& Nanocasting

A Study on the Growth of Cr2O3 in Ordered Mesoporous Silica andIts Replication

Tobias Grewe, Xiaohui Deng, and Harun T�ys�z*[a]

Abstract: A systematic study on the growth of Cr2O3 inthree-dimensional cubic ordered mesoporous silica (KIT-6)and its replication through nanocasting is reported. Bychanging the loading time and amount of precursor, thesize and shape of the obtained replica could be controlledto some extent. More interestingly, in contrast to previouslypublished studies, when KIT-6 with an aging temperature of100 8C, which has a high degree of interconnectivity, wasused as a hard template, a cubic ordered mesoporous Cr2O3

replica with an open uncoupled subframework structure and

reduced symmetry was obtained. Formation of a replicawith different symmetry and uncoupled subframework struc-ture is not only related to the degree of interconnectivity ofthe parent, but also strongly depends on the type of metaloxide and its growth mechanism in the silica template.Nanocasting of Cr2O3 with a low loading results in a replicawith monomodal pore size distribution that has same sym-metry as the hard template, whereas increasing the loadingamount alters the symmetry of the replica and yields a repli-ca with bimodal distribution.

Introduction

Hard templating (nanocasting) has been intensively used tocreate crystalline ordered mesoporous materials after its dis-covery,[1] and it has been covered in several excellent reviews.[2]

The nanocasting process is based on replication of a porousmold. In the first step, the pores of a template are filled witha suitable precursor. After a treatment (thermal, redox) that re-sults in the desired composition and template removal bycombustion in air or with an effective leaching agent such asNaOH or HF, the desired replica is obtained. Parameters thatmay affect the success of a replication procedure include ge-ometry, morphology, and texture of the hard template, chemi-cal and physical properties of the precursor, loading time andamount of precursor, impregnation and drying conditions, thecontainer effect, oxidation and reduction process, heating rateand calcination temperature, and template-removal method.[3]

Cubic ordered mesoporous silica KIT-6 has two types of mes-oporous channels, and these channels are connected to eachother through micropores, which is also called interconnectivi-ty.[4] The interconnectivity of the KIT-6 can be easily controlledby changing the aging temperature during synthesis. A lowaging temperature (around 40 8C) results in a material withthicker silica wall, smaller pore size, and lower degree of inter-connectivity, whereas a higher aging temperature (usuallyabove 100 8C) allows formation of a silica that has thinnerwalls, larger pores, and higher degree of interconnectivity. If

the silica is used as a hard template to create a negative repli-ca, geometry and textural parameters of the replica can betuned by modifying the textural parameters and interconnec-tivity of the parent hard template. For instance, in the replica-tion of Pt,[5] Co3O4,[3a] and NiO,[6] KIT-6 with lower interconnec-tivity was used as a hard template, the obtained replicas hadmostly open, uncoupled subframework structure with bimodalpore size distributions, and an additional 110 reflection peak inthe low-angle XRD pattern was observed,[3a, 5] which indicatesreduced symmetry. This phenomenon was attributed togrowth mechanism of the metal or metal oxide in the sparselyinterconnected channels of the template. If metal or metaloxide grows homogenously in both channel systems of a KIT-6template with high interconnectivity, a perfect replica witha coupled subframework that has same cubic symmetry as thetemplate and a monomodal pore size distribution that is equalto the wall thickness of template is produced. On other hand,when the KIT-6 has lower interconnectivity, metal or metaloxide can also grow in only one of the channel systems of KIT-6 to give a replica with different symmetry and bimodal poresize distribution.

Cr2O3 has several industrial applications as pigment and cat-alyst. Ordered mesoporous Cr2O3 has been fabricated from var-ious templates and precursors by hard and soft templating.[7]

Furthermore, He and co-workers proposed a growth mecha-nism of Cr2O3 in the pores of KIT-6.[7a] They studied the struc-ture of the Cr2O3 replica with low-angle XRD, TEM, and N2 sorp-tion, and concluded that a negative replica of KIT-6 was suc-cessfully prepared, although it did not show small-angle XRDbelow 2 q= 0.8, and TEM and N2 sorption analyses revealed for-mation of pores larger than 10 nm.

Herein, we demonstrate for the first time that the symmetryand structure of Cr2O3 replicas can be controlled by changing

[a] T. Grewe, X. Deng, Dr. H. T�ys�zMax-Planck-Institut f�r KohlenforschungKaiser-Wilhelm-Platz 1, 45470 M�lheim an der Ruhr (Germany)E-mail : tueysuez@kofo.mpg.de

Supporting information for this article is available on the WWW underhttp ://dx.doi.org/10.1002/chem.201402301.

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Full PaperDOI: 10.1002/chem.201402301

the total degree of pore-volume filling of the hard template.Moreover, we show that formation of different symmetry anduncoupled subframework structure is not only related to thedegree of interconnectivity of the parent silica, but also strong-ly depends on type of metal oxide and its growth mechanismin the silica template.

Results and Discussion

A series of ordered mesoporous Cr2O3 materials was synthe-sized through nanocasting by using KIT-6 with 100 8C agingtemperature (KIT-6-100) as hard template and CrO3 as precur-sor according to our recently published recipe.[7b] Briefly,a 0.5 m aqueous solution of CrO3 was added to the silica andstirred at room temperature for 2 h; subsequently it was driedat 70 8C and finally calcined at 400 8C for 6 h. The impregnationsolution has pH<2. At this pH value CrO3 forms singly depro-tonated chromic acid (HCrO4

�) and dichromate anions (Cr2O72�)

in water,[8] whereas silica is below its isoelectric point and posi-tively charged. Despite the addition of the silica to the impreg-nation solution, the pH value does not change considerably.Although interaction between anionic CrVI species and the pos-itively charged silica surface is likely, it does not hinder Cr spe-cies entering the pores of the silica freely, as we could isolatereplicated Cr2O3 from the silica template after impregnation,calcination, and silica-removal steps. After the first cycle of KIT-6-100 impregnation with CrO3 solution, 3.75 % of the totalpore volume of the KIT-6 was loaded with Cr2O3. A portion ofthe composite was separated and silica was removed by usinghot 2 m NaOH solution, and the obtained replica was denotedCr2O3-1c. The rest of the composite material was further im-pregnated with CrO3 precursor in three impregnation cycles,which resulted in Cr2O3 replicas with total degrees of pore-volume filling of 7.5, 15, and 22.5 %, respectively. After silica re-moval, these samples were denoted Cr2O3-2c, Cr2O3-3c, andCr2O3-4c.

After each cycle of impregnation, development of the Cr2O3

particles in mesoporous silica was investigated by TEM; the mi-crographs and particle size distributions are shown in Figure 1.The silica and the Cr2O3 with high electron density can beeasily distinguished from each other. The first cycle of impreg-nation yielded particles in the range of 10 to 150 nm witha particle size distribution centered around 70 nm. After thesecond and third impregnation cycles, average particle sizes ofabout 100 and 150 nm were obtained. Moreover, smaller parti-cles that were observed after the first impregnation cycle wereno longer visible anymore, which indicates that, in the secondand third impregnation cycles, Cr2O3 grows around preformedCr2O3 particles instead of forming new particles. Increasing thetotal degree of pore-volume filling to 22.5 % after four impreg-nation cycles produced particles with an average particle sizeof around 180 nm, and the smallest particles observed were inthe range of 70–80 nm, which was the average particle size ofthe initial Cr2O3 particles after first impregnation cycle. In addi-tion to the particle size, an obvious change was also observedin the shape of the Cr2O3. Lower loadings allow formation ofmostly elliptical particles (rice shape), and increasing the load-

ing favors formation of more spherical particles. Furthermore,increasing the loading to 22.5 % also results in formation ofsome bulk Cr2O3, as seen in Figure 1 d, which indicates thatsuch a high loading causes some diffusion problems in thesilica matrix. Further increasing the degree of pore volume fill-ing produces large amounts of non-mesostructured bulk Cr2O3.

A systematic investigation was also carried out on thechanges in textural parameters of the composite material aftereach impregnation cycle. The N2 sorption isotherms and poresize distributions of KIT-6 and the composite sample after eachimpregnation cycle and their textural parameters are presentedin Figure S1 (Supporting Information) and Table 1. The KIT-6hard template has a type IV isotherm, characteristic of mesopo-rous materials, a BET surface area of 788 m2g�1, and a totalpore volume of 0.99 cm3g�1. After each cycle of impregnation,the surface area and pore volume decreased continuously andreached values of 215 m2 g�1 and 0.20 cm3 g�1, respectively,after four impregnation cycles. According to our theoreticalcalculation, the composite after the first cycle (SiO2-Cr2O3-1c)should have a pore volume of 0.80 cm3 g�1. For the calculation,we assumed 3.75 % of the pore volume of 1 g of KIT-6-100 tobe filled with Cr2O3, which corresponds to 0.037 cm3 or 0.194 gof Cr2O3. This gives a total pore volume for the composite of

Figure 1. TEM images of the SiO2-Cr2O3 composites after each impregnationcycle with a) 3.75, b) 7.5, c) 15, d) 22.5 % total pore-volume filling, and e)particle size distribution of Cr2O3 inside KIT-6-100, observed by TEM aftereach impregnation cycle; for each sample the lengths and widths of 100particles were measured. The values were added and divided by two to givea mean diameter of the particle. The formation of bulk Cr2O3 after 22.5 %loading is marked with red arrows in d).

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0.953 cm3 [(0.99�0.037) cm3] with a total mass of 1.194 g (start-ing with 1 g of KIT-6-100 and filling 3.75 % of the pore volumewith Cr2O3), or a specific pore volume of 0.80 cm3 g�1

(0.953 cm3 divided by 1.194 g). Following this calculation, wedetermined the theoretical pore volumes of all KIT-6-Cr2O3

composite samples (Table 1). The measured value is in goodagreement with the calculated pore volume for SiO2-Cr2O3-1c.With further impregnation cycles, the measured values areabout 10 % lower than the calculated values. For SiO2-Cr2O3-4cthe measured value deviates by 44 % from the theoretically ex-pected value. This mismatch can be related to blocking ofsome pores of the silica template by Cr2O3 particles, whichmakes them inaccessible for N2 adsorption and the formationof non-mesoporous bulk Cr2O3 outside the silica template, asseen in TEM image in Figure 1 d. The major differences in thecomposite sample after the fourth impregnation cycle are con-sistent with the TEM images of the composite, in which somebulk Cr2O3 was observed.

To gain more precise insight into the composition of theSiO2–Cr2O3 samples, the Cr/Si ratio was determined after eachcycle of impregnation. Filling the pore volume by 3.75 % gavea Cr/Si ratio of 0.16. After the second, third, and fourth cycles,the Cr/Si ratio increased to 0.31, 0.61, and 0.93 respectively.

After removal of the silica template with hot NaOH solution,the structure of the nanocast Cr2O3 was investigated by TEM(Figure 2). The ordered mesoporous structure is observable forall samples, but their particle sizes and shapes are different.The lowest degree of pore filling results in elliptical particlesthat have densely coupled subframework structure. In otherwords, at such a low loading, the Cr2O3 grows in both channelsystems of KIT-6 simultaneously to give a replica that has thesame symmetry as the hard template, as is expected for a repli-ca that is produced from KIT-6-100, which has a sufficientdegree of interconnectivity. At higher loadings, the replicasconsist of a mixture of coupled and uncoupled subframeworkstructures (Figure 2 c–h), that is, Cr2O3 grows in both pore sys-tems simultaneously and independently. This behavior becamemore pronounced after the third and fourth impregnationcycles. As shown in Figure 2 e and f, in addition to the smallpores, some larger pores (ca. 11 nm) were observed as well,

which reveals that some of Cr2O3 particles continue to grow inone of the channel systems of the silica hard template. Increas-ing the total degree of pore filling to 22.5 % results also insome bulk Cr2O3 (Figure 2 g). The higher loading blocks someof the pore entrances of the hard template, as confirmed byTEM and N2 sorption analysis. This prevents impregnation ofthe chromium precursor into the pores of the silica hard tem-plate and results in bulk Cr2O3 particles. The crystallite size ofthe ordered mesoporous Cr2O3 was estimated from TEM analy-sis to be around 8 nm and fits well with the value that was de-termined from the wide-angle XRD pattern (not shown) byusing the Sherrer equation.

Replication of the KIT-6 template is illustrated in Scheme 1.Low loadings of chromium precursor (3.75 %) yield a replicawith coupled subframework structure and the same cubic sym-metry as the silica template. In other words, at such low load-ings Cr2O3 grows homogenously in both channel systems ofthe silica template to give a replica with a coupled subframe-work structure, monomodal pore size distribution, and thesame symmetry as the template. Increasing the loading alsopermits growth of Cr2O3 in one of the channel systems of thetemplate, which results in a replica with uncoupled subframe-

Table 1. Overview of the textural parameters of KIT-6 and compositesamples after each impregnation cycle and their corresponding replicas.

Sample SBET [m2 g�1] Pore volume [cm3 g�1] Pore size [nm]

KIT-6-100 788 0.99 7.2SiO2-Cr2O3-1c 545 0.78 (0.80[a]) 7.4SiO2-Cr2O3-2c 461 0.61 (0.66[a]) 7.3SiO2-Cr2O3-3c 324 0.42 (0.47[a]) 7.2SiO2-Cr2O3-4c 215 0.20 (0.36[a]) 7.2Cr2O3-1c 111 0.21 2.7Cr2O3-2c 130 0.27 2.8Cr2O3-3c 125 0.31 2.7Cr2O3-4c 96 0.22 2.5

[a] Calculated pore volume of the composite sample. The reduced porevolume of KIT-6-100 (pore volume of KIT-6 minus volume of the incorpo-rated Cr2O3) was divided by the theoretical mass of the composite (SiO2-Cr2O3).

Figure 2. TEM and HRTEM images of ordered mesoporous Cr2O3 with totalpore-volume filling of a, b) 3.75; c, d) 7.5; e, f) 15; and g, h) 22.5 %. Scale bars:100 nm.

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work structure, bimodal pore size distribution, and reducedsymmetry.

To obtain a 3D projection of the material, one of the sampleof the series (Cr2O3-4c) was further investigated with high-reso-lution (HR) SEM. As shown in Figure 3, the ordered mesopo-rous Cr2O3 particle consists of the coupled and uncoupled sub-framework structures (with larger pore size around 11 nm),which confirms that Cr2O3 was replicated in some part fromboth pore systems and in some region only from one of thechannel systems of the silica hard template.

The structure of the KIT-6 hard template and the obtainedCr2O3 replicas were further investigated by small-angle X-rayscattering (SAXS), and their profiles in the range of q = 0.3–2.0 nm�1 are shown in Figure 4. KIT-6 shows characteristic 211and 220 reflections that can be assigned to a cubic orderedstructure with Ia�3d symmetry. As shown in Figure 4, a totaldegree of pore-volume filling of 3.75 % is enough to obtain anordered structure of Cr2O3; the SAXS profile shows sharp 211

and weak 220 reflections that look identical to those of KIT-6.This confirms that Cr2O3 is replicated from both channels ofthe double-gyroid structure. When the loading amount is en-hanced to 7.5 % after two cycles of impregnation, SAXS showsan additional 110 peak around q = 0.35 nm�1 that is not ob-served in the profile of original KIT-6 with 100 8C aging temper-ature. This phenomenon is unexpected and has not been re-ported or studied for the replication of Cr2O3. Thus, after twocycles of loading, Cr2O3 grows more in one channel system ofthe KIT-6 silica and this results in a material with a lower sym-metry that can be assigned as I4132.[5] After the third andfourth cycles of impregnation, loadings of 15 and 22.5 % of thetotal pore volume were reached. As shown in Figure 4, with in-creasing loading the degree of the ordering and the intensityof the 110 peak increases as well, which confirms that Cr2O3

keeps growing mostly in one of the channel systems of KIT-6.The mesoporosity and textural parameters of the nanocast

Cr2O3 were determined by means of N2 sorption (Table 1). Thesurface area and total pore volume of the materials are in therange of 96–130 m2 g�1 and 0.21–0.31 cm3 g�1, respectively. Asshown in Figure 5 a, all samples show type IV isotherms withtypical hysteresis loops for mesoporosity. The changes in themorphology of the nanocast Cr2O3 can also be noticed in thepore size distributions. A pore filling of 3.75 % yields a replicawith monomodal pore size distribution centered on 3 nm.When the pore filling is increased to 15 %, an additional peakaround 11 nm appears (Figure 5 b). Further increasing the porevolume to 22.5 % makes these larger pores more abundant.Formation of these large pores indicates that, after a certaindegree of pore filling, Cr2O3 grows predominantly in one of thechannel systems of the silica to result in larger pores, the sizeof which is equal to the sum of the pore size of the silica tem-plate and its wall thickness.

Nanocasting of Cr2O3 in several impregnation steps includingdispersal in water, drying at 70 8C, and calcining at 400 8Cmight alter the textual parameters of the silica template, whichcan influence the quality and symmetry of the replica. To rule

Scheme 1. Schematic pathway of replica formation with coupled and uncou-pled subframework structure from KIT-6, depending on the loading of Cr2O3.

Figure 3. SEM image of Cr2O3 replica with 22.5 % total pore-volume fillingshowing both coupled and uncoupled subframework structure.

Figure 4. SAXS profiles of KIT-6-100 silica template (inset) and ordered meso-porous Cr2O3 with various degrees of total pore-volume filling.

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out the effect of template alteration, we prepared a series ofCr2O3 samples with the same pore loadings (3.75, 7.5, 15, and22.5 %) in KIT-6-100 by one-step impregnation. The characteri-zation results of this series are given in the Supporting Infor-mation (Figure S2). The SAXS profile (Supporting Information,Figure S2 a) of the series with one impregnation step showsthat Cr2O3-1c (3.75 %) has cubic symmetry like KIT-6-100, andwith increasing loading the 110 reflection becomes more pro-nounced, which indicates the presence of reduced symmetryin the replica. N2 physisorption (Supporting Information, Fig-ure S2 b) showed that all samples have typical type IV iso-therms for mesoporous materials. Increasing the loadingchanges the pore systems of the replica, whereby bimodalpore size distribution is observed for higher loadings (inset inFigure S2 b of the Supporting Information). The larger poresare indicators for replicas with open, uncoupled subframeworkstructures. TEM imaging (Supporting Information, Figure S2 c–f)of the single-cycle series of Cr2O3 samples shows that 3.75 %loading of the pore volume of KIT-6-100 results in elliptical or-dered mesoporous particles with a coupled subframeworkstructure. Increasing the loading in one cycle to 7.5 % gaveparticles with larger domains that have coupled and uncou-pled ordered mesoporous structure, while 15 % loading in onestep led to formation of some bulk Cr2O3 besides mesoporousuncoupled subframework structure. The high pore-volumeloading of 22.5 % led to a large quantity of bulk Cr2O3 in thereplica, which indicates the importance of replication in severalimpregnation steps. The ordered mesoporous particles haveuncoupled subframework structures similar to that of Cr2O3-4cprepared in four impregnation steps. The results for this seriesof Cr2O3 samples indicate that the number of impregnationcycles does not alter the structure of the silica template, but itcan noticeably influence the growth mechanism of the Cr2O3.

In contrast to previous studies, we demonstrated that theCr2O3 replica grows only in one of the channel systems of KIT-6in some part of the template, although the silica that wasaged at 100 8C has high interconnectivity. This indicates that in-teractions of the metal precursor and metal oxides with thesilica template may play a critical role in the growth path of

metal oxides. To explore this effect, we also prepared orderedmesoporous Co3O4 by using KIT-6 as hard template by fillingits total pore volume to about 15 %. The SAXS, N2 sorption,and TEM data of the nanocast Co3O4 are shown in Figure S3 ofthe Supporting Information. Unlike the replication of Cr2O3, theSAXS profile of the Co3O4 showed 211 and 220 reflections,which are typical for cubic ordered structures with Ia�3d sym-metry. This confirmed that Co3O4 was replicated from bothpore systems of the double-gyroid mesostructure. In addition,TEM illustrated an ordered mesoporous coupled subframeworkstructure, and N2 sorption showed a monomodal pore size dis-tribution. These findings corroborate that the type and natureof metal precursors have a critical effect on the replication pro-cedure.

Conclusion

We have demonstrated that the growth mechanism of metaloxides in the channels of double-gyroid silica is not only relat-ed to the interconnectivity of the template, but also dependson the type and nature of the precursor. By changing the load-ing of Cr2O3, a series of replicas with various particle sizes andmorphologies was prepared. Low loadings result in a perfectreplica of the hard template with the same symmetry, whereasincreasing the loading amount produces a replica with re-duced symmetry and bimodal pore size distribution. We foundthat the structure and the symmetry of a replica could beeasily modified by changing the loading of the starting precur-sor. The method reported here can be used to modify thestructure of other nanocast metal oxides, which may be of in-terest in a range of applications, especially heterogeneous cat-alysis.

Experimental Section

KIT-6 was synthesized according to the literature.[4] Briefly, cubic or-dered mesoporous KIT-6 was prepared under acidic conditions byusing the triblock copolymer poly(ethylene glycol)–block-poly-(propylene glycol)-block-poly(ethylene glycol) (Pluronic P123,EO20PO70EO20, Sigma-Aldrich) as structure-directing agent. First,13.5 g of Pluronic P123 was dissolved in a mixture of 487.5 g ofdistilled water and 26.1 g of concentrated HCl (37 %). The mixturewas stirred at 35 8C and 13.5 g of n-butanol was added to the ho-mogeneous solution. After 1 h of stirring, 29 g of TEOS (31 mL) wasquickly added to the solution, followed by stirring at 35 8C for 24 h.Then, the mixture was heated at 100 8C for another 24 h understatic conditions. The solid product was collected by filtration with-out washing, dried at 90 8C, and finally calcined in air at 550 8C for6 h.

Mesoporous Cr2O3 was synthesized by the hard-templatingmethod with KIT-6 (100 8C aging temperature) as parent silica andCrO3 as precursor. A series of samples was prepared with differentloadings of Cr2O3 in KIT-6. Relative loadings of silica template werecalculated according to the molar amount of Cr2O3 within the totalpore volume of the silica template that was determined from N2

sorption measurements, by considering the bulk density of Cr2O3

(5.22 cm3 g�1) and assuming that all Cr2O3 grows in the pores ofthe template. Four cycles of impregnation and calcination wereperformed. After each cycle, the KIT-6-Cr2O3 composite was divided

Figure 5. a) N2 sorption isotherms of Cr2O3 replicas and b) their pore size dis-tributions. The offsets for isotherms are 100 cm3 g�1.

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into two equal parts. One part was kept for analysis, and the otherpart was reimpregnated. In the first two cycles, 3.75 % of the porevolume of KIT-6 was loaded with Cr2O3, and in the last two cycles7.5 % of the pore volume was filled. In a typical experiment, 2 g ofKIT-6 was dispersed in 0.5 m aqueous CrO3 solution (10 mL) andstirred for 2 h. The mixture was dried at 70 8C and subsequentlycalcined at 400 8C for 6 h with a temperature ramp of 2 8C min�1.The solid was divided into two parts, one of which was kept foranalysis and the other dispersed in 5 mL aqueous CrO3 solution(0.5 m). After stirring for 2 h the material was dried and calcined.The sample was split again, with one part being kept for analysisand the other part re-impregnated. For this, the composite wasdispersed for 2 h with stirring in 5 mL of aqueous CrO3 solution(0.5 m). Subsequently, the material was dried and calcined. After di-viding the sample, one part was kept for analysis and the otherwas impregnated in a final step. 2.5 mL of 0.5 m aqueous CrO3 solu-tion was used to disperse the composite. After 2 h, the sample wasdried and calcined.

The silica template was removed with a 2 m aqueous sodium hy-droxide solution. The Cr2O3–silica composite was dispersed in hot2 m NaOH (25 mL for 1 g silica), shaken for 2 h, and kept in anoven for another 2 h. After sedimentation, the solution was deca-nted and fresh hot NaOH solution was added. It was shaken againfor 1 h and left in an oven at 60 8C for 12 h. The Cr2O3 was filteredoff, washed with water until neutrality, and dried at 50 8C. Theleached samples are denoted Cr2O3-Xc, where X is the number ofcycles (X = 1–4). Composites in which Cr2O3 is still inside KIT-6 aredenoted as SiO2-Cr2O3-Xc.

Single-cycle Cr2O3 samples with 3.75, 7.5, 15, and 22.5 % pore-volume filling in KIT-6 were synthesized by dispersing 1 g of KIT-6in 3.9 mL of aqueous solutions of CrO3 with concentrations of 0.5,1.0, 2.0, and 2.5 m, respectively, for 2 h at room temperature. Sub-sequently, the samples were dried at 70 8C and calcined at 400 8Cfor 6 h (ramp 2 8C min�1). The silica was removed as describedabove. Ordered mesoporous Co3O4 was prepared according to theliterature.[7b]

The XRD patterns were recorded on a Stoe theta/theta diffractom-eter in Bragg–Brentano geometry (CuKa radiation) at room temper-ature. SAXS data were obtained with Anton Paar SAXSess. Nitrogenadsorption isotherms were measured with an ASAP 2010 adsorp-tion analyzer (Micromeritics) at liquid-nitrogen temperature. Priorto the measurements, the samples were degassed at 150 8C for10 h. Total pore volumes were determined by using the adsorbedvolume at a relative pressure of 0.97. BET surface area was deter-mined in the relative pressure range of 0.06–0.2. Pore size distribu-tion curves were calculated from the adsorption branch by the Bar-rett–Joyner–Halenda method. TEM images of samples were ob-

tained with an H-7100 electron microscope (100 kV) from Hitachi.Samples were prepared on carbon-film-coated grids. HRSEMimages of the samples were taken with a Hitachi S-5500 ultrahigh-resolution cold field emission scanning electron microscope oper-ated at 30 kV.

Acknowledgements

We thank our EM department for SEM analysis. This work wasfinancially supported by Max-Planck-Society and Fonds derChemischen Industrie (FCI). T.G. thanks FCI for a ChemiefondsFellowship.

Keywords: chromium oxide · mesoporous materials ·nanocasting · template synthesis

[1] R. Ryoo, S. H. Joo, S. Jun, J. Phys. Chem. B 1999, 103, 7743 – 7746.[2] a) Y. Ren, Z. Ma, P. G. Bruce, Chem. Soc. Rev. 2012, 41, 4909 – 4927; b) M.

Tiemann, Chem. Mater. 2008, 20, 961 – 971; c) A.-H. Lu, F. Sch�th, Adv.Mater. 2006, 18, 1793 – 1805; d) H. T�ys�z, F. Sch�th, Adv. Catal. 2012, 55,127.

[3] a) H. T�ys�z, C. W. Lehmann, H. Bongard, B. Tesche, R. Schmidt, F. Sch�th,J. Am. Chem. Soc. 2008, 130, 11510 – 11517; b) X. Sun, Y. Shi, P. Zhang, C.Zheng, X. Zheng, F. Zhang, Y. Zhang, N. Guan, D. Zhao, G. D. Stucky, J.Am. Chem. Soc. 2011, 133, 14542 – 14545; c) A. Rumplecker, F. Kleitz, E. L.Salabas, F. Sch�th, Chem. Mater. 2007, 19, 485 – 496; d) H. J. Wang, H. Y.Jeong, M. Imura, L. Wang, L. Radhakrishnan, N. Fujita, T. Castle, O. Terasa-ki, Y. Yamauchi, J. Am. Chem. Soc. 2011, 133, 14526 – 14529; e) H. T�ys�z,Y. Liu, C. Weidenthaler, F. Sch�th, J. Am. Chem. Soc. 2008, 130, 14108 –14110.

[4] F. Kleitz, S. H. Choi, R. Ryoo, Chem. Commun. 2003, 2136 – 2137.[5] Y. Doi, A. Takai, Y. Sakamoto, O. Terasaki, Y. Yamauchi, K. Kuroda, Chem.

Commun. 2010, 46, 6365 – 6367.[6] F. Jiao, A. H. Hill, A. Harrison, A. Berko, A. V. Chadwick, P. G. Bruce, J. Am.

Chem. Soc. 2008, 130, 5262 – 5266.[7] a) K. Jiao, B. Zhang, B. Yue, Y. Ren, S. X. Liu, S. R. Yan, C. Dickinson, W. Z.

Zhou, H. Y. He, Chem. Commun. 2005, 5618 – 5620; b) H. T�ys�z, C. Wei-denthaler, T. Grewe, E. L. Salabas, M. J. Benitez Romero, F. Sch�th, Inorg.Chem. 2012, 51, 11745 – 11752; c) H. Liu, X. W. Du, X. R. Xing, G. X. Wang,S. Z. Qiao, Chem. Commun. 2012, 48, 865 – 867; d) A. K. Sinha, K. Suzuki,Angew. Chem. 2005, 117, 275 – 277; Angew. Chem. Int. Ed. 2005, 44, 271 –273; e) A. H. Hill, A. Harrison, C. Dickinson, W. Z. Zhou, W. Kockelmann,Microporous Mesoporous Mater. 2010, 130, 280 – 286.

[8] A. F. Holleman, E. Wiberg, N. Wiberg, in Lehrbuch der AnorganischenChemie (in German), 102 ed., de Gruyter, Berlin, 2007, pp. 1567 – 1571.

Received: February 21, 2014Published online on && &&, 0000

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& Nanocasting

T. Grewe, X. Deng, H. T�ys�z*

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A Study on the Growth of Cr2O3 inOrdered Mesoporous Silica and ItsReplication

Perfect replicas of cubic ordered meso-porous silica template KIT-6 with thesame symmetry and coupled subframe-work structure were obtained by nano-casting of Cr2O3 at low precursor load-

ing, whereas increasing the precursorloading resulted in a replica with re-duced symmetry, uncoupled subframe-work structure, and bimodal pore sizedistribution (see figure).

Chem. Eur. J. 2014, 20, 1 – 7 www.chemeurj.org � 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim7 &&

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