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Microporous and Mesoporous Materials 89 (2006) 132–137

Small angle X-ray scattering study on the pore characteristicsof U-substituted MCM-48 metallosilicates

D. Kumar a, P.U. Sastry b, D. Sen b, N.M. Gupta a,*

a Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, Indiab Solid State Physics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India

Received 27 January 2005; received in revised form 5 October 2005; accepted 9 October 2005Available online 23 November 2005

Abstract

The technique of small angle X-ray scattering (SAXS) has been employed to monitor the influence of uranium substitution on thepore characteristics of MCM-48 silica mesoporous material. The incorporation of uranium, as uranyl ion, at the framework silica sitesresulted in the progressive increase in average pore size, the extent of which depended upon uranium loading. These results are in agree-ment with powder XRD results, showing a progressive increase in unit cell constant (a0) with increasing U-loading. The changes in thepore structure and the unit cell parameter are attributed to the perturbations arising due to bonding of (O@U@O)2+ units to „Si–O�

units, thus forming a local uranate type structure. However, the increase in uranium content beyond 3 wt.% resulted in partial precip-itation of uranium oxide crystallites, thus causing blocking/narrowing of some of the pores in the host material. SAXS results reveal that,in spite of these structural changes, the fractal nature of the host material remained by-and-large unchanged. These findings are in har-mony with the results of the parallel studies conducted using N2-adsorption and TEM studies.� 2005 Elsevier Inc. All rights reserved.

Keywords: MCM-48 metallosilicates; Pore characteristics; SAXS study

1. Introduction

The incorporation of uranium moieties into mesoporoussilicate hosts is of considerable interest, both because ofpotential use in catalysis and also for possible applicationin the area of nuclear waste management [1,2]. We demon-strated recently that the oxides of uranium may beanchored in the channels of MCM-41 and MCM-48 molec-ular sieves, and these materials exhibit unique catalytic andphotocatalytic properties for oxidation reactions, depend-ing upon the mode of binding and the oxidation state ofU [3–8]. It has also been demonstrated that a largeramount of uranium could be loaded into MCM-48, as

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.10.006

* Corresponding author. Present address: Catalysis Division, NationalChemical Laboratory, Homi Bhabha Road, Pune 411 008, India. Tel.:+91 20 25902008; fax: +91 20 25893761.

E-mail addresses: gupta1611@gmail.com, nm.gupta@ncl.res.in (N.M.Gupta).

compared to MCM-41, because of three-dimensional cubic(Ia3d space group) pore system and the variable pore sizein the range of 2–10 nm [9,10]. The mode of uranium incor-poration, i.e., either at the framework sites, inside the poresystem or at the external surface of host matrix, is knownto influence the pore characteristics quite significantly.For instance, in case of a sample synthesized throughhydrothermal route, mere 1–6 wt.% of uranium substitu-tion at Si sites resulted in significant changes in the poregeometry and in the increase in the degree of cross linkingof silica defect sites [11,12]. In continuation of these previ-ous studies, we have now employed the technique of smallangle X-ray scattering (SAXS) to monitor the subtlechanges in the pore structure and the fractal behavior ofMCM-48 samples as a result of incorporation of uraniumduring their hydrothermal synthesis. The use of SAXStechnique enabled us to investigate the angle region of2h = 0.05–3�, well below the detection limit of the earlierreported XRD measurements [11,12].

Fig. 1. TEM images of MCM-48 samples: (a) S1 (no uranium), along(110); (b) S3 (3% U) along (100); and (c) S3 (6% U) along (110) direction.

D. Kumar et al. / Microporous and Mesoporous Materials 89 (2006) 132–137 133

2. Experimental

MCM-48 samples without and with the uranium load-ing of 1, 3 and 6 wt.% (referred to as samples S1, S2, S3and S4, respectively) were synthesized following a hydro-thermal procedure [11]. Tetraethyl orthosilicate (TEOS)(Merck) as a silicon source, n-cetyl trimethylammoniumbromide (CTAB) (Lancaster) as a surfactant, NaOH(S.D. Fine Chem) as a base, and uranyl acetate monohy-drate (Merck) as uranium metal precursor were utilizedfor sample preparation. The detailed characteristics ofthese S1–S4 samples are reported in our previous publica-tions [11,12].

SAXS measurements were carried out on samplepowders with the help of a Rigaku small angle goniometermounted on a 12 kW rotating anode X-ray generator withNi-filtered CuKa radiation. The scattered intensity I(q) wasrecorded in steps of the scattering angle 2h by transmissionmethod using a scintillation counter with pulse heightanalyzer. Symbol q is for the scattering vector equal to4p Æ sin (h)/k, and k is the wavelength of incident X-rays.The measurements were carried out in the q-range of0.04–2.1 nm�1, the step size of 2h was 0.02� and the recordtime per step was 1 min. We may mention that no measure-ments were possible out of this range due to the limitationsof the instrument. The recorded intensities were correctedfor sample absorption and smearing effects of collimatingslits [13].

With a view to supplement the SAXS results, physicaladsorption–desorption isotherms of nitrogen wererecorded using a Quantachrome AUTOSORB-1 instru-ment. About 100 mg of sample was pretreated under vac-uum for this purpose, initially at 373 K for 1 h and thenat 473 K for 7 h. The pore dimensions were obtained fromthe desorption data using Barrett–Joyner–Halenda (BJH)method. The TEM studies were performed on a Jeol 2000FX microscope, operating at 120 kV. The samples for thispurpose were prepared by ultrasonicating the samples inethanol and then dispersing on a carbon film supportedon a copper grid.

3. Results and discussion

3.1. Sample characteristics

29Si MAS-NMR, FTIR spectroscopy, and DR UV–Visspectroscopy studies confirmed that the mode of uraniumbinding depended upon loading, as described in Ref. [11]in detail. In general, a part of uranyl groups were foundto be incorporated in the framework positions while therest were anchored on to the wall of the host silica matrix.At framework silicon sites uranium existed in the hexava-lent state forming a local uranate type structure in theordered cubic-phase of MCM-48. Powder XRD resultsrevealed that the mesoporous structure of the host matrixwas conserved after uranium loading, even though somedistortion was noticed in the pore structure [11]. The

formation of nanocrystallites of a-U3O8 was observed inthe case of samples having U-loading of �6 wt.% or moreand subjected to calcination in air [11].

In order to exhibit these morphological changes as aresult of U-loading, micrographs (a), (b) and (c) in Fig. 1present the typical TEM pictures of S1, S3 and S4 samples,recorded along [110], [100] and [110] directions, respec-tively. Fig. 1(a) depicts a distorted hexagonal pore arrange-ment, typical of pristine MCM-48 mesoporous materialalong [110] direction. TEM micrograph in Fig. 1(b) forS3 sample, viewed along [100] direction, shows porearrangement in a highly ordered square grid like pattern,again typical of cubic phase MCM-48. This picture revealsthat for the U-loading of up to 3 wt.% no extraneous ura-nium oxide crystallites are formed in the sample and mostof the uranium is either in the framework position or isoccluded within the pore system. With the increase in ura-nium loading to 6 wt.%, the presence of the extraneousU3O8 crystallites (XRD evidence, Ref. [11]) of 3–4 nm sizemay be noticed in Fig. 1(c).

Fig. 2. X-ray scattering profiles of pure and U-doped MCM samples,plotted on log–log scale. Intensities are scaled for clarity.

134 D. Kumar et al. / Microporous and Mesoporous Materials 89 (2006) 132–137

The nitrogen sorption results showed that the averagepore radius of MCM-48 increased from a value of 1.4–1.6 nm on substitution of merely 1 wt.% uranium. Also,pores with radius of �1.8 nm were also found to developon U-loading. The number of these pores with 1.8 nm sizegrew considerably with the increase in loading. At the sametime, the presence of two additional kinds of pores havingradius of 1.1 and 1.3 nm were noticed in the sample S4 con-sisting of 6 wt.% of U. This decrease in the size may beattributed to the filling of some of the pores with U3O8

nanosize particles at higher loading. The relative abun-dance of different pores, generated due to U-loading insamples S1–S4, is given in Table 1. Commensurate to theseresults, the value of the unit cell constant (a0) calculatedfrom powder XRD patterns was found to increase progres-sively with the increasing loading of uranium. The varia-tion observed in the value of a0 as a function of Ucontent is presented in Table 1, to enable us a comparison.These results are given in more detail in Ref. [11].

3.2. SAXS results

Fig. 2 shows the observed intensity profiles of the foursamples, plotted on log–log scale. In low q region, the pro-files exhibit a power law behavior over a wide (more than adecade) q-range with negative power law exponent of mag-nitude varying between 2.0 and 3.0. These values indicatethat the structure of these pores in the host matrix relatesto the fractal behavior i.e., the scattering entities posses aself-similarity over a wide range of length scale. Our infer-ence is in conformity with an earlier study by Tismaneanuet al. [14], where the fractal character of MCM samples hasbeen established.

In the case of scattering from a typical fractal system (asin Fig. 2), the intensity of the scattered X-ray profile, i.e.,I(q), can be expressed as [15]

IðqÞ ¼ k � q�a ð1ÞIn this expression, a is a non-integer related to the fractaldimension and k is a constant. For mass fractals withdimension Dm, a(=Dm) 6 3 whereas for surface fractalswith dimension Ds, 3.0 6 a(=6 � Ds) 6 4.0 hence2 6 Ds 6 3 [15]. For smooth or non-fractal surface of thescattering objects, the value of a at higher q-range equals

Table 1Structural parameters of the pure and U-doped MCM-48 samples

U-content(wt.%)

Average pore radius,measured by SAXS (nm)

Power-lawexponent (Dm)

0 1.21 (0.07) 2.951 1.33 (0.06) 2.873 1.39 (0.06) 2.826 1.35 (0.05) 2.91

a Numbers given in the parentheses represent percentage fraction of an indib Ref. [11].

4. Thus, the slope of the scattering curve on log–log scaleindicates the type of the fractal. Usually, the power-lawscattering (Eq. (1)) is observed in a limited q-range deter-mined by upper and lower cut-off lengths between whichthe system behaves as a fractal. Details on the functionalityof I(q) and SAXS investigations on the fractal systems withpore morphology are described in literature [15–20].

In general, the intensity I(q) of the scattering profile foran ensemble of mono-disperse medium of particles is givenby the expression [21,22],

IðqÞ ¼ C � P 21ðqÞ � SðqÞ ð2Þ

where C is the scale factor that depends on the scatteringdensity contrast and the volume of the scatterers, P1(q) isthe form factor and S(q) is the structure factor of thescattering centers. For a fractal object, P1(q) representsthe form factor of the basic unit. In MCM type substances,the basic pore units are known to be cylindrical in shape[23]. The form factor for cylindrical objects of radius r0and length L with an angle a between their long axis andnormal to the incident beam, is given by [22,24]

P 1ðqÞ ¼Z p=2

0

2J 1ðqr0 sin aÞqr0 sin a

sinðqL cos aÞ=2ðqL cos aÞ=2

� �2

� sin ada ð3Þ

Average pore radius,measured by N2

adsorptiona,b (nm)

Unit cell parameter(a0), measuredby powder XRDb (nm)

1.40 (100) 8.421.6 (95.8), 1.8 (4.2) 8.481.6 (94), 1.8 (6) 8.771.1 (22.5), 1.3 (23.5) 8.951.6 (30), 1.8 (24)

vidual size pore in a sample.

D. Kumar et al. / Microporous and Mesoporous Materials 89 (2006) 132–137 135

where J1(x) is the first order Bessel function with argu-ment x. For a mass fractal system, S(q) can be expressedas [25]

SðqÞ ¼ 1þ�DmCðDm � 1Þ

.�ðqr0Þ

d ½1þ 1=ðq2n2Þ�ðDm�1Þ=2��

� sin½ðDm � 1Þ � tan�1ðqnÞ� ð4Þ

where n and 2r0 are the upper and lower cut-offs of thefractal region. The fractal features of larger aggregatesare reflected in the relatively smaller q-region of the SAXSprofiles whereas the information about the constituentbasic scattering units is contained in the higher q-region.The data recorded in this study on the samples S1–S4 wereanalyzed using the above-described formalism.

In Fig. 2, the correlation peaks at larger q-region indi-cate that the mass units along with pores are well ordered.The calculated d spacings corresponding to these peaks arein the range 3.5–3.1 nm for the pure, 1 and 3 wt.% U-dopedsamples. The d values are consistent with the values forcubic MCM-48 samples, as reported earlier [11]. Interest-ingly, for the sample with 6 wt.% U, the correlation peakis found to be considerably weaker in the q-range of study,which may be due to masking by the significant scatteringcontribution in the high-q region arising from the precipi-tated uranium oxide crystallites, the presence of which isrevealed by the TEM results of Fig. 1(c).

In order to investigate the fractal nature of these sam-ples, we have used the scattering data prior to the correla-tion peaks. Fig. 3 shows the observed intensity profiles ofthe samples plotted on log–log scale. It may be noticed thatthe profiles of S1, S2 and S3 samples have a distinct singleslope where as that of S4 exhibits an additional weakhump-like feature in the intermediate q-range of the data.Accordingly, the intensities of the former three sampleswere fitted to Eq. (2), using Eqs. (3) and (4). During the

Fig. 3. Observed and fitted SAXS profiles of pure and U-doped samples,as plotted on log–log scale. Intensities are scaled for clarity.

analysis, the initial value of the length L of the cylinderwas taken to be the order of magnitude of the interplanedistance (d) and is varied in the subsequent fittings. The fit-ted profiles are shown along with the measured ones inFig. 3. From the fitted profiles, the power-law exponentand the size of the entities are obtained. The values of frac-tal dimension (Dm), represented by the slopes of the SAXSdata in Fig. 3, are listed in Table 1. They are in the range2.85–2.95 indicating a mass fractal nature of the samples,with the fractal dimensions equal to the magnitude of theslopes. The insignificant variation in Dm from one sampleto other confirms that the fractal nature of the mass-fractalaggregates remains almost unchanged in spite of U-doping.The length of the cylinders obtained from SAXS is in therange 3–3.1 nm. From Table 1 it may also be noticed thatthe diameter of the cylindrical pores in these samples are inthe range 2.4–2.8 nm. Errors in the fitted radii are shownin the parenthesis. These values are quite close to theresults obtained by nitrogen adsorption and TEM methods(Fig. 1). Also, the mass fractal nature, as well as the orderof magnitude of the size of the basic pores, matches closelywith the values reported by other workers for U-loadedmesoporous MCM samples. For instance, in a parallelSAXS study reported by Tismaneanu et al. [14], the incor-poration of actinides (Th, U) in the framework of MCM-41resulted in the mass fractal dimensions (Dm) in the range of2.78–2.95, in agreement with the present study.

Interestingly, a careful observation of the TEM image(Fig. 1(c)) of the sample S4 indicates that the mesoporoussystem is coexistent with uranium particles, not seen inthe TEM pictures of the rest of the samples. This resultsuggests that, after increasing U-content to 6%, a part ofthe occluded species appear at the surface as a precipitate.This feature is also reflected in the scattering profile of S4,which shows an additional hump-like feature in the inter-mediate q-range as compared to the rest of the samples.In other words, the scattering intensity of S4 resulted fromthe mesoporous medium coexisting with polydisperseensemble of uranium particles. Hence, assuming a dilutesystem of particle assembly, we have fitted the intensitydata of S4 to the expression

IðqÞ ¼ C1 � P 21ðqÞ � SðqÞ þ C2

ZP 22ðq;RÞDðRÞR6 dR

� �ð5Þ

where P2(q,R) is the form factor of the particles with radiusR. For spherical shaped particles the form factor is givenby [21,22]

P 2ðq;RÞ ¼ 3½sinðqRÞ � qR cosðqRÞ�=ðqRÞ3 ð6Þ

C1 and C2 are constants which depend on the scatteringdensity contrast whereas D(R) is the particle size distribu-tion. D(R)dR represents the probability of the number ofparticles having a radius between R and R + dR. For aWeibull-type distribution, which is used presently, the dis-tribution function is given by [26,27]

DðRÞ ¼ ½R=a�ðb�1Þ exp½�ðR=aÞb� ð7Þ

Fig. 4. Size distribution of the precipitated UOx crystallites in the S3 (6% U) sample, as estimated from SAXS data.

136 D. Kumar et al. / Microporous and Mesoporous Materials 89 (2006) 132–137

The parameter �a� gives the mean radius of the particleswhereas �b� is related to the symmetry of the distribution.It may be noted that the chosen shape of the scatteringentities, as mentioned above, was based on the presenceof cylindrical pores in MCM host matrix and the occludedUOx particles are taken as spherical on the basis of ourTEM results.

From the fitted parameters, the mass fractal dimensionDm and diameter of the basic pores in the sample S4 arenoticed to be around 2.9 and 2.7 nm, respectively. Also,the average radius (R) of the precipitated uranium particlesis obtained to be 3.8 nm. The distribution in the size of theU-particles, as obtained by fitting, is shown in Fig. 4. Abroad distribution indicates a significant dispersity in thesize of the particles. Furthermore, noting that unlike inN2-adsorption and TEM methods, SAXS gives the statisti-cally averaged values of the size of the entities, the order ofmagnitude of R obtained by SAXS may be considered to befairly consistent with the average of the particle sizes evalu-ated with the help of alternative methods (Fig. 1, Table 1).

The value of (C2/C1), which represents the ratio ofsquare of electron density difference and number of scatter-ing objects for precipitated U-particles and rest of scatter-ing mass, is found to be about 0.05.

In conclusion, the mass fractal nature of the pure andthe U-doped MCM-48 samples, consisting of cylindricalpores, is characterized with the help of SAXS technique.The increase observed in pore dimension as a result offramework substitution (Table 1) is a phenomenon akinto that reported earlier for incorporation of various guestspecies, such as Ti, Ce, V and Sn etc. into the silicate frame-work (for example, see [28–31]). In line with these earlierreported studies, it is surmised that the insertion of a het-ero-atom metal radical such as U–O with larger M–O bonddistance of �2.3 A in place of Si–O (�1.65 A) would leadto an increase in the unit cell parameter and hence in theincrease of pore size [30,31]. The increase in the unit cell

parameter can also be accounted by the increase in the wallthickness of MCM-48 because of uranium incorporation,as has been substantiated by 29Si MAS–NMR resultsreported in Ref. [11]. The N2 adsorption results (Table 1)further reveal that such morphological changes are local-ized and may occur exclusively in the vicinity of the substi-tution sites, leading thereby to a multi-pore system whileleaving a majority of the pores un-affected. The poredimensions measured by SAXS technique thus representthe statistical average of all the individual pore sizes inthe host matrix. The sample doped with relatively higherconcentration (6%) of uranium showed an additional con-tribution to the SAXS profile from the UOx precipitates ofaverage diameter 7.6 nm with a broad distribution in thesize of the particles. These UOx precipitates are identifiedas U3O8 crystallites, as described in our earlier publications[11,12]. The narrowing of the pores in such host materialsdue to inclusion of certain foreign species is again a widelyreported phenomenon. It is also noteworthy that the size ofthe pores as well as that of the uranium oxide crystallites,as measured by the SAXS method, are quite consistentwith those derived from TEM pictures and also calculatedusing other complementary techniques such as powderXRD and N2 adsorption [1].

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