ZIF-8 Heterometallic Nanoparticles: Control of Nanocrystal Size

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Co/ZIF-8 heterometallic nanoparticles: control of the nanocrystal size and properties by mixed-metal approach Jan K. Zar#ba, Marcin Nyk, and Marek Samoc Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01090 • Publication Date (Web): 26 Sep 2016 Downloaded from http://pubs.acs.org on September 30, 2016

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Crystal Growth & Design

Co/ZIF-8 heterometallic nanoparticles: control of the nanocrystal size and properties by mixed-metal approach Jan K. Zaręba,* Marcin Nyk and Marek Samoć Advanced Materials Engineering and Modelling Group, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland.

ABSTRACT

A mixed-metal approach has been used to control the size and physicochemical properties of heterometallic Co/ZIF-8 nanomaterials. Intentional substitution of zinc with cobalt in a broad concentration range (from 0 to 100 molar percent with 10 percent step) provided a series of Co/ZIF-8 nanoparticles, whose sizes could be tuned in the range from 20 to over 500 nm in diameter. Zinc ions from ZIF-8 matrix were found to be uniformly substituted with the cobalt ions. The increase of nanoparticles size resulted in change of their nitrogen sorption-desorption characteristics due to decreasing participation of the external surface area in the total surface area. Insights from UV-Vis-NIR and IR spectroscopies, as well as remarks on nonlinear optical properties are also provided.

ACS Paragon Plus Environment

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INTRODUCTION Zeolitic imidazolate frameworks (ZIFs), a subclass of metal organic frameworks (MOFs), are porous crystalline solids in which tetrahedral divalent metal ions (Zn2+, Co2+) are connected via coordination bonds to imidazole derivative.1, 2 These materials are topologically similar to typical inorganic zeolites, as the structural arrangement of metal ions and ditopic imidazole linker closely resembles that of aluminosilicate framework. Although in fact ZIFs and zeolites are members of inherently different groups of materials, they also share exceptional features like catalytic properties,3, 4 high thermal and chemical stability,2 and porous structure.5 It is probably the porosity of ZIFs which brings the greatest attention to this class of MOFs.6, 7 Indeed, this feature was investigated beginning from classical gas sorption, through gas separation on membranes8-11, ending up with entrapping of small molecules like dyes,12, 13 pharmaceuticals14-17 or even nanostructures18-20 within the pores. Since all the aforementioned functionalities can be tailored by the size of nanoparticles, it is important to investigate novel methods for shaping the growth of MOF nanocrystals in a controlled manner. The growth of ZIF nanomaterials can be controlled by a number of synthetic conditions, influencing kinetic and thermodynamic factors.21-23 The most commonly used approach relies on changing of the concentration and/or the ratio of substrates, in particular the metal : imidazole ligand : solvent proportion.24 Growth of nanocrystals can be readily controlled by shifting the deprotonation equilibrium of 2-methylimidazole with the use of additional bases such as alkylamines25-27 or weak acids27; ligand deprotonation is also dependent on the solvent used for synthesis. There are three main solvents used for synthesis ZIF nanoparticles which are known to


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be good media for room temperature nanoparticle synthesis: water, methanol, and dimethylformamide (DMF).24, 26, 28, 29 External additives can modify the kinetics of nanocrystal formation by capping the nanoparticles surface, such additives can be also of polymeric type.30 The size of ZIF nanocrystals was controlled with the use of the microemulsion method, in which each micelle serves as a nanoreactor for nanoparticle formation.31 The literature contains relatively few reports on heterometallic ZIFs and virtually none on the effect of metal dopant on ZIF nanoparticle growth. The present data concern either the postsynthetic SAME (Solvent-assisted metal exchange) process itself32, or focus on the catalytic properties of mixed-metal ZIF materials.3,

33-36

However, a closer examination of available

results provides a hint that the growth of nanoparticles can be influenced by partial substitution of one metal salt precursor by another. As an example, Schejn et. al. demonstrated ZIF-8 nanoparticles for catalytic applications, in which copper(II) was added to synthesis at up to 25% of that metal content.35 Although it was not central to their work, they have noticed that upon copper(II) doping the mean size of nanoparticles was increased. It is thus interesting if modification of nanoparticles’ size upon metal ratio variation, presented to date as a side effect, may indeed exist in the broad concentration range and be of practical utility. Following this hypothesis, we conducted an initial experiment in which we modified only zinc and cobalt(II) salt content in the substrates mixture, while keeping the rest of conditions unchanged. In this manner we prepared ZIF-8 nanoparticles, ZIF-67 nanoparticles, by replacing of all zinc nitrate with cobalt(II) nitrate, and ZIF-8 having substituted 60% of zinc centers with cobalt ones (60Co/ZIF-8). Indeed, upon such a replacement we noted 26-fold and 3-fold increase of nanoparticle TEM mean size for completely and partially substituted material, respectively. In consequence, ZIF-67 and 60Co/ZIF-8 nanocrystals afforded in this manner


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were found to have altered sorption-desorption characteristics and optical properties with respect to ZIF-8 nanoparticles (vide infra). With these results in hand, confirming a new possibility for design of size and properties of Co/ZIF-8 nanoparticles, we expanded our studies to the whole concentration range of cobalt(II) doping into ZIF-8 matrix (from 10% to 90%, with 10% step). As a result, in this contribution we present the effect of gradual cobalt(II) doping on the size, gas sorption and spectroscopic properties of Co/ZIF-8 nanomaterials.

EXPERIMENTAL Materials and synthetic procedures. Zinc nitrate hexahydrate (≥98%), cobalt(II) nitrate hexahydrate (≥98%) and 2-methylimidazole (99%) were purchased from Sigma-Aldrich. Methanol of analytical grade was obtained from POCh and used without further purification. Nanoparticles were prepared using modification of previously known procedure.24 The synthesis protocol for 50Co/ZIF-8 will be provided as an example. To 100 mL of methanol 1.620 g (1.97·10-2 mol) of 2-methylimidazole was added and stirred to dissolution. Separately, to 100 mL of methanol 0.3666 g (1.232·10-3 mol) of Zn(NO3)2·6H2O and 0.3583 g (1.232·10-3 mol) of Co(NO3)2·6H2O were added. Both solutions were mixed, immediately obtaining violet colour. The turbidity developed after a couple of minutes. The mixture was stirred at room temperature (294K) for 45 minutes. After that time, the mixture was transferred to tubes and centrifuged at 11,000 RPM for 15 minutes. Supernatants were discarded, solids were combined and redispersed in methanol (30 mL per each vial), sonicated, and centrifuged at 11,000 RPM for 15 minutes. The target materials were stored as methanol dispersion or dried to powder depending on the method of characterization.


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Methods. Powder X-ray diffraction (PXRD) patterns of the ZIF bulk samples were measured on a Rigaku Ultima IV X-ray diffractometer equipped with a Cu-Kα radiation source (λ=1.54182 Å) at 40 kV and 30 mA in the range 5 – 40 degrees. Energy dispersive X-ray spectroscopy (EDS) analysis on bulk samples was performed on coupled with SEM (Scanning electron microscope, JEOL JSM-6610LV) Oxford Aztec Energy detector at an acceleration voltage of 20 kV and a working distance of 10 mm. Morphology of nanoparticles studied with a FEI Tecnai G2 20 X-TWIN transmission electron microscope (TEM). Line-scan EDS analyses on single nanoparticles and groups of nanoparticles were conducted on the same TEM instrument with attached EDS probe, in the STEM (Scanning transmission electron microscope) measurement mode. UV–Vis–NIR extinction spectra were acquired using a Jasco V-670 spectrophotometer. Dynamic light scattering (DLS) experiment was conducted using 444.38 nm (28 mW) laser radiation, on a Photocor Complex instrument (Photocor Instruments, Estonia), the scattering angle was set at 90 degrees. Dispersions of Co/ZIF-8 materials in methanol were placed in 14.75 mm × 45 mm × 8 mm vials, submerged in decalin as a refractive index-matching liquid. Midinfrared and far-infrared spectra (MIR and FIR, respectively) were obtained on a VERTEX 70V FT-IR spectrometer (Bruker Optik GmbH, Ettlingen, Germany) in attenuated total reflection measurement mode. The sorption isotherms for nitrogen were measured with an automatic volumetric adsorption apparatus (Micrometrics ASAP 2010) at 77 K. The as-synthesized samples (weight 40 – 60 mg) were dried in air and subsequently dried in dessicator under vacuum over KOH. Next, samples were placed in the quartz tube and activated under high vacuum at 373 K for 2 h to remove methanol and other volatile compounds prior to measurements.


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RESULTS AND DISCUSSION

Synthesis and nanoparticle composition. Presented herein synthesis of Co/ZIF-8 materials is an adaptation and modification of Cravillon et al. method.24 In comparison to their protocol, we have increased the metal nitrate hexahydrate : 2-methylimidazole : methanol molar ratio to 1 : 8 : 2000 (from 1 : 8 : 700) and the reaction time was reduced to 45 minutes (from 1 hour). The contents of cobalt(II) in materials described in this work are expressed as molar percentages, and refer to the molar ratio of salts taken for synthesis: Co content = nCo salt/( nCo salt + nZn salt)·100% We have chosen Cravillon et al. approach as a basis for our investigations for several reasons: they have obtained ZIF-8 nanoparticles of small size polydispersity at room temperature using a simple precipitation method. No additional capping agent nor external base such as triethylamine is required. Moreover, the synthesis is conducted in methanol which is an economical and relatively safe solvent. From the point of view of gas sorption studies, methanol volatility and small kinetic diameter greatly simplifies activation of target material, which would be more complicated if DMF was used, as it could require additional solvent exchange step. In ZIF-8-type materials 2-methylimidazole acts not only as a linker but also as a capping agent.24 Therefore, two reactions can be proposed, for metal ions residing on the surface of nanoparticle (Scheme 1a) and for those inside the nanoparticle (Scheme 1b). Metal ions inside the nanoparticle are bound to four bicoordinating (bridging) 2-methylimidazole ligands, while metal ions on the surface are surrounded by three bridging 2-methylimidazole ligands and one


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monocoordinating (terminal) ligand. Part of 2-methylimidazole molecules serve as proton acceptors, because of the formation of nitrate salt.

Scheme 1. Reactions and coordination environments for zinc and cobalt(II) centers a) inside the heterometallic ZIF-8 b) residing on the surface of heterometallic ZIF-8. In reactions above the formulae C4H5N2-, C4H6N2, C4H7N2+ denote deprotonated, neutral, and protonated 2methylimidazole, respectively. In green are shown terminating 2-methylimidazole molecules. EDS measurements were conducted on two separate series of bulk Co/ZIF-8 samples to verify whether the molar content of cobalt in the obtained materials is the same as the molar ratio of metal salts used for synthesis (Figure S1 and Table S1, Supporting Information). By plotting the determined cobalt(II) molar content versus the content determined by synthetic conditions, a nearly linear correlation is obtained in the whole concentration range, as presented in Figure S2, Supporting Information. It should be noted that the determined content of cobalt(II) is systematically slightly higher than can be expected from calculations, especially in the middle range of cobalt(II) concentrations. We believe that this trend can be attributed to highly deliquescent properties of cobalt(II) and zinc nitrates hexahydrates, which real composition may contain some additional water, depending on the age and storage conditions of metal reactants. Having this obstacle in mind, we draw a conclusion that the content of cobalt(II) can be predefined at proposed herein synthetic conditions. Free substitution of zinc ions by cobalt ones can be a result of similar lengths of metal-nitrogen coordination bonds: for ZIF-8 and ZIF-67 the


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Zn-N and Co-N bond lengths are equal to 1.987(6)Å and 1.976(6) Å at 153 K, respectively.2 Another contributing factor to this can be a flexibility of ZIF-8 structure,37,

38

which can

additionally compensate the network strains induced by Zn-N and Co-N bond lengths difference. Line-scan EDS analysis proved to be a valuable tool for determining element distributions within nanostructures.39,

40

Investigation with this technique on agglomerates of 60Co/ZIF-8

nanoparticles (Figure S3, a) and b), Supporting Information) revealed that the cobalt to zinc ratio is approximately on the same level for separate nanoparticles. Line-scan EDS analysis of single 60Co/ZIF-8 nanoparticle (Figure S3, c), Supporting Information) strongly suggests that cobalt(II) ions are uniformly distributed within the ZIF-8 matrix.

Nanocrystal size distribution. X-ray powder diffraction study of bulk ZIF-8, ZIF-67 and Co/ZIF-8 nanoparticle powders shows presence of a single phase. Despite the increasing content of cobalt(II), no shift of diffraction peak maxima is observable under our experimental conditions. This is probably due to the small difference between unit cell parameters (a = 16.959(3) Å for ZIF-67 and a = 16.991(3) Å for ZIF-8).2 A comparison of PXRD spectra (Figure S4, Supporting Information) shows a dependence of nanocrystal size upon gradual cobalt(II) doping. The broadening of PXRD signals from (222) crystallographic plane served as a first probe of nanocrystal size, as estimated using the Scherrer’s equation (Table 1).41 The DLS analysis also confirmed the systematic increase of hydrodynamic diameter upon cobalt(II) doping (Figure S5, Supporting Information).

Table 1. A comparison of average diameters calculated from PXRD (dXRD) and mean diameters from the TEM images (dTEM) for ZIF-8, ZIF-67 and Co/ZIF-8 samples.


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Molar content of Co

dXRDa

dTEM

TEM PdIb

%

nm

nm

-

ZIF-8

0

18 ± 2

21 ± 4

0.190

10Co/ZIF-8

10

22 ± 2

24 ± 4

0.167

20Co/ZIF-8

20

24 ± 3

31 ± 6

0.193

30Co/ZIF-8

30

28 ± 3

33 ± 5

0.151

40Co/ZIF-8

40

29 ± 4

38 ± 7

0.184

50Co/ZIF-8

50

36 ± 4

48 ± 9

0.187

60Co/ZIF-8

60

45 ± 5

63 ± 13

0.206

70Co/ZIF-8

70

58 ± 7

100 ± 17

0.170

80Co/ZIF-8

80

82 ± 10

153 ± 31

0.203

90Co/ZIF-8

90

c

304 ± 59

0.194

ZIF-67

100

c

555 ± 142

0.256

Sample

a

uncertainty of dXRD calculated from total differential; b(PdI) polydispersity index defined as PdI=σ/m, in which d – dispersity, σ – standard deviation from mean value, m – mean size of nanoparticles; csize beyond the limitation of the Scherrer formula. Transmission electron microscopy (TEM) was used for detailed investigation of size, morphology and polydispersity of the obtained nanoparticles. Figure 1 shows representative TEM pictures for ZIF-8, ZIF-67 and Co/ZIF-8 nanoparticles doped with 20, 40, 60, and 80 percent of cobalt(II). In Figure S6, Supporting Information, TEM pictures of Co/ZIF-8 nanoparticles doped with 10, 30, 50, 70, and 90 percent of cobalt(II) are presented. The nanoparticles have the shape of rhombic dodecahedra. Upon deposition on the TEM grid they tend to align along the [001] direction, and for this reason mostly hexagonal shapes are observed.24


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Figure 1. Representative TEM pictures along with size distributions histograms for a) ZIF-8, b) 20Co/ZIF-8, c) 40Co/ZIF-8, d) 60Co/ZIF-8, e) 80Co/ZIF-8, f) ZIF-67. TEM mean nanoparticle sizes are compiled in Table 1, and a corresponding plot in the function of cobalt(II) concentration is presented in Figure 2. By fitting of biexponential function through the points, one obtains a purely phenomenological expression, which allows to calculate the expected mean size of Co/ZIF-8 for any given molar concentration of cobalt(II) taken for


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synthesis. Analysis of the given plot shows that the mean size of nanoparticles rises monotonically, however there is a noticeable acceleration of nanocrystal growth above 60% 70% of the cobalt(II) dopant concentration.

Figure 2. A plot of nanoparticle mean diameter in the function of molar percentage of cobalt(II) content. In the provided biexponential formula y stands for nanoparticle mean diameter expressed in nanometers, while x stands for percentage cobalt(II) molar content used for synthesis. The error bars denote the standard deviation of the mean nanoparticle size. In Figure S7, Supporting information, a plot in the semilog scale is provided which emphasizes the obtained dependency for lower concentration range of cobalt(II) dopant. These data suggest that cobalt(II) salt addition results in slowing down of nanocrystal nucleation process. Less nucleating centers, in turn, allows for growing of larger nanoparticles. While the TEM mean diameter visibly increases with doping, no tendency could be found for the polydispersity parameter (Table 1), which remains in the range 0.15 – 0.26. The results obtained by us need to be compared to previous work. Very recently, Wang et al. reported heterobimetallic Zn-Co ZIF-8 nanoparticles with concentration of cobalt(II) reaching up


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to 10%,42 but, contrary to our results, they have not observed any significant impact of cobalt(II) on the nanocrystal size. This seemingly contradictory observation probably results from the microemulsion synthetic method that has been applied by them. Most likely, the growth of ZIF nanoparticles was confined by the size of emulsion nanodroplets, and for this reason all nanoparticles had the same mean size, regardless of cobalt(II) doping level.

Gas sorption. Nitrogen sorption experiments at 77K were conducted to evaluate the trends of porosity change upon varying level of Co doping. As presented in Figure 3, nitrogen uptake for all samples abruptly reaches 400 – 500 cm3/g at relative pressure of 0.1 which is the proof of the microporous structure of obtained materials. For undoped material, ZIF-8, a significant hysteresis of the adsorption-desorption process is clearly noticeable. This shape of isotherm is indicative of capillary mechanism of adsorbate-adsorbent interaction (isotherm of type-IV), typical for nanomaterials.43 For such materials, at higher relative pressures (p/p0 > 0.7), multilayer physisorption of nitrogen takes place in the pores formed in-between nanoparticles.44 When moving to higher cobalt concentrations, and therefore, to larger nanoparticle sizes the hysteresis becomes gradually less pronounced and finally disappears for 80Co/ZIF8 and ZIF-67 materials. Increase of nanoparticles size results in lowering of their surface-to-volume ratio, and also in reducing of the available interparticle space. As a consequence of decreasing participation of meso- and macroporosity, the shape of the isotherm smoothly transits from type-IV to type-I. The latter type of isotherm is expected for purely microporous MOFs, that is, those whose porosity arises from filling of crystallographic cavities.45


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Figure 3. Nitrogen adsorption-desorption isotherms at 77K for ZIF-8, ZIF-67 and Co/ZIF-8 samples. There is no clear trend of BET (Brunauer–Emmett–Teller) surface area change with the increase of nanoparticles size, as these areas take values from 1300 to slightly above 1500 m2/g (Table 2), however, the nanosized nature of the materials is reflected in the share of external


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surface area. For undoped material meso- and macroporosity is responsible for 25% of all surface area, for 20Co/ZIF-8 this share falls to 11.8%, while for ZIF-67 this type of porosity is negligible (2.5%). This trend is of high relevance to catalysis applications (e.g. for Knoevenagel condensation), because larger external surface areas translate into enhanced catalytic properties, as demonstrated elsewhere.22 Surface areas of the investigated materials are smaller than that for ZIF-8 synthesized by Yaghi et al. (SBET: 1630 m2g-1, Vmicro 0.64 cm3g-1) in the single crystal form. The obtained values are within the range (960 – 1600 m2g-1) determined for other nanoscale ZIF-8 and ZIF-67 materials. 24, 30, 46-49

High surface areas can also point to high crystallinity and low amount of residual

species within the pores of cobalt(II)-doped ZIFs.

Table 2. A comparison of BET surface (SBET), external surface (Sext), microporous volume (Vmicro) for ZIF-8, ZIF-67 and Co/ZIF-8 nanoparticles. SBET

Sext

Vmicro

m2g-1

m2g-1

cm3g-1

ZIF-8

1410

359

0.546

20Co/ZIF-8

1331

157

0.613

40Co/ZIF-8

1513

130

0.721

60Co/ZIF-8

1319

108

0.636

80Co/ZIF-8

1308

49

0.661

ZIF-67

1341

33

0.687

Sample


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Optical properties. UV-Vis-NIR extinction spectra of ZIF-8, ZIF-67, and Co/ZIF-8 materials are presented in Figure 4. The scattering component dominates in all the spectra and increases with the size of nanoparticles, as expected from the Mie scattering theory.50

Figure 4. Extinction spectra of ZIF-8, ZIF-67 and Co/ZIF-8 nanoparticles in methanol dispersion. In the ZIF-8 spectrum, the absorption transition is located in the UV region51, and for this reason the extinction spectrum is virtually featureless. For cobalt-doped samples two absorption bands are present. The first, structured absorption band centered at 580 nm with clearly at least three components in the visible region, is due to d-d transition (4A2 → 4T1(P)) of the tetrahedral cobalt(II) ions. The second d-d band is found in the near-infrared region at 1125 nm from 4A2 → 4

T1(F) transition. Inspection of the former band shows decrease of intensity of ‘middle’

component (approximately at 568 nm) and slight shifting to longer wavelengths of the maximum of this transition with increasing concentration of cobalt(II) ions. A probable explanation for these observations is connected with the decrease of relative share of surface cobalt(II) ions with increasing size of nanoparticles. Cobalt(II) ions on the surface of ZIF nanoparticles, as depicted


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in Scheme 1, are coordinated by three bridging 2-methylimidazole molecules and one terminating, resulting in local C3v symmetry. Inner cobalt(II) ions, are in turn coordinated by four deprotonated bridging 2-methylimidazole molecules (symmetry Td). This subtle coordination difference may lead to the modification of the crystal field of cobalt(II) ions, and therefore to the change of relative intensity of d-d band components. Moreover, the coordination of surface cobalt(II) ions by solvent molecules cannot be excluded and may also have influence on the absorption band structure. Dispersions of Co/ZIF nanoparticles are much more stable in DMSO than in methanol; also scattering is lower for this solvent which indicates that the refractive index of ZIF-8 and related cobalt-doped materials is closer to that of DMSO (nD=1.477).52 It has been shown that MOFs can be prospective third-order nonlinear optical materials.53 Thus, one of our initial aims was to investigate whether ZIF-8 and Co/ZIF-8 nanomaterials also manifest two-photon absorption properties. In parallel to our investigations, Cleuvenbergen et al. demonstrated that ZIF-8 microcrystals reveal second harmonic generation (SHG) and twophoton excited fluorescence.54 However, while SHG properties were discussed in detail in their work, the extent of the latter process was not quantified in absolute values. In our attempts we used the femtosecond Z-scan technique on 1.1% DMSO dispersions. The measurement of ZIF-8 nanoparticles at 600 nm did not give any clearly discernible open aperture signal, which would indicate two-photon absorption. Similarly, measurements of 50Co/ZIF-8 nanoparticles at around 950 nm did not detect measureable nonlinear absorption. Basing on the scatter of Z-scan open aperture measurement points, we estimated that two-photon cross section is lower than 1.8·10−49 cm4s at 600 nm and lower than 8·10−50 cm4s at 950 nm, for ZIF-8 and 50Co/ZIF-8, respectively.


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This may result from lack of intense metal charge-transfer transitions as well as from no π-π interaction between ligands, which would enhance the polarizability of the system. The gradual doping of ZIF-8 nanoparticles with cobalt(II) ions was also monitored using farinfrared spectroscopy (450 – 100 cm-1), as presented in Figure 5. Increasing the content of Co results in a decrease of intensity of 292 cm-1 ν(Zn-N) band, while intensity of ν(Co-N) band at 313 cm-1 increases. In this manner the stretching vibrational modes Zn-N and Co-N can be unambiguously ascribed. Vibrational features in MIR spectra are not noticeably affected upon doping, except for a shoulder in a complex band at 1450 cm-1, which decreases in intensity. This band results from C=N stretching within imidazole ring (Figure S8, Supporting Information).

Figure 5. FIR spectra in the 450 – 100 cm-1 range presenting increase and decrease of intensity of ν(Co-N) and ν(Zn-N) vibrational modes upon various cobalt(II) doping level. CONCLUSIONS In our contribution we have presented a series of heterometallic Co/ZIF-8 nanomaterials, which were obtained by substituting a part of zinc precursor with the cobalt(II) one. An analysis of the results revealed two aspects of this design. Firstly, we found that the actual amount of


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cobalt(II) dopant in Co/ZIF-8 nanoparticles could be simply predefined by the reaction stoichiometry, owing to nearly linear correlation between determined and synthetic molar content of metal precursors. Secondly, the proposed herein doping strategy allowed to slow down the nucleation rate, and in consequence resulted in growing of bigger nanoparticles. By plotting TEM mean sizes in the function of percentage molar cobalt(II) content an expression was derived, which can be used as a guide for synthesis of nanoparticles of predefined size. Engineering of nanoparticle size was reflected in the properties such as gas sorption. While the surface areas for materials were unaffected in the range from 1300 - 1500 m2/g, the absorptiondesorption isotherms were smoothly changing from type-IV to type-I. The observed behavior is explained by decreasing participation of macroporosity. We believe that the presented “mixed-metal” or “cobalt(II)-doping” approach not only paves the way for shaping size and properties of mixed-metal ZIF nanoparticles, but can be also transferrable to other heterometallic nanoMOF materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: EDS analyses, PXRD spectra, DLS plots, TEM pictures of 10Co/ZIF-8, 30Co/ZIF-8, 50Co/ZIF-8, 70Co/ZIF-8, and 90Co/ZIF-8 nanoparticles along with size distribution histograms, and MIR spectra.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge financial support from the Polish National Science Centre under “Maestro” DEC-2013/10/A/ST4/00114 grant and the Faculty of Chemistry, Wrocław University of Science and Technology. We also gratefully acknowledge the instrumental grant 6221/IA/119/2012 from the Polish Ministry of Science and Higher Education, which supported our Integrated Laboratory of Research and Engineering of Advanced Materials where IR and EDS measurements were performed. We thank Dr. S. Ronka for nitrogen sorption measurements. We thank Dr. A. Kiersnowski for access to a PXRD instrument.

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For Table of Contents Use Only Title: Co/ZIF-8 heterometallic nanoparticles: control of the nanocrystal size and properties by mixed-metal approach Authors: Jan K. Zaręba,* Marcin Nyk, and Marek Samoć

A strategy of gradual cobalt(II) doping has been used to control the size and physicochemical properties of heterometallic Co/ZIF-8 nanomaterials. By substitution of zinc with cobalt(II) in a broad concentration range a series of Co/ZIF-8 nanoparticles has been obtained. Metal ratio variation allowed for tuning of nanoparticle size in the range from 20 to over 500 nm in diameter. The increase of nanoparticles size resulted in modification, amongst others, of their nitrogen sorption-desorption characteristics.


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ZIF-8 Heterometallic Nanoparticles: Control of Nanocrystal Size

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