Insight and Control of the Crystal Growth of Zeolitic ... - ACS Publications

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Insight and control of the crystal growth of zeolitic imidazolate framework ZIF-67 by atomic force microscopy and mass spectrometry Raghidah Wagia, Ilya Strashnov, Michael W. Anderson, and Martin P. Attfield Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01058 • Publication Date (Web): 28 Dec 2017 Downloaded from http://pubs.acs.org on December 31, 2017

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Crystal Growth & Design

Insight and control of the crystal growth of zeolitic imidazolate framework ZIF-67 by atomic force microscopy and mass spectrometry Raghidah Wagia, # § Ilya Strashnov, § Michael W. Anderson, # § Martin P. Attfield * # § #

Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Brunswick Street, Manchester, M13 9PL, United Kingdom.

§

School of Chemistry, The University of Manchester, Brunswick Street, Manchester, M13 9PL, United Kingdom. ABSTRACT: Combination of electrospray ionization mass spectrometry (ESI-MS) and in situ atomic force microscopy (AFM) are applied to provide the first nanoscopic study of the crystal growth of zeolitic imidazolate framework ZIF-67. ZIF-67 is found to form through a process of nucleation and spreading of meta-stable unenclosed sub-steps to form stable surface steps of the enclosed framework structure and demonstrates that isostructural MOFs, ZIF-67 and ZIF-8, undergo identical crystal growth mechanisms. The information on the crystal growth species obtained from the AFM experiments correlates well with the solution species identified by ESI-MS indicating that the species involved in the growth under low supersaturation conditions are methylimidazole/ methylimidazolate, monomeric non-methylimidazole/ methylimidazolate complexed Co2+ ions and monomeric complexed [Co(methylimidazole/ methylimidazolate)1 - 2] ions. Combination of the use of low supersaturation growth solutions and in situ AFM has also allowed the successful extraction of the synthetic conditions necessary for formation of ZIF-67 nanodots that possesses a maximum vertical dimension of 1.2 nm which is the smallest dimension reported for a stable ZIF-67 entity. This methodology may be expanded to understand the formation of, and to form, complex crystal forms of other MOFs for new or improved functionality.

*Dr Martin P. Attfield Centre for Nanoporous Materials, School of Chemistry, The University of Manchester, Brunswick Street, Manchester, M13 9PL, United Kingdom. Tel: +44-161-306-4467 Fax: +44-161-275-4598 Email: [email protected] Web page: http://www.chemistry.manchester.ac.uk/people/staff/profile/?ea=M.Attfield

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Insight and control of the crystal growth of zeolitic imidazolate framework ZIF-67 by atomic force microscopy and mass spectrometry Raghidah Wagia, # § Ilya Strashnov, § Michael W. Anderson, # § Martin P. Attfield * # § #

Centre for Nanoporous Materials, School of Chemistry, The University of Manchester,

Brunswick Street, Manchester, M13 9PL, United Kingdom. §

School of Chemistry, The University of Manchester, Brunswick Street, Manchester, M13 9PL,

United Kingdom. *E-mail: [email protected] KEYWORDS Crystal growth: Metal-organic framework: ZIF-67: Nanostructures: Core-shell structures: Mass spectrometry: Atomic force microscopy; MOFs.


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ABSTRACT Combination of electrospray ionization mass spectrometry (ESI-MS) and in situ atomic force microscopy (AFM) are applied to provide the first nanoscopic study of the crystal growth of zeolitic imidazolate framework ZIF-67. ZIF-67 is found to form through a process of nucleation and spreading of meta-stable unenclosed sub-steps to form stable surface steps of the enclosed framework structure and demonstrates that isostructural MOFs, ZIF-67 and ZIF-8, undergo identical crystal growth mechanisms. The information on the crystal growth species obtained from the AFM experiments correlates well with the solution species identified by ESIMS indicating that the species involved in the growth under low supersaturation conditions are methylimidazole/ methylimidazolate, monomeric non-methylimidazole/ methylimidazolate complexed Co2+ ions and monomeric complexed [Co(methylimidazole/ methylimidazolate)1 - 2] ions. Combination of the use of low supersaturation growth solutions and in situ AFM has also allowed the successful extraction of the synthetic conditions necessary for formation of ZIF-67 nanodots that possesses a maximum vertical dimension of 1.2 nm which is the smallest dimension reported for a stable ZIF-67 entity. This methodology may be expanded to understand the formation of, and to form, complex crystal forms of other MOFs for new or improved functionality.


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INTRODUCTION The metal-organic framework cobalt 2-methylimidazolate [Co(C4H5N2)2 or Co(MeIm)2 where HMeIm 1

–

2-methylimidazole]

zeolitic

imidazolate

framework

ZIF-67

is one of the archetypal zeolitic imidazole frameworks (ZIFs). ZIF-67 is constructed from

corner-sharing Co(MeIm)4 tetrahedral units in which the MeIm- ligands bridge the Co2+ ions to form a three-dimensional framework with the sodalite framework topology (SOD) (space group I-43m, a = 16.9589Å) and a pore size of 3.4 Å (see Figure S1 in the Supporting Information).1 The attractive properties of ZIF-67, including its high stability, have accelerated its introduction in a wide variety of potential applications on its own2,

3, 4, 5

or, often with the structurally

isomorphous material ZIF-8 [Zn(MeIm)2], as a composite component.6,

7

Indeed, mixed-

component metal-organic frameworks (MC-MOFs)8 based upon SOD topology ZIFs containing different metal ions, or organic linkers in some region of the crystalline assembly have been successfully synthesised and their properties reported. Such crystalline assemblies include formation of isomorphously substituted metal ion [Zn/Co(MeIm)2] (ZIF-8/ 67),9, 10,

11

formation

of isomorphously substituted organic linker [Co(MeIm/BIm)2] (ZIF-67/ 9 where HBIm – benzimidazole),12 growth of ZIF-8 polycrystalline layers on ZIF-67 polycrystalline layers and vice versa,6 growth of ZIF-8 single crystalline shells on ZIF-67 single crystals and vice versa,13, 14, 15

and formation of hollow isomorphously substituted metal ion [Zn/Co(MeIm)2] (ZIF-8/ 67)

shells.14,

15

The performance of these MC-MOFs materials for particular applications is often

better than that of the pure MOF components. 6, 11 Development of synthetic routes to produce MOFs and MC-MOFs with specific crystal properties is a necessity for enhanced application or producing new applications of MOFs.16,

17

Observation and gaining understanding of the crystallization of MOFs at the nanoscale will aid


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the design of such syntheses18 but currently relatively little has been reported in this area, particularly relating to the identification of the solution species that can be correlated directly with the real-time crystallization processes of the MOF.19 In a previous study, we reported the use of in situ atomic force microscopy (AFM) to conduct the first detailed nanoscopic study on the crystal growth of ZIF-8 and provided insights on its crystal growth mechanisms and 2-dimensional nucleation.20 In this work, we use a combination of electrospray ionization mass spectrometry (ESI-MS) and in situ AFM to provide the first detailed nanoscopic study of the crystal growth of the structurally related, but chemically different, ZIF-67 including identification of the solution species involved in the crystal growth phase of this material. The combination of the use of low supersaturation growth solutions and in situ AFM are also shown to enable the formation and observation of ZIF-67 nanodots on ZIF-8 core crystals. The presented ZIF-67 nanodots possess a maximum vertical dimension of 1.2 nm that is the smallest dimension reported for a stable ZIF-67 entity and represents two thirds of the height of the constituent sodalite cages of ZIF-67. EXPERIMENTAL SECTION The ZIF-67 substrate crystals used in the AFM experiments were synthesized based upon the procedure reported by Banerjee et al.1 Co(NO3)2.6H2O (≥98% Sigma Aldrich) (5.343g, 18.35 mmol) and HMeIm (99% Sigma Aldrich) (2.985g, 36.4 mmol) were placed in a 60 ml glass reagent bottle and dissolved in anhydrous N,N-dimethylformamide (DMF) (99.8% Sigma Aldrich) (45 ml, 581 mmol) with the aid of stirring. The molar composition of the synthesis solution was: 1 Co: 2 HMeIm: 32 DMF. A glass cover slide was then put into the solution and the reagent bottle was capped tightly before it was heated in an oven at 120°C for three days. After heating, the glass cover slide with the attached substrate purple ZIF-67 crystals was


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removed from the solution and dipped several times in methanol to wash away any remaining growth solution from the surface of crystals. The ZIF-8 substrate crystals used in the AFM experiments were synthesized based upon the procedure reported by Park et al.21 Zn(NO3)2·4H2O (3.92 g, 15 mmol) (≥98.5% Merek) and HMeIm (99% Sigma Aldrich) (2.46 g, 30 mmol) were placed in a 120 ml glass reagent bottle and dissolved in DMF (99.8% Sigma Aldrich) (50 ml, 646 mmol) with the aid of stirring. The molar composition of the synthesis solution was: 1 Zn: 2 HMeIm: 43.1 DMF. A glass cover slide was then put into the solution and the reagent bottle was capped tightly before it was heated in an oven at 100°C for two days. The majority of the mother solution was decanted and replaced with methanol (≥99.9% Sigma Aldrich) (50 ml, 1236 mmol) after heating. The cover slide-attached ZIF-8 substrate crystals were left in this solution for one day at ambient temperature to allow further slow growth of the ZIF-8 crystals.20 After this treatment, the glass cover slide with the attached substrate ZIF-8 crystals was removed and dipped in methanol several times to wash away any remaining growth solution from the surface of crystals. In situ growth experiments were performed by gluing a seed crystal covered slide into a home-made open fluid cell which was then attached to the AFM (Nanowizard II, JPK Instruments AG). Only crystals with facets approximately parallel to the glass slide surface were scanned to minimize artifacts derived from the non-linearity of the piezo displacement on the zaxis. Two types of static growth solution were placed over the substrate ZIF-67 and ZIF-8 crystals during the in situ AFM experiments. The first growth solution was prepared by dissolving Co(NO3)2.6H2O (1.781 g, 6.12 mmol) and HMeIm (0.995 g, 12.1 mmol) in methanol (15 ml, 371 mmol) contained within a 20 ml scintillation vial. The molar composition of this


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solution was: 1 Co: 2 HMeIm: 61 MeOH. The mixture was stirred for an hour at ambient temperature in which time some polycrystalline ZIF-67 began to form. 1 ml of crystal-free solution was taken and mixed with a further 49 ml of MeOH resulting in growth solution A with a molar composition of 1 Co: 2 HMeIm: >3029 MeOH. The second growth solution, solution B, was prepared in the same manner as solution A except that an additional 50 ml of MeOH was added in the last stage resulting in growth solution B having a molar composition of 1 Co: 2 HMeIm: >6059 MeOH. All AFM experiments were conducted in contact mode using silicon nitride rectangularshaped cantilevers with a radius of curvature of the tip

Insight and Control of the Crystal Growth of Zeolitic ... - ACS Publications

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