Langmuir 1995,11, 3795-3804
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Mechanically Activated Moos. 3. Characterizationby Vibrational Spectroscopy G. Mestl,?T. K. K. Srinivasan,$and H. Knozinger" Znstitut fur Physikalische Chemie, Uniuersitat Miinchen, Sophienstrasse 11, 80333 Miinchen, Germany Received February 27, 1995. I n Final Form: June 21, 1995@ Mechanically activated MOO3 has been characterized by infrared and Raman spectroscopy. A drastic decrease in the Raman intensity connected with particle size reduction is observed when Moo3 is ground in a planetary mill for 600 min. The broadening of Raman bands is linearly correlated with the increasing BET surface area. The observed changes in Raman intensity ratios are related to the known lattice contraction and expansion of Moos-, and thus arise from known distortions in suboxides. Mechanical activation induces only minor internal lattice strain; thus, only small shifts of the bands assigned to rigid chain modes are observed. New small bands observed below 50 cm-' are attributed to backfolded phonon modes due to the formation ofshear superstructures, which were detected by XRD and HRTEM. A resonance effect is indicated by a shift of these bands upon changing the excitation wavelength from 457.9 to 487.9 and 514.5 nm, respectively. This resonanceeffect is confirmedusing a laser line at 621.9 nm, which results in a much broader Rayleigh wing, and a multitude of bands below 116 cm-', a reversed intensity of the pair ofwagging modes at 2831290 cm-', and additional shoulders at 621,639,990, and 1005cm-l. Further confirmation is found in a resonance Raman experiment,where the bands observed are suggested to arise from Mo5+=0 stretching vibrations of defect sites, which were also detected by DR-UVlvis and ESR spectroscopies. The transmission IR spectra in the far-infrared region (200-35 cm-l) are affected in a complex way by particle size reduction and the increasing influence of grain surfaces. This behavior of the FIR bands is connected with the complex, stepwise particle size reduction revealed by XRD. The observed spectral changes in the far-infrared (450-200 cm-l) and mid-infrared (12007450 cm-l) regions as compared to single-crystal data, are explained by TO-LO splitting of the B3* modes. DRIFTS is found to be more sensitive toward spectral changes due t o LO-TO splitting and toward minority species, like molybdenum hydrates, as compared to transmission IR spectroscopy, probably due to the higher surface sensitivityof this technique. Combination modes above 1010cm-l lose intensity upon particle size reduction, thus reflecting the destruction of the Moo3 lattice. A drastic increase in the intensity of bands that are assigned to OH vibrations indicates the presence of water interconnected by H bonds in microcrystalline Moos. The detection of the OH0 deformation mode at 1425 cm-l and additional signals at 770 and 955 cm-l reveals an increasing formation of molybdenum hydrates upon mechanical activation.
1. Introduction There has been growing interest in materials for which physical dimensions are very ~ m a l l . l - Several ~ nanoscopic semiconductor materials have been investigated and studies on the vibrational structure of such materials have been r e p ~ r t e d . ~Theoretical -~ considerations show that the vibrational spectra, infrared and Raman, of powdered samples contain information not only on the phases but also on the size, shape, and state of aggregation of the particles that constitute the powder.1°-14 The position, relative intensity, and bandwidth of the Raman and infrared bands can be influenced
* To whom correspondence may be addressed. Present address: Abt. Oberflachenchemie und Katalyse, Universitat Ulm, Albert-Einstein-Strasse 11, 89081 Ulm/Donau, Germany. On leave from Regional Sophisticated Instrumentation Centre, Indian Institute of Technology, 30333 Madras, India. Abstract published inAdvance ACSAbstracts, August 15,1995. (1)Mestl, G.; Herzog, B.; Schlogl, R.; Knozinger, H. Langmuir, in press. (2) Uzhida, Y.; Pfander, N.; Weinberg, G.; Herein, D.; Schlogl, R.; Mestl, G.; Knozinger, H. To be published. (3)Mestl, G.;Verbruggen, N.F. D.; Knozinger, H. Langmuir, in press. (4)Brus, C. E. J . Phys. Chem. 1986,90,2555. Vreprek, S. J . Phys. C: Solid State Phys. 1982,15,377. (5)Iqbal, Z.; (6) Onaka, T.; Iwaku, T.; Yamamoto, K.; Kasakara, H.; Abe, K. Solid State Commun. 1984,49,809. (7)Ocana, M.; Serna, C . J. Spectrochim. Acta. 1991, 765. (8)Ocana, M.; Fornes, V.; Garcia-Sanios, J. V.; Serna, C. J. J.A m . Chem. Soc. 1985,75,973. (9)Ishikawa, K.; Fujuma, N.; Komura, H. J . Appl. Phys. 1986,57, 973. (10)Genzel, L.; Martin, T. P. Surf. Sci. 1973,34,33. (11)Hayashi, S.;Nakamon, N.; Kanamiro, H . J . Opt. Soc. Jpn. 1979, 46,176. +
@
by these properties due to polarization changes induced by the external electromagnetic field. Experimental observations of these effects have been reported for the infrared and Raman spectra of several microcrystalline solids. 15- l8 Raman and infrared spectroscopies have been utilized in the characterization of powdered samples of Ti02 (rutile),16J9which is used as an oxide support in catalysis. Infrared spectra showed a strong dependence on particle size, shape, and state of aggregation. Surface effects related to particle size were observedin the Raman spectra by the appearance of new bands as the surface-to-volume ratio increases. The nanophase Ti02 stoichiometry was recently investigated by Raman scattering measurements.20,21 Particle size studies of CeOz in automotive exhaust gas catalysts illustrated a strong correlation between the Raman line width and inverse particle size.22 (12)Hayashi, S.;Hirono, J.;Kanamiro, H.; Ruppin, R. J . Phys. SOC. Jpn. 1979,46,1602. (13)Serna, C . J.;Ocana, M.; Iglesias, J.E. J . Chem.Phys. 1987,C20, 47.1 -.-.
(14) Ruppin, R.; Engelmann, R. Rep. Prog. Phys. 1970,33,149. (15) Scott, J. F.; Damen, T. C. Opt. Commun. 1972,5,410. (16)Ocana, M.; Fornes, V.; Garcia Ramos, J. V.; Serna, J. C. J . Solid State Chem. 1988,75,364. (17)Clippe, R.; Evard, R.; Lucas, A. A. Phys. Rev. B 1976,14,1715. (18)Hayashi, S.;To, M. I.; Kanamiro, H. Solidstate Commun. 1982,
44,75. (19)Vovk, S.M.; Tsenter, M. Ya.; Bobovich, Ya. S.; Sharygin, L. M. Opt. Spectrosc. (USSR) 1983,55,476. (20) Parker, J. C.; Siegel, R. W. Appl. Phys. Lett. 1990,57,943. (21)Melenres, C. A.; Narayanaswamy, A,; Maroni, V. A.; Siegel, R. W. J . Mater. Res. 1989.4. 1246. (22)Graham, G. W.; Weber, W. H.; Peters, C. R.; Usman, R. J . Catal. 1991,130,310.
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Moo3 is extensively used as a n active oxide phase in supported catalysts for various a p p l i ~ a t i o n s . ~Activity ~-~~ and selectivity of these materials depend on the kind of crystallographic planes exposed on the ~ u r f a c e Mo.~~~~~ lybdenum oxide exhibits a layered s t r ~ c t u r e This . ~ ~ type of structure indicates the existence of weak interlayer and strong intralayer bonding. The BET surface area increased from about 1.3 to 32 m2/g, when Moo3 was milled in a planetary mill for 600 min.l Calculating the mean particle size from BET data revealed a decreasing particle size from about 1pm to 50 nm.' The particle size reduction was confirmed by SEM.l The primary crystallite size as calculated from X-ray diffraction line broadening gave decreasingcoherent X-ray ranges from 180to about 70 nm.l The presence ofultrafine amorphous material in the 600-min milled sample was indicated by this difference between the mean particle size and the primary crystallite size. An amorphous X-ray scattering background confirms the presence of amorphous I material and HRTEM2detected amorphous surface layers _._. (Beilby layer) on the longest milled Moo3 particles. 250 350 450 550 650 750 Anomalous X-ray diffraction intensity ratios, line shapes, Wavelength [nm] and the disappearance or appearance of X-ray diffraction lines indicated the formation of shear defects during Figure 1. UV/vis diffuse reflectance spectra of MOO3 milled mechanical activati0n.l These shear defects were also for different times: a) unmilled, b) 10 min, c) 20 min, d) 60 min, e) 120 min, f) 180 min, g) 240 min, h) 420 min, and i) 600 min. observed by HRTEM2and ESR spectroscopies3. A close Positions oflaser lines used for excitation of Raman spectra a r e inspection of the primary crystallite size reduction, indicated. connected with the varying X-ray diffraction pattern quality, suggested a stepwise process of disintegration Raman spectra should be affected by the decreasing during ball mi1ling.l particle sizes and/or the introduction of different defects Defects induced by mechanical activation were charinto MOO3 by mechanical Furthermore, IR acterized using DR-UV/vis, ESR, and IR spectroscopies of spectra of centrosymmetric solids show longitudinaladsorbed GO probe molecule^.^ The DR-UV/vis spectra transverse (LO-TO) splitting effects, which depend on (see also Figure 1)exhibited considerable changes with the size and morphology of the crystallites in the powder decreasing particle size. The shape of the valence-tospecimen.32 Moo3 belongs to D2h symmetry and, hence, conduction band transition was strongly altered by should show changing IR spectra with particle size excitonic absorptions which, in turn, were affected by variations. In addition, IR spectroscopy provides inforsmaller particle sizes, altered crystallite surfaces, and the mation on the presence of OH groups which may be formed formation of defect clusters. Both the increasing intensity by adsorption ofwater on coordinatively unsaturated sites and the shift of the polaron band a t 620 and below 800 created by mechanical stress. nm revealed that a substoichiometric Moo3-, was formed The present work reports the relationship between the upon mechanical treatment. ESR spectroscopy showed changes in the IR and Raman spectra with decreasing that Moo3milled for 600 min, and unmilled Moo3although particle size and the introduction of defects into Moo3 in much smaller concentration, contained Mo5+centers. upon mechanical activation. The main part of these Mo5+ions had CzUor Clusymmetry. Both samples also contained Mo5+centers interacting with 2. Experimental Section protons in close vicinity, again for unmilled Moo3in much MOO3 (Merck, p.a.1 was ground in a planetary mill at 145 rpm smaller concentrations. Adsorption of 0 2 did not lead to over a n extended period of time as described previously.' In the paramagnetic broadening; hence, these Mo5+centers are used planetary mill, the agate cylinder, filled with agate balls located within the bulk Moo3. In addition, a signal in the and the material to be milled, is oriented vertically. It is ESR spectra was assigned to free electrons a t the simultaneously rotating around a n externally lying vertical axis crystallite surfaces as revealed by paramagnetic broadenand additionally, in the opposite direction, around its own rotation ingupon 0 2 adsorption. One Mo5+defect species, however, axis. During the milling process, samples were drawn at distinct was exclusively detected in milled Moo3 and attributed times (namely, 10,20,60, 120, 180,240,420, and 600 min; they are denoted MoOO and Mol0 to Mo600). The BET surface area to the precursor structure of shear defects, thus corwas measured by Nz adsorption in a Quantasorb Jr. apparatus. roborating the reported XRD and HRTEM results.lI2 IR After reaching a constant BET surface area, the milling process spectroscopy of adsorbed GO revealed the formation of was discontinued. coordinatively unsaturated Mo*+surface states in Moo3 All Raman spectra were recorded on a n O W S 89 (Dilor) samples which were mechanically activated. (23) Pope, M. T. Heteropoly and Isopoly Oxometallates; Springer: Berlin. 1993. (24)Trifiro, F.; Caputo, G.; Villa, P. L. J.Less Common. Met. 1974, 36, 305. (25) Ozkan, U. S.; Schrader, G. L. J. Catal. 1985, 95, 120. (26) Grasselli, R. K.; Burrington, J. D. Adu. Catal. 1981, 30, 133. (27) Ruiz,P.;Zhou, B.;Remy,M.;Machej,T.;Aoun,F.; Doumain,B.; Delmon, B. Catal. Today, 1987, 1 , 181. (28) Tong, Y.; Lunsford, J. H. J.Am. Chem. SOC.1991, 113, 4741. (29) Farneth, W. E.; McCarron, E. M.; Sleight, A.W.; Staley, R. H. Langmuir 1987, 3, 217. (30) Baiker, A.; Dollenmeier, P.; Reller, A. J.Catal. 1987,103,394. (31) Kihlborg, L. Ark. Kemi 1963,21, 357.
spectrometer operated in the subtractive mode. The entrance slit widths were set to 50 pm, which corresponds to a spectral resolution of 2.5 cm-l. The spectrometer was equipped with a n optical multichannel diode array of512 diodes which was cooled to 250 K to reduce thermal noise. The stepping motor, the detector controller, and the software package were from Spectroscopy Instruments. Figure 1shows diffuse reflectance spectra in the W / v i s regime. For a detailed discussion of the spectral changes induced by mechanical activation, see ref 3. The numbers in Figure 1give
(32) Decius, J. C.; Hexter, R. M. Molecular Vibrations i n Crystals; McGraw-Hill: New York, 1977.
Mechanically Activated Moo3 the wavelength position of the laser lines used for excitation of Raman spectra, namely, the lines at 514.5,487.9, and 457.9 nm of an Ar+ ion laser (Spectra Physics, Type 2020). For excitation ofthe Raman spectrum at 621.9nm, a linear dye laser (Rhodamin 6G;Spectra Physics)was used. All Raman spectra were recorded in backscattering geometry. Laser power for all lines was set to 50 mW measured at the sample position. All Raman spectra were recorded in the conventional multichannel technique. Hence, the spectra are convoluted by the diode array characteristics and the spectrometer function. The real band shapes cannot be observed using this t e ~ h n i q u eThus, .~~~ band ~ ~shifts between the different batches of mechanically activated Moos, as well as changing band profiles, are not discussed in the following,except variations in doublets ofvicinal bands. Hence, detailed information about internal strain35(band shifts) that may change with mechanical activation cannot be obtained. Fourier transform transmission spectra in the far-infrared (FIR) region (400-30 cm-') were recorded on a IFS66V FTIR spectrometer (Bruker)under the usual vacuum conditions. In this wavenumber regime, polyethylene pellets, mylar beam splitters of different thicknesses, and a DTGSdetector were used. The spectral resolution was 4 cm-l. The spectra shown are the sum over 128 single spectra. Fourier transform transmission spectra in the mid-infrared (MIR) region of KBr pellets were also recorded on the IFS66V FTIR spectrometer (Bruker)using the DTGS detector. A KT3r beam splitter was used. Under the conventional vacuum conditions, 64 single spectra were recorded and added. The spectral resolution was also 4 cm-l. DiffusereflectanceFourier transform IR (DRIFT)spectra were obtained on a Bruker IFS 66 spectrometer. For MIR spectra, a MCT detector was used. An in situ DRIFTS cell (Spectra Tech, Model 0030-100)equippedwith KBr windows was used. All MIR spectra were recorded with 1000scans and a frequencyresolution of 2 cm-' in the range between 4000 and 400 cm-l. The spectra were converted into Schuster-Kubelka-Munk units.
Langmuir, Vol. 11, No. 10, 1995 3797 167 pm
n 225 D m mp*
tbI
173 p m 195 p m /
233 p m
C
Figure 2. Crystal structure of Moos. Top: coordination of 0 atoms (large circles) around the Mo center (small circle); bottom: crystal structure of Moo3 formed by chains of edgelinked MOO4 tetrahedra running along the c axis. (after ref 49). 0 min
I
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3. Results and Discussion 3.1. Laser Raman Spectra of Mechanically Activated MoO3. Moo3 must be considered as a layered material in which single layers are built up from two halflayers of moderately coupled chains of Moo4 tetrahedra running along the c axis.31 The layered crystal structure of Moo3 as reported in the literature3I is reproduced in Figure 2 in order to facilitate the following discussion. The Raman spectrum of MoOO exhibited the characteristic bands between 82 and 995 cm-'. The observed bands were assigned following Py et al.,who carried out a singlecrystal and valence force-field calculation^.^^ According to these authors, most of the 45 optical modes are described by atomic displacements either parallel or perpendicular to the chains of tetrahedra along the c axis (Figure 2). Their calculations reproduced the experimental frequencies and grouping in doublets and quartets, which reflect interchain and interlayer interactions. I n Figure 3, the frequencyranges are reproduced, which cover the Raman spectra of ball-milled Moo3with excitation at 487.9 nm. The Raman spectra shown are normalized to the accumulation time. First of all, the general drastic decrease in signal intensity with increased milling time is striking. I t may be suggested that the deeper grayhlue color of Mo6003 is the reason for the intensity reduction due to a larger absorption coefficient. If an increased absorption coefficient gives rise to this effect, (33) Knoll, P.; Singer, R.; Kiefer, W. Appl. Spectrosc. 1990,44,776. (34) Deckert, V.; Kiefer, W. Appl. Spectrosc. 1992,46,322. (35) Tsang, J. C. Light Scattering in Solids. V. In Topics in Applied Physics; Cardona, M., Giintherodt, G., Eds.; Springer: New York,1989; Vol. 66, p 236. (36) Py, M. A,; Schmid, Ph. E.; Vallin, J.T. I1 Nouvo Cimento, 1977, 38B,271. (37) Py, M. A,; Maschke, K. A.Physica 1981,105B, 376.
20 min 60 min
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. 180 min
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Figure 3. Raman spectra of ball milled Moos: effect of milling time (excitation wavelength at 487.9 nm).
differences should be observed between excitations at 458 nm, almost located in the absorption minimum, and at 62 1,9 nm, which coincides with the UV/vis absorptionband at about 620 nm (Figure 1). However, the decrease in intensity is observed to be identical for all exciting laser lines. A plot of the intensity of the most prominent Raman band of Moo3 at 820 cm-l normalized to the recording time against the BET surface area indicates a very steep decrease ofthe intensity with increasing BET surface area for all laser lines used (not shown). An increasing absorption coefficient thus cannot be the only reason for the loss in intensity. This reduction of the band intensity resembles very much the reduction in particle size during the mechanical activation which is also most pronounced in the first minutes ofmechanical activati0n.l Hence, the intensity reduction seems to be affected by the particle size of the samples. In Figure 4, the normalized intensity of the Raman band at 820 cm-' is plotted against the mean particle size as determined from the BET surface areas. Although the data scatter for larger mean particle
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3798 Langmuir, Vol. 11, No. 10, 1995
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Figure 4. Correlation of the intensity of the most prominent Raman band of Moo3 at 820 cm-' with the mean particle size as calculated from the BET surface area data. sizes, a linear correlation seems to exist, which points to a reduced effective scattering volume of microcrystalline Moo3. Raman band shapes in single-crystal studies are determined by the lifetime of the excited phonon, which depends on the decay into andor scattering on other phonons in the solid (homogeneous line width).38 In addition, band shapes are affected by defect-induced scattering of phonons as well as by the breakdown of the k conservation (inhomogeneous line Several factors such as spatial correlations of phonons changing from bulk to nanocry~tallites~~ and strong phonon damping in micro~rystallites~~ have been shown theoretically to account for the shift to lower energy and broadening of Raman bands of silicon microcrystallites. Although microcrystalline silicon and molybdenum trioxide belong to different symmetry groups, the phenomenon of broadening due to decreasing crystallite size may well be due to the effect of the dependence of phonon damping on the coherence length (grain boundaries, dislocations, defects, etc.) in MOOSmicrocrystallites. Graham et a1.22found a strong correlation between the bandwidth and the inverse particle size of CeO2. Raman line broadening is also reported by Weber et for powdered PdOz, and Iqbal et al.,43 from their XRD and Raman measurements, attribute the spectral broadening in polycrystalline silicon to a progressive lifting of the k 0 selection rule. Also for W03, a connection between the bandwidth of the 807cm-l peak and the crystallinity of the sample was reported.44 There is a general broadening observed for all Raman bands of Moo3 with increased mechanical activation. In Figure 5 , the full width at half-maximum (FWHM) of the band a t 820 cm-l is plotted against the BET surface area. Since all spectra were recorded a t the
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(38)Weinstein, B. A.;Zallen, R. Light Scattering in Solids IV. In Topics in Applied Physics; Cardona, M., Guntherodt, G . , Eds.; Springer: New York, 1984;Vol. 54, p 463. (39)Brodsky, M. H.Light Scattering in Solids I. In Topics znApplied Physics; Cardona, M., Ed.; Springer: New York, 1983;Vol. 8,p 205. (40)Ohtani, N.; Kawamura, K. Solid State Commun. 1990,75,711. Fauchet, P. M. Solid State Commun. 1986,58, (41)Campbell, I. H.; 7.19
.I-.
(42)Weber, W. H.; Baird, R. J.;Graham, G. W. J.Raman Spectrosc. 1988,19, 239. (43)Iqbal, 2.; Veprek, S.; Webb, A. P.; Capezutto, P. Solid State Commun. 1981,37.993. (44)Shigesato, Y . ; Murayama, A.; Kamimori, T.; Matsuhiro, K. Reports Res. Lab. Asahi Glass Co. Ltd. 1988,1, 38.
0
a
16
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BET-Surface Area [m2/g]
Figure 5. Correlation of the FWHM of the most prominent Raman band at 820 cm-l with the BET surface area.
same temperature (300 K), homogeneous thermal broadening is excluded. Hence, line broadening arises from the destruction of crystalline Moo3 during the milling process. In contrast to the line intensity behavior, the bandwidth seems to be linearly correlated with increasing BET surface area and not with the mean particle size. Since the BET surface area also includes contributions of pores and cracks within the crystallites, as well as of the surface roughness, it seems to be a better measure for the particle destruction and defect generation during mechanical activation. Consistent with the literature, line broadening is thus not only a function of particle size but also of defect concentration. Parker and Siege120observed a dependence of the Raman spectra of nanocrystalline Ti02 on the TU0 ratio. Nonstoichiometry especially affected FWHM, band positions, and intensities. The authors referred to the formation of Magnkli phases which may be responsible for the observed changes. The above-described alterations in the Raman spectra may thus also be connected with a certain nonstoichiometry of the Moo3 ~ a r t i c l e s . l -The ~ changes in the Raman spectra being discussed in the following seem to arise from nonstoichioimetry in mechanically activated MOOS. In Figure 6A, the doublet of the translatidnal rigid chain modes in the a direction (Ag, 82 cm-l; Blg, 98 cm-l) is reproduced. The band intensities and positions are normalized to the &mode a t 82 cm-l. Hence, only relative changes in intensity and position can be discussed. First of all, a considerable increase in relative intensity of the B1, mode a t 98 cm-l with increasing BET surface is observed. The relative band position also shifts toward the Ag mode by 1-2 cm-l. This observation is in line with the VFF calculation^,^^ showing that the two translational modes fall together to one Ag mode for an isolated MOOS layer. A similar behavior is observed for the translational rigid chain modes along the c direction (Bzg,116; Bzg, 129 cm-l). This corresponding doublet is reproduced in Figure 6B, where the band intensities and positions were normalized to the B3gmode at 129cm-l. While in unmilled Moo3 the two bands have almost the same intensity, the B2gmode a t 116 cm-l is of considerably lower intensity in microcrystalline MOO3. In addition, this band seems to shift toward the Bsg mode. Again according to the VFF calculation^,^^ these two modes are degenerate for a n isolated Moo3 layer. The doublet for the translational
Mechanically Activated Moo3
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Figure 6. Raman spectra of ball-milled MOO3 with varying BET-surface areas: a) 1.3 mVg; b) 1.7 mz/g: c) 1.8 m2/g;d) 3 m2/g;.e) 10 m2/g;D 18 mz/g;g) 2 1 mz/g;h) 30 m2/g,.andi) 32 m2/g. A) Transverse rigid chain modes in the a-direction (Ag, B1,) of Moos wlth increasing mechanical activation. Band intensities and positions are normalized to the band at 82 cm-'. For better visualization, spectra are vertically shifted. Exitation at 487.9 nm. B) Transverse rigid chain modes in the c-direction (Bzg,B3g) of Moo3 with increasing mechanical activation. Band intensities and positions are normalized to the band at 129 cm-l. For better visualization, spectra are shifted vertically. Exitation at 514.5 nm. C) Wagging modes parallel to the c-direction of the terminal Mo=O groups (Bzg,Bsg) for MOO3 samples with increasing mechanical activation. Band intensities and positions are normalized to the band at 284 cm-'. For better visualization, spectra are shifted vertically. Exitation at 457.9 nm.
rigid chain modes along the b axis is not resolved experimentally (band a t 159 cm-'1; hence, changes in this band profile are not discussed. The observed changes of the rigid chain modes are thus interpreted in terms of more weakly interacting Moo3 layers in mechanically activated Moo3. Kihlborgobserved a 3%contraction along the a direction (100) and a 1% expansion along the c axis (001) in M018052 relative to and the introduction of defect structures into Moo3 leads to stress along the (100) and (001) direction^.^^ A Warren-Averbach analysis of the XRD profiles1 revealed a n increased strain of about 1% for Mo600, this being consitent with Kihlborg's observation. Internal strain, e.g., due to lattice mismatch, can result in shifts of Raman f r e q ~ e n c i e s .Hence, ~~ the observed relative band shifts, about 1%ofthe unperturbed band position, and the changing intensity ratios of the rigid chain modes are connected with the reported contraction and expansion of the Moo3lattice in suboxides. In Figure 6C, the two wagging modes of the terminal Mo=O groups a t 283 cm-' (Bzg) and 290 cm-l (B3&, polarized parallel to the c axis, are shown for the Moo3 samples with increasing BET surface area. The band intensities and positions are normalized to the Bzg mode a t 283 cm-l. In the single-crystal the Bsg wagging mode has only one-third of the intensity of the Bzgmode. For unmilled Moo3, the Bsgmode a t 290 cm-l is about three-fourths as intense as the Bzgmode. In Moos, having a BET surface area of 32 m2/g,the two modes are hardly resolved and the Bsgmodes is a n intense as the Bzg mode. In molybdenum suboxides4' the distance between the metal atoms connected through oxygens generally increases toward the shear plane with increasing distortion of the linkage. This should affect all modes polarized parallel to the c axis. Actually, the wagging modes are polarized parallel to this d i r e ~ t i o n ,and ~ ~ hence, the (45) Kihlborg, L. Ark. Kemi 1963,21,443. (46)Gai, P. L.; Thoeni, W.; Hirsch, P. B. J.Less Common Met. 1979, 54, 263. (47) Kihlborg, L.Ark. Kemi 1963,21,471.
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Figure 7. Raman band at 49 cm-' arising from mechanical activation. Intensities are normalized t o the band at 82 cm-'. For better visualization, spectra are shifted vertically. Excitation at 457.9 nm. BET-surface areas: a) 1.3 m2/g;b) 1.7 mVg, c) 1.8 m2/g;d) 3 mVg; e) 10 mVg; 0 18 m2/g;g) 21 m2/g;h) 30 mVg, and i) 32 mVg.
changing intensity ratio also seems to reflect the distortions along the c axis induced by mechanical stress. Again, this observation is consistent with VFF c a l ~ u l a t i o n , ~ ~ which show that these modes fall together for isolated Moo3 layers (vide supra). Considerable changes in the Raman spectra of Moos arising from the milling process are especially found in the very-low-frequencyrange reproduced in Figure 7. The observed broadening of the Rayleigh wing with increasing BET surface area is ascribed to quasi-elastic light scatterine8in amorphous portions of the samples (e.g., Beilby layers)which are generated during mechanical activation.
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Actually, XRDl showed that ball-milled Moos contains a large amount of X-ray amorphous material, and HRTEM images2 revealed that Mo600 particles had amorphous overlayers. For the three samples (Mo240, Mo420, Mo600) having high BET surface area (21,30, and 32 m2/g),a n additional small band is observed a t 49 cm-l on top of the Rayleigh wing (Figure 7). Since Py and M a ~ c h k observed e~~ bands a t 53,49, and 44 cm-l in FIR spectra which were assigned to rigid layer modes, one may suggest that a weakening of the spectroscopic exclusion rule may lead to the observation of normally IR-allowed modes. However, not a single additional band is observed in the whole Raman spectrum of disintegrated Moo3. Thus, a weakening of the spectroscopic exclusion rule, which would lead to the detection of additional Raman-forbidden modes, must be ruled out. The introduction of shear defects can lead to crystallographic shear superstructure^^^ in which the orientation and density ofthe shear planes depend on the degree of reduction. Raman experiments on superstruct u r e revealed ~ ~ ~ the ~ ~backfolding ~ of acoustical phonons due to a smaller Brilliouin zone (larger unit cell in real space). Optical phonons are also folded back, however, 70 120 170 220 due to their small dispersion; this backfolding is not detected. Hence, one may suggest that the small band at Raman Shift [cm-'1 49 cm-I arises from such backfolded acoustical phonons. Figure 8. Raman spectra of ball-milled MOOSexcited at 621.9 This assignment can also explain the drastic increase of nm, bottom spectrum excited with 457.9 nm. Intensities are the band intensity after 180 min ofmechanical activation. normalized to the band at 160 cm-l. For better visualization, While the BET surface is hardly increasing between 18 spectra are shifted vertically. BET-surface areas: a) 1.3m2/g; m2/g(180 min) and 21 m2/g(240 m i d , the density of shear b) 1.7 mVg, c) 1.8 mVg; d) 3 mVg; e) 10 m2/g;f) 18 m2/g;g) 21 defects may have strongly increased. In fact, XRD datal m2/g;h) 30 m2/g,and 1) 32 m2/g. and HRTEM imagesz1 reveal the presence of crystallographic shear structures, which had also been detected the considerable increase in the BET surface area from by ESR.3 21 to 30 m2/g. The different spectral behavior when excited with 621.9 nm can be understood in terms of resonant Exciting the Raman spectra a t 487.9 or 515.4 nm shifts coupling to Mo5+defects. Their concentration especially this band to 35 and 33 cm-l, respectively. The differences should be high in the amorphous portions of milled Moo3. observed in position for different laser lines may probably This resonance effect is confirmed in the pair of wagging be ascribed to a resonance effect (see Figure 1, an excitation a t 621.9 nm is in resonance with the polaron band at 620 modes at 283/290 cm-' (not shown). In contrast to the nm3). A size dependence of the Raman spectra of excitation with 457.9,487.9, and 514.5nm, in the spectrum amorphous graphite51 and microcrystalline C ~ S , S ~ I - , ~ ~of Moo3 having 32 mz/g,the Bsgmode a t 290 cm-l is about 10% more intense as compared to the Bzg mode a t 283 was observed upon changing the excitation wavelengths. cm-I when excited a t 621.9 nm. In addition, in this Such a resonance effect for ball-milled Moo3 is confirmed when a laser line a t 621.9 nm is used. In Figure 8, the spectrum (not shown),the antisymmetric stretching mode of the Mo-0-Mo bridge exhibits two shoulders a t 639 series of low-frequency Raman spectra of Moo3 with and 621 cm-l, and even the stretching mode ofthe terminal increasing BET surface area from 1.3 to 32 m2/g is reproduced. For comparison,the Raman spectrum excited oxygen atoms along the c axis shows two shoulders a t 990 with 457.9 nm is shown a t the bottom. First of all, the and 1005 cm-l (not shown). Again, these differences much broader Rayleigh wing is striking when the spectra observed between the Raman spectra excited with 620 are excited at 621.9 nm. For milling times of 120 min and nm and with 457.9, 488.0, and 514.5 nm are attributed longer, it overwhelms almost all bands below 117 cm-l. to Mo5+ defects in mechanically activated Moos. This The reason for this broadening is also found in the resonance effect was further corroborated by an experiincreasing amorphization of Moo3 upon milling. In ment where Moo3 milled for 600 min was diluted with addition, for Mo420 and Mo600,the shape of the Rayleigh KBr in a ratio of 1/1000 (Figure 9) and where the laser wing has been significantly altered and a multitude of beam was hitting the sample in a glancing angle. The bands below 116 cm-I are recorded. This change in the signal intensity of spectra a and b of Figure 9 is normalized shape of the wing between spectrag and h correlates with to the strong FQmode of BaF2at 241 cm-l, recorded under identical conditions, since the purely white BaFz should not show significant wavelength dependence in this (48)Cardona, M. Light Scattering in Solids 11. In Topics in Applied Physics; Cardona, M., Guntherodt, G.,Eds.; Springer: New York, 1982; spectral region.42 In the lower part of Figure 9, the Raman Vol. 50, p 19. spectra of undiluted Moo3 are reproduced, which were (49) Jusserand, B.; Cardona, M. Light Scattering in Solids V. In excited a t 457.9,488.0,514.5, and621.9nm. In spectrum Topics in Applied Physics; Cardona, M., Guntherodt, G., Eds.; Springer: New York, 1989; Vol. 66, p 49. a of Figure 9, KBr mixture after dehydration a t 390 K and (50) Merlin, R. Light Scattering in Solids V. In Topics in Applied excited at 621.9 nm, three bands or shoulders are observed Physics; Cardona, M., Guntherodt, G., Eds.; Springer: New York, 1989; at 995,1003, and 1012 cm-l, while only a relatively broad Vol. 66, p 214. band at 995 cm-l is seen in spectrum b of the dehydrated (51)Yoshikawa, M.; Nagai, N.; Matsuki, M.; Fukuda, H.; Katagiri, G.; Ishida, H.; Ishitani, A. Phys. Rev. B 1992,46, 7169. sample excited at 457.9 nm. Acomparison with the spectra (52) Baranov, A. V.; Petrov, V. I.; Bobovich, Ya. S. Proc. XIII Intern. of the undiluted, hydrated sample shows that excitation Conf on Raman Spectrosc., Wiirzburg;Kiefer, W., Cardona, M., Schaack, a t 457.9 nm results only in normal Raman scattering (band G., Schneider, F. W., Schrotter, H. W., Eds.; J. Wiley & Sons: New a t 997 cm-l). A comparison of spectrum a, dehydrated York, 1992; p 989.
Mechanically Activated MOOS
Langmuir, Vol. 11, No. 10, 1995 3801
iz
900
950 Raman Shift
I
1000
[cm-'1
Figure 9. Resonance Raman spectra of Moos with a BET surface area of 32 m2/gdiluted (VlOOO) in KBr. (Exitation at 621.9 nm; 90" experiment, glancing incidence). a) dehydrated sample (390 K); b) dehydrated sample (390 K), exitation at 457.9 nm. At the bottom non-resonant bulk spectra. Band intensities are normalized to a BaFz-reference. For better visualization, spectra are shifted vertically.
sample, with the bulk (hydrated)Raman spectrum excited at 621.9 nm reveals that the major band in the latter remains only as a small shoulder a t 995 cm-l. On the other hand, the shoulder a t 1005 cm-l in the latter is the major band in resonance spectrum a. Wachs and cow o r k e r ~have ~ ~ , shown ~ ~ that V=O, W=O, Mo=O, etc., terminal stretching modes shift to higher energies upon dehydration, which was explained by a strengthening of the M=O bond due to a changed coordination sphere. DRW / v i s and ESR spectroscopies3 revealed the presence of Mo5+ defects interacting with protons (e.g., Mo5+-OH groups). These OH groups are partially lost upon dehyd r a t i ~ n .IR ~~ spectroscopy of CO adsorbed on evacuated Mo6003also revealed the presence of Mo4+surface defects. Hence, the Raman bands observed in the resonance experiment (spectrum a of Figure 9) are attributed to Mo-0 stretching vibrations of coordinatively unsaturated defects in mechanically activated Moo3. 3.2. Fourier Transform Transmission Infrared Spectroscopy. There is considerable discrepancy between the IR band positions reported for Moo3 powder samples by different research group^.^^-^^ Py and Maschke37calculated the vibrational frequencies of MOOSand (53) Vuurman, M. A.; Wachs, I. E. J . Phys. Chem. 1992, 96, 5008. (54) Chan, S. S.; Wachs, I. E.; Murrell, L. L.; Wang, L.; Hall, W. K. J . Phys. Chem. 1984,88, 5831. (55) Mestl, G.; Verbruggen, N.; Knozinger, H. Langmuir, to be submitted. (56) Barraclough, C. G.; Lewis, J.;Nyholm, R. S. J . Chem. SOC.1959, 3552. (57) Barraclough, C. G. A u t . J . Chem. 1966, 19, 741. (58) Mattes, R.; Schroder, F. Z . Naturforsch. B: Anorg. Chem., Org. Chem. 1969,24, 1095. (59) Beattie, I. R.; Gilson, T. R. J . Chem. SOC.A 1969, 2322. (60) Beattie, I. R.; Cheetham, N.; Gardner, M.; Rogers, D. E. J . Chem. SOC.A 1971, 2240. (61) Camelot, M. Rev. Chim. Miner. 1969, 6 , 853. (62) Ravikumar, K. G.; Rajaraman, S.; Mohan, S. Proc. Ind. Nutl. Sei. Acad., Part A 1986, 51, 368.
evaluated the VTO and VLO splittings ofeach IR-active mode in a Moo3 single-crystal Thus, the reason for the unreproducible band positions is found in the LO-TO splitting which is affected by particle size and morphology of the crystallites. In addition, the LO-TO splitting depends on the square of the derivative (a,/%)2;32 hence, vibrations generating large dynamic dipole moments exhibit a large LO-TO splitting as well as large intensities, since the latter also depends on In powder samples, the inhomogeneousparticle size distribution and the randomly oriented crystallites further complicate the spectra. Eda carried out a high-temperature DRIFTS in which he utilized the LO-TO splitting concept in deconvoluting the observed spectra and assigning the bands. Eda found that the changes in the IR spectrum with heat treatment are mainly concerned with the BZ,, B3,, and Bz, stretching modes of the terminal Mo=O groups, with the B3, Mo-0-Mo stretching plus bending mode, and with the B3, Mo=O scissoring mode. In this study, we apply the assignment of modes according to Py et a1.36!37 and the concept of LO-TO splitting. Absorption in the Far-IR (200-35 em-'). In Figure 10A, the FIR transmission spectra of Moo3 are shown in dependence on the duration of the mechanical activation. Drastic changes in the frequency range accompany the increasing BET surface area or decreasing particle size of the material under investigation. Even the spectrum of MoOO (Figure 10A, spectrum a ) shows considerable differences compared to the single-crystal study of Py et al.36 In this single-crystal study, bands were observed at 44 sh, 53 sh, and 192 sh cm-'. This observation is in line with the XRD characterization of MoOO, revealing the presence of lattice imperfections in this materia1.l Obviously the lattice mode regime in the spectra of Moo3 is very sensitive to the presence of these crystal imperfections, which is indicated by the drastic changes in the series of FIR spectra of Moo3with increasing BET surface area. Low-frequency phonons react very sensitively on the changes in the crystallinity (coherence length of phonons) and the nature ofthe particle surface. Although a detailed assignment of the bands observedis not possible, the changes in the spectra seem to be connected with the complex stepwise process of particle size reduction found by XRD.'
Absorption in the FIR (450-200 em-') and MIR (450-1200 cm-'1. The transmission spectra in the FIR (450-200 cm-l), Figure 10B, and MIR (450-1200 cm-'), Figure lOC, regimes do not show comparably drastic changes as those observed in the low-frequency regime (Figure 10A). This observation again indicates that the spectral variations in the regime between 200 and 35 cm-' are arising from size and morphology effects on long wavelength phonons. The main changes in the series of transmission spectra upon ball milling (Figure 10B, C) are observed for the broad band of the B1, TO wagging mode ofthe Mo-0-Mo bridges at 292 cm-l, which changes its shape, for the LO mode of the B1, wagging vibration, which increases in intensity, and for the band a t 608 cm-l, which shows a varying shape and position with the milling time. All these bands are assigned to vibrations of Mo0-Mo bridges, and hence, these observed variations may reflect the introduction of shear defects (compressionalong the c axis). Especially, the shape ofthe latter band a t 608 cm-l in spectrum f of Mol80 is striking. This observation is probably connected with the anomalies detected in the X-ray diffraction patterns a t this stage of mechanical activation,l which were attributed to a stepwise particle size reduction. (63) Eda, K. J . Solid State Chem. 1991, 95, 64.
3802 Langmuir, Vol. 11, No. 20, 1995
Mestl et al.
450
400
350
300
250
1200
200
150
100
800
600
Wovenumbers [cm-'1
Wavenumbers [cm-'1
200
1000
50
Wovenumbers [cm-'1
Figure 10. FTIR-transmission spectra of mechanically activated MOO3 with varying BET surface areas: a) 1.3 m2/g;b) 1.7m2/g, c) 1.8 mVg; d) 3 mz/g;e) 10m2/g;D 18 mz/g;g) 2 1 m2/g;h) 30 m2/g,and i) 32 m2/g. A) Far infrared spectra: 50-200 cm-l. B) Far infrared spectra: 200-450 cm-l. Intensities normalized to band at 370 cm-l. C) Mid infrared spectra: 450- 1200cm-l. Intensities normalized to band at 993 cm-l.
1
IT200
d
I T bi
A~
2100 1300
1100
900
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500
1900
1700
1500
Wavenumbers [cm-'1
Wovenumbers [cm-'1
C
1300 4000
3500
3000
2500
Wovenumbers [cm-'1
Figure 11. Driftspectra of mechanically activated MOO3 with varying BET surface areas: a) 1.3 m2/g,b) 1.7 m2/g,c) 1.8 mVg, d) 3 m2m/g,e) 10 mVg, D 18 m2/g,g) 21 mVg, h) 30 m2/g,and i) 32 m2/g. A) Fundamental regions (450-1200 cm-l). Intensities normalized to band at 985 cm-l. B) Spectral region 1300-2100 cm-l. Intensities normalized to combination mode at 2000 cm-l. C) Hydroxyl stretching regions (2100-4000 cm-'1. Intensities normalized t o band at 985 cm-l.
3.3. Diffuse Reflectance Infrared Spectroscopy of Activated MOOS. In Figure 1lA, the diffise reflectance Fourier transform (DRIFT) MIR spectra of MOOSmilled for different times are reproduced. There are drastic changes in the spectral features compared to the transmission MIR spectra (Figure 1OC). The very intense band at 608 cm-l observed in Figure 1OCis present as a shoulder at 622 cm-l in the MoOO spectrum (Figure 11A, a). The small band a t 819 cm-l in Figure 1OC (a)shows a splitting into two bands a t 810 and 825 cm-' in spectrum a ofFigure 11A. The broad intense band at 877 cm-l of Figure 1OC can be recognized in spectrum a of Figure 11A as a broad band of medium intensity. In addition, the combination mode regime between 1300 and 1010 cm-l is of stronger intensity in the DRIFT spectrum. These differences between the transmission IR and DRIFT spectra may be
explained by the different mechanisms leading to the observed spectra. In transmission, the spectral features are mainly controlled by the absorption coefficient, while in diffuse reflectance, both the absorption and the scattering coefficient determine the spectral appearance. Moreover, in transmission, light is penetrating the whole sample (pressed wafer), and hence, information is gained throughout the randomly oriented bulk crystallites,while the information depth in diffuse reflectance may be significantly smaller, and the nature of the surface of the powder sample may also affect the spectral appearance. These effects may lead to different LO-TO splittings in transmission and diffuse reflection, thus changing the spectral features. In the series of spectra in Figure 11A, certain changes are observed with increasing BET surface area. The pair
Langmuir, Vol. 11, No. 10, 1995 3803
Mechanically Activated MOOS
of bands a t 450 and 526 cm-l that are assigned to the Bzu and Bsumodes of the stretching plus bending vibration of the Mo-0-Mo bridges lose resolution and are observed as a broad band a t 480 cm-l in spectrum i. The weak shoulder at 620 cm-l in spectrum a of Figure 11A loses intensity with increasing BET surface area, while a new band of medium intensity, not present in spectrum a, arises at 770 cm-l, which can be assigned to a Mo-OH stretch of molybdenum hydrate in accordancewith the 1iteratu1-e.~~ The doublet at 810 and 825 cm-l in spectrum a, attributed to the TO and LO mode of the Bau stretching vibration, also loses resolution and can be seen a t 820 cm-l in spectrum i. The broad band a t 870 cm-l, probably being identical with the band a t 880 cm-l (TO, B3J in Figure lOC, loses intensity and is identifiedas a broad asymmetry in spectrum i. During mechanical activation, a new band arises a t 955 cm-l which also can be\ assigned to a molybdenum hydrate.63 The band a t 985 cm-l in spectrum a, probably identical to the band a t 988 cm-l (LO, B3u)in Figure lOC, remains only as a shoulder in spectrum i. Hence, both the TO and the LO modes of the Bsuvibration of the terminal Mo=O groups lose intensity. The intense band a t 1026 cm-l shifts to 1009 cm-l and gains intensity with increasing BET surface area. Interestingly, Cariati et al.65report a band a t the latter position for Mo18052. The two high-frequency shoulders a t 1074 and 1130 cm-l observed in spectrum a almost disappear during mechanical activation. These bands, also observed in transmission, arise from combination modes of optical phonons of intermediate energy, and their disappearance reflects the increasing disintegration of Moo3 particles. In conclusion, DRIFTS, as compared to transmission IR spectroscopy, seems to be more sensitive toward spectral changes due to LO-TO splitting induced by mechanical activation (smaller particles) and toward minority species, like suboxides and molybdenum hydrates which may predominantly exist in near surface layers. In Figure 11B, the combination mode regime is reproduced. Five bands or shoulders a t 1990,1970,1955,1933, and 1885cm-l are attributed to combinationmodes arising from stretching fundamentals. Two bands are observed a t 1630 and 1425 cm-l, strongly gaining intensity with increasing BET surface area. The band a t 1630 cm-l can be assigned to the deformation mode of H20. The position, above the gas-phase value of 1595 cm-l and below that of liquid water (1640 cm-l) indicates H20 molecules in a network of H bridges. The band a t 1425 cm-l can be assigned to the OH0 bending vibration of molybdenum hydrates. 63 In Figure 1lC, the OH stretching regime is reproduced. The band intensities are normalized to the intensity of the stretching fundamental a t 985 cm-l. One clearly recognizes the drastic increase in intensity of the bands in this regime with increasing BET surface area. Chemisorption of H20 leads to H bonding of all surface OH groups and, thus, to a broad absorption between 3650 and 3200 cm-l. Therefore, the broad absorption in the spectra of Figure 11C is assigned to OH groups and adsorbed water molecules interconnected by H bonds. In the spectrum of MoOO, the signal maximum is a t about 3220 cm-l, while two shoulders are observed at 3320 and 3470 cm-l. In the spectrum of Mo600, maximum signal intensity is recorded a t 3350 cm-l, while the former most intense band remains as a shoulder a t 3240 cm-l. A second shoulder is observed a t 3500 cm-l. The position of OH stretching (64) Sheik Saleem, S.;Arulghas, G. Pramana 1983,21, 283. (65) Cariati, F.; Bart, J. C. J.;Sgamelotti, A. Inorg. Chim.Acta 1981, 8, 97. (66) Morrow, B. A. In Studies in Surface Science; Fierro, J. L., Ed.; Elsevier: Amsterdam, 1990; Vol. 57, p 161.
100
80
60
IR Intensity [% M06001
40
20
BET Surface Area [m2/g1
0
IR Bands at:
...[cm-I1
c
Figure 12. Plot of the fractional band intensities of all IR bands related to Mo hydrates or OH vibrations against BET surface areas. Band intensitiesnormalized to band at 985 cm-l and value for sample Mo600 taken as 100%. 0: 770 cm-', 0: 949 cm-', 0: 1630 cm-l, V: 3240 cm-l.
modes in that regime is sensitive to the degree and strength of hydrogen bonding between OH groups and water molecules. Thus, the changes observed in intensity and position between MoOO and Mo600 reflect changes in concentration and bonding between H20 molecules and OH groups. Interestingly, even after heat treatment a t 673 K in flowing N2 for 1h, OH bands of lower intensity are still observed.55 This is in line with in situ ESR c h a r a ~ t e r i z a t i o nand ~ ~ indicates a stable incorporation of protons (HzO, OH groups) into Moo3 by mechanical activation. In Figure 12, the normalized intensities of all bands related to molybdenum hydrates or OH vibrations are plotted as fractional values with the values for Mo600 taken as 100% against the BET surface areas. Such a plot may be considered because normalization to Mo=O stretching fundamentals accounts for changing concentrations of absorbing centers in differently sized powders. The resulting correlation between normalized band intensities and BET surface areas may suggest that molybdenum hydrates are formed in the amorphous surface layers. 4. Conclusions
It could be shown that the vibrational powder spectra of Moo3 are affected by mechanical activation in a planetary mill. The drastic decrease in the Raman scattering efficiency correlateslinearly with the decreasing mean particle size. Hence, the intensity reduction may be explained by a reduced effective scattering volume of microcrystallites. The general broadening of all Raman bands observed with increased mechanical activation, on the other hand, linearly correlates with the increasing BET surface,which is a measure of the particle destruction and defect generation during mechanical activation. The observed changes of intensity ratios of the translational rigid chain modes a t 82 (A,) and 98 cm-l (B1,) and 116 (B2,) and 129 cm-l (B3,) are connected with the known contraction along the a axis and expansion along the c axis of the MOOSlattice in suboxides. The small shifts observed for these doublets are in line with more weakly interacting1aye1-s~~ or may arise from minor internal strain induced by mechanical activation. The intensity changes of the wagging modes a t 283 (Bag)and 290 cm-l (B3,) of the terminal Mo=O groups are suggested to arise from distortions along the c axis induced by mechanical stress. The observed broadening of the Rayleigh wing with increasing BET surface area is ascribed to quasi-elastic light scattering induced by increasing amorphization of Moo3. The creation of shear d e f e ~ t s l -leads ~ to the
3804 Langmuir, Vol. 11, No. 10,1995 formation of superstructures and, thus, to smaller Brillouin zones. The additional small band, observed a t 49 cm-l for the three samples having the highest BET surface areas, is attributed to backfolded acoustic phonons. The shift of this band with the excitation wavelength points to a resonance effect. Such a resonance effect is confirmed using an excitation laser line at 621.9 nm which results in considerable changes of the Raman spectra. This resonance effect is further corroborated by a resonance Raman experiment. The bands observed in the resonance Raman experiment were suggested to arise from Mo=O stretching vibrations of defect sites. The drastic changes in transmission IR spectra (20035 cm-l) point to a considerable influence of the reduced particle size upon the lattice modes and reflect the suggested stepwise particle size reduction upon ball mi1ling.l The changes in the transmission IR spectra in the region between 250 and 1200 cm-I are ascribed to variations in the TO-LO splittings of Bsu modes of the Mo=O scissoring and stretching vibrations and of the B3" mode of the Mo-0-Mo bridge. The differences between transmission IR and DRIFT spectra are suggested to arise from the different mechanisms (absorption and absorption plus scatteringlreflection), leading to the observed spectra, and DRIFTS seems to be more sensitive toward spectral changes due to LO-TO splitting induced by mechanical activation as compared to transmission IR spectroscopy.
Mestl et al. In DRIFTS, additional signals appear a t 770 and 955 cm-l which are attributed to molybdenum hydrates. A drastic increase in intensity of the bands that are assigned to OH vibrations indicates the presence of water in microcrystalline MOOS. The band at 1630 cm-I, assigned to the deformation mode of H20,indicates HzO molecules in a network of H bonds. The band a t 1425 cm-I can be assigned to the OH0 bending vibration in molybdenum hydrates. These observations are confirmed by a broad absorption centered about 3300 cm-l which is also indicative of OH groups and adsorbed water molecules that are interconnected by H bonds. The observedchanges in intensity and position between the sample having only 1.3mz/gand the one having 32 mz/gare suggested to reflect changes in concentration and bonding between HzO molecules and OH groups.
Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and the Fonds der Chemischen Industrie. The research stay of T. K. K. Srinivasan was made possible by the Deutsche Forschungsgemeinschaft. We thank Dr. T. Beutel for his assistance with the DRIFTS measurements. LA950147H
