Langmuir 1996,11,3027-3034
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Mechanically Activated MOOS. 1. Particle Size, Crystallinity, and Morphology G. Mestl,t$*B. Herzog,e R. Schlogl,§ and H. Knozinger*?+ Institut fur Physikalische Chemie, Universitat Miinchen, Sophienstrasse 11, 80333 Miinchen, Germany, and Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Received February 20, 1995. I n Final Form: May 17, 1995@ Physicochemicalchanges induced in MOO3 by mechanical activation in a planetary mill were investigated. During the milling process, the BET surface area increases from about 1.3 to 32 mzlg. Energy dispersive X-ray (EDX)analysis and X-ray photoelectron spectroscopy (XPS)of the powdered MOO3 samples reveal that no agate is abrased during the milling process. An estimation of the mean particle size using the BET data or scanning electron microscopic (SEM)images indicates a decrease from about 1pm to about 50 nm. The primary crystallite size calculated from X-ray diffraction (XRD) line broadening also shows a decreasing size from about 160 nm to about 80 nm. The differencebetween the particle size of unmilled M003, as determined from the BET surface area or by scanning electron microscopy (SEM), and the calculated primary crystallite size using X-ray line broadening is explained by Moo3 particles consisting of smaller primary crystallites. The smaller average particle size of Moo3 milled for 600 min calculated from BET data, on the other hand, as compared to the XRD primary crystallite size is ascribed to the presence of ultrafine amorphous material which is X-ray amorphous and, therefore, does not contribute to the X-ray line broadening. This formation of amorphous material is also indicated by an increasing amorphous scattering background in the X-ray diffraction patterns. In SEM pictures, these particles appear to have an amorphousoverlayer. The strange behavior of both the X-ray diffraction pattern quality and the diffuse scattering background, which do not coincide, during mechanical activation probably indicates a complex process of particle size reduction. Variation of X-ray reflection profiles, intensity ratios, and additional X-ray reflections may point toward the formation of shear defects during this process. A Warren-Averbach analysis of the most intense X-ray reflections of milled Moo3 reveals that internal strain is only marginally enhanced by mechanical activation.
1. Introduction The class of molybdenum oxygen compounds is of great technical interest. Thus, pure molybdenum oxide and mixtures with tungsten oxide were considered as being relevant as future display materials, since both compounds changed their color reversibly under irradiation with Molybdenum bronzes, H,Mo03, have electrochromic properties and are also relevant for display systems: while the analogously composed LLMo03bronzes were investigated in connection with their application in nonqueous high-power b a t t e r i e ~ . ~Moreover, -~ alkali and earth alkali bronzes as well as molybdenum chalcogenides are transition-metal semiconductors8and a series of rare earth molybdates have ferroelectric proper tie^.^ Alkali molybdates are most commonly used in corrosion inhibition.1° Moo3 and polymolybdates are also technically highly important compounds in catalysis, as catalysts or as
* To whom correspondence should be addressed. Institut fur Physikalische Chemie, Universitat Munchen. Present address: Abt. Oberflachenchemie und Katalyse, Universitat Ulm, Albert-Einstein-Alee 11, 89081 Ulm, Germany. Fritz-Haber-Institutder Max-Planck-Gesellschaft. Abstract published in Advance ACS Abstracts, July 15, 1995. (1)Yao,J. N.; Loo,B. H.; Hashimoto,K.; Fujishima,A.Ber.Bunsenges. @
Phys. Chem. 1991,95, 557. (2) Yao,J.N.; Loo, B. H.; Hashimoto,K.; Fujishima,A.Ber.Bunsenges. Phys. Chem. 1991,95, 554. (3) Faughnan, B. R.; Crandall, R. S. Appl. Phys. Lett. 1977,31,834. (4) Dickens, P. G.; Reynolds, G. J. Solid State Zonics 1981, 5, 331. ( 5 ) Dampier, F. W. J. Electrochem. SOC.1974, 121, 656. (6) Colton, R. J.; Guzman, A. M.; Rabalais, J. W. Acc. Chem. Res. 1978, 11, 170. (7) Besenhard, J. 0.;Schollhorn,R. J.Power Sources 1976, I, 267. (8)Pichat, P.; Mozzanega, M. N.; Hoang-Van,C.J.Phys. Chem. 1988, 92, 467. (9) McDonald, R. E.; Vogel, J. J.; Brookman, J. W. ZBM J.Res. Dev. 1962, 6 , 363. (10)Vukasovich, M. S.; Farr, J. P. G. Polyhedron 1986, 5, 559.
catalyst precursors.l' Supported molybdenum chalcogenides are relevant materials for hydrodesulfurization (HDS),I2 hydrodenitrogenation (HDN),13 and hydrodemetalation (HDM).14 Furthermore, molybdenum oxides together with group Va oxides are used as oxidation c a t a l y ~ t s l 5 and - ~ ~in oxidative coupling reaction^.^^ Moo3 supported on T i 0 2 is a selective catalyst in photooxidationz0 and additionally highly active for selective catalytic reduction (SCR) of NO, with NH3.21 It is reported in the literature that catalytic properties are affected by the morphology and/or crystallinity of Moo3. Thus, the structure sensitivity of the selective oxidation of propene was shown for different crystallographic faces of M003.22 While total oxidation occurs on the (010) surface, the (100) surface catalyzes acrolein formation. The recently described monoclinic P-phase of Mo0323exhibits a higher catalytic activity in the partial alcohol oxidation as compared to the a - p h a ~ eand , ~ ~also (11)Pope, M. T. Heteropoly and Isopoly Oxometullates; Springer: Berlin, 1993. (12) Grange, P. Catal. Rev.-Sci. Eng. 1980,21, 135. (13) Katzer, J. R.; Sivasubramanian, R. Cutal. Rev.-Sci. Eng. 1979, 20, 155. (14) Vielhaber, B.; Knozinger, H. Appl. Cutul. 1986,26, 375. (15) Trifiro, F.; Caputo, G.; Villa, P. L. J.Less Common Met. 1974, 36, 305. (16) Ozkan, LJ.S.; Schrader, G. L. J. Cutal. 1986,95, 120. (17) Grasselli, R. K.; Burrington, J. D. Adu. Catal. 1981, 30, 133. (18)Ruiz,P.;Zhou,B.;Remy,M.;Machej,T.;Aoun,F.;Doumain,B.; Delmon, B. Catal. Today 1987, 1 , 181. (19) Tong, Y.; Lunsford, J. H. J.A m . Chem. SOC.1991, 113, 4741. (20) Liu, Y. C.; Griffin, G. L.; Chan, S. S.;Wachs, I. E. J.Cutal. 1986, 94, 108, and references therein. (21) Bosch, H.; Jansen, F. Catal. Today 1988, 2, 369. (22) Abon, M.; Mossardier, J.; Mingot, B.; Volta, J. C.; Floquet, N.; Bertrand, 0. J. Catal. 1992, 134, 542. (23)McCarron, E. M. J. Chem. SOC.,Chem. Commun. 1986, 336. (24) Fameth, W. E.; McCarron, E. M.; Sleight, A. W.; Staley, R. H. Langmuir 1987,3, 217. (25) Baiker, A.; Dollenmeier, P.; Reller, A. J.Cutal. 1987, 103,394.
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the catalytic reduction of NO with ammonia is affected by the Moo3 grain morphology.25 Recently, a new preparation route was describedz6-28 toward supported molybdenum catalysts comparable to the technical synthesis of SCR catalyst^.^^ According to this, Moo3 spreads over the carrier oxide a t elevated temperatures. The exact mechanism of this spreading process is not yet clarified, but surface melting of Moo3 (unrolling carpet m e ~ h a n i s m ) similar ,~~ to plastic flow (creep mechanism) at interfaces during ~ i n t e r i n gmight ,~~ partially explain the phenomenon of spreading. The first step of this dry synthesis in laboratory dimensions is the intense mixing of the Moo3 phase with the carrier oxide and subsequent grinding in a mortar. Immediately, the question arises which physicochemical properties (composition, structure, morphology, porosity, etc.) ofMo03are changed by the mechanical stress applied during this process. Moreover, MOOSexhibits an intrinsic variability in the oxygen content and a whole series of ordered defect structures is known to exist in the range between Moo3 and MoOz. Thus, defect structures, such as point defects and point defect clusters, shear and block structures etc., and highly amorphous surface layers may be produced in Moo3under mechanical stress and interfere with the mechanical as well as electronic properties of the material. Besides decreasing crystallite sizes and altered particle morphologies,interatomic,distances and therefore binding force constants as well as lattice symmetries may be affected. In this first article ofa series ofcontributions, the effect of the mechanical activation of Moo3 in a planetary mill upon particle size and morphology as well as the generation of defects and amorphous material is investigated by X-ray diffraction, BET surface area measurements, and scanning electron microscopy.
2. Experimental Section Moo3 (Merck, p.a.) was ground in a planetary mill with 145 rpm over an extended period of time in order to investigate physicochemical properties of Moo3 that change with particle size, morphology, and crystallinity and with the nature and concentration of defects, either on the surface or in the bulk. In the planetary mill used, the agate cylinder (9 cm id.) contained six agate balls (2 cm diameter) and the material to be milled. The container rotation axis is oriented vertically. It is simultaneouslyrotating around an externally lying vertical axis (10.5cm radius) and, additionally,in the opposite directionaround its own rotation axis. Accelerations up to 60g can be created in mills of this geometric design.32 The loading of the mill (70 g of MoO3) with starting material corresponded to that used in spreading experiments of P0lz.3~During the milling process, samples were drawn at distinct times (10,20,60,120, 180,240, 420, and 600 min, leading to the notation MoOO, MolO, ...,Mo600 used in this paper) and the surface area was measured using the BET method34by Nz-adsorption in a quantasorb jr. apparatus. (26) Leyrer, J.;Zaki, M. I.; Knozinger, H. J . Phys. Chem. 1986,90, A775
_ . . I .
(27) Margraf, M.; Leyrer, J.; Knozinger, H.; Taglauer, E. Surf. Sei. 1987,1891190,842. (28) Knozinger, H. Mat. Sci. Forum 1988,25126, 223. (29) Knozinger, H. In Pundamental Aspects of Heterogeneous Catalysis Studied by Particle Beams; Brongersma, H. H., van Santen, R. A.. Eds.: Plenum Press: New York. 1991: D 7. '(30)hazinger, H.; Taglauer, E. In Catuhsis; Spivey, J. J.; Ed.;The Royal Society of Chemistry: Cambridge, 1993; Vol. 10,p 1. (31) Doe, A.; Seidel, B. R.; Johnson, D. L. In Sintering and Related Phenomena; Kucynski, G. C.,Ed.;Plenum Press: New York, 1973; Vol. 6, p 247. (32) Heinecke, G. Tribochemistry; Carl Hanser Verlag: Miinchen, 1984. (33) Polz, J. Dissertation Univ. Munchen, 1992. (34) Brunauer, S.;Emmett, P. H.;Teller, E. J . A m . Chem. SOC.1938, 60, 309.
After a constant BET surface area was reached, the milling process was discontinued. X-ray diffraction was measured on a STADI P powder diffraction system (STOE & Cie) in focusing Guinier geometry (transmission). The diffractometer is equipped with a Ge monochromator to obtain monochromatic Cu Kal radiation (1= 154.056 pm). Flat, rotating powder samples were used. The diffracted radiation was measured using both a small and a large position sensitive detector covering respectively 2 0 ranges of 7.2"with a resolution of 0.08" or 40.2"with a resolution of 0.1". The whole X-ray investigation was carried out without any interruption of the sequence of different measurements. All difiactograms were recorded using the same parameters, changing neither the flat sample holder nor the experimental setup. Scanning electron microscopy (SEMIwas carried out on a Jeol 6400 microscope. The acceleration voltage was set between 15 and 25 kV and the probe current was about 10-l' A. A working distance between 6 and 12 mm and an objective aperture of 70 pm were used. The powder samples were directly supported on the carrier. Since a suspension in HzO resulted in recrystallization of needle-like specimen, as revealed by SEM images, gold was evaporated onto the sample surface for electric conductance. In addition, SEM was carried out independently on a Hitachi S-400microscope. For this experiment, the samples were diluted in ethanol using an ultrasonic bath and spread on a Si carrier which itself was mounted onto an Al carrier by an electrically conducting glue. The SEM images showed that the starting material consisted of a majority of platelet-like particles in the size range of 1pm, but well-developed platelets of dimensions exceeding 10 pm are also identified. After 600 min of milling, particles of dimensions between 50 and 100 nm forming larger agglomerates were observed. Energy dispersive X-ray analysis (EDX) was carried out in the range between 0 and 20 keV in order to test whether agate abrasion occurred during the grinding process. The detector for X-ray microanalysis had a resolution of 110 eV at 5.9 keV. A significant agate abrasion could not be detected using EDX. In order t o confirm the EDX results, X-ray photoelectron spectra were recorded on a modified Vacuum Science Workshop ESCA 100 spectrometer with a hemispherical analyzer HAC 100/285 mm. The system was equipped with a twin anode X-ray gun for Al K a (1468 eV) or Mg K a (1253.6 eV) excitation, respectively. The gun was operated at 180 W (12 kV, 15 mA). The spectra were recorded in the fixed analyzer transmission (FAT) mode, the pass energy being 44 eV for survey scans and 22 eV for highresolution scans. The powders were pressed into stainless steel sample holders. There was no further in situ treatment of the samples except outgassing in UHV at room temperature. The base pressure of the UHV chamber was loT9mbar. The XP spectra of Mo600 did not show any evidence for a surface contamination by agate abrasion.
3. Results and Discussion 3.1. Change in the BET Surface Area during the Milling Process. In Figure 1 (line a) the BET surface area is plotted against the milling time. One can clearly distinguish three different regimes: in the first part, until 60 min of milling, the BET surface area is hardly increasing. In the second part, from 60 to 420 min milling, there is a steep rise of the BET surface area, and in the third regime, the BET surface area levels off to reach a plateau. After 240 min of milling, the ascendence of a n ultrafine dust was observed when the mill was opened to draw a sample. During mechanical activation increasingly smaller particles are generated. The concentration of fractures and bulk defects per particle and the internal strain are decreasing while homogeneity and stiffness are increasing with decreasing particle size.35Thus, continuously greater forces must be applied to further disperse the material. This finally leads to the particle size approaching a certain limit, the so-called milling e q ~ i l i b r i u m .In ~ ~this case, (35)Rumpf, H. Chem. Zng. Techn. 1959,31, 333.
Langmuir, Vol. 11, No. 8, 1995 3029
Mechanically Activated Moo3
30
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Figure 1. Enhancement of the BET surface area of MOOSwith the duration of the milling process: (a)measured BET surface area; (b)theoretically calculated BET surface area using eq 1. the change in surface area is described by the following equation32
(1)
where S is the surface area, S, is the limit value of the surface area at the milling equilibrium, K , is the milling constant for this material and the mill used, and t is time. According to eq 1, the value of the BET surface area becomes constant after a certain milling time. The theoretical increase in the surface area calculated by eq 1is shown as line b in Figure 1. In this calculation, the final BET surface area (Sg)of 32 m2and the total milling time t (600 min) were kept constant, while the milling constant was fitted to the experimental data. It is obvious from Figure 1 that eq 1 correctly describes the actual behavior of the BET surface area only a t milling times t I60 min. The discrepancy between a n isotropic homogeneous medium assumed in the derivation of eq 1 and the anisotropic binding forces of real solids (e.g., layer structure of Mood may explain the difference between the observed and calculated BET surface areas in the first 60 min of milling. The well-shaped platelet-like Moo3 particles in the original sample (MoOO)have dimensions of about 1-15 pm (as shown by SEM images). Thus, one may speculate that only larger particles or agglomerates are separated during the first 60 min of milling. This would only lead to a relatively small increase in the surface area while a significant decrease in the mean particle size is expected. The mean particle sizes as calculated from the BET surface area according to the formula given by Whyte36 is plotted against the BET surface area in Figure 2. In this calculation the MOO3 particles are approximated by (36) Whyte, T.E.,Jr. Catal. Rev.-Sci. Eng. 1973,8, 117.
5
10
15
20
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BET surface area [m2]/g
Milling time [min.]
S = S,[1 - exp(-k,t)l
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Figure 2. Mean particle size calculated from BET surfacearea according t o W h ~ t e . ~ ~ regular cubes where all six faces are exposed to the adsorbing gas phase. The real shape of Moo3 crystallites not being cubic, certainly leads to some error in this particle size calculation. Thus, the surface area of an infinitesimally thin slab (one side of the slab has the same dimension as the reference cube, while the other two sides are increased or decreased by the same factor; the mean particle dimension, however, stays constant) is about 30% smaller as compared to a regular cube. During the first ca. 120 min of the milling process, a drastic decrease in the mean particle dimensions is observed in Figure 2. While increasingly smaller particles are generated, the surface to volume ratio per particle is increasing. Thus large changes in the BET surface area (3-30 m2/g) are accompanied by increasingly smaller changes in the particle size. Finally, the particle size asymptotically approaches its lower limit value. Another serious simplification in this calculation of the mean particle size is the assumption ofa constant density during the milling process. The density ofmaterials under mechanical stress may change due to compressionbetween the milling tools. Furthermore, the introduction of defects into the solid leads also to a changing d e n ~ i t y . ~This ' uncertainity in the particle size determination can be reduced by determining the X-ray densities (vide infra). Another uncertainity in the particle size calculation arises from the fact that under isotropic, statistical stress, e.g., in a planetary mill, an amorphous layer (Beilby layer) may be formed on the surface.3s Comparing the particle size, as determined by different methods like BET, and SEM, and the primary crystallite sizes, using X-ray data, can help to restrict these errors. 3.2. Characterization of MechanicallyActivated Moo3 by X-ray Powder Diffraction. Earlier workers (37) Greenwood, N.N.Ionic Crystals, Lattice Defects and Nonstoichiometry; Butterworth: London, 1968. (38) Brendel, H.Wissensspeicher Tribotechnik;Leipzig, 1978; p 308.
3030 Langmuir, Vol. 11, No. 8, 1995
Mestl et al.
167 pm
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Figure 3. Coordinationaround the Mo centers in M003~'and section of the layered MOOS structure: chains of MOO4 tetrahedra running along the c axis and forming halflayers in the a-c plane. Two halflayers build up one MOOSlayer. These are stapled along the b axis.
30
50
70
Diffraction Angle 2 0 Figure 4. X-ray diffraction patterns of Mooa: dotted line (A) calculated diffraction pattern of Moo3 from single crystal data; X-ray patterns (solidlines)of (B)MoOO and (C) Mo600, measured under identical geometrical conditions.
Table 1. d-Values, Relative Intensities, and Indices of interpreted the complicated layer structure of Moo3 as Reflections Observed for MOOS being built up by distorted Moo6 octahedra connected in dlA int hkl dlA int hkl the c-direction by common edges and corners so as to form 6.921 36 020 1.9812 8 200 zigzag rows. In the perpendicular a-direction the octa77 110 1.9585 12 061 hedra are linked to each other via common ~ o r n e r s . ~ ~ , ~3.808 ~ 3.464 38 040 1.9349 1 151 An alternative description is based on molybdenum atoms 3.440 31 120 1.9051 1 220 in that structure having a tendency toward 5-fold coor3.259 100 021 1.8483 15 002 d i n a t i ~ n .In ~ ~a n X-ray structure analysis, Kihlborg42 3.008 7 130 1.8206 9 230 redetermined the crystal structure of Moo3 more ac2.703 15 101 1.7860 2 022 2.653 26 111 curately (Figure 3). It crystallizes in the orthorhombic 1.7705 2 170 2.608 3 140 1.7559 4 161 lattice (space group P b n m - D ~ 6with ~ ~ )four formula units 2.5267 7 041 1.7324 13 211 per unit cell of dimensions 396.28 pm (a-axis), 1385.5 pm 2.3329 8 131 1.7203 2 240 (b-axis), and 369.64 pm (c-axis). The structure of Moo3 2.3088 21 060 1.6932 5 221 represents a transitional stage between octahedral and 2.2707 13 150 1.6626 10 112 tetrahedral coordination and may be considered as being 2.1311 7 141 1.6301 9 042 built up by Moo4 tetrahedra sharing two oxygen corners 1.9952 2 160 1.6285 9 122 with two neighboring tetrahedra to form chains running in the direction of the c-axis. Crystals of Moo3 usually show that the initial substance is a uniform material. grow in the form ofneedles or platelets along this direction. However, there are some differences in the ratios of peak intensities indicating that the starting material already The chains are condensed in the a-c-plane to form the contains some reduction of crystalline perfection. Thus, layered structure of Moo3. The Moo3 layers are stapled the intensity of the (002)reflection is too high relative to along the b-axis by van der Waals interactions with a n interlayer distance of about 700 pm. the 100% reflection (021). This can be explained by a A simulation of the powder diffraction pattern of Moo3 higher perfection along the b-axis (stacks of Moos layers) from this single crystal data is shown in Figure 4A (dotted with defects being located in the a-c plane (defects in the line). Table 1lists the indices of the observed reflections Moo3 layers). In addition, a thermal diffise background in the range 10" I2 0 I60". A close inspection of the concentrating around the reflections also points to a loss simulated and measured data revealed slight differences in crystallinity and therefore to lattice imperfections. to the common JCPDS data43which are based on the same Calculating the primary crystallite size from line single crystal study by K i h l b ~ r g .Also ~ ~ in Figure 4 broadening of some selected reflections using a silicon difiactograms ofMoOO (B)and Mo600 (C)are reproduced. standard results in a dependence of the mean primary The general loss in the intensity observed for all Bragg crystallite sizes with milling time (Figure 5). The primary reflections induced by mechanical activation is obvious. crystallite size decreases with milling time to about 60Reference measurements of other Moo3 samples and the 80 nm, except the one calculated from the (002)reflection comparison with a simulation using single crystal data which is correlated with the extension of the crystallite along the c-axis. The corresponding dimension remains (39)Brakken, H.2. Krist. 1931,78,484. considerably larger ( ~ 1 1 nm) 0 as compared to the exten(40)Wooster, N.2. Krist. 1931,80, 504. sions parallel to the a- and b-axes. This observation would (41)Anderson, G.;MagnBli, A. Acta Chem. Scand. 1960,4 , 793. correlate with a needle-like shape of the microcrystallites (42) Kihlborg, L.Ark. Kemi 1963,21,357. (43)JCPDS 35-609. after grinding. Actually, SEM images (not shown) reveal
Langmuir, Vol. 11, No. 8, 1995 3031
Mechanically Activated Moo3
200
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50 0
120
240
360
480
600
Milling Time [min.] Figure 5. Decrease in particle size of mechanically activated Moos as calculated from XRD data: W, (002); 0, (200);0,(020); 0,(111);*, (110) reflections.
these needle-shaped particles. Furtheremore, after 240 min of milling, the primary crystallite size as calculated from the (110) reflection remains almost constant, while the dimensions calculated from all additional reflections still are reduced during the mechanical activation. A comparison of the primary crystallite sizes with the mean particle sizes as calculated from the BET data (Figure 2) reveals that in the beginning of the milling process large particles or agglomerates are disintegrated, since the primary crystallite size (about 180 nm for (002) reflection) remains more or less constant for the first 60 min of milling (Figure 5). On the other hand, the mean particle size calculated from the BET surface area for Mo600 (40 nm) is smaller as compared to the primary crystallite size derived from XRD data obtained for the same sample (Figure 5). This observation confirms the presence of X-ray amorphous material, e.g., an amorphous Beilby layer, in the microcrystalline sample as already assumed from the increased diffuse background in the XRD patterns. The smallest particles (%40 nm) in SEM images (not shown) exhibit this amorphous surfacewhich could be excellently resolved in a very recent HRTEM study.& The grinding process, thus, seems to proceed via two steps: in the first hour of milling mainly larger particles or agglomerates seem to be disintegrated; in the second step, when the BET surface area drastically increases, the primary crystallites are broken down, and, in addition, the mechanical activation leads to an amorphization of their surface. Another reason for line broadening which must be considered is due to nonuniform lattice strain. Structural defects give rise to distortions and tensions in their (44) Uzhida, Y.; Pfander, N.; Weinberg, G.; Herein, D.; Schlogl, R.; Mestl, G.; Knozinger, H. To be published.
mechanical activation (0). surroundings and are the reason for intrinsic strain which is determined by the nature, concentration, and distribution of defects in the solid.4S By use of the method of Warren and Averbach,46a separation between line broadening due to primary crystallite size reduction and lattice strain is possible. A Warren-Averbach analysis of fitted and not manipulated reflection profiles reveals that only a minor contribution (> 1%) to the profile broadening of Mo600 arises from internal stress. Therefore, the broadening of reflection profiles with grinding is mainly related to the decreasing primary crystallite sizes. A closer inspection of the diffraction patterns recorded after each step of the grinding certainly gives a more detailed insight into the complex process of particle and primary crystallite size reduction: Already after 10 min of milling, distinct changes are observed in the diffractogram as compared to the one of MoOO. For example, the above mentioned thermal diffuse background around the reflections is now hardly detectable. This can be explained by recrystallization of the amorphous portion in the sample due to the addition of mechanical energy. On the other hand, the reduced intensity of all observed Bragg reflections and the increase in the general background indicates an increasing amount of amorphous material in the sample. These two, a t first glance contradicting observations can be explained by the assumption of two different portions in this sample, of which one consists of crystalline material, being responsible for the Bragg reflections, the second of defect-rich material,47generating the general scattering background. This effect of an increasing background is continuing with the duration of mechanical activation (Figure 6A). Since the X-ray scattering background is attributed to amorphous material in the sample, its increase reflects an increasing portion of defect-rich, X-ray amorphous material which is generated during the milling process. (45) Schmidt, C.; Lower, H.; Peger, F.; van Oi, H.; Hannemann, M.; Liebscher, H. XIII. Int. Wiss. Kol1oq.-Tech. Hochsch. Ilmenau 1988. (46) Warren, B. E.; Averbach, B. L.J.Appl. Phys. 1959,21, 595. (47) Chung, D. D. L.; De Haven, P. W.; Arnold, H.; Gosh, D. X-Ray Diffraction at Elevated Temperatures;Verlag Chemie: Weinheim, 1993.
3032 Langmuir, Vol. 11, No. 8, 1995
The total XRD intensity, that is attributed to the crystalline portion in the sample, shows a different behavior. With increasing milling, fractures are proceeding through the crystals and smaller particles are generated whereby internal strain should be released. In addition, this should lead to a reduction of the defect concentration per crystallite volume and result in diffractograms of again higher quality. Actually, this is observed for the samples Mol0 through Mol80 (Figure 6B). Interestingly, the calculations of the primary crystallite dimensions using XRD data (Figure 5) reveal that after 180 min of milling, the crystallite size along the c-axis (the direction in the Moo3 lattice having the strongest chemical bonds) almost stays constant, while the primary crystallite dimension along the other directions are further reduced (vide supra). With the duration of the milling process, defects must again be generated in the now smaller particles or crystallites resulting in X-ray diffraction patterns of poorer quality as observed for the sample milled for 240 min. Between 240 and 420 min of milling, the X-ray diffraction pattern quality increases (Figure 6B),while after 600 min again a diffraction pattern of poorer quality is obtained. As before, a n inspection of Figure 5 shows that after 240 min of milling the primary crystallite dimension calculated from the (110) reflection remains almost constant while the other dimensions are further reduced. This changing X-ray diffraction pattern quality may thus also point to a stepwise destruction of the crystalline Moo3. First, defects and fractures are generated (decreasing diffraction pattern quality), while in the following step these fractures have proceeded through the crystallites along certain directions leaving back smaller ones whose defect concentration per volume is lower a t the beginning of this next step of crystallite size reduction, again resulting in a n increased X-ray diffraction pattern quality. This process of defect generation and crystallite size reduction is perhaps also reflected in the diffractogram (Figure 7A) of the sample milled for 120 min (b) which in contrast to samples being milled for shorter or longer periods (a, c), shows a broad reflection a t about 2 0 = 17", whose appearance is already pronounced in samples with a shorter milling time. In addition, a weak, sharp reflection is simultaneously observed a t 2 0 = 26.7"(Figure 7B). As will be discussed in a follow-up paper,48transmission Fourier transform spectra in the far-infrared region also undergo drastic variations in the lattice mode regime due to the mechanical activation. The varying concentrations of fractures and defects and strain release during this stepwise crystallite size reduction are supposed to explain these observations. Each solid which is in contact with the vapor phase of one of its components is potentially nonstoichiometric. The larger the internal disorder of a stoichiometric compound, the smaller is the change in the partial pressure needed for creation of nonst~ichiometry.~~ Four different types of nonstoichiometric compounds are distinguished: (a) quasistatistic homogeneous defect distribution, (b) submicroscopic heterogeneity in the single phase regime, (c)intermediate phases with a defect superstructure, and (d) intermediate phases with shear or block structures, and pentagonal columns. In the latter two mentioned cases, the variation in stoichiometry is reached by a change of the relative amounts of the participating phases.37 In general, the type of shear plane is changing with increasing degree of reduction. The higher the degree of reduction, the more easily ordered nonstoichiometric phases can be detected. (48)Mestl, G . ; Srinivasan, T. K. K.; Knozinger, H. Submitted to Langmuir.
Mestl et al. A t c
d
..~ . .* . . _ --. 13.0
15.0
17.0
19.0
21.0
23
Diffroction Angle 2 8 B h
26.3
26.5 26.7 26.9 Diffroction Angle 2 0
Figure 7. (A) X-ray powder difiactograms of (a) MoOO, (b) Mo120, and (c) Mo600 (patterns are vertically shifted). Intensities are normalized to the (020) reflection at 2 0 = 12.8". Solid lines: Iteratively smoothed powder difiactograms (differencebetween measured and smoothed difiactogram in each step was smaller than lo-' counts s-l). (B)Solid lines: X-ray powder difiactograms of (a) Mo60, (b) Mo120, and (c) Mol80 (patterns are vertically shifted). Intensities are normalized to the (020) relfection at 2 0 = 12.8'. Dotted lines: iteratively smoothed powder difiadograms (differencebetween measured and smoothed difiactogram in each stepwas smaller than 10-1 CPS).
The loss of an oxygen atom from Moo3 initially leads to the formation of a Mo-Mo bond across the anion vacancy.49 Under further reduction these defects are ordered (anion vacancies are highly mobile in Moo3-,) to form extended defect structure^.^^ These extended defect structures are stabilized by a great variability of possible Mo positions between the center of symmetry and an interstitial site.51 Between Moo3and MoOz a whole series of defined oxygen-deficient structures is identified.52-56 (49) Haber, J. J.Less. Common Met. 1977, 54, 243. (50)Kihlborg, L. Acta Chem. S c a d . 1959, 13, 954. (51)Catlow, C. A. In Non-stoichiometric Oxides; S@renson,0. T., Ed.;Academic Press: 1981;p 178.
Langmuir, Vol. 11, No. 8, 1995 3033
Mechanically Activated Moo3
lower
22.5
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23.5
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Diffraction Angle
24.5
[e]
25.0
32.0
33.0
34.0
Diffraction Angle 28
35.0
28.0
29.0
30.0
31.0
32.0
Diffraction Angle 28
Figure 8. (A) Changing profile (appearanceof a shoulder at lower d-values)of the (110)reflection of Moos with the duration of mechanical activation. (b) Moo3 X-ray reflections [( 101),(lll),(140)l.For better visualization, patterns are vertically shifted and normalized to the intensity of the (111)reflection. (C) Changingprofile ofthe Moo3 X-ray reflection(130)with mechanicalactivation. For better visualization patterns are vertically shifted.
One may speculate whether suboxides having shear defect planes are produced during the grinding. The observation of variations in the XRD intensity, width, and profile of single reflections is known to show the startingformation of a new phase.37 ESR spectra of Mo600 show a considerably increasing intensity of the signal attributed to free e l e ~ t r o n sthus, ;~~ F-centers are introduced into MOOSby the milling process. If F-centers are generated in a solid, vacancies must be incorporated into the bulk and, therefore, its density must decrease.58 Actually the calculated X-ray density shows a tendency from 4.7276 g/cm3 toward a lower value of 4.7056 g/cm3for the most extensively milled sample. This would point to a generation of oxygen vacancies in the solid. However, the experimental error of the X-ray densities is comparable to the estimated difference and, thus, does not allow a n unequivocal conclusion. On the other hand, this minor change in X-ray density proves that the error in the mean particle size calculated from the BET surface area (Figure 21, due to varying densities, is negligible. If oxygen vacancies are not generated in a concentration detectable by XRD, the question arises whether the above mentioned shear structures are formed during the milling. Actually, the (110) reflection (Figure 8A) seems to shift to smaller d-values with increasing milling time. Additionally, the (110) reflection of the samples milled for 420 and 600 min, respectively, becomes increasingly asymmetric and shows a shoulder at higher 2Wsmaller d-values. A close inspection of the patterns shows that the (140) and the (141) reflections (Figure 8B) have vanished, while a new small reflection seems to arise a t about 28 = 29" (Figure 8C). These changing reflection (52) Kihlborg, L. Ark. Kemi 1963,21, 443. (53) Kihlborg, L. Ark. Kemi 1963,21, 461. (54) Kihlborg, L. Ark. Kemi 1963,21, 471. (55) Cornaz, P. F.; van Hooff, J. H. C.; Pluijm, F. J.; Schuit, G. C. A. Discuss. Faraday SOC.1966, 41, 290. (56) Guyot, H.; Al Khoury, E.; Marcus, J.; Schlenker, C.; Banville, M.; Jandl, S. Solid State Commun. 1991, 79,307. (57) Mestl, G.;Verbruggen,N. F. D.; Knozinger, H. Langmuir 1995, 11, 3035. (58) Tilley, R.J. D. Defect Crystal Chemistry and Its Applications; Blackie & Son: London, 1987.
profiles and intensities were observed independently using different diffraction techniques (e.g., varied counting statistics and detectors). The above mentioned varying diffraction patterns are traced back to the changing profile of the (130)reflection. Comparable results were reported for the poorly ordered Moo3 obtained from the decomposition of heteropoly acid.59 Since the (110) and (130) reflections are more easily resolved due to no overlap with other diffraction lines, this effect is assumed to be general for all the (h,K f 0; I = 0) reflections. These observations are most probably connected with the changing intensity ratios of the Raman bands polarized parallel and perpendicular to the c - a 2 ~ i s .The ~ ~ intensity ratios of Bragg reflections belonging to diagonal planes (101, 110, 111) show drastic effects (Figure 9) with decreasing particle size. In contrast to the intensity ratio1(110)/1(111)which is hardly changing with increasing milling time, the intensity ratios 1(101)/1(110)and1(101)/1(111)are changing dramatically within the first minutes of mechanical activation (comparethe drastic increase in the amorphous scattering background during the first 10 min). It is known60that under mildly reducing conditions (vacuum, temperatures below 353 K), screw dislocations are formed of disklike ordered anion vacancies in the (101) planes. These structures are considered as the precursor of crystallographic shear planes. The changes in the intensity ratios may be correlated with the formation of such defects. Actually in ESR spectra of Mo6OO a signal was detected that is attributable to such precursor structure^.^^ Similar variations in the reflection intensities, however, can also arise from the mechanically induced gliding of Moo3 layers against each other. These changes in the XRD patterns cannot be unequivocally attributed to the formation of a crystallographically defined suboxide, e.g., MoI8O52.However, the changes in reflection profiles and the presence of new reflections may be explained by the formation of shear defects in Moo3 due to mechanical activation. (59) Ilkenhans, T.; Herzog, B.; Braun, T.; Schlogl, R. J. Catal., in
press.
(60) Gai, P. L.; Thoeni, W.; Hirsch, P. B. J.Less. Common Met. 1979, 54, 263.
3034 Langmuir, Vol. 11, No. 8, 1995
Mestl et al.
110 100 90
-
80
u
70
6\"
% -i! 0,
0,
60 50
t
2
40
0
30 20 10
0 0
.,
120
240
360
480
600
Milling Time [min]
Figure 9. Change of the X-ray reflection intensity ratios with increasing BET surface area relative to unmilled MOO$ 0, z(lol)/z(llo); z(lol~/z~lll); 0,z~llo~/z~lll~.
4. Conclusions Mechanical activation of Moo3 in a planetary mill for 600 min results in considerable effects upon the morphology and particle dimensions of the treated powder. The BET surface area increases from 1.3 to 32 m21g during this treatment. An estimation of the mean particle size of mechanieally disintegrated Moo3 using the BET data indicates a reduction ofthe particle dimensions from about 1 pm to about 50 nm. SEM images of untreated and treated Moo3corroborate these particle sizes. Calculating the crystallite sizes from XRD line broadening reveals a decreasing primary crystallite size from about 160 nm to about 80 nm. The difference in the particle sizes of MoOO, calculated from BET data or shown by SEM relative to the primary crystallite size calculated from XRD data, is explained by the fact that untreated Moo3particles consist of smaller primary crystallites. The smaller particle size of Mo600 calculated from BET data as compared to the
XRD primary crystallite size is ascribed to the presence of ultrafine amorphous material. This amorphous material is also indicated by a n amorphous scattering background in the XRD diffraction patterns. SEM images of Mo600 additionally show that the particles have an amorphous overlayer.44 EDX analysis and XPS measurements reveal that no detectable amount ofagate is abrased during the milling process. The amorphous surface layer observed in SEM images, therefore, is connected with the amorphous Moo3 phase observed in XRD. The behavior of the XRD pattern quality and of the amorphous scattering background, as well as differences in the reduction of the crystallite dimensions as calculated from X-ray reflection broadening during mechanical activation, suggests a complex, stepwise process of particle size reduction. Changes in the reflection profiles, anomalous X-ray reflection intensities, and the disappearance of original as well as the appearance of new reflections may point to the formation of shear defects. Thus, in the first step of particle size reduction, large Moo3 particles or agglomerates are suggested to be disintegrated according to their grain boundaries. In the second step of disintegration the remaining Moo3 particles are separated perpendicular to the b-axis, i.e., weak van der Waals interactions between the Moo3 double layers, and a-axis, i.e., weakly interacting Moo4 chains. Between the different steps of particle size reduction, the concentration of defects in the lattice is increasing, as evidenced by the decreasing XRD pattern quality. Such a mechanism of particle size reduction should lead to needle-like particles in the resulting material. Actually, this is what was observed in SEM images. Internal strain, on the other hand, is only marginally enhanced by mechanical activation. The large variability of Mo positions in Mo oxides and, thus, the easily possible formation of shear defects in the Moo3lattice explain this small internal strain. The Moo3 lattice can avoid internal strain by formation of shear defects.
Acknowledgment. This work was financially supported by the Deutsche Forschungsgemeinschaft (SFB 338) and by the Fonds der Chemischen Industrie. We also thank Dr. H. Zeilinger (Papiertechnische StiRung) and Dr. B. Tesche (Max-Planck-Institut fur Kohleforschung) for SEM pictures and EDX analysis, as well as Dr. F. Lange for XPS measurements. LA9501243
