Morphology Design of IRMOF-3 Crystal by Coordination Modulation

Sep 18, 2014 - E-mail: [email protected]. ... A new synthetic protocol was devised where poly(vinylpyrrolidone) (PVP), noble metal source (AgNO3), m...
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Morphology Design of IRMOF‑3 Crystal by Coordination Modulation Di Li,† Hai Wang,‡ Xin Zhang,*,† Hui Sun,† Xiaoping Dai,† Ying Yang,† Lei Ran,† Xinsong Li,† Xingyu Ma,† and Daowei Gao† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Fu Xue Road 18#, Changping, Beijing 102249, China National Institute of Metrology, Beijing 100013, China

‡

S Supporting Information *

ABSTRACT: A one-pot synthesis design on shape-controlled growth of Zn-based isoreticular metal−organic framework (i.e., IRMOF-3) was carried out in this work with the controllable crystal morphological evolution from simple cubes to several complex shapes. A new synthetic protocol was devised where poly(vinylpyrrolidone) (PVP), noble metal source (AgNO3), mixed solvents (N,N-dimethylformamide (DMF)−ethanol mixture) and tetramethylammonium bromide (TMAB) were mutually introduced to the reaction medium as surfactant, adjuvant, accelerator, and structure-directing agent (SDA), respectively. Meanwhile, the crystallization process was investigated by a series of time-dependent experiments. Indeed, the added modulators and crystallization time were able to regulate the growth and thus the morphology of the final products. The resulting homogeneous IRMOF-3-Ag-n materials with unique and novel crystal morphologies were characterized via scanning electron microscopy (SEM), X-ray powder diffraction (XRD), thermogravimetric and differential thermal analyses (TG-DTA), transmission electron microscopy (TEM), infrared spectrum (IR), and optical microscope characterizations. Various shapes of IRMOF-3-Ag-n crystals as sorbents for capturing dibenzothiophene (DBT) were evaluated. Among all the morphology-controlled samples, IRMOF-3-Ag-5 with hollow sphere morphology was demonstrated to show the highest DBT capture capacity due to its unique morphology.

1. INTRODUCTION Metal−organic frameworks (MOFs) are an important subclass of hybrid porous materials. Over the past decade, MOF materials have attracted numerous interests in both scientific research and industrial fields. Because of their diversity of frameworks, feasibility of postsynthetic modifications,1−4 large surface area, and exceptional thermal and chemical stability,5 MOFs have been attracting considerable interests in the application of gas sorption,6,7 separations,8 catalysis,9−11 drug delivery,12,13 optics,14,15 sensing,16,17 adsorbents,18 and fillers of hybrid membranes.19−21 Compared to the considerable works on the synthesis and their applications, morphology-controlled growth (or morphogenesis) of MOFs has attracted great interest. However, because of the lack of knowledge about their crystal growth mechanisms, the control of crystal morphology of MOFs still remains a great challenge. At the current stage of development, it is not an exaggeration to conclude that the morphology design of MOFs remains an art rather than a science. Despite these difficulties, pioneering works still have been carried out.22−36 For instance, Vivek Polshettiwar et al. reported the modulation of crystal size and shape under microwave irradiation and other synthesis methods.22,23 Umemura et al. designed the morphology-controlled synthesis of [Cu3(btc)2]n octahedron, cuboctahedron, and cubic microcrystals with a mean size of about 2 μm by changing the © XXXX American Chemical Society

concentration of modulator (n-dodecanoic acid or lauric acid).24 Cho et al. synthesized rod-, lump-, and disk-shaped porous coordination polymers (PCPs) in the presence of pyridine as blocking agent.25 To the best of our knowledge, the design of crystal morphology has enabled the development of MOFs without changing material compositions, material attributes, and oriented film formation.24 Moreover, MOFs have been used to remove thiophenic compounds from simulated oils recently, which represent a type of alternative adsorbents that can be potentially used in the cost-effective production of low-sulfur fuels by selective adsorption.37,38 Herein, we developed a one-pot synthesis morphology design of IRMOF-3-Ag-n crystals by the coordination modulation method and explored their growth mechanism. Synthetic parameters were variable including surfactant (PVP), noble metal source (AgNO3), mixed solvents (N,N-dimethylformamide (DMF)−ethanol mixture), structure-directing agent (SDA), which used tetramethylammonium bromide (TMAB), and crystallization time. In this study, IRMOF-3-Ag-n crystals were successfully synthesized under the same solvothermal Received: July 20, 2014 Revised: September 5, 2014

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Table 1. Preparative Parameters of IRMOF-3-Ag-n Crystals ID

sample

DMF (mL)

1 2 3 4 5 6 7 8 9 10 11 12 13

IRMOF-3-Ag-1 IRMOF-3-Ag-2 IRMOF-3-Ag-3 IRMOF-3-Ag-4 IRMOF-3-Ag-5 IRMOF-3-Ag-6 IRMOF-3-Ag-7 IRMOF-3-Ag-8 IRMOF-3-Ag-9 IRMOF-3-Ag-10 IRMOF-3-Ag-11 IRMOF-3-Ag-12 IRMOF-3-Ag-13

8 8 8 8 3 1 3 1 8 8 8 8 8

ethanol (mL)

5 7 5 7

PVP (mmol) 1.82 1.82 2.73 2.73 2.73 2.73 2.73 2.73 2.73 2.73 2.73 2.73 2.73

× × × × × × × × × × × × ×

10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2 10−2

AgNO3 (mmol) 0.044 0.014 0.044 0.014 0.044 0.044 0.014 0.014 0.044 0.044 0.044 0.044 0.044

TMAB (mmol)

morphologies

0.17 0.26 0.40 0.65 0.99

cube cuboctahedron truncated cube truncated octahedron hollow sphere core−shell sphere hollow chuzzle sphere flower-like sphere further truncated cube truncated octahedron further truncated octahedron octahedron column

Scheme 1. Strategies and Corresponding Models for Morphology-Controlled IRMOF-3-Ag-n Crystals by One-Pot Synthesis of Coordination Modulation Method

temperature of 100 °C for 18 h. After the reaction mixture was cooled to room temperature, the products were isolated by centrifugation and washed thoroughly thrice with DMF, ethanol, and chloroform, respectively. Finally, the products were dried in vacuum at 65 °C for 3 h for further characterizations. In parallel, a series of crystals with various morphologies were prepared by altering the preparation parameters such as PVP, AgNO3, TMAB, and DMF−ethanol mixed solvents. The corresponding samples were named as IRMOF-3-Ag-n (n refers to the number from 1 to 13). In short, the corresponding sample ID was denoted as n. The controllable synthesis parameters and morphologies of IRMOF-3-Ag-n were summarized in Table 1 and Scheme 1. The simulated oil was prepared using the representative sulfur contaminant of dibenzothiophene (DBT) and cyclohexane solvent, and the content of sulfur element in the simulated oil was 480 ppm. The desulfurization behavior using different morphologies of IRMOF3-Ag-n adsorbents was investigated and compared with IRMOF-3 framework. The static adsorption experiments were carried out at 298 K and atmospheric pressure using 40 mg crystals in 1 mL of simulated oil for an hour. 2.3. Characterizations. The morphology and size of the IRMOF3-Ag-n crystals were monitored using scanning electron microscopy (SEM, FEI Quanta 200F). The materials were also characterized via powder X-ray diffraction (XRD) on a Brü ker D8 Advance diffractometer at 40 kV and 40 mA for CuKα, with a scan speed of 10°/min and a step size of 0.02° in 2θ. Thermogravimetric and differential thermal analyses (TG-DTA) were measured on a Mettler Toledo TGA/SDTA 851 instrument in flowing argon (50 mL min−1) with a rate of 10 K min−1. Transmission electron microscopy (TEM) was used to determine the Ag particle sizes analyzed with JEM-2100. The crystal morphologies of the samples were also observed on a

synthesis conditions with a variety of shapes, including core− shell and hollow sphere, cube, octahedron, cuboctahedron, truncated cube, column, etc. Attempts have been made to remove the thiophenic sulfur compounds by using different morphologies of IRMOF-3-Ag-n adsorbents, where Ag nanoparticles (NPs) were dispersed on the surface of the framework39 and acted as a modulator to shape IRMOF-3Ag-n crystals.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, 98%) was purchased from Acros. 2-Aminoterephthalic acid (NH2−H2BDC, 99.0%), cyclohexane, and dibenzothiophene were purchased from Sigma-Aldrich. All other chemicals such as DMF, silver nitrate (AgNO3, 0.1028 mol/L), polyvinylpyrrolidone (PVP, M.W. 30000), tetramethylammonium bromide (TMAB, 98%), and ethanol were purchased from Sinopharm Chemical Reagent Co. Ltd. Solvents and all other chemicals were obtained from commercial sources and used without further purification. 2.2. Morphology-Controllable Synthesis of IRMOF-3-Ag-n Crystals. IRMOF-3-Ag-n crystals were solvothermally synthesized after a modified recipe. The reactants were mixed with different molar ratios of PVP, AgNO3, TMAB, and DMF−ethanol mixed solvents. Typically, the solution was prepared by dissolving Zn(NO3)2·6H2O (1.0 mmol, 299.2 mg), 2-aminoterephthalic acid (0.34 mmol, 60.8 mg), and a certain amount of PVP in 8 mL DMF−ethanol in a 25 mL autoclave. After dissolved thoroughly, a certain amount of AgNO3 solution was added into the autoclave with severe stirring for 30 min. Then, the autoclave was transferred to an oven with a constant B

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DV320 optical microscope. Infrared (IR) spectra were obtained on a Nicolet Nexus 870 FTIR spectrometer with a resolution of 1 cm−1. The surface areas were determined by the Brunauer−Emmett−Teller (BET) method with nitrogen adsorption at 77 K on a Quantachrome instrument. The inductively coupled plasma optical emission spectrometry (ICP-OES) of samples were performed on IRIS Intrepid II XSP (ThermoFisher) with its working parameters of 1150 W RF power, 1.0 LPM auxiliary gas, and 26.0 PSI Nebulizer flow. The sulfur contents were all examined on a Microcoulometric Synthesize Analyzer from Henan Shuangyang Co., Ltd.

(PVP), noble metal source (AgNO3), and other additives (Figure S1, Supporting Information). When AgNO3 was used individually, the morphology of crystals did not change, all of them were unperfected cubes; while the morphology of IRMOF-3 crystals changed via PVP added in individually, with a variety of irregular morphologies. Notably, the XRD patterns of the IRMOF-3-Ag-n (n = 1−4) samples exhibit identical structure with IRMOF-3 (Figure 2), indicating that

3. RESULTS AND DISCUSSION 3.1. Influence of Modulators on the Morphology of IRMOF-3-Ag-n Crystals. Because the early stage of crystallization influences the crystal growth habit, the process starting from nucleation to the production of primary crystals was examined in this study. In order to take the role of modulators into account, we proposed the cooperation routes of surfactant (PVP), noble metal source (AgNO3), mixed solvents (DMF−ethanol mixture), and structure-directing agent (TMAB) to tune the morphologies of IRMOF-3 crystal, for a perfect and regular morphology with higher uniform and better stability. Figure 1 displays representative SEM and optical microscopy images of the resulting various morphologies of IRMOF-3-Ag-n

Figure 2. XRD patterns of IRMOF-3-Ag-n synthesized with different molar ratios of PVP and AgNO3: IRMOF-3-Ag-1 (a), IRMOF-3-Ag-2 (b), IRMOF-3-Ag-3 (c), IRMOF-3-Ag-4 (d), and IRMOF-3 (e).

IRMOF-3-Ag-n (n = 1−4) crystals were successfully obtained in the presence of PVP and AgNO3. No significant diffraction patterns of Ag can be discerned, which is due to the very small Ag NPs and/or low Ag loadings. By the use of PVP as a modulator during nucleation and growth processes of IRMOF-3-Ag-n particles, the coordination between Zn2+-ligand was perturbed by the modulator; PVP could selectively adsorb on solid−liquid interface, altering the interfacial free energy of the system, leading to the anisotropic crystal growth24 and providing steric stabilization to the formation of surfactant-protected particles without aggregations.40 AgNO3 could play an auxiliary role to support PVP for additionally morphology control. The role of AgNO3 could probably induce PVP to chemisorb selectively on a certain fixed crystal face, which would adjust relative growth rates of the crystal face orientations and improve the regularity of the crystal morphology.41 In the process of morphology modulation, Ag+ might interact with the framework and surfactant simultaneously, forming a chemical interaction between MOF precursors and micelles. This interaction led to the positioning of the building blocks of IRMOF-3. The introduction of PVP and AgNO3 could generate a coordination modulation effect, which exhibited a structure-directing effect with relatively uniform sizes and shapes rather than the cubic-shaped crystals attained without PVP and AgNO3 under the typical synthesis conditions of IRMOF-3.42 The coordination modulation effect could play a directing role, making nucleation and crystal growth proceed in the continuous solvent phase between micelles and MOF precursors. The effect of mixed solvents of DMF and ethanol in this approach on morphology of IRMOF-3-Ag-n was further

Figure 1. SEM images of IRMOF-3-Ag-n synthesized with different molar ratios of PVP and AgNO3: IRMOF-3-Ag-1 (a), IRMOF-3-Ag-2 (b), IRMOF-3-Ag-3 (c), and IRMOF-3-Ag-4 (d). The right top inserts illustrate the structure models of corresponding IRMOF-3-Ag-n. The left bottom inserts are the optic microscope photographs of IRMOF-3Ag-n.

crystals by changing the amount of AgNO3 and PVP. When different amount of AgNO3 was added into the system, while keeping the amount of PVP (0.7284 g) constant, the cube (1) and cuboctahedron (2) crystals with an average size of ca. 50 μm were obtained (Figure 1a,b and Scheme 1). On the basis of synthesis conditions of cube (1) and cuboctahedron (2) crystals by solely increasing the amount of PVP to 1.0926 g, the truncated cube (3) and truncated octahedron (4) morphologies were generated, respectively (Figure 1c,d and Scheme 1). Nevertheless, only bulk cubic IRMOF-3 crystals with an average size of 200 μm were obtained without surfactant C

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explored. As shown in Figure 3 and Scheme 1, four kinds of quasi-spherical morphologies of IRMOF-3-Ag-n (n = 5−8)

Figure 4. XRD patterns of IRMOF-3-Ag-n synthesized with different molar ratios of PVP and AgNO3 with the mixture of DMF and ethanol as solvent: IRMOF-3-Ag-5 (a), IRMOF-3-Ag-6 (b), IRMOF-3-Ag-7 (c), IRMOF-3-Ag-8 (d), and IRMOF-3 (e).

to weaken the crystal crystallinity. Again there are no Ag diffraction patterns on all the samples. It was reported that the properties of mixed solvents had significant influence on the control of the nucleation and crystal growth rates through altering the exchange rate between metal ions and organic ligand. Therefore, ethanol as a counter solvent was added into the original solution to accelerate the IRMOF3-Ag-n’s nucleation.24 Moreover, the slight difference in the solubility profile of PVP in ethanol vs DMF potentially impacted how PVP interacts with different crystal surfaces.45 During this crystal growth process in mixed solvents, PVP might selectively adsorb on some specific crystal surfaces of the crystal nuclei and then could effectively decrease the surface energy of particles and induce particles epitaxy and assembly into spherical crystals.40 Thus, we may conclude that different solvents provide different growth environments for crystals, which would change the crystal growth habit and result in various morphologies. The second synthetic route was investigated by the addition of TMAB as a SDA while keeping other synthesis parameters identical with IRMOF-3-Ag-3. As shown in Figure 5 and Scheme 1, by increasing the TMAB amount of 0, 0.17, 0.26, 0.40, 0.65, and 0.99 mmol while keeping other synthetic conditions constant, the morphology of IRMOF-3-Ag-n crystals intriguingly changed from truncated cube (3) to further truncated cube (9), and then to truncated octahedron (10), further truncated octahedron (11), octahedron (12), and column (13), respectively. The XRD patterns of IRMOF-3-Agn (n = 9−13) as shown in Figure 6 were verified as identical structures to those of reported IRMOF-3 crystals, and no significant diffraction of Ag can be found. In this process, the amount of TMAB should be one of the key parameters for the controlling of the morphologies. Generally, SDAs often acted as counterions for charge balance, space-filling molecules, or “true” templates, in the case of welldefined host−guest interactions.46 On the basis of the above morphological transition phenomenon, we suspected that the amount of TMAB was sensitive for the growth kinetics in the one-pot synthesis process, most possibly acting as a charge balancing template.45 With the increasing amount of TMAB,

Figure 3. SEM images of IRMOF-3-Ag-n synthesized with different molar ratios of PVP and AgNO3 with the mixture of DMF and ethanol as solvent: IRMOF-3-Ag-5 (a), IRMOF-3-Ag-6 (b), IRMOF-3-Ag-7 (c), and IRMOF-3-Ag-8 (d). The right top inserts illustrate the structure models of the corresponding IRMOF-3-Ag-n. The left bottom inserts are the optic microscope photographs of IRMOF-3-Agn.

crystals were obtained when using the mixed solvents. On the basis of the synthesis conditions of IRMOF-3-Ag-3, and by introducing ethanol with the DMF−ethanol volume ratio of 3/ 5 (Table 1), the resulted product of IRMOF-3-Ag-5 exhibits hollow sphere with a very narrow size distribution of about 20 μm in diameter (Figure 3a and Figure S2f, Supporting Information). When the volume ratio of DMF−ethanol is tuned to 1/7, the product of IRMOF-3-Ag-6 has a core−shell structure with a smooth surface and a narrow size distribution of about 20 μm in diameter (Figure 3b and Figure S2g, Supporting Information). Similarly, on the basis of the synthesis conditions of IRMOF-3-Ag-4, only by controlling the DMF−ethanol volume ratio of 3/5 and 1/7, respectively, the hollow chuzzle sphere with the mean size of about 5 μm (7) and flower-like sphere crystals (8) with an average size of about 25 μm were observed (Figure 3c,d). Discrepancies in peak intensities of IRMOF-3-Ag-n (n = 5−8) crystals (Figure 4) arise from preferential orientation because we avoided grinding the samples so as to preserve their structural integrity.43 Apparently, IRMOF-3-Ag-6 and IRMOF-3-Ag-8 samples (Figure 4b,d) missed the key peak at 6.92° and showed a new broad peak located at 10.84° in the diffractogram, probably due to the disruption of periodicity induced by solvent molecules and/or Ag NPs that fill in the pores of IRMOF-3.44 However, there is another possibility: an excess amount of ethanol solvent could provide different environments for crystals and change their growth habit; so IRMOF-3-Ag-6 and IRMOF-3-Ag-8 samples might be infinite coordination polymers, the structure of which is amorphous and has the advantages in achieving shape and size control while keeping the inorganic−organic backbone.43 In a word, the addition of ethanol solvent tended to develop spherical morphologies and D

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illustrated in Figure 7. The Ag NPs distributing on IRMOF3-Ag-n (n = 1−9) crystals are spherical-like morphologies. The average sizes of Ag NPs on all IRMOF-3-Ag-n (n = 1−9) samples are extremely small with average sizes of 2.0−3.9 nm, which agree well with the XRD results that no significant diffraction of Ag crystalline was detected. To characterize the thermal stability of the crystals, TG-DTA experiments have been carried out on IRMOF-3-Ag-4, IRMOF3-Ag-6, and IRMOF-3-Ag-13 samples prepared under the above three typical syntheses conditions as shown in Figure 8. IRMOF-3 was used as a reference. It was shown that IRMOF-3Ag-4 and IRMOF-3-Ag-6 featured a similar thermal stability to IRMOF-3 and that their weight loss could be divided into three stages with 40−241, 241−390, and 390−526 °C. The first stage of weight loss was due to the release of physically adsorbed solvent molecules (such as CHCl3, water, ethanol, and DMF), which was accompanied by an endothermic peak centered at ca. 100 °C in the DTA curve. The second stage ranging from 241 to 390 °C was ascribed to the further release of solvent physically absorbed within pores or on the surface of unreacted ligands and surfactant/template. The third stage of weight loss over temperature higher than 390 °C was due to the decomposition of crystal structure, which was accompanied by a broad exothermic peak centered at ca. 500 °C in the DTA curve. Notably, the TG curve of IRMOF-3-Ag-13 showed continual weight loss, and the structure decomposition temperature was ca. 314 °C. It is evident that the thermal stability of IRMOF-3-Ag-4 and IRMOF-3-Ag-6 samples is much higher than that of IRMOF-3-Ag-13 and IRMOF-3. IR spectroscopy was further used to characterize functional groups in IRMOF-3-Ag-4, IRMOF-3-Ag-6, IRMOF-3-Ag-13, and IRMOF-3 samples. For example, as revealed in Figure 9, the IR spectrum of IRMOF-3-Ag-13 is nearly the same as that of IRMOF-3 crystals, confirming that the IRMOF-3 structure was successfully produced. Both samples exhibit the characteristic two bands centered at 3473, 3356 cm−1 of IRMOF-3 and 3450, 3346 cm−1 of IRMOF-3-Ag-13, assigning to the existence of the amino.47 It seems that the N−H stretching on IRMOF3-Ag-13 is the same as IRMOF-3. This result could reveal that the NH2 group in IRMOF-3-Ag-13 is not transformed and that the surfactant (PVP) should be encapsulated on the surface or pores. The bands at 2973 and 2926 cm−1 can be assigned to the vibrations of C−H originated from aromatic and/or formic ligand, this formic may be generated by decomposition of DMF. For both samples, the two sharp bands at 1574 and 1382 cm−1 correspond to asymmetric (υs(C−O)) and symmetric (υs(C−O)) vibrations of carboxyl groups, respectively.48 Moreover, the bands centered at 1660, 1501, and 1432 cm−1 are ascribed to CC stretching vibration of the aromatic, and the band at 1253 cm−1 can be assigned to C−N vibrations for both samples.49−51 The N2 adsorption−desorption isotherm curves of the representative samples of IRMOF-3-Ag-4, IRMOF-3-Ag-6, and IRMOF-3-Ag-13 have been tested, and the results are not attractive as expected, demonstrating really low BET surface areas of 65.62, 26.33, and 14.75 m2/g, respectively. The much lower surface areas compared with IRMOF-352 (750 m2/ g) should be attributed to the closed porosity, which was caused by the difficulty of cleaning off residue PVP filled in the pores of IRMOF-3-Ag-n.53−55 3.2. Growth Mechanism of Various Morphologies of IRMOF-3-Ag-n Crystals. To further gain an insight into the evolution process of IRMOF-3-Ag-n morphologies, we carried

Figure 5. SEM images of IRMOF-3-Ag-n synthesized with different amounts of TMAB when other conditions are identical: IRMOF-3-Ag3 (a), IRMOF-3-Ag-9 (b), IRMOF-3-Ag-10 (c), IRMOF-3-Ag-11 (d), IRMOF-3-Ag-12 (e), and IRMOF-3-Ag-13 (f). The right top inserts illustrate the structure models of corresponding IRMOF-3-Ag-n. The left bottom inserts are the optic microscope photographs of IRMOF-3Ag-n.

Figure 6. XRD patterns of IRMOF-3-Ag-n synthesized with different amounts of TMAB when the amounts of PVP, AgNO3, and DMF are identical: IRMOF-3-Ag-9 (a), IRMOF-3-Ag-10 (b), IRMOF-3-Ag-11 (c), IRMOF-3-Ag-12 (d), IRMOF-3-Ag-13 (e), and IRMOF-3 (f).

the charge density it provided increased accordingly, so as to balance the charges of the IRMOF-3-Ag-n (n = 9−13) framework to realize the morphological transition process. To analyze the size of Ag NPs, the representative TEM images of some IRMOF-3-Ag-n (n = 1−9) crystals are E

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Figure 7. TEM images of IRMOF-3-Ag-n crystals: IRMOF-3-Ag-1 (a), IRMOF-3-Ag-2 (b), IRMOF-3-Ag-3 (c), IRMOF-3-Ag-4 (d), IRMOF-3-Ag5 (e), IRMOF-3-Ag-6 (f), IRMOF-3-Ag-7 (g), IRMOF-3-Ag-8 (h), and IRMOF-3-Ag-9 (i). The insert in every image is the size distribution of Ag NPs distributed in the corresponding image.

Figure 8. TG-DTA curves of IRMOF-3-Ag-4 (a), IRMOF-3-Ag-6 (b), IRMOF-3-Ag-13 (c), and IRMOF-3 (d). Figure 9. IR spectra of IRMOF-3-Ag-4 (a), IRMOF-3-Ag-6 (b), IRMOF-3-Ag-13 (c), and IRMOF-3 (d).

out a series of time-dependent experiments and monitored the solvothermal process by SEM images. In terms of truncated octahedron (4) crystals displayed in Figure 10a−d, when the reaction time was shortened to 6 h, small uniform cutting angles of octahedral crystals were obtained with an average size of 10 μm. As the reaction time was extended to 8 or 10 h, it was found that the cube surface {100} facet became larger and larger. After further prolonging the crystallization duration time

to 18 h, no significant change in size was observed, but perfect truncated octahedron morphology was obtained. Similarly, the spherical and column structures of IRMOF-3-Ag-6 and IRMOF-3-Ag-13 have been formed early by 2 and 6 h, respectively, and the sizes became larger with prolonging the reaction time. F

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Figure 10. SEM images of IRMOF-3-Ag-4 (a, b, c, d), IRMOF-3-Ag-6 (e, f, g, h), and IRMOF-3-Ag-13 (i, j, k, l) synthesized with different crystallization duration times of 2 (e), 4 (f), 6 (a, i), 8 (b, j), 10 (c, g, k), and 18 h (d, h, l).

from 8 to 20 μm. Moreover, the representative SEM images of IRMOF-3-Ag-13 crystals were tiny column averaged crystals of 5.0 and 15 μm at 6 and 8 h, respectively. When the reaction time was extended to 10 h, the average size of column (13) crystals significantly turned to ca. 100 μm with no obvious changes in morphology. As the reaction proceeded, for the sample at 18 h, the average size of crystals increased a lot, and well-defined column shape was obtained. In sum, the process starting from nucleation to the production of primary crystals was totally examined, the most reasonable prediction for the influence of modulators may be that surfactant (PVP) was the surface-active molecule that adsorbed at the intermediate morphologies, altering the interfacial free energy of the system,59 so as to determine the thermodynamic stability of growth facets and their relative growth rates, then a morphological transition was achieved. Moreover, mixed solvents mainly focused on the nucleation rate at the early stage of crystallization through accelerating the nucleation and preventing the crystal growth in the original solution as a counter solvent.24 TMAB was used probably to balance the charges of IRMOF-3-Ag-n framework, and so long as there were sufficient nutrients in the reaction solution, the growth could be continued with the increasing of the reaction time to produce various crystal shapes with edge lengths up to several hundred micrometers.60 Consequently, taking into account of all these modulator effects above, controllable crystal morphological evolution from simple cubes to a series of complex shapes has been achieved via coordination modulation in the one-pot synthesis method. In other words, the habit of a crystal can change if there is a mix of different facets on the surface during crystal growth. In fact, crystal growth is a very dynamic process! Continued crystal growth will result in an

On the basis of this morphology evolution process, we proposed that the Bravais, Friedel, Donnay, and Harker (BFDH56) law may be responsible for the formation of these truncated octahedron morphologies. It has been well established that crystal shapes can be determined under the fundamentals of BFDH law.56 A crystal shape was arranged by the coexistence of slower and faster growth facets. As the crystal grew, the crystal gradually formed the structure surrounded by the facets of slower growth. In the case of fcc nanocrystals, their final morphology was often determined by the consequence of competitive growth of {111} and {100} faces.45 Therefore, in the IRMOF-3-Ag-n system, we assumed that the truncated octahedron morphology was determined by the growth rate on {100} and {111} facets and that the growth rate on each facet was assumed to be constant throughout the growth according to BFDH law. As we know that PVP has a “magic” power to bind selectively to the Ag{100} surface due to their good match in geometric structures,57,58 the reason for morphological transition processes from 6 to 18 h might be that AgNO3 played a role in inducing PVP to chemisorb on the crystal’s {100} facet so as to inhibit the crystal from growing on the {100} facet, which resulted in the crystal gradually exposing the {100} facets. Additionally, to probe the intrinsic factors influencing the formation of IRMOF-3-Ag-n crystals, a further series of timedependent experiments were carried out on the core−shell sphere (6) crystal (Figure 10e−h) and IRMOF-3-Ag-13 (Figure 10i−l). The core−shell sphere (6) crystals were periodically collected from the reaction solution after 2, 4, 10, and 18 h for comparison. After increasing the crystallization duration time from 2 to 18 h, no obvious change in morphology was observed, only with its average size increased G

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4. CONCLUSIONS In summary, a series of IRMOF-3-Ag-n crystals with various morphologies were designed by a facial one-pot coordination modulation approach, and the possible role of modulators in crystal growth mechanism was investigated. This work extends the successful controlling of the morphologies of IRMOF-3-Agn crystals from simple cubes to a variety of novel shapes correlated to an increase in the amount of modulators (modulators = PVP, AgNO3, ethanol, and TMAB). Additionally, the influences of modulators were elucidated in this system in altering the morphology of IRMOF-3-Ag-n crystals both in thermodynamic equilibrium and their reaction kinetics. This comprehensive study will enable us to design and produce welldefined IRMOF-3-Ag-n crystal morphologies in a similar manner controlled by the coordination modulation method. On the basis of these results, a possible growth mechanism of IRMOF-3-Ag-n crystals has been proposed through the detailed time-dependent evolution experiments. IRMOF-3-Ag-n crystals with different morphologies were used as adsorbents to remove the thiophenic compounds, and IRMOF-3-Ag-5 with hollow sphere structure show greatly enhanced sulfur uptake capacity compared with IRMOF-3.

elongation of the slow growing edges at the expense of the faster growing ones. Such a dynamic evolution can occur to selectively enlarge one set of crystallographic facets at the expense of others on a MOF crystal, yielding new shapes.61 3.3. Adsorption Desulfurization with Various Morphologies of IRMOF-3-Ag-n Adsorbents. As our preliminary effort in application of these micrometer crystals, adsorption desulfurization measurement was carried out via a series of different IRMOF-3-Ag-n crystals and the original IRMOF-3 framework. Because of their good thermal stability and well-defined structures, they may be favorable for the selective capture of thiophenic compounds. As shown in Figure 11, it was found that the sulfur uptake capacity of IRMOF-3-

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ASSOCIATED CONTENT

S Supporting Information *

SEM images and optical micrograph of IRMOF-3-Ag-n crystals with various morphologies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: (+86)10-8973-4979. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 11. Sulfur adsorption capacity of a series of IRMOF-3-Ag-n crystals with different morphologies.

ACKNOWLEDGMENTS This work were supported by the National Natural Science Foundation of China (No. 21173269 and 91127040), Ministry of Science and Technology of China (No. 2011BAK15B05), Specialized Research Fund for the Doctoral Program of Higher Education (No. 20130007110003), and the open fund of the State Key Laboratory of Chemical Resource Engineering.

Ag-n crystals with various morphologies were significantly increased in comparison with the original IRMOF-3 framework, probably due to π-complexation between DBT and Ag NPs.39 Specifically, IRMOF-3-Ag-5 with hollow sphere structure featured almost twice the sulfur uptake capacity of IRMOF-3. Various IRMOF-3-Ag-n crystals have different capacities of adsorption desulfurization, which was mainly due to the morphology effect and the properties of Ag NPs (e.g., size and loadings). The introducing of a very small amount of Ag in IRMOF-3Ag-n crystals in a one-pot synthesis, which can significantly improve the sulfur uptake capacity, is highly interesting, and we have investigated the amount of Ag in IRMOF-3-Ag-n (n = 3, 4, 5, 7) samples determined by ICP-OES. It is calculated that the percentage contents of Ag in IRMOF-3-Ag-3 and IRMOF3-Ag-5 are 0.019 and 0.023 wt %, which were much higher than that of 0.012 and 0.010 wt % in IRMOF-3-Ag-4 and IRMOF-3Ag-7, respectively. Despite the morphologies of IRMOF-3-Ag-n crystals, higher loading amounts of Ag results in the better desulfurization capacity of IRMOF-3-Ag-3 and IRMOF-3-Ag-5 than that of IRMOF-3-Ag-4 and IRMOF-3-Ag-7. However, the sulfur uptake capacity of IRMOF-3-Ag-5 (IRMOF-3-Ag-3) was significantly increased in comparison with the IRMOF-3-Ag-7 (IRMOF-3-Ag-4) at the same level of Ag content, which is due to the morphology effect of various IRMOF-3-Ag-n crystals.

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dx.doi.org/10.1021/cg501089f | Cryst. Growth Des. XXXX, XXX, XXX−XXX