Modulated Formation of MOF-5 NanoparticlesA SANS AnalysisR. Nayuk,† D. Zacher,‡ R. Schweins,§ C. Wiktor,‡ R. A. Fischer,*,‡ G. van Tendeloo,∥ and K. Huber*,†

†Chemistry Department, Universitat Paderborn, Warburger Straße100, D-33098 Paderborn‡Faculty of Chemistry and Biochemistry, Inorganic Chemistry, Ruhr-Universitat Bochum, Universitatsstraße 150, D-44801 Bochum§Institut Laue-Langevin, LSS Group, B.P. 156, 6, rue Jules Horowitz, F-38042 Grenoble Cedex 9∥Electron Microscopy for Materials Research (EMAT), University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerpen

*S Supporting Information

ABSTRACT: MOF-5 nanoparticles were prepared by mixing asolution of [Zn4O(C6H5COO)6] with a solution of benzene-1,4-dicarboxylic acid in DMF at ambient conditions. The formerspecies mimics as a secondary building unit (SBU), and the latteracts as linker. Mixing of the two solutions induced the formation ofMOF-5 nanoparticles in dilute suspension. The applied conditionswere identified as suitable for a closer investigation of the particleformation process by combined light and small angle neutronscattering (SANS). Scattering analysis revealed a significant impactof the molar ratio of the two components in the reaction mixture.Excessive use of the building unit slowed down the process. A similar effect was observed upon addition of 4n-decylbenzoic acid,which is supposed to act as a modulator. The formation mechanism leads to initial intermediates, which turn into cubelikenanoparticles with a diameter of about 60−80 nm. This initial stage is followed by an extended formation period, wherenucleation proceeds over hours, leading to an increasing number of nanoparticles with the same final size of 60−80 nm.

■ INTRODUCTIONMetal−organic frameworks (MOFs), also called porouscoordination polymers (PCPs), represent a subclass ofcoordination polymers with a very robust three-dimensionallattice-like structure. This feature generates a large internalsurface, which predestines MOFs for a variety of potentialapplications.1,2 Examples are the use as gas storage tanks withbulk materials, as gas separation materials based on MOFmembranes, and as heterogeneous catalysts2 whereby theirsuperiority is caused by the fact that they can easily beseparated from the products and thus can be recovered for newproduction cycles. The catalyst may preferentially be designedas micrometer or even nanometer sized particles. Besides,MOFs have a great potential to administer drugs on specificsites at a suitable dose.2−5 It is at hand that MOF materialsapplied as drug carriers, or in gas separation membranes,require a rational design of colloidal MOF material, including acontrolled formation of MOF particles. Such a control has toconsider particle size, size distribution, but also the suppressionof interpenetrating networks, a phenomenon especially knownfor framework systems with large pores.6 A prerequisite for anycontrol of the particle morphology is a detailed knowledge ofthe mechanism of nucleation and growth of MOF nano-particles.MOF particles can be formed with a number of different

techniques. Early strategies to obtain high quality crystallinematerial were predominantly based on solvothermal synthesisin a solution of all necessary constituents.3,7 A drawback ofthese procedures is the rather long preparation time of half a

day or longer. Meanwhile, various attempts have beenpublished to speed up the synthesis of MOF material and tobetter control the formation process and the resulting particlesize of nanoscaled MOF material. Application of microwavetreatment during the solvothermal process decreased thepreparation time considerably8−10 and led to a decrease ofcrystal size with either decreasing component concentrations8

or decreasing crystallization time.9 Similar progress has beenachieved with the application of ultrasound to a solutionprepared at ambient conditions.11

Further progress was achieved by discovering that a simplemixing of appropriate components or building units in solutionat ambient conditions may result in MOF materials.6,12−14 Inparticular, choosing metal ion components that mimic thesecondary building units of the final MOF material provedparticularly advantageous. For example, the Be and Co isotypesof the well-known Zn-based MOF-5, [{Zn4O(BDC)3}n] (BDC= benzene-1,4-dicarboxylate), were accessible only by using thisso-called controlled secondary building unit (SBU) approach.15

This discovery did not only minimize the preparation time butalso opened up a new route to the preparation of MOFnanoparticles. Despite this considerable progress, the control ofparticle size and its distribution remains an open issue, whichattracts increasing attention.

Received: January 12, 2012Revised: February 15, 2012Published: February 15, 2012

Article

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The identification of suitable modulators which will influencethe speed of particle formation as well as the shape of thegrowing MOF particles may offer the possibility to captureparticles with a distinct size and morphology. In an earlyfeasibility study, perfluoromethylbenzenecarboxylate (PFMBC)was identified as a promising coordination reagent, which isable to trap MOF-5 nanoparticles if the PFMBC is addedduring the course of particle formation.16 Shortly thereafter,Kitagawa and co-workers17 succeeded to direct the growth of[{Cu2(NDC)2(DABCO)}n] MOF particles (with NDC = 1,4-naphtalene dicarboxylate and DABCO = 1,4-diazabicyclo-[2,2,2]octane) in dependence on the amount of additionaladded acetic acid, which acts as coordination modulator. Thisinfluences the coordination equilibria during the growth,forcing small nanocrystals to an oriented attachment sincefaces of the nanocrystals are selectively blocked by themodulator. A similar strategy was applied by Oh et al.,18 whosucceeded to block the growth rate of distinct crystal facets,thus also directing the growth toward anisotropically shapedMOF crystals. In these contributions,17,18 no direct in situmonitoring of the particles was performed. The characterizationof the particle shape and size was carried out by means of TEMmeasurements.Further progress to a better control of the particle features

clearly needs more detailed investigations on the nucleationand growth processes of MOF (nano) crystals. Varioustechniques are available to monitor and analyze such processesin situ. One promising technique is light scattering, as this isnoninvasive as long as the scattering particles do not absorb thelight. In particular, time-resolved static light scattering (TR-SLS) turned out to offer a powerful tool to follow processesfrom a few minutes on. The first application of this “new”technology in the field of MOF growth demonstrated thatMOF-5 formation under solvothermal conditions exhibited arelatively fast nucleation, which was succeeded by a growth stepof the particles.16 The latter step could successfully be followedby TR-SLS for more then 10 min, and as mentioned above, thegrowth could be stopped upon the addition of PFMBC.16 Twofurther investigations were devoted to the formation ofHKUST-1 [Cu3(BTC)2] (BTC = benzene, 1,3,5-tricarboxylicacid) in a water−ethanol mixture19 and to ZIF-8 inmethanol,13,20 respectively. The formation of HKUST-1 wasinduced by cooling a supersaturated and thermally pretreatedsolution of Cu(NO3)2 and BTC to room temperature. Duringthe first 10 min, particle growth was overlaid by an ongoingnucleation process. While particle growth stopped at a sizeclose to 400 nm, nucleation still produced new particles evenafter 25 min.19 The formation of ZIF-8 was initiated by simplymixing solutions of Zn(NO3)2 and 2-methylimidazole. In thiscase, ZIF-8 nanoparticles with a size of 50 nm formed rapidlyduring the first minutes and TR-SLS could only indicate theincrease of the number of particles. The nanoparticles started toagglomerate after a few minutes, once its content seemed to belarge enough.13

Meanwhile, the technique of mixing component solutions atambient conditions to generate MOF nanoparticles had beensuccessfully extended also to the preparation of MOF-515 andshall be applied in the present work to investigate the impact ofa modulator molecule on the growth process. The twocomponents are [Zn4O(C6H5COO)6], providing the secondarybuilding unit (and henceforth being abbreviated as SBU), andbenzene-1,4-dicarboxylic acid (BDC), which acts as the linker.In addition, 4n-decylbenzoic acid (DBA) is used as a growth

modulator in order to control the formation of MOF-5nanoparticles. The choice of a molecule with a long alkyl-chainas a modulator was based on the large neutron scatteringcontrast of its alkyl-chain moiety, which enables, in principle, aselective addressing of these molecules in the composite systemof MOF-5 and modulator. The particle formation process isinduced simply via mixing a solution of the SBU in DMF with asolution of BDC in DMF. The focus of the present work lies onthe investigation of the resulting growth mechanism of MOF-5particles with and without modulator molecules and of themorphological nature of the nucleated MOF-5 particles. Thework is based on a joint application of TR-SLS and small angleneutron scattering (SANS). Light scattering will enable us tofollow the evolution of global dimensions with time, and SANSwill extend the local resolution of the particle shape analysisdown to less than 1 nm and offer the chance to selectivelyaddress specific compartments of the particles if heterogeneitiesoccur. Selective addressing with SANS is made possible bycontrast variation via deuterium−hydrogen exchange in theorganic residues of MOF-5. These joint experiments shallpresent insight into the particle formation mechanism inducedupon direct mixing of suitable component solutions incomparison to the solvothermally induced process16 and intocharacterization of the morphology of the MOF-5 nano-particles. In addition, they will give a first rationale on how tomodulate or even stop the growth of MOF-5 nanoparticles in acontrolled manner.

■ EXPERIMENTAL PARTMaterials. Benzene-1,4-dicarboxylic acid, C6H4(COOH)2

(BDC) (98+%), was purchased from Alfa Aesar (Massachu-setts), and deuterated C6D4(COOH)2 (d4-BDC) (99+%) waspurchased from Merck (Darmstadt, FRG). The molecularprecursor [Zn4O(C6H5COO)6] of the secondary building unitwas synthesized according to a literature procedure.21 4n-Decylbenzoic acid (DBA) (+99%) was purchased from ABCR(Karlsruhe, FRG). N,N-Dimethylformamide (DMF) served asthe solvent. H7-DMF (99%) was supplied by Merck(Darmstadt, FRG), while D7-DMF (99,5%) was obtainedfrom Deutero (Kastellaun, FRG).

Sample Preparation. All experiments with a SBU to linkerratio of 1:1 were based on two starting solutions: (i) 0.0055 gof benzene-1,4-dicarboxylic acid (BDC) dissolved in 10 mL ofDMF and (ii) 0.03347 g of preformed secondary building unit[Zn4O(C6H5COO)6] (SBU) dissolved in 10 mL of DMF. Themolar concentration for both components in the startingsolutions was 3.33 × 10−3 mol/L. Two milliliters of eachsolution was precleaned by passing it through a 1.2 μm filter toremove any dust particles while successively injecting it into thescattering cell. Addition of SBU solution as the secondcomponent set time zero for the reaction. Five minutes aftermixing BDC and SBU, 20 μL of the third component, 4n-decylbenzoic acid (DBA) dissolved in DMF at a concentrationof 0.667 mol/L, was added. The experiments with a SBU tolinker ratio of 1:3 were carried out with a procedure similar tothe route described for the 1:1 ratio. In the case of the 1:3 ratio,a precleaned solution of 0.005 mmol of SBU solvent in 3 mL ofDMF was added to 3 mL of the precleaned BDC solution(0.015 mmol of BDC in 3 mL of DMF). Again, the addition ofSBU to the BDC solution set time zero for the reaction. Themolar concentrations for the components in the final solutionswere 1.6710−3 mol/L (SBU) and 5 × 10−3 mol/L (BDC),respectively. SANS experiments were performed at SBU/BDC/

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DBA ratios of 1:1:2, 1:1:5, and 1:1:10. Preparation of thesamples for the SANS experiments has been made accordingly,except for the fact that each sample was investigated in H7-DMF and in mixed H7-DMF/D7-DMF. Use of two solventsdiffering in their degree of deuteration enabled us to tune thescattering contrast of MOF-5. An overview on all SANSsamples is given in Table 1.Time-Resolved Static Light Scattering (TR-SLS). TR-

SLS was performed with a home-built multiangle goniometerdescribed by Becker and Schmidt.22 Cylindrical quartz glasscuvettes with a diameter of 20 mm from Hellma (Mulheim,FRG) served as scattering cells. The goniometer was equippedwith a He−Ne laser operating at a wavelength of 632.8 nm. Itenabled simultaneous recording of the scattering intensity at 2times 19 scattering angles arranged symmetrically on both sidesof the laser beam. The angular regime covered a range of 25.84°≤ θ ≤ 143.13°. Recording of an angular dependent curve wascompleted after 2 ms. 1000 successive recordings were added toform one measurement requiring 2 s in total. The time intervalbetween the start of two successive measurements was 10 s. Allexperiments were performed at room temperature.Scattering curves were processed by means of the Rayleigh

ratio ΔRθ of the MOF-5 particles, which was calculated fromthe difference between the Rayleigh ratio of the particledispersion and of the pure solvent at variable scattering anglesθ. It is composed of three factors.

Δ = · ·θR KcM P q S q( ) ( )w (1)

In eq 1, c is the concentration of monomeric units given bythe formula [Zn4O(BDC)3] in g/L. In the case of thenonstoichiometric ratio, the concentration of MOF-5 wascalculated with respect to BDC, the component used insubstoichiometric amounts. Mw is the weight averaged molarparticle mass, P(q) is the z-averaged form factor of the growingparticles, and S(q) describes the influence of interparticleinterferences. The impact of S(q) denoted as the structurefactor in eq 1 increases with the concentration c. However,application of concentrations as low as 3.33 × 10−3 mol/Lenabled us to set S(q) ≅ 1.The contrast factor K in eq 1

= πλ

⎜ ⎟⎛⎝

⎞⎠K

nN

nc

4 dd

2 2

A4

2

(2)

and the momentum transfer q

= πλ

θ⎜ ⎟⎛⎝

⎞⎠q

n4sin

2 (3)

were calculated with the Avogadro number NA, with the laserwavelength λ = 632.8 nm, with the refractive index n = 1.43 ofpure DMF (20 °C), and with dn/dc the refractive index

increment of MOF-5 in DMF. The lack of a dn/dc value forcedus to apply a default value of dn/dc = 0.1 mL/g. This stillreproduces the correct order of magnitude of mass values andgives correct relative values.Scattering curves were evaluated according to the Zimm

approximation23

Δ= + +

θ

⎛

⎝⎜⎜

⎞

⎠⎟⎟

KcR M

Rq A c

11

3w

g2

22

(4)

with A2 ≅ 0 corresponding to S(q) ≅ 1 in eq 1.An example for a run with a molar ratio of SBU/BDC/DBA

of 1:1:0 is given in Figure 1. RelativeMw values can be extracted

from the intercept of eq 4, and values for the z-averagedsquared radius of gyration Rg

2 are accessible from the slope ineq 4.

Dynamic Light Scattering (DLS). Time-resolved DLSexperiments were performed using a model 5000E compactgoniometer system from ALV-Laser Vertriebsgesellschaft(Langen, Germany), equipped with a 200 mW Nd:YAG laseroperating at a wavelength of λ = 532 nm. The concentrations ofthe reagents for SLS and DLS experiments were identical.Correlation functions were detected in single channel

correlation mode during 30 s. The cumulant method24 withlinear and quadratic terms of the correlation time τ was used forevaluation.

τ = − Γτ +μ

τg Aln( ( )) ln212 2

(5)

In eq 5 ln(g1(τ)) is the logarithm of the electric fieldcorrelation function with the z-averaged decay constant Γ, A is

Table 1. Model Independent Parameters Extracted from the SANS Experiments Performed at Variable Contrast and at VariableDBA Content

solvent composition 66% H7-DMF/34% D7-DMF 100% H7-DMF

ratio SBU/BDC/DBA 1:1:2 1:1:5 1:1:10 1:1:2 1:1:5 1:1:10age of sample, h 17.28 21.17 15 5.88 7.13 9.64(dΣ/dΩ)q=0, cm−1 1.83 2.93 1.56 4.74 4.72 4.75Rg, nm 26.4 28.8 26.0 24.6 23.2 27.0invariant Q, nm−3 cm−1 0.00454 0.00600 0.00372 0.01724 0.01846 0.01161(dΣ/dΩ)q=0/Q ∼ Vp 403 488 419 275 256 409105Q2/(dΣ/dΩ)q=0 ∼ N 1.13 1.23 0.89 6.27 7.22 2.84

Figure 1. Representative example of a scattering curve evaluatedaccording to eq 4. The experimental curve was detected at t = 17504 sfor a run with the molar ratio of SBU/BDC/DBA equal to 1:1:0. Datapoints plotted with hollow cubes were omitted for data evaluation.

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the amplitude, and μ2 is the second cumulant providinginformation on the polydispersity of the sample. The z-averaged translational diffusion coefficient Dz was calculatedaccording to eq 6

= ΓD qz2

(6)

Time-resolved DLS experiments forced us to fix thescattering angle at 30°. At this relatively small angle, theextracted diffusion coefficient Dz,θ=30° is expected to be close tothe true value of the diffusion coefficient Dz,θ=0° at θ = 0°, sincethe averaged particle radii remained well below 100 nm.According to the Stokes−Einstein relation, Dz can be

transformed into a hydrodynamically effective radius Rhaccording to

=πη

Rk T

D6hz

B(7)

with kB the Boltzmann constant, T the absolute temperature,and η the viscosity of the solvent in mPa·s.Small Angle Neutron Scattering (SANS). The SANS

experiments were performed at the Institut Laue-Langevin(ILL), Grenoble, France, using the instrument D11. Allexperiments were carried out at a sample-to-detector distanceof 1.2, 8, or 28 m using a neutron wavelength of 6 Å. Thisresults in a range of the scattering vector q of 0.00254 Å−1 ≤ q≤ 0.51836 Å−1. The SANS intensity signals were radiallyaveraged and normalized by using water as a secondarycalibration standard.Radially averaged data Ix were corrected for sample thickness

and transmission via25

ΣΩ

=

− − −

− − −

×− τ

− τ

− τ− τ

− τ

− τ

+ΣΩ

+

+

⎜ ⎟⎛⎝

⎞⎠

( )

I I I I

I I I I

T n

T n d

dd

( ) ( )

( ) ( )

(1 )

(1 )

S

ST nT n

T n

T n

Cd(1 )

(1 ) EC Cd

H O Cd(1 )

(1 ) EC Cd

H O EC H Odd H O

EC EC

S SEC

EC EC

2H2O EC H2O

EC EC

2 22

(8)

leading to scattering curves in units of absolute scattering crosssection. The indices in eq 8 denote standard (H2O), empty cell(EC), cadmium (Cd), and sample (S), corresponding tosolvent or solution, respectively. The cadmium measurementprovides the electronic background and d is the samplethickness. Cells from Hellma (type 404-QS) with d = 1 mmwere used for all measurements. (dΣ/dΩ)H2O = 0.983 cm−1 isthe differential scattering cross section for water at λ = 6 Å. Theterm (1 − nτ) takes into account the correction for dead timelosses, with n representing the integral count rate of eachmeasurement and τ the dead time. The transmission Tx of anysample x was determined by dividing the measured intensity ofthe beam at q = 0 attenuated by any sample (Ix) by themeasured intensity of the incident beam (Ii):

===

TI qI q

( 0)( 0)x

x

i (9)

The excess signal from the MOF-5 particles was isolated bysubtracting the average of the angular independent signal (dΣ/

dΩ)1.2m in the range 0.06177 Å−1 ≤ q ≤ 0.2464 Å−1 (measuredat 1.2 m) from the solution data (dΣ/dΩ)solution. The value of(dΣ/dΩ)1.2m included the scattering signal from nonreactedSBU and linkers as well as from solvent:

ΣΩ

= ΣΩ

− ΣΩ

⎜ ⎟ ⎜ ⎟ ⎜ ⎟⎛⎝

⎞⎠

⎛⎝

⎞⎠

⎛⎝

⎞⎠

dd

dd

ddMOF solution 1.2m (10)

A model independent analysis of the scattering curvesincluded a Guinier approximation26 and the evaluation of theinvariant.27 The scattering curves were approximated byquadratic Guinier fits according to eq 11

ΣΩ

= ΣΩ

+ +=

⎜ ⎟ ⎜ ⎟⎛⎝⎜⎛⎝

⎞⎠

⎞⎠⎟

⎛⎝⎜⎜⎛⎝

⎞⎠

⎞⎠⎟⎟

Rq K qln

dd

lndd 3qMOF 0

g2

20

4

(11)

with (dΣ/dΩ)q=0 the extrapolated intensity at q = 0, Rg2 the

squared z-averaged radius of gyration, and K0 the coefficient ofthe q4-term. The invariant Q was calculated as the area underthe curve 4πq2(dΣ/dΩ)MOF vs q according to

∫= π ΣΩ

⎜ ⎟⎛⎝

⎞⎠Q q q4

dd

dMOF

2(12)

The parameter Q corresponds to the concentration of theMOF-5 particles. The missing initial part of the scattering curvewas approximated by a triangle (see Figure 2).

Deuterated BDC (D4-BDC) and hydrogenated DBA wereused for all SANS experiments. The final concentrations ofBDC and SBU were 1.67 × 10−3 mol/L, corresponding to theconditions identified by SLS and DLS as the most suitable onesfor SANS. Two different types of experiments, each with threedifferent final concentrations of DBA, corresponding to 2-fold,5-fold, or 10-fold excess compared to BDC, were conducted.An H7/D7 solvent mixture was used for the first type ofexperiments, designed to match as closely as possible thescattering contrast of the MOF-5 particles (core is matched). Arationale for the corresponding composition (66% H7-DMFand 34% D7-DMF) is given in the Supporting Information. Forthe second type of experiments, 100% H7-DMF as a solventwas used, which predominantly matches the contrast of thehydrogenated coordination modulator (shell is matched).

Figure 2. Kratky curve of the experiment with SBU/BDC/DBA molarratio 1:1:2 in a H7-DMF(66%)/D7-DMF(34%) solvent mixture. Inset:quadratic Guinier evaluation of the same experiment. The exampleillustrates the calculation of the two model independent parameters Qand Rg

2.26,27

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Transmission Electron Microscopy (TEM). Typically,0.25 mL of a DMF solution of the SBU-like material[Zn4O(C6H5COO)6] (0.005 mmol) was transferred to 2.75mL of a DMF solution of H2BDC (0.005 mmol). Five minutesafter mixing BDC and SBU, the capping agent was added.Afterward, one drop of the reaction mixture was directlytransferred to a TEM grid. After the evaporation of the solvent,one additional drop was placed on the TEM grid. Thisprocedure was repeated 4 times. The preparation was carriedout under inert conditions. All TEM measurements wereperformed on a Philips CM20 equipped with a LaB6 gunoperating at 200 kV at low dose conditions and long exposuretimes.28

Theoretical Form Factor Calculations. Theoretical formfactors of polydisperse cubes P(q) were calculated according toeq 13

∫

∫ =P q

SZ V VP q V

SZ V V V( )

( ) ( ) d

( ) d

VV

V0

0

max

max(13)

where PV(q)

∫ ∫=π

α βα β

×α β

α βα

αα α β

π π ⎡⎣⎢

⎤⎦⎥⎥

P qqa

qa

qaqa

qaqa

( )2 sin( sin cos )

sin cos

sin( sin sin )sin sin

sin( cos )cos

sin d d

VV

V

V

V

V

V

0

/2

0

/2

2

(14)

is the form factor of a cube29 with the volume V and the edgehalf length aV. SZ(V) is the Schultz−Zimm distribution of thecube volume V given as the weight fraction30

=Γ +

+ −⎛⎝⎜

⎞⎠⎟SZ V

zV

Vz

( )e

( 1)n

zz

zV V1 / n

(15)

with Vn the number averaged volume, Γ(z+1) the gammafunction, and z the characteristic polydispersity parameter. Atgiven parameters Vn and z, an upper limiting value Vmax wasapplied in eq 13 such that at least 99.99% of the distribution iscovered by this limit. The parameter z is related to thepolydispersity index DPI according to z = 1/(DPI − 1). Thepolydispersity index DPI is defined as the ratio of the weight-averaged particle volume Vw and the number averaged particlevolume Vn

=VV

DPI w

n (16)

■ RESULTS AND DISCUSSIONUsually, detection of a single scattering curve of dilute systemswith SANS requires minutes or hours. Hence, a sampleinvestigated with SANS must be stable, or at least changeinsignificantly over the measuring time. In the present work,SLS and DLS experiments were carried out to identify suitableconditions for SANS experiments. For this purpose, dilutedsuspensions of MOF-5 nanoparticles were produced by thereaction of BDC and SBU. The concentration of SBU was setto 1.67 × 10−3 mol/L. Proper evaluation of a potential impactof the modulator DBA on the process requires referenceexperiments on pure DBA in DMF. Such reference experimentsare necessary since pure DBA may already self-assemble in

DMF, leading to a significant scattering pattern from DBAaggregates, which overlay with the scattering signal from MOF-5 nanoparticles. Fortunately, corresponding reference experi-ments both with light and neutrons revealed no significantcontribution from DBA below q ∼ 2 nm−1. Details are outlinedin the Supporting Information. Hence, distortion of thesuccessively discussed scattering signals by independent DBAaggregates can be ruled out.In a first set of light scattering experiments, the molar ratio of

the two components SBU/BDC = 1:3 was investigated in theabsence of DBA. A fast growth of particles leading to theformation of a bidisperse system with a rapidly growing fractionof larger particles with Rg up to 1 μm was observed. Theaddition of DBA up to a ratio of 1:3:6 did not significantly slowdown this process. All experiments of this set turned out to bepoorly reproducible and, hence, not to be suitable for anyfurther scattering analysis.Successively, the growth of MOF-5 particles was followed at

a decreased SBU/BDC ratio of 1:1, with the concentration ofSBU being kept the same as in the case of the experiments atthe ratio 1:3. Additionally, experiments without and with DBAat a 2-fold excess compared to BDC were performed. In bothcases, the growth process could successfully be slowed down.Results of the measurements are given in Figures 3 and 4.

From each SLS curve a weight-averaged molecular mass Mw

and a z-averaged squared radius of gyration Rg2 was extracted.

DLS data resulted in an apparent z-averaged hydrodynamicradius Rh, recorded at an angle of 30°. During the initial phase,the mass was 3 times smaller for the experiment carried outwith DBA. Furthermore, the growth remained well below thetrend observed in the absence of DBA, thus indicating that theselected additive modulated the particle formation process.While mass values increased during the first 10 h, the radius ofgyration Rg, as well as the hydrodynamically effective radius Rh,decreased drastically during the first 2 h of the experimentsfollowed by a slight increase during the next 5−7 h if DBA waspresent. Similar plateau values were also reached in the absenceof DBA, albeit much earlier and without showing a minimum,again indicating a modulating effect of DBA. Finally, a constantparticle size was approached after 10 h.

Figure 3. Results of TR-SLS measurements. The symbols representexperiments with a molar ratio SBU/BDC/DBA equal to 1:1:0 (■),i.e. without modulator, and experiments with a molar ratio SBU/BDC/DBA equal to 1:1:2 (□).

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Apparently, a fast process which cannot be resolved by ourexperiments generates large particles during the first minute.Further insight in this initial stage can be expected from a time-resolved SAXS experiment on the process induced with astopped-flow device. The successive decrease of the size mayresult from a densification or from a partial redissolution oflarge particles or simply due to a lowering of the average massvalue by the generation of a large amount of smaller particles.This decrease is followed by a period where the particle sizehardly changed anymore. While in the presence of DBA thefinal plateau value was approached after passing a shallowminimum, this plateau value was approached immediately afterthe 2 h of decrease in the absence of the modulator. Thisindicates an ongoing nucleation of particles, which grow fastand approach a final size close to Rg ≅ 35 nm. A similarbehavior, where particle formation proceeds via an increase ofthe number of particles with a fixed final size, has beenobserved for ZIF-8.13 This pattern had been inferred from thefact that such a formation hardly affects z-averages like Rg

2 or Rhbut still causes an increase of the weight averaged particle massMw . This is due to the fact that z-averages are higher momentsthan weight averages and therefore approach limiting valuesprior to the weight averages.13,20,31

As seen from SLS measurements performed in the presenceof DBA, the apparent mass evolves more slowly compared tothe experiment without additive, whereas the final particle sizeremains unaffected. This suggests that DBA predominantlymodifies the nucleation step. It is an open question in thiscontext whether DBA adheres to the growing nanoparticles,thereby forming a core−shell structure. The excess of the SBU-like material should result in particles which are capped byZn4O units; thus, DBA should favorably interact with thesespecies. In order to support this hypothesis, we applied acontrast variation in SANS, which may enable revelation of theinner structure of these nanosized objects.These were carried out in two types of SANS measurements

(summarized in Table 1). The first type was performed in asolvent mixture (66 mol % H7-DMF, 34 mol % D7-DMF),which is supposed to match the MOF-5 core. The second typewas carried out in pure H7-DMF, which is expected to match apotential shell based on the modifying agent DBA with its alkylchains. Three different modifying agent concentrations,corresponding to a 2-fold, 5-fold, or 10-fold excess comparedto the BDC, were performed at each scattering contrast. Allsamples were measured within a time interval of 5−22 h

established by SLS as suitable because no significant change insize was observed anymore beyond 5 h. The results of theexperiments are given in Figures 5 and 6 and in Table 1. Both

figures clearly indicate a significant decrease of the scatteringintensity in the H7-DMF/D7-DMF mixture in accordance withthe anticipated matching of the MOF-5 contrast. However, aswill be outlined below, this difference does not suffice tounambiguously prove the existence of a DBA shell.A model independent analysis of the scattering curves already

leads to three highly valuable parameters: The z-averaged meansquare radius of gyration Rg

2 and the intercept of the scatteringcurve (dΣ/dΩ)q=0 ∼ Mw at q = 0 established via eq 11 and theinvariant Q established via eq 12. The invariant Q isproportional to the concentration in grams per liter of theforming MOF-5 particles, which is not known a-priori.Knowledge of Q gives access to two further characteristicquantities. The ratio (dΣ/dΩ)q=0/Q ∼ Vp is proportional to theweight averaged particle mass or particle volume, and Q2/(dΣ/dΩ)q=0 ∼ N represents a relative measure for the numberdensity N of MOF-5 particles in solution.27 All relevantparameters are summarized in Table 1.

Figure 4. Hydrodynamically effective radius from DLS at θ = 30° forexperiments with (□) and without (■) modulator performed with thesame two samples as used in Figure 3.

Figure 5. SANS curves at different DBA concentrations. Curvesplotted with hollow symbols represent experiments in H7-DMF-(66%)/D7-DMF(34%) (lowered contrast of MOF-5 cores), andcurves plotted with filled symbols represent experiments in pure H7-DMF (lowered contrast of DBA shells). The symbols denote thefollowing SBU/BDC/DBA ratios: (□, ■) 1:1:2; (○, ●) 1:1:5, (△, ▲)1:1:10. The numbers are multiples of the BDC concentration of 1.67× 10−3 mol/L.

Figure 6. Kratky representation of SANS curves given in Figure 5. Thesymbols denote the following SBU/BDC/DBA ratios: (□, ■) 1:1:2;(○, ●) 1:1:5; (△, ▲) 1:1:10. The numbers are multiples of the DBCconcentration of 1.67 × 10−3 mol/L.

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The size of the particles obtained from the Guinierevaluation was within the narrow regime of 23 nm < Rg < 29nm for all experiments and does not significantly vary duringthe time window of 5 h < t < 22 h. This may indicate that theparticles stop to grow at a similar size in all experiments. Theintercepts of the scattering curves (d∑/dΩ)q=0 at q = 0 are1.6−3 times larger for shell matched experiments than for therespective core matched experiments. The same tendency isobserved for the invariant Q, whose values are 3.0−3.8 timeslarger for shell matched experiments. Under the assumption ofsimilar particle size distributions within each pair of experi-ments performed in the two different scattering contrasts, thisdecrease indeed indicates that the scattering contrast in 66%H7-DMF/34% D7-DMF is weaker than that in pure H7-DMF.However, the decrease may be too small and does not enableunambiguous deduction of the formation of a thin layer ofDBA. A shell of 1 nm layer thickness would lead to a factor of10 rather than 3, to give but an example. Also, we have to keepin mind that the decrease observed with the presentexperiments does not prove entire matching of the scatteringcontrast of MOF-5 cores in the H7-DMF/D7-DMF-mixture.Our theoretical estimation of the match point may only be apoor estimation due to the uncertainty in the established partialspecific volume of MOF-5 particles in DMF and/or due to avariation of the H7-DMF/D7-DMF composition of the solventmixture in the pores of MOF-5 compared to the solventmixture in the bulk phase.Despite the fact that the issue of a DBA shell has to remain

unsettled, the present SANS data recorded at two differentscattering contrasts bear significant insight into the nanoparticleformation and morphology. First and foremost, the parametersoutlined in Table 1 do not seem to correlate with the amountof modifying agent. However, such a correlation may beoverlaid by a variation of those parameters with the age of thesample, which was identical neither in the three experiments in66% H7-DMF/34% D7-DMF nor in the three experiments inpure H7-DMF. In fact, the invariant Q and with it the numberdensity of particles N increases with the age of the sampleexcept for the experiment 1:1:10 in 66% H7-DMF/34% D7-DMF. More importantly, the size and the particle volumeessentially remained constant. This indicates an ongoingnucleation which generates particles approaching their finaldimensions extremely fast. Most likely, the modulator does notinterfere with the growth of individual particles but ratherinfluences the nucleation, in line with the hypothesis inferredalready from the time-resolved light scattering results.Further information on particle morphology can be expected

from a discussion of the overall trends of the scattering curves.A suitable theoretical model to be compared with the presentexperiments is a polydisperse cube, which corresponds to alimiting case of the model of rectangular parallelepipedsdeveloped by Mittelbach and Porod.29 In Figure 7 experimentalform factors of the 1:1 ratio at three different modifierconcentrations in pure H7-DMF (Figure 7A) and mixed H7-DMF/D7-DMF (Figure 7B) are compared with theoreticalform factors of polydisperse cubes at polydispersity indices DPIof 1.2, 1.5, 2, 3, and 10. The form factors are plotted asnormalized Kratky plots u2·P(u) vs u with u = q·Rg, where

=∑ Ω∑ Ω =

P u( )(d /d )(d /d )q

MOF

0 (17)

For compact structures such as spheres, cubes, or highlybranched polymers, Kratky-plots show a characteristic peaklocated close to q ∼ 1/Rg .

32

Several features become transparent. Once normalized, allthree curves fall on top of each other, indicating that theamount of modifying agent has no significant impact on theparticle shape. Also, there is no significant difference betweenthe scattering curves recorded in H7-DMF (66%)/D7-DMF(34%) and the curves recorded in pure H7-DMF. Both series ofexperiments exhibit a fair agreement with the model formfactors of polydisperse cubes, showing the characteristicmaximum. This also supports the finding that the experimentsperformed in the H7-DMF (66%)/D7-DMF (34%) mixture didnot lead to entirely matched MOF-5 particles with only shellsof modulator molecules to be seen. Finally, the experimentallyobserved Kratky peaks seem to be slightly larger than thosepredicted by polydisperse cubes. Even a drastic increase of thepolydispersity does not entirely account for this smalldiscrepancy. A more detailed and quantitative analysis of theform factors is in progress and will be presented in aforthcoming contribution.Besides the characterization by means of light scattering and

neutron scattering techniques, careful TEM measurementswere carried out in order to prove the presence of phase pureMOF-5 material.TEM measurements revealed very strong faceting with strict

rectangular angles (Figure 8a) and a size between 50 and 100nm for the vast majority of the MOF-5 crystals. Rarely, largercrystals with a size up to 150 nm were found. The shape of thecrystals in projection was usually squared or close to squared.The acquired electron diffraction pattern of a single MOF-5crystal in the [001] zone axis orientation (Figure 8b) identifiesthe material as crystalline MOF-5. The crystals were unusuallystable to the electron beam, allowing the observation ofdiffraction spots up to approximately 1 min. Usually, MOF-5reflections disappear within several seconds after exposure tothe electron beam. The electron diffraction pattern of many

Figure 7. Graph A indicates experiments in pure H7-DMF (loweredcontrast of DBA modulator), and graph B represents experiments inH7-DMF (66%)/D7-DMF (34%) (lowered contrast of MOF-5).Experimental curves are compared with theoretical model curves ofpolydisperse cubes.29 The polydispersity index DPI was varied asfollows: 1.2, 1.5, 2, 3, and 10, whereby the values of the curves increasewith the DPI. Curves are plotted as normalized Kratky plots u2P(u) vsu with u = qRg.

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MOF-5 crystals (Figure 8c) shows only reflections that aretypical for a ⟨001⟩-like orientation of the crystals; otherreflections are completely absent. This indicates that thecrystals are lying on {001} facets.These results confirm the formation of a cubelike

morphology for the MOF-5 crystals suggested by the SANSexperiments and prove the formation of phase pure MOF-5material. It has to be mentioned that, during the preparation ofthe TEM samples, changes in the concentration occur.Therefore, a direct comparison of the obtained results is onlypossible to a certain extent.

■ SUMMARY

MOF-5 nanoparticles were prepared at room temperature bymixing a solution of a linker with a solution of a molecularprecursor providing the secondary building unit. The processwas realized by using [Zn4O(C6H5COO)6], which mimics thesecondary building unit, together with BDC, which acts aslinker. The formation process was analyzed by means of time-resolved static and dynamic light scattering and small angleneutron scattering. A crucial aspect of this analysis was theattempt to modulate the formation process with 4n-decylbenzoic acid.

Initial light scattering revealed that a variation of thestoichiometric ratio of the SBU and linker BDC is an efficienttool to control the formation process. A decrease of the amountof linker from the stoichiometric ratio of SBU/BDC = 1:3 tothe ratio SBU/BDC = 1:1 corresponding to an excess of theSBU caused a significant deceleration of the process. Aninstantaneous formation of particles with a radius of gyration of80 nm, followed by a comparatively fast decrease in size,eventually led to particles with an averaged radius of gyration of40 nm. It is noteworthy that the scattering signal and with it theaveraged particle mass increased throughout all this time; albeit,this increase slowed down in the course of the process. Suchtrends of data from scattering experiments (decrease of size orits leveling off while the mass is still increasing) are unexpectedonly at first sight. They can be nicely reconciled with thefollowing particle formation mechanism. Mixing of the twocomponent solutions instantaneously initiates polymerization.The growth of individual particles is fast with respect to thetime of the entire experiments. During the course of formation,the fast growth of individual particles is accompanied by anongoing nucleation, which persistently generates new nano-particles, all having the same final size of 30 nm < Rg < 40 nm.Addition of 4n-decylbenzoic acid to the solution with a ratio

of SBU/BDC = 1:1 weakened the formation of the initial large-sized particles and slowed down the formation process further,as reflected by the evolution of the apparent particle mass.However, no effect could be discerned on the average size ofthe final particles (light scattering) nor on its morphology(SANS). Since the modulator does not modify the nature of thefinal particles, the observed moderation is likely to be due to aninterference with the nucleation of new particles.On the basis of the light scattering results, a time window

between 5 h < t < 20 h was selected to analyze the intermediatestages of the particle formation by means of small angleneutron scattering. To this end, a sample with an SBU/BDCratio of 1:1 was investigated at three different contents of themodulator. Two different scattering contrasts had been appliedfor the MOF-5 particles. The SANS experiments confirmed theformation of stable nanoparticles with a final average size of Rg≈ 30 nm. The H7-DMF/D7-DMF solvent mixture designed tomatch the scattering contrast of MOF-5 indeed exhibited aconsiderably lower scattering signal of the nanoparticles.However, proof of the existence of a layer of modulatorenveloping the MOF-5 nanoparticles could not be given withthe present contrast varying SANS experiment. IndependentTEM studies combined with electron diffraction measurementsof the as-synthesized nanoparticles supported formation ofnanosized cubic MOF-5 crystallites.As already inferred from light scattering results, SANS

indicated that the amount of modulator had no impact on thesize and shape of the resulting particles. Only the age of thesamples achieved during the SANS analysis slightly influencedthe intensity, which increased the older the samples became.The latter again supports the hypothesis of a formationmechanism where an ongoing generation of new nanoparticlesall growing rapidly and all approaching a constant final sizeextends over several hours. A first analysis of the scatteringcurves revealed close similarity to the curves calculated fornanoscopic compact cubes.Similar in situ scattering experiments, where particle

formation had been induced by an extended period of heatingof a solution of Zn(NO3)2 and linker in diethylformamide, ledto an entirely different particle formation pattern.16 In this

Figure 8. (a) BF-TEM of nanosized MOF-5 crystals showing theirstrong rectangular faceting, indicating a cubed morphology. (b)Electron diffraction pattern of a single MOF-5 crystal in its [001] zoneaxis orientation, evidencing the crystallinity and the composition of theframework. (c) Electron diffraction pattern of many MOF crystals atonce. All reflections are typical for a ⟨001⟩ type orientation of thecrystals, indicating that they are all in the [001] zone axis orientationlying on {001} facets.

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solvothermally pretreated solution, particle growth wasextremely slow, taking an extended period of time of up toan hour, and thus differs significantly from the process underpresent consideration. Growth of individual particles in thepresent case is much shorter and not discernible and in additionis overlaid by continuous nucleation. Apparently, theinstantaneous mixing of preformed secondary building unitsprovides monomers “ready-to-react” and generates a largedegree of supersaturation, which causes persistent nucleationand growth of finitely sized particles. The finite size can easilybe explained by the excess of SBU, which eventually leads toparticles covered with SBU on the outermost layer, makingthem inert for further polymerization due to the lack of linkeras comonomers. Contrary to this, the monomers or SBU firsthave to be gradually formed from solvothermally pretreatedZn(NO3)2 solution, which may lead to a much lower content ofpolymerizable material and, hence, to a much lower number ofnuclei. This low number of nuclei successively incorporatenewly formed SBU and thus grow to much larger particles.

■ ASSOCIATED CONTENT*S Supporting InformationReference experiments (SLS and SANS) on pure 4n-decylbencoic acid solutions in DMF; estimation of the solventcomposition in a mixture of H7-DMF/D7-DMF with a match-point for the neutron scattering contrast with MOF-5. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATIONCorresponding Author*E-mail: klaus.huber@upb.de.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSFinancial support by the DFG Priority Program 1362 “PorousMetal-Organic Frameworks” (FI502/21-1, HU807/12-1) andprovision of beam time at D11 by the ILL is gratefullyacknowledged. C.W. and D.Z. are grateful for support by theRuhr University Research School and by the Priority Program1362 “Metal Organic Frameworks” of the German ResearchFoundation (DFG).

■ REFERENCES(1) Yaghi, O. M.; O’Keeffe, M.; Ockwig, N. W.; Chae, H. K.;Eddaoudi, M.; Kim, J. Nature 2003, 423, 705−714.(2) Ferey, G. Chem. Soc. Rev. 2008, 37, 191−214.(3) Horcajada, P.; et al. Nat. Mater. 2010, 9, 172−178.(4) Horcajada, P.; Serre, C.; Vallet-Regi, M.; Sebban, M.; Taulelle, F.;Ferey, G. Angew. Chem., Int. Ed. 2006, 118, 6120−6124.(5) Horcajada, P.; Serre, C.; Maurin, G.; Ramsahye, N. A.; Balas, F.;Vallet-Regi, M.; Sebban, M.; Taulelle, F.; Ferey, G. J. Am. Chem. Soc.2008, 130, 6774−6780.(6) Hafizovic, J.; Bjorgen, M.; Olsbye, U.; Dietzel, P.-C.; Bordiga, S.;Prestipino, C.; Lamberti, C.; Lillerud, K. P. J. Am. Chem. Soc. 2007,129, 3612−3620.(7) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe,M.; Yaghi, O. M. Science 2002, 295, 469−472.(8) Ni, Z.; Masel, R. J. Am. Chem. Soc. 2006, 128, 12394−12396.(9) Jhung, S. H.; Lee, J.-H.; Yoon, J. W.; Serre, C.; Ferey, G.; Chang,J.-S. Adv. Mater. 2007, 19, 121−124.(10) Demessence, A.; Horcajada, P.; Serre, C.; Boissiere, C.; Grosso,D.; Sanchez, C.; Ferey, G. Chem. Commun. 2009, 7149−7151.

(11) Qiu, L.-G.; Li, Z.-Q.; Wu, Y.; Wang, W.; Xu, T.; Jiang, X. Chem.Commun. 2008, 3642−3644.(12) Huang, L.; Wang, H.; Chen, J.; Wang, Z.; Sun, J.; Zhao, D.; Yan,Y. Microporous Mesoporous Mater. 2003, 58, 105−114.(13) Cravillon, J.; Munzer, S.; Lohmeier, S.-J.; Feldhoff, A.; Huber,K.; Wiebcke, M. Chem. Mater. 2009, 21, 1410−1412.(14) Guillerm, V.; Gross, S.; Serre, C.; Devic, T.; Bauer, M.; Ferey, G.Chem. Commun. 2010, 46, 767−770.(15) Hausdorf, S.; Baitalow, F.; Bohle, T.; Rafaja, D.; Mertens, F. J.Am. Chem. Soc. 2010, 132, 10978−10981.(16) Hermes, S.; Witte, T.; Hikov, T.; Zacher, D.; Bahnmuller, S.;Langstein, G.; Huber, K.; Fischer, R. A. J. Am. Chem. Soc. 2007, 129,5324−5325.(17) Tsuruoka, T.; Furukawa, S.; Takashima, Y.; Yoshida, K.; Isoda,S.; Kitagawa, S. Angew. Chem., Int. Ed. 2009, 48, 4739−4743.(18) Cho, W.; Lee, H. J.; Oh, M. J. Am. Chem. Soc. 2008, 130,16946−16946.(19) Zacher, D.; Lin, J.; Huber, K.; Fischer, R. A. Chem. Commun.2009, 1031−1033.(20) Cravillon, J.; Schroeder, C. A.; Nayuk, R.; Gummel, J.; Huber,K.; Wiebcke, M. Angew. Chem., Int. Ed. 2011, 50, 1−6.(21) Clegg, W.; Harbron, D. R.; Homan, C. D.; Hunt, P. A.; Little, I.R.; Straughan, B. P. Inorg. Chim. Acta 1991, 186 (1), 51.(22) Becker, A.; Schmidt, M. Makromol. Chem. Macromol. Symp.1991, 50, 249−260.(23) Zimm, B. J. Chem. Phys. 1948, 16, 1093−1099.(24) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814−4820.(25) Lindner, P. In Neutrons, X-rays and Light: Scattering methodsapplied to soft condensed matter; Lindner, P., Zemb, T., Eds.; Elsevier:Amsterdam, 2002; Chapter 2.(26) Guinier, A.; Fournet, G. Small-Angle Scattering of X-Rays; Wiley:NewYork, 1955; Chapter 4.1.1.(27) Porod, G. Kolloid Z. 1951, 124, 83−114.(28) Turner, S.; Lebedev, O. I.; Schroder, F.; Esken, D.; Fischer, R.A.; Van Tendeloo, G. Chem. Mater. 2008, 20, 5622−5627.(29) Mittelbach, P.; Porod, G. Acta Phys. Austriaca 1961, 14, 185−211.(30) Schultz, G. V. Z. Phys. Chem. 1939, B43, 25−46.(31) Liu, J.; Pancera, S.; Boyko, V.; Shukla, A.; Narayanan, T.; Huber,K. Langmuir 2010, 26, 17405−17412.(32) Burchard, W. Adv. Polym. Sci. 1983, 48, 1−124.

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