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A MAS NMR STUDY OF THE STRUCTURE OF
AMORPHOUS ALUMINA FILMS
R. Dupree, I. Farnan, A. Forty, S. El-Mashri, L. Bottyan
To cite this version:
JOURNAL
DE
PHYSIQUE
Colloque
C8,
supplgment au n012, Tome 46, d6cembre 1985 page C8-113A MAS NMR STUDY OF T H E STRUCTURE OF AMORPHOUS A L U M I N A F I L M S
R. Dupree, I. Farnan, A.J. Forty, S. ~l-~ashri+ and L. ~ o t t ~ a n *
Physics Department, University o f Warwick, Coventry CV4 7AL,
U.
K.RQsum6 - Des f i l m s d ' a l u m i n e a m o r p h e o b t e n u s p a r d 6 p 6 t s a n o d i q u e s o n t Q t 6 6 t u d i k s p a r MAS-RMN de ~ 1La m6thode de p r g p a r a t i o n f a i t v a r i e r l a c o o r d i n a t i o n ~ ~ . de l'aluminium de 4 5 6 , 5 . Environ 20 % de A 1 s e t r o u v e dans des s i t e s comparables 5 ceux de l ' a l u m i n e c r i s t a l l i n e . Le r e s t e de A 1 s e t r o u v e dans des s i t e s fortement d i s t o r d u s localement. La d i s p e r s i o n s u r l a longueur des l i a i s o n s A1-0 e s t de l ' o r d r e de 10%.
Abstract
-
Magic Angle Spinning 2 7 ~ 1 N.M.R. data on some anodically formed amorphous alumina films are presented. The spectra show that aluminium has6,5 and 4-fold coordinations in these films whose proportions depend upon the method of preparation of the film. About 20% of the aluminium is at sites with relatively high symmetry comparable with crystalline aluminas. the remaining aluminium has more distorted surroundings. The data imply an A 1 4 bond length variation of about 10% in these films.
I - INl'RODWCXTON
Despite the considerable practical importance of oxide films formed anodic- ally on aluminium, only limited information about their structure is available. The oxide is thought to have a complex microstructure often consisting of a mixture of amorphous and crystalline phases of varying porosity depending upon the anodising conditions. Relatively uniform films are formed in neutral electrolytes such as sodium tartrate or sodium borate solutions, but films formed in strong acid electro- lytes are highly porous(1). The techniques used to date to obtain information about the structure of these films include X-ray and electron diffraction(2),(3),(4). X-ray fluorescence spectroscopy(5),(6),(7) and electron yield EXAFS(8). The chem- ical shift of the Alga peak in amorphous alumina formed by rf sputtering lies bet- ween that of u-AlyOa and microline and was therefore interpreted as indicating the presence of both tetrahedrally co-ordinated (A104) and octahedrally co-ordinated (A1061 aluminium ions(7). The average A1-0 bond length derived from EXAFS was 0.18 nm for a film formed in phosphoric acid compared with 0.19 nm for one formed in a sodium tartrate solution. By assuming a bimodal distribution of (AlO6) and (A104) bonding El-Mashri et al(8) deduced that 80% of the aluminium ions are in octahedral sites in the tartrate-formed film whereas from the A1-0 distance the phosphoric acid formed film should contain only tetrahedrally coordinated aluminium ions. The X-ray radial distribution of sulphuric acid formed films gave a mean A1-0 distAnce of 0.185 nm and the scattering intensity was modelled by assuming a spinel like structure containing (A106 )
,
(A104) and (A105 ) polyhedra(4). Thus there is general agreement that amorphous alumma consists of differing amounts of (A10 ) and (A10 ) (and possibly (A10 ) ) coordinations with a range of distortions associaeed with ea$h type of polyhedro8, but no direct measurements of the amounts of each coordination exist.'NOW at Libyan Atomic Energy Establihment, Tripoli, Libya.
*NOW at Central Research Institute for Physics, H-1525 Budapest, P.O.B. 49, Hungary.
JOURNAL DE PHYSIQUE
27
The NMR chemical shift of A1 is sensitive to its local environment; however, much detail is lost in the conventional NMR technique because of various mechanisms which broaden the resonances from different environments. The 'magic angle spinning' (MAS) technique(9) eliminates the dipolar contribution to broadening and also red- uces the effect2qf chemical shift anisotropy and quadrupolar broadening. In crystal- line materials A1 octahedrally coordinated to oxygen gives a sharp resonance in the region 0+20 ppm (with reference to Al(H 0)3+ solution) whereas tetrahedrally coordinated A1 gives resonances at 50-80 ppm. 2 6
In this paper we present Magic Angle Spinning NMR data on two amorphous alumina films and also on three polytypes of crystalline alumina for comparison. The prop- ortions of the different coordinations are obtained directly from the spectra as the area under the absorption curves corresponding to each resonance.
I1
-
EXPERIMENTALTwo types of anodising treatment were used. One film was prepared in a bath of 150 gm/1 H2S04. yith a current density 250 A
m2
; the other (HSH) in a bath of 70 gm/l sulphosalicilic acid, 50 gm/l formic acid and 10 gm/1 hydroquinone. Since more than ~ 2 0 mg of material is required fos an NMR experiment several sq. cm of relatively thick film (10pm)was used. The 7 ~ 1 spectra were obtained at 104 MHzon a Bruker MSL spectrometer. Typically 10,000 pulses of length 2psec
(a/6)
tog- ether with a delay between pulses of 0.3 sec were used to acquire the data over a spectral range of 125 KHz. Several samples were run with different pulse lengths and delays to ensure there was no saturation of the resonances, and variable spin- ning speeds in the range 2.5 KHz - 4.0 KHz were used to confirm the contribution of the spinning side-bands to the spectrum. No smoothing or line broadening of the free induction decay was used before Fourier transformation.I11 - RESULTS
Figure 1 shows the spectra of a-alumina, y-alumina& K-alumina.
In a-A120g where all the aluminium is octahedrally coordinated, a single resonance is observed at %13 ppm with symmetrical spinning sidebands as indicated. y-A1203 has both octahedrally and tetrahedrally coordinated aluminium and as well
I
200 100 0 -100 I
PPM
t " : ' ; ' : " :
500 200 0 -200 -500
PPM
Figure 2
-
104 MHz 27~1 NMR spectra of alumina films formed in (a) H SO electrolyte.2 4
and (b) HSH electrolyte. (c) is a simulation of spectrum (b) produced by fitting the experimental data to 3 narrow gaussian peaks plus sidebands and 2 broad gaussians.
as the octahedral peak and sidebands a peak is observed from the tetrahedral alumin- ium at 65 ppm. The relative amounts of tetrahedral and octahedral aluminium can be found by integrating the spectrum. For overlapping peaks the spectra including the sidebands are assumed symmetrical as is expected for quadrupolar sidebands. We find in y-alumina 0.7420.02 (A10 ) in agreement with the measurements of John
6
et al(l1). The X-ray data for the transition aluminas such as y and K alumina are
similar to that of spinel. For y -alumina the 21xA1 ions are distributed over the 24 cation sites, with the position of the vacant sites not being known with cer- tainty(l2),(13). The proportion of octahedral sites determined above is in agree- ment with the Lippens model(l3) provided an ordered arrangement of aluminium atoms in which all the octahedral sites are occupied and the vacancies occur on the tetra- hedral sites, is obeyed. The spectrum of K alumina shows that the octahedral peak has moved to a smaller shift indicating a change in the local environment. We find 0.66+0.03 (A10 ) i.e. a somewhat higher tetrahedral occupancy than for y alumina.
6
The spectra of the two forms of amorphous alumina investigated are shown in Fig.2.
For both samples a broad line with three narrower peaks superposed is observed The narrowed peaks correspond to the more symmetric sites whose quadrupolar broaden- ing is sufficiently small to be reduced by magic angle spinning. The peak at 60-63 ppm is clearly (A104) and that at -5 -3 ppm is (A10 ) ; However, the peak at 27-29 ppm does not correspond with aluminium in either octdedral or tetrahedral coordina- tion in any aluminium oxide or alumino-compound. By varying the spinning speed we have checked that it is not an artefact arising from the super-position of spinn- ing sidebands. We have measured the spectrum of andalusite (a A1 Si 0 polymorph)
2 5.
in which A1 ions are in five-fold coordination and find a chemical shxft of
-
35 ppm and therefore ascribe the peak at 27-29 ppm in amorphous alumina to (A105) coordination sites. The relative proportions of the three narrowed components are rather different in the two samples 50%(A10 ) 30% (A10 ) and 20% (A10 )in the H2S04 sample compared with 35% for both (A106) and6(~105) and 30% (A104) in t%e HSH sample.IV
-
DISCUSSIONC8-116 JOURNAL DE PHYSIQUE
where the oxide has a cellular porous structure in which it is thought that the cell walls may be partially crystalline(l4),(8). Alternatively there is a continuous distribution of distortions of the polyhedra. In either case strongly disordered material with distorted p o l e d r a will give rise to relatively large electric field gradients (e.f.g.) at the A1 site such that the NMR line will not be narrowed by spinning, whilst A1 sites in the more symmetric polyhedra will experience a smaller e.f.g. and be narrowed.
The simulated spectrum for the film formed in H SO4 shown in Fig.2 consists of two broad Gaussians together with a Gaussian fit to $he narrowed peaks and their sidebands. The broad Gaussians arise from [A104] and [A106] in distorted polyhedra.
A n equally good simulation could be made with three broad Gaussians of similar rel- ative amplitudes to those found for the narrowed peaks. A single broad Gaussian is not suitable to simulate the spectrum since the peak position would not be con- sistent with a particular coordination. If just distorted [A104] and [A106] poly- hedra are responsible for the bro d Gaussian peak, then the respective proportions of the 6, 5 and 4 coordinated A1
'+
are 64%. 6% and 30% for the H2S04 film and 49%.8% and 43% for the HSH film. The occurence of 5-fold coordination may be associated with the incorporation of anions into the film, since these films are known to con- tain 10-12 wt% sulphate ions (H SO ) or - 3 wt% carbonate ions (HSH). However, according ,to Thompson and ~ood(l2) h e anion contaminated region extends over most of the film and it is difficult to see why the [A10
]
should preferentially be in5
a relatively symmetric environment. At present we are unable to distinguish between the two alternatives.
The width of the broad line, which is caused by the e.f.g. at the 2 7 ~ 1 nuc- leus, may be used to obtain an estimate of the A 1 4 bond length variation in these films. In crystalline materials a reasonable correlation has been found between the longitudinal strain of [A10 ] polyhedra (defined as Xi(ln(l./lo(, where li is
6
the individual A1-0 bond length and 1, is the ideal bond length $or a perfect poly- hedron of the same volume) and the e.f.g.(15). If such a correlation exists for amorphous aluminas then A1/1*0.10.
V
-
CONCLUSIONSThe MAS NMR spectra show that 4, 6 and 5 coordinated alumiq&um ions are pres- ent in the amorphous alumina films studied. About 20% of the ~ 1is at sites with ~ -small enough e.f.g.'s for MAS to narrow the spectrum implying an environment of relatively high symmetry comparabAe with that found in the crystalline aluminas. The spectrum from the remaining A1 + is consistent with either a roughly equal split
between the three coordinations or with only [ A ~ O ] and [ A ~ O ] being present. In
4.
future experiments we hope to distinguish between t%ese possibil~ties. Acknowledgements
We wish to thank Dr. G. Thompson for providing the 'H2S04' film, Dr. E. Szontagh for the HSH film and the S.E.R.C. for support.
REFERENCES
(1) Venables, J.D., McNamara, D.K., Chen, J.M. and Sun, T.S., Appl.Surf.Sci. Jones. A.M., Phys-Chem. Glasses Abstr.
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(1975) 99A.Popova. I.A., 1norg.Mater. Consultants Bur.Trans1. 14 (1979) 1503.
Oka, Y., Takahashi, T., Okada, K. and Iwai, S., J .~z.~ryst.~ol.
30
(19791349. Takahashi, T., Nagano, T., Ikegaya, M., Tagai. H., J-Metal Finishing Soc. 21 (1970) 612.-
Takahashi, T., Nagano, T., Wada, K.. Ikegaya, M. and Tagai, H., J.Meta1 Finishing
2
(1971) 400.Hanada,
T.,
Aikawa, T. and Soga. N.. J.Non.Cryst.So1.50
(1982) 397. El-Mashri, S.M., Jones, R.G. and Forty, A.J., Phil-Mag.48
(1983) 665. Andrews, E.R.. 1nt.Rev.Phys.Chem.1
(1981) 295.MUller. D., Gessner, W.. Behrens, H.J. and Scheler. G., Chem.Phys.Lett.
