A. Masson, Surface Sci. 100 (1980) 423. J. Dobson, J.M. Neave and B,A. Joyce, Surface Sci. 119 (1982) I_339. K. Hayakawa and N. Aizawa, Japan. J. Appl. Phys. 21 (1982) 1105. L.C. Dufour, O. Bertrand and N. Floquet, Surface Sci. 147 (1984) 396. M. Henzler, in: Electron Spectroscopy for Surface Analysis, Topics in Current Physics, Vol. 4, Ed. H. Ibach (Springer, Berlin, 1977) p. 117. R.H. Milne, Surface Sci. 122 (1982) 474.